Heterobimetallic Activation of Dioxygen

Abstract

Reaction of the known germylene Ge[N(SiMe₃)₂]₂ and a new heterocyclic variant Ge[(NMes)₂(CH)₂] with [L^Me2 Cu]₂ (L^Me2 = the β -diketiminate derived from 2-(2,6-dimethylphenyl)amino-4-(2,6-dimethylphenyl)imino-2-pentene) yielded novel Cu(I)-Ge(II) complexes L^Me2Cu-Ge[(NMes)₂(CH)₂] (1a) and L^Me2Cu-Ge[N(SiMe₃)₂]₂ (1b), which were characterized by spectroscopy and X-ray crystallography. The lability of the Cu(I)-Ge(II) bond in 1a and b was probed by studies of their reactivity with benzil, PPh₃, and an N-heterocyclic carbene (NHC). Notably, both complexes are cleaved rapidly by PPh₃ and the NHC to yield stable Cu(I) adducts (characterized by X-ray diffraction) and the free germylene. In addition, the complexes are highly reactive with O₂ and exhibit chemistry which depends on the bound germylene. Thus, oxygenation of 1a results in scission and formation of thermally unstable L^Me2CuO₂, which subsequently decays to [(L^Me2Cu)₂(μ-O)₂], while 1b yields L^Me2Cu(μ-O)₂Ge[N(SiMe₃)₂]₂, a novel heterobimetallic intermediate having [Cu^III(μ-O)₂Ge^IV]³⁺ core. The isolation of the latter species by direct oxygenation of a Cu(I)-Ge(II) precursor represents a new route to heterobimetallic oxidants comprising copper.

Efforts to understand the mechanisms of oxidation catalysis¹ are aided by studies aimed at isolating and characterizing metal-oxygen intermediates in proteins² and synthetic systems.³ Such intermediates are often derived from reactions of reduced metal sites with the versatile and abundant reagent O₂, and particular attention has been placed on those species that incorporate a single type of metal ion. For instance, extensive examination of the reactions of O₂ with Cu(I) complexes has led to the identification of a variety of binding motifs and activation pathways of relevance to important biological processes.⁴ Mixed metal systems that operate synergistically to activate O₂ also are of interest, as they are implicated in some synthetically and biologically useful catalytic reactions. Notable examples that involve Cu include Cu-Pd species in Wacker-type oxidations⁵ and Cu-Fe intermediates in cytochrome c oxidase.^3d,6

Examples of well-characterized heterobimetallic oxygen intermediates derived from O₂ are rare, however, due in part to the challenges associated with preventing formation of products arising from oxygenation of only one of the two types of metal ions provided in the starting material(s). Nonetheless, mixed heme-Fe/nonheme-Cu species with peroxo and oxo bridges have been isolated from reactions of Fe(II)-Cu(I) precursors.⁶ In addition, heterobimetallic bis(μ-oxo)CuNi or -CuPd species have been generated by reacting isolable 1:1 Cu/O₂ species⁷ or (PPh₃)₂PdO₂ with a mononuclear complex of a reduced metal (e.g. Ni(I) or Cu(I), respectively).⁸ While this “stepwise” approach toward the synthesis of heterobimetallic oxygen intermediates is attractive, we were intrigued by the reported⁹ discoveries that the two metal ions in (PR₃)₂M-Ge[N(SiMe₃)₂]₂ (M = Pd, Pt) acted together to activate O₂ to afford mixed metal peroxo and bis(μ-oxo)MGe complexes, despite the capability of the M-Ge precursors to cleave to reactive monomeric fragments (e.g., the trappable germylene Ge[N(SiMe₃)₂]₂). Reasoning by analogy, we hypothesized that related Cu(I)-Ge(II) compounds might be prepared and that they might react similarly with O₂ to yield novel Cu-Ge oxygen intermediates.

Herein we report the synthesis and structural characterization of Cu(I)-Ge(II) complexes that contain bonds between these metal centers. Studies of the reactivity of these complexes with a variety of reagents show that the nature of the germylene fragment (i.e., the supporting ligands on Ge) is critical for determining whether monomeric fragments are produced or the heterobimetallic nature of the Cu-Ge compound is retained. Notably, we report the characterization of a novel bis(μ-oxo)Cu(III)Ge(IV) species derived from oxygenation of a Cu(I)-Ge(II) precursor, thus demonstrating the feasibility of a new type of direct route to heterobimetallic oxygen intermediates comprising copper.

