Delocalization Of Unpaired Electron Density Research Articles

Strong π-Backbonding Enables Record Magnetic Exchange Coupling Through Cyanide.

The paramagnetic cyano-bridged complex PhB(tBuIm)3Fe-NC-Mo(NtBuAr)3 (Ar = 3,5-Me2C6H3) is readily assembled from a new four-coordinate, high-spin (S = 2) iron(II) monocyanide complex and the three-coordinate molybdenum(III) complex Mo(NtBuAr)3. X-ray diffraction and IR spectroscopy reveal that delocalization of unpaired electron density into the cyanide π* orbitals leads to a reduction of the C-N bond order. Direct current (dc) magnetic susceptibility measurements, supported by electronic structure calculations, demonstrate the presence of strong antiferromagnetic exchange between spin centers, with a coupling constant of J = -122(2) cm-1. To our knowledge, this value represents the strongest magnetic exchange coupling ever to be observed through cyanide. These results demonstrate the ability of low-coordinate metal fragments to engender extremely strong magnetic exchange coupling through cyanide by virtue of significant π-backbonding into the cyanide ligand.

Syntheses, Crystal Structures and Spectroscopic Studies of Bis[1‐methyl‐3‐(methoxycarbonylmethyl)‐benzimidazolium]2+ [CuBr4]2− and [ZnBr4]2− Compounds

AbstractX‐ray diffraction structure of two new bis [1‐methyl‐3‐(methoxycarbonylmethyl) benzimidazolium]2+ [CuBr4]2− and [ZnBr4]2− compounds has been determined. Zn‐crystal is monoclinic with perfect Td symmetry of the ZnBr4 tetrahedron, whereas the Cu‐compound is orthorhombic with D2d symmetry of CuBr4 tetrahedron. The difference in these structures is described as due to the static Jahn‐Teller effect. Vibrational spectroscopy (1800–3200 cm−1) identified hydrogen bond network. NIR‐UV‐Vis spectra (5000–50000 cm−1) are dominated by ligand‐to‐metal bands with d‐d transition contribution. EPR results (Q‐band) analyzed in terms of the MO‐theory showed a very strong delocalization of unpaired electron density on ligands accompanied by the Cu(II) 4pz orbital contribution. This effect is partially compensated by charge transfer transitions to the EPR g‐factor.

EPR and ESE of CuS4 complex in Cu(dmit)2: g-Factor and hyperfine splitting correlation in tetrahedral Cu–sulfur complexes

Pseudotetrahedral CuS4 complexes of Cu(dmit)2 compound in DMF solution were studied by EPR, UV–Vis and electron spin echo methods. After rapid freezing at 77K a good glassy state is formed and the CuS4 complex has a D2d symmetry of a compressed tetrahedron with xy ground state and spin-Hamiltonian parameters g||=2.089, g⊥=2.026, A||=146×10−4cm−1 and A⊥=30×10−4cm−1. The complex is not deformed in the glassy state and is very rigid as indicated by the echo detected spectrum and by electron spin relaxation which is governed by reorientations of methyl groups of surrounding DMF molecules as shown by electron spin echo envelope modulation (ESEEM) spectrum. The g|| and A|| of Cu(dmit)2 and other CuS4 complexes collected in Peisach–Blumberg correlation diagram were analyzed using extended Molecular Orbital theory. We explain why the correlation line for copper–sulfur complexes has larger slope compared to the CuO4 and CuN4 tetrahedra. Along the correlation line the delocalization of unpaired electron density onto ligand is constant and varies from β=0.78–0.83 for g|| in the range 2.06–2.10 of correlation diagram. The slope of the line is determined by the product of MO-coefficients αc1, where α is a parameter characterizing delocalization of unpaired electron in x2–y2 and c1<1 is a mixing parameter decreasing when 4p contribution grows. We found, unexpectedly, that αc1≈0.7 for all CuS4 complexes suggesting a correlation between degree of tetrahedral deformation and MO-parameters. MO-coefficients for Cu(dmit)2 are α=0.753, β=0.752 and c1=0.930 confirming a strong delocalization of unpaired electron in xy and x2–y2 orbitals.

