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

Article abstract of DOI:10.1021/om00016a045

A controlled-potential electrolysis was performed on mononuclear platinum(II) complexes containing aromatic isocyanide (RNC) and diphosphine (diphos) ligands, [Pt(diphos)(RNC)₂](PF₆)₂(diphos = cis-1,2-bis(diphenylphosphino)ethene (dppen) (1), 1,2-bis(diphenylphosphino)ethane (dppe) (2), 1,3-bis(diphenylphosphino)propane (dppp) (3), 1,4-bis(diphenylphosphino)butane (dppb) (4), and 1,2-bis(di-tert-butylphosphino)ethane (dtbpe) (5); R = 2,6-dimethylphenyl or 2,4,6-trimethylphenyl), which were derived from the reaction of PtCl₂(COD) with diphos, RNC, and NH₄PF₆. 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 (diphos = dppen (6), dppe (7), dppp (8), and dppb (9)). The compounds were characterized by IR, electronic, and ¹H and ³¹P{¹H} NMR spectroscopy and X-ray crystallographic and EXAFS (extended X-ray absorption fine structure) analyses. Complex 8b (R = 2,4,6-Me₃C₆H₂, diphos = dppp) crystallizes in the monoclinic system, space group P2₁/c, with a = 14.941(4) ?, b = 19.268(6) ?, c = 26.002(9) ?, β = 101.23(2)°, and Z = 4 (R = 0.059 and R_w = 0.051 for 5704 independent reflections with I > 3σ(I)). Complex 8b consists of two platinum atoms, each coordinated by one isocyanide, one diphosphine as a chelating ligand, and the neighboring platinum atom in a square planar array. The length of the Pt-Pt bond is 2.653(1) ?. EXAFS analyses showed that the Pt-Pt bond lengths of 6-9 fall within the narrow range of 2.625-2.653 ?. 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 dtbpe (15)). Complex 13a (R = 2,6-Me₂C₆H₃) crystallizes in the monoclinic system, space group P2/n, with a = 14.956(4) ?, b = 12.395(2) ?, c = 22.923(7) ?, β = 104.35(2)°, and Z = 2 (R = 0.048 and R_w = 0.046 for 2872 independent reflections with I > 3σ(I)); complex 14a (R = 2,6-Me₂C₆H₃) crystallizes in the monoclinic system, space group P2₁, with a = 15.491(3) ?, b = 22.575(5) ?, c = 10.883(3) ?, β = 107.48(2)°, and Z = 2 (R = 0.056 and R_w = 0.038 for 3156 independent reflections with I > 3σ(I)); and complex 15a (R = 2,6-Me₂C₆H₃) crystallizes in the monoclinic system, space group C2/c, with a = 19.489(6) ?, b = 15.284(5) ?, c = 25.804(6) ?, β = 116.34(2)°, and Z = 4 (R = 0.045 and R_w = 0.033 for 2904 independent reflections with I > 3σ(II). The three platinum atoms were arranged in a linear array with Pt-Pt bond distances of 2.615(1) ? (13a), average 2.640 ? (14a), and 2.6409(8) ? (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, resulting 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 platinum, [PtH(dtbpe)(2,6-Me₂C₆H₃NC)]PF₆ (16a). The potentiostatic electrolysis of 2 consumed 1.5 F mol^-1 to afford the coordinatively saturated triplatinum complex, [{Pt(dppe)(RNC)}₂Pt(RNC)₂](PF₆)₂ (10), instead of the unsaturated trimer. The analogous compounds [{Pt(diphos)(RNC)}₂Pt(RNC)₂](PF₆)₂ (diphos = dppen (11) and dtbpe (12)) were also obtained from the reaction of [Pt₃-(RNC)₈]²⁺ with diphosphines. Complex 11a (R = 2,6-Me₂C₆H₃) crystallizes in the monoclinic system, space group P2₁/c, with a = 13.403(3) ?, b = 16.126(4) ?, c = 21.944(8) ?, β = 98.57(2)°, and Z = 2 (R = 0.054 and R_w = 0.040 for 3064 independent reflections with I > 3σ(I)). The Pt-Pt bond length of 2.655(1) ? is longer by 0.040 ? than that of 13a. A two-electron reduction of 4 at ca. -1.8 V yielded a Hg-Pt mixed-metal cluster, [HgPt₆(dppb)₂(RNC)₈] (17). The length of methylene chain in the diphosphine ligands dramatically influences the structure of the metal-metal bonded platinum clusters. Extended Hückel MO calculations 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.