Ligand Structural Effects on Cu₂S₂ Bonding and Reactivity in Side-On Disulfido-Bridged Dicopper Complexes

Abstract

In order to assess supporting ligand effects on S-S bond activation, a series of [Cu₂(μ-η²:η²-S₂)]²⁺ complexes supported by various β-diketiminate or anilido-imine ligands (L) were synthesized via the reaction of Cu(I) precursors LCu(CH₃CN) with S₈. For the cases where L = β-diketiminate, the syntheses were complicated by formation of clusters [Cu(SR)]₄, where “SR” represents the ligand functionalized by sulfur at the central methine position. The [Cu₂(μ-η²:η²-S₂)]²⁺ products were characterized by X-ray crystallography and electronic absorption and resonance Raman spectroscopy. Correlations among the Cu-S, Cu-Cu, and S-S distances and the ν(S-S) values were observed and interpreted within the framework of a previously described bonding picture. Comparison of these data to those for other relevant species revealed a remarkable degree of S-S bond activation in the compounds supported by the β-diketiminate and anilido-imine ligands, which through strong electron donation increase backbonding from the copper ions into the S-S σ* orbital and cause S-S bond weakening. Reactions of one of the complexes supported by an anilido-imine ligand with PPh₃ and xylyl isocyanide were explored, revealing facile transfer of sulfur to PPh₃ but only displacement of sulfur to yield a LCu(I)-CNAr (Ar = xylyl) complex with the isocyanide.

Introduction

Copper ion(s) coordinated by multiple histidine N-donor and sulfur-containing thiolate or sulfide ligands comprise the active sites of numerous functionally important metalloproteins. Much interest has focused on the mononuclear type 1 and binuclear Cu_A electron transfer sites,^1,2 for which the significance of highly covalent copper(II)-thiolate interactions has been demonstrated through detailed structural, spectroscopic, and synthetic modeling studies.^2,3 Copper(I)-thiolate centers also are prevalent in proteins that store and transport copper, as well as those that regulate gene transcription to ensure proper copper ion homeostasis.^4-6 Although ubiquitous in the biochemistry of iron,⁷ sulfide has only been identified in one copper-containing enzyme, nitrous oxide reductase (N₂OR), which produces N₂ during microbial denitrification.⁸ Recent structural⁹ and spectroscopic^8,10 studies have revealed the catalytic site of N₂OR (“Cu_Z”) to contain a novel (μ₄-sulfido)tetracopper cluster ligated by multiple histidine donors. This cluster can exist in multiple oxidation states, the most important of which appear to be the fully reduced Cu(I)₄ and mixed valent Cu(I)_xCu(II)_4-x forms.^10,11 Provocative mechanistic proposals for the enzyme have been put forth using computational methods,^8,12 key features of which include unusual side-on coordination of N₂O to an edge of the Cu(I)₄ form of the cluster and μ₄-sulfide-mediated electron delocalization in higher oxidation states. Stimulated by the unusual properties of and mechanistic hypotheses put forth for the (μ₄-sulfido)tetracopper site of N₂OR, we have begun to explore the sulfur chemistry of copper complexes supported by N-donor ligands.¹³ We aim to provide fundamental chemical insights into the properties of copper-sulfur species in general and, in particular, less common ones¹⁴ that feature copper in oxidation states greater than +1. An ultimate goal is to prepare useful models of the N₂OR catalytic site with which to evaluate spectroscopic properties and reactivity.¹⁵

Previous synthetic efforts have yielded sulfur-containing Cu(II and/or III) products with either the S-S bond intact (e.g. disulfido(2-), 1 and 2,^16-18 or disulfido(·1-), 3^13c) or broken (e.g. sulfide, 4),^13b depending on the nature of the N-donor ligand and the reaction conditions (Figure 1). The μ-η¹:η¹- and μ-η²:η²-disulfidodicopper complexes 1¹⁶ and 2 (L = Tp^iPr2 or Me₂NPY2)^17,18 are close counterparts of peroxodicopper analogs with identical tetra- or tridentate supporting ligands,^19-21 and the bonding interactions in the Cu₂O₂ and Cu₂S₂ cores are similar as determined from comparative spectroscopic/theoretical studies.²² The side-on μ-η²:η² complexes most pertinent here (2, L = Tp^iPr2, and its peroxo congener) exhibit a pair of intense charge-transfer (CT) electronic absorption bands and low S-S and O-O stretching frequencies of 500 cm^-1 and ∼760 cm^-1, respectively, that indicate weak bonds (i.e., extensive activation of the S₂ and O₂ moieties). These features have been rationalized by a common core bonding model illustrated in Figure 2.²² According to this picture, the O₂^2-/S₂^2- π* orbital that lies in the plane of the Cu₂O₂/Cu₂S₂ core (π*_σ) interacts strongly with the Cu d_xy orbitals, while the out-of-plane O₂^2-/S₂^2- π* orbital (π*_v) remains basically unperturbed (nonbonding with respect to the Cu ions). The Cu-O/S bonding is thus dominated by highly covalent, strong σ-donation from the filled O₂^2-/S₂^2- π*_σ orbital into the Cu d_xy set. An additional interaction occurs between the Cu d_xy orbitals and the empty O₂^2-/S₂^2- σ* orbital. Essentially a backbonding interaction, it results in a lowering of the predominantly Cu-based HOMO energy and rationalizes the weakening of the O₂^2-/S₂^2- bond. The electronic CT absorption bands derive from excitations out of the π*_σ + xy + xy and π*_v orbitals, with the former being more intense and at higher energy. A detailed comparative study²² showed that the Cu₂S₂ core of 2 (L = Tp^iPr2) exhibits greater metal-ligand covalency (π*_σ/Cu) and backbonding (Cu/σ*) than its peroxide analog, corresponding to a more significant weakening (activation) of the S-S bond.²²

Figure 1.

Copper(II and/or III)-sulfur complexes.

Figure 2.

Schematic molecular orbital energy level diagram for the (μ-η²:η²-peroxo/sulfido)dicopper core (adapted from ref. 22).

We recently communicated the synthesis and characterization of two examples of μ-η²:η²-disulfide complexes supported by bidentate N-donor ligands (2, L = b or i, Figure 3).^13a Their S-S bond distances (2.2007(11) and 2.165(3) Å, respectively) are distinctly longer than that of 2, L = Tp^iPr2 or Me₂NPY2, (2.073(4) Å and 2.117(2) Å, respectively), suggesting an even greater degree of S-S bond weakening due to the nature of the supporting ligands. Here we present a more complete study in which we assess the X-ray structures, UV-vis and resonance Raman spectra, and aspects of the reactivity of a series of μ-η²:η²-disulfide complexes supported by bidentate N-donor ligands with variable steric and electronic properties (Figure 3). Interpretations of the spectroscopic properties within the context of the bonding picture illustrated in Figure 2, correlations of Cu₂S₂ core structural parameters and S-S stretching frequencies, and reactivity different from that previously reported for 2, L = Me₂NPY2 (Figure 1)¹⁸ are discussed. Taken together, these results provide new insights into ligand influences on S-S bond activation by copper ions.

Figure 3.

Ligands and abbreviations used in this work.

