D Bond Research Articles

Redox Potential Tuning of Dimolybdenum Systems through Systematic Substitution by Guanidinate Ligands.

We report the synthesis and characterization of a series of dimolybdenum paddlewheel complexes of the type Mo2(DAniF)4-n(hpp)n (n = 1-3), where DAniF is the anion of N,N'-di-p-anisyl-formamidine and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine. The effect on the electronic structure of these tetragonal paddlewheel dimolybdenum compounds was studied upon systematic substitution of formamidinate ligands by the more basic guanidinates. Mo-Mo distances in the paddlewheel structures decreased upon guanidinate ligand substitution, and were found to be 2.0844(6) and 2.0784(6), for Mo2(DAniF)3(hpp) (1) and trans-Mo2(DAniF)2(hpp)2 (2), respectively. Electrochemical studies show that the half-wave potential of the Mo25+/Mo24+ couple shifts cathodically upon ancillary ligand substitution ranging from -0.286 V for the tetraformamidinate complex to -1.795 V for the tetraguanidinate analogue and with redox potentials of -0.75, -1.07, and -1.14 V for 1, 2, and 3 (Mo2(DAniF)(hpp)3), respectively. The presence of a second redox event assigned to the Mo26+/Mo25+ couple was not observed until two guanidinate ligands were introduced. Raman spectroscopy shows that the v(M-M) stretch gets systematically strengthened upon formamidinate ligand substitution by the guanidinate ligand hpp. The induced delta bond destabilization by the basic hpp ligand was measured using DFT calculations by tracking the energy of the frontier orbitals. The decrease in the HOMO-LUMO energy gap was supported by the red shift in the UV-vis spectra of the compounds: 412, 442, and 450 nm for 1, 2, and 3, respectively.

Laser assisted joining of St12 to polycarbonate: Experimental study and numerical simulation

In this study dissimilar joining of St12 to Polycarbonate is accomplished by a Nd: YAG pulsed laser and examined by finite element (FE) model and analysed by statistical method. Several experiments are carried out to materialize a direct joint between St12 to Polycarbonate. To investigate thermal phenomena of the laser joining process a FE model is developed by Abaqus software. To approximate heat source distribution, a Cylindrical-Involution-Normal model is programmed in FORTRAN language. To find out the sensitivity of the FE model to the elements size, a number adjustment tests are used and the FE model is validated by experimental data. Effects of laser power (190−230 W) and laser scanning speed (3.6–7.6 mm/s) on average bond width (AW), delta bond width (DW), and maximum of temperature profile (MT) have been investigated via response surface methodology. Results reveal that power of laser is the determinant variable of average bond width and maximum temperature profile, however, scanning speed of laser is the most effective variable on delta bond width. An appropriate process window required to achieve a sound dissimilar joint (without any decomposition of the polymer) is finally suggested.

Electronic structure and spectroscopy of homoleptic compounds of dimolybdenum using TDDFT

Ten compounds of dimolybdenum are studied using density functional theory and time-dependent density functional theory. The energy of the strongest symmetry-allowed bands is calculated. The results are then compared with experimental data, when available. The PW91 functional gives results for geometry and for the energy of the δ→δ* band that show good agreement with experimental data. However, the B3LYP functional gives more realistic values for the whole spectrum when the results are compared with experimental data. Finally, the different values of energy of these bands are explained analyzing the molecular orbitals involved in these transitions. Some ligands can act as an unsaturated system in conjugation with the delta bond, modifying the energies of the electronic transitions.

Pushing the Limits of Delta Bonding in Metal-Chromium Complexes with Redox Changes and Metal Swapping.

