Synthesis of the First Example of the 12-Vertex-closo/12-Vertex-nido Biscarborane Cluster by a Metal-Free B-H Activation at a Phosphorus(III) Center.

An unusual 12-vertex-closo-C2 B10 /12-vertex-nido-C2 B10 biscarborane cluster was synthesized through an unprecedented regioselective metal-free B-H activation by a sterically hindered P(III) center under mild conditions accompanied by cage-opening rearrangement. A combination of the electron-accepting properties of a carborane cage and steric enforcement of close interatomic contacts represent a new synthetic strategy for the activation of strong B-H bonds in carboranes.

Cage Opening of a Carborane Ligand by Metal Cluster Complexes.

The reaction of Os3 (CO)10 (NCMe)2 with closo-o-C2 B10 H10 has yielded two interconvertible isomers Os3 (CO)9 (µ3 -4,5,9-C2 B10 H8 )(µ-H)2 (1 a) and Os3 (CO)9 (µ3 -3,4,8-C2 B10 H8 )(µ-H)2 (1 b) formed by the loss of the two NCMe ligands and one CO ligand from the Os3 cluster. Two BH bonds of the o-C2 B10 H10 were activated in its addition to the osmium cluster. A second triosmium cluster was added to the 1 a/1 b mixture to yield the complex Os3 (CO)9 (µ-H)2 (µ3 -4,5,9-µ3 -7,11,12-C2 B10 H7 )Os3 (CO)9 (µ-H)3 (2) that contains two triosmium triangles attached to the same carborane cage. When heated, 2 was transformed to the complex Os3 (CO)9 (µ-H)(µ3 -3,4,8-µ3 -7,11,12-C2 B10 H8 )Os3 (CO)9 (µ-H) (3) by a novel opening of the carborane cage with loss of H2 .

Synthesis and Reactivity of Electronically Unsaturated Dirhenium Carbonyl Compounds Containing Bridging Gold-Carbene Groups.

The electronically unsaturated compounds Re2(CO)8[µ-Au(NHC)](µ-Ph), 1, and Re2(CO)8[µ-Au(NHC)]2, 2, were obtained from the reaction of Re2(CO)8[µ-η2-C(H)═C(H)Bun](µ-H) with MeAu(NHC), NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene. Compound 1 was converted to the new compound Re2(CO)8[µ-Au(NHC)](µ-H), 3, by reaction with H2. Addition of CO to 3 yielded the new compound Re2(CO)9[Au(NHC)](µ-H), 4, which contains a terminally coordinated Au(NHC) group on one of the rhenium atoms, and the hydrido ligand was shifted to bridge the Re-Au bond. The mechanism of the formation of 4 was established by DFT computational analyses. Compound 3 also reacted with C2H2 by an addition with insertion into the Re-H bonds to yield the compound Re2(CO)8[µ-Au(NHC)](µ-C2H3), 5, which contains a σ-π coordinated, bridging C2H3 ligand. The stereochemistry of the insertion was found to proceed preferentially with a cis- (syn-) stereochemistry. Compound 1 reacted with HCl to yield Re2(CO)8[µ-Ph](µ-H), 6, and ClAu(NHC) by selective removal of the bridging Au(NHC) group. All new compounds were characterized by single-crystal X-ray diffraction analyses.

Opening of Carborane Cages by Metal Cluster Complexes: The Reaction of a Thiolate-Substituted Carborane with Triosmium Carbonyl Cluster Complexes.

The reaction of Os3(CO)10(NCMe)2 with closo-o-(1-SCH3)C2B10H11 has yielded the complex Os3(CO)9[µ3-η(3)-C2B10H9(SCH3)](µ-H)2, 1, by the loss of the two NCMe ligands and one CO ligand from the Os3 cluster and the coordination of the sulfur atom and the activation of two B-H bonds with transfer of the hydrogen atoms to the cluster. Reaction of 1 with a second equivalent of Os3(CO)10(NCMe)2 yielded the complex Os3(CO)9(µ-H)[(µ3-η(3)-1,4,5-µ3-η(3)-6,10,11-C2B10H8S(CH3)]Os3(CO)9(µ-H)2, 2, that contains two triosmium triangles attached to the same carborane cage. The carborane cage was opened by cleavage of two B-C bonds and one B-B bond. The B-H group that was pulled out of the cage became a triply bridging group on one of the Os3 triangles but remains bonded to the cage by two B-B bonds. When heated to 150 °C, 2 was transformed into the complex Os3(CO)9(µ-H)[(µ3-η(3)-µ3-η(3)-C2B10H7S(CH3)]Os3(CO)9(µ-H), 3, by the loss of two hydrogen atoms and a rearrangement that led to further opening of the carborane cage. Reaction of 1 with a second equivalent of closo-o-(1-SCH3)C2B10H11 has yielded the complex Os3(CO)6)(µ3-η(3)-C2B10H9-R-SCH3) (µ3-η(3)-C2B10H10-S-SCH3)(µ-H)3, 4a, containing two carborane cages coordinated to one Os3 cluster. Compound 4a was isomerized to the compound Os3(CO)6(µ3-η(3)-C2B10H9-R-SCH3)(µ3-η(3)-C2B10H10-R-SCH3)(µ-H)3, 4b, by an inversion of stereochemistry at one of the sulfur atoms by heating to 174 °C.

