Published: October 11, 2011 r2011 American Chemical Society 5649 dx.doi.org/10.1021/cg2012028 | Cryst. Growth Des. 2011, 11, 5649–5658 ARTICLE pubs.acs.org/crystal Crystal Engineering Studies with Monocarboxamidoalkanes Having C- or N-Terminal Pyridine and Their Coordination Complexes Gargi Mukherjee and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India b S Supporting Information ’ INTRODUCTION The exploration and exploitation of various supramolecular synthons are of utmost importance in crystal engineering. 1 The understanding of synthon interference in the crystal structures of molecules containing multiple functional groups is expected to help the prediction of crystal structures of complex molecules. 2 The systematic study of the crystal structures of a class of analogous compounds is a well-recognized strategy employed in crystal engineering to achieve such understandings. 3 We have previously explored the recognition patterns of N,N 0 -bis- (pyridyl)alkanediamides (amides) and N,N 0 -bis(pyridylcarbo- xamido)alkanes (reverse amides), which have the same combi- nation of functional groups at the molecular level and behave very differently at supramolecular level. 4 In the case of amide analogues, it was shown that the pyridine group does not show any interference in amide-to-amide hydrogen bonds. In contrast, the pyridine group in reverse amide analogues does interfere in the amide-to-amide recognition pattern and forms NH 333 N- (py) hydrogen bonds. From these studies, geometric criteria were evolved that the interplanar angle (θ) between the aryl (R) and the amide planes should be above 20° to form amide-to- amide hydrogen bonds. In this contribution, our focus is on a similar class of compounds, monopyridyl amides (amides, 1 and 2, and reverse amides, 3 and 4) that contain one each of pyridine and amide functional groups. It is important to note here that bis amido pyridine derivatives have two H-donors and four H-ac- ceptors, which are capable of forming four hydrogen bonds each (Chart 1), whereas monopyridyl amides (14, Chart 2) have one H-donor and two H-acceptors. The reduced number of hydrogen bonds in monopyridyl amides (14) is expected to reduce the complexities involved in bis amides, which are observed to form a β-sheet and two-dimensional layers. It is interesting to note that in the case of bis-amido pyridines, the β-sheets observed in the organic compounds have been successfully transferred into the coordination networks. 5 Furthermore, the amides and reverse amides were shown to form iso-structural coordination networks in particular when the spacers between the amido-pyridyl groups are larger. 6 Accordingly, here, our studies are aimed at understanding the following aspects in comparison with those of bis-amides: 1 Does the pyridine interference or lack of it follow similar trends as those observed in bis-amides? 2 Does the formation of amide-to-amide hydrogen bonds depend on the interplanar angle (θ) between aryl and pyridine groups? 3 Do the mono-amides and reverse amides form the iso- structural coordination complexes? ’ RESULTS AND DISCUSSION Compounds 14 were synthesized by employing two well- known procedures of amide synthesis. The first method is the reaction of the hydrochloride salt of aliphatic amine with the hydrochloride salt of the corresponding acid chloride in the presence of pyridine. 7 The second method is the coupling of acid chloride and the corresponding amine. The crystallization of the compounds 14 in various solvents resulted in single crystals suitable for X-ray diffraction. The crystal structure of 2a is Received: September 14, 2011 Revised: October 10, 2011 ABSTRACT: Crystal structures of series of monopyridyl amides (14) are analyzed and compared with those of N, N 0 -bis(pyridyl)alkanediamides (amides) and N,N 0 -bis(pyridyl- carboxamido)alkane (reverse amides). The amide analogues (1 and 2) are found to form amide-to-amide NH 333 O hydro- gen bonds, whereas the reverse amide analogues (3 and 4) exhibit NH 333 N hydrogen bonds. Furthermore, complexation reac- tions of 14 have been carried out with various metal salts, and the hydrogen-bonding patterns in these crystal structures have been analyzed to compare with those observed in 14. The analyses and rationalization of these structures and related derivatives in the Cambridge Structural Database suggested that amide-to-amide recognition is mostly observed with halide metal salts.
