Synthesis, characterisation and photochemistry of platinum diselenolenes

Article abstract of DOI:10.1039/b916605j

The reaction between [Pt(PPh₃)₄] and cycloocteno-1,2,3-selenadiazole or bis-cycloocteno-1,4-diselenin in toluene under reflux yielded the poorly soluble mononuclear platinum diselenolene [Pt(Se₂C₈H₁₂)(PPh₃)₂], 1c. Treatment of [Pt(C₂H₄)(PR₃)₂] with a bis-cycloalkeno-1,4-diselenin in a mixture of 1,4-dioxane, THF and toluene under reflux led in good yield to the platinum diselenolenes [Pt(Se ₂C_n+4H_2n+4)(PR₃)₂] (R = Et (2), Bu (3); n = 3 (b), 4 (c)). The analogous complexes [Pt(Se ₂C₈H₁₂)(L)] (L = dppm: 4c; L = dppe: 5c; L = dppp: 6c) were prepared from 1cvia ligand exchange with chelating phosphines. All new compounds have been characterised by multinuclear NMR, IR and UV-visible spectroscopy and mass spectrometry, and their luminescence properties have been examined. The molecular structures of [Pt(Se₂C₇H ₁₀)(PEt₃)₂] (2b), [Pt(Se₂C ₈H₁₂)(PEt₃)₂] (2c) and [Pt(Se ₂C₈H₁₂)(dppm)] (4c) have been determined by X-ray crystallography. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b916605j

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Synthesis, characterisation and photochemistry of platinum diselenolenes†
Christopher P. Morley,*‡a Christopher A. Webster,a Peter Douglas,§a Karen Rofea and Massimo Di Vairab
Received 13th August 2009, Accepted 7th January 2010
First published as an Advance Article on the web 10th February 2010
DOI: 10.1039/b916605j
The reaction between [Pt(PPh3)4] and cycloocteno-1,2,3-selenadiazole or bis-cycloocteno-1,4-diselenin
in toluene under reflux yielded the poorly soluble mononuclear platinum diselenolene
[Pt(Se2C8H12)(PPh3)2], 1c. Treatment of [Pt(C2H4)(PR3)2] with a bis-cycloalkeno-1,4-diselenin in a
mixture of 1,4-dioxane, THF and toluene under reflux led in good yield to the platinum diselenolenes
[Pt(Se2Cn+4H2n+4)(PR3)2] (R = Et (2), Bu (3); n = 3 (b), 4 (c)). The analogous complexes
[Pt(Se2C8H12)(L)] (L = dppm: 4c; L = dppe: 5c; L = dppp: 6c) were prepared from 1c via ligand
exchange with chelating phosphines. All new compounds have been characterised by multinuclear
NMR, IR and UV-visible spectroscopy and mass spectrometry, and their luminescence properties have
been examined. The molecular structures of [Pt(Se2C7H10)(PEt3)2] (2b), [Pt(Se2C8H12)(PEt3)2] (2c) and
[Pt(Se2C8H12)(dppm)] (4c) have been determined by X-ray crystallography.
and palladium18 diselenolenes bearing hydrocarbon substituents
Introduction
may be produced by this route. Palladium diselenolenes can also
Since the 1960s transition metal dithiolenes have attracted the
be prepared from 1,4-diselenins, the products of thermolysis of
attention of chemists, physicists and materials scientists,1 as a
result of their unusual electrochemical and optical properties,
which have many potential applications in fields such as molecular
electronics, infrared dyes, liquid crystals and catalysis.2-4 The lumi-
nescent properties of platinum dithiolenes have received particular
attention.5 By contrast, the selenium analogues of dithiolenes
(diselenolenes) have been little studied,6-12 largely because of the
lack of generally applicable preparative methods.
One possible route to diselenolenes involves the addition of
an activated alkyne to a metal polyselenide,13 as utilised by
us to prepare [Pt{Se2C2(CO2Et)2}(PPh3)2].14 This method is,
however, restricted to diselenolenes bearing electron-withdrawing
substituents. The reactions of 1,2,3-selenadiazoles with low-valent
transition metal compounds have previously been used, by us15 and
others,16 to prepare a wide variety of selenium-containing com-
plexes. In particular we have shown that cyclopentadienylcobalt17
1,2,3-selenadiazoles.19
In 1995 we reported the outcome of the reaction between
[Pt(PPh3)4] and cycloocteno-1,2,3-selenadiazole (4,5,6,7,8,9-
hexahydrocycloocta-1,2,3-selenadiazole).20 We were unable at that
time to characterise the products fully, but have now re-visited
this reaction as the starting point for an investigation of possible
synthetic routes to platinum diselenolenes. We also report here the
results of some preliminary photochemical studies of this class
of compounds, prompted by our recently reported discovery of
near-infrared luminescence from their palladium analogues.21
Results and discussion
Synthesis of [Pt(Se2C8H12)(PPh3)2], 1c
The reaction of [Pt(PPh3)4] with cycloocteno-1,2,3-selenadiazole
in refluxing toluene gave two products, the compound
[Pt(SeC8H12)(PPh3)2] containing a selenaketocarbene ligand,
and a poorly soluble yellow powder with empirical formula
[Pt(Se2C8H12)(PPh3)2]. In addition to poor solubility the latter
product had a high melting point (>300 ◦C) and so was believed,
at the time, to be polymeric. The poor solubility prohibited a
full spectral analysis, but we have now been able to obtain a
MALDI mass spectrum of this species in which the molecular
ion (m/z = 987) for the monomer has high relative abundance. On
the basis of this result, as well as the outcomes of the phosphine
exchange reactions described below, we conclude that the poorly
soluble yellow powder is, in fact, the monomeric diselenolene
[Pt(Se2C8H12)(PPh3)2], 1c (see eqn (1)).
aDepartment of Chemistry, University of Wales Swansea, Singleton Park,
Swansea, SA2 8PP, U.K. E-mail: morleycp@cardiff.ac.uk
bDipartimento di Chimica, Universita` degli Studi di Firenze, Via della
Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy
† Electronic supplementary information (ESI) available: Results of density
functional calculations; further structural details; additional 77Se NMR
and infrared data. CCDC reference numbers 743425–743427. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b916605j
‡ Current address: School of Chemistry, Cardiff University, P. O. Box 912,
Cardiff, CF10 3AT, U.K.
