CLASS-II MIXED-VALENT COMPLEXES FROM OXIDATION OF DOUBLY LINKED (ARENE)CHROMIUM COMPOUNDS

Oxidation of bridged d6-d6 dinuclear (arene)chromium complexes having a second linkage of P2Me4 has been studied by electrochemistry and by IR and ESR spectroscopies. Cyclic voltammetry shows that both (biphenyl)[Cr(CO)2]2(mu-P2Me4) (VII) and (diphenylmethane)[Cr(CO)2]2(mu-P2Me4) (VIII) undergo two separate reversible one-electron oxidations in CH2Cl2, with E-degree's (vs SCE) of +0.32 and +0.69 V (for VII) and +0.23 and +0.52 V (for VIII). A third complex, (diphenylmethane)[Cr(CO)3]2, has a two-electron oxidation wave for which wave-shape analysis yields E-degrees values of +0.845 and +0.890 V. The monocations of VII and VIII were studied at 263 K by IR spectroscopy and shown to have separate Cr(I) and Cr(0) sites. A similar conclusion was drawn from the frozen-solution ESR spectrum of VIII+. These ion are formulated as class II mixed-valent species in the Robin-Day classification scheme. The IR spectrum of {(biphenyl)[Cr(CO)2PPh3]2}+ was reinvestigated (see ref 16) at 203 K, and a spectrum similar to those of VII+ and VIII+ was obtained. All three class II mixed-valent cations showed carbonyl IR shifts for the Cr(0) site in the range 11-20 cm-1 as compared with the neutral starting materials. The structures of VII and VIII were crystallographically determined. For VII, crystal data are as follows: C20H22Cr2O4P2, monoclinic, P2(1)/n, a = 13.489 (4) angstrom, b = 11.800 (3) angstrom, c = 13.726 (3) angstrom, beta = 107.22 (3)-degrees, V = 2086.79 (11) angstrom 3, Z = 4, R(F) = 4.09%. The biphenyl ligand is twisted 25.7-degrees to accommodate the gauche conformation of the tetramethyldiphosphine group. A large Cr-Cr separation [4.773 (1) angstrom] appears to preclude a strong metal-metal interaction. For VIII, crystal data are as follows: C21H24Cr2O4P2, monoclinic, C2/c, a = 14.580 (3) angstrom, b = 10.359 (3) angstrom, c = 14.705 (4) angstrom, beta = 108.56 (2)-degrees, V = 2165 (1) angstrom 3, Z = 4, R(F) = 2.90%.