Partially fluorinated alkanes, arenes, and alkenes can be transformed by a variety of transition metal and lanthanide systems. Although the C–H bond is weaker than the C–F bond regardless of the hybridization of the carbon, the reaction of the C–F bond at the metal is usually more exothermic than the corresponding reaction of the C–H bonds. Both bonds are activated by the metal systems, but the preference for activating these bonds depends on the nature of the hydrocarbon and of the metal system, so that the reaction can be directed exclusively toward C–H or C–F bonds or yield a mixture of products. Additionally, the presence of fluorine differentiates between C–H bonds at different positions resulting in regioselective C–H bond activation; paradoxically, the strongest C–H bond reacts preferentially. The purpose of this review is to describe the field of reactions of partially fluorinated substrates with transition metal atoms, ions, and molecular complexes. The controlling physical properties (thermodynamics and kinetics) are described first, followed by a description of stoichiometric reactions, with the competition between the C–H and C–F activations as focus. A few representative catalytic systems are discussed. The review also highlights the benefit of combining experimental and theoretical studies.

•Observation of evolution of p-H2-derived spin states over a μs-ms timescale.•Analytical model for evolution under both chemical and magnetic inequivalence.•Experimental validation of the model for a range of ruthenium dihydride complexes.•Prediction and observation of polarisation transfer from p-H2 31P.We recently reported a pump-probe method that uses a single laser pulse to introduce parahydrogen (p-H2) into a metal dihydride complex and then follows the time-evolution of the p-H2-derived nuclear spin states by NMR. We present here a theoretical framework to describe the oscillatory behaviour of the resultant hyperpolarised NMR signals using a product operator formalism. We consider the cases where the p-H2-derived protons form part of an AX, AXY, AXYZ or AA′XX′ spin system in the product molecule. We use this framework to predict the patterns for 2D pump-probe NMR spectra, where the indirect dimension represents the evolution during the pump-probe delay and the positions of the cross-peaks depend on the difference in chemical shift of the p-H2-derived protons and the difference in their couplings to other nuclei. The evolution of the NMR signals of the p-H2-derived protons, as well as the transfer of hyperpolarisation to other NMR-active nuclei in the product, is described. The theoretical framework is tested experimentally for a set of ruthenium dihydride complexes representing the different spin systems. Theoretical predictions and experimental results agree to within experimental error for all features of the hyperpolarised 1H and 31P pump-probe NMR spectra. Thus we establish the laser pump, NMR probe approach as a robust way to directly observe and quantitatively analyse the coherent evolution of p-H2-derived spin order over micro-to-millisecond timescales.Download high-res image (174KB)Download full-size image

Control of intermolecular interactions is integral to harnessing self-assembly in nature. Here we demonstrate that control of the competition between hydrogen bonds and halogen bonds, the two most highly studied directional intermolecular interactions, can be exerted by choice of solvent (polarity) to direct the self-assembly of co-crystals. Competitive co-crystal formation has been investigated for three pairs of hydrogen bond and halogen bond donors, which can compete for a common acceptor group. These competitions have been examined in seven different solvents. Product formation has been determined and phase purity has been examined by analysis of powder X-ray diffraction patterns. Formation of hydrogen-bonded co-crystals is favoured from less polar solvents and halogen-bonded co-crystals from more polar solvents. The solvent polarity at which the crystal formation switches from hydrogen-bond to halogen-bond dominance depends on the relative strengths of the interactions, but is not a function of the solution-phase interactions alone. The results clearly establish that an appreciation of solvent effects is critical to obtain control of the intermolecular interactions.

Photochemical reactivity associated with metal–hydrogen bonds is widespread among metal hydride complexes and has played a critical part in opening up C–H bond activation. It has been exploited to design different types of photocatalytic reactions and to obtain NMR spectra of dilute solutions with a single pulse of an NMR spectrometer. Because photolysis can be performed on fast time scales and at low temperature, metal-hydride photochemistry has enabled determination of the molecular structure and rates of reaction of highly reactive intermediates. We identify five characteristic photoprocesses of metal monohydride complexes associated with the M–H bond, of which the most widespread are M–H homolysis and R–H reductive elimination. For metal dihydride complexes, the dominant photoprocess is reductive elimination of H2. Dihydrogen complexes typically lose H2 photochemically. The majority of photochemical reactions are likely to be dissociative, but hydride complexes may be designed with equilibrated excited states that undergo different photochemical reactions, including proton transfer or hydride transfer. The photochemical mechanisms of a few reactions have been analyzed by computational methods, including quantum dynamics. A section on specialist methods (time-resolved spectroscopy, matrix isolation, NMR, and computational methods) and a survey of transition metal hydride photochemistry organized by transition metal group complete the Review.

