Ligand Donor Strength Research Articles

Laser Flash Photolysis Study of Jacobsen Catalyst and Related Manganese(III) Salen Complexes. Relevance to Catalysis

The laser flash photolysis and emission properties of a set of five-coordinate manganese(III) Schiff-base complexes have been examined. In contrast to the intramolecular electron transfer between Mn3+ and the equatorial salen ligand reported to occur in the absence of axial coordination, our laser flash photolysis study has shown that the reactivity of the respective excited states is appreciably influenced by the electron donor strength of the apical ligand at the metal center. In fact, homolytic and heterolytic photocleavage of the metal-ligand apical bond can be the most important processes upon laser excitation, their relative contribution being influenced by medium effects and the sigma-charge donation of the axial ligand. On the other hand, the detection of reactive intermediates such as the oxomanganese(V) salen complex (lambda(max) 530 nm) by laser flash photolysis opens the way to apply this fast detection technique to the study of reaction mechanisms in catalysis by metallic complexes. As a matter of fact, quenching of oxomanganese(V) salen by simple alkenes has been observed by laser flash.

Phosphane lone-pair energies as a measure of ligand donor strengths and relation to activation energies

Using density-functional theory as implemented in the projector-augmented wave method, we have calculated structures, energy levels, structures of the protonated forms, and proton affinities of the phosphanes PH3, PF3, P(CF3)3, PMe3, P(NMe2)3, P(C6H5)3, P(p-C6H4OMe)3, and P(p-C6H4NMe2)3. The donor strengths of the phosphanes are discussed in terms of lone-pair energies and proton affinities. The influence of the donor ability of the phosphane ligands on the protonolytic cleavage of the metal–carbon bond in [NiCl(CH2CH2NH3)(PR3)2]+ complexes has been studied. A linear relationship between the lone-pair energies of the phosphanes and the activation barrier has been established.

Synthesis, structure and electrochemistry of nitrogen base adducts of tetraacetatodiruthenium(II,III): dependence of redox potential and Ru–Ru bond length on axial ligand donor strength

Diadduct complexes of the mixed-valent form of diruthenium tetraacetate, [Ru 2(μ-O 2CCH 3) 4L 2](PF 6), with L=N-heterocyclic axial ligands quinuclidine (quin) ( 1), 4-methylpyridine (4-Mepy) ( 2), pyridine (py) ( 3), 4-cyanopyridine (4-CNpy) ( 4), 3-cyanopyridine (3-CNpy) ( 5) and 4-phenylpyridine (4-Phpy) ( 6) have been synthesized and all but 5 were characterized by X-ray crystallography to study the effect of the variation of the donor number (DN) of L on the Ru–Ru and Ru–L ax bond lengths, the magnetic moment, the electronic spectral properties and the redox potential. When data from previous studies on O-donor adducts was also included a DN range of 18–61 could be established. Over this range the Ru–Ru bond length increases slightly from 2.265(1) to 2.2917(6) Å as the donor number is increased from 18 (in [Ru 2(μ-O 2CCH 3) 4(H 2O) 2](PF 6)) to 61 in 1. UV–Vis measurements show a very slight increase in energy of the π(Ru–O, Ru 2)→π*(Ru 2) transition, however, room temperature magnetic susceptibilty measurements show no change in the magnetic moment over the same range of donor numbers. Electrochemical measurements in 1,2-dichloroethane of the Ru 2 4+/5+ redox couple show a decrease in the E 1/2 of 292 mV on going from complex 5 (weakest N-donor) to complex 1 (strongest N-donor). The E 1/2 range is over 400 mV when the unligated [Ru 2(μ-O 2C(CH 2) 6CH 3) 4] complex is included (DN=1 for dichloromethane). The variation of axial ligand base strength does not effect the near-degeneracy of the (π*δ*) 3 HOMO or the π→π* energy gap, however, the actual (π*δ*) 3 HOMO energy varies significantly and increases as the basicity of the axial ligand increases allowing selective tuning of the redox potential.

Role of the axial ligand in hemeCO backbonding; DFT analysis of vibrational data

The FeCO backbonding correlation, a plot of CO versus FeCO vibrational frequencies, is a useful tool for investigating proximal ligand and distal environmental effects in CO adducts of heme proteins and models. The data fall on parallel lines of negative slope, shifted along the FeCO axis for proximal ligands of different donor strength. However, new resonance Raman spectra of five-coordinate Fe(II)(porphyrins)(CO) with electron-withdrawing or -donating substituents reveal that the backbonding slope is reduced in the absence of a proximal ligand. This phenomenon has been explored with non-local density functional theory on Fe(II)(porphines)(CO) with fluorine, chlorine and amino substituents. The calculations likewise give lines of negative slope, displaced according to trans ligand donor strength. Strikingly, the slopes are nearly the same for pyridine and methylthiolate trans ligands, but halved for five-coordinate complexes, as in the experimental plots. However, the magnitude of the slopes, as well as the displacements of the correlations, are about twice the experimental values. The origin of the backbonding behavior can be seen in the bond-length relationships for the calculated structures. For a given CO length, the FeCO bond is lengthened systematically by increasing the donor strength of the axial ligand, whose sigma donor orbital competes with the CO donor sigma orbital for the vacant Fe d z 2 orbital. For a given proximal ligand, the FeCO and CO bond lengths are negatively correlated, as expected for backbonding. In addition, the slopes increase systematically with increasing ligand donor strength, because the longer FeCO distance decreases the π overlap, and increases the FeCO sensitivity to backbonding changes.

