Copper, Palladium, and Platinum-Containing Complexes of an Asymmetric Dinucleating Ligand

Abstract

The coordination chemistry of an asymmetric dinucleating hexadentate ligand LH₂ comprising neutral alkyltriamine and potentially dianionic dicarboxamido-pyridyl donor sets with copper, palladium, and platinum has been explored. Monometallic, dicopper, and heterodinuclear Cu-Pd and -Pt complexes have been prepared and characterized, including by NMR, EPR, UV-vis, and IR spectroscopy and X-ray crystallography. For example, the monometallic complexes [(LH₂)MCl]X (M = Cu, X = OTf; M = Pd or Pt, X = Cl) were prepared, wherein the metal(II) ions is coordinated to the triamine portion and the pyridyldicarboxamide is unperturbed. Treatment of LH₂ with [MesCu]_x (Mes = mesityl) provided a monocopper(I) complex, again with the metal coordinated only to the trialkylamine donor set. Reaction of [(LH₂)CuCl]OTf with NaOMe resulted in an unexpected migration of the copper(II)-chloride fragment to the pyridyldicarboxamide site to yield Na[LCuCl], from which a dicopper complex LCu₂Cl₂ and mixed-metal complexes LCu(Cl)M(Cl) (M = Pd, Pt) were prepared by addition of CuCl₂ or MCl₂, respectively. The heterodinuclear complexes were also prepared by addition of CuCl₂ to [(LH₂)MCl]Cl.

Many dimetallic protein active sites and catalysts contain different metal ions (cf. FeZn purple acid phosphatases,¹ CuZn superoxide dismutase,² FeCu cytochrome c oxidase,³ Pd/Cu oxidation catalysts⁴) or two of the same metal ions in different coordination environments and/or oxidation states (cf. dopamine β-monooxygenase, peptidylglycine α-hydroxylating monoxygenase⁵). In efforts to understand the structural and functional properties of such active sites and catalysts, heterodinuclear or otherwise asymmetric complexes are important targets for preparation and characterization.⁶ A particularly effective strategy for their synthesis (among others⁷) is to use dinucleating ligands featuring disparate metal binding sites.^8,9 We aim to use this strategy to prepare mixed-valent dicopper and heterodinuclear complexes containing copper adjacent to other metal ions, with the ultimate goal of designing new oxidation catalysts and gaining new mechanistic insights into oxidation catalysis.

A recent report^9c of NiFe models of a key portion of the active site of carbon monoxide dehydrogenase using LH₂ (Figure 1) as a supporting ligand inspired us to explore its copper coordination chemistry. A key objective is to prepare molecules with reactive cores like 1 and 2, which have been considered^10,11 as the active oxidant in particulate methane monoxygenase.¹² Such species are of great interest due to their novelty and the significance of the hydrocarbon functionalization reaction they are proposed to perform. The ligand LH₂ is attractive for use in preparing such species because it features two distinct donor arrays within a single macrocyclic ligand framework, a potentially dianionic, meridional pyridyldicarboxamide and a neutral, flexible trialkyltriamine. Both donor sets have been used in isolation to examine copper chemistry relevant to oxidation catalysis by copper sites in enzymes. For example, the pyridyldicarboxamide 3 was found to stablize a copper(III)-hydroxide complex that rapidly abstracts hydrogen atoms from weak C-H bonds,¹³ and was used to prepare a tetragonal copper(II)-superoxide complex.¹⁴ Trialkyltriamine ligands have been used extensively to support Cu(I)/O₂ chemistry, including for the isolation and characterization of a variety of reactive intermediates, such as peroxodicopper and bis(oxo)dicopper(III,III) complexes.¹⁵ We envisioned that insertion of copper ions into LH₂ could provide novel dicopper species with proximal metal ions in coordination environments that differ significantly with respect to geometry and charge, resulting in unique reactivity.

Figure 1.

Ligand LH₂ and related pyridyldicarboxamide 3.

In addition, heterobimetallic complexes containing copper ions adjacent to other metal ions could be accessible using LH₂, if sequential incorporation of the different metal ions could be implemented. Mixed Pd- or Pt/Cu systems are of particular interest in the context of the Wacker and related oxidation processes,⁴ where, for instance, the role of Cu in the Pd-catalyzed oxidation of olefins continues to be debated.^16,17 New types of heterobinuclear Pd- and Pt/Cu complexes^{16, 18} would be useful for probing mechanistic issues surrounding oxidation catalysis,^{19, 20} with LH₂ being particularly advantageous in view of recent studies of Pd complexes supported by multidentate alkylamine ligands.^21–24

Herein, we report our initial explorations of the coordination chemistry of LH₂ with copper, palladium, and platinum ions. A variety of mono- and dimetallic complexes of these metal ions have been prepared and characterized and migration of copper(II) from one donor site to the other has been observed. These synthetic and structural studies demonstrate the versatility of LH₂ as a supporting ligand for generating asymmetric dinuclear complexes comprising copper, palladium, and/or platinum ions, and provide a foundation for future reactivity and mechanistic work.²⁵

