Mono-, di- and poly-nuclear transition-metal complexes of a bis(tridentate) ligand: Towards p-phenylenediamine-bridged co-ordination polymers

Article abstract of DOI:10.1039/a800602d

The bis(tridentate) ligand N,N,N′,N′-tetrakis(2-pyridylmethyl)benzene-1,4-diamine (1,4-tpbd) is multifunctional in that mono-, di- and poly-nuclear transition-metal complexes as well as bis-co-ordinated complexes can be prepared. A prototype example of each class of complex has been characterized. In [ZnCl₂(2,4-tpbd)] the 1,4-tpbd is co-ordinated via only one of its tridentate ends. Both ends of 1,4-tpbd are bound in a dipalladium complex. [Pd₂Cl₂(1,4-tpbd)][PdCl₃(dmso)] ₂.This structure constitutes the first example of a crystal structure of the counter anion [PdCl₃(dmso)]^-. The chloride salt of the [Pd₂Cl₂(tpbd)]²⁺ cation has also been isolated. In the structures of both [ZnCl₂(1,4-tpbd)] and [Pd₂Cl₂(tpbd)]²⁺ quinoid character of the benzene linker of the 1,4-tpbd is evident. The compound [Ru(1,4-tpbd)₂][PF₆]₂ is an example of a bis-co-ordinated complex. One tridentate end of each ligand is co-ordinated to the ruthenium(II) ion while the other end is unco-ordinated. These three contrasting complexes demonstrate the versatility of 1,4-tpbd as a ligand for transition-metal complexes and the series represent the structural elements required for the construction of homo- and hetero-nuclear co-ordination oligo- or poly-mers. The compositions of the products isolated from the reaction of 1,4-tpbd with iron and nickel are consistent with the polymeric formulations [M(1,4-tpbd)]_nA_2n (M = Fe, A = Cl; M = Ni, A = NO₃).

Full text of DOI:10.1039/a800602d

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Mono-, di- and poly-nuclear transition-metal complexes of a
bis(tridentate) ligand: towards p-phenylenediamine-bridged
co-ordination polymers
Alan Hazell,a Christine J. McKenzie*,†,b and Lars Preuss Nielsenb
a Department of Chemistry, Aarhus University, 8000 Århus C, Denmark
b Department of Chemistry, Odense University, 5230 Odense M, Denmark
The bis(tridentate) ligand N,N,NЈ,NЈ-tetrakis(2-pyridylmethyl)benzene-1,4-diamine (1,4-tpbd) is multifunctional
in that mono-, di- and poly-nuclear transition-metal complexes as well as bis-co-ordinated complexes can be
prepared. A prototype example of each class of complex has been characterized. In [ZnCl2(2,4-tpbd)] the 1,4-tpbd
is co-ordinated via only one of its tridentate ends. Both ends of 1,4-tpbd are bound in a dipalladium complex.
[Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2. This structure constitutes the first example of a crystal structure of the counter
anion [PdCl3(dmso)]Ϫ. The chloride salt of the [Pd2Cl2(tpbd)]2ϩ cation has also been isolated. In the structures of
both [ZnCl2(1,4-tpbd)] and [Pd2Cl2(tpbd)]2ϩ quinoid character of the benzene linker of the 1,4-tpbd is evident.
The compound [Ru(1,4-tpbd)2][PF6]2 is an example of a bis-co-ordinated complex. One tridentate end of each
ligand is co-ordinated to the ruthenium() ion while the other end is unco-ordinated. These three contrasting
complexes demonstrate the versatility of 1,4-tpbd as a ligand for transition-metal complexes and the series
represent the structural elements required for the construction of homo- and hetero-nuclear co-ordination
oligo- or poly-mers. The compositions of the products isolated from the reaction of 1,4-tpbd with iron and
nickel are consistent with the polymeric formulations [M(1,4-tpbd)]nA2n (M = Fe, A = Cl; M = Ni, A = NO3).
The last decade has witnessed a surge of interest in the dis-
covery of simple building blocks capable of forming specific
molecular arrays under certain chemical conditions.1 For
example, the addition of a specific metal ion to appropriate
ligands may induce the formation of one- and two-dimensional
polymers or framework structures as a consequence of the
constraints induced by co-ordination.2 The search for appro-
priate organic building blocks for the construction of one-
dimensional co-ordination polymers has led us to investigate
tetra-N-functionalized p-phenylenediamines as potential
bis(tridentate) bridging ligands. We have recently reported the
preparation and characterization of the new potentially redox-
active hexadentate p-phenylenediamine-based ligand N,N,NЈ,
2n+
py
py
N
M
N
py
py
n
(a)
2n+
py
py
M
N
N
py
py
(b)
n
NЈ-tetrakis(2-pyridylmethylbenzene)-1,4-diamine
(1,4-tpbd)
and its dicopper complexes with the general formulation
[L2Cu(1,4-tpbd)CuL2]nϩ (L = H2O, n = 4; L = Cl or NO3,
n = 0).3 The compound 1,4-tpbd is easily oxidized in the pres-
ence of the redox-active metal ions Fe3ϩ, Cu2ϩ and Mn3ϩ, and
we have characterized, in solution, its one-electron oxidized
Scheme 1 View of a one-dimensional co-ordination polymer con-
structed of 1,4-tpbd and a divalent octahedral metal ion: (a) meridional
co-ordination of the tridentate chelating end and (b) trans-facial co-
ordination of the tridentate chelating end (cis-facial not shown)
form, the purple radical cation 1,4-tpbd ϩ, using cyclic voltam-
᝽
metry, ESR and UV/VIS spectroscopy. If 1,4-tpbd is treated
with 2 or more equivalents of Cu2ϩ radical formation is sup-
pressed and the dinuclear copper complexes can be isolated. We
have found no evidence to suggest any electronic redistribution
(i.e. the stabilization of a charge-separated state) between the
1,4-tpbd and the copper ions to give, for example, formally
mixed-valence radical-bridged complexes of the type [L2CuI-
one or both of 1,4-tpbd’s tridentate ends is possible. Thus the
elements necessary for the construction of one-dimensional
co-ordination polymers using 1,4-tpbd are established. Such a
polymer is depicted by Scheme 1 and our initial foray into the
preparation of such compounds is also described.
