Electromagnetic Modeling of the Propagation Characteristics of Satellite Communications Through Composite Precipitation Layers Science and Technology, 7 (2002) 241-249 © 2002 Sultan Qaboos University Derivatized Pentadentate Macrocyclic Ligands and Their Transition Metal Complexes Muhammad S. Khan* , Nawal K. Al-Rasbi *, Edwin C. Constable **, Adrian R. Dale** and Jack Lewis ** *Department of Chemistry, College of Science, Sultan Qaboos University, P.O.Box 36, Al Khod 123, Muscat, Sultanate of Oman; **Department of Chemistry, University of Cambridge, Lensfield Rd., Cambridge CB2 1EW, UK, *Email: msk@squ.edu.om. مشتقات المركبات الخماسية الحلقة ومعقداتها مع العناصر اإلنتقالية دال وجاك لويس. كونستبل، أدريان ر. الراسبي، أدوين س. خان، نوال ك. محمد س - )هيدروكسي اثيل -'2,( ثنائي -1,11: تفاعالت مجموعة الهيدروكسي اثيل في المركبات خماسية الحلقة : خالصة ) ل2( أكتايين -20,18,14,12,9,6,4,2- رباعي ازا سيكلوهينيكوسا -11,10,2,1- ثي نايترايلو ثال -8,4;16,12;21,17 ثنائي -2,’2-) هيدروكسي اثيل هايدرازينو'2-( ثنائي 6,'6- ثنائي بريدين الدهايد مع 6,2–المشتق من تفاعالت التكاثف من جموعة الهيدروكسيل اثيل في هذه الحلقات مع مركبات الثيونيل بتفاعالت االسترة كانت إيجابية وتفاعل م ) ل1(البيردين = ML3-6.X2.nH2O) M ل ذات الصيغة العامة 6-3المعقدات الفلزية من هذه المركبات . الكلورايد انتج مشتق الكلورواثيل .درست وحضرت أيضاً) فلزات الدورة األولى من العناصر اإلنتقالية ABSTRACT:The reaction of the pendant hydroxyethyl group in the planar pentadentate macrocyclic ligand,1,11-bis(2’-hydroxyethyl)-4,8;12,16;17,21-trinitrilo-1,2,10,11-tetraazacyclohenicosa- 2,4,6,9, 12,14,18,20-octaene (L2), derived from the condensation of 2,6-pyridinedialdehyde with 6,6’-bis(2’- hydroxyethylhydrazino)-2,2’-bipyridine (L1), has been investigated. Esterification reactions are facile, and the reaction of the hydroxyethyl-substituted macrocycle with thionyl chloride yields a chloroethyl derivative. Metal complexes of the new derivatized macrocyclic ligands L3-6 having general formula ML3-6X2.nH2O (M = Mn, Fe, Co, Ni, Cu, Zn) are readily prepared. KEYWORDS: Macrocyclic Ligands, Coordination Complexes, Transition Metals, Esterification, Functional Group Interconversion. 1. Introduction M acrocyclic compounds of biological significance, such as the porphyrin, chlorophyll and corrin complexes have been studied for many years (Smith, 1975). More recently, other macrocyclic ligands and their metal complexes have been synthesized (Constable, 1999). The wide range of interest in the synthetic macrocyclic ligand complexes has arisen partly because their similarity to certain biological macrocyclic systems has enabled them to be used as models in the study of these systems. Interest in the synthetic macrocyclic systems also arises due to the unusual properties they may exhibit in their own right. The tendency for a macrocyclic complex to be more stable with respect to ligand dissociation than a comparable open-chain multidentate ligand complex, over and above the chelate affect, is well documented. This phenomenon, termed the ‘Macrocyclic Effect’ (Cabbinnes and Margerum, 1969) is thought to have both kinetic and thermodynamic origins. Another interesting aspect is the unusual stereochemical arrangement that may be formed on coordination of the macrocyclic ligands to the metal centers. This is particularly true of the pentadentate N5-donor macrocyclic ligands involving π-electron delocalization around the ring. A rigid stereochemical arrangment is often imposed on the metal that would not occur