Theoretical studies of oxidative addition of E-E bonds (E = S, Se, Te) to palladium(0) and platinum(0) complexes

The density functional method has been applied to investigate the mechanism and controlling factors of RE-ER (R = H, Me and E = S, Se, Te) oxidative addition to M(PR3')(2) complexes (where M = Pd, Pt and R' = H, Me), which is proposed to be the first step of Pd(0)- and Pt(0)-catalyzed E-E addition to C=C and C C bonds. In general, it was shown that the energy of E-E activation correlates with the E-E bonding energy and decreases via the sequence E = S > Se > Te, for all R, R' and transition-metal atoms used; the weaker the E-E bond, the smaller the oxidative addition barrier. The exothermicity of this reaction also decreases via the same trend, E = S > Se > Te, and correlates with the decrease in M-ER bond strength. Meanwhile, the E-E activation barrier is found to be higher for M = Pt than for M = Pd, while for all studied R, R', and E the reaction is found to be more exothermic for M = Pt than for M = Pd. It was shown that the more the methyl substitution in the systems (both in substrate and the catalyst), the larger the E-E activation barrier. Calculations of the energetics of the reaction cis-(PR'(3))(2)Pd(ER)(2) -> cis-(PR'(3))Pd(ER)(2) + PR'(3) show that PR'3 dissociation energy from the cis-(PR'3)2Pd(ER)2 complex decreases (a) via the sequence E = S > Se > Te for given M and R = R' and (b) via the trend M = Pt > Pd for given E and R = R'. The exothermicity of dimerization of the cis-(PR'(3))M(ER)(2) intermediate decreases via the sequence E = S > Se > Te and increases via M = Pd < Pt for R = R' = H.