Coordination modes of 2-mercaptonicotinic acid: Synthesis and crystal structures of palladium(II), platinum(II), rhenium(III) and molybdenum(VI) complexes

Article abstract of DOI:10.1039/b206101e

New Pd(II), Pt(II), Re(III), Mo(VI) and Mo(V) complexes with 2-mercaptonicotinic acid (HnicSH), [Pd(PPh₃)(HnicS)₂]·0.5CH₃OH 1, [Pd(HnicS)₂]·CH₃OH 2, [Pt(PPh₃)(HnicS)₂]·NEt_3<

Full text of DOI:10.1039/b206101e

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Coordination modes of 2-mercaptonicotinic acid: synthesis and
crystal structures of palladium(II), platinum(II), rhenium(III)
and molybdenum(VI) complexes
Susana M. O. Quintal,a Helena I. S. Nogueira,*a Vitor Félixa and Michael G. B. Drewb
a Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal.
E-mail: helena@dq.ua.pt
b Department of Chemistry, University of Reading, Whiteknights, Reading, UK RG6 6AD
Received 25th June 2002, Accepted 26th September 2002
First published as an Advance Article on the web 6th November 2002
New Pd(), Pt(), Re(), Mo () and Mo() complexes with 2-mercaptonicotinic acid (HnicSH),
[Pd(PPh3)(HnicS)2]ؒ0.5CH3OH 1, [Pd(HnicS)2]ؒCH3OH 2, [Pt(PPh3)(HnicS)2]ؒNEt3ؒH2O 3, [Pt(HnicS)2] 4,
[Pt(bipy)(HnicS)2]ؒCH3OH 5, [Re(PPh3)(OCH3)(HnicS)2] 6, [ReI2(PPh3)2(HnicS)] 7, [MoO2(CH3NicS)2] 8
and [Mo2O3(CH3nicS)4]ؒDMF 9 have been prepared. The crystal structures of compounds 1, 3, 8 and
[ReI2(PPh3)2(HnicS)]ؒ0.5H2OؒDMF (7ؒ0.5H2OؒDMF) were determined by X-ray diffraction. Complexes 1
and 3 contain two HnicSϪ ligands bonded in two different coordination modes: monodentate (S) and
chelating (N,S ). Complexes 7 and 8 contain only one HnicSϪ ligand which is bidentate (N,S ). Infrared,
1H and 13C-{1H} NMR spectroscopic data for the complexes are presented. The Mo() complex 8 is active
towards oxygen atom transfer reactions and can catalyse the oxidation of benzoin and PPh3 with dmso;
the catalysis occurs via the Mo() complex 9 which has been isolated and characterized.
The coordination chemistry of heterocyclic thiols has been
Introduction
the subject of various studies due to its versatile bonding modes
and also because similar coordination occurs in many redox
active centres of metalloenzymes.1,2,26–28 Molybdenum com-
plexes containing the MoVIO22ϩ group have been used as models
for oxomolybdenum enzymes. These enzymes catalyse oxygen
atom transfer reactions, that are usually described as involving
Heterocyclic thiolates are ambidentate ligands that are capable
of involving either the exocyclic sulfur or the endocyclic
nitrogen atoms of the thioamide group in monodentade,
chelating or bridging coordination.1–3 The introduction of an
acid function next to the thiol group, as in 2-mercaptonicotinic
acid, leads to other coordination possibilities, such as chelating
(O,S) and (N,S ), presented in Scheme 1.
mononuclear MoVIO2 and MoIVO2ϩ species29–35 rather than
2ϩ
dinuclear MoV–O–MoV species. The formation of dinuclear
MoV–O–MoV species is prevented by steric hindrance of the
ligands, as previously reported.31,36
We have been interested in the coordination of aromatic
ambidentate ligands such as dihydroxybenzoic acids37 and
3-hydroxypicolinic acid38,39 to second- and third-row transition
metals. In this work, we synthesised Pd(), Pt(), Re(), Mo()
and Mo() complexes with 2-mercaptonicotinic acid. The
complexes were characterised by vibrational spectroscopy and
NMR. The X-ray crystal structures of [Pd(PPh3)(HnicS)2]ؒ
Scheme 1
Several pyridine-2-thiolate3–15 and thiosalicylate com-
plexes16–19 have been reported but only a few complexes of
2-mercaptonicotinic acid (HnicSH) have been studied.20–25
Complexes of HnicSH containing transition metals, which have
been isolated include [{Os3H(CO)10}2(nicS)],21 [{Os3H(CO)9}-
(nicS){Os3H(CO)10}],21 [M3(HnicS)3Cl3] (M = Pd, Pt),23 [Ag-
(HnicS)]6ؒ4H2O,24 {Na[Ag(nicS)]ؒH2O}n,24 H[Au(HnicS)2],24
Na3[Au(nicS)2]ؒ2H2O,24 [Ag(HnicS)(PPh3)2],25 [Ag(HnicS)-
(PPh3)3]25 and [Au(HnicS)(PPh3)].25 The crystal structures show
different coordination modes to HnicSH: for the osmium
complexes bridging O,O and S was found in [{Os3H(CO)10}2-
(nicS)],21 and bridging O,O and S and monodentate N was
found in [{Os3H(CO)9}(nicS){Os3H(CO)10}];21 in the palladium
complex [Pd3(HnicS)3Cl3]23 each ligand acts as bidentade N,S,
with each sulfur atom bridging two palladium atoms; in the
silver complex [Ag(HnicS)]6ؒ4H2O24 the ligand acts as bidentate
N,S and each sulfur atom is bridging two or three silver atoms;
HnicSH acts as a monodentate S ligand in H[Au(HnicS)2],24
[Ag(HnicS)(PPh3)2]24 and [Ag(HnicS)(PPh3)3].25 Coordination
by the sulfur atom seems to be preferred in all complexes.
0.5CH3OH,
[Pt(PPh3)(HnicS)2]ؒNEt3ؒH2O,
[ReI2(PPh3)2-
(HnicS)]ؒ 0.5H2OؒDMF and [MoO2(CH3nicS)2] were deter-
mined and show two different types of coordination: mono-
dentate (S) and chelating (N, S ). The oxo-transfer properties of
the molybdenum complex were also studied and are consistent
with the formation of a dinuclear MoV–O–MoV species.
