Coordination or Oxidative Addition? Activation of N-H with [Tp′Rh(PMe3)]

Article abstract of DOI:10.1021/acs.inorgchem.8b02752

A thermal reaction of amines, anilines, and amides with Tp′Rh(PMe₃)(CH₃)H (1, Tp′ = tris(3,5-dimethyl-pyrazolyl)borate) is described in this report. No N-H bond cleavage was observed for reactions between ammonia or unsubstituted aliphatic amines with the reactive fragment [Tp′Rh(PMe₃)]. Instead, amine coordination products (κ²-Tp′)Rh(PMe₃)(NHR¹R²) (R¹ = H, R² = H, ⁿPr, ⁱPr, octyl; R¹ = R² = Et; R¹, R²: pyrrolidine) were observed, and the crystal structure of (κ²-Tp′)Rh(PMe₃)(NH₂ⁱPr) is reported. No coordination products were observed when 1 was reacted with 1,1,1,3,3,3-hexafluoropropan-2-amine, anilines, and amides. Instead, the oxidative addition products (κ³-Tp′)Rh(PMe₃)(NHR)H (R = CH(CF₃)₂, C₆H₅, 3,5-dimethylbenzyl, C₆F₅, C(O)CH₃, C(O)CF₃) were observed. Both Rh^I-N coordination products (κ²-Tp′)Rh(PMe₃)(NH₂CH₂CF₃) and Rh^III N-H addition products (κ³-Tp′)Rh(PMe₃)(NHCH₂CF₃)H were generated when 1 was reacted with 2,2,2-trifluoroethylamine. Coordination products dissociate ammonia and amines in benzene much faster than oxidative addition products eliminate anilines and amides. The relative metal-nitrogen bond energies were studied using established kinetic techniques. Analysis of the relationship between the relative M-N bond strengths and N-H bond strengths showed a linear correlation with a slope = R_M-N/N-H of 0.91 (10), indicating that the Rh-N bond strength varies in direct proportion to the N-H bond strength.

Full text of DOI:10.1021/acs.inorgchem.8b02752

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Coordination or Oxidative Addition? Activation of N−H with
[Tp′Rh(PMe3)]
Jing Yuwen, William W. Brennessel, and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: A thermal reaction of amines, anilines, and amides with
Tp′Rh(PMe3)(CH3)H (1, Tp′ = tris(3,5-dimethyl-pyrazolyl)borate) is
described in this report. No N−H bond cleavage was observed for reactions
between ammonia or unsubstituted aliphatic amines with the reactive
fragment [Tp′Rh(PMe3)]. Instead, amine coordination products (κ2-Tp′)-
n
i
Rh(PMe3)(NHR1R2) (R1 = H, R2 = H, Pr, Pr, octyl; R1 = R2 = Et; R1, R2:
pyrrolidine) were observed, and the crystal structure of (κ2-Tp′)Rh(PMe3)-
i
(NH2 Pr) is reported. No coordination products were observed when 1 was
reacted with 1,1,1,3,3,3-hexafluoropropan-2-amine, anilines, and amides. Instead, the oxidative addition products (κ3-
Tp′)Rh(PMe3)(NHR)H (R = CH(CF3)2, C6H5, 3,5-dimethylbenzyl, C6F5, C(O)CH3, C(O)CF3) were observed. Both RhI−N
coordination products (κ2-Tp′)Rh(PMe3)(NH2CH2CF3) and RhIII N−H addition products (κ3-Tp′)Rh(PMe3)(NHCH2CF3)
H were generated when 1 was reacted with 2,2,2-trifluoroethylamine. Coordination products dissociate ammonia and amines in
benzene much faster than oxidative addition products eliminate anilines and amides. The relative metal−nitrogen bond energies
were studied using established kinetic techniques. Analysis of the relationship between the relative M−N bond strengths and
N−H bond strengths showed a linear correlation with a slope = RM−N/N−H of 0.91 (10), indicating that the Rh−N bond
strength varies in direct proportion to the N−H bond strength.
2,6-(OPtBu2)2C6H3) was reported by Brookhart to give
INTRODUCTION
■
octahedral IrIII hydride complexes as products in 2006.11 In
2009, Turculet reported formation of stable anilido hydride
complexes of the type [(Cy-PSiP)Ir(H)(NHR)] (Cy-PSiP =
κ3-(2-Cy2PC6H4)2SiMe) by N−H oxidative addition of
ammonia and anilines.12 Under similar reaction conditions,
the analogous Rh silyl complex reacts with anilines and
ammonia to give only coordination complexes. Oro has
reported reaction of ammonia with {M(μ-OMe)(tfbb)}2 (M =
Rh, Ir; tfbb = tetrafluorobenzobarrelene) to form M-NH2 (M
= Rh, Ir) containing products.13
Activation of N−H bonds by metal complexes has attracted
increasing attention owing to its applicability to the synthesis
of various amino compounds in the pharmaceutical and
agrochemical industries.1,2 However, investigations of the
activation of N−H bonds by late transition metals are less
prevalent than the activation of C−H or O−H bonds.3,4 The
activation of the N−H bonds of amines and ammonia is
complicated by the tendency of amines to bind to vacant
coordination sites on metal complexes via their lone electron
pair, thereby offering an alternative to N−H bond cleavage.
One of the earliest examples of direct activation of N−H
bonds by a single late transition-metal center was reported by
Merola in 1990, who showed oxidative addition of the N−H
bond of pyrroles and indoles by (COD)(PMe3)3Ir+ to yield the
18-electron, octahedral IrIII hydride species.5 Almost at the
same time, Hartwig and Bergman investigated relative Ru−N
bond strengths by activation of the N−H bond of aniline to
the (PMe3)4Ru(C2H4) complex. They found the Ru−O bond
is stronger than the Ru−N in their case.6 Jones reported N−H
oxidative addition of pyrrole and other amines to both
[Cp*Rh(PMe3)] and [Ru(dmpe)2] in 1994.7,8 In 2003,
Goldman and Hartwig observed oxidative addition of the
N−H bond of aniline by (PCP)Ir (PCP = 2,6-(CH2−
PtBu2)2C6H3).9 Later, they found N−H cleavage of ammonia
to form a stable monomeric amido iridium hydride by an
electron-rich pincer complex [tBu2P(CH2)2CH(CH2)2PtBu2]-
IrI.10 The oxidative addition of anilines and benzamides by an
electron-deficient Ir pincer complex (POCOP)Ir (POCOP =
Due to the difficulty in cleaving the N−H bonds of amines
and ammonia based purely on the metal center, in 2010
Milstein introduced metal−ligand bifunctional activation of the
N−H bonds of amines and ammonia. This activation was
promoted by [(tBuPNP*)RuH(CO)] and involves aromatiza-
tion and dearomatization of the ligand. The reaction of
Milstein’s complex with ammonia and other electron-rich
amines leads to reversible activation. Reaction with electron-
poor substrates results in the formation of stable N−H
activation products.14 Following this work, Milstein and co-
workers reported a series of RhI complexes that were able to
activate amine N−H bonds via metal−ligand cooperation.15
PdII and CuI complexes with PNP ligands were reported to
cooperatively activate N−H bonds by van der Vlugt.16 And
Received: September 26, 2018
© XXXX American Chemical Society
A
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N−H activation of amides by metal−ligand cooperation was
Reactions of Aliphatic Amines. Reaction of 1 with
primary alkyl amines RNH2 (R = n-propyl, i-propyl, n-octyl)
overnight at room temperature generated the N coordination
products (κ2-Tp′)Rh(PMe3)(NH2R) (eq 2, 2b−2d) rather
reported by Mun
̃
iz in 2011.17
Our group has reported the activation of C−H, O−H, B−H,
Si−H, and C−X (X = F or Cl) bonds with [Tp′Rh-
(PMe3)].18−21 Since the bond dissociation energies of the
N−H bonds in ammonia and amines are comparable to those
of O−H bonds in water and alcohols,22 we were interested to
examine the activation of N−H bonds by [Tp′Rh(PMe3)].
