Binding methane to a metal centre

Formation and reactivity of a unique M⋯C-H interaction stabilized by carborane cages

Broadening carborane applications has consistently been the goal of chemists in this field. Herein, compared to alkyl or aryl groups, a carborane cage demonstrates an advantage in stabilizing a unique bonding interaction: M⋯C-H interaction. Experimental results and theoretical calculations have revealed the characteristic of this two-center, two-electron bonding interaction, in which the carbon atom in the arene ring provides two electrons to the metal center. The reduced aromaticity of the benzene moiety, long distance between the metal and carbon atom in arene, and the upfield shift of the signal of M⋯C-H in the nuclear magnetic resonance spectrum distinguished this interaction from metal⋯C π interaction and metal-C(H) σ bonds. Control experiments demonstrate the unique electronic effects of carborane in stabilizing the M⋯C-H bonding interaction in organometallic chemistry. Furthermore, the M⋯C-H interaction can convert into C-H bond metallization under acidic conditions or via treatment with t-butyl isocyanide. These findings deepen our understanding regarding the interactions between metal centers and carbon atoms and provide new opportunities for the use of carboranes.

Photochemical Formation and Electronic Structure of an Alkane σ-Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy

C-H bond activation reactions with transition metals typically proceed via the formation of alkane σ-complexes, where an alkane C-H σ-bond binds to the metal. Due to the weak nature of metal-alkane bonds, σ-complexes are challenging to characterize experimentally. Here, we establish the complete pathways of photochemical formation of the model σ-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal-ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photoinduced CO dissociation from Cr(CO)6 to occur within the 100 fs time resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane σ-complex from both the metal and ligand perspectives. Based on clear experimental observables, we find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C-H bond to the undercoordinated Cr center in the Cr(CO)5-alkane σ-complex, and we define this as a general, orbital-based descriptor of the metal-alkane bond. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the stability and reactivity of alkane σ-complexes in their role as the decisive intermediates in C-H bond activation reactions.

An osmium(II) methane complex: Elucidation of the methane coordination mode

The activation of inert C─H bonds by transition metals is of considerable industrial and academic interest, but important gaps remain in our understanding of this reaction. We report the first experimental determination of the structure of the simplest hydrocarbon, methane, when bound as a ligand to a homogenous transition metal species. We find that methane binds to the metal center in this system through a single M···H-C bridge; changes in the ¹J_CH coupling constants indicate clearly that the structure of the methane ligand is significantly perturbed relative to the free molecule. These results are relevant to the development of better C─H functionalization catalysts.

Tracking C-H activation with orbital resolution

Transition metal reactivity toward carbon-hydrogen (C-H) bonds hinges on the interplay of electron donation and withdrawal at the metal center. Manipulating this reactivity in a controlled way is difficult because the hypothesized metal-alkane charge-transfer interactions are challenging to access experimentally. Using time-resolved x-ray spectroscopy, we track the charge-transfer interactions during C-H activation of octane by a cyclopentadienyl rhodium carbonyl complex. Changes in oxidation state as well as valence-orbital energies and character emerge in the data on a femtosecond to nanosecond timescale. The x-ray spectroscopic signatures reflect how alkane-to-metal donation determines metal-alkane complex stability and how metal-to-alkane back-donation facilitates C-H bond cleavage by oxidative addition. The ability to dissect charge-transfer interactions on an orbital level provides opportunities for manipulating C-H reactivity at transition metals.

Weighted AnX multiplets

That the NMR transition of a spin-1/2 nucleus is split into n evenly spaced lines by indirect dipole-dipole (J) coupling to n magnetically equivalent nuclei, whose successive amplitudes follow the n^th row of Pascal's triangle, is an elementary result in NMR. Described here are a family of less well known multiplet structures with different amplitudes for the evenly spaced lines. The amplitudes can be derived from the n^th row of Pascal's triangle by weighting the corresponding value of the row by z or z², where z is related to the summed magnetic quantum number of the magnetically equivalent spins. z¹-multiplets have been described in INEPT experiments. A z²-multiplet can be indirectly observed in HSQC experiments when the decoupling pulse during t₁ is removed, i.e., an F1-coupled HSQC. While not difficult to generate and despite some reported usefulness, to the best of our knowledge, z²-multiplets have not been rigorously described in previous literature.

[Fp(CH4)]+, [η5-CpRu(CO)2(CH4)]+, and [η5-CpOs(CO)2(CH4)]+: A Complete Set of Group 8 Metal-Methane Complexes

Here, we report the NMR spectroscopic analysis of the group 8 transition metal methane σ-complexes [η⁵-CpM(CO)₂(CH₄)][Al(OC(CF₃)₃)₄] (M = Fe, Ru) at -90 °C in the weakly coordinating solvent 1,1,1,3,3,3-hexafluoropropane. The iron(II)-methane complex has a ¹H resonance at δ -4.27, a ¹³C resonance at δ -53.0, and ¹J_C-H = 126 Hz for the bound methane fragment. The ruthenium(II)-methane complex has a ¹H resonance at δ -2.10, a ¹³C resonance at δ -48.8, and a ¹J_C-H = 126 Hz for the bound methane fragment. DFT and ab initio calculations support these experimental observations and provide further detail on the structures of the [η⁵-CpM(CO)₂(CH₄)]⁺ (M = Fe, Ru) complexes of the Group 8 metals. Both the iron centered methane complex, [η⁵-CpFe(CO)₂(CH₄)][Al(OC(CF₃)₃)₄], and the ruthenium centered methane complex, [η⁵-CpRu(CO)₂(CH₄)][Al(OC(CF₃)₃)₄], are significantly less stable than the previously reported osmium-methane complex [η⁵-CpOs(CO)₂(CH₄)][Al(OC(CF₃)₃)₄].

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.