Regioselective versus complete chalcogen transfer reactions of the bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis-(diphenylphosphino)cyclobutane: Full characterization of new, hemilabile ligands and their complexes with palladium(II) and platinum

Article abstract of DOI:10.1039/b305775e

The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been regioselectively oxidized by sulfur to the novel, hemilabile disulfide cis,trans,cis-2,3-bis(diphenylphosphinothioyl)-1,4-bis(diphenylphosphino)cycl

Full text of DOI:10.1039/b305775e

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Regioselective versus complete chalcogen transfer reactions
of the bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis-
(diphenylphosphino)cyclobutane: full characterization of new,
hemilabile ligands and their complexes with palladium(II) and
platinum(II)
Thomas Stampfl,a Rene Gutmann,a Georg Czermak,a Christoph Langes,a Alexander Dumfort,a
Holger Kopacka,a Karl-Hans Onganiab and Peter Brüggeller*a
a Institut für Allgemeine, Anorganische und Theoretische Chemie, Universität Innsbruck,
Innrain 52a, 6020 Innsbruck, Austria
b Institut für Organische Chemie, Universität Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
Received 23rd May 2003, Accepted 3rd July 2003
First published as an Advance Article on the web 24th July 2003
The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been
regioselectively oxidized by sulfur to the novel, hemilabile disulfide cis,trans,cis-2,3-bis(diphenylphosphinothioyl)-
1,4-bis(diphenylphosphino)cyclobutane (2,3-trans-dppcbS2, I). Also the tetrachalcogenides cis,trans,cis-1,2,3,4-
tetrakis(diphenylphosphinoyl)cyclobutane (dppcbO4, II), cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphinothioyl)-
cyclobutane (dppcbS4, III), and cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphinoselenoyl)cyclobutane (dppcbSe4, IV)
have been obtained. Regioselective selenization of the previously prepared disulfide cis,trans,cis-1,3-bis(diphenyl-
phosphinothioyl)-2,4-bis(diphenylphosphino)cyclobutane (1,3-trans-dppcbS2) leads to the unique, mixed
chalcogenide cis,trans,cis-1,3-bis(diphenylphosphinoselenoyl)-2,4-bis(diphenylphosphinothioyl)cyclobutane (trans-
dppcbSe2S2, V). All of the new ligands I–V have been fully characterized by X-ray structure analyses showing folded
cyclobutane rings. The coordination chemistry of the previously prepared hemilabile diselenide cis,trans,cis-1,3-
bis(diphenylphosphinoselenoyl)-2,4-bis(diphenylphosphino)cyclobutane (1,3-trans-dppcbSe2), 1,3-trans-dppcbS2,
I, III, and IV with PdCl2 or PtCl2 has been studied. The novel complexes [Pd2Cl4(1,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)]
(1), [Pt2Cl4(1,3-trans-dppcbS2-P,PЈ,S,SЈ)] (2), [PtCl2(2,3-trans-dppcbS2-P,PЈ)] (3), [Pt2Cl4(dppcbS4-S,SЈ,SЉ,Sٞ)] (4),
[PtCl2(2,3-trans-dppcbSe2-P,PЈ)] (5), and [Pt2Cl4(2,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] (6) have been obtained. 6 has
been fully characterized by an X-ray structure analysis.
group has prepared the five new ligands cis,trans,cis-2,3-
Introduction
bis(diphenylphosphinothioyl)-1,4-bis(diphenylphosphino)-
Though new classes of unsymmetrical bidentate hemilabile
ligands were recently developed and used in organometallic
cyclobutane (2,3-trans-dppcbS2, I),9 cis,trans,cis-1,2,3,4-
tetrakis(diphenylphosphinoyl)cyclobutane
(dppcbO4,
II),
chemistry, which associate the phosphorus atom with donor
atoms such as oxygen, nitrogen or sulfur,1 particularly few
reports exist on selenium. Especially in the case of selenium the
chemistry of hypercoordination continues to be an active
research field.2 Furthermore, selenium would be better suited
than sulfur as a protecting group for PIII when this is required.3
Due to the different donor/acceptor properties of the two Lewis
basic centres of a bidentate hemilabile ligand, its reactions are
highly selective and no isomers of the resulting complexes are
observed.4 Valuable hemilabile ligands have proven usefulness in
homogeneous catalysis, analytical chemistry, cancer and AIDS
research, chemistry of materials, etc.5 Particularly tertiary
phosphine chalcogenides have been used in extraction, catalysis
and biochemical studies and it appears a greater effort is needed
in investigating further applications of these ligands.6
cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphinothioyl)cyclo-
butane (dppcbS4, III), cis,trans,cis-1,2,3,4-tetrakis(diphenyl-
phosphinoselenoyl)cyclobutane (dppcbSe4, IV), and cis,trans,-
cis-1,3-bis(diphenylphosphinoselenoyl)-2,4-bis(diphenylphos-
phinothioyl)cyclobutane (trans-dppcbSe2S2, V) (Scheme 1). The
In this work a comparative study of O, S, and Se transfer
reactions of the bis(bidentate) phosphine cis,trans,cis-1,2,3,4-
tetrakis(diphenylphosphino)cyclobutane (dppcb) and the
resulting complexes is given. Examples of the coordination
chemistry of dppcb itself and its applicability to homogeneous
catalysis and photochemical devices have been given recently.7
Furthermore, the heterodifunctional, hemilabile ligands cis,-
trans,cis-1,3-bis(diphenylphosphinothioyl)-2,4-bis(diphenyl-
phosphino)cyclobutane (1,3-trans-dppcbS2) and cis,trans,cis-
1,3-bis(diphenylphosphinoselenoyl)-2,4-bis(diphenylphos-
phino)cyclobutane (1,3-trans-dppcbSe2), obtained via the first
regioselective sulfurization or selenization of a bis(bidentate)
phosphine, have also been presented only recently.8 Following
these initial reports of S and Se transfer reactions of dppcb, our
Scheme 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
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pure isolation of I from a reaction mixture of seven possible
compounds8 and the production of regioselectively pure V are
unique, where for example the reaction of bis(2,2-bis(diphenyl-
phosphino)ethyl)phenylphosphine with elemental sulfur sim-
ultaneously gives nine from a possible total of 19 different
phosphine sulfides and only the pentasulfide has been
isolated.10 The new ligands II–IV are comparable to
[Ph2P(E)NP(E)Ph2]Ϫ (E = O, S or Se), which have found
important uses as selective metal extractants, NMR shift
reagents and in catalysis.11
sulfur and selenium of purissimum grade quality were
purchased from Aldrich. Dry solvents of purissimum grade
quality and 30% H2O2 were obtained from Fluka. A Schlenk
apparatus and oxygen-free, dry Ar were used in the syntheses
of all ligands and complexes. Solvents were degassed by several
freeze–pump–thaw cycles prior to use.
Instrumentation
1
Fourier-mode 195Pt{1H}, 77Se{1H}, 31P{1H}, 13C{1H} and H
NMR spectra were obtained using
a Bruker DPX-300
Since there seems to be general belief that the ring closure
driving force for six-membered rings is weak and the formation
of seven-membered rings must compete with other reaction
pathways, in particular with the possibility of obtaining poly-
nuclear species,1 a study of the coordination properties of the
ligands presented herein is interesting. This allows the com-
parison of oxygen and sulfur versus selenium as the donor set in
a series of bis(bidentate) chelates, where similar studies for
Ph2P(E)NHP(E)Ph2 (E = O, S or Se) have been presented.12 The
use of comparable ligands as promoters in oxidation, water–gas
shift reaction, and as catalysts in the polymerization of olefins
and hydrogenation reactions, promise further applications of
their complexes.6 Herein, we present the new complexes
[Pd2Cl4(1,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] (1), [Pt2Cl4(1,3-trans-
dppcbS2-P,PЈ,S,SЈ)] (2), [PtCl2(2,3-trans-dppcbS2-P,PЈ)] (3),
[Pt2Cl4(dppcbS4-S,SЈ,SЉ,Sٞ)] (4), [PtCl2(2,3-trans-dppcbSe2-
P,PЈ)] (5), and [Pt2Cl4(2,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] (6)
(Scheme 2). During the formation of 5 or 6 the new, hemilabile
spectrometer (internal deuterium lock). Positive chemical
shifts are downfield from the standards: 1.0 M Na2PtCl6 for
the 195Pt{1H}, (CH3)2Se for the 77Se{1H}, 85% H3PO4 for the
31P{1H}, and TMS for the 13C{1H} and 1H resonances.
