Bimetallic nickel complexes with bridging dithiolato Schiff base ligands: Synthesis, mass spectral characterisation and electrochemistry

Article abstract of DOI:10.1016/S0022-328X(02)01626-1

Schiff base dithiols with thiophene or phenyl linkers were prepared from condensation of the dialdehydes 2,5-(OHC)₂C₄H₂S, 1,3-(OHC)₂C₆H₄, 1,4-(OHC)₂C₆H₄ and

Full text of DOI:10.1016/S0022-328X(02)01626-1

Journal of Organometallic Chemistry 656 (2002) 262ꢁ269
/
www.elsevier.com/locate/jorganchem
Bimetallic nickel complexes with bridging dithiolato Schiff base
ligands: synthesis, mass spectral characterisation and electrochemistry
Richard M. Moutloali a, Funzani A. Nevondo a, James Darkwa a,*,
Emmanuel I. Iwuoha a, William Henderson b
a Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535; South Africa
b Department of Chemistry, University of Waikaito, Private Bag 3105, Hamilton, New Zealand
Received 11 February 2002; accepted 18 May 2002
Abstract
Schiff base dithiols with thiophene or phenyl linkers were prepared from condensation of the dialdehydes 2,5-(OHC)2C4H2S, 1,3-
(OHC)2C6H4, 1,4-(OHC)2C6H4 and 1,4?-(OHC)2(C6H4)2 with 4-H2NC6H4(SH). The Schiff base dithiols readily reacted with Ni(h5-
C5H5)(PR3)Br (Rꢀ
/
Bu or Ph) in the presence of Et3N to form bimetallic complexes (h5-C5H5)(PR3)NiÃ
/
SC6H4NÄ
/
C(H)ArC(H)Ä
/
NC6H4SÃ
/
Ni(PR3)(h5-C5H5) (Arꢀ
/
C4H2S (6), 1,3-C6H4 (7), 1,4-C6H4 (8), 1,4?-(C6H4)2 (9). The bimetallic nature was established
from elemental analysis and mass spectrometry. Cyclic voltammetry of complexes 6, 8 and 9 are indicative of electronic
communication between the nickel centres via the bridging dithiolato Schiff base ligands. # 2002 Elsevier Science B.V. All rights
reserved.
Keywords: Dithiolato Schiff base; Bimetallic nickel complexes; Mass spectrometry; Cyclic voltammetry
1. Introduction
A recent report by Guerchais et al. [7] has demon-
strated that communication between two iron centres
can be effected through bridging thiophene (A) and
biphenyl (B) units when one of the iron centres is
oxidised. Such intramolecular electron transfer should
Compounds that have units with p-conjugation
ability have been investigated for their potential use as
nonlinear optical materials [1], light emitting diodes [2],
and as molecular wires [3]. Homo- and hetero-bimetallic
complexes with p-conjugated bridging ligands are parti-
cularly interesting for their potential application as
molecular wires [3]. Most of these bridged bimetallic
complexes feature polyene bridging units, as polyenes
are known to allow long range electronic coupling
through p-delocalisation [4]. In these polyene-bridged
also be possible if the bridging ligand has a metalÃ
/
sulphur bond. For example a number of phenylethynyl
thiolate molecules inserted into alkyl thiolate matrix are
known to conduct electrons when adsorbed on gold
surfaces [8]. This demonstrates that sulphur bound to a
metal is capable of acting as electron conduit from a
metal if there is a p-conjugated organic molecule
attached to it.
metal complexes, the metalꢁ
/ligand interaction is usually
a metalÃcarbon s-bond, but the bond assumes some
/
double bond character through electron delocalisation
as one of the metal centres is oxidised. This mechanism
allows electron communication in both homometallic
polymers with Ni, Pd and Pt [5,6] and heterobimetallic
(Ru/Pd, Fe/Pd and Fe/Ni) [7] oligomers with the 1,4-
diethynylbenzene unit (Ã
/
CÅ
/
CÃ
/
C6H4Ã
/
CÅ
/
CÃ/).
* Corresponding author
E-mail address: jdarkwa@uwc.ac.za (J. Darkwa).
