Ligand Bond Distances Research Articles

Authentic-Blue Phosphorescent Iridium(III) Complexes Bearing Both Hydride and Benzyl Diphenylphosphine; Control of the Emission Efficiency by Ligand Coordination Geometry

Sequential treatment of IrCl(3) x nH(2)O with 2 equiv of benzyl diphenylphosphine (bdpH) and then 1 equiv of 3-trifluoromethyl-5-(2-pyridyl) pyrazole (fppzH) in 2-methoxyethanol gave formation to three isomeric complexes with formula [Ir(bdp)(fppz)(bdpH)H] (1-3). Their molecular structures were established by single crystal X-ray diffraction studies, showing existence of one monodentate phosphine bdpH, one terminal hydride, a cyclometalated bdp chelate, and a fppz chelate. Variation of the metal-ligand bond distances showed good agreement with those predicted by the trans effect. Raman spectroscopic analyses and the corresponding photophysical data are also recorded and compared. Among all isomers complex 1 showed the worst emission efficiency, while complexes 2 and 3 exhibited the greatest luminescent efficiency in solid state and in degassed CH(2)Cl(2) solution at room temperature, respectively. This structural relationship could be due to the simultaneously weakened hydride and the monodentate bdpH bonding that are destabilized by the trans-pyrazolate anion and cyclometalated benzyl group, respectively.

Combined Mössbauer Spectral and Density Functional Theory Determination of the Magnetic Easy-Axis in Two High-Spin Iron(II) 2-Pyrazinecarboxylate Complexes

A combination of density functional theory (DFT) calculations and Mössbauer spectroscopy has been used to determine that the magnetic easy-axis is coincident with its crystallographic c-axis in [Fe(pca)(2)(py)(2)] x py, where pac is the 2-pyrazinecarboxylate ligand. This easy-axis bisects the approximately axial O-Fe-O coordination axes of molecules adjacent to each other along the b-axis. In {[Fe(pca)(2)(H(2)O)] x H(2)O}(n) the easy magnetic axis is not coincident with any of its crystallographic axes nor with the Fe-O(water) coordination axis, but is coincident with one of the Fe...Fe axes in the crystal structure. The DFT calculations, which use the B3LYP functional and the 6-311++G(d,p) basis set, yield s-electron probability densities and electric field gradient tensors for the iron(II) ion that are in excellent agreement with the observed iron-57 Mössbauer spectral isomer shifts and quadrupole interactions. The gas phase results are very similar for calculations based either on the X-ray structures of the two complexes or on their optimized structures; the optimized structures indicate that the iron to ligand bond distances increase in the absence of any solid-state lattice interactions. The results of a normal coordinate vibrational mode analysis of the two optimized structures are compared with the observed infrared spectra.

Unusual Structural and Spectroscopic Features of Some PNP-Pincer Complexes of Iron

We report a structural, spectroscopic, and computational study of two tBuPNP (2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) complexes of iron, (tBuPNP)FeCl2 (1) and (tBuPNP)Fe(CO)2 (3). Complex 1, (tBuPNP)FeCl2, also independently synthesized by Milstein, has unusually long iron−ligand bond distances. DFT calculations show that these are clearly attributable to its high-spin electronic structure, and in particular to occupancy of the strongly antibonding dx2−y2 orbital. The crystal structure of 3 reveals two unusual aspects. (1) The geometry around the iron atom in 3 is much closer to square pyramidal (SQP; apical CO) than to trigonal bipyramidal (TBP), although five-coordinate Fe(0) complexes are generally expected to be TBP; moreover, Chirik et al. have reported that (iPrPNP)Fe(CO)2 has essentially a perfect TBP structure (iPrPNP = 2,6-bis(di-isopropyl-phosphinomethyl)pyridine). (2) The apical carbonyl ligand in 3 deviates significantly from linearity (Fe−C−O = 171.9°). Additionally, complex 3 is inte...

Geometries of Third-Row Transition-Metal Complexes from Density-Functional Theory.

