Ligands pyrazole-type

Ovejero, P., Mayoral, M.J., Cano, M. and Lagunas, M.C. (2007) Luminescence of neutral and ionic gold(l) complexes containing pyrazole or pyrazolate-type ligands. Journal of Organometallic Chemistry, 692, 1690-1697. 

More recently, a few groups have started to look at pyrazole-type ligands in chelating systems (Schemes 4 and 5),81-83 as these are easily formed84 and also resemble the imidazole ligands in their coordinating behaviour (vide infra). 

More recently, imidazole- and pyrazole-type ligands with a phosphorus atom as the bridge have been reported,97,98 as depicted in formulae (46)-(48). Also rings with imidazolines (49)," or bridged by a —C—OH group (SO),100 have been reported. Boron-bridged polypyrazolylborates and related systems are dealt with in Chapter 13.6. 

The pentamethylcyclopentadienyl (Cp ) derivatives of rhodium- such as [(RhCp )2(OH)3]Cl- and the catalyst system prepared from [(RhCp )2( JL-Cl)2] and pyrazole-type ligands, are used for alkene hydrogenation in 2-propanol. 

Rhodium complexes with pyrazole or pyrazolate-type ligands have been studied as catalysts for hydrogenation of alkenes under ambient conditions in propan-2-ol <87JMOC34l). Hydroquinones... 

Reactions of the metallocene derivatives of molybdenum with pyrazole lead to the mononuclear complexes of the type 22. Structure 22 shows that it cannot be used as a ligand for the preparation of dinuclear complexes owing to geometric constraints [80JOM( 197)291 83JOM(253)53]. In acetone, an unusual complex 23 is formed [83JOM(253)53]. The bidentate ligand is the product of the reaction of pyrazole and acetone. 

ICA 97)19]. Compound 163 (R = Br) and perchloric acid yield 164, where the monodentate pyrazolate ligand is protonated. Potassium hydroxide regenerates 163 (R = Br). Dicationic complexes of the type 164 can alternatively be produced from [Rh2(r/ -Cp )2(/x-0H)3]Cl04 and perchloric acid in the presence of excess pyrazole, 4-bromopyrazole, 3-methylpyrazole, or 3,5-dimethylpyrazole. 

Organonickel derivatives also offer cases of the -coordination of the substituted hydrotrisfpyrazol- l-yl)borate ligand. For the palladium and platinum complexes, the M(II) M(IV) (M = Pd, Pt) transformation is facile. Organopalla-dium chemistry offers anew type of agostic interactions, C—H - - - Pd, where the C—H bond belongs to one of the pyrazolate rings. Cyclopalladation of various pyrazol-l-ylborates and -methanes does not modify their structure. 

---