Experimental Section

General Considerations

All solvents and reagents were obtained from commercial sources and used as received unless noted otherwise. The solvents tetrahydrofuran (THF), pentane, diethyl ether (Et₂O), acetonitrile (CH₃CN), and CH₂Cl₂ were passed though solvent purification columns (Glass Contour, Laguna, CA, or MBraun) prior to use. All metal complexes were prepared and stored in a Vacuum Atmospheres inert atmosphere glove box under a dry nitrogen atmosphere or were manipulated using standard Schlenk line techniques. Labeled elemental sulfur (³⁴S, 99% enrichment) was purchased from Cambridge Isotope Laboratories, Inc. Complexes of the general formula R(R’₂L^R”2)Cu(CH₃CN),²³ as well as (HL’^iPr2)Cu(CH₃CN),²⁴ (MeL’^iPr2)Cu(CH₃CN),²⁵ [H(Me₂L^Et2)Cu]₂(S₂) (2b),^13a and [(HL’^iPr2)Cu]₂(S₂) (2i)^13a were prepared via previously reported methods (for ligand nomenclature, see Figure 3). The complex (HL’^Me2)Cu was prepared similarly to (HL’^iPr2)Cu(CH₃CN), characterized by ¹H NMR spectroscopy, and used directly for the synthesis of [(HL’^Me2)Cu]₂(S₂) (2h) (supporting information).

Physical Methods

NMR spectra were recorded on a Varian VI-300 or VXR-300 spectrometer. Chemical shifts (δ) for ¹H NMR spectra were referenced to residual protium in the deuterated solvent. UV-vis spectra were recorded on a HP8453 (190-1100 nm) diode array spectrophotometer; extinction coefficients were determined from Beers’ Law plots. 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. A Spectra Physics BeamLok 2065-7S Ar Laser provided excitation at 457.9 nm. The spectra were obtained at -196 °C using a backscattering geometry. Samples were frozen in NMR tubes in thermal contact with a Dewar flask containing liquid nitrogen. Raman shifts were externally referenced to liquid indene. Elemental analyses were performed by Robertson Microlit (Madison, NJ).

[H(Me₂L^Me2)Cu]₂(S₂) (2a)

[H(Me₂L^Me2)]Cu(CH₃CN) (43.6 mg, 0.12 mmol) in CH₃CN (4 mL) was added to elemental sulfur (3.8 mg, 0.015 mmol) in CH₃CN (2 mL). The reaction was stirred for 30 min, during which time a brown precipitate formed. The precipitate was collected, washed with CH₃CN (4 mL) and dried under reduced pressure (13.4 mg, 44%). Allowing the reaction to stir for > 1 h results in the conversion of [H(Me₂L^Me2)Cu]₂(S₂) to [Cu(SR)]₄ [R = H(Me₂L^Me2)] (5a); a ¹H NMR spectrum of this residue matched that of 5a prepared independently (see below). ¹H NMR (C₆D₆, 300 MHz): δ 6.90-7.01 (m, 12H), 4.79 (s, 2H), 2.03 (s, 24H), 1.44 (s, 12 H). UV-vis (THF) [λ_max, nm (ε, M^-1cm^-1)]: 211 (18500), 327 (19200), 352 (12000), 427 (8200), 540 (400), 804 (130). Repeated attempts to obtain a satisfactory elemental analysis (CHN) for 2a failed, which we ascribe to variable small amounts of sulfur impurities that we were unable to remove by recrystallization due to competing formation of 5a. Structural assignment thus rests on the X-ray crystal structure and the spectroscopic data.

[H(tBu₂L^iPr2)Cu]₂(S₂) (2c)

[H(tBu₂L^iPr2)]Cu(CH₃CN) (45.3 mg, 0.075mmol) in pentane (3 mL) was added to elemental sulfur (1.2 mg, 0.0047 mmol) in pentane (2 mL). The reaction was stirred for 1 h, filtered though celite, and the filtrate concentrated under reduced pressure to ∼ 2 mL. Storage of the orange colored solution at -20 °C resulted in the formation of yellow and brown crystals in a ratio of about 30:1. The brown colored material was identified as [H(tBu L^iPr2Cu]₂(S₂) by X-ray crystallography on a selected crystal, but a pure bulk sample has not been accessed to date. Raman spectroscopy was performed on an initial reaction solution prepared in benzene, which was frozen in liquid N₂ immediately after preparation.

[Ph(H₂L^Et2)Cu]₂(S₂) (2d)

[Ph(H₂L^Et2)]Cu(CH₃CN) (112.0 mg, 0.22 mmol) in CH₃CN (5 mL) was added to elemental sulfur (7.0 mg, 0.027 mmol) in CH₃CN (2 mL). The reaction was stirred for 3 h, during which time a green precipitate formed. The precipitate was collected, washed with CH₃CN (8 mL) and dried under reduced pressure (88.7 mg, 40%). ¹H NMR (C₆D₆, 300 MHz): δ 7.58 (s, 4H), 6.92-7.11 (m, 22 H), 2.51 (quartet, J = 7.5 Hz, 16H), 1.07 (t, J = 7.5 Hz, 24H). UV-vis (THF) [λ_max, nm (ε, M^-1cm^-1)]: 295 (49385), 387 (34376), 421 (23470), 588 (1260), 813 (310). Anal. Calcd for C₅₈H₆₆Cu₂N₄S₂: C, 68.95; H, 6.58; N, 5.55. Found: C, 68.67; H, 6.43; N, 5.40.

[Ph(H₂L^iPr2)Cu]₂(S₂) (2e)

[Ph(H₂L^iPr2)]Cu(CH₃CN) (46.5 mg, 0.082 mmol) in CH₃CN (3 mL) was added to elemental sulfur (2.6 mg, 0.01 mmol) in CH₃CN (2 mL). The reaction was stirred for 2 h, during which time a green precipitate formed. The precipitate was collected, washed with CH₃CN (8 mL) and dried under reduced pressure (32.0 mg, 70%). ¹H NMR (C₆D₆, 300 MHz): δ 7.66 (s, 4 H), 7.04-7.22 (m, 14 H), 6.98 (d, J = 7.6 Hz, 8 H), 3.26 (septet, J = 6.9 Hz, 8 H), 1.06 (d, J = 6.8 Hz, 24 H), 1.01 (d, J = 6.8 Hz, 24 H). UV-vis (THF) [λ_max, nm (ε, M^-1cm^-1)]: 299 (52760), 386 (32410), 432 (24350), 612 (1290). Anal. Calcd for C₆₆H₈₂Cu₂N₄S₂: C, 70.61; H, 7.36; N, 4.99. Found: C, 70.32; H, 7.62; N, 4.72.

[3,5-(CF₃)₂C₆H₃(H₂L^Me2)Cu]₂(S₂) (2f)

[3,5-(CF₃)₂C₆H₃(H₂L^Me2)]Cu(CH₃CN) (65.2 mg, 0.11 mmol) in CH₃CN (4 mL) was added to elemental sulfur (3.5 mg, 0.014 mmol) in CH₃CN (3 mL). The reaction was stirred for 4 h, during which time a brown precipitate formed. The precipitate was collected, washed with CH₃CN (5 mL) and dried under reduced pressure (38.0 mg, 59%). ¹H NMR (C₆D₆, 300 MHz): δ 7.56 (s, 2H), 7.41 (s, 4H), 7.26 (s, 4H), 6.96 (m, 4H), 6.88 (d, J = 7.4Hz, 8H), 1.99 (s, 24H). UV-vis (THF) [λ_max, nm (ε, M^-1cm^-1)]: 319 (60500), 377 (32900), 421 (25500), 547 (1400), 827 (600). Anal. Calcd for C₅₄H₄₆Cu₂F₁₂N₄S₂: C, 55.43; H, 3.96; N, 4.79. Found: C, 55.16; H, 4.23; N, 4.58.

