Binding of Nitrate to a CuII−Cyclen Complex Bearing a Ferrocenyl

The structure was solved in the space group Pnma (attempts to solve it in a lower ...... Sheldrick, G. M. SHELXS97; Program for Crystal Structure Solu...
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Inorg. Chem. 2007, 46, 3876−3888

Binding of Nitrate to a CuII−Cyclen Complex Bearing a Ferrocenyl Pendant: Synthesis, Solid-State X-ray Structure, and Solution-Phase Electrochemical and Spectrophotometric Studies Gilles Gasser, Matthew J. Belousoff, Alan M. Bond, and Leone Spiccia* School of Chemistry, Monash UniVersity, Clayton, Victoria 3800, Australia Received August 29, 2006

The reaction of Cu(NO3)2‚3H2O with the ligand 1-(ferrocenemethyl)-1,4,7,10-tetraazacyclododecane (L) in acetonitrile leads to the formation of a blue complex, [Cu(L)(NO3)][NO3] (C1). The X-ray structure determination shows an unexpected binding of a nitrate anion in that the CuII center is surrounded by four N atoms of the 1,4,7,10tetraazacyclododecane (cyclen) macrocycle and two O atoms from a chelating nitrate anion, both Cu−O distances being below the sums of the van de Waals radii. Hydrogen-bonding interactions in the crystal lattice and a weak interaction between a second nitrate O and the CuII center in C1 give rise to a highly distorted CuII geometry relative to that found in the known structure of [Cu(cyclen)(NO3)][NO3] (C5). Electrochemical studies in acetonitrile containing 0.1 M [Bu4N][NO3] as the supporting electrolyte showed that oxidation of C1 in this medium exhibits a single reversible one-electron step with a formal potential E°f of +85 mV vs Fc0/+ (Fc ) ferrocene). This process is associated with oxidation of the ferrocenyl pendant group. Additionally, a reversible one-electron reduction reaction with an E°f value of −932 mV vs Fc0/+, attributed to the CuII/I redox couple, is detected. Gradual change of the supporting electrolyte from 0.1 M [Bu4N][NO3] to the poorly coordinating [Bu4N][PF6] electrolyte, at constant ionic strength, led to a positive potential shift in E°f values by +107 and +39 mV for the CuII/I(C1) and Fc0/+(C1) redox couples, respectively. Analysis of these electrochemical data and UV−vis spectra is consistent with the probable presence of the complexes C1, [Cu(L)(CH3CN)2]2+ (C2), [Cu(L)(CH3CN)(NO3)]+ (C3), and [Cu(L)(NO3)2] (C4) as the major species in nitrate-containing acetonitrile solutions. In weakly solvating nitromethane, the extent of nitrate complexation remains significant even at low nitrate concentrations, due to the lack of solvent competition.

Introduction Metal complexes of the 1,4,7,10-tetraazacyclododecane (cyclen) macrocycle and its derivatives have been used in a wide range of medical applications, which include contrastenhancing agents in magnetic resonance imaging,1 radiotherapy,2 models for protein-metal binding sites in biological systems,3 and RNA cleavage catalysts.4 In the sensing area, Aoki and Kimura showed that the ZnII-cyclen complex selectively binds thymine derivatives in aqueous solution at

physiological pH levels.5 As a consequence, the development of potentially specific redox biosensors for DNA bases could be envisioned when anthraquinone6 or ferrocene moieties7,8 were covalently attached to the ZnII-cyclen complex to act as the redox signaling units. Voltammetric oxidation of the ferrocene center has been widely used for sensing purposes,9-14

* To whom correspondence should be addressed. E-mail: [email protected]. Fax: +61 3 9905 4597. (1) Zhang, S.; Jiang, X.; Sherry, A. D. HelV. Chim. Acta 2005, 88, 923935 and references therein. (2) Reichert, D. E.; Lewis, J.; Anderson, C. J. Coord. Chem. ReV. 1999, 184, 3-66 and references therein. (3) Kimura, E. Pure Appl. Chem. 1993, 65, 355. (4) Epstein, D. M.; Chappell, L. L.; Khalili, H.; Supkowski, R. M.; Horrocks, W. D., Jr.; Morrow, J. R. Inorg. Chem. 2000, 39, 21302134 and references therein.

