Large effects of ion pairing and protonic-hydridic bonding on the stereochemistry and basicity of crown-, azacrown-, and cryptand-222-potassium salts of anionic tetrahydride complexes of iridium(III)

Two Synthetic Tools to Deepen the Understanding of the Influence of Stereochemistry on the Properties of Iridium(III) Heteroleptic Emitters

Two complementary procedures are presented to prepare cis-pyridyl-iridium(III) emitters of the class [3b+3b+3b'] with two orthometalated ligands of the 2-phenylpyridine type (3b) and a third ligand (3b'). They allowed to obtain four emitters of this class and to compare their properties with those of the trans-pyridyl isomers. The finding starts from IrH5(PiPr3)2, which reacts with 2-(p-tolyl)pyridine to give fac-[Ir{κ2-C,N-[C6MeH3-py]}3] with an almost quantitative yield. Stirring the latter in the appropriate amount of a saturated solution of HCl in toluene results in the cis-pyridyl adduct IrCl{κ2-C,N-[C6MeH3-py]}2{κ1-Cl-[Cl-H-py-C6MeH4]} stabilized with p-tolylpyridinium chloride, which can also be transformed into dimer cis-[Ir(μ-OH){κ2-C,N-[C6MeH3-py]}2]2. Adduct IrCl{κ2-C,N-[C6MeH3-py]}2{κ1-Cl-[Cl-H-py-C6MeH4]} directly generates cis-[Ir{κ2-C,N-[C6MeH3-py]}2{κ2-C,N-[C6H4-Isoqui]}] and cis-[Ir{κ2-C,N-[C6MeH3-py]}2{κ2-C,N-[C6H4-py]}] by transmetalation from Li[2-(isoquinolin-1-yl)-C6H4] and Li[py-2-C6H4]. Dimer cis-[Ir(μ-OH){κ2-C,N-[C6MeH3-py]}2]2 is also a useful starting complex when the precursor molecule of 3b' has a fairly acidic hydrogen atom, suitable for removal by hydroxide groups. Thus, its reactions with 2-picolinic acid and acetylacetone (Hacac) lead to cis-Ir{κ2-C,N-[C6MeH3-py]}2{κ2-O,N-[OC(O)-py]} and cis-Ir{κ2-C,N-[C6MeH3-py]}2{κ2-O,O-[acac]}. The stereochemistry of the emitter does not significantly influence the emission wavelengths. On the contrary, its efficiency is highly dependent on and associated with the stability of the isomer. The more stable isomer shows a higher quantum yield and color purity.

IrI(η4-diene) precatalyst activation by strong bases: formation of an anionic IrIII tetrahydride

The reaction between [IrCl(COD)]₂ and dppe in a 1 : 2 ratio was investigated in detail under three different conditions. [IrCl(COD)(dppe)], 1, is formed at room temperature in the absence of base. In the presence of a strong base at room temperature, hydride complexes that retain the carbocyclic ligand in the coordination sphere are generated. In isopropanol, 1 is converted into [IrH(1,2,5,6-η²:η²-COD)(dppe)] (2) on addition of KO^tBu, with k₁₂ = (1.11 ± 0.02) × 10^-4 s^-1, followed by reversible isomerisation to [IrH(1-κ-4,5,6-η³-C₈H₁₂)(dppe)] (3) with k₂₃ = (3.4 ± 0.2) × 10^-4 s^-1 and k₃₂ = (1.1 ± 0.3) × 10^-5 s^-1 to yield an equilibrium 5 : 95 mixture of 2 and 3. However, when no hydride source is present in the strong base (KO^tBu in benzene or toluene), the COD ligand in 1 is deprotonated, followed by β-H elimination of an Ir^I-C₈H₁₁ intermediate, which leads to complex [IrH(1-κ-4,5,6-η³-C₈H₁₀)(dppe)] (4) selectively. This is followed by its reversible isomerisation to 5, which features a different relative orientation of the same ligands (k₄₅ = (3.92 ± 0.11) × 10^-4 s^-1; k_5-4 = (1.39 ± 0.12) × 10^-4 s^-1 in C₆D₆), to yield an equilibrated 32 : 68 mixture of 4 and 5. DFT calculations assisted in the full rationalization of the selectivity and mechanism of the reactions, yielding thermodynamic (equilibrium) and kinetic (isomerization barriers) parameters in excellent agreement with the experimental values. Finally, in the presence of KO^tBu and isopropanol at 80 °C, 1 is transformed selectively to K[IrH₄(dppe)] (6), a salt of an anionic tetrahydride complex of Ir^III. This product is also selectively generated from 2, 3, 4 and 5 and H₂ at room temperature, but only when a strong base is present. These results provide an insight into the catalytic action of [IrCl(COD)(LL)] complexes in the hydrogenation of polar substrates in the presence of a base.

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.

