Hydrogen bonded Schiff bases; di-anil of 2-hydroxy-5-methyl-isophthaldehyde

Krzysztof Wozniaka,* , Waclaw Kolodziejskib, Romana Anulewicza, Dorota Pawlaka,Karol Jackowskia, Teresa Dziembowskac, Zbigniew Rozwadowskic

aDepartment of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, PolandbDepartment of Inorganic and Analytical Chemistry, Warsaw Medical Academy, ul. Banacha 1, 02-097 Warsaw, Poland

cTechnical University of Szczecin, ul. Piasto´w 42, 71-065 Szczecin, Poland

Received 14 November 1997; accepted 2 November 1998

Abstract

Structural and spectroscopic properties of solid dianil of 2-hydroxy-5-methyl-isophthaldehyde have been investigated. Thisis a Schiff base containing intramolecular O–H…N hydrogen bonding. Single crystal X-ray diffraction shows that the possiblyequivalent hard structural parameters (bond lengths and the valence angles) of the title molecule are hardly differentiated by anasymmetric hydrogen bonding and different conformations of the anil wings.13C MAS NMR results are more sensitive and theyclearly show the influence of [O–H…N] hydrogen bonding on the chemical shifts of the central ring carbon atoms.q 1999Elsevier Science B.V. All rights reserved.

Keywords:13C MAS NMR; X-ray diffraction; Schiff bases; Anil; Bisanil; Hydrogen bond; Neutral; Ionic

1. Introduction

Imines, .C�N–R, and their derivatives,containing an aryl group at nitrogen or carbon, forma group of compounds known as Schiff bases [1].They are products of condensation of amines withaldehydes and ketones. Schiff bases and their deriva-tives are subject to ca. 500 publications per year. Theyare very popular owing to a wide range of their appli-cations in organic synthesis from reduction to amines[2], addition to.C�N bond [3], radical dimerization[4], to applications in the synthesis of lactams [5] andaddition to the carbonyl atom of ketones [6].

Schiff bases have been examined by several

spectroscopic and diffractometric methods in solutionand the solid state [7–12]. Salicylideneanilines andrelated Schiff bases with the intramolecular O–H…N hydrogen bond have received considerableattention due to their interesting photophysical prop-erties and potential applications as organic materialsfor reversible optical data storage [7,13,14].

The properties of Schiff bases result from thepresence of neutral intramolecular O–H…Nhydrogen bonding and from the conjugative interac-tions between the aromatic part and hydrogenbonding. Schiff bases have two competitive n–pand p–p conjugative systems. The hydrogen bondstrength, proton transfer equilibrium and conforma-tion of Schiff bases are very much dependent on oneanother [9,15,16].

Most of the Schiff bases studied so far have the O–H…N hydrogen bond in the crystalline state with the

Journal of Molecular Structure 478 (1999) 267–274

0022-2860/99/$ - see front matterq 1999 Elsevier Science B.V. All rights reserved.PII: S0022-2860(99)00798-4

* Corresponding author. Tel./Fax:1 48 22 8222892; e-mail:kwozniak@chem.uw.edu.pl

proton localized at the oxygen atom. In some cases theproton position is disordered over two sites—at the Nand O atoms [15,17]. Recently, the presence of ionic[O2…H–N1] H-bond in 5-nitrosalicylidene ethyla-mine has been demonstrated [18]. Correlationbetween the structural parameters and the electronicconfiguration of Schiff bases are discussed in Refs.[15–18].

Schiff bases are widely used as ligands for manytransition metal complexes [19]. The complexes ofSchiff bases derived from 2-hydroxy-isophthaldehydeare among the most extensively investigated oneswith respect to different experimental and theoreticalaspect [20,21]. In the dianils of 2-hydroxy-isophthal-dehyde (hereafter abbreviated BHMA, Fig. 1) twocompetitive basic—CH � N–Ph—centers arepresent, so a conversion of the two possible confor-mers associated with the rotation of the OH group cantake place. This breaks and re-forms the intramole-cular hydrogen bonds as can be seen in the solution(CDCl3) [22]. This reference also contains anevidence of a temperature effect (slowing down offast rotation of the phenolic OH group with thedecrease of temperature) and an estimation of therotational energy barrier for a series of bis-R-anilsof 2-hydroxy-5-methyl-isophthaldehyde (R� H, Meand OMe).

