or Electron DonorâAcceptor (EDA) Interactions - ACS Publications

Article pubs.acs.org/crystal

Adducts of TCNQ with Neutral Nitrogen Bases. Their Rationalization in Terms of Intermolecular Charge-Transfer (CT) or Electron Donor− Acceptor (EDA) Interactions Valerio Bertolasi, Paola Gilli,* and Gastone Gilli Centro di Strutturistica Diffrattometrica e Dipartimento di Chimica, Università di Ferrara, Via Borsari 46, 44121 Ferrara, Italy S Supporting Information *

ABSTRACT: The structures of 8 alternate-stack adducts of TCNQ with neutral N-bases were determined by X-ray diffraction. Their packings contain 96 contacts ≤(sum vdW radii) dividable in CT or EDA classes as follows: (i) 56 σ*←n [12 inter and 5 intra X−H···:Y (X, Y = N, O, Cl); 31 inter and 8 intra C−H···:Y (Y = N,O) H-bonds]; (ii) 2 π*←n C←:N; (iii) 11 σ*←π C−H←(C,N); (iv) 21 π*←π C←(C,O); and (v) 6 σ*←σ C−H←H−C interactions. The σ*←n contacts link molecules in planar ribbons by saturating lone-pair ndonors (:Y) by a maximum of X−H or C−H acceptors. These ribbons are interlinked in alternate vertical stacks by π*←π contacts or in herringbone packings by π*←n or σ*←π ones. In spite of their alternate geometry, 4/8 crystalline adducts are black with metallic luster imputable to delocalized electronic bands, a fact successfully interpreted in terms of specific packing substructures held together by proper EDA interactions. Finally, a simple three-component model of the forces acting in neutral molecular crystals (dispersion, exchange, and CT or EDA attractions) is discussed, showing that it is theoretically well-grounded in the classical perturbation theory and suggesting that it could be profitably used in synergy with other well-established methods of crystal-packing analysis based on electron densities and electrostatic potentials.

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theory.12,13 In MO terms, these interactions can be ascribed to the overlapping of the highest occupied MO of the donor (HOMO) with the lowest unoccupied MO of the acceptor (LUMO), overlapping which induces a HOMO stabilization roughly proportional to the LUMO←HOMO charge transferred. Common electron donors are as follows: (i) n = lonepair donors; (ii) σ = bonding-pair donors from σ bonding MOs; and (iii) π = bonding-pair donors from π bonding MOs associated with localized or delocalized multiple bonds. Common electron acceptors are as follows: (i′) v (or n*) = acceptors on vacant AOs; (ii′) σ* = acceptors on empty σ* antibonding MOs; and (iii′) π* = acceptors on empty π* antibonding MOs associated with localized or delocalized multiple bonds. In a recent paper on the crystal packing of picric acid and 14 of its adducts with nitrogen bases,14 we have found that the artifice of considering D−H←:A and C−H←:A H-bonds as σ*←n EDA interactions contributes to speed up crystal-packing analysis, to unify all intermolecular forces present, and to stress the importance of bonding and nonbonding electron pairs in steering the crystallization process. These conclusions are now challenged by the packing analysis of a quite different set of

INTRODUCTION Since its discovery in 1920, the D−H···:A hydrogen bond (Hbond) has been essentially considered as a proton exchange between a proton donor D−H (Brønsted acid) and a proton acceptor :A (Brønsted base). This concept can be exploited1 by considering the H-bond as a bimolecular proton-transfer (PT) reaction leading from D:···H−A to D−H···:A through the D···H···A transition state, where what we like to call H-bond is actually a minimum (or two minima) along the PT pathway which can adopt a variety of different shapes according to the strength of the H-bond formed.2−5 This model has finally proved to be an easy route to predict the strength of any Hbond in terms of the difference of the donor/acceptor acid dissociation constants, ΔpKa = pKAH(D−H) − pKBH+(A− H+),6,7 as recently reviewed under the heading “the dual Hbond model”.8,9 Though the Brønsted acid−base PT model (exchange of protons) has proved to lead to a consistent H-bond theory, it is by no means the only way to look at the H-bond because the parallel Lewis acid−base model (exchange of electrons) is equally suited. In such a case the D−H···:A H-bond becomes a D−H←:A σ*←n interaction, a particular member of the class of charge-transfer (CT) or electron donor−acceptor (EDA) interactions introduced, at the beginning of the 1950s, by Mulliken in VB terms10,11 and recast some 40 years later by Weinhold in the frame of his natural bond orbital (NBO) © 2012 American Chemical Society

Received: April 1, 2012 Revised: July 25, 2012 Published: July 26, 2012 4758

dx.doi.org/10.1021/cg3004382 | Cryst. Growth Des. 2012, 12, 4758−4770

Crystal Growth & Design

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mixed crystals formed by the association of a typical π* acceptor, 7,7,8,8-tetracyanoquinodimethane (TCNQ), with substituted nitrogen bases (N-bases) as π donors. TCNQ was first synthesized at the beginning of the 1960s15 and became generally known some ten years later16 when its [TTF][TCNQ] crystalline CT complex with tetrathiafulvalene (TTF) was found to behave as an organic semiconductor (or organic metal), a new class of closed-shell molecules that, instead of being the usual insulators, display an electric conductivity which is metallic at room temperature and becomes semiconducting below some 50−80 K. Not surprisingly, these compounds have aroused great interest in the area of molecular electronics, prompting the synthesis, structural determination, and physical testing of a large number of new terms of the series17−21 whose crystal structures have been recently reviewed by Herbstein18 and Herbstein and Kapon.19 The chemical and physical features of TCNQ in its adducts are greatly affected by its EDA properties, as determined by the interplay of the four strongly electron withdrawing −CN: substituents with the p-quinone frame. This prompts TCNQ (Scheme 1) to behave as (i) an n donor, i.e., an electron-pair

