1544
J. Am. Chem. Soc. 2000, 122, 1544-1545
corresponds to a relatively high surface coverage of ∼1.25 and
Surface Confined Ketyl Radicals via
Samarium(II)-Grafted Mesoporous Silicas
1.40 Sm(II)/nm2, respectively.11 For comparison, the maximum
silanol surface sites available for these materials were determined
as 1.67 (2a) and 1.89 SiOH/nm2 (3a) via tetramethyldisilazane
silylation.12 The relatively low extent of concomitant surface
silylation is in accordance with the IR spectra of the Sm(II) hybrid
materials which indicate a relatively small amount of ≡SiOSi-
HMe2 surface sites at 2145 cm-1. However, the SiH vibration
area is dominated by a broad band at 2030 cm-1 (1920 sh)
assignable to metal-bonded silylamide ligands featuring additional
agostic Sm‚‚‚SiH interactions.7 The nitrogen adsorption/desorption
isotherms of material 4 and 5 clearly indicate the filling of the
mesopores. Interestingly, the host-characteristic type-IV isotherm
is retained as shown for the MCM-48 hybrid material 5 in Figure
1.13 Analysis of the Barret-Joyner-Halenda (BJH) pore size
distribution suggests a regular distribution of the surface species
accounting for pore volumes and mean pore diameters reduced
by ∼75 and 42%, respectively, for material 5.
Iris Nagl, Markus Widenmeyer, Stefan Grasser,
Klaus Ko¨hler, and Reiner Anwander*
Anorganisch-chemisches Institut
Technische UniVersita¨t Mu¨nchen
D-85747 Garching, Lichtenbergstrasse 4, Germany
ReceiVed September 8, 1999
Samarium(II) compounds display unique reductive behavior
in organic transformations as evidenced by numerous individual
and sequential one-electron-transfer processes involving func-
tionalized substrate molecules.1 The rate and selectivity of
reactions induced by the standard reagent SmI2 are markedly
affected by the type of solvents and additives such as strong Lewis
bases, for example, HMPA, or metal salts.2 More recently, it was
shown that the stability of ketyl radicals, which are often key
intermediates in the reduction of carbonyl functionalities, depends
on the ancillary anionic ligand bonded to the samarium(II) center,
for example, amide, alkoxide, or cyclopentadienyl ligands.3
We report the grafting of samarium(II) complexes onto the
internal surface of mesoporous silica materials via surface
organometallic chemistry (SOMC).4 The resulting supramolecular
systems featuring mesopores accessible to an extended intraporous
chemistry form stable ketyl surface radicals by one-electron
reduction. Surface confinement seems to direct the reductive
behavior of the Sm(II) centers yielding selectively the alcoholic
product in the fluorenone/fluorenol transformation (no pinacol-
coupling product could be observed).
A heterogeneously performed silylamide route was applied
for the synthesis of the immobilized Sm(II) species.5,6 This
route provides mild reaction conditions and exploits both special-
ized molecular and support components better to monitor the
surface reaction. Accordingly, black Sm[N(SiHMe2)2]2(THF)x (1)
featuring the SiH moiety as a valuable spectroscopic probe was
used as a molecular precursor.7 Samples of structurally well-
ordered and poreexpanded mesoporous silicas of type MCM-41
(2) and MCM-48 (3) were employed as model support materials.8,9
Treatment of dehydrated samples of materials 2 and 3 with excess
of silylamide 1 in n-hexane gave a black reaction mixture, from
which after several n-hexane washings gray-black materials 4 and
5 were isolated (Scheme 1). Caution! These materials ignite
instantaneously and turn white upon air-exposure, indicating the
presence of Sm(II) surface species.10
The presence of Sm(II) surface species in materials 4 and 5
could be proven by their reactivity toward fluorenone. After addi-
tion of an equimolar amount of fluorenone to black suspensions
of materials 4 and 5 in n-hexane, brown materials 6 and 7 could
be isolated.14 According to GC analyses of the supernatants ∼85%
of the ketone was consumed. Assuming a quantitative im-
mobilization of Sm(II) surface species, not all of these sites seem
to be accessible for the conformationally rigid fluorenone.
Moreover, the high carbon contents and apparent total loss of
pore volume of materials 6 and 7 could not be expected (Table
1), pointing out pore blocking and solvent inclusion. The
unequivocal formation of surface ketyl radicals was revealed by
their X-band EPR spectra.15,16 The room-temperature EPR spectra
of the dry powders (Figure 2,a) consist of five lines at g ) 2.0033.
