DFT study of the systematic variations in metal-ligand bond lengths of coordination complexes: the crucial role of the condensed phase

The experimental M-A and M-B distances in several series of [MAnBm-n]-type complexes have been studied by DFT. Many of the structural features of the series, such as trans influences and sterically induced bond elongations, are not reproduced correctly in gas-phase DFT calculations. However, the correct trends are recovered by explicitly including environmental effects via the COSMO solvation model. These observations imply that the condensed-phase environment plays a critical role in determining the geometric structure of coordination complexes. Thus, any apparently satisfactory reproduction of the condensed-phase structure by an in vacuo calculation may mask an incorrect treatment of the interplay between different ligands attached to the same metal center.     

A comparison of calculated and experimental metal-ligand bond lengths in early transition-metal complexes using the method of partial retention of diatomic differential overlap

Basis sets are developed for PRDDO calculations on complexes of the metals Ti, V, Cr, and Mn in high oxidation states Twenty-three unique metal-ligand bond distances were optimized, with an average absolute error of 0.034 Å.     

Structures of molecular complexes of SbCl5 with pyridine and acetonitrile: equal bond lengths,different stability

Molecular structure of crystalline complex SbCl₅ ? Py was determined for the first time and the structure of the complex SbCl₅ ? AN was refined by means of single crystal X-ray diffraction analysis. It is shown that donor-acceptor Sb–N bond lengths for these complexes are equal within the experimental error. Quantum chemical calculations of the complexes in the gas phase reveal that the energies of the donor-acceptor bond Sb–N are 88 and 180 kJ mol^?1 for SbCl₅ ? AN and SbCl₅ ? Py, respectively. It is established that the length of the donor-acceptor bond Sb–N in the crystal cannot serve as a measure of its strength.

     

Automated site-directed drug design: Searches of the Cambridge Structural Database for bond lengths in molecular fragments to be used for automated structure assembly

Summary In this paper a database of small frequently occurring molecular fragments is used for the determination of fragment bond lengths from the Cambridge Structural Database. A large number of bond types are described that have not been reported previously.     

Antiferromagnetic complexes involving metalmetal bonds : I. Synthesis and molecular structure of an antiferromagnetic dimer with a CrCr bond

The binuclear complex (C₅H₅)₂Cr₂(S)(SCMe₃)₂ was prepared by refluxing a solution of chromocene and t-butylmercaptane in heptane. The structure of the product was determined by single crystal X-ray diffraction. The chronium atoms are linked by a sulphide bridge (SCr 2.24 Å;, <CrSCr 74.1° and two SCMe₃ bridges (CrS 2.38 Å;, <CrSCr 68.3–69.3°). The two cyclopentadienyl ligands (CC 1.41 Å;, CrC 2.23 Å;) are in apical positions, their ring planes being parallel to each other. The complex is an antiferromagnet (?2J cm^?1) despite the small CrSCr angles and short chromiumchromium distance (2.689 Å;) indicative of strong CrCr bonding.     

Polymer complexes: supramolecular modeling for determination and identification of the bond lengths in novel polymer complexes from their infrared spectra

Synthesis and characterization of allyl propenyl‐2‐(4‐derivatives phenylazo)butan‐3‐one (HL_n) are described. The monomers obtained contain N?N and carbonyl functional groups in different positions with respect to the allyl group. This structural difference affects the stereochemical structure of the uranyl polymer complexes prepared by the direct reaction of uranyl acetate with the monomers. The polymer complexes are characterized by elemental analyses, ¹H and ¹³C NMR, electronic and vibrational spectroscopy and other theoretical methods. The bonding sites of the hydrazone are deduced from IR and NMR spectra and each of the ligands were found to bond to the UO₂²⁺ ion in a bidentate fashion. The monomers obtained contain N?N and carbonyl functional groups in different positions with respect to the allyl group. IR spectra show that the allyl azo homopolymer (HL_n) acts as a neutral bidentate ligand by coordinating via the two oxygen atom of the carbonyl group, thereby forming a six‐membered chelating ring. The υ₃ frequency of UO₂²⁺ has been shown to be a good molecular probe for studying the coordinating power of the ligands. The υ₃‐values of UO₂²⁺ from IR spectra have been used to calculate the force constant, F_UO (in 10^?8 N/Å) and the bond length R_UO (in Å) of the U? O bond. We adopted a strategy based upon both theoretical and experimental investigations. The theoretical aspects are described in terms of the well‐known theory of 5d–4f transitions. The necessary structural data (coordination geometries and electronic structures) are determined from a framework for the modeling of novel polymer complexes. The Wilson, G. F. matrix method, Badger's formula and the Jones and El‐Sonbati equations were used to determine the stretching and interaction force constants from which the U? O bond distances were calculated. The bond distances of these complexes were also investigated. The effect of Hamett's constant is also discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

