Nitrene-Transfer Chemistry to C–H and C═C Bonds Mediated by Triangular Coinage Metal Platforms Supported by Triply Bridging Pnictogen Elements Sb(III) and Bi(III)

Abstract

Tripodal ligands (TMG₃trphen-E) that feature heavy pnictogen elements (E = Sb(III), Bi(III)) and tetramethylguanidinyl (TMG) arms have been explored in stabilizing Cu(I) and Ag(I) sites and facilitating nitrene-transfer chemistry. Compounds [(TMG₃trphen-E)M₃(μ-X)₃] (M = Cu(I), Ag(I); X = Cl, Br, I) have been generated upon extraction of M₃(μ-X)₃ units from MX sources, exhibiting support of the crown-shaped M₃(μ-X)₃ fragment by M–N_TMG bonds and triply bridging E → M₃ interactions. Orbital interactions between Cu(I) sites and N_TMG residues are more dominant than Sb/Bi → Cu₃ donor interactions between the Sb 5s or Bi 6s orbitals and admixed Cu 4s/3d orbitals, with larger interaction energies computed for Sb → Cu₃. Nonhalogenated copper compounds [(TMG₃trphen-E)₂Cu₂]²⁺2Y⁻ (Y = PF₆, B(C₆F₅)₄) have been synthesized via dechlorination by TlPF₆ or by application of halide-free Cu(I) sources with TMG₃trphen-E ligands. Nitrene-transfer to olefins mediated by [(TMG₃trphen-E)Cu₃(μ-Cl)₃] (E = Sb and Bi) affords aziridines in good yields, primarily for unencumbered styrenes and with the more robust Sb catalyst. Amination of C–H bonds is most effective with sec-benzylic substrates and requires a more electrophilic nitrene (NTces) to achieve practicable yields with halogenated or nonhalogenated copper precursors. Hammett plots indicate that the competitive amination of para-substituted ethylbenzenes enabled by [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] involves stepwise C–H functionalization.

Graphical Abstract

INTRODUCTION

The development of C–N bond construction methodologies encompasses various pathways directed toward inserting nitrogen functionalities into carbonaceous feedstock for the synthesis of a diverse body of commodity and high-value chemicals (pharmaceuticals, agrochemicals, polymers, semiconductors, catalysts, solvents, and household chemicals).¹ Among different approaches, the insertion of nitrenes (NR) or nitrenoids (NR(X)) into C–H bonds or their addition to C═C bonds affords valuable products of amination and aziridination, respectively, as well as derivatives (such as amidines or five-membered N-heterocycles) in the presence of additional substrates (usually unsaturated entities).² This direct functionalization of C–H/C═C feedstock belongs to the general category of atom/group-transfer chemistry,³ pertaining to a wide variety of common atoms (e.g., H/D, N, O, S, and halogen) or groups (e.g., BR₂, CR₂/CR₃, NR/NR₂, and N₃).^4–12 Biological atom/group-transfer analogs have frequently provided inspiration and opportunities for further development via biomimetic approaches and enzyme engineering.¹³ The direct C–H/C═C functionalization obviates the need for constructing energetic C–X precursors (X = leaving or directing group) but raises challenges for achieving acceptable levels of reactivity and selectivity.¹⁴ Taking C–N bond construction from C–H/C═C feedstock as an example, both metal-free¹⁵ and metal-dependent catalytic systems^2,16 have been advanced to overcome these challenges. Notwithstanding the expense and toxicity involved, platinum-group metal catalysts have been frequently invoked,¹⁷ but nonprecious, first-row transition metal reagents¹⁸ have found increasing use. The latter usually raise the possibility of single-electron steps that provide avenues to radical pathways, which may thus offer intriguing alternatives to closed-shell two-electron processes.

In recent work, in our laboratory, we have developed a family of divalent base metals (Mn, Fe, Co, and Ni) and monovalent coinage metal catalysts (Cu and Ag), supported by tripodal trisamido/imino-amine and bipodal bisamido/imino-amine ligands (Chart 1).¹⁹ The weak ligand field and the ability to decorate the equatorial amido/imino residues by a wide range of aryl, acyl, alkyl, and guanidinyl arms has given rise to a plethora of anionic, neutral, and cationic catalysts employed in nitrene transfer to C–H and C═C bonds of hydrocarbons. These tripodal and bipodal ligand scaffolds are analogs of the iconic N(CH₂CH₂NH₂)₃ (TREN) and bipodal N(CH₂CH₂NH₂)₂Me frameworks^20–22 but rely on phenylene rather than ethylene linkers, resulting in metal reagents that tend to be more reactive in atom/group-transfer chemistry. Versions of the anionic tripodal metal compounds have also been nicely explored in bimetallic synthesis and catalysis.²³ In general, anionic versions tend to be more appropriate for aziridination reactions,²⁴ whereas the much more reactive cationic analogs, especially those decorated with tetramethylated guanidinyl arms (extensively studied in TREN chemistry),^25,26 can be effectively employed both in C–H aminations/amidinations (Cu reagents)²⁷ and C═C aziridinations, in tandem with five-membered N-heterocycle synthesis (Mn, Fe, or Co reagents).²⁸

Chart 1.

Previously Explored Reagents Supported by Axial N_amine Coordination

In all cases noted above, the axial coordination site is occupied by an N_amine atom. With the present work, we are launching an effort toward replacing the apical N_amine residue with more weakly electron-donating and potentially electron-accepting Sb(III) and Bi(III) residues.²⁹ It is anticipated³⁰ that electron-deficient apical elements can enhance the electro-philicity of any axially positioned metal-nitrene moiety and, therefore, increase reactivity for challenging C–H substrates. Of course, the pnictogen elements generate significantly more sizable ligand cavities than those examined with the tripodal [N₃N_amine] scaffolds noted above. Hence, in this publication, we aspire to explore the geometric and electronic characteristics of metal reagents that can be accessed with these axial Sb/Bi containing ligands and provide an initial account of their nitrene-transfer reactivity. We chose to first explore Cu(I) and to a lesser extent Ag(I) sites in tandem with equatorial tetramethylguanidinyl (TMG) residues, given our successful application of the analogous [(N₃N_amine)Cu^I]⁺ reagent in the past.²⁷

RESULTS AND DISCUSSION

Synthesis of Ligands.

The tripodal ligands TMG₃trphen-Sb (1) and TMG₃trphen-Bi (2) used in this study are synthesized by a concise method (Scheme 1). First, the trimethylguanidinyl groups (TMG) are installed by coupling 2-bromoaniline and chlorotetramethylformamidinium chloride (prepared from tetramethylurea and oxalyl chloride) in the presence of triethylamine in acetonitrile, followed by low-temperature lithiation with sec-BuLi (or tert-BuLi) in diethyl ether (Sb) or tetrahydrofuran (Bi). The electrophiles SbCl₃ and BiCl₃ are then used in THF (0.33 equiv versus the lithiated product) to install the axial Sb(III) and Bi(III) elements, respectively. Both ligands can be crystallized from dichloromethane to provide pure products as white solids in moderate to good yields. The lower yields observed for 2 reflect the thermal instability of the Bi–C bond. ¹H NMR spectra (in CD₃CN) indicate that all aryl and methyl groups (NMe₂; sharp single peak at δ = 2.57 (1), 2.58 (2)) are equivalent at 298 K, hence no significant rotational restrictions are observed at room temperature.

Scheme 1.

Synthesis of Ligands Employed in this Study

Synthesis of Metal Halide Compounds.

The reaction of ligand 1 with 3 equiv of anhydrous CuCl of high purity (beads, ≥99.99%) in THF provides a white solid, which can be dissolved in dichloromethane and obtained as colorless crystals of [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃]•4CH₂Cl₂ (3) from concentrated solutions (Scheme 2). The analogous reaction of ligand 2 with CuCl in THF affords the corresponding [(TMG₃trphen-Bi)Cu₃(μ-Cl)₃]•4CH₂Cl₂ (4), which can be crystallized from dichloromethane as off-white crystals suitable for X-ray analysis. The corresponding bromo and iodo analogs of 3 can be obtained in a similar manner from the reaction of anhydrous CuX (X = Br, I) with ligand 1 in THF, followed by crystallization from dichloromethane to obtain colorless crystals of [(TMG₃trphen-Sb)Cu₃(μ-Br)₃]•4CH₂Cl₂ (5) and [(TMG₃trphen-Sb)Cu₃(μ-I)₃]•3CH₂Cl₂ (6), respectively.

Scheme 2.

Synthesis of Metal Halide Compounds

A similar trinuclear Ag₃(μ-Cl)₃ cluster can also be extracted by ligand 1 from anhydrous AgCl (beads, 99.998%) upon prolonged extraction times and crystallized from CH₂Cl₂ as off-white crystals of [(TMG₃trphen-Sb)Ag₃(μ-Cl)₃]•3CH₂Cl₂ (7). The same reaction with ligand 2 was not productive even after several weeks of extraction at room temperature or upon heating.

Synthesis of Halide-Free Metal Compounds.

For comparative purposes, the coordination chemistry of the stibine (1) and bismuthine (2) ligand was examined with respect to Cu(I) sites in the absence of any halides. The reaction of either 1 or 2 with 1.5 equiv of [Cu(NCMe)₄](PF₆) in THF/MeCN (Scheme 3) affords colorless crystals of [(TMG₃trphen-E)₂Cu₂](PF₆)₂•2 solv (E = Sb, solv = THF (8a); Bi, solv = Et₂O (9)), upon crystallization from THF/MeCN (8a) or MeCN/Et₂O (9). The Bi compound is only obtained in very low yields, most likely because of the thermal instability of the ligand, as noted above. Incidentally, the corresponding 1:1 ligand/Cu reaction affords intractable mixtures. On one occasion, a few crystals with stoichiometry [(TMG₃trphen-Sb)₂Cu₃](PF₆)₃ (two ligands capping a Cu₃ triangle) were identified by SCXRD analysis but were of poor quality. Compound 8a provides low-quality twinned crystals for single-crystal X-ray diffraction (see below), but the corresponding compound [(TMG₃trphen-Sb)₂Cu₂](B(C₆F₅)₄)₂•2 MeCN (8b) with B(C₆F₅)₄⁻ as counteranion provides single crystals of superior XRD quality from MeCN. Compound 8b can be obtained from 8a upon addition of 2 equiv of KB(C₆F₅)₄, or via the reaction of ligand 1 with 1.5 equiv of [Cu(NCMe)₂][B(C₆F₅)₄]³¹ in MeCN.

Scheme 3.

