Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes

The coordination chemistry of the N-heterocyclic carbene ligand IMes^(NMe2)2, derived from the well-known IMes ligand by substitution of the carbenic heterocycle with two dimethylamino groups, was investigated with d⁶ [Mn(I), Fe(II)], d⁸ [Rh(I)], and d¹⁰ [Cu(I)] transition-metal centers. The redox behavior of the resulting organometallic complexes was studied through a combined experimental/theoretical study, involving electrochemistry, EPR spectroscopy, and DFT calculations. While the complexes [CuCl(IMes^(NMe2)2)], [RhCl(COD)(IMes^(NMe2)2)], and [FeCp(CO)₂ (IMes^(NMe2)2)](BF₄) exhibit two oxidation waves, the first oxidation wave is fully reversible but only for the first complex the second oxidation wave is reversible. The mono-oxidation event for these complexes occurs on the NHC ligand, with a spin density mainly located on the diaminoethylene NHC-backbone, and has a dramatic effect on the donating properties of the NHC ligand. Conversely, as the Mn(I) center in the complex [MnCp(CO)₂ ((IMes^(NMe2)2)] is easily oxidizable, the latter complex is first oxidized on the metal center to form the corresponding cationic Mn(II) complex, and the NHC ligand is oxidized in a second reversible oxidation wave.     

Influence of the central metal on the crystal structures and electronic structures of biferrocene trinuclear complexes

Five metal-bridged biferrocene complexes of the Schiff-base ligand (HL = S-benzyl-N-(ferrocenyl-1-methyl-methylidene)dithiocarbazate) have been studied by single crystal X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy. The crystal structures of the complexes show that the central metal ions are tetra-coordinated by two ligands in two modes: the central d⁸ transition metal ions (Ni²⁺, Pd²⁺, and Pt²⁺) are nearly square-planar coordinated and the d¹⁰ transition metal ions (Zn²⁺ and Cd²⁺) are tetrahedrally coordinated. Interestingly, the isomer shifts in ⁵⁷Fe Mössbauer spectroscopy are also of two kinds: d⁸ transition metal ions (0.097-0.247 mm/s) and d¹⁰ transition metal ions (0.416-0.435 mm/s).     

The effect of the functional,basis set,and solvent in the simulation of the geometry and spectroscopic properties of VIVO2+ complexes. chemical and biological applications

The geometry of 32 V^IVO²⁺ complexes with different donor set, electric charge, geometry, arrangement of the ligands with respect to the V?O bond and type of ligand was calculated by density functional theory methods. 32 V?O, 45 V? O, 16 V? OH, 40 V? N, 24 V? S, and 14 V? Cl bonds were examined. The performance of several functionals (B3LYP, B3P86, B3PW91, HCTH, TPSS, PBE0, and MPW1PW91), keeping constant the Pople triple‐zeta basis sets 6‐311g, was tested. The order of accuracy of the functional in the prediction of the bond distances, expressed in terms of mean of the deviation Δd (Δd = d^calcd ? d^exptl) and absolute deviation |Δd| (|Δd| = |d^calcd ? d^exptl|) from the experimental values and of the corresponding standard deviations (SD(Δd) and SD(|Δd|)), is: B3P86 ～ PBE0 ～ MPW1PW91 > B3PW91 ? TPSS > B3LYP ? HCTH. In the gas phase the prediction of V?O, V? O, V? N bond lengths is rather good, but that of V? OH, V? S and V? Cl distances is by far worse. An improvement in the optimization of V? S and V? Cl lengths is reached by adding polarization and diffuse functions on the sulfur and chlorine atoms. Finally, a general improvement in the prediction of all the calculated bond lengths and angles is obtained by simulating the structures in the solvent where they are isolated within the framework of the polarizable continuum model. The last choice allows also to improve the prediction of structural (the deviation of a penta‐coordinate geometry toward the trigonal bipyramid) and spectroscopic parameters (⁵¹V and ¹⁴N hyperfine coupling constants and ¹⁴N nuclear quadrupolar coupling constant). In most of the cases, the structures optimized in solution closely approach the experimental ones and this can be of great help in the simulations of naturally occurring vanadium compounds and metal site of V‐proteins, like amavadin and the reduced form of vanadium bromoperoxidase (VBrPO). © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Synthesis,Crystal Structure and Spectroscopic Properties of Bis(1‐(E)‐2‐formylpyridine semicarbazone)nickel(II) Diperchlorate Monohydrate