The co-ordination chemistry of 1,4-tpbd may be compared
to that of the polypyridyl bis(tridentate) ligands 2,3,5,6-tetra-
(2-pyridyl)pyrazine (tppz)4 and the ‘back-to-back’ terpy-based
systems,5 e.g. 6Ј,6Љ-di(2-pyridyl)-2,2Ј:4Ј,4Љ:2Љ,2ٞ-quaterpyrid-
ine (dpqtpy). To our knowledge crystal structures of metal com-
plexes in which both ends of these ligands are simultaneously
bound to a metal ion in either di- or poly-nuclear complexes
have been reported in only the case of tppz, for the complexes
[Cu2(tppz)(H2O)4][ClO4]4 and [{Zn2(µ-tppz)(H2O)Cl(µ-ZnCl4)-
(µ-ZnCl2)[µ-ZnCl3(H2O)]}2][ClO4]4.4 Clearly co-ordination of
both tppz and dpqtpy results in a meridional arrangement for
the tridentate ends of these bis-chelating ligands. By contrast,
the presence of a tertiary amine group in 1,4-tpbd gives the
(1,4-tpbd ϩ)CuIIL2]nϩ. In order better to understand the redox
᝽
behaviour of 1,4-tpbd we have extended this work to include
the study of complexes of relatively redox-inert transition-
metal ions. Described here are zinc, palladium and ruthenium
complexes. Each complex is a prototype for complexes of the
following general formulations: [M(1,4-tpbd)Ln]nϩ [LnM(1,4-
tpbd)MLn]nϩ and [(1,4-tpbd)M(1,4-tpbd)]nϩ. This series of
complexes demonstrate that 1,4-tpbd shows ‘schizodentate’
character; co-ordination to transition-metal ions via either
† E-Mail: chk@chem.ou.dk
J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756
1751

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requires C, 46.34; H, 4.15; Cl, 18.24; N, 10.81%). FAB mass
spectrum: m/z 791 ([M ϩ Cl]ϩ, 50), 756 ([M]ϩ, 30), 721
([M Ϫ Cl]ϩ, 20) and 615 ([M Ϫ PdCl]ϩ, 75%).
N
N
N
N
N
N
Dichloro[N,N,NЈ,NЈ-tetrakis(2-pyridylmethyl)benzene-1,4-
diamine]dipalladium(II) chloride, [Pd2Cl2(1,4-tpbd)]Cl2. Pal-
ladium() chloride (100 mg, 0.564 mmol) and KCl (84 mg,
1.127 mmol) were heated under reflux for 20 min; 1,4-tpbd
(133 mg, 0.282 mmol) was added resulting in immediate
precipitation of the light yellow product. Yield 174 mg, 74%
(Found: C, 43.01; H, 3.58; N, 10.35. C15H14Cl2N3Pd requires
C, 43.56; H, 3.41; N, 10.16%). FAB mass spectrum: m/z 791
{[Pd2Cl3(1,4-tpbd)]ϩ, 36}, 756 {Pd2Cl2(1,4-tpbd)]ϩ, 56}, 721
{[Pd2Cl(1,4-tpbd)]ϩ, 24} and 615 {[PdCl(1,4-tpbd)]ϩ, 51%}.
UV/VIS (MeOH): λ/nm (ε/dm3 molϪ1 cmϪ1): 234 (15 460), 261
(9030) and 354 (1010).
1,4-tpbd
N
N
N
N
N
N
N
N
N
N
N
N
Bis[N,N,NЈNЈ-tetrakis(2-pyridylmethyl)benzene-1,4-diamine]-
ruthenium(II) bis(hexafluorophosphate) [Ru(1,4-tpbd)2][PF6]2ؒ
3H2O. The compounds 1,4-tpbd (200 mg, 0.424 mmol) and
[RuCl2(C6H5CN)4] (62 mg, 0.104 mmol) in ethanol (100 cm3)
were heated under reflux under argon for 48 h. The resulting
yellow solution was evaporated to dryness and the residue
redissolved in the minimum volume of water; 2 mol dmϪ3
NaOH was added dropwise until pH 9. The precipitated
unchanged ligand was filtered off and the crude product pre-
cipitated by addition of a saturated aqueous solution of
NH4PF6. Purification was achieved by column chromatography
in silica gel using CH3CN–saturated aqueous KNO3–water
(14:2:1). The main fraction containing [Ru(1,4-tpbd)2]2ϩ
eluted in the last, yellow band. Yellow [Ru(1,4-tpbd)2][PF6]2 was
precipitated by addition of saturated aqueous NH4PF6 and
recrystallized from water–acetonitrile (2:1). Yield 54 mg, 48%
(Found: C, 51.72; H, 3.88; N, 11.93. C60H62F12N12O3RuP2
tppz
dpqtpy
potential for both meridional and facial co-ordination geom-
etries. Thus we envision different topologies in dinuclear and
polymeric co-ordination complexes of these three linear bis(tri-
dentate) ligands. With the present work we have achieved a
more extensive collection of structurally characterized mono-
and di-nuclear transition-metal complexes of 1,4-tpbd com-
pared with those known for tppz and dpqtpy.
Experimental
Infrared spectra were measured as KBr discs using a Hitachi
270-30 spectrometer, UV/VIS absorption spectra on
a
1
Shimadzu UV-3100 spectrophotometer, EI mass spectra on a
Varian MAT311A spectrometer, FAB mass spectra on a Kratos
MS-50 spectrometer and NMR spectra on a Bruker AC 250
spectrometer. Elemental analyses were carried out at the micro-
analytical laboratory of the H.C. Ørsted Institute, Copenhagen.
