NOTE pubs.acs.org/Organometallics
Synthesis and Characterization of the Triple Cluster (Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N and Its Function as a Tripodal Tetradentate Ligand Huei-Fang Dai, Chia-Hsiang Chen, Chi-Shian Chen, and Wen-Yann Yeh* Department of Chemistry and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
bS Supporting Information ABSTRACT: Schiff base condensation of N(CH2CH2NH2)3 and Cp3Fe4(CO)4(C5H4CHdO) (1) produces the tripodal triple cluster (Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N (2) in high yield, which can bind a Cuþ ion to generate the complex [Cu(κ4-(Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N)]þ (3) bearing a trigonal-pyramidal geometry. The structures of 2 and 3 have been determined by an X-ray diffraction study.
he tetrahedral cluster [Cp4Fe4(CO)4], first reported by King,1 has been the subjects of numerous research papers. A unique feature of this stable cluster is that it is electroactive, reversibly undergoing both reduction and oxidation,2 a property essential in performing important functions such as solar energy conversion and multielectron catalysis.3,4 Previously, functionalization of the cyclopentadienyl groups of [Cp4Fe4(CO)4] with alkyl, aryl, formyl, acetyl, thiol, phosphine, and ferrocenyl moieties were reported by Rauchfuss5 and Yeh.6 Ozawa and Okazaki described reductive coupling of the bridging carbonyls by LiAlH4 to generate alkyne species.7 Astruc has incorporated the Fe4 clusters into dendrimers to present unique redox properties.8 We lately reported a Cp4Fe4(CO)4 derivative bearing a terpyridine group and showed its coordination to Ru(II) and Fe(II) ions.9 Recently, to construct higher nuclearity clusters with well-defined dimensions provides a new field of chemistry with prospective application in areas including molecular recognition, ion sensors, and nanotechnology.10 As part of our interest in the design and coordination chemistry of multifunctional ligands,11 herein we present the synthesis of a new tripodal compound of Fe4 clusters, which contains four N-donor atoms and is able to act as a tetradentate ligand to bind transition-metal ions.
T
’ RESULTS AND DISCUSSION Schiff base condensation of N(CH2CH2NH2)3 and 3 equiv of Cp3Fe4(CO)4(C5H4CHdO) (1) in refluxing benzene affords the triple cluster (Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N (2; 94%) as an air-stable, dark green crystalline solid after crystallization from n-hexane (eq 1). A few pellets of 3 Å molecular sieves must be added to remove the water generated from the condensation reaction; otherwise, low yields of 2 resulted. The FAB spectrum of 2 shows the molecular ion peak at m/z 1964. The 1H NMR spectrum presents a singlet signal at δ 8.12 for the imine protons, two broad signals at δ 4.90 and 4.83 for the C5H4 r 2011 American Chemical Society
protons, a sharp singlet signal at δ 4.75 for the Cp protons, and two triplet signals at δ 3.72 and 2.98 for the ethylene protons. The 13C{1H} NMR spectrum presents two signals at δ 290.8 and 289.2 in a 1:3 ratio for the μ3-CO groups and one signal at δ 158.6 for the imine carbons. These spectral data indicate equivalence of the three [Cp3Fe4(CO)4(C5H4CHdNCH2CH2)] units in solution, likely through C3 rotations.
The molecular structure of 2 is shown in Figure 1a, and the tripodal skeleton is illustrated in Figure 1b. Compound 2 contains a N(CH2CH2NdCH)3 segment with each imine terminal linked to a tetrairon cluster. The central amine group is pyramidal with an average CN1C angle of 113°. The average CNdC angle for the three imine groups (N2, N3, and N4) is 117°, and the average NdC distance is 1.46 Å. The four nitrogen atoms are arranged as an irregular tetrahedron, with the nonbonding Namine 3 3 3 Nimine distances from 2.85 to 3.74 Å (average 3.20 Å), and the Nimine 3 3 3 Nimine distances from 4.55 to 5.45 Å (average 4.98 Å), while the N1 atom is ca. 1.41 Å above the (N2, N3, N4) plane. The coordination environments for the three Fe4 clusters are essentially identical and show great resemblance to that of the parent compound Cp4Fe4(CO)4 and its derivatives.5b,6b,9 Compound 2 contains one amine and three imine groups and may serve as a tetradentate ligand. Thus, treatment of 2 with equimolar [Cu(NCMe)4]BF4 in dichloromethane at room temperature affords [Cu(κ4-(Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N)]BF4 (3a) in 90% after crystallization from n-hexane Received: February 9, 2011 Published: April 21, 2011 2889
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Organometallics
Figure 1. Molecular structure (a) and tripodal skeleton (b) of compound 2 with 30% probability ellipsoids. Selected bond distances (Å): N2C15 = 1.26(3), N3C38 = 1.25(3), N4C61 = 1.25(3). Selected bond angles (deg): C13N1C36 = 112(2), C13N1C59 = 113(2), C36N1C59 = 113(2), C14N2C15 = 118(2), C13C14N2 = 112(2), C37N3C38=115(2), C36C37N3=108(2), C60N4 C61 = 117(2), C59C60N4 = 114(2).