Spectroscopic Investigation of Reduced Protocatechuate 3,4-Dioxygenase: Charge-Induced Alterations in the Active Site Iron Coordination Environment.

Chemical reduction of the mononuclear ferric active site in the bacterial intradiol cleaving catecholic dioxygenase protocatechuate 3,4-dioxygenase (3,4-PCD, Brevibacterium fuscum) produces a high-spin ferrous center. We have applied circular dichroism (CD), magnetic circular dichroism (MCD), variable-temperature-variable-field (VTVH) MCD, X-ray absorption (XAS) pre-edge, and extended X-ray absorption fine structure (EXAFS) spectroscopies to investigate the geometric and electronic structure of the reduced iron center. Excited-state ligand field CD and MCD data indicate that the site is six-coordinate where the (5)E(g) excited-state splitting is 2033 cm(-)(1). VTVH MCD analysis of the ground state indicates that the site has negative zero-field splitting with a small rhombic splitting of the lowest doublet (delta = 1.6 +/- 0.3 cm(-)(1)). XAS pre-edge analysis also indicates a six-coordinate site while EXAFS analysis provides accurate bond lengths. Since previous spectroscopic analysis and the crystal structure of oxidized 3,4-PCD indicate a five-coordinate ferric active site, the results presented here show that the coordination number increases upon reduction. This is attributed to the coordination of a second solvent ligand. The coordination number increase relative to the oxidized site also appears to be associated with a large decrease in the ligand donor strength in the reduced enzyme due to protonation of the original hydroxide ligand.

Synthetic, Thermochemical, and Catalytic Studies Involving Novel R2P(ORf) [R = Alkyl or Aryl; Rf = CH2CH2(CF2)5CF3] Ligands

A series of mixed alkyl (or aryl) phosphinite compounds have been prepared from the alcohol CF3(CF2)5CH2CH2OH (RfOH) and an appropriate ClPR2 (R = ORf, Ph, iPr, Cy) in the presence of base. A bidentate analog, (RfO)2PCH2CH2P(ORf)2, was synthesized in a similar manner. The ligands react with [Rh(CO)2Cl]2 yielding the complexes RhCl(CO)(PR3)2. The structure of RhCl(CO)(PPh2ORf)2 (6) is reported. Infrared studies of the carbonyl complexes yield a relative ligand donor strength for this series. Solution calorimetry was performed on the rhodium reaction in order to quantify this donor strength scale. Both approaches lead to the same donor strength scale: PCy2ORf > PiPr2ORf > PPh2ORf > P(ORf)3. These phosphinite ligands can be used as ancillary ligation in rhodium-mediated hydrogenation. A cationic rhodium complex of the chelating ligand displays selected solubility in fluorous media and biphasic catalysis can be performed.

Stereochemistry of hypervalent tin(IV) compounds. NMR and crystallographic studies of organoyltin(IV) complexes with 1,1-dithiolate ligands

Tin-119 NMR data have been used to examine the effective coordination spheres in dichloromethane solution of a series of organoyltin(IV) 1,1-dithiolate compounds RSnX n (SS) 3− n (where R Ph, Me; X  Cl, Br; n=0, 1,2), for SS S 2CNEt 2, S 2COEt, S 2P(OEt) 2. Intermolecular halide and 1,1-dithiolate exchange are slow on the NMR timescale however the tin-119 chemical shifts are temperature dependent due to the intramolecular equilibrium between monodentate and bidentate coordinated 1,1-dithiolate ligand. The magnitude of the temperature dependence of the tin-119 chemical shifts for the series RSn(SS) 3 indicates that the order of 1,1-dithiolate ligand donor strength is Et 2dtc > Etxan > Et 2dtp. Mixed ligand complexes PhSn(SS)(SS) 2 may be formed either by redistribution reactions of PhSn(SS) 3 and PhSn(SS) 3 or by addition of 1,1-dithiolate salt to solutions of PhSnCl n ,(SS) 3− n , (n=1,2). The crystal structure of PhSn(S 2CNEt 2) 3 has been determined. The geometry about tin in PhSn(S 2CNEt 2) 3 is a distorted pentagonal bipyramid with tin-sulfur distances ranging from 2.487(1) to 2.794(2) Å.

