Reactivity of platinum stanna-closo-dodecaborate complexes: First hydroformylation studies

Article abstract of DOI:10.1002/1099-0682(200209)2002:9<2261::AID-EJIC2261>3.0.CO;2-6

The synthesis and characterization of two (dppp)Pt complexes with the stannaborate ligand SnB₁₁H₁₁ is described. In the case of the salt [Bu₃MeN]₂[(dppp)Pt (SnB₁₁H₁₁)₂] the struc

Full text of DOI:10.1002/1099-0682(200209)2002:9<2261::AID-EJIC2261>3.0.CO;2-6

SHORT COMMUNICATION
Reactivity of Platinum Stanna-closo-dodecaborate Complexes:
First Hydroformylation Studies
Lars Wesemann,*[a] Siegbert Hagen,[a] Thiemo Marx,[a] Ingo Pantenburg,[a] Markus Nobis,[b]
and Birgit Drießen-Hölscher*[b]
Dedicated to Professor Dieter Naumann on the occasion of his 60th birthday
Keywords: Boranes / Cluster compounds / Homogeneous catalysis / Platinum / Tin
The synthesis and characterization of two (dppp)Pt com-
plexes with the stannaborate ligand SnB11H11 is described.
In the case of the salt [Bu3MeN]2[(dppp)Pt(SnB11H11)2] the
structure in the solid state was determined by X-ray crystal
structure analysis. As a result of comparative catalysis studies
plexes we found selective hydroformylation of 1-octene cata-
lysed by the [(dppp)PtPh(SnB11H11)]− and [(dppp)Pt-
(SnB11H11)2]2− complexes.
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
between SnCl3 and [SnB11H11]−-substituted platinum com- 2002)
Introduction
Asymmetric hydroformylation using chiral phosphanes
has been carried out successfully with platinum com-
plexes.[11] Most of the platinum complexes studied so far in
hydroformylation experiments are of the type L2PtCl2 and
have to be reacted with SnCl2 to provide hydroformylation
activity;[12,13] tin() chloride inserts into the PtϪCl bond
and forms the trichlorostannyl ligand.[14] The role of this
ligand is the subject of many studies and it appears that the
lability of the SnCl3 group is one reason for the activation
of platinum complexes.[15] Besides these trichlorostannyl
complexes several very interesting alternative platinum cata-
lysts have been developed ([Pt(C2H4)(chelating phos-
phane)]/CH3SO3H;[16] platinum-diphenylphosphinous acid
complex catalyst).[17] The easily accessible stanna-closo-do-
decaborate cluster is resistant towards moisture and air and
reacts with most of the transition metal electrophiles
studied so far by complexation and formation of a metal-
tin bond. Since (phosphane)Pt-(R)SnCl3 complexes are well
known for their catalytic activity in hydroformylation reac-
tions we started a comparative study on the hydroformyl-
ation activity of SnCl3 and SnB11H11 complexes of plat-
inum.
We are interested in the coordination abilities of the het-
ero-closo-dodecaborate clusters [EB11H11]2Ϫ of the group 14
elements.[1,2] The first example of the complexation of these
closo-borates at a transition metal centre was the copper
compound {[Bu4N]2[ClCu(CB11F11)]} reported by Strauss
et al.[3] For E ϭ Sn we have synthesised coordination com-
pounds of the type LnM-(SnB11H11) for a wide variety of
transition metals (Fe, Mo, Rh, Ir, Ni, Pd, Pt).[4Ϫ7] Charac-
terization of the hydride complex [Bu3MeN][trans-
(Et3P)2PtH(SnB11H11)] led us to the conclusion that the
stannaborate ligand has a higher trans influence than the
trichlorostannyl ligand (SnCl3).[7] Furthermore, we found
that the tin ligand [SnB11H11]Ϫ activates PtϪC bonds to-
wards isocyanide insertion.[8] In the field of carbonylation
hydroformylation is one of the most important reactions
catalysed homogeneously by transition metal complexes,
with cobalt and rhodium complexes being amongst the
most important.[9] The mechanistic aspects of platinum-
catalysed hydroformylation have been studied exten-
sively.[10] In particular, the insertion of the carbon
monoxide molecule and the effect of the bite angle of the
chelating phosphane have been investigated.