The association constants and enthalpies for the binding of hydrogen bond donors to group 10 transition metal complexes featuring a single fluoride ligand (trans-[Ni(F)(2-C5NF4)(PR3)2], R = Et 1a, Cy 1b, trans-[Pd(F)(4-C5NF4)(PCy3)2] 2, trans-[Pt(F){2-C5NF2H(CF3)}(PCy3)2] 3 and of group 4 difluorides (Cp2MF2, M = Ti 4a, Zr 5a, Hf 6a; Cp*2MF2, M = Ti 4b, Zr 5b, Hf 6b) are reported. These measurements allow placement of these fluoride ligands on the scales of organic H-bond acceptor strength. The H-bond acceptor capability β (Hunter scale) for the group 10 metal fluorides is far greater (1a 12.1, 1b 9.7, 2 11.6, 3 11.0) than that for group 4 metal fluorides (4a 5.8, 5a 4.7, 6a 4.7, 4b 6.9, 5b 5.6, 6b 5.4), demonstrating that the group 10 fluorides are comparable to the strongest organic H-bond acceptors, such as Me3NO, whereas group 4 fluorides fall in the same range as N-bases aniline through pyridine. Additionally, the measurement of the binding enthalpy of 4-fluorophenol to 1a in carbon tetrachloride (−23.5 ± 0.3 kJ mol–1) interlocks our study with Laurence’s scale of H-bond basicity of organic molecules. The much greater polarity of group 10 metal fluorides than that of the group 4 metal fluorides is consistent with the importance of pπ–dπ bonding in the latter. The polarity of the group 10 metal fluorides indicates their potential as building blocks for hydrogen-bonded assemblies. The synthesis of trans-[Ni(F){2-C5NF3(NH2)}(PEt3)2], which exhibits an extended chain structure assembled by hydrogen bonds between the amine and metal-fluoride groups, confirms this hypothesis.

We report a study of the photocatalytic reduction of CO2 to CO by zinc porphyrins covalently linked to [ReI(2,2′-bipyridine)(CO)3L]+/0 moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C6H4NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C6H4NHC(O)C6H3(OMe)NHC(O), while Dyad 3 has a saturated bridge C6H4NHC(O)CH2; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5:1) is approximately an order of magnitude better than [Dyad 1 pic]PF6 or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re–Br units is very similar to that of the dyads with [Re–pic]+ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO2 into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH2Cl2) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of 3MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.

The effect of fluorine substituents on the regioselectivity of intramolecular reactions of mono- and difluorinated N,N-dimethylbenzylamines (1a–f) at palladium, to form palladacycles di-μ-acetatobis[o-dimethylaminomethyl-n-fluorophenyl-C,N)dipalladium(II) (2a–f) and di-μ-chlorobis[o-dimethylaminomethyl-n-fluorophenyl-C,N)dipalladium(II) (3a–e), has been investigated. When fluorinated substrates with two sites available for the C–H functionalization (1c and 1e) undergo cyclopalladation via a CMD mechanism (acetate-bridged palladacycles), they do not exhibit regioselectivity. In contrast, the same substrates exhibit complete regioselectivity for the C–H functionalization para to fluorine in cyclopalladation reactions that proceed via an SEAr mechanism (involving chloride-bridged palladacycles). X-ray crystal structures were obtained for all the palladacycles synthesized, and a structural analysis showed that the number and the position of the fluorine atoms on the aromatic ring have a marked effect on the “clamshell” structure of the acetate-bridged palladacycles. By contrast, there is no great variation in the structures of the planar chloride-bridged palladacycles.

The photochemical reactions of Tp′Rh(PMe3)H2 (1) and thermal reactions of Tp′Rh(PMe3)(CH3)H (1a, Tp′ = tris(3,5-dimethylpyrazolyl)borate) with substrates containing B–H, Si–H, C–F, and C–H bonds are reported. Complexes 1 and 1a are known activators of C–H bonds, including those of alkanes. Kinetic studies of reactions with HBpin and PhSiH3 show that photodissociation of H2 from 1 occurs prior to substrate attack, whereas thermal reaction of 1a proceeds by bimolecular reaction with the substrate. Complete intramolecular selectivity for B–H over C–H activation of HBpin (pin = pinacolate) leading to Tp′Rh(PMe3)(Bpin)H is observed. Similarly, the reaction with Et2SiH2 shows a strong preference for Si–H over C–H activation, generating Tp′Rh(PMe3)(SiEt2H)H. The Rh(Bpin)H and Rh(SiEt2H)H products were stable to heating in benzene in accord with DFT calculations that showed that reaction with benzene is endoergic. The intramolecular competition with PhSiH3 yields a ∼1:4 mixture of Tp′Rh(PMe3)(C6H4SiH3)H and Tp′Rh(PMe3)(SiPhH2)H, respectively. Reaction with pentafluoropyridine generates Tp′Rh(PMe3)(C5NF4)F, while reaction with 2,3,5,6-tetrafluoropyridine yields a mixture of C–H and C–F activated products. Hexafluorobenzene proves unreactive. Crystal structures are reported for B–H, Si–H, and C–F activated products, but in the latter case a bifluoride complex Tp′Rh(PMe3)(C5NF4)(FHF) was crystallized. Intermolecular competition reactions were studied by photoreaction of 1 in C6F6 with benzene and another substrate (HBpin, PhSiH3, or pentafluoropyridine) employing in situ laser photolysis in the NMR probe, resulting in a wide-ranging map of kinetic selectivities. The mechanisms of intramolecular and intermolecular selection are analyzed.