authors have suggested that they should be unable to display metallic or semiconducting behavior, though some recent measurements seem to suggest that this may actually be the case.25,26 In this paper we report on the crystal-structure determinations of eight alternate-stack 1:2 adducts of TCNQ with neutral N-bases, out of which four are found to display the metallic luster typical of electrically conducting organic compounds (Scheme 2 and Table 1). While attempts to Scheme 2. The Eight N-Bases Cocrystallized with TCNQ

Scheme 1. EDA Interactions in the TCNQ Molecule Table 1. Chemical Name and Crystal Aspect of TCNQ and Compounds 1−8 compd

donor from its four −CN: nitrogens; (ii) a C−H···:X (X = N, O) H-bond donor or, in EDA terms, a σ* acceptor on the antibonding MOs associated with the four C−H bonds; (iii) a π donor from the four CN multiple bonds; and finally, (iv) an efficient π* acceptor on the π* antibonding MO that practically covers the entire molecule. In the absence of a π donor counterpart, the packing of pure TCNQ crystals22 is driven by in-plane C−H←:N σ*←n and perpendicular (C,C)←:N π*←n EDA interactions which connect the molecular units in a typical herringbone arrangement (see Figures 2.TCNQ and 3.TCNQ.a,b). When, however, TCNQ (the A acceptor) is cocrystallized with a suitable π donor D, the π*←π interactions become predominant and the two molecules are induced to crystallize in parallel stacks that can be either of segregated or alternate types, a phenomenon occurring with overwhelming frequency as it can be easily confirmed by a quick screening of the more than 400 TCNQ cocrystals collected in the Cambridge Structural Database23,24 which shows the systematic formation of such variously stacked π*←π interactions. Segregated stacks are typical of the [TTF][TCNQ] complex16 and consist of separated columns of positive D•+D•+D•+D•+ and negative A•−A•−A•−A•− radical ions internally stabilized by formation of delocalized electronic bands, which are the structures imparting to the stack its metallic or semiconducting properties. Conversely, electronic band formation is not well documented in complexes with alternate stacks, such as A•−D•+A•−D•+A•−D•+, and some

chemical name

TCNQ 1 2

7,7,8,8-tetracyanoquinodimethane bis(2-aminopyrimidine)·TCNQ bis[pyridin-2(1H)-one]·TCNQ

3 4

bis[3-hydroxypyridin-2(1H)-one]·TCNQ bis[quinolin-2(1H)-one]·TCNQ

5

bis(1H-indazol-3-ol)·TCNQ

6

bis(4-benzoyl-3-methyl-1-phenyl-pyrazol-5ol)·TCNQ bis[methyl 2-(4carbamoylphenylhydrazono)-3oxobutanoate]·TCNQ bis[1-((2-chlorophenyl)hydrazono)-1Hnaphthalen-2-one]·TCNQ

7 8

crystal color pale yellow red brown-black with metallic luster green brown-black with metallic luster black with metallic luster dark-red dark-ruby green-black with metallic luster

prepare larger crystals suited for conductivity measurements are still in progress, we focus here on their full crystal-packing analysis carried out in terms of intermolecular EDA interactions and aimed, inter alia, at establishing a correlation between the optical crystal features and the presence of specific CT packing arrays (ribbons and columns) which may still be compatible with the formation of an electronic band structure.

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EXPERIMENTAL SECTION

Sample Preparation. All eight adducts of TCNQ with neutral Nbases (Table 1) were prepared with the same method. Equimolecular amounts of base and TCNQ were ground in a mortar with the addition of a few drops of acetonitrile until formation of a deeply colored powder. The resulting powders were dissolved in acetonitrile, and the solutions slowly evaporated (in between one and two days) until precipitation of a mixture of two types of crystals, one deeply colored and one yellow (not tested, but most probably consisting of excess TCNQ). The deeply colored crystals, selected by hand and submitted to X-ray structural determination, resulted to be 2:1 adducts of the N-base with TCNQ. Compound 1 was barely stable at room 4759



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Figure 1. ORTEP33 views of the molecular complexes 1−8 with the thermal ellipsoids at 30% probability. temperature and decomposed shortly after the structural determination. Compounds 2−6 were stable at room temperature; upon slow heating they did not melt but started to lose the N-base by sublimation (around 120 °C for compounds 2, 3, and 5, and 150 °C for compounds 4 and 6) leaving a yellow residue of TCNQ crystals.

