Nickel(II) Complex of a Hexadentate Ligand with Two o-Iminosemiquinonato(1−) π-Radical Units and Its Monocation and Dication

Abstract

Aerobic reaction of a hexadentate redox-active o-aminophenol-based ligand, H4L(3) = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino(diphenyldithio)-ethane, in CH3OH with Ni(II)(O2CCH3)2·4H2O and Et3N afforded isolation of a reddish-brown crystalline solid [Ni(L(3))] 1. Cyclic voltammetry (CV) experiment exhibits two oxidative responses at E1/2 = 0.09 and 0.53 V vs SCE (saturated calomel electrode). Chemical oxidation of 1 in air by [Fe(III)(η(5)-C5H5)2][PF6] and AgBF4 in CH2Cl2 led to the isolation of one-electron oxidized species [1](1+) as purple [1][PF6]·CH2Cl2 and two-electron oxidized species [1](2+) as dark purple [1][BF4]2·CH2Cl2, respectively. X-ray crystallographic analysis at 100(2) K unambiguously established that the ligand is present in [Ni(II){(L(ISQ)O,N)(•-)}{(L(ISQ)O,N)(•-)}{(LS,S)(0)}] 1, [Ni(II){(L(IBQ)O,N)(0)}{(L(ISQ)O,N)(•-)}{(LS,S)(0)}][PF6]·CH2Cl2, and [Ni(II){(L(IBQ)O,N)(0)}{(L(IBQ)O,N)(0)}{(LS,S)(0)}][BF4]2·CH2Cl2, as monoanionic o-iminosemiquinonate(1-) π-radical (Srad = 1/2) (L(ISQ))(•-) and neutral o-iminoquinone (L(IBQ))(0) redox-levels. Complexes 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 possess an S = 2, 3/2, and 1 ground-state, respectively, established by temperature-dependent (2-300 K) magnetic behavior of 1 and [1][PF6]·CH2Cl2, and a μeff value of [1][BF4]2·CH2Cl2 at 300 K. Both 1 and [1][PF6]·CH2Cl2 exhibit ferromagnetic exchange-coupling between the two electrons of Ni(II) and two/one ligand π-radicals, respectively. The redox processes are shown to be ligand-based. Spectroscopic and redox properties, and density functional theory (DFT) calculations at the CAM-B3LYP-level of theory adequately describe the electronic structure of 1, [1](1+), and [1](2+). The observed UV-vis-NIR absorptions for 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 have been assigned, based on time-dependent (TD)-DFT calculations.

Catalytic Synthesis of N-Heterocycles via Direct C(sp3)–H Amination Using an Air-Stable Iron(III) Species with a Redox-Active Ligand

“…The geometry around each Fe metal center is distorted octahedral, with meridionally coordinated NNO ligands. The iron–ligand bond lengths, angles, and interatomic distances within the NNO moieties are very similar for both molecules and suggestive of the ISQ ligand oxidation state. ,− The iron–ligand bond lengths in complex 2 (average bond distances: Fe–O 1.928 Å, Fe–N imino 1.884 Å, Fe–N pyridyl 1.963 Å) are slightly shorter than those in heteroleptic complex 1 (Fe–O 1.957 Å, Fe–N imino 2.014 Å, Fe–N pyridyl 2.102 Å). On the contrary, the ligand-based interatomic distances in 2 (average bond distances: C–O 1.320 Å, C–N imino 1.368 Å) are slightly longer than those in complex 1 (C–O 1.281 Å, C–N imino 1.339 Å).…”

“…The geometry around iron (τ of 0.52) is intermediate between trigonal bipyramidal and square pyramidal. The iron–ligand bond lengths (Fe–O 1.9572(10); Fe–N 2.0136(12) Å) as well as ligand-based interatomic distances (O1–C1 1.2809(17); N1–C6 1.3390(17) Å) are characteristic of the ISQ ligand oxidation state. ,− A metrical oxidation state value of −0.69 (±0.04) was determined for the NNO ISQ ligand of 1 . Density functional theory (DFT) calculations (B3LYP, def2-TZVP) show a broken-symmetry S = 2 spin state (⟨ S 2 ⟩ = 6.8) as the ground state (see Supporting Information).…”

Assigning Ligand Redox Levels in Complexes of 2-Aminophenolates: Structural Signatures

“…13b,25,32,33,35,49,53 Notably, using a tridentate and a hexadentate ligand, complexes have also been synthesized with two coordinated ligands in two different redox levels (tridentate), 89, 33 or two arms in two different redox levels (hexadentate), 91. 49 The hexadentate ligand also afforded 55. 49 Two potentially tridentate ligands afforded only four-coordinate nickel(II) complexes 23 33 and 24 34 and copper(II) complexes 25 35a and 26 35b with dangling −SR arms.…”

Molecular S = 2 High-Spin, S = 0 Low-Spin and S = 0 ⇄ 2 Spin-Transition/-Crossover Nickel(II)-Bis(nitroxide) Coordination Compounds

“…Copper(II)-and nickel(II)-radical complexes having a direct radical coordination bond are the best documented in the 3d-2p heterospin systems [9,[60][61][62][63][64][65][66][67][68][69][70]. There have been an increasing number of compounds showing a ferromagnetic 3d-2p coupling [75][76][77][78][79][80][81]. It is believed for a long time that axial coordination in copper(II)-nitroxide complexes displays relatively weak ferromagnetic interaction whilst equatorial coordination strong antiferromagnetic interaction [9].…”