Triplet organometallic reactivity under ambient conditions: an ultrafast UV pump/IR probe study

Theoretical Investigation of Transient Species Following Photodissociation of Ironpentacarbonyl in Ethanol Solution

Photodissociation of ironpentacarbonyl [1Fe(CO)5] in solution generates transient species in different electronic states, which we studied theoretically. From ab initio molecular dynamics simulations in ethanol solution, the closed-shell parent compound 1Fe(CO)5 is found to interact weakly with the solvent, whereas the irontetracarbonyl [Fe(CO)4] species, formed after photodissociation, has a strongly spin-dependent behavior. It coordinates a solvent molecule tightly in the singlet state [1Fe(CO)4] and weakly in the triplet state [3Fe(CO)4]. From the simulations, we have gained insights into intersystem crossing in solvated irontetracarbonyl based on the distinct structural differences induced by the change in multiplicity. Alternative forms of coordination between 1Fe(CO)4 and functional groups of the ethanol molecule are simulated, and a quantum chemical investigation of the energy landscape for the coordinated irontetracarbonyl gives information about the interconversion of different transient species in solution. Furthermore, insights from the simulations, in which we find evidence of a solvent exchange mechanism, challenge the previously proposed mechanism of chain walking for under-coordinated metal carbonyls in solution.

Photochemistry of transition metal carbonyls

The purpose of this Tutorial Review is to outline the fundamental photochemistry of metal carbonyls, and to show how the advances in technology have increased our understanding of the detailed mechanisms, particularly how relatively simple experiments can provide deep understanding of complex problems. We recall some important early experiments that demonstrate the key principles underlying current research, concentrating on the binary carbonyls and selected substituted metal carbonyls. At each stage, we illustrate with examples from recent applications. This review first considers the detection of photochemical intermediates in three environments: glasses and matrices; gas phase; solution. It is followed by an examination of the theory underpinning these observations. In the final two sections, we briefly address applications to the characterization and behaviour of complexes with very labile ligands such as N₂, H₂ and alkanes, concentrating on key mechanistic points, and also describe some principles and examples of photocatalysis.

Ultraviolet photodissociation of gas-phase iron pentacarbonyl probed with ultrafast infrared spectroscopy

It is well known that ultraviolet photoexcitation of iron pentacarbonyl results in rapid loss of carbonyl ligands leading to the formation of coordinatively unsaturated iron carbonyl compounds. We employ ultrafast mid-infrared transient absorption spectroscopy to probe the photodissociation dynamics of gas-phase iron pentacarbonyl following ultraviolet excitation at 265 and 199 nm. After photoexcitation at 265 nm, our results show evidence for sequential dissociation of iron pentacarbonyl to form iron tricarbonyl via a short-lived iron tetracarbonyl intermediate. Photodissociation at 199 nm results in the prompt production of Fe(CO)₃ within 0.25 ps via several energetically accessible pathways. An additional 15 ps time constant extracted from the data is tentatively assigned to intersystem crossing to the triplet manifold of iron tricarbonyl or iron dicarbonyl. Mechanisms for formation of iron tetracarbonyl, iron tricarbonyl, and iron dicarbonyl are proposed and theoretically validated with one-dimensional cuts through the potential energy surface as well as bond dissociation energies. Ground state calculations are computed at the CCSD(T) level of theory and excited states are computed with EOM-EE-CCSD(dT).

CO Displacement in an Oxidative Addition of Primary Silanes to Rhodium(I)

The rhodium dicarbonyl {PhB(Ox^Me₂)₂Im^Mes}Rh(CO)₂ (1) and primary silanes react by oxidative addition of a nonpolar Si-H bond and, uniquely, a thermal dissociation of CO. These reactions are reversible, and kinetic measurements model the approach to equilibrium. Thus, 1 and RSiH₃ react by oxidative addition at room temperature in the dark, even in CO-saturated solutions. The oxidative addition reaction is first-order in both 1 and RSiH₃, with rate constants for oxidative addition of PhSiH₃ and PhSiD₃ revealing k_H/ k_D ∼ 1. The reverse reaction, reductive elimination of Si-H from {PhB(Ox^Me₂)₂Im^Mes}RhH(SiH₂R)CO (2), is also first-order in [2] and depends on [CO]. The equilibrium concentrations, determined over a 30 °C temperature range, provide Δ H ° = -5.5 ± 0.2 kcal/mol and Δ S ° = -16 ± 1 cal·mol^-1K^-1 (for 1 ⇄ 2). The rate laws and activation parameters for oxidative addition (Δ H^⧧ = 11 ± 1 kcal·mol^-1 and Δ S^⧧ = -26 ± 3 cal·mol^-1·K^-1) and reductive elimination (Δ H^⧧ = 17 ± 1 kcal·mol^-1 and Δ S^⧧ = -10 ± 3 cal·mol^-1K^-1), particularly the negative activation entropy for both forward and reverse reactions, suggest the transition state of the rate-determining step contains {PhB(Ox^Me₂)₂Im^Mes}Rh(CO)₂ and RSiH₃. Comparison of a series of primary silanes reveals that oxidative addition of arylsilanes is ca. 5× faster than alkylsilanes, whereas reductive elimination of Rh-Si/Rh-H from alkylsilyl and arylsilyl rhodium(III) occurs with similar rate constants. Thus, the equilibrium constant K_e for oxidative addition of arylsilanes is >1, whereas reductive elimination is favored for alkylsilanes.