in 241 M.S. KHAN et al. the absence of the macrocyclic ligand. The geometry of unsaturated macrocycles is generally planar due to the more favorable overlap of π-symmetry orbitals. Planar pentadentate N5-donor macrocyclic ligands may form five coordinate planar, six-coordinate pentagonal-based pyramidal and seven-coordinate pentagonal bipyramidal complexes with metal ions (Melson, 1979; Lindoy, 1989). We have been interested in the design, synthesis and coordination chemistry of a series of pentadentate N5-donor macrocycles involving poly-pyridine groups (Figure 1). N N NN R1 NN R1 R2R2 N R1 = H, CH3 ; R2 = H, CH3 Figure 1. Polypyridyl N5-macrocycles. The rationale for choosing pyridine derivatives as components within the macrocycle framework is that they readily reduce to radical ions, thereby imparting interesting redox properties to the macrocyclic complexes. The fused rings of 2,2'-bipyridine/1,10-phenanthroline are also expected to impart additional rigidity to the macrcocyle rings. Pentadentate macrocyclic ligands of this type are readily prepared by template or transient template condensations of hydrazino- substituted 2,2'-bipyridines (Ansell et al 1982a, 1982b, 1983), 1,10-phenanthroline or 2,2':6',2"- terpyridines with 2,6-pyridine dicarbonyl (Constable et al, 1985). We have synthesized pentadentate macrocycle L2 with pendant hydroxyethyl group that allows further structural development of the macrocycle by derivatization and the design of biometic systems (Ansell et al, 1983). In this paper we wish to describe the synthesis and coordination chemistry of some such derivatized macrocyclic ligands L3-6 (Figure 2). 2. Materials and Method All reagents were purchased from Sigma Aldrich and used without further purification. Solvents were dried and distilled before use by standard methods. All reactions were performed under N2. Infrared spectra were recorded in compressed KBr pellet on Perkin Elmer 983 spectrophtometer. 1H NMR spectra were recorded on Bruker WM 250 or AM 400 spectrometers. Fast atom bombardment (FAB) and electron impact (EI) mass spectra were recorded on a Kratos MS 50 mass spectrometer. Microanalysis was performed in the Department of Chemistry, University of Cambridge, U.K. Conductance measurements were made using a Wayne Kerr 242 DERIVATIZED PENTADENTATE MACROCYCLIC LIGANDS Universal Bridge. The macrocyclic ligand salt [H2L2][PF6]2 was prepared as previously described (Chung et al 1990). N N NN CH2CH2X NN XH2CH2C HH N Figure 2. Derivatized N5-macrocylic ligands. L2: X = OH; L3: X = O2CCH3 ; L4: X = O2CCH2CH3; L5: X = O2CC6H5; L6: X = Cl 3. Synthesis of Derivatized Macrocyclic Ligands L3-6 3.1 Synthesis of [H2L3][PF6]2 A solution of [H2L2][PF6]2·H2O (0.064 g, 0.09 mmol) in dry acetonirile (50 mL) was stirred for 12 hours with acetyl chloride (5 mL). After twelve hours the orange solution was concentrated in vacuo to 5 mL volume and treated with water (50 mL) and sufficient acetonitrile to redissolve the initial precepitate. The solution was then filtered and the filtrate treated with saturated aqueous [NH4][PF6] solution (10 mL). The orange solution was then slowly concentrated in vacuo to yield orange-red crystals of [H2L3][PF6]2·H2O (0.07 g, 96%). Anal. Found: C, 36.6; H, 3.4; N, 12.4. Calc. for C25F12H31N7O6P2: C, 36.8; H, 3.6; N, 12.0%. Infrared, 1741 cm-1. EI-MS (m/z): 487. 