Reductive electrochemical study of Ni(II) complexes with N2O2 Schiff base complexes and spectroscopic characterisation of the reduced species. Reactivity towards CO

doi:10.1016/S0277-5387(02)01025-2

Polyhedron

Volume 21, Issue 17, 15 July 2002, Pages 1695-1705

https://doi.org/10.1016/S0277-5387(02)01025-2 Get rights and content

Abstract

Reductive electrochemical properties of series of nickel(II) complexes with salen ligands, which have different diimine bridges and substituents in the aldehyde moiety have been studied in several solvents (CH₃CN, dmf and (CH₃)₂SO). In order to assess the relative importance of the Ni(I) and Ni(II) anion radical species, the reduced species have been characterised by combining EPR and UV–Vis spectroscopy. The results have shown that complexes with aliphatic diimine bridges are reduced to four-coordinate Ni(I) species with a B_1g (d_xy)¹ ground state, whereas those with aromatic diimine bridges are reduced to square–planar Ni(II) anion radical species that rapidly dimerise. None of the reduced species was found to bind pyridine, imidazole and triphenylphosphine, but in the presence of the stronger π-acceptor ligand CO, new Ni(I) species were formed that, and on the basis of EPR data, can be formulated as five-coordinate complexes with a B_1g (d_xy)¹ ground state, [NiL·CO]⁻. These new species are more stable than the parent complexes as confirmed by the more positive E_1/2 values as a consequence of the extensive π delocalisation M→CO.

Reductive electrochemical properties of series of nickel(II) complexes with salen ligands, have been studied in several solvents. The relative importance between Ni(I) and Ni(II) anion radical species have been assessed by combining EPR and UV–Vis spectroscopy. In the presence of the π-acceptor ligand CO, Ni(I) complexes formed new species, which on the basis of EPR data, can be formulated as five-coordinate Ni(I) complexes, [NiL·CO]⁻.

Introduction

The chemistry of polydentate nickel(I) complexes has attracted attention since they can act as powerful catalysts on chemical or electrochemical reduction of electrophiles, such as alkyl and aryl halides [1], [1](a), [1](b), [1](c), [1](d), [1](e), [2], [2](a), [2](b), [2](c), [2](d), [2](e), [3], [3](a), [3](b) and carbon dioxide [4], [4](a), [4](b), [4](c). The ability of the starting nickel(II) complex to form upon one-electron transfer a nickel(I) complex, rather than the anion radical of the ligand, appears as a key point for obtaining an efficient catalysis. Nickel(I) complexes are expected to react with electrophiles by transfer of their metal centred unpaired electron in an inner-sphere fashion, being more efficient and selective than Ni(II) anion radicals, which function as an outer-sphere electron donor due to the delocalised nature of the ligand based unpaired electron.

Salen ligands can easily stabilise low and high oxidation states of nickel and reduced [Ni(salen)] is used in the electro-reduction of alkyl and aryl halides [1](a), [1](c), [1](d). By introducing substituents in the ligand it is possible to modulate the potential at which the reduction occurs, and to control concomitantly the catalytic properties of the complexes. As salen has the desirable characteristic of being readily subject to systematic modification of its electronic and steric properties by synthetic approaches, we have prepared a series of nickel(II) complexes with salen derivatives that have different diimine bridges and substituents in the aldehyde moieties (Scheme 1). The reductive behaviour of the resulting complexes was studied in several solvents and the reduced species characterised by combining electrochemical, EPR and UV–Vis spectroscopy in order to assess the relative importance of the Ni(I) and Ni(II) anion radical species. As our goal is to use these complexes as catalysts, we report also the reactivity of the reduced species towards π acceptor Lewis bases. Some of the complexes have already been prepared and characterised in some of the solvents used, [1](a), [1](c), [1](d), [5] but they are included to provide a coherent framework for the overall study reported.

Section snippets

Cyclic voltammetry of nickel(II) complexes

The complexes studied can be divided in two groups, based on their electrochemical response (Table 1). Complexes with non-aromatic imine bridges (1–7; group A) show, in the potential range used, one reduction process in all solvents used. With increasing scan rates (v), a linear dependence between i_p and v^1/2 is observed (similar slopes for the i_pc vs. v^1/2 and i_pa vs. v^1/2 plots), and the cathodic–anodic peak potential separations are similar to those observed for the couple Fc⁺–Fc, and the

Concluding remarks

Species generated by one-electron electrochemical reduction of the nickel(II) Schiff base complexes reported in this work were characterised by combining EPR and UV–Vis–NIR spectroscopy and the results obtained allow for unambiguous identification of the electron transfer site. Reduction of nickel complexes was found to be solvent independent, but on the other hand the final reduction product depends largely on the ligand diimine bridge. Complexes with aromatic diimine bridges, (Group B),

Reagents, solvents and nickel(II) complexes

The solvents for syntheses were of reagent grade (Merck), and those for spectroscopic and electrochemical measurements were of analytical grade (Merck, pro analysi); all were used as received. Tetra-n-butylammonium perchlorate (TBAP) was prepared by published procedures from tetrabutylammonium chloride (Aldrich) and perchloric acid (Merck, p.a.) and recrystallised twice from ethanol [40] (CAUTION: perchlorates are hazardous and may explode). Carbon monoxide gas was purchased from Praxair.

