Triazoles transition metal complexes

Cycloaddition reactions catalysed by transition metal complexes are an important tool in the construction of a wide range of carbo- and hetero-cyclic systems, such as benzene, pyridines, triazoles, etc. [7]. In general, these reactions are extremely atom-efficient and involve the formation of several C-C bonds in a single step. Among the innumerable possible catalytic systems for the cycloaddition reaction the NHC-metal complexes have received special attention [7c]. 

Infrared data have been tabulated for benzotriazole and a wide range of its transition metal complexes or adducts (172). Far infrared spectra have been recorded for copper(II) benzotriazole adducts and bands at 270-320 cm-1 have been assigned to Cu-N vibrations (172). Infrared absorptions at approximately 825, 800, and 775 cm-1 in the spectra of cobalt(III)/4,5-disubstituted triazolate complexes have been attributed to triazolate ring vibrations (109). Infrared data have been reported and assignments made for palladium and platinum thiatriazoline-5-thionate complexes (37) and for the parent thione (127). Vibrational spectroscopy has been employed in an attempt to determine coordination sites for a range of 8-azapurine complexes (108). 

One of the most important biomolecules for which fluorescent sensing [94-96] is of great importance is nitric oxide [97-101]. Nitric oxide can react with several organic dyes, switching on their fluorescence as a result of a triazole ring closure reaction [94], There are also useful and selective NO optical sensors based on transition metal complexes (Figures 16.22 and 16.23) [94-96],... 

Although the parent compound 1,2,3-triazole has not been studied, the crystal structure of benzotriazole (18) has been determined (74AX(B)1490). Benzotriazole crystallizes in the space group P2i. In association with its remarkable anticorrosion activity (Section 4.11.6.4.2) benzotriazole and its conjugate base form stable transition metal complexes whose crystal structures have, in part, been studied. Some examples are presented in Figures 5-7, with special regard to the molecular dimensions of the benzotriazole moiety. 

In 1988 Lalor reported the synthesis and spectroscopic characterization of some new poly(benzo-triazolyl)borate salts (Scheme 8).180 The regiospecifity of the synthesis of I I B( Btz)4 differs from that of pyrazole/BH4 reaction in that B—N bond formation takes place in a manner that maximizes steric crowding at boron (i.e., at the triazole N(l) atoms). In 1989 Shiu describes some new metal carbonyls of HB(Btz)3.181 The synthesis and spectroscopic characterization of some first-row transition metal complexes was reported by Cecchi,182 whereas Hill described some new Rh183 and Ru184 carbonyl and isonitrile complexes. The formation of isonitrile derivatives was proposed to proceed through an associative mechanism involving an intermediate complex... 

Luminescent ionic transition-metal complexes for light-emitting electrochemical cells (pyridine, phenanthroHne, pyrazole, imidazole, triazole derivatives as Hgands) 12AG(E)8178. 

Bezuidenhout DI, Kleinhans G, Guisado-Barrios G, Liles DC, Ung G, Bertrand G. Isolation of a potassium bis(l,2,3-triazol-5-yhdene)carbazolide a stabilizing pincer Hgand for reactive late transition metal complexes. Chem Commun. 2014 50 2431-2433. 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

Azidorhodium complexes are of considerable structural interest, but limited applicability can be expected from the unusual triazole synthesis illustrated in Scheme 125.190 It is also difficult to envisage synthetic utility from reactions in which organic azides are decomposed by transition metal carbonyls. Thus 2-arylbenzotriazoles are formed in such reactions on... 

Adducts of triazoles with transition metal salts are usually prepared by direct reactions between the two components involved and frequently precipitate or crystallize spontaneously from the reaction mixture (55,172,194, 202). Complexes containing triazolate anions can usually be obtained from the corresponding transition metal halide, carboxylate, nitrate, or perchlorate complex and an alkali metal (146, 147, 172) or thallium(I) triazolate salt (33). Other routes to triazolate complexes include the direct reactions of metal halides with triazoles in the presence of a base (201) and the treatment of triazole/metal halide... 

Molten 1,2,4-triazole has been tried as a solvent in high temperature chemistry. It is merely a solvent for some simple salts such as sodium chloride but transition metal halides are complexed in an unknown manner (66CI(L)60o). For derivatives of 1-phenyltriazole see Section 4.12.5.2.4. 

Polymeric complexes of 1-phenyltriazole with transition metal cyanates or thiocyanates are structurally and stereochemi ly understood (80MI41200) and an antiferromagnetic complex of triazole is of potential technical interest (80ZN(A)1387). 

Other polymeric heterocyclic amines such as imidazole, benzimidazole pyrazole, triazole or tetrazole are also excellent ligands for transition metal ions [1,2,23,24]. A detailed study on the complexation of Ag(I) and Cu(II) with poly(A-vinylimidazole) gave insights into the intrinsic complexation equilibria [88]. 

---