Dinuclear diselenolenes derived from cycloalkeno-1,2,3-selenadiazoles and tetrakis(triphenylphosphine)palladium

Article abstract of DOI:10.1039/a807666i

Reaction of [Pd(PPh₃)₄] with cycloalkeno-1,2,3-selenadiazoles or bis(cycloalkeno)-1,4-diselenines in toluene under reflux led in good yield to the dinuclear diselenolenes [Pd₂{SeC(R¹)=C(R²)Se}₂

Full text of DOI:10.1039/a807666i

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Dinuclear diselenolenes derived from cycloalkeno-1,2,3-
selenadiazoles and tetrakis(triphenylphosphine)palladium
Susan Ford,a Pawan K. Khanna,a Christopher P. Morley*a and Massimo Di Vairab
a Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea,
UK SA2 8PP. E-mail: c.p.morley@swan.ac.uk
b Dipartimento di Chimica, Università degli Studi di Firenze, Via Maragliano 75 77,
/
50144 Firenze, Italy
Received 2nd October 1998, Accepted 15th January 1999
Reaction of [Pd(PPh3)4] with cycloalkeno-1,2,3-selenadiazoles or bis(cycloalkeno)-1,4-diselenines in toluene under
reflux led in good yield to the dinuclear diselenolenes [Pd {SeC(R1)᎐C(R2)Se} (PPh ) ] [R1,R2 = (CH ) ; n = 4, 5, 6]
᎐
2
2
3
2
2 n
which have been characterised by microanalysis, multinuclear NMR, IR, and mass spectroscopy; the molecular
structure of the compound with n = 6 has been determined by X-ray cystallography.
Transition metal dithiolenes have attracted considerable atten-
tion over the past thirty years, as a result of their potentially
useful electrochemical and optical properties.1 Possible areas of
application include molecular electronics, infrared dyes, liquid
crystals and catalysis.2–4 By contrast, studies of the selenium
analogues of dithiolenes (diselenolenes) have been rare.5–9 This
is largely because of the absence of generally applicable syn-
thetic methods. Diselenolenes are, however, attractive synthetic
targets since replacement of sulfur by selenium should lead to a
decrease in the HOMO–LUMO gap, and the enhancement of
intermolecular interactions in the solid state.
The reactions of 1,2,3-selenadiazoles with low-valent transi-
tion metal compounds have previously been used, by us10 and
others,11 to prepare a wide variety of selenium-containing com-
plexes. In particular we have shown that cyclopentadienylcobalt
diselenolenes may be produced by this route [eqn. (1)].12
N
[Pd(PPh3)4]
+
(CH2)n
N
Se
Ia: n = 4; Ib: n = 5; Ic: n = 6
Toluene
Heat, 30-40%
Se
Ph3P
)
(CH
n
Pd
Se
Se
Pd
Se PPh3
1a: n = 4; 1b: n = 5; 1c: n = 6
(2)
(CH2)n
phosphine: no long-lived intermediate appears to be formed.
We believe the first step of the mechanism to be the decom-
position of the 1,2,3-selenadiazole to form a 1,4-diselenine IIa–
IIc [eqn. (3)]. This conversion is normally accomplished by
R
R1
R
R
N
+
+
Sen
N
R
R
L
Co
R2
Se
Se
N
L
Heat
(CH2)n
N
(CH2)n
(CH2)n (3)
R1, R2 = H, Ph,
(CH2)n, etc.
R = H, Me;
L = CO, PPh3
-N2
Se
Se
IIa: n = 4; IIb: n = 5; IIc: n = 6
Ia: n = 4; Ib: n = 5; Ic: n = 6
Toluene
Heat
heating the 1,2,3-selenadiazole to 140 ЊC under reduced pres-
sure.14 We therefore assume that it is catalysed by Pd0 in this
case. Support for our hypothesis comes from the observation
that compounds 1a–1c are formed in slightly higher yield, if, in
place of the 1,2,3-selenadiazoles, the analogous 1,4-diselenines
IIa–IIc are themselves treated with [Pd(PPh3)4] [eqn. (4)]. The
detailed mechanism of this reaction remains unclear but we
R
R
Se
Se
R1
R2
(1)
R
Co
R
R
We now present the results of a study of the reactions of
1,2,3-selenadiazoles with the bis(triphenylphosphine)pal-
ladium(0) moiety, which is isolobal with cyclopentadienyl-
cobalt().13
Se
[Pd(PPh3)4]
+
(CH2)n
(CH2)n
Se
IIa: n = 4; IIb: n = 5; IIc: n = 6
Results and discussion
Toluene
Heat, 35-50%
A toluene solution containing [Pd(PPh3)4] and an excess of one
of the cycloalkeno-1,2,3-selenadiazoles Ia–Ic darkens rapidly
on heating to reflux. Subsequent column chromatography leads
to isolation of the air-stable dark green compounds 1a–1c [eqn.
(2)].
Se
Ph3P
)
(CH
n
Pd
Se
By following the course of the reaction by 31P NMR spectro-
scopy, we have established that the only phosphorus-containing
compounds present in significant concentration at any stage
are the starting material, the final product and free triphenyl-
Se
Pd
Se PPh3
1a: n = 4; 1b: n = 5; 1c: n = 6
(4)
(CH2)n
J. Chem. Soc., Dalton Trans., 1999, 791–794
791

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Table 1 Selected bond lengths (Å) and angles (Њ) in the structure of
Table 2 NMR Spectroscopic dataa (δ) for complexes 1a–1c
[Pd2{SeC(R1)᎐C(R2)Se}2(PPh3)2]ؒC6H5Me[R1,R2 = (CH2)6]
᎐
1a
1b
1c
Pd(1)–P(1)
2.293(8)
2.387(4)
2.413(4)
2.489(4)
3.078(3)
Pd(2)–P(2)
Pd(2)–Se(4)
Pd(2)–Se(3)
Pd(2)–Se(1)
2.281(8)
2.382(4)
2.419(4)
2.481(4)
Pd(1)–Se(2)
Pd(1)–Se(1)
Pd(1)–Se(3)
Pd(1) ؒ ؒ ؒ Pd(2)
1H
m-H of C6H5
o-, p- 2H of C6H5
(CH2)n
7.65–7.60
7.37–7.34
1.86–0.58
7.64–7.59
7.39–7.29
1.94–0.67
7.61–7.56
7.33–7.26
1.76–0.82
13C
(CH2)n
21.52
21.95
34.64
37.03
26.43
26.79
32.83
38.89
42.63
25.75
26.40
29.55
30.41
35.14
Se(2)–Pd(1)–P(1)
P(1)–Pd(1)–Se(3)
Se(3)–Pd(1)–Se(1)
Se(1)–Pd(1)–Se(2)
Pd(1)–Se(1)–Pd(2)
93.6(2)
101.3(2)
77.73(12)
87.59(12)
77.92(12)
Se(4)–Pd(2)–P(2)
P(2)–Pd(2)–Se(1)
Se(1)–Pd(2)–Se(3)
Se(3)–Pd(1)–Se(4)
Pd(1)–Se(3)–Pd(2)
92.3(2)
102.5(2)
77.77(12)
87.88(12)
77.65(11)
39.69
C᎐C
119.91
144.36
127.99
130.35
131.67c
134.93
123.93
150.13
128.18
130.44
131.71c
135.12
122.10
148.55b
127.89
130.19
131.30c
134.91
᎐
C6H5
31P
P(C6H5)3
30.4
30.7
30.4
77Se
terminal
bridging
559.8d
408.8e
576.1d
422.2e
575.7d
386.5e
a In CDCl3 solution. b J(13C-31P) = 13.7 Hz (resolved for 1c only).
c J(13C-31P) = 45.7 (1a), 45.2 (1b), 45.5 Hz (1c). d J(77Se-31P) = 30 Hz (1a,
1b and 1c). e J(77Se-31P) = 110, ≈11 (1a); 107, 15 (1b); 111, 15 Hz (1c).
enough to justify detailed discussion of the molecular para-
meters, but the positions of the heavy atoms are relatively well
defined. Each palladium atom is approximately square planar
co-ordinated; the maximum deviation from the least squares
plane passing through the palladium atom and its four nearest
neighbours is ca. 0.2 Å. In contrast to the situation in [Pd2-
(η3-C3H5)2{Ph2P(O)NP(Se)Ph2-Se}2], where the Pd2Se2 core is
planar,18 there is a relatively acute angle between the two square
planes [75.6(1)Њ]. This brings the two palladium atoms closer
(3.078 Å) than the sum of the van der Waals radii (3.26 Å19).
There is, however, no evidence for any significant interaction
between them. Similar instances of short non-bonding Pd–Pd
contacts have recently been noted.20 There is also considerable
distortion of the co-ordination geometry, imposed by the geo-
metric constraints of the bridging ligands. The Pd–Se bond
lengths in the Pd2Se2 unit are not equal (average: 2.485, 2.416
Å), each selenium being closer to the palladium to which it is
chelating. Both distances are, however, within the range of
previous observed values: cf. 2.409 Å (average) in [NMe4]-
[Pd(C3S3Se2)2]2 6c and 2.401, 2.419 Å in [Pd{SeC(R1)=C(R2)Se}-
(PBu3)2] [R1,R2 = (CH2)5].21 The Se–Pd–Se angles in the
bridge are also far less than 90Њ (average: 77.7Њ).
1
2
᎐
Fig.
1
Molecular structure of [Pd2{SeC(R )᎐C(R )Se}2(PPh3)2]
᎐
[R1,R2 = (CH2)6] 1c.
have observed C–Se bond cleavage in the reactions of 1,4-
diselenines before.15 The formation of diiron diselenolates
from 1,4-diselenines and [Fe2(CO)9] has also been previously
reported [eqn. (5)].16 By comparison, the reaction of [Pt(PPh3)4]
Se
[Fe2(CO)9]
+
(CH2)n
(CH2)n
Se
IIa: n = 4; IIb: n = 5; IIc: n = 6
Hexane
RT
Se
Se
That the dimeric structure demonstrated in the solid state for
complex 1c is retained in solution by all of 1a–1c has been
confirmed by multinuclear NMR spectroscopy (Table 2). The
77Se NMR spectra contain two resonances, corresponding to
the bridging and terminal selenium environments. The first
of these shows coupling to two magnetically inequivalent
phosphorus nuclei: a large coupling of ca. 110 Hz to the trans-
phosphorus, and a small coupling of ca. 15 Hz to the phos-
phorus in the cis position. The second 77Se resonance shows
only a cis coupling (2J ≈ 30 Hz). Analogous features are present
in the satellite structure of the single 31P resonance (Fig. 2),
Fe(CO)3
Fe(CO)3
(5)
(CH2)n
with 1,2,3-selenadiazoles takes a different course, to yield in at
least one case a selenaketocarbene complex [eqn. (6)].17
N
[Pt(PPh3)4 ] +
N
Se
Toluene
Heat
4
analysis of which yields J(31P–31P) = 7.7 Hz. The 13C NMR
spectra contain resonances for each carbon atom in the alicyclic
ring, confirming that the diselenolate ligands are asymmetric-
ally bound to the Pd2(PPh3)2 unit.
The microanalytical and mass spectroscopic data are in
accord with the proposed formulations (Table 3). In the mass
spectra the stability of the dimeric structure is apparent from
the intensity of the molecular ion, and of the fragments derived
from it by loss of triphenylphosphine. To our knowledge 1a–1c
are the first dinuclear diselenolenes to be described. Complexes
PPh3
Pt PPh3
(6)
Se
The crystal structure of the toluene solvate of complex 1c has
been determined by X-ray diffraction (Fig. 1). Selected bond
lengths and angles are listed in Table 1. The data are not good
792
J. Chem. Soc., Dalton Trans., 1999, 791–794

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with bridging unsaturated dithiolate ligands are known,22 but
here too examples containing palladium do not appear to have
been reported.
Table 3 Microanalytical, infrared and mass spectroscopic data for
complexes 1a–1c
1a
44.9
1b
33.1
1c
46.2
It is interesting that complexes 1a–1c are formed in prefer-
ence to mononuclear diselenolenes containing two molecules of
triphenylphosphine, despite the presence of free phosphine in
solution. Palladium dithiolenes and diselenolenes with mono-
dentate ancillary ligands appear to be unknown in the litera-
ture, although we have recently prepared the complexes
[Pd{SeC(R1)=C(R2)Se}(PBu3)2] [R1,R2 = (CH2)n; n = 4, 5, 6.]21
The corresponding platinum species are well established,8,23
so it would appear that the greater lability of palladium()
complexes is partly responsible for this behaviour.
If the reaction between [Pd(PPh3)4] and one of the 1,2,3-
selenadiazoles Ia–Ic is carried out at slightly lower temper-
atures, traces of the azo-compounds 2a–2c are produced [eqn.
(7)]. One of these (2a) has been obtained in sufficient quantity
Yielda (%)
Mp/ЊC
183–184
202Ϫ203
285–287
Microanalysis
C (%)
H (%)
46.57 (47.51)
3.65 (3.82)
49.10 (48.37)
4.33 (4.06)
51.37 (52.04)b
4.84 (4.59)b
Mass spectrumc m/z (%)
Mϩ
1216 (100)
1244 (76)
982 (100)
1272 (78)
1010 (100)
M
ϩ Ϫ PPh3
954 (52)
Infraredd
(cmϪ1
)
3048m
2932vs
2850s
1597m
1478m
1433s
1094s
747, 731s
694vs
527vs
514s
3048m
2924vs
2856s
1598m
1478m
1434s
1093s
746s
3052m
2918vs
2844s
1586m
1480m
1435s
1095s
744s
N
[Pd(PPh3)4]
+
(CH2)n
N
Se
Ia: n = 4; Ib: n = 5; Ic: n = 6
692vs
524vs
513s
693vs
527vs
512s
(CH
Toluene
90oC
)
n
N
N
496m
496m
495m
Se
(7)
1a-c
+
a From diselenine. b Calculated figures are for toluene solvate.
c Recorded using FAB; figures are for isotopomers containing 80Se,
106Pd. d Recorded as KBr disks; selected bands only.
Pd
Se PPh3
2a: n = 4; 2b: n = 5; 2c: n = 6
(CH2)n
and purity to be characterised by NMR and mass spectroscopy.
Compounds 2a–2c are analogous to the products formed when
trialkylphosphinepalladium(0) complexes are used in place of
[Pd(PPh3)4].24 It should be noted, however, that in this case no
dinuclear diselenolenes are formed from Ia–Ic. This we ascribe
to the greater basicity of the trialkylphosphine, allowing oxid-
ative addition to the Se–N bond to compete successfully with
loss of N2 from the 1,2,3-selenadiazole.
Experimental
All reactions were performed using standard Schlenk tech-
niques and pre-dried solvents under an atmosphere of
1
dinitrogen. H and 13C NMR spectra: Bruker AC400; tetra-
methylsilane as internal standard. 31P and 77Se NMR spectra:
Bruker WM250; 85% phosphoric acid or dimethyl selenide as
external standard. IR spectra: Perkin-Elmer 1725X. Mass spec-
tra were recorded by the EPSRC Mass Spectrometry Centre,
using fast atom bombardment (FAB).
Cycloalkeno-1,2,3-selenadiazoles Ia–Ic,14 tetrakis(triphenyl-
phosphine)palladium25 and bis(cycloalkeno)-1,4-diselenines
IIa–IIc14 were prepared by literature methods.
Synthesis of palladium diselenolenes (1a–1c)
In a typical experiment a solution containing tetrakis(tri-
phenylphosphine)palladium (0.30 g, 0.26 mmol) and cyclo-
hexeno-1,2,3-selenadiazole, Ia (0.20 g, 1.06 mmol) in toluene
(100 cm3) was heated under reflux for 1 h. After this time the
solution had darkened considerably. Removal of the solvent
under reduced pressure left an intensely coloured oily solid,
which was purified by column chromatography on alumina
using a 2:1 mixture of toluene and hexane. Collection of the
dark green band and recrystallisation from hexane gave 1a as an
analytically pure solid.
The same product was obtained in slightly higher yield by the
use of bis(cyclohexeno)-1,4-diselenine, IIa, in place of Ia.
Compounds 1b and 1c were prepared by an analogous
procedure using the corresponding 1,2,3-selenadiazole or
Fig. 2 Satellite structure of the 31P NMR resonance for complex 1c.
Analysis of the data gives 2J(31P–77Se) = 111.3, 15.2; 4J(31P–31P) = 7.7
Hz. The lines at 11.2 Hz have enhanced intensity due to coincidence
between satellites arising from coupling to 77Se and 13C.