Pale‐green crystals of the title complex were prepared by reaction of 2‐formylpyridine semicarbazone (HCSpy) and nickel(II) perchlorate in boiling ethanol. The crystals are triclinic with the nickel ion in an octahedral arrangement, coordinated by two nitrogen atoms and one oxygen donor atom from each ligand molecule. The effect of coordination on bond lengths and angles was explored by comparison with the single‐crystal structure data of the free ligand HSCpy, which was collected as well. The assumed coordination mode was supported by ¹H and ¹³C NMR spectroscopic data. A detailed analysis of the electronic properties, including semi‐empirical quantummechanical calculations is presented. Furthermore, the data obtained from magnetic susceptibility and EPR measurements are in accordance with a low‐spin d⁸ nickel(II) complex.     

μ-η-s-trans-Butadieneoctacarbonyldiiron(0) - Structural and spectroscopic properties

The crystal structure of [{Fe(CO)₄}₂(μ-η^2:2-s-trans-C₄H₆)] was determined by single-crystal X-ray diffraction at 90 K. The complex is located on a center of symmetry in the triclinic space group P1‾. The central C-C bond of the s-trans-butadiene ligand is slightly longer compared to non-coordinated s-trans-butadiene. The Fe-C_ax bond lengths are slightly longer than d(Fe-C_eq) in agreement with marginally shorter d(C≡O_ax) compared to d(C≡O_eq). In addition, the title complex was characterized by IR and Raman as well as NMR spectroscopy and the data are interpreted by the aid of results of DFT calculations.     

Synthesis,Structure, and Bonding of d3 Molybdenum–Oxo Complexes

Reduction of d² metal–oxo ions of the form [MO(PP)₂Cl]⁺ (M=Mo, W; PP=chelating diphosphine) produces d³ MO(PP)₂Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)₂Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)₂Cl indicate the d³ compounds are low spin with a ²[(d_xy)²(π*(MoO))¹] configuration. X‐ray crystallographic and vibrational‐spectroscopic studies on d² and d³ [MoO(dppe)₂Cl]^0/+ establish that the d³ compound possesses a reduced M?O bond order and significantly longer Mo?O bond, accounting for its greater reactivity. These results indicate that the oxo‐centered reactivity of d³ complexes may be controlled through ligand variation.     

Synthesis,Structure, and Bonding of d3 Molybdenum–Oxo Complexes

Reduction of d² metal–oxo ions of the form [MO(PP)₂Cl]⁺ (M=Mo, W; PP=chelating diphosphine) produces d³ MO(PP)₂Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)₂Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)₂Cl indicate the d³ compounds are low spin with a ²[(d_xy)²(π*(MoO))¹] configuration. X-ray crystallographic and vibrational-spectroscopic studies on d² and d³ [MoO(dppe)₂Cl]^0/+ establish that the d³ compound possesses a reduced M−O bond order and significantly longer Mo−O bond, accounting for its greater reactivity. These results indicate that the oxo-centered reactivity of d³ complexes may be controlled through ligand variation.     

