Surface organometallic chemistry - Carbonyl complexes of Re(I) with silanolates as models of silica anchored rhenium carbonyl species

Article abstract of DOI:10.1139/v05-104

Reaction of Me₃SiONa with [Re(CO)₅Cl] affords the new complex Na[Re₂(CO)₆(μ-OSiMe₃)₃], which could be generated via formation of [Re(CO)₅OSiMe₃] followed by immediate reaction with Me₃SiO^-. Substitution of some CO ligands by phosphines significantly decreases the electrophilicity of the Re(I) center, and therefore hinders further attack by Me ₃SiO^-. Thus, fac-[Re(CO)₃(Ph ₂PCH₂CH₂PPh₂)OTf] (OTf is the triflate anion) reacts with Me₃SiONa to give fac-[Re(CO) ₃(Ph₂PCH₂CH₂PPh₂) OSiMe₃], a molecular model of silica anchored [Re(CO) ₅OSi≡]. However, substitution of only one CO by triphenylphosphine is not enough to avoid the formation of [Re ₂(CO)₆(μ-OSiMe₃)₃]^-. While fac-[Re(CO)₃(Ph₂PCH₂CH ₂PPh₂)OSiMe₃] is stable towards hydrolysis, [Re₂(CO)₆(μ-OSiMe₃)₃] ^- is readily hydrolyzed to [Re₂(CO)₆(μ-OH) (μ-OSiMe₃)₂]^-, a molecular model of silica anchored [Re₂(CO)₆(μ-OH)(μOSi≡) ₂]^-, whose structure has been determined by single crystal X-ray diffraction.

Full text of DOI:10.1139/v05-104

1017
Surface organometallic chemistry — Carbonyl
complexes of Re(I) with silanolates as models of
silica anchored rhenium carbonyl species1
Giuseppe D’Alfonso, Claudia Dragonetti, Simona Galli, Elena Lucenti,
Piero Macchi, Dominique Roberto, and Renato Ugo
Abstract: Reaction of Me3SiONa with [Re(CO)5Cl] affords the new complex Na[Re2(CO)6(µ-OSiMe3)3], which could
be generated via formation of [Re(CO)5OSiMe3] followed by immediate reaction with Me3SiO–. Substitution of some
CO ligands by phosphines significantly decreases the electrophilicity of the Re(I) center, and therefore hinders further
attack by Me3SiO–. Thus, fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf] (OTf is the triflate anion) reacts with Me3SiONa to
give fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3], a molecular model of silica anchored [Re(CO)5OSiϵ]. However, substi-
tution of only one CO by triphenylphosphine is not enough to avoid the formation of [Re2(CO)6(µ-OSiMe3)3]–. While
fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3] is stable towards hydrolysis, [Re2(CO)6(µ-OSiMe3)3]– is readily hydrolyzed to
[Re2(CO)6(µ-OH)(µ-OSiMe3)2]–, a molecular model of silica anchored [Re2(CO)6(µ-OH)(µ-OSiϵ)2]–, whose structure
has been determined by single crystal X-ray diffraction.
Key words: surface organometallic chemistry, rhenium, silica, silanolate, molecular model.
Résumé : La réaction de Me3SiONa avec [Re(CO)5Cl] fournit le nouveau complexe Na[Re2(CO)6(µ-OSiMe3)3], proba-
blement généré via la formation de [Re(CO)5OSiMe3] suivie par une réaction immédiate avec Me3SiO–. La substitution
de quelques CO par des phosphines diminue fortement l’électrophilicité du centre Re(I) et, par conséquent, empêche
toute attaque ultérieure par Me3SiO–. Ainsi, fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf] (OTf est l’anion triflate) réagit avec
Me3SiONa pour donner fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3], modèle moléculaire de l’espèce ancrée à la silice
[Re(CO)5OSiϵ]. Cependant la substitution d’un seul CO par la triphenylphosphine ne suffit pas à empêcher la forma-
tion de [Re2(CO)6(µ-OSiMe3)3]–. Alors que fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3] est stable en présence d’eau,
[Re2(CO)6(µ-OSiMe3)3]– s’hydrolyse rapidement pour donner [Re2(CO)6(µ-OH)(µ-OSiMe3)2]–, modèle moléculaire de
l’espèce ancrée à la silice [Re2(CO)6(µ-OH)(µ-OSiϵ)2]–, dont la structure a été déterminée par diffraction des rayons X.
Mots clés : chimie organométallique de surface, rhénium, silice, silanolate, modèle moléculaire. D’Alfonso et al.
1024
Introduction
established. A recently developed way for a better under-
standing of the structure and reactivity of molecular metal
carbonyl fragments bound to the silica surface is based on
the structural and chemical characterization of metal car-
bonyl compounds bearing various silanolate ligands that
mimic the functional groups of the silica surface. These
models are a useful tool not only to understand the structural
aspects of surface organometallic species, but also to clarify
Transition-metal carbonyls supported on silica are hybrid
materials of interest as precursors of highly dispersed metals
that may show improved or unusual catalytic properties (1).
Although largely investigated in the last two decades (2), the
surface organometallic chemistry between molecular metal
carbonyl fragments and the surface sites is not always fully
Received 25 November 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 7 September 2005.
In honour of Professor Howard Alper.