Influence of ligand structure and molecular geometry on the properties of d polypyridinic transition metal complexes

Different strategies to improve the excited state properties of polypyridinic complexes by varying ligand structure and molecular geometry are described. Bidentate and tetradentate ligands based on fragments as dipyrido[3,2-a:2′,3′-c]phenazine, dppz, and pyrazino[2,3-f][1,10]-phenanthroline, ppl, have been used. Quinonic residues were fused to these basic units to improve acceptor properties. Photophysical studies were performed in order to test theoretical predictions.     

Photochrome-coupled metal complexes: molecular processing of photon stimuli

The frontiers of novel photoresponsive materials constructed with photochromes and transition metal complexes are surveyed in this review. Strategies to develop new photofunctions are categorized into four types. In the first category, intramolecular electronic interactions between photochromes and metal complexes produce entangled responses such as redox-regulated photochromic reactions or tristable photochromism. In the second, light-induced molecular structural rearrangements of photochromes induce the transformation of flexible and labile coordination structures, which can be applied to complex photomechanics or photoelectron conversion. In the third, the photochromic moiety also acts as a photonic switch, transmitting a metal-metal interaction when it is located between two metal complex moieties. The last category concerns the development of new photochromic reactions, involving metal-ligand bond rearrangements. These reactions potentially induce drastic electronic tuning of the metal center, and can be used to develop light-driven molecular machines.     

Pentadienyls vs cyclopentadienyls and reversal of metal-ligand bonding affinity with metal oxidation state: synthesis, molecular structures, and electronic structures of high-valent zirconium pentadienyl complexes

Molecules of the form Cp(6,6-dmch)ZrX(2) (Cp = eta(5)-cyclopentadienyl, X = Cl, Br, I; 6,6-dmch = eta(5)-6,6-dimethylcyclohexadienyl) have been synthesized, and the molecular and electronic structures have been investigated. These molecules allow direct comparison of the bonding and properties of pentadienyl and cyclopentadienyl ligands in the same high-oxidation-state metal complexes. Unlike the well-known Cp(2)ZrX(2) analogues, these Cp(6,6-dmch)ZrX(2) molecules are intensely colored, indicating significantly different relative energies of the frontier orbitals. Also unusual, the average Zr-C distances to the 6,6-dmch pentadienyl ligand are about 0.1 A longer than the average Zr-C distances to the cyclopentadienyl ligand for these Zr(IV) complexes, opposite of what is observed for the Zr(II) complex Cp(2,6,6-tmch)Zr(PMe(3))(2) (tmch = eta(5)-2,6,6-trimethylcyclohexadienyl), reflecting a dramatic reversal in the favorability of the bonding depending on the metal oxidation state. The experimental and computational results indicate that the color of the Cp(6,6-dmch)ZrX(2) complexes is due to a 6,6-dmch ligand-to-metal charge-transfer band. Compared to the Cp(2)ZrX(2) analogues, the Cp(6,6-dmch)ZrX(2) molecules have a considerably less stable HOMO that is pentadienyl-based and an essentially unchanged metal-based LUMO. Also, the lowest unoccupied orbital of pentadienyl is stabilized relative to cyclopentadienyl and becomes a better potential delta electron acceptor, thus contributing to the differences in structure and reactivity of the low-valent and high-valent metal complexes.     

Basis sets for geometry optimizations of second-row transition metal inorganic and organometallic complexes

Modest-sized basis sets for the second-row transition metal atoms are developed for use in geometry optimization calculations. Our method is patterned after previous work on basis sets for first-row transition metal atoms. The basis sets are constructed from the minimal basis sets of Huzinaga and are augmented with a set of diffuse p and d functions. The exponents of these diffuse functions are chosen to minimize both the difference between the calculated and experimental equilibrium geometries and the total molecular energies for several second-row transition metal inorganic and organon etallic complexes. Slightly smaller basis sets, based on the same Huzinaga minimal sets but augmented with a set of diffuse s and p functions rather than diffuse p and d functions, are also presented. The performance of these basis sets is tested on a wide variety of second-row transition metal inorganic and organometallic complexes and is compared to pseudopotential basis sets incorporating effective core potentials.     