Paramagnetic 19F NMR and electrospray ionization mass spectrometric studies of substituted pyridine complexes of chromium(III): Models for potential use of 19F NMR to probe Cr(III)–nucleotide interaction

The synthesis and characterization of chromium basic carboxylate complexes, [Cr3(O2CR)6L3]+, containing trifluoroacetate, 3-fluoropyridine, 3-trifluoromethylpyridine, and 4-trifluoromethylpyridine are described. The substituted pyridine ligands are used as models of DNA bases to determine whether 19F NMR would be a potentially useful probe of the binding of Cr3+ to DNA. The 19F NMR resonances of the coordinated ligands, while broadened by delocalization of unpaired electron density from the S=3/2 chromic centers, are readily discernable, and the contact shifts are of sufficient magnitude that the signals from coordinated and free ligands can easily be differentiated. Thus, 19F NMR appears to be a potentially useful probe of the binding of Cr3+ to DNA containing F-labeled bases. Additionally, electrospray MS is shown to be a convenient method to establish the identity of chromium basic carboxylate assemblies.

EPR and potentiometric studies of copper(II) binding to nicotinamide adenine dinucleotide (NAD+) in water solution

Coordination of Cu(II) by nicotinamide adenine dinucleotide (NAD+) molecule has been studied in water solutions of various pH by potentiometry and electron paramagnetic resonance (EPR) and electron spin echo (ESE) spectroscopy. Potentiometric results indicate Cu(II) coordination by protonated NAD+ at low pH and by deprotonated NAD+ at high pH. At medium pH value (around pH=7) NAD+ is not able to coordinate Cu(II) ions effectively and mainly the Cu(H2O)6 complexes exist in the studied solution. This has been confirmed by EPR results. Electronic structure of Cu(II)–NAD complex and coordination sites is determined from EPR and ESE measurements in frozen solutions (at 77K and 6K). EPR spectra exclude coordination with nitrogen atoms. Detailed analysis of EPR parameters (g||=2.420, g⊥=2.080, A||=–131×10−4cm−1 and A⊥=8×10−4cm−1) performed in terms of molecular orbital (MO) theory shows that Cu(II)NAD complex has elongated axial octahedral symmetry with a relatively strong delocalization of unpaired electron density on in-plane and axial ligands. The distortion of octahedron is analyzed using A|| vs. g|| diagram for various CuOx complexes. Electron spin echo decay modulation excludes the coordination by oxygen atoms of phosphate groups. We postulate a coordination of Cu(II) by two hydroxyl oxygen atoms of two ribose moieties of the NAD molecules and four solvated water molecules both at low and high pH values with larger elongation of the octahedron at higher pH.

Where Does the Electron Go? Electron Distribution and Reactivity of Peptide Cation Radicals Formed by Electron Transfer in the Gas phase

We report the first detailed analysis at correlated levels of ab initio theory of experimentally studied peptide cations undergoing charge reduction by collisional electron transfer and competitive dissociations by loss of H atoms, ammonia, and N-C alpha bond cleavage in the gas phase. Doubly protonated Gly-Lys, (GK + 2H) (2+), and Lys-Lys, (KK + 2H) (2+), are each calculated to exist as two major conformers in the gas phase. Electron transfer to conformers with an extended lysine chain triggers highly exothermic dissociation by loss of ammonia from the Gly residue, which occurs from the ground ( X ) electronic state of the cation radical. Loss of Lys ammonium H atoms is predicted to occur from the first excited ( A ) state of the charge-reduced ions. The X and A states are nearly degenerate and show extensive delocalization of unpaired electron density over spatially remote groups. This delocalization indicates that the captured electron cannot be assigned to reduce a particular charged group in the peptide cation and that superposition of remote local Rydberg-like orbitals plays a critical role in affecting the cation-radical reactivity. Electron attachment to ion conformers with carboxyl-solvated Lys ammonium groups results in spontaneous isomerization by proton-coupled electron transfer to the carboxyl group forming dihydroxymethyl radical intermediates. This directs the peptide dissociation toward NC alpha bond cleavage that can proceed by multiple mechanisms involving reversible proton migrations in the reactants or ion-molecule complexes. The experimentally observed formations of Lys z (+*) fragments from (GK + 2H) (2+) and Lys c (+) fragments from (KK + 2H) (2+) correlate with the product thermochemistry but are independent of charge distribution in the transition states for NC alpha bond cleavage. This emphasizes the role of ion-molecule complexes in affecting the charge distribution between backbone fragments produced upon electron transfer or capture.

