Synthesis and characterization of {(diene)Rh[μ-OSi(OtBu) 3]}2, a precursor to silica-bound rhodium

Article abstract of DOI:10.1039/b407398c

Reaction of [(diene)RhCl]₂ with 2 equiv of KOSi(O ^tBu)₃ afforded {(diene)Rh[μ-OSi(O^tBu) ₃]}₂, where diene = cod (1) and nbd (2). Multinuclear NMR studies reveal that 1 and 2 have a dimeric structure with bridging tris(tert-butoxy)siloxy ligands. These dimers are folded along the O...O axis. Complexes 1 and 2 reacted with PR₃ (R = Me, Ph) to give monomeric products, the formulae of which depend on the amount of PR₃ added ((diene)Rh[OSi(O^tBu)₃](PR₃) and Rh[OSi(O^tBu)₃](PMe₃)₃). The behavior of 1 towards water and methanol is discussed. Thermogravimetric analyses (TGAs) of 1 and 2 reveal rather sharp conversions to rhodium-containing materials. Thermolysis of 1 in toluene at 180°C resulted in formation of a black precipitate, which contained rhodium nanoparticles with an average diameter of 22 nm, as determined by powder X-ray diffraction (PXRD), after calcination at 300°C for 1 h.

Full text of DOI:10.1039/b407398c

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F U L L P A P E R
Synthesis and characterization of {(diene)Rh[-OSi(OtBu)3]}2,
a precursor to silica-bound rhodium
Jonggol Jarupatrakorna,b and T. Don Tilley*a,b
a
Department of Chemistry, University of California, Berkeley, California, 94720-1460
Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road,
b
Berkeley, CA, 94720. E-mail: tdtilley@socrates.berkeley.edu
Received 14th May 2004, Accepted 14th July 2004
First published as an Advance Article on the web 3rd August 2004
Reaction of [(diene)RhCl]2 with 2 equiv of KOSi(OtBu)3 afforded {(diene)Rh[-OSi(OtBu)3]}2, where diene = cod (1)
and nbd (2). Multinuclear NMR studies reveal that 1 and 2 have a dimeric structure with bridging tris(tert-butoxy)siloxy
ligands. These dimers are folded along the OO axis. Complexes 1 and 2 reacted with PR3 (R = Me, Ph) to give
monomeric products, the formulae of which depend on the amount of PR3 added ((diene)Rh[OSi(OtBu)3](PR3) and
Rh[OSi(OtBu)3](PMe3)3). The behavior of 1 towards water and methanol is discussed. Thermogravimetric analyses
(TGAs) of 1 and 2 reveal rather sharp conversions to rhodium-containing materials. Thermolysis of 1 in toluene at 180 °C
resulted in formation of a black precipitate, which contained rhodium nanoparticles with an average diameter of 22 nm, as
determined by powder X-ray diffraction (PXRD), after calcination at 300 °C for 1 h.
The products were isolated from cold (−30 °C) pentane as yellow–
Introduction
orange crystals in high yield (82% for 1 and 76% for 2); however,
attempts to obtain X-ray quality crystals for structure analysis have
so far proven futile. Both 1 and 2 are dimeric in benzene solution
(by the Signer method16). These electron-rich Rh(I) compounds are
air- and moisture-sensitive.
The study of rhodium complexes is of increasing interest because
of their remarkable reactivity1 and their participation in catalytic
reactions.2 Also, the useful catalytic properties of rhodium have
attracted considerable attention to the surface chemistry of this
metal.3 Silica-supported rhodium complexes and particles were
found to be effective catalysts for the hydroformylation of alkenes,4
the hydrogenation of arenes,5 and the hydrogenolysis of alkanes.6
There has therefore been considerable interest in the characteriza-
tions of the structures of catalytically active rhodium-bound spe-
cies, and the nature of metal-support interactions.7 Although many
spectroscopic techniques are now available, the synthesis and char-
acterization of molecular models are essential for gaining insights
into the nature of rhodium–silica interactions at the molecular level.
Molecular complexes incorporating Rh–O–Si bonds constitute
good models for rhodium complexes supported on a silica surface.
