Unassisted η2-coordination of polycyclic aromatic hydrocarbons to platinum(II)

Article abstract of DOI:10.1021/ic050405q

The coordination of phenanthrene to the d⁸ Pt^II center in (SP-4-2)-[Pt(C₆F₅)₂(CO) (η²-C₁₄H₁₀)] causes a slight pyramidalization at the metal-bound C atoms (C⁹ and C¹⁰), but no perceptible elongation of the corresponding C-C bond [C(13)-C(14) 132.0-(5) vs. 133.8(5) pm in the free ligand].

Full text of DOI:10.1021/ic050405q

Inorg. Chem. 2005, 44, 7265−7267
Unassisted
Platinum(II)
η
2-Coordination of Polycyclic Aromatic Hydrocarbons to
Jesu´s R. Berenguer,† Juan Fornie´s,* L. Francisco Mart´ın, Antonio Mart´ın, and Babil Menjo´n
Instituto de Ciencia de Materiales de Arago´n, Facultad de Ciencias, UniVersidad de
Zaragoza-C.S.I.C., C/Pedro Cerbuna 12, E-50009 Zaragoza, Spain
Received March 18, 2005
The coordination of phenanthrene to the d8 PtII center in (SP-4-
2)-[Pt(C6F5)2(CO)(
2-C14H10)] causes a slight pyramidalization at
the metal-bound C atoms (C9 and C10), but no perceptible
elongation of the corresponding C C bond [C(13) C(14) 132.0-
(5) vs. 133.8(5) pm in the free ligand].
Scheme 1. Synthesis of Compounds 2 and 3
η
−
−
molecule and formation of cis-[Pt(C6F5)2(CO)(η2-C14H10)]
(2). In a similar way, 1 reacts with naphthalene (C10H8) in
CH2ClCH2Cl solution to give cis-[Pt(C6F5)2(CO)(η2-C10H8)]
(3). Compounds 2 and 3 can be isolated as white solids in
moderate yields (see Supporting Information for details).
The molecular structure of 2 was established by single-
crystal X-ray analysis. The Pt center is located in a SP-4
environment (Figure 1) defined by the C-donor atoms of the
C6F5 and CO ligands and the midpoint between the two
metal-coordinated phenanthrene C atoms. The ligands adopt
a cis arrangement as in the parent species so the substitution
reaction in Scheme 1 takes place with stereoretention. Two
strong absorptions assignable to the X-sensitive vibration
modes of the C6F5 groups7 are accordingly observed in the
IR spectra of both 2 and its parent species 1 (C2V local
symmetry; IR-active X-sensitive modes: A1 + B1). Phenan-
threne coordinates to the “cis-Pt(C6F5)2(CO)” moiety in an
essentially symmetric form [Pt-C(13) 234.5(3), Pt-C(14)
231.2(3) pm] through the two C atoms (C9 and C10 in
standard IUPAC notation)8 for which the highest π-electron
density has been calculated by Hu¨ckel MO methods.9
Coordination through C9 and C10 also involves the lower loss
of aromaticity in the phenanthrene ligand measured in terms
Efficient hydrocarbon C-H activation is among the most
pursued goals in modern chemistry.1 Late transition metal
complexes of various kinds (especially of PtII)1,2 are known
to accomplish a number of such processes under unusually
mild conditions, thereby inducing a higher degree of
selectivity. Much effort has been devoted to studying the
key steps of the mechanism involved, a better understanding
of which would enable the design and development of new
systems with enhanced activity and/or selectivity. It is
currently agreed that the first step in PtII-mediated arene C-H
activation entails the formation of an intermolecular η2-arene
complex.3,4 Unless assisted by intramolecular coordination
of a pendant donor substituent5sthus benefiting from the
chelate effectsthis coordination mode is still rare in PtII
chemistry and, in fact, it has only recently been unambigu-
ously established.3,6 Here we report on highly unusual PtII
species containing η2-coordinated condensed arenes.
The square-planar (SP-4) complex cis-[Pt(C6F5)2(CO)(thf)]
(1) reacts with phenanthrene (C14H10) in CH2Cl2 solution
(Scheme 1) under substitution of the weakly coordinated thf
* To whom correspondence should be addressed.
† Current address: Departamento de Qu´ımica, Grupo de S´ıntesis Qu´ımica
de la Rioja, UA-CSIC, Universidad de La Rioja, C/Madre de Dios 51,
E-26006 Logron˜o (La Rioja), Spain.
(1) Labinger, J. A. J. Mol. Catal. A 2004, 220, 27. Labinger, J. A.; Bercaw,
J. E. Nature 2002, 417, 507. Crabtree, R. H. J. Chem. Soc., Dalton
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(2) (a) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471. (b) Fekl, U.;
Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, 259. (c) Stahl, S. S.;
Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37,
2180.
(3) Norris, C. M.; Reinartz, S.; White, P. S.; Templeton, J. L. Organo-
metallics 2002, 21, 5649.
(5) Alonso, E.; Fornie´s, J.; Fortun˜o, C.; Mart´ın, A.; Orpen, A. G.
Organometallics 2003, 22, 2723. Alonso, E.; Fornie´s, J.; Fortun˜o, C.;
Mart´ın, A.; Orpen, A. G. Organometallics 2000, 19, 2690. Wright, L.
L.; Moore, S. J.; McIntyre, J. A.; Dowling, S. M.; Overcast, J. D.;
Grisel, G. R., IV.; Nelson, K. K.; Rupert, P. R.; Pennington, W. T.
Organometallics 1999, 18, 4558. Casas, J. M.; Fornie´s, J.; Mart´ınez,
F.; Rueda, A. J.; Toma´s, M.; Welch, A. J. Inorg. Chem. 1999, 38,
1529. Ara, I.; Falvello, L. R.; Fornie´s, J.; Lalinde, E.; Mart´ın, A.;
Mart´ınez, F.; Moreno, M. T. Organometallics 1997, 16, 5392. Casas,
J. M.; Fornie´s, J.; Mart´ın, A.; Menjo´n, B.; Toma´s, M. J. Chem. Soc.,
Dalton Trans. 1995, 2949. Casas, J. M.; Fornie´s, J.; Mart´ın, A.;
Menjo´n, B. Organometallics 1993, 12, 4376. Fornie´s, J.; Menjo´n, B.;
Go´mez, N.; Toma´s, M. Organometallics 1992, 11, 1187.
(6) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am.
Chem. Soc. 2001, 123, 12724.
(7) Uso´n, R.; Fornie´s, J. AdV. Organomet. Chem. 1988, 28, 219.
(4) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2000, 122, 10846.
10.1021/ic050405q CCC: $30.25
Published on Web 09/20/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 21, 2005 7265