§ Current address: Chemistry Group, Multidisciplinary Nanotechnology
Centre, School of Engineering, Swansea University, Singleton Park,
Swansea SA2 8PP, U.K.
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Due to the moderate yield (26%) reported for 1c (which
was not exceeded in this study) some alternative routes to this
compound were investigated. The reaction of [Pt(PPh3)4] with
bis-cycloocteno-1,4-diselenin in refluxing toluene was found to
give 1c in 33% yield; there was no evidence of the formation
(CH2)4). A similar colour change was observed when [Pt(PPh3)4]
was reacted with cyclohepteno-1,2,3-selenadiazole at room tem-
perature (although the reaction took several days), but not when
cycloocteno-1,2,3-selenadiazole was used. Details of the synthesis
and characterisation of other examples of this type of compound
will be reported separately.25
2
of [Pt(SeC8H12)(PPh3)2]. Reaction of [Pt(h -C2H4)(PPh3)2] with
cycloocteno-1,2,3-selenadiazole in refluxing toluene also gave
Reactions of 1c with trialkylphosphines and chelating diphosphines
1c (eqn (2)). There was again no evidence for the formation
of [Pt(SeC8H12)(PPh3)2] under these conditions, although [Pt(h -
C2H4)(PPh3)2] has been observed to react with cycloocteno-1,2,3-
selenadiazole at room temperature in Et2O to give small amounts
of [Pt(SeC8H12)(PPh3)2].22
Reaction of [Pt(SeC8H12)(PPh3)2] with elemental selenium or
with cycloocteno-1,2,3-selenadiazole in refluxing toluene did
not yield 1c,20 indicating that [Pt(SeC8H12)(PPh3)2] is not an
intermediate in the formation of 1c. On the basis of these
results we postulate that both products of the original reaction
are formed via the well-known thermal decomposition23,24 of
cycloocteno-1,2,3-selenadiazole to generate a selenaketocarbene:
if this intermediate is intercepted by a source of the Pt(PPh3)2
fragment, [Pt(SeC8H12)(PPh3)2] will be formed; otherwise the
selenaketocarbene will dimerise to generate bis-cycloocteno-1,4-
diselenin, which on reaction with “Pt(PPh3)2” will yield 1c
(Scheme 1).
2
Prolonged stirring of 1c with an excess of PR3 or a chelat-
ing diphosphine, L (L = dppm, dppe, dppp), in toluene
at elevated temperatures yielded diselenolenes of the type
[Pt(Se2C8H12)(PR3)2] (R = Et, 2c; Bu, 3c) or [Pt(Se2C8H12)(L)] (L =
dppm, 4c; dppe, 5c; dppp, 6c) (Scheme 2). Since 1c is immobile
on alumina, its separation from the products was straightforward.
The trialkylphosphine derivatives 2c and 3c are more easily and
directly synthesised by an alternative route (see below), and their
full characterisation is discussed later.
Compounds 4c, 5c and 6c were isolated as green-yellow solids
in good yield. They are soluble in a wide range of organic
solvents, and are generally air-stable both in the solid state and
in solution, although standing in CHCl3 does eventually lead to
some decomposition, evidenced by the formation of a blue solid.
Synthesis of [Pt(Se2Cn+4H2n+4)(PR3)2] (n = 3, 4; R = Et, Bu), 2b–c
and 3b–c
Attempted syntheses of [Pt(Se2C6H8)(PPh3)2], 1a, and
[Pt(Se2C7H10)(PPh3)2], 1b
The platinum diselenolenes [Pt(Se2Cn+4H2n+4)(PR3)2] (n = 3,
4; R = Et, Bu) were obtained by refluxing [Pt(C2H4)(PR3)2]
with the corresponding bis-cycloalkeno-1,4-diselenin in a diox-
ane/toluene/THF mixture (eqn (3)). Subsequent column chro-
matography led to the isolation of [Pt(Se2Cn+4H2n+4)(PR3)2] as
pale green solids. The synthesis of 2b–c and 3b–c in this way
provides further evidence that in the reaction of [Pt(PPh3)4]
with cycloocteno-1,2,3-selenadiazole, 1c is produced via bis-
cycloocteno-1,4-diselenin.
Attempts to prepare and isolate pure [Pt(Se2C6H8)(PPh3)2],
1a, and [Pt(Se2C7H10)(PPh3)2], 1b, by analogous procedures
were unsuccessful. Interestingly in the reaction of [Pt(PPh3)4]
2
(or [Pt(h -C2H4)(PPh3)2]) with cyclohexeno-1,2,3-selenadiazole,
the reaction mixture became a purple colour at both room
temperature and at reflux probably due to the formation
1
2
1
2
1
2
=
=
=
of [Pt{SeC(R ) C(R )N NC(R ) C(R )Se}(PPh3)] (R –R
=
(2)
Scheme 1 Postulated mechanism of reaction leading to [Pt(SeC8H12)(PPh3)2] and [Pt(Se2C8H12)(PPh3)2] (1c).
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(3)
Scheme 2 Phosphine exchange reactions of 1c.
◦
˚
The cyclohexeno- derivatives 2a and 3a could not be prepared
by an analogous procedure. It is believed that a cycloalkyne is
generated in the reaction pathway (Scheme 3) and the greater
ring strain of cyclohexyne will inhibit its formation.26 A similar
situation was encountered in the synthesis of [Pd(Se2C6H8)(PBu3)2]
from [Pd2(dba)3]·dba, PBu3 and bis-cyclohexeno-1,4-diselenin,
where a prolonged reflux in xylene was necessary to give the
product;19 in the platinum case the ethereal solvents used in the
in situ synthesis of [Pt(C2H4)(PR3)2] limit the temperature at which
the reaction can be carried out.
We have obtained further indirect evidence for the validity
of this mechanism. The attempted synthesis of diselenolenes of
the type [Pt(Se2Cn+4H2n+4)(L)] (L = dppm, dppe) by reaction of
[Pt(C2H4)(L)] with bis-cycloalkeno-1,4-diselenins was unsuccess-
ful; there was no trace of the desired products. Compounds 4c
and 5c could therefore only be prepared by phosphine exchange
(see above). These results reinforce the proposed inclusion of a
phosphine dissociation step in the reaction pathway, which for
chelating phosphines (dppm, dppe) is not favoured.
