Divalent Germanium and Tin Compounds Stabilized by Sterically Bulky P∧O, PO∧O, PS∧O, and PN∧O Ligands:  Synthesis and First Insights into Catalytic Application to Polyurethane Systems

A sterically bulky chelating phosphine, tBu2P−CH2−C(CF3)2OH (1), was shown to stabilize divalent tin and germanium compounds: M[−O−C(CF3)2CH2PtBu2]2 (6, M = Sn; 14, M = Ge). Oxidizing reagents, e.g., sulfur, pyridine-N-oxide, and 1-azidoadamantane, reacted with divalent tin 6 on the phosphorus ligands exclusively, preserving the divalent state of tin in the resultant compounds Sn[−O−C(CF3)2CH2P(S)tBu2]2 (7), Sn[−O−C(CF3)2CH2P(O)tBu2]2 (11) and Sn[−O−C(CF3)2CH2PtBu2],[−O−C(CF3)2CH2P(N3-Adamantyl)tBu2]2 (12). The divalent germanium compound 14 was found to be more prone to oxidation to the tetravalent state. For example, the reaction with 1-azidoadamantane gave tetravalent germanium compounds Ge[−O−C(CF3)2CH2P(−)tBu],[-O−C(CF3)2CH2P(N-)tBu2] (16) and [Adamantyl],[−O−C(CF3)2CH2P(N−)]Ge−O−Ge[−O−C(CF3)2CH2P(tBu)2(N−)],[Adamantyl] (17). A Ge−O−Ge bridge in 17 is very linear, with an angle Ge−O−Ge of 177.4(2)°. According to X-ray analyses the intramolecular M−P bond lengths are among the longest known for this kind of bonding: 3.228(4) Å for P−Sn in 12 and 2.7585(12) Å for P−Ge in 14. The divalent tin compounds 6 and 7 were found to be efficient catalysts for the formation of polyurethanes. The divalent germanium compound 14 was active in polyurethane formation, but it was an order of magnitude less active than corresponding tin analogue 6.