Direct Characterization of a Reactive Lattice-Confined Ru2 Nitride by Photocrystallography

Article abstract of DOI:10.1021/jacs.6b13357

Reactive metal-ligand (M-L) multiply bonded complexes are ubiquitous intermediates in redox catalysis and have thus been long-standing targets of synthetic chemistry. The intrinsic reactivity of mid-to-late M-L multiply bonded complexes renders these structures challenging to isolate and structurally characterize. Although synthetic tuning of the ancillary ligand field can stabilize M-L multiply bonded complexes and result in isolable complexes, these efforts inevitably attenuate the reactivity of the M-L multiple bond. Here, we report the first direct characterization of a reactive Ru₂ nitride intermediate by photocrystallography. Photogeneration of reactive M-L multiple bonds within crystalline matrices supports direct characterization of these critical intermediates without synthetic derivatization.

Full text of DOI:10.1021/jacs.6b13357

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Communication
Direct Characterization of a Reactive Lattice-
2
Confined Ru Nitride by Photocrystallography
Anuvab Das, Joseph H Reibenspies, Yu-Sheng Chen, and David C Powers
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b13357 • Publication Date (Web): 14 Feb 2017
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Direct Characterization of a Reactive Lattice-Confined Ru2 Nitride by
Photocrystallography
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Anuvab Das,† Joseph H. Reibenspies,† Yu-Sheng Chen,§ David C. Powers†,*
† Department of Chemistry, Texas A&M University, College Station, TX 77843, United States
§ ChemMatCARS, the University of Chicago, Argonne, IL 60439, United States
Supporting Information Placeholder
tions that allow isolation of these structures inevitably
attenuate their reactivity.3 We are interested in directly
ABSTRACT: Reactive metal–ligand (M–L) multiply bond-
ed complexes are ubiquitous intermediates in redox
characterizing reactive M–L multiple bonds without syn-
catalysis and have thus been long-standing targets of
thetic derivatization. Here, we report direct crystallo-
synthetic chemistry. The intrinsic reactivity of mid-to-
graphic characterization of a reactive Ru2 nitride inter-
late M–L multiply bonded complexes renders these
mediate photogenerated within a crystalline matrix.
structures challenging to isolate and structurally charac-
Reactive metal nitride complexes are potential inter-
terize. While synthetic tuning of the ancillary ligand field
mediates in C–H amination and olefin aziridination reac-
can stabilize M–L multiply bonded complexes and result
tions.4 Similar to other reactive M–L multiply bonded
in isolable complexes, these efforts inevitably attenuate
complexes, metal nitrides that are sufficiently reactive
the reactivity of the M–L multiple bond. Here, we report
to functionalize C–H bonds are challenging to structural-
the first direct characterization of a reactive Ru2 nitride
ly characterize because facile oxidative decomposition
intermediate by photocrystallography. Photogeneration
pathways preclude access to the requisite crystalline
of reactive M–L multiple bonds within crystalline matri-
samples.5 The challenges inherent in characterization of
ces supports direct characterization of these critical in-
reactive metal nitrides are illustrated by the chemistry
termediates without synthetic derivatization.
Metal–oxygen and metal–nitrogen multiply bonded
complexes are critical intermediates in both synthetic
and biological oxidation catalysis as well as O2 and N2
reduction schemes.1 As such, complexes featuring met-
al–ligand (M–L) multiple bonds are longstanding targets
for chemical synthesis. High-valent early transition met-
als support strong M–L multiple bonds via efficient
overlap of filled ligand-based π-symmetry orbitals with
vacant metal-based π* orbitals. In contrast, M–L multi-
ple bonds of mid-to-late transition metals, which fea-
ture higher d-electron counts and thus destabilizing
filled π-π* interactions, tend to be reactive.2 The rela-
tive reactivity of mid-to-late M–L multiply bonded com-
plexes renders these structures attractive intermediates
for catalysis but challenging species to isolate and struc-
turally characterize. Synthetic tuning of the ancillary
ligand field and metal coordination geometry has been
Figure 1. Reactive metal nitrides are challenging to struc-
turally characterize because both intra- (i.e. via amination
of pendant C–H bonds (2 to 3; N–N: N,N’-bis(3,5-
dichlorophenyl)-formamidinate)) or N-atom insertion into
pursued extensively to stabilize higher d-electron count
M–L multiple bonds,1a,2 however, the design considera-
ancillary M–L bonds (5 to 6; R: mesityl)) and intermolecular
(i.e. nitride dimerization (8 to 9; R: t-Bu )) decomposition
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Figure 2. Photocrystallographic characterization of reactive Ru2 nitride 2. Photolysis (λ = 365 nm) of a single crystal of Ru2
azide 1 at 95 K results first in a phase transition (P21/c to C2/c) to generate 1linear, which is promoted by photo-induced crys-
tal heating, and subsequently in extrusion of N2 to generate Ru2 nitride complex 2. Thermal ellipsoids are drawn at 50%
probability. H- and Cl-atoms are omitted for clarity. Metrics: 1, Ru1–Ru2, 2.3445(8) Å; Ru1–N1, 2.047(6) Å; Ru1–N1–N2,
152.9(5)°; 1linear, Ru1–Ru2, 2.373(2) Å; Ru1–N1, 2.01(1) Å; Ru1–N1–N2, 165.5(1)°; 2, Ru1–Ru2, 2.408(3) Å; Ru1–N1, 1.72(2)
Å.
