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Self assembly in transition metal complexes: structural characterisation of the zinc carboxylate{bis(2-pyrrolecarboxylato)bis(1-methylimidazole)}zinc(II)
Polyhedron Vol. 17, Nos 13 14, pp. 2199 2206, 1998 ~" 1998 Elsevier Science Ltd All rights reserved. Printed in Great Brilain P l I : S0277-5387(98)00047-3 0277 5387,98 S 19,00- 0.00
Pergamon
Self assembly in transition metal complexes: structural characterisation of the zinc carbo xyla te { bis(2-pyrrolecarbo xylato)bis( 1methylimidazole) } zinc(II) T h o m a s A. Z e v a c o ? * Helmar G6rls b and Eckhard Dinjus c "Projektgruppe "CO2-Chemie" an der Friedrich-Schiller-Universit~itJena, Lessingstral3e 12, 07743 Jena, Germany bInstitut for Anorganische und Analytische Chemie der Universit~it, August-Bebel-StraBe 2, D-07743 Jena, Germany ~Forschungszentrum Karlsruhe GmbH, lnstitut for Technische Chemie-Chemisch-Physikalische Verfahren, Postfach 3640, 76021 Karlsruhe, Germany
(Received 24 September 1997: accepted 23 January 1998)
Abstract--The synthesis, spectral characterisation of the zinc(ll) pyrrolecarboxylates, [Zn(2-Hpyrrc)_4H20)] (1) and [Zn(2-Hpyrrc)2(1-methylimidazole)2] (2) (2-H2pyrrc = 2-pyrrolecarboxylic acid) is reported as well as the X-ray structure determination of the latter compound. The molecular structure of 2 exhibits a distorted tetrahedral geometry around the zinc atom. The pyrrole-2-carboxylic acid acts as a monodentate ligand, o,nbound via one carboxylate oxygen atom. In the crystal structure, the discrete molecules of 2 are organised in dimers linked by a complex network of hydrogen bonds. A comparative IR study confirms the existence of different types of hydrogen bonding. ~ 1998 Elsevier Science Ltd. All rights reserved. Kevwords: crystal structure; zinc carboxylate; imidazole; pyrrole; 1R-spectroscopy; self-assembly.
INTRODUCTION During the course of our studies on new potential carbon dioxide activators, we became interested in systems involving zinc(II) carboxylates since these compounds are known to be effective catalysts for the copolymerisation of CO2 and epoxides [1,2]. The most efficient carboxylates are obtained from dicarboxylic acids and display most of the time polymeric frameworks [3 5]. The structural characterisation of such catalysts is poorly documented and the exact nature of the active site remains controversial. Our interest was attracted by carboxylic acids including a pyrrole moiety which might engineer, through hydrogen
* A u t h o r to w h o m c o r r e s p o n d e n c e s h o u l d be a d d r e s s e d .
bonding, new low-molar-weight oligomeric carboxylates. Molecules having self" complementary arrays of hydrogen bond donors and acceptors are a topic of main interest in organic chemistry for their ability to build supramolecular assemblies [69]. The preparation of hydrogen-bonded aggregates incorporating metal ions is less documented and attracts an ever-growing interest by associating versatile nanoarchitectures and transition metal characteristics [10]. The control of the molecular aggregation should allow, in our case. an easier investigation of the zinccatalyzed reaction of carbon dioxide with epoxides. Surprisingly, despite a number of extensive studies on the co-ordination of pyrrolecarboxylic acids to metal centres performed with spectroscopic [11, 12] and calorimetric methods [13], no crystallographic data are available, to our knowledge, lbr the corresponding
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T. A. Zevaco et al.
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transition metal carboxylates. We decided to undertake as first part of our study the structural characterisation of such carboxylates. We report here the first X-ray structure of a zinc(II) pyrrolecarboxylate, [Zn(2-PyrrcH)2( 1-methylimidazole)2] (2).
Crystallographic stud)' of 2 The molecular structure of 2 with atomic labelling is given in Fig. 1. Selected distances and angles are given in Table 1. [Zn(2-Hpyrrc)2(l-methylimidazole)2] crystallises in the monoclinic space group P2~/c with four discrete molecules per unit cell. Each molecule is linked via two hydrogen bonds with a centrosymmetrically related counterpart to form a dimer as depicted in Fig. 2. The structural characterisation of such hydrogen-bonded dimeric compounds linked by self complementary donor/acceptor groups is well documented in organic chemistry [14~18] but remains scarce for inorganic compounds. We are aware of only a few transition metal carboxylates of, e.g. zinc [19] and platin [20] displaying such pairs of symmetric C = O . . . H - N bridges. The zinc atom is tetracoordinated occupying the center of a tetrahedron characterised by a dihedral
RESULTS AND DISCUSSION The reaction of zinc oxide with two equivalents of the 2-pyrrolecarboxylic acid, noted 2-H2pyrrc, under reflux in aqueous media affords the carboxylato compound, [Zn(2-Hpyrrc)2.H20], (1). This zinc carboxylate reacts with 1-methylimidazole to afford, after recrystallisation from acetonitrile, red parallelepipedic crystals. The analytical and spectral results are consistent with a monomeric compound, [Zn(2-Hpyrrc)2(1-methylimidazole)2] (2), presenting a monodentate co-ordination of the carboxylate ligand.
C14 ~,(~-~
N4 C12
C13
C10
N2
Cll
~
O3
C6 C9
C8
N3
Zn 04
C3
C1
,
C4
C2 02
N1
N5 C15 C17
C16 N6
"d
c18
Fig. 1. Molecular structure of [Zn(2-pyrrolecarboxylato)~(l-methylimidazole)2] (2), with 50% probability thermal ellipsoids for non-hydrogen atoms.
