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Solid-State N-H···O/O-H···N Tautomerism in Resonance-Assisted 1-(Arylazo)-2-Naphthols and its Trough-Space #*## Perturbation in TCNQ Cocrystals. A Variable-Temperature X-Ray Crystal Study Gastone Gilli, Valerio Bertolasi, and Paola Gilli Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg301546v • Publication Date (Web): 08 Mar 2013 Downloaded from http://pubs.acs.org on March 21, 2013

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Crystal Growth & Design

Solid-State N−H···O/O−H···N Tautomerism in Resonance-Assisted 1-(Arylazo)-2-Naphthols and its Trough-Space π*←π Perturbation in TCNQ Cocrystals. A Variable-Temperature XRay Crystal Study Gastone Gilli , Valerio Bertolasi, and Paola Gilli* Centro di Strutturistica Diffrattometrica e Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, Via Borsari 46, 44121 Ferrara, Italy ABSTRACT: Aryl-substituted 1-arylhydrazo-naphthalen-2-ones and 1-arylazo-naphthalen-2-ols display ···HN−N=C−C=O···  ···N=N−C=C−OH··· ketohydrazone-azoenol prototropic tautomerism in the solid state by forming a strong intramolecular resonance-assisted H-bond (RAHB) that can change from pure N−H···O to pure O−H···N through dynamically disordered N−H···O  O−H···N bonds according to the electronic properties of the substituents. The compounds of the series aryl = 4-F-phenyl, 4-CH3-phenyl, 4-Cl-phenyl, and 1-naphthalenyl and their cocrystals with TCNQ have been studied by X-ray crystallography at temperature variable from 100 to 295 K. The packing is extensively discussed in terms of charge-transfer (CT) or electron donor-acceptor (EDA) interactions, showing that the herringbone packing of pure azonaphthols and the columnar one of their TCNQ cocrystals are respectively determined by specific σ*←π and π*←π interactions. It is shown that cocrystallization with TCNQ induces two main effects: (i) in TCNQ itself, a significant change from quinoid to aromatic geometry, from which a CT of some 0.40 e can be estimated; (ii) in azonaphthols, a shift of the N−H···O  N···H−O equilibrium towards the pure N−H···O tautomer, accompanied by a decrease of π-delocalization inside the conjugated ···HN−N=C−C=O··· resonant fragment. These findings suggest that through-space effects caused by TCNQ cocrystallization on the RAHB equilibrium are formally equivalent to through-bond effects caused by the aryl substituents.

+ TCNQ

 TCNQ

*Corresponding Author: Dr. Paola Gilli Dipartimento di Scienze Chimiche e Farmaceutiche Università di Ferrara Via L. Borsari 46, 44121 FERRARA (Italy) Tel. +39-0532 455141 - Fax +39-0532 240709 E-mail: [email protected] 1

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Solid-State N−H···O/O−H···N Tautomerism in Resonance-Assisted 1-(Arylazo)-2-Naphthols and its Trough-Space π*←π Perturbation in TCNQ Cocrystals. A Variable-Temperature XRay Crystal Study Gastone Gilli , Valerio Bertolasi, and Paola Gilli* Centro di Strutturistica Diffrattometrica e Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, Via Borsari 46, 44121 Ferrara, Italy ABSTRACT: Aryl-substituted 1-arylhydrazo-naphthalen-2-ones and 1-arylazo-naphthalen-2-ols display ···HN−N=C−C=O···  ···N=N−C=C−OH··· ketohydrazone-azoenol prototropic tautomerism in the solid state by forming a strong intramolecular resonance-assisted H-bond (RAHB) that can change from pure N−H···O to pure O−H···N through dynamically disordered N−H···O  O−H···N bonds according to the electronic properties of the substituents. The compounds of the series aryl = 4-F-phenyl, 4-CH3-phenyl, 4-Cl-phenyl, and 1-naphthalenyl and their cocrystals with TCNQ have been studied by X-ray crystallography at temperature variable from 100 to 295 K. The packing is extensively discussed in terms of charge-transfer (CT) or electron donor-acceptor (EDA) interactions, showing that the herringbone packing of pure azonaphthols and the columnar one of their TCNQ cocrystals are respectively determined by specific σ*←π and π*←π interactions. It is shown that cocrystallization with TCNQ induces two main effects: (i) in TCNQ itself, a significant change from quinoid to aromatic geometry, from which a CT of some 0.40 e can be estimated; (ii) in azonaphthols, a shift of the N−H···O  N···H−O equilibrium towards the pure N−H···O tautomer, accompanied by a decrease of π-delocalization inside the conjugated ···HN−N=C−C=O··· resonant fragment. These findings suggest that through-space effects caused by TCNQ cocrystallization on the RAHB equilibrium are formally equivalent to through-bond effects caused by the aryl substituents.

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■ INTRODUCTION We have recently suggested[1,2] that the simple artifice of renaming conventional X−H···:Y (X,Y=N,O,Cl,..) hydrogen bonds (H-bonds) and weaker C−H···:Y (Y=N,O) ones (CH-bonds) as X−H←:Y and C−H←:Y σ*←n charge-transfer (CT) or electron donor-acceptor (EDA) interactions[3-7] considerably simplifies and speeds up the crystal-packing analysis of molecular crystals. This choice has several advantages: (i) it organizes all most common interactions (other than dispersion and exchange) according to a well-established nomenclature of EDA-complexes formed by the donor and acceptor groups depicted in Scheme 1:[3,4,7] n donors with σ* and π* acceptors (σ*←n and π*←n); π donors with σ* and π* acceptors (σ*←π and π*←π); and σ donors with σ* acceptors (σ*← σ); (ii) it associates, as a rule, a specific EDA interaction to any short contact of the crystal (i.e., intermolecular contact shorter than the sum of the van der Waals radii, ΣvdW); (iii) it gives interactions a precise and unique physical meaning in terms of localized perturbation of a ‘donor’ doubly occupied MO (HOMO) by an ‘acceptor’ unoccupied MO (LUMO), perturbation causing a small energy lowering through an equally small LUMO←HOMO transfer of charge; and (iv) it makes it possible to establish a global balance between n lone-pair donors and σ* and π* acceptors, finally leading to assess the electron-pair saturation rule for which saturation of the maximum number of n donors by all available acceptors is the main steering force in the formation of molecular crystals.[1] Scheme 1. Common Electron Donors and Acceptor Groups Occurring in the Packing of Organic Molecular Crystals

It can be easily shown that specific EDA interactions selectively affect the packing of molecular crystals. Two recent analyses of 22 cocrystals of planar N-bases (π donors) with picric acid[1] and 7,7,8,8-tetracyanoquinodimethane (TCNQ)[2] (both π acceptors) have shown some interesting regularities. As a rule, σ*←n interactions induce formation of planar ribbons or planes of molecules linked by ordinary H-bonds and reinforced by a net of weaker but more abundant CHbonds. In a second stage, the planar arrangements tend to associate through the π and π* MOs present, giving rise to parallel vertical stacks connected by π*←π interactions or, when these are lacking, herringbone structures linked by π*←n or σ*←π contacts. These two different options cause such an upsetting of the crystal packing that the fact was considered worth studying by comparing couples of structures where a same molecule was put or not in the condition of forming π*←π interactions. The choice fell on the comparison of the crystal structures of pure π-donors with the structures of their cocrystals with π* acceptors, choosing TCNQ (2.I) as fixed π*acceptor and a 3



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set of substituted 1-arylhydrazo-2-naphthone  1-arylazo-2-naphthol tautomeric azo dyes (2.II) as π-donors (Scheme 2 and Table 1). Scheme 2. The Molecules Discussed in This Paper Table 1. The crystal structures discussed in this paper

