Synthesis and characterization of mono-and bimetallic complexes of Zn ( II ) and Cu ( II ) ; new multifunctional unsymmetrical acyclic and macrocyclic phenol-based ligand

Article history: Received March 27, 2013 Received in Revised form August 27, 2013 Accepted 3 September 2013 Available online 4 September 2013 The dicompartmental macrocyclic ligand (L ) was prepared by [1:1] cyclic condensation of N,N′-dimethyacetate-N,N′-ethylene-di(5-methyl-3-formyl-2-hydroxybenzylamine with 1,3diaminopropane. The ligand includes dissimilar N(amine)2O2 and N(imine)2O2 coordination sites sharing two phenolic oxygen atoms and containing two methyl acetate pedant arms on the amine nitrogen donor atoms. A series of monoand bimetallic complexes were synthesized and characterized on the basis of elemental analysis, molar conductance measurement, IR and UVVis spectroscopy techniques. It was found that during the cyclization process the copper (II) displaced from the N(amine)2O2 to the N(imine)2O2 coordination site and one of the methyl acetate pedant arms is dissociated. The heterodinuclear complex of [ZnLCu(-OAc)] was prepared by a transmetallation reaction on the [ZnLZn(-OAc)] by Cu(II). The characterization results showed that the two metal ions are bridged by two phenolic oxygen atoms and an acetate group, providing distorted five-coordination geometries for the both metal ions.


Introduction
In the last decade, great attention was paid to the design and synthesis of dicompartmental ligands capable of forming macrocyclic or acyclic complexes with similar or dissimilar metal ions 1 .Interest in this topic is due partly to their potential applications to mimic bimetallic biosides of the metalloenzymes 1 and their interesting catalytic properties 2 .Among many different types of dicompartmental ligands, phenol-based ligands having two metal binding sites sharing two phenolic oxygen atoms have been proved as more relevant for modeling of the active sites of many metallobiosites to hosting and carrying small molecules 3 .Therefore, synthesis of such compartmental ligands and their metal complexes are desirable.Zinc(II) complexes, which are thermally stable, structurally diverse and easily modified, have been attracted great attention due to their biomimicking roles.Furthermore, some synthetic dinuclear zinc(II) complexes are also known to participate in some bio-related processes 4 .Copper (II) complexes are the beacon for modeling, and they serve as bioinorganic model compounds not only in enzymatic reactions, but also in catalytic synthetic oxidation reactions 5 .
In our previous work we have succeeded in preparing a family of macrocyclic phenol-based ligands of the type 1, and acyclic dicompartmental type 2, which are shown in Scheme 1 in our laboratory and their structures and reactivities were investigated [6][7][8][9][10][11][12] .In present study, we have reported the synthesis of a new multifunctional phenol-based ligand, L 1 H 2 (Scheme 1), with the new methyl propionate arms, which has potentially a tetra-coordinate compartment with an O 4 donor set, comprising of two phenolic and two formyl oxygen atoms and another a hexa-coordinate compartment with an N 2 O 4 donor set, made by oxygen atoms of two phenolic, two carboxylate groups and two aminic nitrogen atoms.The acyclic monometallic complexes [ML 1 ] was cyclized by 1,3diaminopropane to produce dicompartmental macrocyclic ligand [L 2 ] 2-shown in Scheme 1.

