Hydrogen bonded supramolecular assemblies based on neutral square-planar palladium(ii) complexes

Abstract

An isomeric series of seven, square-planar, neutral dichloropalladium(II) complexes cis-PdCl₂{P,P-C₆H₃N(CH₂PPh₂)₂(CO₂H)(OH)} 2a–2g have been synthesised in high yields (70–99%) from PdCl₂(cod) (cod = cycloocta-1,5-diene) and the appropriate ligands C₆H₃N(CH₂PPh₂)₂(CO₂H)–(OH) 1a–1g in CH₂Cl₂. The new ditertiary phosphine C₆H₃N(CH₂PPh₂)₂(3-CO₂H)(4-CO₂H) 3, bearing two remote carboxylic acid groups on the N-arene backbone, was prepared by condensation of H₂NC₆H₃(3-CO₂H)(4-CO₂H) with 2 equiv. of Ph₂PCH₂OH in MeOH. The palladium(II) complex cis-{P,P-PdCl₂C₆H₃N(CH₂PPh₂)₂(3-CO₂H)(4-CO₂H)} 4 was synthesised in an analogous fashion to 2a–2g. Solution NMR (³¹P{¹H}, ¹H), FT-IR, ms and microanalytical data are in full agreement with the proposed structures. Single crystal X-ray studies confirm that, in each case, compounds 2a–2e and 4 are square-planar with normal Pd–P [range 2.2255(11)–2.2529(5) Å]/Pd–Cl [range 2.3292(14)–2.3640(16) Å] bond lengths and P–Pd–P [range 91.586(18)–97.25(4)°] bond angles. These studies also demonstrate the importance of predisposition, with respect to the –N(CH₂PPh₂)₂, –CO₂H and –OH (or a second –CO₂H as in 4) functionalities around the benzene core, and the choice of recrystallisation solvent used with regards to the supramolecular structures observed here for the palladium(II) complexes 2a–2e and 4. Hence in 2a·MeOH, intermolecular O–H⋯Cl hydrogen bonding associates two molecules into a dimer pair with the formation of a 20-membered ring. Two different hydrogen bonded motifs are observed in 2b·0.5Et₂O namely an intermolecular O–H⋯O dimer pair and a 1-D polymeric chain through bifurcated O–H⋯Cl hydrogen bonding. Both 2c·MeOH and 2d·CHCl₃ have 1-D chain structures using intermolecular O–H⋯Cl hydrogen bonding between neighbouring molecules (for 2d·CHCl₃) or a combination of O–H⋯O (with a MeOH solvent) and O–H⋯Cl hydrogen bonding (for 2c·MeOH). The structures of 2e·CHCl₃ and 2a·CHCl₃ have similar features in common namely a classic H-bonded head-to-tail arrangement of carboxylic acid groups between molecules and additional intermolecular O–H⋯Cl H-bonds giving rise to ladder structures. Each ring is either 38- (for 2e·CHCl₃) or 34-membered (for 2a·CHCl₃) in size. In 4·2.5MeOH dimers are connected via MeOH solvates to give a 2-D infinite H-bonded network. The X-ray structures of 4·3DMSO, 2e·2DMSO and 2a·DMSO have also been determined. Whilst 4·3DMSO and 2e·2DMSO form discrete structures, 2a·DMSO forms a 1-D chain through O–H⋯Cl hydrogen bonding. The Pd⋯Pd contacts for all these palladium(II) complexes lie clearly within the non bonding range [7.35–20.69 Å].