Chemistry of Homo- and Heterometallic Bridged-Borylene Complexes

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.