Dihydrogen activation by frustrated carbene-borane lewis pairs: An experimental and theoretical study of carbene variation

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pubs.acs.org/IC

Dihydrogen Activation by Frustrated Carbene-Borane Lewis Pairs:

An Experimental and Theoretical Study of Carbene Variation

Sabrina Kronig, Eileen Theuergarten, Dirk Holschumacher, Thomas Bannenberg, Constantin G. Daniliuc,

Peter G. Jones, and Matthias Tamm*

Institut f€ur Anorganische und Analytische Chemie, Technische Universit€at Braunschweig, Hagenring 30, D-38106 Braunschweig,

Germany

S Supporting Information

b

ABSTRACT: A variety of Lewis acidꢀbase pairs consisting of tris-

(pentaﬂuorophenyl)borane, B(C₆F₅)₃, in combination with sterically de-

manding ﬁve- and six-membered N-heterocyclic carbenes (NHCs) of the

imidazolin-2-ylidene, imidazolidin-2-ylidene, and tetrahydropyrimidin-2-yli-

dene types were investigated with respect to their potential to act as frustrated

Lewis pairs (FLP) by reaction with dihydrogen (H₂) and tetrahydrofuran

(THF). A suﬃcient degree of “frustration” was usually established by

introduction of a 1,3-di-tert-butyl or 1,3-diadamantyl carbene substitution

pattern, which allows an unquenched acidꢀbase reactivity and thus leads to

heterolytic dihydrogen activation and ring-opening of THF. In contrast, 1,3-

bis(2,6-diisopropylphenyl)-substituted carbenes showed ambiguous behavior,

and the corresponding ﬁve-membered imidazolin-2-ylidene formed a stable

carbene-B(C₆F₅)₃adduct, whereas fast CꢀF activation and formation of a

zwitterionic pyrimidinium-ﬂuoroborate was observed for the six-membered tetrahydropyrimidin-2-ylidene. A stable adduct was also

isolated for the combination of the acyclic carbene bis(diisopropylamino)methylene with B(C₆F₅)₃, and consequently no reactivity

toward H₂and THF was observed. To rationalize the reactivity of the carbene-borane Lewis pairs, the thermodynamics of adduct

formation with B(C₆F₅)₃were calculated for 10 diﬀerent carbenes; the stability (or instability) of these adducts can be used as a good

measure of the degree of “frustration”.

Scheme 1

’ INTRODUCTION

Since 2006, when Stephan and co-workers reported the

reversible, metal-free activation of dihydrogen by the phos-

phine-borane system para-Mes₂P(C₆F₄)B(C₆F₅)₂(Mes =

2,4,6-trimethylphenyl, Scheme 1),¹the use of so-called “fru-

strated Lewis pairs” (FLPs) for the activation of small molecules

has developed into an exceptionally rapidly growing area of

research.²In such FLP systems, the combination of sterically

congested Lewis acids and bases prevents traditional adduct

formation and thus allows mutual and unquenched reactivity of

the two antagonistic partners. For instance, the formation of a

stable PꢀB bond is suppressed in the well-studied pair P(tBu)₃/

B(C₆F₅)₃,³which is therefore capable of heterolytic dihydrogen

cleavage to aﬀord cleanly the phosphonium borate salt

[(tBu)₃PH][HB(C₆F₅)₃] (Scheme 1).⁴To date, numerous

phosphine/borane combinations have been established that

can be employed for the activation not only of H₂⁵but also of

other small molecules such as tetrahydrofuran (THF),⁶oleﬁns,⁷

alkynes,⁸carbon dioxide,⁹nitrous oxide,¹⁰isocyanates, azides,¹¹

and singlet dioxygen.¹²In addition, selective SꢀS and PꢀP bond

activation has been achieved.^13,14

N-heterocyclic carbenes (NHCs) have been used extensively for

the replacement of phosphine ligands in organotransition metal

chemistry and catalysis as a consequence of their high basicity

and ensuing ability to form very strong metalꢀcarbon bonds.^16,17

Accordingly, the combination of a sterically demanding carbene

such as 1,3-di-tert-butylimidazolin-2-ylidene (1a) with the bor-

ane B(C₆F₅)₃could be expected to aﬀord a viable FLP system,

which indeed proved capable of HꢀH, CꢀO, CꢀH, and NꢀH

bond activation.^18ꢀ21In fact, this Lewis pair exhibits a particu-

larly strong propensity for heterolytic dihydrogen cleavage,

which can be ascribed to the strongly exergonic nature of the

During the twenty years since the isolation of the ﬁrst stable

Received: June 15, 2011

Published: June 30, 2011

carbenes of the imidazolin-2-ylidene type by Arduengo et al.,¹⁵

r

2011 American Chemical Society

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Inorganic Chemistry

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Scheme 2

Figure 2. ORTEP diagram of 5 with thermal displacement parameters

drawn at 50% probability. Selected bond lengths [Å] and angles [deg]:

C11ꢀN1 1.3631(17), C11ꢀN2 1.3552(17), C12ꢀC13 1.334(2),

C11ꢀC14 1.4946(19), BꢀO 1.447(2), C17ꢀO 1.354(2), OꢀC(17⁰)

1.403(7); N1ꢀC11ꢀN2 107.76(12), OꢀBꢀC40 106.87(13),

OꢀBꢀC50 110.98(13), OꢀBꢀC60 112.32(13). Only one position

of the disordered atoms C16 and C17 is shown.

According to theoretical calculations, the failure to isolate any

normal adducts from mixtures of 1a and 1b with B(C₆F₅)₃

should not be ascribed solely to their thermodynamic instability

but also to the higher stability of the abnormal adduct 4, whereas

the reverse is true for the sterically less demanding carbene 1e

bearing methyl substituents in the 1,3-positions (Figure 1).¹⁸

Accordingly, the related normal adduct [(1f)B(C₆F₅)₃] was

isolated and structurally characterized,²⁷as was [(1d)B(C₆F₅)₃],

indicating that the 1,3-diisopropylphenyl substituents in1d are not

suﬃciently bulky to establish a frustrated Lewis pair (Figure 1, vide

infra).¹⁹In view of the many variations that have been applied to

phosphine-borane Lewis pairs,²⁸the study presented herein was

initiated (i) to elaborate the factors leading to “frustration” in

related carbene-borane Lewis pairs and (ii) to identify possible

deactivation pathways such as normal and abnormal adduct

formation or other thermal rearrangements similar to those that

have been discovered for PꢀB systems.²⁹Therefore, the reactivity

of the carbenes 1c, 1d, and 1gꢀ1k toward B(C₆F₅)₃was studied

and, if possible, these combinations were employed in THF and

H₂activation. In addition, the thermodynamics of normal adduct

formation with B(C₆F₅)₃were calculated for all carbenes 1aꢀ1k

to rationalize the experimental results and to establish a measure

for the degree of “frustration” (Figure 1).

Figure 1. N-Heterocyclic carbenes for the preparation of carbene-

B(C₆F₅)₃Lewis pairs.

H₂splitting reaction as a result of an enhanced cumulative

acidꢀbase strength.²²In the absence of H₂or other substrates,

however, this high reactivity leads to self-destruction and irre-

versible formation of the abnormal carbene-borane adduct 4 by

CꢀH activation, which eliminates mismatching between Lewis

acid and base and enables this FLP to circumvent frustration at

the expense of its activity (Scheme 2).¹⁸Similarly, abnormal

carbene binding was observed upon PꢀP bond cleavage of white

phosphorus by 1a/B(C₆F₅)₃.^23,24

To block this deactivation path, the dihydro congener of 1a, the

imidazolidin-2-ylidene 1b, was employed, which was also success-

fully used for H₂and THF activation in combination with B-

(C₆F₅)₃(Scheme 2).²⁰Surprisingly, however, all attempts to

isolate a normal carbene-borane adduct from solutions of 1b/

B(C₆F₅)₃failed and again aﬀorded the abnormal adduct 4 together

with equimolar amounts of the corresponding imidazolidinium

hydroborate 3b. Although unexpected, the propensity of this

system to undergo self-dehydrogenation indicated the potential

of carbene-borane FLPs to act as stoichiometric dehydrogenation

reagents,²⁵and this has indeed been demonstrated for the pair 1a/

B(C₆F₅)₃through the preparation of N-heterocyclic germylenes

by H₂or, more precisely, H⁺/H^ꢀextraction.²⁶

’ RESULTS AND DISCUSSION

Lewis Pair 1,3-Bis(1-adamantyl)imidazolin-2-ylidene (1c)/

B(C₆F₅)₃. As previously described, the carbenes 1a and 1b

bearing N-tert-butyl substituents cleanly effect heterolytic clea-

vage of THF and H₂in combination with B(C₆F₅)₃, and, for

steric reasons, similar reactivity was expected for 1,3-bis

(1-adamantyl)imidazolin-2-ylidene (1c). Hence, mixing 1c with

B(C₆F₅)₃in THF afforded the imidazolium-borate zwitterion 5

in 93% yield as a white crystalline solid. The formation of 5 can be

rationalized by nucleophilic attack of the carbene at the Lewis

acidꢀbase adduct [(THF)B(C₆F₅)₃], followed by ring-opening

of the THF ligand. The presence of a (CH₂)₄O moiety is

indicated by the four methylene resonances at 1.68, 1.95, 3.23,

and 3.54 ppm in the ¹H NMR spectrum and at 27.6, 28.6, 31.6,

and 62.7 ppm in the ¹³C NMR spectrum. The ¹¹B NMR

resonance at ꢀ1.7 ppm falls in the range observed for similar

compounds obtained from carbene- and phosphine-borane

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Scheme 3

Scheme 4

pairs.^6,18,20The zwitterionic nature of 5 was also confirmed by

X-ray diffraction analysis (Figure 2), and the structural parameters

with C11ꢀN1 = 1.3631(17) Å, C11ꢀN2 = 1.3552(17) Å, and

N1ꢀC11ꢀN2 = 107.76(12)ꢀ are typical of imidazolium salts.¹⁵

The ring-opened THF molecule gives rise to a BꢀO bond of

1.446(2) Å and a C11ꢀC14 bond of 1.494(2) Å. Note however

that part of the (CH₂)₄O chain is disordered (see Experimental

Section).

Figure 3. ORTEP diagram of 8 with thermal displacement parameters

drawn at 50% probability. Selected bond lengths [Å] and angles [deg]:

C1ꢀN1 1.345(3), C1ꢀN2 1.338(3), C2ꢀC3 1.338(4), C1ꢀC28

1.479(3), BꢀO 1.468(3), C31ꢀO 1.412(3); N1ꢀC1ꢀN2 106.8(2),

OꢀBꢀC41 114.7(2), OꢀBꢀC51 104.93(19), OꢀBꢀC61 106.4(2).

The reactivity of 1c/B(C₆F₅)₃indicated the potential of this

Lewis pair for H₂activation, and purging a toluene solution

containing equimolar amounts of 1c and B(C₆F₅)₃with H₂

at ꢀ30 ꢀC resulted in the formation of a white precipitate. After

stirring for 8 h under a static H₂atmosphere, the imidazolium

hydroborate salt 6 was isolated by ﬁltration in 89% yield

(Scheme 3). In comparison with the carbenes 1a and 1b, the

reaction seems to be somewhat less eﬃcient, since almost quanti-

tative amounts of 3a and 3b were usually obtained within 2 h.^18,20

In the ¹H NMR spectrum, the resonances pertaining to the cation

are found at 8.29 and 7.47 ppm forthe twotypesof NCH hydrogen

atoms, along with three resonances at 1.78, 2.10, and 2.30 ppm for

the adamantyl groups, whereas the anion gives rise to a broad

quartet at 3.58 ppm. Accordingly, a doublet is observed in the

In agreement with the abnormal binding mode, the ¹H and ¹³

C

NMR spectra reveal the presence of two diﬀerent imidazole

CꢀH groups with signals at 6.84 and 8.07 ppm and at 123.7 and

128.2 ppm, respectively. These spectroscopic data are in excel-

lent agreement with those established for 4,¹⁸conﬁrming that 7

constitutes another example of an abnormal NHC main group

complex,³⁰which are much rarer than related abnormal transi-

tion metal NHC complexes.³¹

Lewis Pair 1,3-Bis(2,6-diisopropylphenyl)imidazolin-2-yli-

dene (1d)/B(C₆F₅)₃. The Lewis pair 1d/B(C₆F₅)₃had already

been studied and shown to form the normal adduct

[(1d)B(C₆F₅)₃] (9), which proved unreactive toward dihydro-

gen or olefins.¹⁹Independently, we also studied this pair and

initially treated a THF solution of the borane with 1d to isolate

the zwitterion 8 in quantitative yield as a white solid (Scheme 4).

The bridging (CH₂)₄O moiety gives rise to four methylene

resonances in the ¹H (1.19, 1.41, 2.62, 2.84 ppm) and ¹³C NMR

spectra (25.1, 25.7, 33.0, 63.9 ppm), and the ¹¹B NMR resonance

at ꢀ2.6 ppm is similar to those established for 2a, 2b, and 5. The

molecular structure of 8 was additionally established by X-ray

diffraction analysis (Figure 3), confirming its imidazolium-borate

character with CꢀN bond lengths of C1ꢀN1 1.345(3) Å and

C1ꢀN2 1.338(3) Å and with an N1ꢀC1ꢀN2 angle of

106.8(2)ꢀ. The BꢀO bond length is 1.468(3) Å, which is slightly

longer than in 5 [1.447(2) Å].

1

¹¹B NMR spectrum at ꢀ25.2 ppm, with a J(¹H,¹¹B) coupling

constant of 90 Hz. In addition, the HB(C₆F₅)₃anion generates

three ¹⁹F NMR resonances at ꢀ133.8, ꢀ164.6, and ꢀ167.5 ppm

for the ortho, para and meta ﬂuorine atoms, respectively.

Similar to its tert-butyl congener 1a,¹⁸1c does not form an

isolable normal adduct with B(C₆F₅)₃, and attempts to crystallize

such an adduct under various conditions either led to fractional

crystallization of the two components or indicated formation of

the abnormal adduct 7, in which the B(C₆F₅)₃moiety is attached

to the 4-position of the imidazole heterocycle (Scheme 3). 7

could be isolated in good yield (79%) as a white solid from a

toluene solution of 1c/B(C₆F₅)₃after stirring for 12 h and

precipitation with hexane. Solutions of 7 are inert toward H₂, but

slow decomposition was observed in chlorinated solvents and in

THF solution. The ¹¹B NMR resonance at ꢀ15.4 ppm is in

agreement with the presence of a boron atom in a tetrahedral

environment, and the ¹⁹F NMR spectrum of 7 exhibits six signals

each for the ortho and meta ﬂuorine atoms and three signals

for the para ﬂuorine, which can be explained by hindered rota-

tion around all four BꢀC bonds, rendering the 15 ﬂuorine

atoms inequivalent at room temperature on the NMR time scale.

