Mechanistic investigations of the Fe(ii) mediated synthesis of squaraines.

The scission and homologation of CO is a fundamental process in the Fischer-Tropsch reaction. However, given the heterogeneous nature of the catalyst and forcing reaction conditions, it is difficult to determine the intermediates of this reaction. Here we report detailed mechanistic insight into the scission/homologation of CO by two-coordinate iron terphenyl complexes. Mechanistic investigations, conducted using in situ monitoring and reaction sampling techniques (IR, NMR, EPR and Mössbauer spectroscopy) and structural characterisation of isolable species, identify a number of proposed intermediates. Crystallographic and IR spectroscopic data reveal a series of migratory insertion reactions from 1Mes to 4Mes. Further studies past the formation of 4Mes suggest that ketene complexes are formed en route to squaraine 2Mes and iron carboxylate 3Mes, with a number of ketene containing structures being isolated, in addition to the formation of unbound, protonated ketene (8). The synthetic and mechanistic studies are supported by DFT calculations.

Slow magnetic relaxation in Fe(II) m-terphenyl complexes.

Two-coordinate transition metal complexes are exciting candidates for single-molecule magnets (SMMs) because their highly axial coordination environments lead to sizeable magnetic anisotropy. We report a series of five structurally related two-coordinate Fe(II) m-terphenyl complexes (4-R-2,6-Xyl2C6H2)2Fe [R = tBu (1), SiMe3 (2), H (3), Cl (4), CF3 (5)] where, by changing the functionalisation of the para-substituent (R), we alter their magnetic function. All five complexes are field-induced single-molecule magnets, with relaxation rates that are well-described by a combination of direct and Raman mechanisms. By using more electron donating R groups we were able to slow the rate of magnetic relaxation. Our ab initio calculations predict a large crystal field splitting (>850 cm-1) and sizeable zero-field splitting parameters (ca. -60 cm-1, |E| < 0.2 cm-1) for 1-5. These favourable magnetic properties suggest that m-terphenyl ligands have untapped potential as chemically versatile ligands able to impose highly axial crystal fields.

Structural and Electronic Studies of Substituted m-Terphenyl Group 12 Complexes.

The effects of para-substitution on the structural and electronic properties of four series of two-coordinate m-terphenyl Group 12 complexes (R-Ar#)2M (M = Zn, Cd, Hg; R = t-Bu 1-3, SiMe3 4-6, Cl 7-9, CF3 10-12, where R-Ar# = 2,6-{2,6-Xyl}2-4-R-C6H2 and 2,6-Xyl = 2,6-Me2C6H3) have been investigated. X-ray crystallography shows little structural variation across the series, with no significant change in the C-M-C bond distances and angles. However, considerable electronic differences are revealed by heteronuclear nuclear magnetic resonance (NMR) spectroscopy; a linear correlation is observed between the 113Cd, 199Hg, and 1H (2,6-Xyl methyl protons) NMR chemical shifts of the para-substituted complexes and the Hammett constants for the R-substituents. Specifically, an upfield shift in the NMR signal is observed with increasingly electron-withdrawing R-substituents. Density functional theory (DFT) calculations are employed to attempt to rationalize these trends.

Group 11 m-Terphenyl Complexes Featuring Metallophilic Interactions.

A series of group 11 m-terphenyl complexes have been synthesized via a metathesis reaction from the iron(II) complexes (2,6-Mes2C6H3)2Fe and (2,6-Xyl2C6H3)2Fe (Mes = 2,4,6-Me3C6H2; Xyl = 2,6-Me2C6H3). [2,6-Mes2C6H3M]2 (1, M = Cu; 2, M = Ag; 6, M = Au) and [2,6-Xyl2C6H3M]2 (3, M = Cu; 4, M = Ag) are dimeric in the solid state, although different geometries are observed depending on the ligand. These complexes feature short metal-metal distances in the expected range for metallophilic interactions. While 1-4 are readily isolated using this metathetical route, the gold complex 6 is unstable in solution at ambient temperatures and has only been obtained in low yield from the decomposition of (2,6-Mes2C6H3)Au·SMe2 (5). NMR spectroscopic measurements, including diffusion-ordered spectroscopy, suggest that 1-4 remain dimeric in a benzene-d6 solution. The metal-metal interactions have been examined computationally using the Quantum Theory of Atoms in Molecules and by an energy decomposition analysis employing natural orbitals for chemical valence.

