Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H2

Article abstract of DOI:10.1021/jacs.0c00291

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)_x][(^ketguan)Co(N₃)₂] (^ketguan = [(tBu₂CN)C(NDipp)₂]^-, Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride Na(THF)₄{[(^ketguan)Co(N₃)]₂(μ-N)} (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride complex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory supports a Co^III═N═Co^III canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike other group 9 bridging nitride complexes, no radical character is detected at the bridging N atom of 1. Indeed, 1 is unreactive toward weak C-H donors and even cocrystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(^ketguan)Co(N₃)(py)]₂ (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co(II)/Co(III) bridged imido species [(^ketguan)Co(py)][(^ketguan)Co](μ-NH)(μ-N₃) (5) formed from intermolecular hydrogen atom abstraction (HAA) of strong C-H bonds (BDE ～100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C₆H₁₂ or C₆D₁₂ gives a KIE = 2.5 ± 0.1, supportive of a HAA formation pathway. The reactivity of our system was further probed by photolyzing benzene/pyridine solutions of 4a under H₂ and D₂ atmospheres (150 psi), which leads to the exclusive formation of the bis(imido) complexes [(^ketguan)Co(μ-NH)]₂ (6) and [(^ketguan)Co(μ-ND)]₂ (6-D), respectively, as a result of dihydrogen activation. These results provide unique insights into the chemistry and electronic structure of late 3d metal nitrides while providing entryway into C-H activation pathways.

Full text of DOI:10.1021/jacs.0c00291

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Article
Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its
Hydrogen Atom Abstraction Chemistry and Reactivity Towards H2
Debabrata Sengupta, Christian Sandoval-Pauker, Emily C Schueller, Angela M. Encerrado-
Manriquez, Alejandro J Metta-Magana, Wen-Yee Lee, Ram Seshadri, Balázs Pintér, and Skye Fortier
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00291 • Publication Date (Web): 11 Apr 2020
Downloaded from pubs.acs.org on April 13, 2020
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Isolation of a Bimetallic Cobalt(III) Nitride and Examination of
Its Hydrogen Atom Abstraction Chemistry and Reactivity To-
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wards H
2
†
‡
#
Debabrata Sengupta, Christian Sandoval-Pauker, Emily Schueller, Angela M. Encerrado-Man-
†
†
†
#,Δ
,‡
riquez, Alejandro Metta-Magaña, Wen-Yee Lee, Ram Seshadri, Balazs Pinter,* and Skye
,†
Fortier*
†Department of Chemistry and Biochemistry, University of Texas at El Paso, El Paso, Texas 79968, United States
‡Department of Chemistry, Universidad Técnica Federico Santa María, Valparaíso, 2390123, Chile
#Materials Department and Materials Research Laboratory, University of California, Santa Barbara, California 93106,
United States
ΔDepartment of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
ket
ket
ABSTRACT: Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)
x
][( guan)Co(N
] , Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride
)] (μ-N)} (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride com-
3
)
2
] ( guan =
t
–
2
[
( Bu
2
CN)C(NDipp)
ket
Na(THF)
4
{[( guan)Co(N
3
2
plex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory
III
III
supports a Co =N=Co canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike
other Group 9 bridging nitride complexes, no radical character is detected at the bridging N-atom of 1. Indeed, 1 is unreactive
towards weak C-H donors and even co-crystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell
ket-
to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(
guan)Co(N
bridged imido species [( guan)Co(py)][( guan)Co](μ-NH)(μ-N
tion (HAA) of strong C-H bonds (BDE ~ 100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C
12 gives a KIE = 2.5±0.1, supportive of a HAA formation pathway. The reactivity of our system was further probed by
3
)(py)] (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co(II)/(III)
2
ket
ket
3
) (5) formed from intermolecular hydrogen atom abstrac-
6
H12 or
6
C D
photolyzing benzene/pyridine solutions of 4a under H
2
and D atmospheres (150 psi), which leads to the exclusive formation
2
ket
ket
of the bis(imido) complexes [( guan)Co(μ-NH)]
2
(6) and [( guan)Co(μ-ND)] (6-D), respectively, as a result of dihydrogen
2
activation. These results provide unique insights into the chemistry and electronic structure of late 3d-metal nitrides while
providing entryway into C-H activation pathways.
INTRODUCTION
ligand multiple bonds, a concept that has been coined the
5
, 9-10
Complexes containing metal-ligand multiple bonds, both
biological and synthetic, have been intensively investigated
owing to the participation of these moieties in a host of im-
“oxo wall.”
Yet, the oxo wall can be circumvented
through lowering the coordination number and symmetry,
7
, 11-12
employing sterically encumbering ligands,
and reduc-
1
ing the d-electron count through metal oxidation.6, 8, 13
portant reactions including C-H bond activation, olefin me-
2
3
These strategies have been effective for a handful of Rh/Ir
tathesis, cycloaddition, and heteroatom transfer reactiv-
4
compounds such as the iridium(V)oxo Ir(O)(mes) (mes =
ity. Indeed, the Fe=O intermediate formed in the active site
3
of cytochrome P450 is critical for its enzymatic versatility.5
mesityl) synthesized by Wilkinson,14 and nitride species
t
Of note, isolable compounds containing M=E or M≡E bonds
have been dominated by the early to mid-transition metals,
while the synthesis and isolation of late metal complexes
presents a more significant challenge due to the progressive
such as (PNP)Ir(N) (PNP = N(CHCHP Bu
2
)
2
) reported by the
8
15-19
groups of de Bruin and Schneider, amongst others.