(5) Aoki, S.; Kimura, E. Chem. ReV. 2004, 104, 769-787 and references therein. (6) Tucker, J. H. R.; Shionoya, M.; Koike, T.; Kimura, E. Bull. Chem. Soc. Jpn. 1995, 68, 2465-2469. (7) Gasser, G.; Bond, A. M.; Graham, B.; Kosowski, Z.; Spiccia, L. NSTI Nanotech 2005; Proceedings of the NSTI Nanotechnology Conference and Trade Show, Anaheim, CA, May 8-12, 2005; Nano Science and Technology Institute: Cambridge, MA, 2005; pp 412-415. (8) Gasser, G.; Belousoff, M. J.; Bond, A. M.; Kosowski, Z.; Spiccia, L. Inorg. Chem. 2007,, 46, 1665-1674. (9) Plenio, H.; Aberle, C.; Shihadeh, Y. A.; Lloris, J. M.; Martinez-Manez, R.; Pardo, T.; Soto, J. Chem.sEur. J. 2001, 7(13), 2848-2861 and references therein. (10) van Stavaren, D. R.; Metzler-Nolte, N. Chem. ReV. 2004, 104, 59315985 and references therein.

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© 2007 American Chemical Society Published on Web 04/17/2007

Nitrate Binding to a CuII-Cyclen Complex

Figure 1. Representations of the structures of L, 1, C1 (solid state), and C5 (solid state) and C2-C4.

due to its well-defined reversible behavior. Macrocyles bearing a ferrocenyl unit have also been applied in the selective electrochemical recognition of anions, such as dihydrogenphosphate, chloride, hydrogensulfate, or acetate. Generally, this is achieved through hydrogen bonding, electrostatic interaction, or coordination to a metal cation, with anion complexation in close proximity to the redoxactive fragment to allow electrochemical detection of the guest.15 Recently, Saint-Aman and co-workers described a ferrocenyl receptor that combined two of these binding modes.16 To enhance the electrochemical sensing response of anions, Beer and co-workers synthesized receptors that contain two different redox-active moieties so that dihydrogenphosphate, chloride, bromide, hydrogensulfate, nitrate, or acetate binding could be monitored by more than one electrochemical response. Examples of their strategy include ferrocene and an additional electrochemically active Lewis acid transitionmetal center,17 dithiocarbamate-copper(II) complexes that contain two copper ions and exhibit well-separated reversible oxidation waves,18 ferrocene and metalloporphyrins,19 and ruthenium complexes combined with ferrocene, cobaltocene, or an osmium complex.20 However, in all these studies, anion binding does not involve direct coordination to a metal center. With this concept of using two redox-active centers in mind, we have been investigating the coordination chemistry of (11) Kraatz, H.-B. J. Inorg. Organomet. Polym. Mater. 2005, 15 (1), 83106 and references therein. (12) Plenio, H.; Aberle, C. Angew. Chem., Int. Ed. 1998, 37 (10), 13971399 and references therein. (13) Takaneka, S. Polym. J. 2004, 36 (7), 503-512 and references therein. (14) Tucker, J. H. R.; Collinson, S. R. Chem. Soc. ReV. 2002, 31, 147156 and reference therein. (15) Schmidtchen, F. P.; Berger, M. Chem. ReV. 1997, 97, 1609-1646 and references therein. (16) Bucher, C.; Devillers, C. H.; Moutet, J.-C.; Royal, G.; Saint-Aman, E. New J. Chem. 2004, 12, 1584-1589. (17) Kingston, J. E.; Ashford, L.; Beer, P. D.; Drew, M. G. B. J. Chem. Soc., Dalton Trans. 1999, 251-257. (18) Beer, P. D.; Berry, N.; Drew, M. G. B.; Danny Fox, O.; Padilla-Tosta, M. E.; Patell, S. Chem. Commun. 2001, 199-200. (19) Beer, P. D. Chem. Commun. 1996, 689. (20) Beer, P. D.; Szemes, F.; Balzani, V.; Sala`, C. M.; Drew, M. G. B.; Dent, S. W.; Maestri, M. J. Am. Chem. Soc. 1997, 119, 11864.

