[eta]4-HBCC-[sigma],[pi]-Borataallyl Complexes of Ruthenium Comprising an Agostic Interaction

Thermolysis of [Cp*Ru(PPh2(CH2)PPh2)BH2(L2)] 1 (Cp*=η5-C5Me5; L=C7H4NS2), with terminal alkynes led to the formation of η4-σ,π-borataallyl complexes [Cp*Ru(µ-H)B{R-C=CH2}(L)2] (2a-c) and η2-vinylborane complexes [Cp*Ru(R-C=CH2)BH(L)2] (3a-c) (2a, 3a: R=Ph; 2b, 3b: R=COOCH3; 2c, 3c: R=p-CH3-C6H4; L=C7H4NS2) through hydroboration reaction. Ruthenium and the HBCC unit of the vinylborane moiety in 2a-c are linked by a unique η4-interaction. Conversions of 1 into 3a-c proceed through the formation of intermediates 2a-c. Furthermore, in an attempt to expand the library of these novel complexes, chemistry of σ-borane complex [Cp*RuCO(µ-H)BH2L] 4 (L=C7H4NS2) was investigated with both internal and terminal alkynes. Interestingly, under photolytic conditions, 4 reacts with methyl propiolate to generate the η4-σ,π-borataallyl complexes [Cp*Ru(µ-H)BH{R-C=CH2}(L)] 5 and [Cp*Ru(µ-H)BH{HC=CH-R}(L)] 6 (R=COOCH3; L=C7H4NS2) by Markovnikov and anti-Markovnikov hydroboration. In an extension, photolysis of 4 in the presence of dimethyl acetylenedicarboxylate yielded η4-σ,π-borataallyl complex [Cp*Ru(µ-H)BH{R-C=CH-R}(L)] 7 (R=COOCH3; L=C7H4NS2). An agostic interaction was also found to be present in 2a-c and 5-7, which is rare among the borataallyl complexes. All the new compounds have been characterized in solution by IR, 1H, 11B, 13CNMR spectroscopy, mass spectrometry and the structural types were unequivocally established by crystallographic analysis of 2b, 3a-c and 5-7. DFT calculations were performed to evaluate possible bonding and electronic structures of the new compounds.