N,N,NЈ,NЈ-Tetrakis(2-pyridylmethyl)benzene-1,4-diamine was
synthesized as reported.3
CAUTION: Although no problems were encountered in the
preparation of the perchlorate salt, suitable care should be
taken when handling such potentially hazardous compounds.
requires C, 51.84; H, 4.50; N, 12.09%). H NMR (CD3CN):
δ 4.08 (d, 2 H, CH2 co-ordinated, J = 20.1), 4.18 (d, 2 H, CH2
co-ordinated, J = 20.1), 4.27 (d, 2 H, CH2 co-ordinated, J =
19.1), 4.43 (d, 2 H, CH2 co-ordinated, J = 19.1), 4.77 (s, 8 H,
CH2 unco-ordinated), 6.22 (m, 8 H, C6H4), 6.99–7.46 (m, 8 H,
py H3 ϩ H5 co-ordinated ϩ un-coordinated), 7.70–7.94 (m,
8 H, py H4 co-ordinated ϩ unco-ordinated), 8.24 (d, 2 H, py
H6 co-ordinated, J = 5.5), 8.66 (d, 4 H, py H6 unco-ordinated,
J = 4.7) and 9.19 (d, 2 H, py H6 co-ordinated, J = 5.6). 13C
NMR (CD3CN): δ 57.88 (CH2 unco-ordinated), 64.76, 71.91
(CH2 co-ordinated), 112.84 (phenyl CH), 121.28, 122.93,
123.72, 123.95, 125.86, 126.60 (py C3 ϩ C5 co-ordinated ϩ
unco-ordinated), 138.09, 138.66, 140.96 (py C4 co-ordin-
ated ϩ unco-ordinated), 139.30, 146.12 (phenyl C, co-ordin-
ated ϩ unco-ordinated), 149.40, 153.04, 156.13 (py C6 co-
ordinated ϩ unco-ordinated), 159.34 (py C2 unco-ordinated),
162.03, 165.88 (py C2 co-ordinated). UV/VIS (CH3CN): λ/nm
(ε/dm3 molϪ1 cmϪ1): 255 (52 100) and 389 (13 750).
Preparations
Dichloro[N,N,NЈ,NЈ-tetrakis(2-pyridylmethylbenzene)-1,4-
diamine]zinc(II), [ZnCl2(1,4-tpbd)]. Zinc chloride (116 mg, 0.864
mmol) in dmso (5 cm3) was added to a solution of 1,4-tpbd (200
mg, 432 mmol) in dmso (25 cm3). After 7 d the product was
deposited as pale pink crystals. These were collected and dried
in vacuum, yield 132 mg, 50% (Found: C, 59.36; H, 4.79; N,
13.77. C30H28Cl2N6Zn requires C, 59.18; H, 4.64; N, 13.80%).
FAB mass spectrum: m/z 707.9 {[Zn2Cl3(1,4-tpbd)H2]ϩ, 12},
671.0 {[Zn2Cl2(1,4-tpbd)H]ϩ, 50} and 635 {[Zn2Cl(1,4-tpbd)]ϩ,
100%}. 1H NMR [(CD3)2SO]: δ 4.64 (s, 8 H, CH2), 6.55 (s, 4 H,
C6H4), 7.28 (s, 8 H, py H3, H5) and 7.73 (s, 4 H, py H4); after
addition of 3 equivalents of ZnCl2, δ 4.62 (s, 8 H, CH2), 6.73 (s,
4 H, C6H4), 7.52 (m, 8 H, py H3), 7.99 (t, 4 H, py H4, J = 7.4)
and 8.75 (d, 4 H, py H6, J = 4.4 Hz).
Poly{[N,N,NЈNЈ-tetrakis(2-pyridylmethylbenzene)-1,4-
diamine]iron(II) chloride} [Fe(1,4-tpbd)]nCl2n. This synthesis was
carried out under argon using standard Schlenk techniques.
Anhydrous FeCl2 (32 mg, 0.26 mmol) in CH3CN (5 cm3) was
added to a solution of 1,4-tpbd (120 mg, 0.26 mmol) in CH3CN
(25 cm3) together with a few iron turnings. The stirred mixture
was heated under reflux for 1 h, cooled and allowed to crystal-
lize for 2 h and finally filtered under argon. The resulting yellow
crystals were stable in air, yield 85 mg, 56% (Found: C, 60.90;
H, 4.75; Cl, 11.63; N, 14.25. C30H28Cl2FeN6 requires C, 60.12;
H, 4.71; Cl, 11.83; N, 14.02%). FAB mass spectrum: m/z 1035
{[FeCl(1,4-tpbd)2]ϩ, 1}, 619 {[Fe2Cl(1,4-tpbd)]ϩ, 20} and 563
{[FeCl(1,4-tpbd)]ϩ, 60%}.
Dichloro[N,N,NЈ,NЈ-tetrakis(2-pyridylmethyl)benzene-1,4-
diamine]dipalladium(II) trichloro(dimethyl sulfoxide)pallad-
ate(II), [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2. Palladium() chloride
(184 mg, 1.04 mmol) was heated in dmso (12 cm3) at ca. 100 ЊC
for 30 min. The compound 1,4-tpbd (120 mg, 0.26 mmol) was
added and the solution allowed to stand for 2 weeks, over which
time orange crystals, in variable yields, were deposited. These
were collected, washed with dmso and dried in vacuum (Found:
C, 47.01; H, 4.03; Cl, 18.36. N, 10.85. C17H20Cl4N3O2Pd2S
Poly{[N,N,NЈNЈ-tetrakis(2-pyridylmethylbenzene)-1,4-
diamine]nickel(II) perchlorate}, [Ni(1,4-tpbd)]n[ClO4]2nؒnH2O.
The compound Ni(ClO4)2ؒ6H2O (38.7 mg, 0.106 mmol) in hot
absolute ethanol (5 cm3) was slowly added to a stirred solution
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J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756

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of 1,4-tpbd (50 mg, 0.106 mmol) in hot absolute ethanol (10
cm3). The product precipitated immediately as a green solid,
which was filtered off using an extra fine filter, yield 62 mg, 80%
(Found: C, 47.56; H, 4.12; Cl, 9.43; N, 11.47. C30H28Cl2N6NiO8)
requires C, 48.16; H, 4.04; Cl, 9.48; N, 11.23%). FAB mass
spectrum: m/z 1101 {[Ni(1,4-tpbd)2(ClO4)]ϩ, 30}, 1002 {[Ni(1,4-
tpbd)2]ϩ, 25}, 887 {[Ni2(1,4-tpbd)(ClO4)3]ϩ, 28}, 788 {[Ni2(1,4-
tpbd)(ClO4)2]ϩ, 27}, 629 {[Ni(1,4-tpbd)(ClO4)]ϩ, 75} and 530
{[Ni(1,4-tpbd)]ϩ, 100%}.
