Ligands not Containing Phosphorus

The literature about ligands not containing phosphorus is scarce. Most of the articles report nitrogen-containing ligands, especially substituted bipyridine and ethylene diamine N,N,N, N -tetraacetic acid (EDTA), and were recently reviewed [12], The most commonly used functionalities are sulfonate and carboxylic acid groups. The first functionalization consists of various mono- and disulfonated bipyridines, 

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. 

Studying iron models for the FeMo cofactor of FeMo nitrogenases, Sellmann et al. isolated iron complexes with sulfur ligands [58, 59]. Generally, complexes such as 34-36 are only soluble in organic solvents like CH2C12, THF, DMSO, or DMF. 

They succeeded in rendering them water-soluble by introducing carboxylic acid functions. These iron or ruthenium complexes become water-soluble when the carboxylic groups are deprotonated. Various complexes of iron and ruthenium can be obtained according to the method developed. An example is given in Eq. (9) [58]. 

Interesting chiral water-soluble aminosulfonamide ligands containing a phenyl-sulfonic acid substituent have been synthesized and engaged directly with a ruthenium precursor to reduce enantioselectively aromatic ketones to the corresponding alcohols [84]. As concluded by the authors, these ligands should be evaluated in bi-phasic catalysis. 

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