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].

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 (rho = -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.

Single-molecule force spectroscopy of bimolecular reactions: system homology in the mechanical activation of ligand substitution reactions.

The mechanochemistry of the bimolecular nucleophilic substitution of DMSO for substituted pyridines at a square-planar pincer Pd(II) center was investigated using single-molecule force spectroscopy (SMFS). The SMFS data are interpreted in terms of the Bell-Evans model, which gives thermal off-rates for two reactions that agree well with previous, stress-free measurements. The characteristic force dependency of the rupture rate, fbeta, is effectively constant for the two reactions examined (22 +/- 2 and 24 +/- 2 pN), and the system homology in the mechanical response is consistent with expected similarities in the reaction potential energy surfaces.

Small-molecule dynamics and mechanisms underlying the macroscopic mechanical properties of coordinatively cross-linked polymer networks.

Specific metal-ligand coordination between bis-Pd(II) and Pt(II) organometallic cross-linkers and poly(4-vinylpyridine) in DMSO defines a three-dimensional associative polymer network. Frequency-dependent dynamic mechanical moduli of a series of four different bulk materials, measured across several decades of oscillatory strain rates, are found to be quantitatively related through the pyridine exchange rates measured on model Pd(II) and Pt(II) complexes. Importantly, the mechanism of ligand exchange in the networks is found to be the same solvent-assisted pathway observed in the model complexes, and so the bulk mechanical properties are determined by relaxations that occur when the cross-links are dissociated from the polymer backbone. It is how often the cross-links dissociate, independently of how long they remain dissociated, that determines the bulk mechanical properties. The quantitative relationship between bulk materials properties and the kinetics and mechanisms observed in model compounds holds promise for the rational, molecular design of materials with tailored mechanical properties.

Orthogonal control of dissociation dynamics relative to thermodynamics in a main-chain reversible polymer.

A pair of homologous and soluble reversible polymers display nearly identical solution structures but dramatically different dissociation dynamics (1.0 vs ca. 100 s-1) along their main chains. The polymers are formed by organopalladium-pyridine coordination, and steric effects at the metal center control the dynamics. Importantly, the association constant is not significantly affected by the sterics. The resulting orthogonal control over dissociation kinetics provides a general tool for studying the effects of dynamics independent of thermodynamics in supramolecular systems, particularly main-chain reversible polymers.