Experimental Section

All solvents and reagents were obtained from commercial sources and used as received unless noted otherwise. The solvents tetrahydrofuran (THF), toluene, pentane, diethyl ether (Et₂O), and acetonitrile (MeCN) were degassed and passed through solvent purification columns (Glass Contour, Laguna, CA or MBraun) prior to use. The NMR solvents C₆D₆ and THF-d₈ were dried over CaH₂ or Na/benzophenone and distilled under a nitrogen atmosphere. All metal complexes were prepared and stored in a Vacuum Atmospheres inert atmosphere glove box under a dry nitrogen atmosphere or were manipulated under argon using standard Schlenk line techniques. NMR spectra were recorded on a Varian VXR-300 or VI-300 spectrometer. Chemical shifts (δ) for ¹H (300 MHz) and ¹³C (75 MHz) NMR spectra were referenced to residual nuclei in the deuterated solvent. Chemical shifts (δ) for ³¹P{¹H} (121 MHz) NMR spectra were referenced to an external standard (85% H₃PO₄). UV-vis spectra were recorded on a HP8453 (190-1100 nm) diode array spectrophotometer equipped with a Unisoku low temperature cryostat. Resonance Raman spectra were recorded on an Acton 506 spectrometer using a Princeton Instruments LN/CCD-11100-PB/UVAR detector and ST-1385 controller interfaced with Winspec software. The spectra were obtained at -196 °C using a backscattering geometry. Excitation at 457.9 and 488 nm was provided by a Spectra Physics BeamLok 2065-7S Ar Laser. Samples were frozen in a copper cup attached to a coldfinger Dewar filled with liquid N₂. Raman shifts were externally referenced to liquid indene. Elemental analyses were performed by Atlantic Microlab, Inc. (Norcross, GA) and Robertson Microlit (Madison, NJ). GeCl₂·dioxane and benzil were purchased from Aldrich Chemical Corp. and used as received. The complexes L^R2Cu (MeCN) (R = Me, Et, iPr; L^R2 = β-diketiminate derived from respective 2-(2,6-di-R-phenyl)amino-4-(2,6-dimethylphenyl)imino-2-pentene),¹⁰ [L^Me2Cu]₂,¹⁰ L^iPr2CuO₂,^7a 1,3-dimesitylimidazol-2-ylidene (NHC^Mes2),¹¹ N , N ’-dimesitylethanediimine,¹² and Ge[N(SiMe₃)₂]₂ ¹³ were synthesized according to literature procedures.

1,3-dimesityl-1,3,2-diazagermol-2-ylidene (Ge[(NMes)₂(CH)₂])^{Error! Bookmark not defined}

Solid yellow N,N’-dimesitylethanediimine (496 mg, 1.7 mmol) was placed in a 50 ml Schlenk flask with lithium metal (30 mg, 4.3 mmol) under argon, and THF (15 mL) was added via cannula transfer. The mixture immediately turned deep red and was stirred overnight. The resulting red-brown solution was filtered through Celite, and to the filtrate was added GeCl₂·dioxane (393 mg, 1.7 mmol) in 5 ml THF. The solution was stirred for 2 h, after which the solvent was removed in vacuo to give an orange-brown solid. The solid was extracted with pentane (15 ml) and filtered through Celite to give a light orange solution. Removal of the pentane gave Ge[(NMes)₂(CH)₂] as an orange powder. Recrystallization from pentane (4 mL) at -20 °C yielded analytically pure orange crystals (425 mg, 69%). ¹H NMR (C₆D₆): δ 6.87 (s, 4H), 6.58 (s, 2H), 2.23 (s, 12H), 2.19 (S, 6H) ppm. ¹³C{¹H} NMR (THF-d₈): δ 142.0, 134.7, 133.3, 128.4, 125.0, 19.8, 17.2 ppm. Anal. Calcd. for C₂₀H₂₄N₂Ge: C, 65.80; H, 6.63; N, 7.67. Found: C, 65.53; H, 6.48; N, 7.61.

L^Me2Cu-Ge[(NMes)₂(CH)₂] (1a)

To a solution of [L^Me2Cu]₂ (54 mg, 0.073 mmol) in THF (4 mL) was added a solution of Ge[(NMes)₂(CH)₂] (53 mg, 0.15 mmol) in THF (4 mL). An immediate color change from very pale yellow to bright lemon yellow was observed, and the reaction was stirred for 30 min. The solution was filtered through Celite, and removal of the solvent from the filtrate under reduced pressure resulted in the isolation of 1a as a bright yellow powder. Recrystallization from Et₂O (10 ml) at -20 °C yielded analytically pure yellow crystals (83 mg, 78%). X-ray quality crystals were grown from a concentrated pentane solution at -20 °C. ¹H NMR (C₆D₆): δ 6.95 (m, 6H) 6.82 (s, 4H), 6.21 (s, 2H), 4.88 (s, 1H), 2.22 (s, 6H), 1.99 (s, 12H), 1.91 (s, 12H), 1.61 (s, 6H) ppm. ¹³C{¹H} NMR (THF-d₈): δ 161.5, 152.4, 139.9, 135.2, 133.7, 129.1, 128.4, 127.4, 123.5, 122.0, 93.4, 21.1, 19.9, 17.6, 16.9 ppm. Anal. Calcd for C₄₁H₄₉N₄CuGe: C, 67.09; H, 6.73; N, 7.63. Found: C, 66.66; H, 6.76; N, 7.57.

