Synthesis and characterization of novel unsymmetrical macrobicyclic tricompartmental mono and binuclear nickel(II) complexes: Electrochemical and kinetic studies of phosphate hydrolysis

Inorganica Chimica Acta 359 (2006) 3831–3840 www.elsevier.com/locate/ica

Synthesis and characterization of novel unsymmetrical macrobicyclic tricompartmental mono and binuclear nickel(II) complexes: Electrochemical and kinetic studies of phosphate hydrolysis M. Thirumavalavan b

a,b

, P. Akilan a, M. Kandaswamy

a,*

a Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India Departamento de Engenharia Quı´mica, Instituto Superior Te´cnico, Complexo I, Av. Rovisco Pais 1, 1049-002 Lisboa, Portugal

Received 23 November 2005; received in revised form 11 April 2006; accepted 22 April 2006 Available online 12 May 2006

Abstract A series of macrobicyclic mono and binuclear nickel(II) complexes of type [NiL](ClO4) and [Ni2L](ClO4)2, where L is macroyclic ligand derived from the precursor compound 3,4:10,11-dibenzo-1,13[N,N 0 -bis{(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,9-dioxocyclopentadecane, have been synthesized in order to examine electrochemical and catalytic studies on the basis of macrocyclic ring size. The macrocycle consists of three dissimilar compartments arising from ether oxygen, tertiary nitrogen and imine nitrogen atoms. Electrochemical studies have shown that the mononuclear nickel(II) complexes undergo quasireversible single step one electron reduction and oxidation and binuclear nickel(II) complexes undergo two quasireversible one electron reduction and oxidation. The EPR silent nature is ascribed to Ni(II) state and all the nickel(II) complexes have square planar geometry and are diamagnetic in nature. The complexes were subjected to hydrolysis of 4-nitrophenyl phosphate and the catalytic activities of the complexes are found to increase with macrocyclic ring size of the complexes. As the macrocyclic ring size of the complexes increases, the spectral, electrochemical and catalytic studies of the complexes show remarkable variation due to distortion in the geometry around the nickel(II) centre.  2006 Elsevier B.V. All rights reserved. Keywords: Compartmental ligands; Macrocyclic nickel(II) complexes; Electrochemistry; Phosphate hydrolysis; Macrocyclic ring size

1. Introduction Complexes of binucleating compartmental ligands have been reported with a wide range of transition metal and main group metal ions [1–8]. The versatile synthetic pathway of these complexes is via template condensation of the appropriate aldehyde and diamine precursors around the metal centres [1–4]. Synthetic simple binuclear metal complex cores are important to understand the mutual influences of the two metal centres on the electronic, magnetic and catalytic properties of such bimetallic cores [9]. Compartmental macrocyclic ligands having two phenolic oxygens as endogenous bridge have been developed [10–

*

Corresponding author. Fax: +91 44 2230 0488. E-mail address: [email protected] (M. Kandaswamy).

0020-1693/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.04.031

12] for this purpose because they bind two metal centres in close proximity relevant to the active sites of bimetallic enzymes. The development of new polydentate ligands is important for the discovery of new transition metal catalysis, for the advancement of bioinorganic chemistry both in its role of understanding the chemistry of metallobiomolecules and in producing useful new biomimetic and bioinspired reaction and for the continuing development of basic coordination chemistry [13]. Bimetallic macrocyclic complexes represent a helpful tool in the study of metal–metal interactions. In order to obtain binuclear macrocyclic complexes, three main synthetic strategies have been pursued (i) synthesis of large macrocycles or macrobicycles able to incorporate two metal ions [14], (ii) synthesis of bis macrocycles [15,16] and (iii) use of chelating agents bridging two macrocyclic units [17–19]. Each metal centre can interact with the

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other one directly by electrostatic forces or chemical bonding and indirectly via electron delocalization through the macrocyclic framework or bridging ligands. This can lead on account of the structure of the binuclear complex to unusual chemical and physicochemical properties related to its magnetic, electrochemical and thermodynamic behaviour. Several macromonocyclic complexes of ligands, containing ether oxygen and tertiary nitrogen, tertiary nitrogen and phenolic oxygen as coordinating atoms were reported in the literature [20] and only a few bicyclic systems were studied so far [21]. Unsymmetrical compartmental metal ion, together with ligands containing a variety of structural information, has dragged the attentions in the field of supramolecular chemistry [21,22] due to the selfassembly and translocation processes. By varying the redox potential and pH, the metal with variable oxidation states ions can be translocated from one compartment to another compartment. This paper focuses on the synthesis of phenol based macrobicyclic tricompartmental ligands having dissimilar metal binding sites sharing two phenolic oxygens for the study of di-l-phenolato mono and binuclear nickel(II) complexes. This work also brings out the interesting chemistry observed in electrochemical and catalytic studies of nickel(II) complexes. The role of Ni(II) ion in promoting the hydrolysis of phosphate esters [23] has been the subject of considerable study and a great deal is now known. The mechanism of 4-nitrophenyl phosphate (NPP) hydrolysis involves the concerted binding of the substrate molecule on one metal centre and the nucleophilic attack of water or hydroxide ion at the other metal centre [24]. 2. Experimental Elemental analysis was carried out on a Carlo Erba Model 1106 elemental analyzer. 1H NMR spectra were recorded using FX-80-Q Fourier transition NMR spectrometer. Electronic spectral studies were carried out on a Hitachi 320 spectrophotometer in the range 200–800 nm. IR spectra were recorded on a Hitachi 270-50 spectrophotometer on KBr disks in the range 4000–250 cm1. Mass spectra were obtained on JEOL DX-303 mass spectrometer and the electrospray mass spectra of the complexes were recorded on a Micromass Quatro II triple quadrupole mass spectrometer. Molar conductivity was measured by using an Elico model SX 80 conductivity bridge using freshly prepared solution of the complex in CH3CN. Cyclic voltammograms were obtained on CHI600A electrochemical analyzer. The measurements were carried out under oxygen free condition using a three electrode cell in which glassy carbon electrode was the working electrode, saturated Ag/AgCl electrode was the reference electrode and platinum wire was used as the auxiliary electrode. Ferrocene/ferrocenium (1+) couple was used as an internal standard and E1/2 of the ferrocene/ferrocenium (Fc/Fc+) couple under the experimental condition is 470 mV in DMF and DEp for Fc/Fc+ is 70 mV. Tetra(n-butyl)ammonium perchlorate was used as