G. D’Alfonso, C. Dragonetti, and D. Roberto.2 Centro di Eccellenza CIMAINA and Unità di Ricerca di Milano dell’INSTM and
Dipartimento di Chimica Inorganica, Metallorganica ed Analitica dell’Università di Milano, Via Venezian, 21, 20133 Milan, Italy.
S. Galli. Dipartimento di Scienze Chimiche ed Ambientali, Università dell’Insubria, Via Valleggio, 11, 22100 Como, Italy.
E. Lucenti. Centro di Eccellenza CIMAINA and Unità di Ricerca di Milano dell’INSTM dell’Università di Milano, Via Venezian,
21, 20133 Milan, Italy and Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM), via Golgi 19, 20133 Milan, Italy.
P. Macchi. Centro di Eccellenza CIMAINA and Unità di Ricerca di Milano dell’INSTM and Dipartimento di Chimica Strutturale e
Stereochimica Inorganica dell’Università di Milano, via Venezian 21, 20133 Milan, Italy.
R. Ugo. Centro di Eccellenza CIMAINA and Unità di Ricerca di Milano dell’INSTM and Dipartimento di Chimica Inorganica,
Metallorganica ed Analitica dell’Università di Milano, Via Venezian, 21, 20133 Milan, Italy and Istituto di Scienze e Tecnologie
Molecolari del CNR (ISTM), via Golgi 19, 20133 Milan, Italy.
1This article is part of a Special Issue dedicated to Professor Howard Alper.
2Corresponding author (e-mail: dominique.roberto@unimi.it).
Can. J. Chem. 83: 1017–1024 (2005)
doi: 10.1139/V05-104
© 2005 NRC Canada

1018
Can. J. Chem. Vol. 83, 2005
the rather complex chemical behavior of metal carbonyl spe-
cies anchored to silica (2). Many models of silica anchored
carbonyl species of various noble metals (e.g., Rh (3), Os (2,
4),3 and Ru (5)) have been synthesized, structurally charac-
terized, and chemically investigated; however, in the case of
the oxophilic metal Re, just anionic [Re2(CO)6(µ-OH)x(µ-
OSiEt3)3–x]– (x = 0, 1, or 2) (6) and neutral [Re2(CO)8(µ-H)-
(µ-OSiR2R′)] (R = Et, Ph; R′ = Et, Ph, OH, or OSiPh2OH)
(7) species have been obtained and described only recently.
Nevertheless, these few models cannot give a full under-
standing of the surface organometallic chemistry involving
various rhenium carbonyl species anchored to silica. There
is still a need for the synthesis and investigation of novel
rhenium carbonyl species bearing silanolate ligands to fully
clarify the complex surface chemistry occurring when rhe-
nium carbonyl species such as [Re2(CO)10] or [Re(CO)3-
(OH)]4 are chemically interacting with the silica surface.
Indeed, this kind of surface organometallic chemistry is not
yet well known, despite the industrial relevance of Re cata-
lysts supported on inorganic oxides (8).
Recently, in fact, we reported that silica supported
[Re(CO)3(OH)]4 is easily converted into [Re2(CO)10] by
reductive carbonylation under very mild conditions (1 atm
CO, 1 atm = 101.325 kPa). This reaction does not occur in
solution, suggesting that the silica surface plays a unique
role via the formation of the surface anchored species
[Re(CO)5OSiϵ], which could also act as the intermediate in
the silica mediated conversion under N2 of [Re2(CO)10] into
[Re(CO)3(OH)]4 (9). Evidence for an intermediate chemi-
cally bound to the silica surface was based on IR studies and
on its lack of extraction with solvents, together with its reac-
tivity with HCl and HReO4 to afford [Re(CO)5Cl] and
[Re(CO)5OReO3], respectively. This proposed surface spe-
cies anchored via a Re—OSi bond to the surface is quite pe-
culiar since carbonyl species of the type [Re(CO)5OR] have
so far eluded isolation (9). Therefore, to produce additional
evidence for its existence and to enlighthen its chemical be-
havior, we started an investigation on the synthesis and char-
acterization of molecular models of this unusual silica
anchored species.
[1]
2[Re(CO)5Cl] + 3Me3SiO–
→ [Re2(CO)6(µ-OSiMe3)3]– + 2Cl– + 4CO
Reaction of [Re(CO)5Cl] with the anion Me3SiO– reason-
ably generates [Re(CO)5OSiMe3] as a very reactive interme-
diate species, which could not be detected even by working
under CO at 0 °C. This intermediate would be so reactive
towards nucleophiles to react rapidly with the anion
Me3SiO– affording, finally, by a double nucleophilic attack,
[Re2(CO)6(µ-OSiMe3)3]–. Such an easy dimerization by
silanolates bridging and concomitant loss of carbon monox-
ide, even under a CO atmosphere, suggests that the silano-
late anion, as the alkoxide one (10), is an exceptionally good
cis-labilizing ligand, which facilitates a further nucleophilic
attack: the behavior of Me3SiO– towards [Re(CO)5Cl] is thus
parallel to that of the structurally related alkoxide Me3CO–
(11), despite its lower basicity. The quite high stability of the
proposed silica anchored [Re(CO)5OSiϵ] intermediate (9)
can be due to the much lower nucleophilicity of surface
silanols because the lack of dimerization cannot be attrib-
uted to the topological arrangement of surface silanols. In
fact, we produced evidence for covalent anchoring of the
dimer [Re2(CO)6(µ-OH)3]– to surface silanols (6).