The observation that the carbene 1d is not able to replace the

THF ligand in the solvate complex [(THF)B(C₆F₅)₃], but eﬀects

CꢀO bond cleavage with formation of 8, indicated the potential

presence of an FLP system. Mixing the two components in

toluene, however, aﬀorded the normal adduct 9 as a white solid

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Figure 5. ORTEP diagram of 1g with thermal displacement parameters

drawn at 50% probability. Selected bond lengths [Å], bond angles and

torsion angles [deg]: N1ꢀC1 1.3677(10), N2ꢀC1 1.3661(10), C2ꢀC3

1.3548(11), C2ꢀCl1 1.7087(8), C3ꢀCl2 1.7087(8); N1ꢀC1ꢀN2

104.52(7); C1ꢀN2ꢀC8-C10 ꢀ4.1(1), C1ꢀN1ꢀC4-C6 0.9(1).

Figure 4. ORTEP diagram of 9 with thermal displacement parameters

drawn at 50% probability. Selected bond lengths [Å] and angles [deg]:

BꢀC1 1.684(2), C1ꢀN1 1.3672(18), C1ꢀN2 1.3760(17), C2ꢀC3

1.334(2); N1ꢀC1ꢀN2 104.03 (12), C1ꢀBꢀC31 109.59(11),

C1ꢀBꢀC41 108.79(11), C1ꢀBꢀC51 112.85(12).

Scheme 6

Scheme 5

spectroscopy, and the ¹³C NMR resonance of the carbene carbon

atom is found at 213.0 ppm in the typical carbene range,¹⁶ⁱalbeit at

slightly higher field than observed for the related 1,3-diaryl-4,5-

dichloro carbenes.³²Single crystals suitable for an X-ray diffraction

analysis were obtained by sublimation, and the molecular structure

(Figure 5), with approximate C_2vsymmetry (rms deviation

0.03 Å), reveals structural parameters within the imidazole ring

[C1ꢀN1 = 1.3677(10) Å, C1ꢀN2 = 1.3661(10) Å, N1ꢀC1ꢀN2

= 104.52(7)ꢀ] that are in good agreement with those established

for the two related carbenes.³²It should be noted, however,

that the NꢀCꢀN angle in 1g is 3ꢀ larger than in the 1,3-diaryl

congeners, which could be ascribed to a stronger steric repulsion

between the chlorine and tert-butyl substituents. Since the tBu

groups are forcedto adopta staggered conformation with regardto

the CꢀCl axes, one methyl group of each substituent is located in

the imidazole plane with the corresponding C-Me vector aligned

with the carbene lone pair. Consequently, 1g could be rated as a

sterically more demanding carbene ligand than 1a.

Although the steric properties of 1g might aﬀord an FLP

system with a higher degree of frustration, lower reactivity

should be expected, since the electron-withdrawing nature of

the chlorine substituents usually aﬀords a substantially reduced

basicity and proton aﬃnity.³³These factors might account for

the observation that an equimolar THF solution of 1g and

B(C₆F₅)₃does not give any product with a ring-opened THF

molecule; slow decomposition was observed instead. It was

anticipated that blocking the 4,5-positions in 1g might render

the isolation of a carbene-borane adduct [(1g)B(C₆F₅)₃] possi-

ble; however, all attempts have failed so far, and mixing 1g with

the borane in benzene or toluene led to the separation of an

intractable red oil, which could not be suﬃciently characterized

because of rapid decomposition upon dissolution in more polar

solvents. Nonetheless, the FLP 1g/B(C₆F₅)₃proved capable of

heterolytic dihydrogen cleavage, and purging a toluene solution

with H₂resulted in the slow formation of a white precipitate,

containing the imidazolium-hydroborate 10 (Scheme 6). NMR

spectroscopic characterization also indicated the presence of the

[(HO)B(C₆F₅)₃]^ꢀanion, which might stem from moisture and

after precipitation with pentane. As previously described,¹⁹this

compound is not indeﬁnitely stable in solution at room tempera-

ture and is not capable of cleaving dihydrogen. The X-ray crystal

structure of 9 had also been previously determined, at room

temperature in the triclinic space group P1, and a C_carbeneꢀB bond

length of 1.663(5) Å was found.²¹In our hands, the compound

crystallized in the monoclinic space group P2₁/c from a toluene/

pentane mixture at ꢀ35 ꢀC, and the resulting molecular structure

at 100 K is shown in Figure 4 for comparison. The molecular

conformations of both forms are closely similar except for slight

diﬀerences in the orientation of the Dipp groups, but the

C_carbeneꢀB bond length of 1.684(2) Å is longer than previously

determined for 9 (vide supra) and for the adduct [(1f)B(C₆F₅)₃]

[1.6407(16) Å] containing the sterically less demanding carbene

1,3,4,5-tetramethylimidazolin-2-ylidene (1f).²⁷Both forms of 9

are characterized by substantial displacements of the substituents

out of the central ringplane (in our structure ꢀ0.30 Å for B, 0.17 Å

for C4, and 0.33 Å for C16), reﬂecting the steric overcrowding.

Lewis Pair 1,3-Di-tert-butyl-4,5-dichloroimidazolin-2-yli-

dene (1g)/B(C₆F₅)₃. To identify other possibilities of blocking

FLP deactivation by CꢀH activation and abnormal adduct

formation, we aimed at the synthesis of the carbene 1g, bearing

two chlorine atoms in the 4,5-positions. In a similar fashion as

described for the preparation of 1,3-diaryl-4,5-dichloroimidazolin-

2-ylidenes (aryl = 2,4,6-trimethylphenyl, 2,6-diisopropylphenyl),³²

the new carbene 1g could be obtained from the chlorination

reaction of1a with 2 equiv of carbon tetrachloride at 40ꢀC in THF

(Scheme 5). After careful evaporation of the solvent and sublima-

tion at 60 ꢀC under high vacuum, 1g was isolated in moderate yield

(43%). In contrast to its 1,3-aryl congeners, 1g is not air-stable; it

decomposes rapidly upon exposure to air and also appears to be

light-sensitive at room temperature. 1g was characterized by NMR

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Scheme 7

Figure 6. ORTEP diagram of 1h with thermal displacement parameters

drawn at 50% probability. Selected bond lengths [Å], bond angles and

torsion angles [deg]: N1ꢀC1 1.3638(16), N2ꢀC1 1.3643(16), C2ꢀC3

1.3572(19), C2ꢀC8 1.4963(17), C3ꢀC9 1.4990(18); N1ꢀC1ꢀN2

103.40(10); C1ꢀN1ꢀC4-C5 18.4(2), C1ꢀN2ꢀC10-C11 16.1(2).

H₂O cleavage. Therefore, 10 was only isolated in 40% yield.

Optimization of the reaction conditions was not attempted, but

the results are suﬃcient to demonstrate the inferior dihydrogen

activation properties of 1g.

Lewis Pair 1,3-Di-tert-butyl-4,5-dimethylimidazolin-2-yli-

dene (1h)/B(C₆F₅)₃. Another possibility of blocking side reac-

tions at the 4/5-positions of carbenes such as 1a was recently

introduced by Polyakova and co-workers,³⁴who reported a new

synthetic route to 4,5-disubstituted N,N'-di-tert-alkylimidazolin-

2-ylidenes, including the carbene 1h as the dimethyl derivative of

1a. NMR spectroscopic pK_ameasurements revealed that 1h is

the strongest imidazolin-2-ylidene base known to date (pK_a=

24.8).^33,35Thus, the use of 1h together with B(C₆F₅)₃should

afford an FLP with an even higher reactivity than that of 1a/

B(C₆F₅)₃and should additionally prevent deactivation by ab-

normal carbene binding. As previously described,³⁴1h was isolated

in a multistep synthesis as a colorless liquid, which solidifies when

stored at ꢀ30 ꢀC. Single crystals of 1h could be obtained from

pentane solution at ꢀ30 ꢀC, and X-ray diffraction analysis provided

the molecular structure (approximate symmetry C_2vwith rms

deviation 0.2 Å) shown in Figure 6. The structural parameters

within the imidazole ring [N1ꢀC1 = 1.3638(16) Å, N2ꢀC1 =

1.3643(16) Å, N1ꢀC1ꢀN2 = 103.40(10)ꢀ] compare well with

those established for 1a³⁶and 1g (vide supra); the NꢀCꢀN angle

of 103.40(19)ꢀ lies between the values of 102.2(5)ꢀ and

104.52(7)ꢀ found in 1a and 1g, respectively. In a similar fashion

as observed for the solid state structure of 1g and for that of the

corresponding 1,3-di-tert-butyl-4,5-dimethylimidazolium triflate

Figure 7. ORTEP diagram of 11 with thermal displacement parameters

drawn at 50% probability. Selected bond lengths [Å] and angles [deg]:

C1ꢀN1 1.3505(18), C1ꢀN2 1.3553(19), C2ꢀC3 1.356(2), C1ꢀC14

1.4971(19), BꢀO 1.4592(18), C17ꢀO 1.4094(17); N1ꢀC1ꢀN2

108.57(12), OꢀBꢀC30 104.42(11), OꢀBꢀC18 109.34(11),

OꢀBꢀC24 113.81(12).

bonds of 1.3505(18) (C1ꢀN1) and 1.3553(19) Å (C1ꢀN2)

together with an N1ꢀC1ꢀN2 angle of 108.57(12)ꢀ that are typical

of imidazolium salts (Figure 7).¹⁵One of the “protective” tert-butyl

groups retains its position, with C5ꢀC4ꢀN1ꢀC1 13.8(2)ꢀ, but

the other rotates to a position with steric crowding between C9 and

C11; C1ꢀN2ꢀC10ꢀC13 ꢀ53.3(2)ꢀ but C3ꢀN2ꢀC10ꢀC11

6.8(3)ꢀ. The atom C14 lies 0.28 Å out of the ring plane.

In line with the reactivity observed above, 1h/B(C₆F₅)₃is a

viable FLP for heterolytic dihydrogen cleavage; purging an

equimolar toluene solution of the carbene and the borane with

H₂resulted in quantitative formation of the imidazolium-hydro-

borate 12, which, in contrast to other hydroborate salts, did not

precipitate from solution, but was isolated as a white solid in 94%

yield after evaporation of toluene, suspension in pentane, and

ﬁltration. The BꢀH unit of the hydroborate anion gives rise to a

doublet at ꢀ25.1 ppm in the ¹¹B NMR spectrum with ¹J(¹H,¹¹B)

= 93 Hz and consequently to a broad quartet at 3.58 ppm in the

¹H NMR spectrum, which also exhibits singlets at 1.67, 2.41, and

7.99 ppm for the CMe₃, Me, and CH hydrogen atoms of the

imidazolium cation. Unfortunately, single crystals of 12 isolated

from a saturated CH₂Cl₂/hexane solution at ꢀ30 ꢀC suﬀered

from severe disorder of the [HB(C₆F₅)₃]^ꢀion, and determina-

tion of an X-ray crystal structure was not possible.

[1h H][CF₃SO₃],³⁴steric interaction with the methyl groups in

3

the 4,5-positions in 1h enforces a staggered conformation of the

tert-butyl groups that results in a particularly efficient shielding of

the carbene carbon atom. A further symptom of strain in 1h is

furnished by the out-of-plane deviations of the substituents at N1

and N2, by about 0.2 Å to opposite sides of the ring plane.

In view of the steric and electronic properties of 1h, clean THF

activation was anticipated upon addition of 1h to a THF solution

of B(C₆F₅)₃, and indeed, instantaneous formation of the ring-

opened product 11 was observed, which could be isolated in 91%

yield by crystallization from THF/pentane solution (Scheme 7).

The ¹¹B NMR resonance at ꢀ2.4 ppm is in agreement with the

presence of an sp³-hybridized BꢀO moiety; characteristic multi-

plets for the (CH₂)₄O chain are found at 1.61, 1.76ꢀ1.82, 3.21,

and 3.54 ppm together with two singlets at 1.79 and 2.43 ppm for

the CMe₃and 4,5-Me groups, respectively. X-ray diﬀraction

analysis conﬁrmed the formation of zwitterionic 11; the molec-

ular structure features a BꢀO bond of 1.4592(18) Å and CꢀN

Again, attempts to isolate a normal carbene-borane adduct

proved unsuccessful, despite the blocking of the 4,5-positions. In

contrast to 1a/B(C₆F₅)₃mixtures,^18,19toluene solutions of 1h

and B(C₆F₅)₃can be stored at ꢀ30 ꢀC for prolonged periods of

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Scheme 8

Figure 8. ORTEP diagram of 13 with thermal displacement parameters

drawn at 50% probability. Selected bond lengths [Å] and angles [deg]:

N1ꢀC1 1.3365(16), N1ꢀC2 1.4774(16), N2ꢀC1 1.3483(16),

N2ꢀC4 1.4815(15), BꢀO 1.4544(15), OꢀC16 1.4139(14),

C1ꢀC13 1.5059(16), C2ꢀC3 1.5077(19), C3ꢀC4 1.501(2),

N1ꢀC1ꢀN2 121.22(11), OꢀBꢀC23 108.97(9), OꢀBꢀC17

104.11(9), OꢀBꢀC29 114.51(10).

time without marked decomposition, and these solutions re-

tained the capability of H₂cleavage even after two weeks. An

adduct, however, could not be isolated from these solutions,

and fractional crystallization of the borane as a ﬂuﬀy white solid

was observed instead. In contrast, yellowish toluene solutions

of 1h/B(C₆F₅)₃quickly darkened at room temperature, and

a dark red oil separated. After washing with pentane, a dark

red solid was isolated, the NMR spectroscopic characterization

of which indicated the formation of various species. On one

occasion, we were able to isolate red single crystals from a

diﬀusion reaction in pentane at room temperature, and X-ray

diﬀraction analysis revealed the formation of the imidazolium-

The oil solidifies to a colorless crystalline solid in the refrigerator

at ꢀ30 ꢀC, and its ¹³C NMR spectrum exhibits the signal for the

carbene carbon atom at 237.5 ppm, which falls in the expected

range.^40,41

FLP reactivity of the 1i/B(C₆F₅)₃pair was initially checked by

dissolution of both components in THF, and the zwitterion 13

was obtained in satisfactory yield (67%) as a white solid

(Scheme 8). The ¹¹B NMR resonance at ꢀ2.8 ppm is in good

agreement with the chemical shifts observed for the related

imidazolium-borates obtained by ring-opening of THF (vide

supra). Single-crystals suitable for an X-ray diﬀraction study were

obtained from CH₂Cl₂/pentane solution at ꢀ30 ꢀC, and the

molecular structure is shown in Figure 8. The six-membered

pyrimidine ring displays a 1,3-diplanar conformation whereby

the four atoms N1, N2, C1, C2 are approximately coplanar

(mean deviation 0.03 Å), and C3 and C4 lie 0.99 and 0.43 Å

respectively out of the plane in the same direction. The atom C1

and its three direct substituents are coplanar (mean deviation

0.02 Å). The C1ꢀN1 and C1ꢀN2 bond lengths of 1.3365(16)

and 1.3483(16) Å are very similar to those in the corresponding

1,3-di-tert-butylimidazolium species 2a and 2b,^18,20whereas the

N1ꢀC1ꢀN2 angle of 121.22(11)ꢀ is signiﬁcantly more obtuse.