the ligand 1-(ferrocenemethyl)-1,4,7,10-tetraazacyclododecane (L) (see Figure 1) and the ability of the resulting complex to act as an anion recognition agent. This led to the synthesis and X-ray structure of a novel Cu(II) complex of L, [CuL(NO3)]+ (C1) (see Figure 1), in which the nitrate anion adopts an orientation that significantly distorts the copper coordination sphere. Electrochemical studies in acetonitrile and nitromethane as a function of nitrate concentration demonstrate that recognition of the nitrate ion occurs with respect to both the Fc0/+ (Fc ) ferrocene) and the CuII/I redox couples. These data in conjunction with spectrophotometric measurements allow the species present in solution to be probed and compared to those found in the solid state. Experimental Section Materials. All chemicals were of reagent-grade purity or better and were used as obtained from the commercial suppliers. Highpurity nitrogen gas was used to degas solutions used in electrochemical experiments. Deionized water was distilled prior to use. Instrumentation. 1H and 13C NMR spectra were recorded in deuterated solvents on Bruker AC200, Bruker DPX300, or Avance DRX400 Bruker spectrometers at 30 °C. The chemical shifts, δ, are reported in ppm. Tetramethylsilane or the residual solvent peaks were used as internal references. The abbreviations for the peak multiplicities are as follows: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quartet), m (multiplet), and br (broad). Infrared spectra using KBr disks were recorded with a Bruker IFS 55 FTIR spectrophotometer at a resolution of 4 cm-1. UV-vis spectra were recorded in 1 cm quartz cuvettes using either a Varian Cary 300 or a Varian Cary 5G spectrophotometer. All solutions were filtered with nylon filters (0.45 µm) before being analyzed. To fully dissolve the complex, solutions sometimes needed to be warmed in a water bath. Low-resolution electrospray ionization mass spectra (ESIMS) were obtained with a Micromass Platform II Quadrupole mass spectrometer fitted with an electrospray source. High-resolution MS data were recorded on a Bruker BioApex II 47e FT-ICR MS fitted with an Analytica Electrospray Source. Samples were introduced by a syringe pump at a flow rate of 1 µL min-1. The capillary voltage was 200 V. CHN analyses were performed by the Campbell Microanalytical Services, University of Otago, Dunedin, New Zealand. Inorganic Chemistry, Vol. 46, No. 10, 2007

