Isocyanide Ligands Research Articles

Synthesis, Characterization, and Photophysical and Emission Solvatochromic Study of Rhenium(I) Tetra(isocyano) Diimine Complexes

A new series of rhenium(I) tetra(isocyano) diimine complexes ([Re(CNR)4(N–N)]+, where R = 4-IC6H4, 4-BrC6H4, 4-Br-2,6-Me2C6H2, 2,4,6-Cl3C6H2, 2,4,6-Br3C6H2, 2,4-Cl2-6-MeOC6H2 and N–N = bpy, (MeO)2bpy, tBu2bpy, phen, 1-methyl-2-(2′-pyridyl)imidazole) with various isocyanide and diimine ligands of diverse electronic and steric natures has been synthesized and characterized. The X-ray crystal structures of four of the target complexes were also determined. These complexes were found to show orange to red MLCT (dπ(Re) → π*(N–N)) phosphorescence. Given the submicrosecond solution emission lifetime, nanosecond transient absorption spectroscopic studies have been performed. The emission solvatochromic behavior and the electrochemical properties of these complexes and their structural correlation have also been reported.

The synthesis and cyclic voltammetry of select ferrocene piano-stool isocyanide complexes

Ferrocene piano-stool isocyanide complexes ([CpFeL3]+, Cp=η5-C5H5, L=tert-butyl isocyanide (1), cyclohexyl isocyanide (2), and 2,6-dimethylphenyl isocyanide (3)) are formed by chemical oxidation of ferrocene in the presence of a stoichiometric amount of isocyanide ligand (1:3). The complexes are characterized by elemental analysis, routine spectroscopic methods (IR, 1H NMR, 13C NMR, and UV–Vis), and cyclic voltammetry.

Reactivity of Dinuclear Copper(I)/pybox Complexes towards Isocyanides and Phosphanes

AbstractThe dinuclear complexes [Cu2(R‐pybox)2][PF6]2 {R‐pybox = 2,6‐bis[4′‐(S)‐isopropyloxazolin‐2′‐yl]pyridine for (S,S)‐iPr‐pybox (1) and 2,6‐bis[4′‐(R)‐phenyloxazolin‐2′‐yl]pyridine for (R,R)‐Ph‐pybox (2)} are very efficient precursors for the synthesis of dinuclear and mononuclear derivatives containing isocyanide or phosphane ligands. The reaction of 1 and 2 with the isocyanides (CNBn or CNCy) resulted in the synthesis of the dinuclear derivatives [Cu2(R‐pybox)2(CNBn)2][PF6]2 [R‐pybox = (S,S)‐iPr‐pybox (3), (R,R)‐Ph‐pybox (4)] and [Cu2(R‐pybox)2(CNCy)2][PF6]2 [R‐pybox = (S,S)‐iPr‐pybox (5), (R,R)‐Ph‐pybox (6)]. When the reaction was carried out with the monodentate (PMePh2, PPh3) and the bidentate phosphanes [dppm, dppf, (S)‐peap] the mononuclear complexes [Cu(R‐pybox)(PRPh2)2][PF6] [R‐pybox = (S,S)‐iPr‐pybox, R = Me(7), Ph (8); R‐pybox = (R,R)‐Ph‐pybox, R = Ph (9)], [Cu(R‐pybox){(S)‐peap}][PF6] [R‐pybox = (S,S)‐iPr‐pybox (11), (R,R)‐Ph‐pybox (12)], and [Cu(R‐pybox)(dppf)][PF6] [R‐pybox = (S,S)‐iPr‐pybox (13), (R,R)‐Ph‐pybox (14)] and the dinuclear complex [Cu2{(S,S)‐iPr‐pybox}(μ‐dppm)2][PF6]2 (10) were obtained, respectively. The structures of complexes 3, 7, 10, and 13 have been resolved by X‐ray diffraction methods and by 1D, 2D, and DOSY NMR spectroscopy measurements. The 1H, 31P, and 19F DOSY NMR spectroscopic experiments provided evidence that the solid‐state nuclearity of compounds 3, 7, 10, and 13 is maintained in solution and confirmed that these ionic compounds exist in solution as stable, discrete, cationic complexes.