Results and Discussion

Synthesis and Characterization of LH₂ and Cu/Pd, Pt-Chloride Complexes

We prepared and stored LH₂ as its hydrochloride salt as described in the literature, except using a different method for a key reduction step that in our hands was more reliable than the published^9c procedure (see Experimental Section). Treatment of LH₂·2HCl with base followed by Cu(OTf)₂ yielded the monometalled complex 4 (Scheme 1), which was isolated as a blue solid. The formulation of 4 was confirmed by ESI-MS, EPR spectroscopy, and X-ray crystallography. In the ESI mass spectrum obtained for a solution of the complex in MeOH, a parent ion at m/z 584 was observed with an isotope pattern consistent with the presence of a single copper ion (Figure S1). The EPR spectrum exhibits a slightly rhombically distorted axial signal consistent with an isolated N-donor ligated Cu(II) center in a tetragonal coordination environment, but with N-superhyperfine coupling unresolved (Figure 2a, b; g_x = 2.170, g_y = 2.054, g_z = 2.034, A_||(Cu) = 192 ×10⁻⁴ cm⁻¹). Confirmation of the insertion of a single Cu(II) ion into the trialkylamine portion of LH₂ came from its X-ray crystal structure (Figure 3a). The metal ion adopts a coordination geometry slightly distorted from square planar, and exhibits metal-ligand bond distances typical for 4-coordinate Cu(II).

Scheme 1.

Figure 2.

EPR spectra of 4 (a) and 6 (c) with simulations (b) and (d) respectively. Conditions: 9.64 GHz, 2K. See text for parameters determined from the simulations.

Figure 3.

Representations of the X-ray crystals of (a) the cationic portion of [(LH₂)CuCl]OTf (4), (b) [(LH₂)CuCl] (5), (c) [LCu₂Cl₂] (7), (d) the cationic portion of [(LH₂)PdCl]Cl (8), and (e) [LCuPdCl₂] (10) showing all non-hydrogen atoms (except the amide hydrogen atoms in 3, 4 and 8) as 50% thermal ellipsoids.

Treatment of LH₂·2HCl with excess [MesCu]_x (Mes = mesityl) also yielded a mononuclear complex (5), in this case comprising a single Cu(I) ion. In addition to exhibiting a ¹H NMR spectrum consistent with its formulation (Figure S2), the ESI mass spectrum of 5 contained a parent ion isotope pattern identical to that of 4 (Figure S3), indicating that it contains one copper ion and a chloride (apparently derived from the starting HCl salt of the ligand). The X-ray structure of 5 (Figure 3b) also is similar to that of the cationic portion of 4 insofar as it shows a copper-chloride moiety coordinated to the trialkylamine site but no metal ion bound to the pyridyldicarboxamide portion of the ligand. Unlike 4, however, the metal ion in 5 is bound strongly to only one nitrogen donor from the trialkylamine unit [Cu1-N5 = 1.965(2) Å], with the other two at distances that indicate only weak interactions [2.847(2) – 2.797(2) Å]. Thus, not surprisingly for Cu(I), the coordination geometry is essentially linear two-coordinate [N5-Cu-Cl1 = 178.12(7)°].

Addition of NaOMe to a blue solution of 4 in MeOH yielded a forest green solution, from which a green solid was isolated after filtration and removal of solvent. We hypothesize that this solid is best formulated as Na[LCuCl] (6, Scheme 1) on the basis of EPR and ESI-MS data; attempts to obtain crystals suitable for analysis by X-ray crystallography have not been successful. The EPR spectrum (Figure 2c, d) is significantly different from that of 4, and features an axial signal with resolved hyperfine splitting to a single copper ion [A_||(Cu) = 191 ×10⁻⁴ cm⁻¹] and three N atoms [A_⊥(¹⁴N) = 15 ×10⁻⁴ cm⁻¹] as determined by spectral simulation. Similar spectra have been reported for tetragonal complexes of ligand 3,^13,14 consistent with binding of the copper ion in 6 to the pyridyldicarboxamide portion of the macrocycle. In addition, the negative ion ESI mass spectrum of 6 in MeOH exhibited a parent ion at m/z 582 with an isotope pattern consistent with [6-Na]⁻ (Figure S4; we were unable to observe a positive ion spectrum). Taken together, the data support the hypothesis that added base deprotonates the pyridyldicarboxamide portion of 4 and results in migration of the copper(II)-chloride moiety to this fragment to yield 6.