Core modified meso-aryl corrole: first examples of CuII, NiII, CoII and RhI complexes.

A variety of metal complexes of 5,10,15-triphenyl-21-monooxa-corrole 4 have been investigated. This monooxa corrole, where one of the pyrrole ring is replaced by a furan moiety, is synthesized by the alpha-alpha coupling reaction of 16-oxa tripyrrane and dipyrromethane. The single crystal X-ray structure of 4 indicates only small deviation of the inner-core heteroatoms from planarity and this macrocycle arrange themselves into a columnar structure. Insertion of metals further flattens the corrole framework. Specifically, oxacorrole 4 binds to Nil(II), Cu(II), and Co(II) with the participation of all heteroatoms in the coordination. However, Rh(I) ion binds to only one imino and one amino nitrogen of the macrocycle. The bond angles at the metal center in the Ni(II) and Rh(I) complexes reveal square planar geometry completed by two CO molecules for Rh(I). The EPR spectra of the paramagnetic that Cu(II) and Col(II) complexes display significant decreases in the metal hyperfine couplings compared with the corresponding porphyrin complexes. The presence of superhyperfine coupling in the Cu(II) complex suggests delocalization of unpaired electron density into the ligand orbitals. Electrochemical studies reveal easier oxidations and harder reductions relative to the corresponding porphyrin derivatives while, the metallated derivatives did not show their characteristic metal reductions due to the high energy of their LUMO.

Crystal Structure, Physical Properties, and Hydrolysis Kinetics of [(O)(tmpa)V(IV)(&mgr;-O)V(V)(tmpa)(O)](3+).

