Cyanide Ligands Research Articles

Synthesis, structure and spectroscopic properties of bis(triphenylphosphane)iminium (chlorido)(cyanido)argentates(I)

Four new compounds containing the (chlorido)(2−x)(cyanido)xargentate(I) anion (x=2, 1.63, 1 and 0.5) are reported. Their solid-state structures with the monocation PPN, bis(triphenylphosphane)iminium, are described; in a few cases the products also contain solvent molecules. The compounds have the formula (PPN)2[Ag2Cl3(CN)], (PPN)[Ag(CN)Cl](CH2Cl2) and (PPN)[Ag(CN)1.63Cl0.37](hexane)0.5 and (PPN)[Ag(CN)2](CH2Cl2). Apart from the molecular structures and the synthetic process, also the solid-state luminescence has been studied.In the case of (PPN)2[Ag2Cl3(CN)], the silver ions are pseudo-trigonally coordinated, with 2 chloride bridges between the Ag+ ions. The terminal Cl− and terminal CN− ligands are disordered over two positions, but only a single 13C NMR signal is observed for the cyanide ligands of this compound. The compound (PPN)[Ag(CN)2] serves as a reference compound and contains the linear [Ag(CN)2] unit.The compound with CN/Cl=1.0 shows disorder of the [AgCl(CN)] unit, and also in the compounds with other CN/Cl ratios, disorder among the Cl− and CN− ligands is seen. The Ag+ ion is linearly coordinated by two ligands in these two cases, and for cyanides normal Ag–C bond lengths are observed in both cases (1.97–1.99Å). The [Ag(CN)2] anion is linear and uneventful. The PPN cation has normal bond lengths in all 4 compounds and no short contacts with other atoms in the lattice are observed. The luminescence properties of the new compounds were explored as solid powders. Only one of the compounds (compound 2, with x=1) shows luminescence under excitation. The other three compounds do not show emission when irradiated at 320nm, 350nm or 380nm.

Synthesis, crystal structure and magnetic properties of a two-dimensional cyanide-based heterometallic coordination polymer with cationic piperazinium ligands: poly[aquatetra-μ2-cyanido-κ(8)C:N-dicyanido-κ(2)C-(piperazinium-κN(4))cobalt(III)copper(II)

Cyanide as a bridge can be used to construct homo- and heterometallic complexes with intriguing structures and interesting magnetic properties. These ligands can generate diverse structures, including clusters, one-dimensional chains, two-dimensional layers and three-dimensional frameworks. The title cyanide-bridged Cu(II)-Co(III) heterometallic compound, [Cu(II)Co(III)(CN)6(C4H11N2)(H2O)]n, has been synthesized and characterized by single-crystal X-ray diffraction analysis, magnetic measurement, thermal study, vibrational spectroscopy (FT-IR) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS). The crystal structure analysis revealed that it has a two-dimensional grid-like structure built up of [Cu(Hpip)(H2O)](3+) cations (Hpip is piperazinium) and [Co(CN)6](3-) anions that are linked through bridging cyanide ligands. The overall three-dimensional supramolecular network is expanded by a combination of interlayer O-H...N and N-H...O hydrogen bonds involving the coordinated water molecules and the N atoms of the nonbridging cyanide groups and monodentate cationic piperazinium ligands. A magnetic investigation shows that antiferromagnetic interactions exist in the title compound.

Cyanide-bridged mixed-valence copper(II/I) coordination polymers: Unique 7-connected sev-type 3D network versus anionic 2D host network encapsulated with cationic complex

