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Synthesis and hypodentate Cu(II) complexes of new tripodal tetraamine ligands incorporating a long pendant arm
Polyhedron 23 (2004) 97–102 www.elsevier.com/locate/poly
Synthesis and hypodentate Cu(II) complexes of new tripodal tetraamine ligands incorporating a long pendant arm Natasha J. Lundin, Ian G. Hamilton, Allan G. Blackman
*
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received 16 July 2003; accepted 13 September 2003 This paper is dedicated to Professor George Christou on the happy occasion of his 50th birthday
Abstract The synthesis and characterisation of the new aliphatic tripodal amine ligands apba (N -(5-aminopentyl)-N,N-bis(2-aminoethyl)amine) and ahba (N -(6-aminohexyl)-N,N-bis(2-aminoethyl)amine) are reported. The tetrahydrochloride salts of these ligands, as well as that of the previously reported ligand abba (N -(4-aminobutyl)-N,N-bis(2-aminoethyl)amine), react with CuCO3 Cu(OH)2 to give complexes in which the tripodal ligand coordinates to the Cu(II) ion in a hypodentate fashion, with the longest arm of the tripodal ligand remaining protonated and unbound in all cases. The crystal structure of [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH reveals a five-coordinate Cu(II) ion in a slightly distorted square pyramidal geometry. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Hypodentate; Tripodal amine ligand; Cu(II) complexes; Crystal structure; Five-coordinate
1. Introduction The stereochemical features peculiar to tripodal tetradentate amine ligands have led to their use in a wide variety of inorganic systems. Six-coordinate octahedral complexes containing such ligands invariably display cis geometries, and the inequivalence of the remaining two coordination sites in such complexes has been utilised in numerous studies aimed at monitoring the stereochemical outcome of ligand substitution reactions in Co(III) complexes containing these ligands [1–4]. The steric constraints imposed by such ligands often result in trigonal bipyramidal geometries for five-coordinate systems [5–7], while the rare examples of four-coordinate complexes containing tripodal ligands display distorted tetrahedral geometries [8,9], owing to the inability of these ligands to accommodate a square planar geometry. Of the fully aliphatic tripodal tetradentate amine ligands, the C3v symmetric ligands tren (tris(2-aminoethyl)amine) and trpn (tris(3-aminopropyl)amine), in which *
Corresponding author. Tel.: +64-3-479-7931; fax: +64-3-479-7906. E-mail address: [email protected] (A.G. Blackman).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2003.09.030
all three arms are the same length, have been known for many years [10,11], and it is only in relatively recent times that ligands of lower symmetry have been prepared and their complexes studied (Fig. 1). These ligands generally bind to a single transition metal ion using all four nitrogen donor atoms, although examples of hypodentate coordination [12], in which the ligand binds using less than its full complement of donor atoms, have been reported [13–18]. A number of the ligands outlined in Fig. 1 contain 4-aminobutyl or 5aminopentyl arms [19–22] and can potentially form seven- or eight-membered chelate rings on binding to a metal ion, but their coordination chemistry has not been investigated to date. In this paper we report the synthesis of the new tripodal ligands apba (N -(5-aminopentyl)-N,N-bis(2-aminoethyl)amine) and ahba (N -(6aminohexyl)-N,N-bis(2-aminoethyl)amine), as well as an alternative synthesis of abba (N -(4-aminobutyl)-N,Nbis(2-aminoethyl)amine). Although capable of forming complexes containing large chelate rings, these ligands display hypodentate coordination in Cu(II) complexes, with the longest arm remaining protonated and unbound in all cases. These findings are confirmed by a
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Fig. 1. Structures of the known aliphatic tripodal tetraamine ligands. References to their synthesis are as follows: abap [29], baep [31], abba [19], epb [20], apba, this work, ahba, this work, abbp [21], apbb [21], trbn [21], trpa [22].
crystal structure of the complex [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O.
2. Experimental 2.1. General All chemicals used were LR grade or better. 13 C and H NMR spectra were obtained in D2 O solution on a Varian Inova 300 MHz spectrometer at 25 °C. Chemical shifts are reported relative to NaTSP (1 H, 0.00 ppm) or dioxane (13 C, 67.8 ppm) as internal references. Elemental analyses were performed by the Campbell Microanalytical Laboratory, University of Otago. Electrospray mass spectra were recorded on a Shimadzu LCMS-QP800a spectrometer. Diphthaloyldiethylenetriamine was prepared as described [19]. N -(4-bromobutyl)phthalimide and N -(6bromohexyl)phthalimide were prepared by adaptation of the method used for the synthesis of N -(2-bromoethyl)phthalimide [23] (recrystallisation from cyclohexane and ethanol, respectively), while N -(5-bromopentyl) phthalimide was prepared according to a literature method [24]. 1