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Gasser et al. Electrochemical Studies. All voltammetric measurements were performed with a BAS Epsilon potentiostat operated by BASi Epsilon-EC Software, version 1.50.69_XP. A typical three-electrode cell was employed, comprising a Pt counter electrode, an Ag/AgNO3 (acetonitrile, 10 mM AgNO3) reference electrode, and a 3.06 mm diameter glassy-carbon (GC) working electrode. To avoid overlap with the oxidation of the ferrocenyl pendant group, the voltammetrically reversible one-electron decamethylferrocene couple (Cp*2Fe0/+; Cp* ) pentamethylcyclopentadienyl) was used as a secondary internal reference (0.25 mM). Standard potentials were then referenced to the usual Fc0/+ couple using measured values for the Cp*2Fe0/+ process (-505 mV vs Fc0/+ in acetonitrile and -508 mV vs Fc0/+ in nitromethane as determined in the presence of Bu4NPF6 (0.1 M) as the supporting electrolyte and with a scan rate of 100 mV s-1). It was assumed that these potential conversion values are independent of the electrolyte and scan rate. The voltammetry for oxidation of Fc was used to calculate the area of the GC working electrode (0.07 cm2) by using the Randles-Sevcik equation and a diffusion coefficient of Fc of 1.70 × 10-5 cm2 s-1 in acetonitrile (0.5 M [Bu4N][PF6].21,22 All measurements were recorded over a scan-rate range of 10-1000 mV s-1 at (20 ( 2) °C inside a Faraday cage under a N2 atmosphere to minimize electrochemical noise and atmospheric O2/H2O interference, respectively. Synthesis. 1-(Ferrocenemethyl)-1,4,7,10-tetraazacyclododecane (L). L was synthesized following the general procedure published by Sisti et al. and involved a three-step reaction sequence starting from commercially available cyclen.23 However, the following two improvements were introduced. After a first step to obtain the triprotected cyclen compound, 1,4,7-triformyl-1,4,7,10tetraazacyclododecane, the second step involving the synthesis of 1-(ferrocenemethyl)-4,7,10-triformyl-1,4,7,10-tetraazacyclododecane was performed by refluxing the reagents used in Sisti’s procedure (without base) for 16 h in deoxygenated water instead of DMF. Purification was carried out by chromatography on a silica column with CH2Cl2/MeOH in a 20:1 ratio as the eluent (Rf ) 0.45). An increase in the yield from 53 to 65% was obtained via this modified procedure. The deprotection of the formyl groups was then carried out by heating, for 24 h, 1-(ferrocenemethyl)4,7,10-triformyl-1,4,7,10-tetraazacyclododecane at 80 °C in 1 M NaOH instead of 0.2 M NaOH. Purification of the final product L was performed by chromatography on a silica column with MeOH/ diethylamine in a 10:1 ratio as the eluent (Rf ) 0.31). The required compound obtained after chromatography was dissolved in a minimal amount of CH2Cl2 and filtered to remove any silica that might have dissolved in the eluent. The filtered organic phase was then washed with water to remove any inorganic salts that were present. Anal. Calcd for FeC19H30N4‚H2O: C, 58.8; H, 8.3; N, 14.4. Found: C, 59.3; H, 7.8; N, 14.0. Major IR bands (KBr; ν, cm-1): 3422 m br, 3205 m, 3091 m, 2928 s, 2818 s, 1719 w, 1655 m, 1561 m, 1545 m, 1458 s, 1330 m, 1223 m, 1104 s, 1038 s, 925 m, 803 s, 736 m. UV-vis (CHCl3, λ (nm),  (M-1 cm-1)): 433, 134. 1H NMR (CDCl3): δ 2.48-2.63 (m, 13H, CH2 cyclen ring and NH), 2.75-2.82 (m, 5H, CH2 cyclen and NH), 3.49 (s, 1H, NH), 3.56 (s, 2H, Cp-CH2-cyclen), 4.08-4.12 (m, 9H, CH Cp ring). 13C NMR (CDCl3): δ 45.32 (CH2 cyclen), 46.40 (CH2 cyclen), 47.38 (CH2 cyclen), 50.68 (CH2 cyclen), 54.05 (Cp-CH2(21) Bard, A. J.; Faulkner, L. R. Electrochemical MethodssFundamentals and Applications; Wiley: New York, 2002. (22) Mirkin, M. V.; Richards, T. C.; Bard, A. J. J. Phys. Chem. 1993, 97, 7671. (23) Boldrini, V.; Giovenzana, G. B.; Pagliarin, R.; Pamisano, G.; Sisti, M. Tetrahedron Lett. 2000, 41, 6527-6530.