The electronic structure of thorium monoxide: Ligand field assignment of states in the range 0–5 eV

Configuration interaction ligand field theory (CI LFT) calculations of the electronic energy levels of ThO were performed by treating the molecular electronic states as Th ^{2 +} free-ion levels perturbed by the ligand field of O²⁻. Twenty nine experimentally characterized ThO v = 0 energy levels, together with the energy difference between the v = 0 levels of the Y and W states were fitted using a CI LFT model that included Th ^{2 +} 7s ² , 6d7s, 6d², 7s7p, 6d7p, 5f7s, and 7p² configurations. Predictions from these calculations were used to provide tentative assignments for 171 out of 250 ThO band heads listed by Gatterer et al. [“Molecular Spectra of Metallic Oxides”, Specola Vaticana (1957)]. Term energies for 30 electronic states have been determined based on these assignments. Subsequently, the CI LFT model was refined by fitting to a set of 59 electronic term energies. The inclusion of CI effects together with integer valence, atomic-in-molecule, ionic bonding ideas reveals atomic energy level patterns that are multiply replicated in the molecular energy level patterns of six Th ^{2 +} O²⁻ atomic ion configurations (6d7s, 6d², 7s7p, 6d7p, 5f7s, and 7p²) revealing the underlying atomic ion structure that gives rise to the complex and seemingly erratic unassigned bands reported in the Vatican Atlas. © 2018 Wiley Periodicals, Inc.     

Darstellung,Kristallstruktur, Schwingungsspektren und Normalkoordinatenanalyse von (Ph4P)2[OsN(N3)5] und 15N‐NMR‐Verschiebungen von Nitridoosmaten(VI,VIII)

Synthesis, Crystal Structure, Vibrational Spectra, and Normal Coordinate Analysis of (Ph₄P)₂[OsN(N₃)₅] and ¹⁵N NMR Chemical Shifts of Nitridoosmates(VI, VIII) The treatment of (Ph₄P)[OsNCl₄] with NaN₃ yields (Ph₄P)₂[OsN(N₃)₅], which crystal structure has been determined by single crystal X‐ray diffraction analysis (monoclinic, space group P 2₁/a, a = 20.484(6), b = 11.168(1), c = 20.666(4) Å, β = 97.35(3)°, Z = 4). The IR and Raman vibrations were assigned by a normal coordinate analysis based on the molecular parameters of the X‐ray determination. The valence force constants are f_d(Os≡N) = 8.52, f_d(Os–N^α) = 1.99, f_d(N^α–N^β) = 12.42, f_d(N^β–N^γ) = 12.73 and for the azido ligand in trans‐position to the nitrido group f_d(Os–N^{α ·} ) = 1.84, f_d(N^{α ·} –N^{β ·} ) = 11.91, f_d(N^{β ·} –N^{γ ·} ) = 12.18 mdyn/Å. The ¹⁵N NMR spectra of various nitridoosmates reveal the chemical shifts δ(¹⁵N) for K[OsO₃¹⁵N] = 387.6, K₂[Os¹⁵NCl₅] = 446.7, (Ph₄P)[Os¹⁵NCl₄] = 352.9, [(n‐C₆H₁₃)₄N]₂[Os¹⁵N(N₃)₅] = 307.3 and for [(n‐Pr)₄N]₂[Os¹⁵N(¹⁵NCO)₅] = 483,7 (Os≡N), –417,7 (OsNCO_eq) und –392,8 ppm (OsNCO_ax).     

The Importance of Ligand-Induced Backdonation in the Stabilization of Square Planar d10 Nickel π-Complexes

The electronic nature of Ni π-complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π-complexes, Ni(dtbpe)(X) (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane; X=alkene or carbonyl containing π-ligands), is probed using a combination of ³¹P NMR, Ni K-edge XAS, Ni K_β XES, and DFT calculations. These complexes are best described as square planar d¹⁰ complexes with π-backbonding acting as the dominant contributor to M−L bonding to the π-ligand. The degree of backbonding correlates with ²J_PP from NMR and the energy of the Ni 1s→4p_z pre-edge in the Ni K-edge XAS data, and is determined by the energy of the π*_ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π-acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d¹⁰ electronic configuration. This ligand-induced backbonding can be formally described as a 3-centre-4-electron (3c-4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ-donors to the π*_ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.     