Reactions of CuCl2, K2[Ni(CN)4]/K3[Co(CN)6], and 5-amino-1H-tetrazole led to cyanide-bridged mixed-valence Cu(II/I) complexes, [Cu(H2O)3Cu3(CN)5] (1) and [Cu(H2O)4][Cu4(CN)6] (2).The uncommon semi-closed {Cu2(CN)3} dimmers in 1 are bridged by μ3-C,C,N cyanide ligands to afford rare 1D centipedal-like chain motifs, which are further bridged by μ2-C,N and metalloligand “CN–Cu(4)–CN” to finish sophisticated zeolite-like 3-D structure. Topologically, 1 is the first example of uninodal 7-connected sev-like cyanide-bridged network. Compound 2 consists of two-dimensional honeycomb-related anionic layers [Cu4(CN)6]2−, within which the [Cu(H2O)4]2+ cations are intercalated into alternate interlamellar space. Compound 1 shows antiferromagnetic couple arising from pure ground-state configuration of Cu(II) mediated by diamagnetic bridges –CN–CuI–CN−. The inclusion compound 2 shows blue photoluminescence originated from Cu(I) center to the unoccupied π* orbital of the cyanide ligand (MLCT) charge transfer.

Electronic effect of substituents in coupling reactions between para-substituted benzamidoximes and nitrile ligands in cis-[PtCl2(EtCN)2] complex

The rate constants were determined for a number of reactions of nucleophilic addition of para-substituted benzamidoximes 4-RC6H4C(NH2)=NOH to cis-[PtCl2(EtCN)2] at 23°C in acetone. A linear correlation between log (kR/kH) and σpara was found, and reaction constant ρ296(σpara) was calculated. The prepared compounds were characterized by IR, 1H NMR, and HR ESI+-MS.

Evidence of Unusually Long Metal–Aqua Interaction: The Crystal Structure of Tetraphenylphosponium Rhenium(V)aquatetracyanonitrido Monohydrate Tri-terpyridine

The rhenium(V)aquatetracyanonitrido complex with 2,2′,2″-terpyridine (trpy) as adduct has been isolated and characterized with IR and X-ray crystallography. The title compound crystallizes in the monoclinic space group P21/n with unit cell parameters: a = 16.6829(2), b = 25.7198(10), c = 19.7098(3)A, β = 94.3860(10)°, V = 8432.3(2) A3, Z = 4. The rhenium in the anion has a distorted octahedral geometry with the Re(V) atom surrounded by the terminal nitrido ligand, four carbon atoms of the cyanide ligands and water molecule trans (171.33(4)°) to the nitride group. The water molecule is situated between the rhenium atom and one of the trpy nitrogen ligands with an exceptionally long metal aqua interaction equal to 2.658(3)A and with a distance equal to 2.804(5)A between the oxygen atom and the trpy nitrogen atom. Principal dimensions for this compound are; Re ≡ N = 1.655(3), Re-C1 = 2.100(4), Re-C2 = 2.094(5), Re-C3 = 2.094(4), Re-C4 = 2.113(4)A. The water molecule is situated between the rhenium atom and one of the trpy nitrogen ligands with an exceptionally long metal aqua interaction equal to 2.658(3)A and with a distance equal to 2.804(5)A between the aqua group and the nitrogen atom.

Double Stranded Helical Organo-lead 3D-Supramolecular Coordination Polymer Containing Copper Cyanide and Phenanthroline Ligand as Antimicrobial Agent

The supramolecular coordination polymer (SCP) [Cu(CN)2(Me3Pb)phen]; 1 is formed by self-assembly of K3[Cu(CN)4], Me3PbCl and 1,10-phenanthroline (phen) ligand at ambient conditions. The copper center is coordinated to two ordered cyanide ligands and two nitrogen atoms of the phen ligand forming distorted tetrahedral configuration. The structure of 1 consists of repeating units of the tetrahedral Cu(phen)(CN)2 fragments connected by the (Me3Pb)+ cations forming1-D helical chains. The 1-D helical chains are arranged in a unique way to construct a right-helical and a left-helical chain creating a double-helical 3-D network arranged in a unique A···A···A fashion. H-bonds and π–π stacking play an important role in stabilizing the structure of 1. IR, 1H-NMR, electronic and luminescence spectra as well as thermal analysis are also investigated. The SCP 1 has high antimicrobial activity against E. coli and S. aureus and it also exhibits good activity against the fungal strains, C. albicans and A. Niger. The results of growth inhibiting effect indicated that the inhibitory effect is increased with an increase in concentration of the SCP 1.