Syntheses
The acyclic ligand L 1 H 2 , 3, was prepared by the sequence of reactions outlined in Scheme 2. The pre-ligand 2 was synthesized first by reaction of ethylenediamine and acryl amide with mole ratio of 1:2, respectively in solvent of methanol.In the next step, condensation of one part of compound 1 and two parts pre-ligand 2 afforded ligand L 1 H 2 , 3. The IR spectra of L 1 H 2 , showed a broad band at around 3423 cm -1 , which is probably attributed to the OH stretching vibration of the phenolic groups.The mononuclear [Zn II L 1 ] and [Cu II L 1 ] complexes were readily prepared as solid by reaction of L 1 H 2 with the metal acetate and triethylamine in ethanol (Scheme 2, reaction 1).Scheme 2. General method for the preparation of pre-ligand and L 1 H 2 The metal ion in the acyclic [M II L 1 ] complex can bind at either the N(amine) 2 O 2 or at the O coordination site of the ligand.The IR spectra and the visible spectral data as will be discussed later show that the metal ion positioned in the N(amine) 2 O 2 coordination site.However, through characterization and establishment of the purity of these compounds were hindered by their poor solubility in all of the solvent investigated.In the next step, the monometallic acyclic complex [M II L 1 ] was cyclized by addition of 1,3-diaminopropane under acid catalysis as shown in Scheme (reaction 2).The elemental analysis of the copper (II) complex reveals that one of the pendent arm is removed and the Cu(II) ion coordinated into O 2 N 2 (imine) site and the other aminic site, O 2 N 2 (amine) is occupied by a proton.Although the exact nature of this phenomenon is not known, the arm rupture and copper displacement was observed before 8 .However, the formulation [(H + )L 2 Cu]PF 6 and a structure such as that shown in Scheme 3 would be consistence with the characterization results.All attempts to prepare monometallic macrocyclic complex Zn(II) by cyclization of [ZnL 1 ] and 1,3diaminopropane with alteration of reaction conditions were failed and resulted in the formation of bimetallic complex [Zn(L 2 )Zn(μ-OAc)]PF 6 , 7, as depicted in Scheme 3 (reaction 3).

Scheme 3. The synthesis steps for preparation of bimetallic complexes
It seems this phenomenon depends on the nature of the M(II) ion used.However, the analogous ligand system with two pyridine pendant arms instead of the methylpropionate produced the macrocyclic mononuclear complex as [ZnL 2 (H + ) 2 ] 2+ 13 .The removal of a methyl propionate pendant arm in reaction 1 in Scheme 3 that is, the introduction of Cu(OAc) 2 to the free ligand L 1 H 2 , was excluded due to the lack of the N-H vibration signal in the IR spectrum of [CuL 1 ] complex and also by similarity of its IR spectrum with that of the [ZnL 1 ] complex.The driving force in the displacement of the Cu(II) ion may be in part, which is related to the nature of the donor atoms of two coordination site.The Cu(II) migrates to occupy the relatively rigid N(imine) 2 O 2 coordination site, which provides the more preferred square coplanar environment but Zn(II) exists in the more flexible N(amine) 2 O 2 site 14 .
The heterobimetallic complex including a copper (II) ion in the N(imine) 2 O 2 and zinc (II) ion in the N(amine) 2 O 2 compartments were synthesized by two different routes.The monometallic [(H + )L 2 Cu] + complex offers a perfect precursor for the mild preparation of the heterobimetallic complex of [ZnL 2 Cu(-OAc)] + .The reaction 5 depicted in Scheme 3 can be driven to the right by addition of a base and are exceptionally mild since incorporation of the second metal depends on the fast removal of the cavity proton.Under such conditions it would be anticipated that the coordination of the second metal ion would be much faster than the site scrambling of the metal 15 .The heterobimetallic complexes [ZnL 2 Cu(-OAc)]PF 6 were prepared by reaction of monometallic [(H + )L 2 Cu] + complex with Zn(OAc) 2 •2H 2 O as illustrated in Scheme 3 (reaction 5).Alternatively, the heterobimetallic [ZnL 2 Cu(-OAc)]PF 6 complex was prepared through a transmetallation reaction via the second route (reaction 4 in Scheme 3).The reaction was simply carried out by addition of Cu(OAc) 2 to the homobimetallic [ZnL 2 Zn(OAc)] + complex at room temperature with a moderate yield.It was found that the optimum mole ratio of the reactants between [ZnL 2 Zn(-OAc)] + and Cu(OAc) is 1:1.2.