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cyclen), 68.15 (CH Cp), 68.63 (CH Cp), 70.24 (CH Cp), 83.01 (C Cp). ESIMS (m/z): 199 [Fc-CH2]+ (32%), 371 [M + H]+ (100%). 1,4,7-Tris-tert-butoxycarbonyl-1,4,7,10-tetraazacyclododecane. This compound was synthesized following the procedure of Wieghardt et al.24 The analytical data matched that reported previously.24 (Ferrocenylmethyl)trimethylammonium Iodide. This compound was synthesized following the procedure of Lindsay and Hauser.25 The analytical data of the product matched that reported previously.25 1,4,7-Tris-tert-butoxycarbonyl-10-(ferrocenylmethyl)-1,4,7,10-tetraazacyclododecane (1). 1,4,7-Tris-tert-butoxycarbonyl1,4,7,10-tetraazacyclododecane (0.75 g, 1.58 mmol), (ferrocenylmethyl)trimethylammonium iodide (0.61 g, 1.58 mmol), and K2CO3 (1.09 g, 7.90 mmol) were refluxed in anhydrous DMF (20 mL) for 18 h. The solvent was then removed under reduced pressure, and CH2Cl2 (25 mL) was poured onto the black-brown residue. The suspended solid was removed by filtration, and the filtrate was washed with water (3 × 15 mL). The solvent in the organic phase was removed under reduced pressure to give an orange sticky-solid residue. Purification by chromatography on a silica column was performed with CH2Cl2/acetone in a 25:1 ratio as the eluent (Rf ) 0.30). The desired fractions were evaporated to dryness to yield 1 as an orange solid. Yield: 0.45 g (42%). Anal. Calcd for FeC34H54N4O6·1/2CH2Cl2: C, 58.1; H, 7.8; N, 7.9. Found: C, 58.5; H, 7.9; N, 7.9. Major IR bands (KBr; ν, cm-1): 3095 w, 2976 m, 1686 s, 1559 m, 1415 s, 1362 s, 1316 w, 1250 s, 1172 s, 1104 m, 1025 w, 979 w, 920 w, 884 w, 859 w, 820 w, 772 w, 733 w, 645 w. 1H NMR (CDCl3): δ 1.43 (s, 18H, O-C(CH3)3), 1.46 (s, 9H, O-C(CH3)3), 2.48-2.63 (m, 4H, CH2 cyclen ring), 3.15-3.58 (m, 14H, CH2 cyclen ring and Cp-CH2-cyclen), 4.05-4.15 (m, 9H, CH Cp ring). 13C NMR (CDCl3): δ 28.70 (O-C(CH3)3), 28.90 (O-C(CH3)3), 47.38 (CH2 cyclen), 47.76 (CH2 cyclen), 48.24 (CH2 cyclen), 54.24 (CH2 cyclen), 55.75 (Cp-CH2-cyclen), 68.44 (CH Cp), 68.73 (CH Cp), 70.69 (CH Cp), 79.37 (C Cp), 79.56 (OC(CH3)3), 79.65 (O-C(CH3)3), 155.58 (N-COO), 155.84 (NCOO). ESIMS (m/z): 671 [M + H]+ (100%). High-resolution ESI mass determination: found, 671.3464; calcd for FeC34H55N4O6, 671.3471. [Cu(L)(NO3)][NO3] (C1). L (50 mg, 0.14 mmol) and Cu(NO3)2· 3H2O (32 mg, 0.14 mmol) were dissolved in ethanol (15 mL) and refluxed for 30 min. After cooling to room temperature, diethyl ether was added to precipitate the complex, which was filtered, washed with diethyl ether, and dried. Blue crystals suitable for X-ray crystallography were obtained by diffusion of diethyl ether into an acetonitrile solution of C1. Yield: 61 mg (81%). Anal. Calcd for CuFeC19H30N6O6: C, 40.9; H, 5.4; N, 15.1. Found: C, 40.8; H, 5.3; N, 14.8. Major IR bands (KBr; ν, cm-1): 3179 s br, 2918 s, 1633 m, 1382 vs, 1234 m, 1104 s, 1081 s, 999 m, 815 m, 742 w, 592 w, 513 w. ESIMS (m/z): 199 [Fc-CH2]+ (38%), 432 [L + Cu - H]+ (100%), 495 [C1]+ (15%). Nitrato(1,4,7,10-tetraazacyclododecane)copper(II) Nitrate (C5). C5 was synthesized following the procedure described by Styka et al.26 Blue crystals suitable for X-ray crystallography were obtained by diffusion of diethyl ether into a solution of C5 in acetonitrile. The unit cell was the same as that reported in the literature.27 Major IR bands (KBr; ν, cm-1): 3171 m br, 2927 m, 1629 w, 1382 s, (24) Kimura, S.; Bill, E.; Bothe, W.; Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 6025-6039. (25) Lindsay, J. K.; Hauser, C. R. J. Org. Chem. 1957, 22 (4), 355. (26) Styka, M. C.; Smierciak, R. C.; Blinn, E. L.; DeSimone, R. E.; Passariello, J. V. Inorg. Chem. 1978, 17, 82-86.