TCNQ (2.I) was chosen because of the many EDA interactions it may be involved into. In fact, besides being an efficient π* acceptor on the π* antibonding MOs practically covering the entire molecule, it is a n donor from the four −C≡N: nitrogens, a σ* acceptor on the antibonding MOs associated with the four C−H bonds, and a π donor from its four C≡N multiple bonds. Its EDA properties are well known from the many structural determinations of its cocrystals[8,9] which have prompted, in particular, an acknowledged empirical method for assessing the degree of CT from the modification of TCNQ geometry.[10] The substituted hydrazonaphthones (2.II) used as π-donors are well-known azo dyes whose properties are widely determined by the formation of a strong intramolecular N−H···O bond which, given the π-conjugated nature of the interleaving ···H−N−N=C−C=O··· fragment, is to be classified as resonance-assisted H-bond (RAHB), a particular type of H-bond that deserves here some preliminary comments. RAHB[12-14] was proposed since 1989 to account for the abnormally strong intramolecular O−H···O bonds in β-diketone enols (3.I) in terms of a synergism between H-bond strengthening and enhanced delocalization of the ···O=C−C=C−O−H··· π-conjugated fragment. It has been interpreted by different bonding models,[15,16] the simplest of which (Scheme 3) is based on the resonance between the ketoenol (KE) and enolketo (EK) VB canonical forms 3.Ia ↔ 3.Ib, whose increasing mixing causes H-bonds to assume different strengths and shapes of the proton-transfer (PT) profile: (i) moderately strong O−H···O bonds with asymmetric single-well (aSW) profile (3.IIa); (ii) strong H-bonds with symmetric double-well (sDW) profile associated with the O···H−O  O−H···O tautomeric equilibrium (3.IIb  3.IIb’) and frequently called LBHBs (low-barrier H-bonds);[17,18] and (iii) short-strong H-bonds (SSHBs) with symmetric single-well (sSW) profile and fully πdelocalized resonant fragment (3.IIc). Scheme 3. O−H···O RAHB Formation in β-Diketone Enols Scheme 4. Parent Systems Known to Form N−H···O/O−H···N RAHBs

RAHBs can be both intra- and intermolecular and occur for any ···Y=Rn−X−H··· configuration with X,Y= N,O,S, where Rn is a resonant spacer of n atoms (n odd). Typical examples are n = 1 in carboxylic acids (···O=C−O−H···) and amides (···O=C−N−H···), n = 3 in β-diketone 4


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enols (3.I), β-enaminones (···O=C−C=C−N−H···) and ketohydrazones (4.I), and n = 5, 7 in the few cases of δ- and ζ-diketone enols so far known. Irrespective of the spacer length, however, RAHB strength remains determined by the degree of mixing of the two VB resonant forms, which is the greater the smaller their energy difference is. Consequently, very strong RAHBs are to be homonuclear (X = Y) and homomolecular (i.e., chemically symmetric on the two sides of the Hbond), while heteronuclear N−H···O/O−H···N bonds are predicted to be essentially weak because of the much greater proton affinity (PA) of nitrogen with respect to oxygen (actually amines, with a pKa of nearly 40, are much weaker acids than alcohols, having a pKa around 17). The only way for strengthening the N−H···O/O−H···N bond remains, therefore, that of reducing the oxygen/nitrogen PA mismatch by the use of proper substituents, as illustrated in Scheme 4.[19-21] Because of such a mismatch, unsubstituted ketohydrazones 4.I give rather long N−H···O bonds with N···O distances of some 2.67 Å because of the small resonance mixing between the more stable ketohydrazone and the less stable azoenol tautomer. This second form, however, becomes the more stable one after fusion of the H-bonded ring with a phenylene moiety (4.III), because formation of the ketohydrazone tautomer would now require the loss of the large resonance energy of the aromatic ring; accordingly, the azophenol form 4.III is normally observed with rather short N···O distances of 2.53-2.61 Å. Fusion with a naphthalene ring, having an intermediate resonance energy, leads to the more interesting situation of two roughly isoenergetic N−H···O and N···H−O bonds (2.IIa  2.IIb and 4.IIa  4.IIb) whose ketohydrazone  azoenol tautomeric equilibrium, which can be finely tuned by the N-substituent and has already been extensively studied by variable-temperature X-ray crystallography and quantum-mechanical DFT emulation[20-23] for as many as six different psubstituted 1-(4-X-phenylhydrazo)-2-naphthones (X = NO2, H, Cl, F, N(CH3)2, O−). All these compounds give quite short N···O distances of 2.52-2.55 Å and display p(NH):p(OH) population ratios at 100 K which decrease in the order 100:0, 100:0, 69:31, 64:36, 21:79, and 0:100, in parallel with the decreasing value of the mesomeric constant σ°R of the substituent[24] (respectively, 0.17, 0.0, -0.29, -0.40, -0.53, and -0.60), showing that the equilibrium is continuously shifted from N−H···O to N···H−O by the increasing π-donating properties of the p-substituent. A major aim of this work was to assess whether a similar H-bond modification could be produced by a through-space (instead of through-bond) π-donation made possible by the resonanceassisted nature of the H-bonded ring. Accordingly, four arylazonaphthols were selected (1’ and 3’ as previously determined low-temperature crystal structures[20,21] and 2’ and 4’ as newly crystallized compounds) and cocrystallized with TCNQ, a good π* acceptor, to give the four cocrystal 1-4 and to check whether the prototropic equilibrium could actually be shifted towards the N−H···O side by action of the acceptor. To notice that this research project owes much, in its details, to a previous 5



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study by Inabe[25] who was the first to observed that cocrystallization of N-salicilideneanilines with TCNQ was able to modify the PT profile of the N−H···O  O−H···N tautomeric equilibrium. Chemical names of the compounds studied are listed in Table 1. All crystals except 4’ and TCNQ[11] have been studied by variable-temperature X-ray crystallography (from 100 to 295 K) to assess the best estimates of the populations of the disordered proton.

■ EXPERIMENTAL SECTION Sample Preparation. The four azo dyes (Table 1, compounds 1’-4’) were recrystallized from ethanol-ethyl acetate mixture and the crystals obtained were found to melt at the temperatures of 139, 124-125, 157-158, and 224-225 °C, respectively. The four TCNQ adducts (Table 1, compounds 1-4) were prepared by dissolving equimolecular amounts of dye and TCNQ in acetonitrile. The resulting solutions were slowly evaporated (in between 12 hours and one week) till precipitation of a mixture of two types of crystals, one black with metallic luster and the other yellow (most probably consisting of TCNQ in excess). The black crystals, selected by hand and submitted to X-ray structural determination, resulted to be dye-TCNQ adducts in the ratio 2:1 for 13 and 1:1 for 4. Upon slow heating, the four adducts showed different behavior. Compound 1 decomposed at the temperature of 155-157 °C in a ruby-red liquid containing yellow TCNQ crystals. Compound 2 regularly melted at the temperature of 192-194 °C giving a dark-red liquid. Compound 3 sublimated between 155 and 170 °C leaving no residue. Compound 4 started to lose dye by sublimation around 150 °C leaving only a yellow residue of TCNQ crystals at 190 °C. Variable-Temperature Crystal Structure Determination. X-ray diffraction data were collected at variable temperatures on a Nonius Kappa CCD diffractometer with graphite-monochromated MoKα radiation (λ = 0.71069 Å) equipped with a Cryosystem 600 (Oxford Cryosystem) open-flow gas cryostat. No absorption and extinction corrections were applied. Data sets were integrated with the DENZO-SMN package.[26] Structures were solved by direct methods with SIR97[27] and refined (SHELXL97[28]) by full matrix least squares with anisotropic non-H and isotropic H atoms. All other calculations were accomplished using PARST[29] and PLATON[30] implemented in the WinGX[31] program system. ORTEP[32] views of compounds 2’, 4’, and 1-4 are reported in Figure 1. The uncomplexed compound 2’ was collected at 100, 150, 200 and 295 K; at any temperature, the ΔF map was showing diffuse electron densities astride the N and O atoms with two maxima from which two proton positions could be singled out. Refinement of the two hydrogen positions with partial occupancies and isotropic thermal parameters fixed at 1.2 times those of the N and O atoms was successfully attempted, giving final p(NH):p(OH) occupancy factors of 6


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0.75(3):0.25(3), 0.66(3):0.34(3), 0.64(3):0.36(3), and 0.59(3):0.41(3) respectively at 100, 150, 200 and 295 K. Final ΔF maps were computed after least-squares refinement carried out without the Hbonded proton. The uncomplexed compound 4’ was found to be statically disordered and then collected only at room temperature. The molecule lies on a crystal centre of symmetry located on the average point of the N1−N1’ azo bond and was then refined giving occupancies of one half to the O1, H1, and H2 atoms. For the sake of clarity, the two overlapping molecules are shown sideby-side in Figures 1.4’.a and 1.4’.b. The ΔF map displays a single maximum at bonding distance from the N atom, so excluding the presence of proton tautomeric disorder. TCNQ cocrystals 1 and 2 were collected at four temperatures (100, 150, 200, and 295 K) and 3 and 4 at two temperatures (100 and 295 K). All TCNQ cocrystals provided, at any temperature, ΔF maps with a single minimum at bonding distance from the N atom, ruling so out the presence of proton disorder. 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). Figure 1. ORTEP[32] views of the asymmetric units with thermal ellipsoids at 30% probability for compounds 1-4 and 2’ at 100 K, and for the partially disordered compound 4’ at 295 K.

