Assembly of a Dihydrideborate and Two Aryl Nitriles to Form a C,N,N'-Pincer Ligand Coordinated to Osmium.

The C,N,N'-donor aryl-diimineborate pincer ligand of the complexes OsH{κ-,,-[CHRCH=NB(cat)N=CHCHR]}(PPr) (R = H, Me) has been generated in a one-pot procedure, by the reaction of the hexahydride OsH(PPr) with catecholborane (catBH) and two molecules of the corresponding aryl nitrile. The osmium-pincer bonding situation has been analyzed by means of atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analysis coupled with the natural orbitals for chemical valence (EDA-NOCV) methods. According to the results, the complexes exhibit a rather strong electron-sharing Os-C bond, two weaker donor-acceptor N-Os bonds, and two π-back-donations from the transition metal to vacant π* orbitals of the formed metallacycles. In addition, spectroscopic findings and DFT calculations reveal that the donor units of the pincer are incorporated in a sequential manner. First, the central Os-N bond is formed, by the reaction of the dihydrideborate ligand of the intermediate OsH{κ-,-(HBcat)}(PPr) with one of the aryl nitriles. The subsequent oxidative addition of the -C-H bond of the aryl substituent of the resulting κ--(-boryl-arylaldimine) affords the Os-C bond. Finally, the second Os-N bond is generated from a hydride, an ortho-metalated -boryl-arylaldimine, and the second aryl nitrile.