Syntheses
2,3-trans-dppcbS2 (I). Dppcb (0.50 g, 0.63 mmol) was
dissolved in toluene (150 mL) and sulfur (0.040 g, 1.26 mmol)
was added with vigorous stirring. The reaction mixture was
stirred at ambient temperature for 30 h and a clear solution was
obtained. The solution was cooled down to 5 ЊC and after
3 days colourless crystals were filtered off. These crystals
correspond to 1,3-trans-dppcbS2.8b The volume of the filtrate
was reduced to 70 mL. The yellow–greenish solution was
cooled down to 5 ЊC and after 10 days colourless crystals were
filtered off and dried in vacuo. The crystals were recrystallized
from dichloromethane–n-hexane at ambient temperature.
(Yield: 0.089 g, 16%.) Mp 248–252 ЊC. (Found: C, 72.73; H,
5.31; S, 7.60; C52H44P4S2 requires C, 72.89; H, 5.18; S, 7.48%.)
31P{1H} NMR (CH2Cl2): δ 39.9 (dd, P(S)Ph2), Ϫ20.2 (dd,
ligand
cis,trans,cis-2,3-bis(diphenylphosphinoselenoyl)-1,4-
bis(diphenylphosphino)cyclobutane (2,3-trans-dppcbSe2) is
produced. In the case of sulfur, the transfer of sulfur atoms
from tertiary phosphine sulfides to more basic phosphine
molecules is well documented, and finds numerous synthetic
applications.13 However, migration or elimination reactions of
selenium leading to pure compounds like 5 or 6 are rare.14
3
4
PPh2), J(P,P) = 12 Hz, J(P,P) = 6 Hz. Positive ion FAB-MS:
m/z (m/zcalcd) 857.2 (856.9) [M]ϩ, 825.2 (824.8) [M Ϫ S]ϩ, 779.1
(778.8) [M Ϫ H Ϫ Ph]ϩ, 671.0 (670.7) [M Ϫ H Ϫ PPh2]ϩ, 639.0
(638.7) [M Ϫ H Ϫ P(S)Ph2]ϩ. Single crystals suitable for an
X-ray structure analysis with the composition 2,3-trans-
dppcbS2 were obtained from a CH2Cl2 solution of 2,3-trans-
dppcbS2 via gas-phase diffusion of CH2Cl2 into n-hexane under
reduced pressure and at ambient temperature.
dppcbO4 (II). Dppcb (0.10 g, 0.126 mmol) was dissolved
in DMF (4 mL) and 30 mL of 30% H2O2 was added drop by
drop with vigorous stirring at ambient temperature. A white
voluminous precipitate formed. It was filtered off, washed with
DMF and water, and dried in vacuo. A white powder was
recrystallized from a 1 : 2 DMF–water mixture. (Yield: 0.090 g,
83%.) Mp 264–267 ЊC. (Found: C, 72.78; H, 5.23; O, 7.25;
C52H44O4P4 requires C, 72.90; H, 5.18; O, 7.47%.) 31P{1H}
NMR (CH2Cl2): δ 25.9 (s). Positive ion FAB-MS: m/z (m/zcalcd
)
858.0 (857.8) [M ϩ H]ϩ, 780.2 (779.7) [M Ϫ Ph]ϩ, 655.2 (655.6)
[M Ϫ P(O)Ph2]ϩ. Single crystals suitable for an X-ray structure
analysis with the composition dppcbO4ؒtoluene were obtained
from a toluene solution of dppcbO4 at ambient temperature.
dppcbS4 (III). Powdered sulfur (0.022 g, 0.686 mmol) was
dissolved in toluene (20 mL) at 50 ЊC and dppcb (0.100 g,
0.126 mmol) was added with vigorous stirring. The solution
was stirred at 50 ЊC for 48 h. Then the solution was cooled
down to 5 ЊC and after several days colourless crystals formed.
They were filtered off, washed with toluene in order to remove
excess sulfur and dried in vacuo. The volume of the filtrate was
reduced to 10 mL and the crystallization procedure was
repeated. (Overall yield: 0.085 g, 70%.) Mp 318–320 ЊC.
(Found: C, 68.75; H, 5.11; S, 13.35; C52H44P4S4ؒ0.5toluene
requires C, 68.93; H, 5.01; S, 13.26%.) 31P{1H} NMR (toluene):
δ 43.2 (s, Paxial), 32.6 (s, Pequatorial). Positive ion FAB-MS: m/z
(m/zcalcd) 921.3 (921.0) [M]ϩ, 764.8 (764.8) [M Ϫ 2H Ϫ 2Ph]ϩ,
703.2 (702.8) [M Ϫ H Ϫ P(S)Ph2]ϩ. Single crystals suitable for
an X-ray structure analysis with the composition dppcbS4ؒ
Scheme 2
Experimental
Reagents and general procedures
Dppcb, 1,3-trans-dppcbS2, and 1,3-trans-dppcbSe2 were
prepared as described earlier.7f,8 PdCl2, PtCl2, and elemental
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
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0.5toluene were obtained from a toluene solution of dppcbS4 at
ambient temperature.
44.97; H, 3.19; S, 4.62%.) 195Pt{1H} NMR (CH2Cl2): δ Ϫ3942.6
1 2
(dd, J(Pt,P) = 3789 Hz, J(Pt,P) = 116 Hz). 31P{1H} NMR
(CH2Cl2, 310 K): δ 29.3 (dd, P(S)Ph2), Ϫ4.7 (dd, PPh2),
3
3J(P,P)cis = 31 Hz, J(P,P)trans = 7 Hz. Positive ion FAB-MS:
dppcbSe4 (IV). Dppcb (0.100 g, 0.126 mmol) was dissolved in
toluene (20 mL) and powdered selenium (0.060 g, 0.760 mmol)
was added with vigorous stirring. The reaction mixture was
stirred at 45 ЊC for 3 days. The slurry was filtered in order to
remove excess selenium and the volume of the filtrate was
reduced to 10 mL. The yellow–greenish solution was cooled
down to 5 ЊC and after several days colourless crystals formed.
They were filtered off, washed with toluene and dried in vacuo.
The volume of the filtrate was reduced to 8 mL and the crystal-
lization procedure was repeated. (Overall yield: 0.101 g, 69%.)
Mp 295.7 ЊC (DSC under nitrogen). (Found: C, 57.63; H, 4.31;
C52H44P4Se4ؒ0.5toluene requires C, 57.73; H, 4.19%.) 77Se{1H}
m/z (m/zcalcd) 1387.6 (1387.9) [M Ϫ H]ϩ, 1352.6 (1352.4) [M Ϫ
HCl]ϩ, 1316.1 (1316.0) [M Ϫ 2HCl]ϩ, 1281.8 (1281.5) [M Ϫ
HCl Ϫ 2Cl]ϩ.
[PtCl2(2,3-trans-dppcbS2-P,PЈ)] (3). 2,3-trans-dppcbS2 (I,
0.064 g, 0.075 mmol) was dissolved in CH2Cl2 (25 mL) and
PtCl2 (0.020 g, 0.075 mmol) was added with vigorous stirring.
Then compound 3 was prepared in an analogous manner to 2.
(Yield: 0.040 g, 47%.) Mp > 320 ЊC. (Found: C, 55.80; H, 4.12;
S, 5.83; C52H44Cl2P4PtS2 requires C, 55.62; H, 3.95; S, 5.71%.)
1
195Pt{1H} NMR (CH2Cl2): δ Ϫ4470.3 (t, J(Pt,P) = 3666 Hz).
NMR (CH2Cl2): δ Ϫ373.5 (d, Seaxial
,
1J(Se,P) = Ϫ759 Hz),
31P{1H} NMR (CH2Cl2): δ 40.1 (s, P(S)Ph2), 32.4 (s, PPh2).
Positive ion FAB-MS: m/z (m/zcalcd) 1121.6 (1121.9) [M Ϫ H]ϩ,
1086.7 (1086.5) [M Ϫ HCl]ϩ.
Ϫ400.7 (d, Seequatorial
,
1J(Se,P) = Ϫ777 Hz). 31P{1H} NMR
(CH2Cl2): δ 38.2 (s, Paxial), 28.9 (s, Pequatorial). Positive ion
FAB-MS: m/z (m/zcalcd) 1110.9 (1110.7) [M ϩ 2H]ϩ, 1032.2
(1031.7) [M ϩ 2H Ϫ Se]ϩ, 766.8 (766.6) [M ϩ H Ϫ Se Ϫ
P(Se)Ph2]ϩ. Single crystals suitable for an X-ray structure
analysis with the composition dppcbSe4ؒ0.5toluene were
obtained from a toluene solution of dppcbSe4 at 55 ЊC.
