Force-Modulated Reductive Elimination from Platinum(II) Diaryl Complexes

Abstract

<div><p>Coupled mechanical forces are known to drive a range of covalent chemical reactions, but the interplay of mechanical force applied to a spectator ligand and transition metal reactivity is relatively unexplored. Here we report the effect of mechanical force on the rate of C(sp<sup>2</sup>)-C(sp<sup>2</sup>) reductive elimination from platinum(II) diaryl complexes containing macrocyclic bis(phosphine) force probe ligands. Compressive forces decreased the rate of reductive elimination whereas extension forces increased the rate of reductive elimination relative to the strain-free MeOBiphep complex with a 3.4-fold change in rate over a ~290 pN range of restoring forces. The natural bite angle of the free ligand changes with force, but <sup>31</sup>P NMR analysis strongly suggests no significant force-induced perturbation of the ground state geometry of the (P–P)PtAr<sub>2</sub> complexes. Rather, the force/rate behavior observed across this range of forces (from ca. 65 pN in compression to >200 pN in extension) for reductive elimination is attributed to the coupling of force to the elongation of the O<b><sup>…</sup></b>O distance in the transition state for reductive elimination. The results suggest opportunities to experimentally map geometry changes associated with reactions in transition metal complexes and potential strat-egies for force-modulated catalysis. </p></div><br>

Mechanochemical Regulation of Oxidative Addition to a Palladium(0) Bisphosphine Complex

“…For example, tension accelerates the complementary process of reductive elimination from Pt complexes bearing the same family of bisphosphine ligands employed here. 22 The opposite dependence is observed here; oxidative addition is decelerated by tension (accelerated by compression). The inverse dependencies of reductive elimination and oxidative addition are consistent with these two processes traversing a similar transition state in opposite directions and thus experiencing opposite changes in the P−M−P bond angles on the reaction coordinate proceeding to the transition state.…”

“…The magnitude of the effect observed here, ∼6-fold decrease in rate over ∼300 pN of force, is comparable to the 3fold acceleration of reductive elimination across a similar range of forces. 22 The effect of mechanical force on rate can be thought of as arising from two mechanistic extremes: 47,48 (i) "static" mechanical distortions to the geometry of the complex, such as a change in L-M-L bond angle or M-L bond lengths or (ii) "dynamic" mechanochemical coupling of force to the nuclear motions associated with the progress of the reaction from reactant to transition state. The former can alter catalytic activity either by changing the sterics of the active site or adjusting the energy and shape of molecular orbitals that participate in catalytic reactions and are akin to the perturbations generally taken into account when evaluating classic bite angle effects.…”

“…21 More recently, mechanical forces coupled to a bidentate ligand were shown to affect the rate of a common elementary reaction, the reductive elimination from a platinum diaryl complex, with a tensile force accelerating and compressive force inhibiting this reaction. 22 A better understanding of how force applied to ligand spheres alters the reactivity of the metal center offers hitherto unrecognized opportunities in catalysis. Many organometallic reactions occur through multiple microscopic steps between discrete intermediates, and the kinetics of those steps is often affected differentially by ligand geometry.…”