of Ru2 nitride 2, originally reported by Berry and co-
workers (Fig. 1a).6 Photolysis of Ru2 azide complex 17 at
low temperature has been proposed to generate Ru2
nitride 2,6 which upon warming functionalizes a proxi-
mal ligand-based C–H bond to generate Ru2 amide 3.8
Other nitride decomposition pathways, such as inser-
tion into ancillary M–L bonds9 (i.e. conversion of Co(IV)
nitride 5 to Co(II) imine complex 6,9a Fig. 1b) and bimo-
lecular nitride coupling reactions10 to generate N2 (i.e.
conversion of Rh(IV) nitride 8 to N2-bridged Rh2 complex
9,10a Fig. 1c) are common decomposition pathways of
reactive metal nitrides that can preclude crystallization.
In the absence of crystallographic data, IR, EPR, elec-
tronic absorption spectroscopies and EXAFS spectral
fitting have been used, in combination with high-level
calculations, to assign structures for highly reactive M–L
multiply bonded intermediates.11
We considered that direct structural characterization
of highly reactive M–L multiply bonded intermediates
could be achieved if the targeted structures could be
generated within a crystalline matrix. For example, if a
stable molecular precursor of the intermediate of inter-
est could be converted to the reactive species without
disrupting the ordered crystal lattice, X-ray diffraction
techniques could be utilized to establish the structures
of reactive intermediates. Such an experiment is akin to
classical matrix isolation experiments. For example, iso-
lation of FeN,12 RuN, and OsN13 in both Ar and N2 matri-
ces has allowed characterization of these reactive frag-
ments by vibrational spectroscopy, but traditional ma-
trix isolation experiments do not allow the structural
characterization available in crystalline samples. Cop-
pens and Ohashi have pioneered photocrystallography,
an approach for in situ crystallographic characterization
of photochemically generated structures.14 Photocrys-
tallography has been applied to the characterization of
organic radicals,15 carbenes,16 unsaturated metal com-
plexes,17 metastable linkage isomers,18 and molecular
excited states.19 Photocrystallographic experiments are
challenging to apply to irreversible photoreactions and
have not previously been applied to the characteriza-
tion of reactive intermediates in C–H functionalization.
We targeted characterization of Ru2 nitride 2 because
1) in comparison to isolable mononuclear Ru2 nitride
complexes,20 it has been proposed to exhibit an anoma-
lously long Ru–N bond due to the presence of M–M
interactions,6 2) it is reactive towards C–H bonds at low
temperature,8 and 3) it has been characterized by
EXAFS,6 which allows benchmarking of our photocrystal-
lographic results. Ru2 azide 1 crystallizes in a P21/c space
group and the solid-state absorption spectrum of 1 is
well-matched to the solution phase spectrum (Fig. S1).
A goniometer-mounted crystal of 1 was irradiated with
a 365 nm LED source and reaction progress was moni-
tored in real-time by X-ray diffraction at the Advanced
Photon Source (APS) housed at Argonne National La-
boratory (ANL) (Fig. 2). Irradiation of 1 leads initially to a
phase transition from a P21/c to a C2/c space group. The
observed phase transition is accompanied by the linear-
ization of the azide ligand binding mode: the Ru1–N1–
N2 bond angle expands from 152.9(5)° to 165.5(1)°. The
photo-induced phase transition is not reversible if the
light source is removed. Continued irradiation leads to
evolution of nitride complex 2, as evidenced by both
the contraction of the Ru1–N1 vector (2.047(6) Å to
1.72(2) Å) and the observation of a new electron densi-
ty peak above the Ru2 nitride. The electron density peak
integrates as 12 e– and was satisfactorily modeled as a
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Figure 3. The X-band EPR spectra 2 generated in the solid
state (powdered crystals, —) and a frozen CH2Cl2 glass (—)
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Figure 4. Temperature dependence of the Ru1–N1–N2
angle in Ru2 azide 1. Linearization of this angle upon pho-
tolysis at 15 K is consistent with photochemical heating of
the sample.
