SYNTHESIS AND CATALYTIC APPLICATIONS OF IRON PINCER COMPLEXES

SYNTHESIS AND CATALYTIC APPLICATIONS OFIRON PINCER COMPLEXES

PAPRI BHATTACHARYA and HAIRONG GUAN

Department of Chemistry, University of Cincinnati,Cincinnati, Ohio, USA

Transition metal complexes bearing pincer ligands have beenextensively studied during the past decade. One of the less-exploredor perhaps even neglected areas of pincer chemistry is on the reac-tivity of iron complexes that incorporate these enormously popularligand sets. Meanwhile, recent emphasis on homogeneous catalysishas been placed on the developments of catalytic reactions pro-moted by inexpensive and environmentally benign metals, for whichiron is particularly attractive. In that regard, investigating the reac-tivity patterns of well-defined iron pincer complexes can potentiallyprovide valuable guidelines for the rational design of iron catalysis.This review analyzes the challenges and successes of synthesizingiron complexes with different pincer-type ligands, and discussesthe established or potential utility of these complexes for catalyticreactions.

Keywords: C-H bond activation, cross-coupling reactions, first-rowtransition metals, homogeneous catalysis, hydrogenation, hydrosily-lation, iron, pincer complexes

Abbreviations: acac–acetylacetonate, DFT–density functional theory,dmpe–bis(dimethylphosphino)ethane,MAO–methylaluminoxane,Mes–2,4,6–trimethylphenyl, Mes!–2,4,6-tri-tert-butylphenyl, TMEDA-N,N, N0, N0-tetramethylethylenediamine

Address correspondence to Hairong Guan, Department of Chemistry, University of

Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA. E-mail: hairong.guan@

uc.edu

Comments on Inorganic Chemistry, 32: 88–112, 2011

Copyright # Taylor & Francis Group, LLC

ISSN: 0260-3594 print

DOI: 10.1080/02603594.2011.618855

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INTRODUCTION

Since the late nineteenth century when Alfred Werner proposed

octahedral structures for a series of Co(III) complexes,[1] inorganic

chemists have been on a quest for new ligands that confer unique

reactivity on metal ions. Among numerous multidentate ligands

developed along the journey, pincer ligands have gained tremendous

popularity, largely due to their high tunability and their general tendency

to make the metal complexes thermally robust and, in some cases, their

ability to stabilize unusual metal oxidation states. The chemistry of pin-

cer compounds, especially recent applications in stoichiometric bond

activation and catalytic transformations, has been nicely summarized

in a book[2] and a number of reviews.[3–17] However, a brief survey of

the literature tells us that the bulk of pincer research has dealt with

complexes containing precious metals such as ruthenium, iridium, and

palladium.

From the homogeneous catalysis point of view, while new reactions

are still in high demand, there is an urgency to develop non-precious

metal-based catalysts for known chemical transformations.[18–20]

Of particular note is the rapidly growing research area of iron

catalysis,[21–30] which has not only been driven by the low cost of iron

but also by its low toxicity. Many of the recipes for iron catalysts have

involved a simple iron salt combined with some chelating ligand. They

are simple to operate, but usually offer limited mechanistic insights that

can be used to improve the catalyst systems. Therefore, studies of

well-defined iron complexes sometimes could be more valuable in terms

of leading to the discovery of more efficient and selective catalysts, as

exemplified by recent successes from the Morris group on iron-catalyzed

transfer hydrogenation reactions.[26,31–35]

It occurred to us that the marriage between pincer ligands and iron

might open a new chapter of homogeneous catalysis. The versatile pool

of pincer ligands can potentially provide the desired reactivity at the iron

center while preventing the decomposition of iron species, which has

been a problem commonly encountered in iron catalysis. The purpose

of this review is to summarize the chemistry of iron pincer complexes

to date and to highlight our recent developments on this specific research

topic. It should be pointed out that the term ‘‘pincer’’ has never been

explicitly defined in the literature. The consensus is that the first set of

pincer complexes reported (but not originally called ‘‘pincer’’) by Shaw

IRON PINCER COMPLEXES 89

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in 1976 features a C!M bond with two flanking phosphine groups.[36]

