Conjugative interactions in 1-methylpyrrol-2-yl derivatives of the cyclic phosphazenes (NPF₂)_n (n = 3–6): the crystal and molecular structure of 1-methylpyrrol-2-ylheptafluorocyclotetraphosphazene

Abstract

. Can. J. Chem. 60,535 (1982). I-Methylpyrroles, in which the 2-position is occupied by a pentafluorocyclotriphosphazenyl (la), heptafluorocyclotetraphosphazenyl (lb), nonafluorocyclopentaphosphazenyl (Ic), or undecafluorocyclohexaphosphazenyl group (Id), have been prepared from 2-lithio-I-methylpyrrole and the corresponding fluorophosphazene. Crystals of Ib are monoclinic, a = 17.300(2), b = 5.0281(3), c = 16.902(2) A, P = 108.904(4)", Z = 4, space group P2,/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.038 and R, = 0.047 for 1167 observed reflections. The N4P4 ring is nearly planar.The P-N bond lengths (corrected for libration) range from 1.524(5) to 1.562(5) A with a mean value of 1.541(17) A, and the mean ring angles are 122.9(20)" at phosphorus and 146.4(23)" at nitrogen. The geometry of the 1-methylpyrryl group shows no unexpected features, but the P-C bond (1.756(5) A) is significantly shorter than in typical P-phenyl derivatives, and the P-F bond at the same atom is comparatively long (1.561(4) A). Conjugative electron release from the pyrryl group, invo1vingp.-d, bonding, is indicated both by these geometrical features and by the appearance of intramolecular charge transfer bands in the electronic spectra of l(a-d).RAJENDRA D. SHARMA, STEVEN J. RETTIG, NORMAN L. PADDOCK et JAMES TROTTER. Can. J. Chem. 60.535 (1982). L a reaction du lithio-2 methyl-l pyrrole avec un fluorophospazene approprie conduit a des methyl-1 pyrroles dans lesquels on trouve en position 2 soit un pentafluorocyclotriphosphazenyle (la), soit un heptafluorocyclotetraphosphazCnyle (lb), soit un nonafluorocyclopentaphosphaz~yle phazenyl groups are similar to those of cyano-or is strongest for electron-releasing substituents. nitro-groups (I), but the electronic spectrum of a The pyrryl group is a good electron donor, because phenyi group is little changed by attachment to a canonical forms of pyrrole like 2b make an imporphosphazene ring ( 3 , and the results of a photo-tant contribution to its electronic structure; experelectron spectroscopic investigation are in better imentally, the first ionization energies of pyrrole agreement with the transmission of the electronic (8.20 eV) (6) and especially I-methylpyrrole 2 (7.95 effects of a phenyl group through the o-bond eV) (7) are significantly less than that of benzene system (3). (9.25 eV) (8). As a consequence of the conjugative The bonding system in phosphazenes is closely charge transfer to phosphorus, the tri(pyrry1) phosanalogous to that in phosphoryl compounds, in phine oxides 3 show an intense low frequency band which, again, the spectroscopic evidence (4) indi-in their electronic spectra (9), similar to that found cates only weak p,-6, bonding between phenyl in formyl-and acetyl-pyrroles (lo), which is attriband phosphoryl groups. Electronic delocalization uted to p,-d, bonding (9). on to phosphorus is limited by the relatively high.In the expectation that phosphazenyl and phoselectronegativity of carbon, and in ge...

The molecular and electronic structure of 1-pyrazolylphosphazenes

“…Experimentally, strong I T conjugation requires donation from the substituent (16), inducing a planar configuration of the exo-N atoms in alkylamino derivatives, shortening the exo-P-N bond, and, in agreement with CNDO calculations of electron density (17), making a ring-nitrogen atom the basic centre (18). A band in the UV spectra of the l-methyl-2-(fluorophosphazenyl)pyrroles N,,P,,Fh-, -CJ-I,NMe (n = 3-6) is attributed to IT-PIT charge transfer, its variation with ring size and the conformation of the tetrameric molecule being consistent with conjugation to a homomorphic system (19). Later work (20) on the ESR spectrum of a phenalenylphosphazene radical specifically designed to allow spiro conjugation only through the phosphorus d,, orbital showed at least half of the ring nuclei to be ESR active.…”

“…Conjugative interactions are to be expected between pyrazolyl and phosphazenyl rings, though they should be smaller than in pyrrolylphosphazenes (19). They are discussed first in relation to the simpler molecule bis(1 -pyraz01yl)carbonyl 7 (48).…”