Rhodium–silicon molecular model compounds studied previously
are [(diene)Rh(-OSiMe3)]2, where diene = 1,5-cyclooctadiene
(cod)8a,b and norborna-2,5-diene (nbd),9 [(cod)Rh(-OSiPh3)]2,8a,c
[(CO)Rh(-OSiR3)]2, where R = Ph,8c Me,10 and monomeric
[Rh(cod)(PR′3)(OSiR3)], where R = Me, iPr, OtBu; R′ = Cy, Ph.11
Over the past decade, we have been exploring a single-source
molecular precursor approach to metal silicate materials based on
tris(alkoxy)siloxy complexes of the type M[OSi(OtBu)3]n.12 This
approach provides molecular-level control over the stoichiometry
of the resulting materials, and low-temperature (100–200 °C) path-
ways to homogeneous, metal oxide–silica or metal silicate mate-
rials. However, relatively few late-transition metal (alkoxy)siloxy
complexes have been reported, and these are limited to copper13 and
nickel derivatives.14 We are interested in further investigating the
generality of the single-source molecular precursor method by exa-
mining the behavior of related precursors containing electron-rich
metal centers. Here we report the synthesis, characterization, and
reactivity of the rhodium(I) siloxide complexes, {(diene)Rh[-OSi-
(OtBu)3]}2, where diene = cod (1), nbd (2). The thermolytic conver-
sion of 1 to rhodium-containing materials, and reactions of 1 and 2
toward tertiary phosphines, water and methanol are also described.
(1)
1
The H NMR spectrum of 1 exhibits two slightly broadened
singlets at 4.74 and 4.56 ppm, corresponding to chemically inequiv-
alent vinylic protons. The broadening of these resonances is presum-
ably due to unresolved coupling, and not to chemical exchange. The
allylic proton resonances were observed as four broadened multi-
plets at 2.83, 2.42, 1.82, and 1.66 ppm. The tert-butoxy resonance is
a sharp singlet at 1.54 ppm. Given that 1 is a dimer in benzene solu-
tion, these 1H NMR results suggest that the Rh2O2 core of the dimer
is not planar but is bent in a butterfly fashion resulting in a fold
angle of <180° between the two RhO2 planes. Overall, the geometry
of 1 resembles that previously reported for the dimeric compounds
[(cod)Rh(-OSiR3)]2, R = Ph,8a Me,8b which are bent along the O–O
axis (fold angles 109.6 and 120.4°, respectively). This proposed
structure is further substantiated by the NMR spectroscopic char-
acterizations of 2 (vide infra). The 13C NMR spectrum of 1 is also
consistent with a dimeric, folded structure. The vinylic carbon reso-
nances are not equivalent and appear at 76.06 ppm (JRhC = 15.1 Hz)
and 71.96 ppm (JRhC = 14.7 Hz). The allylic carbon resonances ap-
pear as two singlets at 31.80 and 30.53 ppm. The tert-butoxy carbon
resonances appear as singlets at 72.46 and 32.45 ppm.
Palyi et al. suggested that the geometry of [L2Rh(-X)]2 com-
plexes was affected not by the strengths of Rh–Rh interactions,
but rather by the bridging ligands (X) in terms of interactions
between filled p orbitals on X and d orbitals of Rh.8a In addition,
the electronic structure of the folded dimer [(5-C5H5)Ru(-OMe)]2
investigated by using Fenske–Hall molecular orbital calculations
indicated that the folded structure is due to electronic preferences
of the bridging alkoxide ligands.18 The folding allows the lone-pair
orbitals of appropriate symmetry on the bridging oxygen atoms to
Results and discussion
Synthesis of the molecular precursors {(diene)Rh[-OSi-
(OtBu)3]}2 (diene = cod, nbd)
Reaction of [(diene)RhCl]2 with 2 equiv of KOSi(OtBu)315 in diethyl
ether at room temperature for 3 h produced the Rh(I) tert-butoxysi-
loxy complexes {(diene)Rh[-OSi(OtBu)3]}2, according to eqn. (1).
2 8 0 8
D a l t o n T r a n s . , 2 0 0 4 , 2 8 0 8 – 2 8 1 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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maximize their donation into the unoccupied Ru orbitals. A similar
structure was also observed for the tris(tert-butoxy)siloxy-bridged
dimer [(3-C3H5)Ni[-OSi(OtBu)3]}2.14
Reactions of 1 and 2 with phosphines
A commonly observed reactivity pattern for bridged rhodium(I) di-
mers is bridge cleavage by a Lewis base. Both 1 and 2 react readily
with 2 equiv of PR3 (R = Me, Ph) to give sixteen-electron complexes
of the type (diene)Rh[OSi(OtBu)3](PR3) (3–6; Scheme 1). The H,
The 1H NMR spectrum of 2 in benzene-d6 exhibits broad,
overlapping resonances in the 3.4–4.6 ppm region which are
assigned to vinylic and allylic protons. The bridgehead CH2
chemical shift is observed as a singlet at 1.12 ppm, and the tert-
butoxy resonance is a sharp singlet at 1.52 ppm. The 13C NMR
spectrum of 2 also reveals broad resonances for the vinylic and
allylic carbons. These broadened resonances may be attributed to a
fluxional process (vide infra), which may correspond to a rotation
of the nbd ligand around an imaginary Rh–Rh axis. Such dynamic
behavior has been proposed for many other transition-metal
complexes with a cyclodiene ligand.17 In addition, a butterfly-like
inversion (flapping) of the Rh2O2 ring may be responsible for this
fluxional process. Such broadening was not apparent in the room
temperature 1H NMR spectrum of 1, indicating a higher coalescence
temperature for the analogous process.
