Single-Molecule Activation and Quantification of Mechanically Triggered Palladium–Carbene Bond Dissociation

Abstract

Metal-complexed N-heterocyclic carbene (NHC) mechanophores are latent reactants and catalysts for a range of mechanically driven chemical responses, but mechanochemical scission of the metal−NHC bond has not been experimentally characterized. Here we report the single-molecule force spectroscopy of ligand dissociation from a pincer NHC−pyridine−NHC Pd(II) complex. The force-coupled rate constant for ligand dissociation reaches 50 s −1 at forces of approximately 930 pN. Experimental and computational observations support a dissociative, rather than associative, mechanism of ligand displacement, with rate-limiting scission of the Pd−NHC bond followed by rapid dissociation of the pyridine moiety from Pd.

Activation of a Copper Biscarbene Mechano‐Catalyst Using Single‐Molecule Force Spectroscopy Supported by Quantum Chemical Calculations

“…Recent work by Razgoniaev et al [16c] . revealed rupture forces in the range of 0.8–1.0 nN for the dissociation of the Pd carbene bond during SMFS experiments, which is considerably lower than the forces observed in this work.…”

Heterocyclic Mechanophores in Polymer Mechanochemistry

“…These include covalent bonds such as diazo, 23 47 C–S, 48–50 C–C (in tetraarylsuccinonitrile), 25,28 , 51–56 C–O, 50 57 58 S–S, 59 60 61 62 (Figure 2a ), and coordinative bonds such as Ru–carbene, Ag–carbene, or Cu–carbene bonds, 16 63 Fe–Cp (in ferrocene), 64 65 66 Pt–acetylide, 67 Fe–pyridine, 68 Cu–naphthalene, 69 Pd–phosphine, 70 and Pd–carbene (Figure 2b ). 71 Covalent bonds in strained ring structures have lower BDEs than the analogous linear structures and can respond to mechanical forces. Strained homocyclic rings, including gem -dihalocyclopropane, 11 , 72 73 74 75 76 77 78 79 80 81 82 cyclobutane, 24 , 83 84 85 86 87 88 89 90 polyladderene, 30 , 91 92 93 94 95 benzocyclobutene, 32 74 , 96 97 98 99 100 cyclopentadiene-maleimide Diels–Alder (DA) adduct, 101 and norbornene, 27 have been reported to be mechanophores (Figure 2c ).…”

“…These include covalent bonds such as diazo, 23,47 C-S, [48][49][50] C-C (in tetraarylsuccinonitrile), 25,28,[51][52][53][54][55][56] C-O, 50,57,58 S-S, [59][60][61][62] (Figure 2a), and coordinative bonds such as Ru-carbene, Ag-carbene, or Cucarbene bonds, 16,63 Fe-Cp (in ferrocene), [64][65][66] Pt-acetylide, 67 Fe-pyridine, 68 Cu-naphthalene, 69 Pd-phosphine, 70 and Pdcarbene (Figure 2b). 71 Covalent bonds in strained ring structures have lower BDEs than the analogous linear structures and can respond to mechanical forces. Strained homocyclic rings, including gem-dihalocyclopropane, 11,[72][73][74][75][76][77][78][79][80][81][82] cyclobutane, 24,[83][84]…”

Effect of Intramolecular Hydrogen Bonds on the Single-Chain Elasticity of Poly(vinyl alcohol): Evidencing the Synergistic Enhancement Effect at the Single-Molecule Level

“…An individual PVA chain would be a more suitable object to study the effect of intramolecular H-bonds because no intermolecular H-bonds between PVA chains are involved. Single-molecule atomic force microscopy (AFM) has been widely exploited to study the mechanical behaviors of individual macromolecules. − The AFM can work in different environments such as liquids, air, and vacuum, which enables single-molecule AFM as a competent method to investigate the influence of a certain factor on the single-chain behavior of macromolecules in a specific environment. − Herein, the effect of intramolecular H-bonds on the single-chain elasticity of PVA has been studied by single-molecule AFM. The average energy of intramolecular H-bonds has a positive relationship with the DA, which suggests a synergistic enhancement effect between the intramolecular H-bonds within PVA.…”