Conversely, compounds 7 and 8 regularly melted at the temperatures of 202−204 °C and 178−180 °C, respectively. Crystal Structure Determination. Crystal data of compounds 1− 8 were collected using a Nonius KappaCCD diffractometer with graphite-monochromated Mo Kα radiation; data sets were integrated 4760



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with the Denzo-SMN package27 and corrected for Lorentz and polarization. All structures were solved by direct methods (SIR97)28 and refined by full-matrix least-squares with anisotropic non-hydrogen and isotropic hydrogen atoms with two exceptions: (i) in 6, the hydrogens of the C5 methyl were given calculated positions riding on their carrier atom; and (ii) in 7, the non-H atoms of the disordered O2−C9H3 group were still refined anisotropically but over two positions with final occupancies of 69 and 31%; the C9H3 methyl hydrogens were added in calculated positions, riding on their carrying atom. All calculations were performed by the SHELXL-97,29 PARST,30 and PLATON31 programs implemented in the WINGX system.32 ORTEP33 views of compounds 1−8 are reported in Figure 1. Tables of crystal data, selected bond distances and angles, and H-bond and EDA contact distances have been deposited as Tables S1−S4 of the Supporting Information. All crystallographic details (excluding structure factors) have been deposited as Supporting Information in the form of crystallographic information files (CIF files). Data Treatment. The crystal structures of TCNQ22 and its 1−8 adducts have been screened for the occurrence of short intermolecular contacts, defined as contacts shorter than the sum of the Bondi’s34 van der Waals radii (∑vdW), by the use of the Mercury CSD 2.3 program.35 To correct for the well-known X-ray proton positioning errors, all X−H bonds have been renormalized by setting the O−H, N−H, and C−H distances to the reference values of 0.94, 1.03, and 1.08 Å, respectively, whenever shorter than them. Short contacts were quantified in terms of the shortening with respect to ∑vdW (vdWSH, in Å) or by its percent value, vdWSH% = 100 vdWSH/∑vdW. H-bond energies, EHB, of conventional X−H···:Y bonds (X, Y = N, O, Cl) were estimated by the Lippincott and Schroeder (LS) method accounting for the effects of the X−H−Y angle.36−38 As for the C−H···:X bonds (X = N, O; not parametrized by the LS method), the shortest H···O distance is known to be 1.86 Å (vdWSH% ≅ 30%) in the trinitromethane−dioxane adduct39 with a maximum EHB of some 2.0 kcal mol−1 according to the most accurate theoretical calculations.40 Making use of these few reference data and taking into account the known exponential relationship between EHB and H···O distance,41 C−H···:X energies can be roughly estimated to be 2.0, 1.0, 0.5, 0.2 kcal mol−1 for vdWSH% shortenings of 30, 26, 22, 15%. Complete table of correspondences between EHB and vdWSH% is available as Table 1 of ref 14.

Figure 2. Crystallographic unit cells for the structures of TCNQ and compounds 1−8.

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their C2A−H←N1A contact is much longer than ∑vdW. Because of that, the crystals were unstable and, in fact, decomposed into the constituents shortly after the structural determination. Compound 2 (Figure 2.2) is the first to give a planar packing compatible with the expected π*←π A←D stacked complex. The crystals consist of essentially planar ribbons of TCNQ molecules linked by couples of C3A−H←N2A σ*←n interactions (6.9%; Figure 3.2.a) and of N-bases linked by N1−H←O1 and C2−H←O1 σ*←n interactions (37.5 and 10.6%; Figure 3.2.b). The two ribbons are disposed side-by-side in the horizontal plane and vertically connected by two C←C π*←π interactions (; Figure 3.2.c) with formation of ···A···(D−H···D′)···A···(D−H···D′)··· stacks whose electrical continuity (and then ability of forming extended π*←π electronic bands) is ensured by the (D−H···D′) H-bond connection between couples of N-bases. In compound 3 (Figure 2.3) only N-bases make ribbons connected by two N1−H←O1 and O2−H←O1 σ*←n interactions (31.5 and 33.6%; Figure 3.3.a), while TCNQ molecules do not, being practically segregated in the plane by four surrounding N-bases through a net of C3−H←N2A, C4− H←N2A, and C2A−H←O2 σ*←n interactions (4.0, 7.9, and 4.2%; Figure 3.3.b). As TCNQ is also vertically trapped between two N-bases by four C←C π*←π interactions (; Figure 3.3.c), the final packing resembles that of a molecular crystal made up of these trimolecular sandwiches.

CRYSTAL STRUCTURE DESCRIPTION Crystallographic unit cells of TCNQ22 and its 1−8 adducts with N-bases are shown in Figure 2. More than by exchange repulsions, the packing appears to be controlled by a complex net of EDA interactions that, whenever shorter than ∑vdW, are to be considered of more or less attractive nature42 (full list of the interactions ≤∑vdW is given in Table S5 of the Supporting Information). The most significant structure-determining EDA contacts are depicted in Figure 3 and discussed below (the corresponding vdWSH% values are given in parentheses). TCNQ (Figure 2.TCNQ) forms a particularly simple herringbone packing consisting of infinite ribbons of TCNQ molecules linked by C2−H←N2 σ*←n interactions (vdWSH% = 16.4%; Figure 3.TCNQ.a). The ribbons are interjoined in an angular fashion by two C1−H←N1 σ*←n (11.5%) and one (C4,C5)←N1 π*←n (2.2%) interactions (Figure 3.TCNQ.b). Note, in this context, that all interactions involving π MOs (such as π*←n, σ*←π, and π*←π) are systematically longer than the σ*←n ones. This is because p AOs are more diffuse than s ones and then their overlap integrals have maxima at longer interatomic distances. Compound 1 (Figure 2.1) displays a similar herringbone combination of two different planar ribbons, the first made of N-bases linked by couples of N1−H←N2 and N1−H←N3 σ*←n interactions (25.1%; Figure 3.1) and the second of TCNQ molecules placed side by side but not linked because 4761