[3,5-(CF₃)₂C₆H₃(H₂L^iPr2)Cu]₂(S₂) (2g)

[3,5-(CF₃)₂C₆H₃(H₂L^iPr2)]Cu(CH₃CN) (190.0 mg, 0.27 mmol) in CH₃CN (6 mL) was added to elemental sulfur (8.6 mg, 0.034 mmol) in CH₃CN (3 mL). The reaction was stirred for 2 h, during which time a green precipitate formed. The precipitate was collected, washed with CH₃CN (2 × 6 mL) and dried under reduced pressure (172.0 mg, 91%). ¹H NMR (C₆D₆, 300 MHz): δ 7.67 (s, 4H), 7.60 (s, 4H), 7.53 (s, 2H), 7.11 (t, J = 7.9 Hz, 4H), 6.94 (d, J = 7.86 Hz, 8H), 3.19 (septet, J = 6.58 Hz, 8H), 1.02 (apparent t: two overlapping d, J = 6.58 Hz, 48H). UV-vis (THF) [λ_max, nm (ε, M^-1cm^-1)]: 322 (54300), 381 (26200), 434 (22800), 575 (1300), 830 (290). Anal. Calcd for C₇₀H₇₈Cu₂F₁₂N₄S₂: C, 60.29; H, 5.64; N, 4.02. Found: C, 59.99; H, 5.52; N, 3.95.

[(HL’^Me2)Cu]₂(S₂) (2h)

(HL’^Me2)Cu(CH₃CN) (102.0 mg, 0.24 mmol) in CH₃CN (4 mL) was added to elemental sulfur (7.6 mg, 0.03 mmol) in CH₃CN (2 mL). The reaction was stirred for 3 h, during which time a green precipitate formed. The precipitate was collected, washed with CH₃CN (6 mL) and dried under reduced pressure (67.0 mg, 67%). ¹H NMR (C₆D₆, 300 MHz): δ 7.41 (s, 2H), 6.96-7.07 (m, 8H), 6.77-6.89 (m, 8H), 6.36 (d, J= 8.4 Hz, 2H), 6.26 (m, 2H), 2.09 (s, 12H), 1.91 (s, 12 H). UV-vis (THF) [λ_max, nm (ε, M^-1cm^-1)]: 256 (49700), 282 (31300), 435 (29100), 604 (1300), 815 (400). Anal. Calcd for C₄₆H₄₆Cu₂N₄S₂: C, 65.30; H, 5.48; N, 6.62. Found: C, 64.83; H, 5.42; N, 6.49.

[(MeL’^iPr2)Cu]₂(S₂) (2j)

(MeL’^iPr2)Cu(CH₃CN) (51.8 mg, 0.09 mmol) in toluene (5 mL) was added to elemental sulfur (3.0 mg, 0.012 mmol) in toluene (2 mL). The reaction was stirred for 3 h, filtered though celite and the volume reduced to 2 mL. Addition of 20 mL of CH₃CN and storage at -20°C resulted in the precipitation of a dark green solid. The green solid was collected and dried under reduced pressure (17.0 mg, 33%). ¹H NMR (C₆D₆, 300 MHz): δ 7.40 (d, J = 9.6 Hz, 2H), 6.85-7.30 (m, 12 H), 6.80 (m, 2H), 6.41 (d, J = 8.7 Hz, 2H), 6.31 (m, 2H), 3.25 (septet, J = 6.6 Hz, 4H), 2.87 (septet, J = 6.6 Hz, 4H), 1.85 (s, 6H), 1.22 (m, 24H), 1.12 (d, J = 6.9 Hz, 12H), 0.97 (d, J = 6.9 Hz, 12H). UV-vis (THF) [λ_max, nm (ε, M^-1cm^-1)]: 250 (32200), 292 (15800), 437 (17700), 460 (21600), 651 (1000), 877 (120). Anal. Calcd for C₆₄H₈₂Cu₂N₄S₂: C, 69.97; H, 7.52; N, 5.10. Found: C, 69.35; H, 7.06; N, 4.62.

[Cu(SR)]₄ [R = H(Me₂L^Me2)] (5a)

H(Me₂L^Me2)Cu(CH₃CN) (90.0 mg, 0.22 mmol) in a 1:1 toluene/CH₃CN (4 mL) solution was added to elemental sulfur (7.14 mg, 0.028 mmol) in CH₃CN (2 mL). The reaction was stirred for 2 h, causing the development of a tan precipitate. The solid was collected, washed with CH₃CN (3 × 5 mL), and dried under reduced pressure (50mg, 57%). ¹H NMR (CD₂Cl₂, 300 MHz): δ 6.80-7.00 (m, 24H), 3.90 (s, 4H), 2.09 (broad s, 24H), 2.03 (broad s, 12H), 1.96 (broad s, 12 H), 1.73 (broad s, 12 H), 1.65 (broad s, 12H). Anal. Calcd for C₈₄H₁₀₀Cu₄N₈S₄: C, 62.89; H, 6.28; N, 6.99. Found: C, 62.47; H, 5.85; N, 6.88.

Reactions of 2h with PPh₃

Two equiv. PPh₃ (7.1 mg, 0.027 mmol) were dissolved in C₆D₆ (1 mL) and added to [(HL’^Me2)Cu]₂(S₂) (2h) (11.4 mg, 0.013 mmol). After stirring for 1 h, ¹H and ³¹P{¹H} NMR spectra were obtained, which showed formation of 1 equiv. of triphenylphosphine sulfide, 1 equiv. of (HL’^Me2)Cu(PPh₃), and 0.5 equiv. of unreacted 2h. The identity of (HL’^Me2)Cu(PPh₃) was confirmed by independently reacting (HL’^Me2)Cu(CH₃CN) (9.2 mg, 0.021 mmol) with PPh₃ (5.6 mg, 0.021 mmol) in C₆D₆ (1 mL). ¹H NMR (C₆D₆, 300 MHz): δ 7.97 (d, J = 2.7 Hz, 1H), 6.80-7.10 (m, 23H), 6.66 (d, J = 8.7 Hz, 1H), 6.41 (t, J = 7.6 Hz, 1H), 2.23 (s, 6H), 2.03 (s, 6H). ³¹P{¹H} NMR (C₆D₆, 121.372 MHz): δ 6.81. Similar reaction of PPh₃ with 2h but using 4 equiv. of phosphine yielded 2 equiv. of triphenylphosphine sulfide and 2 equiv. of (HL’^Me2)Cu(PPh₃).

