A study of π-complexation of phenol and β-estradiol by 'Cp*M' M = Rh, Ir moieties : syntheses, solution behavior and reactivity ; X-ray molecular structure of [Cp*Rh(η5-C6H5O.H2O)][BF4], Cp* = -C5Me5

Treatment of [Cp * Rh(CH 3 CN) 3 ] [BF 4 ] 2 1a with PhOH at room temperature in dichloroethane afforded the asymmetric trimer [{Cp * Rh(η 6 -PhOH...)} 2 (η 5 -PhO...)RhCp * ] [BF 4 ] 5 2a in 87% yield, where the η-phenoxo species is hydrogen-bonded to the corresponding η-phenolic forms. The analogous iridium complex 2b was obtained from acetone/dichloroethane mixture in 80% yield. These species 2a and 2b were found to be fluxional in solution, for instance, the variable temperature 1 H NMR spectra for the rhodium species 2a show that the η-Cp * signals coalesce at T = 354 K with ΔG# = 18±0.5 kcal/mol. A mechanism of exchange in accord with the experimental data is proposed. Protonation of 2a by HBF 4 .Et 2 O in acetone gave the unstable phenolic compound [Cp * Rh(η 6 -PhOH)] [BF 4 ] 2 3a in 50% yield. On the other hand, 2a can be deprotonated by NEt 3 to give quantitatively the phenoxo derivative [Cp * Rh(η 5 -PhO.H 2 O)][BF 4 ] 4a. Compound 4a crystallizes in the orthorhombic space group Ccm2 1 , a = 17.469(3) A, b = 28.845(4) A, c = 14.115(2) A, V = 7112(2) A 3 , Z = 16. The structure of 4a shows that the -C=O group of the phenyl ring is bent upward with θ = 14°, and different from the structure of the ruthenium analog. The iridium species 2b behaved similarly when treated with NEt 3 to give an off-white compound [Cp * Ir(η 5 -PhO.H 2 O)][BF 4 ] 4b. When a yellow solution of [Cp * Rh(S) 3 ][BF 4 ] 2 (S = coordinated solvent) was treated with 17β-estradiol in acetone/THF, a mixture of four products (α,β)-[Cp * Rh(η 6 -estradiol)][BF 4 ] 2 13ab and (α,β)-[Cp * Rh(η 5 -estradienonyl)] [BF 4 ] 14ab was obtained with α/β ratio 9 :1. In general the phenoxo form was more stable than the phenolic one for both π-bonded systems (phenol, 17β-estradiol). The effect of the counterion (BF 4 and/or CF 3 SO 3 ) on the stability of the π-bonded phenolic forms (both systems) as well as the reactivity of the π-bonded phenoxo forms (both systems) towards electrophiles (MeI, CF 3 SO 3 Me) are compared and discussed.