X-Ray crystal structures and NMR solution studies on 2,2':3',2'':6'',2'''-quaterpyridine and its N-methylated derivative; conformational rigidity in solution arising from an intramolecular electrostatic interaction

Reaction of 2,2':3',2 '': 6 '',2'''-quaterpyridine (QP) with methyl trifluoromethylsulfonate results principally in N-methylation on the first-named (non-primed) ring to give [QP-Me][PF6]. A comparison of the crystal structures of QP and [QP-Me][PF6] shows that in the sold state, N-methylation results in a substantially more folded conformation than occurs for QP, bringing the positive charge of the N-Me group into close proximity with the lone pair of the 2 '',6 ''-disubstituted pyridyl ring (non-bonded N ... N separation, 3.20 Angstrom), The positive charge is thus, in this conformation, stabilised by an electrostatic interaction with a spatially close pyridyl lone pair. LH NMR studies (in particular two-dimensional COSY spectra and selective NOE difference spectral unexpectedly show clearly that the rigid, folded conformation of [QP-Me][PF6] that is apparent in the crystal structure is retained in both acetonitrile and dimethyl sulfoxide solutions.