Cyclopentadienylnickel thiolate complexes: Synthesis, molecular structures and electrochemical detection of sulfur dioxide adducts

Article abstract of DOI:10.1016/j.jorganchem.2003.10.027

Simple reactions between Ni(η⁵-C₅ H₅)(PR₃)Br and the Schiff-base thiols, 4-HSC₆H₄NC(H)C₄H₂ SBr-4′ (1) and 4-HSC₆H₄NC(H)C ₄H₃S (2), or organothiols, HSC₆ H₄F-4 and HSC₆H₄NH₂-4, produced cyclopentadienylnickel thiolates of the formulae, Ni(η⁵-C₅H₅)(PR₃) (SC₆H₄NC(H)C₄H₂SBr-4) (3), Ni(η⁵-C₅H₅)(PR₃) (SC₆H₄NC(H)C₄H₃S) (4) or Ni(η⁵-C₅H₅)(PR₃) (SC₆H₄X-4) (R=Ph, X=F (6) or NH₂ (7) and R=Bu, X=F (5) or NH₂ (8)) which were characterized by a combination of analytical techniques. Complexes 3, 6 and 7 were structurally characterized by X-ray crystallography, showing that they possess the familiar trigonal geometry around the nickel center. These complexes react with sulfur dioxide, with 5, 6, 7 and 8 exhibiting substantial differences between the redox potentials of the pre- and post-SO₂ compounds to suggest that these complexes can be developed as potentiometric SO₂ sensors.

Full text of DOI:10.1016/j.jorganchem.2003.10.027

Journal of Organometallic Chemistry 689 (2004) 387–394
www.elsevier.com/locate/jorganchem
Cyclopentadienylnickel thiolate complexes: synthesis,
molecular structures and electrochemical detection
of sulfur dioxide adducts
a
Makwena J. Moloto , Simphiwe M. Nelana , Richard M. Moutloali ,
a
a
b
Ilia A. Guzei , James Darkwa
a,
*
a
Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA
b
Received 30 September 2003; accepted 22 October 2003
Abstract
Simple reactions between Ni(g5-C5H5)(PR3)Br and the Schiff-base thiols, 4-HSC6H4NC(H)C4H2SBr-40 (1) and 4-HSC6H4
NC(H)C4H3S (2), or organothiols, HSC6H4F-4 and HSC6H4NH2-4, produced cyclopentadienylnickel thiolates of the formulae,
Ni(g5-C5H5)(PR3)(SC6H4NC(H)C4H2SBr-4) (3), Ni(g5-C5H5)(PR3)(SC6H4NC(H)C4H3S) (4) or Ni(g5-C5H5)(PR3)(SC6H4X-4)
(R ¼ Ph, X ¼ F (6) or NH2 (7) and R ¼ Bu, X ¼ F (5) or NH2 (8)) which were characterized by a combination of analytical tech-
niques. Complexes 3, 6 and 7 were structurally characterized by X-ray crystallography, showing that they possess the familiar
trigonal geometry around the nickel center. These complexes react with sulfur dioxide, with 5, 6, 7 and 8 exhibiting substantial
differences between the redox potentials of the pre- and post-SO2 compounds to suggest that these complexes can be developed as
potentiometric SO2 sensors.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Nickel thiolate complexes; Synthesis; Crystal structures; Sulfur dioxide adducts; Electrochemical detection
1. Introduction
ample of sulfur dioxide insertion into the S–S bond. The
third reaction type involves the binding of sulfur dioxide
to a soft atom in a ligand, usually sulfur [4] but other
atom-binding sites are known [5]. The soft atom in a
ligand binds the sulfur dioxide by donating electrons
into empty d-orbitals of the sulfur atom of the sulfur
dioxide, in a reaction where the soft atom in the ligand
behaves as a Lewis base. Such bonds are inherently
weak and generally lead to unstable sulfur dioxide ad-
ducts compared to compounds formed via type 1 and
type 2 reactions.
