LIGAND SUBSTITUTION PROCESSES ON CARBONYLMETAL DERIVATIVES .1. REACTION OF TETRACARBONYLHYDRIDOFERRATES WITH PHOSPHINES

Ligand substitution processes on KHFe(CO)4 (1) have been demonstrated for the first time by reaction with various phosphines (2 equiv). The reaction times and the nature of the reaction products strongly depend on (i) the nature of the solvent, (ii) the cone angle of the phosphine, and (iii) the reaction conditions. In protic media (e.g. EtOH), phosphines with small cone angles (P(n-Bu)3, PMe2Ph) react with 1 below room temperature to give the newly characterized H2Fe(CO)2(PR3)2 in good yield, whereas phosphines with larger cone angles react only at higher temperature and afford the disubstituted Fe(CO)3(PR3)2 derivatives in quantitative yield. In aprotic medium (THF), phosphines (P(n-Bu)3, PPh3) react only slowly with 1 at room temperature but do so at reflux temperature to yield K2Fe(CO)4 (50%) and bis- or tris-(phosphine)carbonyliron derivatives. The reaction mechanism involves the formation of a monosubstituted K+[HFe(CO)3(PR3)- derivative with a rate strongly dependent on the Tolman cone angle of the phosphine. In THF, this basic hydrido carbonyl anion reacts with 1 to yield K2Fe(CO)4 and H2Fe(CO)3(PR3). The latter further reacts to give bis- or tris(phosphine)carbonyliron derivatives. In ethanol, the monosubstituted K+[HFe(CO)3(PR3)]- derivative is protonated to give the neutral dihydride H2Fe(CO)3(PR3), which, depending on the reaction conditions, is converted either to H2Fe(CO)2(PR3)2 by CO substitution (at low temperature) or to Fe(CO)3(PR3)2 by H-2 elimination (at higher temperature). For phosphines exhibiting small cone angles, the disubstituted dihydride may react further with an excess of phosphine to yield the trisubstituted Fe(CO)2(PR3)3 derivative in good yield.