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Figure 3. continued

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Figure 3. The most characteristic EDA interactions occurring in the structures of TCNQ and compounds 1−8. σ*←n in black, π*←n, σ*←π, and π*←π in red, and σ*←σ in cyan.

linked by C3A−H←N2A σ*←n interactions (1.0%; see Figure 3.2.a) and N-bases connected, besides the normal N1−H←O1

The packing of compound 4 (Figure 2.4) is strictly similar to that of 2. It consists of separate ribbons of TCNQ molecules 4763


2 4 3 4 3 4 6 7 5 38

TCNQ 1 2 3 4 5 6 7 8 sum

0 1 0 1 0 0 0 1 0 3

UNS(nD)

2 7 7 7 9 8 14 15 13 82

σ*A

0 3 4 3 6 4 8 8 8

A−D 0 2 1 2+ 1 2 1+ 3+ 0+ 17 1 1 2

1

XH←:Y σ*←n 24−26 38 13−34 32 20−39 6−37 3−31 13−36

vdW SH% 0 0 0 0 0 0 0 0 0

UNS (XH) 2 1 4 4 3 2 2+3 7+3 6+2 39

CH←:X σ*←n

vdW SH% 11−16 6 0−11 0−8 1−10 9−10 2−16 3−15 0−12

CH←:X

2 0 0 0 0 0 0 0 0 2

A←:D π*←n

C←:N

2−3

vdW SH%

π* acceptors

4 3 5 7 4 4 7 14 10 58

∑INT (nD) 2.0 0.75 1.7 1.7 1.3 1.0 1.2 2.0 2.0

∑INT /D

σ* accs.

0 0 0 0 0 1 6 2 2 11

A←:D σ*←π

1 3−6 1−2 1−3

vdW SH%

CH←(C,N)

π* accs.

0 0 2 4 3 4 0 3 5 21

1−4 1−6

2−3 1−5 1−3 0−1

vdW SH%

C←(C,O) A←:D π*←π

π donors

0 0 0 0 2 0 2 0 2 6

A←:D σ*←σ

5−7

6−12

4−8

vdW SH%

C−H←H−C

σ* accs.

σ donors

vdW

a

nD = electron-pair donors; UNS(D) = unsaturated electron-pair donors; σ*A = σ* electron acceptors; A − D = σ* acceptors minus n donors; XH←:Y = conventional H-bonds; CH←:X = CH H-bonds; SH% = shortening of the A···D distance in per cent of ∑vdW; A←:D = π*←n, σ*←π, π*←π, σ*←σ CT or EDA interactions; ∑INT(nD) = total number of interactions with n donors; occurrences in the (x + y) form refer to intermolecular and intramolecular interactions, respectively.

nD

COMP

XH←:Y

σ* acceptors (σ*A)

n donors (nD)

Table 2. Summary of the Interatomic Contacts Occurring in Compounds 1−8 Which Are Shorter than ∑vdW, the Sum of van der Waals Radii (Data from Table S5 in the Supporting Information)a

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and C6−H←O1 σ*←n interactions (32.3 and 10.4%; Figure 3.4.a), by two rather unusual C2−H←H−C6 and C3−H←H− C7 σ*←σ interactions (4.6, and 7.4%; Figures 3.4.a,b) between the 2-pyridinone and phenylene moieties of the 2-quinolinone molecule. Finally, the two types of ribbons are vertically connected in stacks by three C←(C,O) π*←π interactions (; Figure 3.4.c) according to the ···A···(D− H···D′)···A···(D−H···D′)··· scheme discussed above. Compound 5 (Figure 2.5) displays a second type of π*←π stacking which, though different from 2 and 4, seems equally compatible with the formation of π*←π electronic bands. Here, TCNQ molecules are joined in planar ribbons by C3A−H← N1A σ*←n interactions (9.2%) according to the pattern already observed in TCNQ crystals (Figure 3.TCNQ.a), while N-bases form separate ribbons interlinked by O1−H←N1 and C4−H←O1 σ*←n interactions (39.0 and 9.4%; Figure 3.5.a). Alternating ribbons form planes joined by N2−H←N2A σ*←n and C6−H←(C5A,N1A) σ*←π interactions (20.1 and ; Figure 3.5.b) while, in the perpendicular direction, they are vertically stacked with ···A···D···D···A··· sequence connected by four C←C π*←π interactions (; Figure 3.5.c). The packing of compound 6 (Figure 2.6) is unique in this series. Geometrically, TCNQ molecules are always arranged in ribbons but, being reciprocally tilted, are not able to form short contacts. Also the N-bases would be in the correct position for giving the usual parallel π*←π stacking but, since the C12− C17 phenyl is rotated by some 42° for effect of the C5-methyl hindrance, this stacking cannot take place and is replaced by a net of herringbone contacts of the σ*←π type connecting the two phenyls of the base by C11−H←(C12,C13) interactions (; Figure 3.6.a) and the rotated phenyl with the pyrazole moiety through C17−H←N1 ones (4.6%; Figure 3.6.b). The network is completed by the C15−H···H−C5 and C7−H···H−C7 contacts (12.0 and 6.2%; Figure 3.6.c) which, given the entity of the shortening, are to be classified as σ*←σ attractive interactions. In the crystals of compound 7 (Figure 2.7) TCNQ molecules do not form ribbons but are surrounded in a plane by four Nbases to which are bound by four N1−H←N1A, C2−H←N1A, C6−H←N2A, and C3A−H←O1 σ*←n interactions (3.1, 15.1, 6.5, and 5.5%; Figure 3.7.a). The N-bases, which are joined in centrosymmetric dimers by N3−H←O4 amidic interactions (30.7%), are interlinked by two further N3−H←O3 and C3− H←O3 σ*←n interactions (18.7 and 13.1%). Finally, the planes formed are connected in ···A···(D−H···D′)···A···(D− H···D′)··· stacks by three C←(C,O) π*←π interactions (; Figure 3.7.b). Crystals of compound 8 (Figure 2.8) have a packing similar to that of 5. They form the same TCNQ ribbons linked by C2A−H←N2A σ*←n interactions (9.3%; Figure 3.TCNQ.a) and N-base ribbons linked by C13−H←O1 and couples of C15−H←Cl1 σ*←n interactions (11.5 and 0.0%; Figure 3.8.a). Alternating ribbons form planes joined by a net of TCNQ to Nbase contacts, including C4−H←N1A, C8−H←N1A, and C10−H←N1A σ*←n () and C9−H←(N2A,C6A) σ*←π interactions (; Figure 3.8.c). The two N-bases are further linked by two C7−H←H−C3 and C8−H←H−C4 σ*←σ interactions (5.0 and 6.6%). Rows of side-by-side ribbons form planes, which are vertically connected in ···A···D···D···A··· stacks by five C←(C,O) π*←π interactions (; Figure 3.8.b).