Ligand bound SO2 is generally found for late-
transition metal thiolato complexes. These are either
SO2 bound to the sulfur atom of a bidentate thiolato
ligand complex [4b,4c] or SO2 bound to the sulfur atom
of a monodentate thiolato ligand complex [4a,6]. The
latter are cyclopentadienylmetal complexes, which our
group [6] and others [4a] have studied. Shaver was the
first to report the reversible binding of SO2 to the sulfur
atom of cyclopentadienylmetal thiolato phosphine
Sulfur dioxide coordination to metal complexes
continues to attract attention primarily because of en-
vironmental concerns about sulfur dioxide and the po-
tential use of such reactions to develop sulfur dioxide
scrubbers or sensors. Three main reaction types of sulfur
dioxide with metal complexes are known. The best
studied of these three reactions is the coordination of
sulfur dioxide to a metal centre [1]. The second reaction
type is insertion of sulfur dioxide into a bond formed by
a central metal atom and another atom [1a–1e,2]. This
can either be insertion into a metal–metal bond [1a] or
insertion into a metal–ligand bond [1a–1e,2]. Recently
Shaver and co-workers [3] reported the insertion of
sulfur dioxide into S–S bonds in Mo(g5-C5H5)2S2 to
form Mo(g5-C5H5)2S3O2. This constitutes the first ex-
* Corresponding author. Tel.: +27219593053; fax: +27219593055.
E-mail address: jdarkwa@uwc.ac.za (J. Darkwa).
0022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.10.027

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M.J. Moloto et al. / Journal of Organometallic Chemistry 689 (2004) 387–394
1
complex in 1992 [4a] and showed that the weak sulfur–
sulfur interaction is responsible for the reversibility. The
isoelectronic nickel analogues [6] show similar revers-
ibility but are too labile to allow solids to be isolated and
characterized. As such unequivocal characterization of
the NiS(SO2)R adducts is still lacking. It is however
possible to use analytical techniques to establish the
formation of labile SO2 adducts. Previously we have
used 1H NMR spectroscopy as a tool for detecting SO2-
adduct formation [6]. This is based on electronic changes
that occur upon adduct formation and hence changes in
chemical shift. It is therefore possible that electronic
changes that occur when SO2 adducts are formed can
give a different electrochemical response and allow SO2
to be detected electrochemically. Literature reports
show that SO2 can be detected indirectly by ampero-
metric methods, which involves reduction of the SO2
and the measurement of the current necessary to main-
tain a constant concentration of titrant as SO2 reduces
the titrant [7]. Although this method is limited by the
presence of other compounds that react with the titrant,
it nevertheless demonstrates that electrochemistry can
be used to establish the presence of SO2. A more direct
method in which an electrochemical response is pro-
duced by stable SO2-adduct via a change in potential
would be more desirable since it would not suffer from
such interferences. We report here nickel thiolato com-
pounds and their SO2 adducts that demonstrate that
these nickel thiolato compounds can be used as poten-
tiometric SO2 sensors.
referenced to residual CHCl3 (d 7.26) internally for H
spectra and externally to PPh3 (d )5.00) for 31P spectra.
Electrochemistry experiments were performed on a BAS
CV-50W electrochemical analyzer using
a three-
electrode system that consist of platinum working elec-
trode (1.5 mm diameter), a silver/silver chloride (Ag/
AgCl) reference electrode and a platinum wire as a
counter electrode. All experiments were carried out un-
der a dry nitrogen atmosphere. Cyclic voltammetry ex-
periments were performed in dichloromethane solutions
(10ꢀ3 M) with [n-Bu4N][BF4] as supporting electrolyte
(0.10 M). Redox potentials were referenced internally to
the potential of the oxidation of ferrocene at 0.44 V [11].
2.2. Synthesis of Schiff-base ligands
Schiff-base ligands that were used in synthesising cy-
clopentadienylnickel complexes were prepared following
the procedure described recently [12]. In a typical reac-
tion 4-aminothiophenol was reacted with the aldehyde
in ethanol in a 1:1 mole ratio to give compounds 1 and 2
as analytically pure yellow solids.
2.2.1. 4-HSC6H4N@C(H)C4H2SBr-40 (1)
Yield 68%. Anal. Calc. for C11H8BrNS2: C, 44.30; H,
1
2.70; N, 4.70. Found: C, 44.55; H, 2.60; N 4.80%. H
NMR(CDCl3): d 8.48 (s, 1H, N@CH); 7.52 (d, 2H,
JHH ¼ 8:6 Hz, SC6H4N@C(H)C4H2SBr-40); 7.39 (m,
2H, SC6H4N@C(H)C4H2SBr-40); 7.16 (d, 2H, JHH ¼ 8:6
Hz, SC6H4N@C(H)C4H2SBr-40). IR (nujol mull cmꢀ1
m(C@N): 1604.