Transcript
Published: October 11, 2011
r 2011 American Chemical Society 5649 dx.doi.org/10.1021/cg2012028 | Cryst. Growth Des. 2011, 11, 5649–5658
ARTICLE
pubs.acs.org/crystal
Crystal Engineering Studies with Monocarboxamidoalkanes HavingC- or N-Terminal Pyridine and Their Coordination ComplexesGargi Mukherjee and Kumar Biradha*
Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
bS Supporting Information
’ INTRODUCTION
The exploration and exploitation of various supramolecularsynthons are of utmost importance in crystal engineering.1 Theunderstanding of synthon interference in the crystal structures ofmolecules containing multiple functional groups is expected tohelp the prediction of crystal structures of complex molecules.2
The systematic study of the crystal structures of a class ofanalogous compounds is a well-recognized strategy employedin crystal engineering to achieve such understandings.3 We havepreviously explored the recognition patterns of N,N0-bis-(pyridyl)alkanediamides (amides) and N,N0-bis(pyridylcarbo-xamido)alkanes (reverse amides), which have the same combi-nation of functional groups at the molecular level and behavevery differently at supramolecular level.4 In the case of amideanalogues, it was shown that the pyridine group does not showany interference in amide-to-amide hydrogen bonds. In contrast,the pyridine group in reverse amide analogues does interfere inthe amide-to-amide recognition pattern and forms N�H 3 3 3N-(py) hydrogen bonds. From these studies, geometric criteriawere evolved that the interplanar angle (θ) between the aryl (R)and the amide planes should be above 20� to form amide-to-amide hydrogen bonds. In this contribution, our focus is on asimilar class of compounds, monopyridyl amides (amides, 1 and2, and reverse amides, 3 and 4) that contain one each of pyridineand amide functional groups. It is important to note here that bisamido pyridine derivatives have two H-donors and four H-ac-ceptors, which are capable of forming four hydrogen bonds each(Chart 1), whereas monopyridyl amides (1�4, Chart 2) haveone H-donor and two H-acceptors. The reduced number ofhydrogen bonds in monopyridyl amides (1�4) is expected toreduce the complexities involved in bis amides, which are
observed to form a β-sheet and two-dimensional layers. It isinteresting to note that in the case of bis-amido pyridines, theβ-sheets observed in the organic compounds have beensuccessfully transferred into the coordination networks.5
Furthermore, the amides and reverse amides were shown toform iso-structural coordination networks in particular whenthe spacers between the amido-pyridyl groups are larger.6
Accordingly, here, our studies are aimed at understandingthe following aspects in comparison with those of bis-amides:1 Does the pyridine interference or lack of it follow similartrends as those observed in bis-amides?
2 Does the formation of amide-to-amide hydrogen bondsdepend on the interplanar angle (θ) between aryl andpyridine groups?
3 Do the mono-amides and reverse amides form the iso-structural coordination complexes?
’RESULTS AND DISCUSSION
Compounds 1�4 were synthesized by employing two well-known procedures of amide synthesis. The first method is thereaction of the hydrochloride salt of aliphatic amine with thehydrochloride salt of the corresponding acid chloride in thepresence of pyridine.7 The second method is the coupling of acidchloride and the corresponding amine. The crystallization of thecompounds 1�4 in various solvents resulted in single crystalssuitable for X-ray diffraction. The crystal structure of 2a is
Received: September 14, 2011Revised: October 10, 2011
ABSTRACT: Crystal structures of series of monopyridylamides (1�4) are analyzed and compared with those of N,N0-bis(pyridyl)alkanediamides (amides) and N,N0-bis(pyridyl-carboxamido)alkane (reverse amides). The amide analogues(1 and 2) are found to form amide-to-amide N�H 3 3 3O hydro-gen bonds, whereas the reverse amide analogues (3 and 4) exhibitN�H 3 3 3N hydrogen bonds. Furthermore, complexation reac-tions of 1�4 have been carried out with various metal salts, andthe hydrogen-bonding patterns in these crystal structures havebeen analyzed to compare with those observed in 1�4. Theanalyses and rationalization of these structures and relatedderivatives in the Cambridge Structural Database suggested that amide-to-amide recognition is mostly observed with halidemetal salts.