1
13C and 31P NMR spectra of these rhodium compounds support
a monomeric square planar structure. The complexes (diene)Rh-
[OSi(OtBu)3](PR3) exhibited only one doublet in their 31P NMR
spectra, with rhodium–phosphorus coupling constants in the range
observed for monomeric rhodium(I) species.9a,19 An increase in 
back-bonding between Rh and the diene double bonds is observed
when the other ligands become better  donors.20 Substitution of one
of the -OSi(OtBu)3 ligands in 1 or 2 with PR3 increases the electron
density on the rhodium atom; as a result, the metal→diene  back-
bonding character is increased and a deshielding of the trans-Pvinylic
carbon is observed. In addition, we found that the shielding of the
trans-P vinylic carbons increases slightly on decreasing the basicity
of the phosphine21 from PMe3 to PPh3 ( 105.2 for 3 vs. 104.6 for 5 and
 89.3 for 4 vs. 86.4 for 6). These are opposite to the results reported
by Platzer and coworkers for complexes (cod)Rh(Cl)PR3 (R = Ph,
Cy),20 but are similar to those observed by Bodner et al. for carbonyl
derivatives.22 Additionally, compounds 3–6 exhibited JRhP coupling
constant values of 158, 162, 162, and 184, respectively. Thus, the JRhP
value increases as the basicity of the phosphine decreases.23
1
Variable-temperature H NMR studies of 2 in toluene-d8 were
performed (Fig. 1). The room temperature spectrum shows only
small differences compared to that in benzene-d6 ( < 0.02 ppm).
All resonances were temperature dependent. The tert-butoxy and
bridgehead CH2 resonances shifted to lower fields as the tempera-
ture decreased, and appeared as sharp singlets down to −80 °C. The
broad, overlapping resonances for the allylic and vinylic protons
were eventually resolved at −20 °C into four singlets at 4.40, 4.00,
3.93, and 3.58 ppm with an integral ratio of 2:1:2:1. These results
are consistent with the folded structure proposed for 1. Further cool-
ing led to a collapse of the peaks in the 4.0 ppm region (Fig. 1). In
addition, the 13C NMR spectrum of 2 in toluene-d8 at −60 °C reveals
two types of allylic and vinylic carbon atoms, in agreement with
the proposed structure. The inequivalency of the allylic and vinylic
resonances recorded in the 1H and 13C NMR spectra is apparently due
to the folded angle about the edge formed by the intersection of the
two square planar rhodium fragments in dimeric 1 and 2.
The addition of
1 equiv of PMe3 to a benzene-d6
solution of (diene)Rh[OSi(OtBu)3](PMe3) (3 or 4; Scheme 1) led
to a 1:1 mixture of the starting complex (diene)Rh[OSi(OtBu)3]-
(PMe3) and the new tristrimethylphosphinerhodium(I) complex
Rh[OSi(OtBu)3](PMe3)3 (7). No evidence for the dimeric species
{(PMe3)2Rh(OSi(OtBu)3]}2 was observed. In contrast, Marciniec
and coworkers reported that the rhodium silyloxy complexes
(diene)Rh[OSiMe3](PPh3) react with PPh3 to form dimeric
[(PPh3)2Rh(-OSiMe3)]2.9 The reaction of 1 with 6 equiv of PMe3
in pentane afforded 7 in quantitative yield. The 31P NMR spectrum
of 7 consists of four lines at high field ( −13.9) due to the mutually
trans phosphorus atoms, and six lines at lower field ( 5.59) due to
the cis phosphorus atom. The structure of these lines arises from a
large rhodium–phosphorus coupling (JRhPc = 164 Hz, JRhPt = 144 Hz)
superimposed upon the smaller phosphorus–phosphorus coupling of
the nonequivalent phosphorus atoms (JPP = 50.2 Hz). These chemical
shifts are in close agreement with those observed for surface-bound
rhodium–phosphinecomplexes. Scott etal. havereportedthatthesur-
face species (SiO)Rh(PMe3)3 prepared by grafting CH3Rh(PMe3)3
onto silica exhibits two 31Presonances at −1.4 and −12.0 ppm, which
were assigned to cis and trans phosphine ligands, respectively.24
As expected, the 29Si NMR spectra of 1 and 2 in benzene-d6
reveal only one resonance at very high field (−94.59 ppm for 1
and −91.06 ppm for 2). For comparison, the 29Si NMR spectrum
of [(cod)Rh(-OSiPh3)]2 appears at 10.46 ppm.8a
Scheme 1
The addition of equimolar amounts of PPh3 to a benzene-d6
solution of (diene)Rh[OSi(OtBu)3](PPh3) (5 or 6; Scheme 1) led
to no reaction even after prolonged heating at 110 °C. In contrast,
rhodium silyloxy complexes of the type (diene)Rh[OSiMe3](PPh3)
react with PPh3 to form dimeric [(PPh3)2Rh(OSiMe3)]2.9 The
absence of reaction between 5 or 6 and PPh3 may result from the
steric bulk of the phenyl and tert-butoxy groups, and/or the lower
donating nature of PPh3 (compared to PMe3).
