High-yield synthesis of medium and large platinacycloalkanes from bis(alkenyl) precursors

Article abstract of DOI:10.1002/anie.200604979

Coming full circle: Bis(alkenyl)-platinum(II) complexes cis-[L ₂Pt-{(CH₂)_nCH=CH₂}₂] (L = PPh₃ or L₂ = Ph₂P(CH₂) ₃PPh₂; n = 3-6) have been prepared and fully characterized. Ring-closing metathesis leads to new platinacycloalkenes, which can be hydrogenated to platinacycloalkanes with 9-, 11-, 13-, and 15-membered rings (see picture for 15-membered ring; Pt green, P orange, C blue, H white). (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200604979

Angewandte
Chemie
DOI: 10.1002/anie.200604979
Platinum Metallacycles
High-Yield Synthesis of Medium and Large Platinacycloalkanes from
Bis(alkenyl) Precursors**
Akella Sivaramakrishna, Hong Su, and John R. Moss*
Metallacycloalkanes are an important class of compounds
which have been known for many years to be key intermedi-
ates in useful catalytic reactions.[1] For example, metallacy-
clobutanes are known to be present in the alkene metathesis
reaction.[2] More recently, metallacycloheptanes were first
implicated,[3] then proven[4] to be involved in ethylene
trimerization. There has been interesting discussion[5–7] and
a belief that the formation of nine-membered and other larger
metallacycloalkane rings is unlikely.
In 2005, selective tetramerization of ethylene was discov-
ered, which implicated metallacyclononanes as intermedi-
ates,[8] and Gibson and co-workers showed that larger metal-
lacycloalkanes were involved in the catalytic formation of
higher ethylene oligomers.[9] In spite of the demonstrated
importance of medium to large metallacycloalkanes as
intermediates, very little is known about such compounds.[10]
The elegant synthesis of “molecular gyroscopes” through
intramolecular alkene metathesis reactions using the Grubbs
catalyst has been described by Gladysz and co-workers.[11]
Recently, we introduced a new approach for the synthesis of
small metallacycloalkane complexes using ring-closing meta-
thesis (RCM) with the Grubbs catalyst.[12] Herein, we use this
route to synthesize new medium to large ring metallacycloal-
kanes (Scheme 1), that is, exactly the ones that appear
difficult to make by conventional methods.
Scheme 1. Preparation of platinacycloalkanes.
The bis(alkenyl)platinum(II) complexes 2a–d were
obtained by the transmetalation reaction of 1-alkenyl
Grignard reagents with the corresponding dichloroplat-
inum(II) precursors 1. Complexes 2 were then readily
converted into the platinacycloalkenes 3 using the RCM
reaction with the Grubbs catalyst in CH2Cl2, and were
obtained as colorless solids in high yield after recrystallization
from CH2Cl2/diethyl ether. The hydrogenation of these
compounds yielded the platinacycloalkanes 4 in high yield.
The 31P{1H} NMR spectra of 4 in C6D6 displayed a singlet at
d = 3.4 ppm with Pt satellites (1JPt-P = 1620 Hz), which is quite
similar to the precursor compounds 2 and 3.[13] These and all
new compounds described in Scheme 1 were fully character-
ized. Crystals of 2c and 3d (L2 = dppp = 1,3-bis(diphenyl-
phosphino)propane) suitable for X-ray diffraction were
obtained by recrystallization from CH2Cl2/hexane at T=
08C. To our knowledge, this is the first example of a
structurally characterized larger metallacycloalkene com-
pound reported in the literature.
The molecular structures of 2c and 3d are illustrated in
Figures 1 and 2, respectively. The structural analysis confirms
that 3d is the RCM product. The most striking structural
features of complexes 2c and 3d are the differences in some
ꢀ
ꢀ
of the bond angles and bond lengths. The Pt C and Pt P bond
lengths in the two compounds are almost identical and
comparable to the literature values[14] (2.118(3)–2.110(3) Š
=
and 2.263(9)–2.293(7) Š, respectively), while the C C dis-
=
tances range from 1.229(7) to 1.341(7) Š. The C C distance in
[*] Dr. A. Sivaramakrishna, Dr. H. Su, Prof. J. R. Moss
Department of Chemistry, University of Cape Town
Rondebosch 7701, Cape Town (South Africa)
Fax: (+27)21-689-7499
3d is comparatively long. The P-Pt-P, P-Pt-C, and C-Pt-C
bond angles in 2c and 3d vary from 92.53(3) to 96.72(3),
92.52(8) to 89.49(8), and 85.44(11) to 84.45(11)8 respectively.
