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

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 D2h/D2d/C2v 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.


■ 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 1,2,4,5tetrakis(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.
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 1 H and 31 P 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 1 H and 31 P nuclei, respectively, or a Bruker Ascend 400 spectrometer operating at 162.11 MHz for 31 P. The 1 H NMR spectra were referenced to the solvent signal, while an external aqueous H 3 PO 4 solution was employed as a reference for all 31 P 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-C 6 H 4 ) 3 6 ]. 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 [ n Bu 4 N][PF 6 ] as the supporting electrolyte. Under these conditions, the Cp 2 Fe + / Cp 2 Fe (Fc + /Fc) couple consistently occurred at 0.436 V in CH 2 Cl 2 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.

((NC) 2 C 2 S 2 )Ni(μ-tpbz)Au(μ-tpbz)Ni(S 2 C 2 (CN) 2 )]-[CF 3 SO 3 ], [6][CF 3 SO 3 ].
Under an atmosphere of N 2 , 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(PPh 3 )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[CF 3 SO 3 ] (0.0128 g, 0.0498 mmol) in a 20:1 CH 2 Cl 2 / 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][CF 3 SO 3 ] as a yellow-brown solid. The supernatant was removed by cannula filtration, and the residual solid was washed with hexanes (8 mL) and then Et 2 O (2 × 8 mL) and dried under vacuum. Further purification was achieved on a silica chromatography column eluted with a 5:95 THF/CH 2 Cl 2 mixture, which moved [((MeO-p-C 6 H 4 ) 2 C 2 S 2 )Ni(μ-tpbz)Au(μ-tpbz)Ni-(S 2 C 2 (C 6 H 4 -p-OMe) 2  [

(Ph 2 C 2 S 2 )Pt(μ-tpbz)Au(μ-tpbz)Pt(S 2 C 2 Ph 2 )][CF 3 SO 3 ], [12][CF 3 SO 3 ].
The same procedure and scale as described for the synthesis of [10][CF 3 SO 3 ] were employed but with [(Ph 2 C 2 S 2 )Pt(tpbz)] (0.125 g, 0.0998 mmol) used in place of [(Ph 2 C 2 S 2 )Ni(μ-tpbz)]. The purification procedure described for [10][CF 3  [((NC) 2 C 2 S 2 )Ni(μ-tpbz)Re(CO) 3 Br], 13. Under a N 2 atmosphere, a 50 mL Schlenk flask was charged with solid [(mnt)Ni(tpbz)] (0.111 g, 0.109 mmol) and [Re(CO) 5 Br] (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 Et 2 O (2 × 8 mL) and then dried in vacuo. This crude product was further purified on a silica column eluted with a 90:10 CH 2 Cl 2 /THF mixture, which moved 13 as a brownorange band. Single crystals of diffraction quality were grown by the diffusion of Et 2 O vapor into a solution in CH 2 Cl 2 . Yield: 0.091 g of yellow-brown solid, 67%. 31  [(Ph 2 C 2 S 2 )Pt(μ-tpbz)Re(CO) 3 Br], 15. The same procedure and scale as described for the synthesis of 13 were employed but with [(Ph 2 C 2 S 2 )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 CH 2 Cl 2 /hexanes mixture and collected as a dark orange-yellow band. Crystallization was accomplished by the diffusion of Et 2 O vapor into a CH 2 Cl 2 solution. Yield: 0.098 g of brown solid, 62%. 31 5 Br] (0.022 g, 0.054 mmol). Dry cis,transdecalin (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 Et 2 O (2 × 5 mL) and then dried in vacuo. The crude product was partially dissolved in CH 2 Cl 2 (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 Et 2 O (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 Et 2 O vapor into a solution of this product mixture in nitrobenzene produced an initial crop of 18·4(PhNO 2 ) as orange plate crystals. Following the collection of 18·4(PhNO 2 ), the residual solid was purified on a silica column eluted with a 4:1 CH 2 Cl 2 /hexanes mixture and collected as a yellow-orange band. Orange column crystals of [19][Br] were grown by the same PhNO 2 /Et 2 O vapor diffusion method. Yield of 18: 0.030 g of yellow brown solid, 22%. 31 [19][Br]: 0.018 g of yellow-brown solid. 31  As its triflate salt, the tetrakis(propionitrile)platinum(II) cation provides clean substitution reactions in yields of 75−85% with [(R 2 C 2 S 2 )M(tpbz)], where the identity of R and M can be independently varied as CN, Ph, or p-anisyl and Ni 2+ , Pd 2+ , or Pt 2+ , respectively (Scheme 1). Once prepared, these cations are stable to air and moisture. Use of a Ni 2+ or Pd 2+ 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 PF 6 − or [BArF 24 ] − 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 Figure 1. Graphical definitions of the interplanar angles that decide the core topology of the trimetallic, tpbz-linked assemblies.