[Pt2Cl4(dppcbS4-S,SЈ,SЉ,Sٞ)] (4). DppcbS4 (III, 0.068 g,
0.074 mmol) was dissolved in CH2Cl2 (10 mL) and PtCl2
(0.0394 g, 0.148 mmol) was added with vigorous stirring. The
reaction mixture was stirred at 40 ЊC for 10 days. Then the
solvent was completely removed, and the residue was washed
with toluene and dried in vacuo. DMF (10 mL) was added to
the greyish brown powder and the slurry was stirred at ambient
temperature for 20 h. The slurry was filtered, and the solvent of
the filtrate was completely removed producing a brown powder.
It was recrystallized from DMF/Et2O. (Yield: 0.059 g, 55%.)
Mp > 320 ЊC. (Found: C, 43.12; H, 3.25; S, 9.01; C52H44Cl4-
P4Pt2S4 requires C, 42.98; H, 3.05; S, 8.83%.) 31P{1H} NMR
(CH2Cl2): δ 33.9 (s, 2J(Pt,P) = 115 Hz). Positive ion ESI-MS: m/z
(m/zcalcd) 1454.3 (1454.0) [M ϩ H]ϩ, 1416.2 (1416.6) [M Ϫ
HCl]ϩ, 1187.2 (1187.0) [M Ϫ PtCl2]ϩ, 1018.2 (1018.5) [M Ϫ
2P(S)Ph2]ϩ.
trans-dppcbSe2S2 (V). Powdered selenium (0.039 g, 0.500
mmol) was suspended in toluene (20 mL) and 1,3-trans-
dppcbS2 (0.121 g, 0.141 mmol) dissolved in toluene (20 mL) was
added with vigorous stirring. The reaction mixture was stirred
at 65 ЊC for 5 days. The slurry was filtered in order to remove
excess selenium and the volume of the filtrate was reduced to 10
mL. Then colourless crystals were obtained according to the
procedure described in the case of IV. (Overall yield: 0.096 g,
64%.) Mp 310–314 ЊC. (Found: C, 62.70; H, 4.71; S, 6.15;
C52H44P4Se2S2ؒ0.5toluene requires C, 62.83; H, 4.56; S, 6.04%.)
1
77Se{1H} NMR (CH2Cl2): δ Ϫ401.0 (d, Seaxial, J(Se,P) = Ϫ740
Hz), Ϫ424.7 (d, Seequatorial, 1J(Se,P) = Ϫ790 Hz). 31P{1H} NMR
(CH2Cl2, 193 K): δ 40.9 (dd, P(S)axial), 38.2 (dd, P(Se)axial), 32.6
(d, P(S)equatorial), 28.7 (d, P(Se)equatorial), 3J(P(Se)axial,P(S)axial) = 28
[PtCl2(2,3-trans-dppcbSe2-P,PЈ)] (5).
1,3-trans-dppcbSe2
(0.114 g, 0.120 mmol) was dissolved in CH2Cl2 (30 mL) and
PtCl2 (0.032 g, 0.120 mmol) was added with vigorous stirring.
The reaction mixture was stirred at ambient temperature for
4 days. Then compound 5 was prepared in an analogous
manner to 1. (Yield: 0.044 g, 30%.) Mp 294–296 ЊC. (Found: C,
51.15; H, 3.73; C52H44Cl2P4PtSe2 requires C, 51.33; H, 3.65%.)
3
3
Hz, J(P(Se)axial,P(S)equatorial) = J(P(S)axial,P(Se)equatorial) = 9 Hz.
Positive ion FAB-MS: m/z (m/zcalcd) 1016.0 (1015.9) [M ϩ H]ϩ,
984.1 (983.8) [M ϩ H Ϫ S]ϩ, 952.1 (951.8) [M ϩ H Ϫ 2S]ϩ.
Single crystals suitable for an X-ray structure analysis with
the composition trans-dppcbSe2S2ؒ0.5toluene were obtained
from a toluene solution of trans-dppcbSe2S2 at ambient
temperature.
1
195Pt{1H} NMR (CH2Cl2): δ Ϫ4473.9 (t, J(Pt,P) = 3664 Hz).
1
31P{1H} NMR (CH2Cl2): δ 33.2 ( s, P(Se)Ph2, J(Se,P) = Ϫ764
Hz), 32.5 (s, PPh2). Positive ion FAB-MS: m/z (m/zcalcd) 1217.1
(1216.7) [M]ϩ, 1181.1 (1181.3) [M Ϫ Cl]ϩ, 1103.2 (1103.3) [M ϩ
H Ϫ Cl Ϫ Se]ϩ, 1022.0 (1022.3) [M Ϫ HCl Ϫ 2Se]ϩ.
[Pd2Cl4(1,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] (1).
1,3-trans-
dppcbSe2 (0.143 g, 0.150 mmol) was dissolved in CH2Cl2 (30
mL) and PdCl2 (0.053 g, 0.300 mmol) was added with vigorous
stirring. The reaction mixture was stirred at 40 ЊC for 5 days
with prevention of light. The slurry was filtered, and the solvent
of the filtrate was completely removed. The brown residue was
washed with toluene and dried in vacuo. A brown powder was
recrystallized from CH2Cl2. (Yield: 0.088 g, 45%.) Mp > 190 ЊC
dec. (Found: C, 47.96; H, 3.31; C52H44Cl4P4Pd2Se2 requires C,
47.85; H, 3.40%.) 31P{1H} NMR (CH2Cl2): δ 58.1 (s), 33.4 (s,
1J(Se,P) = Ϫ583 Hz). Positive ion FAB-MS: m/z (m/zcalcd) 1232.2
(1232.4) [M Ϫ 2HCl]ϩ, 1127.1 (1127.0) [M Ϫ HCl Ϫ Cl Ϫ Pd]ϩ,
1093.1 (1092.6) [M Ϫ 3Cl Ϫ Pd]ϩ, 871.0 (870.8) [M Ϫ HCl Ϫ
3Cl Ϫ 2Pd Ϫ Se]ϩ.
[Pt2Cl4(2,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] (6). DppcbSe4 (IV,
0.150 g, 0.135 mmol) was dissolved in CH2Cl2 (20 mL) and
PtCl2 (0.0718 g, 0.270 mmol) dissolved in CHCl3 (20 mL) was
added with vigorous stirring. The solution was stirred at 50 ЊC
for 6 days. Then the solvent was completely removed, and the
dark brown residue was washed with toluene and dried
in vacuo. A dark brown powder was recrystallized from CH2Cl2.
(Yield: 0.072 g, 36%.) Mp > 320 ЊC dec. (Found: C, 41.97; H,
3.10; C52H44Cl4P4Pt2Se2 requires C, 42.12; H, 2.99%.) 31P{1H}
NMR (CH2Cl2): δ 12.5 (d, P(Se)Ph2, 1J(Se,P) = Ϫ604 Hz,
2J(Pt,P) = 121 Hz), 6.9 (d, PPh2, 1J(Pt,P) = 3780 Hz), 3J(P,P)cis
= 26 Hz. Positive ion ESI-MS: m/z (m/zcalcd) 1483.9 (1483.7)
[M ϩ H]ϩ, 1445.9 (1446.2) [M Ϫ HCl]ϩ, 1404.8 (1404.7) [M ϩ
H Ϫ Se]ϩ. Solutions of the dark brown residue in DMF contain
a small amount of [Pt2Cl4(dppcb)], which is not soluble in
CH2Cl2. Single crystals suitable for an X-ray structure analysis
with the composition [Pt2Cl4(2,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)]ؒ
0.5[Pt2Cl4(dppcb)]ؒ5.5DMF were obtained by gas-phase
diffusion of Et2O into a solution of the dark brown residue in
DMF at 4 ЊC.
[Pt2Cl4(1,3-trans-dppcbS2-P,PЈ,S,SЈ)] (2). 1,3-trans-dppcbS2
(0.080 g, 0.093 mmol) was dissolved in CH2Cl2 (15 mL) and
PtCl2 (0.0495 g, 0.186 mmol) was added with vigorous stirring.