Also consistent with the proposed formulation of 6, addition of M^IICl₂ (M^II = Cu, Pd, or Pt) to 6 yielded dinuclear complexes 7, 10, and 11. Compounds 7, 10, and 11 were fully characterized, with ESI-MS, CHN analysis, and X-ray crystallographic data being most informative. A peak envelope in the ESI mass spectrum for 7 is seen at m/z 705 with the appropriate isotope pattern for [M+Na]⁺ (Figure S6). In the X-ray structure of 7 (Figure 3c), two Cu(II) ions are bound to the macrocycle 4.4847(13) Å apart. The coordination geometry for Cu1 is square planar, with metal-ligand distances similar to that in [(3)CuCl]⁻.13 Because chloride Cl1 weakly coordinates to Cu2 at a distance of 2.693(2) Å, the resulting coordination geometry for Cu2 is different than in 5 (τ = 0.28, where a value of 0 corresponds to square pyramidal and 1 denotes trigonal bipyramidal) and is best described as distorted square pyramidal (τ = 0.56) with Cl1 occupying the axial position.²⁶ The 2K X-band EPR spectrum is broad and difficult to interpret without further detailed study (Figure S5).

The overall topologies of 10 (Figure 3e) and 11 (Figure S10) are similar to that of 7, albeit with Pd(II) or Pt(II) instead of Cu(II) in the trialkyltriamine site. In contrast to 7 (τ = 0.56), the M(II) (M = Pd or Pt) geometries in 10 (τ = 0.35) and 11(τ = 0.31) deviate little from square planar, and Cl2 does not interact significantly with the M(II) ion (Pd1-Cl2A and Pt1-Cl2b > 3.5 Å).²⁷ This missing interaction accompanies Cu-Pd/Pt distances of 4.956(1) Å and 5.070(1) Å, respectively, for 10 and 11 that are ~0.5 Å longer than the Cu-Cu separation in 7 (4.485(1) Å). These differences illustrate the ability of the supporting ligand to enable disparate metal-metal interactions. Consistent with their formulations, the EPR spectra of 10 and 11 exhibit similar axial signals with resolved hyperfine splitting to copper and nitrogen atoms (Figure S11 and S14; 10: g_|| = 2.197, g_⊥ = 2.053, A_||(Cu) = 178 ×10⁻⁴ cm⁻¹, A_⊥(¹⁴N) = 13 ×10⁻⁴ cm⁻¹; 11: g_|| = 2.188, g_⊥ = 2.045, A_||(Cu) = 178 ×10⁻⁴ cm⁻¹, A_⊥(¹⁴N) = 12 ×10⁻⁴ cm⁻¹). The complexes also exhibit peak envelopes in their ESI mass spectra with isotope patterns clearly attributable to heterodinuclear CuPd and CuPt complexes (Figures 4/S12 and S13, respectively).

Figure 4.

The negative ion ESI-MS peak envelope pattern for [M+Cl]⁻ for 10 (top, black) and its simulation (bottom, red). The full spectrum is shown in Figure S12.

In addition to being accessible from addition of PdCl₂ to 6, complex 10 can also be prepared from the monopalladium complex 8, which is isolated by treating LH₂·2HCl with base and PdCl₂. An analogous complex 9 was prepared using PtCl₂ as starting material. The formulations of 8 and 9 are supported by ¹H NMR spectroscopy, CHN analysis, ESI-MS, and X-ray crystallography. Notably, the ESI mass spectra contain parent ions with isotope patterns indicative of the presence of a single Pd or Pt atom (Figures S7 and S8). The X-ray crystal structures (Figure 3e for 8 and Figure S9 for 9) are similar to that of 4, with the exception of a Pd or Pt ion bound to the trialkyltriamine portion of the ligand instead of Cu(II). Addition of base and CuCl₂ to 8 generates 10, illustrating how heterodinuclear complexes can be accessed by insertion of the different metal ions in either order (6 + PdCl₂ or 8 + CuCl₂).

Conclusions

Ligand LH₂ serves as a versatile platform for the construction of a variety of mono- and dinuclear complexes comprising Cu, Pd, and Pt ions that have been thoroughly characterized by spectroscopy and X-ray crystallography. The ability to selectively insert Cu ions into either the pyridinedicarboxamide or trialkyltriamine portions of LH₂ is a unique feature, with the migration of Cu(II) from the latter to the former upon treatment with base being notable. Importantly, selective incorporation of divergent metal ions into the LH₂ frame yields new heterodinuclear Cu/Pd or Cu/Pt complexes that we envision will be useful for future reactivity studies, including ones aimed at understanding and developing new oxidation catalysts. The divergent supporting ligand environments in the dicopper complex 7 also are intriguing, for example for developing new routes to dicopper-oxygen models of metalloenzyme active sites. With the coordination chemistry foundation now in hand, pursuit of new reactions promoted by disparate Cu₂, CuPd, and CuPt sites within the dinucleating ligand framework LH₂ can commence.

