η3-Allyl carbonyl complexes of group 6 metals: structural aspects, isomerism, dynamic behaviour and reactivity

Transition metal complexes with π-allylic ligands remain an attractive topic in organometallic chemistry, given the numerousreports of a wide variety of synthetic routes, dynamic behaviour and reactivity, structural (including isomerism),spectroscopic and redox properties, and applications in organic synthesis and catalysis. Surprisingly, despite the considerableinterest in the rich and varied chemistry of this family of organometallic compounds, there is no recent review. This review is focused on π-allylic representatives of low-cost Group-6 metals bearing one or more carbonyl ligand, the coordination sphere being complemented with η5-cyclopentadienyl (Section 2), chelating ligands, including redox-active α-diimines and various complementary diphosphines (Section 3), and novel anionic amidinate or pyrazolate ligands (Section 4). In Section 1, particular attention is paid to rearrangements of the π-allylic ligand, namely exo and endo isomerism, interconversion mechanisms, fluxionality, and agostic interactions. In addition, the application of multinuclear NMR spectroscopy to the elucidation of such isomerism, and the effect of the metal-centre oxidation state on the bonding, dynamic behaviour and reactivity of the π-allylic ligand are described. The detailed mechanistic description of the synthetic routes and dynamic behaviour of selected representatives of α-diimine complexes in Section 2 is followed by a description of the [M(CO)2(η3-allyl-H,R)(α-diimine)]0/+ fragment as a convenient scaffold for diverse monodentate ligands participating in a range of substitution, insertion, intramolecular migration and C–C coupling reactions – frequently involving also the π-allylic ligand, such as allylic alkylation. Special attention is devoted to selected examples of redox and acid-base reactivity of the α-diimine complexes with emphasis on prospects in electrocatalysis. The amidinate (and related pyrazolate) ligands treated in Section 4 may directly replace the π-allylic ligand in some cyclopentadienyl complexes (Section 2) or the α-diimine ligand in some dicarbonyl π-allylic complexes (Section 3). The brief description of their synthetic routes is complemented by intriguing examples of fluxionality and characteristic reactivity encountered for these unusual four-electron donor ligands

Solvent and ligand substitution effects on electrocatalytic reduction of CO2 with [Mo(CO)4(x,xʹ-dimethyl-2,2′-bipyridine)] (x = 4-6) enhanced at a gold cathodic surface

A series of molybdenum tetracarbonyl complexes with dimethyl‐substituted 2,2′‐bipyridine (dmbipy) ligands were investigated by cyclic voltammetry (CV) combined with infra‐red spectroelectrochemistry (IR‐SEC) in tetrahydrofuran (THF) and N‐methyl‐2‐pyrrolidone (NMP) to explore their potential in a reduced state to trigger electrocatalytic CO2 reduction to CO. Addressed is their ability to take advantage of a low‐energy, CO‐dissociation two‐electron ECE pathway available only at an Au cathode. A comparison is made with the reference complex bearing unsubstituted 2,2ʹ‐bipyridine (bipy). The methyl substitution in the 6,6ʹ‐positions has a large positive impact on the catalytic efficiency. This behaviour is ascribed to the advantageous positioning of the steric bulk of the methyl groups, which further facilitates CO dissociation from the 1e‐ reduced parent radical anion. In the contrary, the substitution in the 4,4′‐positions appears to have a negative impact on the catalytic performance, exerting a strong stabilizing effect on the π‐accepting CO ligands and, in THF, preventing exploitation of the low‐energy dissociative pathway

Elucidating the structure of chiral molecules by using amplified vibrational circular dichroism: from theory to experimental realization

Recent experimental observations of enhanced vibrational circular dichroism (VCD) in molecular systems with low-lying electronically excited states suggest interesting new applications of VCD spectroscopy. The theory describing VCD enhancement through vibronic coupling schemes was derived by Nafie in 1983, but only recently experimental evidence of VCD amplification has demonstrated the extent to which this effect can be exploited as a structure elucidation tool to probe local structure. In this Concept paper, we give an overview of the physics behind vibrational circular dichroism, in particular the equations governing the VCD amplification effect, and review the latest experimental developments with a prospective view on the application of amplified VCD to locally probe biomolecular structure