X-Ray crystallography
Crystals suitable for X-ray diffraction studies were isolated
directly from reaction mixtures. Details of structure deter-
minations are listed in Table 1. Intensities were measured using
a Huber four-circle diffractometer, at room temperature for
[Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2. At room temperature crys-
tals of [ZnCl2(1,4-tpbd)] decayed rapidly in the X-ray beam,
and the crystal was therefore cooled to 120 K, by means of a
Crystostream cooler,6 which reduced the problem. Cell dimen-
sions were determined from reflections measured at 2θ. Data
were corrected for background, Lorentz-polarization effects,
and absorption. The structures were determined using SIR 927
and from subsequent difference electron-density maps and were
refined by the minimization of Σw(|Fo| Ϫ Fc|)2 using a modifi-
cation of ORFLS.8 Crystals of [ZnCl2(1,4-tpbd)] were twinned
on (001) so that reflections hkl with l = 2, 9 and 13 were partly
overlapped and were rejected, those with l = 0, 11 were almost
totally overlapped and could therefore be unscrambled, and the
reflections of remaining layers were not overlapped and could
be used unaltered. Non-hydrogen atoms were refined aniso-
tropically; hydrogen atoms of the ligand were kept at calculated
positions (C᎐H 0.95 Å) with isotropic displacement parameters
20% larger than the equivalent isotropic displacement para-
meters of the atoms to which they were attached. Atomic
scattering factors and anomalous dispersion corrections (for Zn
and Pd) were from ref. 9.
Fig. 1 An ORTEP10 drawing of the cation in Zn(1,4-tpbd)Cl2
plets were observed for the signals due to the aromatic protons.
A singlet at δ 4.64 due to the eight methylene protons indicates
that they are chemically equivalent despite the crystal structure
which shows that only one end of the ligand is co-ordinated.
This result can be interpreted such that the complex is still labile
in the presence of excess of zinc and/or dinuclear in solution.
Insolubility in solvents appropriate for low-temperature NMR
studies has prevented investigation of this issue.
The crystal structure of neutral [ZnCl2(1,4-tpbd)] is shown in
Fig. 1. Selected distances and angles are listed in Table 2. The
zinc atom is five-co-ordinated to three nitrogen atoms of
the ligand and two chlorine atoms, with Zn᎐Cl(1) 2.277(4),
Zn᎐Cl(2) 2.300(4), average Zn᎐Npy 2.15(1) and Zn᎐Namine
2.26(1) Å. The co-ordination geometry is intermediate between
that of a square pyramid and a trigonal bipyramid, but is
closest to a tetragonal-pyramidal arrangement with Cl(1) at the
apex. The sum of the C᎐N᎐C angles around the unco-ordinated
phenylenediamine nitrogen, N(2), is 359.8Њ and C(4)᎐N(2) is
only 1.37(1) Å. In contrast the sum of the C᎐N᎐C angles
around the co-ordinated N(1) is 338.5Њ and C(1)᎐N(1) 1.49(1)
Å. The planar geometry about the unco-ordinated amine nitro-
gen atom as well as the double-bond character for C(4)᎐N(2)
indicates a π delocalization of this amine nitrogen atom lone
pair with the aromatic system. A result of this delocalization
is the quinoid character evident in the bond lengths of the
aromatic linker.
CCDC reference number 186/924.
See http://www.rsc.org/suppdata/dt/1998/1751/ for crystallo-
graphic files in .cif format.
Results and Discussion
Our investigations with the bis(tridentate) ligand 1,4-tpbd
demonstrate that this ligand shows ‘schizodentate’ character;
co-ordination to transition-metal ions via either one or both of
the tridentate ends is possible in 1:1, 1:2 and 2:1 metal:ligand
complexes.
A dinuclear complex
2Ϫ
The reaction of 1,4-bpbd with PdCl4 in methanol–water
A mononuclear complex
yields the dinuclear palladium complex [Pd2Cl2(1,4-tpbd)]Cl2 as
a relatively insoluble pale yellow microcrystalline material.
However the cation of this complex could be structurally
characterized in the orange crystalline compound [Pd2Cl2-
(1,4-tpbd)][PdCl3(dmso)]2 which was obtained from the reaction
of 1,4-tpbd with PdCl2 in dmso. The cation and the anion in the
structure of [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2 are shown in Fig.
2. An ORTEP diagram of the counter anion [PdCl3(dmso)]Ϫ is
shown since to our knowledge this is the first report of a crystal
structure containing this particular anion, although there are
several examples known for its platinum analogue.11 Selected
distances and angles for [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2 are
listed in Table 3. In [Pd2Cl2(1,4-tpbd)]2ϩ the 1,4-tpbd bridges
the two palladium atoms which are each co-ordinated to three
nitrogen atoms and one chlorine atom in a square-planar
arrangement with Pd᎐Cl 2.290(1), Pd᎐Npyridyl 2.009(3) and
Pd᎐Namine 2.047(4) Å. Even though meridional co-ordination
of this ligand might be expected to produce a more strained
system these bond distances are shorter than the correspond-
