Structural changes in the crystal packing of highly hindered symmetricalvicinal bis-amides

Claudio A. Jimenez,*a Julio B. Belmar,a Fernando S. Delgado,bc Miguel Julved and Catalina Ruiz-Perezb

Received 4th December 2006, Accepted 20th April 2007

First published as an Advance Article on the web 17th May 2007

DOI: 10.1039/b617605d

The crystal structures of 1,4-bis(3,5-di-tert-butyl-2-hydroxybenzamido)butane (1) and 1,3-bis(3,5-

di-tert-butyl-2-hydroxybenzamido)-2,2-dimethylpropane (2) have being characterized by single

crystal X-ray diffraction. Their crystal packing is discussed in terms of the different interactions

they exhibit. A brief discussion, based on hydrogen bonds, on the structural features of bis-(3,5-

di-tert-butyl-2-hydroxybenzamide) compounds is carried out.

Introduction

The complementary recognition between functional groups in

molecules is the first step in crystallization. Because molecular

recognition is complementary in nature,1 a functional group

approach to crystal engineering is, at least currently, not

possible,2 and prediction of crystal structures is correspond-

ingly difficult.3 Current systematic synthetic strategies in

purely organic species attempt to tackle this issue by

identifying systems where the crystal construction may be

taken up in a modular fashion and where interaction inter-

ference is minimal. Thus the effect of any particular func-

tionality can be identified.4 Indeed, this is one of the well

accepted goals of crystal engineering.

Functional groups exert both steric and electronic effects on

the structure and reactivity, but even in solution, where the

functional group approach is broadly applicable, separating

these two effects is quite difficult.5 This issue is more or less

intractable in the solid state because the effects of the

functional groups are in themselves implicitly connected to

the nature and position of the other functional groups in the

molecule. In the context of crystal engineering, steric and

electronic effects are synonymous with geometrical and

chemical effects in the crystal packing.6 While the origin of

all intermolecular interactions is electrostatic7 and there is no

rigorous basis for such a distinction, it is still convenient to

distinguish these components on the basis of the distance

dependence of the corresponding interactions.8

In this framework, amide functions are the robust synthons

of wide consensus and they have become the choice of a great

number of research groups.9 This functional group has

numerous applications. In particular, bulky amides and their

lithium salts are widely used as deprotonating reagents in

organic synthesis due to their relatively low nucleophilicity

and strong Brønsted basicity,10,11 as well as reagents for

asymmetric synthesis of compounds containing C–C bonds.12

Furthermore, amides have been found to be promising

precursors to synthesize a variety of complexes with main

group elements, transition metal ions, and lanthanide cations,

many of them well-characterized.13 Networking of molecules

through interactions such as hydrogen bonds and metal

coordination can be exploited to generate a wide range of

fascinating solid-state structures.14,15

In this contribution, we wish to present the changes in the

crystal packing when a conformational modification in the

structure is incorporated. The crystal structures of 1,4-bis(3,5-

di-tert-butyl-2-hydroxybenzamido)butane (1) and 1,3-bis(3,5-

di-tert-butyl-2-hydroxybenzamido)- 2,2- dimethylpropane (2)

(Scheme 1) show the occurrence of different chain motifs

which are mainly monitored by a different hydrogen bonding

pattern. Their structures are analyzed in terms of hydrogen

bond and weaker supramolecular interactions and they are

compared with the crystal structures of homologous series

which were already reported.16

Experimental

Ligands

The full synthesis of 1,4-bis(3,5-di-tert-butyl-2-hydroxybenz-

amido)butane (1) and 1,3- bis(3,5-di-tert-butyl-2-hydroxybenz-

amido)-2,2-dimethylpropane (2) are already reported.17 X-Ray

quality crystals of 1 and 2 were obtained by slow evaporation

of their ethanolic solution.

aDpto. Quımica Organica. Facultad de Ciencias Quımicas. Universidadde Concepcion. Casilla 160-C, Concepcion, Chile.E-mail: cjimenez@udec.clbLaboratorio de Rayos X y Materiales Moleculares. Dpto. de FısicaFundamental II. Facultad de Fısica. Universidad de La Laguna. Avda.Astrofısico Francisco Sanchez s/n, 3820-La Laguna, Tenerife, SpaincBM16 – LLS European Synchrotron Radiation Facility 6 Rue JulesHorowitz - BP 220 38043 Grenoble CEDEX 9, FrancedICMol/Departament de Quımica Inorganica de la Universitat deValencia, Polıgono La Coma s/n, 46980 Paterna (Valencia), Spain

Scheme 1 Structures of 1,4-bis(3,5-di-tert-butyl-2-hydroxybenz-

amido)butane (1) and 1,3-bis(3,5-di-tert-butyl-2-hydroxybenzamido)-

2,2-dimethylpropane (2).