We report pump–probe experiments employing laser-synchronized reactions of para-hydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump–probe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single radio frequency (rf) probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3)3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution. Reaction with para-H2 then regenerates 1 and 2 in a well-defined nuclear spin state. The rf probe pulse produces a high-resolution, single-scan 1H NMR spectrum that can be recorded after a pump–probe delay of just 10 μs. The evolution of the spectra can be followed as the pump–probe delay is increased by micro- or millisecond increments. Due to the sensitivity of this para-H2 experiment, the resulting NMR spectra can have hydride signal-to-noise ratios exceeding 750:1. The spectra of 1 oscillate in amplitude with frequency 1101 ± 3 Hz, the chemical shift difference between the chemically inequivalent hydrides. The corresponding hydride signals of 2 oscillate with frequency 83 ± 5 Hz, which matches the difference between couplings of the hydrides to the equatorial 31P nuclei. We use the product operator formalism to show that this oscillatory behavior arises from a magnetic coherence in the plane orthogonal to the magnetic field that is generated by use of the laser pulse without rf initialization. In addition, we demonstrate how chemical shift imaging can differentiate the region of laser irradiation thereby distinguishing between thermal and photochemical reactivity within the NMR tube.

Pt(PCyp3)2 (Cyp = cyclopentyl) undergoes C–O oxidative addition with 2,3,5,6-tetrafluoro-4-methoxypyridine, pentafluoroanisole, 2,3,5,6-tetrafluoroanisole and 2,3,6-trifluoroanisole yielding platinum methyl derivatives. The reactions occur in preference to C–H or C–F activation.

A computational approach (DFT-B3PW91) is used to address previous experimental studies (Chem. Commun. 2009, 6801) that showed that transfer hydrogenation of a cyclic imine by Et3N·HCO2H in dichloromethane catalyzed by 16-electron bifunctional Cp*RhIII(XNC6H4NX′) is faster when XNC6H4NX′ = TsNC6H4NH than when XNC6H4NX′ = HNC6H4NH or TsNC6H4NTs (Cp* = η5-C5Me5, Ts = toluenesulfonyl). The computational study also considers the role of the formate complex observed experimentally at low temperature. Using a model of the experimental complex in which Cp* is replaced by Cp and Ts by benzenesulfonyl (Bs), the calculations for the systems in gas phase reveal that dehydrogenation of formic acid generates CpRhIIIH(XNC6H4NX′H) via an outer-sphere mechanism. The 16-electron Rh complex + formic acid are shown to be at equilibrium with the formate complex, but the latter lies outside the pathway for dehydrogenation. The calculations reproduce the experimental observation that the transfer hydrogenation reaction is fastest for the nonsymmetrically substituted complex CpRhIII(XNC6H4NX′) (X = Bs and X′ = H). The effect of the linker between the two N atoms on the pathway is also considered. The Gibbs energy barrier for dehydrogenation of formic acid is calculated to be much lower for CpRhIII(XNCHPhCHPhNX′) than for CpRhIII(XNC6H4NX′) for all combinations of X and X′. The energy barrier for hydrogenation of the imine by the rhodium hydride complex is much higher than the barrier for hydride transfer to the corresponding iminium ion, in agreement with mechanisms proposed for related systems on the basis of experimental data. Interpretation of the results by MO and NBO analyses shows that the most reactive catalyst for dehydrogenation of formic acid contains a localized Rh–NH π-bond that is associated with the shortest Rh–N distance in the corresponding 16-electron complex. The asymmetric complex CpRhIII(BsNC6H4NH) is shown to generate a good bifunctional catalyst for transfer hydrogenation because it combines an electrophilic metal center and a nucleophilic NH group.

