A seven-membered N,N ′-diamidocarbene

Article abstract of DOI:10.1021/om1007665

Condensation of N,N′-dimesitylformamidine with phthaloyl chloride afforded 1?HCl, which, upon treatment with base, afforded 2,4-dimesitylbenzo[e][1,3]diazepin-1,5-dione-2-ylidene (1), a seven-membered N,N′-diamidocarbene (DAC), in high yield (85%). The free DAC was used to synthesize four new, late transition metal complexes: [Rh(cod)(1)Cl] (2a) (cod = 1,5-cyclooctadiene), [Ir(cod)(1)Cl] (2b), [Rh(CO)₂(1)Cl] (3a), and [1-AuCl] (5). The Tolman electronic parameter (TEP) of 1 was calculated to be 2047 cm^-1 from the IR spectrum of 3a. This TEP value is approximately 10 cm^-1 lower than known DACs and 5 cm^-1 lower than known imidazol-2-ylidenes, indicating that DAC 1 is a relatively strong electron donor. Additionally, electrochemical analyses of 2a and 2b corroborated the IR data obtained on 3a and revealed E_1/2 values that were shifted cathodically by ca. 0.16 V when compared to analogous complexes supported by N-heterocyclic carbenes. The gold complex 5 was found to catalyze the hydration of phenylacetylene, affording acetophenone in yields up to 78% after 12 h at 80 °C at a catalyst loading of 2 mol %. Treatment of 1 with 2,6-dimethylphenylisocyanide afforded N,N′-diamidoketenimine 4 as a thermally robust, crystalline solid.

Full text of DOI:10.1021/om1007665

Organometallics 2010, 29, 4569–4578 4569
DOI: 10.1021/om1007665
A Seven-Membered N,N0-Diamidocarbene
Todd W. Hudnall, Andrew G. Tennyson, and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin,
Texas 78712, United States
Received August 6, 2010
Condensation of N,N0-dimesitylformamidine with phthaloyl chloride afforded 1 HCl, which, upon
3
treatment with base, afforded 2,4-dimesitylbenzo[e][1,3]diazepin-1,5-dione-2-ylidene (1), a seven-membered
N,N0-diamidocarbene (DAC), in high yield (85%). The free DAC was used to synthesize four new,
late transition metal complexes: [Rh(cod)(1)Cl] (2a) (cod = 1,5-cyclooctadiene), [Ir(cod)(1)Cl] (2b),
[Rh(CO)2(1)Cl] (3a), and [1-AuCl] (5). The Tolman electronic parameter (TEP) of 1 was calculated
to be 2047 cm-1 from the IR spectrum of 3a. This TEP value is approximately 10 cm-1 lower than
known DACs and 5 cm-1 lower than known imidazol-2-ylidenes, indicating that DAC 1 is a rela-
tively strong electron donor. Additionally, electrochemical analyses of 2a and 2b corroborated the IR
data obtained on 3a and revealed E1/2 values that were shifted cathodically by ca. 0.16 V when com-
pared to analogous complexes supported by N-heterocyclic carbenes. The gold complex 5 was found to
catalyze the hydration of phenylacetylene, affording acetophenone in yields up to 78% after 12 h at
80 °C at a catalyst loading of 2 mol %. Treatment of 1 with 2,6-dimethylphenylisocyanide afforded N,
N0-diamidoketenimine 4 as a thermally robust, crystalline solid.
Introduction
N-Heterocyclic carbenes (NHCs), comprising a divalent carbene
nucleus flanked by one or two nitrogen atoms in particular,3
have led to great advances in synthetic chemistry, especially
in homogeneous catalytic transformations that are mediated
by transition metal complexes4 and organic compounds.5 In
contrast, the utility of NHCs and other stabilized carbenes to
activate industrially important small molecules remains largely
undeveloped.6 Bertrand reported the first examples of splitting
both dihydrogen (H2) and ammonia (NH3) via H-X bond
polarization using alkyl amino carbenes (AACs) I and II
(Figure 1).6a Although likely to be more electrophilic than
NHCs, the remarkable reactivities exhibited by the AACs are
believed to be due to an increased nucleophilic character.6a
Recently, we discovered that N,N0-diamidocarbenes (DACs)
IIIa and IIIb (Figure 1) also display an unusual combination
of electrophilic and nucleophilic characteristics.7 In addition
to their abilities to coordinate a range of transition metals
and couple with electrophilic organic substrates (e.g., CS2),
DACs III were found to engage in reactions atypical of NHCs,
including intramolecular C-H insertion,7a coupling with iso-
cyanides to form ketenimines,7b coupling with carbon mon-
oxide in a reversible manner,6b,7a,8 and activation of NH3.6b
Despite forcing conditions, however, III was unable to activate
Heteroatom-stabilized singlet carbenes were first described
by Breslow and later by Wanzlick as reactive intermediates
more than half a century ago;1 however, the field lay fallow
until Bertrand’s landmark report of a stable, free carbene
several decades later.2 Since then, in the chemistry of these
once-elusive divalent carbon species has entered a renaissance.3
*To whom correspondence should be addressed. E-mail: bielawski@
cm.utexas.edu.
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r
2010 American Chemical Society
Published on Web 09/29/2010
pubs.acs.org/Organometallics

4570 Organometallics, Vol. 29, No. 20, 2010
Hudnall et al.
Scheme 1. Synthesis of DAC 1a
a Conditions: (i) phthaloyl chloride, Et3N, CH2Cl2, 0 °C, 1 h; (ii) Na-
HMDS, C6H6, 25 °C, 30 min. Isolated yields are indicated.
NHCs have been shown to be capable of inserting into
benzylic C-H bonds, whereas smaller-ring NHCs generally
do not exhibit this reactivity.9k In addition to increased
nucleophilicities, NHCs with wider N-C-N angles often
impose steric constraints on transition metals to which they
are ligated, features that have led to the development of
chiral NHC-supported asymmetric catalysts9a-c,i,j and novel
Ru-based olefin metathesis catalysts.9f Inspired by these reports,
and with the aims of (1) enhancing the nucleophilicities of
DACs to expand their reactivities and (2) incorporating
sterically imposing DACs into catalytically active metal com-
plexes in mind, we herein disclose the syntheses, character-
ization, and applications of the first seven-membered DAC.
Figure 1. Examples of alkyl amino carbenes I and II, N,N0-di-
amidocarbenes III, and seven-membered diaminocarbenes IV,
V, and VI.
dihydrogen, possibly due to insufficient polarization of the
H-H bond. Analogous to AACs, we reasoned that increas-
ing the nucleophilic characteristics of DACs would expand
their reactivity toward small molecules.
Results and Discussion
One method for increasing the electron donicity and, by
extension,6a,9 the nucleophilicity of a cyclic diaminocarbene
is to widen its N-C-N angle.9d,e,g,l,m For example, Fallis
demonstrated that seven-membered diazepine-2-ylidenes,
such as IV, and derivatives thereof, which exhibit average
N-C-N angles ranging from 113.4-116.6°,9e,g are stronger
electron donors than imidazolin-2-ylidenes and tetrahydro-
pyrimidenes, whose average N-C-N angles are in the range
102-105° and 112-114°, respectively. Similarly, other seven-
membered NHCs, including V and VI, have also been shown
to be relatively strong nucleophiles9 electron donors, in some
cases comparable to the cyclic alkyl amino carbenes (CAACs) II
reported by Bertrand.10
Synthesis and Structure of DAC 1. Condensation of N,N0-
dimesitylformamidine with phthaloyl chloride in CH2Cl2,
followed by solvent removal under reduced pressure and
extraction of the product with toluene, afforded the desired
DAC precursor 1 HCl in 88% yield (Scheme 1).6b,7c Although
3
consistent with the formation of a neutral chloroaminal, the
1H NMR spectrum recorded for 1 HCl exhibited a NCHN
3
resonance at δ=6.41 ppm (CDCl3) that was upfield compared
to the analogous signals observed at δ=6.81 and 6.85 ppm
(CDCl3) for the HCl adducts of IIIa and IIIb, respectively.6b,7c
The different chemical shifts observed may be rationalized
by the disparity in ring sizes, where the larger ring in 1 HCl
3
The strong donicities observed in seven-membered NHCs
may be attributed to an increase in the degree of sp-hybridi-
zation at the carbene nucleus, which results in localization
of the carbene lone pair in a more diffuse orbital with greater
p-character.4,9d,9e,9g,9l,9m For example, seven-membered
forces the N-mesityl substituents closer to the NCHN group
and results in increased shielding when compared to IIIa HCl.11
Regardless, the IR spectrum of 1 HCl revealed carbonyl
3
3
stretching energies (νCO=1672 cm-1, KBr) that were in accord
with the HCl adducts of DACs III (IIIa HCl: 1682 cm-1
,
3
ATR; IIIb HCl: 1685 cm-1, KBr) and consistent with
3
νCO exhibited by prototypical amide functional groups
(9) (a)Scarborough, C. C.; Grady, M. J. W.; Guzei, I. A.;Gandhi, B. A.;
Bunel, E. E.; Stahl, S. S. Angew. Chem., Int. Ed. 2005, 44, 5269–5272.
