Benzene hydrogenation by silica-supported catalysts made of palladium nanoparticles and electrostatically immobilized rhodium single sites

Organometallics 2008, 27, 2809–2824

2809

Benzene Hydrogenation by Silica-Supported Catalysts Made of

Palladium Nanoparticles and Electrostatically Immobilized Rhodium

Single Sites

Pierluigi Barbaro,^†Claudio Bianchini,*^,†Vladimiro Dal Santo,^‡Andrea Meli,^†

Simonetta Moneti,^†Claudio Pirovano,^§Rinaldo Psaro,*^,‡Laura Sordelli,^‡and

Francesco Vizza^†

ICCOM-CNR, Area di Ricerca CNR di Firenze, Via Madonna del Piano 10, 50019 Sesto Fiorentino,

Firenze, Italy, ISTM-CNR, Via C. Golgi 19, 20133 Milano, Italy, and Dipartimento CIMA, UniVersità degli

Studi di Milano, Via G. Venezian 21, 20133 Milano, Italy

ReceiVed March 10, 2008

The complex [Rh(cod)(dppp)]OTf (Rh(cod)) has been immobilized onto silica-supported palladium

nanoparticles (Pd/SiO₂) via a dual H-bond/ionic interaction (dppp ) 1,3-bis(diphenylphosphino)propane;

cod ) cycloocta-1,5-diene). The product obtained, Rh(cod)-Pd/SiO₂, has been employed to catalyze the

hydrogenation of benzene to cyclohexane, showing much higher activity as compared to Pd/SiO₂, while

Rh(cod) grafted on bare silica (Rh(cod)/SiO₂) is totally inactive. The catalyst generated by Rh(cod)-Pd/

SiO₂exhibits a remarkable stability and can be recycled several times with no loss of activity, even if

exposed to air. In situ and ex situ EXAFS and DRIFTS measurements, batch catalytic reactions under

different conditions, deuterium labeling experiments, and model organometallic studies, taken altogether,

have provided valuable mechanistic information. The reduction of benzene to cyclohexa-1,3-diene occurs

with the cooperation of the two metals, while the rhodium single sites are more effective than the palladium

nanoparticles in the hydrogenation of cyclohexa-1,3-diene to cyclohexane.

Scheme 1

Introduction

Tethered complexes on supported metals (TCSM) constitute

a particular class of hybrid catalysts that can be used in a number

of heterogeneous processes such as arene hydrogenation,^1–4

hydrodeﬂuorination of ﬂuoroarenes,⁵alkene hydroformylation,⁶

hydrodechlorination of chlorophenols,⁷and direct reduction of

CdO functions to CH₂without the intermediacy of carbynols.⁸

While the supported metals are generally obtained by standard

procedures, involving impregnation of porous oxides with metal

* To whom correspondence should be addressed. E-mail:

claudio.bianchini@iccom.cnr.it.

^†ICCOM-CNR.

salts followed by calcination/reduction, the immobilization of

the metal complexes onto the support material has been so far

achieved by three distinct methods: (a) Covalent grafting of a

remote part of the ligand;^1,3,5,6(b) H-bond grafting of a remote

part of the ligand;^1,2(c) sol–gel entrapment of both metal

complexes and metal particles.^4,7,8Simpliﬁed sketches of TCSM

catalysts of the types a and b, containing palladium nanoparticles

and Rh^I(cod) moieties (cod ) cycloocta-1,5-diene) immobilized

onto silica, are shown in Scheme 1.^1–3,5,6

Irrespective of the preparation procedure and of the combina-

tion of metals, the known TCSM catalysts show higher activity

in a number of hydrogenation reactions as compared to the

separate components under comparable experimental conditions.

Angelici proposed that the enhanced activity of TCSM catalysts

may be due to a hydrogen spillover process,⁹promoted by the

supported metallic phase, that would enhance speciﬁcally the

activity of the molecular catalyst.³A study of the hydrogenation

of arenes with a catalyst obtained by silica sol–gel coentrapment

of metallic palladium and [Rh(cod)(µ-Cl)]₂disagreed with the

^‡ISTM-CNR.

^§Dip. CIMA.

(1) Bianchini, C.; Meli, A.; Vizza, F. In The Handbook of Homogeneous

Hydrogenation; de Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim,

Germany, 2007; Vol. 1, pp 455–488.

(2) (a) Barbaro, P.; Bianchini, C.; Dal Santo, V.; Meli, A.; Moneti, S.;

Psaro, R.; Scafﬁdi, A.; Sordelli, L.; Vizza, F. J. Am. Chem. Soc. 2006,

128, 7065. (b) Bianchini, C.; Dal Santo, V.; Meli, A.; Moneti, S.; Moreno,

M.; Oberhauser, W.; Psaro, R.; Sordelli, L.; Vizza, F. Angew. Chem., Int.

Ed. 2003, 42, 2636.

(3) (a) Yang, H.; Gao, H.; Angelici, R. J. Organometallics 2000, 19,

622. (b) Gao, H.; Angelici, R. J. Organometallics 1999, 18, 989. (c) Perera,

M. A. D. N.; Angelici, R. J. J. Mol. Catal. A: Chem. 1999, 149, 99. (d)

Gao, H.; Angelici, R. J. J. Mol. Catal. A: Chem. 1999, 149, 63. (e) Gao,

H.; Angelici, R. J. J. Mol. Catal. A: Chem. 1999, 145, 83. (f) Gao, H.;

Angelici, R. J. New J. Chem. 1999, 23, 633. (g) Gao, H.; Angelici, R. J.

J. Am. Chem. Soc. 1997, 119, 6937.

(4) Abu-Reziq, R.; Avnir, D.; Miloslavski, I.; Schumann, H.; Blum, J.

J. Mol. Catal. A: Chem. 2002, 185, 179.

(5) Yang, H.; Gao, H.; Angelici, R. J. Organometallics 1999, 18, 2285.

(6) Gao, H.; Angelici, R. J. Organometallics 1998, 17, 3063.

(7) (a) Bovkun, T. T.; Sasson, Y.; Blum, J. J. Mol. Catal. A: Chem.

2005, 242, 68. (b) Ghattas, A.; Abu-Reziq, R.; Avnir, D.; Blum, J. Green

Chem. 2003, 5, 40.

(8) Abu-Reziq, R.; Avnir, D.; Blum, J. J. Mol. Catal. A: Chem. 2002,

187, 277.

(9) Delmon, B.; Fromont, G. F. Catal. ReV. Sci. Eng. 1996, 38, 69.

10.1021/om800223d CCC: $40.75

2008 American Chemical Society

Publication on Web 05/14/2008

2810 Organometallics, Vol. 27, No. 12, 2008

Barbaro et al.

Scheme 2

Scheme 3

hydrogen spillover hypothesis and suggested that the action of

both metals is caused by a sort of synergic effect for which no

clear-cut explanation was provided, however.⁴The existence

of a cooperative effect was later demonstrated and rationalized

through an in-depth study of benzene hydrogenation to cyclo-

hexane using the Rh^I-Pd⁰catalyst precursor Rh(cod)(sulfos)-

Pd/SiO₂shown in Scheme 1b.²It was proposed that the Rh^I

single sites and the Pd⁰particles form a unique, stable entity

throughout the catalytic cycle. Besides decreasing the extent of

cyclohexa-1,3-diene disproportionation at palladium, the com-

bined action of the two metals would activate the arene so as

to allow the rhodium sites to enter the catalytic cycle and speed

up the overall hydrogenation process by rapidly reducing

benzene to cyclohexa-1,3-diene.

In this paper is reported a much simpler method to prepare

TCSM catalysts, which can be extended to any cationic metal

complex with no modiﬁcation of the primitive molecular

structure of the ligand, provided the counteranion is able to form

H-bonds to the support material. To this purpose, an effective

counteranion is the triﬂate ion (CF₃SO₃^-) OTf^-), as it contains

up to three oxygen atoms suitable for H-bonding the surface

hydroxyl groups of supports such as silica or alumina.^10b,11The

new procedure simply requires stirring a solution of a cationic

metal complex bearing the triﬂate counteranion in the presence

of silica-supported metal particles. It is noteworthy that the

immobilization of the metal complex involves two different

bonding interactions: the triﬂate ions form a net of H-bonds to

the surface silanol groups of silica, thus generating a layer of

negative charges, each of which attracts electrostatically the

complex cation (Scheme 2).

measurements, batch catalytic reactions under different condi-

tions, deuterium labeling experiments, and model organometallic

reactions. In light of the results obtained, a reaction mechanism,

containing a number of unusual intermediates as well as

interactions between metal species and the support, is proposed.

Experimental Section

General Considerations. All reactions and manipulations were

routinely performed under a nitrogen or argon atmosphere by using

standard Schlenk techniques, unless stated otherwise. CH₂Cl₂(im-

pregnation procedures) was dried over molecular sieves under argon.

Benzene and n-pentane (catalytic reactions) were distilled prior to use

over Na and LiAlH₄, respectively. The rhodium complex [Rh(cod)(µ-

Cl)]₂was prepared as previously described.¹²All the other reagents

and chemicals were reagent grade and were used as received from

commercial suppliers. The Davison 332 (Grace) silica employed in

this work was a high-surface-area hydrophilic mesoporous nonordered

material. The support was ground, washed with 1 M HNO₃and then

with distilled water to neutrality, and dried overnight in an oven at

100 °C. The porosimetry and the surface area were determined by

nitrogen adsorption. Nitrogen adsorption/desorption isotherms at liquid

nitrogen temperature were measured on a Micromeritics ASAP 2010

instrument. All silica samples were routinely preoutgassed at 300 °C.

The average pore radius (9.70 nm) and speciﬁc pore volume (1.43

cm³/g) were calculated according to the Barret-Joyner-Halenda

(BJH) theory.¹³The speciﬁc surface area (295 m²/g) was obtained

using the Brunauer–Emmett–Teller (BET) equation.¹⁴The palladium

and rhodium contents in the tethered catalysts were determined by

inductively coupled plasma atomic emission spectroscopy (ICP-AES)

with a Intrepid Iris instrument (Thermo Elemental). Each sample

(20–50 mg) was treated in a microwave-heated digestion bomb

(Milestone, MLS-200) with concentrated HNO₃(1.5 mL), 98% H₂SO₄

(2 mL), and a pellet (0.4 g) of a digestion aid reagent (0.1% Se in

K₂SO₄). After the silica particles were ﬁltered off, the solutions were

analyzed. The addition of selenium was necessary to get an effective

digestion of the phosphine ligand, which was hardly achievable by

usual acid dissolution procedures. The same digestion method was

employed to determine the metal contents in the products recovered

after catalysis and in the organic solutions. Batch reactions under a

controlled pressure of hydrogen were performed with a stainless steel

Parr 4565 reactor (100 mL) equipped with a Parr 4842 temperature

and pressure controller and a paddle stirrer. GC analyses of the

solutions were performed on a Shimadzu GC-14 A gas chromatograph

equipped with a ﬂame ionization detector and a 30 m (0.25 mm i.d.,

0.25 µm ﬁlm thickness) SPB-1 Supelco fused silica capillary column.

