Coordination chemistry of CNH, the simplest isocyanide

Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)8 ]4+ (M=MoIV , WIV ) from Superacids

Octacyanometalates K₄[Mo(CN)₈] and K₄[W(CN)₈] are completely protonated in superacidic mixtures of anhydrous hydrogen fluoride and antimony pentafluoride. The resulting hydrogen isocyanide complexes [Mo(CNH)₈]⁴⁺ [SbF₆]⁻₄ and [W(CNH)₈]⁴⁺ [SbF₆]⁻₄ are the first examples of eight‐coordinate homoleptic metal complexes containing hydrogen isocyanide (CNH) ligands. The complexes were crystallographically characterized, revealing hydrogen‐bonded networks with short N⋅⋅⋅H⋅⋅⋅F contacts. Low‐temperature NMR measurements in HF confirmed rapid proton exchange even at −40 °C. Upon protonation, ν(C≡N) increases of about 50 cm⁻¹ which is in agreement with DFT calculations.

MnIII-FeIII Heterometallic Compounds within Hydrogen-Bonded Supramolecular Networks Promoted by an [Fe(CN)5(CNH)]2- Building Block: Structural and Magnetic Properties

The reaction of [Fe(CN)₆]^3- and [Mn(acacen)]⁺ (H₂acacen = N, N'-bis(acetylacetone)ethylenediamine) building units in the presence of supramolecular cations, [(F-Anil)(18-crown-6)]⁺ (F-Anil⁺ = 3-fluoroanilinium) or [(Me-F-Anil)(18-crown-6)]⁺ (Me-F-Anil⁺ = 3-fluoro-4-methylanilinium), affords two new bimetallic compounds, [(F-Anil)(18-crown-6)][Mn(acacen)Fe(CN)₅(CNH)]·MeOH (1) and [(Me-F-Anil)(18-crown-6)][Mn(acacen)(MeOH)Fe(CN)₅(CNH)]·MeOH (2), respectively. Compound 1 exhibits a one-dimensional topology, while compound 2 is a dinuclear discrete system due to the coordination of a MeOH molecule at the axial position of the [Mn(acacen)]^- unit. For both systems, the acidity of the corresponding supramolecular cation triggers the protonation of the Fe^III moiety as [Fe(CN)₅(CNH)]^2-. Moreover, the resulting -CNH ligand induces hydrogen bonding interactions connecting the chains for 1 or the molecules for 2 into higher dimensional supramolecular networks. Magnetic properties of compounds incorporating these [Fe(CN)₅(CNH)]^2- building blocks were, for the first time, thoroughly investigated, indicating a three-dimensional antiferromagnetic order of single-chain magnets for 1 and an antiferromagnetically interacting S = 3/2 spin ground state for 2.

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.

Acylated cyanoimido-complexes trans-[Mo(NCN){NCNC(O)R}(dppe)(2)]Cl and their reactions with electrophiles: chemical, electrochemical and theoretical study

Treatment of a dichloromethane solution of trans-[Mo(NCN){NCNC(O)R}(dppe)(2)]Cl [R = Me (1a), Et (1b)] (dppe = Ph(2)PCH(2)CH(2)PPh(2)) with HBF(4), [Et(3)O][BF(4)] or EtC(O)Cl gives trans-[Mo(NCN)Cl(dppe)(2)]X [X = BF(4) (2a) or Cl (2b)] and the corresponding acylcyanamides NCN(R')C(O)Et (R' = H, Et or C(O)Et). X-ray diffraction analysis of 2a (X = BF(4)) reveals a multiple-bond coordination of the cyanoimide ligand. Compounds 1 convert to the bis(cyanoimide) trans-[Mo(NCN)(2)(dppe)(2)] complex upon reaction with an excess of NaOMe (with formation of the respective ester). In an aprotic medium and at a Pt electrode, compounds 1 (R = Me, Et or Ph) undergo a cathodically induced isomerization. Full quantitative kinetic analysis of the voltammetric behaviour is presented and allows the determination of the first-order rate constants and the equilibrium constant of the trans to cis isomerization reaction. The mechanisms of electrophilic addition (protonation) to complexes 1 and the precursor trans-[Mo(NCN)(2)(dppe)(2)], as well as the electronic structures, nature of the coordination bonds and electrochemical behaviour of these species are investigated in detail by theoretical methods which indicate that the most probable sites of the proton attack are the oxygen atom of the acyl group and the terminal nitrogen atom, respectively.

