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 (2), Me₂C(C₅H₄)₂]Hf(SnHMes₂)NMe₂ (3) and CpCp^∗Hf(SnHMes₂)OMe (4) with Ph₂SnH₂ or ⁿBu₂SnH₂ afforded poly- and oligostannanes of varying molecular weights. The reaction of 2 with 1.2 equiv. of Ph₂SnH₂ produced the oligostannyl complexes CpCp^∗Hf(SnPh₂SnHMes₂)Cl (6, 68%), CpCp^∗Hf(SnPh₂SnHPh₂)Cl (7, 15%), and CpCp^∗Hf(SnPh₂SnPh₂SnHPh₂)Cl (8, 7%), which may be intermediates in the dehydropolymerization process. Compounds 7 and 8 were observed in higher yields in the reaction of CpCp^∗Hf(H)Cl (1) with 2 equiv. of Ph₂SnH₂. Possible mechanisms for the formation of 6, 7, and 8 are discussed. Two trialkylstannyl complexes, CpCp^∗Hf(SnMe₃)Cl (11) and CpCp^∗Hf(SnⁿBu₃)Cl (12), were synthesized in good yields from the reaction of 1 with R₃SnH (R=Me, ⁿBu). When a solution of 11 was heated to 100 °C for 1 h, CpCp^∗Hf(SnMe₂SnMe₃)Cl (13) and CpCp^∗Hf(SnMe₂SnMe₂SnMe₃)Cl (14) 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.