Over the years, the scope of pincer ligands has been extended in various

ways (Scheme 1), with the broadest definition of ‘‘pincer’’ including any

tridentate ligand that binds a metal in a mer configuration.[16] We shall all

be reminded that bis(imino)pyridine iron complexes[37,38] described by

Brookhart[39,40] and Gibson[41,42] have been extensively studied as

excellent catalysts for the polymerization or oligomerization of olefins.

Chirik and others have recently shown other catalytic applications of

these complexes such as in hydrogenation,[43–47] hydrosilylation,[43,48]

[2" 2] cycloaddition,[49–51] and reductive cyclization reactions.[52]

Although bis(imino)pyridine ligands coordinate to iron in a mer fashion,

the metal complexes are rarely referred as pincer complexes, and there-

fore they will not be discussed in this paper. Also excluded from our

discussions here are closely related iron complexes bearing

pyridine-bis(oxazoline),[53] terpyridine,[54] and other neutral N, N, N-type

ligands.[55–57]

SYNTHESIS OF IRON PINCER COMPLEXES

The synthetic routes to iron pincer complexes are highly dependent on the

nature of the pincer donor groups. For instance, iron complexes of the

original type pincer ligands (also known as PCP-pincer ligands) contain

C!Fe bonds, which may be constructed via cyclometalation reactions.

Compared to other d-block transition metals, particularly those in Group

10, cyclometalation with iron are, however, less common.[19,58] To

Scheme 1. Variation of pincer complexes.

90 P. BHATTACHARYA AND H. GUAN

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promote such reactions, one can employ either low-valent metal species

or metal precursors bearing basic ligands such as alkoxide and alkyl

groups.

The synthesis of an iron PCP-pincer complex was accomplished by

Creaser and Kaska, who demonstrated that in the presence of dmpe and

30% sodium amalgam, a structurally ill-defined polymer formed by mix-

ing FeCl2 with 1,3-C6H4(CH2PMe2)2 was converted to an iron hydride

species (Scheme 2).[59] Perhaps at the polymeric stage, the C!H bond

remained intact; however, the cyclometallation event took place once

Fe(II) was reduced to Fe(0).

A more recent example with a phosphinite-based pincer ligand

(commonly abbreviated as POCOP-pincer to distinguish it from PCP-

pincer) involves the use of Fe(Me)2(PMe3)4 as the iron precursor, which

contains basic methyl groups to facilitate the C!H bond activation

(Scheme 3).[60] Surprisingly, the isolated pincer complex was not an iron

methyl complex as expected from methane elimination, but rather an

Scheme 2. The synthesis of an iron PCP-pincer complex.

Scheme 3. Synthesis of an iron hydride complex bearing a POCOP-pincer ligand.


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iron hydride species with the methyl group incorporated into the pincer

backbone. Li and coworkers proposed that C!C reductive elimination

would yield a transient Fe(0) intermediate en route to the second

C!H bond activation step.

Our research group has been interested in homogeneous catalysis

with resorcinol-derived pincer complexes of first-row transition metals.

Our previous efforts have been focused on reduction and cross-coupling

reactions catalyzed by nickel pincer complexes.[61–65] Encouraged by the

result shown in Scheme 3, we recently utilized Fe(PMe3)4 as a Fe(0)

source to accomplish the synthesis of new iron pincer complexes

(Eq. (1)).[66] We found that the size of the substituents on the phosphorus

donors was critical to the success of synthesis; with more bulky groups

(e.g., R #tBu) on the P’s, the pincer ligand was almost unreactive toward

Fe(PMe3)4, producing a negligible amount of a new iron hydride species

with only one PMe3 bonded to the iron center.

An analogous approach to forming C!Fe bonds with pincer-type

ligands is via the oxidative addition of carbon-halogen bonds to Fe(0),

as demonstrated in a recent report on the synthesis of iron complexes

with a bis(oxazolinyl)phenyl ligand (Eq. (2)).[67] Of the commercially

available iron carbonyl reagents, Fe2(CO)9 proved to be the best choice.

A similar reaction of the achiral ligand (in Eq. (2)) with Fe3(CO)12yielded 5% of the desired iron pincer complex, whereas the reaction of

the same ligand with Fe(CO)5 failed.


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Transmetalation from reactive metals such as lithium and magnes-

ium to iron is another efficient way to prepare iron pincer complexes

(Eq. (3)), provided that lithium- or magnesium-based organometallic

reagents are readily accessible from the neutral pincer preligands.

Figure 1 summarizes the reagents that have been successfully used to

synthesize the corresponding iron pincer complexes.[68–73] Having amine

groups as pincer arms allows direct lithiation at the center carbon bynBuLi,[74] rather than at the benzylic positions as observed for the

PCP-pincer preligands.[4] Lithiation of POCOP-pincer precursors could

be problematic as the most vulnerable sites for nBuLi attack is probably

at the phosphorus centers, resulting in the cleavage of P!O bonds. For

NH-centered pincer preligands, generating the desired organolithium

and Grignard reagents is more realistic due to the relatively acidic hydro-

gens. Similarly, pyridine-linked bis(aniline) can be doubly deprotonated

and then subsequently react with FeCl2 to give a four-coordinate iron

pincer complex (Eq. (4)).[75] Although all these transmetalation reactions

seem straightforward, a competing process involving the reduction of

metal salts by the organolithium or Grignard reagents sometimes can

occur. To circumvent this problem, van Koten has recommended con-

verting the reactive organometallic reagents to the thermally more stable

and less reducing organogold(I) reagents before transferring to the

targeted metal center.[76]

Recent studies on metal complexes of 2,6-bis(phosphino)pyridine

ligands have shown that these compounds behave similarly to the

PCP-pincer complexes, providing legitimate reasons to be included in

the extended pincer family. Under this broader definition of ‘‘pincer,’’


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pincer chemistry can be traced back to the days even before Shaw’s first

report on cyclometalated phosphine-based pincer complexes.[36] As early

as in 1971, Nelson and coworkers already disclosed the synthesis and mag-

netic properties of Fe(II), Co(II), and Ni(II) complexes of 2,6-bis(diphe-

nylphosphinomethyl)pyridine.[77] Recent interests in iron complexes with

these PNP-type pincer ligands have been shifted to those with alkyl groups

on the phosphorus donors,[78,79] and those with a different linkage between

the phosphorus donors and the pyridine ring (Figure 2).[80,81] The synthetic

advantage of iron pincer complexes with these neutral pincer ligands lies in

the fact that the ligands are ‘‘ready’’ for use without any activation.

As N-heterocyclic (NHC) carbenes resemble phosphines in many

ways, pyridine-linked dicarbene ligands (C-N-C) represent a new class

of neutral pincer ligands.[11] In the synthesis of iron complexes, mixing

a free C-N-C ligand with FeCl2(THF)1.5 or FeCl2(PPh3)2 led to a species

with the formulation of f[(C-N-C)FeClL]2(FeCl4)g. A similar reaction

with [FeCl2(TMEDA)]2 gave rise to an iron complex with one ligand

adopting the normal pincer coordination mode and a second ligand

cyclometalated at the C-5 of one NHC ring.[82] However, the synthesis

of much simpler (C-N-C)FeX2 is possible if Fe[N(SiMe3)2]2 is used to

react with pyridyl-bis(imidazolium) salts (Eq. (5)).[82,83]

Figure 1. Organolithium and Grignard reagents used for the synthesis of iron pincer

complexes.


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CATALYTIC APPLICATIONS

Many of the reported iron pincer complexes, particularly those

five-[72,77–79,84] and four-coordinate ones,[69–71,75,81,85–87] exhibit unusual

electronic structures and interesting magnetic properties. Iron(0) com-

plexes supported by pyridyl-dicarbene ligands bind N2 and silanes, show-

ing great promise of activating small molecules.[88,89] When exposed to

CO, five-coordinate iron(II) complexes bearing 2,6-diaminopyridine-

derived PNP-pincer ligands lead to completely different chromic beha-

viors in solid state versus in solution, suggesting that these compounds

may be developed into efficient molecular sensors and switches.[90–92]

The focus of our discussions here, however, is on the iron pincer

complexes that have been subjected to catalytic studies.

Hydrosilylation of Aldehydes and Ketones

As our initial efforts to establish the catalytic applications of the iron

POCOP-pincer hydride complexes prepared in our laboratory, we

explored the insertion chemistry of these compounds with various sub-

strates containing C=O bonds. Such information could be very useful

in designing catalytic systems for the reduction of carbonyl function-

alities with first-row transition metals.[20] In contrast to the rapid,

room-temperature insertion of aldehydes[61] and CO2[62,65] observed

for nickel POCOP-pincer hydrides, similar reactions with the iron

hydrides did not take place (Scheme 4).

The inability of the iron pincer hydrides to stoichiometrically reduce

C=O bonds can be rationalized by the way of ligand dissociation from

the coordinatively saturated iron center. As shown in the substitution

reaction of 1 with CO (Scheme 5), PMe3 trans to the hydride ligand gets

Figure 2. Neutral pincer ligands used for the synthesis of iron pincer complexes.


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displaced, producing a new iron hydride species 2.[66] This implies that

aldehydes and CO2 would also be placed at the trans position with

respect to the hydride ligand, preventing the substrates to interact with

the hydride. We also discovered that at 60$C 2 was isomerized to the

thermodynamically more stable 20; however, this process took 7 d to

complete.

Nevertheless, the vacant coordination site generated by PMe3 dis-

sociation can activate organic carbonyl substrates or activate reducing

reagents such as silanes and boranes, providing an alternative solution

to the reduction of carbonyl functionalities. In fact, 1 is capable of cat-

alyzing the hydrosilylation of aldehydes and ketones bearing various

functional groups (Eq. (6)).[66] We have proposed two possible catalytic

Scheme 4. Attempted insertion reactions with iron POCOP-pincer hydride complexes.

Scheme 5. Displacement of PMe3 by CO from an iron POCOP-pincer hydride complex.


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cycles that share the same 16-electron intermediate A (Scheme 6).

Experimental evidence supporting these mechanistic hypotheses

includes the observation that our hydrosilylation reactions were

inhibited by added PMe3. Moreover, we have used the isotope-labeling

method recently developed by Nikonov[93] to probe the role of the

hydride in the catalytic reactions. The 1:1:1 mixture of PhCDO,

Ph2SiD2, and 1 gave silyl products without any H incorporated, suggest-

ing that the hydride ligand does not directly participate in the reduction.

Asymmetric hydrosilylation of ketones catalyzed by chiral iron pin-

cer complexes are also known in the literature. The Gade group has

reported modest to excellent enantioselectivity (50–93% ee) for their iron

catalytic system involving chiral bis(pyridylimino)isoindole ligands

(Eq. (7)).[94] A more recent study by Nishiyama et al. has shown that a

chiral bis(oxazolinyl)phenyl iron complex catalyzes the conversion of

ketones to secondary alcohols with ee’s ranging from 21% to 66%

Scheme 6. Two possible catalytic cycles for the hydrosilylation reactions.


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(Eq. (8)).[67] Although the mechanistic details are not clear for either

system, iron hydride intermediate has been suggested in the Nishiyama

system.[67] The additive Na(acac) was found to enhance the rates of

the reactions, presumably due to its ability to facilitate the transfer of

hydride from Si to Fe.

Hydrogenation of Ketones

Milstein and coworkers have recently established a pyridine-based pincer

complex as an efficient catalyst for the hydrogenation of ketones

(Eq. (9)).[95] They have reported turnover numbers up to 1880 for the

reactions with a catalyst loading as low as 0.05mol%. One of the main

reasons for the effectiveness of this iron catalyst is believed to be the

aromatization-dearomatization of the pincer ligand, which presumably

facilitates the binding of ketone, or the activation of H2, or both. Based

on NMR studies of relevant stoichiometric reactions, they have outlined

a mechanistic cycle involving a 16-electron intermediate B with a dearo-

matized pincer ligand (Scheme 7). The coordination of a ketone to B


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would place it trans to the hydride ligand, assuming that B is configura-

tionally stable. The authors have proposed that geometric isomerization

at the iron center would render ketone and hydride cis to each other, a

necessity for the insertion of the C=O bond. The catalytic cycle is then

completed with the activation of H2 followed by the release of the alcohol

product. To account for the fact that only alcoholic solvents can be used

in the catalytic reactions, they have suggested that alcohols would stabi-

lize B by reversibly forming an 18-electron iron species. The use of apro-

tic solvents such as THF, benzene, and toluene led to the decomposition

of the catalyst.

In a recent Highlight paper,[96] Bauer and Kirchner have pointed out

that the Milstein system shares some similarity with two previously

Scheme 7. Mechanism for the hydrogenation of ketones catalyzed an iron PNP-pincer

complex.


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known bifunctional iron catalytic systems for the hydrogenation of

ketones: Casey’s hydroxycyclopentadienyl iron dicarbonyl hydride[97]

and Morris’ cationic iron complexes with a chiral tetradentate P-N-N-P

ligand (Figure 3).[98] All these three well-defined iron catalytic systems

take the advantage of the relatively acidic CH, OH, or NH (possibly

resulting from the hydrogenation of the C=N bonds in Morris’ com-

pounds) moiety, whether or not the transfer of such acidic hydrogen is

synchronized with the transfer of the hydridic hydrogen from iron.

Hydrogenation and Hydrosilylation of Olefins

The Chirik group has demonstrated that bis(imino)pyridine iron com-

plexes are efficient catalysts for the hydrogenation and hydrosilylation

of olefins.[43] They have also compared these complexes with bis(pho-

sphino)pyridine iron complexes in order to understand how the ancillary

ligand impacts on the catalytic activity.[99] Although the iron pincer dihy-

dride complex is not as effective as bis(imino)pyridine iron complexes in

catalytic hydrogenation of 1-hexene, the reaction can still finish in several

hours with a low catalyst loading (Eq. (10)). The hydrogenation of cyclo-

hexene with the same catalyst has been affected by catalyst degradation,

resulting in an incomplete reaction. The analogous iron pincer silyl

hydride complex is not suitable for catalytic hydrosilylation of 1-hexene

or cyclohexene with PhSiH3.

Figure 3. Bifunctional iron catalytic systems for the hydrogenation of ketones.


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Coupling Aldehydes with a-Diazo Esters

In addition to catalyzing various reduction reactions, iron pincer com-

plexes have been used to promote the coupling of aldehydes with a-diazoters (Eq. (11)). Although simple Lewis acids (e.g., ZnCl2, SnCl4) or

BrØnsted acids (e.g., HBF4 %Et2O) are known to catalyze this process,

the obtained products are often mixtures of b-keto esters and 3-

hydroxy-2-aryl acrylates.[100] Kirchner and coworkers have shown that

the catalytic reactions with cationic iron PNP-pincer complexes

(Figure 4) selectively produce the 3-hydroxy-2-aryl acrylates, underscor-

ing the benefit of employing well-defined metal catalysts.[101,102] Even

though DFT studies (on a truncated model) have suggested that energeti-

cally aryl migration is more favorable than hydride migration from the

expected intermediate (Scheme 8),[102] the origin of the selectivity is still

not fully understood.

Cross-Coupling Reactions

Among many other iron systems,[103–106] pyridine-linked bis(carbene)

iron complexes have been tested for catalytic cross-coupling of alkyl

Figure 4. Iron PNP-pincer complexes for the coupling of aldehydes with a-diazo esters.

Scheme 8. Migration of aryl group and hydride leading to two different products.


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halides with Grignard reagents.[107] In the presence of 5mol% of iron

catalyst, the reaction of 4-tolylmagnesium bromide with alkyl halides

bearing b-hydrogens afforded the coupling products in good GC yields

(71–94%) without any byproducts stemming from b-hydride elimination

(Eq. (12)). Radical intermediates have been invoked based on mechan-

istic studies that were conducted on similar reactions catalyzed by FeCl3with phosphine ligands. Consistent with the proposed radical pathway, a

very small amount of homo-coupling product from chlorocyclohexane

was detected in the coupling reaction.

Attempted or Computated Reactions

We would like to conclude our discussions with two attempted catalytic

reactions with iron pincer complexes: one has been carried out exper-

imentally and the other one has been calculated computationally.

Although neither of them has given encouraging results, these studies

demonstrate the increasing interests in utilizing iron pincer complexes

for catalysis.

Motivated by the successful stories on the polymerization of

olefins catalyzed by bis(imino)pyridine iron complexes and their

derivatives,[37–42] Rieger et al. have examined the catalytic performance

of iron and cobalt complexes with bis(phosphino)pyridine ligands in eth-

ylene polymerization reactions.[108] When activated by MAO, the cobalt

pincer complexes did catalyze the polymerization of ethylene. Surpris-

ingly, the analogous iron complexes showed no catalytic activity due to

the rapid decomposition of the complexes after mixing with MAO.

Pincer complexes [Fe(H)2(H2)(PXP)] have been modeled (using

DFT calculations) as potential catalysts for the synthesis of NH3 from


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N2 and H2.[109] Unfortunately, the transition states for the envisioned

hydrogen transfer to N2 (Eqs. (13) and (14)) have yet to be located.

SUMMARY AND OUTLOOK

In conclusion, we have summarized different methods for the synthesis

of iron pincer complexes. Oxidative addition of C!H or C!X bonds

of pincer preligands to low-valent iron species is an efficient way to

construct pincer complexes featuring C!Fe bonds. When the C!H moi-

ety of pincer preligands can be deprotonated by an internal base (on Fe)

or an external base, cyclometalation or transmetalation strategy can be

used to install the pincer scaffold. A similar approach is applicable to

pincer preligands with more acidic NH centers. For complexes with a

neutral pincer ligand, the synthesis is often accomplished by direct coor-

dination of the ligand to iron.

We have also presented the catalytic activities of iron pincer com-

plexes. Significant progress has been made in iron pincer catalyzed

reduction (hydrogenation and hydrosilylation) of aldehydes, ketones,

and olefins. In addition, iron pincer complexes have been shown to

catalyze C!C bond forming reactions. Although the polymerization

of olefins and activation of N2 with iron pincer complexes have not

yet yielded positive results, more iron pincer systems could be

tested.

Looking forward, we expect that the interests in iron pincer

complexes will continue to grow, as these types of compounds have

great potentials for numerous catalytic applications. They can be devel-

oped as efficient bifunctional catalysts or selective Lewis acid catalysts.

When chiral pincer ligands are employed, some of the reactions are

amenable to asymmetric catalysis. If indeed radical processes are


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involved (as suggested in cross-coupling reactions[107]), the use of

iron pincer catalysts could be advantageous as the redox potentials of

iron complexes can be readily modulated by varying the pincer

ligands.

ACKNOWLEDGMENTS

We thank the National Science Foundation (CHE-0952083) and the

donors of the American Chemical Society Petroleum Research Fund

(49646-DNI3) for generous support of our research. P. B. is grateful to

the University of Cincinnati University Research Council for a graduate

student research fellowship.

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SYNTHESIS AND CATALYTIC APPLICATIONS OF IRON PINCER COMPLEXES

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