Comparative measure of the electronic influence of highly substituted aryl isocyanides

Article abstract of DOI:10.1021/ic5030845

To assess the relative electronic influence of highly substituted aryl isocyanides on transition metal centers, a series of C_4v-symmetric Cr(CNR)(CO)₅ complexes featuring various alkyl, aryl, and m-terphenyl substituents have been prepared. A correlation between carbonyl-ligand ¹³C{¹H} NMR chemical shift (δ_CO) and calculated Cotton-Kraihanzel (C-K) force constant (k_CO) is presented for these complexes to determine the relative changes in isocyanide σ-donor/π-acid ratio as a function of substituent identity and pattern. For nonfluorinated aryl isocyanides possessing alkyl or aryl substitution, minimal variation in effective σ-donor/π-acid ratio is observed over the series. In addition, aryl isocyanides featuring strongly electron-releasing substituents display an electronic influence that nearly matches that of nonfluorinated alkyl isocyanides. Lower σ-donor/π-acid ratios are displayed by polyfluorinated aryl isocyanide ligands. However, the degree of this attenuation relative to nonfluorinated aryl isocyanides is not substantial and significantly higher σ-donor/π-acid ratios than CO are observed in all cases. Substituent patterns for polyfluorinated aryl isocyanides are identified that give rise to low relative σ-donor/π-acid ratios but offer synthetic convenience for coordination chemistry applications. In order to expand the range of available substitution patterns for comparison, the syntheses of the new m-terphenyl isocyanides CNAr^Tripp2, CNp-MeAr^Mes2, CNp-MeAr^DArF2, and CNp-FAr^DArF2 are also reported (Ar^Tripp2 = 2,6-(2,4,6-(i-Pr)₃C₆H₂)₂C₆H₃); p-MeAr^Mes2 = 2,6-(2,4,6-Me₃C₆H₂)₂-4-Me-C₆H₂); p-MeAr^DArF2 = 2,6-(3,5-(CF₃)₂C₆H₃)₂-4-Me-C₆H₂); p-FAr^DArF2 = 2,6-(3,5-(CF₃)₂C₆H₃)₂-4-F-C₆H₂).

Full text of DOI:10.1021/ic5030845

Article
pubs.acs.org/IC
Comparative Measure of the Electronic Influence of Highly
Substituted Aryl Isocyanides
Alex E. Carpenter, Charles C. Mokhtarzadeh, Donald S. Ripatti, Irena Havrylyuk, Ryo Kamezawa,
Curtis E. Moore, Arnold. L. Rheingold, and Joshua S. Figueroa*
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla,
California, 92093-0358, United States
S
* Supporting Information
ABSTRACT: To assess the relative electronic influence of
highly substituted aryl isocyanides on transition metal centers,
a series of C4v-symmetric Cr(CNR)(CO)5 complexes featuring
various alkyl, aryl, and m-terphenyl substituents have been
prepared. A correlation between carbonyl-ligand 13C{1H}
NMR chemical shift (δCO) and calculated Cotton−Kraihanzel
(C−K) force constant (kCO) is presented for these complexes
to determine the relative changes in isocyanide σ-donor/π-acid
ratio as a function of substituent identity and pattern. For
nonfluorinated aryl isocyanides possessing alkyl or aryl
substitution, minimal variation in effective σ-donor/π-acid
ratio is observed over the series. In addition, aryl isocyanides
featuring strongly electron-releasing substituents display an
electronic influence that nearly matches that of nonfluorinated alkyl isocyanides. Lower σ-donor/π-acid ratios are displayed by
polyfluorinated aryl isocyanide ligands. However, the degree of this attenuation relative to nonfluorinated aryl isocyanides is not
substantial and significantly higher σ-donor/π-acid ratios than CO are observed in all cases. Substituent patterns for
polyfluorinated aryl isocyanides are identified that give rise to low relative σ-donor/π-acid ratios but offer synthetic convenience
for coordination chemistry applications. In order to expand the range of available substitution patterns for comparison, the
syntheses of the new m-terphenyl isocyanides CNArTripp2, CNp-MeArMes2, CNp-MeArDArF2, and CNp-FArDArF2 are also reported
(ArTripp2 = 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3); p-MeArMes2 = 2,6-(2,4,6-Me3C6H2)2-4-Me-C6H2); p-MeArDArF2 = 2,6-(3,5-
(CF3)2C6H3)2-4-Me-C6H2); p-FArDArF2 = 2,6-(3,5-(CF3)2C6H3)2-4-F-C6H2).
σ-donor/π-acid ratio,13,14,16−19 which therefore serves as a
strategy to approach the electronic influence inherent to CO.
Notably, however, a systematic quantification of the relative
electronic influence of isocyanides as a function of substituent
INTRODUCTION
■
The isolobal analogy between carbon monoxide (CO) and
organoisocyanides (CNR) has long been recognized in
coordination chemistry.1−7 In general, isocyanides have been
identity and pattern is only available for a small set of
used as surrogates to CO when more sterically encumbering,
cylindrically symmetric π-acidic ligands are desired. However,
there is substantial evidence available demonstrating that, while
substituents.13,14,20,21 In addition, the limits of this approach,
specifically in the context of the degree and type of substitution
necessary for an organoisocyanide to fully mimic the electronic
properties of CO, have not been clearly articulated.
Over the past several years, our group has explored the
qualitatively isolobal, there are significant quantitative electronic
differences present between CO and CNR as ligands.3,6−8
These differences stem from the attenuated “group” electro-
negativity of NR relative to O, which renders organoisocyanides
both stronger σ-donors and weaker π-acids than CO. This
coordination chemistry of m-terphenyl isocyanides for use as
isolobal surrogates to CO.22−35 This ligand class takes
advantage of the encumbering steric properties of the m-
greater σ-donor/π-acid ratio has been used to justify the
terphenyl group36−44 as a means of generating coordinatively
observed tendency of organoisocyanides to stabilize metal
centers in higher formal oxidation states relative to CO.9−11
unsaturated isocyanide complexes that formally mimic the
binary unsaturated metal carbonyls.45−48 Recently, we reported
Despite these fundamental differences, organoisocyanides
allow for electronic and steric modulation in a manner
inaccessible to CO. For electronic modulation, it has been
the synthesis of the fluorinated m-terphenyl isocyanide ligand
CNArDArF2 (ArDArF2 = 2,6-(3,5-(CF3)2C6H3)2C6H3).33 Using
zerovalent mer-Mo(CO)3(CNR)3 complexes as a coordination
well established that the σ-donor/π-acid ratio of an organo-
isocyanide can be modified by changes in substituent
pattern.2,12−15 Accordingly, it has been shown that the presence
Received: December 29, 2014
of electron withdrawing substituents can result in a diminished
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
platform, we noted that CNArDArF2 gave rise to significantly
blue-shifted νCO bands relative to the m-terphenyl isocyanide
CNArMes2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3),22,23 which
features electron-releasing alkyl-substituted flanking rings.
While only a crude estimate, this result indicated to us that
CNArDArF2 possessed a lower σ-donor/π-acid ratio than
CNArMes2, despite the fact that its flanking-ring CF3 groups
are quite distal from the isocyano unit. Given this observation,
we became interested in systematically assessing the relative
electronic influence of highly substituted m-terphenyl iso-
cyanides on transition metal centers. Our aim was to identify
both flanking-ring and backbone substitution patterns that
either significantly alter the σ-donor/π-acid ratio of the
isocyano unit or exert a purely steric influence relative to
less-substituted aryl isocyanide ligands. In addition, we aimed to
understand the degree and type of substitution necessary for an
isocyanide to most accurately mimic the electronic properties of
CO.