Reaction of 2h with Xylyl Isocyanide

Two equiv. of xylyl isocyanide (3.7 mg, 0.028 mmol) were dissolved in C₆D₆ (1 mL) and added to [(HL’^Me2)Cu]₂(S₂) (2h) (12.0 mg, 0.014 mmol). After stirring for 1 h, the only species observed by ¹H NMR spectroscopy was the Cu(I)-xylyl isocyanide adduct. The identity of this adduct was confirmed by independently reacting (HL’^Me2)Cu(CH₃CN) (11.4 mg, 0.026 mmol) with xylyl isocyanide (3.5 mg, 0.026 mmol) in C₆D₆ (1 mL). ¹H NMR (C₆D₆, 300 MHz): δ 7.96 (s, 1H), 6.90-7.30 (m, 8H), 6.68 (d, J = 8.7 Hz, 1H), 6.59 (t, J = 7.5 Hz, 1H), 6.43 (t, J = 7.0 Hz, 1H), 6.39 (d, J = 7.8 Hz, 2H), 2.48 (s, 6H), 2.31 (s, 6H), 1.65 (s, 6H).

X-ray Crystallography

Crystals of the appropriate size were chosen and placed on the tip of a 0.1 mm diameter glass fiber and mounted on a Siemens SMART Platform CCD diffractometer for data collection at 173(2) K. Data collections were carried out using MoKα radiation (graphite monochomator) with a detector distance of 4.9 cm. The intensity data were corrected for absorption and decay (SADABS).²⁶ Final cell constants were calculated from the xyz centroids of strong reflections from the actual data collection after integration (SAINT).²⁷ Please refer to the attached CIFs for additional crystal and refinement information. The structures were solved by direct methods using SHELXL-97²⁸ software. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. Pertinent details for each structure are noted below; see Table S1 for a summary of crystallographic data and the CIFs for full crystallographic information.

[H(Me₂ L^Me2)Cu]₂(S₂) (2a)

Red crystals suitable for X-ray crystallography were grown by vapor diffusion of pentane into a toluene solution at -20 °C. The final full-matrix least-squares refinement converged to R1 = 0.0321 and wR2 = 0.0870 (F², all data).

[H(tBu₂L^iPr2)Cu]₂(S₂) (2c)

Red crystals suitable for X-ray crystallography were grown from pentane at -20° C. The carbon atoms of one isopropyl group were found to be disordered over two positions, with a 79:21 occupancy ratio. A pentane solvent molecule was also disordered over two positions, as well as over a two-fold symmetry axis. Attempts to model the disorder were unsuccessful so the reflection contributions of the solvent were removed using the program PLATON, function SQUEEZE,²⁹ which calculated there to be 171 electrons in a volume of 852 ų per unit cell. The final full-matrix least-squares refinement converged to R1 = 0.0450 and wR2 = 0.1031 (F², all data).

[Ph(H₂L^Et2)Cu]₂(S₂) (2d)

Green crystals suitable for X-ray crystallography were grown from pentane at -20 °C. The final full-matrix least-squares refinement converged to R1 = 0.0386 and wR2 = 0.0825 (F², all data).

[Ph(H₂L^iPr2)Cu]₂(S₂) (2e)

Green crystals suitable for X-ray crystallography were grown by vapor diffusion of pentane into a toluene solution at -20 °C. The carbon atoms of one isopropyl group were found to be disordered over two positions, with a 52:48 occupancy ratio. Two molecules of toluene are also present within the asymmetric unit. The final full-matrix least-squares refinement converged to R1 = 0.0348 and wR2 = 0.0747 (F², all data).

[3,5-(CF₃)₂C₆H₃(H₂L^Me2)Cu]₂(S₂) (2f)

Brown crystals suitable for X-ray crystallography were grown from vapor diffusion of pentane into a CH₂Cl₂ solution at -20 °C. The fluorine atoms of one of the CF₃ groups were found to be disordered over two positions, with an 84:16 occupancy ratio. The final full-matrix least-squares refinement converged to R1 = 0.0419 and wR2 = 0.0962 (F², all data).

[3,5-(CF₃)₂C₆H₃(H₂L^ipr2)Cu]₂(S₂) (2g)

Green crystals suitable for X-ray crystallography were grown from pentane at -20 °C. The fluorine atoms of one of the CF₃ groups were found to be disordered over two positions, with a 90:10 occupancy ratio. Highly disordered solvent was found that could not be modeled appropriately. The reflection contributions of the solvent were removed using the program PLATON, function SQUEEZE,²⁹ from which it was determined that there were 143 electrons in a volume of 578.3 ų per unit cell. The final full-matrix least-squares refinement converged to R1 = 0.0380 and wR2 = 0.1039 (F², all data).

[(HL’^Me2)Cu]₂(S₂) (2h)

Green crystals suitable for X-ray crystallography were grown by vapor diffusion of pentane into a toluene solution at -20 °C. Two molecules of toluene are present within the asymmetric unit. The final full-matrix least-squares refinement converged to R1 = 0.0410 and wR2 = 0.0997 (F², all data).

[(MeL’^iPr2)Cu]₂(S₂) (2i)

Green crystals suitable for X-ray crystallography were grown from pentane at -20° C. Highly disordered solvent was found that could not be modeled appropriately. The reflection contributions of the solvent were removed using the program PLATON, function SQUEEZE,²⁹ from which it was determined that there were 283 electrons in a volume of 1484 ų per unit cell. The final full-matrix least-squares refinement converged to R1 = 0.0446 and wR2 = 0.1183 (F², all data).

[Cu(SR)]₄ [R = H(Me₂L^Me2)] (5a)

Yellow crystals suitable for X-ray crystallography were grown by vapor diffusion of pentane into a CH₂Cl₂ solution at -20 °C. Highly disordered solvent was found that could not be modeled appropriately. The reflection contributions of the solvent were removed using the program PLATON, function SQUEEZE,²⁹ from which it was determined that there were 110 electrons in a volume of 1819 ų per unit cell. The final full-matrix least-squares refinement converged to R1 = 0.0570 and wR2 = 0.1326 (F², all data).

Results

Synthesis of Complexes

With one exception, the series of μ-η²:η²-disulfidodicopper(II) complexes 2a-j were isolated as either brown or green colored solids from reaction of the Cu(I) precursors (a-j)Cu(CH₃CN) with S₈ in CH₃CN, toluene, or pentane (Scheme 1).³⁰ The products were characterized by ¹H NMR, UV-vis, and resonance Raman spectroscopy, as well as by CHN analysis and X-ray crystallography. The exceptional case was [H(tBu₂L^iPr2)Cu]₂(S₂) (2c, Figure 2), which we were unable to isolate as an analytically pure bulk sample, although crystals suitable for an X-ray structural determination were obtained. Isolation of samples of the complexes lacking a substituent on the central methine position of the β-diketiminate ligand (e.g. 2a and b) required shorter reaction times in order to prevent conversion to the Cu(I) clusters [Cu(SR)]₄ (5a,b; Scheme 1), which were fully characterized in two instances (R = H(Me₂L^Et2), 5b,^13a and H(Me₂L^Me2), 5a). Exclusive formation of 5a was possible by using a CH₃CN/toluene mixture as solvent and allowing the solution to stir for > 1 h.

Scheme 1.

Synthesis of complexes.

X-ray Structures. (a) Disulfido Complexes

X-ray crystal structures of 2a-j were determined, with 2b and i having been reported previously.^13a Due to their general similarity, only that of 2d is shown here (Figure 4); thermal ellipsoid representations of 2a,b,c-h and j are provided as supporting information (Figures S1-S7). Selected interactomic distances and angles for all of the complexes are summarized in Table 1.

