Oxidation of organoplatinum(II) by coordinated dimethylsulfoxide: Metal-metal bonded, dinuclear, liquid-crystalline complexes of platinum(III)

Article abstract of DOI:10.1021/ja105469a

Reaction of [PtCl₂(dmso)₂] with 2,5-(dialkoxyphenyl) pyridine in HOAc leads to a dinuclear, acetate-bridged, metal-metal bonded complex of platinum(III); dmso in the presence of acid is found to be responsible for the oxidation. The dimer is analogous structurally to Pd ^III dimers implicated in catalytic acetoxylation. Platinum dimers with longer alkoxy chains are shown to be unique examples of liquid crystals of platinum(III).

Full text of DOI:10.1021/ja105469a

Published on Web 07/20/2010
Oxidation of Organoplatinum(II) by Coordinated Dimethylsulfoxide:
Metal-Metal Bonded, Dinuclear, Liquid-Crystalline Complexes of Platinum(III)
Amedeo Santoro,† Marcin Wegrzyn,† Adrian C. Whitwood,† Bertrand Donnio,‡ and
Duncan W. Bruce*,†
Department of Chemistry, UniVersity of York, Heslington, York YO10 5DD, U.K. and CNRS-UniVersite´ de
Strasbourg (UMR 7504), Institut de Physique et Chimie des Mate´riaux de Strasbourg, 23 rue du Loess BP 43,
67034 Strasbourg Cedex 2, France
Received June 22, 2010; E-mail: db519@york.ac.uk
of palladium-catalyzed reactions. Then, both because of its inherent
Abstract: Reaction of [PtCl2(dmso)2] with 2,5-(dialkoxyphenyl)py-
ridine in HOAc leads to a dinuclear, acetate-bridged, metal-metal
bonded complex of platinum(III); dmso in the presence of acid is
found to be responsible for the oxidation. The dimer is analogous
structurally to PdIII dimers implicated in catalytic acetoxylation.
Platinum dimers with longer alkoxy chains are shown to be unique
examples of liquid crystals of platinum(III).
interest and because of the above-mentioned proposals for the
involvement of PdIV, organoplatinum(IV) chemistry has enjoyed a
great deal of attention recently, particularly from the groups of
Sanford and of Rourke. For the most part, PtIV complexes were
realized Via the oxidation of PtII precursors using peroxide, IIII
reagents (e.g., PhICl2), or N-chlorosuccinimide (NCS). Sanford’s
group also reported two examples of organoplatinum(III) com-
plexes. Thus, oxidation of [Pt(Bzq)(acac)] (here Bzq is orthometa-
lated benzo[h]quinoline) with PhI(OAc)2 in acetic acid led to PtIII
cation [2]+,10 characterized crystallographically as its [BF4]- salt,
Palladium-catalyzed transformations of organic substrates are
ubiquitous1 and responsible for major developments in synthetic
methodology.2 Catalyst precursors have normally been complexes
of Pd0 or PdII (or occasionally PdI dimers3), and it is these oxidation
states that have been implicated predominantly in mechanistic
rationalizations based mainly on classical oxidative addition/
reductive elimination cycles. More recently, it has been postulated
that certain palladium-catalyzed reactions involve the intermediacy
of PdIV complexes, for example in oxidative functionalization of
C-H bonds by the groups of Sanford4 and of Rourke.5 The
emergence of PdIV chemistry in organic transformations has recently
been reviewed comprehensively.6 However, in 2009, Powers and
Ritter,7 and then Deprez and Sanford,8 postulated the involvement
of dinuclear complexes of PdIII in catalytic cycles and complex 1
was characterized crystallographically. In both pieces of work, PdIII
complexes were prepared by oxidation of a PdII precursor by IIII
reagents (PhICl2 or [R-I-R′]+). Ritter’s group then went on to
provide evidence that an analogue of complex 1, in which the
orthometallating group was 2-phenylpyridine and the axial chloride
was replaced by acetate, was a catalyst precursor for oxidative
acetoxylation of 2-phenylpyridine using PhI(OAc)2.9
while, on other occasions, NCS oxidation of cis-[PtII(2-PhPy)2] or
IIII oxidation of cis-[PtMe2(bipy)] led to unsupported, metal-metal
bonded dimers (here 2-PhPy is orthometalated 2-phenylpyridine).11
Previously, oxidation of cis-[PtII(2-PhPy)2] with elemental bromine
had been found to lead to cis-[PtIVBr2(2-PhPy)2],12 while NCS-
oxidation of PtII has also led to PtIV species.13
Supported dimeric PtIII complexes are reasonably well-known
in the literature and normally consist of a metal-metal bonded
arrangement supported by four bridging ligands, for example
14
15
OAc-, SO4
,
pyrophosphites,16 and formamidinates.17 In all
2-
cases, they were obtained from PtII precursors using an external
oxidant.
