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

By M. Stol, D.J.M. Snelders, M.D. Godbole, R.W.A. Havenith, D.M. Haddleton, G. Clarkson, M. Lutz, A.L. Spek, G.P.M. van Klink and G. van Koten

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 [NiBr2(PEt3)2], yielded not only complexes 2 with an 3-mer-N,C,N'-bonded Phebox ligand but also [NiBr(R,R'-Phebox)(PEt3)2], 7, where the nickel center is 1-C bonded to the intra-annular Cipso 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, 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 NiII-Phebox complexes, which excludes the (reversible) formation of a d7-NiIII-Phebox complex, a crucial condition for subsequent reactions. Cyclovoltammetry (CV) experiments ((n-Bu)4NBr as supporting electrolyte) showed no electrochemical waves between -1.00 and +1.50 V (Ag/AgCl reference electrode, (n-Bu)4NBr as supporting electrode). Theoretical calculations showed that the energy (Eox) 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)

Year: 2007
OAI identifier: oai:dspace.library.uu.nl:1874/26821
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