A novel one-dimensional metal-organic framework with a μ-cyanido-argentate group: catena-poly[[(5,5'-dimethyl-2,2'-bipyridyl-κ(2)N,N')silver(I)]-μ-cyanido-κ(2)N:C

The design and synthesis of metal coordination and supramolecular frameworks containing N-donor ligands and dicyanidoargentate units is of interest due to their potential applications in the fields of molecular magnetism, catalysis, nonlinear optics and luminescence. In the design and synthesis of extended frameworks, supramolecular interactions, such as hydrogen bonding, π-π stacking and van der Waals interactions, have been exploited for molecular recognition associated with biological activity and for the engineering of molecular solids.The title compound, [Ag(CN)(C12H12N2)]n, crystallizes with the Ag(I) cation on a twofold axis, half a cyanide ligand disordered about a centre of inversion and half a twofold-symmetric 5,5'-dimethyl-2,2'-bipyridine (5,5'-dmbpy) ligand in the asymmetric unit. Each Ag(I) cation exhibits a distorted tetrahedral geometry; the coordination environment comprises one C(N) atom and one N(C) atom from substitutionally disordered cyanide bridging ligands, and two N atoms from a bidentate chelating 5,5'-dmbpy ligand. The cyanide ligand links adjacent Ag(I) cations to generate a one-dimensional zigzag chain. These chains are linked together via weak nonclassical intermolecular interactions, generating a two-dimensional supramolecular network.

Monocyclometalated Gold(III) Complexes Bearing π-Accepting Cyanide Ligands: Syntheses, Structural, Photophysical, and Electrochemical Investigations.

The synthesis, structural, photophysical, and electrochemical investigations of a series of gold(III) monocyclometalated complexes bearing ancillary ligands with π-accepting properties is reported. Complexes of the type [(C(∧)N)Au(C≡N)2] [C(∧)N = 2-phenylpyridine (ppy) (1), 2-(p-tolyl)pyridine (tpy) (2), 2-(2-thienyl)-pyridine (thpy) (3), 2-(5-methyl-2-thienyl)pyridine (5m-thpy) (4), 1-phenylisoquinoline (piq) (5)], and [(N(∧)N)Au(C≡N)2] [N(∧)N = 3,5-bis(phenyl)-2-(2'-pyridyl)pyrrole (pyrpy) (6)] were prepared, and the influence of both the cyanide as an ancillary ligand as well as the different electronic properties of the cyclometalating ligands (1-5) and the chelating bidentate (6) on the triplet emission properties were studied. The physicochemical properties were evaluated by a variety of physical methods, and the structure of selected complexes was further confirmed by X-ray diffraction studies. Complexes 1-5 display long-lived emission in solution, neat solid, spin coated PMMA films, and at 77 K in 2-MeTHF. The emission energies were strongly dictated by the cyclometalating ligands independent of the cyanide ligand, which is in quite a contrast to the previously reported dicyano complexes of iridium(III) and the isoelectronic platinum(II) complexes. The nonemissive behavior of complex 6 in any medium further highlights the importance that the good σ-donating properties of the cyclometalating ligand alone is not decisive in rendering the gold complexes emissive, but also the appropriate placement of the energy level of the ligand orbitals is also important. Detailed photophysical studies in conjunction with density functional theory and time-dependent density functional theory calculations support the origin of the emission to be a metal perturbed intra ligand (3)IL (π-π*) delocalized over the cyclometalating ligand. The stability of the complexes combined with good emission quantum yields and tunability of the emission energies makes these complexes suitable alternatives to the relatively less stable monocyclometalated gold(III) diaryl or dialkyne complexes for organic light emitting device applications.

