Reductive electrochemical study of Ni(II) complexes with N2O2 Schiffbase complexes and spectroscopic characterisation of the reduced

species. Reactivity towards CO

Fernando Azevedo, Cristina Freire *, Baltazar de Castro

CEQUP/Departamento de Quımica, Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal

Received 20 November 2001; accepted 16 April 2002

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 B1 g (dxy )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 p-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 B1g (dxy )1 ground state, [NiL �/CO]�. These new species are more stable than the

parent complexes as confirmed by the more positive E1/2 values as a consequence of the extensive p delocalisation M0/CO. # 2002

Elsevier Science Ltd. All rights reserved.

Keywords: Salen ligands; Ni(I) complexes; Nickel(II) complexes; EPR

1. 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�/3] and

carbon dioxide [4]. 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

[1a,1c,1d]. By introducing substituents in the ligand it is

possible to modulate the potential at which the reduc-

tion 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 alde-

hyde 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 p acceptor Lewis bases.

Some of the complexes have already been prepared and

* Corresponding authors. Tel.: �/351-22-6082-890; fax: �/351-22-

6082-959

E-mail address: acfreire@fc.up.pt (C. Freire).

Polyhedron 21 (2002) 1695�/1705

www.elsevier.com/locate/poly

0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 0 2 5 - 2

characterised in some of the solvents used, [1a,1c,1d,5]

but they are included to provide a coherent framework

for the overall study reported.

2. Results and discussion

2.1. 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 ip and v1/2 is

observed (similar slopes for the ipc vs. v1/2 and ipa vs. v1/2

plots), and the cathodic�/anodic peak potential separa-

tions are similar to those observed for the couple Fc��/

Fc, and the ratios ipa/ipc are solvent independent,

constant and equal to 1 for all the scan rates used.

Electrochemical data for complexes of group A are thus

consistent with one electron, diffusion-controlled, rever-

sible reduction process [6].

Complexes with aromatic imine bridges (8 and 9;group B) present, in all solvents and for scan rates in the

range 0.02�/0.5 V s�1, cyclic voltammograms with one

cathodic wave at 1.210 V B/�/EpcB/1.378 V and one

anodic wave at 0.282 V B/�/EpaB/0.630 V (Fig. 1(a)).

Despite their large separation (:/0.75 V), the two waves

are related, as no anodic wave could be detected when

the potential is cycled from 0 to �/1.0 V.

With increasing scan rate a linear dependence betweenip and v1/2 is also observed, but with quite different

slopes for the ipc versus v1/2 and ipa versus v1/2 plots,

which implies different diffusion coefficients for the

reduced and oxidised species, and suggests significant

structural changes upon reduction.

Peak potential dependence with complex concentra-

tion at a constant scan rate in dmf was also studied. For8 and 9, the values of Epa are independent of complex

concentration, whereas those of Epc are shifted to more

negative potentials when the complex concentration is

reduced. For a scan rate of 200 mV s�1, Epc is shifted by

21 mV for [Ni(saloph)] (8) and 20 mV for [Ni(saloph-

Cl2)] (9), when the complex concentration is tenfold

reduced.

Similar results have been described in literature fordmf solutions of complex 8 [5,7] and have been

interpreted as a one-electron reversible reduction pro-

cess followed by a fast chemical reaction leading to the

formation of a stable new product, which is oxidised to

the original complex.

Floriani et al. [8,9] have isolated the product of

sodium reduction in the solution of [Ni(saloph)]. X-ray

and magnetic data show that the product, formulated as[Ni(saloph)]2Na2, is a diamagnetic Ni(II) dimeric com-

pound which is composed of two monomeric units

linked by one of the carbon imine atoms that has been

reduced to an amide carbon. Vianello et al. [5], based on

the similarity between the cyclic voltamograms of

[Ni(saloph)]2Na2 and [Ni(saloph)] in dmf, suggested

that the electro-reduction of [Ni(saloph)] in this solvent

produces in a first step a nickel(II) anion radical due toreduction of the imine bond to an amide bond, followed

by a fast dimerisation reaction to yield the Ni(II) dimeric

species, [Ni(saloph)]22�. These species are then reversi-

bly oxidised to the parent complex by cleavage of the

bonding between the two monomer units. We presup-

pose the same mechanism for reduction of complexes 8

and 9 in all solvents used, and experimental evidence for

the formation of the radical intermediate species havebeen obtained by EPR (see below).

