Ring asymmetry in 4-morpholinobenzenediazonium tetrafluoroborate: crystallographic and SCF studies

Abstract

Contrary to previous reports, we find the aromatic ring in the 4-morpholinobenzenediazonium cation in (II) to be considerably distorted compared with that in the benzenediazonium ion (I), very much in the manner in which substitution of a 4-dialkylamino-group distorts the ring in nitrobenzene. These new data are used in a critique of the frequently expressed view that the electronic ground-state of a 4-dialkylaminobenzenediazonium cation is quinoidal in nature rather than aromatic: SCF calculations by several methods, while predicting a shortening of the C-2 to C-3 bond of (II), fail to reproduce the observed bond distances. Crystals of (II) are monoclinic, space group P2₁/c, a= 13.675(4), b= 10.686(4), c= 8.982(2)Å, β= 106.72(2)°, Z= 4. The structure was refined to R 0.065 for 1 674 observed reflections. Mean bond distances are: C(1)–C(2)(systematic numbering) 1.395(1), C(2)–C(3) 1.371(1), C(3)–C(4) 1.424(1), C(1)–N(diazonium) 1.381(1), C(4)–N(morpholino) 1.352(1), and N–N 1.100(1)Å.