However, unlike previous studies, the active fragment [Tp′Rh-
(PMe3)] was not able to cleave the N−H bonds of ammonia
and unsubstituted aliphatic amines; instead, κ2-Tp′ amine
coordination products (κ2-Tp′)Rh(PMe3)(NHR1R2) (R1 = H,
R2 = H, C3H7, C8H17, CH2Ph, CH2CF3; R1 = R2 = C2H5;
pyrrolidine) were observed. No coordination products were
observed when the active fragment [Tp′Rh(PMe3)] was
reacted with arylamines and amides; instead the oxidative
addition products Tp′Rh(PMe3)(NR1R2)H (R1 = H, R2 =
C6H5, C8H9, C6F5, NC(O)CH3, NC(O)CF3) were observed.
than the N−H addition product (κ3-Tp′)Rh(PMe3)(NHR)H
or a C−H activation product. Similar to the reaction of
ammonia, no hydride signal was observed between δ −10 and
1
−17 in the H NMR spectra. For n-propyl amine, the methyl
RESULTS AND DISCUSSION
group of the propylamine features a triplet at δ 0.52 (1JH−H = 7
Hz) and two multiplets at δ 1.75 and δ 1.98 for the methylene
hydrogens. A doublet displayed at δ −1.27 with a coupling
constant of 198.7 Hz in the 31P{1H} NMR spectrum and a
large broad resonance at δ −6.55 in the 11B{1H} NMR
spectrum are consistent with the formation of 2b. (A smaller
resonance at δ −9.40 also observed in the 11B{1H} NMR
spectrum and is consistent with the shift observed for a κ3-Tp′
complex, assigned as Tp′Rh(PMe3)H2 (3) which is generated
during the synthesis of 1 and would not react with 2b
thermally.) Leaving isolated 2b in benzene-d6, 65% of 2b was
converted to Tp′Rh(PMe3)(C6D5)D at room temperature
after 24 h.
■
The thermally labile Tp′Rh(PMe3)(CH3)H (1) complex was
prepared in situ to generate the active fragment [Tp′Rh-
(PMe3)] as previously reported.18 Thermolysis of 1 with
hydrocarbons and alcohols has been reported as a general
method for the selective activation of C−H and O−H bonds.
Here we examine its reactivity with organic substrates
containing an N−H bond.
Reaction of Ammonia with [Tp′Rh(PMe3)]. Addition of
ammonia to a solution of complex 1 in hexane at ambient
temperature resulted in the formation of a N-coordination
product (κ2-Tp′)Rh(PMe3)(NH3) (2a). No hydride signal was
1
observed between δ −10 and −17 in the H NMR spectrum,
indicating that the N−H addition product (κ3-Tp′)Rh(PMe3)-
1H NMR signals of 2c are very similar to those of 2b. The
two methyl groups of isopropylamine appear as two doublets at
δ 0.65 and δ 1.13, and the methane hydrogen appears as a
multiplet at δ 2.55. A doublet is seen at δ −1.44 with a
coupling constant of 200.0 Hz in the 31P{1H} NMR spectrum
and a broad resonance at δ −6.50 in the 11B{1H} NMR
spectrum are consistent with the presence of the κ2-Tp′
complex. Leaving 2c in C6D6 for 1 day (24 h) at room
temperature led to ∼38% conversion of 2c to Tp′Rh(PMe3)-
(C6D5)D.
(NH2)H was not formed. A doublet at δ 0.80 with 2JP−H = 7.6
1
Hz was observed in the H NMR spectrum is for the protons
of the PMe3 ligand. The phosphine-hydrogen coupling
constant is slightly smaller than it is in Tp′Rh(PMe3)(R)H,
which are from 8.5 to 10 Hz18 and Tp′Rh(PMe3)(OR)H,
which are from 10 to 11 Hz.21 The NH3 protons could not be
clearly identified, although a broad signal was observed at δ
1
2.36 (14N is quadrupolar). A doublet at δ −0.38 with JRh−P
=
193.1 Hz was observed in the 31P{1H} NMR spectrum. This
large rhodium-phosphine coupling constant is consistent with
the presence of a (κ2-Tp′)RhI(PMe3)L species.23
A
11B{1H}
Small yellow crystals of 2c were obtained by slow
evaporation of a pentane solution over the course of several
weeks. Single-crystal X-ray diffraction confirmed the structure
of 2c, in which the amine group was bound through the N
atom (Figure 1). The RhI atom adopts a square-planar
structure with a κ2-Tp′ ligand. The twist angle between the
N1−Rh1−P1 and N2−Rh1−N4 planes is 13.3(1)°. The
dissociated pyrazole ring lies above and face-to-face with the
square plane but canted at an angle of 43.86(9)° to the N1−
Rh1−P1 plane and 48.01 (8)° to the N2−Rh1−N4 plane. The
two pyrazole rings bound to the RhI atom are not coplanar but
are folded at an angle of 72.54 (6)° with respect to each other.
Reaction of 1 with the long chain aliphatic amine octylamine
also gave (κ2-Tp′)Rh(PMe3)(NH2(CH2)7CH3) (2d) as the
major product. The hydrogens from the alkyl chain are hard to
NMR spectrum showed a large broad resonance at δ −6.52, a
shift known to be characteristic of κ2-Tp′ complexes24 and is
consistent with the results of the 1H and 31P{1H} NMR
spectra. Leaving isolated 2a in C6D6 for 1 day (24 h) at room
temperature (22 °C) led to ∼70% conversion of 2a to
Tp′Rh(PMe3)(C6D5)D. This indicates that the ammonia is
dissociating from the Rh metal center and the C6D6 is
oxidatively adding to the Rh, giving the phenyl hydride which
is thermodynamically preferred.
1
identify in the H NMR spectrum due to overlap. A doublet
displayed at δ −1.27 with a coupling constant of 198.8 Hz in
the 31P{1H} NMR spectrum and a broad resonance at δ −6.35
in the 11B{1H} NMR spectrum were observed, confirming the
formation of 2d. When isolated 2d was dissolved in benzene-
B
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to the N coordination products could be increased by
decreasing the basicity of the aniline with electron-withdrawing
aryl substituents.11 To push the reaction at hand toward the
N−H activation product, we tried a similar approach, using
electron-poor trifluoromethyl substituted amines. The elec-
tron-withdrawing CF3 group lowers the basicity of the amines,
which we hypothesized would help with the N−H cleavage, as
well increase the Rh−N bond strength in the product, which is
a conclusion from earlier studies of C−H and O−H activation
that electron-withdrawing substituents affect stability.21,25
Addition of 2,2,2-trifluoroethylamine to 1 resulted in the
generation of both the N coordination product (κ2-Tp′)Rh-
(PMe3)(NH2CH2CF3) (2g) and the N−H addition product
(κ3-Tp′)Rh(PMe3)(NHCH2CF3)H (4a) in about a 1:1 ratio
1
(eq 4). In the H NMR spectrum, a doublet of doublets at δ
i
Figure 1. Thermal ellipsoid drawing of (κ2-Tp′)Rh(PMe3)-(NH2 Pr)
(2c). Ellipsoids are shown at the 50% probability level, and hydrogen
atoms (except BH and NH2) were omitted for clarity.
d6, 62% was converted to Tp′Rh(PMe3)(C6D5)D at room
temperature after 24 h.