Cyclic voltammograms recorded in dmf using very

high scan rates, typically in the 1�/60 V s�1 range exhibit

in the reverse scan a new anodic wave at potentials

similar to those of the cathodic wave, whereas the

anodic wave at more positive potentials looses intensity

as the scan rate is increased (Fig. 1(b)). When the scan

rate reaches 60 V s�1, the cyclic voltammograms, in allsolvents, resemble those of a reversible type system with

E1/2�/1/2(Epa�/Epc)�/�/1.380 for 8 and E1/2�/�/1.298

V for 9, and the ratio ipa/ipc reaches 1. The new anodic

wave is attributed to the oxidation of the reduced

monomeric species, in the absence of dimerisation; by

using very fast scan rates the monomeric reduced species

are oxidised before the dimerisation reaction takes

place.An estimate of the dimerisation rate constant, kdim,

can be obtained from the difference between Epc

(obtained for a scan rate of 50 mV s�1, where the

system is kinetically controlled by the dimerisation

Scheme 1. Structure of nickel(II) complexes.

F. Azevedo et al. / Polyhedron 21 (2002) 1695�/17051696

reaction) and E1/2 for a scan rate of 60 V s�1, where the

system is approximately reversible and diffusion con-

trolled. Under the experimental conditions in which the

system is kinetically controlled by the dimerisation

reaction, Epc is expressed by the equation [5,7]:

Epc�E1=2�0:058�0:0197 log kdim�0:0197 log(c=v)

For complex solutions with c�/2.00�/10�3 mol

dm�3 in dmf and using the E1/2 calculated at 60 V

s�1, kdim was found to be: 4.089/0.67�/105 mol�1 dm3

s�1 (Epc�/�/1.355 V) for [Ni(saloph)] (8) and 3.239/

0.53�/105 mol�1 dm3 s�1 (Epc�/�/1.275 V) for [Ni(sa-

loph-Cl2)] (9). The presence of electron withdrawing Cl

groups in the imine bridge reduces the electronic density

in the imine bond which would hinder the dimerisation

process.

2.2. Coulometry of nickel(II) complexes

Electrochemical reduction of complexes of group A

produces a colour change from reddish to dark blue (1�/

5) or dark green (6 and 7), whereas reduction of 8 and 9

(group B) led to brown�/green solutions, implying the

formation of new species that depend on the precursor.

Plots of the cathodic current, ic, against the time of

electrolysis exhibit exponential decay typical of first-

order processes for all complexes. On the other hand,

plots of ic versus q (charge transferred during the

electrolysis) are quasi-linear, which suggests the trans-

ference of a constant number of electrons during

electrolysis [6]. The ic�/q plots were used to obtain the

values of q (�) by extrapolation to ic�/0, which has

enabled the estimation of the number of electrons, n ,

involved in the reduction process (Table 1) [6]. The

observation that the resulting n values are :/1 for

complexes 1�/7 (Table 1), provides additional confirma-

tion that only one electron is involved in the reduction

process of these complexes. For complexes 8 and 9, the

values of n are also :/1, which indicates that only one

imine bond is involved in the reduction and dimerisation

reactions. The values of n were not corrected for blank

solutions, but reduction of these solutions under the

experimental conditions used produces only a small

contribution for the cathodic current. Thus, the small

deviations from unity can be attributed to residual

charge from oxidation of traces of molecular oxygen

or support electrolyte reductions.

Table 1

Electrochemical data for nickel(II) complexes a

Complex Solvent Epc (V) Epa (V) �DE (V) E1/2 (V) b �ipc(mA) ipa (mA) �ipc/ipa q (�)/C c n c

