Lina M. Epstein

New types of hydrogen bonding in organometallic chemistry

The review summarizes the experimental and theoretical advances in the elucidation of the nature, the structure, the spectral and energetic characteristics of the new types of hydrogen bonds (HB) specifically for organometallic compounds: cationic hydride as a proton donor ([MH]+⋯OP(X−)) and a metal atom (M⋯HX) or hydride ligand (MH⋯HX) as a proton acceptor. The main features of new unconventional and classical HB appear to be similar. The influence of different factors on HB and full proton transfer is analyzed. Investigations of HB participation in proton transfer reactions as intermediates as well as approaches to experimental energy profiles of protonation are presented.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

Novel types of hydrogen bonding with transition metal π-complexes and hydrides

Abstract This review deals with the results of investigations of novel types of hydrogen bond involving transition metal complexes, namely H-bonds with a metal atom (XH ⋯ M) and those with the hydride hydrogen atom (XH ⋯ HM) as a base, as well as those with cationic hydrides as proton donors with organic bases ([MH] + ⋯ B). Spectral, thermodynamic and structural characteristics of intra- and intermolecular hydrogen bonds are described. The influence of the nature of the metal atom, and of electronic and steric properties of ligands on the proton donor ability of the metal atom and hydride hydrogen are discussed. Studies of proton transfers occurring through intermediate hydrogen bonds are also analysed.

Structural and energetic aspects of hydrogen bonding and proton transfer to ReH2(CO)(NO)(PR3)2 and ReHCl(CO)(NO)(PMe3)2 by IR and X-ray studies

Abstract The interaction of rhenium hydrides ReHX(CO)(NO)(PR3)2 1 (X=H, R=Me (a), Et (b), iPr (c); X=Cl, R=Me (d)) with a series of proton donors (indole, phenols, fluorinated alcohols, trifluoroacetic acid) was studied by variable temperature IR spectroscopy. The conditions governing the hydrogen bonding ReH⋯HX in solution and in the solid state (IR, X-ray) were elucidated. Spectroscopic and thermodynamic characteristics (−ΔH=2.3–6.1 kcal mol−1) of these hydrogen bonded complexes were obtained. IR spectral evidence that hydrogen bonding with hydride atom precedes proton transfer and the dihydrogen complex formation was found. Hydrogen bonded complex of ReH2(CO)(NO)(PMe3)2 with indole (2a–indole) and organyloxy-complex ReH(OC6H4NO2)(CO)(NO)(PMe3)2 (5a) were characterized by single-crystal X-ray diffraction. A short NH⋯HRe (1.79(5) A) distance was found in the 2a–indole complex, where the indole molecule lies in the plane of the Re(NO)(CO) fragment (with dihedral angle between the planes 0.01°).

Crown Compounds for Anions: Sandwich and Half-Sandwich Complexes of Cyclic Trimeric Perfluoro-o-phenylenemercury with Polyhedralcloso-[B10H10]2− andcloso-[B12H12]2− Anions

It has been shown by IR and NMR spectroscopy that cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4-Hg)3 (1) is capable of binding closo-[B10H10]2- and closo-[B12H12]2- anions to form complexes [[(o-C6F4Hg)3](B10-H10)]2- (2), [[(o-C6F4Hg)3]2(B10H10)]2-(3), [[(o-C6F4Hg)3](B12H12)]2- (4), and [[(o-C6F4Hg)3]2(B12H12)]2- (5). According to IR data, the bonding of the [B10H10]2- and [B12H12]2- ions to the macrocycle in these complexes is accomplished through the formation of B-H-Hg bridges. Complexes 2, 3, and 5 have been isolated in analytically pure form and have been characterized by spectroscopic means. X-ray diffraction studies of 3 and 5 have revealed that these compounds have unusual sandwich structures, in which the polyhedral di-anion is located between the planes of two molecules of 1 and is bonded to each of them through two types of B-H-Hg bridges. One type is the simultaneous coordination of a B-H group to all three Hg atoms of the macrocycle. The other type is the coordination of a B-H group to a single Hg atom of the cycle. According to X-ray diffraction data, complex 2 has an analogous but half-sandwich structure. The obtained complexes 2-5 are quite stable; their stability constants in THF/acetone (1:1) at 20 degrees C have been determined as 1.0 x 10(2)Lmol(-1), 2.6 x 10(3)L(2)mol(2), 0.7 x 10(2)Lmol(-1), and 0.98 x 10(3)L(2)mol(-2), respectively.

In situ IR and NMR study of the interactions between proton donors and the Re(I) hydride complex [{MeC(CH2PPh2)3}Re (CO)2H]. ReH…H bonding and proton-transfer pathways

Abstract The reactions of various proton donors (phenol, hexafluoro-2-propanol, perfluoro-2-methyl-2-propanol, monochloroacetic acid, and tetrafluoroboric acid) with the rhenium (I) hydride complex [(triphos)Re(CO)2H] (1) have been studied in dichloromethane solution by in situ IR and NMR spectroscopy. The proton donors from [(triphos)Re(CO)2H…HOR] adducts exhibiting rather strong H…H interactions. The enthalpy variations associated with the formation of the H-bonds (−ΔH = 4.4–6.0 kcal mol−1) have been determined by IR spectroscopy, while the H…H distance in the adduct [(triphos)Re(CO)2H…HOC(CF3)3] (1.83 A) has been calculated by NMR spectroscopy through the determination of the T1min relaxation time of the ReH proton. It has been shown that the [(triphos)Re(CO)2H…HOR] adducts are in equilibrium with the dihydrogen complex [(triphos)Re(CO)2(η2-H2)]+, which is thermodynamically more stable than any H-bond adduct.