Results and discussion
Preparations
Suspensions of trans-[Pd(PPh3)2Cl2] or cis-[Pt(PPh3)2Cl2] and
HnicSH in an ethanol/methanol mixture were stirred in the
presence of triethylamine to give the complexes [Pd(PPh3)-
(HnicS)2]ؒ0.5CH3OH 1 and [Pt(PPh3)(HnicS)2]ؒNEt3ؒH2O 3.
When K2[MCl4] (M = Pd, Pt) was used as starting material,
the compounds obtained were [Pd(HnicS)2]ؒCH3OH 2 and
[Pt(HnicS)2] 4. The reaction of the 2,2Ј-bipyridine (bipy)
complexes [M(bipy)Cl2] (M = Pd, Pt) with HnicSH gave
DOI: 10.1039/b206101e
J. Chem. Soc., Dalton Trans., 2002, 4479–4487
This journal is © The Royal Society of Chemistry 2002
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Table 1 Bond lengths (Å) and angles (Њ) in the metal coordination
sphere of complexes 1 and 3
M = Pd() 1
M = Pt() 3
M–N(11)
M–S(1)
M–S(2)
M–P(1)
2.123(10)
2.326(4)
2.340(4)
2.229(3)
2.120(8)
2.343(3)
2.322(3)
2.216(4)
N(11)–M–P(1)
P(1)–M–S(1)
P(1)–M–S(2)
N(11)–M–S(1)
N(11)–M–S(2)
S(1)–M–S(2)
169.5(3)
99.2(1)
90.5(1)
70.4(3)
99.7(3)
169.1(1)
170.1(3)
100.9(1)
92.0(1)
69.2(3)
97.9(3)
166.2(1)
[Pt(bipy)(HnicS)2]ؒCH3OH 5 with platinum, but no palladium
analogue was obtained.
The complexes [Re(PPh3)(OCH3)(HnicS)2] 6 and [ReI2-
(PPh3)2(HnicS)] 7 were synthesised by refluxing HnicSH with
[ReO2I(PPh3)2] in methanol and [ReOI2(PPh3)2(OCH2CH3)] in
acetone, respectively. In these reactions there was formation
of rhenium() complexes from the rhenium() precursors. The
reduction of rhenium can be interpreted40–42 as a result of
oxygen transfer from the precursor to free PPh3, giving OPPh3
and a rhenium() complex. Similar results were reported40
for the synthesis of a 4,6-dimethylpyrimidine-2-thiolate com-
plex of rhenium, trans-[ReIIIX2(C6H7N2S)(PPh3)2]ؒCH3COCH3,
from [ReVOX3(PPh3)2] (X = Cl, Br). There is also a possible
explanation40 for the reduction of rhenium(), as a result of
partial oxidation of the HnicSH ligand in the thiol form, to
yield the corresponding disulfide; though in this case a lower
yield should have been obtained as a result of the consumption
of part of the ligand.
The reaction of [MoO2(acac)2] and HnicSH in refluxing
methanol originated the esterification of the COOH group in
the ligand, with formation of the complex [MoO2(CH3nicS)2] 8.
The reaction of 8 in DMF and in the presence of PPh3 origin-
ated a Mo() complex, [Mo2O3(CH3nicS)4]ؒDMF 9.
Suitable crystals of 1, 3, 7ؒ0.5H2OؒDMF and 8 for the X-ray
diffraction studies were obtained by vapour diffusion of diethyl
ether into: a dichloromethane solution of complex 1, an
ethanol solution of complex 3, a DMF solution of complex 7
and a chloroform solution of complex 8.
Crystal structures
The crystal structures of [Pd(PPh3)(HnicS)2]ؒ0.5CH3OH 1,
[Pt(PPh3)(HnicS)2]ؒNEt3ؒH2O 3, [ReI2(PPh3)2(HnicS)]ؒ0.5H2Oؒ
DMF 7ؒ0.5H2OؒDMF and [MoO2(CH3nicS)2] 8 were deter-
mined using single crystal X-ray diffraction.
Fig. 1 ORTEP67 views of (a) [Pd(PPh3)(HnicS)2]ؒ0.5CH3OH 1 and (b)
[Pt(PPh3)(HnicS)2]ؒNEt3ؒH2O
3
showing the overall molecular
geometry. Ellipsoids are drawn at 40% probability level. The labelling
scheme used for the carbon atoms is omitted for clarity as are the
solvent molecules.
ORTEP diagrams of the complexes 1 and 3 together with the
labelling scheme adopted are shown in Fig. 1 (a) and (b),
respectively. Selected bond lengths and angles are listed in Table
1, and indicate that the metal centre in both complexes is
coordinated to two sulfurs, one nitrogen and one phosphorus
atom in a distorted square planar coordination environment.
One of the two HnicSϪ ligands displays a bidentate coordin-
ation fashion leading to the formation of a four membered N,S
chelate ring with a small S–M–N bite angle of 70.4(3) and
69.2(3)Њ for 1 and 3, respectively. The second HnicSϪ ligand is
coordinated to the metal centre via the sulfur atom only, with a
M–S distance of 2.340(4) Å in 1 and 2.322(3) Å in 3. In both
complexes the ligands adopt an identical disposition with two
sulfur atoms trans to each other with S–M–S angles of 169.1(1)Њ
in 1 and 166.2(1)Њ in 3. The dihedral angle between the least
squares plane determined by the carbon atoms of the aromatic
rings is 53.1(5)Њ and 58.2(5)Њ for 1 and 3, respectively. In add-
ition, the molecular dimensions associated with the metal
coordination sphere in these two complexes are similar suggest-
ing that they are isostructural. In fact the match between the
two structures gives a root mean square (RMS) deviation of
0.145 Å only, for 40 non-hydrogen atoms. The RMS value is
reduced to 0.07 Å when the atomic coordinates of the two
MPN2S2 fragments are considered. Protonation of the N,S-
chelating HnicSϪ ligand, in complexes 1 and 3, occurs at O(19).
