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CRYSTAL GROWTH & DESIGN

Structural Influence of Cations on the Topology of Ferrocenemonosulfonate Salts Jingli Xie,*,†,‡ Brendan F. Abrahams,† and Anthony G. Wedd*,†,‡ School of Chemistry, The UniVersity of Melbourne, ParkVille, Victoria 3010, Australia, and Bio21 Molecular Science and Biotechnology Institute, The UniVersity of Melbourne, ParkVille, Victoria 3010, Australia

2008 VOL. 8, NO. 9 3193–3199

ReceiVed October 16, 2007; ReVised Manuscript ReceiVed May 19, 2008

ABSTRACT: The structures of salts containing the ferrocenemonosulfonate anion and a range of nitrogen base cations are reported. It is found that electrostatic interactions, sometimes enhanced by hydrogen bonds, generally lead to layer-type structures with the sulfonate group playing a key role in the association with the cations. Introduction Substituted ferrocenes with a low barrier to internal rotation have been employed to construct a variety of crystallographic architectures.1 This work is propelled by useful electrochemical, magnetic, and optical properties as well as the growing field of bio-organometallic chemistry.2 Ferrocenemonosulfonic acid has been used as an electron mediator in redox enzyme catalysis and in doped polyaniline electrodes, but there have been few reports documenting the structural features of this kind of functional material.3,4 The highly complementary interaction of sulfonates with the trigonal guanidinium cation has been exploited by Ward to generate a wide range of compounds, all based upon a twodimensional (2D) charge-assisted hydrogen-bonded network (Scheme 1).5 This work included structural determination of the guanidinium ferrocenemonosulfonate salt [C(NH2)3][Fe(η5C5H5)(η5-C5H4SO3)] which forms the expected sheet structure.4a As part of an investigation into redox-active 2D sheets that may provide a coating on electrode surfaces, we have explored the formation of networks generated through the formation of hydrogen bonds between the ferrocenedisulfonate/ferrocenemonosulfonate anion and nitrogen base cations.4b,c In the present work, we extend the study to a broader range of cations in an attempt to clarify the structural influence of the cation with the aim of providing more fundamental insights into the factors governing the arrangement of redox-active ionic species. The ability of the guanidinium ion to form three pairs of strong hydrogen bonds is one of the reasons that the symmetric 2D sheets form so readily. We have used cations that have fewer N-H bonds in an attempt to assess the role of hydrogen bonding in these sheet structures. Salts 1-7 (Table 1) were isolated with protonated forms of piperazine, 4-aminopyridine, 4-(dimethylamino)pyridine, trans-1,2-bis(4-pyridyl)ethylene, 2,2′-bipyridine, 1,4-diazabicyclo[2.2.2]octane (DABCO) as well as the quaternary cation, tetramethylammonium. In addition, reaction of ferrocenemonosulfonic acid with 2,4,6tri(pyridine-4-yl)-1,3,5-triazine (tpt) in dimethylformamide (DMF) solution provided unanticipated products [H2N(CH3)2][Fe(η5* To whom correspondence should be addressed. (J.X.) Tel.: (61 3) 8344 2377 (office). Fax: (61 3) 9347 5180. E-mail: [email protected]. Web: http:// www.chemistry.unimelb.edu.au/people/xij.php. (A.G.W.) Tel.: (61 3) 8344 2383 (office). Fax: (61 3) 9347 5180. E-mail: [email protected]. Web: http:// www.chemistry.unimelb.edu.au/people/wedd.php. † School of Chemistry. ‡ Bio21 Molecular Science and Biotechnology Institute.

Scheme 1. Quasihexagonal (6,3) Net Formed by Guanidinium and Sulfonate Ionsa

a

Each ion is involved in six charge-assisted N-H...O hydrogen bonds.

