Solvent Control in the Protonation of [Cp*Mo(dppe)H₃] by CF₃COOH

Abstract

The outcome of the reaction between [Cp*Mo(dppe)H3] [dppe = 1,2‐bis(diphenylphosphanyl)ethane] and trifluoroacetic acid (TFA) is highly dependent on the solvent and the TFA/Mo ratio. The dihydride compound [Cp*Mo(dppe)H2(O2CCF3)] is obtained selectively when the reaction is carried out in aromatic hydrocarbons (benzene, toluene) when using less than one equivalent of TFA. The dihydride is also the end product when THF or MeCN are used as solvent, independent of the TFA/Mo ratio. In benzene/toluene the use of excess acid has a profound effect, resulting in the formation of the tetrahydrido complex [Cp*Mo(dppe)H4]+, which did not further evolve into the dihydrido product. Monitoring of the reaction by NMR and IR spectroscopy under different conditions (solvent, temperature, TFA/Mo ratio) reveals the rapid establishment of an equilibrium between the dihydrogen‐bonded adduct, [Cp*Mo(dppe)H3]···HO2CCF3, the ion‐paired proton‐transfer product, [Cp*Mo(dppe)H4]+···–O2CCF3, and the separated ions, followed by a slower irreversible transformation to the final dihydride product with H2 evolution. The activation parameters of the H2 evolution and M–OR product formation were determined. Excess TFA in low‐polarity solvents stabilizes the separated charged species by forming the homoconjugate anion [CF3COO(CF3COOH)n]–. The effect of the solvent on the course of the reaction can be interpreted in terms of the different polarity, H‐bonding ability, and coordinating power of the various solvent molecules.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Modeling the platinum-catalyzed intermolecular hydroamination of ethylene: The nucleophilic addition of HNEt2 to coordinated ethylene in trans-PtBr2(C2H4)(HNEt2)

“…S11). It did not reveal a n NH band attributable to Et 3 NH þ , although the formation of ionic species is evident from the presence of the CF 3 COO À anion n as OCO band at 1673 cm À1 [45]. Hence, this experiment justifies the lack of observation of a spectroscopic signature for the ammonium ion of 4 in the IR spectrum.…”

Effect of the Nature of the Metal Atom on Hydrogen Bonding and Proton Transfer to [Cp*MH3(dppe)]: Tungsten versus Molybdenum

“…The investigation of hydrogen bonding and proton transfer to complex [Cp*WH 3 A C H T U N G T R E N N U N G (dppe)] followed a combined spectroscopic and theoretical protocol, similar to that used previously for complexes [Cp*MA C H T U N G T R E N N U N G (dppe)H] (M = Fe, Ru), [24,25,27] [Cp*MoA C H T U N G T R E N N U N G (dppe)H 3 ], [29,30] and [CpRuH(CO)A C H T U N G T R E N N U N G (PCy 3 )]. [34] Study of the starting trihydride complex: The synthesis and full characterization of this compound has been reported previously.…”

“…[24][25][26][27][28] Given the above scenario, it may be reasonable to expect that the kinetic preference for attack at an M-H site versus the metal lone pair should be greater for a polyhydride compound than for a monohydride compound. Recently, we reported detailed investigations [29,30] on the hydrogen bonding and proton transfer to complex [Cp*MoA C H T U N G T R E N N U N G (dppe)H 3 ], an electron-rich half-sandwich trihydride complex of Mo IV . [31] The protonation product observed by the use of either strong (HBF 4 , CF 3 COOH) or moderate (perfluoro-tert-butanol (PFTB), hexafluoroisopropanol (HFIP), p-nitrophenol) acids is the classical tetrahydrido derivative, [Cp*Mo-A C H T U N G T R E N N U N G (dppe)H 4 ] + , without detection of any nonclassical intermediate, even upon working at 200 K. On the other hand, use of even weaker proton donors (2,2,2-trifluoroethanol (TFE), 2fluoroethanol (MFE)), and low temperatures (200 K), backed up by a theoretical investigation, revealed that the most likely site of attack is a hydride ligand.…”

Electrochemical study of the role of a H-bridged, unsymmetrically disubstituted diiron complex in proton reduction catalysis

“…Dihydrogen bonded and ionpaired adducts are intermediates along the proton transfer path eventually producing a dihydrogen ligand that may evolve towards a dihydride through the cleavage of the H-H bond [33][34][35][36][37][38][39][40][41][42]. The effect of the metal centre and of the electronic properties of the ligands on the acidity of the dihydrogen ligand [46], and the effect of the solvent on the nature of the protonation product [47] have been investigated.…”