The spectra are symmetric (no g or hf anisotropy) and can well
be interpreted as due to the hyperfine (hf) interaction of the
unpaired electron with four groups of two equivalent protons (two
of them are not resolved) as expected according to the literature.16
Computer simulation using the hyperfine coupling constants
derived from EPR/ENDOR experiments of a fluorenone radical
anion solution16 reproduce the experimental spectrum very well
(ai (1H): 0.31, 0.2, 0.07, and 0.01 mT, line width ∆Bpp ) 0.18
mT, the last two couplings are not resolved). No qualitative
changes were observed in the spectra for recording temperatures
(8) The mesoporous silicas were synthesized according to slightly modified
literature procedures: (a) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz,
M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard,
E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc.
1992, 114, 10834-10843. (b) Van Der Voort, P.; Morey, M.; Stucky, G. D.;
Mathieu, M.; Vansant, E. F. J. Phys. Chem. 1998, 102, 585-590.
(9) For a recent review article on the modification of mesoporous silica
materials, see: Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950-2963.
(10) Sm(II)-doped silicate glasses prepared according to a sol gel process
and subsequent heating in the presence of hydrogen were reported to be
colorless; see: Nogami, M.; Abe, Y. Appl. Phys. Lett. 1994, 65, 1227-1229.
(11) For comparison, homoleptic Ln(III) silylamides yield a metal surface
coverage of approximate 0.8 Ln/nm2, while the alkyl amide Nd(NiPr2)3(THF)
produces a similar surface coverage.5
The hybrid materials were characterized by FTIR spectroscopy,
elemental analysis, and nitrogen physisorption (Table 1). Ap-
proximately 2.1 and 2.4 mmol of complex 1 could be grafted
onto 1 g of MCM-41 and MCM-48 material, respectively. This
* Corresponding author. Fax: +49 89 289 13473. E-mail: reiner.
(1) For a recent review, see: Molander, G. A.; Harris, C. R. Chem. ReV.
1996, 96, 307-338.
(2) Kagan, H. B.; Namy, J.-L. Top. Organomet. Chem. 1999, 2, 155-198.
(3) (a) Hou, Z.; Fujita, A.; Zhang, Y.; Miyano, T.; Yamazaki, H.;
Wakatsuki, Y. J. Am. Chem. Soc. 1998, 120, 754-766 and references therein.
(b) Takats, J. J. Alloys Compd. 1997, 249, 52-55. (c) Clegg, W.; Eaborn, C.;
Izod, K.; O’Shaughnessy, P.; Smith, J. D. Angew. Chem. 1997, 109, 2925-
2926; Angew. Chem., Int. Ed. Engl. 1997, 36, 2815-2817.
(4) Basset, J.-M.; Gates, B. C.; Candy, J. P.; Choplin, A.; Leconte, M.;
Quignard, F.; Santini, C. C. Surface Organometallic Chemistry: Molecular
Approaches to Surface Catalysis; Kluwer: Dordrecht, 1988.
(5) Anwander, R.; Roesky, R. J. Chem. Soc., Dalton Trans. 1997, 137-
138.
(12) Anwander, R.; Palm, C.; Stelzer, J.; Groeger, O.; Engelhardt, G. Stud.
Surf. Sci. Catal. 1998, 117, 135-142.
(13) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti,
R. A.; Rouque´rol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603-
619.
(14) Similarly O, N-ligated mononuclear Sm(II) complexes reveal the
formation of dinuclear Sm(III) pinacolate complexes after change of solvent
from THF to hexane or after removal of any solvent; in contrast, C5Me5-
supported Sm(III) ketyl complexes stay intact in apolar solvents.3a
(15) Molecular mono ketyl systems were reported to display no EPR signal
due to antiferromagnetic superexchange interaction between the ketyl radical
and the lanthanide(II) spins and/or spin-lattice relaxation of the ketyl radical
via paramagnetic lanthanide(III) ions; in contrast, Sm(III) tris(ketyl) showed
a strong EPR signal with g ) 2.0027.3a
(6) Preliminary studies of the reactivity of SmI2 with siliceous support
materials in THF indicated that in the absence of any base molecules and
upon prolonged contacting complete oxidation of the samarium centers
occurred; Nagl, I.; Anwander, R., unpublished results.
(7) Nagl, I.; Scherer, W.; Tafipolsky, M.; Anwander, R. Eur. J. Inorg.
Chem. 1999, 1405-1407.
(16) (a) Dehl, R.; Fraenkel, G. K. J. Chem. Phys. 1963, 39, 1793-1802.
(b) Evans, J. C.; Rowlands, C. C.; Herold, B. J.; Empis, J. M. A. J. Chem.
Soc., Perkin Trans. 2 1984, 389-394.
10.1021/ja9932535 CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/05/2000