ing metal–donor bonds in [ZnCl2(1,4-tpbd)]. The cation has
approximate mm2 symmetry with one mirror plane coincident
The reaction of 1,4-tpbd with an excess of ZnCl2 in dmso
results in pale pink crystals of [ZnCl2(1,4-tpbd)]. This slight
colouration is probably due to the presence of a trace amount
of oxidized ligand (the one-electron oxidized form of the lig-
and, 1,4-tpbd ϩ, is purple3). Preparations using other solvents
᝽
gave products which precipitated much faster yielding white
microcrystalline materials. The solubility properties of the neu-
tral [ZnCl2(1,4-tpbd)] might be the reason for the precipitation
of a mono- rather than a di- or poly-zinc complex. Preparations
with large excesses of zinc did not alter the outcome. However
electronic grounds cannot be excluded; co-ordination of one
end of the ligand may withdraw electron density from the other
end with the result that once the first metal is bound it is more
difficult to bind the second. The 1H NMR spectrum of
[ZnCl2(1,4-tpbd)] was recorded in (CD3)2SO in both the pres-
ence and absence of extra zinc chloride. In the absence of add-
itional ZnII the signals in the spectrum of [ZnCl2(1,4-tpbd)] are
broad indicating, not surprisingly, rapid ligand exchange on the
NMR timescale. When 3 equivalents of zinc chloride were
added to the solution the signals sharpened and indeed multi-
J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756
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Table 1 Crystallographic data and experimental details*
[Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2ؒ0.5H2O
[ZnCl2(1,4-tpbd)]
Formula
M
C34H41Cl8N6O2.5Pd4S2
1347.62
C30H28Cl2N6Zn
608.88
Space group
a/Å
b/Å
c/Å
P21/n
P21
8.822(1)
36.340(5)
14.125(1)
92.086(6)
4525(1)
8.369(2)
13.448(3)
13.330(4)
111.19(1)
1399(1)
β/Њ
U/Å3
Z
4
294
1.978
2
120
1.446
T/K
Dc/g cmϪ3
F(000)
Colour
2637.20
Orange
Lath
0.50 × 0.15 × 0.03
2.170
Empirical
0.705–1.210
2 < 2θ < 50; h, ϩk, ϩl
8892
628
Pale pink
Tabular
0.45 × 0.40 × 0.16
1.100
Integration
0.727–0.851
3 < 2θ < 55; h, ϩk, ϩl
5288
Crystal shape
Crystal size/mm
µ(Mo-Kα)/mmϪ1
Absorption correction
Transmission factors
Data collection range/Њ
No. reflections measured
Rint
0.032
0.170
No. unique reflections
No. observed reflections
No. variables
R
7996
5575 [I > 3σ(I)]
515
0.036
0.046
3352
1976 [I > 2σ(I)]
352
0.065
0.073
RЈ
Goodness of fit
(∆/σ)max
1.094
0.035
Ϫ0.8(1), 0.7(1)
1.395
0.028
Ϫ1.1(1), 1.2(1)
ρmin, ρmax/e ÅϪ3
* Details in common: monoclinic; 30 reflections centred; graphite-monochromated Mo-Kα radiation (λ 0.710 73 Å); 0% decay of standards;
¹
¹
R = Σ||Fo| Ϫ |Fc ||/Σ|Fo|, RЈ = [Σw2(|Fo| Ϫ |Fc|)2/Σw2(|Fo|]², w = 1/{[σCS(F2) ϩ 1.03F2]² Ϫ |F|}.
Table 2 Selected bond distances (Å) and angles (Њ) of [ZnCl2(tpbd)]
Zn᎐Cl(1)
Zn᎐Cl(2)
Zn᎐N(21)
Zn᎐N(11)
Zn᎐N(1)
N(1)᎐C(17)
N(1)᎐C(1)
N(1)᎐C(27)
C(1)᎐C(6)
2.277(4)
2.300(4)
2.115(11)
2.178(9)
2.261(10)
1.466(16)
1.488(14)
1.489(16)
1.386(16)
C(1)᎐C(2)
C(2)᎐C(3)
C(3)᎐C(4)
C(4)᎐N(2)
C(4)᎐C(5)
C(5)᎐C(6)
N(2)᎐C(47)
N(2)᎐C(37)
1.411(15)
1.397(15)
1.401(15)
1.372(14)
1.424(18)
1.386(18)
1.416(19)
1.442(17)
N(11)᎐Zn᎐N(21)
N(1)᎐Zn᎐N(21)
Cl(1)᎐Zn᎐N(21)
Cl(2)᎐Zn᎐N(21)
N(1)᎐Zn᎐N(11)
Cl(1)᎐Zn᎐N(11)
Cl(2)᎐Zn᎐N(11)
Cl(1)᎐Zn᎐N(1)
Cl(2)᎐Zn᎐N(1)
Cl(1)᎐Zn᎐Cl(2)
C(1)᎐N(1)᎐C(17)
150.9(4)
75.4(4)
99.7(3)
98.1(3)
76.5(4)
97.1(3)
96.1(3)
109.9(3)
135.3(3)
114.8(2)
111.5(10)
Zn᎐N(1)᎐C(27)
C(2)᎐C(1)᎐C(6)
N(1)᎐C(1)᎐C(6)
N(1)᎐C(1)᎐C(2)
C(1)᎐C(2)᎐C(3)
C(2)᎐C(3)᎐C(4)
C(3)᎐C(4)᎐N(2)
C(5)᎐C(4)᎐N(2)
C(3)᎐C(4)᎐C(5)
C(4)᎐C(5)᎐C(6)
C(1)᎐C(6)᎐C(5)
C(4)᎐N(2)᎐C(47)
C(4)᎐N(2)᎐C(37)
99.9(8)
119.6(11)
121.7(11)
118.6(10)
120.4(10)
121.2(9)
121.8(11)
121.7(11)
116.4(10)
122.9(11)
119.2(11)
120.3(12)
118.6(11)
C(17)᎐N(1)᎐C(27) 113.7(11)
Zn᎐N(1)᎐C(17)
C(1)᎐N(1)᎐C(27)
Zn᎐N(1)᎐C(1)
105.7(8)
113.3(11)
112.0(7)
C(37)᎐N(2)᎐C(47) 120.8(11)
with the plane of the phenyl ring, and the other perpendicular
to it and bisecting C(2)᎐C(3) and C(5)᎐C(6). The geometry of
the palladium ion in the anion is square planar with Pd᎐Cl
2.300(1) and Pd᎐S 2.250(1) Å. There is a partially occupied
water site. Again conjugation of the amine-based electrons with
those of the linking benzene ring is reflected by a quinoid char-
acter for the bond distances of the phenylenediamine moiety
and the planar geometry of the amine nitrogen atoms. The
‘bite’ angles of the tridentate ends of 1,4-tpbd in [Pd2Cl2(1,4-
tpbd)]2ϩ are very similar to those measured for the related terpy
complex [PdCl(terpy)]ϩ. The N᎐Pd᎐N angles reported for the
distorted [PdCl(terpy)]ϩ cation are 79 and 82Њ,12 similar to those
for [Pd2Cl2(1,4-tpbd)]2ϩ of 83Њ. The similarities between these
Fig. 2 An ORTEP10 drawing of the cation (a) and the anion (b) in
[Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2
structures also support the less aliphatic nature of the tertiary
amine donor in 1,4-tpbd.