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D. J.; Knight, J. C.; Ooi, L.-L.; Stasch, A.; Coles, S.; Male, L.; Hursthouse, M. B.;
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(1630-1695 cm-1).6b,7c
Deprotonation of 1 HCl by NaN(SiMe3)2 (NaHMDS) in
3
benzene, followed by solvent removal under reduced pres-
sure and washing with cold hexanes, afforded the desired
seven-membered DAC 1 in 85% yield as a bright yellow
powder. The 13C NMR spectrum of 1 exhibited a signal at
δ = 268.4 ppm (C6D6, Table 1), which was assigned to the
carbene nucleus and in good agreement with signals dis-
played by analogous seven-membered NHCs (δ=255-269
ppm),9 but still upfield from signals observed in the six-
membered DAC analogues IIIa and IIIb (δ=278 ppm).6b,7a
The observed chemical shift differences between 1 and III
may result from the increased shielding due to the larger
seven-membered ring, in accord with the aforementioned 1H
NMR spectroscopic data obtained for 1 HCl and III HCl.
3
3
Although at a lower energy than the νCO observed in
(11) Despite our best efforts, we were unable to grow X-ray quality
crystals of 1 HCl.
3

Article
Organometallics, Vol. 29, No. 20, 2010 4571
Table 1. Summary of Spectroscopic, Structural, and Electrochemical Data
1
2a
2b
3a
4
5
13C NMR (ppm)a
268.4
1681
244.0
1669
þ 0.84
230.3
1676
þ 0.84
222.6
1699, 2002, 2070e
88.9, 184.1d
1663, 1675
202.1
1715
ν
CO (cm-1 b
)
E1/2 (V)c
Epa (V)c
þ 1.27
122.9(3)
2.093(3)
1.202
1.355
1.430
N-C-N (deg)
122.6(2)
119.7(5)
2.051(6)
1.211
1.366
1.431
119.7(4)
2.043(4)
1.210
1.381
1.416
118.05(14)
123.4(2)
2.002(3)
1.204
1.355
1.449
˚
M-C (A)
av CacyldO (A)
˚
1.209
1.354
1.423
1.221
1.435
1.384
˚
av N-Ccarbene (A)
˚
av N-Cacyl (A)
a 13C NMR spectra were obtained in C6D6 (1 and 4) or CD2Cl2 (2, 3, and 5), and the listed value corresponds to the carbene nucleus in the respective
compound or complex. b IR data obtained in KBr matrices. c Electrochemical analyses were obtained in dry CH2Cl2 with 0.1 M nBu4NPF6 electrolyte
using a 100 mV s-1 scan rate and referenced to saturated calomel electrode (SCE). d The first and second values correspond to the carbenoid (CCN) and
ketenoid (CCN) signals, respectively. e The three values represent the DAC and the Rh trans- and cis-carbonyl groups, respectively.
known DACs (1709-1714 cm-1, KBr),6b,7a the IR spectrum of
1 revealed a νCO at 1681 cm-1 (KBr), which was consistent
with the νCO values typically displayed by amides.
To obtain additional structural information, single crystals
of 1 were grown from a saturated C6D6 solution and sub-
jected to an X-ray diffraction analysis. As shown in Figure 2,
the N-C-N angle measured in 1 (122.6(2)°) was signifi-
cantly wider than the analogous angle measured in IIIa
(114.83(19)°)6b and nearly 2° more obtuse than the widest
N-C-N angle for any NHC reported to date (1,3-neopen-
tylferrocenophan-2-ylidene: 120.5°).12 In addition, the obtuse
N-C-N angle observed in the solid-state structure of 1 sup-
ported the aforementioned 13C NMR data, which indicated
that the carbene nucleus was effectively more shielded when
compared to its six-membered analogue (i.e., IIIa).
Coordination Chemistry and Assessment of the Ligand Donat-
Figure 2. ORTEP representation of 1, rendered using POV-ray,
ing Properties of DAC 1. Upon the synthesis and charac-
with 50% thermal ellipsoids and H atoms and solvent molecules
terization of 1, its coordination chemistry was explored.
˚
omitted for clarity. Distances (A) and angles (deg): C1-N1,
Initial efforts were directed toward the preparation of the
1.3536(17); N1-C2, 1.423(2); C2-O1, 1.209(2); N1-C1-N1A,
M(cod)(1)Cl and M(CO)2(1)Cl complexes (cod=1,5-cyclo-
122.6(2); C1-N1-C2, 136.35(13); N1-C2-O1, 117.43(14).
octadiene; M=Rh or Ir), because related complexes contain-
this assessment, a 13C NMR signal assigned to the carbene
ing NHCs are often stable and have been used to compare
to other ligands reported in the literature.13 As shown in
Scheme 2, combination of DAC 1 with 0.5 molar equiv of
[Rh(cod)(Cl)]2 in toluene resulted in the precipitation of 2a,
which was isolated via filtration in 61% yield as a brick red
nucleus was observed at δ=243.9 ppm (1JRh-C =47.98 Hz;
CD2Cl2), a value upfield from the analogous signal exhibited
by free DAC 1 (268.7 ppm, C6D6) and downfield by approxi-
mately 20 ppm from the analogous signals exhibited by known
[Rh(cod)Cl] complexes supported by NHCs (δ = 180.6-
225.8 ppm; JRhC = 41-76 Hz).9g,13a,13c,13d,14 However, the
low-field signal assigned to the carbenoid nucleus in 2a is con-
sistent with the analogous signal displayed by a [Rh(cod)Cl]
complex supported by IIIa (δ=245.2 ppm; d, 1JRh-C=49.1 Hz;
CDCl3).7c The IR spectrum of 2a revealed a νCO=1669 cm-1
1
solid. The H NMR spectrum of 2a displayed signals at
δ=7.14 and 6.99 ppm as well as δ=2.59, 2.39, and 2.16 ppm
(CD2Cl2), which were assigned to inequivalent mesityl aryl-
CH and methyl groups, respectively. Consistent with other
[Rh(cod)Cl] complexes supported by seven-membered NHCs,
the inequivalency of the mesityl substituents observed in 2a
was presumably due to increased steric encumbrance at
the carbene nucleus imposed by 1.9d,e,g,l,m In support of
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4572 Organometallics, Vol. 29, No. 20, 2010
Hudnall et al.
Scheme 2. Synthesis of 2 and 3a
a Conditions: (i) [M(cod)Cl]2, C7H8, 25 °C, 16 h; (ii) 1 atm CO(g),
CH2Cl2, 25 °C, 30 min. M = Rh or Ir. Isolated yields where applicable
are indicated.