‘2+1’ Tricarbonyltechnetium(I)/tricarbonylrhenium(I) mixed-ligand complexes with methyl thiosalicylate and isocyanide ligands as potential precursors of radiopharmaceuticals

The complexes of [(99m)Tc]-tricarbonyltechnetium(I) and [(188)Re]-tricarbonylrhenium(I), of the '2+1' type, with methyl thiosalicylate as an anionic bidentate ligand and with either tert-butyl 3-isocyanopropionate or glycine-glycine-tyrosine 4-isocyanobutyrate as monodentate ligands, have been prepared on the n.c.a. scale. The complexes exhibit different lipophilicity and high stability in neutral aqueous solutions. Based on the results of the challenge experiment with histidine, cysteine and glutathione the studied complexes might be considered to be promising models for radiopharmaceutical precursors.

Synthesis of a New Family of Heteronuclear Thiolate Iron Complexes that Contain Isocyanide Ligands

AbstractTreatment of complex [Cp*Fe(μ‐SEt)3FeCp*] (1; Cp* = pentamethylcyclopentadienyl) with isocyanides CNR affords the mononuclear complexes [Cp*FeSEt(CNR)2] (2a, R = tBu; 2b, R = Ph; 2c, R = Cy; 2d, R = Bn) in moderate yield. The heterobinuclear complexes [Cp*FeCNR(μ‐CNR)(μ‐SEt)PdCl(PPh3)][PF6] (3a, R = tBu; 3b, R = Ph; 3c, R = Cy; 3d, R = Bn), [Cp*Fe(CNtBu)2(μ‐SEt)NiCl(PPh3)][PF6] (4), and heterotrinuclear complexes [{Cp*Fe(CNtBu)2(μ‐SEt)}2M][PF6] (5, M = Au; 6, M = Ag; 7, M = Cu) have been prepared by the reaction of 2 with complexes [PdCl2(PPh3)2], [NiCl2(PPh3)2], and [(Ph3P)M][X] (X = PF6 or OTf, OTf = OSO2CF3), respectively. These complexes have been spectroscopically and crystallographically characterized.

Photochemical Synthesis of Intensely Luminescent Isocyano Rhenium(I) Complexes with Readily Tunable Structural Features

A new class of readily tunable isocyano rhenium(I) diimine luminophores, cis,cis-[Re(CO)(2)(CNR)(2)(N-N)](+) (R=2,4,6-Cl(3)C(6)H(2), 4-ClC(6)H(4), 4-Br-2,6-(CH(3))(2)C(6)H(2), 2,6-(CH(3))(2)C(6)H(3), 4-[(CH(3))(2)N]C(6)H(4), 4-(C(6)H(5))C(6)H(4), 4-nBuC(6)H(4), tBu), has been synthesized in high yield by a highly selective photochemical substitution reaction. These complexes were characterized by (1)H NMR and IR spectroscopy, mass spectrometry, and elemental analysis. The X-ray crystal structures of one of the complexes and one of the precursor complexes for the photosubstitution reaction were also determined. As the isocyanide ligands are readily tunable, complexes with excellent solubility in benzene or other nonpolar solvents could be designed through slight modification of the isocyanide ligands with a short nBu substituent. With the characteristic strong infrared absorptions of the carbonyl (C≡O) and isocyanide (C≡N) stretches as well as the high solubility of the reactant and product in benzene, which is the solvent for the photoreaction, the photosubstitution reaction of [Re(CO)(3)(nBuC(6)H(4)NC)(2)Br] with 4,4'-di-tert-butyl-2,2'-bipyridine was also studied by in situ IR spectroscopy. The photophysical and electrochemical properties of these complexes were also investigated. Except for the complex with [(CH(3))(2)N]C(6)H(4)NC ligands, all complexes displayed intense luminescence with quantum yields of up to 0.37 in degassed CH(2)Cl(2) solution at room temperature. These emissions were assigned as the phosphorescence derived from the metal-to-ligand charge transfer [dπ(Re)→π*(N-N)] excited state. The emissive excited states of these complexes have also been characterized by transient absorption spectroscopic studies. The capability of tuning the emissive excited-state energy through the modification of the isocyanide ligands could be reflected by the significant shifting of the phosphorescence from 530 to 620 nm with the same phenanthroline ligand.