Synthesis of Halide-free Metal Compounds

We have also attempted to remove bridging chlorides from [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3) under various conditions. When TlPF₆ (3 equiv) was employed in MeCN, the resulting needle-shaped crystalline material proved to be the same as compound 8a, albeit solvated by MeCN. On the other hand, the reaction of AgPF₆ (3 equiv) with compound 3 in MeCN, provided a poorly soluble and difficult to crystallize white solid, whose NMR signature was identical to that obtained from a reaction of ligand 1 with AgPF₆ in MeCN, thus indicating ligand transmetalation from Cu to Ag. We have been unable thus far to crystallize and fully characterize this product. We have also tried to use high-purity Ag(OTf) (3 equiv) for extracting chlorides from 3 in CH₂Cl₂, which led to the isolation of low yields of mononuclear [(TMG₃H₃trphen-Sb)Cu(OTf)₃](OTf) (10), exhibiting protonation and detachment of the TMG arms, although the protonated ligand is still coordinated to Cu(I) via the Sb(III) element (Scheme 4). The supernatant affords an intractable mixture that also indicates partial ligand transformation. The provenance of protons, whether from the solvent or the ligand, is currently unknown, but notably, we have observed facile C–H activation of the NMe₂ residues in the past.²⁷ Instances in which metal triflate salts act as masked TfOH sources are also known in the literature.³²

Scheme 4.

Extraction of Chlorides from 3 by Silver Triflate

Finally, the reaction of 3 equiv of KB(C₆F₅)₄ with 3 in CH₂Cl₂ afforded an intriguing colorless product, [(TMG₃trphen-Sb)Cu₃(μ₃-Cl)₃Cu]₂[B(C₆F₅)₄]₂•10CH₂Cl₂ (11), exhibiting partial chloride extraction (Scheme 5). While the crown-shaped Cu₃(μ-Cl)₃ fragment is largely retained, extraction of Cu(I) sites is also evident, contributing to the assembly of a new {[Cu₃(μ₃-Cl)₃]₂Cu₂} core structure. The mechanism of formation is certainly complex, as we cannot generate the compound from the reaction of 3 with 2 equiv of [Cu(NCMe)₂][B(C₆F₅)₄].

Scheme 5.

Partial Extraction of Chloride from [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3)

Solid-State Structures.

Ligands TMG₃trphen-Sb (1) and TMG₃trphen-Bi (2) are essentially isostructural (Figure 1), featuring a rigorous C₃-symmetric geometry that places all three guanidinyl groups toward the same side as the endo located Sb/Bi element. This preorganized cavity provides similar coordination space for both ligands given the longer Bi–C edges but more acute angles for the BiC₃ pyramid.

Figure 1.

ORTEP Diagrams of TMG₃trphen-Sb•3CH₂Cl₂ (1), TMG₃trphen-Bi•3CH₂Cl₂ (2) drawn with 40% thermal ellipsoids. Selective interatomic distances [Å] and angles [°] for 1: Sb(1)–C(1) = 2.169(5), C(1)–Sb(1)–C(1) = 95.09(18). For 2: Bi(1)–C(1) = 2.2508(18), C(1)–Bi(1)–C(1) = 93.14(18).

Upon metalation with CuCl, both ligands extract a Cu₃(μ-Cl)₃ cluster in a manner similar to that previously noted in two rare cases, either with the assistance of ligand (o(ⁱPr₂P)-C₆H₄)₃E (E = Sb, Bi)³³ or with tris(2(2-pyridyl)ethyl)-phosphine.³⁴ The resulting compounds (3, 4) feature an approximate C₃-symmetric geometry and are distinguished by a crown-shaped Cu₃(μ-Cl)₃ unit embedded in the ligand framework. By contrast, the gas-phase structure of CuCl encompasses planar, D_3h-symmetric Cu₃Cl₃ rings (Cu–Cu = 2.627 ± 0.012, Cu–Cl = 2.166 ± 0.008 Å, and Cu–Cl–Cu = 73.9 ± 0.6°, at 689 K).³⁵

For compounds 3 and 4 (Figures 2 and 3), the Cu₃(μ-Cl)₃ cluster is supported by a Sb(III) (3) or Bi(III) (4) axial element, capping all three Cu sites (Sb–Cu_3,cent = 2.187, Bi–Cu_3,cent = 2.241 Å), as well as by three guanidinyl residues, each coordinated to a separate edge of the Cu₃ triangle (ave. Cu–N = 2.023 ± 0.016 (3), 2.018 ± 0.003 (4) Å). The average Sb–Cu (2.657 ± 0.012 Å) and Bi–Cu (2.785 ± 0.013 Å) bond distances are shorter than those reported by Ke and Gabbaï (Sb–Cu = 2.802(2), Bi–Cu 2.934(2) Å) with (o(ⁱPr₂P)C₆H₄)₃E (E = Sb and Bi) ligands,³³ and very similar to the sum of covalent radii for Sb/Bi and Cu atoms (Sb–Cu = 2.71(6), Bi–Cu = 2.80(6) Å).³⁶

Figure 2.

ORTEP diagrams of [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃]•4CH₂Cl₂ (3), [(TMG₃trphen-Sb)Cu₃(μ-Br)₃]•4CH₂Cl₂ (5), [(TMG₃trphen-Sb)Cu₃(μ-I)₃]•3CH₂Cl₂ (6) drawn with 40% thermal ellipsoids. Selective interatomic distances [Å] and angles [°] for 3: Sb(1)–Cu(1) = 2.652(2), Sb(1)–Cu(2) = 2.646(2), Sb(1)–Cu(3) = 2.673(2), Cu(1)–Cu(2) = 2.628(3), Cu(1)–Cu(3) = 2.594(3), Cu(2)–Cu(3) = 2.615(3), Cu(1)–Cl(1) = 2.320(4), Cu(1)–Cl(2) = 2.366(4), Cu(1)–N(7) = 2.027(11), Cu(2)–Cl(2) = 2.328(4), Cu(2)–Cl(3) = 2.337(4), Cu(2)–N(4) = 2.040(11), Cu(3)–Cl(1) = 2.342(4), Cu(3)–Cl(3) = 2.366(4), Cu(3)–N(1) = 2.001(11), Cu(2)–Cu(1)–Cu(3) = 60.11(7), Cu(1)–Cu(2)–Cu(3) = 59.29(7), Cu(1)–Cu(3)–Cu(2) = 60.60(7), Cu(1)–Cl(1)–Cu(3) = 67.60(11), Cu(1)–Cl(2)–Cu(2) = 68.09(12), Cu(2)–Cl(3)–Cu(3) = 67.57(11), Cu(1)–Sb(1)–Cu(2) = 59.48(6), Cu(1)–Sb(1)–Cu(3) = 58.29(6), Cu(2)–Sb(1)–Cu(3) = 58.91(6). For 5: Sb(1)–Cu(1) = 2.6430(5), Sb(1)–Cu(2) = 2.6510(5), Sb(1)–Cu(3) = 2.6676(5), Cu(1)–Cu(2) = 2.6203(7), Cu(1)–Cu(3) = 2.5958(7), Cu(2)–Cu(3) = 2.6012(7), Cu(1)–Br(1) = 2.4502(6), Cu(1)–Br(2) = 2.4785(6), Cu(1)–N(7) = 2.032(3), Cu(2)–Br(2) = 2.4488(6), Cu(2)–Br(3) = 2.4523(6), Cu(2)–N(4) = 2.032(3), Cu(3)–Br(1) = 2.4670(6), Cu(3)–Br(3) = 2.4719(6), Cu(3)–N(1) = 2.020(3), Cu(2)–Cu(1)–Cu(3) = 59.822(18), Cu(1)–Cu(2)–Cu(3) = 59.621(18), Cu(1)–Cu(3)–Cu(2) = 60.557(18), Cu(1)–Br(1)–Cu(3) = 63.728(18), Cu(1)–Br(2)–Cu(2) = 64.252(18), Cu(2)–Br(3)–Cu(3) = 63.773(18), Cu(1)–Sb(1)–Cu(2) = 59.335(15), Cu(1)–Sb(1)–Cu(3) = 58.522(15), Cu(2)–Sb(1)–Cu(3) = 58.558(15). For 6: Sb(1)–Cu(1) = 2.6619(5), Cu(1)–Cu(#1) = 2.6332(8), Cu(1)–I(1) = 2.6313(5), Cu(#1)–I(1) = 2.6410(5), Cu(1)–N(1) = 2.035(3), Cu(1)#1–Cu(1)–Cu(1)#2 = 60.0, Cu(1)–I(1)–Cu(1)#1 = 59.925(19), Cu(1)–Sb(1)–Cu(1)#1 = 59.289(17).

Figure 3.

ORTEP diagrams of [(TMG₃trphen-Bi)Cu₃(μ-Cl)₃]•4CH₂Cl₂ (4) and [(TMG₃trphen-Sb)Ag₃(μ-Cl)₃]•3CH₂Cl₂ (7) drawn with 40% thermal ellipsoids. Selective interatomic distances [Å] and angles [°] for 4: Bi(1)–Cu(1) = 2.7710(12), Bi(1)–Cu(2) = 2.7823(12), Bi(1)–Cu(3) = 2.8024(13), Cu(1)–Cu(2) = 2.8851(17), Cu(1)–Cu(3) = 2.8262(17), Cu(2)–Cu(3) = 2.8838(17), Cu(1)–Cl(1) = 2.259(3), Cu(1)–Cl(2) = 2.394(3), Cu(1)–N(7) = 2.021(7), Cu(2)–Cl(2) = 2.271(3), Cu(2)–Cl(3) = 2.339(3), Cu(2)–N(4) = 2.018(7), Cu(3)–Cl(1) = 2.360(3), Cu(3)–Cl(3) = 2.295(3), Cu(3)–N(1) = 2.015(8), Cu(2)–Cu(1)–Cu(3) = 60.64(4), Cu(1)–Cu(2)–Cu(3) = 58.67(4), Cu(1)–Cu(3)–Cu(2) = 60.69(4), Cu(1)–Cl(1)–Cu(3) = 75.42(8), Cu(1)–Cl(2)–Cu(2) = 76.36(8), Cu(2)–Cl(3)–Cu(3) = 76.95(8), Cu(1)–Bi(1)–Cu(2) = 62.60(4), Cu(1)–Bi(1)–Cu(3) = 60.94(3), Cu(2)–Bi(1)–Cu(3) = 62.18(4). For 7: Sb(1)–Ag(1) = 2.9174(6), Sb(1)–Ag(2) = 2.9505(7), Sb(1)–Ag(3) = 2.9331(7), Ag(1)–Ag(2) = 2.9256(7), Ag(1)–Ag(3) = 2.9227(7), Ag(2)–Ag(3) = 2.8822(7), Ag(1)–Cl(1) = 2.6443(18), Ag(1)–Cl(3) = 2.509(2), Ag(1)–N(1) = 2.266(6), Ag(2)–Cl(1) = 2.5078(17), Ag(2)–Cl(2) = 2.630(2), Ag(2)–N(4) = 2.274(6), Ag(3)–Cl(2) = 2.524(2), Ag(3)–Cl(3) = 2.6209(19), Ag(3)–N(7) = 2.271(5), Ag(2)–Ag(1)–Ag(3) = 59.054(17), Ag(1)–Ag(2)–Ag(3) = 60.423(18), Ag(1)–Ag(3)–Ag(2) = 60.523(18), Ag(1)–Cl(1)–Ag(2) = 69.14(4), Ag(2)–Cl(2)–Ag(3) = 67.97(5), Ag(1)–Cl(3)–Ag(3) = 69.43(5), Ag(1)–Sb(1)–Ag(2) = 59.808(16), Ag(1)–Sb(1)–Ag(3) = 59.941(17), Ag(2)–Sb(1)–Ag(3) = 58.664(17).

