IS INTERMOLECULAR HYDROGEN-BONDING TO UNCHARGED METAL CENTERS OF ORGANOMETALLIC COMPOUNDS WIDESPREAD IN SOLUTION - A SPECTROSCOPIC INVESTIGATION IN HYDROCARBON, NOBLE-GAS, AND SUPERCRITICAL-FLUID SOLUTIONS OF THE INTERACTION BETWEEN FLUORO ALCOHOLS AND (ETA-5-C5R5)ML2 (R = H, ME, M = CO, RH, IR, L = CO, C2H4, N2, PME3) AND ITS RELEVANCE TO PROTONATION

IR spectroscopy has been used to investigate the interaction of fluoro alcohols with neutral group 9 transition metal half-sandwich complexes (eta5-C5R5)ML2 (R = H, Me; M = Co, Rh, Ir; L = CO, C2H4, N2, PMe3) in fluid solution. The interaction involves hydrogen-bonding by the acidic alcohol to the basic metal center, (eta5-C5R5)L2M...H-ORF, i.e., with the metal center acting as a proton acceptor. The H-bonding has been studied via nu(C-O) and nu(O-H) vibrations in both room temperature solvents-n-heptane, supercritical xenon (scXe), and supercritical ethane (scC2H6)-and cryogenic solutions-liquid xenon (lXe) and krypton (lKr). A detailed study of Cp*Ir(CO)2 (1) has been used to establish that (a) a lower pK(a) of the fluoro alcohol gives a stronger interaction, (b) the shift in the nu(O-H) vibration on deuteration, using (CF3)3COH (PTFB) and (CF3)3COD, is similar to the shift of other nonmetallic H-bonded systems, (c) lower temperatures favor H-bonding, including the formation of a second complex {[Cp*Ir-(CO)2]...(HORF)2} and its Rh analog, (d) neat fluoro alcohol solution leads to the protonation of the Ir center in the case of PTFB and (CF3)2CHOH (HFIP), (e) the interaction of HFIP with 1 in scXe demonstrates the relationship between protonation and H-bonding to the metal center in scXe solution, (f) H-bonding is disfavored by increasing solvent density at constant concentration in scC2H6, (g) similar H-bonding occurs with both Co and Rh compounds, but the strength of interaction increases down the group, (h) H-bonding is weaker for C5H5 complexes than for C5Me5, (i) H-bonding occurs with a range of compounds where one or both of the CO groups are replaced (i.e. by C2H4, N2, or PMe3, and (j) in lXe solution, the wavenumber shift in the nu(O-H) vibration of the H-bonded complex relative to that of free PFTB mirrors the strength of the H-bonded interaction. Finally, the chemical implications of this type of H-bonding are briefly discussed and it is suggested that this shift in the nu(O-H) band could be used as a semiquantitative measure of the basicity of the metal centers.