C5H5)(η5-C5H4SO3)] (8) and all-trans-{FeII[Fe(η5-C5H5)(η5C5H4SO3)]2(H2O)2(DMF)2} (9). Experimental Section Materials and General Procedures. All starting materials and solvents were purchased from commercial sources and used without further purification. The IR spectra in the range 4000-400 cm-1 were recorded on a PerkinElmer Spectrum One FT-IR spectrometer. Elemental analyses were carried out in Chemical & MicroAnalytical Services Pty. Ltd., Belmont, VIC, Australia. 1H NMR spectra were recorded on an INOVA 400 spectrometer (400 MHz, (CD3)2SO, 25 °C). The mass spectrometric measurements were recorded in positive ion and negative mode (cone voltage, 30 V; m/z range, 0-800) using a QuattroII triple-quadrupole spectrometer equipped with an ESI interface (Micromass, Manchester, U.K.) and the mobile phase was MeOH/H2O (1:1) delivering solvent at a rate of 0.03 mL/min (selected 1 H NMR and mass spectral figures can be found in Supporting Information). Deep blue ferrocenemonosulfonic acid Fe(η5-C5H5)(η5C5H4SO3H) was synthesized from ferrocene and chlorosulfonic acid (yield ∼ 60%).6 Synthesis of Complexes. Piperazinium Bis(ferrocenemonosulfonate) [H2N(CH2CH2)2NH2][Fe(η5-C5H5)(η5-C5H4SO3)]2 (1). Piperazine (0.043 g, 0.50 mmol) was added to a stirred solution of ferrocenemonosulfonic acid (0.133 g, 0.50 mmol) in DMF (10 mL). After 10 min, the yellow solution was filtered and exposed to diffusion of diethyl ether vapor. Single crystals of X-ray quality appeared within one week. Yield: 0.12 g, 77%. Anal. Calcd for C12H15FeNO3S: C, 46.62; H, 4.89; N, 4.53. Found: C, 46.54; H, 4.81; N, 4.57%. IR spectrum (cm-1): 2976 (w), 2798 (w), 2481 (m), 1609 (m), 1445 (m), 1231 (s), 1181 (s), 1149 (vs), 1108 (m), 1086 (m), 1040 (vs), 1012 (vs), 959 (m), 883 (m), 819 (s). 1H NMR: δ 2.85 (s, (CH2)2, 4H), 3.95 (s, C2H2, 2H), 4.05 (s, C5H5, 5H), 4.18 (s, C2H2, 2H). ESI-MS: positive ion, m/z 87.0, [(CH2)4N2H3]+; negative ion, m/z 265.0, [Fe(η5-C5H5)(η5C5H4SO3)]- (Figure S1, Supporting Information). Single crystals of the following salts were synthesized with similar procedures:

10.1021/cg701017e CCC: $40.75  2008 American Chemical Society Published on Web 08/05/2008

formula fw cryst syst space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z dcalc/g cm-3 GOF R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

1

C12H15Fe NO3S 309.16 monoclinic C2/c 31.180(8) 8.357(2) 9.340(3) 90 105.215(4) 90 2348.4(11) 8 1.749 1.069 0.0264 0.0600

3 C17H20Fe N2O3S 388.26 monoclinic Cc 11.1073(16) 20.669(3) 8.0145(11) 90 119.199(2) 90 1606.1(4) 4 1.606 1.016 0.0364 0.0842

2

C15H16Fe N2O3S 360.21 monoclinic P21/c 6.8594(5) 39.014(3) 16.7190(12) 90 92.5210(10) 90 4469.9(5) 12 1.606 1.085 0.0442 0.1085

C32H30Fe2 N2O6 S2 714.40 monoclinic P21/n 5.7372(15) 36.042(9) 7.3979(19) 90 98.367(4) 90 1513.5(7) 2 1.568 1.088 0.0341 0.0835

4 C15H14Fe NO3S 344.18 triclinic P1j 5.992(2) 7.741(3) 15.673(6) 99.243(7) 99.969(7) 100.382(7) 690.3(5) 2 1.656 1.031 0.0567 0.1399

5

6 C16H22Fe N2O3S 378.27 triclinic P1j 6.0161(5) 10.1579(9) 12.9981(11) 91.769(2) 99.971(2) 96.2220(10) 776.72(11) 2 1.617 1.009 0.0329 0.0869

