Application of electron-withdrawing coordinatively unsaturated η6-arene β-diketiminato-Ruthenium complexes in Lewis acid catalyzed Diels-Alder reactions

ARTICLE

pubs.acs.org/Organometallics

Application of Electron-Withdrawing Coordinatively Unsaturated

η⁶-Arene β-DiketiminatoꢀRuthenium Complexes in Lewis Acid

Catalyzed DielsꢀAlder Reactions

Dominique F. Schreiber, Yannick Ortin, Helge M€uller-Bunz, and Andrew D. Phillips*

School of Chemistry and Chemical Biology, University College Dublin, Belﬁeld Dublin 4, Ireland

S Supporting Information

b

ABSTRACT: Utilizing the aza-Wittig reaction involving the ylid 3,5-(CF₃)₂C₆H₃NPPh₃and

1,1,1,5,5,5-hexaﬂuoro-2,4-pentanedione, a highly ﬂuorinated and electron-withdrawing

β-diketiminate was obtained. Using strong bases, nBuLi, Ag₂O, or TlOEt, the corresponding

β-diketiminato-Li, -Ag, or -Tl chelated complexes were prepared. Subsequent in situ

transmetalation with (Ru(η⁶-C₆H₆)Cl₂)₂or (Ru(η⁶-p-cymene)Cl₂)₂aﬀorded the half-

sandwich chloro-substituted Ru(II) β-diketimino complexes in high yield. The synthesis

of the Lewis acidic catalysts featuring a vacant coordination site at the metal center

was accomplished using [Na]BArF (BArF = tetrakis[3,5-bis(triﬂuoromethyl)phenyl]boron).

These complexes are active for the Lewis acid catalyzed DielsꢀAlder reaction between α,β-

unsaturated aldehydes, that is, methacrolein, acrolein, and dienes, that is, cyclopentadiene and

2,3-dimethyl-1,3-butadiene, with conversions in the range of 66ꢀ98% under mild conditions.

Whereas the herein described catalysts generally promote exo selectivity of the [4 + 2]

cycloaddition between methacrolein and cyclopentadiene, the reaction involving acrolein

shows predominantly the formation of the endo adduct, similar to that observed for the noncatalyzed reaction. Importantly, the

coordinatively unsaturated complexes demonstrate moderate Lewis acidity, which allows for the controlled reaction between

methacrolein and 2,3-dimethyl-1,3-butadiene to 1,3,4-trimethyl-3-cyclohexene-1-carboxaldehyde without further isomerization to

the bicyclic ketone, which is in contrast to strong Lewis acidic catalysts based on transition metals or main-group elements reported

in the literature.

’ INTRODUCTION

η⁵-cyclopentadienyl-substituted Ru(II) complex bearing a che-

lating diphosphinite with strongly electron-withdrawing ﬂuorine

substituents on the aryl groups. In all reactions, excellent conversion

and selectivity was achieved, with predominant exo selectivity for

the reaction between methacrolein and cyclopentadiene.⁵Further-

more, K€undig and co-workers studied the eﬀect of diﬀerent anions

associated with the kinetics of ruthenium-catalyzed DielsꢀAlder

reactions. If the organometallic-cationic component of the cataly_ꢀst

Organic-based heterocycles containing at least one ring com-

ponent now represent the vast majority of commercially available

pharmaceuticals.¹Concurrently, the number of synthetic metho-

dologies for preparing cycloadducts has signiﬁcantly increased.

One widely utilized method to obtain functionalized six-mem-

bered rings is the highly atom eﬃcient Lewis acid catalyzed Dielsꢀ

Alder reaction. A number of metal- and nonmetal-based catalysts

have been developed to promote the cyclization process and

importantly aﬀord selectivity between exo and endo products.

Initial reports in the ﬁeld describe the potential of Ru(II)-based

Lewis acids in the catalysis of DielsꢀAlder type [4 + 2] cyclo-

additions.^2,3The coordination complex [Ru(salen)(NO)-

(H₂O)]SbF₆(salen = N,N⁰-ethylenebis(salicylimine)) showed

increased selectivity and turnover numbers for the [4 + 2]

cycloaddition between α,β-unsaturated aldheydes/ketones and

various dienes, as compared to noncatalyzed cycloadditions.³

The rationale behind introducing ruthenium as the active metal

is based on high water tolerance,⁴moderate oxygen stability,

increased rate of substrateꢀproduct exchange, and decreased

side-product formation, as compared to the conventional simple,

but strong, Lewis acids. More recently, K€undig et al. reported

Lewis acid catalyzed DielsꢀAlder reactions of methacrolein and

bromoacrolein with diﬀerent types of dienes, employing a chiral

was combined with a weakly coordinating anion, such as SbF₆

,

higher activity was observed than if the stronger coordinating

OTf ^ꢀand BF₄counterions were employed.^5ꢀ7More recently,

ꢀ

Oro et al.⁸reported the development of a potent asymmetric

Ru(II)ꢀpyridylamino half-sandwich complex. Interestingly, it was

shown that, if the steric bulk of the η⁶-arene capping group was

increased from C₆H₆to C₆Me₆, the enantiomeric excess of the

formed product changed from (R) to (S).⁸Other important

contributions to the ﬁeld include the development of Ru(II) η⁶-

arene complexes with 4-isopropyl-2-(2-pyridyl)-1,3-oxazoline as a

chiral ligand.⁹Currently, a wide range of chiral and achiral Ru(II)

complexes capable of catalyzing the DielsꢀAlder reaction are

known, including ligands, such as imidazolines,¹⁰bisoxazolines,¹¹

Received: June 29, 2011

Published: September 29, 2011

r

2011 American Chemical Society

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N-heterocyclic carbenes,¹²and chiral piano-stool complexes, with

and without tethering of the capping arene group to the supporting

chelating ligand.^12ꢀ15To investigate the conformational stability

of three-legged piano-stool complexes, Alezra et al. studied the

temperature-dependent inversion of the geometry at the metal

center of the diastereomeric [CpRu((R-BINOP-F)((CH₃)₂CO)]-

SbF₆complex, an active Lewis acid catalyst for the reaction between

than 90°.^30,33ꢀ35As a consequence, the substitution pattern of

the ﬂanking N-aryl substituents directly inﬂuences the steric

environment around the metal center, forming a partially en-

closed protective metal pocket with a structural analogy to the

active sites found in metallo enzymes. Moreover, the β-diketi-

minate scaﬀold incorporates a wide range of steric and electronic

modularity. Typically, ﬂanking N-aryl groups are introduced

through condensation of functionalized anilines with 2,4-

pentanediones.^22,36ꢀ38The backbone structure of this ligand

class also oﬀers three modular sites, two α- and one β-position.

Recently, we have demonstrated that cationic η⁶-arene β-dike-

timinatoꢀruthenium complexes 1 (Scheme 1) are well suited

for the catalytic hydrogenation of highly substituted oleﬁns,

such as 1-methylcyclohexene. Moreover, the turnover activity

is dependent on the substitution pattern at the α-positions; that

is, electron-donating groups, such as methyl (2) and t-butyl,

increase reactivity, whereas complexes containing ligand 3 with

multiple electron-withdrawing CF₃groups were catalytically

inert (Scheme 1).³⁹The latter ﬁnding suggests that the

nature of 3 signiﬁcantly reduces the electron density when bound

to a Ru(II) metal center, leading to an overall Lewis acidic

complex.

methacrolein and cyclopentadiene.¹⁶Moreover, Diꢀaz-Alvarez et al.

ꢀ

prepared mono- and dicationic Ru(II) half-sandwich complexes

bearing a tricyclic β-iminophosphine P,N-donor ligand demonstrat-

ing moderate to good enantio- and diastereoselectivity.¹⁷Further

information regarding ruthenium Lewis acid catalyzed DielsꢀAlder

reactions is available from the corresponding reviews.^18ꢀ20

Initially, 1,3,5-triaza- and 1,5-diazapentadienyl systems were

introduced as strategic ligands for the stabilization of low-

coordinate and unusually bonded transition-metal and main-

group complexes.^21ꢀ23In the past two decades, however, it

became apparent that these ligands possess a high degree of

tunable steric and electronic properties that naturally impart

application in catalysis. Consequently, there is an increasing

number of reports describing the use of complexes bearing

1,3,5-triaza or 1,5-diazapentadienyl (β-diketiminate) ligands in

homogeneous catalysis, especially in the ﬁelds of carbene and

nitrene transfer,²⁴polymerization,^25,26hydroamination,^27,28CꢀC

bond formation,²⁹hydrogenation,^30,31and recently atom-transfer

radical reactions.³²In comparison to other chelating diazo

ligands, such as amidinates, three-membered 1,2,3-triphenylgua-

nidine, and four-membered α-diimines, the metal-coordinated

β-diketiminate adopts a relatively unconstrained planar six-

membered ring, resulting in a relatively smaller bite angle of less

The research presented herein describes the ﬁrst catalytic

application of complexes featuring the heavily ﬂuorinated β-

diketiminate ligand 3. In particular, emphasis is placed on the

synthesis and characterization of novel cationic η⁶-arene β-

diketiminatoꢀruthenium(II) complexes and the evaluation of

the eﬃciency and selectivity toward the Lewis acid mediated

catalysis of DielsꢀAlder reactions between α,β-unsaturated

aldehydes and various types of dienes.

’ RESULTS AND DISCUSSION

Scheme 1

Synthesis of Ligand 3 and Corresponding Ru(II) Com-

plexes. One advantage associated with β-diketiminate ligands is

the straightforward synthesis, particularly when nonsterically

demanding electron-donating groups are substituted at the flank-

ing N-aryls and the α-positions, that is, ligand 2 (Scheme 2).

The preparation of 2 and related analogues involves acid-

catalyzed thermal condensation of two equivalents of aniline

derivatives onto 2,4-pentanedione.^36,40ꢀ42However, the parti-

cular substitution pattern of 2 does not afford β-diketiminatoꢀ

Ru(II) complexes, that is, 1, with the required Lewis acidity at

the metal center to promote DielsꢀAlder-type cycloadditions.

Therefore, the supporting ligands must be endowed with

strongly electron-withdrawing substituents, such as fluorine or

Scheme 2

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Figure 1. Two diﬀerent synthetic pathways for accessing the heavily ﬂuorinated β-diketiminate 3, using a highly Lewis acidic metal mediated coupling

method (top) or through an aza-Wittg ylid substitution reaction (bottom).