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Self assembly in transition metal complexes Table 1. Selected bond lengths[,~] and angles [ ] l\~r [Zn(2-pyrrolecarboxylatoL,(1methylimidazole):] Bond lengths
Angles and intermolecular distances
Zn-O(1} Zn-O(2) Zn-O(3) Zn-O{4) Zn-N{3) Zn-N(5) C1-O{I) C1-0(2) C6-O(3) C6-O{4)
1,969(2) 2.717{2) 1,950(2) 3.037(2) 2.012(2) 2.002(2) 1.288(3) 1.243(3) 1.285(3) 1.226(3)
N(5)-Zn-N(3) O( 1)-Zn-O(3) O(1)-Zn-N(3) N(5)-Zn-O(3) O(2}-C{ 1)-O(1) O{3)-C(6)-O(4}
112.250) 113.71(8) 112.07(8) 101.95(9) 122.4{2) 124.8(3)
0(2). , N(la) O(4a). • N(2b} O(la). • N(2b)
2.801(3) 2.994(3) 3.114(3)
/
..
>,T
-
Fig. 2. Representation illustrating the hydrogen bondings within and between the adjacent pairs of 2. Hydrogen atoms arc omitted for clarity. Symmetry transformations used: # a ) : - x , l - y 2 - z : # b ) : - x , - 0 . 5 + y , 1.5-z: #c):x, 0.5--y, 0.5+z. lmermolecular bond lengths (A): 0 ( 2 ) . . . N(la) = 2.801(3), O(4a)... N(2b)-2.994(3), O(la)... N{2b) - 3.11412)A.
angle of 90,70(8) between the planes defined by N(3)Zn-N(5) and O(3)-Zn-O(1). The angles within the tetrahedron are shifted from the theoretical value and range from 98.62(8) to 116.44(8) ~. The carboxylate groups are bound in a syn-monodentate way. This geometry is commonly found in zinc aminocarboxylates and was already described for, e.g. the dichlorobis(L-proline)zinc(II) [21] and bis(L-pyroglutamat)zinc(II) dihydrate. [22] The ligand-metal distances are 1.969(2)A and 1.950(2)A for the Zn-O bonds and 2.002(2),~ and 2.012(2)A for the Zn-N bonds. The Zn-O bond
lengths are within the range reported for tetrahedral environments [21, 22, 23]. The Zn-N bond lengths are similar to those found in other tetrahedral zinc complexes of imidazole (Zn-N: 1.995-2.020,~) [21, 24] and are shorter than those found in octahedral imidazole complexes [25, 26] (distances ranging from 2.144 to 2.199(4) A). The C-O~p2 distances measured in the carboxylates groups suggest unequivalent values with 1.226(3)A and 1.243(3)~,. The latter distance may indicate a weakening of the double bond character and can be compared to those usually found in monodentate zinc
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%
Fig. 3. Unit cell of [Zn(2-pyrrolecarboxylato)2(1-methylimidazole)2] 2, viewed along the c axis.
carboxylates stabilised by hydrogen-bonded networks [19, 22, 27]. As expected, hydrogen bonding plays a significant role in the crystal structure and takes place on two levels, giving rise to interesting structural patterns. Firstly, a "strong"t hydrogen bond is found implicating the pendant oxygens 0(2) and the nitrogen atoms N(la) of pyrrolecarboxylate fragments in two adjacent molecules [ 0 ( 2 ) . . • N(1 a) = 2.801 (2) A], this interaction accounts for the formation of the abovementioned dimer. These distant contacts are comparable to hydrogen bond distances found in the crystal structures of many secondary amides involving dimeric arrangements [14]. A second interaction is detected between the oxygen O(4a), the metal-bonded oxygen O(3a) and a pyrazolic nitrogen atom N(2b) belonging to an other adjacent hydrogen-bonded dimer as illustrated in Fig. 2 [ O ( 4 a ) . . . N ( 2 b ) = 2.994(2) A; O(1 a). • • N(2b) = 3.114(2) A]. These contacts can be considered as "weak" hydrogen bonds owing to the fact that the carboxylic group O(4a),C(6a),O(3a) is located in the same plane as the pyrazolic nitrogen (deviation ca. 0.025 A) and directed towards the orbital lobes of the spLhybridised oxygen O(4a) [28]. Likewise, the interaction between O(la) and N(2b) can be regarded as a weak hydrogen bonding assuming an sp 3 hybridisation for the metalbonded O(la) atom [angles within the O(la)-centred
t The terms "strong" and "weak" are used to distinguish the two-centred hydrogen bond (2.801(2)A) from the threecentred hydrogen bond (2.994(2) and 3.114(2) A).
tetrahedron ranging from 105.3(8) to 109.8(8) ~'] [29]. This kind of secondary "weak" hydrogen bonds may explain some of the features found in the molecular structure such as: i) the different conformations of the pyrrolecarboxylate ligands, whose amino groups display cis and trans geometries in respect to the pendant carbonyl oxygens and, ii) the distortions observed in the co-ordination geometry of the carboxylato groups to the zinc atom [Zn-O(2): 3.037(2) and Zn-O(4): 2.717(2) A]. The complex hydrogen-bonded network resulting from the combination of these "strong" and "weak" hydrogen bondings links the pseudo-dimers of 2 into staggered sheets which propagate parallel to the [011] plane as depicted in Fig. 3. The methylation at N(4) and N(6) precludes the formation of further hydrogenbonded networks between the distinct layers.
