Tuning the electronic properties of carbenes: A systematic comparison of neighboring amino versus amido groups

Article abstract of DOI:10.1021/om3001586

A related series of six-membered carbenes featuring adjoining amino and/or amido groups (i.e, a diaminocarbene, a monoamido-aminocarbene (3), and a a diamidocarbene (6)) were systematically compared using crystallographic, spectroscopic, electrochemical, and density functional theory methods. The solid-state structure of 3 was found to exhibit inequivalent nitrogen-carbon bond lengths (C_carbene-N_amide = 1.395(4) A vs C _carbene-N_amine = 1.323(4) A). Moreover, the C _carbene-N_amide distance was longer than that measured in the solid-state structure of 6 (1.371(3) A), while the C _carbene-N_amine distance was similar to that measured in the solid-state structure of a cyclic alkyl-aminocarbene (1.315(3) A). Iridium complexes of the aforementioned carbenes were also evaluated, and the collected data revealed that the introduction of carbonyl groups to the carbene-containing scaffold had a nearly linear, additive effect on the E _1/2 potential of the carbene-ligated iridium I/II redox couple (+165 mV per carbonyl added) as well as the Tolman electronic parameter value of the corresponding carbene-Ir(CO)₂Cl complex (ca. 7 cm^-1 per carbonyl added). Beyond attenuated ligand donicity, the introduction of carbonyl groups was found to broaden the chemical reactivity: unlike prototypical N-heterocyclic carbenes, including diaminocarbenes, the monoamido-aminocarbene was found to couple to isonitriles to form the respective ketenimines.

Full text of DOI:10.1021/om3001586

Article
pubs.acs.org/Organometallics
Tuning the Electronic Properties of Carbenes: A Systematic
Comparison of Neighboring Amino versus Amido Groups
Garrett A. Blake, Jonathan P. Moerdyk, and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, United States
S
* Supporting Information
ABSTRACT: A related series of six-membered carbenes featuring
adjoining amino and/or amido groups (i.e, a diaminocarbene, a
monoamido-aminocarbene (3), and a a diamidocarbene (6)) were
systematically compared using crystallographic, spectroscopic, electro-
chemical, and density functional theory methods. The solid-state structure of 3 was found to exhibit inequivalent nitrogen−
carbon bond lengths (Ccarbene−Namide = 1.395(4) Å vs Ccarbene−Namine = 1.323(4) Å). Moreover, the Ccarbene−Namide distance was
longer than that measured in the solid-state structure of 6 (1.371(3) Å), while the Ccarbene−Namine distance was similar to that
measured in the solid-state structure of a cyclic alkyl-aminocarbene (1.315(3) Å). Iridium complexes of the aforementioned
carbenes were also evaluated, and the collected data revealed that the introduction of carbonyl groups to the carbene-containing
scaffold had a nearly linear, additive effect on the E1/2 potential of the carbene-ligated iridium I/II redox couple (+165 mV per
carbonyl added) as well as the Tolman electronic parameter value of the corresponding carbene−Ir(CO)2Cl complex (ca. 7 cm−1
per carbonyl added). Beyond attenuated ligand donicity, the introduction of carbonyl groups was found to broaden the chemical
reactivity: unlike prototypical N-heterocyclic carbenes, including diaminocarbenes, the monoamido-aminocarbene was found to
couple to isonitriles to form the respective ketenimines.
molecule activation10 and other applications, such as catalysis,11
INTRODUCTION
■
understanding how appropriately positioned carbonyl groups
influence the chemical reactivity displayed by these reagents is
warranted.12 To address this fundamental question, a
monoamido-aminocarbene (MAAC, 3) was envisioned as a
structural intermediate of DACs and NHCs and anticipated to
be an effective probe for quantifying the effect of incorporating
carbonyl groups into the parent diaminocarbene heterocyclic
scaffold. We hypothesized that there would be a direct increase
in the electrophilicity of the carbenes as additional carbonyls
were incorporated into the carbene scaffold. A comparison to
Bertrand’s 2-azaspiro[4.5]dec-1-ylidene,2-[2,6-bis(1-
methylethyl)phenyl]-3,3,9-trimethyl-6-(1-methylethyl),13
which is often called a cyclic alkyl-aminocarbene (CAAC), or
more simply a monoaminocarbene, was also pursued to provide
additional insight into understanding how the adjoining
nitrogen donors stabilize the carbene nucleus.
