Rhodium and iridium phosphinothiolato complexes. Synthesis and crystal structures of mononuclear [M(cod)(S,P-SC2B10H10PPh2)] and dinuclear [M-2(CO)(2)(S,P-mu-SC2B10H10PPh2)] (M = Rh, Ir) and their performance in catalytic carbonylation

Abstract

The synthesis of new group 9 metal complexes containing the P,S-chelate ligands P,S-[SC2B10H10(CH2)(n)PPh2] (n = 0, Cab(P,S) (2a); n = 1, Cab(CH2P,S) (2b)) is described. Two new phosphinothiolato complexes of the type [M(Cab(P,S))(cod)] (M = Rh (3a), Ir (3b); cod = 1,5-cyclooctadiene) have been synthesized by the reaction of chloride-bridged dimers [M(mu-Cl)-(cod)](2) with 2 mol equiv of the corresponding lithium phosphinothiolato ligand LiCab(P,S) (2a). The X-ray crystal structure determination of 3 shows a mononuclear, square-planar cis-P,S metal complex with a cod ligand. Carbonylation of the cod complexes 3 yields the dinuclear complexes [M(mu-S-Cab(P,S))(CO)](2) (where M = Rh (4a), Ir (4b)). The molecular structures reveal that the two metal atoms are bridged by the two thiolato ligands, and the carbonyls complete the coordination of the metal atoms. Dinuclear complexes 4 were also formed upon reaction of carbonyl dimers [M(mu-Cl)(CO)(2)](2) with 2 equiv of 2a. Addition reactions of the dinuclear complexes 4 with triethylphosphine give the monosubstituted mononuclear metal complexes [M(Cab(P,S))(CO)(PEt3)] (where M = Rh (5a), Ir (5b)), which are obtained as trans isomers. The molecular structure of complex 5a has been determined by X-ray diffraction methods. In an analogous manner, reaction of 2b with [Rh(CO)(2)(acac)] (acac = acetylacetone) produces a dinuclear thiolato-bridged complex, [Rh(mu-S-Cab(CH2P,S))(CO)](2) (6). Complex 6 was characterized by single-crystal X-ray analysis. Reacting 6 with 2 equiv of PEt3 gave the trans-phosphino P,S-chelate complex [Rh(Cab(CH2P,S))(CO)(PEt3)] (7), analogous to 5. The mono- and dinuclear P,S-chelates 4 and 5 catalyze the carbonylation of methanol to acetic acid. In particular, the dinuclear rhodium carbonyl complex 4a is much more effective in catalyzing the carbonylation of methanol to acetic acid than the previously known catalyst [RhI2(CO)(2)](-).