In Crystallo Snapshots of Rh2-Catalyzed C-H Amination

Article abstract of DOI:10.1021/jacs.0c09842

While X-ray crystallography routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, elucidation of the structures of transient reactive intermediates, which are intimately engaged in bond-breaking and -making

Full text of DOI:10.1021/jacs.0c09842

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In Crystallo Snapshots of Rh2‑Catalyzed C−H Amination
Anuvab Das, Chen-Hao Wang, Gerard P. Van Trieste III, Cheng-Jun Sun, Yu-Sheng Chen,
Joseph H. Reibenspies, and David C. Powers*
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ABSTRACT: While X-ray crystallography routinely provides structural characterization of kinetically stable pre-catalysts and
intermediates, elucidation of the structures of transient reactive intermediates, which are intimately engaged in bond-breaking and
-making during catalysis, is generally not possible. Here, we demonstrate in crystallo synthesis of Rh2 nitrenoids that participate in
catalytic C−H amination, and we characterize these transient intermediates as triplet adducts of Rh2. Further, we observe the impact
of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh2 nitrenoids. By providing
structural characterization of authentic C−H functionalization intermediates, and not kinetically stabilized model complexes, these
experiments provide the opportunity to define critical structure−activity relationships.
-ray and electron diffraction-based experiments are
Xroutinely employed to elucidate the chemical structures
that are required for the rational design and optimization of
small-molecule catalysts.1,2 While crystallography is routinely
utilized to examine ligand-induced structural variation in
kinetically stable coordination complexes and pre-catalysts,
application to the characterization of reactive intermediates,
which are intimately involved in the bond-making and
-breaking in catalysis, is typically stymied by the kinetic lability
of these species.3−7 As a result, experimental evaluation of the
impact of ligand structure and coordinating additives present
during catalysis is typically not possible.
Nitrene transfer catalysis represents a leading approach for
the construction of C−N bonds via C−H amination and olefin
aziridination reactions (Figure 1a).8−25 Despite the ubiquity of
Rh2 nitrenoid intermediates in nitrene transfer catalysis, the
structures of Rh2 nitrenoids that participate in C−H amination
have eluded experimental scrutiny. We have been interested in
the characterization of transient intermediates involved in C−
H functionalization reactions enabled by in crystallo syn-
thesis.26 This approach represents a crystalline analogue of
classic matrix isolation experiments.27−32 In 2019, we
demonstrated that in crystallo N2 extrusion from a Rh2
adamantyl azide complex enabled structural characterization
Figure 1. (a) Rh2-catalyzed intramolecular nitrene transfer via
transient Rh2 nitrenoid intermediates. (b) In situ generation of a
of a transient triplet nitrenoid adduct of Rh2; however,
rearrangement of the nitrene fragment prevented observation
of C−H amination in this system (Figure 1b).33 We
hypothesized that generation of a Rh2 nitrenoid bearing a
proximal C−H bond would enable characterization of transient
nitrene intermediates that were competent intermediates for
intramolecular C−H amination (Figure 1c). Here, we report
the characterization of a pair of transient Rh2 nitrenoids that
participate in solid-state C−H amination. These structural
snapshots of a C−H amination reaction provide an
opportunity to directly evaluate the impact of coordinating
ligands on the structure of reactive intermediates in C−H
functionalization.
Rh2 adamantyl nitrenoid enables crystallographic characterization;
rapid rearrangement prevents C−H functionalization. (c) Character-
ization of transient Rh2 nitrenoids enables observation of the impact
of coordinating ligands on reactive amination intermediates.
