Facile activation of carbon-fluorine bonds in saturated fluoroalkyl ligands by coordinated water in fluoroalkyl aqua complexes of rhodium [6]

Full text of DOI:10.1021/ja9723388

11544
J. Am. Chem. Soc. 1997, 119, 11544-11545
Facile Activation of Carbon-Fluorine Bonds in
Saturated Fluoroalkyl Ligands by Coordinated
Water in Fluoroalkyl Aqua Complexes of Rhodium
Russell P. Hughes* and Danielle C. Lindner
Department of Chemistry, 6128 Burke Laboratory
Dartmouth College, HanoVer, New Hampshire 03755
Arnold L. Rheingold and Louise M. Liable-Sands
Department of Chemistry, UniVersity of Delaware
Newark, Delaware 19716
ReceiVed July 14, 1997
Figure 1. Molecular structure of 3; fluorines are shown as spheres
for clarity. A full ORTEP appears in the Supporting Information.
Selected bond distances (Å) and angles (deg): Rh-O, 2.164(7); Rh-
P, 2.338(2); Rh-C(17), 2.113(9); C(17)-F(7), 1.388(10); C(17)-F(6),
1.412(10); F(7)-C(17)-F(6), 101.8(6); C(17)-Rh-O, 89.5(3); O-Rh-
P, 90.4(2); C(17)-Rh-P, 88.1(2).
Activation of normally inert carbon-fluorine bonds in
saturated fluorocarbons is a topic of considerable recent interest.1
The methodology of choice usually involves strongly reducing
conditions; the fate of the fluorine is usually fluoride ion,
stabilized as an alkali metal salt. An alternative method for
activation of CF2 groups is hydrolysis, with strong HsF and
CdO bonds providing driving force for the reaction. However,
conditions necessary to hydrolyze a CF2 group in saturated
fluorocarbons often require a combination of strong bases or
acids, heat, and long reaction times, depending on the position
of the hydrolyzable group in the molecule.2 Hydrolysis of CF2
or CF3 groups bound to transition metal centers is sometimes
more facile, but initial activation of an R-CsF bond still requires
a strong Lewis acid, such as BF3,3 SbF5,4 or SiMe3+,5 although
there are some indications that a proton can act as the initial
fluoride acceptor.6 We have reported remarkably facile hy-
drolysis of a RhsCF2 group in a coordinatively unsaturated
perfluorometallacyclobutene complex by traces of moisture
present on glassware surfaces; a coordinatively saturated
analogue was inert, leading to the hypothesis that the vacant
coordination site on the metal could bind and activate water in
the hydrolysis mechanism.7 Here, we describe the synthesis
and structures of two cationic complexes containing adjacent
fluoroalkyl and water ligands, both of which undergo hydrolysis
of a CF2 group, the facility of which depends strongly on the
hydrogen-bonding ability of the counterion.
Figure 2. Molecular structure of 4; fluorines are shown as spheres
for clarity. A full ORTEP appears in the Supporting Information.
Selected bond distances (Å) and angles (deg): Rh-O, 2.219(5); Rh-
P, 2.319(2); Rh-C(11), 2.086(7); C(11)-F(1), 1.396(8); C(11)-F(2),
1.389(8); F(1)-C(11)-F(2), 102.1(6); C(11)-Rh-O, 85.2(3); O-Rh-
P, 85.8(2); C(11)-Rh-P, 92.8(2).
Addition of perfluorobenzyl complex 18 or perfluoropropyl
water molecule with the tetrafluoroborate counterion, with
OsF(9A) and OsF(11) distances of 2.640 and 2.855 Å in 3,
and 2.723 and 2.636 Å in 4; such interactions are well
precedented.11 Close approaches of OsF(1) [2.749 Å] in 3 and
OsF(2) [2.767 Å] in 4 are also well within potential hydrogen-
bonding distance.
-
analogue 29 to AgBF4 in moist CH2Cl2 affords BF4 salts of
the cationic aqua complexes 3 and 4,9 the structures of which
were confirmed by X-ray crystallographic studies.10 ORTEP
diagrams for the two ion pairs, with selected bond distances
and angles, are presented in Figures 1 and 2. Each molecule
exhibits close hydrogen-bonding interactions of coordinated
Solution spectra of 3 are consistent with the solid state
structure. Asymmetric and symmetric stretches of coordinated
(1) For leading recent references, see: Kiplinger, J. L.; Richmond, T.
G. J. Am. Chem. Soc. 1996, 118, 1805. Burdeniuc, J.; Chupka, W.; Crabtree,
R. H. J. Am. Chem. Soc. 1996, 118, 2525. Burdeniuc, J.; Jedlicka, B.;
Crabtree, R. H. Chem. Ber. (Recueil) 1997, 130, 145. Saunders, G. C.