Although it is possible to synthesise mononuclear or dinuclear
palladium diselenolenes (Scheme 4),18,19,27 there is no evidence
for the formation of dinuclear platinum diselenolenes in these
reactions. This is probably due to the platinum–phosphine sto-
ichiometry of 1 : 2 imposed by the starting material; with the
palladium compounds the palladium–phosphine stoichiometry
could be altered to give either the mononuclear or dinuclear
product.
Table 1 Selected bond lengths (A) and angles ( ) for 2b
Pt–P(1)
Pt–Se(1)
Se(1) ◊ ◊ ◊ Se(2)
P(1)–Pt–P(2)
P(1)–Pt–Se(1)
2.289(2)
2.4167(8)
3.318(1)
95.72(8)
89.20(5)
Pt–P(2)
2.296(2)
2.4136(9)
Pt–Se(2)
Se(1)–Pt–Se(2)
P(2)–Pt–Se(2)
86.77(3)
88.58(6)
Compounds 2b–c and 3b–c are generally air-stable and soluble
in hexane, toluene, EtOAc, acetone and dichloromethane; they
do however decompose within a few hours on exposure to air in
CHCl3 solution.
X-Ray crystallography†
It was possible to obtain crystals of 2b, 2c, 3b and 4c suitable for
X-ray diffraction by recrystallisation from hexane–dichloro-
methane. However, the crystals of 3b diffracted poorly, and
although it was possible to confirm that the molecular structure
was analogous to that of 2b, satisfactory geometric parameters
were not obtained. The structure of 2c is affected by the disordered
arrangement of several ethyl chains and of the cyclooctene ring
in one of the two independent molecules forming the asymmet-
ric unit. The molecular structures of 2b, 2c and 4c are shown in
Fig. 1, 2 and 3 respectively. Selected bond lengths and angles are
listed in Tables 1, 2 and 3 respectively.
As expected, each molecule has a square-planar PtP2Se2 core.
In the case of 4c the square plane is quite distorted as a result
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◦
◦
˚
˚
Table 3 Selected bond lengths (A) and angles ( ) for 4c
Table 2 Selected bond lengths (A) and angles ( ) for 2c
Pt(1)–P(1)
Pt(1)–P(2)
Pt(1)–Se(1)
Pt(1)–Se(2)
Se(1) ◊ ◊ ◊ Se(2)
P(1)–Pt(1)–P(2)
Se(1)–Pt(1)–Se(2)
P(1)–Pt(1)–Se(1)
P(2)–Pt(1)–Se(2)
2.289(2)
2.283(2)
2.417(1)
2.422(1)
3.321(1)
96.94(6)
86.67(2)
90.03(4)
86.53(5)
Pt(2)–P(3)
Pt(2)–P(4)
Pt(2)–Se(3)
Pt(2)–Se(4)
Se(3) ◊ ◊ ◊ Se(4)
P(3)–Pt(2)–P(4)
Se(3)–Pt(2)–Se(4)
P(3)–Pt(2)–Se(3)
P(4)–Pt(2)–Se(4)
2.282(2)
2.287(2)
2.414(1)
2.429(1)
3.325(1)
98.39(6)
86.73(2)
89.58(4)
85.47(4)
Pt–P(1)
Pt–Se(1)
Se(1) ◊ ◊ ◊ Se(2)
P(1)–Pt–P(2)
P(1)–Pt–Se(1)
2.257(3)
2.395(1)
3.357(2)
73.92(9)
98.30(7)
Pt–P(2)
Pt–Se(2)
2.253(2)
2.404(1)
Se(1)–Pt–Se(2)
P(2)–Pt–Se(2)
88.79(4)
99.18(7)
P angles formed by the monodentate phosphines in the other
two compounds. This will probably result in a poorer Pt–P
orbital overlap, which may be reflected in the lower 31P–195Pt and
31P–77Se coupling constants observed for 4c (see below). Possibly
as a consequence of such a small P–Pt–P angle in 4c the opposite
of the acute P–Pt–P angle (73.92(9)◦) imposed by the dppm
ligand, which is 23◦ smaller than the average of the P–Pt–
Scheme 3 Postulated mechanism of reaction leading to 2b–c and 3b–c (R = Et, Bu; n = 3, 4).
Scheme 4 Preparation of mono- and dinuclear palladium diselenolenes.19
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Fig. 1 View of the molecule in the structure of 2b with the atomic numbering scheme adopted. Thermal ellipsoids are drawn at the 30% probability
level. Hydrogen atoms are omitted for clarity.
Fig. 2 Views of the two independent molecules in the structure of 2c with the atomic numbering scheme adopted. The disordered arrangement of part of
the cyclooctene ring in one of the molecules is shown, but only the major fraction of each of the disordered methyl or ethyl groups is shown for simplicity.
Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.
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respectively for the seven-membered and the eight-membered ring;
these arrangements match those previously verified for the Pd
derivatives (ESI†).19
As a further note to the comparison between closely related Pt
and Pd complexes, the mean value of the present Pt–P distances
˚
˚
in 2c, 2.285(4) A, is 0.045 A smaller than the mean of Pd–
P distances in the previous derivative with the cyclooctene-1,2-
diselenolate ligand,19 characterised by the same unsymmetrical
setting of phosphines as 2c. The Pt–Se distances in 2c, spanning the
˚
range 2.414(1)–2.429(1) A, are closer to the Pd–Se ones (2.4034(8)
˚
and 2.4223(9) A) in the other compound. If, on the other hand,
the Pt–Se bond lengths in all of the present compounds 2b, 2c and
˚
4c (spanning the range 2.395(1)–2.429(1) A, only marginally wider
than that of the 2c molecules) are compared with those presented
by a broader selection of diselenolene derivatives, they are found to
be slightly longer than in the anionic complex [Pt{Se2C2(CF3)2}2]-
(average 2.375(6))28,29 or in the neutral one [Pt(Se2C6H4)(bipy)]
30
˚
(average 2.367(7) A, bipy = 2,2¢-bipyridyl), but agree with
those of the dianionic [Pt{Se2C2(CN)2}2]2- (2.401(1) A).31 The Se–
˚
Pt–Se angles in 2b and 2c (average 86.7(1)◦) are smaller than
those in the [Pt{Se2C2(CF3)2}2]- salts (88.27(7)–91.64(7)◦) and
[Pt(Se2C6H4)(bipy)] (89.6(2)◦); this is probably a result of the large
P–Pt–P angles of 95.72(8)–98.39(6)◦ in 2b and 2c, which are due
to the steric repulsions between ethyl groups. As already said, the
larger Se–Pt–Se angle of 4c is rationalised by the presence of a
small P–Pt–P angle in that compound.