Received: September 19, 2020
https://dx.doi.org/10.1021/jacs.0c09842
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We initiated our studies by examining the coordination
chemistry of Rh2(esp)2 (1) with o-biphenyl azide (2) (esp =
α,α,α′,α′-tetramethyl-1,3-benzenedipropionate). These sub-
strates were selected because Rh2(esp)2 (1) is among the
most widely used catalysts for nitrene transfer chemistry, and
o-biphenyl azides have been shown to participate in intra-
molecular amination.24 An equimolar mixture of 1 and 2 in
CDCl3 displays 1H NMR spectral features consistent with a 1:1
adduct (i.e., 3a, Figure 2a, Figures S1 and S2). IR analysis of 3a
in a KBr pellet displays azide stretching modes at 2129 and
2101 cm−1 (for comparison, 2125 and 2089 cm−1 in 2) (Figure
S3). Single crystals of 3a were obtained by slow evaporation of
a CH2Cl2 solution of a 1:1 mixture of 1 and 2 at −35 °C
(Figure 2b). The metrical parameters of the Rh2 fragment and
the biphenyl azide moiety in 3a are similar to those of
Rh2(esp)2 (Rh−Rh: 1 = 2.3817(9) Å; 3a = 2.3850(4) Å) and
2, respectively.20,34
Complex 3a participates in solid-state, intramolecular C−H
amination. Either photolysis (335 < λ < 610 nm) or
thermolysis (60 °C) of a KBr pellet of complex 3a results in
the formation of carbazole complex 4a (Figure 2b).35 Solid-
state conversion was evidenced by the disappearance of the
stretching frequencies at 2129 and 2101 cm−1 and the
appearance of peaks at 726, 575, and 508 cm−1 (Figures S6
and S7).36 The final IR spectrum overlays with that of an
authentic sample of 4a prepared by treatment of Rh2(esp)2
with carbazole (Figures S11 and S12). The formation of 4a
1
was further confirmed by H NMR following extraction of
photolyzed or thermolyzed KBr pellets with CDCl3 (Figures
S13 and S14).
Intramolecular C−H amination within the coordination
sphere of 3a to generate 4a presumably proceeds via Rh2
nitrenoid 5a (Figure 3a). Evidence of facile N2 elimination
from 3a was obtained from flux-dependent matrix-assisted laser
desorption/ionization mass spectrometry (MALDI-MS) meas-
Figure 2. (a) Synthesis and reaction chemistry of Rh2 complex 3a. (b)
Displacement ellipsoid plots of 3a and 4a (50% probability). H-atoms
and solvent are omitted. Selected metrics: for 3a, Rh(1)−N(1) =
2.244(3) Å, N(1)−C(1) = 1.441(5) Å, Rh(1)−Rh(2) = 2.3850(4) Å,
Rh(1)−N(1)−C(1) = 125.0(2)°; for 4a, Rh(1)−N(1) = 2.341(3) Å,
Rh(1)−Rh(2) = 2.3835(3) Å.
+
urements (Figure 3b). A signal corresponding to Rh2(esp)2
(m/z = 758.3599, calcd 758.0828) was observed at low laser
power; as the flux of the ablation laser was increased, a new
signal corresponding to the nitrenoid fragment
Rh2(esp)2(C12H10N)+ (m/z = 924.5538, calcd 925.1563)
emerged.
Figure 3. (a) Conversion of Rh2 azide complex 3a to carbazole adduct 4a proceeds via transient Rh2 nitrenoid 5a. (b) Flux-dependent MALDI-MS
shows an ion corresponding to Rh2(esp)2(C12H10N)+; ablation laser flux increased from (i) to (iii). (iv) Simulated isotopic distribution for
Rh2(esp)2(C12H10N)+. (c) Left: Displacement ellipsoid plot of 5a (50% probability). H-atoms, N2, and solvent are omitted. Right: Rotated ellipsoid
plot of 5a (50% probability). H-atoms, esp ligand, and solvent are omitted. Selected metrics: Rh(1)−N(1) = 2.055(4) Å, N(1)−C(1) = 1.335(7)
Å, Rh(1)−Rh(1) = 2.3953(4) Å, Rh(1)−N(1)−C(1) = 140.6(4)°. (d) Rh K-edge EXAFS data (spectral fitting range 1.0−3.0 Å, blue trace) of
Rh2(esp)2(C12H9N) (5a, experimental, black trace, and fit data, red trace).
