C O M M U N I C A T I O N S
is not the case, however, possibly because of the steric effect (see
above) that is counter-directing and particularly significant for the
small H ligand. Entropic effects may also differ in this case.16
Acknowledgment. This is DuPont CRD Contribution No. 8901.
J.G. thanks Heriot-Watt University for support.
Supporting Information Available: Experimental and computa-
tional details and X-ray analysis data (CIF) for 1 (two polymorphs), 2,
and [(Ph3P)2Rh(CO)(F)] and complete ref 13. This material is available
Table 2. Selected Computed Natural Atomic Charges (q) for
[(H3P)3Rh(X)] Reactants and Transition Statesa
reactants
transition states
X
q(Rh)
q(CR/X)
q(Pav)
q(Rh)
q(CR/X)
q(Pav)
References
CF3
Me
Ph
H
CN
Cl
-0.52
-0.48
-0.48
-0.63
-0.49
-0.45
+0.79
-0.96
-0.22
-0.09
-0.02
-0.52
+0.23
+0.23
+0.24
+0.22
+0.25
+0.25
-0.26
-0.24
-0.20
-0.45
-0.17
-0.08
+0.93
-0.73
+0.03
+0.14
+0.14
-0.33
+0.08
+0.10
+0.08
+0.10
+0.10
+0.10
(1) Jardine, F. H. Prog. Inorg. Chem. 1981, 28, 63.
(2) (a) Keim, W. J. Organomet. Chem. 1967, 8, P25. (b) Keim, W. J.
Organomet. Chem. 1968, 14, 179.
(3) Dewhirst, K. C.; Keim, W.; Reilly, C. A. Inorg. Chem. 1968, 7, 546.
(4) Strauss, S. H.; Diamond, S. E.; Mares, F.; Shriver, D. F. Inorg. Chem.
1978, 17, 3064.
(5) Price, R. T.; Andersen, R. A.; Muetterties, E. L. J. Organomet. Chem. 1989,
376, 407.
a Other centers display only minor changes in computed charge.10
(6) (a) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2004, 126, 3068.
(b) Macgregor, S. A.; Roe, D. C.; Marshall, W. J.; Bloch, K. M.;
Bakhmutov, V. I.; Grushin, V. V. J. Am. Chem. Soc. 2005, 127, 15304.
(7) Brothers, J. P.; Roper, W. R. Chem. ReV. 1988, 88, 1293.
(8) For recent well-defined examples, see: (a) Huang, D.; Koren, P. R.; Folting,
K.; Davidson, E. R.; Caulton, K. G. J. Am. Chem. Soc. 2000, 122, 8916.
(b) Huang, D.; Caulton, K. G. J. Am. Chem. Soc. 1997, 119, 3185.
(9) The formation of 1 and 2 upon treatment of [(Ph3P)3RhCl] with [M(CF3)2]
(M ) Cd, Hg) has been proposed, but neither complex was detected because
of facile hydrolysis leading to Rh carbonyl species. See: Burrell, A. K.;
Clark, G. R.; Jeffrey, J. G.; Rickard, C. E. F.; Roper, W. R. J. Organomet.
Chem. 1990, 388, 391.
Our computed data also allow us to address the factors underpin-
ning the strong trans influence of the CF3 ligand.19,21 Ligand trans
influence is thought to be predominantly or exclusively controlled
by field effects,22 so a strong trans influence implies strong electron
donation. In complete contrast, the CF3 group is widely recognized
in organic chemistry as a powerful electron acceptor.23
The computed charges in Table 2 confirm that the Rh atom in
2′ does indeed bear a large negative charge (-0.52). This actually
exceeds that in the CH3 analogue (-0.48), in spite of the opposite
strong charges on the carbon atoms of the CF3 (+0.79) and the
CH3 (-0.96) ligands (Figure 3). In fact, the charge distribution in
2′ parallels the long-known24 ꢀ-effect in fluorinated organic
molecules: substitution of F for H on a carbon atom increases the
negative charge on the next C (or H) atom. The explanation of the
ꢀ-effect in terms of π-donation from the F atoms24a,c accounts for
the apparent flow of electrons toward the metal center in 2′ (Figure
3). The charge data therefore shed light on the previously perplexing
strong trans influence of CF3 and other perfluoroalkyl ligands.19,21
Furthermore, an electrostatic attraction resulting from such
(10) See the Supporting Information for details.
(11) NMR data for 2 (C6D6, 20 °C, δ): 19F:-2.4 (dq, JF-P ) 32 Hz, JF-Rh
11.5 Hz). 31P: 29.4 (dq, JP-F ) 32 Hz, JP-Rh ) 158 Hz).
)
(12) Second-order 31P NMR spectra have been previously observed for similar
Rh(I) σ-aryls [(Me3P)3Rh(Ar)] (Ar ) Ph, Tol).5
(13) Frisch, M. J.; et al. Gaussian 03, revision D.01; Gaussian, Inc.: Pittsburgh,
PA, 2001. Calculations used the BP86 functional. Rh, P, and Cl centers
were described with SDD RECPs and basis sets, with extra polarization
functions on P and Cl; 6-31G** basis sets were used for all other atoms.
