Reactions of Rh 2(O 2CCH 3) 2{Ph 2P(C 6H 4)} 2 with monothiocarboxylic acids: facile synthesis of Rh 2(OSCR) 4·2PPh 3 molecules and X-ray crystal structure of Rh 2(OSCC(CH 3) 3) 4·2PPh 3

Abstract

The reaction of Rh 2(O 2CCH 3) 2(Ph 2P(C 6H 4)) 2·2THF with an excess of the monothio acids HOSCR (RCH 3, C 6H 5,-C(CH 3) 3) gives a series of products Rh 2(OSCR) 4·2PPh 3 in which the initially bridging orthometallated triphenylphosphine ligands have been protonated and are now coordinated as neutral ligands to the rhodium(II) ions. In addition the two bridging acetate groups are displaced and the rhodium-to-rhodium bond is bridged by four monothiocarboxylate groups in the final product. The reaction products were characterized by elemental analyses, infrared and 1H NMR spectroscopy. The structure of Rh 2(OSCC(CH 3) 3) 4·2PPh 3 was unequivocally established by a single crystal X-ray diffraction experiment. This compound crystallizes in the monoclinic space group P2 1/ n with one half of the dimer constituting the asymmetric unit. The cell dimensions are: a=12.508(8), b=12.976(7), c=17.582(5) Å, β=92.31(3)°, V=2851(2) Å 3 and Z=2. The structure was refined to R=0.086 ( R′=0.070). The Rh-Rh distance is 2.584(1) Å and the Rh-P distance 2.475(2) Å. The length of this metal-metal bond is appreciably greater than that observed for Rh 2(OSCCH 3) 4·2HOSCCH 3 (2.550(3) Å), the only other tetramonothiocarboxylato complex of rhodium(II) to have been structurally characterized. The electron transfer behaviour of these compounds has been studied by cyclic voltammetry in several solvents. The potential at which the dinuclear molecule undergoes oxidation was found to be dependent on the identity of the substituent group on the monothiocarboxylate ligands, with the oxidation becoming easier as the electron-donating power of the substituent group was increased.