Trapping an unprecedented octacoordinated iron(II) complex with neutral bis-tetrazolylpyridyl ligands and solvent molecules.

Iron(II) can show a very rich coordination chemistry with concomitant modulation of its properties as promising functional materials. Metalation of the neutral tridentate nitrogen-donor mer-coordinating ligand 2,6-bis(2-(methyl)-2H-tetrazol-5-yl)pyridine (Me2btp) with Fe(ClO4)2·6H2O through accurate solvent polarity control enables the selective crystallization of [FeHS/LS(Me2btp)2](ClO4)2·MeCN·2.75H2O (2HS/LS·MeCN·2.75H2O) as red rods, where half of the iron(II) centres resides in the low spin (LS, S = 0) state and the other half is in the high spin (HS, S = 2) state. The red rods spontaneously convert into yellow crystals once removed from the mother liquor and exposed to air due to solvent rearrangement within the crystal packing; these new crystals can be assigned to [FeHS(Me2btp)2](ClO4)2·solvent (2HS·solvent) where all the iron(II) centres are now blocked in the HS state, as confirmed by magnetic measurements. The polarity of the crystallization solvent, together with the maintenance of the crystals within the mother liquor, are pivotal for the reactivity and interconversion of different species. Indeed, upon long standing in solution, 2HS/LS·MeCN·2.75H2O converts to another form of red crystals belonging to [FeLS(Me2btp)2][FeHS(Me2btp)(MeCN)2(H2O)](ClO4)4·MeCN (2LS·3HS·MeCN), as confirmed by single crystal X-ray diffraction data. In this co-crystal, the iron(II) in 2 resides in the LS state at all temperatures while the iron(II) in 3 is blocked in the HS state. Well-formed yellow crystals could be also isolated among the red crystals of 2HS/LS·MeCN·2.75H2O, and they could be identified as the unprecedented octacoordinated species [Fe(Me2btp)2(MeCN)(H2O)](ClO4)2·H2O (1·H2O) by single-crystal X-ray diffraction. These yellow crystals are stable in the air, but slowly convert into 2LS·3HS·MeCN if kept in the mother liquor for about one week. 1·H2O can be considered the trapped intermediate in the solid state during the conversion of [FeHS(Me2btp)2]2+ into [FeHS(Me2btp)(MeCN)2(H2O)]2+ in solution, where the two tridentate ligands in the starting species can unfold to accommodate coordinated MeCN and H2O molecules, as confirmed by theoretical calculations, and eventually one of the two Me2btp is completely replaced by the solvent.

Molecular Ruthenium Cyclopentadienone Bifunctional Catalysts for the Conversion of Sugar Platforms to Hydrogen.

Molecular ruthenium cyclopentadienone complexes were employed for the first time as pre-catalysts in the homogeneously catalysed Aqueous Phase Reforming (APR) of glucose. Shvo's complex resulted the best pre-catalyst (loading 2 mol %) with H2 yields up to 28.9 % at 150 °C. Studies of the final mixture allowed to identify the catalyst's resting state as a mononuclear dicarbonyl complex in the extracted organic fraction. In situ NMR experiments and HPLC analyses on the aqueous fraction gave awareness of the presence of sorbitol, fructose, 5-hydroxymethylfurfural and furfural as final fate or intermediates in the transformations under APR conditions. These results were summarized in a proposed mechanism, with particular emphasis on the steps where hydrogen was obtained as the product. Benzoquinone positively affected the catalyst activation when employed as an equimolar additive.

A proficient multivariate approach for iron(II) spin crossover behaviour modelling in the solid state.

Iron(II) bis-pyrazolilpyridyl (bpp-R) complexes [Fe(bpp-R)2](X)2·solvent, R = substituent and X- = anion, can undergo a spin transition from high (S = 2, HS) to low spin (S = 0, LS), being spin crossover (SCO) in the solid state. The distortion of the octahedral coordination environment around the metal centre is governed by crystal packing, i.e. the intermolecular interactions among the substituent R of the bpp-R ligands, the anion X-, and the co-crystallized solvent, and this modulates the SCO behaviour. In this work, an innovative multivariate approach, through the combination of the chemometric tools Principal Component Analysis and Partial Least Squares regression, was applied on the coordination bond distances and angles and selected torsional angles of the available HS structures. The obtained results can efficiently model and rationalize the structural data distinguishing between SCO-active and HS-blocked complexes bearing different R groups, X- anions, and co-crystallized solvents and help predict the spin transition temperature T1/2.