It was thus clear that we could follow two strategies to ob-
tain mononuclear Re(I) carbonyl silanolates: to decrease ei-
ther the nucleophilic power of the silanolate anion or the
electrophilic character of the Re(I) center. In this work we
report on the results obtained by following the second op-
tion.
Substitution of some CO ligands of [Re(CO)5Cl] by phos-
phine ligands should decrease the electrophilicity of the
Re(I) center and therefore hinder further nucleophilic attacks
by Me3SiO– with formation of anionic dimeric species.
Therefore, we treated fac-[Re(CO)3(Ph2PCH2CH2PPh2)Cl]
with a stoichiometric amount of Me3SiONa, at room temper-
ature in dichloromethane. No reaction took place because of
the large increase of the electronic density on the Re(I) cen-
ter, as it can be inferred from the IR spectrum. This effect
obviously inhibits even the first nucleophilic attack by the
trimethylsilanolate anion. However, it is well-known that in
Re(I) carbonyl species the triflate anion is a much better
leaving group than the chloride one. Thus, we studied the re-
action of fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf] (OTf is the
triflate anion), easily obtained by reaction of AgOTf with
fac-[Re(CO)3(Ph2PCH2CH2PPh2)Cl].
Addition of Me3SiONa to a dichloromethane solution of
fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf] slowly affords fac-
[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3]. The reaction, which
can be easily followed by IR and 31P NMR spectroscopies
(see the Experimental section), is complete in 2 days. Inter-
estingly, although alkoxide complexes such as fac-
[Re(CO)3(R′2PCH2CH2PR′2)OR] (R = Me, Et, or i-Pr; R′ =
Et or Ph) were prepared by treatment of the triflate complex
with the sodium salt of the appropriate primary or secondary
alcohol (10, 12), it was reported that by reaction with the an-
ion Me3CO–, structurally related to Me3SiO– but of in-
creased nucleophilicity, the corresponding tert-butoxide
complex was not isolated (10).
Results and discussion
Attempts to prepare the molecular model of silica an-
chored [Re(CO)5OSiϵ] by reaction of [Re(CO)5Cl] with
Me3SiONa invariably failed. The expected compound,
[Re(CO)5OSiMe3], was not obtained upon addition of
Me3SiONa to a solution of [Re(CO)5Cl] in dichloromethane
(1:1 molar ratio), working under nitrogen at room temperature.
Under these conditions, only some Na[Re2(CO)6(µ-OSiMe3)3]
(ν(CO) = 2007 (s, br) and 1879 (vs, br) cm–1) is quickly
formed. With a 1.5:1 molar ratio Me3SiONa:[Re(CO)5Cl],
Na[Re2(CO)6(µ-OSiMe3)3] is quantitatively obtained
(eq. [1]). Similarly, Et3SiONa leads to the known anion
[Re2(CO)6(µ-OSiEt3)3]–, previously prepared by reaction of
an anisole solution of [Re2(CO)6(µ-OH)3]– with an excess of
triethylsilanol at 200 °C (6).
3 E. Lucenti, F.J. Feher, and J.W. Ziller. Submitted for publication.
© 2005 NRC Canada

D’Alfonso et al.
1019
Fig. 1. ORTEP representation (at 20% probability) of [Re2(CO)6-
(µ-OH)(µ-OSiMe3)2]–. The labels adopted for the structural de-
scription are depicted.
fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3] reacts readily
with HCl(aq) to give fac-[Re(CO)3(Ph2PCH2CH2PPh2)Cl], a
behavior that mimics the formation of [Re(CO)5Cl] by treat-
ment of the proposed silica anchored [Re(CO)5OSiϵ] spe-
cies with HCl(aq). As a matter of fact, it was already
reported that addition of gaseous HCl to the related alkoxide
complex fac-[Re(CO)3(Et2PCH2CH2PEt2)OEt] results in a
similar cleavage of the metal–oxygen bond with formation
of the corresponding chloride (13).
It appears that the balance of the electron density on the
rhenium atom is of paramount relevance for the stabilization
of monomeric silanolate complexes. Indeed, by decreasing
the electronic density on the rhenium center, such as when
replacing a chelating diphosphine with one triphenyl-
phosphine as in cis-[Re(CO)4(PPh3)OTf], reaction with the
anion Me3SiO– still gives [Re2(CO)6(µ-OSiMe3)3]– with re-
lease of CO and even PPh3, as evidenced by 29Si and 31P
NMR spectroscopy, also when working at 0 °C (see the Ex-
perimental section). This latter result suggests that the coor-
dination to rhenium of only one phosphorous donor atom is
not sufficient to avoid aggregation by bridging silanolates
when working in the normal range of temperatures between
0 and 25 °C.