However, this can lead to non-trivial canonical forms, which
complicates electronic structure interpretations and formal
8
, 20
population of M=E/M≡E π*-orbitals, leading to high reac-
oxidation state assignments.
tivity and instability.6-8
Extension of these synthetic strategies to cobalt has af-
1
3
In specific regard to Group 9 – 11 metals in tetragonal lig-
and fields, it is considered the case that d-orbital occupation
with electron counts of n ≥ 4 is incompatible with metal-
forded a number of imido-complexes and recently led to
the isolation and structural characterization of the first ter-
t
minal cobalt(III)-oxo species [PhB( BuIm)
idazol-2-ylidene).
3
]Co(O) (Im = im-
21
Notably, though, terminal cobalt-
1
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Scheme 1. Synthetic overview and HAA reactivity with alkanes and H .
2
nitrides still remain elusive. In 2010, Chirik et al. reported
products formed as the result of passing Co-N-Co formation.
Building upon this approach of utilizing bimetallic systems
to stabilize Group 9 metal-ligand multiple bonds, we have
targeted the synthesis of isolable homobimetallic bridging
Co=N=Co complexes through steric and geometric control
to gain access and insight into elusive cobalt-nitride bonds.
As part of our ongoing effort to utilize bulky guanidinate
ligands to stabilize 3d-metal complexes with rare and reac-
iPr
iPr
that thermolysis or photolysis of ( PDI)CoN
3
( PDI = 2,6-
i
(
2,6- Pr
2
-C
6
H
3
-N═CMe)
2
C
5
H N) leads to intramolecular N-
3
atom insertion, attributed to the formation of a fleeting ter-
minal cobalt nitride.22 Later, Meyer and co-workers were
able to trap an intermediate nitride complex through the
use of the bis(N-heterocyclic carbene)-mono(phenolate)
chelated compound (BIMPNMes,Ad,Me)Co(N) at 10 K, which
upon warming undergoes N-atom insertion into a Co-C
2
7-28
tive bonding motifs,
tion and the reactivity studies of a mid-valent, four-coordi-
herein, we demonstrate the isola-
2
3
bond. Consequently, controlled reactivity studies of the
cobalt-nitride moiety remain unknown.
nate, bimetallic Co(III)-nitride complex, namely
ket
ket
It has come to light in recent years that the lifetime of
Co=O complexes can be extended through the coordination
of redox-inactive Lewis acids to the oxo moiety.24-25 These
compounds are still short-lived, but under this cooperative
model, the second metal can alleviate charge density that
would otherwise occupy antibonding orbitals, thereby ex-
tending lifetimes. Notably, Tomson and co-workers have
capitalized upon this strategy utilizing macrocyclic pyridyl-
diimine ligands to generate the putative dinuclear cobalt ni-
[Na(THF)
( Bu
4
][( guan)Co(μ-N)(N
3
)]
2
(1)
( guan
=
t
2
CN)C(NDipp)
2
, Dipp = 2,6-diisopropylphenyl). While 1
is stable as a solid and in solution, chemical activation of the
nitride moiety can be achieved by perturbation of the ligand
field through a coordination switch attained by simple ad-
dition of pyridine, resulting in intermolecular hydrogen
atom abstraction (HAA) chemistry. Impressively, 1 can per-
form HAA on strong, unactivated C-H bonds (BDE ~ 100
kcal/mol), generating the parent-imido, mixed-valent com-
n
3+ 26
ket
ket
tride [( PDI
2
)Co
2
(μ‐N)(PMe
3
)
2
] . This nitride is not ob-
plex [( guan)Co(py)][( guan)Co](μ-NH)(μ-N
3
) (5). More-
bond
served but its formation is indicated through the isolation
over, 1 is reactive towards dihydrogen, cleaving the H
2
of phosphinimide and intramolecular C-H amination
2
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a.
b.
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c.
d.
Figure 1. ORTEP representation of compounds (a) 1·CHD (b) 3b·THF (c) 5·C
ellipsoids. The [Na(THF)
for clarity. Asterisks denote symmetry generated atoms.
6
H
14 and (d) 6·2C
6
H
14·2C
5
H12 with 30% probability
+
4
] cation of 1, the hydrogen atoms (exception N-H of 5 and 6), and co-crystallized solvents are omitted
(
BDE ~ 105 kcal/mol) to give the parent bis(imido) [(ket-
guan)Co(μ-NH)]
The combined stability of 1 and its solvent-triggered ni-
tride activation provides a unique platform and opportunity
to study the chemistry of cobalt-nitrides and multiply
bonded late 3d-metal complexes.
low temperature region of the μeff vs T and χT vs T plots for
3b with a maximum at 7 K (9.32 μ
dicative of ferromagnetic coupling (Figure S29).
Compound 2 is isolated as a dark blue material, whereas
3a and 3b are green solids, all air-sensitive but stable indef-
initely under dinitrogen atmosphere. Complex 3a is primar-
3
-1
2
(6).
B
; 10.84 cm ·K·mol ) in-
3
2-35
ily stable to NaN loss in THF solution, but typically gives
3
RESULTS AND DISCUSSION
mixtures of 3a and 3b upon attempted isolation in the solid
state. Regardless, the presence of 3a in solution is easily de-
tected as it is spectroscopically distinct from 3b as indicated
by NMR and UV-vis spectroscopies.