Herein we report a systematic comparison of the first
isolable14 monoamido-aminocarbene (MAAC) (3) to analo-
gous diamino- and diamidocarbenes (5 and 6, respectively; see
Figure 1). These carbenes as well as their Ir(COD)Cl and
Ir(CO)2Cl complexes were collectively analyzed using a variety
of crystallographic, spectroscopic, electrochemical, and density
functional theory techniques. The data obtained enabled a
quantitative assessment of carbenes stabilized by adjoining
amino versus amido functional groups.
Although carbenes have been utilized as reactive intermediates
for nearly 150 years,1 the first isolable derivatives were not
realized until the turn of the 1990s, when Bertrand reported the
isolation of the phosphinosilyl carbenes2 and Arduengo
disclosed a crystalline 1,3-diadamantylimidazol-2-ylidene.3
Derivatives of the latter, which are often called N-heterocyclic
carbenes (NHCs),4 have found exceptional utility as ligands for
transition metal catalysts5 and as organocatalysts.6 NHCs are
stabilized primarily by the donation of the nitrogen lone pairs
into the empty p-orbital of the carbene nucleus to which they
are bound. However, this stabilization phenomenon renders the
carbenes singlet in character and imposes limitations on the
electronic properties as well as the reactivity displayed by these
species, especially compared to more electrophilic triplet
carbenes (e.g., methylene)4d and transient singlet carbenes
(e.g., dimethoxycarbene).7
Recently our group launched a program aimed at reducing
the nucleophilicity of NHCs and thereby expanding their
chemical reactivity and utility. Through a variety of
spectroscopic, crystallographic, and other techniques, we
showed that N,N′-diamidocarbenes (DACs) exhibit significantly
reduced nucleophilicity compared to NHCs and other
diaminocarbenes and engage in reactions frequently associated
with more electrophilic carbenes.8 For example, DACs have
been found to facilitate N−H and intramolecular C−H bond
activation processes, reversible couplings to carbon monoxide,
irreversible couplings with isonitriles to form ketenimines, and
[2+1] cycloadditions of olefins, aldehydes, alkynes, and
nitriles.8a,9 Given the broad reactivity displayed by DACs as
well as the potential utility of such compounds in small-
Received: February 26, 2012
Published: April 9, 2012
© 2012 American Chemical Society
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Figure 1. Structures of the six-membered diaminocarbene 5, the
MAAC 3, and the DAC 6 (Mes = 2,4,6-trimethylphenyl).
RESULTS AND DISCUSSION
■
As summarized in Scheme 1, the monoamido-aminocarbene 3
was synthesized in three steps from readily available starting
materials in good overall yield. Condensation of N,N′-
mesitylformamidine and 3-chloropivaloyl chloride in the
presence of excess triethylamine at 0 °C afforded 1, as
indicated by the appearance of the 1H NMR signals observed at
δ = 1.20 and 3.70 ppm (assigned to the backbone methyl and
methylene protons) and the appearance of the amide carbon
observed at 176.1 ppm in the 13C NMR spectrum (C6D6) of
the crude reaction mixture. Removal of the solvent followed by
the addition of toluene, filtration, and concentration of the
filtrate afforded pure 1 in excellent yield (94%). To induce
intramolecular cyclization, a toluene solution of 1 was heated to
reflux for 16 h, which resulted in the precipitation of a white
solid, which was later determined to exhibit a 1H NMR signal at
10.24 ppm (CDCl3). Diagnostic of a dihydropyrimidinium
species,15 the aforementioned NMR data supported the
structure of the isolated compound as 2, which was ultimately
isolated in 91% yield via filtration. Subsequent treatment of 2
with sodium hexamethyldisilazane (NaHMDS) in benzene for
30 min at 25 °C followed by trituration with pentane afforded
the free carbene 3, as evidenced by the disappearance of the
dihydropyrimidinium proton signal in the compound’s 1H
NMR spectrum and the appearance of a new signal at 260.6
ppm in the 13C NMR spectrum (C6D6), which was assigned to
the carbene nucleus. On a practical note, the monoamido-
aminocarbene 3 was found to be a high-melting solid (126−130
°C) and stable in anhydrous benzene for at least three days.
