Thermodynamics of Pyridine Coordination in 1,4-Phenylene Bridged Bimetallic (Pd, Pt) Complexes Containing Two N,C,N‘ Motifs, 1,4-M2-[C6(CH2NR2)4-2,3,5,6]

Abstract

The thermodynamics of pyridine coordination in 1,4-phenylene-bridged binuclear palladium and platinum organometallic complexes [1,4-(MOTf)₂-{C₆(CH₂NR₂)₄-2,3,5,6}] (11, M = Pd, Pt; R = CH₃, C₂H₅, R₂ = −(CH₂)₅−) are measured by ¹H NMR in DMSO-d₆. The coordination of substituted pyridines by bimetallic complexes 11 or 12 in DMSO is found to proceed via two effectively independent metalligand binding events, and the association constants for pyridine coordination and rate constants for pyridine exchange are nearly identical to those measured previously on monometallic analogs. A linear free energy relationship between the association constant for pyridine coordination and the inductive Hammett constant of the pyridine substituent is observed, and the sensitivity (ρ = −1.7 to −2.1) in DMSO depends only slightly on metal (Pd vs Pt) and spectator ligand (pincer dialkylamine vs triarylphosphine). The association constant for a particular pyridine ligand, however, varies by roughly 3 orders of magnitude across the series of metal complexes. The effective independence of the two coordination sites and the range of available thermodynamic and kinetic behaviors of the coordination guide the use of these versatile building blocks in metallosupramolecular applications.