Evaluation of the Rh(II)–Rh(II) Bond Dissociation Enthalpy for [(TMTAA)Rh]2 by 1H NMR T2 Measurements: Application in Determining the Rh–C(O)– BDE in [(TMTAA)Rh]2CO

Toluene solutions of the rhodium­(II) dimer of dibenzotetramethylaza[14]­annulene ([(TMTAA)­Rh]2; (1)) manifest an increase in the line widths for the singlet methine and methyl 1H NMR resonances with increasing temperature that result from the rate of dissociation of the diamagnetic RhII–RhII bonded dimer (1) dissociating into paramagnetic RhII monomers (TMTAA) Rh (2). Temperature dependence of the rates of RhII–RhII dissociation give the activation parameters for bond homolysis ΔH⧧app = 24(1) kcal mol–1 and ΔS⧧app = 10 (1) cal K–1 mol–1 and an estimate for the RhII–RhII bond dissociation enthalpy (BDE) of 22 kcal mol–1. Thermodynamic values for reaction of 1 with CO to form (TMTAA)­Rh–C­(O)–Rh­(TMTAA) (3) ΔH1° = −14 (1) kcal mol–1, ΔS1°= −30(3) cal K–1 mol–1) were used in deriving a (TMTAA)­Rh–C­(O)– BDE of 53 kcal mol–1.