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 6 2 6 - 1

R.M. Moutloali et al. / Journal of Organometallic Chemistry 656 (2002) 262ꢁ
/269
263
platinum disc working electrode (1.5 mm diameter), a
silver/silver chloride (Ag/AgCl) reference electrode and a
silver wire counter electrode. Supporting electrolyte was
tetrabutylammonium tetrafloroborate (TBA-TFB) [13]
as 0.1 M solution in CH2Cl2, and the concentration of
the analyte was 0.03ꢃ
10ꢂ3M. The potential measure-
ments were referenced internally to the ferrocene/ferro-
cenium redox peak at 0.441 V.
/
Other complexes featuring metal s-acetylides like
Ni(CÅCÃ
C6H5)(PPh3)(h5-C5H5) have oxidisable metal
centres and show nonlinear optical properties, because
the donorꢁacceptor composition is enhanced by the p-
/
/
2.2. Ligand synthesis
/
conjugation in the ligand [9]. We envisage that by using
Schiff base ligands with two end sulphur atoms that
bridge the two metal centres, we could effect the transfer
of electrons via the bridging ligands from one metal to
the other. Such dithiolato complexes should be easy to
prepare since alkyldithiols are known to form bridged
2.2.1. OHCC6H4C6H4CHO (1)
Acetone (60 ml) was added to a mixture of 4-
formylboronic acid (0.5 g, 3.3 mmol), 4-bromobenzal-
dehyde (0.61 g, 3.3 mmol), potassium carbonate (1.2 g,
6.6 mmol) and Pd(dppf)Cl2 (0.5% mol equivalent) and
refluxed for 48 h. The resultant mixture was filtered and
the solvent removed in vacuo to give an off-white
residue, which was chromatographed on silica gel (60
cyclopentadienyl complexes like (h5-C5H5)(PBu3)NiÃ
2, 4, 6) [10]. This
/
S(CH2)nSÃ
/
Ni(PBu3)(h5-C5H5) (nꢀ
/
illustrates that the (h5-C5H5)(PBu3) fragment is capable
of binding dithiolato ligands. We report here the
synthesis of cyclopentadienylnickel complexes with
bridging dithiolato Schiff base ligands. We have also
investigated their electrochemistry to demonstrate that
the p-conjugated dithiolato Schiff base ligands allow
electronic communication between the two nickel atoms.
mesh) using 1:1 CH2Cl2ꢁ
evaporated to dryness and the residue recrystallised
from CH2Cl2ꢁhexane. Yieldꢀ0.5 g (73%). Anal. Calc.
/
hexane eluent. The eluate was
/
/
for C14H10O2, C, 79.98; H, 4.79. Found: C, 81.14; H,
1
4.30%. H-NMR (CDCl3) d 10.09 (s, 2H, CHO), 8.01
(d, 4H, JHH
8.2 Hz, C6H4C6H4).
ꢀ/8.6 Hz, OHCC6H4), 7.80 (d, 4H, JHH
ꢀ
/
2. Experimental
2.2.2. HSC6H4NC(H)-2-C4H2S-5-C(H)NC6H4SH (2)
To a mixture of 4-aminothiophenol (1.79 g, 14.27
mmol) and 2,5-thiophenedicarboxyaldehyde (0.95 g,
7.13 mmol) was added ethanol (40 ml) and the mixture
stirred overnight at room temperature (r.t.). An inso-
luble yellow precipitate was isolated by suction filtration
2.1. Materials and instrumentation
The starting material 4-aminothiophenol was ob-
tained from Fluka and used as received, whilst
NiBr(PR3)(h5-C5H5) (Rꢀ
phenylphosphino)ferrocenepalladium dichloride
PdCl2(dppf) [12] were prepared by the literature meth-
ods. Reagent grade hexane was distilled and stored over
molecular sieves, while toluene was distilled over sodium
and CH2Cl2 from P2O5. All reactions were performed
using standard Schlenk techniques under a dinitrogen
atmosphere.
The 1H- and 31P{1H}-NMR spectra were acquired on
a Varian Gemini 2000 spectrometer at 200 and 80.96
MHz, respectively, and referenced to residual CHCl3 (d
/
Bu, Ph) [11a,11b] and bis(di-
and washed with ethanol. Yieldꢀ84%. Anal. Calc. for
/
C18H14N2S2, C, 67.05; H, 4.38; N, 8.68. Found: C,
67.15; H, 4.43; N, 8.71%.
Compounds 3, 4 and 5 were synthesised in a similar
manner to compound 2 using two equivalents of 4-
aminothiophenol and one equivalent of the appropriate
dicarboxyaldehyde. The products were all insoluble in
common organic solvents but gave good elemental
analyses.