2114 and 2118 cm−1, respectively. These spectroscopic
signatures are similar to those found for CNArMes2 and the
2,6-di-isopropylphenyl derivative CNArDipp2 (Table 1; ArDipp2
=
2,6-(2,6-(i-Pr)2C6H3)2C6H3).22,23
Table 1. Infrared (Solution and Solid State) and 13C{1H}
NMR Spectroscopic Data for Free m-Terphenyl Isocyanides
13C NMR
compound
CNArMes2
CNp-MeArMes2
CNArDipp2
CNArTripp2
CNArClips2
CNp-MeArDArF2
CNArDArF2
CNp-FArDArF2
νCN (C6D6, cm−1
)
νCN (KBr, cm−1
)
δCiso (C6D6, ppm)
2118
2118
2118
2118
2119
2118
2112
2113
2120
2121
2124
2114
2132
2118
2119
2118
170.7
170.0
171.9
171.7
172.0
174.8
175.4
175.1
Accordingly, in this report, we present a series of
monoisocyanide Cr(CNR)(CO)5 complexes featuring an
extensive range of alkyl, aryl, and m-terphenyl substitution
The m-terphenyl isocyanides CNp-MeArMes2, CNp-
MeArDArF2, and CNp-FArDArF2 were prepared by palladium-
catalyzed cross coupling of para-substituted 2,6-dibromoani-
lines with either mesityl boronic acid or 3,5-bis-
(trifluoromethyl)phenyl boronic acid, followed by formylation
and dehydration (Schemes 2 and 3).53 Whereas the cross-
patterns, including the new m-terphenyl derivatives CNArTripp2
,
=
CNp-MeArMes2, CNp-MeArDArF2, and CNp-FArDArF2 (ArTripp2
2,6-(2,4,6-(i-Pr)3C6H2)2C6H3); p-MeArMes2 = 2,6-(2,4,6-
Me3C6H2)2-4-Me-C6H2); p-MeArDArF2 = 2,6-(3,5-
(CF3)2C6H3)2-4-Me-C6H2); p-FArDArF2 = 2,6-(3,5-
(CF3)2C6H3)2-4-F-C6H2). Using vibrational and NMR spec-
Scheme 2
troscopy, we present a correlation between Cotton−Kraihanzel
49
CO force constant (kCO
)
and the 13C chemical shift (δ13C
)
that elucidates the electronic influence of each isocyanide in the
series. Notably, kCO/δ13C correlations have been used
previously to provide insight into the electronic influence of
coordinated phosphines, phosphites, and arsines,50 as well as a
few organoisocyanides.20,51 Our results indicate that substituent
variation can alter the σ-donor/π-acid ratio of an isocyanide
ligand, but not to a significant extent. In addition, our data
suggest that practical limitations exist for the use of isocyanides
with σ-donor/π-acid ratios matching that of CO.
RESULTS AND DISCUSSION
■
1. Synthesis of the m-Terphenyl Isocyanide Ligands
CNArTripp2, CNp-MeArMes2, CNp-MeArDArF2, and CNp-
coupling step proceeded smoothly for the p-MeArDArF2 and p-
FArDArF2 derivatives, construction of the sterically hindered p-
MeArMes2 framework required more forcing conditions and led
to a low overall yield of CNp-MeArMes2 (18%). The solution
FArDArF2. The synthetic routes to CNArTripp2, CNp-MeArMes2
,
CNp-MeArDArF2, and CNp-FArDArF2 are outlined in Schemes
52
1−3. Formylation of the known aniline, H2NArTripp2
,
with
and solid-state IR spectra for CNp-MeArMes2, CNp-MeArDArF2
,
formylacetic anhydride provided the formamide, HC(O)N-
(H)ArTripp2, which was subsequently dehydrated with OPCl3/
Na2CO3 to produce CNArTripp2 as a colorless, nonvolatile solid
in 95% isolated yield (Scheme 1). The IR spectra of CNArTripp2
in the solid state (KBr) and C6D6 solution feature νCN bands at
and CNp-FArDArF2 all exhibit νCN bands between 2113 and
Scheme 3
Scheme 1
B
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Article
Figure 1. Molecular structures of m-terphenyl isocyanide Cr(CNR)(CO)5 complexes.
Table 2. Selected Bond Lengths for m-Terphenyl Isocyanide Cr(CNR)(CO)5 Complexes
a
Cr(CNArR2)(CO)5
d(Cr−Ciso) (Å)
axial d(Cr−CCO) (Å)
average equatorial d(Cr−CCO
)
(Å)
CNArR2 d(N−Cipso) (Å)
b
b
b
b
Cr(CNArTripp2)(CO)5 (3)
Cr(CNp-MeArMes2)(CO)5 (6)
Cr(CNArDipp2)(CO)5 (8)
Cr(CNArClips2)(CO)5 (12)
Cr(CNArMes2)(CO)5 (14)
Cr(CNp-MeArDArF2)(CO)5 (15)
Cr(CNArDArF2)(CO)5 (16)
Cr(CNp-FArDArF2)(CO)5 (17)
1.968(2)
1.902(2)
1.904 ( 0.009)
1.396(2)
1.976(2)
1.966(6)
1.974(2)
1.967(2)
1.970(2)
1.968(4)
1.967(2)
1.885(2)
1.874(6)
1.894(2)
1.894(2)
1.891(2)
1.895(5)
1.894(2)
1.908 ( 0.008)
1.908 ( 0.011)
1.910 ( 0.009)
1.908 ( 0.003)
1.911 ( 0.010)
1.906 ( 0.003)
1.399(2)
1.392(6)
1.391(2)
1.395(2)
1.399(3)
1.395(5)
1.397(2)
1.908 ( 0.006)
a
b
Errors reported as the standard deviation of the average equatorial Cr−CCO bond distances. Average of two crystallographically independent
molecules.
2121 cm−1, which is consistent with those observed for
CNArMes2 and CNArDArF2 (Table 1).
2,6-xylyl (Xyl),57 and 2,6-(iPr)2C6H3 (Dipp),58 respectively,
have been prepared previously by the same or other synthetic
routes. Complexes 3, 6, 8, 12, and 14−17, which feature m-
terphenyl isocyanides, were structurally characterized by X-ray
diffraction (Figure 1). As shown in Table 2, substituent
variation on either the central or flanking rings of the m-
terphenyl framework has little effect on the Cr−C bond lengths
of the carbonyl ligands or isocyano unit. In addition, there is
negligible change in the CN or NCipso bond lengths as a
2. Synthesis and Molecular Structures of Cr(CNR)(CO)5
Complexes. In order to directly compare the electronic
influence of isocyanides over a range of substitution patterns,
we sought a conveniently accessed coordination platform that
featured (i) carbonyl ancillary ligands, (ii) a single isocyanide
group, and (iii) well-defined IR and NMR spectroscopic
signatures. These criteria stemmed from the established
inverse-linear correlation between carbonyl 13C chemical shift
(δ) and CO force constant (kCO) as a metal center becomes
more effective at π-back-donation.20,50,51 In addition, the
presence of a single isocyanide ligand was deemed necessary
to isolate and quantify the effects of substitution pattern and
identity. Accordingly, we targeted C4v-symmetric Cr(CNR)-
(CO)5 complexes, which are known to meet these
criteria.8,12,49,51
function of substituent variation.
33
As we previously noted for CNArDArF2
,
the presence of
flanking 3,5-(CF3)2C6H3 rings on the m-terphenyl framework
qualitatively presents the greatest steric encumbrance in the
vicinity of the metal center. This is also the case for the new
derivatives CNp-MeArDArF2 and CNp-FArDArF2 (Figure 1). For
the parent CNArDArF2 ligand, this encumbrance resulted in
significant steric pressures59 that affected the isomeric
The Cr(CNR)(CO)5 complexes 1−18 were prepared from
the addition of CNR to photogenerated Cr(THF)(CO)5 in
THF solution and isolated in low to moderate yields (20−40%)
by crystallization from n-pentane at −35 °C. Complexes 1, 2, 4,
7, 10, 11, and 13, where R = t-Bu,8,54 cyclohexyl (Cy),55 benzyl
(Bn),56 phenyl (Ph),55 p-methylphenyl (p-MePh, i.e., tolyl),12
preference of its Mo(CO)3(CNR)3 complexes relative to
33
CNArMes2 and CNArClips2
.