As part of our abiding interest in metallomesogens, we have
recently reported series of emissive PtII complexes with liquid
crystal properties18,19 for potential applications in OLED fabrications.
One of these series is represented by 3-n, normally obtained
directly from reaction of a di-µ-Cl dimer of an orthometalated
2-phenylpyridine complex of platinum(II) with Na(acac). In this
case, however, the elongated ligands rendered this dimer rather
insoluble, and so it was found to be better to proceed through the
S-dmso complex 4-n. However, preparing 4-n in two steps is
inefficient, and so its preparation directly from cis-[PtCl2(dmso)2]
was investigated. Thus, in one set of conditions, cis-[PtCl2(dmso)2]
was heated under reflux in AcOH with 2,5-bis(4-ethoxyphenyl)py-
ridine. However, the product obtained after workup was not the
characteristic yellow of 4-n, but rather an orange material.
In general, third-row transition elements form more inert
complexes than their second-row congeners, and so it is not
uncommon for the third-row element to be used in mechanistic
investigations. As such, a good deal of organoplatinum chemistry
has been developed with the aim of establishing mechanistic details
† University of York.
‡ CNRS-Universite´ de Strasbourg.
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J. AM. CHEM. SOC. 2010, 132, 10689–10691 10689

C O M M U N I C A T I O N S
Chart 3. Small-Angle X-ray Diffraction Pattern for 6-14,1 at 100 °C
(a) and 180 °C (b)
Fortunately, the complex crystallized, and upon determination of
the structure, it emerged that the product, 6-n,m (here n refers to
the number of carbons in the ligand chain, while m refers to the
number of carbons in the chain bound to the bridging carboxylate
function), was a dinuclear PtIII complex (Charts 1 and 2). The two
Pt centers were supported by two µ2-acetato ligands at a distance
of 2.5730(3) Å, consistent with a Pt-Pt single bond (cf. 2.5647(2)
Å in 2); the diamagnetic nature was also evidenced by obtaining a
high-resolution 1H NMR spectrum. Each platinum was also bound
to a diphenylpyridine ligand, which lay in the opposite orientation
with respect to that bound to the other Pt center. The structure is
unsymmetric in that there are two dPt-Cl at 2.465(1) and 2.442(1)
Å and the two square planes are offset by ∼18.8°. The two ligands
are not coparallel, and planes defined by the two five-membered
Pt-ligand rings are at an angle of 17.03°.
Chart 1. Preparation and Structure of the New Dinuclear PtIII
Complex, 6-n,m
Chart 2. Molecular Structure of 6-2,1a
Thus, oxidation of PtII by coordinated dmso was established first
in 1968 upon treatment of cis-[PtCl2(dmso)2] with concentrated
aqueous HCl to give trans-[PtCl4(SMe2)2].21 It is now a more widely
recognized reaction.22 More recent and detailed mechanistic studies
have shown the necessity for acidic conditions and implied that
the reaction involves dissociation of dmso and then recoordination
of [dmsoH]+ through oxygen, followed by successive intramolecular
electron transfers and elimination of the oxygen as H2O.23 However,
in all previous cases that we can find, the Pt product was in the +4
oxidation state, and while dinuclear PtIII intermediates have been
either proposed24 or isolated25 for other oxidation routes, none has
been reported with coordinated dmso. Clearly then, in accord with
the findings of Dick et al.,10 the presence of acetate promotes a
dinuclear, bridged motif which appears to constrain the Pt centers
to the +3 oxidation state.
The difference in thermal stability of 6-n,m and 1 has been noted
already and so other examples were prepared to investigate the
thermal properties more thoroughly, especially given that similar
complexes with this extended bis-2,5-diphenylpyridine ligand
(3-n and 4-n) were liquid crystalline. Thus, investigations of 6-14,1
by hot-stage polarizing optical microscopy and small-angle X-ray
diffraction (Chart 3) showed that, from room temperature to
170 °C, the complex existed in a liquid-crystalline mesophase that
could be indexed into the centered rectangular plane group c2mm,
identified clearly from indexation of the reflections shown in Chart
3a; full indexation is found in Table S1. Above 170 °C, the
mesophase transformed to show a lamellar (smectic) arrangement
with two orders of reflection, d001 and d002, corresponding to a repeat
a Molecules of CHCl3 of crystallization are omitted for clarity (Figure
S1).