I R spectroscopic studies
Evidence of hydrogen bonding can also be found in the IR spectra of both zinc carboxylates. Infrared spectra of compounds 1, 2 and of the ionic sodium pyrrolecarboxylate were run and comparisons were made between the frequencies of the N-H stretching bands in the solid state and the frequency found for the free pyrrolecarboxylic acid in solution (CHCI3/CH3CN, v(N-H): 3432 c m - ~). Selected IR frequencies are summarised in Table 2. The spectrum of 1 shows one strong band assigned to the N-H stretching mode at 3350 c m - ' comparable
Self assembly in transition metal complexes
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Table 2. Selected vibrational frequencies (KBr, _+2 cm~ ~) of compounds 1, and 2 and of the sodium pyrrole-2-carboxylate. (s = strong, m = medium, w = weak. br = broad, sh = shoulder) Compound
v(OH2)
v(N-H)
[Zn(2-Hpyrrc).,(H2 0 ) ]
3 4 1 8 m , b r 3348s
el(OH2)
v:,~(CO())
v(C=C), v(C-N)+ fi(CN-H)
v(C-C)-+ v~(COO)
1636sh.br
1562s
1510vs
-
1608vs 1574vs
1546s,
1462m.sh 1442s 1390w,sh Xv,.= 120 1436s. 1414s 1350vs
I
A,, = - 82 [Zn(2-Hpyrrc)21methylimidazole):]
3338m 3234s
2
A,,= -94,
Na(2-Hpyrrc) 3
-
3262s A,, = -
-198
~,v,. = 258, 224
1556s
1528vs
1426m 1468s 13~m.sh
170
.~,v,. = 88
A,, shift defined as: v(N-H) of the free acid in solution -v(N-H) of the carboxylate in the solid state. Av, differencedefined as: V~ym(COO)-V~ym(COO)of the carboxylate in the solid state.
to the vibration found in the solid-state spectrum of the free 2-pyrrolecarboxylic acid. The absence of band shift indicates that the amine of the 2-pyrrolecarboxylic acid is not co-ordinated to the zinc atom [30]. The comparison with the solution spectrum of the free acid reveals a bathochromic shift of 82 cmindicating the presence in compound 1, as in the free carboxylic acid, of a "weak" hydrogen bonding implicating aminogroups and carboxylic oxygens (entry l, Tab. 2). This bonding might also implicate the water molecule as suggested by the presence of a broad v(O-H) band at 3418cm -~ [32]. Compound 2 exhibits in the region characteristic of the N-H stretching vibrations two strong well resolved bands at 3338 and 3234cm ~ (bathochromic shift of 94 and 198 cm - ~, respectively) consistent with the presence of two amino groups engaged in hydrogen bonding of different intensities (entry 2, Tab. 2). In comparison, the spectrum of the ionic sodium pyrrolecarboxylate, 3, displays only one strong v(N-H) band at 3262 cm (shift of 170cm ~) which would be concordant with a dimeric structure involving strong hydrogen bonds between cis-aminogroups and carbonyl oxygens (entry 3, Tab. 2). Differences in the spectra of 1 and 2 are also observed in the 1700-1350cm ~ range in which the stretching frequencies of the carboxylate group must appear. An unequivocally attribution is arduous owing to the extensive hydrogen bonding and the presence in the same spectral region of stretching vibrations of the pyrrole ring and deformation modes of the amine. However it is possible, according to comprehensive literature studies [30-39], to propose some band attributions. Thus, in compound 1, the asymmetric v(COO) vibration can be found at
1562 cm-~ whereas the symmetric ones appear, combined with stretching vibrations of the pyrrole rings, at 1442 and 1390cm ~[34, 35]. The medium bands at 1510 and 1462cm t can be attributed, most probably, to the v(C = C) band and a combination of the v(C-N) and 6(CN-H) bands [33]. The carboxylate 2 displays a similar pattern with a larger splitting of the different absorption bands due to the presence of both terminal monodentate carboxylate and terminal carboxylate engaged in strong directive hydrogen bonding. The asymmetric v(COO) bands are located at 1608 and 1574cm -~, confirming the presence of two distinct carboxylate groups [32]. The symmetric v(COO) bands, associated with stretching modes of both pyrrole and imidazole rings, can be localised, at 1414, 1436 and 1350cm ~. The strong absorption band noticed at 1546cm ~ can be attributed to the stret~ ching mode of the double bonds [33]. The separation Av, defined as v a ~ ( C O O ) - v ~ ( C O 0 ) , provides some useful information on the bonding mode of the carboxylate group [37, 38, 39]. Deacon and Phillips have established, in the case of acetato compounds, some correlations between the Av values and the different modes of bonding of the carboxylato moiety found in the molecular structure: i) monodentate complexes display a greater Av than the corresponding ionic complexes, ii) chelating complexes exhibit lower Av values than the ionic compounds, and iii) bridging carboxylates give raise to Av values greater than those of chelating carboxylates and close to those found in ionic compounds. An unambiguous calculation of Av is more difficult than usually reported in the literature due to the mixing of the CC and C O 0 stretching modes which affords more than one symmetric v(COO) band.[34-39] However, this
T. A. Zevaco et al.
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/f--~
trans /
OH2
I,o .......
s •
o
Z /2.s
.
monomer
.
.
.
,o
........
o\
oligomer
Fig. 4. Potential structures of [Zn(2-pyrrolecarboxylato)2(OH2)], 1.
relationship can supply, at least in general terms, some information on the structure of compound 1. The A value found for the compound 1 (120cm -~) is bigger than that found for the ionic sodium pyrrolecarboxylate 3 (90 cm ~) and is significantly lower than the values found for the monodentate carboxylate 2 (ca. 225 and 260 cm-~). This suggests for compound 1 a near-to-symmetrical bonding of the carboxylate groups to the zinc atom. Two types of solid state structure are therefore conceivable for compound 1 (Fig. 4): on one hand, a mononuclear complexe with carboxylato groups nearly bound in a chelate way such as in [Zn(O2CCH3)(H20)2 ] [40] or, on the other hand, an oligomer presenting an O,O'bridging mode for the carboxylato groups comparable to the bonding described for the anhydrous zinc 2chlorobenzoate [41] and zinc crotonate [42]. The latter compound was reported to react with an N-donor base to afford low-molar-weighted oligomeric structures such as, e.g.[Zn2(CH3CH=CHCO2h(quinoline)2] [43]. Such a mechanism might be also conceivable in the case of the pyrrolecarboxylates 1 and 2, the monomer 2 being the result of the fragmentation of the oligomer 1. Without an X-ray crystal structure for compound 1 it is not possible to rule out one of these geometries. Such a structural characterisation would also define the nature of the hydrogen bonding in compound 1, a participation of a further hydrogen bonding occurring between pendant carbonyl oxygens and water molecules cannot be excluded. This interaction can also compensate the asymmetry produced by a monodentate co-ordination of the carboxylate and can consequently decrease the Av value [40]. More work is planned to elucidate precisely the solid state structure of compound 1 and to rationalise the dependence of the hydrogen-bonded network found in compound 2 on the nature of the present ligands.