7.26) and externally to PPh3 (d ꢂ5.00), respectively.
/
2.2.3. HSC6H4NC(H)C6H4-3-C(H)NC6H4SH (3)
A yellow solid was obtained in 87% yield. Anal. Calc.
for C20H16N2S2, C, 68.96; H, 4.59; N, 8.04. Found: C,
68.91; H, 4.66; N, 7.09%.
Elemental analysis was performed on a Carlo Erba NA
1500 in the Department of Chemistry at the University
of the Western Cape. Fast Atom Bombardment
(FABMS) were run on a Fannigan MAT GCQ GCꢁ
/
MS and Electrospray mass spectra (ESMS) were run on
a VG Platform II instrument in a positive ion mode.
Electrochemical measurements were performed on a
BAS 50W instrument using a standard three electrode
electroanalytical cell configuration equipped with a
2.2.4. HSC6H4NC(H)C6H4-4-C(H)NC6H4SH (4)
A yellow solid was isolated in 84% yield. Anal. Calc.
for C20H16N2S2, C: 68.96; H: 4.59; N: 8.04. Found: C,
68.99; H, 4.73; N, 8.10%.

264
R.M. Moutloali et al. / Journal of Organometallic Chemistry 656 (2002) 262ꢁ269
/
2.2.5. HSC6H4NC(H)C6H4C6H4C(H)NC6H4SH (5)
A yellow precipitate was isolated in a yield of 82%.
Anal. Calc. for C26H20N2S2, C, 73.55; H, 4.75; N, 6.60.
Found: C, 73.16; H, 4.74; N, 5.97.
C6H4C6H4); 7.74 (d, 4H, JHH
ꢀ8.4 Hz, C6H4C6H4);
/
7.64 (d, 4H, JHH 8.6 Hz, NC6H4); 6.90 (d, 4H, JHH
ꢀ
/
ꢀ
/
8.4 Hz, SC6H4); 5.28 (s, 10H, C5H5); 1.58ꢁ1.34 (m, 36H,
/
PBu3); 0.93 (t, 18H, PBu3). 31P{1H}-NMR (CDCl3) d
22.41 (PBu3).
2.3. Complexation of ligands with NiBr(PR3)(h5-C5H5)
(Rꢀ
/
Bu or Ph)
2.3.5. [Ni(h5-C5H5)PPh3]2(SC6H4NC(H)C4H2S-5-
C(H)NC6H4S) (10)
All the complexes were synthesised in a similar
manner to the procedure described for complex 6.
A brown solid was isolated in a yield of 70%. Anal.
Calc. for C64H52N2S3P2Ni2, C, 68.32; H, 4.62; N, 2.49.
Found: C, 67.78; H, 4.59; N, 2.47. H-NMR (CDCl3) d
1
2.3.1. [Ni(h5-C5H5)PBu3]2(SC6H4NC(H)C4H2S-5-
C(H)NC6H4S) (6)
8.55 (s, 2H, NÄ
/
C(H)), 7.69 (m, 12H, PPh3), 7.35 (m,
24H, PPh3, SC6H4, C4H2S), 6.89 (d, 4H, JHH
SC6H4), 5.15 (s, 10H, C5H5). 31P{1H}-NMR (CDCl3) d
35.33 (s, PPh3).
ꢀ8.2 Hz,
/
A mixture of 2 (0.24 g, 0.69 mmol) and NiBr(PBu3)-
(h5-C5H5) (0.53 g, 1.38 mmol) was suspended in toluene
to which Et3N (1.5 ml) was added. The maroon colour
changed to dark green after several hours and stirring
continued for 72 h. After filtration, the filtrate was
evaporated to dryness and the residue recrystallised
2.3.6. [Ni(h5-C5H5)PPh3]2(SC6H4NC(H)C6H4-3-
C(H)NC6H4S) (11)
A black crystalline solid was isolated in 51% yield.