The presence of cuprophilic (d¹⁰–d¹⁰) interactions³⁷ within the essentially equilateral Cu₃ triangle of the Cu₃(μ-Cl)₃ unit is more nuanced, especially for the Bi supported compound 4, since the observed Cu–Cu distances (2.612 ± 0.014 (3), 2.865 ± 0.027 (4) Å) are either within range (3) or exceed (4) the sum of covalent radii (Cu–Cu = 2.64(6) Å)³⁶ and may also be at the limit of the sum of the vdW radii (Cu–Cu = 2.80 Å), although the latter value may be an underestimation.³⁸ Similar metallophilic Cu–Cu interactions were suggested and computationally justified, in the previously reported cases of [(o(ⁱPr₂P)C₆H₄)₃ECu₃(μ-Cl)₃] (Cu–Cu = 2.660(2) (E = Sb), 2.853(2) (E = Bi) Å)³³ and [(2(2-Py)-CH₂CH₂)₃PCu₃(μ-Cl)₃] (Cu–Cu = 2.542 ± 0.049 Å).³⁴ Single chloride bridges between pairs of Cu(I) sites (Cu–Cl = 2.343 ± 0.018 (3), 2.320 ± 0.049 (4) Å); Cu–Cl–Cu = 67.75 ± 0.24 (3), 76.24 ± 0.63 (4)°) complete the metal coordination sphere, with Cu–Cl distances that are very similar to those previously reported for the aforementioned Cu₃(μ-Cl)₃ units.^33,34

The bromo- and iodo-bridged congeners [(TMG₃trphen-Sb)Cu₃(μ-X)₃] (X = Br (5), I (6)) are essentially isostructural to the chloro analog 3. Compound 6 features a strict C₃ axis penetrating the middle of the equilateral Cu₃ triangle and the apical Sb element (Figure 2). Interestingly, metrical parameters associated with their [(L-Sb)Cu₃] framework (Cu–Cu = 2.6058 ± 0.0105 (5), 2.6332(8) (6), Sb–Cu = 2.6539 ± 0.0102 (5), 2.6619(5) (6) Å) are similar to those of 3. The size of the halide is only reflected in the longer and slightly asymmetric halide bridges (Cu–Cl = 2.343 ± 0.018 (3), Cu–Br = 2.4615 ± 0.0116 (5), Cu–I = 2.6362 ± 0.0049 (6) Å) and the corresponding acute Cu–X–Cu angles (67.75 ± 0.24 (3), 63.918 ± 0.237 (5), 59.925(19) (6)°). The related Cu₃X₃ cluster embedded in tris(2(2-pyridyl)ethyl)phosphine (L-P) exhibits a [(L-P)Cu₃] framework that is more tightly coordinated (Cu–Cu ranges narrowly from 2.528 to 2.542 Å) but is highly asymmetric in the cases of X = Cl and I, since one Cu–P bond is longer than the other two by a significant margin (0.40 (Cl), 0.37 (I) Å).³⁴

The Ag₃(μ-Cl)₃ unit (7, Figure 3) is embedded in the cavity of ligand TMG₃trphen-Sb in a similar fashion to that observed for Cu₃(μ-Cl)₃, as evidenced by the pseudo C₃-symmetric attachment of the cluster to the tripodal Sb ligand via the three guanidinyl residues (ave. Ag–N = 2.270 ± 0.003 Å) and the axial Sb (ave. Sb–Ag = 2.934 ± 0.014 Å). Within the Ag₃(μ-Cl)₃ cluster, the average Ag–Ag (2.910 ± 0.020 Å) and Ag–Cl (2.573 ± 0.060 Å) bond distances are comparable to the sum of the corresponding covalent radii (Ag–Ag = 2.90(7), Ag–Cl = 2.47(6) Å)³⁶ and shorter than the values reported for two independent molecules in the unit cell of [(o(ⁱPr₂P)-C₆H₄)₃SbAg₃(μ-Cl)₃] (Ag–Ag = 3.0288(8), 3.1559(8); Ag–Cl = 2.5791(14), 2.6443(16) Å).³³ One distinguishing feature of the Ag₃(μ-Cl)₃ cluster is the somewhat asymmetric chloride bridging (Δ(Ag–Cl) = 0.118 ± 0.013 Å) presumably due to distortion arising from the accommodation of a larger cluster in the ligand cavity.

The structures of the halide-free compounds [(TMG₃trphen-E)₂Cu₂](X)₂•2 solv (E = Sb (8a, 8b), Bi (9); solv = THF (8a), MeCN (8b), Et₂O (9); X = PF₆ (8a, 9), B(C₆F₅)₄ (8b)) are all very similar (Figures 4 and S1), although significant differences in metrical parameters are observed. They all possess a Cu₂E₂ parallelogram with a short and a long Cu–E bond and exhibit a local inversion center applicable to the entire compound. Each stibine (1) or bismuthine (2) ligand is coordinated to both Cu sites via two separate TMG-substituted arms, whereas the third TMG residue remains noncoordinated, at a significant distance (>4 Å) from the closest Cu. The ligand moieties that bridge the shorter Cu–Sb bonds are oriented axially with respect to the Cu₂E₂ ring, whereas those spanning the longer Cu–Sb bonds are located equatorially. Since crystal data for 8a were of low quality (derived from a twinned specimen), we rely on the solid-state structure of 8b for further discussion of metrical parameters. Each symmetry-related Cu(I) site of 8b is coordinated by two Sb(III) elements (Cu–Sb = 2.5925(4), 2.8121(4) Å; Sb–Cu–Sb = 112.319(13)°) and two N_TMG residues (Cu–N = 2.060(2), 2.022(2) Å; N–Cu–N = 122.35(9)°). Houser’s four-coordinate geometry index, τ₄ (0.73),³⁹ places the coordination of the Cu(I) site between trigonal pyramidal and seesaw. The Cu···Cu interatomic distance (3.0152(7) Å) is rather long to denote any significant cuprophilic interaction. The bismuth analog 9 is geometrically equivalent to 8a/8b yet the Cu₂Bi₂ parallelogram is characterized by metrical parameters (Cu–Bi = 2.9025(8), 3.3075(8) Å; Bi–Cu–Bi = 83.41(2)°) that include a bonafide covalent Cu–Bi bond and a longer Cu···Bi interaction that at best denotes a weak contact at the van der Waals sum of radii limit (3.47 Å).^38,40 The Cu···Cu interatomic bond distance (4.644 Å) also precludes any cuprophilic contacts. Therefore, the Cu(I) sites of 9 are essentially three-coordinate with shorter Cu–N bonds (both 1.896(5) Å) and more obtuse N–Cu–N angle (155.0(2)°) than those observed with the stibine analogs (8a, 8b).

Figure 4.

ORTEP diagrams of [(TMG₃trphen-Sb)₂Cu₂][B(C₆F₅)₄]₂•2MeCN (8b) and [(TMG₃trphen-Bi)₂Cu₂](PF₆)₂•2Et₂O (9) drawn with 40% thermal ellipsoids. Selective interatomic distances [Å] and angles [°] for 8b: Sb(1)–Cu(1) = 2.5925(4), Sb(1)–Cu(1)#1 = 2.8121(4), Cu(1)–Cu(1)#1 = 3.0152(7), Cu(1)–N(4) = 2.060(2), Cu(1)–N(7) = 2.022(2), Cu(1)–Sb(1)–Cu(1)#1 = 67.681(13), N(4)–Cu(1)–Sb(1) = 88.00(6), N(4)–Cu(1)–Sb(1)#1 = 126.81(6), N(4)–Cu(1)–N(7) = 122.35(9), N(7)–Cu(1)–Sb(1) = 129.86(6), N(7)–Cu(1)–Sb(1)#1 = 82.21(6). For 9: Bi(1)–Cu(1) = 2.9025(8), Bi(1)–Cu(1)#1 = 3.3075(8), Cu(1)–N(1) = 1.896(5), Cu(1)–N(4) = 1.896(5), Cu(1)–Bi(1)–Cu(1)#1 = 96.59(2), N(1)–Cu(1)–Bi(1) = 88.26(14), N(1)–Cu(1)–Bi(1)#1 = 120.96(16), N(1)–Cu(1)–N(4) = 155.0(2), N(4)–Cu(1)–Bi(1) = 112.73(15), N(4)–Cu(1)–Bi(1)#1 = 76.64(15).

A version of the coordination mode noted above was most recently unraveled in a comprehensive publication by Wright and co-workers⁴¹ for [{(2-Me-8-quinolyl)₃Sb}₂Cu₂](PF₆)₂. In this case, the compound lacks an inversion center, and the Cu₂Sb₂ ring is essentially rhombic (Cu–Sb = 2.5451(19)–2.5884(19) Å) bridged by an ax/ax/eq/eq suite of ligand moieties rather than the alternating ax/eq/ax/eq motif observed in our work. Interestingly, the corresponding bismuth ligand (2-Me-8-quinolyl)₃Bi did not provide compounds with Cu–Bi bonds.⁴¹

The structure of compound [(TMG₃H₃trphen-Sb)Cu(OTf)₃](OTf) (10) (Figure S2) demonstrates an unusual decomplexation of the TMG residues of ligand 1 due to protonation (Cu–N interatomic distances vary from 3.561 to 3.902 Å), accompanied by retention of a strong Sb(III)–Cu(I) bond (2.4259(9) Å).⁴² The pseudotetrahedral ligand field around the Cu(I) site also includes three coordinated triflates (Cu–O = 2.090 ± 0.012 Å) with less obtuse O–Cu–O (99.30 ± 2.60°) than Sb–Cu–O (118.37 ± 2.83°) angles, reflecting the repulsive effect of the Cu–Sb bond and the bulk of the protonated ligand.

Finally, compound [(TMG₃trphen-Sb)Cu₃(μ₃-Cl)₃Cu]₂[B(C₆F₅)₄]₂ (11) (Figure 5) relates two [(TMG₃trphen-Sb)-Cu₃Cl₃] units via a central chloro-bridged Cu₂ element by means of inversion. This unique structure demonstrates metrical parameters for the essentially equilateral Cu^I₃ triangle and its coordination to the antimony ligand that are very similar to those demonstrated by 3, save for the shorter Cu–N bond distances (Cu–N = 1.957 ± 0.044 Å) due to the weaker Cu–Cl bonds in 10. The μ₂-Cl bridges in 3 become triply bridged in 10 by incorporating an additional Cu^I site from the central Cu₂ element. As a result, the Cu₃Cl₃ fragment in 10 exhibits longer Cu–Cl bond distances (2.413 ± 0.055 Å) and more acute Cu–Cl–Cu angles (65.27 ± 0.52°). Each Cu site of the Cu₂ dimer is coordinated by three chlorides in a trigonal planar arrangement (Cu–Cl = 2.272 ± 0.080 Å, Cl–Cu–Cl = 118.7 ± 9.9°) and also features a cuprophilic contact (Cu–Cu = 2.772(7) Å).