PAPER www.rsc.org/crystengcomm | CrystEngComm

746 | CrystEngComm, 2007, 9, 746–752 This journal is � The Royal Society of Chemistry 2007

Crystal data collection and structure determination

Single crystals of 1 and 2 were mounted on a Bruker-Nonius

KappaCCD diffractometer. Orientation matrix and lattice

parameters were obtained by least-squares refinement of the

reflections obtained by a h–x scan (Dirax/lsq method).

Diffraction data for all compounds were collected at 293(2) K

using graphite-monochromated Mo Ka radiation (l =

0.71073 A). Data collection and data reduction were done

with the COLLECT18 and EVALCCD19 programs. Empirical

absorption corrections were carried out using SADABS20 for

all compounds. The indexes of data collection were 212 ¡ h

¡ 12, 214 ¡ k ¡ 14 and 220 ¡ l ¡ 20 for 1 and 226 ¡ h ¡

16, 226 ¡ k ¡ 29 and 211 ¡ l ¡ 14 for 2. Of the 7124 (1)

and 5432 (2) measured independent reflections in the h range

4.14–27.50 (1) and 6.48–24.50 (2), 3915 (1) and 3845 (2) have

I ¢ 2s(I). All calculations for data reduction, structure

solution, and refinement were done by standard procedures

(WINGX).21 The structures were solved by direct methods and

refined with full-matrix least-squares technique on F2 using the

SHELXS-97 and SHELXL-97 programs.22 All hydrogen

atoms bound to carbon and those of the phenol groups were

placed in calculated positions and refined isotropically. The

hydrogen atoms for all compounds were refined with isotropic

temperature factors. The final Fourier-difference maps

showed maximum and minimum height peaks of 0.297 and

20.225 e A23 (1) and 0.134 and 20.116 e A23 (2). A summary

of the crystallographic data and structure refinement is

given in Table 1. The final geometrical calculations and the

graphical manipulations were carried out with PARST9723

and DIAMOND24 and MERCURY25 programs, respectively.

Supramolecular interactions are listed in Tables 2 (1) and 4 (2)

whereas the main bond distances and angles are given in

Tables 3 (1) and 5 (2).

CCDC reference numbers 629529 (1) and 629530 (2. For

crystallographic data in CIF or other electronic format see

DOI: 10.1039/b617605d

Results and discussion

The structure of 1 is built up from neutral bis-amide units

which are held together by means of hydrogen bonds, C–H…p

and CH3…CH3 hydrophobic interactions affording a 3-D

structure. Intramolecular hydrogen bonds involving the phenol

and amide oxygens help to stabilize each branch of the

molecule (Fig. 1a and Table 4) and they make the amide and

phenol groups roughly coplanar. The values of the dihedral

angle between the mean planes of the amide group and the

phenyl ring within each half of the molecule are 12.7(2) [C(1)–

C(6)/O(3)–C(7)–N(1)] and 7.49(15)u [C(13)–C(18)/O(4)–C(12)–

N(2)]. The separation between the amide nitrogen atoms

N(1)…N(2) is 5.127(3) A, a value which is longer than that

reported for other related compounds11,16,26 because of the

longer size of the organic substituent in 1 compared to that

reported previously. Within the bis-amide unit, the values of

the centroid…centroid separation and that of the dihedral

angle between the planes defined by phenyl rings from each

half of the molecule are 9.423(5) A and 60.79(7)u, respectively.

The torsion angle of the alkyl bridge is 265.5(8)u [N(1)–C(8)–

C(12)–N(2)]. It deserves to be noted that the bis-amide group

adopts the anti conformation, as observed in other related

compounds.16,26 Each bis-amide unit is connected to other

two centrosymmetrically related molecules through hydrogen

Table 1 Crystallographic data for compounds 1 and 2

Compound 1 2

Formula C34H52N2O4 C35H54N2O4

M 552.78 566.80Crystal system Triclinic OrthorhombicSpace group P1 Iba2a/A 10.0092(6) 23.029(4)b/A 11.2895(7) 25.201(5)c/A 15.5109(8) 12.087(3)a/u 98.90(4) —b/u 102.04(4) —c/u 104.00(4) —V/A3 1623.85(16) 7015(3)Z 2 8rcalc(Mg m23) 1.131 1.073m(Mo Ka)/mm21 0.71073 0.71073l(Mo Ka)/mm21 0.073 0.069R1a, I . 2s(I) (all) 0.065 (0.132) 0.046(0.087)wR2b, I . 2s(I) (all) 0.147 (0.175) 0.085(0.098)Measured reflections 15426 15705Independent reflections (Rint) 7124 (0.029) 5432 (0.042)a R1 = S | |Fo| 2 |Fc| |/S |Fo| b wR2 = {S [w(Fo

