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 tr ifluoromethylsulfonate results principally in N-methylation on the fir st-named (non-primed) ring to give [QP-Me][PF6]. A comparison of the c rystal structures of QP and [QP-Me][PF6] shows that in the sold state, N-methylation results in a substantially more folded conformation tha n occurs for QP, bringing the positive charge of the N-Me group into c lose proximity with the lone pair of the 2 '',6 ''-disubstituted pyrid yl ring (non-bonded N ... N separation, 3.20 Angstrom), The positive c harge is thus, in this conformation, stabilised by an electrostatic in teraction 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.