Quoted energies include a correction for zero-point energies.10
(14) Triplet [(H3P)3Rh(Cl)] (E ) +22.7 kcal mol-1) is more stable than the
singlet TS. This represents the lower limit for the barrier to phosphine
exchange via a spin-crossover mechanism but is still too high to be accessed
at room temperature. Triplet [(H3P)3Rh(CF3)] (E ) +27.4 kcal mol-1) is
much higher in energy than singlet TS(2′-I′) and I′.
(15) Fernandes, M. A.; Circu, V.; Weber, R.; Varnali, T.; Carlton, L. J. Chem.
Crystallogr. 2002, 32, 273.
(16) Relating ∆Hqcalc to fluxionality rates assumes similar entropic changes for
all X. Entropy changes are difficult to address accurately in the present
calculations because of the absence of solvent effects. Generally, ∆Scqalc
was a small positive value, possibly because of the greater space available
to the PPh3 ligands in the TS. One exception, however, was [(Ph3P)3Rh(H)],
where a small negative value of ∆Sqcalc was computed.10
Mδ- δ+CF3 polarization could contribute to the M-C bond
-
shortening in related polyfluoroalkyl complexes19,21 compared to
their nonfluorinated counterparts.25 The positive charge on the
carbon atom, however, is probably stabilized by the same pπ back-
donation mechanism from the fluorines.24,25b,c
(17) Macgregor, S. A.; Wondimagegn, T. Organometallics 2007, 26, 1143.
(18) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. ReV. 1973, 10,
335.
(19) For reviews, see: (a) Hughes, R. P. AdV. Organomet. Chem. 1990, 31, 183.
(b) Morrison, J. A. AdV. Organomet. Chem. 1993, 35, 211.
(20) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in
Chemistry; John Wiley & Sons: New York, 1985.
(21) For discussions of the trans influence and trans effect of CF3 and other
polyfluoroalkyl ligands, see: (a) Bennett, M. A.; Chee, H.-K.; Robertson,
G. B. Inorg. Chem. 1979, 18, 1061. (b) Bennett, M. A.; Chee, H.-K.; Jeffery,
J. C.; Robertson, G. B. Inorg. Chem. 1979, 18, 1071. (c) Hughes, R. P.;
Meyer, M. A.; Tawa, M. D.; Ward, A. J.; Williamson, A.; Rheingold, A. L.;
Zakharov, L. N. Inorg. Chem. 2004, 43, 747. (d) Grushin, V. V.; Marshall,
W. J. J. Am. Chem. Soc. 2006, 128, 4632.
Figure 3. Natural atomic charges computed for [(H3P)3Rh(CH3)] and 2′10
and a resonance structure accounting for the ꢀ-effect.
(22) Landis, C. R.; Firman, T. K.; Root, D. M.; Cleveland, T. J. Am. Chem.
Soc. 1998, 120, 1842.
(23) See, for example: Uneyama, K. Organofluorine Chemistry; Blackwell:
Oxford, U.K., 2006.
In conclusion, the mechanism of the unusual intramolecular ligand
exchange in complexes of the type [(R3P)3Rh(X)] has been elucidated
by experimental and computational methods. The rearrangement occurs
via a distorted trigonal TS with the anionic ligand X in an axial position
trans to a vacant site. Our results explain why certain [(R3P)3Rh(X)]
complexes (X ) Cl,1 Br, I, F,6 CN,15 OR,26 NR227) are stereochemi-
cally rigid in solution, whereas others (X ) Alk,2,5 Ar,2,6b H3,4) are
fluxional under similar conditions. Exchange is governed by a
combination of steric and electronic factors and is facilitated by bulkier
ligands on the Rh as well as by strongly donating anionic ligands X
that can stabilize the TS.28 Our studies of the most fluxional new
complex 2 (X ) CF3) have also clarified the previously puzzling strong
trans influence and trans effect of Rf ligands as well as the nature of
the bonding in perfluoroalkyl complexes of late transition metals.
(24) (a) Pople, J. A.; Gordon, M. J. Am. Chem. Soc. 1967, 89, 4253. (b) Davis,
D. W.; Banna, M. S.; Shirley, D. A. J. Chem. Phys. 1974, 60, 237. (c)
Holmes, S. A.; Thomas, T. D. J. Am. Chem. Soc. 1975, 97, 2337.
(25) (a) Notably, the overall charge on the CF3 ligand (-0.30) is roughly the
same as on the CH3 group (-0.29; Figure 3). Hence, the CF3 ligand should
not be viewed as carbocationic.25b,c (b) Christe, K. O.; Hoge, B.; Boatz,
J. A.; Prakash, G. K. S.; Olah, G. A.; Sheehy, J. A. Inorg. Chem. 1999,
38, 3132. (c) Christe, K. O.; Zhang, X.; Bau, R.; Hegge, J.; Olah, G. A.;
Prakash, G. K. S.; Sheehy, J. A. J. Am. Chem. Soc. 2000, 122, 481.
(26) (a) Kuznetsov, V. F.; Yap, G. P. A.; Bensimon, C.; Alper, H. Inorg. Chim.
Acta 1998, 280, 172. (b) Osakada, K.; Ishii, H. Inorg. Chim. Acta 2004,
357, 3007. (c) Zhao, P.; Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc.
2006, 128, 3124.
(27) Zhao, P.; Krug, C.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 12066.
(28) No evidence has been obtained for substantial contributions of the proposed
H-bonding interactions of the Rh complexes with solvent molecules. See:
Carlton, L. Magn. Reson. Chem. 2004, 42, 760.
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