[AuII([12]anS4)]2+ – X‐ und Q‐Band‐EPR‐spektroskopischer Nachweis einer neuen monomeren Gold(II)‐Verbindung

[Au^II([12]anS₄)]²⁺ – X‐ and Q‐Band EPR Evidence of a New Monomeric Gold(II) Compound The reaction of [Au^IIICl₄]^– with the thiacrown ether [12]aneS₄ leads to an instable [Au^II([12]anS₄)]²⁺ complex (5d⁹, S = 1/2) which was characterized by X‐ and Q‐ band EPR. The spin Hamiltonian parameters g , A ^Au and P ^Au were derived using a program package allowing an exact diagonalisation of the spin‐Hamiltonian‐Matrix. The EPR parameters suggest the coordination of only one thiacrown ether ligand in the new Au^II complex.     

Correspondence of RuIIIRuII and RuIVRuIII Mixed Valent States in a Small Dinuclear Complex

The diruthenium(III) compound [(μ‐oxa){Ru(acac)₂}₂] [ 1 , oxa^2?=oxamidato(2?), acac^?=2,4‐pentanedionato] exhibits an S=1 ground state with antiferromagnetic spin‐spin coupling (J=?40 cm^?1). The molecular structure in the crystal of 1? 2 C₇H₈ revealed an intramolecular metal–metal distance of 5.433 Å and a notable asymmetry within the bridging ligand. Cyclic voltammetry and spectroelectrochemistry (EPR, UV/Vis/NIR) of the two‐step reduction and of the two‐step oxidation (irreversible second step) produced monocation and monoanion intermediates (K_c=10^5.9) with broad NIR absorption bands (ε ca. 2000 M ^?1 cm^?1) and maxima at 1800 ( 1 ^?) and 1500 nm ( 1 ⁺). TD‐DFT calculations support a Ru^IIIRu^II formulation for 1 ^? with a doublet ground state. The 1 ⁺ ion (Ru^IVRu^III) was calculated with an S=3/2 ground state and the doublet state higher in energy (ΔE=694.6 cm^?1). The Mulliken spin density calculations showed little participation of the ligand bridge in the spin accommodation for all paramagnetic species [(μ‐oxa){Ru(acac)₂}₂]ⁿ, n=+1, 0, ?1, and, accordingly, the NIR absorptions were identified as metal‐to‐metal (intervalence) charge transfers. Whereas only one such NIR band was observed for the Ru^IIIRu^II (4d⁵/4d⁶) system 1 ^?, the Ru^IVRu^III (4d⁴/4d⁵) form 1 ⁺ exhibited extended absorbance over the UV/Vis/NIR range.     

Synthesis and characterization of tetramethylammonium trifluorosulfate

[Me₄N]⁺[SO₂F₃]^?, the first example of a [SO₂F₃]^? salt, has been prepared from Me₄NF and SO₂F₂. The colorless, microcrystalline solid was characterized by its infrared and Raman spectra. The trigonal bipyramidal structure of C_2v symmetry of the [SO₂F₃]^? anion is predicted by ab initio calculations. Two oxygen atoms with d(SO)=143.2 pm and one fluorine atom with d(SF)=157.9 pm occupy the equatorial plane. The two fluorine atoms in the axial position with d(SF)=168.5 pm are repulsed by the two oxygen atoms forming a bent axis with ?(F_axSF_ax)=165.2°.     

Steric Control of the Electronic Ground State in Six-Coordinate Copper(II) Complexes

Replacing the 3- and 3′′-protons of the ligand 2,6-di(pyrazol-1-yl)pyridine L by mesityl groups changes the electronic ground state of [Cu(L)₂]²⁺ complexes from {d}¹ to {d}¹. This is the best example so far for a “homoleptic” Jahn–Teller-compressed six-coordinate Cu^II complex.     

