| Abstrakt: |
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&dbd;C and C&tbd1;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-(PR3)2Pd(ER)2 → cis-(PR3)Pd(ER)2 + PR3 show that PR3 dissociation energy from the cis-(PR3)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-(PR3)M(ER)2 intermediate decreases via the sequence E = S > Se >Te and increases via M = Pd < Pt for R = R = H. |