[Ni(salen)] (1) CH3CN �1.715 �1.650 0.065 �1.682 8.50 7.75 1.10 5.44 1.07

dmf �1.633 1.565 0.068 �1.599 5.00 4.90 1.02

(CH3)2SO �1.627 �1.563 0.064 �1.595 2.48 2.22 1.12

[Ni(saltMe)] (2) CH3CN �1.781 �1.710 0.071 �1.735 5.60 5.60 1.00 5.46 1.07

dmf �1.707 �1.636 0.071 �1.672 4.18 3.84 1.09

(CH3)2SO �1.704 �1.639 0.065 �1.672 1.53 1.32 1.16

[Ni(Cl2-saltMe)] (3) CH3CN �1.488 �1.413 0.075 �1.450 3.18 3.12 1.02 5.15 1.01

dmf �1.408 �1.330 0.078 �1.369 4.04 4.00 1.01

(CH3)2SO �1.406 �1.328 0.078 �1.367 4.70 4.70 1.00

[Ni(salhd)] (4) CH3CN �1.760 �1.685 0.075 �1.722 3.18 3.12 1.02 5.20 1.12

dmf �1.662 �1.594 0.068 �1.628 4.04 4.00 1.01

(CH3)2SO �1.676 �1.584 0.092 �1.630 3.55 3.20 1.11

[Ni(a,a?-Me2salen)] (5) CH3CN d

dmf �1.740 �1.675 0.065 �1.707 3.28 3.10 1.06 3.75 1.08

(CH3)2SO �1.727 �1.669 0.058 �1.698 2.38 2.21 1.08

[Ni(salpd)] (6) CH3CN d

dmf �1.386 �1.330 0.056 �1.358 3.36 3.40 0.99 3.09 1.23

(CH3)2SO �1.400 �1.322 0.078 �1.361 3.70 3.70 1.00

[Ni(salbd)] (7) CH3CN d

dmf �1.402 �1.335 0.067 �1.369 3.10 2.92 1.06 3.12 1.20

(CH3)2SO �1.435 �1.352 0.083 �1.393 2.50 2.35 1.06

[Ni(saloph)] (8) CH3CN �1.378 �0.631 0.747 7.61 1.89 4.02 2.94 1.02

dmf �1.300 �0.555 0.745 �1.380 e 5.24 1.40 3.74

(CH3)2SO �1.298 �0.555 0.743 5.56 2.11 2.63

[Ni(saloph-Cl2)] (9) CH3CN �1.292 �0.364 0.928 10.5 3.94 2.66 3.00 1.01

dmf �1.210 �0.282 0.928 �1.298 e 7.00 2.80 2.50

(CH3)2SO �1.210 �0.284 0.926 6.47 2.71 2.39

a The values were obtained at 0.05 V s�1.b The E1/2 values are calculated as 1/2(Epc�Epa).c Refers to electrolysis.d Complexes have low solubility in this solvent.e Obtained at 60 V s�1, see text for details.

F. Azevedo et al. / Polyhedron 21 (2002) 1695�/1705 1697

2.3. EPR and electronic spectroscopy of reduced species

Analysis of electrochemical data has shown that all

complexes of group A and B are reduced by an electron

charge transfer process, but that different species are

produced by complexes with aromatic and non-aromatic

imine bridges, a consequence of different reduction

processes. Coupling EPR and electronic spectroscopy

provides further characterisation of the reduced species.

It must be pointed out that all complexes were also

reduced with Na(Hg) and in the same solvents used for

coulometric reductions. As the resulting solutions have

the same colours and identical frozen solution EPR

spectra it is possible to conclude that the same reduced

species are formed by chemical and electrochemical

reduction. The following description is thus applicable

to chemically and electrochemically reduced nickel(II)

complexes.

Frozen solution EPR spectra of reduced species from

group A in any of the solvents are very similar, and

show large g tensor anisotropy with gav:/2.12�/2.15,

typical of metal centred reduced species, in this case

nickel(I) complexes. The spectra of reduced solutions of

1�/5 are rhombic and exhibit hyperfine splittings in the

two high magnetic field regions (a :/0.6�/1.0 mT) due to

the interaction of the unpaired electron with two

nitrogen atoms of the polydentate ligand (Fig. 2(a)).

For reduced 6 and 7, the EPR spectra are apparently

axial, with no detectable hyperfine splittings in any g

region (Fig. 2(b)).In the absence of single crystal EPR measurements for

these complexes, the observed similarity between the g

features from both type of spectra with those of other

Ni(I) complexes with similar ligands and with tetraaza

macrocyclic ligands, [10�/13] and of analogous isoelec-

tronic d9 Cu(II) complexes, [14�/16] can be taken as

support for the following orientation scheme for tensor

axes in complexes 1�/7: g1�/gz , g2�/gx and g3�/gy ,where g1 and g3 refer to the lowest and highest magnetic

field values, respectively (obviously, a1�/az ; a2�/ax and

a3�/ay ). The observation that gz �/gx ; gy implies a

B1g(dxy)1 ground state for all the Ni(I) complexes [17�/

21]. All EPR spectra were manipulated and simulated as

described in the Section 4; EPR parameters are sum-

marised in Table 2. Spectra of each complex are

practically independent of the solvent used, thus provid-ing an indication that the reduced nickel complexes keep

their four-coordinate geometry.

Analysis of the g and a (N) values show that

complexes 1�/5, which possess an ethylenediimine based

bridge, exhibit very similar values, but that their g

values are smaller and their a(N) are larger than those of

complexes with three and four atoms in the imine bridge

(6 and 7). By taking into account data for Cu(II)complexes, [22] this observation provides an indication

that the Ni(I) complexes with a long diimine bridge

exhibit larger tetrahedral distortions than those with

ethylenediimine based bridges (1�/5). Additional indirect

evidence for the larger tetrahedral distortion of Ni(I)

complexes 6 and 7, can be gathered from the existence of

larger distortions in the corresponding Ni(II) complexes,

as observed in the crystallographic structures of thelatter complexes [23,24].