Fig. 3 View of the molecule in the structure of 4c with the atomic
numbering scheme adopted. Thermal ellipsoids are drawn at the 30%
probability level. Hydrogen atoms are omitted for clarity.
Se–Pt–Se angle in that molecule is larger, by ca. 2◦, than the
corresponding angles in 2b and 2c. Also the value of the Se ◊ ◊ ◊ Se
˚
˚
separation in 4c (3.357(2) A) is higher, by at least 0.04 A,
than those found for the other structures. These differences in
the geometry of the metal environment, between 4c and the
other compounds, exceed those existing between 2b and 2c, in
spite of the fact that the latter compounds are formed from
ligands having different cycloalkene ring sizes. The most significant
differences between the coordination geometries of 2b and 2c
are in their P–Pt–Se angles: both molecules of 2c are slightly
unsymmetrical, each with a 3–4◦ difference in the values of its
opposite P–Pt–Se angles, whereas only a 0.6◦ difference is found
for the 2b molecule, which is essentially symmetrical. Also the
arrangements of the triethylphosphine ligands are different in
the two compounds, as may be appreciated from Fig. 1 and 2:
that of 2b is symmetrical with respect to a plane bisecting the
molecule, normal to the coordination plane, whereas those of both
molecules of 2c are unsymmetrical. Asymmetry of the metal atom
environment and of the phosphine arrangement, similar to that
of the present 2c molecules, was previously detected for a set of
palladium diselenolene complexes having tributylphosphine co-
ligands.19 These aspects are considered in detail in the ESI,† with
the support of the results of quantum mechanical calculations:
the same factors that were considered to be responsible for the
asymmetry in the palladium complexes appear to be effective also
in the present compounds.
NMR spectroscopy
The NMR spectroscopic data for compounds 2b–c, 3b–c, 4c, 5c
and 6c are shown in Tables 4, 5 and 6, and are in accord with the
proposed structures.
The 77Se NMR spectra show the patterns expected for the X
part of an AA¢X spin system (a result of the 31P nuclei being
chemically but not magnetically equivalent). These consist of five
lines (ignoring the satellites due to 77Se–195Pt coupling) from which
the coupling constants 1J(77Se–195Pt), 2J(77Se–31Pcis), 2J(77Se–31Ptrans
)
and J(31P–31P) can be established by calculation based on the
relative intensities of the lines (these parameters are usually more
precisely accessible by analysis of the 77Se satellite structure of the
single 31P resonance, but difficulties were encountered in obtaining
31P NMR spectra with sufficiently good signal-to-noise ratios). All
five lines of the AA¢X pattern were observed for 2b–c, 3b–c and
5c; as an example Fig. 4 shows the 77Se resonance of 3c.
2
There are large differences between the cis- and trans- coupling
constants; the small 77Se–31Pcis coupling in these complexes is
in part the reason why the AA¢X pattern was not always fully
resolved. For 2b–c and 3b–c the two 31P–77Se coupling constants
2
2
(avg. J(77Se–31Ptrans) = 65 Hz; avg. J(77Se–31Pcis) = 16 Hz) are
almost identical to those of the analogous palladium complexes
[Pd(Se2Cn+4H2n+4)(PBu3)2] (avg. J(77Se–31Ptrans) = 66 Hz, J(77Se–
2
2
31
In all present structures the metal and ligand atoms, as well as
the nearest four carbon atoms of the cycloalkene ring, lie in a plane,
P ) = 16 Hz).19 The calculated 31P–31P coupling constants are
cis
quite different however: avg. 2J(31P–31P) = 19 Hz, cf . 2J(31P–31P) =
with largest deviations from the plane of 0.151(2) A (P(2) in 2b),
44 Hz in [Pd(Se2Cn+4H2n+4)(PBu3)2]. The calculated 2J(77Se–31Ptrans
)
˚
˚
˚
0.124(2) A and 0.295(7) A (P(2) and, respectively, C(28) in the two
coupling constant of 86 Hz for compound 5c is greater than
those for 2b–c and 3b–c; this could be a result of the geometrical
constraints placed on the PtSe2P2 core by the chelating phosphine,
or reflect the change from a trialkyl- to a diarylalkylphosphine.
˚
molecules of 2c) and 0.27(1) A (C(8) in 4c). In all the structures,
the other atoms deviate considerably from such molecular planes,
with ring conformations of the chair and open envelope types,
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Table 4 1H and 13C NMR spectroscopic data for 2b–c, and 3b–c in d6-acetone solution
Compound
2b
2c
3b
3c
1H: d (ppm)
a-CH2
b-CH2
2.29–2.50 (4H, m)
1.27–1.35 (4H, m)
1.75–1.83 (2H, m)
1.78–1.90 (12H, m)
—
2.42–2.56 (4H, m)
1.24–1.34 (4H, m)
1.41–1.57 (4H, m)
1.90–2.10 (12H, m)
—
2.40–2.50 (4H, m)
1.25–1.60 (4H, m)
1.85–2.10 (2H, m)
1.85–2.10 (12H, m)
1.25–1.60 (24H, m)
0.81 (18H, t)a
2.41–2.46 (4H, m)
1.24–1.50 (4H, m)
1.90–2.01 (4H, m)
1.90–2.01 (12H, m)
1.24–1.50 (24H, m)
0.80 (18H, t)a
g-CH2
CH2P
CH2CH2CH2
CH3
0.89 (18H, dt)a
0.99 (18H, dt)a
13C: d (ppm)
a-CH2
b-CH2
g-CH2
CH2P
39.3f
33.8
27.7
17.9c
—
37.0f
31.2
26.7
18.0c
—
39.3f
33.8
37.0f
31.2
27.7
26.7
25.9c
24.5d
26.7e
13.9
26.0c
24.5d
26.7e
14.0
CH2CH2CH2
CH3CH2
CH3
—
—
8.2b
131.1
8.2b
130.2
=
C
C
130.8
130.0
a
3J(1H–1H) = 7.6 Hz (2b–c, 3c), 7.2 Hz (3b), 3J(1H–31P) = 16 Hz (2b–c). b 3J(13C–195Pt) = 20 Hz (2b), 24 Hz (2c). c AA¢X system, avg. J(13C–31P) = 16 Hz
(2b), 18 Hz (2c, 3b–c), 2J(13C–195Pt) = 26 Hz (2b–c), 25 Hz (3b–c). d AA¢X system, avg. J(13C–31P) = 7 Hz. e 4J(13C–195Pt) = 19 Hz. f 2J(13C–195Pt) = 45 Hz
(2b), 44 Hz (2c), 47 Hz (3b), 46 Hz (3c).
