Digallane with Redox-Active Diimine Ligand: Dualism of Electron-Transfer Reactions

The reactivity of digallane (dpp-Bian)­Ga–Ga­(dpp-Bian)
(1), which consists of redox-active ligand 1,2-bis­[(2,6-diisopropylphenyl)­imino]­acenaphthene
(dpp-Bian), has been studied. The reaction of 1 with
I2 proceeds via one-electron oxidation of each of two dpp-Bian
ligands to a radical-anionic state and affords complex (dpp-Bian)­IGa–GaI­(dpp-Bian)
(2). Dissolution of complex 2 in pyridine
(Py) gives monomeric compound (dpp-Bian)­GaI­(Py) (3) as
a result of a solvent-induced intramolecular electron transfer from
the metal–metal bond to the dpp-Bian ligands. Treatment of
compound 3 with B­(C6F5)3 leads to removal of pyridine and restores compound 2. The reaction of compound 1 with 3,6-di-tert-butyl-ortho-benzoquinone (3,6-Q) proceeds with
oxidation of all the redox-active centers in 1 (the Ga–Ga
bond and two dpp-Bian dianions) and results in mononuclear catecholate
(dpp-Bian)­Ga­(Cat) (4) (Cat = [3,6-Q]2–). Treatment of 4 with AgBF4 gives a mixture
of [(dpp-Bian)2Ag]­[BF4] (5) and
(dpp-Bian)­GaF­(Cat) (6), which both consist of neutral
dpp-Bian ligands. The reduction of benzylideneacetone (BA) with 1 generates the BA radical-anions, which dimerize, affording
(dpp-Bian)­Ga–(BA–BA)–Ga­(dpp-Bian) (7). In this case the Ga–Ga bond remains unchanged. Within 10
min at 95 °C in solution compound 7 undergoes transformation
to paramagnetic complex (dpp-Bian)­Ga­(BA–BA) (8) and metal-free compound C36H40N2 (9). The latter is a product of intramolecular addition
of the C–H bond of one of the iPr groups to
the CN bond in dpp-Bian. Diamagnetic compounds 3, 5, 6, and 9 have been characterized
by NMR spectroscopy, and paramagnetic complexes 2, 4, 7, and 8 by ESR spectroscopy.
Molecular structures of 2–7 and 9 have been established by single-crystal X-ray analysis.