Di-anil of 2-hydroxy-5-methyl-isophthaldehyde isa very good model compound to study intramolecularelectronic and structural (conformational) effects, forexample, possible asymmetry of this molecule. Thepresence of the two possibly competitive acceptorsites in BHMA can create in the solid state either amulticentre disordered [N…H(partial)…O…H(par-tial)…N] hydrogen bonding or localized O–H…Nhydrogen bonding. The BHMA molecule is a goodmodel compound, with well defined structural pertur-bations to study structural and spectroscopic conse-quences of possible asymmetry.

2. Experimental

Bisanil of 2-hydroxy-5-methyl-isophthaldehydewas obtained by condensation of 2-hydroxy-5-methyl-isophthaldehyde with aniline [22]. Solid-state 13C NMR with cross-polarization (CP) andmagic-angle spinning (MAS) was conducted at50.3 MHz on a Varian Unity-200 spectrometer usinga high-speed double bearing probe and silicon nitriderotors spun in dry air. The single-contact13C CP/MASspectra were recorded at 300 K with an optimized-contact time of 4 ms (OCT) and with a short-contacttime of 50ms (SCT). We usedp/2 pulses of 5ms,recyle delays of 3 s and an MAS rate of 7 kHz. The13C dipolar–dephased spectra [23] (DD) wererecorded with a contact time of 4 ms and a 50msperiod without decoupling prior to acquisition. TheDD experiments expose quaternary carbon peaksand the SCT experiments highlight the peaks fromcarbons with adjacent protons. Ordinary OCT spectracontain all these peaks.

2.1. X-ray diffraction

The X-ray measurements were conducted on a KM-4 KUMA diffractometer with graphite monochro-mated CuKa radiation. The data were collected atroom temperature using av-2u scan technique. Theintensity of the control reflections varied by less than3%, and the linear correction factor was applied toaccount for this effect. The data were also correctedfor Lorentz and polarization effects, but no absorptioncorrection was applied. The structure was solved bydirect methods [24] and refined using SHELXL [25].The refinement was based onF2 for all reflections,except those with very negativeF2. Weighted Rfactors wR and all goodness-of-fitS values are basedon F2. ConventionalR factors are based onF, with Fset to zero for negativeF2. TheF2

o . 2s�F2o� criterion

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Fig. 1. Schematic illustration of (a) anils; and (b) dianils.

was used only for calculatingR factors and is notrelevant to the choice of reflections for the refinement.TheR factors based onF2 are about twice as large asthose based onF. All hydrogen atoms were locatedfrom a differential map and refined isotropically. Scat-tering factors were taken from Tables 6.1.1.4 and4.2.4.2 in Ref. [26]. Experimental details concerningthe collection and refinement of data are listed inTable 1.

3. Results and discussion

3.1. X-ray data

The bisanil of 2-hydroxy-isophthaldehyde (BHMA)crystallizes in a general position in the monoclinicP 21/c space group with four molecules in the unit cell(Table 1). The labelling of atoms in the asymmetricunit and an ORTEP illustration of atomic displacementparameters are given in Fig. 2. The positional para-meters are shown in Table 1, the bond lengths andangles in Table 2 and the numerical values of atomicdisplacement parameters in Table 3 of the Supplemen-tary Materials. The packing of the BHMA molecules inthe crystal lattice is shown in Fig. 3.

K. Wozniak et al. / Journal of Molecular Structure 478 (1999) 267–274 269

Fig. 2. The labelling of atoms in the asymmetric unit and the atomic displacement parameters. An ORTEP drawing of BHMA.