Hydrolysis and alcoholysis of 1
Most alkoxy(siloxy) derivatives of transition metals are hydro-
lyzed irreversibly by even traces of moisture. To characterize the
Fig. 1 Variable-temperature 1H NMR spectra of 2 in toluene-d8.
D a l t o n T r a n s . , 2 0 0 4 , 2 8 0 8 – 2 8 1 3
2 8 0 9

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hydrolytic stability of the Rh–O–Si linkages in 1, we examined its
reaction with water. A sample of 1 and a ferrocene standard were
dissolved in benzene-d6 and hydrolyzed by the addition of 5 L of
water. The hydrolysis products, identified by 1H NMR spectroscopy,
were HOSi(OtBu)325 (2.0 equiv) and the hydroxo-bridged complex
[(cod)Rh(-OH)]2.26 The facile hydrolysis of the Rh–O–Si bonds
suggests that the presence of water could influence the stability of
highly dispersed rhodium species on a silica surface. This tendency
is also supported by the reported behavior of the hydroxo complex
Cp*Ir(OH)(Ph)(PMe3),27 which easily binds to a silica surface by a
reversible interaction with a surface silanol group.
scanning calorimetry (DSC) under flowing nitrogen revealed the
presence of two endotherms at 210 and 230 °C followed by a broad
exotherm above 250 °C. The first endotherm is attributed to the
decomposition of 1, but the second one has not been characterized.
The broad exotherm is probably associated with elimination of the
organic components in 1.
As might be expected, thermolysis of 2 proceeded similarly to
that of 1. The TGA trace for 2 under a flow of nitrogen (Fig. 3)
reveals an abrupt weight loss above 190 °C and a yield of 41.4% at
260 °C corresponding to the loss of silanol. Further heating led to a
gradual weight loss and a final yield of 24.0% at 1000 °C, which is
considerably less than the theoretical yield for RhO0.5·SiO2 (37.3%),
but lies between those for RhO0.5 and Rh (24.2 and 22.5%, respec-
tively). Under oxygen, the decomposition is more gradual with
precipitous weight loss initiating at ca. 180 °C. This decomposition
is complete by 400 °C, resulting in a yield of 24.2% correspond-
ing to the formation of RhO0.5. Further heating under oxygen to
1000 °C led to a weight gain with a final yield of 27.3%, which is
in good agreement with the theoretical yield for RhO1.5 (27.7%).
The differential thermal analysis (DTA) curve for 2 reveals a highly
exothermic reaction associated with decomposition under oxygen.
This phenomenon may be explained by the combustion of organic
residues under oxygen, catalyzed by the presence of Rh species.
Reaction of 1 with excess MeOH (0.02 mL) in benzene-d6 led
to a complete displacement of –OSi(OtBu)3 by –OMe ligands, and
28
formation of [(cod)Rh(-OMe)]2 and HOSi(OtBu)3.25 This is in-
teresting in light of the enhanced acidity of silanols over alcohols,
which has been demonstrated theoretically and experimentally.29
In addition, some metal tert-butoxysiloxy complexes were synthe-
sized from silanolysis reactions of metal alkoxide complexes and
HOSi(OtBu)3. For example, [CuOSi(OtBu)3]4 was prepared from
[Cu(-OtBu)]4,13a and (tBuO)3CrOSi(OtBu)3 was synthesized from
Cr(OtBu)4.12h Moreover, the –OMe ligands in [(cod)Rh(-OMe)]2
are known to be labile and susceptible to exchange reactions with
other alcohols (nPrOH, nBuOH and tBuOH) under mild conditions.30
Thus, the facile reaction of 1 with methanol may be the result of the
excess amount of methanol, which drives the reaction towards the
formation of [(cod)Rh(-OMe)]2.