(KBr), a lower energy compared to the analogous signal
exhibited by [Rh(cod)(IIIa)Cl]) (νCO = 1719 cm-1; KBr),
reflecting the greater amide-like character in the former.7c
Crystals of 2a suitable for X-ray diffraction analysis were
obtained by vapor diffusion of n-pentane into a saturated
CH2Cl2 solution. As shown in Figure 3, the DAC ligand
occupied a coordination site cis to the chloride ligand and
was nearly orthogonal to the metal-ligand plane (dihe-
dral angle = 84.7(5)°), features that are consistent with the
solid-state structures of known NHC-Rh(cod)Cl com-
plexes.9g,13a,13c,13d,14 In contrast to previously reported seven-
membered NHC-supported [M(cod)Cl] complexes, which fea-
Figure 3. ORTEP representation of 2a, rendered using POV-ray,
with 50% thermal ellipsoids and H atoms and solvent molecules
˚
omitted for clarity. Relevant distances (A) and angles (deg):
C1-Rh1 2.051(6), Rh1-Cl1 2.3822(17), N1-C1 1.375(7), N2-C1
1.356(7), N1-C2 1.433(7), N2-C5 1.429(7), C2-O1 1.208(7),
C5-O2 1.214(7), N1-C1-N2 119.7(5), C1-N1-C2 132.7(5),
C1-N2-C5 132.6(5), N1-C2-O1 117.6(6), N2-C5-O2
118.7(6).
ture large CAr-N N-CAr torsion angles R (see Scheme 4)
3 3 3
of 15.9-46.0°,9d,e,g,l,m the R measured in the solid-state struc-
ture of 2a was 9.8°.15 This relatively acute angle was surpris-
ing given the obtuse N-C-N angle observed in the solid-
state structure of 1. As such, we reasoned that the steric
congestion about the carbene nucleus was not significantly
increased upon coordination to the rhodium center, partic-
ularly when compared to known seven-membered NHC-
supported [Rh(cod)Cl] complexes.9d,e,g,l,m Regardless, the
N-C-N angle measured for 2a (119.7(5)°) agreed well with
analogous angles measured in known seven-membered NHC-
supported [M(cod)Cl] complexes (118.7-122.3°).9d,g,l,m To
investigate the steric properties of 1 further, its percent
buried volume (%Vbur)
16 was calculated from the solid-state
structure of 2a and compared to analogous complexes. Using
the method developed by Cavallo,16b,c the %Vbur of 1 was cal-
culated16d at 41.2%, which is significantly greater than the
analogous values reported for 1,3-dimesitylimidazol-2-ylidene
(33.5%)16a and DAC IIIa (38.4%)7c as well as the seven-
membered NHC IV (40.6%; Ar = mesityl).9g
As noted above, 2a (M = Rh) precipitated during the
course of its synthesis, which facilitated its isolation. The Ir
congener 2b, however, could not be isolated cleanly under
similar conditions, and its formation was concomitant with
an unidentified decomposition product. It was empirically
determined that the purity and isolated yield of the desired
product significantly improved when the metalation reac-
tion was conducted in the absence of light.17 In partic-
ular, if the metalation reaction was carried out using toluene
Figure 4. Illustration describing torsion angle R.
as a solvent at room temperature in a foil-wrapped vial, the
1H NMR spectrum (CD2Cl2) of the crude reaction mixture
obtained after 16 h revealed the formation of 2b as the major
product (with ca. 90% purity; 85% isolated yield).
The NMR spectral data recorded for 2b were nearly
identical to that obtained for 2a, and inequivalent 1H
NMR resonances assigned to the mesityl substituents were
observed (δ = 7.14 and 6.99 ppm for the aryl-CH groups
and δ = 2.59, 2.39, and 2.16 ppm for the methyl protons;
CD2Cl2). Similar to the conclusions derived when [Ir(cod)-
(IIIb)Cl] (δ=231.3 ppm, C6D6)7a was analyzed by 13C NMR
spectroscopy, the signal assigned to the carbene nucleus in 2b
(δ=230.3 ppm, CD2Cl2) resonated downfield from analogous
signals exhibited by NHC-supported [Ir(cod)Cl] complexes
(δ = 179.6-208.2 ppm), likely due to the electron-with-
drawing nature of the carbonyl groups of the respective
DAC ligands.9g,13a,13c,13d,14 The IR spectrum of 2b exhibited
a νCO = 1676 cm-1 (KBr), which was consistent with the
signal measured for 2a, but at a lower frequency than that
exhibited by [Ir(cod)(IIIb)Cl] (νCO = 1711 cm-1), again
correlating with the greater amide-like character of the
seven-membered NHC in the former.7a
(15) The torsion angle R is ∼0° in the X-ray crystal structures of
known seven-membered NHCs as well as DAC 1.9d,e,g,l,m
(16) (a) Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841–861.
(b) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.;
Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759–1766. (c) http://www.molnac.
unisa.it/OMtools/sambvca.php. (d) The %VBur values were calculated with
Bondi radii scaled by 1.17, 3.5 Å radius of the sphere, and a M-Ccarbene
distance of 2.00 Å.
(17) For an example of an Ir(I) complex that was shown to decom-
pose photochemically see: Dahlenburg, L.; Ernst, M.; Dartiguenave,
M.; Dartiguenave, Y. J. Organomet. Chem. 1993, 463, C8–C10.

Article
Crystals of 2b that were suitable for X-ray diffraction
Organometallics, Vol. 29, No. 20, 2010 4573
analysis were obtained by slow vapor diffusion of n-pentane
into a saturated CH2Cl2 solution. As shown in Figure 5,
complex 2b was found to be isostructural with 2a, where the
DAC ligand in the former was oriented orthogonal to the
Ir-square plane (dihedral angle = 85.6(3)°) and a nearly
linear (8.0°) torsion angle R was observed. Additionally,
the N-C-N angle measured in the solid-state structure of
2b (119.7(7)°) was nearly identical to that measured in 2a
(119.7(5)°), both of which were in accord with analogous
metal complexes containing seven-membered NHCs (118.7-
122.3°).9d,e,g,l,m From these crystallographic data, we con-
cluded that a significant amount of steric strain was not
enforced to the carbene upon metalation of 1, as evidenced
by the relatively acute N-C-N (compared to 1) and acute
R angles measured in the aforementioned solid-state struc-
tures of 2a and 2b.
To gauge the donicity of DAC 1 in greater detail, efforts
were directed toward converting the aforementioned
[M(cod)(1)Cl] complexes into their corresponding dicarbon-
yl complexes. The Rh dicarbonyl complex 3a was prepared in
excellent yield (98%) by sparging a CH2Cl2 solution of 2a
with CO(g) for 30 min (Scheme 2). Complex 3a was found to
be stable at room temperature for extended periods of time,18
which facilitated its characterization. Single crystals were
obtained by slow vapor diffusion of n-pentane into a satu-
rated CH2Cl2 solution and subjected to an X-ray diffraction
analysis, thus enabling more detailed structural confirmation.
As shown in Figure 6, the square-planar geometry of the Rh
center in 3a wasconserved upon carbonylation, with the DAC
oriented perpendicular to the metal-ligand plane (dihedral
angle = 89.2(2)°). Consistent with 2a and 2b, the steric
constraint enforced on the carbene in 3a was not significant
when compared to other seven-membered NHCs, as evidenced
by the linear torsion angle R observed (0.0°).
Figure 5. ORTEP representation of 2b, rendered using POV-ray,
with 50% thermal ellipsoids and H atoms omitted for clarity.
Relevant distances (A) and angles (deg): C1-Ir1 2.043(4), Ir1-Cl1
2.3807(12), N1-C1 1.375(6), N2-C1 1.387(5), N1-C2 1.422(6),
N2-C5 1.409(6), C2-O1 1.209(6), C5-O2 1.211(6), N1-C1-
N2 119.7(4), C1-N1-C2 132.5(4), C1-N2-C5 132.8(4), N1-
C2-O1 117.5(4), N2-C5-O2 118.4(4).