reported earlier, and its coordinates were taken from theCambridge Structural Database (CSD) for comparative analysis.8
We were not successful in synthesizing the compounds 1b and 2b.Furthermore, complexation reactions of 1�4 with various metalsalts have been carried out with the goal of understanding thesubtleties in transfer of molecular recognition information fromorganic materials to their coordination complexes and also tostudy the robustness of the hydrogen-bonding patterns in thepresence of counteranions. However, crystals suitable for singlecrystal X-ray diffraction studies were obtained only in the case of3a and 3b. Six coordination complexes (5�10) of 3a and 3bwere synthesized by mixing the methanolic solution of the metalsalt with the methanolic solution of the ligand. Crystallinecomplexes were obtained by slow evaporation of the solutionat room temperature. The CSD Studies on molecules containing
monoamido pyridines have been carried out for both organicsand inorganic structures to compare with the results obtainedhere. The pertinent crystallographic details, geometrical para-meters of strong hydrogen bonds involving amide functionality,and interplanar angles of compounds 1�4 and complexes 5�10are given in Tables 1�5, respectively.Amide-to-Amide Hydrogen Bonds in 1a and 2a. Com-
pound 1a crystallizes in the triclinic crystal system with spacegroup, P1. The molecules are assembled via amide-to-amidehydrogen bonds, indicating no interference from the pyridinemoiety (Figure 1). The hydrogen-bonding pattern of 1a is similarto the one observed in 2a. These two structures differ insecondary interactions: In 1a, the pyridine�N is involved inhydrogen bonding with the methyl protons, while in 2a, it ishydrogen bonded to pyridine �CH group.Interference of Pyridine in 3 and 4. Both 3a and 3b crystal-
lize in the monoclinic, P21/n space group (Figure 2). Theasymmetric unit of 3a contains two molecules each of 3a andwater, while that of 3b has two molecules of 3b and one water.The inclusion of water in the crystal lattice in 3a and 3b is inagreement with our previous observation that among all fourclasses of compounds only 4-pyridyl derivatives of bis-amides(reverse) includes water. Compound 3a contains one water
Chart 1. Hydrogen-Bonding Patterns: (a) Ribbon in Mono-amides and (b) β-Sheet in Bis-amides
Chart 2
Table 1. Crystallographic Parameters for Compounds 1a, 3a, 3b, 4a, and 4b
compds 1a 3a 3b 4a 4b
formula C7H8N2O C7H10N2O2 C16H20N4O3 C7H8N2O C8H10N2O
mol. wt. 136.15 154.17 316.36 136.15 150.18
T (K) 293(2) 293(2) 293(2) 293(2) 293(2)
system triclinic monoclinic monoclinic monoclinic monoclinic
space group P1 P21/n P21/n C2/c P21/n
a (Å) 8.409(4) 11.936(2) 7.252(3) 15.070(9) 13.6085(2)
b (Å) 9.724(4) 9.768(2) 26.884(1) 7.742(4) 8.1708(9)
c (Å) 9.984(4) 14.267(3) 8.724(3) 12.065(7) 15.5985(2)
molecule per formula unit and does not exhibit an amide-to-amide hydrogen bond in its crystal structure, as both the CdOand the N�H groups of the amide are engaged in hydrogen-bonding interactions with water. The water molecules join theamide molecules into 2D layers. These 2D layers are furtherpacked via C�H 3 3 3N and C�H 3 3 3π interactions in an alter-nate fashion.The water exhibits identical 3-coordination with respect to