Table 5 1H and 13C NMR spectroscopic data for 4c, 5c and 6c in CDCl3 solution
Compound
4c
5c
6c
1H: d (ppm)
a-CH2
2.73–2.90 (4H, m)
1.56–1.69 (4H, m)
1.32–1.46 (4H, m)
4.61 (2H, t)a
2.55–2.70 (4H, m)
1.29–1.59 (4H, m)
1.29–1.59 (4H, m)
2.20–2.45 (4H, m)
—
7.25–7.45 (12H, m)
7.70–7.80 (8H, m)
2.45–2.70 (4H, m)
1.20–1.60 (4H, m)
1.20–1.60 (4H, m)
1.20–1.60 (4H, m)
0.76-0.90 (2H, m)
7.15–7.45 (12H, m)
7.50–7.65 (8H, m)
b-CH2
g-CH2
CH2P
CH2CH2CH2
C6H5-o, p
C6H5-m
—
7.25–7.42 (12H, m)
7.72–7.78 (8H, m)
13C: d (ppm)
a-CH2
b-CH2
37.3b
31.4
36.7b
30.3
37.6b
31.2
g-CH2
27.1
26.1
27.1
CH2P
48.0c
28.5c
—
26.5c
CH2CH2CH2
C6H5-o
C6H5-ipso
C6H5-p
C6H5-m
=
—
24.5
133.6d
132.6
131.9
129.5e
Not observed
133.1d
130.9
130.7
128.2e
139.2
134.1d
Not observed
131.2
128.4e
C
C
Not observed
a
2J(1H–31P) = 10 Hz, 3J(1H–195Pt) = 42 Hz. b 3J(13C–195Pt) = 49 Hz (4c), 47 Hz (5c, 6c). c 1J(13C–31P) = 31 Hz (4c), for 5c complex multiplet with avg.
J(13C–31P) = 21 Hz, for 6c coupling unresolved. d AA¢X system, avg. J(13C–31P) = 6 Hz (4c, 5c), 5 Hz (6c). e AA¢X system, avg. J(13C–31P) = 6 Hz (4c),
5 Hz (5c, 6c).
One point of note is that the 77Se resonances of compounds
2b and 3b are at much lower field (approximately 30 ppm)
than those of 2c and 3c. This phenomenon is also observed in
other diselenolenes,18,19 1,2,3-selenadiazoles and 1,4-diselenins (see
ESI†): the number of carbon atoms in the aliphatic ring appears
to have an influence on the electronic environment of the selenium
atoms, with the C7 ring giving lower field 77Se NMR resonances
than the C6 and C8 ring analogues. It is not clear why this is
the case, nor whether it is limited to the C6–C8 ring sizes. This
effect of ring size upon chemical shift is also observed in the
13C NMR spectra of these compounds:32 the resonances of the
aliphatic ring carbons are all at lower field for 2b and 3b. The
UV-visible spectroscopy
The colours of 2b–c and 3b–c (pale green/yellow) differ signifi-
cantly from those of their palladium analogues [Pd(Se2Cn+4H2n+4)-
(PBu3)2] and [Pd2(Se2Cn+4H2n+4)2(PBu3)2] (purple).24 The UV-
visible spectra of the palladium diselenolenes show absorption
maxima in both the UV and visible regions; by contrast com-
pounds 2b–c, 3b–c, 4c, 5c and 6c absorb relatively weakly in the
visible region with absorption maxima in the UV region only
(see Table 7). This indicates that the lowest excited state in these
compounds is unlikely to have MLCT character, and is therefore
different from that in a related series of bis(phosphine)platinum
dithiolenes.33 There are two significant differences between the two
systems: firstly, the change from sulfur to selenium makes the lig-
and more electron-rich; secondly, the dithiolenes concerned carry
=
resonances associated with the C C group were weak, and as such
the coupling constants associated with these atoms could not be
determined.
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Table 6 31P and 77Se NMR spectroscopic data for 2b–c, 3b–c in d6-acetone solution and 4c, 5c and 6c in CDCl3 solution
Compound
2b
2c
3b
3c
4c
5c
6c
31P
d (ppm)
2.5
2767
2.4
2781
-5.3
2767
-5.1
2780
-47.4
2377
44.2
2784
-6.6
2676
1J(31P–195Pt)/Hz
77Se
d (ppm)
481
268
41
14
67
441
268
40
16
63
474
264
40
16
64
454
256
41
18
64
457
294
35
452
268
46
6
86
7
466
260
47
1J(77Se–195Pt)/Hz
avg. 2J(77Se–31P)/Hz
2J(77Se–31Pcis)/Hz
2J(77Se–31Ptrans)/Hz
2J(31P–31P)/Hz
a
a
—
—
—
—
a
a
a
a
20
20
19
18
—
—
a AA¢X pattern not fully resolved.
The red shift in the lowest energy absorption band for com-
pound 4c is intriguing. We speculate that the distortion at platinum
imposed by the four-membered chelate ring is responsible, as com-
plexes with monodentate phosphines resemble the dppp complex
6c, and the behaviour of the dppe complex 5c is intermediate.
Photochemistry
Compounds 1c, 2c, 3b, 4c, 5c and 6c were examined for lumines-
cence under steady-state (usually 380 nm) and pulsed laser UV
(355 nm) excitation in: the solid state at room temperature and
at 77 K; aerated and nitrogen-purged room temperature (RT)
toluene solution (except 1c because of solubility problems); a
toluene–diethyl ether–ethanol (1 : 2 : 1) (TDE) glass at 77 K (again
excepting 1c because of solubility problems). Table 8 collects
photochemical data.