2 2 Fc2)2]/S

[w(Fo2)2]}1/2; w = 1/[s2(Fo)2 + (aP)2 + bP], where P = [2Fc

2 +Max(Fo

2,0)]/3.

Fig. 1 Supramolecular association in 1 mediated by N–H…O

interactions (a). It leads to the formation of chains along the a-axis

(b). The atom numbering is also shown.

This journal is � The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 746–752 | 747

bonds, involving the amide groups: 2.990(3) A for N(1)…O(4)i

[(i) 2x 2 1, 2y + 1, 2z + 1] and 3.003(3) A for N(2)…O(3)ii

[(ii) 2x, 2y + 1, 2z + 1.] leading to regular belt-like chains

that run along the a-axis (Fig. 1b). These chains are held

together by means of C–H…p interactions [H…centroid

separations ranging from 2.94(5) to 3.54(5) A] along the

b- and c-axes (see Fig. 2), affording a 3-D network. In

addition, weak CH3…CH3 hydrophobic interactions contri-

bute to stabilize the whole structure, the shortest C…C

distances varying in the range 3.678(5)–3.895(3) A (Table 2).

The crystal packing in 1 (Fig. 3) is very similar to that observed

in the related bis-amide compounds with an n-alkyl bridge

connecting the two halves:11 hydrogen bonds in the a direction

and CH…p type interactions in the bc-plane, build up the 3-D

structure (see Fig. S1).

As in 1, the structure of 2 consists of neutral bis-amide units

which are held together by means of hydrogen bonds, C–H…p

and CH3…CH3 interactions leading to a 3-D structure.

Similarly to 1, intramolecular hydrogen bonds involving the

phenol and amide oxygens help to stabilize each branch of the

molecule (Fig. 4 and Table 5). However in 2, contrary to 1,

Table 2 Interaction data for compounds (1)

H Bond Typea D…A/A H…Aa/A D-H…A/u

O(1)–H(1)O…O(3) 2.520(3) 1.63(3) 157(3)O(2)–H(2)O…O(4) 2.519(3) 1.57(4) 158(3)N(1)–H(1)N…O(4)i 2.990(3) 2.21(2) 149(2)N(2)–H(2)N…O(3)ii 3.003(3) 2.20(3) 152(2)

CH…pb Catom C…Cg/A H…Cg/A C– H…Cg/u

C(26)– H(26)A…Cg1iii C(26)iii 3.634(4) 2.99(5) 150 (2)C(32)–H(32A)…Cg2iv C(18)iv 3.928(5) 3.04(5) 149 (3)C(25)–H(25A)…Cg2iii C(14)iii 3.916(4) 3.54(5) 114 (2)

CH3…CH3 C…C/A

(Hydrophobic C(22)…C(32)v 3.863(5)Interactions) C(28)…C(20)vi 3.678(5)

C(30)…C(25)vii 3.895(3)C(34)…C(20)viii 3.830(7)

a Symmetry operator: (i) 2x 2 1, 2y + 1, 2z + 1; (ii) 2x, 2y + 1,2z + 1; (iii) 2x, 2y + 2, 2z + 1; (iv) 2x, 2y + 1, 2z + 2; (v) x 21, y, z 2 1; (vi) 2x 2 1, 2y + 2, 2z + 1; (vii) x, y, z + 1; (viii) 2x,2y + 2, 2z + 1. b Centroids: Cg1:C(1)–C(6); Cg2:C(13)–C(18).

Fig. 2 Chains of 1 projected on the bc -plane showing its connection

to four adjacent ones through C–H…p type interactions (broken lines).