Table 1. Crystal Parameters for Complexes 1-9 7 C14H21Fe NO3S 339.23 orthorhombic P212121 6.2786(6) 7.3112(7) 32.877(3) 90 90 90 1509.2(2) 4 1.493 1.049 0.0351 0.0812

8 C12H17Fe NO3S 311.18 orthorhombic Pca21 14.927(5) 5.6746(19) 29.526(10) 90 90 90 2501.0(14) 8 1.653 0.914 0.0451 0.0679

9 C26H36Fe3 N2O10S2 768.24 monoclinic P21/c 6.107(5) 11.998(10) 20.849(16) 90 93.926(14) 90 1524(2) 2 1.674 1.026 0.0901 0.1951

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Figure 1. The structure of 1. (a) The 2-D hydrogen bonded network. (b) A representation of the layer structure viewed from a direction parallel to the plane of the hydrogen-bonded network.

4-Aminopyridinium Ferrocenemonosulfonate [H2NC5H4NH][Fe(η5-C5H5)(η5-C5H4SO3)] (2). Yield: 78%. Anal. Calcd for C15H16FeN2O3S: C, 50.02; H, 4.48; N, 7.78; O, 13.32; S, 8.90. Found: C, 49.98; H, 4.50; N, 7.68; O, 13.17; S, 8.63%. IR spectrum (cm-1): 3297 (m), 3159 (m), 3092 (m), 2934 (w), 1675 (s), 1644 (vs), 1606 (m), 1533 (s), 1244 (vs), 1198 (m), 1186 (s), 1145 (vs), 1058 (m), 1037 (vs), 1023 (s), 1010 (s), 890(m), 851 (m), 819 (vs), 693 (s). 1H NMR: δ 4.05 (s, C2H2, 2H), 4.18 (s, C5H5, 5H), 4.30 (s, C2H2, 2H), 6.75 (d, J ) 7.1 Hz, C2H2, 2H), 8.00 (s, NH2, 2H), 8.10 (d, J ) 7.1 Hz, C2H2, 2H). ESI-MS: positive ion, m/z 94.9, [H2NC5H4NH]+;

Topology of Ferrocenemonosulfonate Salts

Figure 2. The structure of 2. (a) The layer-type structure of 2. (b) The 2-D hydrogen-bonded network present in 2. negative ion, m/z 264.9, [Fe(η5-C5H5)(η5-C5H4SO3)]- (Figure S2, Supporting Information). 4-(Dimethylamino)pyridinium Ferrocenemonosulfonate [(CH3)2NC5H4NH][Fe(η5-C5H5)(η5-C5H4SO3)] (3). Yield: 75%. Anal. Calcd for C17H20FeN2O3S: C, 52.60; H, 5.20; N, 7.20. Found: C, 52.87; H, 5.01; N, 6.86%. IR spectrum (cm-1): 3229 (w), 3082 (m), 1644 (vs), 1558 (vs), 1440 (m), 1402 (m), 1227 (m), 1208 (s), 1156 (vs), 1104 (m), 1043 (vs), 1030 (s), 1010 (s), 991 (m), 819 (vs). 1H NMR: δ 3.12 (s, (CH3)2, 6H), 4.04 (s, C2H2, 2H), 4.16 (s, C5H5, 5H), 4.28 (s, C2H2, 2H), 6.86 (d, J ) 7.1 Hz, C2H2, 2H), 8.16 (d, J ) 7.1 Hz, C2H2, 2H). ESI-MS: positive ion, m/z 123.0, [(CH3)2NC5H4NH]+; negative ion, m/z 265.0, [Fe(η5-C5H5)(η5-C5H4SO3)]- (Figure S3, Supporting Information). (E)-4,4′-(Ethene-1,2-diyl)dipyridinium Bis(ferrocenemonosulfonate) [HNC5H4CHCHC5H4NH][Fe(η5-C5H5)(η5-C5H4SO3)]2 (4). A stoichiometric ratio of acid: base of 2:1 was used for salts 4-6. Yield: 68%. Anal. Calcd for C32H30Fe2N2O6S2: C, 53.80; H, 4.23; N, 3.92. Found: C, 53.70; H, 4.31; N, 4.02%. IR spectrum (cm-1): 3076 (m), 2479 (w), 2111 (m), 1629 (s), 1515 (m), 1383 (m), 1226 (s), 1158 (vs), 1107 (m), 1039 (vs), 996 (s), 898 (m), 816 (vs), 734 (m). 2,2′-Dipyridinium Bis(ferrocenemonosulfonate) [HNC5H4C5H4NH][Fe(η5-C5H5)(η5-C5H4SO3)]2 (5). Yield: 75%. This salt is very hygroscopic and elemental analysis was not attempted. However, its

