2,6-Bis(oxazolinyl)phenylnickel(II) Bromide and 2,6-Bis(ketimine)phenylnickel(II) Bromide:  Synthesis, Structural Features, and Redox Properties

Abstract

2,6-Bis(oxazolinyl)phenylnickel bromide complexes [NiBr(R,R‘-Phebox)] ( 2 ) were synthesized via two synthetic routes (A and B). In route A, selective bis- ortho lithiation of [R,R‘-PheboxBr], followed by a transmetalation reaction with [NiBr 2 (PEt 3 ) 2 ], yielded not only complexes 2 with an η 3 - mer - N, C, N ‘-bonded Phebox ligand but also [NiBr(R,R‘-Phebox)(PEt 3 ) 2 ], 7, where the nickel center is η 1 - C bonded to the intra-annular C ipso of the Phebox ligand. Coordination of two PEt 3 ligands completes the square-planar coordination sphere of the Ni center in 7 . When R = t- Bu, R‘ = H, only complex 7c was formed. Alternatively, when route B (oxidative addition with [Ni(cod) 2 ], cod = cyclooctadiene) was followed, selective formation of complexes 2 was observed. X-ray crystal structures were obtained for [NiBr-( i -Pr,H-Phebox)] ( 2b ) and [NiBr(bis(ketimine)phenyl)] ( 3 ). The Ni centers have square-planar geometries with a planar, η 3 - mer - N, C, N ‘ coordination of the terdentate ligand systems. Complexes 2 were found to be inactive as catalysts in the atom-transfer radical polymerization (ATRP) reaction of methyl methacrylate (MMA) and in the atom-transfer radical addition (ATRA, Kharasch addition) reaction of CCl 4 to MMA. This is ascribed to the relatively high oxidation potential of Ni II −Phebox complexes, which excludes the (reversible) formation of a d 7 -Ni III −Phebox complex, a crucial condition for subsequent reactions. Cyclovoltammetry (CV) experiments (( n -Bu) 4 NBr as supporting electrolyte) showed no electrochemical waves between −1.00 and +1.50 V (Ag/AgCl reference electrode, ( n -Bu) 4 NBr as supporting electrode). Theoretical calculations showed that the energy ( E ox ) needed for the oxidation reaction occurring during the CV experiments is considerably higher for [NiBr(Me,Me-Phebox)] (1.87 eV) and [NiBr(bis(ketimine)phenyl)] (1.90 eV) than for [NiBr(NCN)] ( 1 ) (1.45 eV).