Into the metalloligand Cr[N(o-(NCH2P((i)Pr)2)C6H4)3] (1, CrL) was inserted a second chromium atom to generate the dichromium complex Cr2L (2), which is a homobimetallic analogue of the known MCrL complexes, where M is manganese (3) or iron (4). The cationic and anionic counterparts, [MCrL](+) and [MCrL](-), respectively, were targeted, and each MCr pair was isolated in at least one other redox state. The solid-state structures of the [MCrL](+,0,-) redox members are essentially the same, with ultrashort metal-metal bonds between 1.96 and 1.74 Å. The formal shortness ratios (r) of these interactions are between 0.84 and 0.74 and are interpreted as triple to quintuple metal-metal bonds with the aid of theory. The trio of (d-d)(10) species [Cr2L](-) (2(red)), MnCrL (3), and [FeCrL](+) (4(ox)) are S = 0 diamagnets. On the basis of M-Cr bond distances and theoretical calculations, the strength of the metal-metal bond across the (d-d)(10) series increases in the order Fe < Mn < Cr. The methylene protons in the ligand are shifted downfield in the (1)H NMR spectra, and the diamagnetic anisotropy of the metal-metal bond was calculated as -3500 × 10(-36), -3900 × 10(-36), and -5800 × 10(-36) m(3) molecule(-1) for 2(red), 3, and 4(ox) respectively. The magnitude of diamagnetic anisotropy is, thus, affected more by bond polarity than by bond order. A comparative vis-NIR study of quintuply bonded 2(red) and 3 revealed a large red shift in the δ(4) → δ(3)δ* transition energy upon swapping from the (Cr2)(2+) to the (MnCr)(3+) core. Complex 2(red) was further investigated by resonance Raman spectroscopy, and a band at 434 cm(-1) was assigned as the Cr-Cr bond vibration. Finally, 4(ox) exhibited a Mössbauer doublet with an isomer shift of 0.18 mm/s that suggests a primarily Fe-based oxidation to Fe(I).

ChemInform Abstract: The Magic of the Delta Bonds: How to Modulate Electronic Communication in Pairs‐of‐Pairs Containing Multiple Bonded Mo2 Species

AbstractReview: 45 refs.

From gas-phase ionization energies to solution oxidation potentials: Dimolybdenum tetraformamidinate paddlewheel complexes

The gas-phase ionization energies of a series of Mo2(DPhF)4 paddlewheel complexes (DPhF is the N,N′-diphenylformamidinate anion with p-CH3, p-Cl, m-Cl, p-CF3, or m-CF3 phenyl substituents) have been measured by ultraviolet photoelectron spectroscopy (UPS) and compared with the solution oxidation potentials measured by cyclic voltammetry (CV) reported by Ren and coworkers. A linear relationship was found between the gas-phase ionization energies and the solution oxidation potentials. Density functional theory (DFT) computations clarify the individual electronic and thermodynamic factors that contribute to the correlation. The metal–metal delta bond electron energy is the largest factor in determining the solution oxidation potential. The substituents shift the metal–metal orbital energies by changing the through-space field potential at the metals rather than by an inductive change in charge at the metals or orbital overlap effects. The cation solvation energies determine the extent that the potential shifts are attenuated in solution. The results show that substituent field effects and solvation have major roles in determining the dimetal redox chemistry even when the dimetal unit is protected from direct interaction with the substituent and the solvent.

Fitting the Pieces of the Puzzle: The Delta Bond

The first compound with a quadruple bond was described by Al Cotton et al on August of 1964 (Cotton, F. A.; Curtis, N. F.; Harris, C. B.; Johnson, B. F. G.; Lippard, S. J.; Mague, J. T.; Robinson, W. R.; Wood, J. S. Science 1964, 145, 1305.). Since that time X-ray crystallography along other techniques such as EPR, electrochemistry, NMR, PES, electronic spectra, circular dichroism, magnetism, theoretical calculation have played an important role understanding of the delta bond. Here we will discuss some of the latest advances in the field.

A Magic Equation: Delta Bonds Plus Bicyclic Guanidinates Equals Strong Reducing Agents

Reactions of bicyclic guanidinates with dimolybdenum and ditungsten precursors having quadruple bonded units with a σ2π4δ2 (Q) electronic configuration have generated the most easily ionized, chemically stable species and very strong reducing agents. Analogous rhenium compounds have led to the formation of species having dimetal units in unusually high oxidation states. Here we review this chemistry and the accounts that explain such behaviour that has been attributed to the interaction of the π electrons of the guanidinate C(N)3 core with the electrons that give rise to the delta bond of the dimetal units.