Table 1Crystal data and structure refinement details for BHMA

Empirical formula C21H18N2OFormula weight 314.37Temperature (K) 293(2)Wavelength (A˚ ) 1.54178Crystal system MonoclinicSpace group P 21/cUnit cell dimensions a � 8.703(2) Aa � 908

b � 17.984(4) Ab � 90.13(3)c � 10.556(2) Ag � 908

Volume (A3) 1652.2(6)Z 4Density (calculated) (mg/m3) 1.264Absorption coefficient (mm21) 0.617F(000) 664Crystal size (mm3) 0.20× 0.30× 0.25Theta range for data collection(8)

4.86 to 80.72

Index ranges 0# h # 10, 2 19 # k Q 0, 2

12 # l # 12Reflections collected 3691Independent reflections 3402 (R(int)� 0.0179)Refinement method Full-matrix least-squares onF2

Data/restraints/parameters 3378/0/290Goodness-of-fit onF2 1.051Final R indices [l . 2s(l)] R1 � 0.0438, wR2 � 0.1252R indices (all data) R1 � 0.0548, wR2 � 0.1466Extinction coefficient 0.0068(8)Largest diff. peak and hole(eA23)

0.249 and2 0.164

The crystal and molecular structure of BHMAreveal a few interesting findings. It appears thatthere are two asymmetric structural perturbations inthe structure of BHMA. One is the ordered asym-metric intramolecular O(1)–H(1)…N(1) hydrogenbonding typical of Schiff bases. The structural para-meters of this bonding, as well as some other shortintermolecular contacts which can be treated as weakhydrogen bonds are given in Table 2. This bondingenforces co-planarity of the central aromatic ring and

the region of hydrogen bonding. In practice, the wholeanil wing engaged in the intramolecular hydrogenbonding is co-planar with the central aromatic frag-ment. The second anil fragment—not being involvedin the intramolecular hydrogen bond—adopts thetrans conformation relative to the C(2)O(1) bondand this is the second cause of the asymmetry of theBHMA molecule.

Although there are two structural perturbationsdifferentiating the right- and the left-hand sides of

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Table 2Structural parameters of hydrogen bonds and close intermolecular contacts

Hydrogen bond Donor-H Acceptor…H Donor…Acceptor D-H…AD–H…A D–H A…H D…A (8)Sym. code (A˚ ) (A) (A)

O1–H1…N1 01–H1 H1…N1 01…N1 O1–H1…N10.97(2) 1.64(2) 2.560(3) 158(1)

C83–H83…N1 C83–H83 H83…N1 C83…N1 C83–H83…N12-X, 0.51 Y, 0.5-Z 1.03(3) 2.98(3) 3.608(2) 120(1)C83–H83…01 C83–H83 H83…01 C83…01 C83–H83…N12-X, 0.51 Y, 0.5-Z 1.03(3) 2.85(3) 3.637(2) 133(1)C76–H76…N2 C76–H76 H76…N2 C76…N2 C76–H76…N21-X, Y-0.5, 0.5-Z 1.01(3) 2.90(3) 3.892(2) 166(1)C82–H82…01 C82–H82 H82…01 C82…01 C82–H82…01X, -Y 1 1.5, 0.51 Z 1.01(3) 2.74(3) 3.327(2) 118(1)C86–H86…N2 C86–H86 H86…N2 C86…N2 C86–H86…N2X, -Y 1 1.5, -0.51 Z 0.99(3) 2.88(3) 3.778(2) 151(1)

Table 3Assignment of the13C NMR spectra of BHMA in solution and in the solid state (CPIMAS). Chemical shifts in ppm relative to TMS

Solutions

Systematic C-1 C-4 C-7 C-8 C-9 C-10 C-11designation CH3 C-3 C-2 C-6 C-5 C-70 C-80 C-90 C-100 C-110

CDCl3a 300 K 20.3 — 159.7 126.5 128.3 — — 121.1 129.3 126.5

CDCl3a 250 K 20.3 119.3 159.5 131.1 128.3 162.2 147.6 121.1 129.1 127.2

123.5 135.9 155.9 152.2 129.4 125.9CD3CN 300 K 20.5 122.8 160.5 134.6 129.5 160.2 151.2 122.2 130.4 127.6Solid stateb

Crystallographic C(1) C(4) C(7) C(71) C(72) C(73) C(74)designation C(51) C(3) C(2) C(6) C(5) C(8) C(81) C(76) C(75) C(84)

C(82) C(83)C(86) C(85)