Moreover, the compound was successfully stored under a dry
nitrogen atmosphere in the solid state for months without
decomposition, as determined by NMR spectroscopy.
Figure 2. ORTEP diagram of 3 with thermal ellipsoids drawn at 50%
probability and H atoms omitted for clarity. Selected distances (Å) and
angles (deg): C1−N1 1.395(4), C1−N2 1.323(4), N1−C2 1.411(5),
C2−O1 1.225(4), N1−C1−N2 114.0(3).
than the analogous angle measured in the solid-state structure
of 6 (114.8°). Regardless, the Ccarbene−Namine distance measured
in 3 was comparable to that reported for the solid-state
structure of Bertrand’s CAAC (1.315(3) Å),13 which suggested
to us that donation by the amino group in the MAAC scaffold
dominated the stabilization of the carbene nucleus.
Scheme 2. Synthesis of the Amino-Amido-Ketenimines and
a
the Dithioate Adduct 3-CS2
a
Conditions: (i) CS2, C6H6, 25 °C, 1 h. (ii) RNC, C6H6, 25 °C, 12 h.
Isolated yields are indicated.
Having isolated and characterized the free carbene 3, our
attention shifted to exploring its chemical reactivity. As
diaminocarbenes are known to condense with electrophiles,
we first turned our attention to determine if 3 would react with
the electrophilic agent CS2 (Scheme 2). The addition of CS2
(1.05 equiv) to a benzene solution of 3 ([3]0 = 14 mM) at
ambient temperature resulted in the formation of a deep red
color. Concentration and subsequent addition of pentane
induced precipitation, which enabled isolation of the expected
product 3-CS2 in excellent yield (94%), as evidenced by the
observation of a 13C NMR resonance diagnostic of a dithioate
species (δ = 158.7 ppm; C6D6) and an upfield carbenoid 13C
NMR resonance (i.e., 222.3 ppm in 3-CS2 vs 260.6 ppm in 3;
To gain additional structural information, single crystals of 3
were grown by cooling a saturated hexane solution (Figure 2)
to −40 °C. Single-crystal X-ray diffraction analysis revealed that
the Ccarbene−Namide distance in 3 (C1−N1 = 1.395(4) Å) was
significantly elongated compared to the Ccarbene−Namine distance
(C1−N2 = 1.323(4) Å). Additionally, the Ccarbene−Namide
distance was longer than that measured in the previously
reported8a solid-state structure of 6 (1.371(3) Å), signifying
weaker donation from the amide nitrogen in the former. The
differential donation may explain the difference in bond angles
observed: the N−C−N angle in 3 (114.0°) was slightly smaller
a
Scheme 1. Synthesis of the MAAC 3
a
Conditions: (i) 3-chloropivaloyl chloride, Et3N, CH2Cl2, 0 °C, 30 min; (ii) toluene, 16 h, 110 °C; (iii) NaHMDS, C6H6, 25 °C, 30 min. Isolated
yields are indicated.
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a
C6D6). After demonstrating 3 may function as a nucleophile,
we next turned our attention to exploring its ability to also act
as an electrophile. Upon the addition of n-butyl isocyanide
(1.05 equiv) to 3 ([3]0 = 23 mM) in benzene at ambient
temperature, a color change from light tan to bright yellow was
observed. Subsequent stirring of the reaction mixture for 12 h
ultimately yielded the corresponding ketenimine 4a in 82%
isolated yield (see Supporting Information), as indicated by the
new 13C NMR resonances observed at 210.1 and 114.0 ppm
(C6D6), which were diagnostic of the CCN and CCN carbons,
respectively.9b Benzyl- and 2,6-diisopropylphenylisonitrile were
found to react with 3 in a similar manner and afforded the
corresponding ketenimines 4b and 4c in good yield (74−82%,
Scheme 2).16 Although structurally analogous to the
diamidoketenimines derived from 6 and the corresponding
isonitriles, 4a−c were found to slowly decompose over time.
