“Criss-Crossed” Dinucleating Behavior of an N4 Schiff Base Ligand: Formation of a μ-OH,μ-O₂ Dicobalt(III) Core via O₂ Activation

Abstract

We report the synthesis and structural characterization of a dicobalt(III) complex with a μ-OH,μ-O2 core, namely μ-OH,μ-O2-[Co(enN4)]2(X)3 [1(ClO4)3 and 1(BF4)3]. The dinuclear core is cross-linked by two N4 Schiff base ligands that span each cobalt center. The formally Co(III)-Co(III) dimer is formed spontaneously upon exposure of the mononuclear Co(II) complex to air and exhibits a ν(O-O) value at 882 cm(-1) that shifts to 833 cm(-1) upon substitution with (18)O2. The CV of 1(BF4)3 exhibits a reversible {Co(III)-Co(III)}↔{Co(III)-Co(IV)} redox process, and we have investigated the oxidized {Co(III)-Co(IV)} species by EPR spectroscopy (g = 2.02, 2.06; S = 1/2 signal) and DFT calculations.

Water Oxidation Catalyzed by a Dinuclear Cobalt–Polypyridine Complex

“…Controlled potential electrolysis indicates that the oxidation at 1.34 V (Figure S3) is a one‐electron process (Co III Co III →Co IV Co III ). An EPR spectrum of an aliquot from the bulk electrolysis shows a signal centered at g =2.03 that is very similar to the EPR signals of two reported dinuclear peroxo‐bridged Co IV Co III complexes7, 10d (Figure S4). The reductive process at −0.20 V is assigned to a Co III Co III /Co II Co III couple.…”

“…After one laser flash, the EPR spectrum at 10 K shows a signal at g =2.03 (Figure 4) that is very similar to that found after bulk electrolysis in MeCN and that is therefore assigned to a low spin S =1/2 mixed‐valent Co III Co IV species 7. 10d This is the same EPR‐signal that is obtained when 1 is chemically oxidized by [Ru(bpy) 3 ] 3+ in 1:1 borate buffer/CH 3 CN solution (Figure S7).…”

“…The doubly bridging μ‐OH/μ‐O 2 motif has been observed in other cobalt complexes10 and is suggested to enhance the kinetic stability of the compounds. The CoCo distance in 1 is 3.265 Å, and the O2O3 bond is 1.412 Å, which is in the typical peroxo range of 1.4–1.5 Å 10d…”

From Mesocates to Helicates: Structural, Magnetic and Chiro‐Optical Studies on Nickel(II) Supramolecular Assemblies Derived from Tetradentate Schiff Bases

“…In the same way and following the same tendencies, both forms seem to be equally favored for the quinolyl ligand L7 with an ethyl spacer, whereas the helicate is exclusively formed for the previously reported [38][39][40] most flexible ligandsL 5a nd L6. Then, is not surprising that the mesocate arrangement is the preferred form for 4R,w hereas both helicate and mesocate forms seem to be equally preferred for L3.…”

“…In this sense, the first-rowt ransition metals have been speciallys tudied, although structures with other transition metals or even quadruple helicates with rare earths have also been reported. These systems, in whichL corresponds to the bis-bidentatep yridyl/imine Schiff bases L5 or L6 (Scheme 2) and Xi sab ridging ligand,a re scarce and have only been reported for Co II cationsw ith X = oxo, or peroxo, [39] and for Ni II cations with X = azido or cyanate. Double Ni II helicates with the cations in octahedral environment and with two coordination sites occupiedb yo ne bidentate ligand and employing the L2 Schiff base (Scheme 2), exhibit similarc oordination sites for the N-donors andN CCN torsion angles in the same range (Scheme 1c).…”