Figure 2. General synthetic transmetalation route for accessing chloro-η⁶-arene β-diketiminato ruthenium complexes (6ꢀ8), using lithiated

β-diketiminates (4 and 5) obtained through deprotonation of 2 or 3 using n-BuLi in pentane.

trifluoromethyl groups.^5,6The heavily fluorinated β-diketiminate

3, first reported by Sadighi et al., represents an ideal candidate

for formulating a series of Lewis acidic Ru(II) complexes

(Scheme 2).⁴³

ligand with an η⁶-arene ruthenium(II) fragment is achieved fol-

lowing the methodology previously established by Phillips et al.³⁰

In this step, ((η⁶-C₆H₆)RuCl₂)₂is reacted with either the

electron-donating ligand 4 or the electron-withdrawing ligand

5 over a 12 h period at room temperature in dichloromethane,

aﬀording the chloro-substituted complexes 6 and 7 in moderate

to high yields (Figure 2). In contrast, the corresponding trans-

metalation reaction between the β-diketiminatoꢀlithium deriva-

tive 5 and thep-cymenesubstituteddimer ((η⁶-(C₁₀H₁₄)RuCl₂)₂

proceeds to form complex 8, albeit at low yield. We hypothesize

that, in this reaction, a competing nucleophilic attack on the

coordinated η⁶-p-cymene ligand by the anionic β-diketiminate 5

occurs in parallel with the transmetalation step, as evidenced by

the recovery of a high amount of the reconstituted protonated

In contrast to 2, the synthesis of 3 requires a diﬀerent synthetic

methodology, where activation of the electron-deﬁcient bis-3,5-

triﬂuoromethyl-aniline is necessary prior to coupling with the

1,1,1,5,5,5-hexaﬂuoro-2,4-pentanedione. Thisis achieved through

the use of the highly Lewis acidic TiCl₄(Figure 1),^38,44or

through a stoichiometric aza-Wittig reaction involving a ylid

derivative (3,5-(CF₃)₂C₆H₃)NdPPh₃and hexaﬂuoroacetylace-

tone (Figure 1).⁴³In our hands, both procedures aﬀorded 3 in

high yield. However, the TiCl₄-based method has disadvantages,

including the use of a large excess of aniline and the required

strict anhydrous conditions. Using the slow vapor diﬀusion

method, single crystals of 3 were obtained and characterized by

X-ray diﬀraction techniques; see the Supporting Information for

a more detailed discussion.

The previously reported synthesis of β-diketiminatoꢀRu(II)

complexes bearing an η⁶-arene ligand utilized a transmetalation

procedure with the lithium complex 4.^37,38,45,46Similarly, lithia-

tion of the heavily ﬂuorinated β-diketiminate 3 is accomplished

by the addition of 1.6 M n-BuLi at low temperatures, resulting in

the precipitation of complex 5 from n-pentane (Figure 2).^47,48The

bright orange solid 5 is highly air- and moisture-sensitive,

soluble in dry and degassed Et₂O, and stable in chlorinated

hydrocarbons. Characterization of 5 was performed using solu-

tion ¹H, ¹³C, and ¹⁹F NMR. Coordination of the β-diketiminato

1

ligand 3, as observed by solution H NMR. A similar reaction

with a coordinated metal arene has been observed between a β-

diketiminatoꢀlithium complex and the η⁶-CF₃C₆H₅-substituted

β-diketiminatoꢀruthenium(II) complex.⁴⁷

Subsequently, two alternative synthetic routes were explored

for preparing 8 with increased yields. The ﬁrst method involved

initial formation of the dimeric acetonitrile β-diketiminato silver

complex 14 as ﬁrst characterized by Chiong et al., which does not

require use of the lithiated β-diketiminate 5.⁴⁹Using a modiﬁed

procedure, employing a microwave reactor, metalation of 3

with 0.5 equiv of Ag₂O in acetonitrile aﬀorded 14 with

yields of up to 69%. Subsequent transmetalation between 14

and ((η⁶-C₁₀H₁₄)RuCl₂)₂in dichloromethane, with strict exclu-

sion of light, provided complex 8 in 80% yield (Figure 3).

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Figure 3. Synthesis of the dimeric silver β-diketiminate derivative 14 (Ar = (CF₃)₂C₆H₃) from 3 and the subsequent transmetalation to aﬀord the

corresponding η⁶-p-cymene ruthenium half-sandwich chloride complex 8.

Figure 4. Synthesis of the thallium β-diketiminate derivative 15 from the in situ deprotonation of 3. Subsequent transmetalation aﬀorded the

corresponding η⁶-p-cymene ruthenium half-sandwich chloride complex 8.

However, considering the entire synthetic route from 3 to 8, the

overall yield is only moderate (55%).

byproduct. The corresponding red-colored triﬂate complex 11 is

obtained in high yields of 87% (Figure 5), whereas complexes

featuring the BArF counterion were prepared in yields of 60ꢀ

70%. All of the cationic η⁶-areneꢀruthenium(II) β-diketiminate

species demonstrated moderate sensitivity toward both oxygen

and water and thus required storage and manipulation under an

inert atmosphere. Complexes 9 and 10 are highly soluble in

chlorinated solvents, such as CH₂Cl₂and CHCl₃, over a wide

temperature range. In contrast, especially 12 and, to some extent,

13 show reduced solubility at ꢀ20 to ꢀ50 °C. In particular, the

triﬂate complex 11 is practically insoluble in dichloromethane,

even at ambient temperatures.

A third method was devised that involves the formation of the

neutral β-diketiminatoꢀthallium complex 15. Transmetalation

using Tl complexes represents one of the mildest routes for

ligand transfer, and a number of precedents in the literature

involving β-diketiminates have recently been reported. Although

a limited number of thallium complexes are known, most syn-

thetic procedures involve use of an alkaline metal precursor.^50ꢀ53

In contrast, we have employed a modiﬁed method based on the

work reported by the groups of Chang et al., Tonzetich et al., and

Dias et al.^4,54,55TlOEt is used to quantitatively deprotonate the

β-diketiminate 3 and directly form complex 15 in situ (Figure 4).

In a separate reaction, the identity of 15 was validated using

Characterization in Solution. All isolated η⁶-arene chloro-β-

diketiminatoꢀRu(II) complexes were characterized by solution

NMR (Table 1). The spectra reveal separate resonances for the

o-CH and o-CH⁰aryl protons associated with the Cl-substituted

complexes 6ꢀ7, bearing ligand 3, consistent with C_ssymmetry,

whereas complex 6 with the m-CH₃aryl-substituted ligand 2

adopts C₂symmetry. The latter observation suggests that 6 is

subject to either fast rotation (compared to NMR time scale) of

the flanking aryl groups or an associationꢀdissociation process

of the Cl group. As complexes 6ꢀ8 possess a similar geometry, it

is concluded that the lower symmetry observed in 6 originates

from the difference in electronic character between ligands 2 and

3. Hence, the complexes bearing the electron-withdrawing β-

diketiminate 3 possess a stronger RuꢀCl bond, therefore, retain-

ing the observed solid-state C_ssymmetry even in solution. In

particular, the C_sgeometry found for the p-cymene-substituted

complex 7 is even more defined in solution, as confirmed by a

δ(¹³C) difference for the two aryl o-C(H) positions. In contrast,

1

solution H, ¹³C, and ¹⁹F NMR. Afterward, puriﬁcation of 8

required column chromatography, using an eluent mixture of

n-pentane, dichloromethane, and ethyl acetate (ratio of 4.5:4.5:1).

The resulting dark red complex 8 was obtained in 86% yield. All

of the isolated η⁶-arene chloro-ruthenium(II) species bearing the

heavily ﬂuorinated β-diketiminate, 7 and 8, were found to be air

stable both in solution and in the solid state, in contrast to

complex 6, which decomposes to as yet an unidentiﬁed black

powder within days upon exposure to air.

To obtain a catalytically active species, a vacant coordination

site at the ruthenium center was generated via an anion metathe-

sis reaction of the Cl substituent. This was accomplished using

the corresponding sodium salt of a weakly coordinating anion,

such as [Na]OTf or [Na]BArF (BArF = [(3,5-(CF₃)₂C₆H₃)₄-

B]^ꢀ),⁵⁶aﬀording the coordinatively unsaturated complexes 9,

10, 12, and 13 in yields of up to 70% (Figure 5).^30,31In contrast

to complex 9, the Cl-anion metathesis reaction of 7 does not

proceed if [Na]OTf is used, as the incorporation of 3 leads to a

signiﬁcantly stronger RuꢀCl bond. Therefore, Cl abstraction

and subsequent anion exchange required the stronger metathesis

reagent Me₃SiOTf, with the highly volatile Me₃SiCl formed as a

1

the H and ¹³C NMR spectra of the cationic complexes 9ꢀ13

indicate the expected C_2vsymmetry. A measure of the Lewis

acidity associated with the Ru(II) complexes bearing β-diketi-

minate ligands 2 or 3 is provided by the highly diagnostic δ(¹H)

associated with the β-H(C) group (Table 1). A comparison of

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Figure 5. Cl-anion metathesis of the β-diketiminatoꢀchloro-ruthenium complexes 6ꢀ8 with Na[OTf] or Na[BArF] to aﬀord complexes 9, 10, 12, and

13. Use of the strong chlorine abstraction reagent Me₃SiOTf is required to obtain complex 11 in high yield.

Table 1. Selected Solution ¹H, ¹³C, and ¹⁹F NMR^aChemical Shifts (ppm) for Complexes 6ꢀ13

compound

6

7

8

9

11

10

12

13

¹H NMR

β-CH

4.48

4.56

7.02

5.40

4.57

7.61

7.77

8.3

5.46

3.79

7.60

8.56

7.85

6.37

5.20

6.98

6.11

6.39

7.53

7.41

arene-CH

aryl o-CH

aryl o-CH⁰

aryl p-CH

¹³C NMR

β-CH

5.31

8.35

5.11

6.92

5.41

8.08

4.67ꢀ4.69

8.01

6.84

7.09

8.13

7.11

8.24

8.25

94.8

86.9

87.4

104.2

81.7

104.5

96.3

96.0

aryl o-CH

aryl o-CH⁰

aryl p-CH

CdN

124.3

126.4

124.5

130.0

120.5

151.3

156.9

121.8

127.7

121.6

125.2

125.3

126.9

160

120.1

150.9

156.5

129.1

163.6

160.4

121.9

152.1

157.2

129.4

164

124.1

154.8

157.2

162.3

123.8

154.3

157.0

162.3

i-C

159.8

160.4

162.4

BArF C_B

¹⁹F NMR

α-CF₃

ꢀ59.25

ꢀ63.27

ꢀ63.24

ꢀ58.95

ꢀ63.37

ꢀ63.30

ꢀ57.50

ꢀ63.77

79.13

ꢀ57.97

ꢀ63.34

ꢀ57.96

ꢀ63.36

m-CF₃

0

m-CF₃

ꢀ78.81

OTf CF₃

79.13

BArF CF₃

ꢀ62.88

ꢀ62.91

ꢀ62.92

^aSpectra were recorded in CD₂Cl₂, except complex 11, which was measured in acetone-d₆.

the chloro-substituted complexes 6 and 7 shows increased shield-

ing at the β-carbon position, consistent with an increase in

electron density, which is in agreement with the observed δ(¹H)

value in the uncoordinated ligand. The cationic β-diketiminato

complexes 9ꢀ13 show substantially deshielded values for β-

H(C), which corresponds to increased electron donation from

the ligand to the Ru(II) center. Interestingly, a greater δ(¹H)

difference was observed for the various types of anions used. For

example, complexes with the weakly coordinating BArF, that is,

10, 12, and 13, show δ(¹H) values greater than 7 ppm. Moreover,

use of the electron-withdrawing ligand 3 further deshields β-

H(C) and indicates an overall reduction of electron density in the

core of the β-diketiminate. It is noteworthy that the chemical

shifts for 9 and 10 are very similar, indicating that, in solution,

both the triflate and the BArF components behave as noncoor-

dinating anions. As indicated by the unchanged ¹³C and ¹¹B

chemical shifts of the BArF β-C and B center for complexes 10,

12, and 13, it can be assumed that BArF is fully dissociated from

the cationic complexes, leading to catalytic active species with the

required vacant metal coordination site. In the context of hydro-

genation reactions involving complexes similar to 10, solution

1

19

heteronuclear Hꢀ F correlation and diffusion NMR studies

revealed that the heavily fluorinated anion BArF weakly interacts

with the cationic organometallic complex through a number

of diffuse contacts involving all F groups. In contrast, the OTf

and BF₄counterions show strong localized interactions with

specific regions of the cation, which can potentially interfere with

substrateꢀmetal coordination.³¹

Characterization in the Solid State. Using the liquid vapor-

diffusion technique at either low temperatures, less than ꢀ10 °C,

or at room temperature, single crystals suitable for X-ray dif-

fraction studies were obtained for the majority of complexes

(Figure 6). Indicative structural parameters are given in Table 2.