L^Me2Cu-Ge[N(SiMe₃)₂]₂ (1b)

Ge[N(SiMe₃)₂]₂ (71 mg, 0.18 mmol) in THF (4 mL) was added to a solution of [L^Me2Cu]₂ (67 mg, 0.09 mmol) in THF (4 mL), which immediately turned deep orange. The reaction was stirred for 30 min, filtered through Celite, and the solvent removed from the filtrate under reduced pressure to give 1b as an orange solid in approximately quantitative yield (135 mg). X-ray quality crystals were grown from a concentrated MeCN/Et₂O solution at -20 °C. ¹H NMR (C₆D₆): δ 6.98 (m, 6H), 4.95 (s, 1H), 2.33 (s, 12H), 1.66 (s, 6H), 0.14 (s, 36H) ppm. ¹³C{¹H} NMR (THF-d₈): δ 162.7, 152.3, 130.0, 127.9, 122.5, 94.0, 21.8, 19.0, 4.5 ppm. Anal. Calcd for C₃₃H₆₁N₄Si₄CuGe: C, 51.99; H, 8.06; N, 7.35. Found: C, 49.50; H, 7.51; N, 6.68. Repeated attempts to obtain satisfactory elemental analysis were unsuccessful due to small amounts of impurities present, as evidenced by ¹H NMR spectroscopy (see text).

Benzil Trapping Experiments

1b (5 mg, 0.0066 mmol) was mixed with four equiv. of benzil (5.5 mg, 0.026 mmol) in C₆D₆ (0.5 mL) and the solution was transferred to a screwcap NMR tube. ¹H NMR spectra were monitored over the course of six hours and the conversion to trapped germylene was followed by the growth of a resonance at 0.36 ppm for the Me₃Si hydrogens of the germanium(IV) species 2. The extent of the conversion to 2 was determined by comparing the integration of the peak at 0.14 ppm for 1b and that at 0.36 ppm for 2.

L^Me2Cu(PPh₃) (3)

Independent synthesis: A solution of PPh₃ (36 mg, 0.14 mmol) in Et₂O (4 mL) was added to a suspension of [L^Me2Cu]₂ (50 mg, 0.068 mmol) in Et₂O (5 mL). The solution was stirred for 30 min, filtered through Celite, and the solvent was removed from the filtrate under reduced pressure to yield 3 as a tan powder (82 mg, 95%). X-ray quality crystals were grown from a concentrated pentane solution at -20 °C. ¹H NMR (C₆D₆): δ 6.88 (m, 21H), 5.05 (s, 1H), 2.12 (s, 12H), 1.73 (S, 6H) ppm. ¹³C{¹H} NMR (THF-d₈): δ 161.7, 152.3, 133.5, 133.0, 132.9, 132.7, 129.9, 128.9, 128.1, 127.9, 127.7, 122.0, 93.5, 21.6, 17.8 ppm. ³¹P{¹H} NMR (C₆D₆, 121.5 MHz): 5.45 ppm. Anal. Calcd for C₃₉H₄₀N₂PCu: C, 74.20; H, 6.39; N, 4.44. Found: C, 74.06; H, 6.57; N, 4.36. From 1a: To a solution of 1a (10 mg, 0.014 mmol) in 0.5 mL C₆D₆ was added one equivalent of PPh₃ (3.6 mg). The solution immediately changed from a bright to a pale yellow. ¹H NMR spectroscopy revealed quantitative formation of 3 and the free germylene Ge[N(SiMe₃)₂]₂. From 1b: To a solution of 1b (10 mg, 0.013 mmol) in 0.5 mL C₆D₆ was added one equivalent of PPh₃ (3.4 mg). The solution immediately changed from deep orange to pale yellow. ¹H NMR spectroscopy revealed quantitative formation of 3 and free germylene Ge[N(SiMe₃)₂]₂.

L^Me2Cu(NHC^Mes2) (4)