In nitrogen-purged toluene solution all the soluble platinum
diselenolenes show a weak and narrow emission band at ca. 650–
660 nm (see Fig. 6 for an example) with a shoulder at ca. 700 nm
which is perhaps a vibronic feature. Emission decays are acceptable
fits to single exponentials, the lifetimes are quite long (ca. 50–
90 ms), and quantum yields are low. We assign this RT emission
to molecular phosphorescence because of the long lifetime and
the observation that it is quenched by oxygen at the diffusion-
controlled rate.
In the solid state at 77 K all the complexes exhibit long-lived
intense emission. Emission decays are acceptable fits to single
exponentials with lifetimes of ca. 14–20 ms. The emission spectra
are all broad and structureless (Fig. 5) but show emission maxima
ranging from ca. 550–680 nm depending upon the ligands. Only 1c
and 4c show strong emission in the solid state at RT, and this is even
broader than that at 77 K and is somewhat reduced in intensity
(Fig. 5). Examination of the emission decay kinetics across the
emission bands for these two compounds, i.e. 1c and 4c, showed
no wavelength dependence of emission decay at either 77 K or RT.
For 2c and 6c a combination of a small sample and relatively weak
emission made collection of data difficult. For all the compounds
solid state room temperature decay lifetimes are ca. 1.4–3.2 ms,
significantly shorter than those at 77 K. It is interesting to note
that while the emission quantum yield for 4c increases by perhaps
only a factor of 1.5 in going from RT to 77 K the lifetime increases
by a factor of 10.
Fig. 4 77Se NMR spectrum of 3c in CDCl3 solution.
Table 7 Solution phase UV-visible data for compounds 2b–c, 3b–c, 4c,
5c and 6c
Compound
l
max/nm (e/cm-1 M-1)
2ba
2ca
3ba
3ca
4cb
5cb
6cb
220 (26 400), 235 (29 500), 320 (4500)
215 (26 300), 240 (29 200), 320 (4700)
220 (28 100), 240 (36 900), 320 (5100)
215 (37 500), 240 (37 500), 320 (5500)
365 (3700)
350 (3050)
325 (4000)
a Recorded in CH2Cl2 solution (10-5 M) between 200 and 800 nm.
b Recorded in MeCN solution (10-4 M) between 300 and 800 nm; typically
there is a 5 nm bathochromic shift on changing the solvent to toluene.
When examined at 77 K in a TDE glass the general behaviour of
all the compounds is similar in that an intense broad emission band
with emission maximum in the 580–620 nm range is observed.
electron-withdrawing substituents, making them much better
p-acceptors.
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Table 8 Photophysical data for compounds 1c, 2c, 3b, 4c, 5c, and 6c
Emission in nitrogen-purged room temperature
toluene solution
Emission in toluene–diethyl
ether–ethanol (1 : 2 : 1) glass at 77 K
Emission in the solid state
298 K 77 K
max/nm t/10-6 max/nm t/10-6
Compound lmax/nm t/10-6
s
kq/1010 M-1 s-1 Uphos
l
max/nm
t/10-6
s
l
s
l
s
1c
675
3.21 0.02 675
16.7 0.7
2c
3b
4c
650
647c
647
50
80
2
6
600
614
22.5 1.1 Weakb
18.3 1.7 Weakb
10.8 1.2 630
60.1 0.7f
672d
1.89 0.04 619
14.2 0.5
20.1 1.5
1.08a
0.0007 0.0001e 600 (broad)
645 (sharp)
581
610
5c
6c
662
649
87
55
7
5
22.8 2.1 Weakb
22.7 2.5 Weakb
600
14.7 0.9
14.1 0.6
1.51 0.05 556
a From a comparison of emission lifetimes in nitrogen-purged and air-equilibrated solutions. b Weak emission and broad noisy spectrum made assignment
of lmax difficult. c Weak emission. d Excitation at 470 nm. e Excitation at 416 nm. f Long-lived component from a double exponential fit.
Fig. 5 Emission spectra of 4c in the solid state at room temperature and 77 K. Insets show emission decay curves, with best-fit single exponentials (solid
lines).
However for 4c (Fig. 6) there is also a distinct feature at 645 nm
which, by comparison with the solution phase emission spectrum
we assign to molecular phosphorescence. Decay kinetics for the
broad emission, measured at 600 nm, and the sharp band, at
645 nm, are different, with the latter showing a long-lived compo-
nent (Table 8). Because of the presence of this phosphorescence
band in the emission spectrum of 4c and the two decay times, we
tentatively suggest that both the broad emission component seen
in TDE glasses at 77 K and the broad band solid state emission
are intermolecular in origin, from either the solid state lattice,
or aggregates in the glass. Initially we thought that the emissions
might have a similar origin to the broad band emission seen in
stacked platinum complexes such as the tetracyanoplatinates,34
but the X-ray diffraction data for 2b, 2c and 4c show the closest
intermolecular distances to be greater than 500 pm which is too
large for this to be the case.
structure. A similar band can be observed with varying clarity
when any of the complexes is irradiated with UV light under any
of the conditions we have used, i.e. aerated or nitrogen-purged
solution, solid state at RT or 77 K, or TDE glass at 77 K, even
when the broad orange/red emission is absent. The short lifetime
and position suggest this feature may be fluorescence.
We are currently examining the photochemistry of these com-
plexes in more detail in an attempt to identify the origins of these
various emission bands
Conclusion
The poorly soluble product of the reaction between cycloocteno-
1,2,3-selenadiazole and [Pt(PPh3)4] has been confirmed as the
mononuclear platinum diselenolene [Pt(Se2C8H12)(PPh3)2], 1c. The
phosphine ligands in 1c may be exchanged to generate more
soluble species, including those containing the chelating bidentate
phosphines dppm, dppe and dppp. The complexes containing
trialkylphosphines are best prepared by the reaction between
[Pt(C2H4)(PR3)2] (R = Et, Bu) and a bis-cycloalkeno-1,4-diselenin,
Fig. 7 shows the emission from 3b in TDE glass at 77 K. We
include this spectrum because it most clearly shows an additional
emission band in the blue spectral region which is short lived (t <
ca. 25 ns, the time resolution of our ns laser system) with vibronic
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Fig. 6 Emission spectra of 4c in room temperature nitrogen-purged toluene solution (dashed line) and in a toluene–diethyl ether–ethanol (1 : 2 : 1) glass
at 77 K (solid line).