Data Treatment. The nine structures of Table 1, with particular regard to the seven determined at 100 K, were systematically screened by the Mercury CSD 2.0 program[33] for the occurrence of short contacts, defined as the intermolecular contacts which are equal to or shorter than the sum of the Bondi’s[34] van der Waals radii (ΣvdW). The rather formal ΣvdW limit was chosen to restrain all contacts within an uniform reference distance. It must be noticed, however, that this limit has no real energetic implications because, due to the smooth form of atom-atom potentials, interatomic interactions keep to be attractive considerably beyond the ΣvdW distance. To correct for X-ray proton positioning errors, all X−H bonds were previously 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 whenever shorter than them. Short contacts were quantified by their shortening with respect to ΣvdW (vdWSH, in Å), normally expressed as its percent value, vdWSH%. H-bond energies, EHB, of conventional X−H···:Y bonds (X,Y = N, O, Cl) were estimated by the Lippincott and Schroeder (LS) method[35-37] and those of the C−H···:Y bonds (Y = N, O; not parameterized by the LS method) roughly estimated to be 2.0, 1.0, 0.5, 0.2 kcal mol-1 for vdWSH% shortenings of 30, 26, 22, 15%.[1] A global π-delocalization index, , of the ···HN−N=C1−C2=O··· conjugated fragment (Scheme 2) was calculated as = [(n1 - 1) + (2 - n2) + (n3 - 1) + (2 - n4)]/4, a parameter which 7



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assumes formal values of = 0.0, 0.5, 1.0 for the pure ketohydrazone 2.IIa, fully π-delocalized, and pure azoenol 2.IIb forms, respectively. Here, the ni are the bond numbers of bonds di (as labeled in 2.IIa) and are calculated from the Pauling’s formula[38] d(n) = d(1) - c log10n, where d(n) and d(1) are the bond lengths for n = n and n = 1, and c is a constant to be evaluated from the pure single- and double-bond distances: (1.49; 1.33), (1.38; 1.20), (1.41; 1.27) and (1.39; 1.24) Å, respectively for the C(sp2)-C(sp2), C(sp2)-O, C(sp2)-N(sp2) and N(sp2)-N(sp2) bonds. To account for naphthalene aromaticity, the C1-C2 bond number was assigned the reference value of n = 1.646 derived from the C−C distance of 1.375 Å in the naphthalene structure.[39] DFT calculations for 2’ were performed, in analogy with the other azonaphthols previously studied,[20,21] by using the Gaussian 98 package[40] at the B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory. Full geometry optimization of both N−H···O and N···H−O tautomers of 2’ and of its radical cation 2’(●+) were carried out in their singlet and doublet state, respectively, using the restricted and unrestricted scheme for closed and open shell calculations. In structures 1’-3’, the H-bond proton is dynamically disordered between the N−H···O and N···H−O positions with proton population ratios p(NH):p(OH) changing monotonically with the four temperatures investigated. Accordingly, the thermodynamic parameters of the tautomeric equilibrium can be calculated by the van’t Hoff linear regression, lnK (T) = ΔS°/R – ΔH°/R (1/T), where K(T) = p(NH)/p(OH) = p(NH)/(1- p(NH)) is the equilibrium constant at the temperature T. Calculated standard enthalpies and entropies, ΔH in cal mol-1 and ΔS° in cal mol-1 K-1, are given in Table 2 below.

■ CRYSTAL PACKING ANALYSIS In Figure 2, the unit cells of the four ketohydrazone-TCNQ complexes (2.1-4; right column) are compared with those of the pure ketohydrazone-azoenol tautomers (2.1’-4’; left column) and of TCNQ itself (2.TCNQ). The packing appears to be controlled by a number of weak EDA interactions having contact distances around or just below ΣvdW, whose full list is reported in Table S5 of the Supporting Information. The most significant of these structure-determining EDA contacts are depicted in Figure 3 and discussed below (with the relative vdWSH% values given in parentheses). Both ketohydrazone/azoenol (the π donors) and TCNQ (the π acceptor) molecules are almost perfectly planar. The former, in particular, are kept planar by a strong intramolecular N2−H···O1/N2···H−O1 bond (36.1  vdWSH%  39.9) sided by two weak C−H···N1 ones (7.512.2%) and, accordingly, are unable to form other X−H···:Y intermolecular H-bonds except through the p-F and p-Cl phenyl substituents in 1’ and 3’. The overall packing could then be imagined as a 8


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3-D composition of just one or two different types of these planar objects that, being sterically rather similar, might be supposed to give comparable arrangements. It is, therefore, rather surprising to observe what a profound transformation is induced by the formation of the π-π* adduct, transformation apparently following an identical rule when going from the left to the right column of Figure 2 and whose possible meaning will now be shortly investigated. Figure 2. Crystallographic unit cells for compounds 1’-4’, 1-4, and TCNQ. Figure 3 (Part 1 and Part 2). The most characteristic EDA interactions  ΣvdW occurring in the structures of compounds 1’-4’, 1-4, and TCNQ.

Compound 1’ (Figure 2.1’) forms a particularly simple herringbone packing of planar double ribbons which run along the [-101] direction and are interlinked by one C12−H←:O1 (14.3%) and three C−H←:F1 () σ*←n interactions (Figure 3.1’.a). Since the remaining C−H σ* acceptors at the border of the ribbons are unable to find other n donors suited to extend the plane, the ribbons become connected in an angular fashion by just one C16−H←C8 σ*←π interaction (10.5%; Figure 3.1’.b) which is sufficient to produce the typical herringbone packing so frequently observed, for similar reasons, in condensed aromatics. In compound 2’ (Figure 2.2’) the F atom is substituted by a p-methyl group, so that the molecule no longer contains n-donors enough to form the planar ribbon above. For this reason, molecules stack in columns linked by C←C π*←π interactions (; Figure 3.2’.a) by taking advantage of the different donor-acceptor properties in different parts of the molecule. These stacks pack by simple vdW forces on the methyl side but are rich of C−H σ* acceptors on the opposite side, which are used to join adjacent columns with herringbone geometry by three C−H←C σ*←π interactions (; Figure 3.2’.b) supported by a weak C9−H←H−C12 σ*←σ one (≈0 %; Figure 3.2’.a). In spite of being p-Cl substituted, compound 3’ (Figure 2.3’) does not form H-bonded ribbons but keeps forming columnar stacks which are connected by a single C←C π*←π interaction (≈0 %; Figure 3.3’.a) and interlinked on both sides by C−H←C σ*←π interactions (; Figure 3.3’.b) reinforced by C−H←H−C σ*←σ ones (0.0, 6.3. and 7.2%) to give the usual herringbone junction. The same packing pattern is essentially maintained in compound 4’ (Figures 2.4’ and 3.4’) by means of an identical set of contacts, i.e., one C←C π*←π (1.5%), two C−H←C σ*←π (5.8 and 6.3%), and two C−H←H−C σ*←σ ones (0.4 and 4.0%). For comparison, Figure 2.TCNQ shows the structure of pure TCNQ which also forms the simple herringbone packing based on TCNQ planar ribbons linked by C2−H←N2 σ*←n interactions (16.4%; Figure 3.TCNQ.a), ribbons which are then interjoined in an angular fashion by 9



