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

Abstract

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 solid 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 Å). The positive charge is thus, in this conformation, stabilised by an electrostatic interaction with a spatially close pyridyl lone pair. ¹H NMR studies (in particular two-dimensional COSY spectra and selective NOE difference spectra) 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.