Heterotrimetallic Assemblies with 1,2,4,5-Tetrakis(diphenylphosphino)benzene Bridges: Constructs for Controlling the Separation and Spatial Orientation of Redox-Active Metallodithiolene Groups

Abstract

Metallodithiolene complexes of the type [(R₂C₂S₂)M(η²-tpbz)] [R = CN, Ph, or p-anisyl; M = Ni²⁺, Pd²⁺, or Pt²⁺; tpbz = 1,2,4,5-tetrakis(diphenylphosphino)benzene] chelate transition metals ions to form trimetallic arrays [[(R₂C₂S₂)M(tpbz)]₂M′]ⁿ⁺, where M′ is square planar Pt²⁺, tetrahedral Cu⁺, Ag⁺, or Au⁺, or octahedral {ReBr(CO)}/{Re(CO)₂}⁺. Forcing conditions (190 °C reflux in decalin, 72 h) are demanded for the Re⁺ compounds. With third-row metals at the nexus, the compounds are stable to air. Twelve members of the set have been characterized by X-ray diffraction and reveal dithiolene centroid–centroid distances ranging from 22.4 to 24.0 Å. Folding around each tpbz intrachelate P···P axis such that the MP₂/M′P₂ planes meet the tpbz P₂C₆P₂ mean plane at non-zero values gives rise to core topologies that appear “S-like” or herringbone-like for M′ = Pt²⁺ or {ReBr(CO)}/{Re(CO)₂}⁺. Calculations reveal that departure from idealized D_2h/D_2d/C_2v symmetries is induced by steric crowding between Ph groups and that dynamic, fluxional behavior is pertinent to the solution phase because multiple, lower-symmetry minima of comparable energy exist. Spectroscopically, the formation of the trimetallic arrays is marked by a shift of the open end ³¹P nuclear magnetic resonance signal from approximately −14.5 ppm to approximately +41, approximately +20.5, and approximately +28.5 ppm for M′ = Pt²⁺, Au⁺, and {ReBr(CO)}/{Re(CO)₂}⁺, respectively. Electrochemically, dithiolene-based oxidations are observed for the R = Ph and M′ = Pt²⁺ or Au⁺ compounds but at potentials that are anodically shifted relative to charge-neutral [[(R₂C₂S₂)M]₂(μ-tpbz)]. The compounds reported clarify the possibilities for the synthesis of assemblies in which weakly coupled spins may be created in their modular (R₂C₂S₂)M and M′ parts.

Short abstract

Metallodithiolene complexes [(R₂C₂S₂)M(η²-tpbz)] [R = CN, Ph, or p-anisyl; M = Ni²⁺, Pd²⁺, or Pt²⁺; tpbz = 1,2,4,5-tetrakis(diphenylphosphino)benzene] form trimetallic arrays [[(R₂C₂S₂)M(tpbz)]₂M′]ⁿ⁺, where M′ is square planar Pt²⁺, tetrahedral Cu⁺, Ag⁺, or Au⁺, or octahedral {ReBr(CO)}/{Re(CO)₂}⁺. Twelve members of the set have been crystallographically characterized. Electrochemically, dithiolene-based oxidations are observed for R = Ph and M′ = Pt²⁺ or Au⁺ at potentials anodically shifted relative to [[(R₂C₂S₂)M]₂(μ-tpbz)]⁰, with J ≈ 0 because of interspin separations of ∼23 Å.

Introduction

The arena of quantum information sciences is now widely recognized as strategically significant ground for the economy and security of the future.¹ This broadly ranging theme includes, inter alia, development of a robust and scalable physical platform that can support coherent quantum states that form the basis for memory and logic operations. Among the various scaffolds that have been considered for quantum bits (qubits) in quantum computing and quantum memory applications, electron spins hosted within discrete molecules or coordination complexes have some distinct advantages over other systems.² One such advantage is the power of synthetic chemistry to vary the separation between spins such that the coupling between them is strong enough to support a coherent quantum state but weak enough to move coherence lifetimes into the time domain necessary for the execution of logic operations. Another advantage is a capacity to change the relative spatial orientation of two or more spins within a host molecule such that one spin might be manipulated selectively over another by an appropriately directed pulse.³ This selectivity is a basis for addressability.⁴

Bearing the forgoing general ideas in mind, we have elaborated upon the coordination chemistry of group 10 dithiolene diphosphine complexes, which generally sustain one, or sometimes two, reversible redox processes at the dithiolene ligand. When the diphosphine used is 1,2,4,5-tetrakis(diphenylphosphino)benzene (tpbz), centrosymmetric dimetallic compounds that support identical, very nearly concurrent oxidations at the dithiolene end groups can be prepared.^5,6 The diradical thus created, with dipolar coupling of ∼16 cm^–1, forms the basis for a diqubit system.⁷ In the course of these studies, we noted that the synthesis conditions can be modified to favor the formation of “open-ended” compounds (Scheme 1, left).⁸ Among other possibilities, these open-ended compounds provide for the introduction of asymmetry with different dithiolene ligands featuring different redox potentials. They also may be implemented as “ligands” in their own right, thereby arraying about some central ion two, or possibly three, metallodithiolene moieties in a well-defined geometry. Because such multimetal assemblies enable still further possibilities for weakly coupled distal spins, possibly but not necessarily involving the ensconced central ion, we have targeted the synthesis of a variety of such complexes and report herein the leading results of a broad synthetic foray into such compounds with attending structural, electrochemical, and spectroscopic characterization.

Scheme 1. Trimetallic Assemblies with Metallodithiolene End Groups and tpbz Bridging Ligands.

Work from the Freedman group has highlighted the great promise of dithiolene coordination compounds as hosts for electron spin qubits. In systems designed to minimize the presence of spin-active nuclei that contribute to the decoherence of qubit superposition states, coherence lifetimes of ∼1 ms have been achieved.^9,10 The work reported here differs from that of the Freedman group in that the electron spins can be reversibly generated at the organic dithiolene ligands rather than existing within the d-electron manifold of the transition metal. More recently, possibilities for selective optical addressability of ground state spins in discrete molecules have been disclosed.¹¹ The results described herein regarding both the incorporation of photoactive third-row transition metal ions into metallodithiolene arrays and the orthogonal disposition of dithiolene-based electron spins relate to this very powerful design concept in qubit engineering.

Experimental Section

Physical Methods

All ¹H and ³¹P nuclear magnetic resonance (NMR) spectra were recorded at 25 °C with either a Bruker Avance spectrometer operating at 300.13 and 121.49 MHz for the ¹H and ³¹P nuclei, respectively, or a Bruker Ascend 400 spectrometer operating at 162.11 MHz for ³¹P. The ¹H NMR spectra were referenced to the solvent signal, while an external aqueous H₃PO₄ solution was employed as a reference for all ³¹P spectra. Ultraviolet–visible (UV–vis) spectra were recorded at ambient temperature with a Hewlett-Packard 8455A diode array spectrometer, while IR spectra were recorded as KBr pellets using a Thermo Nicolet Nexus 670 Fourier transform infrared instrument in absorption mode. Mass spectra (ESI+) were recorded with a Bruker micrOTOF II mass spectrometer. All observed and calculated masses represent the most intense peak in the mass envelope pattern. The [8]³⁺ and [12]³⁺ cation radicals were generated by chemical oxidation of [8]⁺ and [12]⁺ using 2 equiv of [(Br-p-C₆H₄)₃N][SbCl₆]. X-Band electron paramagnetic resonance (EPR) spectra were recorded on a Bruker ELEXSYS E500 spectrometer, and simulations were performed using XSophe.¹² Electrochemical measurements were taken with a CH Instruments 620C electroanalyzer workstation using a Ag/AgCl reference electrode, a platinum disk working electrode, Pt wire as the auxiliary electrode, and [ⁿBu₄N][PF₆] as the supporting electrolyte. Under these conditions, the Cp₂Fe⁺/Cp₂Fe (Fc⁺/Fc) couple consistently occurred at 0.436 V in CH₂Cl₂ and at 0.500 V in N,N-dimethylformamide. Procedural details regarding crystal growth, X-ray diffraction data collection, data processing, and structure solution and refinement are available in the Supporting Information. Unit cell data and selected refinement statistics for the compounds that have been structurally identified are listed in Table 1; more complete crystallographic data are summarized in Tables S1–S4. A description of the methodologies employed, the theory level implemented, and other details pertinent to the computations is deposited in the Supporting Information.

Table 1. Unit Cell and Refinement Data for Crystallographically Characterized Trimetallic Compounds.

	[1][CF3SO3]2	[3][CF3SO3]2	[4][CF3SO3]2	[5][CF3SO3]2
formula	C144H119N7O14F6P8S6Ni2Pt	C147H129N3O9F6P8S6Ni2Pt	C148.5H128.5N3.5O11.5F6P8S6Pd2Pt	C150H136N4O12F6P8S6Pt3
fw (g/mol)	3038.08	2944.12	3108.05	3326.01
crystal system	triclinic	monoclinic	monoclinic	monoclinic
space group	P1̅	P21/n	P21/n	P21/n
a (Å)	13.592(3)	18.0711(13)	18.1595(15)	18.176(3)
b (Å)	14.064(3)	16.1273(12)	15.9710(13)	15.956(3)
c (Å)	21.188(5)	24.8605(18)	24.785(2)	24.813(4)
α (deg)	73.458(3)	90	90	90
β (deg)	89.326(3)	94.710(2)	92.570(2)	92.568(2)
γ (deg)	63.186(3)	90	90	90
volume (Å3)	3432.9(13)	7220.8(9)	7181.0(10)	7189(2)
Z	1	2	2	2
no. of independent reflections	14975	10390	11475	8843
goodness of fita	1.015	1.028	1.028	1.054
R1,b,c wR2c,d	0.0677, 0.1614	0.0761, 0.2093	0.0820, 0.2350	0.0790, 0.1671
R1,b,e wR2d,e	0.1041, 0.1771	0.1327, 0.2575	0.1208, 0.2798	0.1365, 0.1992