ChemInform Abstract: The Selective Synthesis of Acetonitrile from Carbon Monoxide, Hydrogen, and Ammonia Over Mo/SiO2.

Abstract The selective synthesis of acetonitrile from CO/H2/NH3 over Mo/SiO2 was investigated. The effects of temperature, space velocity, and reactant compositions on conversion and selectivity were explored with a two-level factorial designed experiment. High selectivities (37–96% excluding CO2) were achieved at reasonably high conversions (29–64%). The product's carbon number distribution differed significantly from that obtained from a CO/H2 feed under identical conditions. The results are explained in terms of three competing reactions: water-gas shift, Fischer-Tropsch synthesis, and nitrile synthesis. Other experiments are described that indicate a nitrile synthesis mechanism involving a CN intermediate which further reacts with a carbon species to form the acetonitrile. A mechanistic hypothesis involving an isocyanide ligand is proposed.

ChemInform Abstract: Coordination and Reduction of Isocyanide Ligands in Hydrido-Carbonyl Rhenium Cluster Chemistry. Synthesis and Characterization of the Anion (Re3(μ-H)4(CO)9(CNR))- and of the Formimidoyl Derivative (Re3(μ- H)3(CO)9(.mu±eta.2-C(H)=NR)(P

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ChemInform Abstract: Reductive Coupling of Carbon Monoxide and Alkyl Isocyanide Ligands in Early Transition Metal Complexes: A Review

ChemInform Abstract: Reductive Coupling of Carbon Monoxide and Alkyl Isocyanide Ligands in Early Transition Metal Complexes: A Review

ChemInform Abstract: Cycloaddition of Nitrones to Metal‐Activated Nitriles and Isocyanides

AbstractReview: 69 refs.

Enantiomerically selective vapochromic sensing

The double salt materials platinum(II)tetrakis-R-β-methylphenethylisocyanide tetracyanoplatinate(II) ( R-1) and platinum(II)tetrakis-S-β-methylphenethylisocyanide tetracyanoplatinate(II) ( S-1) have been synthesized with highly enantiomerically pure isocyanide ligands. The vapochromic behavior of R-1 and S-1 has been studied in the presence of a chiral probe vapor to determine if enantiomerically selective sensing is possible with these materials. The wavelength of maximum emission values ( λ max) for solid-state vapoluminescence spectra of R-1 and S-1 in the presence of enriched R- and S-2-butanol vapor differ by approximately 10 nm while the λ max values for R-1 and S-1 under nitrogen are nearly identical. Principal component analysis has been performed on datasets that consist of a series of vapoluminescence spectra of R-1 and S-1 as a function of the R/S-2-butanol ratio. Plots of principal component one versus R/S-2-butanol ratio show mirror image trends for R-1 relative to S-1. While care must be taken to control water vapor and monitor R-1 and S-1 for possible decomposition, the reported results nevertheless show that R-1 and S-1 are capable of enantiomerically selective vapochromic sensing.

Efficient preparation of 99mTc(III) ‘4+1’ mixed-ligand complexes for peptide labeling with high specific activity

An improved labeling procedure for peptides attached to organometallic (99m)Tc(III) '4+1' mixed-ligand complexes in which the radiometal is coordinated by a tripodal tetradentate chelator 2,2',2''-nitrilotriethanethiol (NS(3)) and a monodentate isocyanide ligand is presented. The labeling procedure was evaluated by the synthesis of [(99m)Tc(NS(3))(L2-RGD)]. The containing radiopharmaceutically interesting RGD-peptide cyclo[Arg-Gly-Asp-D-Tyr-Lys] was modified with 4-isocyanobutanoic acid (L2) as linker conjugated to N(6)-Lys to get the monodentate ligand L2-RGD. The structural identity of the (99m)Tc-conjugate was confirmed by comparison to a Re reference compound. The Tc- and Re-conjugates had matching retention times under identical HPLC conditions. The (99m)Tc-labeling was performed in a novel one-step procedure using the eluate of a (99)Mo/(99m)Tc generator, NS(3), the isocyanide modified peptide, SnCl(2), Na(2)EDTA, mannitol and ascorbic acid in the reaction mixture. Using optimized reagents it is possible to label 50nmol peptide with (99m)Tc within 60min at room temperature with a radiochemical yield higher than 95% and a specific activity of approximately 20GBq/micromol.