The complexes 6ꢀ13, depending on the substituent pattern of

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Figure 6. ORTEP diagrams of the chloro-substituted η⁶-arene β-diketiminatoꢀRu(II) complexes 6, 7, and 8; the triﬂate-coordinated complex 11; and

the coordinatively unsaturated species 13. Thermal ellipsoids are drawn at 50% probability. Anions, solvates, and internal disorder have been omitted for

clarity. In the case of complexes 9 and 11, two crystallographically independent molecules are present within the unit cell, and the less disordered

molecule is shown.

the β-diketiminate ligand, display either the standard three-

coordinate piano-stool geometry about the Ru(II) center or, in

cases where the organometallic component is cationic, a two-

coordinate piano-stool geometry.

((2,6-(CH₃)₂C₆H₃NC(CH₃))₂CH complex,³⁰which features

an ortho-methyl N-aryl substitution pattern. In contrast, the

heavily ﬂuorinated complex 7 showed a considerably shortened

RuꢀCl bond of 2.414(1) Å, which is increased slightly in 8,

2.429(2) Å, where the sterically more demanding and electron-

donating η⁶-p-cymene is employed. However, the Ru(II)ꢀCl

bond lengths of 7 and 8 are considerably shorter than those

reported in other Lewis acidic (η⁶-arene)Ru(II)Cl com-

plexes.^8,10,15,57Another measure of the Lewis acidity of the above

complexes is the distance between the centroid of the η⁶-arene

and the metal center. Complex 6 with electron-donating methyl

groups has the shortest distance, whereas both 7 and 8 feature a

longer η⁶-areneꢀmetal distance. Moreover, smaller RuꢀNꢀC-

(ipso) angles were observed for 7 and 8 as a result of the greater

steric interaction between the larger CF₃substituents at the

A comparison of complexes 6ꢀ8 featuring either ligand 2 or 3

reveals some key structural diﬀerences. Importantly, for ligand 2,

meta-substituted methyl groups on the ﬂanking N-aryls were

selected to minimize steric eﬀects, thus highlighting the electro-

nic properties derived from the substituent patterns of ligands

and, in particular, the donor strength of the coordinated β-

diketiminate. The most striking diﬀerence between 6 and 7 is the

RuꢀCl bond length, which is a valuable measure of the Lewis

acidity associated with the complex. Species 6 features an extre-

mely long RuꢀCl bond of 2.453(1) Å; however, this bond is

shorter than the 2.521(1) Å observed for the (η⁶-C₆H₆)RuCl-

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Table 2. Selected Bond Lengths (Å) and Angles (°) for η⁶-Areneꢀβ-diketiminatoꢀRu(II) Complexes, 6ꢀ9, 11, and 13

complexes

6

7

8

9^a

11^a

13^b

RuꢀN

2.091(2)

2.098(2)

2.108(2)

2.106(2)

2.106(3)

2.118(3)

2.005(2)

2.008(3)

2.011(2)

2.008(2)

2.091(4)

2.101(4)

2.088(4)

2.092(4)

2.247(4)

2.252(4)

1.690(2)

1.686(3)

88.9(2)

2.022(3)

RuꢀCl/O

RuꢀC^c

2.453(1)

1.683(1)

88.2(1)

2.414(1)

1.703(1)

87.8(1)

2.429(2)

1.700(2)

88.7(1)

1.704(2)

1.698(1)

89.1(1)

1.714(1)

89.0(1)

NꢀRuꢀN

NꢀC_αꢀC_β

88.8(1)

89.5(2)

124.0(2)

124.6(2)

126.1(2)

126.4(2)

126.8(3)

126.6(4)

122.4(2)

122.7(3)

122.4(2)

122.8(2)

126.1(5)

126.4(6)

125.5(6)

125.7(5)

84.9(2)

125.0(3)

NꢀRuꢀCl/O

85.4(1)

84.2(1)

83.3(1)

82.4(1)

83.7(1)

83.0(1)

84.1(2)

81.8(2)

76.7(2)

Cl/OꢀRuꢀC^c

C^cꢀRuꢀN^d

RuꢀN^dꢀC_β

126.3(1)

150.8(1)

171.6(1)

126.0(1)

154.0(1)

170.5(1)

126.5(1)

152.8(1)

173.4(2)

123.4(1)

128.8(2)

154.3(2)

156.4(2)

179.2(3)

170.0(3)

177.2(1)

178.8(1)

175.5(1)

176.2(2)

180.0^e

^aTwo crystallographically independent molecules are present in the unit cell. ^bDisorder is present within the molecule. ^cRefers to the centroid point of

e

the η⁶-arene ligand. ^dRefers to the midpoint distance between the two nitrogen centers of the β-diketiminato ligand. No esd is possible due to

placement of atoms on special positions.

α-position of the β-diketiminate and the ﬂanking N-aryls. Intere-

stingly, the NꢀRuꢀN bond angle (Table 2), remains constant in

the entire series and is independent of chloro substitution or the

substituent pattern of the β-diketiminate or η⁶-arene.

Structural analysis of the β-diketiminato fragment in the

above-discussed complexes shows an overall shortening of the

NꢀC_αbonds for those species featuring 3, as compared with

those with 2. An important measure of the nucleophilic character

of the β-C position is given by the C_αꢀC_βꢀC_αbond angle,

which is decreased by 2.4° in complexes bearing 3, indicating

decreased s-orbital contribution to the sp²σ-hybridization of the

β-C carbon.⁴⁷The presented solid-state structures demonstrate

that the choice of substituents on the β-diketiminate ligand can

be used to eﬀectively modulate the Lewis acidity of the metal

center. Complexes with an electron-withdrawing β-diketiminate

3, as opposed to 2, show signiﬁcantly increased Lewis acidity.

This is well illustrated by the fact that, for 11, a covalent RuꢀO-

(SO₂CF₃) interaction is observed in the solid state, which is

absent in the analogous complex 9, featuring the electron-

donating ligand 2. The RuꢀO bond length is 2.250(4) Å and

is slightly longer than the mean RuꢀO(SO₂CF₃) distance of

2.234 Å reported for the 66 currently known examples.⁵⁸In

comparison, the shortest RuꢀO bond involving a triﬂate group is

2.099(2) Å, as reported by Krause et al.⁵⁹In contrast to the 18-

electron conﬁguration of complex 11, the 16-electron conﬁgura-

tion of the RuCl(dCH-iPr-OC₆H₄)(IMesH₂)(OTf) species

results in a very short metalꢀoxygen bond. On the other hand,

the closest cationꢀtriﬂate interaction in 9 is 2.399 Å, which

involves a close contact between a triﬂate oxygen and a hydrogen

associated with the η⁶-C₆H₆group.

For complexes bearing the meta-methyl-substituted β-diketi-

minate 2, a comparison of the covalently bonded chloro-sub-

stituted species 6 with the cationic species 9 containing a

noncoordinating triﬂate counterion shows a number of impor-

tant diﬀerences. These include an increased η⁶-areneꢀmetal

interaction and shortened RuꢀN bonds in the cationic species 9,

consistent with increased β-diketiminato σ- and π-bonding inter-

actions to support the electron-deﬁcient metal center. Similarly, a

comparison of 8 and 13 shows that the latter has increased η⁶-p-

cymeneꢀmetal interaction in parallel with a shortening of the

RuꢀN bond distances. A key structural diﬀerence associated

with the β-diketiminato ligand between the chloro complexes 6

and 7 and the cationic complexes 9 and 13 is the folding back

of the β-diketiminate component, as indicated by η⁶-arene-

(centroid)ꢀRuꢀN(midpoint) angles of 154.0(1), 152.8(1),

and 154.1(2)°, respectively, for 6, 7, and 8. Thus, the ﬂanking

N-aryl groups rotate in order to minimize steric interaction with

the η⁶-arene. In contrast, the coordinatively unsaturated com-

plexes 9 and 13 with weakly coordinating anions show angles of

178.8(1)° and 180.0°, respectively; and thus the centroid point

η⁶-arene group, the Ru center, and the β-diketiminate backbone

atoms are positioned within a common plane.

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Figure 7. Catalytic DielsꢀAlder [4 + 2] cycloaddition between methacrolein or acrolein (0.5 mmol) and cyclopentadiene (9 equiv) mediated by

5 mol % of either complex 12 or 13 to yield the bicyclic exo- or endo-2-methyl-bicyclo[2.2.1]hept-5-ene-2-carboxaldehyde 16 and exo- or endo-

bicyclo[2.2.1]hept-5-ene-2-carboxaldehyde 17.

Table 3. Comparison of Conversions in DielsꢀAlder

Cycloaddition Reactions Using HC(O)C(R)CdCH₂, Where

R = H is Acrolein and R = Me is Methacrolein as the

Dieneophile and Cyclopentadiene^a

substrate time conversion yield^bexo/

catalyst

arene

η⁶-C₆H₆

η⁶-ⁱPrC₆H₄Me

R

(h)

(%)

endo

12

13

CH₃

H

2

80^c

99^d

83

70

81

74

84

88:12^c

17:83^d

89:11

26:74

CH₃

H

99

^aReaction conditions: Dienophile (0.5 mmol) and cyclopentadiene

(9 equiv) using 2 mol % of catalysts 12 and 13 at 20 °C in dichloro-

methane (6 mL) at 20 °C. ^bIsolated yield after column chromatography.

^cWithout catalyst added, the conversion to 16 is 16% after 24 h with an

exo/endo ratio of 78:22. ^dWithout catalyst added, the conversion to 17

is 44% after 2 h with an exo/endo ratio of 23:77.

Figure 8. Plot of product conversion 16 (including exo/endo isomer

ratio), as a function of reaction time catalyzed by complex 12 (0) and 13

(O), with methacrolein (0.5 mmol) and cyclopentadiene (9 equiv) at

ꢀ20 °C for 24 h.

To further quantify the ability of 12 and 13 to catalyze a range

of [4 + 2] cycloadditions, reactions with diﬀerent dienophiles

(i.e., methacrolein and acrolein) and cyclopentadiene were per-

formed at 20 °C using a lower catalyst loading of 2 mol % (see

Table 3).

Lewis Acid Mediated DielsꢀAlder Catalysis Using β-

DiketiminatoꢀRu(II) Complexes. To determine if complexes

9, 10, 12, and 13 feature the required Lewis acidity to efficiently

catalyze the [4 + 2] cycloaddition between methacrolein and

cyclopentadiene (Figure 7), a series of room-temperature NMR

experiments were performed. Employingsimilarconditionsused in

the literature,^5,60ꢀ62species 9 and 10, bearing the β-diketiminate

2, are catalytically inert. In contrast, when methacrolein was

added to CH₂Cl₂solutions containing 5 mol % of either complex

12 or 13, an immediate color change from brown-yellow to red

was observed. This change is consistent with the color of the

triflate complex 11 in CH₂Cl₂, indicating the presence of a

Ru(II)ꢀoxygen bond. Subsequent addition of excess cyclopen-

tadiene to the complexes 12 and 13 in the presence of metha-

crolein resulted in the efficient formation of 2-methyl-bicyclo-

[2.2.1]hept-5-ene-2-carboxaldehyde (16) within a period of 0.5 h

at 20 °C. Product 16 was obtained with a exo/endo selectivity of

88ꢀ92% regardless of which catalyst was employed.

