METAL-METAL MULTIPLY-BONDED COMPLEXES OF TECHNETIUM .1. SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF PHOSPHINE COMPLEXES THAT CONTAIN A TC-TC MULTIPLE BOND

A series of triply metal-metal bonded ditechnetium(II) phosphine complexes with the general formula Tc2Cl4(PR3)4 (PR3 = PEt3, PPrn3, PMePh2, PMe2Ph) have been prepared from mononuclear Tc(IV) precursors and fully characterized. Two-electron reduction of the Tc(IV) bis(phosphine) complexes TcCl4(PR3)2 (PR3 = PEt3, PPrn3, PMePh2, PMe2Ph) with finely divided zinc in aromatic solvents or tetrahydrofuran results in the formation of the corresponding electron-rich triply bonded compounds TC2Cl4(PR3)4 in high yield. These are the first phosphine complexes of technetium that possess a metal-metal bond. The solid-state structures of the PEt3, PMe2Ph, and PMePh2 derivatives have been investigated by X-ray crystallography and are described in detail. The crystallographic parameters for these compounds are as follows: TC2Cl4(PEt3)4 (1), I43mBAR with a = 12.3261(9) angstrom, V = 1872.7(2) angstrom3, and Z = 2; TC2Cl4(PMe2Ph)4 (3), C2/c with a = 17.613(5) angstrom, b = 9.957(5) angstrom, c = 23.861(6) angstrom, beta = 117.19(2)-degrees, V = 3722 (2) angstrom3, and Z = 4; Tc2Cl4(PMePh2)4.C6H6 (4.C6H6), P2(1)/c with a = 22.910(2) angstrom, b = 12.286(2) angstrom, c = 21.550(2) angstrom, beta = 11 5.867(6)-degrees, V = 5458(l) angstrom3, and Z = 4. Similar to the analogous dirhenium(II) complexes, the molecules adopt an eclipsed M2L8 conformation with approximate D2d symmetry. The Tc-Tc bond lengths are 2.133(3), 2.127(1), and 2.1384(5) angstrom for TC2Cl4(PEt3)4, TC2Cl4(PMe2Ph)4, and Tc2Cl4(PMePh2)4, respectively. Structural and spectroscopic evidence indicates that these dimers contain an electron-rich Tc-Tc triple bond with a sigma2pi4delta2delta*2 ground-state electronic configuration. Electrochemical studies reveal that each compound undergoes two reversible one-electron oxidation processes, which presumably produce the corresponding Tc2(5+) and TC2(6+) dinuclear species. H-1 NMR, P-31{H-1} NMR, and UV-vis spectroscopic data are presented for each compound.