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New Types of Constrained Geometry Group 4 Metal Complexes Derived from the Aminomethyldicarbollyl Ligand System:  Synthesis and Structural Characterization of Mono-dicarbollylamino and Bis-dicarbollylamino Group 4 Metal Complexes

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journal contribution
posted on 03.04.2020, 14:24 by Jong-Dae Lee, Young-Joo Lee, Ki-Chul Son, Minserk Cheong, Jaejung Ko, Sang Ook Kang
A series of constrained geometry group 4 metal complexes containing the (N,N‘-dimethylaminomethyl) dicarbollyl ligand DcabNH [nido-7-NHMe2(CH2)-8-R-7,8-C2B9H10] (3) was prepared. New types of constrained geometry titanium complexes with the formula (DcabN)TiCl2, [{(η5-RC2B9H9)(CH2)(η1-NMe2)}TiCl2] (R = H, 4a; Me, 4b), were produced by the reaction of the potassium salt of 3 with titanium tetrachloride. The reaction of 3 with Ti(NMe2)4 in toluene afforded (DcabN)Ti(NMe2)2, [{(η5-RC2B9H9)(CH2)(η1-NMe2)}Ti(NMe2)2] (5) (R = H, a; Me, b), which readily reacted with Me3SiCl to yield the corresponding chloride complexes (DcabN)TiCl2 (4). However, the reaction of 3 with Zr(NMe2)4 in toluene afforded (DcabN)Zr(NMe2)2(HNMe2) [{(η5-RC2B9H9)(CH2)(η1-NMe2)}Zr(NMe2)2(HNMe2)] (6) (R = H, a; Me, b). The structures of the diamido complexes were established by X-ray diffraction studies of 5a and 5b, which authenticated an η51-bonding mode derived from the dicarbollylamino ligand functional group. The corresponding bis-chelate complexes, (DcabN)2Ti (7), were synthesized from the reaction of 4a with an additional dicarbollyl ligand 3a. Bis-chelated complexes (DcabN)2M (M = Ti, 7; Zr, 8; Hf, 9) were also formed upon the reaction of 3a with MX4 (M = Ti, Zr, Hf) in a 2:1 molar ratio. The NMR spectra revealed that intramolecular M−N coordinations were preserved in solution, resulting in the formation of bis-chelated complexes (DcabN)2M (79). The tetrahedral coordination of the metal center was proven by single-crystal X-ray determination of the complex (DcabN)2Zr (8). New types of titanium alkoxides, (DcabN)Ti(OiPr)2 (10), were synthesized from the reaction of 3a with Ti(OiPr)4. Sterically less-demanding phenols such as C6H5OH and 2-MeC6H4OH replaced the coordinated amido ligands on (DcabN)Ti(NMe2)2 (5a) to yield the aryloxy-stabilized CGC complexes (DcabN)Ti(OPh)2 (Ph = C6H5, 11; 2-Me-C6H4, 12). The NMR spectra suggested that an intramolecular Ti−N coordination was intact in solution, resulting in a stable piano-stool structure whereby two aryloxy ligands resided in the leg positions. The aryloxy coordinations were further confirmed by single-crystal X-ray diffraction studies on the complexes (DcabN)Ti(OPh)2 (Ph = C6H5, 11; 2-Me-C6H4, 12).