˚
The IR spectrum of 3a revealed two distinct carbonyl
stretching frequencies at νCO = 2070 and 2002 cm-1 (KBr),
which were assigned to the trans- and cis-carbonyl moieties,
respectively.19 These values correlated well with other known
NHC-rhodium carbonyl complexes (trans: 2057-2093 cm-1
,
Upon synthesis and solid-state characterization of 3a,
efforts shifted toward examining this complex using a variety
of spectroscopic and electrochemical techniques. The 13C NMR
signal attributed to the carbene nucleus in 3a was observed
at δ = 222.6 ppm (1JRh-C = 43.6 Hz, CD2Cl2), a value
intermediate of the analogous signals observed for 2a (δ =
cis: 1984-2006 cm-1).9d,g,i,14 Using methods developed by
Plenio and Nolan,13a,b,f the average νCO (2036 cm-1) was
used to calculate a Tolman electronic parameter (TEP) of
2048 cm-1 for DAC 1. This value was in agreement with the
TEP (2051 cm-1) calculated for 1 from 3b (νCO = 2066 and
1984 cm-1; average νCO = 2025 cm-1), which was prepared
in situ (by treating 2b with CO(g) in the absence of light). On
the basis of these calculated TEP values, DAC 1 appeared to
1
244.9 ppm, JRh-C = 47.98 Hz; CD2Cl2) and other NHC-
supported [Rh(CO)2Cl] complexes (δ = 186.2-206.2 ppm,
1JRh-C = 37.7-53.8 Hz, CD2Cl2).9d,g,i,14 The relatively
downfield chemical shift exhibited by the carbene nucleus
in 3a compared to its NHC analogues may be attributed to
the electron-withdrawing carbonyl groups. In contrast, the
13C NMR signals assigned to the CO ligands in 3a were ob-
served at δ=186.0 ppm (1JRh-C =55.4 Hz) and 182.7 ppm
(1JRh-C = 76.6 Hz), and were in agreement with those record-
ed for other NHC complexes containing seven-membered
be a stronger donor than IIIa (TEP=2055 cm )
-1 7c as well as
1,3-dimesitylimidazol-2-ylidene (IMes) and its saturated ana-
logue 1,3-dimesitylimidazolin-2-ylidene (SIMes) (TEP=2050.7
and 2051.5 cm-1, respectively),14b but not as strong as II
(TEP = 2041 cm )
-1 10a or other known seven-membered NHCs
(e.g., IV: TEP=2043 cm-1).9d These results could be consid-
ered surprising given that the electron-withdrawing carbonyl
groups should decrease the carbene donicity; however, the
observed donicity for 1 supported our hypothesis that increasing
the DAC ring size from six to seven atoms increased its ligand
donicity and, by extension, its nucleophilicity (vide infra).6a,20
1
NHCs and DACs (δ = 184.0-188.2 ppm, JRh-C = 42.3-
54.7 Hz and δ = 182.0-184.9 ppm, 1JRh-C = 72.7-75.5 Hz,
respectively). Collectively, these data suggested to us that while
the DAC ligand exhibited a downfield-shifted 13C NMR reso-
nance for the carbene nucleus, it did not sufficiently perturb
the metal’s electron density to alter the 13C NMR resonances
attributed to the carbonyl ligands and was more akin to known
seven-membered carbenes in this regard.9d,g,i,14
(19) Because there is no symmetry operation that exchanges the two
carbonyl ligands in (NHC)MX(CO)2-type complexes, there is no projec-
tion operator that mediates combination. Hence, the carbonyl ligands
cannot be coupled and must be energetically independent. For similar
reasons, the use of symmetric/asymmetric nomenclature to describe
these complexes is not advised.
(20) (a) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239–
2246. (b) Herrmann, W. A.; Mihalios, D.; Ofele, K.; Kiprof, P.; Belmedjahed,
F. Chem. Ber. Recl. 1992, 125, 1795–1799.
(18) Over a period of approximately 2 days, 3a was found to change
from pale yellow to dark green. Although the identity of this discolored
species was not identified, subsequent treatment with CO resulted in re-
formation of the desired [Rh(CO)2(1)Cl] complex (3a).

4574 Organometallics, Vol. 29, No. 20, 2010
Hudnall et al.
Scheme 3. Synthesis of Diamidoketenimine 4a
a Conditions: (i) CtN-2,6-(Me)2-C6H3, C7H8, 25 °C, 4 h. Isolated
yield indicated.
of values observed for other NHC-supported [M(CO)2Cl]
complexes (0.88-1.16 V).14b Regardless, we concluded from
the collective electrochemical data that the coordination of 1
to a transition metal renders the metal more electron rich
when compared to analogous complexes supported by six-
membered analogues III, a result consistent with the former
being a relatively stronger electron donor.
Exploring the Reactivity of DAC 1: Applications in Small-
Molecule Activation. Having evaluated the ligand donicity of
1 and finding that this carbene is a stronger donor than
previously reported DACs, we next turned our attention to
utilizing 1 to activate small molecules. Recent results from
our group have shown that DACs are effective at engaging
in coupling reactions with nucleophilic substrates such as
organic isocyanides.6b,7b Indeed, these coupling reactions
have led to the synthesis and characterization of the first
N,N0-diamidoketenimines, which are thermally robust and
crystalline materials. Given that DACs react with nucleo-
philic isocyanides, whereas more typical imidazol-2-ylidenes
do not, we believe that the enhanced electrophilicities of
DACs makes them susceptible to nucleophilic attack.6b,7b
To test whether 1 engaged in comparable nonylidene-
type reactivities, we explored its reaction with an organic
isocyanide. As shown in Scheme 3, treatment of a toluene
solution of 1 with 2,6-dimethylphenylisocyanide resulted in a
color change from bright orange to yellow, and the desired
N,N0-diamidoketenimine 4 was obtained in excellent yield
(92%) upon removal of the solvent after 4 h. The 1H NMR
spectrum of 4 revealed the resonances expected for the
desired product. Whereas the signal corresponding to the
methyl groups at the 2,6-positions for the isocyanide aryl
group appeared as a sharp singlet (δ=1.84 ppm; C6D6), the
mesityl methyl groups resonated as broad signals (δ =
2.02-2.29 ppm; C6D6), indicative of steric congestion about
the ketenimine fragment. Additionally, the 13C NMR
spectrum of 4 revealed diagnostic signals at δ = 88.90 and
184.07 ppm (C6D6), which were assigned to the carbe-
noid (CCN) and ketenoid (CCN) nuclei of diamidoketenimines,
respectively.6b,7b The IR spectrum of 4 exhibited a signal at 2015
Figure 6. ORTEP representation of 3a, rendered using POV-ray,
with 50% thermal ellipsoids and H atoms omitted for clarity.
Relevant distances (A) and angles (deg): C1-Rh1 2.093(3),
Rh1-Cl1 2.3420(10), Rh1-C10 1.883(4), Rh1-C11 1.846(4),
C10-O3 1.113(5), C11-O4 1.117(5), N1-C1 1.355(2), N1-C2
1.430(3), C2-O1 1.202(3), N1-C1-N2 122.9(3), C1-N1-C2
133.1(2), N1-C2-O1 117.6(2).
˚
Despite its widespread use in organometallic chemistry for
the pseudoquantification of NHC electron-donating ability,
the synthesis of [(NHC)M(CO)2Cl] complexes and measure-
ment of their respective νCO values is an indirect measure of
NHC donicity.21 Moreover, the measured NHC donicity via
the determination of its TEP can be influenced by trans-
directing ligands, whereby the carbonyl trans to the ligand of
interest will experience a weakening of the metal-carbon
bond (and thus attenuation of π-back-bonding), indepen-
dent of the electron-richness of the metal. To complement the
IR spectroscopic analyses and to circumvent the inherent
limitations thereof, we performed cyclic voltammetry (CV)
of the [M(cod)Cl] complexes supported by 1 (see Figure 7
and Table 1). The RhI/II couple in 2a was measured at 0.84 V
vs SCE,22 which is below the range of values previously
observed in NHC-supported [Rh(cod)Cl] complexes (ca.