EXPERIMENTAL
Reagents and physical measurements 2-pyrrolecarboxylic acid was purchased from Aldrich and used as received. Solvents and reagents were purified by literature methods [44]. 1-methylimidazole was distilled in vacuo from calcium hydride (50°C at 0.1 bar). ~H and uC N M R data were recorded on a Bruker AC-200 N M R spectrometer. Infrared spectra were measured from KBr pellets in the range of 4000400cm -~ on a Perkin Elmer PE-16 FT-IR spectrometer. Microanalysis for carbon, hydrogen and nitrogen was carried out by the Institut ffir Organische und Makromolekulare Chemie of Jena.
Synthesis Preparation of [Zn(2-Hpyrrc)2.H20] (1): zinc oxide (0.5g, 5.1 mmol) was added to a solution of 2-pyrrolecarboxylic acid (1.37g, 12.3mmol) in 20ml of deionized water. The white suspension was refluxed for ca. 12 hours. The reaction mixture was cooled down to room temperature, concentrated (ca. 5ml), filtered off and washed with acetone. The white precipitate is soluble in MeOH, Acetonitrile, T H F and fairly soluble in water. The solid was recrystallised from hot water (white needles) (1.8g, 92%) [Zn(2-Hpyrrc)2(H20)] Anal. Calc. for Ct0H~0N2OsZn: C, 39.55;H, 3.29; N, 9.23. Found: C, 39.37; H, 3.38; N, 9.26. Attempts to isolate singlecrystals of this hydrated carboxylate were not successful. Spectral characterisation of 1 IR: Table 2; ~H-NMR in T H F d8, 6 [ppm] (multiplicity, assignment): 11.01 (s, sharp, N-H); 6.85(s,
Self assembly in transition metal complexes pyrrole H3); 6.79 (s, pyrrole H5); 6.10 (s, pyrrole H4); 4.62(s, very broad, H20) (The singulets at 11.01 and 4.62 ppm disappear after D_,O addition); ~3C-NMR, 6 [ppm]: 166.30 ( C = O ) ; 126.76, 122.46, 115.30, 109.66, (respectively C2, C5, C3 and C4 of the pyrrole ring). Preparation of[zn(2-hpyrrc)2( 1-methylimidazole)2] (2): l-methylimidazole (4ml, 50 mmole) was added to an acetonitrile solution of compound 1 (0.5g, 1,65 mmole). The colourless solution was refluxed for 10 minutes, filtered and allowed to stand at room temperature. Over a period of ca. 2 weeks, the solution turned slowly to red and deep red crystals suitable for an X-ray structure determination formed (0.2 g, 27%). The crystals are fairly soluble in dmso.[Znl2Hpyrrc):(1-methylimidazole)2] Anal. Calc. for C~sH20N~O4Zn: C, 48.07; H, 4.48; N, 18.68. Found: C, 48.03: H, 4.55; N, 18.79. Spectral characterisation 0[2 1R: Table 2; 1H-NMR in dmso d6, 6 [ppm] (multiplicity, assignment): 11.12 (s, N-H); 8.16(s, imidazole H2); 7.29 (s, 1-Me-ira. H4); 7.23 (s, 1-Me-im. H5); 6.76(s, pyrrole H3); 6.56 (s, pyrrole H5); 6.03 (s, pyrrole H4); 3.72(s, CH3); 13C-NMR, 6 [ppm]: 166.57(COO): 121.36, 120.11, 111.97, 108.25, (C2, C5, C3 and C4 of the pyrrole ring);139.43, 127.93, 127.45 (1Me-ira. C2, C4 and C5 sp-~), 30,74 (1-Me-ira. C sp3). Crystalloqraphy 0/ compound 2 X-ray diffraction data: CAD4 diffractometer using graphite-monochromated Mo-Kc~ radiation. The crystals were mounted in a cold nitrogen stream. Data were corrected for Lorentz and polarisation effects, but not for absorption [45]. The structure was solved by direct methods (SHELXS)[46] and refined by fullmatrix least squares techniques against F0 (SHELXL93)[47]. The hydrogen atoms were located by difference Fourier synthesis and refined isotropically. Finally all non-hydrogen atoms were refined anisotropically. XP (SIEMENS Analytical X-ray Instruments, Inc.) was used for structure representations. CIsH~_0N604Zn, Mr =449.77 g.mol 1, monoclinic, space group P2dc, a =9.106(1), b =21.127(3), c = 10.468(2)z~,, fl = 102.70(1), V = 1964.6(4) A3, Z =4, D~,~d. = 1.521 g.cm 3, T = 18TK, 2 =0.71069,~, (Mo-K~) =10.39cm ~, red parallelepipeds, size 0.40 x 0.38 x 0.36 mm 3, F(000) = 928, 4730 reflections in +h, - k , +1, measured in the range 2,39 '~ < ® _< 27.44 ~', 4488 independent reflections, Rm~ = 0.024, 2527 reflections with F,, >4a(Fo), 342 parameters, R = 0.0347, wR 2 = 0.079, G O F = 1.004, largest difference peak: 0.417 e A ~. Acknowledgements--We gratefully acknowledge the Max-
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Planck-Gesellschaft for financial support and for a postdoctoral fellowship (T.A.Z.).