Independent synthesis: A solution of NHC^Mes2 (50 mg, 0.16 mmol) in THF (4 mL) was added to a solution of [L^Me2Cu]₂ (60 mg, 0.08 mmol) in THF (5 mL). The solution was stirred for 30 min, filtered through Celite, and the solvent was removed from the filtrate under reduced pressure to yield 4 as a tan powder (100 mg, 91 %). X-ray quality crystals were grown from a concentrated pentane solution at -20 °C. ¹H NMR (C₆D₆): δ 7.01 (m, 6H), 6.77 (s, 4H), 5.78 (s, 2H), 4.81 (s, 1H), 2.18 (S, 6H), 1.96 (s, 12H), 1.63 (s, 12H), 1.53 (s, 6H) ppm. ¹³C{¹H} NMR (THF-d₈): δ 185.3, 160.0, 153.7, 137.7, 136.9, 135.6, 130.3, 128.5, 127.2, 121.1, 120.9, 92.3, 21.4, 20.1, 18.7, 17.5 ppm. Anal. Calcd for C₄₂H₄₉N₄Cu: C, 74.91; H, 7.33; N, 8.32. Found: C, 75.17; H, 6.95; N, 8.27. From 1a: To a solution of 1a (10 mg, 0.014 mmol) in 0.5 mL C₆D₆ was added one equivalent of NHC^Mes2 (4.3 mg). The solution immediately changed from bright to pale yellow. ¹H NMR spectroscopy revealed quantitative formation of 4 and the free germylene Ge[(NMes)₂(CH)₂]. From 1b: To a solution of 1b (10 mg, 0.013 mmol) in 0.5 mL C₆D₆ was added one equivalent of NHC^Mes2 (4.0 mg). The solution immediately changed from deep orange to pale yellow. ¹H NMR spectroscopy revealed quantitative formation of 4 and the free germylene Ge[N(SiMe₃)₂]₂.

Oxygenation of 1b to form L^Me2Cu(μ-O)₂Ge[N(SiMe₃)₂]₂

A solution of 1b (20 mg, 0.026 mmol) in toluene (1.2 mL) was cooled to -80 °C in a MeOH/liquid N₂ bath. Dry O₂ was bubbled through the cold solution for 10 min, during which time very little change was observed. The solution was stirred at -80 °C for four hours, during which time the color became deep orange-brown. The UV-vis spectrum was measured by transfer of a known volume of the oxygenated solution using a pre-cooled gas-tight syringe to a UV-vis cuvette filled with a known volume of toluene at -80 °C (λ_max ∼ 440 nm, ε ∼ 4400 M^-1cm^-1, as approximated from the initial concentration of 1b). To prepare the ¹⁸O-isotopomer, the solution of 1b was frozen by immersion in liquid N₂, the head space was evacuated, and ¹⁸O₂ gas was transferred from a glass bulb. The solution was allowed to warm to -80 °C, and it was stirred for 4 h, during which time the solution turned deep orange-brown. Resonance Raman spectra were obtained by micropipet transfer of the solution to a copper cup on a coldfinger and frozen with liquid N₂.

Reaction of L^iPr2CuO₂ with Ge[N(SiMe₃)₂]₂ to form L^ipr2 Cu(μ-O)₂Ge[N(SiMe₃)₂]₂

Dry O₂ was passed through solution of L^iPr2Cu(MeCN) (10 mg, 0.019 mmol) in toluene (1.0 mL) at -80 °C for 10 min to form L^iPr2CuO₂. The solution was freeze-pump-thaw degassed and purged with argon to remove excess O₂. One equivalent of Ge[N(SiMe₃)₂]₂ (7.4 mg, 0.019 mmol) was added in 0.2 mL of toluene by syringe, upon which time the solution immediately turned deep orange-brown. The ¹⁸O-isotopomer was prepared similarly, except the solution of L^iPr2Cu(MeCN) (10 mg, 0.019 mmol) in toluene (1.0 mL) was frozen in a 10 mL Schlenk flask by immersion in liquid N₂, the headspace was evacuated, ¹⁸O₂ gas was transferred from a glass bulb, and the solution was allowed to thaw to -80 °C and stirred for 30 min. The solution was freeze-pump-thaw degassed and purged with argon to remove excess O₂. One equivalent of Ge[N(SiMe₃)₂]₂ (7.4 mg, 0.019 mmol) was added in 0.2 mL of toluene by syringe, resulting in an immediate color change to deep orange-brown. Resonance Raman spectra were obtained by transfer of the solutions to a copper cup on a coldfinger and frozen with liquid N₂. The UV-vis spectrum was measured by addition of one equivalent of Ge[N(SiMe₃)₂]₂ in toluene by syringe to a toluene solution of degassed L^iPr2CuO₂ in a UV-vis cuvette at -80 °C (λ_max ∼ 463 nm, ε ∼ 4100 M^-1cm^-1, as approximated from the initial concentration of L^iPr2Cu(MeCN)).

Oxygenation of 1a to form L^Me2CuO₂

A 0.1 mM solution of L^Me2Cu-Ge[(NMes)₂(CH)₂] (1a) in THF was cooled to -80 °C in a UV-vis cuvette, and dry O₂ was bubbled for approximately 10 seconds. The addition of O₂ was followed by the immediate disappearance of the 402 nm band in the UV-vis spectrum and the growth of a new shoulder at 390 nm (ε ∼ 2400 M^-1cm^-1, approximated based on the initial concentration of 1a) over approximately 10 min. Warming the solution to -65 °C resulted in the growth of a new feature at 422 nm. Likewise, addition of O₂ to more concentrated solutions of 1a in THF at -80 °C (5-20 mM) resulted in the rapid formation of a dark yellow-brown intermediate with a UV-vis feature at 422 nm. Resonance Raman experiments on the concentrated solutions (λ_ex = 457.9 nm) revealed a strong peak at 608 cm^-1, consistent with the formation of [(L^Me2Cu)₂(μ-O)₂] and the observed UV-vis spectrum.