We believe these steric pressures
are a consequence of the 3,5-disubstitution of the flanking m-
terphenyl rings, which combine with the three-atom CNC
linker to provide a more congested steric environment near the
primary coordination sphere of a metal center. However, from a
C
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Article
Table 3. Carbonyl 13C{1H} NMR Chemical Shifts (δ, ppm; C6D6), Stretching Frequencies (ν, cm−1; C6D6), and Calculated C−K
a
Force Constants (kn, mdyn/Å) for Cr(CNR)(CO)5 Complexes 1−20 and Cr(CO)6
axial CO 13C NMR δ
equatorial CO
A1
A1
B1
E
k1
(mdyn/Å) (mdyn/Å) (mdyn/Å)
k2
ki
(1)
(2)
Cr(CNR)(CO)5
CNt-Bu (1)
(ppm)
13C NMR δ (ppm)
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
)
217.6
217.6
216.8
217.2
216.7
217.2
216.8
216.7
217.3
217.0
216.7
216.8
216.9
217.0
215.4
215.2
215.1
215.6
215.6
215.7
214.3
215.4
214.9
214.4
215.0
214.2
215.3
215.1
214.8
214.2
215.1
214.3
213.7
213.7
213.5
214.4
2063
2063
2056
2064
2058
2055
2057
2055
2057
2058
2056
2052
2057
2054
2045
2045
2044
2046
1950
1948
1952
1952
1956
1954
1954
1955
1955
1955
1956
1957
1957
1958
1960
1963
1965
1965
1980
1990
1996
1993
1997
1996
1999
1995
1994
1997
1998
1996
1996
1999
2002
2002
2003
2002
1950
1948
1952
1952
1956
1954
1954
1955
1955
1955
1956
1957
1957
1958
1960
1963
1965
1965
15.54
15.55
15.56
15.56
15.56
15.58
15.59
15.60
15.60
15.60
15.61
15.62
15.63
15.64
15.65
15.69
15.72
15.72
15.76
16.36
k2 (mdyn/Å)
15.91
15.89
15.89
15.93
15.94
15.91
15.92
15.92
15.93
15.94
15.94
15.93
15.95
15.95
15.93
15.96
15.98
15.99
16.00
16.77
0.28
0.28
CNCy (2)
CNArTripp2 (3)
0.25
CNBn (4)
0.27
CNp-FPh (5)
CNp-MeArMes2 (6)
0.25
0.25
CNPh (7)
0.25
CNArDipp2 (8)
CNMes (9)
0.24
0.25
CNp-MePh (10)
0.25
CNXyl (11)
0.24
CNArClips2 (12)
CNDipp (13)
CNArMes2 (14)
CNp-MeArDArF (15)
CNArDArF2 (16)
CNp-FArDArF2 (17)
CN(3,5-(CF3)2C6H3) (18)
CNC6F5 (19)
0.23
0.24
0.23
0.21
0.20
0.19
0.20
b
b
c
c
c
214.6
213.3
2041
2020
T1u (cm−1
1968
1950
1968
1950
0.18
d
d
e
e
e
e
CNCF3 (20)
211.5
211.7
2000
Eg (cm−1
0.31
CO 13C NMR δ (ppm)
A1g (cm−1
2100
)
)
)
ki (mdyn/Å)
Cr(CO)6
211.7
1985
2020
16.49
0.22
a
b
c
d
Complexes are listed by increasing k1 value. Measured in CDCl3 (ref 19). Measured in n-pentane (ref 19). Measured in CDCl3 (ref 16).
e
Measured in the gas phase (ref 16).
qualitative perspective, CNArTripp2, with its 4-iso-propyl group,
also displays a significant degree of spatial extension near the Cr
center in Cr(CNArTripp2)(CO)5 (3; Figure 1) in a manner
absent for CNArMes2, CNp-MeArMes2, CNArDipp2, and
CNArClips2. Accordingly, CNArTripp2 may be reasonably
expected to exert significant steric pressures as a ligand in a
manner rivaling the CNArDArF derivatives.
3. Correlation between the CO Force Constant (kCO)
and 13C Chemical Shift (δ) for Cr(CNR)(CO)5 Complexes.
As demonstrated by Gansow, the correlation between CO force
constant and carbonyl 13C chemical shift serves as a particularly
good quantitative reporter of the electronic influence of
coordinated ligands L in heteroleptic M(L)(CO)5 complexes.59
The correlation follows from the fact that CO force constants
increase as π-backbonding from a metal center becomes less
effective. Correspondingly, carbonyl 13C chemical shifts show
an upfield progression as the L ligand becomes an increasingly
competitive π-acid.51 This latter effect is due to the dominance
of the paramagnetic shielding term (σ) in determining the 13C
chemical shift of carbonyl ligands in strongly π-backbonding
coordination environments.20,60,61
Figure 2. Carbonyl 13C{1H} NMR chemical shifts (δ, ppm) versus
calculated force constants for axial CO ligands (k1, mdyn/Å) for
Cr(CNR)(CO)5 complexes 1−20.
force constants k1 and k2, respectively).49 Ostensibly, the axial
(k1) and equatorial (k2) CO force constants in these
Cr(CNR)(CO)5 complexes report on different aspects of the
isocyanide σ-donor/π-acid ratio. While neither the σ-donor nor
π-acid properties of an isocyanide can be completely isolated,
the axial CO force constant is expected to be more sensitive to
changes in σ-donor ability of a trans-CNR ligand, whereas the
equatorial CO force constant (k2) reports more fully on the
combined isocyanide σ-donor/π-acid properties. On this basis,
several trends reflective of isocyanide electronic influence can
be deduced from both Figures 2 and 3.
The CO stretching frequencies, CO force constants (kCO
)
calculated using the Cotton−Kraihanzel (C−K) approxima-
tion,49,62 and carbonyl 13C{1H} NMR data (δC) for complexes
1−18 are listed in Table 3. Plots of δC vs kCO for the axial and
equatorial CO ligands are shown in Figures 2 and 3,
respectively. Also included are the data and calculated C−K
force constants for Lentz’s complexes Cr(CNC6F5)(CO)5
(19)19 and Cr(CNCF3)(CO)5 (20),16 which feature the
perfluorinated, but unencumbering, isocyanides CNC6F5 and
CNCF3, respectively. As shown in Figures 2 and 3, a reasonably
good correlation between δC vs kCO values is observed for both
the axial and equatorial carbonyl ligands (corresponding to the
First, it is important to note that the relative spread of CO
force constants (both k1 and k2) and 13C chemical shifts is very
D
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Article
for polyfluorinated aryl isocyanides may originate more
significantly from an attenuation of σ-donor, rather an
enhancement of π-acid, capability. However, relative to
nonfluorinated aryl isocyanides, the magnitude of the change
in σ-donor/π-acid ratio upon polyfluorination is not substantial.
This suggestion follows from the fact that CNC6F5 (in 19),
which possesses the highest degree of fluorination proximal to
the isocyano group, does not lead, in absolute terms, to
significantly different δC vs kn values than high σ-donor/π-acid
ratio aryl isocyanides (e.g., CNArTripp2 (in 3) and CNArDipp2 (in
8)). In addition and most importantly, the σ-donor/π-acid ratio
of CNC6F5 (in 19), as measured by δC vs k2 (i.e., equatorial
force constant; Figure 3),49 is still significantly higher than that
of CO,64 despite substantial fluorination of the aryl backbone.
This observation indicates that the aryl ring itself mitigates the
ability to significantly control the σ-donor/π-acid ratio of the
isocyanide unit, especially for the goal of using substituent
variation to achieve a σ-donor/π-acid ratio rivaling CO.
Accordingly, we contend that the δC vs kn correlation shown
in Figures 2 and 3 begins to define a lower limit for the ability
of an aryl isocyanide to function as an electronic mimic of CO.
Furthermore, as indicated from Figures 2 and 3, only
polyfluorinated alkyl isocyanides, such as the difficult to
prepare and handle derivative CNCF3 (bp = −80 °C),18 may
be expected to display σ-donor/π-acid ratios rivaling or
exceeding that of CO.
Despite this important limitation of aryl isocyanides, it is
noteworthy that fluorination of the aryl ring directly attached to
the isocyano unit is not the only means of achieving relatively
low σ-donor/π-acid ratios. As indicated by Figures 2 and 3,
distal fluorination of the type present in the m-terphenyl
derivatives CNp-XArDArF2 (X = Me, H, F) can also effectively
lower the σ-donor/π-acid ratio of the isocyano unit relative to
nonfluorinated aryl isocyanides. As the most extreme example,
Figures 2 and 3 show that the m-terphenyl derivative CNp-
FArDArF2 (in 17) gives rise to δC vs kn values nearly matching
those of CNC6F5 (in 19). Accordingly, the combination of
distal, flanking-ring CF3 groups and a single central-ring fluoro
substituent can be viewed as equivalent to perfluorination in
terms of its effect on the aryl isocyanide σ-donor/π-acid ratio.