Fig. 7 Emission spectrum of 3b in a toluene–diethyl ether–ethanol (1 : 2 : 1) glass at 77 K.
and this represents an efficient and potentially versatile route to
platinum diselenolenes. These compounds display unusual lumi-
nescent properties, which are the subject of ongoing investigation
in our laboratories.
molten sodium (1,4-dioxane, toluene) or CaH2 (CHCl3, CH2Cl2,
MeCN). Solvents were degassed by repeated boiling under reduced
pressure followed by saturation with inert gas.
Cycloalkeno-1,2,3-selenadiazoles,23 bis-cycloalkeno-1,4-dise-
lenins,24 [Pt(PPh3)4],35 [Pt(Se2C8H12)(PPh3)2] (1c),20 [PtCl2(PEt3)2]36
and [PtCl2(PBu3)]37 were synthesised by literature procedures
(slightly adapted in some cases22). Platinum salts were obtained on
loan from Johnson Matthey plc; PEt3 was obtained from Strem
Chemicals Inc.; cyclohexanone, cycloheptanone, cyclooctanone,
dppm, dppe, dppp, ethene, naphthalene, PBu3, P(i-C5H11)3,
P(OMe)3, potassium, and sodium were obtained from Aldrich
Chemical Company. All chemicals were used as supplied.
Experimental
Materials and general procedures
All the reactions below were performed using standard Schlenk
techniques under an atmosphere of dry argon. Dry solvents, when
required, were distilled over molten potassium (hexane, THF),
3186 | Dalton Trans., 2010, 39, 3177–3189
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Physical measurements
toluene (8 mL). The reaction mixture was heated to reflux for 1 h
after which time it was pale green in colour; concentration in vacuo
gave a green oil, which was purified by column chromatography on
alumina with a 3 : 1 mixture of toluene and ethyl acetate. Collection
of the pale green band gave 2b–c and 3b–c as analytically pure
yellow solids. Yields: 2b, 218 mg, 80%; 2c, 192 mg, 69%; 3b,
201 mg, 59%; 3c: 218 mg, 63%. NMR spectroscopic data for 2b–c
and 3b–c are summarised in Tables 4 and 6. FAB MS, m/z (relative
abundance, assignment): 2b, 685 (50, [M]+), 431 (100, [Pt(PEt3)2]+);
2c, 699 (20, [M]+), 431 (100, [Pt(PEt3)2]+); 3b, 839 (30, [M]+), 599
(100, [Pt(PBu3)2]+); 3c, 867 (30, [M]+), 599 (100, [Pt(PBu3)2]+).
Crystals of 2b and 2c suitable for an X-ray diffraction study were
obtained by recrystallisation from toluene–hexane at -20 ◦C.
1H, 13C and 31P NMR spectra were recorded at 400.1 MHz,
100.6 MHz and 162.0 MHz respectively using a Bruker AC400
with tetramethylsilane as internal standard (1H, 13C) or 85%
phosphoric acid as external standard (31P). 77Se NMR spectra
were recorded at 47.7 MHz on a Bruker WM250 with dimethyl
selenide as external standard. All 13C, 31P and 77Se NMR spectra
were 1H decoupled. IR spectra were recorded using a Perkin-
Elmer Spectrum One FTIR spectrometer with ATR; the data
obtained are included in the ESI. UV-visible spectra were recorded
on either a Unicam UV300 spectrometer or an HP 8452A diode
array spectrophotometer, using quartz cuvettes. Mass spectra were
recorded by the EPSRC Mass Spectrometry Service Centre using
fast atom bombardment (FAB) or MALDI (for 1c); m/z values
have been rounded to the nearest integer; assignments are based
X-Ray crystallography†
on isotopomers containing H, 12C, 31P, 80Se and 195Pt; expected
isotope distribution patterns were observed.
1
X-Ray diffraction data for 2b, 2c and 4c were collected at
room temperature on an Oxford Diffraction Xcalibur 3 CCD
diffractometer, using Mo Ka radiation. Crystal data and the main
data collection and structure refinement parameters are given in
Table 9.
Syntheses of 2c and 3c from [Pt(Se2C8H12)(PPh3)2], 1c
Under argon in a pre-dried Schlenk tube, 1c (50 mg, 0.05 mmol)
Lattice constants were determined from the settings of 6990 (2b),
13 233 (2c) and 1962 (4c) reflections. Corrections for absorption
were applied with SADABS.38 The structures of 2b and 2c were
solved by direct methods39 and that of 4c by heavy-atom methods;
they were all refined by full-matrix least-squares on F2 values,
with SHELXL.40 In the case of the acentric 4c the absolute
structure was assigned on the basis of R values and Flack’s test;41
a correction for the presence of a small twinning component
generated by rotation around [100] was also applied. All non–
hydrogen atoms were refined anisotropically and hydrogens were
placed in idealized positions, each riding on the respective carrier
atom, with UH = 1.2UCeq (UH = 1.5UCeq for methyl groups). Effects
of disorder in the chains of 2c were accounted for. Programs used
in the crystallographic calculations included PARST for analysis
of geometries,42 and ORTEP for graphics.43
was slurried in dry toluene (4 mL); the slurry was treated with 1 mL
of PEt3 (2c) or PBu3 (3c) and stirred at approximately 70 C for
◦
3 days. After this time a green/yellow solution had formed. This
was concentrated in vacuo and the residue chromatographically
separated on Al2O3 with toluene–EtOAc (1 : 1) elution; collection
of the green/yellow fraction and subsequent concentration in
vacuo gave 2c (yellow solid, 32 mg, 93% yield) or 3c (yellow solid,
40 mg, 90% yield).
Syntheses of 4c, 5c and 6c from [Pt(Se2C8H12)(PPh3)2], 1c
Under argon in a pre-dried Schlenk tube, a mixture of 1c (60 mg,
0.06 mmol) and dppm (230 mg, 0.6 mmol), dppe (240 mg,
0.6 mmol) or dppp (250 mg 0.6 mmol) in dry toluene (8 mL)
◦
was heated to 90 C for 4 days. After this time a green solution
had formed. This was concentrated in vacuo, and the residue
chromatographically separated on alumina with toluene–CH2Cl2
(1 : 1) elution; collection of the green fraction and subsequent
concentration in vacuo gave 4c (yellow solid, 44 mg, 86% yield), 5c
(yellow solid, 36 mg, 69% yield) or 6c (yellow solid, 36 mg, 68%
yield). NMR spectroscopic data for 4c, 5c and 6c are summarised
in Tables 5 and 6. FAB MS, m/z (relative abundance, assignment):
4c, 847 (100, [M]+); 5c, 861 (100, [M]+); 6c, 875 (100, [M]+). Crystals
of 4c suitable for an X-ray diffraction study were obtained from a
hexane–CH2Cl2 solution at -20 ◦C.