þ1.0 V).14 Similarly, the IrI/II couple in 2b was also measured
at 0.84 V and was within the range of values previously
observed in NHC-supported [Ir(cod)Cl] complexes (0.65-
0.92 V).23 Replacing the cod ligand with two carbonyl ligands
afforded a significant anodic shift in the RhI/II and IrI/II
oxidation potentials in complexes 3a and 3b (the latter was
prepared in situ by bubbling CO(g) into the electrochemical
cell after obtaining the CV for 2b).24 The carbonyl complexes
exhibited irreversible oxidation processes (Epa = 1.27 and
1.90 V for 3a and 3b, respectively) but were within the range
cm-1, as expected for the ketenimine stretching mode (νCdCdN
,
KBr), as well as bands at 1675 and 1663 cm-1, which were
assigned to the amide carbonyl stretching modes.
Whereas X-ray diffraction analysis confirmed the struc-
ture of 4 (see Figure 8), a torsion angle R = 44.5° was mea-
sured, which deviated significantly from the analogous angles
measured in the solid-state structures of the Rh and Ir com-
plexes 2a, 2b, and 3a (R=9.8°, 8.0°, and 0.0°, respectively).
The disparity in the torsion angles may be explained by the
C1-E distance (where E=a carbon atom or a metal atom).
In the case of 4, the C1-C2 distance was measured to be
(21) For efforts detailing the use of 13C NMR spectroscopy to
determine NHC donor strength see: Huynh, H. V.; Han, Y.; Jothibasu,
R.; Yang, J. A. Organometallics 2009, 28, 5395–5404.
(22) All potentials were referenced to SCE.
(23) (a) Tennyson, A. G.; Rosen, E. L.; Collins, M. S.; Lynch, V. M.;
€
Bielawski, C. W. Inorg. Chem. 2009, 48, 6924–6933. (b) Leuthausser, S.;
Schwarz, D.; Plenio, H. Chem.;Eur. J. 2007, 13, 7195–7203.
(24) The electrochemical analysis of 3b was obtained in situ, as we
were unable to isolate this complex without decomposition.
˚
1.323(2) A (consistent with the formation of a CdC double
bond of a ketenimine),6b,7b which is nearly 0.8 A shorter than
˚

Article
Organometallics, Vol. 29, No. 20, 2010 4575
Figure 7. CVs of (A) [Rh(cod)(1)Cl] (2a); (B) [Rh(CO)2(1)Cl] (3a); (C) [Ir(cod)(1)Cl] (2b); and (D) [Ir(CO)2(1)Cl] (3b). Electrochemical
analyses were obtained in dry CH2Cl2 with 0.1 M nBu4NPF6 electrolyte using a 100 mV s-1 scan rate and referenced to saturated
calomel electrode (SCE) by shifting (Fc*)0/þ to -0.057 V.25.
as we have previously found that N,N0-diamidoketenes
(prepared from III) are typically blue to purple.6b,7a Regard-
less, an IR spectrum recorded for a toluene solution of
DAC 1 treated with CO(g) revealed stretching frequencies
at 2112 and 1729 cm-1, which were consistent with the
ν
CdCdO and νCdO stretching modes, respectively, measured
for the ketene obtained from IIIa (2092 and 1681 cm-1).6b
1
Although H NMR analysis of the aforementioned reaction
was also promising (broad signals were observed at δ=7.92,
7.75, 6.99, and 2.22 ppm; C6D6), attempts to isolate the
desired product were unsuccessful.
In addition to exploring the ability of 1 to couple to an
isonitrile and CO, efforts were also directed toward activat-
ing NH3 and H2. Unlike the aforementioned results obtained
with CO, our preliminary experiments, which involved treat-
ing solutions of 1 with NH3 or H2, resulted predominately in
the formation of unidentified decomposition products, even
under forcing conditions (elevated temperatures and pres-
sures of H2 as high as 1000 psi). Collectively, these results sug-
gested to us that while DAC 1 exhibited ylidene and carbene-
like reactivities, the carbene or its adducts also engaged in
reactions that led to decomposition, an indicator of a loss of
controlled reactivity.
Exploring the Potential for DAC 1 to Support Catalytically
Active Complexes. Given that DAC 1 exhibited a unique set
of steric and electronic properties when compared to known
seven-membered diaminocarbenes as well as other DACs,
efforts shifted toward investigating its use as a ligand in a
catalytically active NHC complex. Preliminary efforts were
directed toward gold complexes supported by NHCs, such
as IPr-AuCl (IPr=1,3-(2,6-diisopropylphenyl)imidazol-2-
ylidene), which have recently been shown to catalyze a variety
of chemical reactions, ranging from the hydration of alkynes
to the carboxylation of arenes.26 As shown in Scheme 4,
treatment of a solution of 1 in toluene with stoichiometric
Figure 8. ORTEP representation of 4, rendered using POV-ray,
with 50% thermal ellipsoids and H atoms and solvent molecules
˚
omitted for clarity. Relevant distances (A) and angles (deg):
C1-N1 1.429(2), C1-N2 1.440(2), C1-C2 1.323(2), C2-N3
1.217(2), N1-C3 1.375(2), N2-C6 1.392(2), C3-O1 1.224(2),
C6-O2 1.218(2), N1-C1-N2 118.05(14), C1-C2-N3 165.52(17),
C1-N1-C3 119.58(14), C1-N2-C6 119.61(14), N1-C3-O1
122.50(15), N2-C6-O2 120.99(16).
the analogous distances observed in the solid-state structures
˚
˚
of 2a, 2b, and 3a (C1-Rh1=2.051(6) A, C1-Ir1=2.043(4) A,
˚
and C1-Rh1=2.093(3) A, respectively). Thus, a more obtuse
torsion angle R may be required to alleviate steric conges-
tion in the ketenimine. Regardless, the NCO fragments in 4
exhibited short N-Cacyl distances concomitant with long
˚
˚
Cacyl-O distances (N1-C3=1.375(2) A, C3-O1=1.224(2) A,
˚
˚
N2-C6 = 1.392(2) A, C6-O2 = 1.218(2) A) and were con-
sistent with the IR data, which indicated that these groups
are characteristic of prototypical amides.
Because carbon monoxide (CO) is isolobal to isocyanides,
we next investigated whether 1 could react with CO to
generate the respective N,N0-diamidoketene (structure not
shown).6b,7a,8 Upon bubbling CO through a C6D6 (or C7H8)
solution of 1, an instantaneous color change from dark orange-
red to yellow was observed. This color change was unexpected,
(25) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay,
P. A.; Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713–6722.

4576 Organometallics, Vol. 29, No. 20, 2010
Hudnall et al.
Scheme 4. Synthesis of DAC Gold(I) Chloride Complex 5a
a Conditions: (i) THT-AuCl (THT = tetrahydrothiophene), C7H8,
25 °C, 2 h. Isolated yield indicated.
[Au(THT)Cl] (THT = tetrahydrothiophene) resulted in the
immediate precipitation of the desired DAC-AuCl complex
(5) in near-quantitative yield as a pale yellow solid. Complex
5 was found to be thermally stable (mp 216-220 °C, dec) and
could be stored and manipulated under ambient conditions
without detriment. In contrast to the aforementioned Rh and
Ir complexes containing 1, the 1H NMR spectrum of 5
(CD2Cl2) exhibited equivalent mesityl substituents in solu-
tion at 25 °C, indicating relatively minimal steric congestion
at the carbene nucleus. Upon coordination of [AuCl], the 13C
NMR signal assigned to the carbene nucleus significantly
shifted upfield from δ = 268.4 ppm (C6D6) in 1 to 202 ppm
(CD2Cl2) in 5, consistent with analogous signals observed in
other known NHC-supported AuCl complexes (δ = 166-
195 ppm).27 To confirm the structure of 5, single crystals
suitable for an X-ray diffraction analysis were obtained by
slow evaporation of a saturated CH2Cl2 solution. As shown
in Figure 9, the solid-state structure of 5 was found to be iso-
structural with reported NHC-supported [AuCl] complexes,
with a nearly linear CDAC-Au-Cl angle (179.29(8)°)
Figure 9. ORTEP representation of 5, rendered using POV-ray,
with 50% thermal ellipsoids and H atoms omitted for clarity.
