Coordination Chemistry and Reactivity of Bis(aldimino)pyridine Nickel Complexes in Four Different Oxidation States

Abstract

A series of nickel complexes with potentially redox active bis(aldimino)pyridine ligands [NNN] ([NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-arylmethanimine), where aryl = 2, 6-diisopropylphenyl, mesityl, 4-methoxyphenyl, 4-trifluoromethylphenyl, and 3,5-bis-(trifluoromethyl)phenyl) were synthesized, and their properties and reactivities were investigated as a function of the overall oxidation state of the system. (Ni[NNN]) 2+ complexes of ligands featuring bulky electron-rich substituents (1a-Br 2 and 1b-Br 2 , [NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-(2,6-diisopropylphenyl)methanimine) and 1,1′-(pyridine-2,6-diyl)bis(N-mesitylmethanimine), respectively) demonstrated five electrochemical reduction events, the first three of which were quasi-reversible. In contrast, only two quasi-reversible reductions were observed for the less bulky and electron-deficient N-aryl substituents 4-(trifluoromethyl)phenyl and 3,5-bis(trifluoromethyl)phenyl. Chemical reduction of 1a-Br 2 and 1b-Br 2 with 1 equiv of KC 8 or CoCp* 2 forms (Ni[NNN]) + complexes of the general formula Ni[NNN]Br (2a-Br and 2b-Br). Structural, spectroscopic, and theoretical studies reveal that these complexes feature significant unpaired spin density on the metal, consistent with "nickel(I)" character. This behavior is in contrast with previously reported bis(ketimino)pyridine systems, in which at the (Ni[NNN]) + state the unpaired electron resided exclusively in the ligand. Further reduction forms a series of (Ni[NNN]) 0 complexes, in which all of the potentially tridentate [NNN] ligands bind via only one iminopyridine unit; the second arm is left unbound in most complexes. Variable temperature NMR spectroscopy demonstrates that bound and unbound arms exchange via a postulated tridentate intermediate. Electrochemical reduction, via three sequential one-electron reductions, of 1a-Br 2 and 1b-Br 2 in the presence of CO 2 /H + forms an active catalyst for H 2 evolution at a glassy-carbon electrode surface, again emphasizing the unique redox chemistry of the bulky bis(aldimino)pyridine nickel complexes.

Electrochemical Water Oxidation and CO2 Reduction with a Nickel Molecular Catalyst

“…where F is the Faraday constant (96,485 C•mol −1 ), v is the scanning rate used (0.1 V s −1 ), n p is the number of electrons involved in the non-catalytic oxidation-reduction reaction (n p = 1), R is the gas constant (8.314 J•K −1 •mol −1 ), T is the temperature (298.15 K), n cat = 2 indicates the reduction of CO 2 to CO, and i p and i cat are identified as peak currents under Ar and CO 2 , respectively. The TOF is separately calculated as 7.84 s −1 (i cat /i p = 6.3) in CO 2 -saturated 0.1 M n Bu 4 NPF 6 /DMF solution with 0.0175 M CH 3 COOH at −1.99 V, which is comparable to those reported nickel-based homogeneous catalysts (Table S4) [76][77][78][79][80][81]. However, due to the process being controlled by diffusion, for most molecular catalysts, the product of the electrocatalytic CO 2 is carbon monoxide, which is very different from other heterogeneous catalysts for producing formic acid and formate with Ni-based anodes [82][83][84][85].…”

Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine–Benzophenone Ligand in Alkyne Cyclotrimerization

“…While stable tricoordinate Ni(0) alkyne complexes with formally 16 valence electrons analogous to Ph 6-Ph are well documented, 53,54 compound p -tol 5-Ph represents an unusual example of tetracoordinate, 18 VE Ni(0)–alkyne complex, to the best of our knowledge the first to be structurally characterized. Spectroscopic and structural data point toward a considerably weaker activation of the C≡C triple bond in this geometry (Table 1).…”

Versatile Coordination and C–C Coupling of Diphosphine-Tethered Imine Ligands with Ni(II) and Ni(0)

“…the side-on bound and the metallo-aza-cycle extreme. Limited examples of an η 2 (C,N)-bound benzophenone-imine to Ni(0) are reported, as well as a recently published bimetallic Ni complex with a chelating NNN-bis­(imino)­pyridine ligand . Monometallic complexes with a chelating ligand with an η 2 (C,N)-coordinating imine functionality based on precious metals Pd, Rh, and Ir are described as well. − …”

“…This latter binding mode can be described by the Dewar-Chatt-Duncanson (DCD) model for coordination of π-ligands, i. as well as a recently published bimetallic Ni complex with a chelating NNN-bis(imino)pyridine ligand. 18 Monometallic complexes with a chelating ligand with an η 2 (C,N)coordinating imine functionality based on precious metals Pd, Rh, and Ir are described as well. 19−24 Here, we investigate the versatile binding of an imine ligand flanked by two ortho-diphenylphosphino-substituted phenyl rings to nickel.…”