The bridging THF fragment exhibits an OꢀB bond length of

1.4544(15) Å and a C1ꢀC13 bond length of 1.5059(16) Å.

Since the ring-opening of THF provided evidence for the

presence of an FLP system, heterolytic dihydrogen cleavage with

1i/B(C₆F₅)₃was performed. A solution of B(C₆F₅)₃in toluene

was cooled to ꢀ78 ꢀC, purged with H₂for some minutes, and a

toluene solution of carbene 1i was subsequently added. The

reaction mixture was kept under one atm of H₂and quickly

brought to room temperature. After 1 h, the pyrimidinium

hydroborate 14 was isolated in 70% yield (Scheme 8). We

noticed that the FLP 1i/B(C₆F₅)₃showed a particularly high

propensity to form the corresponding hydroxyborate salt con-

taining the [(HO)B(C₆F₅)₃]^ꢀanion. Therefore, it is mandatory

to carry out these reactions in silylated glassware and to employ

solvents dried over potassiumꢀsodium alloy; without these

precautions, the pyrimidinium hydroxyborate was formed as

the predominant species. The hydroborate salt 14 shows the

ﬂuoroborate [1h H][FB(C₆F₅)₃], indicating that CꢀF activa-

3

tion reactions might be involved in mutual carbene-borane

deactivation (vide infra, see Supporting Information for a pre-

sentation of the X-ray crystal structure). The [FB(C₆F₅)₃]^ꢀion

can also be detected spectroscopically as the main ﬂuorine-

containing compound, and the ¹¹B NMR resonance at ꢀ2.1 ppm

1

(in THF-d₈) with a J(¹¹B,¹⁹F,) coupling constant of 65 Hz

and also the ¹⁹F NMR resonances are in good agreement with

those previously established.³⁷Nonetheless, the complexity of

the NMR spectra suggested the interference of several compet-

ing reactions and precluded us from assigning a conclusive

deactivation path.

Lewis Pair 1,3-Di-tert-butyltetrahydropyrimidin-2-yli-

dene (1i)/B(C₆F₅)₃. Recently, six-membered NHCs of the

tetrahydropyrimidin-2-ylidene type have received considerable

interest as ligands in organotransition metal chemistry,^38ꢀ40

since these so-called ring-expanded N-heterocyclic carbenes^40,41

exhibit higher basicities than their five-membered counterparts.³³

It has been suggested that, in the absence of other influences such

as electron-donating or -withdrawing substitutents, the widening

of the NCN angle has the strongest impact on the increase of the

basicity, rather than electron delocalization. At the same time, six-

membered NHCs can be considered as sterically more demand-

ing ligands than five-membered NHCs for simple geometric

reasons. Thus, we aimed at the synthesis of the previously

unknown 1,3-bis-tert-butyltetrahydropyrimidin-2-ylidene (1i)

to obtain a bulkier and more basic analogue of 1a (Scheme 8).

The corresponding pyrimidinium bromide was prepared accord-

ing to the procedure published by Buchmeiser et al.,³⁹and the

free carbene could be isolated as a colorless oil in good yield by

deprotonation with sodium hexamethyldisilazide, NaN(SiMe₃)₂.

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Figure 9. ORTEP diagram of 14 with thermal displacement parameters

drawn at 50% probability. Selected bond lengths [Å] and angles [deg]:

N1ꢀC1 1.3164(19), N2ꢀC1 1.3165(19), N1ꢀC2 1.4758(19),

N2ꢀC4 1.475(2), C2ꢀC3 1.512(2), C3ꢀC4 1.511(2), N1ꢀC1ꢀN2

126.02(14), C13ꢀBꢀC25 114.82(12), C13ꢀBꢀC19 112.52(12),

C25ꢀBꢀC19 107.40(12).

Figure 10. ORTEP diagram of 15 with thermal displacement param-

eters drawn at 50% probability. Selected bond lengths [Å] and angles

[deg]: N1ꢀC1 1.3187(14), N2ꢀC1 1.3173(15), BꢀO 1.4617(14),

OꢀC31 1.4197(13), N1ꢀC2 1.4740(14), N2ꢀC4 1.4771(14),

C2ꢀC3 1.5179(17), C3ꢀC4 1.5161(16), C31ꢀC32 1.5207(16),

OꢀBꢀC19 108.46(8), OꢀBꢀC13 106.24(8), OꢀBꢀC25 114.38(9),

N2ꢀC1ꢀN1 125.75(10).

characteristic doublet at ꢀ25.2 ppm in the ¹¹B NMR spectrum

1

with a J(¹H,¹¹B) coupling constant of 90 Hz, together with a

1

broad quartet at 3.59 ppm in the H NMR spectrum. The

resonance for N₂CH hydrogen atom is observed at 7.82 ppm.

14 was additionally characterized by X-ray diﬀraction analysis

(Figure 9), conﬁrming the presence of a pyrimidinium cation and

a tetrahedral [HB(C₆F₅)₃]^ꢀanion, in which the CꢀBꢀC angles

range between 107.40(12)ꢀ and 114.82(12)ꢀ. The six-membered

pyrimidinium ring now displays a 1,2-diplanar (“sofa”) confor-

mation, in which C3 lies 0.68 Å out of the plane of the other ﬁve

atoms (mean deviation 0.02 Å), and thus has approximate mirror

symmetry (rms deviation 0.12 Å). In the cation, the C1ꢀN1

and C1ꢀN2 bond lengths and the N1ꢀC1ꢀN2 angle of

126.02(14)ꢀ are in agreement with those established for other

pyrimidinium salts.^{38a,40,41e,42}Similar to 3a and 3b^18,20and

¹¹B NMR does not display the characteristic doublet at about

ꢀ25 ppm, and a resonance at ꢀ2.9 ppm is observed instead,

which can be assigned to a tetrahedral [(CH₃CH₂O)B(C₆F₅)₃]^ꢀ

anion. The formation of the pyrimidinium ethoxyborate 15

(Scheme 8) was also conﬁrmed by X-ray diﬀraction analysis.

The molecular structure is shown in Figure 10; the structural

parameters of the cation in 15 are similar to those established for

the cationin14. The borate anion exhibits OꢀBꢀC bondangles in

the range from 106.2ꢀ to 114.3ꢀ. In addition to various CꢀH

F

3 3 3

contacts, the oxygen atom of the ethoxy group displays O HꢀC

3 3 3

contacts to hydrogen atoms at C2 and C4 of 2.33 and 2.71 Å,

respectively. Since 15 was isolated in 71% yield from diethyl ether

solution, we propose that its formation proceeds via deprotonation

of the etherate [(Et₂O)B(C₆F₅)₃] by 1i in the β-position, followed

by release of ethylene. In principle, ethylene could also be captured

by the FLP; however, the large excess of the solvent diethyl ether

prevents the formation of a CH₂CH₂-bridged zwitterion as was

previously observed for PꢀB Lewis pairs.^7a

unlike the BH HP contact of 2.75 Å observed for the salt

3 3 3

[(tBu)₃PH][HB(C₆F₅)₃],⁴the BꢀH and CꢀH units in 14 are

not oriented toward each other. Instead, both ions display

numerous interionic CꢀH F contacts ranging from 2.25 to

3 3 3

2.63 Å. In contrast to 3a and 3b, no intermolecular contacts

involving the BꢀH moiety are observed.

As mentioned above, the combinations 1a/B(C₆F₅)₃and

1b/B(C₆F₅)₃do not aﬀord stable Lewis adducts, but formation

of the abnormal adduct 4 and, in case of 1b, dehydrogenation

is observed instead.^18,20To investigate whether the pair 1i/

B(C₆F₅)₃exhibits similar reactivity, equimolar amounts of both

compounds were dissolved in toluene. Within minutes, a yellow

oil formed, and the reaction mixture was stirred for 1 h. After

evaporation, the residue was examined by NMR spectroscopy,

revealing that the pyrimidinium hydroborate 14 had formed as

the main product, presumably by self-dehydrogenation. Unfor-

tunately, we were able to identify neither any dehydro species nor

Lewis Pair 1,3-Bis(2,6-diisopropylphenyl)tetrahydropyrimidin-

2-ylidene (1j)/B(C₆F₅)₃. The Dipp-substituted imidazolin-2-yli-

dene 1d was shown to form the normal adduct 9 with B(C₆F₅)₃,

despite its ability to activate the CꢀO bond in [(THF)B(C₆F₅)₃]

(vide supra, Scheme 4). Since the corresponding six-membered

pyrimidin-2-ylidene 1j can be expected to confer a higher steric

impact toward the Lewis acid, this carbene was also employed for

THF and H₂activation. 1j was prepared from the corresponding

pyrimidinium tetrafluoroborate by deprotonation with KOtBu,

whereas previous protocols involve KN(SiMe₃)₂or LiN(SiMe₃)₂

as bases.^40,41aThe free carbene was crystallized from a saturated

solution in THF at ꢀ35 ꢀC to obtain crystals suitable for an X-ray

diffraction analysis. The molecule displays crystallographic mirror

symmetry, whereby, for example, the N1ꢀC1ꢀN2moiety and the

ipso- and para-carbon atoms of the Dipp-substituents (Figure 11)

lie in the mirror plane. However, the methylene groups at C2, C3,

and C4 are disordered and located either below or above the

crystallographic mirror plane, leading to two alternative sofa

conformations of the ring. The phenyl groups adopt a perfectly

perpendicular orientation toward the NꢀCꢀN plane. The

1

any other byproduct, since the H, ¹⁹F and ¹¹B NMR spectra

indicated the presence of an intractable mixture of compounds.

Therefore, we chose to perform the reaction in a diﬀerent

solvent; 1i and B(C₆F₅)₃were dissolved in diethyl ether. Within

30 min, a white precipitate had formed, which was collected by

1

ﬁltration. The H NMR spectrum indicated the presence of a

pyrimidinium cation, with an N₂CH resonance at 7.83 ppm, and

of an OCH₂CH₃moiety with a triplet at 1.02 ppm (³J_HH= 6.8 Hz)

together with a broad quartet at 3.14 ppm. The latter two signals

are shifted to higher ﬁeld compared to the Et₂O solvent. The

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Figure 12. ORTEP diagram of 16 with thermal displacement param-

eters drawn at 50% probability. Selected bond lengths [Å] and angles

[deg]: N1ꢀC1 1.3255(18), N1ꢀC2 1.4780(18), N2ꢀC1 1.3314(18),

N2ꢀC4 1.4817(18), BꢀO 1.4581(19), OꢀC32 1.4078(17), C1ꢀC29

1.5040(19), C2ꢀC3 1.508(2), C3ꢀC4 1.509(2), N1ꢀC1ꢀN2

120.99(13), OꢀBꢀC33 105.02(12), OꢀBꢀC39 108.71(12),

OꢀBꢀC45 114.83(12).

Figure 11. ORTEP diagram of 1j with thermal displacement param-

eters drawn at 30% probability. Selected bond lengths [Å] and angles

[deg]: N1ꢀC1 1.338(3), N1ꢀC2 1.488(4), C2ꢀC3 1.531(13),

C3ꢀC4 1.528(14), N2ꢀC4 1.478(3), N2ꢀC1 1.347(3), N1ꢀC1ꢀN2

115.48(19). The carbon atoms C2, C3, and C4 are disordered across a

mirror plane; only one conformation is displayed.

Scheme 9

ﬂuoroborate 17. In addition, contamination with [(HO)B-

(C₆F₅)₃]^ꢀsalts was observed, despite careful silylation of the

glassware.

In absence of H₂, the formation of a gel-like precipitate was

observed, and 17 could be isolated in 75% yield by ﬁltration and

subsequent extraction of byproduct with Et₂O and hexane

1

(Scheme 9). The H NMR spectrum displays relatively broad

resonances, indicating hindered rotation, presumably around the

N₂CꢀC₆F₄bond. In the ¹³C NMR spectrum, the resonance for

the N₂C carbon atom is found at 158.0 ppm and, as expected,

shifted to signiﬁcantly higher ﬁeld compared to the free carbene

1j (244.9 ppm). The ¹⁹F NMR spectrum exhibits two signals for

the bridging C₆F₄unit at ꢀ133.0 and ꢀ135.3 ppm, partially

overlapping with the resonance assigned to the ortho-C₆F₅

ﬂuorine atoms, whereas the meta- and para-C₆F₅peaks are

observed at ꢀ166.4 and ꢀ161.9 ppm. The broad resonances

for the BꢀF moiety appear at ꢀ193.0 and ꢀ0.7 ppm in the ¹⁹F

and ¹¹B NMR spectra, respectively. The latter resonances are in

good agreement with those reported for related phosphonium

borates that have been isolated from the reactions of B(C₆F₅)₃

with sterically encumbered phosphines PR₃(R = Et, nBu, iPr, Cy,

Ph, FC₆H₄, MeOC₆H₄) and R₂PH (R = tBu, Cp, Cy, Ph

C₆H₂Me₃-2,4,6).^1,21,28aIn a similar fashion as described for these

species, the formation of 17 can be rationalized by nucleophilic

attack of the carbene at the para-position of a C₆F₅ring, followed

by ﬂuoride migration to boron.

The molecular structure of 17 was also conﬁrmed by X-ray

diﬀraction (Figure 13); the pyrimidinium ring again displays the

sofa conformation, and the BꢀF1 and BꢀC32 bond lengths of

1.4185(15) and 1.6579(18) Å fall in the expected range com-

pared to related phosphonium ﬂuoroborates.^1,21,28aThe angle

between the planes N1,C1,N2,C29 and C29ꢀ34 is 63ꢀ, corre-

sponding to a more perpendicular rather than a coplanar

orientation between the carbene moiety and the C₆F₄ring.

Lewis Pair Bis(diisopropylamino)methylene (1k)/B(C₆F₅)₃.

According to theoretical calculations,³³acyclic diaminocarbenes

such as 1k are among the most basic N,N-substituted carbenes,

and therefore, the reactivity of this so-called Alder-carbene toward

B(C₆F₅)₃was also studied.⁴⁵Mixing of 1k and B(C₆F₅)₃in toluene

afforded the normal adduct [(1k)B(C₆F₅)₃] (18) as an air-stable

white solid (Scheme 10). The ¹¹B NMR resoance at ꢀ14.1 ppm is

in good agreement with the chemical shift (ꢀ15.6 ppm) found for

corresponding N1ꢀC1ꢀN2 angle of 115.48(19)ꢀ and the

C1ꢀN1 and C1ꢀN2 bond lengths of 1.338(3) and 1.347(3) Å

are in good agreement with the structures of related free six-

membered carbenes,⁴¹butthedimensionsofdisorderedstructures

should be interpreted with caution.

As expected, the reaction of 1j with B(C₆F₅)₃in THF led to

the formation of the zwitterionic compound 16, which was

isolated in 76% yield (Scheme 9). The NMR spectroscopic data

and also the structural parameters of 16 that were established by

X-ray diﬀraction analysis of the dichloromethane hemisolvate

(Figure 12) are consistent with those of 13, even though the

presence of sterically less demanding Dipp-substituents aﬀords a

signiﬁcantly more acute N1ꢀC1ꢀN2 angle of 120.99(13) Å.