Single crystals of 3b can be grown from a saturated
THF/hexanes (1:1) solution stored at -30 °C for one day. X-
ray diffraction analysis of 3b confirms the formation of a
four-coordinate, diamond core cobalt center (Figure 1b).
ket
Synthesis and Characterization. Addition of K[ guan]
ket-
to CoBr
2
(DME) in THF affords monomeric
(
guan)Co(Br)(THF) (2) which is readily converted to the co-
baltate bis(azide) complex [Na(THF)
3a) upon treatment with excess NaN
In the absence of any structural data for 3a, it is ostensibly
formulated as a monomeric species. Upon dissolution in ar-
omatic or non-polar solvents, the NaN
the neutral dimer [( guan)Co(N
clearly observed through H NMR spectroscopy, 2 – 3 are
paramagnetic complexes (Figures S9 – S11). While pristine
samples of 3a are complicated by the facile loss of NaN ,
SQUID magnetometry measurements on solid-state sam-
ples of 3b reveal a room temperature effective magnetic
moment of 7.3 μ
than expected for the spin-only value for two non-interact-
ing high-spin Co(II) centers (5.48 μ ) but not beyond the
range typically encountered for systems with high magnetic
ket
x
][( guan)Co(N
in THF (Scheme 1).
3
)
2
]
(
3
The molecule crystallizes in the P2 /c space group and con-
1
3
is rapidly lost to give
(3b) (Scheme 1). As
ket
tains one-half of the dimer in the asymmetric unit which
generates the full molecule through inversion symmetry.
The bond metrics of 3b (Co1-N1 = 1.994(2) Å, Co1-N2 =
1.988(2) Å, Co1-N4 = 1.997(2) Å) are comparable to those
3
)]
2
1
3
found in the related dinuclear cobalt complex
i
{[( Pr
2
N)C(NDipp)
2
]Co(μ-N
3
)} (Co-N = 1.991(3) – 1.994(3)
2
Å, Co-Nazide = 2.010(3) Å).36 Inspection of the azide units in
B
(Figure S29). This value is slightly higher
3b reveals inequivalent N-N distances (N4-N5 = 1.227(3) Å,
N5-N6 = 1.135(4) Å) consistent with azide activation,
feature also present in {[( Pr
Furthermore, treatment of 3a with a C H /py (10:1) so-
lution produces
3
7
a
B
i
2
N)C(NDipp)
2
]Co(μ-N
3
2
)} .
2
9-30
anisotropies.
Interestingly, bridging μ-1,1-N
3
azide
6
6
ket-
units are known to facilitate ferromagnetic interactions in
binuclear nickel systems,31 and a sharp rise is visible in the
a
mixture of the dimer [(
(4a) and the mononuclear
]} (4b) (Figures S5 and
guan)Co(N )(py)]
3
2
ket
{
[Na(THF)
2
][( guan)Co(py)(N
3
)
2
n
3
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S6), with both complexes featuring five-coordinate cobalts. radical as discerned through electronic structure calcula-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
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2
3
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3
3
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3
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3
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5
5
5
5
5
5
5
6
In the latter case, the sodium cations bridge the azide
groups to give a 1D-coordination polymer. Similar solubil-
ity properties complicate separation; however, 4a is selec-
tively obtained from the addition of pyridine to benzene so-
lutions of 3b, which occurs instantaneously (Scheme 1). Of
further note, the solid-state molecular structure of
tions. However, high electron delocalization obfuscates
definitive oxidation state assignments, and the rhodium di-
41
1
7
mer may be more appropriately presented as {Rh N} uti-
2
lizing Enemark-Feltham notation, taking into consideration
the number of nitride N-atom based π-symmetry electrons.
1
6
Extension of this notion to 1 would provide a {Co N} core,
2
4
a·THF·C
5
H
12 (Figure S5) shows azide rearrangement,
where we assert the geometry and lower electron count
aids in avoiding population of Co-N π*-orbitals (vide infra).
The room-temperature effective magnetic moment of 1 is
adopting end-on coordination modes, with the cobalt cen-
ters in distorted square pyramidal geometries. Parkin and
co-workers have found that endo oriented azides, such as
3.60 μ
a Co(III)/Co(III) system containing two non-interacting, S =
2 ions (6.9 μ ). The lower than expected μeff can be readily
B
(Figure S28), substantially lower than predicted for
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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0
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3
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5
6
7
8
9
0
3
8
those in 4a·THF·C
5
H
12, are more sensitive to N loss, sug-
2
gesting the end-on arrangement may be a key step in the ni-
tride formation of 1. Yet, poor resolution of the diffraction
data only permits a qualitative structural analysis of
B
explained by electron pairing through the nitride bridge
(see electronic structure analysis). Moreover, the χT vs T
3
-1
4
a·THF·C
5
H
12, preventing assessment of azide N-N dis-
plot slopes from 1.57 cm ·K·mol at 300 K to 0.08
3
-1
tances and their degree of activation.
cm ·K·mol at 2 K (or μeff vs T from 3.60 μ
B
to 0.82 μ , re-
B
Based upon the azide activation observed for 3b, at-
spectively) (Figure S28), indicating increased antiferromag-
netic coupling to a singlet ground state at low temperature.
Interestingly, a small peak with a local maxima value of 0.23
tempts were undertaken to initiate N loss to generate co-
2
balt nitrides. Photolyzing C
6
D
6
solutions of 3b leads to de-
ket
3
-1
composition and formation of protonated guanH as the
only identifiable product. On the other hand, room temper-
ature photolysis (365 nm) of THF solutions of 3a gradually
gives way to the formation of a new paramagnetic species.
