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-C ²,N)₂] 1, [Pd₂(µ-C₅H₄N-C ²,N)₂(µ-dppm)₂]Br₂ 2, [Pd₂(η¹-dppp)₂(µ-C₅H₄N-C ²,N)₂(µ-dppp)]Br₂ 3, trans-[{PdBr(η¹-C₅H₄N-C ²)(µ-dppb)}_n] 4, trans-(N,P)-[Pd₂Br₂(µ-C₅H₄N-C ²,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-C₉H₆N-C ²,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.