Recent advances in metallabenzene chemistry

Reviews

M. M. Haley and C. W. Landorf

DOI: 10.1002/anie.200504358

Metallabenzenes

Recent Advances in Metallabenzene Chemistry

Christopher W. Landorf and Michael M. Haley*

Keywords:

Dedicated to Professor Peter Vollhardt

aromaticity · iridium · metallacycles ·

osmium · platinum · ruthenium

on the occasion of his 60th birthday

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Chemie

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Metallacycles

Chemie

R

esearch into aromatic metallacycles, though discussed in the

From the Contents

1. Introduction

2. Historical Perspectives

3. Osmium

literature over the last quarter century, has undergone a major

expansion since 2000. A wide variety of new metallabenzenes,

encompassingnew synthetic methods and new metal centers, is now

available. New aromatic metallacycle topologies (iridanaphthalene,

osmabenzynes) have been isolated and characterized. The first

metallabenzene valence isomers (iridabenzvalenes, rhodabenzva-

lenes) and constitutional isomers (isoosmabenzenes) are now

known. This review discusses the synthesis, chemistry, and physical

properties of these intriguing aromatic compounds.

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4. Ruthenium

5. Iridium

6. Rhodium

7. Platinum

1. Introduction

affect the aromatic-

ity and thus stabil-

ity, reactivity, prop-

erties, etc.of the

resultant compounds.

8. Conclusions

Ever since KekulØ proposed a cyclic six-membered ring

with rapidly alternating single and double bonds to describe

the structure and stability of benzene, chemists have been

debating the nature of aromaticity and “aromatic” mole-

cules.^[1,2]New aromatic molecules are constantly being

synthesized, some of which continue to stretch and expand

our definition of this concept.How does one qualify an

aromatic compound?^[1–3]An undergraduate chemist is usually

taught that a compound is either aromatic, antiaromatic, or

non-aromatic based on the Hückel [4n+2] p-electron rule,

but it does not take much imagination to think of a compound

that does not neatly fall into one of these categories.These

“intermediate” compounds may exhibit some properties

normally associated with aromaticity, such as downfield

¹H NMR signals, but completely lack other facets, such as

stability towards dienophiles.For these reasons, it is risky to

attempt to define aromaticity in fixed terms.Instead, it may

be best to remove arguments that attempt to rigidly define the

concept of “aromaticity” by considering the characteristics

commonly attributed to aromaticity separately.^[3]

The replacement of a CH group within a benzenoid ring

with an isolobal heteroatomic fragment is well-known and

exemplified by molecules such as pyridine, phosphabenzene,

pyrillium, and thiabenzene.^[4–6]Benzenoid aromatic com-

pounds of even heavier elements such as silicon and gallium

are also known.^[5–8]Where the lines begin to blur is with one of

these classes of “intermediate” compounds known as metal-

labenzenes.Metallabenzenes are six-membered metallacycles

analogous to benzene for which one CH unit has been

replaced by an isolobal transition-metal fragment {ML_n}.^[9,10]

These metallacycles differ from regular aromatic compounds

in that the p bonding requires involvement of the metal

d orbitals since the metal p orbitals participate in s bonding to

the ligands.Thorn and Hoffmann were the first to consider

the application of the Hückel rule to metallabenzenes.^[11]In

their analysis, four electrons come from the p orbitals in the

five-carbon backbone, while two electrons come from the

filled d_xzorbital of the metal fragment; therefore, metal-

labenzenes obey the Hückel definition of aromaticity.None-

theless, with such a fundamental difference in bonding, one

cannot but wonder how incorporation of the d orbitals might

In the quarter century since Thorn and Hoffmannꢀs initial

proposal, over 30 varieties of metalla-aromatic species have

been synthesized and/or characterized.Although most of

these metallacycles have been isolated examples, a majority

exhibit properties normally associated with aromatic systems,

such as relatively deshielded proton resonances in the

¹H NMR spectrum.^[12]Whereas the ring protons in metal-

labenzenes are also deshielded, the anisotropy of the metal

center can severely affect this analysis, especially at the

position ortho to the metal.^[9]NICS (Nucleus Independent

Chemical Shift) calculations have often been invoked to

determine the aromaticity of a compound;^[13]however, NICS

values are also subject to the anisotropy of metal centers.^[14]

Structurally, bond lengths and ring planarity can be

analyzed to determine if the molecule in question is

aromatic.^[15]An aromatic ring typically exhibits bond lengths

intermediate between a single and double bond, and this

property holds true to a reasonable extent for all metal-

labenzenes characterized to date.The bulkiness of the metal

fragment, however, tends often to distort the six-membered

ring from planarity.

Unique reactions such as electrophilic aromatic substitu-

tion (EAS) are occasionally invoked to demonstrate aroma-

ticity.Owing in part to the reactivity of the metal center, EAS

reactions have rarely been observed for metallabenzenes.^[9,16]

Another reaction unique to aromatic compounds is the

formation of arene-coordination complexes.For metallaben-

zenes, these complexes are formed readily and often stabilize

metallacycles that would otherwise be too short-lived to be

observed.^[9,17,18]

[*] C. W. Landorf, Prof. Dr. M. M. Haley

Department of Chemistry

University of Oregon

Eugene, OR 97403-1253 (USA)

Fax : (+1)541-346-0487

E-mail: haley@uoregon.edu

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M. M. Haley and C. W. Landorf

Aromatic compounds are often noted for their stability

and lack of reactivity under ordinary conditions.Metal-

labenzenes, however, undergo some very unusual reactions

that are unheard of for classically aromatic compounds.For

example, cycloadditions with dienophiles and rearrangements

to form cyclopentadienyl complexes suggest that these

molecules are not as aromatic—that is, as stable—as their

all-carbon analogues.^{[9,17,19–22]}

While metallabenzenes have been known since 1982^[23]

and were the subject of an excellent review in early 2001,^[9]

these metallacycles have been synthesized at an ever-increas-

ing pace over the last few years.More importantly, new

aromatic topologies (metallabenzynes, metallanaphthalenes)

as well as new, versatile syntheses of such metallacycles have

begun to emerge.As opposed to being an all-encompassing

document, this review will highlight the numerous advances

from 2000 on.We will focus predominantly on the syntheses

and properties of metallabenzenes and related aromatics

where one metal atom and five carbon atoms comprise the

metallacycle.Some attention will be paid to metal-coordi-

nated metallabenzenes, for which the uncoordinated metal-

labenzenes, specifically of the 4d metals, were until very

recently not known.Although there are numerous examples

of hetero-metallabenzenes, such as a metallathiabenzene, that

are worthy of inclusion, they do not fall under the scope of this

review.^[24,25]

the metal center increases, the three peaks at d = 7.28

resonated in the typical range for aromatic protons.Addi-

ꢀ

tionally, C C bond-length alternation was shown to be

negligible by X-ray crystallography.

Work reported by Hughes and co-workers in 1986

demonstrated the first use of vinylcyclopropenes to give six-

membered metallacycles.Platinacyclohexadiene 2 could be

made from treating [Pt(h²-C₂H₄)(PPh₃)₂] with 1,2,3-triphenyl-

3-vinylcycloprop-1-ene.^[27]A year later this group reported

that reaction of the same vinylcyclopropene with [MCl-

(PMe₃)₂] (M = Rh, Ir) also produced the corresponding

metallacyclohexadienes, one of which was crystallographi-

cally characterized as the acetylacetonate (acac) complex 3.^[28]

On heating, these metallacyclohexadienes were found to

generate 1,2,3-triphenylcyclopentadiene.Recent studies have

demonstrated that elimination to form cyclopentadienyl

complexes is a common decomposition pathway of metal-

labenzenes, although other pathways could be responsible for

this transformation.^[22]

2. Historical Perspectives

Originally postulated to be a stable species by Thorn and

Hoffman in their seminal paper in 1979,^[11]the first unambig-

uous isolation of a metallabenzene was reported by Roper

and co-workers in 1982.^[23,26]Through inspiration from the

synthesis of metallacyclopentadienes from the [2+2+1] cyclo-

addition of alkynes with transition-metal complexes, arene 1

was isolated from the formal [2+2+2] cycloaddition of two

ethyne molecules to [Os(CO)(CS)(PPh₃)₃].The ¹H NMR

spectrum of the resulting six-membered metallacycle exhib-

ited a peak at d = 13.95 ppm and several overlapping peaks at

d = 7.28 ppm. The unusual peak at d = 13.95 ppm was attrib-

uted to the proton ortho to the metal, and its deshielding was

attributed to the anisotropy of the metal.Since the effects of

the metal are expected to drop sharply as the distance from

In 1989, Bleeke et al.reported the synthesis of the first

stable iridabenzene 4 utilizing 2,4-dimethylpentadienide as a

source for the carbon backbone.^[17,29]Using the resultant

iridabenzene as a starting point, the Bleeke group extensively

examined the chemistry of such complexes.^[17]When exposed

to EAS conditions, it was found that the electrophiles react

preferentially with the electron-rich metal center.Addition-

ally, it was found that when exposed to dienophiles, these

iridabenzenes behave more like cyclohexatrienes and

undergo cycloaddition reactions.^[19,20,30]

The first ruthenabenzene 5 to be observed spectroscopi-

cally was reported by Jones, Allison, and co-workers in

1995.^[31]Utilizing a method similar to what Ferede and Allison

had previously reported for a suspected ferrabenzene,^[32]

a

Michael M. Haley was born in 1965 in Lake

Charles, LA. After growing up in Tulsa,

Oklahoma, he carried out his Bachelor’s and

Ph.D. studies with Prof. W. E. Billups at Rice

University. In 1991 he received an NSF

Postdoctoral Fellowship to work with Prof.

K. P. C. Vollhardt at the University of Cal-

ifornia, Berkeley. In 1993 he joined the

faculty at the University of Oregon where he

is currently Professor of Chemistry and

member of the Materials Science Institute.

His current research focuses on the chemis-

try of dehydroannulenes and dehydroben-

zoannulenes, metallabenzenes, and other

novel aromatic systems.

Christopher W. Landorf was born in

Summit, New Jersey in 1972. After receiving

his B.S. degree in chemistry at Iowa State

University in 1995, he studied at Southern

Illinois University at Carbondale where he

investigated the synthesis of hydrogen-

bonded liquid crystals and earned his M.S.

degree in chemistry in 2001. He is currently

completinghis Ph.D. studies at the Univer-

sity of Oregon under the direction of Profes-

sor Haley investigating platina- and irida-

benzenes.

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Chemie

¹³C NMR spectrum of the ¹³C-enriched complex exhibited a

label-enhanced singlet at d = 291 ppm, which was attributed

to the ring carbon atom in the {RuC(OEt)} moiety.Complex 5

subsequently decomposed above ꢀ508C to an h¹-cyclopenta-

dienyl complex.^[31]Additionally, a similar reaction sequence

was proposed to proceed through a ruthenaphenanthrene

complex, although no direct observation of the metallaphen-

anthrene was made.This was the first, and to date remains

one of the very few, second-row metalla-aromatic molecules

to be reported.