Results and Discussion
At the beginning of our stannaborate complex synthesis
we decided to use the 1,3-bis(diphenylphosphanyl)propane
(dppp) as the chelating ligand. Following our standard pro-
cedure two stannaborate complexes were synthesised in
high yield by nucleophilic substitution of the chloride anion
with the stannaborate dianion (Scheme 1 and 2).
[a]
Institut für Anorganische Chemie, Universität zu Köln
Greinstr. 6, 50939 Köln, Germany
Fax: (internat.) ϩ 49-(0)221/470-4899
E-mail: lars.wesemann@uni-koeln.de
[b]
Institut für Makromolekulare und Technische Chemie der
Technischen Hochschule Aachen,
Worringerweg 1, 52074 Aachen, Germany
Eur. J. Inorg. Chem. 2002, 2261Ϫ2265  WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ1948/02/0909Ϫ2261 $ 20.00ϩ.50/0
2261

L. Wesemann, B. Drießen-Hölscher et al.
SHORT COMMUNICATION
Scheme 1
Scheme 2
The salts 1 and 2 were characterised by elemental ana-
Due to steric crowding around the platinum centre the
lysis, NMR spectroscopy and in the case of 2 by a single- square-planar coordination is slightly distorted towards a
crystal structure analysis. It is well established that the 2JSnP tetrahedral
arrangement with torsion angle
a
coupling constant permits unambiguous assignment of the (PϪPЈϪSnϪSnЈ) of 17°. The interatomic distances are sim-
stereochemistry in PtII complexes. Indirect coupling be- ilar to those in other stannaborate complexes.
tween ligand atoms is known to be much larger for trans
Hydroformylation experiments were carried out with 1-
2
than for cis related ligands.[18] Here the JSnP of the signal octene in CH2Cl2 with four different (dppp)Pt complexes:
at δ ϭ 2.40 ppm (trans to Sn) is around 10 times larger than 1, 2, [(dppp)Pt(Ph)SnCl3] (3) and [(dppp)Pt(SnCl3)2] (4).
the respective coupling of the signal at δ ϭ Ϫ5.40 ppm (cis Due to isomerization of 1-octene during the catalysis four
to Sn).
different aldehydes are the expected products of this conver-
Single crystals of complex 2 were obtained at room tem- sion (Scheme 3).
perature from acetone and methanol solution. The salt
crystallises as yellow-green octahedra in the orthorhombic
space group Pbcn with the dianion [(dppp)Pt(SnB11H11)]2Ϫ
lying on a twofold rotation axis.[19] Figure 1 shows the mo-
lecular structure of the dianion of 2.
Scheme 3. Products of the hydroformylation of 1-octene and the
olefins 2-, 3- and 4-octene, which result from preceding isomeriz-
ation (isomerized olefins were drawn without considering the ste-
reochemistry).
