Svetozar R. Niketić

Conformational analysis of copper(II) chelates with epimeric amino acids

Abstract The method of consistent force field (CFF) was applied to both natural and allo -isomers of Cu(II) chelates with epimeric amino acids ( L -Ile and L -Thr). The influence of ligand water, as well as hydrophilic interactions, were neglected. The strain and Gibbs energies of 53 and 43 conformational minima of Cu(IIe) 2 and Cu(Thr) 2 , respectively, were computed. Difference in average Gibbs energies of epimeric chelates is in rough accordance with thermodynamic results.

Synthesis and structure of aqua-2′,‴-(2,6-pyridindiyldiethylidene) dioxamohydrazide-copper(II) hydrate

Abstract In a template synthesis from copper(II) acetate, 2,6-diacetylpyridine, and semioxamazide (NH2CO CONHNH2), two copper(II) complexes, [Cu(dapsox)(H2O)]·H2O and [Cu(Hdapsox)](H2O)]ClO4, (where H2dapsox = 2′,2‴-(2,6-pyridindiyldiethylidene)dioxamohydrazide) were obtained and characterized. The structure of the former complex was determined by a single-crystal X-ray analysis. CuII is located in a square pyramidal environment. The polydentate ligand, dapsox2− is coordinated in dianionic form, as an unsymmetrical quadridentate planar system forming one-membered and two-five-membered metal-chelate rings. The fifth coordination site is occupied by a water molecule.

Conformational analysis of octa- and tetrahalogenated tetraphenylporphyrins and their metal derivatives

Abstract A new maximally diagonal force field for molecular modelling of metalloporphyrins is developed and optimized on the crystal structures of nickel(II) porphine, nickel(II) mono- tert -butylporphyrin and nickel(II) di- tert -butylporphyrin. It is then used to investigate non-planar distortions of octa- and tetrachloro tetraphenylporphyrins (TPP) and their Ni(II) and Tb(III) complexes. Molecular mechanics (MM) calculations reproduced very well the structure of Tb(III) octachloro-TPP (so far the only example of a crystallographically characterized chloro TPP metal derivative). Normal-coordinate structural decomposition (NSD) analysis was performed on the equilibrium structures obtained by MM calculations. As expected, sad distortion dominates in octachloro structures irrespective of the presence or the size of the central metal atom; dom distortion dominates in tetrachloro structures with large Tb(III) central atom, while sad , ruf , wav and pro distortions are present in various amounts in other tetrachloro structures (TPP free base and Ni(II) complex) depending on the pattern of peripheral chloro substitution on the pyrrole rings. Other observed regularities are: reduction of the conformational flexibility of the porphyrin core upon metallation, and increase of the dihedral angle between the phenyl groups and the mean LSQ plane of the porphyrin core, as well as the overall increase in structural regularity upon the increase of the size of the central metal atom.

Crystal structure and conformational analysis of s-cis-(acetylacetonato)(ethylenediamine-N,N′-diacetato)-chromium(III)

AbstractThe crystal structure of [Cr(edda)(acac)] (edda = ethylediamine-N,N′-diacetate; acac = acetylacetonato) has been determined by a single crystal X-ray diffraction study at 150 K. The chromium ion is in a distorted octahedral environment coordinated by two N and two O atoms of chelating edda and two O atoms of acac, resulting in s-cis configuration. The complex crystallizes in the space group P21/c of the monoclinic system in a cell of dimensions a = 10.2588(9), b = 15.801(3), c = 8.7015(11) Å, β =101.201(9)° and Z = 4. The mean Cr-N(edda), Cr-O(edda) and Cr-O(acac) bond distances are 2.0829(14), 1.9678(11) and 1.9477(11) Å while the angles O-Cr-O of edda and O-Cr-O of acac are 171.47(5) and 92.72(5)°, respectively. The crystal structure is stabilized by N–H⋯O hydrogen bonds linking [Cr(edda)(acac)] molecules in distinct linear strands. The visible electronic and IR spectroscopic properties are also discussed. An improved, physically more realistic force field, Vibrationally Optimized Force Field (VOFF), capable of reproducing structural and vibrational properties of [Cr(edda)(acac)] was developed and its transferability demonstrated on selected chromium(III) complexes with similar ligands. Figure1D strand of hydrogen-bonded ···Δ -[Cr(R,R-edda)-(acac)]···Λ-[Cr(S,S-edda)(acac)]···in the crystal structure of [Cr(edda)(acac)]