GC/MS analyses were performed on a Shimadzu QP5000 apparatus

equipped with an identical capillary column. Deuterated solvents for

NMR measurements (Aldrich) were dried over molecular sieves. ¹H

(400.13 MHz), ¹³C{¹H} (100.613 MHz), ²H{¹H} (61.423 MHz), and

³¹P{¹H} (161.98 MHz) NMR spectra were recorded on a Bruker

Avance DRX-400 spectrometer equipped with a temperature controller

accurate to (0.1 °C. Chemical shifts are relative to external TMS (¹H

The resulting H-bond/ionic immobilization of metal com-

plexes is very robust in aprotic media and has been successfully

applied to enantioselective hydrogenation reactions by chiral

catalysts tethered to bare silica.^10b,11

Herein, we speciﬁcally describe the synthesis and character-

ization of a new TSCM catalyst obtained by H-bond/ionic

grafting the Rh^Icomplex [Rh(cod)(dppp)]OTf (Rh(cod)) to

silica-supported palladium nanoparticles (Pd/SiO₂) (dppp ) 1,3-

bis(diphenylphosphino)propane). The product obtained, Rh(cod)-

Pd/SiO₂(Scheme 3a), has been employed to catalyze the

hydrogenation of benzene to cyclohexane, showing much higher

activity as compared to Pd/SiO₂, while Rh(cod) grafted on bare

silica (Rh(cod)/SiO₂, Scheme 3b) was totally inactive.

The existence of a synergic action involving the rhodium

single sites and the palladium particles has been proved and

rationalized by means of in situ and ex situ EXAFS and DRIFTS

(10) (a) Bianchini, C.; Dal Santo, V.; Meli, A.; Moneti, S.; Moreno,

M.; Oberhauser, W.; Psaro, R.; Soredelli, L.; Vizza, F. J. Catal. 2003, 213,

47. (b) Bianchini, C.; Barbaro, P.; Dal Santo, V.; Gobetto, R.; Meli, A.;

Oberhauser, W.; Psaro, R.; Vizza, F. AdV. Synth. Catal. 2001, 343, 41. (c)

Bianchini, C.; Dal Santo, V.; Meli, A.; Oberhauser, W.; Psaro, R.; Vizza,

F. Organometallics 2000, 19, 2433. (d) Bianchini, C.; Burnaby, D. G.;

Evans, J.; Frediani, P.; Meli, A.; Oberhauser, W.; Psaro, R.; Sordelli, L.;

Vizza, F. J. Am. Chem. Soc. 1999, 121, 5961.

(12) Bianchini, C.; Frediani, P.; Sernau, V. Organometallics 1995, 14,

5458.

(11) (a) Raja, R.; Thomas, J. M.; Jones, M. D.; Johnson, B. F. G.;

Vaughan, D. E. W. J. Am. Chem. Soc. 2003, 125, 14982. (b) de Rege,

F. M.; Morita, D. K.; Ott, K. C.; Tumas, W.; Broene, R. D. Chem. Commun.

2000, 1797.

(13) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc.

1951, 73, 373.

(14) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938,

60, 309.

Benzene Hydrogenation by Silica-Supported Catalysts

Organometallics, Vol. 27, No. 12, 2008 2811

and ¹³C) or 85% H₃PO₄(³¹P) with downﬁeld values reported as

positive. The stereochemistry of the benzene hydrogenation was

assessed by ¹³C{¹H} and ²H{¹H} NMR analysis of the reaction

mixtures obtained by hydrogenation of C₆H₆-d₆(ca. 80% conversion)

with Pd/SiO₂(1.93 wt % Pd) or Rh(cod)-Pd/SiO₂(0.75–1.91 wt %

Rh-Pd). Experimental ¹³C{¹H} NMR spectra were computer simu-

lated using the gNMR program.^{15 1}H (200.13 MHz) and ³¹P{¹H}

(81.01 MHz) high-pressure NMR (HPNMR) experiments were carried

out on the Bruker ACP 200 instrument. The high-pressure 10 mm

OD sapphire NMR tube was purchased from Saphikon (Milford, NH),

while the titanium high-pressure charging head was constructed at the

ICCOM-CNR.¹⁶Note: Since high gas pressures are inVolVed, safety

precautions must be taken at all stages of studies inVolVing high-

pressure NMR tubes. Elemental analyses (C, H, N) were performed

using a Carlo Erba model 1106 elemental analyzer. Infrared spectra

of molecular compounds were recorded on a Perkin-Elmer Spectrum

BX FT-IR.

Preparation of Pd/SiO₂. Silica-supported palladium nanopar-

ticles, usually 5 g batches (Pd content ranging from 1.66 to 2.08

wt %), were prepared from PdCl₂/SiO₂by the following treatment:

(1) calcination at 500 °C for 1 h in O₂ﬂow (40 mL/min) at a heating

rate of 10 °C/min, followed by cooling in argon ﬂow (40 mL/min)

to room temperature; (2) reduction at 300 °C for 1 h in H₂ﬂow

(40 mL/min) at a heating rate of 10 °C/min followed by cooling in

argon ﬂow (40 mL/min) to room temperature. PdCl₂/SiO₂was, in

turn, prepared by impregnation of 5 g of silica with a solution of

PdCl₂(0.14–0.18 g) in deionized water (50 mL) and 36% HCl (2

mL). The suspension was maintained under stirring for 4 h, and

then the solvent was removed by evaporation under reduced

pressure. The solid residue was washed with a small amount of

water and dried under vacuum overnight.

temperature. The reaction was followed by variable-temperature

³¹P{¹H} and ¹H NMR spectroscopy. At room temperature, Rh(cod)

transformed selectively in 30 min into Rh(C₆H₆), which was the

only NMR-visible species at 40 °C for 1 h. The ¹H NMR spectrum

showed the formation of coa, while no trace of benzene hydroge-

nated products was detected by GC/MS. Signiﬁcant decomposition

of Rh(C₆H₆) occurred above 80 °C to give several unidentiﬁed

products, among which, a hydride compound in a trace amount

(¹H NMR multiplet at δ -22.71). However, no benzene hydrogena-

tion occurred even at the latter temperature.

Preparation of Rh(C₆H₆). A solution of Rh(cod) (0.40 g, 0.52

mmol) in a 3:1 (v/v) THF/benzene mixture (40 mL) was introduced

by suction into a 100 mL Parr reactor previously evacuated by a

vacuum pump. After pressurization with H₂to 30 bar, the reaction

mixture was stirred at room temperature for 30 min. Afterward,

the reactor was depressurized and vented under a nitrogen stream

and the contents were transferred into a Schlenk-type ﬂask.

Evaporating the solvent under vacuum gave Rh(C₆H₆) as a yellow

solid in 70% yield. Anal. Calcd (found) for C₃₄H₃₂F₃O₃P₂RhS

(742.53): C, 55.00; H 4.34. Found: C, 54.89; H 4.38. ¹H NMR

(CD₂Cl₂, 20 °C): δ 7.48 (m, 20H, P(C₆H₅)), 6.01 (s, 6H, C₆H₆),

2.55 (m, 4H, PCH₂CH₂), 1.93 (m, 2H, PCH₂CH₂). ³¹P{¹H}

(CD₂Cl₂, 20 °C): δ 25.32 (d, J_RhP) 191.1 Hz).

Preparation of [Rh(CO)₂(dppp)]OTf (Rh(CO)₂).¹⁸A solution

of Rh(cod) (0.40 g, 0.52 mmol) in THF (40 mL) was introduced

by suction into a 100 mL autoclave previously evacuated by a

vacuum pump. After pressurization with 5 bar of CO, the reaction

mixture was stirred for 30 min at room temperature. After the

reactor was depressurized and vented under a nitrogen stream, the

contents were transferred into a Schlenk-type ﬂask. Evaporating

the solvent under vacuum gave a yellow solid of Rh(CO)₂(90%

yield), which was washed with n-pentane. Anal. Calcd for

C₃₀H₂₆F₃O₅P₂RhS (720.44): C, 50.02; H 3.64. Found: C, 49.85; H

Preparation of [Rh(cod)(dppp)]OTf (Rh(cod)). Solid AgOTf

(0.11 g, 0.4 mmol) was added to a stirred solution of [Rh(cod)(µ-

Cl)]₂(0.10 g, 0.2 mmol) and dppp (0.17 g, 0.4 mmol) in CH₂Cl₂

(20 mL). After 15 min, AgCl was eliminated by ﬁltration and

ethanol (50 mL) was added. Partial evaporation of the solvents

under a steady stream of nitrogen gave yellow-orange crystals of

Rh(cod) in 90% yield, which were collected by ﬁltration and washed

with ethanol and n-pentane. Anal. Calcd for C₃₆H₃₈F₃O₃P₂RhS

1

3.70. H NMR (CD₂Cl₂, 20 °C): δ 7.48 (m, 20 H, P(C₆H₅)), 2.71

(m, 4H, PCH₂CH₂), 2.15 (m, 2H, PCH₂CH₂). ³¹P{¹H} (CD₂Cl₂,

20 °C): δ 3.33 (d, J_RhP) 113 Hz). IR (KBr): ν_CO2100, 2056 cm^-1

.

Preparation of [Rh(cod)(dppp)]OTf/SiO₂(Rh(cod)/SiO₂).

Samples with rhodium contents spanning from 0.5 to 1.5 wt %

were prepared following a known procedure.^10bIn particular, for

catalysts with a metal loading of ca. 1 wt %, 1 g of pretreated

silica was added to a stirred solution of 80 mg of Rh(cod) in 30

mL of anhydrous CH₂Cl₂under argon. After 5 h, the grafted

material was collected by ﬁltration on a Pyrex Büchner ﬁltering

funnel under argon. The solid product was washed three times with

10 mL portions of anhydrous CH₂Cl₂and kept under vacuum (10^-6

bar) overnight at room temperature. The samples were stored under

argon or nitrogen prior to use.

1

(772.60): C, 55.97; H, 4.96. Found: C, 55.70; H, 5.05. H NMR

(CD₂Cl₂, 20 °C): δ 7.5 (m, 20H, P(C₆H₅)), 4.62 (br s, 4H, CH cod),

2.72 (br s, 4H, CH₂cod), 2.5 (br s, 4H CH₂cod), 2.40 (m, 4H,

PCH₂CH₂), 2.07 (m, 2H, PCH₂CH₂). ³¹P{¹H} (CD₂Cl₂, 20 °C): δ

10.1 (d, J_RhP) 140 Hz).

In Situ HPNMR Study of the Reaction of Rh(cod) with H₂.

A 10 mm sapphire tube was charged with a solution of Rh(cod)

(0.05 g, 0.065 mmol) in THF-d₈(2 mL) under nitrogen. After

³¹P{¹H} and ¹H NMR spectra were recorded, the tube was

pressurized with 30 bar of H₂at room temperature. The reaction

was followed at room temperature by ³¹P{¹H} and ¹H NMR

spectroscopy. Rh(cod) disappeared in 30 min with the formation

of cyclooctane (coa) and several rhodium-dppp species, among

which there was the bis-solvento complex [Rh(solv)₂(dppp)]OTf

(solv ) THF, adventitious water; ³¹P{¹H} NMR: δ 40.1 (d, J_RhP

) 190.5 Hz); 35%).¹⁷

In Situ HPNMR Study of the Reaction of Rh(cod) with H₂in

the Presence of Benzene. Synthesis of [Rh(η⁶-C₆H₆)(dppp)]OTf

(Rh(C₆H₆)). A 10 mm sapphire tube was charged with a THF-d₈

(2 mL) solution of Rh(cod) (0.05 g, 0.065 mmol) and a 10-fold

excess of C₆H₆under nitrogen. After ³¹P{¹H} and ¹H NMR spectra

were recorded, the tube was pressurized with 30 bar of H₂at room

Preparation of [Rh(cod)(dppp)]OTf-Pd/SiO₂(Rh(cod)-Pd/SiO₂).

