Umpolung of a metal-carbon bond: A potential route to porphyrin-based methane functionalization catalysts [6]

Full text of DOI:10.1021/ja001380r

J. Am. Chem. Soc. 2000, 122, 8569-8570
8569
Scheme 1. Proposed Mechanism for Hydrocarbon
Umpolung of a Metal-Carbon Bond: A Potential
Route to Porphyrin-Based Methane Functionalization
Catalysts
Functionalizationa
Andrew P. Nelson and Stephen G. DiMagno*
Department of Chemistry, UniVersity of Nebraska-Lincoln
Lincoln, Nebraska 68588-0304
ReceiVed April 20, 2000
a
The four steps shown are: (1) oxidative addition, (2) proton transfer,
(3) oxidation of a metal center, and (4) nucleophilic attack on an
organometallic species. M ) complexed metal ion, RH ) hydrocarbon,
Selective functionalization of otherwise unactivated alkanes is
an important goal, since alkyl chains are ubiquitous and constitute
one of the few functional groups for which there is not a highly
developed chemistry.1,2 While several homogeneous late transition
metal catalysts activate alkanes under relatively mild conditions,
well-defined systems capable of efficient alkane functionalization
are rare.3-7 At the most basic level, metal-based alkane function-
alization is complicated by the need to catalyze heterolytic (ionic)
and homolytic bond-making and bond-breaking events at a single
metal center. A typical sequence, such as that proposed for the
Pt-based Shilov oxidation process, is outlined in Scheme 1.8
The reactions outlined in eq 1-4 differ radically in their
ionicity. Thus, if one can effectively change the electronegativity
of a metal center while maintaining a model catalyst’s coordina-
tion geometry, the effects on the thermodynamics of the relevant
reactions can be predicted from first principles. In Pauling’s
valence-bond treatment, the strength of a chemical bond, as
measured by its homolytic bond dissociation energy (BDE), is
determined by covalent and ionic terms.9 According to this
analysis, BDE is a function of the difference in electronegativity
(∆ø) of the two atoms; effectively decreasing ∆ø will decrease
BDE. Since transition metal atoms are more electropositive than
hydrogen or carbon, it is expected that M-C and M-H BDEs
will decrease as electron density is withdrawn from a given metal
center’s coordination sphere, decreasing the enthalpy for oxidative
addition into a C-H bond (and M-C and M-H BDEs).
However, from inspection of the thermodynamic cycle shown in
Scheme 2, one can also conclude that M-C and M-H BDEs
will be less sensitive, in terms of energy, than their respective
heterolytic (or ionic) bond dissociation energies (eqs 2 and 4
above) as transition metal atoms are rendered more electrophilic.10
Elegant studies from Wayland’s laboratories have shown that
Rh(II) porphyrins activate methane reversibly.11,12 However,
nucleophilic functionalization of the resulting methylrhodium
complexes has not been demonstrated. Recently, the first syntheses
and characterization of the extremely electron-deficient porphyrin
F28TPP13 and its transition metal chelates were reported.14-16
Electron-deficient porphyrin macrocycles offer the opportunity
to stabilize the Rh(I) oxidation state dramatically, potentially
B- ) base, Ox(m) ) oxidant, Red(m-2) ) reduced oxidant, Y-
nucleophile.
)
Scheme 2. Thermodynamic Cycle for Bond Heterolysisa
a BDE ) bond dissociation energy, IP(H) ) ionization potential for
hydrogen atom, EA is the electron affinity of the metal (M) in the
oxidation state (n - 1), and ∆H(i) ) is the enthalpy for the ionic
(heterolytic) reaction shown.
switching on a key SN2 reaction featuring an alkylrhodium
electrophile.17 In accord with the above discussion, activating this
ionic pathway need not compromise C-H activation, since the
Rh-H and Rh-C BDEs should be less dependent upon metal
electrophilicity than heterolysis of these same bonds. Therefore,
the series of transformations outlined in Figure 1 should be
feasible.
Syntheses of the rhodium complexes CH3Rh(F28TPP), HRh-
(F28TPP), Rh(F28TPP), and [Rh(F28TPP)]- relied on methodology
established for analogous nonfluorinated derivatives.18 All of these
compounds are stable to water, and only [Rh(F28TPP)]- is air
sensitive.