	[6][CF3SO3]	[7][Cl]	[10][CF3SO3]	[11][CF3SO3]
formula	C134H107.5N4.5O5F3P8S5Ni2Au	C142.5H117NO4.5P8S4ClNi2Au	C149H129N4O5.5F3P8S5Ni2Au	C137H104O3F3P8S5Pd2Au
fw (g/mol)	2640.19	2641.20	2843.00	2673.02
crystal system	monoclinic	trigonal	monoclinic	tetragonal
space group	C2	P31c	P21/n	I41/a
a (Å)	36.6060(12)	28.9658(4)	16.9691(7)	40.930(3)
b (Å)	20.5956(7)	28.9658(4)	26.4603(10)	40.930(3)
c (Å)	21.1856(7)	35.2990(15)	32.2338(12)	34.252(4)
α (deg)	90	90	90	90
β (deg)	123.408(1)	90	92.246(1)	90
γ (deg)	90	120	90	90
volume (Å3)	13333.2(8)	25648.6(18)	14462.1(10)	57382(11)
Z	4	6	4	16
no. of independent reflections	36107	26413	36255	24484
goodness of fita	1.066	1.050	1.159	1.100
R1,b,c wR2c,d	0.0463, 0.1036	0.0400, 0.1076	0.1198, 0.2876	0.0762, 0.1944
R1,b,e wR2d,e	0.0614, 0.1105	0.0451, 0.1122	0.1504, 0.3049	0.1210, 0.2537

	16	[17][Br]	18	[19][Br]
formula	C117H84N4OP8S4Ni2BrRe	C118H84N4O2P8S4Ni2BrRe	C161H124N4O9P8S4BrRePt2	C161H114N3O9P8S4BrRePt2
fw (g/mol)	2321.41	2349.42	3290.92	3266.83
crystal system	monoclinic	monoclinic	monoclinic	monoclinic
space group	P21/n	P21/n	P21/c	P21/c
a (Å)	14.0537(10)	13.924(3)	14.2385(7)	14.0974(14)
b (Å)	24.4013(17)	23.264(5)	21.6579(11)	21.564(2)
c (Å)	20.3377(14)	19.354(4)	23.3488(12)	23.225(2)
α (deg)	90	90	90	90
β (deg)	108.650(2)	92.803(2)	96.601(2)	96.407
γ (deg)	90	90	90	90
volume (Å3)	6608.1(8)	6262(2)	7152.5(6)	7016.4(12)
Z	2	2	2	2
no. of independent reflections	8112	2180	14769	17463
goodness of fita	1.011	1.072	1.119	1.009
R1,b,c wR2c,d	0.0716, 0.2030	0.1056, 0.2776	0.0701, 0.1558	0.0789, 0.1596
R1,b,e wR2d,e	0.1068, 0.2363	0.1265, 0.3023	0.1028, 0.1786	0.2013, 0.2086

^aGoodness of fit = {∑[w(F₀² – F_c²)²]/(n – p)}^1/2, where n is the number of reflections and p is the total number of parameters refined.

^bR1 = ∑||F₀| – |F_c||/∑|F₀|.

^cR indices for data cutoff at I > 2σ(I).

^dwR2 = {∑[w(F₀² – F_c²)²]/∑[w(F₀²)²]}^1/2; w = 1/[σ²(F₀²) + (xP)² + yP], where P = [2F_c² + max(F₀², 0)]/3.

^eR indices for all data.

General Considerations

Literature methods were implemented for the syntheses of the tpbz ligand,¹³ all [(R₂C₂S₂)M(η²-tpbz)] complexes,⁸ [Pt(N≡CCH₂CH₃)₄][CF₃SO₃]₂,¹⁴ [(MeCN)₄Cu][PF₆],^15a and [(MeCN)₄Ag][BF₄].^15b All other reagents were purchased from commercial sources and used as received. Solvents were either dried with a system of drying columns from the Glass Contour Company (CH₂Cl₂, n-pentane, hexanes, Et₂O, THF, C₆H₆, and toluene) or freshly distilled according to standard procedures (MeOH, CH₃CN, and 1,2-dichloroethane).¹⁶ All reactions described below were conducted under an atmosphere of N₂, unless otherwise indicated, while silica columns were run in the open air using 60–230 μm silica (Dynamic Adsorbents). Abbreviations used throughout the text are as follows: mnt = maleonitriledithiolate(2−) = [(NC)₂C₂S₂]^2–; pdt = [Ph₂C₂S₂]^2– = 1,2-diphenyl-1,2-ethylenedithiolate(2−); adt = [(MeO-p-C₆H₄)₂C₂S₂]^2– = 1,2-di-p-anisyl-1,2-ethylenedithiolate(2−); dppb = 1,2-bis(diphenylphosphino)benzene; [BArF₂₄]⁻ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate(1−).

Syntheses

[((NC)₂C₂S₂)Ni(μ-tpbz)Pt(μ-tpbz)Ni(S₂C₂(CN)₂)][CF₃SO₃]₂, [1][CF₃SO₃]₂

Under a N₂ atmosphere, a solution of [(mnt)Ni(η²-tpbz)] (0.100 g, 0.0986 mmol) in CH₂Cl₂ (20 mL) was transferred dropwise via cannula to a solution of [Pt(NCCH₂CH₃)₄][CF₃SO₃]₂ (0.0352 g, 0.0493 mmol) dissolved in CH₂Cl₂ (5 mL). This reaction mixture was stirred for 12 h at 25 °C. The light brown precipitate that was produced was separated by filter cannulation from the colorless supernatant, washed with CH₂Cl₂ (2 × 5 mL) and then Et₂O (2 × 5 mL), and dried in vacuo. Crystallization was accomplished by diffusion of Et₂O vapor into a filtered solution of [1][CF₃SO₃]₂ in a minimal volume of C₆H₅NO₂. Yield: 0.103 g, 84%. ¹H NMR (δ, DMSO-d₆): 7.75–7.72 (m, 7 H, aromatic C–H), 7.65–7.56 (m, 19 H, aromatic C–H), 7.48–7.43 (m, 32 H, aromatic C–H), 7.24–7.19 (m, 14 H, aromatic C–H), 7.10–7.05 (m, 12 H, aromatic C–H). ³¹P{¹H} NMR (δ, DMSO-d₆): 59.15 (s), 42.41 (s). UV–vis [DMF, λ_max, nm (ε)]: 369 (7170), 423 (2330). IR (KBr, cm^–1): 2203 (asym v_C≡N), 2216 (sym v_C≡N). MS (ESI⁺) calcd for [C₁₁₆H₈₄N₄Ni₂P₈PtS₄]²⁺: m/z 1111.0928. Observed: m/z 1111.0855. Error (δ): 6.57 ppm.

[((MeO-p-C₆H₄)₂C₂S₂)Ni(μ-tpbz)Pt(μ-tpbz)Ni(S₂C₂(C₆H₄-p-OMe)₂)][CF₃SO₃]₂, [2][CF₃SO₃]₂

The same procedure and scale as described for the synthesis of [3][CF₃SO₃]₂ (vide infra) were employed but with [((MeO-p-C₆H₄)₂C₂S₂)Ni(tpbz)] (0.117 g, 0.0995 mmol) used in place of [(Ph₂C₂S₂)Ni(tpbz)]. Yield: 0.095 g of brown solid, 75%. ¹H NMR (δ, DMSO-d₆): 7.73–7.38 (m, 57 H, aromatic C–H), 7.23–7.12 (m, 15 H, aromatic C–H), 7.05–7.00 (m, 7 H, aromatic C–H), 6.90–6.85 (m, 12 H, aromatic C–H), 6.63–6.60 (m, 9 H, aromatic C–H), 3.63 (s, 12 H, -OCH₃). ³¹P{¹H} NMR (δ, DMSO-d₆): 54.59 (s), 41.03 (s). MS (ESI⁺) calcd for [C₁₄₀H₁₁₂Ni₂O₄P₈PtS₄]²⁺: m/z 1273.1855. Observed: m/z 1273.1641. Error (δ): 16.84 ppm.

[(Ph₂C₂S₂)Ni(μ-tpbz)Pt(μ-tpbz)Ni(S₂C₂Ph₂)][CF₃SO₃]₂, [3][CF₃SO₃]₂

Under an atmosphere of N₂, a 25 mL Schlenk flask was charged with [(Ph₂C₂S₂)Ni(tpbz)] (0.111 g, 0.0995 mmol) in 20 mL of CH₂Cl₂, while a solution of [Pt(NCCH₂CH₃)₄][CF₃SO₃]₂ (0.035 g, 0.050 mmol) in 5 mL of CH₂Cl₂ was composed in a separate 50 mL Schlenk flask. The green solution of [(Ph₂C₂S₂)Ni(tpbz)] was transferred dropwise via cannula to the colorless [Pt(NCCH₂CH₃)₄][CF₃SO₃]₂ solution. The resulting reaction mixture was stirred overnight (14 h) at ambient temperature, during which time a dark brown solution developed. The reaction mixture was thereupon reduced to a volume of ∼3 mL, and hexanes (∼10 mL) were added via syringe to precipitate [3][CF₃SO₃]₂ as a brown powder. The supernatant was removed by cannula filtration, and the solid residue was then washed with Et₂O (2 × 8 mL) and dried under vacuum. Purification of [3][CF₃SO₃]₂ was accomplished by layering a dry 0.040 g sample onto a 0.50 in. thick Celite pad in a filter pipet. Dichloromethane (2 × 3 mL) was passed through the crude complex to remove any unreacted open-ended [(Ph₂C₂S₂)Ni(tpbz)], dimetallic [(Ph₂C₂S₂)Ni(μ-tpbz)Ni(S₂C₂Ph₂)], and other soluble impurities until the filtrate was colorless. The settled thin brown layer of trimetallic [3][CF₃SO₃]₂ on the Celite pad was recovered by extraction into N,N-dimethylformamide. X-ray quality crystals of [3][CF₃SO₃]₂·4DMF·2Et₂O were grown by diffusion of Et₂O vapor into a filtered concentrate in N,N-dimethylformamide. Yield: 0.100 g of brown solid, 79%. ¹H NMR (δ, DMSO-d₆): 7.55–7.38 (m, 52 H, aromatic C–H), 7.27–7.17 (m, 20 H, aromatic C–H), 7.07–7.01 (m, 20 H, aromatic C–H), 6.95–6.90 (m, 12 H, aromatic C–H). ³¹P{¹H} NMR (δ, DMSO-d₆): 55.87 (s), 41.05 (s). MS (ESI⁺) calcd for [C₁₃₆H₁₀₄Ni₂P₈PtS₄]²⁺: m/z 1213.1486. Observed: m/z 1213.1643. Error (δ): 12.95 ppm.

