The first crystallographically characterized transition metal buckybowl compound: C30H12 carbon-carbon bond activation by Pt(PPh3)2

Full text of DOI:10.1021/ja973454d

J. Am. Chem. Soc. 1998, 120, 835-836
835
The First Crystallographically Characterized
Transition Metal Buckybowl Compound: C30H12
Carbon-Carbon Bond Activation by Pt(PPh3)2
Raef M. Shaltout, Renata Sygula, Andrzej Sygula,
Frank R. Fronczek, George G. Stanley,* and
Peter W. Rabideau*
Department of Chemistry, Louisiana State UniVersity
Baton Rouge, Louisiana 70803-1804
ReceiVed October 3, 1997
Semibuckminsterfullerene (C30H12, 1) represents half of the
buckminsterfullerene (C60) molecule with the rim carbons capped
by hydrogen atoms.1-3 Unlike C60, however, C30H12 has a readily
accessible internal surface and π-system for the potential coor-
dination of transition metal complexes. We have been studying
the metal coordination chemistry of curved polyaromatic hydro-
carbons such as corannulene (C20H10) and other buckybowls and
report here the first structural characterization of the unusual
product from the reaction of Pt(CH2dCH2)(PPh3)2 and 1.
Figure 1. (top) ORTEP plot of 2. (bottom) Stick figure representation
of 2 with the phenyl rings on the PPh3 ligands omitted for clarity.
C30H12 and Pt(CH2dCH2)(PPh3)2 were reacted together at room
temperature in toluene under inert atmosphere conditions for 15
h and then refluxed for 1 h. Pt(η2-σ-C30H12)(PPh3)2, 2, was
isolated in low yield (∼5-10%) via column chromatography and
recrystallized by slow evaporation of a benzene/cyclohexane
solution.4 A single-crystal X-ray structural determination revealed
that the Pt(PPh3)2 unit has indeed coordinated to the buckybowl
(Figure 1).5,6 But instead of binding to one of the localized CdC
double bonds on either the inner or outer surface the Pt(0) center
has oxidatively added to and broken one of the rim C-C bonds
to form an η2-σ-bonded Pt(II)-buckybowl compound.
3, prepared from the dilithioorganic precursor and not from a C-C
bond breaking reaction, is the most analogous model compound
that has been structurally characterized.7
The structure of 2 is shown in Figure 1 along with selected
bond distances and angles in Table 1. As one might qualitatively
expect, the Pt center is folded 42° outward (from the mean plane
defined by atoms C1-C5) to the less sterically hindered exo-
face of the C30H12 ligand (Figure 1b). The breaking of the C1-
C5 σ-bond in 1 will naturally splay the resulting sp2 hybrid orbitals
to the position of the Pt center shown in Figure 1. The 9,10-
dihydroplatina-anthracene system, Pt(PR3)2(σ-η2-C6H4CH2C6H4),
The bond distances and angles about the metal centers in 2
and 3 are quite similar, except for the C1-Pt-C5 angle, which
is opened up 6° in 2 (87.9(5)° vs 81.3(4)° for 3) and the larger
C1-C5 separation (2.84 Å in 2 vs 2.67 Å in 3). This reflects
the considerably greater ring strain present in the curved structure
of precursor 1. The other structural differences present are also
related to the larger and more sterically demanding structure of
2 relative to 9,10-dihydroplatina-anthracene. There is a distinct
16° rotation of the PPh3-Pt-PPh3 plane relative to the C-Pt-C
plane caused by steric hindrance between the PPh3 and the C30H12
ligands. The higher flexibility of the 9,10-dihydroplatina-
anthracene moiety also leads to greater folding of the anthracene
environment about the Pt away from square planar (average C6
dihedral angle of 37.5°) relative to 2 (23.5°).
(1) Rabideau, P. W.; Abdourazak, A. H.; Folsom, H.; Marcinow, Z.; Sygula,
A.; Sygula, R. J. Am. Chem. Soc. 1994, 116, 7891-2.
(2) Rabideau, P. W.; Sygula, A. Acc. Chem. Res 1996, 29, 235-42.