Nitrate Binding to a CuII-Cyclen Complex Table 1. Crystallography Data for C1 crystal empirical formula M/g mol-1 cryst syst space group a/Å b/Å c/Å V/Å3 Z T/K λ/Å Dc/g cm-3 µ(Mo KR)/mm-1 no. of data measd unique data (Rint) obs data [I > 2(σ)I] restraints no. of params final R1, wR2 (obs data) final R1, wR2 (all data) Fmin, Fmax/e Å-3 a

C1 C19H30CuFeN6O6 557.88 orthorhombic Pnma 16.6366(11) 9.9935(7) 13.6266(9) 2265.5(3) 4 123(2) 0.71073 1.636 1.628 12 015 2602 (0.0322) 2286 6 208 0.0555,a 0.1286b 0.0633, 0.1331 -0.879, 0.926

R1 ) ∑||Fo| - |Fc||/∑|Fo|. bwR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.

1307 m, 1240 w, 1124 w, 1081 m, 1035 w, 1012 m, 983 m, 909 w, 868 w, 807 w, 582 w. X-ray Crystallography. Intensity data for C1 (0.15 mm × 0.10 mm × 0.08 mm) were measured at 123 K on a Bruker Apex 2 CCD fitted with a graphite-monochromated Mo KR radiation (0.71073 Å) source. The data were collected to a maximum 2θ value of 55° and processed using the Bruker Apex 2 software. (Collection and refinement parameters are summarized in Table 1.) The structure was solved using Patterson heavy-atom search methods and expanded using standard Fourier routines in the SHELX9728,29 software package. The structure was solved in the space group Pnma (attempts to solve it in a lower symmetry space group were unsuccessful, giving an unreasonably high Flack parameter). There was disorder about the cyclen ring with three of the methylene C atoms and the tertiary cyclen N atom located in two distinct positions. These disordered components had their respective site occupancies set to 50%. The coordinating nitrate was also found to be in two discrete positions, each modeled at 50% occupancy. The nitrate N was initially located on the special position (x, 0.75, z), but this gave unreasonable N-O distances. Consequently, it was refined in two positions on either side of the special position and isotropic restrictions were placed on its anisotropic refinement. All H atoms were placed in idealized positions while all non-H atoms were refined anisotropically.

Results and Discussion Synthesis and Characterization. The ligand 1-(ferrocenemethyl)-1,4,7,10-tetraazacyclododecane (L) was obtained via modifications of the synthesis reported by Sisti et al., as described in the Experimental Section.23 However, a potentially more efficient route to L also was attempted in initial experiments. Briefly, following the facile formation of 1,4,7tris-tert-butoxycarbonyl-10-(ferrocenylmethyl)-1,4,7,10-tetraazacyclododecane (1) (see Figure 1) by reaction of 1,4,7tris-tert-butoxycarbonyl-1,4,7,10-tetraazacyclododecane24 with (27) Clay, R.; Murray-Rust, P.; Murray-Rust, J. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1979, 35, 1894. (28) Sheldrick, G. M. SHELXS97; Program for Crystal Structure Solution; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (29) Sheldrick, G. M. SHELXL97; Program for Crystal Structure Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

Figure 2. Thermal ellipsoid plot of the complex cation of C1. H atoms, a nitrate counterion, and disorder are omitted for clarity (thermal ellipsoids drawn at 50%).