Samples of this bimetallic catalyst (0.07–1.07 wt % Rh and

1.58–1.91 wt % Pd) were prepared following a procedure analogous

to that reported above for the silica-tethered Rh^Icomplex Rh(cod)/

SiO₂using, as support material, Pd/SiO₂, previously passivated

under O₂ﬂow (40 mL/min) at room temperature for 30 min

followed by ﬂushing in argon ﬂow (40 mL/min) for 30 min, instead

of pretreated silica.

Preparation of [Rh(η⁶-C₆H₆)(dppp)]OTf/SiO₂(Rh(C₆H₆)/SiO₂).

This compound (1.05 wt % Rh) was prepared following a procedure

analogous to that reported above for Rh(cod)/SiO₂, using Rh(C₆H₆)

in place of Rh(cod) and a solvent mixture comprising anhydrous

CH₂Cl₂and benzene in a 4:1 (v/v) ratio.

Preparation of [Rh(η⁶-C₆H₆)(dppp)]OTf-Pd/SiO₂(Rh(C₆H₆)-

Pd/SiO₂). This bimetallic compound (0.70 wt % Rh and 1.86 wt

% Pd) was prepared following a procedure analogous to that

reported above for Rh(cod)-Pd/SiO₂, using Rh(C₆H₆) in place of

Rh(cod) and a solvent mixture comprising anhydrous CH₂Cl₂and

benzene in a 4:1 (v/v) ratio.

(15) Budzelaar, P. H. M. gNMR V4.0; Cherwell Scientiﬁc Publishing:

Oxford, 1995–1997 (Ivory Soft).

(16) Bianchini, C.; Meli, A.; Traversi, A. Ital. Pat. FI A000025, 1997.

(17) (a) Slack, D. A.; Greveling, I.; Baird, M. C. Inorg. Chem. 1979,

18, 3125. (b) Slack, D. A.; Baird, M. C. J. Organomet. Chem. 1977, 142,

C69.

(18) Betley, T. A.; Peters, J. C. Angew. Chem., Int. Ed. 2003, 42, 2385.

2812 Organometallics, Vol. 27, No. 12, 2008

Barbaro et al.

Table 1. Best-Fit EXAFS Data at the Rh K-Edge^a

entry

1

sample

shell

N

r (Å)

σ_DW(Å)

∆E₀

Rh(cod)

C

P

C

P

O

P

C

P

C

P

4

2

4

2

2.7

2

4

2

4

2

3.4

6

2

3.4

2

1.6

4

2

2.213

2.308

2.057

2.252

2.33

0.09

2.7

2

3.2

-4

13

0.059

0.064

0.082

0.078

0.063

0.065

0.091

0.061

0.078

0.073

0.092

0.086

0.077

0.098

0.069

0.067

0.11

2

3

4

5

6

7

Rh(cod)/SiO₂(0.71 wt % Rh)

Rh(cod)/SiO₂(0.71 wt % Rh) heated in situ under H₂(ﬂow rate of 50

mL/min)up to 120 at 5 °C/min and then at 120 °C for 1 h

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd)

2.26

6.6

4

2.087

2.246

2.084

2.251

2.275

2.741

2.088

2.272

2.74

-4

3.6

-2.3

0.2

-2.6

5.9

-1.7

0

1.7

-1.7

-1

0.7

1.3

-9

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) exposed to air at rt overnight

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) heated in situ under H₂(ﬂow

rate of 50 mL/min) up to 120 at 5 °C/min and then at 120 °C for 1 h

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) after catalysis to 33%

benzene conv.

P

Pd

C

P

Pd

O

P

Pd

C

P

8

9

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) after catalysis to 33%

benzene conv., exposure to air at rt overnight

1.98

2.246

2.783

2.07

2.3

2.655

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) after catalysis to 33%

benzene conv. and exposure to air at rt overnight, catalysis to 68%

benzene conv. (24 h)

0.068

0.087

0.099

Pd

3.8

^aSample sealed inside sample holder under argon.

Table 2. Best-Fit EXAFS Data at the Pd K-Edge^a

entry

1

sample

shell

N

r (Å)

σ_DW(Å)

∆E₀

d_mean(Å)

Pd/SiO₂(1.93 wt % Pd)

O

Pd

O

Pd

O

Pd

O

Pd

O

Pd

O

Pd

1.2

6

1.8

6

1.4

7.8

1.2

7.9

2

5.7

1.8

6

2.037

2.722

2.037

2.718

2.037

2.724

2.076

2.723

2.03

2.715

2.04

2.715

2.725

0.071

0.079

0.074

0.081

0.063

0.082

0.066

0.087

0.072

0.098

0.068

0.09

7.9

-1.2

8.4

2.5

7.9

1

15

2

3

4

5

6

7

Pd/SiO₂(1.93 wt % Pd) exposed to O₂ﬂow at rt for 30 min

15

18

15

30

Pd/SiO₂(1.93 wt % Pd) exposed to O₂ﬂow at rt for 30 min followed

by wetting in CH₂Cl₂

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd)

11

8

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) exposed to air at rt overnight

7.4

0.6

8.4

2.7

1.4

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) after catalysis to 33%

benzene conv., exposed to air at rt overnight

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) after catalysis to 33%

benzene conv. and exposure to air at rt overnight, catalysis to 68%

benzene conv. (24 h)

9.5

0.9

^aSample sealed inside sample holder under argon.

Diffuse Reﬂectance Infrared Fourier Transform Spectros-

copy (DRIFTS). Free and silica-supported metal complexes and their

derivatives obtained after appropriate treatments, (i.e., ex situ catalytic

runs, in situ reduction, etc.) were investigated by DRIFTS, using CO

as probe molecule. The spectra were recorded on a FTS-60A

spectrophotometer equipped with a homemade reaction chamber¹⁹and

a online quadrupole mass spectrometer (HPR-20 QIC gas analysis mass

spectrometer system/Hiden Analytical Ltd.). Unless otherwise stated,

all samples were handled under an argon atmosphere in a drybox (water

and oxygen levels below 0.1 ppm) containing a homemade reaction

chamber, which was closed and connected to the gas lines of a gas-

feeding apparatus. Before performing any CO chemisorption experi-

ment, the samples were maintained under He. CO chemisorption was

performed at the desired temperature for the desired time, followed

by purging in He ﬂow.

room temperature over a range of 1000 eV after the edge, with a

sampling step of 2 eV and 2–4 s integration per point. Each spectrum

was repeated three or four times for the signal-to-noise ratio optimiza-

tion and for the error bars evaluation. The data analysis was performed

with the FEFF7 software package.²⁰Experimental ꢀ(k) functions were

extracted from the absorption spectra with a standard procedure and

Fourier transformed over a k range 2.5–15 Å^-1. The main peaks of

the Fourier transformed modulus were ﬁltered and analyzed with a

nonlinear least-squares ﬁt program, which provided the atomic species,

the number N of nearest neighbors, their distance R from the absorber

atom, and the disorder σ_DWfactor for each shell. The single scattering

approximation was used in the theoretical signal modeling of the

oxygen and palladium shells at the Rh K-edge. Multiple scattering

was employed in the ﬁtting procedure of the ligands bound to the

rhodium atoms.

Extended X-ray Absorption Fine Structure Spectroscopy

(EXAFS). Measurements were performed at the Rh and Pd K-edges

on the samples reported in Tables 1 and 2. Palladium and rhodium

foils were employed as reference standards for the extraction of the

phase and amplitude experimental functions. The experiments were

carried out at the XAFS beamline of the Elettra synchrotron in Trieste,

with a double-crystal Si(311) monochromator and the ring operating

at 2.4 GeV. The spectra were recorded in transmission mode, on

samples sealed inside sample holders under a nitrogen atmosphere, at

Heterogeneous Benzene Hydrogenation Reactions Using Rh-

(cod)-Pd/SiO₂as Catalyst Precursor. The autoclave was charged

with the appropriate amount of a selected sample of Rh(cod)-Pd/

SiO₂, the desired amount of benzene, and n-pentane (complement

to 30 mL total volume). The ensemble was pressurized with H₂to

the desired pressure, heated to 40 °C, and then stirred at 1500 rpm.

After 2 h, the autoclave was cooled to ambient temperature and

depressurized. The liquid contents were analyzed by GC and GC/

(20) (a) Zabinsky, S. I.; Rehr, J. J.; Ankuninov, A.; Albers, R. C.; Eller,

M. J. Phys. ReV. B 1995, 52, 2995. (b) Mustre de Leon, J.; Rehr, J. J.;

Zabinsky, S. I.; Albers, R. C. Phys. ReV. B 1991, 44, 4146.

(19) Dal Santo, V.; Dossi, C.; Fusi, A.; Psaro, R.; Mondelli, C.; Recchia,

S. Talanta 2005, 66, 674.

Benzene Hydrogenation by Silica-Supported Catalysts

Organometallics, Vol. 27, No. 12, 2008 2813

Table 3. Hydrogenation of Benzene to Cyclohexane Catalyzed by Rh(cod)-Pd/SiO₂: Dependence on the Rhodium Content^a

cat. amnt

(mg)

Pd

Rh

benzene

benzene/Pd benzene/Rh

d

entry

cat. precursor

Pd/SiO₂

Rh(cod)-Pd/SiO₂

Rh(C₆H₆)-Pd/SiO₂

Rh(cod)/SiO₂

Pd wt % Rh wt % µmol µmol (mL/mmol)

ratio

yield (%)^bTOF^cTOF_Rh

1

2

3

4

5

6

7

8

9

47

1.93

1.87

1.91

1.86

8.5

8.3

8.4

8.2

0.430/4.818

0.417/4.672

0.426/4.773

0.430/4.818

0.417/4.672

0.426/4.773

565

566

587

3

21

23

25

27

35

0

72

491

537

597

644

843

0

0.07

0.27

0.75

1.07

0.70

0.65

0.08

0.70

0.3

1.2

3.4

4.9

3.2

3.0

0.4

3.2

14616

3789

1394

977

1507

1623

12789

1493

1535

436

174

132

264

0

Rh(cod)-Pd/SiO₂

9.86

43.5

107

110

36

52

841

1241

2302

388

^aExperimental conditions: n-pentane, complement to 30 mL total volume; H₂pressure, 30 bar; temperature, 40 °C; time, 2 h; stirring rate, 1500 rpm.

^bAverage values over at least three runs. ^cµmol of product h^{-1 d}mol of product (mol of Rh)^-1h^-1

.