The usefulness of EPR spectroscopy in the study of compounds with metal–metal multiple bonds

This Perspective reviews some of the contributions that EPR spectroscopy has made for the understanding of the bonding and electronic structure of molecular species with metal-to-metal multiple bonds. The goal is to show how useful this technique can be in (a) elucidating whether unpaired electrons reside in metal-based or ligand-based molecular orbitals, (b) providing information on the metal center's total electronic spin, an aspect that offers support to its oxidation state, (c) informing about the distribution of the unpaired electrons between the metals and the organic ligand, (d) detecting molecular dynamics and phase transitions to complement X-ray studies and (e) using high-frequency, high-field EPR for studying compounds that might be EPR-silent when only commonly available spectrometers are used. A brief comparison with NMR is provided in the ESI where it is noted that for typical laboratory magnetic fields, the energy gap between resonant states in EPR is in the microwave region of the electromagnetic spectrum (GHz) while for NMR it is in the radiofrequency range (MHz). Our aim is to encourage the inorganic chemistry community and more broadly other chemists and physicists to collaborate while characterizing new paramagnetic inorganic complexes. Enough basic background is provided as ESI to allow senior-level undergraduate and graduate students to understand the simplicity as well as the power of EPR spectroscopy with a view to encourage them to use this technique in their own research activities. Finally, it should be noted that this report commemorates the fiftieth anniversary of the description in Science of the first species with a quadruple bond.

Evidence of Disruption of Conjugation Involving Delta Bonds in Intramolecular Electronic Coupling

A dimer of dimers containing two quadruply bonded [Mo(2)(DAniF)(3)](+) units (DAniF = N,N'-di(p-anisyl)formamidinate) linked by the S-donor linker, dimethyldithiooxamidate was synthesized, structurally characterized, and electronic communication was probed. The core of [Mo(2)(DAniF)(3)](2)(C(2)S(2)N(2)Me(2)), 1, formed by the Mo(2)NSC(2)SNMo(2) atoms shows two fused but non planar six-membered rings, which differs from that of the beta form of dimethyloxamidate analogue that has a heteronaphthalene-type structure (Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Villagran, D.; Wang, X. J. Am. Chem. Soc. 2004, 126, 14822). For these two analogous compounds electronic coupling between the two [Mo(2)] units, as determined by electrochemical measurements, diminishes considerably upon replacement of O-donor by S-donor atoms (DeltaE(1/2) = 531 mV and 440 mV, respectively). This suggests that the non planar conformation of the linker in 1 hampers a pathway leading to pi conjugation. Density functional theory (DFT) calculations show that the highest occupied molecular orbitals HOMO-HOMO-1 energy gap of 0.12 eV for 1 is much smaller than that of 0.61 eV for the O-donor analogue, which is consistent with the electrochemical data.

Journey from Mo−Mo Quadruple Bonds to Quintuple Bonds

Heating K(4)Mo(2)Cl(8) and 2 equiv of Li[RC(NDipp)(2)] (R = H, Ph; Dipp = 2,6-i-Pr(2)C(6)H(3)) in tetrahydrofuran (THF) at 60 degrees C gives two paddlewheel type quadruply bonded dimolybdenum complexes, Mo(2)(mu-Cl)[Cl(2)Li(OEt(2))][mu-eta(2)-RC(N-2,6-i-Pr(2)C(6)H(3))(2)](2) (R = H (1), Ph (2)). The Mo-Mo bond lengths of 1 and 2 are 2.0875(4) and 2.0756(8) A, respectively, indicating typical Mo-Mo quadruple bonds. Reduction of 1 and 2 by two electrons results in the isolation of their corresponding Mo-Mo quintuple bonded complexes, Mo(2)[mu-eta(2)-RC(N-2,6-i-Pr(2)C(6)H(3))(2)](2) (R = H (3), Ph (4)), and the Mo-Mo bond lengths dramatically decrease to 2.0187(9) A (3) and 2.0157(4) A (4), a consequence of the formation of the second delta bond and representing the shortest metal-metal bonds beyond the first row elements. The Mo-Mo quintuple bonding characters are corroborated by DFT calculations at the level of BP86/def2-TZVP and BP86/def2-TZVPP.

