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

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.