Spin–Spin Interactions Across the “Cove” in the (Z) and (E) Isomers of 1,1′‐Difluoro‐9,9′‐bifluorenylidene

Abstract

In the (Z) isomer, but not in the (E) isomer of title compound 1, moderate 19F,19F spin–spin coupling across the “cove” is observed [J(F,F) = 11.0 Hz], which is deemed to obey a through‐space mechanism. The coupling between the isochronous fluorine nuclei of 1Z causes second‐order effects in the 1H, 13C and 19F NMR spectra of this compound and allows its easy distinction from the (E) isomer. A B3LYP/6‐31+G(d) calculation of 1Z as an isolated molecule was carried out which gave a non‐bonded F,F distance d(F,F) of 2.711 Å and an angle between the two C–F bond vectors of 34.1°. The computed F···F compliance constant of 6.96 Å/mdyn points to a very weak interaction. According to our previous correlation between d(F,F) and through‐space J(F,F), the F,F distance in 1Z would be expected to go along with a coupling constant of ca. 46 Hz. This discrepancy between prediction and experiment gives support to our earlier presumption that a simple d(F,F)/J(F,F) correlation may not sufficiently describe through‐space couplings and that an angular factor should also be taken into account. An X‐ray diffraction study of 1Z resulted in an F,F distance of 2.638 Å and an angle between the C–F bonds of 36.4°, in fair agreement with the MO calculation. The rigidity of the (Z) isomer was studied using theoretical compliance constants. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The Effects of Fluorine and Chlorine Substituents across the Fjords of Bifluorenylidenes: Overcrowding and Stereochemistry

“…Table 2 gives the relative energies of a series of homodesmotic reactions of the studied BFs. Table 3 gives representative optimized geometrical parameters of the twisted conformations of the various BFs under study [at the B3LYP/ 6-311++G(d,p) level] and the respective experimental parameters of Z11ЈF2, [24] Z11ЈCl2, [31] and 1,3,6,8,1Ј,3Ј,6Ј,8Ј-octachlorobifluorenylidene (13681Ј3Ј6Ј8ЈCl8) [26] derived from their molecular (X-ray) structures. The following geometrical parameters were included: pure ethylenic twist angle [4] (ω) around C 9 =C 9Ј , defined as the average of the two torsion angles C 9a -C 9 -C 9Ј -C 9aЈ and C 8a -C 9 -C 9Ј -C 8aЈ ; folding dihedral angle of the tricyclic (fluorenylidene) moiety (A-B), defined as the dihedral angle between the leastsquare planes of the atoms C 1 , C 2 , C 3 , C 4 , C 4a , C 9a and C 5 , C 6 , C 7 , C 8 , C 8a , C 4b ( Figure 2) of the benzene rings and reflecting the nonplanarity of the tricyclic moieties; twisting dihedral angle between the fluorenylidene moieties (AEB-CFD), defined as the dihedral angle between the leastsquare planes of all the untagged and all the tagged carbon atoms; pyramidalization angles [4] (χ) at C 9 and C 9Ј , defined as the improper torsion angle C 9a -C 9 -C 9Ј -C 8a subtracted from 180°, bond lengths r(…”

“…Kresmar et al, [24] on the basis of analysis of the 1 H NMR spectrum, reported 11.0 Hz for the 7 J F-F coupling, but did not report the details of the 19 F spectrum. The 13 C data differ slightly from ours, with Kresmar reporting 1 J C-F = 260.2 Hz for E11ЈF2 and 265.2 or 255.5 Hz for Z11ЈF2, compared to our value of 262.3 Hz.…”

“…[25] This result is consistent with the experimental ratio (Z)/(E) = 69:31 (2.2:1, ∆G 298 = -2.0 kJ/ mol) observed in the 1 H NMR spectrum of a mixture of Z11ЈF2 and E11ЈF2 in solution. [24] In the chlorine series, the (Z)/(E) ratio is reversed: Z11ЈCl2 is less stable than E11ЈCl2: ∆H 298 = 2.2 kJ/mol, ∆G 298 = 2.4 kJ/mol. Bucking chlorine atoms and bucking hydrogen atoms in the fjord regions of the (Z) diastereomer are indeed sterically more demanding than a pair of bucking chlorine and hydrogen atoms in the fjord regions of the (E) diastereomer.…”

“…[17] Dibenzo[a,l]pyrene, [18,19] one of the worst environmental hazards, [20,21] has a cove region and is chiral, while the notorious carcinogen benzo[a]pyrene [22] has a bay region and is planar. [23] Kresmar et al [24] have mistakenly used the term cove instead of the preferred term fjord to denote the overcrowding regions in bifluorenylidenes. There seems to be no need for more "catchy" nomenclature when the older "fjord" can aptly describe such regions in a range of molecules.…”

“…Through-space 19 F-19 F coupling was reported in the literature. [24,25] Special emphasis is given to the effects of fluorine vs. chlorine substituents across the fjord regions of bifluorenylidenes and to (E) vs. (Z) diastereomers. In particular, the unexpected higher stability of the (Z)-1,1Ј-difluoro diastereomer relative to the (E)-1,1Ј-difluoro diastereomer is highlighted and analyzed.…”