η4-HBCC-σ,π-Borataallyl Complexes of Ruthenium Comprising an Agostic Interaction

Thermolysis of [Cp*Ru(PPh2(CH2)PPh2)BH2(L-2)] 1 (Cp*=eta(5)-C5Me5; L=C7H4NS2), with terminal alkynes led to the formation of eta(4)-sigma,pi-borataallyl complexes [Cp*Ru(mu-H) B{R-C=CH2}(L)(2)] (2a-c) and eta(2)-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 eta(4)-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 s-borane complex [Cp*RuCO(mu-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 eta(4)-sigma,pi-borataallyl complexes [Cp*Ru(mu-H)BH{R-C=CH2}(L)] 5 and [Cp*Ru(mu-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 eta(4)-sigma,pi-borataallyl complex [Cp*Ru(mu-H) BH{R-C=CH-R}(L)] 7 (R=COOCH3; L=C(7)H(4)NS2). 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, H-1, B-11, C-13 NMR 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.