Polymer complexes. LXXVIII. Synthesis and characterization of supramolecular uranyl polymer complexes: Determination of the bond lengths of uranyl ion in polymer complexes

The UO₂(II) polymer complexes (1–5) of azo dye ligands 5(4`‐derivatives phenylazo)‐8‐hydroxy‐7‐quinolinecarboxaldehyde (HL_x) were prepared and characterized by elemental analysis, ¹H NMR, IR spectra, thermal analysis and X‐ray diffraction analysis (XRD). The molecular geometrical structures and quantum chemical of the ligands (HL_x) and their tautomeric forms (D and G) were calculated. Molecular docking between the HL_x ligands and their tautomeric form with two receptors of the breast cancer (1JNX) and the prostate cancer (2Q7K) was discussed. From the histogram of the HOMO–LUMO energy gap (ΔE) and the estimated free energy of binding of the receptors of prostate cancer (2Q7K) and breast cancer (1JNX) for the ligands (HL_x), it is observed that the ΔE values of the ligands (HL_x) increases in the order HL₂ < HL₃ < HL₄ < HL₁ < HL₅. The electronic structures and coordination were determined from a framework for the modeling of the formed polymer complexes. From the IR spectra of the polymer complexes, the symmetric stretching frequency υ₃ values of UO₂²⁺ were used for the determination of the force constant (F_U‐O (in 10^?8 N/?)) and the bond length (R_U‐O (?)) of the U–O bond by using Wilson, G. F. matrix method, McGlynn & Badger's formula and El‐Sonbati equations. The plot of the bond distance r_U‐O (r₁, r₂, r₃, and r_t) vs. υ₃ was showed straight lines with increase in the value of υ₃ and decrease in r_U‐O.     

Grafting and anchoring of molecular complexes as metal precursors for alumina-supported Pd catalysts

Abstract

The surface coordination chemistry of Pd complexes on alumina has been studied in the framework of synthesizing Pd/γ-Al₂O₃ catalytic materials. Two methodologies were explored: the direct grafting of Pd complexes on hydroxyl functions present at the alumina surface and the anchoring of the precursors via amine-bearing silanes previously grafted on the support. Suitable conditions to graft and anchor Pd complexes on alumina surface were found and experimental proofs of grafting and anchoring processes are provided. The results show that covalent grafting indeed took place for samples prepared in acetonitrile with [Pd(CF₃CO₂)₂(bipy)] and [PdCl₂(PhCN)₂] complexes or with [Pd(OAc)₂] and [Pd(CF₃CO₂)₂] in acetone. The anchoring was successful for catalysts prepared in acetone with 1 wt.% of [Pd(CF₃CO₂)₂] loading. Grafting and anchoring were found to stabilize palladium in its Pd(II) oxidation state. This has an adverse effect on the activation step that should lead to reduction of the complex to give the metallic catalytic supported active phase.     

Optimization and validation of an automated voltammetric stripping technique for ultratrace metal analysis

A new automated batch method for the determination of ultratrace metals (nanogram per liter level) was developed and validated. Instrumental and chemical parameters affecting the performance of the method were carefully assessed and optimized. A wide range of voltammetric methods under different chemical conditions were tested. Cadmium, lead and copper were determined by anodic stripping voltammetry (ASV), while nickel, cobalt, rhodium and uranium by adsorptive cathodic stripping voltammetry (AdCSV). The figures of merit of all of these methods were determined: very good precision and accuracy were achieved, e.g. relative percentage standard deviation in the 4-13% for ASV and 2-5% for AdCSV.The stripping methods were applied to the determination of cadmium, lead, copper, nickel, cobalt, rhodium and uranium in lake water samples and the results were found to be comparable with ICP-MS data.     

Modification of immobilized metal complexes toward the design and synthesis of functional materials for nitric oxide delivery

Functional polymeric materials containing immobilized metal complexes with the potential to bind nitric oxide (NO) are described. The materials were synthesized using template copolymerization techniques and contain immobilized metalloporphyrin and metallosalen (N,N′‐bis(salicylidene)‐1,2‐ethylenediaminato(2‐)) sites where NO binding occurs. A potential drawback of current materials designed for NO‐delivery is the rebinding of NO to the material after release. To minimize this undesirable rebinding event, materials were prepared in which immobilized sites contain additional covalently‐linked functional groups that can bind to the metal centers after NO dissociation. A model system with a covalently attached Co^IIsalen complex and pyridine groups confirmed that the immobilized sites bind NO in the presence of the additional ligands. Attempts to immobilize an iron porphyrin, containing two axial imidazole ligands, were unsuccessful because the lability of the iron center prevented the formation of a stable template. Instead, the desired material was achieved by selecting Co^III porphyrin with axially coordinated imidazole ligands as the template complex. Once immobilized, a procedure was developed to remove the cobalt ions and incorporate iron into the sites. The site structure of these new immobilized sites was examined spectroscopically. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2282–2292, 2006     