A set of 41 metal-ligand bond distances in 25 third-row transition-metal complexes, for which precise structural data are known in the gas phase, is used to assess optimized and zero-point averaged geometries obtained from DFT computations with various exchange-correlation functionals and basis sets. For a given functional (except LSDA) Stuttgart-type quasi-relativistic effective core potentials and an all-electron scalar relativistic approach (ZORA) tend to produce very similar geometries. In contrast to the lighter congeners, LSDA affords reasonably accurate geometries of 5d-metal complexes, as it is among the functionals with the lowest mean and standard deviations from experiment. For this set the ranking of some other popular density functionals, ordered according to decreasing standard deviation, is BLYP > VSXC > BP86 ≈ BPW91 ≈ TPSS ≈ B3LYP ≈ PBE > TPSSh > B3PW91 ≈ B3P86 ≈ PBE hybrid. In this case hybrid functionals are superior to their nonhybrid variants. In addition, we have reinvestigated the previous test sets for 3d- (Bühl M.; Kabrede, H. J. Chem. Theory Comput. 2006, 2, 1282-1290) and 4d- (Waller, M. P.; Bühl, M. J. Comput. Chem. 2007, 28, 1531-1537) transition-metal complexes using all-electron scalar relativistic DFT calculations in addition to the published nonrelativistic and ECP results. For this combined test set comprising first-, second-, and third-row metal complexes, B3P86 and PBE hybrid are indicated to perform best. A remarkably consistent standard deviation of around 2 pm in metal-ligand bond distances is achieved over the entire set of d-block elements.

X-ray K-absorption studies of cobalt(II) semicarbazones

X-ray K-absorption edges along with their extended X-ray absorption fine structure (EXAFS) have been recorded using a laboratory XAFS spectrometer for the cobalt(II) semicarbazones viz., Co(emsc) 2Br 2, Co(emsc) 2SO 4, Co(mchsc) 2Br 2, Co(mchsc) 2SO 4 and Co(mchsc) 2(NCS) 2. The chemical shifts for these samples have been reported. The total phase shifts δ 1 and phase parameters β 1 for these samples have been estimated using the modified graphical method for the determination of bond distances. The variations in the values of chemical shift, phase parameter β 1 and average metal–ligand bond distances have been explained. The total phase shift values for the complexes indicate a linear variation with wave vector k.

Structural, Thermodynamic, and Mesomorphic Consequences of Replacing Nitrates with Trifluoroacetate Counteranions in Ternary Lanthanide Complexes with Hexacatenar Tridentate Ligands

The promesogenic hexacatenar tridentate ligands L3(Cn) (I shape) and L4(Cn) (V shape) react with trivalent lanthanide trifluoroacetates, Ln((CF3CO2)3, to give either monometallic [Ln(Li(Cn))(CF3CO2)3] or trifluoroacetato-bridged bimetallic [Ln(Li(Cn))(CF3CO2)3]2 complexes in the solid state, as exemplified by the crystal structures of [Lu(L4(CO))(CF3CO2)3(H2O)], [Lu(L4(CO))(CF3CO2)3]2, and [La(L3(C4))(CF3CO2)3]2. Although the dimerization process is influenced by the competiting complexation of anions or solvent molecules, the coordination of CF3CO2- instead of NO3- to Ln(III) produces a significant lengthening of the Ln-N(ligand) bond distances. This translates into a considerable decrease of the affinity of the Li(C12) (i = 3, 4) ligands for Ln(CF3CO2)3 in solution, thus leading to significant dissociation of the [Ln(Li(C12))(CF3CO2)3] complexes at millimolar concentrations. The thermal properties of these complexes also suffer from their limited thermodynamic stability, and the thermotropic liquid crystalline phases produced at high temperatures reflect mixtures of different species. However, a hexagonal columnar organization characterizes the main component in the mesophases obtained with [Ln(L3(C12))(CF3CO2)3] at high temperature. A tentative interpretation of the small-angle X-ray scattering (SAXS) profiles suggests that disklike dimers of [Ln(L3(C12))(CF3CO2)3]2 are packed along the columnar axes. For [Ln(L4(C12))(CF3CO2)3], SAXS profiles are compatible with a lamellar organization in the mesophases originating from the existence of rodlike dimers of [Ln(L4(C12))(CF3CO2)3]2 as the major component in the liquid-crystal state.