B
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Photolysis (λ = 365 nm) of a single crystal of 3a under
cryogenic conditions (100 K) promoted in crystallo synthesis
of Rh2 nitrenoid 5a·N2 (Figure 3c). Reaction progress was
monitored by periodic collection of X-ray crystal structures
(synchrotron radiation, λ = 0.41328 Å). Refinement of the
resulting data indicated that the extrusion of N2 generates Rh2
nitrenoid 5a·N2 with 90% chemical conversion (Figure 3c).37
Elimination of N2 from 3a is accompanied by the contraction
of Rh(1)−N(1) from 2.244(3) Å (3a) to 2.055(4) Å (5a·N2).
Concurrently, contraction of N(1)−C(1) from 1.441(5) Å to
1.335(7) Å and expansion of the Rh(1)−N(1)−C(1) bond
angle from 125.0(2)° to 140.6(4)° are observed.38 The
Rh(1)−Rh(2) bond distance remained unchanged (3a:
2.3850(4) Å, 5a·N2: 2.3953(4) Å). The evolved N2 was
refined at 90% occupancy, which is consistent with the single-
crystal-to-single-crystal chemical conversion of the azide
fragment in 3a to the nitrene fragment in 5a. In crystallo
conversion of 3a to 5a·N2 could also be accomplished more
slowly by sustained exposure to synchrotron radiation (λ =
0.41328 Å) without photolysis. This observation is similar to
X-ray-stimulated N2 extrusion reactions previously observed in
both Rh2-33 and Co-azide39 complexes, and other X-ray
stimulated reactions.2 We were not able to observe conversion
of 5a·N2 to 4a via a second single-crystal-to-single-crystal
transformation; slow warming of a crystal of 5a·N2 to promote
C−H amination resulted in sample amorphization.
To further probe the structure of 5a, Rh K-edge extended X-
ray absorption fine structure (EXAFS) analysis was pursued.
Data obtained for nitrenoid 5a (generated by sustained (2 h)
exposure of a boron nitride pellet of 3a to synchrotron
radiation (λ = 0.41328 Å)) was fit with four and half Rh−N/O
interactions (2.03(1) Å) and one Rh−Rh interaction
(2.401(6) Å) (Figure S16 and Table S6).40,41 The first-shell
Rh−N/O scatters were not distinguishable due to the limited
resolution (0.22 Å = π/(2Δk)). These metrical parameters are
consistent with those obtained from in crystallo synthesis of 5a·
N2. Comparison of the X-ray absorption near-edge structure
(XANES) data of Rh2(esp)2, 3a, and 5a suggests that all
complexes feature a Rh2[II,II] core (Figure S17).
The experimental metrical parameters of 5a are in excellent
agreement with density functional theory (DFT) optimization
of the triplet electronic configuration (i.e., 3[5a]) carried out at
the wB97XD42/SDD (Rh)43−45 and 6-31G(d) (light
atoms)46,47 levels of theory.48 Consistent with the assigned
electronic configuration, ΔEtriplet‑singlet = −12.2 kcal·mol−1.
Comparison of various computational strategies that have been
used in the past to examine Rh2-catalyzed reactions revealed
that while the optimized structures of 1 and 3a were well
captured by several methods, the optimized geometry of
reactive intermediate 5a displayed significant variation (Tables
S7−S9).
During catalysis, substrate 2 is present in significant excess
with respect to Rh2 catalyst 1.24 Slow evaporation of a CHCl3
solution of 1 that contained excess 2 afforded single crystals of
3b, in which each Rh center is coordinated by an azide ligand
(Figure 4). The metrical parameters of complexes 3a and 3b
are similar except the Rh(1)−N(1) bond distance in 3b
(2.280(4) Å) is longer than the corresponding distance in 3a
(2.244(3) Å), which is consistent with a significant structural
trans influence of the coordinated azide ligand.
