-Dialkyl aroylimidothiocarbonates are structural analogues of S
-dialkyl aroylimidothiocarbonates, which have been widely used as synthetic starting materials (Augustín et al.
; Sato et al.
; Fukada et al.
; Insuasty et al.
). We report here the structures of two O
-ethyl aroylthiocarbamates, (I)
), and two O
-diethyl aroylimidothiocarbonates, (III)
), and we compare these structures with those of two closely related S
-ethyl aroyldithiocarbamates, (V)
) (Low et al.
). The structure of (I)
has been briefly reported previously [Cambridge Strutural Database (Allen, 2002
) refcode GIFSUK; Arslan et al.
)], but the authors’ primary concern was the comparison of the experimental geometry and vibrational frequencies with those calculated from first principles at various levels of theory. The hydrogen bonding was described extremely briefly in terms of only an N—H
O interaction. Accordingly, we have thought it worthwhile to report here a more complete description of the hydrogen bonding in (I)
Figure 1 The molecular structures of (a) (I), (b) (II), (c) (III) and (d) (IV), showing the atom-labelling schemes. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii.
were prepared by the addition of ethanol to the corresponding intermediate aroylisothiocyanate, (B
) (see scheme
), itself prepared by reaction of potassium thiocyanate with the aroyl chloride, (A
). Deprotonation of the O
-ethyl aroylthiocarbamates yields the ionic intermediate, (C
), reaction of which with bromoethane gives (III)
. The corresponding reactions of the appropriate aroylisothiocyanate, (B
), with ethanethiol rather than with ethanol produces the dithiocarbamate esters (V)
(Low et al.
). It is interesting to note that in the conversion of (A
) to (B
), the thiocyanate anion reacts with the aroyl chloride exclusively via
the harder N terminus, while in the formation of (III)
), this anionic component of (C
) reacts with bromoethane exclusively at the softer S centre.
Despite the close pairwise similarities between the molecular constitutions within the pairs of compounds (I)
, and (V)
, and the overall similarities within the group comprised of (I)
, no two of these compounds are isomorphous. Thus, for example, while compounds (I)
crystallize in the space groups Pna
, respectively, both with Z
′ = 1, their close analogues (V)
crystallize in, respectively, C
′ = 2 and P
′ = 1. The unit-cell dimensions of (II)
, which could well have been isomorphous and isostructural are, in fact, significantly different, particularly for the cell repeat distance a
and the cell angle β. Also, (III)
, which might plausibly have been expected to be isomorphous, crystallize with triclinic unit cells in which the real and reduced cell angles are very different: the cell angles in (III)
are all significantly greater than 90°, while those in (IV)
are all well below 90°. Hence the unit cell for (III)
can be assigned as belonging to type II, designation 4R (Buerger, 1956
), while that of (IV)
can be assigned as type I, designation 1R.
In thiocarbamate esters (I)
, the molecular skeletons adopt chain-extended conformations which, apart from the aryl substituents, are close to planarity, as shown by the leading torsion angles (Table 1). The most striking feature of the conformations of thiocarbonates (III)
is the orientation of the S
-ethyl groups (Figs. 1
). Whereas in (III)
this ethyl group is almost coplanar with the rest of the skeleton, in (IV)
this unit is almost orthogonal to the rest of the molecule (Table 1). Also, whereas in thiocarbamate esters (I)
the formal C=O and C=S double bonds are mutually cisoid
, in dithiocarbamate esters (V)
(Low et al.
) these units adopt a transoid
Table 1 Selected geometric parameters (Å, °) for (I)–(IV)
In each of compounds (I)
, the C—O bond distances involving atom O3, namely C3—O4 in (I)
, and C3—O41 in (III)
, are all the same within experimental uncertainty. In addition, the N2—C3 bonds in (I)
are long for their type (Allen et al.
). These observations, taken together, indicate that in (I)
there is very little electronic delocalization from atoms N2 and O4 on to atom S3. By contrast, the C=S bond distances are 1.6586 (18) and 1.6552 (18) Å in (V)
, where Z
′ = 2 (Low et al.
), and 1.631 (3) Å in (VI)
(Low et al.
), typical of such distances in compounds containing >N—C(=S)—S– fragments (Allen et al.
) and consistent with the occurrence of conjugative delocalization. In (III)
, the C—N distances provide a clear distinction between the formal C1—N2 single bonds and the formal N2=C3 double bonds. Despite the apparent lack of any significant polarization of the electronic structures in (I)
, the bond angles at atoms C1, N2 and C3 are certainly consistent with a strongly repulsive nonbonded interaction between atoms O1 and S3. A similar pattern of bond angles is apparent in (III)
The supramolecular aggregation in (I)
is dominated by a combination of N—H
O and C—H
O hydrogen bonds, utilizing the same acceptor O1 atom in each compound (Table 2). These interactions are augmented, in the case of (I)
only, by two C—H
S interactions. In (I)
, molecules related by the a
-glide plane at y
are linked by a combination of a two-centre N—H
O hydrogen bond and a weaker three-centre C—H
(O,S) hydrogen bond to form a ribbon containing alternating
(6) (Bernstein et al.