)
2. Experimental
2.2.2. 4-HSC6H4N@C(H)C4H3S (2)
Yield 85%. Anal. Calc. for C11H9NS2: C, 60.24; H,
4.14; N, 6.39. Found: C, 59.65; H, 4.30; N, 6.50%. H
1
2.1. Materials and instrumentation
NMR(CDCl3): d 8.53 (s, 1H, N@CH); 7.47 (m, 4H,
SC6H4N@C(H)C4H3S); 7.16 (m, 3H, SC6H4N@C(H)C4
H3S). IR (nujol mull cmꢀ1) m(C@N): 1609.
Analytical grade solvents were dried by standard
procedures and stored over activated molecular sieves.
However, dichloromethane used for electrochemical
experiments was refluxed twice over P2O5 for 24 h,
distilled under nitrogen and stored over activated
molecular sieves. The commercially available chemicals
4-fluorothiophenol, 4-aminothiophenol, 4-bromo-2-
thiophenecarboxyaldehyde and 2-thiophenecarboxyal-
dehyde were obtained from Aldrich and used as
received. The nickel starting materials, Ni(g5-C5H5)
(PBu3)Br [8], Ni(g5-C5H5)(PPh3)Br [9] and [Ni(g5-
C5H5)(l-SC6H4 NH2-4)]2 [10] were prepared by the
literature procedures. All reactions were performed
under a nitrogen atmosphere, but the air and moisture
stable complexes that were formed were worked-up in
air.
2.3. Synthesis of nickel thiolato complexes
2.3.1. Ni(g5-C5H5)(PBu3)(SC6H4N@C(H)C4H2SBr-
40) (3)
To a mixture of 4-HSC6H4N@C(H)C4H2SBr-40 (0.10
g; 0.34 mmol) and Ni(g5-C5H5)(PBu3)Br (0.18 g, 0.34
mmol) was added degassed CH2Cl2 (50 mL) followed by
excess Et3N (1.0 mL). The maroon solution gradually
turned brownish-green after stirring the mixture for 2 h.
The reaction mixture was stirred for 24 h at room
temperature to ensure that the reaction was complete.
The solvent was removed in vacuo and the residue ex-
tracted with toluene (50 mL). The toluene extract was
evaporated to dryness and the residue recrystallised
from CH2Cl2/hexane (1:1) to give dark green X-ray
quality crystalline solid. Yield 0.14 g (67%). Anal. Calc.
for C28H39BrNSPNi: C, 53.95; H, 6.31; N, 2.26. Found:
IR spectra were recorded on a Perkin–Elmer Paragon
1
1000 PC FTIR spectrometer as nujol mulls. H and 31P
NMR spectra were recorded on a Varian Gemini 2000
spectrometer at 200.00 and 80.96 MHz, respectively, and

M.J. Moloto et al. / Journal of Organometallic Chemistry 689 (2004) 387–394
389
C, 53.62; H, 5.83; N, 2.13%. 1H NMR(CDCl3): d 8.51 (s,
1H, N@CH); 7.62 (d, 2H, JHH ¼ 8:8 Hz, SC6H4N@
C(H)C4H3SBr-40); 7.33 (s, 1H, SC6H4N@C(H)C4H3
SBr-40); 7.31 (s, 1H, SC6H4N@C(H)C4H3SBr-40); 6.95
(d, 2H, JHH ¼ 8:4 Hz, SC6H4N@C(H)C6H3SBr); 5.27 (s,
Anal. Calc. for C35H26NPSNi ꢁ 1/2CH2Cl2: C, 66.71; H,
1
5.31; N, 2.63. Found: C, 66.87; H, 5.46; N, 2.95%. H
NMR(CDCl3): d 7.71 (t, 6H, PPh3), 7.37 (m, 9H, PPh3),
7.12 (d, 2H, JHH ¼ 8:4 Hz, SC6H4NH3-4), 6.35 (d, 2H,
JHH ¼ 8:4 Hz, SC6H4NH2-4), (s, 2H, NH2), 5.03 (s, 5H,
C5H5); 3.77 (s, br, 2H, NH2).
5H, C5H5); 1.42 (m, 18H, PBu3); 0.92 (t, 9H, JHH
7:5 Hz, PBu3). IR (nujol mull cmꢀ1) m(C@N): 1595.