This complex is a direct analogue of the PdIII dimer reported by
Powers and Ritter7 containing an orthometalated Bzq ligand, except
that their complex was stable only below -30 °C in the solid state;
above this temperature it eliminated 10-chlorobenzo[h]-quinoline
leaving a mixture of undefined PdII complexes. Complexes 6-n,m
are shown by TGA analysis to be stable to over 200 °C. This type
of complex has not been reported previously for PtIII, the closest
analogue being a di-µ2-acetato complex described by Steele and
Vrieze (Figure S2).20
However, as indicated above, previously MIII and MIV (M ) Pd,
Pt) complexes were obtained by direct oxidation of MII precursors,
but in this case the identity of the oxidizing agent was not
immediately apparent as the reaction was carried out with the
rigorous exclusion of atmospheric oxygen. Attention therefore
turned to the coordinated dmso, and its role as the oxidizing agent
was established when the reaction was repeated using cis-
[PtCl2(S(Me)Et)2] as starting material, which led simply to 5-n with
coordinated methylethylsulfide with no trace of 6. Further evidence
was provided (Chart 1) by the observation that reaction of 4-12
with acetic acid led smoothly to 6-12,1, consistent with the need
for both dmso and acidic conditions as now described.
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C O M M U N I C A T I O N S
distance of 37.5 Å. The lower-temperature phase is assigned as a
ribbon phase with a structure analogous to the SmA phase (Figure
References
˜
(1) Maitlis, P. M. The Organic Chemistry of Palladium; Academic Press:
London, 1971; Vols. 1 and 2.
S3),26 except that this is much more ordered. This is preferred over
the alternative assignment as a Colr phase both because the
molecular shape is not conducive to columnar phase formation and
because of the similarity in the rectangular b dimension (35.7 Å)
and the lamellar spacing in the higher-temperature phase (37.5 Å).
However, this lamellar spacing is less than the molecular length
(max. ca. 51 Å), which suggests either a tilted phase or one that is
interdigitated, or both. Interdigitation would allow efficient filling
of space as the complexes cannot pack well together side by side
owing to the lateral carboxylate group, although a strongly tilted
arrangement could achieve the same end. In this case, the
correspondence between the rectangular b-parameter and d001
spacing would suggest a tilted ribbon phase, reducing the symmetry
below c2mm, which is not excluded by the indexation. Microscopy
shows that the ribbon phase is extremely well organized, while the
smectic phase becomes quite fluid. However, there is strong
paramorphosis, and so a characteristic texture was not obtained and
so the data presently do not allow unequivocal resolution of this
question.
(2) For example, see: Tsuji, J. Palladium Reagents and Catalysts: New
PerspectiVes for the 21st Century; Wiley: Chichester, 2004. Palladium in
Organic Synthesis; Tsuji, J., Ed.; Springer: Berlin, 2005.
(3) Murahashi, T.; Kurosawa, H. Coord. Chem. ReV. 2002, 231, 207–28.
(4) Dick, A. R.; Hull, K. L.; Sanford, M. S. J. Am. Chem. Soc. 2004, 126,
2300–01. Kalyani, D.; Deprez, N. R.; Desai, L. V.; Sanford, M. S. J. Am.
Chem. Soc. 2005, 127, 7330–31. Hull, K. L.; Anani, W. Q.; Sanford, M. S.
J. Am. Chem. Soc. 2006, 128, 7134–3.
(5) Newman, C. P.; Casey-Green, K.; Clarkson, G. J.; Cave, G. W. V.;
Errington, W.; Rourke, J. P. Dalton Trans. 2007, 3170–82. Mamtora, J.;
Crosby, S. H.; Newman, C. P.; Clarkson, G. J.; Rourke, J. P. Organome-
tallics 2008, 27, 5559–65. Crosby, S. H.; Clarkson, G. J.; Deeth, R. J.;
Rourke, J. P. Organometallics 2010, 29, 1966–1976.
(6) Sehnal, P.; Taylor, R. J. K.; Fairlamb, I. J. S. Chem. ReV. 2010, 110, 824–
89.
(7) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302–309.
(8) Deprez, N. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 11234–41.
(9) Powers, D. C.; Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T. J. Am. Chem.
Soc. 2009, 131, 17050–17051.
(10) Dick, W. R.; Kampf, J. W.; Sanford, M. S. Organometallics 2005, 24,
482–485.
(11) Whitfield, S. R.; Sanford, M. S. Organometallics 2008, 27, 1683–89. Canty,
A. J.; Gardiner, M. G.; Jones, R. C.; Rodemann, T.; Sharma, M. J. Am.
Chem. Soc. 2009, 131, 7236–7237.
(12) Chassot, L.; Mu¨ller, E.; von Zelewsky, A. Inorg. Chem. 1984, 23, 4249–
53.
(13) Scollard, J. D.; Day, M.; Labinger, J. A.; Bercaw, J. E. HelV. Chim. Acta
2001, 84, 3247. Zhang, F.; Broczkowski, M. E.; Jennings, M. C.;
Puddephatt, R. J. Can. J. Chem. 2005, 83, 595.