Peculiarities of the Complexation of Copper and Silver Adducts of a 3, 5-Bis (trifluoromethyl) pyrazolate Ligand with Organoiron Compounds

Interaction of the copper, {[3,5-(CF(3))(2)Pz]Cu}(3), and silver, {[3,5-(CF(3))(2)Pz]Ag}(3), macrocycles [3,5-(CF(3))(2)Pz = 3,5-bis(trifluoromethyl)pyrazolate] with cyclooctatetraeneiron tricarbonyl, (cot)Fe(CO)(3), was investigated by IR and NMR spectroscopy for the first time. The formation of 1:1 complexes was observed at low temperatures in hexane. The composition of the complexes (1:1) and their thermodynamic characteristics in hexane and dichloromethane were determined. The π-electron system of (cot)Fe(CO)(3) was proven to be the sole site of coordination in solution and in the solid state. However, according to the single-crystal X-ray data, the complex has a different (2:1) composition featuring the sandwich structure. The complexes of ferrocene with copper and silver macrocycles have a columnar structure (X-ray data).

Crown compounds for anions. A polymeric complex of cyclic trimeric perfluoro-o-phenylenemercury with thiocyanate anion containing an infinite helical chain of alternating molecules of mercury-containing macrocycle and SCN− ions

Abstract It has been shown by IR spectroscopy that cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4Hg)3 is capable of binding thiocyanate anions in acetone solution, forming complexes whose composition is dependent on the ratio of reagents. In the interaction of (o-C6F4Hg)3 with an equimolar amount of [nBu4N]+ SCN− in ethanol the complex formulated as [(o- C 6 F 4 Hg ) 3 ( SCN )] − [ n Bu 4 N ] + has been isolated. An X-ray diffraction study of the complex has revealed that in the solid state it is polymeric and contains an infinite helical chain of alternating (o-C6F4Hg)3 molecules and thiocyanate anions. Every SCN− ion in the complex is bonded to the mercury atoms of two neighbouring molecules of the macrocycle through the sulphur atom and forms with each of these molecules two relatively short Hg … S bonds and one considerably longer Hg … S bond.

Solvent‐Dependent Dihydrogen/Dihydride Stability for [Mo(CO)(Cp*)H2(PMe3)2]+[BF4]− Determined by Multiple Solvent⋅⋅⋅Anion⋅⋅⋅Cation Non‐Covalent Interactions

Low-temperature (200 K) protonation of [Mo(CO)(Cp*)H(PMe(3))(2)] (1) by Et(2)OHBF(4) gives a different result depending on a subtle solvent change: The dihydrogen complex [Mo(CO)(Cp*)(eta(2)-H(2))(PMe(3))(2)](+) (2) is obtained in THF, whereas the tautomeric classical dihydride [Mo(CO)(Cp*)(H)(2)(PMe(3))(2)](+) (3) is the only observable product in dichloromethane. Both products were fully characterised (nu(CO) IR; (1)H, (31)P, (13)C NMR spectroscopies) at low temperature; they lose H(2) upon warming to 230 K at approximately the same rate (ca. 10(-3) s(-1)), with no detection of the non-classical form in CD(2)Cl(2), to generate [Mo(CO)(Cp*)(FBF(3))(PMe(3))(2)] (4). The latter also slowly decomposes at ambient temperature. One of the decomposition products was crystallised and identified by X-ray crystallography as [Mo(CO)(Cp*)(FHFBF(3))(PMe(3))(2)] (5), which features a neutral HF ligand coordinated to the transition metal through the F atom and to the BF(4) (-) anion through a hydrogen bond. The reason for the switch in relative stability between 2 and 3 was probed by DFT calculations based on the B3LYP and M05-2X functionals, with inclusion of anion and solvent effects by the conductor-like polarisable continuum model and by explicit consideration of the solvent molecules. Calculations at the MP4(SDQ) and CCSD(T) levels were also carried out for calibration. The calculations reveal the key role of non-covalent anion-solvent interactions, which modulate the anion-cation interaction ultimately altering the energetic balance between the two isomeric forms.

Dihydrogen bonding of decahydro-closo-decaborate(2−) and dodecahydro-closo-dodecaborate(2−) anions with proton donors: experimental and theoretical investigation

Abstract The interactions of [Bu 4 N] 2 [B 10 H 10 ] and [Bu 4 N] 2 [B 12 H 12 ] with various proton donors (MeOH, EtOH, Pr i OH, PhOH, 4-FC 6 H 4 OH, 4-NO 2 C 6 H 4 OH, CF 3 CH 2 OH, (CF 3 ) 2 CHOH, (CF 3 ) 3 COH) in low polarity media were investigated. The site of coordination for [B 10 H 10 ] 2− and [B 12 H 12 ] 2− was found to be hydride hydrogen. Spectral (IR, NMR) evidences for the BH⋯HO hydrogen bonding between the boron hydrides and the OH proton donors in solution are presented. Spectral (Δ ν , Δ ν 1/2 , ΔA) and thermodynamic (Δ H °, Δ S °) characteristics of the H-complexes were determined. The BH⋯HO bonding strength increases from [B 12 H 12 ] 2− to [B 10 H 10 ] 2− . The geometry, energy, as well as electron distribution in the [B 10 H 10 ] 2− ·HOCH 3 , [B 10 H 10 ] 2− ·HOCF 3 , [B 10 H 10 ] 2− ·HCN, and [B 12 H 12 ] 2− ·HOCH 3 complexes were studied using ab initio HF/6-31G approximation. It was shown that increase of the proton donor ability of acids leads to formation of bifurcate H-bonds.