¼
2.3.6. Ni(g5-C5H5)(PBu3)(SC6H4NH2-4) (8)
2.3.2. Ni(g5-C5H5)(PBu3)(SC6H4N@C(H)C4H3S) (4)
A similar procedure as for 3 was followed using
HSC6H4N@C(H)C4H3S (0.1 g, 0.46 mmol) and Ni(g5-
C5H5)(PBu3)Br (0.18 g, 0.34 mmol). A green oily
product was obtained from which green crystals were
isolated after allowing a solution of the product in a 1:1
mixture of CH2Cl2/hexane to stand at )15 °C for several
days. Yield 0.15 g (60%). Anal. Calc. for C18H40NSPNi:
C, 61.77; H, 7.41; N, 2.57. Found: C, 61.27; H, 7.56; N,
2.73%. 1H NMR(CDCl3): d 8.58 (s, 1H, N@C(H)); 7.61
(d, 2H, JHH ¼ 8:4 Hz, SC6H4NC(H)C4H3S); 7.43 (t, 2H,
SC6H4NC(H)C4H3S); 7.12 (t, 1H, SC6H4NC(H)C4
H3S); 6.95 (d, 2H, JHH ¼ 8:6 Hz, SC6H4NC(H)C4H3S);
5.26 (s, 5H, C5H5); 1.44 (m, 18H, PBu3), 0.91 (t, 9H,
JHH ¼ 7:5 Hz PBu3). IR (nujol mull, cmꢀ1) m(C@N):
1590.
Starting with (0.50 g, 3.99 mmol) 4-aminothiophenol
and (1.86 g, 3.99 mmol) Ni(g5-C5H5)(PBu3)Br in
toluene (60 mL), brown-green oil of Ni(g5-C5H5)
(PBu3)(SC6H4NH2-4) was obtained in a yield of 1.37 g
1
(76%). H NMR(CDCl3): d 7.33 (d, 2H, JHH ¼ 8:1 Hz,
SC6H4NH2-4); 6.38 (d, 2H, JHH ¼ 8:1 Hz, SC6H4NH2-
4), 5.15 (s, 5H, C5H5); 2.95 (s, br, 2H, NH2); 1.49 (m,
18H, PBu3); 0.91 (t, 9H, JHH ¼ 7:8 Hz, PBu3).
2.3.7. X-ray structural determination
Crystal evaluation and data collection were per-
formed on a Bruker CCD-1000 diffractometer with Mo
ꢀ
Ka (k ¼ 0:71073 A) radiation and the diffractometer to
crystal distance of 4.9 cm. Crystal data, data collection,
and refinement parameters are listed in Table 1. The
initial cell constants were obtained from three series of x
scans at different starting angles. The reflections were
successfully indexed by an automated indexing routine
built in the SMART program. These highly redundant
datasets were corrected for Lorentz and polarization
effects. The absorption correction was based on fitting a
function to the empirical transmission surface as sam-
pled by multiple equivalent measurements [13]. The
structures were solved by direct methods and refined by
least-squares techniques using the SHELXTL program
[14]. All non-hydrogen atoms were refined with aniso-
tropic displacement coefficients. All hydrogen atoms
were included in the structure factor calculation at ide-
alized positions and were allowed to ride on the neigh-
bouring atoms with relative isotropic displacement
coefficients.
2.3.3. Ni(g5-C5H5)(PBu3)(SC6H4F-4) (5)
A similar procedure as for 3 was followed using
Ni(g5-C5H5)(PBu3)Br (0.37 g, 0.79 mmol) and
HSC6H4F-4 (0.12 g, 0.90 mmol). The product isolated
was as dark green crystals. Yield 0.17 g, (52%). Anal.
Calc. for C23H36FPSNi ꢁ 1/2CH2Cl2: C, 56.94; H, 7.59.
Found: C, 56.99; H, 8.00%. 1H NMR(CDCl3): d 7.69 (t,
2H, JHF ¼ 8:6 Hz, SC6H4F-4), 7.04 (t, 2H, JHF ¼ 8:6
Hz, SC6H4F-4); 5.22 (s, 5H, C5H5), 1.57 (m, 18H), 0.94
(t, 9H, JHH ¼ 8:0 Hz, PBu3). 31P{1H} NMR(CDCl3): d
22.2 (s, PBu3).