Thermolytic conversions of 1 and 2
Thermolyses of 1 and 2 were initially examined by thermogravimetric
analysis (TGA). Complexes 1 and 2 exhibit a rather high thermal sta-
bility relative to most other tert-butoxysiloxy complexes, perhaps due
to the presence of strong Rh–C bonds. The TGA trace for 1 under a
flow of nitrogen (Fig. 2) reveals an onset temperature at ca. 180 °C.
This decomposition resulted in a dark-gray solid in a yield of 22.1%
at 1000 °C. This solid was determined by powder X-ray diffraction
to contain crystallites of Rh. For comparison, the theoretical ceramic
yields for RhO0.5·SiO2, Rh·SiO2, RhO0.5, and Rh are 36.0, 34.4, 23.4,
and 21.7%, respectively. The reduction in the observed yield may
involve the loss of silanol from the precursor during the thermolysis.
Fig. 3 TGA trace of 2 under N2 with a heating rate of 10 °C min−1.
Thermolysis of 1 in toluene solution was carried out at 180 °C
in a sealed Parr reactor under a nitrogen atmosphere. Heating the
solution for 20 h led to formation of a black solid precipitate in a
brownish solution. The brownish solution contained HOSi(OtBu)3
(ca. 1.5 equiv), cod (ca. 0.2 equiv), and unidentified compound(s).
These results suggest that some of the silicon may be incorporated
into the solid precipitate. The solid precipitate was washed with
pentane and air-dried for 3 days. The solid was then ground into a
fine powder and placed under vacuum overnight at 120 °C to pro-
vide the as-prepared material, which is amorphous as determined by
powder X-ray diffraction (PXRD; Fig. 4). This as-prepared material
has a very low surface area (6 m2 g−1). Elemental analysis revealed
the rhodium to silicon ratio to be 1.23, which is consistent with the
loss of some silanol during the thermolysis. The organic content of
the as-prepared sample was relatively high (C: 14.5; H: 1.34%).
Calcination of this material under a flow of oxygen at 300 °C
(10 °C min−1) for 1 h removed most of the organic material and in-
duced the crystallization of cubic rhodium with an average particle
size of 22 nm, as approximated by the Scherrer equation.31
At this point, it is difficult to speculate insightfully about the
mechanism by which 1 and 2 undergo thermolytic elimination to
rhodium-containing materials. This process clearly involves the
elimination of silanol as it is formed as a byproduct in the thermo-
lyses of 1 and 2. These results indicate the distinct nature of 1 and 2
compared to that of tert-butoxysiloxy complexes of early-transition
metals such as Zr12b and Cr,12h which convert cleanly to metal
silicates via isobutylene elimination. Furthermore, the presence of
Fig. 2 TGA trace of 1 under N2 with a heating rate of 10 °C min−1.
1
The chemistry of thermolysis for 1 was further studied by H
NMR spectroscopy. The compound was heated at 200 °C under
vacuum in a J-Young tube, and the volatile decomposition products
were then vacuum-transferred to another NMR tube. Analysis of
1
these products by H NMR spectroscopy revealed the presence
of HOSi(OtBu)3 and HOtBu in a ratio of 6:1. These results sug-
gest that 1 undergoes pyrolytic elimination to rhodium-containing
materials by a different pathway from those responsible for the
clean conversions of electropositive metal tert-butoxysiloxy com-
plexes via isobutylene elimination.12 Analysis of 1 by differential
2 8 1 0
D a l t o n T r a n s . , 2 0 0 4 , 2 8 0 8 – 2 8 1 3

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(C, H). The starting material KOSi(OtBu)3 was prepared according
to the literature procedure.15 [(cod)RhCl]2 and [(nbd)RhCl]2 were
purchased from Strem and recrystallized from CH2Cl2/Et2O before
use. All other reagents were purchased from Aldrich and used as
received, unless stated otherwise.
