Synthesis and Characterization of Pt(II) Complexes with Pyridyl Ligands: Elongated Octahedral Ion Pairs and Other Factors Influencing 1H NMR Spectra

Our goal is to develop convenient methods for obtaining trans-[PtII(4-Xpy)2Cl2] 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-[PtII(DMSO)2Cl2] (DMSO = dimethyl sulfoxide) with 4-Xpy in acetonitrile allowed isolation of a new series of simple trans-[PtII(4-Xpy)2Cl2] complexes. A side product with very downfield H2/6 signals led to our synthesis of a series of new [PtII(4-Xpy)4]­Cl2 salts. For both series in CDCl3, 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-Me2Npy. 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 [PtII(4-Xpy)4]­Cl2 salts in CDCl3 (a solvent with low polarity) and the very downfield shift of the [PtII(4-Xpy)4]­Cl2 H2/6 signals for the solutions provide evidence for the presence of strong {[PtII(4-Xpy)4]2+,2Cl–} ion pairs that are stabilized by multiple CH···Cl contacts. This conclusion gains considerable support from [PtII(4-Xpy)4]­Cl2 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 [PtII(4-Xpy)4]­Y2 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.