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Figure 3. The structure of 3. (a) The packing arrangement of the anions and cations. (b) A representation of the hydrogen bonding between the sulfonate groups and the cation. composition was confirmed by X-ray crystallography. IR spectrum (cm-1): 3361 (w), 3081 (m), 1642 (s), 1604 (m), 1467 (m), 1444 (m), 1384 (m), 1181 (vs), 1151 (vs), 1107 (m), 1041 (vs), 1028 (s), 824 (m), 771 (s), 734 (m), 687 (m). 1H NMR: δ 3.96 (s, C2H2, 2H), 4.06 (s, C5H5, 5H), 4.18 (s, C2H2, 2H), 7.29 (s, CH, 1H), 7.42 (s, CH, 1H), 8.12 (s, CH, 1H), 8.76 (s, CH, 1H). ESI-MS: positive ion, m/z 78.8, [C5H4NH]+; negative ion, m/z 264.9, [Fe(η5-C5H5)(η5-C5H4SO3)](Figure S4, Supporting Information). 4-Aza-1-azonia-bicyclo[2.2.2]octane Ferrocenemonosulfonate [HN(CH2CH2)3N][Fe(η5-C5H5)(η5-C5H4SO3)] (6). Yield: 80%. Anal. Calcd for C16H22FeN2O3S: C, 50.80; H, 5.86; N, 7.41. Found: C, 50.81; H, 5.82; N, 7.38%. IR spectrum (cm-1): 3079 (m), 3000 (w), 2324 (w), 1649 (m), 1415 (m), 1234 (s), 1181 (m), 1141 (vs), 1106 (m), 1038 (vs), 1010 (s), 977 (m), 890 (m), 817 (s). 1H NMR: δ 2.88 (s, (CH2)6, 12H), 3.99 (s, C2H2, 2H), 4.12 (s, C5H5, 5H), 4.23 (s, C2H2, 2H). ESI-MS: positive ion, m/z 112.9, [(CH2)6N2H]+; negative ion, m/z 265.0, [Fe(η5-C5H5)(η5-C5H4SO3)]- (Figure S5, Supporting Information)). Tetramethylammonium Ferrocenemonosulfonate [N(CH3)4][Fe(η5-C5H5)(η5-C5H4SO3)] (7). A procedure similar to the earlier preparations was employed here, but methanol was used in place of DMF as the solvent. Yield: 82%. Anal. Calcd for C14H21FeNO3S: C, 49.57; H, 6.24; N, 4.13. Found: C, 49.55; H, 6.18; N, 4.19%. IR spectrum (cm-1): 3029 (m), 1487 (m), 1413 (m), 1194 (vs), 1177 (vs), 1105 (m), 1041 (vs), 1025 (m), 950 (s), 814 (s). 1H NMR: δ 3.04 (s, (CH3)4, 12H), 3.99 (s, C2H2, 2H), 4.13 (s, C5H5, 5H), 4.23 (s, C2H2, 2H). ESI-MS: positive ion, m/z 74.0, [(CH3)4N]+; negative ion, m/z 265.0, [Fe(η5-C5H5)(η5-C5H4SO3)]- (Figure S6, Supporting Information). Dimethylammonium Ferrocenemonosulfonate [H2N(CH3)2][Fe(η5-C5H5)(η5-C5H4SO3)] (8) and all-trans-{FeII[Fe(η5-C5H5)(η5C5H4SO3)]2(H2O)2(DMF)2} (9). A 1:1 mixture of tpt and ferrocenemonosulfonic acid in DMF was stirred at room temperature for 10

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Figure 5. The structure of 5. (a) The layer type structure present in 5. (b) A representation of the interactions between sulfonate groups and the cations in 5. The dashed lines represent O · · · C contacts between the sulfonate group and the cation.