Frozen EPR spectra of electro-reduced complexes of

group B (8�/9) are quite different: they exhibit a low

intensity pseudo isotropic signal at g :/2.007�/2.010

(Fig. 2(c)), assigned to a radical, which provides an

indication that reduction of these complexes involves

ligand based processes. The low intensity isotropic

signal must thus be due to monomeric Ni(II) radicalspecies present in solution, probably a consequence of

the existence in solution of an equilibrium between

monomeric and dimeric species.

Electronic spectra of the Ni(I) species obtained by

reduction of the corresponding Ni(II) complexes can

also be divided into two sets. Spectra of reduced

complexes 8�/9 are similar to those of the parent Ni(II)

complexes, thus suggesting a similar electronic structurein dimeric complex.

On the other hand, electronic spectra in dmf of all

Ni(I) species obtained by reduction of 1�/7, are practi-

cally identical for all complexes and comparable to those

Fig. 1. Cyclic voltammograms of the complex [Ni(saloph)] (8) (group

B) in dmf at scan rates in the range: (a) 0.02, 0.05, 0.10, 0.20 and 0.5 V

s�1 and (b) 5, 10, 20, and 40 V s�1.

F. Azevedo et al. / Polyhedron 21 (2002) 1695�/17051698

of similar Ni(I) compounds described in literature

[13,25�/29]; they show a medium intensity band at l�/

800�/830 nm (o�/1100�/1800 dm�3 mol�1 cm�1) with a

shoulder at higher energies l�/637�/760 nm (o�/3200�/

5100 dm�3 mol�1 cm�1), and high-intensity bands in

the UV region lB/400 (o�/10 000 dm�3 mol�1 cm�1)

(Fig. 3(a) and Table 2). On the basis of their extinction

coefficients the low energy band can be tentatively

assigned to a d�/d transition, albeit with a large

contribution from ligand orbitals, and the shoulder at

higher energy to charge transfer bands. The wavelength

of the d�/d band in spectra of reduced species is

bathochromic shifted by about 30% relative to the d�/d

transitions of the precursor Ni(II) complexes, which

occur in the range l�/550�/500 nm (Table 2) [13,25�/29].

This indicates a weakening in the ligand field caused by

the decrease in effective nuclear charge of the metal

centre concomitant with its reduction. The extinction

coefficients for Ni(I) complexes are usually much larger

than those observed for similar Ni(II) complexes which

is attributed to a large degree of covalency in the metal�/

ligand bonds (see below) [13,25�/29].Using the splitting scheme proposed by Nishida et al.

[30,31] for the d-orbitals in isoelectronic square�/planar

copper(II) complexes with di-negatively charged tetra-

dentate ligands, dx2�/y2B/dz2B/dxz 5/dyz B/dxy , and as-

suming the same d-splitting for the isoelectronic Ni(I)

complexes used in this work, the low energy band in

each spectrum can be assigned to the transition dxz :/

dyz 1/dx2.

Confirmation for this assignment is provided by EPR

data. Ni(I) complexes are at best of D2h symmetry and

using the g equations deduced for d9 systems with B1g

ground state, the values of s and p-out-of plane

covalency parameters, a2 and d ?2, respectively, can be

estimated from the gx and gy values [18�/21]. This

calculation was made using the methodology outlined in

[13], with the further assumption that the low-energy

electronic band corresponds to the transition dxz :/

dyz 1/dz2. The values of these parameters, summarised

Fig. 2. Frozen solution EPR spectra of the reduced species in dmf (�/120 8C): (a) [Ni(a,a?-Me2salen)]�; (b) [Ni(salbd)]� and (c) [Ni(saloph)]22� in

N2 atmosphere and (a?) [Ni(a,a?-Me2salen)]� in CO atmosphere.

F. Azevedo et al. / Polyhedron 21 (2002) 1695�/1705 1699

Table 2

EPR parameters and electronic data for nickel reduced species

Complex Solvent gz gx gy gava Az(N) b Ax(N) b Ay(N) b lmax (o /M�1 cm�1) c) lmax nm (o /M�1 cm�1) d a2 d ?2

[Ni(salen)]� CH3CN 2.264 2.084 2.043 2.131

dmf 2.250 2.081 2.040 2.124 0.82 0.94 0.98 806 (1700); 637 (5100) 540 (230) 0.81 0.48

(CH3)2SO 2.261 2.086 2.044 2.130

[Ni(saltMe)]� CH3CN 2.264 2.084 2.046 2.132

dmf 2.253 2.081 2.041 2.125 0.82 0.94 0.96 820 (1800); 668 (5000) 545 (160) 0.80 0.49

(CH3)2SO 2.250 2.081 2.040 2.124

[Ni(Cl2-saltMe)]� CH3CN 2.273 2.088 2.047 2.136

dmf 2.252 2.080 2.040 2.124 0.82 0.93 0.96 819 (1750); 668 (4700) 545 (180) 0.79 0.49