Relevant distances (A) and angles (deg): C1-Au1 2.002(3), Au1-
Cl1 2.2828(7), N1-C1 1.358(4), N2-C1 1.352(4), N1-C2 1.443(4),
N2-C5 1.454(4), C2-O1 1.205(4), C5-O2 1.203(4), C1-Au1-
Cl1 179.29(8), N1-C1-N2 123.4(2), C1-N1-C2 132.4(2), C1-
N2-C5 132.2(2), N1-C2-O1 117.7(3), N2-C5-O2 118.2(3).
˚
negligible formation of acetophenone (<5% yield by gas
chromatography) was observed at temperatures at or below
25 °C and catalyst decomposition was observed at tempera-
tures above 40 °C, the best results were obtained when the
reaction was performed at 80 °C for 12 h, which afforded
acetophenone in good yield (78%).
˚
and C-Au and Au-Cl distances (2.002(3) and 2.2828(7) A,
respectively) that were within range (1.942-1.998 and
27
˚
2.275-2.306 A).
Upon the synthesis and characterization of 5, efforts then
shifted toward studying its catalytic activity. To explore the
potential of 5 as a viable catalyst for the hydration of termi-
nal alkynes in a manner similar to other known complexes,26f,g
the hydration of phenylacetylene in the presence of 2 mol %
of 5 under a variety of conditions was explored (see eq 1).
Using a previously reported procedure, 1 mmol of phenyl-
acetylene in methanol (2.5 mL) was treated with solid 5
(0.02 mmol) followed by the addition of water (2.5 mL).26f
The resulting solution was then heated to 80 °C in a 40 mL
Teflon-capped vial for 3 h. A 1H NMR spectrum (CD2Cl2) of
the crude reaction mixture recorded after removal of solvent
revealed the formation of the desired product, albeit in low
yield (∼36%), which was accompanied with starting material
and a minor amount (∼2%) of an unidentified product
(presumably due to catalyst decomposition, as the formation
of a gold mirror was observed). To explore conditions that
resulted in minimal catalyst decomposition, the hydration
reaction was repeated at various temperatures. Although
Conclusions
In summary, we have synthesized and characterized the
first example of a seven-membered N,N0-diamidocarbene
(1). X-ray crystallographic analysis revealed that this car-
bene contained the widest N-C-N angle (122.6(2)°)
reported to date as well as a large calculated %Vbur (41.2%).
However, various Rh and Ir metal complexes supported by 1
exhibited acute torsion angles (R < 10°), which are atypical
for analogous complexes supported by seven-membered
NHCs and may reflect minimal steric constraint being
enforced on the carbene upon metalation. Electrochemical
and IR spectroscopic analyses of 2a, 2b, and 3a revealed that
DAC 1 is a stronger electron donor than six-membered
analogues (III) as well as IMes and SIMes. While coupling
of 1 to 2,6-dimethylphenylisocyanide afforded the N,N0-
diamidoketenimine 4 as a thermally robust, crystalline com-
pound, treatment of 1 with CO, NH3, or H2 resulted in the
formation of decomposition products. Hence, the unique
steric and electronic properties inherent to DAC 1 appeared
to result in a loss of controlled reactivity toward many small
molecules. Conversely, DAC 1 was successfully used to support
a catalytically active Au(I) complex, which was shown to
facilitate the hydration of phenylacetylene to acetophenone
in good yield. We conclude from these studies that modulat-
ing the ring size effectively alters orbital hybridization at the
carbene nucleus and the attendant singlet-triplet gap, and
(26) (a) Boogaerts, I. I. F.; Nolan, S. P. J. Am. Chem. Soc. 2010, DOI:
10.1021/ja103429q. (b) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem.,
Int. Ed. 2006, 45, 7896–7936. (c) Li, Z.; Brouwer, C.; He, C. Chem. Rev.
2008, 108, 3239–3265. (d) Arcadi, A. Chem. Rev. 2008, 108, 3266–3325.
ꢀ
~
(e) Jimenez-Nꢀunez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326–
ꢀ
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3350. (f) Almassy, A.; Nagy, C. E.; Benyei, A. C.; Joo, F. Organometallics
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2010, 29, 2484–2490. (g) Marion, N.; Ramon, R. S.; Nolan, S. P. J. Am.
Chem. Soc. 2009, 131, 448–449.
ꢀ
(27) (a) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P.
Organometallics 2005, 24, 2411–2418. (b) Gaillard, S.; Slawin, A. M. Z.;
Bonura, A. T.; Stevens, E. D.; Nolan, S. P. Organometallics 2010, 29, 394–
402.

Article
Organometallics, Vol. 29, No. 20, 2010 4577
that the ring size of cyclic carbenes is an important variable
to consider in determining carbene character and reactiv-
ity (i.e., ylidene vs carbene). As such, new discoveries may
result from further detailed investigations of the structure-
reactivity relationship of cyclic carbenes as a function of their
ring size.
atoms. Key details of the crystal and structure refinement data
are summarized in Table S1. Further crystallographic details
may be found in the respective CIF files, which were deposited at
the Cambridge Crystallographic Data Centre, Cambridge, UK.
The CCDC reference numbers for 1 (C6H6), 2a (CH2Cl2), 2b,
3
3
3a, 4 (C6H6), and 6 and were assigned as 782378, 782379, 782380,
3
782381, 782383, and 782382, respectively.
Synthesis of 1 HCl. Phthaloyl chloride (2.00 g, 9.85 mmol,
3
1.05 equiv) was added dropwise to a stirred solution of N,N0-
dimesitylformamidine (2.63 g, 9.38 mmol) and triethylamine
(2.00 mL, 14.1 mmol, 1.50 equiv) in CH2Cl2 (100 mL) at 0 °C.
The solution was stirred at 0 °C for 1 h, whereupon the volatiles
were removed under reduced pressure. The resulting solid was
taken up into toluene (100 mL), inducing the precipitation of
triethylammonium chloride, which was then removed via filtra-
tion over a plug of dry Celite. After removal of the solvent under
reduced pressure, the residue was washed with hexanes (3 ꢀ 50 mL).
Subsequent drying of the residual solid under reduced pressure
afforded the desired compound as an off-white solid (3.69 g,
88.1% yield). m.p.: 158-160 °C (dec). 1H NMR (CDCl3, 400.27
MHz): δ 2.13 (br s, 12H, Mes-o-CH3), 2.26 (s, 6H, Mes-p-CH3),
6.41 (s, 1H, NCHN), 6.90 (s, 4H, Ar-CH), 7.75 (m, 2H, Ar-CH),
8.11 (m, 2H, Ar-CH). 13C NMR (CDCl3, 75.47 MHz): δ 19.28
(br s), 20.86, 85.33 (N-CHCl-N), 130.19, 130.92 132.40, 132.73,
135.51, 137.15, 138.64, 167.97 (CdO). IR (KBr): νCO = 1672
cm-1. HRMS: [M]þ calcd for C27H27N2O2Cl: 446.1761, found
446.1757. Anal. Calcd for C27H27N2O2Cl: C, 72.55; H, 6.09; N,
6.27. Found: C, 72.49; H, 6.27; N, 6.17.
Experimental Section
General Considerations. All procedures were performed using
standard Schlenk techniques under an atmosphere of nitrogen
or in a nitrogen-filled glovebox unless otherwise noted. N,N0-
Dimesitylformamidine was synthesized using previously re-
ported procedures.28 Phthaloyl chloride and 2,4,6-trimethylani-
line were obtained from TCI America and used as received.
Triethylorthoformate and 2,6-dimethylphenylisocyanide were
purchased from Alfa Aesar and used as received. Sodium bis-
(trimethylsilyl)amide (NaHMDS) was purchased from ACROS
and used as received. [Rh(cod)Cl]2 and [Ir(cod)Cl]2 were ob-
tained from Strem Chemical and used as received. THT-AuCl
(THT = tetrahydrothiophene) was prepared as previously de-
scribed.29 Benzene was dried over molecular sieves and distilled
prior to use. Dichloromethane (CH2Cl2), hexanes, and toluene
were dried and degassed by a Vacuum Atmospheres Company
solvent purification system and stored over molecular sieves in
a nitrogen-filled glovebox. Infrared (IR) spectra were recorded
on a Perkin-Elmer Spectrum BX FTIR spectrophotometer.