(ferrocenylmethyl)trimethylammonium iodide,25 it was anticipated that L could be obtained by removing the tertbutoxycarbonyl protecting groups with trifluoroacetic acid or aqueous solutions of HCl. Unfortunately, this approach was unsuccessful, as ligand L could not withstand these cleavage conditions, as previously found by Metzler-Nolte et al. for other ferrocenyl derivatives.30 Consequently, it was not possible to isolate a stable hydrochloride (or hydrobromide) salt of L. C1 was then obtained by reacting equimolar amounts of Cu(NO3)2‚3H2O and L in ethanol. The complex was precipitated with diethyl ether. IR spectroscopy on C1 shows characteristic bands at 2918 cm-1 corresponding to aromatic C-H stretching. The band at 3179 cm-1 could be due to N-H stretching, and the lower than expected frequency could be indicative of hydrogen bonding. The presence of the nitrate groups is confirmed by a strong band at 1382 cm-1. Definitive evidence of the formation of C1 was obtained by X-ray structural analysis as described below and by the concordance of the microanalysis data with expected values. X-ray Crystal Structure of C1. The X-ray crystal structure of C1 (see Figure 2) reveals a highly distorted Cu(II) geometry. Formally, the C1 coordination sphere is occupied by five bonding atoms, the four N atoms from the cyclen macrocycle and one of the O donors from the nitrate (for bond angles and distances, see Table 2). If we consider the Cu(II) geometry to be distorted square pyramidal, then the basal plane is defined by the N donors, and the Cu(II) resides out of the plane defined by the N4 ring by 0.513(3) Å, a value similar to that found for the Cu(II) complex of the parent macrocycle, [Cu(cyclen)(NO3)]+ (C5) (see Figure 1).27 However, the N-Cu-O(1) angles in C1 vary substantially, viz., 105.0(3)°, 99.3(3)°, 122.9(3)°, 104.2(3)°, and 89.0(4)° for N(1)/N(1)a/N(2)/N(3)/N(2)a-Cu(1)-O(1), respectively (symmetry operator a: x, 3/2 - y, z) (see Table (30) Sehnert, J.; Hess, A.; Metzler-Nolte, N. J. Organomet. Chem. 2001, 637-639, 349-355 and references therein.

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Gasser et al. Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for O(1)-Cu(1) O(2)-Cu(1) Cu(1)-N(1) Cu(1)-N(2) Cu(1)-N(2)a Cu(1)-N(3) Cu(1)-N(1)a

a

2.266(8) 2.730(4) 2.069(6) 1.999(5) 1.999(5) 2.006(5) 2.069(6)

N(2)-Cu(1)-O(1) N(2)a-Cu(1)-O(1) N(3)-Cu(1)-O(1) N(1)a-Cu(1)-O(1) N(1)-Cu(1)-O(1) N(2)-Cu(1)-N(2)a N(2)-Cu(1)-N(3) N(2)a-Cu(1)-N(3) N(2)-Cu(1)-N(1)a N(2)a-Cu(1)-N(1) N(3)-Cu(1)-N(1)a N(3)-Cu(1)-N(1)

C1a

122.9(3) 89.0(4) 104.2(3) 99.3(3) 105.0(3) 148.0(2) 86.63(13) 86.63(13) 95.0(2) 95.0(2) 150.8(2) 150.8(2)

Figure 3. View of hydrogen-bonding interaction connecting adjacent cations in C1. The ferrocenyl moiety is omitted for clarity.

Symmetry operator a: x, 3/2 - y, z.