(eq 2). Compound 3a forms an air-stable, dark green crystalline solid. The FAB mass spectrum displays the ion peak at m/z 2027 corresponding to the combination of 2 with one Cuþ ion. The 1 H NMR spectrum shows that the imine proton signal at δ 8.20 and the ethylene proton signals at δ 3.98 and 3.32 are shifted downfield compared to those in 2, while the 13C resonances are only slightly affected. We then investigated if the neutral complex CuCl((Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N) could be prepared by treating 2 with CuCl. However, this reaction leads to 3b (86%), which displays the FAB mass and 1H NMR spectral patterns identical with those of 3a, suggesting the same configuration for the two compounds. Crystals of 3b found suitable for an X-ray diffraction study were obtained by adding n-hexane to a dichloromethane solution at room temperature. Compound 3b consists of discrete cations [Cu(κ4-(Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N)]þ and Cl/ CuCl2 counterions; the ORTEP drawing for the cationic part is illustrated in Figure 2a. The Cuþ ion shows an uncommon trigonalpyramidal coordination (Figure 2b),12 which is bound by three imine N2, N3, and N4 atoms at the basal position with the CuN
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bond distances ranging from 2.002(7) to 2.020(7) Å (average 2.01 Å) and by the amine N1 atom at the apical position with a substantially longer bond distance (2.237(7) Å). The mean Nimine—CuNimine bond angle of 119.2° is close to the ideal value of 120°. The copper atom is located 0.18 Å below the basal plane (away from the N1 atom), giving rise to the slightly acute N1—Cu1 —Nimine angles from 84.4(3) to 85.3(3)°. Coordination of the copper ion leads to a conformation change of the tripodal skeleton, such that the mean distances between the N2, N3, and N4 atoms are shortened by 1.5 Å and the distance between the N1 atom and the (N2, N3, N4) plane is lengthened by 0.65 Å in comparison with 2. Moreover, the sterically demanding Fe4 clusters would preclude the approach of incoming ligands from the basal side, so that treating 3a with Br, NO2, N3, S8, NCPh, or PPh3 in dichloromethane solution gave no reactions. The UVvis spectrum of 2 displays one main absorption at 401 nm and is attributed to a MLCT transition (dfπ*Cp) for the Fe4 cluster. Almost identical spectra were recorded for the cationic complexes 3a,b, indicating negligible charge effects by the ligated Cuþ ion. These compounds contain redox-active Fe4 centers, and the electrochemical properties of 2 and 3a were measured by cyclic voltammetry in dry, oxygen-free CH2Cl2 solution at 28 °C (Figure 3). The parent cluster compound Cp4(CO)4Fe4 exists in four electrochemically reversible oxidation states, [Cp4(CO)4Fe4]2þ/þ/0/.2 However, compounds 2 and 3a each display one irreversible reduction wave at 1.94 V and two irreversible oxidation waves at 0.88 and 0.22 mV for the Fe4 clusters, while the broad wave at 0.71 V for 3a likely has a contribution from irreversible reduction of the Cuþ ion. In summary, a new tripodal triple cluster has been synthesized and structurally characterized. Compound 2 may act as a tetradentate ligand to bind a Cuþ ion in an uncommon trigonal-pyramidal coordination, which presents a hybrid of organometallic and coordination chemistry and shows prospects for applications in pollutant sequestration and metal ion transport.10e For instance, we have found that compound 2 can precipitate Hg2þ and Pb2þ ions quantitatively from the solution. However, the use of 2 as a versatile tetradentate chelating agent is restricted by its rigidity and ease of undergoing a hydrolysis reaction in wet solvents. We are currently investigating the hydrogenation reaction of 2 in attempts to prepare the more flexible, robust tetraamine derivative (Cp3Fe4(CO)4(C5H4CH2NHCH2CH2))3N and to study its coordination chemistry.