[(Ph₂C₂S₂)Pd(μ-tpbz)Pt(μ-tpbz)Pd(S₂C₂Ph₂)][CF₃SO₃]₂, [4][CF₃SO₃]₂

The same procedure and scale as implemented for the synthesis of [3][CF₃SO₃]₂ were employed but with [(Ph₂C₂S₂)Pd(tpbz)] used in place of [(Ph₂C₂S₂)Ni(tpbz)]. The same purification protocol as given for [3][CF₃SO₃]₂ was also followed. Yield: 0.089 g of purple solid, 70%. ¹H NMR (δ, DMSO-d₆): 7.53–7.44 (m, 52 H, aromatic C–H), 7.24–7.16 (m, 17 H, aromatic C–H), 7.04–6.93 (m, 35 H, aromatic C–H). ³¹P{¹H} NMR (δ, DMSO-d₆): 48.92 (s), 40.75 (s). MS (ESI⁺) calcd for [C₁₃₆H₁₀₄P₈Pd₂PtS₄]²⁺: m/z 1260.6343. Observed: m/z 1260.6213. Error (δ): 10.36 ppm.

[(Ph₂C₂S₂)Pt(μ-tpbz)Pt(μ-tpbz)Pt(S₂C₂Ph₂)][CF₃SO₃]₂, [5][CF₃SO₃]₂

The same procedure and scale as implemented for the synthesis of [3][CF₃SO₃]₂ were employed but with [(Ph₂C₂S₂)Pt(tpbz)] (0.125 g, 0.0998 mmol) used in place of [(Ph₂C₂S₂)Ni(tpbz)]. The same purification protocol as given for [3][CF₃SO₃]₂ was also followed. Yield: 0.081 g of red solid. ¹H NMR (δ, DMSO-d₆): 7.97–7.95 (m, 4 H, aromatic C–H), 7.56–7.44 (m, 51 H, aromatic C–H), 7.25–7.18 (m, 17 H, aromatic C–H), 7.07–7.02 (m, 24 H, aromatic C–H), 6.96–6.92 (m, 8 H, aromatic C–H). ³¹P{¹H} NMR (δ, DMSO-d₆): 43.69 (s, J_Pt–P = 2744 Hz), 40.50 (s, J_Pt–P = 2345 Hz). UV–vis [DMF, λ_max, nm (ε)]: 437 (3150), 510 (2850). MS (ESI⁺) calcd for [C₁₃₆H₁₀₄P₈Pt₃S₄]²⁺: m/z 1349.1956. Observed: m/z 1349.1987. Error (δ): 2.32 ppm.

[((NC)₂C₂S₂)Ni(μ-tpbz)Au(μ-tpbz)Ni(S₂C₂(CN)₂)][CF₃SO₃], [6][CF₃SO₃]

Under an atmosphere of N₂, a red solution of [(mnt)Ni(tpbz)] (0.102 g, 0.101 mmol) in dry THF (5 mL) was transferred dropwise via cannula to a colorless solution of Au(PPh₃)Cl (0.025 g, 0.051 mmol) in THF (5 mL). During the course of this addition, immediate formation of a yellow brown precipitate was observed. The reaction mixture was stirred for 8 h at ambient temperature, after which time the solvent was removed by cannula filtration, and the solid residue was dried under vacuum. To this dried crude solid was added Ag[CF₃SO₃] (0.0128 g, 0.0498 mmol) in a 20:1 CH₂Cl₂/MeOH solvent (21 mL). This mixture was stirred overnight (12 h) at ambient temperature in the dark. The solution was filtered through a 1 in. Celite pad on a glass frit to remove AgCl. The filtrate was reduced to a volume of ∼3 mL, and 10 mL of hexanes were then added via syringe to precipitate [6][CF₃SO₃] as a yellow-brown solid. The supernatant was removed by cannula filtration, and the residual solid was washed with hexanes (8 mL) and then Et₂O (2 × 8 mL) and dried under vacuum. Further purification was achieved on a silica chromatography column eluted with a 5:95 THF/CH₂Cl₂ mixture, which moved [6][CF₃SO₃] as a yellow-brown band. Recrystallization was accomplished by the diffusion of Et₂O into a filtered PhNO₂ or 1,3-dimethyl-2-imidazolidinone solution of the complex. Yield: 0.093 g of yellow-brown solid, 79%. ¹H NMR (δ, CD₂Cl₂): 7.51–7.40 (m, 14 H, aromatic C–H), 7.34–7.29 (m, 10 H, aromatic C–H), 7.35–7.21 (m, 30 H, aromatic C–H), 7.03–6.98 (m, 15 H, aromatic C–H), 6.86–6.83 (m, 15 H, aromatic C–H). ³¹P{¹H} NMR (δ, CD₂Cl₂): 56.30 (s), 21.21 (s). UV–vis [CH₂Cl₂, λ_max, nm (ε)]: 365 (sh, 18600), 405 (sh, 9680), ∼575 (210). IR (KBr, cm^–1): 2127 (asym v_C≡N), 2156 (sym v_C≡N). MS (ESI⁺) calcd for [C₁₁₆H₈₄AuN₄Ni₂P₈S₄]⁺: m/z 2223.1871. Observed: m/z 2223.1795. Error (δ): 3.45 ppm.

[((MeO-p-C₆H₄)₂C₂S₂)Ni(μ-tpbz)Au(μ-tpbz)Ni(S₂C₂(C₆H₄-p-OMe)₂)][Cl], [7][Cl]

The same procedure and scale as described for the synthesis of [10][CF₃SO₃] (vide infra) were employed but with [((MeO-p-C₆H₄)₂C₂S₂)Ni(tpbz)] (0.117 g, 0.0995 mmol) implemented in place of [(Ph₂C₂S₂)Ni(μ-tpbz)]. The purification procedure described for [10][CF₃SO₃] was also applied. Although [CF₃SO₃]⁻ was the intended counteranion, the anion in the isolated product was Cl^–, as determined by X-ray crystallography. Yield: 0.093 g of brown solid, 69%. R_f = 0.19 (9:1 DCM/THF). ¹H NMR (δ, CD₂Cl₂): 7.51–7.37 (m, 42 H, aromatic C–H), 7.35–7.30 (m, 16 H, aromatic C–H), 7.23–7.03 (m, 24 H, aromatic C–H), 6.96–6.83 (m, 12 H, aromatic C–H), 6.57–6.54 (m, 6 H, aromatic C–H), 3.65 (s, 12 H, -OCH₃). ³¹P{¹H} NMR (δ, CD₂Cl₂): 53.76 (s), 20.13 (s). UV–vis [CH₂Cl₂, λ_max, nm (ε)]: 490 (3070), ∼625 (1090). MS (ESI⁺) calcd for [C₁₄₀H₁₁₂AuNi₂O₄P₈S₄]⁺: m/z 2547.3724. Observed: m/z 2547.3735. Error (δ): 0.43 ppm.

[(Ph₂C₂S₂)Ni(μ-tpbz)Cu(μ-tpbz)Ni(S₂C₂Ph₂)][BArF₂₄], [8][BArF₂₄]

In a scintillation vial, [(Ph₂C₂S₂)Ni(tpbz)] (0.100 g, 0.0896 mmol) was combined with [Cu(N≡CCH₃)₄][PF₆] (0.018 g, 0.048 mmol) in CH₂Cl₂ (10 mL) and stirred at ambient temperature for 1 h. The dark green reaction mixture was treated with NaBArF₂₄ (0.043 g, 0.054 mmol) and left to stir overnight. The mixture was then filtered to remove the insoluble residue. The dropwise addition of MeOH induced precipitation of [8][BArF₂₄], which was collected on a frit, washed with MeOH and then Et₂O, and dried under vacuum. Yield: 0.113 g, 80%. ³¹P{¹H} NMR (δ, CD₂Cl₂): 54.46 (s), 7.46 (br, s). UV–vis [CH₂Cl₂, λ_max, nm (ε)]: 464 (3480). 620 (950). MS (ESI⁺) calcd for [C₁₃₆H₁₀₄CuNi₂P₈S₄]⁺: m/z 2294.2937. Observed: m/z 2294.3171. Error (δ): 10.2 ppm. MS (ESI⁺) calcd for [(Ph₂C₂S₂)Ni(μ-tpbz)Cu]⁺, [C₆₈H₅₂CuNiP₄S₂]⁺: m/z 1179.1089. Observed: m/z 1179.1023. Error (δ): 5.6 ppm. MS (ESI⁺) calcd for [(Ph₂C₂S₂)Ni(tpbz)]⁺, [C₆₈H₅₂NiP₄S₂]⁺: m/z 1114.1809. Observed: m/z 1114.1501. Error (δ): 27.6 ppm.

[(Ph₂C₂S₂)Ni(μ-tpbz)Ag(μ-tpbz)Ni(S₂C₂Ph₂)][BF₄], [9][BF₄]

In a 20 mL glass scintillation vial, [(Ph₂C₂S₂)Ni(tpbz)] (0.117 g, 0.105 mmol) was dissolved in CH₂Cl₂ (5 mL). To this solution was added solid [(MeCN)₄Ag][BF₄] (0.0188 g, 0.052 mmol), and the reaction mixture was then diluted with MeCN (5 mL). After the mixture had been stirred for 2 h at ambient temperature, all volatiles were removed under reduced pressure. The residual solid was triturated with MeOH (∼20 mL), collected as an olive-green powder by filtration onto a fine porosity glass frit, and dried in the open air. Yield: 0.0823 g, 33%. ³¹P{¹H} NMR (δ, CD₂Cl₂): 54.74 (s), −0.68 (d, J_{³¹P-¹⁰⁷Ag} = 234 Hz), −0.69 (d, J_{³¹P-¹⁰⁹Ag} = 262 Hz). MS (ESI⁺) calcd for [C₂₇₂H₂₀₈Ag₂Ni₄P₁₆S₈]²⁺: m/z 2339.2686. Observed: m/z 2339.2595. Error (δ): 3.87 ppm. The ESI-MS fit is best described as a mixture of a monocation and a dimeric dication.