Table 4. Hydrogenation of Benzene to Cyclohexane Catalyzed by Rh(cod)-Pd/SiO₂: Dependence on the Substrate Concentration, the Hydrogen

Pressure, and the Catalyst Amount^a

cat. amnt

(mg)

Pd

Rh

benzene

(mL/mmol)

benzene/Pd benzene/Rh

d

entry

Pd wt % Rh wt % µmol µmol

ratio

pH₂(bar) yield (%)^bTOF^cTOF_Rh

1

2

3

4

5

6

7

8

47

23

47

94

1.59

1.58

1.00

0.65

7.0

3.4

7.0

14.0

4.6

1.5

3.0

5.9

0.430/4.818

0.860/9.636

2.000/22.410

4.000/44.821

6.000/67.231

8.000/89.641

0.430/4.818

686

1055

2110

4907

9814

14721

19628

3317

1623

812

1.5

5

10

4

12

24

27

28

38

26

18

13

9.6

7.4

14

27

57

96

289

578

650

675

21

63

127

142

148

200

274

442

638

707

726

232

219

231

16

30

48

30

686

915

1372

3191

6383

9574

12765

1411

690

1253

2017

2913

3227

3317

337

9

10

11

12

13

14

650

1373

345

^aExperimental conditions: n-pentane, complement to 30 mL total volume; temperature, 40 °C; time, 2 h; stirring rate, 1500 rpm. ^bAverage values

over at least three runs. ^cµmol of product h^{-1 d}mol of product (mol of Rh)^-1h^-1

.

Table 5. Hydrogenation of Benzene to Cyclohexane Catalyzed by Rh(cod)-Pd/SiO₂: Recycling Tests^a

run

type

cat. amnt

(mg)

Pd

Rh

benzene

(mL/mmol)

benzene/Pd benzene/Rh

d

entry

Pd wt % Rh wt % µmol µmol

ratio

yield (%)^bTOF^cTOF_Rh

1

2

3

4

cycle

59

1.50

0.73

8.3

4.2

0.400/4.482

539

1071

27

29

32

41

605

650

717

919

145

155

171

220

recycle 1

recycle 2

recycle 3^e

^aExperimental conditions: n-pentane, complement to 30 mL total volume; H₂pressure, 30 bar; temperature, 40 °C; time, 2 h; stirring rate, 1500 rpm.

^bAverage values over at least three runs. ^cµmol of product h^-1

.

^dmol of product (mol of Rh)^-1h^{-1 e}The catalyst recovered from recycle 2 was

.

exposed to air for 2 days at room temperature before being employed in recycle 3.

previous paper.²In this work, only samples with Pd contents

of ca. 2 wt % have been used for both spectroscopic studies

and catalytic reactions. EXAFS (Table 2, entry 1) and HRTEM²

analyses showed these samples to contain highly dispersed

palladium nanoparticles (mean diameter of ca. 15 Å). Rh(cod)/

SiO₂(0.71 wt % Rh) and Rh(cod)-Pd/SiO₂(0.07–1.07 wt %

Rh and 1.58–1.91 wt % Pd) are new compounds (Scheme 3),

yet the synthetic procedure to either species closely resembles

that developed to anchor cationic Rh^I-diphosphine complexes

with triﬂate counteranions.^10b,11The heterogenization of Rh(cod)

was achieved by stirring a solution of this cationic complex in

anhydrous CH₂Cl₂in the presence of either bare silica or Pd/

SiO₂. Following this simple procedure, the triﬂate anions get

anchored to the surface silanol groups of silica via H-bonding,

while the complex cations are kept close to the functionalized

silica by ionic interaction. Provided the use of the resulting

material in aprotic solvents, the overall support-anions-cations

interaction is quite robust and no leaching has been ever

observed even under harsh catalytic reactions.^10b,11The only

precaution required to prepare the hybrid material Rh(cod)-Pd/

SiO₂is to mildly passivate Pd/SiO₂with oxygen so as to avoid

any undesired chemical interaction of the palladium particles

MS. Catalytic data are reported in Tables 3 and 4. The stability of

the immobilized catalyst against leaching from the support was

established by recycling experiments (Table 5). To this purpose,

after a catalytic run, the liquid phase was decanted under nitrogen

in a drybox and the grafted rhodium/palladium product was washed

with n-pentane until no trace of benzene or cyclohexane was

detected by GC/MS in the decanted solvent (generally 3 × 20 mL).

The residue was reused for a second run. After the liquid phase

was analyzed by GC, the solvent was removed under reduced

pressure and the residue was analyzed by both ³¹P{¹H} NMR

spectroscopy and ICP-AES. No trace of phosphorus was seen by

NMR spectroscopy in all cases, while the amounts of rhodium

detected by ICP-AES were <1 ppm. For comparative purposes,

some hydrogenation reactions were also carried out using Pd/SiO₂,

Rh(cod)/SiO₂, Rh(C₆H₆)/SiO₂, and Rh(C₆H₆)-Pd/SiO₂as catalyst

precursors (Table 3) as well as cyclohexa-1,3-diene and cyclohexene

as substrates (Table 6).

Results and Discussion

Synthesis of Rhodium Complexes and Silica-Supported

Catalyst Precursors. The preparation and characterization of

Pd/SiO₂with different metal contents have been described in a

2814 Organometallics, Vol. 27, No. 12, 2008

Barbaro et al.

Table 6. Hydrogenation of Cyclohexa-1,3-diene and Cyclohexene by Miscellaneous Catalysts^a

cat. amnt

(mg)

Pd

Rh

substrate/Pd substrate/Rh

B

%

HDE HE HA

bc b,c bc bc

d

entry cat. precursor

Pd wt % Rh wt % µmol µmol substrate^b

ratio

%

TOF_PdTOF_Rh

1

2

3

4

5

6

7

Pd/SiO₂

Rh(cod)/SiO₂

Rh(cod)-Pd/SiO₂

Pd/SiO₂

58

33

60

58

33

60

2.00

10.9

HDE

HE

963

4

30

4

66 3851

96

95 3838 18 463

539

98 3774

18

0.72

0.39

2.3

4546

4616

18 183

1.94

2.00

10.9 2.3

10.9

960

963

5

7

e

86

7

2

72

Rh(cod)/SiO₂

Rh(cod)-Pd/SiO₂

0.72

0.39

2.3

HE

4546

4616

3273

1.94

10.9 2.3

960

100 3838 18 463

^aExperimental conditions: substrate, 10.495 mmol (cyclohexa-1,3-diene, 1.00 mL; cyclohexene; 1.05 mL); n-pentane, complement to 30 mL total

volume; H₂pressure, 30 bar; temperature, 40 °C; time, 15 min; stirring rate, 1500 rpm. ^bB ) benzene, HDE ) cyclohexa-1,3-diene, HE )

cyclohexene, HA ) cyclohexane. ^cAverage values over at least three runs. ^dmol of total product (mol of M)^-1h^{-1 e}Under nitrogen.

.

Figure 1. DRIFTS spectra, in the O-H stretching region, of (a) Pd/SiO₂(1.93 wt % Pd) and (b) Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd).

with the impregnation solvent.^2a,21The thin layer of surface

oxide is readily removed by treatment with H₂. At variance with

the TMCS catalysts obtained with the methods illustrated in

Scheme 1, the complex cations in Rh(cod)-Pd/SiO₂have a much

higher degree of freedom, as their only constraint is to lie in

proximity of the anions, which in turn can walk over the support

by making and cleaving H-bonds.^10b,11

Consistent with the H-bond anchoring of the triﬂate anions,

the DRIFTS spectrum of Rh(cod)-Pd/SiO₂(0.75–1.91 wt %

Rh-Pd) showed a strong decrease in the intensity of the isolated

silanols band at 3740 cm^-1, as compared to Pd/SiO₂(1.93 wt

% Pd), together with the occurrence of a broad band centered

at ca. 3440 cm^-1due to the silanols in H-bond interaction with

the sulfonate tail of the triﬂate groups (Figure 1).

local surroundings of the absorbing atom, performed by least-

squares reﬁnement, are reported in Tables 1 and 2.

The main peak in the spectrum of Rh(cod) at the Rh K-edge

consists of four carbon atoms from the cod ligand and two

phosphorus atoms from dppp (Table 1, entry 1). The average

Rh-P distance of 2.308 Å is comparable to that (2.315_avÅ)

obtained by a single-crystal X-ray diffraction analysis of the

hexaﬂuorophosphate derivative [Rh(cod)(dppp)]PF₆, while the

average Rh-C distance of 2.213 Å is slightly shorter (2.250_av

Å).²²The same coordinative environment around the rhodium

atom has been determined for Rh(cod)/SiO₂(0.71 wt % Rh),

which conﬁrms that the present way of anchoring Rh(cod) onto

silica fully preserves the structure of the complex cation (Table

1, entry 2). The only difference observed upon immobilization

of Rh(cod) was the shortening of the Rh-C distances (2.057

Å), which may be an artifact of the EXAFS measurement, as

the standard Gaussian description of the radial pair distribution

underestimates distances in the presence of a high conforma-

tional disorder. Indeed, the electrostatically tethered complex

cation [Rh(cod)(dppp)]⁺is expected to exhibit a higher degree

of freedom as compared to the same cation packed in the crystal

lattice of Rh(cod).^10b,11The damping of the signal from C shells

belonging to the PPh₂groups can be due to the high σ_DWfactor

for the carbon atoms at distances greater than 3 Å, which reﬂects

their thermal and conformational disorder. Exactly the same

local structure, i.e., four equidistant C atoms and two phosphorus

atoms (Table 1, entry 4), was found for the complex cation in

Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd). No additional

Others detectable bands (C-H stretching of the dppp phenyls

and of the cod ligand) showed only minor changes as compared

to those of unsupported Rh(cod), suggesting that the structure

of the rhodium complex remains unchanged after its grafting.

EXAFS Characterization of Rhodium Complexes and

Silica-Supported Catalyst Precursors. Figure 2 shows the

Fourier spectra at the Rh K-edge of Rh(cod), Rh(cod)/SiO₂(0.71

wt % Rh), and Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd), while

the Fourier spectra at the Pd K-edge of Rh(cod)-Pd/SiO₂

(0.75–1.91 wt % Rh-Pd) and Pd/SiO₂(1.93 wt % Pd) are

reported in Figure 3. Figures 2 and 3 show also the spectra of

materials recovered after various treatments of Pd/SiO₂, Rh(cod)/

SiO₂, and Rh(cod)-Pd/SiO₂. The spectra are not phase-corrected.

The results of the spherical wave curve ﬁtting analysis of the

(22) Kempe, R.; Spannenberg, A.; Heller, D. Z. Kristallogr.–New Cryst.

Struct. 2001, 216, 153.

(21) Solymosi, F.; Rasko, J. J. Catal. 1995, 155, 74.

Benzene Hydrogenation by Silica-Supported Catalysts

Organometallics, Vol. 27, No. 12, 2008 2815

Figure 2. Rh K-edge FFT EXAFS spectra (not phase-corrected) of (---) Rh(cod); (^__) Rh(cod)/SiO₂(0.71 wt % Rh); ( · · · ) Rh(cod)-Pd/SiO₂

(0.75–1.91 wt % Rh-Pd); and ()) Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) after exposure to air.