Binuclear Aromatic C-H Bond Activation at a Dirhenium Site.

The electronically unsaturated dirhenium complex [Re2(CO)8(µ-H)(µ-Ph)] (1) has been found to exhibit aromatic C-H activation upon reaction with N,N-diethylaniline, naphthalene, and even [D6]benzene to yield the compounds [Re2(CO)8(µ-H)(µ-η(1)-NEt2C6H4)] (2), [Re2(CO)8(µ-H)(µ-η(2)-1,2-C10H7)] (3), and [D6]-1, respectively, in good yields. The mechanism has been elucidated by using DFT computational analyses, and involves a binuclear C-H bond-activation process.

Host-guest behavior of a heavy-atom heterocycle Re4(CO)16(µ-SbPh2)2(µ-H)2 obtained from a palladium-assisted ring opening dimerization of Re2(CO)8(µ-SbPh2)(µ-H).

The heavy-atom heterocycle Pd[Re2(CO)8(µ-SbPh2)(µ-H)]2 (5) has been synthesized by the palladium-catalyzed ring-opening cyclodimerization of the three-membered heterocycle Re2(CO)8(µ-SbPh2)(µ-H) (3). The Pd atom occupies the center of the ring. The Pd atom in 5 can be removed reversibly to yield the palladium-free heterocycle [Re2(CO)8((µ-SbPh2)(µ-H)]2 (6).

Facile C-H bond formation by reductive elimination at a dinuclear metal site.

The electronically unsaturated dirhenium complex [Re2(CO)8(µ-AuPPh3)(µ-Ph)] (1) was obtained from the reaction of [Re2(CO)8{µ-η(2)-C(H)=C(H)nBu}(µ-H)] with [Au(PPh3)Ph]. The bridging {AuPPh3} group was replaced by a bridging hydrido ligand to yield the unsaturated dirhenium complex [Re2(CO)8(µ-H)(µ-Ph)] (2) by reaction of 1 with HSnPh3. Compound 2 reductively eliminates benzene upon addition of NCMe at 25 °C. The electronic structure of 2 and the mechanism of the reductive elimination of the benzene molecule in its reaction with NCMe were investigated by DFT computational analyses.

Tetrarhena-heterocycle from the palladium-catalyzed dimerization of Re2(CO)8(µ-SbPh2)(µ-H) exhibits an unusual host-guest behavior.

The six-membered heavy atom heterocycles [Re(2)(CO)(8)(µ-SbPh(2))(µ-H)](2), 5, and Pd[Re(2)(CO)(8)(µ-SbPh(2))(µ-H)](2), 7, have been prepared by the palladium-catalyzed ring-opening cyclo-dimerization of the three-membered heterocycle Re(2)(CO)(8)(µ-SbPh(2))(µ-H), 3. The palladium atom that lies in the center of the heterocycle 7 was removed to yield 5. The palladium removal was found to be partially reversible leading to an unusual example of host-guest behavior. A related dipalladium complex Pd(2)Re(4)(CO)(16)(µ(4)-SbPh)(µ(3)-SbPh(2))(µ-Ph)(µ-H)(2), 6, was also formed in these reactions of palladium with 3.

Reversible water activation driven by contraction and expansion of a 12-vertex-closo-12-vertex-nido biscarborane cluster.

The activation of O-H bonds of water at room temperature driven by the cage-opening rearrangement of a biscarborane-based cluster is reported. The reaction of the 12-vertex-closo-12-vertex-nido biscarborane cluster with water led to the quantitative and selective two-vertex decapitation of a carborane cluster and formation of the pendent boronic acid hydride B(H)(OH) group. Remarkably, this transformation can be quantitatively reversed with the release of water and re-formation of the starting biscarborane cage. The flexibility of cage decapitation/expansion and its influence on the reactivity of an exohedral substituent represent a new approach to cluster-induced organic transformations.