Figure 4.

X-ray structure of 2d, showing all nonhydrogen atoms as 50% thermal ellipsoids.

Table 1.

Selected Interatomic Distances (Å) and Angles (deg).^a

[H(Me2LMe2)Cu]2(S2) (2a)
Cu1-N1	1.8964(17)	S1-S1A	2.2140(10)
Cu1-N2	1.8994(17)	Cu1···Cu1A	3.7687(5)
Cu1-S1	2.1842(6)	S1-Cu1-S1A	60.87(2)
Cu1-S1A	2.1868(6)	N1-Cu1-S1-S1A	3.84(17)
[H(Me2LEt2)Cu]2(S2)b (2b)
Cu1-N1	1.9065(18)	S1-S1A	2.2007(11)
Cu1-N2	1.9101(18)	Cu1···Cu1A	3.7991(5)
Cu1-S1	2.1930(6)	S1-Cu1-S1A	60.16(3)
Cu1-S1A	2.1974(6)	N-Cu1-S1-S1A	-0.07(18)
[H(tBu2LiPr2)Cu]2(S2) (2c)
Cu1-N1	1.942(2)	S1-S2	2.1242(13)
Cu1-N2	1.936(2)	Cu1···Cu1A	3.9950(7)
Cu1-S1	2.2572(6)	S1-Cu1-S2	56.00(3)
Cu1-S2	2.2674(6)	N-Cu1-S1-S2	-32.40(18)
[Ph(H2LEt2)Cu]2(S2) (2d)
Cu1-N1	1.910(2)	S1-S1A	2.2138(13)
Cu1-N2	1.909(2)	Cu1···Cu1A	3.7899(12)
Cu1-S1	2.1951(10)	S1-Cu1-S1A	60.58(3)
Cu1-S1A	2.1940(10)	N2-Cu1-S1-S1A	-5.0(2)
[Ph(H2LiPr2)Cu]2(S2) (2e)
Cu1-N1	1.9054(16)	S1-S1A	2.2007(10)
Cu1-N2	1.9127(16)	Cu1···Cu1A	3.8143(5)
Cu1-S1	2.1984(6)	S1-Cu1-S1A	59.97(2)
Cu1-S1A	2.2051(6)	N2-Cu1-S1-S1A	-6.27(17)
[3,5-(CF3)2C6H3(H2LMe2)Cu]2(S2) (2f)
Cu1-N1	1.912(2)	S1-S1A	2.2013(15)
Cu1-N2	1.906(2)	Cu1···Cu1A	3.8045(7)
Cu1-S1	2.1978(8)	S1-Cu1-S1A	60.11(4)
Cu1-S1A	2.1976(8)	N1-Cu1-S1-S1A	-4.6(3)
[3,5-(CF3)2C6H3(H2LiPr2)Cu]2(S2) (2g)
Cu1-N1	1.9213(18)	S1-S1A	2.2060(12)
Cu1-N2	1.9047(17)	Cu1···Cu1A	3.8072(10)
Cu1-S1	2.2060(9)	S1-Cu1-S1A	60.18(2)
Cu1-S1A	2.1941(9)	N1-Cu1-S1-S1A	-6.3(2)
[(HL’Me2)Cu]2(S2) (2h)
Cu1-N1	1.925(2)	S1-S1A	2.2130(15)
Cu1-N2	1.893(2)	Cu1···Cu1A	3.7858(7)
Cu1-S1	2.1936(8)	S1-Cu1-S1A	60.62(3)
Cu1-S1A	2.1916(8)	N1-Cu1-S1-S1A	-6.6(2)
[(HL’iPr2)Cu]2(S2)b (2i)
Cu1-N1	1.880(5)	S1-S1A	2.165(3)
Cu1-N2	1.922(5)	Cu1···Cu1A	3.8446(16)
Cu1-S1	2.2011(18)	S1-Cu1-S1A	58.78(8)
Cu1-S1A	2.2113(18)	N-Cu1-S1-S1A	-3.9(5)
[(MeL’iPr2)Cu]2(S2) (2j)
Cu1-N1	1.889(2)	S1-S2	2.1691(13)
Cu1-N2	1.929(2)	Cu1···Cu1A	3.8857(8)
Cu1-S1	2.2278(6)	S1-Cu1-S2	58.34(3)
Cu1-S2	2.2224(6)	N2-Cu1-S1-S2	1.5(2)

^aEstimated standard deviations in parentheses. “A” refers to symmetry-related atoms.

^bThese structures have been reported previously.^13a

In each complex, the copper centers are coordinated to two nitrogen atoms of the supporting ligand and both sulfur atoms of the μ-η²:η²-disulfido bridge in a square planar geometry. Some deviation of the Cu(II) ion from a square planar shape is observed for 2c, presumably due to the large steric constraints of the supporting ligand. This deviation is evident by the significant distortion of the N-Cu-S-S torsion angle (N2-Cu-S1-S2 = -32.4°) from the idealized value of ∼ 0°. Overall, the complexes in the series exhibit similar planar Cu₂S₂ core geometries. Nonetheless, structural variation is evident upon comparison of the Cu₂S₂ core parameters (Cu-Cu, S-S, and Cu-S distances) for 2a-j and other examples from the literature (Table 2, Figure 4). The plot in Figure 5 shows that the core parameters for most of the β-diketiminate complexes are clustered close to one another (lower left), but across the entire set there is a correlated trend, such that as the Cu-Cu and Cu-S distances increase, the S-S distance decreases. This trend implies that as the supporting N-donor ligand is varied, the core bonding and extent of S-S bond activation changes; stronger bonding of the copper centers to the S₂^2- moiety accompanies a decreased S-S bond order. The relationship between the Cu₂S₂ bonding and the supporting ligand attributes is analyzed below (Discussion).

Table 2.

Cu₂S₂ Core Distances (Å) and Vibrational Frequencies (cm^-1) for (μ-η²:η²-disulfido)dicopper complexes

	S-S	Cu···Cu	avg. Cu-S	ν(S-S)	Δν(34S)	Ref
2a	2.214(10)	3.7687(5)	2.186	442	9	this work
2b	2.2007(11)	3.7991(5)	2.195	443	10	13a
2c	2.1242(13)	3.9950(7)	2.262	454	13	this work
2d	2.2138(13)	3.7899(12)	2.195	424	10	this work
2e	2.2007(10)	3.8143(5)	2.202	435	8	this work
2f	2.2013(15)	3.8045(7)	2.198	441	9	this work
2g	2.2060(12)	3.8072(10)	2.200	428	12	this work
2h	2.2130(15)	3.7858(7)	2.193	432	8	this work
2i	2.165(3)	3.8446(16)	2.206	440	13	13a
2j	2.1691(13)	3.8857(8)	2.225	443	11	this work
2 (L = TpiPr2)	2.073(4)	4.028(3)	2.264	500	12	17, 22
2 (L = Me2NPY2)	2.117(2)	3.9336(10)	2.233	--	--	18

Figure 5.

Plot of Cu-Cu, S-S, and Cu-S distances (Å, from Table 2) determined by X-ray crystallography for μ-η²:η²-disulfidodicopper complexes (blue = complexes supported by tri- and tetradentate ligands; green = complexes supported by bidentate β-diketiminate and anilido-imine ligands). For label nomenclature, see Figures 1 and 3.