The mechanistic investigations of photochemical decarbonylations and oxidative addition reactions for M(CO)5 (M = Fe, Ru, Os) complexes

The mechanisms for the photochemical CO-dissociation and the oxidative addition reactions are studied theoretically using three model systems: M(CO)₅ (M = Fe, Ru, and Os) and the CASSCF/Def2-SVP (fourteen-electron/ten-orbital active space) and MP2-CAS/Def2-SVP//CASSCF/Def2-SVP methods. The structures of the intersystem crossings and the conical intersections, which play a decisive role in these CO photo-extrusion reactions, are determined. The intermediates and the transition structures in either the singlet or triplet states are also computed, in order to explain the reaction routes. These model studies suggest that after the irradiation of Fe(CO)₅ with UV light, it quickly loses one CO molecule to generate a 16-electron iron tetracarbonyl, in either the singlet or the triplet states. It is found that the triplet Fe(CO)₄ plays a vital role in the formation of the final oxidative addition product, Fe(CO)₄(H)(SiMe₃), but the singlet Fe(CO)₄ plays a relatively minor role in the formation of the final product. However, its vacant coordination site interacts weakly with solvent molecules ((Me₃)SiH) to yield the alkyl-solvated iron complexes, which are detectable experimentally. The theoretical observations show that Ru(CO)₅ and Os(CO)₅ have similar photochemical and thermal potential energy profiles. In particular, this study demonstrates that the oxidative addition yield for Fe is much greater than those for its Ru and Os counterparts, under the same chemical conditions.

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.

Applying a Nonspin-Flip Reaction Scheme to Explain for the Doublet Sulfide Oxides SMO2 Observed for the Reactions of SO2 with V(4F), Nb(6D), and Ta(4F)

Energy profiles linking the reactants M + SO2 (M = V(4F), Nb(6D;4F), and Ta(4F)) with the products observed for these reactions under matrix-isolation conditions, mainly the oxide complex OV(η2-SO) and the sulfide oxides SVO2, SNbO2, and STaO2, have been obtained from DFT and CASSCF-MRMP2 calculations. For each of these interactions, the radical fragments MO + SO can be reached from the lowest-lying quadruplet electronic states of the reactants. As the quadruplet and doublet radical asymptotes that vary only in the spin of the unpaired parallel electrons of the nonmetallic fragment are degenerated, a second reaction leading to the rebounding of the radical fragments can take place through both multiplicity channels. Reaction along the doublet pathway leads in each case to the most stable structure for the oxide SMO2. For the vanadium interaction, recombination of the radical species through the quadruplet channel explains for the oxide product OV(η2-SO).

A model study on the photodecarbonyl reaction of (η5-C5H5)M(CO)2 (M = Co, Rh, Ir)

The group 9 organometallic complexes η⁵-CpM(CO)₂ (M = Co, Rh, and Ir) and Si(CH₃)₃(H) have been considered as a model system to study their photochemical decarbonyl reactions as well as the Si–H bond activation reactions using the CASSCF and MP2-CAS computational methods. For the cobalt complex, three kinds of reaction pathways, which result in the same oxidative addition product, are investigated. Our theoretical finding demonstrated that after the photoirradiation, η⁵-CpCo(CO)₂ loses one CO ligand without any difficulty to form either the triplet ([η⁵-CpCo(CO)]³) or singlet ([η⁵-CpCo(CO)]¹) species. The former plays a decisive role in the formation of the final oxidative addition product. On the other hand, the latter plays no role in the production of the final product molecule, but its singlet cobalt center interacts weakly with solvent molecules ((Me₃)SiH) to produce an alkyl-solvated organometallic complex, which is experimentally detectable. The present works reveal that both η⁵-CpRh(CO)₂ and η⁵-CpIr(CO)₂ should adopt the conical intersection mechanism after they are irradiated by light. Moreover, our theoretical examinations strongly suggest that for the 16-electron monocarbonyl η⁵-CpM(CO) (M = Rh and Ir) species, the insertion into a Si–H bond by the Ir system is much more facile and more exothermic than that for the Rh counterpart.