The reaction mixture was stirred at 40 ЊC for 10 days. The slurry
was filtered, and the solvent of the filtrate was completely
removed producing a brown residue. A brown powder was
recrystallized from CH2Cl2. (Yield: 0.081 g, 63%.) Mp > 320 ЊC.
(Found: C, 45.10; H, 3.03; S, 4.75; C52H44Cl4P4Pt2S2 requires C,
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
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Table 1 Crystallographic data for 2,3-trans-dppcbS2 (I), dppcbO4 (II), and dppcbS4 (III)
Compound
I
II
III
Formula
Fw
C52H44P4S2
856.92
C52H44O4P4ؒC7H8
948.94
C52H44P4S4ؒ0.5C7H8
967.11
Crystal system, space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
Triclinic, P1
12.6057(2)
13.0547(2)
15.4843(2)
98.1899(9)
107.8746(8)
109.6632(9)
2196.22(6)
2
Orthorhombic, Pna21
31.5748(3)
13.0352(1)
Tetragonal, I41/a
38.638(3)
24.7531(3)
13.102(4)
γ/Њ
U/Å3
10188.0(2)
8
19560(7)
16
Z
T/K
218(2)
17807
5546
0.0413, 0.1111
223(2)
43171
14543
0.0396, 0.0999
183(2)
12402
8402
0.0436, 0.0993
Measured reflections
Independent reflections
Final R1, wR2 [I > 3σ(I )]
Table 2 Crystallographic data for dppcbSe4 (IV), trans-dppcbSe2S2 (V), and [Pt2Cl4(2,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] (6)
Compound
Formula
IV
V
6
C52H44P4Se4ؒ0.5C7H8
C52H44P4S2Se2ؒ0.5C7H8
C52H44Cl4P4Pt2Se2ؒ0.5C52H44Cl4P4Pt2ؒ
5.5C3H7NO
2547.12
Fw
1154.71
1060.91
Crystal system, space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
Tetragonal, I41/a
38.730(3)
Tetragonal, I41/a
38.688(1)
Triclinic, P1
14.9398(5)
18.8634(6)
19.2668(6)
68.799(2)
80.929(2)
87.253(2)
4998.6(3)
2
13.281(2)
13.1777(3)
γ/Њ
U/Å3
19921(4)
16
19724(1)
16
Z
T/K
183(2)
10844
6281
0.0474, 0.0600
213(2)
17829
6042
0.0425, 0.0879
223(2)
20732
10537
0.0355, 0.0780
Measured reflections
Independent reflections
Final R1, wR2 [I > 3σ(I ) IV, 6; I > 2σ(I ) V]
X-Ray crystallography
13C{1H} and 1H NMR and IR spectroscopies
Details of the crystals and data collections are summarized
in Tables 1 and 2. In the cases of I, II, V, and 6, the data
collections were performed on a Nonius Kappa CCD diffract-
ometer using combined ꢀ–ω-scans. Cell refinement, data
reduction, and the empirical absorption correction were done
by Denzo and Scalepack programs.15 In the cases of III and IV,
all data were collected on a Siemens P4 diffractometer using
ω-scans. Cell refinement and data reduction were done by the
software of the Siemens P4 diffractometer,16 and the empirical
absorption corrections were based on ψ-scans of nine
reflections, respectively (χ = 78 to 102Њ, 360Њ scans in 10Њ steps in
ψ).17
Within the classes of compounds I–V and 1–6, the 13C{1H} and
1H NMR spectra in CH2Cl2-d2 are rather insensitive to struc-
tural variations. The 13C{1H} NMR resonances of the phenyl
rings occur in the range 128–136 as multiplets. The cyclobutane
rings show broad 13C{1H} NMR signals centred at 39. In the
1H NMR spectra the phenyl rings show multiplets in the range
6.5–8.0. Broad peaks centred at 4.3 are attributed to the cyclo-
butane hydrogen atoms, where the integrated relative intensities
are correct. For all compounds I–V and 1–6, the ν(P᎐E; E = O,
᎐
S, Se) vibrations in the IR spectra (KBr) are obscured by lattice
vibrations of dppcb.
All structure determination calculations were carried out
using SHELXTL NT 5.10 including SHELXS-9718 and
SHELXL-97.19 Final refinements on F 2 were carried out with
anisotropic thermal parameters for all non-hydrogen atoms in
the cases I–V and 6. Except for IV, the protons attached to the
cyclobutane rings were located and isotropically refined with
fixed U. All other hydrogen atoms were included using a riding
model with isotropic U values depending on the Ueq of the
adjacent carbon atoms. The toluene solvent molecules of III–V
are disordered. In the case of 6, one DMF solvent molecule is
also disordered. In compound V additional disorder occurs
with respect to the selenium and sulfur positions, where the
occupancy factors have been refined. Tables 3 and 5 contain
selected bond distances and bond angles of I–V and 6,
respectively.
Results and discussion
Dppcb is the first case of a bis(bidentate) phosphine containing
four phosphorus atoms, where a regioselective sulfurization or
selenization leading to pure products is possible.8 For compar-
able polyphosphorus ligands like bis(2,2-bis(diphenylphos-
phino)ethyl)phenylphosphine, different phosphine chalco-
genides are only present in slightly unequal proportions,
indicating a small degree of steric control over the chalcogen
transfer reactions.10 Nevertheless also for this phosphine, two
sulfurization rules acting in concert reduce the number of
actual reaction products from a possible 19 to just nine. In the
case of the oxidation of dppcb by S or Se as many as seven
different species could be present in the reaction system: dppcb,
its monochalcogenide, three isomers of the dichalcogenides, the
trichalcogenide, and the tetrachalcogenide. Also the reaction
between 1,1,2-tris(diphenylphosphino)ethane and an atomic
equivalent of elemental selenium in refluxing benzene yields a
CCDC reference numbers 211317–211322.
See http://www.rsc.org/suppdata/dt/b3/b305775e/ for crystal-
lographic data in CIF or other electronic format.
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
3428

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Table 3 Selected bond lengths (Å) and angles (Њ) for 2,3-trans-
dppcbS2 (I), dppcbO4 (II), dppcbS4 (III), dppcbSe4 (IV), and
trans-dppcbSe2S2 (V)
dppcbS2 is not complete, and the by-product 2,3-trans-dppcbS2
(I) is presented as clean substance in this work. Trans-
dppcbSe2S2 (V) is only obtainable starting from 1,3-trans-
dppcbS2, since the rate of chalcogen atom transfer between
phosphines is seen to increase in the order S Ӷ Se < Te.13
Comparable to this, the importance of the rate of chalcogen
transfer is also illustrated by the preparation of the mixed
dichalcogenide derivative diphenylphosphinoselenoyldiphenyl-
phosphinothioylmethane (dppmSeS).3 It is only possible to
produce dppmSeS from dppmS and elemental selenium. Due
to the more efficient steric control over the regioselective
formation of 1,3-trans-dppcbSe2 than over 1,3-trans-dppcbS2,
the ligand 2,3-trans-dppcbSe2 was not obtained in pure form.
However, using PtCl2 as template this ligand can be prepared
from 1,3-trans-dppcbSe2 as well as from dppcbSe4 (IV). Via
regioselective selenium migration or extrusion the unique
complexes [PtCl2(2,3-trans-dppcbSe2-P,PЈ)] (5) and [Pt2Cl4(2,3-
trans-dppcbSe2-P,PЈ,Se,SeЈ)] (6) are produced.