Addition of Ge[N(SiMe₃)₂]₂ to L^Me2CuO₂ to form L^Me2Cu(μ-O)₂Ge[N(SiMe₃)₂]₂

A 0.1 mM solution of 1a in THF was cooled to -80 °C in a UV-vis cuvette and dry O₂ was bubbled for approximately 10 s. The addition of O₂ was followed by the immediate disappearance of the 402 nm band in the UV-vis spectrum and the growth of a new shoulder at 390 nm over approximately 10 min. The solution was purged with argon for 30 min to remove excess O₂, and one equivalent of Ge[N(SiMe₃)₂]₂ in 0.1 mL THF was added by syringe. The rapid formation of an absorption feature was observed (λ_max ∼ 440 nm, ε ∼ 4000 M^-1cm^-1, approximated based on the initial concentration of 1a), consistent with that observed previously for L^Me2Cu(μ-O)₂Ge[N(SiMe₃)₂]₂ obtained upon oxygenation of L^Me2Cu-Ge[N(SiMe₃)₂]₂.

X-ray Crystallography

Data were collected on either a Bruker or Siemens SMART Platform CCD diffractometer at 173(2)K. Data collections were carried out using MoKα radiation (graphite monochoromator) at a distance of 4.9 cm. The intensity data were integrated using SAINT¹⁴ and were corrected for absorption and decay using SADABS.¹⁵ The structures were solved by direct methods using SHELXL-97¹⁶ software. All non-hydrogen atoms were refined with anisotropic displacement parameters, and all hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. X-ray crystallographic data and tables (Table S1) with pertinent details for each structure are located in the Supporting Information.

Results and Discussion

Synthesis and Characterization of Cu(I)-Ge(II) Complexes

We chose as starting material a β-diketiminate complex of Cu(I) previously described by Warren and coworkers,¹⁷ [L^Me2Cu]₂ (Scheme 1), which we prepared in a slightly different fashion by vacuum drying of L^Me2Cu(MeCN)¹⁰ and recrystallization from pentane. An X-ray crystallographic analysis (Figure S1) revealed a dimeric structure similar to that which was reported, although variations in the crystal system and slight differences in some bond distances were observed. Two germylenes were selected for reaction with the Cu(I) reagent, the known compound Ge[N(SiMe₃)₂]₂¹³ and a previously unreported heterocyclic germylene Ge[(NMes)₂(CH)₂]. The latter compound was synthesized by the reductive dimetalation of the α-diimine N₂^{Mes2 11} with Li metal followed by metathesis with GeCl₂·dioxane, a procedure similar to that used previously to synthesize related heterocyclic stannylenes¹⁸ and germylenes.¹⁹ Studies of the reactivity and stability of related heterocyclic germylenes^19,20 suggested that Ge[(NMes)₂(CH)₂] would exhibit different electronic properties than Ge[N(SiMe₃)₂]₂, which we hypothesized might be manifested in reactivity differences for the targeted Cu(I)-Ge(II) complexes.

Scheme 1.

Addition of one equivalent (per copper) of Ge[N(SiMe₃)₂]₂ or Ge[(NMes)₂(CH)₂] to a pale yellow THF solution of [L^Me2Cu]₂ resulted in the instantaneous formation of deep orange or bright lemon-yellow solutions, respectively. In the case of the reaction with Ge[(NMes)₂(CH)₂], the product was isolated analytically pure upon filtration and crystallization from Et₂O and identified on the basis of UV-vis and ¹H NMR spectroscopy and X-ray crystallography as L^Me2Cu-Ge[(NMes)₂(CH)₂] (1a, Scheme 1). For the reaction with Ge[N(SiMe₃)₂]₂, the exceedingly high solubility of the product (1b) in common organic solvents hindered the isolation of an analytically pure sample, and ¹H NMR spectra in both C₆D₆ and THF-d₈ revealed a small amount of free Cu(I) complex in all batches synthesized (∼2-3% relative to 1b). Nevertheless, we were able to obtain X-ray quality crystals of 1b from a concentrated acetonitrile/Et₂O solution at -20 °C that enabled determination of its structure.