demonstrate that nitride 2 generated in the solid state is
spectroscopically indistinguishable from that generated in
a frozen glass.
molecule of N2 (85% occupancy, see Figure S2 for re-
considered two possibilities to account for the observed
linearization of the azide ligand during photolysis. First,
local photo-induced sample heating is common during
condensed-phase photoreactions,21 and thus the ob-
served phase transition could be thermally promoted.
Second, we considered that the phase transition may be
due to evolution of a thermally trapped molecular ex-
cited state.22
To explore the origin of the observed phase transi-
tion, X-ray diffraction data for Ru2 azide 1 were collect-
ed between 15–250 K; metrical parameters and unit cell
data as a function of temperature are collected in Tables
S2 and S3. The same P21/c to C2/c phase transition ob-
served at early irradiation times is observed between
150 K and 200 K in the VT crystallography experiment.
In addition, the Ru(1)–N(1)–N(2) bond angle is highly
temperature dependent, displaying a 150.9(4)° bond
angle at 15 K and 168.8(6)° bond angle at 250 K (Fig. 4).
In contrast, the Ru–Ru distance, which is sensitive to
changes in molecular spin state,23 is invariant with tem-
perature.
The size of thermal ellipsoids of atoms that are not in-
volved in the primary photoreaction can be used as an
in situ probe of the effective temperature of crystalline
samples.21 Comparison with the thermal ellipsoids of a
crystal photolyzed at 15 K with the thermal ellipsoid
obtained from VT crystallography reveals that photo-
induced heating renders the effective sample tempera-
ture closer to 200 K, which is in good agreement with
the measured linearization of the azide ligand (data col-
lected in Fig. S4). Together, these data suggest that the
phase transition that precedes nitride evolution is driv-
en by photo-induced crystal heating. Similar analysis
indicates that the effective sample temperature during
95 K irradiation is approximately 350 K (Fig. S5; see
Supporting Information for additional discussion of
sample temperature).
finement details). The N2 molecule is located in a bowl-
shaped void space that is defined by the aromatic sub-
stituents on the formamidinate ligands.
The metrical parameters of 2 determined by photo-
crystallography (Ru1–Ru2: 2.408(3) Å; Ru1–N1: 1.72(2)
Å) are in good agreement with values extracted from
EXAFS spectral fitting (Ru1–Ru2: 2.42 Å; Ru1–N1: 1.76
Å).6 In addition to bond length information, photocrys-
tallographic structure determination provides direct
measurement of bond angles, which are challenging to
obtain by other experimental methods. Of note is the
linearity of the Ru–Ru–N unit. Comparison of the struc-
ture of 2 with a refinement in a triclinic unit cell con-
firms that the metrical parameters are not symmetry
enforced (Table S1).
Ru2 nitride 2 generated by photolysis of a solid-state
powdered-crystalline sample of 1 is spectroscopically
identical to 2 generated by photolysis of a frozen CH2Cl2
glass of 1. As shown in Figure 3, photolysis of 1, as ei-
ther a powder or as a frozen glass, results in the evolu-
tion of an axial EPR signal characteristic of the δ*
ground state of nitride 2.6 Powder X-ray diffraction
(PXRD) analysis of the solid-state sample confirmed that
the powder used in the EPR experiment exhibits the
same crystal phase as the sample utilized for photocrys-
tallographic analysis (Fig. S3).
As described above, photolysis of Ru2 azide 1 at 95 K
leads first to a P21/c to C2/c phase transition that is
characterized by linearization of the Ru1–N1–N2 bond
angle (1linear), and subsequently to the evolution of ni-
tride 2. An identical phase transition is observed when
photolysis is carried out at 15 K, but at this temperature
we do not observe substantial formation of nitride 2,
even after extended photolysis. Because substantial
conversion of 1 to 2 was not observed when photolysis
was carried out at 15 K, examination of the structural
changes associated with the observed phase transition
was pursued with this lower-temperature data set. We
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We thank Greg Wylie for help with EPR measurements,
We further confirmed that the phase transition ob-
served crystallographically is not due to a change in
electronic structure by photolyzing both solution-phase
and powdered samples of 1 in the cavity of an EPR spec-
trometer maintained at 15 K. No signals were observed
other than those attributable to azide 1 and a minor
amount of 2 (Fig. S6). After photolysis at 15 K, the sam-
ple was warmed to 95 K in the dark and no evolution of
the EPR spectrum was observed. Taken with the VT X-
ray diffraction data, these experiments suggest that
photo-induced crystal heating is responsible for the lin-
earization of the azide ligand prior to evolution of ni-
tride 2.