Table 3 Selected bond lengths (A) and angles (u) for 1

C(1)–C(2) 1.399(3) C(2)–C(1)–C(7) 119.1(2)C(1)–C(7) 1.483(3) O(1)–C(2)–C(1) 120.8(2)C(2)–O(1) 1.355(2) O(3)–C(7)–N(1) 119.9(2)C(7)–O(3) 1.251(2) O(3)–C(7)–C(1) 121.0(2)C(7)–N(1) 1.332(3) N(1)–C(7)–C(1) 119.1(2)C(8)–N(1) 1.462(3) N(1)–C(8)–C(9) 113.9(2)C(8)–C(9) 1.522(4) C(10)–C(9)–C(8) 114.0(2)C(9)–C(10) 1.497(4) C(9)–C(10)–C(11) 113.4(2)C(10)–C(11) 1.517(4) N(2)–C(11)–C(10) 114.4(2)C(11)–N(2) 1.457(3) O(4)–C(12)–N(2) 119.5(2)C(12)–O(4) 1.252(2) O(4)–C(12)–C(13) 121.1(2)C(12)–N(2) 1.332(3) N(2)–C(12)–C(13) 119.4(2)C(12)–C(13) 1.479(3) C(14)–C(13)–C(12) 119.4 (2)C(13)–C(14) 1.405(3) O(2)–C(14)–C(13) 120.6(2)C(14)–O(2) 1.355(2)

Fig. 3 View of the packing of 1 along the a-axis.

Fig. 4 The molecular structure of 2 with labelling scheme of relevant

atoms showing intra and intermolecular H-bonds

748 | CrystEngComm, 2007, 9, 746–752 This journal is � The Royal Society of Chemistry 2007

there exist intramolecular bifurcated hydrogen bonds invol-

ving the nitrogen and oxygen atoms from the amide groups

which favor the bending of the molecule (Table 3).

Table 4 Interaction data for compounds (2)

H Bond Typea D…A/A H…Aa/A D–H…A/u

N(1)–H(1)N…O(4) 2.943(3) 2.14(3) 154(3)O(1)–H(1)O…O(3) 2.541(3) 1.51(4) 153(3)O(2)–H(2)O…O(4) 2.613(3) 1.90(3) 147(3)N(2)–H(2)N…O(3)i 2.876(3) 1.97(2) 164(2)

CH…pb Catom C…Cg/A H…Cg/A C–H…Cg/u

C(22)–H(22A)…Cg1ii C(6)ii 3.772(4) 3.40(7) 112(2)C(27)–H(27C)…Cg1iii C(2)iii 3.724(5) 2.96(8) 145(3)C(30)–H(30C)…Cg2iv C(15)iv 3.876(4) 3.35(6) 128(2)

CH3…CH3 C…C/A

(Hydrophobic C(33)…C(21)v 3.811(5)Interactions)a Symmetry operator: (i) 2x + 1/2, 2y + 1/2, z + 1/2; (ii) x, 2y, z 21/2; (iii) x, 2y, z + 1/2; (iv) x, y, z + 1/2; (v) 2x + 1/2, y + 1/2, z.b Centroids: Cg1:C(1)–C(6); Cg2:C(14)–C(19).

Table 5 Selected bond lengths (A) and angles (u) for 2

C(1)–C(2) 1.399(3) C(2)–C(1)–C(7) 118.4(2)C(1)–C(7) 1.478(3) O(1)–C(2)–C(1) 120.5(2)C(2)–O(1) 1.353(3) O(3)–C(7)–N(1) 120.8(2)C(7)–O(3) 1.252(3) O(3)–C(7)–C(1) 120.9(2)C(7)–N(1) 1.327(3) N(1)–C(7)–C(1) 118.2(2)C(8)–N(1) 1.450(3) N(1)–C(8)–C(9) 114.4(2)C(8)–C(9) 1.528(3) C(12)–C(9)–C(8) 109.9(2)C(9)–C(12) 1.527(3) N(2)–C(12)–C(9) 114.5(2)C(12)–N(2) 1.454(3) O(4)–C(13)–N(2) 121.1(2)C(13)–O(4) 1.248(3) O(4)–C(13)–C(14) 121.2(2)C(13)–N(2) 1.330(3) N(2)–C(13)–C(14) 117.7(2)C(13)–C(14) 1.476(3) C(15)–C(14)–C(13) 119.1(2)C(14)–C(15) 1.394(3) O(2)–C(15)–C(14) 120.8(2)C(15)–O(2) 1.355(3)

Fig. 5 Supramolecular association along c-axis mediated by N–H…O

interactions leading to the formation of a helical chain. The aromatic

rings have been omitted for clarity.