A structural study on uranyl (VI) nitrate complexes with cyclic amides: N- n-butyl-2-pyrrolidone, N-cyclohexylmethyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidone

Structural analyses of UO 2(NO 3) 2L 2 [L = N- n-butyl-2-pyrrolidone (NBP), N-cyclohexylmethyl-2-pyrrolidone (NCMeP), and 1,3-dimethyl-2-imidazolidone (DMI)] have been carried out using X-ray diffraction method. These uranyl complexes were found to have a hexagonal bipyramidal structure. The bond distances (Å) of U O and U–O(ligand), and bond angles (°) of U–O–C(carbonyl) are determined as follows: 1.774(2), 2.374(2), and 137.6(2) for UO 2(NO 3) 2(NBP) 2; 1.770(1), 2.383(2), and 135.3(1) for UO 2(NO 3) 2(NCMeP) 2; 1.771(2), 2.361(2), and 143.3(2) for UO 2(NO 3) 2(DMI) 2. In uranyl nitrate complexes with cyclic amides such as 2-pyrrolidone, urea, and caprolactam derivatives, a linear correlation was found to hold between U–O(ligand) bond distances and U–O–C(carbonyl) bond angles. Vibrational frequencies of UO 2(NO 3) 2L 2 have also been measured by IR and Raman spectrophotometers. Using relationships between vibrational frequencies of O U O bonds and donor numbers (DNs) of ligands, it was found that donicities of N-substituted-2-pyrrolidones (Me, Et, Bu, cyclohexyl, and cyclohexylmethyl) are in the range of 26–29, and the DN of 1,3-dimethyl-2-imidazolidone was estimated as 27.8.

Comparative structural studies of iodide complexes of uranium(III) and lanthanide(III) with hexadentate tetrapodal neutral N-donor ligands.

The syntheses, the solution structures, and the crystal structures of the two new tetrapodal N-donor ligands N,N,N',N'-tetrakis(2-pyrazylmethyl)-1,3-trimethylenediamine (tpztn), 1, and N,N,N',N'-tetrakis(2-pyrazylmethyl)-trans-1,2-cyclohexanediamine (tpzcn), 2, are described. Two different geometric isomers of the cation [La(tpztn)I(2)](+) were isolated in which the ligand adopts two different conformations leading to strong differences in the metal-ligand bond distances. The crystal structure of isostructural complexes of La, U, Ce, and Nd were determined by X-ray diffraction studies for the ligands tpztn and tpzcn. In both series of complexes the two methylpyrazyl arms and the diamine spacer (trimethylene or cyclohexane) around each aliphatic nitrogen adopt the same helical configuration. The complexes crystallize as a racemic mixture of Lambda,Lambda and Delta,Delta enantiomers with distorted square antiprism geometries. In these complexes the M-N(pyrazine) distances show a decrease from La to Ce and from La to Nd which corresponds well to the decrease in ionic radius as expected in a purely ionic bonding model. Conversely the mean value of the U-N(pyrazine) distances is shorter (0.043(3) A for tpztn and 0.054(11) A for tpzcn) than the mean value of the La-N(pyrazine) distances. These differences are significantly larger than the decrease expected from the variation of the ionic radii and can be interpreted in terms of a stronger M-N interaction for U(III). Previously reported extraction studies have shown that while the tripod tris[(2-pyrazyl)methyl]amine (tpza) containing three pyrazyl nitrogens extracts An(III) preferentially to Ln(III), tpztn and tpzcn display no selectivity despite the presence of four pyrazyl groups connected to a different spacer. The structural studies described here show that despite the lack of selectivity observed in the extraction conditions, the arrangement of pyrazyl nitrogens in the tetrapodal architectures of tpztn and tpzcn allows for metal-ligand interaction similar to that observed for tpza.