(b) Clusters [Cu(SR)]₄ (R = H(Me₂L^Me2), 5a, or H(Me₂L^Et2), 5b)

The new X-ray structure of 5a is presented in Figure 6a, and its core is compared to that of the previously reported complex^13a 5b in Figure 6b. The atom connectivity in 5a and 5b is the same; both complexes feature 3-coordinate Cu(I) ions bridged by thiolate sulfur atoms derived from functionalization of the original β-diketiminate ligand at the central methine carbon. The complexes adopt significantly different geometries, however, and thus can be envisioned as conformational isomers (notwithstanding the different Me vs. Et substituents). As indicated in Figure 6b, the [Cu₄(SR)₄] core of 5a (which features a C₄ molecular axis of symmetry) is expanded relative to the folded core of 5b, such that the former features Cu···Cu distances ∼0.7 Å longer than in the latter. The puckering of the core in 5b is accompanied by significantly more acute Cu-S-Cu angles (∼75°) relative to those in 5a (∼100°), a variation that indicates significant flexibility in the bonding of the thiolates to the Cu(I) ions.

Figure 6.

(a) X-ray structure of [Cu(SR)]₄ (R = H(Me₂L^Me2)), showing all nonhydrogen atoms as 50% thermal ellipsoids. (b) Comparison of the cores of the structures of [Cu(SR)]₄ (left, R = H(Me₂L^Me2); right, H(Me₂L^Et2)) with selected interatomic distances shown (Å). Color key: green, Cu; blue, N; yellow, S; white, C. Note that the complex on the left features a C₄ axis, so only one set of unique distances are listed.

Spectroscopy on Disulfido Complexes. (a) Absorption

The disulfido complexes 2a-j are deeply colored due to multiple low energy features in their absorption spectra. Plots of the spectra are presented in Figure S8 for those complexes that could be isolated analytically pure in bulk; the data for 2f and 2g are presented for illustrative purposes in Figure 7(a). In general, similar spectral features are observed for all the complexes, which we assign by reference to the detailed analysis published for 2 (L = Tp^iPr2).²² The spectrum of 2 (L = Tp^iPr2) contains two intense features at ∼28,000 cm^-1 (ε ∼ 31,200 M^-1cm^-1) and ∼21,000 cm^-1 (ε ∼3700 M^-1cm^-1) attributed to π* → Cu(II) CT transitions originating from the π*_σ + xy + xy (hereafter designated as “π*_σ”) and π*_v orbitals, respectively (Figure 2). In addition, weaker d → d transitions at ∼15,000 cm^-1 (ε ∼230 M^-1cm^-1) and ∼10,300 cm^-1 (ε ∼ 130 M^-1cm^-1) were reported. The spectra for complexes 2a-j also exhibit intense bands between 20,000-28,000 cm^-1. In the case of the β-diketiminate complexes exemplified by 2f and 2g (Figure 7(a)), these are clearly resolved; for example, the maxima for 2g are at ∼23,000 and ∼26,400 cm^-1. By analogy to the assignments for 2 (L = Tp^iPr2), the features are assigned as the out-of-plane disulfide π*_v → Cu(II) and the in-plane disulfide π*_σ → Cu(II) CT transitions, respectively. The spectra also contain less intense features between 13,000-18,000 cm^-1 that may be assigned as Cu d → d transitions. In general, the absorption features for the β-diketiminate complexes occur at higher energy than those of 2 (L = Tp^iPr2). Thus, the lower energy π*_v → Cu(II) CT has a maximum at an energy >23,000 cm^-1 for 2a-g, which is ≥ 2,000 cm^-1 greater than that of the corresponding feature at ∼21,000 cm^-1 for 2 (L = Tp^iPr2). Similarly, the d→ d transitions for the β-diketiminate compounds appear ∼1,000-2000 cm^-1 higher in energy than the ∼15,000 cm^-1 band for 2 (L = Tp^iPr2). These discrepancies may be traced to differences in the strength of the Cu-S bonding, as described below (Discussion). Such Cu-S bonding differences also are likely to be the cause of more subtle variation of absorption spectral features seen when data for complexes with similar supporting ligands are compared. For example, the disulfide π* → Cu(II) CT and d → d bands for 2f are shifted to higher energy relative to 2g (Figure 7(a)).

Figure 7.

UV-vis absorption spectra of (a) β-diketiminate disulfido complexes 2f (dashed line) and 2g (solid line), and (b) anilido-imine disulfido complexes 2h (long dashed line), 2i (solid line), and 2j (short dashed line). All spectra were measured in THF at ambient temperature. Proposed assignments are indicated (see text).

The absorption spectra for the anilido-imine disulfido complexes 2h-j (Figure 7(b)) are unique, insofar as their π* → Cu(II) CT bands appear to overlap and to lie at lower energy than the β-diketiminate analogs. The d → d features also are shifted to lower energy. Proper assignments of these spectra and rationales for their differences await more complete spectroscopic studies, which have yet to be performed.

(b) Resonance Raman

Using an excitation wavelength of 457.9 nm that falls within the π*_v → Cu(II) CT transition, resonance Raman spectra were obtained of the complexes 2a-j prepared with ³²S₈ or ³⁴S₈. The spectra encompassing the region where sulfur-isotope sensitive features were observed (300-550 cm^-1) are provided as Figures S9-S18; for illustration, the spectrum for 2d is shown in Figure 8. All the spectra contain a sharp sulfur-isotope sensitive peak (Δν(³⁴S) = 8-13 cm^-1) that on the basis of precedent²² is assigned to a predominantly ν(S-S) vibrational mode. The peak positions and isotope shifts are listed in Table 2. The ν(S-S) values for 2a-j fall within a narrow range (424-454 cm^-1) notably lower than ν(S-S) = 500 cm^-1 reported for 2 (L = Tp^iPr2). This latter value is similar to those reported for a wide range of disulfido complexes of transition metals (Table S2). Thus, the data indicate an especially high degree of S-S bond activation for the series of compounds 2a-j.

Figure 8.

Resonance Raman spectra (λ_ex = 457.9 nm ≃ 21,900 cm^-1) of frozen benzene solutions (77K) of [Ph(H₂L^Et2)Cu]₂(S₂) (2d, solid line for ³²S, dashed line for ³⁴S).

Reactivity

For the purposes of drawing some preliminary comparisons to reactivity reported for 2 (L = Me₂NPY2),¹⁸ reactions of one of the disulfido complexes (2h) with PPh₃ and xylyl isocyanide were explored (Scheme 2). Monitoring by ¹H and ³¹P NMR spectroscopy showed that, as seen for 2 (L = Me₂NPY2),¹⁸ treatment of 2h with 4 equiv. PPh₃ yielded 2 equiv. of S=PPh₃ and 2 equiv. of the Cu(I)-phosphine adduct (HL’^Me2)Cu(PPh₃). This complex was identified on the basis of comparison to other known examples with β-diketiminate supporting ligands,³¹ as well as by independent synthesis from (HL’^Me2)Cu(CH₃CN). Similar reaction of 2h with 2 equiv. PPh₃ resulted in incomplete conversion to 1 equiv. of the phosphine adduct and 1 equiv. S=PPh₃, with half of 2h remaining. This suggests similar efficiency for S-atom transfer to PPh₃ and trapping of Cu(I) by PPh₃, which differs from the results reported for 2 (L = Me₂NPY2), where 2 equiv. PPh₃ were reported to yield 2 equiv. S=PPh₃.^18,32

Scheme 2.