Figure 4. The structure of 4. (a) The layer type structure present in 4. (b) A representation of the interactions between sulfonate groups and the cations in 4. The dashed lines represent O · · · C contacts between the sulfonate group and the cation. min. Following filtration, the solution was exposed to diethyl ether vapor. The dark green cuboid single crystals of 8 and yellow needle single crystals of 9 suitable for X-ray analysis were separated manually. Removal of 9 from the mother liquor led to a change of color from yellow to dark-green over five minutes. Microanalysis was not attempted but the compositions of 8 and 9 were confirmed by X-ray crystallography. Crystallography. Crystal data, details of the data collection, and refinement are summarized in Table 1. Crystal structures were solved using direct methods (SHELXTL V5.17) from single-crystal data collected at 130 K on a Bruker SMART/CCD area detector diffractometer fitted with Mo KR radiation (λ ) 0.71073 Å) and a graphite monochromator. Structure refinements were performed using the SHELX97 program,8 which uses a full-matrix least-squares refinement based on F2. All non-hydrogen atoms in 1-9 were refined with anisotropic thermal displacement parameters. The hydrogen atoms attached to carbon atoms and those to nitrogen atoms were placed at geometrically estimated positions. Absorption corrections were performed using the SADABS program.9 CCDC reference numbers:

621213 (1), 621210 (2), 621211 (3), 621214 (4), 621215 (5), 621212 (6), 621216 (7), 621218 (8), and 621217 (9). ORTEP drawing (30% thermal ellipsoid probability) of all complexes can be found in Supporting Information.

Results and Discussion Reaction of ferrocenemonosulfonic acid with protonated nitrogen base cations in DMF or methanol at room temperature provided a series of crystalline salts 1-7 (Table 1). In contrast, the reaction of ferrocenemonosulfonic acid with tpt led unexpectedly to a mixture of [H2N(CH3)2][Fe(η5-C5H5)(η5C5H4SO3)] (8) and all-trans-{FeII[Fe(η5-C5H5)(η5-C5H4SO3)]2(H2O)2(DMF)2} (9). The sulfonate group of ferrocenemonosulfonate acts as a uninegative monodentate ligand in the latter compound. The diprotonated piperazine cation has the potential to act as a quadruple hydrogen bond donor, and indeed, in the structure of [H2N(CH2CH2)2NH2][Fe(η5-C5H5)(η5-C5H4SO3)]2 (1), each N-H hydrogen atom participates in an hydrogen bond with a sulfonate oxygen atom. A 2D hydrogen bonded network is formed between the sulfonate centers and the cation which extends in the b-c plane (Figure 1a). The sulfonate groups lie

Topology of Ferrocenemonosulfonate Salts

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Figure 7. The layer type structure present in 7.

Figure 6. The structure of 6. (a) The hydrogen bonding interaction between DABCOH+ and the sulfonate group in 6. (b) The layer type structure present in 6.

in two distinct planes, a lower plane in which they are attached to ferrocenes which lie below the mean plane of the network and an upper plane in which they are attached to ferrocenes which extend in a direction above the mean plane (Figure 1b). Each cation links to four separate sulfonate groups (two up and two down), while each sulfonate is bound to two cations. From a topological perspective, each cation serves as a 4-connecting node which is bridged by sulfonate groups to equivalent 4-connecting nodes resulting in the formation of a 4,4 net. This contrasts with the 6,3 net which forms between guanidinium and sulfonate ions (Scheme 1). The 2D networks stack in the a-direction, yielding the layer type structure depicted in Figure 1b. Polar regions involving the cations and sulfonate groups are separated by layers of nonpolar ferrocene units. A layer structure is also apparent in the salt [H2NC5H4NH][Fe(η5C5H5)(η5-C5H4SO3)] (2) which contains the 4-aminopyridinium monocation. The morphology of the plate-like single crystals is shown in the Supporting Information. However, the three N-H protons are bound to three separate sulfonate centers. The