Experimental Section

General Procedures

All reactions and manipulations were performed under a pure argon atmosphere using Schlenk techniques or in a nitrogen atmosphere box unless otherwise noted. Solvents were passed through a solvent purification system (Glass Contour, Laguna CA) and degassed before use. Anhydrous grade N, N-dimethylformamide (DMF) was purchased from Aldrich, degassed and dried over activated 3Å molecular sieves. Tetrahydrofuran (THF) was dried by reflux, under an argon atmosphere, over sodium metal with a benzophenone ketyl indicator and distilled prior to use. All chemicals were purchased from Aldrich and used without purification unless listed below. 3-N-Methylaminopentane-1,5-diamine was purchased from TCI Chemicals and used without further purification. Mesitylcopper(I) was prepared as described in the literature.²⁸ 1,3-N,N″-Di(3-nitrobenzyl)-N,N′,N″-trimethyldiethylenetriamine and LH₂·2HCl were prepared according a published procedure, except using an alternate synthesis of intermediate 1,3-N,N″-Di(3-aminobenzyl)-N,N′,N″-trimethyldiethylenetriamine (see below).^9c All of the compounds except for the Cu(I) complex 5 are air stable and appear to be stable in solution, as evinced by UV-vis and EPR spectroscopy.

Physical methods

¹H and ¹³C{¹H} NMR spectra were recorded on a Varian VI-300 MHz spectrometer at ambient temperature. UV-vis spectra were collected on a HP8453 (190–1000 nm) diode array spectrophotometer. Electrospray ionization mass spectrometry (ESI-MS) was performed on a Bruker Bio-TOF II instrument. Elemental analyses were performed by Robertson Microlit Laboratory (Ledgewood, NJ). Infrared spectra were collected on a Nicolet Avatar 370 FT-IR equipped with a Smart OMNI Sampler. Perpendicular-mode X-band (9.62 GHz) EPR spectra were recorded on a Bruker Elexsys E500 spectrometer at 2 K. Simulations were performed using Bruker SimFonia software (version 1.25).

1,3-N,N″-di(3-aminobenzyl)-1,3,5-N,N′,N″-trimethylethylenetriamine

To a solution of 1,3-N,N″-di(3-nitrobenzyl)-N,N′,N″-trimethyldiethylenetriamine (3.00 g, 7.23 mmol) in EtOH (150 mL) was added SnCl₂·2H₂O (20 g, 89 mmol) and aqueous HCl (55 mL, 35%). The resulting suspension was heated in an oil bath to 80 °C after which stirring started. A clear solution formed within ~5 min. The solution was refluxed for 12 h, during which precipitation of a white solid occurred. The suspension was allowed to cool to room temperature and the solids were collected by filtration. The filtrate was concentrated to half of its original volume under reduced pressure, cooled for an hour in an ice bath, and the additional solid precipitate collected by filtration. The filtered solids were combined and washed with aqueous HCl (2 x 10 mL, 35 %) and EtOH (2 x 20 mL) and dried in vacuo to obtain the product as a Sn₂Cl₁₀·2HCl adduct (5.50g, 73%). IR (solid, cm⁻¹): 3070(s), 3008 (s), 2900 (w), 2860 (vw), 1625 (w), 1568 (m), 1500 (s), 1492 (vs), 1392 (s), 1238(w), 1107 (m), 1016 (w), 988 (w), 965 (m), 942 (m), 897 (s), 795 (vs), 750 (s), 681 (vs). ¹H NMR (300 MHz, DMSO-d₆): δ 7.66 (d, J = 7.70 Hz, 2H), 7.54 (t, J = 7.70 Hz, 4H), 7.41 (d, J = 8.10 Hz, 2H), 4.45 (br, s, 4H), 3.59 (br. m, 8H), 2.75 (s, 3H), 2.69 (s, 6H) ppm. Anal. Calcd for C₂₁H₄₁Cl₁₂N₅O₂Sn₂ (Sn₂Cl₁₀·2HCl adduct + 2H₂O): C, 23.83; H, 3.90; Cl, 40.19; N, 6.62; Sn, 22.43, Found: C, 23.43; H, 3.89; Cl, 40.83; N, 6.18; Sn, 21.20. The adduct (5 g, 4.9 mmol) in MeOH (50 mL) was slowly (5 min) added to a saturated methanolic solution of NaOH (20 mL) in a 250 mL beaker and stirred for 30 min. The mixture was filtered and the filtrate was completely dried under reduced pressure, avoiding heating. The residue was mixed with Et₂O (20 mL) and filtered. The filtrate was dried in vacuo to yield the product as a light yellowish colored oil. This Et₂O extraction procedure was repeated until no residual solids were seen (1.04 g, 60% yield). The ¹H NMR data matched that reported previously.^9c

[(LH₂)CuCl]OTf (4)

