On the peculiarities of hydrogen bonding and proton transfer equilibria for organic vs organometallic bases

Abstract

This review summarizes experimental and theoretical results on hydrogen bonding (HB) and proton transfer processes involving organometallic bases (especially transition metal hydrides) in comparison with organic bases. The competition between different sites for HB and protonation, the proton accepting ability and the HB enthalpy dependence on the atom position in the Periodic Table, proton affinity values and features of proton transfer potential energy profiles are displaying peculiarities of the organometallic bases as proton acceptors.

Hydrogen bonding to carbonyl hydride complex Cp*Mo(PMe3)2(CO)H and its role in proton transfer

“…The latter is slightly lower than PA values calculated for other transition metal hydride complexes (1074-1132 kJ mol -1 ). 4 When CF 3 COOH (PA = 1338 kJ mol -1 ) was used as proton source the [Cp*MoH(COH)(PMe 3 ) 2 ] + OCOCF 3 ion pair product could not be optimized, all attempts leading back to the hydrogen bonded complex. These data are in support of the notion that the CO protonation pathway is not feasible.…”

“…With the exception of bridging carbonyl ligands, 3 protons from strong acids usually add to the metal atom or to the hydride ligand. 4 On the other hand, much bulkier Lewis acids generally add to carbonyl oxygens. 2 Weak proton donors (mostly alcohols) interact with either the hydride ligand or the CO-oxygen.…”

“…2 Weak proton donors (mostly alcohols) interact with either the hydride ligand or the CO-oxygen. 4,5 Very recently we have reported the results of the protonation of the carbonyl hydride complex Cp*Mo(PMe3)2(CO)H (1) by the strong acid HBF4. At low temperatures proton transfer in THF yields the dihydrogen complex [Cp*Mo(PMe3)2(CO)( 2 -H2)]BF4 as a sole observable protonation product, however when the same reaction is carried out in CH2Cl2 the only product of the reaction is the cationic dihydride complex…”

Steric and Acidity Control in Hydrogen Bonding and Proton Transfer to trans-W(N2)2(dppe)2

“…53 The effect was explained 53 by the changes in the rates of key elementary steps leading to the suppressed formation of an off-path borohydrido−hydrido species characterized as the resting state for the KC 8 and [H(OEt 2 ) 2 ][BAr F 4 ] system. 54 Since proton transfer to either organic or organometallic bases and transition metals is typically preceded by the formation of hydrogen-bonded species, 55 we expected this should also be the case in proton transfer to dinitrogen complexes. Indeed, the results reported herein show for the first time that rather weak acids (fluorinated alcohols, p-nitrophenol, pK a (DMSO) = 17.9−…”

“…Since proton transfer to either organic or organometallic bases and transition metals is typically preceded by the formation of hydrogen-bonded species, we expected this should also be the case in proton transfer to dinitrogen complexes. Indeed, the results reported herein show for the first time that rather weak acids (fluorinated alcohols, p- nitrophenol, p K a (DMSO) = 17.9–10.7) form hydrogen bonds to coordinated nitrogen [W]–NN···H–X ( 1a ).…”