Reactions of 2h (L = HL’^Me2, h).

The reactions of 2h with xylyl isocyanide, O₂, and CO also proceeded differently than the same reactions with 2 (L = Me₂NPY2). While S-atom transfer was reported for reaction of 2 (L = Me₂NPY2) with xylyl isocyanide to yield ArN=C=S,¹⁸ we observed clean conversion of 2h to the Cu(I) adduct of the unfunctionalized isocyanide, (HL’^Me2)Cu(CNAr) (no effort to determine the fate of the S₂^2- fragment was made). This complex was identified by comparison to other examples,⁴⁵ as well as by independent synthesis. Complex 2h was unreactive with O₂ (1 atm, ∼1 h, room temperature, THF, UV-vis) or CO (1 atm, 20 min, room temperature, C₆D₆, NMR), in contrast to 2 (L = Me₂NPY2), which yielded peroxodicopper and Cu(I)-CO complexes, respectively.¹⁸

Discussion

Reaction of S₈ with Cu(I) complexes of a series of anionic, bidentate N-donor ligands (Figure 3) yielded a series of (μ-η²:η²-disulfido)dicopper complexes 2a-j. In the case of the β-diketiminate ligands, in particular those that contain an unsubstituted methine position (a-c), the syntheses are complicated by further reaction to yield the clusters 5, which were isolated for ligands a and b. Related sensitivity of β-diketiminate ligands toward reaction at the methine position has been reported previously.³³ The ligand functionalization is avoided entirely when anilido-imine ligands h-j are used. X-ray structures of 5a and 5b reveal that, despite their different alkyl substituents, they are conformational isomers. Their cyclic Cu₄(SR)₄ topologies differ with respect to their Cu-Cu distances and Cu-S-Cu angles, such that 5b is puckered relative to 5a.

A primary goal of this work is to assess the structural and spectroscopic properties of the disulfido complexes 2a-j in order to understand supporting ligand effects on S-S bond activation. The relevant experimental data are best understood by recourse to the bonding picture for the [Cu₂(μ-η²:η²-S₂)]²⁺ core (and peroxo analogs) shown in Figure 2.²² According to this picture, increased Cu-S bonding interactions results in greater backbonding from the filled Cu d_xy orbitals into the empty S₂^2- σ* orbital and a lowering of the S-S bond order (i.e., greater S-S bond activation). Stronger bonding interactions between the Cu ions and the disulfide fragment also cause an increase in the energy splitting of the frontier orbitals, notably a lowering of the π*_σ +d_xy + d_xy and the HOMO concomitant with a raising of the LUMO (π*_σ +d_xy - d_xy). Experimentally, these shifts are indicated by increases in the energies of the π*_v → Cu(II) and π*_σ → Cu(II) CT transitions in absorption spectra.²² In addition, X-ray crystallographic and vibrational spectroscopic data enable the extent of Cu-S bond strengthening and S-S bond weakening to be directly discerned. Observation of appropriate correlations among the experimental parameters provides support for the bonding picture and enables the specific role of ligand structural variation on S-S bond activation to be determined.

The X-ray structural and resonance Raman data show that the degree of S-S bond activation in the [Cu₂(μ-η²:η²-S₂)]²⁺ complexes of bidentate, anionic β-diketiminate and anilido-imine ligands is generally greater than in other disulfido complexes of copper and other transition metals. As illustrated in Figure 5 (Table 2), with one exception (2c), the complexes 2a-j feature shorter Cu-S, longer Cu-Cu, and longer S-S distances than 2 (L = ii or Me₂NPY2). The correlated trends are consistent with the bonding picture in Figure 2; shorter Cu-S distances indicate stronger Cu-S bonds, which result in shorter Cu-Cu distances and a lowering of the S-S bond order (longer S-S distance) through greater backbonding into the S₂^2- σ* orbital. At one extreme in Figure 5 are the complexes 2c, 2j, and 2 (L = Tp^iPr2 or Me₂NPY2), which exhibit Cu-S > 2.21 Å, Cu-Cu > 3.85 Å, and S-S < 2.17 Å. Notably, the S-S distance in 2 (L = Tp^iPr2) of 2.073(4) Å is close to that of H₂S₂ (2.055 Å), both of which fall within the range of values reported for a large sampling of transition metal disulfido complexes (∼2.00-2.08 Å, Table S2).³⁴ Complexes 2a,d and h are at the other extreme, with Cu-S < 2.195 Å, Cu-Cu < 3.79 Å, and S-S > 2.21 Å that are indicative of an extraordinary degree of S-S bond activation. Indeed, a search of the CSD³⁵ revealed only a few examples of complexes with S₂^2- ligands featuring S-S bond distances > 2.2 Å. The S₂^2- ligand is bound to 3 or more metal ions in several of these,³⁶ leaving three cases with μ-η²:η²-disulfido moieties bridging two Ni (S-S = 2.298 and 2.209 Å)³⁷ or Re ions (S-S = 2.228 Å).³⁸

The generally high degree of S-S bond activation in 2a-j is further supported by the resonance Raman spectra, which feature peaks attributed to predominantly ν(S-S) modes on the basis of their positions and ³⁴S-isotope shifts. The peak positions for 2a-j are >45 cm^-1 lower than that of 1 or 2 (L = Tp^iPr2), consistent with lower S-S bond orders for the former class. The data may be further analyzed by application of “Badger’s Rule” (eq. 1), an empirical relationship

$$r_e=\frac{C_{ij}}{{\nu }_e^{2∕3}}+d_{ij}$$ (1)

between an equilibrium bond distance (r_e) and its associated stretching frequency (ν_e) in a series of related species.³⁹ Usually applied to small polyatomic molecules, it has also recently been found to be useful for assessing heme and non-heme iron-oxygen bonds.⁴⁰ A plot of S-S distance versus 1/ν^2/3 is shown in Figure 9 for 2a-j (red circles), 2 (L = Tp^iPr2) (black triangle), 1 (green triangle), and a large sampling from the large class of known transition metal disulfido complexes (purple circles, listed in Table S2). Also shown are data for Na₂S₂ and S₂,⁴¹ taken as representative of S-S single and S=S double bonds, respectively (black circles). Further perspective is provided by a data point (green circle) corresponding to the μ-1,2-disulfido(·1-) moiety in a recently reported dicopper complex.^13c The data fit reasonably well to eq. 1 with parameters C = 63.95 and d = 1.063 (dashed line; R = 0.93), although the scatter about the line is greater than that reported for a similar plot of Fe-O bonds in heme species over similar ranges of distances (Δ ∼ 0.3 Å) and stretching frequencies (Δ ∼ 300-350 cm^-1).⁴⁰ We speculate that this may be due to the fact that the ν(S-S) features are not always pure S-S stretches, a notion supported by calculations reported for 2 (L = Tp^iPr2) that indicate that its 500 cm^-1 mode has 63% S-S and 37% Cu-S character.²² With this caveat in mind, a number of broad conclusions can nonetheless be drawn from the linear correlation in Figure 9. Although tightly clustered, the data for 2a-j lie at one extreme, indicative of S-S bond orders even smaller than that in Na₂S₂ (bond order formally equal to one, but with an S-S bond considered to be slightly elongated due to lone pair-lone pair repulsions). The complexes 1 and 2 (L = Tp^iPr2) fall within the regime of the sampling of typical transition metal disulfido complexes (purple circles, bond order formally equal to 1). These generally feature weaker S-S bonds than that of S ^-₂ (green circle, bond order ∼ 1.5) and S₂ (bond order = 2).

