Synthesis and electrospray mass spectrometry of palladium(II) diphosphine complexes from oxidative addition of 2-bromopyridine to Pd0

Abstract

Oxidative addition reactions of palladium(0) phosphine complexes with 2-bromopyridine gave rise to a series of structurally distinctive complexes, namely trans-(N,P)-[Pd2Br2(PPh3) 2(μ-C5H4N-C2,N)2] 1, [Pd2(μ-C5H4N-C2,N) 2-(μ-dppm)2]Br2 2, [Pd2(η1-dppp)2-(μ-C5H 4N-C2,N)2(μ-dppp)]Br2 3, trans-[{PdBr(η1-C5H4N-C 2)(μ-dppb)}n] 4, trans-(N,P)-[Pd2Br2(μ-C5H 4N-C2,N)2(μ-dppb)] 5 and cis-[PdBr(η1-C5H4NH-C 2)(η2-dppf)]Br 6 [Ph2P(CH2)nPPh2, n = 1(dppm), 3(dppp) or 4(dppb); dppf= Fe(Ph2PC5H4)2]. Similarly, trans-(N,P)-[Pd2Cl2(μ-C9H 6N-C2,N)2(μ-dppb)] 7 has been obtained from 2-chloroquinoline and [Pd(dppb)2]. An array of structural possibilities is envisaged based on the different co-ordination modes of the pyridine (C or/and N bonded; terminal or bridging; pyridyl, pyridine or pyridinium), phosphine (terminal, bridging or chelating) bromide (terminal or ionised) ligands. Complexes 2 and 3, but not the others, can be obtained from phosphine exchange reactions of 1. Complexes 5 and 6 were analysed by X-ray single-crystal crystallographic methods. The former reveals a dinuclear structure with a dppb ligand bridging diagonally two metals that are juxtaposed by two syn-bridging pyridyl groups. It represents an unusual dinuclear core stabilised by two types of bridging ligands of contrasting steric and geometric demands. The latter shows a cationic and mononuclear square planar palladium(II) complex containing a chelating dppf, terminal bromide and an unusual C-bonded pyridyl group with the N-site protonated. The fragmentation of these complexes was investigated by electrospray mass spectrometry under different cone voltages. Breakdown of the dinuclear framework is facilitated by addition of H-Br to the N-Pd bonds of the bridging pyridyl group.