COMPLEXES OF MOLYBDENUM(V) AND LANTHANUM(III) WITH 2′,2″′-(2,6-PYRIDINEDIYLDIETHYLIDENE)-DIOXAMOHYDRAZIDE (H2DAPSOX)

Abstract In a template synthesis from ethanolic solution of MoOCl3, 2,6-diacetylpyridine (dap), and semioxamazide (sox), in the molar ratio 1:1:2, a dimeric molybdenum(V) complex [Mo2O2(H2dapsox)2]Cl6 · 4H2O (where H2dapsox = 2′,2″′-(2,6-pyridinediyldiethylidenedioxa-mohydrazide) was obtained. In a similar reaction, starting from La(NO3)3 · 6H2O, the complex [La(H2dapsox)(NO3) x ](NO3)3-x · 1/2EtOH (x = 1,2), having coordination number 9, was obtained. In the latter complex two NO3 Groups were bidentately coordinated in the solid state, but only one in the solution. Besides [Mo2O2(TPP)2] (TPP = tetraphenylporphyrin), the molybdenum(V) complex [Mo2O2 (H2dapsox)2]Cl6 · 4H2O is the only other known example of a dimeric μ-oxodimolybdenum(V) species that is paramagnetic (μ = 0.95 BM). One of the Mo atoms has pentagonal bipyramidal coordination, and the other pentagonal pyramidal coordination. In aqueous solution a rare example of a pentagonal pyramidal ion [MoO(dapsox)]+ is presumably present. Solution EPR spectra (at 77K) cannot be related to either of the two known types of Mo(V) species based on the extent of g anisotropy. The substances also were characterized by IR and electronic spectroscopy, and by thermal analysis.

Structural analysis of conformational flexibility in (aqua)(propanediamine- N , N ′-diacetato- N -propionato)chromium(III) dihydrate. Crystal structure of cis -polar, trans (H2O,O5)-[Cr(1,3-pddap)(H2O)]·2H2O

The quinquedentate complex trans(H2O,O5)-[Cr(1,3-pddap)(H2O)] · 2H2O (where 1,3-pddap is the 1,3-propanediamine-N,N ′-diacetate-N-3-propionate ion) was prepared and its structure established by X-ray diffraction method. It crystallizes in the orthorhombic space group Pna21, a = 17.290(2), b = 10.821(2), c = 7.872(1) Å, Z = 4. The metal atom is surrounded octahedrally with two nitrogen and three oxygen donors of (1,3-pddap)3−, forming two six-membered and two five-membered metal chelate rings, and with one water molecule occupying the trans position with respect to the oxygen of the axial glycinate ring. Conformational analysis of the five geometrical isomers of [Cr(1,3-pddap)(H2O)], performed with the Consistent Force Field (CFF) program and recently developed edta force field, revealed that the global minimum is indeed the trans(H2O,O5) isomer with the geometry in a very good agreement with the crystallographic structure. General patterns for the conformational preferences of edta-type complexes of trivalent first-row transition metals are exposed and discussed.

The anisotropic π-effect of the nitro group in ammine-nitro cobalt(III) complexes

Abstract The analysis of crystal structures of the series of ammine-nitro cobalt(III) complexes has shown that CoNO2 bond lengths can be successfully rationalized on the basis of the anisotropic π-effect of nitro groups. This effect governs both the orientation of the nitro group and the CoNO2 length, and explains the irregularities in the increase of the latter with the increasing number of NO, groups present in the coordination sphere.