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one (C4,C5)←N1 π*←n (2.2%) and two C1−H←N1 σ*←n (11.5%) interactions (Figure 3.TCNQ.b). To be noticed, at this point, that all contacts involving π MOs (e.g., π*←n, σ*←π , and π*←π) are systematically longer than the σ*←n ones, a fact normally explained by saying that p AOs are more diffused than s ones and, therefore, have maximum overlap at longer interatomic distances. The crystal structures of the four TCNQ adducts (1-4) are quite different, being totally dominated by the many TCNQ←ketohydrazone π*←π interactions established (on average, 12 short contacts per structure). Compound 1 (Figure 2.1) consists of planar ribbons of ketohydrazone dimers connected by C15−H←F1 σ*←n interactions (10.3%) and intercalated by TCNQ molecules linked through C4−H←N1A σ*←n (2.4%) and C7−H7←(N1A,C5A) σ*←π () contacts (Figure 3.1.a). These parallel ribbons are slightly stepped and laterally interconnected by one C3A−H←H−C3 σ*←σ (1.3%) and three C16−H←N2A, C2A−H←O1, and C9−H←F1 σ*←n interactions (6.0, 9.0, and 5.4%, respectively). Finally, the different ribbons are vertically connected by 12 C←C π*←π interactions (; Figure 3.1.b) to form ···A···D···D···A··· stacks. Compound 2 (Figure 2.2) form identical ribbons (Figure 3.2.a) where TCNQ and ketohydrazone are still connected by the C4−H←N2A σ*←n (6.7%) and C7−H←(N2A,C6A) σ*←π () interactions, but the two ketohydrazones of the dimer are disjointed because of the substitution of the p-F by the p-methyl group. Parallel and stepped ribbons are still laterally connected by two C16−H←N1A and C2A−H←O1 σ*←n (7.8 and 11.0%, respectively) and one C3A−H←H−C3 σ*←σ interactions (2.5%), and then vertically connected by 12 C←C π*←π interactions (; Figure 3.2.b) able to stabilize the packing of interrupted ribbons by formation of ···A···D···D···A··· stacks. Compound 3 (Figure 2.3) is isostructural with 2, the only difference being the replacement of the p-methyl by the isovolumetric p-Cl group. The ribbons (Figure 3.3.a) are still internally linked by C4−H←N1A σ*←n (8.4%) and C7−H←(N1A,C5A) σ*←π () interactions, interlinked by other three C16−H←N2A, C2A−H←O1, and C8−H←Cl1 σ*←n (6.9, 12.4, and 2.1%, respectively) and one C3A−H←H−C3 σ*←σ interactions (2.5%), and then perpendicularly connected in ···A···D···D···A··· stacks by 12 C←C π*←π interactions (; Figure 3.3.b). The structure of compound 4 (Figure 2.4) is considerably different. It consists of two parallel ribbons (Figure 3.4.a), the first built up of TCNQ molecules linked by C3A−H←N1A and C6A−H←N4A σ*←n interactions () and the second of ketohydrazones connected only by two C13−H←H−C17 and C14−H←H−C18 σ*←σ interactions (), the two ribbons being joined in a plane by inter-crossed C4−H←N2A, C7−H←N2A, C14−H←N3A, and C18−H←N3A σ*←n interactions (). These planes are connected by 11 π*←π interactions which, however, 10


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do not give rise to the usual infinite stacks but rather to discrete A···D···D···A packets where the central couple of dyes is linked by only two weak C←C contacts (0.2%) while the two external TCNQ molecules bind to the dye through 10 C←C or (C,C)←C interactions (; Figure 3.4.b).

■ LOW- AND VARIABLE-TEMPERATURE X-RAY DIFFRACTION In addition to crystal lattice changes, TCNQ cocrystallization induces quite interesting modifications of the intramolecular RAHBs which have a tautomeric DW structure (therefore, with the exclusion of the non-tautomeric 4’-4 couple). This effect is illustrated in Figure 4 reporting the 3-D ΔF maps of compounds 1’-3’ (upper part) and 1-3 (lower part) at the two temperatures of Figure 4. 3-D ΔF maps along the N···H···O reaction pathway for compounds 1’-3’ at 100 (4.1’-3’.a) and 295 K (4.1’-3’.b) and their TCNQ adducts 1-3 at 100 (4.1-3.a) and 295 K (4.1-3.b). Vertical scale in e/Å3; heavy atoms on the borderline of the maps.

100 and 295 K. All maps have been computed in the mean plane of the H-bonded resonant ring by least-squares refinement carried out after having removed the H-bond proton. Maps were plotted using the WinGX[31] system of programs; the O and N atoms are located on the left and right border lines of the plot. The first row compares the three maps 1’- 3’ at 100 K, all indicative of a N−H···O  N···H−O ketohydrazone-azoenol tautomeric equilibrium strongly shifted towards the N−H···O form (on the right of the maps) at 100 K but increasingly populated on the N···H−O side (on the left of the maps) at room temperature (second row). The third and forth rows illustrate the effect of TCNQ cocrystallization on the tautomeric equilibrium, which consists in switching it towards the pure N−H···O form at both temperatures. These rather qualitative findings are a first indication that azocompound-to-TCNQ π*←π donation can induce significant perturbations in the H-bond of the dye, most probably mediated by the resonant nature of the π-conjugated RAHB fragment. These results can be better quantified in terms of the per cent proton population ratios, p(NH)%:p(OH)%, computed by least-squares refinement (Table 2). In pure azocompounds 1’-3’ these ratios undergo Table 2. NH and OH proton population ratios, p(NH)%:p(OH)%, equilibrium constants, K = p(NH)/p(OH), and thermodynamic equilibrium parameters, ΔH° and ΔS° at 298.15 K (obtained by van’t Hoff regression on four different temperatures; standard deviations in parentheses) Figure 5. Van’t Hoff plot lnK(T) = ΔS°/R – ΔH°/R(1/T) for compound 2’. K(T) = p(NH)/p(OH) = p(NH)/(1- p(NH)) is the equilibrium constant at the temperature T; calculated standard enthalpies, ΔH°, and entropies, ΔS°, in cal mol-1 and cal mol-1 K-1, respectively. Figure 6. Variations of the experimental H-bond distances, N2−H and O1···H, caused by a change of temperature from 100 to 295 K (confidence intervals drawn at 3 σ level). Comparison of compounds 1 and 2. 11



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significant variations while the temperature changes from 100 to 295 K, confirming that the H-bond proton is dynamically disordered between the two tautomeric N−H···O and O−H···N positions. Equilibrium thermodynamic parameters computed by van’t Hoff regression over at least four temperatures (see Figure 5 as an example) are also listed in the table. Standard enthalpies, ΔH°, decrease in the correct order -120, -146, -215 cal mol-1 while the tautomeric equilibrium is increasingly shifted towards the N−H···O form, while the ΔS° values are never significantly different from zero, as it may be expected for any intramolecular process. On the contrary, refinement of adducts 1-4 indicates an essentially non-tautomeric N−H···O bond with 100:0 proton population ratio, in agreement with the refined N2−H distances of compounds 2-4 which are nearly 1.0 Å in the full range of temperatures. Rather surprisingly, the N2−H bond and O1···H contact distances of compound 1 are seen to respectively increase and decrease with the increasing temperature (Figure 6). This ‘thermal proton migration’[41] is believed to occur in association with H-bonds having strongly dissymmetric SW potential. In such a potential, the centre of the mean vibrational level (i.e., the proton position) will move while the level itself is raised by the temperature, so inducing the changes of distances actually observed. Further evidence of the effect exerted by TCNQ in shifting the tautomeric equilibrium comes from the analysis of the bond distances within the ···O=C−C=N−NH··· ↔ ···HO−C=C−N=N··· resonant fragment (Table S6 of the Supporting Information). The parameter to be evaluated is, in this case, the global π-delocalization index, , which is seen to change from 0.52-0.55 for the pure azocompounds 1’-3’ to 0.46-0.49 for the TCNQ-adducts 1-4, so indicating a decrease of RAHB delocalization which is coherent with the shift of the tautomeric system towards the localized N−H···O form described above. Scheme 5. Effect of TCNQ-to-TCNQ− Reduction Table 3. Reduction of TCNQ to TCNQ−modifies the a-e bond distances according to the quinoid-toaromatic transition depicted in Scheme 5. A systematic analysis ………………………..

