Activation of M—H bond upon the complexation of transition metal hydrides with acids and bases

Abstract

Features of the electronic structure of adducts of transition metal hydride complexes (Cp*M(dppe)H, dppe is the 1,2 (diphenylphosphino)ethane, M = Fe, Ru, Os; CpM(CO) 3 H, M = Mo, W) with acids and bases were analyzed with the ADF2014 program using energy decomposition analysis (EDA) by the Ziegler-Rauk method combined with the natural orbitals for chemical valence theory (ETS NOCV). The nature of orbital interactions in the complex determines the reaction pathway:  MH  * OH interaction leads to the proton transfer to hydride ligand, n M  * OH leads to the metal atom protonation, n N  * MH im plies the metal hydride deprotonation, and  MH  n* B corresponds to the hydride transfer to Lewis acid. It was shown that M-H bond polarization change has the similar character upon the formation of complexes with Brønsted and Lewis acids. The ease of polarization of M-H bonds in complexes CpM(CO) 3 H determines their reactivity as proton and hydride ion donors.Key words: quantum chemical calculations, hydrogen bonds, non covalent interactions, transition metal hydrides.Neutral transition metal hydrides can demonstrate dif ferent reactivity being formally the sources of hydrogen atom Н • , hydride ion Н -, or proton Н + . 1,2 This kind of reactivity is not merely typical of metal hydrides as a class of compounds, but is well known in some cases when, depending on the conditions, one and the same hydride complex participates in the reactions of all three types. For example, СрM(CO) 3 Н (М = Cr, Mo, W; Ср =  5 С 5 Н 5 ), НМ(СО) 5 (М = Mn, Re) and CpM(CO) 2 H (M = Fe, Ru, Os) exibit unique reactivity. 3-7The hydride transfer from the metal complexes is the key stage of the ionic hydrogenation reactions both in stoichiometric and in catalytic variants. Hydride carbonyl complexes СрM(CO) 3 Н (М = Cr, Mo, W) are used as hydride donors in combination with proton donor, tri fluoromethyl sulfonic acid CF 3 SO 3 H, in reactions of hydrogenation of substituted alkenes, 8 and also aldehydes and ketones. 9 The proton transfer to an organic substrate competes with the metal hydride protonation with the formation of classical or non classical hydride. 10 Half sandwich ruthenium hydride complexes Cp´Ru(P-P)H (Cp´ =  5 C 5 R 5 ) are the catalysts of the ionic hydrogena tion of imines, imine salts, ketones and aziridinium cations. 11-13 In this case the cationic complexes [Cp´Ru(P-P)H 2 ] + serve as proton sources, and neutral hydrides Cp´Ru(P-P)H serve as hydride sources. 11

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

“…DFT calculations , have shown that the MH bond polarization in CpMH­(CO) 3 is similar to that in HCF 3 . As expected, the decrease in the MH bond polarization [Δpol­(MH), Table ] caused by hydrogen bonding reflects an increase in positive charge on the metal-bound hydrogen (by 0.053–0.084 units), as expected for a proton donor.…”

“…Similarly, in XH···HM bonds, the transfer of electron density takes place from the interacting site of the base σ MH orbital to the σ* XH orbital of an acid. In complex transition-metal hydrides, the σ MH orbitals need not correspond to the HOMO (contrary to the majority of simple main-group hydrides) but specifically these orbitals are involved in charge transfer to σ* XH . − …”

“…The σ MH orbital is centered mainly on hydrogen, which typically has over 45% of the total electron density. In dihydrogen-bonded complexes of Cp*MH­(dppe) species with CF 3 CH 2 OH, pol­(MH) increases, whereas pol­(OH) decreases, reflecting the increase of the alcohol acidity and MH bond hydricity . Hence, DHB formation results in a change in the polarizations of XH and MH bonds (see Table ), preparing both for subsequent transformations.…”

“…The interaction with bases caused the appearance of low-frequency shoulders/bands of ν CO and ν MH vibrations (Δν CO = 4–12 cm –1 and Δν MH = 9–12 cm –1 ). These are very weak bonds with Δ H values of less than −3.5 kcal·mol –1 , and the calculated MH···Y distances (2.51–2.48 Å) , are comparable to the CH···Y contacts in hydrogen-bonded complexes. ,− …”

Z−H Bond Activation in (Di)hydrogen Bonding as a Way to Proton/Hydride Transfer and H2 Evolution

“…The easy polarization of M−H bond and relatively low electron density on the hydride ligand in free CpMH(CO) 3 complexes allow the interaction not only with bases but also with Lewis acids. The calculations on their adducts with BH 3 as a mild Lewis acid revealed the σ M−H to LP* B donation (Figure ) leading to the increase of the electron density on the metal bound hydrogen (Table ) . In a sense, the [M]H⋅⋅⋅BH 3 interaction is similar to the dihydrogen bonding.…”