Anal. Calc. for C66H54N2P2S2Ni2, C, 70.87; H, 4.83; N,
2.50. Found: C, 70.56; H, 4.81; N, 2.21. 1H-NMR
from CH2Cl2ꢁhexane to give a maroon solid in a yield
/
of 69%. Anal. Calc. for C52H76N2P2S3Ni2, C, 64.92; H,
1
7.81; N, 2.86. Found. C, 65.49; H, 8.32; N, 2.91. H-
(CDCl3) d 8.54 (s, 2H, NÄ
8.15 (d, 2H, JHH 8.6 Hz, C6H4), 7.98 (d, 1H, JHH
Hz, C6H4), 7.71 (m, 12H, PPh3), 7.35 (m, 22H, PPh3,
/C(H)), 8.28 (s, 1H, C6H4),
NMR (CDCl3) d 8.57 (s, 2H, NÄ
/
C(H)), 7.62 (d, 4H,
8.4 Hz, NC6H4), 7.40 (s, 2H, C4H2S), 6.99 (d, 4H,
ꢀ
/
ꢀ/8.4
JHH
JHH
ꢀ
ꢀ
/
/
8.4 Hz, SC6H4), 5.28 (s, 10H, C5H5), 1.54ꢁ
/
1.34
SC6H4,), 6.89 (d, 4H, JHH
10H, C5H5).
ꢀ 8.6 Hz, SC6H4), 5.16 (s,
/
(m, 36H, PBu3), 0.92 (t, 18H, PBu3). 31P{1H}-NMR
(CDCl3) d 22.40, (s, PBu3).
2.3.7. [Ni(h5-C5H5)PPh3]2(SC6H4NC(H)C6H4-4-
C(H)NC6H4S) (12)
A brown solid with a yield of 70% was obtained.
Anal. Calc. for C66H54P2S2N2Ni2, C, 68.48; H, 4.53; N,
2.32. Found: C, 68.34; H, 4.68; N, 2.28. 1H-NMR
2.3.2. [Ni(h5-C5H5)PBu3]2(SC6H4NC(H)C6H4-3-
C(H)NC6H4S) (7)
A dark green solid was isolated in a yield of 58%.
Anal. Calc. for C54H78N2P2S2Ni2, C, 64.92; H, 7.81; N,
2.80. Found: C, 64.90; H, 7.83; N, 2.80. 1H-NMR
(CDCl3) d 8.54 (s, 2H, NÄ
7.65 (m, 14H, SC6H4, PPh3), 7.42 (m, 18H, PPh3), 6.92
(d, 4H, JHH 8.6 Hz, SC6H4), 5.16 (s, 10H, C5H5).
/C(H)), 7.97 (s, 4H, C6H4),
(CDCl3) d 8.55 (s, 2H, NÄ
7.98 (d, 2H, JHH 8.0 Hz, iso-C6H4); 7.64 (d, 4H,
JHH 8.4 Hz, NC6H4); 7.53 (t, 1H, iso-C6H4); 6.98 (d,
4H, JHH 8.0 Hz, SC6H4); 5.28 (s, 10H, C5H5); 1.52ꢁ1.34
/
C(H)), 8.35 (s, 1H, iso-C6H4),
ꢀ
/
ꢀ
/
ꢀ
/
/
(m, 36H, PBu3); 0.92 (t, 18H, PBu3).
3. Results and discussions
2.3.3. [Ni(h5-C5H5)PBu3]2(SC6H4NC(H)C6H4-4-
C(H)NC6H4S) (8)
3.1. Synthesis of complexes
A brown solid with a yield of 64% was isolated. Anal.
Calc. for C54H78N2P2S2Ni2, C, 64.92; H, 7.81; N, 2.80.
Dithiol Schiff base compounds (2ꢁ5) were prepared
/
by the condensation of two equivalents of 4-aminothio-
phenol and one equivalent of the appropriate dialdehyde
(Scheme 1). The dialdehyde, OHCC6H4C6H4CHO (1),
was prepared by the Suzuki coupling reaction of 4-
formylboronic acid with 4-bromobenzaldehyde. Com-
1
Found: C, 65.01; H, 7.83; N, 2.91. H-NMR (CDCl3) d
8.52 (s, 2H, NÄ
/
C(H)), 7.95 (s, 4H, C(H)C6H4C(H)),
8.4 Hz, NC6H4), 7.00 (d, 4H, JHH
7.65 (d, 4H, JHH
ꢀ
/
ꢀ
/
8.4 Hz, SC6H4), 5.28 (s, 10H, C5H5), 1.54ꢁ1.27 (m, 36H,
/
PBu3), 0.92 (t, 18H, PBu3).31P{1H}-NMR (CDCl3) d
22.39, (s, PBu3).
pounds 2ꢁ5 were isolated as yellow insoluble solids,
/
which appeared to be stable to air and moisture.
