Pip (Aldrich), was used after vacuum distillation, (pK_b = 2.8). Gaseous CO₂, was dried by passage through a 10 cm column of Drierite. PhNO₂, was distilled from P₂O₅, and kept over 4 Å molecular sieves (K_f = 7.0˚C/molal, d = 1.25). CH₂Cl₂ was washed with concentrated sulphuric acid, dried over Na₂CO₃, refluxed over P₂O₅, then distilled and stored over anhydrous Na₂CO₃. DMP was purified by sublimation, (m.p. 46˚C - 47˚C). Dinitrogen gas was deoxygenated by passage through a column of Alfa-DE-Ox solid catalyst and dried by passage through a 60 cm column of dehydrated silica gel and 30 cm column of (Calcium chloride and molecular sieves). Copper(I) halides were prepared as described in literature (CuCl and CuBr) [19] .

UV-vis spectrophotometer model 160A (Shimadzu) was used to record the electronic spectra of the investigated complexes. FT-IR spectra of the free ligands and their complexes were performed as KBr discs using Perkin Elmer System 2000 FT-IR spectrophotometer. Calibration of wave numbers was made with a polystyrene film. EPR spectra for the investigated copper complexes were measured using a Radiopan varian spectrometer at 100.0000 KHz at different G modulation amplitude with rectangular TE 102 cavity and 100 KHz modulation field Resonance conditions were found at 9.7 GHz (X-band) at room temperature. The field was calibrated with a powder of diphenylpicrylhydrazyl (DPPH; g = 2.0037) [20] . Cyclic voltammetery (CV) measurements were carried out using a bioanalytical system BAS-27 electrochemistry analyzer connected with BAS, X-Y recorder and in conjugation with a three electrodes cell fitted with a purged dinitrogen gas inlet and outlet. Three electrodes were a Beckman Pt working electrode at room temperature (5 mm diameter) and a Pt wire auxiliary electrode. All potentials of Cu-complexes (1.0 × 10⁻³ M) were determined using Ag/Ag⁺ as a reference electrode (1.0 × 10⁻³ M AgNO₃ in a 0.1 M TBAP (tetrabutylammonium perchlorate) in CH₂Cl₂ under N₂ gas at room temperature [21] . Molecular weight determination was performed via freezing point depression of nitrobenzene solution containing a known amount of solute using Eutechnics precision temperature, model 4600 thermometer [22] . The elemental analyses for Cu and X (Cl, Br) were estimated using the same protocols reported before [23] . CHNS analysis was obtained using LECO CHNS-932 Elemental Analyzer.

CH₂Cl₂ Solutions of [(Pip)_4nCu₄X₄(CO₃)₂] complexes were added to various samples of 100 fold excess of DMP in CH₂Cl₂. O₂ was then streamed through each solution for 20 min. DPQ was characterized at 431 nm by comparison with an authentic sample.

[(Pip)_4nCu₄X₄] complexes are oxidized by stoichiometric amount of O₂ under N₂ condition to form [(Pip)_4n Cu₄X₄O₂], Equation 1, followed by rapid reaction with CO₂ in accordance with Equation 2 under N₂ [24] - [28] .

In this reaction, CO₂ acted as a Lewis acid for the accessible basic µ-oxo copper(II) centers [24] - [28] . Solid [(Pip)_4nCu₄X₄(CO₃)₂] products formed strong effervescence with dilute HCl to confirm the presence of carbonato moiety. The molar mass and analytical results for the prepared complexes are illustrated in Table 1. The molar mass determination confirmed that all [(Pip)_4nCu₄X₄(CO₃)₂] complexes are stable tetranuclear species.