Figure 9.

Plot of the S-S distance (Å) versus 1/ν^2/3 (cm^2/3), where ν = S-S vibrational mode. The various S₂^2- complexes shown as purple circles are listed in Table S2. The dashed line is a linear least squares fit of the data to eq. 1, with slope = C_S-S = 63.95 and intercept = d_S=S = 1.063 (R = 0.93).

The weak S-S bonds in 2a-j may be attributed to the presence of the strongly electron donating β-diketiminate and anilido-imine supporting ligands, which are effective at inducing back-donation from the copper ions into the S-S σ* orbital. Accordingly, this effect is decreased in 2 (L = Tp^iPr2 or Me₂NPY2) because the supporting ligands are poorer electron donors, resulting in shorter and stronger S-S bonds. The differences in electron donating capabilities of the supporting ligands are also manifested in differences in the Cu-S bonding, as revealed experimentally by the energies of the π → Cu(II) CT and d → d transitions in electronic absorption spectra. These transition energies are generally higher in 2a-j than in 2 (L = Tp^iPr2 or Me₂NPY2), reflecting greater splitting of the frontier molecular orbitals and stronger Cu-S bonding (Figure 2).

The relative electron donating power of β-diketiminate/anilido-imine vs. tris(pyrazolyl)hydroborate ligands was identified previously as a key determinant of the electronic structures of 1:1 Cu/O₂ adducts (peroxo-Cu(III) vs. superoxo-Cu(II))⁴² and Cu-thiolate models of type 1 copper electron transfer sites.⁴³ Differences in the relative stability of bis(μ-oxo)- vs. μ-η²:η²-peroxodicopper isomers have also been linked to the electron donating capabilities of these supporting ligands.⁴⁴ A finding of particular significance here is the sole observation of bis(μ-oxo)dicopper complexes with the β-diketiminate ligands a,b,d-g (which stabilize the formal Cu(III) state).⁴⁵ In contrast, analogous bis(μ-sulfido)dicopper(III) cores appear not to be accessible, which has been verified by theoretical calculations.^13a Despite the S-S bond weakening evident in 2a-j, the lower electronegativity of sulfur relative to oxygen renders cleavage of the S-S bond in the disulfidodicopper core less favorable than that of the O-O bond in the peroxodicopper congener.

Differences in the degree of S-S bond activation within the series 2a-j are most clearly evident from the bond distances (Figure 5)⁴⁶ and can generally be attributed to steric effects. For those ligands with relatively small aryl substituents (R” = Me or Et; a,b,d,f,h) or with larger iPr groups but which feature R’ = H (thus allowing the aryl group to ‘bend back’; e,g,i), the full electronic donor influence of the ligands is manifested by longer S-S, shorter Cu-S, and shorter Cu-Cu distances. The ligands c and j are significantly more sterically congested due to the presence of iPr aryl (R”) substituents and Me or tBu backbone (R’) groups.⁴⁷ We postulate that this congestion is the underlying cause of the relatively longer Cu-S and Cu-Cu distances and the decreased level of S-S bond activation in their disulfido complexes. These steric effects for c and j have precedent in Cu/O₂ chemistry, as reactions of O₂ with Cu(I) complexes of these ligands yield 1:1 adducts^42b,48 instead of bis(μ-oxo) complexes seen with a,b,d-g.⁴⁵

Preliminary investigation of the reactivity of one of the disulfido complexes (2h) showed some similarity to that previously reported for 2 (L = Me₂NPY2),¹⁸ although significant differences also were seen (Scheme 2). Thus, S-atom transfer to PPh₃ with trapping of the resulting Cu(I) sites by excess phosphine was observed for both, but the results with only 2 equiv. PPh₃ differed. For 2 (L = Me₂NPY2), complete S-atom transfer to yield 2 equiv. S=PPh₃ was reported,¹⁸ but we found that 2h yielded 1 equiv. S=PPh₃ and 1 equiv. of the Cu(I)-phosphine adduct, with half of 2h left unreacted. These results imply that S-atom transfer from 2 (L = Me₂NPY2) outpaces trapping of the Cu(I) complex by PPh₃, wheareas for 2h the two processes are more closely competitive. Poorer S-atom transfer capabilities for 2h relative to 2 (L = Me₂NPY2) are also suggested by the results of reactions with xylyl isocyanide, which for the former gave the Cu(I) adduct of the isocyanide, but for the latter yielded the product of S-atom incorporation, ArN=C=S. In addition, the lack of reactivity of 2h with O₂ and CO contrasts with reactions of these reagents with 2 (L = Me₂NPY2), which yielded peroxodicopper or Cu(I)-CO species, respectively.

Conclusions

Through the synthesis and detailed structural and spectroscopic characterization of the series of complexes 2a-j that feature the [Cu₂(μ-η²:η²-S₂)]²⁺ core, the degree of S-S bond activation as a function of supporting ligand was assessed. In general, the strong electron donating power of the β-diketiminate and related anilido-imine ligands results in strong Cu-S interactions that are correlated with weak S-S bonds and short Cu-Cu distances. In addition to structural correlation of the Cu-S, S-S, and Cu-Cu distances within the [Cu₂(μ-η²:η²-S₂)]²⁺ core (Figure 5), the S-S distances also correlate inversely with ν(S-S) measured by resonance Raman spectroscopy (Badger’s rule, Figure 9). A comparison to a range of other species that feature S ^n-₂ units places 2a-j at the extreme of weak S-S bonding (long S-S and low ν(S-S)) that corresponds to particularly powerful S-S bond activation. These phenomena, as well as the observation of increased energies for π → Cu(II) CT and d → d transitions in electronic absorption spectra, may be understood in terms of a previously proposed bonding picture (Figure 2). According to this picture, large frontier orbital energy splittings result from strong Cu-S bonding, with the powerful electron donating capabilities of the β-diketiminate and anilido-imine ligands underlying increased backbonding into the S₂^2- σ* orbital, which weakens the S-S bond. These ligand effects are mitigated by steric hindrance provided by bulky substituents, which yield complexes with longer Cu-Cu and shorter S-S distances.

The [Cu₂(μ-η²:η²-S₂)]²⁺ complexes supported by β-diketiminate or anilido-imine ligands also exhibit unique reactivity. Sulfur transfer to the central methine carbon of the β-diketiminate ligands yields [Cu(SR)]₄ clusters 5, which may adopt different conformations depending on the ligand substituents. This reaction is prevented when the anilido-imine ligands are used (2h-j). A preliminary reactivity study of 2h revealed facile sulfur transfer to added PPh₃, but only displacement without sulfur insertion upon reaction with xylyl isocyanide, and no reactions with O₂ or CO. In comparison to 2 (L = Me₂NPY2), the sulfur moiety in 2h appears less reactive, with trapping of the copper ions to yield Cu(I)-X (X = PPh₃ or ArNC) complexes being more facile.