However, they gradually turned reddish maroon on
exposure to light over 2 weeks, though their elemental
analysis remained the same. We suggest that the colour
change from yellow to reddish maroon could be due to
isomerisation between cis and trans forms of the
compounds about the imine bonds. A lack of solubility
of these compounds in common organic solvents such as
2.3.4. [Ni(h5-C5H5)PBu3]2(SC6H4NC(H)-
C6H4C6H4C(H)NC6H4S) (9)
A green solid was isolated in a yield of 68%. Anal.
Calc. for C60H82N2P2S2Ni2, C, 67.06; H, 7.69; N, 2.61.
1
Found: C, 67.14; H, 7.81; N, 2.78. H-NMR (CDCl3) d
8.53 (s, 2H, NÄ/C(H)); 7.95 (d, 4H, JHH
ꢀ8.4 Hz,
/

R.M. Moutloali et al. / Journal of Organometallic Chemistry 656 (2002) 262ꢁ
/269
265
chemical shifts of 5.28 and 5.16 ppm for the PBu3 and
PPh3 complexes, respectively, were slightly more down-
field compared with 5.27 and 5.13 ppm observed for the
mononuclear complexes Ni(PBu3)(SC6H4X-4)(h5-C5H5)
and Ni(PPh3)(SC6H4X-4)(h5-C5H5) (Xꢀ
/
F, Cl, Br),
respectively, [14a]. The similarities in the cyclopentadie-
nyl chemical shifts in complexes 6ꢁ12 and those
/
reported by us recently [14], points to the similar
electronic environment around the nickel centres in the
bimetallic and the monometallic complexes. We expect
this similarity in electronic environment in the bimetallic
and the monometallic nickel complexes to result in
similar electrochemical behaviour as will be discussed
later. There is no obvious effect of the aryl or thiophene
linkers on the electronic environment of the nickel
centres.
In spite of the considerable solubility of the nickel
complexes prepared, particularly the PBu3 analogues,
Scheme 1.
our attempts to grow crystals of complexes 6ꢁ12 were
/
unsuccessful. We have thus used mass spectrometry to
establish the bimetallic nature of these complexes.
CH2Cl2, toluene and THF made it impossible to prove
this isomerisation; though a closer examination of the
1H-NMR spectrum of complex 6 (Fig. 1) suggest that a
similar isomerisation about the imine might exist when
the ligands form a complex with nickel. Although a
plausible explanation for the change in colour when
3.2. Mass spectral analysis of complexes
Two mass spectra ionisation techniques were used in
characterising the bimetallic complexes isolated in this
study, namely FAB and ES ionisation. Both of these
ionisation sources are perceived to be soft, however,
compounds 2ꢁ5 were exposed to sunlight could be
/
oxidation to disulfides, which will also give the same
elemental analysis, the possibility of these compounds
oxidising to disulfides was not investigated.
The dithiol compounds reacted with two equivalents
of NiBr(PR3)(h5-C5H5) to form toluene soluble pro-
ducts when excess Et3N was added to this mixture
(Scheme 1). The 1H-NMR spectra of the bimetallic
nickel products are typical of cyclopentadienylnickel
phosphino thiolato complexes [14]. Cyclopentadienyl
only complex 8 gave a molecular ion with m/zꢀ998
/
when the ionisation source was FAB (Fig. 2). This
shows that FAB ionisation is harsher for these com-
plexes as they readily fragment. Even the softer ESMS
gave generally weak peaks and the spectra had a lot of
fragmentation. For example while complex 9 gave no
molecular ion in its FABMS, ESMS of this complex at a
cone voltage of 5 V yielded both [Mꢄ
/
H]ꢄ and [Mꢄ
/
Fig. 1. 1H-NMR spectrum of complex 9.

266
R.M. Moutloali et al. / Journal of Organometallic Chemistry 656 (2002) 262ꢁ269
/
In general mass spectroscopy gave structural informa-
tion about the bimetallic nature of the PBu3 complexes
suggested by 1H-NMR and elemental analysis. How-
ever, we were unable to have structural confirmation of
the PPh3 complexes (except complex 8) from mass
spectroscopy. It is quite clear from the fragmentation
pattern (Scheme 2) that the observation of molecular
ions in the PBu3 complexes can be attributed to the
better s-donor ability of PBu3 compared with PPh3
which contribute to stabilising complexes of the former.