We report the synthesis, crystal structure, physical properties and hydrolysis kinetics of [(O)(tmpa)V(IV)(&mgr;-O)V(V)(tmpa)(O)](3+) (tmpa = tris(2-pyridylmethyl)amine), the first cation in its class for which both solid state and solution-phase characteristics are fully consistent with expectations for a Robin-Day type III mixed-valence complex. [V(2)(tmpa)(2)(O)(3)](ClO(4))(3).CH(3)CN crystallizes in the triclinic, P&onemacr; (No. 2) space group with a = 10.719(2) Å, b = 11.609(1) Å, c = 19.516(2) Å, alpha = 87.876(9) degrees, beta = 79.295(9) degrees, gamma = 68.743(9) degrees, and V = 2222.6(6) Å(3); Z = 2. The two vanadium centers are crystallographically equivalent in a linear, oxo-bridged cation with V-O(b) and V=O bond lengths of 1.800(1) and 1.597(4) Å, respectively. A 15-hyperfine line EPR spectrum (g = 1.9704, A = 46.4 x 10(-4) cm(-1)) was observed for [V(2)(tmpa)(2)(O)(3)](3+), consistent with coupling of a single electron to two equivalent (51)V atoms. The mixed-valence complex exhibits an intervalence transfer (IT) band at 1.05 x 10(4) cm(-1) (epsilon = 2.1 x 10(3) M(-1) cm(-1), CH(3)CN) whose energy is insensitive to variations in the solvent; d-d transitions are observed at 1.25 x 10(4) cm(-1) ((2)E <-- (2)B(2)) and 1.44 x 10(4) cm(-1) ((2)B(1) <-- (2)B(2)). [V(2)(tmpa)(2)(O)(3)](2+), [V(2)(tmpa)(2)(O)(3)](3+), and [V(2)(tmpa)(2)(O)(3)](4+) comprise a redox family related by vanadium (IV,V/IV,IV) and (V,V/IV,V) half-wave reduction potentials of 0.25 and 1.62 V vs NHE (25 degrees C, CH(3)CN, I = 0.1 M), respectively. The hydrolysis reaction: [V(2)(tmpa)(2)(O)(3)](3+) + H(2)O --> [V(IV)(tmpa)(O)(OH)](+) + [V(V)(tmpa)(O)(2)](+) + H(+) proceeds with an acid-independent rate constant of 2.37 x 10(-3) s(-1) (25 degrees C; DeltaH() = 71 kJ/mol, DeltaS() = -59 J/mol K). [V(4)O(5)(tmpa)(4){Fe(CN)(6)}](ClO(4))(4).H(2)O was isolated as an unexpected product from the reaction of [V(2)(tmpa)(2)(O)(3)](ClO(4))(2).2H(2)O with equimolar K(3)[Fe(CN)(6)] in aqueous solution. It is proposed that pairs of V(IV)=O and V(V)=O units aggregate with [Fe(CN)(6)](4-) into a charge transfer complex. Hexacyanoferrate(II)-to-vanadium IT charge transfer bands are observed at 1.98 x 10(4) cm(-1) (epsilon = 5.3 x 10(3) M(-1) cm(-1)) and 2.72 x 10(4) cm(-1) (epsilon = 9.3 x 10(3) M(-1) cm(-1)); a 29-line EPR spectrum (g = 1.9737, A = 22.0 x 10(-4) cm(-1)) demonstrates equal delocalization of unpaired electron density over all four V atoms (CH(3)CN solution).

Analysis of the hyperfine-shifted nitrogen-15 resonances of the oxidized form of Anabaena 7120 heterocyst ferredoxin.

Hyperfine-shifted nitrogen signals have been detected in one-dimensional 15N NMR spectra of oxidized Anabaena 7120 heterocyst ferredoxin labeled uniformly with 15N. Several of these have been classified by amino acid type by reference to results from selective 15N-labeling studies. Remarkable agreement is seen between a dipole-dipole analysis of the 15N T1 relaxation and the distances of several of the nitrogens from the irons of the cluster as derived from the X-ray structure of this protein [Jacobson, B. L., Chae, Y. K., Markley, J. L., Rayment, I., & Holden, H. M. (1993) Biochemistry 32, 6788-6793]. The agreement is within experimental error for hyperfine-shifted nitrogens that are at least 4.2 A distant from either of the irons of the cluster; however, the simple model appears to fail for hyperfine-shifted nitrogens that are closer to the cluster. The failure of the model for short distances may stem either from a breakdown of the point-dipole approximation and/or from neglect of delocalization of unpaired electron density from the iron ions to other atoms. Even with the above limitations, dipolar analysis of 15N relaxation should provide useful distance constraints for solution-state studies of iron-sulfur proteins.

Nuclear magnetic resonance studies of multinuclear chromium assemblies

1H NMR and 2H NMR resonance of a series of oxo-centred trinuclear chromium complexes of the form [Cr 3O(O 2CR) 6(L) 3] + (R = —CH 3, —CH 2CH 3, —Ph, -4-MePh; L = H 2O, py, 2-Mepy, 3-Mepy, 4-Mepy) have been observed and assigned. The role of π- and σ-bonding frameworks in the delocalization of unpaired electron density onto the ligand protons is examined. The π-delocalization mechanism dominates where ligands contain aromatic substituents or carboxylate functionalities, consistent with the presence of a metal centre where unpaired electrons reside solely in t 2g orbitals. A comparison of the importance of these mechanisms with that in trinuclear manganese and iron complexes is reported. These results suggest that 1H and 2H NMR studies could be useful probes of proteins containing coupled multinuclear chromium assemblies.

Delocalization of Unpaired Electron Density of π Radicals into Substituent Phenyl Groups

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