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39. Catterick, J. and Thornton, P., Adv. Inorg. Chem. Radiochem, 1977, 20, 291. 40. Van Niekerk, J. N., Schoening, F. R. L. and Talbot, J.H., Acta Cryst., 1953, 6, 720. 41. Nakacho, Y., Misawa, T., Fujiwara, T., Wakahara, A. and Tomita, K., Bull. Chem. Soc. Jpn.~ 1976, 49, 58. 42. Clegg, W., Little, I. R. and Straughan, B.P., Acta. Crvst., 1986, C42, 919. 43. Clegg, W., Little, I. R. and Straughan, B. P., J. Chem. Soc., Dalton Trans., 1986, 1283. 44. Perrin, D. D. and Armarego, W. L., Purification of Laboratory Chemicals, 3rd edn., Pergamon, New York, 1988. 45. MOLEN, An Interactive Structure Solution Procedure, Enraf-Nonius, Delft, The Netherlands, 1990. 46. Sheldrick, G. M., Acta Cryst., 1990, A46, 467. 47. Sheldrick, G. M., University of G6ttingen, Germany, 1993.
Pergamon
Self assembly in transition metal complexes: structural characterisation of the zinc carbo xyla te { bis(2-pyrrolecarbo xylato)bis( 1methylimidazole) } zinc(II) T h o m a s A. Z e v a c o ? * Helmar G6rls b and Eckhard Dinjus c "Projektgruppe "CO2-Chemie" an der Friedrich-Schiller-Universit~itJena, Lessingstral3e 12, 07743 Jena, Germany bInstitut for Anorganische und Analytische Chemie der Universit~it, August-Bebel-StraBe 2, D-07743 Jena, Germany ~Forschungszentrum Karlsruhe GmbH, lnstitut for Technische Chemie-Chemisch-Physikalische Verfahren, Postfach 3640, 76021 Karlsruhe, Germany
(Received 24 September 1997: accepted 23 January 1998)
Abstract--The synthesis, spectral characterisation of the zinc(ll) pyrrolecarboxylates, [Zn(2-Hpyrrc)_4H20)] (1) and [Zn(2-Hpyrrc)2(1-methylimidazole)2] (2) (2-H2pyrrc = 2-pyrrolecarboxylic acid) is reported as well as the X-ray structure determination of the latter compound. The molecular structure of 2 exhibits a distorted tetrahedral geometry around the zinc atom. The pyrrole-2-carboxylic acid acts as a monodentate ligand, o,nbound via one carboxylate oxygen atom. In the crystal structure, the discrete molecules of 2 are organised in dimers linked by a complex network of hydrogen bonds. A comparative IR study confirms the existence of different types of hydrogen bonding. ~ 1998 Elsevier Science Ltd. All rights reserved. Kevwords: crystal structure; zinc carboxylate; imidazole; pyrrole; 1R-spectroscopy; self-assembly.
INTRODUCTION During the course of our studies on new potential carbon dioxide activators, we became interested in systems involving zinc(II) carboxylates since these compounds are known to be effective catalysts for the copolymerisation of CO2 and epoxides [1,2]. The most efficient carboxylates are obtained from dicarboxylic acids and display most of the time polymeric frameworks [3 5]. The structural characterisation of such catalysts is poorly documented and the exact nature of the active site remains controversial. Our interest was attracted by carboxylic acids including a pyrrole moiety which might engineer, through hydrogen
* A u t h o r to w h o m c o r r e s p o n d e n c e s h o u l d be a d d r e s s e d .
bonding, new low-molar-weight oligomeric carboxylates. Molecules having self" complementary arrays of hydrogen bond donors and acceptors are a topic of main interest in organic chemistry for their ability to build supramolecular assemblies [69]. The preparation of hydrogen-bonded aggregates incorporating metal ions is less documented and attracts an ever-growing interest by associating versatile nanoarchitectures and transition metal characteristics [10]. The control of the molecular aggregation should allow, in our case. an easier investigation of the zinccatalyzed reaction of carbon dioxide with epoxides. Surprisingly, despite a number of extensive studies on the co-ordination of pyrrolecarboxylic acids to metal centres performed with spectroscopic [11, 12] and calorimetric methods [13], no crystallographic data are available, to our knowledge, lbr the corresponding
2199
T. A. Zevaco et al.
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transition metal carboxylates. We decided to undertake as first part of our study the structural characterisation of such carboxylates. We report here the first X-ray structure of a zinc(II) pyrrolecarboxylate, [Zn(2-PyrrcH)2( 1-methylimidazole)2] (2).
Crystallographic stud)' of 2 The molecular structure of 2 with atomic labelling is given in Fig. 1. Selected distances and angles are given in Table 1. [Zn(2-Hpyrrc)2(l-methylimidazole)2] crystallises in the monoclinic space group P2~/c with four discrete molecules per unit cell. Each molecule is linked via two hydrogen bonds with a centrosymmetrically related counterpart to form a dimer as depicted in Fig. 2. The structural characterisation of such hydrogen-bonded dimeric compounds linked by self complementary donor/acceptor groups is well documented in organic chemistry [14~18] but remains scarce for inorganic compounds. We are aware of only a few transition metal carboxylates of, e.g. zinc [19] and platin [20] displaying such pairs of symmetric C = O . . . H - N bridges. The zinc atom is tetracoordinated occupying the center of a tetrahedron characterised by a dihedral
RESULTS AND DISCUSSION The reaction of zinc oxide with two equivalents of the 2-pyrrolecarboxylic acid, noted 2-H2pyrrc, under reflux in aqueous media affords the carboxylato compound, [Zn(2-Hpyrrc)2.H20], (1). This zinc carboxylate reacts with 1-methylimidazole to afford, after recrystallisation from acetonitrile, red parallelepipedic crystals. The analytical and spectral results are consistent with a monomeric compound, [Zn(2-Hpyrrc)2(1-methylimidazole)2] (2), presenting a monodentate co-ordination of the carboxylate ligand.
C14 ~,(~-~
N4 C12
C13
C10
N2
Cll
~
O3
C6 C9
C8
N3
Zn 04
C3
C1
,
C4
C2 02
N1
N5 C15 C17
C16 N6
"d
c18
Fig. 1. Molecular structure of [Zn(2-pyrrolecarboxylato)~(l-methylimidazole)2] (2), with 50% probability thermal ellipsoids for non-hydrogen atoms.