Similar to the reactions employing 5 mol % of either complex

12 or 13, the latter proved more active for [4 + 2] cycloadditions.

Interestingly, 12 shows an increased endo selectivity in the

reaction involving acrolein and cyclopentadiene, as compared

to 13. To study the interaction between α,β-unsaturated alde-

hydes and 13, a series of solution NMR studies were performed.

In contrast to other catalytically active Ru(II) species used in

1

DielsꢀAlder reactions, both H and ¹⁹F NMR measurements

provided no evidence of the formation of a methacrolein-13

adduct, even in the presence of an excess of methacrolein (1ꢀ

50 equiv).^5,60ꢀ62However, the addition of an excess of acrolein

(10 equiv) to a solution of 13 in CH₂Cl₂resulted in the

formation of complex 18. Figure 9 shows the acrolein concen-

tration-dependent formation of 18, as monitored by ¹⁹F NMR.

Upon closer examination of the reaction between 13 and

acrolein, the splitting pattern observed in the ¹⁹F NMR asso-

ciated with the 3,5-(CF₃)₂C₆H₃ﬂanking aryl groups reveals that

complex 18 does not feature the C_2vsymmetry associated with

13. Moreover, ¹H NOE measurements of 18 did not correspond

to a presumed structure 18⁰, which, based on literature prece-

dents, should feature a RuꢀO(CH(CHCH₂)) bond; see the

Supporting Information for further details. In particular, the

expected NOE interaction between the β-CH hydrogen and

the aldehyde CHO proton was absent. Furthermore, the NOE

experiment indicated a close proximity of the H(CO) group and

the aromatic hydrogens of p-cymene. Fortunately, single crystals

were obtained from the mixture of 13 and 18 when an excess of

To understand the eﬀect of using diﬀerent η⁶-arene ligands;

benzene for 12 and p-cymene for 13, the above reaction was

repeated at ꢀ20 °C with NMR aliquots sampled over a period of

24 h (Figure 8). A noteworthy observation is the diﬀerence in the

conversion rate between complexes 12 and 13 at ꢀ20 °C, which

was not observed at room temperature. The increased activity of

13 is particularly surprising. On the basis of the structural data,

increased Lewis acidity would be attributed to 12. Furthermore,

12 shows a slight increased exo selectivity as compared to the

reaction at 20 °C. Interestingly, the exo selectivity induced by 13

shows minor diﬀerences with changes in temperature, suggesting

that 13 is inherently a highly rigid complex.

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Figure 9. Solution ¹⁹F NMR spectrum of complex 13, and spectra of

mixtures containing 13 with diﬀerent ratios of acrolein in CD₂Cl₂.

Complex 18 represents the product formed from the reaction of 13 and

acrolein.

Figure 11. Solution ¹H NMR of 10 in CD₂Cl₂with (top) and without

(bottom) an excess of acrolein added. The change in symmetry is

indicated by the m-CH₃signals that form multiple inequivalent reso-

nances upon formation of the acrolein-10 adduct.

η¹-metalꢀoxygen coordination of such CdO containing sub-

strates, only one other coordination mode of α,β-unsatuated

aldehydes to transition metals is known from the literature.

Hiraki and co-workers described the insertion of 2-methyl-2-

propen-1-ol onto a [RuClH(CO)(PPh₃)₃] complex, forming a

(η⁴-enone)Ru(0) complex. The latter insertion is accompanied

by the reductive elimination of hydrogen.⁶⁴

The rate of appearance of the yellow-colored adduct 18 is

correlated with the concentration of acrolein; that is, the greater

the excess of acrolein, the faster the formation of 18. The latter

observations strongly suggest that the formation of 18 is subject

to an equilibrium process involving 13 and acrolein. Importantly,

no red-to-yellow color change was observed during the catalysis

reaction of 13 involving acrolein and cyclopentadiene. Hence,

the formation of 18 probably represents a resting state of the

catalyst. To conﬁrm the latter hypothesis, an excess of acrolein

(25 equiv) was added to the catalytically inactive complex 10,

bearing the electron-donating ligand 2 with α-methyl substitu-

ents. As shown in Figure 11, the formation of an adduct acrolein-

10, structurally similar to 18, was observed after a short period

of time.

Figure 10. ORTEP diagram of the acrolein-13 adduct, 18. Thermal

ellipsoids are drawn at 50% probability. The anion, BArF, was omitted

for clarity. Selected bond distances (Å) and angles (°): RuꢀN(1),

2.090(2); RuꢀN(2), 2.100(2); RuꢀC(23), 2.220(2); C(11)ꢀC(22),

2.553(4); C(22)ꢀC(23), 1.509(4); RuꢀC^b, 1.767(1); N(1)ꢀRuꢀN-

(2), 82.8(1); N(1)ꢀC(9)ꢀC(11), 120.2(2); N(2)ꢀC(12)ꢀC(11),

119.4(2); N(1)ꢀRuꢀC(23), 81.9(1); N(2)ꢀRuꢀC(23), 86.0(1); C-

(23)ꢀRuꢀC^b, 125.2(1); C^bꢀRuꢀN^c, 152.8(1); RuꢀN^cꢀC(11),

127.8(1). C^bcentroid formed between atoms C(25)ꢀC(30) of the p-

cymene ring. N^ccentroid formed between atoms N(1) and N(2).

By similarity, these ﬁndings suggest that complex 18 is not the

catalytically active species in Lewis acid mediated cycloadditions.

It is proposed that only the Ru(II)ꢀO coordinated adduct 18⁰is

catalytically active in the herein described DielsꢀAlder reactions,

as reported in the literature (Figure 12).⁶⁵

Lewis acidic Ru(II) complexes are known to catalyze a range

of various inter- and intramolecular DielsꢀAlder reactions.⁶⁶

To explore the applicability of complex 13 toward additional

types of dienes, cycloaddition reactions between methacro-

lein and 1,3-cyclohexadiene or 2,3-dimethyl-buta-1,3-diene were

performed using 2 mol % catalyst loadings at 20 °C in CH₂Cl₂

(Figure 13).

The reaction between methacrolein and 1,3-cyclohexadiene

does not aﬀord the bicyclic product 2-methyl-bicyclo[2.2.2]oct-

5-ene-2-carboxaldehyde 19 in the presence of 13, even after

an extended period of 24 h at room temperature. In contrast to

cyclopentadiene, 1,3-cyclohexadiene is a bulky, less strained

cyclic dienophile, which is generally considered a poor substrate

for cycloadditions,⁶⁷especially when the Lewis acidity of the

acrolein (25:1) was added. X-ray diﬀraction techniques identiﬁed

18 as a metalꢀligand bridged metallocycle adduct, whereby the

vinyl group of acrolein has undergone a [4 + 2] cycloaddition

with the Ru(II) center and the β-carbon of the β-diketiminate

(Figure 10).⁶³This particular reversible mode of bonding has

been observed between alkenes, such as ethylene or styrene, and

η⁶-arene Ru(II) complexes bearing electron-rich β-diketiminate

ligands, similar in nature to complex 10.^30,47Nevertheless, it is

surprising to observe the formation of 18 in the presence of an

electron-withdrawing ﬂuorinated β-diketiminate ligand, in which

the nucleophilicity of the β-carbon is signiﬁcantly reduced.⁴⁷

Moreover, this particular bonding mode of an α,β-unsaturated

aldehyde has not been reported to date. Besides the common

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Figure 12. Proposed species and associated equilibria involved in the DielsꢀAlder reaction of α,β-unsaturated aldehydes and cyclopentadiene in the

presence of 13, involving formation of complex 18.

Figure 13. (top) The reaction between methacrolein (0.5 mmol) and 1,3-cyclohexadiene (9 equiv) in the presence of 13 (2 mol %) does not yield 19.

(bottom) Reaction of methacrolein (0.5 mmol) and 2,3-dimethyl-buta-1,3-diene (9 equiv) in the presence of 13 (2 mol %) shows formation of 20

without isomerization to 21.

catalyst is mild, and the active space for substrate binding is

sterically restricted, as in the case of 12 and 13. In contrast, the

reaction between methacrolein and 2,3-dimethyl-buta-1,3-diene

proceeds readily at 20 °C to aﬀord 1,3,4-trimethyl-3-cyclohex-

ene-1-carboxaldehyde 20 in 24 h with 66% conversion. In the

presence of strongly Lewis acidic catalysts, such as [Ir(CO)-

(Me)(DIM)((R)-(+)-BINAP)]²⁺, where DIM is diethyl isopro-

pylidenemalonate, 20 is a transient intermediate, and the catalyst

actively isomerizes this substrate to the bicycle 1,3,3,4-tetra-

methyl-bicyclo[2.2.1]heptan-2-one 21 (Figure 13).^68,69Solution

¹H NMR data recorded with an aliquot of the catalytic reaction

mediated by 13 in the presence of methacrolein and 2,3-dimethyl-

buta-1,3-diene showed exclusively formation of the mono-

cyclic product 20. In fact, no trace formation of 21 was detected,

even after a prolonged reaction period of 24 h. Thus, 13 appears

to be of similar Lewis acid strength compared to [Ru(salen)-

(NO)(H₂O)]SbF₆, which also exclusively forms 20.³However,

the RuꢀCl bond length measured for [Ru(salen)(NO)(Cl)]

SbF₆is 2.354(2) Å, considerably shorter than the 2.429(2) Å

measured for 13.⁷⁰As previously described, the RuꢀCl bond

length is an appropriate measure of the Lewis acidity at the

Ru(II) center. Therefore, 13 is clearly one of the mildest Lewis

acid catalysts known that can still eﬃciently catalyze a variety of

[4 + 2] cycloadditions.

’ CONCLUSION

Employing the heavily ﬂuorinated β-diketiminate ligand 3, a

series of diﬀerent substituted η⁶-arene ruthenium(II) complexes

have been prepared using a variety of synthetic routes. The

corresponding coordinatively unsaturated cationic complexes 12

and 13 are obtained through Cl abstraction using the weakly

coordinating anion BArF. Consequently, these complexes were

found to be robust Lewis acidic catalysts for the DielsꢀAlder

reaction between α,β-unsaturated aldehydes (methacrolein, ac-

rolein) and dienes (cyclopenatdiene, 2,3-dimethyl-1,3-butadiene).

The structurally rigid β-diketiminate ligand 3 not only provides the

required Lewis acidity at the metal center but also enhances the

selectivity of the herein discussed DielsꢀAlder reactions. In the

presenceof 5 mol % of 13, methacrolein and cyclopentadieneform

the bicyclic carboxaldehyde product 16 almost quantitatively with

an increase in exo selectivity, as compared to the reaction in the

absence of catalyst. Importantly, species 12 and 13 enable the use

of mild reaction conditions, reducing the risk of side reactions. As a

direct result, the catalyzed formation of 1,3,4-trimethyl-3-cyclo-

hexene-1-carboxaldehyde 20 proceeds with good yield and with-

out subsequent Lewis acid mediated isomerization of the product.

Further types of DielsꢀAlder reactions with heteronuclear sub-

strates are currently being explored.

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’ EXPERIMENTAL SECTION

found [calculated]: C, 53.10 [53.07]; H, 4.68 [4.93]; N, 4.33 [4.42].