Figure 4. (a) Synthesis and photochemistry of Rh2 bis-azide 3b. (b)
Displacement ellipsoid plots of 3b and 5b (50% probability). H-atoms
and solvent are omitted. Selected metrics: for 3b, Rh(1)−N(1) =
2.280(4) Å, N(1)−C(1) = 1.436(6) Å, Rh(1)−Rh(1) = 2.3872(6) Å,
Rh(1)−N(1)−C(1) = 125.1(3)°; for 5b, Rh(1)−N(1) = 2.10(4) Å,
N(1)−C(1) = 1.36(4) Å, Rh(1)−Rh(1) = 2.4061(8) Å, Rh(1)−
N(1)−C(1) = 127(2)°.
spectra obtained during either photolysis or thermolysis of a
KBr pellet of 3b revealed the disappearance of signals at 2129
and 2101 cm−1 and the evolution of new peaks at 726, 575,
1
and 508 cm−1 (Figures S18 and S19). H NMR analysis
following extraction of either the photolyzed or thermolyzed
KBr pellet with CDCl3 revealed a 2:1 ratio of carbazole to
Rh2(esp)2 (Figures S13 and S20), which indicates that both
azide ligands undergo cyclization to carbazole under these
conditions.
In crystallo expulsion of N2 from 3b enabled characterization
of Rh2 nitrenoid 5b·N2, which differs from 5a·N2 by the
presence of a coordinated o-biphenylazide ligand on the distal
Rh center (Figure 4b). In crystallo reaction progress was
monitored by periodic collection of X-ray crystal structures.
Refinement of the resulting data indicated that the extrusion of
N2 generated Rh2 nitrenoid 5b·N2 in 43% chemical conversion
(Figure 4).49 The Rh centers in both 3b and 5b·N2 are
symmetry equivalent, and thus loss of N2 results in positional
disorder of the unreacted biphenyl azide and newly generated
biphenyl nitrene moieties (i.e., each Rh center modeled as 50%
C12H10N3 and C12H10N occupancy). Extrusion of N2 from 3b
resulted in the contraction of Rh(1)−N(1) from 2.280(4) Å
(3b) to 2.10(4) Å (5b·N2). Concurrent with the expulsion of
N2 and the contraction of the Rh(1)−N(1) bond, N(1)−C(1)
contracted from 1.436(6) Å to 1.36(4) Å and the Rh(1)−
N(1)−C(1) bond angle expanded from 125.1(3)° to
127(2)°.38 The Rh(1)−Rh(1) bond distance remained
essentially unchanged (i.e., 2.3872(6) Å (3b); 2.4061(8) Å
(5b·N2)). We were unable to locate the evolved N2 in the
structure of 5b due to disorder with a lattice CHCl3.
Crystallinity was compromised at higher chemical conversions,
which prevented higher precision data from being obtained for
5b.
Similar to 3a, bis-azide complex 3b participates in
photochemically (335 < λ < 610 nm) and thermally (60 °C)
promoted solid-state, intramolecular C−H amination. IR
DFT geometry optimization of 3[5b] is well-matched to the
experimental data and ΔEtriplet‑singlet = −16.8 kcal·mol−1. The
C
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Communication
only significant deviation between experiment and theory is the
bond angle, which is measured to be 127(2)° and computed to
be 148.4° (consistent across computational methods examined,
Tables S10−S12). We hypothesize that the observed deviation
arises from crystal packing restrictions on in crystallo structural
reorganization, which prevent full relaxation of the Rh−N−C
angle in 5b. Consistent with this hypothesis, the energy to
bend the nitrene linkage on the triplet surface is <2 kcal·mol−1
(Table S13).
graphic Data Center under CCDC 2025847−2025851)
(TXT)
AUTHOR INFORMATION
Corresponding Author
■
David C. Powers − Department of Chemistry, Texas A&M
University, College Station, Texas 77843, United States;
orcid.org/0000-0003-3717-2001; Email: powers@
chem.tamu.edu
Comparison of the experimental structures of 5a·N2 and 5b·
N2 reveals the impact of coordinated axial ligands on the
reactive intermediate responsible for intramolecular C−H
amination. In the absence of a trans axial ligand to the nitrene
in 5a, the Rh(1)−N(1) bond is 2.055(4) Å; in the presence of
a trans axial azide ligand to the nitrene in 5b, the Rh(1)−N(1)
bond distance is 2.10(4) Å. The long Rh−N linkage in 5b
suggests a significant structural trans effect through the M−M
bond (Table S14).50−54 In addition to elongating the Rh−
nitrenoid bond, the presence of an axial ligand increases the
spin density on the nitrene nitrogen atom: natural bond order
Authors
Anuvab Das − Department of Chemistry, Texas A&M
University, College Station, Texas 77843, United States;
orcid.org/0000-0002-9344-4414
Chen-Hao Wang − Department of Chemistry, Texas A&M
University, College Station, Texas 77843, United States;
orcid.org/0000-0003-3416-4452
Gerard P. Van Trieste III − Department of Chemistry, Texas
A&M University, College Station, Texas 77843, United
States
Cheng-Jun Sun − Advanced Photon Source, Argonne National
Laboratory, Argonne, Illinois 60439, United States
Yu-Sheng Chen − ChemMatCARS, University of Chicago,
Argonne, Illinois 60439, United States
3
3
(NBO) analysis of [5a] and [5b] indicates the N-centered
spin density increases upon azide binding (5a, 1.16 e−; 5b,
1.48 e−) (Figure 5, Tables S15 and S16).
Joseph H. Reibenspies − Department of Chemistry, Texas
A&M University, College Station, Texas 77843, United
States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c09842
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Yohannes H. Rezenom for assisting with
the MALDI experiment and Debmalya Ray for helpful
discussion. X-ray diffraction data of 3a, 3b, 5a·N2, and 5b·N2
were collected at ChemMatCARS Sector 15 housed at
Advanced Photon Source (APS) at Argonne National
Laboratory (ANL), which is supported by the National
Science Foundation (NSF), Division of Chemistry (CHE),
and Materials Research (DMR) (NSF/CHE-1346572). Use of
the PILATUS X CdTe 1M detector is supported by NSF
(NSF/DMR-1531283). XANES/EXAFS data of 1, 3a, and 5a
were collected at Beamline 20-ID housed at APS. This research
used resources of the Advanced Photon Source, an Office of
Science User Facility operated for the U.S. Department of
Energy (DOE) Office of Science by Argonne National
Laboratory, and was supported by the U.S. DOE under
Contract No. DE-AC02-06CH11357, and the Canadian Light
Source and its funding partners. The authors acknowledge the
U.S. DOE, Office of Science, Office of Basic Energy Sciences,
Catalysis Program (DE-SC0018977), the Welch Foundation
(A-1907), and an Alfred P. Sloan Fellowship to D.C.P. for
financial support.
Figure 5. Calculated spin density plots for Rh2 nitrenoid complexes
3
3[5a] and [5b]; isolevel = 0.004.
In summary, we describe the structural characterization of a
pair of Rh2 nitrenes that mediate intramolecular C−H
amination chemistry. In crystallo confinement enables direct
characterization of the species that are intimately involved in
bond-breaking and -making in catalysis. Importantly, by having
access to experimentally derived metrical parameters, we were
able to evaluate the fidelity of various commonly employed
computational approaches to the description of Rh2-catalyzed
processes, which provides critical but previously unavailable
insight into the fidelity of these models to experiment. These
observations demonstrate in situ crystallography to be a
powerful tool for characterizing authentic C−H activation
intermediates.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c09842.
■
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Communication
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