These differences may indicate that the variance in the
structural features depends on the degree of strain on the
E-mail: jrm@science.uct.ac.za
[**] The authors thank Johnson Matthey (London), Anglo Platinum
Corporation, C* Change, NRF, and UCT for the financial support,
Ian Rogers for laboratory assistance, and Feng Zheng for the
thermal decomposition studies.
=
bridging C C bonds, which is necessary to stabilize the ring.
The above RCM reactions were found to depend on the
concentration, temperature, solvent, length of alkenyl chains,
tertiary phosphine ligand, and catalyst used. The concentra-
tion of 2 had a noticeable effect on the RCM rate, thus at high
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
monomer to dimer, or it could be due to one of the fragments
of the dimeric species. The same trends were obtained when
the RCM was carried out with 2d in a concentrated solution;
the mass spectra showed a mixture of monomer 3d and its
corresponding dimeric species. Owing to the steric, electronic,
and chelating effects of ligands, the donor-ligand system in 2
plays an important role in the RCM reactions. PPh3 enhances
the rate of reaction with the Grubbs catalyst compared to the
diphosphine ligands. There was no RCM observed with the
first-generation Grubbs catalyst when PtBu3 was used as
ligand. However, the reaction proceeds smoothly with the
second-generation Grubbs catalyst. The thermal stability of 2
increases along the series: PPh3 < PtBu3 < diphosphine. Using
10 mol% rather than 5 mol% of the Grubbs catalyst causes a
dramatic enhancement (ca. 10-fold) of the rate of conversion
2!3 (Scheme 1). It is interesting to note that the length of
alkenyl chain also has a substantial effect on the rate of the
RCM reaction. When the bis(butenyl) complex with PPh3 was
used, the catalyzed reaction was completed in 90 minutes. The
same reaction with the bis(heptenyl) complex 2c, however,
took 9 h to go to completion.
Figure 1. ORTEP diagram of the molecular structure of 2c (L2 =dppp);
thermal ellipsoids set at 30%.
The RCM reaction discussed here is not general for all
metal systems, since the terminal alkenyl groups in metal–
alkenyl complexes are reactive and may undergo a variety of
reactions, depending on the nature of metal center, such as
conversion to h1,h2-alkenyl complexes. 31P NMR examination
of products derived from RCM reactions to form 3 indicates
the presence of different conformations of platinacycloal-
kenes in solution. Each spectrum shows a multiplet, which
displays only one coupling constant (1JPt-P = 1620 Hz). It was
found that the 31P NMR spectra of solutions of platinacy-
cloalkenes 3 changed after aging and showed sharp singlets.
This aging could be due to the conversion of possible isomers
to the most stable conformation. The aging also indicates that
such structural changes are sensitive to light and temperature.
The alkene protons in 3d were observed to be mutually trans.
The same results were observed with the other platinacy-
cloalkene compounds (3a–c).
Figure 2. ORTEP diagram of the molecular structure of 3d
(L2 =dppp); thermal ellipsoids set at 30%.
concentrations dimeric products such as 5 were observed
[Eq. (1)].
Thermal decomposition of the platinacycloalkanes in the
solid state generated a mixture of alkenes, which consisted
mainly of 1-alkene derived from the metallacyclic moiety
through b-hydride elimination [Eq. (2)], which is consistent
with the literature data for small ring compounds.[15] This
result suggests that these compounds can be useful models for
metallacyclic intermediates in ethylene oligomerization reac-
tions.
In conclusion, we have demonstrated that it is possible, at
least with platinum, to prepare medium to large ring metal-
lacycloalkanes in high yield through a ring-closing metathesis
reaction, and that such compounds are quite thermally stable.
