Bis-Catecholate, Bis-Dithiocatecholate, and Tetraalkoxy Diborane(4) Compounds: Aspects of Synthesis and Electronic Structure

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Abstract

The synthesis and characterization of a series of bis-catecholate diborane(4) compounds, B2(1,2-O2C6H4)2 (3), B2(1,2-O2-3-MeC6H3)2 (6), B2(1,2-O2-4-MeC6H3)2 (7), B2(1,2-O2-4-ButC6H3)2 (8), B2(1,2-O2-3,5-But2C6H2)2 (9), B2(1,2-O2-3-MeOC6H3)2 (10), bis-dithiocatecholate diborane(4) compounds, B2(1,2-S2C6H4)2 (13), B2(1,2-S2-4-MeC6H3)2 (14), and tetraalkoxy diborane(4) compounds, B2(OCH2CMe2CH2O)2 (11) and B2(OCMe2CMe2O)2 (12) from B2(NMe2)4 (1) is described, as are the bis(NHMe2) adducts of 3 and 9, namely [B2(1,2-O2C6H4)2(NHMe2)2] (4) and [B2(1,2-O2-3,5-But2C6H2)2(NHMe2)2] (5). The latter two compounds are intermediates in the formation of 3 and 9 from 1. Compound 1 is synthesized by reductive coupling of BCl(NMe2)2, which in turn is prepared from reaction of BCl3 with B(NMe2)3 in a 1:2 stoichiometry. We have also characterized [B2Cl4(NHMe2)2] (15) formed from addition of HCl to 1 prior to complete reaction with diols, and the salt, [NH2Me2][B(1,2-O2C6H4)2] (16), which arises from addition of catechol to B(NMe2)3. Thus, any B(NMe2)3 impurity present after the preparation of 1 needs to be removed by distillation prior to reaction with alcohols. The dimer, [BCl2(μ-NMe2)]2 (17) has also been characterized. This is formed from reaction of BCl3 with B(NMe2)3 if a 2:1 rather than 1:2 stoichiometry is used. Photoelectron spectra of 1, 3, 8, 11, and 12 are reported along with those of the corresponding diols, catechol, 4-But-catechol, 2,2-dimethyl-1,3-propanediol, and pinacol. The ionization energies of the B2(OR)4 compounds follow the series 8 < 3 < 12 < 11. Replacement of O for N in the B2N4 framework increases the IE by ca. 1.65 eV, whereas the presence of an aromatic ring rather than an aliphatic chain decreases the IE by ca. 1.50 eV. The presence of electron donating But-groups also decreases the IE.

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