Expanding the family of C,N-chelated organotin(IV) pseudohalides: Synthesis and structural characterization

A set of tri- and diorganotin(IV) pseudohalides bearing the 2-(N,N-dimethylaminomethyl)phenyl-as a C,N-chelating ligand (LCN) has been prepared and structurally characterized. Triorganotin(IV) pseudohalides of the type LCNR2SnX [R = n-Bu, X = NCO (1a), NCS (1b), NCSe (1c), CN (1d); R = Ph, X = NCO (2a), NCS (2b), NCSe (2c), CN (2d)] and (LCN)2(n-Bu)SnX [X = NCO (3a), NCS (3b), NCSe (3c), CN (3d)] are monomeric both in solution and the solid state. The central tin atom in these species is five-coordinated in the solid state with distorted trigonal bipyramidal geometry. Monomeric diorganotin(IV) pseudohalides of the type (LCN)2SnX2 [X = NCO (4a), NCS (4b), NCSe (4c), CN (4d)] contain six-coordinated tin atoms with heavily distorted octahedral geometry both in solution and solid state due to the presence of two LCN units. Finally, we have found that LCN(n-Bu)Sn(NCS)2 (5) is presumably dimeric in CDCl3 and monomeric in THF-d8 or DMSO-d6 solution while it forms infinite linear polymer in the solid state via thiocyanate bridges. According to IR, multinuclear NMR spectroscopy and XRD analysis, it was found that the pseudohalide ligands of the NCY type (Y=O, S, Se) are bound via nitrogen while the CN substituent act as a common cyanide ligand (i.e. binds via carbon). DFT calculations were performed for selected compounds (2a, 2b, 2d and 4d) to further support our conclusions about the nature of the Sn-pseudohalide bonds. Compound LCNSnPhI2 (6), which was prepared in the scope of this work as well has a trigonal bipyramidal structure with non-equivalent iodine atoms.

Is the Hexacyanoferrate(II) Anion Stable in Aqueous Solution? A Combined Theoretical and Experimental Study.

A combined theoretical and experimental study was performed to elucidate the structural and dynamical properties of the isolated aqueous hexacyanoferrate(II) ion as well as in the presence of potassium counterions. It is shown that in absence of counterions, the highly negatively charged hexacyanoferrate(II) complex is not stable in aqueous solution. However, if the high negative charge is compensated by potassium counterions, a stable complex is observed, which is proven by theoretical simulations as well as by extended X-ray absorption fine structure (EXAFS) experiments. From the simulation it is found that potassium ions surrounding the complex are highly mobile and thus cannot be observed via EXAFS experiments. The structure of aqueous hexacyanoferrate(II) in the presence of potassium ions is identical to that of the solid-state structure, but the mobility of potassium ions is significantly increased in the liquid. These highly mobile potassium ions circulating the complex are the reason for the very short lifetime of hydrogen bonds between solvent water molecules and cyanide ligands being on the femtosecond scale.

Rhenium(V)-nitrido complexes with different nitrogen donor bidentate ligands

Rhenium(V)-nitridocyano complexes with 2,2′bipyridine (bipy), 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10-phenanthroline (Me2phen) are isolated for the first time and characterized by IR and X-ray crystallography as (AsPh4)[ReN(CN)3(2,2′bipy)]·2H2O·0.5bipy, 2{(AsPh4)[ReN(CN)3(phen)]·0.5H2O·0.54EtOH·0.46MeOH and (PPh4)[ReN(CN)3(Me2phen]·H2O·Me2phen. The rhenium anions in all three structures have a distorted octahedral geometry with the Re(V) atom coordinated by a terminal nitrido ligand, three carbon atoms of the cyanide ligands and bidentate ligands that are bonded bidentally via two pyridine nitrogen atoms. The bidentate ligands coordinate to the Re(V) metal centre via two nitrogen atoms and form five-membered chelate rings with bite angles of 71.37(9)°, 72.7(2)° and 70.7(3)° for the bipy, phen and Me2phen complexes, respectively. Principal dimensions are: bipy: Re≡N = 1.651(3), Re–N(11) = 2.144(1), Re–N(12) = 2.374(2), Re–C(1) = 2.025(4), Re–C(2) = 2.142(4) and Re–C(3) 2.080(3) A; phen: Re≡N = 1.674(5), Re–N(11) = 2.164(5) A, Re–N(12) = 2.374(5) A, Re–C(1) = 2.112(6), Re–C(2) = 2.082(6) and Re–C(3) 2.102(6) A; Me2phen: Re≡N = 1.634(9), Re–N(11) = 2.190(7) A, Re–N(22) = 2.454(8), Re–C(1) = 2.074(9) A, Re–C(2) = 2.095(10) A and Re–C(3) 2.094(10) A. One of the major differences between the complexes of phen and Me2phen is that the latter exhibits a fluxional behaviour, while the phenanthroline plane in the phen complex is almost coplanar with the coordination plane. This pronounced distortion in the Me2phen complex is illustrated by the dihedral angle of 117°(2) between the plane of the chelating moiety and that of the square-planar coordination plane.