Remarkably Efficient Photocurrent Generation Based on a [60]Fullerene-Triosmium Cluster/Zn-Porphyrin/Boron-Dipyrrin Triad SAM

A new artificial photosynthetic triad array, a [60]fullerene-triosmium cluster/zinc-porphyrin/boron-dipyrrin complex (1, Os(3)C(60)/ZnP/Bodipy), has been prepared by decarbonylation of Os(3)(CO)(8)(CN(CH(2))(3)Si(OEt)(3))(mu(3)-eta(2):eta(2):eta(2)-C(60)) (6) with Me(3)NO/MeCN and subsequent reaction with the isocyanide ligand CNZnP/Bodipy (5) containing zinc porphyrin (ZnP) and boron dipyrrin (Bodipy) moieties. Triad 1 has been characterized by various spectroscopic methods (MS, NMR, IR, UV/Vis, photoluminescence, and transient absorption spectroscopy). The electrochemical properties of 1 in chlorobenzene (CB) have been examined by cyclic voltammetry; the general feature of the cyclic voltammogram of 1 is nine reversible one-electron redox couples, that is, the sum of those of 5 and 6. DFT has been applied to study the molecular and electronic structures of 1. On the basis of fluorescence-lifetime measurements and transient absorption spectroscopic data, 1 undergoes an efficient energy transfer from Bodipy to ZnP and a fast electron transfer from ZnP to C(60); the detailed kinetics involved in both events have been elucidated. The SAM of triad 1 (1/ITO; ITO=indium-tin oxide) has been prepared by immersion of an ITO electrode in a CB solution of 1 and diazabicyclo-octane (2:1 equiv), and characterized by UV/Vis absorption spectroscopy, water contact angle, X-ray photoelectron spectroscopy, and cyclic voltammetry. The photoelectrochemical properties of 1/ITO have been investigated by a standard three-electrode system in the presence of an ascorbic acid sacrificial electron donor. The quantum yield of the photoelectrochemical cell has been estimated to be 29 % based on the number of photons absorbed by the chromophores. Our triad 1 is unique when compared to previously reported photoinduced electron-transfer arrays, in that C(60) is linked by pi bonding with little perturbation of the C(60) electron delocalization.

Isocyano Analogues of [Co(CO)4]n: A Tetraisocyanide of Cobalt Isolated in Three States of Charge

The encumbering m-terphenyl isocyanide ligand, CNAr(Mes2) (Mes = 2,4,6-Me(3)C(6)H(2)), is used to stabilize homoleptic tetraisocyanide complexes of cobalt in the 1-, 0, and 1+ charge state. Most importantly, these complexes serve as isolable analogues of the binary carbonyl complexes [Co(CO)(4)](-), Co(CO)(4), and [Co(CO)(4)](+). Sodium amalgam reduction of CoCl(2) in the presence of CNAr(Mes2) provides the salt Na[Co(CNAr(Mes2))(4)], which can be oxidized with 1 equiv of ferrocenium triflate (FcOTf) to the neutral complex, Co(CNAr(Mes2))(4). X-ray diffraction, FTIR spectroscopy, and low-temperature EPR spectroscopy reveal that Co(CNAr(Mes2))(4) modulates between D(2d)- and C(2v)-symmetric forms. DFT calculations are used to rationalize this structural modulation in terms of thermal access to low-energy b(2)-symmetric C-Co-C bending modes. Treatment of Na[Co(CNAr(Mes2))(4)] with 2 equiv of FcOTf, followed by addition of Na[BAr(F)(4)], provides the salt [Co(CNAr(Mes2))(4)]BAr(F)(4), which contains a diamagnetic, square planar monovalent cobalt center. The molecular and electronic structures of [Co(CNAr(Mes2))(4)]BAr(F)(4) are compared and contrasted to the reported properties of the carbonyl cation, [Co(CO)(4)](+).