In both cases the hydrogen positions were tried alternatively in
O(18) and O(19) and the structures shown in Fig. 1 (a) and (b)
correspond to the lower R values. Furthermore the inter-
molecular interactions involving the proton bonded to O(19)
have dimensions within the usual values; if the proton is
bonded to O(18) it would result in senseless H ؒ ؒ ؒ H short
intermolecular contacts. The lengths for the metal centres are
within the expected values.43
A striking structural feature is apparent from the molecular
diagrams presented in Fig. 1 (a) and (b) for complexes 1 and 3,
respectively; in both species the nitrogen atom of the HnicSϪ
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Table 2 Bond lengths (Å) and angles (Њ) subtended at the metal centre
ligand, with monodentate coordination behaviour, is directed
towards the metal centre leading to short intermolecular dis-
tances [Pd ؒ ؒ ؒ N of 2.982(11) and Pt ؒ ؒ ؒ N of 3.060(14) Å]
which suggest a weak bonding interaction consistent with a
[4ϩ1] coordination type. A comparable situation was found38
for the related complexes [M(PPh3)2Cl(picOH)] (M = Pd() or
Pt()). In both cases the 3-hydroxypicolinate (picOHϪ) ligand is
coordinated in N-monodentate fashion with the carboxylate
group pointing to the metal centre; the distances Pd ؒ ؒ ؒ O
2.773 Å and Pt ؒ ؒ ؒ O 2.734(4) Å were obtained. The spatial
disposition adopted by the ligands in this type of complex may
reflect a delicate balance between the electronic ground states of
the metal centre and the steric requirements of the ligands. The
d8 electronic configuration of Pd() and Pt() favours struc-
tures with coordination number 4, while the bulky PPh3 ligand
forces the pyridine derivative to adopt an orientation with
a long M ؒ ؒ ؒ N interaction, in order to minimize the steric
interactions within the complex.
for complexes 7ؒ0.5H2OؒDMF and 8
Complex 7
Re–N(11)
Re–P(2)
Re–I(1)
2.111(12)
2.494(5)
2.711(3)
Re–S(1)
Re–P(1)
Re–I(2)
2.425(4)
2.495(5)
2.721(3)
N(11)–Re–S(1)
S(1)–Re–P(2)
S(1)–Re–P(1)
N(11)–Re–I(1)
P(2)–Re–I(1)
N(11)–Re–I(2)
P(2)–Re–I(2)
I(1)–Re–I(2)
66.6(3)
91.0(2)
90.0(2)
92.4(3)
87.7(1)
165.4(3)
92.2(1)
102.2(1)
N(11)–Re–P(2)
N(11)–Re–P(1)
P(2)–Re–P(1)
S(1)–Re–I(1)
P(1)–Re–I(1)
S(1)–Re–I(2)
P(1)–Re–I(2)
88.6(3)
88.9(3)
176.7(1)
159.1(1)
90.3(1)
98.8(1)
90.8(1)
Complex 8
Mo–O(1)
Mo–N(11)
Mo–S(2)
1.697(3)
2.309(4)
2.445(2)
Mo–O(2)
Mo–N(21)
Mo–S(1)
1.701(4)
2.317(5)
2.452(2)
ORTEP diagrams of 7 and 8, including the atomic notation
scheme used, are presented in Fig. 2 and 3, respectively. Selected
bond lengths and angles within the metal coordination
sphere are given in Table 2. The Re() complex 7 contains one
O(1)–Mo–O(2)
O(2)–Mo–N(11)
O(2)–Mo–N(21)
O(1)–Mo–S(2)
N(11)–Mo–S(2)
O(1) –Mo–S(1)
N(11)–Mo–S(1)
S(2)–Mo–S(1)
106.8(2)
90.2(2)
155.4(2)
107.4(1)
90.0(1)
91.7(1)
64.6(1)
146.3(1)
O1–Mo–N(11)
O(1)–Mo–N(21)
N(11)–Mo–N(21)
O(2)–Mo–S(2)
N(21)–Mo–S(2)
O(2)–Mo–S(1)
N(21)–Mo–S(1)
154.6(1)
90.2(2)
80.5(2)
92.7(2)
64.8(1)
108.4(1)
88.3(1)
HnicSϪ unit while the Mo() complex 8 presents two methyl
ester derivatives of the HnicSH acid. The complexes are
six-coordinated and both HnicSϪ and its derivative CH3nicSϪ
ligands exhibit again the N,S coordination mode rather than
O,S coordination.
In complex 7 the Re() centre shows a distorted octahedral
coordination with the equatorial plane determined by two
iodine atoms, one sulfur and one nitrogen atom both from
the HnicSϪ ligand. The axial positions are occupied by two
phosphorus atoms from the PPh3 ligands. The HnicSϪ ligand
is approximately perpendicular to the PReP vector leading
to an angle between this vector and the plane defined by the
carbon atoms of the aromatic ring of 89.4(4)Њ and a sandwich
containing the HnicSϪ ligand between two PPh3 ligands is
formed. This geometric arrangement may be determined by
minimisation of the steric interactions between the bulky PPh3
ligands and the HnicSϪ ligand. A slightly differential trans
effect is apparent from the values of the Re–N, Re–S and Re–I
distances. The Re–I(1) bond [2.711(3) Å] trans to the Re–S(1)
bond [2.425(4) Å] is 0.01 Å shorter than the Re–I(2) bond
[2.721(3) Å] trans to the Re–N(11) bond [2.111(12) Å]. These
distances are comparable to those reported in the literature for
Re() complexes containing a I2SN donor set.43
Fig. 2 An ORTEP67 view of [ReI2(PPh3)2(HnicS)]ؒ0.5H2OؒDMF
7ؒ0.5H2OؒDMF showing the overall molecular geometry. Ellipsoids are
drawn at 40% probability level. The labelling scheme used for the
carbon atoms is omitted for clarity.
In complex 8 the coordination environment of the Mo()
centre can be described as a distorted octahedron. The two
CH3nicSϪ ligands are in different coordination planes: one is
axial and the other is equatorial; consequently the aromatic
rings are almost perpendicular making an angle of 89.5(1)Њ,
which is near the ideal value of 90Њ. On the other hand, the
bond angles between trans donor atoms N–Mo᎐O and S–Mo–S
᎐
display a significant deviation of ca. 30Њ from 180Њ. The steric
demands of the N–Mo-S bite angle of the CH3nicSϪ ligand
precludes the sulfur and nitrogen donor atoms from achieving
the ideal positions of an octahedron. Indeed all four complexes
reported here present acute N–M–S chelating angles. Thus in
8 the two N–Mo-S angles are 64.6(1) and 64.8(1)Њ while in 7 a
slightly larger value for the N–Re–S chelating angle of 66.6(3)Њ
was found.
The crystal structure of 1 is built up from an asymmetric unit
composed of one [Pd(PPh3)(HnicS)2] discrete molecule and one
CH3OH solvent molecule, while the crystal structure of 3 is
made up from one [Pt(PPh3)(HnicS)2] molecule and two solvate
Fig. 3 An ORTEP67 view of [MoO2(CH3nicS)2] 8 showing the overall
molecular geometry. Ellipsoids are drawn at 40% probability level. The
labelling scheme used for the carbon atoms is omitted for clarity.