Inorganic Chemistry pubs.acs.org/IC
Article P···P axis such that the MP 2 plane is not coincident with the P 2 C 6 P 2 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 P 2 C 6 P 2 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) 2 M] 2+ [tdpttf = tetrakis(diphenylphosphino)tetrathiafulvalene; 18 M = Pd 2+ or Pt 2+ ], which, although monometallic species, are comparable in size because of the greater length of the tdpttf ligand. 19 In [1] 2+ , while the local coordination geometry at each group 10 M 2+ ion is square planar, the tpbz ligand displays a rotation about the intrachelate P(1)···P(2) axis such that the S 2 NiP 2 mean plane presents an angle of 33.3°(φ 1 ) to the P 2 C 6 P 2 mean plane ( Figure 2 and Table 1). A similar fold in the same direction between the P 2 C 6 P 2 mean plane and the P 2 Pt plane disposes them at an angle of 28.3°(ψ 1 ) such that the S 2 NiP 2 and P 2 Pt planes meet at 61.6°(τ 1 ), 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 Sshaped core topology. The OTf − counteranions are ensconced within the cavities defined by this particular shape ( Figure S7) 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 [Ph 2 C 2 S 2 ] 2− ligands, the OTf − anions are not as closely associated with these dications as in  (Tables 1 and 2), the principal difference between them and the M−Pt−M set being the orthogonal disposition of the [(R 2 C 2 S 2 )M(tpbz)] end groups that is enforced by the d 10 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 C 2 axis a θ 1 is the angle between the S 2 M and P 2 M planes, left side. b θ 2 is the angle between the S 2 M(2) and P 2 M(2) planes, right side. c θ 3 is the angle between the P 2 M planes at the central ion. d ϕ 1 is the angle between the S 2 MP 2 (left side) and P 2 C 6 P 2 (left side) mean planes. e ϕ 2 is the angle between the S 2 MP 2 (right side) and P 2 C 6 P 2 (right side) mean planes. f ψ 1 is the angle between the P 2 C 6 P 2 mean plane (left side) and P 2 M plane (central ion or Re). g ψ 2 is the angle between the P 2 C 6 P 2 (right side) and P 2 M plane (central ion). h τ 1 is the angle between S 2 MP 2 (left side) and P 2 M (central ion). i τ 2 is the angle between S 2 MP 2 (right side) and P 2 M (central ion). j d is the distance between the centroids of the dithiolene C� C bonds. k The right half of the compound is symmetry-related to the left half. l Two independent half cations of [6] + occur in the asymmetric unit. 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°a ngle defines the juncture between the S 2 NiP 2 and P 2 C 6 P 2 mean planes at its left half (φ 1 ), but its effect is offset by a 9.2°fold of  Table 2). Compound 13 reveals a fold angle (ψ 1 ) between the P 2 C 6 P 2 and P 2 Re 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 τ 1 (∼31°) is essentially the difference between ψ 1 and φ 1 (∼20°), while a boat conformation at the tpbz bridge in 14 and 15 causes φ 1 (∼13°) and ψ 1 to be additive in producing τ 1 (∼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 [(R 2 C 2 S 2 )M(tpbz)] with 0.5 equiv of [ReBr(CO) 5 ], leads to separable mixtures of [(R 2 C 2 S 2 )M(tpbz)ReBr(CO)(tpbz)M-(S 2 C 2 R 2 )] and [(R 2 C 2 S 2 )M(tpbz)Re(CO) 2 (tpbz)M(S 2 C 2 R 2 )] 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, 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. 