(CH3)2SO 2.273 2.088 2.047 2.136

[Ni(salhd)]� CH3CN 2.266 2.083 2.044 2.131

dmf 2.254 2.081 2.041 2.125 0.81 0.92 0.96 803 (1880); 663 (4900) 540 (200) 0.81 0.49

(CH3)2SO 2.267 2.085 2.045 2.132

[Ni(a,a?-Me2salen)]� CH3CN 2.263 2.077 2.042 2.127

dmf 2.255 2.076 2.041 2.124 0.78 0.88 0.96 828 (1500); 674 (4600) 545 (190) 0.74 0.53

(CH3)2SO 2.263 2.077 4.042 2.127

[Ni(salpd)]� CH3CN e

dmf 2.300 2.083 2.055 2.146 0.71 0.73 0.76 807 (1200); 672 (3500) 500 (140) 0.83 0.65

(CH3)2SO 2.304 2.084 2.058 2.149

[Ni(salbd)]� CH3CN e

dmf 2.314 2.088 2.070 2.157 0.60 0.63 0.69 805 (1100); 763 (3200) 505 (140) 0.88 0.79

(CH3)2SO 2.322 2.094 2.076 2.164

[Ni(saloph)]22� CH3CN 2.010 f g

dmf 2.007 f 622 (60); 462 (1200) 575 (240); 475 (8400) g

(CH3)2SO 2.010 f g

[Ni(saloph-Cl2)] 22 CH3CN 2.009 f g

dmf 2.006 f 620 (75); 465 (1500) 565 (260); 485 (9200) g

(CH3)2SO 2.010 f g

a gav is calculated as 1/2(gx�gy�gz ).b AN(N) are expressed in mT and have been obtained as described in Section 4.2; only in dmf the spectra have the appropriate resolution.c The electronic bands have been deconvoluted with the program ORIGIN.d Refers to parent Ni(II) complexes.e The complexes are not soluble in this solvent.f The signal is pseudo isotropic and the value corresponds to gav .g No metal reduction occurs.

F.

Azeved

oet

al.

/P

oly

hed

ron

21

(2

00

2)

16

95�

/17

05

17

00

in Table 2, compare with those observed for other Ni(I)

complexes with similar ligands [11�/13,32,33] which

provides indirect support for the assignment. The values

indicate that covalency is greater for the p-out-of plane

bonding, suggesting large p delocalisation between theligand and the metal centre. The lower planarity of the

Ni(I) complexes with a long aliphatic diimine bridge (6,

7) is also evidenced by the larger values calculated for

d ?2.

2.4. Reactivity of reduced species towards Lewis bases

No changes in the frozen EPR spectra or electronic

spectra were observed upon addition of pyridine,

imidazole or triphenylphosphine (in stoichiometric or

excess molar ratio) to nickel reduced solutions, either

freshly electrolysed or chemically reduced, an indication

that neither these Ni(I) species interact with the strong

Lewis bases, nor do the nickel(II) dimeric species.Probably the monodentate N and P donors ligands act

as weaker p acceptors than the tetradentate ligand in

stabilising the Ni(I) species.

2.5. Reactivity of reduced species towards carbon

monoxide

The binding of CO, a stronger p acceptor ligand, with

complexes 1�/9 was assessed by studying the reduction

process by cyclic voltammetry and coulometry under a

CO atmosphere, and the reduced species were charac-

terised by EPR and electronic spectroscopy.

2.6. Cyclic voltammetry and electrolysis

No changes were observed in the cyclic voltammo-

grams of complexes of group B (8, 9) obtained under a

CO atmosphere when compared with those under a

nitrogen atmosphere. This suggests no interactionbetween CO and the nickel species, and corroborates

further that reduction of group B complexes does not

involve the metal centre. Contrastingly, the voltammo-

grams of group A complexes are slightly different from

those in a nitrogen atmosphere: a reduction process is

always observed, but (i) the DE values are larger; (ii) the

values of DE increase significantly with increasing scanrates; and (iii) the ipa/ipc ratios are smaller than 1, which

suggests a slow reaction between the nickel species and

CO [6,7].

In order to get further information on the interaction

of CO with reduced complexes of 1�/7, electrolysis of the

Ni(II) complexes were made in a CO atmosphere.