J. Chem. Soc., Dalton Trans., 2002, 4479–4487
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Table 3 Analytical and spectroscopic data for complexes of 2-mercaptonicotinic acid and the free ligand
Analysisa(%) Vibrational spectrab/cmϪ1
Compound
C
H
N
S
ν(C᎐O)
ν(C᎐N)
ν(C–O)
ν(C᎐S)
ν(M–N)
ν(M–S)
᎐
᎐
᎐
2-Mercaptonicotinic acid
[Pd(PPh3)(HnicS)2]ؒ0.5CH3OH 1
[Pd(HnicS)2]ؒCH3OH 2
[Pt(PPh3)(HnicS)2]ؒNEt3ؒH2O 3
[Pt(HnicS)2] 4
1679 vs
1680(3)
1706 s
1710(1)
1677 vs
1714(1)
1700 s
1710(1)
1677 vs
1714(1)
1713 vs
1709(1)
1717 vs
1597 s
1332 s
1142 m
1140(3)
1127 s
1131(3)
1132 vs
1133(4)
1129 s
1132(3)
1132 vs
1132(4)
1134 s
1133(4)
1132 vs
1580(4)
1590 vs
1586(5)
1563 vs
1572(4)
1595 vs
1588(5)
1567 vs
1572(5)
1580 vs
1571(5)
1570 vs
1332(1)
1377 vs
1377(2)
1398 vs
1399(1)
1377 vs
1379(2)
1400 vs
1400(1)
1380 s
53.6
(52.9)
34.8
(34.9)
49.1
(48.9)
27.1
(28.6)
35.7
(39.9)
46.2
(47.3)
45.5
(45.1)
35.2
3.5
(3.6)
2.3
(2.7)
5.1
(4.6)
2.1
(1.6)
3.2
(2.9)
3.5
(3.3)
3.7
(3.1)
3.1
4.1
(4.0)
6.5
(6.3)
4.7
(4.7)
5.7
(5.6)
7.9
(8.1)
3.2
(3.6)
1.2
(1.3)
6.2
9.0
371 m
356(3)
384 m
385(1)
387 w
360(1)
392 w
395(1)
387 w
375(1)
447 w
317 vw
312(1)
345 m
344(1)
–
306(1)
354 w
352(1)
350 w
344(1)
425 w
(9.3)
14.6
(14.3)
6.6
(7.2)
13.1
(12.7)
9.2
(9.3)
8.7
(8.1)
2.7
(2.9)
14.9
(13.8)
13.3
(13.0)
[Pt(bipy)(HnicS)2]ؒCH3OH 5
[Re(PPh3)(OCH3)(HnicS)2] 6
[ReI2(PPh3)2(HnicS)] 7
1399(1)
1398 vs
1706 vs
–
1725 vs
1720(3)
1725 vs
1724(3)
1563 s
1384 s
1129 vs
1133(1)
1131 s
1139(3)
1134 s
449 w
501(1)
372 s
365(3)
374 w
–
416 w
425(1)
352 s
345(1)
342 w
326(6)
1572(2)
1567 vs
1570(4)
1568 s
1396(1)
1402 vs
1404(1)
1400 vs
1400(1)
[MoO2(CH3nicS)2] 8
(36.2)
36.4
(37.8)
(2.6)
3.9
(3.2)
(6.0)
6.8
(7.1)
[Mo2O3(CH3nicS)4]ؒDMF 9
1569(9)
1135(1)
a Calculated values in parentheses. b Raman data in italics.
molecules, one of NEt3 and one of H2O. In spite of the different
contents of the unit cells of these two compounds they crystal-
lize in the same chiral space group P21. Furthermore, self-
assembly of [M(PPh3)(HnicS)2] results in both cases in the
formation of a 1-D polymeric structure anchored by hydrogen
bonding interactions between the free carboxylic groups of two
neighbouring molecules, as illustrated in Fig. 4 for the Pd
suggested by the short O(W1) ؒ ؒ ؒ O(19) distance of 2.49 Å
[H(19) ؒ ؒ ؒ O(W1) 2.16 Å, O(19)–H(19) ؒ ؒ ؒ O(W1) 104.1Њ]
and with triethylamine molecules thorough C–H ؒ ؒ ؒ O(H)C
hydrogen bonds [H(81B) ؒ ؒ ؒ O(19) [Ϫx, 1/2 + y, Ϫz] 2.56 Å,
C(81)–H(81B) ؒ ؒ ؒ O(19) 137.4Њ, C(81) ؒ ؒ ؒ O(19) 3.34(3) Å].
In the crystal structure of 7ؒ0.5H2OؒDMF the molecules of
[ReI2(PPh3)2(HnicS)] and DMF are connected via a hydrogen
bonding interaction with the following dimensions:
H(18) ؒ ؒ ؒ O(90) 1.83 Å, O(18)–H(18) ؒ ؒ ؒ O(90) 166.4Њ,
O(18) ؒ ؒ ؒ O(90) 2.63(2) Å. The analysis of intermolecular
interactions for complex 8 did not show any relevant structural
features.
Vibrational spectra
Infrared and Raman spectroscopic data for HnicSH and its
complexes are shown in Table 3; tentative assignments are
based on those found in the literature for HnicSH,23,24,44 hetero-
cyclic thiones45–47 and thiosalicylic acid complexes.16,19
2-Mercaptonicotinic acid shows the characteristic thione–
thiol tautomerism1 (Scheme 2). The absence, in the infrared
Scheme 2
Fig.