Cleavage of the N−H bond was not observed for secondary
aliphatic amines either (eq 3). Reaction of 1 with diethylamine
−17.135 (1JRh−H = 23.0 Hz, 2JP−H = 31.0 Hz) was seen for the
hydride of 4a. The rhodium-hydride coupling constant and
phosphorus-hydride coupling constant are similar to those seen
in the alkyl and alkoxide analogs. A triplet at δ −0.29 was
assigned to the Rh−NH of 4a. A doublet at δ 1.12 with a
2
coupling constant of JP−H= 9.8 Hz was assigned to the PMe3
ligand of complex 4a. The phosphine-hydrogen coupling
constant is slightly larger than it is in 2a−g (7−8 Hz). Two
doublets were observed in the 31P{1H} NMR spectrum, one at
δ 5.31 with a rhodium-phosphine coupling constant of 139.6
Hz for 4a and one at δ −2.12 with a rhodium-phosphine
coupling constant of 192.7 Hz for 2g. Two broad resonances in
a 1:1 ratio were also observed in the 11B{1H} NMR spectrum,
at δ −6.49 for complex 2g and δ −9.49 for complex 4a. These
results were confirmed by the 19F NMR spectrum, which
showed two triplets at δ −72.80 (JH−F = 9.8 Hz) and δ −74.35
(JH−F = 9.3 Hz).
generated the N coordination product (κ2-Tp′)Rh(PMe3)-
(NHEt2) (2e). No hydride signal was observed between δ −10
1
and −17 in the H NMR spectrum, and two triplets at δ 0.81
and δ 1.65 correspond to the protons from the two methyl
groups of the diethylamine. The methylene groups are
observed as multiplet-peaks appearing from δ 1.29 to δ 1.45
and from δ 0.98 to δ 1.09. A doublet is seen at δ −5.26 with a
coupling constant of 208.0 Hz in the 31P{1H} NMR spectrum,
and a broad resonance was observed at δ −6.58 in the 11B{1H}
NMR spectrum, confirming the formation of 2e.
Clean conversion to the N−H addition product (κ3-
Tp′)Rh(PMe3)(NHCH(CF3)2)H (4b) was observed in the
reaction of the less basic amine, 1,1,1,3,3,3-hexafluoropropan-
2-amine with 1 at room temperature after standing overnight
Similarly, pyrrolidine yielded the complex (κ2-Tp′)Rh-
(PMe3)(NHC4H8) (2f). A doublet at δ −3.24 with a coupling
constant of 202.2 Hz was observed in the 31P{1H} NMR
spectrum, and a broad resonance at δ −6.37 was seen in the
11B{1H} NMR spectrum. After leaving 2f in benzene-d6 for 24
h at room temperature, 54% of 2f was converted to
Tp′Rh(PMe3)(C6D5)D.
1
(eq 5). The N coordination product was not observed. H
In contrast to hydrocarbons and alcohols which undergo C−
H or O−H oxidative addition, N-coordinated products are
preferred over N−H activated products for ammonia and
unsubstituted aliphatic amines. Although the N−H bond
strengths in amines are very similar to the O−H bond
strengths in alcohols, the covalent Rh−N bond strengths of the
N−H addition products may be too weak to make them
favorable compared to the N coordination products, leading to
the observation of only the N coordination products. An
electronic preference for d8 square planar RhI also contributes
to the stability of the N-bound isomer. In 2006, Brookhart
reported that the ratio of the N−H oxidative addition products
NMR signals of 4b are very similar to those of 4a. The hydride
resonance appeared at δ −16.496 (dd, 1JRh−H= 20.8 Hz, 2JP−H
=
30.0 Hz), and the Rh−NH signal appeared as a broad peak at δ
−3.30. The signal from the methine may overlap with signals
from the PMe3 or methyl groups on the Tp′, and was not
observed. A typical doublet at δ 7.10 (1JRh−P = 136.5 Hz) was
observed in the 31P{1H} NMR spectrum. A broad resonance at
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δ −9.39 in the 11B{1H} NMR spectrum indicates the formation
of a RhIII complex. The 19F NMR spectrum showed only one
doublet at δ −79.60 (JH−F = 6.9 Hz). All of these data are
consistent with the formulation of 4b as N−H the oxidative
addition product.
broad singlet at δ 2.413 is assigned to the two methyl groups. A
doublet at δ 4.00 (1JRh−P = 132.9 Hz) appeared in the 31P{1H}
NMR spectrum, and a broad resonance at δ −9.47 in the
11B{1H} NMR spectrum is consistent with the formation of a
RhIII complex.
Activation of Arylamines. In anilines, the lone pair of
electrons on the nitrogen delocalizes into the ring, resulting in
decreased basicity.26 N−H activation is more likely to be
observed in the reactions with anilines than the reactions with
unsubstituted aliphatic amines. Reaction of 1 with aniline gave
N−H activated product (κ3-Tp′)Rh(PMe3)(NHPh)H (4c) as
well as a series of aryl C−H activation products (eq 6). No (κ2-
Activation of Amides. Amides are very weak bases when
compared to amines, due to the electron-withdrawing nature of
the carbonyl group. The lone pair of electrons on the nitrogen
is delocalized by resonance, which may result in an easy
cleavage of the N−H bond. From previous C−H activation
studies on [Tp′Rh(PMe3)], it was determined that the
complex Tp′Rh(PMe3)(CH2C(O)CH3)H is more stable
than Tp′Rh(PMe3)(CH3)H and Tp′Rh(PMe3)(n-pentyl)H.18
Therefore, N−H activation products of amides should be more
stable than N−H activation products of unsubstituted aliphatic
amines.
Addition of acetamide to 1 yielded (κ3-Tp′)Rh(PMe3)-
(NHC(O)CH3)H (4f); no RhI complex was observed (eq 8).
Tp′)Rh(PMe3) complex was observed. Compared to aliphatic
amine products 4a and 4b, the hydride resonance for 4c was
shifted upfield to δ −15.773 with typical coupling constants of
2
1JRh−H = 22.0 Hz and JP−H = 29.0 Hz. The resonance for the
2
PMe3 ligand was located at δ 1.04 with JP−H = 10.0 Hz; it is
similar to the phosphine-hydrogen coupling constant of 4a and
4b. The resonances for the aryl hydrogens were located from δ
6.2 to 7.3. Several additional hydride species located were
observed around δ −17.05 and δ −17.55 and assigned as C−H
activation products of isomeric o-, m-, and p-Tp′Rh(PMe3)-
(C6H4NH2)H complexes. The results are similar to what was
observed when we reacted 1 with phenol.21
The 1H NMR signals of 4f are very similar to those of 4c−4e.
The hydride resonance of 4f is located at δ −15.54 (dd, 1JRh−H
2
= 18.7 Hz, JP−H = 28.9 Hz). Seven singlets were observed
from δ 1.97 to δ 2.43, six of them belonging to the methyl
groups of Tp′ ligand, and one of them belonging to the methyl
group of acetamide. The 31P{1H} NMR and 11B{1H} NMR
spectra are also consistent with the formation of a Rh(III)
complex. No evidence for activation of the methyl C−H bonds
was observed.
To avoid aryl C−H activation, 2,3,4,5,6-pentafluoroaniline
was chosen as a substrate and reacted with 1. N−H addition
product (κ3-Tp′)Rh(PMe3)(NHC6F5)H (4d) was generated
after overnight reaction at room temperature. No coordination
product was generated. A typical hydride resonance at δ
1
2
−15.367 (dd, JRh−H= 17.3 Hz, JP−H= 26.2 Hz) was observed
in the 1H NMR spectrum. The 31P{1H} NMR, 11B{1H} NMR,
and 19F NMR spectral results are consistent with formulation
of 4d as a RhIII product.
Cleavage of the N−H bond was also observed in the
reaction of 1 with 2,2,2-trifluoroacetamide (eq 8). The N−H
addition product (κ3-Tp′)Rh(PMe3)(NHC(O)CF3)H (4g)
was generated. A broad resonance at δ 4.744 was observed in
the 1H NMR spectrum and assigned to the proton of Rh−NH.