Figure 5.

ORTEP diagram of [(TMG₃trphen-Sb)Cu₃(μ₃-Cl)₃Cu]₂[B(C₆F₅)₄]₂•10CH₂Cl₂ (11) drawn with 40% thermal ellipsoids. Selective interatomic distances [Å] and angles [°]: Sb(1)–Cu(1) = 2.661(3), Sb(1)–Cu(2) = 2.649(3), Sb(1)–Cu(3) = 2.645(3), Cu(1)–Cu(2) = 2.648(4), Cu(1)–Cu(3) = 2.601(4), Cu(2)–Cu(3) = 2.564(4), Cu(4)–Cu(4)#1 = 2.772(7), Cu(1)–Cl(2) = 2.328(5), Cu(1)–Cl(3) = 2.465(6), Cu(1)–N(1) = 2.004(12), Cu(2)–Cl(1) = 2.386(5), Cu(2)–Cl(3) = 2.398(5), Cu(2)–N(4) = 1.968(14), Cu(3)–Cl(1) = 2.404(5), Cu(3)–Cl(2) = 2.497(6), Cu(3)–N(7) = 1.899(14), Cu(4)–Cl(1) = 2.375(7), Cu(4)–Cl(2) = 2.261(6), Cu(4)–Cl(3) = 2.180(6), Cu(2)–Cu(1)–Cu(3) = 58.48(11), Cu(1)–Cu(2)–Cu(3) = 59.83(10), Cu(1)–Cu(3)–Cu(2) = 61.69(9), Cu(2)–Cl(1)–Cu(3) = 64.72(14), Cu(2)–Cl(1)–Cu(4) = 103.8(2), Cu(3)–Cl(1)–Cu(4) = 74.62(19), Cu(1)–Cl(2)–Cu(3) = 65.12(15), Cu(1)–Cl(2)–Cu(4) = 101.9(2), Cu(3)–Cl(2)–Cu(4) = 74.8(2), Cu(1)–Cl(3)–Cu(2) = 65.97(14), Cu(1)–Cl(3)–Cu(4)#1 = 101.9(2), Cu(2)–Cl(3)–Cu(4)#1 = 97.9(2), Cu(4)#1–Cu(4)–Cl(1) = 82.2(2), Cu(4)#1–Cu(4)–Cl(2) = 98.8(2), Cu(4)#1–Cu(4)–Cl(3) = 105.0(2).

Summaries of crystallographic data for compounds 1–11 are collected in Tables S1–S4.

Solution Behavior.

¹H NMR data for compounds [(TMG₃trphen-Sb)Cu₃(μ-X)₃] (X = Cl (3), Br (5), I (6)) in CD₂Cl₂ at room temperature (298 K) exhibit broad peaks attributed to the four methyl groups of the TMG residues (fully resolved only for X = I), extending from δ 1.5 to 3.5 ppm for all compounds (Figure S3). The degree of resolution of the four distinct methyl peaks (all broad at 298 K) is enhanced with increasing size of the halide presumably due to restricted rotation of the TMG branches. Low-temperature ¹H NMR data (243–298 K, Figure S4) indicate that all four methyl groups are progressively resolved with decreasing temperature, to afford four sharp peaks at −30 °C (for 3: δ (ppm, CD₂Cl₂, −30 °C) = 3.100 (H(23)), 2.758 (H(21/22)), 2.582 (H(21/22), 1.672 (H(20), see Figure 2 for atom labeling). Peak assignments were facilitated by computational (B3LYP/6–311+G(2d,p)) single point NMR property determination at the B3LYP/6–31G(d)-optimized geometry of 3, which indicates that the most upfield peak is due to protons residing in the shielded region of the phenylene rings, whereas the most downfield peak arises from protons pointing endo with respect to the ligand cavity. With respect to [(TMG₃trphen-Bi)Cu₃(μ-Cl)₃] (4), a featureless broad band at room temperature progressively resolves to four distinct Me peaks at −30 °C in CD₂Cl₂. A similar broad band, assigned to TMG methyl groups, is also observed for [(TMG₃trphen-Sb)Ag₃(μ-Cl)₃] (7). Apparently, for this more sizable ligand cavity, the rotational restrictions for the TMG arms are more relaxed than those applying to compounds 1–3 at room temperature. Notably, the ¹H NMR spectra of the [(TMG₃trphen-Sb)₂Cu₂]²⁺ compounds (8a, 8b) exhibit only a single peak for all Me groups of the three TMG arms (as well as equivalent aryl groups) in CD₃CN at 298 K. On the other hand, the corresponding [(TMG₃trphen-Bi)₂Cu₂]²⁺ (9) compound distinguishes the TMG arms (and the related aryl groups) in a 2:1 ratio, most likely reflecting the coordinated (two arms) and noncoordinated (one arm) TMG moieties observed in the solid-state structure of 9. Fast exchange between coordinated and noncoordinated TMG groups may account for the single TMG peak observed with the Sb analogs (8a, 8b), although dissociation to monomers with possible MeCN coordination cannot be excluded. The protonation of the TMG arms in compound 10 can easily be detected by ¹H NMR. Finally, compound 11 demonstrates a ¹H NMR spectrum not unlike that of 3 at 298 K, featuring broad overlapping peaks for the Me groups of TMG.

The metal coordination of the strongly superbasic TMG arms results in charge delocalization over the CN₃ unit, as demonstrated by the essentially equivalent C–N bond distances (SCXRD data) and downfield shift of the central C peak (>160 ppm, ¹³C NMR data).

Computational Studies.

The structures of 3 and 4 were optimized and found to have an overall generally good agreement with the experimental crystallographic data. EDA calculations⁴³ were conducted to decompose the interaction energy and give a more detailed view of the bonding for these complexes (Table 1). The results show that complexes 3 and 4 have similar bonding energetic patterns, as evidenced by the same percent contribution of attraction interactions and energy values. The leading attractive interaction is electrostatic interaction between two charge-neutral moieties, mainly arising from the Cu–N coordination bonds between two fragments. The steric effect of Sb (3) and Bi (4) to the Cu₃Cl₃ fragment accounts for the Pauli repulsion seen in both complexes, whereas the transfer of electrons synergistically increases their electrostatic attraction. Also, the Sb complex 3 has a slightly larger electrostatic (−287.8 kcal/mol) and orbital interaction energies (−141.7 kcal/mol) vs those for Bi complex 4 (−275.8 kcal/mol and −133.5 kcal/mol, respectively). The higher orbital interaction energy in the Sb complex 3 vs the Bi analogue 4, an attractive force, is offset by the correspondingly higher Pauli repulsion energy, resulting in the Bi complex 4 possessing a stronger overall interaction energy than that in the Sb analogue 3.

Table 1.

Energy Decomposition Analysis Results of Compounds 3 and 4^a

compound	ΔEelstat	ΔEPauli	ΔEOrbInt	ΔEdisp	ΔEint
3	−287.8 (63%)	344.5	−141.7 (31%)	−24.4 (6%)	− 109.4
4	−275.8 (63%)	323.5	−133.5 (31%)	−25.4 (6%)	−111.2

^aUnit: kcal/mol, values in parentheses are the energy percentage to the total attractive interaction. Total interaction energy (ΔE_int) is decomposed into electrostatic interaction (ΔE_alstat), Pauli repulsion (ΔE_Pauli), orbital interaction (ΔE_OrbInt), and dispersion energy (ΔE_disp).

As the Cu–N coordination bonds are broken during the fragmentation procedure for the EDA analysis, the total ΔE_int does not directly reflect the interaction strengths between the Cu₃Cl₃ and ligand with Sb/Bi moieties. To overcome this limitation, we further divide ΔE_OrbInt into pairwise interactions to reveal the detailed bonding pictures between fragments under the ETS-NOCV theoretical framework.⁴³ The broken-down ΔE_OrbInt is denoted as ΔE_OrbInt,i (i = integer) in the energy strength order. NOCV results are consistent with Sb/Bi → Cu₃ dative bonding in both complexes 3 and 4. The NOCV pairwise orbital deformation density maps are illustrated in Figure 6; the electron flows from the white region to the brown region comprising the bonding formation. The largest contributing NOCV pairwise orbitals (ΔE_OrbInt,1) of 3 and 4 are the bonding interactions between Sb or Bi, respectively, and the Cu₃Cl₃ moieties, with interaction energies of −32.6 kcal/mol and −23.6 kcal/mol, respectively. The electrons that contribute to the bonding behavior originate from the filled Sb 5s or Bi 6s orbitals to the admixed Cu 4s^0+x/3d^10−x partially unoccupied/partially occupied orbitals, confirming the directional Sb/Bi→Cu₃ dative bonding character (Figure 6a/c). The ΔE_OrbInt,2–4 values for 3 and 4 clearly suggest Cu–N coordination bonds (Figure 6b/d), with comparable attractive bond strengths of −40.3 and −43.6 kcal/mol, respectively. The rest of the NOCV pairwise orbitals possess negligible interaction energies (<4 kcal/mol in total) suggest that the major bonding in these species is due to the Cu–N coordination, followed by Sb/Bi→Cu₃ donor contributions. Any residual backdonation (Sb/Bi→Cu₃) cannot be excluded at the present time, but this contribution is likely to be very small.

Figure 6.

NOCV deformation density maps of (a/c) ΔE_OrbInt,1 and (b/d) ΔE_OrbInt,2–4 of 3 and 4, respectively. Isovalue: 0.002 au 3 (for (a) ΔE_OrbInt,1 = −32.6 kcal/mol; for (b) ΔE_OrbInt,2–4 = −40.3 kcal/mol), 4 (for (c) ΔE_OrbInt,1 = −23.6 kcal/mol; for (d) ΔE_OrbInt,2–4 = −43.6 kcal/mol).

The independent gradient model based on the Hirshfeld partition (IGMH)⁴⁴ analyzes noncovalent interactions between Sb/Bi and Cu₃Cl₃ moieties (Figure 7). The results correspond to the above NOCV results in that a stronger interfragment bonding character exists in 3 and 4. The plots confirm the attractive interaction between Sb/Bi atoms and Cu atoms, as illustrated by the blue region in Figure 7a–d. Comparing the IGMH results of 3 and 4, the blue color between Sb and Cu₃ is deeper than that between Bi and Cu₃, which suggests a stronger interaction in 3, in agreement with the larger interaction energy between Sb → Cu₃ as indicated by the NOCV results (Figure 6a/c). These calculations are consistent with a decrease in electron-donating ability of the pnictine lone pair from Sb to Bi, concomitant with an increase in s-character. Also, the calculated average bond distance of 2.924 Å for Bi–Cu as compared to the average bond distance of 2.746 Å for Sb–Cu (2.68–2.84 Å) suggests that a slightly shorter bond contributes to the Sb → Cu₃ interaction, given the rather similar van der Waals radii of Sb (2.06 Å) and Bi (2.07 Å).⁴⁰

Figure 7.