Physical Characterization
The IR spectra of ligand L 1 H 2 and all complexes show a band at 1729 cm -1 associated to the presence of the carbonyl group of the pendent arm(s) and it remains unchanged in all the complexes, indicating that the carbonyl group(s) is free of coordination in complexes.The vibration band around 1680 cm -1 is due to the presence of (C=O) aldehyde groups in the ligand L 1 H 2 .This band appears at 1606 cm -1 for the mononuclear copper complexes, [ML 1 ].This displacement to the lower wave numbers is due to coordination of the M(II) ion into the N 2 O 2 coordination site.Appearance of a band at near 1550 cm -1 for all of the complexes is assigned to be the skeletal vibration of the aromatic rings 16 .Further, the appearance of new bands at 845 and 559 cm -1 in [(H + )L 2 Cu]PF 6 and in the bimetallic complexes correspond to the vibration modes of PF 6 -ion 17 .Also the cyclization was confirmed by the disappearance of the aldehyde C=O stretching band at around 1606 cm -1 and the emergence of a strong band at around 1629 cm -1 that is assigned to C=N stretching vibration mode 18,19 .The N-H vibration of the quaternized amine appears at near 3240 cm -1 in the complexes [(H + )L 2 Cu]PF 6 , [ZnL 2 Zn(μ-OAc)]PF 6 and [ZnL 2 Cu(μ-OAc)]PF 6 indicate the breakage of an methyl propionate pendant arm and meanwhile the presence of the (C=O) vibration at 1730 cm -1 point out existence of a propionate pendant pendent arm.These results reveal that the only one of the pendent arms is removed.A strong evidence for the coordination of the second metal in the N(imine) 2 O 2 or N(amine) 2 O 2 coordination site is the appearance of the phenolic stretching band at 1320-1334 cm -1 , which is absent in this region in the monometallic complexes 19 .The IR spectra of the bimetallic complexes show the symmetric and anti-symmetric (COO) vibration bands of the acetate group at around 1572 and 1462 cm -1 , respectively.The small separation between the two vibration bands (less than 150 cm -1 ) is in agreement with the bridging function of the acetate group in the complex 20 .The electronic absorption spectra of the mono-and bimetallic complexes were run in acetonitrile solutions over the range 200-900 nm.The spectra of the complexes are dominated by charge transfer band and other ligand-based transitions.Azomethine π→π* transitions appear at around 280 nm for monometallic complexes consistent with the other monometallic related complexes reported 21 .These bands typically shift to about 350-360 nm in the bimetallic complexes.In the copper complexes the charge transfer bands overlap with the d-d band.The bands at 689 nm in [CuL 1 ] and 590 nm in [ZnL 2 Cu(μ-OAc)]PF 6 complexes are clearly due to the d-d transition of the copper(II) center 14 .The variation in the position of the d-d band undoubtedly reflects a difference in the coordination geometry and perhaps coordination number of the Cu(II) ion in the respective coordination site.The d-d band of [CuL 1 ] complex is also compared to those of related copper(II) complexes with propionitrile pendent arm that have similar structures (665 nm) 8 .These results supports that the copper(II) ion displaced from N(amine) 2 O 2 coordination site to the N(imine) 2 O 2 coordination site upon the cyclization of [CuL 1 ] and the N(amine) 2 O 2 coordination site is free from coordination or occupied by zinc(II) ion (see Scheme 2).In [(H + )L 2 Cu] + the charge transfer band appears as a lower energy shoulder on the more intense azomethine transition, which may entirely obscure the d-d band.
Molar Conductivity measurement of the [CuL 1 ] and [ZnL 1 ] complexes showed lack of conductivity in DMSO solution.Molar conductance of all the ionic complexes fall within the range reported for 1:1 electrolytes (123-155 Ω -1 cm 2 mol -1 ) in acetonitrile solutions at 25°C 20 .A two-ion electrolyte observed in the mononuclear macrocyclic complex [(H + )L 2 Cu]PF 6 , confirms the presence of one proton in the N 2 O 4 coordination site.

Conclusions
The aim of the present study involves the synthesis of novel unsymmetrical mono-and homo/heterobimetallic Cu(II) and Zn(II) complexes utilizing new dicompartmental acyclic and macrocyclic ligands possessing two contagious hexa-and tetra-coordination sites.Although, the synthetic objective of this research has been achieved, the characterization results showed that the propionate pendant pendent arms are free from coordination.An interesting result observed when acyclic monometallic copper (II) complex was cyclizied by 1,3-diaminopropan, which the copper ion was displacement from N(amine) 2 O 2 to the N(imine) 2 O 2 coordination sites along with dissociation of an pendent arm.To achieve our primarily objective that is the preparation of dicompartmental ligands possessing contiguous six-and four-coordination sites the ligand system requires to be modified in order to bring the pedant arms in coordination to the metal ion.In view of the present results, the ester moieties of the pedant arms are not able to coordinate to the metal ions but it seems probable to hydrolyze the -COOMe to -COO -group which contain better donor set atoms.This transformation is currently in progress in our lab.