Finally, a similar comparison of TCNQ geometries at 100 K can be taken advantage of to quantify the amount of CT from the azocompound to the TCNQ molecule (Table 3). The method[10] relies on the fact that the electron transfer needed to reduce TCNQ0 to TCNQ− induces significant bond-distance variations which are associable with the quinoid-to-aromatic transition of Scheme 5 and quantifiable in terms of the b-a and c-d distance differences. By analyzing the structures of more than 100 TCNQ complexes, Herbstein and Kapon[9] have singled out the reference topquinoid (5.Q) and top-aromatic (5.A) geometries associated with those adducts where TCNQ has exact 0 and -1 charges and from which the formal CT (FCT, in fractions of one e) transferred in any 12


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other compound can be evaluated by a simple proportion. Though of semiempirical nature this method is believed to give reasonable CT estimates at a qualitative level. Table 3 applies this method to pure TCNQ and its four adducts 1-4, showing that the fraction of charge transferred from one molecule to another in adduct formation amounts to 0.39-0.43 e, values which are to be compared with that of the pure TCNQ crystal, which surprisingly amounts to 0.25 e in spite of the lack of the π*-π couples. The origin of this particular CT contribution can be reasonably traced back to the (C4,C5)←N1 π*←n interaction shown in Figure 3.TCNQ.b. The complexation of TCNQ with the azo dyes is then seen to increase the charge transferred to TCNQ by some 0.15 e (from 0.25 to 0.40 e) in relation to the formation of the new π*←π interactions depicted in Figures 3.1-4.b.

■ RESULTS AND DISCUSSION Molecular interaction analysis. All together, the eight structures studied contain 146 contacts ΣvdW, which are fully listed in Table S5 of the Supporting Information and depicted, for the most interesting cases, in Figure 3. Their main feature is that all of them are formally associable with one or another type of attractive EDA interaction, ruling so out any clear evidence of repulsive interactions. The contacts observed are summarized in Table 4 where they are divided in three main groups, according to whether the donor is of n, π, or σ type (68, 66, and 12 cases, respectively), any group being further subdivided according to the σ* or π* acceptor types. Before starting to analyse the data, some preliminary considerations may be useful. This is the third paper of a series[1,2] intended to exploit the use of CT or EDA interactions in interpreting the crystal packing of molecular cocrystals. To circumvent the basic problem that the strongest interactions arise from conventional σ*←n H-bonds while the most structure-perturbing ones come from π*←π interactions, different sets of cocrystals were chosen where these forces were differently represented: (i) planar N-bases with picric acid[1] (dominating H-bonds; moderate π*←π potentiality); (ii) planar N-bases with TCNQ[2] (moderate H-bonds; increased π*←π potentiality); and, presently, (iii) planar azocompounds with TCNQ (no H-bonds; dominating π*←π interactions). Hence, the present set represents a system which, being unable to form the intermolecular H-bonds that normally dominate the packing, is particularly suited to study the role of weaker interactions, such as σ*←n C−H···:Y bonds and σ*←π and π*←π interactions. The problem of the relative interaction strengths of the different classes of interactions has been treated by Mulliken.[3,4] Having classified donors and acceptors as increvalent (ivl; when the number of bonds increases because of the interaction) and sacrificial (scr; when at least one bond disappears because of it), he noticed that CT-adduct stabilities decrease in the order icv–icv > icv–scr ≈ scr–icv 13



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> scr–scr. In our case only n donors are increvalent while both π and σ donors, as well as σ* and π* acceptors, are sacrificial. Accordingly, all interactions can be predicted to be weak except for the case of moderate σ*←n ones, that is conventional H-bonds and weaker CH-bonds. This is what actually observed in Table 4 where X−H···:Y, C−H···:Y, and the remaining π- and σ-donor interactions have, according to expectation, decreasing average vdW shortenings of 37.6 > 8.0 > 3.5%. The interactions observed can be arranged in a number of different groups: Group 1A. X−H←:Y and C−H←:Y σ*←n EDA interactions (66 cases of which 42 intermolecular). Since in aromatic rings all carbons are sp2 hybridized, both C−H bonds (σ* acceptors) and lone pairs of the C−X: bonds (n donors) necessarily lie in the ring plane. Consequently, all σ*←n interactions remain localized within (or quite close to) the horizontal plane of both ketohydrazone and TCNQ molecules, which become so connected in ribbons or, sometimes, planes. In present structures 41 C−H←:Y and just one X−H←:Y bonds are formed. Comparison with a previous study[2] where 31 C−H←:Y and 12 X−H←:Y bonds were found to form quite similar planar structures seems to indicate that stronger X−H←:Y bonds, though certainly determinant, are not really indispensable for the crystal packing because, anyhow, they can always be replaced by a greater number of C−H···:Y ones which still assure the degree of dominance of σ*←n EDA interactions (columns 210 of Table 4). Including intramolecular bonds, the crystals contain 33 n donors (D: = −C≡N, >C=O, ≡N, −C−F, −C−Cl ) and 119 σ* acceptors (A = O−H, N−H, C−H), all compounds but TCNQ having a considerable excess of acceptors. These 33 n donors are involved in 66 σ*←n interactions (9 of the X−H←:Y and 57 of the C−H←:Y type) which leaves unsaturated 0/33 n donors with an average donor coverage of 66/33 = 2.0 acceptors per donor. The effort to saturate all possible n donors by one or another of the available σ* acceptors is, therefore, quite apparent, suggesting that it may be considered one of the basic steering forces in the formation of molecular crystals. This observations are in agreement with the electron-pair saturation rule established in our previous papers[1,2] and already discussed in the introduction. Group 1B. (C,C)←:N π*←n EDA interactions (2 cases). The interaction of a n lone pair with π* MOs localized on aromatic or multiple bonds gives rise to contacts of strongly angular geometry which are frequently observed in the herringbone packing occurring in many crystals (e.g., in the interaction of NO2 with the aromatic ring of picric acid).[1] In the present compounds it is observed only once in the structure of TCNQ (Figure 3.TCNQ.b).

14


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Group 2A. C−H←(C,N) σ*←π EDA interactions (14 cases). These interactions take two main forms according to the nature of the π donor. When the π MO belongs to an aromatic ring, it becomes the well-known determinant of the herringbone crystal packing of fused-ring aromatics[42,43] and of the benzene←I2 adduct which was the first CT complex ever reported by Benesi and Hildebrand in 1948.[44,45] Likewise, it exerts such a herringbone promotion role in TCNQ itself (Figure 3.TCNQ.b) and in all the pure azo dyes 1’-4’ (Figures 2.1’-4’; Figures 3.1’-3’.b and 3.4’). When the π MO is on a triple bond, however, the C−H acceptor becomes just perpendicular to the triple bond.[46] In the three TCNQ adducts 1-3, such an interaction develops in a planar arrangement linking TCNQ to the azo dyes in planar ribbons (Figures 3.1-3.a). Group 2B. C←:C π*←π EDA interactions (52 cases). In present compounds π*←π interactions occur in two different circumstances. In pure azocompounds 2’-4’, which are unable to form planar ribbons for lack of n donors, they are used (5 occurrences) to preorganize the molecules in homomolecular π*←π stacks that will successively be arranged in herringbone fashion by σ*←π or σ*←σ interactions (Figures 2.2’-4’; Figures 3.2’.a,b, 3.3’.a,b, and 3.4’). In the more general case of the TCNQ adducts (1-4; 47 occurrences) they are employed to connect π donors (D) and π* acceptors (A) in ···A···D···D···A··· stacks (1-3) or discrete A···D···D···A packets (4) by an average of 12 short contacts per crystal, causing a complete upsetting of the structures of pure azocompounds 1’-4’ from herringbone (Figures 2.1’-4’) to columnar packing (Figures 2.1-4, Figures 3.1-4.b) and a radical change of crystal color from red to black with metallic luster (Table 1) imputable to the formation of electronic bands. To notice that, in all stacks of aromatic rings, adjacent molecules are essentially parallel but not exactly juxtaposed, the rings being mutually shifted as shown in their ORTEP views (Figure 1). According to Mulliken,[4] this is due to the fact that the overlap integral of two orthogonal bonding and antibonding π MOs belonging to parallel hexagonal rings can be different from zero only when the two rings are mutually shifted. Table 4. Summary of the interatomic contacts occurring in compounds TCNQ, 1’-4’, and 1-4 which are  ΣvdW, the sum of van der Waals radii……………………………..

