Influence Of Coordination Geometry Research Articles

Influence of Coordination Geometry on the Ring Opening of Benzoxazine: Oxidovanadium(V), Dioxidomolybdenum(VI) and Dioxidotungsten(VI) Complexes of Benzoxazine Based Ligands and their Bio Inspired Catalytic Applications

AbstractMono benzoxazine appended N‐capped amino bis(disubstitutedphenol) ligands [II(a–c)] upon reaction with VVO(OEt)3 in a 1 : 1 molar ratio in EtOH/MeOH give [{VVO}en(3,5‐dtbb)3] (1), [{VVO}en(3‐tb,5‐mb)3] (2) and [{VVO}en(3,5‐dmb)3] (3). During the reaction, the benzoxazine ring opens with the loss of methylene group and the newly formed ligands, N,N‐bis(2‐hydroxy‐3,5‐disubstitutedbenzyl)‐N’‐2‐hydroxy‐3,5‐disubstituted benzyledene‐1,2‐diaminoethane [III(a–c)], behave as tribasic pentadentate in these complexes. Under similar conditions, when [MVIO2(acac)2] (M=Mo or W; Hacac=acetylacetone) reacts with II(a–c), these ligands retain their identity and form cis‐[MVIO2] complexes, [{MoVIO2}{en(3,5‐dtbb)2(6,8‐dtbbenzox)}] (4), [{MoVIO2}{en(3‐tb,5‐mb)2(6‐tb,8‐mbbenzox)}] (5) and [{MoVIO2}{en(3,5‐dmb)2(6,8‐dmbenzox)}] (6), [{WVIO2}{en(3,5‐dtbb)2(6,8‐dtbbenzox)}] (7), and [{WVIO2}{en(3‐tb,5‐mb)2(6‐tb,8‐mbbenzox)}] (8). However, the benzoxazine ring ruptures in case of ligand IIc under these conditions and form [{WVIO2}{en(3,5‐dmb)3}] (10), similar to complexes 1–3. Complex [{WVIO2}{en(3,5‐dmb)2(6,8‐dmbenzox)}] (9), having structure similar to 4–8, could only be obtained when the reaction was carried out in toluene. Not only 9, even complexes 4–8 can be isolated in toluene. Rupturing of both benzoxazine rings has also been experienced when ligands 1,2‐bis(6,8‐disubstitutedbenzo[e][1,3]oxazin‐3(4H)‐yl)ethane [I(a–c)] react with [MVIO2(acac)2] (M=Mo or W) in MeOH and give salan type complexes [(MVIO2)en(3,5‐dtbb)2] [M=Mo (11), M=W (14)], [(MVIO2)en(3‐tb,5‐mb)4] [M=Mo (12), M=W (15)] and [(MVIO2)en(3,5‐dmb)4] [M=Mo (13), M=W (16)]. Complexes 1–9 have been used as catalyst for the multicomponent Biginelli reaction for the synthesis of 3,4‐dihydropyrimidin‐2(1H)‐ones (DHPMs) and oxidative bromination of phenol derivatives.

Ruthenium catalysts for water oxidation involving tetradentate polypyridine-type ligands.

A series of Ru(II) complexes that behave as water oxidation catalysts were prepared involving a tetradentate equatorial ligand and two 4-substituted pyridines as the axial ligands. Two of these complexes were derived from 2,9-di-(pyrid-2'-yl)-1,10-phenanthroline (dpp) and examine the effect of incorporating electron-donating amino and bulky t-butyl groups on catalytic activity. A third complex replaced the two distal pyridines with N-methylimidazoles that are more electron-donating than the pyridines of dpp and potentially stabilize higher oxidation states of the metal. The tetradentate ligand 2-(pyrid-2'-yl)-6-(1'',10''-phenanthrol-2''-yl)pyridine (bpy-phen), possessing a bonding cavity similar to dpp, was also prepared. The Ru(II) complex of this ligand does not have two rotatable pyridines in the equatorial plane and thus shows different flexibility from the [Ru(dpp)] complexes. All the complexes showed activity towards water oxidation. Investigation of their catalytic behavior and electrochemical properties suggests that they may follow the same catalytic pathway as the prototype [Ru(dpp)pic2](2+) involving a seven-coordinated [Ru(IV)(O)] intermediate. The influence of coordination geometry on catalytic performance is analyzed and discussed.

The influence of coordination geometry and valency on the K-edge absorption near edge spectra of selected chromium compounds

Abstract X-ray absorption spectra at the chromium K-edge are reported for a number of selected chromium compounds of known chemical structure. The spectra were obtained with use of synchrotron radiation available at the EL ectron S tretcher A ccelerator ELSA in Bonn. The compounds studied include the tetrahedrally coordinated compounds Ca2Ge0.8Cr0.2O4, Ba2Ge0.1Cr0.9O4, Sr2CrO4, Ca2(PO4)x(CrO4)1−xCl (x=0.25,0.5), Ca5(CrO4)3Cl, CrO3, the octahedrally coordinated compounds Cr(II)-acetate, CrCl3, CrF3, Cr2O3, KCr(SO4)2 · 12H2O, CrO2 and cubic coordinated metallic chromium. In these compounds chromium exhibits a wide range of formal oxidation states (0 to VI). The absorption features in the near edge region are shown to be characteristic of the spatial environment of the absorbing atom. The occurrence of a single pre-edge line easily allows one to distinguish between tetrahedral and octahedral coordination geometry, whereas the energy position of the absorption edge is found to be very sensitive to the valency of the excited chromium atom. Calculations of the ionisation potential of Cr in different oxidation states using the non-relativistic Hartree–Fock method (Froese–Fischer) confirm that the ionisation limit shifts to higher energy with increasing Cr valency. More detailed information on the electronic structure of the different compounds is gained by real-space full multiple scattering calculations using the FEFF8 code.

EHT Model Studies on Coordination‐Sphere Effects in the Elementary steps of d0 transition‐metal‐catalyzed Ziegler‐Natta polymerization

AbstractIn a theoretical investigation on Ziegler‐Natta catalysis, the influence of the coordination number and ligand type of model compounds in relevant reaction steps were studied. Thus, by using the MEHT method, insertion reactions of ethylene into Zr–CH3 and Zr–H bonds were analyzed in systems of the type [Zr(C2H4)R4R′]−[Zr(Cp)(C2H4)R2R′], and [Zr(Cp)(C2H4)R3R′]−(R=R′=CH3, R=CH3, R′=H). It was found that all processes do not have significant kinetic barriers, whereas the reverse reactions in particular the β‐hydride elimination have relatively high ones. The influence of coordination geometry and number on these transformations was found to be insignificant. While studying related conversions starting from [Zr(L)(C2H4)R3R′], [Zr(Cp)(C2H4)RR′(L)]+, and [Zr(Cp)(C2H4)R2R′(L)] (L = π‐donor, R=R′=CH3 or R=CH3, R′=H) compounds a pronounced π‐donor effect was observed. Methyl insertions in these cases showed a higher computed activation barrier than hydride migrations. An orbital basis for this phenomenon was provided and conclusions concerning chain‐length control in Ziegler‐Natta catalysis were drawn.