COMMUNICATION
Scheme 2. Isomerization of (cis) 1 to (trans) 4
more marked basicity of the remaining ligands. It is also
interesting to note that considerably shorter Pt-C distances
[221(1)-227(1) pm] have been found in other PtII com-
pounds containing η2-arene ligands, [PtH(HTp′)(η2-RH)]+
[RH ) benzene, p-xylene; Tp′ ) hydridotris(3,5-dimeth-
ylpyrazolyl)borato].3,6 In these structurally characterized
precedents, a slight lengthening of the η2-coordinated C-C
bond was also observed.
The presence of two sets of C6F5 signals in the 19F NMR
spectrum of 2 in [2H]dichloromethane solution between -80
and 25 °C denotes the presence of two chemically inequiva-
lent C6F5 groups, in agreement with the cis arrangement
observed in the solid-state structure. Despite the negligible
structural changes observed in the C framework with respect
to the free ligand, the coordination of phenanthrene to the
“cis-Pt(C6F5)2(CO)” moiety produces a distinct upfield shift
in the resonance signal assigned to H9 and H10, which appear
Figure 1. Thermal ellipsoid diagram of 2. Selected distances (pm) and
angles (deg): Pt-C(1), 203.4(3); Pt-C(7), 204.0(3); Pt-C(13), 234.5(3);
Pt-C(14), 231.2(3); Pt-C(27), 193.7(3); C(13)-C(14), 132.0(5); C(27)-
O, 111.8(4); C(1)-Pt-C(7), 88.64(11); C(1)-Pt-C(27), 89.14(12); C(7)-
Pt-C(27), 177.78(11); Pt-C(27)-O, 177.9(3).
of resonance energy.10 The C(13)-C(14) line forms an angle
of 21.2° with the normal to the coordination plane and adopts
a staggered conformation with respect to the trans-standing
C6F5 group. It is interesting to note that the C9-C10 bond
distance in 2 [C(13)-C(14) 132.0(5) pm] is statistically
indistinguishable from the corresponding one in the free
ligand [133.8(5) pm];11 hence, no elongation at all is observed
upon coordination. This structural feature denotes that little
π-back-donation, if any, from the “cis-Pt(C6F5)2(CO)” moiety
to the antibonding π* aromatic system of phenanthrene
actually occurs. This can be considered to be a consequence
of the marked electron-withdrawing character of the C6F5
groups,12 as well as the well-known ability of CO to behave
as a poor σ-donor/good π-acceptor ligand. The high νCO value
observed in the IR spectrum of 2, νCO ) 2125 cm-1, can be
taken as further evidence of the lack of electron density at
the metal center.13 In this context, it is interesting to note
that a significant lengthening of the coordinated C-C bond
[C9-C10 146.0(5) pm] was observed in [Rh(η5-C5Me5)-
(PMe3)(η2-C14H10)].15 This observation would be in agree-
ment with the higher electron density available for the Rh
center considering: (1) its lower oxidation state and (2) the
1
in the H NMR spectrum of 2 as a singlet with platinum
satellites [2J(195Pt,H) ) 48 Hz] at δ ) 7.51 ppm (cf. δ )
7.78 ppm for free phenanthrene). This fact can be related to
the slight pyramidalization observed at the coordinated C
atoms, whose corresponding C-H bonds deviate more than
10° from the phenanthrene ring, implying a higher contribu-
tion of the 2p orbitals in the spn hybridization of C9 and C10
atoms. However, no significant modification in the corre-
1
sponding J(13C,H) value has been observed (167 Vs. 160
Hz in the free ligand).
