Photochemical generation of nickel(I) complexes and their reaction with hydrogen: nickel hydride catalysed hydrogenation of 1,5-cyclo-octadiene

Abstract

In benzene under an atmosphere of hydrogen, triplet excited-state xanthone or acetone sensitizes the photoreduction of bis(acety1acetonato) nickel(ii), Ni(acac),, to transient nickel(i) complexes which can be detected b y ESR spectroscopy in the presence of stabilizing ligands. The kinetics of these ESR signal decays prove that these nickel ( I) complexes are derived from moderately co-ordinating ligands, such as tetrahydrofuran and 1,5-cyclo-octadiene (1,5-COD), and react with hydrogen in the dark t o form nickel hydride complexes. A n excess of 1,5-COD in the photolysate was sequentially and catalytically isomerized and hydrogenated under the irradiation conditions; i.e., 1,5-COD-+I ,4-COD+l,3-COD+COE-+COA.The nickel hydride complexes are proposed as the key species in the initiation of the catalytic cycle in which steps involving the addition and elimination of nickel hydride bonds and the hydrogenolysis of nickel complexes are assumed. As these alkene reactions cease in the dark and are quenched b y a high concentration of 1,3-COD, it can be concluded that one or more steps in the catalytic cycle requires sensitized photoexcitation.We have shown that triplet excited-state ketones with high triplet-state energy (e.g. xanthone and benzophenone) can sensitize the photoreduction of bis(acetylacetonato)nickel(rr), Ni(acac),, to give nickel(r) complexes and acetylacetone (acacH) in an H-atom donating solvent which is itself oxidized.' The Ni' complexes generally disproportionate to give Ni" compounds and Nio as metallic precipitates (or its complexes in the presence of suitable ligands, L). Ni(acac), + RCH,OH-Ni(acac)L, + acacH + RCHO Scheme 1.

A Proton Directly Attacks 1,5-Cyclooctadiene in Bis(1,5-cyclooctadiene)nickel(0) in the Formation of a Keim Type Oligomerization Catalyst

“…As only one carbon signal, F, related to a single proton signal, 8, remains, these signals belong to the σ-bonded carbon and its hydrogen. (This proton assignment contradicts some literature suggestions. 2e,h, ) Assignments of coupling constants (as shown in the second column of Table ) and connectivity within the cyclooctenyl ligand were unambiguously established by extensive decoupling experiments, simulations of nonoverlapping 1 H signals, and a homonuclear COSY study…”

Hydrosilylation of Aldehydes and Ketones Catalyzed by Nickel PCP-Pincer Hydride Complexes

“…Nickel hydride complexes are of great importance in the research areas of homogeneous catalysis, coordination chemistry, and enzymatic reaction mechanisms. They are often postulated as key intermediates in a variety of nickel-catalyzed organic transformations. − These hypothesized nickel hydrides are usually too reactive to allow direct observation of reaction intermediates or thorough investigation of reaction mechanisms. Details of their existence during catalytic processes largely rely on computational studies. ,, In contrast, a number of discrete and stable nickel hydride complexes have been prepared and many stoichiometric transformations involving these hydrides have been reported. ,− However, very few of these complexes are catalytically competent; of the known well-defined catalytic systems, nickel hydride complexes are solely used as catalysts for olefin isomerization and oligomerization. , This has prompted us to study new reactivity of nickel hydride complexes and to explore their potential as relatively inexpensive metal catalysts for various organic reactions.…”