Attempts to observe conversion of lattice-confined ni-
tride 2 to amido complex 3 were unsuccessful. Con-
sistent with experimentally determined activation pa-
rameters,8 C–H insertion does not proceed at 95 K. Slow
warming of crystalline samples of 2 to 200 K led to sam-
ple cracking, manifest as diminished diffraction intensity
and quality, which precluded observation of C–H func-
tionalization. We speculate that the observed sample
cracking may be due to diffusion of lattice-bound N2 at
higher temperature.
The results reported here demonstrate photocrystal-
lography to be a viable approach to directly interrogat-
ing the structures of highly reactive M–L multiply bond-
ed intermediates relevant to C–H amination. The suc-
cess of these experiments, which do not require syn-
thetic stabilization of the reactive fragment of interest,
are likely due to a combination of low-temperature pho-
togeneration of the targeted reactive intermediates and
lattice confinement of those structures, which restricts
motions of reactive intermediates. Experimental defini-
tion of metrical parameters is critical to rigorous corre-
lation of structure, reactivity, and electronic structure.
We anticipate that photocrystallography will contribute
to these efforts and may find application as a tool for
the direct structure elucidation of additional reactive
intermediates.
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Marcetta Darensbourg for an EPR finger dewar, François
Gabbaï for a Xe lamp, as well as Texas A&M University and
the Welch Foundation (A-1907) for financial support.
ChemMatCARS Sector 15 is principally supported by the
NSF under grant number NSF/CHE-1346572. Use of the
APS was supported by the U.S. DOE under Contract No.
DE-AC02-06CH11357.
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ASSOCIATED CONTENT
Supporting Information. Experimental procedures and
spectroscopic data, VT X-ray diffraction data (PDF, cif).
Crystallographic data deposited in Cambridge Crystal
Structure Database (1523480-1523482). The Supporting
Information is available free of charge on the ACS Publica-
tions website.
AUTHOR INFORMATION
Corresponding Author
*david.powers@chem.tamu.edu
Funding Sources
No competing financial interests have been declared.
ACKNOWLEDGMENT
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Reactive metal nitride complexes are challenging to structurally characterize because both intra- (i.e. via
amination of pendant C–H bonds (2 to 3; N–N: N,N’-bis(3,5-dichlorophenyl)formamidinate)) or N-atom
insertion into ancillary M–L bonds (5 to 6; R: mesityl)) and intermolecular (i.e. nitride dimerization (8 to 9;
R: t-Bu )) decomposition pathways can be facile.
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Photocrystallographic characterization of reactive Ru2 nitride 2. Photolysis (λ = 365 nm) of a single crystal of
Ru2 azide 1 at 95 K results first in a phase transition (P21/c to C2/c) to generate 1linear, which is promoted
by photoꢀinduced crystal heating, and subsequently in extrusion of N2 to generate Ru2 nitride complex 2.
Thermal ellipsoids are drawn at 50% probability. Hꢀ and Clꢀatoms are omitted for clarity. Metrics: 1, Ru1–
Ru2, 2.3445(8) Å; Ru1–N1, 2.047(6) Å; Ru1–N1–N2, 152.9(5)°; 1linear, Ru1–Ru2, 2.373(2) Å; Ru1–N1,
2.01(1) Å; Ru1–N1–N2, 165.5(1)°; 2, Ru1–Ru2, 2.408(3) Å; Ru1–N1, 1.72(2) Å.
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The X-band EPR spectra 2 generated in the solid state (powdered crystals, —) and a frozen CH2Cl2 glass (—
) demonstrate that nitride 2 generated in the solid state is spectroscopically indistinguishable from that
generated in a frozen glass.
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(a) Temperature dependence of the Ru1–N1–N2 angle in Ru2 azide 1. Linearization of this angle upon
photolysis at 15 K is consistent with photochemical heating of the sample. (b) Principle mean square atomic
displacements (X ( ), Y ( ), and Z ( )) obtained by VT crystallography. Atomic displacements obtained during
irradiation (gray box) provide an in situ probe of crystal temperature during photolysis.
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