Synthesis, structure, and reactivity of iridium perfluorocarbene complexes: regio- and stereo-specific addition of HCl across a metal carbon double bond

Abstract

Reductive activation of an α-fluorine in the perfluoroalkyl complexes Cp*(L)(I)Ir–CF₂R_F using Mg/graphite leads to perfluorocarbene complexes Cp*(L)IrCFR_F (L = CO, PMe₃; R_F = CF₃, C₂F₅, C₆F₅). New complexes E-Cp*(PMe₃)IrCFC₂F₅ and E-Cp*(CO)IrCFC₆F₅ have been characterized by single crystal X-ray diffraction studies, and a comparison of metric parameters with previously reported analogues is reported. Experimental NMR and computational DFT (B3LYP/LACV3P**++) studies agree that for IrCFR_F complexes (R_F = CF₃, CF₂CF₃) the thermodynamic preference for the E or Z isomer depends on the steric requirements of ligand L; when L = CO the Z-isomer (F cis to Cp*) is preferred and for L = PMe₃ the E-isomer is preferred. When reduction of the precursors is carried out in the dark the reaction is completely selective to produce E- or Z-isomers. Exposure of solutions of these compounds to ambient light results in slow conversion to a photostationary non-equilibrium mixture of E and Z isomers. In the dark, these E/Z mixtures convert thermally to their preferred E or Z equilibrium geometries in an even slower reaction. A study of the temperature dependent kinetics of this dark transformation allows ΔG^‡₂₉₈ for rotation about the IrCFCF₃ double bond to be experimentally determined as 25 kcal mol⁻¹; a DFT/B3LYP/LACV3P**++ calculation of this rotation barrier is in excellent agreement (27 kcal mol⁻¹) with the experimental value. Reaction of HCl with toluene solutions of Cp*(L)IrCFR_F (L = CO, PMe₃) or Cp*(CO)IrC(CF₃)₂ at low temperature resulted in regiospecific addition of HCl across the metal carbon double bond, ultimately yielding Cp*(L)Ir(CHFR_F)Cl and Cp*(CO)Ir[CH(CF₃)₂]Cl. Reaction of HCl with single E or Z diastereomers of Cp*(L)IrCFR_F gives stereospecific cis-addition to give single diastereomers of Cp*Ir(L)(CHFR_F)Cl; addition of HCl to several different E/Z ratios of Cp*(L)IrCFR_F affords ratios of diastereomeric products Cp*(L)Ir(CHFR_F)Cl identical to the original ratio of starting material isomers. The addition of HCl is therefore demonstrated to be unambiguously regio- and stereo-specific. The observed product regiochemistry of addition of HCl to IrCF₂, IrCFR_F, and IrC(CF₃)₂ ligands is the same and is not dependent on the ground state energy preference (singlet or triplet) for the free perfluorocarbene. DFT calculations on model HCl addition reactions indicate that this regiochemistry is strongly preferred thermodynamically, but predict that in H^δ+–Cl^δ− addition to Cp(PH₃)IrCF₂, H^δ+ attack at Ir has a lower energy transition state, while for Cp(PH₃)IrCFCF₃ and Cp(PH₃)IrC(CF₃)₂, H^δ+ attack at C is the kinetically preferred pathway. The carbene carbon atoms in IrCFCF₃ and IrC(CF₃)₂ complexes are unambiguously basic towards HCl, while in the IrCF₂ analogues the carbene carbon is less basic than its Ir partner, and the eventual regiochemistry of HCl addition arises from thermodynamic control.