Unusual Reaction between (Nitrile)Pt Complexes and Pyrazoles: Substitution Proceeds via Metal-Mediated Nitrile−Pyrazole Coupling Followed by Elimination of the Nitrile

The reaction of platinum(IV) complex trans-[PtCl4(EtCN)2] with pyrazoles 3,5-RR‘pzH (R/R‘ = H/H, Me/H, Me/Me) leads to the formation of the trans-[PtCl4{NHC(Et)(3,5-RR‘pz)}2] (1−3) species due to the metal-mediated nitrile−pyrazole coupling. Pyrazolylimino complexes 1−3 (i) completely convert to pyrazole complexes cis-[PtCl4(3,5-RR‘pzH)2] by elimination of EtCN upon reflux in a CH2Cl2 solution or upon heating in the solid state; (ii) undergo exchange at the imino C atom with another pyrazole different from that contained in the pyrazolylimino ligand. The reaction of trans-[PtIICl2(EtCN)2] and 3,5-RR‘pzH, conducted under conditions similar to those for trans-[PtIVCl4(EtCN)2], is much less selective, and the composition of the products strongly depends on the pyrazole employed: (a) with pzH, the reaction gives a mixture of three products, i.e., [PtCl2{NHC(Et)pz-κ2N,N}] (4), [PtCl(pzH){NHC(Et)pz-κ2N,N}]Cl (5), and [Pt(pzH)2{NHC(Et)pz-κ2N,N}]Cl2 (6) (complexes 5 and 6 are rather unstable and gradually transform to trans-[PtCl2(pzH)2] and [Pt(pzH)4]Cl2 and free EtCN); (b) with 3,5-Me2pzH, the reaction leads to the formation of [PtCl2{NHC(Et)(3,5-Me2pz)-κ2N,N}] (7) and [PtCl(3,5-Me2pzH)3]Cl (8); (c) in the case of asymmetric pyrazole 3(5)-MepzH, which can be added to EtCN and/or bind metal centers by any of the two nonequivalent nitrogen sites, a broad mixture of currently unidentified products is formed. The reduction of 1−3 with Ph3PCHCO2Me in CHCl3 allows for the formation of corresponding platinum(II) compounds trans-[PtCl2{NHC(Et)(3,5-RR‘pz)}2] (9−11). Ligands NHC(Et)(3,5-RR‘pz) (12−14) were almost quantitatively liberated from 9−11 with 2 equiv of 1,2-bis-(diphenylphosphino)ethane in CDCl3, giving free imines 12−14 in solution and the precipitate of trans-[Pt(dppe)2](Cl)2. Pyrazolylimines 12−14 undergo splitting in CDCl3 solution at 20−25 °C for ca. 20 h to furnish the parent propiononitrile and the pyrazole 3,5-RR‘pzH, but they can be synthetically utilized immediately after the liberation.