Advances in Catalytic Routes for the Homogeneous Green Conversion of the Bio-Based Platform 5-Hydroxymethylfurfural.

5-Hydroxymethylfufural (HMF) is an intriguing platform molecule that can be obtained from biomasses and that can lead to the production of a wide range of products, intermediates, or monomers. The presence of different moieties in HMF (hydroxy, aldehyde, furan ring) allows to carry out different transformations such as selective oxidations and hydrogenations, reductive aminations, etherifications, decarbonylations, and acetalizations. This is a great chance in a biorefinery perspective but requires the development of active and highly selective catalysts. In this view, homogeneous catalysis can lead to efficient conversion of HMF at mild reaction conditions. This Review discussed the recent achievements in homogeneous catalysts development and application to HMF transformations. The effects of metal nature, ligands, solvents, and reaction conditions were reported and critically reviewed. Current issues and future chances have been presented to drive future studies toward more efficient and scalable processes.

Bringing Homogeneous Iron Catalysts on the Heterogeneous Side: Solutions for Immobilization.

Noble metal catalysts currently dominate the landscape of chemical synthesis, but cheaper and less toxic derivatives are recently emerging as more sustainable solutions. Iron is among the possible alternative metals due to its biocompatibility and exceptional versatility. Nowadays, iron catalysts work essentially in homogeneous conditions, while heterogeneous catalysts would be better performing and more desirable systems for a broad industrial application. In this review, approaches for heterogenization of iron catalysts reported in the literature within the last two decades are summarized, and utility and critical points are discussed. The immobilization on silica of bis(arylimine)pyridyl iron complexes, good catalysts in the polymerization of olefins, is the first useful heterogeneous strategy described. Microporous molecular sieves also proved to be good iron catalyst carriers, able to provide confined geometries where olefin polymerization can occur. Same immobilizing supports (e.g., MCM-41 and MCM-48) are suitable for anchoring iron-based catalysts for styrene, cyclohexene and cyclohexane oxidation. Another excellent example is the anchoring to a Merrifield resin of an FeII-anthranilic acid complex, active in the catalytic reaction of urea with alcohols and amines for the synthesis of carbamates and N-substituted ureas, respectively. A SILP (Supported Ionic Liquid Phase) catalytic system has been successfully employed for the heterogenization of a chemoselective iron catalyst active in aldehyde hydrogenation. Finally, FeIII ions supported on polyvinylpyridine grafted chitosan made a useful heterogeneous catalytic system for C-H bond activation.

Application of the SMALP technology to the isolation of GPCRs from low-yielding cell lines.

The ability of styrene-maleic acid (SMAc) co-polymers to spontaneously insert into biological membranes can be exploited to extract G protein-coupled receptors (GPCRs) embedded in styrene-maleic acid lipid particles (SMALPs), preserving the native environment around the protein and thus enhancing the feasibility of functional studies. So far, the SMALP technology has been primarily employed on non-mammalian cells and protocols are not optimized for adherent human cell lines, which cannot be harvested in large amounts. In this work, a fine investigation of key parameters affecting the formation of SMALPs was undertaken with the purpose of maximizing the yield of extraction of a recombinant form of human ß2-adrenergic receptor (rhß2AR) from HEK293T cells. The study highlighted an important influence of ionic strength on the membrane solubilization efficiency and GPCR purification yield of SMAc co-polymers: by lowering the salt concentration of all buffers used in previously published SMALP protocols, the water solubility and extraction efficiency of the selected SMAc co-polymer (commercially supplied as a potassium salt) were enhanced. In-line combination of size-exclusion chromatography (SEC) with immobilized metal affinity chromatography (IMAC) allowed further improvement of the final rhß2AR yield by reducing the loss of SMALP-embedded GPCRs during the fractionation and purification of SMALPs. The overall findings of this study show that the available SMALP protocols can be significantly optimized in several aspects in order to increase the efficiency of GPCR solubilization and isolation from low-yielding expression systems.

Thermal Growth of Au-Fe Heterometallic Carbonyl Clusters Containing N-Heterocyclic Carbene and Phosphine Ligands.

