New Structural Motifs in Metallaborane Chemistry. Synthesis, Characterization, and Solid-State Structures of (Cp*W)₃(μ-H)B₈H₈, (Cp*W)₂B₇H₉, and (Cp*Re)₂B₇H₇ (Cp* = η-C₅Me₅)

Abstract

Hydrogen loss from the 7 skeletal electron pair (sep) nido 2-Cp*H3WB4H8, 1 (Cp* = η-C5Me5), the metal analogue of pentaborane (9), has been examined as a potential source of unsaturated, reactive species. Pyrolysis of 1 leads to 6 skeletal electron pair (Cp*W)2B5H9, 2 (45%), 7 sep Cp*3W3(μ-H)B8H8 3 (19%), and 7 sep (Cp*W)2B7H9, 4 (low yield), whereas photolysis gives 2 (52%) and 4 (7%). Compound 2 is known, and 3 and 4 have been spectroscopically and crystallographically characterized. Further, the isoelectronic and nearly isostructural analogue of 4, (Cp*Re)2B7H7, 5, has been prepared from the reaction of Cp*ReCl4 and BH3·THF in 66% yield. It is demonstrated that 3 exhibits a skeletal structure corresponding to a highly capped W3 bonded triangle and is analogous to a known multinuclear Ru11 cluster with a hexagonal close packed metal core. As such it constitutes an example of a closed, boron-rich metallaborane cluster with (n − 4) sep. Likewise, it is shown that 4 and 5 possess unusual structures and constitute examples of closed metallaborane clusters with (n − 2) sep. Fenske−Hall MO calculations show that the observed cluster shapes are appropriate for the observed sep's. These two new cluster types demonstrate that transition metal fragments can be used to manipulate the cluster bonding network of a borane, effectively collapsing a single cage into a more closely packed network.

Synthesis of [(Cp*Re)2BnHn]n=8-10: Metal Boride Particles That Stretch the Cluster Structure Paradigms

“…These compounds establish a new set of cluster geometries remarkably different from the canonical deltahedra of the [B n H n ] 2À boranes that underpin the clusterelectron counting rules. [19,20] The oblate cluster shapes, high metal-coordination numbers, and cross-cluster bonds featured support the idea that variation in the transition metal can drive a change from borane-to boride-like bonding.Subsequent to the isolation of [(Cp*Re) 2 B 7 H 7 ] from the reaction of [Cp*ReCl 4 ] and monoboranes, [15] we found that it is formed in a more straightforward manner from the reaction of [(Cp*ReH 2 ) 2 B 4 H 4 ] with BH 3 …”

“…Subsequent to the isolation of [(Cp*Re) 2 B 7 H 7 ] from the reaction of [Cp*ReCl 4 ] and monoboranes, [15] we found that it is formed in a more straightforward manner from the reaction of [(Cp*ReH 2 ) 2 B 4 H 4 ] with BH 3…”

“…[11±14] Recently we discovered a few unusual compounds that do not fit the mold of the borane paradigm. [15] Compared with compounds containing later transition metal fragments, these compounds exhibit structures characterized by high metalcoordination number, cross-cluster bonds, and unexpected stability for low valent Group 6 and 7 metal compounds. In trying to understand the cause of this behavior we adopted an approach which has been usefully applied to borides of intermediate stoichiometries.…”

Molecular Models of Solid State Metal Boride Structures

“…The convenient synthesis of Cp*WH B H in good yield, mentioned above, permitted an examination of its reaction chemistry (34,35). This tungstaborane obeys the electron counting rules*the three hydrides on tungsten providing the three electrons required to make it analogous to CpCoB H (36).…”

“…Hence, we were interested in the consequences of removing one or more of the potentially labile hydrogens. Whatever the initial product, it is not stable and rapidly produces three compounds: Cp We have discussed elsewhere (35) the connection between these compounds and &&hypoelectronic'' clusters found in zintl phases (42) as well as those with large transition metal clusters containing close-packed metal atom cores. Essentially, reduction of the metal electron count collapses a closo structure into a highly capped core of lower vertex number.…”