Irradiation of CpRh(PMe3)(C2H4) (1; Cp = η5-C5H5) in the presence of pentafluoropyridine in hexane solution at low temperature yields an isolable η2-C,C-coordinated pentafluoropyridine complex, CpRh(PMe3)(η2-C,C-C5NF4) (2). The molecular structure of 2 was determined by single-crystal X-ray diffraction, showing coordination by C3–C4, unlike previous structures of pentafluoropyridine complexes that show N-coordination. Corresponding experiments with 2,3,5,6-tetrafluoropyridine yield the C–H oxidative addition product CpRh(PMe3)(C5NF4)H (3). In contrast, UV irradiation of 1 in hexane, in the presence of 4-substituted tetrafluoropyridines C5NF4X, where X = NMe2, OMe, results in elimination of C2H4 and HF to form the metallacycles CpRh(PMe3)(κ2-C,C-CH2N(CH3)C5NF3) (4) and CpRh(PMe3)(κ2-C,C-CH2OC5NF3) (5), respectively. The X-ray structure of 4 shows a planar RhCCNC-five-membered ring. Complexes 2–5 may also be formed by thermal reaction of CpRh(PMe3)(Ph)H with the respective pyridines at 50 °C.

The formation of halogen bonds from iodopentafluorobenzene and 1-iodoperfluorohexane to a series of bis(η5-cyclopentadienyl)metal hydrides (Cp2TaH3, 1; Cp2MH2, M = Mo, 2, M = W, 3; Cp2ReH, 4; Cp2Ta(H)CO, 5; Cp = η5-cyclopentadienyl) is demonstrated by 1H NMR spectroscopy. Interaction enthalpies and entropies for complex 1 with C6F5I and C6F13I are reported (ΔH° = −10.9 ± 0.4 and −11.8 ± 0.3 kJ/mol; ΔS° = −38 ± 2 and −34 ± 2 J/(mol·K), respectively) and found to be stronger than those for 1 with the hydrogen-bond donor indole (ΔH° = −7.3 ± 0.1 kJ/mol, ΔS° = −24 ± 1 J/(mol·K)). For the more reactive complexes 2–5, measurements are limited to determination of their low-temperature (212 K) association constants with C6F5I as 2.9 ± 0.2, 2.5 ± 0.1, <1.5, and 12.5 ± 0.3 M–1, respectively.

Three ruthenium complexes Λ-[cis-Ru((R,R)-Me-BPE)2(H)2] Λ-R,R-Ru1H2, Δ-[cis-Ru((S,S)-Me-DuPHOS)2(H)2] Δ-S,S-Ru2H2, and Λ-[cis-Ru((R,R)-Me-DuPHOS)2(H)2] Λ-R,R-Ru2H2 (1 = (Me-BPE)2, 2 = (Me-DuPHOS)2) were characterized by multinuclear NMR and CD spectroscopy in solution and by X-ray crystallography. The chiral ligands allow the full control of stereochemistry and enable mechanistic studies not otherwise available. Oxidative addition of E–H bonds (E = H, B, Si, C) was studied by steady state and laser flash photolysis in the presence of substrates. Steady state photolysis shows formation of single products with one stereoisomer. Solid state structures and circular dichroism spectra reveal a change in configuration at ruthenium for some Δ-S,S-Ru2H2/Λ-R,R-Ru2H2 photoproducts from Λ to Δ (or vice versa) while the configuration for Λ-R,R-Ru1H2 products remains unchanged as Λ. The X-ray structure of silyl hydride photoproducts suggests a residual H(1)···Si(1) interaction for Δ-[cis-Ru((R,R)-Me-DuPHOS)2(Et2SiH)(H)] and Δ-[cis-Ru((R,R)-Me-DuPHOS)2(PhSiH2)(H)] but not for their Ru(R,R-BPE)2 analogues. Molecular structures were also determined for Λ-[cis-Ru((R,R)-Me-BPE)2(Bpin)(H)], Λ-[Ru((S,S)-Me-DuPHOS)2(η2-C2H4)], Δ-[Ru((R,R)-Me-DuPHOS)2(η2-C2H4)], and trans-[Ru((R,R)-Me-DuPHOS)2(C6F5)(H)]. In situ laser photolysis in the presence of p-H2 generates hyperpolarized NMR spectra because of magnetically inequivalent hydrides; these experiments and low temperature photolysis with D2 reveal that the loss of hydride ligands is concerted. The reaction intermediates [Ru(DuPHOS)2] and [Ru(BPE)2] were detected by laser flash photolysis and have spectra consistent with approximate square-planar Ru(0) structures. The rates of their reactions with H2, D2, HBpin, and PhSiH3 were measured by transient kinetics. Rate constants are significantly faster for [Ru(BPE)2] than for [Ru(DuPHOS)2] and follow the substrate order H2 > D2 > PhSiH3 > HBpin.