(7) rings and running parallel to the  direction (Fig. 2). A second C—H
S interaction links molecules related by the n
-glide plane at x
into a simple C
(7) chain running parallel to the [01
] direction (Fig. 3). The combination of the ribbon along  and the chain along [01
] generates a hydrogen-bonded sheet parallel to (011), but there are no direction-specific interactions between adjacent sheets. In particular, both C—H
π(arene) hydrogen bonds and aromatic π–π stacking interactions are absent from the crystal structure of (I)
Table 2 Hydrogen bonds and short intermolecular contacts (Å, °) for (I)–(IV)
Figure 2 Part of the crystal structure of (I), showing the formation of a ribbon along  containing (6) and (7) rings. For the sake of clarity, H atoms not involved in the motifs shown have been omitted. Atoms marked with an asterisk (*), a hash symbol (#), (more ...)
Figure 3 Part of the crystal structure of (I), showing the formation of a C(7) chain along . For the sake of clarity, H atoms not involved in the motifs shown have been omitted. Atoms marked with an asterisk (*), a hash symbol (#) or a dollar sign ($) are (more ...)
, a combination of N—H
O and C—H
O hydrogen bonds, similar to the corresponding combination in (I)
, links molecules related by the c
-glide plane at y
into a chain of
(7) rings running parallel to the  direction (Fig. 4). The closest intermolecular contacts between atom S3 and the H atoms in (II)
corresponding to the C—H
S interactions in (I)
all have H
S distances well in excess of 3 Å, and hence they cannot be regarded as structurally significant. Although significant C—H
S interactions are absent from the crystal structure of (II)
, the hydrogen-bonded chains are linked by an aromatic π–π stacking interaction. The aryl rings in the molecules at (x
) and (1 − x
, 1 − y
, 1 − z
) are parallel by symmetry, with an interplanar spacing of 3.479 (2) Å. The corresponding ring-centroid separation is 3.746 (2) Å and the ring-centroid offset (slippage) is 1.389 (2) Å. The effect of this stacking interaction is to link the hydrogen-bonded chains into a sheet parallel to (100) (Fig. 5).
Figure 4 Part of the crystal structure of (II), showing the formation of a chain of (7) rings along . For the sake of clarity, H atoms not involved in the motifs shown have been omitted. Atoms marked with an asterisk (*), a hash symbol (#), a dollar sign (more ...)
Figure 5 A stereoview of part of the crystal structure of (II), showing the π-stacking of hydrogen-bonded chains along  to form a sheet parallel to (100). For the sake of clarity, H atoms not involved in the motifs shown have been omitted.
Since no N—H bonds are present in (III)
, the modes of supramolecular aggregation in these compounds necessarily differ substantially from those observed in (I)
. In (III)
, a single C—H
π(arene) hydrogen bond links pairs of molecules into centrosymmetric dimers (Fig. 6). However, π–π stacking interactions are absent and there are no direction-specific interactions between the dimeric units. By contrast, the crystal structure of (IV)
contains no C—H
π(arene) hydrogen bonds, but instead chains built from C—H
O hydrogen bonds are linked in pairs to form a ladder-type structure, in which molecules related by translation form C
(6) chains running parallel to the  direction. The aryl rings of the molecules at (x
) and (1 − x
, 1 − y
, 1 − z
) are strictly parallel, with an interplanar spacing of 3.428 (2) Å. The ring-centroid separation and offset are 3.787 (2) and 1.609 (2) Å, respectively, so that pairs of antiparallel C
(6) chains are weakly linked (Fig. 7).
Figure 6 Part of the crystal structure of (III), showing the formation of a centrosymmetric dimer by means of symmetry-related C—Hπ(arene) hydrogen bonds. For the sake of clarity, H atoms bonded to C atoms which are not involved in the (more ...)
Figure 7 A stereoview of part of the crystal structure of (IV), showing a π-stacked pair of antiparallel hydrogen-bonded chains along . For the sake of clarity, H atoms not involved in the motif shown have been omitted.
It is of interest briefly to compare the supramolecular aggregation in (V)
(Low et al.
) with that in (I)
. Compound (V)
crystallizes with Z
′ = 2 in space group C
(Low et al.
) and each type of molecule independently forms a cyclic
(8) dimer. These dimers are built using two symmetry-related N—H
S hydrogen bonds, with the thione-type S atoms as the acceptors in both types of dimer. One type of dimer contains molecules related by inversion and the other contains molecules related by a twofold rotation axis, and the two independent types of dimer are linked into chains by a single C—H
π(arene) hydrogen bond. It is striking that there is no participation by the amidic O atom in the hydrogen bonding in (V)
. In compound (VI)
, on the other hand, where Z
′ = 1 (Low et al.
), a combination of N—H
O and C—H
O hydrogen bonds generates a chain of
(7) rings along , and chains of this type are linked into a sheet by a single aromatic π–π stacking interaction. Thus, while (II)
are not isomorphous, and while their molecules adopt different conformations, nonetheless their crystal structures exhibit very similar patterns of intermolecular interaction.
The only hydrogen bonds in this series of compounds, viz.
, which involve S atoms as the acceptors utilize the thione-type S atoms in (I)
; the two-coordinate S atoms in compounds (III)
do not participate in any hydrogen-bond formation. This observation is certainly consistent with the deductions drawn from database analyses (Allen et al.
) that, while thione-type S atoms can in some circumstances act as effective hydrogen-bond acceptors, two-coordinate S atoms are, in general, very poor acceptors.