Dipolar metal complexes

In this section, selected examples are described of dipolar second-order NLO chromophores in which an organometallic or coordination moiety may act as the donor (D), the acceptor (A) or even as the n-bridge connecting D to A. Representative examples of noncentrosymmetric bimetallic complexes containing metal-based fragments used as electron-donating and electron-accepting moieties, are also discussed. 

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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]. 

Merlic demonstrated the direct, non-photochemical insertion of carbon monoxide from acylamino chromium carbene complexes 14 to afford a presumed chromium-complexed ketene 15 <00JA7398>. This presumed metal-complexed ketene leads to a munchnone 16 or munchnone complex which undergo dipolar cycloaddition with alkynes to yield the pyrroles 17 upon loss of carbon dioxide. 

The proton ENDOR study of the chromyl ethyleneglycolate anion in ethanol reported by Mohl et al.293 presents the first successful adaptation of the ENDOR technique to a transition metal complex in liquid solutions. The aim of this work was to characterize the ENDOR relaxation behavior and to find optimum conditions for ENDOR detection. Two proton ENDOR lines with a hf splitting of ajj, = 1.74 MHz were observed. This is in agreement with a previous EPR study294 which had shown that all eight protons are equivalent. The optimum microwave and rf fields are both proportional to (Tr(g - geO rAj 2)1/2, where A denotes the dipolar part of the proton hfs tensor. For the chromyl ethyleneglycolate anion these two values have been calculated to — 8 10-6T and B = 2.7 mT. According to Mohl et al.293, successful proton... 

A variety of strategies for effecting dipolar cycloadditions from stable, neutral precursors, including carbon-carbon bond heterolysis induced by Lewis acidic metal complexes and chiral organic catalysts ... 

Fullerenes, among which the representative and most abundant is the 4 symmetrical Cgg with 30 double bonds and 60 single bonds, are known to behave as electron-deficient polyenes rather than aromatic compounds [7]. The energy level of the triply degenerate LUMO of Cgg is almost as low as those of p-benzoquinone or tetracyanoethylene. Thus, a wide variety of reactions have been reported for Cgg such as nucleophilic addition, [4-1-2] cycloaddition, 1,3-dipolar addition, radical and carbene additions, metal complexation, and so on [7]. Fullerene Cgg also undergoes supramolecular complexation with various host molecules having electron-donating ability and an adequate cavity size [8]. 

The reactivity of peroxo metal complexes as nucleophilic oxidants is a known process ". To visualize this type of reactivity one has to refer to peroxo metal complexes as a 1,3-dipolar reagent M+-0-0 interacting in a bimolecular fashion with electrophilic dipo-larophiles such as electron-poor olefins " (equation 15), to form peroxymetallacycle intermediates. 

The enantioselective inverse electron-demand 1,3-dipolar cycloadditions of nitrones with alkenes described so far are catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminium complexes. However, the glyoxylate-derived nitrone 256 favors abidentate coordination to the catalyst, and this nitrone is an interesting substrate, since the products that are obtained from the reaction with alkenes are masked ot-amino acids (Scheme 12.81). 

The J0rgensen and Desimoni groups have also carried out bis(oxazoline)-metal complex-catalyzed 1,3-dipolar cycloadditions with nitrones. Cycloaddition of a,(3-unsaturated oxazolidinones such as 69, 80a, and 130 with nitrone 127 in the presence of phe-box ligands 6 and ent-6 provided quantitative yields of cycloadducts. Selectivities of up to 100 0 endo/exo ratio and corresponding endo ee as high as 82% were achieved (Table 9.22, Fig. 9.41a). 

As anticipated in Sections 2.2.2 and 3.1, the unpaired electrons should not be considered as point-dipoles centered on the metal ion. They are at the least delocalized over the atomic orbitals of the metal ion itself. The effect of the deviation from the point-dipole approximation under these conditions is estimated to be negligible for nuclei already 3-4 A away [31]. Electron delocalization onto the ligands, however, may heavily affect the overall relaxation phenomena. In this case the experimental Rm may be higher than expected, and the ratios between the Rim values of different nuclei does not follow the sixth power of the ratios between metal to nucleus distances. In the case of hexaaqua metal complexes the point-dipole approximation provides shorter distances than observed in the solid state (Table 3.2) for both H and 170. This implies spin density delocalization on the oxygen atom. Ab initio calculations of R m have been performed for both H and 170 nuclei in a series of hexaaqua complexes (Table 3.2). The calculated metal nucleus distances in the assumption of a purely metal-centered dipolar relaxation mechanism are sizably smaller than the crystallographic values for 170, and the difference dramatically increases from 3d5 to 3d9 metal ions [32]. The differences for protons are quite smaller [32]. 

For other very useful reactions of alkynyl-A3-iodanes involving Diels-Alder reactions, 1,3-dipolar cycloadditions, and reactions with transition metal complexes, see the excellent reviews of Koser [39] and Stang [208,209]. 

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