This review describes recent developments in photocatalytic and electrocatalytic CO2 reduction. On the electrocatalytic side, there have been advances in optimization of known rhenium motifs sometimes in conjunction with silicon photoelectrodes giving enhanced catalytic current and stability. Complexes of copper capable of absorbing atmospheric CO2 have been incorporated into an electrocatalytic cycle and metal-free electrocatalysis of CO2 to methanol has been achieved with pyridinium ions. A complete cell with two photo-electrodes, one for water oxidation and the other for CO2 reduction to formate has been set up successfully. The cathode employs ruthenium catalysts on InP. Progress in photocatalytic CO2 reduction has been made with osmium complexes exhibiting good selectivity and stability. The separation between Ru and Re centers in light-harvesting donor–acceptor dyads has been investigated providing some inspiration for design. A ruthenium catalyst has been sensitized by tantalum oxide particles. Metalloporphyrin-rhenium dyads have also been studied for photocatalytic CO2 reduction. In the biological arena, a ruthenium complex has been used to sensitize carbon monoxide dehydrogenase on titanium dioxide particles.Highlights► Development of electrocatalysts that absorb atmospheric CO2. ► Electrocatalysts are composed of earth-abundant materials, some totally organic. ► Photocatalytic CO2 reduction is coupled to water oxidation. ► CO2 reducing enzymes are powered by visible light. ► Low energy visible light is harvested by porphyrins and utilized for CO2 reduction.

Photocatalytic CO2 reduction has been studied for two dyads with porphyrin covalently attached to rhenium tricarbonyl bipyridine moieties, and on separate components consisting of [Re(CO)3(Picoline)Bpy]+ and either zinc porphyrin or zinc chlorin. TONs decrease in the order: zinc porphyrin + Re > long spacer dyad > zinc chlorin + Re > short spacer dyad.

The mechanism of the hydrofluoroarylation of alkynes, RC≡CR, by nickel phosphine complexes, described by Nakao et al. ( Dalton Trans. 2010, 39, 10483), was studied by density functional theory (DFT) calculations. The oxidative addition of a C–H bond of partially fluorinated benzenes, C6FnH6–n (n = 0–5) to a Ni(0) phosphine complex is reversible, but the oxidative addition of a C–F bond yields a stable product via a high-energy barrier. A pathway via the Ni(II) hydride complex is eliminated on the basis of a calculated H/D kinetic isotope effect (KIE) that does not agree with the measured value. An alternate pathway was determined, using as reactant a Ni(phosphine)(alkyne) complex that is shown to be the major species in the reactive media under the catalytic conditions. This pathway is initiated by arene coordination to the Ni alkyne complex followed by proton transfer from the σ-C–H bond of the coordinated arene to the alkyne as the C–H activation step. Analysis of the charge distribution shows that the alkyne is strongly negatively charged when coordinated to the Ni(phosphine) species, which favors a C–H activation as a proton transfer, similar to that in CMD and AMLA but not previously seen between hydrocarbyl ligands for electron rich metals. The C–H activation step thus represents an example of a general class of mechanism that we term ligand-to-ligand hydrogen transfer (LLHT). The product of this reaction is a nickel(vinyl)(aryl) complex, which rearranges to place the aryl and vinyl groups cis to one another before undergoing reductive elimination of the arylalkene. An analysis of the calculated turnover frequencies shows that the rate-determining states that control the energy span are the alkyne complex + free arene and the transition state for the vinyl-aryl complex trans-to-cis rearrangement. The calculated KIE agrees with the observed lack of isotope effect. Analysis of the effects of fluorine substituents shows that the Ni–C(aryl) bond energies control the energy barriers for the arene C–H activation step and the energy spans. A correlation between bond dissociation energies for the Ni–C(aryl) bond and the arene C–H bond follows the behavior presented previously ( J. Am. Chem Soc. 2009, 131, 7817), in which the effects of ortho fluorine substituents are dominant. Consequently, fluorine substitution of the arene, especially at the ortho positions, strengthens the Ni–C bond and increases the TOF. The LLHT mechanism described here may also apply to nickel-catalyzed C–H activation reactions with other substrates.

In this Account, we describe the transition metal-mediated cleavage of C−F and C−H bonds in fluoroaromatic and fluoroheteroaromatic molecules.The simplest reactions of perfluoroarenes result in C−F oxida tive addition, but C−H activation competes with C−F activation for partially fluorinated molecules. We first consider the reactivity of the fluoroaromatics toward nickel and platinum complexes, but extend to rhenium and rhodium where they give special insight. Sections on spectroscopy and molecular structure are followed by discussions of energetics and mechanism that incorporate experimental and computational results. We highlight special characteristics of the metal−fluorine bond and the influence of the fluorine substituents on energetics and mechanism.Fluoroaromatics reacting at an ML2 center initially yield η2-arene complexes, followed usually by oxidative addition to generate MF(ArF)(L)2 or MH(ArF)(L)2 (M is Ni, Pd, or Pt; L is trialkylphosphine). The outcome of competition between C−F and C−H bond activation is strongly metal dependent and regioselective. When C−H bonds of fluoroaromatics are activated, there is a preference for the remaining C−F bonds to lie ortho to the metal.An unusual feature of metal−fluorine bonds is their response to replacement of nickel by platinum. The Pt−F bonds are weaker than their nickel counterparts; the opposite is true for M−H bonds. Metal−fluorine bonds are sufficiently polar to form M−F···H−X hydrogen bonds and M−F···I−C6F5 halogen bonds.In the competition between C−F and C−H activation, the thermodynamic product is always the metal fluoride, but marked differences emerge between metals in the energetics of C−H activation. In metal−fluoroaryl bonds, ortho-fluorine substituents generally control regioselectivity and make C−H activation more energetically favorable. The role of fluorine substituents in directing C−H activation is traced to their effect on bond energies. Correlations between M−C and H−C bond energies demonstrate that M−C bond energies increase far more on ortho-fluorine substitution than do H−C bonds.Conventional oxidative addition reactions involve a three-center triangular transition state between the carbon, metal, and X, where X is hydrogen or fluorine, but M(d)−F(2p) repulsion raises the activation energies when X is fluorine. Platinum complexes exhibit an alternative set of reactions involving rearrangement of the phosphine and the fluoroaromatics to a metal(alkyl)(fluorophosphine), M(R)(ArF)(PR3)(PR2F). In these phosphine-assisted C−F activation reactions, the phosphine is no spectator but rather is intimately involved as a fluorine acceptor. Addition of the C−F bond across the M−PR3 bond leads to a metallophosphorane four-center transition state; subsequent transfer of the R group to the metal generates the fluorophosphine product. We find evidence that a phosphine-assisted pathway may even be significant in some apparently simple oxidative addition reactions.While transition metal catalysis has revolutionized hydrocarbon chemistry, its impact on fluorocarbon chemistry has been more limited. Recent developments have changed the outlook as catalytic reactions involving C−F or C−H bond activation of fluorocarbons have emerged. The principles established here have several implications for catalysis, including the regioselectivity of C−H activation and the unfavorable energetics of C−F reductive elimination. Palladium-catalyzed C−H arylation is analyzed to illustrate how ortho-fluorine substituents influence thermodynamics, kinetics, and regioselectivity.

Manganese propane and manganese butane complexes derived from CpMn(CO)3 were generated photochemically at 130−136 K with the alkane as solvent and characterized by FTIR spectroscopy and by 1H NMR spectroscopy with in situ laser photolysis. Time-resolved IR spectroscopic measurements were performed at room temperature with the same laser wavelength. The ν(CO) bands in the IR spectra of the photoproducts in propane are shifted to low frequency with respect to CpMn(CO)3, consistent with formation of CpMn(CO)2(propane). The 1H NMR spectra conform to the criteria for alkane complexes: a high-field resonance for the η2-CH protons that shifts substantially on partial deuteration of the alkane and exhibits a coupling constant JC−H on 13C-labeling of ca. 120 Hz. The NMR spectrum of each system exhibits two diagnostic product resonances in the high-field region for the η2-CH protons, corresponding to CpMn(CO)2(η2-C1−H−alkane) and CpMn(CO)2(η2-C2−H−alkane) isomers. Partial deuteration of the alkane at C1 results in characteristic strong isotopic perturbation of equilibrium of the η2-CH resonance of CpMn(CO)2(η2-C1−H−alkane). With propane-13C1, the η2-CH resonance of CpMn(CO)2(η2-C1−H−alkane) isomer exhibits 13C satellites with JC−H = 119 Hz. The corresponding resonance of CpMn(CO)2(η2-C2−H−alkane) is identified by use of propane-2,2-d2. The lifetimes of the (η2-C1−H−alkane) isomers of the manganese complexes were determined by NMR spectroscopy as 22 ± 2 min at 134 K (propane) and 5.5 min at 136 K (butane). The corresponding spectra and lifetimes of the CpRe(CO)2(alkane) complexes were measured for reference (CpRe(CO)2(propane) lifetime ca. 60 min at 161 K; CpRe(CO)2(butane) 13 min at 171 K). The lifetimes determined by IR spectroscopy were similar to those determined by NMR spectroscopy, thereby supporting the assignments. These measurements extend the range of alkane complexes characterized by NMR spectroscopy from rhenium and rhodium derivatives to include less stable manganese derivatives.