hydrogen bonds in both of the structures. However, both of thestructures differ significantly in their hydrogen-bonding patterns,probably due to the difference in number of water moleculespresent in the crystal lattice. The two symmetry independentmolecules of 3b (A and B) have different set of interactions.Molecule A is hydrogen bonded to two water molecules and oneamide �NH group, whereas molecule B is hydrogen bonded toone water and one pyridyl group (Figure 2d). Thus, unlike 3a,the crystal structure of 3b contains N�H 3 3 3N hydrogen bondsalong withN�H 3 3 3Ow andOw�H 3 3 3O interactions (w-water).Overall, 3a forms flat 2D layers, and 3b forms corrugated 2D layersvia strong hydrogen bonds. These layers are further connectedwith adjacent layers via C�H 3 3 3O hydrogen bonds betweenamide carbonyl and pyridine �CH groups.Compounds 4a and 4b crystallize in monoclinic C2/c and
P21/n space groups, respectively. In both of the cases, themolecules are assembled via N�H 3 3 3N hydrogen bonds(Figure 3). These hydrogen-bonding patterns are reminiscentof those observed in 3-pyridyl analogues of bis-amides (reverse).The amide carbonyls are engaged in C�H 3 3 3Ohydrogen bondswith pyridine C�H from the neighboring 1D chains, leading to a2D layer. The adjacent layers are packed opposite to one anothervia edge-to-edge and C�H 3 3 3π interactions in 4a and 4b,respectively.Hydrogen-Bonding Patterns in the Coordination Com-
plexes of 3a and 3b. The reaction of 3a with AgNO3 in
methanol resulted in crystals of the complex [Ag(3a)2](NO3),5 (Figure 4). In this complex, the asymmetric unit is constitutedby one each of Ag(I), NO3
�, and two units of 3a. Two pyridinemoieties are linearly coordinated to Ag atoms and two such unitscome closer via Ag 3 3 3Ag interaction (Ag 3 3 3Ag, 3.159 Å), givingrise to a dimeric aggregate. These dimeric aggregates are linked toeach other through amide-to-amide hydrogen bonds forming astair caselike 1D chain. Adjacent chains are linked through nitrateanions via C�H 3 3 3O hydrogen bonds with pyridine �CH.Furthermore, face-to-face aromatic interactions between thepyridine moieties also involved in governing the 3D packing. Itis interesting to note here thatN-(4-pyridyl)benzamide (NPBA)exhibits similar coordination to Ag(I) in [Ag(NPBA)2](NO3).
9
In this complex, one nitrate anion is coordinated to silver, and thedimers are assembled through N�H 3 3 3O interactions betweenthe coordinated nitrate and the amide proton. In contrast to 5, noamide-to-amide contact is observed in this structure.Complex [Cu(3a)2(NO3)2], 6 (Figure 5), is obtained from 3a
and copper nitrate in methanol. In 6, Cu(II) adopts the distortedoctahedral geometry, but interestingly, here, the ligands are syncoordinated, generating a V-shaped geometry. These V-shaped
Table 4. Crystallographic Parameters of Complexes (5�10)
compds 5 6 7 8 9 10
formula C28H32Ag2N10O10 C14H16CuN6O8 C18H22N4O6Cu C20H24N4O6Cl2Cu C18H24N4O7Cd C14H16N4O2I2Cd
units are assembled via N�H 3 3 3O hydrogen bonds betweenamide �NH and O-atom of nitrate ions to form a 1D chain.Furthermore, the adjacent chains interact with each other viadipole�dipole interactions between amide carbonyls (C 3 3 3Odistance, 3.297 Å) to form a 2D layer in bc-plane. The methylgroups of acetyl are not involved in C�H 3 3 3O interactions withthe CO groups of acetyl (H 3 3 3O, C 3 3 3O, and C�H 3 3 3O:3.208 Å, 3.530 Å, and 101�).We note here that a similar type of cis-coordination around
Zn(II) was observed in Zn(NPBA)2(OAc)2.9 In the Zn(II)
complex, π 3 3 3π stacking interactions of pyridine rings also takepart in the formation of 1D chain in addition to the aforemen-tioned N�H 3 3 3O hydrogen bonds between amide �NH andO-atom of anion.When the counteranion is changed from nitrate to acetate, 3a