Treatment of 4g with carbon tetrachloride yielded (κ3-
Tp′)Rh(PMe3)(NHC(O)CF3)Cl (4g-Cl). The identity of
4g-Cl was confirmed by X-ray diffraction (Figure 2). As with
many Tp′Rh(L)(X)(Y) analogs, the crystal structure of 4g-Cl
displays an octahedral geometry around the metal center. The
bond distance for Rh−NH is 2.036(2) Å, longer than Rh−OH
2.007(4) Å and Rh−OCH3 1.965(3) Å in Tp′Rh(PMe3)(OH)
Br and Tp′Rh(PMe3)(OCH3)Br,21 but slightly shorter than
Rh−CH3 2.062(6) Å in Tp′Rh(PMe3)(CH3)Cl.18 It is also
∼0.1 Å shorter than the Rh(1)−N(1) distance in 2e,
consistent with sp2 hybridization at N(1) and the prediction
of more s-character in the bond to rhodium as the electron-
withdrawing groups on N become stronger, in accord with
Bent’s Rule.27
Clean conversion to the N−H addition product (κ3-
Tp′)Rh(PMe3)(NH-3,5-dimethylphenyl)H (4e) was observed
in the reaction of the 3,5-dimethylaniline with 1 at room
temperature after stirring overnight (eq 7). No aryl C−H
addition products were generated, presumably because the
methyl groups on the 3,5-dimethylaniline blocked the rhodium
X-ray crystallography was used to identify the hapticity of
the Tp′ ligand. Square-planar geometry around the metal
center was observed for (κ2-Tp′)Rh(L)(X), while octahedral
geometry around the metal center was observed for (κ3-
Tp′)Rh(L)(X)(Y) analogs.23,28−30 However, not all metal
complex from activating the aryl C−H bonds. A typical
1
hydride resonance was observed at δ −15.765 (dd, JRh−H
=
2
1
20.6 Hz, JP−H= 29.4 Hz) in the H NMR spectrum, and a
D
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Table 1. Rates of Reductive Elimination of Amines and
Amides (RNH2) from Tp′Rh(PMe3)(NHR)H in C6D6 at
a
60.0°C
Figure 2. Thermal ellipsoid drawing of (κ3-Tp′)Rh(PMe3)(NHC-
(O)CF3)Cl (4g-Cl). Ellipsoids are shown at the 50% probability level,
and hydrogen atoms (except NH and BH) have been omitted for
clarity.
a
Errors in kre(RNH2) are reported as standard deviations. Errors in
⧧
ΔGre are calculated from k as propagated errors, using σG = (RT/
kre)σk. The errors are small because G is a log function of rate.
Systematic errors are probably larger and can be estimated as 0.1
kcal mol−1 assuming 10% error in k.
complexes crystallize satisfactorily for X-ray crystallography. 1H
NMR spectroscopy is ambiguous since the nature of the Tp′
ligand around the metal center is similar. Our group has
reported that chemical shifts of the 11B NMR data can be used
to determine the hapticity of the Tp′ ligand.24 In this work, we
found the coupling constant of the phosphine-rhodium in the
31P{1H} NMR spectrum can also be used to determine the
hapticity of the Tp′ ligand. A large phosphine-rhodium
coupling constant of ∼200 Hz indicates a (κ2-Tp′)RhI(PMe3)
product, and small phosphine-rhodium coupling constant of
130−150 Hz represents a (κ3-Tp′)RhIII(PMe3) product.
Reductive Elimination of (κ3-Tp′)Rh(PMe3)(NHR)H.
Complexes 4b and 4d−g undergo first-order reductive
elimination as determined by the following the disappearance
of the hydride resonance of each compound in C6D6 at 60.0
Table 2. Kinetic Selectivity Data Determined from
Competition Experiments vs C6H6 (10 min irradiation at
10°C).
a
1
°C by H NMR spectroscopy (see SI, Tables S2−S11). The
rate constants kre(RNH2) and the activation energies for the
reductive eliminations are summarized in Table 1. The barriers
for reductive elimination of amines and amides in 4b and 4d−
g are generally higher than the barrier for methanol elimination
in Tp′Rh(PMe3)(OCH3)H by 0.5−6.3 kcal·mol−1. The barrier
for elimination of benzene from Tp′Rh(PMe3)(C6H5)H at a
range of temperatures was reported earlier.18
a
Competitive Selectivity Experiments. Photolysis of
Tp′Rh(PMe3)H2 (3) in a mixture of an amine substrate and
C6H6 allowed for determination of the relative selectivity of the
fragment [Tp′Rh(PMe3)] for N−H activation vs benzene C−
Errors in rate ratio estimated at 5% for proton NMR integration,
giving σG = (RT/ratio)σratio = 0.05RT ≈ 0.03 kcal mol−1 (see Figures
b
S43−47). A positive ΔΔG⧧ means that benzene is kinetically
favored.
1
H activation. H NMR spectroscopic analysis of the mixture
before irradiation allowed determination of the ratio of the two
substrates. To ensure that kinetic product ratios could be
determined, competition experiments were carried out with
only 10 min of photolysis at 10 °C. The product distribution
was determined on the basis of the relative areas of the
10. Table 2 shows the results of competition between 4b, 4d−
g, and benzene.
k1/k2 = (I /I2)(n2/n1)
(9)
1
1
corresponding hydride resonances by H NMR spectroscopy
⧧
ΔΔGoa = RTln(k1/k2)
(10)
in deuterated solvent (THF-d8 or C6D12). The relative
competitive rate ratios k1/k2 reported in Table 2 were
calculated using eq 9, where I1/I2 is the integrated area of
the hydride resonances and n1/n2 is the mole ratio of the two
substrates (subscript 1 refers to benzene and subscript 2
represents the other competing substrate). The differences in
Analysis of Rhodium−Nitrogen Bond Strengths.
These kinetic barriers to reductive elimination (ΔGre⧧) can
be combined with the kinetic selectivities (ΔΔGoa⧧) to allow
the calculation of the driving force for N−H activation relative
to benzene activation (Figure 3, eq 11). Nitrogen−hydrogen
bond energies (from DFT) can then be used to find the
⧧
free energies of activation ΔΔGoa can be calculated using eq
E
DOI: 10.1021/acs.inorgchem.8b02752
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Inorganic Chemistry
Article
RNH2. As in C−H activation and O−H activation, activation
of a strong N−H bond will also give a strong M−N bond in
the product. A slope of 0.91 (10) (slope = RM−N/N−H) was
observed, indicating that the Rh−N bond strength varies in
direct proportion to the N−H bond strength. This slope is
similar to the slope seen with alcohols, RM−O/O−H.
21 By putting
those two correlations together, we can see both correlations
show almost identical slopes with the line for relative Rh−N
bond strengths lying ∼5 kcal mol−1 above the line for the
relative Rh−O bond strengths. This offset indicates that Rh−N
bonds are stronger than Rh−O bonds, in general, for a specific
X−H bond strength (X = N, O). Note that the N−H bond
strengths of NH3 and MeNH2 fall in between those of the
other substrates, but no N−H activation is observed. It appears
that the higher basicity of these amines leads to a preference
for N-coordination over oxidative addition. This same
preference is seen in reaction of [Tp′Rh(PMe3)] with PMe3;
a σ-adduct with P is formed rather than C−H oxidative
addition to a methyl group.31
It is also worth pointing out that in 1986, Bryndza and
Bercaw performed a critical study of relative bond strengths of
M−N, M−O, and M−C bond strengths for a ruthenium and a
platinum complex and found a good 1:1 correlation of H−X
and LnM−X bond strengths.32 This study has stood as the
benchmark for many years in guiding organometallic chemists
as to the energetics of exchange of these types of bonds.
Bryndza and Tam have also reviewed exchange reactions of
late transition-metal alkoxides and amides.33
Figure 3. Free energy diagram for activation of N−H bonds with
fragment [Tp′Rh(PMe3)].
relative metal−nitrogen bond energies in the activation
products Tp′Rh(PMe3)(NHR)H using eqs 12 and 13. Note
here that the statistical contribution has been included in the
calculation to account for the different number of hydrogen
atoms available to activate (6 for benzene, 2 for amines and
amides), which represents an entropic correction to ΔG°.