Structural Requirements of Cholesterol for Binding to Vibrio cholerae Hemolysin

Cholesterol is necessary for the conversion of Vibrio cholerae hemolysin (VCH) monomers into oligomers in liposome membranes. Using different sterols, we determined the stereochemical structures of the VCH-binding active groups present in cholesterol. The VCH monomers are bound to cholesterol, diosgenin, campesterol, and ergosterol, which have a hydroxyl group at position C-3 (3betaOH) in the A ring and a C-C double bond between positions C-5 and C-6 (C-C Delta(5)) in the B ring. They are not bound to epicholesterol and dihydrocholesterol, which form a covalent link with a 3alphaOH group and a C-C single bond between positions C-5 and C-6, respectively. This result suggests that the 3betaOH group and the C-CDelta(5) bond in cholesterol are required for VCH monomer binding. We further examined VCH oligomer binding to cholesterol. However, this oligomer did not bind to cholesterol, suggesting that the disappearance of the cholesterol-binding potential of the VCH oligomer might be a result of the conformational change caused by the conversion of the monomer into the oligomer. VCH oligomer formation was observed in liposomes containing sterols with the 3betaOH group and the C-C Delta(5) bond, and it correlated with the binding affinity of the monomer to each sterol. Therefore, it seems likely that monomer binding to membrane sterol leads to the assembly of the monomer. However, since oligomer formation was induced by liposomes containing either epicholesterol or dihydrocholesterol, the 3betaOH group and the C-C Delta(5) bond were not essential for conversion into the oligomer.

A Front-end Desaturase from Chlamydomonas reinhardtii Produces Pinolenic and Coniferonic Acids by ω13 Desaturation in Methylotrophic Yeast and Tobacco

Pinolenic acid (PA; 18:3Delta(5,9,12)) and coniferonic acid (CA; 18:4Delta(5,9,12,15)) are Delta(5)-unsaturated bis-methylene-interrupted fatty acids (Delta(5)-UBIFAs) commonly found in pine seed oil. They are assumed to be synthesized from linoleic acid (LA; 18:2Delta(9,12)) and alpha-linolenic acid (ALA; 18:3Delta(9,12,15)), respectively, by Delta(5)-desaturation. A unicellular green microalga Chlamydomonas reinhardtii also accumulates PA and CA in a betain lipid. The expressed sequence tag (EST) resource of C. reinhardtii led to the isolation of a cDNA clone that encoded a putative fatty acid desaturase named as CrDES containing a cytochrome b5 domain at the N-terminus. When the coding sequence was expressed heterologously in the methylotrophic yeast Pichia pastoris, PA and CA were newly detected and comparable amounts of LA and ALA were reduced, demonstrating that CrDES has Delta(5)-desaturase activity for both LA and ALA. CrDES expressed in the yeast showed Delta(5)-desaturase activity on 18:1Delta(9) but not 18:1Delta(11). Unexpectedly, CrDES also showed Delta(7)-desaturase activity on 20:2Delta(11,14) and 20:3Delta(11,14,17) to produce 20:3Delta(7,11,14) and 20:4Delta(7,11,14,17), respectively. Since both the Delta(5) bond in C18 and the Delta(7) bond in C20 fatty acids are 'omega13' double bonds, these results indicate that CrDES has omega13 desaturase activity for omega9 unsaturated C18/C20 fatty acids, in contrast to the previously reported front-end desaturases. In order to evaluate the activity of CrDES in higher plants, transgenic tobacco plants expressing CrDES were created. PA and CA accumulated in the leaves of transgenic plants. The highest combined yield of PA and CA was 44.7% of total fatty acids, suggesting that PA and CA can be produced in higher plants on a large scale.

