Redox-Induced Change in the Ligand Coordination Mode

Abstract

The reaction of cobalt(II) pivalate with a spin-labeled Schiff base (HL(1)) in organic solvents formed trinuclear complex [Co3(Piv)2L(1)2L(2)2]·Solv (Solv is Me2CO and/or C7H16 and CH3CN) containing both nitroxide L(1) and the product of its single-electron reduction, nitrone L(2). The formation of [Co3(Piv)2L(1)2L(2)2] was a consequence of an unusual phenomenon, which we called "redox-induced change in the ligand coordination mode". A reduction of L(1) to L(2) led to a change in the set of donor atoms and even in the size of the metallocycle. This phenomenon was also found for mononuclear [CrL(1)2L(2)] and [FeL(1)2L(2)]·Me2CO.

A New Method for the Synthesis of Heterospin Complexes

“…In [Co5(Piv)4L were never recorded. This is fully consistent with the data of [23][24][25][26][27], where the authors also used the metals capable of changing the oxidation level.…”

“…An interesting manganese(II)-nitroxide-hydroxylamine complex was known, as well [36]. In the case of the interaction of transition metal compounds with L 2 [23] and of Co(II) pivalate with L 1 , we found that heterospin complexes could be obtained, which contained both the starting nitroxide and the product of its reduction as a result of the transition metal-catalyzed transformation of the starting radical.…”

“…where L 2 is the imino nitroxide 2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl anion and L 3 is the corresponding amidine oxide anion (Scheme 1), was described in [23]. The isolation of this compound was a nontrivial problem, because [Сo3(Piv)2L 2 2L…”

“…While reacting with a transition metal, imino nitroxide HL 3 2] containing both the starting imino nitroxide and its reduced diamagnetic analog was isolated. The redox process led to a change not only in the electronic state of the ligand, but also in its coordination mode [23]. The very fact of the formation of [Сo3(Piv)2L 2 2L 3 2] indicates that there exists an individual class of metal compounds with nitroxides whose ligand shell contains both the starting radical and the product of its reduction.…”

Simultaneous Introduction of Two Nitroxides in the Reaction: A New Approach to the Synthesis of Heterospin Complexes

“…In this reaction, some molecules of the spin-labeled Schiff base HA 2 were reduced to the amidine oxide 4-nitro-2-(4,4,5,5-tetramethyl-3-oxy-4,5-dihydro-1 H -imidazol-2-yl)­phenol (HA 3 ), and the solid product was the [Co 3 (A 2 ) 2 (A 3 ) 2 (Piv) 2 ] mixed-ligand complex containing both the starting imino nitroxide and its reduced diamagnetic analogue. The redox process provoked a change not only in the electronic state of the ligand but also in its coordination mode . If the diamagnetic HA 3 is replaced by the structurally related nitronyl nitroxide HA 1 , we can obtain [Co 3 (A 1 ) 2 (A 2 ) 2 (Piv) 2 ], whose nontrivial peculiarity is the presence of the two paramagnetic ligands in the coordination sphere: nitronyl nitroxide A 1 and imino nitroxide A 2 …”