Compound I
P(2)–S(1)
P(3)–S(2)
P(1)–C(1)
P(2)–C(2)
P(3)–C(3)
1.9546(15)
1.9575(15)
1.894(3)
1.845(3)
1.830(3)
P(4)–C(4)
C(1)–C(2)
C(1)–C(4)
C(2)–C(3)
C(3)–C(4)
1.877(3)
1.571(4)
1.564(4)
1.540(4)
1.561(4)
C(4)–C(1)–C(2)
C(3)–C(2)–C(1)
87.92(19)
88.38(19)
C(2)–C(3)–C(4)
C(3)–C(4)–C(1)
89.16(19)
87.88(19)
Compound II
P(1)–O(1)
P(2)–O(2)
P(3)–O(3)
P(4)–O(4)
P(1)–C(1)
P(2)–C(2)
1.485(3)
1.482(3)
1.480(3)
1.481(3)
1.831(4)
1.848(3)
P(3)–C(3)
P(4)–C(4)
C(1)–C(2)
C(1)–C(4)
C(2)–C(3)
C(3)–C(4)
1.819(3)
1.812(3)
1.561(5)
1.587(5)
1.561(5)
1.558(5)
Crystal structures of 2,3-trans-dppcbS2 (I), dppcbO4 (II),
dppcbS4 (III), dppcbSe4 (IV), and trans-dppcbSe2S2 (V)
C(2)–C(1)–C(4)
C(1)–C(2)–C(3)
86.5(2)
87.4(2)
C(4)–C(3)–C(2)
C(3)–C(4)–C(1)
87.5(2)
86.7(2)
A view of I is shown in Fig. 1 and Table 3 contains selected bond
lengths and angles. The crystal structure of I consists of a pair
of MMMP- and MPPP-enantiomers per unit cell. The chirality
of I is induced by the axes of the folded cyclobutane ring,7f,8b
where in the case of the MMMP-enantiomer the sign of the
torsion angles along P(1)–C(1)–C(2)–P(2), P(2)–C(2)–C(3)–
P(3), and P(3)–C(3)–C(4)–P(4) is negative, thus three times
minus (MMM) and the sign of the torsion angle along P(4)–
C(4)–C(1)–P(1) is positive, thus plus (P) (see Fig. 1). The
dihedral angle between the planes through C(1), C(2), C(3) and
C(1), C(3), C(4), respectively, is 152.6Њ. This folding of the
cyclobutane ring is nearly identical with the corresponding
value of 151.8Њ in [Pd2Cl4(1,3-trans-dppcbS2-P,PЈ,S,SЈ)] (7) and
consistent with the usual range from 145 to 160Њ.8b The C–C
bond lengths of the cyclobutane ring in I (see Table 3) are also
comparable with the typical range of 1.545–1.607 Å.7c,d,f,8 Inter-
estingly, the significantly different cyclobutane C–C bond
lengths of I indicate that the phenyl rings of all four phos-
phorus moieties are involved in steric interactions. This is a
consequence of the “anti” conformation of the P(S)Ph2 groups,
thus giving rise to the possibility of the observed regioselective
oxidation of dppcb due to steric hindrance. The solid state
structure of I shows the isomer with two equatorial P(S)Ph2 and
Compound III
P(1)–S(1)
P(2)–S(2)
P(3)–S(3)
P(4)–S(4)
P(1)–C(1)
P(2)–C(2)
1.9514(14)
1.9426(14)
1.9425(16)
1.9544(14)
1.856(4)
P(3)–C(3)
P(4)–C(4)
C(1)–C(2)
C(1)–C(4)
C(2)–C(3)
C(3)–C(4)
1.822(4)
1.845(4)
1.575(5)
1.584(5)
1.548(5)
1.580(5)
1.834(4)
C(2)–C(1)–C(4)
C(3)–C(2)–C(1)
86.7(3)
88.3(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(1)
87.8(3)
86.9(3)
Compound IV
P(1)–Se(1)
P(2)–Se(2)
P(3)–Se(3)
P(4)–Se(4)
P(1)–C(1)
P(2)–C(2)
2.120(3)
2.096(3)
2.096(3)
2.128(3)
1.875(9)
1.847(9)
P(3)–C(3)
P(4)–C(4)
C(1)–C(2)
C(1)–C(4)
C(2)–C(3)
C(3)–C(4)
1.854(8)
1.826(9)
1.564(11)
1.603(11)
1.522(10)
1.570(11)
C(2)–C(1)–C(4)
C(3)–C(2)–C(1)
86.1(6)
88.7(6)
C(2)–C(3)–C(4)
C(3)–C(4)–C(1)
88.8(6)
85.6(6)
Compound V
P(1)–S(1)
P(2)–Se(2)
P(3)–S(3)
P(4)–Se(4)
P(1)–C(1)
P(2)–C(2)
1.977(17)
2.073(3)
1.983(9)
2.132(4)
1.850(4)
1.836(4)
P(3)–C(3)
P(4)–C(4)
C(1)–C(2)
C(1)–C(4)
C(2)–C(3)
C(3)–C(4)
1.830(4)
1.856(4)
1.563(5)
1.573(5)
1.547(5)
1.570(5)
two axial PPh groups, respectively (see Fig. 1). The P᎐S dis-
᎐
2
tances of 1.9546(15) and 1.9575(15) Å are within the usual
range of 1.89–1.97 Å for non-coordinating P᎐S groups,8b,22
᎐
being shorter than the sum of the covalent radii of the P and S
C(2)–C(1)–C(4)
C(3)–C(2)–C(1)
86.9(3)
88.1(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(1)
87.6(3)
86.9(3)
mixture of products together with some unreacted material,
producing ten different phosphorus environments in the mix-
ture.20 Like dppcb, this phosphine shows considerable steric
crowding, and consequently certain conformations are likely
to be strongly favoured. However, in the case of bis(diphenyl-
phosphino)methane (dppm) the monochalcogenides (dppmE;
E = S, Se) are easily obtained, which is in contrast to 1,2-bis-
(diphenylphosphino)ethane (dppe).13,21 This difference is
because the more electron deficient PV atom affects the activity
of the PIII atom in dppmE, but not in the monochalcogenides
of dppe where the phosphorus atoms are separated by two
carbon atoms like in dppcb. Furthermore, since both sides of
dppcb are equal, no preference for the monochalcogenides of
dppcb is possible, and they have not been obtained in pure
form. The production of 1,3-trans-dppcbE2 (E = S, Se) is
favoured by steric hindrance, and a subtle choice of solvents
makes it possible to synthesize both ligands in pure form.8
However, the steric control over the formation of 1,3-trans-
Fig. 1 Molecular structure of the MMMP-enantiomer of 2,3-trans-
dppcbS2 (I). For clarity only the first atoms of the phenyl rings are
shown.
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
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atoms (2.12 Å)23 and corresponding to a double-bond.6 Since
only 1,3-trans-dppcbS2 and I are formed of a total of three
possible disulfides of dppcb, the sulfurization of dppcb shows
that through steric substituent effects, the geometry of tetra-
tertiary phosphines can be tailored such that access to a reactive
centre is only possible from certain directions leading to
regioselectivity.24
A view of II is presented in Fig. 2 and Table 3 contains
selected bond lengths and angles. Two independent MMMP-
and MPPP-enantiomers are present in the crystals of II. The
folding angles of the cyclobutane rings are 142.2 and 143.3Њ,
respectively. This means that this cyclobutane folding is even
more pronounced in II than in the “free” ligand dppcb showing
145.6Њ.7f Nevertheless, the C–C bond lengths of the cyclobutane
rings in II (see Table 3) remain within the usual range given
above. Typically, all axial P–Ccyclobutane bond lengths are longer
in II than the corresponding equatorial ones.7f,8b The P᎐O dis-
᎐
tances ranging from 1.478(3) to 1.496(3) Å in II are comparable
to the corresponding parameters of 1.465(5) and 1.478(4) Å
for P-unidentate Ph2PNHP(O)Ph2 in trans-[Pt(CH3)Cl{Ph2-
PNHP(O)Ph2-P}2],21 but shorter than in trans-[Hg{Ph2-
PNP(O)Ph2-P,O}2] showing 1.5035(15) Å.25 Nevertheless, these
Fig.
3
Molecular structure of the MMMP-enantiomer of
dppcbS4 (III). For clarity only the first atoms of the phenyl rings are
shown.
P᎐O bond lengths are consistent with typical double-bonds,
dppcb of 145.6Њ.7f Within statistical significance all C–C bond
lengths of the cyclobutane rings in III–V (see Table 3) are again
typical. Also all axial P–Ccyclobutane bond lengths are longer in III
and V than the equatorial ones.7f,8b However, the equatorial
P–Ccyclobutane bond lengths of IV are elongated (see Table 3) as a
consequence of attractive interactions between the equatorial
selenium atoms and hydrogen atoms attached to adjacent
phenyl groups (Se(2) ؒ ؒ ؒ H = 3.081 Å, Se(3) ؒ ؒ ؒ H = 2.895 Å).
᎐
since the sum of the covalent radii of the P and O atoms is 1.85
Å.26 Tertiary phosphine oxide ligands play an important role in
transition metal chemistry due to the fact that the steric and
electronic properties of the ligands can be tuned by modifying
the substituents bonded to the phosphorus atom.27 In the case
of II its conformation changes, when water molecules are
incorporated in the lattice.28 This fine tuning of the conform-
ation of II leads to perfect chelating orientations of the two
cis-diphenylphosphinoyl groups attached to each side of the
cyclobutane rings via hydrogen bonding to the water molecules,
which is not present in the actual conformation of II presented
in this work (see Fig. 2).
