Homogeneous oxidative coupling catalysts: stoichiometry and characterization of the first stable oxotetranuclear solids [(Pip)nCuX]4O2 (n=1 or 2, Pip=piperidine, X=Cl, Br, I)

Synthesis and Characterization of Novel <i>μ</i>-Carbonato Tetranuclear Copper Complexes [(Pip)<sub>4n</sub>Cu<sub>4</sub>X<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>] in Aprotic Media

“…In the FTIR spectrum of the free Pip ligand, a peak appeared at 3445 cm −1 assigned as ν NH which was shifted to 3281 cm −1 in the spectra of [(Pip) 4n Cu 4 X 4 (CO 3 ) 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) 4n Cu 4 X 4 O 2 ] 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].…”

“…It is noticed that the values of ε (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 electronic spectral data of [(Pip) 4n Cu 4 X 4 (CO 3 ) 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 ε (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).…”

“…From (900 -800) cm −1 , the carbonato bridge has ν 2 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 − change of carbonato bridge from structure a to structure b as in (Scheme 1) [24]- [28].…”

“…[(Pip) 4n Cu 4 X 4 ] complexes are oxidized by stoichiometric amount of O 2 under N 2 condition to form [(Pip) 4n Cu 4 X 4 O 2 ], Equation 1, followed by rapid reaction with CO 2 in accordance with Equation 2 under N 2[24]-[28]. CO 2 acted as a Lewis acid for the accessible basic µ-oxo copper(II) centers[24]-[28]. Solid [(Pip) 4n Cu 4 X 4 (CO 3 ) 2 ] products formed strong effervescence with dilute HCl to confirm the presence of carbonato moiety.…”