Currently, we are exploring the interesting properties and
reactivity of these compounds to determine the reaction
The mass spectrum of compound 5 also displayed a peak
corresponding to the molecular ion of monomeric platina-
cycle 3. This peak could be a result of interconversion of
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[5] Z.-X. Yu, K. N. Houk, Angew. Chem. 2003, 115, 832 – 835;
pathways that are available to metallacycloalkanes. We are
also investigating similar compounds with other metals as well
as even-membered ring metallacycloalkanes and cross ring-
closing metathesis reactions to prepare larger metallacycloal-
kanes.
Angew. Chem. Int. Ed. 2003, 42, 808 – 811.
[6] A. N. J. Blok, P. H. M. Budzelaar, A. W. Gal, Organometallics
2003, 22, 2564 – 2570.
[7] A. Bollmann, K. Blann, J. T. Dixon, F. M. Hess, E. Killian, H.
Maumela, D. S. McGuinness, D. H. Morgan, A. Neveling, S.
Otto, M. Overett, A. M. Z. Slawin, P. Wasserscheid, S. Kuhl-
mann, J. Am. Chem. Soc. 2004, 126, 14712 – 14713.
[8] M. J. Overett, K. Blann, A. Bollmann, J. T. Dixon, D. Haasbroek,
E. Killian, H. Maumela, D. S. McGuinness, D. H. Morgan, J. Am.
Chem. Soc. 2005, 127, 10723– 10724.
[9] A. K. Tomov, J. J. Chirinos, D. J. Jones, R. J. Long, V. C. Gibson,
J. Am. Chem. Soc. 2005, 127, 10166 – 10167.
[10] B. Blom, H. Clayton, M. Kilkenny, J. R. Moss, Adv. Organomet.
Chem. 2006, 54, 149 – 205.
[11] A. J. Nawara, T. Shima, F. Hampel, J. A. Gladysz, J. Am. Chem.
Soc. 2006, 128, 4962 – 4963, and references therein.
[12] K. Dralle, N. L. Jaffa, T. L. Roex, J. R. Moss, S. Travis, N. D.
Watermayer, A. Sivaramakrishna, Chem. Commun. 2005, 3865 –
3866.
Experimental Section
2 c (L2 = dppp): A solution of 1-heptenyl Grignard reagent (2.3mL,
1.22m, 2.806 mmol) in diethyl ether was added to a solution of
[(cod)PtCl2] (296 mg, 0.791 mmol, cod = 1,5-cyclooctadiene) in
diethyl ether (25 mL) cooled to ꢀ788C. The solution was brought
to 08C and stirred until the solution became clear. Solid dppp
(327 mg, 0.793 mmol) was added and the resulting mixture was stirred
for 36 h until a clear solution was formed. The excess Grignard
reagent was removed by hydrolyzing the reaction mixture with
saturated aqueous NH4Cl (5 mL) at ꢀ788C. The aqueous layer was
washed with dichloromethane (2 ” 5 mL) and the organic layer was
separated. The solvent was removed under reduced pressure, and the
[13] a) Structure determination and crystallographic data for 2c:
crystals were obtained by slow diffusion of n-hexane into a
solution of 2c in dichoromethane. C41H52P2Pt1, Mr =
residue was recrystallized from
a
CH2Cl2/hexane mixture
(3mL:5 mL) at ꢀ108C for 48 h. The colorless crystalline solid was
isolated by decanting the mother liquor and dried under vacuum.
Yield: 95%; m.p. 80–828C (decomp); 1H NMR (300 MHz, CDCl3):
801.86 gmolꢀ1, triclinic, space group P1, a = 12.1550(1), b =
¯
12.3649(1), c = 14.4431(2) Š, a = 108.061(1), b = 95.214(1), g =
114.631(1)8, V= 1814.14(3) Š3, Z = 2, 1calcd = 1.468 gcmꢀ3, T=
113(2) K, m = 3.982 mmꢀ1, F(000) = 812, 3.378 < 2q < 26.378, l-
(MoKa) = 0.71073Š, crystal size 0.15 ” 0.17 ” 0.21 mm 3. Intensity
data were collected with a Nonius KappaCCD diffractometer.
