The selective reduction of benzene to cyclohexene mediated by platinum metal complexes: X-ray crystal structure of [(η5-C5Me5)Ir(η5-C6H6CH2NO2)][BF4]

Abstract

The dicationic η⁶-benzene complexes [(C_nMe_n)M(C₆H₆)]²⁺[M = Rh, n= 5 (2a); M = Ir, n= 5 (2b); M = Ru, n= 6 (2c)] are reduced by tetrahydroborate to the η⁵-cyclohexadienyl complexes [(C_nMe_n)M(C₆H₇)]⁺(3) which are further reduced to the η⁴-cyclohexadiene complexes [(C_nMe_n)M(C₆H₈)](4). Reaction of complexes (4) with acid gives cyclohexene with 100% selectivity; when the reaction is carried out with tetrafluoroboric acid in benzene, the initial benzene complexes (2) are regenerated. The overall reaction consists of addition of two hydrides followed by two protons to co-ordinated benzene, and a cycle catalytic in the platinum metal can be constructed. The efficiency of the overall reduction of benzene to cyclohexene decreases in the order Ir > Ru > Rh. The reaction is compared to the reduction of benzene to cyclohexane mediated by rhodium, iridium, and ruthenium trichloride hydrates and tetrahydroborate in ethanol. Other nucleophiles also attack the benzene ring in (2b): these include methyl-lithium (which gives two dimethylcyclohexadiene complexes from which isomeric dimethylcyclohexenes can be obtained with acid), methoxide (which gives the methoxycyclohexadienyl and, with an excess, two dimethoxycyclohexa-1,3-diene complexes), and nitromethane which in the presence of base gives the nitromethylcyclohexadienyl complex [(C₅Me₅)Ir(C₆H₆CH₂NO₂)][BF₄](9). The single-crystal X-ray structure determination of (9) shows the C₅Me₅ and the C₆ ring both to be η⁵-bonded, and the CH₂NO₂ substituent to, be in the exo position on the η⁵-cyclohexadienyl ring. Analysis of the high-field¹H n.m.r. spectra of the complexes showed that exo attack on the C₆ ring occurred with all the nucleophiles. In the presence of base, (9) reacted further with (2b) to give [{(C₅Me₅)Ir(C₆H₆)}₂CHNO₂]²⁺. These reactions show the versatility of the reduction procedure.