In the FTIR spectrum of the free Pip ligand, a peak appeared at 3445 cm⁻¹ assigned as n_NH which was shifted to 3281 cm⁻¹ in the spectra of [(Pip)_4nCu₄X₄(CO₃)₂] indicating the coordination of Cu-centres to piperidyl nitrogen, Figure 1. For n = 2 complexes, these bands were broad or splitted which may be attributed to the fact that each Cu centre is surrounded by two Pip ligands, in which the hydrogen of one Pip ligand is free, while the other hydrogen in pip ligand is hydrogen bonded with basic centre existing in carbonato complexes as described before in similar reported cases for the oxo [(Pip)_4nCu₄X₄O₂] complexes [24] - [28] . The spectrum of Pip showed also two bands at 1652 cm⁻¹ and 1542 cm⁻¹ due to δ_NH, which became overlapped, broad and shifted to 1610 cm⁻¹ on complexation, Figure 1 [24] - [28] . The carbonato bridge has characteristic vibrational bands, n₃ at 1600 - 1500 cm⁻¹ and at 1490 - 1350 cm⁻¹, n₂ (900 - 800) cm⁻¹ and n₄ (750 - 700) cm⁻¹ [26] [27] . The increase in the intensity of n₃ band at 1450 cm⁻¹, for the [(Pip)₄Cu₄Cl₄(CO₃)₂], Figure 1 confirmed the fact that the bridge is tridentate to fulfil the 6-coordinated Cu(II) centres, (Scheme 1(a)). While in the [(Pip)₈Cu₄Cl₄(CO₃)₂], the bridge switches to bidentate ligand (Scheme 1(b)). From (900 - 800) cm⁻¹, the carbonato bridge has n₂ while Pip has three bands; 805, 830 and a very strong one at 863 cm⁻¹. Therefore, the strong band at 863 cm⁻¹ will be moved to 877 cm⁻¹ for [(Pip)₄Cu₄Cl₄(CO₃)₂] and its intensity at the same wave number, 877 cm⁻¹, becomes weak by adding one extra Pip per each Cu(II) centre as in [(Pip)₈Cu₄Cl₄(CO₃)₂], Figure 1, supporting the

Scheme 1. Proposed molecular core structures for [(Pip)_4nCu₄X₄(CO₃)₂].

change of carbonato bridge from structure a to structure b as in (Scheme 1) [24] - [28] .

The electronic spectral data of [(Pip)_4nCu₄X₄(CO₃)₂] complexes are presented in Table 2 and Figure 2. A splitted peak within (740 - 840) nm range for the studied complexes are observed indicating the presence of at least 3 halo ligands per each Cu(II) centre which indicates a tetranuclear cuban core structured complexes (Scheme 1) [24] - [28] . It is noticed that the values of e (M⁻¹ cm⁻¹) of the splitted peak in case of bromo-complexes are 1.7 times greater than those of the chloro-complexes for similar n. To maintain the coordination number 6 for Cu(II), the only possible change is the conversion of carbonato bridging ligand from tridentate as in structure a to bidentate as in structure b (Scheme 1). A previous work showed a similar spectral behavior for the comparable complexes, of which the electronic spectra were attributed to LMCT between a minimum of 3 halo ligands and a Cu(II) site [24] - [31] .

The solid state EPR spectra of [(Pip)_4nCu₄X₄(CO₃)₂], Figure 3 and Table 2 show axial spectra with g_|| > g_^ > 2.04, confirming the d_x2-y2 ground state for elongated tetragonal octahedral geometry [32] [33] .

The CV measurements, Table 3 and Figure 4 for [(Pip)_4nCu₄X₄(CO₃)₂] complexes are conducted in CH₂Cl₂ made of 0.1 M TBAP using a Pt-working electrode with non-aqueous reference electrode (Ag/Ag⁺, 1.0 ´ 10⁻³ M AgNO₃ in 0.1 M TBAP in CH₂Cl₂) and Pt-wire auxiliary electrode. The electrode potentials are measured against Ag/Ag⁺ as a reference electrode. The Ferrocene (Fc) internal standard was used against Ag/Ag⁺ under similar experimental conditions to correlate the electrode potentials to NHE. The formal electrode potential of a reversible one-electron standard Fc/Fc⁺ against NHE is 0.4 volt [34] [35] . The CV of [(Pip)_4nCu₄X₄(CO₃)₂] complexes are irreversible in character and show two cathodic peaks as in Table 3. The above systems are quite similar to [(Pip)_4nCu₄X₄O₂] cyclic voltammograms in which the electrolysis at −1.3 volt, indicated four electrons to reduce four copper(II) [34] [35] . In [(Pip)_4nCu₄X₄(CO₃)₂], the reduction occurs in two steps separated by about 0.35 volt, due to the morphology of the tetranuclear cuban structure on the electrode surface.

CH₂Cl₂ solutions of [(Pip)_4nCu₄X₄(CO₃)₂] complexes were added to various samples of a 100-fold excess of DMP in CH₂Cl₂. O₂ was then streamed through each solution for 20 min. The DPQ which was characterized at

^*0.4 volt is the formal electrode potential of a reversible one-electron standard couple (Fc/Fc+) versus NHE [21] .

431 nm by making comparison with an authentic sample (e = (5.05 ± 0.01) ´ 10⁴ M⁻¹・cm⁻¹) [36] was the only product in all reactions, Scheme 2.

After 3 days, the yield of (DPQ) formed was in the range of (55% ± 5%), the same yield was also observed for [(Pip)₄Cu₄Cl₄O₂] [24] .