gives a crystalline complex, [Cu(3a)2 (OAc)2], 7 (Figure 6).Single crystal X-ray analysis reveals that the Cu(II) has a squareplanar geometry, but here, the two pyridyl ligands are co-ordinated trans to each other, while the two acetate anionsoccupy the other two positions in trans fashion. The amideprotons are hydrogen bonded to one of the O-atom of acetate(N�H 3 3 3O), resulting in a 1D chain. Pyridyl groups areinvolved in face-to-face aromatic interaction and have a centroidto centroid distance of 3.322 Å. The amide carbonyls are engagedin C�H 3 3 3O hydrogen bonds with pyridine C�H groups fromthe neighboring chains to form a 2D layer. Overall, these 2Dlayers are further assembled via hydrophobic interactions be-tween the methyl groups of acetates from adjacent layer
(Figure 6e). The coordination around Cu(II) in 7 resemblesthat in trans-[Cu(NPBA)2(OAc)2], and in both of the cases, the1D chains are bound via C�H 3 3 3O interactions between theamide keto and the pyridine ring protons.9
Complex [Cu(3b)2(ClCH2CO2)2], 8, exhibits a similar typeof 1D chain as observed in complex 7 (Figure 6b). In thiscomplex, the 1D chains are joined together via hydrophobicinteractions between ethyl groups of 3b and C�H 3 3 3Cl inter-actions. No Cl 3 3 3Cl interactions were observed (Figure 6d).Complexation of 3a with cadmium acetate under similar
conditions to above reactions afforded complex [Cd(3a)2-(OAc)2(H2O)], 9 (Figure 7). Cd(II) exhibits pentagonal bipyr-amid geometry as the equatorial plane contains two acetates andone water molecule, while the axial positions are coordinated bypyridine units. Each cluster is hydrogen bonded to five otherclusters via eight noncovalent interactions: two amide-to-amideN�H 3 3 3O, two acetate to amide N�H 3 3 3O, and four acetateto water O�H 3 3 3O hydrogen bonds (Figure 7b). Joining themetal centers of the neighboring hydrogen bonded units reveals a5-connected 3D network with 44.66-sqp topology (Figure 7c).10
The nodes exhibit a distorted square pyramidal geometry, withvaried Cd 3 3 3Cd distances of 14.907, 14.907, 9.405, 5.973, and5.973 Å. The base of the pyramid is rather a rectangle than asquare. We note here that the networks with five-connectednodes are very rare, and so far, only two usual varieties are knownto exist. One of which is the bnn network that was observedin Mn[C(CN)3]2 L, where L=1,2-bis(4-pyridyl)ethane-N,N0-dioxide,10a whereas the second type is the one observed here,
Figure 1. Illustrations for the crystal structures of 1a and 2a: geometry of the ligand in (a) 1a and (b) 2a; 2D layer via N�H 3 3 3O and C�H 3 3 3Nhydrogen bonds in (c) 1a and (d) 2a; packing of the adjacent layers (e) via alternate C�H 3 3 3Nhydrogen bond and hydrophobic interaction and (f) viahydrophobic interactions.
and recently, it was found to occur in the complex [Cu14I14-(dabco)5(pyridine)]n.
10b However, in that example, the net-works were found to doubly interpenetrate to fill the channels.In the present case, the hexagonal channels of the network areoccupied by the methyl groups. Recently, coordination polymerscontaining a new variety of five connected Archimedean 2D layertopology are reported.10f,g
Because the nitrate and acetate anions are interfering intoamide to amide hydrogen bonds, we thought to usemetal halides,which are less prone to interfere in amide-to-amide recognitionpatterns.The crystalline complex of 3a was obtained with cadmium
iodide as [Cd(3a)2I2], 10 (Figure 8). Cd(II) exhibits tetrahedralcoordination geometry and is coordinated to two iodine and twopyridine units. Each discrete unit is hydrogen bonded to twoneighboring units via N�H 3 3 3 I hydrogen bond forming a 1Dchain (Figure 8a). Furthermore, these units interact with eachother through edge-to-edge aromatic interactions (closest CCdistance, 3.536 Å) (Figure 8c). The 3D packing is governed bybifurcated iodide anions through weakN�H 3 3 3 I and C�H 3 3 3 I(3.065, 133.74, and 3.768 Å�) interactions leading to a 3Dstructure. No amide-to-amide recognition is observed in thiscomplex also.