3.2 Synthesis of [H2L4][PF6]2 The procedure was analogous to that described above for [H2L3][PF6]2. Yield: 75% as orange-red crystals of [H2L4][PF6]2·H2O. Anal. Found: 39.5; H, 3.6; N, 11.9. Calc. for C27F12H33N7O5P2: C, 39.3; H, 3.5; N, 11.9%. Infrared, 1737 cm-1. EI-MS (m/z): 515. 3.3 Synthesis of [H2L5][PF6]2 A solution of [H2L2][PF6]2·H2O (0.10 g, 0.14 mmol) in dry acetonirile (50 mL) was heated to reflux for 36 hours with benzoyl chloride (0.7 mL). The resulting orange solution was concentrated in vacuo to 5 mL volume when an oil formed. Water (25 mL) was added and the mixture heated to 90ºC for 10 minutes to hydrolyse excess benzoyl chloride. The solution was then extracted with diethyl ether (2 x 20 mL) and the resulting aqueous suspension treated with sufficient acetonitrile to give a clear solution. This was then filtered and the filtrate treated with saturated aqueous [NH4][PF6] solution (10 mL). The orange solution was then slowly concentrated in vacuo to yield orange-red crystals of [H2L5][PF6]2·H2O (0.05 g, 40%). Anal. Found: C, 68.46 ; H, 5.18 ; N, 15.89. Calc. for C35F12H31N7O5P2: C, 68.6; H, 5.1; N, 16.0%. Infrared, 1720 cm-1. 243 M.S. KHAN et al. Table 1: Analytical data for transition metal complexes of L3-6 . Compound Found % Calc. % C H N C H N [MnL3(H2O)2][PF6]2·H2O 34.04 3.56 11.21 33.89 3.53 11.07 [FeL3(H2O)2][PF6]2·H2O 34.12 3.63 10.98 33.86 3.52 11.06 [CoL3][PF6]2·H2O 34.99 3.64 11.39 35.16 3.66 11.48 [NiL3(H2O)2][PF6]2·MeCN 35.58 3.46 12.21 35.53 3.42 12.28 [CuL3(H2O)2][PF6]2·H2O 33.65 3.52 11.4 33.57 3.49 10.96 [ZnL3(H2O)2][PF6]2·H2O 33.28 3.42 10.89 33.50 3.49 10.94 [MnL4(H2O)2][PF6]2·H2O 35.41 3.53 10.69 35.48 3.61 10.73 [FeL4(H2O)2][PF6]2·H2O 35.38 3.68 10.63 35.45 3.61 10.72 [CoL4][PF6]2·H2O 36.71 3.72 11.09 36.77 3.75 11.12 [NiL4(H2O)2][PF6]2·MeCN 36.98 3.48 11.82 37.02 3.51 11.91 [CuL4(H2O)2][PF6]2·H2O 35.28 3.47 10.72 35.15 3.58 10.63 [ZnL4(H2O)2][PF6]2·H2O 36.27 3.65 10.89 36.18 3.71 10.94 [MnL5(H2O)2][PF6]2·H2O 29.97 2.57 11.67 30.10 2.53 11.70 [FeL5(H2O)2][PF6]2·H2O 29.98 2.58 11.74 30.07 2.52 11.69 [CoL5][PF6]2·H2O 31.25 2.72 12.18 31.30 2.63 12.17 [NiL5(H2O)2][PF6]2·MeCN 32.07 2.47 13.01 31.94 2.45 12.96 [CuL5(H2O)2][PF6]2·H2O 29.85 2.44 11.63 29.79 2.50 11.59 [ZnL5(H2O)2][PF6]2·H2O 28.09 2.28 11.01 28.14 2.36 10.94 [MnL6(H2O)2][PF6]2·H2O 50.23 3.69 11.63 50.17 3.73 11.70 [FeL6(H2O)2][PF6]2·H2O 50.19 3.68 11.79 50.11 3.73 11.69 [CoL6][PF6]2·H2O 52.23 3.94 12.24 52.16 3.88 12.17 [NiL6(H2O)2][PF6]2·MeCN 48.56 3.59 13.05 48.61 3.61 12.96 [CuL6(H2O)2][PF6]2·H2O 49.72 3.71 11.67 49.66 3.69 11.59 [ZnL6(H2O)2][PF6]2·H2O 46.94 3.52 11.01 46.89 3.49 10.94 3.4 Synthesis of [H2L6][PF6]2 A solution of [H2L2][PF6]2·H2O (0.05 g, 0.07 mmol) in dry acetonirile (25 mL) was heated to reflux for 3 hours with thionyl chloride (1 mL). After this period the orange solution was evaporated to dryness in vacuo and the residue dissolved in acetonitrile (0.5 mL). Diffusion of diethyl ether vapour into this solution yielded orange-red needles of [H2L6][PF6]2 (0.03 g, 67%). Anal. Found: 34.34; H, 2.97; N, 13.62. Calc. for C21Cl2F12H21N7P2: C, 34.42; H, 2.93; N, 13.68%. FAB-MS (m/z): 439, 441, 443. 244 DERIVATIZED PENTADENTATE MACROCYCLIC LIGANDS 4. Synthesis of Metal Complexes of Derivatized Macrocyclic Ligands 4.1 Metal Complexes of L3 A solution of [H2L3][PF6]2·2H2O in 3:1 acetonitrile/methanol was treated with a solution of 1 equivalent of an appropriate metal(II) acetate dissolved in the minimum volume of methanol. The mixture was then heated to reflux for 15 minutes, concentrated to 1/5th volume in vacuo and treated with saturated [NH4][PF6] solution. The products were collected by filtration and dried to yield the desired complexes in a good yield (typically > 75%). Analytical data are presented in Table 1. 