Click-Type PtII -Mediated Hydroxyguanidine-Nitrile Coupling Provides Useful Catalysts for Hydrosilylation Cross-Linking.

The nitrile complexes trans-[PtCl2 (RCN)2 ] (R=Et (NC1), tBu (NC2), Ph (NC3), p-BrC6 H4 (NC4)) and cis-[PtCl2 (RCN)2 ] (R=Et (NC5), tBu (NC6), Ph (NC7)) react with 1 equiv of the hydroxyguanidine OC4 H8 NC(=NOH)NH2 (HG) furnishing the mono-addition products trans- and cis-[PtCl2 (RCN){NH=C(R)ON=C(NH2 )NC4 H8 O}] (1-4 and 9-11; 7 examples; 54-74 % yield). Treatment of any of the nitrile complexes NC1-NC7 with HG in a 1:2 molar ratio generated the bis-addition products trans- and cis-[PtCl2 {NH=C(R)ON=C(NH2 )NC4 H8 O}2 ] (5-8 and 12-14; 7 examples; 69-89 % yield). The PtII -mediated coupling between nitrile ligands and HG proceeds substantially faster than the corresponding reactions involving amid- and ketoximes and gives redox stable products under normal conditions. Complexes 1, 6⋅4 CH2 Cl2 , 7⋅4 CH2 Cl2 , 8⋅2 CH2 Cl2 , and NC4 were studied by X-ray crystallography. Platinum(II) species 1-3, 10, 11, and especially 9, efficiently catalyze the hydrosilylation cross-linking of vinyl-terminated poly(dimethylsiloxane) and trimethylsilyl-terminated poly(dimethylsiloxane-co-ethylhydrosiloxane) giving high-quality thermally stable silicon resins with no structural defects. The usage of these platinum species as the catalysts does not require any inhibitors and, moreover, the complexes and their mixtures with vinyl- and trimethylsilyl-terminated polysiloxanes are shelf-stable in air.

A combined FTIR and infrared emission spectroscopy investigation of layered double hydroxide as an effective electron donor

A novel method has been presented to characterize electron transfer in layered double hydroxides (LDHs) utilizing an investigation combing FTIR and infrared emission spectroscopy. At room temperature, electron could transfer to interlayer Fe3+ through monodentate ligand cyanide, and resulted in a reduction of 40% Fe3+ to Fe2+. When the environmental temperature increased from 25 to 300°C, reduction of Fe3+ and Ni2+ increased to 94% and 42%. Furthermore, electron also transferred to interlayer cation through multidentate ligand EDTA. As a result, LDHs has been proven to be an effective electron donor, and FTIR was a feasible tool in characterizing this property by monitoring the valence state of cations. It was also concluded that octahedral units with OH− groups in LDH layer functioned as electron donor centers. Driving force for electron transfer is attributed to the charge density difference between cation layer and probe anion. These results could help to explain the mechanism of various applications of LDHs in catalysis and photocatalysis.