The UV-vis spectra of 1a and b in THF exhibit an intense band at 342 or 345 nm (ε ∼ 27000 M^-1cm^-1) and a weaker shoulder at 402 or 409 nm (ε ∼ 7700 or ∼ 4700 M^-1cm^-1), respectively (Figure 1). On the basis of analogy to spectra previously reported for L^R2Cu(NCR’) complexes,^7a,11 the former intense features may be assigned as β-diketiminate-based π → π* transitions and the latter shoulders as Cu → Ge[N(SiMe₃)₂]₂/Ge[(NMes)₂(CH)₂] charge transfer (MLCT) transitions. Both complexes exhibit sharp ¹H NMR spectra that are shifted relative to the starting materials. Thus, for example, the ¹H NMR spectrum of 1a in C₆D₆ shows the β-diketiminate backbone hydrogen peak at 4.88 ppm, downfield from the value of 4.75 ppm for the Cu(I) starting material, and the olefinic hydrogens of the germylene ligand at 6.21 ppm shifted upfield relative to their position in free Ge[(NMes)₂(CH)₂] (6.58 ppm). Resonances for the germylene olefin and the 2,6-methyl groups of the mesitylene ring for 1a are somewhat broadened, suggestive of a fluxional process in solution that we have yet to define.

Figure 1.

UV-vis spectra of L^Me2Cu-Ge[(NMes)₂(CH)₂] (1a, solid line) and L^Me2Cu-Ge[N(SiMe₃)₂]₂ (1b, dashed line) in THF at 25 °C.

The X-ray structures of 1a and b (Figure 2) confirm their formulations and are consistent with the presence of direct Cu(I)-Ge(II) bonds, with Cu-Ge distances of 2.2138(4) Å and 2.2492(4) Å, respectively. In the structure of 1b, the N(SiMe₃)₂ groups were found to be disordered over two positions (supporting information). Only a few other complexes with Cu-Ge bonds are known,²¹ and these feature Ge(IV) centers with longer Cu-Ge distances in the range 2.33-2.38 Å. While the Cu(I)-Ge(II) distances in 1a and b are similar to each other, the relative orientation of the N-M-N planes in the two complexes differ significantly. In 1a, the N1-Cu1-N2 plane forms an angle of 44° relative to the N3-Ge1-N4 plane, whereas in 1b these respective planes are nearly perpendicular. The latter arrangement would maximize backbonding interaction between the filled copper d_xy orbital and the empty germanium p_y orbital, where the x axis is defined by the Cu-Ge vector. Due to its involvement in the heterocyclic aromatic system this p orbital may be less available for this interaction in 1a than in 1b,²² helping to rationalize the observed conformational preferences. Similar backbonding has been invoked in related Cu(I)-carbene²³ and other metal-germylene complexes,²⁴ but given the relatively poor backbonding capabilities of Cu(I) and the steric differences between the supporting ligands, other influences such as crystal packing could also be important.

Figure 2.

Molecular structures of (a) 1a and (b) 1b with heteroatoms labeled, all atoms as 50% thermal ellipsoids, and hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): (a) Cu1-N1, 1.9194(17); Cu1-N2, 1.9153(17); Cu1-Ge1, 2.2138(4); N1-Cu1-N2/N3-Ge1-N4, 44.03(7). (b) Cu1-N1, 1.956(3); Cu1-N2, 1.938(3); Cu1-Ge1, 2.2492(4); N1-Cu1-N2/N3A-Ge1-N4A, 88.15(21). For 1b, only one conformation of the disordered N(SiMe₃)₂ groups is shown.

Reactivity with Benzil, PPh₃, and a N-Heterocyclic Carbene

In order to probe the propensity of the Cu(I)-Ge(II) bond in 1a and b to dissociate we explored reactions of these compounds with reagents that are known to trap germylenes (benzil) or Cu(I) (PPh₃ and N-heterocyclic carbenes). The results are summarized in Scheme 2.

Scheme 2.

Benzil is known to be reduced rapidly by Ge[N(SiMe₃)₂]₂ to yield the Ge(IV)-enediolate (Ph₂C₂O₂)Ge[N(SiMe₃)₂]₂ so it has been used as a trapping agent to assess the degree of germylene dissociation from (PPh₃)₂M-Ge[N(SiMe₃)₂]₂ complexes, where M = Ni, Pd, and Pt.^9b,25 We envisioned similar use of benzil to probe germylene dissociation from 1a and b. However, addition of excess benzil to a C₆D₆ solution of Ge[(NMes)₂(CH)₂] resulted only in slow decomposition and no isolable Ge(IV) product. Thus, only the reaction of benzil with 1b was explored. Addition of four equivalents of benzil to a C₆D₆ solution of 1b and monitoring by ¹H NMR spectroscopy revealed the slow formation of (Ph₂C₂O₂)Ge[N(SiMe₃)₂]₂ and L^Me2Cu(C₆D₆)¹⁷ to the extent of approximately 90% conversion after 5 h (t_1/2 ∼ 70 min). In comparison, the complexes (PPh₃)₂M-Ge[N(SiMe₃)₂]₂ were reported to generate the same Ge(IV) product within 1 min for M = Ni, and to reach ∼90% conversion after 4 h and 94 h for M = Pd and Pt, respectively.^9b Thus, by this crude measure, the Cu(I)-Ge(II) bond in 1b is less prone to dissociation than the Ni(I)-Ge(II) compound, is similarly labile compared to the Pd(II)-Ge(II) bond, and is more reactive than the Pt(II)-Ge(II) case.