Structural and electronic studies of substituted m-terphenyl lithium complexes.

The effect of para-substitution upon the structural and electronic properties of a series of m-terphenyl lithium complexes [R-Ar#-Li]2 (R = t-Bu 1, SiMe32, H 3, Cl 4, CF35; where R-Ar# = 2,6-{2,6-Xyl}2-4-R-C6H2 and 2,6-Xyl = 2,6-Me2C6H3) has been investigated. X-ray crystallography reveals the complexes to be structurally similar, with little variation in C-M-C bond lengths and angles across the series. However, in-depth NMR spectroscopic studies reveal notable electronic differences, showing a linear correlation between the 7Li{1H} NMR chemical shifts of the para-substituted complexes and their Hammett constants. The flanking methyl protons exhibit a similar electronic shift in the 1H NMR spectra, which has been rationalised by the presence of through-space LiH interactions, as evidenced by two-dimensional 7Li-1H heteronuclear Overhauser spectroscopy (HOESY). In both cases, electron-withdrawing substituents are found to cause an upfield peak shift. A computational analysis is employed to account for these trends.

Mapping blood biochemistry by Raman spectroscopy at the cellular level.

We report how Raman difference imaging provides insight on cellular biochemistry in vivo as a function of sub-cellular dimensions and the cellular environment. We show that this approach offers a sensitive diagnostic to address blood biochemistry at the cellular level. We examine Raman microscopic images of the distribution of the different hemoglobins in both healthy (discocyte) and unhealthy (echinocyte) blood cells and interpret these images using pre-calculated, accurate pre-resonant Raman tensors for scattering intensities specific to hemoglobins. These tensors are developed from theoretical calculations of models of the oxy, deoxy and met forms of heme benchmarked against the experimental visible spectra of the corresponding hemoglobins. The calculations also enable assignments of the electronic transitions responsible for the colour of blood: these are mainly ligand to metal charge transfer transitions.

A transition metal-gallium cluster formed via insertion of "GaI".

The reaction between a two-coordinate Co(ii) diaryl complex and "GaI" affords 2,6-Pmp2C6H3CoGa3I5, in a new geometry for a heavier group 13-transition metal cluster. Experimental and computational investigations show that this compound is best described as a nido metalla-group 13 cluster.

Carbene-Induced Rescue of Catalytic Activity in Deactivated Nitrite Reductase Mutant.

The role of His145 in the T1 copper center of nitrite reductase (NiR) is pivotal for the activity of the enzyme. Mutation to a glycine at this position enables the reconstitution of the T1 center by the addition of imidazole as exogenous ligands, however the catalytic activity is only marginally rescued. Here, we demonstrate that the uptake of 1,3-dimethylimidazolylidene as N-heterocyclic carbene (NHC) by the H145G NiR mutant instead of imidazole yields a significantly more active catalyst, suggesting a beneficial role of such C-bonding. Spectroscopic analyses of the formed H145G≈NHC variant as well as an analogue without the catalytic T2 copper center reveal no significant alteration of the T1 site compared to the wild type or the variant containing imidazole as exogenous N-bound surrogate of H145. However, the presence of the carbene doubles the catalytic activity of the mutant compared to the imidazole variant. This enhanced activity has been attributed to a faster electron transfer to the T1 center in the NHC variant and a concomitant change of the rate-limiting step.

Influence of molecular design on radical spin multiplicity: characterisation of BODIPY dyad and triad radical anions.

A strategy to create organic molecules with high degrees of radical spin multiplicity is reported in which molecular design is correlated with the behaviour of radical anions in a series of BODIPY dyads. Upon reduction of each BODIPY moiety radical anions are formed which are shown to have different spin multiplicities by electron paramagnetic resonance (EPR) spectroscopy and distinct profiles in their cyclic voltammograms and UV-visible spectra. The relationship between structure and multiplicity is demonstrated showing that the balance between singlet, biradical or triplet states in the dyads depends on relative orientation and connectivity of the BODIPY groups. The strategy is applied to the synthesis of a BODIPY triad which adopts an unusual quartet state upon reduction to its radical trianion.

Imidazolylidene Cu(II) Complexes: Synthesis Using Imidazolium Carboxylate Precursors and Structure Rearrangement Pathways.