It has been recently pointed out14 that metal-promoted
C-H activation processes should be favored by the enhanced
Lewis-basicity of the “MLn” metal fragment. On the other
hand, the required coordination of the organic substrate RH
would be expected to preferentially occur at highly acidic
metal complex species. A mismatch in properly balancing
these two opposite electronic requirements in the “cis-Pt-
(C6F5)2(CO)” fragment could well be the reason 2, containing
an η2-coordinated phenanthrene molecule, is sufficiently
stable to allow its isolation and extensive characterization
while, in turn, C-H activation has not yet been observed to
occur. These results can be considered to exemplify the
electrophilic behavior of electron-poor metal centers.2a
(8) Rigaudy, J.; Klesney, S. P. Nomenclature of Organic Chemistry;
Pergamon Press: Oxford, UK, 1979.
(9) Cotton, F. A.; Dikarev, E. V.; Petrukhina, M. A. J. Am. Chem. Soc.
2001, 123, 11655.
(10) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.; Jones, W.
D.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 7685.
(11) Petrˇ´ıcˇek, V.; C´ısarˇova´, I.; Hummel, L.; Kroupa, J.; Bˇrezina, B. Acta
Crystallogr., Sect. B 1990, 46, 830.
(12) Sheppard, W. A. J. Am. Chem. Soc. 1970, 92, 5419. Tolman, C. A. J.
Am. Chem. Soc. 1970, 92, 2953.
(13) The νCO value in 2 is even higher than that found in the cationic
complexes [PtMe(RNdCR′-CR′dNR-κ2N)(L)][BF4] (L ) CO; R )
an aryl group; R′ ) H, Me; νCO ) 2103-2113 cm-1), whose
corresponding solvates (L ) H2O, CF3CH2OH) have been found to
effectively promote the activation of benzene C-H bonds.14
(14) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002,
124, 1378.
In contrast to the previously dicussed “electrophilic
behavior” observed for the “cis-Pt(C6F5)2(CO)” fragment
toward the polycyclic aromatic hydrocarbons, it has been
found that benzene is not to accomplish clean substitution
of thf in complex 1. Thus, heating 1 in benzene solution at
50-60 °C for 1 h results in an isomerization process giving
trans-[Pt(C6F5)2(CO)(thf)] (4) in 50% yield together with
extensive decomposition to platinum metal (Scheme 2). The
same isomerization process has been found to occur using
cyclohexane as solvent. In this saturated hydrocarbon,
however, the yield of isolated 4 increases to 70% and no
(15) Jones, W. D.; Dong, L. J. Am. Chem. Soc. 1989, 111, 8722.
7266 Inorganic Chemistry, Vol. 44, No. 21, 2005

COMMUNICATION
decomposition to Pt0 is observed (even though the temper-
ature was maintained in this case at 90-95 °C). In view of
the different outcome, it might be considered that the solvent
plays a noninnocent role.
Acknowledgment. This work was supported by the
Spanish MCYT (DGI)/FEDER (Project No. BQU2002-
03997-CO2-02) and Promerus LLC.
Supporting Information Available: Crystallographic data of
2 (CIF) and experimental details for the preparation of 1-4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Further studies aiming to exploit the synthetic potential
of the cis and trans “Pt(C6F5)2(CO)” moieties as highly
acidic, single-site metal fragments enabling unusual coor-
dination patterns are currently underway.
IC050405Q
Inorganic Chemistry, Vol. 44, No. 21, 2005 7267