The product is sparingly soluble and deposits as a crystal-
line solid from the photoreaction mixture, even when per-
formed in dilute solutions (ca. 5 mmol). The X-ray deter-
mined solid-state molecular structure revealed the for-
mation of the dinuclear cobalt nitride 1·THF (Figure S2) but
poor X-ray data precludes accurate parameter evaluation.
Surprisingly, higher quality single crystals suitable for X-ray
diffraction analysis can be obtained through photolysis of
3a in THF solutions containing excess 1,4-cyclohexadiene
cm ·K·mol appears in the χT vs T plot at 13 K (Figure S28).
The reproducibility of this feature in both zero-field cooled
(ZFC) and field cooled (FC) magnetization measurements
indicates that this is a magnetic transition associated with
the onset of ferromagnetic coupling, not unlike that ob-
served for 3b. Indeed, mixed antiferromagnetic and ferro-
magnetic coupling has been noted for other multimetallic
42-46
3d-metal systems.
The groups of Holland and Betley have demonstrated
that geometric perturbation of the three-coordinate Fe(III)
Me
Me
and Co(III) imidos L Fe(NAd) (L = 2,4-bis(DippN)pent-3-
dipy
dipy
yl)
Ph
tetrahedral (L)M(NR)(py) leads to ligand-accelerated C-H
and ( L)Co(NR) (
L = 5-mesityl-1,9-(2,4,6-
3
C
6
H
2
)dipyrrin), respectively, by pyridine ligation to give
(
CHD) to give 1·CHD (Scheme 1)(Figures 1a and S1). The
electronic absorption profiles of 1·THF and 1·CHD are iden-
tical by UV-vis spectroscopy (Figure S32).
It should be noted that in the solid-state, 1·CHD forms a
4
7-48
bond amination chemistry.
wards C-H bonds in the presence of pyridine with the
(salen)ruthenium(VI)–nitride complex [(salen)Ru-
(N)(MeOH)]PF has also been reported. In these cases, re-
Enhanced reactivity to-
4
9
1
D-coordination polymer formed through bridging sodium-
6
to-azide contacts. Addition of 18-crown-6 to suspensions of
activity promotion is ascribed in part to coordination de-
pendent, spin-state changes that lead to the population of
M-N π*-orbitals upon pyridine binding.
1
·CHD or 1·THF fail to solubilize the material.
To the best of our knowledge, 1 represents the first ex-
47-48
ample of a low nuclearity cobalt nitride complex as the clos-
est related systems are nitride-encapsulated homo- and
Investigating the role of pyridine coordination in our sys-
tem, addition of pyridine to 1·CHD leads to the quantitative
-3 39
ket-
heterometallic clusters of the type [Co10Rh(N)
2
(CO)21] .
formation of the bimetallic bridging imido [(
̅
ket
Compound 1·CHD crystallizes in the P1 space group dis-
playing one-half molecule in the asymmetric unit with the
full dimer generated through inversion symmetry, and con-
sequently, the Co1-N4-Co1* bond angle is perfectly linear.
Examination of the crystallographically unique cobalt-ni-
tride distance (Co1-N4 = 1.678(1) Å) shows that it falls
guan)Co(py)][( guan)Co](μ-NH)(μ-N
3
) (5) where its N-H
proton is derived as the product of HAA from the co-crystal-
lized CHD (eq 1). Accordingly, this reaction is accompanied
1
by the formation of benzene as revealed by H NMR spec-
troscopy (Figure S36). Compound 5 does form upon addi-
tion of pyridine to 1·THF; however, the product mixture is
ket
within the typical range of cobalt-imido Co=NR distances
marked by the presence of significant amounts of guanH.
i
(
cf., [( Pr
2
N)C(NDipp)
2
]Co(=NAd): Co-N
=
1.621(3)
3
6
aryl
40
Å; [(TIMEN )Co(=NAr)](BPh ): Co-N = 1.675(8) Å).
4
The metrical parameters of 1 are suggestive of a dimer
III
III
with a core Co =N=Co unit. Yet, formal charge assign-
ments can be complicated by a number of factors in mole-
cules of this type. For example, the related Rh dimer
(PNN)Rh(N·)Rh(PNN) (PNN
phinomethylene-2,2’-bipyridine) is formulated as
Rh(II)/Rh(II) complex with a reactive, bridging nitridyl
=
6-di-(tert-butyl)phos-
a
4
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Journal of the American Chemical Society
Similarly, photolysis of 4a in C
generates 5; though, when conducted in the absence of H-
atom donors such as CHD, the yield is considerably lower
6
H
6
/py (10:1) solution also
Conversely, 1·THF is reactive in C
wards several hydrocarbon substrates, including those with
strong C-H bonds, giving 5 as the sole product in nearly
quantitative yields. Similarly, photolysis of 4a in C
solution with the same hydrocarbon substrates generates 5
in equivalent yield. In particular, HAA is observed with cy-
clohexene (BDE: 87 kcal/mol) and fully saturated cyclohex-
ane (BDE: 99 kcal/mol) (Figure 2a), yielding primarily ben-
zene but also bicyclohexenyl and 1,4 cyclohexadiene in
78:17:5 and 85:8:4 ratios, respectively, as indicated by
GC/MS analysis of the product mixtures (Table S4).
