Direct Observation of a Transition Metal Alkane Complex, CpRe(CO)2(cyclopentane), Using NMR Spectroscopy

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.

[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.

Binding methane to a metal centre

The σ-alkane complexes of transition metals, which contain an essentially intact alkane molecule weakly bound to the metal, have been well established as crucial intermediates in the activation of the strong C-H σ-bonds found in alkanes. Methane, the simplest alkane, binds even more weakly than larger alkanes. Here we report an example of a long-lived methane complex formed by directly binding methane as an incoming ligand to a reactive organometallic complex. Photo-ejection of carbon monoxide from a cationic osmium-carbonyl complex dissolved in an inert hydrofluorocarbon solvent saturated with methane at -90 °C affords an osmium(II) complex, [η⁵-CpOs(CO)₂(CH₄)]⁺, containing methane bound to the metal centre. Nuclear magnetic resonance (NMR) spectroscopy confirms the identity of the σ-methane complex and shows that the four protons of the metal-bound methane are in rapid exchange with each other. The methane ligand has a characteristically shielded ¹H NMR resonance (δ -2.16), and the highly shielded carbon resonance (δ -56.3) shows coupling to the four attached protons (¹J_C-H = 127 Hz). The methane complex has an effective half-life of about 13 hours at -90 °C.

Inverse Isotope Effects in Single-Crystal to Single-Crystal Reactivity and the Isolation of a Rhodium Cyclooctane σ-Alkane Complex

The sequential solid/gas single-crystal to single-crystal reaction of [Rh(Cy₂P(CH₂)₃PCy₂)(COD)][BAr^F₄] (COD = cyclooctadiene) with H₂ or D₂ was followed in situ by solid-state ³¹P{¹H} NMR spectroscopy (SSNMR) and ex situ by solution quenching and GC-MS. This was quantified using a two-step Johnson–Mehl–Avrami–Kologoromov (JMAK) model that revealed an inverse isotope effect for the second addition of H₂, that forms a σ-alkane complex [Rh(Cy₂P(CH₂)₃PCy₂)(COA)][BAr^F₄]. Using D₂, a temporal window is determined in which a structural solution for this σ-alkane complex is possible, which reveals an η²,η²-binding mode to the Rh(I) center, as supported by periodic density functional theory (DFT) calculations. Extensive H/D exchange occurs during the addition of D₂, as promoted by the solid-state microenvironment.

A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane

Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H-C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H-C interactions with the geminal C-H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]- anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.

Recent Advances in the Synthesis and Reactivity of Transition Metal σ-Borane/Borate Complexes

The coordination of an element-element σ bond to a transition metal (TM) is both a fundamentally intriguing binding mode and of critical importance to metal-mediated bond activation mechanisms and catalysis, particularly the hotly contested field of C-H activation. TM σ complexes of dihydrogen (i.e., H-H) and silanes (H-SiR₃) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H-CR₃). TM σ complexes of hydroboranes and hydroborates (i.e., H-BR₂, H-BR₃, (H-)₂BR₂) are somewhat less well studied but similarly have relevance to catalytic borylation reactions that are of high current interest to organic synthesis. Our two research groups have made significant contributions to elaborating the family of σ-borane/-borate complexes using two distinct approaches: while the Ghosh group generally starts from hydrogen-rich tetracoordinate boron species such as borates, the Braunschweig group starts from hypovalent and/or hypocoordinate boron building blocks. Through these two approaches, a wide range of species containing one or two σ-bound B-H ligands have been prepared, some with additional chelating donor sites. Over the past 2 years, the body of work on σ-borane/-borate complexes from our two research groups has significantly expanded, with a combined nine published articles in 2019-2020 alone. Very recent work from the Braunschweig group has led to the synthesis of the first bis(σ)-borane complexes of group 6 metals, as well as the synthesis of a series of novel bis(σ)-borane and bis(σ)-borate complexes of ruthenium and iridium, the former being useful precursors for pentacoordinate borylene complexes of Ru. Recent work from the Ghosh group has uncovered a remarkable diversity of structures with σ(B-H)-bound ligands from the combination of borohydrides and nitrogen/chalcogen-containing groups and heterocycles. These reactions, while in some cases producing conventional scorpionate-type chelating products, more frequently undergo fascinating rearrangements with unpredictable outcomes. This Account aims to highlight this recent acceleration of research progress in this area, particularly the distinct but related approaches of-and complexes produced by-our two research groups, in addition to relevant works from other groups where appropriate.