It is worth pointing out that the Re-OSiMe3 bond in fac-
[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3] is quite stable to hy-
drolysis in the presence of water at room temperature, as in
some other neutral rhenium silanolate models such as
[Re2(CO)8(µ-H)(µ-OSiR2R′)] (R = Et, Ph; R′ = Et, Ph, OH,
or OSiPh2OH) (7), but in sharp contrast with the fast hydro-
structure for the anionic dimer, as in the present case, was
obtained only for the anion [Re2(CO)6(µ-OH)(µ-OPh)2]–
(14f ), heavy disorder affecting the alkoxy groups in all the
other cases (14a, 14e).
lysis of anionic species such as [Re2(CO)6(µ-OH)x(µ-
–
Within each [Re2(CO)6(µ-OH)(µ-OSiMe3)2]– dimer, the
metal center, hexa-coordinated in distorted octahedral stereo-
chemistry, is bound by three terminal carbonyl ligands, oc-
cupying facial positions, by two bridging -OSiMe3 moieties
and a bridging hydroxy group. The metal atom thus satisfies
the valence bond electron rule without invoking a bonding
metal···metal interaction, whose distance (3.111(1) Å) is
anyway only slightly greater than that of 3.02 Å found in
[Re2(CO)10] (15), as occurs in other RO– supported Re–Re
dimers (see Table 2). Comparable Re—O(H) and Re—O(Si)
bond distances (2.142(5) and 2.141(3) Å, respectively) find
correspondence in similar Re-O(H)-Re and Re-O(Si)-Re
bond angles (93.1(3)° and 93.2(2)°, respectively). Both inde-
pendent carbonyl groups reveal almost normal features, with
Re—C bond distances of 1.888(6) and 1.904(9) Å, C—O
bond distances of 1.161(7) and 1.166(11) Å, and Re-C-O an-
gles of 177.3(6)° and 176.9(9)°.
OSiEt3)3–x
]
(x = 0, 1, or 2) (6). Indeed, addition of a
stoichiometric amount of [NEt4]Cl to an acetonitrile solution
of Na[Re2(CO)6(µ-OSiMe3)3], followed by evaporation of
the solvent and extraction of the organometallic anion with
dry dichloromethane, afforded [NEt4][Re2(CO)6(µ-OSiMe3)3].
However, by slow crystallization from dichloromethane–n-
pentane at –20 °C, only crystals of the product of the first
step of the hydrolysis (6), namely [NEt4][Re2(CO)6(µ-
OH)(µ-OSiMe3)2], were obtained because a partial hydrolysis
arose even from very minor traces of water. The related
complex [Re2(CO)6(µ-OH)(µ-OSiEt3)2]– has already been
characterized spectroscopically (6), but has never been struc-
turally investigated, although it is of interest as a model of the
silica anchored surface species [Re2(CO)6(µ-OH)(µ-OSiϵ)2]–
proposed to be thermally formed by condensation of
[Re2(CO)6(µ-OH)3]– with surface silanols (6).
The crystal structure of [NEt4][Re2(CO)6(µ-OH)(µ-
OSiMe3)2] (orthorhombic Cmcm) is composed by dimeric
[Re2(CO)6(µ-OH)(µ-OSiMe3)2]– anionic units (Fig. 1), lying
about a crystallographic m2m position (C2v), counterbal-
anced by disordered tetraethylamonium cations, lying on a
2/m symmetry operator. Crystallographic data as well as data
collection and treatment details are gathered in Table 1. A
synoptic collection of bond lengths and angles is reported in
Table 2. To our knowledge, this is the first example of a
[Re2(CO)6(µ-OH)x(µ-OR)3–x]– structure (x = 0, 1, or 2) in
which R is a silyl group. Previous characterizations regard-
ing both homo- and hetero-bridged dimers of this kind in-
variably involved alkoxy or aryloxy bridges (14). Moreover,
The steric hindrance of the trimethylsilanolate group
seems not to induce significant molecular rearrangements:
by inspecting Table 2, the major sterical effect seems to oc-
cur when PhO– is acting as the bridge (see, in particular, the
Re—O and Re···Re distances).
The dimeric [Re2(CO)6(µ-OH)(µ-OSiMe3)2]– units pack to
generate rows parallel to a which stack, staggered, along b
with minor intermolecular interactions (Fig. 2). Consecutive
stacks of rows are separated, along c, by the counterions.
Conclusions
among the known X-ray structures of hetero-bridged
Due to (i) the high cis-labilizing effect of the silanolate
ligand, (ii) the lability of CO ligands in “Re(CO)5” and
–
[Re2(CO)6(µ-OH)x(µ-OR)3–x
]
complexes, a well-defined
© 2005 NRC Canada

1020
Can. J. Chem. Vol. 83, 2005
Table 1. Crystallographic data and structure refinement parame-
ters for compound [NEt4][Re2(CO)6(µ-OH)(µ-OSiMe3)2].
The increased stability of carbonyl ligands and the lower
electrophilicity of the rhenium center, which is the origin of
the stability of the monomeric silanolate complexes towards
dimerization by further nucleophilic attack by the trimethyl-
siloxy anion, are quite dependent on the number of phospho-
rous donor atoms coordinated to the rhenium center. Whereas
the complex fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3] does
not react with excess Me3SiO–, cis-[Re(CO)4(PPh3)OSiMe3],
reasonably formed as intermediate species by reaction of cis-
[Re(CO)4(PPh3)OTf] with Me3SiO–, still shows a great
lability of a cis carbonyl and even a phosphine ligand giving
place to [Re2(CO)6(µ-OSiMe3)3]–.
In conclusion, this work has indirectly suggested that the
relative stability of the proposed “Re(CO)5(OSiϵ)” species
anchored to the silica surface may be due to the extremely
low nucleophilicity of the silanols of the silica surface.
Therefore, work is in progress to synthesize [Re(CO)5-
OSiR3] species where R3 better represents the tetrahedral ox-
ygen sphere of silica silanols, which, being quite acidic, dis-
play a very low basicity either as such or even as silanolates.