CSD Studies. The CSD studies11 were performed to ratio-nalize the results obtained here and to understand the followingaspects.a. Probability of cis and trans Coordination of Nitrate and
Acetate Anions. CSD (version 5.32) was searched for discretecoordination complexes containing ML2A2 or ML2A2S2 formu-las with any transition metal (M), pyridine, or with 4-substitutedpyridines (L), methanol, or water (S) and acetate or nitrate (A).For the ML2A2 formula, after manual screening of large
subsets, five complexes were found with nitrate and 11complexes were found with acetate ions. The ligands foundto exhibit cis geometry in three {NIPYZN [Zn(II)], TABBII[Zn(II)], ICUVEH [Pt(II)]} out of four nitrate complexesand in four {ZZZPKM01 [Zn(II)], MOHRAC [Zn(II)],FABFAR [Zn(II)], and LEVPAE [Zn(II)]} out of 11 acetatecomplexes.9,12,13
Similarly, for the ML2A2S2 formula, it was found that there aretotal of 13 structures with nitrate and nine structures with acetate.Interestingly, all of these nitrate and acetate complexes were found tocontain trans coordination of the ligands or anions around metals[Cu(II), Ni(II), Zn(II), Co(II), and Mn(II)].9,14,15 These resultssuggest that complex 6 is the first example of a complex containingcis-coordination of ligands and as well as nitrate anions havingCu(II)
Figure 2. Illustrations for the crystal structures of 3a and 3b: geometry of the ligands in (a) 3a and (b) 3b; 2D layer involving water molecules in (c) 3aand (d) 3b; packing of the adjacent layers through C�H 3 3 3N and C�H 3 3 3π interactions in (e) 3a and (f) 3b; 3D packing in (g) 3a and (h) 3b.
as themetal atom, as all of the related cis complexes are reportedwithZn(II), except ICUVEH.11 We note here that CSD studies revealthat cis coordination is more common with Zn(II) (five out of five)but not with Cu(II) (zero out seven).b. Occurrence of Amide-to-Amide Hydrogen Bonds in Deri-
vatives Containing Pyridine and Amide One Each. A CSDsearch was performed on the molecules having the fragments
I�IV (Chart 3) with single amide functionality to analyze theinterference of pyridine on amide-to-amide hydrogen bond inthe published crystal structures of monoamido-pyridyl deriva-tives. Molecules having interfering groups and bulky substituentswere not considered. The CSD contains five, three, three, andseven crystal structures for fragments I�IV, respectively. All ofthe structures containing fragments I and II were found to form
Figure 3. Illustrations for the crystal structures of 4a and 4b: geometry of the ligand (a) 4a and (b) 4b; 2D layer via N�H 3 3 3N and C�H 3 3 3Ohydrogen bonds in (c) 4a and (d) 4b; 3D packing (e) in 4a through edge-to-edge aromatic interactions and (f) in 4b through C�H 3 3 3π interactions.
Figure 4. Illustrations for the crystal structures of 5: (a) dimeric units assembled via amide-to-amide hydrogen bonds and (b) 3D packing throughpyC�H 3 3 3O (nitrate) hydrogen bonds.
amide-to-amide hydrogen bonds.16,17 Furthermore, all of thesecompounds were found to exhibit interplanar angle (θ) of above20� between amide and pyridine planes. For fragment III, twoform amide-to-amide hydrogen bonds out of three structures andhave θ values greater than 20�. FOVCUP forms N�H 3 3 3Nhydrogen bonds and also has θ value (16.62�) of less than 20�.18In contrast to the structures of fragments I�III, the crystalstructures of IV prefer to assemble via N�H 3 3 3N hydrogenbonds (five out of seven) over amide-to-amide hydrogen bondsand also was found to have θ values less than 20�.19
From the CSD analyses, it clearly appears that for fragments Iand II, whatever be the substituent on the amide �NH, theinterplanar angle (θ) is greater than 20�, and consequently,amide-to-amide contacts are favored in all of the structures,even in some cases in the presence of strong hydrogen-bondinggroups also. However, this is not the case with the fragments IIIand IV. With the fragments III and IV, all of the moleculesavailable in the literature have aromatic substituents on theamide �NH, and depending on the θ values, the moleculesexhibit N�H 3 3 3N hydrogen bonds (out of 10, six involve
Figure 6. Illustrations for the crystal structures of 7 and 8: tapelike 1D chain viaN�H 3 3 3O(acetate) hydrogen bonds in (a) 7 and (b) 8; 2D layer (c) viaC�H 3 3 3O hydrogen bonds in 7 and (d) via hydrophobic interactions between ethyl groups in 8; (e) 3D packing through hydrophobic interactionsbetween the methyl groups of acetates in 7.
