A Re(CO)5 fragment bonded to a polyhedral oligomeric hydroxysilsesquioxane as a model which gives further evidence to the existence of an unexpected Re(CO)5 surface species

Article abstract of DOI:10.1021/ja0630588

Reaction of Re(CO)₅O₃SCF₃ with (c-C₆H₁₁)₇Si₈O₁₂O^-Li⁺ at 273 K under a CO atmosphere affords the [Re(CO)₅OR] (R = (c-C₆H₁₁)₇Si₈O₁₂) derivative (1). 1 is the first example of a rhenium pentacarbonyl bearing an OR ligand (R = alkyl, aryl, or silyl) stable enough to be characterized, and it represents also the first molecular model of the surface [Re(CO)₅OSi≡] species formed by reductive carbonylation of silica-supported [Re(CO)₃OH]₄. At room temperature, 1 loses one carbonyl ligand and dimerizes to afford {Re(CO)₄[(μ-O)O₁₂Si₈(c-C₆H₁₁)₇]}₂ (2), which has been characterized by X-ray diffraction and is the first reported example of a rhenium tetracarbonyl μ-oxo-bridged dimer of the type [Re(CO)₄(μ-OR)]₂. Copyright

Full text of DOI:10.1021/ja0630588

Published on Web 08/29/2006
A Re(CO)
5
Fragment Bonded to a Polyhedral Oligomeric
Hydroxysilsesquioxane as a Model which Gives Further Evidence to the
Existence of an Unexpected Re(CO)
5
Surface Species
Elena Lucenti,*,†,§ Giuseppe D’Alfonso,‡,§ Piero Macchi, Mario Maranesi, Dominique Roberto,
§,¶
§
‡,§
§,¶
†,‡,§
Angelo Sironi, and Renato Ugo
Institute of Molecular Science and Technology of CNR, Via Golgi 19, 20133 Milan, Italy, Department of Inorganic,
Metallorganic, and Analytical Chemistry, UniVersity of Milan, Via Venezian 21, 20133 Milan, Italy, CIMAINA
Center, Via Celoria 16, 20133 Milan, Italy, and Department of Structural Chemistry and Inorganic Stereochemistry,
UniVersity of Milan, Via Venezian 21, 20133 Milan, Italy
Received May 10, 2006; E-mail: e.lucenti@istm.cnr.it
Most of the significance of surface organometallic chemistry
relies on reaction paths rather selective or otherwise forbidden and
on the stabilization of species too labile to survive in solution.1
The origin of such stabilization is one of the key aspects still not
completely understood.
6 11 7 8
hydroxysilsesquioxane (c-C H ) Si O12OH (Figure 1A), which has
been recently developed as an effective homogeneous model for
13
isolated silanols in partially dehydroxylated silica.
Deprotonation of the hydroxy group of (c-C
6
H
11
)
7
Si
8
-
O
12OH by
+ 14
a molar amount of n-BuLi affords [(c-C Si O12O Li ] that
6
H
11
)
7
8
2
15
Very recently, a notable example was provided by the unex-
reacts with [Re(CO)
atmosphere at 273 K to give {Re(CO)
2
Scheme S1 in Supporting Information), characterized by νCO (CH -
5
(O
3
SCF
3
)] in CH
2
Cl
2
solution under CO
pected detection of a surface-anchored pentacarbonyl rhenium(I)
5
[OO12Si
8
(c-C ]} (1,
6 11 7
H )
3
species, during the reductive carbonylation of silica-supported [Re-
-
1
13
(CO)
3
OH]
4
, which finally afforded [Re
2
(CO)10] under mild condi-
Cl
2
) at 2038(vs) and 1970(m) cm and C NMR signals (CD
2
-
tions. This reductive carbonylation does not occur in solution,
suggesting that the silica surface plays a unique role “via” the
Cl
2
) at δ 180.3 (4CO) and 179.3 (1CO) ppm, typical of rhenium
16
pentacarbonyl complexes. The νCO frequencies are slightly shifted
to lower frequency with respect to the [Re(CO) OSit] surface
species (νCO in Nujol 2049(vs) and 1991(m) cm ), probably due
to both the higher electrophilicity and the topological effects of
the silica surface.17 Both “surface” and “solution” species show
formation of the [Re(CO)
5
OSit] intermediate. In solution, pen-
5
-1
-
tacarbonyl rhenium(I) species bearing an OR ligand have so far
eluded isolation,4 due to the strong cis-labilizing power of OR
-6
-
groups and to their great tendency to assume a bridging coordina-
tion, resulting in the fast formation of dimeric complexes containing
the same reactivity toward HCl, affording easily [Re(CO)
expected for a “Re(CO) ” moiety covalently bound to oxygen donor
groups of rather low nucleophilicity.