Figure 3. Pd K-edge FFT EXAFS spectra (not phase-corrected) of (s) Pd/SiO₂(1.93 wt % Pd); ( · · · ) Pd/SiO₂(1.93 wt % Pd) after

exposure to oxygen; ()) Pd/SiO₂(1.93 wt % Pd) after exposure to oxygen and treatment with CH₂Cl₂; (gray s) Rh(cod)-Pd/SiO₂(0.75–1.91

wt % Rh-Pd); and (---) Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) after exposure to air.

contribution from surface atoms or other adjacent species was

detected for Rh(cod)/SiO₂and Rh(cod)-Pd/SiO₂.

the same packing and cubooctahedric geometry, a mean diameter

of ca. 18 Å was estimated for the palladium particles in Rh(cod)-

Pd/SiO₂(0.75–1.91 wt % Rh-Pd). This value is slightly larger

than that measured before anchoring Rh(cod) (N_Pd-Pd) 6,

corresponding to ca. 15 Å), which suggests that the impregnation

procedure modiﬁes somewhat the dispersion of the preformed

palladium particles (Table 2, entry 1). The same behavior at

the Pd K-edge was observed for Pd/SiO₂(1.93 wt % Pd) after

exposure to oxygen (Table 2, entry 2) followed by treatment

with anhydrous CH₂Cl₂(Table 2, entry 3), the solvent used to

anchor Rh(cod), which conﬁrms that a palladium particle

The Pd K-edge spectrum of Rh(cod)-Pd/SiO₂(0.75–1.91 wt

% Rh-Pd) showed a shell structure that is characteristic of fcc

palladium metal particles, with a Pd-Pd nearest neighbors value

(N_Pd-Pd) of 7.9 (Pd-Pd distance of 2.723 Å; Table 2, entry 4),

while the peak trend for the next shells indicated the formation

of small metal particles dispersed on the support with a small

contribution from the surface oxygen atoms (N_Pd-O) 1.2). As

the palladium foil has a fcc crystal frame with 12 nearest

neighbors at 2.75 Å, on the assumption of particle growth with

2816 Organometallics, Vol. 27, No. 12, 2008

Barbaro et al.

Figure 5. DRIFTS spectrum of Rh(cod)-Pd/SiO₂(0.75–1.91 wt %

Rh-Pd) after CO exposure (1 bar ﬂow, room temperature, 15 min)

and purging in He ﬂow.

Figure 4. DRIFTS spectra of (a) Rh(cod) and (b) Rh(cod)/SiO₂

(0.71 wt % Rh) after CO exposure (1 bar ﬂow, room temperature,

15 min) and purging in He ﬂow.

Scheme 4

the two compounds. The former is a classical cationic metal

complex where cations and anions are packed in a crystal lattice.

In contrast, in Rh(CO)₂/SiO₂the triﬂate anions are anchored to

the silica surface by H-bonding and the complex cations are

kept close to the triﬂate-functionalized silica surface exclusively

by electrostatic interaction, which allows for a relatively high

degree of mobility and ﬂexibility. Consistently, the IR spectrum

of Rh(CO)₂in a solvent containing hydroxyl groups such as

ethylene glycol showed ν(CO) bands exactly at 2098 and 2054

cm^{-1 23}

It is noteworthy that two IR bands in the range from

.

2038 to 2096 cm^-1are typical for a number of surface-

constrained Rh^Icomplexes, whereas Rh⁰particles generally

exhibit a single band from 2040 to 2066.²⁴The bands observed

for Rh(CO)₂/SiO₂are therefore in line with the literature data.

Upon carbonylation, Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-

Pd) was converted to Rh(CO)₂-Pd(CO)_x/SiO₂(Scheme 4)

containing electrostatically tethered [Rh(CO)₂(dppp)]⁺cations,

as shown by the appearance of IR bands at 2099 and 2055 cm^-1

(Figure 5). In addition, to these bands, the spectrum also

contained a band at 1925 cm^-1, which can be ascribed to

bridging CO groups adsorbed on palladium metal particles.²⁵

The band due to linear CO on palladium nanoparticles, usually

located at 2100 cm^-1, was not detected, as it falls in the same

region of the more intense symmetric ν(Rh-CO) absorption.

enlargement takes place irrespective of the presence of rhodium

in the palladium surroundings. As reported in Table 2, all

samples showed the presence of oxygen atoms at a bonding

distance to palladium atoms. These oxygen atoms generally

belong to the silanols of silica, but a palladium oxide contribu-

tion is present in just-impregnated Rh(cod)-Pd/SiO₂(see

Experimental Section) as well as in the samples exposed to air/

oxygen.

EXAFS Studies of Rh(cod)-Pd/SiO₂Exposed to Air. The

effects caused by the exposure of Rh(cod)-Pd/SiO₂to air, hence

to O₂and moisture, has been checked in an attempt to obtain

information on the stability of this catalytic system as well as

detect possible structural changes caused by the use in catalysis

of recycled catalysts recovered without the protection of an inert

gas (vide infra).

The Fourier spectra at the Rh K-edge of Rh(cod)-Pd/SiO₂

(0.75–1.91 wt % Rh-Pd) before and after being exposed to air

overnight at room temperature are reported in Figure 2 (Table

1, entries 4 and 5). No relevant modiﬁcation of the rhodium

coordination sphere apparently occurred, which demonstrates

the excellent stability of the electrostatically immobilized

[Rh(cod)(dppp)]⁺complex cation.

DRIFTS Spectra of Carbonylated Rhodium Complexes

and Silica-Supported Catalyst Precursors. Due to the absence

of functional groups readily identiﬁable by IR spectroscopy,

CO was used as a probe molecule to get structural and chemical

information on the supported compounds by DRIFTS.

Solid Rh(cod), diluted with anhydrous KBr powder, was

reacted with CO (1 bar ﬂow) at room temperature for 15 min

to yield a species with an IR spectrum containing two intense

bands located at 2093 and 2047 cm^-1(Figure 4a), which were

readily ascribed to the symmetric and antisymmetric stretch

vibrations of the cis-dicarbonyl complex Rh(CO)₂.¹⁸The

supported form of the latter complex, Rh(CO)₂/SiO₂, was

obtained by carbonylation of Rh(cod)/SiO₂(0.71 wt % Rh), as

shown by the IR spectrum of the resulting product, containing

two analogous bands at 2098 and 2054 cm^-1(Figure 4b).

EXAFS Studies of the Hydrogenation Products of Rh(cod)/

The replacement of the cod ligand in Rh(cod)/SiO₂by two

CO molecules also modiﬁed the C-H stretching region,

allowing us to determine some absorptions bands of the C-H

vibrations of cod (3014, 3067 cm^-1). The small difference in

wavenumbers and line width between Rh(CO)₂and Rh(CO)₂/

SiO₂can be accounted for by the different physical status of

(23) Lucenti, E.; Roberto, D.; Roveda, C.; Ugo, R.; Cariati, E. J. Cluster

Sci. 2001, 12, 113.

(24) Stanger, K. J.; Tang, Y.; Anderegg, J.; Angelici, R. J. J. Mol. Catal.

A: Chem 2003, 202, 147, and references therein.

(25) Liotta, L. F.; Martin, G. A.; Deganello, G. J. Catal. 1996, 164,

322, and references therein.

Benzene Hydrogenation by Silica-Supported Catalysts

Organometallics, Vol. 27, No. 12, 2008 2817

Figure 6. Rh K-edge FFT EXAFS spectra (not phase-corrected) of ( · · · ) Rh(cod)/SiO₂(0.71 wt % Rh); (---) Rh(cod)/SiO₂(0.71 wt % Rh)

after in situ reduction with H₂ﬂow at 120 °C; ()) Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd); and (s) Rh(cod)-Pd/SiO₂(0.75–1.91 wt

% Rh-Pd) after in situ reduction with H₂ﬂow at 120 °C.

Scheme 5

SiO₂and Rh(cod)-Pd/SiO₂. Upon in situ treatment with H₂at

120 °C (Figure 6; Table 1, entry 3), the cod ligand was removed

from Rh(cod)/SiO₂(0.71 wt % Rh) as coa (temperature-

programmed reductive decomposition/quadrupole mass spec-

troscopy (TPRD/QMS) evidence). The EXAFS analysis of this

sample showed each rhodium atom to be surrounded by two

phosphorus atoms with Rh-P distances almost identical to those

in Rh(cod) and by ca. three oxygen atoms at a distance of 2.330

Å, presumably from surface silanols interacting with the metal

center (Scheme 5a). No further contribution was visible in the

coordination shell of rhodium. This result, however, does not

exclude the presence of hydride/dihydrogen ligands that cannot

be detected by EXAFS.^17,26

The EXAFS spectrum at the Rh K-edge of Rh(cod)-Pd/SiO₂

(0.75–1.91 wt % Rh-Pd) after in situ reduction with H₂at 120

°C (coa formation) showed two distinct peaks (Figure 6; Table

1, entry 6). In addition to two phosphorus atoms with Rh-P

distances a little longer than those in the precursor, a relevant

contribution from a palladium shell was actually detected

(N_Rh-Pd) 3.4). In principle, it is not possible to discriminate

palladium from rhodium as photoelectron scatterer, but the

attribution of the Rh-metal shell to palladium neighbors was

2818 Organometallics, Vol. 27, No. 12, 2008

Barbaro et al.

Figure 7. Variable–temperature ³¹P{¹H} HPNMR study (THF-d₈, 81.01 MHz) of the hydrogenation reaction of Rh(cod) in the presence

of 10-fold excess of C₆H₆: (a) under nitrogen at room temperature; (b) under H₂(30 bar) after 5 min at room temperature; (c) after 30 min

at room temperature; (d) at 40 °C.

unambiguously supported by the evidence that Rh(cod)/SiO₂

did not exhibit any Rh-Rh contribution even upon treatment

with H₂up to 300 °C. The Rh-Pd distance of 2.741 Å, only

slightly longer than that reported for a Rh-Pd alloy (2.72 Å),

and the number of Rh-Pd nearest neighbors of 3.4 (lower than

the corresponding Pd-Pd value at the Pd K-edge in Rh(cod)-

Pd/SiO₂; Table 2, entry 4) point to rhodium and palladium atoms

in close proximity but not alloyed. Besides leading to the

conversion of cod to coa, H₂would react with the 12e^-fragment

[Rh(dppp)]⁺, formed upon the last hydride-alkyl reductive

migration, to give [Rh(H)_x(H₂)_y(dppp)]⁺species containing

either classical or nonclassical hydride ligands. In turn, the

palladium particles will react with H₂to form surface Pd-H

species. It is therefore likely that the observed Rh-Pd vicinity

involves the formation of hydride-bridged Rh(µ-H)_xPd species,

featured by a metal-metal bond (Scheme 5b), as previously

reported for the hydrogenation of Rh(cod)(sulfos)-Pd/SiO₂

(Scheme 5c).²

The low propensity of dppp to form stable rhodium hydride

complexes is witnessed by the lack of such complexes in the

relevant literature.^17,26Therefore, we were not surprised to ﬁnd

that the in situ homogeneous reaction of Rh(cod) with H₂(30

bar) in THF-d₈gave coa and several unidentiﬁed products,

among which, only one hydride compound in trace amount (vide

infra).

Homogeneous Modeling Studies. Hydrogenation Reac-

tion of Rh(cod). The independent reaction of Rh(cod) with H₂

was studied by ¹H and ³¹P{¹H} HPNMR spectroscopy in THF-

d₈in a 10 mm sapphire tube pressurized with 30 bar of H₂.