The ring again displays a sofa conformation.

In contrast to THF activation, ambiguous reactivity of 1j/

B(C₆F₅)₃toward dihydrogen was observed. Purging a toluene

solution of this Lewis pair with H₂produced the corresponding

pyrimidinium hydroborate in only small amounts as indicated by a

characteristic doublet in the ¹¹B NMR spectrum at ꢀ25.2 ppm

(¹J(¹¹B,¹H) = 90 Hz) together with a N₂CH ¹H NMR resonance

at 7.63 ppm. Although the yield of this product could be increased

by performing the reaction at ꢀ78 ꢀC, the NMR spectra are

dominated by resonances pertaining to the zwitterionic pyrimidinium

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Figure 14. ORTEP diagram of 18 with thermal displacement param-

eters drawn at 50% probability. Selected bond lengths [Å] and angles

[deg]: BꢀC1 1.711(2), C1ꢀN1 1.3585(17), C1ꢀN2 1.3736(16);

N1ꢀC1ꢀN2 120.69(12), N1ꢀC1ꢀB, N2ꢀC1ꢀB, C1ꢀBꢀC14

104.47(11), C1ꢀBꢀC20 114.61(11), C1ꢀBꢀC26 114.67(11).

Figure 13. ORTEP diagram of 17 with thermal displacement param-

eters drawn at 50% probability. Selected bond lengths [Å] and angles

[deg]: BꢀF1 1.4185(15), BꢀC32 1.6579(18), N1ꢀC1 1.3307(15),

N2ꢀC1 1.3324(16), C1ꢀC29 1.4942(16), N1ꢀC1ꢀN2 122.68(11),

N1ꢀC1ꢀC29ꢀC30 ꢀ65.18(16), N2ꢀC1ꢀC29ꢀC34 ꢀ62.36(15).

Table 1. Selected Energies [kcal mol^ꢀ1], Bond Lengths [Å],

3

Scheme 10

and Angles [deg] for the Formation of the Normal Carbene-

Borane Adducts [(1)B(C₆F₅)₃]^a

ΔE_el

ΔE_0K

ΔH_298K

ΔG_298K

d(CꢀB)

—(NCN)

1a

1b

1c

1d

1e

1g

1h

1i

ꢀ23.8

ꢀ20.1

ꢀ25.1

ꢀ51.5

ꢀ59.3

ꢀ10.6

ꢀ18.6

ꢀ11.5

ꢀ36.9

ꢀ57.6

ꢀ20.2

ꢀ16.4

ꢀ21.1

ꢀ47.8

ꢀ55.9

ꢀ7.5

ꢀ21.1

ꢀ17.3

ꢀ21.7

ꢀ48.8

ꢀ56.5

ꢀ8.4

ꢀ2.5

1.7

1.714

1.727

1.720

1.682

1.646

1.714

1.708

1.711

1.734

1.708

105.0

108.4

104.9

104.6

105.3

106.7

105.9

113.3

116.5

116.7

ꢀ2.2

ꢀ26.2

ꢀ40.6

10.6

the normal adduct [(1d)B(C₆F₅)₃] (9). At room temperature,

however, the ¹H, ¹³C, and ¹⁹F NMR spectra exhibit broad

resonances, indicating fluxional behavior on the NMR time scale,

which could be ascribed to hindered rotation around the CꢀN and

CꢀB bonds. A variable-temperature NMR study revealed that sharp

spectra can be recorded at ꢀ30 ꢀC, and for instance, the ¹H NMR

spectrum displays eight individual doublets for the eight methyl

groups, two of which show additional through-space coupling to

one ¹⁹F nucleus. In a similar fashion, 15 well-separated signals are

observed in the ¹⁹F NMR spectrum for the fluorine atoms of the

three C₆F₅rings (see Supporting Information for a presentation of

these spectra).

Recrystallization of 18 from a CH₂Cl₂/Et₂O/pentane solution

at 3 ꢀC aﬀorded single-crystals suitable for an X-ray diﬀraction

analysis. The carbene carbon atom C1 resides in an almost

perfectly trigonal-planar environment with an N1ꢀC1ꢀN2

angle of 120.69(12)ꢀ, which together with the C1ꢀN bond

distances of 1.3585(17) and 1.3736(16) Å only slightly deviates

from the structural parameters found for the free carbene 1k

[121.0(5)ꢀ, 1.363(6) Å, 1.381(6) Å].^45aIn contrast, the boron

atom is coordinated in a strongly distorted tetrahedral fashion

(Figure 14). Strikingly, a very long C1ꢀB bond length of

1.711(2) Å is observed, which signiﬁcantly exceeds the

C_carbeneꢀB distances found for the adducts [(1d)B(C₆F₅)₃]

[1.684(2) Å] and [(1f)B(C₆F₅)₃] [1.6407(16) Å] and falls in

the same range as the CꢀB distance calculated for the elusive

normal adduct [(1a)B(C₆F₅)₃].¹⁸These structural features

suggested that the formation of the adduct 18 might be reversible

and that suﬃcient amounts of the carbene and the borane might

be present in solution to observe FLP reactivity. However,

various attempts to eﬀect dihydrogen cleavage by a solution of

18 proved unsuccessful, and this observation is in line with its

calculated stability (vide infra, Table 1).

ꢀ14.4

ꢀ8.1

ꢀ15.4

ꢀ8.9

3.5

10.6

1j

ꢀ32.5

ꢀ53.6

ꢀ33.7

ꢀ54.6

ꢀ9.4

ꢀ35.1

1k

^aΔE_el= zero-point uncorrected M05-2X/6-311G(d,p) electronic en-

ergies, ΔE_0K= energies at 0 K, ΔH_298K= enthalpies at 298 K, ΔG_298K

Gibbs free energies at 298 K.

=

Theoretical Calculations of Carbene-Borane Adduct For-

mation. To rationalize the reactivity of the carbene-borane Lewis

pairs, the thermodynamics of adduct formation with B(C₆F₅)₃

were calculated for all carbenes that have been previously

employed (1a, 1b)^18ꢀ21or were used in this study (1c, 1d,

1gꢀ1k). In addition, adduct formation was also calculated for the

sterically least demanding carbene 1e, for which normal adduct

formation can be expected, since the analogous adduct

[(1f)B(C₆F₅)₃] is known (Figure 1). For all calculations, the

M05-2X functional developed by Zhao and Truhlar was applied,

which conveniently describes noncovalent interactions that can

be expected to significantly contribute to the overall binding

energies in these sterically encumbered systems.

46

Table 1

summarizes the zero-point uncorrected energies (ΔE_el) and also

the values for the energies at 0 K (ΔE_0K) and for the enthalpies

(ΔH_298K) and Gibbs free energies at 298 K (ΔG_298K) calculated

by statistical thermodynamics. Since the energetic differences

between the carbene-borane adducts is reproduced in a similar

fashion for all these energies, the following discussion will focus

on the ΔH_298Kand ΔG_298Kvalues only, and for clarity, a bar

diagram of these values is shown in Figure 15. Presentations of all

structures together with their Cartesian coordinates can be found

in the Supporting Information.

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(ꢀ33.7, ꢀ9.4 kcal mol^ꢀ1). Adduct [(1i)B(C₆F₅)₃] exhibits a

particularly high degree of frustration, since the adduct is

unstable for steric reasons, allowing full exploitation of the high

reactivity of the FLP combination B(C₆F₅)₃and 1i, which

represents the most basic N-heterocyclic carbene in this study.

Therefore, 1i/B(C₆F₅)₃showed a particularly high propensity

for THF, H₂, and even Et₂O activation, whereas in the absence of

substrates fast and uncontrolled decomposition was observed. In

contrast, the only moderately frustrated Lewis pair 1j/B(C₆F₅)₃

seems to undergo self-deactivation in a more orderly fashion, allowing

unambigous identiﬁcation of a CꢀF activation process, which might,

inter alia, also be relevant for other carbene-borane Lewis pairs.

Finally, the structurally characterized adduct [(1k)B(C₆F₅)₃]

containing the acyclic diaminomethylene 1k is conﬁrmed to be

thermodynamically stable (ꢀ54.6, ꢀ35.1 kcal mol^ꢀ1). It is

interesting to note that the formation of this adduct is almost

as exothermic as calculated for the adduct of the sterically least

hindered carbene 1e, although a signiﬁcantly longer C_carbeneꢀB

bond length of 1.708 Å is obtained, which nicely matches the

experimental value of 1.711(2) Å (Figure 14).

Figure 15. Bar diagram showing the enthalpies at 298 K (ΔH_298K) and

Gibbs free energies at 298 K (ΔG_298K) for the formation of the normal

carbene-borane adducts [(1)B(C₆F₅)₃].

As previously reported, normal adduct formation for the tBu-

substituted NHCs 1a and 1b is calculated to be exothermic by

ꢀ21.1 and ꢀ17.3 kcal mol^ꢀ1, and our inability to isolate the

adducts [(1a)B(C₆F₅)₃] and [(1b)B(C₆F₅)₃] could be ascribed

not only to their thermodynamic instability, but to the higher

stability of the abnormal adduct 4, which is formed in both cases.

Given that adduct formation is reversible, the small absolute values

of ΔG_298K(ꢀ2.5 and 1.7 kcal mol^ꢀ1) indicate that suﬃcient

amounts of free carbene and borane are present in solution to

observe FLP reactivity, for example, THF and H₂activation. As

expected from very similar spatial requirements, the values calcu-

lated for the diadamant-1-yl-substituted carbene 1c are almost

identical with those of 1a. In contrast, adduct formation is

calculated to be signiﬁcantly more strongly exothermic and

exergonic (ꢀ48.8/ꢀ26.2 kcal mol^ꢀ1) for the Dipp-derivative

1d, and consequently, the adduct [(1d)B(C₆F₅)₃] can be isolated

and structurally characterized (vide supra).^19,21The C_carbeneꢀB

bond length of 1.684(2) Å established in this publication is also

nicely reproduced (1.682 Å). This trend is followed upon further

reduction of the steric requirements, as is the case for 1e, and

[(1e)B(C₆F₅)₃] represents the most stable adduct in this series

(ꢀ56.5/ꢀ40.6 kcal mol^ꢀ1) and exhibits the shortest C_carbeneꢀB

bond distance (1.646 Å), which is also in excellent agreement with

the experimental value of 1.6407(16) Å in [(1f)B(C₆F₅)₃].²⁷

The structural characterization of the carbenes 1g and 1h

bearing chlorine or methyl groups in the 4,5-positions indicated

that these carbenes can be regarded as sterically more demanding

ligands in comparison with 1a and should therefore furnish less stable

adducts. This is theoretically conﬁrmed for [(1g)B(C₆F₅)₃] (ꢀ8.4, +

10.6 kcal mol^ꢀ1) and [(1h)B(C₆F₅)₃] (ꢀ15.4, +3.5 kcal mol^ꢀ1),

and the diﬀerences between these two adducts can be easiliy

ascribed to the smaller basicitiy and nucleophilicity of 1g as a

result of the electron-withdrawing chlorine substituents. Conse-

quently, the combination 1g/B(C₆F₅)₃showed an inferior

perfomance in THF and H₂activation compared to 1h/

B(C₆F₅)₃.

’ CONCLUSIONS

This study conﬁrmed the potential of frustrated carbene-

borane Lewis pairs for metal-free small molecule activation,

and various combinations containing N-heterocyclic carbenes

(NHCs) and B(C₆F₅)₃were shown to eﬀect CꢀO and HꢀH

bond cleavage. The degree of “frustration” and the ensuing

reactivity of these Lewis pairs can be eﬃciently tuned by variation

of the NHC type and its respective steric and electronic properties.

For most of these FLP systems, however, the cumulative Lewis

acidꢀbase strength leads to self-destruction in the absence of

substrates, and possible decomposition pathways involving CꢀH

and CꢀF bond activationwerediscovered. Thisbehavior might be

regarded as a drawback, but particularly reactive combinations

might nonetheless become useful stoichiometric reagents, for

example, for dehydrogenation. Furthermore, the reactivity of

carbene-borane Lewis pairs could be moderated by employing

less Lewis acidic boranes or other Lewis acids,^22,51and such

combinations are currently under investigation in our laboratories.

’ EXPERIMENTAL SECTION

Synthetic Details. Chemicals and Instrumentation. All opera-

tions with air and moisture-sensitive compounds were performed in a

glovebox under a dry argon atmosphere (MBraun 200B) or on a high

vacuum line using Schlenk techniques. Hydrogen gas was purchased

from Westfalen AG and passed through a P₄O₁₀column prior to use. All

solvents were purified by a solvent purification system from MBraun and

stored over molecular sieve (4 Å) prior to use. The ¹H, ¹³C, ¹¹B, ¹⁹F and

³¹P NMR spectra were recorded on Bruker DPX 200 (200 MHz),

Bruker AV 300 (300 MHz), Bruker DRX 400 (400 MHz), and Bruker

DPX 600 (600 MHz) devices. The chemical shifts are expressed in parts

per million (ppm) using tetramethylsilane (TMS) as internal standard.

Coupling constants (J) are reported in Hertz (Hz), and splitting patterns

are indicated as s (singlet), d (doublet), t (triplet), q (quartet), m

(multiplet), sept (septet) and br (broad). Elemental analysis (C, H, N)

succeeded by combustion MS on a Finnigan MAT 95 (EI), Finnigan MAT

95 XL (ESI), respectively and HR-MS from Bruker-Demo QTOF micro.

Synthesis and Characterization. Unless otherwise indicated, all

starting materials were obtained from Sigma-Aldrich and were used without

further purification. 1,3-Di-tert-butylimidazolin-2-ylidene (1a),³⁶1,3-di-tert-

butylimidazolidin-2-ylidene (1b),⁴³1,3-di-1-adamantylimidazolin-2-ylidene

Although the six-membered tBu- and Dipp-substituted NHCs

1i and 1j can be regarded as being more basic and more strongly

nucleophilic than their ﬁve-membered counterparts 1a and 1d,

the ring enlargement aﬀords sterically more demanding Lewis

bases, resulting in thermodynamically less stable adducts

[(1i)B(C₆F₅)₃] (ꢀ8.9, +10.6 kcal mol^ꢀ1) and [(1j)B(C₆F₅)₃]

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(1c),⁴⁴1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (1d),^32b1,3-di-

tert-butyl-4,5-dimethylimidazolin-2-ylidene (1h),³⁴bis(diisopropylamino)

[1c H][HB(C₆F₅)₃] (6). A 1:1 mixture of B(C₆F₅)₃(0.78 mmol,

3

400 mg) and 1c (0.78 mmol, 263 mg) was dissolved in 20 mL of toluene

at ꢀ30 ꢀC under an atmosphere of H₂. The yellow solution was purged with

H₂for 10 min, resulting in the formation of a milky white suspension. The

reaction was stirred under a static atmosphere of H₂for 8 h at room

temperature. The solvent was removed in vacuo, and the remaining solid

was stirred for 10 min with 20 mL of a 1:10 toluene/pentane mixture. The

white precipitate was isolated by filtration, immediately washed three times

with 10 mL of pentane and dried in vacuo. The product 6 was collected as a

47

carbene (1i),⁴⁵1,3-di-tert-butylhexahydropyrimidine⁴²and B(C₆F₅)₃

were prepared according to literature procedures.

