Hydrazone–diacetyl platinum(II) complexes: Substituent effect on intramolecular N–H⋯O hydrogen-bond strength

Bis(benzylamine)diacetylplatinum(II) (3) reacted with 2-pyridyl-functionalized hydrazones and with diacetyl dihydrazone to yield diacetyl platinum(II) complexes [Pt(COMe)2(2-pyCRNNH2)] (R = H, 4a; Me, 4b; Ph, 4c) and [Pt(COMe)2(H2NNCMe–CMeNNH2)] (5). These complexes showed weak intramolecular N–H⋯O hydrogen bonds where the hydrazone and the acetyl ligand act as H donor and H acceptor, respectively. Using hydrazones 2-pyCRNNHR′ substituted with electron-withdrawing groups R′ resulted in complexes [Pt(COMe)2(2-pyCRNNHR′)] (R/R′ = H/C6H4-p-F, 6d; Me/C6H4-p-F, 6e; H/COMe, 7a; Me/COMe, 7b; H/COPh, 7c; Me/COPh, 7d; H/CO(C6H4-p-F), 7e; Me/CO(C6H4-p-F), 7f) with stronger intramolecular N–H⋯O hydrogen bonds. The isolation of the analogous phenylhydrazone complexes (R′ = Ph) failed on this way, but reactions of the 1D coordination polymer [{Pt(COMe)2}n] (2) with phenylhydrazones resulted in the formation of the desired complexes [Pt(COMe)2(2-pyCRNNHR′)] (R/R′ = H/Ph, 6a; Me/Ph, 6b; Ph/Ph, 6c; H/C6F5, 6f). The constitution of all complexes was unambiguously confirmed analytically, spectroscopically and, in part, by single-crystal X-ray diffraction analyses. Structural and NMR parameters gave evidence that the strength of the N–H⋯O hydrogen bond is increased in the order 5 ≈ 4a–c < 6a–e < 6f ≈ 7a–f. This goes parallel with an activation of the acetyl ligand, but in no case the reaction with amines resulted in the formation of iminoacetyl platinum(II) complexes as it was found in analogous oxime–diacetyl complexes [Pt(COMe)2(2-pyCRNOH)] which have stronger (even than in type 6f/7a–f complexes) intramolecular O–H⋯O hydrogen bonds.