Figure 5. Illustrations for the crystal structure of 6: (a) assembling of hydrogen-bonded chains via dipole�dipole interactions between amide carbonylsand (b) 3D packing via C�H 3 3 3O hydrogen bonds.
pyridine interference). These results are in agreement with ourprevious studies on bis-amide- and bis-pyridine-containingderivatives.c. Role of Anions in Amide-to-Amide Recognition. A CSD
search on the fragments I �IV coordinated to any transitionmetal was carried out to explore the role of anions in amide-to-amide recognition. Similar constraints to the searches that aredescribed in section a were applied.A total of eight and six crystal structures were found containing
acetate and nitrates, respectively.9,20,21 Among the eight structureswith acetates, only one entry, namely, MOHRIK (N�H 3 3 3O =3.334Å), exhibits amide-to-amide recognition,while in the remainingseven structures, amide proton hydrogen bonds to the noncoordi-nated O of acetate. Nitrate anions also were found to disrupt theamide-to-amide recognition in a similar fashion; six out of six contain
anion interference. However, in one of these six (DUQTOZ),amide-to-amide hydrogen bond as well as anion interference wasfound.21 In contrast, amide-to-amide hydrogen bond was found tobe more prominent in the structures containing halide ions (threeout of six). Two of the three structures (ALARAH and ALAQUA)do not form amide-to-amide hydrogen bonds due to the presence
Figure 8. Illustrations for the crystal structure of 10: (a) 1D chain via N�H 3 3 3 I hydrogen bonds, (b) 2D layer via CH 3 3 3O hydrogen bonds, and (c)3D packing via N�H 3 3 3 I and C�H 3 3 3O hydrogen bonds.
Figure 7. Illustrations for the crystal structure of 9: (a) five-coordinated node formation; (b) top view of the sqp net; and (c) schematic representationof 3D 44.66-sqp topology.
Chart 3. CSD Search on Organic Molecules Having Frag-ments I�IV and Their Coordination Complexes
of DMF in the crystal lattice.22 In HEBVIU, the amide groupexclusively exhibits N�H 3 3 3Cl hydrogen bonding with amideN�H, and similar hydrogenbondingwas also found inALARAH.22,23
Some of the common patterns observed in the above-describedcrystal structures are shown inChart 4. TheCSD analysis indicatesthat nitrate and acetate have more propensities to interfere inamide-to-amide recognition as compared to halide ions.
’CONCLUSION
The studies show that monoamides follow the same structuraltrend as observed previously in case of bis-amides. Here also, onlythe amide derivatives of monoamides exhibit amide-to-amidehydrogen bonding. The 3-pyridyl reverse amides derivatives areexclusively assembled via N�H 3 3 3N hydrogen bonding, whereas4-pyridyl analogues exhibit the tendency to include water mol-ecules in the crystal lattice. As of bis-amides, monoamides havealso exhibited similar preference for angle (θ < 20�) to amide-to-amide hydrogen bond. Only deviation occurs in 1a that has twomolecules in the asymmetric unit and thus two θ values, one ofwhich is greater than 20�. From the CSD data, it is observed thatgenerally reverse amides prefer planar geometry as compared totheir amide analogues, and in most of the cases, the θ value is lessthan 20� (36 out of 45). Also, the results observed here clearlyindicate that amide-to-amide hydrogen bond formation occursonly if the θ value is greater than or equal to 20�.
In the case of coordination complexes depending upon the metalatom and counteranions, 3a can adopt a variety of supramolecularstructures. The amide-to-amide hydrogen bonding is not observed in6, 7, 8, and 10 as the anions, that is, nitrate, acetate, or iodide arehydrogen bondingwith amide functionality. However, in 5 and 9, theinterference of anions is partial as one ligandparticipates in the amide-to-amide hydrogen bond, while the other interacts with anions.
’ASSOCIATED CONTENT
bS Supporting Information. Experimental details, IR spec-tra, elemental analyses, and CSD tables. This material is availablefree of charge via the Internet at http://pubs.acs.org.
We gratefully acknowledge the DST for the financial supportand DST-FIST for the single-crystal X-ray facility. G.M. thanksCSIR for a research fellowship.
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