supporting electrolyte. The kinetic investigation of hydrolysis of 4-nitrophenyl phosphate was carried out spectrophotometrically by choosing the strongest absorbance at 420 nm and monitoring the increase in the absorbance at this wavelength as a function of time. A plot of log (A/ A  At) vs. time was made for each complex and the rate constant of hydrolysis of 4-nitrophenyl phosphate was calculated. All the kinetic studies were carried out in DMF solvent at room temperature (25 C). Constant ionic strength (0.1 M) was maintained throughout the kinetic studies by using tetraethylammonium perchlorate (TEAP) salt. Inhibition studies were performed by adding an equivalent amount of inhibitory agent and 4-nitrophenyl phosphate. 2.1. Materials 5-Methylsalicylaldehyde [25], 3-chloromethyl-5-methylsalicylaldehyde [26] and 3,4:10,11-dibenzo-1,13-diaza-5,9dioxocyclopentadecane [27,28] were prepared from the literature methods. TBAP used as supporting electrolyte in electrochemical measurement was purchased from Fluka and recrystallized from hot methanol. (Caution! TBAP is potentially explosive; hence care should be taken in handling the compound). DMF and CH3CN were obtained from E. MERCK. All other chemicals and solvents were of analytical grade and were used as received without any further purification. 2.2. Synthesis of precursor compound 2.2.1. 3,4:10,11-dibenzo-1,13[N,N 0 -bis{(3-formyl-2hydroxy-5-methyl)benzyl}diaza]-5,9-dioxocyclopentadecane (PC) A mixture of 3,4:10,11-dibenzo-1,13-diaza-5,9-dioxocyclopentadecane (0.998 g, 3.2 mmol) and triethylamine (1.32 g, 6.4 mmol) in THF was added slowly to a stirred solution of 3-chloromethyl-5-methylsalicylaldehyde (1.16 g, 6.4 mmol) in THF. After the addition was over, stirring was continued further for one more hour. The whole solution was then refluxed on a water bath for 3 h and was allowed to cool on standing at room temperature. Copious water was added to this solution to dissolve any salt obtained. Then the required compound was extracted in organic medium using chloroform. The extraction was repeated two to three times. A pale yellow compound was obtained on evaporation of the solvent at room temperature (25 C). Light yellow crystals were obtained on recrystallization from chloroform. Yield: 1.60 g (82%), m.p.: 85 C. Anal. Calc. for C37H40O6N2: C, 73.02; H, 6.57; N, 4.60. Found: C, 73.21; H, 6.68; N, 4.78%. IR data (m cm1): 1674 (mC@O, s), 3442 (mOH, br). 1H NMR (d ppm in CDCl3): 10.0 (s, 2H, CHO protons), 7.2 (m, 12H, aromatic protons), 4.7 (m, 4H, methylene protons attached to oxygen atom), 3.7 (m, 8H, benzylic protons), 3.3 (s, 4H, methylene protons attached to nitrogen atom), 2.3 (s, 6H, aromatic CH3 protons), 2.1 (s, 2H, CH2 protons) attached to alkyl chain. Mass (EI) m/z: 608 (m+).

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2.3. Synthesis of macrobicyclic mononuclear nickel(II) complexes 2.3.1. [NiL1](ClO4) A methanolic solution of nickel(II)perchlorate hexahydrate (0.694 g, 1.9 mmol) was added to the hot solution of PC (1.155 g, 1.9 mmol) in CHCl3 followed by the addition of 1,2-diaminoethane (0.128 g, 1.9 mmol) in methanol. The solution was refluxed on a water bath for 24 h. After the reaction was over, the solution was filtered at hot condition and allowed to stand at room temperature. After slow evaporation of the solvent at 25 C, the orange red compound was obtained, washed with methanol and dried in a vacuum. Yield: 1.25 g (79%). Anal. Calc. for C41H46O8N5ClNi: C, 59.27; H, 5.54; N, 8.43. Found: C, 59.35; H, 5.62; N, 8.51%. Selected IR (KBr): 1630 (s), 1082 (s), 627 (s) cm1. 1H NMR (d ppm in CH3CN): 8.1 (s, 2H, protons in CH@N), 6.6–7.1 (m, 12H, aromatic protons), 3.9 (m, 4H, methylene protons attached to oxygen atom), 2.1 (m, 2H, methylene protons attached to CH2–O), 4.5 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.6 (s, 4H, benzylic protons attached to N in the amine compartment), 3.3 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.8 (s, 2H, methylene protons attached to N in the amine compartment), 2.3 (s, 6H, aromatic CH3 protons), 3.9 (s, 4H, CH2 protons attached to alkyl chain in the imine compartment). 13C NMR (d ppm in CH3CN): 163 (s, carbon in CH@N), 124, 128, 130 and 133 and 155 (m, aromatic carbons in the imine compartment), 114, 120, 123, 126, 129, 159 (m, aromatic carbons in ether oxygen compartment), 68.9 (m, 2C, carbons in CH2–O), 30.9 (m, 1C, carbon in CH2–CH2–O), 49 (s, 4C, benzylic carbons attached to +NH and N in the amine compartment), 53 (s, 1C, methylene carbon in CH2–+NH), 49 (s, 1C, methylene carbon in CH2–N), 21 (s, 2C, carbons in Ar–CH3), 163 (s, 2C, carbons in Ar–CH@N), 34 (s, 2C, carbons in @NACH2A CH2AN@). Conductance (Km/S cm2 mol1) in CH3CN: 134. kmax (nm) (e/M1 cm1) in CH3CN: 540 (168), 387 (15 100), 276 (37 800). 2.3.2. [NiL2](ClO4) [NiL2](ClO4) was synthesized by following the previously described procedure for [NiL1](ClO4), using 1,3-diaminopropane instead of 1,2-diaminoethane. The compound obtained was orange red. Yield: 1.23 g (76%). Anal. Calc. for C42H48O8N5ClNi: C, 59.71; H, 5.68; N, 8.29. Found: C, 59.84; H, 5.76; N, 8.38%. Mass (EI) m/z: 803 (m+). Selected IR (KBr): 1629 (s), 1084 (s), 629 (s) cm1. 1H NMR (d ppm in CH3CN): 8.1 (s, 2H, protons in CH@N), 6.7–7.1 (m, 12H, aromatic protons), 3.9 (m, 4H, methylene protons attached to oxygen atom), 2.1 (m, 2H, methylene protons attached to CH2–O), 4.3 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.6 (s, 4H, benzylic protons attached to N in the amine compartment), 3.4 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.5 (s, 2H,