During the electrolysis a change in colour was observed

from red to crimson, which differs from the dark blue�/

green colour of the reduced species in a nitrogen

atmosphere, indicating the presence of different species

in solution. Cyclic voltammograms obtained after

electrolysis show one reduction process with DE�/60�/

70 mV at n�/50 mV s�1, and with ipa/ipc�/1, but with

E1/2 at slightly more positive potentials than those

observed in a N2 atmosphere. In Table 3 are summarised

the electrochemical data for the complexes in COatmosphere. The new electrochemical process corre-

sponds to reduction of nickel species with axial coordi-

nated CO; the more positive E1/2 values imply that

coordination of CO originates more stable Ni(I) species.

The difference between E1/2(N2) and E1/2(CO) can be

taken as a measure of the extent of CO coordination.

From data in Tables 1 and 3 it is possible to infer that

complexes with large tetrahedral distortions (6 and 7)show the weaker interactions with CO, and that com-

plexes with ethylenic bridges, except those with axial

substituents (2, 3 and 4), exhibit the strongest interac-

tions. These results suggest that the planarity of the

coordination sphere [(NiI)N2O2]� plays an important

role in the CO interaction with the nickel centre.

2.7. EPR and electronic spectroscopy of reduced species

with bound CO

Frozen EPR spectra of nickel species with bound CO

have g features similar to those obtained in a N2

atmosphere, but with larger anisotropy and rhombicity

(gav:/2.15, Dxy :/0.06�/0.07). EPR data are summarised

in Table 4, and an analysis of data in this table shows

that the reduced species are still Ni(I) complexes with aB1g(dxy)1 ground state, as gz �/gx , gy . Furthermore, the

observation that gz and gx have been shifted to lower

magnetic field values and that no hyperfine splitting

could be detected in any of the g regions (Fig. 2(a))

suggests that only one CO molecule is bound per nickel

centre, leading to a square pyramidal geometry for the

Ni(I) complexes, as a six-coordinate complex would

have the gz and gx shifted to higher magnetic fields[14,22]. EPR spectra of the reduced species that interact

less extensively with CO (6,7) show a sobreposition of

the parent Ni(I) complex and the one with bound CO.

Fig. 3. UV�/Vis spectra in dmf of [Ni(salen)]� (1 mmol dm�3) in: (a)

N2 and (b) CO atmospheres.

F. Azevedo et al. / Polyhedron 21 (2002) 1695�/1705 1701

Electronic spectra of these CO bound Ni(I) complexesare quite different from those of their parent complexes

and show one intense electronic band at lmax�/546 nm

(o�/5000�/6900 dm�3 mol�1 cm�1) and several high

intense bands in the UV region (Fig. 3). The new band in

the visible region occurs at the same energy for all the

complexes and taking in account their extinction

coefficient they can be assigned to a M0/CO charge

transfer band and is indicative of the high p acceptorcapacity of CO. The non-observation of any d�/d band

can be accounted for by a combination of two factors.

First, the energy of the d�/d bands is increased due to the

high ligand field of CO; second, that of the CT bands is

decreased due to the high p acceptor capacity of CO. As

a consequence the CT bands that occur in the same

region mask less intense d�/d bands.

3. Concluding remarks

Species generated by one-electron electrochemical

reduction of the nickel(II) Schiff base complexes re-

ported 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), yielded ligand-centred reduced

species, whereas those with aliphatic imine bridges,

(group A), exhibit metal-centred reduction processes.

Furthermore, the observation that the E1/2 values for all

complexes are practically solvent independent implies

that the solvent does not participate in the charge

transfer process, and indicates that the reduced nickel

species must keep their tetracoordinate geometry. This

result must be contrasted with the oxidation process of

the same complexes, for which the solvent has a marked

influence on oxidation site (metal vs. ligand), with good

donor solvents favouring formation of Ni(III) com-

plexes [34�/39].

As referred above the extent of unsaturation not only

is critical in controlling the ultimate reduction site, but

influences also the E1/2 values, as complexes with greater

unsaturation exhibit the more positive values. Further-

more, the size of the imine bridge and the substituents in

the ligand were also found to influence E1/2 values.

There is a pronounced positive shift in E1/2 values when

the number of carbon atoms in the aliphatic chain of the

Table 3

Electrochemical data for nickel reduced complexes in CO atmosphere a

Complex Epc (V) Epa (V) �DE (V) E1/2 (V) �ipc (mA) ipa (mA) �ipc/ipa

[Ni(salen)]� �1.610 �1.540 0.070 �1.575 4.02 3.94 1.02

[Ni(saltMe)]� �1.672 �1.598 0.075 �1.635 3.85 3.66 1.05

[Ni(Cl2-saltMe)]� �1.363 �1.287 0.076 �1.325 4.12 4.03 1.02

[Ni(salhd)]� �1.639 �1.561 0.078 �1.600 3.75 3.53 1.06

[Ni(a,a?-Me2salen)]� �1.698 �1.631 0.067 �1.665 4.32 4.29 1.00

[Ni(salpd)]� �1.374 �1.296 0.078 �1.335 3.06 2.75 1.11

[Ni(salbd)]� �1.410 �1.330 0.080 �1.370 2.94 2.68 1.10

[Ni(saloph)]22� b

[Ni(saloph-Cl2)]22� b

a Data obtained in dmf at 0.050 V s�1.b No changes were observed in the voltammogram obtained under a CO atmosphere.