2.3.4. Ni(g5-C5H5)(PPh3)(SC6H4F-4) (6)
The reaction of Ni(g5-C5H5)(PPh3)Br (0.36 g, 0.79
mmol) and HSC6H4F-4 (0.09 g, 0.08 mmol) was per-
formed in a similar manner to that of 3. Yield 0.25 g
(77%). Anal. Calc. for C29H24FPSNi ꢁ 1/4CH2Cl2: C,
65.73; H, 4.59. Found: C, 65.79; H, 4.87%. 1H
NMR(CDCl3): d 7.67 (m, 6H, PPh3), 7.37 (m, 9H,
PPh3), 7.26 (t, 2H, JHF ¼ 8:8 Hz, SC6H4F), 6.59 (t, 2H,
JHF ¼ 8:8 Hz, SC6H4F), 5.09 (s, 5H, C5H5). 31P{1H}
NMR(CDCl3): d 35.4 (s, PPh3).
3. Results and discussion
3.1. Synthesis of ligands and metal complexes
Two Schiff-base thiols (1 and 2) were synthesized via
the condensation of 4-aminothiophenol and different
thiophene carboxyaldehydes. Compounds 1 and 2
readily precipitated from ethanol as pure products; thus
these sparingly soluble products in CH2Cl2 did not need
further purification. Infrared spectroscopic analysis was
a quick way to establish the formation of the Schiff-base
ligands. This revealed the absence of the carbonyl
functionality of the aldehyde used in preparing each
compound and the presence of a m(C@N) peak charac-
teristic of imines at 1604 cmꢀ1 for 1 and 1609 cmꢀ1 for 2.
2.3.5. Ni(g5-C5H5)(PPh3)(SC6H4NH2-4) (7)
A mixture of [Ni(g5-C5H5)(l2-SC6H4NH2-4)]2 (0.40
g, 0.81 mmol) and PPh3 (0.42 g, 1.62 mmol) in CH2Cl2
(50 mL) was stirred at room temperature for 4 h. The
resultant dark brown solution was concentrated to
about 20 mL and hexane (20 mL) added. The solution
was cooled at )15 °C overnight to form green crystals of
Ni(g5- C5H5)(PPh3)(SC6H4NH2-4). Yield 0.35 g, (85%).

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M.J. Moloto et al. / Journal of Organometallic Chemistry 689 (2004) 387–394
Table 1
Crystal and structural refinement data for 3, 6 and 7
3
6
7
Empirical formula
Formula weight
Temperature (K)
C28H39BrNPS2Ni
623.31
173(2)
C29H24FPSNi
513.22
173(2)
C29H26NPSNi
510.25
100(2)
ꢀ
Wavelength (A)
0.71073
Triclinic
0.71073
Triclinic
0.71073
Triclinic
Crystal system
Space group
Unit cell dimensions
ꢁ
ꢁ
ꢁ
P1
P1
P1
ꢀ
a (A)
12.4741(9)
15.8951(14)
17.0772(13)
113.4480(10)
91.976(3)
9.6530(5)
9.6530(5)
9.3599(19)
11.535(2)
12.002(2)
ꢀ
b (A)
ꢀ
c (A)
11.4173(6)
79.7450(10)
83.4110(10)
74.2320(10)
1184.08(11)
2
a (°)
b (°)
c (°)
79.566(3)
69.056(3)
106.477(2)
2936.8(4)
81.047(3)
1184.4(4)
3
ꢀ
V (A )
Z
4
1.410
2
1.431
Dcalc: (mg mꢀ3
)
1.439
0.999
532
Absorption coefficient (mmꢀ1
F ð000Þ
)
2.235
1296
0.993
532
Crystal size (mm)
Reflections collected
0.42 ꢃ 0.26 ꢃ 0.14
19,272
96.1%
0.40 ꢃ 0.30 ꢃ 0.30
10,520
99.3%
0.42 ꢃ 0.25 ꢃ 0.21
19,610
99.1%
Completeness to h ¼ 26:36%
Data/Restraints/Parameters
R indices (all data)
11,524/0/612
R1 ¼ 0:0965, wR2 ¼ 0:1315
R1 ¼ 0:0503, wR2 ¼ 0:133
0.28 and )0.692
4802/0/298
R1 ¼ 0:0340, wR2 ¼ 0:0780
R1 ¼ 0:0283, wR2 ¼ 0:0753
0.330 and )0.233
4840/0/298
R1 ¼ 0:0320, wR2 ¼ 0:0838
R1 ¼ 0:0305, wR2 ¼ 0:0826
0.597 and )0.558
Final R indices ½I > 2rðIÞꢂ
Largest difference peak and
ꢀ3
ꢀ
hole (e A
)
NMR data and elemental analysis confirmed the for-
mation of these compounds.
