Ytterbium complex

Ytterbium, trinitratotris(dimethyl sulfoxide)-structure, 1, 97 Ytterbium, tris(acetylacetone)(4-ammo-3-penten-stereochemistry, 1,81 Ytterbium complexes acetylacetone, 2,373 dipositive oxidation state hydrated ions, 3,1109 polypyrazolylborates, 2,255 Ytterbium(III) complexes ethyl glycinate, diacetate... 

Figure 6 LMCT-mediated energy transfer in an ytterbium complex.

Scheme 28 explains the stereochemical outcome from the tandem radical cyclization in the presence of the [Yb(Ph-pybox)(OTf)3] (pybox = 2,6-bis(2-oxazolin-2-yl)pyridine). The ytterbium complex 107 is shown in an octahedral geometry (with one triflate still bound to the metal) where re-face cyclization is favored due to the steric interactions of the substrate and the ligand s phenyl groups. The 6-endo cyclization takes place via a chair-like transition state to yield a tertiary radical 108 followed by a ring flip and... 

More recently reported was a bridged bis(amidinate)-isopropoxy ytterbium complex 85 (Fig. 14), which displayed excellent polymerization activity in conjunction with a good control of the polymerization of L-lactide with a linear increases of polymer molecular weights (M ) with [M]o/[I]o [103]. It was also evidenced that 85 was even more active in the polymerization of lactide than its structural analog with bridging phenoxide group, as an isopropoxide is intrinsically more nucleophilic than a phenoxide. 

Beeby, A. Clarkson, I. M. Dickins, R. S. Faulkner, S. Parker, D. Royle, L. de Sousa, A. S. Williams, J. A. G. Woods, M. Non-radiative deactivation of the excited states of europium, terbium and ytterbium complexes by proximate energy-matched OH, NH and CH oscillators an improved luminescence method for establishing solution hydration states. J. Chem. Soc., Perkin Trans. 2 1999, (3), 493-503. 

CjHjS, Thiophene, tetrahydro-gold complexes, 26 85-87 C4H,NO, 2-Propenamide, 2-methyl-nickel complex, 26 205 C4H1()02, Ethane, 1,2-dimethoxy-solvates of chromium, molybdenum, and tungsten carbonyl cyclopentadienyl complexes, 6 343 tungsten complex, 26 50 ytterbium complex, 26 22 C4H i02.NaC5H5, Ethane, 1,2-dimethoxy-compd. with cyclopentadienylsodium, 26 341... 

The basis for applying the LIS quantitatively to problems in stereochemistry depends upon expressions including the term (3 cos2 — l)r-3, where r is the distance from the carbon to the lanthanide ion and the angle d is defined by the symmetry axis of the complex and the vector from the lanthanide ion to the carbon in question. This application depends on a LIS imposed entirely by the pseudocontact mechanism. It has been shown that the contact mechanism is important for europium and praseodymium complexes in 13C NMR for distances up to four bonds from the site of complexation, and that ytterbium complexes interact with 13C nuclei largely, if not entirely, by the pseudocontact process. (12, 13)... 

Chen, R., Qian, C. and de Vries, J. G. Asymmetric epoxidation of a,/ -unsaturated ketones catalyzed by chiral ytterbium complexes, Tetrahedron Lett., 2001, 42, 6919-6921. 

A similar reaction between diphenyl ditellurium and bisfpentamethylcyclopentadienyl] diamineytterbium in toluene produced the ytterbium complex with one benzenetellurolate, one amine ligand, and two cyclopentadienyl groups in a pscudotetrahedral arrangement1. 

The transamination of the anionic amido ytterbium complex LiYb (N Pr2)4 with aryloxo-functionalised NHC imidazolium salt precursors affords bis-aryloxo-NHC monoamido ytterbium(III) complexes (Scheme 32)71 The complexes are isostructural with one another in the solid state and exhibit average ytterbium-NHC bond distances of 2.487 A (R = Me) and 2.535 A (R = Pr). These are comparable to the monodentate lanthanide(III)-NHC bond lengths where the ligand possesses and amido tether, though a direct comparison with like-for-like metal centre is not possible. The longer ytterbium-NHC bond length where R = Pr is likely as a result of the increased bulk of the ligand. 

Scheme 32 Synthesis of bis-aryloxo-NHC monoamido ytterbium complexes.

The complex has a tetrahedral configuration with Lu-C bonds of 2.42-2.50 A bond length. The bulky 2,6-dimethylphenyl group provides steric limitations in the complex. Ytterbium complex isomorphous with lutetium is also known. The electronic structure and the nature of chemical bonding of the lutetium complex was studied by the INDO method [26]. The MO s of Lu(CgH9)4 ion and the charge distribution are shown in Figs 5.4 and 5.5, respectively. 

The contact contribution to the shift induced by the ytterbium complex is minimal and a similar observation has been made for proton shifts in Tm(DPA)3 and Yb(DPA)3 complexes in aqueous solution [23]. 

Some salient points to note are (i) the model is in accord with the experimental results, (ii) nuclei at a and yS positions to the coordinating atom show deviations from the model due to contributions of other shift mechanisms. Thus the best procedure for the elucidation of molecular structure by using lanthanide reagents consists of (i) to obtain the relative magnitude of geometrical function, G for different ligand nuclei from the slopes of A bT versus 1 / T plots, (ii) since temperature dependence of ytterbium complexes conforms to the model, use of ytterbium complexes is prudent, (iii) in cases where the temperature dependence is interfered with effects due to chemical equilibrium or exchange, data for a number of lanthanides at room temperature may be obtained and plots of equation... 

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