The NMR analysis showed the complete disappearance of the

starting complex already after 30 min at room temperature with

formation of coa. Several Rh-dppp species were formed, among

which there was the bis-solvento complex [Rh(solv)₂(dppp)]OTf

(solv ) THF, adventitious water; ³¹P{¹H} NMR: δ 40.1 (d,

J_RhP) 190.5 Hz)) in 35% yield.¹⁷Notably, only a trace amount

1

of a hydride compound, featured by a H NMR multiplet at δ

–22.02, was formed. The behavior of Rh(cod) under hydrogena-

tion conditions reﬂects the elusive nature of polyhydride

rhodium complexes with chelating diphosphine ligands.^17,26

These compounds, which may also exhibit nonclassical dihy-

drogen (η²-H₂) structure,²⁹have a great tendency to lose H₂

upon reaction with mild nucleophiles.²⁶

Noticeably, the Rh-Pd bond was observed by EXAFS only

upon in situ hydrogenation of Rh(cod)-Pd/SiO₂and maintaining

the sample under a H₂atmosphere during the spectral acquisi-

tion. Even very fast transfer of the product obtained by ex situ

hydrogenation of Rh(cod)-Pd/SiO₂to the EXAFS cell led to

the disappearance of the hydride-mediated rhodium-palladium

moiety and the [Rh(dppp)]⁺fragments were stabilized by

interacting with the surface OH groups (Scheme 5b), as occurs

when the cod ligand in Rh(cod)/SiO₂is hydrogenated to coa

(Scheme 5a).

Hydrogenation Reaction of Rh(cod) with H₂in the Pre-

sence of Benzene. The reaction of Rh(cod) with H₂was

performed in THF-d₈also in the presence of benzene and

1

followed in situ by H and ³¹P{¹H} HPNMR spectroscopy.

Figure 7 shows the ³¹P{¹H} NMR spectra obtained at different

temperatures and acquisition times. At room temperature,

Rh(cod) was completely transformed into Rh(C₆H₆) within 30

min. The latter compound was the only NMR-visible species

at 40 °C for 1 h. The ¹H NMR spectra showed also the formation

of coa, while no trace of benzene-hydrogenation products was

detected by GC/MS. Signiﬁcant decomposition of Rh(C₆H₆)

(26) Douglas, T. M.; Brayshaw, S. K.; Dallanegra, R.; Kociok-Köhn,

G.; Macgregor, S. A.; Moxham, G. L.; Weller, A. S.; Wondimagegn, T.;

Vadivelu, P. Chem.-Eur. J. 2008, 14, 1004.

(27) (a) Buisman, G. J. H.; van der Veen, L. A.; Kamer, P. C. J.; van

Leeuwen, P. W. N. M. Organometallics 1997, 16, 5681. (b) Buisman,

G. J. H.; Vos, E. J.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Dalton

Trans. 1995, 409. (c) Kiss, G.; Horváth, I. Organometallics 1991, 10, 3798.

(d) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Frediani, P.; Ramirez,

J. A. Organometallics 1990, 9, 226.

(28) Chen, C. S.; Lin, J. H.; Chen, H. W. Appl. Catal., A 2006, 298,

161.

(29) Bakhmutov, V. I.; Bianchini, C.; Peruzzini, M.; Vizza, F.;

Vorontsov, E. V. Inorg. Chem. 2000, 38, 1655.

Benzene Hydrogenation by Silica-Supported Catalysts

Organometallics, Vol. 27, No. 12, 2008 2819

Scheme 6

Chart 1

Figure 8. Hydrogenation reactions of benzene catalyzed by

Rh(cod)-Pd/SiO₂(Table 4, entries 1–6). Dependence of the rate

on H₂pressure.

To get information on the mechanism of benzene hydrogena-

tion to cyclohexane by Rh(cod)-Pd/SiO₂catalysis, especially

as regards the synergic action of the rhodium single sites and

the palladium nanoparticles, we decided to perform a study

where H₂pressure, substrate concentration, rhodium to pal-

ladium ratio, and catalyst amount were systematically varied

(Tables 3 and 4). Partial benzene hydrogenation products were

not detected by GC/MS at any stage of the reactions. The

catalytic activity, in terms of dependence of benzene conversion

on H₂pressure, benzene concentration, and catalyst amount,

was evaluated by using two different Rh(cod)-Pd/SiO₂catalyst

precursors (1.00–1.59 wt % Rh-Pd and 0.65–1.58 wt %

Rh-Pd). The dependence of substrate conversion on the

rhodium content in the catalyst precursor was evaluated with

samples of Rh(cod)-Pd/SiO₂containing from 0.07 to 1.07 wt

% Rh (1.87–1.91 wt % Pd).

Figure 9. Hydrogenation reactions of benzene catalyzed by

Rh(cod)-Pd/SiO₂(Table 4, entries 5, 7–11). Dependence of the rate

on benzene concentration.

occurred only above 80 °C, to give several unidentiﬁed products,

among which we noticed a hydride compound in a trace amount

(¹H NMR multiplet at δ -22.71).

Synthesis of Rh(C₆H₆). A solution of Rh(cod) in 40 mL of

a 3:1 (v/v) THF/benzene mixture was hydrogenated (30 bar H₂)

in an autoclave at room temperature for 30 min with stirring.

Evaporation of the reaction mixture under reduced pressure gave

the benzene complex Rh(C₆H₆) as a yellow solid in analytically

pure form. Unambiguous identiﬁcation of the latter complex

was achieved by NMR spectroscopy in CD₂Cl₂, also by

comparison with the spectra of [Rh(benzene)(diphos)]⁺(diphos

) 1,2-bis(diphenylphosphino)ethane).³⁰Noteworthy, dissolving

Rh(C₆H₆) in THF-d₈or MeOH-d₄under nitrogen generated bis-

solvento complexes analogous to those described above, which

reﬂects the great oxophilicity of the [Rh(dppp)]⁺fragment.¹⁷

Catalytic Hydrogenation of Benzene by Rh(cod)-Pd/SiO₂.

The hydrogenation reactions were generally carried out with

0.43 mL of benzene under 30 bar H₂in n-pentane (complement

to 30 mL total volume) at 40 °C for 2 h. Under these

experimental conditions, Rh(cod)/SiO₂did not catalyze the

hydrogenation of benzene (Table 3, entry 7), while Pd/SiO₂

yielded cyclohexane with a TOF of 72 µmol of product h^-1

(Table 3, entry 1). A much higher TOF was obtained using the

hybrid catalyst Rh(cod)-Pd/SiO₂even at very low rhodium

content (Table 3, entries 2–5). The same TOF as in entry 1 was

obtained by using a mechanical mixture of Pd/SiO₂(47 mg,

1.93 wt % Pd) and Rh(cod)/SiO₂(47 mg, 0.65 wt % Rh), which

conﬁrmed the occurrence of enhanced activity only when the

Rh^Isingle sites and the palladium particles are concomitantly

anchored to the support.

From a perusal of the TOFs obtained by varying the amount

of rhodium (Table 3, entries 2–5), one may readily infer that,

in the interval of rhodium loading examined, the hydrogenation

rate of benzene shows a little dependence on the amount of the

tethered single site component (the TOF increases from 491 to

644 for a rhodium amount increasing from 0.07 to 1.07 wt %).

On the other hand, rhodium contents signiﬁcantly lower than

0.07 wt % in Rh(cod)-Pd/SiO₂were not investigated because

of the poor synthetic reproducibility and the unreliable elemental

analysis.

On the assumption of palladium nanoparticles with a mean

diameter of ca. 18 Å (Table 2), there would be ca. 40 palladium

atoms per nanoparticle with a percentage of superﬁcial atoms

between 50% and 80%.^2,31Therefore, in a sample of Rh(cod)-

Pd/SiO₂containing 0.07 wt % Rh and 1.87 wt % Pd, every

palladium nanoparticle would have a maximum of ca. 3

potentially interacting [Rh(dppp)]⁺fragments. This number

would increase to 12 already in Rh(cod)-Pd/SiO₂with 0.27 wt

% Rh and to 50 in Rh(cod)-Pd/SiO₂with 1.07 wt % Rh. At the

latter rhodium loadings, the steric and electrical (cation–cation

repulsion) crowding may be excessive for 18 Å metal particles.

(31) (a) Sergeev G. B. Nanochemistry; Elsevier: Amsterdam, 2006. (b)

Beck, A.; Horváth A.; Sárkány, A.; Guczi. L. In Nanotechnology in

Catalysis; Zhou, B., Hermans, S., Somorjai, G. A., Eds.; Springer: Berlin,

2004; Vol. 1, Chapter 5. (c) Haw, J. F.; Marcus, D. M. In Nanotechnology

in Catalysis; Zhou, B., Hermans, S., Somorjai, G. A., Eds.; Springer: Berlin,

2004; Vol. 1, Chapter 13.

(30) Landis, C. R.; Halpern, J. Organometallics 1983, 2, 840.

2820 Organometallics, Vol. 27, No. 12, 2008

Barbaro et al.

Figure 10. NMR spectra (21 °C, C₆H₆-d₆) in the cyclohexane region of the mixtures of products obtained by hydrogenation of C₆H₆-d₆in

the presence of Pd/SiO₂(1.93 wt % Pd) (¹³C{¹H} NMR, 100.613 MHz, a) and Rh(cod)-Pd/SiO₂(0.75–1.91 wt % Rh-Pd) (¹³C{¹H} NMR,

100.613 MHz, b; ²H{¹H} NMR, 61.423 MHz, c). Representative resonances attributed to all-cis, cis–trans mixture, and disproportionation

products are marked with #, *, and °, respectively.

Figure 11. Rh K-edge FFT EXAFS spectra (not phase-corrected) of recovered catalysts derived from Rh(cod)-Pd/SiO₂(0.75–1.91 wt %

Rh-Pd): ( · · · ) after a catalytic run to 33% benzene conversion (30 bar, 40 °C, 2 h); (s) after a catalytic run to 33% benzene conversion

(30 bar, 40 °C, 2 h) and exposure to air; (---) after a catalytic run to 33% benzene conversion (30 bar, 40 °C, 2 h) and exposure to air

followed by a second catalytic run to 68% benzene conversion (30 bar, 40 °C, 24 h). As a comparison ()) Rh(cod)-Pd/SiO₂(0.75–1.91

wt % Rh-Pd) after in situ reduction with H₂ﬂow at 120 °C.