Theoretical observations show that the triplet cobalt photoproduct [η⁵-CpCo(CO)] activates Si–H bonds much more readily than its singlet counterpart. The conical intersection mechanism may play a decisive role in photodecarbonyl reactions of (η⁵-C₅H₅)M(CO)₂ (M = Rh and Ir).

Using a non-spin flip model to rationalize the irregular patterns observed in the activation of the C-H and Si-H bonds of small molecules by CpMCO (M = Co, Rh) complexes

The activation of the C-H and Si-H bonds of CH(CH₃)₃ and SiH(CH₃)₃ molecules by organometallic compounds CpMCO (M = Co, Rh) has been investigated through DFT and CASSCF-MRMP2 calculations. In particular, we have analyzed the pathways joining the lowest-lying triplet and singlet states of the reactants with the products arising from the insertion of the metal atom into the C-H or Si-H bonds of the organic molecules. Channels connecting the reactants with the inserted structure Cp(CO)H-M-C(CH₃)₃ through the oxidative addition of the C-H bond of the organic molecule to the metal fragment were found only for the reaction CpRhCO + CH(CH₃)₃. However, inserted structures could also be obtained for the interactions of SiH(CH₃)₃ with CpCoCO and CpRhCO by two sequential reactions involving the formation and rebounding of the radical fragments Cp(CO)H-M + Si(CH₃)₃. According to this two-step reaction scheme, the complex CpCoCO is unable to activate the C-H bond of the CH(CH₃)₃ molecule due to the high energy at which the radical fragments Cp(CO)H-M + C(CH₃)₃ are located. The picture attained for these interactions is consistent with the available experimental data for this kind of reaction and allows rationalization of the differences in the reactivity patterns determined for them without using spin-flip models, as has been proposed in previous studies.

Identification of the dominant photochemical pathways and mechanistic insights to the ultrafast ligand exchange of Fe(CO)5 to Fe(CO)4EtOH

We utilized femtosecond time-resolved resonant inelastic X-ray scattering and ab initio theory to study the transient electronic structure and the photoinduced molecular dynamics of a model metal carbonyl photocatalyst Fe(CO)5 in ethanol solution. We propose mechanistic explanation for the parallel ultrafast intra-molecular spin crossover and ligation of the Fe(CO)4 which are observed following a charge transfer photoexcitation of Fe(CO)5 as reported in our previous study [Wernet et al., Nature 520, 78 (2015)]. We find that branching of the reaction pathway likely happens in the (1)A1 state of Fe(CO)4. A sub-picosecond time constant of the spin crossover from (1)B2 to (3)B2 is rationalized by the proposed (1)B2 → (1)A1 → (3)B2 mechanism. Ultrafast ligation of the (1)B2 Fe(CO)4 state is significantly faster than the spin-forbidden and diffusion limited ligation process occurring from the (3)B2 Fe(CO)4 ground state that has been observed in the previous studies. We propose that the ultrafast ligation occurs via (1)B2 → (1)A1 → (1)A' Fe(CO)4EtOH pathway and the time scale of the (1)A1 Fe(CO)4 state ligation is governed by the solute-solvent collision frequency. Our study emphasizes the importance of understanding the interaction of molecular excited states with the surrounding environment to explain the relaxation pathways of photoexcited metal carbonyls in solution.

