• Gouré, Eric
• Fortage, Jérôme
• Collomb, Marie-Noëlle
• Cleŕac, Rodolphe
• Pećaut, Jacques
• Gerey, Bertrand
• Thomas, Fabrice
• Molton, Florian

Abstract

We report on the synthesis and structural characterization of the cobalt pentanuclear helicate complex from the rigid tetradentate bis(2-pyridyl)-3,5-pyrazolate ligand bpp − , namely, [{Co II (μ-bpp) 3 } 2 Co II 3 (μ 3-OH)] 3+ (1 3+), in which a trinuclear {Co II 3 (μ 3-OH)} core is wrapped by two {Co II (μ-bpp) 3 } units. The cyclic voltammogram of 1 3+ in CH 3 CN revealed seven successive reversible one-electron waves, in the 0 and −3.0 V potential range, highlighting the remarkable stability of such architecture in several redox states. Two mixed-valent states of this complex, the two-electron-oxidized Co II 3 Co III 2 (1 5+) and the one-electron-reduced species Co I Co II 4 (1 2+), were generated by bulk electrolyses and successfully characterized by single-crystal X-ray diffraction among the eight redox levels between Co I 5 and Co II 3 Co III 2 that can be accessed under electrochemical conditions. Because of the crystallographic characterization of 1 5+ and 1 2+ , the five reduction processes located at E 1/2 values of −1.63 (1 3+/2+), −1.88 (1 2+/+), −2.14 (1 +/0), −2.40 (1 0/−), and −2.60 V (1 −/2−) versus Ag/ AgNO 3 were unambiguously assigned to the successive reduction of each of the five Co(II) ions to Co(I), starting with the three ions located in the central core followed by the two apical ions. The two other redox events at E 1/2 values of −0.21 (1 4+/3+) and −0.11 V (1 5+/4+) are assigned to the successive oxidation of the apical Co(II) ions to Co(III). The Co(I) complexes are rare, and the stabilization of a Co(I) within a trinuclear μ-hydroxo core in the reduced species, 1 2+ , 1 + , 1 0 , 1 − , and 1 2− , is probably the result of the particular structure of this complex in the presence of the two apical sites that maintain the trinuclear core through the six bridging bpp − ligands. The spectroscopic characteristics of 1 2+ , 1 3+ , and 1 5+ (ultraviolet−visible and X-band electron paramagnetic resonance) are also described as well as their magnetic properties in the solid state. ■ INTRODUCTION The self-assembly of polynuclear coordination complexes has attracted considerable attention over the past few decades. 1−3 It is now possible to direct synthesis toward structures with well-defined nuclearity and geometry by altering the ligand properties (rigidity, number and orientation of coordination sites, and nature of donor atoms) and the nature of the metal ions. 4 Among bridging ligands, heterocycles with two adjacent nitrogen donors such as pyrazolates, pyridazines, triazoles, and derivatives have been extensively used to design polynuclear complexes. Beyond their ability to maintain two metal ions in the proximity of each other, they indeed allow an electronic communication between these centers providing peculiar electronic properties and reactivities. 5−8 More particularly, the rigid tetradentate 3,5-bis(pyridyn-2-yl)-pyrazole (Hbpp) ligand 9 can form, by self-assembly with various first-row transition metal ions at high temperatures, remarkable bis(triple) helicate pentanuclear architectures of the type [{M(μ-bpp) 3 } 2 M 3 (μ 3-X)] n+ (M = Mn or Fe with X = O, and M = Ni or Zn with X = OH). 10−13 The five metal ions are arranged in a trigonal bipyramidal topology with two axial {M II (μ-bpp) 3 } units connecting a central μ 3-oxo or hydroxo trinuclear core {M 3 (μ 3-X)} through the bridging bpp − ligands. Interesting structure-specific physicochemical properties have been observed for this family of complexes, such as spin frustration for the copper complex, 11 while for the iron derivative, it has been shown that the spin states of the two

Topics

• impedance spectroscopy
• x-ray diffraction
• Nitrogen
• copper
• cobalt
• iron
• self-assembly