4.2 Metal Complexes of L4 The procedure was analogous to that used for metal complexes of L3. The desired products were isolated in > 80% yield. Analytical data are presented in Table 1. 4.3 Metal Complexes of L5 The procedure was analogous to that used for metal complexes of L3. The products were isolated in > 80% yield. Analytical data are presented in Table 1. 4.4 Metal complexes of L6 The procedure was analogous to that used for metal complexes of L3. The products were isolated in respectable yields (typically > 65%). Analytical data are presented in Table 1. 5. Results and Discussion Planar pentadentate macrocyclic ligands such as L2 may be prepared by transient template condensations of the appropriate hydrazines L1 with 2,6-pyridinedialdehyde or 2,6 diacetylpyridine. These ligands, which are of particular interest in bearing a functionalized substituent that may be further derivatized to yield encapsulating or capped macrocycles. The open-chain bishydrazines are readily prepared by the reaction of a suitable dihalo compound with α-hydroxyethylhydrazine. The free macrocyclic ligand L2 is best prepared as the hydrochloride or other salt by transient template condensations of 6,6’-bis(2’-hydroxyethylhydrazino)-2,2’- bipyridine (L1) with 2,6- pyridinedialdehyde in the presence of chromium(III) chloride (Scheme 1). N N NN CH2CH2OH H2NNH2 HOH2CH2C HH N O O + Cr(III)Cl3.6H2O NH4PF6 [H2L2][PF6]2 L1 Scheme 1. Transient template synthesis of L2 . 245 M.S. KHAN et al. As an initial investigation into the further functionalization of these macrocyclic ligands we studied acylation reactions of L2. Solutions of [H2L2][PF6]2 in acetonitrile changed in color from red to orange upon stirring overnight with excess of acetyl chloride. Treatment of this orange solution with water followed by [NH4][PF6] resulted in the formation of an orange crystalline product. In the presence of an excess of acetyl chloride we anticipated the formation of the bisacetylated ligand L3, and microanalysis was in accord with the orange product being formulated [H2L3][PF6]2·2H2O. The conversion proceeds in a near-quantitative manner, and isolated yields of 96% of the salt were obtained. The EI mass spectrum of the salt exhibits a parent ion for {L3}+ at m/z 487. The infrared spectrum of the compound showed a single strong carbonyl absorption at 1741 cm-1 as expected for the bisacetylated compound. The isolated hexafluorophosphate salt was insufficiently soluble in D2O or CD3OD to obtain 1H NMR spectra in these solvents, but broadened spectra could be obtained in CD3CN or CD3COCD3. Unfortunately, the resonances assigned to the acetyl groups were obscured in these solvents, and it was necessary to use CD3SOCD3 solution, and this effect was again observed with [H2L3][PF6]2. The acetyl protons appeared as a singlet (δ 1.91) and integration confirmed that a bisacetylated derivative had been obtained. A similar smooth reaction occurred when an acetonitrile solution of [H2L2][PF6]2 was stirred with propionyl chloride, and after the usual work up, [H2L4][PF6]2 was obtained as an orange crystalline solid in 75% yield. Once again, microanalysis was consistent with this formulation, and the EI mass spectrum exhibited a parent ion at m/z 515. The infrared spectrum showed a singlet strong