The hypothetical reaction involving migratory insertion of

CO instead of CS was also studied.^[33]In addition to higher

transition-state energies, it was found that the final step,

rearrangement of the 2-osma-3,5-cyclohexadiene-1-one to the

osmabenzene, would not take place owing to the strain in the

resulting three-membered ring.The larger size of the sulfur

atom reduces the ring strain sufficiently to allow formation of

7.

While the structural properties of metallabenzenes are

clearly consistent with the “aromatic” label, it is less clear

with regard to the chemistry of these compounds since most

metallabenzenes tend to react in ways that are atypical of

conventional aromatic molecules.^[9,17]Recently, however, the

first demonstration of electrophilic aromatic substitution was

observed for osmabenzene 8 (Scheme 2).^[16]The nitration of 8

3. Osmium

3.1. Osmabenzenes

The first metallabenzene to be isolated and characterized

was osmabenzene 1.^[23,26]Recently, van der Boom, Martin,

and co-workers calculated the energies of the various

transition states in the formation of 1 by using model 6 as

the starting complex to prepare 7 (Scheme 1).^[33]Their study

Scheme 2. Electrophilic aromatic substitution of osmabenzene 8.

was achieved with a solution of Cu(NO₃)₂in Ac₂O.Analysis

of the resultant purple crystals showed that nitration had

occurred on the position para to the thioether to give 9.

Similarly, the addition of Br₂and Fe powder to a solution of 8

resulted in bromination at the para position of the metalla-

cycle to give 10 (partial exchange of the osmium-bound iodide

was also observed).Chlorination of 8 with PhICl₂gave the

corresponding metallacycle 11.Since electrophilic substitu-

tion occurred at the position predicted by the directing effects

of the thioether, it is unclear what, if any, the directing effects

of the metal fragment are.

A second pathway to stable osmabenzenes was reported

by Jia and co-workers in 2004.^[35]This group found that

reaction of [OsCl₂(PPh₃)₃] with 1,4-pentadiyn-3-ol led to the

isolation of metallacycle 12, the phosphonium salt of an

osmabenzene.The mechanism proposed for formation of this

unusual species involves the substitution of a coordinated

phosphine ligand with an h²-coordinated alkyne to give

species 13 (Scheme 3).This step is followed by addition of

the phosphine at the 2-position on the coordinated alkyne to

afford intermediate 14, which could be isolated.A second

molecule of PPh₃then attacks the resulting coordinated

alkyne to give intermediate 15, which then eliminates a

hydroxide ion to yield osmabenzene 12.Treatment of 12 with

excess PMe₃and Bu₄NCl furnished osmabenzene 16.

Scheme 1. Calculated pathway to osmabenzene by Roper and co-work-

ers. ^[23,26]Energies are given in kcalmol^ꢀ1. TS=transition state.

revealed that the first step in osmabenzene formation is loss

of a phosphine ligand, followed by coordination to an

acetylene molecule.The lowest-energy pathway was found

to proceed by migratory insertion of the CS ligand into the

s bond of the osmacyclopropene (Dewar–Chatt–Duncanson

model).^[34]The first insertion step opens up another coordi-

nation site for a second equivalent of acetylene, but the

pathway leading to the resulting osmabenzene requires that

the CO ligand rearranges to a position cis to the thiocarbonyl

ligand prior to coordination with a second equivalent of

acetylene.Coordination followed by insertion leads to the 2-

osma-3,5-cyclohexadiene-1-thione, which then rearranges to

afford osmabenzene 7.

Spectroscopic characterization of 12 displayed the char-

1

acteristic downfield shift of the ortho proton in the H NMR

spectrum at d = 23.13 ppm. This shift is significantly farther

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C

_phenylbond lengths are similar, thus indicating that these two

resonance structures are not major contributors to the overall

structure.

Nucleophiles other than PPh₃were also found to react

with intermediate 14 to produce osmabenzenes.^[35]Addition

of NaI to a solution of 14 gave iodobenzene 17 (Scheme 5).

Scheme 5. Synthesis of iodoosmabenzene 17.

The ortho protons of 17 resonate at d = 20.1 ppm (OsCHCI)

and d = 19.0 ppm (OsCHCPPh₃) and the para proton reso-

1

nates at d = 8.1 ppm in the H NMR spectrum.This upfield

Scheme 3. Synthesis of osmabenzene 12.

shift in resonances in comparison to analogous protons in

cationic 12 reflects the electronic effects of having one

phosphonium group fewer.

downfield from the ortho proton in 1, which resonates at d =

13.95 ppm.^[23,26]The lower-field signal in 12 could, in part, be

explained by the electron-deficient phosphonium substituents

on the ring.The para proton gave a signal at d = 8.57 ppm.

Once again, compared to 1 (d = 7.28 ppm), the shift is

comparatively deshielded.A similar trend was observed for

the ortho and para carbons in the ¹³C NMR spectrum, in

which the respective signals were observed at d = 239.7 and

160.5 ppm. The meta carbon is shifted upfield (d = 112.7 ppm)

relative to benzene (d = 128.4 ppm).

Similar reactivity was observed upon treatment of [OsBr₂-

(PPh₃)₃] with 1,4-pentadiyn-3-ol, which formed intermediate

18.Subsequent treatment with PPh produced osmabenzene

3

19 after metathesis with Bu₄NBr (Scheme 6).^[35]

An X-ray diffraction analysis of 12 revealed that the

metallacycle is a planar ring structure similar to osmabenzene

ꢀ

1, except that the Os C bond lengths are shorter than in 1

(1.97 vs. 2.00 ). Contributing resonance structures 12c

and 12d may explain this difference (Scheme 4).The mean

Scheme 6. Synthesis of osmabenzene 19.

3.2. Osmabenzynes

Addition of strong bases such as KNH₂to a solution of

chlorobenzene produces benzyne, a well-known but transient

species.^[3a,36]Benzyne is a relative of benzene with two

neighboring sp-hybridized carbons.The six-membered ring

puts enormous strain on the ordinarily linear carbon–carbon

triple bond, thus making this species extremely unstable.^[37]

For this reason, it seems almost unthinkable that a metal-

labenzyne could be isolated and fully characterized.

Scheme 4. Resonance contributors for osmabenzene 12.

In 2001, the Jia group was able nonetheless to isolate the

first stable osmabenzyne 20 (Scheme 7).^[38–40]Treatment of

[OsCl₂(PPh₃)₃] with an excess of trimethylsilylacetylene in

wet benzene produced a brown solution, from which 20 was

isolated in 30% yield.As confirmed by X-ray structural

analysis, 20 contains an essentially planar six-membered

metallacycle with a maximum deviation from the least-

deviation from the least-squares plane of the ring is 0.075 .

ꢀ

All C C bond lengths are intermediate between bond lengths

typical for single and double bonds, which range from 1.363

to 1.448 . Two of the contributing resonance structures (12c

ꢀ

and 12d) are not cyclically conjugated.Comparison of the P

ꢀ

C bond lengths, however, shows that the P C_osmabenzeneand P

squares plane of 0.047 . The Os C(sp) bond length is

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Chemie

27 ([Os] = OsCl₂(PH₃)₂), this angle is constricted to 79.88, and

ꢁ

the optimal [Os] C-C_Meangle of 1808 is constricted to 148.38.

The resulting increase in energy on going from 26 to 27 was

only 9.6 kcalmol^ꢀ1.

Several additional osmabenzynes have been synthesized

by derivatizing 20 (Scheme 8).^[43]Reaction with two equiva-

lents of aqueous HBF₄first led to substitution of one chloride

Scheme 7. Proposed mechanism for the synthesis of osmabenzyne 20.

1.815 , shorter than that observed in both 1 and 12, but

longer than that for typical osmium–carbyne complexes.The

ꢀ

ring C C bond lengths range from 1.376 to 1.420 . Not

2

ꢀ

surprisingly, the Os C(sp ) bond length is 1.939 , only

slightly shorter than those in the osmabenzenes (see Section

3.1).

A possible mechanism for the formation of osmabenzyne

20 is given in Scheme 7.Initial formation of a vinylidene

complex followed by hydrolysis of the trimethylsilyl group

should produce 21.Subsequent [2 +2] cycloaddition with a

second equivalent of trimethylsilylacetylene would afford

cyclobutene 22.The remaining ring carbons then come from

insertion of a third equivalent of trimethylsilylacetylene to

give the six-membered metallacycle 23, which aromatizes by

protonation at the terminal methylene group and subse-

quently loses a proton at the carbon atom ortho to the

Os center to furnish aryne 20.

The sole aromatic proton on the benzyne ring of 20

resonates at d = 13.83 ppm, a value very similar to those of the

osmabenzenes.^[9,23,38]In the ¹³C NMR spectrum, the sp carbon

atom resonates at d = 306.6 ppm. The remaining ortho carbon

atom produces a signal at d = 227.8 ppm. The carbon atoms

meta to the metal center produce signals at d = 136.1 and

113.0 ppm, and the para carbon atom resonates at d =

188.6 ppm.

Initially, it may seem surprising that such a metallacycle

might be stable; however, there are several factors that likely

contribute to the stability of this species.The main reason for

the instability of benzyne is the strain placed on the two

sp carbon atoms by constricting the bond angles from 1808 to

1278.^[41,42]With 2-butyne as a model, calculations showed that

going from linear 24 to constrained 25 leads to a 51.8 kcal

mol^ꢀ1increase in energy.In a metallabenzyne, one of the sp-

hybridized carbon atoms is replaced by a 14-electron tran-

sition-metal fragment.For this metal unit, the optimized Me-

Scheme 8. Derivatization of osmabenzyne 20.

ion with water to produce the osmabenzyne salt 28.The

mechanism probably involves loss of one of the chloride

ligands followed by trapping of the resultant cation with

water.Reaction of the salt with six equivalents of acid

resulted in hydrolysis of the trimethylsilyl groups to give

cationic complex 29.When 20 was treated with acid and NaCl,

or when NaCl was added to the hydrolyzed complex 29, the

neutral species 30 was produced.Reaction of either 20 or 30

with deuterated triflic acid resulted in quantitative, regiospe-

cific deuteration of the b-carbon atoms.Similarly to normal

silylarenes, treatment of 20 with Br₂resulted in bromodesi-

lylation to give tetrabromide 31.^[43]

Attempts to synthesize osmabenzynes directly from

[OsCl₂(PPh₃)₃] and other terminal alkynes have not been

successful.^[44,45]The proposed intermediate metallacyclobu-

tene 22 (Scheme 7) was not isolable; however, cycloaddition

of vinylidene 32 with phenylacetylene produced the related

compound 33 (Scheme 9).^[46]Based on NMR and structural

data, this compound is best described as an h³-allenylcarbene

complex with three contributing resonance forms.

ꢁ

[Os] CMe angle in the acyclic fragment 26 ([Os] = OsCl₂-

(PH₃)₂) was calculated to be 93.88.In the osmabenzyne model

Reaction of 33 with phenylacetylene resulted mainly in

polymerization and/or produced a mixture of species.Only a

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M. M. Haley and C. W. Landorf

3.3. Isoosmabenzene

The tautomer of benzene 1,2,4-cyclohexatriene (isoben-

zene) is a highly reactive, short-lived species proposed as a

key intermediate in Diels–Alder reactions of enynes.^[47]

Interestingly though, like the metallabenzynes, isometalla-

benzenes have proven to be isolable compounds.Reaction of

38 with phenylacetylene and HBF₄·OEt₂produces the

alkenyl–alkynyl–carbyne complex 39 (Scheme 11), which

Scheme 9. Synthesis and contributing resonance structures of allenyl-

carbene complex 33.

trace of osmabenzyne could be observed.Interestingly, when

the reaction was carried out in the presence of NEt₃,

osmabenzyne 34 was produced as the major product after a

day, thus suggesting that an acetylide intermediate may be

involved in the formation of the osmabenzyne.This hypoth-

ꢁ

esis was tested by treating 33 with [(PPh₃)AuC CPh] in the

Scheme 11. Synthesis of isoosmabenzene 40.

presence of HNEt₃Cl, which quickly proceeded to form

osmabenzyne 34.Exposure of 33 to three other gold

acetylides under analogous conditions produced metallaben-

zynes 35–37 (Scheme 10).^[46]

decomposes in solution at temperatures above ꢀ308C.In

the presence of excess NaCl, the decomposition process is

controlled and produces isoosmabenzene 40 in 64% yield.^[48]

The mechanism involved in producing 40 probably starts

with migration of the a-alkenyl carbon atom to the a-carbyne

carbon atom.Next, the b-alkynyl carbon atom couples with

the b-alkenyl carbon atom.In the presence of CO instead of

Cl^ꢀ, it was found that the carbyne is destabilized so that

(E,Z)- and (E,E)-1,4-diphenyl-1,3-butadiene and the alkynyl–

tricarbonyl osmium complex 41 are isolated.

Scheme 10. Synthesis of a series of osmabenzynes.

3.4. Osmabenzofuran

Consistent with the mechanism calculated by Martin, van

der Boom, and co-workers,^[33]Elliott and Roper have found

that the reaction of diphenylacetylene with [Os(CS)(CO)-

(PPh₃)₃] produces the alkyne-substituted complex 42 and an

osmacyclohexadienethione.^[49]Unlike in the case calculated

for acetylene, a second equivalent of diphenylacetylene does

not add to generate the osmabenzene.Reaction of 42 with

methyl propiolate, an activated acetylene, does indeed afford

a metallacycle.^[50]Interestingly, an osmabenzene did not result

from insertion of the thiocarbonyl into the ring, but instead a

second equivalent of methyl propiolate inserted to create the

bicyclic species 43 (Scheme 12; the numbering scheme is used

in Table 1).This molecule is best described as an osmaben-

zofuran in which the Os center is shared by both rings.

Reaction of 43 with ethanolic HCl results in transesterifica-

tion of the free ester functionality to afford 44 while leaving

the metal-bound ester unchanged.Reaction of 43 with

pyridinium tribromide results in bromination at C6 to give

The mechanism for formation of these complexes is still

not completely known.^[46]Reaction of 33 with [(PPh₃)AuC

ꢁ

CR] may initially produce the osmium–acetylide complexes

2

ꢁ

=

= =

[OsCl(C CR)( CPh-h -CH C CHPh)(PPh₃)₂].Alterna-

tively, the gold acetylide may coordinate to the osmium–

allenylcarbene complexes.The b-carbon atom of the acetylide

attacks the g-carbon atom of the allenyl carbene in the

cyclization reaction.It is unclear if the cyclization reaction

takes place before or after protonation of the terminal

CHPh group.This newer work calls into question the

mechanism outlined in Scheme 7; however, direct comparison

may not be valid, as phenylacetylene and trimethylsilylace-

tylene can often exhibit different modes of reactivity with

metal complexes.

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Table 1: Bond lengths for osmabenzofurans 43–45, osmabenzene 47,

and structurally related Ru (48) and Fe (49) complexes.

43

44

45

47

48

49

M

Os

Ru

Fe

M-C1

M-C5

M-O

C1-C2

C2-C3

C3-C4

C4-C5

C5-C6

C6-C7

C7-O

2.068

2.142

2.218

1.367

1.442

1.359

1.436

1.371

1.416

1.333

2.068

2.162

2.232

1.406

1.45

2.055

2.135

2.234

1.389

1.433

1.368

1.438

1.363

1.438

1.244

1.996

2.103

2.221

1.45

1.374

1.438

1.375

1.507

1.495

1.226

2.003

2.107

2.26

1.367

1.448

1.34

1.477

1.369

1.442

1.239

1.908

1.994

2.073

1.364

1.43

1.366

1.467

1.363

1.421

1.242

1.38

1.466

1.375

1.424

1.264

significantly.In all of these complexes, the trans influence of

ꢀ

the thiocarbonyl ligand lengthens the Os C5 bond.

The X-ray structure for cationic complex 47 reveals

deviations from the trends in the neutral series.In this

ꢀ

structure, C1 C2, C3 C4, and C5 C6 have bond lengths that

Scheme 12. Synthesis of osmabenzofurans 43–45 and osmabenzenes

46–47.

ꢀ

are larger than the C2 C3 and C4 C5 bond lengths (Table 1).

ꢀ

Additionally, the Os C5 bond is slightly shortened.This

combination of structural features suggests that 47 has

significantly more delocalization than 43–45.Consistent

45.Finally, C6 can also be protonated with either trifluoro-

acetic acid or HI/I₂to generate complexes 46 and 47,

respectively.While the triiodide salt 47 could be isolated,

attempts to isolate 46 only resulted in regeneration of 43.The

reversible nature of this reaction at C6 is strikingly similar to

recent iridabenzene chemistry reported by Chin et al.(see

Section 5.2).^[51,52]

1

with this observation is the downfield shift of the H NMR

resonance signal of the ortho proton upon acidification with

HI/I₂or CF₃CO₂H (from d = 6.99 ppm for 43 to d = 8.36 ppm

for 46).

Similar Ru (48) and Fe (49) complexes have been

reported previously but were not initially recognized as

Three resonance structures account for the overall bond-

ing picture in these complexes.Structures 43a and 43b

(Scheme 13) account for the aromatic character of these

metallabenzofurans.^[53–56]These molecules are structurally

quite similar to 43–45.It is interesting to note that the

authors did not report these complexes as aromatic, though

they do have some characteristics suggesting that delocaliza-

tion is present.Nonetheless, bond alternation is more

pronounced in 48 and 49 (Table 1) and thus these may be

borderline cases of metalla-aromatic compounds.

Scheme 13. Contributing resonance structures of 43.

4. Ruthenium

molecules, while structure 43c contains an osmacyclohexa-

diene system.^[50]The crystal structures of compounds 43–45

show, unlike in other metallabenzenes, that there is some

While osmium has been shown to be involved in a number

of thermally stable metalla-aromatic compounds, the same

cannot be said about chemically similar ruthenium.Most of

the ruthenabenzenes that have been synthesized are either

short-lived at room temperature or can be isolated only if

coordinated to a transition-metal center to which some of the

electron density can be donated.^[18,31]This stability trend

between the 4d and 5d metallabenzenes is generally true, in

ꢀ

apparent bond alternation (Table 1).The C1 C2, C3 C4, and

ꢀ

C5 C6 bonds are all shorter than the C2 C3 and C4 C5

bonds.This bond alternation suggests that the dominant

contributing resonance structure may be 43c; however, the

ꢀ

C5 C6 and the C6 C7 bonds are similar enough in length to

suggest that resonance forms 43a and 43b also contribute

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M. M. Haley and C. W. Landorf

that the 4d metals do not usually produce isolable

metallabenzenes while the 5d metals do.

4.1. Coordinated Ruthenabenzenes

One recent example of a metal-coordinated

ruthenabenzene was reported by Bruce et al.^[57]The

authors found that when the Ru cluster 50 was mixed

ꢁ

with two equivalents of HC CFc in CH₂Cl₂

(Scheme 14), complex 51 was formed in 10% yield,

2

ꢀ

as identified by X-ray crystallography.The Ru C(sp )

bond lengths in the metallacycle are 2.083 and

ꢀ

2.054 , and the C C bond lengths within the ring

range from 1.407 to 1.437 . The C-Ru-C angle is

88.48, and the remaining ring angles lie between 120.8

and 128.48.Despite the structural similarities to

previous metallabenzenes, the two protons meta to

the metal resonate as a singlet at 5.31 ppm in the

¹H NMR spectrum.Similar upfield shifts for metal-

coordinated arenes are commonly observed.^[9,34]The

mechanism for formation of ruthenabenzene 51

ꢁ

probably involves double insertion of HC CFc into

ꢀ

the Ru C bonds of the complex and subsequent loss

of water.

Scheme 15. Synthesis of triple-decker ruthenabenzene 52.

the Ru-C(sp²) bond lengths are 2.043 and 2.055 . The

C(sp²)-Ru-C(sp²) angle is 82.28, and the remaining ring angles

range between 120.4 and 132.98.As in the previously

described coordinated ruthenabenzene 51, the ¹H NMR

spectrum of 52, with signals ranging from d = 5.26 to

8.20 ppm, exhibits signals shifted upfield from those of

other metallabenzenes.

Scheme 14. Synthesis of ruthenium-coordinated ruthenabenzene 51.

Fc =ferrocenyl.

Other examples of metal-coordinated ruthenabenzenes

are the homoleptic Ru sandwich complexes 58 and 59

reported by Salzer, Kaupp, and co-workers (Scheme 16).^[59]

These complexes were produced by treating pentadienyl

sandwich complexes with two equivalents of [Ru₃(CO)₁₂].

Interestingly, reaction with only one equivalent of

[Ru₃(CO)₁₂] afforded pentadienyl ruthenabenzene complex

60 in very low yield.Complex 58 produced signals at d =

6.23 ppm (para) and d = 5.62 ppm (ortho) in the ¹H NMR

spectrum.The related complex 59 produced a signal at d =

5.39 ppm for the protons ortho to the metal center.In both 58

and 59, the most stable conformation is fully eclipsed with the

ring plane tilted 18.28 (58) toward one another at the side of

the metals, such that the ring plane is defined by the four

ortho and meta carbon atoms.The Ru atoms in the metalla-

cycles of 58 are bent slightly away from each other by 14.38

Jia obtained the triple-decker ruthenabenzene 52 by

treating [Cp*Ru(H₂O)(nbd)]BF₄(53; nbd = norbornadiene)

with sodium formate to produce first the binuclear Ru cluster

54 (Scheme 15).This cluster was then treated with HBF ₄, and

the resultant solution was allowed to stand for several days at

room temperature to give ruthenabenzene 52.^[58]The initial

ꢀ

complex 54 is probably produced by C C bond cleavage of

the norbornadiene ligand and subsequent elimination to

produce the bridging vinylcyclopentadiene ligand.Protona-

tion of 54 leads to the intermediate complex 55, which

rearranges to form the triple-decker ruthenabenzene 52, as

well as ruthenacene 56 and the triple-decker complex 57.

While the details of these rearrangements are not well

ꢀ

understood, it is interesting since it must involve C C bond

activation under mild conditions.

ꢀ

and 16.38 with Ru Ru distances (3.38 ) between the rings

X-ray diffraction revealed that the six-membered ruth-

enacycle of 52 is h⁶-bonded to the lower Ru center but only h⁵-

bonded to the upper Ru center.The bridging hydride ligand

allows the Ru center of the ring to maintain its 18-electron

that are smaller than the sum of the van der Waals radii.

These structural features indicate that there is more than a

superficial relationship between them.Reaction of 58 with

HBF₄produced unstable complex 61, which revealed a

¹H NMR resonance (d = ꢀ16.89 ppm) typical for a metal

hydride.

ꢀ

count.The C C bonds of the metallacycle range between

1.358 and 1.463 with no significant bond alternation while

3922

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Metallacycles

Chemie

ligands.Our group has recently generated a similar result

via a lithiated vinylcyclopropene.^[61]Based on related chemis-

try from our lab (see below), ruthenabenzene 64 is presumed

as an unstable intermediate which undergoes facile carbene

insertion to give 65.

Given the instability of 5 and the elusiveness of 64, it

would seem that synthetic efforts to prepare an isolable

metallabenzene of a 4d metal which is not coordinated to

another metal fragment would be futile.Very recently, Jia,

Xia, and co-workers proved this theory wrong by isolating the

first stable ruthenabenzene by a method analogous to the

synthesis of 12.Treatment of [RuCl ₂(PPh₃)₃] with 1,4-

pentadiyn-3-ol, PPh₃, and Bu₄NCl afforded cationic ruthena-

benzene 66 in 55% yield (Scheme 18).^[62]This complex can

Scheme 16. Synthesis of ruthenabenzene complexes 58–61.

4.2. Ruthenabenzenes Revisited

Early work by Hughes and Robinson suggested that

ruthenium would be a good candidate for producing metal-

labenzenes from vinylcyclopropene derivatives.^[60]The Dart-

mouth group demonstrated that mixing 3-vinylcyclopropenes

with either [Cp*RuCl(cod)] or [{Cp*Ru(m³-Cl)}₄] produced

ruthenacenes such as 62 (Scheme 17) which are potential

Scheme 18. Synthesis and derivatization of stable ruthenabenzene 66.

2,2’-bipy=2,2’-bipyridine.

also be produced directly from RuCl₃·3H₂O, PPh₃, and 1,4-

pentadiyn-3-ol in 20% yield.Metallacycle 66 is stable in air at

temperatures up to 1008C.The authors attribute this unusual

stability to the bulky phosphine substituents and/or ligands.

Complex 66 has similar spectral characteristics to other

metallabenzenes.The ortho proton resonates in the ¹H NMR

spectrum at d = 17.5 ppm, which is reasonably expected for a

Ru carbene.The para proton appears in the aromatic region

at d = 8.2 ppm. The carbon atoms ortho, meta, and para to the

metal center resonate at d = 284.3, 108.3, and 146.0 ppm,

respectively, in the ¹³C NMR spectrum.Confirmation of this

structure was provided by X-ray diffraction, which showed

that 66 is basically planar with only small deviations (0.043 )

from the root-mean-square plane defined by the six-mem-

bered metallacycle.The electrons in the ruthenacycle are fully

Scheme 17. Reactions of vinylcyclopropenes with ruthenium com-

plexes. cod=cyclooctadiene; Cp*= C₅Me₅.

ꢀ

delocalized with the C C bond lengths ranging from 1.378 to

1.395 , thus showing no significant bond alternation.

Derivatization of 66 afforded three additional ruthena-

decomposition products of ruthenabenzenes.It was presumed

that these complexes were formed by loss of HCl from the

intermediate olefin complexes such as 63 and subsequent

carbene insertion to produce the new cyclopentadienyl

benzene complexes (Scheme 18).Reaction with PMe

3

resulted in substitution of the metal-bound phosphines to

produce 67.The ligand sphere could also be modified by

reaction of 66 with tBuNC, resulting in 68 in which one

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M. M. Haley and C. W. Landorf

chlorine ion is exchanged for the isonitrile.Complex 69 could

be produced by reaction of 66 with 2,2’-bipyridine.The

structural and spectroscopic properties of 67 and 68 are

analogous to those of 66; however, the solid-state structure of

69 reveals that the Ru atom is situated outside of the plane of

the ring (defined by the five carbon atoms) by 0.672 .

Nonetheless, 69 retains its aromaticity, as the para proton

ꢀ

resonates at d = 8.2 ppm and the C C and Ru C bond lengths

indicate delocalization.

Scheme 19. Iridabenzene synthesis from a nucleophilic 3-vinylcyclopro-

pene.

5. Iridium

Table 2: Product ratios for the synthesis of iridabenzenes 71 with various

phosphines.

One of the earliest metallabenzenes to be isolated and

characterized was iridabenzene 4, which Bleeke et al.syn-

thesized by the reaction of 2,4-dimethylpentadienide with

[IrCl(PEt₃)₃].^[17,29]A number of iridabenzene derivatives

could be prepared from 4; for example, reactions with

phosphines that are more electron-rich usually resulted in

substitution of one or more PEt₃ligand, depending on the

sterics of the ligand.Carbon monoxide could substitute one

ligand at room temperature, while at reflux a second

equivalent would insert into the metallacycle to produce an

iridium–phenoxide complex.Reaction with bromine or

iodine resulted in oxidation of the Ir center rather than

electrophilic aromatic substitution.^[9,29]

Cmpd

Phosphine

a [8]^[a]

Yield [%]

71:72^[b]

71

72

72!71

71a

71b

71c

71d

71e

71 f

71g

PPh₃

PMe₃

PMe₂Ph

PEt₃

PMePh₂

PiBu₃

145

118

122

132

136

143

145

100:0

0:100

2:98

0:100

30:70

35:65

100:0

66

–

58

–

56

49

61

–

54

–

51

–

97

–

95

–

P(p-MeOC₆H₄)₃

[a] Cone angle, reference [70]. [b] Initial product ratio, determined by

¹H NMR spectroscopy.

A key step in the synthesis of 4 is deprotonation of an

intermediate iridacyclohexadiene.^[17,29]This deprotonation

was not found to occur when using [IrCl(PMe₃)₃] as the

metal source.^[63]As a result, synthesis of an iridabenzene was

only feasible from [IrCl(PEt₃)₃], such that all additional

metallacycle examples must be derived from 4.

Phosphines with smaller cone angles^[70]tended to afford

more of the iridabenzvalene product.Comparison of PMe

3

and PiBu₃shows that the smaller PMe₃ligand forms 100%

iridabenzvalene, while the larger PiBu₃forms only 65%.

Additionally, ligands possessing electron-rich groups resulted

in an initial product ratio favoring the iridabenzvalene.For

example, while PPh₃led to exclusive formation of the

iridabenzene, PiBu₃led to an initial product ratio of about

2:1 iridabenzvalene to iridiabenzene.Since the cone angles on

these two ligands are 145 and 1438, respectively, it is likely

that electronic effects are primarily responsible for this

difference.Steric effects, which seem to be secondary in this

study, do eventually come into play as a solution of the PiBu₃

product mixture in C₆D₆does convert to all iridabenzene over

eight hours at room temperature, while a solution of the PMe₃

iridabenzvalene in C₆D₆is stable indefinitely at room temper-

ature.

5.1. Iridabenzenes from Nucleophilic 3-Vinylcyclopropenes

Recently, our group reported the direct synthesis of a

series of iridabenzenes from nucleophilic 3-vinylcyclopro-

penes.^[64–68]In this case, several different ligands and metal

complexes were used to produce a variety of structurally

related iridabenzenes.This method of synthesis, which was

inspired by the aforementioned studies from Hughes and co-

workers, is derived from the well-known reactivity of cyclo-

propenes to form vinyl carbenes.^[69]For

a bond-fixed

“KekulØ” structure of a metallabenzene, the ring can be

seen as a vinyl carbene that is tethered to the metal center by

a vinyl s bond.Retrosynthetic analysis suggested that a ( Z)-3-

(2-iodoethenyl)cyclopropene such as 70 would be a good

synthon for iridabenzenes like 71 (Scheme 19).Lithium–

halogen exchange of vinyl iodide 70 and subsequent addition

of Vaskaꢀs complex resulted in the isolation of iridabenzene

71a (R = Ph).^[64]When less bulky phosphines were used, the

s-vinyl/h²-cyclopropene complex 72 (for example, R = Me)

was isolated.This “iridabenzvalene”, a valence isomer of 71,

then could be converted into the corresponding iridabenzene

in nearly quantitative yield either by heating in solution or by

treatment with Ag^Isalts.^[65]

There are two potential mechanisms for the formation of

iridabenzene 71 from iridabenzvalene 72 (Scheme 20).^[33,67,68]

One pathway begins by dissociation of the cyclopropene from

the metal center and subsequent oxidative addition to

generate Dewar benzene 74, which quickly rearranges into

71.Evidence for this pathway lies in the ability of donor

solvents to rapidly increase the rate of isomerization.Similar

types of reactivity with cyclopropenes have been reported by

Hughes and co-workers.^[71,72]In these cases the intermediate

metallacyclobutenes were isolated from the reaction of

perfluorinated cyclopropenes with transition-metal com-

plexes.Additionally, a metal-assisted mechanism has been

proposed for the opening of 3,3-diphenylcyclopropenes, as

based on the isolation of an iridium-coordinated iridacyclo-

butene.^[73]

Varying the phosphine ligand gave insight into the factors

determining which isomer would be isolated (Table 2).^[67]

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Chemie

Scheme 20. Proposed mechanisms for formation of iridabenzenes and

valence isomers from a nucleophilic 3-vinylcyclopropene (R’=Ph). For

the purpose of calculations, R=R’=H. The numbering scheme is

used in Table 3.

over the meta-phenyl isomer 77.In addition to the iridacycles,

cyclopentadienyl complexes such as 78 and 81 were also

isolated.As in previous examples, these complexes are

thought to come from migratory carbene insertion of the

corresponding iridacycle.

The series derived from 1-alkyl-2-phenylcyclopropenes

82a–d proved to be the best “behaved” of the various systems

explored, with 82d allowing easy access for complete study of

the iridabenzene manifold.^[68]Reaction of lithiated 82d with

Vaskaꢀs complex furnished iridabenzvalene 75d (Scheme 21).

Recent DFT calculations reported by van der Boom,

Martin, and co-workers, however, suggest that the more

feasible pathway is through direct rearrangement of the

iridabenzvalene by twisting of the iridacyclopropane ring into

the planar metallacycle.^[33]For the purpose of these calcu-

lations, an Ir complex 73 for which R = R’ = H was used.

Under these circumstances, two isomers of 73 were distin-

guishable by the orientation of the cyclopropene ring.The

syn isomer, in which the cyclopropene ring is oriented

towards the Ir center, gave a calculated energy of 1.8 kcal

mol^ꢀ1higher than the anti isomer.When this lower-energy

s complex is defined as having an energy of zero, the

transition state to iridabenzvalene 72 carries an energy of

only 3.5 kcalmol^ꢀ1.The iridabenzvalene itself lies in an

energy well at ꢀ23.1 kcalmol^ꢀ1.From the iridabenzvalene, a

barrier of 39.5 kcalmol^ꢀ1was found in forming the iridaben-

zene 71 directly.While no transition state was found for the

formation of Dewar benzene 74, its energy was calculated to

be 38.3 kcalmol^ꢀ1above the iridabenzvalene, thus suggesting

that the Dewar benzene model would be considerably higher

in energy.

Scheme 21. Synthesis of tBu/Ph-substituted iridacycles.

None of the experimental evidence so far indicates

definitively either the Dewar benzene or the direct-insertion

pathway.While the calculations are generally reliable, they

may not take into account any associative mechanism(s) in

which the solvent may play a role.Since the calculations were

only able to approximate solvation effects by using the

polarized continuum model, no direct interactions such as

solvent coordination or a metal-assisted mechanism would be

predicted.It is also possible that no one of these mechanisms

operates to the exclusion of the others.

Although stable in the solid state, solutions of 75d in C₆D₆at

208C immediately began to isomerize, which after four days

afforded a 3:1 mixture of iridabenzene 76d and cyclopenta-

dienyl complex 78d in 94% combined yield.The regiochem-

istry of 76d was confirmed by X-ray analysis to be the meta-

tBu isomer, and none of the corresponding ortho-tBu

regioisomer 77d was detected.Unlike 71a–g, 76d proved to

be extremely labile as it converted quantitatively into 78d in

C₆D₆solution at 508C.The kinetics of the rearrangement

were shown to be first-order with respect to 76d.

In addition to isolating iridabenzenes with different

phosphine ligands, we have successfully synthesized

a

number of iridabenzenes with different ring substituents

from unsymmetrically substituted cyclopropene precur-

sors.^[66,68]As in the previous work, iridabenzvalenes such as

75 and 79 are formed in addition to iridabenzenes depending

not on the phosphine, but on the nature of the R group.In

certain cases, isomerization to the iridabenzene was found to

be highly regioselective, forming the ortho-phenyl isomer 76

Initially, it was presumed that the ortho-tBu regioisomer

77d had quickly rearranged and thus was the source of 78d.

To test this theory, kinetic studies on the isomerization of 75d

and 76d to the cyclopentadienyl complex 78d were per-

formed.The data showed that at 20 8C the rate of isomer-

ization of 76d to 78d was faster than the corresponding rate of

isomerization of 75d to 78d.This was taken to mean that the

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M. M. Haley and C. W. Landorf

cyclopentadienyl complex 78d originated by the rearrange-

ment of the meta-tBu isomer 76d and not from ortho-tBu

isomer 77d.It therefore appears that the valence isomer-

ization of unsymmetrical iridabenzvalene 75d to iridabenzene

76d is highly regioselective.

Reactions of Vaskaꢀs complex with cyclopropenes 82a–c

showed the influence of alkyl substituent on both the

formation of 75 and its transformation to 76. ¹H NMR

spectroscopy showed that each crude reaction mixture was

composed of the corresponding iridabenzvalene and irida-

benzene.Iridabenzvalenes 75a–c were not isolated from the

reaction mixture because of their relatively rapid isomer-

ization to 76a–c, which were isolated in about 25–30% yield.

After eight hours and 30 hours at 208C in C₆D₆solution,

75a,b and 75c, respectively, had isomerized completely to

76a–c, and very minor amounts of the ortho-alkyl regioisomer

77a–c were detected by NMR spectroscopy of the crude

reaction mixtures.Unlike 76d, solutions of 76a–c were stable

for over 48 hours at 758C.

Scheme 22. Synthesis of Ph/SiMe₃-substituted complexes 75e–78e.

In contrast, reaction of vinylcyclopropenes 82b,d with the

less bulky and more electron-rich complex [IrCl(CO)-

(PMe₃)₂] yielded the corresponding iridabenzvalenes 79b,d

as the only products, which were stable at 208C.Isomerization

at 758C converted 79b completely into the meta-Et regioiso-

mer 80b over four hours.Under the same conditions, how-

ever, the tBu analogue 79d gave a mixture composed of

unreacted iridabenzvalene, iridabenzene 80d, and cyclopen-

tadienyl complex 81d in a ratio of 5:10:1, as well as partial

decomposition.Prolonged heating of this mixture led to

complete decomposition to unidentified materials.Interest-

ingly, evidence for formation of the ortho-alkyl regioisomer of

80 was never observed; thus, the lower steric hindrance of

PMe₃than PPh₃does not decrease the regioselectivity of the

isomerization.The influence of the Et and tBu substituents on

the stability and isomerization rate of 79 is similar to that

observed for the corresponding PPh₃analogues 75.Addition-

ally, the greater stability of meta-tBu substituted 80d com-

pared to the corresponding PPh₃analogue 76d indicates that

the PMe₃ligand does stabilize the iridabenzene.Regarding

the whole “alkyl” series, the trend of the isomerization rate of

alkyl-substituted iridabenzvalenes to iridabenzenes is 75a =

75b > 75c > 75d > 79b > 79d, which is in agreement with the

increase in both electronic-donating ability and steric hin-

drance of the alkyl group as well as with increased electronic-

donating ability and/or reduced steric hindrance of the

phosphine ligand.On the other hand, iridabenzene stability

is ordered as 80b > 76a = 76b = 76c > 80d > 76d, indicating

that a decrease in steric hindrance of the alkyl group as well as

a decrease in electronic-donating ability and/or sterics of the

phosphine ligand enhance the stability of the iridabenzene.

Replacement of the alkyl group with a trimethylsilyl unit,

as in 82e, furnished interesting but more perplexing results.In

this case, the reaction of the lithiated vinylcyclopropene with

Vaskaꢀs complex furnished a 47% yield of a mixture of 75e,

77e, and 78e in a 10:2:3 ratio (Scheme 22).Iridabenzvalene

75e and cyclopentadienyl 78e were isolated cleanly by

treatment of the purified mixture with MeI or by heat,

respectively; unfortunately, pure iridabenzene 77e could not

be isolated.In contrast to 75a–d, the Ph/SiMe₃-substituted

iridabenzvalene 75e is stable at room temperature.Heating a

solution of pure 75e to 758C, however, did not lead to

isomerization of 77e as expected, but instead afforded the

meta-silyl

regioisomer

76e

(confirmed

by

¹H,²⁹Si gHMQC NMR experiments).While 76e is more

stable than the ortho-silyl isomer 77e, it also undergoes

carbene migratory insertion to give complex 78e.It is

reasonable to suspect that the ortho-silyl isomer 77e is also

an intermediate in the isomerization of 75e to 78e; however,

77e was not detected by ¹H NMR during the isomerization of

75e.

These results suggest that regioisomers 76 and 77 are

formed by different mechanisms.One possible pathway is that

the initial s-vinyl complex 83 forms an intermediate such as

84 (Scheme 23, R = SiMe₃).This intermediate is plausible for

R = SiMe₃since the carbocation would be stabilized by both

the a-Ph and the b-SiMe₃moieties.Subsequent cleavage of

the three-membered ring could then lead to the selective

formation of iridabenzene 77e.Possibly a result of steric

congestion by the bulky trimethylsilyl group, 77e easily

undergoes carbene migratory insertion and subsequent dis-

sociation of PPh₃to give cyclopentadienyl complex 78e.The

lower stability to 84 which results from the corresponding

alkyl groups essentially shuts down this pathway and thus

leads to detection of 77a–c only by NMR.Instead, the major

pathway consists of concerted opening of 75 to yield 76.

Nonetheless, the exact origin of this preferential pathway to

furnish regioisomer 76 is still uncertain and is subject to

further interpretation and debate.

One additional system studied by our group is derived

from the bis(trimethylsilyl)cyclopropene 85.By employment

of the usual synthetic method with Vaskaꢀs complex, 86 could

be isolated in 40% yield (Scheme 24).^[74]Compound 86 has

proven to be one of the most stable iridabenzvalenes isolated

to date, requiring prolonged heating at 758C to isomerize/

rearrange to cyclopentadienyl complex 87 in essentially

quantitative yield.Unfortunately, iridabenzene 88 has not

been detected in this process, though its intermediacy is

presumed based on the related studies.Incorporation of two

trimethylsilyl groups seems to have two effects: the s-

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Angewandte

Metallacycles

Chemie

Scheme 23. Proposed mechanism for isomerization/rearrangement to 78.

scopic properties.All the iridabenzenes are easily identified

by the NMR resonance of the proton ortho to the metal,

which varies between d = 10.41 ppm for 71a and d =

11.30 ppm for 71d (Table 3).^[67]The downfield shift of these

characteristic resonances is attributed to the anisotropy of the

neighboring metal.This effect quickly dissipates for the

protons meta and para to the metal center (d = 7.38–7.90 ppm

and d = 7.69–8.78 ppm, respectively). These latter two reso-

nances are more in line with aromatic ring currents in

electron-withdrawn systems such as pyridine.Interestingly, in

C₆D₆the meta and para protons are 0.1–0.5 ppm downfield

compared to the same protons in CD₂Cl₂, but the analogous

comparison of the ortho proton shows that it generally

resonates 0.1–0.2 ppm upfield in C₆D₆.

Scheme 24. Synthesis of bis(trimethylsilyl) complexes 86 and 87.

donating ability of the Si atoms appears to stabilize the

formation of iridabenzvalene, yet the p-accepting nature of

the Si atoms destabilizes the corresponding iridabenzenes.

Ongoing computational efforts tend to support this hypoth-

esis.^[75]

The ¹³C NMR spectra demonstrate similar anisotropy

trends (Table 3).Carbon atoms neighboring the Ir center

resonate between d = 174 and 190 ppm with the resonance of

the phenyl-substituted carbon atom C1 appearing generally

downfield from unsubstituted C5.The remaining carbons

resonate in the standard aromatic region.While the resonat-

ing frequency of the carbon atom meta to the metal center

remains relatively unaffected by changes to the ligand sphere

of 71 (Dd < 1.4 ppm), carbon atoms C5 (ortho) and C3 (para)

5.1.1. Spectroscopic Properties

The synthesis of a family of structurally related iridaben-

zenes has allowed for detailed comparison of their spectro-

Table 3: Selected NMR data for iridabenzenes 71, 76, and 80.^[a]

Compound Phosphine

Substituent

R

d(¹H) [ppm]

H4 H5

d(¹³C) [ppm]

C3 C4

d(³¹P) [ppm]

H3

C1

C2

C5

C6

71a^[b]

71a^[c]

71b^[b]

71b^[c]

71c^[b]

71d^[b]

71e^[b]

71 f^[b]

71g^[b]

PPh₃

Ph

7.99 7.69 10.41

8.44 7.79 10.79

7.75 7.49 11.10

8.25 7.91 11.00

7.82 7.53 11.05

7.69 7.40 11.30

7.98 7.66 10.93

7.71 7.51 10.89

7.96 7.38 10.43

187.60 141.93 139.99 127.76 187.43 201.09

187.74 142.83 141.16 127.64 187.49 203.26

189.25 140.49 135.53 128.65 176.58 189.44

189.95 141.49 136.74 129.06 176.04 189.44

188.56 140.53 136.49 128.85 179.68 190.77

189.87 140.53 135.67 128.07 176.45 192.36

187.36 141.47 137.61 128.37 181.93 197.14

190.54 141.45 136.14 128.01 177.28 194.09

188.51 141.70 138.92 128.50 187.11 198.18

16.44

17.88

PMe₃

PMe₂Ph

PEt₃

PMePh₂

PiBu₃

ꢀ40.54

ꢀ40.27

ꢀ26.90

ꢀ2.08

ꢀ5.20

ꢀ0.29

13.01

P(p-MeOC₆H₄)₃Ph

76a^[c]

76b^[c]

76c^[c]

76d^[c]

76e^[c]

PPh₃

Me

Et

iPr

tBu

SiMe₃

8.30 7.75 10.62

8.29 7.79 10.61

8.41 7.86 10.60

187.46 140.47 133.93 129.07 183.92 204.97

18.03

18.65

18.79

18.24

17.13

187.31 140.84 139.99 129.48 184.95 204.17

[d]

187.65 145.52

–

129.85 185.52 203.90

8.69 7.83 10.54

186.68 145.61 138.89 129.63 186.55 208.32

[d]

[e]

8.78

–

10.85

188.00 141.37 137.15 128.95 188.93

–

80b^[c]

80d^[c]

PMe₃

Et

tBu

8.13 7.90 10.92

8.46 7.88 10.87

188.83 138.55 135.38 129.70 174.20 181.02

ꢀ38.56

ꢀ38.79

[e]

–

[a] Atom labeling as shown in Scheme 20. [b] In CD₂Cl₂. [c] In C₆D₆. [d] Resonance obscured by other signals. [e] Resonance not assigned.

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M. M. Haley and C. W. Landorf

vary considerably.Replacement of an alkyl group with a

phenyl group on the phosphine results in a deshielding of the

para carbon atom C3 (Dd = 4.5 ppm) and the ortho carbon

atom C5 (Dd = 11 ppm).The resonance of the carbonyl group

varies in a similar way (C6: Dd = 11.5 ppm). The substituted

carbon atom C1 does not show this dependence on the ligand

sphere (Dd = ꢀ1.4 ppm) and actually shifts in the opposite

direction.For the “alkyl” series 76/80, replacement of PPh₃

with PMe₃on the Ir center leads to an upfield shift of the

resonances for C3 (Dd = ꢀ5.5 ppm), C5 (Dd = ꢀ10.8 ppm),

and C6 (Dd = ꢀ23.2 ppm), which can be rationalized in terms

of electronic influences.The stronger donating ability of PMe ₃

compared to PPh₃results in a more electron-rich metal

center, which in turn increases the electron density of the

iridabenzene ring and thus results in the upfield carbon shifts.

On the other hand, these effects are much smaller for C1

(Dd = ꢀ1.5 ppm), C2 (Dd = ꢀ1.4 ppm), and C4 (Dd =

ꢀ0.2 ppm).

Figure 1. Molecular structure of iridabenzene 76b. The thermal ellip-

soids are set at 25% probability.

The ³¹P NMR spectra reveal sharp singlets for the

phosphines, even when samples were cooled to ꢀ808C, thus

demonstrating that the axial and basal phosphines exchange

rapidly in solution by the well-known Berry pseudorotation

process. ³¹P-¹³C coupling is observed for the substituted

carbon atom C1 but not for the unsubstituted C5.This

observation demonstrates that while the ligand sphere in

solution is dynamic, the smaller CO ligand spends a dispro-

portionate amount of time near substituted C1, probably as a

result of steric crowding.Analogous behavior was demon-

strated previously by Bleeke et al.through careful correlation

of ³¹P-¹H coupling constants in a similar system.^[17]

Table 4: Selected bond lengths [] and bond angles [8] from X-ray

structural analyses of iridabenzenes.

71a

71d

76a

76b

76d

Ir-C1

Ir-C5

2.021

2.025

1.409

1.410

1.377

1.334

1.383

2.047

2.004

1.427

1.372

1.386

1.381

1.392

2.020

2.012

1.423

1.335

1.382

1.408

1.387

2.029

2.023

1.413

1.390

1.393

1.360

1.389

2.054

2.000

1.407

1.382

1.427

1.343

1.390

C1-C2

C2-C3

C3-C4

C4-C5

1(C-C)

The ¹H NMR signals of the other species observed in

these studies offer a quick and easy diagnostic handle.For the

iridabenzvalenes, the bridgehead cyclopropyl proton appears

as a characteristic broad singlet in the range of d = 3.2–

3.8 ppm. Two complex multiplets in the ranges d = 6.0–

6.8 ppm and d = 6.7–7.2 ppm (the latter was sometimes

obscured by additional peaks from Ph groups) correspond

to the alkene resonances H4 and H5, respectively.For

cyclopentadienyl complexes, sharp signals for the three ring

hydrogens appear around d = 4.2–5.4 ppm. The unreacted

lithiated vinylcyclopropene that is protonated in the reaction

work up has a characteristic signal (ddd) that appears at about

Ir-C1-C2

128.8

123.6

124.2

125.5

130.7

86.9

127.1

123.1

127.9

122.5

130.4

88.1

129.1

123.5

125.9

126.6

126.4

88.5

130.0

122.1

126.2

124.3

130.3

87.0

130.8

119.5

128.2

123.9

130.2

87.4

C1-C2-C3

C2-C3-C4

C3-C4-C5

C4-C5-Ir

C1-Ir-C5

sum of angles

719.7

719.1

720.0

719.9

720.0

C5-Ir-C6

P1-Ir-P2

P1-Ir-C1

P2-Ir-C1

tilt angle^[a]

165.0

99.2

112.6

148.2

3.7

167.8

104.8

101.9

153.0

6.4

172.5

102.5

119.8

137.7

1.7

171.8

102.4

121.0

136.3

1.2

172.1

105.6

137.5

116.5

1.4

[a] The angle between the C1-Ir-C5 plane and the C1-C2-C3-C4-C5 plane.

1

d = 6.0–6.2 ppm. Thus, by H NMR analysis, it is relatively

easy to ascertain what products are present in the crude

reaction mixtures.

ꢀ

to 1.392 . The Ir C1 and Ir C5 bond lengths (average:

ꢀ

5.1.2. Solid-State Structures

2.02 ) are intermediate between typical Ir C single- and

double-bond lengths.The metallacycles are basically planar

(sum of bond angles: 719.1–720.08) with the Ir center tilted

out of the five-carbon backbone from 1.28 (76a) up to 6. 48

(71d).The X-ray analyses verify that the alkyl groups of

76a,b,d are at the meta position to the Ir center, thus in

agreement with the spectroscopic assignments.Contrary to

our expectations based on reactivity, the bulky tBu group

does not induce greater torsion strain in the iridabenzene ring

of 76d, at least not in the solid state.

X-ray diffraction studies of five iridabenzenes synthesized

in our group illustrate some key structural features of

metallabenzenes.The molecular structure of 76b is shown

in Figure 1.Comparison of the bond lengths and bond angles

of 71a,d and 76a,b,d is given in Table 4.The metallacycles

possess a distorted coordination geometry that can be

classified as somewhere between square pyramidal and

ꢀ

trigonal bipyramidal.The C C bond lengths in the central

ring (1.334–1.427 ) are essentially equal and thus can be

The solid-state structures of the iridabenzvalenes are also

of considerable interest.The molecular structure of 75d is

shown in Figure 2, and selected bond lengths and bond angles

regarded as evidence of delocalization of the p-electrons.The

ꢀ

mean C C bond lengths are very close and range from 1.383

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Chemie

5.2. Iridabenzenes by [2+2+1] Cycloaddition

ꢀ

C C bond-forming reactions of alkynes with transition-

metal complexes is an area of considerable interest because of

the synthetic utility in assembling highly conjugated organic

products.^[76]One common intermediate in such reactions is a

metallacyclopentadiene.During recent investigations, Chin

et al.found that reaction of [Ir(CO)(NCMe)(PPh )₂]⁺with

3

two equivalents of acetylene and subsequent addition of an

equivalent of a terminal arylacetylene, produced alkynyl

iridacyclopentadienes such as 89 (Scheme 25).^[77]Upon

acidification with HBF₄, 89 rearranged cleanly into iridacy-

clohexadiene 90, the product of a formal [2+2+1] alkyne

cyclization.Treatment of 90 with Lewis bases such as CO and

MeCN caused the aryl group to rotate outward to a position

trans to the Ir center to afford 91 and 92, respectively.

Although zwitterionic iridabenzene resonance structures such

as 90’ and 91’ were invoked to help explain these types of

rearrangements, metalla-aromatic species were not

observed.^[76]

Figure 2. Molecular structure of iridabenzvalene 75d. Thermal ellip-

soids are set at 30% probability. Only the ipso carbon atoms of the

PPh₃ligands are shown.

of 72b,d, 75d, and 86 are given in Table 5.The iridabenzva-

lenes possess a torsion trigonal-bipyramidal coordination

sphere composed of a carbonyl ligand, the cyclopropene

double bond (bound in h²mode), two phosphine ligands, and

More recently though, Chin and Lee found that if MeCN

is present during acidification of 90 with HBF₄, cationic

iridabenzene 93 is then isolated.^{[51] 1}H NMR analysis of 93 is

consistent with the classification of this compound as an

iridabenzene.The proton ortho to the Ir center in 93 (Ar=

Ph) resonates at d = 13.99 ppm, considerably downfield from

where the corresponding proton resonates in 90 (R = H, d =

6.64 ppm). On going from 90 (R = Me) to 93 (Ar= p-Tol), a

similar trend is observed for the protons meta to the Ir center:

from d = 5.79 ppm and d = 5.55 ppm to d = 6.88 ppm and d =

7.3–7.6 ppm (with overlapping signals from PPh₃), respec-

tively.

Unique to 93 is its ability to reversibly react with weak

Lewis bases such as CO and Et₃N to form iridacyclohexadiene

complexes 91 and 92, respectively.^[51]It is particularly unusual

that MeCN (ordinarily a more labile ligand) would replace

the carbonyl ligand on conversion of 91 back into 93.The

facile interconversion of cyclohexadienes 91 and 92 and

iridabenzene 93 supports the presumed intermediacy of

iridabenzenes in the original studies.^[76]This reactivity could

be a result of the amphiprotic benzylic carbon atom con-

nected to the carbon atom ortho to the Ir center.Resonance

structures 90’ and 91’ could at least partly explain this

behavior.While the ¹H NMR data of the iridacyclohexa-

dienes suggest that 91’ is not a dominant resonance structure,

it may increase the lability of the CO ligand by reducing the

ability of the Ir center to backbond efficiently.Additional

evidence for the amphiprotic nature of the benzylic carbon

atom is found on treatment of 90 with HCl, which protonates

this carbon atom and cleanly produces iridabenzene 94.^[51]

This unusual reactivity parallels that observed in the pre-

viously mentioned osmabenzofuran studies (see Section

3.4).^[50]

Table 5: Selected X-ray bond lengths [] and bond angles [8] for

iridabenzvalenes.

72b

72d

75d

86

Ir-P1

Ir-P2

Ir-C1

Ir-C2

2.306

2.318

2.146

2.143

2.095

1.447

1.529

1.526

1.466

1.322

2.330

2.326

2.172

2.159

2.092

1.440

1.519

1.533

1.481

1.328

2.353

2.189

2.220

2.184

1.405

1.555

1.566

1.401

1.375

2.367

2.381

2.173

2.189

2.074

1.446

1.539

1.543

1.457

1.330

Ir-C5

C1-C2

C1-C3

C2-C3

C3-C4

C4-C5

C1-Ir-C2

C5-Ir-C6

P1-Ir-P2

P1-Ir-C1

39.4

178.9

104.1

109.4

105.9

110.4

116.3

38.8

176.5

109.4

106.5

104.0

109.9

116.2

37.1

174.0

110.4

100.5

109.8

108.8

120.2

38.7

172.6

106.5

105.0

109.2

108.4

129.6

P2-Ir-C2

dihedral angle^[a]

dihedral angle^[b]

[a] Angle between the C1-C2-C3 plane and the Ir-C1-C2 plane. [b] Angle

between the C1-C2-C3 plane and the C7-C1-C2-C13 plane.

ꢀ

the s-vinyl ligand around the Ir atom.The Ir C1 and Ir C2

ꢀ

bond lengths (2.143–2.220 ) are typical of Ir C single bonds.

ꢀ

The C1 C2 bond lengths (1.405–1.447 ) are characteristic of

transition-metal–olefin complexes that have significant back-

bonding contribution from the metal center, that is, metal-

ꢀ

lacyclopropanes.The longer Ir C bond lengths, the shorter

ꢀ

C1 C2 bond length, and the smaller C1-Ir-C2 bond angle in

75d all suggest that the h²interaction of the cyclopropene

p bond with the Ir center is considerably weaker than in the

analogous complexes listed in Table 5.This weaker bonding is

corroborated by the fact that 75d isomerizes at room

temperature to 76d whereas the other iridabenzvalenes are

stable under similar conditions (see Section 5.1).

The addition of an arylacetylene to (h²-acetato)iridacycle

95 produced an unexpected difference in the key intermediate

that nonetheless led to iridabenzene synthesis.^[52]Unlike 89,

the alkynyl(buta-1,3-dien-1-yl)iridium complex 96 was iso-

lated in this experiment (Scheme 26).Addition of triflic acid

to a CHCl₃solution of 96 furnished cationic iridabenzene 97,

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M. M. Haley and C. W. Landorf

Scheme 25. Formation of iridabenzenes by [2+2+1] cycloaddition.

tion of diene 98 with propene at 608C furnished iridabenzene

99 (Scheme 27).It is believed that 99 results from the

coordination of the alkene and subsequent isomerization to

the propylidene species 100, in which the propylidene ligand

undergoes carbene migratory insertion into the iridacyclo-

pentadiene moiety.Subsequent a-hydride elimination from

101 produces iridabenzene 99.Support for this mechanism is

provided by the reaction of 99 with MeCN to yield a 6:1

kinetic mixture of 102 and 103.These two products are

explained by hydride migration back to the six-membered

ring.Upon heating, 102 rearranges to the more thermody-

namically favored isomer 103, likely via 99, which is similar to

reactivity first observed by Hughes et al.in their studies of the

reaction of 3-vinylcyclopropenes with Ir complexes.^[79]

Replacement of 98 with the related diene complex 104

illustrates the subtleties of the system, as this analogous

reaction affords iridabenzene 105, which bears a Me group

bound to the Ir center.^[78]As before, the first step in the

mechanism is alkene coordination; however, to explain the

different outcome, a 1-methylethylidene unit must be favored

over the linear propylidene in the previous example.Carbene

migratory insertion would then be followed by the unusual a-

methyl elimination to give iridabenzene 105.Even more

surprisingly, the reaction with MeCN completely reverses the

process, such that loss of propene regenerates the original

iridacyclopentadiene ring in 106.

Scheme 26. Formation of iridabenzene 97. Ar=Ph, p-Tol.

which was characterized as its OTf^ꢀsalt by NMR and IR

spectroscopy and by elemental analysis.Iridabenzene 97

could also be produced by displacement of both MeCN

ligands in complex 93 when it was treated with NaOAc.^[51,52]

The probable mechanism for the formation of 97 from 96

involves proton attack on the b-carbon atom of the alkynyl

group in 96 to give a cationic alkylidene, which is then

attacked at the a-carbon atom by the terminal diene.

Tautomerization of the resulting iridacycle affords 97.Deu-

terium-labeling studies corroborate this hypothesis.Reaction

of 96 with D⁺produces iridabenzene [6-D]-97 (here, C6 is the

benzylic carbon).Similarly, reaction of 95 with deuterated

acetylene and subsequently with D⁺produced [4,6-D₂]-97,

thus confirming that it is the terminal carbon atom of the

diene that attacks the a-carbon atom of the alkylidene

X-ray structure analyses of 99 and 105 reveal two

interesting features.First, the molecules are decidedly non-

planar, with the Ir centers deviating 0.57 and 0.70 out of the

plane defined by the five C atoms of each iridacycle.Second,

the molecules both exhibit a surprising degree of bond

alternation.In contrast to most metallabenzenes, the differ-

ences between the “single” and “double” bonds in the two

complex.This chemistry is related to the known reactions of

+

ꢀ

alkynyl complexes with H which lead to C C coupling with

neighboring unsaturated ligands.^[76,77]

Paneque, Poveda, and co-workers have recently reported

an interesting variation of the [2+2+1] protocol by the

coupling of an alkene and an iridacyclopentadiene.^[78]Reac-

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Chemie

naphthalene.The Ir atom is tilted

somewhat out of the plane of the

metallacycle carbon atoms by

ꢀ

0.76 . The two Ir C bond lengths

(1.930 and 1.981 ) are intermediate

between single- and double-bond

lengths and are in good agreement

with those in Ir^IIImetallabenzenes.

Spectroscopically, the aromatic pro-

tons of 107 resonate between d =

6.60 and 8.12 ppm and are increas-

ingly deshielded the nearer they are

to the metal center.While no protons

are attached directly to the primary

metallacycle, ¹³C NMR spectroscopy

demonstrates the typical anisotropic

effects of the metal, as the outside

C atom produces a signal at d =

255.0 ppm, and the bridging C atom

gives a signal at d = 177.9 ppm.

Column chromatography of 107

on silica gel results in the destruction

of its aromaticity and affords 113

Scheme 27. Formation of iridabenzenes 99 and 105.

structures are on the order of 0.040–0.057 and 0.21–0.36 for

99 and 105, respectively.These features reflect the diminished

aromaticity of these iridabenzenes and thus could explain in

part the unusual reactivity noted above.

5.3. Iridanaphthalene

Although the existence of higher homologues of metal-

labenzenes had only been inferred from their decomposition

products and the comparable reactivity of the metallabenzene

analogues, one of the most exciting developments in recent

years is the isolation and characterization of iridanaphthalene

107 by Paneque et al.^[80]Synthesis of 107 begins by treating

108 with dimethyl acetylenedicarboxylate (DMAD), which

yields isomeric iridacycloheptatrienes 109 and 110

(Scheme 28).^[81]Even though no intermediates could be

detected, these products are likely produced by initial

ꢀ

insertion of one molecule of DMAD into one of the Ir C_aryl

ꢀ

bonds.C H activation of the ortho proton in the resulting

phenylalkenyl ligand and subsequent reductive elimination of

benzene would lead to an iridacyclopentadiene complex.

ꢀ

Insertion of a second equivalent of DMAD into either Ir C

bond and coordination of an adventitious water molecule

would result in 109 or 110.Oxidation of isomer 109 with

tBuOOH produces the coordinated oxoacetyl product 111.If

excess tBuOOH is added, then iridanaphthalene 107 is

produced.^[80]Conversion of 111 into 107 likely starts as a

simple Baeyer–Villiger oxidation of the oxoacetyl group.The

Scheme 28. Synthesis of iridanaphthalene 107.

ꢀ

newly generated O C_aliphaticbond in 112 can then be cleaved

irreversibly to form 107.

(Scheme 28),^[80]thus reminiscent of an internal “neutral”

Jackson–Meisenheimer complex.^[82]Hydrolysis of the carbox-

ylate ligand in 107 would result in an intermediate hydroxyl

complex, which likely undergoes intramolecular nucleophilic

Structurally, the iridanaphthalene framework of 107 is

similar to naphthalene in many respects.The bond lengths in

the six-membered carbocycle are within 0.02 of those in

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M. M. Haley and C. W. Landorf

attack at the g-carbon atom to furnish 113.Reaction of this

Second-row metals are known to backbond less efficiently.

ꢀ

new complex with ClC(O)CO₂Me in the presence of pyridine

regenerates the iridanaphthalene, thus proving the viability of

the proposed final step in the initial formation of 107.

Similar to the method used to generate iridanaphthalene

107, treatment of 114 with excess tBuOOH produced

iridabenzene 115 (Scheme 29).^[80]When Thorn and Hoffman

This was reflected in a shorter C1 C2 bond length (1.414 in

116 vs.1.447 in 72b).The dihedral angle between the plane

of the three-membered ring and the plane formed by the

phenyl ipso carbon atoms and the cyclopropene double bond

was greater (119.38 in 116 vs.1158. 8 in 72b).The weaker

coordination also translated into a much larger P1-M-P2

angle, which was nearly 78 larger in 116.Formation of the h²-

cyclopropene–rhodium moiety was unique, as oxidative

addition to the strained s bond had been the exclusive

reactivity observed in previous studies of cyclopropenes with

Rh complexes.Equally rare was the s-vinyl Rh^Ilinkage as

such complexes generally display poor thermal stability and

require sterically demanding ligands for stabilization.Given

these facts, successful preparation of 116 was attributed to the

synergistic combination of intramolecular p bonding of the

cyclopropene unit and s-vinylic linkage to the Rh atom, both

of which in turn inhibit decomposition of the other respective

moiety.

Scheme 29. Synthesis of iridabenzene 115.

first predicted the stability of metallabenzenes, it was

suggested that because of the low-lying LUMO of the

ꢀ

C₅R₅fragment, p donors, especially those ortho and para

In 1999, Hughes et al.reported the reaction of a 3-

vinylcyclopropene with the {RhCl(PMe₃)₂} fragment to fur-

nish isomeric complexes 117 and 118.In solution, a third

to the metal, would be most suited for stabilizing the

metallabenzene.^[11]Although a number of metallabenzenes

have been synthesized with p donors ortho to the metal

fragment,^[23,31]iridanaphthalene 107 and iridabenzenes 99,

105, and 115 are the first examples of metalla-aromatic

species substituted with electron-withdrawing groups, namely

CO₂Me.Other examples of metallabenzenes bearing elec-

tron-withdrawing groups include the phosphonium-substi-

tuted osmabenzenes and ruthenabenzenes reported by

Jia,^[35,62]but these moieties are located on the less influential

meta positions.

compound, 119, was detected.^[85]There is a striking similarity

between cation 119 and 120,^[29]the precursor to Bleekeꢀs

iridabenzene 4, which suggested that deprotonation may lead

to a rhodabenzene.Complex 119, however, either decom-

posed or failed to react with several strong bases.

