Reactions of hafnocene stannyl complexes with stannanes: implications for the mechanism of the metal-catalyzed dehydropolymerization of secondary stannanes

Reactions of the hydrostannyl complexes CpCp∗Hf(SnHMes2)Cl (), [Me2C(C5H4)2]Hf(SnHMes2)NMe2 () and CpCp∗Hf(SnHMes2)OMe () with Ph2SnH2 or nBu2SnH2 afforded poly- and oligostannanes of varying molecular weights. The reaction of with 1.2 equiv. of Ph2SnH2 produced the oligostannyl complexes CpCp∗Hf(SnPh2SnHMes2)Cl (, 68%), CpCp∗Hf(SnPh2SnHPh2)Cl (, 15%), and CpCp∗Hf(SnPh2SnPh2SnHPh2)Cl (, 7%), which may be intermediates in the dehydropolymerization process. Compounds and were observed in higher yields in the reaction of CpCp∗Hf(H)Cl () with 2 equiv. of Ph2SnH2. Possible mechanisms for the formation of , , and are discussed. Two trialkylstannyl complexes, CpCp∗Hf(SnMe3)Cl () and CpCp∗Hf(SnnBu3)Cl (), were synthesized in good yields from the reaction of with R3SnH (R=Me, nBu). When a solution of was heated to 100 °C for 1 h, CpCp∗Hf(SnMe2SnMe3)Cl () and CpCp∗Hf(SnMe2SnMe2SnMe3)Cl () were formed, probably via Me2Sn insertion into Hf–Sn bonds. Based on the known influence of catalyst structure on the molecular weight of polystannanes, and the observations reported herein, it is proposed that the Sn–Sn bond-forming mechanism may involve R2Sn insertions into M–Sn bonds.