Synthesis, Characterization, and Oxidation of New POCN_imine-Type Pincer Complexes of Nickel

Abstract

This report describes the synthesis and characterization of new pincer-type complexes of divalent and trivalent nickel. Refluxing toluene mixtures of NiBr2(CH3CN) n and the unsymmetrical pincer-type ligands Ph-POCHNimine-R gave (Ph-POCNimine-R)Ni(II)Br in good yields (Ph-POCHNimine-R = 1-(Ph2PO),3-(CHNR)C6H4; R = Bn, 1; Ph, 2; t-Bu, 3; Cy, 4). The C–H nickelation step involved in these syntheses is facilitated in the presence of NEt3. Treating compounds 1–4 with Br2, I2, N-bromosuccinimide (NBS) and N-chlorosuccinimide appears to bring about the desired one-electron oxidation of the Ni(II) center in these complexes, but the putative trivalent species decompose over time and could not be isolated. One of the decomposition products obtained in the Br2 reaction was identified as the zwitterionic compound [Br3Ni(II){κO-(E)-N-(3-((diphenylphosphoryl)oxy)benzylidene)benzenaminium}], 5. To identify the factors that are important for isolation of stable trivalent derivatives, we prepared the divalent precursors (i-Pr-POCNimine-Ph)NiX (X= Br, 6, and NCS, 9) and studied their reactions with various oxidants. Whereas treatment of 6 with Br2 or NBS gave the target 17-electron complex (i-Pr-POCNimine-Ph)Ni(III)Br2 (7), no mixed-ligand trivalent complexes of the type (i-Pr-POCNimine-Ph)Ni(III)Br(X) could be isolated from reactions of (i-Pr-POCNimine-Ph)Ni(NCS) (9) with NBS or of 6 with N-chlorosuccinimide. The latter reaction gave a complex mixture from which was isolated a paramagnetic, trinuclear compound (8) composed of octahedral Ni(II) units featuring μ2-Cl and μ2,κO,κN-succinimide fragments. The solid state structures of all new complexes 2-9 have been elucidated by X-ray crystallography.

Recent Advances in Chemistry of Unsymmetrical Phosphorus-Based Pincer Nickel Complexes: From Design to Catalytic Applications

“…Since then, many other nickel complexes containing unsymmetrical POCN ligands have been described [ 82 ], including various substituted analogues of complex 3a , such as compound 7 ( Figure 4 ) [ 80 ]. Optically active species are also known, for example, a complex containing the imidazole fragment ( 8 and 9 ) ( Figure 4 ), which is successfully used in the asymmetric Suzuki–Miyaura cross-coupling reaction [ 83 , 84 , 85 , 86 ], as well as complex 10 [ 87 ], containing diisopropylphosphinite and imine fragments, which can be oxidized with bromine or N -bromosuccinimide to complex 11 ( Figure 4 ). It is interesting to note that complex 12 also obtained by Zargarian and co-workers, containing the diphenylphosphinite fragment, undergoes two-electron oxidation of the phosphinite fragment with the formation of complex 13 upon interaction with bromine ( Figure 4 ).…”

“…It is interesting to note that complex 12 also obtained by Zargarian and co-workers, containing the diphenylphosphinite fragment, undergoes two-electron oxidation of the phosphinite fragment with the formation of complex 13 upon interaction with bromine ( Figure 4 ). Thus, we can conclude that the choice of the P-substituent is very important for the stabilization of Ni (III) particles [ 87 ].…”

Photoreductive Chlorine Elimination from a Ni(III)Cl2 Complex Supported by a Tetradentate Pyridinophane Ligand

“…Herein, we report the isolation, characterization, and photoreactivity study of a stable Ni III dichloride complex supported by the tetradentate pyridinophane ligand Me N4, [ Me N4Ni III Cl2] + (1 + ). Although several N-donor supported Ni III monochloride species, 33,34 porphyrin Ni III dibromide complexes, 35 and mixed N/C-donor supported Ni III halide complexes 30,[36][37][38][39][40][41][42][43] were reported, none of them were studied in terms of their photochemistry. To the best of our knowledge, 1 represents the first isolated all N-donor supported Ni III chloride complex for which its photochemistry was investigated.…”

Photoreductive Chlorine Elimination from a Ni(III)Cl2 Complex Supported by a Tetradentate Pyridinophane Ligand

“…Herein, we report the isolation, characterization, and photoreactivity study of a stable Ni III dichloride complex supported by the tetradentate pyridinophane ligand Me N4, [ Me N4Ni III Cl2] + (1 + ). Although several N-donor supported Ni III monochloride species, 33,34 porphyrin Ni III dibromide complexes, 35 and mixed N/C-donor supported Ni III halide complexes 30,[36][37][38][39][40][41][42][43] were reported, none of them were studied in terms of their photochemistry. To the best of our knowledge, 1 represents the first isolated all N-donor supported Ni III chloride complex for which its photochemistry was investigated.…”