Rh(F28TPP) is sufficiently soluble in benzene (Bz) and chlo-
robenzene (ClBz) for 19F NMR experiments. The â-fluorine
resonances show a substantial isotropic shift [Rh(F28TPP) δ )
-98.3, Co(F28TPP) δ ) -124.8, Zn(F28TPP) δ ) -145.4 ppm]16
and the line widths of the broad signals are insensitive to changing
temperatures (20-60 °C), indicating that the metalloradical is
largely monomeric in solution. This behavior contrasts with that
observed for typical rhodium porphyrins, which readily form
dimers possessing Rh-Rh bonds.19 The reversible activation of
(13) Abbreviations used in the paper are: F28TPP - 5,10,15,20-tetrakis-
(pentafluorophenyl)-2,3,7,8,12,13,17,18-octafluoroporphyrin, Rh(F28TPP)-
[5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octafluoro-
porphinato]rhodium(II), HRh(F28TPP)-[5,10,15,20-tetrakis(pentafluorophen-
yl)-2,3,7,8,12,13,17,18-octafluoroporphinato][hydrido]rhodium(III), CH3Rh-
(F28TPP)-[5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octa-
fluoroporphinato][methyl]rhodium(III), [Rh(F28TPP)]-- [5,10,15,20-tetrakis-
(pentafluorophenyl)-2,3,7,8,12,13,17,18-octafluoroporphinato]rhodium(I) an-
ion, Rh(OEP) - [2,3,7,8,12,13,17,18-octaethylporphinato]rhodium, Rh(TMP)
- [5,10,15,20-tetramesitylporphinato]rhodium, CH3Rh(OETAP) - [5,10,15,-
20-tetraaza-2,3,7,8,12,13,17,18-octaethylporphinato][methyl]rhodium(III).
(14) Woller, E. K.; DiMagno, S. G. J. Org. Chem. 1997, 62, 1588-1593.
(15) Leroy, J.; Bondon, A.; Toupet, L.; Rolando, C. Chem. Eur. J. 1997,
3, 1890-1893.
(16) Smirnov, V. V.; Woller, E. K.; DiMagno, S. G. Inorg. Chem. 1998,
37, 4971-4978.
(17) Nucleophilic attack on neutral transition metal species is most common
with anionic metal centers acting as nucleophiles. See: Collman, J. P.;
Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of
Organotransition Metal Chemistry; University Science Books: Mill Valley,
1987.
(1) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879-2932.
(2) Bergman, R. G. Science 1984, 223, 902-908.
(3) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; H., P. T. Acc. Chem.
Res. 1995, 28, 154-162.
(4) Shilov, A. E. ActiVation of Saturated Hydrocarbons by Transition Metal
Complexes; D. Reidel: Dordrecht, 1984.
(5) Hill, C. J. In ActiVation and Functionalization of Alkanes; Hill, C. J.,
Ed.; Wiley: New York, 1989.
(6) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii,
H. Science 1998, 280, 560-564.
(7) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000,
287, 1995-1997.
(8) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998,
37, 2180-2192.
(9) Pauling, L. The Nature of the Chemical Bond; Cornell University
Press: Ithaca, 1960.
(10) Pearson, R. G. Chem. ReV. 1985, 85, 41-49.
(11) Zhang, X.-X.; Wayland, B. B. J. Am. Chem. Soc. 1994, 116, 7897-
7898.
(18) Full experimental procedures are given in the Supporting Information.
(19) Wayland, B. B.; Coffin, V. L.; Farnos, M. D. Inorg. Chem. 1988, 27,
2745-2747.
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10.1021/ja001380r CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/18/2000

8570 J. Am. Chem. Soc., Vol. 122, No. 35, 2000
Communications to the Editor
this metrical parameter is relatively insensitive to large changes
in porphyrin electronic structure, as is expected for a covalent
bond with little ionic character.
In Bz or ClBz at 40 °C, the potent nucleophile triphenylphos-
phine (PPh3) (0.1 M) attacks CH3Rh(F28TPP) at the methyl
group, quantitatively forming methyltriphenylphosphonium cation
[CH3PPh3]+ and displacing the [Rh(F28TPP)]- leaving group
(reaction 6 in Figure 1). Remarkably, ion pair formation is
sufficiently favorable for the reaction to occur in Bz, a relatively
nonpolar solvent, although the reaction required 10 d to reach
completion (12 h in ClBz). Evidence for extensive ion-paring of
the product salt is apparent in the 1H NMR spectrum, where there
are substantial upfield shifts of the phenyl and methyl proton
resonances; the methyl doublet is shielded to the greatest extent
(δ ) -1 ppm). (The product assignments were confirmed by
NMR spectroscopy of independently synthesized [CH3PPh3]+
[Rh(F28TPP)]- and of [CH3PPh3]+ Cl- extracted from the reaction
mixture with NaCl/D2O.) The success of this nucleophilic
displacement confirms that the Rh(I) oxidation state is sufficiently
stabilized by the perfluorinated porphyrin to make the metal
complex a relatively good leaving group in comparison to typical
Rh(I) porphyrins. To our knowledge, this is the first example of
a bona fide nucleophilic displacement on a neutral organometallic
complex leading to a stable ion pair product.
Figure 1. Proposed reaction sequence for methane functionalization. The
net reaction is homogeneous alkane-hydrogen metathesis.