(3) Hagen, S.; Bratcher, M. S.; Erickson, M. S.; Zimmermann, G.; Scott,
L. T. Angew. Chem., Int. Ed. Engl. 1997, 36, 406-408.
(4) Spectroscopic data: 1H NMR (250 MHz, CDCl3) 4.2 (td, 1H, JX-H
)
7 Hz, JPt-H ) 110 Hz, X coupling could be to a P or H), 5.7 Hz (d, 1H, JX-H
) 7 Hz, X coupling could be to a P or H), 6.6 (t, 1H, JX-H ) 7 Hz, X coupling
could be to a P and/or H), 6.9-7.3 (m, PPh3 phenyl H), 7.5-7.8 (m); 31P{1H}
NMR (250 MHz, CDCl3, H3PO4 ref) 26.1 (pseudo-singlet, JPt-P ) 3876 Hz,
the Pt satellites show a doublet-doublet 31P pattern, indicating that the primary
31P peak at 26.1 is not a first-order singlet, consistent with the formal lack of
symmetry and presence of magnetic inequivalence between the two PPh3
groups in 2).
The structural features of the C30H12 ligand in 2 are surprisingly
similar to that predicted from ab inito calculations for 1,
particularly with respect to the C-C bond distances, which, at
first glance, appear to be somewhat randomly distributed as short
and longer distances.6 The main difference, of course, is that
(5) Crystal Data: monclinic P21/n, Z ) 4, a ) 14.646(6) Å, b ) 16.392(3)
Å, c ) 28.02(1) Å, b ) 95.78(3)°, V ) 6691(1) Å.3 Data collected at 150K.
Refinement on F2 converged at R ) 7.4%, Rw ) 6.2%, with GOF(F2) ) 1.383.
There were three benzene and one cyclohexane solvent molecules present in
the unit cell. One of the benzenes is disordered with a likely partial substitution
of cyclohexane at that site.
(7) Alcock, N. W.; Bryars, K. H.; Pringle, P. G. J. Chem. Soc., Dalton
Trans. 1990, 1433-1439.
(6) Supporting Information. See masthead for instructions.
S0002-7863(97)03454-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/16/1998

836 J. Am. Chem. Soc., Vol. 120, No. 4, 1998
Communications to the Editor
Table 1. Selected Bond Distances (Å) and Angles (deg) for 2a
reactions in biphenylene and find that Cp*Rh(CH2dCH2)2 ac-
complishes this by initial C-H bond activation followed by C-C
bond cleavage (and subsequent reformation of the C-H bond).14
They have tried the same reaction with biphenylene and M(PEt3)3
(M ) Pd, Pt) and also see C-C bond cleavage reactions, but in
this case they believe that C-C bond breaking is probably
preceded by simple η2-π-arene coordination.15 To our knowledge,
this is the first example of metal induced C-C bond cleavage in
a nonheterocyclic five-membered ring. This is probably related
to the fact that 1 possesses very unusual, highly strained five-
membered rings. We believe that the cleavage is proceeding via
simple π-coordination of Pt(PPh3)2 to one of the rim CdC bonds
adjacent to the five-membered-ring site in 1, followed by C-C
oxidative addition.
That 1 should possess strained C-C bonds along the rim of
the molecule is not surprising. The synthesis of 1 requires high-
temperature pyrolysis to provide enough thermal energy for the
more “open” precursor species to close the rim bonds to form
the curved buckybowl.1 The site of C-C bond cleavage is also
where one would predict the maximum release of steric strain,
i.e., the five-membered, formally nonaromatic ring sites, which
are the source of curvature in 1. These also are the longest and
most strained C-C bonds in the structure (1.51 Å from the X-ray
structure on 2, 1.53 Å from ab initio calculations on 1). The
3-fold symmetry in 1, of course, means that C-C bond breaking
can occur at any of the three equivalent rim five-membered-ring
exo C-C bonds.
The coordination of transition metal centers to these unusual
curved polyaromatic hydrocarbon ligands is a new area of
organometallic chemistry and several researchers are actively
working on preparing new complexes and studying their reaction
chemistry. Of course, the coordination of metals to C60 has also
garnered considerable interest, although the coordination is
generally limited to η2-π-coordination.17 Rubin and co-workers
were the first to demonstrate the Co initiated C-C bond cleavage
in a derivatized version of C60.18 Considerably less common is
the use of fullerene subunits as ligands for transition metal centers.
O’Connor, Siegel, and co-workers recently published a paper on
the coordination of Cp*Ru to corannulene, the smallest curved
polyaromatic hydrocarbon ligand, in an η6-coordination mode.19
Although they did not have an X-ray structure, it is likely that
the Ru is coordinated to an exo-face of one of the rim C6
corannulene rings. Balch and co-workers20 have reported an
X-ray structure on the more highly curved polyaromatic C36H12,
first prepared by Scott and co-workers, and are working on
preparing transition metal compounds of this ligand.21 It is clear
that interest in these novel ligands will continue to grow as
improved syntheses are reported for these and other curved
polyaromatic hydrocarbon compounds.
