The ‘reverse transmetalation’ reaction of the pyrrole-based PNP pincer Ni(II) complexes: X-ray structures of binuclear silver(I) and thiocyanate nickel(II) complexes

Abstract

Treatment of [{C4H2N-2,5-(CH2PPh2)2-κ3PNP}NiX] (X = Cl (1a), Br (1b), and I (1c)) with the silver salt AgBF4 containing the weakly coordinating anion BF4− yielded two types of complexes: monohalide ion bridged cationic binuclear silver(I) complexes, [Ag2(μ-X){μ-C4H2NH-2,5-(CH2PPh2)2-κ2PP}2][BF4] [X = Cl (2a) and Br (2b)] and the dihalide ions bridged neutral binuclear silver(I) complex, [Ag(μ-I){μ-C4H2NH-2,5-(CH2PPh2)2-κ2PP}]2 (3c). The analogous reaction with AgOTF afforded complex [Ag2(μ-Cl){μ-C4H2NH-2,5-(CH2PPh2)2-κ2PP}2]OTf (2a′). Interestingly, the 31P NMR spectrum of the reaction mixture showed the formation of the cationic complex, [Ni(NCCH3){C4H2N-2,5-(CH2PPh2)2-κ3PNP}][BF4], at the beginning of the reaction, which slowly changes to give the binuclear silver(I) complexes containing the neutral PNP pincer ligand upon crystallization in open atmosphere. Interestingly, the nickel complex 1a reacts with AgSCN containing the coordinating anion SCN− to give both the chlorine-substituted N-bonded thiocyanate neutral complex [Ni(NCS){C4H2N-2,5-(CH2PPh2)2-κ3PNP}] (4) and the dichloride bridged complex [Ag(μ-Cl){μ-C4H2NH-2,5-(CH2PPh2)2-κ2PP}]2 (3a). The structures of all these complexes were determined by single crystal X-ray diffraction studies. The Ni-NNCS bond distance of 1.849(3) Å in 4 is considerably shorter than those reported in the literature.