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Self assembly in transition metal complexes Table 1. Selected bond lengths[,~] and angles [ ] l\~r [Zn(2-pyrrolecarboxylatoL,(1methylimidazole):] Bond lengths
Angles and intermolecular distances
Zn-O(1} Zn-O(2) Zn-O(3) Zn-O{4) Zn-N{3) Zn-N(5) C1-O{I) C1-0(2) C6-O(3) C6-O{4)
1,969(2) 2.717{2) 1,950(2) 3.037(2) 2.012(2) 2.002(2) 1.288(3) 1.243(3) 1.285(3) 1.226(3)
N(5)-Zn-N(3) O( 1)-Zn-O(3) O(1)-Zn-N(3) N(5)-Zn-O(3) O(2}-C{ 1)-O(1) O{3)-C(6)-O(4}
112.250) 113.71(8) 112.07(8) 101.95(9) 122.4{2) 124.8(3)
0(2). , N(la) O(4a). • N(2b} O(la). • N(2b)
2.801(3) 2.994(3) 3.114(3)
/
..
>,T
-
Fig. 2. Representation illustrating the hydrogen bondings within and between the adjacent pairs of 2. Hydrogen atoms arc omitted for clarity. Symmetry transformations used: # a ) : - x , l - y 2 - z : # b ) : - x , - 0 . 5 + y , 1.5-z: #c):x, 0.5--y, 0.5+z. lmermolecular bond lengths (A): 0 ( 2 ) . . . N(la) = 2.801(3), O(4a)... N(2b)-2.994(3), O(la)... N{2b) - 3.11412)A.
angle of 90,70(8) between the planes defined by N(3)Zn-N(5) and O(3)-Zn-O(1). The angles within the tetrahedron are shifted from the theoretical value and range from 98.62(8) to 116.44(8) ~. The carboxylate groups are bound in a syn-monodentate way. This geometry is commonly found in zinc aminocarboxylates and was already described for, e.g. the dichlorobis(L-proline)zinc(II) [21] and bis(L-pyroglutamat)zinc(II) dihydrate. [22] The ligand-metal distances are 1.969(2)A and 1.950(2)A for the Zn-O bonds and 2.002(2),~ and 2.012(2)A for the Zn-N bonds. The Zn-O bond
lengths are within the range reported for tetrahedral environments [21, 22, 23]. The Zn-N bond lengths are similar to those found in other tetrahedral zinc complexes of imidazole (Zn-N: 1.995-2.020,~) [21, 24] and are shorter than those found in octahedral imidazole complexes [25, 26] (distances ranging from 2.144 to 2.199(4) A). The C-O~p2 distances measured in the carboxylates groups suggest unequivalent values with 1.226(3)A and 1.243(3)~,. The latter distance may indicate a weakening of the double bond character and can be compared to those usually found in monodentate zinc
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T . A . Zevaco et al.
%
Fig. 3. Unit cell of [Zn(2-pyrrolecarboxylato)2(1-methylimidazole)2] 2, viewed along the c axis.
carboxylates stabilised by hydrogen-bonded networks [19, 22, 27]. As expected, hydrogen bonding plays a significant role in the crystal structure and takes place on two levels, giving rise to interesting structural patterns. Firstly, a "strong"t hydrogen bond is found implicating the pendant oxygens 0(2) and the nitrogen atoms N(la) of pyrrolecarboxylate fragments in two adjacent molecules [ 0 ( 2 ) . . • N(1 a) = 2.801 (2) A], this interaction accounts for the formation of the abovementioned dimer. These distant contacts are comparable to hydrogen bond distances found in the crystal structures of many secondary amides involving dimeric arrangements [14]. A second interaction is detected between the oxygen O(4a), the metal-bonded oxygen O(3a) and a pyrazolic nitrogen atom N(2b) belonging to an other adjacent hydrogen-bonded dimer as illustrated in Fig. 2 [ O ( 4 a ) . . . N ( 2 b ) = 2.994(2) A; O(1 a). • • N(2b) = 3.114(2) A]. These contacts can be considered as "weak" hydrogen bonds owing to the fact that the carboxylic group O(4a),C(6a),O(3a) is located in the same plane as the pyrazolic nitrogen (deviation ca. 0.025 A) and directed towards the orbital lobes of the spLhybridised oxygen O(4a) [28]. Likewise, the interaction between O(la) and N(2b) can be regarded as a weak hydrogen bonding assuming an sp 3 hybridisation for the metalbonded O(la) atom [angles within the O(la)-centred
t The terms "strong" and "weak" are used to distinguish the two-centred hydrogen bond (2.801(2)A) from the threecentred hydrogen bond (2.994(2) and 3.114(2) A).
tetrahedron ranging from 105.3(8) to 109.8(8) ~'] [29]. This kind of secondary "weak" hydrogen bonds may explain some of the features found in the molecular structure such as: i) the different conformations of the pyrrolecarboxylate ligands, whose amino groups display cis and trans geometries in respect to the pendant carbonyl oxygens and, ii) the distortions observed in the co-ordination geometry of the carboxylato groups to the zinc atom [Zn-O(2): 3.037(2) and Zn-O(4): 2.717(2) A]. The complex hydrogen-bonded network resulting from the combination of these "strong" and "weak" hydrogen bondings links the pseudo-dimers of 2 into staggered sheets which propagate parallel to the [011] plane as depicted in Fig. 3. The methylation at N(4) and N(6) precludes the formation of further hydrogenbonded networks between the distinct layers.