Table 1, while most relevant bond angles and distances
provided in Tables 2 and 3.
The reactions of these Schiff-base thiols, 1 and 2, as well
as the commercially available thiols, 4-fluorothiophenol
and 4-aminothiophenol, with Ni(g5-C5H5) (PBu3)Br and
Ni(g5-C5H5)(PPh3)Br are described in Scheme 1. The
scheme represents a facile route to the synthesis of cy-
clopentadienylnickel thiolato compounds. Complexes 7
and 8 on the other hand were also prepared by reacting
[Ni(g5-C5H5)(l2-SC6H4NH2-4)]2 with PR3 [Eq. (1)], a
reaction we have used previously to prepare Ni(g5-
C5H5)(PR3)(C6H4X-4)(X ¼ Cl, Br; R ¼ Ph, Bu) [6].
In each of the three complexes the Ni atom exhibits a
pseudo-trigonal geometry, if the cyclopentadienyl ligand
is considered a point ligand. The Ni–ligand bond dis-
tances (Table 2) are rather typical and compare well to
the structural data reported to the Cambridge Crystal-
lographic Database (CSD) [15]. For 12 compounds
similar to 3, 6, and 7 in which the central Ni is bound to
phosphorus, sulfur, and a cyclopentadienyl ligand only,
the following bond length ranges have been determined:
Ni–C(g5-C5H5 ring) 2.054–2.177 A, Ni–Centroid(g -
5
ꢀ
ꢀ
C5H5 ring) 1.717–1.780 A, Ni–P 2.135–2.183 A, and Ni–
ꢀ
½Niðg5-C5H5Þðl2-SC6H4NH2-4Þꢂ þ 2PR3
ꢀ
S 2.144–2.203 A. In general, distances of these types
2
5
ꢀ
! 2Niðg5-C5H5ÞðPR3ÞðSC6H4NH2-4Þ
ðR ¼ Phð7Þ or Buð8ÞÞ
vary in wider ranges: Ni–C(g -C5R5 ring) 1.721–2.705 A
5
ꢀ
(2570 bonds, av. 2.12(5) A), Ni–Centroid(g -C5R5 ring)
ꢀ
1.628–2.031 A (514 bonds, av. 1.76(3) A), Ni–P 2.145–
ð1Þ
ꢀ
ꢀ ꢀ
The nickel thiolate complexes were characterized by a
combination of infrared and NMR spectroscopy and
showed characteristic signals of the various functional
groups in the complexes. Single crystal X-ray crystal-
lography of 3, 6 and 7 were used to confirm the for-
mation of the nickel thiolate complexes deduced from
the spectroscopic data.
2.522 A (73 bonds, av. 2.20(6) A), and Ni–S 2.144–2.52
ꢀ
A (110 complexes, av. 2.30(12) A).
ꢀ
A Density Functional Theory (DFT) geometry opti-
mization of a simplified analog of 3, 6, and 7, mono-
nuclear complex Ni(g5-C5H5)(PMe3)(SMe) has been
performed at the B3LYP/LANL2DZ level of theory
[16]. By performing the DFT calculations we are able to
compare the literature data that encompass a wide range
of bond distances and angles and the theoretical values
for the trigonal geometry found in our nickel complexes.
The theoretically calculated bond angles (Table 3) are in
good agreement with the experiment, however the
3.2. Molecular structures of 3, 6 and 7
The solid-state structures of 3, 6 and 7 are shown in
Figs. 1–3. The crystallographic data are tabulated in

M.J. Moloto et al. / Journal of Organometallic Chemistry 689 (2004) 387–394
391
Et3N
+ HS
X
-Et3NHBr
Ni
Ni
X
S
R3P
Br
R3P
Complex
3
X
R
Bu
Br
N
S
H
Bu
4
N
S
H
F
F
Bu
Ph
Ph
5
6
7
NH2
Scheme 1.
