Kinetics and Mechanism of the Proton Transfer to Cp*Fe(dppe)H:  Absence of a Direct Protonation at the Metal Site

Abstract

The reaction between CpFe(dppe)H and a number of different proton donors (2-fluoroethanol, MFE; 2,2,2-trifluoroethanol, TFE; hexafluoro-2-propanol, HFIP; perfluoro-tert-butyl alcohol, PFTB; and trifluoroacetic acid, TFA) has been investigated spectroscopically by variable-temperature infrared, UV-visible, and NMR spectroscopy, and has been measured kinetically by the stopped-flow technique with UV-visible detection. The low-temperature IR study shows the establishment of hydrogen-bonding interactions which involve the hydride ligand as the proton accepting site. This investigation quantifies the thermodynamics of the hydrogen-bonding interaction and the basicity factor (E(j)) of the hydride complex. All techniques agree in indicating an equilibration process, after the immediate hydrogen-bond formation, between the hydride complex and an intermediate dihydrogen complex, [CpFe(dppe)(H(2))](+). The equilibrium is shifted toward the dihydrogen complex to a greater extent for the stronger alcohols and for higher alcohol/Fe ratios. The observed equilibration rate constant is linearly dependent on the alcohol concentration, in agreement with the involvement of two alcohol molecules and the formation of a homoconjugate pair. The rate constant increases with the acidity of the proton donor (TFE < HFIP < PFTB < TFA). The rate of the subsequent irreversible isomerization leading to the classical dihydride complex, [CpFe(dppe)H(2)](+), is first order, and the rate constant does not depend on the proton donor nature. The reaction continues, if conducted in CH(2)Cl(2), with a third, slower step leading to the paramagnetic [CpFe(dppe)Cl](+) product. The kinetic data are in accord with an isomerization mechanism consisting of an intramolecular reorganization, leading in one step from the dihydrogen complex to the classical dihydride species, and disagree with the occurrence of a proton-transfer process at the metal site.

Experimental and Computational Studies of Hydrogen Bonding and Proton Transfer to [Cp*Fe(dppe)H]

“…[25] Using weaker acids, we have not only confirmed the results of Hamon et al [5,6] for the protonation with HBF 4 whereby the proton transfer is faster at the hydride site to give an intermediate dihydrogen complex, [Cp*Fe(H 2 )(dppe)]…”

“…[25] This conclusion followed the key observation that the rate of isomerization is independent of the nature of the proton donor, whereas the rate of proton transfer at the hydride site increases with the hydrogen bond strength.…”

“…[25] Upon lowering the temperature, the band of the free phenol decreases and those of hydrogen-bonded phenol (340 nm) and hydrogenbonded ion pair (380 nm) increase ( Figure 4). Note that neither the free phenolate band, expected at 420-430 nm, nor the band of homoconjugated anion [ArOHOAr] À , expected at approximately 400 nm, is observed, suggesting that hydrogen-bonded ion pair essentially does not dissociate below 260 K. These qualitative observations are in agreement with the equilibria depicted in Scheme 2 (R = p-O 2 N À C 6 H 4 ) and with the exothermicity for both the hydrogen bond formation and for the proton transfer steps.…”

“…A vant Hoff analysis of the K 1 constants derived from the previously published [25] UV/Vis data yields DH = À5.5 AE 0.3 kcal mol À1 and DS = À13.0 AE 0.6 cal mol À1 K…”

“…[25] By carrying out IR studies in the n OH region at low temperatures (200-280 K) according to our established protocol, we have now obtained the thermodynamic parameters for the [Cp*Fe(dppe)H]-HFIP hydrogen bonding interaction (DH8 = À6.5 AE 0.4 kcal mol Figure 6). These parameters indicate stronger bonding for HFIP relative to TFE (DH8À5.4 AE 0.3 kcal mol À1 and DS8 = À13.6 AE 0.9 cal mol À1 K…”