Theoretical analysis of the [Mn2(μ-oxo)2(μ-carboxylato)2]+core

The first example of a dinuclear manganese complex containing two oxo and two carboxylate bridges, [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ (where bpy = 2,2′-bipyridine, and ArTolCO2− = 2,6-di(p–tolyl)benzoate), was reported recently (J. Am. Chem. Soc. 2003, 125, 13010). X-ray crystallographic analysis performed on this complex reveals a trapped mix-valence species as evidenced, for example, by very different metal–ligand bond distances at the MnIII and MnIV centers. The fact that there are rather bulky bridging carboxylate ligands present in this recently reported dinuclear species raises the question as to whether they affect the extent of valence trapping and the metrical parameters in general. Specifically, it was thought that intramolecular nonbonded contacts could play an important role. In the work reported here, density functional theory calculations were used to address this issue. Structural parameters obtained from calculations on a model compound bearing sterically small bridging carboxylates, [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+, are in good agreement with the experimentally determined single crystal X-ray structure. Thus, the sterically large carboxylate bridges in [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ appear not to have a significant effect on the metal–ligand bond distances and angles. There is calculated to be minimal Mn⋯Mn bonding despite contraction of the Mn⋯Mn distance relative to related complexes. In addition to calculations on the mixed-valence MnIIIMnIV complex, various electronic configurations of the corresponding MnIIIMnIII and MnIVMnIV complexes are explored. Although our calculations support assignment of [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+ as a valence-trapped MnIIIMnIV configuration involving high-spin MnIII, a delocalized configuration arising from low-spin MnIII is calculated to lie very close in energy. The energetic proximity of the delocalized configuration is attributed to an effective crossed-exchange mechanism, which permits mixing of an eg-based orbital (nominally on high-spin MnIII) with a vacant t2g-based orbital (nominally on MnIV).

Quantum chemical analysis of the bond lengths in fn and fn−1d1 states of Ce3+, Pr3+, Pa4+, and U4+ defects in chloride hosts

It is widely believed that impurity–ligand bond distances in lanthanide (Ln) and actinide (An) doped crystals, are larger in the fn−1d1 energy levels than in the fn ones. This idea, which was not justified and is probably based on the fact that Ln 5d (An 6d) orbitals have a radial extent much larger than Ln 4f (An 5f ) orbitals, has been neither confirmed nor rejected experimentally in spite of the fact that a very large number of absorption/emission spectroscopic studies on f-element doped hosts exist, because the band shapes depend on the square of the bond length offsets between initial and final electronic states. Recent quantum chemical calculations on Ln and An impurities in fluoride and chloride cubic hosts, which considered host embedding, dynamic electron correlation, and relativistic spin–free and spin–orbit coupling effects, have shown that impurity–ligand bond distances are classified in three sets according to their configuration, with the following trend: Re[fn−1d(t2g)1]<Re[fn]<Re[fn−1d(eg)1], in contradiction with the assumed expectations. In this paper we give an interpretation of this, on the basis of a constrained space orbital variation analysis of the chemical bond in states of the fn, fn−1d(t2g)1, and fn−1d(eg)1 configurations of four model systems: Cs2NaYCl6:Ce3+, Cs2NaYCl6:Pr3+, Cs2ZrCl6:Pa4+, and Cs2ZrCl6:U4+. The analysis shows that the basic difference between fn and fn−1d1 configurations regarding bond effects which are responsible for the bond distance is that, in the former, all the open-shell electrons are shielded from the ligands by the 5p (6p) filled shell and the bond length is determined by closed-shell interactions between the outermost Ln 5p6 (An 6p6) shell and the ligands, whereas in the latter one electron has crossed the 5p (6p) barrier and is much more exposed to bonding interactions with the ligands, at the same time that an internal 4f (5f ) hole has been created which induces ligand to Ln (An) charge transfer, all of it resulting in the shown trends.