Besides other criteria the importance of olefin hydro-
formylation arises, of course, from the selectivity of the al-
dehyde formation. The n/iso ratio, which is always an im-
portant parameter in hydroformylation catalysis, refers to
the amount of n-nonanal (5) and iso-nonanal (6), which is
usually determined by gas chromatography. Table 1Ϫ4
show the results of the catalysis experiments with complexes
Figure 1. Molecular structure of the [(dppp)Pt(SnB11H11)2]2Ϫ dian-
˚
ion in the crystal structure of 2; bond lengths [A] and angles [°]
(with estimated standard deviations in parentheses): PtϪSn
2.596(1), PtϪP 2.279(2), SnϪB2 2.288(8), SnϪB3 2.322(8), SnϪB4
2.285(8), SnϪB5 2.308(9), SnϪB6 2.299(9), PϪC1 1.821(7),
PϪPtϪPЈ 91.07(8), SnϪPtϪSnЈ 88.87(2), PϪPtϪSn 91.32(4)
2262
Eur. J. Inorg. Chem. 2002, 2261Ϫ2265

Reactivity of Platinum Stanna-closo-dodecaborate Complexes
SHORT COMMUNICATION
Table 4. Hydroformylation results of 1-octene with complex 2[a]
and their dependence on the CO/H2 pressure at 140 °C
1Ϫ4. From comparative catalysis studies between SnCl3-
and [SnB11H11]Ϫ-substituted platinum complexes we found
the highest activity at 100 °C for the SnCl3-substituted de-
rivative. However, at 120 °C the activity of the stannaborate
complex 1 is the highest and rises upon increasing the tem-
perature. Thus, complexes 1 and 2 are thermally stable at
140 °C whereas their SnCl3 counterparts decompose at this
temperatures. In the range of temperature studied the n/iso
selectivity is much higher with the borate complex, al-
though upon increasing the temperature the amount of iso-
merized olefins also increases. Only a slight pressure de-
Pressure [bar]
n/iso
TOF [hϪ1
]
iso. Olefins[b]
20
30
40
50
8.9
10.1
9.7
9.2
25.4
41.4
63.7
73.9
43.0%
24.8%
27.6%
26.7%
[a]
Reactions were carried out in 10 mL of CH2Cl2 under 50 bar of
CO/H2 with an octene/Pt catalyst ratio of 250 (0.02 mmol cat.) and
[b]
reaction times of 2 h.
The isomerized olefins were detected by
pendence of the n/iso ratio in the case of the reactions with GC analysis.
complex 2 was observed (Table 4). In our opinion it is very
interesting that the isomerized olefins (Table 3) which were
In order to study the olefin selectivity, in a separate ex-
periment the hydroformylation of trans-4-octene at 140 and
160 °C was investigated with complex 2. Here the formation
of 3-propyl-1-hexanal was not detected; olefin isomeri-
zation took place. The internal olefins thus formed did not
undergo hydroformylation, and the only aldehydes obtained
were n-nonanal (1-nonanal, 5) and iso-nonanal (2-methyl-
1-octanal, 6).
formed during catalysis were not hydroformylated by the
stannaborate complexes. Instead 1-octene is selectively
transformed into the aldehydes 5 and 6. The olefin selectiv-
ity observed is presumably due to the higher steric crowding
in the cluster compounds 1 and 2. Furthermore, no reaction
products from aldol condensation of the produced alde-
hydes were detected in the experiments with 1 and 2.
Table 1. Initial TOF[a] [hϪ1] for the hydroformylation of 1-octene
with (dppp)Pt complexes 1؊4 at different temperatures[b]
Conclusion
Pt compound
100 °C
120 °C
140 °C
160 °C
To conclude, with the stanna-closo-dodecaborate dianion
we have found a new ligand for the activation of platinum
complexes for hydroformylation. The Pt(SnB11H11) deriva-
tives studied here have a higher thermal stability and are
more selective in hydroformylation experiments than their
1
2
3
4
11.0
19.3
50.5
31.6
62.1
72.8
23.6[c]
34.1
73.9
52.7
46.6[c]
10.2
[c]
[c]
[c]
[c]
[a] Turnover frequencies were calculated as mol of aldehyde per mol SnCl3 analogues. n- and iso-nonanal (5, 6) are the only
[b]
of Pt per hour. Reactions were carried out in 10 mL of CH2Cl2
aldehydes formed during the catalysis.
under 50 bar of CO/H2 with an octene/Pt catalyst ratio of 250
[c]
(0.02 mmol cat.) and reaction times of 2 h.
the catalyst.