ferrocene units all lie on the same side of the 2D hydrogenbonded network (Figure 2a). Both cation and anion serve as 3-connecting nodes in this 2D network and so, topologically, the hydrogen-bonded network is a 6,3 net (the same as that found for the guanidinium sulfonate networks in Scheme 1). In this case, however, the appealing symmetry of Ward’s 6,3 net is lost as one oxygen of a sulfonate group interacts with two cations, another interacts with one cation, and the third does not participate in the hydrogen-bonded network (Figure 2b). In 2, a symmetry-related inverted network sits directly above this hydrogen-bonded sheet in a manner that generates a layer structure that resembles that observed for 1 (compare Figures 1b and 2a). Despite this similarity, an important distinction between 1 and 2 is that the opposed sulfonate groups are not part of the same hydrogen-bonded network. Cyclic voltammetry of complexes 1 and 2 revealed the oneelectron reversible behavior expected for ferrocene derivatives (Figure S16, Supporting Information).4c When 4-(dimethylamino)pyridinium is used instead of 4-aminopyridinium, the layer structure is not preserved (Figure 3a). The only hydrogen bonding present in [(CH3)2NC5H4NH][Fe(η5C5H5)(η5-C5H4SO3)] (3) involves the pyridinium proton which forms a bifurcated hydrogen bond with two sulfonate groups (Figure 3b). When the cation is diprotonated (E)-4,4′-(ethene-1,2-diyl)dipyridinium, a layer structure is again generated in [HNC5H4CHCHC5H4NH][Fe(η5-C5H5)(η5-C5H4SO3)]2 (4), but, in this case, the hydrogen bonding is not as extensive. The dipyridinium cation provides a hydrogen-bonded bridge between symmetryrelated ferrocene units (Figure 4a). Although each sulfonate group participates in only one hydrogen bond, a second oxygen of the sulfonate group interacts with the R-carbon of a neighboring cation (2.900(3) Å) (Figure 4b). The hydrogen bonding in combination with the close OsC intermolecular contact appears to play a key role in the assembly of this structure with clearly defined layers of cations and anions. While the 2,2′-bipyridinium cation is geometrically different from the (E)-4,4′-(ethene-1,2-diyl)dipyridinium, the structure of [HNC5H4C5H4NH][Fe(η5-C5H5)(η5-C5H4SO3)]2 (5; Figure 5a) nevertheless shares common structural features with 4. The 2,2′-

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Figure 9. The structure of 9. (a) The Fe(II) complex of 9. (b) A representation of the association between the Fe(II) complexes in 9. Striped bonds indicate hydrogen-bonded interactions.

Figure 8. The structure of 8. (a) The layer type structure present in 8. (b) Parallel hydrogen-bonded helices formed between layers of the ferrocenemonosulfonate groups. The helices run in a direction normal to the page in (a).

bipyridinium cation adopts a centro-symmetric conformation and bridges two ferrocene units through hydrogen bonding. Once again, an O · · · C intermolecular contact involving a sulfonate oxygen atom and a carbon atom adjacent to the protonated nitrogen (3.041(4) Å) (Figure 5b) assists in the formation of this aesthetically pleasing layer structure. Thus, although the pyridyl groups in 1,2-bis(4-pyridyl)ethylene are oriented differently to those in 2,2′-bipyridine, there are significant similarities in the way each of these cations interacts with the anionic layer as can be seen by inspection of Figures 4b and 5b.