Fig. 1. Molecular structure of 3, drawn with 40% probability ellipsoids. The H atoms are omitted for clarity.
theoretical bond distances (Table 2) are considerably
longer (by ꢄ4.8%) than those determined by X-ray
single crystal analysis. Nonetheless the relative ratio of
the calculated bond distances (Ni–P:Ni–S:Ni–Centroid)
is very similar to the experimental value.
readily reacted with SO2, changing from greenish-brown
to dark red solutions. We first established SO2-adduct
formation by monitoring reactions using 1H NMR
spectroscopy. Chemical shifts associated with the cy-
clopentadienyl ligand, except for 3, which had no
change in the cyclopentadienyl resonance, showed that
SO2 adducts were formed. Generally a chemical shift
difference of at least 0.25 ppm was observed between the
thiolato complex and its SO2 adduct (Table 4). Sulfur
dioxide reactions with 5, 6, 7 and 8 were found to be
reversible. These were established by first running the
1H NMR spectrum of the thiolato complex before re-
acting SO2, and then the spectrum was re-run after the
3.3. Sulfur dioxide adducts
3.3.1. Spectroscopic characterization of sulfur dioxide
adducts
In order to investigate the use of electrochemistry as
a detection technique for SO2 complexes 3, 5, 6, 7 and 8
were selected for reactions with SO2. These complexes

392
M.J. Moloto et al. / Journal of Organometallic Chemistry 689 (2004) 387–394
Fig. 2. Molecular structure of 6, drawn with 30% probability ellipsoids. The H atoms are omitted for clarity.
Fig. 3. A molecular drawing of 7, shown with 50% probability ellipsoids. All H atoms except amino hydrogens have been omitted for clarity.
Table 2
Selected bond distances in complexes 3, 6 and 7, and Ni(g5-C5H5)(PMe3)(SMe)
3a
6
7
Ni(g5-C5H5)(PMe3)(SMe) (DFT B3LYP/LANL2DZ data)
ꢀ
Bond lengths (A)
Ni–P
Ni–S
av. 2.1399(17)
av. 2.1884(11)
av. 1.744(3)
2.1356(5)
2.1995(2)
1.752(2)
2.1430(5)
2.1881(6)
1.767(2)
2.25
2.28
Ni–Centroid
Ni–C(av.)
1.857
2.24(6)
av. 2.122(10)
2.122(16)
2.138(15)
a There are two symmetry independent molecules in the asymmetric unit in the lattice of 3, thus the table contains values averaged between the two
molecules.
SO2 reaction. The solution of the SO2 reaction product
was then purged with dinitrogen for several minutes and
the 1H NMR spectrum was found to be exactly the
same as the thiolato complex before reaction with SO2.
These spectral changes followed the same pattern as
observed for Ni(g5-C5H5)(PBu3)(SC6H4X-4) (X ¼ Cl or
Br) [6].
Notwithstanding the same chemical shifts for 3 and
its SO2-adduct, sulfur dioxide reactions of complex 3
resulted in clear colour change from brownish-green to

M.J. Moloto et al. / Journal of Organometallic Chemistry 689 (2004) 387–394
393
Table 3
Selected bond angles in complexes 3, 6 and 7
Bond angles (°)
3a
6
7
Ni(g5-C5H5)(PMe3)(SMe) (DFT B3LYP/LANL2DZ data)
P–Ni–S
P–Ni–Centroid
S–Ni–Centroid
av. 92.03(8)
av. 135.55(11)
av. 132.45(4)
90.608(19)
136.2(1)
133.1(1)
92.2(5)
136.7(1)
132.4(1)
91.0
136.4
132.0
a There are two symmetry independent molecules in the asymmetric unit in the lattice of 3, thus the table contains values averaged between the two
molecules.
Table 4
1H NMR data of selected complexes and their SO2 adducts
dioxide to ensure that any observed electrochemical
activity is not from the interaction of the electrolyte and
Complex
Without SO2
Cp
SO2 adducts
Cp
sulfur dioxide. In the control experiments we found no
electrochemical response in the potential window that
was used.
SC6H4
SC6H4
6
7
8
5.09
5.25
5.16
6.59(d)
–
6.40(d)
5.41
5.53
5.53
6.73(d)
–
6.57(d)
Complexes 5 and 8 exhibited quasi-reversible redox
behaviour, whilst complexes 6 and 7 had irreversible
redox behaviour. This is reminiscent of the electro-
chemical properties of Ni(g5-C5H5)(PR3)(SC6H4X-4)
(X ¼ Cl, Br; R ¼ Ph, Bu) [6].
