THEORETICAL AND EXPERIMENTAL-STUDY OF DIAMAGNETIC AND PARAMAGNETIC PRODUCTS FROM THERMAL AND LIGHT-INDUCED ALKYL TRANSFER BETWEEN ZINC OR MAGNESIUM DIALKYLS AND 1,4-DIAZA-1,3-BUTADIENE SUBSTRATES

Ab initio pseudopotential calculations were used to study species formed in thermal and light-induced alkyl transfer reactions between organometallic compounds R'2M (M = Zn, Mg; R' = alkyl, aryl) and 1,4-diaza-1,3-butadienes RN = CHCH = NR (R-DAB). Geometries of the first isolable intermediates, the chelate complexes MR'2.R-DAB, have been optimized. The X-ray structure of the crystalline complex ZnMe2.t-Bu-DAB is presented. Its main features are well-reproduced by the calculations on systems with R = H, Me. The primary complexes exhibit a destabilized HOMO (antisymmetric M-C sigma-bond combination) and a low-lying LUMO (DAB pi* orbital). The energy of the HOMO and the intense long-wavelength HOMO-LUMO transition are strongly influenced by the electronegativity of the carbanionic alkyl group and by the electronegativity of the metal. N,N'-Dimethyl substitution destabilizes the LUMO compared to R = H. Geometries of the lowest triplet-excited states and the radical anions of the primary chelate complexes can be rationalized in terms of the nodal properties of the frontier orbitals, and indicate M-C bond labilization as a prerequisite for alkyl transfer by intramolecular electron transfer in the ground or electronically excited state. In agreement with experiments the M-C bond labilization is generally more pronounced for M = Mg than for the M = Zn analogues. Calculated vertical and adiabatic electron affinities of the primary complexes are both higher for M = Mg than for M = Zn. Calculations on the synthetically useful N- and C-alkylated products formed from the primary complexes by the alkyl transfer reaction indicate a rather small energy difference between these two possible collapse products. ESR data of several thermally or photochemically generated paramagnetic products, the anion radicals of the primary chelate complexes and the neutral radicals MR'.DAB, are in agreement with the calculated geometrical and electronic structures. The electronic structures of all organometallic species are studied by means of localized and canonical MOs as well as Mulliken charges and calculated dipole moments. The dependence of reactivity and product selectivity on substituent characteristics (R, R'), solvation, temperature, and excitation wavelength with respect to the LLCT transition is discussed.