6
H /py solution to-
6
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
and is also accompanied by substantial amounts of proto-
6
6
H /py
ket
nated guanH. Moreover, photolyzing C
6
H
6
/py (10:1) solu-
15
tions of the isotopomer 4a- N (made with 98% enriched
1
5
14 14
Na N N N) in the presence of excess CHD gives a 50:50
ket
14
15
14
mixture of [( guan)Co] (μ- NH)(μ- NNN) (5- NH) and
2
ket
15
15
15
[
( guan)Co]
2
(μ- NH)(μ- NNN) (5- NH) which show re-
1
4
15
spective N-H and N-H stretching frequencies of 3418 and
-1
3
400 cm in the infrared spectrum (KBr pellet) (Figure
S24).
Complex 5 is a dark brown, paramagnetic compound that
is soluble in non-polar solvents. Single crystals of 5·C
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Notably, dissolution of 1·THF in py-d
5
/cyclohexane-d12
ket-
solution yields the deuterated parent imido [(
ket
6
H
14
guan)Co(py)][( guan)Co](μ-ND)(μ-N
3
) (5-D) that exhibits
are obtained from concentrated hexanes solution, and its
solid-state molecular structure (Figure 1c) reveals an asym-
metric, bimetallic complex with four- and five-coordinate
cobalt centers possessing bridging NH and N
cobalt-imido bond distances of 5·C 14 (Co1-N4 = 1.963(3)
Å, Co2-N4 = 1.925(3) Å) are not informative as they fall
an N-D stretching frequency in the infrared spectrum at
-1
2248cm (Figure S23), within the expected isotopic mass
-1
shift range of ca. 2500 cm as compared to 5-H (υN-H = 3424
-1
3
groups. The
cm ). This result is important as it corroborates an inter-
molecular HAA mechanism, eliminating the ligand as the H-
atom source.
6
H
within
the
CH NH
range
of
2
both
amido
Co-N
(cf.,
=
Interestingly, performing the reaction in the presence of
5
2
{
[(NH
2
CH
2
2
2
)
2
Co] (μ-NH
2
)(μ-OH)}[NO
3
]
4
:
excess cyclooctane (BDE: 95 kcal/mol) generates 1,5-cy-
clooctadiene almost exclusively with a trace amount of cy-
clooctene in a 97:2 ratio (Figure S45)(Table S4). In our
studies of the HAA chemistry, we do not observe reactivity
with benzene (BDE: 112.4 kcal/mol), pegging the upper
limit of the C-H bond abstraction between 99.5 and 112.4
kcal/mol.
5
0
1
.947(5) – 1.948(5) Å) and imido-bridged cobalt com-
pounds (cf., {[HC(MeCNDipp)]
.983(3) – 1.988(3) Å).51
SQUID magnetometry measurements of 5 give a room
temperature effective magnetic moment of 3.97 μ . This
2
Co(μ-NAr)} Co-N =
2
:
1
B
value is considerably lower than expected for two, non-in-
teracting high-spin Co(II) (S = 1.5) and Co(III) (S = 2) cen-
The reactivity of our system towards cyclohexane af-
forded us the opportunity to easily examine the KIE upon
ters (6.24 μ
B
) but reasonable if partial antiferromagnetic in-
teractions are present. Accordingly, the χT vs T plot is also
switching from C
tolysis of 4a (100 μM) in C
6
H
12 to C
6
D
6
12 in the reaction mixtures. Pho-
/py (10:1) in the presence of
3
-1
observed to slope, dropping from 1.99 cm ·K·mol at 300 K
H
6
3
-1
to 0.42 cm ·K·mol at 2K (or μeff vs T from 3.97 μ
B
to 1.81 μ
B
,
100 equiv of cyclohexane was followed by UV-vis absorp-
tion spectroscopy (Figure 2b). As seen from the rate plot
presented in Figure 2c, there is a clear difference in rate
with a KIE = 2.5±0.1. This value is within the KIE range es-
tablished for the related Fe=O mediated HAA reactions (KIE
respectively) (Figure S30), consistent with increased anti-
ferromagnetic coupling to a S = 0.5 ground state (calcd. 0.37
3
-1
cm ·K·mol or 1.73 μ
B
) at low temperature. Notably, a small,
ZFC/FC-independent peak is also observed at low tempera-
3
-1
53
ture, appearing at 18.5 K (1.00 cm ·K·mol ), indicating fer-
romagnetic contributions as seen with 1 and 3b.
= 2.5 – 5.7) and in complete accordance with the HAA
2
+
chemistry observed for the Co=O complex [(N Py)Co(O)]
4
As in the cases of L Fe(NAd), (dipyL)Co(NR), and
Me
(N
Py = N,N‐bis(2‐pyridylmethyl)‐N‐bis(2‐pyridyl)methyl‐
4
54
[
(salen)Ru(N)(MeOH)]PF
6
, the addition of pyridine to 1
amine) (KIE = 2.1±0.1). This reaction can be performed on
a preparative scale, giving the isotopomers 5-H and 5-D
(Figure S23).
clearly engages new reactivity where the coordinating base
acts as a solvent-switch to greatly enhance the reactivity of
the bridging nitride atom. Based upon our electronic struc-
ture analysis of 1 (vide infra) and the above mentioned ex-
amples, we contend that pyridine binding leads to a spin-
state promotion resulting in population of Co-N-Co π*-or-
bitals. Attempts to identify a [Co(N·)Co(py)] intermediate
by EPR spectroscopy via addition of pyridine to solutions of
Understanding the reactivity of molecular metal nitrides
towards hydrogen is very important for providing chemical
insights into the formation of ammonia through the Haber-
5
5
Bosch process. Despite this, only a few examples of nitride
8
, 19, 56-59
reactivity towards H
2
have been reported,
with the
majority using terminal nitride species. To date, only the
1
were unsuccessful.