Given that CNC6F5 is also difficult to prepare and
spontaneously polymerizes when pure,18,65,66 we believe that
distal-fluorination of an m-terphenyl or other polyaryl group67
can therefore serve as an effective and more convenient strategy
for obtaining aryl isocyanides with relatively low σ-donor/π-
acid ratios. However, it is likely that the σ-donor/π-acid ratios
of such polyfluorinated aryl isocyanides will remain substan-
tially higher than that of CO.
Figure 3. Carbonyl 13C NMR chemical shifts (δ, ppm) versus
calculated force constants for equatorial CO ligands (k2, mdyn/Å) for
Cr(CNR)(CO)5 complexes 1−20.
narrow over the series (Δkn ≅ 1.0 mdyn/Å; ΔδC ≅ 5 ppm).
Accordingly, in absolute terms, significant changes to both the
ligand topology and substituent pattern do not have a dramatic
effect on the electronic influence of a single isocyanide ligand.
Second, while alkyl isocyanides can be interpreted as having the
highest σ-donor/π-acid ratio, as expected, this ratio is only
marginally higher than that of monoaryl or polyaryl isocyanides
featuring electron-releasing substituents. Indeed, as indicated by
the δC vs kn plots in Figures 2 and 3, the m-terphenyl isocyanide
ligands CNArDipp2 and CNArTripp2, which are heavily substituted
with electron-releasing iso-propyl groups in the flanking-ring
positions, possess σ-donor/π-acid ratios matching those of alkyl
isocyanides.
Within the set of aryl isocyanides considered here, only
polyfluorination leads to a measurable modulation of the
isocyano σ-donor/π-acid ratio. However, the degree of
modulation is again not large. On the basis of the δC vs kn
plots in Figures 2 and 3, roughly two groupings of aryl
isocyanides are apparent. One group consists of aryl isocyanides
possessing electron releasing alkyl- or alkyl-substituted aryl
groups, which all show nearly identical δC vs kn values (n = 1 or
2) irrespective of substitution identity or pattern. The most
notable comparison within this group is between the m-
terphenyl isocyanide CNArMes2 (in 14) introduced22 by our
group and the commonly used aryl isocyanide CNXyl (in 11;
Xyl = 2,6-Me2C6H3).63 On the basis of the δC vs kn values of
their complexes (Figures 2 and 3, insets), it is clear that the
electronic influence of these two isocyanides is largely similar.
Therefore, only differences in steric encumbrance should be
expected to dictate their relative properties as ancillary ligands.
In addition, it is noteworthy that the monofluorinated aryl
isocyanide, CNp-FC6H4 (in 5), exhibits similar δC vs kn values
to those of nonfluorinated aryl isocyanides, thereby illustrating
that a single fluoro substituent does not dramatically alter the σ-
donor/π-acid ratio of aryl isocyanides. A similar observation
was made by Treichel when comparing the electrochemical
properties of homoleptic [M(CNp-XC6H4)6]n complexes (X =
H, Me, F; M = Cr, n = 0; M = Mn, n = 1+).13,14
CONCLUSIONS
■
In coordination chemistry, organoisocyanides have long been
viewed as sterically and electronically tunable surrogates to
carbon monoxide. While it has been recognized that organo-
isocyanides are both stronger σ-donors and less effective π-acids
than CO, the extent of these differences has been less clearly
established. This is particularly true for aryl isocyanides, where
substituent modulation has been thought to afford control over
the electronic influence of the isocyano unit. However, the
results presented here for 20 different isocyanides demonstrate
that the degree to which substituent variation can affect the
electronic influence of an isocyano unit is substantially limited.
For aryl isocyanides, the aryl group itself seemingly mitigates
the ability of electron-withdrawing substituents to significantly
The second group of aryl isocyanides is those featuring
polyfluorinated substituents. As illustrated by the δC vs kn values
(n = 1 or 2), polyfluorination can be considered an effective
means for lowering the σ-donor/π-acid ratio of an aryl
isocyanide. Furthermore, as indicated by Figure 2 (i.e., axial
CO k1 values), the lower isocyano σ-donor/π-acid ratios found
E
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
mmol, 100 mL) over the course of 1 h. The reaction mixture was then
heated to 60 °C for 4 days. Thereafter, the reaction mixture was
allowed to cool to room temperature and the volatile components
removed via rotary evaporation. The crude residue was then washed
with H2O (3 × 100 mL) to remove residual acetic acid. The resulting
solid was then dissolved in Et2O (200 mL), dried over MgSO4, and
filtered. All volatile materials were then removed by rotary evaporation,
and the resulting solid was suspended in EtOH (100 mL) and cooled
to 0 °C. The mixture was then filtered to provide HC(O)NHArTripp2
as a colorless solid, which was dried in vacuo. Yield: 3.55 g, 6.76 mmol,
78.2%. 1H NMR (400.1 MHz, CDCl3, 20 °C): δ = 7.66 (d, 1H, J = 11
Hz, HC(O)NH), 7.22 (t, 1H, p-Ph), 7.17 (d, 2H, J = 7 Hz, m-Ph),
7.087 (s, 4H, m-Tripp), 6.7 (br d, 1H, J = 11 Hz, HC(O)NH), 2.93
(septet, 2H, J = 7 Hz, p-(CH(CH3)2)), 2.61 (septet, 4H, J = 7 Hz, p-
(CH(CH3)2)), 2.22 (br s, 2H, NH2), 1.30 (d, 12 H, J = 7 Hz,
(CH(CH3)2)), 1.12 (d, 12H, J = 7 Hz, (CH(CH3)2), 1.10 (d, 12H, J =
7 Hz, (CH(CH3)2) ppm. 13C{1H}NMR (125.7 MHz, CDCl3, 20 °C):
δ = 162.8 (HC(O)N), 149.5, 146.4, 133.8, 132.4, 131.6, 131.0, 124.4,
121.9, 34.4 ((CH(CH3)2), 30.8 ((CH(CH3)2), 25.0 ((CH(CH3)2),
24.2 ((CH(CH3)2), 23.4 ((CH(CH3)2) ppm. FTIR (KBr window,
CDCl3): νNH = 3352 (w) cm−1, νCO = 1677 (s) cm−1, also 2964 (m),
2929 (w), 2870 (w), 1606 (w), 1461 (w), 1448 (w), 1410 (w), 1364
(w), 1318 (w), 1278 (w), 1070 (w) cm−1. HRMS (ESI pos. ion;
NCMe): m/z Calcd, 548.3863; m/z Found, 548.3864 [M]+.
lower the effective σ-donor/π-acid ratio of the isocyano unit.
Furthermore, the presence of strongly electron-releasing
substituents readily raises the σ-donor/π-acid ratio of an aryl
isocyanide ligand to values similar to alkyl isocyanides. The
analysis presented here also suggests that σ-donor/π-acid ratios
matching or exceeding that of CO may only be achieved by
perfluorinated alkyl isocyanides, which are known to be difficult
to prepare and handle. However, distal polyfluorination, as in
the CNp-XArDArF2 derivatives presented here, can effectively
lower the σ-donor/π-acid ratio of an isocyano group in a
synthetically and operationally convenient manner. Accord-
ingly, we believe the results presented here will be useful for the
design of transition-metal isocyanide complexes and contribute
to a more detailed understanding of their electronic structure
properties.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out
under an atmosphere of dry dinitrogen using standard Schlenk and
glovebox techniques. Solvents were dried and deoxygenated according
to standard procedures.68 Unless otherwise stated, reagent-grade
starting materials were purchased from commercial sources and
purified by standard methods when necessary.69 Benzene-d6 (Cam-
bridge Isotope Laboratories) was distilled from NaK/benzophenone
and stored over 4 Å molecular sieves for 2 days prior to use.