2.2. Ligand syntheses: general procedure An equimolar mixture of diphthaloyldiethylenetriamine and the appropriate bromoalkylphthalimide was heated at 190 °C for 30 min with constant stirring. Following cooling, the glassy brown solid was ground to a powder and heated under reflux in 8 M HCl overnight. The solution was cooled and the precipitated phthalic acid removed by filtration. The filtrate was reduced to a yellow oil (rotary evaporator), which was diluted to 1 L with water and loaded onto a Dowex 50W-X2 (Hþ form) column. This was washed with 1 M HCl and the product was removed on elution with 3 M HCl. Removal of the solvent (rotary evaporator) gave the crude product which was recrystallised from hot ethanol (a small amount of ether was added to the cooled solution to induce crystallisation if necessary) to give the pure ligand tetrahydrochloride as a white powder. 2.2.1. N-(4-aminobutyl)-N,N-bis(2-aminoethyl)amine tetrahydrochloride (abba 4HCl) Diphthaloyldiethylenetriamine (31.0 g, 85.3 mmol), N -(4-bromobutyl)phthalimide (24.6 g, 87.2 mmol), 8 M HCl (600 mL). Yield: 11.0 g (40.3%). Found: C, 29.6; H, 8.3; N, 16.9; Cl, 43.9. Calc. for C8 H26 N4 Cl4 : C, 30.0; H,
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8.2; N, 17.5; Cl, 44.3%. 1 H NMR (D2 O), dH : 1.8 (m, 4H), 3.08 (t, J ¼ 6 Hz, 2H), 3.36 (t, J ¼ 6 Hz, 2H), 3.56 (m, 8H) ppm. 13 C NMR (D2 O), dC : 21.8, 25.0, 35.1, 40.1, 50.9, 54.5 ppm. MS (ESI): m=z 175 (MHþ ). 2.2.2. N-(5-aminopentyl)-N,N-bis(2-aminoethyl)amine tetrahydrochloride (apba 4HCl) Diphthaloyldiethylenetriamine (23.1 g, 63.6 mmol), N -(5-bromopentyl)phthalimide (19.2 g, 64.8 mmol), 8 M HCl (190 mL). Yield: 8.02 g (37.7%). Found: C, 32.2; H, 8.4; N, 16.4; Cl, 42.0. Calc. for C9 H28 N4 Cl4 : C, 32.3; H, 8.4; N, 16.8; Cl, 42.4%. 1 H NMR(D2 O), dH : 1.51 (m, 2H), 1.78 (m, 2H), 1.87 (m, 2H), 3.06 (t, J ¼ 7:5 Hz, 2H), 3.39 (t, J ¼ 8:7 Hz, 2H), 3.56 (m, 4H), 3.66 (m, 4H) ppm. 13 C NMR (D2 O), dC : 23.3, 23.4, 26.9, 34.3, 39.8, 50.2, 54.4 ppm. MS (ESI): m=z 189 (MHþ ). 2.2.3. N-(6-aminohexyl)-N,N-bis(2-aminoethyl)amine tetrahydrochloride (ahba 4HCl) Diphthaloyldiethylenetriamine (4.57 g, 12.6 mmol), N -(6-bromohexyl)phthalimide (3.99 g, 12.9 mmol) 8 M HCl (100 mL). Yield: 1.14 g (26.0%). Found: C, 34.7; H, 9.1; N, 15.9; Cl, 40.7. Calc. for C10 H30 N4 Cl4 : C, 34.5; H, 8.7; N, 16.1; Cl, 40.7%. 1 H NMR (D2 O), dH : 1.47 (m, 4H), 1.72 (m, 2H), 1.83 (m, 2H), 3.03 (t, J ¼ 7:8 Hz, 2H), 3.35 (t, J ¼ 8:4 Hz, 2H), 3.57 (m, 4H), 3.64 (m, 4H) ppm. 13 C NMR (D2 O), dC : 23.5, 25.78, 25.81, 27.1, 34.3, 39.9, 50.1, 54.6 ppm. MS (ESI): m=z 203 (MHþ ). 2.3. Complex syntheses Caution! Although we have experienced no difficulties with the perchlorate complexes described below, the usual care afforded all perchlorate species should be taken in their preparation and handling. 2.3.1. [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH A suspension of abba 4HCl (0.50 g, 1.6 mmol) and CuCO3 Cu(OH)2 (0.17 g, 0.77 mmol) in acetonitrile (50 mL) was heated under reflux for 2 h. The solution was allowed to cool, and the resulting green powder was removed by filtration. The powder was dissolved in the minimum volume of methanol and slow diethyl ether diffusion over three days gave bright blue needles suitable for X-ray diffraction. These crystals lose solvent readily and the analytical data thus refer to the desolvated material [Cu(abbaH)Cl2 ]Cl 2H2 O (0.21 g, 36%). Found: C, 24.7; H, 7.0; N, 14.2; Cl, 28.2. Calc. for C8 H27 N4 Cl3 CuO2 : C, 25.2; H, 7.1; N, 14.7; Cl, 27.9%. 2.3.2. [Cu(apbaH)Cl(ClO4 )]ClO4 A suspension of apba 4HCl (0.50 g, 1.5 mmol) and CuCO3 Cu(OH)2 (0.17 g, 0.77 mmol) in MeOH (20 mL) was heated under reflux for 2 h to give a deep green solution which was then hot filtered. Addition of LiClO4
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to the filtrate gave rapid formation of blue microcrystals. Following cooling in the freezer, the product was filtered off, washed with diethyl ether and dried in a vacuum desiccator. (0.29 g, 40%). Found: C, 22.2; H, 5.3; N, 11.2; Cl, 22.3. Calc. for C9 H25 N4 Cl3 CuO8 : C, 22.2; H, 5.2; N, 11.5; Cl, 21.8%. 2.3.3. [Cu(ahbaH)Cl(ClO4 )]ClO4 A suspension of ahba 4HCl (0.50 g, 1.4 mmol) and CuCO3 Cu(OH)2 (0.15 g, 0.68 mmol) in MeOH (20 mL) was heated under reflux for 2 h to give a deep green solution which was then hot filtered. Addition of LiClO4 to the filtrate and storage in the freezer gave the product as blue microcrystals, which were then washed with diethyl ether and dried in a vacuum desiccator (0.27 g, 40%). Found: C, 24.1; H, 5.7; N, 11.0; Cl, 21.7. Calc. for C10 H27 N4 Cl3 CuO8 : C, 24.0; H, 5.4; N, 11.2; Cl, 21.2%. 2.4. X-ray crystallography Data for [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH were collected at )115 °C on a Siemens SMART system using graphite monochromated Mo Ka radiation with exposures over 0.3°. Data were corrected for Lorentz and polarisation effects using S A I N T [25]. The structure was solved by direct methods using SIR-97 [26] running within the WinGX package [27], with the resulting Fourier map revealing the location of all non-hydrogen atoms. Weighted full matrix refinement on F 2 was carried out using S H E L X L -97 [28] with all non-hydrogen atoms being refined anisotropically. Hydrogen atoms were included in calculated positions and were refined as riding atoms with individual (or group, if appropriate) isotropic displacement parameters. O3 was found to be disordered over two sites, with approximately 60:40 occupancy. Disorder of O2 was also evidenced by large thermal parameters, but this was not investigated further. Details of the crystal, data collection and structure refinement are given in Table 1.
