Redox-active zwitterionic ZCp ligands for stable RhI/IrI complexes and hyperpolarisation applications

Air-stable Ir and Ru complexes of a chelating pyrimidine-functionalized N-heterocyclic carbene were synthesized. The complexes were characterized by NMR spectroscopy and single-crystal X-ray diffraction and were found to be catalytically active for transfer hydrogenation, β-alkylation of secondary alcohols with primary alcohols, and N-alkylation of amines with primary alcohols. Notably, the Ir complexes were found to catalyze the N-alkylation of amines using the mild base NaHCO3.

Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst

The unique dinuclear Ir(III) complex (Cp*IrCl)(μ-H)[μ-(η1:η3-C6H6S)](IrCp*) (1) has been synthesized and characterized by NMR spectroscopy (1H and 13C), elemental analysis, and single crystal X-ray diffraction. It is the first structurally determined complex in which an activated thiophene ligand displays an η3-allylic interaction. 1 appears to form from successive C-H bond activations of 2,5-dimethylthiophene, resulting in its bridging the two iridium centers. The η3-allylic interaction occurs with one of the Ir centers and has Ir–Cthio bond lengths ranging from 2.133(5)-2.207(5) A; the C–C double bond involved in the interaction has a bond length of 1.438(7) A compared to 1.348(8) A for the uncoordinated C–C double bond. The 3-carbon of the thiophene ring bridges both iridium centers with bond lengths of 2.036(5) A and 2.208(5) A. 1 crystallizes in space group P−1 with cell constants a = 8.6303(6) A, b = 9.0153(6) A, c = 18.1089(12) A, α = 84.728(1)°, β = 87.534(1)°, γ = 64.373(1)°, and Z = 2. The structure was solved by direct methods and refined to R = 0.0363 (F 2 > 2σ(F 2)) and wR = 0.0851 (F 2). The NMR data indicate the solution state structure is consistent with the solid state structure. The reaction of 2,5-dimethylthiophene with [Cp*IrHCl]2 leads to the formation of a unique double C–H activation product displaying a unique μ-η1:η3 coordination of the thiophene

Generating parahydrogen-induced polarization (PHIP) of nuclear spins with immobilized transition metal complexes as hydrogenation catalysts allows one to produce pure hyperpolarized substances, which can open new revolutionary perspectives for PHIP applications. A major drawback of immobilized complexes is their low stability under reaction conditions. In the present work we studied an immobilized iridium complex, Ir/SiO2 P, synthesized by a covalent anchoring of Vaska’s complex on phospine-modified silica gel. This complex was used to obtain hyperpolarized gasses in the gas phase hydrogenation of propene, propyne and 1-butyne with parahydrogen in PASADENA and ALTADENA experiments. It was found that, in contrast to other immobilized complexes, Ir/SiO2 P is stable under reaction conditions at up to 140°C, and the reduction of iridium does not occur according to XPS analysis. Moreover, the application of Ir/SiO2 P catalyst allowed us to generate continuous flow of hyperpolarized propene and 1-butene with (300–500)-fold NMR signal enhancement which is significantly higher than commonly observed for most supported metal catalysts. The shape of polarized propene signals in PASADENA experiment has indicated that parahydrogen addition to propyne occurs non-stereospecifically, i.e. PHIP was observed for all protons of the vinyl fragment of propene. The analysis of the polarized signals has shown that syn pairwise addition dominates, which was confirmed by spectra simulations. It was found that storage of Ir/SiO2 P under Ar atmosphere leads to a decrease in PHIP amplitude and an increase in the activity of the catalyst. This observation is discussed in terms of the interaction of Ir/SiO2 P with trace amounts of oxygen in Ar, leading to partial oxidation of triphenylphosphine ligand to triphenylphosphine oxide accompanied by the activation of the immobilized complex. It was also found that the interaction of Ir/SiO2 P with alkenes likely leads to formation of stable monohydride complexes, decreasing the production of PHIP in hydrogenations. At the same time, stable substrate complexes are likely formed in alkyne hydrogenations, leading to a significant decrease in the monohydride complex formation and to an increased production of PHIP.