Table 6 Selected geometrical parameters that define the geometry of the bis-amide unitsa

N…N/A Cg1–Cg2b/A Ph1…Ph2c/u CNO(amide)-Phd/u N–C…C–Ne/ub CO–CO(amide)f/u CN–CN(amide)g/uCrystal PackingThrough H-Bonds

2.943(3) 7.417(4) 88.21(9) 15.5(2)/ 60.5(3) 61.2(2) 89.5(2) Alternatingbelt-like chain14.30(10)

3.384(3) 8.146(2) 18.54(8) 25.88(13)/ 95.9(2) 84.0(2) 82.2(2) Zig-zag chain30.70(14)

5.127(3) 9.427(5) 60.79(7) 12.7(2)/ 65.5(8) 17.3(2) 73.5(2) Regularbelt-like chain7.5(2)

2.876(2)/ 7.105(7)/ 78.37(8)/ 6.02(13) 2.5(3)/ 85.88(14)/ 82.0(2)/ Dimers12.64(10)

2.878(3) 6.964(8) 77.80(7) 11.93(15) 3.2(4) 85.03(14) 84.1(2)11.24(12)

2.802(4) 5.207(4) 24.00(13) 17.6(2) 5.4(3) 55.7(2) 65.7(3) Monomers2.4(4)

a Two crystallographically independent bis-amide units are present in the fourth compound (phenylene acts as bridge). b Separation betweenthe centroids belonging to the phenyl rings from each half of the molecule. c Dihedral angle between the mean planes defined by phenyl ringsfrom each branch of the bis-amide unit. d Dihedral angle between the mean planes defined by a phenyl ring and the CNO group within thesame branch of the molecule. e Torsion angle defined by the amide nitrogen atoms and the carbons belonging to the central bridge bonded tothem. f Angle between the vectors defined by the amide carbonyl groups from each branch of the molecules. g Angle between the vectorsdefined by the amide CN groups from each branch of the molecule.

This journal is � The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 746–752 | 749

The separation between nitrogen atoms N(1)…N(2)

[3.383(3) A] is shorter than in 1, as expected, due to the

shorter length of the organic substituent in 2 in comparison to

that of 1. Within the bis-amide unit, the values of the dihedral

angles between the mean planes of the amide group and the

phenyl ring within each half of the molecule are 25.70(13)u[C(1)–C(6)/O(3)–C(7)–N(1)] and 30.70(15)u [C(14)–C(19)/

O(4)–C(13)–N(2)]. The value of the centroid…centroid separa-

tion and that of the dihedral angle between the planes defined

by phenyl rings from each branch of the bis-amide unit are

8.146(2) A and 18.54(8)u, respectively. The torsion angle of the

alkyl bridge is 295.9(2)u [N(1)–C(8)–C(12)–N(2)]. As in 1, the

bis-amide group adopts the anti conformation. Each bis-amide

unit is connected to other ones through hydrogen bonds

[N(2)…O(3)i; (i) x 2 0.5, 2y + 0.5, z + 0.5] affording helical

chains that run along the c-axis (Fig. 5). The period of the

helical chain is 12.087(4) A [C(2)…C(2)(x + 1, y, z)]. In

addition each helix is interconnected with other four ones

through C–H…p interactions [the values of the H…centroid

separations ranging from 2.96(8) to 3.40(7) A] (Fig. 6a). These

interactions run in the same directions that the hydrogen

bonded helix that is along the c-axis leading to a zip-like struc-

ture (see Fig. 6b). Finally methyl…methyl hydrophobic inter-

actions connect adjacent helixes and contribute to stabilize a

3-D packing, the CH3…CH3 separations being 3.811(5) A.

Let us carry out a little survey of the structural features of

the bis-(3,5-di-tert-butyl-2-hydroxybenzamides) focusing on

hydrogen bonds. Some structural parameters of compounds

1 and 2 together with those of the previously reported related

compounds16 are compared in Table 6. Scheme 2 shows the

different hydrogen patterns exhibited by these compounds.

It deserved to be noted that ligands are able to pack,

affording hydrogen bonding chains only when the central

organic bridge is non-aromatic. The aromatic groups make the

molecule more rigid (see the values of the N–C…C–N torsion

angles in Table 6) and introduce steric effects that preclude the

formation of chains through hydrogen bonds.

Fig. 6 Two adjacent chains of 2 running along the c-axis showing (a)

the C–H…p interactions and (b) space-filling.

Scheme 2 Packing through hydrogen bonds of the bis-amide units for

each compound belonging to bis-(3,5-di-tert-butyl-2-hydroxybenz-

amides). Hydrogen bonds are drawn as blue broken lines. For the

sake of clarity, tert-butyl groups have been omitted.