⧧
⧧
⧧
ΔG° = ΔGRh−Ph + ΔΔGoa − ΔGRh−NHR
ΔH° = [DPh−H + DRh−NHR] − [DRHN−H + DRh−Ph
(11)
(12)
(13)
]
Equilibration Experiments. An experiment was con-
ducted to confirm that the results obtained from the above
analysis is in agreement with direct equilibration measure-
ments. Reaction of Tp′Rh(PMe3)(OEt)H with 10 equiv of
C6F5NH2 in C6D12 solvent was examined (eq 14). A slow
D
= (DRh−NHR − DRh−Ph) = ΔH° + DRHN−H − DPh−H
rel
= −ΔG° + RTln(6/#H) + DRHN−H − DPh−H
These relative Rh−N bond strengths can be plotted vs the
N−H bond strengths to give a good linear relationship for all
amines and amides which give N−H oxidative additions
products when reacted with 1 (Figure 4). The correlation
indicates that the Rh−N bond strengths could be inferred from
the corresponding N−H bond strengths in the substrate
reaction occurs at RT, and after 2 weeks we observe complete
conversion to Tp′Rh(PMe3)(NHC6F5)H (81%) plus some
Tp′Rh(PMe3)H2 (19%, from slow β-hydrogen elimination of
ethoxide). No Tp′Rh(PMe3)(OEt)H remained. The reaction
is calculated (from the data in Table 3) to have Keq > 30,000.34
Attempts to directly equilibrate substrates failed to give
reliable data. For example, reaction of 4e with perfluoroaniline
to give 4d and dimethylaniline is predicted from the data in
Table 3 to have Keq = 0.014.35 Reaction of 4e in a 10:1 neat
mixture of 3,5-dimethylaniline and perfluoroaniline only
occurred slowly over several weeks at 60 °C (the latter is a
solid, but is soluble in the former, which is a liquid). During
this time, substantial quantities of TpRh(PMe3)2 are observed
by 31P NMR spectroscopy. Reliable Keq data could not be
obtained.
A final comment to be made is that the diagram in Figure 3
is not meant to imply mechanism, but only thermodynamics.
The mechanism by which one hydrido amido complex
exchanges with benzene or another amine could be a simple
reductive elimination/oxidative addition process, or there
could be intermediate steps where binding of the amine via
Figure 4. Plot of relative M−N (red)/M−O (blue) bond strengths vs
N−H/O−H bond strengths for Tp′Rh(PMe3)(NHR/OR)H. The
solid line is fit to the amines and amides, and the dashed line is fit to
the alcohols. Experimental O−H bond strengths were used for all
substrates22 except benzyl alcohol. O−H bond in benzyl alcohol was
estimated from the calculated (B3LYP/M06-2X) bond energy, since
the experimental values are unavailable. Calculated N−H bond
strengths were used for all amines and amides (B3LYP/6-31G**).
F
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Inorganic Chemistry
Article
were dried over activated alumina and distilled prior to use.
Isopropylamine and aniline were purchased from Fisher Scientific.
1,1,1,3,3,3-Hexafluoropropan-2-amine was purchased from Apollo
Scientific. 2,2,2-Trifluoroethylamine was purchased from Matrix
Scientific. Propylamine, diethylamine, pyrrolidine, and bis-
(cyclopentadienyl)zirconium dihydride were purchased from Alfa
Aesar. Octylamine, 2,3,4,5,6-pentafluoroaniline, acetamide, and
trifluoroacetamide were purchased from Aldrich Chemical Co. 3,5-
Dimethylaniline and potassium tris(3,5-dimethylpyrazol-1-yl)-
borohydride were purchased from Tokyo Chemical Industry Co.
Ammonia was dried over calcium oxide prior to use, and n-
propylamine, isopropylamine, octylamine, diethylamine, pyrrolidine,
2,2,2-trifluoroethylamine, 1,1,1,3,3,3-hexafluoropropan-2-amine, ani-
line, and 3,5-dimethylaniline were dried over activated molecular
sieves (3 Å) prior to use. 2,3,4,5,6-Pentafluoroaniline, acetamide, and
2,2,2-trifluoroacetamide were vacuum-dried overnight on a Schlenk
line prior to use. THF-d8, benzene-d6, and cyclohexane-d12 were
purchased from Cambridge Isotopes, distilled under vacuum from
CaH2, and stored in an ampule with a Teflon valve. Preparation of
Tp′Rh(PMe3)(CH3)H and Tp′Rh(PMe3)H2 (3) has been previously
reported.18
Table 3. Kinetic and Thermodynamic Data for
Tp′Rh(PMe3)(NHR)H
†
All photolysis experiments were performed using a 200 W Hg(Xe)
arc lamp purchased from Oriel, which was fitted with a water-filled IR
filter and a 310−380 nm low pass filter. All 1H, 31P{1H}, 11B{1H}, and
19F NMR spectra were collected on an Avance 400 or Avance 500
†
a
All values in kcal/mol. N−H bond strengths were calculated with
b
density functional B3LYP level of theory with 6-31G** basis set. C−
H bond strength is from experimental data.22 Systematic errors are
probably larger and can be estimated as 0.1 kcal mol−1.
1
NMR spectrometer. All H chemical shifts are reported in ppm (δ)
relative to the chemical shift of residual solvent (benzene-d6 (δ
7.160)). 19F NMR spectra were referenced to external α,α,α-
trifluorotoluene in benzene-d6 (δ 0.0). 31P{1H} NMR spectra were
referenced to external H3PO4 (δ 0.0). 11B{1H} NMR spectra were
referenced to external BF3·OEt2 in benzene-d6 (δ 0.0). While 1H
chemical shifts are given to 3 decimal places ( 0.4 Hz), these values
can vary slightly with concentration and temperature. All kinetic plots
and least-squares error analysis were done using Microsoft Excel. A
Bruker-AXS SMART platform diffractometer equipped with an APEX
II CCD detector was used for X-ray crystal structure determination.
In Situ Preparation of Tp′Rh(PMe3)(CH3)H. Approximately 10 mg
(0.019 mmol) of 1 was synthesized in situ from Tp′Rh(PMe3)(CH3)
Cl and Cp2ZrH2 in THF. The zirconium-free form of 1 was purified
by flash chromatography with trace amounts (5−10%) of Tp′Rh-
(PMe3)(tetrahydrofuranyl)H being present. This solution was then
evaporated to dryness, and the residue used in the following reactions
with amines.
nitrogen precedes N−H oxidative cleavage, similar to the way
alkanes form sigma-C−H complexes prior to C−H oxidative
cleavage.36 Another possibility is that exchange occurs via an
amine coordination followed by deprotonation to make the
amido ligand, with subsequent proton transfer to the metal.
For the thermodynamic arguments made here, these
intermediates are kinetically irrelevant, as long as the exchange
reaction of a given substrate occurs by the same mechanism in
the different experiments. While a mechanism cannot be
proven, the internal agreement of the data presented here (to
give a linear correlation) is in agreement with the notion that
the same mechanism is followed by the substrates examined.
CONCLUSIONS
■
Thermolysis of Tp′Rh(PMe3)(CH3)H in the presence of
trifluoromethyl-substituted amines, anilines, and amides
resulted in N−H activation products of the type Tp′Rh-
(PMe3)(NHR)H (R = CH2CF3, CH(CF3)2, Ph, C6F5, 3,5-
dimethyphenyl, CH3C(O), CF3C(O)). Activation of other
unsubstituted aliphatic amines resulted in the formation of the
N coordination products (κ2-Tp′)Rh(PMe3)(NHR1R2) (R1 =
H, R2 = H, nPr, iPr, C8H17; R1 = R2 = Et; R1, R2 = pyrrolidine).
Reductive elimination of RNH2 from Tp′Rh(PMe3)(NHR)H
in C6D6 allows for the measurement of activation barriers.