Independent gradient model based on Hirshfeld partition (IGMH) analysis results of 3 and 4 in (a/c) top view and (b/d) side view, respectively. The value of the surface is set as 0.01 au.

Catalytic and Mechanistic Nitrene-Transfer Studies.

Aziridination of para-Substituted Styrenes.

An initial evaluation of nitrene addition to olefins was conducted with the assistance of imidoiodinane PhINTs (1 equiv, Ts = tosyl) and a panel of para-substituted styrenes (8 equiv) in the presence of different catalytic systems (5 mol % with respect to Cu) at 30 °C (drybox temperature) over 12 h (Table 2). The choice of the solvent (MeCN) was suggested by an initial screening of various chlorinated and nonchlorinated solvents (MeCN, benzene, CH₂Cl₂, 1,2-dichloroethane, and chlorobenzene) in catalytic reactions with styrene/PhINTs mediated by [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3). Acetonitrile has an edge over all other solvents, even if the catalyst is more soluble in CH₂Cl₂. Molecular sieves (5 Å) are also needed for obtaining good yields, thus denoting sensitivity to any adventitious water.

Table 2.

Yields of Aziridination Products of para-Substituted Styrenes Catalyzed by [(TMG₃trphen-E)Cu₃(μ-Cl)₃] (E = Sb (3), Bi (4))^a

Entry No.	Substrate	Products		Yield (%) [(L-Sb)Cu3Cl3](3)	Yield (%) 3/KPFe (3 equiv.)	Yield (%) [(L-Bi)Cu3Cl3] (4)
1.			R = H	92	80	85
2.			R = Me	73	77	80
3.			R = tBu	77	63	82
4.			R = OMe	28	polymer	polymer
5.			R = OtBu	53	polymer	polymer
6.			R = F	77	74	83
7.			R = Cl	88	79	89
8.			R = CF3	87	96	82
9.			R=NO2	83	87	73

^aConditions: catalyst, 0.0125 mmol with respect to Cu (5 mol %); PhINTs, 0.25 mmol; olefin, 2.0 mmol; KPF₆, 0.0125 mmol; MS 5 Å, 20 mg; solvent (MeCN) 0.250 g; 30 °C; 12 h.

Gratifyingly, both catalysts 3 and 4 provide good to excellent yields of the corresponding aziridines as evaluated by ¹H NMR spectra of the purified products (with respect to an internal standard). Both 3 (87%) and 4 (62%) can be recovered at the end of the reaction, albeit to a much lower extent for 4, potentially reflecting the fragility of the Bi–C bond.⁴⁵

To evaluate the effect of partial removal of chlorides as suggested by the synthesis of compound 11 (see above), KPF₆ (3 equiv) was added to the catalytic mixture containing [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3). Yields of aziridines (Table 2) are not unlike those obtained in the absence of KPF₆, save for the most electron-withdrawing styrenes (CF₃, NO₂), for which unusually high yields are observed. Interestingly, SbAg₃(μ-Cl)₃ reagent 7 is totally unproductive.

Aziridination of Other Alkenes.

The more robust catalyst 3 was further employed in the aziridination of a wider panel of aromatic and aliphatic olefins under the conditions noted above (Table 3). In the presence of ortho styrene substitution (entry 1), significantly lower yields are obtained, due to steric and electronic reasons (orthogonal orientation of the aromatic/olefinic planes).⁴⁶ Benzylic amination was not observed in this case. Nitrene transfer to α-substituted styrenes (Me, Ph; entries 2, 3) is also sterically hampered, especially for the α-Ph congener. The α-methylstyrene affords substantial allylic amination in almost 1:1 ratio with respect to aziridination (entry 2). Surprisingly, the major product for α-phenylstyrene is a hydroxyamine rather than the more common enamine (entry 3), even under conditions that are purported to exclude water. In this case, aziridine is known to undergo facile ring opening (and rearrangement).⁴⁷ The corresponding cis- and trans-β-substituted styrenes (Me, Ph; entries 4–7) are less hampered by sterics and afford good yields of the corresponding cis- and trans-aziridines. A minor amount of allylic amination is only observed for the Me-substituted analog. On the other hand, the retention of stereochemistry for aziridination of the mechanistically diagnostic cis-β-R-styrenes is rather modest (cis:trans = 1:1.3, R = Me; 1.8:1, R = Ph). Amination of allylic/benzylic C–H sites is very competitive with styrenyl aziridinations (entries 8, 9), provided that acyclic trans substrates are involved (entry 8) that usually restrict aziridination yields. Allylic aminations are also dominant with cycloalkenes (entries 10, 11), although the more electron-rich cyclooctene (entry 12) favors aziridination by a significant margin since H atom abstraction from the allylic C–H bonds is associated with large energy barriers, due to modest σ_C–H/π_C=C orbital overlap.⁴⁸ Other aliphatic alkenes (entries 13–15) exhibit significantly lower yields than those encountered with aromatic congeners.

Table 3.

Yields of Aziridination/Amination Products of Various Olefins Catalyzed by [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3)^a

Entry No.	Substrate	Products	Yields (%)
1.			22
2.			24, 23
3.			tr, 2, 17
4.			28, 36, 3
5.			49, 2, 4
6.			30, 17
7.			45
8.			25, 37
9.			48, nd
10.			4, 14
11.			2, 42
12.			51, nd
13.			8, 5
14.			19, 12
15.			28

^aConditions: 3, 0.0125 mmol with respect to Cu (5 mol %); PhINTs, 0.25 mmol; olefin, 2.0 mmol; MS 5 Å, 20 mg; solvent (MeCN) 0.250 g; 30 °C; 12 h.

Amination of Benzylic Substrates.

The more challenging amination of C–H-bond containing substrates was subsequently examined. The benchmark substrate ethylbenzene (1 equiv) was initially explored with PhINTs (2 equiv) in the presence of catalytic amounts of [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3) (5 mol % with respect to Cu) in several solvent matrices. Under all circumstances, benzylic amination yields did not exceed 10%. The more electrophilic nitrene source PhINTces (Tces = 2,2,2-trichloroethoxysulfonyl) provided significantly better yields and was further applied in the amination of ethylbenzene. An initial screening of chlorinated and nonchlorinated solvents indicated that PhCl and PhCF₃ are the most productive solvents, especially if a small amount of HFIP (10%) is added. Presynthesized PhINTces⁴⁹ is significantly more soluble than PhINTs but is unstable in halogenated solvents. The selection of PhCF₃/HFIP (10:1 v/v) for further experimentation was indicated by the superior stability it offered to PhINTces at 30 °C over a period of 24 h in the absence of light, according to ¹H NMR experiments.

The amination of a panel of benzylic substrates (Table 4) was pursued most effectively with a 2:1 ratio of PhINTces/substrate in PhCF₃/HFIP (10:1 v/v) at 30 °C, over 24 h. Several catalysts (5 mol % with respect to Cu) were explored, namely, [(TMG₃trphen-Sb)Cu₃(μ-X)₃] (X = Cl (3), Br (5), I (6)), [(TMG₃trphen-Bi)Cu₃(μ-Cl)₃] (4), and [(TMG₃trphen-Sb)₂Cu₂](PF₆)₂ (8a). As a general characteristic, all catalysts afford moderate to good amination yields specifically for secbenzylic sites. Prim-benzylic C–H bonds are unaffected (entry 11), as are C–H bonds adjacent to O/N atoms and tert/sec/prim C–H bonds of unactivated alkanes and cycloalkanes (yields ≤10%, not shown). Notably, amination of ethylbenzene does not occur in the presence of catalytic amounts (5 mol %) of TMG₃trphen-Sb (1) or CuCl alone.

Table 4.

Yields of Benzylic Amination of Various Substrates Mediated by [(TMG₃trphen-Sb)Cu₃(μ-X)₃] (X = Cl (3) Br (5), I (6)), [(TMG₃trphen-Bi)Cu₃(μ-Cl)₃] (4), and [(TMG₃trphen-Sb)₂Cu₂](PF₆)₂ (8a)^a

Entry No.	Substrate	Product	Yield (%) [(L-Sb)Cu3X3]			Yield (%) [(L-Bi)Cu3Cl3] (4)	Yield (%) [(L-Sb)2Cu2]2+ (8a)
			X = Cl (3)	X = Br (5)	X = I (6)
1.			50	35	49	49	46
2.			54	41	42	41	32
3.			60	29	56	50	41
4.			48	44	42	50	49
5.			49	67	63	43	54
6.			61	39	37	57	45
7.			52	46	31	51	41
8.			13	18	10	20	13
9.			31	12	17	26	9
10.			12	31	34	20	26
11.			5	7	2	6	8
12.			23	34	36	31	20
13.			26	50	36	29	26
14.			30	22	25	24	23
15.			7	9	10	13	25
16.			26	2	5	25	25

^aConditions: catalyst, 0.0125 mmol with respect to Cu (5 mol %); PhINTces, 0.50 mmol; benzylic substrate, 0.25 mmol; MS 5 Å, 20 mg; solvent (PhCF₃/HFIP 10:1 v/v), 0.300 g (0.500 g for 8a); 30 °C; 24 h.

A cursory look at the yields afforded for the amination of all benzylic sites shown in Table 4 by catalysts 3 and 4 indicates that no significant variations exist in the operation of these axially supported Sb and Bi reagents. This is likely consistent with nitrene docking in the Cu₃ plane (cis with respect to the E–Cu₃,_cent axis) rather than on the encumbered side of the halide crown. Inspection of the solid-state structures of 3 and 4 provides evidence of ample coordination space externally to the Cu₃ ring (in-plane). Additionally, the difference between Sb(III) and Bi(III) in terms of electron donicity to the Cu₃ cluster may be insufficient to significantly alter the metal-nitrene electrophilicity.

