Bis(phosphanyl)monosulfide platinum(II) complexes for hydroformylation reactions: catalytic activity and high pressure NMR mechanistic study

The neutral complexes of formula [κ2P,S-(dppmS)Pt(CH 3)-(Cl)] (1), [κ2P,S-(dppeS)Pt(CH3)(Cl)] (2) and [κ2P,S-(dpppS)-Pt(CH3)(Cl)] (3) [dppmS = Ph2PCH2P(S)Ph2; dppeS = Ph2P(CH 2)2P(S)Ph2; dpppS = Ph2P(CH 2)3P(S)Ph2] are active catalyst precursors for the hydroformylation of 1-octene in methyl isobutyl ketone. The order of reactivity found is 3 > 2 > 1. Surprisingly, the cationic complexes [κ2P,S-(dppeS)Pt(CH3)(CH3CN)]BF 4 (4a) and [{κP,μ-κS-(dppeS)-Pt(CH3)} 2][BF4]2 (5) are less active than the analogous neutral complex 2. High-pressure NMR studies revealed that, at 20°C under 1 to 50 bar of syngas, and in the presence of SnCl2, both 2 and 4a react immediately to form the same acetyl complex (8). However, in the absence of SnCl2, the methyl carbonyl complex [κ2P,S-(dppeS) Pt(CH3)(CO)]+ (9), which is formed from 4a and CO, does not undergo insertion to give the acetyl complex, even under 50 bar of syngas. Thus, the role of SnCl2 is not only to create a vacant site for CO coordination and to lower the energy barrier for the hydrogenolysis, but also to assist migration of the alkyl group in the CO insertion step. High-pressure NMR studies of the working reaction solution, under steady-state conditions, found no evidence for intermediates in which the phosphane sulfide group shows hemilabile behaviour during catalysis