Brønsted-Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes

Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Brønsted-Lowry acids. A simple additive ligand acidity constant equation (LAC for short) allows the estimation of the acid dissociation constant Ka(LAC) of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including catalytic cycles where these reactions are proposed or observed. There are links between pKa(LAC) and pKa(THF), pKa(DCM), pKa(MeCN) for neutral and cationic acids. For the groups from chromium to nickel, tables are provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pKa(LAC) -18) to least acidic (pKa(LAC) 50). Figures are constructed showing metal acids above the solvent pKa scales and organic acids below to summarize a large amount of information. Acid-base features are analyzed for catalysts from chromium to gold for ionic hydrogenations, bifunctional catalysts for hydrogen oxidation and evolution electrocatalysis, H/D exchange, olefin hydrogenation and isomerization, hydrogenation of ketones, aldehydes, imines, and carbon dioxide, hydrogenases and their model complexes, and palladium catalysts with hydride intermediates.

Non-classical hydrogen bonding in [K(1-aza-18-crown-6)]BH4 and its 18-crown-6 counterpart

The extended structures of [K(1-aza-18-crown-6)]BH(4) and its 18-crown-6 analogue exhibits significantly different primary and secondary stabilizing interactions. However, their respective ion pairs display similar cation-to-anion interactions, in spite of the differences in the nature of the crown ether ligand.

PGSE NMR diffusion overhauser studies on [Ru(Cp*)(eta6-arene)][PF6], plus a variety of transition-metal, inorganic, and organic salts: an overview of ion pairing in dichloromethane

PGSE diffusion, 19F, 1H HOESY and 13C NMR studies for a series of [Ru(Cp*)(eta6-arene)][PF6] (1) salts are presented. The solid-state structure of [Ru(Cp*)(eta6-fluorobenzene)][PF6] (1 c) is reported. The extent of the ion pairing and the relative positions of the ions are shown to depend on the arene. For the solvent dichloromethane, new and literature PGSE data for PF6(-) salts of transition-metal, inorganic, and organic salts are compared. Taken together, these new results show that the charge distribution and the ability of the anion to approach the positively charged positions (steric effects due to molecular shape) are the determining factors in deciding the amount of ion pairing. DFT calculations of the charges in four salts of type 1, as well as in a variety of other salts, using a natural population analysis (NPA), support this view. This represents the first attempt, using experimental data, to understand, correlate, and partially explain the various degrees of ion pairing in a widely different collection of salts.

Properties of the Polyhydride Anions [WH5(PMe2Ph)3]- and [ReH4(PMePh2)3]- and Periodic Trends in the Acidity of Polyhydride Complexes

The new anionic complexes [K(18-crown-6)][WH5(PMe2Ph)3], [K(1,10-diaza-18-crown-6)][WH5(PMe2Ph)3], [K(2,2,2-crypt)][ReH4(PMePh2)3], and [K(1,10-diaza-18-crown-6)][ReH4(PMePh2)3] were prepared by reaction of KH/crown or KH/crypt with the appropriate neutral polyhydride WH6(PMe2Ph)3 or ReH5(PMePh2)3. The rate of deprotonation of the rhenium hydride in THF is much greater for the reaction involving crypt compared with that of crown. The structure of [ReH4(PMePh2)3]- is distorted pentagonal bipyramidal as determined by an X-ray diffraction study of the crypt salt. No hydridic-protonic M-H...HN bonding is detected between the hydrides of the anionic hydrides and the amino hydrogens of the cations [K(1,10-diaza-18-crown-6)]+ suggesting that stronger M-H...K interactions are present. Acid dissociation constants Ka of polyhydride complexes in THF, approximately corrected for ion pairing, are determined by NMR in order to better understand the periodic trends of metal hydrides. The pKalphaTHF of (WH6(PMe2Ph)3/[WH5(PMe2Ph)3]-) is 42+/-4 according to the equilibrium set up by reacting WH6(PMe2Ph)3 with [K(2,2,2-crypt)][ReH6(PCy3)2]. The pKalphaTHF for ReH5(PMePh2)3 can be estimated as greater than the pKalphaTHF of 38 for HNPh2 and less than the pKalphaTHF of 41 for ReH7(PCy3)2. Reaction of the phosphazene base P4-tBu with ReH7(PCy3)2 gave an equilibrium with [HP4-tBu]+[ReH6(PCy3)2]- whereas reaction with WH6(PMe2Ph)3 gave an equilibrium with [HP4-tBu]+[WH5(PMe2Ph)3]-. From these and a related equilibrium, the pKalphaTHF of [HP4-tBu]+ is found to be 40+/-4. In general, neutral complexes MHx(PR3)n (M=W, Re, Ru, Os, Ir; n=3, 2) studied to date have pKalphaTHF values from 30 to 44 on going from phenyl-substituted to alkyl-substituted phosphine ligands whereas MHx(PR3)n+ (M=Re, Fe, Ru, Os, Co, Rh, Ni, Pd, Pt; n=4, 3), including diphosphine ligands ((PR3)2=PR2-PR2), have values from 12 to 23. From the equilibrium established from the reaction of [HP2-tBu][BPh4] and [K(2,2,2-crypt)][OP(OEt)2NPh], [HP2-tBu]+ was calculated to have a pKalphaTHF of 30+/-4. The equilibrium constant for the similar deprotonation reaction with [K(18-crown-6)][{ReH2(PMePh2)2}2(mu-H)3] confirmed this value.