A suspension of LH₂·2HCl (85 mg, 0.15 mmol) in THF (10 mL) was treated with a solution of sodium methoxide in methanol (0.70 mL, 0.35 mmol). Subsequent addition of a solution of Cu(OTf)₂ (54 mg, 0.15mmol) in MeOH (2 mL) resulted in a deep blue colored mixture. The reaction mixture was stirred for 4 h and filtered, and the filtrate was evaporated to leave a blue-green solid. The solid was washed with isopropanol (2×5 mL) and dried in vacuo to yield the product as a blue solid (57 mg, 53%). X-ray quality crystals were obtained by slow evaporation of a solution of the complex in methanol solution at −20 °C. UV-Vis (THF/DMF 2:1): λ_max (ε, M⁻¹cm⁻¹) 285 (~11000), 530 (~200), 665 (~140). ESI-MS (CH₃OH, m/z) calcd 584.17 (M-OTf), found 584.17. IR (Solid, cm⁻¹): 1695 (s), 1681 (s), 1610 (m), 1552 (s), 1492 (s), 1438 (w), 1324 (w), 1263 (vs), 1222 (w), 1147 (m), 1035 (s), 1006 (w), 968 (w), 871 (w), 844 (s), 811 (s), 775 (s), 701 (m). EPR (9.64 GHz, toluene, 2K): g_x = 2.170, g_y = 2.054, g_z = 2.034, A_||(Cu) = 192 ×10⁻⁴ cm⁻¹. Despite repeated attempts, satisfactory CHN analysis results have not been obtained, which we hypothesize is due to the presence of salt coproducts in the blue solid that we were unable to fully separate.

[(LH₂)CuCl] (5)

To a solution of LH₂·2HCl (50 mg, 0.09 mmol) in CH₂Cl₂ (3 mL) was added mesitylcopper(I) (56 mg, 0.3 mmol) and the mixture was stirred for 2 h. The mixture was then filtered and Et₂O was allowed to slowly diffuse into the filtrate at −20 °C to yield the product as colorless crystals (23 mg, 0.04 mmol, 43 % yields). UV-Vis (DCM): λ_max (ε, M⁻¹cm⁻¹) 285 (~12000). IR (Solid, cm⁻¹): 3315 (w), 2865 (w), 1683 (s), 1614 (s), 1589 (w), 1575 (w), 1542 (s), 1490 (s), 1450 (m), 1430 (m), 1370 (m), 1313 (m), 1265 (w), 1217 (w), 1180 (w), 1132 (w), 1047 (s), 1010 (w), 1000 (w), 983 (w), 950 (w), 904 (w), 883 (s), 842 (w), 832 (w), 781 (s), 731 (m), 670 (s), 670 (s). ¹H NMR (300 MHz, DMSO-d₆): δ 11.21 (s, 2H), 8.38 (br, 5H), 7.65 (s, 2H), 7.34 (t, J = 7.70 Hz, 2H), 7.01 (d, J = 7.00 Hz, 2H), 3.50 (s, 4H), 2.54 (br, s, 8H), 2.24 (br, s, 3H), 2.12 (br s, 6H). ESI-MS (CH₃OH) m/z (%): calcd 584.17 (M⁺), found 584.12. Despite repeated attempts, satisfactory CHN analysis results have not been obtained, which we hypothesize is due to the presence of salt coproducts that we were unable to fully separate.

Na[LCuCl] (6)

To a blue solution of 4 (50 mg, 0.068 mmol) in methanol (2 mL) was added 1.1 eq of NaOMe (0.071 mL, 0.071 mmol, 1M in methanol) to form a dark green mixture after 4 h stirring at room temperature. The mixture was filtered and the solvent was removed. The residue was solved in THF (2 x 3 mL), filtered and dried in vacuo to yield compound 6 as a green solid (32 mg, 80%). UV-Vis (THF/DMF 2:1): λ_max (ε, M⁻¹cm⁻¹) 260 (~6500 M⁻¹cm⁻¹), 662 (~200 M⁻¹cm⁻¹). IR (Solid, cm⁻¹): 2962 (w), 2860 (w), 2794 (w), 1610 (s), 1587 (s), 1575 (s), 1558 (m), 1538 (w), 1506 (w), 1484 (m), 1456 (m), 1434 (m), 1386 (m), 1375 (m), 1286 (s), 1260 (s), 1228 (m), 1164 (s), 1081 (w), 1037 (s), 1018 (w), 985 (w), 941 (w), 898 (w), 840 (w), 823 (w), 786 (w), 759 (s), 731 (w), 703 (m), 686 (s). ESI-MS (CH₃OH, m/z): calcd 582.15 [M-Na]⁻, found 582.15. EPR (9.64 GHz, toluene, 2K): g_|| = 2.227, g_⊥ = 2.053, A_||(Cu) = 191 ×10^{− 4} cm⁻¹, A_⊥(¹⁴N) = 15 ×10⁻⁴ cm⁻¹. Despite repeated attempts, satisfactory CHN analysis results have not been obtained, which we hypothesize is due to the presence of salt coproducts that we were unable to fully separate.