With the notable exception of various dimetal tetracarboxylates
of Cu, Rh, Ru, and Mo,27 dinuclear metallomesogens are rare and
with metal-metal bonded species being rarer still. There is one
previous example of a metal-metal bonded complex showing
nematic or smectic phases that uses ruthenium,28 but no previous
examples with platinum. Neither are there examples of a PtIII liquid
crystal. The closest analogous structures are found in dinuclear
acetato-bridged dipalladium(II) complexes of azines,29 which are
similar to 6-n,m except that there is no metal-metal bond and no
axial Cl ligands.30 As such, the present complexes are novel and
without precedent.
However, of equal significance is the fact that oxidation of PtII
to PtIII is demonstrated using coordinated dmso rather than using
IIII-based oxidants. The resulting complex is directly analogous to
that proposed in palladium chemistry as having involvement in
catalytic oxidative acetyloxylation and as such suggests alternative
approaches in terms of both catalytic reactivity and mechanistic
study.7,9
(14) Appleton, T. G.; Byriel, K. A.; Garrett, J. M.; Hall, J. R.; Kennard, C. H. L.;
Mathieson, M. T.; Stranger, R. Inorg. Chem. 1995, 34, 5646–5655.
(15) Cotton, F. A.; Falvello, L. R.; Han, S. Inorg. Chem. 1982, 21, 2889–91.
(16) Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989, 22, 55–
61.
(17) Cotton, F. A.; Matonic, J. H.; Murillo, C. A. Inorg. Chem. Scta 1997, 264,
61–65.
(18) Kozhevnikov, V. N.; Donnio, B.; Bruce, D. W. Angew. Chem., Int. Ed.
2008, 47, 6286–6289.
(19) Santoro, A.; Whitwood, A. C.; Williams, J. A. G.; Kozhevnikov, V. N.;
Bruce, D. W. Chem. Mater. 2009, 21, 3871–3882.
(20) Steele, B. R.; Vrieze, K. Transition Met. Chem. 1977, 2, 169–174.
(21) Kukushkin, Yu. N.; Vyazmenskii, Yu. E.; Zorina, L. I.; Pazukhina, Yu. L.
Zh. Neorg. Khim. 1968, 13, 1595–99.
(22) Kukushkin, Yu. N. Coord. Chem. ReV. 1995, 139, 375–407.
(23) Alexandrova, L.; D’yachenko, O. G.; Kazankov, G. M.; Polyakov, V. A.;
Samuleev, P. V.; Sansores, E.; Ryabov, A. D. J. Am. Chem. Soc. 2000,
122, 5189–5200. Ryabov, A. D.; Otto, S.; Samuleev, P. V.; Polyakov, V. A.;
Alexandrova, L.; Kazankov, G. M.; Shova, S.; Revenco, M.; Lipkowski,
J.; Johansson, M. H. Inorg. Chem. 2002, 41, 4286–4294.
(24) Bonnington, K. J.; Jennings, M. C.; Puddephatt, R. J. Organometallics 2008,
27, 6521–6530.
(25) Bandoli, G.; Caputo, P. A.; Intini, F. P.; Sivo, M. F.; Natile, G. J. Am.
Chem. Soc. 1997, 119, 10370–10376.
ˇ
(26) Sepelj, M.; Lesac, A.; Baumeister, U.; Diele, S.; Nguyen, H. L.; Bruce,
D. W. J. Mater. Chem. 2007, 17, 1154–1165.
(27) Described in: Donnio, B.; Guillon, D.; Deschenaux, R.; Bruce, D. W. In
ComprehensiVe Coordination Chemistry II; McCleverty, J. A., Meyer, T. J.,
Eds.; Elsevier: Oxford, U.K., 2003; Vol. 7, Chapter 7.9, pp 357-627.
(28) Deschenaux, R.; Donnio, B.; Rheinwald, G.; Stauffer, F.; Suss-Fink, G.;
Velker, J. J. Chem. Soc., Dalton Trans. 1997, 4351–4355.
Acknowledgment. We thank the University of York for
financial support (A.S.) and Professor Ian Fairlamb for helpful
discussions.
(29) Espinet, P.; Lalinde, E.; Marcos, M.; Perez, J.; Serrano, J. L. Organome-
tallics 1990, 9, 555–560.
Supporting Information Available: Details of the preparation of
the complexes and their spectral and analytical data; cif file for complex
6; representation of ribbon phase. This material is available free of
charge via the Internet at http://pubs.acs.org.
(30) For a recent discussion of PdII dimers of this general type, see: Bercaw,
J. E.; Durrell, A. C.; Gray, H. B.; Green, J. C.; Hazari, N.; Labinger, J. A.;
Winkler, J. R. Inorg. Chem. 2010, 49, 1801–1810.
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