Anodic electrochemistry of mono- and dinuclear aminophenylferrocene and diphenylaminoferrocene complexes

Two related three-membered series of nonlinear aminophenylferrocene and diphenylaminoferrocene complexes were prepared and characterized by 1H and 13C NMR spectroscopy. The first series consists of 4-(diphenylamino)phenylferrocene (TPA-Fc, 1a), its dimethoxy-substituted tetraphenylphenylenediamine derivative (M2TPPD-Fc, 1c), and the triphenylamine-bridged bis(ferrocenyl) complex (Fc-TPA-Fc, 1b). The second series involves bis(4-methoxyphenyl)aminoferrocene (M2DPA-Fc, 1d), 4-methoxyphenylaminoferrocene (MPA-Fc) with N-phenyl-appended terminal TPA (1e), and the corresponding bis(MPA-Fc) complex with bridging TPA (1f). The structure of complex 1d was further confirmed by single crystal X-ray diffraction. Combined investigations, based on anodic voltammetry, UV-vis-NIR spectroelectrochemistry and density functional theory (DFT) calculations, were conducted to illustrate the influence of the integration of multiple redox-active components on the sequential oxidation of these complexes. The first anodic steps in 1a–1f are localized preferentially on the ferrocenyl units, followed by oxidation of the TPA or TPPD moieties (absent in 1d). Irreversible oxidation of the ferrocene-appended strong donor DPA/MPA units in 1d–1f terminates the anodic series. The one-electron oxidation of the triphenylamine-bridged diferrocenyl (1b) and bis(phenylaminoferrocenyl) (1f) complexes triggers their facile redox disproportionation to dicationic bis(ferrocenium) product

Biomimics of [FeFe]-hydrogenases with a pendant amine: Diphosphine complexes [Fe₂(CO)₄{μ-S(CH₂)nS}{κ²-(Ph₂PCH₂)₂NR}] (n = 2, 3; R = Me, Bn) towards H₂ oxidation catalysts

We report the synthesis and molecular structures of [FeFe]-ase biomimics [Fe2(CO)4{µ-S(CH2)nS}{κ2-(Ph2PCH2)2NR}] (1–4) (n = 2, 3; R = Me, Bn) and a comparative study of their protonation and redox chemistry, with the aim of assessing their activity as catalysts for H2 oxidation. They are prepared in good yields upon heating the hexacarbonyls and PCNCP ligands in toluene, a minor product of one reaction (n = 3, R = Bn) being pentacarbonyl [Fe2(CO)5(µ-pdt){Ph2PCH2N(H)Bn}] (5). Crystal structures show short Fe-Fe bonds (ca. 2.54 Å) with the diphosphine occupying basal-apical sites. Each undergoes a quasi-reversible one-electron oxidation and IR-SEC shows that this results in formation of a semi-bridging carbonyl. As has previously been observed, protonation products are solvent dependent, nitrogen being the favoured site of protonation site upon addition of one equivalent of HBF4.Et2O in d6-acetone, while hydride formation is favoured in CD2Cl2. However, the rate of N to Fe2 proton-transfer varies greatly with the nature of both the dithiolate-bridge and amine-substituent. Thus with NMe complexes (1–2) N-protonation is favoured in acetone affording a mixture of endo and exo isomers, while for NBn complexes (3–4) proton-transfer to afford the corresponding μ-hydride occurs in part (for 3 edt) or exclusively (for 4 pdt). In acetone, addition of a further equivalent of HBF4.Et2O generally does not lead to hydride formation, but in CD2Cl2 dications [Fe2(CO)4{µ-S(CH2)nS}(μ-H){κ2-(Ph2PCH2)2NHR}]2+ result, in which the diphosphine can adopt either dibasal or basal-apical positions. Proton-transfer from Fe2 to N has been previously identified as a required transformation for H2 oxidation, as has the accessibility of the all-terminal carbonyl isomer of cations [Fe2(CO)4{µ-S(CH2)nS}{κ2-(Ph2PCH2)2NR}]+. We have carried out a preliminary H2 oxidation study of 3, oxidation by Fc[BF4] in the presence of excess P(o-tolyl)3 affording [HP(o-tol)3][BF4], with a turnover of ca. 2.3 ± 0.1 mol of H2 consumed per mole of