[LCu₂Cl₂] (7)

To a solution of 6 (50 mg, 0.068 mmol) in DMF (1 mL) was added CuCl₂ (9.4 mg, 0.07 mmol) in MeOH (4 mL), and the mixture was stirred for 12 h, filtered, and Et₂O was slowly diffused into the deep green filtrate over one week to afford the product as green crystals (18 mg, 37%). In an alternative, more direct method, compound 7 was synthesized by slow (10 min) addition of (2.2 eq, 0.41 mmol) Bu₄NOH (1 M in MeOH) to the blue mixture of CuCl₂ (38 mg, 0.28 mmol) and LH₂ (70 mg, 0.14 mmol) in DMF (2 mL). The mixture stirred for 12 h, after which the solvent was removed in vacuo to yield a greenish powder. This powder was dissolved in CH₂Cl₂, filtered, and solvent was removed from the filtrate. The product was then obtained in crystalline form via the slow evaporation of a MeOH solution of the product (20 mg, 41%). UV-Vis (MeOH/DMF 3:1) λ_max (ε, M⁻¹cm⁻¹): 297 (6500 M⁻¹cm⁻¹), 632 (167 M⁻¹cm⁻¹). ESI-MS (CH₃OH, m/z): calcd 705.04 (M+Na), found 705.03. IR (Solid, cm⁻¹): 2960 (w), 2923 (w), 2854 (vw), 1629 (s), 1618 (s), 1602 (w), 1589 (s), 1571 (s), 1486 (s), 1438 (m), 1375 (s), 1365 (vs), 1311 (w), 1159 (w), 1137 (w), 1076 (w), 1054 (w), 1039 (w), 997 (w), 962 (m), 952 (m), 939 (w), 904 (w), 839 (w), 812 (m), 770 (vs), 704 (m), 668 (m). EPR [9.64 GHz, MeOH/CH₂Cl₂ (1:7), 2K]. g_|| = 2.227, g_⊥ = 2.075, A_||(Cu) = 173 ×10⁻⁴ cm⁻¹, A_⊥(¹⁴N) = 19 ×10⁻⁴ cm⁻¹. Anal. Calcd for C₂₈H₃₂Cl₂Cu₂N₆O₂: C, 49.27; H, 4.73; N, 12.31, Found: C, 50.06; H, 4.73; N, 12.16.

[(LH₂)PdCl]Cl (8)

A solution of PdCl₂ (41 mg, 0.24 mmol) in CH₃CN (5 mL) was added to a solution of LH₂·2HCl (100 mg, 0.22 mmol) and NEt₃ (46 mg, 0.044 mmol) in CH₃CN (5 mL). The mixture were stirred for 1 h and then warmed at 50 °C for 5 min. The yellow-brown mixture was filtered and Et₂O diffused slowly into the filtrate to afford the product as bright-yellow crystals (130 mg, 78%). ESI-MS (CH₃OH, m/z): calcd 627.12 (M⁺-Cl), found 627.13. UV-Vis (methanol) λ_max (ε, M⁻¹cm⁻¹): 230 (12300), 281 (5700). IR (Solid, cm⁻¹): 2958 (w), 2919 (w), 2850 (vw),1629 (s), 1618 (s), 1602 (w), 1590 (s), 1571 (s), 1484 (m), 1465 (w), 1438 (w), 1423 (vw), 1373 (s), 1363 (vs), 1315 (m), 1282 (vw), 1159 (vw), 1141 (w), 1076 (w), 1052 (w), 1039 (w), 997 (w), 966 (m), 952 (m), 937 (w), 906 (w), 836 (w), 811 (m), 768 (s), 755 (s), 702 (m), 686 (m). ¹H NMR (300 MHz, CDCl₃): δ 11.08 (br, s), 8.72 (d, J = 8.40 Hz, 2H), 8.39 (t, J = 8.80 Hz, 2H), 8.09 (s, 2H), 7.48 (t, J = 7.70 Hz, 2H), 7.10 (d, J = 7.30 Hz, 2H), 3.65 (s, 4H), 3.19 (m, J = 7.00 Hz, 8H), 2.47 (s, 6H), 2.03 (s, 3H). Anal. Calcd for C₂₈H₃₄Cl₂N₆O₂Pd: C, 50.65; H, 5.16; N, 12.66, Found: C, 49.86; H, 5.00; N, 12.30.

[(LH₂)PtCl]Cl (9)

A solution of PtCl₂ (37 mg, 0.14 mmol) in DMSO (1 mL) was added to a solution of LH₂ (70 mg, 0.14 mmol) in DMSO (1 mL). The mixture was stirred for 4 h. The solvent was removed in vacuo and the residue washed with dry MeOH (2 × 2 mL) to yield the product as a yellowish solid (72 mg, 67%). X-ray quality crystals were obtained by slow evaporation of the complex in CH₂Cl₂. ESI-MS (CH₃OH, m/z): calcd 716.21 (M-Cl)⁺, found 716.21 and calcd 794.22 [(M+DMSO)-Cl)]⁺, found 794.20. IR (Solid, cm⁻¹): 3500 (m), 3250 (m), 2960 (w), 2915 (w), 2855 (vw), 1681 (s), 1612 (s), 1552 (vs), 1492 (s), 1454 (m), 1330 (m), 1290 (w), 1263 (w), 1226 (w), 1172 (w), 1123 (w), 1027 (vs), 1000 (m), 950 (m), 883 (m), 840 (w), 815 (m), 800 (m), 790 (m), 763 (s), 746 (s), 701 (s), 682 (s). ¹H NMR (300 MHz, DMSO-d₆): δ 10.85 (s, 2H), 7.88 (d, J = 8.42 Hz, 2H), 7.57 (s, 2H), 7.50 (s, 2H), 6.64 (t, J = 7.92 Hz, 3H), 6.27 (d, J = 7.32 Hz, 2H), 3.52 (br, s, 8H), 2.69 (br, s, 6H), 2.60 (s, 3H), 2.58 (s, 6H). Anal. Calcd for C₂₈H₃₄Cl₂N₆O₂Pt+(2 DMSO): C, 42.29; H, 5.10; N, 9.25, Found: C, 42.52; H, 4.81; N, 9.52.