3. Results and discussion 3.1. Synthesis of the tripodal ligands The tetrahydrochloride salts of the ligands abba, apba and ahba were prepared in moderate yields in an analogous fashion to similar tripodal tetraamine ligands, by the solventless reaction of a melt of the appropriate bromoalkylphthalimide with diphthaloyldiethylenetriamine, followed by deprotection in refluxing HCl (Scheme 1) [19,20,29–31]. In our hands, this appears to be an easier method of preparing tripodal ligands than the alternative procedure involving synthesis of the appropriate trinitrile and subsequent reduction with LiAlH4 / H2 SO4 [32,33]. Purification of the crude ligands was
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Table 1 Crystal data and structure refinement for [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O Empirical formula Formula weight Temperature (K) ) Wavelength (A Crystal system Space group Unit cell dimensions ) a (A ) b (A ) c (A b (°) 3 ) Volume (A Z Density (calculated) (Mg/m3 ) Absorption coefficient (mm1 ) F (0 0 0) Crystal size (mm) Theta range for data collection (°) Index ranges Reflections collected Independent reflections Completeness to theta ¼ 26.42° (%) Absorption correction Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F 2 Final R indices [I > 2rðIÞ] R indices (all data) 3 ) Largest differential peak and hole (e A
C9 H27 Cl3 CuN4 O3 409.24 158(2) 0.71069 monoclinic P 2(1)/n 6.247(5) 20.122(5) 15.729(5) 92.734(5) 1974.9(18) 4 1.376 1.521 852 0.44 0.38 0.28 2.40–26.42 )7 6 h 6 4, )24 6 k 6 25, )19 6 l 6 19 24 811 3999 [Rint ¼ 0.0216] 98.7 semi-empirical from equivalents 1.000 and 0.911 full-matrix least-squares on F 2 3999/0/184 1.150 R1 ¼ 0.0566, wR2 ¼ 0.1671 R1 ¼ 0.0636, wR2 ¼ 0.1704 1.196 and )0.685
3.2. Synthesis of metal complexes
Scheme 1. The synthetic method used to prepare apba 4HCl and ahba 4HCl.
achieved by chromatography on Dowex 50W-X2 cation exchange resin (elution with 3M HCl) and the tetrahydrochloride salts of the pure ligands were obtained as white powders on recrystallisation from ethanol. Elemental analysis, mass spectral and 1 H and 13 C NMR data were consistent with the structures given in Fig. 1.
The tetrahydrochloride salts of abba, apba and ahba react with half a mole equivalent of CuCO3 Cu(OH)2 in either acetonitrile (abba) or methanol (apba, ahba), to give dissolution of the metal salts and production of bright green solutions. A green abba complex precipitated from the acetonitrile solution on cooling and the blue complex [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH was isolated following recrystallisation from methanol. The green solutions obtained from the reaction of both apba and ahba in methanol slowly turned blue on cooling, and addition of LiClO4 to these solutions gave the complexes [Cu(apbaH)Cl(ClO4 )]ClO4 and [Cu(ahbaH)Cl(ClO4 )] ClO4 , respectively. Microanalytical data for all complexes were consistent with the presence of three counterions (Cl or ClO 4 ), strongly suggesting that the tripodal ligands were coordinating in a hypodentate fashion with a protonated pendant arm remaining uncoordinated to the metal ion. This was confirmed by the X-ray structure of [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O. 3.3. Crystal structure of [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O Crystallographic data for [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O are given in Table 1, bond lengths and
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Table 2 ) and angles (°) for [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH Bond lengths (A Bond lengths Cu(1)–N(1) Cu(1)–N(3) Cu(1)–Cl(3) N(3)–C(1) N(3)–C(5) N(4)–C(8) C(7)–C(8) N(1)–C(2) C(5)–C(6) Bond angles N(1)–Cu(1)–N(2) N(2)–Cu(1)–N(3) N(2)–Cu(1)–Cl(2) N(1)–Cu(1)–Cl(3) N(3)–Cu(1)–Cl(3) C(4)–N(2)–Cu(1) C(1)–N(3)–C(5) C(1)–N(3)–Cu(1) C(5)–N(3)–Cu(1) C(8)–C(7)–C(6) N(4)–C(8)–C(7) N(3)–C(1)–C(2) N(3)–C(3)–C(4)
1.997(5) 2.095(4) 2.541(2) 1.477(7) 1.491(6) 1.491(7) 1.521(7) 1.475(8) 1.522(7) 160.20(18) 83.81(17) 92.40(13) 95.86(13) 101.10(11) 111.6(3) 111.2(4) 105.2(3) 111.8(3) 111.8(4) 110.5(4) 109.8(4) 109.4(4)
angles are given in Table 2, and Fig. 2 shows an ORTEP [34] diagram of the [Cu(abbaH)Cl2 ]þ cation. The structure consists of a five-coordinate Cu(II) ion coordinated to two chloride ligands and three nitrogen atoms of the abba ligand in a slightly distorted square pyramidal geometry (s ¼ 0:03) [35], with the three nitrogen atoms and one chloride ion occupying the basal plane. The remaining N atom of the abba ligand is protonated, as evidenced by the presence of three chloride ions in the structure, and remains uncoordinated to the metal ion. The copper ion lies approximately 0.32 A above the basal plane defined by the atoms N1, Cl2, N2 ) is sigand N3. The axial Cu1–Cl3 bond (2.541(2) A ), nificantly longer than the Cu1–Cl2 bond (2.3221(14) A as has been found in a number of other five-coordinate Cu(II) complexes [36]. The Cu1–N1–C2–C1–N3 chelate ring adopts a symmetric skew k conformation while the
Fig. 2. ORTEP diagram of the [Cu(abbaH)Cl2 ]þ cation. Ellipsoids are drawn at the 50% probability level.
