Trapping a difluorocarbene-platinum fragment by base coordination

Article abstract of DOI:10.1002/anie.200907031

Caught in the act: Evidence for the intermediacy of difluorocarbene- platinum species along the acidic degradation of Pt-CF₃ bonds was gained by trapping a base-stabilized Pt=CF₂ moiety (see scheme). Attack of an N nucleophile (py*) to the highly electrophilic and extremely reactive difluorocarbene carbon atom takes place with C-N bond formation. (Figure Presented)

Full text of DOI:10.1002/anie.200907031

Communications
DOI: 10.1002/anie.200907031
Carbenes
Trapping a Difluorocarbene–Platinum Fragment by Base
Coordination**
Sonia Martꢀnez-Salvador, Babil Menjꢁn, Juan Forniꢂs,* Antonio Martꢀn, and Isabel Usꢁn
Dedicated to Prof. Dr. H. W. Roesky on the occasion of his 75th birthday
The trifluoromethyl group (CF3) when bound to an element E
of medium to high electronegativity behaves as a monovalent
substituent with a high thermal stability and a marked
chemical inertness. This low-reactivity profile together with
its unique combination of electronic and steric properties
have definitely encouraged the increasing use of CF3 as a
robust terminal group in modern organic chemistry.[1] When
bound to an electropositive atom, however, CF3 becomes
more reactive: a-fluoride elimination and the formation of a
difluorocarbene unit (Scheme 1) is a general reaction path-
Scheme 2. Hydrolytic processes undergone by homoleptic tetrakis(tri-
fluoromethyl) derivatives of A) main-group element boron (with K+ as
counterion) or B) transition-metal element platinum (with [NBu4]+ as
counterion).
Scheme 1. a-Fluoride elimination process operating in trifluoromethyl
derivatives of electropositive elements E.
reactivity made possible by the availability of d orbitals,
À
À
way for such [E] CF3 species. This kind of reaction may occur
makes them attractive as potential agents leading to C F
spontaneously or require the action of an acid. The failure of
preparations of LiCF3 (cLi = 0.97)[2] has been attributed to the
ready decomposition into LiF and DCF2 even at very low
temperatures.[3] At the other extreme is the case of the highly
stable tetrahedral anion [B(CF3)4]À, which required treatment
with concentrated H2SO4 (Scheme 2A)[4] to be brought into
reaction (cB = 2.01).[2] The process presumably goes through
the unstable difluorocarbene intermediate B(CF3)3(CF2),
which undergoes subsequent hydrolysis to give eventually
the unusual carbonyl derivative B(CF3)3(CO).
bond activation,[5] which is the reason why the chemistry of
trifluoromethyl–transition-metal derivatives is receiving
much current attention.[6,7]
We have now observed that [NBu4]2[Pt(CF3)4] (1)[8]
undergoes a hydrolytic process to give the monocarbonyl
derivative [NBu4][Pt(CF3)3(CO)] (2) in high yield (Sche-
me 2B; for experimental details, see the Supporting Infor-
mation). The reaction takes place under mild conditions and
is effected simply by moisture. The ease with which one of the
Pt-bound CF3 groups in 1 is transformed into CO contrasts
with other known precedents that require treatment of the
appropriate trifluoromethylplatinum complex with acids as
strong as HBF4/Et2O or HClO4/H2O.[9] In this context, it is
also interesting to note that, although a number of compounds
Transition metals (TMs) belong to the class of moderately
electropositive elements (cTM = 1.22–1.75)[2] and are therefore
intermediate cases. This feature, together with the richness in
=
containing the [TM] CF2 unit are known for Group 8 and 9
[*] Dipl.-Chem. S. Martꢀnez-Salvador, Dr. B. Menjꢁn, Prof. Dr. J. Forniꢂs,
Dr. A. Martꢀn
metals,[10] none have been isolated for Pt, probably because of
the high electrophilic character of the [Pt] CF2 moiety. The
=
Instituto de Ciencia de Materiales de Aragꢁn (I.C.M.A.)
Universidad de Zaragoza—C.S.I.C.
C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
Fax: (+34)976-761-187
transformation of one of the anionic CF3 groups within the
square-planar (SP-4), homoleptic unit [Pt(CF3)4]2À into the
neutral, poorly s-donating CO ligand deactivates the result-
ing [Pt(CF3)3(CO)]À ion towards further hydrolysis. Com-
pound 2 is, in fact, a fairly stable white solid, which can be
handled in the air without alteration.
E-mail: juan.fornies@unizar.es
Dr. I. Usꢁn
Institut de Biologia Molecular de Barcelona—C.S.I.C.