3.3. Electrochemical studies
Fig. 2. FABMS of complex 6.
In order to establish whether the dithiolato Schiff
base bridging ligands can act as electron conduit
between the nickel centres, cyclic voltammetry experi-
ments were performed on complexes (6, 8 and 9). A
major consideration for choosing complexes 6, 8 and 9
for electrochemical studies was to see how a variation of
spacer (Scheme 1) could affect electron conduction. Fig.
4 represents the voltammograms of complexes 6, 8 and 9
as well as one of the ligands (compound 2). For example
the nature of the redox couples in complex 6 is
reminiscent of the coupling of strong adsorption of
NH4]ꢄ molecular ions. On increasing the cone voltage
to 20 and 40 V, the former gave only [Mꢄ
NH4]ꢄ
/
molecular ion (Fig. 3) while the latter had no molecular
ion peak, but only fragmentation peaks. These observa-
tions illustrate how harsher conditions easily decompose
these bimetallic complexes. We observed no [2Mꢄ
/
H]ꢄ
or [2Mꢄ
/
NH4]ꢄ ions even at a low cone voltage of 5 V,
which is a further indication of how readily molecular
ions formed fragment.
Fig. 3. ESMS of complex 9.

R.M. Moutloali et al. / Journal of Organometallic Chemistry 656 (2002) 262ꢁ
/269
267
Scheme 2.
products (C/D: E1/2
diffusion controlled electrochemical process (A/B:
E1/2 0.294 V) [15]. However, the peak separation
ꢀ
/
0.590 V) on electrode surface to a
showed an irreversible oxidation peak at 0.699 V (Fig.
4). Table 1 shows that the redox behaviour of complexes
6, 8 and 9 differ from that of the nickel starting
materials and ligand 2. Thus based on the electrochem-
istry of the ligand 2, the nickel starting material and of 6,
8 and 9 the redox activities of 6, 8 and 9 are two one-
electron redox processes.
From Table 1, it is apparent that the introduction of
the electron rich sulphur ligands lowers the oxidation
potentials of the nickel starting material and that of the
ligands. Compared with Ni(PBu3)(SC6H4X-4)(h5-C5H5)
ꢀ
/
(DEpC/D) for the second redox process is 0.094 V, a
value greater than the upper limit peak separation of
0.065 V for surface-bound species undergoing rapid one
electron transfer [16]. In addition the voltammetric
peaks in complexes 6, 8 and 9 are not as symmetrical
as expected for adsorbed species. Hence the source of
the redox couple C/D cannot be adsorbed species. The
peak separation for the first redox couple ((DEpA/B) is
0.104 V. Since peak separation for both redox couples
for 6, and also for 8 and 9 (Table 1) are greater than
0.060 V, the value for the peak separation of non-
equilibrium one electron transfer process, the redox
couples in 6, 8 and 9 are quasi-reversible.
(Xꢀ
Ni(PBu3)(SC6H4NÄ
/
Cl (0.36 V), Br (0.32 V)) [14a] and
/
C(H)C6H4X-4)(h5-C5H5)
(Xꢀ/F
(0.35 V), Cl (0.37 V), Br (0.36 V), Me (0.35 V)) [14b]
oxidation potentials for 6, 8 and 9 are also lower. The
quasi-reversible redox couples found for 6, 8 and 9 are
indicative of stabilisation of the oxidised species by the
presence of the electron rich thiolato ligands. This
observation is in line with those of the mononickel
complexes reported by us recently [14]. Literature
reports on the electrochemical oxidation of m-thiolato
nickel complexes, [(h5-C5H5)Ni(m-SR)]2, show they
form unstable radical cations if the thiolato ligand has
an alkyl group [17], however, the radical cations are
stabilised by aryl thiolato ligands [18]. Hence the quasi-
reversibility associated with the thiolato Schiff base
complexes (6, 8 and 9) must be the result of p-
conjugated units that link the sulphur ends of the
bridging ligands.
A Tafel analyses (lnjijꢀ
/
ln ioꢂaFE/RT) of the oxida-
/
tion wave A/B and the reduction wave of C/D gave a
Tafel slope of 0.120 V per decade in each case, as
expected for one-electron transfer process. Thus the A/B
and C/D redox couples in 6, 8 and 9 represent two quasi-
reversible process which involve the coupling of diffu-
sion controlled electron transfer at the electrode via
electron-hopping along the bridging ligands in the
bimetallic complexes [15,16].