Synthesis and Characterization of Diiron Thiadithiolate Complexes Related to the Active Site of [FeFe]‐Hydrogenases

AbstractOn the basis of preparation of the known complex [Fe2(μ‐SCH2)2S(CO)6] (A) by a new method involving condensation of [(μ‐LiS)2Fe2(CO)6] with excess S(CH2Br)2, twelve new diiron thiadithiolates as mimics of the [FeFe]‐hydrogenase active site have been synthesized by substitution of the CO ligand and coordination at the central S atom of complex A with appropriate reagents. Treatment of A with 1 equiv. of the monodentate ligands PPh3 and [(η5‐C5H5)(η5‐Ph2PC5H4)Fe] in the presence of Me3NO and with 1 equiv. tBuNC and cyclohexyl isocyanide gave the single [2Fe3S]‐cluster‐containing monosubstituted complexes [Fe2(μ‐SCH2)2S(CO)5(L1)] (1, L1 = PPh3; 2, L1 = (η5‐C5H5)(η5‐Ph2PC5H4)Fe; 3, L1 = tBuNC; 4, L1 = C6H11NC), whereas the double [2Fe3S]‐cluster‐containing disubstituted complexes [{Fe2(μ‐SCH2)2S(CO)5}2(L2)] [5, L2 = 4,4′‐(Ph2P)2(C6H4)2; 6, L2 = trans‐Ph2PCH=CHPPh2; 7, L2 = 1,4‐(CN)2C6H4; 8, L2 = (η5‐Ph2PC5H4)2Fe; 9, L2 = (η5‐Ph2PC5H4)2Ru] were produced by reaction of A with 0.5 equiv. of the corresponding bidentate ligands in the presence of Me3NO. In addition, the single [2Fe3S]‐cluster‐containing complexes in which the central S atom is coordinated, [{Fe2(μ‐SCH2)2S(CO)6}{(η5‐MeC5H4)(CO)2Fe}(BF4)] (10), [{Fe2(μ‐SCH2)2S(CO)6}{Cr(CO)5}] (11), and [{Fe2(μ‐SCH2)2S(CO)6}{W(CO)5}] (12), could be obtained by reaction of complex A with the in situ prepared [{(η5‐MeC5H4)(CO)2Fe}(BF4)], [Cr(CO)5(thf)], and [W(CO)5(thf)], respectively. While complex 3 was found to be able to reduce the proton of the weak acid Et3NHCl to give H2, the X‐ray crystallographic study confirmed that (i) each P atom of the phosphane ligands in 1 and 8 occupies an apical position at the Fe atoms, (ii) the isocyanide ligand in 3 lies in a basal position of the Fe atom, and (iii) the (η5‐MeC5H4)(CO)2Fe, Cr(CO)5, and W(CO)5 units in 10–12 are linked to the central S atom of complex A by an equatorial bond from the two fused six‐membered rings of their [2Fe3S]‐cluster cores.

Cleaving carbon–carbon bonds by inserting tungsten into unstrained aromatic rings

The cleavage of C-H and C-C bonds by transition metal centres is of fundamental interest and plays an important role in the synthesis of complex organic molecules from petroleum feedstocks. But while there are many examples for the oxidative addition of C-H bonds to a metal centre, transformations that feature oxidative addition of C-C bonds are rare. The paucity of transformations that involve the cleavage of C-C rather than C-H bonds is usually attributed to kinetic factors arising from the greater steric hindrance and the directional nature of the sp(n) hybrids that form the C-C bond, and to thermodynamic factors arising from the fact that M-C bonds are weaker than M-H bonds. Not surprisingly, therefore, most examples of C-C bond cleavage either avoid the kinetic limitations by using metal compounds in which the C-C bond is held in close proximity to the metal centre, or avoid the thermodynamic limitations by using organic substrates in which the cleavage is accompanied by either a relief of strain energy or the formation of an aromatic system. Here, we show that a tungsten centre can be used to cleave a strong C-C bond that is a component of an unstrained 6-membered aromatic ring. The cleavage is enabled by the formation of an unusual chelating di(isocyanide) ligand, which suggests that other metal centres with suitable ancillary ligands could also accomplish the cleavage of strong C-C bonds of aromatic substrates and thereby provide new ways of functionalizing such molecules.