Preparation of the siloxide complexes
{(cod)Rh[-OSi(OtBu)3]}2 (1). A solid mixture of [(cod)RhCl]2
(0.500 g, 1.01 mmol) and KOSi(OtBu)3 (0.614 g, 2.03 mmol) was
dissolved in Et2O (50 mL). The reaction mixture was stirred at
ambient temperature for 3 h. The volatile materials were removed
under vacuum and the product was extracted from the KCl with
pentane (3 × 20 mL). Concentration and cooling to −30 °C
afforded 1 as orange crystals (0.790 g, 82%). Anal. found: C, 50.8;
H, 8.47. C40H78O8Rh2Si2 requires: C, 50.6; H, 8.28%. IR (Nujol
mull, CsI, cm−1): 1363s, 1327m, 1300m, 1242s, 1213s, 1196s,
1157sh, 1056br s, 1038br s, 1001s, 968br s, 914sh, 893w, 879w,
872sh, 823m, 804w, 783w, 721sh, 696s, 621w, 588w, 517s, 501s,
Fig. 4 Room temperature PXRD patterns of Rh/Si/O materials derived
from 1, before and after calcination for 1 h under a flow of oxygen at
300 °C.
1
465sh, 432m. H NMR (500 MHz, benzene-d6, 20 °C):  4.74 (s,
rhodium–carbon bonds may complicate the pyrolytic conversion.
The appearance of both rhodium metal and silanol indicates that
Rh–O homolysis may be involved. This mechanistic step was also
suggested by Terry et al. in studies of copper tert-butoxysiloxy
complexes.13a
4H, CH), 4.56 (s, 4H, CH), 2.83 (m, 4H, exo CH2), 2.42 (m,
4H, exo CH2), 1.82 (m, 4H, endo CH2), 1.66 (m, 4H, endo CH2),
1.54 (s, 54H, OCMe3). 13C{1H} NMR (125.8 MHz, benzene-d6,
20 °C):  76.1 (d, JRhC = 15.1 Hz, CH), 72.5 (s, OCMe3), 72.0
(d, JRhC = 14.7 Hz, CH), 32.5 (s, OCMe3), 31.8 (s, CH2), 30.5
(s, CH2). 29Si NMR (99.35 MHz, benzene-d6, 20 °C):  −94.59 (s,
OSi(OtBu)3).
Concluding remarks
We have described the syntheses of the rhodium(I) siloxide com-
plexes, {(diene)Rh[-OSi(OtBu)3]}2, where diene = cod (1), nbd (2).
These compounds provide good models for dinuclear rhodium(I)
species on silica. Both 1 and 2 are dimeric and their Rh2O2 cores
are bent in a butterfly fashion along the O–O axis as inferred from
their NMR spectroscopic data. The monomeric siloxide complexes
3–7 were also prepared by reactions of 1 or 2 with various tertiary
phosphines. These compounds may also serve as useful models for
mononuclear rhodium(I) complexes on a silica surface, owing to
their Rh–O–Si linkage. The behavior of 1 towards water and metha-
nol was also studied. The facile hydrolysis of the Rh–OSi bond sug-
gests that the presence of water could greatly influence the stability
of highly dispersed rhodium particles on the silica surface.
{(nbd)Rh[-OSi(OtBu)3]}2 (2). A solid mixture of [(nbd)RhCl]2
(0.250 g, 0.543 mmol) and KOSi(OtBu)3 (0.329 g, 1.09 mmol) was
dissolved in Et2O (50 mL). The reaction mixture was stirred at am-
bient temperature for 3.5 h. The volatile materials were removed
under vacuum and the product was extracted from the KCl with
pentane (3 × 20 mL). Concentration and cooling to −30 °C af-
forded 2 as orange crystals (0.378 g, 76%). Anal. found: C, 49.9;
H, 7.82. C38H70O8Rh2Si2 requires: C, 49.8; H, 7.70%. IR (Nujol
mull, CsI, cm−1): 1362s, 1302m, 1240s, 1215s, 1196s, 1174s,
1155m, 1060br s, 1028s, 991sh, 966br s, 918m, 887w, 854w,
823m, 795m, 775w, 721sh, 696s, 623w, 569m, 522m, 492m,
1
476sh, 461sh, 434m. H NMR (500 MHz, benzene-d6, 20 °C): 
The high-yielding, convenient syntheses of 1 and 2 make these
complexes good candidates for single-source molecular precursors,
via thermolytic molecular precursor conversions, to Rh/Si/O
materials. However, the thermolyses of 1 and 2 led to Rh–O–Si
cleavage and formation of rhodium metal at low temperature. Even
though 1 and 2 appear to be poor single-source precursors for
highly dispersed mixed-element materials, they may prove useful
for deposition of rhodium onto silica surfaces. Preliminary results
suggested that 1 reacted with the hydroxyl groups on mesoporous
SBA-15 surface via siloxide displacement to give a 2.6 wt.% Rh
loading, or 0.19 Rh nm−2. Much of the interest in materials of this
type centers around their use in catalysts. Further investigations
on the nature of these rhodium surface species, and their catalytic
properties, are ongoing.
