The reaction of M(CO)3(Ph2PCH2CH2PPh2) (M = Fe, Ru) with parahydrogen: probing the electronic structure of reaction intermediates and the internal rearrangement mechanism for the dihydride products

Iron-Catalyzed Parahydrogen Induced Polarization

Parahydrogen induced polarization (PHIP) can address the low sensitivity problem intrinsic to nuclear magnetic resonance spectroscopy. Using a catalyst capable of reacting with parahydrogen and substrate in either a hydrogenative or nonhydrogenative manner can result in signal enhancement of the substrate. This work describes the development of a rare example of an iron catalyst capable of reacting with parahydrogen to hyperpolarize olefins. Complexes of the form (MesCCC)Fe(H)(L)(N2) (L = Py (Py = pyridine), PMe3, PPh3) were synthesized from 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 hydrogenation of olefins and demonstrated high activity toward the hydrogenation of monosubstituted terminal olefins. Reactions with p-H2 resulted in the first PHIP effect mediated by iron which requires diamagnetism throughout the reaction sequence. This work represents the development of a new PHIP catalyst featuring iron, unlocking potential to develop more PHIP catalysts based on first-row transition metals.

Effects of a Tridentate Pincer Ligand on Parahydrogen Induced Polarization

The role of ligands in rhodium- and iridium-catalyzed Parahydrogen Induced Polarization (PHIP) and SABRE (signal amplification by reversible exchange) chemistry has been studied in the benchmark systems, [Rh(diene)(diphos)]⁺ and [Ir(NHC)(sub)₃ (H)₂ ]⁺ , and shown to have a great impact on the degree of hyperpolarization observed. Here, we examine the role of the flanking moieties in the electron-rich monoanionic bis(carbene) aryl pincer ligand, ^Ar CCC (Ar=Dipp, 2,6-diisopropyl or Mes, 2,4,6-trimethylphenyl) on the cobalt-catalyzed PHIP and PHIP-IE (PHIP via Insertion and Elimination) chemistry that we have previously reported. The mesityl groups were exchanged for diisopropylphenyl groups to generate the (^Dipp CCC)Co(N₂ ) catalyst, which resulted in faster hydrogenation and up to 390-fold ¹ H signal enhancements, larger than that of the (^Mes CCC)Co-py (py=pyridine) catalyst. Additionally, the synthesis of the (^Dipp CCC)Rh(N₂ ) complex is reported and applied towards the hydrogenation of ethyl acrylate with parahydrogen to generate modest signal enhancements of both ¹ H and ¹³ C nuclei. Lastly, the generation of two (^Mes CCC)Ir complexes is presented and applied towards SABRE and PHIP-IE chemistry to only yield small ¹ H signal enhancements of the partially hydrogenated product (PHIP) with no SABRE hyperpolarization.

Mechanistic in situ investigation of heterogeneous hydrogenation over Rh/TiO2 catalysts: selectivity, pairwise route and catalyst nature

We report a catalyst with the highest selectivity toward pairwise hydrogen addition of 7% among supported metal catalysts, found as a result of variation of Rh/TiO₂ catalyst preparation procedures.

Impact of the Novel Z-Acceptor Ligand Bis{(ortho-diphenylphosphino)phenyl}zinc (ZnPhos) on the Formation and Reactivity of Low-Coordinate Ru(0) Centers

The preparation and reactivity with H₂ of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(o-C₆H₄PPh₂)₂) are described. Ru(ZnPhos)(CO)₃ (2) and Ru(ZnPhos)(IMe₄)₂ (4; IMe₄ = 1,3,4,5-tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh₃)(C₆H₄PPh₂)₂(ZnMe)₂ (1) or Ru(PPh₃)₃HCl/LiCH₂TMS/ZnMe₂ with CO and IMe₄, respectively. Structural and electronic structure analyses characterize both 2 and 4 as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in 2, Ru adopts an octahedral coordination, while 4 displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2 loses CO to give Ru(ZnPhos)(CO)₂ that then adds H₂ over the Ru-Zn bond to form Ru(ZnPhos)(CO)₂(μ-H)₂ (3). In contrast, 4 reacts directly with H₂ to set up an equilibrium with Ru(ZnPhos)(IMe₄)₂H₂ (5), the product of oxidative addition at the Ru center. DFT calculations rationalize these different outcomes in terms of the energies of the square-pyramidal Ru(ZnPhos)L₂ intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H₂ to add across the Ru-Zn bond, but for L = IMe₄, this species is kinetically inaccessible and reaction can only occur at the Ru center. This difference is related to the strong π-acceptor ability of CO compared to IMe₄. Steric effects associated with the larger IMe₄ ligands are not significant. Species 4 can be considered as a Ru(0)L₄ species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)L₄ species devoid of strong π-acceptor ligands.

