Derivates of 1,2-diborabenzene: II. (1,2-Dibora-3,5-cyclohexadiene)metal complexes. Crystal structure of (C5Me5)Rh(C4H4B2Cl2)

Abstract The 1,2-diboratabenzene derivative Li 2 [C 4 H 4 B 2 (NMe 2 ) 2 ] (I) reacts with various transition metal compounds to form sandwich and triple-decked complexes such as (C 6 Me 6 RuL (III), (C 5 Me 5 )RhL (IV), (μ-L)[Rh(COD)] 2 (V), NiL 2 (VI), Fe(CO)L 2 (VII), and Co 2 (CO) 4 L 2 (VIII), where L is the 1,2-bis(dimethylamino)-1,2-dibora3,5-cyclohexadiene ligand. Barriers to internal rotation around the BN bond are 48(1), 56(1), and 38(1) kJ/mol for III, IV, and V, respectively, while VII and VIII are rigid on the NMR time scale. Nucleophilic substitution reactions at boron were examined for the rhodium complex IV. The Lewis acids BF 3 ·OEt 2 and BCl 3 bring about substitution to give (C 5 Me 5 )Rh[C 4 H 4 B 2 F(NMe 2 · BF 3 )] (XI) and (C 5 Me 5 )Rh(C 4 B 2 Cl 2 ) (XII), respectively. Attempted substitution with Al 2 Me 6 and with AlH 3 in THF effected a novel ring contraction to produce the borole complexes (C 5 Me 5 )Rh(C 4 H 4 BR) (XIII: R = Me; XIV: R = H); similarly, traces of air transform XII into (C 5 Me 5 )Rh(C 4 H 4 BCl) (XV). The structure of XII determined by X-ray diffraction confirms the η 6 -bonding mode of the diboracyclohexadiene ligand.