The

Acknowledgements

This work was partially supported by the ‘Fundação para a Ciência e Tecnologia’, Lisboa, Portugal, through Project POCTI/32831/QUI/2000.

References (41)

A Bakac et al.
J. Am. Chem. Soc.
(1986)
A Bakac et al.
J. Am. Chem. Soc.
(1986)
A Bakac et al.
J. Am. Chem. Soc.
(1986)
M.S Ram et al.
Inorg. Chem.
(1986)
N Sadler et al.
Inorg. Chem.
(1989)
B Fischer et al.
J. Am. Chem. Soc.
(1980)
M Beley et al.
J. Am. Chem. Soc.
(1986)
J.B Collin et al.
Inorg. Chem.
(1988)
A.A Isse et al.
Electrochim. Acta
(1992)
M Chikira et al.
Bull. Chem. Soc. Jpn.
(1974)
C. Freire, B. de Castro, J. Chem. Soc., Dalton Trans. (1998)...
I.C Santos et al.
Polyhedron
(2000)
J.R Pilbrow et al.
Mol. Phys.
(1973)
(a)C. Gosden, K.P. Healy, D. Pletcher, J. Chem Soc. Dalton Trans. (1978)...C Gosden et al.
J. Organomet. Chem.
(1980)
K.P Healey et al.
J. Organomet. Chem.
(1978)
C Gosden et al.
J. Electroanal. Chem.
(1981)
J.Y Becker et al.
J. Electroanal. Chem. Interf. Electrochem.
(1981)
G.K Lahiri et al.
Inorg. Chem.
(1992)
K.M Kadish et al.
Inorg. Chem.
(1992)
A.J Bard et al.
Electrochemical Methods. Fundamentals and Applications
(2001)

R.S Nickolson

Anal. Chem.

(1965)

S. Gambarotta, C. Floriani, A. Chiesi-Villa, C. Guastini, J. Chem. Soc., Chem. Commun. (1982)...

S Gambarotta et al.

Inorg. Chem.

(1983)

F Azevedo et al.

Inorg. Chim. Acta

(1994)

G.A Bowmarker et al.

Inorg. Chem.

(1982)

G.A. Bowmaker, G.A. Boyd, G.K. Campbell, M. Zvagulis, J. Chem. Soc., Dalton Trans. (1986)...

M. Valente, C. Freire, B. de Castro, J. Chem. Soc., Dalton Trans. (1998)...

J Maroney et al.

Inorg. Chem.

(1984)

A.D. Toy, M.D. Hobday, P.D.W. Boyd, J. Chem. Soc., Dalton Trans. (1973)...

A.H Maki et al.

J. Am. Chem. Soc.

(1964)

Cited by (32)