Pt-P1
2.348(4)
2.04(2)
1.37(2)
1.45(2)
1.49(2)
1.47(2)
1.39(2)
1.36(2)
1.48(2)
1.44(2)
1.39(2)
1.35(2)
1.45(2)
1.43(2)
1.42(2)
1.41(2)
1.37(2)
1.34(2)
1.44(2)
1.51(2)
1.42(2)
96.0(1)
89.9(4)
164.8(4)
125(1)
113.7(9)
115(1)
115(1)
133(1)
106(1)
111(1)
106(1)
125(1)
113(1)
126(1)
127(1)
136(1)
127(1)
103(1)
105(1)
125(1)
Pt-P2
2.323(4)
2.05(1)
1.44(2)
1.45(2)
1.41(2)
1.37(2)
1.41(2)
1.42(2)
1.33(2)
1.50(2)
1.36(2)
1.41(2)
1.37(2)
1.43(2)
1.43(2)
1.41(2)
1.43(2)
1.42(2)
1.37(2)
1.38(2)
1.38(2)
87.9(5)
88.0(4)
171.9(4)
120(1)
130(1)
114(1)
114(1)
106(1)
111(1)
125(1)
112(1)
115(1)
130(1)
109(1)
105(1)
112(1)
115(1)
138(1)
135(1)
Pt-C1
Pt-C5
C1-C2
C1-C6
C2-C3
C2-C9
C3-C4
C3-C12
C4-C5
C4-C15
C5-C18
C6-C7
C7-C8
C8-C9
C9-C10
C10-C11
C11-C12
C11-C20
C12-C13
C13-C14
C13-C19
C14-C15
C14-C26
C15-C16
C16-C17
C16-C27
C17-C18
C17-C30
C19-C20
C19-C24
C20-C21
C21-C22
C22-C23
C23-C24
C24-C25
C25-C26
C26-C27
C27-C28
C28-C29
C29-C30
P1-Pt-P2
P1-Pt-C5
P2-Pt-C5
Pt-C1-C2
Pt-C5-C4
C2-C1-C6
C4-C3-C12
C10-C11-C20
C11-C12-C13
C15-C14-C26
C14-C15-C16
C17-C16-C27
C16-C17-C30
C5-C18-C17
C20-C19-C24
C11-C20-C21
C23-C24-C25
C14-C26-C27
C16-C27-C26
C28-C29-C30
C1-Pt-C5
P2-Pt-C1
P1-Pt-C1
Pt-C1-C6
Pt-C5-C18
C3-C2-C9
C4-C5-C18
C12-C11-C20
C12-C13-C19
C4-C15-C16
C15-C16-C27
C16-C17-C18
C18-C17-C30
C13-C19-C20
C11-C20-C19
C19-C24-C23
C14-C26-C25
C25-C26-C27
C26-C27-C28
a Only angles for the buckybowl portion of the structure that deviate
more than (4° from 120° are listed. Values in parentheses are esd’s.
the curvature of the bowl is less in 2 due to the breaking of one
of the rim five-membered-ring C-C bonds. The calculated
average quaternary carbon atom pyramidalization angle (POAV1)
for 1 is 4.8° versus 2.9° for the X-ray structure on 2.6,8-11
Homogeneous C-C σ-bond breaking reactions are quite
difficult and usually only occur with strained rings such as
cyclopropane,12 biphenylene,13-15 or when a C-C σ-bond is held
in close proximity to a metal center combined with the generated
aromaticity of the product.16 In the case of saturated hydrocarbons
such as cyclopropane, C-H bond activation is believed to be the
first step, followed by C-C bond cleavage.12 In the case of
unsaturated π-systems, coordination of the metal center to the
π-system followed by oxidative addition of the C-C bond to the
metal center represents an alternate mechanistic possibility. Jones
and co-workers have carefully studied the C-C bond breaking
Acknowledgment. We thank the National Science Foundation (CHE-
9420908) and the Division of Chemical Sciences, Office of Basic Energy
Research, U.S. Department of Energy for financial support.
Supporting Information Available: X-ray experimental details with
positional parameters and full bond distances and angles; comparison of
X-ray bond lengths for 2 with ab initio calculated bond lengths for 1;
calculated pyramidalization values for 2 (11 pages). See any current
masthead page for ordering and Internet access instructions.
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JA973454D
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