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The research supported by this PRF grant broadened from its initial focus of reversible binding of ligands to iron and cobalt dithiolene complexes to include a systematic exploratory synthesis of multimetallic dithiolene complexes and their properties. This synthetic work focused on the group 10 metals nickel, palladium and platinum because of the nearly ideal square planar corrdination geometries these metals adopt with dithiolene ligands. The fundamental motivation for this work was an interest in further developing access to and understanding of systems capable of supporting multiple electron transfer processes.

Dimetallic compounds [(P-P)M(S₂C₆H₂S₂)M(P-P)] (M = Ni, Pd; P-P = chelating bis(phosphine), 3a-3f) were prepared from O=CS₂C₆H₂S₂C=O or ⁿBu₂SnS₂C₆H₂S₂SnⁿBu₂, which are protected forms of 1,2,4,5-benzenetetrathiolate, a type of bis(dithiolene) ligand. Selective monodeprotections of O=CS₂C₆H₂S₂C=O or ⁿBu₂SnS₂C₆H₂S₂SnⁿBu₂ led to [(P-P)Ni(S₂C₆H₂S₂C=O)] or [(P-P)Ni(S₂C₆H₂S₂SnⁿBu₂)]; the former was used to prepare trimetallic compounds [(dcpe)Ni(S₂C₆H₂S₂)M(S₂C₆H₂S₂)Ni(dcpe)] (M = Ni (6a) or Pt (6b) (Figure 1); dcpe = 1,2-bis(dicyclohexylphosphino)ethane). Compounds 3a-3f are redox active and display two oxidation processes, of which the first is generally reversible (Figure 2). Dinickel compound [(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)] (3d) reveals two reversible oxidation waves with ΔE_½ = 0.66 V, corresponding to K_c of 1.6 x 10¹¹ for the mixed valence species. Electrochemical behavior is unstable to repeated scanning in the presence of [Bu₄N][PF₆] electrolyte but indefinitely stable with Na[BArF₂₄] (BArF₂₄ = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), suggesting that the radical cation generated by oxidation is vulnerable to reaction with PF₆^1-. Chemical oxidation of 3d with [Cp₂Fe][BArF₂₄] led to formation of [3d][BArF₂₄]. Structural identification of [3d][BArF₂₄] revealed appreciable shortening and lengthening of C–S and C–C bond distances, respectively, within the tetrathioarene fragment compared to charge-neutral 3d, indicating this to be the redox active moiety. Attempted oxidation of [(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)] (3c) (dppb = 1,2-bis(diphenylphosphino)benzene) with AgBArF₂₄ produced [[(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)]₂(µ-Ag)₂][BArF₂₄]₂, [4c][BArF₂₄]₂, in which no redox chemistry occurred. Near IR spectroscopy upon cationic [3d]⁺ (Figure 2) and neutral 6a revealed multiple intense absorptions in the 950-1400 nm region. Time-dependent DFT calculations on a 6a model compound indicated that these absorptions are transitions between ligand-based π-type orbitals that have significant contributions from the sulfur p orbitals.

Tetraphosphinobenzene-linked dimetallic compounds with dithiolene capping ligands were prepared in yields of 60-70% from the corresponding dimetal tetrachloro compounds by transmetalation reactions with dialkytin protected forms of the dithiolene ligands (Method 1, Scheme 1). We observed this approach to be generally cleaner and higher yielding than an alternative route proceeding by halide displacement from X₂M(tpbz)MX₂ (X = halide; tpbz = 1,2,4,5-tetrakis(diphenylphos-phino)benzene) with fully reduced ene-1,2-dithiolate salts (Method 2, Scheme 1). The resulting dimetal compounds are air-stable and soluble enough to be readily crystallized and interrogated by a variety of physical methods. Related monometallic dithiolene bis(phosphine) compounds with the 1,2-bis(diphenylphosphino)benzene ligand (dppb) were similarly synthesized.

Diffraction quality crystals of the dipalladium compound bearing the p-anisyl substituted dithiolene ligand (adt) (7), were grown by vapor diffusion techniques. Compound 7 crystallized with an unusual asymmetric unit in  consisting of three independent molecules on general positions and two half-molecules resting on inversion centers such that Z = 8. The differences between these several molecules, which presumably obviate a smaller asymmetric unit and higher symmetry crystal system, are variations in the degree of tetrahedralizing distortions at the palladium centers and differences in the magnitude of the folding angle between the mean plane of the central tetraphosphinobenzene ligand and the pseudo square planar enviroment of each metal atom. One of these molecules is illustrated in Figure 3 (a). The tetrahedralization at each metal atom may be quantified by the angle between the S₂Pd and P₂Pd planes in the immediate S₂PdP₂ coordination environment of Pd. These values range from 3.2°-15.7°, the average being 9.0°. Angles between the S₂PdP₂ and P₂C₆P₂ mean planes, which express the magnitude of the chair-type conformation visible to the molecule in Figure 3, range from 13.4°-31.0°. These deviations from idealized D_2h symmetry contrast with the structure observed for [(dcpe)Pd(S₂C₆H₂S₂)Pd(dcpe)] (dcpe = 1,2-bis(dicyclohexylphosphino)ethane), which crystallized with near perfectly planar P₂Pd(S₂C₆S₂)PdP₂ core. The monopalladium compound [(adt)Pd(dppb)] (8) was also characterized structurally (Figure 3 (b)) but was observed to have the square planar coordination geometry that is more typical for Pd(II) in this strong ligand field environment. Average C–S and C–C_chelate bond lengths for 7 are 1.771[1] and 1.348[3] Å, respectively, values consistent with fully reduced ligand and a Pd(II) oxidation state.

The tpbz ligand has been observed to be an effective insulator between redox active metal centers. Thus, the cyclic voltammetry of 7 was anticipated to reveal a single reversible oxidation wave corresponding to the simultaneous one-electron oxidation of both metallodithiolene end groups. By analogy to the related compound [(Ph₂C₂S₂)Ni(dppe)] (dppe = 1,2-bis(diphenylphosphino)ethane), this oxidation was expected to be an ene-1,2-dithiolate to thienyl radical monoanion oxidation (Scheme 2, (a) → (b)). Surprisingly, the electrochemistry for 7 revealed two well-separated and reversible oxidation waves at 0.00 V and 0.50 V vs Fc⁺/Fc (Figure 4 (a)). This observation implied either a non-insulating quality to the tpbz ligand or a capacity of the Pd(adt) fragment to sustain a second oxidation such that each end of 7 is doubly oxidized and produces overall a tetracation.

Recourse to compound 8 was indispensible in interpreting the electrochemical data for 7. Cyclic voltammetry upon an equimolar solution of 8 under the same conditions as employed for 7 revealed two reversible oxidations that were nearly superimposable upon those of 7 but at half the current amplitude (Figure 4, (b)). This observation proved that 7 is essentially a mere sum of parts (twice 8) While the first of these oxidations may be plausibly assumed to be dithiolene ligand oxidation ((a) → (b) in Scheme 2), the nature of the second oxidation was less immediately clear. Both Pd(II) to Pd(III) oxidation and thienyl radical monoanion to dithioketone oxidation ((b) → (c), Scheme 2), which would mean that the three-member, ligand-based electron transfer series is fully traversible in this compound, are possibilities. The latter possibility of completely ligand-based redox chemistry was strongly indicated by a complete invariance of the voltammetry when the Ni and Pt analogues of 7 were measured (Figure 4 (c)).