Empirical formula
C20H39NO9Re2Si2
Formula weight (g mol–1)
Crystallographic system
Space group
a (Å)
b (Å)
c (Å)
V (Å3)
Z
865.96
Orthorhombic
Cmcm, No. 63
11.062(1)
11.887(2)
23.530(4)
3 094.1(8)
4
F(000)
1604
Calculated density (Mg m–3)
Absorption coefficient (mm–1)
Temperature (K)
Crystal morphology
Crystal size (mm3)
Integration range (°)
Indexes range
1.859
7.934
299
Needle
0.12 × 0.14 × 0.22
3.5 ≤ 2θ ≤ 61.1
–15 ≤ h ≤ 15;
–16 ≤ k ≤ 16;
–33 ≤ l ≤ 33
26 963
2 537
2 098
0.047, 0.025
Experimental section
General comments
Measured reflections
Unique reflections
Observed reflections for I > 2σ(I)
a
Rint, Rσ
[Re(CO)5Cl] and [Re(CO)4Cl]2 were prepared according
to the literature, the first complex by reaction of [Re2(CO)10]
(commercially available from Acros Organics) with Cl2 (16),
the second by heating under reflux a heptane solution of
[Re(CO)5Cl] (17). AgOTf and Me3SiONa (1 mol/L solution
in dichloromethane) were purchased from Sigma-Aldrich.
Organic solvents were freshly distilled before use (Et2O and
THF from sodium/benzophenone; CH2Cl2 from CaH2). All
reactions were carried out under N2 (dried by flowing over
drierite) in anhydrous conditions. Products were characterized
by IR and NMR (1H, 29Si, 31P) spectroscopies, by mass
spectrometry, and by elemental analyses, which were carried
out at the Dipartimento di Chimica Inorganica, Metallor-
ganica ed Analitica of the Università di Milano.
Data / Restraints / Parameters
Goodness-of-fitb
Figures of merit for I > 2σ(I)c
Figures of merit for all data
Largest difference peak and hole
(e Å–3)
2 537 / 3 / 80
S(F2) 1.225
R(F) 0.037, wR(F2) 0.089
R(F) 0.050, wR(F2) 0.092
1.24 and –2.36
aRint = Σ|Fo – Fmean2|/Σ|Fo |; Rσ = Σ|σ(Fo )|/Σ|Fo |.
2
2
2
2
bS(F2) = [Σw(Fo – Fc2)2/(n – p)]1/2, where n is the number of reflec-
2
tions, p is the number of parameters, and w = 1/[σ2(Fo ) + (0.019P)2 +
2
2
1.88P] with P = (Fo + 2Fc2)/3.
cR(F) = Σ||Fo| – |Fc||/Σ|Fo| and wR(F2) = [Σw(Fo – Fc2)2/ΣwFo ]1/2
.
2
4
“Re(CO)4” moieties containing mutually trans carbonyls,
and (iii) the strong tendency of OSiR3 groups to function as
bridging
ligands
(4g),
the
molecular
model
Synthesis of Na[Re2(CO)6(µ-OSiMe3)3]
[Re(CO)5OSiMe3] of the proposed surface species
[Re(CO)5OSiϵ] can not be obtained by reaction of
[Re(CO)5Cl] with the trimethylsilanolate anion, despite the
lower nucleophilicity of this latter compound when com-
pared with alkoxy or aryloxy anions. The reaction product,
under different reaction conditions, is invariably the bridged
anion [Re2(CO)6(µ-OSiMe3)3]–, as it occurs with alkoxy an-
ions (11).
Only substitution of CO groups with a chelating
diphosphine to increase the electron density on the Re(I)
complex enhances the stability of CO ligands and thus al-
lows the preparation of neutral rhenium silanolate mono-
nuclear species. Thus, the easy reaction of Me3SiONa with
fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf] provides a pathway to
the novel complex fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3],
characterized by an increased stability towards hydrolysis of
In a typical synthesis, a solution of Me3SiONa in CH2Cl2
(0.65 mL of a 1 mol/L solution) was added to a solution of
[Re(CO)5Cl] (150 mg, 0.415 mmol, molar ratio
[Re(CO)5Cl]:Me3SiONa = 1:1.6) in freshly distilled anhydr.
CH2Cl2 (35 mL) under N2 in a 100 mL two-neck round bot-
tomed flask equipped with a magnetic stirring bar. After
24 h under stirring at room temperature, a white precipitate
was present and the reaction was complete as evidenced by
IR spectroscopy of the solution, which showed that no
[Re(CO)5Cl] (ν(CO) in CH2Cl2 = 2046 (m), 1985 (s) cm–1)
was left. Evaporation to dryness of the solvent afforded a
white residue that was dissolved in freshly distilled anhydr.
THF (5 mL). Anhydr. Et2O was slowly added to the latter
solution, at ca. –5 °C (ice and salt bath), to precipitate the
NaCl formed during the reaction. The solution was then sep-
arated from the precipitate using a syringe. Evaporation of
the solvent followed by washing with a few mL of anhy-
drous pentane afforded pure Na[Re2(CO)6(µ-OSiMe3)3] as a
white powder (162 mg, 0.195 mmol, 94% yield). IR (in
a Re—OSi bond in sharp contrast with the easy hydrolysis
of anionic species such as [Re2(CO)6(µ-OH)x(µ-OSiR3)3–x
(x = 0, 1, or 2; R = Me, Et) (6).
