Exceptional Sensitivity of Metal−Aryl Bond Energies to <i>ortho</i>-Fluorine Substituents: Influence of the Metal, the Coordination Sphere, and the Spectator Ligands on M−C/H−C Bond Energy Correlations

Abstract

DFT calculations are reported of the energetics of C−H oxidative addition of benzene and fluorinated benzenes, ArFH (ArF = C6FnH5−n, n = 0−5) at ZrCp2 (Cp = η5-C5H5), TaCp2H, TaCp2Cl, WCp2, ReCp(CO)2, ReCp(CO)(PH3), ReCp(PH3)2, RhCp(PH3), RhCp(CO), IrCp(PH3), IrCp(CO), Ni(H2PCH2CH2PH2), Pt(H2PCH2CH2PH2). The change in M−C bond energy of the products fits a linear function of the number of fluorine substituents, with different coefficients corresponding to ortho-, meta-, and para-fluorine. The values of the ortho-coefficient range from 20 to 32 kJ mol−1, greatly exceeding the values for the meta- and para-coefficients (2.0−4.5 kJ mol−1). Similarly, the H−C bond energies of ArFH yield ortho- and para-coefficients of 10.4 and 3.4 kJ mol−1, respectively, and a negligible meta-coefficient. These results indicate a large increase in the M−C bond energy with ortho-fluorine substitution on the aryl ring. Plots of D(M−C) vs D(H−C) yield slopes RM−C/H−C that vary from 1.93 to 3.05 with metal fragment, all in excess of values of 1.1−1.3 reported with other hydrocarbyl groups. Replacement of PH3 by CO decreases RM−C/H−C significantly. For a given ligand set and metals in the same group of the periodic table, the value of RM−C/H−C does not increase with the strength of the M−C bond. Calculations of the charge on the aryl ring show that variations in ionicity of the M−C bonds correlate with variations in M−C bond energy. This strengthening of metal−aryl bonds accounts for numerous experimental results that indicate a preference for ortho-fluorine substituents