Photo-assisted electrocatalytic reduction of CO2: A new strategy for reducing catalytic overpotentials

Electrochemical and photochemical reduction of CO2 are both well-established, independent catalytic routes toward producing added-value chemicals. The potential for any cross-reactivity has, however, hardly been explored so far. In this report, we assess a system primarily using spectroelectrochemical monitoring, where photochemistry assists the cathodic activation of precursor complexes [Mn(CO)3(2,2ʹ-bipyridine)Br] and [Mo(CO)4(6,6ʹ-dimethyl2,2ʹ-bipyridine)] to lower the catalytic overpotential needed to trigger the electrocatalytic reduction of CO2 to CO. Following the complete initial 1e‒ reduction of the parent complexes, the key photochemical cleavage of the Mn‒Mn and Mo‒CO bonds in the reduction products, [Mn(CO)3(2,2ʹ-bipyridine)]2 and [Mo(CO)4(6,6ʹ-dimethyl-2,2ʹbipyridine)]•‒, respectively, generates the 2e‒-reduced, 5-coordinate catalysts, [Mn(CO)3(2,2ʹ-bipyridine)]‒ and [Mo(CO)3(6,6ʹ-dimethyl2,2ʹ-bipyridine)]2‒ appreciably closer to the initial cathodic wave R1. Experiments under CO2 confirm the activity of both electrocatalysts under the photoirradiation with 405-nm and 365-nm light, respectively. This remarkable achievement corresponds to a ca. 500 mV positive shift of the catalytic onset compared to the exclusive standard electrocatalytic activation

Group 6 complexes as electrocatalysts of CO2 reduction: strong substituent control of the reduction path of [Mo(η3-allyl)(CO)2(x,x′- dimethyl-2,2′-bipyridine)(NCS)] (x = 4-6)

A series of complexes [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)(NCS)] (dmbipy = dimethyl-2,2ʹ-bipyridine; x = 4-6) have been synthesized and their electrochemical reduction investigated using combined cyclic voltammetry (CV) and variable-temperature spectroelectrochemistry (IR/UV-vis SEC) in tetrahydrofuran (THF) and butyronitrile (PrCN), at gold and platinum electrodes. The experimental results, strongly supported by DFT calculations, indicate that the general cathodic path of these Group-6 organometallic complexes is closely related to that of the intensively studied class of Mn tricarbonyl α-diimine complexes, themselves recently identified as important smart materials for catalytic CO2 reduction. The dimethyl substitution on the 2,2ʹ-bipyridine ligand backbone has presented new insights into this emerging class of catalysts. For the first time, the 2e‒ reduced 5-coordinate anions [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)]‒ were directly observed with IR SEC. The role of steric and electronic effects in determining the reduction-induced reactivity was also investigated. For the 6,6′-dmbipy, the primary 1e‒ reduced radical anions exert unusual stability radically changing the follow up cathodic path. The 5-coordinate anion [Mo(η3-allyl)(CO)2)(6,6′-dmbipy)]‒ remains stable at low temperature in strongly coordinating butyronitrile and does not undergo dimerization at elevated temperature, in sharp contrast to reactive [Mo(η3-allyl)(CO)2)(4,4′-dmbipy)]‒ that tends to dimerize in a reaction with the parent complex. The complex with the 5,5′-dmbipy ligand combines both types of reactivity. Under aprotic conditions, the different properties of [Mo(η3-allyl)(CO)2)(x,x′-dmbipy)]‒ are also reflected in their reactivity towards CO2. Preliminary CV and IR SEC results reveal differences in the strength of CO2 coordination at the free axial position. Catalytic waves attributed to the generation of the 5-coordinate anions were observed by CV, but only a modest catalytic performance towards the production of formate was demonstrated by IR SEC. For 6,6′-dmbipy, a stronger catalytic effect was observed for the Au cathode compared to Pt

