Dicarbollylamine ligand as a tunable template for sigma,sigma- and pi,sigma-bonding modes: Syntheses, structures, and theoretical studies of eta(5):eta(1)-coordinated constrained-geometry group 13 metal complexes

Abstract

A series of group 13 main group complexes with pi,sigma-type bonding interaction of the formula [{(eta(5)-RC(2)B(9)H(9))(CH(2))(eta(1)-NMe(2))}MMe] (M = Al, R = H 5, Me 6; Ga, R = H 7, Me 8; In, R - H 9, Me 10) was produced by the reaction of group 13 metal alkyls (MMe(3); M = Al, Ga, In) with the dicarbollylamine ligands, nido-8-R-7,8-C(2)B(9)H(10)-7-(CH(2))NHMe(2) (R = H 1, Me 2). The reactions of 1 and 2 with AlMe(3) in toluene initially afforded tetra-coordinated aluminum complexes with sigma,sigma-type bonding interaction, [{(eta(1)-RC(2)B(9)H(10))(CH(2))(eta(1)-NMe(2))}AlMe(2)] (R = H 3, Me 4), which readily underwent further methane elimination to yield the corresponding constrained geometry complexes (CGCs, 5 and 6) of aluminum with pi,sigma-bonding interaction. However, the reactions between I and 2 and MMe3 (M = Ga, In) in toluene produced gallium and indium pi,sigma-CGCs of 7 and 10 directly, not proceeding through sigma,sigma-intermediates. The structures of group 13 metal CGCs were established by X-ray diffraction studies of 5, 6, and 8, which authenticated a characteristic eta(5):eta(1)-coordination mode of the dicarbollylamino ligand to the group 13 metals. A similar pi,sigma-bonding interaction was also established in ethylene-bridged dicarbollylethylamine series. Thus, aluminum pi,sigma-CGCs of dicarbollylethylamine, [{(eta(5)-RC(2)B(9)H(9))(CH(2))(2)(eta(1)-NBZ(2))}AlMe] (R = H 17, Me 18), were prepared by the trans-metalation of the [{(eta(5)-RC(2)B(9)H(9))(CH(2))(2)(eta(1)-NBZ(2))}Ti(NMe(2))(2)] (R = H 15, Me 16) with AlMe(3). However, only sigma,sigma-bonded complexes of the formula [{(eta(1)-RC(2)B(9)H(9))(CH(2))(2)(eta(1)-NBz(2))}AlMe(2)] (R = H 13, Me 14) were isolated by the reaction between [nido-7-8-R-7,8-C(2)B(9)H(10)-(CH(2))(2)HNBz(2)] (R = H 11, Me 12) and AlMe(3). When methane-elimination reactions between metal alkyls; and dicarbollylamines were carried out with either the gallium atom or monobenzyl aminoethyl tethered ligands, [nido-7-H(2)NBz(CH(2))(2)-8-R-7,8-C(2)B(9)H(10)] (R = H 21, Me 22), desired pi,sigma-CGCs, [{(eta(5)-RC(2)B(9)H(9))(CH(2))(2)(eta(1)-NBz(2))}GaMe] (R = H 19, Me 20) or [{(eta(5)-RC(2)B(9)H(9))(CH(2))(2)(eta(1)-NHBz)}AlMe] (R = H 23, Me 24), were generated, respectively. DFT calculation on 5 provides evidence of existence of pi,sigma-bonding of dicarbollylamine ligand to the aluminum atom: g-bonding interaction of a dicarbollyl unit becomes intensified in the presence of a weak a-bonding amine-tethered group. Furthermore, preference for the formation of pi,sigma-bonding was predicted by optimizing a reaction profile including sigma,sigma- and pi,sigma-structures as well as transition state structures for each methylene-and ethylene-spaced ligand system, 3-5 and 14-18, to reveal that pi,& siga;-bonding interaction is more favorable in the case of a methylene-tethered ligand system.