Abstract
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.
Highlights
Transition metal complexes stabilized by pincer ligands are having a great impact on modern chemistry because of their high stability and the broad range of their applications,1 which extend from catalysis2 and medicine3 to materials science.4 platinum group metal pincer complexes are currently situated at the forefront of important fields such as homogeneous catalysis5 or photophysics.6 the disposition of their donor atoms at the metal coordination sphere allows them to develop a noticeable ability to form uncommon coordination polyhedra and favor unusual metal oxidation states
This paper describes the discovery of a new class of formally dianionic pincer ligands coordinated to osmium, aryl-diimineborate (C,N,N), which are generated on the metal coordination sphere by a novel procedure involving the coupling of a coordinated dihydrideborate anion with two aryl nitrile molecules and the activation of an ortho-CH bond of the aryl group of one of them
We were studying the reactions of the hexahydride complex OsH6(PiPr3)2 (1) with boranes16 and nitriles17 as a part of our work on the chemistry of polyhydride complexes of platinum group metals18 and we observed that while complex 1 promoted the catalytic addition of the B-H bond of catecholborane and pinacolborane to alkyl nitriles to give borylimines, which under the reaction conditions evolved to diborylamines,19 it was inactive for the same reactions with aryl nitriles
Summary
Transition metal complexes stabilized by pincer ligands are having a great impact on modern chemistry because of their high stability and the broad range of their applications,1 which extend from catalysis2 and medicine3 to materials science.4 platinum group metal pincer complexes are currently situated at the forefront of important fields such as homogeneous catalysis5 or photophysics.6 the disposition of their donor atoms at the metal coordination sphere allows them to develop a noticeable ability to form uncommon coordination polyhedra and favor unusual metal oxidation states. The central Os-N bond is formed, by the reaction of the dihydrideborate ligand of intermediate OsH3{κ2-H,H(H2Bcat)}(PiPr3)2 with one of the aryl nitriles.
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