Group 3A. C−H←H−C σ*←σ EDA interactions (12 cases). Essentially, two broad classes of H···H interactions have been so far described. The first, extensively investigated from the mid-1990s, concerns the binding of conventional H-bond donors, X−H, to partially covalent hydrides H−M (M = B, Al, Ga, or transition metals) to give strongly ionic Xδ−−Hδ+···Hδ−−Mδ+ bonds, which are habitually named dihydrogen bonds[47] though, in the present context, could be more appropriately called ionic or polar σ*←σ interactions. The second class, conversely, deals with weak closed-shell 15



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interactions between hydrogens which are close to electrical neutrality, typically the C−H←H−C σ*←σ interactions observed 12 times in structure 1’-4’ and 1-4 (Table 4). Such bonds started to be investigated by pure QM computational methods some ten years ago and, to be distinguished from the previous ones, were called hydrogen-hydrogen bonds by Matta and Bader[48,49] or dihydrogen contacts by Shaik and Alvarez,[50] who respectively studied intramolecular (in fused-ring aromatics) or intermolecular (in alkanes) cases (we would rather suggest the alternative name neutral or nonpolar σ*←σ interactions for its grater generality). The interaction energies involved were estimated for n-hexane, for which both crystal structure at 90 K[51] and interaction-energy calculations at the MP2 level with large basis sets[52] are available. In the calculations, the most stable dimer is identical to that observed in the crystal and consists of a side-by-side antiparallel association of two monomers linked by ten C−H←H−C with H···H contacts of 2.407-2.421 Å. The total interaction energy of -4.58 kcal mol-1 has been partitioned in electrostatic (0.14), repulsion (4.37), and charge transfer (-9.09 kcal mol-1) energies, suggesting that the contribution per C−H←H−C interaction is not irrelevant, being some -4.58/10 ≈ -0.5 kcal mol-1. Through-space perturbation of the N−H···O/O−H···N RAHB equilibrium. Besides causing a considerable reorganization of the crystal packing, the cocrystallization with TCNQ induces a number of changes in the chemical structure of both associating partners which can be traced back to the formation of the π*←π adduct and summarized as follows: i) in TCNQ itself, bond distances undergo small but significant variations from quinoid to aromatic geometry (Scheme 5) from which the charge transferred from the azo dye to TCNQ can be estimated to be of the order of 0.40 e (Table 3); ii) in the azo dye, the N−H···O/O−H···N RAHB equilibrium is displaced towards the pure N−H···O tautomer (Table 2 and Figure 4), this displacement being practically complete in the couples 2-2’ and 3-3’ but only partial in the couple 1-1’ where some additional shift of the proton with the temperature is still allowed (Figure 6); iii) finally, the delocalization index, , of the interleaving and dynamically disordered ···O=C−C=N−N−H···/ ···H−O−C=C−N=N··· resonant fragment is decreased, on average, from 0.54 in 1’-3’ to 0.47 in 1-4 (Table 6 of the Supporting Information), so confirming the shift towards the pure N−H···O form. It is of particular interest to stress that the effect caused by TCNQ cocrystallization on the azo dye is formally analogous to that exerted by p-phenyl substitution by groups having increasing electronattracting properties, groups which, by increasing the proton affinity of the H-bonded nitrogen, decrease its viability of being involved in the N−H···O/O−H···N tautomerism.[22,23,53] This analogy is illustrated in Figure 7(a), which reports the X-ray proton populations [p(NH) and p(OH) in per cent] of all p-substituted 1-(4-X-phenylazo)-2-naphthols so far studied [X = NO2, H, CH3, Cl, 16


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F, N(CH3)2; all data at 100 K except for p-NO2][54] as a function of the mesomeric constant, σ°R,[24] a well-known LFER indicator of electronic π-conjugative effects. In pure azo dyes, a σ°R decrease from 0.17 (for p-NO2) to -0.53 (for p-NMe2) is seen to modify the H-bond from the pure N−H···O to prevalent O−H···N form, while the effect of TCNQ cocrystallization (indicated by the vertical arrows) is such to take back the three tautomeric 1’-3’ bonds to the fully N−H···O localized 1-3 ones. This suggests that the π*←π CT discussed above can induce a through-space effect which is quantitatively equivalent to the through-bond effect induced by a σ°R change of some 0.57, i.e. from p-F to p-NO2. Similar conclusions can be drawn from the zero-point corrected (ZPC) DFTcalculated reaction energies (ΔrE, i.e. the energy differences between the N−H···O and O−H···N minima in the PT DW potential) which are available from our previous papers[20-22] except for compound 2’ which has been calculated here. The ΔrE versus σ°R plot of Figure 7(b) shows that the progressive decrease of σ°R is associated with a parallel increase of ΔrE from -1.56 for p-NO2 (pure N−H···O), to -0.29 for p-F (tautomeric with almost isoenergetic minima), and finally to 0.55 kcal mol-1 for p-NMe2 (reversed O−H···N population). The effect of TCNQ cocrystallization on 1’-3’ (see vertical arrows) is that of canceling out any tautomerism by bringing back ΔrE to the value for the non-tautomeric p-NO2 derivative. This interpretation is supported by similar ZPC DFT calculations carried out on the 2’(●+) radical cation in its doublet state, a virtual model system of compound 2’ having transferred one electron. The ΔrE value, which was -0.58 kcal mol-1 for 2’ (a typical small value for tautomeric systems) is now increased to -1.52 kcal mol-1 for the radical cation, a value which compares well with that of -1.56 kcal mol-1 obtained for the neutral p-NO2 derivative in its singlet state, the compound that better represents the pure N−H···O non-tautomeric bond. Figure 7. X-ray proton populations [p(NH) and p(OH) in per cent] against mesomeric constant of the substituent, σ°R,[24] for all p-substituted 1-(4-X-phenylazo)-2-naphthols …………..

■ ASSOCIATED CONTENT Supporting Information. Tables of crystal data, selected bond distances and angles, and H-bond and EDA contact distances for compounds 2’, 4’, and 1-4 (Tables S1-S4). Full list and per cent shortening values (vdWSH%) of H-bonds and EDA contacts  ΣvdW (Table S5). Bond distances and numbers within the H-bond resonant fragment (Table S6). X-ray crystallographic information files (CIF) for structures 2’, 4’, and 1-4 (deposited at the CCDC with deposition numbers 903350903367). This information is available free of charge via the Internet at http://pubs.acs.org/. 17



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■ AUTHOR INFORMATION Corresponding Author. *E-mail: [email protected] Tel. +39-0532 455141. Fax +39-0532 240709 Notes. The authors declare no competing financial interest. ■ AKNOWLEDGMENT This work was supported by COFIN-2008 (MIUR Rome) and by the University of Ferrara with local research funds (FAR).

■ REFERENCES (1) Bertolasi, V.; Gilli, P.; Gilli G. Cryst. Growth Des., 2011, 11, 2724-2735. (2) Bertolasi, V.; Gilli, P.; Gilli G. Cryst. Growth Des., 2012, 12, 4758-4770. (3) Mulliken, R. S. J. Phys. Chem., 1952, 56, 801-822. (4) Mulliken, R. S.; Person, W.B. Molecular Complexes. A Lecture and Reprint Volume, John Wiley & Sons, Inc.: New York, 1969. (5) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev., 1988, 6, 899-926 (6) Weinhold, F; Landis, C. R. Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective, Cambridge University Press: Cambridge, 2005. (7) Gilli, P.; Gilli, G. Noncovalent Interactions in Crystals. In: Supramolecular Chemistry: From Molecules to Nanomaterials, J.W. Steed and P.A. Gale, Eds., John Wiley & Sons: Chichester, 2012, pp. 2829-2868. (8) Herbstein, F. H., Crystalline Molecular Complexes and Compounds: Structure and Principles, Vols. 1,2, Oxford University Press: Oxford, 2005. (9) Herbstein, F. H.; Kapon, M. Cryst. Rev., 2008, 14, 3-74. (10) Kristenmacher, T. J.; Phillips, T. E.; Cowan, D. O. Acta Crystallogr., 1974, B30, 763-768. (11) Long, R. E.; Sparks, R. A.; Trueblood, K. N. Acta Cryst., 1965, 18, 932-939. (12) Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023-1028. (13) Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1991, 113, 4917-4925. (14) Gilli, G.; Bertolasi, V.; Ferretti, V.; Gilli, P. Acta Crystallogr. 1993, B49, 564-576.