Structural controlled pure metallo-triangular assembly through bisterpyridinyl Dibenzo[b,d]thiophene,Dibenzo[b,d]furan and Dibenzo[b,d]carbazole

A novel family of metallocycles was constructed by a one-pot self-assembly of three analogous bis(terpyridine) ligand monomers L1-L3, having different bent angles, with metal ions (Zn²⁺ or Cd²⁺). The dibenzo[b,d]thiophene-containing ligand L3 assembled with the metal ions to form a single trimer, whereas the dibenzo[b,d]furan-containing ligand L2 and dibenzo[b,d]carbazole-containing ligand L1 formed a mixture of trimers and tetramers. Heteroatoms (N, O, S) significantly contributed to the molecular size of the assemblies, owing to the bent angle of the bis-terpyridines ligands.     

Calix[4]arene bisphosphite ligands bearing two distal 2,2′-biphenyldioxy or 2,2′-binaphthyldioxy moieties: conformational flexibility and allyl-palladium complexes

Achiral and chiral calix[4]arene bisphosphite ligands (2 and 3) bearing two distal 2,2′-biphenyldioxyphosphinoxy and 2,2′-binaphthyldioxyphosphinoxy moieties, respectively, have been synthesized. Each of these ligands exists in two pairs of interconverting conformations in solution. The partial cone conformer (A) of the (bis)biphenyldioxyphosphinoxy ligand 2 has been separated by fractional crystallization and its structure established by X-ray crystallography. The mechanism of interconversion of the pairs of conformers (A/B and C/D) has been probed by two-dimensional NMR spectroscopy. The ¹H and ³¹P NMR evidence strongly supports a similar kind of exchange mechanism for ligand 3. Freezing of the cone conformer from the interconverting C/D pair of conformers of ligand 2 has been achieved by complexation with (allyl)palladium moieties. The methyl-allyl complex (2d) is moderately effective for catalytic regioselective allylic alkylation of crotyl acetate.     

Molecular orbital calculations on transition metal complexes : XIX. π-cyclopentadienyl-π-cyclopropenylnickel

INDO SCF molecular orbital calculations for π-cyclopentadienyl-π-cyclopropenylnickel indicate a formally d¹⁰ configuration for the metal. Calculations of the ionisation energies show that electron loss should take place first from the occupied closely grouped set of dominantly d-orbitals, and then from a mainly π-cyclopentadienyl e orbital, this being the highest occupied ligand level. This latter level shows however only a slight mixing with the metal d-orbitals, resulting in a small ligand→metal electron donation; the dominant interaction is that between the higher lying π-cyclopropenyl e level and the metal 3d_xz and 3d_yz orbitals which leads to a substantial metal→ligand charge donation. The behaviour of the π-cyclopropenyl ligand is discussed using the calculated charge distributions.     

Metal–Metal Interactions in Heterobimetallic Complexes with Dinucleating Redox‐Active Ligands

The tuning of metal–metal interactions in multinuclear assemblies is a challenge. Selective P coordination of a redox‐active PNO ligand to Au^I followed by homoleptic metalation of the NO pocket with Ni^II affords a unique trinuclear Au–Ni–Au complex. This species features two antiferromagnetically coupled ligand‐centered radicals and a double intramolecular d⁸–d¹⁰ interaction, as supported by spectroscopic, single‐crystal X‐ray diffraction, and computational data. A corresponding cationic dinuclear Au–Ni analogue with a stronger d⁸–d¹⁰ interaction is also reported. Although both heterobimetallic structures display rich electrochemistry, only the trinuclear Au–Ni–Au complex facilitates electrocatalytic C?X bond activation of alkyl halides in its doubly reduced state. Hence, the presence of a redox‐active ligand framework, an available coordination site at gold, and the nature of the nickel–gold interaction appear to be essential for this reactivity.     