’ EXPERIMENTAL SECTION General Methods. All manipulations were carried out under an atmosphere of purified dinitrogen with standard Schlenk techniques. Cp3Fe4(CO)4(C5H4CHdO) (1)5b and [Cu(NCMe)4][BF4]13 were prepared as described in the literature. N(CH2CH2NH2)3 (Fluka) and CuCl (Strem) were used as received. Solvents were dried over appropriate reagents under dinitrogen and distilled immediately before use. Infrared spectra were recorded on a Jasco FT/IR-4100 IR spectrometer. 2890
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Figure 3. Cyclic voltammograms for 2 and 3a in dichloromethane. The potential was scanned at 100 mV s1 at 27 °C, with arrows indicating the direction of current. The potentials are vs the Fc/Fcþ couple.
Figure 2. Molecular structure (a) and trigonal-pyramidal skeleton (b) for the cationic part of 3b with 30% probability ellipsoids. Selected bond distances (Å): Cu1N1 = 2.237(7), Cu1N2 = 2.009(8), Cu1N3 = 2.020(7), Cu1N4 = 2.002(7), N2C15 = 1.28(1), N3 C38 = 1.26(1), N4C61 = 1.26(1). Selected bond angles (deg): N1 Cu1N2 = 85.3(3), N1Cu1N3 = 84.4(3), N1Cu1N4 = 85.2(3), N2Cu1N3 = 121.1(3), N2Cu1N4 = 119.1(3), N3 Cu1N4 = 117.5(3), C13N1C36 = 114.2(8), C13N1C59 = 115.9(7), C36N1C59 = 113.4(7), C14N2C15 = 116.5(8), C37N3C38 = 118.2(8), C60N4C61 = 116.2(8). NMR spectra were obtained on a Varian Unity INOVA-500 spectrometer. Fast-atom-bombardment (FAB) mass spectra were recorded on a JEOL JMS-SX102A mass spectrometer. UVvis spectra were recorded by using a 1.0 cm quartz cell with an Agilent 8453 spectrophotometer. Elemental analyses were performed at the National Science Council Regional Instrumentation Center at National Chen-Kung University, Tainan, Taiwan. Synthesis of 2. Cp3Fe4(CO)4(C5H4CHdO) (1; 139 mg, 0.222 mmol) and molecular sieves (3 Å, 400 mg) were placed in an oven-dried 50 mL Schlenk flask equipped with a condenser, under a dinitrogen atmosphere. A benzene (20 mL) solution of N(CH2CH2NH2)3 (11 μL, 0.074 mmol) was introduced into the flask via a syringe, and the mixture was then heated to reflux for 24 h. The solution was cooled to room
temperature, and n-hexane (20 mL) was carefully added to the top of the solution. An air-stable, dark green crystalline solid of (Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N (2; 142 mg, 94%) was obtained. Anal. Calcd for C81H72N4Fe12O12 3 CH2Cl2: C, 48.08; H, 3.64: N, 2.73. Found: C, 48.58; H, 3.95; N, 2.61. MS (FAB): m/z 1964 (Mþ). IR (KBr, ν(CO)): 1627 (br) cm1. 1H NMR (CDCl3, 25 °C): δ 8.12 (s, 3H, HCdN), 4.90 (br, 6H, C5H4), 4.83 (br, 6H, C5H4), 4.75 (s, 45H, Cp), 3.72 (t, JHH = 14 Hz, 6H, CH2), 2.98 (t, JHH = 14 Hz, 6H, CH2). 13 C{1H} NMR (CDCl3, 25 °C): δ 290.8 (μ3-CO, 3C), 289.2 (μ3-CO, 9C), 158.6 (HCdN), 104.8 (C5H4), 99.1 (C5H5), 98.7 (C5H4), 96.8 (C5H4), 60.8 (CH2), 55.4 (CH2). Reaction of 2 and [Cu(NCMe)4]BF4. Compound 2 (25 mg, 0.013 mmol), [Cu(NCMe)4]BF4 (4 mg, 0.013 mmol), and dichloromethane (5 mL) were placed in a 25 mL Schlenk tube under dinitrogen. The solution was stirred at room temperature for 24 h and then layered with n-hexane (10 mL) to yield a dark green crystalline solid of [Cu(κ4-(Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N)]BF4 (3a; 24 mg, 90%). Anal. Calcd. for C81H72N4CuBF4Fe12O12: C, 46.02; H, 3.43; N, 2.65. Found: C, 45.97; H, 4.09; N, 2.63. MS (FAB): m/z 2027 ([M BF4]þ). IR (KBr, ν(CO)): 1633 (br) cm1. 