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 (P2 1 /c, P2 1 /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 τ 1 essentially being the difference between ψ 1 and φ 1 , 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 theory 26−29 finds that D 2h -[1] 2+ is a highorder saddle point, more than 50 kcal/mol above any potential minimum (Table 3). Lowering the symmetry to D 2 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 , C 2 , and C 1 symmetry were located (Figure 4), with the C 1 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 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 C 2 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 C 2 symmetry of the calculated minimum. The calculated D 2 and C 1 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 31 P NMR spectra, which revealed a substantial downfield shift of the signal attributed to the open phosphines in [(R 2 C 2 S 2 )M(η 2 -tpbz)] to positions that depended upon the identity of the central metal ion: approximately +55 ppm for central Pt 2+ , 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) 2 M] + (M = Cu +22, 30,31 or Ag +32−34 ) cations (cf. Figures S85 and S91). The more downfield-shifted 31 P signal in [8] + , which is due to the phosphines that are chelating Ni 2+ , has the sharp character typical of most of these complexes ( Figure S85). The 31 P NMR signal for the phosphines chelating to Cu + in [8] + , however, is broad to a degree that is not observed for [(P-P) 2 Cu] + 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 [(Ph 2 C 2 S 2 )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.
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., [(Ph 2 C 2 S 2 )Ni(tpbz)Cu] + from [(Ph 2 C 2 S 2 )Ni(tpbz)Cu(tpbz)Ni(S 2 C 2 Ph 2 )] + ( 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) 2 Au] + 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][21][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) 2 Ni-(dithiolene) fragments, which are likely lower in energy than the emitting state of the Au center. However, the isostructural [(Ph 2 C 2 S 2 )Pt(tpbz)Au(tpbz)Pt(S 2 C 2 Ph 2 )] + 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) 2 Pt(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 [(Ph 2 C 2 S 2 )Pt-(tpbz)Au(tpbz)Pt(S 2 C 2 Ph 2 )] + because the dithiolene ligand can serve as an acceptor.
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.  Figure S109). 7 Electrochemistry. When R = alkyl or aryl, dimetallic compounds of the type [(R 2 C 2 S 2 )M(tpbz)M(S 2 C 2 R 2 )] support reversible, concurrent oxidations of the terminal dithiolene ligands ([R 2 C 2 S 2 ] 2− − e − → [R 2 C 2 S − 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 [(Ph 2 C 2 S 2 )Pt(tpbz)Pt-(S 2 C 2 Ph 2 )] 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 [(Ph 2 C 2 S 2 )Pt(tpbz)-Pt(S 2 C 2 Ph 2 )] arises from its dipositive charge. The LUMO for [5] 2+ is the σ* combination of the central Pt d x 2 −y 2 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 Pt 0 , a redox level with ample precedent in a tetraphosphine environment, 38−41 is reversible because the geometry is maintained at that point.
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 [R 2 C 2 S 2 ] 2− − e − → [R 2 C 2 S − 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* 2 Fe 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     (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 − → Au 0 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.
(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 Pt 2+ or Re + . The Au + -linked compounds are spiro-like with an orthogonal disposition of the [(R 2 C 2 S 2 )-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 C 1 , C 2 , and/or C i symmetries are likely pertinent to the solution phase.
(5) 31 P 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 Pt 2+ , Re 1+ and Au 1+ , respectively, at the center.
(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 radicals 43 provide a potential platform for optically addressing the central d 10 ion and generating an array of spin coupling options in a well-characterized ensemble. Demonstrating this design concept would represent a stepchange 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. ■ ASSOCIATED CONTENT
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]