J. Chem. Soc., Dalton Trans., 1999, 791–794
793

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Allg. Chem., 1996, 622, 1979.
1,4-diselenine. Yields, spectroscopic and analytical data are
summarised in Tables 2 and 3.
By conducting the reaction of [Pd(PPh3)4] with Ia at a lower
temperature (ca. 90 ЊC) the by-product 2a could be isolated.
This eluted as a deep purple band just before the dark green
band corresponding to 1a. 4a: mp 196–197 ЊC; 1H NMR δ 7.69
(m, 6 H), 7.48–7.40 (m, 9 H), 3.07 (m, 2 H), 2.98 (m, 2 H), 2.75
(m, 4 H), 1.93 (m, 2 H) and 1.78–1.70 (m, 6 H); 13C NMR δ
164.2, 157.8, 143.9, 143.0 [J(13C–31P) 6], 134.9 [J(13C–31P) 12],
130.9, 130.8 [J(13C-31P) 51] and 128.0 [J(13C–31P) 11 Hz]; 31P
NMR δ 30.4; 77Se NMR δ 604 [J(77Se–31P) 25] and 432 [J(77Se–
31P) 24]; MS m/z 715 (Mϩ).
7 W. B. Heuer, P. J. Squattrito, B. M. Hoffman and J. A. Ibers, J. Am.
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Dalton Trans., 1998, 2467.
X-Ray crystallography
Crystal data. C59H62P2Pd2Se4 1c, M = 1361.66, crystal size
0.15 × 0.30 × 0.50 mm, monoclinic, space group P21/n (alterna-
tive setting of no. 14), a = 12.614(4), b = 22.037(15), c =
21.567(4) Å, β = 92.79(2)Њ, U = 5988(5) Å3 (by least squares
refinement on setting angles of 24 reflections, Z = 4),
F(000) = 2696, Dc = 1.510 g cmϪ3, µ(Mo-Kα) = 3.12 mmϪ1
.
Data collection (Enraf-Nonius CAD4, graphite-mono-
chromated Mo-Kα radiation, λ = 0.71069 Å, T = 295 K), ω–2θ
scans, 2.5 < θ < 22Њ, 6784 measured reflections ( h, ϩk, ϩl),
6334 unique. Structure solution by direct methods, with SIR,26
and heavy-atom procedures with SHELXL 93.27 Empirical
absorption correction (ψ scan; minimum, maximum correction
factors 0.84, 1.00). Final refinement cycles performed against
F2 with all non-hydrogen atoms anisotropic and hydrogen
atoms in calculated positions. Refinement on 521 variables
converged at R1 = 0.082 (based on 2978 reflections with
Fo > 4(Fo), R1 = 0.221 (on all reflections), wR2 = 0.399, good-
ness of fit = 1.415. Maximum, minimum peaks in the final
difference map = 1.23, Ϫ0.99 e ÅϪ3
CCDC reference number 186/1317.
.
19 A. Bondi, J. Phys. Chem., 1964, 68, 441.
20 T. W. Hambley, Inorg. Chem., 1998, 37, 3767.
21 S. Ford, C. P. Morley and M. Di Vaira, Chem. Commun., submitted
for publication.
Acknowledgements
We thank EPSRC for the provision of a postdoctoral research
assistantship to P. K. K. and a research studentship to S. F.,
Johnson Matthey plc for the loan of palladium salts, and the
Ministero dell’ Università e della Ricerca Scientifica e Techno-
logica for financial support to M. di V.
22 D. C. Bravard, W. E. Newton, J. T. Huneke, K. Yamanouchi and
J. H. Enemark, Inorg. Chem., 1982, 21, 3795; R. Weberg, R. C.
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Acta, 1986, 116, 15; M. Concepción Gimeno, P. G. Jones,
A. Laguna, M. Laguna and R. Terroba, Inorg Chem., 1994, 33,
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Commun., 1997, 2211.
23 C. E. Johnson, R. Eisenberg, T. R. Evans and M. S. Burbery, J. Am.
Chem. Soc., 1983, 105, 1795.
24 S. Ford, C. P. Morley and M. Di Vaira, Chem. Commun., 1998, 1305.
25 D. R. Coulson, Inorg. Synth., 1972, 13, 120.
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