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Concerning the intramolecular weak interactions, in all

complexes, the phenol and amide oxygens are involved in

hydrogen bonds within each moiety of the molecule, making

the phenyl and amide groups roughly coplanar [the dihedral

angle between the two groups ranging from 2.4(4)–30.70(14)u].On the other hand, when the central organic bridge is short

enough, amide nitrogens are involved in intramolecular

interactions connecting the two moieties of the bis-amide

molecules. The size of the organic bridge is directly related

with the N…N distance and the separation between the

centroids belonging to the aromatic rings of each branch of the

molecule. It deserves to be noted that the shortest N…N

separation is observed for the naphthylidene group although

there are two shorter bridges (ethylene and phenylene groups).

The explanation for this feature is the formation of a different

intramolecular hydrogen bond, involving the two amide

nitrogens in this compound. In the case of 2, the long N…N

separation avoids the formation of intramolecular interactions

involving the amide nitrogens, thus being the key factor in the

formation of the hydrogen–bonded regular belt-like chains in

this compound, because of the freedom of the two amide

groups to form intermolecular interactions. Also in 2, the bis-

amide groups seem to arrange in a quasi-parallel fashion, the

angle between the carbonyl groups belonging to the two

different moieties of the molecules being 17.3(2)u. It deserves to

be pointed out that this angle ranges from 55.7(2) to 85.88(14)ufor the other compounds where regular belt-like pattern have

not been observed and it has a value for 7.55(11) and 30.6(2)ufor two related compounds18 [N,N9-bis(benzoyl)-(1R,2R)-

diaminocyclohexane and N,N9-bis(isonicotinoyl)-(1R,2R)-dia-

minocyclohexane, respectively].

Regarding intermolecular hydrogen bonds, amide nitrogen

and oxygen atoms are always involved. Interestingly, belt-like

hydrogen bonds involving amide nitrogens and oxygens are

the main intermolecular pattern that appears in these families

of compounds. The steric effects, due to the nature of the

central organic substituent, are responsible for the non-

formation of this hydrogen pattern in the naphthylidene and

phenylene compounds. As commented above, a 1-D hydrogen

pattern appears when the central organic bridge is non-

aromatic. When the ethylene group acts as an organic bridge,

the phenol oxygen participates in the intermolecular interac-

tions leading to alternating belt-like chains. For the 2,29-

dimethylpropylene compound, zigzag chains are observed.

Finally, for the tetramethylene compound regular belt-like

chains are observed as for other related compounds where no

intramolecular interactions connecting the amide branch’s of

the molecule appear.26

In conclusion, the nature and the size of the central moiety

which bridges the amide branches became a key factor for the

formation of intra- and intermolecular hydrogen bonds and

consequently is the main factor responsible for the crystal

packing of the molecule.

Summary

In this paper, we tried to present the changes in the crystal

packing issued from a conformational modification in the

structure. In some cases, one may predict the connectivity

pattern and thus the geometry, topology, and directionality of

the molecular networks. Our experience showed that, as one

may expect, with our present level of knowledge and under-

standing, we are unable to predict the conformation

adopted by flexible ligands. However, as stated by Dunitz,1b

by considering a crystal as a ‘‘supermolecule par excellence’’,

using concepts and principles developed in crystal engineer-

ing,3 one may predict some of the connectivity patterns and

thus the formation of molecular networks, through the control

of intermolecular interactions. After many years of intense

research efforts by several groups, nowadays one may design

molecular networks in the crystalline phase with an acceptable

degree of precision. So far the description of molecular

networks is based on geometrical features. A further important

issue is obviously to use this knowledge for the design of

molecular networks with predicted properties. In other terms,

the molecular tectonics approach must extend structural

networks to functional networks. This aspect is of outstanding

importance in terms of applications and it remains a challenge.

Acknowledgements

This work was supported by University of Concepcion (PDI

205.023.041-1.0 and 205.023.042-1.0). CONICYT provided a

doctoral scholarship for Mr. C.A.J. Financial support from

the Spanish Ministerio de Educacion y Ciencia through

projects MAT2004-03112, CTQ2004-03633 and ‘‘Factorıa de

Cristalizacion’’ (Consolider-Ingenio2010, CSD2006-0015) and

the Generalitat Valenciana (Grupos 03/197) is gratefully

acknowledged. F.S.D acknowledges a postdoctoral fellowship

(‘‘Grandes Instalaciones’’) from the Spanish Ministerio de

Educacion y Ciencia.

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