Expanding upon the observed pyridine promoted HAA
bimetallic nitride complexes (L
3
)Fe(μ-N)Fe(L
3
) (L
] (μ‐N)} have
3
=
-
t
[PhB(CH
2
PPh
2
)
3
] ) and {Cs[U(OSi(O Bu)
3
)
3
2
chemistry of 1 towards CHD, its reactivity with other hydro-
carbons was studied. Interestingly, addition of pyridine to
been shown capable of heterolytically cleaving dihydrogen
5
6, 59
to give bridging M
In light of the observed HAA reactivity of our dicobalt ni-
tride, its reactivity towards H was investigated. Suspen-
sions of 1·THF in a mixture of benzene and pyridine under
(150 psi) gradually gives way to a new paramagnetic
product. Solutions of 4a in C /py mixtures are unreactive
towards H (150 psi); however, upon exposure to UV-light,
the solution gradually changes from emerald green to dark
2
(μ-NH)(μ-H) products.
1
or photolysis of 4a in C
6
H
6
/py in the presence of 9,10-di-
52
hydroanthracene (DHA) (BDE: 76.3 kcal/mol) fails to give
anthracene. We presume this is a consequence of steric hin-
drance as the two hydrocarbon substrates possess compa-
rably weak C-H bonds (Figure 2a). A similar steric argu-
ment was invoked to explain the unreactivity of
2
H
2
6
H
6
2
Me
L Fe(NAd)(py) towards DHA.
5
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Journal of the American Chemical Society
Page 6 of 12
asymmetric unit, providing the complete dimers upon sym-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
metry generation. The metrical parameters between the
monomers in the asymmetric unit are nearly identical and
reveal the cobalt imido distances to range from Co-N =
1.982(2) – 1.996(2) Å, only slightly longer than those found
in 5·C
6
H
14. The room temperature effective magnetic mo-
ment of 6·4THF is 3.20 μ
B
, similar to that found for 1, thus
supporting a Co(III)/Co(III) assignment for 6 possessing
partial antiferromagnetic coupling contributions. Indica-
tive of such, the χT vs T plot decreases in a near linear fash-
(
a)
3
-1
3
-1
ion from 1.25 cm ·K·mol at 300 K to 0.04 cm ·K·mol at 2
K (Figure S31). Yet, as 6 is lacking bridging azide groups, no
ferromagnetic coupling features are observed at low tem-
peratures as seen with 1, 3b, and 5.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
000
3000
1
Following the formation of 6 by H NMR spectroscopy
shows nearly quantitative conversion and the product is
isolated in 73% yield. Whether 6 forms from the heterolytic
2
1
000
000
0
or homolytic cleavage of H
the preference in our system for HAA of strong C-H bonds
suggests the latter. Conducting the experiment in C /py-
with H exclusively gives 6-H with a N-H stretching fre-
2
is not known at this time, though
6
D
6
d
5
2
-1
quency appearing at 3422 cm (Figure S18), indicating no
HAA reactivity with the solvent. Conversely, performing the
identical experiment in C
6
H
6
/py-h
5
under a D atmosphere
2
2
95
350
405
460
515
leads to the exclusive formation of the deuterium isoto-
Wavelength (nm)
ket
(b)
pomer [( guan)Co(μ-ND)] (6-D) as indicated by IR spec-
2
-1
troscopy (νN-D = 2318 cm ) (Figure S27), further eliminating
the solvent or ligand as the hydrogen atom sources. Finally,
photolyzing C
6
D
6
/py-d
5
solutions of 5 under H (150 psi)
2
gradually gives way to 6, suggesting the formation of the
bis(imido) occurs stepwise.
Computed Geometry and Electronic Structure. To
scrutinize the electronic structure of 1, we turned to density
functional theory (DFT). Using a non-truncated model of 1,
+
in an anionic form without the [Na(THF)
4
] counterion, the
empirical X-ray structure was remarkably reproduced by
dispersion-corrected DFT calculations in the quintet spin
state (see Supporting Information for details). This quintet
Q
(
1 ), with two unpaired electrons at each cobalt center,
agrees with the measured μeff = 3.60 μ at room tempera-
B
ture. A broken-symmetry single state was found to be more
stable by 22.64 kcal/mol than the quintet and, accordingly,
(c)
S
it represents the ground state of 1 at 0 K (1 ). As pure func-
tionals, like the PBE used, have the tendency to over stabi-
lize low spin states, the quintet state is plausibly much
closer in energy to an antiferromagnetically derived singlet
Figure 2. (a) Substrate screening for the HAA chemistry of 1
and respective C-H BDEs. (b) UV-vis spectra of the photocon-
version of 4a (green) to 5 (orange) in C
presence of excess CHD. (c) KIE rate plot of the photolysis of 4a
in C /py (10:1) in the presence of 100 equiv of C 12 (blue)
and C 12 (red).
6
H
6
/py solution in the
S
than predicted by DFT. The structural parameters of 1 are
Q
6
H
6
6
6
H
similar to 1 , except the Co-N-Co angle becomes slightly
D
bent at 162°, which facilitates the antiferromagnetic cou-
pling of the metal-centered radicals through the nitride cen-
ter.
green concomitant with the formation of the same paramag-
netic product after 3 days. In either case, starting with 1 or
Clearly, the quintet state description is most pertinent as
it best represents the observed character of 1. Thus, we
used quasi-restricted orbitals (QROs) (Figure 3) to rational-
4
a, the conversion is nearly quantitative by NMR spectros-
copy. The product crystallizes from hexane/pentane solu-
tion or concentrated THF stored at -30 °C, to give the parent
Q
ize and characterize the electronic structure of 1 . QROs ex-
ket
press unrestricted wavefunctions (with different  and 
orbital subsets) through the intuitive conceptual picture of
the restricted open-shell (RO) solution with doubly and sin-
gly occupied orbitals (i.e. through identical  and  spatial
orbitals).
bis(imido) [( guan)Co(μ-NH)]
ates 6·2C 12 and 6·4THF, respectively.