Mechanistic investigations of CO-photoextrusion and oxidative addition reactions of early transition-metal carbonyls: (η(5)-C5H5)M(CO)4 (M = V, Nb, Ta)

The mechanisms for the photochemical Si-H bond activation reaction are studied theoretically using a model system of the group 5 organometallic compounds, η(5)-CpM(CO)4 (M = V, Nb, and Ta), with the M06-2X method and the Def2-SVPD basis set. Three types of reaction pathways that lead to final insertion products are identified. The structures of the intersystem crossings, which play a central role in these photo-activation reactions, are determined. The intermediates and transitional structures in either the singlet or triplet states are also calculated to provide a mechanistic explanation of the reaction pathways. All of the potential energy surfaces for the group 5 η(5)-CpM(CO)4 complexes are quite similar. In particular, the theoretical evidence suggests that after irradiation using light, η(5)-CpM(CO)4 quickly loses one CO ligand to yield two tricarbonyls, in either the singlet or the triplet states. The triplet tricarbonyl 16-electron intermediates, ([η(5)-CpM(CO)3](3)), play a key role in the formation of the final oxidative addition product, η(5)-CpM(CO)3(H)(SiMe3). However, the singlet counterparts, ([η(5)-CpM(CO)3](1)), play no role in the formation of the final product molecule, but their singlet metal centers interact weakly with solvent molecules ((Me3)SiH) to produce alkyl-solvated organometallic complexes, which are observable experimentally. This theoretical evidence is in accordance with the available experimental observations.

Dinitrogen binding, P4-activation and aza-Büchner ring expansions mediated by an isocyano analogue of the CpCo(CO) fragment

Synthetic studies targeting an m-terphenyl isocyanide analogue of the unstable 16e⁻, S = 1 complex CpCo(CO) are reported (Cp = η⁵-C₅H₅).

Non-decarbonylative photochemical versus thermal activation of Bu4N[Fe(CO)3(NO)] - the Fe-catalyzed Cloke-Wilson rearrangement of vinyl and arylcyclopropanes

The base metal complex Bu4N[Fe(CO)3(NO)] (TBA[Fe]) catalyzes the rearrangement of vinyl and arylcyclopropanes both under thermal or photochemical conditions to give the corresponding vinyl or aryldihydrofurans in good to excellent yields. Under photochemical conditions the reaction is performed at room temperature. Spectroscopic investigations show that the metal carbonyl catalyst is not decarbonylated. The best performance was observed at a wavelength of 415 nm. icMRCI+Q analysis of the excited singlet and triplet states of the [Fe(CO)3(NO)] anion was performed and used to calculate the vertical excitation energies which are in good agreement with the experimental data. CASSCF analysis indicates that the Fe center in all excited states of the ferrate becomes more electrophilic while adopting a distorted tetrahedral configuration. Both aspects have a positive synergistic effect on the formation of the initial π-complex with the incoming organic substrate.

Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution

Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site that need to be controlled to optimize complexes for photocatalytic hydrogen production and selective carbon-hydrogen bond activation. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst with an electron deficiency at the Fe centre, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16 - 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.

Mechanistic studies of photoinduced spin crossover and electron transfer in inorganic complexes

Electronic excited-state phenomena provide a compelling intersection of fundamental and applied research interests in the chemical sciences. This holds true for coordination chemistry, where harnessing the strong optical absorption and photocatalytic activity of compounds depends on our ability to control fundamental physical and chemical phenomena associated with the nonadiabatic dynamics of electronic excited states. The central events of excited-state chemistry can critically influence the dynamics of electronic excited states, including internal conversion (transitions between distinct electronic states) and intersystem crossing (transitions between electronic states with different spin multiplicities), events governed by nonadiabatic interactions between electronic states in close proximity to conical intersections, as well as solvation and electron transfer. The diversity of electronic and nuclear dynamics also makes the robust interpretation of experimental measurements challenging. Developments in theory, simulation, and experiment can all help address the interpretation and understanding of chemical dynamics in organometallic and coordination chemistry. Synthesis presents the opportunity to chemically engineer the strength and symmetry of the metal-ligand interactions. This chemical control can be exploited to understand the influence of electronic ground state properties on electronic excited-state dynamics. New time-resolved experimental methods and the insightful exploitation of established methods have an important role in understanding, and ideally controlling, the photophysics and photochemistry of transition metal complexes. Techniques that can disentangle the coupled motion of electrons and nuclear dynamics warrant emphasis. We present a review of electron localization dynamics in charge transfer excited states and the dynamics of photoinitiated spin crossover dynamics. Both electron localization and spin crossover have been investigated by numerous research groups with femtosecond resolution spectroscopy, but challenges in experimental interpretation have left significant uncertainty about the molecular properties that control these phenomena. Our Account will emphasize how tailoring the experimental probe, femtosecond resolution vibrational anisotropy for electron localization, and femtosecond resolution hard X-ray fluorescence for spin crossover can make a significant impact on the interpretability of experimental measurements. The emphasis on thorough and robust interpretation has also led to an emphasis on simpler molecular systems. This enables iteration between experiment and theory, a requirement for the development of a more predictive understanding of electronic excited-state phenomena and an essential step to the development of design rules for solar materials.