The amination yields for the functionalization of the benzylic substrates by catalysts [(TMG₃trphen-Sb)Cu₃(μ-X)₃] (X = Cl (3), Br (5), I (6)) vary more appreciably (Table 4). For the panel of the para-substituted ethylbenzenes (entries 1–9), each catalyst provides good yields not only with substrates bearing ED substituents (entries 1–3) but also with para-X (X = F, Cl, Br, I) substituted congeners (entries 4–7), a trend that usually denotes significant benzyl radical stabilization via resonance effects. Only strongly EW substituents (entries 8, 9) are associated with low yields, as anticipated for electrophilic metal nitrene-driven reactions. The yields for the amination of sec-benzylic sites (entry 10) are modest most likely due to steric encumbrance, and those for prim-benzylic sites (entry 11) are below ≤10% given the higher aliphatic C–H bond energy of toluene (90 kcal/mol⁵⁰). Steric hampering is also reflected in the moderate yields for the benzylic amination of 1,2,3,4-tetrahydronaphthalene and derivatives (entries 12, 13). Competitions between benzylic and tert-C–H sites (entries 14, 15) singularly result in benzylic aminations with yields increasing with increasing distance between the two sites. Finally, acetate not only deactivates adjacent C–H sites (entry 16), as anticipated, but curiously seems to interfere with the operation of the Br- and I-possessing catalysts 5 and 6. Lateral comparisons between catalysts 3, 5, and 6 do not reveal any particular trends possibly because the effect of the halide is likely to be multivariate in regulating (i) the electrophilicity of the putative metal nitrene; (ii) the approach of the substrate to the active catalytic site; and (iii) the stability offered to any substrate-centered carboradical (or carbocation) upon H atom abstraction. Finally, the halide-free catalyst [(TMG₃trphen-Sb)₂Cu₂](PF₆)₂ (8a) affords similar yields and trends to those encountered with the aforementioned reagents but also shows signs of more tolerance to steric encumbrance and/or electron-withdrawing functionalities (entries 14–16).

Direct comparisons to the halide-rich reagents 3–6 cannot be made at the present time given the structural uniqueness of catalyst 8a. On average, the previously explored catalyst [(TMG₃trphen)Cu](PF₆),²⁷ featuring N_amine apical coordination, is more active than the present reagents, but this may be due to its mononuclear character. Incidentally, an interesting mechanistic distinction between Cu(I) catalysts that carry halide ligands and others that are halide-free, has been recently reported by Peréz and co-workers.⁵¹

Hammett Plots.

To start placing these reagents in a mechanistic context, Hammett plots were constructed for the competitive amination (PhINTces) of ethylbenzene versus seven para-X-ethylbenzenes (X = MeO, Me, F, Br, I, CF₃, and NO₂) as mediated by catalyst 3 (5 mol % with respect to Cu) in PhCF₃/HFIP (10:1 v/v) at 30 °C (5 h). Reasonably, good linear free-energy correlations of log (k_X/k_H) versus the polar substituent parameter σ_p (ρ_p = −1.70, R² = 0.95) and, even better, versus the resonance-sensitive σ ⁺ (ρ⁺ = −1.22, R² = 0.98) can be obtained (Figure 8 and Table S5). The negative ρ_p and ρ⁺ coefficients indicate that significant positive charge develops en route to the transition state, presumably at the benzylic carbon, by means of an incipient hydrogen-atom abstraction by an electrophilic oxidant. These sizable ρ values are more consistent with stepwise benzylic C–H functionalization ([Ru₂(hp)₄Cl], ρ⁺ = −0.90; [Ru₂(esp)₂SbF₆], ρ⁺ = −1.49)⁵² by comparison to the more modest values found in Rh amination chemistry (−0.47,^17g −0.55,^17b −0.66,⁵³ −0.73,^17a −0.90^17f); the latter is characterized by a concerted asynchronous C–H insertion pathway.

Figure 8.

Linear free energy correlation of log(k_X/k_H) as a function of σ_p (left) (ρ_p = −1.70, R² = 0.95) and σ⁺ (right) (ρ⁺ = −1.22, R² = 0.98) for the competitive amination of para-substituted ethylbenzenes versus ethylbenzene catalyzed by [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3).

CONCLUSIONS

The following are the major findings and insights of the present study:

1. The synthesized tripodal ligands TMG₃trphen-E (E = Sb (1) and Bi (2)) generate extended cavities that permit capturing of M₃(μ-X)₃ fragments (M = Cu, Ag; X = Cl, Br, I) extracted from anhydrous MX precursors. Single-crystal crystallographic analysis indicates that the crown-shaped M₃(μ-X)₃ units are supported by axial Sb(III) and Bi(III) elements, triply bridging the M₃ triangle, and equatorial N-donor residues provided by the superbasic TMG ligand arms. ¹H NMR data suggest that the rotation of the CN₃ branches of TMG is partially restricted at ambient temperatures to various degrees, commensurate with the bulk of the halide, with resolution of all four methyl groups emerging at low temperatures.

2. Although abstraction of the halides in select cases can best be achieved by means of TlPF₆, the most convenient path for obtaining halide-free copper(I) compounds with ligands 1 and 2 involves the use of [Cu(NCMe)₄](PF₆) or [Cu(NCMe)₂][B(C₆F₅)₄]. The resulting complexes [(TMG₃trphen-E)₂Cu₂]²⁺ (E = Sb and Bi) are dimeric in the solid state and possess an asymmetric Cu₂(μ-E)₂ core that is retained in solution for E = Bi and tentatively for E = Sb, according to ¹H NMR data. Other efforts to dechlorinate [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] by means of AgPF₆ and AgOTf lead to ligand transmetalation and protonation, respectively. Partial dechlorination and cluster rearrangement is achieved with KB(C₆F₅)₄.

3. Energy decomposition analysis (EDA) studies performed on [(TMG₃trphen-E)Cu₃(μ-Cl)₃] (E = Sb (3) and Bi (4)) indicate that among the attractive interactions, the electrostatic component contributions (63%) far exceed those associated with orbital interactions (31%) and dispersion energy (6%) for both compounds. The natural orbitals for chemical valence (NOCV) analysis, further supported by the independent gradient model based on Hirshfeld partition (IGMH) method, suggest that the dominant orbital interaction is that between the Cu(I) sites and N_TMG residues, followed by the Sb/Bi → Cu₃ donor interaction between the Sb 5s or Bi 6s orbitals and admixed Cu 4s^0+x/3d^10−x orbitals. The attractive Sb → Cu₃ interaction is more pronounced than that of Bi, given the increased s-character of the 6s lone pair. Little, if any, π-backdonation can be extracted from the analysis, suggesting that 1 and 2 are largely L-type ligands.⁵⁴

4. An initial evaluation of [(TMG₃trphen-E)Cu₃(μ-Cl)₃] (E = Sb (3), Bi (4)) as nitrene-transfer agents in the aziridination of various p-substituted styrenes by PhINTs afforded very good to excellent product yields, although catalyst 4 showed signs of fragility. Further evaluation of 3 as the catalyst for the aziridination of other aromatic and aliphatic alkenes provided good to modest yields and selectivities, respectively, in response to steric (aromatic alkenes) and electronic (aliphatic alkenes) challenges.

5. The amination of C–H bonds is best accomplished by the more electrophilic PhINTces in PhCF₃/HFIP (10:1 v/v). An extensive study of catalysts [(TMG₃trphen-E)Cu₃(μ-X)₃] (E = Sb, X = Cl (3) Br (5), I (6)); E = Bi, X = Cl (4) and [(TMG₃trphen-Sb)₂Cu₂](PF₆)₂ (8a) demonstrates good yields, almost exclusively for the amination of electron-rich and unencumbered secbenzylic-C–H bonds (mostly para-substituted ethyl-benzenes). Although no discernible patterns of reactivity emerge that can be correlated with catalyst characteristics, reagents 3 and 8a are more consistent in their behavior. Their activity is still inferior to the previously explored [(TMG₃trphen)Cu](PF₆) catalyst²⁷ presumably because the latter catalyst is strictly mononuclear. Hammett plots for the competitive p-X-ethylbenzene/ethylbenzene amination (PhINTces) mediated by 3, provide evidence that a stepwise C–H activation takes place by an electrophilic oxidant, featuring an intermediate benzylic carboradical.

Future studies will address more precisely the interaction between these reagents and the N-donor sources, to explore the location of nitrene docking and further establish the mode of operation of the resulting active oxidant responsible for C–H aminations. In addition, catalyst modifications that are purported to incorporate halide substituents in the ligand framework will be pursued, given the known role of halide substituents in Z-type ligands.^29,55

EXPERIMENTAL SECTION

Safety Warning.

Antimony and thallium compounds can be highly toxic and should be handled and disposed carefully.

Synthesis and Characterization of Ligands and Metal Compounds.

TMG₃trphen-Sb•3CH₂Cl₂ (1).

2-(2′-bromophenyl)-1,1,3,3-tetramethylguanidine (1.10 g, 4.08 mmol) was dissolved in 20 mL of Et₂O in a Schlenk flask. The solution was cooled in a dry ice/acetone bath to −78 °C, after which a cyclohexane solution of sec-butyllithium (1.4 M, 6.4 mL, 8.99 mmol) was added, effecting a color change to bright orange. After stirring for 1 h while being kept cold, SbCl₃ (0.307 g, 1.34 mmol) dissolved in THF (10 mL) was added at −78 °C via cannula. The reaction mixture was kept at −78 °C for 2 h and then was allowed to warm to room temperature and stirred overnight. After the addition of 1.0 mL of methanol, the volatiles were removed in vacuo, producing a yellow-orange residue. The crude was dissolved in EtOAc (20 mL) and washed with water (10 mL). The aqueous phase was extracted with additional EtOAc (20 mL). The combined organic phases were dried over sodium sulfate and concentrated by rotary evaporation to give an oily mass. The oily mass was dissolved in 10 mL of CH₂Cl₂ and chilled to 0 °C for 1 h, separating colorless crystals. The white solid was collected by filtration, washed with cooled CH₂Cl₂ (5 mL), and dried in vacuum (yield = 0.580 g, 62%). ¹H NMR (CD₃CN, 1.94 ppm): δ 7.08 (t, 3H, aryl), 6.97(d, 3H, aryl), 6.60 (t, 3H, aryl), 6.41(d, 3H, aryl), 2.57 (s, 36H, CH₃).¹³C NMR (CD₃CN, 118.26, 1.32 ppm): δ 159.0, 157.5, 137.6, 135.3, 129.1, 120.6, 120.2, 39.9. IR (KBr, cm⁻¹): ν 3031, 2932, 2889, 1539,1452, 1419, 1378, 1136, 1018, 791, 723, 535. UV–vis (CH₃CN): λ_max (ε (M⁻¹ cm⁻¹)) 220(76263). Elem. Anal. calcd for C₃₆H₅₄Cl₆N₉Sb (1): C, 45.64; H, 5.75, N, 13.31. Found: C, 45.71; H, 5.76, N, 13.29.

TMG₃trphen-Bi•3CH₂Cl₂ (2).