Table 3. Hydrogen Bonds for C1 (Å and deg) D-H‚‚‚A

d(D-H)

d(H‚‚‚A)

d(D‚‚‚A)

∠(DHA)

N(2)-H(2)‚‚‚O(2) N(2)-H(2)‚‚‚O(2)b N(2)-H(2)‚‚‚O(3)b N(3)-H(3)‚‚‚O(4)a N(3)-H(3)‚‚‚O(4)

0.93 0.93 0.93 0.93 0.93

2.64 1.69 2.51 2.23 2.23

3.010(9) 2.607(1) 3.083(6) 3.067(6) 3.067(6)

100 170 120 150 150

a Symmetry operators a: x, 3/ -y, z. b Symmetry operators b: -x + 1, 2 -y + 2, -z + 1.

2), whereas in C5 they were all quite similar (107.8(2)°, 105.9(2)°, 100.6(2)°, and 104.9(2)°). This variation in the N-Cu-O angles in C1 coupled with the fact that the adjacent N-Cu-N angles vary from 95.0(2)° for N(1)-Cu(1)-N(2)a to 86.6(1)° for N(2)-Cu(1)-N(3) (cf. ∼85° in C5) leads us to consider the origin of this distortion in the geometry of Cu(1). Given that in C5 the nitrate is unidentate and binds perpendicularly to the plane formed by the four cyclen N atoms, the orientation of the second O atom on the nitrate ion toward the CuII center in C1 was unexpected. A weak interaction of the second nitrate O with the CuII center, as well as a hydrogen-bonding interaction between O(2) and the proton on N(2), contribute to this distortion. The Cu(1)-O(1) distance of 2.266(8) Å is typical of apical Cu-O distances for CuII complexes, which normally exhibit axial elongation. This distance is slightly longer than that found for C5 (2.183(4) Å).20 In the case of O(2), the interaction with Cu(1) may be mainly electrostatic rather than coordinative in nature, since the Cu(1)-O(2) distance of 2.730(4) Å is 0.2 Å shorter than the sum of the van der Waals radii of the two atoms (2.92 Å); cf. 0.65 Å for Cu(1)-O(1). The positioning of O(2) may also be affected by a relatively weak hydrogen bond with H(2) reflected by a N‚‚‚O distance of 3.010(9) Å (see Table 3; H atoms not located). There is a further stronger hydrogen bond between the secondary N atoms on the cyclen rings and the nitrate anions on an adjacent complex (viz., N‚‚‚O distance of 2.607(1) Å), which may be helping to position the second nitrate O near the CuII center (Figure 3). A further weaker interaction with the N(3) hydrogen generates a hydrogen-bonded “chelate” ring with the noncoordinating nitrate (N‚‚‚O 3.067(6) Å). It is noteworthy that long Cu-O distances have been reported for nitrates chelating to CuII.31-33 For example, a (31) Bernarducci, E. E.; Bharadwaj, P. K.; Lalancette, R. A.; KroghJespersen, K.; Potenza, J. A.; Schugar, H. J. Inorg. Chem. 1983, 22, 3911-3920.

3880 Inorganic Chemistry, Vol. 46, No. 10, 2007

Figure 4. Stick representation plot of the Cu coordination sphere, highlighting the pseudo-trigonal-prismatic (TP) geometry (blue planes show planes north and south of the prism; red hexagons represent the O positions of ideal TP geometry).

Cu-O distance of 2.568(3) Å was reported by Wisniewski et al.34 for bis(2-(4′-thiazolyl)benzimidazole-N,N′)nitratocopper(II) nitrate and of 2.674(1) Å by Casellas et al. for a trinuclear CuII complex.35 Reedijk and co-workers reported an analysis of the structure of a series of complexes of tripodal tetradentate ligands (CuII, NiII, and CdII) in which they proposed three bonding modes for nitrate anions: monodentate for CuII [difference in two Cu-O distances >0.6 Å); anisodentate (asymmetric) for CdII (difference in two Cd-O distances 0.3-0.6 Å); and bidentate for NiII (difference in two Ni-O distances