C/Jordi Girona 18–26, 08034 Barcelona (Spain)
The structure of the (SP-4)-[Pt(CF3)3(CO)]À ion
(Figure 1) was established by X-ray diffraction methods[11]
on single crystals of the salt [PPh4][Pt(CF3)3(CO)] (2’), which
was obtained by following a simple metathetical process. The
[**] This work was supported by the Spanish MICINN (DGPTC)/FEDER
(Project CTQ2008-06669-C02-01/BQU) and the Gobierno de
Aragꢁn (Grupo de Excelencia: Quꢀmica Inorgꢁnica y de los Com-
puestos Organometꢁlicos).
À
Pt C(CF3) bond lengths in 2’ are insensitive to the trans-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200907031.
standing ligand (CO vs. CF3)[12] and do not appreciably differ
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Chemie
pound [NBu4][Pt(CF3)2(CF2NC5H4S-kC,kS)] (5) in good yield
(Scheme 4; upper path). The reaction involves not only
replacement of the CO ligand at 2 as observed in the
Figure 1. Thermal ellipsoid diagram (50% probability) of the [Pt-
(CF3)3(CO)]À ion in 2’. Selected bond lengths [pm] and angles [8] with
estimated standard deviations: Pt–C(1) 207.5(4), Pt–C(2) 206.3(3), Pt–
C(3) 206.5(3), Pt–C(4) 191.3(4), C(4)–O 113.4(4), average C–F
136.8(4), C(1)-Pt-C(2) 91.1(1), C(1)-Pt-C(3) 175.9(1), C(1)-Pt-C(4)
89.7(1), C(2)-Pt-C(3) 89.5(1), C(2)-Pt-C(4) 171.9(1), C(3)-Pt-C(4)
90.3(1), Pt-C(4)-O 172.3(3), average Pt-C-F 115.1(2), average F-C-F’
103.3(3).
Scheme 4. Experimentally observed (upper path) formation of com-
pound 5 (see the Supporting Information for details) together with the
suggested reaction mechanism (lower path).
À
À
preceding cases, but also entails C F bond activation and C
N coupling. This complex reaction can be rationalized by
means of the following simple steps (Scheme 4; lower path):
1) ONMe3-assisted replacement of the CO ligand at 2 by the
thione tautomer of C5H5NS; 2) a-fluoride elimination in one
of the CF3 groups cis to the S-donor atom promoted by the
neighboring and moderately acidic pyridinium moiety,[15,16]
and 3) attack at the highly electrophilic C atom of the
resulting difluorocarbene fragment by the N-donor atom of
the anchored pyridine ligand.
À
from those observed in the precursor species 1 (average Pt
C(CF3) bond length 205.0(4) pm).[8] The n(CO) frequency in
2’ is the highest within the [PtR3(CO)]À series: 2117 (R =
CF3) > 2084 (R = C6F5) > 2073 cmÀ1 (R = C6Cl5),[13] thus
denoting the lower electron density on the metal center
where R = CF3. This lack of electron density on the metal
appears to be the main reason for the enhanced stability
against hydrolysis of 2 (or 2’) with respect to its parent species
1.
The crystal and molecular structures of 5 have been
established by single-crystal X-ray diffraction methods.[11] The
Pt center is in an approximate SP-4 environment (Figure 2)
formed by two terminal CF3 groups and the S- and C-donor
Compound 2 reacts with a number of neutral (L) or
anionic (XÀ) ligands in the presence of trimethylamine N-
oxide (ONMe3), undergoing efficient replacement of CO by
the incoming ligand L or XÀ (Scheme 3). Following this
procedure, a series of mono- or dianionic complexes of
formula [NBu4][Pt(CF3)3(L)] (L = CNCMe3 (3a), PPh3
(3b),[14] P(2-MeC6H4)3 (3c)) and [NBu4]2[Pt(CF3)3X] (X = Cl
(4a), Br (4b), I (4c)) can be easily obtained. Treatment of 2
with pyridin-2-thiol (C5H5NS) in the presence of ONMe3
unexpectedly gave the gem-difluorinated metallacyclic com-
À
atoms of the metallacycle. The Pt C(CF3) bond lengths are
Figure 2. Thermal ellipsoid diagram (50% probability) of one of the
two disordered [Pt(CF3)2(CF2NC5H4S-kC,kS)]À ions in 5. Selected bond
lengths [pm] and angles [8] with estimated standard deviations: Pt–
C(1) 200.1(9), Pt–C(2) 202.9(9), Pt–C(3) 185(2), Pt–S 231.0(5), average
C–F(CF3) 137.8(14), C(3)–N(1) 160(3), C(3)–F(7) 142.2(15), C(3)–F(8)
141.3(16), S–C(4) 174(4), C(1)-Pt-C(2) 89.7(3), C(1)-Pt-C(3) 93.5(7),
C(1)-Pt-S 178.2(3), C(2)-Pt-C(3) 176.1(6), C(2)-Pt-S 88.6(3), C(3)-Pt-S
88.2(6), Pt-C(3)-F(7) 122.5(12), Pt-C(3)-F(8) 122.9(12), F(7)-C(3)-F(8)
98.0(13).