[(Ph₂C₂S₂)Ni(μ-tpbz)Au(μ-tpbz)Ni(S₂C₂Ph₂)][CF₃SO₃], [10][CF₃SO₃]

Under N₂, a green solution of [(Ph₂C₂S₂)Ni(μ-tpbz)] (0.111 g, 0.0995 mmol) in dry THF (5 mL) was added dropwise via cannula to a colorless solution of [AuCl(PPh₃)] (0.025 g, 0.051 mmol) in THF (5 mL), which immediately induced formation of a dark brown color. This reaction mixture was stirred for 8 h at 25 °C and then reduced to dryness. To the crude solid residue were added Ag[CF₃SO₃] (0.0128 g, 0.0498 mmol) and a 20:1 CH₂Cl₂/MeOH mixture (21 mL). Stirring was continued for 12 h at 25 °C in the dark. The resulting suspension of AgCl was removed by filtration through a Celite pad, and the filtrate was reduced in vacuo to a volume of 3 mL. Addition of hexanes (10 mL) induced precipitation of [10][CF₃SO₃] as a brown solid, which then was separated by filtration, washed with hexanes (8 mL) followed by Et₂O (2 × 8 mL), and dried in vacuo. Further purification was accomplished on a silica column eluted with (95:5 CH₂Cl₂/THF) followed by crystallization by diffusion of Et₂O vapor into a 1,3-dimethyl-2-imidazolidinone solution. Yield: 0.096 g, 75%. R_f = 0.26 (9:1 CH₂Cl₂/THF). ¹H NMR (δ, CD₂Cl₂): 7.52–7.50 (m, 4 H, aromatic C–H), 7.42–7.31 (m, 33 H, aromatic C–H), 7.25–7.20 (m, 16 H, aromatic C–H), 7.08–6.99 (m, 36 H, aromatic C–H), 6.88–6.82 (m, 15 H, aromatic C–H). ³¹P{¹H} NMR (δ, CD₂Cl₂): 54.15 (s), 20.57 (s). UV–vis [CH₂Cl₂, λ_max, nm (ε)]: 470 (4280), ∼625 (1020). MS (ESI⁺) calcd for [C₁₃₆H₁₀₄AuNi₂P₈S₄]¹⁺: m/z 2427.3323. Observed: m/z 2427.3263. Error (δ): 2.5 ppm.

[(Ph₂C₂S₂)Pd(μ-tpbz)Au(μ-tpbz)Pd(S₂C₂Ph₂)][CF₃SO₃], [11][CF₃SO₃]

The same procedure and scale as described for the synthesis of [10][CF₃SO₃] were employed but with [(Ph₂C₂S₂)Pd(tpbz)] (0.116 g, 0.0997 mmol) used in place of [(Ph₂C₂S₂)Ni(tpbz)]. The purification procedure described for [10][CF₃SO₃] was also applied. Yield: 0.098 g of purple-brown solid, 74%. R_f = 0.32 (9:1 CH₂Cl₂/THF). ¹H NMR (δ, CD₂Cl₂): 7.63–7.58 (m, 4 H, aromatic C–H), 7.42–7.31 (m, 35 H, aromatic C–H), 7.25–7.20 (m, 16 H, aromatic C–H), 7.07–6.99 (m, 35 H, aromatic C–H), 6.87–6.84 (m, 14 H, aromatic C–H). ³¹P{¹H} NMR (δ, CD₂Cl₂): 47.59 (s), 20.48 (s). UV–vis [CH₂Cl₂, λ_max, nm (ε)]: 480 (3950). MS (ESI⁺) calcd for [C₁₃₆H₁₀₄AuP₈Pd₂S₄]⁺: m/z 2523.2453. Observed: m/z 2523.2707. Error (δ): 10.07 ppm.

[(Ph₂C₂S₂)Pt(μ-tpbz)Au(μ-tpbz)Pt(S₂C₂Ph₂)][CF₃SO₃], [12][CF₃SO₃]

The same procedure and scale as described for the synthesis of [10][CF₃SO₃] were employed but with [(Ph₂C₂S₂)Pt(tpbz)] (0.125 g, 0.0998 mmol) used in place of [(Ph₂C₂S₂)Ni(μ-tpbz)]. The purification procedure described for [10][CF₃SO₃] was also applied. Yield: 0.118 g of orange-red solid, 83%. R_f = 0.31 (9:1 CH₂Cl₂/THF), ¹H NMR (δ, CD₂Cl₂): 7.68–7.65 (m, 4 H, aromatic C–H), 7.45–7.32 (m, 45 H, aromatic C–H), 7.25–7.16 (m, 20 H, aromatic C–H), 7.04–6.99 (m, 24 H, aromatic C–H), 6.88–6.86 (m, 11 H, aromatic C–H). ³¹P{¹H} NMR (δ, CD₂Cl₂): 42.61 (s, J_Pt–P = 2746 Hz), 20.48 (s). UV–vis [CH₂Cl₂, λ_max, nm (ε)]: 465 (5290). MS (ESI⁺) calcd for [C₁₃₆H₁₀₄AuP₈Pt₂S₄]⁺: m/z 2701.3687. Observed: m/z 2701.3918. Error (δ): 8.53 ppm.

[((NC)₂C₂S₂)Ni(μ-tpbz)Re(CO)₃Br], 13

Under a N₂ atmosphere, a 50 mL Schlenk flask was charged with solid [(mnt)Ni(tpbz)] (0.111 g, 0.109 mmol) and [Re(CO)₅Br] (0.040 g, 0.098 mmol). Dry mesitylene (10 mL) was transferred to the reaction mixture via syringe. The reaction mixture was heated to reflux for 48 h, during which time it assumed a dark brown-red color. Once the mixture had cooled, a solid residue was deposited. The solvent was removed by cannula filtration, and the solid thus isolated was washed with hexanes (8 mL) followed by Et₂O (2 × 8 mL) and then dried in vacuo. This crude product was further purified on a silica column eluted with a 90:10 CH₂Cl₂/THF mixture, which moved 13 as a brown-orange band. Single crystals of diffraction quality were grown by the diffusion of Et₂O vapor into a solution in CH₂Cl₂. Yield: 0.091 g of yellow-brown solid, 67%. ³¹P{¹H} NMR (δ, CDCl₃): 54.81 (s), 28.91 (s). UV–vis [CHCl₃, λ_max, nm (ε)]: 425 (sh, 3890). IR (KBr, cm^–1): 2038 (asym v_C≡N), 2053 (sym v_C≡N). MS (ESI⁺) calcd for [C₆₁H₄₂BrN₂NiO₃P₄ReS₂ + H⁺]⁺: m/z 1364.9735. Observed: m/z 1364.9630. Error (δ): 7.66 ppm.

[(Ph₂C₂S₂)Pd(μ-tpbz)Re(CO)₃Br], 14

The same procedure and scale as described for the synthesis of 13 were employed but with [(Ph₂C₂S₂)Pd(tpbz)] (0.116 g, 0.0997 mmol) used in place of [(mnt)Ni(μ-tpbz)]. Compound 14 was purified on a silica column eluted with a 90:10 CH₂Cl₂/hexanes mixture and collected as a brown-red band. Crystallization was accomplished by diffusion of Et₂O vapor into a CH₂Cl₂ solution. Yield: 0.080 g of brown solid, 53%. ³¹P{¹H} NMR (δ, CDCl₃): 46.65 (s), 28.51 (s). MS (ESI⁺) calcd for [C₇₁H₅₂BrO₃P₄PdReS₂]⁺: m/z 1514.0144. Observed: m/z 1514.0112. Error (δ): 2.1 ppm.

[(Ph₂C₂S₂)Pt(μ-tpbz)Re(CO)₃Br], 15

The same procedure and scale as described for the synthesis of 13 were employed but with [(Ph₂C₂S₂)Pt(tpbz)] (0.125 g, 0.0998 mmol) in place of [(mnt)Ni(tpbz)]. Compound 15 was purified on a silica column eluted with a 70:10 CH₂Cl₂/hexanes mixture and collected as a dark orange-yellow band. Crystallization was accomplished by the diffusion of Et₂O vapor into a CH₂Cl₂ solution. Yield: 0.098 g of brown solid, 62%. ³¹P{¹H} NMR (δ, CDCl₃): 38.06 (s), 28.65 (s), 28.57 (s). MS (ESI⁺) calcd for [C₇₁H₅₂BrO₃P₄PtReS₂]⁺: m/z 1602.0690. Observed: m/z 1602.0676. Error (δ): 0.82 ppm. Anal. Calcd for C₇₁H₅₂O₃P₄S₂BrRePt: C, 53.22; H, 3.27. Found: C, 53.02; H, 3.29.

[((NC)₂C₂S₂)Ni(μ-tpbz)ReBr(CO)(μ-tpbz)Ni(S₂C₂(CN)₂)], 16, and [((NC)₂C₂S₂)Ni(μ-tpbz)Re(CO)₂(μ-tpbz)Ni(S₂C₂(CN)₂)]Br, [17]Br

The same procedure and scale as described for the synthesis of 18 and [19]Br (vide infra) were employed but with [(mnt)Ni(tpbz)] used in place of [(Ph₂C₂S₂)Pt(μ-tpbz)]. Following extraction of the crude solid with CH₂Cl₂ and filtering of the extract through a Celite pad, the filtrate was reduced to dryness under vacuum to afford a light red-orange residue. Diffusion of Et₂O vapor into a 1,3-dimethyl-2-imidazolidinone solution of this crude solid produced orange plate crystals of 16. After the crystals of 16 had been separated, the remaining crude material was reduced to dryness and further washed with Et₂O. Crystallization by the same method that was used for 16 yielded [17]Br as orange-brown column-shaped crystals. Yield of 16: 0.080 g, 68%. ³¹P{¹H} NMR (δ, CD₂Cl₂): 58.08 (s), 28.71 (s). IR (KBr, cm^–1): 2125 (asym v_C≡N), 2156 (sym v_C≡N). MS (ESI⁺) calcd for [C₁₁₇H₈₄BrN₄Ni₂OP₈ReS₄]⁺: m/z 2320.0861. Observed: m/z 2320.1266. Error (δ): 17.47 ppm. Yield of [17]Br: 0.023 g. ³¹P{¹H} NMR (δ, CD₂Cl₂): 54.98 (s), 29.10 (s). MS (ESI⁺) calcd for [C₁₁₈H₈₄N₄Ni₂O₂P₈ReS₄]⁺: m/z 2269.1639. Observed: m/z 2269.1619. Error (δ): 0.87 ppm.