Synthesis, Crystal Structure, and Spectroscopy of cis-Dichlorotetrakis(2,6-Dimethylphenyl Isocyanide)Iron(II)

The compound cis-FeCl2(DMPI)4 (DMPI = 2,6-dimethylphenyl isocyanide) was synthesized and characterized by X-ray crystallography, IR spectroscopy, and 1H-NMR. The structure of the hexanes solvate was solved in space group C2/c with a = 16.4160(2), b = 15.8028(2), c = 14.1263(2) A, β = 92.345(1), V = 3661.56(8) A3, Z = 4, with R = 0.0385. The coordination geometry about the Fe(II) center was octahedral with the two chlorides cis disposed. This is the first crystal structure of a cis-dihalotetrakis(isocyanide)iron(II) complex, and the isocyanide ligands trans to the chlorides are shown to participate in π-backbonding while those cis to the chlorides do not. IR spectroscopy confirms these observations, and 1H-NMR is consistent with the presence of two different types of isocyanide ligands. The structure and spectroscopy of cis-dichlorotetrakis(2,6-dimethylphenyl isocyanide)iron(II) revealed a complex with two types of isocyanide ligands: those trans to chloride which participate in π-backbonding from iron, and those cis to chloride which do not participate in π-backbonding.

The 1,4-phenylenediisocyanide dimer: gas-phase properties and insights into organic self-assembled monolayers

The 1,4-phenylenediisocyanide (PDI) dimer serves as an intriguing case of the substituted benzene dimer, as well as a prototype system for self-assembled monolayers of organic isocyanide complexes. Structures and binding energies are explored using recently developed dual-basis second-order Møller-Plesset perturbation theory energies and gradients. The structures are dictated by a combination of dispersion and electrostatics, a combination not properly treated with local or gradient-corrected density functionals. The PDI dimer binds more than twice as strongly as unsubstituted benzene dimers in several configurations, and greater directional specificity between parallel-displaced and T-shaped structures is observed. A rotated-parallel structure is the predicted lowest-energy, gas-phase configuration, in which the isocyanide ligands are staggered on the monomers. Relevant potential energy curves of the dimer are also presented, and insights into PDI monolayer formation on metal surfaces are explored via simple two-body models. Based on the adsorbate interaction alone, a high-coverage configuration and non-vertical tilt are predicted to be favorable, although the total binding for PDI in these configurations is still insufficient to form ordered monolayers, a result consistent with previous experimental findings. Additional phenyl rings (biphenyldiisocyanide, triphenyldiisocyanide) significantly stabilize the interaction and provide the additional dispersion necessary for an ordered monolayer.

Synthesis of cis- and trans-[M(CN)2(1-CH3-C6H10-NC)4] (M=Fe, Ru) and crystal structure of trans-[Ru(CN)2(1′-CH3-C6H10-NC)4

Abstract The reaction of the metal carbonyls Fe2(CO)9 or Ru3(CO)12 with 1-isocyano-1-methyl-cyclohexane produces the octahedral metal(II) cyano complexes [M(CN)2(1-CH3-C6H10-NC)4] in good yields. The cis-isomer is not isolated as a pure compound due to rearrangement processes that yield the sterically less crowded trans-isomer. The molecular structure of the trans-isomer of the ruthenium complex shows the expected almost ideal octahedral coordination of the central ruthenium ion with two cyanide and four isocyanide ligands. This reaction again indicates that isocyanides exhibiting quarternary carbon atoms in α-position with respect to the isocyano substituent in general react with di- or trinuclear metal carbonyls to yield cyano complexes with oxidized metal centres and concomitant reductive cleavage of the respective isocyanide.

Synthesis of a Ruthenium(II) Complex Containing an [11]ane-P2CNHC(NHC = Imidazolidin-2-ylidene) Macrocycle

Reaction of [RuCl(Cp)(PPh3)2] with bis[di(2-fluorophenyl)phosphino]benzene, 3, yields complex [RuCl(Cp)(3)], [4]. The chloro ligand in [4] can be exchanged for the 2-azidoethyl isocyanide ligand 2,...