Metallocycle synthesis accelerated by high pressure

Reaction of cis-[FeH₂(dmpe)₂](1) with diphenylbutadiyne results in an insertion into both of the iron-hydride bonds to form an iron metallocycle. Spectroscopic and crystallographic data of [Fe(PhHCC₂CHPh)(dmpe)₂] (3) show 1,4-diphenylbutatriene is symmetrically bound to the metal via the central double bond. The reaction to form the metallocyclic complex is greatly accelerated by application of external pressure. A 41% yield of (3) is isolated after two days at atmospheric pressure or after approximately 75 min at 800MPa.     

Structural measures of element-oxygen bond covalency from the changes to the delocalisation of the carboxylate ligand

The data set of more than 40,000 crystal structures containing the carboxylate group that have been deposited in the CSD has been used to examine the structural changes that occur in the carboxylate C-O bond lengths upon binding to different elemental centres. We report here quantifiable structural changes that are dependent on the elemental centre with which the group is interacting. For the main-group elements the trends are entirely periodic and follow those traditionally associated with covalency; elements exhibiting electronegativity closest to that of oxygen exhibit the largest structural change. In addition, we find the measure is extendable to both the transition metals and the lanthanoids and actinoids. Amongst the transition metals the trends of Pauling neutrality are not only maintained, but are quantifiable. The difference between the two C-O bond lengths increases with oxidation state and decreases with an increase in coordination number. All of the lanthanoids exhibit covalency within error of each other and the bonds to the actinoids are found to be more covalent than those to the lanthanoids. From the data analysis we are able to derive a correlation between the lengths of the two carboxylate arms that allows us to quantify percentage covalent character defined in terms of the resonance contributions to the carboxylate group.     

Strain energy minimization study of the mechanism of,and the barrier to,conformational interconversion in five-membered diamine chelate rings

The method of Lagrangian multipliers is used to constrain torsion angles during molecular mechanics refinement for the purpose of plotting strain energy against a reaction coordinate. A complete two-dimensional analysis of the conformational interconversion from δ- to λ-[Co(ethane-1,2-diamine) (NH₃)₄]³⁺ reveals a mechanism in which the transition state geometry has an envelope conformation and an inversion barrier of 15.7 kJ mol^?1. Substitution at the carbon atoms, variation of the metal-nitrogen distance, and replacement of the amine ligands with bidentate amines only slightly alters the inversion barrier. Substitution at the nitrogen atoms of the bidentate ligand increases the inversion barrier significantly to 24.6 kJ mol^?1 for (N,N,N′,N′-tetramethylethane-1,2-diamine) [(NH₃)₄]³⁺.     

XAFS studies of anti-inflammatory dinuclear and mononuclear Zn(II) complexes of indomethacin

Zinc K-edge X-ray absorption fine structure (XAFS) experiments were performed in the solid and solution states at low temperature (10 K), on dimeric and monomeric anti-inflammatory Zn(II) complexes of indomethacin [1-(4-chlorobenzoyl)-5-methoxy-2-methyl-1H-indole-3-acetic acid=IndoH] of the formula [Zn2(Indo)4L2] [L=pyridine (Py), N,N-dimethylacetamide (DMA)], [Zn(Indo)2L2] [L=ethanol (EtOH), methanol (MeOH)], and Zn(II) acetate dihydrate [Zn(OAc)2(OH2)2]. The bond distances and angles obtained from multiple-scattering fits to the XAFS data of the Zn(II) dimeric complexes in the solid and solution states exhibit excellent correspondence with those obtained from single crystal diffraction studies. The Zn...Zn separations of 2.97 and 2.96 A and carboxylate group O-C-O angles of 125 degrees for powdered [Zn2(Indo)4(Py)2] and [Zn2(Indo)4(DMA)2] agree well with the XRD values of 2.969(1) and 2.9686(6) A and 125.8(4) degrees and 126.1(2) degrees, respectively. The calculated Zn-O(RCOO) and Zn-L bond distances of 2.03 and 2.04 A, or 2.02 and 1.98 A for Py or DMA complexes, respectively, also agree well with crystallographic data. The X-ray powder diffraction data on samples of the monomers exhibited additional reflections apart from those due to the crystallographically characterized cis-[Zn(eta2-O,O'-Indo)2L2], but microanalyses were consistent with this formulation. Therefore, mixed models that contained the cis complex and a second component consisting of a trans-six-coordinate complex, a five-coordinate complex, or a four-coordinate complex were used to model the XAFS. The best fits to the XAFS data were obtained with a mixture of the cis-six-coordinate complex and a four-coordinate complex containing two monodentate Indo ligands. The bond lengths for the six-coordinate structure were consistent with those determined on a single crystal, and those for the four-coordinate complexes were consistent with related four-coordinate structures with two monodentate carboxylate ligands. Dissolution of the dimer (DMA adduct) in DMF resulted in a mixture of dimer and monomer species as shown by MS XAFS fitting. This is the first time that solution structures have been determined for anti-inflammatory Zn(II) complexes, and this is an important first step in understanding the pharmacology of the complexes.     

Studies on pyridazine azide cyclisation reactions

Reaction of sodium azide with 4-methyl-3,5,6-tribromopyridazine results in the formation of 3,5,6-triazide intermediate which could cyclise to give two possible bicyclic products while ab initio calculations show that the formation of a tricyclic compound is extremely energetically unfavourable. However, experimentally, only one major product is isolated. The structure of this unstable product has been conclusively established by X-ray crystallography as 3,5-diazido-4-methyl[1,5-b]tetrazolopyridazine confirming theoretical predictions.     

The preparation and characterization of trans-platinum(IV) complexes with unusually high cytotoxicity

The physical and biological properties have been determined for three Pt(IV) complexes with trans amine ligands: trans,trans,trans-[PtCl(2)(OH)(2)(dimethylamine)(isopropylamine)] (1(IV)), trans,trans,trans-[PtCl(2)(OH)(2)(dimethylamine)(methylamine)] (2(IV)) and trans,trans,trans-[PtCl(2)(OH)(2)(isopropylamine)(methylamine)] (3(IV)). The crystal structures of 2(IV) and 3(IV) reveal substantial strain resulting from repulsion between the amine ligands and the chlorido and hydroxido ligands. All three complexes have reduction potentials in the range -666 to -770 mV, values usually associated with high resistance to reduction and low cytotoxicity. However, the complexes all demonstrate surprisingly high cytotoxicity with values and trends that closely follow those seen for the Pt(II) congeners of these complexes. These results are consistent with more rapid reduction of the Pt(IV) complexes than would be expected based on the reduction potentials, perhaps associated with the trans arrangement of the chlorido ligands.