–
]
© 2005 NRC Canada

D’Alfonso et al.
1021
Table 2. Synoptic collection of significant geometrical parameters (distances, Å; angles, °) in the known dimeric anionic [Re2(CO)6(µ-
OR)x(µ-OR′)y]– units (x + y = 3).
Dimeric unit
Re···Re (Å)
Re—O(H) (Å)
Re—O (Å)
Re-O(H)-Re (°)
Re-O-Re (°)
Ref.
[Re2(CO)6(µ-OSiMe3)2(µ-OH)]–
[Re2(CO)6(µ-OMe)x(µ-OEt)y]–a
[Re2(CO)6(µ-OMe)x(µ-OEt)y]–a
3.111
3.085
3.117
2.141
n.a.
2.142
2.065
2.123
93.2
n.a.
n.a.
93.1
This work
14a
95.65
94.51
n.a.
14c
2.121
94.59
95.65
[Re2(CO)6(µ-OPh)3]–
3.154
n.a.
2.107, 2.149b,c
n.a.
14b
2.133, 2.145b,c
2.128, 2.154b,c
n.a.
94.98
94.89
n.a.
[Re2(CO)6(µ-OH)3]–
3.103
3.117
2.063d
2.130d
n.a.
97.55
14d
14e
93.53
n.a.
[Re2(CO)6(µ-OMe)(µ-OEt)2]–
2.108, 2.109b,e
95.32
2.145, 2.157b,f
2.132, 2.149b,f
2.162, 2.163b,g
92.86
93.46
93.55
[Re2(CO)6(µ-OH)(µ-OPh)2]–
Note: n.a. = not applicable.
3.152
2.119, 2.138b
95.51
14f
2.148, 2.151b,g
94.29
aSevere disorder affecting the alkoxo groups impeded a definite assessment of stoichiometry.
bThe two values on each line refer to the two independent Re atoms present.
cThe values refer to one of the three independent phenoxy ligands.
dThe values refer to one of the two independent hydroxy ligands.
eThe values refer to the methoxy ligand.
fThe values refer to one of the two independent ethoxy ligands.
gThe values refer to one of the two independent phenoxy ligands present.
Fig. 2. ORTEP representation (at 20% probability) of the packing motif of the dimeric units in [NEt4][Re2(CO)6(µ-OH)(µ-OSiMe3)2]
viewed down [1 0 0]. Horizontal axis, c; vertical axis, b. Disordered tetraethylammonium cations and hydrogen atoms have been omit-
ted for clarity.
© 2005 NRC Canada

1022
Can. J. Chem. Vol. 83, 2005
1
CH2Cl2, cm–1): ν(CO) 2007 (s, br), 1879 (vs, br). H NMR
(in CD3CN, ppm) δ: 0.02. 29Si NMR (in CD3CN, ppm) δ:
0.59; the 29Si NMR signal of Me3SiONa in CD3CN lies at
–13.09 ppm. In the FAB– mass spectrum, the ion peak corre-
sponding to [Re2(CO)6(µ-OSiMe3)3]– lies at m/e = 807. Ele-
mental anal. calcd.: C 21.67, H 3.25; found: C 21.79, H
3.41.
syringe in a round-bottomed flask. Then Me3SiONa
(1 mol/L solution in CH2Cl2, 0.152 mmol, molar ratio
Re:Me3SiONa = 1:2) was added at 0 °C, and the resulting
solution was stirred under nitrogen at room temperature for
48 h, evaporated to dryness, and the residue extracted with
anhydrous pentane to give, after evaporation to dryness of
the solvent, fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3] in
75% yield (43.75 mg, 0.058 mmol). IR (in CH2Cl2, cm–1):
1
ν(CO) 2014 (vs), 1927 (s), 1873 (s). H NMR (in CD2Cl2,
Synthesis of Na[Re2(CO)6(␮-OSiEt3)3]
ppm) δ: –0.61 (s, 9H, OSiMe3), 2.43 (m, 2H, CH2), 3.04 (m,
2H, CH2), 7.21–7.83 (m, 20H, 2PPh2). 31P NMR (in CD2Cl2,
ppm) δ: 31.36. 29Si NMR (in CD2Cl2, ppm) δ: 6.05. Mass
spectrometry (EI, m/e): 758 (M = Molecular ion peak), 730
[M – CO], 702 [M – 2CO], 674 [M – 3CO]. Elemental anal.
calcd.: C 50.71, H 4.35; found: C 50.32, H 4.30.
Synthesis of Et3SiONa
An excess of metallic Na (3.82 g, 166 mmol, cut in small
pieces) was added to a solution of Et3SiOH (0.98 mmol,
0.15 mL, d = 0.846 g/mL) in anhydr. Et2O (15 mL) under
N2. After stirring at room temperature for ca. 4 h no more
gas was evolved, the solution was separated with a syringe
and evaporated to dryness affording pure Et3SiONa. 1H
NMR (in CD3CN, ppm) δ: 0.36 (q, 2H, CH2), 0.93 (t, 3H,
CH3). 29Si NMR (in CD3CN, ppm) δ: –3.19.