Cu(1)–N(2) Cu(1)–Cl(2) N(2)–C(4) N(3)–C(3) O(1)–C(9) C(4)–C(3) C(7)–C(6) C(2)–C(1)
N(1)–Cu(1)–N(3) N(1)–Cu(1)–Cl(2) N(3)–Cu(1)–Cl(2) N(2)–Cu(1)–Cl(3) Cl(2)–Cu(1)–Cl(3) C(1)–N(3)–C(3) C(3)–N(3)–C(5) C(3)–N(3)–Cu(1) N(2)–C(4)–C(3) C(2)–N(1)–Cu(1) N(1)–C(2)–C(1) N(3)–C(5)–C(6) C(7)–C(6)–C(5)
2.015(4) 2.3221(14) 1.482(6) 1.487(7) 1.414(8) 1.509(7) 1.515(7) 1.517(9)
85.05(19) 93.19(14) 162.05(11) 102.31(13) 96.85(5) 113.8(4) 112.7(4) 101.4(3) 108.5(4) 109.2(4) 107.2(4) 115.8(4) 111.2(4)
Cu1–N3–C3–C4–N2 chelate ring exists in an asymmetric envelope d conformation [37]. The dangling aminobutyl chain displays an anti conformation about all C–C and C–N bonds, and the protonated terminal nitrogen atom is involved in hydrogen-bonding to both Cl3 and Cl4 of a neighbouring molecule (N4 Cl3 ¼ , N4 Cl4 ¼ 3.106(6) A ) and the oxygen atom 3.209(5) A of the methanol of crystallisation (N4 O1 ¼ 2.827(7) ). N1 and N2 also participate in hydrogen-bonding A interactions with chloride counterions. 3.4. Discussion The stability of five- and six-membered chelate rings in coordination complexes is usually explained in terms of the chelate effect, in which a positive entropy term renders chelation thermodynamically favourable [38]. However, the process of chelation becomes less entropically favourable for systems in which chelate rings containing more than six atoms can be formed, and such rings are also inherently strained. In agreement with this, a search of the Cambridge Structural Database [39] revealed 9891 metal complexes containing the chelated –N(CH2 )2 N– fragment and 2762 containing the chelated –N(CH2 )3 N– fragment, but only 140, 59 and 6 containing the chelated –N(CH2 )4 N–, –N(CH2 )5 N– and –N(CH2 )6 N– fragments, respectively. In the majority of these latter complexes, the large chelate rings are incorporated in macrocyclic or cryptand ligands, and this preorganisation undoubtedly aids chelation. The moderate steric constraints afforded by the tripodal ligands abba, apba and ahba are apparently not sufficient to
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enforce chelation of the longest pendant arm of these ligands, and they thus coordinate to Cu(II) in a hypodentate fashion. However, it is interesting to note in this respect that when abba is incorporated into a Schiffbase macrocycle, all four N atoms coordinate to Cu(II) in the resulting complex [20]. X-ray quality crystals of [Cu(apbaH)Cl(ClO4 )]ClO4 and [Cu(ahbaH)Cl(ClO4 )]ClO4 could not be obtained and thus the exact geometries of these complexes remain uncertain. However, the presence of three counterions, as shown by elemental analysis, strongly suggests both complexes to be hypodentate, with either a five-coordinate (coordinated chloride and perchlorate) or four-coordinate (coordinated chloride) Cu(II) ion and a non-coordinated protonated aminoalkyl arm. The presence of non-coordinated nitrogen atoms in the hypodentate complexes described herein affords several synthetic opportunities, as these complexes can potentially be used as building blocks for the synthesis of polymetallic species. The non-coordinated nitrogen atom can bind to another metal ion following deprotonation, or it can be used to prepare novel Schiff-base ligands through condensation with an appropriate carbonyl functionality. We are currently investigating these possibilities.
4. Supplementary material Crystallographic data for [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O (CCDC 214721) have been deposited with the Cambridge Crystallographic Data Centre. A copy of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-3360333; e-mail: [email protected]. ac.uk; www: http://www.ccdc.cam.ac.uk). Acknowledgements We thank Professor Ward Robinson and Dr. Jan Wikaira (University of Canterbury) for X-ray data collection. References [1] N.E. Brasch, D.A. Buckingham, Chem. 37 (1998) 4865. [2] N.E Brasch, D.A. Buckingham, Chem. 35 (1996) 7728. [3] D.A. Buckingham, C.R. Clark, (1991) 466. [4] D.A. Buckingham, C.R. Clark, (1988) 293.
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[5] D.B. Leznoff, B.-Y. Xue, R.J. Batchelor, F.W.B. Einstein, B.O. Patrick, Inorg. Chem. 40 (2001) 6026. [6] R.T. Stibrany, J.A. Potenza, H.J. Schugar, Acta Cryst. C 49 (1993) 1561. [7] M.J. Scott, S.C. Lee, R.H. Holm, Inorg. Chem. 33 (1994) 4651. [8] E.V. Rybak-Akimova, A.Y. Nazarenko, S.S. Silchenko, Inorg. Chem. 38 (1999) 2974. [9] X. Xu, C.S. Allen, C.-L. Chuang, J.W. Canary, Acta Cryst. C 54 (1998) 600. [10] E. Ristenpart, Ber. 20 (1896) 2526. [11] F.G. Mann, W.J. Pope, J. Chem. Soc. (1926) 489. [12] E.C. Constable, in: K.D. Karlin (Ed.), Progress in Inorganic Chemistry, vol. 42, Wiley, New York, 1994, pp. 95–100. [13] The cation of the complex [Co(tren)2 ][Co(CO)4 ]2 contains two hydodentate tren ligands, but the pendant arms are not protonated J.C. Mareque Rivas, L. Brammer, Acta Cryst. C 54 (1998) 757. [14] K. Sakai, Y. Yamada, T. Tsubomura, Inorg. Chem. 35 (1996) 3163. [15] M.S. Davies, R.R. Fenton, F. Huq, E.C.H. Ling, T.W. Hambley, Aust. J. Chem. 53 (2000) 451. [16] C.-H. Chen, J. Cai, X.-M. Chen, Acta Cryst. C 58 (2002) m59. [17] R.L. Fanshawe, A. Mobinikhaledi, C.R. Clark, A.G. Blackman, Inorg. Chim. Acta, 307 (2000) 26. [18] I.P.Y. Shek, T.-C. Lau, W.-T. Wong, J.