Synthesis and reactivity of 3,5-dimethyl-4-aminomethylpyrazole ligands. An entry to new water-soluble pyrazolate rhodium(I) complexes

The new pyrazoles 3,5-dimethyl-4-(ethylamino)methylpyrazole (HL1) and 3,5-dimethyl-4-(isopropylamino)methylpyrazole (HL2), both containing aminoalkyl groups at position 4 have been prepared by aminoalkylation of 3,5-dimethylpyrazole and by the reaction between 1-chloromethyl-3,5-dimethylpyrazolium chloride and NH2R amines. The reaction between HL1, HL2 and [RhCl(COD)]2 resulted in complexes of formula [Rh2(HL1)2(COD)2]Cl2 (1) and [Rh2(HL2)2(COD)2]Cl2 (2), which contained the pyrazole ligands in the ‘zwitterionic’ pyrazolate-ammonium forms. The X-ray structure analysis of 2 confirmed the neutral nature of bridging-pyrazolate ligands and revealed that 1 and 2 belonged to the [Rh2(Pz)2L2] family of compounds. The same reaction with two equivalents of NaOMe resulted in neutral pyrazolate complexes [Rh2(L1)2(COD)2] (5) and [Rh2(L2)2(COD)2] (6). The reaction between both cationic and neutral pyrazolate complexes and a 1:1 CO–H2 mixture (20 atm) led to the dinuclear pyrazolate-bridged tetracarbonyl compounds 3, 4, 7 and 8 in good yields. Tetracarbonyl complexes 3 and 4 were not isolated in pure state. All the complexes synthesized are soluble in polar solvents such as water.