Reactivity of fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3]
Addition, at room temperature, of a drop of aq. HCl to
fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3] (4 mg) dissolved
in CH2Cl2 (5 mL) led to the immediate formation of fac-
[Re(CO)3(Ph2PCH2CH2PPh2)Cl] as confirmed by IR and 31P
NMR spectroscopies. On the contrary, no reaction occurred
upon stirring a CH2Cl2 solution of fac-[Re(CO)3(Ph2-
PCH2CH2PPh2)OSiMe3] in the presence of a few drops of
water for ca. 48 h at room temperature.
Synthesis of Na[Re2(CO)6(␮-OSiEt3)3]
A solution of Et3SiONa in anhydr. CH2Cl2 (6.5 mL of a
0.0981 mol/L solution) was added to a solution of
[Re(CO)5Cl] (150 mg, 0.415 mmol, molar ratio
[Re(CO)5Cl]:Et3SiONa = 1:1.5) in freshly distilled anhydr.
CH2Cl2 (35 mL) under N2 in a 100 mL two-neck round bot-
tomed flask equipped with a magnetic stirring bar. After
24 h under stirring at room temperature, a white precipitate
was present and the reaction was complete as evidenced by
IR spectroscopy of the solution. Evaporation to dryness of
the solvent afforded a white residue that was dissolved in
freshly distilled anhydr. THF (5 mL). Anhydr. Et2O was
slowly added to the latter solution, at –5 °C, to precipitate
the NaCl formed during the reaction. The solution was then
separated from the precipitate using a syringe. Evaporation
of the solvent followed by washing with a few mL of anhy-
drous pentane (to remove traces of Et3SiOSiEt3 and
Et3SiOH) afforded the known complex Na[Re2(CO)6(µ-
OSiEt3)3] (6) as a white powder (188 mg, 0.197 mmol, 95%
yield).
Reactivity of cis-[Re(CO)4(PPh3)OTf] with Me3SiONa
Addition of AgOTf (20.6 mg, 0.084 mmol) to a solution
of cis-[Re(CO)4(PPh3)Cl] (19) (40 mg, 0.067 mmol; IR (in
CH2Cl2, cm–1): ν(CO) 2107 (m), 2005 (vs), 1947 (s); 31P
NMR (in CD2Cl2, ppm) δ: 4.28) in anhydr. CH2Cl2 (8 mL),
at room temperature, led to the precipitation of AgCl and the
formation of cis-[Re(CO)4(PPh3)OTf] as confirmed by IR
and 31P NMR spectroscopies (IR (in CH2Cl2, cm–1): ν(CO)
2117 (m), 2016 (vs), 1961 (s); 31P NMR (in CD2Cl2, ppm) δ:
11.60). After 2.5 h under stirring in the dark at room temper-
ature, the reaction was complete, as evidenced by IR spec-
troscopy, and the solution containing cis-[Re(CO)4-
(PPh3)OTf] was filtered on Celite under nitrogen. Then
Me3SiONa (1 mol/L solution in CH2Cl2, 0.067 mL,
0.067 mmol, molar ratio Re:Me3SiONa = 1:1) was added,
leading to the precipitation of some product. After 1.5 h,
evaporation of the solvent afforded a residue that contained
Na[Re2(CO)6(µ-OSiMe3)3] and free PPh3 as evidenced
by 29Si NMR (signal at δ 0.59 ppm) and 31P NMR (signal at
δ –4.91 ppm) spectroscopy (in CD3CN).
Synthesis of fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3]
The new complex fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3]
was prepared from fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf]
(12), easily obtained by the reaction of AgOTf with fac-
[Re(CO)3(Ph2PCH2CH2PPh2)Cl] (18) recovered in quantita-
tive yield by refluxing under N2 for 14 h a CHCl3 solution of
[Re(CO)4Cl]2 (0.154 mmol) with a stoichiometric amount of
PPh2CH2CH2PPh2 (0.308 mmol).
Thus, addition of AgOTf (19.8 mg, 0.073 mmol) to a
solution of fac-[Re(CO)3(Ph2PCH2CH2PPh2)Cl] (54 mg,
0.077 mmol, IR (in CH2Cl2 cm–1): ν(CO) 2034 (vs), 1954
(s), 1906 (s); 31P NMR (in CD2Cl2, ppm) δ: 32.65) in
anhydr. CH2Cl2 (10 mL) led to the precipitation of AgCl and
the formation of fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf] as
confirmed by IR and 31P NMR spectroscopies (IR (in
CH2Cl2, cm–1): ν(CO) 2044 (vs), 1966 (s), 1919 (s); 31P
NMR (in CD2Cl2, ppm) δ: 38.68). After 2.5 h under stirring
in the dark at room temperature, the reaction was complete,
as evidenced by IR spectroscopy, and the solution containing
fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf] was transferred with a
Formation of [NEt4][Re2(CO)6(␮-OH)(␮-OSiMe3)2]
Addition of [NEt4]Cl (stoichiometric amount) to a solu-
tion of Na[Re2(CO)6(µ-OSiMe3)3] in acetonitrile, followed
by evaporation of the solvent to dryness, extraction of the
residue with CH2Cl2 and filtration, afforded a solution of
[NEt4][Re2(CO)6(µ-OH)(µ-OSiMe3)2] because of the pres-
ence of traces of water. Addition of pentane to this solution
led, at –20 °C, to the formation of colorless crystals of
[NEt4][Re2(CO)6(µ-OH)(µ-OSiMe3)2] suitable for X-ray
structure determination. IR (in CH2Cl2, cm–1): ν(CO) 1995
1
(s, br), 1876 (vs, br). H NMR (in CD3CN, ppm) δ: 0.22 (s,
18H, 2Si(CH3)3), 1.22 (t, J = 7.2 Hz, 12H, 4NCH2CH3),
1.76 (s, 1H, OH), 3.18 (q, J = 7.2 Hz, 8H, 4NCH2CH3). 29Si
© 2005 NRC Canada

D’Alfonso et al.