Geometric and Electronic Structure of the Heme−Peroxo−Copper Complex [(F8TPP)FeIII−(O22-)−CuII(TMPA)](ClO4)

The geometric and electronic structure of the untethered heme-peroxo-copper model complex [(F(8)TPP)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](ClO(4)) (1) has been investigated using Cu and Fe K-edge EXAFS spectroscopy and density functional theory calculations in order to describe its geometric and electronic structure. The Fe and Cu K-edge EXAFS data were fit with a Cu...Fe distance of approximately 3.72 A. Spin-unrestricted DFT calculations for the S(T) = 2 spin state were performed on [(P)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](+) as a model of 1. The peroxo unit is bound end-on to the copper, and side-on to the high-spin iron, for an overall mu-eta(1):eta(2) coordination mode. The calculated Cu...Fe distance is approximately 0.3 A longer than that observed experimentally. Reoptimization of [(P)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](+) with a 3.7 A Cu...Fe constrained distance results in a similar energy and structure that retains the overall mu-eta(1):eta(2)-peroxo coordination mode. The primary bonding interaction between the copper and the peroxide involves electron donation into the half-occupied Cu d(z)2 orbital from the peroxide pi(sigma) orbital. In the case of the Fe(III)-peroxide eta(2) bond, the two major components arise from the donor interactions of the peroxide pi*(sigma) and pi*(v) orbitals with the Fe d(xz) and d(xy) orbitals, which give rise to sigma and delta bonds, respectively. The pi*(sigma) interaction with both the half-occupied d(z)2 orbital on the copper (eta(1)) and the d(xz) orbital on the iron (eta(2)), provides an effective superexchange pathway for strong antiferromagnetic coupling between the metal centers.

Diuranium inverted sandwiches involving naphthalene and cyclooctatetraene.

Treatment of IU(DME)(NC[(t)Bu]Mes)(3) (2-I-DME) with 4 equiv of KC(8) and 0.5 equiv of naphthalene in DME allowed the isolation of a naphthalene-bridged compound, K(2)(mu-eta(6),eta(6)-C(10)H(8))[U(NC[(t)Bu]Mes)(3)](2) (K(2)-2(2)-mu-C(10)H(8)), in 60% yield as a dark brown powder. The twelve U-C distances are rather short, varying from 2.565(11) to 2.749(10) A. Treatment of M(2)-2(2)-mu-C(10)H(8) (M = Na, K) with 2 equiv of 1,3,5,7-cyclooctatetraene afforded a mixture of two products: M-2-COT and 2(2)-mu-COT. Compound 2(2)-mu-COT can be assembled independently in 90% yield by salt elimination upon reaction of M-2-COT with iodide 2-I-DME. The U-C(arene) distance in compound 2(2)-mu-COT is longer than that in its naphthalene counterpart K(2)-2(2)-mu-C(10)H(8)(2.822 vs 2.634 A), in accord with bonding considerations. A DFT study performed on model compounds for both M(2)-2(2)-mu-C(10)H(8) and 2(2)-mu-COT indicates that the delta bonds present in the former compound show better covalent overlap.

The sterol C-14 reductase encoded by the Neurospora crassa erg-3 gene: essential charged and polar residues identified by site-specific mutagenesis.

Sterol C-14 reductase catalyses the reduction of the Delta(14,15) bond in intermediates in the sterol biosynthesis pathway using NADPH as a cofactor. We have undertaken a systematic site-directed mutational analysis of all the conserved charged and potentially proton-donating residues of the sterol C-14 reductase from Neurospora crassa. The effect of each mutation was determined using an in vivo assay based on the complementation of the corresponding N. crassa mutant ( erg-3). The non-complementing mutations were also tested in the erg24 mutant of Saccharomyces cervisiae. The results are discussed with reference to the predicted topology of the enzyme and to its proposed catalytic mechanism, which involves addition of a proton from an appropriately positioned charged or polar residue to the substrate double bond, followed by addition of hydride ion from NADPH.

Analytic environment-dependent tight-binding bond integrals: application to MoSi2.