4.36–3.40 (br m, 12H, CH, –CH), 1.52 (s, 54H, OCMe3), 1.12
(s, 4H, CH2). 13C{1H} NMR (125.8 MHz, benzene-d6, 20 °C): 
72.0 (s, OCMe3), 60.2 (d, JRhC = 7.55 Hz, CH), 49.4 (br, CH,
CH2), 44.9 (br, CH), 32.3 (s, OCMe3). 29Si NMR (99.35 MHz,
benzene-d6, 20 °C):  −91.06(s, OSi(OtBu)3). 1H NMR (500 MHz,
toluene-d8, −20 °C):  4.40 (s, 4H, CH), 4.00 (s, 2H, CH), 3.93
(s, 4H, CH), 3.58 (s, 2H, CH), 1.53 (s, 54H, OCMe3), 1.15 (s, 4H,
CH2). 13C{1H} NMR (125.8 MHz, toluene-d8, −60 °C):  71.8 (s,
OCMe3), 60.1 (m, CH), 49.9 (m, CH2), 49.7 (s, CH), 48.8 (s, CH),
44.7 (m, CH), 32.0 (s, OCMe3).
(cod)Rh[OSi(OtBu)3](PMe3) (3). To a 25 mL pentane solution
of 1 (0.188 g, 0.198 mmol) was added 0.040 mL (0.39 mmol)
of PMe3. The mixture was stirred at room temperature for 1 h.
The volatile materials were removed in vacuo to afford a yellow
powder, which was redissolved in pentane. Concentration and cool-
ing to −78 °C afforded 3 as yellow crystals (0.165 g, 77%). Anal.
found: C, 50.4; H, 8.87. C23H48O4PRhSi requires: C, 50.2; H, 8.79%.
1H NMR (400 MHz, benzene-d6, 20 °C):  6.00 (s, 2H, CH), 2.82
(s, 2H, CH), 2.27 (m, 2H, exo CH2), 2.24 (m, 2H, endo CH2), 1.82
(m, 2H, exo CH2), 1.80 (m, 2H, endo CH2), 1.59 (s, 27H, OCMe3),
Experimental
All manipulations were performed under an atmosphere of nitrogen
with standard Schlenk techniques and/or in a Vacuum Atmospheres
dry-box. Dry oxygen-free solvents were used throughout. Calcina-
tions were performed with a Lindberg 1200 °C three-zone tube
furnace. NMR spectra were recorded on Bruker AVQ400 and
DRX500 instruments. Infrared spectra were acquired as Nujol
mulls with CsI cells on a Mattson Infinity Series FTIR spectrometer.
Surface area and pore volume analyses were measured by using the
BET method on a QuantaChrome Autosorb-1 surface area analyzer
with all samples heated at 120 °C, under vacuum, for a minimum of
2 h immediately prior to data collection. Powder X-ray diffraction
was performed on a Siemens D5000 diffractometer with Cu  radia-
tion of wavelength 1.54056 Å. Elemental analyses were performed
by Galbraith Laboratories (Rh, Si) or by the College of Chemistry’s
Microanalytical Facility at the University of California, Berkeley
13
0.90 (d, JPH = 9.50 Hz, 9H, PMe3). C{1H} NMR (125.8 MHz,
benzene-d6, 20 °C):  105.2 (m, CH), 70.9 (s, OCMe3), 61.0 (d,
JRhC = 14.1 Hz, CH), 33.8 (s, CH2), 32.3 (s, OCMe3), 28.6 (s,
CH2), 12.9 (d, JPC = 25.2 Hz, PMe3). 31P{1H} NMR (162.0 MHz,
benzene-d6, 20 °C):  −8.60 (d, JRhP = 158 Hz, PMe3).
(nbd)Rh[OSi(OtBu)3](PMe3) (4). To a 25 mL pentane solution
of 2 (0.089 g, 0.097 mmol) was added 0.020 mL (0.19 mmol) of
PMe3. The mixture was stirred at room temperature for 3 h. The
volatile materials were removed in vacuo to afford a yellow powder
D a l t o n T r a n s . , 2 0 0 4 , 2 8 0 8 – 2 8 1 3
2 8 1 1

View Article Online
(0.075 g, 73%). 1H NMR (500 MHz, benzene-d6, 20 °C):  5.78 (s,
2H, CH), 3.49 (s, 2H, CH), 2.73 (s, 2H, CH), 1.58 (s, 27H,
OCMe3), 1.42 (br, 2H, CH2), 0.74 (d, JPH = 9.00 Hz, 9H, PMe3).
