Pt(II) Rollover Complexes with Poor-Donor Bipyridines: Synthesis, Reactivity, and Preliminary Antimicrobial Evaluation

Abstract

This study describes the synthesis, characterization, and preliminary antimicrobial investigation of a series of rollover platinum complexes derived from 6-chloro-2,2′-bipyridine (bpyCl) and 6-bromo-2,2′-bipyridine (bpyBr) ligands. The electron-withdrawing halogen substituents in the 6-position significantly influence the coordination behavior of the ligands, favoring “rollover” cyclometalation rather than conventional N^N chelation. A series of platinum complexes, including [Pt(bpyX-H)(DMSO)Me], [Pt(bpyX-H)(PPh3)Me], [Pt(bpyX-H)(DMSO)Cl], [Pt(bpyX-H)(PPh3)Cl] (where X = Br or Cl), were successfully synthesized and characterized using multinuclear NMR spectroscopy and, in two cases, X-ray crystallography. The ligands’ stereoelectronic profile was evaluated based on the steric hindrance of the substituents and the electronic properties of the nitrogen donors. Electronic donor properties were correlated with NMR coupling constant data of the complexes. The reduced Lewis basicity of these ligands results in a noticeable reluctance to adopt the traditional N^N chelation motif; this is substantiated by the X-ray crystal structure of the complex [Pt(bpyBr)Cl2], which reveals a severe distortion induced by the bromine substituent. Conversely, a pronounced inclination toward rollover cyclometalation was observed. Selected complexes were thoroughly characterized and subsequently evaluated in a preliminary investigation of their antimicrobial activity against a panel of Gram-positive and Gram-negative multidrug-resistant bacteria, as well as a representative yeast strain, selected among clinically relevant and high-priority pathogens. Antibiofilm activity was also assessed, revealing moderate broad-spectrum antimicrobial effects and promising antibiofilm performance. These findings provide a useful basis for future structure–activity relationship studies and further optimization of the compounds. Furthermore, the reactivity of these species was also explored, leading to the isolation of various derivatives, including rare acetimine complexes obtained via room-temperature activation of ammonia and acetone.