Scheme 3. Synthesis of the mono- or dianionic tris(trifluoromethyl)pla-
tinate(II) derivatives: [NBu4][Pt(CF3)3(L)] (L=CNCMe3 (3a), PPh3 (3b),
P(C6H4Me-2)3 (3c)) and [NBu4]2[Pt(CF3)3X] (X=Cl (4a), Br (4b), I
(4c)). See the Supporting Information for experimental details.
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Communications
Received: December 14, 2009
Revised: March 8, 2010
Published online: May 6, 2010
virtually identical and slightly shorter than those found in the
related species 1 and 2’. The 5-membered metallacycle has a
bite angle that is large enough not to induce heavy angular
distortions in the overall SP-4 geometry around the Pt atom.
À
Keywords: carbene ligands · C N coupling · fluorinated ligands ·
À
The Pt S bond length [231.0(5) pm] is typical of a terminal
.
metallacycles · platinum
thiolatoplatinum(II) complex (ca. 232 pm).[17] At first sight,
this metallacycle could appear as a Pt derivative of a
fluorinated N-ylide ligand. However, a closer look reveals
geometric parameters associated with the CF2 unit that are
[1] I. Kieltsch, P. Eisenberger, K. Stanek, A. Togni, Chimia 2008, 62,
260; N. Shibata, S. Mizuta, H. Kawai, Tetrahedron: Asymmetry
2008, 19, 2633; T. Billard, B. R. Langlois, Eur. J. Org. Chem.
2007, 891; J.-A. Ma, D. Cahard, J. Fluorine Chem. 2007, 128, 975;
M. Schlosser, Angew. Chem. 2006, 118, 5558; Angew. Chem. Int.
Ed. 2006, 45, 5432; B. R. Langlois, T. Billard, Synthesis 2003, 185;
G. K. S. Prakash, M. Mandal, J. Fluorine Chem. 2001, 112, 123;
R. P. Singh, J. M. Shreeve, Tetrahedron 2000, 56, 7613; G. K. S.
Prakash, A. K. Yudin, Chem. Rev. 1997, 97, 757; T. Umemoto,
Chem. Rev. 1996, 96, 1757; M. A. McClinton, D. A. McClinton,
Tetrahedron 1992, 48, 6555.
[2] J. E. Huheey, E. A. Keiter, R. L. Keiter, Inorganic Chemistry, 4th
ed., Harper Collins, New York, 1993, pp. 182 – 199; E. J.
Little, Jr., M. M. Jones, J. Chem. Educ. 1960, 37, 231; A. L.
Allred, E. G. Rochow, J. Inorg. Nucl. Chem. 1958, 5, 264.
[3] D. J. Burton, L. Lu, Top. Curr. Chem. 1997, 193, 45; D. J. Burton,
Z.-Y. Yang, Tetrahedron 1992, 48, 189.
[4] M. Finze, E. Bernhardt, H. Willner, Angew. Chem. 2007, 119,
9340; Angew. Chem. Int. Ed. 2007, 46, 9180; M. Finze, E.
Bernhardt, A. Terheiden, M. Berkei, H. Willner, D. Christen, H.
Oberhammer, F. Aubke, J. Am. Chem. Soc. 2002, 124, 15385; A.
Terheiden, E. Bernhardt, H. Willner, F. Aubke, Angew. Chem.
2002, 114, 823; Angew. Chem. Int. Ed. 2002, 41, 799.
[5] H. Amii, K. Uneyama, Chem. Rev. 2009, 109, 2119; H. Torrens,
Coord. Chem. Rev. 2005, 249, 1957; T. G. Richmond, Angew.
Chem. 2000, 112, 3378; Angew. Chem. Int. Ed. 2000, 39, 3241;
T. G. Richmond, Top. Organomet. Chem. 1999, 3, 243; J.
Burdeniuc, B. Jedicka, R. H. Crabtree, Chem. Ber. 1997, 130,
145; H. Plenio, Chem. Rev. 1997, 97, 3363; J. L. Kiplinger, T. G.