Table 4

EPR values and electronic bands in CO atmosphere in dmf

Complex gz gx gy gava lmax/nm (o /M�1 cm�1) b

[Ni(salen)]� 2.307 2.104 2.044 2.152 546 (6800)

[Ni(saltMe)]� 2.302 2.106 2.039 2.149 546 (6000)

[Ni(Cl2-saltMe)]� 2.303 2.107 2.040 2.150 546 (6900)

[Ni(salhd)]� 2.305 2.086 2.056 2.149 546 (6200)

[Ni(a,a?-Me2salen)]� 2.308 2.111 2.035 2.151 546 (6800)

[Ni(salpd)]� 2.295 2.114 2.052 2.154 545 (5000)

[Ni(salbd)]� 2.294 2.121 2.044 2.153 546 (4600)

[Ni(saloph)]22� 2.006 c

[Ni(saloph-Cl2)]22� 2.007 c

a gav is calculated as 1/2(gx�gy�gz ).b The electronic bands have been deconvoluted with the program ORIGIN.c There is no interaction of the reduced species with CO, the signal corresponds to that observed in the absence of CO.

F. Azevedo et al. / Polyhedron 21 (2002) 1695�/17051702

imine bridge is increased. The larger hole sizes and

larger tetrahedral distortions of [Ni(salpd)] (6) and

[Ni(salbd)] (7), [23,24] make it easier to accommodate

the size increase expected upon reduction of the metaland thus the more positive E1/2 values.

The number, ring position and electronic character-

istics of the substituents also influence the reductive

properties of the complexes: introduction of chloride

atoms increases the stability of the reduced state

(comparison between 2 and 3), whereas the introduction

of methyl groups decreases the stability of reduced

complexes (comparison of 1 and 2 with 7). Thisbehaviour is attributed to changes in the electronic

density in the metal centre induced by the substituents.

In the absence of steric effects, electron-withdrawing

substituents decrease electron density on the metal

making it easier to reduce, whereas electron donating

groups will have the opposite effect. The position of the

methyl groups in the ligand is also important, as

introduction of two methyl groups in the imine carbonfragment, �/C�/N, shifts the E1/2 to more negative values

than four methyl groups in the aliphatic imine bridge.

Due to the large p delocalisation between the Ni(I)

and thepolydentate ligands (as quantified by s and p-

out-of plane covalency parameters, a2 and d ?2), no

interaction with monodentate N and P donor Lewis

bases could be observed. However, with a stronger pacceptor like CO, a strong interaction was observed andmore stable Ni(I) species were formed as proved by their

more positive E1/2 values and lower energy charge

transfer bands.

4. Experimental

4.1. Reagents, solvents and nickel(II) complexes

The solvents for syntheses were of reagent grade

(Merck), and those for spectroscopic and electrochemi-

cal measurements were of analytical grade (Merck, pro

analysi); all were used as received. Tetra-n-butylammo-

nium perchlorate (TBAP) was prepared by published

procedures from tetrabutylammonium chloride (Al-

drich) and perchloric acid (Merck, p.a.) and recrystal-lised twice from ethanol [40] (CAUTION: perchlorates

are hazardous and may explode). Carbon monoxide gas

was purchased from Praxair.

The nickel(II) complexes [Ni(salen)] (1), [Ni(saltMe)]

(2), [Ni(salhd)] (4), [Ni(salpd)] (5) and [Ni(a,a?-Me2sa-

len)] (7), [Ni(saloph)] (8), [Ni(saloph-Cl2)] (9) have been

prepared and fully characterised in [35,36]. The com-

plexes [Ni(Cl4saltMe)] (3) and [Ni(salbd)] (6) have beenprepared by standard methods [35,36] and characterised

by elemental analysis: [Ni(Cl4saltMe)], bis (3,5-tetra-

chlorosalicylaldehyde)tetramethylethylenediamine:

NiC16H18N2O2Cl4. Anal . Calc.: C, 40.8; H, 3.9; N, 5.9.

Found: C, 40.2; H, 3.8; N, 5.5%.

[Ni(salbd)], bis (salicylaldehyde)butylenediamine:

NiC18H18N2O2. Anal . Calc.: C, 61.2; H, 5.1; N, 7.9.Found: C, 61.1; H, 5.0; N, 8.1%.