1,3-Di-tert-butyl-4,5-dichloroimidazol-2-ylidene (1g). To a solu-

tion of 1,3-di-tert-butylimidazolin-2-ylidene 1a (30.0 mmol, 5.40 g) in

THF (100 mL) was added CCl₄(80.0 mmol; 7.60 mL) under an inert

atmosphere. The reaction is slightly exothermic and was stirred for 6 h at

40 ꢀC. Subsequent removal of the volatiles under reduced pressure at

10 ꢀC gave a violet solid. Sublimation of the solid at 50 ꢀC and 0.02 atm

afforded a white oily raw product, which was dissolved in 5 mL of

pentane. The solvent was removed in vacuo. Sublimation of the

remaining solid gave 1g in 43% (3.20 g) yield. Crystals suitable for

X-ray diffraction analysis were obtained by sublimation. Note: 1g is a

light and air-sensitive solid. ¹H NMR (200 MHz, C₆D₆): δ/ppm 1.57

(s, 18H, (C(CH₃)₃); ¹³C NMR (50 MHz, C₆D₆): δ/ppm 30.3 (CH₃),

59.5 (C(CH₃)₃), 115.2 (NCCl), 213.0 (NCN), melting point: 60 ꢀC.

1,3-Di-tert-butyltetrahydropyrimidin-2-ylidene (1i). 1,3-Di-tert-

butyltetrahydropyrimidinium bromide (18.0 mmol, 5.00 g) was sus-

pended in THF (100 mL), and a solution of NaN(SiMe₃)₂(18.0 mmol,

3.18 g) in THF (50 mL) was added at ꢀ78 ꢀC. The reaction mixture was

stirred for 3 h at ꢀ78 ꢀC and then allowed to warm to room temperature.

The solvent was then reduced to 100 mL, and the suspension was filtered

through Celite. The solvent was removed in vacuo to yield the free

white solid in 89% yield (603 mg). ¹H NMR (400 MHz, CD₂Cl₂): δ/ppm

3

0

1.78 (dd, J_{AA BB}= 42.93 Hz, J_HH= 12.8 Hz, 12 H, Ad), 2.10 (d, J_HH= 2.6

Hz, 12H, Ad), 2.30 (br s, 6H, Ad), 3.58 (q, ¹J_HB= 90 Hz, 1H, BH), 7.47 (d,

J_HH= 1.8 Hz, 2H, CH), 8.29 (t, J_HH= 1.7 Hz, 1H, N₂CH); ¹¹B NMR (96

MHz, CD₂Cl₂): δ/ppm ꢀ25.2 (s); ¹³C NMR (100 MHz, CD₂Cl₂):

δ/ppm 29.9 (CH, Ad), 35.5 (CH₂, Ad), 43.2 (CH₂, Ad), 61.5 (C, Ad),

119.7 (N₂(CH)₂);¹⁹FNMR(376MHz, CD₂Cl₂):δ/ppm ꢀ133.8 (m, ³J_FF

=21.8Hz, 6F, o-C₆F₅), ꢀ164.5 (m, ³J_FF=20.0Hz, 3F, p-C₆F₅), ꢀ167.5 (m,

6F, m-C₆F₅); Elemental analysis (%) calcd. for C₄₁H₃₄BF₁₅N₂: N 3.29, C

57.90, H 4.03, found: N 3.20, C 57.08, H 4.23; MS (ESI, MeOH), m/z (%):

positive 373.2 (100, [AdImH]⁺), negative 512.9 (100, [HꢀB(C₆F₅)₃]^ꢀ).

Abnormal Adduct 7. A 1:1 mixture of B(C₆F₅)₃(0.98 mmol, 500

mg) and 1c (0.98 mmol, 328 mg) was dissolved in 40 mL of toluene. The

yellow solution was stirred for 24 h at room temperature, resulting in the

formation of a yellowish solution with some precipitation. The solvent

was removed in vacuo, and the remaining solid was stirred for 10 min

with 20 mL of pentane. The white precipitate was isolated by filtration,

immediately washed several times with 10 mL of pentane and recrystal-

lized from CHCl₃at ꢀ35 ꢀC. The product was dried in vacuo and

collected as a white solid in 79% yield (523 mg). Note: 7 decomposes at

room temperature in chlorinated solvents and THF. ¹H NMR (300 MHz,

CDCl₃): δ/ppm 1.34ꢀ2.24 (m, 30H, Ad), 6.84 (br m, 1H, CHCB), 8.07

(d, ⁴J_HH= 1.9 Hz, 1H, N₂CH); ¹¹B NMR (96 MHz CD₂Cl₂): δ/ppm

ꢀ15.4 (s); ¹³C NMR (75 MHz, CDCl₃): δ/ppm 29.2 (CH) 29.7 (CH),

35.1 (CH₂), 35.4 (CH₂), 42.1 (CH₂), 42.8 (CH₂), 30.7 (CH₃), 58.4 (C),

63.5 (C), 123.7 (CH), 128.2 (CH), the C₆F₅resonances were too broad

for assignment; ¹⁹F NMR (178 MHz, CDCl₃): δ/ppm ꢀ124.7 (br s, 1F,

o-C₆F₅), ꢀ129.0 (br s, 1F, o-C₆F₅), ꢀ130.2 (m, 1F o-C₆F₅), ꢀ134.1 (m,

1F o-C₆F₅), ꢀ135.3 (m, 2F, o-C₆F₅), ꢀ160.0 (m, 1F, p-C₆F₅), ꢀ160.2

(m, 1F, p-C₆F₅), 161.1 (m, 1F, p-C₆F₅), ꢀ164.1 (m, 1F, m-C₆F₅), ꢀ164.7

(m, 1F,m-C₆F₅),ꢀ165.0 (m, 1F, m-C₆F₅),ꢀ165.9 (m, 1F, m-C₆F₅),ꢀ166.3

(m, 2F, m-C₆F₅); elemental analysis (%) calcd. for C₄₁H₃₂BF₁₅N₂: N

3.30, C 58.04, H 3.80, found: N 3.12, C 58.64, H 3.94.

1

carbene as a colorless oil (2.46 g, 72%). H NMR (200 MHz, C₆D₆):

3

δ/ppm 1.27 (br s, 18H, C(CH₃)₃), 1.36 (m, CH₂), 2.61 (br t, J_HH

=

6.0 Hz, 4H, N₂(CH₂)₂); ¹³C NMR(50MHz, C₆D₆): δ/ppm23.4(CH₂),

29.6 (C(CH₃)₃), 36.1 (N₂(CH₂)₂), 58.5 (C(CH₃)₃), 237.5 (N₂C).

1,3-Di-tert-butyltetrahydropyrimidinium Bromide. To a solution of

1,3-di-tert-butylhexahydropyrimidine⁴²(223 mmol, 44.2 g) in 100 mL of

abs. dimethoxyethane was slowly added solid N-bromosuccinimide. The

suspension was stirred for 3 h at room temperature. The white precipitate

was filtered off, washed with pentane, and dried in vacuo to yield the

product as a white powder (34.3 g, 55%). ¹H NMR (200 MHz, CDCl₃):

δ/ppm 1.54 (s, 18H, C(CH₃)₃), 2.16 (m, 2H, CH₂), 3.59 (br t, ³J_HH

=

5.8 Hz, 4H, N₂(CH₂)₂), 8.04 (s, N₂CH); ¹³C NMR (50 MHz,

CDCl₃): δ/ppm 20.2 (CH₂), 28.1 (C(CH₃)₃), 40.5 (N₂(CH₂)₂),

61.5 (C(CH₃)₃), 147.0 (N₂CH); ¹H NMR (400 MHz, DMSO):

δ/ppm 1.38 (s, 18H, C(CH₃)₃), 2.50 (m, 2H, CH₂), 3.41 (br t, ³J_HH

=

5.7 Hz, 4H, N₂(CH₂)₂), 7.82 (s, 1H, N₂CH); ¹³C NMR (100 MHz,

DMSO): δ/ppm 19.4 (CH₂), 27.0 (C(CH₃)₃), 40.1 (N₂(CH₂)₂),

60.3 (C(CH₃)₃), 146.5 (N₂CH).

[(1c)(CH₂)₄OB(C₆F₅)₃] (5). A 1:1 mixture of B(C₆F₅)₃(0.98 mmol,

500 mg) and 1c (0.98 mmol, 328 mg) was dissolved in 20 mL of THF.

The yellow solution was stirred for 12 h at room temperature, resulting

in the formation of a colorless solution. The solvent was removed in

vacuo, and the remaining solid was stirred for 10 min with 20 mL of

pentane. The white precipitate was isolated by filtration, immediately

washed three times with 10 mL of pentane, and dried in vacuo yielding

the product as a white solid (0.77 g, 93%). Crystals suitable for X-ray

analysis of 5 were obtained from a mixture of CH₂Cl₂and Et₂O at 0 ꢀC.

¹H NMR (300 MHz, acetone-d₆): δ/ppm 1.68 (m, 2H, N₂CCH₂CH₂),

1.78 (br s, 12 H, Ad), 1.95 (m, 2H, CH₂CH₂O), 2.26 (br s, 6H, Ad) 2.40

[(1d)(CH₂)₄OB(C₆F₅)₃] (8). Compound 8 was prepared according to

the procedure described for 5. B(C₆F₅)₃(0.98 mmol, 500 mg) and 1d

(0.98 mmol, 381 mg); yield 0.77 g (93%). Crystals suitable for X-ray

analysis of 8 were obtained from a mixture of THF, toluene and pentane

at 20 ꢀC; ¹H NMR (400 MHz, THF-d₈): δ/ppm 1.19 (m, 2H,

3

N₂CCH₂CH₂), 1.21 (d, J_HH= 6.7 Hz, 12H, CH(CH₃)₂), 1.26 (d,

³J_HH= 6.8 Hz, 12H, CH(CH₃)₂), 1.41 (m, 2H, CH₂CH₂O), 2.34 (sept,

4H, CH(CH₃)₂), 2.62 (m, 2H, N₂CCH₂), 2.84 (m, 2H, CH₂O), 7.49

(m, 4H, m-CH_Dipp), 7.60 (m, 2H, p-CH_Dipp), 8.00 (s, 2H, N₂(CH)₂);

¹³C NMR (100 MHz, THF-d₈): δ/ppm 22.7 (CH(CH₃)₂), 25.1

(N₂CCH₂CH₂), 25.3 (CH(CH₃)₂), 25.7 (CH₂CH₂O), 30.2 (CH-

3

(m, 12H, Ad), 3.23 (t, J_HH= 5.3 Hz, 2H, N₂CCH₂), 3.54 (m, 2H,

(CH₃)₂), 33.0 (N₂CCH₂), 63.9 (CH₂O), 125.8 (N₂(CH)₂), 125.9

CH₂O), 7.85 (s, 2H, N₂CH); ¹¹B NMR (96 MHz, acetone-d₆): δ/ppm

ꢀ1.7 (s); ¹³C NMR (75MHz, acetone-d₆): δ/ppm 27.6 (N₂CCH₂CH₂),

28.6 (OCH₂CH₂), 29.8 (CH, Ad), 31.6 (N₂CCH₂), 34.7 (CH₂, Ad), 41.7

(CH₂, Ad), 62.7 (CH₂O), 65.6 (C, Ad), 119.8 (N₂(CH)₂), 137.1 (dm,

¹J_CF= 251.9 Hz, m-C₆F₅), 138.9 (dm, ¹J_CF= 247.2 Hz, p-C₆F₅), 149.0

(m-CH_Dipp), 130.4 (i-C_Dipp) 133.1 (p-CH_Dipp), 137.1 (dm J_CF

=

1

244.2 Hz, m-C₆F₅), 138.9 (dm J_CF= 240.7 Hz, p-C₆F₅), 146.0

(o-C_Dipp), 148.8 (dm ¹J_CF= 242.5 Hz, o-C₆F₅), 151.6 (N₂C); ¹⁹F NMR

(178 MHz, THF-d₈): δ/ppm ꢀ132.6 (m, ³J_FF= 22.0 Hz, 6F, o-C₆F₅),

ꢀ164.5 (m, ³J_FF= 20.0 Hz, 3F, p-C₆F₅), ꢀ167.3 (m, ³J_FF= 19.5 Hz 6F,

m-C₆F₅); ¹¹B NMR (96 MHz, THF-d₈): δ/ppm ꢀ2.6 (s); elemental

analysis (%) calcd. for C₄₉H₄₄BF₁₅N₂O: C 60.51, H 4.50, N 2.88,

found: C 59.85, H 4.36, N 2.82.

1

(dm, J_CF= 242.5 Hz, o-C₆F₅), 149.5 (N₂C); ¹⁹F NMR (178 MHz,

acetone-d₆): δ/ppm ꢀ132.8 (m, ³J_FF= 22.35 Hz, 6F, o-C₆F₅), ꢀ163.7

3

(m, J_FF= 20.23 Hz, 3F, p-C₆F₅), ꢀ167.1 (m, J_FF= 19.24 Hz 6F,

m-C₆F₅); elemental analysis (%) calcd. for C₄₅H₄₀BF₁₅N₂O: N 3.04, C

58.71, H 4.38, found: N 2.92, C 59.16, H 5.07.

[(1d)B(C₆F₅)₃] (9). A 1:1 mixture of B(C₆F₅)₃(0.98 mmol, 500 mg)

and 1d (0.98 mmol, 379 mg) was dissolved in 10 mL of toluene. The

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reaction was stirred for 6 h at room temperature. Further addition of

30 mL pentane resulted in the formation of a white precipitate, which

was isolated by filtration and dried in vacuo. Yield 0.75 g (86%). Crystals

of 9 were obtained from a toluene/pentane mixture at ꢀ35 ꢀC. Note:

Compound 9 decomposes at room temperature. NMR data are con-

sistent with those previously established.¹⁹Elemental analysis (%)

calcd. for C₄₅H₃₆BF₁₅N₂: C 60.02, H 4.03, N 3.11, found: C 60.85, H

4.21, N 2.97.

ꢀ133.81 (m, ³J_FF= 21.69 Hz, 6F, o-C₆F₅), ꢀ164.54 (m, ³J_FF= 19.86 Hz,

3F, p-C₆F₅), ꢀ167.51 (m, 6F, m-C₆F₅); Elemental analysis (%) calcd.

for C₁₃H₂₇BF₁₅N₂: N 3.87, C 51.47, H 3.76, found: N 3.55, C 51.29,

H 3.55.