Nevertheless, the equatorial P᎐E distances (E = S or Se) in III
᎐
and IV are significantly smaller than the axial ones. Since no
short intermolecular contact approaches are present in the
lattice of III and steric differences are negligible within
isomorphous crystal structures, this must be attributed to an
electronic effect, being more pronounced in the case of selen-
ium than for sulfur. Of course, this effect is obscured by the
disorder in V. Unsymmetrical ligands like V or [Ph2P(O)N-
P(E)Ph2]Ϫ (E = S or Se), bearing a mismatch in donor atoms,
have been remarkably poorly studied.11 The dissimilar donor
properties of asymmetrical ligands could play a role in catalysis
and in the synthesis of heterobimetallic compounds. Com-
parable to the P᎐S bonds in I and III, also the P᎐Se bond
᎐
᎐
distances in IV are shorter than the sum of covalent radii of
P and Se atoms (2.27 Å).23 Furthermore, the lattices of II–V
contain incorporated toluene solvent molecules. In the case of
the diselenide of dppe (dppeSe2), its marked selective inclusion
behaviour is of interest with respect to the separation of solvent
mixtures.30 When a mixture of two isotopic liquid guests is
equilibrated with a host like dppeSe2 capable of forming a
clathrate phase with them the mole ratio of the guests in the
clathrate phase differs from that in the coexisting liquid.31 It
seems possible that other sulfur or selenium derivatives of
ditertiary phosphines of these types like the bis(bidentate)
ligands III–V might also show significant inclusion behaviour.
Fig. 2 Molecular structure of the MPPP-enantiomer of dppcbO4 (II).
For clarity only the first atoms of the phenyl rings are shown.
Investigation of I–V in the solution state
There is a formal analogy between the addition of sulfur or
other chalcogens to phosphorus(III) and its coordination to
a metal atom, both in terms of the change in phosphorus
hybridization which occurs and in the 31P NMR parameters.10
The crystal structures of III–V are isomorphous, thus allow-
ing a comparison of the electronic influences of sulfur versus
selenium.7f,29 As an example a view of III is shown in Fig. 3.
Table 3 contains selected bond lengths and angles for III–V.
Pairs of MMMP- and MPPP-enantiomers are present in the
lattices. In the lattice of V disorder occurs with respect to the
sulfur and selenium positions due to site sharing of two flipped
configurations. Selenium produces a slightly larger folding
of the cyclobutane rings than sulfur, where the corresponding
folding angles for III–V are 145.7, 145.2 and 145.5Њ, respect-
ively. These values are nearly identical with the folding angle of
This leads to a pronounced down-field shift of the P᎐E (E = O,
᎐
S, Se) 31P resonances compared with the related phosphines. In
the case of I the 31P{1H} NMR signals for the P(S)Ph2 and PPh2
groups occur at 39.9 and Ϫ20.2, respectively, indicating that the
folding of the cyclobutane ring is fast on the NMR time-scale at
3
ambient temperature. The J(P,P) value of 12 Hz is typical,
since it is conformationally dependent and the variations in
3J(P,P) where at least one phosphorus atom is trivalent arise
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Table 4 Activation energy (∆G‡), activation entropy (∆S‡) and coalescence temperature (Tc) for the folding of the cyclobutane ring
Compound
∆G‡/kJ molϪ1
∆S‡/J molϪ1 KϪ1
Tc/K
Dppcba
37.4
40.0
51.1
61.2
63.5
62.2
41.8
38.0
57.9
348.2
99.9
179.1
132.6
132.6
65.2
200
220
273
333
343
338
223
203
2,3-Trans-dppcbS2 (I)
DppcbO4 (II)
DppcbS4 (III)
DppcbSe4 (IV)
Trans-dppcbSe2S2 (V)
[Pt2Cl4(1,3-trans-dppcbS2-P,PЈ,S,SЈ)] (2)
[Pd2Cl4(1,3-trans-dppcbS2-P,PЈ,S,SЈ)] (7)
614.0
a Data from ref. 7f.
Table 5 Selected bond lengths (Å) and angles (Њ) for [Pt2Cl4(2,3-trans-
dppcbSe2-P,PЈ,Se,SeЈ)] (6)
produces the largest down-field shift with respect to dppcb
at Ϫ19.0, followed by selenium and oxygen. A typical example
for the temperature-dependent 31P{1H} NMR spectra in this
series of ligands is given in Fig. 4 for the case of IV. For II the
decoalescence of the single peak occurs below 273 K resulting
in resonances at 27.2 for the axial P(O)Ph2 groups and at 19.5
for the equatorial ones at 253 K. The kinetic parameters for
II–IV are summarized in Table 4. The ∆G‡ values clearly show
that heavier elements (Se > S ӷ O) slow down the folding of the
cyclobutane rings. Nevertheless, around room temperature all
the phosphorus atoms of dppcb and II–V readily invert
between axial and equatorial positions, thus adopting the
conformation of the cyclobutane ring to the stereochemical
requirements of a complexed metal. By contrast, the high
pyramidal inversion barrier of tervalent phosphorus implies
that a macrocyclic polyphosphine exists at room temperature as
a complex mixture of diastereomers.35 This is an advantage of
dppcb and its derivatives, since in a macrocycle all the lone pairs
may point away from each other, so that no chelation is possible
in that conformation.35
Pt(1)–Se(1)
Pt(1)–P(1)
Pt(1)–Cl(1)
Pt(1)–Cl(2)
2.4063(12)
2.237(3)
2.359(3)
2.320(3)
Pt(2)–Se(2)
Pt(2)–P(4)
Pt(2)–Cl(3)
Pt(2)–Cl(4)
2.3975(14)
2.237(3)
2.319(3)
2.359(3)
P(2)–Se(1)
P(1)–C(1)
P(2)–C(2)
C(1)–C(2)
C(1)–C(4)
2.162(3)
1.886(10)
1.813(9)
1.570(12)
1.557(12)
P(3)–Se(2)
P(3)–C(3)
P(4)–C(4)
C(2)–C(3)
C(3)–C(4)
2.153(3)
1.820(10)
1.908(10)
1.552(13)
1.541(13)
P(1)–Pt(1)–Se(1)
Cl(1)–Pt(1)–Se(1)
Cl(2)–Pt(1)–Se(1)
P(1)–Pt(1)–Cl(1)
P(1)–Pt(1)–Cl(2)
Cl(1)–Pt(1)–Cl(2)
P(2)–Se(1)–Pt(1)
103.93(7)
79.93(8)
167.78(8)
174.66(10)
87.80(10)
88.60(11)
102.74(8)
P(4)–Pt(2)–Se(2)
Cl(3)–Pt(2)–Se(2)
Cl(4)–Pt(2)–Se(2)
P(4)–Pt(2)–Cl(3)
P(4)–Pt(2)–Cl(4)
Cl(3)–Pt(2)–Cl(4)
P(3)–Se(2)–Pt(2)
103.67(8)
169.46(8)
80.92(9)
86.87(10)
175.38(11)
88.55(11)
103.68(9)
C(4)–C(1)–C(2)
C(3)–C(2)–C(1)
88.3(7)
88.1(7)
C(4)–C(3)–C(2)
C(3)–C(4)–C(1)
89.5(7)
89.0(7)
The 77Se chemical shifts of phosphine selenides occur at the
extreme high-shielding end of the selenium chemical shift
range.32e This has been attributed to major contributions from
the polar structure R3Pϩ–SeϪ, where the 77Se{1H} NMR reson-
ances of IV and V occurring from Ϫ373.5 to Ϫ424.7 are located
within the typical range.6,8a,32e,36 It has been suggested that when
the two P(Se)Ph2 groups are close, the polar form R3Pϩ–SeϪ is
somewhat disfavoured.32e Since IV and V show identical con-
formations in the solid state and an analogous variable-temper-
ature solution behaviour, the mean values for the chemical
shifts of Ϫ387.1 in IV and Ϫ412.9 in V confirm this effect. The
smaller van der Waals radius of sulfur versus selenium favours
R3Pϩ–SeϪ in V. It is well established that the sign of 1J(Se,P) is
negative in the type of compound IV or V, where their mag-
nitudes are located within the typical range.2,6,20,32a,e,37 Interest-
ingly, in IV and V 1J(Se,P) is larger for the equatorial P(Se)Ph2
mainly from lone-pair orientation effects determined by steric
interactions between adjacent diphenylphosphino groups.10,20,32
4
Also J(P,P) of 6 Hz is reasonable, because it is expected that
long-range couplings are observed in certain rigid systems like
I where particular geometrical relationships prevail.32e In the
case of I the folding of the cyclobutane ring leads to an
exchange between equatorial and axial positions of the P(S)Ph2
and PPh2 groups, respectively. On cooling, the 31P{1H} NMR
signals for both groups broaden and at 193 K the signal for
the PPh2 groups is split into two peaks at Ϫ21.6 and Ϫ27.5.