4
Hydrogen bonded 1-D polymer of [Pd(PPh3)(HnicS)2]ؒ
0.5CH3OH 1, viewed along the z axis.
spectrum of the free HnicSH ligand, of a ν(S–H) absorption
band at ca. 2500 cmϪ1 16,47
and the presence of the ν(N–H)
,
complex. The dimensions of the OH ؒ ؒ ؒ O(H)C hydrogen
bonds in 1 are H(28) ؒ ؒ ؒ O(19) [1 Ϫ x, 1/2 ϩ y, 1 Ϫ z] 2.06 Å,
O(28)–H(28) ؒ ؒ ؒ O(19) 120.9Њ, O(28) ؒ ؒ ؒ O(19) 2.58(2). The
absorption46–48 at 3186 cmϪ1 and ν(C᎐S) at 1142 cmϪ1, suggest
᎐
that the ligand in the solid state exists in the thione rather
than the thiol form. This is in agreement with what was found
for similar ligands, such as pyrimidine-2-thione,45 2-mercapto-
benzothiazole,47 2-mercaptopyridine13,49,50 and its derivatives.51
dimensions of the OH ؒ ؒ ؒ O᎐C hydrogen bonds in 3 are
᎐
H(28) ؒ ؒ ؒ O(18) [1 Ϫ x, 1/2 ϩ y, Ϫz] 1.76 Å, O(28)–
H(28) ؒ ؒ ؒ O(18) 164.3Њ, O(28) ؒ ؒ ؒ O(18) 2.55(2) Å. In add-
ition, in 1 some of the carboxylic groups of the N,S coordinated
HnicSϪ ligand, interact with CH3OH molecules via one
The position of the bands assigned to the ν(C᎐O) carboxyl
᎐
stretch, in the vibrational spectra of complexes 1 to 9 (with the
exception of 2 and 4), at wavenumbers over 1700 cmϪ1, clearly
indicates37–39,52 that in these compounds the carboxyl group is
protonated (or esterification has occurred, as in the case of
compounds 8 and 9) and it is not coordinated to the metal
centres, as shown in the crystal structures of 1, 3 and 7. The
infrared and Raman spectra of the free HnicSH ligand show
C᎐O ؒ ؒ ؒ H–O hydrogen bond [O(18) ؒ ؒ ؒ H(91) 1.90 Å,
᎐
O(18) ؒ ؒ ؒ H(91)–O(91) 172.5Њ, O(18) ؒ ؒ ؒ O(91) 2.71(3) Å].
A comparable situation happens with the supramolecular
structure of 3, in which the complex molecules interact with
mother liquor waters through CO–H ؒ ؒ ؒ O hydrogen bonds as
4482
J. Chem. Soc., Dalton Trans., 2002, 4479–4487

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Table 4 1H and 13C-{1H} NMR spectroscopic data for 2-mercaptonicotinic acid and its complexesa
Compound
H4
8.15
H5
H6
8.51
C2
C3
C4
C5
C6
C7
Cester
2-Mercaptonicotinic acidb
7.12
6.76
165.3
168.5
148.1
149.8
142.8
152.9
129.6
124.7
129.4
130.8
129.2
124.0
143.3
144.9
139.7
141.6
140.8
139.6
115.1
117.2
116.6
118.9
113.4
117.2
143.9
147.9
129.6
135.8
133.9
144.7
173.2
171.7
168.7
169.5
167.6
170.4
[Pd(PPh3)(HnicS)2]ؒ0.5CH3OH 1
7.96
8.13
[Pd(HnicS)2]ؒCH3OH 2b
8.20
7.95
8.17
7.07
6.94
6.87
8.55
8.23
8.17
[Pt(PPh3)(HnicS)2]ؒNEt3ؒH2O 3
148
129.1
139.0
141.9
140.7
138.3
116.9
119.1
113.5
118.0
130.0
169.4
169.1
168.0
170.2
149.9
141.9
153.0
130.7
135.6
e
e
[Pt(HnicS)2] 4b
8.28
8.27
7.22
7.11
6.98f
7.23
7.16
7.61
7.11
8.72
8.33
125.2
138.3
8.09f
7.87
8.19f
9.09
[Pt(bipy)(HnicS)2]ؒCH3OH 5c
7.65
11.53
8.34
8.81
11.88
8.71
[ReI2(PPh3)2(HnicS)] 7b
[MoO2(CH3nicS)2] 8
143.3
164.0
164.4
163.6
154.8
154.1
127.6
125.1
124.0
125.9
116.5
115.1
134.8
142.4
146.7
140.4
139.2
138.2
115.5
119.3
121.4
120.9
108.3
107.0
132.6
148.1
150.2
148.3
132.9
131.0
173.2
170.0
169.2
52.7
53.7
53.0
[Mo2O3(CH3nicS)4]ؒDMF 9d
8.30
8.01
7.01
6.78
8.68
8.50
168.2
160.8
e
e
a Spectra in CDCl3 solution unless otherwise stated; CPMAS solid state NMR data in italics. b In (CD3)2SO. c In CD3OD. d In CD2Cl2 e Peak
obscured. f New species in solution.
ν(C᎐O) at a low wavenumber (1679 cmϪ1), when compared to
the spectra of the complexes; this is possibly due to hydrogen
bonding in HnicSH, also shown by the presence of several
bands in the 3012–2812 cmϪ1 region. A similar explanation can
The 1H NMR spectra of HnicSH in dmso-d6, shows a triplet
at 7.12 ppm assigned to the H5 proton, a double-doublet at 8.15
ppm assigned to H4 and another double-doublet at 8.51 ppm
assigned to H6. The absence of any evidence for the thiol proton
(SH, δ = 2–5 ppm)45 confirms the thione form as the dominant
species of HnicSH in dmso-d6 solution, as in the solid state
(shown by the infrared and Raman spectra). Also, a broad
signal at about 14.6 ppm may be assigned51 to the NH proton;
it is absent in the spectra of the complexes. In general, the
resonances of the ring protons of the coordinated HnicSϪ lig-
᎐
be given for the values of ν(C᎐O) in complexes 2 and 4 (1677
᎐
cmϪ1), very similar to that in the free ligand.
In the free ligand IR and Raman spectra, the band at 1597
cmϪ1 is mainly assigned to the ν(C᎐N) vibration.51 In general,
᎐
the spectra of the complexes exhibit the ν(C᎐N) stretching
᎐
mode shifted to lower wavewnumber, with an increase in inten-
sity. The band assigned to ν(C᎐S)48 at 1142 cmϪ1 shifts to lower
and, show small upfield shifts in the H NMR spectra of the
1
᎐
wavenumber on coordination.
complexes, with the exception of complex 7. In complex 7 the
resonances of the ring protons of the ligand shift downfield,
possibly as a result of the unpaired electrons in the rhenium()
centre. In contrast with complexes 1 and 3, a large downfield
shift is also observed for the protons of the PPh3 phenyl rings in
7 (two broad signals at 14.1 and 8.9 ppm with an intensity ratio
of 2 : 3, were assigned to the ortho, 12H, and para and meta
protons, 18H). In the [MoO2(CH3nicS)2] 8 1H NMR spectra the
signal of the methyl ester group is observed at 3.98 ppm; and
in the case of [Mo2O3(CH3nicS)4]ؒDMF 9 at 3.93 ppm. The 1H
NMR spectra in dmso-d6 of complexes 2 and 4 show two differ-
ent sets of peaks corresponding to the two ligand molecules in
each compound; a third set of peaks in the case of 4, with
different intensity, indicates the formation of another species in
solution. However, for 2 and 4 only one set of peaks could be
clearly identified in the solid state 13C NMR spectra.
