Bivalent, Trivalent, and Tetravalent Nickel Complexes with a Common Tridentate Deprotonated Pyridine Bis-Amide Ligand. Molecular Structures of Nickel(II) and Nickel(IV) and Redox Activity

Abstract

Using a tridentate bis-amide ligand 2,6-bis[ N -(phenyl)carbamoyl]pyridine (H 2 L), in its deprotonated form, nickel complexes in three consecutive oxidation states [Et 4 N] 2 [Ni II L 2 ]·H 2 O ( 1 ), [Et 4 N][Ni III L 2 ]·H 2 O ( 2 ), and [Ni IV L 2 ]·0.75H 2 O ( 3 ) have been prepared, and 1 and 3 structurally characterized. These X-ray structures represent first crystallographically characterized NiN 6 coordination sphere, with a common pyridine bis-amide ligand. Complex 1 crystallizes in the orthorhombic space group Pccn, a = 10.175(2) Å, b = 20.834(3) Å, c = 23.765(4) Å, Z = 4, and 3 crystallizes in the monoclinic space group P 2 1 / a, a = 14.874(7) Å, b = 13.300(4) Å, c = 16.604(5) Å, β = 99.678(3)°, Z = 4. Considerable distortion is observed with the average distances being Ni−N amide 2.131(8) Å and Ni−N py 1.994(7) Å for 1 and Ni−N amide 1.946(8) Å and Ni−N py 1.846(8) Å for 3 . The observation of short axial M−N py and long equatorial M−N amide bonds (tetragonally compressed octahedral geometry) is caused by the steric requirement of the ligand. Magnetic susceptibility measurements (63−300 K) reveal that the spin states of nickel centers in 1 and 2 are S = 1 and S = 1 / 2, respectively. Complex 3 is diamagnetic. In their absorption spectra (MeCN), 1 exhibits a d−d transition at 854 nm; 2 and 3 display LMCT transitions at 449 nm with a shoulder at 636 nm and at 480 nm with a shoulder at 730 nm, respectively. The nickel(III) complex 2 exhibits a rhombic EPR signal ( g values: 2.149, 2.115, and 2.034), showing that the metal center is the primary residence site of the unpaired electron. Cyclic voltammetric measurements of 1 in MeCN solution at a glassy carbon electrode exhibit two chemically reversible ( i p,a / i p,c ≈ 1) and electrochemically quasireversible (Δ E p = 100 mV) oxidative responses: a Ni III −Ni II couple ( E 1/2 = 0.05 V vs SCE) and a Ni IV −Ni III couple ( E 1/2 = 0.51 V vs SCE). A one-electron chemical oxidation of yellowish brown 1 was achieved in a two-phase solvent mixture H 2 O−CH 2 Cl 2 with [Fe(η 5 -C 5 H 5 ) 2 ][PF 6 ], which led to the isolation of reddish brown 2 . A two-electron chemical oxidation of 1 was readily achieved in MeCN with ceric ammonium nitrate to afford dark violet crystals of 3 . For 1 a linear correlation between the Ni III −Ni II reduction potentials and the reciprocal of solvent dielectric constants is obtained.