Copper(II) complexes of type (NHC)CuX2 (X = OAc, Cl, Br, BF4, and NO3) bearing monodentate N-heterocyclic carbenes (NHCs) were prepared by in situ decarboxylation of imidazolium carboxylates as a new synthetic methodology for Cu(II)-NHC complexes. In contrast to the classical deprotonation method, the decarboxylation protocol does not require anaerobic conditions and provides access to complexes with NHCs that are unstable as free carbenes such as N,N'-diisopropyl-imidazolylidene and N,N'-dimethyl-imidazolylidene. Spectroscopic evidence of the formation of the Cu-CNHC bond is provided by UV-vis and EPR, in particular by the 44 MHz carbene hyperfine coupling constant using a 13C-labeled imidazolylidene ligand. A variation of the nature of the carbene N-substituents and the anions bound to the Cu(II) center is possible with this methodology. These variations strongly influence the stability of the complexes. Structural rearrangement and ligand reorganization was observed during recrystallization, which are comprised of heterolytic Cu-CNHC bond dissociation for unstable NHC ligands as well as homolytic Cu-X bond cleavage and disproportionation reactions depending on the nature of the anion X in the copper complex.

Formation of a Copper(II)-Tyrosyl Complex at the Active Site of Lytic Polysaccharide Monooxygenases Following Oxidation by H2O2.

Hydrogen peroxide is a cosubstrate for the oxidative cleavage of saccharidic substrates by copper-containing lytic polysaccharide monooxygenases (LPMOs). The rate of reaction of LPMOs with hydrogen peroxide is high, but it is accompanied by rapid inactivation of the enzymes, presumably through protein oxidation. Herein, we use UV-vis, CD, XAS, EPR, VT/VH-MCD, and resonance Raman spectroscopies, augmented with mass spectrometry and DFT calculations, to show that the product of reaction of an AA9 LPMO with H2O2 at higher pHs is a singlet Cu(II)-tyrosyl radical species, which is inactive for the oxidation of saccharidic substrates. The Cu(II)-tyrosyl radical center entails the formation of significant Cu(II)-(●OTyr) overlap, which in turn requires that the plane of the d(x2-y2) SOMO of the Cu(II) is orientated toward the tyrosyl radical. We propose from the Marcus cross-relation that the active site tyrosine is part of a "hole-hopping" charge-transfer mechanism formed of a pathway of conserved tyrosine and tryptophan residues, which can protect the protein active site from inactivation during uncoupled turnover.

Selective reduction and homologation of carbon monoxide by organometallic iron complexes.

Carbon monoxide is a key C1 feedstock for the industrial production of hydrocarbons, where it is used to make millions of tonnes of chemicals, fuels, and solvents per annum. Many transition metal complexes can coordinate CO, but the formation of new C-C bonds in well-defined compounds from the scission and subsequent coupling of two or more CO moieties at a transition metal centre remains a challenge. Herein, we report the use of low-coordinate iron(II) complexes for the selective scission and homologation of CO affording unusual squaraines and iron carboxylates at ambient temperature and pressure. A modification of the ligand framework allows for the isolation and structural characterisation of a proposed metallacyclic Fe(II) carbene intermediate. These results indicate that, with the appropriate choice of supporting ligands, it is possible to cleave and homologate carbon monoxide under mild conditions using an abundant and environmentally benign low-coordinate, first row transition metal.

Carbene in Cupredoxin Protein Scaffolds: Replacement of a Histidine Ligand in the Active Site Substantially Alters Copper Redox Properties.

N-heterocyclic carbene (NHC) ligands have had a major impact in homogeneous catalysis, however, their potential role in biological systems is essentially unexplored. We replaced a copper-coordinating histidine (His) in the active site of the redox enzyme azurin with exogenous dimethyl imidazolylidene. This NHC rapidly restores the type-1 Cu center, with spectroscopic properties (EPR, UV/Vis) that are identical to those from N-coordination of the His in the wild type. However, the introduction of the NHC markedly alters the redox potential of the metal, which is a key functionality of this blue copper protein. These results suggest that C-bonding for histidine is plausible and a potentially relevant bonding mode of redox-active metalloenzymes in their (transient) active states.