NMR Investigation of the Dihydrogen-Bonding and Proton-Transfer Equilibria between the Hydrido Carbonyl Anion [HRe2(CO)9]− and Fluorinated Alcohols

The interaction of fluorinated alcohols with the anionic hydrido complex [HRe2(CO)9]- (1) has been investigated by NMR spectroscopy. According to the acidic strength of the alcohols, the interaction may result not only in the formation of dihydrogen-bonded ROH...[HRe2(CO)9]- adducts 2, but also in proton transfer to give the neutral species [H2Re2(CO)9] (3). With the weaker acid trifluoroethanol (TFE) evidence for the occurrence of the dihydrogen-bonding equilibrium was obtained by 2D 1H NOESY. The dependence of the hydride chemical shift on TFE concentration at different temperatures provided values for the constants of this equilibrium, from which the thermodynamic parameters were evaluated as deltaH(degrees) = -2.6(2) kcal mol(-1), deltaS(degrees) = -9.3(2) cal mol(-1) K(-1). This corresponds to a rather low basicity factor (E(j) = 0.64). Variable-temperature T1 measurements allowed the proton-hydride distance in adduct 2 a to be estimated (1.80 angstroms). In the presence of hexafluoroisopropyl alcohol (HFIP) simultaneous occurrence of both dihydrogen-bonding and proton-transfer equilibria was observed, and the equilibria shifted versus the protonated product 3 with increasing HFIP concentration and decreasing temperature. Reversible proton transfer between the alcohol and the hydrido complex occurs on the NMR timescale, as revealed by a 2D 1H EXSY experiment at 240 K. For the more acidic perfluoro-tert-butyl alcohol (PFTB) the protonation equilibrium was further shifted to the right. Thermal instability of 3 prevented the acquisition of accurate thermodynamic data for these equilibria. The occurrence of the proton-transfer processes (in spite of the unfavorable pK(a) values) can be explained by the formation of homoconjugated RO...HOR- pairs which stabilize the alcoholate anions.

Chemistry of Ruthenium(II) Monohydride and Dihydride Complexes Containing Pyridyl Donor Ligands Including Catalytic Ketone H2-Hydrogenation1

In this study we determine the changes to the properties of dihydride catalysts for ketone H2-hydrogenation by successively replacing the amine donors in the known dach complex RuH2(PPh3)2(dach) (2a), dach = 1,2-(R,R)-diaminocyclohexane, with one pyridyl group in the corresponding 2-(aminomethyl)pyridine (ampy) complexes RuH2(PPh3)2(ampy) (2b) and with two pyridyl groups in the complexes RuH2(PPh3)2(bipy) (2c) and RuH2(PPh3)2(phen) (2d). The ruthenium monohydride complex, (OC-6-54)-RuHCl(PPh3)2(ampy), (1b with Cl trans to H) was prepared by the addition of 1 equiv of ampy to RuHCl(PPh3)3 in THF. Treatment of the monohydride complex with K[BH(sec-Bu)3] in THF or KOtBu/H2 in toluene resulted in the formation of a mixture of at least two isomers of the highly reactive, air-sensitive ruthenium dihydride complex 2b. One is the cis dihydride (OC-6-14)-2b or more simply c,t-2b with trans PPh3 groups and another is the cis dihydride c,c-2b (OC-6-42) that has PPh3 trans to H and PPh3 trans to N(pyridyl). The isomer c,c-2b slowly converts to c,t-2b in solution. The reaction of 1b with KOtBu under Ar results in the formation of a mixture that includes a complex with an imino ligand HN=CH-2-py while the same reaction under H2 leads to c,c-2b and then c,t-2b. The dach complex c,t-2a, reacts with ampy, 2,2'-bipyridine (bipy), and 1,10-phenanthroline (phen) in refluxing THF to form the substituted cis-dihydride complexes c,t-2b, (OC-6-13)-RuH2(PPh3)2(bipy) (c,t-2c with trans PPh3 groups) and (OC-6-13)-RuH2(PPh3)2(phen), c,t-2d, respectively. The dihydrides containing amino groups and cis-PPh3 groups, i.e., c,c-2a or c,c-2b, are active precatalysts for the H2-hydrogenation of acetophenone (neat or in benzene) under mild reaction conditions, whereas those with trans-PPh3 groups, c,t-2a and c,t-2b are much less active. The combination of ampy complex 1b and KOtBu also provides a catalyst in benzene that is more active than the corresponding dach system. The complexes without amino groups c,t-2c and c,t-2d are air-stable and inactive as hydrogenation catalysts under comparable conditions. The mechanism of hydrogenation of ketones catalyzed by isomers of 2a,b is thought to be similar and to proceed via a trans-dihydride complex, t,c-2a or t,c-2b, and an amido complex, neither of which are directly observed for the ampy complexes. The dihydride complex c,t-2b reacts with formic acid to give (OC-6-45)-RuH(OCHO)(PPh3)2(ampy), 3b, with formate trans to hydride. The structures of 1b, c,t-2b, c,t-2c, and 3b have been determined by single-crystal X-ray diffraction.