Synthesis, Structures, and Bonding of Metal-Rich Metallaboranes Comprising Triply Bridging Borylene and Boride Moieties

A series of metal-rich metallaboranes of groups 7 and 9 comprising triply bridging borylene and boride units have been isolated and structurally characterized. Thermolysis of nido-[(RhCp*)(2)B3H7] (1; Cp* = eta(5)-C5Me5) with [Co-2(CO)(8)] led to the isolation of tetrametallic [mu(3)-BH)(RhCp*)(2)(mu-CO)(mu(3)-CO){Co-2})(CO)(4)}] (2), featuring a triply bridging borylene unit, and the trimetallic cluster [(mu(3)-BH)(mu-H)(RhCp*)(2)(mu-CO)(3){Co(CO)}] (3) that contains a triply bridging hydrido(borylene) unit. The borylene {BH} unit of 2 is coordinated to a deltahedral face of a tetrametallic tetrahedron in a mu 3 fashion. Cluster 3 is a rare example of a tetrahedral metallaborane featuring a hydrido(borylene) unit. In an attempt to synthesize the Mn analogues of 2 and 3, a similar reaction was carried out with [Mn-2(CO)(10)] that afforded the trimetallic cluster [(mu(3)-BH)(RhCp*)(2)(mu-CO)(3){MnH(CO)(3)}] (5) having a triply bridging borylene moiety, the two heterometallic mu(9)-boride clusters [(RhCp*)(3){Rh(CO)}(3)(mu 4-CO)(3){MnH(CO)(3)}B3H2] (6) and [(RhCp*)(3){Mn(CO)(3)}(2)Rh(CO)(2)B4H3] (7) and the unusual tetrametallic complex [(RhCp*)(2)(mu(4)-CO)(2)(mu(3)-eta(3)-CO2){Mn-2(CO)(9)}] (8). Clusters 6 and 7 are both unusual heterometallic metal-rich boride clusters, where the boride boron atom is encapsulated inside a tricapped trigonal prism depicting a mu(9)-bonding mode. Compound 8 is a unique example of a metal carbonyl compound in which a CO2 group is bridging two Rh atoms and one Mn atom in a mu(3)-eta(3) fashion. To explore this chemistry with a heavier transition metal, we have carried out the thermolysis of arachno-[IrCp*H-2(B3H7)] (9) with [Mn-2(CO)(10)], which afforded the face-fused iridaborane cluster [(IrCp*)(3){Ir(CO)(2)}(3)(mu(4)-CO)(mu(3)-CO)B] (10). Compound 10 can also be viewed as a boride cluster, where the naked boron is coordinated to iridium centers in a unique mu(5) coordination mode. All of the compounds have been characterized by H-1, B-11, and C-13 NMR spectroscopy and mass spectrometry, and the structures of 2, 3, 5, 6, 8, and 10 have been unambiguously established by crystallographic analyses. Computational studies show that a substantial amount of overlap occurs between the metal frameworks and borylene/boride units.

Automated summary

A series of metal-rich metallaboranes of groups 7 and 9 with triply bridging borylene and boride units have been isolated and structurally characterized. Experimental and computational studies reveal a substantial amount of overlap between the metal frameworks and borylene/boride units.