Synthesis and structural characterization of isomeric ‘lantern-shaped’ platinum(III) complexes of formula [Pt 2(PPh 3)X{ N(H)C(R) O} 4](NO 3) 2 (X=PPh 3, H 2O)

The platinum(III) lantern type complexes [Pt 2(PPh 3) 2{ N(H)C(R) O} 4](NO 3) 2 [R=Me ( 1), Bu t ( 2)], and [Pt 2(H 2O)(PPh 3){ N(H)C(Bu t) O} 4](NO 3) 2 ( 3) were synthesized and characterized by 1H NMR and X-ray crystallography ( 2 and 3). The compounds can give rise to formation of isomers differing for the sets of equatorial donor atoms around each platinum, N 3O/NO 3 or N 2O 2, and, in the case of N 2O 2, for the cis or trans geometry. The effect of the anion upon the chemical shifts of NH protons was studied for NO 3 − , BF 4 − , and ClO 4 − . The stability of phosphine axial ligands in the complexes N 3O/NO 3–[Pt 2(PPh 3) 2{ N(H)C(R) O} 4](NO 3) 2 as a function of the set of donor atoms was also studied. The complex N 3O/NO 3– 3 is the fist non-symmetric lantern-type platinum dimer to be characterized by X-ray diffraction. Comparison of the platinum/axial ligand bond distances in different complexes of this type allows to conclude that two factors contribute to the lengthening of axial bonds: the strong trans labilizing effect of the intermetallic bond and the trans-influence of the axial ligand on the second platinum unit.

The Preparation, Characterization, and X-Ray Structural Analysis of Dichlorobis[1-methyl-3-(2-propyl)-2(3H)-imidazolethione]mercury(II)

A new compound of mercury(II) chloride complexed to 1-methyl-3-(2-propyl) -2(3H)-imidazolethione (mipit) has been prepared and characterized via standard methods and X-ray crystallography. The structural significance of this study is that it shows one of the few monomeric examples of a mercury(II) chloride thione complex reported to date. The compound crystallizes in space group P21/c with a = 17.143(6) A, b = 17.047(6) A, c = 14.759(5) A, β = 105.899(5)°, V = 4148(2) A3, Z = 8. The coordination sphere is distorted tetrahedral with Hg–S bonds and Hg–Cl bond distances falling within the normally expected ranges. Bond angles ranged from 108.11(4)° to 115.51(4)° with the widest angle being observed for the S–Hg–S linkage. Ligand bond distances and angles including the C=S distance are within the normally expected values observed for this compound.

Synthesis, characterisation and crystal structure of cis-dioxomolybdenum(VI) complexes of some potentially pentadentate but functionally tridentate (ONS) donor ligands

Neutral cis-dioxomolybdenum(VI) complexes with potentially pentadentate ONSNO donor Schiff bases of thiocarbodihydrazone of salicylaldehyde (H 3L 1), 5-bromo (H 3L 2), 5-nitro salicylaldehyde (H 3L 3) and 2-hydroxyacetophenone (H 3L 4) acting as tridentate ONS donor ligands have been synthesised. The complexes are found to be of the form MoO 2LH(ROH) (R=CH 3) where, LH=L IH, L 2H, L 3H and L 4H. The complexes were characterised by elemental analyses, UV, IR, and 1H NMR spectroscopy, magnetic susceptibility measurement, molar conductivities in solution and by cyclic voltammetry. Two of the complexes [MoO 2(L 2H)(MeOH)] and [MoO 2(L 4H)(MeOH)] were crystallographically characterised. The structures reveal that the molybdenum acceptor centre is present in a distorted octahedral NO 4S donor environment. The presence of a substituent either on the aromatic ring of the salicylaldehyde moiety or on the carbon atom of its carbonyl group is found to exhibit little effect on the corresponding metal ligand bond distances and angles. The sixth coordination site of the complexes harboring the weakly coordinated ROH moiety is found to act as the binding site for various neutral monodentate Lewis bases.

The role of Glu39 in MnII binding and oxidation by manganese peroxidase from Phanerochaete chrysoporium.