UV-visible spectra were recorded using a Perkin-Elmer Lambda
35 UV/vis spectrophotometer. High-resolution mass spectra
(HRMS) were obtained with a VG analytical ZAB2-E instru-
ment (CI). NMR spectra were recorded on Varian UNITYþ
300, Varian Mercury 400, and Varian INOVA 500 spectro-
meters. Chemical shifts (δ) are given in ppm and are referenced
to the residual solvent (1H: CDCl3, 7.24 ppm; C6D6, 7.15 ppm;
C7D8, 2.09 ppm; 13C: CDCl3, 77.0 ppm; C6D6, 128.0 ppm, C7D8,
128.3 ppm). Elemental analyses were performed at Midwest
Microlab, LLC (Indianapolis, IN). Melting points were obtained
using a Mel-Temp apparatus and are uncorrected.
Synthesis of 1. A 20 mL vial was charged with 1 HCl (250 mg,
3
0.561 mmol), NaHMDS (105 mg, 0.560 mmol), benzene (15 mL),
and a stir bar. The solution was stirred at 25 °C for 30 min and
then filtered through a PTFE filter. After removing the residual
solvent under reduced pressure, the solid residue was washed
with cold hexanes, decanted, and then dried under vacuum to
afford the desired product as a yellow solid (195 mg, 84.9%
yield). Single crystals suitable for an X-ray diffraction analysis
were grown from a concentrated solution of 1 in C6D6. m.p.:
75-77 °C (dec). 1H NMR (C6D6, 400.27 MHz): δ 2.13 (s, 12H,
Mes-o-CH3), 2.14 (s, 6H, Mes-p-CH3), 6.79 (s, 4H, Ar-CH) 7.02
(m, 2H, Ar-CH), 8.14 (m, 2H, Ar-CH). 13C NMR (C6D6, 75.47
MHz): δ 18.34, 20.94, 129.53, 130.49, 132.57, 133.82, 133.99,
Electrochemistry. Electrochemical experiments were conducted
on CH Instruments electrochemical workstations (series 660D)
using a gastight, three-electrode cell under an atmosphere of dry
nitrogen. The electrochemical cells employed were equipped
with gold working, tungsten counter-, and silver quasi-reference
electrodes. Measurements were performed in dry CH2Cl2 with
0.1 M nBu4NPF6 electrolyte and decamethylferrocene (Fc*)
internal standard. Data deconvolution and fitting were per-
formed with the Origin 8.0 software package. All reported
potentials were determined at 100 mV s-1 scan rate and refer-
enced to saturated calomel electrode (SCE) by shifting (Fc*)0/þ
to -0.057 V (CH2Cl2).25
136.69, 143.79, 165.56 (CdO), 268.42 (NCN). IR (KBr): νCO
=
1681 cm-1. HRMS: [M þ H]þ calcd for C27H27N2O2 411.2073,
found 411.2069.
Synthesis of [Rh(cod)(1)Cl] (2a). A 20 mL vial was charged
with a toluene (5 mL) solution of 1 (104 mg, 0.252 mmol) and a
stir bar. To this solution was added a toluene solution (5 mL) of
[Rh(cod)Cl]2 (62.0 mg, 0.126 mmol). The color of the latter
changed from bright orange to deep red upon mixing, and the
resulting solution was stirred at 25 °C for 16 h. During the course
of the reaction a red precipitate formed, which was isolated via
decantation of the supernatant solution, washed with hexanes
(3 ꢀ 5 mL), and dried under reduced pressure to afford the
desired product (45.1 mg). The hexanes washes were added to
the toluene mother liquor, and the combined solution was
concentrated to dryness. The resulting residue was dissolved in
dichloromethane (2 mL) and precipitated into a solution of
hexanes (20 mL) under sonication. The precipitated solid was
isolated via decantation, washed with hexanes (3 ꢀ 5 mL), and
dried to afford the desired product as a brick-red solid (55.9 mg,
combined yield: 101 mg, 61.2%). Single crystals suitable for an
X-ray diffraction analysis were grown by slow vapor diffusion of
n-pentane into a CH2Cl2 solution saturated with 2a. m.p.:
176-178 °C. 1H NMR (CD2Cl2, 400.28 MHz): δ 1.38-1.52
(br s, 5H, COD-CH2), 1.64-1.74 (br s, 3H, COD-CH2), 2.16
(s, 6H, Mes-CH3), 2.39 (s, 6H, Mes-CH3), 2.59 (s, 6H, Mes-CH3),
2.80-2.84 (br s, 2H, COD-CH), 4.58-4.64 (br s, 2H, COD-
CH), 7.00 (s, 2H, Mes-CH), 7.14 (s, 2H, Mes-CH), 7.81-7.84
(m, 2H, Ar-CH), 7.91-7.94 (m, 2H, Ar-CH). 13C NMR
(CD2Cl2, 125.59 MHz): δ 19.99, 21.01, 21.54, 27.79, 32.06, 75.34
Crystallography. All crystallographic measurements were
carried out on a Rigaku Mini CCD area detector diffractometer
˚
using graphite-monochromated Mo KR radiation (R=0.71073 A)
at 150 K using an Oxford Cryostream low-temperature device.
A sample of suitable size and quality was selected and mounted
onto a nylon loop. Data reductions were performed using
DENZO-SMN.30 The structures were solved by direct methods,
which successfully located most of the non-hydrogen atoms.
Subsequent refinements on F2 using the SHELXTL/PC package
(version 5.1)31 allowed location of the remaining non-hydrogen
(28) Kuhn, K. M.; Grubbs, R. H. Org. Lett. 2008, 10, 2075–2077.
(29) Uson, R.; Laguna, A.; Laguna, M.; Briggs, D. A.; Murray,
H. H.; Fackler, J. P., Jr. Inorg. Synth. 1989, 26, 85–91.
(30) Otwinowski, Z.; Minor, W. In Methods in Enzymology in Macro-
molecular Crystallography; Carter, C. W., Jr., Sweets, R. M., Eds.; Academic
Press: New York, 1997; Part A, Vol. 276, pp 307-326.
(31) Sheldrick, G. M. SHELXL/PC package (version 5.1), Program for
the Refinement of Crystal Structures; University of Gottingen: Germany,
2003.

4578 Organometallics, Vol. 29, No. 20, 2010
Hudnall et al.
1
1
(d, JRh-C = 13.7 Hz), 98.77 (d, JRh-C = 6.9 Hz), 128.94,
130.65, 130.87, 131.64, 134.23, 136.80, 137.30, 139.20, 140.49,
166.81 (CO), 166.83 (CO), 243.95 (d, 1JRh-C=47.98 Hz, NCN).
IR (KBr): νCO = 1669, 1718 cm-1. HRMS: [M]þ calcd for
C35H38N2O2ClRh 656.1677, found 656.1667. Anal. Calcd for
C35H38N2O2ClRh: C, 63.98; H, 5.83; N, 4.26. Found: C, 63.62;
H, 5.96; N, 3.95.
130.36, 131.48, 135.57, 135.67, 135.83, 135.88, 139.91, 140.63,
167.60 (ligand-CO), 167.62 (ligand-CO), 182.73 (d, JRh-C =
1
1
76.61 Hz, Rh-CO), 185.95 (d, JRh-C = 55.39 Hz, Rh-CO),
1
222.56 (d, JRh-C = 43.58 Hz, NCN). IR (KBr): νCO = 1699,
2002, 2070 cm-1. HRMS: [M - Cl]þ calcd for C29H26N2O4Rh
569.0948, found 569.0941. Anal. Calcd for C29H26N2O4ClRh:
C, 57.58; H, 4.33; N, 4.63. Found: C, 57.55; H, 4.50; N,
4.59.