Nickel(I)‐Komplexe mit 1,1′‐Bis(phosphino)ferrocenen als Liganden

Nickel(I) Complexes with 1,1′‐Bis(phosphino)ferrocenes as Ligands The thermically stable monomeric Nickel(I) complexes [(d^tbpf)Ni(acac)] ( 1 ) and [(dⁱppf)NiCl] ( 2 ) were synthesized and characterized by elemental analyses, EPR spectroscopy, and by X‐ray crystal structure analyses of single crystals (d^tbpf: 1,1′‐bis(di‐tertbutylphosphino)ferrocene; dⁱppf: 1,1′‐bis(diisopropylphosphino)ferrocene). 1 is formed by reduction of Ni(acac)₂ with triethylaluminium in the presence of d^tbpf, together with the nickel(0) complex [(d^tbpf)Ni(C₂H₄)]. 1 contains a Ni^I atom surrounded of two O‐ and two P donor atoms in a distorted tetrahedral coordination. 2 was obtained by reduction of [(dⁱppf)NiCl₂] with NaBH₄. In 2 the nickel(I) atom adopts trigonal planar coordination.     

Application of a Structure/Oxidation‐State Correlation to Complexes of Bridging Azo Ligands

Based on data from more than 40 crystal structures of metal complexes with azo‐based bridging ligands (2,2′‐azobispyridine, 2,2′‐azobis(5‐chloropyrimidine), azodicarbonyl derivatives), a correlation between the N? N bond lengths (d_NN) and the oxidation state of the ligand (neutral, neutral/back‐donating, radical‐anionic, dianionic) was derived. This correlation was applied to the analysis of four ruthenium compounds of 2,2′‐azobispyridine (abpy), that is, the new asymmetrical rac‐[(acac)₂Ru1(μ‐abpy)Ru2(bpy)₂](ClO₄)₂ ([ 1 ](ClO₄)₂), [Ru(acac)₂(abpy)] ( 2 ), [Ru(bpy)₂(abpy)](ClO₄)₂ ([ 3 ](ClO₄)₂), and meso‐[(bpy)₂Ru(μ‐abpy)Ru(bpy)₂](ClO₄)₃ ([ 4 ](ClO₄)₃; acac^?=2,4‐pentanedionato, bpy=2,2′‐bipyridine). In agreement with DFT calculations, both mononuclear species 2 and 3 ²⁺ can be described as ruthenium(II) complexes of unreduced abpy⁰, with 1.295(5)<d_NN<1.320(3) Å, thereby exhibiting effects from π back‐donation. However, the abpy ligand in both the asymmetrical diamagnetic compound 1 ²⁺ (d_NN=1.374(6) Å) and the symmetrical compound 4 ³⁺ (d_NN=1.360(7), 1.368(8) Å) must be formulated as abpy^.?. Remarkably, the addition of [Ru^II(bpy)₂]²⁺ to mononuclear [Ru^II(acac)₂(abpy⁰)] induces intracomplex electron‐transfer under participation of the noninnocent abpy bridge to yield rac‐[(acac)₂Ru1^III(μ‐abpy^.?)Ru2^II(bpy)₂]²⁺ ( 1 ²⁺) with strong antiferromagnetic coupling between abpy^.? and Ru^III (DFT (B3LYP/LANL2DZ/6‐31G*)‐calculated triplet–singlet energy separation E_S=1?E_S=0=11739 cm^?1). Stepwise one‐electron transfer was studied for compound 1 ⁿ, n=1?, 0, 1+, 2+, 3+, by UV/Vis/NIR spectroelectrochemistry, EPR spectroscopy, and by DFT calculations. Whereas the first oxidation of compound 1 ²⁺ was found to mainly involve the central ligand to produce an (abpy⁰)‐bridged Class I mixed‐valent Ru1^IIIRu2^II species, the first reduction of compound 1 ²⁺ affected both the bridge and Ru1 atom to form a radical complex ( 1 ⁺), with considerable metal participation in the spin‐distribution. Further reduction moves the spin towards the {Ru2(bpy)₂} entity.