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RESULTS AND DISCUSSION

Short-Contact Analysis. All together, TCNQ and its 8 adducts contain 96 different contacts with distances ≤∑vdW, which are fully listed in Table S5 in the Supporting Information and depicted, for the most representative cases, in Figure 3. Their main feature is that all of them can, at least formally, be associated with one or another type of attractive EDA interaction, there not being any clear evidence of intermolecular repulsions. The interactions have been divided (Table 2) in three main groups, according to whether the donor is of n, π, or σ type (58, 32, and 6 cases, respectively), any group being further subdivided according to the type of acceptor. The final classification is as follows: Group 1A. This group includes X−H←:Y σ*←n EDA interactions, or conventional X−H···:Y (X, Y = N, O) H-bonds (5 intra- and 12 intermolecular cases; 3.3 ≤ vdWSH% ≤ 39.0). The intermolecular interactions are mostly localized in the horizontal plane of the aromatic π donor (N-base) and π* acceptor (TCNQ) molecules and connect them in planes or ribbons. Their energies, as evaluated by the LS method,36−38 range from weak to moderate (1 ≤ EHB ≤ 6.5 kcal mol−1) but, in spite of their relative weakness, are sufficient to induce the saturation of all molecular X−H functionalities by a proper N or O H-bond acceptor, so fulfilling the Donahue and Etter tenet43,44 that all conventional H-bond donors occurring in molecular crystals are, as a rule, involved in H-bond formation. Group 1B. This group includes C−H←:X σ*←n EDA interactions, or weak C−H···:X (X = N, O) H-bonds (8 intraand 31 intermolecular cases; 0 ≤ vdWSH% ≤ 16). Also these interactions are essentially arranged in the horizontal plane, contributing to complete and strengthen the two-dimensional net formed by conventional H-bonds. Of particular interest are the couples of σ*←n contacts linking TCNQ molecules according to the two schemes of Figures 3.TCNQ.a and 3.2.a, with formation of planar ribbons that contribute to the optical properties of the crystals (see below). The energies involved are, anyway, considerably small and, judging from the maximum vdWSH% value observed (some 16%), are not expected to exceed the 0.2−0.3 kcal mol−1 per single interaction. Group 1C. This group includes (C,C)←:N π*←n EDA interactions (2 cases; 2 ≤ vdWSH% ≤ 3). The interaction of n lone pairs with C...C aromatic bonds gives a strongly angular geometry that is a well-known determinant of the herringbone packing occurring in many crystals (e.g., in the picric acid structure).14 In the present series of compounds, dominated by the parallel packing induced by the interplanar π*←π CT, it occurs only once in the structure of TCNQ (Figure 3.TCNQ.b). Group 2A. This group includes C−H←(C,N) σ*←π EDA interactions, or weak CH/π H-bonds45 (11 cases; 1 ≤ vdWSH% ≤ 6). Different π donors give rise to three types of contacts: (i) planar C−H←(CN) (Figures 3.5.b and 3.8.c), where CN is the TCNQ nitrile; (ii) angular C−H←N (Figure 3.6.b) where N belongs to the pyrazole ring; (iii) angular C− H←(C...C) (Figure 3.6.a) where C...C is a delocalized bond of the aromatic base. This last interaction is known to be the main determinant of herringbone packing in fused-ring aromatics,46,47 and it is then not surprising that its occurrence in 6 can totally hinder the pattern of parallel π*←π interactions. Group 2B. This group includes C←(C,O) π*←π EDA interactions (21 cases; 0 ≤ vdWSH% ≤ 6). Since TCNQ and N4765



