Thermal Stability of Alkylcopper(I) Complexes Coordinated with Tertiary Phosphines

Abstract

Abstract Thermolysis of isolated alkylcopper(I) complexes having tertiary phosphine ligand(s) RCuLn (R=CH3, C2H5, n-C3H7, i-C4H9) in the solid state and in toluene has been studied in order to clarify the role of the tertiary phosphine ligand in stabilizing the alkylcopper(I) complexes. The distribution of the thermolysis products shows that the thermolysis of the alkylcopper complexes having ethyl or longer alkyl chain proceeds through a β-elimination mechanism. The rate of the thermolysis obeys the first order rate law, −d[RCuLn]/dt=k[RCuLn]. The activation parameters of the thermolysis were obtained from the temperature dependence of k and a compensation effect between ΔH\eweq and ΔS\eweq was demonstrated. The activation energy Ea of the thermolysis of RCu(PPh3)2 increases in the order of R=C2H5<n-C3H7<i-C4H9. Coordination of electron-donating tertiary phosphine decreases Ea and a linear relationship between ν(Cu–CH3) frequencies and Ea for thermolysis of the methylcopper complexes was observed. Addition of PPh3 into the toluene solutions of CH3Cu(PPh3)2(diethyl ether)0.5 and C2H5Cu(PPh3)2 causes some retardation of the thermolysis. The major role of the tertiary phosphine ligand in stabilizing the alkylcopper complexes is proposed to strengthen the Cu–C bond by changing the electronic state of copper atom. An energetic consideration based on Chatt and Shaw’s theory is given in order to account for the results concerning the stabilities of the alkylcopper complexes. Photolysis of the alkylcopper complexes takes a quite different decomposition pathway producing mainly RH from RCuLn.