I R spectroscopic studies
Evidence of hydrogen bonding can also be found in the IR spectra of both zinc carboxylates. Infrared spectra of compounds 1, 2 and of the ionic sodium pyrrolecarboxylate were run and comparisons were made between the frequencies of the N-H stretching bands in the solid state and the frequency found for the free pyrrolecarboxylic acid in solution (CHCI3/CH3CN, v(N-H): 3432 c m - ~). Selected IR frequencies are summarised in Table 2. The spectrum of 1 shows one strong band assigned to the N-H stretching mode at 3350 c m - ' comparable
Self assembly in transition metal complexes
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Table 2. Selected vibrational frequencies (KBr, _+2 cm~ ~) of compounds 1, and 2 and of the sodium pyrrole-2-carboxylate. (s = strong, m = medium, w = weak. br = broad, sh = shoulder) Compound
v(OH2)
v(N-H)
[Zn(2-Hpyrrc).,(H2 0 ) ]
3 4 1 8 m , b r 3348s
el(OH2)
v:,~(CO())
v(C=C), v(C-N)+ fi(CN-H)
v(C-C)-+ v~(COO)
1636sh.br
1562s
1510vs
-
1608vs 1574vs
1546s,
1462m.sh 1442s 1390w,sh Xv,.= 120 1436s. 1414s 1350vs
I
A,, = - 82 [Zn(2-Hpyrrc)21methylimidazole):]
3338m 3234s
2
A,,= -94,
Na(2-Hpyrrc) 3
-
3262s A,, = -
-198
~,v,. = 258, 224
1556s
1528vs
1426m 1468s 13~m.sh
170
.~,v,. = 88
A,, shift defined as: v(N-H) of the free acid in solution -v(N-H) of the carboxylate in the solid state. Av, differencedefined as: V~ym(COO)-V~ym(COO)of the carboxylate in the solid state.
to the vibration found in the solid-state spectrum of the free 2-pyrrolecarboxylic acid. The absence of band shift indicates that the amine of the 2-pyrrolecarboxylic acid is not co-ordinated to the zinc atom [30]. The comparison with the solution spectrum of the free acid reveals a bathochromic shift of 82 cmindicating the presence in compound 1, as in the free carboxylic acid, of a "weak" hydrogen bonding implicating aminogroups and carboxylic oxygens (entry l, Tab. 2). This bonding might also implicate the water molecule as suggested by the presence of a broad v(O-H) band at 3418cm -~ [32]. Compound 2 exhibits in the region characteristic of the N-H stretching vibrations two strong well resolved bands at 3338 and 3234cm ~ (bathochromic shift of 94 and 198 cm - ~, respectively) consistent with the presence of two amino groups engaged in hydrogen bonding of different intensities (entry 2, Tab. 2). In comparison, the spectrum of the ionic sodium pyrrolecarboxylate, 3, displays only one strong v(N-H) band at 3262 cm (shift of 170cm ~) which would be concordant with a dimeric structure involving strong hydrogen bonds between cis-aminogroups and carbonyl oxygens (entry 3, Tab. 2). Differences in the spectra of 1 and 2 are also observed in the 1700-1350cm ~ range in which the stretching frequencies of the carboxylate group must appear. An unequivocally attribution is arduous owing to the extensive hydrogen bonding and the presence in the same spectral region of stretching vibrations of the pyrrole ring and deformation modes of the amine. However it is possible, according to comprehensive literature studies [30-39], to propose some band attributions. Thus, in compound 1, the asymmetric v(COO) vibration can be found at
1562 cm-~ whereas the symmetric ones appear, combined with stretching vibrations of the pyrrole rings, at 1442 and 1390cm ~[34, 35]. The medium bands at 1510 and 1462cm t can be attributed, most probably, to the v(C = C) band and a combination of the v(C-N) and 6(CN-H) bands [33]. The carboxylate 2 displays a similar pattern with a larger splitting of the different absorption bands due to the presence of both terminal monodentate carboxylate and terminal carboxylate engaged in strong directive hydrogen bonding. The asymmetric v(COO) bands are located at 1608 and 1574cm -~, confirming the presence of two distinct carboxylate groups [32]. The symmetric v(COO) bands, associated with stretching modes of both pyrrole and imidazole rings, can be localised, at 1414, 1436 and 1350cm ~. The strong absorption band noticed at 1546cm ~ can be attributed to the stret~ ching mode of the double bonds [33]. The separation Av, defined as v a ~ ( C O O ) - v ~ ( C O 0 ) , provides some useful information on the bonding mode of the carboxylate group [37, 38, 39]. Deacon and Phillips have established, in the case of acetato compounds, some correlations between the Av values and the different modes of bonding of the carboxylato moiety found in the molecular structure: i) monodentate complexes display a greater Av than the corresponding ionic complexes, ii) chelating complexes exhibit lower Av values than the ionic compounds, and iii) bridging carboxylates give raise to Av values greater than those of chelating carboxylates and close to those found in ionic compounds. An unambiguous calculation of Av is more difficult than usually reported in the literature due to the mixing of the CC and C O 0 stretching modes which affords more than one symmetric v(COO) band.[34-39] However, this
T. A. Zevaco et al.
2204
/f--~
trans /
OH2
I,o .......
s •
o
Z /2.s
.
monomer
.
.
.
,o
........
o\
oligomer
Fig. 4. Potential structures of [Zn(2-pyrrolecarboxylato)2(OH2)], 1.
relationship can supply, at least in general terms, some information on the structure of compound 1. The A value found for the compound 1 (120cm -~) is bigger than that found for the ionic sodium pyrrolecarboxylate 3 (90 cm ~) and is significantly lower than the values found for the monodentate carboxylate 2 (ca. 225 and 260 cm-~). This suggests for compound 1 a near-to-symmetrical bonding of the carboxylate groups to the zinc atom. Two types of solid state structure are therefore conceivable for compound 1 (Fig. 4): on one hand, a mononuclear complexe with carboxylato groups nearly bound in a chelate way such as in [Zn(O2CCH3)(H20)2 ] [40] or, on the other hand, an oligomer presenting an O,O'bridging mode for the carboxylato groups comparable to the bonding described for the anhydrous zinc 2chlorobenzoate [41] and zinc crotonate [42]. The latter compound was reported to react with an N-donor base to afford low-molar-weighted oligomeric structures such as, e.g.[Zn2(CH3CH=CHCO2h(quinoline)2] [43]. Such a mechanism might be also conceivable in the case of the pyrrolecarboxylates 1 and 2, the monomer 2 being the result of the fragmentation of the oligomer 1. Without an X-ray crystal structure for compound 1 it is not possible to rule out one of these geometries. Such a structural characterisation would also define the nature of the hydrogen bonding in compound 1, a participation of a further hydrogen bonding occurring between pendant carbonyl oxygens and water molecules cannot be excluded. This interaction can also compensate the asymmetry produced by a monodentate co-ordination of the carboxylate and can consequently decrease the Av value [40]. More work is planned to elucidate precisely the solid state structure of compound 1 and to rationalise the dependence of the hydrogen-bonded network found in compound 2 on the nature of the present ligands.