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methylene protons attached to N in the amine compartment), 2.3 (s, 6H, aromatic CH3 protons), 3.6 (s, 4H, CH2 protons attached to CH@N), 2.1 (s, 2H, CH2 protons attached to CH2AN@CH). Conductance (Km/S cm2 mol1) in CH3CN: 148. kmax (nm) (e/M1 cm1) in CH3CN: 590 (140), 365 (16 000), 270 (33 800). 2.3.3. [NiL3](ClO4) [NiL3](ClO4) was synthesized by following the previously described procedure for [NiL1](ClO4), using 1,4diaminobutane instead of 1,2-diaminoethane. The compound obtained was orange red. Yield: 1.31 g (80%). Anal. Calc. for C43H50O8N5ClNi: C, 60.13; H, 5.82; N, 8.15. Found: C, 60.24; H, 5.94; N, 8.23%. Selected IR (KBr): 1625 (s), 1088 (s), 629 (s) cm1. 1H NMR (d ppm in CH3CN): 8.1 (s, 2H, protons in CH@N), 6.7–7.1 (m, 12H, aromatic protons), 3.9 (m, 4H, methylene protons attached to oxygen atom), 2.1 (m, 2H, methylene protons attached to CH2–O), 4.5 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.4 (s, 4H, benzylic protons attached to N in the amine compartment), 3.1 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.8 (s, 2H, methylene protons attached to N in the amine compartment), 2.3 (s, 6H, aromatic CH3 protons), 3.6 (s, 4H, CH2 protons attached to CH@N), 1.8 (s, 4H, CH2 protons attached to CH2AN@CH). Conductance (Km/S cm2 mol1) in CH3CN: 138. kmax (nm) (e/ M1 cm1) in CH3CN: 615 (136), 355 (15 000), 272 (32 500). 2.3.4. [NiL4](ClO4) [NiL4](ClO4) was synthesized by following the previously described procedure for [NiL1](ClO4), using o-phenylenediamine instead of 1,2-diaminoethane. The compound obtained was orange red. Yield: 1.29 g (77%). Anal. Calc. for C45H46O8N5ClNi: C, 61.50; H, 5.23; N, 7.97. Found: C, 61.62; H, 5.31; N, 8.08%. Selected IR (KBr): 1627 (s), 1086 (s), 629 (s) cm1. 1H NMR (d ppm in CH3CN): 8.1 (s, 2H, protons in CH@N), 6.9–7.1 (m, 12H, aromatic protons), 3.9 (m, 4H, methylene protons attached to oxygen atom), 2.1 (m, 2H, methylene protons attached to CH2– O), 4.5 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.6 (s, 4H, benzylic protons attached to N in the amine compartment), 3.3 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.8 (s, 2H, methylene protons attached to N in the amine compartment), 2.3 (s, 6H, aromatic CH3 protons), 7.3 (s, 4H, aromatic amine protons in the imine compartment). Conductance (Km/S cm2 mol1) in CH3CN: 155. kmax (nm) (e/ M1 cm1) in CH3CN: 610 (180), 460 (17 100), 365 (16 300), 355 (16 000), 270 (37 200). 2.3.5. [NiL5](ClO4) [NiL5](ClO4) was synthesized by following the previously described procedure for [NiL1](ClO4), using 1,8-diaminonaphthalene instead of 1,2-diaminoethane. The compound obtained was orange red. Yield: 1.33 g (75%).