The inability to isolate a rhodabenzene is congruent with

the DFT calculations reported by van der Boom, Martin, and

co-workers.^[22,33]According to their computations, after initial

phosphine loss, the hypothetical rhodabenzene [C₅H₅Rh-

(PH₃)₃] has a barrier of 20.5 kcalmol^ꢀ1for decomposition to

the cyclopentadienyl complex, while the analogous decom-

position of the corresponding iridabenzene gives a much

higher barrier of 44.4 kcalmol^ꢀ1.For that reason it can be

expected that, in the absence of stabilizing functionalities,

rhodabenzenes are too reactive to be isolated.

6. Rhodium

As with other 4d transition metals, uncoordinated rhoda-

benzenes are probably unstable, and no rhodabenzene has yet

been reported.Reaction of lithiated cyclopropene 70 with the

Rh analogue of Vaskaꢀs complex provided spectroscopic

evidence of a rhodabenzene, but all attempts to isolate the

material furnished either uncharacterizable decomposition

products or very low yields of cyclopentadienyl complexes

similar to 78.^[83]However, analogous reaction with

[RhCl(CO)(PMe₃)₂] resulted in the isolation of rhodabenz-

valene 116 (Scheme 30).^[84]Solutions of this complex decom-

posed at room temperature to an unidentified mixture of

products, but at ꢀ308C are stable for several weeks.

X-ray data of a crystal of 116 which was kept at low

temperature showed that, while most bond lengths and bonds

angles were similar to those of 72b, the main differences 7. Platinum

between the two structures were a consequence of the

differing binding affinities for the cyclopropene p bond.

The most recent transition metal to be incorporated into

stable metallabenzene complexes is platinum.^[86,87]Using the

same chemistry that was used to prepare iridabenzenes, we

reported the synthesis of platinabenzene 121 by the addition

of lithiated 70 to [PtCl₂(cod)], albeit in only 7% yield

(Scheme 31).^[86]One particularly intriguing aspect of this

reaction is that both the metallacycle and cyclopentadienyl

moiety are derived from the starting 3-vinylcyclopropene.

Scheme 30. Synthesis of rhodabenzvalene 116.

3932

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Angewandte

Metallacycles

Chemie

the cyclopropene moiety does not coordinate to afford a

platinabenzvalene.Protonolysis of 122 with HBF₄opened a

coordination site by removal of the secondary leaving group,

Me^ꢀ.Similarly, substitution with chloride to give 123 and

subsequent Cl^ꢀabstraction with TlPF₆generated the analo-

gous reactive 14-electon Pt^IIspecies.In both cases, only

complex 124 was isolated.This complex is an interesting

molecule itself, which in the solid state possesses a slipped

Cp ring with h³coordination.Since platinabenzene inter-

mediates were suspected, protonolysis was performed at

ꢀ608C with trifluoroacetic acid to yield the cis complex 125.

While 125 was stable at low temperatures, warming to room

temperature resulted in the production of a 3:2 mixture of

124-O₂CCF₃and the trans isomer of 125.Even though

Scheme 31. Synthesis of the first platinabenzene 121.

Based on related experiments (see below), it is possible that

formation of the cyclopentadienyl ligand results from the

decomposition of an intermediate platinabenzene before

addition of the second equivalent of lithiated 70.In addition

to being the first platinabenzene, this example demonstrates

the versatility of the 3-vinylcyclopropene approach to metal-

labenzenes, as this is the first method that has produced stable

metallabenzenes for more than one transition metal, while

Jiaꢀs 1,4-pentadiyn-3-ol cyclization route now represents a

second such method (see Sections 3.1 and 4.2).

After this initial success, attempts were made to synthe-

size platinabenzenes by using an “asymmetric” [L₂Pt(X)Y]

complex in which only ligand X would be substituted for the

vinylcyclopropene unit while Y would remain unchanged.In

the following step, Y could then be cleaved from the s-vinyl

compound, thus allowing for coordination of the cyclopro-

pene double bond.Depending on stability, either the resultant

cationic platinabenzvalene or rearranged products like a

cationic platinabenzene would be expected.Addition of

lithiated 70 to [Pt(Me)I(PEt₃)₂] resulted in displacement of

the primary leaving group, I^ꢀ, and thus formation of s-vinyl

complex 122 (Scheme 32).^[88]Given that Pt^IIis found prefer-

entially in a square-planar geometry, it is not surprising that

1

monitoring the reaction by H NMR spectroscopy provided

no direct evidence for a platinabenzene, it is reasonable to

assume the intermediacy of unstable cationic platinabenzenes

such as 126, which readily undergo carbene migratory

insertion to give Cp complexes like 124.

The results of the above two studies suggested that the

next set of experiments should begin with a Cp unit already

[89]

affixed to the Pt center.Reaction of [Cp*Pt(CO)Cl]

with

the lithiated vinylcyclopropene 70 resulted only in trace

amounts of platinabenzene, recovered Pt starting complex,

and the protonated vinylcyclopropene.A more labile leaving

group was next examined, and thus, the new complex

[Cp*Pt(CO)I] was synthesized from the known platinum

dimer [{Cp*Pt(CO)}₂].^[87,89]Reaction of lithiated 70 with

[Cp*Pt(CO)I] afforded a mixture of s complex 127a and

platinabenzene 128a (Scheme 33).Solutions of pure 127a in

C₆D₆rearrange over 2–3 days at room temperature to give

quantitative conversion to 128a.Presumably, 128a is formed

by metathesis of the iodide with the lithiated vinylcyclopro-

pene and subsequent loss of CO with fast rearrangement to

the platinabenzene.Unlike in the case with iridium, monitor-

1

ing this process by H NMR spectroscopy did not provide

evidence for an intermediate platinabenzvalene.Extension of

this chemistry has also led to phenyl(alkyl)platinabenzenes

Scheme 32. Unsuccessful attempts at the synthesis of platinaben-

zenes.

Scheme 33. Synthesis of platinabenzenes 128.

Angew. Chem. Int. Ed. 2006, 45, 3914 – 3936

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3933

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M. M. Haley and C. W. Landorf

ꢀ

128b,c.Additional examples are currently under investigation

The Pt C bond lengths are 1.926–1.937 and 1.951–1.975 ,

along with general studies of platinabenzene reactivity.^[61]

The NMR spectroscopic data for 121 and 128 are similar

to those of other metallabenzenes.The proton ortho to the

Pt atom produces a ¹⁹⁵Pt-coupled signal at d = 12.76 ppm for

121 and in the range d = 11.83–12.09 ppm for 128a–c.The

upfield shift for the latter complexes illustrates the greater

contribution of electron density by Cp* to the metal center.

The meta and para protons produce signals in the ranges d =

7.4–7.6 and 8.2–8.5 ppm, respectively. Two ¹³C NMR signals in

the ranges d = 200–204 and 188–195 ppm are attributed to the

phenyl-substituted and unsubstituted ortho carbon atoms,

respectively.

which compare well with Pt C bond lengths in other Pt

II

ꢀ

carbene complexes.The C C bond lengths in the metallacycle

have an average of 1.382–1.387 with no appreciable bond

alternation.The Cp ring in each system is h⁵-coordinated to

ꢀ

the Pt center (Pt C_Cpbond lengths: 2.257–2.342 ).

Van der Boom, Martin, and co-workers also performed

calculations on the stability of several platinabenzene com-

plexes.^[22,33]Based on their results, the cyclopentadienyl ligand

does stabilize the platinabenzene by increasing the energy

barrier for the occurrence of carbene migratory insertion.In

the case of [C₅H₅Pt(PH₃)₂]⁺, the computational analogue of

126, the transition state to forming [(h⁵-Cp)Pt(PH₃)₂]⁺was

only 24.0 kcalmol^ꢀ1, which corroborates the experimental

results shown in Scheme 32.For the Cp complex [C ₅H₅PtCp],

the computational analogue of 121, the energy of the

transition state for forming [(h³-Cp)₂Pt] was considerably

higher (45.9 kcalmol^ꢀ1).De Proft and Geerlings recently

utilized the isomerization method to predict the aromatic

stabilization energy of the platinabenzene ring in the model

system [Pt(C₅H₃Me₂)(h³-C₅H₃Me₂)].^[90]They obtained a cal-

culated value of 23.4 kcalmol^ꢀ1, approximately two thirds of

the aromatic stabilization energy calculated for benzene.

Based on these computational results, it is not surprising that

platinabenzenes such as 121 and 128 are stable even at

elevated temperatures.

The solid-state structures of the platinacycles are of

considerable interest.The molecular structure of 128b is

shown in Figure 3, and selected bond lengths and angles for

8. Conclusions

Aromaticity is a concept that many chemists take for

granted as well-defined.In fact, like so many things in

chemistry, the truth is multifaceted and thus considerably

more complicated.Metallabenzenes and their higher ana-

logues display a number of contradictory behaviors that place

them in a delicate gray area between aromatic and non-

aromatic.While their structures are in general planar and

exhibit negligible bond alternation, they also undergo

[4+2] cycloadditions as if they were simple metallacyclohex-

atrienes.The NMR spectroscopic properties suggest that the

ring substituents are deshielded; however, this could simply

be an effect of the transition metal.Unlike benzene, many

metallabenzenes rearrange readily to form cyclopentadienyl

complexes.Like benzene, they readily form h⁶-metal com-

plexes, through which the metallabenzenes are in fact clearly

stabilized.

Over the last few years, the field of metalla-aromatics has

expanded significantly.Several new methods to generate

metallabenzenes have been developed.In addition to syn-

thesis from nucleophilic 3-vinylcyclopropenes, they can be

prepared from a number of alkyne precursors by generally

unrelated mechanisms.For the first time, a metallanaphtha-

lene has been isolated and characterized.Platinum has now

been added to the list of transition metals capable of yielding

a metallabenzene.At the current rate of progress, there is

little doubt that the coming years will be equally exciting for

metallabenzene chemistry.

Figure 3. Molecular structure of platinabenzene 128b. Thermal ellip-

soids are set at 30% probability.

121 and 128a,b are given in Table 6.Unlike most metal-

labenzenes, the platinabenzenes have planar structures (for

121: deviation from mean plane 0.02 , sum of angles 7208).

Table 6: Selected X-ray bond lengths [] and bond angles [8] for

platinabenzenes.

121

128a

128b

Pt-C1

Pt-C5

1.959

1.929

1.387

1.392

1.381

1.364

1.382

1.951

1.937

1.401

1.395

1.364

1.387

1.975

1.926

1.406

1.389

1.387

1.350

1.383

C1-C2

C2-C3

C3-C4

C4-C5

1(C-C)

Pt-C1-C2

129.2

122.6

124.8

124.1

130.0

89.3

129.2

122.3

125.2

124.5

128.9

89.7

129.8

124.1

126.9

119.8

129.8

89.4

C1-C2-C3

C2-C3-C4

C3-C4-C5

C4-C5-Pt

C1-Pt-C5

sum of angles

720.0

719.8

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Angewandte

Metallacycles

Chemie

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our work on metallabenzene chemistry possible. The National

Science Foundation and the University of Oregon are grate-

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Profs. John Bleeke, Guochen Jia, Warren Roper, and James

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Recent advances in metallabenzene chemistry

DOI: 10.1002/anie.200504358

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