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(15) Gilli, G.; Gilli P. The Nature of the Hydrogen Bond. Outline of a Comprehensive Hydrogen Bond Theory; Oxford University Press: Oxford, 2009. (16) Gilli, P.; Bertolasi, V.; Pretto, L.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 2004, 126, 3845. (17) Cleland, W.W.; Kreevoy, M.M Science 1994, 264, 1887–1890. (18) Frey, P.A.; Whitt, S.A.; Tobin, J.B. Science 1994, 264, 1927–1930. (19) Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 2000, 122, 10405-10417. (20) Gilli, P.; Bertolasi, V.; Pretto, L.; Lycka, A.; Gilli, G. J. Am. Chem. Soc. 2002, 124, 1355413567. (21) Gilli, P.; Bertolasi, V.; Pretto, L.; Antonov, L.; Gilli, G. J. Am. Chem. Soc. 2005, 127, 49434953. (22) Gilli, P.; Bertolasi, V.; Pretto, L.; Gilli, G. J. Mol. Struct. 2006, 790, 40-49. (23) Bertolasi, V.; Gilli, P.; Gilli, G. Curr. Org. Chem. 2009, 13, 250-268. (24) Chapman, N.B.; Shorter, J. (Eds) Correlation Analysis in Chemistry, Plenum Press: New York, 1978. (25) Inabe, T. New J. Chem. 1991, 15, 129-136. (26) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326. (27) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna, R. SIR97, J. Appl. Crystallogr. 1999, 32, 115-119. (28) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Refinement, University of Gőttingen, Germany, 1997. (29) Nardelli, M. J. Appl. Crystallogr. 1995, 28, 659. (30) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool, Utrecht University, The Netherlands, 2002. (31) Farrugia, L. J. WINGX, J. Appl. Crystallogr. 1999, 32, 837. (32) Burnett, M. N.; Johnson, C.K. ORTEP III, Report ORNL-6895, Oak Ridge National Laboratory, Oak Ridge, TN, 1996. (33) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A. J. Appl. Cryst. 2008, 41, 466-470. (34) Bondi, A., J. Phys. Chem., 1964, 68, 441-451. (35) Lippincott, E. R.; Schroeder, R. J. Chem. Phys. 1955, 23, 1099-1106. (36) Schroeder, R.; Lippincott, E. R. J. Phys. Chem. 1957, 61, 921-928. (37) Gilli, P.; Gilli, G. LSHB. A Computer program for performing Lippincott and Schroeder HB calculations, University of Ferrara, Italy, 1992. (38) Pauling, L. J. Am. Chem. Soc. 1947, 69, 542-553. 19



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(39) Brock, C.P.; Dunitz, J.D. Acta Crystallogr. 1982, B38, 2218-2228. (40) (a) Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Zakrzewski, V.G.; Montgomery, Jr., J.A.; Stratmann, R.E.; Burant, J.C.; Dapprich, S.; Millam, J.M.; Daniels, A.D.; Kudin, K.N.; Strain, M.C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G.A.; Ayala, P.Y.; Cui, Q.; Morokuma, K.; Rega, N.; Salvador, P.; Dannenberg, J.J.; Malick, D.K.; Rabuck, A.D.; Raghavachari, K.; Foresman, J.B.;. Cioslowski, J.; Ortiz, J.V.; Baboul, A.G.; Stefanov, B.B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.L.; Fox, D.J.; Keith, T.; Al-Laham, M.A.; Peng, C.Y.; Nanayakkara, A.; Challacombe, M.; Gill, P.M.W.; Johnson, B.; Chen, W.; Wong, M.W.; Andres, J.L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E.S.; Pople, J.A. GAUSSIAN 98 (Revision A.11.3); Gaussian, Inc.: Pittsburgh, PA, 2002. (41) Wilson, C.C. Acta Cryst., 2001, B57, 435-439. (42) Gavezzotti, A.; Desiraju, G. Acta Cryst., 1988, B44, 427-434. (43) Desiraju, G.; Gavezzotti, A. Acta Cryst., 1989, B45, 473-482. (44) Benesi, H.A.; Hildebrand, J.H. J. Am. Chem. Soc. 1948, 70, 3978-3981. (45) Benesi, H.A.; Hildebrand, J.H. 1949, 71, 2703-2707. (46) Legon, A.C.; Millen, D.J. Acc. Chem. Res. 1987, 20, 39-46. (47) For a short account see ref 15, pp 49-56. (48) Matta, C.F.; Hernandz-Trujillo, J.; Tang, T.-H.; Bader, R.F.W. Chem. Eur. J. 2003, 9, 19401951. (49) Matta, C.F., Hydrogen−Hydrogen Bonding: The Electrostatic Limit of Closed-Shell Interaction between Two Hydrogen Atoms. A Critical Review. In: Hydrogen Bonding – New Insights, S. J. Grabowski, Ed., Spriger: Dordrech, The Netherland, 2006. (50) Echeverrìa, J.; Aullòn, G.; Danovich, D.; Shaik, S.; Alvarez, S. Nature Chem 2011, 3, 323-330. (51) Boese, R.; Weiss, H.-C.; Blaser, D. Angew. Chem. Int. Ed., 1999, 38, 988-992. (52) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M. J. Phys. Chem. A, 2004, 108, 10311-10316. (53) Gilli, P,; Gilli, G. J. Mol. Struct. 2010, 972, 2-10. (54) Data are from: p-F at 100 K, ref 20; p-Cl and p-N(CH3)2 at 100 K, ref 21; p-CH3 at 100 K, present work; p-H at 100 K from a recent redetermination (V. Bertolasi, private communication) of the r.t. structure of ref 55); p-NO2 at 295 K, ref 56. (55) Olivieri, A.C.; Wilson, R.B.; Paul, I.C.; Curtin, D.Y. J. Am. Chem. Soc. 1989, 111, 5525-5532. (56) Whitaker, A. Z. Kristallogr. 1980, 152, 227.

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■ TABLES, FIGURES, AND SCHEMES

Table 1. The crystal structures discussed in this paper Compd TCNQ

Chemical name

Temp

7,7,8,8-tetracyanoquinodimethane

295 K

1-(4-fluoro-phenylhydrazo)-naphthalen-2-one

1’

bis[1-(4-fluoro-phenylhydrazo)-naphthalen-2-one)]·TCNQ

1

1-(4-methyl-phenylhydrazo)-naphthalen-2-one

2’

bis[1-(4-methyl-phenylazo)-naphthalen-2-one)]·TCNQ

2

Crystal color Pale yellow

[11]

100-295 K

a

Red orange

[20]

100-295 K

b

Black, metallic luster

pw

a

Red

pw

100-295 K

a


pw

a

Red

[21]


pw

Dark garnet

pw


pw

100-295 K

3’

1-(4-chloro-phenylhydrazo)-naphthalen-2-one

100-295 K

3

bis[1-(4-chloro-phenylhydrazo)-naphthalen-2-one)]·TCNQ

100, 295 K

4’

1-(naphthalenylhydrazo)-naphthalen-2-one)] (disordered) 1-(naphthalenylhydrazo)-naphthalen-2-one)·TCNQ

4 a

Ref

295 K 100, 295 K

b

Temperatures of 150 and 200 K also available; temperatures of 150, 200, and 275 K also available

21



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Table 2. NH and OH proton population ratios, p(NH)%:p(OH)%, equilibrium constants, K = p(NH)/p(OH), and thermodynamic equilibrium parameters, ΔH° and ΔS° at 298.15 K (obtained by van’t Hoff regression on four different temperatures; standard deviations in parentheses) Comp

