Fluxionality in a paramagnetic seven-coordinate iron(II) complex: a variable-temperature, two-dimensional NMR and DFT study.

The preparation and detailed characterizations of the high-spin seven-coordinate complexes [M(kappa(7)N-L)](ClO(4))(2) (M = Mn(II), Fe(II); L = N,N,N',N'-tetrakis(2-pyridylmethyl)-2,6-bis(aminomethyl)pyridine) are described. The X-ray crystal structures reveal seven-coordinate metal complex ions. Consideration of continuous shape measures reveals that the coordination environments about the metal ions are better described as having (C(s)) face-capped trigonal prismatic symmetry than (C(2)) pentagonal bipyramidal symmetry. The (S = (5)/(2)) Mn(II) species shows complicated X-band electron paramagnetic resonance (EPR) spectra and broad, unrevealing (1)H NMR spectra. In contrast, the (S = 2) Fe(II) complex is EPR-silent and shows completely interpretable (1)H NMR spectra containing the requisite number of paramagnetically shifted peaks in the range delta +150 to -60. The (13)C NMR spectra are likewise informative. Variable-temperature (1)H NMR spectra show coalescences and decoalescences indicative of an intramolecular process that pairwise-exchanges the nonequivalent pyridylmethyl "arms" of the two bis(pyridylmethyl)amine (bpa) domains. A suite of NMR techniques, including T(1) relaxation measurements and variable-temperature (1)H-(1)H correlation spectroscopy, (1)H-(1)H total correlation spectroscopy, (1)H-(1)H nuclear Overhauser effect spectroscopy/exchange spectroscopy, and (1)H-(13)C heteronuclear multiple-quantum coherence experiments, has been used to assign the NMR spectra and characterize the exchange process. Analysis of the data from these experiments yields the following thermodynamic parameters for the exchange: DeltaH++ = 53.6 +/- 2.8 kJ mol(-1), DeltaS++ = -10.0 +/- 9.8 J K(-1) mol(-1), and DeltaG++ (298 K) = 50.6 kJ mol(-1). Density functional theory (B3LYP) calculations have been used to explore the energetics of possible mechanistic pathways for the underlying fluxional process. The most plausible mechanism found involves dissociation of a pyridylmethyl arm to afford a strained six-coordinate species followed by rebinding of the arm in a different position to afford a new seven-coordinate transition state in which the pyridylmethyl arms within each bpa domain are essentially equivalent; the calculated energy barrier for this process is 53.5 kJ mol(-1), in good agreement with the observations.

Rhodium, palladium and platinum complexes of tris(pyridylalkyl)amine and tris(benzimidazolylmethyl)amine N4-tripodal ligands.

To investigate the influence of a potentially N4-tripodal amine ligand on the structure and internal exchange processes of its complexes with late transition metals, five rhodium, six palladium and two platinum complexes have been prepared from seven alkyl-bridged N-heterocyclic amine tripodal ligands: tris(2-pyridylmethyl)amine, (2-(2-pyridylethyl))bis(2-pyridylmethyl)amine, bis(2-(2-pyridylethyl))-2-pyridylmethylamine, bis(2-(2-pyridylethyl))amine, ((6-(hydroxymethyl)-2-pyridyl)methyl)bis(2-pyridylmethyl)amine, tris(2-benzimidazolylmethyl)amine (tbima) and tris(3-ethyl-2-benzimidazolylmethyl)amine. Single-crystal X-ray diffraction studies were completed for ten complexes: the d6-rhodium(III) complexes are octahedral with kappa 4 N-bound ligands, whereas the d8-palladium(II) and d8-platinum(II) complexes are square planar, kappa 3 N-bound by the tripodal ligand with a dangling N-donor leg, except for the unusual [Pd2(tbima)2Cl2]Cl2 dimer in which each palladium(II) ion is square planar and bound by two benzimidazole legs from one tbima ligand, one leg from the other tbima ligand and a chloride ancillary ligand. Cation bilayers are a common structural motif in the crystal structures. Variable-temperature 1H NMR studies reveal exchange occurs between the coordinated and dangling N-donor legs in the palladium and platinum complexes. Exchange free energy (Delta G++ c) values have been calculated and some general rules governing the favoured complex structures and exchange pathways elucidated. The palladium(II) and platinum(II) complexes of a ligand with an pyridylethyl leg are unstable with respect to elimination of vinylpyridine.

Novel one-dimensional structures and solution behaviour of copper(II) bromide and chloride complexes of a new pentapyridyldiamine ligand.

Copper(II) bromide and chloride complexes of the new heptadentate ligand 2,6-bis(bis(2-pyridylmethyl)amino)methylpyridine (L) have been prepared. For the bromide complexes, chains of novel, approximately C2-symmetric, chiral [Cu2(L)Br2]2+ 'wedge-shaped' tectons are found. The links between the dicopper tectons and the overall chirality and packing of the chains are dictated by the bromide ion content, not the counter anion. In contrast, the chloride complexes exhibit linked asymmetric [Cu2(L)Cl3]+ tectons with distinct N3CuCl2 and N4CuCl2 centres in the solid. The overall structures of the dicopper bromide and chloride units persist in solution irrespective of the halide. The redox chemistry of the various species is also described.

17O quantitative nuclear magnetic resonance spectroscopy of gasoline and oxygenated additives.

17O Nuclear magnetic resonance (NMR) spectroscopy allows exclusive detection and direct quantification of oxygenates in gasoline unaffected by its hydrocarbon content, using the internal standard quantitative NMR (QNMR) method. Chemical shifts of 24 oxygen-containing compounds as potential additives and contaminants have been measured in gasoline and corrected values of deltaO 18.1 and 3.9 determined for neat methyl tert-butyl ether (MTBE) and neat di-n-butyl ether, respectively. Quantification of ethanol in gasoline can be readily achieved by 17O QNMR with dimethyl sulfone as an internal standard reference material, at the levels currently used in retail gasolines (1-20%). In addition, the simultaneous detection and quantification of the oxygenates methanol, ethanol, 2-propanol, tert-butyl alcohol, and MTBE in gasoline has been established to further demonstrate the specificity of the method. 17O NMR has distinct advantages over 1H and 13C QNMR methods, and although it cannot reliably differentiate 1-propanol, 1-butanol, 1-pentanol, and isopentyl alcohol, 17O NMR does allow the rapid and unambiguous identification of unexpected oxygenates such as acetates and ketones found as contaminants in some retail gasoline.