X-ray crystallographic investigations on polarized twisted ethylenes

Abstract

The thesis entitled “X-ray crystallographic investigations on polarized twisted ethylenes” deals mainly with the studies carried out by the candidate on the details of molecular geometry and conformation of six push–pull ethylenes (Chapter I to V) as part of an ongoing programme in this laboratory. Also are discussed in Chapter VI the geometrical features of a molecule in which the donor and acceptor groups are present at vicinal positions of a C = N bond instead of a C = C bond. Chapter VII presents the results obtained on the crystal and molecular structures of two nitroenamines. Chapter I is a brief account of general features of the molecular geometry and conformation of polarized twisted ethylenes. Ethylenes substituted with electron-donating and electron-accepting groups in vicinal positions are known as polarized or push–pull ethylenes. The most noteworthy feature of the push–pull ethylene molecule is the lengthening of the C = C bond, the degree of lengthening depending upon the strength of the donors and acceptors. This results in a significant lowering of the rotational barrier about the C = C bond in comparison with that of ethylene. The important conformational properties of this class of molecules, dictated mainly by steric and electronic effects, are discussed in this chapter. A brief mention is made of the role of the push–pull effect as a stabilizing force in the synthesis of new aromatic compounds or otherwise unstable systems. In addition, how Nature incorporates this effect is also elucidated with an interesting example. Chapter II deals with the crystal and molecular structures of 3-[bis(dimethylamino)methylene]-3-phenyl-2-propanone (A) (Fig. 1) and methyl 2-[bis(dimethylamino)methylene]-3-oxobutyrate trihydrate (B) (Fig. 2). In order to study the effects of donors of the type –NMe? and acceptors such as –COMe, –Ph, –COMe and –COOMe, molecules (A) and (B) have been investigated. The crystals of (A) are orthorhombic, space group Pbca with a = 8.605(2), b = 14.845(3), c = 21.445(4) Å and Z = 8. The compound (B) is triclinic, space group P1, a = 6.872(1), b = 8.707(1), c = 12.341(1) Å, ? = 91.13(1)°, ? = 92.82(1)°, ? = 100.48(2)° and Z = 2. Both the structures were solved by direct methods and refined to an R value of 0.057 for 1563 significant reflections for molecule (A) and an R value of 0.055 for 1960 significant reflections for molecule (B). The most significant observation in the geometry of these molecules is the lengthening of the C = C bond [1.409(4) Å in (A) and 1.461(2) Å in (B)]. This shows that the acceptor combination –COMe and –COOMe in (B) is more powerful than –Ph and –COMe in (A). The twist angles about the C = C bond are 34.8(3)° and 56.9(2)° in (A) and (B), respectively. The changes in the C = C bond are accompanied by significant changes in the distances of the adjacent N–C(sp²) (donor) and C(sp²)–C(sp²) bonds. The intermolecular hydrogen-bonding scheme in the crystals of (B) is discussed in detail. Chapter III is devoted to a discussion of a push–pull ethylene having donor (–SMe) and acceptor (–NO? and –CN) groups. The molecule studied is 3,3-bis(methylthio)-2-nitro-2-propene-1-nitrile (C) (Fig. 3), which crystallizes in the space group P2?/n with a = 4.123(1), b = 13.096(3), c = 15.126(3) Å, ? = 93.83(2)° and Z = 4. The structure was solved by direct methods to an R value of 0.054 with 1119 significant reflections. The C = C bond length of 1.376(4) Å indicates only a moderate polarization, showing that the donor and acceptor groups in this molecule are weak. The molecule is twisted about the C = C bond by 12.8(3)°. An interesting intramolecular feature of the molecule is the presence of a non-bonded interaction of the S···O type. Chapter IV reports the crystal and molecular structures of 3-dimethylamino-3-methylthio-2-phenylacrylonitrile (D) (Fig. 4) and (1,3-dimethyl-2-imidazolidinylidene)phenylacetonitrile (E) (Fig. 5). The cell dimensions are [text omitted in original; see figures and structural tables]. (D) and (E) have the same acceptor groups but differ in donor combinations. Both molecules (D) and (E) crystallize in the