The thermal reactions of [NEt4][Fe(CO)4(AuNHC)] [NHC = IMes ([NEt4][1]) or IPr ([NEt4][2]); IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3iPr2)2], Fe(CO)4(AuNHC)2 [NHC = IMes (3) or IPr (4)], Fe(CO)4(AuIMes)(AuIPr) (5), and Fe(CO)4(AuNHC)(AuPPh3) [NHC = IMes (6) or IPr (7)] were investigated in different solvents [CH2Cl2, CH3CN, dimethylformamide, and dimethyl sulfoxide (dmso)] and at different temperatures (50-160 °C) in an attempt to obtain higher-nuclearity clusters. 1 and 2 completely decomposed in refluxing CH2Cl2, resulting in [Fe2(CO)8(AuNHC)]- [NHC = IMes (10) or IPr (11)]. Traces of [Fe3(CO)10(CCH3)]- (12) were obtained as a side product. Conversely, 6 decomposed in refluxing CH3CN, affording the new cluster [Au3{Fe(CO)4}2(PPh3)2]- (15). The relative stability of the two isomers found in the solid state structure of 15 was computationally investigated. 4 was very stable, and only after prolonged heating above 150 °C in dmso was limited decomposition observed, affording small amounts of [Fe3S(CO)9]2- (9), [HFe(CO)4]- (16), and [Au16S{Fe(CO)4}4(IPr)4]n+ (17). A dicationic nature for 17 was proposed on the basis of density functional theory calculations. All of the other reactions examined led to species that were previously reported. The molecular structures of the new clusters 11, 12, 15, and 17 were determined by single-crystal X-ray diffraction as their [NEt4][11]·1.5toluene, [Au(IMes)2][15]·0.67CH2Cl2, [NEt4][12], and [17][BF4]n·solvent salts, respectively.

Polymerization Isomerism in [{MFe(CO)4} n] n- (M = Cu, Ag, Au; n = 3, 4) Molecular Clusters Supported by Metallophilic Interactions.

Triangular clusters [{MFe(CO)4}3]3- (M = Cu, 4; Ag, 5; Au, 6) were selectively obtained by heating Fe(CO)4(MIMes)2 (M = Cu, 1; Ag, 2; Au, 3; IMes = C3N2H2(C6H2Me3)2). 1-3 were synthesized by reacting Na2[Fe(CO)4]·2thf with 2 equiv of M(IMes)Cl. As previously described, the direct reactions of Na2[Fe(CO)4]·2thf with one equivalent of M(I) salts resulted in the triangular cluster [{CuFe(CO)4}3]3- for Cu, whereas the square clusters [{MFe(CO)4}4]4- were formed for Ag and Au. Thus, depending on the synthetic protocol adopted, both the triangular [{MFe(CO)4}3]3- and square [{MFe(CO)4}4]4- polymerization isomers can be selectively obtained, at least for Ag and Au. Polymerization isomerism, that is two compounds having the same elemental compositions but different molecular weights, was investigated in [{MFe(CO)4} n] n- ( n = 3, 4; M = Cu, Ag, Au) by means of structural and theoretical methods and the role of metallophilic interactions was computationally studied by means of the atoms-in-molecules (AIM) approach.

Click-Derived Triazolylidenes as Chelating Ligands: Achievement of a Neutral and Luminescent Iridium(III)-Triazolide Complex.

Versatility in the synthesis of triazole derivatives was exploited to obtain convenient mesoionic carbenes working as chelating or cyclometalating ligands for the preparation of cationic or neutral iridium(III) complexes. We present the synthesis and characterization of three new cationic cyclometalating iridium(III) complexes (1-3-BF4) and a neutral one (4), equipped with functionalized triazolylidene ligands. All the complexes are obtained in good yields, present irreversible or quasi-reversible oxidation and reduction processes, and display good photophysical stability. The complexes emit from 3MLCT or 3LC states, depending on the nature of the ancillary ligand. Compounds 1-3-BF4 display very low photoluminescence quantum yields (PLQY ≈ 1% in acetonitrile solution). Density functional theory calculations show that the luminescence of these three complexes is quenched by the presence of low-lying 3MC states, leading to a reversible detachment of the neutral ancillary ligands from the metal coordination sphere. On the contrary, this nonradiative deactivation pathway is not present in the case of the neutral complex 4, which in fact shows PLQYs above 10% and is the best emitter of the series. Moreover, complex 4 represents the first reported example of a photochemically and thermally stable neutral triazolide iridium(III) complex.

Mechanistic Insight into Electrocatalytic H2 Production by [Fe2(CN){µ-CN(Me)2}(µ-CO)(CO)(Cp)2]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models.