Combination with kinetic competition experiments allows for
the relative M−N bond energies to be determined vs the
known C−H bond energy in benzene. The correlation of
RM−N/N−H demonstrates good linearity with a slope of 0.91
(10) for CF3 substituted amines, anilines, and amides. By
comparing relative Rh−N bond strengths and Rh−O bond
strengths, it is seen that Rh−N bonds are stronger for a given
X−H bond strength (X = N, O). Simple N-coordination is
seen with more basic alkyl amines.
Activation of Ammonia. Approximately 10 mg (0.019 mmol) 1
was dissolved in 0.5 mL of hexane and transferred to a resealable 5
mm NMR tube. One atm of NH3 was transferred into the solution
through Schlenk techniques. The reaction was complete after standing
overnight at room temperature. The volatiles were removed in vacuo,
and the resulting solids were dissolved in C6D6. The NMR spectra of
the crude products showed (κ2-Tp′)Rh(PMe3)(NH3) (2a) as major
1
2
product. For 2a, H NMR (500 MHz, C6D6): 0.805 (d, JP−H = 7.6
Hz, 9 H, PMe3), 2.088 (s, 3 H, pzCH3), 2.212 (s, 3 H, pzCH3), 2.303
(s, 6 H, 2 × pzCH3), 2.394 (s, 6 H, 2 × pzCH3), 5.600 (s, 1 H, pzH),
5.717 (s, 1 H, pzH), 5.996 (s, 1 H, pzH). 31P{1H} NMR (202 MHz,
1
C6D6): δ −0.382 (d, JRh−P = 193.1 Hz, RhPMe3). 11B{1H} NMR
(160 MHz, C6D6): δ −6.522 (br, s)
Activation of n-Propylamine. Approximately 10 mg (0.019 mmol)
of 1 was synthesized in situ, dissolved in 0.5 mL of n-propylamine, and
transferred to a resealable 5 mm NMR tube. The reaction was
complete after standing overnight at room temperature. The volatiles
were removed in vacuo, and the resulting solids were dissolved in
C6D6. The NMR spectra of the crude products showed (κ2-
n
1
Tp′)Rh(PMe3)(NH2 Pr) (2b) as major product. For 2b, H NMR
(500 MHz, C6D6): 0.517 (t, 1JH−H = 7 Hz, 3 H, CH3), 0.897 (d, 2JP−H
= 7.5 Hz, 9 H, PMe3), 1.747 (m, 2 H, CH2), 1.978 (m, 2 H, CH2),
2.119 (s, 3 H, pzCH3), 2.291 (s, 3 H, pzCH3), 2.347 (s, 3 H, pzCH3),
2.370 (s, 3 H, pzCH3), 2.383 (s, 3 H, pzCH3), 2.395 (s, 3 H, pzCH3),
5.607 (s, 1 H, pzH), 5.726 (s, 1 H, pzH), 6.034 (s, 1 H, pzH).
EXPERIMENTAL SECTION
■
General Procedures. All operations and routine manipulations
were performed under a nitrogen atmosphere, either on a high-
vacuum line using modified Schlenk techniques or in a Vacuum
Atmospheres Corp. Dri-Lab. Hexane, pentane, THF, and benzene
1
31P{1H} NMR (202 MHz, C6D6): δ −1.27 (d, JRh−P = 198.7 Hz,
RhPMe3). 11B{1H} NMR (160 MHz, C6D6): δ −6.549 (br, s).
G
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Article
1
Activation of Isopropylamine. Approximately 10 mg (0.019
mmol) of 1 was synthesized in situ, dissolved in 0.5 mL of
isopropylamine, and transferred to a resealable 5 mm NMR tube.
The reaction was complete after standing overnight at room
temperature. The volatiles were removed in vacuo, and the resulting
solids were dissolved in C6D6. The NMR spectra of the crude
(202 MHz, C6D6): δ −2.117 (d, JRh−P = 192.7 Hz). 19F NMR (376
MHz, C6D6): δ −74.347 (t, JF−H = 9.3 Hz). 11B{1H} NMR (160
3
MHz, C6D6): δ −6.486 (br, s). For 4a, 1H NMR (500 MHz, C6D6): δ
−17.135 (dd, 1JRh−H = 23.0 Hz, 2JP−H = 31.0 Hz, 1 H, RhH), − 0.294
(t, 1JH−H = 10.5 Hz, 1 H, NH), 1.116 (d, 2JP−H = 9.8 Hz, 9 H, PMe3),
1.996 (s, 3 H, pzCH3), 2.052 (s, 3 H, pzCH3), 2.097 (s, 3 H, pzCH3),
2.318 (s, 3 H, pzCH3), 2.462 (s, 3 H, pzCH3), 2.742 (s, 3H, pzCH3),
5.500 (s, 1H, pzH), 5.529 (s, 1H, pzH), 5.573 (s, 1H, pzH). 31P{1H}
i
products showed (κ2-Tp′)Rh(PMe3)(NH2 Pr) (2c) as major product.
1
3
For 2c, H NMR (400 MHz, C6D6): 0.645 (d, JH−H = 6.4 Hz, 3 H,
CH3), 0.859 (d, 2JP−H = 7.5 Hz, 9 H, PMe3), 1.134 (d, 3JH−H = 6.4 Hz,
3 H, CH3), 2.189 (s, 3 H, pzCH3), 2.329 (s, 3 H, pzCH3), 2.351 (s, 6
H, 2 × pzCH3), 2.378 (s, 3 H, pzCH3), 2.385 (s, 3 H, pzCH3), 2.545
NMR (202 MHz, C6D6): δ 5.307 (d, JRh−P = 139.6 Hz). 19F NMR
1
(376 MHz, C6D6): δ −72.804 (t, JF−H = 9.8 Hz). 11B{1H} NMR
3
(160 MHz, C6D6): δ −9.487 (br, s).
(m, 1 H, CH), 5.590 (s, 1 H, pzH), 5.703 (s, 1 H, pzH), 6.040 (s, 1
Activation of 1,1,1,3,3,3-Hexafluoropropan-2-amine. Approxi-
mately 10 mg (0.019 mmol) of 1 was synthesized in situ. It was then
dissolved in 0.05 mL (0.435 mmol) of 1,1,1,3,3,3-hexafluoropropan-2-
amine and 0.5 mL of hexane in a resealable 5 mm NMR tube. The
reaction was complete after standing overnight at room temperature.
The solvents were removed in vacuo, and the resulting white residue
was dissolved in C6D6. The NMR spectra of the crude reaction
showed (κ3-Tp′)Rh(PMe3)(NHCH(CF3)2)H (4b) as the major
product. For 4b, 1H NMR (400 MHz, C6D6): δ −16.496 (dd,
1
H, pzH). 31P{1H} NMR (162 MHz, C6D6): δ −1.436 (d, JRh−P
=
199.8 Hz, RhPMe3). 11B{1H} NMR (160 MHz, C6D6): δ −6.495 (br,
s). Twenty mg of 2c was synthesized by above method and dissolved
in 0.5 mL pentane, and the pentane slowly evaporated at −22 °C for
weeks; a small yellow crystal was obtained.