Manganese peroxidase (MnP) is a heme-containing enzyme produced by white-rot fungi and is part of the extracellular lignin degrading system in these organisms. MnP is unique among Mn binding enzymes in its ability to bind and oxidize Mn(II) and efficiently release Mn(III). Initial site-directed mutagenesis studies identified the residues E35, E39, and D179 as the Mn binding ligands. However, an E39D variant was recently reported to display wild-type Mn binding and rate of oxidation, calling into question the role of E39 as an Mn ligand. To investigate this hypothesis, we performed computer modeling studies which indicated metal-ligand bond distances in the E39D variant and in an E35D--E39D--D179E triple variant which might allow Mn binding and oxidation. To test the model, we reconstructed the E35D and E39D variants used in the previous study, as well as an E39A single variant and the E35D--E39D--D179E triple variant of MnP isozyme 1 from Phanerochaete chrysosporium. We find that all of the variant proteins are impaired for Mn(II) binding (K(m) increases 20--30-fold) and Mn(II) oxidation (k(cat) decreases 50--400-fold) in both the steady state and the transient state. In particular, mutation of the E39 residue in MnP decreases both Mn binding and oxidation. The catalytic efficiency of the E39A variants decreased approximately 10(4)-fold, while that of the E39D variant decreased approximately 10(3)-fold. Contrary to initial modeling results, the triple variant performed only as well as any of the single Mn ligand variants. Interestingly, the catalytic efficiency of the triple variant decreased only 10(4)-fold, which is approximately 10(2)-fold better than that reported for the E35Q--D179N double variant. These combined studies indicate that precise geometry of the Mn ligands within the Mn binding site of MnP is essential for the efficient binding, oxidation, and release of Mn by this enzyme. The results clearly indicate that E39 is a Mn ligand and that mutation of this ligand decreases both Mn binding and the rate of Mn oxidation.

Substituent Effect on Organotin Tp* Compounds as the Tp* Reagent for the Preparation of Mono Tp* Complexes of Group 4—6 Metals (Tp* = Tris(3,5-dimethylpyrazol-1-yl)hydroborate)

Abstract Organotin compounds [Tp*SnCl3-nBun] (2: n = 1; 3: n = 2) having a Tp* ligand (Tp* = tris(3,5-dimethylpyrazol-1-yl)hydroborate) are useful reagents for introducing a Tp* ligand on Group 4 and 5 metals and chromium. The test reactions of these organotin compounds along with [Tp*SnCl3] (1) with ZrCl4, affording a known complex [Tp*ZrCl3] (4), were examined. The reaction rates were in the order 3 > 2 1. This tendency was understood by the average Sn-N(Tp* ligand) bond distance being longer in the order 3 > 2 > 1 as revealed by the crystallographic studies and the orbital interactions between the Tp* ligand and the SnCl3-nBun fragments estimated by the ab initio calculations for 1—3. On the basis of the above findings, we applied the compound 3 to prepare Tp* complexes of Group 4—6 metals. Reactions of TiCl4 with an equimolar amount of 3 afforded [Tp*TiCl3] (7) in good yields after a simple rinse to remove the resulting tin compound, SnBu2Cl2. [Tp*HfCl3] (8) was prepared in moderate yield by reaction of [HfCl4(THF)2] with 3 under severe conditions. Similarly, the reactions of 3 with a stoichiometric amount of [NbCl4(THF)2] or [NbCl5(OEt)2] and TaCl5 in toluene gave [Tp*NbCl3] (9) and [Tp*TaCl3][TaCl6] (10), respectively. We also applied our synthetic method to prepare Tp* complexes of vanadium, [Tp*VCl2(THF)] (11), [Tp*VCl2(DNAP)] (12) (DMAP = 4-dimethylaminopyridine), and [Tp*VCl2(=NC6H3Me2-2,6)] (13) and those of chromium, [Tp*CrCl2(L)] (14: L = THF; 15: L = H2O; 16: L = DMAP). Some of these Tp* complexes, 4, 8, 12, 13, 15, and 16 were crystallographically characterized to have discrete octahedral geometry containing the facial coordination of the Tp* ligand.