Chloroform-d (Cambridge Isotope Laboratories) was distilled from
Synthesis of CNArTripp2. Diisopropyl amine (HN(i-Pr)2; 4.52 g,
44.63 mmol, 7 equiv) was added via syringe to a stirring CH2Cl2
solution of HC(O)NHArTripp2 (3.55 g, 6.38 mmol, 250 mL) under a
N2 atmosphere. The resulting mixture was cooled to 0 °C where
POCl3 (4.89 g, 31.9 mmol, 5 equiv) was added via syringe over 20
min. The resulting mixture was allowed to warm to room temperature
and then stir for 12 h. Thereafter, aqueous Na2CO3 (1.5 M, 200 mL)
was added and the reaction mixture was stirred for 1 h. The aqueous
and organic layers were then separated and the aqueous layer washed
with CH2Cl2 (3 × 100 mL). The combined organic extracts were then
dried over MgSO4, filtered, and dried by rotary evaporation. The
resulting residue was then suspended in NCMe (100 mL), filtered,
washed with cold (0 °C) MeCN (2 × 25 mL), and dried in vacuo to
afford CNArTripp2 as a colorless solid. Yield: 3.07 g, 6.05 mmol, 94.8%.
1H NMR (400.1 MHz, C6D6, 20 °C): δ = 7.25 (S, 4H, m-Tripp), 7.02
(d, 2H, J = 7 Hz, m-Ph), 6.94 (t, 1H, J = 7 Hz, p-Ph), 2.86 (septet, 2H,
J = 7 Hz, p-(CH(CH3)2)), 2.78 (septet, 4H, J = 7 Hz, o-
((CH(CH3)2)), 1.37 (d, 12H, J = 7 Hz, (CH(CH3)2)), 1.24 (d,
12H, J = 7 Hz, (CH(CH3)2)), 1.18 (d, 12H, J = 7 Hz, (CH(CH3)2))
ppm. 13C{1H} NMR (125.7 MHz, C6D6, 20 °C): δ = 171.7 (CNR),
149.7, 146.7, 139.7, 132.9, 129.9, 128.5, 127.8, 121.4, 34.8 ((CH-
(CH3)2), 31.6 ((CH(CH3)2), 24.7 ((CH(CH3)2), 24.4 ((CH(CH3)2),
24.3 ((CH(CH3)2) ppm. FTIR (KBr window, C6D6): νCN = 2118 (s)
cm−1, also 2961 (m), 2930 (w), 2870 (w), 1464 (w), 1383 (w), 1363
calcium hydride and stored over 4 Å molecular sieves for 2 days prior
52
to use. The m-terphenyl aniline, H2NArTripp2
,
and isocyanides
CNArMes2, CNArDipp2, CNArDArF2, and CNArClips2 were prepared
according to published procedures.22,23,33 The isocyanides CN-t-Bu,
CNCy, CN-p-FPh, CNPh, and CNBn (Cy = cyclohexyl; Bn = benzyl)
were purchased from commercial sources and used as received. The
isocyanides CNMes, CNXyl, and CN-p-Tol (Mes = 2,4,6-Me3C6H2;
Xyl = 2,6-Me2C6H3; p-Tol = 4-MeC6H4) were prepared by
formylation of the corresponding aniline with HC(O)OH in toluene,
followed by dehydration with OPCl3/NEt3/Na2CO3 in CH2Cl2. The
known isocyanide CN(3,5-(CF3)2C6H3)70 was prepared by the
modified procedure described below. Full characterization data for
Cr(CNR)(CO)5 complexes 1−18 are provided in the Supporting
Information.
1
Solution H and 13C{1H} NMR spectra were recorded on Varian
Mercury 300 and 400 spectrometers, a Varian X-Sens500 spectrom-
1
eter, or a JEOL ECA-500 spectrometer. H and 13C{1H} chemical
shifts are reported in ppm relative to SiMe4 (1H and 13C, δ = 0.0 ppm)
with reference to residual solvent resonances of 7.16 ppm (1H) and
128.06 ppm (13C) for benzene-d6 and 7.26 ppm (1H) and 77.16 ppm
(13C) for chloroform-d.71 FTIR spectra were recorded on a Thermo-
Nicolet iS10 FTIR spectrometer. Samples were prepared as C6D6
solutions injected into a ThermoFisher solution cell equipped with
KBr windows or as KBr pellets. For solution FTIR spectra, solvent
peaks were digitally subtracted from all spectra by comparison with an
authentic spectrum obtained immediately prior to the sample. The
following abbreviations were used for the intensities and characteristics
of important IR absorption bands: vs = very strong, s = strong, m =
medium, w = weak, vw = very weak, b = broad, vb = very broad, sh =
shoulder. High-resolution mass spectrometry (HRMS) was performed
using either an Agilent 6230 ESI-TOFMS instrument running in either
positive or negative ion mode or a Thermo MAT900XL Electron
Impact MS instrument running in negative ion mode. Combustion
analyses were performed by Robertson Microlit Laboratories of
Madison, NJ.
(w), 1104 (w), 879 (w), 760 (w) cm−1. FTIR (KBr pellet): νCN
=
2114 (s) cm−1, also 2962 (s), 2929 (s), 2904 (s), 2868 (s), 1666 (w),
1608 (m), 1570 (m), 1459 (s), 1422 (m, sh), 1380 (m), 1363 (m),
1316 (m), 1240 (m), 1105 (m), 1068 (m), 1054 (m), 1011 (w), 946
(m), 924 (w, sh), 876 (w, sh), 823 (m), 803 (m, sh), 757 (m), 652
(m) cm−1. HRMS (ESI pos. ion; NCMe): m/z Calcd, 508.3938; m/z
Found, 508.3941 [M]+.
Synthesis of H2N(p-MeArMes2). A resealable ampoule was charged
with 2,6-dibromo-4-methylaniline (0.954 g, 3.60 mmol), mesityl
boronic acid (MesB(OH)2; 1.30 g, 7.92 mmol, 2.2 equiv), Na2CO3
(1.68 g, 15.84 mmol, 4.4 equiv), and toluene (30 mL). To this mixture
was then added a toluene solution (10 mL) containing Pd2(dba)3
(0.165 g, 0.18 mmol, 5 mol %; dba = dibenzylideneacetone), PPh3
(0.094 g, 0.36 mmol, 10 mol %), followed by EtOH (10 mL) and H2O
(5 mL). The ampoule was sealed under an argon atmosphere and
heated to 90 °C for 72 h. The resulting mixture was then cooled to
room temperature and filtered through a medium porosity fritted
funnel packed with Celite. Brine (50 mL) was added to the filtrate, and
the resulting biphasic mixture was neutralized to pH ≈ 7 using HCl
(0.1 M). The aqueous and organic phases were then separated and the
aqueous phase extracted with Et2O (3 × 10 mL). The organic extracts
were combined and dried over MgSO4. Volatile materials were then
Synthesis of HC(O)NHArTripp2. Acetic anhydride (Ac2O; 17.7 g,
0.173 mol, 20 equiv) was cooled to 0 °C under a N2 atmosphere and
formic acid (7.96 g, 0.173 mol, 20 equiv) was added dropwise, via
syringe, over 20 min. The mixture was stirred at 0 °C for 20 min and
then heated to 50 °C for 5 h. The mixture, now containing
formylacetic anhydride,72 was then cooled to room temperature and
added, via cannula, to a THF solution of H2NArTripp2 (4.30 g, 8.65
F
DOI: 10.1021/ic5030845
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
removed by rotary evaporation and the resulting oil purified by column
chromatography (silica gel) using hexanes to elute principle
contaminants followed by a 0.5% EtOAc/hexanes solution to elute
H2N(p-MeArMes2). Fractions containing H2N(p-MeArMes2) were
combined and concentrated to a solid by rotary evaporation. Yield:
0.222 g, 0.648 mmol, 18.0%. 1H NMR (499.8 MHz, C6D6, 20 °C): δ =
6.91 (s, 4H, m-Mes), 6.75 (s, 2H, m-Ph), 2.91 (s, 2H, NH2), 2.22 (s,
6H, p-Mes), 2.19 (s, 3H, p-MeAr), 2.17 (s, 12H, o-Mes) ppm. 13C{1H}
NMR (125.7 MHz, C6D6, 20 °C): δ = 138.9, 137.1, 136.8, 136.1,
129.5, 128.9, 127.3, 126.4, 21.2 (p-MeAr), 20.8 (p-Mes), 20.4 (o-Mes)
ppm. FTIR (C6D6, KBr window): νNH = 3472 (m) and 3378 (m)
cm−1, also 3006 (m), 2970 (w), 2942 (w), 2915 (m), 2859 (w), 2713
(vw), 1610 (m), 1585 (m), 1480 (w), 1463 (s), 1377 (v), 1299 (vw),
1258 (m), 1097 (vw), 1036 (w), 1014 (w), 991 (vw), 866 (w), 852
(m), 775 (vw), 694 (vw) cm−1. HRMS (ESI pos. ion; NCMe): m/z
Calcd, 344.2373; m/z Found, 344.2372 [M]+.