EXPERIMENTAL
Reagents and physical measurements 2-pyrrolecarboxylic acid was purchased from Aldrich and used as received. Solvents and reagents were purified by literature methods [44]. 1-methylimidazole was distilled in vacuo from calcium hydride (50°C at 0.1 bar). ~H and uC N M R data were recorded on a Bruker AC-200 N M R spectrometer. Infrared spectra were measured from KBr pellets in the range of 4000400cm -~ on a Perkin Elmer PE-16 FT-IR spectrometer. Microanalysis for carbon, hydrogen and nitrogen was carried out by the Institut ffir Organische und Makromolekulare Chemie of Jena.
Synthesis Preparation of [Zn(2-Hpyrrc)2.H20] (1): zinc oxide (0.5g, 5.1 mmol) was added to a solution of 2-pyrrolecarboxylic acid (1.37g, 12.3mmol) in 20ml of deionized water. The white suspension was refluxed for ca. 12 hours. The reaction mixture was cooled down to room temperature, concentrated (ca. 5ml), filtered off and washed with acetone. The white precipitate is soluble in MeOH, Acetonitrile, T H F and fairly soluble in water. The solid was recrystallised from hot water (white needles) (1.8g, 92%) [Zn(2-Hpyrrc)2(H20)] Anal. Calc. for Ct0H~0N2OsZn: C, 39.55;H, 3.29; N, 9.23. Found: C, 39.37; H, 3.38; N, 9.26. Attempts to isolate singlecrystals of this hydrated carboxylate were not successful. Spectral characterisation of 1 IR: Table 2; ~H-NMR in T H F d8, 6 [ppm] (multiplicity, assignment): 11.01 (s, sharp, N-H); 6.85(s,
Self assembly in transition metal complexes pyrrole H3); 6.79 (s, pyrrole H5); 6.10 (s, pyrrole H4); 4.62(s, very broad, H20) (The singulets at 11.01 and 4.62 ppm disappear after D_,O addition); ~3C-NMR, 6 [ppm]: 166.30 ( C = O ) ; 126.76, 122.46, 115.30, 109.66, (respectively C2, C5, C3 and C4 of the pyrrole ring). Preparation of[zn(2-hpyrrc)2( 1-methylimidazole)2] (2): l-methylimidazole (4ml, 50 mmole) was added to an acetonitrile solution of compound 1 (0.5g, 1,65 mmole). The colourless solution was refluxed for 10 minutes, filtered and allowed to stand at room temperature. Over a period of ca. 2 weeks, the solution turned slowly to red and deep red crystals suitable for an X-ray structure determination formed (0.2 g, 27%). The crystals are fairly soluble in dmso.[Znl2Hpyrrc):(1-methylimidazole)2] Anal. Calc. for C~sH20N~O4Zn: C, 48.07; H, 4.48; N, 18.68. Found: C, 48.03: H, 4.55; N, 18.79. Spectral characterisation 0[2 1R: Table 2; 1H-NMR in dmso d6, 6 [ppm] (multiplicity, assignment): 11.12 (s, N-H); 8.16(s, imidazole H2); 7.29 (s, 1-Me-ira. H4); 7.23 (s, 1-Me-im. H5); 6.76(s, pyrrole H3); 6.56 (s, pyrrole H5); 6.03 (s, pyrrole H4); 3.72(s, CH3); 13C-NMR, 6 [ppm]: 166.57(COO): 121.36, 120.11, 111.97, 108.25, (C2, C5, C3 and C4 of the pyrrole ring);139.43, 127.93, 127.45 (1Me-ira. C2, C4 and C5 sp-~), 30,74 (1-Me-ira. C sp3). Crystalloqraphy 0/ compound 2 X-ray diffraction data: CAD4 diffractometer using graphite-monochromated Mo-Kc~ radiation. The crystals were mounted in a cold nitrogen stream. Data were corrected for Lorentz and polarisation effects, but not for absorption [45]. The structure was solved by direct methods (SHELXS)[46] and refined by fullmatrix least squares techniques against F0 (SHELXL93)[47]. The hydrogen atoms were located by difference Fourier synthesis and refined isotropically. Finally all non-hydrogen atoms were refined anisotropically. XP (SIEMENS Analytical X-ray Instruments, Inc.) was used for structure representations. CIsH~_0N604Zn, Mr =449.77 g.mol 1, monoclinic, space group P2dc, a =9.106(1), b =21.127(3), c = 10.468(2)z~,, fl = 102.70(1), V = 1964.6(4) A3, Z =4, D~,~d. = 1.521 g.cm 3, T = 18TK, 2 =0.71069,~, (Mo-K~) =10.39cm ~, red parallelepipeds, size 0.40 x 0.38 x 0.36 mm 3, F(000) = 928, 4730 reflections in +h, - k , +1, measured in the range 2,39 '~ < ® _< 27.44 ~', 4488 independent reflections, Rm~ = 0.024, 2527 reflections with F,, >4a(Fo), 342 parameters, R = 0.0347, wR 2 = 0.079, G O F = 1.004, largest difference peak: 0.417 e A ~. Acknowledgements--We gratefully acknowledge the Max-
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Planck-Gesellschaft for financial support and for a postdoctoral fellowship (T.A.Z.).
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