Activation of Octylamine. Approximately 10 mg (0.019 mmol) of
1 was synthesized in situ. 0.05 mL (0.30 mmol) of octylamine and 1
were then dissolved in 0.5 mL of hexane and transferred to a
resealable 5 mm NMR tube. The reaction was complete after standing
overnight at room temperature. The volatiles were removed in vacuo,
and the resulting solids were dissolved in C6D6. The NMR spectra of
the crude products showed (κ2-Tp′)Rh(PMe3)(NH2(CH2)7CH3)
(2d) as major product. For 2d, 1H NMR (500 MHz, C6D6): 0.932 (d,
2JP−H = 7.5. Hz, 9 H, PMe3), 2.131 (s, 3 H, pzCH3), 2.338 (s, 3 H,
pzCH3), 2.360 (s, 3 H, pzCH3), 2.373 (s, 3 H, pzCH3), 2.399 (s, 3 H,
pzCH3), 2.409 (s, 3 H, pzCH3), 5.612 (s, 1 H, pzH), 5.735 (s, 1 H,
pzH), 6.043 (s, 1 H, pzH). 31P{1H} NMR (202 MHz, C6D6): δ
1JRh−H = 20.8 Hz, JP−H = 30.0 Hz, 1 H, RhH), − 3.296 (br, s, 1 H,
2
NH), 1.207 (d, 2JP−H = 10.3 Hz, 9 H, PMe3), 2.043 (s, 3 H, pzCH3),
2.139 (s, 3 H, pzCH3), 2.227 (s, 3 H, pzCH3), 2.358 (s, 3 H, pzCH3),
2.653 (s, 3 H, pzCH3), 2.721 (s, 3 H, pzCH3), 5.494 (s, 1 H, pzH),
5.558 (s, 1 H, pzH), 5.810 (s, 1 H, pzH). 31P{1H} NMR (162 MHz,
toluene-d8): δ 7.100 (d, JRh−P = 136.5 Hz). 19F NMR (376 MHz,
1
toluene-d8): δ −79.598 (d, JF−H = 6.9 Hz). 11B{1H} NMR (160
3
MHz, C6D6): δ −9.390 (br, s).
Activation of Aniline. Approximately 10 mg (0.019 mmol) of 1
was synthesized in situ and was then dissolved in 0.05 mL (0.548
mmol) of aniline. 0.5 mL of hexane was added in a resealable 5 mm
NMR tube. The reaction was complete after standing overnight at
room temperature. The solvents were removed in vacuo, and the
resulting white residue was dissolved in C6D6. The NMR spectra of
the crude products showed (κ3-Tp′)Rh(PMe3)(NHPh)H (4c) and
minor species from aryl C−H activation. For 4c, 1H NMR (500 MHz,
C6D6): δ −15.772 (dd, 1JRh−H = 22.0 Hz, 2JP−H = 29.0 Hz, 1 H, RhH),
1.036 (d, 2JP−H = 10.0 Hz, 9 H, PMe3), 2.080 (s, 3 H, pzCH3), 2.087
(s, 3 H, pzCH3), 2.202 (s, 3 H, pzCH3), 2.307 (s, 3 H, pzCH3), 2.341
(s, 3 H, pzCH3), 2.442 (s, 3 H, pzCH3), 5.445 (s, 1 H, pzH), 5.552 (s,
1 H, pzH), 5.783 (s, 1 H, pzH), 6.352 (d, 3JH−H= 8.5 Hz, 2 H, arylH),
6.552 (t, 3JH−H = 7.0 Hz, 2 H, arylH), 6.718 (d, 3JH−H = 7.5 Hz, 1 H,
1
−1.266 (d, JRh−P = 198.8 Hz, RhPMe3). 11B{1H} NMR (160 MHz,
C6D6): δ −6.346 (br, s).
Activation of Diethylamine. Approximately 10 mg (0.019 mmol)
of 1 was synthesized in situ, dissolved in 0.5 mL of diethylamine, and
transferred to a resealable 5 mm NMR tube. The reaction was
complete after standing overnight at room temperature. The volatiles
were removed in vacuo, and the resulting solids were dissolved in
C6D6. The NMR spectra of the crude products showed (κ2-
1
Tp′)Rh(PMe3)(NHEt2) (2e) as major product. For 2e, H NMR
3
(500 MHz, C6D12): 0.809 (t, JH−H = 7.0 Hz, 3 H, CH3), 0.914 (d,
3
2JP−H = 7.5 Hz, 9 H, PMe3), 1.650 (t, JH−H = 7.0 Hz, 3 H, CH3),
1.871 (s, 3 H, pzCH3), 2.085 (s, 3 H, pzCH3), 2.252 (s, 3 H, pzCH3),
2.289 (s, 3 H, pzCH3), 2.309 (s, 3 H, pzCH3), 2.363 (s, 3 H, pzCH3),
5.522 (s, 1 H, pzH), 5.617 (s, 1 H, pzH), 5.655 (s, 1 H, pzH).
arylH). 31P{1H} NMR (202 MHz, C6D6): δ 4.555 (d, JRh−P = 132.8
1
1
31P{1H} NMR (202 MHz, C6D12): δ 5.265 (d, JRh−P = 208.0 Hz,
Hz).
RhPMe3). 11B{1H} NMR (160 MHz, C6D6): δ −6.580 (br, s).
Activation of Pyrrolidine. Approximately 10 mg (0.019 mmol) of 1
was synthesized in situ, dissolved in 0.5 mL of pyrrolidine, and
transferred to a resealable 5 mm NMR tube. The reaction was
complete after standing overnight at room temperature. The volatiles
were removed in vacuo, and the resulting solids were dissolved in
C6D6. The NMR spectra of the crude products showed (κ2-
Activation of 2,3,4,5,6-Pentafluoroaniline. Approximately 10 mg
(0.019 mmol) of 1 was synthesized in situ and was then dissolved in
0.5 mL of hexane followed by addition of 4 mg (0.022 mmol) of
2,3,4,5,6-pentafluoroaniline. The solution was transferred to a
resealable 5 mm NMR tube. After the reaction was complete
(overnight at room temperature), the volatiles were removed in vacuo,
and the resulting yellow solids were dissolved in C6D6. The NMR
spectra of the crude products showed a major product of (κ3-
Tp′)Rh(PMe3)(NHC6F5)H (4d). For 4d, 1H NMR (500 MHz,
C6D6): δ −15.367 (dd, 1JRh−H = 17.5 Hz, 2JP−H = 26.1 Hz, 1 H, RhH),
1
Tp′)Rh(PMe3)(NHC4H8) (2f) as major product. For 2f, H NMR
(500 MHz, C6D6): 0.943 (d, 2JP−H = 7.5 Hz, 9 H, PMe3), 2.094 (s, 3
H, pzCH3), 2.231 (s, 3 H, pzCH3), 2.375 (s, 3 H, pzCH3), 2.403 (s, 3
H, pzCH3), 2.423 (s, 6 H, 2 × pzCH3), 5.611 (s, 1 H, pzH), 5.672 (s,
1 H, pzH), 6.071 (s, 1 H, pzH). 31P{1H} NMR (202 MHz, C6D6): δ
2
0.955 (d, JP−H = 10.1 Hz, 9 H, PMe3), 2.034 (s, 6 H, 2 × pzCH3),
2.151 (s, 3 H, pzCH3), 2.243 (s, 3 H, pzCH3), 2.287 (s, 3 H, pzCH3),
2.295 (s, 3 H, pzCH3), 5.415 (s, 1 H, pzH), 5.481 (s, 1 H, pzH),
5.726 (s, 1 H, pzH). 31P{1H} NMR (202 MHz, C6D6): δ 5.957 (d,
1JRh−P = 130.0 Hz). 19F NMR (376 MHz, C6D6): δ −188.94 (m, 1 F,
Hpara), − 167.25 (br s, 1 F, Hortho), − 166.50 (br s, 2 F, Hmeta), −
162.90 (br s, 1 F, Hortho′) (the broad resonances of meta-F and ortho-F
may due to hindered rotation of the aryl ring; the two ortho-F appear
as two separate broad peaks; the two meta-F appear as a broad singlet
at −167.25; the para-F remains sharp, as its environment does not
change with aryl rotation. See Figure S31). 11B{1H} NMR (160 MHz,
C6D6): δ −9.493 (br, s).
1
−3.242 (d, JRh−P = 202.2 Hz, RhPMe3). 11B{1H} NMR (160 MHz,
C6D6): δ −6.373 (br, s).
Activation of 2,2,2-Trifluoroethylamine. Approximately 10 mg
(0.019 mmol) of 1 was synthesized in situ, dissolved in 0.5 mL of
2,2,2-trifluoroethylamine, and transferred to a resealable 5 mm NMR
tube. The reaction was complete after standing overnight at room
temperature. The volatiles were removed in vacuo, and the resulting
solids were dissolved in C6D6. The NMR spectra of the crude
products showed a mixture of (κ2-Tp′)Rh(PMe3)(NH2CH2CF3)
(2g) and (κ3-Tp′)Rh(PMe3)(NHCH2CF3)H (4a) in about 1:1 ratio.