[(Ph₂C₂S₂)Pt(tpbz)Re(CO)Br(μ-tpbz)Pt(S₂C₂Ph₂)], 18, and [(Ph₂C₂S₂)Pt(μ-tpbz)Re(CO)₂(μ-tpbz)Ph(S₂C₂Ph₂)][Br], [19][Br]

Under an atmosphere of N₂, a 50 mL Schlenk flask was charged with [(Ph₂C₂S₂)Pt(μ-tpbz)] (0.125 g, 0.0998 mmol) and [Re(CO)₅Br] (0.022 g, 0.054 mmol). Dry cis,trans-decalin (10 mL) was transferred to the reaction mixture via syringe. The reaction mixture was heated to reflux for 72 h. At an early stage (∼30 min), the reaction mixture turned a dark brown-red color, and solid material was observed around the wall of the flask. After the reaction mixture had cooled to ambient temperature, the solvent was removed by cannula filtration, and the solid residue was washed with hexanes (5 mL) and Et₂O (2 × 5 mL) and then dried in vacuo. The crude product was partially dissolved in CH₂Cl₂ (10 mL) and filtered through a Celite pad. The filtrate was reduced to a volume of 3 mL whereupon 10 mL of hexanes was added via syringe to precipitate the product as an orange powder. The supernatant was removed by cannula filtration, and the solid residue was washed with Et₂O (2 × 8 mL) and dried under vacuum. Mass spectrometric analysis (ESI) of the crude solid indicated the presence of both 18 and [19][Br]. Diffusion of Et₂O vapor into a solution of this product mixture in nitrobenzene produced an initial crop of 18·4(PhNO₂) as orange plate crystals. Following the collection of 18·4(PhNO₂), the residual solid was purified on a silica column eluted with a 4:1 CH₂Cl₂/hexanes mixture and collected as a yellow-orange band. Orange column crystals of [19][Br] were grown by the same PhNO₂/Et₂O vapor diffusion method. Yield of 18: 0.030 g of yellow brown solid, 22%. ³¹P{¹H} NMR (δ, CDCl₃): 41.92 (s), 28.31 (s). UV–vis [CHCl₃, λ_max, nm (ε)]: 434 (140). MS (ESI⁺) calcd for [C₁₃₇H₁₀₄BrOP₈Pt₂ReS₄]⁺: m/z 2798.2917. Observed: 2798.2405. Error (δ): 18.28 ppm. Yield of [19][Br]: 0.018 g of yellow-brown solid. ³¹P{¹H} NMR (δ, CDCl₃): 42.03 (s, J_Pt–P = 2734 Hz), 28.47 (s). MS (ESI⁺) calcd for [C₁₃₈H₁₀₄O₂P₈Pt₂ReS₄]⁺: m/z 2746.3694. Observed: m/z 2746.3220. Error (δ): 17.28 ppm.

Discussion

Syntheses and Structures

Well-defined coordination compounds featuring two chelating diphosphine ligands have broad precedent among the group VI–XI elements. In general, when implemented using open-ended [(R₂C₂S₂)M(tpbz)] complexes (M = Ni²⁺, Pd²⁺, or Pt²⁺; R = CN, Ph, or p-anisyl) as “ligands,” modifications of these procedures are effective in placing two such metallodithiolene diphosphine complexes around a third, central ion. Hexacationic [(phen)₂Os(tpbz)Ni(tpbz)Pd(dppb)]⁶⁺ [dppb = 1,2-bis(diphenylphosphino)benzene], designed by Fox and Zahavy as a redox-gated, long-range photoelectron transfer device, is the only related trimetallic assembly thus far reported.¹⁷ This cation was not characterized structurally.

As its triflate salt, the tetrakis(propionitrile)platinum(II) cation provides clean substitution reactions in yields of 75–85% with [(R₂C₂S₂)M(tpbz)], where the identity of R and M can be independently varied as CN, Ph, or p-anisyl and Ni²⁺, Pd²⁺, or Pt²⁺, respectively (Scheme 1). Once prepared, these cations are stable to air and moisture. Use of a Ni²⁺ or Pd²⁺ precursor in place of [Pt(NCEt)₄]²⁺ for the central ion was less successful in providing tractable products. Although subject to some degree of static disorder in the crystalline state, the triflate anion (OTf⁻) repeatedly proved to be more amenable than anions such as PF₆^– or [BArF₂₄]⁻ for the formation of diffracting crystals.

The crystalline state structures of these trimetallic compounds composed of divalent group 10 ions are subject to two distinctive distortions that operate independently but with cumulative effects that can produce rather different core topologies. The first of these distortions is minor tetrahedralization at the square planar sites, which is quantified as the angle, θ, between the two planes defined by M with the donor atoms of each ligand chelate (Figure 1 and Table 2). The second deformation is folding of the tpbz ligand along an intrachelate P···P axis such that the MP₂ plane is not coincident with the P₂C₆P₂ mean plane. The folding along the P···P axis on the two different ends of each tpbz ligand, designated as φ and ψ (Figure 1), may bend the appended fragments to the same side, or to opposite sides, of the P₂C₆P₂ mean plane, thereby conferring a “boat-like” or “chair-like” local geometry at the tpbz ligand. Analogous structural features with similar variation are seen in [(tdpttf)₂M]²⁺ [tdpttf = tetrakis(diphenylphosphino)tetrathiafulvalene;¹⁸ M = Pd²⁺ or Pt²⁺], which, although monometallic species, are comparable in size because of the greater length of the tdpttf ligand.¹⁹

Figure 1.

Graphical definitions of the interplanar angles that decide the core topology of the trimetallic, tpbz-linked assemblies.

Table 2. Selected Angles (degrees) and Distances (angstroms) of All Structurally Characterized Compounds.

		θ1a	θ2b	θ3c	ϕ1d	ϕ2e	ψ1f	ψ2g	τ1h	τ2i	tpbz conformation	dj
M–Pt–M compounds	[1]2+	1.1	–k	0.0	33.3	–k	28.3	–k	61.6	–k	boat	22.38
	[3]2+	16.6	–k	0.0	19.4	–k	18.8	–k	7.1	–k	chair	23.58
	[4]2+	13.3	–k	0.0	19.7	–k	17.9	–k	5.9	–k	chair	23.95
	[5]2+	13.3	–k	0.0	19.7	–k	18.2	–k	5.7	–k	chair	23.95
M–Au–M compounds	[6]+l	6.5	–k	83.4	5.5	–k	3.9	–k	3.0	–k	–	23.77
		10.4		87.1	8.8		3.5		6.6			23.66
	[7]+	13.0	9.2	87.5	8.3	13.1	9.2	4.9	5.3	8.2	–	23.72
	[10]+	8.1m	5.0	83.9	16.0m	32.7	13.8m	11.8	4.6	21.5	–	23.25
	[11]+	5.7	1.1	86.3	18.4	21.1	12.2	19.3	8.9	5.5	–	23.86
M–Re compounds	13n	2.8	–	–	18.5	–	50.6	–	32.1	–	chair	–
	13o	1.6p	–	–	19.9p	–	50.8	–	31.4p	–	chair	–
	14	6.1	–	–	12.4	–	25.1	–	37.5	–	boat	–
	15	5.1	–	–	13.1	–	25.3	–	38.4	–	boat	–
M–Re–M compounds	16	3.5	–k	0.0	18.1	–k	21.6	–k	39.6	–k	boat	23.46
	[17]+	8.9	–k	0.0	21.2	–k	34.9	–k	55.6	–k	boat	22.89
	18	4.7	–k	0.0	22.5	–k	25.4	–k	4.2	–k	chair	23.97
	[19]+	2.0	–k	0.0	23.9	–k	22.6	–k	3.3	–k	chair	24.02

^aθ₁ is the angle between the S₂M and P₂M planes, left side.

^bθ₂ is the angle between the S₂M(2) and P₂M(2) planes, right side.

^cθ₃ is the angle between the P₂M planes at the central ion.

^dϕ₁ is the angle between the S₂MP₂ (left side) and P₂C₆P₂ (left side) mean planes.

^eϕ₂ is the angle between the S₂MP₂ (right side) and P₂C₆P₂ (right side) mean planes.

^fψ₁ is the angle between the P₂C₆P₂ mean plane (left side) and P₂M plane (central ion or Re).

^gψ₂ is the angle between the P₂C₆P₂ (right side) and P₂M plane (central ion).

^hτ₁ is the angle between S₂MP₂ (left side) and P₂M (central ion).

ⁱτ₂ is the angle between S₂MP₂ (right side) and P₂M (central ion).

^jd is the distance between the centroids of the dithiolene C=C bonds.

^kThe right half of the compound is symmetry-related to the left half.

^lTwo independent half cations of [6]⁺ occur in the asymmetric unit.

^mOne P atom of the tpbz ligand is disordered over two sites. The position giving the smaller distortion from square planarity is used to evaluate the angle.

ⁿPseudopolymorph 1, data set JPD979.

^oPseudopolymorph 2, data set JPD1046.

^pThese values were determined using the positional variant of the disordered mnt(2−) ligand with a higher site occupancy.

In [1]²⁺, while the local coordination geometry at each group 10 M²⁺ ion is square planar, the tpbz ligand displays a rotation about the intrachelate P(1)···P(2) axis such that the S₂NiP₂ mean plane presents an angle of 33.3° (φ₁) to the P₂C₆P₂ mean plane (Figure 2 and Table 1). A similar fold in the same direction between the P₂C₆P₂ mean plane and the P₂Pt plane disposes them at an angle of 28.3° (ψ₁) such that the S₂NiP₂ and P₂Pt planes meet at 61.6° (τ₁), and an accentuated concave shape results for each half of [1]²⁺. The dication occurs on a crystallographic inversion center that is coincident with Pt(1) and thus causes the cation to undulate with a pronounced S-shaped core topology. The OTf^– counteranions are ensconced within the cavities defined by this particular shape (Figure S7), suggesting that the exaggerated conformation displayed by [1]²⁺ arises from felicitous charge-pairing/ion-packing effects for the crystalline state. The distance between the centroids of the dithiolene C=C bonds in [1]²⁺ is 22.38 Å.