Heterobimetallic [NiFe] Complexes Containing Mixed CO/CN- Ligands: Analogs of the Active Site of the [NiFe] Hydrogenases.

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging, and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN- ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni( N2 S2)Fe(CO)2(CN)2], [Ni( S4)Fe(CO)2(CN)2], and [Ni( N2 S3)Fe(CO)2(CN)2] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)2(CN)2} unit. X-ray crystallographic studies on [Ni( N2 S3)Fe(CO)2(CN)2], supported by DFT calculations, are consistent with a solid-state structure containing distinct molecules in the singlet ( S = 0) and triplet ( S = 1) states. Each cluster exhibits irreversible reduction processes between -1.45 and -1.67 V vs Fc+/Fc and [Ni( N2 S3)Fe(CO)2(CN)2] possesses a reversible oxidation process at 0.17 V vs Fc+/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a NiIIIFeII formulation for [Ni( N2 S3)Fe(CO)2(CN)2]+. The singly occupied molecular orbital (SOMO) in [Ni( N2 S3)Fe(CO)2(CN)2]+ is based on Ni 3dz2 and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a NiIIIFeII formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni( N2 S3)Fe(CO)2(CN)2] and its [Ni( N2 S3)] precursor, together with calculations on the oxidized [Ni( N2 S3)Fe(CO)2(CN)2]+ and [Ni( N2 S3)]+ forms suggests that the binding of the {Fe(CO)(CN)2} unit to the {Ni(CysS)4} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.

Author Correction: The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes.

This corrects the article DOI: 10.1038/ncomms14137.

Thionated naphthalene diimides: tuneable chromophores for applications in photoactive dyads.

Varying the degree of thionation of a series of naphthalene diimide (NDI) and naphthalic imide (NI) phenothiazine dyad systems affords a systematic approach for tuning the system's donor-acceptor energy gap. Each dyad was compared to model NDI/NI systems and fully characterised through single crystal X-ray diffraction, NMR, cyclic voltammetry, electron paramagnetic resonance (EPR), transient absorption spectroscopy (TA), time-resolved infra-red spectroscopy (TRIR) and DFT. The measurements reveal that thionation increases both electron affinity of the NDI/NI acceptor dyad component and accessibility of the singly or doubly reduced states. Furthermore, FTIR and TA measurements show that excited state behaviour is greatly affected by thionation of the NDI and induces a decrease in the lifetime of the excited states formed upon the creation of charge-separated states.

The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes.

Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal-ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle.

Core-Substituted Naphthalene Diimides: Influence of Substituent Conformation on Strong Visible Absorption.

Functionalization of the aromatic core of naphthalene diimide (NDI) chromophores with morpholine substituents leads to molecules with strong absorbance in the visible spectrum. The shift of absorption maxima to lower energy is determined not only by the degree of substitution but also by the relative conformation and orientation of the tertiary amine with respect to the plane of the NDI.

Dynamic nuclear polarisation by thermal mixing: quantum theory and macroscopic simulations.

A theory of dynamic nuclear polarisation (DNP) by thermal mixing is suggested based on purely quantum considerations. A minimal 6-level microscopic model is developed to test the theory and link it to the well-known thermodynamic model. Optimal conditions for the nuclear polarization enhancement and effects of inhomogeneous broadening of the electron resonance are discussed. Macroscopic simulations of nuclear polarization spectra displaying good agreement with experiments, involving BDPA and trityl free radicals, are presented.

High-pressure studies of three polymorphs of a palladium(II) oxathioether macrocyclic complex.

The three reported phases of the mononuclear macrocyclic Pd(II) complex [PdCl2([9]aneS2O)] [(1); [9]aneS2O = 1-oxa-4,7-dithiacyclononane] were each studied up to pressures exceeding 9 GPa using high-pressure single-crystal X-ray diffraction. The α- and γ-phases both exhibit smooth compression of the unit-cell parameters with third-order Birch-Murnaghan bulk moduli of 14.4 (8) and 7.6 (6) GPa, respectively. Between 6.81 and 6.87 GPa ß-[PdCl2([9]aneS2O)] was found to undergo a reversible transition to a phase denoted as ß' and characterized by a tripling of the unit-cell volume. Across the phase transition, rearrangement of the conformation of the bound macrocycle at two of the resulting three unique sites gives rise to an extensively disordered structure.