In the structures of the di- and tetra-positive cations
[Pd2Cl2(1,4-tpbd)]2ϩ (Fig. 2) and [Cu2(1,4-tpbd)(H2O)4]4ϩ 3 the
1754
J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756

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Table 3 Selected bond distances (Å) and angles (Њ) of [Pd2Cl2(tpbd)]-
[PdCl3(dmso)]2
Cation
Pd(1)᎐N(11)
Pd(1)᎐N(21)
Pd(1)᎐N(1)
Pd(1)᎐Cl(1)
Pd(2)᎐N(31)
Pd(2)᎐N(41)
Pd(2)᎐N(2)
Pd(2)᎐Cl(2)
N(1)᎐C(1)
2.005(5)
2.004(5)
2.051(5)
2.294(2)
2.011(5)
2.016(5)
2.042(5)
2.286(2)
1.480(7)
1.511(8)
N(1)᎐C(27)
N(2)᎐C(4)
N(2)᎐C(37)
N(2)᎐C(47)
C(1)᎐C(2)
C(1)᎐C(6)
C(2)᎐C(3)
C(3)᎐C(4)
C(4)᎐C(5)
C(5)᎐C(6)
1.508(8)
1.480(7)
1.503(9)
1.504(8)
1.399(8)
1.376(8)
1.378(9)
1.374(9)
1.383(8)
1.395(8)
Fig. 3 Proton spectrum of [Ru(1,4-tpbd)2][PF6]2
N(1)᎐C(17)
N(11)᎐Pd(1)᎐N(21)
N(1)᎐Pd(1)᎐N(21)
Cl(1)᎐Pd(1)᎐N(21)
N(1)᎐Pd(1)᎐N(11)
Cl(1)᎐Pd(1)᎐N(11)
Cl(1)᎐Pd(1)᎐N(1)
N(31)᎐Pd(2)᎐N(41)
N(2)᎐Pd(2)᎐N(31)
Cl(2)᎐Pd(2)᎐N(31)
N(2)᎐Pd(2)᎐N(41)
Cl(2)᎐Pd(2)᎐N(41)
Cl(2)᎐Pd(2)᎐N(2)
C(1)᎐N(1)᎐C(17)
C(1)᎐N(1)᎐C(27)
166.4(2)
83.2(2)
97.1(1)
83.3(2)
96.5(1)
171.8(1)
167.1(2)
83.3(2)
96.7(2)
83.8(2)
96.2(2)
177.0(2)
111.6(4)
112.3(5)
C(17)᎐N(1)᎐C(27)
C(4)᎐N(2)᎐C(37)
C(4)᎐N(2)᎐C(47)
C(37)᎐N(2)᎐C(47)
C(2)᎐C(1)᎐C(6)
N(1)᎐C(1)᎐C(6)
N(1)᎐C(1)᎐C(2)
C(1)᎐C(2)᎐C(3)
C(2)᎐C(3)᎐C(4)
C(3)᎐C(4)᎐C(5)
N(2)᎐C(4)᎐C(3)
N(2)᎐C(4)᎐C(5)
C(4)᎐C(5)᎐C(6)
C(1)᎐C(6)᎐C(5)
112.2(5)
111.4(5)
111.9(5)
112.4(5)
119.6(5)
121.2(5)
119.2(5)
119.5(6)
121.0(6)
119.7(5)
120.0(5)
120.3(5)
119.9(5)
120.2(5)
ligand are equivalent and appear as a singlet at δ 4.77. The
methylene protons of the co-ordinated end are constrained
and show geminal coupling patterns. The four doublets due to
these protons are shifted slightly upfield. The signals for the
α-pyridine protons are split in a corresponding fashion. The
α-protons of the unco-ordinated pyridines are seen as a doublet
at δ 8.66, whereas the α-protons of the co-ordinated pyrid-
ines are assigned to two doublets at δ 8.24 and 9.19. From the
NMR results it is impossible to determine the co-ordination
geometry of the tridentate ends of the ligand in [Ru(tpbd)2]2ϩ
,
i.e. the ligand might be co-ordinated in either a cis- or trans-
facial or in a meridional fashion. The apparent trigonal geom-
etry around the amine nitrogen atoms in the structures of the
complexes of Zn, Pd and Cu3 suggest a preference for the tri-
dentate end of the ligand for meridional co-ordination. How-
ever one indirect piece of evidence we have to support a possible
facial co-ordination of the tridentate ends of the two 1,4-tpbd
ligands in [Ru(1,4-tpbd)2]2ϩ is the crystal structure of a mono-
nuclear ruthenium() complex of a related phenyl-substituted
bis(picolyl)amine ligand, [Ru(bpba)2][PF6]2 [bpba = N,N-bis-
(2-pyridylmethyl)aniline] which shows cis-facial ligand co-
Anion
Pd(3)᎐S(1)
Pd(3)᎐Cl(3)
Pd(3)᎐Cl(4)
Pd(3)᎐Cl(5)
Pd(4)᎐S(2)
Pd(4)᎐Cl(6)
Pd(4)᎐Cl(7)
2.245(2)
2.311(2)
2.284(2)
2.296(2)
2.256(2)
2.312(2)
2.312(2)
Pd(4)᎐Cl(8)
S(1)᎐O(1)
S(1)᎐C(7)
S(1)᎐C(8)
S(2)᎐O(2)
S(2)᎐C(9)
S(2)᎐C(10)
2.283(2)
1.467(5)
1.759(9)
1.757(8)
1.470(6)
1.763(8)
1.770(9)
Cl(4)᎐Pd(3)᎐S(1)
Cl(5)᎐Pd(3)᎐S(1)
Cl(3)᎐Pd(3)᎐S(1)
Cl(4)᎐Pd(3)᎐Cl(5)
Cl(3)᎐Pd(3)᎐Cl(4)
Cl(3)᎐Pd(3)᎐Cl(5)
Cl(8)᎐Pd(4)᎐S(2)
Cl(7)᎐Pd(4)᎐S(2)
Cl(6)᎐Pd(4)᎐S(2)
Cl(7)᎐Pd(4)᎐Cl(8)
Cl(6)᎐Pd(4)᎐Cl(8)
Cl(6)᎐Pd(4)᎐Cl(7)
94.01(7)
86.79(8)
176.66(6)
177.13(9)
88.01(8)
91.32(8)
90.8(1)
88.89(8)
178.02(9)
172.44(9)
90.3(1)
Pd(3)᎐S(1)᎐O(1)
Pd(3)᎐S(1)᎐C(7)
Pd(3)᎐S(1)᎐C(8)
Pd(4)᎐S(2)᎐O(2)
Pd(4)᎐S(2)᎐C(9)
Pd(4)᎐S(2)᎐C(10)
O(1)᎐S(1)᎐C(8)
O(1)᎐S(1)᎐C(7)
C(7)᎐S(1)᎐C(8)
O(2)᎐S(2)᎐C(9)
O(2)᎐S(2)᎐C(10)
C(9)᎐S(2)᎐C(10)
114.9(2)
114.4(3)
109.3(4)
113.9(3)
114.5(4)
111.3(3)
109.7(5)
107.5(4)
100.1(5)
107.7(4)
108.8(5)
99.5(5)
1
ordination.14 The H NMR signals arising from the aromatic
and aliphatic protons of the co-ordinated end of [Ru(tpbd)2]2ϩ
show similar patterns to those assigned to the corresponding
1
protons of bpba in the H NMR spectrum of [Ru(bpba)2]2ϩ 14
,
supporting the assignment of similar cis-facial co-ordination
geometries.