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe2(CN){µ-CN(Me)2}(µ-CO)(CO)(Cp)2] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN- and amino-carbyne ligands have been elucidated. After the first reduction, CN- works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN- that forms a Fe(CNH) moiety: i.e., the acidic source for the H2 generation. The hydride (formally 2e/H+), necessary to the heterocoupling with H+ is thus provided by the µ-CN(Me)2 ligand and not by Fe centers, as occurs in typical L6Fe2S2 derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN- plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN- to CN(Me)2 is highlighted. The incompetence for the HER of the related species [Fe2{µ-CN(Me)2}(µ-CO)(CO)2(Cp)2]+ (2+) has been investigated and attributed to the loss of proton responsiveness caused by CN- replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe2 core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe2 scaffold, could be exploited to develop more active and robust catalysts for the HER.

Negatively charged Ir(iii) cyclometalated complexes containing a chelating bis-tetrazolato ligand: synthesis, photophysics and the study of reactivity with electrophiles.

The bis-tetrazolate dianion [1,2 BTB](2-), which is the deprotonated form of 1,2 bis-(1H-tetrazol-5-yl)benzene [1,2-H2BTB], is for the first time exploited as an ancillary N^N ligand for negatively charged [Ir(C^N)2(N^N)](-)-type complexes, where C^N is represented by cyclometalated 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)pyridine (F2ppy). The new Ir(iii) complexes [Ir(ppy)2(1,2 BTB)]- and [Ir(F2ppy)2(1,2 BTB)]- have been fully characterised and the analysis of the X-ray structure of [Ir(ppy)2(1,2 BTB)]- confirmed the coordination of the [1,2 BTB](2-) dianion in a bis chelated fashion through the N-atoms adjacent to each of the tetrazolic carbons. Both of the new anionic Ir(iii) complexes displayed phosphorescence in the visible region, with intense sky-blue (λmax = 460-490 nm) or aqua (λmax = 490-520 nm) emissions originating from [Ir(F2ppy)2(1,2 BTB)]- and [Ir(ppy)2(1,2 BTB)]-, respectively. In comparison with our very recent examples of anionic Ir(iii)tetrazolate cyclometalates, the new Ir(iii) tris chelate complexes [Ir(F2ppy)2(1,2 BTB)]- and [Ir(ppy)2(1,2 BTB)]-, display an improved robustness, allowing the study of their reactivity toward the addition of electrophiles such as H(+) and CH3(+). In all cases, the electrophilic attacks occurred at the coordinated tetrazolate rings, involving the reversible - by a protonation deprotonation mechanism - or permanent - upon addition of a methyl moiety - switching of their global net charge from negative to positive and, in particular, the concomitant variation of their photoluminescence output. The combination of the anionic complexes [Ir(F2ppy)2(1,2 BTB)]- or [Ir(ppy)2(1,2 BTB)]- with a deep red emitting (λmax = 686 nm) cationic Ir(iii) tetrazole complex such as [IrTPYZ-Me]+, where TPYZ-Me is 2-(2-methyl-2H-tetrazol-5-yl)pyrazine, gave rise to two fully Ir(iii)-based soft salts capable of displaying additive and O2-sensitive emission colours, with an almost pure white light obtained by the appropriate choice of the ionic components.

Straightforward synthesis of iron cyclopentadienone N-heterocyclic carbene complexes.

Novel iron complexes bearing both cyclopentadienone and N-heterocyclic carbene ancillary ligands were obtained by a straightforward synthesis from Fe2(CO)9. The preparation represents a rare example of silver transmetallation involving iron. The reaction is general and occurs in the presence of variously functionalized NHC and cyclopentadienones.

Substrate and product role in the Shvo's catalyzed selective hydrogenation of the platform bio-based chemical 5-hydroxymethylfurfural.

The bio-based substrate and target product 2,5-bishydroxymethylfuran (BHMF) demonstrated to influence the reaction kinetics in the homogeneous reduction of 5-hydroxymethylfurfural (HMF) catalyzed by the Ru-based Shvo's catalyst. A combined experimental and computational study supports an important role of the -CH2OH moiety which may be involved in the catalytic cycle toward the formation of different intermediates from HMF and BHMF. The reaction is selective and leads to quantitative formation of BHMF working under mild conditions. Furthermore, an optimized recycling procedure which avoids the use of water, allows recover and reuse of the catalyst without loss of activity. The mechanistic insights from this work may be extended to provide a general description of the chemistry of the Shvo's catalyst feeding further bio-based molecules.

