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9 Surface, Supported, and Cooperative Catalysis

9 Surface, Supported, and Cooperative Catalysis

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252



Homogeneous Catalysis



products of the reaction. For example, polystyrene (P) beads can be

functionalized with –CH2PPh2 groups, allowing attachment of a variety

of catalysts, including (P–CH2PPh2)RhCl(PPh3)2.The bead swells in organic

solvents to admit substrate and the catalytic cycle proceeds normally.65a

Leaching of metal from the support is often a problem, however.

A catalyst can also be supported in a separate liquid phase if the

catalyst is made soluble in that liquid by appending solubilizing

groups, SG, to a ligand, as in P(C6H4(SG))3. Solubility in water can

be induced with –SO3Na solubilizing groups and in fluorocarbons

with –CH2CH2(CF2)nCF3. The reaction is run in a mixed solvent such

that the substrate and products concentrate in the organic phase and

the catalyst in the water or fluorocarbon layer; in the case of mixed

fluorocarbon–hydrocarbon solvents, the two layers become miscible on

heating but separate on cooling.65b

Surface Organometallic Chemistry

Catalysts functionalized with siloxane anchors can be attached to SiO2

nanoparticles (NPs) via [Si]–O–M links involving surface silanol groups,

denoted [Si]OH. They thus benefit from their high surface area of NPs

and relatively easy separability. Similarly, catalysts supported on magnetic Fe3O4 nanoparticles can be magnetically separated from the reaction medium for reuse.66 Other advantages accrue: catalysts that are

insoluble in a given solvent become viable when supported on NPs;

two different catalysts that might otherwise mutually interfere in conventional cooperative catalysis can be kept out of contact on separate

NPs.

A variety of organometallics has been covalently anchored to a silica

surface at single sites by [Si]–O–M links involving [Si]OH groups. The

oxophilic early metals are particularly well suited to this approach.

Once bound to the surface, many of the usual solution characterization

methods no longer apply. A combination of EXAFS (extended X-ray

absorption fine structure: see Chapter 16), solid-state NMR, and IR

spectroscopy, however, can often give sufficient information. Unusual

reactivity can be seen, probably as a result of site isolation, which prevents the formation of inactive M(μ-OR)nM dimers.67 Many such species

are catalytically active. Cp*ZrMe3 on Al2O3 gives an ethylene polymerization catalyst in the presence of the usual MAO activator ([MeAlO]n;

see Section 12.2); [([Si]O)Re(≡CtBu)(=CHtBu)(CH2tBu)] is active in

alkene metathesis. Remarkably, a number of these species carry out

alkane conversion reactions unknown in heterogeneous and very rare

in homogeneous catalysis. For example, ([Si]–O)3TaH causes disproportionation of acyclic alkanes into lower and higher homologs, such as of



References



253



ethane into methane and propane. A number of commercially important catalysts consist of organometailic compounds covalently attached

to surfaces. In the Phillips alkene polymerization catalyst,68 for example,

CrCp2 is supported on silica.

Cooperative Catalysis

If two or more catalysts operate within the same reactor to bring about

a tandem reaction that relies on them both, we have cooperative or

tandem catalysis. Common cases involve a metal complex and an

organocatalyst, the latter often supplying the asymmetric aspect.69a In

another example, light alkanes were first dehydrogenated to alkenes

with a pincer Ir catalyst (Section 12.4) and the resulting olefins were

then upgraded to heavier hydrocarbons by a Cp*TaCl2(C2H4) alkene

dimerization catalyst (Section 12.2).69b

Hidden Acid Catalysis

If a reaction is catalyzed by a proton acid, a metal-catalyzed version

may also be possible. Such is the case for addition of alcohols or carboxylic acids to alkenes and alkynes catalyzed by silver salts such as

AgOTf. In hidden acid catalysis,70 the metal may liberate free protons

that are the true catalyst. Careful control experiments are needed to

test this possibility.



• Catalysis, a key organometallic application, goes by the steps discussed in Chapters 6–8.

• Directed and asymmetric catalysis (Section 9.3) can lead to high

selectivity.

• Intermediates must be kinetically competent (Section 9.3); apparent intermediates may in fact be off-loop species (Fig. 9.1).



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PROBLEMS

It can be useful to work backwards from the product by identifying

reactant-derived fragments to see how they might be assembled by

standard organometallic steps.

9.1. Compound 9.31 is hydrogenated with a number of homogeneous

catalysts. The major product in all cases is a ketone, C10H16O, but



Problems



257



small amounts of an acidic compound C10H12O, 9.32, are also

formed. What is the most reasonable structure for 9.32, and how

could it be formed?





9.2. Would you expect Rh(triphos)Cl to be a hydrogenation catalyst

for alkenes (triphos = Ph2PCH2CH2CH2PPhCH2CH2CH2PPh2)?

How might the addition of BF3 or TlPF6 affect the result?

9.3. Predict the steps in the hydrocyanation of 1,3-pentadiene to

1,5-pentanedinitrile with HCN and Ni{P(OR)3}4.

9.4. Write out a mechanism for arene hydrogenation with (η3-allyl)­

Co{P(OMe)3}3, invoking initial propene loss. Why do you think

arene hydrogenation is so rare for homogeneous catalysts? Do

you think that diphenyl or naphthalene would be more or less

easy to reduce than benzene? Explain your answer.

9.5. Suggest plausible mechanisms for the reactions shown below,

which are catalyzed by a Rh(I) complex, such as RhCl(PPh3)3.





9.6. Comment on the possibility of finding catalysts for each of the

following:





9.7. What do you think is the proper structural formulation for

H2PtCl6? Why do you think the compound is commonly called

chloroplatinic acid? Make sure that your formulation gives a

reasonable electron count and oxidation state.

9.8. In some homogeneous alkyne hydrosilations, a second product

(B) is sometimes found in addition to the usual one (A). How do

you think B is formed? Try to write a balanced equation for the



258



Homogeneous Catalysis



reaction, assuming an A/B ratio of 1 : 1 and you will see that A

and B cannot be the only products. Suggest the most likely identity for a third organosilicon product C, which is always formed

in equimolar amounts with B.



9.9. The

following

reaction,

catalyzed

by

(η6-C6H6)­

Ru(CH2=CHCO2Et)2/Na[C10H8] (Na[C10H8] is simply a reducing

agent), has been studied by workers at du Pont as a possible route

to adipic acid, an important precursor for Nylon. Suggest a mechanism. How might you use a slightly modified substrate to test

your suggestion?



9.10.  (η6-C6H6)Mo(CO)3 is a catalyst for the reduction of 1,3-dienes to

cis monoenes with H2; suggest how this might work, why the cis

product is formed, and why the alkene is not subsequently reduced

to alkane.



9.11. A Pd(II) precatalyst with tBuPPh2 as supporting ligand gives a

catalyst that, with trimethylsilyl iodide and NEt3 as coreactants,

converts styrene to PhCH=CH(SiMe3). Propose a mechanism

and explain the role of the amine. (R. McAtee, S. E. S. Martin, D.

T. Ahneman, K. A. Johnson, and D. A. Watson, Angew. Chem. Int.

Ed., 51, 3663, 2012.)



10

PHYSICAL METHODS



We now look at spectroscopic and crystallographic methods for identifying a new complex, assigning its stereochemistry, and learning about

its properties.*

10.1  ISOLATION

Isolation and purification procedures closely resemble those of organic

chemistry. Most organometallics are solids at 20°, although some are

liquids, for example, CH3C5H4Mn(CO)3, or even volatile liquids, such as

Ni(CO)4. Numerous organometallics are air and water stable and can be

handled exactly like organic compounds, but inert atmosphere work is

sometimes required, notably for the electropositive f-block, and early dblock metals. In those cases, air and water must be completely absent.

Typical methods involve flasks and filter devices fitted with ground joints

for making connections and vacuum taps for removing air or admitting

* Undergraduates taking this course may not have had a physical chemistry course. The

material on spectroscopy has therefore been gathered together here, so that instructors

have the option of omitting all or part of it without losing the narrative flow of the rest

of the book.

The Organometallic Chemistry of the Transition Metals, Sixth Edition.

Robert H. Crabtree.

© 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.



259



260



PHYSICAL METHODS



nitrogen. In this Schlenk glassware, all operations can be carried out

under an inert atmosphere on an ordinary benchtop. As an alternative,

operations can be carried out in a N2-filled inert atmosphere box.

Details of these techniques are available in comprehensive monographs.1

10.2 



1



H NMR SPECTROSCOPY



Of all spectroscopic techniques,2 organometallic chemists tend to rely

heavily, perhaps too heavily, on NMR spectroscopy. The commonest

situation involves observing I = ½ nuclei with sufficient isotopic abundance, such as 1H (∼100% abundance), 13C (∼1%), 31P (100%), and 19F

(100%). Each chemically different nucleus in a molecule normally gives

a distinct signal. Any J coupling to adjacent inequivalent I = ½ nuclei

can provide evidence about the local environment of the atom in question. Beyond identifying the organic ligands, the 1H NMR technique3

is particularly useful for metal hydrides, which resonate in an otherwise

empty spectral region (0 to −40δ). This unusual chemical shift is ascribed

to shielding by the metal d electrons, and the shifts indeed become

more negative for higher dn configurations. The number of hydrides

present may be determined by integration or, if phosphines are also

present, from 2J(P,H) coupling in the 31P NMR spectrum (Section 10.4),

where the term nJ(X,Y) refers to the coupling of nucleus X and Y

through n bonds. For the 2J(P,H) coupling of M–H to adjacent PR3

groups, the fact that trans couplings (90–160  Hz) are larger than cis

(10–30 Hz) often allows full stereochemical assignment, as seen in Fig.

10.1 for some Ir(III) hydrides. Similar cis < trans coupling relationships

hold for other pairs of NMR-active donor atoms. The 5-, 7-, 8-, and

9-coordinate hydrides are often fluxional so that the ligands exchange

positions within the complex sufficiently fast to become equivalent on

the NMR timescale (∼10−2 s). We look at some consequences of fluxionality later (Section 10.5).

2

H NMR spectroscopy is useful for following the fate of deuterium

in mechanistic experiments. Even though D is an I = 1 nucleus, the 2H

spectrum is still obtainable but has broader resonances than for 1H. The

chemical shifts are essentially identical to those seen in the 1H NMR

spectrum, however, which greatly simplifies the interpretation, but all

J coupling to 2H are reduced by a factor of 6.5 versus 1H because of the

lower gyromagnetic ratio for 2H.

Virtual Coupling

Virtual coupling in the 1H NMR spectrum can help geometry assignments for complexes involving phosphines such as PMe3 or PMe2Ph. If



H NMR SPECTROSCOPY



1



261



FIGURE 10.1  The 1H NMR spectra of some iridium hydrides (hydride

region). Each stereochemistry gives a characteristic coupling pattern.



two such phosphines are cis, they behave independently, and we see a

2

J(P,H) doublet for P–Me. If they are trans, the 2J(P,P′) coupling coupling becomes so large that the 1H NMR of the P–Me unit is affected.

Instead of a simple doublet, we see a distorted triplet with a broad

central peak giving the appearance that the P–Me is coupled to P and

P′ about equally (Fig. 10.2a). Intermediate P–M–P angles between 90°

and 180° give intermediate patterns (Fig. 10.2b and 10.2c).

Diastereotopy

The 1H NMR spectrum of a PMe2Ph ligand in 10.1 and 10.2 can provide

stereochemical assignments from symmetry (Fig. 10.3). In 10.1, a mirror

plane containing M, X, Y, and the PMe2Ph phosphorus reflects one

P–Me group into the other and makes them equivalent; 10.2 lacks such

a plane of symmetry, and the inequivalent Me′ and Me″ groups are

termed diastereotopic.3a In general, two groups will be inequivalent if

no symmetry element of the molecule exchanges one with the other.



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PHYSICAL METHODS



FIGURE 10.2  Virtual coupling in the PMe proton resonance of methylphosphine complexes. Each methyl group shows coupling both to P and to P′ as

long as 2J(P, P′) is large enough. As the MeP–M–P′ angle decreases from 180°,

the virtual coupling decreases, until at an angle of 90°, we see a simple doublet,

owing to coupling of the PMe protons only to P, not to P′. At intermediate

angles the spectrum takes up a ghostly appearance (case b).



Diastereotopic groups are inequivalent and generally resonate at different chemical shifts. We will therefore see a 2J(P,H) doublet for 10.1

and a pair of 2J(P,H) doublets for 10.2. The appearance of the spectrum

changes on moving to a higher field spectrometer (Fig. 10.3) because

the diastereotopic resonances differ by a certain chemical shift in parts



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