Biomedical subjects
Publications and source records attributed to Peter Sirsch.
The bonding in Mn(eta2-SiH) complexes is interpreted in terms of an asymmetric oxidative addition whose extent is controlled by the substitution pattern at the hypercoordinate silicon centre, and especially by the ligand trans to the eta2-coordinating SiH moiety.
A series of heteroleptic scandium SALEN complexes, [(SALEN)Sc(mu-Cl)]2 and (SALEN)Sc[N(SiHMe2)2] is obtained via amine elimination reactions using [Sc(N(i)Pr2)2(mu-Cl)(THF)]2 and Sc[N(SiHMe2)2]3(THF) as metal precursors, respectively. H(2)SALEN ligand precursors comprising H2Salen [(1,2-ethandiyl)bis(nitrilomethylidyne)bis(2,4-di-tert-butyl)phenol], H2Salpren [(2,2-dimethylpropanediyl)bis(nitrilomethylidyne)bis(2,4-di-tert-butyl)phenol], H2Salcyc [(1R,2R)-(-)-1,2-cyclohexanediyl)bis(nitrilomethylidyne)bis(2,4-di-tert-butyl)phenol] and H2Salphen [((1S,2S)-(-)-1,2-diphenylethandiyl)bis(nitrilomethylidyne)bis(2,4-di-tert-butyl)phenol] are selected according to solubility and ligand backbone variation ("=N-(R)-N=" bite angle) criteria. Consideration is given to the feasibility of [Cl --> NR2] and [N(SiHMe2)2--> OSiR3] secondary ligand exchange reactions. X-ray crystal structure analyses of donor-free (Salpren)Sc(N(i)Pr2), (R,R)-(Salcyc)Sc[N(SiHMe2)2], (Salen)Sc(OSi(t)BuPh2) and (Salphen)Sc(OSiH(t)Bu2) reveal (i) a very short Sc-N bond distance of 2.000(3) A, (ii) weak beta(Si-H)(amido)-Sc agostic interactions and (iii) an exclusive intramolecularly tetradentate and intrinsically bent coordination mode of the SALEN ligands with angle(Ph,Ph) dihedral angles and Sc-[N(2)O(2)] distances in the 124.27(9)-127.7(3) degrees and 0.638(1)-0.688(1) A range, respectively.
Reduction of [(triphos)NiCl2] (1) with an excess of NaBH4 in THF produces the paramagnetic Ni(I) complex [(triphos)Ni(eta2-BH4)] (2). X-ray crystallography shows 1 to be a square-planar Ni(II) species in which the phosphine ligand is bidentate, whereas 2 has pseudotetrahedral geometry at the Ni(I) center, with a tridentate phosphine and the borohydride ligand occupying a single coordination site. Density functional theory calculations show the unpaired electron in 2 to reside in an orbital located mainly on the Ni atom.
Ruthenium trihydrides containing N-heterocyclic carbene ligands display large quantum mechanical exchange couplings in their 1H NMR spectra: DFT calculations are used to explore this phenomenon and to compare them to their phosphine congeners.
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In this paper we describe a range of model d(0) metal ethyl compounds and related complexes, studied by DFT calculations and high resolution X-ray diffraction. The concept of ligand-opposed charge concentrations (LOCCs) for d(0) metal complexes is extended to include both cis-and trans-ligand-induced charge concentrations (LICCs) at the metal, which arise as a natural consequence of covalent metal-ligand bond formation in transition metal alkyl complexes. The interplay between locally induced sites of increased Lewis acidity and an ethyl ligand is crucial to the development of a beta-agostic interaction in d(0) metal alkyl complexes, which is driven by delocalization of the M-C bonding electrons. Topological analysis of theoretical and experimental charge densities reveals LICCs at the metal atom, and indicates delocalization of the M-C valence electrons over the alkyl fragment, with depletion of the metal-directed charge concentration (CC) at the alpha-carbon atom, and a characteristic ellipticity profile for the C(alpha)-C(beta) bond. These ellipticity profiles and the magnitude of the CC values at C(alpha) and C(beta) provide experimentally observable criteria for assessing quantitatively the extent of delocalization, with excellent agreement between experiment and theory. Finally, a concept is proposed which promises systematic control of the extent of C-H activation in agostic complexes.
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In this paper we present the results of density functional theory (DFT) calculations on the ethyl ligand and some related organic moieties; we then proceed to consider a range of alkyllithium complexes studied by DFT calculations and high-resolution X-ray and neutron diffraction. Topological analysis of the charge density is used to follow changes in the electronic structure of the organic fragment. The charge concentrations (CCs) in the valence shell at the alpha and beta atoms reveal faithfully the delocalization of the lone pair at the Calpha atom or of the Li-C bonding electrons. Negative hyperconjugation is thus shown to arise from delocalization of the lone pair or the Li-C bonding electrons over the alkyl fragment, with depletion of the metal-directed charge concentration at Calpha, and characteristic ellipticity profiles for the bonds involved in hyperconjugative delocalization. In the case of so-called lithium agostic complexes, we show that close Li.H contacts are a consequence of this delocalization and further secondary interactions, with Li.H-C agostic interactions, playing only a minor role. The ellipticity profiles and the magnitude of the CCs at Calpha provide a quantitative measure of the extent of delocalization, and show excellent agreement between experiment and theory.