-L. Zuoc, New J. Chem. 23 (1999) 1049. [19] H. Keypour, S. Salehzadeh, Trans. Met. Chem. 25 (2000) 205. [20] H. Keypour, S. Salehzadeh, R.G. Pritchard, R.V. Parish, Inorg. Chem. 39 (2000) 5787. [21] M. Niitsu, H. Sano, K. Samejima, Chem. Pharm. Bull. 40 (1992) 2958. [22] F.J. Weigert, J. Org. Chem. 43 (1978) 622. [23] G.G. Wubbels, A.M. Halverson, J.D. Oxman, V.H. De Bruyn, J. Org. Chem. 50 (1985) 4499. [24] J.F. Constant, B.M. Carden, J. Lhomme, J. Heterocyclic Chem. 22 (1985) 1035. [25] SAINT, Version 4, Siemens Analytical X-ray Systems, Inc., Madison, WI, 1996. [26] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Cryst. 32 (1999) 115. [27] L.J. Farrugia, J. Appl. Cryst. 32 (1999) 837. [28] G.M. Sheldrick, S H E L X L -97, Program for the Refinement of Crystal Structures, University of G€ ottingen, 1997. [29] R.L. Fanshawe, A.G. Blackman, Inorg. Chem. 34 (1995) 421. [30] M. Streater, P.D. Taylor, R.C. Hider, J. Porter, J. Med. Chem. 33 (1990) 1749. [31] R.J. Geue, A.M. Sargeson, R. Wijesekera, Aust. J. Chem. 46 (1993) 1021. [32] A.M. Dittler-Klingemann, F.E. Hahn, Inorg. Chem. 35 (1996) 1996. [33] C. Ochs, F.E. Hahn, T. L€ ugger, Eur. J. Inorg. Chem. (2001) 1279. [34] ORTEP3 for Windows L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565. [35] A. Addison, T. Rao, J. Reedijk, J. van Rijn, G. Verschoor, J. Chem. Soc., Dalton Trans. (1984) 1349. [36] B.J. Hathaway, in: G. Wilkinson, R.D. Gillard, J.A. McCleverty (Eds.), Comprehensive Coordination Chemistry, vol. 5, Pergamon, Oxford, 1987, p. 606, Ch. 53. [37] C.J. Hawkins, Absolute Configurations of Metal Complexes, Wiley, New York, 1971. [38] A.E. Martell, R.D. Hancock, R.J. Motekaitis, Coord. Chem. Rev. 133 (1994) 39. [39] Cambridge Structural Database, Version 5.24, November 2002.
Synthesis and hypodentate Cu(II) complexes of new tripodal tetraamine ligands incorporating a long pendant arm Natasha J. Lundin, Ian G. Hamilton, Allan G. Blackman
*
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received 16 July 2003; accepted 13 September 2003 This paper is dedicated to Professor George Christou on the happy occasion of his 50th birthday
Abstract The synthesis and characterisation of the new aliphatic tripodal amine ligands apba (N -(5-aminopentyl)-N,N-bis(2-aminoethyl)amine) and ahba (N -(6-aminohexyl)-N,N-bis(2-aminoethyl)amine) are reported. The tetrahydrochloride salts of these ligands, as well as that of the previously reported ligand abba (N -(4-aminobutyl)-N,N-bis(2-aminoethyl)amine), react with CuCO3 Cu(OH)2 to give complexes in which the tripodal ligand coordinates to the Cu(II) ion in a hypodentate fashion, with the longest arm of the tripodal ligand remaining protonated and unbound in all cases. The crystal structure of [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH reveals a five-coordinate Cu(II) ion in a slightly distorted square pyramidal geometry. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Hypodentate; Tripodal amine ligand; Cu(II) complexes; Crystal structure; Five-coordinate
1. Introduction The stereochemical features peculiar to tripodal tetradentate amine ligands have led to their use in a wide variety of inorganic systems. Six-coordinate octahedral complexes containing such ligands invariably display cis geometries, and the inequivalence of the remaining two coordination sites in such complexes has been utilised in numerous studies aimed at monitoring the stereochemical outcome of ligand substitution reactions in Co(III) complexes containing these ligands [1–4]. The steric constraints imposed by such ligands often result in trigonal bipyramidal geometries for five-coordinate systems [5–7], while the rare examples of four-coordinate complexes containing tripodal ligands display distorted tetrahedral geometries [8,9], owing to the inability of these ligands to accommodate a square planar geometry. Of the fully aliphatic tripodal tetradentate amine ligands, the C3v symmetric ligands tren (tris(2-aminoethyl)amine) and trpn (tris(3-aminopropyl)amine), in which *
Corresponding author. Tel.: +64-3-479-7931; fax: +64-3-479-7906. E-mail address: [email protected] (A.G. Blackman).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2003.09.030
all three arms are the same length, have been known for many years [10,11], and it is only in relatively recent times that ligands of lower symmetry have been prepared and their complexes studied (Fig. 1). These ligands generally bind to a single transition metal ion using all four nitrogen donor atoms, although examples of hypodentate coordination [12], in which the ligand binds using less than its full complement of donor atoms, have been reported [13–18]. A number of the ligands outlined in Fig. 1 contain 4-aminobutyl or 5aminopentyl arms [19–22] and can potentially form seven- or eight-membered chelate rings on binding to a metal ion, but their coordination chemistry has not been investigated to date. In this paper we report the synthesis of the new tripodal ligands apba (N -(5-aminopentyl)-N,N-bis(2-aminoethyl)amine) and ahba (N -(6aminohexyl)-N,N-bis(2-aminoethyl)amine), as well as an alternative synthesis of abba (N -(4-aminobutyl)-N,Nbis(2-aminoethyl)amine). Although capable of forming complexes containing large chelate rings, these ligands display hypodentate coordination in Cu(II) complexes, with the longest arm remaining protonated and unbound in all cases. These findings are confirmed by a
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Fig. 1. Structures of the known aliphatic tripodal tetraamine ligands. References to their synthesis are as follows: abap [29], baep [31], abba [19], epb [20], apba, this work, ahba, this work, abbp [21], apbb [21], trbn [21], trpa [22].
crystal structure of the complex [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O.