It is therefore likely that the majority of the [Rh(dppp)]⁺

fragments will not have any chance to interact with the

palladium particle. In this eventuality, the small dependence of

the TOF on the rhodium amount in the catalyst might be simply

due to a saturation effect, as only the rhodium single sites

interacting with the palladium atoms are catalytically active for

benzene hydrogenation (vide infra). In searching for further

evidence supporting this concept, two Rh(cod)-Pd/SiO₂samples

with Rh loadings of 0.08 and 0.70 wt % were prepared using a

Pd/SiO₂support with a metal loading of 9.86 wt %. HRTEM

and EXAFS analyses of the latter material showed a metal

dispersion quite comparable to that of the analogous material

used to synthesize the catalysts of entries 1–6 in Table 3.²The

TOFs of benzene hydrogenation catalyzed by these new

Rh(cod)-Pd/SiO₂compounds were 841 for the sample containing

0.07 wt % Rh (entry 8) and 1241 for the sample with 0.70 wt

% Rh (entry 9). While the increased Pd loading in the catalyst

of entry 8, as compared to the catalyst of entry 2, can well

account for the higher TOF (841 vs 491), the much higher TOF

observed for the catalyst of entry 9, as compared to the catalyst

of entry 4 (1241 vs 597), cannot be solely justiﬁed by the

increased Pd loading. Again, a substantial role in enhancing

the catalytic activity seems to be played by a more effective

interaction between the rhodium single sites and the surface

palladium atoms, whose number increases by increasing the

palladium loading at constant metal dispersion. In other words,

Benzene Hydrogenation by Silica-Supported Catalysts

Organometallics, Vol. 27, No. 12, 2008 2821

Scheme 7

silica-supported Pd nanoparticles has been reported by Guczi

et al.³²According to this author, the treatment in air at high

temperature does not change the particle dimensions, whereas

it contributes to restructure the particle surface as a consequence

of impurities removal as well as the PdO/Pd transition in the

subsequent hydrogen treatment. In the present case, a further

complication might be provided by the presence of the tethered

rhodium complex. In an attempt to rationalize the enhancing

effect of air exposure, EXAFS analyses of recycled catalysts,

prior and after exposure to air, have been carried out (vide infra).

Pd/SiO₂, Rh(cod)/SiO₂, and Rh(cod)-Pd/SiO₂were also

scrutinized as catalysts for the hydrogenation of either cyclo-

hexa-1,3-diene or cyclohexene, which are possible intermediates

along the reduction of benzene to cyclohexane. The results

obtained are given in Table 6. From a perusal of the data relative

to cyclohexa-1,3-diene hydrogenation (entries 1–3), one may

readily realize that Rh(cod)/SiO₂is more effective than Pd/SiO₂

and comparable to the hybrid system Rh(cod)-Pd/SiO₂. A

comparison of the data (yields and TOFs) reported in Tables

3-6 also indicates that the rate-determining step in the

hydrogenation of benzene to cyclohexane by Rh(cod)-Pd/SiO₂

is just the disruption of the substrate aromaticity upon addition

of the ﬁrst H₂molecule. Even by taking into account the intrinsic

ability of palladium nanoparticles to disproportionate cyclohexa-

1,3-diene into benzene and cyclohexene (entry 4),²the much

larger amount of residual cyclohexene in the reaction catalyzed

by Pd/SiO₂(entry 1), as compared to the reaction promoted by

Rh(cod)/SiO₂(entry 2), is a clear indication that the single-site

catalyst is more efﬁcient also for the reduction of adsorbed

cyclohexene. This result, corroborated by the deuterium labeling

study reported below, suggests that the hydrogenation of

cyclohexa-1,3-diene to cyclohexane by Rh(cod)/SiO₂proceeds

via an associative mechanism similar to that reported by

Muetterties for the Co^Icatalyst [CoH(PR₃)₂] (PR₃) phosphite,

phosphine) (Scheme 6).³³

the [Rh(dppp)]⁺fragments which have no chance to interact

with the palladium particles in the catalyst with 1.91–0.75 wt

% Pd-Rh seem to be operative in the catalyst with 9.86–0.70

wt % Pd-Rh, thus leading to increased benzene conversion.

Although kinetically irrelevant, the study of the TOF depen-

dence on the rhodium loading has conﬁrmed that Rh^Isingle

sites and not Rh⁰metal particles take part in the catalytic

process. Indeed, the fact that the TOF does not increase by

increasing the rhodium loading from 0.07 to 1.07 wt % rules

out any role of Rh⁰metal particles in the hydrogenation of

benzene (as shown also by the EXAFS studies on Rh(cod)-Pd/

SiO₂and Rh(cod)/SiO₂upon treatment with H₂). In fact, were

the formation of Rh⁰metal particles responsible for the

hydrogenation of the substrate, then one would have observed

an increase of the TOF by increasing the rhodium amount in

the precursor.²⁴

A plot of TOF vs H₂pressure gave an almost a linear cor-

relation for H₂pressures lower than 10 bar, while a saturation

effect apparently occurred at higher H₂pressures, featured by

a plateau of activity (Table 4; Figure 8).

The TOF tended to plateau also at substrate concentrations

higher than 0.7 mol/L (Table 4; Figure 9), while an almost linear

correlation was observed at lower catalyst amounts.

Only when cyclohexa-1,3-diene was replaced by cyclohexene

as initial substrate (entries 5–7) was Pd/SiO₂more active (entry

5) than Rh(cod)/SiO₂(entry 6), which reﬂects the troublesome

coordination of cyclic oleﬁns to sterically hindered transition

metal fragments such as the [Rh(dppp)]⁺ones.³⁴

Finally, a linear correlation between TOF and catalyst amount

was found keeping H₂pressure (30 bar) and benzene concentra-

tion (0.161 M) constant (Table 4, entries 12–14).

Benzene Hydrogenation Regiochemistry. The regiochem-

istry of C₆H₆-d₆hydrogenation in the presence of either Pd/

SiO₂(1.93 wt % Pd) or Rh(cod)-Pd/SiO₂(0.75–1.91 wt %

The physical and chemical stability of the TCSM catalyst

derived from Rh(cod)-Pd/SiO₂against leaching from the support

was proved by a series of recycling experiments reported in

Table 5. In all cases, after the liquid phase was separated from

the solid catalyst, the solvent was removed under vacuum and

the residue was analyzed by both ³¹P{¹H} NMR spectroscopy

and ICP-AES. No trace of phosphorus was seen by NMR

spectroscopy, while the amount of rhodium detected by ICP-

AES was invariably <1 ppm. After performing a catalytic run

with Rh(cod)-Pd/SiO₂(0.73–1.50 wt % Rh-Pd) (Table 5, entry

1), the solvent was decanted under nitrogen in a drybox and

the remaining solid was washed with n-pentane until no trace

of benzene or cyclohexane was detected by GC/MS in the liquid

phase; then the catalyst was reused for a second run (entry 2).

This recycling procedure was repeated for a third run (entry 3).

Finally, the catalyst recovered from the second recycle was

exposed to air for 2 days at room temperature before being

employed in a further reaction (entry 4). Consistent with a great

stability of the TCSM catalyst, comparable TOFs were obtained

in the ﬁrst three reactions. When, however, the recycled catalyst

was exposed to air prior to a further use, the TOF increased

from 717 to 919 (entries 3 and 4). Evidence for a signiﬁcant

activity enhancement in benzene hydrogenation by air-treated

2

Rh-Pd) was examined by ¹³C{¹H} and H{¹H} NMR spec-

troscopy. Previous reports in the literature for heterogenized

single-site catalysts, speciﬁcally organoactinides and organozir-

conium complexes, showed the formation of mixtures of all-

cis and cis–trans isotopomers, with a prevalence of the latter.³⁵

In contrast, the homogeneous hydrogenation of benzene cata-

lyzed by (η³-C₃H₅)Co(PR₃)₃(PR₃) phosphite, phosphine) has

been found to give exclusively the all-cis product.³⁶

(32) Horváth, A.; Beck, A.; Sárkány, A.; Koppány, Zs.; Szucs, A.;

Dékány, I.; Horváth, Z. E.; Guczi, L. Solid State Ionics 2001, 141–142,

147.

(33) (a) Bleeke, J. R.; Muetterties, E. L. J. Am. Chem. Soc. 1981, 103,

556. (b) Stuhl, L. S.; Rakowski Du Bois, M. C.; Hirsekorn, F. J.; Bleeke,

J. R.; Stevens, A. E.; Muetterties, E. L. J. Am. Chem. Soc. 1978, 100, 2405.

(c) Rakowski, M. C.; Hirsekorn, F. J.; Stuhl, L. S.; Muetterties, E. L. Inorg.

Chem. 1976, 15, 2379. (d) Muetterties, E. L.; Hirsekorn, F. J. J. Am. Chem.

Soc. 1974, 96, 4063. (e) Muetterties, E. L.; Hirsekorn, F. J. J. Am. Chem.

Soc. 1974, 96, 7920.

(34) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1976, 98, 4450.

(35) (a) Ahn, H.; Nicholas, C. P.; Marks, T. J. Organometallics 2002,

21, 1788. (b) Eisen, M. S.; Marks, T. J. J. Am. Chem. Soc. 1992, 114,

10358.

(36) Muetterties, E. L.; Rakowski, M. C.; Hirsekorn, F. J.; Larson, W. D.;

Bauss, V. J.; Anet, F. A. L. J. Am. Chem. Soc. 1975, 97, 1266.

2822 Organometallics, Vol. 27, No. 12, 2008

Barbaro et al.

Figure 12. Pd K-edge FFT EXAFS spectra (not phase-corrected) of ()) Rh(cod)-Pd/SiO₂; ( · · · ) Rh(cod)-Pd/SiO₂after a catalytic run to

33% benzene conversion (30 bar, 40 °C, 2 h) and exposure to air; and (s) Rh(cod)-Pd/SiO₂after a catalytic run to 33% benzene conversion

(30 bar, 40 °C, 2 h) and exposure to air followed by a second catalytic run to 68% benzene conversion (30 bar, 40 °C, 24 h).

Scheme 8

after 33% conversion of benzene to cyclohexane in the presence

of Rh(cod)-Pd/SiO₂in n-pentane. The sample was transferred

from the autoclave into the EXAFS sample holder under argon

in a drybox. Under these conditions, a Rh-Pd shell was clearly

evident at the same distance (Figure 11; Table 1, entry 7) as in

the sample hydrogenated under solid–gas conditions (Table 1,

entry 6), which means that a surface species with rhodium single

sites and palladium particles bonded to each other is formed

upon quenching the reaction. Notably, this species exhibits a

signiﬁcant C contribution, up to six neighbors, which suggests

that benzene or some of the possible hydrogenation intermedi-

ates (cyclohexene, cyclohexa-1,3-diene) are coordinated to the

rhodium sites in the quenched catalyst.

The former hypothesis, i.e., the coordination of benzene by

the Rh-Pd moiety, is much more likely than any other in view

of the fact that the η⁶-C₆H₆adduct Rh(C₆H₆) is an isolable

product (vide supra). Scheme 7 shows a possible structure for

the product recovered after quenching the benzene hydrogena-

tion reaction at 33% conversion.

The ¹³C{¹H} NMR spectrum of the product obtained with

Pd/SiO₂(Figure 10a) showed the formation of several regioi-

somers C₆H_6+xD_6-x, consistent with the hydrogenation of ben-

zene to cyclohexane and with the disproportionation of cyclohexa-

1,3-diene to benzene and cyclohexene.^2a,35b

This means that also C-H/C-D exchange can be achieved.

The major product was the C₆H₆D₆cis–trans/all-cis isotopomer

mixture (Chart 1).

The ¹³C{¹H} and ²H{¹H} NMR spectra of the product

obtained with Rh(cod)-Pd/SiO₂showed a much higher propor-

tion of the all-cis isotopomer (Figure 10, b and c, respectively).