Richmond, C. E. Osterberg, Chem. Rev. 1994, 94, 373.
À
inconsistent with this bonding scheme. Thus, the N C(CF2)
bond [160(3) pm] is much longer than that previously
2
[18]
À
observed for an N(sp ) C(CF2) single bond (ca. 145 pm).
The Pt-C(3)-F(7) and Pt-C(3)-F(8) bond angles are approx-
imately 1208 and significantly wider than the Pt-C-F angles
associated with a terminal CF3 group (ca. 1158). Moreover,
À
the Pt C(CF2) bond length [185(2) pm] is much shorter than
À
the usual Pt C(CF3) bonds (ca. 205 pm) and is comparable to
À
the Pt C(CO) bond length found in 2’ [191.3(4) pm]. In fact,
À
À
the Pt C(CF2) bond length is quite similar to the Ir C(CF2)
ones found in structurally characterized difluorocarbene–
=
iridium compounds, such as [Ir(CF3)( CF2)(CO)(PPh3)2]
5
[187.4(7) pm],[19]
[185.5(13) pm],[20]
[Ir(h -C5Me5)( CF2)(CO)]
[Ir(h -C5Me5)( CF2)(PMe3)]
=
5
=
and
[185.4(11) pm].[16] All these structural features suggest that
compound 5 can be more appropriately described as a
pyridine-stabilized difluorocarbene–platinum derivative.
Related structural patterns had been observed in pyridine-
stabilized silylene–metal derivatives[21] in which the longer Si
À
À
N bond (ca. 195 pm vs. 170–176 pm for a normal Si N single
bond) was taken as evidence of a dative bond.[22,23] In view of
its structural properties, compound 5 can be considered a
valid model for the initial step operating in the solvolysis
(including hydrolysis) of difluorocarbene–element derivatives
(Scheme 5).[24] Of particular relevance to the present case is
[6] Selected references: J. Vicente, J. Gil-Rubio, J. Guerrero-Leal,
D. Bautista, Dalton Trans. 2009, 3854; J. Goodman, V. V.
Grushin, R. B. Larichev, S. A. Macgregor, W. J. Marshall, D. C.
Roe, J. Am. Chem. Soc. 2009, 131, 4236; G. G. Dubinina, J.
Ogikubo, D. A. Vicic, Organometallics 2008, 27, 6233; G. G.
Dubinina, W. W. Brennessel, J. L. Miller, D. A. Vicic, Organo-
metallics 2008, 27, 3933; V. V. Grushin, W. J. Marshall, J. Am.
Chem. Soc. 2006, 128, 12644; J. Vicente, J. Gil-Rubio, J.
Guerrero-Leal, D. Bautista, Organometallics 2005, 24, 5634; S.
Balters, E. Bernhardt, H. Willner, T. Berends, Z. Anorg. Allg.
Chem. 2004, 630, 257; E. Bernhardt, M. Finze, H. Willner, J.
Fluorine Chem. 2004, 125, 967; J. Vicente, J. Gil-Rubio, J.
Guerrero-Leal, D. Bautista, Organometallics 2004, 23, 4871; R.
Eujen, B. Hoge, D. J. Brauer, Inorg. Chem. 1997, 36, 1464; J. A.
Schlueter, J. M. Williams, U. Geiser, J. D. Dudek, S. A. Sirchio,
M. E. Kelly, J. S. Gregar, W. H. Kwok, J. A. Fendrich, J. E.
Schirber, W. R. Bayless, D. Naumann, T. Roy, J. Chem. Soc.
Chem. Commun. 1995, 1311; D. Naumann, T. Roy, K.-F. Tebbe,
W. Crump, Angew. Chem. 1993, 105, 1555; Angew. Chem. Int.
Ed. Engl. 1993, 32, 1482; J. A. Morrison, Adv. Organomet. Chem.
1993, 35, 211.
Scheme 5. Suggested reaction path for solvolytic processes undergone
by difluorocarbene element species.
=
the transformation of the [E] CF2 unit into a coordinated
À
isocyanide ([E] CNR), which is effected by reaction with a
primary amine (NH2R; Scheme 5).[19] The reasonable stability
of 5 can be attributed to the lack of H atoms bound to the N-
donor atom as well as to the chelate effect.
In short, the homoleptic compound [NBu4]2[Pt(CF3)4] (1)
was found to undergo stepwise CF3 degradation under mild
conditions. Evidence for the intermediacy of highly reactive
[7] D. Naumann, N. V. Kirij, N. Maggiarosa, W. Tyrra, Y. L.