2. Experimental 2.1. General All chemicals used were LR grade or better. 13 C and H NMR spectra were obtained in D2 O solution on a Varian Inova 300 MHz spectrometer at 25 °C. Chemical shifts are reported relative to NaTSP (1 H, 0.00 ppm) or dioxane (13 C, 67.8 ppm) as internal references. Elemental analyses were performed by the Campbell Microanalytical Laboratory, University of Otago. Electrospray mass spectra were recorded on a Shimadzu LCMS-QP800a spectrometer. Diphthaloyldiethylenetriamine was prepared as described [19]. N -(4-bromobutyl)phthalimide and N -(6bromohexyl)phthalimide were prepared by adaptation of the method used for the synthesis of N -(2-bromoethyl)phthalimide [23] (recrystallisation from cyclohexane and ethanol, respectively), while N -(5-bromopentyl) phthalimide was prepared according to a literature method [24]. 1
2.2. Ligand syntheses: general procedure An equimolar mixture of diphthaloyldiethylenetriamine and the appropriate bromoalkylphthalimide was heated at 190 °C for 30 min with constant stirring. Following cooling, the glassy brown solid was ground to a powder and heated under reflux in 8 M HCl overnight. The solution was cooled and the precipitated phthalic acid removed by filtration. The filtrate was reduced to a yellow oil (rotary evaporator), which was diluted to 1 L with water and loaded onto a Dowex 50W-X2 (Hþ form) column. This was washed with 1 M HCl and the product was removed on elution with 3 M HCl. Removal of the solvent (rotary evaporator) gave the crude product which was recrystallised from hot ethanol (a small amount of ether was added to the cooled solution to induce crystallisation if necessary) to give the pure ligand tetrahydrochloride as a white powder. 2.2.1. N-(4-aminobutyl)-N,N-bis(2-aminoethyl)amine tetrahydrochloride (abba 4HCl) Diphthaloyldiethylenetriamine (31.0 g, 85.3 mmol), N -(4-bromobutyl)phthalimide (24.6 g, 87.2 mmol), 8 M HCl (600 mL). Yield: 11.0 g (40.3%). Found: C, 29.6; H, 8.3; N, 16.9; Cl, 43.9. Calc. for C8 H26 N4 Cl4 : C, 30.0; H,
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8.2; N, 17.5; Cl, 44.3%. 1 H NMR (D2 O), dH : 1.8 (m, 4H), 3.08 (t, J ¼ 6 Hz, 2H), 3.36 (t, J ¼ 6 Hz, 2H), 3.56 (m, 8H) ppm. 13 C NMR (D2 O), dC : 21.8, 25.0, 35.1, 40.1, 50.9, 54.5 ppm. MS (ESI): m=z 175 (MHþ ). 2.2.2. N-(5-aminopentyl)-N,N-bis(2-aminoethyl)amine tetrahydrochloride (apba 4HCl) Diphthaloyldiethylenetriamine (23.1 g, 63.6 mmol), N -(5-bromopentyl)phthalimide (19.2 g, 64.8 mmol), 8 M HCl (190 mL). Yield: 8.02 g (37.7%). Found: C, 32.2; H, 8.4; N, 16.4; Cl, 42.0. Calc. for C9 H28 N4 Cl4 : C, 32.3; H, 8.4; N, 16.8; Cl, 42.4%. 1 H NMR(D2 O), dH : 1.51 (m, 2H), 1.78 (m, 2H), 1.87 (m, 2H), 3.06 (t, J ¼ 7:5 Hz, 2H), 3.39 (t, J ¼ 8:7 Hz, 2H), 3.56 (m, 4H), 3.66 (m, 4H) ppm. 13 C NMR (D2 O), dC : 23.3, 23.4, 26.9, 34.3, 39.8, 50.2, 54.4 ppm. MS (ESI): m=z 189 (MHþ ). 2.2.3. N-(6-aminohexyl)-N,N-bis(2-aminoethyl)amine tetrahydrochloride (ahba 4HCl) Diphthaloyldiethylenetriamine (4.57 g, 12.6 mmol), N -(6-bromohexyl)phthalimide (3.99 g, 12.9 mmol) 8 M HCl (100 mL). Yield: 1.14 g (26.0%). Found: C, 34.7; H, 9.1; N, 15.9; Cl, 40.7. Calc. for C10 H30 N4 Cl4 : C, 34.5; H, 8.7; N, 16.1; Cl, 40.7%. 1 H NMR (D2 O), dH : 1.47 (m, 4H), 1.72 (m, 2H), 1.83 (m, 2H), 3.03 (t, J ¼ 7:8 Hz, 2H), 3.35 (t, J ¼ 8:4 Hz, 2H), 3.57 (m, 4H), 3.64 (m, 4H) ppm. 13 C NMR (D2 O), dC : 23.5, 25.78, 25.81, 27.1, 34.3, 39.9, 50.1, 54.6 ppm. MS (ESI): m=z 203 (MHþ ). 2.3. Complex syntheses Caution! Although we have experienced no difficulties with the perchlorate complexes described below, the usual care afforded all perchlorate species should be taken in their preparation and handling. 2.3.1. [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH A suspension of abba 4HCl (0.50 g, 1.6 mmol) and CuCO3 Cu(OH)2 (0.17 g, 0.77 mmol) in acetonitrile (50 mL) was heated under reflux for 2 h. The solution was allowed to cool, and the resulting green powder was removed by filtration. The powder was dissolved in the minimum volume of methanol and slow diethyl ether diffusion over three days gave bright blue needles suitable for X-ray diffraction. These crystals lose solvent readily and the analytical data thus refer to the desolvated material [Cu(abbaH)Cl2 ]Cl 2H2 O (0.21 g, 36%). Found: C, 24.7; H, 7.0; N, 14.2; Cl, 28.2. Calc. for C8 H27 N4 Cl3 CuO2 : C, 25.2; H, 7.1; N, 14.7; Cl, 27.9%. 2.3.2. [Cu(apbaH)Cl(ClO4 )]ClO4 A suspension of apba 4HCl (0.50 g, 1.5 mmol) and CuCO3 Cu(OH)2 (0.17 g, 0.77 mmol) in MeOH (20 mL) was heated under reflux for 2 h to give a deep green solution which was then hot filtered. Addition of LiClO4
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to the filtrate gave rapid formation of blue microcrystals. Following cooling in the freezer, the product was filtered off, washed with diethyl ether and dried in a vacuum desiccator. (0.29 g, 40%). Found: C, 22.2; H, 5.3; N, 11.2; Cl, 22.3. Calc. for C9 H25 N4 Cl3 CuO8 : C, 22.2; H, 5.2; N, 11.5; Cl, 21.8%. 2.3.3. [Cu(ahbaH)Cl(ClO4 )]ClO4 A suspension of ahba 4HCl (0.50 g, 1.4 mmol) and CuCO3 Cu(OH)2 (0.15 g, 0.68 mmol) in MeOH (20 mL) was heated under reflux for 2 h to give a deep green solution which was then hot filtered. Addition of LiClO4 to the filtrate and storage in the freezer gave the product as blue microcrystals, which were then washed with diethyl ether and dried in a vacuum desiccator (0.27 g, 40%). Found: C, 24.1; H, 5.7; N, 11.0; Cl, 21.7. Calc. for C10 H27 N4 Cl3 CuO8 : C, 24.0; H, 5.4; N, 11.2; Cl, 21.2%. 2.4. X-ray crystallography Data for [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH were collected at )115 °C on a Siemens SMART system using graphite monochromated Mo Ka radiation with exposures over 0.3°. Data were corrected for Lorentz and polarisation effects using S A I N T [25]. The structure was solved by direct methods using SIR-97 [26] running within the WinGX package [27], with the resulting Fourier map revealing the location of all non-hydrogen atoms. Weighted full matrix refinement on F 2 was carried out using S H E L X L -97 [28] with all non-hydrogen atoms being refined anisotropically. Hydrogen atoms were included in calculated positions and were refined as riding atoms with individual (or group, if appropriate) isotropic displacement parameters. O3 was found to be disordered over two sites, with approximately 60:40 occupancy. Disorder of O2 was also evidenced by large thermal parameters, but this was not investigated further. Details of the crystal, data collection and structure refinement are given in Table 1.