Vibrational Stark Effects on Carbonyl, Nitrile, and Nitrosyl Compounds Including Heme Ligands, CO, CN, and NO, Studied with Density Functional Theory

Changes in the matrix electric field in a protein, due for example to mutations or structural fluctuations, can be correlated with changes in the vibrational transition frequencies of suitable chromophores measured by IR spectroscopy through the Stark tuning rate. To make this correlation, the Stark tuning rate must be known from experiment or theory. In this paper, density functional theory at the B3LYP/TZV level of theory is used to compute the Stark tuning rate of adducts of heme porphyrin, namely, -CO, -CN, and -NO+ compounds. The results are compared with the corresponding vibrational frequencies-field dependencies from vibrational Stark spectroscopy of heme-proteins. The zero-field computed Stark tuning rate of 1.3 cm-1/MV/cm for heme-CO is in agreement with experiment, where typically the rate is 2.4/f cm-1/MV/cm for myoglobin, where f is a local field correction between 1.1 and 1.4. Several small nitrile, carbonyl, and dinitrile molecules were studied to rationalize the findings for the heme-adducted models. Here, the higher B3LYP/6-311++G(2d,2p) level of theory could be used so the agreement with recent experimental results is even better than for heme-adducted groups.

Mixed dinitrogen-organocyanamide complexes of molybdenum(0) and their protic conversion into hydrazide and amidoazavinylidene derivatives

Organocyanamides, Ntbd1;CNR(2) (R = Me or Et), react with trans-[Mo(N(2))(2)(dppe)(2)] (1, dppe = Ph(2)PCH(2)CH(2)PPh(2)), in THF, to give the first mixed molybdenum dinitrogen-cyanamide complexes trans-[Mo(N(2))(NCNR(2))(dppe)(2)] (R = Me 2a or Et 2b) which are selectively protonated at N(2) by HBF(4) to yield the hydrazide(2-) complexes trans-[Mo(NNH(2))(NCNR(2))(dppe)(2)][BF(4)](2) (R = Me, 3a, or Et, 3b). On treatment with Ag[BF(4)], oxidation and metal fluorination occur, and the ligating cyanamide undergoes an unprecedented beta-protonation at the unsaturated C atom to form trans-[MoF(NCHNR(2))(dppe)(2)][BF(4)](2) (R = Me, 4a, or Et, 4b) compounds which present the novel amidoazavinylidene (or amidomethyleneamide) ligands. Complexes 4 are also formed from the corresponding compounds 3, with liberation of ammonia and hydrazine. The crystal structure of 2b was determined by single-crystal X-ray diffraction analysis which indicates that the N atom of the amide group has a trigonal planar geometry.

Electrochemical Behaviour of trans-[FeH(CN)(dppe)2] Adducts

Electrochemical behaviour of various metallo cyano adducts of trans-[FeH(CN)(dppe)2], viz. the dinuclear complexes [FeH(dppe)2(μ-CN)PdCl2(PPh3)], [FeH(dppe)2(μ-CN)NiCl2(PCy3)], and trinuclear [{FeH(dppe)2(μ-CN)}2(ReOCl3)], [{FeH(dppe)2(μ-CN)}2PtCl(Ph)] and [{FeX(dppe)2(μ-CN)}2WCl3(OH)] (X = Cl or OH, dppe = 1,2-diphenylphosphinoethane, PCy3 = P(C6H11)3), as well as the benzoylisocyanide mononuclear adduct trans-[FeH(CNCOPh)- (dppe)2], is reported. All of them exhibit FeII/FeIII-based oxidations (which are reversible, except for trans-[FeH(CNCOPh)(dppe)2]). The metallocyanide bridges C≡N-Re-N≡C and C≡N-W-N≡C in [{FeH(dppe)2(μ-CN)}2(ReOCl3)] and [{FeX(dppe)2(μ-CN)}2WCl3(OH)], respectively, allow electronic communication between the iron centres, with possible generation of mixed-valence FeII/FeIII complexes whose comproportionation constant could be estimated in the former case. From the values of the measured oxidation potentials, the electrochemical PL and EL ligand parameters have been estimated for the metallocyanide ligands that were shown to behave as stronger net electron donors than organoisocyanides, although weaker than cyanide itself. Ligand-centred reduction processes were also observed to lead, in the cases of complexes trans-[FeH(CNCOPh)(dppe)2] (CNCOPh-based reduction) and [FeH(dppe)2(μ-CN)PdCl2(PPh3)] (PdII-based reduction in the CNPdCl2(PPh3)- metallocyanide ligand), to the dissociation of the adduct, with regeneration of the parent cyano complex trans-[FeH(CN)(dppe)2], thus reflecting the reductive decrease of the electrophilic (or Lewis acidic) character of the benzoyl and {PdCl2(PPh3)} groups.