A Structurally Characterized Cobalt(I) σ-Alkane Complex

A cobalt σ-alkane complex, [Co(Cy2 P(CH2 )4 PCy2 )(norbornane)][BArF 4 ], was synthesized by a single-crystal to single-crystal solid/gas hydrogenation from a norbornadiene precursor, and its structure was determined by X-ray crystallography. Magnetic data show this complex to be a triplet. Periodic DFT and electronic structure analyses revealed weak C-H→Co σ-interactions, augmented by dispersive stabilization between the alkane ligand and the anion microenvironment. The calculations are most consistent with a η1 :η1 -alkane binding mode.

Diborane(6) and Its Analogues Stabilized by Mono-, Bi-, and Trinuclear Group 7 Templates: Combined Experimental and Theoretical Studies

Irradiation of [Re₂(CO)₁₀] in the presence of BH₃·thf resulted in the formation of several rhenium diborane(6) species, for example, [{(OC)₄Re}{Re(CO)₃}₂(μ₃-η²:η²:η²-B₂H₆)(μ-H)], 1; [{(OC)₄Re}₂{Re(CO)₃}(μ₃-η²:η²:η¹-B₂H₆)(μ-H)], 2; and [{(OC)₄Re}₂(μ-η²:η²-B₂H₆)], 3, comprising diverse coordination modes of the [B₂H₆]^2- ligand. Compound 1 contains a tris(bidentate) [B₂H₆]^2- unit, whereas 2 consists of an unsymmetrically bound [μ₃-η²:η²:η¹-B₂H₆]^2- ligand. In contrast, the irradiation of [Mn₂(CO)₁₀] with BH₃·thf yielded only the Mn analogue of 1, [{(OC)₄Mn}{Mn(CO)₃}₂(μ₃-η²:η²:η²-B₂H₆)(μ-H)], 1'. In an attempt to generate the bimetallic Mn-diborane(6), we have carried out the reaction of 1' with PCy₃ under photolytic conditions. The reaction led to the formation of two single base stabilized unsymmetrical diborane(5) species, [{Mn(CO)₃}{Mn(CO)₂PCy₃}(μ-η²:η²-B₂H₅·PCy₃)(μ-H)], 4, and [{Mn(CO)₂PCy₃}(η³-B₂H₅·PCy₃)], 5. As [B₂H₆]^2- and [B₂H₅·PCy₃]^- are isoelectronic, the bondings in 4 and 5 are analogous to that of diborane(6) species 1-3. All the new species have been characterized spectroscopically, and their structures were further confirmed by single-crystal X-ray diffraction studies. DFT-type quantum chemical calculations were carried out that provided insight into the bonding interaction of [B₂H₆]^2- and [B₂H₅·PCy₃]^- with the M(CO)_n fragments.

σ-H-H, σ-C-H, and σ-Si-H Bond Activation Catalyzed by Metal Nanoparticles

Activation of H-H, Si-H, and C-H bonds through σ-bond coordination has grown in the past 30 years from a scientific curiosity to an important tool in the functionalization of hydrocarbons. Several mechanisms were discovered via which the initially σ-bonded substrate could be converted: oxidative addition, heterolytic cleavage, σ-bond metathesis, electrophilic attack, etc. The use of metal nanoparticles (NPs) in this area is a more recent development, but obviously nanoparticles offer a much richer basis than classical homogeneous and heterogeneous catalysts for tuning reactivity for such a demanding process as C-H functionalization. Here, we will review the surface chemistry of nanoparticles and catalytic reactions occurring in the liquid phase, catalyzed by either colloidal or supported metal NPs. We consider nanoparticles prepared in solution, which are stabilized and tuned by polymers, ligands, and supports. The question we have addressed concerns the differences and similarities between molecular complexes and metal NPs in their reactivity toward σ-bond activation and functionalization.

Chromatographic approach to study the configurational stability of Ni(II) complexes of amino-acid Schiff bases possessing stereogenic nitrogen

Herein, we disclose the design of a model Ni(II) complex of glycine Schiff base possessing single-nitrogen stereogenic center, which was successfully used for high-performance liquid chromatography (HPLC)-assisted assessment of its configurational stability. The major finding is that the configurational stability of the Ni(II)-coordinated nitrogen is profoundly dependent on the reaction conditions used, in particular the solvent, and can range from inconsequential (t_½ less than 5 min) to virtually completely stable (t_½ 90 y). The discovery reported in this study most likely to be of certain theoretical and synthetic value.