In the infrared and Raman spectra of the Mo() complex 8,
the symmetric cis-dioxo stretch, νsym(MoO2), is seen as a very
strong band at 932 cmϪ1, and the asymmetric stretch,
νasym(MoO2), corresponds to a strong band at 903 cmϪ1, weaker
in the Raman. For the Mo() complex 9, both infrared and
Raman spectra show a very strong band at 950 cmϪ1 assigned
to the ν(MoO) stretch of the Mo᎐O terminal group; another
᎐
band at 762 cmϪ1 can possibly be assigned to the ν(Mo2O)
stretch of the bridging Mo–O–Mo group.39 In addition to the
above, PPh3 and bipy bands were observed in all the applicable
spectra.
The proposed formula for complexes 2, 4 to 6 and 9 are based
on elemental analysis and spectroscopic data, considering
analogous complexes that have been reported for chelating N,S
and other bidentade ligands.6,38,45,53
In the 13C NMR spectra of the complexes, the C–S carbon of
the HnicSϪ ligand, C2, shows a downfield shift of over 10 ppm
(only less than this in the case of the molybdenum complex 8).
This C2 shift supports coordination through the sulfur atom.
There is also a considerable downfield shift in C6, shown in the
spectra of the complexes. The significant shifts in the reson-
ances of C2 and C6 together with the fact that the position of
the carboxylic carbon atom C7 is little altered, is consistent with
N,S-chelation in the complexes (shown in the crystal structures
of 1, 3, 7 and 8).
1H and 13C-{1H} NMR spectra
Proton and carbon NMR data and tentative assignments for
HnicSH (see Chart 1 for labelling) and the complexes are given
in Table 4. Tentative assignments are based on those found in
the literature for HnicSH24,25 and similar heterocyclic thionate
complexes.48,51,54
The 13C NMR spectra in CDCl3 of complexes 1 and 3 show
similar chemical shifts, the signals are broad which implies that
in solution, there might be a rapid inter-conversion between the
two HnicSϪ ligands (monodentade (S) and bidentade (N,S )),
possibly via a five-coordinate intermediate.6 In the solid state
13C NMR spectra of both 1 and 3, two different sets of peaks
are observed corresponding to the two ligands.
Chart 1
J. Chem. Soc., Dalton Trans., 2002, 4479–4487
4483

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Oxo-transfer properties of the molybdenum complexes
The oxo-transfer properties of [MoO2(CH3nicS)2] 8 were
studied using benzoin (1,2-diphenyl-2-hydroxyethanone) and
PPh3 (triphenylphosphine) as substrates.30,31,55,56
The catalytic properties of 8 for benzoin oxidation were
investigated using dimethyl sulfoxide (dmso) as the oxo donor.
The complex was able to catalyse the oxidation of benzoin to
benzil (1,2-diphenylethane-1,2-dione) by dmso. The reaction
1
was monitored by H NMR spectroscopy, which allowed the
quantification of benzoin and benzil. The percentage con-
version of benzoin to benzil was 75% after 6 hours, and about
100% after 24 hours. Similar results were previously reported39
for the polynuclear molybdenum complexes (nBu4N)2[Mo4-
O12(picOH)2] and (nHex4N)2[Mo2O6(picOH)2] (HpicOH
3-hydroxypicolinic acid).
=
Complex 8 was tested as catalyst for PPh3 oxidation using
again dmso as the oxo donor. The oxidation of PPh3 using
molybdenum() complexes is usually monitored using UV-
visible absorption,30,31,57 by the intensity decrease in the Mo()
band and the intensity increase in a new Mo() or Mo() band.
In the reaction with complex 8, we did not observe a Mo() or
Mo() band over a reaction period of 3 hours. The reaction
was then monitored by 31P NMR spectroscopy (Fig. 5: PPh3,
Fig.
6
UV-Visible spectra of the molybdenum complexes (a)
[MoO2(CH3nicS)2] 8 and (b) [Mo2O3(CH3nicS)4]ؒDMF 9, in CH2Cl2
solution.
lytic reaction in dmso. These results are consistent with other
oxygen atom transfer reactions involving active dinuclear MoV–
O–MoV species.35,53,57,58 Moreover, following a 1 : 1 addition of
[MoO2(CH3nicS)2] 8 and PPh3 in a CD2Cl2 solution, the 31P
NMR spectra show that after 4 hours a 50% conversion of PPh3
to POPh3 was achieved without further reaction, which corre-
sponds to the complete consumption of 8 according to the
reaction:
2[MoO2(CH3nicS)2] ϩ PPh3
[Mo2O3(CH3nicS)4] ϩ POPh3
The proton NMR of the final solution presents the character-
istic signals of complex [Mo2O3(CH3nicS)4] 9, which strongly
suggests that this is the intermediate Mo() compound in the
catalytic cycle as shown in Scheme 3.
Assuming that the regeneration of the Mo() complex (step
b in Scheme 3) is fast, its concentration should be essentially
constant throughout the reaction. The rate of oxidation of the
substract (benzoin or PPh3), k1 (step a in Scheme 3), can thus be
described as corresponding to a pseudo-first order reaction:
Ϫd[S]/dt = kobs[S], where kobs = k1[catalyst] and S = benzoin or
PPh3. Fig. 7 shows the variation of ln([PPh3]0/[PPh3]) versus
Fig. 5 Evolution of PPh3 oxidation to POPh3 in dmso-d6, using
[MoO2(CH3nicS)2] 8 as catalyst, as monitored by 31P NMR.
Ϫ5.6 ppm; POPh3, 26.7 ppm). The reaction of 8 and 20 equiv-
alents of PPh3 was performed in dmso-d6, at room temperature.
The complex shows catalytic activity and the percentage
conversion of PPh3 to POPh3 was 42% after 6 hours, and 90%
after 24 hours. To observe the visible spectrum of the reduced
form of complex 8, a similar reaction was performed in dichloro-
methane, in the absence of dmso. The reaction was monitored
using the visible spectra and in fact, the intensity of the band at
371 nm characteristic of the Mo() complex 8 (Fig. 6a)
strongly decreased during reaction and new bands at 504 and
540 nm appeared with increasing intensity, corresponding to a
Mo() compound. The final spectrum is similar to that of the
Mo() species isolated and characterised as [Mo2O3(CH3nicS)4]
9 (Fig. 6b).