1H NMR (CDCl3, 25 °C): δ 8.20 (s, 3H, HCdN), 4.90 (s, 6H, C5H4), 4.72 (s, 45H, Cp), 4.59 (s, 6H, C5H4), 3.98 (br, 6H, CH2), 3.32 (br, 6H, CH2). 13C{1H} NMR (CDCl3, 25 °C): δ 289.7 (μ3-CO, 3C), 288.02 (μ3-CO, 9C), 158.1 (HCdN), 104.9 (C5H4), 99.1 (C5H5), 96.7 (s, C5H4), 96.2 (C5H4), 60.2 (CH2), 52.1 (CH2). Reaction of 2 and CuCl. Compound 2 (22 mg, 0.011 mmol), CuCl (2 mg, 0.02 mmol), and dichloromethane (10 mL) were placed in a 25 mL Schlenk tube under dinitrogen. The solution was stirred at room temperature for 24 h and then layered with n-hexane (10 mL) to afford dark green crystals of [Cu(κ4-(Cp3Fe4(CO)4(C5H4CHdNCH2CH2))3N)] [CuCl2þCl]0.5 (3b; 20 mg, 86%). MS (FAB): m/z 2027([M 1 /2CuCl32]þ). IR (KBr, ν(CO)): 1627 (br) cm1. 1H NMR (CDCl3, 25 °C): δ 8.22 (s, 3H, HCdN), 4.91 (s, 6H, C5H4), 4.73 (s, 45H, Cp), 4.59 (s, 6H, C5H4), 4.01 (br, 6H, CH2), 3.36 (br, 6H, CH2). Cyclic Voltammetric Measurements for 2 and 3a. Electrochemical measurements were taken with a CV 50 W system. Cyclic voltammetry was performed with a Pt-button working electrode, a Pt-wire auxiliary electrode, and an Ag/AgCl reference electrode. The experiments were carried out with 1 mM of 2 and 3a, respectively, in dry CH2Cl2 solvent containing 0.1 M (n-C4H9)4NPF6 as the supporting electrolyte. The potential was scanned at 100 mV s1 at 27 °C. Under these conditions, ferrocene shows a reversible one-electron redox wave with E1/2 = 420 mV. 2891
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Organometallics
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Table 1. Crystallographic Data for 2 and 3b 2
3b
chem formula
C82H74Cl2Fe12N4O12
C162H144Cl3Cu3Fe24N8O24
cryst syst
monoclinic
monoclinic
fw
2048.55
4224.22
T, K
200(2)
200(2)
space group
P21/c
P21/c
a, Å
14.5831(13)
21.6248(11)
b, Å
27.378(2)
23.0879(12)
c, Å β, deg
18.3903(15) 90.954(2)
16.7075(9) 100.934(3)
V, Å3
7341.4(11)
8190.1(7)
Z
4
2
Dcalcd, g cm3
1.853
1.713
μ, mm1
2.435
2.545
R1/wR2
0.1504/0.3113
0.0745/0.1845
GOF on F2
1.358
1.012
Structure Determination for 2 and 3b. The crystals of 2 and 3b found suitable for X-ray analysis were each mounted in a thin-walled glass capillary and aligned on the Nonius Kappa CCD diffractometer, with graphite-monochromated Mo KR radiation (λ = 0.710 73 Å). The data were collected at 200 K. The θ range for data collection is 1.4025.02° for 2 and 0.9625.02° for 3b. Of the 47 016 and 53 633 reflections collected, 12 363 and 14 121 reflections were independent for 2 and 3b, respectively. All data were corrected for Lorentz and polarization effects and for the effects of absorption. The structure was solved by direct methods and refined by least-squares cycles. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included but not refined. All calculations were performed using the SHELXTL-97 package.14 The data collection and refinement parameters are collected in Table 1.
