2,6-Bis(oxazolinyl)phenylnickel(II) bromide and 2,6-bis(ketimine)phenylnickel(II) bromide: Synthesis, structural features, and redox properties

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 [NiBr2(PEt3)(2)], yielded not only complexes 2 with an eta(3)-mer-N,C,N'-bonded Phebox ligand but also [NiBr(R,R'-Phebox)(PEt3)(2)], 7, where the nickel center is eta(1)-C bonded to the intra-annular C-ipso of the Phebox ligand. Coordination of two PEt3 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, eta(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 CCl4 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).