In contrast to the slow benzil trapping of free germylene observed for 1b, complexes 1a and b both react instantaneously at room temperature and at -100 °C with PPh₃ and the N-heterocyclic carbene NHC^Mes2,¹² with concurrent bleaching of the solution color and disappearance of the ∼ 400 nm absorption feature in the UV-vis spectra. ¹H NMR spectra revealed the quantitative formation of the respective free germylene and the Cu(I) adducts L^Me2Cu(PPh₃) (3) and L^Me2Cu(NHC^Mes2) (4), each of which was synthesized independently from [L^Me2Cu]₂ and PPh₃ or NHC^Mes2, respectively, and characterized structurally by X-ray crystallography (Figure S2). Overall, the structures resemble those of related molecules reported previously.^24,26,27 Notably, in 4 the N3-C-N4 plane of the carbene is nearly perpendicular to the N1-Cu-N2 plane, most likely due to steric interactions as N-heterocyclic carbenes are known to be weak π-acceptor ligands for Cu(I).²³

The findings that benzil traps Ge[N(SiMe₃)₂]₂ from 1b only slowly and the reactions of 1a and b with PPh₃ and NHC^Mes2 proceed essentially instantaneously suggest that the latter reactions proceed through associative pathways. This hypothesis is consistent with the previously described associative pathway for the oxygenation of L^iPr2Cu(NCR) to yield L^iPr2CuO₂,⁷ as well as for the displacement of O₂ from this product by phosphines.²⁷ Although detailed kinetic studies would be required to more definitively show that associative mechanisms are operative, the data nonetheless suggest that 1a and b might react with O₂ in a similar manner such that substrate and germylene are bound simultaneously to the copper, with the nature of the bound germylene possibly playing a role in the intermediates observed.

Reactivity with Dioxygen

Both 1a and b dissolved in THF or toluene reacted with O₂ at low temperature (-80 °C), but the course of the reactions differ significantly. Considering first 1b, oxygenation in toluene resulted in development of a deep orange-brown color accompanied by a feature at λ_max = 440 nm (ε ∼ 4400 M^-1cm^-1) in the UV-vis spectrum which degraded upon warming of the sample. Resonance Raman experiments using 488 nm laser excitation²⁸ revealed a single strong peak at 578 cm^-1 that shifted to two peaks when the sample was prepared with ¹⁸O₂ (Figure 3). The magnitude of the isotopic shift to the average of the two ¹⁸O₂ peaks is Δ_avg = 26 cm^-1, consistent with attribution of the 578 cm^-1 feature to a bis(μ-oxo)dimetal core vibration that is split into a Fermi doublet upon O-isotope substitution.^8,29 The λ_max and ν(M₂O₂) values (440 nm, 578 cm^-1) differ significantly from those of [(L^Me2Cu)₂(μ-O)₂] (422 nm, 608 cm^-1),¹⁰ thus arguing against dissociation of 1b into germylene and L^Me2Cu(I) fragments that individually oxygenate. Instead, the data indicate formation of the heterobimetallic bis(μ-oxo)copper(III)germanium(IV) complex L^Me2Cu(μ-O)₂Ge[N(SiMe₃)₂]₂ (Scheme 3). Similar results were reported for the oxygenations of (PPh₃)₂M-Ge[N(SiMe₃)₂]₂ (M = Pd, Pt) that yield [M^II(μ-O)₂Ge^IV]²⁺ cores,⁹ but the generation of a heterobimetallic bis(μ-oxo) complex via oxygenation of a reduced heterobimetallic precursor comprising Cu(I) is unprecedented.

Figure 3.

(a) UV-vis spectrum of L^Me2Cu-Ge[N(SiMe₃)₂]₂ (1b) + O₂ in toluene at -80 °C. (b) Resonance Raman spectra (λ_ex = 488 nm) of frozen toluene solutions (77K) of L^MeCu(μ-O)₂Ge[N(SiMe₃)₂]₂ (solid line for ¹⁶O₂, dashed line for ¹⁸O₂, * denotes solvent peaks).

Scheme 3.

A similar [Cu^III(μ-O)₂Ge^IV]³⁺ core is also accessible via a stepwise approach (Scheme 3). Thus, addition of 1 equiv. Ge[N(SiMe₃)₂]₂ to a deoxygenated solution of L^iPr2CuO₂ in toluene at -80 °C resulted in the immediate development of a deep orange-brown color, a UV-vis band at λ_max = 463 nm (ε ∼ 4100), and a peak at 578 cm^-1 in the resonance Raman spectrum (λ_ex = 488 nm) that shifted to a doublet when L^iPrCu¹⁸O₂ was used (Δ_avg = 27 cm^-1) (Figure S3). These data clearly support formation of L^iPr2Cu(μ-O)₂Ge[N(SiMe₃)₂]₂ via a reaction akin to previously reported syntheses involving such a stepwise method,⁸ and provide further confirmation of the similar nature of the product from the direct oxygenation of 1b.

