Triazole complexes rhodium

Relatively little has been reported on the electronic spectra of triazole and triazolate complexes. Copper(II) benzotriazolate adducts and benzotriazolate complexes show ligand-field maxima in the range 12.5-15.9 kK, in agreement with the proposed octahedral coordination geometry (172). Electronic spectra have also been reported for rhodium(I) and iridium(I) benzotriazolate complexes (33). 

Lithium 1,2,4-triazolate with [Rh2( j,-Ph2PCH2PPh2)(CO)2( j.-Cl)]PFj. gives the A-framed complex 177 (L=L = CO) (86IC4597). With one equivalent of terf-butyl isocyanide, substitution of one carbon monoxide ligand takes place to yield 177 (L = CO, L = r-BuNC), whereas two equivalents of rerr-butyl isocyanide lead to the product of complete substitution, 177 (L = L = r-BuNC). The starting complex (L = L = CO) oxidatively adds molecular iodine to give the rhodium(II)-rhodium(II) cationic species 178. 

The NHCs have been used as ligands of different metal catalysts (i.e. copper, nickel, gold, cobalt, palladium, rhodium) in a wide range of cycloaddition reactions such as [4-1-2] (see Section 5.6), [3h-2], [2h-2h-2] and others. These NHC-metal catalysts have allowed reactions to occur at lower temperature and pressure. Furthermore, some NHC-TM catalysts even promote previously unknown reactions. One of the most popular reactions to generate 1,2,3-triazoles is the 1,3-dipolar Huisgen cycloaddition (reaction between azides and alkynes) [8]. Lately, this [3h-2] cycloaddition reaction has been aided by different [Cu(NHC)JX complexes [9]. The reactions between electron-rich, electron-poor and/or hindered alkynes 16 and azides 17 in the presence of low NHC-copper 18-20 loadings (in some cases even ppm amounts were used) afforded the 1,2,3-triazoles 21 regioselectively (Scheme 5.5 Table 5.2). 

Involvement of two nucleophilic nitrogen atoms is thus typical for the amino heterocycles. The mutual disposition of the pyridine and amine nitrogen atoms allows the formation of chelate structures for the cobalt complexes of purine, 221 and 222. Structures with the N, iV -five-membered metal cycles were proven for the tri- and tetranuclear complexes of silver ) with 8-aminoquinoline (223) (92IC4370), and polymeric copper- and rhodium-acetate clusters (224). Another coordination mode can be found in the complexes of 4-amino-3,5-bis(pyridin-2-yl)-l,2,4-triazole, (225 or... 

The rhodium complex 219 gave the naphthoquinone derivative 220 with DMAD182 there was no attack at the triazole ring. 

New pyrazolyl ligands containing an aminomethyl group, in position 4, have been coordinated to rhodium(I) and show a good solubility in water, presumably because the ammonium group does not interact with the metal center. The resulting complexes have been characterized and, in our opinion, they are good candidates for biphasic catalysis [82], such as l,2,4-triazol-2-ium-5-ylidene complexes of Rh(I), Ir(I), Ni(0), Ni(II), Pd(II), and Hg(II) [83]. Due to their ionic character, these carbene complexes are extremely soluble in water. 

Mesoionic bis-NHCs have also given a number of bridging complexes of rhodium and iridium, such as those depicted in Figure 10.9 [89]. Worth mentioning is the example reported by Bertrand and coworkers m which the 1,2-dihapto ligand is an anionic l,2,3-triazole-4,5-dilylidene that is boimd to two different rhodium fragments (31 and 32) [90]. 

Pentamethylcyclopentadienylrhodium (or iridium) azido complexes 38 react with ditrifluoromethylacetylene to give the corresponding 1,2,3-triazole rhodium complex 39. Subsequent treatment with NH4CI opened the route to free NH-triazole 40 [44],... 

Triazoles also serve as a precursor of metal carbene complexes. For instance, Murakami and Fokin independently reported a denitrogenative rearrangement reaction of triazolyl alcohol 69 (Scheme 7.27) [41]. The ring-chain tautomeriza-tion of 69 generates its diazo imine form, which reacts with Rh2(Oct)4 to produce a rhodium carbene complex. The following 1,2-alkyl migration and elimination of rhodium produce enaminone 70. 

Tinnis F, Adolfsson H. Asymmetric transfer hydrogenation of ketones catalyzed by rhodium complexes containing amino acid triazole ligands. Org. Biomol. Chem. 2010 8 4536 539. 

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