Mitochondrial heat-shock protein hsp60 is essential for assembly of proteins imported into yeast mitochondria

A nuclear encoded mitochondrial heat-shock protein hsp60 is required for the assembly into oligomeric complexes of proteins imported into the mitochondrial matrix. hsp60 is a member of the 'chaperonin' class of protein factors, which include the Escherichia coli groEL protein and the Rubisco subunit-binding protein of chloroplast

A novel heteroditopic terpyridine-pincer ligand as building block for mono- and heterometallic Pd(II) and Ru(II) complexes

A palladium-catalyzed Stille coupling reaction was employed as a versatile method for the synthesis of a novel terpyridine-pincer (3, TPBr) bridging ligand, 4'-{4-BrC6H2(CH2NMe2)(2)-3,5}-2,2':6',2 ''-terpyridine. Mononuclear species [PdX(TP)] (X = Br, Cl), [Ru(TPBr)(tpy)](PF6)(2), and [Ru(TPBr)(2)](PF6)(2), synthesized by selective metalation of the NCNBr-pincer moiety or complexation of the terpyridine of the bifunctional ligand TPBr, were used as building blocks for the preparation of heterodi- and trimetallic complexes [Ru(TPPdCl)(tpy)](PF6)(2) (7) and [Ru(TPPdCl)(2)]-(PF6)(2) (8). The molecular structures in the solid state of [PdBr(TP)] (4a) and [Ru(TPBr)(2)](PF6)(2) (6) have been determined by single-crystal X-ray analysis. Electrochemical behavior and photophysical properties of the mono-and heterometallic complexes are described. All the above di- and trimetallic Ru complexes exhibit absorption bands attributable to (MLCT)-M-1 (Ru -> tpy) transitions. For the heteroleptic complexes, the transitions involving the unsubstituted tpy ligand are at a lower energy than the tpy moiety of the TPBr ligand. The absorption bands observed in the electronic spectra for TPBr and [PdCl(TP)] have been assigned with the aid of TD-DFT calculations. All complexes display weak emission both at room temperature and in a butyronitrile glass at 77 K. The considerable red shift of the emission maxima relative to the signal of the reference compound [Ru(tpy)(2)](2+) indicates stabilization of the luminescent (MLCT)-M-3 state. For the mono- and heterometallic complexes, electrochemical and spectroscopic studies (electronic absorption and emission spectra and luminescence lifetimes recorded at room temperature and 77 K in nitrile solvents), together with the information gained from IR spectroelectrochemical studies of the dimetallic complex [Ru(TPPdSCN)(tpy)](PF6)(2), are indicative of charge redistribution through the bridging ligand TPBr. The results are in line with a weak coupling between the {Ru(tpy)(2)} chromophoric unit and the (non)metalated NCN-pincer moiety

Formation of stimuli-responsive cyclophanes by self-assembly: the case of carbazole-based biradicals

Dynamic covalent bonds has recently received lot of attention because of their unique feature to become reversible under mild conditions.[1] In this context, π-conjugated biradical compounds has emerged as essential building blocks.[2] For instance, we have demonstrated that 2,7-dicyanomethylene-9-(2-ethylhexyl)carbazole biradical reversibly converts to a macrocycle cyclophane upon soft stimuli (temperature, pressure, light), showing strong chromic effects.[3] We now extent this study towards longer conjugated carbazole backbone (i.e., indolocarbazole shown in Figure 1), aiming at investigating how the elongation of the conjugated backbone impacts on the formation of stimuli-responsive cyclophanes. The self-assembly process is investigated both in solution and solid state by linking theory and experiments.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech