Synthesis and properties of palladium diselenolenes: X-ray crystal structures of [Pd{SeC(R1)=C(R2)Se}(PBu3) 2] [R1, R2 = (CH2)n, n = 4, 5, 6]

Article abstract of DOI:10.1021/ic040069y

The reaction between [Pd₂(dba)₃] (dba = dibenzylideneacetone), tributylphosphine, and a bis(cycloalkeno)-1,4-diselenin leads to either a mononuclear diselenolene [Pd{SeC(R¹)=C(R ²)Se}(PBu₃)₂] or a dinuclear diselenolene [Pd₂{SeC(R¹)=C(R²)Se}₂(PBu ₃)₂] [R¹, R² = (CH₂) _n, n = 4, 5, 6] depending on the stoichiometry employed. Treatment of the dinuclear diselenolenes with 1,2-bis(diphenylphosphino)ethane (dppe) provides a high-yielding route to the mononuclear species [Pd{SeC(R ¹)=C(R²)Se}(dppe)]. All new compounds have been characterized by standard spectroscopic and analytical techniques, in particular by multinuclear NMR spectroscopy; the structure of each of the mononuclear tributylphosphine complexes has been determined by X-ray crystallography. Computational studies show that the observed asymmetry of the diselenolenes in the solid state is a result primarily of intramolecular repulsive interactions between the ligands.

Full text of DOI:10.1021/ic040069y

Inorg. Chem. 2004, 43, 7101−7110
Synthesis and Properties of Palladium Diselenolenes: X-ray Crystal
Structures of [Pd (CH2)n,
SeC(R1) C(R2)Se (PBu3)2] [R1, R2
4, 5, 6]
{
d
}
)
n
)
Susan Ford,† Christopher P. Morley,*,† and Massimo Di Vaira‡
Department of Chemistry, UniVersity of Wales Swansea, Singleton Park, Swansea, U.K. SA2 8PP,
and Dipartimento di Chimica, UniVersita` degli studi di Firenze, Via della Lastruccia 3,
50019 Sesto Fiorentino (FI), Italy
Received May 12, 2004
The reaction between [Pd2(dba)3] (dba
diselenin leads to either a mononuclear diselenolene [Pd
[Pd2 (CH2)n, n 4, 5, 6] depending on the stoichiometry employed. Treatment
SeC(R1)dC(R2)Se 2(PBu3)2] [R1, R2
of the dinuclear diselenolenes with 1,2-bis(diphenylphosphino)ethane (dppe) provides a high-yielding route to the
mononuclear species [Pd (dppe)]. All new compounds have been characterized by standard
SeC(R1)dC(R2)Se
)
dibenzylideneacetone), tributylphosphine, and a bis(cycloalkeno)-1,4-
{
SeC(R1)dC(R2)Se
}
(PBu3)2] or a dinuclear diselenolene
{
}
)
)
{
}
spectroscopic and analytical techniques, in particular by multinuclear NMR spectroscopy; the structure of each of
the mononuclear tributylphosphine complexes has been determined by X-ray crystallography. Computational studies
show that the observed asymmetry of the diselenolenes in the solid state is a result primarily of intramolecular
repulsive interactions between the ligands.
Introduction
LUMO gap and an increase in the strength of intermolecular
interactions in the solid state.
For many years, the potentially useful electrochemical and
optical properties of transition metal dithiolenes have at-
tracted the attention of chemists, physicists, and materials
scientists.1 Possible areas of application include molecular
electronics, infrared dyes, liquid crystals, and catalysis.2-4
By contrast, the selenium analogues of dithiolenes (disele-
nolenes) have been little studied,5-10 largely because of the
lack of generally applicable preparative methods. These
complexes are, however, attractive synthetic targets as the
incorporation of selenium should lead to a smaller HOMO-
The reactions of 1,2,3-selenadiazoles with low-valent
transition metal compounds have previously been used, by
us11 and others,12 to prepare a wide variety of selenium-
(5) (a) Sandman, D. J.; Stark, J. C.; Allen, G. W.; Acampora, L. A.; Jansen,
S.; Jones, M. T.; Ashwell, G. J.; Foxman, B. M. Inorg. Chem. 1987,
26, 1664. (b) Gautheron, B.; Tainturier, G.; Pouly, S.; Theobald, F.;
Vivier, H.; Laarif, A. Organometallics 1984, 3, 1495. (c) Klapo¨tke,
T. M. Phosphorus, Sulfur, Silicon 1989, 41, 105.
(6) (a) Papavassiliou, G. C. Mol. Cryst. Liq. Cryst. 1982, 86, 159. (b)
Matsubayashi, G.; Yokazawa, A. J. Chem. Soc., Dalton Trans. 1990,
3535. (c) Faulmann, C. Legros, J.-P.; Cassoux, P.; Cornelissen, J.;
Brossard, L.; Inokuchi, M.; Tajima, H.; Tokumoto, M. J. Chem. Soc.,
Dalton Trans. 1994, 249. (d) Zeltner, S.; Olk, R. M.; Pink, M.; Jelonek,
S.; Jorchel, P.; Gelbrich, T.; Sieler, J.; Kirmse, R. Z. Anorg. Allg.
Chem. 1996, 622, 1979.
(7) (a) Heuer, W. B.; Squattrito, P. J.; Hoffman, B. M.; Ibers, J. A. J.
Am. Chem. Soc. 1988, 110, 792. (b) Wudl, F.; Zellers, E. T.; Cox, S.
D. Inorg. Chem. 1985, 24, 2864.
(8) Bollinger, C. M.; Rauchfuss, T. B. Inorg. Chem. 1982, 21, 3947.
(9) (a) Okano, Y.; Sawa, H.; Aonuma, S.; Kato, R. Chem. Lett. 1993,
1851. (b) McCullough, R. D.; Belot, J. A.; Seth, J.; Rheingold, A. L.;
Yap, G. P. A.; Cowan, D. O. J. Mater. Chem. 1995, 5, 1581.
(10) (a) Kajitani, M.; Ochiai, R.; Dohki, K.; Kobayashi, N.; Akiyama, T.;
Sugimori, A. Bull. Chem. Soc. Jpn. 1989, 62, 3266. (b) Miller, E. J.;
Landon, S. J.; Brill, T. B. Organometallics 1985, 4, 533.
* To whom correspondence should be addressed. E-mail: c.p.morley@
swan.ac.uk.
† University of Wales Swansea.
‡ Universita` degli studi di Firenze.
(1) (a) Karlin, K. D., Stiefel, E. I., Eds. Dithiolene Chemistry: Synthesis,
Properties, and Applications; Progress in Inorganic Chemistry Series;
Wiley: New York, 2004; Vol. 52. (b) Mu¨ller-Westerhoff, U. T.;
Vance, B. In ComprehensiVe Coordination Chemistry; Wilkinson G.,
Ed.; Pergamon Press: Oxford, U.K., 1988; Vol. 2, Chapter 16.5, p
595.
(2) Cassoux, P.; Valade, L.; Kobayashi, H.; Clark, R. A.; Underhill, A.
E. Coord. Chem. ReV. 1991, 110, 115.
(3) Bjørnholm, T.; Geisler, T.; Petersen, J. C.; Grieve, D. R.; Schiødt, N.
C. Non-Linear Optics 1995, 10, 129.
(11) (a) Morley, C. P. Organometallics 1989, 8, 800. (b) Dorrity, M. R. J.;
Lavery, A.; Malone, J. F.; Morley C. P.; Vaughan, R. R. Heteroatom
Chem. 1992, 3, 87. (c) Morley, C. P.; Vaughan, R. R. J. Organomet.
Chem. 1993, 444, 219.
(4) Coomber, A. T.; Beljonne, D.; Friend, R. H.; Bre´das, J. L.; Charlton,
A.; Robertson, N.; Underhill, A. E.; Kurmoo, M.; Day, P. Nature 1996,
380, 144.
10.1021/ic040069y CCC: $27.50
Published on Web 09/29/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 22, 2004 7101

Ford et al.
Scheme 1
phosphoric acid or dimethyl selenide as an external standard. UV/
Vis spectra were obtained on a Perkin-Elmer Lambda 9 instrument.
Mass spectra were recorded by the EPSRC Mass Spectrometry
Centre using fast atom bombardment (FAB).
Cycloalkeno-1,2,3-selenadiazoles 1a-c,16 bis(cycloalkeno)-1,4-
diselenins 2a-c,16 and [Pd2(dba)3]‚dba21 were prepared by literature
methods. Tributylphosphine, triethylphosphine, and 1,2-bis(diphen-
ylphosphino)ethane (dppe) were obtained commercially (Aldrich)
and used without further purification.
Synthesis of Tributylphosphine-Palladium Diselenolenes (3a-
c, 4a-c, and 5a). (a) 3a, 4a, and 5a. PBu3 (0.16 g, 0.80 mmol)
was added to a xylene solution (100 cm3) of [Pd2(dba)3]‚dba (0.23
g, 0.20 mmol). After the mixture had been stirred at room
temperature for a short time, bis(cyclohexeno)-1,4-diselenin, 2a
(0.14 g, 0.44 mmol), was added, and the mixture was heated to
reflux overnight. Removal of the solvent under reduced pressure
left a dark colored oil, which was purified by column chromatog-
raphy on an alumina column with toluene as the eluent. Collection
of the deep purple band and recrystallization from hexane gave 3a
as an analytically pure solid. Yield: 0.05 g.
Scheme 2
Use of one-half of the above quantity of PBu3 gave the analogous
dinuclear compound 4a by an identical procedure. In addition, in
this case, a second band was eluted from the chromatography
column using toluene/diethyl ether (5:1). Removal of the solvent
under reduced pressure left a deep red oil, which was recrystallized
from hexane to give 5a as an analytically pure solid. Yield: 0.04
g.
(b) 3b,c and 4b,c. In a typical experiment, PBu3 (0.16 g, 0.80
mmol) was added to a toluene solution (100 cm3) of [Pd2(dba)3]‚
dba (0.23 g, 0.20 mmol). After the mixture had been stirred at room
temperature for a short time, bis(cyclohepteno)-1,4-diselenin, 2b
(0.15 g, 0.43 mmol), was added, and the mixture was heated to
reflux for 1 h. Removal of the solvent under reduced pressure left
a dark colored oil, which was purified by column chromatography
on an alumina column with toluene as the eluent. Collection of the
deep purple band and recrystallization from hexane gave 3b as an
analytically pure solid. Yield: 0.05 g.
Use of one-half of the above quantity of PBu3 gave the analogous
dinuclear compound 4b by an identical procedure. Compounds 3c
and 4c were prepared similarly from 2c. Yields and spectroscopic
and analytical data for 3a-c, 4a-c, and 5a are summarized in
Tables 1-4.
Conversion of 4a-c to 3a-c. In a typical experiment, PBu3
(0.08 g, 0.40 mmol) was added to a toluene solution (50 cm3) of
4a (0.11 g, 0.10 mmol). The mixture was stirred at room
temperature for 1 h. Removal of the solvent under reduced pressure
left a dark colored oil, which was purified by column chromatog-
raphy on an alumina column with toluene as the eluent. Collection
of the deep purple band and recrystallization from hexane gave 3a
as an analytically pure solid. Yield: 0.11 g.
containing complexes. In particular, we have shown that
cyclopentadienylcobalt diselenolenes can be produced by this
route.13
We recently extended this methodology to the synthesis
of dinuclear diselenolenes based on the triphenylphosphine-
palladium fragment, starting from the palladium(0) complex
[Pd(PPh3)3].14 The reactions of trialkylphosphinepalladium-
(0) precursors with cycloalkeno-1,2,3-selenadiazoles proceed
via a different route to give the azo complexes shown in
Scheme 1.15
Despite their ready preparation from 1,2,3-selenadiazoles16
(Scheme 2), little research into the chemistry of 1,4-diselenins
has been documented.17-19 We have now examined the
reactions of bis(cycloalkeno)-1,4-diselenins with a tribu-
tylphosphinepalladium(0) complex, and in this paper, we
present the results of our study. A preliminary report of some
of this work has already been published.20
Experimental Section
All reactions were performed using standard Schlenk techniques
1
and predried solvents under an atmosphere of dinitrogen. H and
13C NMR spectra were obtained on a Bruker AC400 instrument
using tetramethylsilane as an internal standard. 31P and 77Se NMR
spectra were obtained on a Bruker WM250 instrument using 85%
(12) (a) Gilchrist, T. L.; Mente, P. G.; Rees, C. W. J. Chem. Soc., Perkin
Trans. 1 1972, 2165. (b) Schrauzer, G. N.; Kisch, H. J. Am. Chem.
Soc. 1973, 95, 2501. (c) Pannell, K. H.; Mayr, A. J.; Hoggard, R.;
Pettersen, R. C. Angew. Chem., Int. Ed. Engl. 1980, 19, 632. (d) Ba¨tzel,
V.; Bo¨se, R. Z. Naturforsch. B 1981, 38, 172.
Reaction of 4a-c with PEt3. In a typical experiment, PEt3 (0.08
g, 0.68 mmol) was added to a toluene solution (50 cm3) of 4a (0.11
g, 0.10 mmol). The mixture was stirred at room temperature for
1 h. After this time, the 31P NMR spectrum of an aliquot contained
three singlets, two of which corresponded to PEt3 (δ ) -19.2 ppm)
and PBu3 (δ ) -31.7 ppm). The third resonance (δ ) 8.8 ppm)
was assigned to the product [Pd(Se2C6H8)(PEt3)2]. Removal of the
solvent under reduced pressure left a dark colored oil, which was
purified by column chromatography on an alumina column with
toluene/ethyl acetate (4:1) as the eluent. Collection of the deep
(13) Morley, C. P.; Vaughan, R. R. J. Chem. Soc., Dalton Trans. 1993,
703.
(14) Ford, S.; Khanna, P. K.; Morley, C. P.; Di Vaira, M. J. Chem. Soc.,
Dalton Trans. 1999, 791.
(15) Ford, S.; Morley, C. P.; Di Vaira, M. J. Chem. Soc., Chem. Commun.
1998, 1305.
(16) Meier, H.; Voigt, E. Tetrahedron 1972, 28, 187.
(17) Bates, C. M.; Khanna, P. K.; Morley, C. P.; Di Vaira, M. J. Chem.
Soc., Chem. Commun. 1997, 913.
(18) (a) Dorrity, M. R. J.; Lavery, A.; Malone, J. F.; Morley, C. P.;
Vaughan, R. R. Heteroatom Chem. 1992, 3, 87. (b) Chesney, A.;
Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K. J. Chem. Soc., Chem.
Commun. 1997, 2293.
(19) Mayr, A. J.; Lien, H.-S.; Pannell, K. H.; Parkanyi, L. Organometallics
1985, 4, 1580.
(20) Ford, S.; Morley, C. P.; Di Vaira, M. New J. Chem. 1999, 23, 811.
(21) Takahashi, Y.; Ito, T.; Sakai, S.; Ishii, Y. J. Chem. Soc., Chem.
Commun. 1970, 1065.
7102 Inorganic Chemistry, Vol. 43, No. 22, 2004

Synthesis and Properties of Palladium Diselenolenes
Table 1. 1H NMR Spectroscopic Data for 3a-c, 4a-c, and 5a in CDCl3 Solution
3a
3b
3c
4a
4b
4c
5a
CH3
0.92 (t)a
-
1.84 (m)
-
0.92 (t)a
-
1.80 (m)
-
0.92 (t)a
-
1.83 (m)
-
0.92 (t)a
-
1.45-1.75
-
0.92 (t)a
-
1.47-1.72
-
0.92 (t)a
-
1.43-1.81
-
0.92 (t)a
0.97 (t)b
CH2P
1.63 (m)
1.75 (m)
CH2CH2P
CH3CH2CH2
γ-CH2
1.48 (m)
1.42 (m)
-
1.47 (m)
1.41 (m)
1.47 (m)
1.63 (m)
-
1.48 (m)
1.42 (m)
1.48 (m)
1.63 (m)
-
1.45-1.75
1.41 (m)
-
1.47-1.72
1.41 (m)
1.47-1.72
1.47-1.72
-
1.43-1.81
1.38 (m)
1.43-1.81
1.43-1.81
-
1.26-1.57
1.26-1.57
-
1.97 (m, 2H)
2.26 (m, 2H)
2.48 (m, 4H)
2.53-2.82
â-CH2
1.64 (m)
-
1.45-1.75
-
-
-
-
-
-
-
R-CH2
2.50 (m)
2.54 (m)
2.53 (m)
2.38-2.64
2.41-2.65
2.36-2.75
a
b
3J(1H-1H) ) 7.1 Hz. 3J(1H-1H) ) 6.8 Hz.
Table 2. 13C NMR Spectroscopic Data for 3a-c, 4a-c, and 5a in
and the mixture was heated to reflux for 1 h. Removal of the solvent
under reduced pressure left an orange oil, which was purified by
column chromatography on an alumina column. Two yellow bands
were initially obtained with toluene as the eluent. Collection of
the brown third band using a mixture of toluene and ethyl acetate
(4:1) and recrystallization from toluene gave 6a as an analytically
pure, pink solid. Yield: 0.02 g.
CDCl3 Solution
3a
3b
3c
4a
4b
4c
5a
CH3
CH2P
CH2CH2P
13.8
13.7
13.7
13.8 13.8 13.8 13.6, 13.9
25.2a 24.9a 25.1a 24.3d 24.3d 24.2d 21.4,f 24.2g
24.4b 24.4b 24.4b 24.4e 24.4e 24.4e 24.1,e 24.4h
CH3CH2CH2 26.3
CdC
26.2
26.2
26.4 26.5 26.4 26.4
127.5c 131.4c 130.1c 117.4 120.7 119.6 116.7, 120.0
(b) From 4a-c. In a typical experiment, dppe (0.08 g, 0.20
mmol) was added to a toluene solution (50 cm3) of 4a (0.11 g,
0.10 mmol). The mixture was stirred at room temperature for 1 h.
Concentration of the solution by removal of the solvent under
reduced pressure resulted in the precipitation of 6a as a pink solid,
which was collected by filtration and recrystallized from toluene.
Yield: 0.10 g.
-
-
-
145.1 151.0 149.7 140.2, 146.8
36.1 40.6 37.3 35.9, 37.2
38.1 43.6 40.2 38.6, 40.6i
22.3 30.9 30.8 22.8,j 23.2
23.4 33.1 30.8 30.5, 30.7
R-CH2
â-CH2
γ-CH2
35.1
-
39.9
-
37.3
-
22.6
-
-
-
29.7
-
27.3
-
30.8
-
26.4
-
-
-
27.3 26.0
- 26.4
-
-
a Apparent triplet: average J(13C-31P) ) 7 Hz (3a, 3b), 6 Hz (3c).
b Apparent triplet: average J(13C-31P) ) 13 Hz (3a), 12 Hz (3b, 3c).
c Apparent triplet: average J(13C-31P) ) 7 Hz (3a, 3b), 6 Hz (3c).
Yields and spectroscopic and analytical data for 6a-c are
summarized in Tables 5-8.
1J(13C-31P) ) 25 Hz (4a, 4c), 21 Hz (4b). 2J(13C-31P) ) 14 Hz.
d
f
e
X-ray Crystallography. Details of the crystal structure deter-
minations are presented in Table 9. Data for 3b have been reported
previously, as part of a preliminary communication,20 and are
included here for comparison purposes. Measurements were carried
out at room temperature with an Enraf-Nonius CAD4 diffractometer
for 3a and 3b, using graphite-monochromated Mo KR radiation,
and with a Siemens-Bruker P4 diffractometer mounted on a rotating
anode generator for 3c, using graphite-monochromated Cu KR
radiation. Empirical corrections for absorption were applied, based
on ψ scans. Structure solution was by direct methods with SIR9722
and Fourier cycles with SHELXL-93,23 which was also used for
structure refinements. The final refinement cycles were performed
against F2 with all non-hydrogen atoms anisotropic and hydrogen
atoms in calculated positions. Drawings were made with ORTEP.24
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos.
CCDC 250883 (3a), CCDC 440/126 (3b), and CCDC 250882 (3c).
Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. [Fax: int. code
+44 (0)1223 336033; e-mail: deposit@ccdc.cam.ac.uk].
1J(13C-31P) ) 51 Hz. 1J(13C-31P) ) 50 Hz. 2J(13C-31P) ) 16 Hz.
g
h
4J(13C-31P) ) 23 Hz. 5J(13C-31P) ) 5 Hz.
i
j
Table 3. 31P and 77Se NMR Spectroscopic Data for 3a-c, 4a-c, and
5a in CDCl3 Solution
3a
3b
3c
4a
4b
4c
5a
31P
1.4a
0.8a
0.7a
10.9
472
11.2
498
9.6
19.8, 8.9
not observed
418, 391
77Se
512b
533b
519b
493
410c
434c
407c
2J(31P-31P) ) 44 Hz. 2J(77Se-31Ptrans) ) 65 Hz (3a, 3b), 67 Hz
a
b
(3c); 2J(77Se-31Pcis) ) 16 Hz (3a-c). 2J(77Se-31Ptrans) ) 111 Hz (4a,
c
4c), 110 Hz (4b); J(77Se-31Pcis) ) 9 Hz (4a-c).
2
purple band gave an oily solid whose mass spectrum contained
peaks due to 3a, 4a, [Pd(Se2C6H8)(PEt3)(PBu3)] (m/e ) 666), and
[Pd2(Se2C6H8)2(PEt3)(PBu3)] (m/e ) 1012).
Synthesis of dppe-Palladium Diselenolenes (6a-c). (a) From
1,2,3-Selenadiazoles 1a-c or 1,4-Diselenins 2a-c. In a typical
experiment, dppe (0.16 g, 0.40 mmol) was added to a toluene
solution (100 cm3) of [Pd2(dba)3]‚dba (0.23 g, 0.20 mmol). After
the mixture had been stirred at room temperature for a short time,
cyclohexeno-1,2,3-selenadiazole, 1a (0.16 g, 0.86 mmol), or bis-
(cyclohexeno)-1,4-diselenin, 2a (0.14 g, 0.44 mmol), was added,
Computation. Conformational searches, in some cases with
constraints described in the Results and Discussion section, were
Table 4. Yields, Melting Points, and Mass Spectrometric Data for 3a-c, 4a-c, and 5a
3a 3b 3c 4a
4b
4c
5a
yield (%)
17b
99
14b
119
11b
114
15
-
14
-
54
112-113
18
118-119
c
c
melting point (°C)
mass spectrometric dataa
M+
750 (100%)
548 (73%)
764 (60%)
562 (100%)
778 (100%)
576 (30%)
1096 (100%)
894 (31%)
1124 (100%)
922 (82%)
1148 (100%)
946 (71%)
1176 (100%)
-
[M-PBu3]+
a Recorded using FAB; figures are for isotopomers containing 80Se, 106Pd. b Isolated yields from 4a-c: ∼75%. c Compounds not obtained in crystalline
form.
Inorganic Chemistry, Vol. 43, No. 22, 2004 7103

Ford et al.
methylene groups and the 6-31G(d,p) set29 for the other C and H
atoms and for the P atoms. Frequency calculations, to check the
nature of stationary states, were performed for the models not
subject to constraints, and unscaled ZPE corrections were applied
when energy comparisons were of interest. Parts of the results were
interpreted in the light of natural bond orbital (NBO) analysis
procedures.30 For graphics, Molden31 was employed. Atomic
coordinates for the optimized models are available from the authors
on request.
Table 5. 1H NMR Spectroscopic Data for 6a-c in CDCl3 Solution
6a
6b
6c
CH2P
2.37 (m)a
7.36 (m)
7.72 (m)
7.15 (m)
-
2.35 (m)a
7.36 (m)
7.71 (m)
7.16 (m)
1.34 (m)
1.51 (m)
2.50 (m)
2.35 (m)a
7.35 (m)
7.70 (m)
7.14 (m)
1.34 (m)
1.53 (m)
2.51 (m)
o-C6H5
m-C6H5
p-C6H5
γ-CH2
â-CH2
R-CH2
1.57 (m)
2.45 (m)
a Apparent doublet: average J(1H-31P) ) 10.3 Hz (6a), 10.2 Hz
(6b, 6c).
Results and Discussion
Table 6. 13C NMR Spectroscopic Data for 6a-c in CDCl3 Solution
Preparation of 3a-c and 4a-c. When a mixture of
[Pd2(dba)3], tributylphosphine, and one of the bis(cyclo-
alkeno)-1,4-diselenins 2b or 2c is heated to reflux in toluene
for 1 h, the solution becomes a deep purple color. To obtain
the same transformation using 2a, higher temperature (reflux
in xylene) and longer reaction time are required. By
manipulation of the stoichiometry, the product obtained after
column chromatography can be either the mononuclear (3a-
c: P/Pd ) 2) or dinuclear (4a-c: P/Pd ) 1) diselenolene
(Scheme 3).
The detailed mechanism of the reaction is unclear.
However, on the basis of previous research showing cleavage
of the C-Se bond in 1,4-diselenins,17 we postulate a first
step of metal insertion into the heterocyclic ring system,
followed by loss of the cycloalkyne (Scheme 4).
6a
6b
6c
CH2P
28.7a
133.4b
128.8c
131.2
130.5d
129.6e
35.7
28.3a
133.3b
128.8c
131.1
130.8d
132.6e
40.3
28.0a
133.3b
128.8c
131.1
130.7d
131.4e
37.7
o-C6H5
m-C6H5
p-C6H5
ipso-C6H5
CdC
R-CH2
â-CH2
γ-CH2
22.7
-
29.7
27.2
30.7
26.5
a Apparent triplet: average J(13C-31P) ) 24 Hz. b Apparent triplet:
average J(13C-31P) ) 6 Hz. c Apparent triplet: average J(13C-31P) )
5 Hz. d Apparent doublet: average J(13C-31P) ) 10 Hz (6a), 11 Hz (6b,c).
e Apparent triplet: average J(13C-31P) ) 6 Hz.
Table 7. 31P and 77Se NMR Spectroscopic Data for 6a-c in CDCl3
Solution
6a
6b
6c
The forcing conditions required to obtain the products
when bis(cyclohexeno)-1,4-diselenin (n ) 4) is employed
are in accord with the proposed mechanism, as expulsion of
a cycloalkyne fragment would be favorable for n ) 6,
cyclooctyne being an isolable species, but not for n ) 4, the
ring strain in cyclohexyne being significantly larger.
We believe that these are the first palladium diselenolenes
to have been isolated in both mononuclear and dinuclear
forms. Mononuclear palladium diselenolenes containing
ancillary ligands such as phosphines appear to be almost
unknown in the literature. Apart from compounds prepared
in our own laboratory, we are aware of only one other set
31P
48.5
522
47.6
not recorded
46.4a
531b
77Se
2J(31P-31P) ) 38 Hz. 2J(77Se-31Ptrans) ) 71 Hz; 2J(77Se-31Pcis) )
a
b
16 Hz.
Table 8. Yields, Melting Points, and Mass Spectrometric Data for
6a-c
6a
6b
6c
yield (%)
8b
6b
7b
melting point (°C)
mass spectrometric dataa
M+
209-212 (d) 181-183
201-205 (d)
744 (100%)
664 (21%)
758 (100%) 772 (100%)
664 (25%) 664 (61%)
[PdSe2(dppe)]+
a Recorded using FAB; figures are for isotopomers containing 80Se, 106Pd.
(25) Frisch, J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts,
R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C.
Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M., Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision
A.7; Gaussian, Inc.: Pittsburgh, PA, 1998.
(26) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (b) Becke, A. D. J.
Chem. Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G. Phys.
ReV., B 1988, 37, 785.
(27) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(28) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724.
b Isolated yields from 4a-c: ∼70%.
performed on simplified models of 3a, where the phosphine ligands
were replaced by PH3 groups (with no simplification of the
diselenolate ligand). More detailed models, with PR3 groups (R )
Me, Et, Pr, Bu), were also optimized, starting from the experimental
3a geometry, progressively pruned of the outermost methylene
groups in the alkyl chains. Using the Gaussian 9825 suite of
programs, calculations were run at the B3LYP/6-31G(d,p) level26
with the LANL2DZ valence functions and effective core potentials27
for the Pd and Se atoms. For the model with R ) Bu, the limited
3-21G basis set25,28 was used for atoms of the methyl and adjacent
(22) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(23) Sheldrick, G. M. SHELXL-93, Program for Crystal Structure Refine-
ment; University of Go¨ttingen, Go¨ttingen, Germany, 1993.
(24) (a) Johnson, C. K. ORTEPsA Fortran Thermal Ellipsoid Plot
Program; Technical Report ORNL-5138, Oak Ridge National Labora-
tory, Oak Ridge, TN, 1976. (b) Farrugia, L. J. J. Appl. Cryst. 1997,
30, 565.
(29) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980,
102, 939.
(30) (a) Glendening, E. D.; Reed, A. E.; Carpenter, J. A.; Weinhold, F.
NBO Version 3.1; University of Wisconsin: Madison, WI, 1990. (b)
Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211.
(31) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Design 2000,
14, 123.
7104 Inorganic Chemistry, Vol. 43, No. 22, 2004

Synthesis and Properties of Palladium Diselenolenes
Table 9. Crystallographic Data for Compounds 3a-c
3a
3b
3c
formula
Mr
C30H62P2PdSe2
749.06
C31H64P2PdSe2
763.08
C32H66P2PdSe2
777.11
crystal system
space group
monoclinic
P21/c (No. 14)
monoclinic
P21/c (No. 14)
Monoclinic
P21/n (alternative
setting of No. 14)
13.210(2)
22.915(3)
13.362(3)
105.80(1)
3892(1)
a/Å
b/Å
c/Å
10.515(2)
21.196(6)
16.697(3)
96.65(1)
3696(1)
4
10.447(2)
22.124(4)
16.772(2)
94.80(1)
3863(1)
4
â/°
V/Å3
Z
4
F(000)
1544
1.346
0.15 × 0.40 × 0.50
Mo KR
0.71069
2.574
1576
1.312
0.30 × 0.40 × 0.60
Mo KR
0.71069
2.46
1608
1.326
Dc/Mg m-3
crystal size/mm3
radiation
0.40 × 0.60 × 0.80
Cu KR
wavelength/Å
1.54180
µ/mm-1
6.874
ψ scan
0.218, 0.470
+h, -k, (l
ω - 2θ
7-116
6366
absorption correction
min, max correction factors
data collected
scan type
ψ scan
ψ scan
0.734, 0.982
(h, +k, +l
ω - 2θ
5-54
0.711, 0.997
(h, +k, +l
ω - 2θ
5-50
2θ range for data/deg
reflections collected
unique reflections
unique observed reflections
with F0 > 4σ(F0)
no. of parameters
R1 (observed reflections)
R1 (all reflections)
wR2
6374
6036
2933
5924
5458
2858
5233
4864
322
331
341
0.0567
0.1654
0.1456
1.005
0.0538
0.1545
0.1504
1.021
0.0571
0.0598
0.1587
1.037
goodness of fit
largest features (max, min)
0.609, -0.429
0.471, -0.416
0.880, -1.021
in final ∆F map (e Å-3
)
differences in behavior.35 In particular, analogous dinuclear
diselenolenes have yet to be isolated. Perhaps the greater
lability of palladium complexes is partly responsible for this
effect.
Scheme 3
Spectroscopic Characterization of 3a-c and 4a-c. The
molecular formulas of 3a-c and 4a-c have been established
by mass spectrometry (Table 4). The mass spectrum of each
complex shows an intense cluster corresponding to the
molecular ion, with the expected isotope distribution. There
is also a major fragment ion formed by the loss of one
molecule of tributylphosphine.
The NMR spectroscopic data (Tables 1-3) are in accord
with the proposed structures. For the mononuclear species
3a-c, the two phosphorus atoms are chemically, but not
magnetically, equivalent. Analysis of the 77Se satellites in
the 31P NMR spectra allows evaluation of the two different
31P-77Se coupling constants: J(77Se-31Ptrans) ) 65 Hz (3a,
2
Scheme 4
2
3b), 67 Hz (3c); J(77Se-31Pcis) ) 16 Hz (3a-c). There
is also coupling between the two phosphorus nuclei:
(32) (a) Herberhold, M.; Schmalz, T.; Milius, W.; Wrackmeyer, B. Z.
Naturforsch. B 2002, 57, 53. (b) Herberhold, M.; Schmalz, T.; Milius,
W.; Wrackmeyer, B. Z. Anorg. Allg. Chem. 2002, 628, 979.
(33) (a) Davison, A.; Edelstein, N.; Holm, R. H.; Maki, A. H. Inorg. Chem.
1964, 3, 814. (b) Scrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc.
1965, 87, 1483. (c) van Houten, K. A.; Heath, D. C.; Barringer, C.
A.; Rheingold, A. L.; Pilato, R. S. Inorg. Chem. 1998, 37, 4647.
(34) (a) Johnson, C. E.; Eisenberg, R.; Evans, T. R.; Burbery, M. S. J.
Am. Chem. Soc. 1983, 105, 1795. (b) Bollinger, C. M.; Rauchfuss, T.
B. Inorg. Chem. 1982, 21, 3947. (c) McCullough, R. D.; Belot, J. A.;
Seth, J.; Rheingold, A. L.; Yap, G. P. A.; Cowan, D. O. J. Mater.
Chem. 1995, 5, 1581. (d) Giolando, D. M.; Rauchfuss, T. B.;
Rheingold, A. L. Inorg. Chem. 1987, 26, 1636.
of examples.32 Even the corresponding dithiolenes are rare,33
although their platinum analogues are well established.34 It
is interesting to note here that preliminary examination of
the corresponding platinum chemistry has revealed significant
(35) (a) Khanna, P. K.; Morley, C. P. J. Chem. Res. (S) 1995, 64. (b)
Webster, C. A.; Morley, C. P. Unpublished observations.
Inorganic Chemistry, Vol. 43, No. 22, 2004 7105

Ford et al.
Figure 1. 31P NMR spectrum of 3b, showing 77Se satellite structure.
2
2J(31P-31P) ) 44 Hz (cf. cis-[PdCl2(PMe3)2], J ) 8 Hz;
2
cis-[PdCl2{P(OMe)3}2], J ) 80 Hz).36 Figure 1 shows the
relevant portion of the 31P NMR spectrum of 3c as an
example, with the eight lines resulting from the 31P-77Se
interaction highlighted. The other lines in this spectrum arise
from coupling to various 13C nuclei. As expected in these
symmetrical molecules there is only one 13C resonance for
the olefinic carbon atoms in the alicyclic ring, and only one
signal in the 77Se NMR spectrum. As these nuclei constitute
the X part of separate AA′X systems, five lines might be
expected in each case. In practice, the outer two lines are
too weak to be observed, and an apparent triplet is observed,
with the separation of the peaks equal to the average coupling
n
constant J(31P-X) (X ) 13C, n ) 3 or X ) 77Se, n ) 2).
The 77Se NMR spectrum of 3c is shown in Figure 2 as an
example.
For the dinuclear species 4a-c, the selenium NMR spectra
contain two signals corresponding to the terminal and
bridging selenium atoms. The resonance due to the bridging
selenium atoms is a doublet of doublets, showing coupling
to the cis (2J ) 9 Hz) and trans (2J ) 110-111 Hz)
phosphorus atoms. These coupling constants are similar to
those observed in the analogous triphenylphosphine com-
plexes.14 The signal corresponding to the terminal selenium
atoms is an apparent singlet, as the expected small coupling
to the neighboring cis phosphorus atoms is not resolved.
The ultraviolet/visible spectra of 3a-c and 4a-c are all
similar and contain absorption maxima around 560 and
410 nm. The extinction coefficients of these bands are
significantly greater (by a factor of more than 102) for the
dinuclear complexes. For example, for 3c (10-3 mol dm-3
Figure 2. 77Se NMR spectrum of 3b.
solution in hexane), ꢀ ) 140 and 420 dm3 mol-1 cm-1 at
560 and 410 nm respectively; for 4c (10-4 mol dm-3 solution
(36) Berger, S.; Braun, S.; Kalinowski, H. O. NMR Spectroscopy of the
Non-Metallic Elements; Wiley: London, 1997; p 954.
7106 Inorganic Chemistry, Vol. 43, No. 22, 2004

Synthesis and Properties of Palladium Diselenolenes
Figure 3. Molecular structure of 3a with H atoms omitted for clarity. In
this and following ORTEP drawings, 20% probability ellipsoids are shown.
Figure 5. Molecular structure of 3c with H atoms omitted for clarity.
Table 10. Selected Bond Lengths (Å) and Angles (deg) in the
Structures of 3a-c
3a
3b
3c
Pd-P(1)
2.325(2)
2.319(3)
2.4183(12)
2.3965(14)
1.887(10)
1.916(9)
2.318(3)
2.326(3)
2.402(1)
2.419(1)
1.90(1)
2.326(2)
2.334(2)
2.4223(9)
2.4034(8)
1.877(7)
Pd-P(2)
Pd-Se(1)
Pd-Se(2)
Se(1)-C(1)
Se(2)-C(6)
Se(2)-C(7)
Se(2)-C(8)
C(1)-C(6)
C(1)-C(7)
C(1)-C(8)
1.87(1)
1.31(1)
1.900(7)
1.305(12)
1.345(10)
P(2)-Pd-P(1)
99.78(9)
100.2(1)
84.34(7)
86.66(4)
88.81(8)
104.2(4)
99.58(6)
88.52(4)
86.69(3)
85.28(5)
104.8(2)
P(1)-Pd-Se(2)
Se(2)-Pd-Se(1)
Se(1)-Pd-P(2)
Pd-Se(1)-C(1)
Se(1)-C(1)-C(6)
Se(1)-C(1)-C(7)
Se(1)-C(1)-C(8)
C(1)-C(6)-Se(2)
C(1)-C(7)-Se(2)
C(1)-C(8)-Se(2)
C(6)-Se(2)-Pd
C(7)-Se(2)-Pd
C(8)-Se(2)-Pd
88.68(7)
87.07(4)
84.60(7)
104.3(3)
122.9(7)
Figure 4. Molecular structure of 3b with H atoms omitted for clarity.
122.4(8)
121.7(8)
105.0(3)
122.3(5)
120.4(5)
105.4(2)
in hexane), the corresponding values are ꢀ ) 2100 and 8700
dm3 mol-1 cm-1. Complexes 4a-c are deep purple in color,
unlike their triphenylphosphine analogues, which are dark
green. This is a consequence of a small (25-30 nm)
hypsochromic shift in the wavelengths of the two bands in
the visible region of the absorption spectra on changing from
PPh3 to PBu3, accompanied by an increase in the relative
intensity of the higher-wavelength band. We do not believe
that these changes reflect significant differences in electronic
or molecular structure.
X-ray Structures of 3a-c. The molecular structures of
3a-c, determined by X-ray structural investigations, are
shown in Figures 3-5. Selected bond lengths and angles
are listed in Table 10. Each complex consists of a distorted
square-planar PdSe2P2 core, with only the outer atoms of
the hydrocarbon ring and the butyl chains protruding
significantly from this plane. The structural parameters for
the central portions of the three compounds are very similar.
The Pd-Se distances in the three structures, lying in the
range 2.396(1)-2.422(1) Å, are shorter than those to the
monodentate benzeneselenolate SePh- in the compounds
[Pd(SePh)2(dppe)] [2.444(1), 2.480(1) Å]37 and trans-[Pd-
120.9(8)
104.8(3)
(SePh)2(PBu3)2] [2.4609(4) Å];38 they are also shorter than
the distances to the SeCN- anion in the compounds
[Pd(SeCN)2(dppe)] [2.477(1) Å]39 and [Pd(SeCN)2(dppp)]
[2.458(1), 2.488(1) Å].39 On the other hand, the present Pd-
Se distances are comparable to those [mean values 2.406(7),
2.407(1), 2.4022(5), and 2.4023(7) Å, respectively] to the
structurally related bidentate ligands in the diselenolenes
[NMe4][Pd(C3S3Se2)2]2,40 [NBu4][Pd(C3S3Se2)(Se2CNEt2)],41
(37) Singhal, A.; Jain, V. K.; Varghese, B.; Tiekink, E. R. T. Inorg. Chim.
Acta 1999, 285, 190.
(38) Alyea, E. C.; Ferguson, G.; Kannan S. Polyhedron 1998, 17, 2231.
(39) Grygon, C. A.; Fultz, W. C.; Rheingold, A. L.; Burmeister, J. L. Inorg.
Chim. Acta 1988, 144, 31.
(40) Faulmann, C.; Legros, J. P.; Cassoux, P.; Cornelissen, J.; Brossard,
L.; Inokuchi, M.; Tajima H.; Tokumoto M. J. Chem. Soc., Dalton
Trans. 1994, 249.
(41) Olk, R.-M.; Dietzsch, W.; Kahlmeier, J.; Jorchel, P.; Kirmse, R.; Sieler,
J. Inorg. Chim. Acta 1997, 254, 375.
Inorganic Chemistry, Vol. 43, No. 22, 2004 7107

Ford et al.
[K(2.2.2-crypt)]2[Pd{Se2C2(CN)2}],42 and [PPh4]2[Pd{Se2C2-
(CO2Me)2}2].43 Smaller values of Pd-Se distances [mean
2.379(5) Å] in the structure of the similarly related compound
[NBu4]2[Pd(C3S5)(C3Se5)]44 are likely caused by S/Se dis-
order. The values of the Pd-P distances in 3a-c, all in the
2.318(3)-2.334(2) Å range, substantially agree with that
[2.333(1) Å] measured for the centrosymmetric trans-[Pd-
(SePh)2(PBu3)2] complex.38 The Se-Pd-Se bond angles in
3a-c [mean 86.8(2)°] are smaller than those formed by the
above pairs of monodentate ligands [99.11(4)°,37 91.2(1)°,
and 94.1(1)° 39], and they are also smaller than those formed
by the bidentate ones [90.74(9)° and 91.18(9)°,40 93.14(4)°,41
91.61(2)°,42 90.03(2)°,43 91.5(1)° 44]. This might be ascribed
both to geometric requirements of the diselenolene ligands
and to steric repulsions between them and the large adjacent
trialkylphosphine ligands in the present structures (see
below). The coordination geometry in each of the complexes
3a-c is slightly asymmetric. One Pd-Se bond is ca. 0.02
Å longer than the other, and one P-Pd-Se angle is
significantly expanded [mean 88.7(1)° vs 84.7(5)°]. We have
previously observed a similar distortion in the structures of
palladium and platinum dithiolenes.45 It is clear from the
NMR data, however, that the asymmetry is not maintained
in solution
Computational Studies. Quantum mechanical calculations
were undertaken, by the procedures described in the Experi-
mental Section, in an attempt to rationalize some of the
unusual structural features noted above and to achieve a
better understanding of the bonding in mononuclear disele-
nolenes and related complexes. A simplified model for 3a
was used initially, with PH3 groups replacing the tertiary
phosphine ligands. Despite the simplification, the model was
found to reproduce the experimental geometry with reason-
able accuracy, yielding a P-Pd-P angle of 98.4° (vs the
experimental value of 99.8°) and an Se-Pd-Se angle of
86.9° (vs 87.1°), although it suffered from a well-known
limitation of computational approaches of this type, in
producing longer metal-donor atom distances than expected
(by 0.04 Å for Pd-P and as much as 0.08 Å for Pd-Se).
The values of the Se-C and C-C distances from the
calculations were also slightly larger than the experimental
ones, but this could be ascribed to apparent shortening of
the latter distances as a result of thermal motion. The fair
agreement obtained in the value of the Se-Pd-Se angle,
despite the larger Pd-Se bond lengths from the calculations,
was due to an Se‚‚‚Se separation predicted to be moderately
greater (3.427 Å) than that found experimentally [3.316(2)
Å].
Table 11. Values of Selected Interatomic Distances (Å) and Bond
Angles (deg) for Models with Phosphines of Increasing Complexity,
Compared with Experimental Data for 3aa
3a
R ) Me
R ) Et
R ) Pr
R ) Bu
Pd-Se1
Pd-Se2
Pd-P1
Pd-P2
2.418
2.396
2.325
2.319
2.522
2.509
2.375
2.390
2.525
2.514
2.396
2.407
2.527
2.515
2.397
2.408
2.527
2.515
2.394
2.408
P1-Pd-Se2
P2-Pd-Se1
P1-Pd-P2
Se1-Pd-Se2
Se‚‚‚Se
88.7
88.4
85.3
100.3
86.0
88.8
84.7
100.9
85.6
88.7
84.9
100.9
85.6
89.0
84.7
100.7
85.6
84.6
99.8
87.1
3.316
3.431
3.424
3.424
3.427
a Labels as in Figure 6, where conformer geometries for R ) Me are
shown.
P-Pd-P angle in 3a-c, primarily imposed by the bulky
phosphines (see below), is nevertheless consistent with any
requirements that might be set by the diselenolene co-ligand,
particularly through its bite size and the consequent prefer-
ential use of specific metal orbitals.
The simplified model above was used for a set of geometry
optimizations in which the P-Pd-P angle was fixed at the
99.8° experimental value, while the adjacent P-Pd-Se angle
was varied in steps (but fixed in the course of each
optimization). These calculations confirmed that the minimum-
energy conformation indeed exhibits symmetrical geometry,
but showed that the bending deformation was not energeti-
cally demanding: a geometry with ca. 4° difference in the
values of the opposite P-Pd-Se angles, approximately
corresponding to the experimental arrangement, lay only 0.14
kcal mol-1 above the energy of the symmetrical model, and
a 10° difference in the angles merely involved an increase
in energy of 1.2 kcal mol-1.
Calculations on more realistic models, with tertiary phos-
phines PR3 (R ) Me, Et, Pr, Bu) arranged, at the beginning
of the optimizations, as the PBu3 groups in 3a, yielded in
all cases the same type of distortion of the metal environment
as found experimentally (Table 11), suggesting that these
structural features result from interactions between the
innermost parts of the alkyl groups and the selenium atoms.
Indeed, with the orientation of phosphine groups shown in
Figure 6a for R ) Me, but representative of the situation
for all the phosphines considered and corresponding to the
“experimental” setting, the two Se atoms are involved in
different contact interactions with PR3 methyl groups (or
methylene groups for R * Me), as there are two methyls
(or methylene groups) in the proximity of Se1 and only one,
but at a shorter distance, near Se2. Applying 60° rotations
to the PMe3 groups about the respective Pd-P axis produced
the expected interchange of P-Pd-Se angle values. More-
over, two conformers could be identified with symmetrically
oriented PMe3 groups, and their unconstrained geometries
substantially attained C2 symmetry in the course of the
optimizations; these are shown in parts b and c of Figure 6.
Their energies are close (within 0.3 kcal mol-1) to that of
the “unsymmetrical” conformer of Figure 6a; presumably, a
preference for the latter is enhanced under the experimental
conditions by the presence of alkyl chains longer than those
of the models with trimethylphosphines. The substantial
These preliminary results showed that (a) in the absence
of interligand crowding, the system adopts a “symmetrical”
geometry, with identical P-Pd-Se angles, and (b) the large
(42) McLauchlan, C. C.; Robowski, S. D.; Ibers, J. A. Inorg. Chem. 2001,
40, 1372.
(43) McLauchlan, C. C.; Ibers, J. A. Acta Crystallogr. C: Cryst. Struct.