carbonyl absorption at 1737 cm-1 as expected for the desired bisacylated product. This product is considerably more soluble in CD3OD than [H2L4][PF6]2, and sharp well-resolved 1H NMR spectra could be obtained in this solvent. The most evident feature of the spectrum are the triplet (δ 1.01, 6H, J 7.5 Hz) and quartet (δ 2.26, 4H, J 7.5 Hz) assigned to the propionyl group. Similar acylations have been demonstrated for other alkanecarboxylic acids, and the reaction appeared to be general. We also considered the formation of esters with arenecarboxylic acids, and investigated the reaction of [H2L2][PF6]2 with benzoyl chloride under a variety of experimental conditions. The macrocycle was recovered unchanged after stirring a solution of [H2L2][PF6]2 in acetonitrile with benzoyl chloride overnight. We could not obtain benzoylated products from attempted acylation under Schotten-Baumann conditions. Eventually, the desired bisbenzoylated compound was obtained after heating an acetonitrile solution of [H2L2][PF6]2 with an excess of benzoyl chloride to reflux for 36 hours. The product of this reaction tended to be oily, but eventually orange-red crystals of [H2L5][PF6]2 were obtained by repeated crystallization from aqueous acetonitrile. The mass spectrum did not exhibit a molecular ion, but merely showed a fragmentation peak assigned to {L2}+. The infrared spectrum provides firm evidence for the formation of the desired derivatized macrocyclic product and shows strong carbonyl stretching absorption at 1720 cm-1, typical of benzoate esters. The 1H NMR spectrum of the compound in CD3SOCD3 solution is of interest in showing both broadened and sharp resonances in the aromatic region, Figure 3a. Integration of spectra obtained using very long relaxation delays between pulses indicated that the ratio of the sharp peaks to the broadened peaks was 10:11; this is in accord with the resonances assigned to protons in the macrocyclic ring being broadened as previously noted, but those of the phenyl groups being sharp. The 1H NMR spectrum of the solution of [H2L5][PF6]2 shows temperature dependent behaviour, and upon heating to 370 K is significantly sharpened, Figure 3b. In addition to esterification, we have also investigated other reactions of pendant hydroxyethyl substituent in L2. Numerous attempts to prepare the tosylates by reaction with 4-toluenesulphonyl chloride in a wide range of experimental conditions were unsuccessful. Generally, the products were intractable tars. We have had rather more success in the reaction with thionyl chloride to yield L6. Upon heating acetonirile solutions of [H2L2][PF6]2 with thionyl chloride, orange solutions were obtained, from which the salt [H2L6][PF6]2 could be isolated as orange needles. The microanalysis is in accord with this formulation, and the FAB mass spectrum exhibited a parent ion at m/z 439, 441, 443 showing the expected isotopomeric distribution for a compound containing two chlorine atoms. No absorptoions due to hydroxy groups are observed in the infrared spectrum of the 246 DERIVATIZED PENTADENTATE MACROCYCLIC LIGANDS compound. Once again, the complex exhibits a broadened 1H NMR spectra in CD3SOCD3 solution. Figure 3. 