21.2 (p-Ph), 21.1 (p-Mes), 20.5 (o-Mes) ppm. FTIR (KBr pellet): νNH
= 3236 (s) cm−1, νCO = 1664 (m) cm−1, also 3025 (w), 2966 (w),
2943 (vw), 2912 (w), 2868 (w), 1687 (w), 1612 (w), 1597 (vw), 1569
(vw), 1534 (m), 1486 (vw), 1456 (w), 1386 (w), 1271 (sh), 1263 (w),
1034 (vw), 1017 (vw), 903 (vw), 864 (w), 840 (w), 789 (vw), 746
(vw), 707 (w), 701 (w), 625 (w), 585 (vw), 563 (w), 520 (vw) cm−1.
HRMS (ESI pos. ion; NCMe): m/z Calcd, 372.2325; m/z Found,
372.2322 [M]+.
Data for HC(O)NH(p-MeArDArF2). Yield: 0.788 g, 1.41 mmol,
1
72.4%. H NMR analysis HC(O)NH(p-MeArDarF2) as isolated above
indicated a 3:1 mixture of cis- and trans-isomers. This isomeric mixture
was used in the subsequent dehydration step without separation.
1
Spectroscopic data for cis-isomer: H NMR (499.8 MHz, CDCl3, 20
°C): δ = 7.90 (s, 2H, p-ArF), 7.86 (s, 1H, HC(O)NH), 7.84 (s, 4H, o-
ArF), 7.28 (s, 2H, m-Ph), 6.62 (s, 1H, HC(O)NH), 2.48 (s, 3H, CH3)
ppm. 13C{1H} NMR (125.7 MHz, CDCl3, 20 °C): δ = 160.0
2
Synthesis of the m-Terphenyl Anilines H2N(p-RArDArF2) (R =
Me, F). A resealable ampoule was charged with the desired 2,6-
dibromoaniline (3.60 mmol), 3,5-(CF3)2C6H3B(OH)2 (2.04 g, 7.92
mmol, 2.2 equiv), Na2CO3 (1.68 g, 15.84 mmol, 4.4 equiv), and
toluene (30 mL). To this mixture was then added a toluene solution
(10 mL) containing Pd2(dba)3 (0.033 g, 0.036 mmol, 1 mol %; dba =
dibenzylideneacetone), PPh3 (0.019 g, 0.072 mmol, 2 mol %),
followed by EtOH (10 mL) and H2O (5 mL). The ampoule was
sealed under an argon atmosphere and heated to 90 °C for 20 h. Work
up and isolation was conducted in identical fashion to that of H2N(p-
MeArMes2).
(HC(O)NH), 141.1, 139.4, 138.5, 131.9 (q, JCF = 33 Hz, m-ArF),
131.8, 129.1, 127.1, 123.3 (q, 1JCF = 273 Hz, CF3), 121.8, (septet, 3JCF
= 4 Hz, p-ArF), 21.2 (s, p-Me) ppm. 19F NMR (282.3 MHz, C6D6): δ
= −62.80 (s, CF3) ppm. Spectroscopic data for trans-isomer: 1H NMR
(499.8 MHz, CDCl3, 20 °C): δ = 7.91 (s, 2H, p-ArF), 7.83 (s, 4H, o-
ArF), 7.71 (d, 1H, J = 11 Hz, HC(O)NH), 7.30 (s, 2H, m-Ph), 7.01
(d, 1H, J = 11 Hz, HC(O)NH), 2.49 (s, 3H, CH3) ppm. 13C{1H}
NMR (125.7 MHz, CDCl3, 20 °C): δ = 164.1 (HC(O)NH), 140.3,
2
138.7, 136.2, 132.5, 132.6 (q, JCF = 33 Hz, m-ArF), 129.7, 128.1,
123.1 (q, 1JCF = 273 Hz, CF3), 122.1 (septet, 3JCF = 4 Hz, p-ArF), 21.1
(s, p-Me) ppm. 19F NMR (282.3 MHz, C6D6): δ = −62.86 (s, CF3)
ppm. FTIR isomeric mixture (C6D6, KBr windows): νCO = 1704 (s)
cm−1; νNH = 3372 (w), 2979 (w), 2923 (w), 2864 (w), 1360 (m),
1280 (vs), 1186 (s), 1133 (vs), 900 (w), 844 (w), 739 (w), 708 (w),
683 (w), 644 (w) cm−1. Anal. Calcd for C23H11F12NO (bulk sample,
isomeric mixture): C, 50.66; H, 2.03; N, 2.57. Found: C, 50.77; H,
1.84; N, 2.66.
Data for H2N(p-MeArDArF2). Yield: 0.860 g, 1.62 mmol, 45.0%. 1H
NMR (499.8 MHz, C6D6): δ = 7.76 (s, 2H, p-ArF), 7.70 (s, 4H, o-
ArF), 6.50 (s, 2H, m-Ph), 2.59 (s, 2H, NH2), 2.05 (s, 3H, p-MeAr)
ppm. 13C{1H} NMR (125.7 MHz, C6D6): δ = 142.1, 138.2, 132.5 (q,
1
2JCF = 33 Hz, m-ArF), 131.7, 129.7, 128.4, 125.3, 123.9 (q, JCF = 273
3
Hz, CF3), 121.3 (septet, JCF = Hz, p-ArF), 20.2 (p-Me) ppm. 19F
Data for HC(O)NH(p-FArDArF2). Yield: 0.913 g, 1.62 mmol, 83.3%.
1H NMR analysis of HC(O)NH(p-MeArDarF2) as isolated above
indicated a 7:1 mixture of cis- and trans-isomers. This isomeric mixture
was used in the subsequent dehydration step without separation.
NMR (282.3 MHz, C6D6, 20 °C): δ = −63.31 (s, CF3) ppm. FTIR
(C6D6, KBr window: νNH = 3459 (w) and 3378 (w) cm−1, also 2959
(w), 2923 (m), 2854 (w), 1619 (m), 1480 (m), 1460 (w), 1363 (s),
1302 (m), 1277 (vs), 1183 (vs), 1141 (vs), 1105 (m), 899 (s), 841
(m), 811 (s), 708 (m), 680 (m), 641 (w) cm−1. Anal. Calcd for
C23H13F12N: C, 52.00; H, 2.47; N, 2.64. Found: C, 50.83; H, 2.28; N,
2.64.
1
Spectroscopic data for cis-isomer: H NMR (499.8 MHz, CDCl3, 20
°C): δ = 7.93 (s, 2H, p-ArF), 7.86 (s, 1H, HC(O)NH), 7.84 (s, 2H, o-
2
ArF), 7.21 (d, 2H, JFH = 8 Hz, m-Ph), 6.67 (s, 1H, (d, 1H,
Data for H2N(p-FArDArF2). Yield: 1.30 g, 2.43 mmol, 67.5%. H
HC(O)NH) ppm. 13C{1H} NMR (125.7 MHz, CDCl3, 20 °C): δ =
1
NMR (499.8 MHz, C6D6): δ = 7.74 (s, 2H, p-ArF), 7.55 (s, 4H, o-
160.0 (HC(O)NH), 161.3 (d, 1JCF = 253 Hz, p-Ph), 140.8 (d, 3JCF = 7
2
2
Hz, o-Ph), 139.9, 132.2 (q, JCF = 33 Hz, m-ArF), 129.0, 125.9, 123.1
ArF), 6.39 (d, 2H, JFH = 8 Hz, m-Ph), 2.45 (s, 2H, NH2) ppm.