1
red. The major difference in the H NMR spectrum of
complex 3 before and after adding SO2 is a peak at 9.99
ppm, characteristic of aldehydes. The appearance of this
peak suggests some decomposition of 3 to an aldehyde,
but it is not clear how SO2 converts the Schiff-base li-
gand to an aldehyde.
It was interesting to note distinct differences between
the redox behaviour of thiolato complexes before and
after introducing sulfur dioxide. The potentials where
electrochemical activities were found for the complexes
before addition of SO2 shifted to more positive values on
adding sulfur dioxide (Table 5). This indicates that the
SO2 adducts are more difficult to oxidize and corroborate
1
In all other complexes H NMR spectroscopic detec-
tion of SO2-adduct formation was simple. All chemical
shiftsafterSO2-adductformationweredownfieldfrom the
starting nickel thiolate complex. This observation is di-
agnostic of the thiolato sulfur atom acting as a Lewis base
in binding the SO2 [6,17]. The involvement of thiolato
sulfur atoms in SO2-adduct formation is well documented
with solid-state X-ray structures [4a,4b,4d]. There are also
examples of labile SO2 adducts that have only been iden-
tified in solution. Apart from our work on labile SO2-
adduct [6,17], Sadler et al. [4c] and recently Shaver and
co-workers [18] have used NMR spectroscopy to docu-
ment the formation of [n-Bu4N][Au(SC6H3MeS (SO2))]
and Ru(g5-C5H5)(PPh3)2(S(SO2)SiiPr3). Both the Au and
Ru SO2 adducts readily desorb SO2 when attempts were
made to isolate them as solids. The lability of these Au and
Ru SO2 adducts, as well as those reported in this paper, is
clearly linked to the strength of the SO2-thiolate interac-
tion, aninteractionthatinvolveselectrondensitydonation
from the thiolate sulfur. This donor-acceptor relation
between the thiolate sulfur and the SO2 is expected to
reflect in the redox behaviour of such complexes.
1
the conclusion drawn from the H NMR experiments
that SO2 adduct formation draws electron density from
the nickel centre. The reversibility of the redox behaviour
for the SO2 adducts mirrors those of the original com-
plexes. The more basic PBu3 complexes showed quasi-
reversibility (Fig. 4), whilst the PPh3 analogues were
irreversible. What is significant in these studies is the
clear shift in redox potential in all cases between the
complexes and their SO2 adducts. It is therefore quite
clear that cyclic voltammetry can be used as an analytical
technique to detect the absorption of sulfur dioxide by
complexes 5, 6, 7 and 8 hence these compounds can be
used as potentiometric sensors for sulfur dioxide.
As observed in the 1H NMR experiments, cyclic
voltammetry experiments after reacting complex 3 with
SO2 were ill defined. There were no distinct voltamo-
grams that would suggest SO2 adduct formation as
observed for the other complexes. Multiple scan exper-
iments over a five minute period (Fig. 5) illustrates that
any species formed by SO2 on reacting with 3 is unstable
and decomposes during this period. This combined with
3.3.2. Electrochemical detection of sulfur dioxide adducts
The reaction of SO2 with thiolato complexes in this
paper was also monitored by cyclic voltammetry. This
was accomplished by first running a cyclic voltammetry
experiment on a solution of a thiolato nickel complex
without sulfur dioxide, followed by bubbling sulfur di-
oxide through the same solution and repeating the cyclic
voltammetry experiment. It was necessary to run a
control experiment involving the electrolyte and sulfur
Table 5
Oxidation potentials of selected complexes and their SO2 adducts
Complex
Eox (mV) without
SO2
Eox (mV) SO2
adducts
5
6
7
8
411
514
332
283
513
586
585
356

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M.J. Moloto et al. / Journal of Organometallic Chemistry 689 (2004) 387–394
Acknowledgements
0.0
-0.5
-1.0
-1.5
-2.0
We acknowledge help with some of the electro-
chemistry experiments by Ms. Aoife Morrin. Financial
support by the International Foundation for Science
(Sweden) is gratefully acknowledged.
Complex
SO adduct
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4. Supplementary material
Crystallographic data for structures in this paper have
been deposited with the Cambridge Crystallographic
Data Centre with deposition numbers CCDC 221178 (3),
CCDC 221179 (6) and CCDC 221180 (7). Copies of data
can be obtained, free of charge, on application to The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44-1223-336033 or e-mail: deposit@ccdc.cam.
ac.uk or http://www.ccdc.cam.ac.uk).
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