[(dpp-bian)Ga-Ga(dpp-bian)] and [(dpp-bian)Zn-Ga(dpp-bian)]: Synthesis, molecular structures, and DFT studies of these novel bimetallic molecular compounds

1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphthene) (dpp-bian) stabilizes gallium-gallium and zinc-gallium bonds (compounds 1-3). The compound [(dpp-bian)Ga-Ga(dppbian)] (2) was prepared by the reaction of GaCl3 with K-3[dpp-bian] and the heterometallic [(dpp-bian)Zn-Ga(dppbian)] (3) was prepared by a simple one-pot reaction of [{(dpp-bian)Znl}(2)] with GaCl3 and K-4[dpp-bian]. In contrast to [(dpp-bian)Zn-Zn(dpp-bian)] (1) and 3, compound 2 is ESR silent, thus proving the dianionic character of both dpp-bian ligands. The solution ESR spectrum of 3 reveals the coupling of an unpaired electron with the gallium nuclei Ga-69 and Ga-71 (A(Ga-69)=0.97, A (Ga-71) - 1.23 mT), thus confirming the presence of Zn-Ga bonds in solution. According to the results of the X-ray crystal structure analyses the metal-metal bond lengths in 2 (2.3598(3) angstrom) and 3 (2.3531(8) angstrom) are close to that found in 1 (2.3321(2) angstrom). The electronic structures of compounds 2 and 3 were studied by DFT (B3LYP/6-31G* level). The metal-metal pi bond in 2 is mainly formed by overlap of the p orbitals of Ga in the HOMO and HOMO-1, the latter showing a stronger interaction. The s and p orbitals of Ga overlap in the deeper located HOMO-17 producing a Ga-Ga sigma bond. In contrast to the Zn-Zn bond in 1, which has 95 % s character, the NBO (natural bond order) analysis of 2 reveals 67.8% s, 32.0% p, and 0.2% d character for the Ga-Ga bond. Compound 3 has a doublet electronic ground state. ne unpaired electron occupies the a HOMO-1 localized at the Zn-containing fragment. The Ga-Zn bond is mainly formed by overlap of the metal orbitals in the alpha HOMO-6 and beta HOMO-5. According to the results of the NBO analysis, the Zn wave functions are responsible for 28.7 % of the Zn-Ga bond, with 96.7% s, 1.0% p, and 2.3 % d character.