Photochemistry
Steady-state room temperature and 77 K emission measurements
were performed using a Perkin Elmer MPF-44E fluorescence
spectrometer with a 150 W xenon arc lamp and a Hamamatsu
R928 photomultiplier. An excitation wavelength of 380 nm was
generally used, with excitation and emission slitwidths of between
6 nm and 8 nm. A 420 nm cut-off filter on the emission
monochromator was used for studies at wavelengths longer
than 450 nm. At room temperature a 1 cm ¥ 1 cm quartz
fluorescence cell was used; at 77 K an NMR tube cooled by
liquid nitrogen in a phosphorescence Dewar was used. The
quantum yield of room temperature phosphorescence for 4c
was measured using tetraphenylporphyrin in ethanol at 298 K
(Uem = 0.15)44 as emission standard. Nanosecond laser kinetic
measurements were obtained using an Applied Photophysics Laser
Kinetic Spectrometer with the 355 nm emission from a Spectron
frequency tripled Nd/YAG laser as excitation pulse, and emission
slitwidths of 19 nm. The averaged data from 8–16 decay curves
were recorded using a LeCroy 9304AM oscilloscope. For samples
which gave relatively weak emission, amplification of the laser
pulse was used in order to improve the signal-to-noise ratio. Low
Syntheses of 2b–c and 3b–c from [Pt(C2H4)(PR3)2]
All experiments were carried out according to the following
general procedure. Under argon [PtCl2(PR3)2] (0.4 mmol; R =
Et, Bu) was taken up in 1,4-dioxane (20 mL) and the solution
degassed. This solution was put under an ethene atmosphere
and treated dropwise with NaC10H8 (~0.12 M solution in THF),
complete conversion to [Pt(C2H4)(PR3)2] being evidenced by the
solution holding the green colour for approximately 2–3 min
before clearing. The ethene atmosphere was replaced with an
argon atmosphere, and the reaction mixture was treated with a
solution of bis-cycloalkeno-1,4-diselenin (0.6 mmol) in degassed
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Table 9 Crystallographic data collection and refinement parameters for 2b, 2c and 4c
Compound
2b
2c
4c
Empirical formula
Formula weight
T/K
C19H40P2PtSe2
683.46
293(2)
C20H42P2PtSe2
697.48
293(2)
C33H34P2PtSe2
845.55
293(2)
˚
Wavelength/A
0.71069
0.71069
0.71069
Crystal system
Orthorhombic
Pbca
11.584(1)
15.516(2)
27.248(2)
90.00
90.00
90.00
4897.5(8)
8
1.854
8.834
2640
0.45 ¥ 0.40 ¥ 0.36
4.04 to 26.37
34 446
4986 [0.0761]
3813
Triclinic
P1
Orthorhombic
Pc21n
13.504(1)
14.049(2)
16.183(2)
90.00
90.00
90.00
3070.2(6)
4
1.829
7.067
1632
0.40 ¥ 0.40 ¥ 0.25
4.05 to 27.33
27 746
6358 [0.0610]
4882
¯
Space group
˚
a/A
12.836(1)
14.189(1)
16.900(1)
110.62(1)
108.63(1)
99.46(1)
2593.5(2)
4
1.786
8.343
1352
0.70 ¥ 0.60 ¥ 0.40
4.26 to 26.37
18 863
10048 [0.0265]
8049
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
Volume/A
Z
Dc/Mg m-3
Absorption coefficient/mm-1
F(000)
Crystal size/mm
q range for data collection/◦
Reflections collected
Independent reflections [Rint]
Observed reflections [I > 2s(I)]
Parameters [restraints]
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
229
1.001
560 [348]
1.082
344 [1]
1.061
R1 = 0.0490
wR2 = 0.1256
R1 = 0.0649
wR2 = 0.1356
2.310/-2.251
R1 = 0.0324
wR2 = 0.0776
R1 = 0.0454
wR2 = 0.0812
1.197/-1.180
R1 = 0.0489
wR2 = 0.1137
R1 = 0.0596
wR2 = 0.1207
2.457/-1.799
R indices (all data)
-3
˚
Largest diff. peak/hole/e A
temperature emission spectra of frozen solutions were recorded in
a toluene–diethyl ether–ethanol (1 : 2 : 1) glass.45 The kinetic data
P. Jorchel, T. Gelbrich, J. Sieler and R. Kirmse, Z. Anorg. Allg. Chem.,
1996, 622, 1979–1986.
8 S. Currell, P. Deplano, M. L. Mercuri, L. Pilla, A. Serpe, F. Bigoli, M. A.
Pellinghelli, E. Coronado, C. J. Go´mez-Garcia and E. Canadell, J. Solid
State Chem., 2002, 168, 653–660; J. C. Dias, X. Ribas, J. Morgado, J.
Seic¸a, E. B. Lopes, I. C. Santos, R. T. Henriques, M. Almeida, K. Wurst,
P. Foury-Leylekian, E. Canadell, J. Vidal-Gancedo, J. Veciana and C.
Rovira, J. Mater. Chem., 2005, 15, 3187–3199; S. Eid, M. Fourmigue´,
T. Roisnal and D. Lorcy, Inorg. Chem., 2007, 46, 10647–10654.
9 W. B. Heuer, P. J. Squattrito, B. M. Hoffman and J. A. Ibers, J. Am.
Chem. Soc., 1988, 110, 792–803; F. Wudl, E. T. Zellers and S. D. Cox,
Inorg. Chem., 1985, 24, 2864–2865.
were analysed using Jandel Table-CurveTM
.
Acknowledgements
We thank the University of Wales Swansea and EPSRC for
the provision of research studentships to C. A. W. and K. R.
respectively, Johnson Matthey plc for the loan of platinum salts,
and the Ministero dell’ Universita` e della Ricerca Scientifica e
Tecnologica for financial support to M. di V.
10 C. M. Bollinger and T. B. Rauchfuss, Inorg. Chem., 1982, 21, 3947–
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