For 2g, 1H NMR (500 MHz, C6D6): δ 0.775 (d, 2JP−H = 7.5 Hz, 9 H,
PMe3), 2.184 (s, 6 H, 2 × pzCH3), 2.307 (s, 3 H, pzCH3), 2.318 (s,
3H, pzC H3), 2.337 (s, 3 H, pzCH3), 2.349 (s, 3 H, pzCH3), 5.662 (s,
1 H, pzH), 5.809 (s, 1 H, pzH), 5.985 (s, 1 H, pzH). 31P{1H} NMR
Activation of 3,5-Dimethylaniline. Approximately 10 mg (0.019
mmol) of 1 was synthesized in situ and then dissolved in 0.5 mL of
hexane followed by addition of 0.05 mL (0.401 mmol) of 3,5-
dimethylaniline. The solution was transferred to a resealable 5 mm
H
DOI: 10.1021/acs.inorgchem.8b02752
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
NMR tube. After the reaction was complete (overnight at room
temperature), pale yellow precipitates were generated, washed with
hexane (3 × 0.5 mL), and dissolved in C6D6. The NMR spectra of the
crude products showed a clean formation of (κ3-Tp′)Rh(PMe3)(NH-
dissolved in 0.5 mL of cyclohexane-d12 in a resealable 5 mm NMR
tube, then 49 mg (0.268 mmol) of 2,3,4,5,6-pentafluoroaniline was
added into the NMR tube. The reaction was monitored by H NMR
1
and completed after mixing at room temperature for 2 weeks.
Attempted Equilibration with Perfluoroaniline and 3,5-Dime-
thylaniline. Approximately 5 mg (0.010 mmol) of 4e was synthesized
from Tp′Rh(PMe3)(CH3)H. 4e was then dissolved in ∼0.5 mL of
aniline mixture [3,5-dimethylaniline (484 mg, 3.99 mmol) and
2,3,4,5,6-pentafluoroaniline (65.6 mg, 0.358 mmol)]. This sample was
placed into a J-Young NMR tube and heated at 60 °C. Complex 4d
was generated in the reaction, and the equilibration was monitored by
31P{1H} NMR spectroscopy. After 1 week, 4d was observed as well as
1
3,5-dimethylphenyl)H (4e). For 4e, H NMR (400 MHz, C6D6): δ
−15.760 (dd, 1JRh−H = 20.6 Hz, 2JP−H = 29.4 Hz, 1 H, RhH), 1.101 (d,
2JP−H = 10.0 Hz, 9 H, PMe3), 2.079 (s, 3 H, pzCH3), 2.113 (s, 3 H,
pzCH3), 2.203 (s, 3 H, pzCH3), 2.337 (s, 3 H, pzCH3), 2.344 (s, 3 H,
pzCH3), 2.413 (s, 6 H, 2 × arylCH3) 2.481 (s, 3 H, pzCH3), 5.459 (s,
1 H, pzH), 5.563 (s, 1 H, pzH), 5.791 (s, 1 H, pzH), 6.200 (s, 1 H,
1
arylH). 31P{1H} NMR (162 MHz, C6D6): δ 4.004 (d, JRh−P = 132.9
Hz). 11B{1H} NMR (160 MHz, C6D6): δ −9.474 (br, s).
Tp′Rh(PMe3)2. Additional heating led to more bis-phosphine
Activation of Acetamide. Approximately 10 mg (0.019 mmol) of 1
was synthesized in situ as described above. In this instance,
Tp′Rh(PMe3)(Cl)H (∼20%) was also present. It was then dissolved
in 0.5 mL of hexane followed by addition of 3 mg (0.051 mmol) of
acetamide. The solution was stirred in a round bottomed flask for 4
days at room temperature. After the reaction was complete, it was
transferred the solution into a resealable 5 mm NMR tube. The
volatiles were removed in vacuo, and the resulting solids were
dissolved in C6D6. The NMR spectra of the crude products showed a
major product of (κ3-Tp′)Rh(PMe3)(NHC(O)CH3)H (4f). For 4f,
1H NMR (500 MHz, C6D6): δ −15.538 (dd, 1JRh−H = 18.7 Hz, 2JP−H
complex.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02752.
Kinetic treatments for reductive elimination reactions,
competition experiments, NMR spectra, and details of
X-ray analysis (PDF)
2
= 28.9 Hz, 1 H, RhH), 1.383 (d, JP−H = 10.5 Hz, 9 H, PMe3),
1.972(s, 3 H, CH3), 2.106(s, 3 H, pzCH3), 2.109 (s, 3 H, pzCH3),
2.177 (s, 3 H, pzCH3), 2.299 (s, 3 H, pzCH3), 2.333 (s, 3 H, pzCH3),
2.428 (2, 3 H, pzCH3), 5.506 (s, 1 H, pzH), 5.524 (s, 1 H, pzH),
5.823 (s, 1 H, pzH). 31P{1H} NMR (162 MHz, C6D6): δ 2.996 (d,
1JRh−P = 134.3 Hz). 11B{1H} NMR (160 MHz, C6D6): δ −9.528 (br,
s).
Accession Codes
CCDC 1869111−1869112 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Activation of 2,2,2-Trifluoroacetamide. Approximately 10 mg
(0.019 mmol) of 1 was synthesized in situ and was then dissolved in
0.5 mL of hexane followed by addition of 3 mg (0.027 mmol) of
2,2,2-trifluoroacetamide. The solution was transferred to a resealable
5 mm NMR tube. After the reaction was complete (overnight at room
temperature), white precipitates were generated, washed by hexane (3
× 0.5 mL), and dissolved in C6D6. The NMR spectra of the crude
products showed a clean formation of (κ3-Tp′)Rh(PMe3)(NHC(O)-
AUTHOR INFORMATION
Corresponding Author
*E-mail: jones@chem.rochester.edu.
■
1
ORCID
CF3)H (4g). For 4g, H NMR (500 MHz, C6D6): δ −15.224 (dd,
1JRh−H = 17.5 Hz, 2JP−H = 27.9 Hz, 1 H, RhH), 1.140 (d, 2JP−H = 10.6
William W. Brennessel: 0000-0001-5461-1825
William D. Jones: 0000-0003-1932-0963
Notes
Hz, 9 H, PMe3), 2.041 (s, 3 H, pzCH3), 2.045 (s, 3 H, pzCH3), 2.141
(s, 3 H, pzCH3), 2.196 (s, 3 H, pzCH3), 2.294 (s, 3 H, pzCH3), 2.355
(s, 3 H, pzCH3), 4.744 (br, 1 H, NH), 5.404 (s, 1 H, pzH), 5.466 (s, 1
H, pzH), 5.723 (s, 1 H, pzH). 31P{1H} NMR (202 MHz, C6D6): δ
3.259 (d, 1JRh−P = 127.6 Hz). 19F NMR (376 MHz, C6D6): δ −71.828
(s, 3F, CF3). 11B{1H} NMR (160 MHz, C6D6): δ −9.576 (br, s).
Preparation of (κ3-Tp′)Rh(PMe3)(NHC(O)CF3)Cl (4g-Cl). 4g (∼40
mg) was prepared from 1 (∼40 mg, 0.076 mmol) in 0.5 mL of hexane
from the method above. This was followed by dissolving the sample in
0.5 mL of carbon tetrachloride in a resealable 5 mm NMR tube. The
mixture reacted at room temperature for 7 days, and then after leaving
the NMR tube standing for another 7 days, a yellow crystal was
formed.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Acknowledgment is made to the U.S. Department of Energy
Office of Science, Basic Energy Sciences grant FG02-
86ER13569 for their support of this work.
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