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of them (2, 4, 5, 8) are almost black and display the typical metallic luster of electrically conducting organic metals, suggesting the possibility that these molecules form electronic bands in spite of the fact that their crystals contain alternate, instead of segregated, stacks. In the following, we try to investigate such a possibility by making use of the crystallographic information so far collected. Two methods seem practicable, the first consisting in measuring the amount of charge transferred by a bond-distance analysis of the TCNQ moiety,16,18,19 and the second in singling out specific CT packing arrays (ribbons and columns) compatible with electronic band formation. The application of the first method is illustrated in Scheme 3 and Table 3. Two main series of alternate TCNQ derivatives

base constitute an electron acceptor−donor couple, these interactions occur in 6/8 structures giving rise to as many as 21 intermolecular contacts ≤ ∑vdW. In such a π*←π stacking, aromatic rings of adjacent molecules are essentially parallel but not exactly juxtaposed, the rings being mutually shifted as shown in their ORTEP views of Figures 1.2−5 and 1.7−8. For the simple case of two overlapping aromatic hexagons, Mulliken11 has interpreted such a shift as due to the fact that the overlap integral of two orthogonal bonding and antibonding π MOs can differ from zero only when the two hexagons are shifted. The vdW shortening observed for all these π*←π interactions is always small, a fact already accounted for above in terms of longer-range p AO overlap. This suggests that the strength of these interactions would be better judged by other factors than the simple shortening of intermolecular distances, such as the amount of charge transferred, the degree of [D,A] ↔ [D+,A−] resonance mixing, or the frequency and intensity shift of the charge-transfer band, νCT, which is always associated with the formation of these complexes.11,13 Group 3A. This group includes C−H←H−C σ*←σ EDA interactions (6 cases; 4 ≤ vdWSH% ≤ 12). H···H interactions can be divided in two separate classes. The first involves metal hydrides which form strongly ionic Xδ−−Hδ+···Hδ−−Mδ+ bonds known, from the mid-1990s, as dihydrogen bonds (for a short account see ref 1, pp 49−56). The second class concerns stabilizing closed-shell interactions between H atoms which are close to electrical neutrality and that, to distinguish them from the previous ones, have been called hydrogen−hydrogen bonds by Matta and Bader48,49 and dihydrogen contacts by Shaik and Alvarez.50 The 6 cases of C−H←H−C interactions observed in compounds 4, 6, and 8 (Figures 3.4.a,b, 3.6.c, and 3.8.a,c) clearly belong to this second class and are of particular interest for being direct experimental findings while most of the previous work was accomplished by pure QM computational methods. By comparison with these QM calculations, their association energies can be roughly estimated not to exceed 0.5 kcal mol−1 per single interaction. Horizontal σ*←n Interactions and Electron-Pair Saturation Rule. The σ*←n interactions occurring in TCNQ and its 8 adducts are summarized in columns 2−10 of Table 2. The crystals contain 38 n donors (D: = −CN, >CO, C(R)O, N) and 82 σ* acceptors (A = O−H, N−H, C−H), all compounds but TCNQ having a considerable excess of acceptors (A − D). These 38 donors are involved in 56 σ*← n interactions, 17 of the X−H←:Y and 39 of the C−H←:X type, which leaves unsaturated 3/38 donors, that is only the 7.9% of them, with an average donor coverage of 56/38 = 1.47 acceptors per donor. Apparently there are not repulsive interactions, all short contacts being accountable in terms of one or another attractive EDA interaction. Conversely, the effort to saturate all possible n donors by one or another of the available σ* acceptors is evident, suggesting that this effort is to be considered a basic steering force in the formation of molecular crystals. This observation, that we have called electron-pair saturation rule, has been already discussed in connection with the packing analysis of picric acid and 14 of its adducts with N-bases.14 Vertical π*←π Interactions and Crystalline Optical Properties. As reported in Table 1, crystals of compounds 1− 8 differ considerably in their aspect. While TCNQ is practically colorless (pale yellow), the adducts are deeply colored, most probably because of their donor−acceptor CT band, and four

Scheme 3. Quinoid-to-Aromatic Transition in the TCNQ0to-TCNQ− Transformation

are known, neutral TCNQ0−donor0 and ionic TCNQ−− donor+ adducts,19 and the formal CT needed to transform TCNQ0 into TCNQ− was found to induce in the molecule bond-distance changes which are associable with the quinoidto-aromatic transition illustrated in Scheme 3 and normally quantified by the values of the b − a and c − d distance differences (Table 3). As the CT in EDA complexes is normally partial, true TCNQ0 and TCNQ−1 structures are actually extreme forms, respectively indicated in the table as topquinoid0/0 and top-aromatic+/− limit structures. All other structures have intermediate charges, whose entity can be estimated by making a proportion with the full range, Δ, of the b − a and c − d values spanned by the two limit structures. Average formal charges calculated in this way ( in e) are listed in the last column of Table 3. What should be expected is that all compounds displaying metallic luster are endowed with definitely greater values. This is probably true for 2, 4, and 8 (0.34−0.36 e) but certainly not for 5 (0.28 e) whose CT value is not significantly different from those of other nonmetallic compounds (TCNQ, 3, and 7). Moreover, compound 6 shows a large CT (0.38 e) without displaying any metallic luster. It must be concluded that the present method, which was originally developed to interpret the electric properties of segregated crystals, is not equally suited to rationalize metallic behavior in alternated ones. The second method seems more promising. A first comparison of the crystal structures (Figures 2 and 3) indicates that only those displaying metallic luster (2, 4, 5, and 8) are true parallel packings of donors and acceptors, while such a parallelism is considerably perturbed, or just not existing at all, in the other four complexes and in TCNQ itself. Moreover, the metallic subset is endowed with specific substructures putatively suited to propagate the electronic band, which are identical in couples (2, 4) and (5, 8) and are summarized in Figure 4. They consist of ribbons of TCNQ molecules linked by C−H←:N σ*←n interactions (4.1.R and 4.2.R) which are then vertically joined by C←:C π*←π connections to form alternate 4766