non-centrosymmetric space group P2?2?2? and were solved using direct methods and refined to R values of 0.039 (1169 reflections) and 0.041 (875 reflections). The C = C bond lengths in molecules (D) and (E) are 1.369(5) Å and 1.382(4) Å, respectively. The slightly longer length in (E) seems to indicate that the donor capability of the imidazolidinylidene group in (E) is better than the –NMe?, –SMe group contributions in (D). The twist angles about the C = C bond are 22.2(3)° for (D) and 23.1(4)° for (E). Chapter V discusses the molecular geometry of the most interesting structure among the various push–pull ethylenes investigated so far, namely 1,3-dibenzyl-2-(5,5-dimethylcyclohexane-1,3-dithione-2-ylidene)imidazolidine (F). The molecule (F) crystallizes in the space group P2?/c with a = 11.685(2), b = 9.892(2), c = 20.864(3) Å, ? = 102.15(1)° and Z = 4. Investigation of this push–pull system is of interest as it allows us to compare the changes produced in the molecular geometry when the acceptor group with C = O is replaced by C = S. From the X-ray determination of this compound it was found that the C = C bond is as long as 1.482(6) Å with a very large twist about the C = C bond [80.8(5)°]. In this molecule the rotational barrier about the C = C bond was reported to be very large (> 104.6 kJ mol?¹) from dynamic NMR studies. The X-ray results concerning the C = C bond length with such a large angle of rotation provide an experimental basis for assessing the correctness of the quantum chemical calculations for predicting the C = C bond length with an angle of rotation of 90° for ethylene. Chapter VI deals with the molecule N-[bis(methylthio)methylene]cinnamide (G) (Fig. 7) with –SMe as donors and the cinnamoyl group as an acceptor at the vicinal positions of the C = N bond. The molecule (G) crystallizes in space group P2?/n, a = 14.626(3), b = 7.144(1), c = 11.996(2) Å, ? = 90.03(2)° and Z = 4. Based on 1455 significant reflections, the structure has been refined to an R value of 0.035. It is found that the C = N bond does not undergo any significant lengthening in the presence of these donors and acceptors. The molecule provided yet another opportunity to study the directional preferences for O in the weak intramolecular S···O interaction of 2.658(3) Å. Chapter VII is concerned with the molecular structures of two nitroenamines, namely 2-(nitromethylene)thiazolidine (H) (Fig. 8) and N?methyl?(2?nitromethylene)?pyrrolidine (I) (Fig. 9). These have a cyclic donor with a nitrogen atom in the ring attached to one carbon of the C = C bond and the electron-withdrawing nitro group at the other carbon. The molecule (H) crystallizes in space group C2/c, a = 8.821(2), b = 10.884(2), c = 12.870(2) Å, ? = 95.64(1)° and Z = 8. The molecule (I) belongs to the space group P2?/c, a = 6.023(4), b = 15.851(6), c = 7.447(4) Å, ? = 106.93(3)°. The crystal structures of (H) and (I) were solved using direct methods and refined to R values of 0.071 (841 significant reflections) and 0.110 (809 significant reflections), respectively. The C = C bond length in (H) is 1.405(5) Å and in (I) 1.357(8) Å. The molecule (H) is the third example in this set wherein an intramolecular S···O interaction is observed. The structural characteristics of the push–pull ethylene molecules investigated by the candidate and those reported earlier are discussed at length in Chapter VIII. An interesting correlation between C = C bond length and rotational barrier (?G‡) from dynamic NMR data has been noted. Based on the crystallographic evidence, the relative strengths of the donors and acceptors in the molecules studied have been assessed. With the view of investigating the chemical reactivity of push–pull ethylenes, experiments were carried out on a model system, methyl 2?(1,3?dimethyl?2?imidazolidinylidene)?3?oxobutyrate, and the observations are recorded in this Chapter. The references are listed in alphabetical order at the end of each Chapter. The lists of observed and calculated structure factors for all the structures have been placed together at the end of the last Chapter. References to papers based on the work carried out by the author, either already published or in press, are given below: