ARTICLE pubs.acs.org/crystal
Planar Chirality and Helical Polymers: Ferrocenyl-Substituted AmidiniumCarboxylate Salts Martyn P. Coles,* Francesca A. Stokes, Benjamin F. K. Kingsbury, Benjamin M. Day, and Peter B. Hitchcock Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.
bS Supporting Information ABSTRACT: A series of amidiniumcarboxylate salts, [FcC{N(Cy)H][O2CR00 ] (Fc = ferrocenyl, R00 = Me, tBu, Ph, mesityl, CF3) have been synthesized and structurally characterized. In each case, the amidinium cation forms two NH 3 3 3 O hydrogen bonds to carboxylate anions. The extent of aggregation is, however, dependent on the carboxylate substituent. For R00 = Me, a 1:1 salt is observed with an {E,E}-configuration of cyclohexyl groups within the amidinium component, whereas R00 = tBu forms a 2:2 dimeric salt with an {E,Z}-cation. This latter configuration generates a planar chiral cation. When R00 = Ph or mesityl, the hydrogen-bonding forms a helical polymer, with each chain containing a single enantiomeric form of the cation. In contrast, when R00 = CF3 the hydrogen-bonded polymer contains alternating enatiomeric forms of the cation.
’ INTRODUCTION Aminidiniumcarboxylate bridges are important in protoncoupled electron transfer in biological systems1 and have been widely used in the construction of supramolecular assemblies.2 In the latter context, they have been identified in double helices,3,4 a triple stranded “molecular braid”,5 cylindrical complexes,6 molecular capsules,7 and an optically active [2]catenane.8,9 Among the reasons for the widespread occurrence of this linkage are the high association constant for charge-assisted hydrogen bonds (CAHBs, A, Figure 1), and the supposed “well-defined geometry” linking the two components together. The first point has merit,10 but because trisubstituted amidinium cations can adopt three possible configurations (BD, Figure 1),11,12 and hydrogen bonds to carboxylates can be syn, anti, and nonplanar,13 the geometry of the bridge may be considerably more varied. Incorporation of redox-active centers in hydrogen-bonded ion pairs is important in many applications, with most work on amidiniumcarboxylate pairs in the area of electrochemically controllable chemosensors.14 It has been shown that ferrocenecarboxylic acid forms strong hydrogen-bonded bridges with a benzamidine,15 and a series of related [3.3]ferrocenophanes, in which ferrocene is N-bound to a guanidine, have been used in the selective recognition of anions, cations, and amino acids.16 In 1997, the synthesis of N,N0 -dicyclohexyl-C-ferrocenyl amidine was reported (I-H, Scheme 1)17 and a series of coordination compounds incorporating the corresponding amidinate anion, [I], have been described.17,18 The structure of the protonated guanidinium, [I-H2]þ, has not been reported. Herein we describe the synthesis and structure of a series of carboxylate salts and show that, depending on the carboxylate substituent, [I-H2]þ may be planar chiral in the solid-state and involved in the formation of helical polymeric chains. r 2011 American Chemical Society
Figure 1. Charge-assisted hydrogen bond (CAHB, A) and three configurations, BD, of trisubstituted amidinium cations, showing direction of hydrogen bonding.
Scheme 1. Synthesis of AmidiniumCarboxylate Saltsa
a
R00 = Me (1), tBu (2), Ph (3), mes (4), and CF3 (5).
’ EXPERIMENTAL SECTION Materials and Methods. N,N0 -Dicyclohexyl-C-ferrocenyl amidine was synthesized according to literature procedures.17 Acids R00 CO2H, R00 = Me (Aldrich), tBu (Aldrich), Ph (Aldrich), mes (Aldrich), and CF3 (Acros) were purchased from commerical sources and used without further Received: April 11, 2011 Published: April 20, 2011 3206
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452.41
173(2)
0.71073 orthorhombic,
Fdd2
18.9358(4), 81.0567(19),
T (K)
wavelength (Å) cryst syst, space
group
a, b, c (Å)
2
3207
4201 (0.044)
reflns collected
independent reflns (Rint)
R1 = 0.046, wR2 = 0.103
R1 = 0.038, wR2 = 0.093
0.41 and 0.50
R indices (all data)
largest diff. peak/hole (e Å3) 0.73 and 0.54
0.995 R1 = 0.040, wR2 = 0.099
1.13 R1 = 0.032, wR2 = 0.082
5105/30/304
5105 (0.037)
19124
GOF on F2 final R indices [I > 2σ(I)]
params
4201/0/280
9863
θmin/θmax
data/restraints/
3.71/26.02
cryst size (mm3) 3.49/26.12
0.61 0.40 0.30 0.25
0.70
0.35 0.30 0.20
Mg m3) μ (mm1)
1300.82(5), 2 1.26
8986.4(3), 16
1.34
density (calcd
88.804(1), 79.869(1), 69.421(1)
13.0451(3)
9.6024(2), 11.2793(2),
0.71073 triclinic, P1
173(2)
494.49
C28H42FeN2O2
V (Å3), Z
90, 90, 90
R, β, γ (deg)
5.8548(1)
C25H36FeN2O2
fw
1
formula
Table 1
0.39 and 0.42
R1 = 0.073, wR2 = 0.101
1.024 R1 = 0.045, wR2 = 0.090
5193/0/324
5193 (0.052)
15526
3.51/26.02
0.20 0.20 0.10
0.60
1.29
2649.26(15), 4
90, 90.713(2), 90
21.0704(7)
12.1788(4), 10.3248(3),
0.71073 monoclinic, P21/c
173(2)
514.47
C30H38FeN2O2
3
0.28 and 0.29
R1 = 0.051, wR2 = 0.102
1.024 R1 = 0.039, wR2 = 0.094
4130/0/351
4130 (0.042)
13424
3.46/23.00
0.40 0.30 0.25
0.53
1.23
6012.6(3), 8
90, 98.173(1), 90
24.8877(9)
23.8115(8), 10.2500(3),
0.71073 monoclinic, C2/c
173(2)
556.55
C33H44FeN2O2
4
0.66 and 0.44
R1 = 0.047, wR2 = 0.103
0.848 R1 = 0.040, wR2 = 0.097
4325/31/296
4325 (0.043)
18966
3.51/26.01
0.40 0.40 0.35
0.66
1.36
2479.29(10), 4
90, 90, 90
17.7228(5)
12.0543(3), 11.6052(2),
0.71073 orthorhombic, Pna21
173(2)
506.38
C25H33F3FeN2O2
5
Crystal Growth & Design ARTICLE
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Figure 2. Molecular structure of [FcC{N(Cy)H}2][O2CMe] (1). Selected bond lengths (Å) and angles (deg): C11N1 1.323(3), C11N2 1.321(3), C1C11 1.472(4), C25O1 1.262(3), C25O2 1.250(3), N1 3 3 3 O1 2.751(3), N2 3 3 3 O2 2.717(3); N1C11N2 117.0(2), O1C25O2 124.0(3).
t
Figure 3. Molecular structure of [FcC{N(Cy)H}2][O2C Bu] ([2]2). Selected bond lengths (Å) and angles (deg): C11N1 1.316(3), C11N2 1.327(3), C1C11 1.477(3), C24O1 1.249(3), C24O2 1.244(3), N1 3 3 3 O1 2.723(3), N2 3 3 3 O2 2.785(3); N1C11N2 120.36(19), O1C24O2 125.2(2). purification. NMR spectra were recorded using Varian VNMRS 400 MHz spectrometer at 400.1 (1H), 100.4 (13C{1H}), and 376.3 MHz (19F{1H}) from samples at 303 K in CD2Cl2, unless otherwise stated. Elemental analyses were performed by S. Boyer at London Metropolitan University. [FcC{N(Cy)H}2][O2CMe] (1). FcC{NCy}{NHCy} (0.25 g, 0.64 mmol) was dissolved in 20 mL of acetonitrile to afford a clear orange solution. One equivalent of MeCO2H (0.04 mL, 0.64 mmol) was added via syringe, and the resulting orange solution was stirred for 1 h, after which time an orange precipitate had formed. The reaction was warmed to ∼50 C, filtered and allowed to slowly attain room temperature, resulting in formation of orange crystals of 1. Yield 28%. Anal. Calcd for C25H36FeN2O2: C 66.37, H 8.02, N 6.19%. Found: C 66.29, H 7.92, N 6.17. 1H NMR: δ 12.08 (s br, 2H, NH), 4.55 (s br, 2H, C5H4), 4.48 (s, 2H, C5H4), 4.30 (s, 5H, C5H5), 3.99 (m br, 2H, RH-C6H11), 1.88 (s, 3H, CO2Me), 1.79, 1.62, 1.50, 1.24 (m br, 20H, C6H11). 13C{1H} NMR: δ 178.0 (MeCO2), /, 70.8 (br, C5H4 and C5H5), 54.0 (RC-C6H11), 34.3 (br), 25.7 (C6H11), †, 24.6 (CO2Me). *C{NHCy}2 not observed; † remaining carbon resonance for the Cy group is obscured by overlap. [FcC{N(Cy)H}2][O2CtBu] (2). FcC{NCy}{NHCy} (0.25 g, 0.64 mmol) and tBuCO2H (0.07 g, 0.64 mmol) were combined as solids
Figure 4. Molecular structure of [FcC{N(Cy)H} 2][O 2CPh] (3). Selected bond lengths (Å) and angles (): C11N1 1.319(3), C11N2 1.321(3), C1C11 1.471(3), C30O1 1.255(3), C30O2 1.240(3), C24C30 1.522(3), N1 3 3 3 O1 2.733(3), N2 3 3 3 O2 2.715 (3); N1C11-N2 121.3(2), O1C30O2 125.8(2).
Figure 5. Molecular structure of [FcC{N(Cy)H}2][O2Cmes] (4). Selected bond lengths (Å) and angles (): C11N1 1.323(3), C11 N2 1.320(3), C1C11 1.473(3), C30O1 1.226(3), C30O2 1.243(3), C24C30 1.522(4), N1 3 3 3 O1 2.775(3), N2 3 3 3 O2 2.729(3); N1 C11N2 120.5(2), O1C24O2 126.1(3). and dissolved in 20 mL of acetonitrile to afford an orange/brown solution. Crystallization at room temperature afforded 2 as orange crystals. Yield 34%. Anal. Calcd for C28H42FeN2O2: C 68.01, H 8.56, N 5.67%. Found: C 68.08, H 8.60, N 5.65%. 1H NMR: δ /, 4.52 (s br, 2H, C5H4), 4.48 (s, 2H, C5H4), 4.29 (s, 5H, C5H5), 3.92 (m br, 2H, RH-C6H11), 1.79, 1.61, 1.24 (m br, 20H, C6H11), 1.15 (s, 9H, CMe3). *NH resonance not observed. 13C{1H} NMR: δ 184.7 (tBuCO2), †, 70.7 (br, C5H4 and C5H5), 54.0 (RC-C6H11), 39.5 (CMe3), 34.2 (br), 28.8 (CMe3), 25.8, 25.6 (C6H11). †C{NHCy}2 not observed. [FcC{N(Cy)H}2][O2CPh] (3). Compound 3 was synthesized as described for 2, using FcC{NCy}{NHCy} (0.25 g, (0.64 mmol) and PhCO2H (0.08 g, 0.64 mmol). The product was isolated as orange crystals. Yield 32%. Anal. Calcd for C30H38FeN2O2: C 70.04, H 7.44, N 5.45%. Found: C 70.06, H 7.58, N 5.47%. 1H NMR: δ 11.94 (s br, 2H, NH), 8.06 (m br, 2H, C6H5), 7.39 (m br, 3H, C6H5), 4.62 (s br, 2H, C5H4), 4.56 (s, 2H, C5H4), 4.32 (s, 5H, C5H5), 4.11 (m br, 2H, RH-C6H11), 1.82, 1.62, 1.27 (m br, 20H, C6H11). 13C{1H} NMR: δ 171.3 (PhCO2), /, 137.2, 130.1, 129.4, 127.5 (Ph), 70.3, (br, C5H4 and C5H5), 53.3 (RC-C6H11), 33.8 (br), 32.6, 25.4 (C6H11). *C{NHCy}2 not observed [FcC{N(Cy)H}2][O2Cmes] (4). Compound 4 was synthesized as described for 2, using FcC{NCy}{NHCy} (0.25 g, (0.64 mmol) and mesCO2H (0.11 g, 0.64 mmol). The product was isolated as orange 3208
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Crystal Growth & Design
Figure 6. (a) [3]¥ and (b) [4]¥ perpendicular to the hydrogen-bonded system, highlighted in black (H atoms omitted).
ARTICLE
Figure 8. Parameters used to describe the bonding in 15; (R defined looking along the N2CC5 bond toward the ferrocenyl unit. mounted on a Kappa CCD diffractometer. Data was collected at 173(2) K using Mo KR radiation at 0.71073 Å. The structures were refined with SHELXL-97.19 In all cases, the hydrogen atoms on the nitrogens were located on the difference map and freely refined. Additional features of note are described below: [FcC{N(Cy)H}2][O2CtBu] (2): The tBu group is disordered and both component methyl C atoms were left isotropic; SADI restraints were applied to the C25-methyl and methylmethyl distances of the group. [FcC{N(Cy)H}2][O2CCF3] (5): The fluorine atoms are disordered over two orientations and were left isotropic and with geometry restraints (SADI) applied to the CF3 group.
Figure 7. (a) [3]¥ and (b) [4]¥ viewed along the hydrogen-bonded system, highlighted in black (H atoms omitted). crystals. Yield 21%. Anal. Calcd for C33H44FeN2O2: C 71.21, H 7.97, N 5.03%. Found: C 71.15, H 8.03, N 5.04%. 1H NMR: δ 12.15 (s br, 2H, NH), 6.75 (s, 2H, mes-C6H2), 4.64 (s br, 2H, C5H4), 4.56 (s, 2H, C5H4), 4.34 (s, 5H, C5H5), 4.11 (m br, 2H, RH-C6H11), 2.30 (s, 6H, 2,6-Me2), 2.24 (s, 3H, 4-Me), 1.83, 1.62, 1.25 (m br, total 20H, C6H11). 13C{1H} NMR: 177.4 (mesCO2), 165.0 (C{NHCy}2), 141.0, 135.5, 132.6 (mes-C), 127.9 (mes-CH), 71.4, 71.2 (C5H4)*, 70.9 (C5H5), 54.9 (RC-C6H11), 34.4 (br), 25.7, 25.6 (C6H11), 21.3 (4-Me), 20.1 (2,6-Me). *Remaining carbon resonance for the substituted cyclopentadienyl ring obscured by overlap with other peaks. [FcC{N(Cy)H}2][O2CCF3] (5). Compound 5 was synthesized as described for 1, using FcC{NCy}{NHCy} (0.37 g, 1.0 mmol) and CF3CO2H (0.077 mL, 1.0 mmol). The product was isolated as red crystals. Yield 30%. Anal. Calcd for C25H33F3FeN2O2: C 59.29, H 6.52, N 5.53%. Found: C 59.30, H 6.56, N 5.35%. 1H NMR: δ 10.31 (s, br, 1H, NH), 7.08 (s br, 1H, NH), 4.70 (s br, 2H, C5H4), 4.56 (s br, 2H, C5H4), 4.32 (s, 5H, C5H5), 4.13 (m br, 1H, RH-C6H11), 3.67 (m br, 1H, RH-C6H11), 2.06, 1.77, 1.61, 1.49, 1.20 (m br, total 20H, C6H11). 13C{1H} NMR: δ 162.2 (C{NHCy}2), 160.8 (2JCF 33 Hz, CF3CO2), 118.0 (1JCF = 297 Hz, CF3), 74.8 (C5H4), 71.7 (C5H4), 71.1 (C5H5), 70.9 (C5H4), 56.9 (RC-C6H11), 52.0 (RCC6H11), 34.3, 33.0, 25.6, 24.9 (br, C6H11). 19F NMR: δ 75.5 (CF3).
Single-Crystal X-ray Data Collection and Structure Determination. Details of the crystal data, intensity collection, and refinement for complexes 15 are in Table 1. Crystals were covered in an inert oil and suitable single crystals were selected under a microscope and
’ RESULTS AND DISCUSSION Amidinium salts 15 were prepared from the reaction of I-H with the appropriate carboxylic acid (Scheme 1). The 1H NMR spectra for 14 are similar, showing replacement of the distinct resonances for the Nimino- and Namino-cyclohexyl R-protons in I-H with one multiplet, and the appearance of a broad singlet in the range δ 11.9412.15 ppm for the NH protons. These data indicate a symmetrical amidinium component in solution (i.e., B or D) or an {E,Z}-system in which the different nitrogen configurations are interconverting on the NMR time scale. In contrast, the trifluoroacetate derivative 5 showed inequivalent cyclohexyl groups and two broad resonances for the NH protons, consistent with a static {E,Z}-configuration. Single-crystal X-ray diffraction studies of 15 reveal a remarkable variation in the long-range order, depending on the carboxylate substituent. The acetate 1 forms a discrete (1:1) ion pair with an {E,E}-configuration of the amidinium group (Figure 2). This represents the “well-defined geometry” alluded to in previous publications. In contrast, the pivalate 2 is dimeric in the solid state (Figure 3) with the nitrogen substituents of [I-H2]þ in an {E,Z}-configuration. The cations in 3 and 4, incorporating [PhCO2] and [mesCO2], respectively, adopt {E,Z}-configurations similar to that in [2]2 (Figures 4 and 5). As expected,12,20 the 2,6-methyl groups disfavor a coplanar arrangement of the mesityl and carboxylate components in 4 [angle between least-squares planes: 3 5.2(4); 4 79.8(2)], although the C24C30 distance is the same as that in the benzoate 3 suggesting similar π-delocalization. 3209
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Table 2. Geometric Parameters for the Carboxylate Anions and the Cation in 1 and (ss-SP) Cations in [2]2, [3]¥, [4]¥, and [5]¥ 1 [2]2 [3]¥ [4]¥ [5]¥
R, deg
l, Å
41.8(2)
1.472(4)
26.3(1) 28.8(3) 25.4(3) þ40.4(3)
1.477(3) 1.471(3)
jO, deg
dO, Å
jN, deg
dN, Å
O1
114.2(2)
0.298(5)
N1
120.0(2)
0.587(6)
O2
118.4(2)
0.518(5)
N2
118.1(2)
0.245(6)
O1
145.5(2)
0.523(5)
N1
130.6(1)
1.004(6)
O2
141.3(2)
1.400(4)
N2
116.3(1)
1.290(5)
O1
135.3(2)
0.867(5)
N1
131.0(2)
1.647(5)
O2
150.8(2)
1.162(5)
N2
120.1(2)
1.246(5)
1.473(3)
O1
161.7(2)
0.587(6)
N1
125.2(2)
0.785(6)
1.484(5)
O2 O1
133.1(2) 130.8(2)
0.476(5) 0.011(5)
N2 N1
127.9(2) 129.8(2)
0.198(6) 1.255
O2
129.9(2)
0.938(6)
N2
128.8(2)
2.116(6)
Figure 9. Molecular structure of [FcC{N(Cy)H}2][O2CCF3] (5). Selected bond lengths (Å) and angles (deg): C11N1 1.311(5), C11N2 1.316(5), C1C11 1.484(5), C24O1 1.231(6), C24O2 1.227(6), N1 3 3 3 O1 2.843(3), N2 3 3 3 O2 2.906(3); N1C11N2 121.7(3), O1C24O2 132.4(4).
Figure 10. Schematic representation of [5]¥ viewed perpendicular to (top) and along (bottom) the hydrogen-bonded system.
Despite the different steric profiles of the anions, the bridging CAHBs propagate in both crystal structures to form one-dimensional chains
Figure 11. Assignment of the solid-state structures of the cation [I-H2]þ as ss-SP and ss-RP, by prioritizing the E-nitrogen atom.
[3]¥ and [4]¥, in which a central helical core is defined by the hydrogen bonds (Figures 6 and 7). The cation [I-H2]þ from 14 exhibits a range of geometries in compounds containing CAHBs to carboxylate anions (Figure 8). The dihedral angle (R) between the amidinium and Cp group and the CC bond length (l) linking these units show the extent of charge delocalization, which plays a key role in applications involving communication between these groups. In the neutral amidine I-H, which has an {E,Z}-configuration, R = 71 and l = 1.506 Å (average of two molecules in the unit cell). The angle R in 2, 3, and 4 is considerably smaller despite the similar arrangement of Cy groups, with distance l between 1.471(3) and 1.477(3) Å (Table 2). Even in 1, where the {E,E}configuration forces both substituents toward the ferrocenyl group, R = 41.8(2) and l = 1.472(4) Å, providing good evidence for charge delocalization in the C5CN2 bond in [I-H2]þ. The archetypal hydrogen bond at a carboxylateamidinium junction (A, Figure 1) involves ∼120 angles at oxygen and nitrogen (jO and jN, Figure 8), with all participating atoms in plane (dO and dN = 0). As expected, these conditions are most closely met in 1 (Table 2), although there is a notable twist between the amidinium and carboxylate components, resulting in a maximum deviation d for one of the nitrogen atoms of 0.587(6) Å (see Supporting Information for additional views of the carboxylateamidinium components of 15). Analysis of the bridging ions in [2]2, [3]¥, and [4]¥ shows a high sensitivity to the nature of the carboxylate substituent, despite it being remote from the hydrogen-bonding component. Distortion from 3210
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Figure 12. View along the chain of (a) [3]¥ and (b) [5]¥ highlighting the relative position of the ferrocenyl substituents.
synsyn hydrogen bonding to the carboxylate is noted in all cases, with jO up to 161.7(2) for [mesCO2] in [4]¥. The corresponding jN values do not vary to such an extent [max jN = 131.0(2)] because they are restricted by the cyclohexyl substituents. Comparing the position of the donor/acceptor atoms relative to the complementary ion shows that, in addition to the twist described in 1, both d values are commonly of the same sign, representing a staggered junction. There does not appear to be a correlation between either the j or d values and whether the CAHB involves an E- or Z-nitrogen. For example, jN1(E) > jN2(Z) and dO2(Z) > dO1(E) in [2]2 and [3]¥, while the reverse is true in [4]¥. Previous work with Al and Ga carboxylates has characterized the relative size of the anions by the Tolman cone angle.21 Because the size of the CF3 group is intermediate between those of the methyl and tert-butyl groups,22 one would predict that the structure of the salt derived from trifluoroacetic acid (5, Scheme 1) would be analogous to 1 or [2]2. X-ray diffraction data show an {E,Z}-configuration in [I-H2]þ (Figure 9), with a relatively large angle R [40.4(3)]. The jO and jN angles indicate symmetrical CAHBs, with longer O 3 3 3 N distances reflecting the lower basicity of [O2CCF3]. Rather than the expected (1:1) or (2:2) ion pairs, a one-dimensional hydrogenbonded chain, [5]¥, is present (Figure 10). This is significantly different from the chain structure of [3]¥ and [4]¥. To understand the origins of this difference, we note that two CAHBs in an {E,Z}-configured cation makes the structure chiral and the combination of these chiral units can vary in the solid state. Three types of chirality for substituted ferrocene scaffolds have been described previously,23 (a) lateral or central chirality involving a chiral substituent at the C5-ring, (b) planar chirality in 1,2-disubstituted derivatives, and (c) axial chirality induced upon coordination of a 1,10 -disubstituted derivative at a metal center.24 The {E,Z}-configuration of the cyclohexyl substituents and “rotationally locked” C5CN2/CN bonds in solid-state structures 24 impose a form of planar chirality in [I-H2]þ (Figure 11). To distinguish between the two enantiomers we have prioritized the “NCyH” group in the E-configuration, allowing us to assign the two isomers ss-SP and ss-RP, according to the nomenclature developed by Prelog.25 In [2]2 (space group P1), both enantiomeric forms of the cation are present in the dimer, related by the center of inversion. In [3]¥ and [4]¥, the CAHB’s form helical chains containing a single enantiomer of the cation. Because [3]¥ (P21/c) and
[4]¥ (C2/c) crystallize in centrosymmetric space groups, both left and right-hand helices are present, packed in a layered pattern (Figures S17b and S19b, Supporting Information). Within these structures, a correlation exists between the chirality of the cation and the hand of the helix. Thus the ss-SP cations form a righthanded helix and the ss-RP cations generate the corresponding lefthanded helix. This observation integrates with related research on enantiomerically pure benzamidines (incorporating chiral nitrogen substituents) which induce macromolecular helicity4 and on the formation of optically active [2]catenane structures.8 In contrast, [5]¥ crystallized in the polar space group Pna21, in which both (ss-SP) and (ss-RP) forms of [I-H2]þ alternate in the same chain (Figure 10). This prevents a fully helical structure from developing along the axis of the chain. The ferrocenyl and CF3 groups are therefore arranged on opposite sides of the chain, in contrast with [3]¥ and [4]¥ in which the groups repeat every 180 (Figure 12). In conclusion, we have described a novel ferrocenyl-substituted amidinium cation and a series of salts incorporating carboxylate anions. Solid-state structures have shown that the amidinium group frequently adopts an {E,Z}-configuration, which precludes formation of discrete (1:1) ion pairs and suggests that higher order oligomers are accessible in solution. In these cases, the ferrocenyl group induces a new form of planar chirality in the solid state that leads to the formation of helical polymer chains or polar materials. These results highlight that the often assumed (1:1) pairing between amidinium and carboxylate ions in the solution and solid states may be an oversimplification and offer intriguing possibilities in the construction of novel supramolecular structures using this popular linkage.
’ ASSOCIATED CONTENT
bS
Supporting Information. ORTEPs of 15, additional views of the hydrogen-bonding component in 15, schematic representations of the packing of helical chains in the crystal structures of [3]¥ and [4]¥, and crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for the crystal structrues reported in the paper have been deposited with the Cambridge Crystallographic Database (CCDC Nos. 809037;809041). This material can be obtained free of charge at http://www.ccdc.cam.ac. uk/conts/retrieving.html (or from the CCDC, 12 Union Road,
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Crystal Growth & Design Cambridge CB2 1EZ, UK; fax þ44 1223 336033; email: deposit@ ccdc.cam.ac.uk).
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the University of Sussex for financial support, and Dr J. David Smith for helpful discussions during the preparation of this manuscript.
ARTICLE
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