Integration shows that the peak at Ϫ27.5 corresponds to 82.8%
and this main fraction is attributed to equatorial P(S)Ph2
groups consistent with the solid state structure of I (see Fig. 1).
Obviously, the bulkier P(E)Ph2 groups (E = O, S, Se) compared
with PPh2 prefer equatorial positions, since the same conform-
3
groups than for the axial ones. The J(P(Se),P(S)) values in V
are typical for 3J(PV,PV) couplings.32c ∆G‡ for the folding of the
cyclobutane ring in V nicely fits in the corresponding values for
III and IV(see Table 4), clearly indicating that ∆G‡ depends on
the atomic mass of the chalcogen in this class of compounds.
ation is observed for
6 and [Co2X4(2,3-trans-dppcbO2-
P,PЈ,O,OЈ)] (XϪ = ClϪ, BrϪ), where 2,3-trans-dppcbO2 is cis,-
trans,cis-2,3-bis(diphenylphosphinoyl)-1,4-bis(di-
phenylphosphino)cyclobutane.33 Using the variable-temper-
ature 31P{1H} NMR parameters obtained for I in CH2Cl2 and
the DNMR3 program,34 the ∆G‡ and ∆S‡ values of 40.0 kJ
Investigation of [Pd2Cl4(1,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] (1),
[Pt2Cl4(1,3-trans-dppcbS2-P,PЈ,S,SЈ)] (2), [PtCl2(2,3-trans-
dppcbS2-P,PЈ)] (3), [Pt2Cl4(dppcbS4-S,SЈ,SЉ,Sٞ)] (4),
[PtCl2(2,3-trans-dppcbSe2-P,PЈ)] (5), and [Pt2Cl4(2,3-trans-
dppcbSe2-P,PЈ,Se,SeЈ)] (6) in the solution state
molϪ1 and 348.2 J molϪ1
K
Ϫ1, respectively, for the folding of
the cyclobutane ring have been obtained. Compared with the
corresponding data for dppcb (see Table 4), the folding of the
cyclobutane ring has slowed down due to the presence of two
further sulfur atoms in I. ∆S‡ is dramatically increased as a
consequence of the two different conformations of I.
Since especially with respect to their catalytic application,
particularly few reports exist on bis-phosphine monosulfide
complexes,38 an investigation of the related compounds 1–6 is
interesting. Compared with P^O chelating ligands, hemilabile
P^P(E) (E = S, Se) ligands act as much stronger chelates to
transition metals. In the case of [PtPh2(CO)(η1-P^S)] bidentate
phosphorus–sulfur ligands (P^S) are bound to the metal centre
in a monodentate fashion, but these initially formed species
undergo a slow ring closure process with extrusion of carbon
In the series II–IV only the 31P{1H} NMR spectrum of II
remains a single resonance at 25.9 at ambient temperature
comparable to dppcb.7f In both cases III and IV two distinct
signals corresponding to axial and equatorial phosphorus
atoms occur at 43.2 and 32.6 and at 38.2 and 28.9, respectively.
The low-field peaks of III and IV are attributed to the axial
P(E)Ph2 groups (E = S, Se).7f Interestingly, oxidation by sulfur
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
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Fig. 4 Temperature-dependent 31P{1H} NMR spectra for dppcbSe4 (IV) in toluene (296–368 K) and in CH2Cl2 (178–293 K).
monoxide and formation of the chelate [PtPh2(P^S)] products.1
For the ligands 1,3-trans-dppcbE2 (E = S, Se) also the formation
of η2-P^P(E) (E = S, Se) chelate complexes leading to 1, 2, and
behaviour is comparable to the ligand o-Ph2PNHC6H4P(S)Ph2,
where monodentate P-bound or κ2-P,S-bound compounds
result.39 The selenium donor groups of 1,3-trans-dppcbSe2 and
IV can be readily de-coordinated and migration or elimination
of selenium followed by subsequent re-coordination leads to 5
or 6. This de-coordination effect is also typical for hemilabile
7
8b has been achieved (see Scheme 2). However, for the ligands
2,3-trans-dppcbE2 (E = S, Se) both monometallic (3, 5) as well
as bimetallic coordinations (6) are possible. This complexation
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
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P^N ligands.4 In contrast to 5 and 6, the selenium migration
during the reaction of bis(diphenylphosphinoselenoyl)methane
(dppmSe2) with [PtCl{PhP(CH2CH2PPh2)2}]Cl produces no
pure complex.36 Obviously, the ability of the selenium-based
ligand 2,3-trans-dppcbSe2 to fulfil the requirements imposed
by the central atoms is due to its great flexibility and notable
chelating ability as well as the employment of soft donor
atoms.12
properties especially in the latter case leads to the instability
of seven-membered ring systems.44
Crystal structure of 6
Though ligand IV contains only selenium donor atoms, it is
hemilabile because upon coordination the resulting seven-
membered rings are unstable and in the case of 6 de-co-
ordination followed by regioselective extrusion of selenium and
re-coordination occurs.4,14a In order to characterize 6 definitely
an X-ray structure analysis was performed. Views of 6 are
shown in Fig. 5 and 6 and Table 5 contains selected bond lengths
and angles. The crystals containing 6 are a unique example of a
co-crystallization of two different complexes, namely 6 and 8.
As a consequence the unit cell consists of two discrete
[Pt2Cl4(2,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] molecules, one discrete
[Pt2Cl4(dppcb)] molecule, and eleven DMF solvent molecules.
[Pt2Cl4(dppcb)] and one disordered DMF solvent molecule are
located on centres of symmetry. As in the cases I–V the folding
of the cyclobutane ring in 6 (see Fig. 6) induces chirality and a
pair of MMMP- and MPPP-enantiomers is present in the unit
cell. This folding is reduced to 155.9Њ in 6 compared with the
range of 142.2–152.6Њ in I–V and 151.8Њ in 7. Nevertheless, the
C–C bond lengths of the cyclobutane ring in 6 (see Table 5)
The structure type of 1 and 2 (see Scheme 2) has been
confirmed by the X-ray structure analysis of the analogous
compound 7.8b 1J(Se,P) of Ϫ755 Hz for the “free” ligand 1,3-
trans-dppcbSe2 8a is reduced to Ϫ583 Hz in 1, clearly indicating
coordinated P(Se)Ph2 groups.2,6,11,36,37,40 Compared with δ(Pt) of
Ϫ4577 for [Pt2Cl4(dppcb)] (8),7f the corresponding parameter
of Ϫ3942.6 for 2 is shifted to lower field, but remains located
within the usual range for analogous complexes containing
hemilabile ligands.4,11,14a,21,29,37a,b 1J(Pt,P) of 3630 Hz is smaller
in 8 than the same coupling of 3789 Hz in 2, being a con-
sequence of the enlargement of the chelate ring size, since this is
also observed in 6 and comparable compounds.39 Also 2J(Pt,P)
of 116 Hz in 2 is typical.11,14a,21,29,37a,b,39 3J(P,P)cis of 31 Hz is
similar to the corresponding parameter in 1,3-trans-dppcbE2
(E = S, Se), 6, and 7,8 3J(P,P)trans of 7 Hz in 2 being smaller for
this structure type. Due to the good solubility of 2 in CH2Cl2, a
series of temperature-dependent 31P{1H} NMR spectra is
possible. At 193 K the resonances for the axial and equatorial
P(S)Ph2 and PPh2 groups, respectively, are clearly resolved. The
resulting kinetic parameters34 are summarized in Table 4, where
for purposes of comparison also the corresponding data for
7 have been determined. As a consequence of the smaller
square-planar stabilization energy for PdII than for PtII,7d ∆G‡
for the folding of the cyclobutane ring is smaller in 7 than in 2.