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Anal. Calc. for C49H48O8N5ClNi: C, 63.36; H, 5.17; N, 7.54. Found: C, 63.42; H, 5.22; N, 7.61%. Selected IR (KBr): 1620 (s), 1090 (s), 628 (s) cm1. 1H NMR (d ppm in CH3CN): 8.4 (s, 2H, protons in CH@N), 6.6–7.1 (m, 12H, aromatic protons), 3.3 (m, 4H, methylene protons attached to oxygen atom), 2.1 (m, 2H, methylene protons attached to CH2–O), 4.5 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.6 (s, 4H, benzylic protons attached to N in the amine compartment), 3.3 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.8 (s, 2H, methylene protons attached to N in the amine compartment), 2.3 (s, 6H, aromatic CH3 protons), 7.3–7.7 (m, 6H, aromatic amine protons in the imine compartment). Conductance (Km/S cm2 mol1) in CH3CN: 150. kmax (nm) (e/M1 cm1) in CH3CN: 640 (190), 450 (16 200), 375 (14 300), 279 (36 900). 2.4. Synthesis of macrobicyclic binuclear nickel(II) complexes 2.4.1. [Ni2L1](ClO4)2 A methanolic solution of nickel(II)perchlorate hexahydrate (0.694 g, 1.9 mmol) was added to the hot solution of PC (1.383 g, 1.9 mmol) in CHCl3 followed by the addition of 1,2-diaminoethane (0.128 g, 1.9 mmol) and triethylamine (0.265 g, 1.9 mmol) in methanol. After 1 h, nickel(II) perchlorate (0.694 g, 1.9 mmol) was added and the whole reaction mixture was refluxed on a water bath for 24 h. After the reaction was over, the reaction mixture was filtered and allowed to stand at room temperature (25 C). After slow evaporation of the solvent at 25 C, the orange red compound obtained was washed with methanol and dried in a vacuum. Yield: 1.52 g (78%). Anal. Calc. for C43H48O12N6Cl2Ni2: C, 50.19; H, 4.66; N, 8.17. Found: C, 50.28; H, 4.81; N, 8.30%. Selected IR (KBr): 1630 (s), 1100 (m), 627 (s) cm1. 1H NMR (d ppm in CH3CN): 8.1 (s, 2H, protons in CH@N), 6.6–7.0 (m, 12H, aromatic protons), 3.9 (m, 4H, methylene protons attached to oxygen atom), 2.3 (m, 2H, methylene protons attached to CH2–O), 4.3 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.8 (s, 4H, benzylic protons attached to N in the amine compartment), 3.3 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.6 (s, 2H, methylene protons attached to N in the amine compartment), 2.1 (s, 6H, aromatic CH3 protons), 3.9 (s, 4H, CH2 protons attached to alkyl chain in the imine compartment). Conductance (Km/S cm2 mol1) in CH3CN: 280. kmax (nm) (e/M1 cm1) in CH3CN: 580 (260), 368 (12 100), 268 (35 600). 2.4.2. [Ni2L2](ClO4)2 [Ni2L2](ClO4)2 was synthesized by following the previously described procedure for [Ni2L1](ClO4)2, using 1,3diaminopropane instead of 1,2-diaminoethane. The compound obtained was orange red. Yield: 1.58 g (80%). Anal. Calc. for C44H50O12N6Cl2Ni2: C, 50.77; H, 4.79; N, 8.06. Found: C, 50.81; H, 4.90; N, 8.18%. Selected IR

(KBr): 1628 (s), 1099 (m), 628 (s) cm1. 1H NMR (d ppm in CH3CN): 8.0 (s, 2H, protons in CH@N), 6.5–7.1 (m, 12H, aromatic protons), 3.7 (m, 4H, methylene protons attached to oxygen atom), 2.2 (m, 2H, methylene protons attached to CH2–O), 4.1 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.7 (s, 4H, benzylic protons attached to N in the amine compartment), 3.2 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.8 (s, 2H, methylene protons attached to N in the amine compartment), 2.4 (s, 6H, aromatic CH3 protons), 3.6 (s, 4H, CH2 protons attached to CH@N), 2.0 (s, 2H, CH2 protons attached to CH2–N@CH). Conductance (Km/S cm2 mol1) in CH3CN: 298. kmax (nm) (e/ M1 cm1) in CH3CN: 600 (236), 358 (17 200), 280 (32 300). 2.4.3. [Ni2L3](ClO4)2 [Ni2L3](ClO4)2 was synthesized by following the previously described procedure for [Ni2L1](ClO4)2, using 1,4diaminobutane instead of 1,2-diaminoethane. The compound obtained was orange red. Yield: 1.61 g (80%). Anal. Calc. for C45H52O12N6Cl2Ni2: C, 51.13; H, 4.92; N, 7.95. Found: C, 51.31; H, 5.03; N, 8.07%. Selected IR (KBr): 1626 (s), 1100 (m), 628 (s) cm1. 1H NMR (d ppm in CH3CN): 8.1 (s, 2H, protons in CH@N), 6.5–7.1 (m, 12H, aromatic protons), 3.9 (m, 4H, methylene protons attached to oxygen atom), 2.2 (m, 2H, methylene protons attached to CH2–O), 4.3 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.6 (s, 4H, benzylic protons attached to N in the amine compartment), 3.3 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.8 (s, 2H, methylene protons attached to N in the amine compartment), 2.3 (s, 6H, aromatic CH3 protons), 3.6 (s, 4H, CH2 protons attached to CH@N), 1.7 (s, 4H, CH2 protons attached to CH2–N@CH). Conductance (Km/S cm2 mol1) in CH3CN: 313. kmax (nm) (e/M1 cm1) in CH3CN: 625 (238), 362 (18 100), 283 (33 300). 2.4.4. [Ni2L4](ClO4)2 [Ni2L4](ClO4)2 was synthesized by following the previously described procedure for [Ni2L1](ClO4)2, using ophenylenediamine instead of 1,2-diaminoethane. The compound obtained was orange red. Yield: 1.58 g (77%). Anal. Calc. for C47H48O12N6Cl2Ni2: C, 52.41; H, 4.46; N, 7.80. Found (%): C, 52.53; H, 4.58; N, 7.89%. Selected IR (KBr): 1625 (s), 1098 (m), 628 (s) cm1. 1H NMR (d ppm in CH3CN): 8.1 (s, 2H, protons in CH@N), 6.7–7.0 (m, 12H, aromatic protons), 3.9 (m, 4H, methylene protons attached to oxygen atom), 2.2 (m, 2H, methylene protons attached to CH2–O), 4.6 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.5 (s, 4H, benzylic protons attached to N in the amine compartment), 3.1 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.6 (s, 2H, methylene protons attached to N in the amine compartment), 2.2 (s, 6H, aromatic CH3 protons), 7.5 (s, 4H, aromatic amine protons in the imine compartment). Conductance (Km/S cm2 mol1) in CH3CN:

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301. kmax (nm) (e/M1 cm1) in CH3CN: 615 (241), 452 (14 100), 370 (13 200), 310 (14 300), 268 (37 800). 2.4.5. [Ni2L5](ClO4)2 [Ni2L5](ClO4)2 was synthesized by following the previously described procedure for [Ni2L1](ClO4)2, using 1,8diaminonaphthalene instead of 1,2-diaminoethane. The compound obtained was orange red. Yield: 1.68 g (79%). Anal. Calc. for C51H52O12N6Cl2Ni2: C, 54.35; H, 4.44; N, 7.46. Found: C, 54.44; H, 4.59; N, 7.61%. Selected IR (KBr): 1625 (s), 1100 (m), 627 (s) cm1. 1H NMR (d ppm in CH3CN): 8.3 (s, 2H, protons in CH@N), 6.5–7.1 (m, 12H, aromatic protons), 3.5 (m, 4H, methylene protons attached to oxygen atom), 2.4 (m, 2H, methylene protons attached to CH2–O), 4.4 (s, 4H, benzylic protons attached to +NH in the amine compartment), 3.7 (s, 4H, benzylic protons attached to N in the amine compartment), 3.2 (s, 2H, methylene protons attached to +NH in the amine compartment), 2.6 (s, 2H, methylene protons attached to N in

the amine compartment), 2.1 (s, 6H, aromatic CH3 protons), 7.1–7.6 (m, 6H, aromatic amine protons in the imine compartment). Conductance (Km/S cm2 mol1) in CH3CN: 288. kmax (nm) (e/M1 cm1) in CH3CN: 645 (236), 430 (15 200), 372 (12 500), 271 (33 200). In all cases, only microcrystals were obtained and attempts to obtain the crystals suitable for X-ray analysis were unsuccessful. 3. Results and discussion By template method, various macrobicyclic mono and binuclear nickel(II) complexes were synthesized by Schiff’s base condensation of the precursor compounds with diamines in the presence of metal ion. The synthetic pathway of mono and binuclear complexes is shown in Scheme 1. It is proposed that the geometry around nickel(II) ion in mononuclear complex is distorted square planar and one of the tertiary nitrogen atoms is protonated. In binuclear

OH

N

+

CH3

CH3

O

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O

+ HN

O

O

N

Ni(ClO4)2.6H2O (CH2)2

(CH2)3 O

H2N-R-NH2

OH

N

O

O

Ni

(CH2)2

(CH2)3

CH3

N

O

R N

CH3 mononuclear complexes i) Ni(ClO4)2.6H2O , H2N-R-NH2 ii) Triethylamine, Ni(ClO4)2.6H2O

R 2+

CH3

1

L

-(CH2)2-

L2

-(CH2)3-

L3

-(CH2)4-

4

(CH2)3

Ni

(CH2)2 O

N

L

O

N

O

N Ni

O

R

L5

N

CH3 binuclear complexes

Scheme 1. Synthesis of macrobicyclic mono and binuclear nickel(II) complexes.

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nickel(II) complex, both the nickel(II) ions are four coordinated. Conductivity measurements also vindicate that mono and binuclear nickel(II) complexes are 1:1 and 1:2 electrolyte type, respectively. Spectral, electrochemical and catalytic studies of the complexes were carried out. 3.1. Spectral studies The IR spectra of the complexes show m(C@N) peak [29] at 1625–1630 cm1. The formation of this new peak and the disappearance of the m(C@O) peak at 1674 cm1 in the complexes indicate the effective Schiff’s base condensation. The peaks near 1100 cm1 and 620 cm1 are characteristics of the uncoordinated perchlorate ion. Three main transitions were observed in the electronic spectra of the complexes. A weak band observed in the range 540– 645 nm due to d–d transition of the metal ion. On moving from the complexes of ligand L1–L3 and L4–L5, an increase in kmax (red shift) [30] of the d–d transition of Ni(II) ion in the complexes was observed for both mono and binuclear copper(II) complexes. A moderate intensive band observed in the range 355–387 nm is due to ligand to metal charge transfer transition and the strong band observed in the range 268–283 nm is due to intraligand charge transfer transition [31]. Apart from these, additional peaks are observed for the complexes of ligands L4–L5 due to the presence of aromatic ring in the imine nitrogen compartment. All the nickel(II) complexes are diamagnetic [32,33] and ESR silent.

Fig. 1. Cyclic voltammograms of the mononuclear nickel(II) complexes (1 · 103 M) (a) [NiL1](ClO4) (b) [NiL2](ClO4) (c) [NiL3](ClO4).

3.2. Electrochemistry The conductivity measurements [34] of the complexes in acetonitrile indicate that the mononuclear nickel(II) complexes are of 1:1 electrolyte type and binuclear nickel(II) complexes are of 1:2 electrolyte type. Electrochemical properties of the complexes reported in the present work were studied by cyclic voltammetry in the potential range 0 to 1.6 V and 0 to +1.6 V in dimethylformamide containing 101 M tetra(n-butyl)ammonium perchlorate. All the nickel(II) complexes undergo both reduction and oxidation in cathodic and anodic potentials, respectively. Since the macrobicyclic complexes of ditopic ligands contain non-equivalent compartments, the translocation process has also been studied electrochemically using redox-driven mechanism. 3.3. Reduction at cathode The cyclic voltammograms for the mononuclear complexes are shown in Fig. 1. Each voltammogram shows one quasireversible reduction wave at negative potential in the range 0.85 to 1.17 V. Controlled potential electrolysis carried out at 100 mV more negative than the reduction wave reveals the consumption of one electron per molecule (n = 0.94) and the experiment reports that the couple corresponds to one electron transfer process. Fig. 2 represents the cyclic voltammograms of the binu-