Macrocyclic nickel complexes in DNA recognition and oxidation

Abstract

Catalytic de-ligation of bis(arene)iron dications upon electro-reduction

Bis(arene)iron(II) dications (Ar 2Fe 2+) suffer the facile loss of aromatic ligands with Ar  hexamethylbenzene (HMB), durene (DUR), and mesitylene (MES), when acetonitrile solutions are exposed to reducing conditions. Electrocatalysis of the arene deligation is established during the controlled cathodic reduction of Ar 2Fe 2=+ by catalytic turnover numbers of 20, 75 and 150 for Ar  HMB, DUR and MES. Cyclic voltammetry of Ar 2Fe 2+ reveals a pair of waves P 1 and P 2 for the 1-electron couples Ar 2Fe 2+/+ and Ar 2Fe +/0, respectively, the chemical reversibility of which depends on the donor strength of the arene ligand in order: Ar  HMB > DUR > MES. The detailed examination of the dependence of the peak current ratio i p a/ i p c for P 1 with the CV sweep rate points to the monocation Ar 2Fe + as the labile intermediate leading to arene deligation. The mechanistic discussion for electrocatalysis centers around the further oxidation of the metastable Fe(NCCH 3) 6 at the electrode (ECE) or by Ar 2Fe 2+ (HOMO). First-order rate constants for the deligation of the neutral Ar 2Fe 0 are also evaluated from the CV analysis of P 2.

1H-NMR and electrochemical studies on ligated iron(III)perchlorates in acetonitrile-d 3

Half-wave potentials,E1/2, of Fe(ligand)63+/2+, as ClO4− salt, [ligand=N,N-dimethylformamide (dmf), acetamide (aa), N,N-dimethylacetamide (dma), trimethylphosphate (tmp), dimethylsulfoxide (dmso), and acetonitrile (MeCN) are given. A linear correlation betweenE1/2 and Gutmann's donor numbers of the ligands, a parameter which expresses quantitatively the Lewis donor properties towards hard acceptors, was found. Ligand replacement on Fe(ligand)63+ in acetonitrile-d3 was studied by means of1H-NMR spectroscopy at 20°C. An average number of ligands coordinated to Fe3+,ncoord, is given.ncoord increases with the ligand's donor strength; i.e.tmp<dmf<dmso.

The coordination chemistry of UCl 4, UBr 4, ThCl 4 and CeCl 4 in acetone solution and their relationship to the solid complexes

The coordination chemistry of UCl 4, UBr 4, ThCl 4 and CeCl 4 in acetone solution and their relationship to the solid complexes

Electron and ligand transfer reactions between cyclometallated platinum(II) compounds and thallium(III) carboxylates

The relative redox potentials of amine adducts of tetra(p-isopropylphenyl)porphinatocarbonyl-ruthenium(II) have been measured in order to study the dependence of porphyrin ring oxidation on axial base. The potentials observed for aromatic heterocyclic amines vary over a range of 0.09 volts; are dependent on the π-base and hydrogen bond donor strength of the amine; and are independent of base strength and steric bulk of the ligand. These results indicate that tension in the imidazole linkage is not a mechanism for protein control on porphyrin ring oxidations and that changes in the hydrogen bonding of the bound imidazole may be a mechanism for controlling porphyrin ring redox potential. The potentials to within experimental uncertainty and independent of the hydrogen bond donor strength of the axial ligand. The latter result indicates that hydrogen bonding effects are transmitted via the π-bonds of the metal porphyrin complex. A direct correlation between the axial ligand dependence of porphyrin ring and metal center oxidation has been found and this implies that the same mechanisms may be operative in either porphyrin ring or metal center oxidation.

The chemistry of uranium—XVII Ligand donor strength order towards uranium(IV): (Rationalization and application)

Some experimental methods of determining ligand donor strengths are compared. The advantages of a potentiometric method, in which halide ion replacement could be measured directly, are indicated. The ligand donor strength order for UCl 4 and CoCl 2 as Lewis acids thus obtained is rationalized in terms of a simplified electrostatic treatment and the relative significance of the various factors important in determining donor strength for the UCl 4 system, was obtained. The donor strength order, solution equilibria and approximate lattice energy ratios of the species in solution are used to interpret the chemical behaviour of these systems.

Isonitrile complexes of chromium(0) and molybdenum(0): characterisation and reactivity

The preparation of a series of isonitrile complexes, [(CO)6 –xM(CNR)x](x= 1–3; M = Cr, Mo; R = Me, Et, Pri, C6H11, But, p-tolyl, Ph, p-ClC6H4) is reported. The results of spectroscopic (i.r., u.v., mass, and n.m.r.— particularly aromatic solvent induced shifts) and electrochemical measurements are employed to assess the nature of the bonding in these systems. It is shown that the contribution of (d→π*)π-bonding is negligible and that variations in the donor strength of the isonitrile ligand are principally responsible for the changes observed. The compounds (RNC)Cr(CO)5 do not react with weak nucleophiles (amines, alcohols). Ketimine formation does not result from the reaction between (RNC)Cr(CO)5 and a silane in the presence of pyridine. The reaction between (RNC)2Cr(CO)4 and (Me2PCH2)2(dmpe) affords cis-(RNC)2(dmpe)Cr(CO)2 as the principal product, together with (dmpe)2Cr(CO)3.