The cyclic voltammogram of [Ru(1,4-tpbd)2][PF6]2 shows
one reversible wave centered at 1.196 V. This is assigned to a
RuII–RuIII redox process rather than ligand oxidation. Thus lig-
and oxidation is suppressed upon co-ordination to ruthenium.
(A reversible oxidation at 310 mV and an irreversible oxidation
at 670 mV is observed for free 1,4-tpbd.3)
90.15(8)
donor nitrogen atoms and the metal atom to which they are co-
ordinated are almost coplanar with the consequence of a trans
arrangement of two pyridine groups at each metal ion. This
pseudo-meridional co-ordination is expected in the case of the
palladium complex, given the preference of palladium for
square planar geometry. It is, however, a unusual arrangement
for the bis(2-pyridylmethyl)amine ends of the ligand. The fact
that a similar arrangement is present in the copper complex
points towards the fact that a meridional-type arrangement
may be preferred by the ligand. In contrast, the structures of
complexes of bis(picolyl)amine and its N-substituted deriv-
atives show almost exclusively facial co-ordination.13
Polynuclear complexes/co-ordination polymers
The evidence so far suggests that polymeric or oligomeric
nickel() and iron() complexes have been prepared, however in
the absence of crystal structures we have found it difficult
unambiguously to characterize these materials. The relatively
insoluble products obtained from the reaction of 1,4-tpbd
with nickel() and iron() salts are microcrystalline, however
crystals suitable for X-ray crystallography have so far eluded
us. Elemental analyses are consistent with polymeric formul-
ations. The pattern of the pyridine absorptions around 1600
cmϪ1 in the IR spectra of [Fe(1,4-tpbd)]nCl2n and [Ni(1,4-
tpbd)]n[ClO4]2n indicate co-ordination of all the pyridine
groups since they bear greater resemblance to the spectra
obtained for the complexes which show co-ordination of
both tridentate ends of the ligand, i.e. the dinuclear 1,4-tpbd-
bridged palladium() and copper() complexes, compared to
those of the monoco-ordinated zinc() and bis[ruthenium()]-
complex.
A bis-coordinated complex
The mononuclear bis-co-ordinated complex [Ru(1,4-tpbd)2]-
[PF6]2 was prepared from the reaction of 1,4-tpbd and
[RuCl2(C6H5CN)4] in 1:4 proportions in ethanol. The elemental
analysis, mass and NMR spectra confirm the formulation. The
presence of both co-ordinated and unco-ordinated picolyl
groups is clearly evident in the NMR spectra of [Ru(1,4-
1
tpbd)2][PF6]2. The H NMR spectrum is shown in Fig. 3. The
four methylene protons of the unco-ordinated end of the
Mass spectrometry (FAB) supports the assignment of poly-
meric formulations. The spectra of [Fe(1,4-tpbd)]nCl2n and
J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756
1755

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[Ni(1,4-tpbd)]n[ClO4]2n show several peaks consistent with the
mass of the ions expected from decomposition of a polymer,
e.g. peaks can be assigned to ions of the composition 1:2 metal:
ligand and 2:1 metal:ligand ratios. In contrast and as expected,
peaks assignable to 2:1 metal:ligand combinations are
observed while peaks for 1:2 metal:ligand combinations are
absent in the mass spectra of the crystallographically character-
ized dinuclear complexes [Pd2(1,4-tpbd)Cl2][Pd(dmso)Cl3]2 and
[Cu2(1,4-tpbd)(H2O)4][S2O6]2.
References
1 F. Vögtle, Supramolecular Chemistry, Wiley, Chichester, 1991;
Supramolecular Chemistry, eds. V. Balzani and L. De Cola, Kluwer,
Dordrecht, 1992; Transition Metals in Supramolecular Chemistry,
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B. F. Abrahams, M. J. Hardie, B. F. Hoskins, R. Robson and G. A.
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T. Ito and A. Ohto, J. Chem. Soc., Chem. Commun., 1995, 1545;
M. Ohba, N. Maruono, H. Okawa, T. Enoki and J.-M. Latour,
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N. Matsumoto, H. Okawa, N. Re, E. Gallo and C. Floriani, Angew.
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Mater., 1996, 8, 13 and refs. therein.
3 T. Buchen, A. Hazell, L. Jessen, C. J. McKenzie, L. Preuss Nielsen,
J. Z. Pedersen and D. Schollmeyer, J. Chem. Soc., Dalton Trans.,
1997, 2697.
4 M. Graf, B. Greaves and H. Stoeckli-Evans, Inorg. Chim. Acta,
1993, 204, 239; M. Graf and H. Stoeckli-Evans, Acta Crystallogr.,
Sect. C, 1994, 50, 1461.
5 E. C. Constable, A. M. W. Cargill Thompson and S. Greulich,
J. Chem. Soc., Chem. Commun., 1993, 1444; E. C. Constable and
A. M. W. Cargill Thompson, J. Chem. Soc., Dalton Trans., 1995,
1615; New J. Chem., 1992, 16, 855; J. Chem. Soc., Dalton Trans.,
1992, 3467; E. C. Constable, Prog. Inorg. Chem., 1994, 42, 67; M. T.
Indelli, F. Scandola, J.-P. Collin, J.-P. Sauvage and A. Sour, Inorg.
Chem., 1996, 35, 303.
