Ligand substitution, metals/metal complexes

On the coordination chemistry side, ligand substitution on metal complexes in ILs has attracted quite some interest. This is mainly due to the fact that both spectroscopic and catalytic properties are strongly governed by the nature of the ligands and the stability of their bond to a metal center. Begel et al. have studied the role of different ILs on ligand substitution reactions on [Pt(terpy)Cl]+ (terpy = 2,2 6, 2"-terpyridine) with thiourea with stopped-flow techniques. The substitution kinetics show similar trends if compared to conventional solvents with similar polarities. Moreover, much like in conventional solvents, the authors find an associative character of the substitution reaction [205], These results are essentially supporting an earlier study by Weber et al., who found the same behavior [206],... 

The pyridyl substituted 1,4-diazepinone derivative 91 was prepared by photolysis of 2,6-bispyridyl-4-azidopyridine, and it represents a new class of ligand for metal ion complexing <00HCA384>. 

When a metal atom donates electron density to a bound ligand, usually by means of Ji-back bonding, electrophilic substitution reactions may be promoted. This is observed then usually with metals in low oxidation states and is therefore prevalent with organometallic complexes - and less with those of the Werner-type, where the metals are usually in higher oxidation states. Nevertheless there have been detailed studies of electrophilic substitution in metal complexes of P-diketones, 8-hydroxyquinolines and porphyrins. Usually the detailed course of the reaction is unaffected. It is often slower in the metal complexes than in the free ligand but more rapid than in the protonated form. 

Another electrophilic substitution reaction which has been examined for both a free ligand and its metal complexes is mercuration. The rate of mercuration of aromatic compounds can generally be given by a second order expression of the type ... 

These equations show the general theoretical basis for the empirical order of rate constants given earlier for electrophilic attack on an aromatic ligand L, its metal complex ML, and its protonated form HL, one finds kt > n > hl. Conflicting reports in the literature state that coordination can both accelerate electrophilic aromatic substitution (30) and slow it down enormously (2). In the first case the rates of nitration of the diprotonated form of 0-phenanthroline and its Co(III) and Fe(III) complexes were compared. Here coordination prevents protonation in the mixed acid medium used for nitration and kML > h2l. In the second case the phenolate form of 8-hydroxyquinoline-5-sulfonic acid and its metal chelates were compared. The complexes underwent iodination much more slowly, if at all, and kL > kML ... 

Of commercial interest are benzo- and other fused aromatic 1,2,3-diazaborine derivatives which have exhibited good antibacterial activity against a variety of microorganisms (155—157). The reaction of pyrazole or C-substituted pyrazoles with boranes yields the pyrazabole system, a class of exceptionally stable compounds. More than 70 species in this system have been reported and the subject comprehensively reviewed (158). These compounds have been used as ligands in transition-metal complexes (159). 

Zinc chelates of the stoichiometry Z11L2 have been prepared from various substituted monothio-jS-diketones RC(SH)=CHCOCF3(HL).756 The mass spectra of Znl complexes of the monothio-/ -diketones RC(SH)=CHCOPh (R = Me or Ph) have been reported.757 The most interesting feature is the loss of H2S, which does not occur with the free ligands, or with metal complexes of fhiorinated /3-ketones.758... 

The isolation of these closely related thiolate complexes hints at an important role for 172-vinyl ligands in reactions which lead to net ligand substitution at metal. The SR bridge between Cp and W may resemble a snapshot along a reaction path for alkyne insertion into a M—L bond or for transfer of L from an T 2-vinyl to metal (97). A mechanism for alkyne polymerization based on rj2-vinyl intermediates has also been constructed (186). 

Zhang, K., Gonzalez, A. A., Mukerjee, S. L., Chou, S.-J., Hoff, C. D., Kubat-Martin, K. A., Barnhar, D. and Kubas, G. J. (1991). Solution calorimetric and stopped-flow kinetic study of ligand substitution for the complexes M(CO)3(PCy)2(L) (M = Cr, Mo, W). Comparison of first-, second- and third-row transition-metal-ligand bonds at sterically-crowded metal centres, J. Am. Chem. Soc. 113, 9170. 

Hall et al. also observed that the extent of delocalization on the mono-substituted squarate ligand ring, and hence its dimensions, was dependent on electron migration from the substituent 111, 123, 130, 132-137) (Table VI). For example, for the methoxysquarate ligand in its metal complexes A(C—C) = 0.05 A, whereas in the analogous dipheny-laminosquarate complexes the value is 0.02 A (Table VI). This difference was attributed to the smaller size of the oxygen atom relative to... 

An unusual feature of the Beat ligand in metal-Bcat complexes is the resistance of the the B-O(catecholale) bonds to exchange with other alcohols. For example, whereas Ru(Bcat)Cl(CO)(PPh3)2 may be recovered unchanged from EtOH solutions, compound (15), where the Beat function is no longer directly attached to the metal, undergoes rapid ethanolysis.11 The observation of substitution reactions at the boron centre therefore... 

Trialkyl and triphenyl substituted tertiary arsines and stibines are, like the phosphine analogues, suitable as ligands in transition metal complexes in several oxidation states, due to their low reduction potentials (cf Table 7 in Section III.A.l) and relatively high oxidation potentials (cf Table 14 in Section V.A). 

The same concept applies to substitution reactions (e.g., ligand exchange) in metal complexes, for example, trans [PtCl2(py)2] + 2NH3 trans [PtCl2(NH3)2] -F 2 py. The same two substitution mechanisms are operative (see Section 6.9). Coordination chemists speak of associative (or adjunctive) mechanisms and of dissociative (or disjunctive) mechanisms. 

Much of the work on ligand substitution of organometallic complexes has involved metal carbonyls with a trialkyl or a triarylphosphine serving as the replacement ligand. Two main mechanistic pathways exist by which substitution may occur— associative (A) and dissociative (D). 

---