1023
NMR (in CD3CN, ppm) δ: 16.61; the 29Si NMR signal of
Me3SiONa in CD3CN lies at –13.09 ppm.
by P. Braunstein, L. Oro, and P. Raithby. Wiley-VCH Verlag,
Weinheim, Germany. 1999. pp. 782–793.
2. (a) E. Lucenti, D. Roberto, and R. Ugo. Organometallics, 20,
1725 (2001), and refs. therein; (b) E. Cariati, D. Roberto, R.
Ugo, and E. Lucenti. Chem. Rev. 103, 3707 (2003), and
refs. therein.
X-ray crystallography
A suitable colorless needle of [NEt4][Re2(CO)6(µ-OH)(µ-
OSiMe3)2], obtained by slow crystallization of a CH2Cl2–
pentane solution of [NEt4][Re2(CO)6(µ-OSiMe3)3], was
mounted in air on the glass fiber tip of a goniometer head.
Data collection was performed on a Bruker Axs SMART
CCD area-detector equipped with graphite-monochromatized
Mo Kα radiation (λ = 0.710 73 Å), by applying the ω-scan
method. 2500 frames were acquired with ∆ω = 0.3°, t = 25 s
per frame, and sample–detector distance fixed at 3.45 cm.
Data reduction within the hemisphere with 2θ < 61.2° af-
forded 26 963 reflections of which 2537 were unique. An
empirical absorption correction was applied (20). The struc-
ture was solved by direct methods (21) and refined with full-
matrix least-squares calculations on F2 (22). Anisotropic
temperature factors were assigned to all atoms but: (i) hy-
drogens, riding their parent atoms with an isotropic tempera-
ture factor arbitrarily chosen as 1.2 times that of the parent
itself; and (ii) the nitrogen and carbon atoms of the tetra-
ethylammonium cation. To properly describe the heavily dis-
ordered tetraethylammonium cation: (i) some of its carbon
atoms have been “splitted” over two positions, with a fixed
site occupation factor of 0.5; (ii) soft restraints have been
imposed on its C—C bond distances (1.55 Å); (iii) all its
carbon atoms have been given a common isotropic displace-
ment parameter; and (iv) hydrogen atoms have not been as-
signed.4
3. (a) A. Vizi-Orosz, G. Pályi, and L. Markó. J. Organomet.
Chem. 57, 379 (1973); (b) G. Pályi, A. Vizi-Orosz, L. Markó,
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Walzer. Inorg. Chem. 29, 1604 (1990); (g) E. Lucenti, D.
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Acknowledgements
and L. Carlucci. Organometallics, 22, 3271 (2003).
8. (a) P.S. Kirlin and B.C. Gates. Nature (London), 325, 38 (1987);
(b) S.K. Purnell and B.C. Gates. J. Mol. Catal. 94, L1 (1994).
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Sironi. Organometallics, 19, 2564 (2000), and refs. therein.
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(1991).
DR deeply thanks her first master, Professor Howard Alper,
for his magic teaching and guidance during the first years of
her scientific life. We are grateful to Dr. Silvia Malaguti for
NMR experiments and to Dr. Carmen Roveda, Dr. Matteo
Vailati, and Dr. Virna Formaggio for some experimental help.
This work was supported by the Ministero dell’Istruzione,
dell’Università e della Ricerca (PRIN 2003, Research Title:
Proprietà di singole molecole ed architetture molecolari
funzionali supportate: caratterizzazione chimico-fisica,
sviluppo di sintesi chimiche e di sistemi per l’indagine) and
by the Consiglio Nazionale delle Ricerche (CNR, Roma).
13. R.D. Simpson and R.G. Bergman. Organometallics, 11, 3980
(1992).
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replaced by ethoxy ones): G. Ciani, A. Sironi, and A. Albinati.
Gazz. Chim. Ital. 109, 615 (1979); (b) [NEt4][Re2(CO)6(µ-
OPh)3]: T. Beringhelli, G. Ciani, G. D’Alfonso, A. Sironi, and
M. Freni. J. Chem. Soc. Dalton Trans. 1507 (1985);
(c) [NMe4][Re2(CO)6(µ-OCH3)3]: W.A. Herrmann, D. Mihalios,
K. Ofele, P. Kiprof, and F. Belmedjahed. Chem. Ber. 125,
1795 (1992); (d) [NEt4][Re2(CO)6(µ-OH)3]·2H2O: R. Alberto,
A. Egli, U. Abram, K. Hegetschweiler, V. Gramlich, and P.A.
Schubiger. J. Chem. Soc. Dalton Trans. 2815 (1994); (e) [N-
CH3-adamantanium][Re2(CO)6(µ-OCH3)(µ-OCH2CH3)2]: N.E.
Kolobova, V.I. Zdanovich, I.A. Lobanova, V.G. Andrinov,
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