Parahydrogen inducedpolarization (PHIP)can addressthe low sensitivity problem intrinsic to nuclear magnetic resonancespectroscopy. Using a catalyst capable of reacting with parahydrogen and substrate in either a hydrogenative or nonhydrogenativemanner can result in signal enhancement of the substrate. This workdescribes the development of a rare example of an iron catalyst capableof reacting with parahydrogen to hyperpolarize olefins.Complexes of the form (MesCCC)Fe(H)(L)(N2) (L= Py (Py = pyridine), PMe3, PPh3) were synthesizedfrom the reaction of the parent complexes (MesCCC)FeMes(L)(Mes = mesityl) with H2. The isolated low-spin iron(II)hydride compounds were characterized via multinuclear NMR spectroscopy,infrared spectroscopy, and single crystal X-ray diffraction. (MesCCC)Fe(H)(Py)(N2) is competent in the hydrogenationof olefins and demonstrated high activity toward the hydrogenationof monosubstituted terminal olefins. Reactions with p-H2 resulted in the first PHIP effect mediated by ironwhich requires diamagnetism throughout the reaction sequence. Thiswork represents the development of a new PHIP catalyst featuring iron,unlocking potential to develop more PHIP catalysts based on first-rowtransition metals.

We have studied the o/p spin conversion of dihydrogen in contact with frozen solutions of Vaska’s complex Ir(CO)Cl(PPh3)2 (1) in C6D6 and with polycrystalline 1 at 77 K. The main purpose of this study was to elucidate the mechanism of this type of reactions found accidentally previously (Eisenschmid et al JACS 109:8089–8091, 1987 and Eisenberg ACS 24:110–116, 1991). The formation of p-H2 was followed after thawing of the samples by 1H nuclear magnetic resonance (NMR) spectroscopy at 298 K, where the oxidative addition of dihydrogen to 1 occurs leading to Vaska’s dihydride Ir(CO)ClH2(PPh3)2 (2) which is known to exhibit para-hydrogen-induced polarization (PHIP). The PHIP signal was shown to be proportional to the concentration of p-H2 as elucidated from the decrease of the signal of dissolved o-H2. The reaction was found to be faster for the frozen solution as compared to the polycrystalline powder. Optical microscopy showed that small particles of 1 are separated from the solution during the freezing process, exhibiting a larger surface area as compared to the polycrystalline powder. When a mixture of H2 and D2 was exposed to the frozen solutions or to the polycrystalline powder, the formation of HD was observed by 1H NMR. This finding indicates the presence of a chemical spin conversion involving two dihydrogen molecules. Additional 1H NMR experiments of dihydrogen in frozen C6D6 at 110 K indicated the formation of larger pores containing gaseous H2 as well as dihydrogen sites in interstitial sites between benzene molecules. Moreover, in the presence of 1, a signal at −4.5 ppm was observed which was attributed to a dihydrogen in close contact with Ir.

Parahydrogen induced polarisation (PHIP) is often used to enhance the sensitivity of NMR, with the purpose of extending the applicability of the technique. Nuclear spin hyperpolarisation obtained via PHIP is generally localised on the protons derived from the addition of para-enriched hydrogen to an unsaturated substrate. This limitation has been previously addressed by pulse schemes that can spread this hyperpolarised magnetisation through the entire network of J-coupled protons in the product molecule. Here, we extend this approach, by implementing 2D NMR spectroscopy on such network of hyperpolarised protons. This novel approach to 2D acquisition during parahydrogenation allows information to be gained from the entirety of a molecule, quicker and/or at lower concentrations than by conventional NMR. The efficacy of the method is exemplified by performing a 2D TOCSY experiment during hydrogenative PHIP, using 2-pentyn-1-ol as a substrate. A 2D spectrum was obtained in a few minutes at micromolar concentration, demonstrating the applicability of this methodology.