In contrast to the O₂ reaction with 1b, oxygenation of 1a in THF (0.1 mM) at -80 °C resulted in instantaneous disappearance of the 402 nm Cu → Ge[(NMes)₂(CH)₂] MLCT feature in the UV-vis spectrum (intermediate A, Figure 4), followed by the gradual (∼ 10 minutes) development of a new absorption feature at 390 nm (ε ∼ 2400) and a shift of the π → π* bands from 345 to 326 nm (intermediate B). These new features remain unperturbed at -80 °C, even upon purging with argon, but convert further upon warming to -65 °C or when the reaction is performed at higher concentrations (∼ 5-15 mM) at temperatures less than -90 °C to the spectrum previously reported for [(L^Me2Cu)₂(μ-O)₂] (Figure S4). The formation of this species was confirmed by resonance Raman spectroscopy (ν(Cu₂O₂) = 608 cm^-1). On the basis of analogy to the UV-vis spectrum for L^iPr2CuO₂, we assign intermediate B as the previously unobserved, relatively sterically unhindered variant L^Me2CuO₂. Unfortunately, the rapidity of the decay of intermediate B to [(L^Me2Cu)₂(μ-O)₂] at the higher concentrations required to obtain resonance Raman spectra prevented direct confirmation of its structural assignment via this method. Consistent with it being L^Me2CuO₂, however, was our finding that addition of 1 equiv. Ge[N(SiMe₃)₂]₂ to a deoxygenated solution yielded the spectral features for L^Me2Cu(μ-O)₂Ge[N(SiMe₃)₂]₂. Identification of intermediate A is more difficult, as it is metastable and lacks diagnostic spectroscopic features. Thus, we can only speculate that it is some sort of O₂ adduct of 1a that decays to L^Me2CuO₂ and a germanium species, such as the germylene Ge[(NMes)₂(CH)₂], which unlike Ge[N(SiMe₃)₂]₂,³⁰ appears not to react cleanly with available O₂ to yield a [Ge^IV₂(μ-O)₂]⁴⁺ core.

Figure 4.

UV-vis spectra of L^Me2Cu-Ge[(NMes)₂(CH)₂] (1a) in THF (0.1 mM) at -80 °C (solid line), the immediate product of oxygenation (intermediate A, Scheme 4, dotted line), the species resulting after allowing intermediate A to stand for 10 minutes (intermediate B, dashed line), and the product resulting from warming of intermediate B to -65 °C (dot-dash line).

In summary, on the basis of the available evidence we propose that the sequence of reactions shown in Scheme 4 occurs upon oxygenation of 1a. In contrast to 1b, which yields a stable heterobimetallic [Cu^III(μ-O)₂Ge^IV]³⁺ complex, 1a cleaves to yield L^Me2CuO₂ and, presumably, Ge[(NMes)₂(CH)₂]. The relatively unhindered (compared to L^iPr2CuO₂)⁷ L^Me2CuO₂ decays to [(L^Me2Cu)₂(μ-O)₂] via a process facilitated by warming or when high concentrations of 1a are used. Differences in the nature of the germylene fragment are clearly responsible for the divergent O₂ reactivity of 1a and b, with the greater intrinsic stability of the heterocyclic Ge[(NMes)₂(CH)₂] possibly underlying its ejection rather than formation of a heterobimetallic [Cu^III(μ-O)₂ Ge^IV]³⁺ core.

Scheme 4.

Conclusions

A pair of novel Cu(I)-Ge(II) complexes 1a and b that differ with respect to the nature of the germylene fragment have been synthesized and characterized by spectroscopic and X-ray crystallographic methods. Studies of their reactivity with benzil, PPh₃, and an N-heterocyclic carbene (NHC) showed that (a) the lability of the Cu(I)-Ge(II) bond in 1b with respect to trapping of the germylene with benzil is similar to that of a previously reported Pd(II)-Ge(II) analog, and (b) both complexes are cleaved rapidly by PPh₃ and an NHC to yield stable Cu(I) adducts and the free germylene, presumably by associative processes. In addition, the complexes are highly reactive with O₂ and exhibit unique chemistry which depends on the bound germylene. In one case (1a) oxygenation results in the generation of a new 1:1 Cu/O₂ adduct supported by a relatively sterically unhindered β-diketiminate ligand, whereas in another (1b) a heterobimetallic intermediate having a [Cu^III(μ-O)₂Ge^IV]³⁺ core forms. The isolation of the latter species by direct oxygenation of a Cu(I)-Ge(II) precursor represents a new route to heterobimetallic oxidants comprising copper, the reactivity of which will be of great interest in future work aimed at understanding O₂ activation and catalysis by mixed-metal systems.