[LCuPdCl₂] (10)

(Method A) A solution of PdCl₂ (27mg, 0.15mmol) in CH₃CN (5 mL) was added to a solution of 6 (72 mg, 0.12 mmol) in DMF (2 mL). The resulting red-brown mixture was stirred overnight and then filtered. Solvent was removed from the filtrate and the red residue was washed with sequentially with THF (2 × 2 mL) and Et₂O (2 mL) and then dried in vacuo. Slow evaporation from a solution in CH₃CN (5 mL) at 0 °C yielded 10 as purple crystals (28 mg, 32%). (Method B). To a solution of 9 (70 mg, 0.1 mmol) in DMF (2 mL) was added 2.2 equiv. of NaOMe (1 M in MeOH) or 2.2 equiv. of (ⁿBu)₄NOH (1 M in MeOH) and the mixture was stirred for 2 h. A solution of CuCl₂ (13.4 mg, 0.1 mmol) in MeOH (2 mL) was then added and the mixture stirred for an additional 12 h at room temperature. The mixture was filtered and solvent was removed from the filtrate. The residue was washed with THF (2 x 3 mL) and dried in vacuo to yield the product as a brown-green solid (45 mg, 62%). X-ray quality purple crystals were grown by slow evaporation of a methanolic solution of the product. ESI-MS (CH₃OH, m/z): calcd 760.00 (M + Cl)⁻, found 760.01. UV-Vis (THF/DMF 3:1) λ_max (ε, M⁻¹cm⁻¹): 308 (6200), 587 (140). IR (Solid, cm⁻¹): 2964 (w), 2919 (w), 2880 (vw), 1559 (s), 1575 (vs), 1488 (s), 1457 (m), 1436 (m), 1376 (s), 1313 (vw), 1268 (vw), 1162 (vw), 1141 (m), 1081 (w), 1041 (m), 951 (m), 900 (w), 881 (w), 819 (s), 771 (s), 760 (s), 703 (m), 680 (w), 668 (w). EPR (9.64 GHz, CHCl₃, 2K): g_|| = 2.197, g_⊥ = 2.053, A_||(Cu) = 178 ×10⁻⁴ cm⁻¹, A_⊥(¹⁴N) = 13 ×10⁻⁴ cm⁻¹. Anal. Calcd for C₂₈H₃₂Cl₂CuN₆O₂Pd: C, 46.36; H, 4.45; N, 11.58, Found: C, 46.51; H, 4.33; N, 11.16 (note: the analysis sample was prepared by removing crystals manually from co-precipitated powdered material under a microscope).

[LCuPtCl₂] (11)

(Method A) A solution of PtCl₂ (37 mg, 0.14 mmol) in DMSO (1 mL) was added to a solution of LH₂ (70 mg, 0.14 mmol) in DMSO (1 mL). The mixture was stirred for 4 h and to this was added 1.5 equiv. of NaOMe (1 M in methanol) or 1.5 equiv. of Bu₄NOH (1 M in methanol) and a solution of CuCl₂ (19 mg, 0.14 mmol) in DMSO (1 mL). The mixture was stirred for an additional 12 h at room temperature. Solvent was removed from the filtrate and the green residue was washed sequentially with MeOH (2 × 2 mL). The solid was dissolved in CH₂Cl₂ (2 mL), filtered, and solvent removed from the filtrate in vacuo to yield the product (45 mg, 41%). X-ray quality single crystals formed upon the slow evaporation of a solution of 11 in CH₂Cl₂/MeOH (2 mL). (Method B) A solution of CuCl₂ (19 mg, 0.14 mmol) in DMSO (1 mL) was added to a solution of LH₂ (70 mg, 0.14 mmol). The resulting blue solution was stirred for 1h and then 1.5 equiv. of NaOMe or Bu₄NOH (1 M in methanol) was added to form a dark green mixture. A solution of PtCl₂ (37 mg, 0.14 mmol) in DMSO (1 mL) was added to the reaction mixture which was stirred for 12 h and then filtered. Solvent was removed from the filtrate and the green residue was washed with MeOH (2 × 2 mL), filtered, and then solvent removed from the filtrate in vacuo to yield the product (34 mg, 32%). ESI-MS (CH₃OH, m/z): calcd 778.19 (M - Cl)⁺, found 778.12. UV-Vis (MeOH/DMF 1:1) λ_max (ε, M⁻¹cm⁻¹): 305 (8756), 598 (220). IR (Solid, cm⁻¹): 2991 (w), 2957 (w), 1612 (s), 1578 (s), 1483 (m), 1429 (m), 1374 (s), 1313 (w), 1251 (s), 1143 (s), 1096 (s), 1028 (s), 933 (w), 843 (w), 803 (vs), 770 (s), 701 (w), 682 (s), 671 (m). EPR (9.64 GHz, CH₂Cl₂, 2K): g_|| = 2.188, g_⊥ = 2.045, A_||(Cu) = 178 × 10⁻⁴ cm⁻¹, A_⊥(¹⁴N) = 12 ×10⁻⁴ cm⁻¹. Anal. Calcd for C₂₉H₃₆Cl₂CuN₆O₃Pt (10 + MeOH): C, 41.16; H, 4.29; N, 9.93, Found: C, 41.20; H, 4.10; N, 9.93.