The catalytic cycle proposed is shown in Scheme 3. If step b
Fig. 7 Plot of ln([PPh3]0/[PPh3]) versus time using [MoO2(CH3nicS)2] 8
as catalyst.
Scheme 3
time, where [PPh3]0 is the initial concentration of PPh3. The
values of the kobs are 0.0038 sϪ1 (k1 = 19.0 × 10Ϫ2 MϪ1
is fast, the reduced form of the catalyst is rapidly converted to
its oxidised form, complex 8; which can explain the absence
of the Mo() band in the UV-visible spectra, during the cata-
s )
Ϫ1) for benzoin and 0.0017 sϪ1 (k1 = 14.2 × 10Ϫ2 MϪ1 sϪ1
for PPh3. These results are based on the first 6 and 24 hours for
the oxidation of benzoin and PPh3, respectively.
4484
J. Chem. Soc., Dalton Trans., 2002, 4479–4487

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[MoO2(CH3nicS)2] (8). To a methanolic solution (50 cm3) of
Experimental
[MoO2(acac)2] (0.33 g, 1 mmol) was added a methanolic
solution (130 cm3) of HnicSH (0.16 g, 1 mmol) under N2. The
resulting solution was refluxed for 30 hours. After evaporation
of part of the solvent a brownish solid was isolated, washed
with a small volume of methanol and dried over silica gel.
Yield: 0.16 g, 0.34 mmol, 68%. Brownish crystals of 8 suitable
for X-ray diffraction were obtained by vapour diffusion of
diethyl ether into a solution of the compound in chloroform.
Preparation of complexes
All chemicals were of at least reagent grade and used as
supplied by Aldrich. The starting complexes, trans-[Pd(PPh3)2-
Cl2],59 cis-[Pt(PPh3)2Cl2],60 [Pt(bipy)Cl2],61 [ReO2I(PPh3)2],62
[ReOI2(PPh3)2(OCH2CH3)]62 and [MoO2(acac)2]63 were
prepared by the respective literature procedures.
[Pd(PPh3)(HnicS)2]ؒ0.5CH3OH (1). To a stirred suspension
of trans-[Pd(PPh3)2Cl2] (0.35 g, 0.5 mmol) in ethanol (10 cm3)
was added a methanolic suspension (4 cm3) of 2-mercapto-
nicotinic acid (0.16 g, 1 mmol) and triethylamine (0.28 cm3,
2 mmol) giving an orange solution. Addition of diethyl ether
resulted in the formation of orange crystals. The crystals were
filtered off, washed with diethyl ether and dried over silica gel.
Yield: 0.32 g, 0.46 mmol, 92%. Orange X-ray quality crystals of
1 were obtained by vapour diffusion of diethyl ether into a
solution of the compound in CH2Cl2.
[Mo2O3(CH3nicS)4]ؒDMF (9). To a DMF solution (2.5 cm3)
of [MoO2(CH3nicS)2] 8 (0.037g, 0.08 mmol) was added a DMF
solution (2.5 cm3) of PPh3 (0.031 g, 0.22 mmol), under N2. The
resulting pink solution was stirred at room temperature for
5 hours. Diethyl ether was added and after 2 weeks in the fridge,
a pink solid was filtered off, washed with diethyl ether and dried
over silica gel. Yield: 0.013 g, 0.013 mmol, 33%.
Oxo-transfer reactions
The benzoin oxidation reaction was carried out using the
method of Wong et al.56 Complex 8 (0.01 mmol) and 25 equiv-
alents of benzoin were dissolved in deoxygenated CD3SOCD3
(0.5 cm3). The mixture was sealed in NMR tubes under nitrogen
and heated at 100 ЊC. The reaction was monitored by 1H NMR
spectroscopy.
For PPh3 oxidations, complex 8 and 20 equivalents of PPh3
were added to CD3SOCD3, at room temperature. The reaction
was monitored by 31P NMR spectroscopy.
[Pd(HnicS)2]ؒCH3OH (2). To a stirred suspension of HnicSH
(0.16 g, 1 mmol) in methanol (4 cm3) was added an aqueous
solution (5 cm3) of K2[PdCl4] (0.14 g, 0.5 mmol). The resulting
orange suspension was stirred for one day. It was centrifuged
and an orange solid isolated, washed with ethanol and
acetonitrile and dried over silica gel. Yield: 0.16 g, 0.36 mmol,
72%.
[Pt(PPh3)(HnicS)2)]ؒNEt3ؒH2O (3). To a stirred suspension
of cis-[Pt(PPh3)2Cl2] (0.20 g, 0.25 mmol) in ethanol (5 cm3) was
added a methanolic suspension (2 cm3) of HnicSH (0.08 g,
0.5 mmol) and triethylamine (0.14 cm3, 1 mmol), resulting in a
yellow solution. Addition of diethyl ether originated formation
of yellow crystals. The crystals were filtered off, washed with
diethyl ether and dried over silica gel. Yield: 0.16 g, 0.18 mmol,
72%. Yellow crystals of 3 suitable for X-ray diffraction were
obtained by vapour diffusion of diethyl ether into a solution of
the compound in ethanol.
Crystallography
A summary of the crystallographic data together with data
collection and the refinement details for [Pd(PPh3)(HnicS)2]ؒ
0.5CH3OH 1, [Pt(PPh3)(HnicS)2]ؒNEt3ؒH2O 3, [ReI2(PPh3)-
(HnicS)]ؒ0.5H2OؒDMF 7ؒ0.5H2OؒDMF and [MoO2(CH3nicS)2]
8 are given in Table 5. X-Ray data sets for these four complexes
were collected on a MAR research image plate system equipped
with graphite-monochromated Mo-Kα radiation (0.71073 Å).
95 Frames were measured at 2Њ intervals using a counting time
adequate to the crystal under study. Data analysis was per-
formed with the XDS program.64 Intensities for complexes 3
and 7 were corrected empirically for absorption effects with the
DIFABS program,65 using a version modified for the image
plate system, while no absorption correction was applied to
complexes 1 and 8. The structures were solved by direct
methods and refined by full-matrix least-squares methods
against F 2 using SHELXS and SHELXL from the SHELX97
package.66 The occupation factors of the solvent molecules
CH3OH in 1 and H2O in 3 were set to 0.5 in agreement with
their electronic densities in the corresponding Fourier maps. All
non-hydrogen atoms were refined using anisotropic thermal
parameters, except the oxygen atom of the H2O molecule in 3,
which was refined with an isotropic temperature factor. The
hydrogen atoms were inserted in idealised positions and allowed
to refine, riding on the parent C(O) atom with an isotropic
thermal parameter equal to 1.2 times those to which they were
bonded. The hydrogen atoms of the H2O molecule were not
included in the refinement.