(7) (a) Okazaki, M.; Takano, M.; Ozawa, F. J. Am. Chem. Soc. 2009, 131, 1684–1685. (b) Okazaki, M.; Ohtani, T.; Takano, M.; Ogino, H. Organometallics 2004, 23, 4055–4061. (8) Aranzaes, J. R.; Belin, C.; Astruc, D. Angew. Chem., Int. Ed. 2006, 45, 132–136. (9) Yeh, W.-Y.; Chang, H.-M. Organometallics 2009, 28, 5992–5997. (10) (a) Drexler, K. E. Molecular Machinery, Manufacturing and Computation; Wiley: New York, 1992. (b) Shriver, D. F.; Kaesz, H. D.; Adams, R. D. The Chemistry of Metal Cluster Complexes; VCH: New York, 1990. (c) Braunstein, P.; Oro, L. A.; Raithby, P. R. Metal Clusters in Chemistry; Wiley-VCH: Weinheim, Germany, 1999. (d) Hidai, M.; Kuwata, S.; Mizobe, Y. Acc. Chem. Res. 2000, 33, 46–52. (e) Steed, J. W. Chem. Soc. Rev. 2009, 38, 506–519. (11) (a) Yeh, W.-Y.; Hsiao, S.-C.; Peng, S.-M.; Lee, G.-H. Organometallics 2005, 24, 3365–3367. (b) Shiue, T.-W.; Yeh, W.-Y.; Lee, G.-H.; Peng, S.-M. Organometallics 2006, 25, 4150–4154. (c) Chen, C.-S.; Yeh, W.-Y. Chem. Commun. 2010, 46, 3098–3100. (12) (a) Brown, E. C.; Johnson, B.; Palavicini, S.; Kucera, B. E.; Casella, L.; Tolman, W. B. Dalton Trans. 2007, 3035–3042. (b) Raab, V.; Kipke, J.; Burghaus, O.; Sundermeyer, J. Inorg. Chem. 2001, 40, 6964–6971. (13) Kubas, G. J. Inorg. Synth. 1979, 19, 90–92. (14) Sheldrick, G. M. SHELXTL-97; University of G€ottingen, G€ottingen, Germany, 1997.
’ ASSOCIATED CONTENT Supporting Information. CIF files giving X-ray crystal data for 2 and 3a. This material is available free of charge via the Internet at http://pubs.acs.org.
bS
’ ACKNOWLEDGMENT We are grateful for support of this work by the National Science Council of Taiwan. We thank Mr. Ting-Shen Kuo (National Taiwan Normal University, Taipei) for X-ray diffraction analysis. ’ REFERENCES (1) King, R. B. Inorg. Chem. 1966, 5, 2227–2230. (2) Ferguson, J. A.; Meyer, T. J. J. Chem. Soc., Chem. Commun. 1971, 623–624. (b) Ferguson, J. A.; Meyer, T. J. J. Am. Chem. Soc. 1972, 94, 3409–3412. (3) Adams, R. D.; Cotton, F. A. Catalysis by Di- and Polynuclear Metal Cluster Compounds; Wiley-VCH: New York, 1998. (4) Bruce, D. W.; O’Hare, D. Inorganic Materials; Wiley: Chichester, U.K., 1997. (5) (a) Massa, M. A.; Rauchfuss, T. B. Chem. Mater. 1991, 3, 788–790. (b) Westmeyer, M. D.; Massa, M. A.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem. Soc. 1998, 120, 114–123. (6) (a) Yeh, W.-Y.; Wu, C.-Y.; Chiou, L.-W. Organometallics 1999, 18, 3547–3550. (b) Yeh, W.-Y.; Liu, Y.-C.; Peng, S.-M.; Lee, G.-H. J. Organomet. Chem. 2004, 689, 1014–1018. 2892
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