Chemistry of homo- and heterometallic bridged-borylene complexes

Abstract

Thermolysis of [(Cp*RuCO)₂B₂H₆] (1; Cp* = η⁵-C₅Me₅) with [Ru₃(CO)₁₂] yielded the trimetallaborane [(Cp*RuCO)₃(μ₃-H)BH] (2) and a number of homometallic boride clusters: [Cp*RuCO{Ru(CO)₃}₄B] (3), [(Cp*Ru)₂{Ru₂(CO)₈}BH] (4), and [(Cp*Ru)₂{Ru₄(CO)₁₂}BH] (5). Compound 2 is isoelectronic and isostructural with the triply bridged borylene compounds [(μ₃-BH)(Cp*RuCO)₂(μ-CO){Fe(CO)₃}] (6) and [(μ₃-BH)(Cp*RuCO)₂(μ-H)(μ-CO){Mn(CO)₃}] (7), where the [μ₃-BH] moiety occupies the apical position. To test if compound 2 undergoes hydroboration reactions with alkynes, as observed with 6, we performed the reaction of 2 with the same set of alkynes under photolytic conditions. However, neither 2 nor 7 undergoes hydroboration to yield a vinyl–borylene complex. On the other hand, thermolysis of 6 with trimethylsilylethylene yielded the novel diruthenacarborane [1,1,7,7,7-(CO)₅-2,3-(Cp*)₂-μ-2,3-(CO)-μ₃-1,2,3-(CO)-5-(SiMe₃)-pileo-1,7,2,3,4,5-Fe₂Ru₂C₂BH] (8). The solid-state X-ray diffraction results suggest that 8 exhibits a pentagonal -bipyramidal geometry with one additional CO capping one of its faces. Cluster 3 is a boride cluster where boron is in the interstitial position of a square-pyramidal geometry, whereas compound 4 can be described as a tetraruthenium boride in which the Ru4 butterfly skeleton has an interstitial boron atom. Electronic structure calculations of compound 2 employing density functional theory (DFT) generate geometries in agreement with the structure determinations. The existence of a large HOMO–LUMO gap in 2 is in agreement with its high stability. Bonding patterns in the structure have been analyzed on the grounds of DFT calculations. Furthermore, the B3LYP-computed ¹¹B and ¹H chemical shifts for compound 2 precisely follow the experimentally measured values. All the compounds have been characterized by IR and ¹H, ¹¹B, and ¹³C NMR spectroscopy, and the geometries of the structures were unambiguously established by crystallographic analyses of 2–4 and 8.