Reductive and oxidative electrochemical study and spectroscopic properties of nickel(II) complexes with N<inf>2</inf>O<inf>2</inf> Schiff bases derived from (±)-trans-N,N′-bis(salicylidene)-1,2-cyclohexanediamine
2014, Electrochimica Acta
Citation Excerpt :
Considerable difference is observed for ɛ’9 which, together with more significant shift of λ9 [Fig. 3, dashed line], may indicate some contribution of solvent in coordination of Ni(II) ion and, accordingly, a presence of other than square forms of the complex. Octahedral Ni(II) complexes with similar ligands exhibit weak absorption bands slightly over 600 nm [26]. In spectra recorded for CH2Cl2 solutions of complexes substituted in phenolate moiety one can observe shift of absorption bands towards lower energies in comparison to unsubstituted complexes.
Cyclic voltammetry, chronopotentiometry, chronocoulometry, controlled potential electrolysis and UV VIS were used in investigation of ligands and Ni(II) complexes with (±)-trans-N,N′-bis(salicylidene)-1,2-cyclohexanediamine substituted in orto- and/or para- positions of phenolate anions with tert-butyl and/or methoxy groups. Irreversible reduction and oxidation of ligands were shown. No influence of solvent on reduction of complexes, which results in Ni(I) complexes, was observed, however, an effect of solvent and substituents of phenolate anions on oxidation processes was proved. Anodic process in (CH₃)₂SO always results in appropriate Ni(III)-phenolate complex. Oxidation of orto- and para- substituted complexes in CH₂Cl₂ results in Ni(II)-mono-phenoxyl radicals, which undergo delocalisation and subsequently are oxidised to Ni(II)-bis-phenoxyl radicals. Last oxidation step of radicals produce long-lived Ni(II)-bis-phenoxonium cations. Complexes without substituents in para-and/or orto- position having formed Ni(II)-mono-phenoxyl radicals undergo dimerization. Mechanisms of electrode processes were studied and D and E_f values for processes of reduction and I^st step of oxidation of complexes were calculated.
Synthesis, spectroscopy, electrochemistry and thermal study of Ni(II) and Cu(II) unsymmetrical N <inf>2</inf>O <inf>2</inf> Schiff base complexes
2010, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
The new tetradentate unsymmetrical N₂O₂ Schiff base ligands, Ni(II) and Cu(II) complexes were synthesised and chracterized by IR, UV–vis, ¹H NMR and elemental analysis. The electrochemical properties of the Ni(II) complexes were investigated. The thermogravimetry of the Ni(II) and Cu(II) complexes were carried out in the range of 20–700 °C. Decomposition of synthesised complexes is related to the Schiff base characteristics.
Catalytic reduction of 1-iodooctane by nickel(I) salen electrogenerated at carbon cathodes in dimethylformamide: Effects of added proton donors and a mechanism involving both metal- and ligand-centered one-electron reduction of nickel(II) salen
2007, Journal of Electroanalytical Chemistry
In dimethylformamide containing tetramethylammonium tetrafluoroborate, 1-iodooctane is reduced catalytically by nickel(I) salen electrogenerated at a glassy carbon cathode. Cyclic voltammograms for the nickel(II) salen–1-iodooctane system recorded in the absence as well as in the presence of a proton or deuteron donor (1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), phenol, or D₂O) exhibit enhanced cathodic peaks and diminished anodic peaks for the nickel(II) salen–nickel(I) salen couple, along with a new cathodic peak attributed to reduction of a nickel(II) salen species for which an imino bond of the ligand is octylated. Without a proton donor, controlled-potential catalytic reduction of 1-iodooctane by nickel(I) salen affords hexadecane, octane, and 1-octene. For electrolyses performed in the presence of either HFIP or phenol, the yields of hexadecane and octane are decreased and increased, respectively, whereas that of 1-octene remains unchanged. Bulk electrolyses done in the presence of D₂O give a product distribution similar to that obtained when no proton donor is added; none of the octane is deuterated, indicating that octyl radicals (not octyl carbanions) are precursors for the formation of octane. Theoretical calculations involving density functional theory have been employed to establish that nickel(II) salen can undergo either a metal- or ligand-centered one-electron reduction. A mechanistic scheme is proposed that invokes both metal- and ligand-centered reduction of nickel(II) salen to explain the effects of proton donors on the catalytic reduction of alkyl halides as well as the pathway for alkylation of the imino bonds of the ligand.
Ni(II)-Salophen—Comprehensive Analysis on Electrochemical and Spectral Characterization and Biological Studies
2023, Molecules
Mechanistic insights into the electrochemical reduction of CO<inf>2</inf> to CO on Ni(salphen) complexes
2023, Inorganic Chemistry Frontiers
Experimental Electrochemical Potentials of Nickel Complexes
2021, Synlett

View all citing articles on Scopus

View full text

Reductive electrochemical study of Ni(II) complexes with N2O2 Schiff base complexes and spectroscopic characterisation of the reduced species. Reactivity towards CO

Abstract

Introduction

Section snippets

Cyclic voltammetry of nickel(II) complexes

Concluding remarks

Reagents, solvents and nickel(II) complexes

Acknowledgements

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Inorg. Chem.

Inorg. Chem.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Inorg. Chem.

Electrochim. Acta

Bull. Chem. Soc. Jpn.

Polyhedron

Mol. Phys.

J. Organomet. Chem.

J. Organomet. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem. Interf. Electrochem.

Inorg. Chem.

Inorg. Chem.

Electrochemical Methods. Fundamentals and Applications

Anal. Chem.

Inorg. Chem.

Inorg. Chim. Acta

Inorg. Chem.

Inorg. Chem.

J. Am. Chem. Soc.

Reductive electrochemical study of Ni(II) complexes with N₂O₂ Schiff base complexes and spectroscopic characterisation of the reduced species. Reactivity towards CO