Crystals suitable for X-ray diffraction analysis were grown by slow eva-

poration of a saturated n-pentane solution of 3. H NMR (300 MHz,

1

General Procedures. The synthesis of the starting materials and

the catalysts was carried out under a purified N₂atmosphere using

standard Schlenk techniques, whereas subsequent synthesis and mani-

pulations of all products and reagents were performed in an Innovative

Technologies glovebox with a N₂atmosphere containing less than

1 ppm of O₂and H₂O. All glassware was predried, and the flasks under-

went several purge/refill cycles before the introduction of solvents or

reagents. All solvents were dried according to literature procedures

involving distillation over the appropriate drying agents and stored in

Schlenk flasks equipped with a Teflon stopcock.⁷¹Celite for filtration

was kept in an oven at 130 °C and degassed prior to use. All other

reagents and gases (technical grade) were purchased from commercial

sources and used as received if not specified otherwise. The synthesis of

the bis(dichloro(η⁶-arene)ruthenium(II)) (arene = benzene and p-

cymene) dimers was carried out by a slightly modified procedure

according to Bennett et al.,⁷²whereas anhydrous sodium tetrakis(3,5-

bis(trifluoromethylphenyl)borate, [Na]BArF (BArF = B(3,5-(CF₃)₂-

C₆H₃)₄), was synthesized according to Reger et al.⁷³NMR spectra were

recorded using a Varian VNMRS 300, 400 and a Varian INOVA 500

instrument. Where necessary, ¹H (COSY, NOE, NOESY), ¹⁹F, ¹⁰B, and

¹³C (HMBC and HSQC) one- and two-dimensional spectra were used

to assign molecular connectivity and conformation in solution. Deuter-

ated dichloromethane was distilled over CaH₂and stored over 4 Å

molecular sieves. Chemical shifts for ¹H and ¹³C spectra were referenced

to the relevant solvent peaks. ¹⁹F NMR spectra were referenced to the

relevant residual solvent peak and CCl₃F. Infrared spectra were recorded

on a Varian 3100FT-IR Excalibur spectrometer. Samples were prepared

as nujol mulls on KBr discs. Elemental microanalyses were obtained

using an Exeter Analytical EA-1110 elemental analyzer. Mass spectra

were recorded using either a solution or a nano-electrospray ionization

(ESI) technique on a Waters alliance HT Micromass Quattro LCT

(MeOH/H₂O, 60/40) TOF instrument with a cone voltage of 35 V and

a capillary voltage of 2800 V (+) and 2500 V (ꢀ). Mass spectra for 9 and

11 were recorded using electrospray or nanoelectrospray techniques on

a ThermoFinnigan LCQ DECA XP Plus quadrupole ion trap instrument

set in positive mode: flow rate, 5 μL per min; spray voltage, 5 kV;

capillary temperature, 100 °C; capillary voltage, 20 V. Conditions were

used as described previously.⁷⁴Microwave reactions were carried out

using a Biotage Initiator 2.0 operating at 400 MW.

30 °C, CD₂Cl₂) δ(ppm): 6.07 (s, 1H, β-CH), 7.52 (s, 4H, Ar o-CH),

7.75 (s, 2H, Ar p-CH), 11.70 (s, 1H, NHN). ¹³C NMR (30 °C, 101

MHz, CD₂Cl₂) δ(ppm): 90.9 (m, ³J_CF= 4.7 Hz, β-CH), 118.9 (q, ¹J_CF

=

283.2 Hz, α-CF₃), 120.1 (m, Ar p-CH), 123.0 (q, ¹J_CF= 273 Hz, m-CF₃),

123.4 (s, br, Ar o-CH), 133.0 (q, ²J_CF= 33.9 Hz, Ar m-CCF₃), 143.7 (s,

1

Ar i-C), 150.8 (q, J_CF= 30.7 Hz, α-CH₃C). ¹⁹F NMR (30 °C, 188.2

MHz, CD₂Cl₂) δ(ppm): ꢀ63.17 (s, 12F, o-CF₃), ꢀ62.92 (s, 6F, α-

CF₃). TOF MS-ES (25 °C, MeCN), positive mode (m/z): 631.0455

[parent + H⁺, 100%, calcd. 631.0478]. FT-IR (25 °C, nujol mull, KBr

discs), υ(cm^ꢀ1): 2917 (vs), 2856 (vs), 1817 (vw), 1649 (w), 1579 (s),

1328 (m), 1288 (s), 1224 (m), 1175 (s, br), 1134 (m), 947 (w), 899

(vw), 844 (vw), 801 (vw), 703 (vw), 628 (wv).

Synthesis and Characterization of Complex 6 (η⁶-C₆H₆)-

RuCl(3,5-(CH₃)₂C₆H₃NC(CH₃))₂CH. The title complex was synthe-

sized using a method modified from that previously described.³⁰A 1.14 g

(3.74 mmol) portion of (3,5-(CH₃)₂C₆H₃NC(CH₃))₂CH₂was added

to a dried 50 mL Schlenk flask under inert conditions. A 20 mL portion

of dried and nitrogen-saturated n-pentane was added. The solution was

cooled to ꢀ60 °C, and 2.6 mL (4.11 mmol) of a 1.6 M nBuLi solution

was added dropwise over a period of 5 min. After the addition, the

reaction was stirred at ꢀ60 °C for 2 h. Volatile components were

removed in vacuo, leaving a crude off-white solid, which was suspended

in 10 mL of dried and degassed dichloromethane. This suspension

was added to 0.92 g (0.5 equiv) of ((η⁶-C₆H₆)RuCl)₂Cl₂under a flow

of nitrogen. The reaction was stirred overnight under nitrogen. The

precipitated LiCl was removed by filtration over a plug of Celite under

nitrogen, and the solvent was removed in vacuo. After drying under high

vacuum, 1.54 g (80%) of a dark purple solid was obtained. Elemental

analysis found [calculated with 1/4 CH₂Cl₂solvate]: C, 60.71 [60.46];

H, 5.86 [5.87]; N, 5.00 [5.18]. Crystals suitable for X-ray diffraction

analysis were grown by slow vapor diffusion of n-pentane into a saturated

CH₂Cl₂solution of 6 at room temperature. ¹H NMR (300 MHz, 30 °C,

CD₂Cl₂) δ(ppm): 1.67 (s, 6H, α-CH₃), 2.36 (s, 12H, m-CH₃), 4.48 (s,

1H, β-CH), 4.56 (s, 6H, C₆H₆), 6.84 (m, 2H, p-CH), 7.02 (m, 4H, o-

CH). ¹³C NMR (30 °C, 100.6 MHz, CD₂Cl₂) δ(ppm): 21.8 (s, m-CH₃),

24.5 (s, α-CH₃), 86.3 (s, C₆H₆), 94.8 (s, β-CH), 124.3 (s, o-CH), 126.9

(s, p-CH), 138.7 (s, m-CCH₃), 159.8 (s, i-C), 160.0 (s, α-CH₃C). TOF

MS-ES (25 °C, MeCN), positive mode (m/z): 485.1508 [parent M⁺,

100%, calcd. 485.1531]. FT-IR (25 °C, nujol mull, KBr discs), υ(cm^ꢀ1):

2953 (vs), 2925 (vs), 2854 (vs), 2360 (w), 2340 (w), 1601 (vw), 1589

(vw), 1561 (w), 1524 (w), 1461 (s), 1407 (m), 1377 (m), 1308 (w),

1150 (vw), 1026 (vw), 977 (vw), 916 (vw), 850 (vw), 825 (w), 722 (w),

696 (vw).

Synthesis and Characterization of Ligand 2 (3,5-(CH₃)₂-

C₆H₃NC(CH₃))₂CH₂. The synthesis of the title compound was per-

formed using the methodology established for N,N⁰-bis(2,6-dimethyl-

phenyl)-2,4-pentanediimine as published by Feldman et al.³⁶Yield: 1.2 g

(20% based on 2,4-pentanedione). Elemental analysis found [calculated]:

1

Synthesis and Characterization of Complex 7 (η⁶-C₆H₆)-

RuCl(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH. To a 50 mLSchlenk flask, 0.255 g

of ((η⁶-C₆H₆)RuCl₂)₂(orange-red powder) was added to 10 mL of

CH₂Cl₂. Subsequently, 2 equiv of the corresponding β-diketiminate

ligand 3 (0.642 g, 1.02 mmol) was dissolved in 10 mL of pentane. The

solution was cooled to ꢀ78 °C, and 0.7 mL (1.1 equiv) of a 1.6 M nBuLi

solution was added dropwise over a time of 10 min. After the addition,

the reaction was stirred at ꢀ78 °C for 2 h. Volatile components were

removed in vacuo, leaving a crude orange solid, which was suspended in

10 mL of dried and degassed dichloromethane. This suspension was

slowly added to the ((η⁶-C₆H₆)RuCl₂)₂solution over a period of

approximately 0.5 h. The flask was capped, and the reaction mixture

was stirred for 14 h. Afterward, the resulting magenta solution was

filtered through a 1 cm Celite-Schlenk frit combination, and the solution

was reduced under vacuum to a volume of ca. 1 mL. While stirring,

25 mL of n-pentane was added, causing the formation of a magenta color

solid. This solid was collected using a Schlenk frit and washed 3 ꢁ 5 mL

of n-pentane. For further pruification, the complex was dissolved in a

C, 82.28 [82.31]; H, 8.61 [8.55]; N, 9.37 [9.14]. H NMR (30 °C,

400 MHz, CDCl₃) δ(ppm): 1.99 (s, 6H, α-CH₃), 2.28 (s, 12H, o-CH₃),

4.82 (s, 1H, β-CH), 6.58 (m, 2H, Ar p-CH), 6.69 (m, 4H, Ar m-CH),

12.58 (s, 1H, NHN). ¹³C NMR (30 °C, 101 MHz, CDCl₃) δ(ppm):

21.1 (s, α-CH₃), 21.6 (s, m-CH₃), 97.3 (s, β-CH), 120.5 (s, Ar o-CH),

125.0 (s, Ar p-CH), 138.5 (s, Ar m-CCH₃), 146.0 (s, Ar i-C), 159.4

(s, α-CH₃C). TOF MS-ES positive (25 °C, MeCN) (m/z): 307.2177

[parent + H⁺, 100%, calc. 307.2174]. FT-IR (25 °C, nujol mull, KBr

discs), υ(cm^ꢀ1): 2953 (vs), 2923 (vs), 2854 (vs), 1628 (m), 1603 (m),

1550 (vs), 1489 (m), 1463 (s), 1377 (m), 1362 (m), 1313 (m), 1272

(m), 1218 (v), 1144 (m), 1030 (v), 1021 (w), 999 (vw), 960 (vw), 889

(vw), 877 (vw), 865 (vw), 850 (m), 746 (w), 722 (w), 685 (w).

Synthesis and Characterization of Ligand 3 (3,5-(CF₃)₂C₆-

H₃NC(CF₃))₂CH₂. The synthesis was carried out according to a modi-

fied literature procedure as described by Laitar et al.,⁴³which involved

the reaction of 2.1 equiv of 3,5-dis(trifluoromethyl)phenyl azide with

CF₃C(O)CH₂C(O)CF₃in dry and degassed toluene. Yield: 3.60 g (90%

based on 1,1,1,5,5,5-hexafluoro-2,4-pentanedione). Elemental analysis

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minimum amount of acetone, and n-pentane was slowly added until the

majority of the compound had precipitated. The resulting purple micro-

crystalline solid was filtered and washed with 10 mL of n-pentane and

dried for 14 h under high vacuum.Yield: 385 mg (89% based on ((η⁶-

C₆H₆)RuCl)₂Cl₂). Elemental analysis found [calculated]: C, 39.17

[38.43]; H, 1.46 [1.55]; N, 3.23 [3.32]. Crystals suitable for X-ray

diffraction analysis were grown by slow vapor diffusion of n-pentane into

a saturated CH₂Cl₂solution of 7 at room temperature. ¹H NMR (30 °C,

400 MHz, CD₂Cl₂) δ(ppm): 4.57 (s, 6H, C₆H₆), 5.40 (s, 1H, β-CH),

7.61 (s, 2H, Ar o-CH), 7.77 (s, 2H, Ar o-CH⁰), 8.30 (s, 2H, Ar p-CH). ¹³C

NMR (30 °C, 100.6 MHz, CD₂Cl₂) δ(ppm): 86.9 (s, β-CH), 87.0 (s,

C₆H₆), 119.1 (q, ¹J_CF= 285 Hz, α-CF₃), 120.1 (sept, ³J_CF= 3.69 Hz, Ar p-

CH), 123.0 (q, ¹J_CF= 258 Hz, Ar m-CF₃), 126.4 (s, Ar o-CH), 131.6 (m,

²J_CF= 33.9 Hz, Ar m-CCF₃), 150.9 (q, ²J_CF= 26.9 Hz, α-CCF₃), 156.5 (s,

Ar i-C). ¹⁹F NMR (30 °C, 282 MHz, CD₂Cl₂) δ(ppm): ꢀ63.27 (s, 6F, m-

(25 °C, CH₂Cl₂), positive mode (m/z): 809.00 [parent + H⁺, 100%,

calcd. 808.99]. FT-IR (25 °C, nujol mull, KBr discs), υ(cm^ꢀ1): 2953 (vs),

2925(vs), 2854(vs), 1584 (vw), 1557 (vw), 1465 (s), 1375(s), 1312 (w),

1283 (m), 1224 (m), 1196 (m), 1184 (m), 1175 (m), 1137 (m), 1119

(m), 964 (w), 895 (vw), 847 (vw), 829 (vw), 779 (vw), 722 (w), 709

(vw), 683 (w).

The crude solid was washed twice with degassed n-pentane and dried

to afford 196 mg (80%, 55% overall from 3) of the red colored title

compound.

Method C. Using a modified procedure published by Tonzetich

et al.,⁵⁴a solution of thallium ethoxide (79 mg, 0.317 mmol) in dried

and nitrogen-saturated dichloromethane was prepared under inert

conditions. Likewise, 200 mg (0.317 mmol) of 3 dissolved in dichloro-

methane was added dropwise to the TlOEt solution at 0 °C while

stirring. After the addition, the solution was allowed to warm to room

temperature and stirred in the exclusion of light for several hours under

a flow of nitrogen. The clear yellow solution was directly added by

cannula to a solution containing 97 mg (0.158 mmol) of [(η⁶-C₁₀H₁₄)-

RuCl]₂Cl₂in 10 mL of CH₂Cl₂. The reaction was stirred overnight in

the dark. Subsequently, the solvent was removed in vacuo and the crude

product was extracted with dried and nitrogen-flushed Et₂O and filtered

over a plug of Celite. After the removal of solvent under reduced

pressure, the solid was purified by column chromatography on silica gel

using dichloromethane/pentane/ethyl acetate in a ratio of 19:90:0 to

48:48:4. The purified title compound was obtained in 86% yield as a dark

red solid. Yield: 610 mg (86% based on 3). Elemental analysis found

[calculated]: C,: 41.67 [41.37]; H, 2.63 [2.35]; N, 2.88 [3.11]. Suitable

crystals were grown from a concentrated chloroform solution by

0

CF₃), ꢀ63.24 (s, 6F, m-CF₃), ꢀ59.25 (s, 6F, α-CF₃). TOF MS-ES

Synthesis and Characterization of Complex 8 (η⁶-ⁱPrC₆H₄-

Me)RuCl(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH. Method A. The synthesis

of the title compound was carried out following a similar method as for

the η⁶-C₆H₆analogue 7. To a nitrogen-filled 50 mL Schlenk tube

containing 122.4 mg (0.2 mmol) of ((η⁶-C₁₀H₁₄)RuCl)₂Cl₂, a solution

of 275 mg of Li(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH (0.4 mmol) in dichlo-

romethane was added by cannula. The reaction was stirred at room

temperature under nitrogen overnight. Afterward, the mixture was fil-

tered over Celite under nitrogen to separate the product from LiCl. The

solvent was removed in vacuo, and the solid was washed with dried and

degassed pentane. The product was extracted with degassed diethyl

ether and filtered over Celite under nitrogen. The solid was purified by

column chromatography on silica gel using a pentane/dichloromethane

eluent (1:1) and dried under vacuum to afford 20 mg (6%) of the title

compound.

pentane diffusion at ꢀ30 °C. H NMR (30 °C, 400 MHz, CD₂Cl₂)

1

δ(ppm): 1.25 (d, ³J_HH= 6.9 Hz, 6H, 1,4-ⁱPrC₆H₄Me CH(CH₃)₂), 2.05

3

(s, 3H, 1,4-ⁱPrC₆H₄Me CH₃), 2.62 (sept, J_HH= 6.9 Hz, 1H,

1,4-ⁱPrC₆H₄Me CH(CH₃)₂), 3.79 (m, 2H, 1,4-ⁱPrC₆H₄Me CHCMe),

4.22 (m, 1,4-ⁱPrC₆H₄Me CHCⁱPr), 5.46 (s, 1H, β-CH), 7.60 (s, 2H, Ar

o-CH), 7.84 (s, 2H, Ar p-CH), 8.56 (s, 2H, Ar o-CH⁰). ¹³C NMR

(30 °C, 100.6 MHz, CD₂Cl₂) δ(ppm): 19.2 (s, 1,4-ⁱPrC₆H₄Me CH₃),

23.2 (s, 1,4-ⁱPrC₆H₄Me CH(CH₃)₂), 31.3 (s, 1,4-ⁱPrC₆H₄Me CH-

(CH₃)₂), 84.4 (m, 1,4-ⁱPrC₆H₄Me Ar CHCMe), 87.4 (m, β-CH), 89.7

(m, 1,4-ⁱPrC₆H₄Me Ar CHCⁱPr), 103.8 (m, 1,4-ⁱPrC₆H₄Me Ar CⁱPr),

104.8 (m, 1,4-ⁱPrC₆H₄Me Ar CMe), 119.8 (q, ¹J_CF= 284.9 Hz, α-CF₃),

1

0

120.5 (m, Ar p-CH), 123.6 (q, J_CF= 272.9 Hz, m-CF₃), 123.7 (q,

¹J_CF= 272.5 Hz, m-CF₃), 124.5 (s br, Ar o-CH), 130.0 (s br, Ar o-CH⁰),

131.0 (q, ²J_CF= 33.6 Hz, m-CCF₃), 132.6 (q, J_CF= 33.5 Hz, m-CCF₃),

2

0

151.3 (q, ²J_CF= 26.1 Hz, α-CH₃C), 156.9 (s, Ar i-C). ¹⁹F NMR (30 °C,

376 MHz, CD₂Cl₂) δ(ppm): ꢀ63.37 (s, 6F, m-CF₃), ꢀ63.30 (s, 6F, m-

Method B. In modification of the procedure described by Chiong

et al.,⁴⁹the silver complex 14 of (3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH₂(3)

was prepared by a microwave reaction. In a 20 mL microwave vial

equipped with a magnetic stir bar, 92 mg (0.4 mmol) of silver oxide

Ag₂O was added, and the vial was sealed. A 250 mg (0.4 mmol) portion

of 3 was added to a 50 mL Schlenk flask and purged with nitrogen and

dissolved in 10 mL of dried and degassed acetonitrile. The bright yellow

solution was then transferred to the nitrogen-purged microwave vial by

cannula. The microwave reaction was performed at 80 °C for 30 min. A

gradual color change from orange to red was observed. The reaction

mixture was filtered over Celite under nitrogen to remove excess Ag₂O.

The filtrate was evaporated to dryness and washed with dried and

nitrogen-saturated pentane to remove excess 3. The resulting red solid

was dried under vacuum to afford 204 mg (69%) of [Ag(CH₃CN)₂-

(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH₂]₂(14). ¹H NMR (30 °C, 300 MHz,

C₆D₆) δ(ppm): 5.79 (s, 1H, β-CH), 7.24 (s, 4H, Ar o-CH), 7.52 (s, 2H,

Ar p-CH). ¹³C NMR (30 °C, 75 MHz, C₆D₆) δ(ppm): 84.7 (br), 117.0

0

CF₃), ꢀ58.95 (m br, 6H, α-CF₃). TOF MS-ES (25 °C, MeCN),

positive mode (m/z): 865.0504 [parent M⁺, 100%, calcd. 865.0461].

FT-IR (25 °C, nujol mull, KBr discs), υ(cm^ꢀ1): 2924 (vs), 2856 (vs)

1558 (vw), 1462 (s), 1371 (m), 1310 (w), 1280 (m), 1224 (w), 1180

(m), 1134 (m, br), 959 (w), 898 (vw), 848 (vw), 779 (vw), 720 (w),

681 (vw).

Synthesis and Characterization of Complex 9 [(η⁶-C₆H₆)-

Ru(3,5-(CH₃)₂C₆H₃NC(CH₃))₂CH]OTf. To a 100 mL Schlenk flask,

400 mg (0.770 mmol) of [(η⁶-C₆H₆)RuCl(3,5-(CH₃)₂C₆H₃NC-

(CH₃))₂CH]Cl (6) and 159 mg (1.2 equiv) of [Na]OTf were dissolved

in 10 mL of dried and nitrogen-saturated dichloromethane under an

inert atmosphere. The reaction mixture was stirred for 12 h. Afterward,

the solution was filtered to remove NaCl and excess [Na]OTf, and the

solvent was removed in vacuo. The resulting orange-brown crude solid

was washed several times with dried and degassed pentane and dried in

vacuo to afford 390 mg (81%) of the dark brown title compound. Yield:

390 mg (81% based on 6). Elemental analysis found [calculated +3/4

CH₂Cl₂solvate]: C, 49.60 [49.51]; H, 4.65 [4.70]; N, 3.91 [4.02].

Crystals suitable for X-ray diffraction analysis were grown by slow vapor

diffusion of n-pentane into a saturated CH₂Cl₂solution of 9 at room

temperature. ¹H NMR (400 MHz, 30 °C, CD₂Cl₂) δ(ppm): 2.26 (s, 6H,

α-CH₃), 2.47 (s, 12H, m-CH₃), 5.20 (s, 6H, C₆H₆), 6.37 (s, 1H, β-CH),

6.98 (m, 4H, o-CH), 7.09 (m, 2H, p-CH). ¹³C NMR (30 °C, 101 MHz,

CD₂Cl₂) δ(ppm): 21.7 (s, m-CH₃), 24.5 (s, α-CH₃), 83.7 (s, C₆H₆),

104.2 (s, β-CH), 121.8 (s, o-CH), 129.1 (s, p-CH), 139.6 (s, m-CCH₃),

160.4 (s, i-C), 163.6 (s, α-CH₃C). ¹⁹F NMR (30 °C, 376 MHz, CD₂Cl₂)

1

(br), 117.3, 120.8 (q, J_CF= 288.4 Hz), 122.8 (br), 124.3 (q, J_CF

=

272.9 Hz), 132.7 (q, ²J_CF= 33.3 Hz), 153.5 (q, ²J_CF= 25.0 Hz), 154.0.

¹⁹F NMR (30 °C, 282.9 MHz, C₆D₆) δ(ppm): ꢀ59.1 (s, 12F, m-CF₃),

ꢀ56.7 (s, 6F, α-CF₃). Under inert conditions, 83 mg (0.136 mmol) of

[(η⁶-C₁₀H₁₄)RuCl]₂Cl₂and 200 mg (0.136 mmol) of [Ag(CH₃CN)₂-

(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH₂]₂(14) were loaded into a 50 mL

Schlenk flask, and 25 mL of dried and degassed dichloromethane was

added. The reaction mixture was stirred with the exclusion of light for

14 h under nitrogen. The precipitatedAgCl was removedbyfiltrationover

a 1 cmCelite pad. Afterward, the solvent was removedunderhighvacuum.

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δ(ppm): ꢀ78.81 (s, ¹J_FC= 321.6 Hz, CF₃SO₃^ꢀ). TOF MS-ES (25 °C,

MeCN), positive mode (m/z): 485.1521 [parent M⁺, 100%, calcd.

485.1531]. TOF MS-_ꢀES (25 °C, MeCN), negative mode (m/z):

148.9492 [parent OTf , 100%, calcd. 148.9520]. FT-IR (25 °C, nujol

mull, KBr discs), υ(cm^ꢀ1): 2953 (vs), 2925 (vs), 2854 (vs), 2361 (w),

2340 (w), 1605 (vw), 1589 (vw), 1552 (w), 1462 (s), 1377 (m), 1347

(w), 1262 (m), 1223 (w), 1151 (m), 1030 (m), 843 (vw), 722 (w), 697

(vw), 637 (m).

Synthesis and Characterization of Complex 12 [(η⁶-C₆H₆)-

Ru(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH]BArF. A 100 mL Schlenk flask was

charged with 100 mg (0.120 mmol) of complex 7with 111 mg (0.125 mmol,

1.05 equiv) of [Na]BArF in 5 mL of dried and degassed dichloro-

methane. The mixture was stirred overnight and filtered over a plug of

Celite to remove sodium chloride. Subsequently, the solvent was

removed in vacuo. The crude solid was washed three times with dry

and degassed n-pentane and dried under high vacuum for 24 h to afford

120 mg (60%) of a brown-green solid. Elemental analysis found

[calculated]: C, 42.48 [42.39]; H, 1.45 [1.51]; N, 1.63 [1.68]. ¹H

NMR (30 °C, 300 MHz, CD₂Cl₂) δ(ppm): 5.41 (s, 6H, C₆H₆), 7.53 (s,

1H, β-CH), 7.54 (s, 4H, B(ArF)₄p-CH), 7.70 (m, 8H, B(ArF)₄o-CH),

8.08 (s, 4H, Ar o-CH), 8.24 (s, 2H, Ar p-CH). ¹³C NMR (30 °C, 101

MHz, CD₂Cl₂) δ(ppm): 86.4 (s, C₆H₆), 96.3 (m, β-CH), 118.1 (sept,

Synthesis and Characterization of Complex 10 [(η⁶-C₆H₆)-

Ru(3,5-(CH₃)₂C₆H₃NC(CH₃))₂CH]BArF. To a 50 mL Schlenk flask,

48 mg (0.09 mmol) of [(η⁶-C₆H₆)RuCl(3,5-(CH₃)₂C₆H₃NC-

(CH₃))₂CH]Cl (6) and 90 mg (1.1 equiv) of [Na]BArF were dissolved

in 10 mL of dried and nitrogen-saturated dichloromethane under an

inert atmosphere. The reaction mixture was stirred for 12 h, after which

the solution was filtered to remove NaCl and the solvent was removed in

vacuo. The golden-brown crude solid was washed several times with

dried and degassed n-pentane and dried under high vacuum to afford

85 mg (93%) of the dark brown title compound. Elemental analysis found

[calculated +1.5 CH₂Cl₂solvate]: C, 50.18 [49.26]; H, 3.06 [3.14]; N,

1.56 [1.90]. ¹H NMR (400 MHz, 30 °C, CD₂Cl₂) δ(ppm): 2.26 (s, 6H,

α-CH₃), 2.46 (s, 12H, m-CH₃), 5.11 (s, 6H, C₆H₆), 6.39 (s, 1H, β-CH),

6.92 (s br, 4H, o-CH), 7.11 (s br, 2H, p-CH), 7.56 (s br, 4H, B(ArF)₄

p-CH), 7.72 (s br, 8H, B(ArF)₄o-CH). ¹³C NMR (30 °C, 101 MHz,

CD₂Cl₂) δ(ppm): 21.7 (s, m-CH₃), 24.5 (s, α-CH₃), 83.6 (s, C₆H₆),

104.5 (s, β-CH), 118.1 (s br, B(ArF)₄p-CH), 121.6 (s, Ar o-CH), 125.2

1

³J_CF= 4.2 Hz, B(ArF)₄p-CH), 119.1 (q, J_CF= 283.6 Hz, α-CF₃),

122.9 (q, ¹J_CF= 273.6 Hz, m-CF₃), 124.1 (m, p-CH), 125.2 (q, ¹J_CF

=

272.3 Hz, B(ArF)₄m-CF₃), 125.3 (s, o-CH), 129.4 (q, ²J_CF= 31.4 Hz,

2

B(ArF)₄m-CCF₃), 133.2 (q, J_CF= 34.9 Hz, m-CCF₃), 135.4 (s br,

B(ArF)₄o-CH), 154.8 (m, α-CH₃C), 157.2 (s, i-C), 162.3 (q, ¹J_CB

=

49.9 Hz, B(ArF)₄i-C). ¹⁹F NMR (30 °C, 282 MHz, CD₂Cl₂) δ(ppm):

ꢀ63.34 (s, ¹J_CF= 273.6 Hz, 12F, m-CF₃), ꢀ62.91 (s, 24F, B(ArF)₄m-

1

CF₃), ꢀ57.97 (s, J_CF= 283.6 Hz, 6F, α-CF₃). ¹¹B NMR (25 °C,

1

3

128 MHz, CD₂Cl₂) δ(ppm): ꢀ6.64 (m, J_BC= 49.9 Hz, J_BH

=

2.57 Hz, B(ArF)₄). TOF MS-ES positive (25 °C, MeCN) (m/z):

808.9821 [parent M⁺, 100%, calcd. 808.9835]. TOF MS-ES negative

(25 °C, MeCN) (m/z): 863.0640 [parent BArF^ꢀ, 100%, calcd.

863.0649]. FT-IR (25 °C, nujol mull, KBr discs), υ(cm^ꢀ1): 2921

(vs), 2856 (vs), 1612 (vw), 1282 (m), 1189 (m br), 1144 (m br), 961

(vw), 894 (vw), 842 (vw), 719 (w), 680 (vw).

1

(q, J_CF= 272.2 Hz, B(ArF)₄m-CF₃), 129.4 (s, Ar p-CH), 135.4 (s,

1

B(ArF)₄o-CH), 139.8 (s, m-CCH₃), 160.4 (s, i-C), 162.4 (q, J_BC

=

49.4 Hz, B(ArF)₄),164.0(s,α-CH₃C).¹⁹FNMR(30°C, 376 MHz, CD₂Cl₂)

δ(ppm): ꢀ62.88 (s, J_FC= 272.4 Hz, B(ArF)₄m-CF₃). ¹¹B NMR

1

Synthesis and Characterization of Complex 13 [(η⁶-

C₁₀H₁₄)Ru(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH]BArF. In a 100 mL

Schlenk flask, 197 mg (0.219 mmol) of complex 8 and 214 mg (1.1

equiv) of Na[BArF] were dissolved in dried and degassed dichloro-

methane under inert conditions and stirred for 12 h. Afterward, the

reaction mixture was filtered through a 1 cm Celite pad under nitrogen

and the solvent was removed in vacuo from the resulting solution. The

brown-colored crude solid was washed several times with dried and

nitrogen-saturated n-pentane and dried under high vacuum overnight.

The resulting yield was 241 mg (64%) of a dark green-brown solid.

Elemental analysis found [calculated]: C, 44.20 [43.80]; H, 2.00 [1.93];

N, 1.36 [1.62]. X-ray quality crystals were grown from a concentrated

(25 °C, 128.4 MHz, CD₂Cl₂) δ(ppm): ꢀ6.61 (s, J_BC= 49.5 Hz,

B(ArF)₄). TOF MS-ES positive (25 °C, MeCN) (m/z): 484.1515

[parent M⁺, 100%, calcd. 485.1531]. FT-IR (25 °C, nujol mull, KBr

discs), υ(cm^ꢀ1): 2953 (vs), 2925 (vs), 2854 (vs), 1609 (vw), 1555 (vw),

1462 (s), 1377 (m), 1356 (w), 1278 (w), 1124 (w, br), 1043 (vw), 888

(vw), 839 (vw), 722 (w), 682 (vw), 670 (vw).

Synthesis and Characterization of Complex 11 (η⁶-C₆H₆)-

RuOTf(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH. This reaction was performed

in a glovebox. To a 50 mL round-bottom flask, 125 mg (0.148 mmol) of

complex 7 was added and dissolved with 15 mL of dry and degassed

dichloromethane. Rapidly, 0.300 g of Me₃SiO₃SCF₃(colorless liquid)

was added, and within 5 min, the solution rapidly changed to a dark

orange-brown color. After stirring for 2 h, all volatiles were removed

under reduced pressure. The resulting orange-brown solid was trans-

ferred to a frit and washed 3 ꢁ 10 mL with dry and degassed n-pentane,

then dried for 4 h under high vacuum. Yield: 144 mg (0.144 mmol,

97.2%). Elemental analysis found [calculated]: C, 34.14 [35.12]; H, 1.33

[1.37]; N, 2.54 [2.93]. Crystals suitable for X-ray diffraction analysis

were grown by slow vapor diffusion of n-pentane into a saturated ace-

tone solution of 11 at room temperature. ¹H NMR (30 °C, 400 MHz,

acetone-d₆) δ(ppm): 5.61 (s, 6H, C₆H₆), 6.11(s, 1H, β-CH), 8.13 (s,

2H, Ar p-CH), 8.35 (s, 4H, Ar o-CH). ¹³C NMR (25 °C, 101 MHz,

1

dichloromethane solution by diffusion with n-pentane at ꢀ30 °C. H

NMR (30 °C, 500 MHz, CD₂Cl₂) δ(ppm): 1.19 (d, ³J_HH= 6.9 Hz, 6H,

1,4-ⁱPrC₆H₄Me CH(CH₃)₂), 2.04 (s, 3H, 1,4-ⁱPrC₆H₄Me CH₃), 2.36

(sept, 1H, 1,4-ⁱPrC₆H₄Me CH(CH₃)₂), 4.67ꢀ4.69 (m, 2H, 1,4-ⁱPrC₆-

H₄Me CHCⁱPr), 4.76ꢀ4.78 (m, 1,4-ⁱPrC₆H₄Me CHCMe), 7.41 (s, 1H,

β-CH), 7.54 (s, 4H, B(ArF)₄Ar p-CH), 7.70 (m, 8H, B(ArF)₄o-CH),

8.01 (s, 4H, Ar o-CH), 8.25 (s, 2H, Ar p-CH). ¹³C NMR (30 °C, 126

MHz, CD₂Cl₂) δ(ppm): 20.2 (s, 1,4-ⁱPrC₆H₄Me CH₃), 23.0 (s, 1,4-

ⁱPrC₆H₄Me CH(CH₃)₂), 32.3 (s, 1,4-ⁱPrC₆H₄Me CHMe₂), 87.6 (s,

1,4-ⁱPrC₆H₄Me Ar CHCMe), 90.1 (s, 1,4-ⁱPrC₆H₄Me Ar CHCⁱPr), 96.0

acetone-d₆) δ(ppm): 81.7 (s, β-CH), 87.3 (s, C₆H₆), 120.4 (q, ¹J_CF

=

(s, br, β-CH), 96.5 (s, 1,4-ⁱPrC₆H₄Me Ar CCH₃), 106.6 (s, 1,4-

287.1 Hz, α-CF₃), 121.9 (m, Ar p-CH), 124.2 (q, ¹J_CF= 272.3 Hz, m-

CF₃), 127.7 (s br, Ar o-CH), 132.6 (q, ²J_CF= 34.0 Hz, m-CCF₃), 152.1

(q, ²J_CF= 27.4 Hz, α-CH₃C), 157.2 (s, Ar i-C). ¹⁹F NMR (30 °C, 282

MHz, acetone-d₆) δ(ppm): ꢀ79.13 (s, 3F, ¹J_CF= 320.8 Hz, CF₃SO₃^ꢀ),

ꢀ63.77 (s, 12F, m-CF₃), ꢀ57.50 (s, 6F, α-CF₃). TOF MS-ES (25 °C,

MeCN), positive mode (m/z): 808.76 [parent + H⁺, 100%, calcd.

