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