NMR Investigation and Dynamic Behaviour of [2,2′‐Bipyridylbis(pyridine)platinum( II )] 2+ and Related Cationic Complexes − Crystal Structure of [Pt(bipy)(py) 2 ](PF 6 ) 2

Detailed 1 H and 13 C{ 1 H} NMR analysis has been carried out on complexes with the general formulae [Pt(bipy)( n ‐Rpy) 2 ](PF 6 ) 2 and [Pt(4‐Phbipy)( n ‐Rpy) 2 ](PF 6 ) 2 [bipy = 2,2′‐bipyridyl; 4‐Phbipy = 4,4′‐diphenyl‐2,2′‐dipyridyl; n ‐Rpy = pyridine or substituted pyridines]. The resonance assignments and X‐ray structures of these complexes are important steps on our route to: (i) interpretation of the intercalative geometry on the basis of the magnetic changes produced on interaction with DNA, (ii) rational design of new intercalators. The X‐ray structure of the parent complex [Pt(bipy)(py) 2 ](PF 6 ) 2 (py = pyridine) shows pyridine rings quasiperpendicular to the coordination plane. In solution, because of the hindered rotation about Pt−N bonds of ortho ‐ and meta ‐monosubstituted pyridines, this arrangement leads to “head‐to‐head” s‐cis C s and “head‐to‐tail” s‐trans C 2 rotamers in equilibrium. NMR line shape and 13 C relaxation time constant measurements were carried out in order to evaluate activation energies and anisotropic rotational motion of n ‐Rpy. In some of these complexes a second dynamic process could be frozen on the 13 C NMR timescale below 265 K. This lower activation energy process can reasonably be attributed to minimal bipyridyl torsion angle flipping driving a concerted molecular motion about the core of the four N‐donors. Indeed, the X‐ray structure of 1 proves distortion with a slight alternate up and down shift of the four nitrogen atoms about the coordination plane. Significantly, the fast rotation of unhindered pyridine rings does not allow NMR detection of this molecular distortion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)