(q, 1JCF = 273 Hz, CF3), 122.4, (septet, 3JCF = 4 Hz, p-ArF), 118.1 (d,
2JCF = 20 Hz, m-Ph) ppm. 19F NMR (282.3 MHz, C6D6): δ = −62.85
1
13C{1H} NMR (125.7 MHz, C6D6): δ = 156.3 (d, JCF = 239 Hz, p-
Ph), 140.6, 136.9, 132.6 (q, 2JCF = 33 Hz, m-ArF), 129.5, 126.0 (d, 3JCF
2
1
3
(s, CF3), 110.87 (t, JFH = 8 Hz, p-Ph) ppm. Spectroscopic data for
= 7 Hz, o-Ph), 123.7 (q, JCF = 273 Hz, CF3), 121.8 (septet, JCF = 4
trans-isomer: 1H NMR (499.8 MHz, CDCl3, 20 °C): δ = 7.95 (s, 2H,
p-ArF), 7.82 (s, 4H, p-ArF), 7.70 (d, 1H, J = 11 Hz, HC(O)NH), 7.25
(d, 2H, JF−H = 8 Hz, m-Ph), 6.89 (d, 1H, J = 11 Hz, HC(O)NH) ppm.
13C{1H} NMR (125.7 MHz, CDCl3, 20 °C): δ = 163.7 (HC(O)NH),
Hz, p-ArF), 117.7 (d, JCF = 20 Hz, m-Ph) ppm. 19F NMR (282.3
MHz, C6D6, 20 °C): δ = −63.33 (s, CF3), −126.4 (t, JFH = 8 Hz, p-
2
2
Ph) ppm. FTIR (C6D6, KBr windows): 3478 (w), 3384 (w), 3081 (w),
1613 (vw), 1574 (s), 1465 (s), 1349 (s), 1274 (s), 1185 (s), 1141 (s),
1105 (s), 1074 (s), 916 (m), 889 (m) cm−1. Anal. Calcd for
C22H10F13N: C, 49.34; H, 1.89; N, 2.62. Found: C, 50.83; H, 2.28; N,
2.78.
1
3
161.1 (d, JCF = 271 Hz, p-Ph), 139.1, 138.6 (d, JCF = 9 Hz, o-Ph),
132.8 (q, 2JCF = 33 Hz, m-ArF), 129.5, 126.9, 122.9 (q, 1JCF = 273 Hz,
CF3), 122.7 (septet, 3JCF = 4 Hz, p-ArF), 118.8 (d, 2JCF = 20 Hz, m-Ph)
ppm. 19F NMR (282.3 MHz, C6D6): δ = −62.91 (s, CF3), 111.18 (t,
2JFH = 8 Hz, p-Ph) ppm. FTIR isomeric mixture (C6D6, KBr
Synthesis of the m-Terphenyl Formamides H(O)CNH(p-
R′ArR2) (R′ = H, Me, F; R = Mes, DArF). Acetic anhydride (Ac2O;
5.9 g, 0.058 mol, 20 equiv) was cooled to 0 °C under a N2 atmosphere,
and formic acid (3.33 g, 0.073 mol, 25 equiv) was added dropwise, via
syringe, over 10 min. The mixture was stirred at 0 °C for 20 min and
then heated to 50 °C for 5 h. This mixture, now containing formyl
acetic anhydride, was then cooled to room temperature and added, via
syringe, to a toluene solution of H2N(p-R′ArR2) (1.95 mmol). The
resulting mixture was stirred for 16 h. Volatile materials were then
removed by rotary evaporation to afford H(O)CNH(p-R′ArR2) as a
colorless solid, which was used without further purification.
Data for HC(O)NH(p-MeArMes2). Yield: 0.616 g, 1.66 mmol, 85%.
1H NMR (499.8 MHz, CDCl3, 20 °C): δ = 7.59 (d, 1H, J = 12 Hz,
windows): νCO = 1707 (s) cm−1; νNH = 3369 (w), 3239 (vw), 3075
(w), 2884 (w), 2279 (s), 1487 (w), 1361 (m), 1276 (vs), 1176 (s),
1137 (vs), 905 (w), 843 (w), 804 (w) cm−1. Anal. Calcd for
C23H10F13NO (bulk sample, isomeric mixture): C, 49.02; H, 1.78; N,
2.49. Found: C, 49.11; H, 1.84; N, 2.66.
Synthesis of the m-Terphenyl Isocyanides (CNp-R′ArR2) (R′ =
H, Me, F; R = Mes, DArF). Diisopropylamine (HN(i-Pr)2; 0.520 g,
5.13 mmol, 3.5 equiv) was added, via syringe, to a CH2Cl2 solution of
HC(O)NH(p-R′ArR2) (1.50 mmol, 60 mL). The resulting mixture was
stirred vigorously for 5 min and then cooled to 0 °C where POCl3
(0.450 g, 2.93 mmol, 2 equiv) was added dropwise by syringe over 5
min. The reaction mixture was allowed to warm to room temperature
and was then stirred for 3 h. Aqueous Na2CO3 (1.5 M, 40 mL) was
added and the resulting mixture stirred for 1 h. The organic and
aqueous layers were separated and the latter extracted with CH2Cl2 (3
HC(O)NH), 6.94 (s, 6H, m-Mes and m-Ph), 6.49 (d, 1H, J = 12 Hz,
HC(O)NH), 2.37 (s, 3H, p-Ph), 2.31 (s, 6H, p-Mes), 2.01 (s, 12H, o-
Mes) ppm. 13C{1H} NMR (125.7 MHz, CDCl3, 20 °C): δ = 162.8
(HC(O)NH), 137.8, 135.8, 135.7, 134.7, 133.6, 130.8, 129.7, 128.9,
G
DOI: 10.1021/ic5030845
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
× 20 mL). The organic extracts were combined and dried over
MgSO4. Volatile materials were removed by rotary evaporation.
Dissolution of the resulting solid in a minimal amount of NCMe
followed by cooling to −40 °C resulted in the formation of a colorless
precipitate, which was collected by filtration and dried in vacuo.
ArF) ppm. 19F NMR (470.6 MHz, C6D6): δ = −62.7 (s, CF3) ppm.
FTIR (C6D6, KBr window): νCN = 2129 (vs) cm−1, also 3088 (vw),
3062 (vw), 2928 (vw), 1622 (w), 1459 (m), 1369 (vs), 1281 (vs),
1249 (s), 1179 (vs), 1145 (vs), 1107 (m), 1047 (vw), 898 (s), 847
(m), 741 (vw), 723 (w), 685 (m) cm−1. MS (electron impact,
NCMe): m/z Calcd, 239.0; m/z Found, 239.0 [M]−.
1
Data for CNp-MeArMes2. Yield: 0.345 g, 0.975 mmol, 69.6%. H
NMR (499.8 MHz, C6D6, 20 °C): δ = 6.87 (s, 4H, m-Mes), 6.66 (s,
2H, m-Ph), 2.18 (s, 6H, p-Mes), 2.08 (s, 12H, o-Mes), 1.94 (s, 3H, p-
CH3) ppm. 13C{1H} NMR (125.7 MHz, C6D6, 20 °C): δ = 170.0
(CNR), 139.7, 139.5, 137.7, 135.7, 134.9, 129.9, 128.8, 124.1, 21.2 (p-
Mes), 21.1 (p-CH3), 20.3 (o-Mes) ppm. FTIR (C6D6, KBr windows):
νCN = 2118 cm−1, also 2953 (w), 2924 (w), 2855 (w), 1615 (m), 1488
(sh), 1452 (m), 1377 (w), 1035 (w), 870 (w), 852 (m), 791 (w), 770
(w), 707 (w), 629 (w), 566 (w) cm−1. FTIR (KBr pellet): νCN = 2121
cm−1, also 3033 (m), 2973 (m), 2912 (w), 2854 (w), 1614 (m), 1599
(m), 1441 (m), 1271 (w), 1248 (vw), 1034 (m), 1012 (m), 866 (s),
852 (m), 794 (m), 769 (m), 748 (w), 707 (m), 628 (m), 588 (w), 564
(m), 522 (w) cm−1. Anal. Calcd for C26H27N: C, 88.34; H, 7.70; N,
3.96. Found: C, 88.24; H, 7.49; N, 3.90.
Synthesis of Cr(CNR)(CO)5 Complexes (1−18). A 350 mL
resealable ampoule was charged with a THF solution of Cr(CO)6
(1.00 g, 4.54 mmol, 100 mL, 10 equiv). The solution was then
subjected to three freeze−pump−thaw cycles. The ampule was then
sealed and warmed to room temperature where it was irradiated for 2
h using a mercury arc lamp (Oriel Corporation, model 66023, 1000
W). During this time, the reaction mixture turned from colorless to
bright yellow with visible effervescence of CO, indicating the
formation of Cr(THF)(CO)5. The resulting mixture was then
combined with a THF solution (4.54 mM) of isocyanide (1.0
equiv) and allowed to stir for 2 h. All volatile materials were then
removed under reduced pressure. Unreacted Cr(CO)6 was then
recovered from the solid product mixture by sublimation (50 mTorr,
40 °C). The resulting residue was then extracted with C6H6 and
filtered through Celite. The pale yellow to colorless filtrate was then
evaporated to dryness under reduced pressure. The resulting solid was
dissolved in a minimal amount of n-pentane and then stored at −35
°C. The single crystals thereby produced were collected and dried in
vacuo. Full characterization details for all complexes are provided in the
Supporting Information.