The instability of these adducts may reflect a weaker interaction
between the isonitriles and the MAAC 3 as compared to the
DAC 6. Unfortunately, 3 showed no reactivity toward carbon
monoxide8a or ammonia9a and did not react cleanly with
various olefins9c (e.g., acrylonitrile or methyl acrylate) or
aldehydes (e.g., benzaldehyde, 2-methylpropanal, or acrolein).9c
Following an investigation of the chemical reactivity,
subsequent efforts were taken to characterize the electronic
properties of 3. Iridium carbonyl complexes are commonly used
to quantify the electron-donating ability of NHCs and other
ligands through the calculation of the Tolman electronic
parameter (TEP) based on the observed infrared νCO stretching
frequencies.17,18 As such, 3-Ir(CO)2Cl was first synthesized as
shown in Scheme 3. Addition of 3 to a benzene solution of
Scheme 4. Synthesis of 5-Ir(COD)Cl and 5-Ir(CO)2Cl
a
Conditions: (i) 2,4,6-trimethylaniline, pyridine, CH2Cl2, −78 °C, 2 h;
(ii) BH3·SMe2, THF, 85 °C, 12 h; (iii) NH4BF4, CH(OEt)3, n-
heptane, 90 °C, 12 h; (iv) (1) NaHMDS, C6H6, 25 °C, 1 h; (2)
{Ir(COD)Cl}2, C6H6, 25 °C, 12 h; (v) CO, CH2Cl2, 25 °C, 30 min.
Isolated yields are indicated.
by condensing 2,2-dimethylmalonyl dichloride with mesityl
amine in 64% yield and found to display a distinctive amide 13
C
NMR resonance at 172.5 ppm (C6D6). Subsequent reduction
of 5a in a refluxing solution of BH3·SMe2 in THF afforded the
diamine 5b in 82% yield. A new signal was observed at 2.80
1
ppm in the H NMR spectrum acquired for this material, and
the loss of the amide carbon resonances concomitant with the
appearance of a new methylene resonance observed at 58.3
ppm in the 13C NMR spectrum (C6D6) served to confirm the
structure product as indicated. Formylative cyclization of 5b to
5·HBF4 was conducted with CH(OEt)3 in n-heptane and
facilitated with NH4BF4, as indicated by a characteristic
dihydropyrimidinium resonance at 7.62 ppm in the respective
1H NMR spectrum (CDCl3).20 Unfortunately, the free carbene
5 could not be isolated; efforts to concentrate benzene
solutions of the carbene (generated in situ via treatment of
5·HBF4 with NaHMDS) resulted in decomposition, as did
attempts at selectively precipitating the desired compound.
However, in situ deprotonation of 5·HBF4 in the presence of
{Ir(COD)Cl}2 in C6H6 afforded 5-Ir(COD)Cl as an orange
solid in 62% yield following purification via column
chromatography. Bubbling CO through a CH2Cl2 solution of
5-Ir(COD)Cl afforded 5-Ir(CO)2Cl. The formation of this
complex was supported by the appearance of strong CO bands
in the IR spectrum (1976 and 2061 cm−1) and new 13C NMR
signals at 171.3 and 180.7 ppm (C6D6) associated with carbonyl
ligands.
The ligand-donating ability of the aforementioned carbenes
was investigated by analyzing the corresponding Ir carbonyl
complexes using IR spectroscopy. Using Nolan’s method,18 the
TEP for 3 was calculated to be 2050 cm−1; this value was
significantly lower than that calculated for 6 from the analogous
Ir complex (2056 cm−1)11a but markedly higher than that of 5
(2042 cm−1) (Figure 2). While TEPs are commonly used to
analyze the donicity of NHCs, the measurement is indirect, as it
relies upon changes in the stretching frequency of the
coordinated carbonyl groups.11a,21 To directly quantify the
electronic properties of the carbene-ligated metal centers, a
series of cyclic voltammetry and differential pulse voltammetry
measurements were performed (see Supporting Information).
As summarized in Table 1, 3-Ir(COD)Cl displayed an
oxidation potential 170 mV lower than the analogous 6
complex, indicating that 3 is a stronger donor than DAC.
