Selective coordination

Thus it appears that the presence of two soft carbons on the palladium stabilizes the trans coordination of hard ligands and drives the selective coordination of ambidente ligands through their hardest atom. These results, as those described in the previous scheme, constitute other examples of the antisymbiotic effect which can be observed in soft palladium(II) complexes. 

Sanders (14) has exploited the strong and selective coordination of phosphine donor groups to Ru(II) to construct hetero-dimetallic porphyrin dimers (17, Fig. 5). An alkyne-phosphine moiety introduced on the periphery of a free base or metalloporphyrin (M = Zn or Ni) spontaneously coordinates to a Ru(II)(CO) porphyrin when the two porphyrins are mixed in a 1 1 ratio. Coordination is characterized by a downfield shift of the 31P resonance (A<531P = 19 ppm). There is no evidence of self-coordination of the zinc porphyrin at 10 6 m in toluene, there is no shift in the Soret band in the UV-Vis absorption spectrum. The Ni-Ru dimer was observed by MALDI-TOF mass spectrometry. Heating the Ru(II)CO porphyrin with 2 equivalents of the phosphine porphyrins led to quantitative formation of trimeric assemblies. 

Selection of an active-site model almost always leads to truncations of the hydrogen-bond network. Upon optimization of the active-site structure, this may lead to the formation of artificial hydrogen bonds that disrupt the structure. Freezing selected coordinates in the active-site model can prevent some of these hydrogen bonds to form. Another remedy could be to include more residues around the metal center, but larger QM models are much more expensive and there will probably still be truncated hydrogen bonds, although further away from the reaction center. 

The sulfonyl group is achiral in itself but has prochiral oxygens, and sultanas have served as important auxiliaries in chiral reactions. Scheme 4 shows a simple example of selective coordination of a chiral Lewis acid to the sulfonyl oxygen. 

The high E-stereoselectivity can be explained by the face-selective coordination of an allene to an acylpalladium complex (Scheme 16.57). 

To develop an additive that selectively coordinates with salt anions and frees lithium ion for conduction, McBreen and co-workers pursued a molecular design and tailor-synthesis approach that yielded several families of novel compounds based on nitrogen or boron centers with strongly electron-withdrawing substituents. 

For a complete picture, however, some critical remarks have to be added. For medium-sized molecules, the obviously successful combination of theoretical prediction and experimental verification rather should be considered as an individually tailored interplay, which is based on numerous assumptions i.e. so-called chemical intuition. Tobeginwith, any hypersurface design confined to a few selected coordinates will only cover the aspects introduced thereby. Even several independent cuts through the (3n-6) dimensional hyperspace cannot unravel the complexity of a molecule in a specific molecular state. And although semiempirical methods - especially MNDO (10)- are not only fast but to quite an extent also reliable, their numerical accuracy should not be overstressed. Thus semiempirical hypersurfaces might better be considered as supplying trends in essential features, which can be compared to and correlated with measurement data to add the numerical accuracy, and, in return, to test and to substantiate all underlying assumptions. 

Karan, C. Miller, B. L. RNA-selective coordination complexes identified via dynamic combinatorial chemistry. J. Am. Chem. Soc. 2001, 123, 7455-7456. 

In accordance with Emery s retro-hydroxamate ferrichrome, mentioned above, two retro analogs of the linear ferrioxamine G and cyclic desferrioxamine E (129 and 130, respectively) were prepared. The iron-chelating properties were compared to DFO, showing that the linear retro-desferrioxamine G (131) binds iron faster and the cyclic retro desferrioxamine E (132) has improved affinity to iron, compared to the linear DFO. Based on these resnlts, many retro-hydroxamate ferrioxamines were prepared. In a later paper, Akiyama and coworkers reported the attachment of -cyclodextrin, a cyclic oligosaccharide, composed of seven a-D-glucopyranoside units, linked from position 1 to position 4, to linear retro-hydroxamate ferrioxamines (133 and 134), which formed 1 1 iron(III) complexes. Influenced by the chiral -cyclodextrin gronp, 133 and 134 formed A-selective coordination around the metal ion. In addition, Akiyama proposed that the... 

The assembly formation of heterobidentate ligated complexes was studied in detail using bidentate ligand 13"b as a typical example (Figure 8.8). UV/Vis titrations and NMR spectroscopy experiments showed that the pyridyl moiety of b selectively coordinates to thezinc(ii)porphyrinl3withabindingconstantofK(ib) = 3.8x lO m. An increase in the Kfor 13 b to 64.5 X lO was observed in the presence of metal... 

Selective coordination and stabilization of reaction intermediates in solution and in... 

Enantiomer-selective polymerization of MBMA has also been attained by using the reaction products of chiral amine compounds, 168 and 169, with cyclohexylmagnesium bromide as initiator [242,243] and by using the aluminum porphyrin complex 170 in the presence of optically active aluminum alkoxide compounds 171a-e [244], In the latter systems, the enantiomer selection is based on enantiomer-selective coordination of the chiral aluminum compounds to MBMA as revealed by NMR analysis. With 171e as a catalyst, the ee of the unreacted monomer is 40% at 75% monomer conversion ratio in the polymerization at -70°C. 

Balzani, Vogde et al. demonstrated for dendritic cyclam compounds that signalling units are dendritically amplified by increasing the number of peripheral fluorophores. Opportunities for these PET sensor systems exist in the combination of selective coordination sites with the dendritic architecture. The mechanism resembles that described above, but can be influenced by the introduction of dendritic units of different generations into the sensor system. The distance between the terminal naphthalene groups of the dendron and the receptor (ion as guest in the cyclam core) is of crucial importance for electron transfer (ET) (Fig. 8.15) [53],... 

The use of asymmetric bridging ligands, which permits a selective coordination of the different donor atoms to the metal centers and leads to supported gold-thallium interactions forced by the ligand. 

If the individual electrons of a beam have different spin directions which remain constant in a statistical average, one can describe the polarization of this beam by the result of three successive measurements of the relative excess Pt of spin projections into three preselected Cartesian direction i = x, y, and z. This is demonstrated for an electron beam of six electrons with precessing spins as indicated in Fig. 9.2. The intensities Nt for electrons with (+) spin projections along the selected coordinate axes then follow from individual measurements for each of the six precessing spins ... 

Continue to select coordinate pairs until a minimum of three random sampling points are located for each type of bulk PCB remediation waste. 

The y-z plane in Fig. 4.1.18 is defined by the position of A, the center-of-mass position of BC, and the velocity v of A relative to BC with v being parallel to the, i-axis. The origin of the coordinate system is put at the center-of-mass position of BC. This leads to the following initial values of selected coordinates and momenta 0 <34=0 no x-component since we are in the y-z plane ... 

Oxygen-17 Chemical Shifts for Selected Coordination Types3... 

Using the same technique of CP/MASS I3C-NMR, Kamide et al.133) disclosed that in aged solid alkali cellulose the molecular motion is relatively rapid, as compared with solid cellulose intramolecular hydrogen bonding is present, and the sodium ion selectively coordinates to the hydroxyl oxygen at the C2 position of cellulose. 

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