Rocha MV, Hamlin TA, Poater J, Bickelhaupt FM. Understanding the differences between iron and palladium in cross-coupling reactions

We aim at developing design principles, based on quantum chemical analyses, for a novel type of iron-based catalysts that mimic the behavior of their well-known palladium analogs in the bond activation step of cross coupling reactions. To this end, we have systematically explored C-X bond activation via oxidative addition of CH3X substrates (X = H, Cl, CH3) to model catalysts mFe(CO)4q (q = 0, -2; m = singlet, triplet) and, for comparison, Pd(PH3)2 and Pd(CO)2, using relativistic density functional theory at the ZORA-OPBE/TZ2P level. We find that the neutral singlet iron catalyst 1Fe(CO)4 activates all three C-X bonds via barriers that are lower than those for Pd(PH3)2 and Pd(CO)2. This is a direct consequence of the capability of the iron complex to engage not only in π-backdonation, but also in comparably strong σ-donation. Interestingly, whereas the palladium complexes favor C-Cl activation, 1Fe(CO)4 shows a strong preference for activating the C-H bond, with a barrier as low as 10.4 kcal mol-1. Our results suggest a high potential for iron to feature in palladium-type cross-coupling reactions.

The effect of coordination of alkanes, Xe and CO2 (η1-OCO) on changes in spin state and reactivity in organometallic chemistry: a combined experimental and theoretical study of the photochemistry of CpMn(CO)3

A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragment 3CpMn(CO)2 formed upon photolysis of CpMn(CO)3. Experimental data are provided for reactions in n-heptane and perfluoromethylcyclohexane (PFMCH), as well as in PFMCH doped with C2H6, Xe and CO2. In PFMCH we find that the conversion of 3CpMn(CO)2 to 1CpMn(CO)2(PFMCH) is much slower (τ = 18 (±3) ns) than the corresponding reactions in conventional alkanes (τ = 111 (±10) ps). We measure the effect of the coordination ability by doping PFMCH with alkane, Xe and CO2; these doped ligands form the corresponding singlet adducts with significantly variable formation rates. The reactivity as measured by the addition timescale follows the order 1CpMn(CO)2(C5H10) (τ = 270 (±10) ps) > 1CpMn(CO)2Xe (τ = 3.9 (±0.4) ns) ∼ 1CpMn(CO)2(CO2) (τ = 4.7 (±0.5) ns) > 1CpMn(CO)2(C7F14) (τ = 18 (±3) ns). Electronic structure theory calculations of the singlet and triplet potential energy surfaces and of their intersections, together with non-adiabatic statistical rate theory, reproduce the observed rates semi-quantitatively. It is shown that triplet adducts of the ligand and 3CpMn(CO)2 play a role in the kinetics, and account for the variable timescales observed experimentally.

DFT study of isomers of the ruthenium dihydride complex RuH2(CO)2(AsMe2Ph)2

A density functional theory (DFT) study of cct-As, ccc, and cct-CO isomers of the ruthenium dihydride complex RuH₂(CO)₂(AsMe₂Ph)₂ is reported (see Scheme for the labeling isomer 34 structures of RuH₂(CO)₂(AsMe₂Ph)₂). Complex geometries and relative energies of different isomers have been calculated with both B3LYP and M06-2X functionals. The results show that the B3LYP calculated Boltzmann populations of cct-As, ccc, and cct-CO isomers are 65.5, 34.2, and 0.3%, respectively. These are in better agreement with the experimental data than those calculated at the M06-2X level. However, the calculations of ¹H NMR chemical shifts were found to be better described with M06-2X than with B3LYP or with HF level of theories. In addition, a transition state between the two most stable isomers was determined through DFT/(B3LYP or M06-2X) calculations. Graphical Abstract Scheme: Labeling structure of RuH₂(CO)₂(AsMe₂Ph)₂.

Activation of a (cyclooctadiene) rhodium(I) complex supported by a chiral ferrocenyl phosphine thioether ligand for hydrogenation catalysis: a combined parahydrogen NMR and DFT study

The reaction of [RhCl(P,S(t)Bu)(COD)] (1) or [Rh(P,S(t)Bu)(COD)]BF4 (2) where (P,S(t)Bu) is CpFe[η(5)-1,2-C5H3(PPh2)(CH2S(t)Bu)] with H2 in MeOH gives rise to COD hydrogenation and formation of a solvent-stabilized product. The formation of hydride species cannot be observed in view of a very rapid H/D exchange between H2 and the solvent. Introduction of pyridine or acetonitrile slows down this exchange process and allows observation of diastereometric dihydride complexes, [Rh(P,S(t)Bu)(H)2(L)2](+), the stereochemistry of which was fully elucidated. The hydride site exchange rates have been derived from EXSY NMR experiments and used, with assistance from DFT calculation, to elucidate the isomerization and site exchange mechanisms.

