ELECTROCHEMICAL PREPARATION OF PLATINUM ISOCYANIDE CLUSTERS CONTAINING CHELATING DIPHOSPHINES - AN UNPRECEDENTED TRINUCLEAR PLATINUM COMPLEX INVOLVING A COORDINATIVELY UNSATURATED METAL CENTER, [(PT(DIPHOSPHINE)(ISOCYANIDE))2PT](PF6)2

A controlled-potential electrolysis was performed on mononuclear plati num(II) complexes containing aromatic isocyanide (RNC) and diphosphine (diphos) ligands, [Pt(diphos)((RNC)2](PF6)2 diphos = cis-1,2-bis(diph enylphosphino)ethene (dppen) (1), 1,2-bis(diphenylphosphino)ethane (dp pe) (2),1,3-bis(diphenylphosphino)propane (dppp) (3),1,4-bis(diphenylp hosphino)butane (dppb) (4), and 1,2-bis(di-tert-butylphosphino)ethane (dtbpe) (5); R = 2,6-dimethylphenyl or 2,4,6-trimethylphenyl), which w ere derived from the reaction of PtCl2(COD) with diphos, RNC, and NH4P F6. Electrolyses of complexes 1-4 at a mercury-pool electrode consumed 1 F mol-1 in acetonitrile at -1.45 to -1.5 V (vs Cp2Fe/CP2Fe+), which gave dinuclear platinum(I) complexes, [Pt4(diphos)2(RNC)2] (PF6)2 (di phos = dppen (6), dppe (7), dppp (8), and dppb (9)). The compounds wer e characterized by IR, electronic, and H-1 and P-31{H-1} NMR spectrosc opy and X-ray crystallographic and EXAFS (extended X-ray absorption fi ne structure) analyses. Complex 8b (R = 2,4,6-Me3C6H2, diphos = dppp) crystallizes in the monoclinic system, space group P2(1)/c, with a = 1 4.941(4) angstrom, b = 19.268(6) angstrom, c = 26.002(9) angstrom, bet a = 101.23(2)degrees, and Z = 4 (R = 0.059 and R(w) = 0.051 for 5704 i ndependent reflections with I > 3sigma(I)). Complex 8b consists of two platinum atoms, each coordinated by one isocyanide, one diphosphine a s a chelating ligand, and the neighboring platinum atom in a square pl anar array. The length of the Pt-Pt bond is 2.653(1) angstrom. EXAFS a nalyses showed that the Pt-Pt bond lengths of 6-9 fall within the narr ow range of 2.625-2.653 angstrom. Electrolyses of 1, 3, and 5 consumed 1.5 F mol-1 at ca. -1.8 V, which gave trinuclear platinum complexes, [{Pt(diphos)(RNC)}2Pt](PF6)2 (diphos = dppen (13), dppp (14), and dtbp e (15)). Complex 13a (R = 2,6-Me2C6H3) crystallizes in the monoclinic system, space group P2/n, with a = 14.956(4) angstrom, b = 12.395(2) a ngstrom, c = 22.923(7) angstrom, beta = 104.35(2)degrees, and Z = 2 (R = 0.048 and R(w) = 0.046 for 2872 independent reflections with I > 3s igma(I)); complex 14a (R = 2,6-Me2C6H3) crystallizes in the monoclinic system, space group P2(1), with a = 15.491(3) angstrom, b = 22.575(5) angstrom, c = 10.883(3) angstrom, beta = 107.48(2)degrees, and Z = 2 (R = 0.056 and R(w) = 0.038 for 3156 independent reflections with I > 3sigma(I)); and complex 15a (R = 2,6-Me2C6H3) crystallizes in the mono clinic system, space group C2/c, with a = 19.489(6) angstrom, b = 15.2 84(5) angstrom, c = 25.804(6) angstrom, beta = 116.34(2)degrees, and Z = 4 (R = 0.045 and R(w) = 0.033 for 2904 independent reflections with I > 3sigma(II). The three platinum atoms were arranged in a linear ar ray with Pt-Pt bond distances of 2.615(1) angstrom (13a), average 2.64 0 angstrom (14a), and 2.6409(8) angstrom (15a). The outer Pt atoms are coordinated by one isocyanide and one chelating diphosphine, and the central Pt atom is bound only to the two neighboring Pt atoms, resulti ng in a coordinatively unsaturated platinum center. The coordinatively unsaturated triplatinum complexes 13-15 have 40 valence electrons and are extremely electron-deficient. In the electrolysis of 5a, further reduction led to the formation of a mononuclear hydride complex of pla tinum, [PtH(dtbpe)-(2,6-Me2C6H3NC)]PF6 (16a). The potentiostatic elect rolysis of 2 consumed 1. 5 F mol-1 to afford the coordinatively satura ted triplatinum complex, [{Pt(dppe)(RNC)}2Pt(RNC)2](PF6)2 (10), instea d of the unsaturated trimer. The analogous compounds [{Pt(diphos)(RNC) }2Pt(RNC)2](PF6)2 (diphos = dppen (11) and dtbpe (12)) were also obtai ned from the reaction of [Pt3-(RNC)8]2+ with diphosphines. Complex 11a (R = 2,6-Me2C6H3) crystallizes in the monoclinic system, space group P2(1)/c, with a = 13.403(3) angstrom, b = 16.126(4) angstrom, c = 21.9 44(8) angstrom, beta = 98.57(2)degrees, and Z = 2 (R = 0.054 and R(w) = 0.040 for 3064 independent reflections with I > 3 sigma(I)). The Pt- Pt bond length of 2.655(1) angstrom is longer by 0.040 angstrom than t hat of 13a. A two-electron reduction of 4 at ca. -1.8 V yielded a Hg-P t mixed-metal cluster, [HgPt6(dppb)2(RNC)8] (17). The length of methyl ene chain in the diphosphine ligands dramatically influences the struc ture of the metal-metal bonded platinum clusters. Extended Huckel MO c alculations were carried out on the models, [{Pt(PH3)2(HNC)}2Pt]2+ and [{Pt(PH3)2(HNC)}2Pt(HNC)2]2+, for the coordinatively unsaturated and saturated trimers.