2-(2′-bromophenyl)-1,1,3,3-tetramethylguanidine (1.16 g, 4.31 mmol) was dissolved in 20 mL of THF in a Schlenk flask. The solution was cooled in a dry ice/acetone bath to −78 °C, after which a cyclohexane solution of sec-butyllithium (1.4 M, 6.8 mL, 9.48 mmol) was added, effecting a color change to bright orange. After stirring for 1 h while being kept cold, BiCl₃ (0.448 g, 1.42 mmol), dissolved in THF (10 mL), was added at −78 °C via cannula. The reaction mixture was kept at −78 °C for 2 h and then was allowed to warm to room temperature. The reaction mixture was refluxed for 16 h, and subsequently, the solvent was removed to give an oily residue that was redissolved in toluene (20 mL). The reaction mixture was heated again and stirred at 90–95 °C for 3 h. The temperature of the reaction was lowered to ambient conditions, and the solvent was evacuated to dryness to give an oily mass. The residual oily mass was suspended in CH₂Cl₂ (10 mL) and was stirred vigorously at room temperature for 30 min, separating white crystals that were collected by filtration (yield = 0.550 g, 49%). ¹H NMR (CD₃CN, 1.94 ppm): δ 7.49 (d, 3H, aryl), 7.09 (t, 3H, aryl), 6.65 (t, 3H, aryl), 6.51(d, 3H, aryl), 2.58 (s, 36H, CH₃) ¹³C NMR (CD₃CN, 118.26, 1.32 ppm): δ 159.2, 157.6, 139.5, 128.5, 122.7, 120.7, 39.9. IR (KBr, cm⁻¹): ν 3031, 2918, 2791, 1545, 1443, 1419, 1372, 1133, 1012, 781, 747, 724, 534. UV–vis (CH₃CN): λ_max (ε (M⁻¹ cm⁻¹)) 214 (59181). Elem. Anal. calcd for C₃₆H₅₄BiCl₆N₉ (2): C, 41.79; H, 5.26, N, 12.18. Found: C, 41.73; H, 5.23, N, 12.12.

[(TMG₃trphen-Sb)Cu₃(μ-Cl)₃]•4CH₂Cl₂ (3).

To a suspension of CuCl (138 mg, 1.39 mmol) in THF (12 mL) was added a solution of TMG₃trphen-Sb (1) (321 mg, 0.463 mmol) in THF (18 mL) at room temperature. The solution was stirred for 24 h. The volatiles were removed under vacuum, and the residue was redissolved in CH₂Cl₂ (10 mL) and stirred for 6 h. The solution was filtered and concentrated to give an off-white-colored powder (yield = 0.380 g, 83%). The compound was recrystallized from CH₂Cl₂ to obtain X-ray-quality crystals. ¹H NMR (CD₂Cl₂, 5.32 ppm, 243 K): δ 7.17–7.14 (t, 3H, aryl), 7.02–7.01 (d, 3H, aryl), 6.69–6.66 (t, 3H, aryl), 6.46–6.44 (d, 3H, aryl), 3.10(s, 9H, CH₃), 2.76 (s, 9H, CH₃), 2.58 (s, 9H, CH₃), 1.67 (s, 9H, CH₃). ¹³C NMR (CD₂Cl₂, 54.0 ppm): δ 166.2, 156.3, 136.5, 131.7, 126.0, 122.0, 121.7, 40.4, 30.6. IR (KBr, cm⁻¹): ν 3476, 3044, 2929, 2879, 2791, 1519, 1453, 1405, 1392, 1330, 1266, 1155, 1027, 853, 750, 725, 536. UV–vis (CH₂Cl₂): λ_max (ε (M⁻¹ cm⁻¹)) 235 (225820). Elem. Anal. calcd for C₃₃H₄₈Cl₃Cu₃N₉Sb (3 – 4CH₂Cl₂): C, 40.05; H, 4.89, N, 12.74. Found: C, 39.99; H, 4.84, N, 12.80.

[(TMG₃trphen-Bi)Cu₃(μ-Cl)₃]•4CH₂Cl₂ (4).

To a suspension of CuCl (57.2 mg, 0.58 mmol) in THF (12 mL) was added a solution of TMG₃trphen-Bi (2) (150 mg, 0.192 mmol) in THF (18 mL) at room temperature. The solution was stirred for 24 h. The volatiles were removed under vacuum, and the residue was redissolved in 1,2-dichloroethane (10 mL) and stirred for 6 h. The solution was filtered and concentrated to give off-white colored powder (yield = 0.205g, 99%). The compound was recrystallized from CH₂Cl₂ to obtain X-ray-quality crystals. ¹H NMR (CD₂Cl₂, 5.32 ppm, 243 K): δ 7.50–7.48 (d, 3H, aryl), 7.21–7.18 (t, 3H, aryl), 6.80–6.77 (t, 3H, aryl), 6.58–6.56 (d, 3H, aryl), 3.19(s, 9H, CH₃), 2.82 (s, 9H, CH₃), 2.61 (s, 9H, CH₃), 1.69 (s, 9H, CH₃).¹³C NMR (CD₂Cl₂, 54.0 ppm): δ 162.5, 150.2, 143.4, 127.7, 121.9, 118.5, 115.7, 40.7, 39.9. IR: 3040, 2924, 2878,2114, 1518, 1447, 1415, 1406, 1388, 1330, 1260, 1234, 1199, 1152, 1109, 1064, 1041, 1025, 923, 850, 805, 751, 720, 536. UV–vis (CH₂Cl₂): λ_max (ε (M⁻¹ cm⁻¹)) 235 (26520). Elem. Anal. calcd for C₃₃H₄₈BiCl₃Cu₃N₉ (4 – 4CH₂Cl₂): C, 36.81; H, 4.49, N, 11.71. Found: C, 36.77; H, 4.45, N, 11.78.

[(TMG₃trphen-Sb)Cu₃(μ-Br)₃]•4CH₂Cl₂ (5).

To a suspension of CuBr (205 mg, 1.43 mmol) in THF (12 mL) was added a solution of TMG₃trphen-Sb (1) (330 mg, 0.477 mmol) in THF (18 mL) at room temperature. The solution was stirred for 24 h. The volatiles were removed under vacuum, and the residue was redissolved in CH₂Cl₂ (10 mL) and stirred for 6 h. The solution was filtered and concentrated to give off-white colored powder (yield = 0.402 g, 85%). The compound was recrystallized in CH₂Cl₂ to obtain X-ray-quality crystals. ¹H NMR (CD₂Cl₂, 5.32 ppm, 243 K): δ 7.25–7.21 (t, 3H, aryl), 7.08–7.07 (d, 3H, aryl), 6.75–6.71 (t, 3H, aryl), 6.51–6.49 (d, 3H, aryl), 3.20(s, 9H, CH₃), 2.85 (s, 9H, CH₃), 2.65 (s, 9H, CH₃), 1.74 (s, 9H, CH₃). ¹³C NMR (CD₂Cl₂, 54.0 ppm): δ 165.9, 156.0, 136.0, 130.9, 121.5, 120.7, 40.8, 15.0. IR (KBr, cm⁻¹): ν 3043, 2924, 2881,2792, 1517,1453,1416, 1404, 1392, 1330, 1282, 1265, 1202, 1155, 1116, 1063, 1047, 1028, 926, 854, 807, 753, 726, 705, 537, 452. UV–vis (CH₂Cl₂): λ_max (ε (M⁻¹ cm⁻¹)) 230 (225450). Elem. Anal. calcd for C₃₃H₄₈Br₃Cu₃N₉Sb (5 – 4CH₂Cl₂): C, 35.30; H, 4.31, N, 11.23. Found: C, 35.34; H, 4.34, N, 11.19.

[(TMG₃trphen-Sb)Cu₃(μ-I)₃]•3CH₂Cl₂ (6).

To a suspension of CuI (372 mg, 1.95 mmol) in THF (12 mL) was added a solution of TMG₃trphen-Sb (1) (450 mg, 0.650 mmol) in THF (18 mL) at room temperature. The solution was stirred for 24 h. The volatiles were removed under vacuum and the residue was redissolved in CH₂Cl₂ (10 mL) and stirred for 6 h. The solution was filtered and concentrated to give off-white colored powder (yield = 0.530 g, 82%). The compound was recrystallized from CH₂Cl₂ to obtain X-ray-quality crystals. ¹H NMR (CD₂Cl₂, 5.32 ppm, 243 K): δ 7.22–7.18 (t, 3H, aryl), 7.05–7.03 (d, 3H, aryl), 6.73–6.69 (t, 3H, aryl), 6.45–6.43 (d, 3H, aryl), 3.19(s, 9H, CH₃), 2.84 (s, 9H, CH₃), 2.61 (s, 9H, CH₃), 1.70 (s, 9H, CH₃). ¹³C NMR (CD₂Cl₂, 54.0 ppm): δ 166.6, 156.3, 136.1, 130.7, 121.5, 120.3, 40.7, 14.8. IR (KBr, cm⁻¹): ν 3042, 2923,2880, 2792, 1557,1544, 1512, 1454, 1415, 1404, 1392, 1329, 128. UV–vis (CH₂Cl₂): λ_max (ε (M⁻¹ cm⁻¹)) 340(18032). Elem. Anal. calcd for C₃₃H₄₈Cu₃I₃N₉Sb (6 – 3CH₂Cl₂): C, 31.36; H, 3.83, N, 9.97. Found: C, 31.38; H, 3.85, N, 9.93.

[(TMG₃trphen-Sb)Ag₃(μ-Cl)₃]•3CH₂Cl₂ (7).

To a solution of TMG₃trphen-Sb (1) (186 mg, 0.268 mmol) in THF (20 mL), AgCl (115 mg, 0.805 mmol) was added at room temperature. The suspension was stirred for 2 weeks, protected from light. The volatiles were removed under vacuum, and the residue was redissolved in CH₂Cl₂ (20 mL) and stirred for 12 h. The solution was filtered and concentrated to give off-white colored powder (yield = 0.215 g, 71%). The compound was recrystallized from CH₂Cl₂ to obtain X-ray-quality crystals. ¹H NMR (CD₂Cl₂, 5.32 ppm, 243 K): δ 7.25–7.22 (t, 3H, aryl), 7.13–7.11 (d, 3H, aryl), 6.77–6.73(t, 3H, aryl), 6.52–6.50 (d, 3H, aryl), 2.66 (s, 36H, CH₃). ¹³C NMR (CD₂Cl₂, 54.0 ppm): 165.1, 156.6, 137.2, 130.9, 122.1, 120.8, 39.8, 29.6. IR (KBr, cm⁻¹): ν 3020, 2930, 2870, 1519, 1454, 1417, 1406, 191, 1263, 1153, 1026, 852, 758, 725, 536. UV–vis (CH₂Cl₂): λ_max (ε (M⁻¹ cm⁻¹)) 295(12534). Elem. Anal. calcd for C₃₃H₄₈ Ag₃Cl₃N₉Sb (7 – 3CH₂Cl₂): C, 35.31; H, 4.31, N, 11.23. Found: C, 35.36; H, 4.33, N, 11.19.

[(TMG₃trphen-Sb)₂Cu₂](PF₆)₂•2THF (8a).