The still complex isotopomer distribution obtained with Rh(cod)-

Pd/SiO₂can be accounted for by the concomitant presence of

palladium, yet the largely increased proportion of the all-cis

product suggests a prevalent role of the rhodium single sites

along the hydrogenation path down to cyclohexane, once

benzene is reduced to cyclohexa-1,3-diene with the mandatory

assistance of the palladium nanoparticles (vide supra).

The benzene substrate in Scheme 7 has been drawn as a

bridging ligand between the rhodium single site and surface

palladium atoms (dual activation). Even though quite specula-

tive, this representation has the merit of being consistent with

both the EXAFS data and the reactivity of Rh(cod) in

homogeneous phase and of Rh(cod)/SiO₂in heterogeneous

phase. Indeed, neither catalyst is able to hydrogenate benzene

even under drastic experimental conditions, which points

unambiguously to a key role of the palladium nanoparticles in

the activation of benzene. Further support for this hypothesis

was provided by the use of either Rh(C₆H₆)/SiO₂or Rh(C₆H₆)-

Pd/SiO₂as catalyst precursor. The former species was com-

pletely inactive, whereas the latter was even more active than

Rh(cod)-Pd/SiO₂, which may be due to a catalyst precursor

already bearing the substrate (Table 3, entry 6 vs Table 3, entries

3–5 and Table 4 entries 5, 13).

EXAFS and DRIFTS Studies of Recovered Catalysts. The

catalytic materials recovered after quenching selected benzene

hydrogenation reactions were studied by EXAFS and DRIFTS.

The ﬁrst EXAFS analysis was carried out on a product obtained

The surprising increase in productivity observed for catalysts

recovered in the air (Table 5, entry 4 vs entry 3) prompted us

to study by EXAFS the effect of air exposure on a recycled

Benzene Hydrogenation by Silica-Supported Catalysts

Organometallics, Vol. 27, No. 12, 2008 2823

Scheme 9

Scheme 10

catalyst at 33% conversion (Figure 11; Table 1 entry 8). The

two phosphorus atoms of the dppp ligand were still in the ﬁrst

shell, but a contribution of 1.6 palladium atoms appeared at a

distance corresponding to the second neighboring shell of

rhodium. Moreover, an average number of two oxygen atoms

was detected at a rather short bonding distance (1.980 vs 2.330

Å of the Rh-silanol interaction), consistent with the formation

of a Rh-O interaction (Table 1, entries 3 and 8). These ﬁndings

suggest that the [Rh(dppp)]⁺fragments interact with the thin

oxide layer over the palladium nanoparticles, formed upon

exposure to air, so as to originate a Rh-O-Pd array. Within

this context, it is worth restating that the binding afﬁnity of

[Rh(dppp)]⁺moieties to oxygen atoms has been unambiguously

proven in both the solid state (see Scheme 5) and solution (see

homogeneous modeling studies).¹⁷In agreement with the

oxidation of a fraction of surface palladium atoms, the EXAFS

spectrum at the Pd K-edge of the recycled catalyst (Figure 12;

Table 2, entry 6) showed the presence of palladium particles

(N_Pd-Pd) 6) interacting with oxygen atoms at a shorter distance

than that observed in Rh(cod)-Pd/SiO₂(Table 2, entry 4).

Scheme 8 illustrates a tentative structure of the quenched and

air-exposed sample.

The next step was to use the ﬁrst-recycle, air-treated catalyst

in a second catalytic reaction. After 24 h, the reaction was

terminated, yielding 68% conversion, and the catalyst was

transferred into the EXAFS sample holder under argon in a

drybox. The Rh K-edge spectrum showed a rhodium center

surrounded by two phosphorus atoms and four C atoms (Table

1, entry 9; Figure 11). A contribution from palladium surface

atoms of adjacent metal particles was again detected. The

determination of an average contribution of four C atoms,

instead of the expected six as found at 33% benzene conversion

(Table 1, entry 7), may be due to the presence of different

species onto the quenched catalyst that was actually subjected

to a remarkable chemical stress (ﬁrst catalysis run at 33%

conversion/aging in the air/second catalysis run for 24 h/recovery

under argon). Indeed, the spectrum at the Pd K-edge of this

sample (Figure 12; Table 2, entry 7) showed a signiﬁcant

increase in the average size of the palladium particles (d_mean

)

30 Å) with respect to the value measured prior to the exposure

to air. Such a particle enlargement is typical of palladium

catalysts subjected to repeated hydrogenation cycles.³⁷Consis-

tent with the magnitude of the metal particles, no appreciable

oxygen contribution was detected.

2824 Organometallics, Vol. 27, No. 12, 2008

Barbaro et al.

Carbonylation reactions were carried in the DRIFTS apparatus

for all samples, prior to and after repeated catalysis cycles, with

or without air exposure. The treatment of all samples with CO

regenerated the dicarbonyl [Rh(CO)₂(dppp)]⁺, thus conﬁrming

the excellent stability of the electrostatically tethered [Rh-

(dppp)]⁺moiety even when subjected to repeated severe

treatments.

substrate activation and ﬁrst H₂uptake (Scheme 10).²This

difference as well as the fact that, unlike [Rh(sulfos)]⁺, the

[Rh(dppp)]⁺fragment is able to pick up a benzene molecule

can well account for the higher activity of Rh(cod)-Pd/SiO₂vs

Rh(cod)(sulfos)-Pd/SiO₂.²

Concluding Remarks

Overall, the EXAFS and DRIFTS data are fully consistent

with a remarkable chemical stability of the anchored [Rh-

(dppp)]⁺single sites and suggest that the catalytic activity

enhancement observed with the recycled/air-exposed catalysts

may really be due to a restructuring of the Pd particle surface.³²

Besides oxidation/reduction cycles, a role in the surface modi-

ﬁcation process may also be played by the [Rh(dppp)]⁺

fragments through the formation of structures of the type shown

in Scheme 8.

Proposed Catalytic Mechanism. Scheme 9 illustrates a

mechanism for the hydrogenation of benzene by Rh(cod)-Pd/

SiO₂that we propose on the basis of the experimental evidence

accumulated in this work. First of all, we are conﬁdent to state

that the much faster hydrogenation rate of Rh(cod)-Pd/SiO₂as

compared to Pd/SiO₂is due to the combined action of the

rhodium single sites and of the surface palladium atoms on the

substrate. In particular, the contribution of palladium is manda-

tory for the conversion of benzene to cyclohexa-1,3-diene, yet

the hydrogenation of the latter requires the action of the rhodium

complex to be fast and irreversible. The overall mechanism

comprises three distinct reaction paths that reﬂect the concomi-

tant presence on the support material of chemically interacting

Rh^Isingle sites and palladium nanoparticles (a), isolated

palladium nanoparticles (b), and isolated Rh^Isingle sites (c).

Overall, the present mechanism is rather similar to that

previously suggested for benzene hydrogenation by Rh(cod)-

(sulfos)-Pd/SiO₂catalysis, shown in Scheme 10.²However,

some speciﬁc differences exist.

For example, the hydrogenation of cyclohexa-1,3-diene to

cyclohexane has been described in Scheme 9 as occurring with

the exclusive assistance of the rhodium single sites with no

involvement of the palladium particles. Obviously, this is an

extreme representation of the actual process, as no one can rule

out that an interaction between the rhodium sites and the

palladium atoms may persist under catalytic conditions; how-

ever, it is a fact that Rh(cod)/SiO₂is much more active than

Rh(cod)-Pd/SiO₂for the hydrogenation of cyclohexa-1,3-diene

to cyclohexane. Moreover, the largely prevalent proportion of

all-cis cyclohexane (stereoselective H₂cis-additions) in the

reduction of C₆H₆-d₆with H₂on Rh(cod)-Pd/SiO₂, as compared

to Pd/SiO₂, corroborates the hypothesis that the reduction of

cyclohexa-1,3-diene to cyclohexane is prevalently assisted by

the Rh^Isites.

This paper describes the synthesis and characterization of an

unprecedented type of hybrid bimetallic catalyst that combines

silica-supported palladium nanoparticles with an electrostatically

anchored rhodium complex. As such, this synthetic method can

extended to any cationic metal complex with no modiﬁcation

of the primitive molecular structure of the ligand, provided the

counteranion is able to form H-bonds to the support material

holding the metal particles.

The product obtained from the H-bond/ionic immobilization

of [Rh(cod)(dppp)]OTf onto Pd/SiO₂has been employed to

catalyze the hydrogenation of benzene to cyclohexane, showing

much higher activity as compared to Pd/SiO₂, even at extremely

low rhodium loading. In situ and ex situ EXAFS and DRIFTS

measurements, batch catalytic reactions under different condi-

tions, deuterium labeling experiments, and model organometallic

studies, taken altogether, have provided valuable information

on the mechanism by which the rhodium single sites and the

palladium nanoparticles cooperate with each other in promoting

the hydrogenation of benzene. The combined action of the two

metals activates the arene so as to allow the rhodium sites to

enter the catalytic cycle and speed up the overall hydrogenation

process by rapidly reducing benzene to cyclohexa-1,3-diene.

The new hybrid catalyst exhibits an excellent stability to

repeated catalysis cycles with no need of careful handling, which

contrasts with the intrinsic instability of the [Rh(dppp)]⁺

fragment under homogeneous hydrogenation conditions. The

formation of Rh-O bonds involving oxygen atoms from either

the silica support or oxidized Pd particles has been identiﬁed

as the factor that stabilizes the Rh single sites. Notably, the

exposure to air of used catalysts has a beneﬁcial effect on the

hydrogenation activity, which may be due to the modiﬁcation

of the Pd particle surface by the oxidation/reduction cycles as

well as the formation of (dppp)Rh-O-Pd(surface) bonds.

The present protocol for tethered complexes on supported

metals can provide a number of innovative hybrid catalysts with

various applications, including stereoselective reactions provided

chiral metal complexes and/or chiral modiﬁers of metal surfaces

are used.

Acknowledgment. The ELETTRA synchrotron light

laboratory in Basovizza (Trieste, Italy) and the staff of the

XAFS beamline are gratefully acknowledged for their support

and technical assistance. We thank also the Ministry of

University and Research of Italy (MUR) for ﬁnancial support

(FISR project: Nanosistemi inorganici ed ibridi per lo

sviluppo e l’innovazione di celle a combustibile) and the

European Commission for ﬁnancing the Network of Excel-

lence IDECAT (contract no. NMP3-CT-2005-011730).

It is noteworthy that no evidence for the decoordination of a

phosphine arm of dppp was obtained at any stage of the reactions

catalyzed by Rh(cod)-Pd/SiO₂, whereas phosphine arm unfas-

tening was actually required by Rh(cod)(sulfos)-Pd/SiO₂for

(37) Grunes, J.; Zhu, J.; Somorjai, G. A. In Nanotechnology in Catalysis;

Zhou, B., Hermans, S., Somorjai, G. A., Eds.; Springer: Berlin, 2004; Vol.

1, Chapter 1.

OM800223D

Article Doi

Benzene hydrogenation by silica-supported catalysts made of palladium nanoparticles and electrostatically immobilized rhodium single sites

DOI: 10.1021/om800223d

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Article abstract of DOI:10.1021/om800223d

Full text of DOI:10.1021/om800223d

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