[Pt(HnicS)2] (4). A supension of HnicSH (0.06 g, 0.4 mmol)
and K2[PtCl4] (0.08 g, 0.2 mmol) in methanol (10 cm3) was
stirred for three days. The resulting yellow suspension was
centrifuged and a yellow solid isolated, washed with ethanol
and dried over silica gel. Yield: 0.10 g, 0.19 mmol, 96%.
[Pt(bipy)(HnicS)2]ؒCH3OH (5). To a stirred suspension of
[Pt(bipy)Cl2] (0.06 g, 0.15 mmol) in ethanol (5 cm3) was added a
methanolic suspension (3 cm3) of HnicSH (0.05 g, 0.3 mmol).
The resulting orange suspension was stirred for one day. It was
centrifuged and an orange solid isolated, washed with ethanol
and dried over silica gel. Yield: 0.09 g, 0.13 mmol, 87%.
[Re(PPh3)(OCH3)(HnicS)2] (6). A brownish suspension of
[ReO2I(PPh3)2] (0.17 g, 0.2 mmol) and HnicSH (0.06 g,
0.4 mmol) in methanol (20 cm3) was refluxed for 5 hours. To the
dark red solution obtained was added 40 cm3 of water and a
dark suspension was obtained. It was centrifuged and a dark
red solid isolated, washed with water and dried over silica gel.
Yield: 0.11 g, 0.14 mmol, 70%.
Molecular and crystal packing diagrams were drawn with
PLATON graphical software.67
CCDC reference numbers 188470–188473.
See http://www.rsc.org/suppdata/dt/b2/b206101e/ for crystal-
lographic data in CIF or other electronic format.
[ReI2(PPh3)2(HnicS)] (7). A suspension of [ReOI2(PPh3)2-
(EtO)] (0.31 g, 0.3 mmol) and HnicSH (0.05 g, 0.3 mmol)
in acetone (7 cm3) was refluxed under N2 for two hours. The
resulting dark suspension was centrifuged and a dark red solid
isolated, washed with acetone and dried over silica gel. Yield:
0.16 g, 0.14 mmol, 48%. Dark red crystals suitable for X-ray
diffraction were obtained by vapour diffusion of diethyl ether
into a solution of compound 7 in DMF, giving the crystalline
compound [ReI2(PPh3)2(HnicS)]ؒ0.5H2OؒDMF.
Instrumentation
Infrared spectra were measured as KBr pellets on a Mattson
7000 FT instrument. Raman spectra were recorded on a Brüker
RFS100/S FT instrument (Nd:YAG laser, 1064 nm excitation).
1H and 13C NMR spectra were recorded on a Brüker AMX300
J. Chem. Soc., Dalton Trans., 2002, 4479–4487
4485

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Table 5 Room temperature crystal data and the pertinent refinement parameters for metal complexes 1, 3, 7ؒ0.5H2OؒDMF and 8
Compound
1
3
7ؒ0.5H2OؒDMF
8
Empirical formula
M
C30.5H25N2O4.50PPdS2
693.02
C36H39N3O5PPtS2
884.89
C45H42.50I2N2O3.50P2ReS
1201.31
C14H12MoN2O6S2
464.32
Crystal system
Space group
Monoclinic
P21
Monoclinic
P21
Monoclinic
P21/c
Orthorhombic
Pbca
a/Å
b/Å
c/Å
10.177(14)
17.733(27)
11.431(16)
115.72(1)
1859(5)
2
10.390(15)
17.781(20)
11.553(15)
115.54(1)
1926(4)
2
11.068(16)
23.569(26)
18.371(24)
96.26(1)
4764(11)
4
8.128(14)
28.616(35)
14.941(17)
(90.0)
3475(8)
8
β/Њ
V/Å3
Z
Dc/g cmϪ3
1.238
0.687
1.526
3.836
1.675
3.998
1.775
1.028
µ/mmϪ1
F(000)
702
3962
3962
884
6141
6141
2326
8431
1856
11034
3113 (Rint = 0.0226)
Reflections measured
Unique reflections
Final R indices
R1 and wR2 a [I > 2σ(I )]
R1 and wRa (all unique data)
8431 (Rint = 0.0643)
0.0679, 0.1850
0.0923, 0.2045
0.0449, 0.1176
0.0546, 0.1241
0.0973, 0.1750
0.1259, 0.1860
0.0441, 0.0843
0.0584, 0.0909
1
2
–
a
2
2
2
2
2
2
2
2
2
R1 = Σ(∆F)/Σ(Fo), wR2 = {Σ[w∆(F ) ]/Σ[w(Fo ) ]} , w = 1/[σ (Fo ) ϩ (aP) ϩ bP], where P = (max(Fo ,θ) ϩ 2Fc )/3.
spectrometer (1H, 300 MHz; 13C, 75.4 MHz) referenced to
SiMe4 or the solvent. CPMAS 13C NMR spectra were recorded
on a Brüker MSL 400P spectrometer at 100.6 MHz with a 4.5
µs 90Њ pulse, contact time of 2 ms. UV-Visible spectra were
measured on a JASCO V-560 instrument. Microanalyses (C, H,
N and S) were measured by the Department of Chemistry,
University of Aveiro.
18 W. Schneider, A. Bauer and H. Schmidbaur, Organometallics, 1996,
15, 5445.
19 S. Mitra, H. Biswas and P. Bandyopadhyay, Polyhedron, 1997, 16,
447.
20 F. N. Rein and H. E. Toma, Polyhedron, 1998, 17, 1439.
21 E. W. Ainscough, A. M. Brodie, R. K. Coll, T. G. Kotch, A. J. Lees,
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Acknowledgements
The authors would like to acknowledge Fundação para a
Ciência e a Tecnologia (FCT) for financial support. S. M. O.
Quintal thanks the FCT for a postgraduate grant. We also
thank the EPSRC (UK) and the University of Reading (UK)
for funds for the Image Plate System and Mr A. W. Johans for
his assistance with the crystallography.
30 C. Lorber, M. R. Plutino, L. I. Eldind and E. Nordlander, J. Chem.
Soc., Dalton Trans., 1997, 3997.
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