Bimetallic complexes [LW(CO)2(μ-CO)⋯Pd(PCP)] cooperatively activate amine-boranes for their dehydrogenation via N–H proton tunneling at RDS and H2 evolution from two neutral hydrides.
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
The interactions of HA acids {indole, fluorinated alcohols, phenols, and [CpW(CO)3H] (2)]} with the title hydrides [(tBuPCP)MH] [M = Ni (1a), Pd (1b)] have been studied by a combination of spectroscopic (variable‐temperature IR, NMR, UV/Vis) and computational (DFT/M06, AIM) methods in THF and toluene. The formation of the dihydrogen bond (DHB) 1···HA is the first step in a process leading to proton transfer and H2 evolution. The DHBs of 1 with 2 are much weaker than those of 1 with NH or OH acids, but the former complexes are more reactive. Kinetic studies of the reactions of 4‐(4′‐nitrophenylazo)phenol and [CpWH(CO)3] with 1b in THF gave activation enthalpies ΔH≠ = 9.2 ± 0.4 and 6.5 ± 1.5 kcal mol–1 and activation entropies ΔS≠ = –30 ± 1 and –35 ± 6 cal mol–1 K–1, respectively. Calculations revealed the (η2‐H2)‐like TS (ΔE≠ = 9.3 kcal mol–1) for the reaction of 1b with p‐nitrophenol and the [Pd(η2‐H2)]+OAr– complex as a local minimum at around 5 kcal mol–1 above the DHB adduct. In the reaction of 1b with 2, both the acidic WH and hydridic NiH or PdH bonds undergo heterolytic cleavage (ΔE≠ = 7.4 kcal mol–1) to yield the unusual µ,η1:1‐H2 end‐on complex. The µ,η1:1‐H2 molecule transforms easily into the more stable η2‐H2 side‐on tautomer, which eventually gives the bimetallic product after H2 evolution.
Two stereoisomers of pentacoordinate iridium(III) hydridochloride with triptycene-based PC(sp 3 )P pincer ligand (1,8bis(diisopropylphosphino)triptycene), 1 and 2, differ by the orientation of hydride ligand relative to the bridgehead ring of triptycene. According to DFT/B3PW91/def2-TZVP calculations performed, an equatorial Cl ligand can relatively easily change its position in 1, whereas that is not the case in 2. Both complexes 1 and 2 readily bind the sixth ligand to protect the empty coordination site. Variable temperature spectroscopic (NMR, IR, and UV−visible) studies show the existence of two isomers of hexacoordinate complexes 1•MeCN, 2•MeCN, and 2•Py with acetonitrile or pyridine coordinated trans to hydride or trans to metalated C(sp 3 ), whereas only the equatorial isomer is found for 1• Py. These complexes are stabilized by various intramolecular noncovalent C−H•••Cl interactions that are affected by the rotation of isopropyls or pyridine. The substitution of MeCN by pyridine is slow yielding axial Py complexes as kinetic products and the equatorial Py complexes as thermodynamic products with faster reactions of 1•L. Ultimately, that explains the higher activity of 1 in the catalytic alkenes' isomerization observed for allylbenzene, 1-octene, and pent-4-enenitrile, which proceeds as an insertion/elimination sequence rather than through the allylic mechanism.
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