To quantify the stabilization of the Rh(I) oxidation state by
the fluorinated porphyrin, the acidities of HRh(F28TPP) and HRh-
(TPP) were measured in dimethyl sulfoxide against a series of
standard acids.24 At 25 °C the following data were obtained: HRh-
(F28TPP), pKa ) 2.1 ((0.3); HRh(TPP), pKa ) 11.0 ((0.3); ∆pKa
) 8.9. The large difference in acidity between the fluorinated
and nonfluorinated complexes (∆∆G° ) -12.1 kcal/mol) con-
firms that the ionic reactions are much more sensitive to the
electron-withdrawing nature of the perfluorinated porphyrin
than are the Rh-C and Rh-H BDEs. In addition a sample of
[CH3PPh3+] [Rh(F28TPP)]- in benzene was titrated with TFA,
and the relative acidities were measured by 19F NMR spectros-
copy. The rhodium hydride is slightly more acidic than TFA
(∆pKa ) 0.6) under these conditions. This acid-base reaction
formally completes the reaction cycle shown in Figure 1.
Here we have demonstrated that a modified rhodium porphyrin
complex performs each of the reactions shown in Figure 1. The
key design principle, that ligand modification can so increase
metal nucleofugacity that nucleophilic substitution can be acti-
vated without compromising alkane and hydrogen activation, is
drawn directly from fundamental concepts of chemical bonding.
Developing an efficient catalytic system from these or similar
components requires dramatically accelerated rates of the methane
activation and nucleophilic functionalization reactions; studies
aimed at satisfying these kinetic requirements are currently
underway.
H2 (5 atm) by Rh(F28TPP) in Bz was followed by 19F NMR
spectroscopy at 30-60 °C; only the paramagnetic Rh(F28TPP)
and diamagnetic HRh(F28TPP) were observed. There was a
marked solvent dependence upon the rate at which the equilibrium
was established (30 min in ClBz, 1 d in Bz). By monitoring the
equilibrium constant for the reaction as a function of temperature,
the relevant thermodynamic quantities (∆H ) -11 (( 3) kcal/
mol, ∆S ) -20 (( 10) eu, BDE ) 58 ((2) kcal/mol) were
determined. The Rh-H BDE observed with Rh(F28TPP) compares
to Rh(OEP) (Rh-H ) 61.8 kcal/mol)19 and Rh(TMP) (Rh-H ≈
60 kcal/mol),20 indicating that the Rh-H bond strength varies
with metal electron density in the expected manner.
Similarly, the Rh(F28TPP) metalloradical was observed by 19
F
NMR to activate methane (5 atm) to form CH3Rh(F28TPP) at 40
°C. The prohibitively slow rate of this process (15 d to reach
40% conversion in ClBz) and evolution of H2 from the initially
formed rhodium hydride precluded an accurate determination for
the reaction enthalpy.
The CH3Rh(F28TPP) complex is robust and can be isolated by
crystallization or benchtop chromatography. It is stable in toluene
heated at reflux, and Bz solutions are unchanged by iodine,
bromine, trifluoroacetic acid (TFA), or triflic acid (1 M). The
13C-1H coupling constant (JC-H ) 144.5 Hz) is substantially
larger than that reported for [13CH3Rh(TMP)]2, 141.5 Hz.11 The
reluctance of this complex to eliminate methane by protonolysis,
and the large JC-H value testify to the metal center’s electrophi-
licity. The Rh-C bond length for CH3Rh(F28TPP) (2.027(4) Å),
determined from an X-ray diffraction study of CH3Rh(F28TPP)
(toluene solvate),21 is indistinguishable from that of CH3Rh(OEP)
(2.031(6) Å)22 and CH3Rh(OETAP) (2.034(7) Å),23 indicating that
Acknowledgment. We thank Professor Victor Day of the University
of Nebraska, for performing the reported crystal structure analysis,
Crystalytics Co. of Lincoln, NE, for use of its equipment and supplies,
and Professor Richard Shoemaker for assistance with NMR spectroscopy.
We thank the National Science Foundation (CHE-9817247) and the Office
of Naval Research (N00014-00-1-0283) for support.
Supporting Information Available: Experimental procedures, 19F
NMR spectra, and characterization data for Rh(F28TPP) derivatives; X-ray
structural information on CH3Rh(F28TPP) (PDF) and a CIF file. This
material is available free of charge via the Internet at http://pubs.acs.org.
(20) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113,
5305-5311.
(21) Single crystals of [CH3]Rh[N4C20F8(C6F5)4]‚2.5CH3C6H5 obtained from
toluene are, at -85 ( 2 °C, triclinic, space group P-C (No. 2) with a )
13.0624(8) Å, b ) 13.7551(8) Å, c ) 16.1232(10) Å, R ) 91.210(1)°, â )
96.405(1)°, γ ) 106.739(1)°, V ) 2752.5(3) Å3 and Z ) 2 formula units
{dcalcd ) 1.767 gcm-3; µa(Mo K) ) 0.454 mm-1}. A full description of the
structure determination is included in the Supporting Information.
(22) Takenaka, A.; Syal, S. K.; Sasada, Y.; Omura, T.; Ogoshi, H.; Yoshida,
Z. Acta Crystallogr. 1976, B32, 62-65.
JA001380R
(23) Ni, Y.; Fitzgerald, J. P.; Carroll, P.; Wayland, B. B. Inorg. Chem.
1994, 33, 2029-2035.
(24) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463.