Hydroboration via metal-bound σ(B–H) bonds in Ru-(σ-borate) complexes: a pathway to η4-HBCC-σ,π-borataallyl complexes

Abstract

A series of hydroboration reactions involving terminal alkyne, internal alkyne, and alkynylphosphine using Ru-(σ-borate) complexes has been established. Initially, Cp*-based Ru-(σ-borate) complexes, [Cp*Ru(κ³-H,H,E-BH₃L^E)] (L^E = C₅H₄NE, E = S, Se), denoted as 2^E, were synthesized via a modified and efficient route. To assess the impact of secondary interaction mediated by heavier chalcogen atom, the reactions of 2^Se with terminal alkyne were carried out, which resulted in the formation of η⁴-HBCC-σ,π-borataallyl complexes, [Cp*Ru(η⁴-HBCC-κ¹-Se-BH₂(CHCHCO₂Me)L^Se)], 3a–b. To further examine the role of ancillary ligands in the hydroboration process, two distinct Ru-(σ-borate) complexes were selected, i.e., Ru-(σ-borate)(dihydridoborate), fac/mer-4, and Ru-bis(dihydridoborate), cis/trans-5. Treatment of fac/mer-4 with methyl propiolate yielded borataallyl complexes, fac/mer-[(κ³-H,S,S′-BH₂(L^S)₂)Ru(η⁴-HBCC-κ¹-S-BH₂(CHCHCMeO₂)L^S)], 6a–b. The formation of complex 6 was attributed to the addition of the metal-bound σ(B–H) bond across the alkyne CC bond, followed by coordination of the resulting hydroborated olefin unit to the ruthenium centre. In contrast, prolonged thermolysis of fac/mer-4 with an internal alkyne, diphenylacetylene, led to the formation of thiolate-bridged bimetallic ruthenium complex, [(κ²-N,S-L^S)Ru{(μ₂-κ²-N,S-L^S)(μ₂-S-L^S)}]₂ (7). A notable structural feature of complex 7 is the presence of bidentate bridging ligands, observed in two tautomeric forms, i.e., pyridine-2-thiolate and pyridinyl-2-thione. Given that alkynylphosphine possess enhanced electron density at the alkynyl moiety and offer an additional coordination site through the phosphine group, the reaction of fac/mer-4 with Ph₂P–CC–SiMe₃ was subsequently performed. Interestingly, preferential coordination of the soft nucleophilic phosphine group occurred over hydroboration of the CC bond, resulting in the formation of a Ru-(σ-borate) complex, fac/mer-[(Ph₂P–CC–SiMe₃)Ru(κ³-H,S,S′-BH₂(L^S)₂)(κ²-H,S-BH₃L^S)] (8). In an extension, the hydroboration of diphenylacetylene with cis/trans-5 yielded η⁴-HBCC-σ,π-borataallyl complexes, cis/trans-[(κ³-H,H,S-BH₃L^S)Ru(η⁴-HBCC-κ¹-S-BH₂(PhCCHPh)L^S)] (9). In contrast, treatment of cis/trans-5 with an alkynylphosphine substrate led to the formation of mono (10) and bis (11) phosphine-coordinated σ-borate complexes. Density functional theory (DFT) calculations were conducted to assess the thermodynamic feasibility of these hydroboration pathways and to gain bonding insights into the resulting borataallyl complexes.