Coordination bidentate

Symmetrical bidentate coordination (a) has been observed in complexes with 1-6 nitrates coordinated to the central metal, e.g. [Cu(N03)-(PPh3)2] [Cu(N03)2], [Co(N03)2(OPMe3)2] ... 

Titanium and zirconium chemistry has some unique features the chair conformation of the M2N4 metallocycle in the exobidentate complexes and endo-bidentate coordination. 

The assumed transition state of this reaction is shown in Scheme 5.3. Yb(OTf)3, (J )-(-h)-BINOL, and DBU form a complex with two hydrogen bonds, and the axial chirality of (J )-(-h)-BINOL is transferred via the hydrogen bonds to the amine parts. The additive would interact with the phenolic hydrogen of the imine, which is fixed by bidentate coordination to Yb(III). Because the top face of the imine is shielded by the amine, the dienophiles approach from the bottom face to achieve high levels of selectivity. 

In most TiCl2-TADDOLate-catalyzed Diels-Alder and 1,3-dipolar cycloaddition reactions oxazolidinone derivatives are applied as auxiliaries for the alkenoyl moiety in order to obtain the favorable bidentate coordination of the substrate to the catalyst... 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

One of the problems related to the Lewis acid activation of a,/ -unsaturated carbonyl compounds for the reaction with a nitrone is the competitive coordination of the nitrone and the a,/ -unsaturated carbonyl compound to the Lewis acid [30]. Calculations have shown that coordination of the nitrone to the Lewis acid can be more feasible than a monodentate coordination of a carbonyl compound. However, this problem could be circumvented by the application of alkenes which allow a bidentate coordination to the Lewis acid which is favored over the monodentate coordination. 

If the thiomalate ligand is not present in excess, it is completely displaced. When gold binds, the M ions are displaced in the gold metal ratio of 3 2, which suggests bidentate coordination. EXAFS data shows AuS2 coordination with Au—S bond lengths of 229 pm [20, 103]. In the presence of both cadmium and zinc, zinc is preferentially displaced which is possibly a consequence of thermodynamics as Zn7MT and CdyMT react at comparable rates. 

Both 2-formylthiophene thiosemicarbazone, 26, and 2-acetylthiophene thiosemicarbazone, 27, form six coordinate [FeL2A2] complexes (A = Cl, Br) [156], The complexes formed with 26 are low spin, but complexes of 27 are high spin. For both ligands the bidentate coordination is via the azomethine nitrogen... 

The coordination chemistry of MandyPhos and TaniaPhos with rhodium and palladinm has been explained by Knochel (1, Knochel). The chemistry of ligands snch as Duphos or BINAP with all relevant established ratheninm(II) precnrsors is known and was derived from their bidentating coordination behaviour ... 

It was not possible to isolate defined complexes with clear bidentate coordination for the ligands MandyPhos and TaniaPhos using established ruthenium precursors such as [Ru(COD)Cl2]x, [Ru(benzene)Cl2]2, [Ru(p-cymene)Cl2]2,... 

The molecular structures of several [TpBut]ZnX derivatives have been determined by x-ray diffraction. For example, x-ray diffraction studies confirm that the acetate ligand in [TpBut]Zn(r)1-02CMe) is bound to zinc in a unidentate mode, similar to that proposed for [TpBut]Mg(7j1-02CMe), but in contrast to the bidentate coordination proposed for the copper analogue [TpBut]Cu(T)2-02CMe) (86,87). Such a change in coordination mode for copper and zinc derivatives is to be anticipated on the basis of structural studies on the nitrate derivatives [TpBut]M(N03) (M = Co, Ni, Cu, Zn), as described in Section V,B,2,e. The thioacetate [TpPh]-Zn V-SC(0)Me (81), and cyanoacetate [Tp lZnlr -C CCH N) (88) derivatives also exhibit unidentate coordination. 

Secondly, the molecular structure of [TpPr 2]Zn 2(/u,-7)1,7)2-C03), as determined by x-ray diffraction (151,153), reveals that the carbonate ligand is coordinated in an asymmetric manner, with unidentate coordination to one zinc center and bidentate coordination to the other zinc center, i.e., fi-ri1,ri2-C03 (Scheme 24). Such coordination is noticeably distinct from that of [TpBut Me]Zn 2(/x.-r)1,7 1-C03), in which the carbonate ligand bridges in a symmetric unidentate mode. 

The coordination modes of the nitrate ligand in the complexes [TpBut]M(N03) (M = Cu, Ni, Co, Zn) are summarized in Fig. 46. (171, 184). Evidently, the coordination mode varies from unidentate for Zn to symmetric bidentate for Ni and Cu, with the cobalt derivative exhibiting an anisobidentate coordination mode. Moreover, the related cadmium derivative [TpBut,Me]Cd(N03) also exhibits bidentate coordination of the nitrate ligand, with Cd-0 bond lengths of2.272(6) A and 2.295(7) A (91). Such symmetric bidentate coordination contrasts with the significantly different Zn-0 interactions [1.978(3) A and 2.581(3) A] in unidentate [TpBut]Zn(N03). The coordination modes for a variety of [TpRR ]M(N03) complexes are summarized in Table VIII. 

The structural variations observed for [TpRR ]M(N03) (M = Co, Ni, Cu, Zn Cd) reveal that, for a given [Tp1 ] ligand, the preference for bidentate coordination increases across the series Zn < Co Cu, Ni, and Cd. Significantly, these structural preferences of the nitrate ligand correlate with the activity of the metal-substituted enzymes Zinc, the metal with the greatest tendency to exhibit unidentate coordination of the nitrate ligand, is the most active, while nickel, copper, and... 

The ability of cobalt(II), nickel(II), and copper(II) to exhibit a greater tendency than Zn(II) towards bidentate coordination is further illustrated by structural comparisons within a series of bridging carbonate complexes (188). For example, of the complexes [TpPr 2]M 2(/x-C03) (M = Mn, Fe, Co, Ni, Cu, Zn), only the zinc derivative does not exhibit bidentate coordination at both metal centers (151,153). Furthermore, the carbonate ligand in the complexes [TpPr 2]M 2(/x-C03) (M = Mn, Fe, Co, Ni, Cu) also exhibits varying degrees of asymmetry that closely parallel the series of nitrate complexes described earlier (Fig. 47 and Table IX). 

IR spectroscopy is often used for distinguishing between unidentate and bidentate coordination of carboxylate (02CR) ligands. For monomeric carboxylate derivatives the separation between the symmetric and asymmetric C02 stretching bands, At = [t asym(C02) - t syJCOa)], provides a useful indication of the coordination mode complexes which exhibit values of greater than 200 cm-1 invariably possess unidentate coordination. Deacon, G. B. Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. 

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