1D cyanide complexes with 2-pyridinemethanol: Synthesis, crystal structures and spectroscopic properties

Two new one-dimensional coordination polymers, [Cu(hmpH)2Pd(μ-CN)2(CN)2]n (1) and [Cu(hmpH)2Pt(μ-CN)2(CN)2]n (2), (hmpH = 2-pyridinemethanol), have been synthesized and characterized by vibrational (FT-IR and Raman) spectroscopy, single crystal X-ray diffraction, thermal and elemental analyses techniques. Single crystal X-ray diffraction analysis indicates that complexes 1 and 2 are isomorphous and isostructural, and crystallize in the triclinic system and P-1 space group. The Pd(II) or Pt(II) ions are four coordinated with four cyanide-carbon atoms in a square planar geometry. Cu(II) ion displays a distorted octahedral coordination by two N-atoms and two O-atoms of hmpH ligands, two bridging cyanide groups. In one dimensional structure of the complexes, [M(CN)4]2− (M = Pd(II) or Pt(II)) anions and [Cu(hmpH)2]2+ cations are linked via bridging cyanide ligands. In the complexes, the presence of intramolecular C–H⋯M (M = Pd(II) or Pt(II)) interactions with distance values of 3.00–2.95 Å are established, respectively.

New structure, reconstruction, and behaviour of the coordination polymer framework [CuZn(CN)4]– towards hydrated alkali metal ions

A new framework structure of the coordination polymer [CuZn(CN)4]– has been discovered in the newly synthesized [Na(H2O)n][CuZn(CN)4] (n = 12.7; 1). X-ray crystal structure analysis revealed a 3D framework comprised of tetrahedral Cu(I) and Zn(II) centres with bridging cyanide ligands containing both octagonal and square channel cavities; these channels contain hydrated Na+ ions as guests. In open air, complex 1 is extremely unstable and loses water molecules to form [Na(H2O)n][CuZn(CN)4] (n = 5.2–6.8; 2) within 30 min. Powder X-ray diffraction patterns showed that this change results in the conversion of the framework of complex 1 to the same framework observed in [K(H2O)n][CuZn(CN)4]. The [CuZn(CN)4]– framework of [K(H2O)n][CuZn(CN)4] has a tridymite structure with hexagonal channel cavities occupied by hydrated K+ ions. Like 1, 2 is also unstable under atmospheric conditions, yielding [Na(H2O)n][CuZn(CN)4] (n = 3), which has an unknown structure. While the framework of [K(H2O)n][CuZn(CN)4] is structurally flexible and robust, the framework of 2 is unstable and fragile. This contrast comes from the difference between the nature of the hydrated K+ and Na+ ions present in the frameworks.

Identification of an Isothiocyanate on the HypEF Complex Suggests a Route for Efficient Cyanyl-Group Channeling during [NiFe]-Hydrogenase Cofactor Generation.

[NiFe]–hydrogenases catalyze uptake and evolution of H2 in a wide range of microorganisms. The enzyme is characterized by an inorganic nickel/ iron cofactor, the latter of which carries carbon monoxide and cyanide ligands. In vivo generation of these ligands requires a number of auxiliary proteins, the so–called Hyp family. Initially, HypF binds and activates the precursor metabolite carbamoyl phosphate. HypF catalyzes removal of phosphate and transfers the carbamate group to HypE. In an ATP–dependent condensation reaction, the C–terminal cysteinyl residue of HypE is modified to what has been interpreted as thiocyanate. This group is the direct precursor of the cyanide ligands of the [NiFe]–hydrogenase active site cofactor. We present a FT–IR analysis of HypE and HypF as isolated from E. coli. We follow the HypF–catalyzed cyanation of HypE in vitro and screen for the influence of carbamoyl phosphate and ATP. To elucidate on the differences between HypE and the HypEF complex, spectro–electrochemistry was used to map the vibrational Stark effect of naturally cyanated HypE. The IR signature of HypE could ultimately be assigned to isothiocyanate (–N=C=S) rather than thiocyanate (–S–C≡N). This has important implications for cyanyl–group channeling during [NiFe]–hydrogenase cofactor generation.

Hydration of Nitriles to Amides by Thiolate-Bridged Diiron Complexes

A series of nitrile-coordinating complexes [Cp*Fe(μ-SEt)RCN]2[PF6]2 (1, R = alkyl, aryl, vinyl, amine) have been obtained by the reaction of [Cp*Fe(μ-SEt)MeCN]2[PF6]2 (1a) with various nitriles in acetone. Complexes 1 can realize the hydration of a nitrile ligand under ambient conditions. Complexes [Cp*Fe(μ-SEt)2(μ-η1:η1-NH(O)CR)FeCp*][PF6] (2) were successfully isolated as intermediates during the hydration process, with 2b and 2e (R = CH2═CH and Et2N) being characterized by spectrometry and X-ray crystallography. Treatment of 2 with HBF4·Et2O in the presence of nitriles released corresponding amides 3. At the same time, the structural features of the [Fe2S2] scaffold were retained. These results confirmed that the hydration of nitriles was realized by cooperative interaction on diiron centers.