Modulation of σ-Alkane Interactions in [Rh(L2)(alkane)]+ Solid-State Molecular Organometallic (SMOM) Systems by Variation of the Chelating Phosphine and Alkane: Access to η2,η2-σ-Alkane Rh(I), η1-σ-Alkane Rh(III) Complexes, and Alkane Encapsulation

Solid/gas single-crystal to single-crystal (SC-SC) hydrogenation of appropriate diene precursors forms the corresponding σ-alkane complexes [Rh(Cy₂P(CH₂) _nPCy₂)(L)][BAr^F₄] ( n = 3, 4) and [ RhH(Cy₂P(CH₂)₂( CH)(CH₂)₂PCy₂)(L)][BAr^F₄] ( n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as determined by single-crystal X-ray diffraction, have cations exhibiting Rh···H-C σ-interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion microenvironment. These range from chelating η²,η² Rh···H-C (e.g., [Rh(Cy₂P(CH₂) _nPCy₂)(η²η²-NBA)][BAr^F₄], n = 3 and 4), through to more weakly bound η¹ Rh···H-C in which C-H activation of the chelate backbone has also occurred (e.g., [ RhH(Cy₂P(CH₂)₂( CH)(CH₂)₂PCy₂)(η¹-COA)][BAr^F₄]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment, [Rh(Cy₂P(CH₂)₃PCy₂)][COA⊂BAr^F₄], in which the metal center instead forms two intramolecular agostic η¹ Rh···H-C interactions with the phosphine cyclohexyl groups. CH₂Cl₂ adducts formed by displacement of the η¹-alkanes in solution ( n = 5; L = NBA, COA), [ RhH(Cy₂P(CH₂)₂( CH)(CH₂)₂PCy₂)(κ¹-ClCH₂Cl)][BAr^F₄], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO, and NCI calculations, alongside variable temperature solid-state NMR spectroscopy, provide snapshots marking the onset of Rh···alkane interactions along a C-H activation trajectory. These are negligible in [Rh(Cy₂P(CH₂)₃PCy₂)][COA⊂BAr^F₄]; in [ RhH(Cy₂P(CH₂)₂( CH)(CH₂)₂PCy₂)(η¹-COA)][BAr^F₄], σ_C-H → Rh σ-donation is supported by Rh → σ*_C-H "pregostic" donation, and in [Rh(Cy₂P(CH₂) _nPCy₂)(η²η²-NBA)][BAr^F₄] ( n = 2-4), σ-donation dominates, supported by classical Rh(dπ) → σ*_C-H π-back-donation. Dispersive interactions with the [BAr^F₄]^- anions and Cy substituents further stabilize the alkanes within the binding pocket.

Solution, Solid-State, and Computational Analysis of Agostic Interactions in a Coherent Set of Low-Coordinate Rhodium(III) and Iridium(III) Complexes

A homologous family of low-coordinate complexes of the formulation trans-[M(2,2'-biphenyl)(PR3 )2 ][BArF4 ] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT 1 H and 31 P NMR spectroscopy) and solid-state (single crystal X-ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3 >PCy3 >PiPr3 >PPh3 . In addition to these structure-property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2-difluorobenzene vs. CH2 Cl2 ) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M-H-C distances.

Gas phase 1H NMR studies and kinetic modeling of dihydrogen isotope equilibration catalyzed by Ru-nanoparticles under normal conditions: dissociative vs. associative exchange

Exposure of surface H-containing Ru-nanoparticles to D₂ gas produces HD via associative adsorption, surface H-transfer and associative desorption.

A Rhodium-Pentane Sigma-Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques

The pentane σ-complex [Rh{Cy2 P(CH2 CH2 )PCy2 }(η(2) :η(2) -C5 H12 )][BAr(F) 4 ] is synthesized by a solid/gas single-crystal to single-crystal transformation by addition of H2 to a precursor 1,3-pentadiene complex. Characterization by low temperature single-crystal X-ray diffraction (150 K) and SSNMR spectroscopy (158 K) reveals coordination through two Rh⋅⋅⋅H-C interactions in the 2,4-positions of the linear alkane. Periodic DFT calculations and molecular dynamics on the structure in the solid state provide insight into the experimentally observed Rh⋅⋅⋅H-C interaction, the extended environment in the crystal lattice and a temperature-dependent pentane rearrangement implicated by the SSNMR data.

Photochromic switching behaviour of donor–acceptor Stenhouse adducts in organic solvents

We report photochromic donor–acceptor Stenhouse adducts (DASAs) capable of fully reversible photoisomerization with visible light in organic solvents including chloroform, acetonitrile and benzene.

Solid-state synthesis and characterization of σ-alkane complexes, [Rh(L2)(η(2),η(2)-C7H12)][BAr(F)4] (L2 = bidentate chelating phosphine)

The use of solid/gas and single-crystal to single-crystal synthetic routes is reported for the synthesis and characterization of a number of σ-alkane complexes: [Rh(R2P(CH2)nPR2)(η(2),η(2)-C7H12)][BAr(F)4]; R = Cy, n = 2; R = (i)Pr, n = 2,3; Ar = 3,5-C6H3(CF3)2. These norbornane adducts are formed by simple hydrogenation of the corresponding norbornadiene precursor in the solid state. For R = Cy (n = 2), the resulting complex is remarkably stable (months at 298 K), allowing for full characterization using single-crystal X-ray diffraction. The solid-state structure shows no disorder, and the structural metrics can be accurately determined, while the (1)H chemical shifts of the Rh···H-C motif can be determined using solid-state NMR spectroscopy. DFT calculations show that the bonding between the metal fragment and the alkane can be best characterized as a three-center, two-electron interaction, of which σCH → Rh donation is the major component. The other alkane complexes exhibit solid-state (31)P NMR data consistent with their formation, but they are now much less persistent at 298 K and ultimately give the corresponding zwitterions in which [BAr(F)4](-) coordinates and NBA is lost. The solid-state structures, as determined by X-ray crystallography, for all these [BAr(F)4](-) adducts are reported. DFT calculations suggest that the molecular zwitterions within these structures are all significantly more stable than their corresponding σ-alkane cations, suggesting that the solid-state motif has a strong influence on their observed relative stabilities.

Group 9 bimetallic carbonyl permethylpentalene complexes

We describe the synthesis, structure and bonding of the first iridium and rhodium permethylpentalene complexes, syn-[M(CO)₂]₂(μ:η⁵:η⁵-Pn*) (M = Rh, Ir).

Detection of σ-alkane complexes of manganese by NMR and IR spectroscopy in solution: (η5-C5H5)Mn(CO)2(ethane) and (η5-C5H5)Mn(CO)2(isopentane)

Irradiation of CpMn(CO)3 in liquid ethane at 135 K at 355 nm yields a photoproduct that exhibits ν(CO) bands in the IR spectrum shifted to low wavenumber with respect to CpMn(CO)3 that are indicative of a Mn(i) dicarbonyl. Parallel experiments employing in situ irradiation within an NMR probe (133 K, 355 nm photolysis) reveal the 1H NMR signals of this product and confirm its formulation as the σ-ethane complex CpMn(CO)2(η2-C1-H-ethane). The resonance of its coordinated C-H group is observed at δ -5.84 and decays with lifetime of ca. 360 s. Analogous photolysis experiments in isopentane solution with IR detection produce CpMn(CO)2(η2-C-H-isopentane) with similar IR bands to those of CpMn(CO)2(η2-C-H-ethane). 1H NMR spectra of the same species were obtained by irradiation of CpMn(CO)3 in a 60 : 40 mixture of propane and isopentane; three isomers of CpMn(CO)2(η2-C-H-isopentane) were detected with coordination of manganese at the two inequivalent methyl positions and at the methylene group, respectively. The lifetimes of these isomers are ca. 380 ± 20 s at 135 K and do not vary significantly from each other. These σ-complexes of manganese are far more reactive than those of related CpRe(CO)2(alkane) complexes which are stable in solution at 170-180 K. The room temperature lifetimes of CpMn(CO)2(η2-C-H-ethane) and CpMn(CO)2(η2-C-H-isopentane), as determined by TRIR spectroscopy, are 2.0 ± 0.1 and 28 ± 1 μs, respectively.

Synthesis and structures of doubly-bridged dicyclopentadienyl dinuclear rhenium complexes, and their photochemical reactions with aromatic halides in benzene