Figure 2.

Thermal ellipsoid plots of [1]²⁺ (50%), [7]⁺ (50%), 13 (40%), and 18 (35%). For the sake of maximum clarity, all H atoms, counteranions, and interstitial solvent molecules have been omitted and disordered phenyl groups have been edited to present only one positional variant.

The series [(Ph₂C₂S₂)M(μ-tpbz)Pt(μ-tpbz)M(S₂C₂Ph₂)]²⁺ (M = Ni²⁺, Pd²⁺, or Pt²⁺) was prepared in a fashion analogous to that of [1]²⁺ and similarly crystallized by diffusion of Et₂O vapor into solutions composed with a strongly polar organic solvent. While similar to [1]²⁺ in crystallizing upon an inversion center, [3]²⁺–[5]²⁺ contrast with [1]²⁺ by assuming the “chair” conformation at the tpbz ligands, rather than the “boat” conformation, such that their geometry has a staircase-like or herringbone-like appearance (Figure 3). Possibly because of the greater steric profile projected by the [Ph₂C₂S₂]^2– ligands, the OTf^– anions are not as closely associated with these dications as in [1]²⁺.

Figure 3.

Core topologies found in [1]²⁺ and [5]²⁺, which are illustrative of the “S-like” and “herringbone-like” conformations, respectively. Ellipsoids are shown at the 50% level.

Introduction of [Cu(MeCN)₄][PF₆] or [Ag(MeCN)₄][BF₄] to [(Ph₂C₂S₂)Ni(tpbz)] in a 1:2 ratio induced an immediate reaction, as marked by a distinctive darkening in color. Formation of the intended trimetallic monocations [8]⁺ and [9]⁺ was readily corroborated by ³¹P NMR spectroscopy and mass spectrometry (vide infra). However, despite considerable effort, these compounds were not amenable to the formation of crystals suited for X-ray diffraction because of an apparent limited solution stability.

Treatment of [AuCl(PPh₃)] with [(R₂C₂S₂)M(tpbz)] in a 1:2 ratio led to the immediate formation of the trimetallic assembly, again as manifested by a change in the solution color. Possibly because of a greater M′–P_phosphine bond dissociation energy for the heavier metal, the trimetallic constructs with Au⁺ at the nexus ([6]⁺, [7]⁺, and [10]⁺–[12]⁺) were decisively more tractable to crystallization. As with [1]²⁺ and [3]²⁺–[5]²⁺, triflate proved to be the anion most conducive to formation of X-ray diffraction quality crystals, although for [7]⁺, chloride that presumably originated from [AuCl(PPh₃)] was the counteranion identified by crystallography. Four of the trimetallic M–Au–M monocations have been structurally characterized (Tables 1 and 2), the principal difference between them and the M–Pt–M set being the orthogonal disposition of the [(R₂C₂S₂)M(tpbz)] end groups that is enforced by the d¹⁰ Au⁺ ion with its preference for the tetrahedral geometry. The incompatibility of this geometry with a crystallographic inversion center brought most of these cations onto a general position, but [6]⁺ notably occurred on a crystallographic C₂ axis in C2 (No. 5). When the conformations of peripheral substituents and the twist (θ) and fold angles (φ and ψ) are neglected, the idealized point group symmetry in these M–Au–M compounds is D_2d.

The structure of [7]⁺, which is representative of the M–Au–M set in general respects, is presented in Figure 2. A modest 8.3° angle defines the juncture between the S₂NiP₂ and P₂C₆P₂ mean planes at its left half (φ₁), but its effect is offset by a 9.2° fold of the P(5)–Au(1)–P(7) plane in the opposite direction (downward) across the P(5)···P(7) axis with the result that a slight chair conformation to the left-side tpbz ligand results. A qualitatively similar description is pertinent to the right half of [7]⁺, as presented in Figure 2. The slight distortion from D_2d local symmetry at Au(1), as expressed by the modest departure from orthogonality between the P(5)–Au(1)–P(7) and P(6)–Au(1)–P(8) planes by 2.5°, is similar in magnitude to distortions from ideal D_2d symmetry observed in a variety of mononuclear [Au(dppb)₂]⁺ complexes.²⁰⁻²² This minor distortion therefore appears to be unrelated to the size of [7]⁺ and any packing forces to which it is subject.

Bis(diphosphine) complexes of Re(I) can be directly accessed from [ReX(CO)₅] (X = Cl or Br) but demand forcing conditions, such as refluxing for 7 days in mesitylene (∼165 °C). With dppb itself, the organic analogue of open-ended [(R₂C₂S₂)M(tpbz)] complexes, these conditions are reported to produce a cis/trans mixture of [Re(CO)₂(dppb)₂]⁺, as deduced from spectroscopic data.²³ However, when used as the medium for reaction between [(R₂C₂S₂)M(tpbz)] and [ReBr(CO)₅], mesitylene enables only the formation of [(R₂C₂S₂)M(μ-tpbz)ReBr(CO)₃], several examples of which (13–15) have been isolated and structurally identified (Figure 2 and Table 2). Compound 13 reveals a fold angle (ψ₁) between the P₂C₆P₂ and P₂Re planes that is twice the magnitude (∼51°) of the corresponding value in 14 and 15 (∼25°). A further difference is that 13 holds a chair conformation about its tpbz bridge such that τ₁ (∼31°) is essentially the difference between ψ₁ and φ₁ (∼20°), while a boat conformation at the tpbz bridge in 14 and 15 causes φ₁ (∼13°) and ψ₁ to be additive in producing τ₁ (∼38°). Similarly sharp fold angles observed between the tpbz bridge and M(CO)₄ (when M = Mo, ψ_ave = 20.3°; when M = W, ψ = 21.3°)²⁴ and ReBr(CO)₃ (ψ_ave = 32.9°)²⁵ end groups in symmetric dimetallic compounds suggest that this structural feature is a lower-energy configuration arising from enhanced M → CO π overlap at the expense of the M–tpbz interaction.

The somewhat more elevated reflux temperature offered by decalin (∼190 °C), when used as the medium for reactions of [(R₂C₂S₂)M(tpbz)] with 0.5 equiv of [ReBr(CO)₅], leads to separable mixtures of [(R₂C₂S₂)M(tpbz)ReBr(CO)(tpbz)M(S₂C₂R₂)] and [(R₂C₂S₂)M(tpbz)Re(CO)₂(tpbz)M(S₂C₂R₂)]Br in a time frame of 72 h (Scheme 1). The thermal stability demonstrated by these compounds under such forcing conditions is striking. Successful preparation and isolation, including identification by X-ray crystallography, of two representative sets (for 16 and [17]Br, R = CN and M = Ni²⁺; for 18 and [19]Br, R = Ph and M = Pt) support the presumption that other permutations of the identity of R and M would be similarly effective. All four compounds crystallize upon inversion centers in monoclinic space group 14 (P2₁/c, P2₁/n), a situation that imposes disorder between the trans-disposed CO and Br^– ligands in 16 and 18. In 18, despite the disorder, the two ligands were clearly distinguishable in the electron density map and amenable to refinement with minimal restraints. The tpbz conformations within both 18 and [19]⁺ are chairlike, with τ₁ essentially being the difference between ψ₁ and φ₁, which produces a core topology similar to that illustrated for [5]²⁺ in Figure 3. The opposite situation pertains to 16 and [17]⁺, which both have the boat conformations at the tpbz bridges and an overall S-like core topology similar to that of [1]²⁺ (Figure 3) but less pronounced.

The range of structural variation among these sets of trimetallic compounds implies conformational complexity with only modest differences in the energies of the various geometries. A casual inspection of chemical drawings of these compounds suggests that complexes [1]²⁺ and [5]²⁺ possess low-energy conformations of D_2h symmetry, but such structures exhibit severe steric conflicts between the many phenyl substituents. A computational exploration of the energy landscape for [1]²⁺ at the B3PW91/LANL2DZ and B3PW91/Def2SVP levels of theory²⁶⁻²⁹ finds that D_2h-[1]²⁺ is a high-order saddle point, more than 50 kcal/mol above any potential minimum (Table 3). Lowering the symmetry to D₂ relieves most of the strain, but even this structure is a transition state 3–7 kcal/mol higher in energy than various lower-symmetry structures. Low-energy potential minima of C_i, C₂, and C₁ symmetry were located (Figure 4), with the C₁ structure being the ground state by a fraction of a kilocalorie per mole. The B3PW91/LANL2DZ-calculated C_i-symmetric conformation closely resembles the experimental molecular structure, but the small range of energy differences (∼2 kcal/mol) between the local minima, and the likely low barriers between these conformations, suggest a dynamic solution behavior in which the trimetallic core of the molecule fluctuates between broad S-shaped and U-shaped conformations.

Table 3. Calculated Energies of the Conformations of Selected Trimetallic Complexes.

compound	symmetry	core shapea	E (au)b	ΔE (kcal/mol)	E+ZPE (au)b	Δ(E+ZPE) (kcal/mol)	nIc	dxz (Å)
At the B3PW91/LANL2DZ Level
X = Z = Ni2+; Y = Pt2+	D2h	F	–5237.507053	+60.0	–5235.828275	+58.2	11	18.28
	D2	F	–5237.595433	+4.5	–5235.913860	+4.5	1	18.24
	Ci (∼C2h)	S	–5237.600267	+1.5	–5235.918662	+1.5	0	17.90
	C1 (∼C2)	S	–5237.602645	0.0	–5235.921097	0.0	0	17.94
	C2	U	–5237.602268	+0.2	–5235.920752	+0.2	0	17.88
X = Z = Ni2+; Y = Au+	D2	T	–5254.128435	+5.1	–5252.450013	+5.2	0	18.44
	C1 (∼C2)	T	–5254.130774	+3.6	–5252.452486	+3.7	0	18.21
	C2	T	–5254.136544	0.0	–5252.458358	0.0	0	17.70
At the B3PW91/def2SVP Level
X = Z = Ni2+; Y = Pt2+	D2h	F	–12142.070835	+65.7	–12140.405196	+64.3	12	17.98
	D2	F	–12142.164330	+7.1	–12140.496690	+6.9	1	17.91
	Ci (∼C2h)	S	–12142.172096	+2.2	–12140.504095	+2.3	0	17.58
	C1	S	–12142.175578	0.0	–12140.507703	0.0	0	17.50
	C2	U	–12142.174724	+0.5	–12140.507090	+0.4	0	17.53

^aLegend: F, flat; S, S-shaped; U, U-shaped; T, tetrahedral center.

