Azole assisted C-H bond activation promoted by an osmium-polyhydride: Discerning between N and NH

This is an open access article published under a Creative Commons Attribution (CC-BY) License.The capacity of the hexahydride complex OsH6(PiPr3)2 (1) to discern between the nitrogen atom and the NH unit in the azole assisted aryl C−H bond activation has been investigated. Complex 1 reacts with 2-phenylimidazole to give OsH3{κ2-C,N-(C6H4-imidazole)}(PiPr3)2 (2), which has been characterized by X-ray diffraction analysis. The structure proves the higher affinity of the metal center for the N atom in the presence of the NH unit, which remains unchanged, and reveals that in the solid state the molecules of this complex form infinite chains by means of intermolecular asymmetric 3-center bifurcated dihydrogen bonds. In solution, 1HDOSY NMR experiments suggest that the association degree decreases as the temperature increases. The fused six-membered ring of benzimidazole weakens the NH bond, enhancing its reactivity. As a consequence, complex 1 cannot discern between the N atom and the NH unit of 2-phenylbenzimidazole. Thus, the treatment of 1 with this substrate leads to a mixture of OsH3{κ2-C,N-(C6H4-benzimidazole)}(PiPr3)2 (3) and the dinuclear species (PiPr3)2H3Os(C6H4-benzimidazolate)OsH(η2-H2)(PiPr3)2 (4). The latter is the result of a N-assisted ortho-C−H bond activation of the phenyl group promoted by 0.5 equiv of 1 and the N−H bond activation promoted by the remaining 0.5 equiv of hexahydride 1 along with the agostic coordination of the remaining ortho-C−H bond to the metal center of the unsaturated fragment OsH(η 2-H2)(PiPr3)2. The comparison of the redox properties of 3 and 4 suggests that the interaction between the metal centers in the dinuclear compound is negligible. The replacement of the NH group of the azoles by a sulfur atom does not modify the behavior of the substrates. Thus, the reactions of 1 with 2-phenylthiazole and 2-phenylbenzothiazole afford OsH3{κ 2-C,N-(C6H4-thiazole)}(PiPr3)2 (5) and OsH3{κ2-C,N-(C6H4-benzothiazole)}(PiPr3)2 (6). In turn, complexes 2, 3, 5, and 6 are phosphorescent.Financial support from the Spanish MINECO and FEDER (Projects CTQ2013-46459-C2-01-P to M.A.S., CTQ2014-52799-P to M.A.E., CTQ2013-44303-P to I.F., CTQ2014-54071-P to A.L., and CTQ2014-51912-REDC, the DGA (E35), and the European Social Fund (FSE) is acknowledged.Peer Reviewe

Analysis of the aromaticity of osmabicycles analogous to the benzimidazolium cation

Complex OsH2Cl2(PiPr3) 2 (1) reacts with 1,2-phenylenediamine in the presence of triethylamine to give OsH2(κ-N,N-o-HNC6H 4NH)(PiPr3)2 (2), containing an osmabenzimidazolium core. The planarity and length equalization of the bicycle, along with the negative NICS values calculated for both rings and the aromatic MO delocalization, suggest that, as the organic counterpart, the osmabenzimidazolium moiety of 2 is aromatic. The electron Ï€-delocalization through the bicycle has a marked influence on the chemical behavior of the metal center. Thus, in contrast to 1, complex 2 is stable in acetonitrile and reacts with [FeCp2]PF6 to give a 1:1 mixture of [OsH(κ-N,N-o-HNC6H4NH)(NCCH3)(P iPr3)2]PF6 (3) and [OsH 3(κ-N,N-o-HNC6H4NH)(PiPr 3)2]PF6 (4), containing bicycles that also appear to be aromatic. © 2011 American Chemical Society.Financial support from the MICINN of Spain (Projects CTQ2008-00810 and Consolider Ingenio 2010CSD2007-00006) and Departamento de Ciencia, Tecnología y Universidad del Gobierno de Aragón (E35) and the European Social Fund is acknowledged. M.B. thanks the Spanish MICINN/Universidad de Zaragoza for funding through the “Ramón y Cajal” Program.Peer Reviewe