4.2. Physical measurements

Electrochemical measurements were performed using

an EG&G PAR 273 potentiostat�/galvanostat using

solutions 1 mmol dm�3 in nickel(II) complex and 0.1

mol dm�3 TBAP. Cyclic voltammograms were obtained

in a standard one-compartment electrochemical cellusing a platinum-disc electrode (area of 0.0314 cm2) as

working electrode, a platinum gauze as the counter

electrode and an Ag�/AgCl (1 mol dm�3 NaCl)

reference electrode. All voltammograms were obtained

without IR feedback compensation and measured

potentials were not corrected for liquid junction poten-

tials. All potentials are reported relative to that of the

Ag�/AgCl (1 mol dm�3 NaCl) reference electrode and toE1/2 of the ferrocenium�/ferrocene couple. Cyclic vol-

tammetry was performed in Me2SO, dmf and MeCN in

the potential range �/1.0�/�/2.0 V using scan rates 0.02�/

0.5 V s�1; for complexes of group B scan rates in the

range 5�/60 V s�1 were also used. Under the experi-

mental conditions used and for scan rate 0.05 V s�1,

E1/2 (DE ) for Fc��/Fc couple are 0.50 V (0.09 V) in

Me2SO, 0.50 V (0.10 V) in dmf, and E1/2�/0.48 V (0.085V) in MeCN.

Controlled potential electrolysis was carried out

under a N2 or a CO atmosphere at a potential 0.1 V

higher than the cathodic peak potentials. The electro-

chemical cell has two compartments: one for the

platinum-gauze working electrode and the other for

the platinum-gauze counter electrode, which are sepa-

rated by a silica frit. A Ag�/AgCl (1 mol dm�3 NaCl)reference electrode was used.

Room-temperature (r.t.) electronic spectra of

nickel(II)�/(I) complexes were recorded with a Schi-

madzu UV/3101PC spectrometer in the range 1500�/200

nm. Solutions of nickel(II)�/(I) complexes, under an

atmosphere of N2 or CO, were typically 1 mmol dm�3.

The EPR spectra were obtained with an X-band

Bruker ESP 300E spectrometer equipped with BrukerB-VT 2000 temperature accessory controller, in a dual

cavity and using diphenylpicrylhydrazyl (dpph; g�/

2.0037) as standard; the magnetic field was calibrated

by use of Mn2� in MgO. Typical experimental condi-

tions used were: microwave power 10�/15 mW, modula-

tion amplitude 0.5 mT; modulation frequency 100 kHz.

Spectra were recorded at �/120 8C, using sealed quartz

tubes and the reported EPR parameters were obtainedby computer simulation, [41] using a rhombic g tensor.

As the spectra in Me2SO and CH3CN have lower

resolution than in dmf, the values of superhyperfine

F. Azevedo et al. / Polyhedron 21 (2002) 1695�/1705 1703

splittings obtained for spectra in dmf were used to

simulate spectra in the former solvents. Even for spectra

in dmf the superhyperfine couplings with the ligand

nitrogen atoms had to be obtained from second-derivative EPR spectra, after a Fourier transformation

and the subsequent elimination of all high-frequency

components with a cut off filter just above the typical

frequency components of nitrogen hyperfine coupling.

After back-transformation to the magnetic field space,

the noise at high frequencies introduced by differentia-

tion is eliminated.

4.3. Reduction of Ni(II) complexes

Nickel(I) species were generated in-situ by electrolysis

or chemical reduction of 1 mmol dm�3 solutions of the

corresponding nickel(II) complexes under strict anaero-

bic conditions. Electrochemical reduction was per-formed in all solvents used the procedure outlined

above. Chemical reduction was performed in dimethyl-

formamide using an excess of 5% sodium mercury

amalgam. The solution�/amalgam was stirred for 15

min, and a fast solution colour change was observed.

The progress of the reduction was monitored by UV�/

Vis and EPR spectroscopy and was assumed to be

complete (1 mmol dm�3 in Ni(I) species, at approxi-mately 15 min), when no changes in intensity of the EPR

signals and in the absorbance of electronic bands were

detected.

Manipulations of nickel(I) species were carried out in

a nitrogen atmosphere, by use of standard Schlenk

techniques and/or of a glove box. Addition of pyridine,

imidazole and triphenylphosphine to freshly electrolysed

or chemically reduced solutions was performed underanaerobic conditions at r.t. The resulting solutions were

immediately transferred to EPR tubes and frozen in

liquid nitrogen. Adducts with CO were prepared by

making the reduction (electrochemically and chemically)

under a CO atmosphere.

Acknowledgements

This work was partially supported by the ‘Fundacao

para a Ciencia e Tecnologia’, Lisboa, Portugal, through

Project POCTI/32831/QUI/2000.

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