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Table 3. The Transformation from TCNQ0 to TCNQ− Induces Bond-Distance Changes Typical of the Quinoid-to-Aromatic Transition Shown in Scheme 3a a

b

b−a

c

d

c−d

e

%

top-quinoid0/0 (OJIXOU) top-aromatic0/0 (ROJSIS)

1.319 1.343 1.365

1.444 1.439 1.419

0.125 0.096 0.054

1.356 1.379 1.416

1.439 1.431 1.418

−0.083 −0.052 −0.002

1.143 1.142

00 343 964

top-quinoid+/− (JOBSOI) top-aromatic+/− (MCFETC01)

1.332 1.355 1.372

1.426 1.423 1.420

0.108 0.068 0.048

1.369 1.407 1.417

1.434 1.419 1.420

−0.065 −0.012 −0.003

1.134 1.145 1.146

220 817 1000

1.140 1.141 1.140 1.141 1.142 1.141 1.143 1.139 1.153

255 202 362 270 364 282 380 323 344

compd

Δ = top-quinoid0/0 − top-aromatic+/− TCNQ (TCYQME) 1 2, metallic luster 3 4, metallic luster 5, metallic luster 6 7 8, metallic luster

−0.080

0.077 1.346 1.334 1.344 1.341 1.341 1.338 1.348 1.340 1.344

1.448 1.441 1.439 1.444 1.441 1.442 1.442 1.438 1.446

0.102 0.107 0.095 0.104 0.100 0.104 0.095 0.098 0.102

1.374 1.367 1.374 1.374 1.379 1.374 1.379 1.371 1.381

1.441 1.436 1.430 1.435 1.430 1.433 1.432 1.431 1.434

−0.067 −0.069 −0.056 −0.061 −0.051 −0.059 −0.053 −0.060 −0.053

The table summarizes the distances found in 66 neutral TCNQ0−donor0 and 34 TCNQ−−donor+ adducts19 and compares them with the values found in the structures of TCNQ and compounds 1−8. The formal charge transfer values (%, in percent of 1e; standard deviations in subscript) can be estimated from the values of the b − a and c − d differences (see text; note that the aromatic geometries are characterized by smaller b − a and less-negative c − d values. Data for compound 7 (in italics) are of lesser quality because of crystal disorder. a

Figure 4. The four 2, 4, 5, and 8 compounds whose crystals display metallic luster are endowed with specific structures able to propagate the electronic band through the crystal. These structures are identical in the couples (2, 4) and (5, 8) and include C←:C π*←π columnar alternate stacks (4.1.S and 4.2.S) as well as C−H←:N σ*←n ribbons of TCNQ molecules (4.1.R and 4.2.R).

isolated H-bonds. Finally, it seems interesting that in the two TCNQ ribbons 4.1.R and 4.2.R the C−H···N H-bonds are part of a resonance-assisted ···NC−CC−H··· sequence which forms infinite chains in 4.II and closed dimers in 4.III.

molecular stacks (4.1.S and 4.2.S). In (5, 8) these stacks are of the rather common ···A···D···D···A··· sequence type, while in (2, 4) they assume the peculiar ···A···(D−H···D′)···A···(D− H···D′)··· form, where D−H···D′ are couples of 2-pyridinone or 2-quinolinone molecules connected by rather strong N− H···O bonds (vdWSH% of 37.5 and 32.3, respectively). It might be relevant to the establishing of the electronic band that these bonds are actually linked in chains (Scheme 4.I) of ···OC− N−H··· resonance-assisted H-bonds (RAHBs)1,51,52 that can be supposed to provide better electronic conduction than ordinary

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CONCLUSIONS Traditionally, the packing of molecular crystals is interpreted by taking separate account of H-bonded and CT or EDA interactions. Since, however, H-bonds can equally be treated as EDA interactions, we thought it worthwhile to undertake a 4767



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Scheme 4. Formation of Chains or Dimers Connected by N−H···O and C−H···N RAHBs

number of intensive packing analyses (that is, purely empirical case studies) carried out on pure EDA grounds and just see what happens. Without any claim of generality (foreign to the case study method), the two cases so far studied (this paper and ref 14) lead to a considerable number of interesting observations that, for their internal consistency, seem to deserve future attention and, accordingly, are listed below. 1. The previous discussion of practical cases has clearly shown that the crystal packing of molecular crystals can be actually analyzed without introducing the H-bond concept and making exclusive use of the unified CT or EDA approach outlined above. Besides being of general applicability, this method also presents two interesting features of which it can be taken advantage of during the packing analysis. The first is mostly taxonomical, the unified method allowing the association of specific packing substructures to equally specific EDA interactions, such as planes or ribbons to σ*←n, herringbones to σ*←π or π*←n, and columnar packing to π*←π interactions. The second14 is that the unification of all interaction forces as EDA interactions directly leads to formulate the electron-pair saturation rule for which “all electron donors of a closed-shell molecule (mostly nonbonding pairs of lone pairs but also π-bonding pairs of multiple bonds) become engaged in EDA interactions with the electron acceptors (X−H, C−H, or π*) present, as far as they are available; when the acceptors are insufficient, they are saturated in order of decreasing EDA interaction strength”. 2. The main outcome of this unified approach is that, inside any neutral molecular crystal, all contacts around ∑vdW (either shorter or slightly longer) can be (or possibly, are to be) referred to a well-defined type of EDA interaction. This seems to attribute a particular importance to CT phenomena in the overall crystallization process (see point 4 in this list). 3. Simple extension of point 2 provides us with a particularly easy way of looking at the crystal packing of neutral molecular crystals. Let us start from alkanes whose molecules, containing only fully saturated sp3 carbons, can be considered to interact only by rather weak London’s dispersion forces,53 giving so rise to