Complex 1 is the first [Re(CO) OR] species (R ) alkyl, aryl, or
5
Cl], as
units joined by three bridging RO groups.7
-
Re(CO)
3
5
This different behavior might be imputable either to the silica
surface acting as reaction medium, which may modify (or hinder)
5
the process of condensation reactions with respect to what occurs
silyl) stable enough to be characterized, although it is not as stable
-
-
in solution (although [Re
2
(CO)
6
(OH)
n
(OSit)3-n] (n ) 0, 1, or 2)
5
as the anions [W(CO) OR] (R ) Ph, m-CH
3
C
6
H
4
), where the
8
7
species were observed on the silica surface), or to the different
electronic properties of silanolates with respect to alkoxide and
aryloxide ligands, for instance, a lower nucleophilicity of the
silanolate oxygen nonbonding electrons. To confirm this latter
hypothesis, we attempted the synthesis of molecular models bearing
a silanolate ligand.
presence of a negative charge decreases the cis lability. Actually,
complex 1, which can be stabilized only under CO atmosphere, is
unstable above 273 K. At room temperature, IR monitoring (Figure
S1 in Supporting Information) shows that 1 slowly loses one
carbonyl ligand and dimerizes, affording cleanly the novel bridged
dimeric species 2 (Reaction 1 below and Scheme S1).
5
Reaction of [Re(CO) Cl] with trimethylsilanolate reproduced the
known reactivity typical of alkoxides, leading instantaneously to
2
{Re(CO) [OO Si (c-C H ) ]} f
5 12 8 6 11 7
-
anionic dimers containing bridging Me
3
SiO ligands, such as [Re
2
-
{Re(CO) [(µ-O)O Si (c-C H ) ]} + 2 CO (1)
4 12 8 6 11 7 2
-
9
(
6
CO) (µ-OH)
n
(µ-OR)3-n
]
3
(R ) Me Si), even under CO atmo-
10
sphere and at low temperature. However, trimethylsilanol could
not be a good model of surface silanols which are much more
electrophilic than alkyl silanols so that further nucleophilic attacks
To our knowledge, 2 is the first example of a µ-oxo-bridged
1
8
dimer of the type [Re
(µ-SR)
species of the type [Re
proposed (but never detected) as possible intermediates in the
2
(CO)
8
(µ-OR)
or C
(µ-OR) ] (R ) Ph, H ) have been
2
], although related [Re
2
(CO)
8
-
1
9
20
2
] complexes (R ) C
(CO)
6
F
5
6
H
5
,
CF ) are known and
3
2
1
22
could be less facile. By reacting the less nucleophilic aryl silanolate
2
8
2
-
(C
6 5
H )
3
SiO and by changing, at the same time, the chloride with
-
the better leaving group triflate (no reaction occurred with [Re-
CO) Cl]), we in fact observed at 253 K the formation of an elusive
species with spectroscopic features typical for a rhenium pentac-
syntheses of [Re
3
(µ
3
-OMe)(µ-OR)
3
(CO)
9
] . The structure of 2 has
(
5
been unambiguously established by X-ray diffraction analysis of
crystals grown from CH Cl or n-pentane solutions at 253 K (Figure
1B). In both cases, the solvents were co-crystallized, producing
two pseudo-polymorphs (though, with the same space group P2
and two similar cell axes). The structure of 2 is built around the
Re (CO) “core” bridged by the two OR groups, and it is
comparable to that of other double bridged molecules, such as [Re
2
2
11
arbonyl complex. Therefore, we investigated the reaction with
the anion of a more electron-withdrawing silanol, with an electro-
1
12
philicity closer to that of the silica silanol sites, such as the
2
8
†
Institute of Molecular Science and Technology of CNR.
2
-
‡
Department of Inorganic, Metallorganic, and Analytical Chemistry, University
23
of Milan.
(CO)
8 2
(µ-Cl) ]. No formal Re-Re bond is expected from electron
§
CIMAINA Center, University of Milan.
Department of Structural Chemistry and Inorganic Stereochemistry,
¶
counting, and actually, the Re‚‚‚Re distance (3.45 Å) is longer than
the reference unsupported Re-Re bond in Re (CO)10 (3.04 Å).
2
2
4
University of Milan.
12054
9
J. AM. CHEM. SOC. 2006, 128, 12054-12055
10.1021/ja0630588 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
References
(
1) (a) Psaro, R.; Ugo, R. Metal Clusters in Catalysis; Gates, B. C., Guczi,
L., Kn o¨ zinger, H., Eds.; Elsevier: Amsterdam, 1986; p 427. (b) Basset,
J. M., Gates, B. C., Candy, J. P., Choplin, A., Leconte, M., Quignard, F.,
Santini, C., Eds. Surface Organometallic Chemistry: Molecular Ap-
proaches to Surface Catalysis; Kluwer: Dordrecht, The Netherlands, 1988.