Likewise, the oxidation potential of 5-Ir(COD)Cl was 160 mV
lower than that of 3-Ir(COD)Cl. Altogether, the data suggested
a
Scheme 3. Synthesis of 3-Ir(COD)Cl and 3-Ir(CO)2Cl
a
Conditions: (i) {Ir(COD)Cl}2, C6H6, 25 °C, 16 h; (ii) CO, CH2Cl2,
25 °C, 30 min. Isolated yields are indicated.
{Ir(COD)Cl}2 at 25 °C for 16 h resulted in the formation of 3-
Ir(COD)Cl, as indicated by the presence of the cyclooctadiene
1
olefin signals observed at 3.06, 4.56, and 4.69 ppm in the H
NMR spectrum as well as the upfield shift of the carbenoid
resonance to 218.6 ppm in the 13C NMR spectrum (C6D6) of
the crude reaction mixture. Subsequent purification by column
chromatography afforded the desired complex in 75% isolated
yield. Exchange of the cyclooctadiene ligand for two carbonyls
was achieved by bubbling CO through a CH2Cl2 solution of 3-
Ir(COD)Cl at ambient temperature followed by washing the
dry solid with pentane to afford a bright yellow solid. The
product was identified as 3-Ir(CO)2Cl by the absence of NMR
signals associated with the starting material, the appearance of
new 13C NMR signals observed at 170.1 and 179.5 ppm
(C6D6), and the appearance of strong CO bands in the IR
spectrum (1986 and 2066 cm−1), which were attributed to the
carbonyl ligands.
For comparative purposes, a saturated diamino analogue, 5-
Ir(CO)2Cl, was also prepared using a modified version of a
literature procedure (Scheme 4).19 Bisamide 5a was obtained
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Table 1. Summary of Spectroscopic, Electrochemical, and Crystallographic Data
a
b
The TEP values were calculated from the corresponding LIr(CO)2Cl complexes using Nolan’s method.18 The electrochemical data were obtained
from the corresponding LIr(COD)Cl complexes in CH2Cl2 in the presence of 0.1 M [Bu4N][PF6] electrolyte and referenced to a
c
decamethylferrocene (Fc*) internal standard. Carbene−metal bond distance measured in the solid-state structure of the indicated carbene−
d
e
f
Ir(COD)Cl complex. Calculated using the SambVca application.22 In d6-benzene. The carbene was prepared in situ from 5·HBF4 and NaHMDS
at 25 °C. In d8-toluene.23 In CDCl3. Since 7-Ir(COD)Cl is unknown, the NMR data reported for the corresponding Rh complex were used.14 In
g
h
i
j
CD2Cl2. Since 7-Ir(COD)Cl is unknown, the metal−carbene distance shown was obtained from the corresponding Rh complex.14
k
Figure 3. ORTEP diagram of 3-Ir(COD)Cl and 5-Ir(COD)Cl with thermal ellipsoids drawn at 50% probability and H atoms omitted for clarity.
Selected distances (Å) and angles (deg): 3-Ir(COD)Cl: Ir1−Cl1 2.378(5), Ir1−C1 2.040(6), Ir1−C25 2.131(6), Ir1−C28 2.170(5), Ir1−C29
2.207(5), Ir1−C32 2.108(6), N1−C2 1.405(7), C2−O1 1.210(7), N1−C1−N2 114.8(5), C1−Ir1−Cl1 87.67(2). 5-Ir(COD)Cl: Ir1−Cl1 2.365(0),
Ir1−C1 2.067(4), Ir1−C25 2.108(4), Ir1−C26 2.118(4), Ir1−C29 2.154(5), Ir1−C30 2.169(5), N1−C1−N2 116.3(4), C1−Ir1−Cl1 88.18(1).
to us that the donating ability of 3 was intermediate between 5
and 6 and that the incorporation of additional carbonyl groups
had an additive effect on attenuating the parent diaminocarbene
donating ability.