Different Ground Spin States in Iron(III) Complexes with Quadridentate Schiff Bases: Synthesis, Crystal Structures, and Magnetic Properties

The synthesis, structure, and magnetic characterization of [Fe(L1)(HIm)2] Y (Y = ClO4- (1a), PF6- (1b), and BPh4- (1c)), [Fe(L2)(HIm)2]ClO4 (2a), [Fe(L2)(HIm)2]ClO4·H2O (2b), [NaFe(L2)(HIm)2(ClO4)2] (2c), [Fe(L3)(HIm)2]ClO4 (3), [Fe(L4)(HIm)2]ClO4 (4a), and [Fe(L4)(HIm)2] BPh4·H2O (4b) are reported (L1 = N,N‘-3,4-toluenebis(3-ethoxysalicylideneimine), L2 = N,N‘-4-chloro-o-phenylenebis(3-methoxysalicylideneimine), L3 = N,N‘-4-chloro-o-phenylenebis(3-ethoxysalicylideneimine), L4 = N,N‘-1,2-propylenebis(3-methoxysalicylideneimine), and HIm = imidazole). Compound 1a crystallizes in the triclinic system, space group P1̄, Z = 2, with a = 10.609(3) Å, b = 10.762(3) Å, c = 15.043(3) Å, α = 105.13(2)°, β = 90.67(2)°, and γ = 102.09(2)°. Compound 2c crystallizes in the monoclinic system, space group C2/c, Z = 4, with a = 14.969(6) Å, b = 17.324(6) Å, c = 13.050(10) Å, and β = 100.85(6)°. Compound 4a crystallizes in the orthorhombic system, space group P212121, Z = 4, with a = 12.402(2) Å, b = 14.354(2) Å, c = 15.773(2) Å. The structures of 1a and 4a are made up of discrete [Fe(L1)(HIm)2]+ and [Fe(L4)(HIm)2]+ cationic units, while 2c consists of dinuclear neutral entities [NaFe(L2)(HIm)2(ClO4)2]. The iron(III) ion is six-coordinated, the equatorial plane being occupied by the cis tetradentate Schiff base and the axial positions by the imidazole ligands. The metal−ligand bond distances as well as the relative orientation of the imidazole ligands are discussed in relation with the spin state of the title compounds. The magnetic properties reveal that 1a−c, 2a, 3, and 4b are high-spin species while 2c is a low-spin complex, and 2b and 4a display spin-crossover behavior. The thermodynamic model of Slichter and Drickamer was applied to account for the temperature variable high-spin molar fraction deduced from the magnetic behavior. The intermolecular interaction parameter, Γ, the enthalpy, ΔH, and the entropy, ΔS, changes associated with the spin transition of 2b were estimated as Γ = 1.6 kJ mol-1, ΔH = 12 kJ mol-1, and ΔS = 60 J mol-1 K-1.

Metal−Ligand Bond Distances in First-Row Transition Metal Coordination Compounds: Coordination Number, Oxidation State, and Specific Ligand Effects

Mean metal−ligand bond distances for the coordination ligands isothiocyanate, pyridine, imidazole, water, and chloride, bound to the transition metals Mn, Fe, Co, Ni, Cu, and Zn in their 2+ oxidation states, were collected from searches the Cambridge Structure Database. The metal−ligand bond distances were converted to bond orders through the bond distance-bond order technique, as suggested by Pauling. The mean bond order sums at the 2+ metal centers were found to be independent of coordination number or geometry and to be strongly ligand-dependent; the values (by ligand) are as follows: isothiocyanate = 2.56 ± 0.13; imidazole = 2.13 ± 0.04; chloride = 2.12 ± 0.07; pyridine 1.95 ± 0.10; water = 1.88 ± 0.10. The bond order sum for Fe(III) bound to chloride was found to be 3.09, approximately one bond order unit larger than for the 2+ metal centers bound to chloride. Division of the ligand-specific bond order sums by coordination number allows prediction of the M−L bond distance to within 0.017 Å, regardless of the specific coordination geometry. The physical basis for the ligand-specific variation in bond order sum is also discussed.

