A radical pathway in catecholase activity with nickel(ii) complexes of phenol based “end-off” compartmental ligands

Abstract

Seven dinuclear and one dinuclear based dicyanamide bridged polymeric Ni(II) complexes of phenol based compartmental ligands (HL(1)-HL(4)) have been synthesized with the aim to investigate their catecholase-like activity and to evaluate the most probable mechanistic pathway involved in this process. The complexes have been characterized by routine physicochemical studies as well as by X-ray single crystal structure analyses namely [Ni2(L(2))(SCN)3(H2O)(CH3OH)] (), [Ni2(L(4))(SCN)3(CH3OH)2] (), [Ni2(L(2))(SCN)2(AcO)(H2O)] (), [Ni2(L(4))(SCN)(AcO)2] (), [Ni2(L(2))(N3)3(H2O)2] (), [Ni2(L(4))(N3)3(H2O)2] (), [Ni2(L(1))(AcO)2(N(CN)2)]n () and [Ni2(L(3))(AcO)2(N(CN)2)] (), [SCN = isothiocyanate, AcO = acetate, N3 = azide, and N(CN)2 = dicyanamide anion; L(1-4) = 2,6-bis(R2-iminomethyl)-4-R1-phenolato, where R1 = methyl and tert-butyl, R2 = N,N-dimethyl ethylene for L(1-2) and R1 = methyl and tert-butyl, R2 = 2-(N-ethyl) pyridine for L(3-4)]. A UV-vis spectrophotometric study using 3,5-di-tert butylcatechol (3,5-DTBC) reveals that all the complexes are highly active in catalyzing the aerobic oxidation of (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) in methanol medium with the formation of hydrogen peroxide. An EPR study confirms the generation of radicals during the catalysis. Cyclic voltammetric studies of the complexes in the presence and absence of 3,5-DTBC have been performed. Reduction of Ni(II) to Ni(I) and that of the imine bond of the ligand system have been detected at ∼-1.0 V and ∼-1.5 V, respectively. Coulometric separation of the species at -1.5 V followed by the EPR study at 77 K confirms the species as an organic radical and thus most probably reduced imine species. Spectroelectrochemical analysis at -1.5 V clearly indicates the oxidation of 3,5-DTBC and thus suggests that the radical pathway is supposed to be responsible for the catecholase-like activity exhibited by the nickel complexes. The ligand centred radical generation has further been verified by density functional theory calculation.

Investigation of 3d-transition metal acetates in the oxidation of substituted dioxolene and phenols

“…S7) signal is characteristic of formation of organic radical species (semiquinone of DTBC) as the reaction intermediate during the catalytic process. The literature data is also in good agreement with our studies, suggesting formation of a semiquinone intermediate [20,26,61,62,67,80,137,146,151] in presence of the 3d-metals used in those complexes. In order to get further insight into the nature of possible intermediates, electrospray ionization mass spectral (ESI-MS) studies in negative ion mode were performed with the catalytic reaction mixture of M II acetates and DTBC (1:50) in methanol.…”

“…It may be argued that Mn would decompose H 2 O 2 [76] hence it cannot be detected but then we have detected peroxide for Mn based catalyst earlier [31]. For Ni II/III CO mimics, H 2 O 2 is always found to be the end product of DTBC oxidation [26,[80][81][82]146] and the same is true for model complexes of Cu also [24,53]. Co and Fe complexes are also known to decompose H 2 O 2 [147][148][149][150].…”

“…The metals were chosen from the list of 3d-metals used so far to design mimics of CO or PHS using different ligand systems viz. copper [24,, iron [25,[55][56][57], cobalt [58][59][60][61][62][63][64][65], manganese [66][67][68][69][70][71][72][73][74][75][76][77][78][79], nickel [26,68,[80][81][82][83] and zinc [33,84]. PHS mimics cannot be found as much as CO but yet are well studied with 3d-metals viz.…”

Mono- and tetra-nuclear copper complexes bearing bis(imino)phenoxide derived ligands: catalytic evaluation for benzene oxidation and ROP of ε-caprolactone

“…The complex is centrosymmetric with the 4-coordinate square planar Cu 2+ ion residing on the inversion centre. For 1a (collected at 160 K) the Cu-O and Cu-N distances are 1.917(2) and 1.971(2) Å and the N-Cu-O bite angle is 91.73 (9) o . For 1b (collected at 293 K), the analogous values are 1.9129(15) Å, 1.9841(17) Å, and 91.36 (7) o .…”

“…[7] More recently bis(imino)phenoxy N 2 O type ligation has been utilized to isolate complexes that can act as catalysts for the oxidation of 50 cyclohexane and toluene, [8] and in catecholase-like activity. [9] We also note that Pandey et al have, by employingβ-ketoaminato ligands, isolated both mono and tetranuclear copper complexes, the formation of which was dictated by the use of anhydrous conditions or not. [10] Herein, we explore the chemistry of the 55 ligand set 1,3-(2,6-R 2 2 C 6 H 3 N=CH) 2 -5-R 1 C 6 H 2 OH-2 (LH) (where R 1 = Me, tBu, Cl; R 2 = Me, iPr) towards [Cu(OAc) 2 ] and have structurally characterized both tetranuclear and mononuclear complexes (see scheme1), the yield of each product can be controlled by the reaction stoichiometry.…”