rather soft and low-melting crystals. Simple addition of localized or delocalized multiple C−C bonds, however, adds to the molecular surface π-bonding electron pairs and more acidic C−H groups on the sp2 carbons, which can attract one another by C−H···C σ*←π EDA interactions, giving so rise, e.g., to the typical herringbone packing of the naphthalene crystals which are no more plastic and melt at some 80 °C, though still remaining considerably volatile. In a similar way, higher-melting crystals can easily be obtained by adding other chemical functionalities (such as CO, CS, OH, NH2, ringembedded N, O, and S atoms, and halogen atoms) which are able to activate the new class of σ*←n EDA interactions in the form of D−H←:A and C−H←:A Hbonds or C−X←:Y (X = halogen) halogen bonds.14,21 Hence, in this simple packing scheme ever new substituents add to the molecule ever new electron donors and acceptors which, by forming ever new CT contacts, cause the intermolecular interaction network to become more and more strong and complex. The new interactions added, however, can only correspond to one or another member of the well-established set of EDA interactions. 4. Finally, reconsideration of crystal packing in terms of unified CT or EDA approach also leads to reconsider the very nature of the physical forces which are deputed to steer the packing. To start with, we need to specify what is really meant by “CT complex”. The term was introduced by Mulliken10 to describe a form of association between electron donor (D:) and acceptor (A) molecules which is stabilized by the D:···A ↔ D+···A− covalent−ionic VB resonance. CT-complex formation can equally be accounted for in MO terms, where the VB mixing is substituted by the second-order perturbation of a “donor” doubly occupied MO (the HOMO) by an “acceptor” unoccupied MO (the LUMO), perturbation which causes a small LUMO← HOMO transfer of charge, qCT, and lowers by ΔECT the energy of the CT complex by inducing a HOMO stabilization and a LUMO destabilization which are the greater the more the two MOs are energetically close. Notice that the mechanism of CT binding, being purely 4768



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Notes

perturbative and then quantum-mechanical in nature, may be widely independent of the electrostatic charges present on the reacting monomers. A good example of that may be the well-known π→σ* benzene→I2 adduct, the first CT complex ever described in 1948,54,55 which forms in spite of the fact that both isolated monomers lack any net charge or dipole moment. In the words of Herzberg,56 “the resonance supplies the whole of the binding energy” (as quoted from ref 13, p 662). The energy lowering, ΔECT, becomes now a function of the amount of charge, qCT, transferred from the filled to the unfilled orbital and, in the past 25 years, numerous attempts to evaluate these two quantities by both theoretical12,13 and experimental methods57,58 have been undertaken. The most complete series of high-level quantum mechanical calculations is due to Weinhold and co-workers in the frame of their NBO (natural bond orbital) theory.12,13 Testing a large set of σ*←n interactions (i.e., hydrogen and halogen bonds) together with a considerable number of other interactions involving π and π* MOs, these authors obtained the significant result that the shifts of energy and charge are nearly proportional in all types of interactions with a large proportionality constant of ≈1 in atomic units, so that a qCT as small as 0.001 e comes to correspond to a ΔECT = 0.001 au ≈ 0.6 kcal mol−1, a quantity already significant in the field of molecular interactions. In conclusion, the CT or EDA representation of all intermolecular attractions but dispersion is both theoretically consistent and naturally explicative of the physical origin of the attractions themselves, suggesting that a simple threecomponent model of the forces acting in neutral molecular crystals (dispersion, exchange, and CT or EDA interactions), besides being well suited for the present qualitative interpretation of the crystal packing, could also account for the whole of the crystal packing energy as long as a suitable computational apparatus is made available. The greater advantage of such an approach is that it looks at interactions from a considerably different perspective than that adopted by other well-established methods, such as those based on electrostatic potentials59 or electron densities,60−69 so that it can be expected that, in the future, all the three classes of methods could be profitably used in synergistic association for the full understanding of the forces that drive the formation of molecular crystals.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by COFIN-2008 (MIUR Rome) and by the University of Ferrara with local research funds (FAR).

ASSOCIATED CONTENT

S Supporting Information *

Tables of crystal data, selected bond distances and angles, and H-bond and EDA contact distances for compounds 1−8 (Tables S1−S4). Full list and percent shortening values (vdWSH %) of H-bonds and EDA contacts ≤∑vdW (Table S5). X-ray crystallographic information files (CIF) for structures 1−8. This material is available free of charge via the Internet at http://pubs.acs.org. X-ray crystallographic information files (CIF) for structures 1−8 are also deposited at the CCDC with deposition numbers 867258−867265.

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AUTHOR INFORMATION

Corresponding Author

*Dipartimento di Chimica, Università di Ferrara, Via L. Borsari 46, 44121 Ferrara, Italy. Tel: +39-0532 455141. Fax: +39-0532 240709. E-mail: [email protected]. 4769



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