A simple and low-cost device for generating hyperpolarized contrast agents using parahydrogen

A detailed description of the construction and use of a device for hyperpolarization of select contrast agents is presented. The device is based on molecular incorporation of the spin-order inherent to parahydrogen, followed by order transfer to a metastable heteronuclear alignment. Design considerations and experimental results relating to catalyst/solvent choice and handling, solvent heating, efficient gas entrainment and spin-order transfer are described. The resulting degree of hyperpolarization is shown to be substantial, ranging from a few to over 50%, depending on the choice of target molecule. Finally, the use of the hyperpolarized agent is demonstrated in a series of in vivo images.

In situ photochemistry with NMR detection of organometallic complexes

A review focusing on the application of NMR spectroscopy to the study of organometallic photochemistry where the photochemical step is performed in situ, i.e. the irradiation of the sample takes place within the probe of the NMR spectrometer. Various experimental designs, taken from all areas of chemical and biological study, that facilitate in situ irradiation are discussed, paying attention to light sources and light delivery methods. The literature covering the application of the in situ method across the field of organometallic chemistry is then reviewed. There is particular emphasis on studies of reactive organometallic compounds with weakly coordinating ligands such as alkane, xenon and other “solvent” species, as complexes with short lifetimes benefit most from application of the in situ illumination method.

Contrasting photochemical and thermal reactivity of Ru(CO)2(PPh3)(dppe) towards hydrogen rationalised by parahydrogen NMR and DFT studies

The synthesis, characterisation and thermal and photochemical reactivity of Ru(CO)2(PPh3)(dppe) 1 towards hydrogen are described. Compound proved to exist in both fac (major) and mer forms in solution. Under thermal conditions, PPh3 is lost from 1 in the major reaction pathway and the known complex Ru(CO)2(dppe)(H)2 2 is formed. Photochemically, CO loss is the dominant process, leading to the alternative dihydride Ru(CO)(PPh3)(dppe)(H)2 3. The major isomer of 3, viz. 3a, contains hydride ligands that are trans to CO and trans to one of the phosphorus atoms of the dppe ligand but a second isomer, 3b, where both hydride ligands are trans to distinct phosphines, is also formed. On the NMR timescale, no interconversion of 3a and 3b was observed, although hydride site interchange is evident with activation parameters of DeltaH(double dagger) = 95 +/- 6 kJ mol(-1) and DeltaS(double dagger) = 26 +/- 17 J K(-1) mol(-1). Density functional theory confirms that the observed species are the most stable isomeric forms, and suggests that hydride exchange occurs via a transition state featuring an eta2-coordinated H2 unit.

Parahydrogen studies of H2addition to Ir(i) complexes containing chiral phosphine–thioether ligands: implications for catalysis

Ir(CO)[CpFe{eta5-C5H3(PPh2)CH2SR}]Cl [R = Ph and (t)Bu], containing a kappa2:P,S ligand, undergoes H2 addition across the S-Ir-CO axis under kinetic control to form two distinct diastereoisomeric products, which then rearrange via S dissociation in a process that can be hijacked for useful catalysis, but ultimately form a single diastereoisomer of the thermodynamic product where the hydride ligands are trans to chloride and phosphine.

Computational study of the energetics of3Fe(CO)4,1Fe(CO)4and1Fe(CO)4(L), L = Xe, CH4, H2and CO

Large basis CCSD(T) calculations are used to calculate the energetics of 3Fe(CO)4, 1Fe(CO)4 and 1Fe(CO)4(L), L = Xe, CH4, H2 and CO. . The relative energy of the excited singlet state of Fe(CO)4 with respect to the ground triplet state is not known experimentally, and various lower levels of theory predict very different results. Upon extrapolating to the infinite basis set limit, and including corrections for core-core and core-valence correlation, scalar relativity, and multi-reference character of the wavefunction, the best CCSD(T) estimate for the spin-state splitting in iron tetracarbonyl is 2 kcal mol(-1). Calculation of the dissociation energy of 1Fe(CO)4(L) into singlet fragments, taken together with known experimental behaviour of triplet Fe(CO)4, provides independent evidence for the fact that the spin-state splitting is smaller than 3 kcal mol(-1). These calculations highlight some of the challenges involved in benchmark calculations on transition metal containing systems.