^b1 au = 627.503 kcal/mol.

^cnI is the number of imaginary frequencies.

Figure 4.

B3PW91/LANL2DZ-calculated low-energy conformations of compound [1]²⁺. Hydrogen atoms have been omitted for the sake of clarity. C_i-[1]²⁺, C₁-[1]²⁺, and C₂-[1]²⁺ from top to bottom, respectively.

When the central platinum is replaced with tetrahedral gold, the situation is less complex. At the B3PW91/LANL2DZ level, three low-energy conformations were found, with the apparent C₂ ground state resembling the experimental structure of [6]⁺, although the molecule in the crystal lies at a general position and thus does not have the strict C₂ symmetry of the calculated minimum. The calculated D₂ and C₁ minima are significantly higher in energy (5.2 and 3.7 kcal/mol, respectively), and it is likely that fewer than 1 in 100 molecules adopt these conformations in solution.

Spectroscopy

Clean formation of the intended trimetallic arrays was affirmed unequivocally in their ³¹P NMR spectra, which revealed a substantial downfield shift of the signal attributed to the open phosphines in [(R₂C₂S₂)M(η²-tpbz)] to positions that depended upon the identity of the central metal ion: approximately +55 ppm for central Pt²⁺, approximately +35 ppm for Au⁺, and approximately +43 ppm for Re⁺ (Table 4). The Cu⁺- and Ag⁺-linked compounds revealed peak positions and, in the case of Ag⁺, coupling constants very similar to those of the analogous mononuclear [(dppb)₂M]⁺ (M = Cu^+22,30,31 or Ag⁺³²⁻³⁴) cations (cf. Figures S85 and S91). The more downfield-shifted ³¹P signal in [8]⁺, which is due to the phosphines that are chelating Ni²⁺, has the sharp character typical of most of these complexes (Figure S85). The ³¹P NMR signal for the phosphines chelating to Cu⁺ in [8]⁺, however, is broad to a degree that is not observed for [(P-P)₂Cu]⁺ complexes until temperatures of ≤190 K are imposed. This effect may be due to hindered fluxionality about Cu⁺ arising from the appreciably greater size of the [(Ph₂C₂S₂)Ni(tpbz)] “ligands” compared to the smaller, purely organic diphosphines. In all cases, the phosphine signal attributable to tpbz bound to the metallo-dithiolene end groups occurred downfield of the phosphine signal arising from the tpbz end chelating the central ion.

Table 4. Summary of ³¹P NMR Data for tpbz Compounds.

compound	31P NMR signals (solvent)
[(mnt)Ni(tpbz)]	57.2, – 4.5a (CDCl3)
[(adt)Ni(tpbz)]	55.2, −14.2a (CDCl3)
[(pdt)Ni(tpbz)]	55.0, −14.8a (CDCl3)
[(pdt)Pd(tpbz)]	49.4, −14.7a (CDCl3)
[(pdt)Pt(tpbz)]	42.7, −14.7a (CDCl3)
[(mnt)Ni(tpbz)Pt(tpbz)Ni(mnt)]2+, [1]2+	59.14, 42.40b (DMSO-d6)
[(adt)Ni(tpbz)Pt(tpbz)Ni(adt)]2+, [2]2+	54.59, 41.03b (DMSO-d6)
[(pdt)Ni(tpbz)Pt(tpbz)Ni(pdt)]2+, [3]2+	55.87, 41.05b (DMSO-d6)
[(pdt)Pd(tpbz)Pt(tpbz)Pd(pdt)]2+, [4]2+	48.92, 40.75b (DMSO-d6)
[(pdt)Pt(tpbz)Pt(tpbz)Pt(pdt)]2+, [5]2+	43.69, 40.50b (DMSO-d6)
[(dppb)2Cu]+	8.12 (CDCl3)c
[(dppb)2Ag]+	0.28 (CD2Cl2)c
	(d, J31P-107Ag = 230 Hz)
	(d, J31P-107Ag = 265 Hz)
[(dppb)2Au]+	21.43 (CD2Cl2)c
[(mnt)Ni(tpbz)Au(tpbz)Ni(mnt)]+, [6]+	56.30, 21.21b (CD2Cl2)
[(adt)Ni(tpbz)Au(tpbz)Ni(adt)]+, [7]+	53.76, 20.13b (CD2Cl2)
[(pdt)Ni(tpbz)Cu(tpbz)Ni(pdt)]+, [8]+	54.46, 7.46b (br) (CD2Cl2)
[(pdt)Ni(tpbz)Ag(tpbz)Ni(pdt)]+, [9]+	54.74 (CD2Cl2)
	–0.68b (d, J31P-107Ag = 234 Hz)
	–0.69b (d, J31P-109Ag = 262 Hz)
[(pdt)Ni(tpbz)Au(tpbz)Ni(pdt)]+, [10]+	54.15, 20.57b (CD2Cl2)
[(pdt)Pd(tpbz)Au(tpbz)Pd(pdt)]+, [11]+	47.59, 20.48b (CD2Cl2)
[(pdt)Pt(tpbz)Au(tpbz)Pt(pdt)]+, [12]+	42.61, 20.48b (CD2Cl2)
[(mnt)Ni(tpbz)ReBr(CO)3], 13	54.81, 28.91d (CDCl3)
[(pdt)Pd(tpbz)ReBr(CO)3], 14	46.65, 28.51d (CDCl3)
[(pdt)Pt(tpbz)ReBr(CO)3], 15	38.06, 28.65, 28.57d,e (CDCl3)
[(mnt)Ni(tpbz)ReBr(CO)(tpbz)Ni(mnt)], 16	58.08, 28.71b (CD2Cl2)
[(mnt)Ni(tpbz)Re(CO)2(tpbz)Ni(mnt)]+, [17]+	54.08, 29.10b (CD2Cl2)
[(pdt)Pt(tpbz)ReBr(CO)(tpbz)Pt(pdt)], 18	41.92, 28.31b (CDCl3)
[(pdt)Pt(tpbz)Re(CO)2(tpbz)Pt(pdt)]+, [19]+	42.03, 28.47b (CDCl3)

^aSignal due to the open end of tpbz.

^bSignal arising from the end of tpbz coordinated to the central ion.

^cFrom ref (22).

^dSignal assigned to the end of tpbz coordinated to Re(I).

^eThe slight splitting in the signal for the Re-bound end is presumed to arise from the presence of both ¹⁸⁵Re (I = ⁵/₂, 37%) and ¹⁸⁷Re (I = ⁵/₂, 63%).

Mass spectrometry (ESI⁺) unequivocally identifies the trimetallic compounds with their parent masses and their distinctive isotope distribution profiles. Fragment masses arising from the trimetallic assembly {e.g., [(Ph₂C₂S₂)Ni(tpbz)Cu]⁺ from [(Ph₂C₂S₂)Ni(tpbz)Cu(tpbz)Ni(S₂C₂Ph₂)]⁺ (Figure S90)} are, in some instances, observable. Generally darker colors mark the formation of the trimetallic arrays from their monometallic fragments; where absorption maxima are discernible and shoulders estimable, they are indicated (vide supra). The electronic absorption spectra of the trimetallic compounds generally show shifts to higher energy for the assemblies comprised of third-row metals compared to the analogues with lighter metals of the same group. In many instances, multiple broad unresolved bands that absorb across the visible spectrum complicate simple comparisons and interpretation. Rigorous spectral deconvolution and assignment of these transitions with the aid of time-dependent density functional theory calculations have not been attempted here.

When probed with 325–355 nm light, [(dppb)₂Au]⁺ is very brightly luminescent in the solid state with λ_em values that are highly dependent on the presence or absence of an interstitial solvent and the nature and/or identity of the counteranion.^20−22,35 However, its trimetallic homologue, [(pdt)Ni(tpbz)Au(tpbz)Ni(pdt)]⁺, shows no emission under the same conditions. We attribute this observation to intramolecular energy transfer to states localized on the (phosphine)₂Ni(dithiolene) fragments, which are likely lower in energy than the emitting state of the Au center. However, the isostructural [(Ph₂C₂S₂)Pt(tpbz)Au(tpbz)Pt(S₂C₂Ph₂)]⁺ cation displays a relatively long-lived emission of ∼5 μs at ∼610 nm in a frozen 4:1 EtOH/MeOH solution (Figure 5). This emission likely arises from the (phosphine)₂Pt(dithiolene) end groups, as mononuclear complexes of this general type are known to be luminescent.^36,37 The solid state, low-temperature matrix luminescence of these monoplatinum complexes is assigned as arising from a Pt (d π) to dithiolene (π*) transition, with the phosphine ligands influencing the relative energy of the Pt d π levels.³⁷ A similar assignment should apply to [(Ph₂C₂S₂)Pt(tpbz)Au(tpbz)Pt(S₂C₂Ph₂)]⁺ because the dithiolene ligand can serve as an acceptor.

Figure 5.

Emission spectrum of [(Ph₂C₂S₂)Pt(tpbz)Au(tpbz)Pt(S₂C₂Ph₂)]⁺ following 401 nm excitation (77 K, 4:1 EtOH/MeOH).