Crystallographic Structure Determinations. Single-crystal X-
ray structure determinations were carried out using Bruker Platform or
Kappa X-ray Diffractometers equipped with Mo or Cu radiation
sources (sealed tube or rotating anode), low-temperature cryostats,
and CCD detectors (Bruker APEX or Bruker APEX II). All structures
were solved by direct methods using SHElXS73 and refined by full-
matrix least-squares procedures utilizing SHELXL73 within the Olex2
small-molecule solution, refinement and analysis software package.74
Crystallographic data-collection and refinement information are listed
in Table S3.1 (Supporting Information). Full details of disorder
modeling and structure refinement are provided in the Supporting
Information.
1
Data for CNp-MeArDArF. Yield: 0.336 g, 1.11 mmol, 74.4%. H
NMR (499.8 MHz, C6D6, 20 °C): 7.76 (s, 2H, p-ArF), 7.70 (s, 4H, o-
ArF), 6.44 (s, 2H, m-Ph), 1.87 (s, 3H, p-Me) ppm. 13C{1H} NMR
(125.7 MHz, C6D6, 20 °C): δ = 174.8 (CNR), 140.29, 139.3, 136.7,
2
1
132.2 (q, JCF = 33 Hz, m-ArF), 131.0, 129.6, 127.6, 123.7 (q, JCF
=
273 Hz, CF3), 122.5 (septet, 3JCF = 4 Hz, p-ArF), 20.9 (s, p-Me) ppm.
19F NMR (282.3 MHz, C6D6 20 °C): δ = −63.19 (s, CF3) ppm. FTIR
(C6D6, KBr window): νCN = 2118 (s) cm−1, also 3098 (w), 2962 (w),
2926 (w), 1621 (w), 1596 (w), 1463 (w), 1399 (m), 1369 (s), 1310
(w), 1277 (vs), 1247 (w), 1191 (m), 1174 (m), 1161 (m), 1127 (s),
905 (m), 869 (w), 849 (w), 705 (m), 683 (m), 647 (w) cm−1. FTIR
(KBr pellet): νCN = 2118 (s) cm−1, also 3099 (vw), 2927 (vw), 2848
(vw), 1622 (w), 1597 (w), 1465 (w), 1405 (m), 1370 (m), 1314 (m),
1278 (vs), 1246 (w), 1189 (s), 1178 (s), 1165 (s), 1125 (vs), 1114 (s),
1073 (m), 951 (w), 906 (s), 869 (m), 848 (m), 708 (s), 681 (s) cm−1.
Anal. Calcd for C23H9F12N: C, 52.39; H, 1.72; N, 2.66. Found: C,
52.07; H, 1.65; N, 2.70.
Data for CNp-FArDArF. Yield: 0.428 g, 0.790 mmol, 52.4%. 1H
NMR (499.8 MHz, C6D6, 20 °C): δ = 7.75 (s, 2H, p-ArF), 7.57 (s, 4H,
2
o-ArF), 6.28 (d, 2H, JFH = 8 Hz, m-Ph) ppm. 13C{1H} NMR (125.7
MHz, C6D6, 20 °C): δ = 175.1 (CNR), 161.7 (d, 1JCF = 254 Hz, p-Ph),
3
2
139.0 (d, JCF = 9 Hz, o-Ph), 137.9, 132.4 (q, JCF = 33 Hz, m-ArF),
ASSOCIATED CONTENT
129.5, 127.5, 123.5 (q, 1JCF = 273 Hz, CF3), 123.0 (septet, 3JCF = 4 Hz,
■
p-ArF), 119.4, 117.5 (d, JCF = 20 Hz, m-Ph) ppm. 19F NMR (282.3
2
S
* Supporting Information
MHz, C6D6): δ = −62.9 (s, CF3), −107.3 (t, 2JFH = 8 Hz, p-Ph) ppm.
FTIR (C6D6, KBr window): νCN = 2113 (s) cm−1, also 2964 (w), 2920
(w), 2848 (w), 1596 (vw), 1398 (vw), 1364 (m), 1280 (vs), 1186.4
(s), 1143 (s), cm−1. FTIR (KBr pellet): νCN = 2118 (s) cm−1, also
3090 (w), 2926 (w), 2851 (w), 1624 (w), 1596 (m), 1463 (m), 1424.2
(m), 1366 (s), 1285 (s), 1224 (m), 1154 (vs), 1133 (vs), 1069 (s),
908 (s), 874 (m), 847 (m), 736 (m), 633 (m), 683 (s) cm−1. HRMS
(ESI neg. ion; NCMe): m/z Calcd, 544.0376; m/z Found, 544.0384
[M]−.
Characterization data for Cr(CNR)(CO)5 complexes 1−18,
Cotton−Kraihanzel force constant (k) calculations and δ13C vs
kn plots, crystallographic structure determinations, and cif. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
Synthesis of CN(3,5-(CF3)2C6H3). Diisopropylamine (HN(i-Pr)2;
1.06 g, 10.50 mmol, 2.7 equiv) was added, via syringe, to a stirring
CH2Cl2 solution of HC(O)NH(3,5-(CF3)2C6H3) (1.00 g, 3.89 mmol,
60 mL) under a N2 atmosphere. The resulting mixture was cooled to 0
°C and allowed to equilibrate for 5 min. POCl3 (0.656 g, 4.28 mmol,
1.1 equiv) was then added dropwise via syringe over 10 min. The
reaction mixture was then allowed to warm to room temperature and
react for 24 h. Aqueous Na2CO3 (1.5 M, 40 mL) was added and the
resulting mixture stirred for 1 h. The organic and aqueous layers were
separated and the latter extracted with CH2Cl2 (3 × 20 mL). The
organic extracts were combined and dried over MgSO4. Volatile
materials were removed by rotary evaporation to afford a crude oil,
which was purified by column chromatography (silica gel) with
CH2Cl2 as an eluent to afford CN(3,5-(CF3)2C6H3) as a low-melting
solid. Yield: 0.499, 2.09 mmol, 53.7%. 1H NMR (499.8 MHz, C6D6): δ
= 7.32 (s, 1H), 6.76 (s, 2H) ppm. 13C{1H} NMR (125.7 MHz, C6D6):
δ = 171.6 (CNR), 132.7 (q, 2JCF = 34 Hz, m-ArF), 126.6 (m, i-ArF and
*E-mail: jsfig@ucsd.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the U.S. Department of Energy (DE-
SC0008058) for support of this work. The U.S. National
Science Foundation is acknowledged for Graduate Research
Fellowships to A.E.C. and C.C.M. The California Institute for
Telecommunications and Information Technology (CalIT2) is
thanked for an undergraduate summer research scholarship to
D.S.R. J.S.F is a Camille Dreyfus Teacher Scholar (2012-2017).
We also thank Professors William C. Trogler and Mikhail V.
Barybin for very helpful discussions.
1
3
o-ArF), 122.5 (q, JCF = 274 Hz, CF3), 122.4 (septet, JCF = 4 Hz, p-
H
DOI: 10.1021/ic5030845
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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DOI: 10.1021/ic5030845
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