[(1i)(CH₂)₄OB(C₆F₅)₃] (13). Compound 13 was prepared according

to the procedure described for 5. B(C₆F₅)₃(0.39 mmol, 200 mg) and 1i

(0.39 mmol); yield 0.20 g (67%); crystals suitable for X-ray analysis of 11

were obtained from a mixture of CH₂Cl₂/pentane at ꢀ35 ꢀC; ¹H NMR

(300 MHz, CD₂Cl₂): δ/ppm 1.52 (s, 18H, C(CH₃)₃), 1.60 (m, ³J_HH= 5.8

Hz, 2H, N₂(CH₂)₂CH₂), 1.76 (m, 2H, N₂CCH₂CH₂), 1.88ꢀ2.20 (br m,

2H, OCH₂CH₂), 3.19 (br t, ³J_HH= 5.4, N₂CCH₂CH₂), 3.27ꢀ3.41 (br m,

6H, OCH₂CH₂, N₂(CH₂)₂); ¹¹B NMR (96 MHz, CD₂Cl₂): δ/ppm ꢀ2.8

(s); ¹³C NMR (75 MHz, CD₂Cl₂): δ/ppm 25.6 (N₂(CH₂)₂CH₂), 30.5

(C(CH₃)₃), 31.3 (N₂CCH₂CH₂), 36.6 (OCH₂CH₂), 40.1 (N₂CCH₂CH₂),

46.4 (OCH₂CH₂), 63.3 (OCH₂CH₂), 64.6 (C(CH₃)₃), 177.1 (N₂C),

the C₆F₅resonances were too broad for assignment; ¹⁹F NMR (376

MHz, acetone-d₆): δ/ppm ꢀ132.6 (br d, ³J_FF= 21.8 Hz, 6F, o-C₆F₅),

[1g H][HB(C₆F₅)₃] (10). A 1:1 mixture of B(C₆F₅)₃(0.85 mmol, 436

3

mg) and 1g (0.85 mmol, 212 mg) was dissolved in 20 mL of toluene at

ꢀ30 ꢀC under an atmosphere of H₂. The yellow solution was purged

with H₂for 10 min, resulting in the formation of a milky white

suspension. The reaction was stirred under a static atmosphere of H₂

for 1 h at room temperature. The solvent was then removed in vacuo,

and the remaining solid was stirred for 10 min with 20 mL of a 1:10

toluene/pentane mixture. The white precipitate was isolated by filtra-

tion, immediately washed three times with 10 mL of pentane and dried

in vacuo. The raw product contains the hydroxyborate anion as an

impurity, which could be partially removed by recrystallization from

CH₂Cl₂/pentane. Yield: 40% based on the ¹⁹F NMR spectra of the raw

product. Note: A combination of 1g/B(C₆F₅)₃is extremely hygro-

scopic. ¹H NMR (600 MHz, toluene-d₈): δ/ppm 1.01 (s, 18H,

3

ꢀ163.60 (t, 3F, J_FF= 20.1 Hz, p-C₆F₅), ꢀ167.0 (m, 6F, m-C₆F₅);

elemental analysis (%) calcd. for C₃₄H₃₂BF₁₅N₂O: N 3.59, C 52.33, H

4.13, found: N 3.71, C 52.33, H 4.27.

[1i H][HB(C₆F₅)₃] (14). The glassware was silylated with hexam-

3

ethyldisiloxane prior to use, and toluene was dried over sodiumꢀ

potassium alloy! A solution of B(C₆F₅)₃(0.39 mmol, 200 mg) in 3 mL

of toluene was cooled to ꢀ78 ꢀC under an atmosphere of H₂. 1i

(0.39 mmol, 77.0 mg) was added as a solution in 2 mL of toluene, and

the reaction was quickly brought to room temperature. The reaction was

stirred under a static atmosphere of H₂at room temperature for 1 h. The

solvent was removed in vacuo, and the remaining solid was stirred for 10

min in 10 mL of pentane. The white precipitate was isolated by filtration,

immediately washed three times with 10 mL of pentane and dried in

vacuo. The product was collected as a white solid (0.19 g, 70%). Crystals

suitable for X-ray analysis of 12 were obtained from a mixture of

C(CH₃)₃), 3.70 (q, 1H, ¹J_HB= 93 Hz, BH), 8.14 (s, 1H, N₂CH); ¹¹

B

NMR (96 MHz, toluene-d₈): δ/ppm ꢀ24.4 (s); ¹³C NMR (150 MHz,

toluene-d₈): δ/ppm 27.3 (C(CH₃)₃), 65.8 (C(CH₃)₃), 120.7

(N₂(CCl)₂), 131.0 (N₂CH), the C₆F₅resonances were too broad for

assignment; ¹⁹F NMR (178 MHz, toluene-d₈): δ/ppm ꢀ133.1 (m, ³J_FF

= 21.4 Hz, 6F, o-C₆F₅), ꢀ163.0 (m, ³J_FF= 20.4 Hz, 3F, p-C₆F₅), ꢀ166.4

(m, 6F, m-C₆F₅); elemental analysis (%) calcd. for C₂₉H₂₀BCl₂F₁₅N₂:

N 3.67, C 45.64, H 2.64, found: N 3.70, C 46.06, H 2.90.

[(1 h)(CH₂)₄OB(C₆F₅)₃] (11). Compound 11 was prepared accord-

3

ing to the procedure described for 5. B(C₆F₅)₃(0.58 mmol, 300 mg) and

1h (0.58 mmol); yield 0.42 g (91%); crystals suitable for X-ray analysis of

1

CH₂Cl₂/pentane at ꢀ35 ꢀC. H NMR (400 MHz, CD₂Cl₂): δ/ppm

1

1.39 (s, 18H, C(CH₃)₃), 2.04 (m, CH₂), 3.40 (br t, ³J_HH= 6.0 Hz, 4H,

12 were obtained from a mixture of THF/pentane at 5 ꢀC; H NMR

1

N₂(CH₂)₂), 3.58 (br q, J_HB= 90 Hz, 1H, BH), 7.82 (s, N₂CH); ¹¹

B

(400 MHz, THF-d₈): δ/ppm 1.61 (m, 2H, N₂CCH₂CH₂), 1.76ꢀ1.82

(m, 18H, C(CH₃)₃+ 2H, CH₂CH₂O), 2.43 (s, 6H, CH₃), 3.21 (br t,

³J_HH= 4.8 Hz, 2H, N₂CCH₂CH₂) 3.54 (m, 2H, OCH₂CH₂); ¹¹B NMR

(128 MHz, THF-d₈): δ/ppm ꢀ2.4 (s); ¹³C NMR (100 MHz, THF-d₈):

δ/ppm 14.1 (CH₃), 29.7 (N₂CCH₂CH₂), 29.9 (OCH₂CH₂), 31.6

(C(CH₃)₃), 35.1 (N₂CCH₂CH₂), 63.2 (OCH₂CH₂), 65.68

(C(CH₃)₃), 125.8 (br s, i-C₆F₅), 129.5 (NCC(CH₃)); 137.0 (dm,

¹J_CF= 248.0 Hz, m-C₆F₅), 138.8 (dm, ¹J_CF= 248.5 Hz, p-C₆F₅), 148.9

NMR (128 MHz, CD₂Cl₂): δ/ppm ꢀ25.2 (d, J_HB= 90 Hz); ¹³C NMR

(100 MHz, CD₂Cl₂): δ/ppm 19.9 (CH₂), 27.5 (C(CH₃)₃), 40.1

1

(N₂(CH₂)₂), 61.9 (C(CH₃)₃), 136.9 (dm, J_CF= 245 Hz, m-C₆F₅),

138.1 (dm, ¹J_CF= 228 Hz, p-C₆F₅), 148.7 (dm, ¹J_CF= 232 Hz, o-C₆F₅),

146.1 (N₂CH); ¹⁹F NMR (376 MHz, CD₂Cl₂): δ/ppm ꢀ133.8 (br d,

³J_FF= 22.0 Hz, 6F, o-C₆F₅), ꢀ164.4 (t, 3F, J_FF= 20.3 Hz, p-C₆F₅),

3

ꢀ167.4 (m, 6F, m-C₆F₅); elemental analysis (%) calcd. for

1

(dm, J_CF= 242.5 Hz, o-C₆F₅), 151.0 (N₂C), ¹⁹F NMR (376 MHz,

C₃₀H₂₆BF₁₅N₂: N 3.94, C 50.73, H 3.69, found: N 4.02, C 50.24, H 3.72.

THF-d₈): δ/ppm ꢀ132.5 (br d, ³J_FF= 22.6, 6F, o-C₆F₅), ꢀ164.4 (t, 3F,

³J_FF= 20.2, p-C₆F₅), ꢀ167.2 (m, 6F, m-C₆F₅); elemental analysis (%)

calcd. for C₃₅H₃₃BF₁₅N₂O: N 3.53, C 52.98, H 4.19, found: N 3.15, C

52.47, H 4.12.

[1i H][CH₃CH₂OB(C₆F₅)₃] (15). To a solution of B(C₆F₅)₃(0.39

3

mmol, 200 mg) in 2 mL of Et₂O was added a solution of 1i (0.39 mmol,

77.0 mg) in 3 mL of Et₂O. After 30 min, a white precipitate had formed,

which was collected by filtration, washed with pentane, and dried in

vacuo to yield the product as a white solid (0.21 g, 71%). Crystals

suitable for X-ray analysis of 15 were obtained from a mixture of

[1h H][HB(C₆F₅)₃] (12). The experiment was carried out in silylated

3

glassware using toluene that had been dried over sodiumꢀpotassium

alloy. A solution of B(C₆F₅)₃(0.976 mmol, 500 mg) in 10 mL of toluene

was cooled to ꢀ78 ꢀC. To the cooled solution was added via a syringe 1h

(0.976 mmol, 203 mg) dissolved in 5 mL of toluene. After addition,

hydrogen was immediately purged through the mixture for 10 min. The

reaction mixture was brought to room temperature and stirred for an

additional 1.5 h under H₂atmosphere at room temperature. After

removal of the solvent in vacuo the residue was washed with 3 mL of

pentane and dried in vacuo, giving 0.662 g (94%) of a white solid. ¹H

NMR (400 MHz, CD₂Cl₂): δ/ppm 1.67 (s, 18H, CH₃), 2.41 (s, 6H,

CH₃) 3.58 (q, 1H, ¹J_HB= 93 Hz, BH), 7.99 (s, 1H, N₂CH); ¹¹B NMR

(128 MHz, CD₂Cl₂): δ/ppm ꢀ25.1 (d, J_HꢀB= 93 Hz); ¹³C NMR (100

1

CH₂Cl₂/pentane at ꢀ35 ꢀC. H NMR (400 MHz, CD₂Cl₂): δ/ppm

1.02 (t, ³J_HH= 6.8 Hz, 3H, OCH₂CH₃), 1.41 (s, 18H, C(CH₃)₃), 2.05

(m, 2H, CH₂), 3.14 (br q, ³J_HH= 6.8 Hz, 2H, OCH₂CH₃), 3.41 (br t,

³J_HH= 5.8 Hz, 4H, N₂(CH₂)₂), 7.83 (s, N₂CH); ¹¹B NMR (96 MHz,

CD₂Cl₂): δ/ppm ꢀ2.8 (s); ¹³C NMR (100 MHz, CD₂Cl₂): δ/ppm

18.3 (br s, OCH₂CH₃), 19.9 (CH₂), 27.6 (C(CH₃)₃), 40.1 (N₂(CH₂)₂),

59.6 (OCH₂CH₃), 61.9 (C(CH₃)₃), 136.5 (dm, ¹J_CF= 245 Hz, m-C₆F₅),

138.6 (dm, ¹J_CF= 248 Hz, p-C₆F₅), 148.4 (dm, ¹J_CF= 241 Hz, o-C₆F₅),

146.1 (N₂CH); ¹⁹F NMR (376 MHz, CD₂Cl₂): δ/ppm ꢀ133.9

3

(br d, J_FF= 22.8 Hz, 6F, o-C₆F₅), ꢀ163.9 (t, 3F, J_FF= 20.2 Hz,

p-C₆F₅), ꢀ167.3 (m, 6F, m-C₆F₅); elemental analysis (%) calcd. for

C₃₄H₃₄BF₁₅N₂O: N 3.56, C 51.58, H 4.20, found: N 3.86, C 51.41,

H 4.27.

MHz, CD₂Cl₂): δ/ppm 11.7 (CH₃), 29.42 (C(CH₃)₃), 61.84

1

(C(CH₃)₃), 128.7 (N₂CH), 129.9 (N₂CC(CH₃)), 136.9 (dm, J_CF

=

1

245.0 Hz, m-C₆F₅), 138.0 (dm, J_CF= 242.0 Hz, p-C₆F₅), 148.5 (dm,

[(1j)(CH₂)₄OB(C₆F₅)₃] (16). Compound 16 was prepared according

to the procedure described for 5. B(C₆F₅)₃(0.19 mmol, 100 mg) and 1j

¹J_CF= 239.4 Hz, o-C₆F₅); ¹⁹F NMR (376 MHz, CD₂Cl₂): δ/ppm

7355

dx.doi.org/10.1021/ic201290g |Inorg. Chem. 2011, 50, 7344–7359

Inorganic Chemistry

ARTICLE

Table 2. Crystallographic Data

1g

1h

1j

5

8

9

11

empirical formula

a (Å)

C₁₁H₁₈Cl₂N₂

9.2303(3)

11.7158(4)

12.0991(3)

90

C₁₃H₂₄N₂

5.8979(6)

6.4251(8)

17.3670(2)

84.434(10)

80.802(10)

82.488(8)

642.12(13)

2

C₂₈H₄₀N₂

6.141(2)

19.031(7)

11.164(4)

90

C₄₅H₄₀BF₁₅N₂O

7.5356(2)

21.3544(6)

24.8512(6)

90

C₄₉H₄₄BF₁₅N₂O

14.2587(6)

14.9988(8)

21.4354(14)

90

C₄₅H₃₆BF₁₅N₂

12.0311(16)

18.5858(8)

18.8956(8)

90

C₃₅H₃₂BF₁₅N₂O

10.3406(4)

21.7522(8)

15.2061(6)

90

b (Å)

c (Å)

R (deg)

β (deg)

γ (deg)

V (Å³)

90

103.73(6)

90

90.533(2)

90

91.215(4)

90

107.195(9)

90

94.775(4)

90

1308.40(7)

4

1267.6(8)

2

3998.84(18)

4

4583.2(4)

4

4036.3(6)

4

3408.4(2)

4

Z

formula weight

space group

T (K)

249.17

P2₁2₁2₁

100(2)

0.71073

1.265

208.34

404.62

P2₁/m

100(2)

0.71073

1.060

920.60

972.67

900.57

792.44

P1

P2₁/n

P2₁/c

P2₁/n

100(2)

0.71073

1.410

100(2)

0.71073

1.482

100(2)

1.54184

1.544

λ (Å)

1.54184

1.078

0.71073

1.529

D_calcd(g cm^ꢀ3

μ (mm^ꢀ1

reﬂection collected

)

0.469

0.477

0.061

0.139

0.126

0.135

1.330

38844

5887

37120

103670

68169

55754

29655

independent reﬂections 3994 R_int= 0.0220 2655 R_int= 0.0376 2483 R_int= 0.0873 8183 R_int= 0.0635 9325 R_int= 0.0812 10015 R_int= 0.0633 7025 R_int= 0.0212

goodness of ﬁt on F²

1.017

1.027

1.050

0.907

0.991

0.830

1.033

R(F_o), [I > 2σ(I)]

0.0192

0.0454

0.0565

0.0348

0.0623

0.0377

0.0387

2

R_w(F_o

)

0.0531

0.1227

0.1166

0.0488

0.1556

0.0729

0.1046

ΔF [e Å^ꢀ3

]

0.286/ꢀ0.129

0.239/ꢀ0.191

0.169/ꢀ0.168

0.237/ꢀ0.194

0.412/ꢀ0.363

0.309/ꢀ0.236

0.409/ꢀ0.374

(0.19 mmol); yield 0.14 g (76%); crystals suitable for X-ray analysis of 16

were obtained from a mixture of CH₂Cl₂/pentane at ꢀ35 ꢀC; ¹H NMR

(300 MHz, acetone-d₆): δ/ppm 0.85 (m, 2H, N₂(CH₂)₂CH₂), 1.24

(m, 2H, N₂CCH₂CH₂), 1.29 (d, ³J_HH= 6.8 Hz, 12H, CH(CH₃)₂), 1.33

(188 MHz, CD₂Cl₂): δ/ppm ꢀ133.0 (br m, 2F, C₆F₄), ꢀ135.3, (m, 2F,

C₆F₄),ꢀ135.4 (m, 6F, o-C₆F₅),ꢀ161.9 (t, ³J_FF= 20.0 Hz, 2F, p-C₆F₅),ꢀ166.4

(m,4F,m-C₆F₅),ꢀ193.0 (br s, 1F, FB(C₆F₅)₂;¹¹BNMR(96MHz,CD₂Cl₂):

δ/ppm ꢀ0.7 (br s); elemental analysis (%) calcd. for C₄₆H₄₀BF₁₅N₂1/

3

(d, J_HH= 6.8 Hz, 12H, CH(CH₃)₂), 2.12 (m, 2H, OCH₂CH₂), 2.64

2CH₂Cl₂: C 58.57, H 4.55, N 2.88, found: C 58.68, H 4.41, N 2.74.

3

(1k)B(C₆F₅)₃] (18). A 1:1 mixture of B(C₆F₅)₃(0.98 mmol, 500 mg)

and 1j (0.98 mmol, 206 mg) was dissolved in 10 mL of toluene. The

reaction was stirred for 30 min at room temperature. The solvent was

removed in vacuo, and the remaining solid was washed with pentane.

Recrystallization from a saturated solution in CH₂Cl₂/pentane at 3 ꢀC

afforded a white, air-stable solid, which was dried in vacuo. Yield 97%

(686 mg). Crystals suitable for X-ray analysis of 16 were obtained from a

mixture of CH₂Cl₂/Et₂O/pentane at 3 ꢀC. Note: broad NMR reso-

nances at room temperature. ¹H NMR (400 MHz, ꢀ30 ꢀC, CD₂Cl₂):

δ/ppm 0.68 (d, ³J_HH= 6.6 Hz, 3H, CH₃), 1.06 (d, ³J_HH= 6.9 Hz, 3H,

CH₃), 1.18 (d, ³J_HH= 6.8 Hz, 3H, CH₃), 1.27 (m, 3H, CH₃), 1.35 (m,

3H, CH₃), 1.50 (d, ³J_HH= 7.2 Hz, 3H, CH₃), 1.57 (d, ³J_HH= 6.6 Hz, 3H,

CH₃), 1.65 (d, ³J_HH= 6.9 Hz, 3H, CH₃), 4.05 (sept, ³J_HH= 6.6 Hz, 1H,

CH), 4.27 (sept, ³J_HH= 6.4 Hz, 2H, CH), 4.59 (sept, ³J_HH= 6.8 Hz, 1H,

(m, 4H, N₂CCH₂CH₂, OCH₂CH₂), 3.07 (sept, J_HH= 6.8 Hz, 4H,

CH(CH₃)₂), 4.11 (m, 4H, N₂(CH₂)₂), 7.39ꢀ7.42 (m, 4H, m-CH_Dipp),

7.47ꢀ7.52 (m, 2H, p-CH_Dipp); ¹¹B NMR (96 MHz, acetone-d₆):

δ/ppm ꢀ1.9 (s); ¹³C NMR (75 MHz, acetone-d₆): δ/ppm 19.6

(N₂(CH₂)₂CH₂), 22.4 (N₂CCH₂CH₂), 23.1 (CH(CH₃)₂), 26.4

(CH(CH₃)₂), 29.6 (CH(CH₃)₂), 32.2(OCH₂CH₂), 33.2(N₂CCH₂CH₂),

52.1 (N₂(CH₂)₂CH₂), 63.4 (OCH₂CH₂), 126.4 (m-CH_Dipp), 131.7

(p-CH_Dipp), 137.1 (i-C_Dipp), 145.4 (o-C_Dipp), 167.2 (N₂C), the C₆F₅

resonances were too broad for assignment; ¹⁹F NMR (188 MHz,

3

acetone-d₆): δ/ppm ꢀ132.6 (br d, J_FF= 21.4 Hz, 6F, o-C₆F₅),

3

ꢀ163.6 (t, 3F, J_FF= 20.0 Hz, p-C₆F₅), ꢀ167.24 (m, 6F, m-C₆F₅);

elemental analysis (%) calcd. for C₅₀H₄₈BF₁₅N₂O 1.5THF: N 2.73, C

3

60.95, H 5.11, found: N 2.58, C 60.49, H 5.41.

[(1j)C₆F₄{FB(C₆F₅)₂}] (17). To a solution of 1j (0.15 mmol, 60.0 mg)

in 5 mL of toluene a solution of B(C₆F₅)₃(0.15 mmol, 79.0 mg) in 5 mL

of toluene was added, which resulted in the formation of a gel-like,

voluminous precipitate within a couple of minutes. After 1 h, the solvent

was reduced to 5 mL, and the white solid was filtered off, washed with

Et₂O (small portion) and hexane, and dried in vacuo to yield the raw

product as a white solid. To obtain an analytically pure sample, 17 was

recrystallized from CH₂Cl₂/pentane solutions. (0.11 g, 75%). Crystals

suitable for X-ray analysis were obtained from a saturated solution of 17

in CH₂Cl₂at room temperature. ¹H NMR (300 MHz, CD₂Cl₂): δ/ppm

CH); ¹¹B NMR (96 MHz, ꢀ30 ꢀC, CD₂Cl₂): δ/ppm ꢀ14.1 (s); ¹³

C

NMR (150 MHz, ꢀ30 ꢀC, CD₂Cl₂): δ/ppm 20.5 (CH₃), 21.3 (CH₃),

22.0 (CH₃), 22.9 (m, CH₃), 23.0 (CH₃), 24.9 (m, CH₃), 25.0 (CH₃),

25.7 (m, CH₃), 53.3 (m, CH), 59.6 (CH), 63.2 (CH), the NCN

resonance was not observed; ¹⁹F NMR (178 MHz, ꢀ30 ꢀC, CD₂Cl₂):

δ/ppm ꢀ121.8 (m, ³J_FF= 25.7 Hz, 1F, o-C₆F₅), ꢀ126.7 (m, ³J_FF= 25.4

Hz, 1F, o-C₆F₅), ꢀ128.7 (m, 1F, o-C₆F₅), ꢀ129.5 (m, 1F, o-C₆F₅),

3

ꢀ134.7 (m, J_FF= 23.6 Hz, 1F, o-C₆F₅), ꢀ134.9 (m, 1F, o-C₆F₅),

ꢀ158.4 (m, ³J_FF= 20.9 Hz, 1F, p-C₆F₅), ꢀ159.3 (m, ³J_FF= 21.0 Hz, 1F,

3

1.20 (d, ³J_HH= 6.8 Hz, 12H, CH(CH₃)₂), 1.24 (br d, ³J_HH= 6.6 Hz, 12H,

p-C₆F₅), ꢀ159.93 (m, J_FF= 20.7 Hz, 1F, p-C₆F₅), ꢀ163.52 (m, 1F,

3

m-C₆F₅), ꢀ164.2 (m, 1F, m-C₆F₅), ꢀ164.5 (m, 1F, m-C₆F₅), ꢀ165.3

(m, 1F, m-C₆F₅), ꢀ165.8 (m, 1F, m-C₆F₅), ꢀ166.3 (m, 1F, m-C₆F₅);

elemental analysis (%) calcd. for C₃₁H₂₈BF₁₅N₂: N 3.87, C 51.80, H

3.90, found: N 3.78, C 52.29, H 4.22.

CH(CH₃)₂), 2.60 (br m, 2H, N₂(CH₂)₂CH₂), 2.99 (br sept, J_HH

=

6.6 Hz, 4H, CH(CH₃)₂), 4.07 (br m, 4H, N₂(CH₂)₂), 7.16ꢀ7.18 (m,

4 H m-CH_Dipp), 7.33ꢀ7.38 (m, 2H, p-CH_Dipp); ¹³C NMR (75 MHz,

CD₂Cl₂): δ/ppm 19.1 (N₂(CH₂)₂CH₂), 22.4 (br, CH(CH₃)₂), 27.3

(CH(CH₃)₂), 29.5 (br, CH(CH₃)₂), 52.5 (N₂(CH₂)₂CH₂), 125.5

(m-CH_Dipp), 131.6 (p-CH_Dipp), 135.9 (i-C_Dipp), 145.6 (o-C_Dipp), 158.0

(N₂C), the C₆F_4,5resonances were too broad for assignment; ¹⁹F NMR

Theoretical Calculations. All computations were performed

using the density functional method M05-2X as implemented in the

Gaussian09 program.^46,48For all main-group elements (C, H, N, B, F,

7356

dx.doi.org/10.1021/ic201290g |Inorg. Chem. 2011, 50, 7344–7359

Inorganic Chemistry

ARTICLE

Table 3. Crystallographic data

13

14

15

16 0.5CH₂Cl₂

17

18

[1h H][FB(C₆F₅)₃]^a

3

empirical formula

a (Å)

C₃₄H₃₂BF₁₅N₂O C₃₀H₂₆BF₁₅N₂

C₃₂H₃₀BF₁₅N₂O C_50.50H₄₉BClF₁₅N₂O C₄₆H₄₀BF₁₅N₂

C₃₁H₂₈BF₁₅N₂

10.5602(6)

C₃₁H₂₅BF₁₆N₂

16.7801(2)

12.5752(2)

15.5632(2)

16.9021(2)

90

10.1143(2)

14.0410(2)

21.5565(2)

90

8.0521(2)

18.2928(4)

22.2546(4)

90

9.5340(4)

14.5959(6)

17.6549(8)

94.969(2)

96.389(2)

99.508(2)

2394.15(18)

2

11.6425(8)

12.3525(10)

14.7161(8)

82.343(6)

81.076(4)

81.820(6)

2056.1(2)

2

b (Å)

10.7479(8)

14.5654(8)

93.988(4)

97.676(4)

109.782(6)

1529.96(17)

2

10.1136(2)

18.1876(4)

90

c (Å)

R (deg)

β (deg)

γ (deg)

V (Å³)

Z

91.345(2)

90

98.729(2)

90

95.965(2)

90

97.464(2)

90

3307.00(8)

4

3025.89(8)

4

3260.25(12)

4

3060.41(10)

4

formula weight

space group

T (K)

780.43

P2₁/c

710.34

P2₁/n

754.39

P2₁/c

1031.18

P1

916.61

724.36

740.34

P2₁/c

P1

100(2)

1.54184

1.567

100(2)

1.54184

1.559

100(2)

1.54184

1.537

100(2)

1.54184

1.607

λ (Å)

1.54184

1.430

1.54184

1.481

0.17073

1.572

D_calcd(g cm^ꢀ3

μ (mm^ꢀ1

reﬂection collected

)

1.359

1.397

1.357

1.585

1.169

0.156

1.458

50872

39634

47250

35103

33761

37026

55661

independent reﬂections 6866 R_int= 0.0211 6284 R_int= 0.0254 6769 R_int= 0.0297 9896 R_int= 0.0318

8407 R_int= 0.0295 6732 R_int= 0.0426 6360 R_int= 0.0219

goodness of ﬁt on F²

1.042

1.072

1.030

1.034

1.050

0.857

1.028

R(F_o), [I > 2σ(I)]

0.0314

0.0407

0.0312

0.0377

0.0352

0.0327

0.0372

2

R_w(F_o

)

0.0826

0.1090

0.0845

0.0985

0.0938

0.0665

0.1052

ΔF [e Å^ꢀ3

]

0.272/ꢀ0.208

0.617/ꢀ0.222

0.273/ꢀ0.237

0.488/ꢀ0.367

0.337/ꢀ0.246

0.256/ꢀ0.212

0.298/ꢀ0.228

^aAn ORTEP diagram of this structure can be found in the Supporting Information.

and Cl) the all-electron triple-ζ basis set (6-311G**) was used.⁴⁹All

geometry optimizations were performed without any imposed symme-

try constraints. After the relevant stationary points were localized on the

energy surface, they were further characterized as minima by normal-

mode analysis based on the analytical energy second derivatives.

Enthalpic and entropic contributions were calculated by statistical

thermodynamics as implemented in the Gaussian09 set of programs.

Presentations together with Cartesian coordinates can be found in the

Supporting Information.

’ ASSOCIATED CONTENT

S

Supporting Information. Details of the electronic struc-

b

ture calculations, presentations of all calculated structures to-

gether with Cartesian coordinates of their atomic positions and

selected NMR spectra of 18 are provided. See DOI: 10.1039/

b000000x/. This material is available free of charge via the

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

Single-Crystal X-ray Structure Determinations. Crystals

were mounted in inert oil on glass fibres. Intensity measurements were

performed at low temperature on Oxford Diffraction Xcalibur diffract-

ometers using monochromated Mo K_Ror mirror-focused Cu K_R

radiation. Absorption corrections were based on multiscans. Structures

were refined anisotropically on F²using the program SHELXL-97.⁵⁰

Carbenium hydrogen atoms were refined freely; other hydrogens were

included using idealized rigid methyl groups or a riding model. Crystal-

lographic data are summarized in Tables 2 and 3. Exceptions and special

features: Compound 1g was refined as a racemic twin, with minor

component 0.10(3). The structure of compound 1j is disordered in

P2₁/m, with atoms C2, C3, C4 occupying alternative positions on both sides

of the mirror plane. The structure can also be refined without disorder in

space group P2₁, but the refinement is not satisfactory (e.g., it displays

larger and more irregular thermal ellipsoids). It is unlikely that X-ray

methods alone can determine which model is correct, and the quantitative

results should be interpreted with caution, although the qualitative nature

of the compound has been established. For compound 5, the atoms C16

and C17 are disordered over two positions with occupancy 3:1. Despite

the use of similarity restraints, dimensions of the minor component are

not entirely satisfactory. It is possible that C15 is also slightly disordered.

Data for 8 were weak, and 300 restraints to anisotropic displacement

parameters were employed to improve stability of refinement. In 14 the

hydrogen at boron was refined freely. The dichloromethane molecule in

16 is disordered over an inversion center; appropriate similarity restraints

were used to ensure stability of refinement.

’ AUTHOR INFORMATION

Corresponding Author

*E-mail: m.tamm@tu-bs.de. Fax: (+49) 531-391-5387. Phone:

(+49) 531-391-5309.

’ REFERENCES

(1) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.

Science 2006, 314, 1124.

(2) (a) Kenward, A. L.; Piers, W. E. Angew. Chem., Int. Ed. 2008,

47, 38. (b) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535.

(c) Stephan, D. W. Dalton Trans. 2009, 3129. (d) Stephan, D. W.;

Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46. (e) Stephan, D. W. Chem.

Commun. 2010, 46, 8526.

(3) (a) Rokob, T. A.; Hamza, A.; Stirling, A.; Soꢀos, T.; Pꢀapai, I.

Angew. Chem., Int. Ed. 2008, 47, 2435. (b) Grimme, S.; Kruse, H.;

Goerigk, L.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 1402.

(4) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.

(5) (a) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Fr€ohlich, R.;

Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072; (b) Ullrich,

M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 52.

(c) Neu, R. C.; Ouyang, E. Y.; Geier, S. J.; Zhao, X.; Ramos, A.; Stephan,

D. W. Dalton Trans. 2010, 39, 4285.

(6) (a) Welch, G. C.; Masuda, J. D.; Stephan, D. W. Inorg. Chem.

2006, 45, 478. (b) Birkmann, B.; Voss, T.; Geier, S. J.; Ullrich, M.; Kehr,

G.; Erker, G.; Stephan, D. W. Organometallics 2010, 29, 5310.

7357

dx.doi.org/10.1021/ic201290g |Inorg. Chem. 2011, 50, 7344–7359

Inorganic Chemistry

ARTICLE

(7) (a) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem.,

Int. Ed. 2007, 46, 4968. (b) Stirling, A.; Hamza, A.; Rokob, T. A.; Pꢀapai, I.

Chem. Commun. 2008, 3148. (c) Guo, Y.; Li, S. Eur. J. Inorg. Chem.

2008, 2501. (d) Ullrich, M.; Seto, K. S.-H.; Lough, A. J.; Stephan, D. W.

Chem. Commun. 2009, 2335. (e) M€omming, C. M.; Fr€omel, S.; Kehr, G.;

Fr€ohlich, R.; Grimme, S.; Erker, G. J. Am. Chem. Soc. 2009, 131, 12280.

(8) (a) Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009,

131, 8396. (b) Voss, T.; Chen, C.; Kehr, G.; Nauha, E.; Erker, G.;

Stephan, D. W. Chem.—Eur. J. 2010, 16, 3005. (c) M€omming, C. M.;

Kehr, G.; Wibbeling, B.; Fr€ohlich, R.; Schirmer, B.; Grimme, S.; Erker, G.

Angew. Chem., Int. Ed. 2010, 49, 2414. (d) Chen, C.; Fr€ohlich, R.; Kehr,

G.; Erker, G. Chem. Commun. 2010, 46, 3580.

(9) (a) M€omming, C. M.; Otten, E.; Kehr, G.; Fr€ohlich, R.; Grimme,

S.; Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2009, 48, 6643.

(b) Zhao, X.; Stephan, D. W. Chem. Commun. 2011, 47, 1833.

(10) (a) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am. Chem. Soc.

2009, 131, 9918. (b) Neu, R. C.; Otten, E.; Lough, A. J.; Stephan, D. W.

Chem. Sci. 2011, 2, 170.

(23) Holschumacher, D.; Bannenberg, T.; Ibrom, K.; Daniliuc,

C. G.; Jones, P. G.; Tamm, M. Dalton Trans. 2010, 39, 10590.

(24) Recently, normal adduct formation was reported for the

corresponding carbene-alane adduct [(1a)Al(C₆F₅)₃], see: Zhang, Y.;

Miyake, G. M.; Chen, X.-Y. Angew. Chem., Int. Ed. 2010, 49, 10158.

(25) Theuergarten, E.; Schl€uns, D.; Grunenberg, J.; Daniliuc, C. G.;

Jones, P. G.; Tamm, M. Chem. Commun. 2010, 46, 8561.

(26) (a) Jana, A.; Objartel, I.; Roesky, H. W.; Stalke, D. Inorg. Chem.

2009, 48, 7645. (b) Jana, A.; Tavꢂcar, G.; Roesky, H. W.; Schulzke, C.

Dalton Trans. 2010, 39, 6217.

(27) Phillips, A. D.; Power, P. P. Acta Crystallogr., Sect. C 2005, 61, o291.

(28) (a) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda,

J. D.; Wei, P.; Stephan, D. W. Dalton Trans. 2007, 3407. (b) Welch,

G. C.; Prieto, R.; Dureen, M.; Lough, A. J.; Labeodan, S.; Holtrichter-

R€ossmann, T.; Stephan, D. W. Dalton Trans. 2009, 1559.

(29) Welch, G. C.; Holtrichter-R€ossmann, T.; Stephan, D. W. Inorg.

Chem. 2008, 47, 1904.

(30) (a) Graham, T. W.; Udachin, K. A.; Carty, A. J. Chem. Commun.

2006, 2699. (b) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.;

Parameswaran, P.; Frenking, G.; Bertrand, G. Science 2009, 326, 556.

(c) Schmitt, A.-L.; Schnee, G.; Welter, R.; Dagorne, S. Chem. Commun.

2010, 46, 2480.

(11) M€omming, C. M.; Kehr, G.; Wibbeling, B.; Fr€ohlich, R.; Erker,

G. Dalton Trans. 2010, 39, 7556.

(12) Porcel, S.; Bouhadir, G.; Saﬀon, N.; Maron, L.; Bourissou, D.

Angew. Chem., Int. Ed. 2010, 49, 6186.

(13) Dureen, M. A.; Welch, G. C.; Gilbert, T. M.; Stephan, D. W.

Inorg. Chem. 2009, 48, 9910.

(14) Geier, S. J.; Stephan, D. W. Chem. Commun. 2010, 46, 1026.

(15) (a) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem.

Soc. 1991, 113, 361. (b) Arduengo, A. J., III; Dias, H. V. R; Harlow, R. L.;

Kline, M. J. Am. Chem. Soc. 1992, 114, 5530.

(31) (a) Crabtree, R. H. Pure Appl. Chem. 2003, 75, 435. (b) Arnold,

P. L.; Pearson, S. Coord. Chem. Rev. 2007, 251, 596. (c) Albrecht, M.

Chem. Commun. 2008, 3601. (d) Schuster, O.; Yang, L.; Raubenheimer,

H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445.

(32) (a) Arduengo, A. J., III; Davidson, F.; Dias, H. V. R.; Goerlich,

J. R.; Khasnis, D.; Marshall, W. J.; Prakasha, T. R. J. Am. Chem. Soc. 1997,

119, 12742. (b) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.

Tetrahedron 1999, 55, 14523.

(16) For reviews regarding NHC chemistry, see for instance:

(a) Regitz, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 725. (b) Herrmann,

W. A.; K€ocher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162.

(c) Arduengo, A. J., III Acc. Chem. Res. 1999, 32, 913. (d) Bourissou,

(33) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004,

126, 8717.

(34) Grishina, A. A.; Polyakova, S. M.; Kunetskiy, R. A.; Císaꢂrovꢀa, I.;

€

D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39.

(e) Wentrup, C. Science 2001, 292, 1846. (f) Nair, V.; Bindu, S.;

Sreekumar, V. Angew. Chem., Int. Ed. 2004, 43, 5130. (g) Hahn, F. E.

Angew. Chem., Int. Ed. 2006, 45, 1348. (h) Hahn, F. E.; Jahnke, M. C.

Angew. Chem., Int. Ed. 2008, 47, 3122. (i) Tapu, D.; Dixon, D. A.; Roe, C.

Chem. Rev. 2009, 3385. (j) Dr€oge, T.; Glorius, F. Angew. Chem., Int. Ed.

2010, 49, 6940.

Lyapkalo, I. M. Chem.—Eur. J. 2011, 17, 96.

(35) (a) Alder, R. W.; Allen, P. R.; Williams, S. J. J. Chem. Soc., Chem.

Commun. 1995, 1267. (b) Kim, Y.-J.; Streitwieser, A. J. Am. Chem. Soc.

2002, 124, 5757. (c) Chu, Y.; Deng, H.; Cheng, J.-P. J. Org. Chem. 2007,

72, 7790.

(36) Arduengo, A. J., III; Bock, H.; Chen, H.; Denk, M.; Dixon,

D. A.; Green, J. C.; Herrmann, W. A.; Jones, N. L.; Wagner, M.; West, R.

J. Am. Chem. Soc. 1994, 116, 6641.

(37) Alcarazo, M.; Gomez, C.; Holle, S.; Goddard, R. Angew. Chem.,

Int. Ed. 2010, 49, 5788.

(38) (a) Herrmann, W. A.; Schneider, S. K.; Ofele, K.; Sakamoto, M.;

Herdtweck, E. J. Organomet. Chem. 2004, 689, 2441. (b) Binobaid, A.;

Iglesias, M.; Beetstra, D. J.; Kariuki, B.; Dervisi, A.; Fallis, I. A.; Cavell,

K. J. Dalton Trans. 2009, 7099. (c) Fl€ugge, S.; Anoop, A.; Goddard, R.;

Thiel, W.; F€urstner, A. Chem.—Eur. J. 2009, 15, 8558. (d) Armstrong,

R.; Ecott, C.; Mas-Marzꢀa, E.; Page, M. J.; Mahon, M. F.; Whittlesey,

M. K. Organometallics 2010, 29, 991.

(17) For reviews regarding NHC metal complexes, see for instance:

(a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b) Clavier,

H.; Nolan, S. P. Annu. Rep. Prog. Chem. Sect. B 2007, 103, 193.

(c) Kantchev, E. A. B.; O’Brien, C.; Organ, M. Angew. Chem., Int. Ed.

2007, 119, 2824. (d) Díez-Gonzꢀalez, S.; Nolan, S. P. Synlett 2007, 2158.

(e) Pugh, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610.

(f) Lin, I. J. P.; Vasam, C. S. Coord. Chem. Rev. 2007, 251, 642.

(g) Douthwaite, R. E. Coord. Chem. Rev. 2007, 251, 702. (h) Gade,

L. H.; Bellemin-Laponnaz, A. Coord. Chem. Rev. 2007, 251, 718. (i)

Colacino, E.; Martinez, J.; Lamaty, F. Coord. Chem. Rev. 2007, 251, 726.

(j) Dragutan, V.; Dragutan, I.; Delaude, L. Coord. Chem. Rev. 2007,

251, 765. (k) Mata, J. A.; Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007,

251, 841. (l) Sommer, W. J.; Weck, M. Coord. Chem. Rev. 2007, 251, 860.

€

(39) Mayr, M.; Wurst, K.; Ongania, K.-H.; Buchmeiser, M. R. Chem.

—Eur. J. 2004, 10, 1256.

ꢁ

(m) Diez-Gonzꢀalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874.

(40) Kolychev, E. L.; Portnyagin, I. A.; Shuntikov, V. V.; Khrustalev,

V. N.; Nechaev, M. S. J. Organomet. Chem. 2009, 694, 2454.

(41) (a) Iglesias, M.; Beetstra, D. J.; Knight, J. C.; Ooi, L.-L.; Stasch,

A.; Coles, S.; Male, L.; Hursthouse, M. B.; Cavell, K. J.; Dervisi, A.; Fallis,

I. A. Organometallics 2008, 27, 3279. (b) Bazinet, P.; Yap, G. P. A.;

Richeson, D. S. J. Am. Chem. Soc. 2003, 125, 13314. (c) Otto, M.;

Conejero, S.; Canac, Y.; Romanenko, V. D.; Rudzevitch, V.; Bertrand, G.

J. Am. Chem. Soc. 2004, 126, 1016. (d) Bazinet, P.; Ong, T.-G.; O’Brien,

J. S.; Lavoie, N.; Bell, E.; Yap, G. P. A.; Korobkov, I.; Richeson,

D. S. Organometallics 2007, 26, 2885. (e) Alder, R. W.; Blake, M. E.;

Bortolotti, C.; Bufali, S.; Butts, C. P.; Linehan, E.; Oliva, J. M.; Orpen,

A. G.; Quayle, M. J. Chem. Commun. 1999, 241.

(n) Cavell, K. Dalton Trans. 2008, 6676. (o) Arnold, P. L.; Casely, I. J.

Chem. Rev. 2009, 109, 3599. (p) Díez-Gonzꢀales, S.; Marion, N.; Nolan,

S. P. Chem. Rev. 2009, 109, 3612. (q) Poyatos, M.; Mata, J.; Peris, E.

Chem. Rev. 2009, 109, 3677. (r) Samojzowicz, C.; Bieniek, M.; Grela, K.

Chem. Rev. 2009, 109, 3708. (s) Mercsa, L.; Albrecht, M. Chem. Soc. Rev.

2010, 39, 1903.

(18) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.;

Tamm, M. Angew. Chem., Int. Ed. 2008, 47, 7428.

(19) Chase, P. A.; Stephan, D. W. Angew. Chem., Int. Ed. 2008, 47, 7433.

(20) Holschumacher, D.; Taouss, C.; Bannenberg, T.; Hrib, C. G.;

Daniliuc, C. G.; Jones, P. G.; Tamm, M. Dalton Trans. 2009, 6927.

(21) Chase, P. A.; Gille, A. L.; Gilbert, T. M.; Stephan, D. W. Dalton

Trans. 2009, 7179.

(42) Denk, M. K.; Gupta, S.; Brownie, J.; Tajammul, S.; Lough, A. J.

Chem.—Eur. J. 2001, 7, 4477.

(22) Rokob, T. A.; Hamza, A.; Stirling, A.; Pꢀapai, I. J. Am. Chem. Soc.

2009, 131, 2029.

(43) Arentsen, K.; Caddick, S.; Cloke, F. G. N. Tetrahedron 2005,

61, 9710.

7358

dx.doi.org/10.1021/ic201290g |Inorg. Chem. 2011, 50, 7344–7359

Inorganic Chemistry

ARTICLE

(44) Arduengo III., A. J. U.S. Patent 5 077 414, 1991.

(45) (a) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew.

Chem. 1996, 35, 1121. (b) Alder, R. W.; Blake, M. E.; Bufali, S.; Butts,

C. P.; Orpen, A. G.; Sch€utz, J.; Williams, S. J. J. Chem. Soc., Perkin Trans. 1

2001, 1586.

(46) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157.

(47) Wang, C.; Erker, G.; Kehr, G.; Wedeking, K.; Fr€ohlich, R.

Organometallics 2005, 24, 4760.

(48) Frisch, M. J. et al. Gaussian 09, Revision A.1; Gaussian, Inc.:

Wallingford, CT, 2009.

(49) Cao, X.; Dolg, M. J. Chem. Phys. 2001, 115, 7348.

(50) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

(51) Runyon, J.; Steinhof, O.; Dias, H. V. R.; Calabrese, J.; Marshall,

W.; Arduengo, A. J., III. Aust. J. Chem., in press.

7359

dx.doi.org/10.1021/ic201290g |Inorg. Chem. 2011, 50, 7344–7359

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Dihydrogen activation by frustrated carbene-borane lewis pairs: An experimental and theoretical study of carbene variation

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