We present the first derivation of explicit analytic expressions for the environmental dependence of the sigma, pi, and delta bond integrals within the orthogonal two-center tight-binding approximation by using the recently developed bond-order potential theory to invert the nonorthogonality matrix. We illustrate the power of this new formalism by showing that it not only captures the transferability of the bond integrals between elemental bcc Mo and Si and binary C11(b) MoSi2 but also predicts the absence of any discontinuity between first and second nearest neighbors for the ddsigma bond integral even though large discontinuities exist for ppsigma, pppi, and ddpi.

The whole story of the two-electron bond, with the delta bond as a paradigm.

It is shown that the delta bond, as found particularly in the Re(2)(6+) and Mo(2)(4+) cores of hundreds of compounds, provides a paradigm for the behavior of two-electron bonds of all types. By control of the angle of twist around the M-M axis, the strength of the bond can be systematically varied. By means of conventional electronic spectroscopy, nuclear magnetic resonance spectroscopy, and two-photon excitation spectroscopy, the entire picture of the manifold of four states for two electrons bonding two atoms, as first described by Coulson and Fischer in 1949, has been confirmed.

Resonance Raman, X-ray Crystallographic, and Magnetic Susceptibility Studies of Metal-Metal-Bonded MoRu and WOs Porphyrin Dimers. 1. Evidence for an Unusual MO Diagram.

Solution (VT NMR, Evans method magnetic susceptibility, resonance Raman) and solid-state (SQUID magnetic susceptibility, X-ray crystallography) spectroscopic studies of intertriad heterodimeric [(OEP)MoRu(OEP)] (1), [(OEP)WOs(OEP)] (2), and [(OEP)MoRu(TPP)]PF(6) (3(+)) metalloporphyrins are reported (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinato; TPP = 5,10,15,20-tetraphenylporphyrinato). Solution and solid-state magnetic susceptibility data indicate that 1 and 2 contain two unpaired electrons in the ground electronic configuration. The presence of a delta bond in 3(+) has been confirmed by structural characterization. The experimental evidence is consistent with a molecular orbital ordering, sigma < pi < delta < pi < delta, which is different from that seen for homologous metalloporphyrin dimers with homometallic or intratriad heterometallic multiple metal-metal bonds. Resonance Raman data suggest that the heterometallic bonds are slightly stronger than isoelectronic homometallic species.

Phase Transition Behavior and Molecular Structures of Monounsaturated Phosphatidylcholines: CALORIMETRIC STUDIES AND MOLECULAR MECHANICS SIMULATIONS

High resolution differential scanning calorimetric studies were performed to investigate the thermotropic phase behavior of 26 molecular species of sn-1 saturated/sn-2 monounsaturated phosphatidylcholines. In parallel with calorimetric studies, the energy-minimized structures and steric energies of the diglyceride moieties of these monoenoic lipids were determined using a molecular mechanics approach. The combined calorimetric and computational studies led to the following results and conclusions. (i) When a single cis-carbon-carbon double bond (delta) is incorporated into a saturated diacylphosphatidylcholine molecule at any position within the central segment of the long sn-2 acyl chain, the resulting monoenoic lipid molecules will, in excess water, exhibit reduced phase transition temperature (Tm) and transition enthalpy (delta H) as they undergo the gel to liquid-crystalline phase transition. The Tm and delta H-lowering effects of the delta bond can be attributed to a decrease in the chain length of the sn-2 acyl chain, a change in the chain length difference between the sn-1 and sn-2 acyl chains, and a local perturbation of the chain-chain van der Waals interaction in the vicinity of the delta bond. (ii) For a series of positional isomers of 1-stearoyl-2-cis-octadecenoylphosphatidylcholine, C(18):C(18:1 delta n)PC, with a delta bond at different positions along the sn-2 acyl chain, the Tm value depends critically on the position of the delta bond. Specifically, the Tm value is minimal as the delta bond is located at the geometric center of the linear segment of the sn-2 acyl chain, and the Tm value is progressively increased as the delta bond migrates toward either end of the sn-2 acyl chain. (iii) The various monoenoic phosphatidylcholines under study can be divided into two groups. The Tm values of most lipids in each group can be correlated in an identical manner with their structural parameters, yielding a common Tm-structure relationship.