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

Abstract

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] (2 a–c) and η²-vinylborane complexes [Cp*Ru(R-C=CH₂)BH(L) ₂] (3 a–c) (2 a, 3 a: R=Ph; 2 b, 3 b: R=COOCH₃; 2 c, 3 c: R=p-CH₃-C₆H₄; L=C₇H₄NS₂) through hydroboration reaction. Ruthenium and the HBCC unit of the vinylborane moiety in 2 a–c are linked by a unique η⁴-interaction. Conversions of 1 into 3 a–c proceed through the formation of intermediates 2 a–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 2 a–c and 5–7, which is rare among the borataallyl complexes. All the new compounds have been characterized in solution by IR, ¹H, ¹¹B, ¹³C NMR spectroscopy, mass spectrometry and the structural types were unequivocally established by crystallographic analysis of 2 b, 3 a–c and 5–7. DFT calculations were performed to evaluate possible bonding and electronic structures of the new compounds.