(
c) Cariati, E.; Roberto, D.; Ugo, R.; Lucenti, E. Chem. ReV. 2003, 103,
3707. (d) Coperet, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J. M.
Angew. Chem., Int. Ed. 2003, 42, 156. (e) Severn, J. R.; Chadwick, J. C.;
Duchateau, R.; Friederichs, N. Chem. ReV. 2005, 105, 4073.
2) D’Alfonso, G.; Roberto, D.; Ugo, R.; Bianchi, C. L.; Sironi, A.
Organometallics 2000, 19, 2564.
(
(
3) The formulation of the surface species as a covalently bonded pentacar-
bonyl derivative was based on the infrared data, on its lack of extraction
4
with donor solvents, together with its reactivity with HCl and HReO to
2
5 5 3
afford [Re(CO) Cl] and [Re(CO) OReO ], respectively.
(
4) Ioganson, A. A.; Lokshin, B. V.; Kolobova, E. E.; Anisimov, K. N. J.
Gen. Chem. 1974, 20, 20 (translated from Zh. Obshch. Khim. 1974, 44,
2
3).
5) Simpson, R. D.; Bergman, R. G. Organometallics 1993, 12, 781.
6) Complex [(CO) ReOCH CH NMe ] has been suggested as a probable
intermediate species in the reaction of [Re(CO) Br] with Me NCH CH
ONa to generate [Re (µ -OH)(µ-OH) {(µ-O)CH CH NMe H}(CO) ] (Wang
Wenwu Ph.D. Thesis, National University of Singapore, 2002).
7) An analogous behavior was observed for the related zero-valent
(
(
5
2
2
2
2
-
Figure 1. (A) (c-C6H11)7Si8O12OH; Cy ) c-C6H11. (B) Molecular structure
of {Re(CO)4[(µ-O)O12Si8(c-C6H11)7]}2 (2).
5
2
2
3
3
2
2
2
2
9
(
-
[
5
M(CO) OR] anionic complexes (M ) Cr, Mo, W), but in that case (at
This distance is shorter than with four-electron donor bridges of
the third period or below (3.8-4.0 Å) but is in agreement with the
least for M ) W and R ) Ph), the presence of a negative charge made
it possible to avoid easy CO elimination by nucleophilic attack, for a time
4 5
long enough to grow crystals of [NEt ][W(CO) OPh] (Darensbourg, D.
2 n 2
other Re (CO)8-nL (OR) (n ) 1, 2) species known (L ) any non-
J.; Sanchez, K. M.; Reibenspies, J. H.; Rheingold, A. L. J. Am. Chem.
Soc. 1989, 111, 7094).
25
carbonyl ligand), meaning that the oxo bridge requires a much
smaller Re-O-Re angle.26 The observed stereochemistry around
the oxygen bridge is close to a (distorted) trigonal coordination
rather than pyramidal as observed for S-bridged dimers20 (<Re-
O-Si> ) 126°; <Re-O-Re> ) 106°; O, Si, and the two Re
(8) Roberto, D.; D’Alfonso, G.; Ugo, R.; Vailati, M. Organometallics 2001,
20, 4307.
(
9) D’Alfonso, G.; Dragonetti, C.; Galli, S.; Lucenti, E.; Macchi, P.; Roberto,
D.; Ugo, R. Can. J. Chem. 2005, 83, 1017.
(
10) Neutral mononuclear ReL
two phosphines were substituted for two carbonyls, to avoid dimerization,
as previously shown for the analogous alkoxide complexes.
(11) See details in Supporting Information.
5
(OR) complexes could be obtained only when
9
atoms lie almost on the same plane). Each O12Si
8
(c-C
H
6 11
)
7
group
5
can be regarded as a slightly distorted cube, whose vertexes are
occupied by Si atoms (Si‚‚‚Si in the range 3.03-3.17 Å), attached
(
12) (a) Feher, F. J.; Newman, D. A.; Walzer, J. F. J. Am. Chem. Soc. 1989,
1
11, 1741. (b) Feher, F. J.; Rahimian, K.; Budzichowski, T. A.; Ziller, J.
to the µ
2
bridging oxygen along one of its 3-fold axes. The two
W. Organometallics 1995, 14, 3920.
(
13) Lorenz, V.; Fischer, A.; Edelmann, F. T. J. Organomet. Chem. 2002, 647,
“
cubanes” approximately share the same orientation, and they
245.
27
assume an eclipsed mutual conformation.
(
14) Duchateau, R.; Abbenhuis, H. C. L.; Van Santen, R. A.; Thiele, S. K. H.;
Van Tol, M. F. H. Organometallics 1998, 17, 5222.
Contrary to its pentacarbonyl precursor 1, complex 2 is stable
at room temperature and does not lose CO ligands even after
(
(
15) Schmidt, S. P.; Nitschke, J.; Trogler, W. C. Inorg. Synth. 1989, 26, 113.