A study is presented of the thermodynamics of the halogen-bonding interaction of C6F5I with a series of structurally similar group 10 metal fluoride complexes trans-[Ni(F)(2-C5NF4)(PCy3)2] (2), trans-[Pd(F)(4-C5NF4)(PCy3)2] (3), trans-[Pt(F){2-C5NF2H(CF3)}(PR3)2] (4a, R = Cy; 4bR = iPr) and trans-[Ni(F){2-C5NF2H(CF3)}(PCy3)2] (5a) in toluene solution. 19F NMR titration experiments are used to determine binding constants, enthalpies and entropies of these interactions (2.4 ≤ K300 ≤ 5.2; −25 ≤ ΔHo ≤ −16 kJ mol–1; −73 ≤ ΔSo ≤ −49 J K–1 mol–1). The data for −ΔHo for the halogen bonding follow a trend Ni < Pd < Pt. The fluoropyridyl ligand is shown to have a negligible influence on the thermodynamic data, but the influence of the phosphine ligand is significant. We also show that the value of the spin–spin coupling constant JPtF increases substantially with adduct formation. X-ray crystallographic data for Ni complexes 5a and 5c are compared to previously published data for a platinum analogue. We show by experiment and computation that the difference between Pt–X and Ni–X (X = F, C, P) bond lengths is greatest for X = F, consistent with F(2pπ)–Pt(5dπ) repulsive interactions. DFT calculations on the metal fluoride complexes show the very negative electrostatic potential around the fluoride. Calculations of the enthalpy of adduct formation show energies of −18.8 and −22.8 kJ mol–1 for Ni and Pt complexes of types 5 and 4, respectively, in excellent agreement with experiment.

The photochemistry and photophysics of the cationic molecular dyad, 5-{4-[rhenium(I)tricarbonylpicoline-4-methyl-2,2′-bipyridine-4′-carboxyamidyl]phenyl}-10,15,20-triphenylporphyrinatopalladium(II) ([Re(CO)3(Pic)Bpy-PdTPP][PF6]) have been investigated. The single crystal X-ray structure for the thiocyanate analogue, [Re(CO)3(NCS)Bpy-PdTPP], exhibits torsion angles of 69.1(9)°, 178.1(7)°, and 156.8(9)° between porphyrin plane, porphyrin-linked C6H4 group, amide moiety, and Bpy, respectively. Steady-state photoexcitation (λex = 520 nm) of [Re(CO)3(Pic)Bpy-PdTPP][PF6] in dimethylformamide (DMF) results in substitution of Pic by bromide at the Re(I)Bpy core. When [Re(CO)3(Pic)Bpy-PdTPP][PF6] is employed as a photocatalyst for the reduction of CO2 to CO in DMF/NEt3 solution with λex > 420 nm, 2 turnovers (TNs) CO are formed after 4 h. If instead, a two-component mixture of PdTPP sensitizer and mononuclear [Re(CO)3(Pic)Bpy][PF6] catalyst is used, 3 TNs CO are formed. In each experiment however, CO only forms after a slight induction period and during the concurrent photoreduction of the sensitizer to a Pd(II) chlorin species. Palladium(II) meso-tetraphenylchlorin, the hydrogenated porphyrin analogue of PdTPP, has been synthesized independently and can be substituted for PdTPP in the two-component system with [Re(CO)3(Pic)Bpy][PF6], forming 9 TNs CO. An intramolecular electron transfer process for the dyad is supported by cyclic voltammetry and steady-state emission studies, from which the free energy change was calculated to be ΔGox* = −0.08 eV. Electron transfer from Pd(II) porphyrin to Re(I) tricarbonyl bipyridine in [Re(CO)3(Pic)Bpy-PdTPP][PF6] was monitored using time-resolved infrared (TRIR) spectroscopy in the ν(CO) region on several time scales with excitation at 532 nm. Spectra were recorded in CH2Cl2 with and without NEt3. Picosecond TRIR spectroscopy shows rapid growth of bands assigned to the π–π* excited state (2029 cm–1) and to the charge-separated state (2008, 1908 cm–1); these bands decay and the parent recovers with lifetimes of 20–50 ps. Spectra recorded on longer time scales (ns, μs, and seconds) show the growth and decay of further species with ν(CO) bands indicative of electron transfer to Re(Bpy).