Cleavage of carbon-carbon bonds of diphenylacetylene and its derivatives via photolysis of Pt complexes: tuning the C-C bond formation energy toward selective C-C bond activation

Carbon-carbon bond activation of diphenylacetylene and several substituted derivatives has been achieved via photolysis and studied. Pt0-acetylene complexes with eta2-coordination of the alkyne, along with the corresponding PtII C-C activated photolysis products, have been synthesized and characterized, including X-ray crystal structural analysis. While the C-C cleavage reaction occurs readily under photochemical conditions, thermal activation of the C-C bonds or formation of PtII complexes was not observed. However, the reverse reaction, C-C reductive coupling (PtII --> Pt0), did occur under thermal conditions, allowing the determination of the energy barriers for C-C bond formation from the different PtII complexes. For the reaction (dtbpe)Pt(-Ph)(-CCPh) (2) --> (dtbpe)Pt(eta2-PhCCPh) (1), DeltaG was 32.03(3) kcal/mol. In comparison, the energy barrier for the C-C bond formation in an electron-deficient system, that is, (dtbpe)Pt(C6F5)(CCC6F5) (6) --> (dtbpe)Pt(eta2-bis(pentafluorophenyl)acetylene) (5), was found to be 47.30 kcal/mol. The energy barrier for C-C bond formation was able to be tuned by electronically modifying the substrate with electron-withdrawing or electron-donating groups. Upon cleavage of the C-C bond in (dtbpe)Pt(eta2-(p-fluorophenyl-p-tolylacetylene) (9), both (dtbpe)Pt(p-fluorophenyl)(p-tolylacetylide) (10) and (dtbpe)Pt(p-tolyl)(p-fluorophenylacetylide) (11) were obtained. Kinetic studies of the reverse reaction confirmed that 10 was more stable toward the reductive coupling [the term "reductive coupling" is defined as the formation of (dtbpe)Pt(eta2-acetylene) complex from the PtII complex] than 11 by 1.22 kcal/mol, under the assumption that the transition-state energies are the same for the two pathways. The product ratio for 10 and 11 was 55:45, showing that the electron-deficient C-C bond is only slightly preferentially cleaved.     

Synthesis of uranium(VI) terminal oxo complexes: molecular geometry driven by the inverse trans-influence

Oxidation of our previously reported uranium(V) oxo complexes, supported by the chelating ((R)ArO)(3)tacn(3-) ligand system (R = tert-butyl (t-Bu), 1-t-Bu; R = 1-adamantyl (Ad), 1-Ad), yields terminal uranium(VI) oxo complexes [(((R)ArO)(3)tacn)U(VI)(O)]SbF(6) (R = t-Bu, 2-t-Bu; R = Ad, 2-Ad). These complexes differ in their molecular geometry in that 2-t-Bu possesses pseudo-C(s) symmetry in solution and solid state as the terminal oxo ligand lies in the equatorial plane (as defined by the three aryloxide arms of the ligand) in order to accommodate the thermodynamic preference of high-valent uranium oxo complexes to have a σ- and π-donating ligand trans to the oxo (vis-à-vis the ubiquity of the linear UO(2)(2+) moiety). The distortion of the ligand--which stands in contrast to all other complexes of uranium supported by the ((R)ArO)(3)tacn(3-) ligand, including 2-Ad--is most clearly seen in the structures of 2-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)]SbF(6), and 3-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)(OC(O)CF(3))(ax)]. The solid-state structure of 3-t-Bu reveals that the trans U-O(ArO) bond length is shortened by 0.1 ? in comparison to the cis U-O(ArO) bonds and the trans U-O-C(ipso) angle is linearized (157.67° versus 147.85° and 130.03°). Remarkably, the minor modification of the ligand to have Ad groups at the ortho positions of the aryloxide arms is sufficient to stabilize a C(3v)-symmetric terminal uranium(VI) oxo complex (2-Ad) without a ligand trans to the oxo. These experimental results were reproduced in DFT calculations and allow the qualitative bracketing of the relative thermodynamic stabilization afforded by the inverse trans-influence as ～6 kcal mol(-1).