16) Kaesz, H. D.; Bau, R.; Hendrickson, D.; Smith, J. M. J. Am. Chem. Soc.
1967, 89, 2844.
vacuum treatment. It reacts with HCl to give the expected [Re
] derivative.28 Interestingly, reaction 1 is not reversible.
CO) (µ-Cl)
No evidence for the formation of the pentacarbonyl derivative 1 or
of [Re (CO)10] was obtained by treating a CH Cl solution of 2
2
-
(17) An analogous trend in shifting ν
surface” species [Os (CO)10(µ-OSit)(µ-H)] (Roberto, D.; Pizzotti, M.;
Ugo, R. Gazz. Chim. Ital. 1995, 125, 133) and its “solution” molecular
model {Os (CO)10[(µ-O)Si 10(c-C ](µ-H)} (Liu, J.; Scott, W. R.;
Shapley, J. R.; Feher, F. J. Inorg. Chem. 1990, 29, 5138).
18) In the literature, a species of the type [Re (CO) (OSit) ] is suggested as
(CO)10] on the
CO frequencies was observed for the
(
8
2
“
3
3
7
O
6 11 7
H )
2
2
2
under CO (up to 100 atm, either at 273 or 298 K) for 22 h.
In conclusion, this work has shown that hydroxysilsesquioxane,
(
2
8
2
one of the products formed during the photolysis of [Re
2
silica surface, but it has never been clearly characterized (McKenna, W.
(
c-C
6
H
11
)
7
Si
8
O12OH, owing to the very low nucleophilicity of its
P.; Higgins, B. E.; Eyring, E. M. J. Mol. Catal. 1985, 31, 199).
anion, can stabilize for the first time a [Re(CO)
5
OR] species,
(19) Osborne, A. G.; Stone, F. G. A. J. Chem. Soc. (A) 1966, 1143.
(
(
20) King, R. B.; Welcman, N. Inorg. Chem. 1969, 8, 2540.
21) Jiang, C.; Wen, Y.-S.; Liu, L.-K.; Hor, T. S. A.; Yan, Y. K. J. Organomet.
Chem. 1997, 543, 179.
previously detected on the silica surface only but never isolated in
solution. On the other hand, the chemical and thermal stability of
the molecular model is lower than that of the corresponding surface
pentacarbonyl species, thus confirming the unique role of the silica
surface as an unusual reaction medium.
(22) Wang, W.; Yan, Y. K.; Hor, T. S. A.; Vittal, J. J.; Wheaton, J. R.; Hall,
I. H. Polyhedron 2002, 21, 1991.
(
(
(
23) Vega, A.; Calvo, V.; Manzur, J.; Spodine, E.; Saillard, J.-Y. Inorg. Chem.
2002, 41, 5382.
24) Churchill, M. R.; Amoh, K. N.; Wasserman, H. J. Inorg. Chem. 1981,
20, 1609.
Acknowledgment. This work was supported by the Ministero
dell’Istruzione, dell’Universit a` e della Ricerca (PRIN 2003, Re-
search Title: Propriet a` di singole molecole ed architetture mole-
colari funzionali supportate: caratterizzazione chimico-fisica, svi-
luppo di sintesi chimiche e di sistemi per l’indagine), and by the
Consiglio Nazionale delle Ricerche (CNR, Roma). The authors
25) Cambridge Crystallographic Data Centre (http://www.ccdc.cam.ac.uk/)
2005.
(26) If a two-electron donor, such as a hydride, a metal, a carbonyl, or a
carbonyl isolobal ligand, is supporting the Re-Re bond (thus inducing
delocalization through the bridge), much shorter distances are found (2.9-
25
3
.3 Å), often in agreement with the formal electron counting schemes
that predict a direct Re-Re bond.
27) The planarity of the Re (OSi) core and the steric crowding of the axial
carbonyls of 2 suggest that dimerization to a structure such as the
(
2
2
6 11 7 7 9 3
thank professor Frank J. Feher for a gift of (c-C H ) Si O (OH) .
1
8
previously proposed [Re
silica surface, even in the presence of vicinal silanol sites.
(28) (a) Abel, E. W.; Hargreaves, G. B.; Wilkinson, G. J. Chem. Soc. 1958,
149. (b) Dolcetti, G.; Norton, J. R. Inorg. Synth. 1976, 16, 35.
2
(CO)
8
(OSit)
2
]
should not be favored on the
Supporting Information Available: Figure S1, Scheme S1, full
synthetic details, spectroscopic data, and crystallographic data in CIF
format. This material is available free of charge via the Internet at http://
pubs.acs.org.
3
JA0630588
J. AM. CHEM. SOC.
9
VOL. 128, NO. 37, 2006 12055