A computational study of the C(methyl)–O bond activation of fluorinated aryl methyl ethers by a platinum(0) complex Pt(PCyp3)2 (Cyp = cyclopentyl) (N. A. Jasim, R. N. Perutz, B. Procacci and A. C. Whitwood, Chem. Commun., 2014, 50, 3914) demonstrates that the reaction proceeds via an SN2 mechanism. Nucleophilic attack of Pt(0) generates an ion pair consisting of a T-shaped platinum cation with an agostic interaction with a cyclopentyl group and a fluoroaryloxy anion. This ion-pair is converted to a 4-coordinate Pt(II) product trans-[PtMe(OArF)(PCyp3)2]. Structure-reactivity correlations are fully consistent with this mechanism. The Gibbs energy of activation is calculated to be substantially higher for aryl methyl ethers without fluorine substituents and higher still for alkyl methyl ethers. These conclusions are in accord with the experimental results. Further support was obtained in an experimental study of the reaction of Pt(PCy3)2 with 2,3,5,6-tetrafluoro-4-allyloxypyridine yielding the salt of the Pt(η3-allyl) cation and the tetrafluoropyridinolate anion [Pt(PCy3)2(η3-allyl)][OC5NF4]. The calculated activation energy for this reaction is significantly lower than that for fluorinated aryl methyl ethers.

Control of intermolecular interactions is integral to harnessing self-assembly in nature. Here we demonstrate that control of the competition between hydrogen bonds and halogen bonds, the two most highly studied directional intermolecular interactions, can be exerted by choice of solvent (polarity) to direct the self-assembly of co-crystals. Competitive co-crystal formation has been investigated for three pairs of hydrogen bond and halogen bond donors, which can compete for a common acceptor group. These competitions have been examined in seven different solvents. Product formation has been determined and phase purity has been examined by analysis of powder X-ray diffraction patterns. Formation of hydrogen-bonded co-crystals is favoured from less polar solvents and halogen-bonded co-crystals from more polar solvents. The solvent polarity at which the crystal formation switches from hydrogen-bond to halogen-bond dominance depends on the relative strengths of the interactions, but is not a function of the solution-phase interactions alone. The results clearly establish that an appreciation of solvent effects is critical to obtain control of the intermolecular interactions.

On reaction of IrI(CO)(PPh3)21 with para-hydrogen (p-H2), Ir(H)2I(CO)(PPh3)22 is formed which exhibits strongly enhanced 1H NMR signals for its hydride resonances. Complex 2 also shows similar enhancement of its NMR spectra when it is irradiated under p-H2. We report the use of this photochemical reactivity to measure the kinetics of H2 addition by laser-synchronized reactions in conjunction with NMR. The single laser pulse promotes the reductive elimination of H2 from Ir(H)2I(CO)(PPh3)22 in C6D6 solution to form the 16-electron precursor 1, back reaction with p-H2 then reforms 2 in a well-defined nuclear spin-state. The build up of this product can be followed by incrementing a precisely controlled delay (τ), in millisecond steps, between the laser and the NMR pulse. The resulting signal vs. time profile shows a dependence on p-H2 pressure. The plot of kobs against p-H2 pressure is linear and yields the second order rate constant, k2, for H2 addition to 1 of (3.26 ± 0.42) × 102 M−1 s−1. Validation was achieved by transient-UV-vis absorption spectroscopy which gives k2 of (3.06 ± 0.40) × 102 M−1 s−1. Furthermore, irradiation of a C6D6 solution of 2 with multiple laser shots, in conjunction with p-H2 derived hyperpolarization, allows the detection and characterisation of two minor reaction products, 2a and 3, which are produced in such low yields that they are not detected without hyperpolarization. Complex 2a is a configurational isomer of 2, while 3 is formed by substitution of CO by PPh3.

Pt(PCyp3)2 (Cyp = cyclopentyl) undergoes C–O oxidative addition with 2,3,5,6-tetrafluoro-4-methoxypyridine, pentafluoroanisole, 2,3,5,6-tetrafluoroanisole and 2,3,6-trifluoroanisole yielding platinum methyl derivatives. The reactions occur in preference to C–H or C–F activation.

We report a study of the photocatalytic reduction of CO2 to CO by zinc porphyrins covalently linked to [ReI(2,2′-bipyridine)(CO)3L]+/0 moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C6H4NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C6H4NHC(O)C6H3(OMe)NHC(O), while Dyad 3 has a saturated bridge C6H4NHC(O)CH2; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5:1) is approximately an order of magnitude better than [Dyad 1 pic]PF6 or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re–Br units is very similar to that of the dyads with [Re–pic]+ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO2 into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH2Cl2) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of 3MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.

Photocatalytic CO2 reduction has been studied for two dyads with porphyrin covalently attached to rhenium tricarbonyl bipyridine moieties, and on separate components consisting of [Re(CO)3(Picoline)Bpy]+ and either zinc porphyrin or zinc chlorin. TONs decrease in the order: zinc porphyrin + Re > long spacer dyad > zinc chlorin + Re > short spacer dyad.