Reaction of the doubly-bridged biscyclopentadienes (C5H4(EMe2))(C5H4(SiMe2)) (E = Si(1a), or C(1b)) with Re2(CO)10 in refluxing mesitylene gave the corresponding dirhenium carbonyl complexes [(η(5)-C5H3)2(EMe2)(SiMe2)][Re(CO)3]2 (trans-2a,b and cis-2a,b), and the desilylated products [(η(5)-C5H4)2(EMe2)][Re(CO)3]2 (3a,b). Photolysis of trans-2a,b with a series of aryl halides in benzene results in the formation of biphenyl, together with the corresponding rhenium dihalide complexes trans-[(η(5)-C5H3)2(EMe2)(SiMe2)][Re(CO)3][Re(CO)2X2] (X = Cl, Br, I) (4, 6, 8, or 10) and trans-[(η(5)-C5H3)2(EMe2)(SiMe2)][Re(CO)2X2]2 (5, 7, or 9). Additional experiments indicate that one phenyl ring in the resulting biphenyl comes from aryl halides and the other phenyl ring comes from the solvent benzene. However, photolysis of trans-2a with benzyl chloride and n-hexyl chloride in benzene afforded the corresponding bibenzyl and n-dodecane, as well as the rhenium dichlorides 8 and 9. In addition, complex trans-2a can also activate the C-F bond on C6F6 to form the pentafluorophenyl rhenium fluoride trans-[(η(5)-C5H3)2(SiMe2)2][Re(CO)3][Re(CO)2(C6F5)F] (11). Molecular structures of cis-2a, cis-2b, trans-2b, 3b, 6, 7, 8, 11, and 12 determined by X-ray diffraction are also presented.

High-pressure-low-temperature cryostat designed for use with fourier transform infrared spectrometers and time-resolved infrared spectroscopy

The design for a new high-pressure-low-temperature infrared (IR) cell for performing experiments using conventional Fourier transform infrared or fast laser-based time-resolved infrared spectroscopy, in a range of solvents, is described. The design builds upon a commercially available compressor and cold end (Polycold PCC(®) and CryoTiger(®)), which enables almost vibration-free operation, ideal for use with sensitive instrumentation. The design of our cell and cryostat allows for the study of systems at temperatures from 77 to 310 K and at pressures up to 250 bar. The CaF2 windows pass light from the mid-IR to the ultraviolet (UV), enabling a number of experiments to be performed, such as Raman, UV-visible absorption spectroscopy, and time-resolved techniques where sample excitation/probing using continuous wave or pulsed lasers is required. We demonstrate the capabilities of this cell by detailing two different applications: (i) the reactivity of a range of Group V-VII organometallic alkane complexes using time-resolved spectroscopy on the millisecond timescale and (ii) the gas-to-liquid phase transition of CO2 at low temperature, which is applicable to measurements associated with transportation issues related to carbon capture and storage.

Stability and dynamic processes in 16VE iridium(III) ethyl hydride and rhodium(I) σ-ethane complexes: experimental and computational studies

Iridium(I) and rhodium(I) ethyl complexes, (PONOP)M(C2H5) (M = Ir (1-Et), Rh (2-Et)) and the iridium(I) propyl complex (PONOP)Ir(C3H7) (1-Pr), where PONOP is 2,6-(tBu2PO)2C5H3N, have been prepared. Low-temperature protonation of the Ir complexes yields the alkyl hydrides, (PONOP)Ir(H)(R) (1-(H)(Et)(+) and 1-(H)(Pr)(+)), respectively. Dynamic (1)H NMR characterization of 1-(H)(Et)(+) establishes site exchange between the Ir-H and Ir-CH2 protons (ΔG(exH)(‡)(-110 °C) = 7.2(1) kcal/mol), pointing to a σ-ethane intermediate. By dynamic (13)C NMR spectroscopy, the exchange barrier between the α and β carbons ("chain-walking") was measured (ΔG(exC)(‡)(-110 °C) = 8.1(1) kcal/mol). The barrier for ethane loss is 17.4(1) kcal/mol (-40 °C), to be compared with the reported barrier to methane loss in 1-(H)(Me)(+) of 22.4 kcal/mol (22 °C). A rhodium σ-ethane complex, (PONOP)Rh(EtH) (2-(EtH)(+)), was prepared by protonation of 2-Et at -150 °C. The barrier for ethane loss (ΔG(dec)(‡)(-132 °C) = 10.9(2) kcal/mol) is lower than for the methane complex, 2-(MeH)(+), (ΔG(dec)(‡)(-87 °C) = 14.5(4) kcal/mol). Full spectroscopic characterization of 2-(EtH)(+) is reported, a key feature of which is the upfield signal at -31.2 ppm for the coordinated CH3 group in the (13)C NMR spectrum. The exchange barrier of the hydrogens of the coordinated methyl group is too low to be measured, but the chain-walking barrier of 7.2(1) kcal/mol (-132 °C) is observable by (13)C NMR. The coordination mode of the alkane ligand and the exchange pathways for the Rh and Ir complexes are evaluated by DFT studies. On the basis of the computational studies, it is proposed that chain-walking occurs by different mechanisms: for Rh, the lowest energy path involves a η(2)-ethane transition state, while for Ir, the lowest energy exchange pathway proceeds through the symmetrical ethylene dihydride complex.