3. Results and discussion 3.1. Synthesis of the tripodal ligands The tetrahydrochloride salts of the ligands abba, apba and ahba were prepared in moderate yields in an analogous fashion to similar tripodal tetraamine ligands, by the solventless reaction of a melt of the appropriate bromoalkylphthalimide with diphthaloyldiethylenetriamine, followed by deprotection in refluxing HCl (Scheme 1) [19,20,29–31]. In our hands, this appears to be an easier method of preparing tripodal ligands than the alternative procedure involving synthesis of the appropriate trinitrile and subsequent reduction with LiAlH4 / H2 SO4 [32,33]. Purification of the crude ligands was
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Table 1 Crystal data and structure refinement for [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O Empirical formula Formula weight Temperature (K) ) Wavelength (A Crystal system Space group Unit cell dimensions ) a (A ) b (A ) c (A b (°) 3 ) Volume (A Z Density (calculated) (Mg/m3 ) Absorption coefficient (mm1 ) F (0 0 0) Crystal size (mm) Theta range for data collection (°) Index ranges Reflections collected Independent reflections Completeness to theta ¼ 26.42° (%) Absorption correction Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F 2 Final R indices [I > 2rðIÞ] R indices (all data) 3 ) Largest differential peak and hole (e A
C9 H27 Cl3 CuN4 O3 409.24 158(2) 0.71069 monoclinic P 2(1)/n 6.247(5) 20.122(5) 15.729(5) 92.734(5) 1974.9(18) 4 1.376 1.521 852 0.44 0.38 0.28 2.40–26.42 )7 6 h 6 4, )24 6 k 6 25, )19 6 l 6 19 24 811 3999 [Rint ¼ 0.0216] 98.7 semi-empirical from equivalents 1.000 and 0.911 full-matrix least-squares on F 2 3999/0/184 1.150 R1 ¼ 0.0566, wR2 ¼ 0.1671 R1 ¼ 0.0636, wR2 ¼ 0.1704 1.196 and )0.685
3.2. Synthesis of metal complexes
Scheme 1. The synthetic method used to prepare apba 4HCl and ahba 4HCl.
achieved by chromatography on Dowex 50W-X2 cation exchange resin (elution with 3M HCl) and the tetrahydrochloride salts of the pure ligands were obtained as white powders on recrystallisation from ethanol. Elemental analysis, mass spectral and 1 H and 13 C NMR data were consistent with the structures given in Fig. 1.
The tetrahydrochloride salts of abba, apba and ahba react with half a mole equivalent of CuCO3 Cu(OH)2 in either acetonitrile (abba) or methanol (apba, ahba), to give dissolution of the metal salts and production of bright green solutions. A green abba complex precipitated from the acetonitrile solution on cooling and the blue complex [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH was isolated following recrystallisation from methanol. The green solutions obtained from the reaction of both apba and ahba in methanol slowly turned blue on cooling, and addition of LiClO4 to these solutions gave the complexes [Cu(apbaH)Cl(ClO4 )]ClO4 and [Cu(ahbaH)Cl(ClO4 )] ClO4 , respectively. Microanalytical data for all complexes were consistent with the presence of three counterions (Cl or ClO 4 ), strongly suggesting that the tripodal ligands were coordinating in a hypodentate fashion with a protonated pendant arm remaining uncoordinated to the metal ion. This was confirmed by the X-ray structure of [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O. 3.3. Crystal structure of [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O Crystallographic data for [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O are given in Table 1, bond lengths and
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Table 2 ) and angles (°) for [Cu(abbaH)Cl2 ]Cl 2H2 O CH3 OH Bond lengths (A Bond lengths Cu(1)–N(1) Cu(1)–N(3) Cu(1)–Cl(3) N(3)–C(1) N(3)–C(5) N(4)–C(8) C(7)–C(8) N(1)–C(2) C(5)–C(6) Bond angles N(1)–Cu(1)–N(2) N(2)–Cu(1)–N(3) N(2)–Cu(1)–Cl(2) N(1)–Cu(1)–Cl(3) N(3)–Cu(1)–Cl(3) C(4)–N(2)–Cu(1) C(1)–N(3)–C(5) C(1)–N(3)–Cu(1) C(5)–N(3)–Cu(1) C(8)–C(7)–C(6) N(4)–C(8)–C(7) N(3)–C(1)–C(2) N(3)–C(3)–C(4)
1.997(5) 2.095(4) 2.541(2) 1.477(7) 1.491(6) 1.491(7) 1.521(7) 1.475(8) 1.522(7) 160.20(18) 83.81(17) 92.40(13) 95.86(13) 101.10(11) 111.6(3) 111.2(4) 105.2(3) 111.8(3) 111.8(4) 110.5(4) 109.8(4) 109.4(4)
angles are given in Table 2, and Fig. 2 shows an ORTEP [34] diagram of the [Cu(abbaH)Cl2 ]þ cation. The structure consists of a five-coordinate Cu(II) ion coordinated to two chloride ligands and three nitrogen atoms of the abba ligand in a slightly distorted square pyramidal geometry (s ¼ 0:03) [35], with the three nitrogen atoms and one chloride ion occupying the basal plane. The remaining N atom of the abba ligand is protonated, as evidenced by the presence of three chloride ions in the structure, and remains uncoordinated to the metal ion. The copper ion lies approximately 0.32 A above the basal plane defined by the atoms N1, Cl2, N2 ) is sigand N3. The axial Cu1–Cl3 bond (2.541(2) A ), nificantly longer than the Cu1–Cl2 bond (2.3221(14) A as has been found in a number of other five-coordinate Cu(II) complexes [36]. The Cu1–N1–C2–C1–N3 chelate ring adopts a symmetric skew k conformation while the
Fig. 2. ORTEP diagram of the [Cu(abbaH)Cl2 ]þ cation. Ellipsoids are drawn at the 50% probability level.
