Synthesis and Characterization of Pt(II) Complexes with Pyridyl Ligands: Elongated Octahedral Ion Pairs and Other Factors Influencing H NMR Spectra

Our goal is to develop convenient methods for obtaining trans-[Pt(4-Xpy)Cl] complexes applicable to 4-substituted pyridines (4-Xpy) with limited volatility and water solubility, properties typical of 4-Xpy, with X being a moiety targeting drug delivery. Treatment of cis-[Pt(DMSO)Cl] (DMSO = dimethyl sulfoxide) with 4-Xpy in acetonitrile allowed isolation of a new series of simple trans-[Pt(4-Xpy)Cl] complexes. A side product with very downfield H2/6 signals led to our synthesis of a series of new [Pt(4-Xpy)]Cl salts. For both series in CDCl, the size of the H2/6 Δδ [coordinated minus "free" 4-Xpy H2/6 shift] decreased as 4-Xpy donor ability increased from 4-CNpy to 4-MeNpy. This finding can be attributed to the greater synergistic reduction in the inductive effect of the Pt(II) center with increased 4-Xpy donor ability. The high solubility of [Pt(4-Xpy)]Cl salts in CDCl (a solvent with low polarity) and the very downfield shift of the [Pt(4-Xpy)]Cl H2/6 signals for the solutions provide evidence for the presence of strong {[Pt(4-Xpy)],2Cl} ion pairs that are stabilized by multiple CH···Cl contacts. This conclusion gains considerable support from [Pt(4-Xpy)]Cl crystal structures revealing that a chloride anion occupies a pseudoaxial position with nonbonding (py)C-H···Cl contacts (2.4-3.0 Å). Evidence for (py)C-H···Y contacts was obtained in NMR studies of [Pt(4-Xpy)]Y salts with Y counterions less capable of forming H-bonds than chloride ion. Our synthetic approaches and spectroscopic analysis are clearly applicable to other nonvolatile ligands.