STRUCTURE OF THE RADICAL CATIONS OF N,N'-POLYMETHYLENE-SYN-1,6/8,13-DIIMINO[14]ANNULENES - AN ESR AND ENDOR STUDY

N,N'-Polymethylene-syn-1,6:8,13-diimino[14]annulenes, 1-7, in which the number, m, of CH2 groups varies from 1 to 7, and N,N'-dimethyl-syn-1,6:8,13-diimino[14]annulene (8) were synthesized. The radical cations of 1-8, as well as those of several derivatives of 1-4, have been studied by ESR, ENDOR, and TRIPLE resonance spectroscopy. In contrast to the corresponding anions 1.--8.-, which are bridged pi-perimeter radicals, the cations 1.+-8.+ must be considered as N-centered radicals with the bulk of the spin population residing at the two heteroatoms. The structure and properties of these radical cations critically depend on the length of the polymethylene chain linking the N atoms. When the number, m, of the CH2 groups in this chain is 1 or 2, the N lone pairs are directed "outward", so that their interaction is relatively weak. Thus, the radical cations 1.+ (m = 1) and 2.+ (m = 2) are thermodynamically and kinetically rather unstable, and their N-14 coupling constant, a(N), is only 0.6-0.7 mT. On the other hand, with m = 3-7 the N lone pairs point "inward", an arrangement that favors the formation of an N-N three-electron sigma-bond. The pertinent radical cations 3.+-7.+ (m = 3-7) exhibit unusual thermodynamic and kinetic stabilities which, in two cases (3.+ and 4.+), allowed X-ray crystallographic structure analyses to be carried out. The coupling constant a(N) amounts to 1.70 mT for 3.+, and it increases further to an almost constant value of 2.57-2.72 mT for 4.+-7.+; an a(N) value of 2.66 mT is also observed for 8.+, which may be regarded as having its N atoms linked by a very long polymethylene chain. The changes in the coupling constant a(N) along the 1.+-7.+ (8.+) series can be rationalized in terms of the varying "s-character" of the singly occupied orbital centered at the spin-bearing N atoms.