Table 1.

Selected interatomic distances (Å) and angles (deg) for the indicated X-ray crystal structures.

[(LH2)CuCl]OTf (4)
Cu1-N4	2.058(4)	N5-Cu1-N4	85.83(16)
Cu1-N5	1.998(4)	N6-Cu1-N4	150.26(16)
Cu1-N6	2.043(4)	N5-Cu1-N6	86.12(16)
Cu1-Cl1	2.200(1)	Cl1-Cu1-N5	164.03(13)
		Cl1-Cu1-N4	97.37(12)
		Cl1-Cu1-N6	98.19(11)
[(LH2)CuCl] (5)
Cu1-N5	1.965(2)	N5-Cu1-Cl1	178.12(7)
Cu1-Cl1	2.136 (1)
Cu1-N6	2.847(2)
Cu1-N4	2.797(2)
[LCu2Cl2] (7)
Cu1-N1	1.911(6)	N1-Cu1-N2	80.0(3)
Cu1-N2	2.007(7)	N1-Cu1-N3	80.7(3)
Cu1-N3	1.999(6)	N2-Cu1-N3	158.9(3)
Cu1-Cl1	2.216(2)	N1-Cu1-Cl1	170.9(2)
Cu1···Cu2	4.484(1)	N2-Cu1-Cl1	100.29(19)
Cu2-N5	2.082(7)	N3-Cu1-Cl1	100.09(18)
Cu2-N6	2.097(7)	N5-Cu2-N6	85.1(3)
Cu2-N4	2.122(6)	N5-Cu2-N4	85.5(3)
Cu2-Cl2	2.241(2)	N6-Cu2-N4	139.8(2)
Cu2···Cl1	2.693(2)	N5-Cu2-Cl2	174.23(19)
Cu1···Cu2	4.4847(13)	N6-Cu2-Cl2	93.39(18)
		N4-Cu2-Cl2	92.08(18)
		N5-Cu2-Cl1	89.02(19)
		N6-Cu2-Cl1	109.27(17)
		N4-Cu2-Cl1	109.57(17)
		Cl2-Cu2-Cl1	96.72(7)
[(LH2)PdCl]Cl (8)
N4-Pd1	2.031(2)	N3-Pd1-N3A	159.8(3)
N3-Pd1	2.109(1)	N4-Pd1-N3	85.73(3)
Cl1-Pd1	2.286(1)	N4-Pd1-Cl1	174.98(2)
		N3-Pd1-Cl1	95.05(5)
[(LH2)PtCl]Cl (9)
N5-Pt1	2.0081(3)	N5-Pt1-N6	86.38(6)
N6-Pt1	2.080(1)	Cl1-Pt1-N5	175.38(1)
Cl1-Pt1	2.2998(3)	Cl1-Pt1-N6	94.24(1)
[LCuPdCl2] (10)
N1-Cu1	1.895(6)	Cl1-Pd1-N5	177.0(3)
N2-Cu1	1.977(8)	Cl1-Pd1-N6	94.3(2)
N3-Cu1	1.967(7)	Cl1-Pd1-N4	94.6(2)
N5-Pd1	2.081(7)	N5-Pd1-N6	86.3(3)
N6-Pd1	2.095(8)	N5-Pd1-N4	86.1(3)
N4-Pd1	2.086(8)	N6-Pd1-N4	155.6(3)
Pd1-Cl1	2.298(3)	Cl2B-Cu1-N3	99.0(3)
Cl2A-Cu1	2.095(10)	N3-Cu1-N2	158.4(3)
Cl2B-Cu1	2.314(16)	N3-Cu1-N1	81.3(3)
Pd1···Cu1	4.9561(9)	N2-Cu1-N1	79.6(3)
[LCuPtCl2] (11)
N5-Pt1	2.087(9)	N2-Cu1-Cl2B	98.7(4)
N1-Cu1	1.899(6)	N1-Cu1-Cl2B	178.7(5)
N2-Cu1	1.959(7)	Cl1-Pt1-N5	174.7(3)
Cl1-Pt1	2.284(3)
Pt1···Cu1	5.070(1)