The diamine DABCO has the potential to form a dication, but in the 2:1 reaction of ferrocenesulfonic acid with DABCO, the product [HN(CH2CH2)3N][Fe(η5-C5H5)(η5-C5H4SO3)] (6) contains DABCOH+ cations only. The cation forms a single hydrogen bond with a sulfonate group to form a layered structure (Figure 6a). However, the layers are not as clearly defined as in some of the previous structures as the cations form an undulating sheet that extends in the a-b plane (Figure 6b). Structure 6 resembles that of [N(CH3)4][Fe(η5-C5H5)(η5C5H4SO3)] (7) which involves the quaternary tetramethylammonium cation (Figure 7). In this case, there is no possibility of hydrogen bonding, yet a layer structure is still apparent. The cation sheet is even more undulating than in the case of the DABCOH+ structure 6. In both cases, it appears that the electrostatic attraction between the cation and sulfonate anions promotes the formation of the layers. Interestingly, the crystal adopts the chiral space group P212121. As part of a broader examination of cations, attempts were made to form structures with protonated tpt but without success.

Topology of Ferrocenemonosulfonate Salts

Unanticipated reactions took place at room temperature and two products crystallized from the reaction solution (see Figure S17, Supporting Information). The salt [H2N(CH3)2][Fe(η5-C5H5)(η5C5H4SO3)] (8) contains the dimethylammonium cation derived from DMF by hydrolysis. This conversion has been observed before but under forcing conditions.10 Structure 8 (Figure 8a) shows a resemblance to that of 7. Sulfonate units are surrounded by dimethylammonium cations and vice versa. An important distinction is the presence of hydrogen-bonded helices that extend in the b-direction in 8 (Figure 8b). Each sheet contains helices of alternating handedness. As was the case with 1, the diprotonated nitrogen atoms provide a hydrogen bonded link from the downward pointing sulfonate groups to the upward pointing sulfonate groups. The second product was the complex all-trans-{FeII[Fe(η5C5H5)(η5-C5H4SO3)]2(H2O)2(DMF)2} (9; Figure 9a) in which ferrocenemonosulfonate acts as a uninegative monodentate ligand via its sulfonate group. Apparently, the reaction conditions led to decomposition of some Fe(η5-C5H5)(η5-C5H4SO3H) leading to liberation of Fe(II). As indicated in Figure 9b, hydrogen bonding involving sulfonate groups results in the formation of chains. Conclusions The beautiful complementary interactions apparent in Ward’s guanidinium ferrocenemonosulfonate crystals lead to the generation of 2D hydrogen bonded sheets.4a Similarly in the structures of compounds 1 and 2 (Figures 1 and 2), 2D hydrogen bonded networks are formed within layer-type structures. While these results may suggest that hydrogen bonding between cations and anions is a key reason for the formation of the layer-type structures, the current work indicates that hydrogen bonding is of secondary importance only. Electrostatic interactions between cations and anionic sulfonate groups appear to be largely responsible for the cation-anion layers. For example, in the layer-type structures of compounds 4, 5, 6, and 8, there is some hydrogen bonding between cations and the sulfonate groups, but 2D hydrogenbonded networks are not formed. Further support for the cation-anion electrostatic interaction being the dominant influence in layer formation is provided by the structure of compound 7, in which the cation, tetramethylammonium, encourages layer formation despite its inability to participate in classical hydrogen bonding. The magnitude of the electrostatic interaction (Coulombic interaction) among the ions depends on the sizes, absolute charges, and shapes of the ions; thus the 2D (or 3D)

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arrangements of these ionic-type structures may be controlled by those variable parameters. These results point to the possibility of engineering layer-type structures by exploiting electrostatic interactions even though there may be little or no support from hydrogen bonding. Acknowledgment. The authors are grateful for financial support by the Australian Research Council under Grant Nos. DP0770585 (J.X.) and DP0450134 (A.G.W.). J.X. thanks Dr. Arindam Mukherjee for helpful discussions, and Mrs. Sioe See Volaric for mass spectrometric measurements. Supporting Information Available: X-ray crystallographic file in CIF format for 1-9, selected 1H NMR and mass spectrum figures, ORTEP drawings of all complexes, cyclic voltammetry of 1 and 2, photos of crystals 2 and mixtures 8/9. This information is available free of charge via the Internet at http://pubs.acs.org.

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