A Step further towards polymer synthesis, condensation was carried out between a bifunctional aromatic amine, 3,3',4,4’-biphenyltetramine 58 and monofunctional ferrocenyl diketone, ferrocenylphenyl-1,2-diketone,
40a, in another reaction bifunctional ferrocenyl diketone, ferrocene-1,1'- diylbis(phenylethanedione) 48 and monofunctional 1,2-diamine, 1,2- diaminobenzene 49 were also condensed together to give ferrocene- containing bisquinoxalines.
3.1.2.1 Reaction of 40a with 3,3*?4,4*-biphenyItetramine.
The condensation of 3,3’,4,4'-biphenyltetramine 58 and 40a in the molar ratio of 1 : 2 was carried out in dioxane solution as well as in
aqueous ethanol containing some K2CO3 to help the availability of free-
amine: three regio-isomers of bisquinoxalyl compound namely 2,2'-
diferrocenyl-3,3'-diphenyl-6,6'-diquinoxaline 59a, 2,2'-diphenyl-3,3'- diferrocenyl-6,6’-diquinoxaline 59b, and 2,3'-diferrocenyl-3,2'-diphenyl- 6,6'-diquinoxaline 59c, were the expected product. The TLC shows the
presence of the two purple products of very close Rf, and it is assumed that two of these isomers are appearing at the same position. Of the proposed isomers, 59a and 59b both have two-fold symmetry while 59c
lacks any symmetry, and this may be the reason that the quinoxalyl product is appearing as two spots i.e. those having two fold symmetry may be behaving similarly on TLC. These isomers were not found separable either on silica gel or on alumina. However, elemental analysis and NMR studies of the isomeric mixture of the quinoxaline confirms the identity of the product as 59. The IH NMR spectrum revealed the relative ratio of the three isomers as 9:6:5, by examining the integration of the ferrocene region, and use of resolution enhancement technique in the NMR spectrum enabled resolution of all 36 peaks in the aromatic region.
The ferrocenyl region was also readily characterised with few peak overlaps.
The bisquinoxalyl product in above reaction was also accompanied with the two isomers of monoquinoxalyldiamine 60 as a minor product, an intermediate in the reaction pathway. To confirm the identity of the intermediate, a reaction in the aqueous ethanol was carried out using excess 58, where both isomers of product 60 were isolated in good yield and characterised by NMR and elemental analysis along with only traces of 59. Ph 59a Fc 59b Ph P Ph 59c Fc Fc/P Ph/Fc 60
3.1.2.2 Reaction of 48 with 1,2-diaminobenzene.
When the bifunctional ferrocenyl diketone, ferrocene-1,1 '-diylbis- (phenylethanedione) 48 and the monofunctional 1,2-diamine, 1,2- diaminobenzene 49 were reacted in the molar ratios of 1:2 respectively a bis-quinoxalyl compound, l,r-bis(3-phenyl-2-quinoxalyl)ferrocene
61
was the only product. The reaction was carried out in aqueous ethanol in the presence of K2CO3 and NaiSOs in order to have the free-amineavailable.
^^^rcocoph
Ee
COCOPh
(48)
The reaction was followed by TLC, early in the reaction, two new spots of different Rf appeared on the TLC. The lower Rf value spot, possibly corresponding to condensation of the diketone in one ring, disappeared slowly during the reaction, leaving only one spot. The product
61
was purified on silica gel column; the diamine in this reaction is unsubstituted hence there is no possibility of having regioisomers of this compound. The compound was crystallized from dichloro- methane/diethylether, to produce red, thin plate like crystals suitable for single-crystal X-ray analysis. The X-ray structure showed 0.9 mol of dichloromethane present in the crystal lattice, hydrogen bonded to a quinoxaline nitrogen. The iH NMR spectrum shows two peaks of equalFe
intensity arising from two sets of CH protons in Cp rings in the ferrocene region, which integrate for 4 protons each, while the phenyl region integrates for 18 protons. The l^C spectrum shows three non equivalent ferrocenyl carbons including one quaternary carbon and the aromatic region shows all 12 carbons of which 5 are quaternary. Elemental analysis when calculated for C38H26FeN4.0.9CH2Cl2, as revealed from X-ray analysis was found to be satisfactory. The X-ray structure of 1,1'- bis(3-phenyl-2-quinoxalyl)-ferrocene 61 is shown in Figure 3.4; and section 3.1.2.3 contains a discussion of its crystal structure.
3.1.2.3 Crystal and molecular structure of compound 61.
Compound 61 crystallized from diethylether/dichloromethane as a partial dichloromethane solvate,C38H26FeN4.0.9CH2Cl2, in which the solvent molecule is hydrogen-bonded to one of the quinoxaline fragments by means of a C-H N hydrogen bond (Figure 3.3). Within the molecules of compound 61 the Fe-Cgl and Fe-Cg2 distances are 1.656(1) and
1.655(1) Â and the Cgl-Fe-Cg2 angle is 177.8(1)°; (Cgl and Cg2
represent the centroids of the cyclopentadienyl rings C ln and C2n
respectively, for n = 1 - 5). The mean value of the C-C bond distances in these rings is typical of those observed in other mono-substituted
fe r r o c e n y lq u in o x a lin e s . 1 >2 The quinoxalinyl fragments in 61 both exhibit
the bond fixation typical of the heteroaromatic s y s t e m . L 2 , 7 The
conformation of 61 and the molecular packing are intimately connected.
The independent quinoxalinyl groups are both nearly coplanar with the neighbouring cyclopentadienyl ring, with interplanar angles between quinoxalinyl and cyclopentadienyl rings of 5.2(3) and 7 .8(3)° for the
a 62
C32 C33
<
Figure 3 .3 Single crystal X-ray structure of 1,1 '-bis(3-phenyl-2-quinox-alyl)- ferrocene 61 (top) view of the molecule with our numbering scheme
fragments containing C il and C21 respectively, while the dihedral angles between the two quinoxalinyl planes is 5.4(2)°. Moreover, the mean value of the torsion angles C1 n-Cg 1 -Cg2-C2n (n = 1 - 5) is -20.9(9)°, where a value of zero would indicate perfect eclipsing of both the cyclopentadienyl rings and the exocyclic C-C bonds: consequently the two independent quinoxalinyl groups within the molecule exhibit considerable overlap (Figure 3.3). The mean perpendicular distance between the groups is ca, 3.4Â, so that intramolecular tz-k stacking interactions are significant, and indeed are possibly die dominant factor underlying die observed conformation: this is ahnost certainly associated with the fact that the typical inter-plane spacing in k - k stacked aromatic systems is very similar to the inter-ring separation in ferrocenes. Furthermore, adjacent molecules are stacked in columns along the [100] direction, such that there are intermolecular tz- k stacking interaction
between the C2n ring in the molecule at (x, y, z) and the 10-membered heteroaromatic ring containing N1 and N4 at (-1 + x, y, z), with a mean perpendicular distance between these rings of ca. 3 .5Â, so that the intra- and inter-molecular spacings are very similar. As in 2-ferrocenyl-3- phenyIquinoxaline,2 the neighbouring phenyl and quinoxalinyl groups are nearly normal to one another: the twists of the phenyl groups (76.5(1), 77.0(1)° away from co-planarity with the quinoxalinyl groups is forced by the presence of the adjacent bulky ferrocenyl group.