Intramolecular NH···Pt interactions of platinum(II) diimine complexes with phenyl ligands.

Pt(pipNC)(2)(phen) [pipNC(-) = 1-(piperidylmethyl)phenyl anion; phen = 1,10-phenanthroline] was prepared by the reaction of cis-Pt(pipNC)(2) with phen. Crystallographic and (1)H NMR data establish that the phen ligand is bidentate, whereas each piperidyl ligand is monodentate and bonded to the platinum at the ortho position of the phenyl group. Acidic conditions allowed for isolation of the salts of diprotonated Pt(pipNHC)(2)(diimine)(2+) adducts (diimine = phen, 2,2'-bipyridine, or 5,5'-ditrifluoromethyl-2,2'-bipyridine). Crystallographic and spectroscopic data for the diprotonated complexes are consistent with H···Pt interactions (2.32-2.51 Å) involving the piperidinium groups, suggesting that the metal center behaves as a Brønsted base. Metal-to-ligand (diimine) charge-transfer states of Pt(pipNHC)(2)(phen)(2+) in solution are strongly destabilized (>2500 cm(-1)) relative to Pt(pipNC)(2)(phen), in keeping with the notion that NH···Pt interactions effectively reduce the electron density at the metal center. Though N···Pt interactions in Pt(pipNC)(2)(phen) appear to be weaker than those found for outer-sphere two-electron reagents, such as Pt(pip(2)NCN)(tpy)(+) [pip(2)NCN(-) = 1,3-bis(piperidylmethylphenyl anion; tpy = 2,2':6',2'-terpyridine], each of the Pt(pipNC)(2)(diimine) complexes undergoes diimine ligand dissociation to give back cis-Pt(pipNC)(2) and free diimine ligand. Electrochemical measurements on the deprotonated complexes suggest that the piperidyl groups help to stabilize higher oxidation states of the metal center, whereas protonation of the piperidyl groups has a destabilizing influence.