808.99]. ESI-MS (25 °C, CH₂Cl₂), (m/z) positive mode 557.231

[parent, 100%], negative mode 149.214 [parent, 100%]. FT-IR (25 °C,

nujol mull, KBr discs), υ(cm^ꢀ1): 2953 (vs), 2925 (vs), 2854 (vs), 1566

(vw), 1462 (s), 1376 (s), 1284 (m, br), 1226 (m), 1184 (m), 1138 (m),

1030 (w), 1020 (w), 965 (w), 906 (vw), 849 (w), 802 (vw), 723 (w), 683

(vw), 637 (w).

ⁱPrC₆H₄Me Ar CⁱPr), 118.0 (s br, B(ArF)₄p-CH), 119.2 (q, J_CF

=

1

283.6 Hz, α-CF₃), 122.9 (q, ¹J_CF= 273.5 Hz, m-CF₃), 123.8 (m, Ar p-

CH), 125.2 (q, ¹J_CF= 272.4 Hz, B(ArF)₄m-CF₃), 125.3 (s br, Ar o-CH),

129.4 (q br, ²J_CF= 31.5 Hz, B(ArF)₄m-CCF₃), 132.9 (q, ²J_CF= 34.9 Hz,

Ar m-CCF₃), 135.4 (s, B(ArF)₄Ar o-CH), 154.3 (q, ²J_FC= 30.0 Hz, α-

CH₃C), 157.0 (s, Ar i-C), 162.3 (q, ¹J_CB= 49.8 Hz, B(ArF)₄i-C). ¹⁹

F

NMR (30 °C, 282 MHz, CD₂Cl₂) δ(ppm): ꢀ63.36 (s, 12F, m-CF₃),

ꢀ62.92 (s, 24F, B(ArF)₄m-CF₃), ꢀ57.96 (s, 6F, α-CF₃). ¹¹B NMR

(25 °C, 128 MHz, CD₂Cl₂) δ(ppm): ꢀ6.64 (m, ¹J_BC= 49.9 Hz, ³J_BH

=

2.57 Hz, B(ArF)₄). TOF MS-ES (25 °C, MeCN), positive mode (m/z):

865.0440 [parent M⁺, 100%, calcd. 865.0461]. FT-IR (25 °C, nujol mull,

KBr discs), υ(cm^ꢀ1): 2918 (vs), 2856 (vs), 1612 (vw), 1459 (m), 1372

5393

dx.doi.org/10.1021/om200566a |Organometallics 2011, 30, 5381–5395

Organometallics

ARTICLE

(m), 1282 (m), 1132 (m br), 962 (vw), 891 (vw), 843 (vw), 719 (w),

681 (vw).

for full spin recovery of the methacrolein aldehyde proton and the

product aldehyde proton. The corresponding T₁was measured by a

standard inversion recovery experiment, which showed both com-

pounds to have a similar T₁of approximately 10.4 ( 0.2 s. For full

characterization of the resulting products, n-pentane or n-hexanes was

added to precipitate the catalyst and the mixture was filtered over a small

plug of Celite and the solvent was removed in vacuo. The crude products

were purified over a short column of silica gel using ratios of n-pentane/

dichloromethane of either 3:1 or 2:1.

Synthesis and Characterization of Complex 18 {[(η⁶-

C₁₀H₁₄)Ru(3,5-(CF₃)₂C₆H₃NC(CF₃))₂CH][CH₂CH(CHO)]}BArF.

In a 4 mL vial, 30 mg (0.017 mmol) of [(η⁶-C₁₀H₁₄)Ru(3,5-(CF₃)₂-

C₆H₃NC(CF₃))₂CH]BArF (13) and 30 μL (25 equiv, 0.43 mmol) of

acrolein were dissolved in 1.5 mL of CH₂Cl₂. After a reaction period of

48 h, crystals suitable for X-ray analysis were grown by diffusion of

pentane into the above described mixture at ꢀ10 °C. Solution NMR

data were obtained by repeating the above procedure in an NMR sample

1

tube, using only CD₂Cl₂as a solvent. H NMR (30 °C, 500 MHz,

’ ASSOCIATED CONTENT

3

CD₂Cl₂) δ(ppm): 1.03 (d, J_HH= 6.9 Hz, 3H, 1,4-ⁱPrC₆H₄Me CH-

3

i

0

(CH₃)₂), 1.22 (d, J_HH= 6.9 Hz, 3H, 1,4- PrC₆H₄Me CH(CH₃)₂),

1.35 (dd, ³J_HH= 9.7 Hz, ²J_HH= 12.6 Hz, 1H, CH_acrolein,β), 2.09 (sept,

1H, 1,4-ⁱPrC₆H₄Me CH(CH₃)₂), 2.14 (s, 3H, 1,4-ⁱPrC₆H₄Me CH₃),

S

Supporting Information. An in-depth discussion regard-

b

ing the 2D NOE experiments for complex 18 is provided.

Structural details concerning β-diketiminate 3 and supplemen-

tary kinetic data for the DielsꢀAlder reactions with the catalysts

are also given. Crystallographic details (CIF) for all relevant

complexes are provided. This material is available free of charge

via the Internet at http://pubs.acs.org.

3

2.29ꢀ2.37 (m, 1H, CH⁰_acrolein,β), 4.06 (d, J_HH= 6.3 Hz, 1H,

1,4-ⁱPrC₆H₄Me Ar CHCMe), 4.19 (m, 1H, 1,4-ⁱPrC₆H₄Me Ar

3

CHCCMe₂), 4.21 (m, 1H, CH_acrolein,α), 5.14 (d, J_HH= 6.3 Hz,

1,4-ⁱPrC₆H₄Me Ar CH⁰CCHMe₂), 5.35ꢀ5.40 (m, 1H, β-CH), 5.48

(d, ³J_HH= 6.4 Hz, 1,4-ⁱPrC₆H₄Me Ar CH⁰CMe), 7.33 (s, 1H, Ar o-CH),

7.54 (s, 4H, BArF Ar p-CH), 7.58 (s, 1H, Ar o-CH⁰), 7.71 (m, 8H, BArF

Ar o-CH), 7.81 (s, 1H, Ar o-CH⁰⁰), 8.02 (s, 1H, Ar p-CH), 8.11 (s br, 1H,

’ AUTHOR INFORMATION

Ar o-CH⁰⁰⁰), 8.11 (s br, 1H, Ar p-CH⁰), 9.93 (s, br, 1H, COH_acrolein). ¹³

C

Corresponding Author

*E-mail: andrew.phillips@ucd.ie.

NMR (30 °C, 126 MHz, CD₂Cl₂) δ(ppm): 18.8 (s, 1,4-ⁱPrC₆H₄Me

CH₃), 20.7 (s, C_acrolein,β), 22.3 (s, 1,4-ⁱPrC₆H₄Me CH(C⁰H₃)₂), 23.1

(s, 1,4-ⁱPrC₆H₄Me CH(CH₃)₂), 31.2 (s, 1,4-ⁱPrC₆H₄Me CHMe₂), 31.7

(s, C_acrolein,α), 47.4 (s, β-CH), 87.2 (s, 1,4-ⁱPrC₆H₄Me Ar CHCMe),

87.5 (s, 1,4-ⁱPrC₆H₄Me Ar CH⁰CMe), 88.9 (s, 1,4-ⁱPrC₆H₄Me

Ar CHCCHMe₂), 90.1 (s, 1,4-ⁱPrC₆H₄Me Ar C⁰HCCHMe₂), 110.3

(s, 1,4-ⁱPrC₆H₄Me Ar CCH₃), 117.1 (q, ¹J_CF= 284.9 Hz, α-CF₃), 117.4

’ ACKNOWLEDGMENT

This research was supported by a start-up grant from the

School of Chemistry and Chemical Biology at the University

College Dublin (UCD). A.D.P. thanks the Science Foundation

Ireland (SFI) for a Stokes Lectureship at UCD. We thank

Dr. Rosario Scopelitti at the Institute of Chemical Sciences and

Engineering (ISIC), Ecole Polytechnique Fꢁedꢁerale de Lausanne

(EPFL), CH-1015 Lausanne, Switzerland, for assistance with the

X-ray data collection.

1

0

(q, J_CF= 285.6 Hz, α-CF₃), 118.1 (s br, BArF Ar p-CH), 120.6

(s, 1,4-ⁱPrC₆H₄Me Ar CCH(CH₃)₂), 121.4 (s br, Ar o-CH), 121.7 (s br,

Ar o-C⁰H), 122.2 (s br, Ar o-C⁰⁰H), 122.9 (s br, Ar o-C⁰⁰⁰H), 123.8 (m, Ar

p-CH), 124.0 (m, Ar p-C⁰H), 125.2 (q, ¹J_CF= 272.0 Hz, BArF m-CF₃),

129.5 (q br, ²J_CF= 31.3 Hz, BArF Ar m-CCF₃), 132.33ꢀ134.43 (m, Ar

m-CCF₃), 135.4 (s, BArF Ar o-CH), 153.0 (s, Ar i-C), 153.2 (s, Ar i-C⁰),

162.4 (q, ¹J_CB= 50.1 Hz, BArF BC), 166.0 (q, ²J_FC= 33.0 Hz, α-CH₃C),

166.8 (q, ²J_FC= 32.7 Hz, α-CH₃C⁰), 198.5 (s, OdC_acrolein). The m-CF₃

groups were not observed due to overlapping signalsbetween 120 and 130

ppm. ¹⁹F NMR (30 °C, 282 MHz, CD₂Cl₂) δ(ppm): ꢀ63.57 (s, 3F, m-

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dx.doi.org/10.1021/om200566a |Organometallics 2011, 30, 5381–5395

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Application of electron-withdrawing coordinatively unsaturated η6-arene β-diketiminato-Ruthenium complexes in Lewis acid catalyzed Diels-Alder reactions

DOI: 10.1021/om200566a

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Full text of DOI:10.1021/om200566a

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