1H NMR spectrum of a CD3SOCD3 solution of [H2L5][PF6]2 at: a) 298 K and b) 370 K. We have made preliminary studies of the coordination behaviour of the derivatized ligands. The salts [H2L3-6][PF6]2 react smoothly with metal acetates in boiling methanolic solution to yield metal complexes, which may be recrystallized from aqueous acetonitrile to yield the crystalline complexes [ML3-6][PF6]2·nH2O (M = Mn, Fe, Co, Ni, Cu, Zn). In the absence of any structural data we propose that these complexes possess pentagonal bipyramidal geometries, in which the two axial sites are occupied by coordinated water. The infrared spectra exhibited sharp absorptions at 3604 cm-1 due to the coordinated water molecules. In the solid state, considerable shifts of the carbonyl stretching frequency occurred upon coordination of L3-5, Table 2. This suggests that there is significant hydrogen-bonding interaction between the coordinated water and the carbonyl groups. Conducivity in nitromethane (see Table 2) indicates that all the complexes are 2:1 electrolytes (Geary, 1971) corresponding to the presence of two counter ions per macrocyclic complex. Taken in conjunction with the analytical data and infrared spectra, conductivity measurements provide very strong evidence for pentagonal bipyramidal coordination geometry for transition metal complexes of the derivatized macrocyclic ligands. 247 M.S. KHAN et al. Table 2: Carbonyl stretching frequency and conductivity of the complexes. νCO (cm–1) Λ (Ohm–1cm2 mole–1) [MnL3(H2O)2][PF6]2·H2O 1716 165 [FeL3(H2O)2][PF6]2·H2O 1714 160 [CoL3][PF6]2·H2O 1707 167 [NiL3(H2O)2][PF6]2·MeCN 1723 162 [CuL3(H2O)2][PF6]2·H2O 1739 158 [ZnL3(H2O)2][PF6]2·H2O 1723 164 [MnL4(H2O)2][PF6]2·H2O 1715 164 [FeL4(H2O)2][PF6]2·H2O 1716 163 [CoL4][PF6]2·H2O 1710 166 [NiL4(H2O)2][PF6]2·MeCN 1721 165 [CuL4(H2O)2][PF6]2·H2O 1737 160 [ZnL4(H2O)2][PF6]2·H2O 1724 163 [MnL5(H2O)2][PF6]2·H2O 1718 161 [FeL5(H2O)2][PF6]2·H2O 1715 163 [CoL5][PF6]2·H2O 1710 165 [NiL5(H2O)2][PF6]2·MeCN 1724 167 [CuL5(H2O)2][PF6]2·H2O 1735 162 [ZnL5(H2O)2][PF6]2·H2O 1725 161 [MnL6(H2O)2][PF6]2·H2O - 160 [FeL6(H2O)2][PF6]2·H2O - 162 [CoL6][PF6]2·H2O - 164 [NiL6(H2O)2][PF6]2·MeCN - 165 [CuL6(H2O)2][PF6]2·H2O - 164 [ZnL6(H2O)2][PF6]2·H2O - 160 6. Conclusion We have shown that derivatized planar pentadentate ligands may be prepared by esterification and functional group interconversions of suitable hydroxyethyl substituted macrocyclic ligand. These ligands appear to possess a coordination chemistry which parallels that of related unsubstituted ligand. 7. Aknowlegments We would like to thank the Department of Chemistry, College of Science, Sultan Qaboos University, Oman and the Engineering and Physical Sciences Research Council (EPSRC), U.K. for financial support. 248 DERIVATIZED PENTADENTATE MACROCYCLIC LIGANDS References SMITH, K.M. 1975. Porphyrins and Metalloporphyrins; Elsevier, Amsterdam. CONSTABLE, E.C. 1999. Coordination Chemistry of Macrocyclic Compounds, Oxford Science Publications. CABBINNES, D.K. and MARGERUM, D.W. 1969. Macrocyclic effect on the stability of copper (II) tetramine complexes. J. Amer. Chem. Soc. 91: 6540-6541. LINDOY, L.F. 1989. 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Received 27 June 2001 Accepted 20 November 2002 249 Derivatized Pentadentate Macrocyclic Ligands and Their Transition Metal Complexes Muhammad S. Khan* , Nawal K. Al-Rasbi *, Edwin C. Constable **, Adrian R. Dale** and Jack Lewis ** *Department of Chemistry, College of Science, Sultan Qaboos University, P.O.Box 36, Al Khod 123, Muscat, Sultanate of Oman; **Department of Chemistry, University of Cambridge, Lensfield Rd., Cambridge CB2 1EW, UK, *Email: msk@squ.edu.om. ãÍãÏ Ó. ÎÇä¡ äæÇá ß. ÇáÑÇÓÈí¡ ÃÏæ Compound Found % C H N C H N References