C-C bond formation in diiron complexes.

The growing effort to design new sustainable synthetic methodologies, based on readily available and environmentally friendly transition metals, has boosted research on iron complexes. This review article focuses on C-C-bond-forming reactions occurring at bridging ligands in diiron complexes, aimed at evidencing distinctive aspects and advantages associated with the presence of two adjacent iron centres. A number of diiron-mediated C-C-bond-forming reactions reported in the literature, including nucleophilic and electrophilic additions and insertion and cycloaddition reactions, have been accumulated over the years, which, together with more recent developments, indicate that diiron complexes might provide promising alternatives to precious metals in the challenging field of metal-promoted C-C bond formation.

[3+2+1] cycloaddition involving alkynes, CO and bridging vinyliminium ligands in diiron complexes: a dinuclear version of the Dötz reaction?

The vinyliminium complex [Fe2{mu-eta(1):eta(3)-C(SiMe3)=CHC=N(Me)2}(mu-CO)(CO)(Cp)2][SO3CF3] reacts with HC[triple bond]CR (R = CPh2OH), affording a mixture of the 2,4,6-trisubstituted oxo-eta(5)-cyclohexadienyl complex [Fe{eta(5)-C6H2O(NMe2)(SiMe3)(R)}(Cp)], the 2,6-disubstituted phenol C6H3R(NMe2)OH (R = CHPh2) and 1,2,4-trisubstituted ferrocene [1-NMe2-2-R-4-SiMe3-Fc]. The corresponding reaction with HC[triple bond]CR (R = CMe2OH) yields analogous products: [Fe{eta(5)-C6H2O(NMe2)(SiMe3)(R')}(Cp)] (R' = CMe=CH2), the phenol C6H3R'(NMe2)OH together with [1-NMe2-2-R-4-SiMe3-Fc].

Enzyme electrodes based on sono-gel containing ferrocenyl compounds.

An amperometric-mediated glucose sensor has been developed by employing a silica sono-gel carbon composite electrode (SCC). The chosen mediators, ferrocene (Fc) and 1,2-diferrocenylethane (1), have been immobilized in the sono-gel composite matrix. The complex 1 has been employed for the first time as an electron transfer mediator for signal transduction from the active centre of the enzyme to the electrode conductive surface. After the optimisation of the construction procedure the best operative conditions for the analytical performance of the biosensor have been investigated in terms of pH, temperature and applied potential. Cyclic voltammetric and amperometric measurements have been used to study the response of both the glucose sensors, which exhibit a fast response and good reproducibility. The sensitivity to glucose is quite similar (6.7+/-0.1 microA/mM versus 5.3+/-0.1 microA/mM) when either Fc or 1 are used as mediators as are the detection limit ca. 1.0 mM (S/N=3) and the range of linear response (up to 13.0 mM). However, the dynamic range for glucose determination results wider when using 1 (up to 25.0 mM). The apparent Michaelis-Menten constants, calculated from the reciprocal plot under steady state conditions, are 27.7 and 31.6 mM for SCC-Fc/GOx and SCC-1/GOx electrodes, respectively, in agreement with a slightly higher electrocatalytic efficiency for the mediator 1.

Synthesis of new poly(propylenimine) dendrimers DAB-dendr-[NH(O)COCH2CH2OC(O)C5H4Rh(NBD)](n) [n=4, 8, 16, 32, 64] functionalized with alkoxycarbonylcyclopentadienyl complexes of rhodium(I).

The reaction between [Rh[C5H4CO2(CH)2OH](NBD)] (1) and 1,1'-carbonyldiimidazole (CDI) leads to the new CO2-imidazole functionalized alkoxycarbonylcyclopentadienyl complex [Rh[C5H4CO2(CH2)2O2C-Im](NBD)] (2) (Im=imidazole). The latter was treated with five generations of poly(propylenimine) dendrimers DAB-dendr-(NH2)(n) [n=4, 8, 16, 32, 64] (DAB=diaminobutane) to accomplish the synthesis of the new organometallic dendritic macromolecules DAB-dendr-[NH(O)COCH2CH2OC(O)C5H4Rh(NBD)](n) [n=4 (4), 8 (5), 16 (6), 32 (7), 64 (8)] based on flexible poly(propylenimine) dendrimer cores, built up to the fifth generation. Spectroscopic characterization of all the new compounds will be presented and discussed.