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 OsH2{κ3-C,N,N-[C6H3RCHNB­(cat)­NCHC6H4R]}­(PiPr3)2 (R
= H, Me) has been generated in a one-pot procedure, by the reaction
of the hexahydride OsH6(PiPr3)2 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 OsH3{κ2-H,H-(H2Bcat)}­(PiPr3)2 with one of the aryl nitriles. The subsequent oxidative
addition of the o-C–H bond of the aryl substituent
of the resulting κ1-N-(N-boryl-arylaldimine) affords the Os–C bond. Finally, the second
Os–N bond is generated from a hydride, an ortho-metalated N-boryl-arylaldimine, and the second aryl nitrile.