This flexibility of the Pd–P bonds if reflected in the large
positive ∆S‡ for 7, allowing easy access to different conform-
ations of the six-membered rings.8b
The complexes 3, 5, and 6 contain the ligands I and 2,3-trans-
dppcbSe2 (see Schemes 1 and 2). The P,PЈ-coordination of
these ligands leads to the monometallic species 3 and 5, whereas
the P,PЈ,Se,SeЈ-coordination of 2,3-trans-dppcbSe2 produces
the bimetallic compound 6. This is an intriguing example of the
versatile coordination behaviour of these hemilabile ligands.
The structure type of 3 and 5 has been confirmed by the X-ray
structure analysis of the analogous complex [PtCl2(2,3-trans-
dppcbO2-P,PЈ)]41 and the X-ray structure of 6 is given below.
Compared with δ(Pt) of Ϫ3942.6 for 2, the corresponding
parameters of Ϫ4470.3 for 3 and Ϫ4473.9 for 5 are shifted to
higher fields and are nearly identical with δ(Pt) of Ϫ4577 for 8,7f
clearly indicating the five-membered ring P,PЈ-coordination for
3 and 5. This is confirmed by the reduction of 1J(Pt,P) of 3789
Hz in 2 to 3666 Hz in 3 and 3664 Hz in 5, where the latter two
couplings are again in line with 1J(Pt,P) of 3630 Hz in 8.7f
Fig. 5 Molecular structure of the MMMP-enantiomer of [Pt2Cl4(2,3-
trans-dppcbSe2-P,PЈ,Se,SeЈ)] (6).
1
Furthermore, J(Se,P) of Ϫ764 Hz in 5 is comparable to the
magnitudes of the same parameter in IV and V and typical for
non-coordinating P(Se)Ph2 groups,2,6,20,32a,e,37 thus also con-
firmimg its structure type shown in Scheme 2. By contrast,
1J(Se,P) of Ϫ604 Hz in 6 is reduced similar to 1 as a con-
sequence of coordination.2,6,11,36,37,40 As in 2, the so formed
six-membered rings of 6 produce an enlargement of 1J(Pt,P) of
3780 Hz.39 Also J(Pt,P) of 121 Hz in 6 is comparable to the
2
same parameter of 116 Hz in 2 and 115 Hz in 4 and located
within the usual range.11,14a,21,29,37a,b,39 In the case of 4 (see
Scheme 2) the occurrence of two seven-membered rings joined
by cyclobutane is confirmed by the X-ray structure analyses of
[Co2X4(dppcbO4-O,OЈ,OЉ,Oٞ)] (XϪ = ClϪ, BrϪ, IϪ).42 4 is a rare
example of seven-membered chelate ring formation among
metal–phosphine sulfides or selenides, where the seven-
membered metallacyclic rings are interestingly stable and
flexible similar to complexes of the disulfide of dppe.43 How-
ever, since the relative donor- versus acceptor-strength follows
the order P(O)Ph2 < P(S)Ph2 ≤ P(Se)Ph2, the lack of acceptor
Fig.
6
Views of the MMMP-enantiomer of [Pt2Cl4(2,3-trans-
dppcbSe2-P,PЈ,Se,SeЈ)] (6) with the least-squares plane through the
cyclobutane ring perpendicular to the projection plane. For clarity only
the first atoms of the phenyl rings are shown.
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
3433

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remain located within the typical range of 1.545–1.607 Å.7c,d,f,8
Since 2,3-trans-dppcbSe2 is analogous to I, the X-ray structures
of I and 6 reveal, how the conformation of this type of ligand
changes upon coordination neglecting packing differences. In I
the torsion angle along P(1)–C(1)–C(2)–P(2) of 22.9(3)Њ (see
Fig. 1) is significantly different from the corresponding par-
ameter along P(3)–C(3)–C(4)–P(4) of 35.6(3)Њ. However, in 6
(see Fig. 5) the same parameters are identical within statistical
significance leading to a mean value of 23.0(8)Њ. The same effect
is observed for the cis phosphorus ؒ ؒ ؒ phosphorus distances in
I and 6. In I P(1) ؒ ؒ ؒ P(2) of 3.430(2) Å is significantly shorter
than P(3) ؒ ؒ ؒ P(4) of 3.647(2) Å, whereas in 6 the analogous
parameters are again identical within statistical significance
showing a mean value of 3.511(3) Å. Thus, the conformation
of the 2,3-trans-dichalcogenides of dppcb is easily adjusted to
fulfil the requirements of a hemilabile bis(bidentate) ligand. In
the case of the tetrasulfide of bis[{(diphenylphosphino)ethyl}-
phenylphosphino]methane two pairs of sulfur atoms point in
opposite directions and such an arrangement would preclude
chelation to a metal.45 The six-membered Pt–Se–P–C–C–P ring
conformations in 6 can be most readily described as pseudo-
boats (see Fig. 5 and 6) with P(2) or P(3) at the prow (0.976 and
0.926 Å above the plane, respectively) and P(1) or P(4) at the
stern (0.327 and 0.274 Å above the plane, respectively) of the
“boats”.11 These conformations are comparable to the pseudo-
boat structure of the six-membered Pd–O–P–N–P–S ring
in [Pd{Ph2P(O)NP(S)Ph2-O,S}(tmeda)]PF6, where tmeda is
N,N,NЈ,NЈ-tetramethylethylenediamine.11 The two coordin-
ation planes of 6 include an angle of 11.2Њ. Interestingly, the
atoms Pt(2), Cl(3), Cl(4), P(4), and Se(2) are completely co-
planar within statistical significance (see Fig. 6). The same
near-perfect square-planar geometry at the metal centre is
evident in trans-[Pt{Ph2PNHC6H4P(S)Ph2-P,S}2][ClO4]2, pre-
sumably because of the lack of strain imposed at the platinum
centre by the seven-membered rings.39 However, the largest
deviation of 0.083 Å from the least-squares plane through
Pt(1), Cl(1), Cl(2), P(1), and Se(1) in 6 is similar to the largest
deviation of 0.068 Å from the least-squares coordination plane
in co-crystallized 8. Thus, since packing differences are
negligible within the same crystal, the degree of co-planarity of
the coordination planes for five- and six-membered rings
depends on the specific steric requirements of the chelating
ligands, being different for the two chelating moieties of
2,3-trans-dppcbSe2. In 6 this leads to a significantly longer
Pt(1)–Se(1) bond length of 2.4063(12) Å than Pt(2)–Se(2)
(1, 5, 6) are unique,8 where 5 and 6 show the novel ligand 2,3-
trans-dppcbSe2 and 6 has also been characterized definitely.
Typically, dppcb has been converted to all respective chalco-
genides with retention of stereochemical configuration.47 It has
often been observed that the non-phosphorus donor E (E = S,
Se) of the ligands in complexes like 1, 2, and 6 is weakly co-
ordinated in solution to the metal centre, generating potential
catalytically active systems, by providing, under mild condi-
tions, a vacant site for the coordination and activation of
organic substrates.1 A further application of this class of
ligands is that they are able to form complexes with unusual
structures.25,46b Among the methods for the synthesis of trans-
ition-metal clusters containing bridging chalcogenido ligands,
that involving tertiary phosphine chalcogenide R3PE (E = S, Se
or Te) has been proved to be one of the most effective.48 Also
the considerable difference between BINAP and BINAP(O)
complexes bears important implications for catalysis.49 As in
the cases 1–6, for an increase in the potential catalytic activity
of mono- and bimetallic systems it is important to constrain the
chelating groups in mutually cis positions.45 The chirality of 1–6
is also a desirable feature.50 Our future studies will focus on the
catalytic applications of these systems.
Acknowledgements
We thank the Fonds zur Förderung der wissenschaftlichen
Forschung, Austria, for financial support.
References and notes
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᎐
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Summary
We have reported the preparation and full characterization in
the solid as well as in solution state of five new ligands,
2,3-trans-dppcbS2 (I), dppcbO4 (II), dppcbS4 (III), dppcbSe4
(IV), and trans-dppcbSe2S2 (V), derived from a bis(bidentate)
phosphine via regioselective or complete chalcogen transfer
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(E = S, Se) have been presented. They correspond to
[Pd2Cl4(1,3-trans-dppcbSe2-P,PЈ,Se,SeЈ)] (1), [Pt2Cl4(1,3-trans-
dppcbS2-P,PЈ,S,SЈ)] (2), [PtCl2(2,3-trans-dppcbS2-P,PЈ)] (3),
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The compounds containing hemilabile selenium-based ligands
[PtCl2(2,3-trans-dppcbSe2-
D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
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D a l t o n T r a n s . , 2 0 0 3 , 3 4 2 5 – 3 4 3 5
3435