X-ray crystallographic analysis of 6·2C
ure 1d) shows the compound crystallizes in the P1 space
group with three independent half-molecules in the
2
(6) (Scheme 1) as the solv-
6
H
14·2C H
5
6
H
14·2C H12 (Fig-
5
̅
6
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Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. Molecular orbital diagram of the Co-N-Co core of 1Q.
Q
According to the features of the revealed molecular
QROs, 1 can be formally described as possessing two, high-
To better envisage the bonding interactions in 1 , a sim-
Q
ket
plified substructure of the “( guan)Co(μ-N)(N )” unit can
3
spin Co(III) centers exhibiting both antiferromagnetic and
weak ferromagnetic coupling through two two-electron in-
teractions (Figure 3). In particular, the formal partition of
these delocalized MOs to individual cobalt contributions
gives four half-filled d-orbitals, dxz, dyz, dxy, and dx2-y2 with a
doubly occupied dz2-orbital at each metal. At each cobalt,
two of the metal-centered radicals pair through the π-sub-
space (MO5 and MO6 in Figure 3), whereas the couplings of
unpaired electrons with local σ- and δ-symmetries through
the nitrogen remain weak and ferromagnetic in nature
(MO7 – MO10 in Figure 3).
be defined by the cobalt center, its azide group, the bridging
ket-
N
nitride, and one of the nitrogen contact atoms from each [
–
guan] ligand, giving an ML
3
structural approximation for
each metal which is highlighted in blue in Figure 3 (see in-
set). This yields a planar arrangement where the ligand at-
°
oms are separated by nearly 120 , affording a D3h symmetry
approximation for each cobalt fragment with the out-of-
ket
–
plane nitrogen atom of the [ guan] ligand acting as an
asymmetric axial perturbation on the idealized electronic
structure at the metal. Through this perspective, the d-or-
bital arrangement represents the most ideal orientation for
7
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Journal of the American Chemical Society
Page 8 of 12
the schematic representation of the cobalt-nitride and co-
balt-cobalt interactions.
Therefore, the in-plane Co(dxy) orbitals are perfectly ori-
ented on both sides to form an appreciable three-center in-
electron within the {Rh
embodies the most significant formal difference between its
2
N}17 core of (PNN)Rh(N·)Rh(PNN)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
6
Q
electronic structure and the {Co N} core of 1 , thus giving
2
way to the observed nitridyl radical character in
Q
teraction with the p
y
orbital of the Nnitride atom as seen in the
(PNN)Rh(N·)Rh(PNN) that is absent in 1 .
molecular orbital picture, MO1, in Figure 3. An almost iden-
tical three-center -interaction (MO2) evolves in the per-
CONCLUSION
pendicular xz-plane through the Co(dxz)+N(p
z
)+Co(dxz
)
In conclusion, while late metal complexes of the type
M=E/M≡E can be difficult to access and stabilize owing to
“oxo wall” considerations, we show here that surrogates for
such chemistry can be obtained through systems of the type
M=E=M. In this case, we demonstrate that the first example
of a bimetallic cobalt nitride, compound 1, is accessed
through photolysis of a guanidinate cobalt-azide precursor.
According to DFT calculations, conforming also to experi-
atomic orbital combination. Both of these molecular orbit-
als formally represent Nnitride-based lone pairs; however, the
spatial distributions clearly expose delocalization to the
metals resulting in the formation of -type bonding interac-
tions of covalent character. The two other lone pairs of the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-3
formally N bridge are represented by low lying orbitals
with N(p ) and N(s) character (not shown in Figure 3). The
interactions along the Co-N-Co axis are, in fact, classical
x
Q

mental observations, the calculated quintet 1 possesses
three-center four-electron interactions, giving way to the
Co(dxy)-Co(dxy) (MO5) and Co(dxz)-Co(dxz) (MO6) antisym-
metric non-bonding, metal-centered orbital combinations
(Figure 3). Being doubly occupied, MO5 and MO6 represent
the π-subspace, d-electron pairing.
two high-spin cobalt(III) centers coupled through a formal
III
III
Co =N=Co core with covalent Co-N -bonds. The pseudo-
III
III
planar arrangement about the Co =N=Co unit facilitates
the pairing of -symmetry metal-centered radicals (two at
each Co) across the bridging nitride, whereas the inefficient
through-space interaction of - and -symmetry d-orbitals
leads to the weak ferromagnetic coupling of the corre-
sponding unpaired electrons (also two at each Co), eventu-
In the -subspace, the Co(dx2-y2) orbitals are arranged
with the N(s) orbital to form a direct Co-N-Co σ-interaction,
leading to a low-lying orbital combination (not depicted in
Q
Figure 3). Along these lines, the Co(dx2-y2)+N(p
x
)+Co(dx2-y2
)
ating in an S=2, quintet spin-state of 1 . The geometry and
and Co(dx2-y2)-N(p )-Co(dx2-y2) arrangements also leads to
x
electron count of the cobalt centers avoids population of Co-
N π*-orbitals, thereby avoiding radical character at the N-
atom. Consequently, 1 is stable in solution and as a solid.