Oxidative addition of bulky primary, secondary, and tertiary silanes to PNP (PNP = [N(2-P(i)Pr(2)-4-Me-C(6)H(3))(2)](-)) iridium complexes (PNP)IrH(2) and (PNP)Ir(COE) (11) afforded iridium silyl hydride complexes (PNP)Ir(H)(SiRR'R'') (3-8). Addition of 2 equiv of PhSiH(3) or (3,5-Me(2)C(6)H(3))SiH(3) to (PNP)IrH(2) or 11 yielded disilyl complexes (PNP)Ir(SiH(2)R)(2) (R = Ph (9), 3,5-Me(2)C(6)H(3) (10)). Hydride abstraction from (PNP)Ir(H)(SiH(2)R) (R = Trip (5), Dmp (6)) by [Ph(3)C][B(C(6)F(5))(4)] afforded iridium silylene complexes [(PNP)(H)Ir=SiR(H)][B(C(6)F(5))(4)] (R = Trip (12), Dmp (13)) exhibiting downfield (29)Si NMR resonances (234 ppm (12), 226 ppm (13)) and downfield (1)H NMR resonances for the Si-H group (10.76 ppm (12), 9.76 ppm (13)). Thermally stable disubstituted silylene complexes [(PNP)(H)Ir=SiPh(2)][A] (A = (-)B(C(6)F(5))(4) (14), (-)CB(11)H(6)Br(6) (16)) were isolated via hydride abstraction from (PNP)Ir(H)(SiHPh(2)). The X-ray structure of 16 confirmed sp(2) hybridization at silicon and revealed a short Ir-Si bond of 2.210(2) A. Catalytic hydrosilation of alkenes by hydrogen-substituted silylene complexes [(PNP)(H)Ir=SiMes(H)][B(C(6)F(5))(4)] (1) and 14 exhibited anti-Markovnikov regioselectivity with an array of alkene substrates. Addition of H(3)SiMes to complex 1 afforded [(PNP)(SiH(Mes)(Hex))IrH(SiH(2)Mes)][B(C(6)F(5))(4)] (19), featuring a beta-agostic interaction demonstrated by a J(SiH) of 102 Hz for the N-SiH hydrogen. Similarly, addition of H(2)SiPh(2) to 16 afforded the structurally characterized Ir(V) disilyl complex [(PNP)(SiPh(2))Ir(SiPh(2)H)(H)(2)][CB(11)H(6)Br(6)] (20). Complex 20 was found to be catalytically active for the hydrosilation of alkenes, which is consistent with its intermediacy in the catalytic cycle.

A series of five heteroleptic Ir(III) complexes of the general form Ir(ppy)2(C∧C:) have been prepared (C∧C represents a bidentate cyclometalated phenyl-substituted imidazolylidene ligand). The five complexes arise from the cyclometalated phenyl ring of the NHC ligand being unsubstituted or having electron-donating (OMe and Me) or electron-withdrawing (Cl and F) groups at the 2- and 4-positions of the ring. The synthesized phenyl-substituted imidazole precursors, imidazolium salts, and Ir(III) complexes have been characterized by elemental analysis, NMR spectroscopy, cyclic voltammetry, and electronic absorption and emission spectroscopy. The molecular structures for two imidazolium salts and two Ir(III) complexes were determined by single-crystal X-ray diffraction. Each of the Ir(III) complexes exhibited intense photoluminescence in acetonitrile solution at room temperature with quantum yields (ϕp) ranging from 42% to 68% and excited-state lifetimes on the order of 2 μs. Voltammetric experiments revealed ...

A novel coordination polymer, {[Cd4(Dccbp)4]·H2O} (1) (Dccbp = 3,5-dicarboxy-1-(3-carboxybenzyl)pyridin-1-ium) was synthesized under hydrothermal conditions by a zwitterionic organic ligand and characterized by single crystal X-ray diffraction, infrared spectrum (IR), thermogravimetric analysis (TG), powder X-ray diffraction (PXRD) and luminescence. Complex 1 with a pyridine cation basic skeleton has the potential to serve as the first case of a luminescent material based on the zwitterionic type of organic ligand for selective, sensitive, and recyclable sensing of 2,4,6-trinitrophenol in the aqueous phase.