34532 reflections, 7379 independent (Rint = 0.0309), empirical
absorption correction (SADABS), structure solution using
direct methods (SHELXS-97), structure refinement on F2
using full-matrix least-squares procedures (SHELXL-97), R1 =
0.0229 [I > 2s(I)], wR2 = 0.0498 (all data), GOF = 1.043, max/
min residual electron density = 1.899/ꢀ0.977 eŠꢀ3; b) Structure
determination and crystallographic data for 3d: crystals were
obtained by slow diffusion of n-hexane into a solution of 3d in
dichoromethane. C41H52P2Pt1, Mr = 801.86 gmolꢀ1, monoclinic,
space group P21/c, a = 17.0142(2), b = 14.4679(1), c =
=
d = 7.61–7.23(m, 20H; Ph), 5.77–5.57 (m, 2H; CH), 4.91–4.69 (m,
=
ꢀ
ꢀ
4H; CH2), 2.66–2.41 (m, 6H; P CH2), 2.12–0.84 ppm (m, 20H;
CH2); 31P{1H} NMR (300 MHz, CDCl3): d = 3.32 ppm (s, JPt-P
=
1612 Hz). Elemental analysis (%) calcd for C41H52P2Pt: C 61.41,
H 6.54; found: C 61.53, H 6.58.
3d (L = dppp): Compound 2d (L = dppp; 366 mg, 0.441 mmol)
and the first generation Grubbs catalyst (9 mg, 0.0109 mmol,
2.5 mol%) were added to dichloromethane (30 mL). The solution
was heated at reflux at 508C. After 3h, additional catalyst (9 mg,
0.0109 mmol, 2.5 mol%) was added. After another 3h, the solvent
was removed in vacuo. The residue was recrystallized from a CH2Cl2/
hexane mixture (3mL:5 mL) to give 3d (L = dppp) as a white solid.
Yield: 91%; m.p. 163–1668C (decomp); 1H NMR (300 MHz, CDCl3):
=
d = 7.81–7.28 (m, 20H; Ph), 5.40–5.21 (m, 2H; CH), 2.64–2.34 (m,
ꢀ
ꢀ
6H; P CH2), 2.12–0.70 ppm (m, 12H;
CH2); 31P{1H} NMR
15.9256(1) Š, b = 111.499(1)8, V= 3647.49(5) Š3, Z = 4, 1calcd
=
(300 MHz, CDCl3): d = 3.08 (m, JPt-P = 1610 Hz; a mixture of iso-
mers). Elemental analysis (%) calcd for C41H52P2Pt: C 61.41, H 6.54;
found: C 61.23, H 6.69. MS: m/z = 802.1 [M]+, 606.8 [(dppp)Pt]+. The
experimental procedures as well as the characterization data of the
remaining compounds 2a,b,d, 3a–c, 4a–d, and 5 are given in the
Supporting Information.
1.460 gcmꢀ3 T= 113(2) K, m = 3.962 mmꢀ1
,
,
F(000) = 1624,
3.098 < 2q < 25.698, l(MoKa) = 0.71073Š, crystal size 0.06 ”
0.10 ” 0.12 mm3. Intensity data were collected with a Nonius
KappaCCD diffractometer. 103989 reflections, 6891 independ-
ent (Rint = 0.0670), empirical absorption correction (SADABS),
structure solution using direct methods (SHELXS-97), structure
refinement on F2 using full-matrix least-squares procedures
(SHELXL-97), R1 = 0.0258 [I > 2s(I)], wR2 = 0.0547 (all data),
GOF = 1.066, max/min residual electron density = 1.512/
ꢀ0.861 eŠꢀ3. CCDC-629152 (2c) and CCDC-629153( 3d) con-
tain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/
data_request/cif.
Received: December 8, 2006
Published online: March 30, 2007
Keywords: metallacycles · metathesis · platinum ·
structure elucidation · thermal decomposition
.
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