Fig. 2. Cyclic voltammograms of the binuclear nickel(II) complexes (1 · 103 M) (a) [Ni2L1](ClO4)2 (b) [Ni2L2](ClO4)2 (c) [Ni2L3](ClO4)2.

clear complexes. Binuclear complexes show two quasireversible reduction waves. The first reduction potential ranges from 0.98 to 1.31 V and the second reduction potential falls in the range 1.31 to 1.59 V. Tables 1 and 2 represent the electrochemical data of nickel(II) complexes in cathodic and anodic potential, respectively. Controlled potential electrolysis was also carried out and the experiment reports that each couple correspond to one electron transfer process. So, the two redox processes are assigned as follows: NiII NiII NiII NiI NiI NiI The conproportionation constant for the equilibrium K con

NiII NiII þ NiI NiI 2NiII NiI was calculated using the relationship log Kcon = DE/0.0591.

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Table 1 Electrochemical dataa of mono and binuclear nickel(II) complexes in DMF medium (reduction) Complexes 1

[NiL ](ClO4) [NiL2](ClO4) [NiL3](ClO4) [NiL4](ClO4) [NiL5](ClO4) [Ni2L1](ClO4)2 [Ni2L2](ClO4)2 [Ni2L3](ClO4)2 [Ni2L4](ClO4)2 [Ni2L5](ClO4)2

E1pc ðVÞ

E1pa ðVÞ

E11=2 ðVÞ

DE (mV)

E2pc ðVÞ

E2pa ðVÞ

E21=2 ðVÞ

DE (mV)

Kcon

0.94 0.88 0.85 1.17 1.11 1.28 1.21 0.98 1.31 1.22

0.75 0.70 0.68 1.04 0.99 1.03 0.97 0.87 1.16 1.13

0.85 0.79 0.77 1.11 1.05 1.16 1.09 0.93 1.24 1.18

190 180 170 130 120 250 240 110 150 90

1.59 1.44 1.31 1.58 1.39

1.35 1.25 1.17 1.41 1.25

1.47 1.35 1.24 1.50 1.32

240 190 140 170 140

1.76 · 105 2.50 · 104 1.75 · 105 2.51 · 105 2.33 · 102

a Measured by CV at 25 mV/s. E vs. Ag/AgCl conditions: GC working and Ag/AgCl reference electrodes; supporting electrolyte TBAP; concentration of complex 1 · 103 M, concentration of TBAP 1 · 101 M.

Table 2 Electrochemical dataa of mono and binuclear nickel(II) complexes in DMF medium (oxidation) Complexes 1

[NiL ](ClO4) [NiL2](ClO4) [NiL3](ClO4) [NiL4](ClO4) [NiL5](ClO4) [Ni2L1](ClO4)2 [Ni2L2](ClO4)2 [Ni2L3](ClO4)2 [Ni2L4](ClO4)2 [Ni2L5](ClO4)2

E1pa ðVÞ

E1pc ðVÞ

E11=2 ðVÞ

DE (mV)

E2pa ðVÞ

E2pc ðVÞ

E21=2 ðVÞ

DE (mV)

0.91 0.71 0.85 0.72 0.75 0.96 0.96 0.97 0.65 0.85

0.81 0.60 0.72 0.61 0.62 0.86 0.86 0.89 0.55 0.74

0.86 0.65 0.79 0.67 0.69 0.91 0.91 0.93 0.60 0.80

100 110 130 110 130 100 100 80 100 110

1.15 1.17 1.19 1.05 1.15

1.04 1.06 1.07 0.96 1.05

1.10 1.12 1.13 1.01 1.10

110 110 120 90 100

a Measured by CV at 25 mV/s. E vs. Ag/AgCl conditions: GC working and Ag/AgCl reference electrodes; supporting electrolyte TBAP; concentration of complex 1 · 103 M, concentration of TBAP 1 · 101 M.

It is of interest to compare the electrochemical behaviour of the complexes heedfully on the basis of macrocyclic ring size. The reduction potential of the mononuclear nickel(II) complexes of ligands L1–L3 and L4–L5 shifts anodically from 0.94 V to 0.85 V and from 1.17 V to 1.11 V, respectively, as the macrocyclic ring size increases due to the presence of bulky group. This imparts more flexibility to the macrocycle [33,35–37] and tries to stabilize the nickel(I) complex. In this case, the observed DE values for quasireversible waves are generally larger and this may be due to the occurrence of electrochemical interconversion (translocation) [38–41] of reduced cation Ni(I) from the rigid imine nitrogen compartment (N2O2) to flexible tertiary nitrogen compartment (N2O2), which favours the tetrahedral geometry. The larger DE values vividly explain that the reduction process is associated with ECE mechanism. This is only based on electrochemical studies and not on structural analysis. This phenomenon is associated with the decrease in DE value as the chain length between two imine nitrogens increases, which really eases the translocation process [42–44] moving from the complexes of ligands L1–L3 and L4–L5. In order to assess translocation (ECE mechanism), controlled potential electrolysis was carried out at 100 mV more negative than the reduction potential (1.04 V). We also made an attempt to carry out the UV–Vis and ESR spectral studies for the one electron reduced (electrolyzed) solution of complex [NiL1]-