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To gain additional structural information, crystals of 3-
Ir(COD)Cl and 5-Ir(COD)Cl were grown by the slow vapor
diffusion of hexanes and pentane into concentrated benzene
solutions of the respective compounds (Figure 3).24 The solid-
state structure of 6-Ir(COD)Cl has been previously reported.8a
The observed carbene−iridium bond lengths of 1.985(6),8a
2.040(6), and 2.067(4) Å for 6-, 3-, and 5-Ir(COD)Cl,
respectively, fall within the expected range for carbene−iridium
complexes (1.95−2.10).20,21 However, the inverse relationship
between the observed M−Ccarbene bond distances and the ligand
donicity derived from the aforementioned TEP and oxidation
potentials was attributed to enhanced π-back-bonding between
the electron-rich metal centers and the less donating carbene
ligands.11a,12,25
properties. Per carbonyl introduced, the E1/2 potential of the
corresponding ligated iridium I/II redox couple increased by an
average of 165 mV and the TEP value increased by 7 cm−1.
Considering that similar trends were observed between five-
and six-membered carbene scaffolds,28 we believe the results
will help guide the general design of other stable, electron-
deficient carbenes.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic procedures as well as additional crystallographic,
electrochemical, spectroscopic, and computational data are
available free of charge via the Internet at http://pubs.acs.org.
To determine if the effects outlined above were general, our
attention shifted toward analyzing the five-membered analogues
of the aforementioned carbenes (Figure 3): the prototypical
NHC 1,3-dimesityldihydroimidazol-2-ylidene (SIMes), Lav-
igne’s26 monoamido-aminocarbene 7, and Ganter’s diamido-
carbene 8.12a Contrary to the six-membered series, the TEP
value measured for 7 (2058 cm−1) was found not to be the
median of SIMes (2053 cm−1) and 8 (2068 cm−1), which may
be explained by the partial formation of the more donating enol
tautomer.14,26 Regardless, akin to their six-membered ana-
logues, increasing the number of carbonyl groups incorporated
into the five-membered diaminocarbene scaffold resulted in the
reduction of carbene donicity, as evidenced by the increasing
TEP values and decreasing carbene−metal bond distances.
In addition to the aforementioned spectroscopic and
electrochemical analyses, computational methods were em-
ployed to help elucidate the observed chemical reactivity and
electronic properties displayed by the free diaminocarbenes as
compared to their carbonyl-containing derivatives. As summar-
ized in Figure S9, the HOMO−LUMO gap (ΔHS−T),27
calculated at the B3LYP 6-31+G(d) level of theory for the
N-methyl analogues of 3, 5, and 6 decreased relatively
minimally between 5-Me (116.06 kcal/mol) and 3-Me
(112.27 kcal/mol); however, a large difference was calculated
between 6-Me (98.19 kcal/mol) and 3-Me. This observation,
coupled with the asymmetry in the observed C−N bond
lengths measured in the solid-state structure of 3, suggested to
us that the amino group effectively compensated for the
relatively weakly donating amido group. Likewise, the relatively
small ΔHS−T calculated for 6-Me was consistent with the
electrophilic character displayed by DAC 6, particularly
compared to prototypical NHCs.8,9,11,12
AUTHOR INFORMATION
Corresponding Author
*E-mail: bielawski@cm.utexas.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation (CHE-
0645563), the Robert A. Welch Foundation (F-1621), and the
Sloan Foundation for their generous support.
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CONCLUSION
■
The first isolable monoamido-aminocarbene carbene (3) and
corresponding iridium complexes (3-Ir(COD)Cl and 3-
Ir(CO)2Cl) were synthesized and characterized. The MAAC
3 was found to exhibit an asymmetric electronic structure and
exhibited reactivity intermediate of typical NHCs and DACs: 3
coupled with typical organic electrophiles (i.e., CS2 and
coordinatively unsaturated metal centers) but also nucleophilic
isonitriles. In support of this observation, spectroscopic and
electrochemical characterization of the various carbene−iridium
compounds revealed the electronic properties of 3 were the
intermediate of prototypical NHCs and DACs. Importantly,
these findings provided a quantitative assessment of the effect
of incorporating one versus two carbonyl moieties into an
NHC scaffold and that increasing the number of carbonyls had
an additive effect on the carbene’s electronic and chemical
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Organometallics
Article
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