Mechanical Force Modulated Organometallic Transformations

Mechanical forces are known to drive a range of covalent chemical reactions and have a number of applications, including access to new reaction pathways, polymer transformations, degradable polymers, stress/strain sensing in bulk materials, and the release of small molecules/protons. In switchable catalysis, mechanical forces have been mainly exploited to activate latent catalysts by unplugging inhibiting ligands. However, mechanical forces offer more opportunities beyond breaking bonds due in part to the reversibility and continuous/wide adjustability. As an complementary strategy, force may be applied to a spectator ligand to toggle the structure and reactivity of the transition metal complex incrementally and reversibly among multiple states, without incurring scissile events. In this dissertation, we study force-activity relationships of elementary steps and isolated catalytic transformations under this strategy, to build knowledge toward such multi-state mechanocatalysts. We introduce force probe ligands, a series of macrocyclic bis(phosphine) ligands containing a stiff-stilbene photoswitch, as tools to quantify force effect (Chapter 2). Each force probe ligand has a known force applied to the bis(phosphine), which is quantified by DFT methods. When employed in transition metal complexes, force probe ligands enables the measurements of force-dependent properties. In Chapter 3, we quantify the rate of C(sp2)–C(sp2) reductive elimination from platinum(II) diaryl complexes containing force probe ligands as a function of mechanical force applied to these ligands, as our first step toward force-dependent elementary step reactivity. DFT computations reveal complex dependence of mechanochemical kinetics on the structure of the force-transducing ligand. We experimentally validated the computational finding for the most sensitive of the ligand designs, based on MeOBiphep, by coupling it to a macrocyclic force probe ligand. Consistent with the computations, compressive forces decreased the rate of reductive elimination whereas extension forces increased the rate relative to the strain-free MeOBiphep complex with a 3.4-fold change in rate over a ~290 pN range of restoring forces. The calculated natural bite angle of the free macrocyclic ligand changes with force, but 31P NMR analysis and calculations strongly suggest no significant force-induced perturbation of ground state geometry within the first coordination sphere of the (P–P)PtAr2 complexes. Rather, the force/rate behavior observed across this range of forces is attributed to the coupling of force to the elongation of the O…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 strategies for force-modulated catalysis. In Chapter 4, we move forward to mechanistic study on how forces are coupled to reactions by kinetic experiments on the stilbene isomerization (E to Z) of force probe ligand with/without platinum(II) coordination. We obtained the activation energies of free force probe ligands and (P–P)PtCl2 complexes, and results reveal the energy difference between free ligand and coordinated complex increases with restoring force, with ~ 6 kcal/mol activation energy difference change over ~ 120 pN ranges of forces on force probe ligands. We further simulated the activation energies of untethered stiff-stilbene under different tension, and found a decent consistency of computational data with empirical activation energies for free ligand. Taking the simulated energy/force relationship as a calibration curve, we estimated force experienced by stilbene in (P–P)PtCl2 complexes, which showed > 100 pN can be generated through Pt(II) coordination. The results suggest an allosteric effect by distal metal-ligand coordination can generate large forces and could drive orders-of-magnitude (up to ~104) changes in the rate of a coupled unimolecular trans/cis alkene isomerization. In Chapter 5, we quantify the rate of C(sp3)–C(sp2) reductive elimination of N,N,4-trimethylaniline from palladium(II) methyl aryl complexes employing force probe ligands, as an effort to explore other useful scopes and metal influence in force sensitivity. Analysis of the resulting first-order rate constants revealed that the rate of reductive elimination was largely invariant of ligand restoring force, as kobs varied by < 20% across the series of ligands employed. Different from the aforementioned (P–P)PtCl2 complexes, (P–P)PdArMe complexes are not stable at ambient conditions and thus generated in situ for kinetic experiments, which introduced 2 equiv. of bromide. We propose the formation of anionic complex [(P–P)PdBrArMe]- under this condition deactivates force coupling to the reductive elimination pathway. Finally, in Chapter 6, we close the catalytic cycle by demonstrations of isolated catalytic transformations of Rh(I)-catalyzed hydroformylation of 1-octene/styrene and Cu–H-catalyzed hydrosilylation of acetophenone, likewise employing force probe ligands. Over a range of ~230 pN, we found the linear to branch regioselectivity of 1-octene hydroformylation changed ~1.7 folds, and the enantioselectivity of styrene hydroformylation changed ~ 10% in ee. Cu–H-catalyzed hydrosilylation of acetophenone showed ~ 20% increase in ee as force decreases over ~ 290 pN. Low temperature NMR studies indicate the structure of (P–P)RhH(CO)2 complexes, the key intermediate that determines selectivity, remains the same across the series of ligands applied with regard to equatorial/equatorial or equatorial/apical bis(phosphine) coordination modes, while vast changes in selectivity are often resulted from changes in the coordination modes. Therefore, we propose force couples to mentioned reactions as a dynamic effect, in contrast to toggling the intermediate structure. With the limited force range accessible with this series of force probe ligand, observed changes in reaction selectivity are also limited. However, this research provides a bridge from elementary step study to polymeric matrix-supported switchable mechanocatalysis, in which wider range of forces can be achieved to provide opportunities in better force-regulated transformations.