A solution of [Cu(CH₃CN)₄](PF₆) (108 mg, 0.288 mmol) in acetonitrile/THF (5 mL each) was added to a prestirred solution of TMG₃trphen-Sb (1) (200 mg, 0.288 mmol) in acetonitrile (20 mL) at room temperature. The suspension was stirred for 24 h. The reaction mixture was filtered, reduced to 5 mL, and stored at −35 °C for 2 weeks to obtain X-ray quality crystals (yield = 0.145 g, 51%). ¹H NMR (CD₃CN, 1.94 ppm): δ 7.45–7.44 (d, 6H, aryl), 7.17–7.13 (t, 6H, aryl), 6.80–6.76 (t, 6H, aryl), 6.42–6.39 (d, 6H, aryl), 2.67 (s, 72H, CH₃). ¹³C NMR (CD₃CN, 118.26, 1.32 ppm): 163.9, 157.0, 137.2, 131.6, 130.0, 122.3, 121.6, 120.9, 40.1. IR: 3672, 2935, 2883, 1515, 1454, 1416, 1394, 1324, 1281, 1265, 1231, 1201, 1142, 1111, 1062, 1046, 1021, 875, 834, 783, 762, 750, 726, 710, 556, 536, 467, 448. UV–vis (CH₃CN): λ_max (ε (M⁻¹ cm⁻¹)) 275(70831). Elem. Anal. calcd for C₆₆H₉₆Cu₂F₁₂N₁₈P₂Sb₂ (8a – 2THF): C, 43.99; H, 5.37, N, 13.99. Found: C, 44.02; H, 5.38, N, 13.92.

[(TMG₃trphen-Sb)₂Cu₂][B(C₆F₅)₄]₂•2MeCN (8b).

An acetonitrile solution (10 mL) of [Cu(CH₃CN)₂][B(C₆F₅)³¹ (64.4 mg, 0.078 mmol) was added to a prestirred solution of TMG₃trphen-Sb (1) (36.05 mg, 0.052 mmol) in acetonitrile (10 mL) at room temperature. The suspension was stirred for 24 h. The reaction mixture was filtered, reduced to 5 mL under vacuum, and stored at −35 °C for 1 week to obtain X-ray quality colorless crystals (yield = 0.054 g, 70%). ¹H NMR (CD₃CN, 1.94 ppm): δ 7.50–7.46 (d, 6H, aryl), 7.19–7.15(t, 6H, aryl), 6.82–6.78 (t, 6H, aryl), 6.44–6.42 (d, 6H, aryl), 2.69 (s, 72H, CH₃). ¹³C NMR (CD₃CN, 118.26, 1.32 ppm): δ 165.07, 157.0, 137.1, 129.5, 121.9, 121.7, 120.3, 52.1, 39.7. IR (cm⁻¹): 2932, 1643, 1513, 1460, 1420, 1396, 1324, 1274, 1156, 1085, 1021, 978, 850, 807, 774, 755, 727, 683, 661, 611, 574, 537. UV–vis (CH₃CN): λ_max (ε (M⁻¹ cm⁻¹)) 215(147250). Elem. Anal. calcd for C₁₁₈H₁₀₂B₂Cu₂F₄₀N₂₀Sb₂ (8b): C, 48.01; H, 3.48, N, 9.49. Found: C, 48.07; H, 3.50, N, 9.45.

[(TMG₃trphen-Bi)₂Cu₂](PF₆)₂•2Et₂O (9).

A solution of [Cu(CH₃CN)₄](PF₆) (143.4 mg, 0.385 mmol) in acetonitrile (5 mL) was added to a prestirred solution of TMG₃trphen-Bi (2) (200 mg, 0.256 mmol) in acetonitrile (20 mL) at room temperature. The suspension was stirred for 24 h. The reaction mixture was filtered, reduced to 5 mL under vacuum, and carefully layered with diethyl ether to obtain X-ray quality crystals at −35 °C over a period of 2 weeks (yield = 0.035 g, 6.4%). ¹H NMR (CD₃CN, 1.94 ppm): δ 7.85–7.83 (d, 2H, aryl), 7.26–7.22 (t, 6H, aryl), 7.19–7.15 (t, 2H, aryl), 6.95–6.91 (t, 4H, aryl), 6.88–6.84 (d, 2H, aryl), 6.83–6.80 (d, 6H, aryl), 6.56–6.54 (d, 2H, aryl), 2.75 (s, 42H, CH₃), 2.67(s, 30H, CH₃). ¹³C NMR (CD₃CN, 118.26, 1.32 ppm): 164.9, 157.0, 139.3, 130.0, 129.3, 124.0, 123.1, 122.1, 40.3. IR: 3677, 3367, 2937, 1630, 1526, 1485, 1472, 1451, 1424, 1398, 1322, 1263, 1233, 1205, 1158, 1066, 1038, 831, 750, 696, 606, 556, 442. UV–vis (CH₃CN): λ_max (ε (M^{− 1} cm^{− 1} )) 265(58035). Elem. Anal. calcd for C₆₆H₉₆Bi₂Cu₂F₁₂N₁₈P₂ (9 – 2Et₂O): C, 40.11; H, 4.90, N, 12.76. Found: C, 40.08; H, 4.88, N, 12.80.

[(TMG₃H₃trphen-Sb)Cu(OTf)₃](OTf)•1.21CH₂Cl₂ (10).

A methylene chloride solution (15 mL) of [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3) (64.1 mg, 0.065 mmol) was added to solid Ag(OTf) (49.93 mg, 0.195 mmol), and the reaction was stirred for 12 h in the absence of light. The resulting mixture was filtered to remove AgCl and reduced to 5.0 mL under vacuum. The clear solution was kept at −35 °C over a period of 1 week, affording large colorless crystals of the titled compound (yield = 0.036 g, 14%; based on Cu). ¹H NMR (CD₂Cl₂, 5.32 ppm): δ 7.36–7.32 (t, 3H, aryl), 7.25–7.23 (d, 3H, aryl), 6.93–6.89 (t, 3H, aryl), 6.61–6.59 (d, 3H, aryl), 2.85 (bs, 36H, CH₃), 1.26 (s, 3H, = NH). ¹³C NMR (CD₂Cl₂, 54.0 ppm): 166.5, 155.6, 136.5, 132.5, 128.0, 122.6, 122.0, 40.7, 30.2. IR: 3225, 3053, 2930, 1630, 1547, 1520, 1456, 1419, 1398, 1328, 1262, 1222, 1154, 1028, 855, 753, 636, 517, 447. UV–vis (CH₂Cl₂): λ_max (ε (M⁻¹ cm⁻¹)) 235(10664). Elem. Anal. calcd for C₃₇H₅₁CuF₁₂N₉O₁₂S₄Sb (10 – 1.21CH₂Cl₂): C, 32.79; H, 3.79, N, 9.30. Found: C, 32.75; H, 3.78, N, 9.35.

[(TMG₃trphen-Sb)Cu₃(μ₃-Cl)₃Cu]₂[B(C₆F₅)₄]₂•10CH₂Cl₂ (11).

A methylene chloride solution (20 mL) of [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃] (3) (47.0 mg, 0.047 mmol) was added to K[B(C₆F₅)₄] (102.3 mg, 0.142 mmol), and the reaction mixture was stirred for 24 h. The resulting mixture was filtered to remove KCl and reduced to 5.0 mL under vacuum. The solution was kept at −35 °C over a period of 1 week, to provide off-white crystals of the titled compound (yield = 0.046 g, 75%; based on Cu). ¹H NMR (CDCl₃, 7.26 ppm): δ 7.27–7.24 (t, 6H, aryl), 7.19–7.17 (d, 6H, aryl), 6.85–6.82 (t, 6H, aryl), 6.49–6.47 (d, 6H, aryl), 3.15 (bs, 12H, CH₃), 2.81 (bs, 40H, CH₃), 1.90 (bs, 20H, CH₃). ¹³C NMR (CDCl₃, 77.2 ppm): 166.0, 155.4, 137.8, 135.4, 132.2, 122.4, 121.5, 53.9, 40.0. IR: 2928, 1643, 1513, 1457, 1420,1407, 1396, 1330, 1270, 1233, 1158, 1085, 1031, 977, 854, 809,774, 755, 725, 683,661, 611, 574, 537, 450. UV–vis (CH₃CN): λ_max (ε (M⁻¹ cm⁻¹)) 230(14172). Elem. Anal. calcd for C₁₁₄H₉₆B₂Cl₆Cu₈F₄₀N₁₈Sb₂ (11 – 10CH₂Cl₂): C, 39.53; H, 2.79, N, 7.28. Found: C, 39.49; H, 2.78, N, 7.31.

Catalytic and Mechanistic Studies.

General Catalytic Olefin Aziridination Procedure.

In a typical experiment, a 20 mL screw-cap vial containing a small magnetic bar was charged in sequence with the catalyst (0.0125 mmol with respect to Cu), N(p-tolylsulfonyl)-imido]phenyliodinane (93.3 mg, 0.25 mmol), molecular sieves (5 Å) (20 mg), olefin (2.0 mmol), and solvent (0.200 g) (acetonitrile, unless otherwise stated). The reaction mixture was stirred vigorously at 30 °C for 24 h (unless otherwise stated). After completion of the reaction, the products were isolated by column chromatography (silica gel) and quantified by ¹H NMR (in CDCl₃) versus an internal standard (4′-methoxyacetophenone). All aziridination and/or allylic/benzylic amination products are known compounds and have been identified with the assistance of ¹H by comparison to spectroscopic features reported for authentic samples in the literature.

General Catalytic Benzylic Amination Procedure.

In a standard method for copper-catalyzed amination, a tiny magnetic stirrer was placed in a flame-dried 20 mL screw cap vial. To this vial, catalyst (0.0125 mmol with respect to copper), 2,2,2-trichloroethyl(phenyl-λ₃-iodanylidene)sulfamate (PhINTces) (214.4 mg, 0.500 mmol), and molecular sieves (5 Å) (20 mg) were added sequentially. A solution of the substrate (0.250 mmol) in a mixture of trifluorotoluene and HFIP (10:1 v/v, 300 mg) was added to the vial that contained the solids. The reaction was stirred at 30 °C for 24 h. After completion of the reaction, the products were isolated by column chromatography (silica gel) using a mixture of hexane and ethyl acetate and were quantified by ¹H NMR (in CDCl₃) versus an internal standard (4′-methoxyacetophenone).

Hammett Plots.

The general procedure for the amination of benzylic substrates was conducted with the assistance of the catalyst [(TMG₃trphen-Sb)Cu₃(μ-Cl)₃]•4CH₂Cl₂ (3) (0.0125 mmol with respect to Cu). The reaction mixture was composed of 1.0 mmol of ethylbenzene, 1.0 mmol of p-X-ethylbenzene (X = Me, MeO, F, Cl, Br, CF₃, and NO₂), and PhINTces (2 mmol) in the PhCF₃:HFIP mixture (10:1 v/v). The reaction vial was sealed tightly and wrapped with aluminum foil to protect the reaction mixture from light. The reactions were stirred for 6 h. At the end of the reaction, the mixture was flash chromatographed on silica gel (CH₂Cl₂) in order to recover the benzylic aminated products and evaluate their ratio by quantitative ¹H NMR analysis in CDCl₃.