Yagupolskii, M. S. Wickleder, Z. Anorg. Allg. Chem. 2004, 630,
746.
[8] B. Menjꢀn, S. Martꢁnez-Salvador, M. A. Gꢀmez-Saso, J. Forniꢂs,
L. R. Falvello, A. Martꢁn, A. Tsipis, Chem. Eur. J. 2009, 15, 6371.
[9] R. A. Michelin, R. Ros, G. Guadalupi, G. Bombieri, F. Bene-
tollo, G. Chapuis, Inorg. Chem. 1989, 28, 840; T. G. Appleton,
R. D. Berry, J. R. Hall, D. W. Neale, J. Organomet. Chem. 1989,
=
[Pt] CF2 species was attained by the isolation of the ligand-
stabilized adduct 5, in which the CF2 unit still preserves much
of its original carbene nature.
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Chemie
364, 249; R. A. Michelin, G. Facchin, R. Ros, J. Organomet.
Chem. 1985, 279, c25.
[18] G. Bissky, G.-V. Rꢄschenthaler, E. Lork, J. Barten, M. Mꢂde-
bielle, V. Staninets, A. A. Kolomeitsev, J. Fluorine Chem. 2001,
109, 173; R. M. Schoth, E. Lork, A. A. Kolomeitsev, G.-V.
Rꢄschenthaler, J. Fluorine Chem. 1997, 84, 41; A. Kolomeitsev,
R.-M. Schoth, E. Lork, G.-V. Rꢄschenthaler, Chem. Commun.
1996, 335.
[10] D. Huang, P. R. Koren, K. Folting, E. R. Davidson, K. G.
Caulton, J. Am. Chem. Soc. 2000, 122, 8916; P. J. Brothers,
W. R. Roper, Chem. Rev. 1988, 88, 1293; M. A. Gallop, W. R.
Roper, Adv. Organomet. Chem. 1986, 25, 121; W. R. Roper, J.
Organomet. Chem. 1986, 300, 167.
[11] CCDC 749438 (2’) and 749437 (5) contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[19] P. J. Brothers, A. K. Burrell, G. R. Clark, C. E. F. Rickard, W. R.
Roper, J. Organomet. Chem. 1990, 394, 615.
[20] C. J. Bourgeois, R. P. Hughes, J. Yuan, A. G. di Pasquale, A. L.
Rheingold, Organometallics 2006, 25, 2908.
[21] H. Sakaba, M. Tsukamoto, T. Hirata, C. Kabuto, H. Horino, J.
Am. Chem. Soc. 2000, 122, 11511; H. Tobita, T. Sato, M. Okazaki,
H. Ogino, J. Organomet. Chem. 2000, 611, 314.
[22] A. Haaland, Angew. Chem. 1989, 101, 1017; Angew. Chem. Int.
Ed. Engl. 1989, 28, 992.
[23] The presence of the referred dative bond can be taken as the
main reason to explain the upfield shift of the CF2 signal in the
13C{19F} NMR spectrum of 5 (d = 145.69 ppm) with respect to the
CO signal in 2 (d = 174.21 ppm). This difference notwithstand-
ing, similar values of 1J(195Pt,13C) are observed in both cases:
1067 Hz for the CF2 unit in 5 vs. 1103 Hz for the CO ligand in 2.
[24] In a broader sense, this kind of adduct has been suggested to be
involved as a transition state (saddle point in the calculations) in
the reaction of a nucleophile with an electrophilic carbene
carbon atom: M. A. Sierra, I. Fernꢃndez, F. P. Cossꢁo, Chem.
Commun. 2008, 4671.
[12] On the basis of NMR data, the CF3 group was assigned a
markedly higher trans influence than the CO ligand: T. G.
Appleton, M. A. Bennett, Inorg. Chem. 1978, 17, 738.
[13] R. Usꢀn, J. Forniꢂs, M. Tomꢃs, I. Ara, B. Menjꢀn, J. Organomet.
Chem. 1987, 336, 129.
[14] The molecular structure of the [Pt(CF3)3(PPh3)]À ion in 3b
would be expected to be quite similar to that observed in its
tetramethylammonium salt: see Ref. [7].
[15] a-Fluoride elimination at a CF3 group promoted by moderately
acidic pyridinium salts has been documented in the Ir chemistry:
see Ref. [16].
[16] R. P. Hughes, R. B. Laritchev, J. Yuan, J. A. Golen, A. N. Rucker,
A. L. Rheingold, J. Am. Chem. Soc. 2005, 127, 15020.
[17] A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard, D. G.
Watson, R. Taylor, J. Chem. Soc. Dalton Trans. 1989, S1.
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