MIXED-VALENT INTERACTIONS IN RIGID DINUCLEAR SYSTEMS - ELECTROCHEMICAL AND SPECTROSCOPIC STUDIES OF CR-I-CR-O IONS WITH CONTROLLED TORSION OF THE BIPHENYL BRIDGE

Three dinuclear complexes were prepared in which Cr(CO)(2)(PPh(3)) moi eties are connected through biphenyl linkages. The dihedral (twist) an gle of the two arenes in the biphenyl decreases from 100 to 17 to 0 de grees as the bridging ligand is changed from biphenyl-2,2'-dicarboxyli c acid dimethyl ester (in compound 2) to dihydrophenanthrene (3) to fl uorene (4). All three compounds are oxidized in two reversible one-ele ctron steps with a voltage separation of ca. 260 mV. Each monocation i s shown by IR spectroscopy to be trapped-valent. ESR spectroscopy in f rozen glasses gives the same result. IR sampling of the dications is a chieved by low-temperature flow electrochemical experiments. The asymm etric stretching frequency of the CO pair assigned to the Cr(0) site i n the mixed-valent ion is measurably shifted from that of the original (CrCr0)-Cr-0 complex, and the amount of shift increases significantly as the biphenyl twist diminishes. It is proposed that although the ha lf-filled orbital is localized on one metal site in the monocations, t here is significant transmission of charge between the metals through fined orbitals of the biphenyl linkage. The results support a through- space model for electronic (mixed-valent) delocalization in Cr-I(mu-bi phenyl)Cr-0 linkages. Results of studies of the oxidation of (arene)Cr (CO)(2)L, where L = CO or PPh(3), and of (biarene)Cr-2(CO)(6), prepare d as part of this work, are briefly reported.