X-Ray Absorption Studies of NdCo Intermetallic Compounds

X-ray K-absorption edges along with their extended x-ray absorption fine structure (EXAFS) were recorded using a Cauchois-type bent crystal spectrograph of 0.4 m radius in the transmission mode for three intermetallic compounds, Nd2Co17, NdCo5 and NdCo3. Using the graphical method for the determination of bond distances, total phase shifts δ1 and phase parameter β1 were estimated for the three intermetallics. A linear variation was observed between δ1 and k. Also, the values of average metal–ligand bond distances estimated by the different methods were in good agreement. © 1997 by John Wiley & Sons, Ltd.

A missing link in cupredoxins: crystal structure of cucumber stellacyanin at 1.6 A resolution.

Stellacyanins are blue (type I) copper glycoproteins that differ from other members of the cupredoxin family in their spectroscopic and electron transfer properties. Until now, stellacyanins have eluded structure determination. Here we report the three-dimensional crystal structure of the 109 amino acid, non-glycosylated copper binding domain of recombinant cucumber stellacyanin refined to 1.6 A resolution. The crystallographic R-value for all 18,488 reflections (sigma > 0) between 50-1.6 A is 0.195. The overall fold is organized in two beta-sheets, both with four beta-stands. Two alpha-helices are found in loop regions between beta-strands. The beta-sheets form a beta-sandwich similar to those found in other cupredoxins, but some features differ from proteins such as plastocyanin and azurin in that the beta-barrel is more flattened, there is an extra N-terminal alpha-helix, and the copper binding site is much more solvent accessible. The presence of a disulfide bond at the copper binding end of the protein confirms that cucumber stellacyanin has a phytocyanin-like fold. The ligands to copper are two histidines, one cysteine, and one glutamine, the latter replacing the methionine typically found in mononuclear blue copper proteins. The Cu-Gln bond is one of the shortest axial ligand bond distances observed to date in structurally characterized type I copper proteins. The characteristic spectroscopic properties and electron transfer reactivity of stellacyanin, which differ significantly from those of other well-characterized cupredoxins, can be explained by its more exposed copper site, its distinctive amino acid ligand composition, and its nearly tetrahedral ligand geometry. Surface features on the cucumber stellacyanin molecule that could be involved in interactions with putative redox partners are discussed.

Aminolysis of Dicationic Ruthenium Thiophene Complexes

Dicationic sandwich complexes containing thiophene, 2-methylthiophene, 2,5-dimethylthiophene, and tetramethylthiophene react with ammonia to give salts of the formula [(ring)Ru(SC{sub 4}R{sub 4}NH{sub 2})]X where ring = C{sub 6}Me{sub 6} or cymene. The thiophene, 2-methylthiophene, and 2,5-dimethylthiophene complexes undergo C-S cleavage to give iminium-thiolato derivatives. In the case of the 2,5-dimethylthiophene complex, a kinetic isomer was isolated which slowly isomerized to a thermodynamic isomer. The ammonia adducts of the tetramethylthiophene complexes [(cymene)Ru(C{sub 4}Me{sub 4}S)]{sup 2+} and [(C{sub 5}Me{sub 5})Rh(C{sub 4}Me{sub 4}S)]{sup 2+} do not undergo C-S cleavage. These 2-NH{sub 2}C{sub 4}Me{sub 4}S complexes react with protic acids to regenerate the starting dication [(ring)M(C{sub 4}Me{sub 4}S)]{sup 2+}. The aniline adducts of thiophene, 2-methylthiophene, and 2,5-dimethylthiophene are similar to the ammonia derivatives. The structures of the kinetic isomer of [(C{sub 6}Me{sub 6})Ru(SC{sub 3}H{sub 2}MeCHNHPh)]PF{sub 6} and the thermodynamic isomer of [(cymene)Ru(SC{sub 3}H{sub 2}MeCMeNHPh)]OTf were established by single-crystal X-ray diffraction. The crystallographic study proves that the isomerization in this family of compounds arises from the relative configuration at the terminal carbon of the alkenyl thiolate ligand Bond distance data indicate an interaction between the iminium carbon center and the Ru atom in the kinetic isomer. 20 refs., 2 figs., 5 tabs.