However, its N-atom reactivity is accessible through addi-
tion of pyridine, triggering HAA chemistry to give the
bridged imido 5. Impressively, this HAA reactivity extends
to strong C-H bonds such as those found in cyclohexane
(BDE ~ 100 kcal/mol). Moreover, this chemistry can be fur-
appreciable σ- and σ*-combinations; though, Co(dx2-y2)/(px)
orbital hybridizations give rise to the predominantly non-
bonding, metal-based combinations MO7 and MO10 (Figure
3
). Together with the symmetric and antisymmetric -sym-
metry oriented d-orbital combinations, Co(dyz)+Co(dyz) and
Co(dyz)-Co(dyz), respectively, with practically zero overlap,
the σ and -symmetry d-orbitals appear as quasi-degener-
ate orbitals (MO7 – MO10) with each hosting one electron
according to Hund’s maximum multiplicity rule.
ther extended to include H , leading to a rare example of hy-
2
drogen activation by a molecular metal nitride to generate
the bis(imido) 6. At present, investigations are underway
Owing to the strong delocalization of the nitride lone-
pairs to the metals (MO1 and MO2), the central Co-N inter-
actions have a notable -bond character in addition to the
to understand the observed H activation chemistry while
2
working to apply our synthetic strategy to other late metal
systems.
Q
dative -bond. Accordingly, the electronic structure of 1
III
III
can be best represented by the Co =N=Co canonical form,
with the data indicating no radical character at the nitride
atom. Mayer bond orders of 1.33 calculated for the Co-N in-
teractions support this notion. The spin density value of 1.5
at each cobalt also conforms to the partial paring of the d-
electrons. As the unpaired electrons occupy - and -sym-
metry metal orbitals that cannot mix with the nitrogen’s -
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website. Crystallographic details (CIF) and
the supporting Information is available free of charge on the
ACS Publication Website at DOI:
symmetry p
x
and p atomic orbitals, the radical character
z
cannot delocalize to the bridging nitrogen, which is re-
flected by the negligible atom-condensed spin density value
of 0.1 at the nitrogen.
The lowest-lying unoccupied molecular orbitals, MO11
and MO12, correspond to the antibonding combinations of
the Co=N=Co -interactions and have notable amplitude at
the central nitrogen due to the covalent nature of the Co-N
Experimental procedures, spectral data for all the
complexes, magnetic data and computational details
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*asfortier@utep.edu,

-bonds. In comparison, the reported spin density distribu-
1
5
tion of the related dimer (PNN)Rh(N·)Rh(PNN) closely re-
*balazs.pinter@usm.cl
Q
sembles the out of plane antibonding combination of 1 ,
MO11, with one electron occupancy of this orbital in the dir-
hodium nitride case. As a matter of fact, this additional
ORCID
Debabrata Sengupta: 0000-0001-7212-3761
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Ram Seshadri: 0000-0001-5858-4027
Journal of the American Chemical Society
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5d-Reactivity and Fate of Terminal Nitrido Complexes of Rhodium
and Iridium. Eur. J Inorg. Chem. 2010, (31), 4911-4915.
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I.; de Bruin, B., Steric Protection of Rhodium-Nitridyl Radical
Species. Eur. J Inorg. Chem. 2019, (39-40), 4249-4255.
Balazs Pinter: 0000-0002-0051-5229
Skye Fortier: 0000-0002-0502-5229
Notes
The authors declare no competing financial interests.
13. Saouma, C. T.; Peters, J. C., M E and M=E complexes of iron
ACKNOWLEDGMENT
and cobalt that emphasize three-fold symmetry (E O, N, NR). Coord.
Chem. Rev. 2011, 255 (7-8), 920-937.
We are grateful to the NSF (CHE-1664938 and DMR-1827745;
S.F.) and the Welch Foundation (AH-1922-20170325; S.F) for
financial support of this work. S.F. is an Alfred P. Sloan Foun-
dation research fellow and is thankful for their support. We
also wish to acknowledge the NSF-MRI program (CHE-
14. Hay-Motherwell, R. S.; Wilkinson, G.; Hussain-Bates, B.;
Hursthouse, M. B., Synthesis and x-ray crystal structure of
oxotrimesityliridium(V). Polyhedron 1993, 12 (16), 2009-12.
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15. Gloaguen, Y.; Rebreyend, C.; Lutz, M.; Kumar, P.; Huber, M.;
van der Vlugt, J. I.; Schneider, S.; de Bruin, B., An Isolated Nitridyl
Radical-Bridged {Rh(N.)Rh} Complex. Angew. Chem. Int. Ed. 2014,
53 (26), 6814-6818.
16. Scheibel, M. G.; Wu, Y.; Stueckl, A. C.; Krause, L.; Carl, E.;
Stalke, D.; de Bruin, B.; Schneider, S., Synthesis and Reactivity of a
Transient, Terminal Nitrido Complex of Rhodium. J. Am. Chem. Soc.
1
827875) for providing funding for the purchase of an X-ray
diffractometer. Magnetic measurements were performed at the
shared experimental facilities of the NSF Materials Research
Science and Engineering Center (MRSEC) at UC Santa Barbara
(
DMR 1720256). The UCSB MRSEC is a member of the NSF-
supported Materials Research Facilities Network
(www.mrfn.org).
2
013, 135 (47), 17719-17722.
7. Angersbach-Bludau, F.; Schulz, C.; Schoeffel, J.; Burger, P.,
1
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