Angew. Chem., Int. Ed. Engl. 1996, 35, 2615.
(10) Crystal data for 3: C20H26BF11OPRh, red block, triclinic, P1h, a )
8.682(3), b ) 11.033(2), and c ) 14.376(3) Å, R ) 74.50(1), â ) 76.11-
(3), and γ ) 73.02(3)°, V ) 1249.4(7) Å3, Z ) 2, Dx ) 1.691 g cm-3, T
) 298 K, R(F) ) 6.74%, R(wF2) ) 19.49%. Crystal data for 4: C16H26-
BF11OPRh, orange block, monoclinic, P21/n, a ) 10.115(1), b ) 16.270-
(2), and c ) 14.070(2) Å, â ) 92.74(1)°, V ) 2312.7(7) Å3, Z ) 4, Dx )
1.689 g cm-3, T ) 298 K, R(F) ) 4.81%, R(wF2) ) 8.41%. Details of the
crystallographic determinations are provided in the Supporting Information.
(11) For a compilation of some crystallographically characterized orga-
nometallic complexes containing water ligands and a discussion of hydrogen-
bonding to counterions, see: Kubas, G. J.; Burns, C. J.; Khalsa, G. R. K.;
Van Der Sluys, L. S.; Kess, G.; Hoff, C. D. Organometallics 1992, 11,
3390. For other more recent references, see: Veltheer, J. E.; Burger, P.;
Bergman, R. G. J. Am. Chem. Soc. 1995, 117, 12478. Canty, A. J.; Jin, H.;
Skelton, B. W.; White, A. H. J. Organomet. Chem. 1995, 503, C16. Bodige,
S.; Porter, L. C. J. Organomet. Chem. 1995, 487, 1. Dadci, L.; Elias, H.;
Frey, U.; Ho¨rnig, A.; Koelle, U.; Merbach, A. E.; Paulus, H.; Schneider, J.
S. Inorg. Chem. 1995, 34, 306. Carmona, D.; Cativiela, C.; Garc´ıa-Correas,
R.; Lahoz, F. J.; Lamata, M. P.; Lo´pez, J. A.; Lo´pez-Ram de V´ıu, M. P.;
Oro, L. A.; San Jose´, E.; Viguri, F. J. Chem. Soc., Chem. Commun. 1996,
1247. Fries, A.; Green, M.; Mahon, M. F.; McGrath, T. D.; Nation, C. B.
M.; Walker, A. P.; Woolhouse, C. M. J. Chem. Soc., Dalton Trans. 1996,
4517.
(2) See: Chemistry of Organic Fluorine Compounds II; Hudlicky, M.,
Pavlath, A. E., Eds.; ACS Monograph 187; American Chemical Society:
Washington, DC, 1995; Chapter 4. Chemistry of Organic Fluorine
Compounds, 2nd ed.; Hudlicky, M., Ed.; John Wiley & Sons: New York,
NY, 1976; Chapter 5. Chemistry of Organic Fluorine Compounds; Hudlicky,
M., Ed.; Macmillan; New York, 1962.
(3) Richmond, T. G.; Crespi, A. M.; Shriver, D. F. Organometallics 1984,
3, 314.
(4) Reger, D. L.; Dukes, M. D. J. Organomet. Chem. 1978, 153, 67.
(5) Koola, J. D.; Roddick, D. M. Organometallics 1981, 10, 591.
(6) Clark, B. R.; Hoskins, S. V.; Roper, W. R. J. Organomet. Chem.
1982, 234, C9. Appleton, T. G.; Berry, R. D.; Hall, J. R.; Neale, D. W. J.
Organomet. Chem. 1989, 364, 249. Burrell, A. K.; Clark, G. R.; Rickard,
C. E. F.; Roper, W. R. J. Organomet. Chem. 1994, 482, 261.
(7) Hughes, R. P.; Rose, P. R.; Rheingold, A. L. Organometallics 1993,
12, 3109.
(8) Hughes, R. P.; Lindner, D. C.; Rheingold, A. L.; Yap, G. P. A.
Organometallics 1996, 15, 5678.
(9) Experimental details and spectroscopic data for this compound are
provided in the Supporting Information.
S0002-7863(97)02338-X CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 47, 1997 11545
Scheme 1
salt of cation 3 from 6 using this method in the presence of
water afforded very rapid hydrolysis to give the B{ArF}4- salt
79 with no signs of any intermediate water complex analogous
to 3.
