Coordination to transition metal

The aromatic shifts that are induced by 5.1c, 5.If and S.lg on the H-NMR spectrum of SDS, CTAB and Zn(DS)2 have been determined. Zn(DS)2 is used as a model system for Cu(DS)2, which is paramagnetic. The cjkcs and counterion binding for Cu(DS)2 and Zn(DS)2 are similar and it has been demonstrated in Chapter 2 that Zn(II) ions are also capable of coordinating to 5.1, albeit somewhat less efficiently than copper ions. Figure 5.7 shows the results of the shift measurements. For comparison purposes also the data for chalcone (5.4) have been added. This compound has almost no tendency to coordinate to transition-metal ions in aqueous solutions. From Figure 5.7 a number of conclusions can be drawn. (1) The shifts induced by 5.1c on the NMR signals of SDS and CTAB... 

Monomeric thiazyl halides can be stabilized by coordination to transition metals and a large number of such complexes are known (Section 7.5). In addition, NSX monomers undergo several types of reactions that can be classified as follows (a) reactions involving the n-system of the N=S bond (b) reactions at the nitrogen centre (c) nucleophilic substitution reactions (d) halide abstraction, and (e) halide addition. Examples of each type of behaviour are illustrated below. 

Many variations on these synthetic routes have been devised and the field is still being actively developed. The reactions of NO coordinated to transition metals have been extensively reviewed. ... 

Stabilization of quinone methides by coordination to transition metals might have various applications, beyond the important structural and spectroscopic studies of these coordinated compounds. For example, it is possible to manipulate their structure while coordinated to the metal center and affect their controlled release and trapping, leading to new synthetic procedures. 

The second ligand type consists of a large group of cyclic compounds incorporating numbers of ether functions as donors. Structure (22) illustrates a typical example. Such crown polyethers usually show strong complexing ability towards alkali and alkaline earth ions but their tendency to coordinate to transition metal ions is less than for the above... 

Generally, cyclohexyne is an unstable molecule because of its ring strain. However, it can be stabilized by coordination to transition metals.35 The reduction of 1,2-dibromocyclohexene by sodium/mercury in the presence of a nickel-bromide complex afforded the Ni-alkyne complex 66 as a thermally stable and isolable compound (Scheme 22).36 Complex 66 smoothly reacted with C02 under atmospheric pressure to give nickelacycle 67 in good yield. Dimethyl acetylenedicarboxylate was inserted into the vinyl-nickel bond in 67 to give the seven-membered oxanickelacycle 68. 

Benzynes are highly strained molecules, which are recognized as useful intermediates in organic synthesis.44 They can be isolated by coordination to transition metals.45 Similar to the reaction of the cyclohexyne species 66, Ni-benzyne complex such as 85 reacted with C02 to give the corresponding five-membered oxanickelacyle complex 86 (Scheme 31).46... 

Herein we have reported the synthesis, characterization, and reactivity of both transition metal and actinide elements supported by tripodal ligand scaffolds. The carbon-anchored ligands TIME normally form polynuclear species while the nitrogen-anchored ligands TIMEN can coordinate to transition metals in a 1 1, fashion, rendering the coordinated metal centers in well-protected pockets. 

Several examples of direct ring phosphorus interaction with transition metals are now known [279-287]. For example, reaction of N3P3CI6 with Na2Fe2(CO)g affords a novel spirocyclic diiron octa carbonyl derivative (Scheme 25) [282, 283]. The diiron spiro derivative acts as a template for the construction of several transition metal clusters. Some of the other examples where ring phosphorus atom is involved in interaction with transition metals are summarized in Scheme 25. The ring phosphorus atoms in hydrido phos-phazenes, N3P3R4R H also coordinates to transition metals. This has been discussed in an earlier section (vide supra). 

McCleverty, J. A. (1979). Reactions of nitric oxide coordinated to transition metals. Chem. Rev. 79, 53-76. 

Carbon monoxide, like hydrogen, readily coordinates to transition metal centers. Consequently, cycles such as 18.10 can be used to catalyze the addition of CO (carbonylation) as well as hydrogen to a terminal —CH=CH2 function, giving an aldehyde ... 

Fan, J.,Whitehold, J.A., Olenyuk, B., Levin, M.D., Stang, P.J. and Fleischer, E.B. (1999) Self assembly of porphyrin arrays via coordination to transition metal bisphosphine complexes and the unique spectral properties of the product metallacyclic ensembles. J. Am. Chem. Soc., 121 (12), 2741-2752. 

We started this templated approach to catalyst encapsulation using tris(mefa-pyridyl)phosphine and zinc(ii)TPP (TPP = tetraphenylporphyrin) [7]. From NMR and UV/Vis titration experiments we found a selective assembly process via coordination of the nitrogen to the zinc(ii)TPP, rendering the phosphine donor atom completely encapsulated by the three porphyrin components (Figure 8.4). The phosphine center is still available for coordination to transition metals, providing... 

Hydride abstraction from alkylamines forms the corresponding imi-nium ions, whose coordination to transition metals gives either a ir-complex or cr-bonded three-membered ring (Scheme 15) (26). Ligation of the cationic dehydro amines to Rh is aided by substantial electron donation from the metal to the electron-deficient carbon atom to produce the Rh(IH) complex with a covalent C—Rh bond and an N—Rh dative bond, consistent with the long C—N bond (1.467 A) and the small H— C( 1)-—N—C(2) dihedral angle (124.6°) as well as the noncoplanarity of the CH2—CH bond and a possible CH—NH2 plane seen in the allylam-ine oxidative addition product (24). 

Three strategies for the stabilization of these highly reactive heterocarbonyl species have proven to be successful (a) electronic stabilization by 7T interaction with heteroatoms such as nitrogen, oxygen, and sulfur via unsaturated linking groups, (b) kinetic stabilization by bulky substituents, and (c) coordination to transition metals. 

The stabilization of these heteroaldehydes and -ketones by coordination to transition metals is the subject of this review. Many problems connected with the high reactivity of these heterocarbonyl compounds can be circumvented by using their transition metal complexes. The chemistry of organo-sulfur and organoselenium transition metal complexes in more general terms30,31 and some aspects of thio- and selenoaldehydes and -ketones as... 

The steric effects of the substituents, such as in 3,5-dialkylpyrazoles, may induce the coordination of small ligands or counterions that usually do not coordinate to transition metal ions. This behaviour is believed to be responsible for the unusual decomposition of transition metal tetra-fluoroborates in the presence of such ligands.43 Otherwise, the coordination number changes, resulting in, for example, square-planar Ni11 (low-spin) complexes, even with rather weak ligands such as 1,2-dimethylimidazole.44... 

Despite the fact that carbon dioxide (C02) is used in a great number of industrial applications, it remains a molecule of low reactivity, and methods have still to be identified for its activation. Both thermodynamic and kinetic problems are connected with the reactivity of C02, and few reactions are thermodynamically feasible. A very promising approach to activation is offered by its coordination to transition metal complexes, as both stoichiometric reactions of C-C bond formation and catalytic reactions of C02 are promoted by transition metal systems. Efforts to enhance the yield of hydrogen in water gas-shift (WGS) reactions have also been centered on C02 interactions with transition metal catalysts. The coordination on metal centers lowers the activation energy required in further reactions with suitable reactants involving C02, making it possible to convert this inert molecule into useful products. 

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

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