Powder 20.4 118.2 w 159.6 wat 300 K 121.1 s 161.9 s 134-140 b 129.9 157 b 144 b 114 b 128 s,b 128 s,b

123.9 s 130.9 155.0 120 b126.9 w

a Ref. [22].b w, weak; s, strong; b, broad.

the BHMA molecule, there are practically no signifi-cant differences (except one) between the possiblysymmetric bond lengths of the BHMA molecule(Fig. 4a). Only the double N(1)C(7) bond [1.283(2)A] seems to be slightly longer than N(2)C(8)[1.271(2) A]. The difference between these bondsis in the limit of significance [ca. 4ESD(Diff.)].The elongation of the N(2)C(7) bond results fromthe involvement of the accepting N(1) atom in theO(1)–H(1)…N(1) hydrogen bonding. The strongerthe hydrogen bonding is, the more elongated theN(1)C(7) bond becomes. This is due to the fact thata part of electron density from this bond is becomingincreasingly involved in the hydrogen bond, whichdecreases the double character of the N(1)C(7)bond. In the very extreme case of the ionic hydrogenbonding [O2…H–N1] the N(1)C(7) bond becomessingular [18].

Similar symmetry exists for the valence angles(Fig. 4b). Except for those responsible for the confor-mational differences and those associated with theintramolecular hydrogen bonding, the other valenceangles of the left and right parts of the BHMA mole-cule have the same values (within the level of errors).The first group of the other anil wing—not involvedin the hydrogen bond—is also co-planar with thecentral aromatic ring. This seems to be due tosome weak intramolecular C(8)–H(8)…O(1) andC(4)–H(4)…N(2) hydrogen bonds. The presence ofthis kind of weak interaction is confirmed by thevalues of C(3)C(8)H(8) and C(3)C(4)H(4) angles[115.8(9)8 and 117.0(9)8, respectively]. These angles

are ca. 28 smaller than their almost symmetricalcounterparts [C(1)C(7)H(7) and C(1)C(6)H(6),respectively].

The most significant differences between possiblyequivalent valence angles caused by the intra-molecular hydrogen bonding are the following:O(1)C(2)C(1) � 121.2(1)8 and O(1)C(2)C(3)�119.3(1)8, C(2)C(1)C(7) � 120.9(1)8 andC(2)C(3)C(8)� 119.0(1)8, etc. Of course, due todifferent conformations of the anil fragments thereare significant differences in the values of torsionalangles describing these different conformers. Thewhole anil fragment engaged in the intramolecularhydrogen bonding is co-planar with the centralaromatic ring [C(72)C(71)N(1)C(7)� 0.1(1)8],whereas the other anil fragment has the equivalenttorsion angle C(82)C(81)N(2)C(8) equal to2141.9(1)8. The dihedral angles between the best(least-squares) planes of the central aromatic ringand the other aromatic fragments are equal to 2.08and 41.48 for the C(7i) and C(8i) aromatic rings,respectively. This asymmetry is not transmitted tothe central aromatic ring.

Because of hydrogen bonding located in asym-metric manner the whole BHMA molecule has adipole moment and the neighbouring molecules arelocated in such a way as to maximize attractionbetween the dipoles—centrosymmetric dimers areformed which are additionally supported byp–pinteractions in the stack. We also want to stress therole of stacking interactions in the formation of the 3Dmolecular structure of BHMA (Fig. 3).

K. Wozniak et al. / Journal of Molecular Structure 478 (1999) 267–274 271

Fig. 3. 3D-packing of molecules: (a) view along Z-axis; (b) view along X-axis.

3.2. Solid-state NMR

The assignment of the13C NMR resonances (Fig. 5)was carried out with reference to the spectra from theCDCl3 and CD3CN solutions (Table 3).

The peaks of the proton bearing carbons are wellseen in the SCT spectrum. The peaks are broadenedby 13C–1H dipolar interactions and the joint C(7)/C(8)peak by the proximate quadrupolar14N nuclei. Thelarge width of the peaks precludes more specific

K. Wozniak et al. / Journal of Molecular Structure 478 (1999) 267–274272

Fig. 4. Selected bond lengths [A˚ ] (a) and valence angles [8] (b) in BHMA.

assignment of the 140 –122 ppm region containingpeaks from C(4), C(6), andmeta and para carbonsof the side aromatic rings. There are two partlyresolved broad peaks at 114 and 120 ppm from theortho carbons of the side aromatic rings. At144 ppm there is a residual peak corresponding tothat in the DD spectrum.

