Dehydrogenation of alcohols by bis(phosphinite) benzene based and bis(phosphine) ruthenocene based iridium pincer complexes

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Dehydrogenation of Alcohols by Bis(phosphinite) Benzene Based

and Bis(phosphine) Ruthenocene Based Iridium Pincer Complexes

Alexey V. Polukeev,^†Pavel V. Petrovskii,^†,§Alexander S. Peregudov,^†Mariam G. Ezernitskaya,^†

and Avthandil A. Koridze*^,†,‡

^†A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Street, 119991 Moscow,

Russian Federation

^‡Institute of Organometallic Chemistry, I. Javakhishvili Tbilisi State University, 3 Chavchavadze Avenue, 0128 Tbilisi, Georgia

S

* Supporting Information

ABSTRACT: Dehydrogenation of alcohols by three iridium

pincer complexes, IrH(Cl)[2,6-(^tBu₂PO)₂C₆H₃] (1), {IrH-

(acetone)[2,6-(^tBu₂PO)₂C₆H₃]}{BF₄} (2), and IrH(Cl)[{2,5-

(^tBu₂PCH₂)₂C₅H₂}Ru(C₅H₅)] (3), is reported, in both the

presence and the absence of a sacriﬁcial hydrogen acceptor.

Dehydrogenation of secondary alcohols proceeds in a catalytic

mode with turnover numbers up to 3420 (85% conversion) for

acceptorless dehydrogenation of 1-phenylethanol. Primary

alcohols are readily decarbonylated even at room temperature

to give catalytically inactive 16e Ir−CO adducts. The

mechanism of this transformation was studied in detail, especially for EtOH; new intermediates were isolated and characterized.

compounds,¹³and alkenes¹⁴being used as hydrogen acceptors.

INTRODUCTION

■

The most desired acceptorless alcohol dehydrogenation is a less

Hydrogen is potentially an ideal energy carrier, as it is

nonpolluting and has a high energy density by weight. In

view of global concerns regarding the environment and

sustainable energy resources, hydrogen is often considered as

common case.¹⁰Several systems capable of acceptorless

dehydrogenation of alcohols have been developed using

rhodium,¹⁵ruthenium,¹⁶and iridium¹⁷catalysts. While some

of them appeared promising toward “green” synthesis,¹⁸their

the fuel of the future, a promising candidate to solve the

activity should be further improved to produce a commercial

problems caused by the use of fossil fuels.¹The production,

process for hydrogen generation and storage based on

storage, and transportation of hydrogen have attracted careful

transformations of organic molecules.⁶

attention in recent decades.²To preserve the important

advantages of liquid fuels such as gasoline and diesel, namely

relative safety, fast reﬁlling times, and high energy density, it

would be of great interest to achieve hydrogen storage in liquid

Furthermore, in the case of primary alcohols, the

dehydrogenation reaction is often complicated by decarbon-

ylation of the corresponding aldehyde,^14b,19which leads to

catalyst deactivation. Decarbonylation of secondary alcohols is

materials. Signiﬁcant eﬀorts are devoted to the development of

rarely observed.²⁰These processes are generally believed to

catalysts able to dehydrogenate some hydrogen-rich liquids

(“organic hydrides”) such as alkanes,³formic acid,⁴and

proceed via formation of acyl complexes followed by migratory

deinsertion of CO;^19c,d,g,20however, few intermediates were

nitrogen heterocycles.⁵In this respect, alcohols seem to hold

isolated and characterized.^19c,g

considerable promise.⁶

Earlier, a family of benzene-, anthracene-, and metallocene-

On the other hand, dehydrogenation may be considered as a

based iridium pincer complexes have been shown to be the

kind of oxidation of alcohols to aldehydes and ketones, which is

an important transformation in synthetic organic chemistry.⁷

most productive catalysts for the dehydrogenation of

alkanes.^3,21One of these catalysts, namely IrH₂[2,6-

Transition-metal-catalyzed dehydrogenative oxidation of alco-

(^tBu₂PCH₂)₂C₆H₃], was successfully applied to the dehydro-

hols is tantalizing in view of the development of environ-

mentally friendly, high-atom-economy processes.⁸⁻¹⁰Carbonyl

genation of alcohols. Thus, it catalyzed the transfer

dehydrogenation of primary and secondary alcohols with tert-

compounds generated usually have a much wider range of

reactivity and are of higher value. They can be functionalized in

situ to form a substrate for hydrogenation, to which catalyst

butylethylene (tbe) as a hydrogen acceptor to form the

corresponding aldehydes and ketones and tert-butylethane in

excellent yields (however, with low turnover numbers,

returns taken hydrogen (so-called borrowing hydrogen

TONs),^14bindicating that iridium pincer complexes have

strategy),^8,9or alternatively hydrogen can be liberated or

transferred to a sacriﬁcial hydrogen acceptor.¹⁰A number of

transition-metal complexes able to provide transfer dehydro-

genation of alcohols are known, with O₂,¹¹H₂O₂,¹²carbonyl

Received: September 28, 2012

Published: February 8, 2013

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potential in this area. Indeed, a recently developed

dibenzobarrelene-based complex (not tested for alkane

dehydrogenation) has shown excellent activity in acceptorless

dehydrogenation of alcohols.^17eBearing these results in mind,

we decided to test counterparts of IrH₂[2,6-

(^tBu₂PCH₂)₂C₆H₃], which showed higher activity in alkane

dehydrogenation, in alcohol dehydrogenation.

In this paper we report catalytic dehydrogenation of alcohols

by bis(phosphinite) benzene based and bis(phosphine)

ruthenocene based pincer complexes. Some of these complexes

have shown high activity in the acceptorless dehydrogenation of

secondary alcohols; remarkably, the reaction proceeds slowly

even at room temperature. Primary alcohols readily undergo

decarbonylation. The mechanism of the reaction with alcohols

was studied in detail, which allowed identiﬁcation of some new

intermediates. We hope that the results obtained will shed

additional light on the processes of dehydrogenation and

decarbonylation of alcohols by transition-metal complexes.

The results are summarized in Table 1. At temperatures

above 200 °C for 1 (entry 1) and about 200 °C for 3 (entry 4)

Table 1. Transfer Dehydrogenation of Isopropyl Alcohol by

a

Catalysts 1 and 3

entry

cat.

substrate

T, °C

time, h

conversion, %

TON

1

2

3

4

5

6

7

8

1

3

ⁱPrOH

240

200

150

200

150

110

1

76

100

13

3040

4000

500

8

38

1520

1770

370

16

8

44

9

40

26

1030

a

t

Conditions: 0.025 mol % of catalyst, 1.5 equiv of BuONa, 1/1

substrate/acceptor ratio.

RESULTS

■

decarbonylation of isopropyl alcohol or acetone occurs to give

the catalytically inactive carbonyl compounds Ir(CO)[2,6-

(^tBu₂PO)₂C₆H₃] (4)^3gand Ir(CO)[{2,5-(^tBu₂PCH₂)₂C₅H₂}-

Ru(C₅H₅)] (5),^3hrespectively, which limits the TON.

Decarbonylation as a pathway for deactivation of iridium

dehydrogenation catalysts is not unexpected;^14b,17eindeed,

control experiments revealed that complex 4 possesses

negligible activity. Lowering the temperature allowed us to

avoid decarbonylation; thus, at 200 °C complex 1 rapidly

aﬀords 4000 TON with formation of acetone in the

quantitative yield (entry 2), while at 150 °C, it takes

approximately 8 h for the reaction to be completed. It should

be noted that the TON obtained is 2 orders of magnitude

higher than that for the related complex IrH₂[2,6-

(^tBu₂PCH₂)₂C₆H₃].^14bComplex 3 was found to be suﬃciently

less active than 1 under any conditions employed; lowering the

temperature again allowed us to avoid decarbonylation;

however, it did not much improve the results.

Acceptorless Dehydrogenation of Alcohols. Surpris-

ingly, in the presence of 1 equiv of the base, complexes 1 and 3

are able to provide catalytic acceptorless alcohol dehydrogen-

ation even at room temperature, although the reaction rate does

not exceed 2−3 TON per day. To our knowledge, no catalyst

operates under such mild conditions. As was mentioned earlier,

heating is expected to improve the performance. Taking into

consideration the results of transfer dehydrogenation, we

limited temperatures used to 190 °C for 1 and to 150 °C for

3 to prevent decarbonylation.

Complex 1 has been shown to be an eﬀective catalyst for

acceptorless dehydrogenation of secondary alcohols (Table 2),

providing approximately 2000 equiv of hydrogen/mol of

catalyst for alcohols with high boiling points (entries 1 and

6). A small excess of base always present in catalytic

experiments with 1 and 3 causes the formation of some

amounts of Guerbet-type products in addition to the target

ketones. The Guerbet reaction can be considered as

dehydrogenation of an alcohol to a carbonyl compound

followed by condensation of the latter and rehydrogenation.¹⁰

Thus, under these conditions some quantity of alcohol is

involved into transfer dehydrogenation and the amount of

hydrogen evolved actually is smaller than the TON. The

addition of more than 1.5 equiv of a base has never caused any

serious changes, except for the increased formation of

condensation products. In order to identify catalytically active

Synthesis of Catalysts. The known complexes 1^3fand 3

(the latter as a mixture of H-endo and H-exo isomers^3h) (Chart

1) were synthesized according to the literature procedures.

Chart 1. Catalysts Tested in This Work

Cationic complex 2 was obtained by chloride abstraction from

1 using AgBF₄in acetone. Its counterpart with the B(C₆F₅)₄

−

anion was described in the literature as a catalyst for the

reduction of alkyl halides by triethylsilane;²²complexes with

other anions were generated in situ and used for hydrogen

isotope exchange reactions²³without characterization. Complex

2 reveals broadened signals in the H, ³¹P, and ¹⁹F NMR

1

−

spectra due to ﬂuxional coordination of BF₄and acetone to

the metal center in solution.²⁴The IR spectrum of 2 in KBr

pellets contains two ν(CO) bands at 1653 and 1637 cm⁻¹of

coordinated acetone, which can be explained by solid-state

interactions rather than by the existence of isomers. A solution

of 2 in CHCl₃reveals only the band at 1710 cm⁻¹

corresponding to free acetone, indicating that, despite a

broadened signal in the ¹H NMR spectrum, in solution acetone

is for the most part not bound to the iridium.

Transfer Dehydrogenation of Isopropyl Alcohol. The

thermodynamics of acceptorless alcohol dehydrogenation

demand elevated temperatures and removal of H₂from the

equilibrium.^25,26We ﬁrst tested catalysts 1 and 3 for more

favorable transfer dehydrogenation using tbe as a hydrogen

acceptor. The latter was found to isomerize into the ineﬃcient

hydrogen acceptors 2,3-dimethyl-2-butene and 2,3-dimethyl-1-

butene if acidic species are present in the reaction mixture;²⁷

this is why complex 2, which we wanted to test without basic

additives, was not used. According to the literature data,^3fin the

case of hydrido chloride complexes 1 and 3, 1 equiv of a base is

required to remove HCl from the iridium atom to give

catalytically active species.

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a

Table 2. Acceptorless Dehydrogenation of Alcohols by Catalysts 1−3 and 6

b

c

entry

cat.

substrate

T, °C

time, h

conversion, %

TON

amt of H₂evolved, equiv

d

1

6

1

3

2

PhCH(OH)CH₃

CyOH

190

175

150

190

90

18

36

18

8

74

2960

2000

2200

1120

350

2070

2000

2200

1120

350

1780

90

2

50

55

28

9

3

4

5

e

6

89

3560

7

ⁱPrOH

2

90

2

8

PhCH(OH)CH₃

ⁱPrOH

23

24

8

0.1

0

2

9

23

3

10

11

12

13

14

15

16

17

EtOH

90

0

MeOH

90

8

0

d

0

d

PhCH(OH)CH₃

EtOH

150

90

18

8

48

1920

1130

0

f

ⁱPrOH

23

24

18

36

18

g

PhCH(OH)CH₃

190

175

71

2530

3420

1720

2530

3420

1720

g

85

g

43

a

b

c

Conditions: 0.025 mol % of catalyst, 1.5 equiv of ^tBuONa (except for 2 and 6). Oil bath temperature. Calculated from stoichiometry on the basis

of analysis of products. When this number diﬀers from the TON, products other than the appropriate carbonyl compound are formed, leading to

d

e

some contribution of transfer dehydrogenation. 1,3-Diphenyl-1-butanone and 1,3-diphenyl-1-butanol are formed as byproducts. 2-Cyclohexenyl-1-

cyclohexanol and 3-cyclohexenyl-1-cyclohexanol are formed as major products. In C₆D₆. Trace amounts of bis(1-phenylethyl) ether and

f

g

ethylbenzene are formed.

Scheme 1. Reaction of Complex 1 with EtOH

species, we synthesized dihydride IrH₂[2,6-(^tBu₂PO)₂C₆H₃]

(6)^3gand tested it in the dehydrogenation of PhCH(OH)CH₃

(entries 2−5). This complex was found to operate under

neutral conditions and gave the same results as 1 did. Thus,

complex 6 is the compound actually involved into the catalytic

cycle, as it was the case for alkane dehydrogenation.^3gHowever,

from an experimental point of view, it is more convenient to

use the air-stable hydrido chloride 1 as a catalyst precursor.

Since catalysis by 6 proceeds without side reactions, we used it

to optimize the reaction conditions. Thus, it was found that

high temperature is required to obtain a reasonable reaction

rate, which becomes negligible after 18 h of heating at 190 °C

and 2000 TON obtained.

Cationic complex 2 was used without addition of base. It is

inactive at ambient temperature (entry 14), but at high

temperatures its activity is higher than that of 1 or 6 (entries

15−17) and diminishes slowly. Thus, after 36 h of heating it

showed 3420 TON with 85% conversion in comparison to

2200 and 55% for 6. We speculate that, at elevated

temperatures, 2 is probably able to lose reversibly HBF₄and

operates via the same mechanism as 6 does. In this respect, it is

worth noting that trace amounts of bis(1-phenylethyl) ether

and ethylbenzene are detected in the reaction mixture when 2 is

used, presumably as a result of an acid-catalyzed reaction. The

improved activity of 2 may be ascribed to the destruction of

some inhibiting side products: for example, products of

metalation of ketones by the acid evolved. Thus, the reaction

of an analogue of 6, IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃], with

acetophenone in the presence of norbornene was reported²⁸

to give the iridium acetylphenyl hydride IrH(C₆H₄C(O)CH₃)-

[2,6-(^tBu₂PCH₂)₂C₆H₃], which is suﬃciently stable (apart from

isomerization), at least at 135 °C.

Complex 3 again gave signiﬁcantly poorer results than 1 did.

Primary alcohols appeared to be unsuitable substrates because

they are readily decarbonylated at room temperature by both 1

and 3.

Decarbonylation of Primary Alcohols. Decarbon-

t

ylation of EtOH. In the presence of EtOH and BuONa,

complex 1 is rapidly converted to dihydride complex 6.

Stoichiometric considerations require the formation of 1 equiv

of acetic aldehyde, which, however, was not detected by NMR,

presumably due to condensation. Further, dihydride 6 slowly

reacts with ethanol at room temperature to form the new

carbonyl hydrido methyl complex 7 and its isomer 8 (Scheme

1). No base is required at this stage; independently prepared 6

can be used instead.

Several days are required for the reaction to be completed

and to provide a mixture of 7, 8, and carbonyl complex 4. The

addition of tbe as a hydrogen acceptor accelerates the reaction

with alcohol; a mixture of 7 and 8 in a 98/2 ratio is observed

after 0.5−1 h, with no starting material or decomposition

products present. Compounds 7 and 8 (the latter in mixtures

with 7 or 4 due to further transformations; see below) were

characterized by means of NMR and IR spectra and elemental

analysis. Thus, the ³¹P{¹H} NMR spectrum of 7 reveals a

singlet at 169.4 ppm and the characteristic hydride signal

appears as triplets of quadruplets at −10.93 ppm (²J_P−H= 18.6

Hz, ³J_H−H= 1.5 Hz), while in the ¹³C{¹H} NMR spectrum, the

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upﬁeld resonance corresponding to the iridium-bound methyl

group is observed at −26.7 ppm. Isomeric complex 8 gives

similar signals at 163.7, −9.71 (²J_PH= 17.6 Hz, ³J_HH= 1.5 Hz),

and −47.8 ppm (t, J_CP= 3.8 Hz) in the ³¹P{¹H}, H, and

¹³C{¹H} NMR spectra, respectively. Unfortunately, we could

not determine the structures of 7 and 8 using X-ray

crystallography because of molecular disorder in the crystals

obtained. For this reason, the conclusions on the relative

conﬁguration of the substituents at the iridium atom were made

using NMR spectra. Thus, in the case of 7, ¹H NOESY

spectrum displayed a cross peak between the Ir-H and one pair

of diastereotopic tert-butyl groups, while the methyl group

bound to iridium showed a cross peak with another pair, which

is consistent with a structure where the hydride and the methyl

group are arranged at the opposite sides of the plane formed by

the benzene ring and two ﬁve-membered chelating metalla-

cycles. Additional proof was provided by the ¹³C NMR

spectrum without decoupling, where the CO signal exhibited a

2

1

Figure 2. IR spectra of a hexane solution of 8 (a) and its deuterated

analogue (b).

for complex 8 indicates a strong coupling of ν(CO) and ν(Ir−

H) modes leading to the redistribution of band intensities,

which was earlier observed for the trans arrangement of CO

and H ligands.²⁹Upon deuteration of the hydride position of 7

(Figure 1b), the band at 2046 cm⁻¹is shifted to the low-

frequency range. conﬁrming its Ir−H nature with the ν(CO)

band being virtually unchanged, while deuteration of 8 results

in the disappearance of both intense ν(CO) and ν(Ir−H)

bands. This happens because the M−H stretch moves to the

low-frequency region, violating the conditions for coupling, and

a new absorption at 1987 cm⁻¹appears corresponding to the

uncoupled C−O stretch (Figure 2b).

1

small H,¹³C spin−spin coupling of ca. 4 Hz, which is strongly

indicative of a mutually cis arrangement of CO and hydride

ligands.²⁴For complex 8, there is also a cross peak between the

Ir-H and one pair of diastereotopic tert-butyl groups, but in this

case, Ir−CH₃interacts with both “upper” and “lower” pairs of

tert-butyls and also with hydride, thus conﬁrming a trans Ar−

Ir−CH₃and cis H−Ir−CH₃arrangement (with the methyl

group lying in the plane of the benzene ring). In the

nondecoupled ¹³C NMR spectrum, the carbonyl signal

1

displayed a large H,¹³C coupling constant of ca. 45 Hz, thus

indicating that the carbonyl and hydride ligands are arranged in

a trans conﬁguration.²⁴

When a solution of complex 7 is kept at room temperature, 7

slowly isomerizes to 8, where the methyl group and the hydride

are in the cis conﬁguration, and 8 slowly loses methane to form

carbonyl complex 4 (Scheme 2). These two processes proceed

The IR spectrum of 7 in hexane reveals a strong ν(CO) band

at 1997 cm⁻¹and a very weak band of Ir−H stretch at 2046

cm⁻¹(Figure 1a). This assignment is conﬁrmed by the Raman

Scheme 2. Transformations of Complex 7: Rearrangement

to 8 Followed by Methane Loss To Give 4

with comparable rates (Figure 3); therefore, it is impossible to

obtain a pure sample of 8, and it was characterized in mixtures

with 7 or 4. The kinetic curves of the tranformations are

satisfactorily described by a scheme involving the two

consecutive irreversible ﬁrst-order reactions 7 → 8 → 4. The

assumption that the elimination of methane is a true ﬁrst-order

reaction seems realistic, while the isomerization of 7 to 8 is

rather a pseudo-ﬁrst-order process actually involving several

stages. The extracted rate constants k_1,obs= 0.0046 0.0001

h⁻¹and k₂= 0.0033 0.0001 h⁻¹correspond to almost equal

activation free energies of 25.5 0.1 and 25.7 0.1 kcal/mol,

respectively, at room temperature.

How does the rearrangement of 7 to 8 occur? Both CO and

phosphine “arms” are fairly strongly bound to the iridium (we

are unaware of examples where bis(phosphinite) iridium pincer

complexes lose these ligands, even under harsh conditions of

alkane dehydrogenation), for this reason, any kind of

dissociative mechanism does not seem likely under mild

conditions employed. For the related isomerization of cis-

Ir(H)₂CO[2,6-(ⁱPr₂PCH₂)₂C₆H₃] to trans-Ir(H)₂CO[2,6-

(ⁱPr₂PCH₂)₂C₆H₃],³⁰a nondissociative trigonal twist mecha-

Figure 1. IR spectra of a hexane solution of 7 (a) and its deuterated

analogue (b).

spectrum of solid 7, where in this region two bands with

virtually the same frequencies but opposite intensities are

observed. For its isomer 8, along with the ν(CO) band of

residual 7, two overlapping intense bands at 1972 and 1981

cm⁻¹are observed in the IR spectrum of a hexane solution

(Figure 2a). In addition, the band at 1949 cm⁻¹is always

present associated with the carbonyl complex 4. In the Raman

spectrum of solid 8, the bands at 1976 and 1983 cm⁻¹are also

present, but the intensity ratio is opposite to that observed in

the IR spectrum. While the IR spectrum of 7 is typical of a

transition-metal complex containing CO and hydride ligands

(strong ν(CO) and weak ν(Ir−H)), the pattern in this region

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have been shown to operate via a dissociative 16e−14e

mechanism.²¹In the ﬁrst step, hydrogen from [Ir]H₂([Ir] =

Ir[2,6-(^tBu₂PCH₂)₂C₆H₃], Ir[2,6-(^tBu₂PO)₂C₆H₃]) is trans-

ferred to a sacriﬁcial hydrogen acceptor or removed thermolyti-

cally to give the reactive 14e species [Ir]. The latter undergoes

oxidative addition of a C−H bond, and subsequent β-hydride

elimination produces the oleﬁn product and regenerates [Ir]H₂.

This is not the case for dehydrogenation of alcohols by the

systems studied. Thus, dihydride complex 6, which is the

particle actually involved in the catalytic cycle, was shown to be

thermally stable up to 200 °C^3f,g,21and does not lose hydrogen

readily. Apparently, alcohol assistance is required for this

process to proceed at room temperature. Some possible

pathways for the ﬁrst steps of this reaction include (a)

coordination of alcohol to 6 followed by hydrogen loss, (b)

oxidative addition of the O−H bond of alcohol to give an Ir(V)

complex with subsequent reductive elimination, and (c)

protonation of hydride with alcohol acting as a Brønsted acid

(Scheme 3).

Figure 3. ³¹P{¹H} NMR monitoring of the rearrangement of 7 to 8

and formation of 4. Dots correspond to the experimental points and

continuous lines to the simulated curves.

In attempt to clarify this question, the reaction of 6 with

C₂D₅OD was conducted, which led to deuterium scrambling

and formation of a mixture of 6, 6-D, 6-D₂, 7-H,CD₃and 7-

D,CD₃(Scheme 4).

nism was calculated³¹to be the favored one. It should be noted

i

that in calculations the Pr groups were replaced by hydrogen

Scheme 4. Reaction of 6 with C₂D₅OD

atoms. In our case, given the higher degree of steric crowding at

the iridium atom (^tBu instead of ⁱPr, CH₃instead of H), such a

twist does not seem likely; reversible insertion of CO into the

Ir−CH₃bond followed by rotation of the acetyl group and

deinsertion of CO look more realistic. It should be noted that

the related isomerization of the isonitrile complex IrH(CNR)-

Me[2,6-(^tBu₂PCH₂)₂C₆H₃] to IrH(Me)CNR[2,6-

(^tBu₂PCH₂)₂C₆H₃] was reported; however, the mechanism

was not discussed.³²

Decarbonylation of alcohols by iridium pincer complexes is

not a common case;^{14b,17e,19f,g}formation of methyl hydrido

carbonyl complexes from ethanol is even more rare, with only

two examples reported,^19g,33both on iridium compounds. Since

a deeper understanding of alcohol decarbonylation at metal

centers may be relevant to a range of catalytic processes, it was

in our interest to investigate the formation of 7 and 8 in detail.

Iridium pincer complexes which were used for catalytic

alkane dehydrogenation (including those used in this work)

Deuterium exchange at the hydridic positions of 6 was found

to be much more facile than the formation of 7, with deuterium

being incorporated from the O−D group. For example, the

reaction of 6 with a ca. 8-fold excess of C₂D₅OD approaches

the equilibrium point with respect to deuterium distribution

Scheme 3. Possible Pathways of the Reaction of 6 with EtOH

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within 4 min (ca. 77% of total deuteration of the hydridic

positions was observed with only trace amounts of 7-D,CD₃).

It is obvious that pathway (a) cannot explain deuterium

scrambling and should be discarded, while the choice between

pathways (b) and (c) is ambiguous. In this respect, it is of

interest to perform the reaction of 6 with a substrate which

would hamper the formation of an analogue of the over-

Scheme 5. Equilibrium between Two Forms of Complex 6

t

crowded Ir(V) intermediate III, such as BuOD. This bulky

to formation of hydrogen bonds, other conclusions would be

too speculative.

alcohol is not able to enter the coordination sphere of the Ir

atom. If the IrH(O^tBu)[2,6-(^tBu₂PO)₂C₆H₃] particle (analogue

of II) was formed, it would be stable due to the absence of β-

hydrogen and certainly detected by NMR (for example,

IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃] reacts with PhOH to give stable

IrH(OPh)[2,6-(^tBu₂PCH₂)₂C₆H₃]³⁴); however, this was not

We conducted special IR experiments aimed at detecting H-

t

bonded species between BuOH and dihydride 6. tert-Butyl

alcohol was selected as an acid because it is inert to dihydride.

Its concentration in hexane solution was 0.04 mol/L. In the IR

spectrum in the ν(OH) region (Figure 4a), only the band for

t

the case. Isotopic exchange with BuOD proceeds much more

slowly (ca. 7% of total deuteration of the hydridic positions

under the same conditions as for the reaction with C₂D₅OD;

on approaching the equilibrium point its rate becomes

comparable with deuterium scrambling into phosphorus-

bound tert-butyl groups) but clearly points to pathway (c) as

being the only possible route for this substrate. It should be

noted that a hydrido dihydrogen complex similar to IV with a

noncoordinating counteranion, {IrH(H₂)[2, 6-

(^tBu₂PO)₂C₆H₃]}{BAr^f₄}, was reported in the literature.³⁵

It was proved that the protonation of the transition-metal

hydride is a multistep process occurring via hydrogen-bonded

intermediates and ion pairs;³⁶thus, detection of such species

would give additional proof for pathway (c). Several NMR and

IR spectroscopic techniques have been used to establish the

formation of hydrogen-bonded species.³⁷In particular, the low-

frequency shift of the OH stretching vibration in the IR spectra

Figure 4. IR spectra in the ν(OH) region in hexane solution: (a)

t

^tBuOH (c = 0.04 M); b) BuOH in the same concentration in the

1

and high-ﬁeld shift of the hydride resonance in H NMR upon

presence of 6; (c) the same mixture as in (b) at 0 °C.

addition of the proton donor are among the reliable indicators

of hydrogen-bond formation between a metal hydride and an

alcohol.³⁷Indeed, when an alcohol is added to a benzene-d₆or

toluene-d₈solution of 6, a high-ﬁeld shift of hydride resonance

the free OH stretch at 3623 cm⁻¹was observed. When this

solution was added to solid 6 (a saturated solution of 6 was

obtained, which provided a large excess of the base in the

mixture), the intensity of the band at 3623 cm⁻¹decreased and

a new broad band with a center of gravity at ca. 3420 cm⁻¹

(Figure 4b) appeared in the spectrum. These features (low-

frequency shift of the ν(OH) band and intensity growth) are

indicative of H-bond formation between dihydride 6 and

^tBuOH. Lowering the temperature to 0 °C resulted in a further

increase of the intensity of the H-bonded band.

t

occurs, being slightly larger for EtOH versus BuOH and

increasing with the excess of alcohol. The highest value

obtained was 0.4 ppm for ca. 50-fold excess of EtOH. Addition

of ca. 4 equiv of EtOH to a solution of 6 in toluene-d₈induces a

shift of 0.08 ppm, which turns to 0.36 ppm upon cooling to 213

K. However, the hydridic resonance of complex 6 itself

undergoes a signiﬁcant high-ﬁeld shift at low temperatures³⁸

and thus, rather surprisingly, the actual diﬀerence between 6

and 6 + EtOH remains near the initial 0.08 ppm within

experimental error at all ranges of temperatures screened.

Quantitative interpretation of these data remains elusive, due

to the very complex nature of compound 6. A thorough NMR

spectroscopic analysis of the deuterium-labeled isotopomers of

6 and their analogues with substituents in para positions by

Heinekey and Brookhart revealed signiﬁcant temperature- and

solvent-dependent isotopic shifts and HD coupling constants.³⁸

These observations, supported by T₁measurements, were

interpreted by a model involving an equilibrium between the

solvated Ir(III) dihydride 6a with nonequivalent hydride sites

and the elongated dihydrogen complex of Ir(I) 6b (Scheme 5).

This equilibrium appeared to be fast on the NMR time scale,

even at low temperatures, and only one resonance is present in

We estimated the H-bond enthalpy from the ν(OH) low-

frequency shift value of 203 cm⁻¹using the empirical

formula^37a,39

18Δν_OH

ΔH° =

Δν_OH+ 720

This estimation gave an enthalpy value of about 4 kcal/mol.

The basicity factor was estimated using the same ideology^37a,39

by the formula

ΔH°_ij

E_j=

ΔH°₁₁P_i

where ΔH°_ijis the experimental enthalpy and ΔH°₁₁is the

value for the pair phenol/diethyl ether taken as a standard

(measured in the corresponding solvent; 5.7 kcal/mol for

hexane^37a). For this standard, the basicity and acidity factors are

taken as unity. The basicity factor E_jis considered as an

objective characteristic of the basic properties of the substance,

1

the hydride region of the H NMR spectrum of 6. While at

t

least for bulky BuOH, which is not expected to coordinate to

the Ir atom and inﬂuence the equilibrium in Scheme 5, the

shifts of hydride resonances observed can be clearly attributed

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independent of the nature of proton donors and the medium,

and can be used for comparison of these properties in a series

of related compounds.

spectrum revealed a singlet at 165.5 ppm, while the hydride

appeared as a triplet at −30.38 ppm (²J_P−H= 12.8 Hz) in the

¹H NMR spectrum. It should be noted that in the case of

iridium pincer complexes, both neutral and cationic, a hydride

which is rigorously trans to a vacant coordination site reveals a

signal at ca. −40 ppm,^3f,g,40while a signal near −30 ppm is

typical for a hydride which is mutually trans to a low-ﬁeld

ligand, especially oxygen.^28,34,41Therefore, the presence of the

η²-acetyl group in 9a may be suggested. In the IR spectrum, we

observed a band at 1546 cm⁻¹assigned to an acetyl C−O

stretch. This frequency is lower than that typically observed for

terminal Ir acyls⁴²(near 1600 cm⁻¹) and falls in the range

observed for complexes with η²-acyls.⁴³Low-ﬁeld (>200 ppm)

signals are usually observed in ¹³C NMR spectra for terminal

acyls bound to transition metals,⁴⁴especially for those

demonstrating low C−O stretching frequencies of the acyl

groups, due to the contribution of M⁺=C(O⁻)R structures.⁴⁵

Nevertheless, the signal of the CH₃CO group is observed at

183.4 ppm, indicating that complex 9a does not have any

signiﬁcant carbene character. It is remarkable that 9a does not

demonstrate any trend to isomerization to 7 or 8. Thus, 9a is

stable at ambient temperature, while heating a solution of 9a in

C₆D₆for 9 h at 80 °C results in decomposition of ca. 5% of 9a

to give 4. Complex 9a readily adds CO to form compound 10

(Scheme 7). In the ³¹P{¹H} NMR spectrum, complex 10

Our estimation of E_jgave a value of 1.23, indicating that

dihydride 6 is quite basic and its proton-accepting properties

can be compared with those of hydride ligands in RuH₂[P-

(CH₂CH₂PPh₂)₃] (1.33)^37aor CpRuH(CO)PCy₃(1.0).^37a

As for the basic center of H bonding, no distinct evidence

could be derived from the IR spectra. We conducted low-

temperature IR experiments with a hexane solution of pure

dihydride 6. In the ν(Ir−H) region, dihydride 6 has four bands

instead of the two expected for two Ir−H bonds. When the

temperature is lowered, the intensity ratio of these bands

changes; however, on returning to room temperature, the initial

pattern is not restored, thus indicating that in the solution of 6,

complicated processes can occur. For this reason, IR spectra do

not provide proof that it is the hydride that is involved in H

1

bonding; however, the high-ﬁeld shift of the hydride in the H

NMR points out that it is the most reasonable basic partner for

^tBuOH as an acid.

To sum up this part of our research, we presented evidence

that the reaction of 6 with alcohols under ambient conditions

begins with the formation of hydrogen-bonded species,

followed by proton transfer to give the hydrido dihydrogen

intermediate IV and, presumably, coordination of the counter-

anion and liberation of dihydrogen to form II. However, the

contribution of an alternative pathway through the formation of

Ir(V) to a reaction with small alcohols cannot be completely

excluded.

Scheme 7. Addition of CO to Complex 9a

Alcohol decarbonylation is generally believed to proceed

through dehydrogenation to give the appropriate carbonyl

compounds followed by formation of acyl complexes and

migratory deinsertion of CO.^19c,d,g,20In our case, we have never

observed species other than 6 and 7 (and products of its

subsequent transformations) in the reaction of 6 with EtOH.

To elucidate the further destiny of the putative IrH(OEt)[2,6-

(^tBu₂PO)₂C₆H₃] particle, we performed a reaction of 6 with

acetaldehyde. When a slight excess of CH₃CHO was used (∼10

equiv), the expected mixture of 7 and 8 was formed within

minutes; however, according to the ³¹P{¹H} NMR spectra, it

contained a small amount of a new compound. The use of a

large excess of CH₃CHO (∼500 equiv) resulted in almost

selective formation of this new compound, which appeared to

be acetyl hydride complex 9a (Scheme 6). This complex was

characterized by NMR and IR spectra and elemental analysis

and turned out to be a tricky compound. The ³¹P{¹H} NMR

resonates at 161.7 ppm. In the ¹H NMR spectrum, the hydride

signal appears as a triplet at −8.31 ppm (²J_P−H= 16.7 Hz). The

¹³C NMR spectrum reveals a singlet at 176.2 ppm

corresponding to CH₃CO and a triplet at 182.4 ppm (²J_C−P

=

5.0 Hz) from carbonyl. A strong absorption at 2035 cm⁻¹is

clearly attributable to the CO ligand, and another strong band

at 1590 cm⁻¹corresponds to the acetyl C−O stretch. Also,

there is a weak absorption at 2200 cm⁻¹from the Ir−H stretch.

The conﬁguration of 10 was conﬁrmed by preparing a ¹³CO-

1

enriched sample, which demonstrated a large H,¹³C coupling

constant of 52.4 Hz indicative of a trans conﬁguration of

carbonyl and hydride ligands.²⁴Complex 10 slowly decom-

poses at room temperature to give 4 and acetaldehyde. The

same trends were observed for the reaction with propionalde-

hyde (see Experimental Section). It is worth noting that in this

case only the analogue of 8, namely complex 11, where H and

C₂H₅are in a cis arrangement (on the basis of a very close

chemical shift in ³¹P{¹H} NMR spectrum), was selectively

formed when small quantities of aldehyde were used,

presumably due to steric reasons.

Scheme 6. Reaction of Dihydride 6 with CH₃CHO

To summarize, the reaction with CH₃CHO suggests that,

surprisingly, acetyl hydrido complex 9a is not an intermediate

on the way from 6 to 7. As the use of small quantities of

CH₃CHO leads to the formation of 7, it is likely that

acetaldehyde is actually involved in the sequence of reactions of

6 with EtOH and is formed upon β-hydride elimination of the

Ir(H)OEt particle. Direct addition of the C−C bond of

CH₃CHO to Ir is unlikely (no precedents for Ir pincers are

known), and thus it may be suggested that two diﬀerent

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Scheme 8. Decarbonylation of MeOH by Complex 1

isomers (rotamers) of acetyl hydrido complex are formed when

6 reacts with CH₃CHO, depending on the concentration of the

latter. One rotamer, 9b, is able to isomerize to 7, while for the

other, complex 9a, this possibility is blocked by coordination of

oxygen to Ir and/or the necessity of rotation hindered by bulky

tert-butyl groups. The reaction of CH₃CHO with 6 probably

proceeds via an associative mechanism, as was the case for

EtOH. However, aldehyde is a much weaker acid compared to

alcohol and therefore the mechanisms involving coordination of

aldehyde as a Lewis base (similar to pathway (a)) or formation

of an Ir(V) intermediate (similar to pathway (b)) seem more

probable than protonation.

ferrocenylmethanol aﬀorded some aldehyde but did not

prevent decarbonylation at all (Scheme 10). Three unstable

intermediates were observed by NMR spectroscopy during this

reaction. They tend to decompose during chromatography, and

all our attempts to separate them by crystallization failed; the

thermal stability of these species is suﬃciently lower than that

of 7 and 8, and after several days at room temperature complex

4 is the only substance in the reaction mixture observed by

³¹P{¹H} NMR. The ﬁrst intermediate can be obtained in a

mixture with formylferrocene by the reaction of 1 with

ferrocenylmethanol in the presence of tbe. It reveals a singlet

at 183.3 ppm in the ³¹P{¹H} NMR spectrum and a triplet (²J_PH

1

Decarbonylation of Other Alcohols. Decarbonylation of

= 14.2 Hz) at −36.97 ppm in the H NMR spectrum; this

spectrum is consistent with structure 13 (Chart 2), where, in

methanol proceeds in the way similar to that of EtOH. Thus,

t

reaction of 1 with MeOH and BuONa in the presence of tbe

produces a mixture of trans dihydride 12⁴⁶and carbonyl

complex 4 in a 75:25 ratio after 30 min (Scheme 8).

Presumably, this reaction lacks the stereoselectivity observed

for formation of 7, because 12 is stable enough under these

conditions to exclude its decomposition to give 4. Thus, the

presence of 4 may be a result of formation of unobserved cis

dihydride which loses dihydrogen to give 4. In contrast to

CH₃CHO, paraformaldehyde gives a mixture of 12 and 4 when

reacted with 6 rather than hydrido formyl complex (Scheme 9).

Chart 2. Proposed Intermediates for Ferrocenylmethanol

Decarbonylation by Complex 1

contrast to complex 9a, oxygen is not coordinated to the Ir

atom. In the IR spectrum in C₆H₆, the band at 1666 cm⁻¹was

ascribed to the ketone ν(CO) in 13; also, an absorption at 1685

cm⁻¹from formylferrocene was observed. Thus, 13 may be an

analogue of the unobserved rotamer of the acetyl hydrido

complex, which is responsible for the formation of 7. Upon

standing, a new peak at 161.9 ppm in the ³¹P{¹H} NMR

Scheme 9. Reaction of Dihydride 6 with Paraformaldehyde

spectrum appears (along with the peak from 4), which

2

This result can be ascribed to the poor solubility of

paraformaldehyde in hexane, in view of the huge excess of

CH₃CHO required for the formation of 9a. Complex 12 was

characterized by NMR and IR spectra. It displays a singlet at

183.5 ppm in ³¹P{¹H} NMR and a triplet at −9.48 ppm (²J_P−H

= 15.2 Hz) in its ¹H NMR spectrum. The IR spectrum in C₆D₆

reveals an intense absorption at 1995 cm⁻¹corresponding to

CO and a weaker band at 1827 cm⁻¹which is characteristic of

mutually trans hydride ligands.^19f,30

Bearing in mind the steric crowding at the iridium atom in 1,

we attempted to use sterically hindered primary alcohols in

eﬀort to avoid decarbonylation. We supposed that for bulky

substrates the cleavage of a C−C bond to form compounds

analogous to 7 and 8 would be inhibited. Indeed, the use of

corresponds to the hydride signal at −9.65 ppm (t, J_PH

=

17.7 Hz). These values are close to those of 8; thus, the

structure 14 (Chart 2) where H is mutually cis to the ferrocenyl

group was proposed for this compound. Finally, at high

conversions small amounts of the third intermediate can be

detected, which resonates at 184.8 ppm in the ³¹P{¹H} NMR

2

spectrum and has a hydride signal at −12.29 ppm (t, J_PH

=

21.4 Hz). These data suggest that it is a six-coordinate Ir(III)

complex, for which the P−H coupling of 21.4 Hz is larger than

that typically observed for related iridium pincers, indicating

signiﬁcant structural perturbations.

In general, the ruthenocene-based complex 3 demonstrates

similar behavior in its reactions with primary alcohols. Thus, it

decarbonylates ethanol with formation of the methyl hydrido

Scheme 10. Decarbonylation of Ferrocenylmethanol by Complex 1

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Scheme 11. Decarbonylation of EtOH by Ruthenocene-Based Complex 3

Scheme 12. Reaction of Dihydride 15 with C₂H₅CHO

carbonyl 16, which further transforms into isomer 17 and

carbonyl complex 5 (Scheme 11). This reaction is slower than

in the case of 1, and even in the presence of tbe, it requires

several days for the initially formed dihydride 15^3hto transform

into 16. Spectral features of 16 and 17 are similar to those of

their benzene-based counterparts and are not discussed in detail

(see Experimental Section).

between [Ir]H₂and CH₃CHO was shown to give [Ir](H)-

(CO)CH₃; no formation of hydrido acyl compounds was

detected. Irradiation is required to drive reductive elimination

of methane, presumably resulting in the formation of the cis

isomer.

Decarbonylation of methanol was reported for two iridium

pincer complexes. Thus, the IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃]

pincer complex promotes methanol decarbonylation in the

presence of tbe to generate the corresponding Ir−CO

complex,^14bwhile the treatment of {IrH(C₆H₅)[2,6-

(^tBu₂PCH₂)₂C₅H₃N]}PF₆with methanol upon heating results

in stereoselective formation of the trans [Ir](H)₂CO.^19fFor the

latter reaction, the suggested mechanism involves reductive

elimination of benzene and oxidative addition of the O−H

bond followed by β-hydride elimination, leading to form-

aldehyde and the dihydride complex {IrH₂[2,6-

(^tBu₂PCH₂)₂C₅H₃N]}PF₆. In the ﬁnal step, the aldehyde

activation is followed by deinsertion of CO (migration of the

hydride) to produce speciﬁcally the trans dihydride complex.

This last step of the reaction is intriguing, as it appears to be a

stereoselective migration resulting in the formation of the trans

rather than the cis dihydride complex. The formation of Ir(I)

carbonyl in the case of IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃] implies

that the migratory deinsertion proceeds stereospeciﬁcally to the

cis dihydride isomer, since reductive elimination of H₂to

generate the iridium(I) carbonyl must occur from the cis

dihydride. Remarkably, IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃] is compat-

ible with primary alcohols other than CH₃OH even at elevated

temperatures and is able to catalyze dehydrogenation with

formation of aldehydes. This probably results from the high

barrier for decarbonylation caused by steric reasons, as CO

bonding energies to the model particles Ir[2,6-

(Me₂PCH₂)₂C₆H₃] and Ir[2,6-(Me₂PO)₂C₆H₃] were calcu-

lated to be quite similar (62.2 vs 63.1 kcal/mol),^3cindicating an

almost equal aﬃnity of the catalysts for CO. In our case,

presumably both cis and trans dihydrides are formed in the

reaction of 1 with methanol, with the unobserved cis isomer

immediately decomposing to carbonyl 4. It is worth noting that

The reaction of dihydride 15 with a large excess of

C₂H₅CHO leads to the formation of propionyl hydride

complex 18, which resembles benzene-based complex 9a

(Scheme 12). Remarkably, only one of the two possible

stereoisomers is formed; the conﬁguration of 18 was conﬁrmed

by a NOESY spectrum. Complex 18 adds CO to give the 18e

adduct 19. The arrangement of the ligands in 19 was conﬁrmed

by preparing a ¹³CO-enriched sample, which demonstrated a

1

large H,¹³C coupling constant of 52.4 Hz, indicative of a

mutually trans arrangement of carbonyl and hydride ligands.²⁴

DISCUSSION

■

Decarbonylation of Alcohols. It is of interest to compare

our results with the literature data. The known examples of

formation of carbonyl hydrido methyl complexes include

generation of [IrH(CO)(CH₃)Cyttp]BPh₄(Cyttp = C₆H₅P-

(CH₂CH₂CH₂PCy₂)₂) by metalation of the Cyttp ligand in the

presence of ethanol³²and reaction of IrH₂{N[2-(ⁱPr₂P)-4-

MeC₆H₃]₂} with ethanol in the presence of norbornene with

formation of the [Ir](H)(CO)CH₃complex (H is trans to

CH₃), which undergoes photolytically induced reductive

elimination of methane to generate an Ir−CO adduct.^19gIn

the latter case, some mechanistic details were provided. The

proposed mechanism includes generation of the Ir(I) particle

Ir{N[2-(ⁱPr₂P)-4-MeC₆H₃]₂} followed by oxidative addition of

the O−H bond of the alcohol. Further steps presumably consist

of β-hydride elimination to give acetaldehyde and IrH₂{N[2-

(ⁱPr₂P)-4-MeC₆H₃]₂} with subsequent reaction between these

species accompanied by evolution of hydrogen and formation

of the [Ir](H)(CO)CH₃complex. Indeed, the reaction

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Scheme 13. Proposed Overall Mechanism for EtOH Decarbonylation by Complex 1

better selectivity is observed for ethanol, where trans complex 7

is formed almost exclusively. In this respect, it is interesting that

the cis dihydride Ir(H)₂CO[2,6-(ⁱPr₂PCH₂)₂C₆H₃] isomerizes

to the more stable trans dihydride Ir(H)₂CO[2,6-

(ⁱPr₂PCH₂)₂C₆H₃] upon heating,³¹while trans complex 7

appears to be less stable than cis complex 8 (presumably for

steric reasons). Such diﬀerent behaviors of related compounds

toward alcohols point out the importance of steric factor and

potentially open the possibility of impeding the formation of

intermediates leading to decarbonylation.

The proposed overall mechanism for decarbonylation of

EtOH by 1 is depicted in Scheme 13. Presumably,

dehydrochlorination of 1 leads to the formation of the active

14e particle, which oxidatively adds the O−H bond of EtOH

followed by β-hydride elimination to give complex 6. The 1

equiv of CH₃CHO formed might undergo rapid condensation

catalyzed by excess base. This is conﬁrmed by the reaction with

ferrocenylmethanol, where the corresponding aldehyde is not

able to undergo condensations due to the absence of α-

hydrogens and remains in the reaction mixture. The further

steps are of special interest because they may be relevant to the

catalytic cycle of acceptorless dehydrogenation. Complex 6

reacts with EtOH to give hydrogen-bonded intermediate V,

which after proton transfer aﬀords the cationic complex IV. As

was proved by deuterium exchange experiments, these stages

are reversible and rapid. The next step involving dihydrogen

dissociation from IV is slow and may be rate determining on

the way to 7. It should be noted that the analogue of IV,

namely the complex {IrH(H₂)[2,6-(^tBu₂PO)₂C₆H₃]}{BAr^f₄}, is

relatively stable and sustains mild heating.³⁶β-Hydride

elimination from II leads to complex 6 and CH₃CHO, which

presumably does not leave the coordination sphere of iridium

and undergoes further reaction before reacting with the base,

forming adduct VI and/or VII. It is possible that both are

formed depending on the aldehyde concentration, with one of

them being responsible for the formation of complex 9a and

the other for the unobserved intermediate 9b. Migratory

deinsertion of CO might give complex 7 as the kinetic product

and complex 8 as the thermodynamic product.

Comparison among the Various Catalysts. Complexes

IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃], 6, and 15 are well-known

catalysts for alkane dehydrogenation. Complex 6 is approx-

imately by 1 order of magnitude more active than its

bis(phosphine) counterpart IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃]. This

diﬀerence is primarily attributable to the fact that the metal

center of 6 is much less sterically hindered than that of

IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃].^3h,21Complex 15 is slightly more

active than 6, presumably due to a combination of electronic

and steric factors. Indeed, the geometry of the ligands around

iridium for 15 (P−Ir−P angle, conformation of chelating

metallacycles) is rather close to that of 6, while the electronic

properties of pincer ligand in 15 resemble those of IrH₂[2,6-

(^tBu₂PCH₂)₂C₆H₃].^3h,47As expected, in the case of alcohols

complex 6 outperformed its counterpart IrH₂[2,6-

(^tBu₂PCH₂)₂C₆H₃]. Thus, the quantitative yield with a TON

of 7 was reported for dehydrogenation of secondary alcohols by

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IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃] in the presence of tbe,^14bwhile 6

under similar conditions demonstrated TON values as high as

4000. Remarkably, the attempts to conduct acceptorless

dehydrogenation of alcohols using IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃]

failed.

The superiority of 15 over 6 was not reproduced for alcohol

dehydrogenation; moreover, under any conditions employed

15 showed signiﬁcantly poorer results.

IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃] in comparison to that for 6 may

result in stronger hydrogen binding.

The comparison of our results with other catalysts for alcohol

dehydrogenation is not straightforward, because there is no

benchmark reaction and several characteristics of the catalyst

should be considered. Thus, on the one hand, the state-of-the-

art catalysts for acceptorless alcohol dehydrogenation signiﬁ-

cantly outperform the complexes reported here with respect to

turnover frequencies and total turnover numbers. The best

examples developed by Beller and co-workers include a mixture

of [RuCl₂(p-cymene)]₂and tetramethylethylenediamine^16fand

a mixture of HN(CH₂CH₂PⁱPr₂)₂and RuH₂(PPh₃)₃CO,^16k

which showed 17215 TON after 11 days and an impressive

40000 TON after 12 h, correspondingly, in the dehydrogen-

ation of ⁱPrOH at 90 °C. However, taking into consideration 4

ppm catalyst loading, these results correspond to conversions of

only 7% and 16%, respectively. On the other hand, a

dibenzobarrelene iridium pincer complex developed by Gelman

and co-workers^17eand a Cp*Ir complex with a hydroxypyr-

idine-based C,N chelating ligand introduced by Fujita and

Yamaguchi^17fprovide excellent conversions (for example, for

PhCH(OH)CH₃94%, 940 TON, solution in p-xylene, reﬂux, 6

h, and 96%, 960 TON, solution in p-xylene, reﬂux, 20 h,

respectively) with moderate TON. Performance of the most

active catalyst reported here, complex 2 (85% conversion, 3420

TON, 190 °C, 32 h, neat PhCH(OH)CH₃) is roughly

comparable to those of the last two systems concerning

conversion and TON. Higher temperatures are required for 2,

while it takes advantage of carrying out catalysis in neat

substrate instead of organic solvent.

To rationalize such a behavior of the catalysts, it is necessary

to know the mechanism that is operating. It was found that, in

the case of EtOH and complex 6, the mechanism is clearly

associative and involves proton transfer from the alcohol. It

seems reasonable that, for secondary alcohols, at room

temperature the mechanism is the same. However, since at

some steps of the associative mechanism the entropy change

should be negative (formation of one molecule from two), it is

possible that under optimum conditions (near 190 °C) the

contribution of the dissosiative 16e−14e mechanism may be

signiﬁcant or even predominating. In any event, the primary

reason for the diﬀerence in catalytic activity seems to be steric,

as it was for alkane dehydrogenation. Moreover, if the proposed

mechanism is the one actually operating at elevated temper-

atures, some peculiar features should be considered. Thus,

dissociation of dihydrogen from a cationic complex is expected

to be the rate-determining step. As was mentioned earlier, such

compounds with non-nucleophilic counteranions are relatively

stable and the assistance of an alkoxide anion may be required

to accelerate dihydrogen evolution. It is obvious from simple

geometrical considerations that a front attack of the alkoxide

anion will result in deprotonation, while a bottom attack on the

Ir atom may result in nucleophilic assistance (Scheme 14). This

bottom attack can be hindered by the unsubstituted cyclo-

pentadienyl ring for 15 and unsuitable arrangement of the tert-

butyl groups in IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃]. In addition,

increased electron density on the Ir atom⁴⁷for 15 and

CONCLUSION

■

The reactivity of three iridium pincer complexes toward

alcohols has been reported. Bis(phosphinite) complex 1

demonstrated good catalytic activity for the dehydrogenation

of secondary alcohols, both with and without a sacriﬁcial

hydrogen acceptor. Mechanistic investigations showed that

dihydride 6 is the particle actually involved in the catalytic

cycle. Remarkably, dehydrogenation with 6 can be carried out

under basic, neutral, and acidic conditions, depending on the

catalyst precursor (1, 6, or 2); the last appeared to be the

preferable choice presumably due to suppression of out-of-cycle

species formation. Thus, the activity of 2 in acceptorless alcohol

dehydrogenation is comparable to that of the state-of-the art

catalysts with respect to the combination of conversion and

TON, though higher temperatures are required for 2 to achieve

satisfactory performance.

Scheme 14. Proposed Interactions of Cationic Hydrido

Dihydrogen Intermediates with Alkoxide Counteranion:

Deprotonation or Coordination

In the case of primary alcohols, rapid decarbonylation leads

to the formation of catalytically inactive species. A detailed

study of reaction of 1 with EtOH allowed us to trace the

unprecedented sequence of rearrangements on the way from 1

to 4 and to gain some insight into the factors inﬂuencing the

reactivity of Ir pincers with alcohols. Steric factors appeared to

be of primary importance and are responsible for a ﬁne balance

between a number of possible reaction pathways; thus, the

more open geometry of complex 6 in comparison to that of

IrH₂[2,6-(^tBu₂PCH₂)₂C₆H₃] results in a signiﬁcantly improved

reactivity toward secondary alcohols, while for the same reason

6 is not compatible with primary alcohols due to a lower barrier

to decarbonylation.

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Organometallics

Article

(vt, 18H, J = 7.4 Hz, 2 ^tBu), 1.22 (vt, 18H, J = 7.2 Hz, 2 ^tBu), 0.62 (td,

3H, ³J_PH= 3.8 Hz, ³J_HH= 1.5 Hz, Ir-CH₃), −9.71 (tq, 1H, ²J_PH= 17.6

EXPERIMENTAL SECTION

■

General Considerations. All manipulations were conducted

under an argon atmosphere using standard Schlenk techniques unless

otherwise stated. All solvents (including deuterated) were distilled

under an argon atmosphere from the appropriate drying agents.

Commercially available reagents were used as received. C₂D₅OD

contained ca. 90% atom D, while ^tBuOD contained 96%. Compounds

1,^3f3,^3hand 6^3gwere prepared according to the literature procedures.

NMR spectra were recorded on Bruker Avance 400 and 600 MHz

3

Hz, J_HH= 1.5 Hz, Ir-H). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ

163.7. ¹³C{¹H} NMR (150.93 MHz, C₆D₆): δ 186.0 (m, Ir-CO), 164.2

(vt, J = 4.5 Hz, C2 and C6), 125.9 (s, C4), 123.2 (vt, J = 4.6 Hz, C1),

104.7 (vt, J = 5.2 Hz, C3 and C5), 41.7 (two overlapping vt, J₁= 12.2

Hz, J₂= 13.6 Hz, 4 C(CH₃)₃), 28.6 (vt, J = 3.0 Hz, 2 C(CH₃)₃), 28.5

(vt, J = 2.6 Hz, 2 C(CH₃)₃), −47.8 (vt, J = 3.8 Hz, Ir-CH₃). IR

(hexane, cm⁻¹): 1981 (s), 1972 (s).

Reaction of 6 with Deuterated Alcohols. Formation of 7-

D,CD₃and 8-D,CD₃. Complex 1 (0.0172 g, 0.027 mmol) and

^tBuONa (0.0070 g, 0.073 mmol) were placed into a NMR tube, and

the ampule was evacuated and reﬁlled with Ar. Then, C₆D₆(ca. 0.7

mL) was added via syringe using a rubber septum and hydrogen was

bubbled through the reaction mixture via a needle until it became pale

orange. A slow ﬂow of Ar was bubbled using the same technique until

the reaction mixture became red, indicating the formation of complex

6. Then, C₂D₅OD (0.50 mL, 0.858 mmol) was added and the ampule

was shaken. After ca. 0.5 h, the reaction mixture consisted of an 88:12

mixture of isotopomers of 6 and 7, where “6” consisted of 3% of 6,

27% of 6-D, and 70% of 6-D₂while “7” contained 10% of 7-H,CD₃

and 90% of 7-D,CD₃. After this, tert-butylethylene (0.02 mL, 0.155

mmol) was added and 7-D,CD₃and 8-D,CD₃were isolated following

the procedure described above in almost quantitative yield. The

reactions with other amounts of C₂D₅OD as well as with ^tBuOD were

conducted in a similar manner.

1

spectrometers. H and ¹³C{¹H} NMR chemical shifts are reported in

parts per million downﬁeld from tetramethylsilane; the residual signals

of deuterated solvents were used as references (7.26 ppm for CDCl₃,

7.16 ppm for C₆D₆). In ¹³C{¹H} NMR measurements the signals of

C₆D₆(128.1 ppm) and CDCl₃(77.2) were used as a reference. Some

resonances demonstrate additional splitting due to incomplete {¹H}

decoupling of the hydride signals and thus appear as multiplets rather

than virtual triplets. ³¹P{¹H} NMR chemical shifts are reported relative

to an external 85% solution of phosphoric acid in D₂O. ¹⁹F{¹H} NMR

chemical shifts are reported relative to external CFCl₃. Assignments of

signals were conﬁrmed using HMQC, HSQC, and HMBC spectra

where necessary. FTIR spectra were recorded on a Nicolet Magna-IR

750 Fourier spectrometer with a resolution of 2 cm⁻¹. Elemental

analyses were performed at the A. N. Nesmeyanov Institute of

Organoelement Compounds of the RAS.

Synthesis of {IrH(acetone)[2,6-(^tBu₂PO)₂C₆H₃]}{BF₄} (2). Com-

plex 1 (0.058 g, 0.093 mmol) was partially dissolved in 15 mL of

acetone in a light-protected Schlenk ﬂask, and a solution of AgBF₄

(0.019 g, 0.097 mmol) in 5 mL of acetone was added dropwise. The

reaction mixture was stirred for 0.5 h and then kept for several minutes

without stirring. The yellow-orange solution was decanted and

evaporated, and the residue was recrystallized from an acetone/

hexane mixture. After it was washed with hexane and dried under

vacuum 2 (0.058 g, 85%) was obtained as an orange powder. Anal.

Calcd for C₂₅H₄₆BF₄IrO₃P₂: C, 40.82; H, 6.30. Found: C, 40.44; H,

Reaction of 6 with a Small Excess of CH₃CHO with

Formation of IrH(CO)CH₃[2,6-(^tBu₂PO)₂C₆H₃] (7) and IrH(CH₃)-

CO[2,6-(^tBu₂PO)₂C₆H₃] (8). Complex 1 (0.063 g, 0.101 mmol) and

^tBuONa (0.031 g, 0.323 mmol) were suspended in 10 mL of hexane in

a rubber septum capped Schlenk ﬂask while the ﬂask was purged with

hydrogen. The suspension was stirred for several hours under a slow

ﬂow of hydrogen until it became pale orange, and then the reaction

mixture was ﬁltered through a thin layer of Celite into another Schlenk

ﬂask. The stirred solution was purged with Ar until it became red,

indicating the formation of complex 6, and then 1 drop of CH₃CHO

was added via syringe. The color changed from red to yellow within

several minutes. The solution was stirred for 1 h and then evaporated

to dryness, giving a mixture of 7 and 8 (0.054 g, 84%) as a yellow

powder.

1

3

6.42. H NMR (400.13 MHz, CDCl₃): δ 6.81 (t, 1H, J_HH= 7.9 Hz,

3

H4), 6.54 (d, 2H, J_HH= 7.9 Hz, H3 and H5), 2.29 (br s, 6H,

t

(CH₃)₂CO), 1.36−1.32 (m, 36H, 4 Bu), −42.93 (br s, 1H, Ir-H).

³¹P{¹H} NMR (161.98 MHz, CDCl₃): δ 175.3 (br s). ¹⁹F{¹H} NMR

(376.50 MHz, CDCl₃): −151.5 (very br s). IR (KBr, cm⁻¹): 1653 (s),

1637 (s).

Reaction of 6 with a Large Excess of CH₃CHO with

Formation of Ir(H)COCH₃[2,6-(^tBu₂PO)₂C₆H₃] (9a). Complex 1

Decarbonylation of Ethanol. Formation of IrH(CO)CH₃[2,6-

(^tBu₂PO)₂C₆H₃] (7) and IrH(CH₃)CO[2,6-(^tBu₂PO)₂C₆H₃] (8).

t

(0.061 g, 0.098 mmol) and BuONa (0.030 g, 0.313 mmol) were

t

Complex 1 (0.0159 g, 0.025 mmol) and BuONa (0.0085 g, 0.089

suspended in 10 mL of hexane in a rubber septum capped Schlenk

ﬂask while the ﬂask was purged with hydrogen. The suspension was

stirred for several hours under a slow ﬂow of hydrogen until it became

pale orange, and then the reaction mixture was ﬁltered through a thin

layer of Celite into another Schlenk ﬂask. The stirred solution was

purged with Ar until it became red, indicating the formation of

complex 6, and then CH₃CHO (2 mL, 35 mmol) was added in one

portion via syringe. The color changed from red to yellow

immediately. The solution was stirred for 10 min and then evaporated

to dryness, giving complex 9a (0.057 g, 92%) as a yellow powder. Anal.

Calcd for C₂₄H₄₃IrO₃P₂: C, 45.48; H, 6.84. Found: C, 45.41; H, 6.88.

¹H NMR (400.13 MHz, C₆D₆): δ 6.78 (t, 1H, ³J_HH= 7.8 Hz, H4), 6.69

mmol) were placed into a J. Young NMR tube, and the ampule was

evacuated and reﬁlled with Ar. After this, C₆D₆(ca. 0.7 mL) and EtOH

(0.04 mL, 0.686 mmol) were added via syringe. The ampule was

vigorously shaken for several minutes, accompanied by dissolution of 1

and formation of a red solution containing 6 and some quantity of 7.

Then tert-butylethylene (0.02 mL, 0.155 mmol) was added, and the

ampule was shaken and left for 0.5−1 h. After this period, the reaction

was complete and only 7 and 8 in a 98:2 ratio were present, according

to NMR. The reaction mixture was evaporated, and the residue was

extracted with pentane. The solvent was removed under vacuum to

give mixture of 7 and 8 as a yellow powder in almost quantitative yield.

Anal. Calcd for C₂₄H₄₃IrO₃P₂: C, 45.48; H, 6.84. Found: C, 45.59; H,

6.91.

3

(d, 2H, J_HH= 7.8 Hz, H3 and H5), 1.80 (s, 3H, COCH₃), 1.40 (vt,

t

18H, J = 7.0 Hz, 2 Bu), 1.30 (vt, 18H, J = 7.3 Hz, 2 Bu), −30.38 (t,

1

1H, J_PH= 12.8 Hz, Ir-H). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ

2

Characterization of 7. H NMR (400.13 MHz, C₆D₆): δ 6.82 (t,

1H, ³J_HH= 7.8 Hz, H4), 6.74 (d, 2H, ³J_HH= 7.8 Hz, H3 and H5), 1.31

(vt, 18H, J = 7.2 Hz, 2 ^tBu), 1.26 (vt, 18H, J = 7.1 Hz, 2 ^tBu), 0.16 (td,

3H, ³J_PH= 3.1 Hz, ³J_HH= 1.6 Hz, Ir-CH₃), −10.93 (tq, 1H, ²J_PH= 18.6

165.5. ¹³C{¹H} NMR (150.93 MHz, C₆D₆): 181.4 (s, Ir-COCH₃),

166.2 (vt, J = 6.0 Hz, C2 and C6), 124.1 (s, C4), 109.5 (m, C1), 104.9

(vt, J = 5.2 Hz, C3 and C5), 42.1 (vt, J = 12.2 Hz, 2 C(CH₃)₃), 40.1

(vt, J = 12.1 Hz, 2 C(CH₃)₃), 28.5 (vt, J = 3.2 Hz, 2 C(CH₃)₃), 27.4

(br s, 2 C(CH₃)₃), 25.5 (s, Ir-COCH₃). IR (CHCl₃, cm⁻¹): 1546 (s, Ir-

COCH₃).

3

Hz, J_HH= 1.5 Hz, Ir-H). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ

169.4. ¹³C{¹H} NMR (150.93 MHz, C₆D₆) 181.3 (m, Ir-CO), 163.3

(vt, J = 6.0 Hz, C2 and C6), 134.9 (m, C1), 125.5 (s, C4), 105.8 (vt, J

= 5.6 Hz, C3 and C5), 43.7 (m, 2 C(CH₃)₃), 42.9 (vt, J = 11.6 Hz, 2

C(CH₃)₃), 29.2 (vt, J = 2.8 Hz, 2 C(CH₃)₃), 28.0 (vt, J = 2.3 Hz, 2

C(CH₃)₃), −26.7 (m, Ir-CH₃). IR (hexane, cm⁻¹): 2046 (w, Ir−H),

1997 (s, Ir-CO).

Synthesis of IrH(COCH₃)CO[2,6-(^tBu₂PO)₂C₆H₃] (10). Complex

9a (0.043 g, 0.068 mmol) was dissolved in C₆D₆(0.6 mL) in a

septum-capped NMR tube, and CO was bubbled through the resulting

solution via a needle for 30 min accompanied by a color change from

yellow to pale yellow. Evaporation of the volatiles gave complex 10 in

almost quantitative yield as a pale yellow powder. Anal. Calcd for

1

Characterization of 8. H NMR (400.13 MHz, C₆D₆): δ 6.88 (t,

1H, ³J_HH= 7.8 Hz, H4), 6.78 (d, 2H, ³J_HH= 7.8 Hz, H3 and H5), 1.33

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1

C₂₅H₄₃IrO₄P₂: C, 45.37; H, 6.55. Found: C, 45.31; H, 6.38. H NMR

(400.13 MHz, C₆D₆): δ 6.75 (t, 1H, ³J_HH= 7.9 Hz, H4), 6.60 (d, 2H,

³J_HH= 7.9 Hz, H3 and H5), 2.26 (s, 3H, COCH₃), 1.41 (vt, 18H, J =

a J. Young NMR tube and kept at room temperature. The ³¹P{¹H}

spectra were recorded immediately and after speciﬁed intervals (see

Figure 3). Alternatively, decomposition can be achieved by placing an

NMR tube with solution of 7 and 8 in C₆D₆into a bath with reﬂuxing

EtOH for 3 h. Complex 4 and CH₄were observed as the only

products of decomposition. Ca. 0.125 equiv of CH₄was present in the

solution according to ¹H NMR. Taking into consideration the

solubility of methane in toluene,⁴⁸it seems reasonable that the rest of

methane was present in the gas phase; a similar situation was reported

in ref 32.

Thermolysis of 9a. Complex 9a (0.005 g, 0.008 mmol) was

dissolved in C₆D₆(0.3 mL) in a J. Young NMR tube, and the tube was

immersed into a bath with reﬂuxing EtOH. Periodically, the ampule

was cooled to room temperature and the NMR spectra were recorded.

After 9 h, ca. 5% of complex 9a decomposed with formation of

complex 4.

Thermolysis of 10. Complex 10 (0.006 g, 0.009 mmol) was

dissolved in C₆D₆(0.3 mL) in a J. Young NMR tube, and the tube was

immersed into a bath with reﬂuxing EtOH. Periodically, the ampule

was cooled to room temperature and the NMR spectra were recorded.

After 1 h, ca. 50% of complex 10 decomposed with formation of

complex 4 and acetaldehyde. After 10 h, the reaction was almost

complete. The amount of acetaldehyde produced corresponded well

with the amount of complex 4.

7.7 Hz, 2 ^tBu), 1.26 (vt, 18H, J = 7.4 Hz, 2 ^tBu), −8.31 (t, 1H, ²J_PH

=

16.7 Hz, Ir-H). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ 161.7. ¹³C{¹H}

NMR (100.61 MHz, C₆D₆): 182.4 (m, Ir-CO), 176.2 (s, Ir-COCH₃),

164.0 (vt, J = 4.3 Hz, C2 and C6), 126.0 (s, C4), 109.0 (vt, J = 2.7 Hz,

C1), 105.9 (vt, J = 4.9 Hz, C3 and C5), 42.5 (vt, J = 12.8 Hz, 2

C(CH₃)₃), 41.0 (vt, J = 11.7 Hz, 2 C(CH₃)₃), 28.5 (vt, J = 3.1 Hz, 2

C(CH₃)₃), 27.7 (vt, J = 2.8 Hz, 2 C(CH₃)₃), 21.3 (s, Ir-COCH₃). IR

(CHCl₃, cm⁻¹): 2196 (w, Ir−H), 2033 (s, Ir-CO), 1591 (s, Ir-

COCH₃).

Synthesis of IrH(COCH₃)CO[2,6-(^tBu₂PO)₂C₆H₃] (10) with ¹³C-

Labeled CO. Complex 9a (0.005 g, 0.008 mmol) was dissolved in

C₆D₆(0.6 mL) in a rubber septum capped NMR tube, and ¹³CO (86.4

atom % ¹³C) was slowly bubbled through the resulting solution via a

needle for 30 min. A mixture of 10 and 10-¹³CO was formed in almost

quantitative yield according to NMR. Selected signals for 10-¹³CO are

as follows. ¹H NMR (400 MHz, C₆D₆): −8.29 (dt, 1H, ²J_CH= 53.9 Hz,

²J_PH= 16.7 Hz, Ir-H). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ 161.7.

2

¹³C NMR (100.61 MHz, C₆D₆) 182.4 (dt, J_CH= 53.9 Hz, J_CP= 6.3

Hz, Ir-CO).

Reaction of 6 with a Small Excess of C₂H₅CHO with

Formation of IrH(C₂H₅)CO[2,6-(^tBu₂PO)₂C₆H₃] (11). The reaction

was conducted in a manner similar to the aforementioned reaction

with acetaldehyde. Starting from complex 1 (0.023 g, 0.037 mmol),

^tBuONa (0.009 g, 0.094 mmol) and 1 drop of C₂H₅CHO, complex 11

Decarbonylation of Methanol. Formation of trans-IrH₂(CO)-

[2,6-(^tBu₂PO)₂C₆H₃] (12) and IrCO[2,6-(^tBu₂PO)₂C₆H₃] (4). Com-

t

plex 1 (0.020 g, 0.032 mmol) and BuONa (0.010 g, 0.104 mmol)

were placed into a NMR tube, and the ampule was evacuated and

reﬁlled with Ar. Then, C₆D₆(ca. 0.7 mL) and MeOH (0.04 mL, 0.998

mmol) were added via syringe using a rubber septum. The ampule was

vigorously shaken for several minutes, accompanied by dissolution of 1

and formation of a red solution containing 6 and some quantity of 12.

Then tert-butylethylene (0.02 mL, 0.155 mmol) was added, and the

ampule was shaken and left for 0.5−1 h. For this time, 12 and 4 in a

75:25 ratio were present, along with trace amounts of a compound

with signal at 183.0 ppm in the ³¹P{¹H} NMR assigned to

IrD(C₆D₅)[2,6-(^tBu₂PO)₂C₆H₃].^3g¹H NMR (400.13 MHz, C₆D₆): δ

(0.021 g, 87%) was obtained as a yellow powder. Anal. Calcd for

1

C₂₅H₄₅IrO₃P₂: C, 46.35; H, 7.00. Found: C, 46.39; H, 7.07. H NMR

(400.13 MHz, C₆D₆): δ 6.89 (t, 1H, ³J_HH= 7.8 Hz, H4), 6.77 (d, 2H,

³J_HH= 7.8 Hz, H3 and H5), 2.21 (t, ³J_HH= 7.8 Hz, Ir-CH₂CH₃), 1.78

(m, 2H, Ir-CH₂CH₃), 1.35 (vt, 18H, J = 7.3 Hz, 2 ^tBu), 1.22 (vt, 18H,

J = 7.2 Hz, 2 ^tBu), −9.57 (tt, 1H, ²J_PH= 18.0 Hz, ³J_HH= 1.4 Hz, Ir-H).

³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ 162.3 IR (hexane, cm⁻¹): 1991

(m), 1981 (s), 1966 (s).

Reaction of 6 with a Large Excess of C₂H₅CHO with

Formation of Ir(H)COC₂H₅[2,6-(^tBu₂PO)₂C₆H₃]. The reaction was

conducted in a manner similar to the aforementioned reaction with

acetaldehyde. Starting from complex 1 (0.092 g, 0.147 mmol),

^tBuONa (0.045 g, 0.469 mmol), and C₂H₅CHO (2 mL, 28 mmol),

3

6.83 (t, 1H, J_HH= 8.0 Hz, H4), 6.72 (d, 2H, J_HH= 8.0 Hz, H3 and

H5), 1.34 (vt, 36H, J = 7.4 Hz, 4 ^tBu), −9.48 (t, 1H, ²J_PH= 15.2 Hz, Ir-

H). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ 183.5. IR (C₆D₆, cm⁻¹):

1995 (s, Ir-CO), 1827 (m, Ir-H).

complex Ir(H)COC₂H₅[2,6-(^tBu₂PO)₂C₆H₃] (0.081 g, 85%) was

obtained as a yellow powder. Anal. Calcd for C₂₅H₄₅IrO₃P₂: C, 46.35;

Reaction of 6 with (HCHO)_n. Complex 1 (0.052 g, 0.083 mmol)

t

and BuONa (0.027 g, 0.281 mmol) were suspended in 10 mL of

1

H, 7.00. Found: C, 46.19; H, 7.09. H NMR (400.13 MHz, C₆D₆): δ

hexane in a rubber septum capped Schlenk ﬂask while the ﬂask was

purged with hydrogen. The suspension was stirred for several hours

under a slow ﬂow of hydrogen until it became pale orange, and then

the reaction mixture was ﬁltered through a thin layer of Celite into

another Schlenk ﬂask. The stirred solution was purged with Ar until it

became red, indicating the formation of complex 6, and then

(HCHO)_n(2.01 g, 67 mmol) was added in one portion. The

suspension was stirred for 1 h and then allowed to precipitate. The

yellow solution was decanted and ﬁltered through a thin pad of Celite.

Evaporation of the volatiles aﬀorded 0.045 g of a mixture of 12 and 4

(ca. 75:25).

3

6.77 (t, 1H, J_HH= 7.8 Hz, H4), 6.68 (d, 2H, J_HH= 7.8 Hz, H3 and

H5), 2.18 (q, 2H, ³J_CH= 7.6 Hz, COCH₂CH₃), 1.38 (vt, 18H, J = 7.0

t

3

Hz, 2 Bu), 1.29 (vt, 18H, J = 7.2 Hz, 2 Bu), 1.12 (t, 3H, J_CH= 7.6

Hz, COCH₂CH₃), −30.43 (t, 1H, J_PH= 12.7 Hz, Ir-H). ³¹P{¹H}

2

NMR (161.98 MHz, C₆D₆): δ 165.5. ¹³C{¹H} NMR (100.61 MHz,

C₆D₆) 186.4 (s, Ir-COCH₂CH₃), 166.2 (vt, J = 6.0 Hz, C2 and C6),

124.1 (s, C4), 109.6 (m, C1), 104.8 (vt, J = 5.4 Hz, C3 and C5), 42.2

(vt, J = 12.2 Hz, 2 C(CH₃)₃), 40.1 (vt, J = 12.1 Hz, 2 C(CH₃)₃), 32.4

(s, COCH₂CH₃), 28.5 (vt, J = 3.4 Hz, 2 C(CH₃)₃), 27.4 (vt, J = 2.8

Hz, 2 C(CH₃)₃), 9.5 (s, Ir-COCH₂CH₃). IR (CHCl₃, cm⁻¹): 1536 (s,

Ir-COCH₂CH₃).

Decarbonylation of Ferrocenylmethanol. Complex 1 (0.0095

Synthesis of IrH(CO)COCH₂CH₃[2,6-(^tBu₂PO)₂C₆H₃]. The re-

action was conducted in a manner similar to the aforementioned

synthesis of 10. Starting from complex Ir(H)COC₂H₅[2,6-

(^tBu₂PO)₂C₆H₃] (0.031 g, 0.048 mmol), IrH(COC₂H₅)CO[2,6-

(^tBu₂PO)₂C₆H₃] was obtained in almost quantitative yield as a pale

yellow powder. Anal. Calcd for C₂₆H₄₅IrO₄P₂: C, 46.21; H, 6.71.

Found: C, 46.09; H, 6.93. ¹H NMR (400 MHz, C₆D₆): δ 6.76 (t, 1H,

³J_HH= 8.0 Hz, H4), 6.61 (d, 2H, ³J_HH= 8.0 Hz, H3 and H5), 2.57 (q,

t

g, 0.015 mmol), BuONa (0.0045 g, 0.047 mmol), and ferrocenylme-

thanol (0.0164 g, 0.076 mmol) were placed into a NMR tube, and the

ampule was evacuated and reﬁlled with Ar. Then, C₆D₆(ca. 0.7 mL)

was added via syringe. The ampule was vigorously shaken for several

minutes, and the NMR spectra were recorded periodically. After 10

days, the volatiles were evaporated under vacuum and column

chromatography on silica aﬀorded 4, ferrocene, and formylferrocene

(along with nonreacted ferrocenylmethanol) as the major products.

The quantity of formylferrocene corresponds to 1.6 TON.

3

t

2H, J_CH= 7.6 Hz, COCH₂CH₃), 1.40 (vt, 18H, J = 7.7 Hz, 2 Bu),

1.33 (t, 3H, ³J_CH= 7.6 Hz, COCH₂CH₃), 1.26 (vt, 18H, J = 7.4 Hz, 2

Decarbonylation of Ethanol by 3. Complex 3 (0.016 g, 0.021

^tBu), −8.30 (t, 1H, J_PH= 16.7 Hz, Ir−H). ³¹P{¹H} NMR (161.98

2

t

mmol) and BuONa (0.008 g, 0.083 mmol) were placed into a J.

MHz, C₆D₆): δ 161.6. IR (CHCl₃, cm⁻¹): 2202 (w, Ir−H), 2032 (s, Ir-

CO), 1601 (s, Ir-COCH₃).

Young NMR tube, and the ampule was evacuated and reﬁlled with Ar.

Then, C₆D₆(ca. 0.4 mL) and EtOH (0.02 mL, 0.343 mmol) were

added via syringe. The ampule was vigorously shaken for several

minutes, accompanied by dissolution of 3 and formation of a brown-

Kinetics of Thermolysis of 7 and 8. A mixture of 7 and 8

(87.7:12.3; 0.0221 g, 0.035 mmol) was dissolved in C₆D₆(0.74 mL) in

1012

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Article

red solution containing 15. Then tert-butylethylene (0.02 mL, 0.155

mmol) was added, the ampule was shaken, and the NMR spectra were

recorded periodically. After 4 days, consumption of 15 was almost

complete and 16 was the main substance present in the system.

Further, the formation of 17, 5, and small amounts of unidentiﬁed

compounds was observed.

time, the Schlenk ﬂask was warmed to room temperature. The reaction

mixtures were analyzed using ¹H NMR spectra. The side products 1,3-

diphenyl-1-butanol,⁴⁹1,3-diphenyl-1-butanone,⁵⁰(1-phenylethyl)

ether,⁵¹and 2-cyclohexenyl-1-cyclohexanol⁵²were identiﬁed by

comparison of spectra with literature data. To our knowledge 3-

cyclohexenyl-1-cyclohexanol was not reported previously. It was

isolated by means of column chromatography on silica using

hexane/CH₂Cl₂1/1 as eluent in the form of a slightly yellowish oil;

the sample contained a small amount of unidentiﬁed impurity which

we were unable to separate; its signals are excluded from the spectrum.

1

Characterization of 16. H NMR (400.13 MHz, C₆D₆): δ 4.73 (s,

2

5H, C₅H₅), 4.50 (s, 2H, C₅H₂), 2.79 (dt, 2H, J_HH= 16.0 Hz, J_PH

=

2

2.8 Hz, CH_ACH_BP), 2.39 (dt, 2H, J_HH= 16.0 Hz, J_PH= 4.3 Hz,

t

CH_ACH_BP), 1.25 (vt, 18H, J = 6.5 Hz, 2 Bu), 1.13 (vt, 18H, J = 6.3

1

Hz, 2 ^tBu), 0.55 (td, 3H, ³J_PH= 3.4 Hz, ³J_HH= 1.5 Hz, Ir-CH₃), −11.67

MS: m/z 91.1 (M + 2H). H NMR (600.22 MHz, CDCl₃): δ 5.54−

(tq, 1H, J_PH= 14.8 Hz, J_HH= 1.5 Hz, Ir-H). ³¹P{¹H} NMR (161.98

2

3

4.53 (m, 1H, −CHC<), 3.36 (td, 1H, J₁= 10.3 Hz, J₂= 4.2 Hz,

>CH(OH)−), 2.01−1.97 (m, 3H), 1.90−1.87 (m, 2H), 1.77−1.69 (m,

2H), 1.64−1.49 (m, 6H), 1.28−1.14 (m, 4H). ¹³C{¹H} NMR (150.93

MHz, CDCl₃) 138.52, 124.49, 70.52, 55.09, 34.14, 30.03, 25.90, 25.31,

25.02, 24.98, 23.00, 22.73.

MHz, C₆D₆): δ 69.6.

Characterization of 17. ¹H NMR (400.13 MHz, C₆D₆): δ 0.58 (td,

3H, ³J_PH= 3.7 Hz, ³J_HH= 1.5 Hz, Ir-CH₃), −10.83 (tq, 1H, ²J_PH= 17.0

Hz, ³J_HH= 1.5 Hz, Ir-H). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ 64.5.

Reaction of 15 with a Large Excess of C₂H₅CHO with

Formation of 18. The reaction was conducted in a manner similar to

the aforementioned reaction of 6 with acetaldehyde. Starting from

ASSOCIATED CONTENT

* Supporting Information

NMR spectra of compounds 7 and 8. This material is available

■

S

t

complex 3 (0.039 g, 0.050 mmol), BuONa (0.015 g, 0.156 mmol),

and C₂H₅CHO (2 mL, 28 mmol), complex 18 (0.033 g, 83%) was

free of charge via the Internet at http://pubs.acs.org.

obtained as a yellow-orange powder. Anal. Calcd for C₃₁H₅₃IrOP₂Ru:

C, 46.72; H, 6.70. Found: C, 46.80; H, 6.51. H NMR (400.13 MHz,

1

C₆D₆): δ 4.80 (s, 2H, C₅H₂), 4.62 (s, 5H, C₅H₅), 2.65 (dt, 2H, ²J_HH

=

AUTHOR INFORMATION

Corresponding Author

*E-mail: koridze14@hotmail.com.

■

16.6 Hz, ²J_PH= 2.8 Hz,CH_ACH_BP), 2.39 (dt, 2H, ²J_HH= 16.6 Hz, ²J_PH

3

= 4.6 Hz, CH_ACH_BP), 2.33 (q, 2H, J_HH= 7.7 Hz, Ir-COCH₂CH₃),

t

3

1.36 (vt, 18H, J = 6.5 Hz, 2 Bu), 1.23 (t, 3H, J_HH= 7.7 Hz, Ir-

COCH₂CH₃), 1.19 (vt, 18H, J = 6.3 Hz, 2 ^tBu), −30.27 (t, 1H, ²J_PH

=

Notes

13.8 Hz, Ir-H). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ 76.2. IR

(CHCl₃, cm⁻¹): 1538 (s, Ir-COCH₂CH₃).

The authors declare no competing ﬁnancial interest.

^§This author deceased during the preparation of this paper.

Synthesis of Complex 19. The reaction was conducted in a

manner similar to the aforementioned synthesis of 10. Starting from

complex 18 (0.025 g, 0.031 mmol), 19 was obtained in almost

quantitative yield as a yellow powder. Anal. Calcd for C₃₂H₅₃IrO₂P₂Ru:

ACKNOWLEDGMENTS

■

This work was partly supported by the Russian Academy of

Sciences, P-8 Program. A. P. thanks the Academician K. I.

Zamaraev Foundation for fellowship.

1

C, 46.59; H, 6.48. Found: C, 46.19; H, 6.83. H NMR (400.13 MHz,

C₆D₆): δ 4.56 (s, 5H, C₅H₅), 4.46 (s, 2H, C₅H₂), 2.61 (dt, 2H, ²J_HH

16.4 Hz, J_PH= 2.8 Hz, CH_ACH_BP), 2.55 (q, 2H, J_HH= 7.6 Hz, Ir-

=

2

3

2

COCH₂CH₃), 2.30 (dt, 2H, J_HH= 16.4 Hz, J_PH= 4.5 Hz,

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CH_ACH_BP), 1.32 (t, 3H, J_HH= 7.6 Hz, Ir-COCH₂CH₃), 1.16 (vt,

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Dehydrogenation of alcohols by bis(phosphinite) benzene based and bis(phosphine) ruthenocene based iridium pincer complexes

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