The cyclic voltammetry of the two starting materials
that were used to prepare the complexes were run to
establish the centre of the electrochemical activity. The
nickel complex, NiBr(PBu3)(h5-C5H5), had two irrever-
sible peaks at 0.338 and 0.996 V. Because the ligand is
insoluble, its cyclic voltammogram was run by deposit-
ing a paste of the compound on the working electrode. It
Cyclic voltammetry can be used to establish electronic
communication between two metal centres with a
bridging ligand [19]. Usually this involves observing

268
R.M. Moutloali et al. / Journal of Organometallic Chemistry 656 (2002) 262ꢁ269
/
Fig. 4. Cyclic voltammogram of 2, 6, 8 and 9 at a scan rate of 300 mV sꢂ1
.
two reversible redox couples that are separated. For
example redox couple separation, DE, for [(h5-
6, 8 and 9 are indicative of electronic communication
between the two nickel centres in these complexes. As in
the [(h5-C5H5)Ni(m-SR)]2 systems [18], electrochemical
activities in 6, 8 and 9 are two sequential one electron
processes. Thus the mechanism of electronic commu-
nication after the first oxidation could be p-electron
delocalisation as observed for the diiron polyene systems
C5H5)FeL]2(m-CHÄ
/
CHÄ
/
CH) (LꢀCO, PMe3, PPh3)
/
varies from 0.41 to 0.45 V [20] whereas for [(h5-
C5H5)FeCO]2(m-1,4-C6F4) has a redox couple separa-
tion, DE of 0.28 V [21]. We observe DE values of 0.296,
0.296 and 0.300 V, respectively, for the quasi-reversible
redox peaks in 6, 8 and 9 and suggest that DE values of
[4,19ꢁ21].
/
Table 1
Cyclic voltammetry data of complexes NiBr(PBu3)(h5-C5H5), 6, 8 and 9
Complex
E1/2 (V)
Ip,a/Ip,c
DEp (V)
DE (V)
A/B
C/D
A/B
C/D
A/B
C/D
NiBr(PBu3)(h5-C5H5)
6
8
9
0.338
0.294
0.298
0.285
0.996
0.590
0.594
0.583
1.10
1.00
1.13
0.85
0.85
0.77
0.104
0.104
0.103
0.094
0.094
0.087
0.296
0.296
0.300
DEpꢀjEp,oxꢂEp,redj, DEꢀjE1/2,C/DꢂE1/2,A/Bj.

R.M. Moutloali et al. / Journal of Organometallic Chemistry 656 (2002) 262ꢁ
/269
269
(d) O. Lavastre, J. Plass, P. Bachmann, S. Guesmi, C. Moinet,
P.H. Dixneuf, Organometallics 16 (1997) 184.
In summary, we have successfully prepared binuclear
nickel complexes that have bridging dithiolato ligands
from Schiff base dithiols. The bimetallic nature of the
complexes synthesised was established by mass spectro-
metry, mainly ESMS. Electronic communication be-
tween the nickel centres was established by cyclic
voltammetry. Thus electronic coupling of the two nickel
[5] K. Sonogashira, K. Ohga, S. Takahashi, N. Hagihara, J.
Organomet. Chem. 188 (1980) 237.
[6] S. Takahashi, K. Sonogashira, H. Morimoto, E. Murata, S.
Kataoka, N. Hagihara, J. Polym. Sci. Polym. Chem. Ed. 20 (1982)
565.
[7] J. Kiessewetter, G. Poignant, V. Guerchais, J. Organomet. Chem.
595 (2000) 81.
centres was possible even though the nickelÃ
/
sulphur
[8] (a) L.A. Bumm, J.J. Arnold, M.T. Cygan, T.D. Dunbar, T.P.
Burgin, L. Jones, II, D.L. Allara, J.M. Tour, P.S. Weiss, Science
271 (1996) 1705;
bonds in these complexes is predominantly a s-bond.
(b) M.T. Cygan, T.D. Dunbar, J.J. Arnold, L.A. Bumm, N.F.
Shedlock, T.P. Burgin, L. Jones, II, D.L. Allara, J.M. Tour, P.S.
Weiss, J. Am. Chem. Soc. 120 (1998) 2721.
Acknowledgements
[9] O. Lavastre, M. Even, P.H. Dixneuf, A. Pacreau, J.P. Vairon,
Organometallics 15 (1996) 1530.