13C{1H} NMR (125.8 MHz, benzene-d6, 20 °C):  89.3 (m, CH),
70.8 (s, OCMe3), 63.3 (s, CH), 50.8 (s, CH2), 41.9 (m, CH), 32.3 (s,
OCMe3), 12.5 (d, JPC = 25.2 Hz, PMe3). 31P{1H} NMR (162.0 MHz,
benzene-d6, 20 °C):  −6.78 (d, JRhP = 162 Hz, PMe3).
DE-AC03-76SF00098. J. J. thanks the Royal Thai Government for
support with a DPST scholarship. We thank Professor A. Stacy for
the use of instrumentation (PXRD).
References
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1
requires: C, 61.9; H, 7.39%. H NMR (400 MHz, benzene-d6,
20 °C):  7.93 (m, 6H, PPh3), 7.07 (m, 9H, PPh3), 6.47 (br m, 2H,
CH), 2.76 (br s, 2H, CH), 2.41 (m, 2H, exo CH2), 2.26 (m,
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PPh3), 130.0 (s, PPh3), 104.6 (m, CH), 70.8 (s, OCMe3), 64.4
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JRhP = 162 Hz, PPh3).
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yellow powder (0.107 g, 69%). Anal. found: C, 61.9; H, 7.21.
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1
C37H50O4PRhSi requires: C, 61.7; H, 6.99%. H NMR (400 MHz,
benzene-d6, 20 °C):  7.81 (m, 6H, PPh3), 7.05 (m, 9H, PPh3), 6.12
(s, 2H, CH), 3.43 (s, 2H, CH), 2.49 (s, 2H, CH), 1.51 (s, 27H,
OCMe3), 1.13 (d, JHH = 8.4 Hz, 1H, CH2), 1.01 (d, JHH = 8.4 Hz,
1H, CH2). 13C{1H} NMR (100.6 MHz, benzene-d6, 20 °C):  135.0
(d, JPC = 12.1 Hz, PPh3), 132.8 (d, JPC = 39.2 Hz, PPh3), 129.9 (s,
PPh3), 86.4 (m, CH), 70.9 (s, OCMe3), 63.0 (s, CH), 52.1 (s,
CH2), 44.4 (d, JRhC = 11.1, CH), 32.3 (s, OCMe3). 31P{1H} NMR
(162.0 MHz, benzene-d6, 20 °C):  28.7 (d, JRhP = 184 Hz, PPh3).
Rh[OSi(OtBu)3](PMe3)3 (7). To a 30 mL pentane solution of 1
(0.090 g, 0.095 mmol) was added 0.060 mL (0.58 mmol) of PMe3.
The mixture was stirred at room temperature for 3 h. The volatile
materials were removed in vacuo. The resulting product was then
redissolved in pentane. Concentration and cooling to −78 °C af-
forded orange crystals (0.121 g, 91%).Anal. found: C, 42.6; H, 9.39.
C21H54O4P3RhSi requires: C, 42.4; H, 9.15%. 1H NMR (400 MHz,
benzene-d6, 20 °C):  1.68 (s, 27H, OCMe3), 1.31 (m, 18H, PMe3),
1.01 (d, JPH = 8.00 Hz, 9H, PMe3). 13C{1H} NMR (125.8 MHz,
benzene-d6, 20 °C):  70.5 (s, OCMe3), 32.7 (s, OCMe3), 22.2
(d, JPC = 25.2 Hz, PMe3), 18.2 (d, JPC = 26.4 Hz, PMe3), 17.6 (d,
JPC = 69.2 Hz, PMe3). 31P{1H} NMR (162.0 MHz, benzene-d6,
20 °C):  5.59 (td, JPP = 50.2, JRhP = 164 Hz, cis-PMe3), −13.9 (dd,
JPP = 50.2, JRhP = 144 Hz, trans-PMe3).
Solution thermolysis of 1
A 20 mL Parr reactor was charged with a toluene (5.0 mL) solu-
tion of 1 (0.401 g, 0.423 mmol) in a dry-box. The reactor was
then placed in a preheated oven at 180 °C for 20 h. The resulting
precipitate was isolated by filtration, washed with pentane (3 ×
25 mL) and air-dried for 3 days, yielding 0.120 g (30%) of a solid
powder.
Acknowledgements
This work was supported by the Director, Office of Energy
Research, Office of Basic Energy Sciences, Chemical Sciences
Division, of the U. S. Department of Energy under Contract No.
2 8 1 2
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D a l t o n T r a n s . , 2 0 0 4 , 2 8 0 8 – 2 8 1 3
2 8 1 3