Picosecond time-resolved infrared spectroscopy of rhodium and iridium azides

Picosecond time-resolved infrared spectroscopy was used to elucidate early photochemical processes in the diazido complexes M(Cp*)(N3)2(PPh3), M = Rh (), Ir (), using 266 nm and 400 nm excitation in THF, CH2Cl2, MeCN and toluene solutions. The time-resolved data have been interpreted with the aid of DFT calculations on vibrational spectra of the singlet ground states and triplet excited states and their rotamers. While the yields of phototransformations via N2 loss are low in both complexes, cleaves a N3 ligand under 266 nm excitation. The molecular structure of is also reported as determined by single crystal X-ray diffraction.

Bis(imino)pyridine cobalt-catalyzed dehydrogenative silylation of alkenes: scope, mechanism, and origins of selective allylsilane formation

The aryl-substituted bis(imino)pyridine cobalt methyl complex, ((Mes)PDI)CoCH3 ((Mes)PDI = 2,6-(2,4,6-Me3C6H2-N═CMe)2C5H3N), promotes the catalytic dehydrogenative silylation of linear α-olefins to selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C-H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.

Ultrafast infrared studies of the role of spin states in organometallic reaction dynamics

The importance of spin state changes in organometallic reactions is a topic of significant interest, as an increasing number of reaction mechanisms involving changes of spin state are consistently being uncovered. The potential influence of spin state changes on reaction rates can be difficult to predict, and thus this class of reactions remains among the least well understood in organometallic chemistry. Ultrafast time-resolved infrared (TRIR) spectroscopy provides a powerful tool for probing the dynamics of spin state changes in organometallic catalysis, as such processes often occur on the picosecond to nanosecond time scale and can readily be monitored in the infrared via the absorptions of carbonyl reporter ligands. In this Account, we summarize recent work from our group directed toward identifying trends in reactivity that can be used to offer predictive insight into the dynamics of coordinatively unsaturated organometallic reaction intermediates. In general, coordinatively unsaturated 16-electron (16e) singlets are able to coordinate to solvent molecules as token ligands to partially stabilize the coordinatively unsaturated metal center, whereas 16e triplets and 17-electron (17e) doublets are not, allowing them to diffuse more rapidly through solution than their singlet counterparts. Triplet complexes typically (but not always) undergo spin crossover prior to solvent coordination, whereas 17e doublets do not coordinate solvent molecules as token ligands and cannot relax to a lower spin state to do so. 16e triplets are typically able to undergo facile spin crossover to yield a 16e singlet where an associative, exothermic reaction pathway exists. The combination of facile spin crossover with faster diffusion through solution for triplets can actually lead to faster catalytic reactivity than for singlets, despite the forbidden nature of these reactions. We summarize studies on odd-electron complexes in which 17e doublets were found to display varying behavior with regard to their tendency to react with 2-electron donor ligands to form 19-electron (19e) adducts. The ability of 19e adducts to serve as reducing agents in disproportionation reactions depends on whether the excess electron density localized at the metal center or at a ligand site. The reactivity of both 16e and 17e complexes toward a widely used organic nitroxyl radical (TEMPO) are reviewed, and both classes of complexes generally react similarly via an associative mechanism with a low barrier to these reactions. We also describe recent work targeted at unraveling the photoisomerization mechanism of a thermal-solar energy storage complex in which spin state changes were found to play a crucial role. Although a key triplet intermediate was found to be required for this photoisomerization mechanism to proceed, the details of why this triplet is formed in some complexes (those based on ruthenium) and not others (those based on iron, molybdenum, or tungsten) remains uncertain, and further exploration in this area may lead to a better understanding of the factors that influence intramolecular and excited state spin state changes.

Stepwise addition of difluorocarbene to a transition metal centre

The Ruppert–Prakash reagent (Me₃SiCF₃) is used to introduce difluorocarbene (CF₂) and tetrafluoroethylene (TFE) ligands to cobalt(i) metal centres, whereby the TFE ligand is generated via [2+1] cycloaddition between [Co]CF₂ and CF₂.

Direct observation of photoinduced bent nitrosyl excited-state complexes

Ground-state structures with side-on nitrosyl (eta (2)-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited-state structures with bent-NO ligands have been proposed for years but never directly observed. Here, we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO) 3(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and eta (2)-NO structural isomers, but we have observed two bent-NO complexes. DFT modeling of the ground- and excited-state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO) 3(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited-state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.