Synthesis of [Ir(cod)(1)Cl] (2b). A 20 mL vial wrapped in
aluminum foil was charged with a toluene (5 mL) solution of 1
(47.6 mg, 0.116 mmol) and a stir bar. To this solution was added
a toluene solution (5 mL) of [Ir(cod)Cl]2 (39.0 mg, 0.0580
mmol). The resulting solution was stirred in the dark at 25 °C
for 16 h. The resulting dark red solution was then transferred to
a round-bottom flask that was wrapped in aluminum foil, and
the solvent was removed by rotary evaporation. The residue was
dissolved in dichloromethane (1 mL) and added dropwise to
n-pentane (35 mL) under sonication, which resulted in the
precipitation of a dark orange solid. The solid was isolated by
filtration, dissolved in dichloromethane (1 mL), and added
dropwise to a sonicated solution of hexanes (35 mL), which
resulted in the precipitation of the desired product as a red-
brown solid, which was isolated by filtration (36.7 mg) The
n-pentane and hexanes washes were combined and concentrated
under reduced pressure. To the residue was added hexanes
(40 mL), and the resulting suspension was sonicated for 1 h.
The precipitated solid was collected via filtration and dried to
yield an additional crop of the desired product as a red-brown
solid (37.0 mg; combined yield: 73.7 mg, 84.8%). Single crystals
suitable for an X-ray diffraction analysis were grown by slow
vapor diffusion of n-pentane into a CH2Cl2 solution saturated
with 2b. m.p.: 205-208 °C (dec). 1H NMR (CD2Cl2, 400.28
MHz): δ 1.31-1.42 (br s, 4H, COD-CH2), 1.50-1.62 (br s, 4H,
COD-CH2), 2.21 (s, 6H, Mes-CH3), 2.36 (s, 6H, Mes-CH3), 2.43
(s, 6H, Mes-CH3), 2.56-2.58 (br m, 2H, COD-CH), 4.39-4.41
(br m, 2H, COD-CH), 6.98 (s, 2H, Mes-CH), 7.05 (s, 2H, Mes-
CH), 7.80-7.84 (br m, 2H, Ar-CH), 7.90-7.93 (br m, 2H, Ar-
CH). 13C NMR (CD2Cl2), 125.59 MHz): δ 20.11, 20.99, 21.36,
27.94, 33.19, 60.27, 87.62, 128.69, 130.59, 130.60, 131.82, 134.12,
136.58, 137.04, 139.10, 139.86, 167.85 (CO), 230.27 (NCN). IR
(KBr): νCO = 1676 cm-1. HRMS: [M - Cl]þ calcd for C35H38-
N2O2Ir 711.25570, found 711.25594. Anal. Calcd for C35.5H39-
Synthesis of 4. An 8 mL vial was charged with toluene (5 mL),
2,6-dimethylphenylisocyanide (31.5 mg, 0.242 mmol), 1 (100 mg,
0.244 mmol), and a stir bar. The resulting mixture was stirred at
25 °C for 4 h. Removal of the residual solvent under reduced
pressure afforded the desired product as a yellow solid (120 mg,
92.3% yield). Slow evaporation of a benzene solution of 4
afforded crystals suitable for X-ray diffraction analysis. m.p.:
164-166 °C. 1H NMR (C6D6, 400.27 MHz): δ 1.84 (s, 6H,
Ar-(CH3)2), 2.02 (s, 6H, Mes-p-CH3), 2.00-2.28 (br s, 12H,
Mes-o-CH3), 6.59-6.62 (m, 6H, Ar-CH), 6.69-6.72 (m, 1H, Ar-
CH), 7.14 (m, 2H, Ar-CH), 8.31 (m, 2H, Ar-CH). 13C NMR
(C6D6, 75.47 MHz): δ 18.76, 19.58 (br s), 20.75, 88.90 (CCN),
128.73, 129.03, 129.95 (br s), 131.42, 131.85, 133.75, 134.70,
137.43, 138.35, 170.30 (CO), 184.07 (CCN). IR (KBr): νCdN
=
2015 cm-1; νCO=1675, 1663 cm-1. HRMS: [M]þ calcd for C36H35-
N3O2 541.2729, found 541.2733. Anal. Calcd for C35H36N3O2:
C, 79.82; H, 6.51; N, 7.76. Found: C, 79.85; H, 6.62; N, 7.65.
Synthesis of [1-AuCl] (5). A vial was charged with a toluene
(3 mL) solution of 1 (63.4 mg, 0.154 mmol) and a stir bar. To this
solution was added a toluene suspension (3 mL) of THT-AuCl
(THT=tetrahydrothiophene) (50.0 mg, 0.154 mmol). The latter
solution changed color from bright orange to pale yellow (with
some THT-AuCl present as a solid) upon mixing. Upon
stirring, a yellow precipitate formed, resulting in a yellow
suspension, which was stirred for 2 h at 25 °C. The precipitated
product was then isolated via decantation of the supernatant
solution. The resulting pale yellow solid was washed with
hexanes (3 ꢀ 5 mL) and dried under reduced pressure to afford
the desired product as a pale yellow powder (94.9 mg, 95.8%).
Single crystals suitable for an X-ray diffraction analysis were
grown by slow evaporation of a CH2Cl2 solution saturated with
1
7. m.p.: 216-220 °C (dec). H NMR (CD2Cl2, 400.28 MHz):
δ 2.21 (s, 12H, Mes-CH3), 2.39 (s, 6H, Mes-CH3), 7.08 (s, 4H,
Mes-CH), 7.94-7.99 (m, 2H, Ar-CH), 8.08-8.13 (m, 2H, Ar-CH).
13C NMR (CD2Cl2, 125.59 MHz): δ 19.50, 21.27, 130.00,
130.25, 131.91, 134.68, 135.93, 140.22, 141.66, 167.69 (CO),
202.10 (NCN). IR (KBr): νCO = 1715, 1729 cm-1. HRMS: [M]þ
calcd for C27H26N2O2ClAu 642.1348, found 642.1350. Anal.
N2O2Cl2Ir (2b 0.5 CH2Cl2): C, 54.05; H, 4.98; N, 3.55. Found:
C, 53.97; H, 5.41; N, 3.19.
3
Synthesis of [Rh(CO)2(1)Cl] (3a). Carbon monoxide (CO) was
bubbled through a solution of 1-Rh(cod)Cl (40.0 mg, 0.0611
mmol) in dichloromethane (5 mL) until the solvent had evapo-
rated. The remaining light yellow residue was then dissolved in
dichloromethane (0.5 mL) and added dropwise to a rapidly
stirring solution of hexanes (20 mL), which resulted in the
precipitation of a pale yellow solid. The solid was isolated via
decantation of the supernatant solution, washed with hexanes
(3 ꢀ 5 mL), and dried under reduced pressure to afford the desired
product as a pale yellow solid (35.4 mg, 0.0591 mmol, 96.3%
yield). Single crystals suitable for an X-ray diffraction analysis
were grown by slow vapor diffusion of n-pentane into a CH2Cl2
Calcd for C27.5H27N2O2Cl2Au (5 0.5CH2Cl2): C, 48.19; H,
3.97; N, 4.09. Found: C, 47.75; H, 3.95; N, 4.00.
3
Acknowledgment. We are grateful to the National
Science Foundation (NSF) (CHE-0645563) and the Robert
A. Welch Foundation (F-1621) for their generous financial
support. X-ray data were collected using instrumentation
purchased with funds provided by the NSF (0741973).
1
solution saturated with 3a. m.p.: 113-115 °C (dec). H NMR
(CD2Cl2, 400.27 MHz): δ 2.18 (s, 6H, Mes-CH3), 2.37 (s, 12H,
Mes-CH3), 6.98 (s, 2H, Mes-CH), 7.04 (s, 2H, Mes-CH),
7.90-7.94 (m, 2H, Ar-CH), 7.99-8.21 (m, 2H, Ar-CH). 13C
NMR (CD2Cl2, 125.59 MHz): δ 19.13, 20.18, 21.20, 129.59, 130.10,
Supporting Information Available: X-ray crystallographic
data in CIF format for compounds 1, 2a, 2b, 3a, 4, and 5 as
well as crystal tables and NMR spectra are available free of
charge via the Internet at http://pubs.acs.org.