The inability to isolate the perfluorobenzyl aqua cation using
-
the B{ArF}4 counterion suggests that an anion incapable of
either displacing water, or of hydrogen-bonding to it, might
strongly enhance the ability of coordinated water to hydrolyze
the adjacent CF2 group. Loss of hydrogen bonding to the
counterion will allow the proton of coordinated water a stronger
interaction with the adjacent CF2 group. In agreement with this
hypothesis, the perfluoropropyl complex 2 reacts with silver
triflate to afford 8;9 treatment of 8 with NaB{ArF}4 in the
presence of water results in rapid hydrolysis to give the
perfluoroethyl carbonyl complex 9.9 The BF4- analogue 10 was
prepared unambiguously by the reaction of [Rh(η5-C5Me5)-
(C2F5)I(PMe3)] with AgBF4 in the presence of CO.
water are observed in the IR spectrum at 3630 and 3370 cm-1 12
19F NMR resonances for the diastereotopic fluorines of the CF2
group are broad at room temperature, but sharpen when cooled
to -50 °C to give a typical AX pattern. This temperature
dependence may reflect dissociation of water13 with inversion
at rhodium, thereby equilibrating the CF2 environments; 103Rh
coupling to 31P is retained indicating that phosphine dissociation
is not responsible for this phenomenon. In contrast, perfluo-
ropropyl analogue 49 shows a sharp AX pattern for the R-CF2
resonances at room temperature indicating that the molecule is
not fluxional on the NMR time scale.
.
While perfluoropropyl complex 4 appears to be indefinitely
stable on standing in solution at room temperature, perfluo-
robenzyl analogue 3 is cleanly transformed on standing over-
night in CDCl3 solution into pentafluorophenyl carbonyl
complex 5;9 the CO stretch is observed in the IR spectrum at
2074 cm-1, and the pentafluorophenyl ligand exhibits five
resonances in the 19F NMR spectrum due to restricted rotation,
as previously observed for other pentafluorophenyl rhodium
complexes.14 HF can be observed in the volatiles, after vacuum
transfer, as previously noted.8 A suggested mechanism is shown
in Scheme 1; hydrolysis of the CF2 group affords a coordina-
tively unsaturated pentafluorobenzoyl complex, which undergoes
pentafluorophenyl migration into the vacant coordination site
to afford 5. The initial step is suggested to involve a proton
from coordinated water acting as a fluoride acceptor with loss
of fluoride enhanced by resonance stabilization from Rh;
presumably the difference in reactivity of the CF2 groups toward
hydrolysis in perfluorobenzyl complex 3 compared to per-
fluoropropyl analogue 4 rests in additional enhancement of the
leaving group ability of the benzylic fluoride.
This transformation does not take place in acetone-d6, which
displaces water from the coordination sphere of 3,13 suggesting
that it is a reaction of coordinated water with the adjacent CF2
group which is important; addition of more water to an acetone
solution failed to accomplish hydrolysis. In addition, reaction
of 1 with silver triflate affords the triflate complex 6;9 the sharp
AX pattern observed for the CF2 group at room temperature
indicates the triflate ligand is not labile. The triflate ligand is
not displaced by water alone, and no hydrolysis of the CF2 group
is observed. Bergman has shown that addition of the sodium
salt of weakly coordinating anion tetrakis{3,5-bis(trifluoromethyl)-
phenyl}borate (B{ArF}4) will displace triflate from iridium as
In summary, we provide evidence that the hydrolysis reaction
of CF2 groups in perfluoroalkyl aqua complexes is a reaction
of coordinated water, which can be extremely facile if the
counterion is incapable of hydrogen bonding. This represents
an unusually mild activation of saturated C-F bonds and
suggests that other reactions of water in the coordination sphere
of transition metals, particularly those in which it acts as a protic
acid, might also benefit from the presence of a non-hydrogen-
bonding counterion.
Acknowledgment. R.P.H. acknowledges support of this research
by the National Science Foundation and by the Donors of the Petroleum
Research Fund, administered by the American Chemical Society.
-
sodium triflate;15 however, attempts to prepare the B{ArF}4
(12) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th ed.; Wiley: New York, NY, 1986.
(13) The water ligands in [Rh(C5Me5)(OH2)32+][OTf-]2 are labile to
exchange and substitution: Eisen, M. S.; Haskel, A.; Chen, H.; Olmstead,
M. M.; Smith, D. P.; Maestre, M. F.; Fish, R. H. Organometallics 1995,
14, 2806.
(14) Jones, W. D.; Partridge, M. G.; Perutz, R. N. J. Chem. Soc., Chem.
Commun. 1991, 264. Belt, S. T.; Helliwell, M.; Jones, W. D.; Partridge,
M. G.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 1429.
(15) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970.
Supporting Information Available: Details of syntheses and
characterizations of complexes 2-9, and the X-ray structure determina-
tions, tables of atomic coordinates and anisotropic thermal parameters,
bond lengths and bond angles, anisotropic displacement parameters,
and hydrogen atom coordinates for 3 and 4 (24 pages). See any current
masthead page for ordering and Internet access instructions.
JA9723388