Decomposition of
Experimental Section
Table 2. Influence of the temperature of the hydroformylation
experiments on the n/iso ratio of nonanal[a]
General: All manipulations were carried out under dry N2 in
Schlenk glassware; solvents were dried and purified by standard
methods and were stored under N2; NMR Bruker AC 200 (1H:
200 MHz, int. TMS. 31P{1H}: 81 MHz, ext. H3PO4. 11B{1H}:
64 MHz, ext. BF3·Et2O); Elemental analysis: Institut für Anorgani-
sche Chemie der Universität zu Köln, Heraeus C,H,N,O-Rapid ele-
mental analyser.
Pt compound
100 °C
120 °C
140 °C
160 °C
1
2
3
4
7.8
8.8
1.7
1.4
8.4
8.2
6.0[b]
9.1
9.2
7.8
1.6[b]
1.6
[b]
[b]
[b]
[b]
[Bu3MeN][(dppp)PtPh(SnB11H11)] (1): At room temperature
(dppp)Pt(Ph)Cl (360 mg, 0.50 mmol) was dissolved in 10 mL of
[a]
Reactions were carried out in 10 mL of CH2Cl2 under 50 bar of
CO/H2 with an octene/Pt catalyst ratio of 250 (0.02 mmol cat.) and
CH2Cl2 and reacted with
a
CH2Cl2 solution of [Bu3-
[b]
reaction times of 2 h. Decomposition of the catalyst.
MeN]2[SnB11H11] (341 mg, 0.53 mmol). The solution turned yellow
immediately and was stirred for 24 h. The solvent was then evapor-
ated and the remaining solid was washed with water to give 1
(493 mg, 87%). Since we were not able to recrystallise this salt, an
Table 3. Degree of isomerization of 1-octene in hydroformylation
reaction with complex 1 and 2[a][b]
1
elemental analysis of the crude product was carried out. H NMR
Pt compound
100 °C
120 °C
140 °C
160 °C
(CD2Cl2, 200 MHz, TMS, without the signals for [Bu3MeN]ϩ): δ ϭ
2.00 (m, 2 H, dppp), 2.50 (m, 4 H, dppp), 6.40 (m, 1 H, Pt-Ph),
6.53 (m, 2 H, Pt-Ph), 6.92 (m, 2 H, Pt-Ph), 7.2Ϫ7.4 (m, 12 H, P-
Ph), 7.8Ϫ7.9 (m, 8 H, P-Ph) ppm. 11B{1H} NMR (CD2Cl2,
64 MHz, ext. BF3Et2O): δ ϭ Ϫ16.1 (s, B2ϪB11), Ϫ10.9 (s, B12)
ppm. 31P{1H} NMR (CD2Cl2, 81 MHz, ext. H3PO4): δ ϭ Ϫ5.4 (d,
1
2
2.5%
3.9%
13.7%
8.5%
27.5%
26.7%
55.3%
46.1%
[a]
Reactions were carried out in 10 mL of CH2Cl2 under 50 bar of
CO/H2 with an octene/Pt catalyst ratio of 250 (0.02 mmol cat.) and
1
2
2
2JP-P ϭ 26.2, JP-Pt ϭ 1733.0, JP-Sn ϭ 204.9 Hz), 2.4 (d, JP-P
ϭ
[b]
reaction times of 2 h. In the case of complexes 3 and 4 the iso-
merized olefins were partially hydroformylated.
26.2, 1JP-Pt ϭ 3167.3, 2JP-Sn ϭ 2419.5 Hz) ppm. C46H72B11NP2PtSn
Eur. J. Inorg. Chem. 2002, 2261Ϫ2265
2263

L. Wesemann, B. Drießen-Hölscher et al.
SHORT COMMUNICATION
(1133.7): calcd. C 48.73, H 6.40, N 1.24; found C 49.51, H 6.01,
N 1.59.