T (K)

p(NH)%: p(OH)%

K = p(NH)/ p(OH)

ΔH° (cal mol-1)

ΔS° (cal mol-1 K-1)

Ref

1’(p-F)

100 295

64:36 54:46

1.78 1.17

-120(15)

0.0(1)

[20]

3’(p-Cl)

100 295

69:31 58:42

2.23 1.38

-146(25)

0.19(17)

[21]

2’(p-Me)

100 295

75:25 59:41

3.00 1.44

-215(20)

0.01(13)

pw

4’

295

100:0

-

-

-

pw

1-4

100

100:0

-

-

-

pw

Table 3. Reduction of TCNQ to TCNQ− modifies the a-e bond distances according to the quinoid-to-aromatic transition depicted in Scheme 5. A systematic analysis of the TCNQ-adduct structures[10] allows to identify the limit top-quinoid (5.Q) and top-aromatic (5.A) structures where TCNQ is endowed with the exact 0 and -1 charges and from which the formal CT (FCT, in fractions of one e) of any other compound can be evaluated by a simple proportion over the b-a and c-d quantities. The table reports the reference distances (in Å) of the two top-complexes and compares them with those found in the structures of TCNQ[11] itself and in the present TCNQ adducts 1-4. Compound Top-Quinoid0/0 (OJIXOU) = Q Top-Aromatic+/− (MCFETC01) = A Δ = Top-Quinoid0/0 − Top-Aromatic +/−

a 1.319 1.372

b 1.444 1.420

b-a 0.125 0.048 0.077

c 1.356 1.417

d 1.439 1.420

c-d -0.083 -0.003 -0.080

e 1.143 1.146

FCT (e) 0.0 1.0

TCNQ (TCYQME) (r.t.) 1. (100 K) 2. (100 K) 3. (100 K) 4. (100 K)

1.346 1.352 1.352 1.352 1.351

1.448 1.445 1.445 1.442 1.443

0.102 0.093 0.093 0.090 0.092

1.374 1.381 1.380 1.384 1.383

1.441 1.438 1.435 1.435 1.435

-0.067 -0.055 -0.055 -0.051 -0.052

1.140 1.152 1.150 1.146 1.150

0.25 0.39 0.39 0.43 0.41

22


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Table 4. Summary of the interatomic contacts occurring in compounds TCNQ, 1’-4’, and 1-4 which are  ΣvdW, the sum of van der Waals radii, as indicated by the quantity vdW SH% = shortening of the contact distance in per cent of ΣvdW; data from Table S5. nD = electron-pair donors; UNS(D) = unsaturated electron-pair donors; σ*A = σ* electron acceptors; A−D = σ* acceptors minus n donors; XH←:Y = σ*←n conventional H-bonds; CH←:Y= σ*←n CH-bonds; A←:D = π*←n, σ*←π, π*←π and C−H←H−C = σ*←σ interactions; ΣINT(nD) = number of interactions with n donors; occurrences in the (x + y) form refer to intermolecular and intramolecular interactions, respectively.

COMP

n donors (nD) σ* acceptors (σ*A) XH←:Y CH←:Y nD UNS σ*A A−D XH←:Y vdW UNS CH←:Y vdW (nD) σ*←n SH% (XH) σ*←n SH%

π* acceptors C←:N A←:D vdW π*←n SH%

ΣINT (nD)

ΣINT /D

π donors σ* accs. π* accs. CH←(C,N) C←C A←:D vdW A←:D vdW σ*←π SH% π*←π SH%

σ donors σ* accs. C−H←H−C A←:D vdW σ*←σ SH%

TCNQ

2

0

2

0

0

-

-

2+0

11-16

2

2-3

4

2.0

0

-

0

-

0

-

1’

4

0

12

8

1+1

36-38

0

4+2

3-14

0

-

8

2.0

1

11

0

-

1

0

2’

2

0

14

12

0+1

36

0

3+2

1-13

0

-

6

3.0

3

3-6

3

1-3

1

0

3’

3

0

11

8

0+1

40

0

2+2

4-11

0

-

5

1.7

2

0-3

1

0

3

0-7

4’

2

0

13

11

0+1

40

0

1+2

5-12

0

-

4

2.0

2

5-6

1

2

2

0-4

1

5

0

16

9

0+1

37

0

7+2

3-10

0

-

10

2.0

1

3-4

12

0-4

1

1

2

4

0

18

14

0+1

38

0

7+2

4-11

0

-

10

2.5

1

3-4

12

0-5

1

1

3

5

0

15

10

0+1

36

0

5+2

2-12

0

-

8

1.6

2

2-6

12

0-5

1

1

4

6

0

18

12

0+1

39

0

10 + 2

0-11

0

-

13

2.2

2

10-12

11

0-6

2

10-12

Sum

33

0

119

84

1+8 =9

0

41 + 16 = 57

68

14

2

52

12

23



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Figure 1. ORTEP[32] views of the asymmetric units with thermal ellipsoids at 30% probability for compounds 1-4 and 2’ at 100 K, and for the partially disordered compound 4’ at 295 K.

24


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Figure 2. Crystallographic unit cells for compounds 1’-4’, 1-4, and TCNQ. 25



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Figure 3 (Part 1). The most characteristic EDA interactions  ΣvdW occurring in the structures of compounds 1’-4’, 14, and TCNQ. 26


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Figure 3 (Part 2). The most characteristic EDA interactions  ΣvdW occurring in the structures of compounds 1’-4’, 14, and TCNQ.

27



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Figure 4. 3-D ΔF maps along the N···H···O reaction pathway for compounds 1’-3’ at 100 (4.1’-3’.a) and 295 K (4.1’3’.b) and their TCNQ adducts 1-3 at 100 (4.1-3.a) and 295 K (4.1-3.b). Vertical scale in e/Å3; heavy atoms on the borderline of the maps. 28


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Figure 5. Van’t Hoff plot lnK(T) = ΔS°/R – ΔH°/R(1/T) for compound 2’. K(T) = p(NH)/p(OH) = p(NH)/(1- p(NH)) is the equilibrium constant at the temperature T; calculated standard enthalpies, ΔH°, and entropies, ΔS°, in cal mol-1 and cal mol-1 K-1, respectively.

Figure 6. Variations of the experimental H-bond distances, N2−H and O1···H, caused by a change of temperature from 100 to 295 K (confidence intervals drawn at 3 σ level). Comparison of compounds 1 and 2.

Figure 7. (a) X-ray proton populations [p(NH) and p(OH) in per cent] against substituent mesomeric constant, σ°R,[24] for all p-substituted 1-(4-X-phenylazo)-2-naphthols so far studied [X = NO2, H, CH3, Cl, F, N(CH3)2; data at 100 K except p-NO2].[56] (b) Zero-point corrected DFT-calculated reaction energies (ΔrE = energy differences between the N−H···O and O−H···N minima in the PT DW potential) plotted against the correspomding σ°R values. Data from ref 21 except for compound 2’ which has been calculated here. 29



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For Table of Contents Use Only Solid-State N−H···O/O−H···N Tautomerism in Resonance-Assisted 1-(Arylazo)-2-Naphthols and its Trough-Space π*←π Perturbation in TCNQ Cocrystals. A Variable-Temperature XRay Crystal Study Gastone Gilli, Valerio Bertolasi, and Paola Gilli* Centro di Strutturistica Diffrattometrica e Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, Via Borsari 46, 44121 Ferrara, Italy

TOC Graphic

+ TCNQ

 TCNQ

Synopsis Aryl-azonaphthol dyes display ···HN−N=C−C=O···  ···N=N−C=C−OH··· ketohydrazone-azoenol tautomerism modulated by the aryl substituents. The solid-state N−H···O  O−H···N tautomerism in four different azo dyes and their TCNQ cocrystals is studied by variable-temperature X-ray crystallography, showing that the TCNQ←dye π*←π interactions can cause through-space perturbations of the tautomeric equilibrium which are formally equivalent to the through-bond effects caused by the substituents.

32


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O–H···N Tautomerism in ... - ACS Publications

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