Protonation of Transition Metal Hydrides to Give Dihydrogen Complexes

Papish, Elizabeth T.; Magee, Matthew P.; Norton, Jack R.

doi:10.1016/b978-044450733-4/50002-4

Cited by 31 publications

(34 citation statements)

References 128 publications

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“…[1] It has now been quite firmly established that when both a hydride ligand and a metal-based electron pair are present in the same complex, proton donors show a kinetic preference for the hydride site, [2][3][4][5][6][7][8] though exceptions have recently been reported from studies carried out in our laboratories. [9,10] It has also been established that hydrogenbonded adducts are well-defined intermediates along the Abstract: The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA).…”

Section: Introductionmentioning

confidence: 93%

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

Belkova

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et al. 2005

Chemistry A European J

133

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The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed by IR spectroscopy when using a deficit of the proton donor. An excess leads to the formation of the homoconjugated anion. The interaction with hexafluoroisopropanol (HFIP) was investigated quantitatively by IR spectroscopy and by 1H and 31P NMR spectroscopy at low temperatures (200-260 K) and by stopped-flow kinetics at about room temperature (288-308 K). The hydrogen bond formation to give [Cp*Fe(dppe)H]HA is characterized by DeltaH degrees =-6.5+/-0.4 kcal mol(-1) and DeltaS degrees = -18.6+/-1.7 cal mol(-1) K(-1). The activation barrier for the proton transfer step, which occurs only upon intervention of a second HFIP molecule, is DeltaH(not equal) = 2.6+/-0.3 kcal mol(-1) and DeltaS(not equal) = -44.5+/-1.1 cal mol(-1) K(-1). The computational investigation (at the DFT/B3 LYP level with inclusion of solvent effects by the polarizable continuum model) reproduces all the qualitative findings, provided the correct number of proton donor molecules are used in the model. The proton transfer process is, however, computed to be less exothermic than observed in the experiment.

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Section: Introductionmentioning

confidence: 93%

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

Belkova

Collange

Dub

et al. 2005

Chemistry A European J

133

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“…(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) portance for catalysis and in biochemistry. [1,2] The kinetic preference for hydride versus metal protonation is now quite firmly established, [3][4][5][6][7][8][9] though exceptions have recently been reported from studies carried out in our laboratories. [10,11] The proton transfer occurs via intermediates, for which characteristic spectroscopic signatures have been established, that contain hydrogen bonds between the proton donor and the proton acceptor (the metal center or a hydride ligand) (see Scheme 1).…”

Section: Introductionmentioning

confidence: 94%

Hydrogen Bonding and Proton Transfer to the Trihydride Complex [Cp*MoH₃(dppe)]: IR, NMR, and Theoretical Investigations

et al. 2006

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Keywords: Dihydrogen bonding / Hydride ligands / Molybdenum / Proton-transfer mechanism / DFT calculationsThe interaction between [Cp*MoH 3 (dppe)] (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) and a variety of proton donors has been investigated by a combination of experiments and DFT calculations. Weak proton donors [2-monofluoroethanol (MFE) and trifluoroethanol (TFE)] allow the determination of basicity factor (E j = 1.42 ± 0.02) and thermodynamic parameters for the hydrogen bond formation (∆H HB = -4.9 ± 0.2 and -6.1 ± 0.3 kcal mol -1 ; ∆S HB = -15.7 ± 0.7 and -20.4 ± 1 cal mol -1 K -1 for MFE and TFE, respectively). For TFE, a stable low-temperature proton-transfer equilibrium (220-240 K) with the cationic classical tetrahydrido derivative [Cp*MoH 4 (dppe)] + could be investigated independently by UV/Vis (∆H°P T = -2.8 ± 0.4 kcal mol -1 and ∆S°P T = -15 ± 2 cal mol -1 K -1 ) and 1 H NMR (∆H°P T = -2.7 ± 0.5 kcal mol -1 and ∆S°P T = -11 ± 2 cal mol -1 K -1 ) spectroscopy. Upon warming, however, the tetrahydride evolves by dihydrogen loss and formation of a hydride-free diamagnetic product. Stronger proton donors [hexafluoroisopropanol (HFIP), p-nitrophenol (PNP), perfluoro-tert-butyl alcohol (PFTB), and HBF 4 ·OEt 2 ] lead to more extensive proton transfer at lower donor/Mo ratios. A 1:1 proton-transfer stoichiometry is indicated independently by a titration experiment with UV/Vis monitoring for the [Cp*MoH 3 (dppe)]-PNP reaction, and by a stopped-flow kinetics investigation for the

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“…[26] While these studies made it clear that the dihydride form is thermodynamically favored, kinetics studies by Norton and co-workers led to the conclusion that the protonation at the MÀH bond (to give a dihydrogen complex) is faster than direct protonation at the metal to give the dihydride. [27] Protonations of this type [18] represent the microscopic reverse of deprotonations of dihydrogen complexes, which are known to be kinetically preferred over deprotonation of the corresponding dihydrides. [11] For the Mo analogue, Poli and coworkers observed prompt evolution of H 2 when [Mo(Cp)-(CO) 2 (PMe 3 )H] was protonated at À78 8C; they suggested the formation of an unstable molybdenum±dihydrogen complex to account for these results.…”

Section: (Pme 3 )(H) 2 ]mentioning

confidence: 99%

“…In contrast to hydrosilanes such as HSiEt 3 , which immediately evolve hydrogen upon protonation, many transition-metal hydrides can be protonated to give stable products. Protonation at the MÀH bond [18] produces a dihydrogen complex [11] in which an H 2 ligand is bound to the metal, while protonation at the metal gives a dihydride complex (Scheme 2).…”

Section: Stoichometric Ionic Hydrogenations With An External Acid As mentioning

confidence: 99%

Catalytic Ionic Hydrogenations

Bullock

2004

Chemistry A European J

287

206

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Catalytic ionic hydrogenation of ketones occurs by proton transfer to a ketone from a cationic metal dihydride, followed by hydride transfer from a neutral metal hydride. This contrasts with traditional catalysts for ketone hydrogenation that require binding of the ketone to the metal and subsequent insertion of the ketone into a M-H bond. Ionic hydrogenation catalysts based on the inexpensive metals molybdenum and tungsten have been developed based on mechanistic understanding of the individual steps required in the catalytic reaction.

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Protonation of Transition Metal Hydrides to Give Dihydrogen Complexes

Cited by 31 publications

References 128 publications

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

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

Hydrogen Bonding and Proton Transfer to the Trihydride Complex [Cp*MoH₃(dppe)]: IR, NMR, and Theoretical Investigations

Catalytic Ionic Hydrogenations

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Protonation of Transition Metal Hydrides to Give Dihydrogen Complexes

Cited by 31 publications

References 128 publications

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

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

Hydrogen Bonding and Proton Transfer to the Trihydride Complex [Cp*MoH3(dppe)]: IR, NMR, and Theoretical Investigations

Catalytic Ionic Hydrogenations

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Hydrogen Bonding and Proton Transfer to the Trihydride Complex [Cp*MoH₃(dppe)]: IR, NMR, and Theoretical Investigations