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[Bu3MeN]2[(dppp)Pt(SnB11H11)2] (2): At room temperature a solu-
tion of [(dppp)PtCl2] (250 mg, 0.37 mmol) in 20 mL of CH2Cl2 was
added to a solution of [Bu3MeN]2[SnB11H11] (479 mg, 0.74 mmol)
also in 20 mL of CH2Cl2. The colour of the reaction mixture turned
orange immediately and after 14 h stirring the solvent was evapor-
ated. The resulting solid was washed with water (two times with
30 mL). Crystallization from acetone/methanol at room temper-
ature yielded yellow-green crystals of 2 (498 mg, 90%). 1H NMR
(CD2Cl2, 200 MHz, TMS, without the signals for [Bu3MeN]ϩ): δ ϭ
2.08 (m, 2 H, dppp), 2.42 (m, 4 H, dppp), 7.48 (m, 12 H, Ph), 7.85
(m, 8 H, Ph) ppm. 11B{1H} NMR (CD2Cl2, 64 MHz, ext.
BF3Et2O): δ ϭ Ϫ15.8 (s, B2ϪB11), Ϫ9.5 (s, B12) ppm. 31P{1H}
NMR (CD2Cl2, 81 MHz, ext. H3PO4): δ ϭ Ϫ1.1 (s, 1JPt-P ϭ 2820.7,
2JP-Sn ϭ 2127.5 Hz)] ppm. C53H108B22N2P2PtSn (1387.0): calcd. C
42.28, H 7.23, N 1.86; found C 42.11, H 7.32, N 2.12.
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evacuated and filled with argon three times. The catalyst solution,
prepared previously in a Schlenk vessel, was then introduced (the
exact amount was determined by a syringe). The exact amounts of
1-octene and of GC standard (di-n-butyl ether) were transferred to
the dropping funnel. The autoclave was pressurized with synthesis
gas (CO/H2) and was heated to reaction temperature by placing it
in an oil bath. The reaction mixture was stirred at 750 rpm and
kept for 30 minutes at the reaction temperature in order to ensure
the preformation of the active catalyst species before the mixture
of 1-octene and GC standard was added. Finally the autoclave was
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Column: 50m Pona-HP-FS; temperature program: 5 min isotherm
at 80 °C; 8 °C per minute to 25 °C; temperature of the evaporator:
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[19]
X-ray crystal structure analysis of 2: C53H108B22N2P2PtSn2;
Mr ϭ 1505.64; orthorhombic space group Pbcn (no. 60); a ϭ
˚
20.2352(6), b ϭ 19.5630(7), c ϭ 17.9094(7) A, V ϭ 7089.6(4)
A , Z ϭ 4, ρcalcd. ϭ 1.411 g cmϪ3; µlin. ϭ 2.748 mmϪ1; Stoe
3
˚
˚
IPDS II diffractometer; Mo-Kα (λ ϭ 0.71073 A); graphite mon-
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scans (0 Յ ω Յ 180°, ϕ ϭ 0°; 0 Յ ω Յ 180°, ϕ ϭ 60°; 0 Յ ω
Յ 180°, ϕ ϭ 90°; 0 Յ ω Յ 106°, ϕ ϭ 180°; ∆ω ϭ 2°, exposition
time of 1 min) in the 2θ range of 2.3 to 59.5° on a single crystal
0.7 ϫ 0.4 ϫ 0.5 mm; 233603 reflections measured, 9587 inde-
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and polarisation factors; numerical absorption correction,
max./min. transmission 0.3371/0.1674; structure solution with
direct methods[20] and difference Fourier synthesis, F2
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CCDC-177990 contains the supplementary crystallographic
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2264
Eur. J. Inorg. Chem. 2002, 2261Ϫ2265

Reactivity of Platinum Stanna-closo-dodecaborate Complexes
SHORT COMMUNICATION
[22]
[23]
Cambridge CB2 1EZ, UK; Fax: (internat.) ϩ44-1223/336-033;
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