Sterically Protected Titanium (Aminoethyl)dicarbollides: Synthesis of Novel Constrained-Geometry Complexes Showing an Unusual Cage B<i>,</i>N-Cyclization

Abstract

The syntheses of group 4 metal complexes containing the ((dibenzylamino)ethyl)dicarbollide ligand DcabHC(2)N (nido-7-HNBz2(CH2)2-8-R-7,8-C2B9H10; 2) are reported. Various new types of constrained-geometry complexes [{(η5-RC2B9H9)(CH2)2(η1-NBz2)}MCl2] (M = Ti (3), Zr (4); R = H (a), Me (b)) were prepared by the reaction of the potassium salt of 2 with titanium or zirconium tetrachloride. The reaction of 2 with Ti(NMe2)4 in toluene affords [{(η5-RC2B9H9)(CH2)2(η1-NBz2)}Ti(NMe2)2] (5), which readily reacts with Me3SiCl to yield the corresponding chloride complex 3. The structure of the constrained-geometry complex [{(η5-C2B9H10)(CH2)2(η1-NBz2)}Ti(NMe2)2] (5a) was established by X-ray diffraction, which showed an η5:η1 bonding mode derived from the dicarbollylamino ligand fuctional group. In contrast, the reaction of 2 with Zr(NMe2)4 produced the untethered half-metallocene complexes [{(η5-RC2B9H9)(CH2)2(NBz2)}Zr(NMe2)2(NHMe2)] (6). The identity of 6 was confirmed by single-crystal X-ray diffraction. Due to coordination of free dimethylamine to the zirconium metal center, the sterically hindered dibenzylamine sidearm leaves the coordination sphere to form the corresponding untethered complexes. Furthermore, the titanium(IV) CGC complexes 3 exhibited unusual B,N-cyclization when reacted with O2, leading to the production of novel exocyclic dicarbollides (7). The crystallographic data were consistent with the formation of an unusual five-membered exocyclic ring, presumably due to steric interactions between the dibenzyl units and the dicarbollyl group. Finally, the sterically less bulky zirconium(IV) ((benzylamino)ethyl)dicarbollyl complex [{(η5-CH3C2B9H9)(CH2)(η1-NBz)}ZrCl2(THF)] (10b), in which the pendant amine is coordinated to the metal center, was prepared by the reaction of ZrCl4 with the potassium salt of [nido-7-NH2Bz+(CH2)-8-H-7,8-C2B9H10-] (9b) in toluene.