Synthesis, photoluminescence properties of novel cationic Ir(III) complexes with phenanthroimidazole derivative as the ancillary ligand

Parahydrogen-induced polarisation (PHIP) is a nuclear spin hyperpolarisation technique employed to enhance NMR signals for a wide range of molecules. This is achieved by exploiting the chemical reactions of parahydrogen (para-H2), the spin-0 isomer of H2. These reactions break the molecular symmetry of para-H2 in a way that can produce dramatically enhanced NMR signals for reaction products, and are usually catalysed by a transition metal complex. In this review, we discuss recent advances in novel homogeneous catalysts that can produce hyperpolarised products upon reaction with para-H2. We also discuss hyperpolarisation attained in reversible reactions (termed signal amplification by reversible exchange, SABRE) and focus on catalyst developments in recent years that have allowed hyperpolarisation of a wider range of target molecules. In particular, recent examples of novel ruthenium catalysts for trans and geminal hydrogenation, metal-free catalysts, iridium sulfoxide-containing SABRE systems, and cobalt complexes for PHIP and SABRE are reviewed. Advances in this catalysis have expanded the types of molecules amenable to hyperpolarisation using PHIP and SABRE, and their applications in NMR reaction monitoring, mechanistic elucidation, biomedical imaging, and many other areas, are increasing.

Heating a mixture of Ir(4)(CO)(9)(PPh(3))(3) (1) and 2 equiv of C(60) in refluxing chlorobenzene (CB) affords a "butterfly" tetrairidium-C(60) complex Ir(4)(CO)(6){mu(3)-kappa(3)-PPh(2)(o-C(6)H(4))P(o-C(6)H(4))PPh(eta(1)-o-C(6)H(4))}(mu(3)-eta(2):eta(2):eta(2)-C(60)) (3, 36%). Brief thermolysis of 1 in refluxing chlorobenzene (CB) gives a "butterfly" complex Ir(4)(CO)(8){mu-k(2)-PPh(2)(o-C(6)H(4))PPh}{mu(3)-PPh(2)(eta(1):eta(2)-o-C(6)H(4))} (2, 64%) that is both ortho-phosphorylated and ortho-metalated. Interestingly, reaction of 2 with 2 equiv of C(60) in refluxing CB produces 3 (41%) by C(60)-assisted ortho-phosphorylation, indicating that 2 is the reaction intermediate for the final product 3. On the other hand, reaction of Ir(4)(CO)(8)(PMe(3))(4) (4) with excess (4 equiv) C(60) in refluxing 1,2-dichlorobenzene, followed by treatment with CNCH(2)Ph at 70 degrees C, affords a square-planar complex with two C(60) ligands and a face-capping methylidyne ligand, Ir(4)(CO)(3)(mu(4)-CH)(PMe(3))(2)(mu-PMe(2))(CNCH(2)Ph)(mu-eta(2):eta(2)-C(60))(mu(4)-eta(1):eta(1):eta(2):eta(2)-C(60)) (5, 13%) as the major product. Compounds 2, 3, and 5 have been characterized by spectroscopic and microanalytical methods, as well as by single-crystal X-ray diffraction studies. Cyclic voltammetry has been used to examine the electrochemical properties of 2, 3, 5, and a related known "butterfly" complex Ir(4)(CO)(6)(mu-CO){mu(3)-k(2)-PPh(2)(o-C(6)H(4))P(eta(1)-o-C(6)H(4))}(mu(3)-eta(2):eta(2):eta(2)-C(60)) (6). These cyclic voltammetry data suggest that a C(60)-mediated electron transfer to the iridium cluster center takes place for the species 3(3)(-) and 6(2)(-) in compounds 3 and 6. The cyclic voltammogram of 5 exhibits six well-separated reversible, one-electron redox waves due to the strong electronic communication between two C(60) cages through a tetrairidium metal cluster spacer. The electrochemical properties of 3, 5, and 6 have been rationalized by molecular orbital calculations using density functional theory and by charge distribution studies employing the Mulliken and Hirshfeld population analyses.