Financial support from the National Research Foun-
dation (South Africa) (RMM, FAN, JD) and ESKOM
(South Africa) is gratefully acknowledged. We also wish
to thank Mr. R.M. Mampa (University of the North,
South Africa) for 31P{1H)-NMR data.
[10] F. Sato, T. Yoshida, M. Sato, J. Organomet. Chem. 37 (1972)
381.
[11a] K.W. Barnett, J. Chem. Educ. 51 (1974) 422.
[11b] J. Darkwa, Organometallics 13 (1994) 4734.
[12] O.L. Caagrande, Jr., A.E. Gerbase, F.C. Atedile, F.O.V. da
Cunha, Polyhedron 16 (1997) 171.
[13] A.J. Fry, in: P.T. Kissinger, W.R. Heineman, (Eds.), Laboratory
Techniques in Electrochanalytical Chemistry, second ed., Revised
References
and Expanded, Marcel Dekker, New York, pp. 469ꢁ
/
485,
[1] (a) J. Mata, S. Uriel, E. Peris, R. Llusar, S. Hiubrechts, A.
Persoons, J. Organomet. Chem. 562 (1998) 197;
(b) I.R. Whitall, A.M. McDonagh, M.G. Humphrey, Adv.
Organomet. Chem. 42 (1998) 291.
(Chapter 15).
[14] (a) J. Darkwa, R.M. Moutloali, T. Nyokong, J. Organomet.
Chem. 564 (1998) 37;
(b) F.A. Nevondo, A.M. Crouch, J. Darkwa, J. Chem. Soc.
Dalton Trans. (2000) 43.
[2] (a) F. Geiger, M. Stoldt, P. Bauerle, H. Schweizer, E. Umbach,
Adv. Mater. 5 (1993) 922;
[15] R.H. Wopschall, I. Shain, Anal. Chem. 39 (1967) 1515; Ref: R.H.
Wopschall, I. Shain, Anal. Chem. 39 (1967) 1527.
[16] R.W. Murray, in: A.J. Bard (Ed.), Electroanalytical Chemistry,
(b) G. Horowitz, P. Delannoy, H. Bouchriha, F. Deloffe, J.-L.
Fave, F. Garnier, R. Hajlaoui, M. Heyman, F. Kouki, P. Valat,
V. Wittgens, A. Yassar, Adv. Mater. 6 (1994) 752;
(c) K. Uchiyama, H. Akimichi, S. Hotta, H. Noge, H. Sakaki,
Synth. Met. 63 (1994) 57.
Marcel Dekker, New York, 1984, pp. 191ꢁ368.
/
[17] R.E. Dessy, R. Kornmann, C. Smith, R. Haytor, J. Am. Chem.
Soc. 90 (1968) 2001.
[18] (a) P.D. Frisch, M.K. Lloyd, J.A. McCleverty, D. Seddon, J.
Chem. Soc. Dalton Trans. (1973) 2268.;
[3] (a) D. Touchard, P.H. Dixneuf, Coord. Chem. Rev. 178 (1998)
409 (and references therein);
(b) F. Paul, C. Lapinte, Coord. Chem. Rev. 178 (1998) 431 (and
references therein).
(b) N.-F. Ho, T.C.W. Mak, J. Chem. Soc. Dalton Trans. (1990)
359.
[4] (a) S. Woitellier, J.P. Launay, C.W. Spangler, Inorg. Chem. 28
(1989) 758;
[19] S. Guesmi, D. Touchard, P.H. Dixneuf, J. Chem. Soc. Chem.
Commun. (1996) 2773.
(b) J.A. Thomas, C.J. Jones, J.A. McCleverty, D. Collison, F.E.
Mables, C.J. Harding, M.G. Hutchings, J. Chem. Soc. Chem.
Commun. (1992) 1796.;
[20] B.A. Etzenhouser, M.D. Cavanaugh, H.N. Spurgeon, M.B.
Sponsler, J. Am. Chem. Soc. 116 (1994) 2221.
[21] R. Chukwu, A.D. Hunter, B.D. Santarisiero, S.G. Bott, J.L.
Atwood, J. Chassaignac, Organometallics 11 (1992) 589.
(c) S.R. Marder, C.B. Gorman, B.G. Tieman, L.-T. Cheng, J. Am.
Chem. Soc. 115 (1993) 3006;