Next, consider quaternary carbon peaks in the DDspectrum. At the highest frequency of the aromaticspectral region one might expect a C(2) peak and atthe lowest frequency two peaks from crystallographi-cally inequivalent C(1) and C(3) carbons. Unexpect-edly, we found that each such peak is accompanied bya weak neighbour, shifted by 2–3 ppm. Furthermore,the C(5) peak is also split. The additional weak peaksfrom the central aromatic ring indicate that in the solidstate this ring exists in two forms, one much morepopulated than the other. From the XRD studies weknow that the hydroxyl group does not donate itsproton to nitrogen N(1) (Fig. 2) in this and almostall known Schiff bases. The nitrogen atom N(2) is inthe trans position to carbon C(2), thus rendering theN(2)…H(1)–O(1) hydrogen bond impossible, even ifthe hydroxyl group would point in that direction.

Therefore, the solid-state splitting has anotherpurpose to that observed in the CDCl3 solutions,where the OH rotation was assumed [22]. A possiblereason would be that the unpolarized N(1)…H(1)–O(1) hydrogen bond is considerably favoured overthe polarized ionic N(1)1–H(1)…O(1)2 hydrogenbond. The former corresponds to much strongerquaternary peaks from the central aromatic ring thanthe latter. However, the C(2) chemical shift usuallyincreases when proton transfer takes place [27,28].The remaining quaternary peaks at 144.2 and155.0 ppm are from the crystallographically nonequi-valent C(71) and C(81) sites, and they are broadbecause they are close to the nitrogen atoms. Thesevalues are close to those observed for CDCl3 solution(Table 3). This assignment agrees well with the thatpublished in Refs. [15] and [18]. Note that these peaksare 11 ppm apart and that their mean position is at149.6 ppm, which is reasonably close to the value of151.2 ppm, measured for the CD3CN solution. Weinfer that the liquid state chemical shift at 300 K isaveraged by the rotation of the side rings, while thesolid-state molecular interactions impose particularside ring orientations, thus inducing marked chemical

K. Wozniak et al. / Journal of Molecular Structure 478 (1999) 267–274 273

Fig. 5. 13C CP/MAS NMR spectra of BHMA with the optimized-contact time (OCT), short-contact time (SCT) and dipolar dephasing (DD).Sidebands are outside the region shown. For assignment see Table 3.

shift differences of the corresponding carbons. Thisconvinced us that the simple picture of orderedhydrogen bonding in solid Schiff bases should be re-examined to take into account possible disorder andformation of ionic [O2…H–N1] hydrogen bonding.In particular, in the case of X-ray results, electrondensity maps for this region should carefully beproduced without hydrogen bonded proton. Veryoften they will show not only strong polarization ofelectron density towards the nitrogen atom (which isin the case of BHMA) but also two distinct maximanext to both electronegative atoms where a disorderedhydrogen atom is located.

4. Conclusions

We want to stress that in the crystal lattice thebisanil of 2-hydroxy-5-methyl-isophthaldehydeadopts the conformation with one ordered (accordingto the X-ray data) intramolecular O–H…N hydrogenbonding enforcing co-planarity with the centralaromatic fragment of this anil wing which is involvedin the hydrogen bonding. The other anil fragmentadopts conformation trans with respect to theC(2)O(1) bond and the aromatic ring of this fragmentis not co-planar with the remaining parts of theBHMA molecule. There is no significant differencebetween the bond lengths of the left- and right-handsides of the BHMA molecule, although some smalldifferences between the possibly equivalent valenceangles are observed. Some of them are due to O(1)–H(1)…N(1) hydrogen bonding and the differences intorsional angles are caused by different conformationsof the anil fragments.

Acknowledgements

KJ and KW were supported by grant No. P303 00707 from the Polish State Committee for ScientificResearch.

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