The activation of CO2 by chemical, electrochemical, and photochemical means is discussed. Binuclear transition metal complexes mediate oxygen atom transfers from CO2 by three distinct chemical pathways: (i) deoxygenation of CO2, (ii) multiple bond metathesis, and (iii) disproportionation. The complex Ir2 (μ‐CNR)2(CNR)2(dmpm),(dmpm = bis(dimethylphosphino)methane) undergoes double cycloaddition of CO2 to its μ‐CNR ligands. A subsequent reaction produces the bis(carbamoyl) complex [Ir2(μ‐CO)(μ‐H)(CONHR)2(CNR)2(dmpm)2]Cl. Isotope labelling studies show that the μ‐CO ligand results from net deoxygenation of CO2. In contrast, the binuclear nickel complex Ni2(μ‐CNMe)(CNMe)2(dppm)2 (dppm = bis‐(diphenylphosphino)methane) reacts with liquid CO2 to give the tricarbonyl complex Ni2(μ‐CO)(CO)2(dppm)2. Isotope labelling indicates that the carbonyl ligands are not derived from CO2 deoxygenation, but from C/CO triple bond metathesis. The reaction of CO2 with the Ir(0) complex Ir2(CO)3(dmpm)2 leads to CO2 disproportionation by formation of the carbonate, Ir2(CO3)(CO)2(dmpm)2, and tetracarbonyl, Ir2(CO)4(dmpm)2, complexes. The complex Ir2(CO3)(CO)2 (dmpm)2 undergoes reversible O‐atom transfers from its carbonate ligand. The electrochemical activation of CO2 by the binuclear Ni2(μ‐CNMe)(CNMe)2(dppm)2 and trinuclear [Ni3(μ‐CNMe)(μ‐I)(dppm)3][PF6] species is described. The triangular nickel complex [Ni3(μ3‐CNMe)(μ3‐I)(dppm)3][PF6] is an electrocatalyst for the reduction of CO2. The cluster exhibits a reversible single electron reduction at E0(+/0) = −1.09 V vs. Ag/AgCl. In the presence of CO2, the cluster reduces CO2 by an EC' electrochemical mechanism. The reduction products correspond to the disproportionation and H‐atom abstraction products of CO2*−, with a partitioning ratio of 10:1. Isotope labelling studies with 13CO2 indicate that 13CO2*− disproportionation produces 13CO and 13CO32−.Studies of the photochemical activation of CO2 by Ni2(μ‐CNMe)(CNMe)2(dppm)2 are described. The bimolecular photochemical addition of CO2 to the complex was examined by laser transient absorbance spectroscopy. Photolysis at 355nm in the presence of CO2 (1 atm) leads to cycloaddition of CO2 to the μ‐CNMe ligand and the complex Ni2(μ‐CN(Me)C(O)O)(CNMe)2(dppm)2 with Φ355=0.05. The triplet excited state of Ni,(μ‐CNMe)(CNMe)2(dppm)2 was determined to react with CO2 with the bimolecular reaction rate constant k = 1 × 104 M−1 s−1. Bridging ligand substituent effects and solvent dependence of the lowest energy electronic absorption spectral bands of the series of complexes, Ni2(μ‐L)(CNMe)2(dppm)2, L = CNMe, CNC6H5, CN‐p‐C6H4Cl. and CN‐p‐C6H4Me, confirm the assignment of di‐metal to bridging ligand charge transfer (M2→μ‐LCT). This assignment is supported by results of extended Hückel calculations which indicate a LUMO of predominantly μ‐isocyanide π* character. A systematic study of the nature of the lowest excited states of related d10–d10 binuclear complexes of the type Ni2(μ‐L)(CNMe)2(dppm)2, where L = CNMe(Ph)+, CNMe2+, CNMe(C5H11)+, CNMe(H)+, CNMe(CH2C6H5)+, and NO+ reveals dramatic differences in the lowest excited states of the three classes of complexes: μ‐isocyanide, μ‐aminocarbyne, and μ‐nitrosyl. Spectroscopic and extended Hückel MO studies confirm that the μ‐isocyanide complexes are characterized by di‐metal to bridging ligand charge transfer (M2 → μ‐LCT) excited states. However, the μ‐aminocarbyne and μ‐nitrosyl complexes exhibit bridging ligand to metal charge transfer (μ‐L→M2) and intraligand (IL) lowest excited states, respectively.

The synthesis and behaviour of pyrazine mononuclear carbonyl complexes of Rh(I), Ir(I), Ru(II) and Os(II)