(ClO4). The UV–Vis spectral analysis reveals that the electrolyzed solution shows no d–d band around 550 nm and it shows a peak at 395 nm. The disappearance of d–d around 550 nm clearly indicates that the Ni(II) ion is reduced to Ni(I) ion. Obtaining the ESR spectra of the electrolyzed solution can further evidence this. The solution state ESR spectra of electrolyzed solution is ESR active and shows splittings with gk ¼ 2:24 and g^ = 2.05. This gk > g? is characteristic of Ni(I) ion [45] present in the tertiary nitrogen compartment and this also confirms the electrolysis of Ni(II) ion in the complex. Busch et al. [45] have reported the ESR spectra of Ni(I) ion with gk ¼ 2:26 and g^ = 2.06. The binuclear nickel(II) complexes are associated with the interesting feature of shifting of both first and second reduction potentials towards anodic as the size of the macrocycle is increased. For example, the complex [Ni2L1](ClO4)2 has E1pc ¼ 1:28 V and E2pc ¼ 1:59 V, which is more negative compared to the complex [Ni2L2](ClO4)2 ðE1pc ¼ 1:21 V; E2pc ¼ 1:44 VÞ and the same observation is inferred for the complexes of ligand L45 also. Thus due to the presence of bulky group in the imine nitrogen compartment, the whole macrocyclic ring becomes more flexible and hence undergoes easy reduction. Thus the large size of the cavity easily holds the reduced cation and stabilizes the formation of Ni(I) in both the compartments. Concern in the calculation of Kcon values of binuclear Ni(II) complexes leads to another interesting feature that

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the variation of Kcon is random and not uniform as the macrocyclic ring size increases, which has no explanation. For all the binuclear Ni(II) complexes, the first reduction takes place at a more negative potential than the reduction of the mononuclear Ni(II) complexes. This is probably due to the fact that the second Ni(II) centre rigidifies the ligand structure. From the cyclic voltammetric studies, it can be stated that the complexes containing aromatic diimines get reduced at higher negative potential than that of the complexes containing aliphatic diimines. The reason may be due to the increased stability of the systems due to aromatic and less deviation from planarity. 3.4. Oxidation at anode The cyclic voltammograms of nickel(II) complexes obtained at positive potential region show one electron transfer waves. The oxidation process is also quasireversible in nature. It is obvious to say that the increase in macrocyclic ring size influences the oxidation potential to less extend to only and no uniform variation is observed. Hence, the influence of macrocyclic ring size is unpredictable. The oxidation process can be assigned as follows:

Fig. 3. Catalytic activity of mononuclear nickel(II) complexes for hydrolysis of 4-nitrophenylphosphate (a) [NiL1](ClO4) (b) [NiL2](ClO4) (c) [NiL3](ClO4).

NiII NiII NiII NiIII NiIII NiIII 3.5. Kinetic studies of hydrolysis of 4-nitrophenyl phosphate Emphasis has been laid to investigate the kinetic studies of the nickel(II) complexes and the study implies that the complexes act as a model for phosphate hydrolase. The kinetic investigation for the hydrolysis of 4-nitrophenyl phosphate was carried out using an equal concentration of the complex catalyst, and tetramethylammonium hydroxide (103 M) and 4-nitrophenyl phosphate (101 M). The chromophore leaving group 4-nitrophenolate, which is the hydrolysis product of the substrate, was monitored at 420 nm. The course of the reaction was followed at 420 nm for nearly 45 min at regular time intervals. We have carried out the reaction in the absence of the metal complex. No detectable hydrolysis of test substrate was observed when the metal complex was absent. We have also carried out the hydrolysis of 4-nitrophenyl phosphate for various concentrations of the metal complex. A linear relationship for initial rate and the complex concentration obtained for the complex shows a pseudo first order dependence on the complex concentration. This clearly shows that this is truly catalytic. The slope was determined by monitoring the growth of 420 nm band of the product 4-nitrophenolate ion. Plots of log (A/A  At) vs. time for catalytic activity of the mono and binuclear complexes hydrolysis for 4-nitrophenyl phosphate are obtained and shown in Figs. 3 and 4, respectively. The rate constant values are also reported in Table 3. The first apparent result was that the reactivities of the complexes differ significantly, as the size of the macrocycle increases. The binuclear complexes have higher

Fig. 4. Catalytic activity of binuclear nickel(II) complexes for hydrolysis of 4-nitrophenylphosphate (a) [Ni2L1](ClO4)2 (b) [Ni2L2](ClO4)2 (c) [Ni2L3](ClO4)2.

Table 3 Catalytic activitya of mono and binuclear nickel(II) complexes for hydrolysis of 4-nitrophenyl phosphate Complexes 1

[NiL ](ClO4) [NiL2](ClO4) [NiL3](ClO4) [NiL4](ClO4) [NiL5](ClO4) [Ni2L1](ClO4)2 [Ni2L2](ClO4)2 [Ni2L3](ClO4)2 [Ni2L4](ClO4)2 [Ni2L4](ClO4)2 a

Rate constant (min1) 1.13 · 103 1.52 · 103 1.93 · 103 1.09 · 103 1.14 · 103 1.77 · 102 2.51 · 102 3.19 · 102 1.54 · 102 2.05 · 102

Measured spectrophotometrically in DMF medium. Concentration of the complexes and tetramethylammonium hydroxide: 1 · 103 M, concentration of 4-nitrophenyl phosphate: 1 · 101 M.