Synthesis and molecular structure of [Cu(NH3)4][Ni(CN)4]: A missing piece in the [Cu(NH3)n][Ni(CN)4] story

Reaction of CuCl2·2H2O and K2[Ni(CN)4]·2H2O in aqueous ammonia gave blue rod-like crystals of [Cu(NH3)4][Ni(CN)4]. An X-ray crystallographic reveals that square-planar anions and cations are weakly associated through coordination of a cis pair of cyanide ligands to copper, with one short and one long contact and thus the copper centre is best described as a square-based pyramid. Crystals lose ammonia readily upon removal from the solvent and this has been probed by TGA and DSC measurements. For comparison we have also re-determined the structure of the related ethylenediamine (en) complex [Cu(en)2][Ni(CN)4] at 150 K. This consists of a 1D chain in which a trans pair of cyanide ligands bind to copper such that the latter has an overall tetragonally distorted octahedral coordination geometry.

Bis(cyano) Iron(III) Porphyrinates: What Is the Ground State?

The synthesis of six new bis(cyano) iron(III) porphyrinate derivatives is reported. The anionic porphyrin complexes utilized tetraphenylporphyrin, tetramesitylporphyrin, and tetratolylporphyrin as the porphyrin ligand. The potassium salts of Kryptofix-222 and 18-C-6 were used as the cations. These complexes have been characterized by X-ray structure analysis, solid-state Mössbauer spectroscopy, and EPR spectroscopy, both in frozen CH2Cl2 solution and in the microcrystalline state. These data show that these anionic complexes can exist in either the (dxz,dyz)(4)(dxy)(1) or the (dxy)(2)(dxz,dyz)(3) electronic configuration and some can clearly readily interconvert. This is a reflection that these two states can be very close in energy. In addition to the effects of varying the porphyrin ligand, subtle effects of the cyanide ligand environment in the solid state and in solution are sufficient to shift the balance between the two electronic states.

Correlations between the Electronic Properties of Shewanella oneidensis Cytochrome c Nitrite Reductase (ccNiR) and Its Structure: Effects of Heme Oxidation State and Active Site Ligation.

The electrochemical properties of Shewanella oneidensis cytochrome c nitrite reductase (ccNiR), a homodimer that contains five hemes per protomer, were investigated by UV-visible and electron paramagnetic resonance (EPR) spectropotentiometries. Global analysis of the UV-vis spectropotentiometric results yielded highly reproducible values for the heme midpoint potentials. These midpoint potential values were then assigned to specific hemes in each protomer (as defined in previous X-ray diffraction studies) by comparing the EPR and UV-vis spectropotentiometric results, taking advantage of the high sensitivity of EPR spectra to the structural microenvironment of paramagnetic centers. Addition of the strong-field ligand cyanide led to a 70 mV positive shift of the active site's midpoint potential, as the cyanide bound to the initially five-coordinate high-spin heme and triggered a high-spin to low-spin transition. With cyanide present, three of the remaining hemes gave rise to distinctive and readily assignable EPR spectral changes upon reduction, while a fourth was EPR-silent. At high applied potentials, interpretation of the EPR spectra in the absence of cyanide was complicated by a magnetic interaction that appears to involve three of five hemes in each protomer. At lower applied potentials, the spectra recorded in the presence and absence of cyanide were similar, which aided global assignment of the signals. The midpoint potential of the EPR-silent heme could be assigned by default, but the assignment was also confirmed by UV-vis spectropotentiometric analysis of the H268M mutant of ccNiR, in which one of the EPR-silent heme's histidine axial ligands was replaced with a methionine.