The application of these multimetal assemblies as prototype two-qubit gates was investigated using EPR spectroscopy. Samples of [8]⁺ and [12]⁺ were oxidized in situ converting each terminal dithiolene ligand into its monoanionic radical form. This action generates tricationic spin-triplet species, whose spectra recorded at ambient temperature provide a measure of the exchange interaction between the ligand radicals. Most notable is the absence of exchange coupling, as the spectra are indicative of uncoupled S = ¹/₂ entities. The three-line spectrum of [8]³⁺ was simulated with g = 2.0131 and A_P = 4.5 × 10^–4 cm^–1 for coupling of the dithiolene radical with the two ³¹P (I = ¹/₂, 100%) nuclei of the μ-tpbz ligand (Figure 6). These parameters are identical to those of [(dppb)Ni(S₂C₂Ph₂)]⁺ [dppb = 1,2-bis(diphenylphosphino)benzene].⁷ The absence of additional hyperfine lines demonstrates J ≈ 0 because of the sizable interspin distance exceeding 20 Å and the orthogonal orientation of each dithiolene spin center caused by the Cu(I) tetrahedral node (vide supra). The spectrum of [12]³⁺ gave a similar result with isolated S = ¹/₂ spin centers localized to the terminal dithiolene ligands with negligible exchange coupling. Simulation gave g = 1.9911 and includes additional ¹⁹⁵Pt (I = ¹/₂, 33.4%) coupling of A_Pt = 12.0 × 10^–4 cm^–1 commensurate with a coordinated dithiolene radical (Figure S109).⁷

Figure 6.

Comparison of the X-band EPR spectra of [8]³⁺ (A) and [12]³⁺ (B) recorded in a CH₂Cl₂ solution at 293 K. Experimental data are shown by the black lines, and simulations are depicted by the dashed red traces. Spin Hamiltonian parameters are given in the text.

Electrochemistry

When R = alkyl or aryl, dimetallic compounds of the type [(R₂C₂S₂)M(tpbz)M(S₂C₂R₂)] support reversible, concurrent oxidations of the terminal dithiolene ligands ([R₂C₂S₂]^2– – e^– → [R₂C₂S^–S^•]⁻) at ∼0 V versus Fc⁺/Fc. In the cathodic direction, at approximately −2.0 V versus Fc⁺/Fc, reversible reduction of the tpbz ligand is found but at half the current amplitude.⁶ The electrochemistry of [5]²⁺ differs from that of its dimetallic analogue [(Ph₂C₂S₂)Pt(tpbz)Pt(S₂C₂Ph₂)] in that it shows an anodically shifted quasireversible oxidation at approximately +0.16 V and two reduction processes, the first of which reveals a distinctive offset between its peak maxima (Figure 7, top, and Table 5). The anodic process arises from dithiolene ligand oxidation, as corroborated by geometry optimization and inspection of its frontier MOs (Figure S49). Its positive shift relative to [(Ph₂C₂S₂)Pt(tpbz)Pt(S₂C₂Ph₂)] arises from its dipositive charge. The LUMO for [5]²⁺ is the σ* combination of the central Pt d_x²–y² orbital with the phosphine lone pairs. Reduction to Pt⁺ induces a change in geometry from square planar to tetrahedral, which is affirmed computationally and accounts for the quasireversible nature of this feature. Subsequent reduction to Pt⁰, a redox level with ample precedent in a tetraphosphine environment,³⁸⁻⁴¹ is reversible because the geometry is maintained at that point.

Figure 7.

Cyclic voltammograms of [5]²⁺ in DMF (top) and [10]⁺ in CH₂Cl₂ (bottom) with [Et₄N][PF₆] as the supporting electrolyte and a Pt disk as the working electrode.

Table 5. Cyclic Voltametric Data (volts vs Fc⁺/Fc) for Selected Compounds.

	solvent	oxidation	reductions
[(mnt)Ni(tpbz)Pt(tpbz)Ni(mnt)]2+	DMF	–	–1.62 (Pt2+ → Pt0)
[(pdt)Pt(tpbz)Pt(tpbz)Pt(pdt)]2+	DMF	+0.16	–1.02a (Pt2+ → Pt+)	–1.34 (Pt+ → Pt0)
[(mnt)Ni(tpbz)Au(tpbz)Ni(mnt)]+	CH2Cl2	–	–1.40
[(pdt)Ni(tpbz)Cu(tpbz)Ni(pdt)]+	CH2Cl2	+0.11	–1.46 (irr)	–1.63
[(pdt)Ni(tpbz)Au(tpbz)Ni(pdt)]+	CH2Cl2	+0.16	–1.92 (tpbz)

^aAttended by a change in geometry from square planar to tetrahedral.

In contrast to [5]²⁺, cyclic voltammetry of [1]²⁺ reveals no reversible oxidation because the highly electron-withdrawing nature of the nitrile groups of the mnt(2−) ligand renders it unable to sustain the [R₂C₂S₂]^2– – e^– → [R₂C₂S^–S^•]⁻ oxidation that is supported by most dithiolene ligands. A surprising point of difference between [5]²⁺ and [1]²⁺ is that the latter shows a reversible reduction at −1.62 V versus Fc⁺/Fc, which is confirmed as a two-electron process by use of 1 equiv of Cp*₂Fe as an internal standard (Figure S55). A related complex with zerovalent Pt, [Pt(dppeb)₂] {dppeb = 1,2-bis[(diphenylphosphino)ethynyl]benzene}, is reported to undergo a reversible, two-electron oxidation to the corresponding dication.⁴² It is unclear why [5]²⁺ and [1]²⁺, which appear to share a similarly composed LUMO and which differ only in the identity of the peripheral metallodithiolene groups, diverge in the nature of the cathodic process that they undergo.

Cation [10]⁺ sustains a quasireversible oxidation at +0.16 V and a reversible reduction at −1.92 V versus Fc⁺/Fc (Figure 7, bottom), again with an anodic potential shift compared to its charge-neutral homodimetallic analogue. The current amplitude is the same for these two processes. Inspection of the frontier MOs for [10]⁺ following a geometry optimization shows both HOMO and LUMO to be essentially degenerate orbital pairs (Figure S50), therefore suggesting that both the oxidation wave and the reduction wave are two simultaneous one-electron processes involving the two distal dithiolene ligands and the two bridging tpbz ligands, respectively. The rather negative −1.92 V potential seen for the reduction in [10]⁺ is consistent with its assignment as a tpbz-based process, as similar potentials attributable to tpbz reduction are seen in the charge-neutral dimetallic compounds (vide supra).⁶ In contrast, cation [6]⁺, which differs from [10]⁺ only in having mnt(2−) as the terminal ligand in place of pdt(2−), shows a single reversible reduction at −1.40 V. The 0.5 V less negative potential for this process compared to that in [10]⁺, and its apparent one-electron nature compared to Cp*Fe as the standard, point toward a locus for reduction other than the tpbz ligands. We tentatively attribute this feature to Au⁺ + e^– → Au⁰ reduction but note that no reversible reduction is reported for the related [Au(dppb)₂]⁺ cation.²²

Conclusions

The principal conclusions that emerge from this work are as follows.

(1) Trimetallic complexes of the general form [(R₂C₂S₂)M(μ-tpbz)M′(μ-tpbz)M(S₂C₂R₂)] are readily prepared in moderate to good yields from open-ended [(R₂C₂S₂)M(μ-tpbz)] and an appropriate precursor for M′ [M′ = Pt²⁺, Cu⁺, Ag⁺, Au⁺, or ReBr(CO)/{Re(CO)₂}⁺] in a 2:1 ratio.

(2) Trimetallic assemblies with third-row transition metals at the nexus reveal a greater stability that is possibly the result of stronger M–L bonds compared to those of analogous first- and second-row complexes.

(3) Structural authentication of numerous compounds of the type reveals centrosymmetric “S” and herringbone core topologies arising from boat and chair conformations of the tpbz connectors about Pt²⁺ or Re⁺. The Au⁺-linked compounds are spiro-like with an orthogonal disposition of the [(R₂C₂S₂)M(μ-tpbz)] end groups.

(4) A computational evaluation of the conformational energetics of [1]²⁺ and [6]⁺ finds that Ph···Ph clashing greatly disfavors idealized point group symmetries and that multiple, near isoenergetic minima of C₁, C₂, and/or C_i symmetries are likely pertinent to the solution phase.

(5) ³¹P NMR identifies the trimetallic assemblies by downfield shifts of ∼35–55.5 ppm of the open-ended phosphine signal upon chelation. These shifts are ∼55 ppm, ∼43 ppm and ∼35 ppm for the compounds with Pt²⁺, Re¹⁺ and Au¹⁺, respectively, at the center.

(6) A limited electrochemistry survey shows that trimetallic [[(R₂C₂S₂)M(μ-tpbz)]₂M′]^2+/+ compounds with R = Ph support dithiolene oxidation, but at anodically shifted potentials owing to their cationic charge. Cathodic scanning reveals processes that appear to be, on the collective bases of potential, current amplitude, and computational assessment, either tpbz-based {e.g., [[(Ph₂C₂S₂)Ni(μ-tpbz)]₂Au]⁺} or metal-based at the central ion {e.g., [[(Ph₂C₂S₂)Pt(μ-tpbz)]₂Pt]²⁺}.

(7) The trimetallic assemblies reported here are electrically activated two-qubit prototypes in which the negligible exchange coupling allows for convenient switching between the singlet and triplet states. The spin coherence lifetimes offered by these organic ligand radicals⁴³ provide a potential platform for optically addressing the central d¹⁰ ion and generating an array of spin coupling options in a well-characterized ensemble. Demonstrating this design concept would represent a step-change in multi-qubit design.

In continuing work, we target the synthesis of related multimetal systems in which either the linking metal ion(s), the organic ligands, or both sustain reversible electrochemistry leading to the creation of isolable multispin states in which spins are weakly coupled and selectively addressable.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c03112.

• Procedures for crystal growth, X-ray diffraction data collection, and structure solution and refinement; description of computational procedures; tables summarizing unit cell and refinement data (Tables S1–S4); thermal ellipsoid plots with complete atom labeling (Figures S1–S48); MO energy level diagrams for [5]²⁺ and [10]⁺ (Figures S49 and S50, respectively); spectroscopic, electrochemical, and analytical data for compounds reported (Figures S51–S126); and coordinates for geometry-optimized [1]²⁺ and [6]⁺ (PDF)

Author Present Address

^§ Xavier University of Louisiana, 1 Drexel Dr., New Orleans, LA 70125

Author Present Address

^∥ University of Jaffna, Vavuniya Campus, Vavuniya, Sri Lanka

Author Present Address

^⊥ Air Force Research Laboratory, Dayton, OH 45433

The authors declare no competing financial interest.