Commun. 1999, 55, 30.
(44) Matsubayashi, G.; Tanaka, S.; Yokozawa, A. J. Chem. Soc., Dalton
Trans. 1992, 1827.
(45) Ford, S.; Lewtas, M. R.; Morley, C. P.; Di Vaira, M. Eur. J. Inorg.
Chem. 2000, 933.
7108 Inorganic Chemistry, Vol. 43, No. 22, 2004

Synthesis and Properties of Palladium Diselenolenes
Figure 6. Geometries of conformers of the model system with methylphosphines: (a) unsymmetrical setting (data from column 3 of Table 11); (b, c)
symmetrical arrangements, with unique values for Pd-P and Pd-Se distances. Distances are in angstroms, angles in degrees.
Scheme 5
Figure 7. Possible structures of 5a.
constancy in the value of the P-Pd-P angle for the models
of Table 11 indicates, however, that conclusions drawn on
for determination of the structure by X-ray crystallography
were not obtained.
the basis of the one with PMe3 are likely to be general. In
particular, the similarity in energies of the conformers of
Figure 6a-c and the easy interconversions between them,
verified in the course of the calculations, rationalize the time-
averaged symmetrization occurring in solution, according to
the NMR spectra. More details of the results of calculations
on these systems and a discussion of the similarities and
differences between factors controlling the geometries of the
present diselenolene complexes and those of the previous
dithiolene derivatives45 are provided with the Supporting
Information.
Although the mechanism for the formation of 5a is
unknown, we have established that there is no reaction
between 4a and red selenium, which might have been
generated by thermal decomposition of the 1,4-diselenin
under these conditions. Attempts to prepare compounds
analogous to 5a from 2b and 2c were unsuccessful. The
reactions with [Pd2(dba)3]/PBu3 lead to 3a,b and 4a,b even
when conducted in xylene at reflux over extended periods.
The addition of red selenium to the reaction mixture also
fails to have the desired effect.
Preparation and Characterization of 5a. In the prepara-
tion of 4a, a second palladium species, 5a, can be isolated
in reasonable yield. Mass spectrometry reveals that this red
compound has a formula corresponding to the addition of a
selenium atom to the dinuclear diselenolene. The 31P NMR
spectrum of 5a contains two singlets, corresponding to two
independent, inequivalent tributylphosphine ligands. In the
1H and 13C NMR spectra, there is a corresponding doubling
of all resonances for the phosphorus-bound alkyl groups.
Similarly, each atom in the two alicylic rings gives rise to a
separate signal. It is clear, therefore, that the C2 symmetry
of 4a has been lost. Structures for 5a that are consistent with
these observations are shown in Figure 7. Alternatives, such
as those incorporating a phosphine selenide ligand, can be
excluded on the basis of the NMR spectroscopic data, but
distinguishing between the four remaining possibilities has
not been possible. Unfortunately, crystals of sufficient quality
Reactions of 4a-c. The only previously reported ex-
amples of dinuclear diselenolenes are the triphenylphosphine-
palladium complexes prepared in our own laboratory.14
A
difference between these two systems is that no mononuclear
product containing triphenylphosphine could be isolated from
the reaction of [Pd(PPh3)3] with 2a-c, even though excess
phosphine was present in solution. Furthermore the dinuclear
complexes 4a-c do not react with PPh3, whereas their
treatment with excess PBu3 in toluene solution at room
temperature results in rapid cleavage of the Pd-Se-Pd
bridge and formation of 3a-c (Scheme 5). These differences
presumably reflect the greater basicity of the trialkylphos-
phine.
Complexes 4a-c also react rapidly with an excess of
triethylphosphine (Scheme 5). 31P NMR spectroscopy shows
that the products are not the expected mixed-phosphine
Inorganic Chemistry, Vol. 43, No. 22, 2004 7109

Ford et al.
Scheme 6
derivatives; instead, there is quantitative conversion to species
with triethylphosphine as the only phosphorus-containing
ligand. These are presumed to be the mononuclear disele-
nolenes [Pd{SeC(R1)dC(R2)Se}(PEt3)2] [R1, R2 ) (CH2)n,
n ) 4, 5, 6]. That phosphine exchange takes place in addition
to bridge cleavage indicates that the phosphine ligands in
these compounds are quite labile. Indeed, under these
conditions, it was not possible to isolate the products free
of contamination by tributylphosphine complexes.
Mononuclear diselenolenes containing the chelating diphos-
phine bis(diphenylphosphino)ethane (dppe) can be prepared
by the reaction of a mixture of [Pd2(dba)3] and dppe with a
bis(cycloalkeno)-1,4-diselenin. The yields of the products
6a-c are, however, much lower than those of the analogous
tributylphosphine complexes 3a-c. Use of the corresponding
1,2,3-selenadiazole in place of the 1,4-diselenin results in
the same species being formed, as the reaction pathway
shown in Scheme 1, which leads to azo complexes, does
not compete. Nevertheless, the yields of 6a-c are still poor.
This suggests that a key step in the synthesis of diselenolenes
by this route involves the formation of an intermediate
containing a single monodentate phosphine ligand, a process
that is inhibited by the chelate effect in the case of dppe.
Compounds 6a-c are most efficiently prepared by a
ligand-exchange reaction starting with the dinuclear species
4a-c (Scheme 6). This involves cleavage of the Pd-Se-
Pd bridge, accompanied by displacement of the PBu3 ligand
on subsequent ring closure.
The molecular formulas of 6a-c have been established
by mass spectrometry (Table 8). The mass spectrum of each
complex shows an intense cluster corresponding to the
molecular ion, with the expected isotope distribution. In
contrast to the behavior of 3a-c, loss of the phosphine is
not a major fragmentation pathway: the principal fragment
ion is formed by elimination of the cycloalkyne from the
diselenolene.
The NMR spectroscopic data (Tables 5-7) are in accord
with the proposed structures and resemble those for the
analogous tributylphosphine complexes 3a-c. There are also
similarities with the compounds [M{E2C2(CO2Et)2}(dppe)]
(M ) Pd, E ) S; M ) Pt, E ) S, Se), which we have
prepared in a complementary study via addition of activated
alkynes to the tetrachalcogenide complexes [ME4(dppe)].45
Because of the delocalization of the electrons present in
the metal-ligand bonding system, diselenolenes, like their
sulfur analogues, are expected to display rich and varied
chemical behavior. The properties and reactivity of 3a-c,
4a-c, and 6a-c are currently being investigated further.
Acknowledgment. We thank EPSRC for the provision
of a research studentship to S.F., Johnson Matthey plc for
the loan of palladium salts, and the Ministero dell’ Universita`
e della Ricerca Scientifica e Tecnologica for financial support
to M.V.
Supporting Information Available: More details and discus-
sion of the results of the computational studies of these systems;
crystallographic data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC040069Y
7110 Inorganic Chemistry, Vol. 43, No. 22, 2004