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activities than the mononuclear complexes [46–50]. For example, the binuclear complexes [Ni2L1](ClO4)2, [Ni2L2](ClO4)2, [Ni2L3](ClO4), [Ni2L4](ClO4)2 and [Ni2L5](ClO4)2 have the rate constant values 1.77 · 102, 2.51 · 102, 3.19 · 102, 1.54 · 102 and 2.05 · 102 min1, respectively, whereas the mononuclear complexes [NiL1](ClO4), [NiL2](ClO4), [NiL3](ClO4), [NiL4](ClO4) and [NiL5](ClO4) have the rate constant values 1.13 · 103, 1.52 · 103, 1.93 · 103, 1.09 · 103 and 1.14 · 103 min1, respectively. This simply shows that the presence of two metal ions in close proximity enhances the catalytic activity in binuclear complexes than that of mononuclear complexes, which contain single metal ion and hence the rate constant values of binuclear complexes are comparatively high. This is due to the fact that the mononuclear complexes [24] proceed via SN1 type of reaction mechanism, wherein the external hydroxide ion coordinates to the metal ion, followed by deprotonation of OH and formation of an intermediate with the phosphate group and then cleavage of the bond to form the phenolate anion, along with hydroxylation of the phosphate group. The binuclear nickel(II) complexes [24] follow a concerted SN2 type of mechanism where the nucleophile OH adds to one of the Ni ions and the phosphate is bound to the other metal ion. Here, bond formation and bond breakage occur at the same time. The mechanism proposed for hydrolysis is shown in Scheme 2. The catalytic activities of both mono and binuclear complexes are found to increase as the macrocyclic ring size increases due to the intrinsic flexibility. The catalytic activity of the complexes containing aromatic diimines is found to be less than that of the complexes containing aliphatic diimines. This can be explained that the planarity, which is associated with aromatic ring, imparts less catalytic efficiency due to the rigidity of the systems as observed in the case of electrochemical reduction of the complexes. The kinetic studies suggested that the reported Ni(II) complexes significantly accelerate the hydrolysis of 4-nitrophenyl phosphate. In order to check the inhibition activity of inhibitors, we have also carried out inhibition studies for complexes of ligand L1 using sodium methyl phosphate and 4-nitrophenol as inhibitors. Inhibition studies show a reduction of 28% in rate by added sodium methyl phosphate for mononuclear complex and for the binuclear comO-

R

P

Ni N

O

O

O mononuclear

OH P

H

O

O: N

OH

O-

O H

O2N

O

O: N N

O Ni

Ni O

N N

binuclear

Scheme 2. Mechanism for hydrolysis of 4-nitrophenyl phosphate.

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plex the reduction rate is 22%. But no inhibitory effect was observed for the added 4-nitrophenol. 4. Conclusion Various mono and binuclear nickel(II) complexes of tricompartmental ligands containing aromatic and aliphatic units in imine nitrogen compartment have been synthesized and their spectral, electrochemical and catalytic studies were carried out. These studies vividly indicate that a small variation in macrocyclic ring size in the ligand framework influences the spectral, electrochemical and catalytic properties of the complexes. Acknowledgements The author M.T. gratefully acknowledges the financial support received from CSIR and UGC major project sanctioned to M.K., New Delhi. References [1] R. Robson, N.H. Pilkington, Aust. J. Chem. 23 (1970) 2225. [2] C.L. Spiro, S.L. Lambert, T.J. Smith, E.N. Duesler, R.R. Gagne, D.N. Hendrickson, Inorg. Chem. 20 (1981) 1229. [3] M. Tadokoro, H. Sakiyama, N. Matisumoto, M. Kodera, H. Okawa, S. Kido, J. Chem. Soc., Dalton Trans. (1992) 313. [4] J. Atkins, A.J. Blake, M. Scroder, J. Chem. Soc., Chem. Commun. (1993) 353. [5] H. Okawa, S. Kida, Bull. Chem. Soc. Jpn. 45 (1972) 1759. [6] K.K. Nanda, R. Das, L.K. Thompson, K. Venkatasubramanian, P. Paul, K. Nag, Inorg. Chem. 33 (1994) 1188. [7] P.A. Vigato, S. Tamburini, Coord. Chem. Rev. 248 (2004) 1717. [8] C. Bucher, J. Moutet, J. Pecaut, G. Royal, E.S. Aman, F. Thomas, S. Torelli, M. Ungureanu, Inorg. Chem. 42 (2003) 2242. [9] A.M. Valentine, S.J. Lippard, J. Chem. Soc., Dalton Trans. (1997) 3295. [10] H. Okawa, Y. Aratake, K. Motoda, M. Ohba, H. Sakiyama, W. Matsumotot, Supramol. Chem. 6 (1996) 293. [11] B. Bosnich, Inorg. Chem. 38 (1999) 2554. [12] H. Okawa, H. Frutachi, D.E. Fenton, Coord. Chem. Rev. 174 (1998) 51. [13] M.J. Goldcamp, S.E. Edison, L.N. Squires, D.T. Rasa, N.K. Vowels, N.L. Coker, J.A. Bauer, M.J. Baldwin, Inorg. Chem. 42 (2003) 717. [14] B. Dietrich, M.W. Hosseini, J.M. Lehn, R.B. Session, Helv. Chim. Acta 66 (1983) 1262. [15] J. Murase, S. Ueno, S. Kida, Inorg. Chim. Acta 111 (1986) 57. [16] M. Ciampolini, L. Fabbrizzi, A. Perotti, A. Poggi, B. Seghi, F. Zanobini, Inorg. Chem. 26 (1987) 3527. [17] N.F. Curtis, J. Chem. Soc. A (1968) 1584. [18] D. Cook, E.D. Mckenzie, Inorg. Chem. 31 (1978) 54. [19] M.D. Duggan, E.K. Barefield, D.N. Hendrickson, Inorg. Chem. 12 (1973) 985. [20] M. Yonemura, N. Usuki, Y. Nakamura, M. Ohba, H. Okawa, J. Chem. Soc., Dalton Trans. (2000) 3624. [21] (a) S. Subramanian, T.M. Barclay, K.R. Coulter, A. McAuley, Coord. Chem. Rev. 245 (2003) 65; (b) M.A. Hossain, J.M. Llinares, N.W. Alcock, D. Powell, K.B. James, J. Supramol. Chem. 2 (2002) 143; (c) F.C. Van Voggel, W. Verboom, D.N. Reinhoudt, Chem. Rev. 94 (1994) 279. [22] J.-P. Collin, C.D. Buchecker, P. Gavina, M.C.J. Molero, J.-P. Sauvage, Acc. Chem. Res. 34 (2001) 477.

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