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.