Cu(1)–N(2) Cu(1)–Cl(2) N(2)–C(4) N(3)–C(3) O(1)–C(9) C(4)–C(3) C(7)–C(6) C(2)–C(1)
N(1)–Cu(1)–N(3) N(1)–Cu(1)–Cl(2) N(3)–Cu(1)–Cl(2) N(2)–Cu(1)–Cl(3) Cl(2)–Cu(1)–Cl(3) C(1)–N(3)–C(3) C(3)–N(3)–C(5) C(3)–N(3)–Cu(1) N(2)–C(4)–C(3) C(2)–N(1)–Cu(1) N(1)–C(2)–C(1) N(3)–C(5)–C(6) C(7)–C(6)–C(5)
2.015(4) 2.3221(14) 1.482(6) 1.487(7) 1.414(8) 1.509(7) 1.515(7) 1.517(9)
85.05(19) 93.19(14) 162.05(11) 102.31(13) 96.85(5) 113.8(4) 112.7(4) 101.4(3) 108.5(4) 109.2(4) 107.2(4) 115.8(4) 111.2(4)
Cu1–N3–C3–C4–N2 chelate ring exists in an asymmetric envelope d conformation [37]. The dangling aminobutyl chain displays an anti conformation about all C–C and C–N bonds, and the protonated terminal nitrogen atom is involved in hydrogen-bonding to both Cl3 and Cl4 of a neighbouring molecule (N4 Cl3 ¼ , N4 Cl4 ¼ 3.106(6) A ) and the oxygen atom 3.209(5) A of the methanol of crystallisation (N4 O1 ¼ 2.827(7) ). N1 and N2 also participate in hydrogen-bonding A interactions with chloride counterions. 3.4. Discussion The stability of five- and six-membered chelate rings in coordination complexes is usually explained in terms of the chelate effect, in which a positive entropy term renders chelation thermodynamically favourable [38]. However, the process of chelation becomes less entropically favourable for systems in which chelate rings containing more than six atoms can be formed, and such rings are also inherently strained. In agreement with this, a search of the Cambridge Structural Database [39] revealed 9891 metal complexes containing the chelated –N(CH2 )2 N– fragment and 2762 containing the chelated –N(CH2 )3 N– fragment, but only 140, 59 and 6 containing the chelated –N(CH2 )4 N–, –N(CH2 )5 N– and –N(CH2 )6 N– fragments, respectively. In the majority of these latter complexes, the large chelate rings are incorporated in macrocyclic or cryptand ligands, and this preorganisation undoubtedly aids chelation. The moderate steric constraints afforded by the tripodal ligands abba, apba and ahba are apparently not sufficient to
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enforce chelation of the longest pendant arm of these ligands, and they thus coordinate to Cu(II) in a hypodentate fashion. However, it is interesting to note in this respect that when abba is incorporated into a Schiffbase macrocycle, all four N atoms coordinate to Cu(II) in the resulting complex [20]. X-ray quality crystals of [Cu(apbaH)Cl(ClO4 )]ClO4 and [Cu(ahbaH)Cl(ClO4 )]ClO4 could not be obtained and thus the exact geometries of these complexes remain uncertain. However, the presence of three counterions, as shown by elemental analysis, strongly suggests both complexes to be hypodentate, with either a five-coordinate (coordinated chloride and perchlorate) or four-coordinate (coordinated chloride) Cu(II) ion and a non-coordinated protonated aminoalkyl arm. The presence of non-coordinated nitrogen atoms in the hypodentate complexes described herein affords several synthetic opportunities, as these complexes can potentially be used as building blocks for the synthesis of polymetallic species. The non-coordinated nitrogen atom can bind to another metal ion following deprotonation, or it can be used to prepare novel Schiff-base ligands through condensation with an appropriate carbonyl functionality. We are currently investigating these possibilities.
4. Supplementary material Crystallographic data for [Cu(abbaH)Cl2 ]Cl CH3 OH 2H2 O (CCDC 214721) have been deposited with the Cambridge Crystallographic Data Centre. A copy of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-3360333; e-mail: [email protected]. ac.uk; www: http://www.ccdc.cam.ac.uk). Acknowledgements We thank Professor Ward Robinson and Dr. Jan Wikaira (University of Canterbury) for X-ray data collection. References [1] N.E. Brasch, D.A. Buckingham, Chem. 37 (1998) 4865. [2] N.E Brasch, D.A. Buckingham, Chem. 35 (1996) 7728. [3] D.A. Buckingham, C.R. Clark, (1991) 466. [4] D.A. Buckingham, C.R. Clark, (1988) 293.
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