6 J. Cosier and A. M. Glazer, J. Appl. Crystallogr., 1986, 19, 105.
7 A. Altomare, B. Cascarano, C. Giacovazzo, A. Guagliardi, A. M. C.
Burla, G. Polidori and M. Camalli, J. Appl. Crystallogr., 1994, 27,
435.
An effective magnetic moment of ca. 6 µB (µB ≈ 9.27 × 10Ϫ24 J
T
Ϫ1) per iron() at room temperature was obtained for [Fe(1,4-
tpbd)]nCl2n at room temperature. The Mössbauer spectrum at
room temperature shows a doublet with an isomer shift, δ, of
0.803(2) mm sϪ1 and a quadrupole splitting, ∆EQ, of 2.963(4)
mm sϪ1. These results are consistent for high-spin iron().
If co-ordination polymers with 1,4-tpbd, tppz and dpqtpy
are eventually structurally characterized they are expected to
show quite different topologies: the structures of the dinuclear
complexes [Pd2Cl2(1,4-tpbd)]2ϩ and [Cu2(1,4-tpbd)(H2O)4]4ϩ
reveal that the ligand exists in two different conformations. This
is reflected in the steric arrangement of the two ends; the metal
planes in the dipalladium complex are located on the same side
of the benzene linker, whilst a trans arrangement is evident in
the case of the dicopper complex. The cause of these ‘cis’ and
‘trans’ arrangements is probably due simply to crystal-packing
effects. However the consequence of these conformations in
oligomeric systems will be a puckering of the linear molecules.
In the case of dpqtpy, a one-dimensional polymer is expected to
be rod-like; only rotation about the interannular C᎐C bond
linking the terpy-based ends is feasible. The ligand tppz is even
less flexible, although it was shown to be highly twisted in the
structures reported for the dinuclear copper and decanuclear
zinc complex.4
Conclusion
8 W. T. Busing, K. O. Martin and H. A. Levy, ORFLS, Report
ORNL-TM-305, Oak Ridge National Laboratory, Oak Ridge, TN,
1962.
9 International Tables for X-Ray Crystallography, Kynoch Press,
Birmingham, 1974, vol. 4, p. 72.
10 M. N. Burnett and C. K. Johnson, ORTEP III, Report ORNL-6895,
Oak Ridge National Laboratory, Oak Ridge, TN, 1996.
11 R. Melanson, J. Hubert and F. D. Rochon, Acta Crystallogr., Sect.
B, 1976, 32, 1914; N. Veldman and A. L. Spek, Acta Crystallogr.,
Sect. C, 1994, 32, 1572; P. P. Khodadad and N. Rodier, Acta
Crystallogr., Sect. C, 1987, 43, 1690; P. B. Viossat, P. Toffoli,
P. P. Khodadad and N. Rodier, Acta Crystallogr., Sect. C, 1988, 44,
92.
The isolation of a mononuclear complex [ZnCl2(1,4-tpbd)],
and a bis complex, [Ru(1,4-tpbd)2][PF6]2, of 1,4-tpbd opens up
fascinating possibilities, for example the co-ordination of a
second and different metal by the non-co-ordinated tridentate
ends. Co-ordination of a second ligand to starting materials like
the dinuclear [Pd2Cl2(1,4-tpbd)]2ϩ and [Cu2(1,4-tpbd)(H2O)4]4ϩ 3
may lead to similar species. In fact these complexes represent
well characterized examples of the types of building blocks
needed to carry out the ‘complexes-as-metals, complexes-as-
ligands’ approach to the assembly of oligomers proposed by
Constable and Balzani and co-workers.15 Future work will
include the reaction of [ZnCl2(1,4-tpbd)] and [Ru(1,4-tpbd)2]-
[PF6]2 with a second transition-metal ion to give hetero-
12 G. M. Intille, C. E. Pfluger and W. A. Baker, jun., J. Cryst. Mol.
Struct., 1973, 3, 47.
13 S. Larsen, K. Michelsen and E. Pedersen, Acta Chem. Scand., Sect.
A, 1986, 40, 63; J. Glerup, P. A. Goodson, D. J. Hodgson,
K. Michelsen, K. M. Nielsen and H. Wiehe, Inorg. Chem., 1992, 31,
4611; R. J. Butcher and A. W. Addison, Inorg. Chim. Acta, 1989,
158, 211; S. Pal, M. K. Chan and W. H. Armstrong, J. Am. Chem.
Soc., 1992, 114, 6398; S. Pal, M. M. Olmstead and W. H. Armstrong,
Inorg. Chem., 1995, 34, 4708; M. Palaniandavar, T. Pandiyan,
M. Laksminarayanan and H. Manohar, J. Chem. Soc., Dalton
Trans., 1995, 455; A. Hazell, K. B. Jensen, C. J. McKenzie,
O. Simonsen and H. Toftlund, Inorg. Chim. Acta, 1997, 257, 163.
14 A. Hazell, C. J. McKenzie and L. Preuss Nielsen, unpublished
work.
15 G. Denti, S. Serroni, S. Campagna, A. Junis, M. Ciano and
V. Balzani, in Perspectives in Coordination Chemistry, eds. A. F.
Williams, C. Floriani and A. E. Merbach, VCH, Basel, 1992, p. 153;
E. C. Constable, in Transition Metals in Supramolecular Chemistry,
eds. L. Fabbrizzi and A. Poggi, NATO ASI Series, 1994, p. 81.
dinuclear complexes of the type [LnM(1,4-tpbd)ZnCl2]nϩ
,
[LnM(1,4-tpbd)Ru(1,4-tpbd)]nϩ or heterotrinuclear complexes
of the type [LnM(1,4-tpbd)Ru(1,4-tpbd)MLn]nϩ. In summary,
we have now characterized transition-metal complexes with
M:1,4-tpbd ratios of 1:1, 2:1 and 1:2. Using combinations
of these units the strategic build-up of linear heteronuclear
complexes can be envisioned.
Acknowledgements
We are grateful for support from the Danish Natural Science
Council (grant no. 9503162 to C. J. M.). Dr. Thomas Buchen,
Johannes-Gutenberg-Universität-Mainz, Germany, is thanked
for the magnetic susceptibility measurement and Mössbauer
spectrum of the iron complex.
Received 22nd January 1998; Paper 8/00602D
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J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756