Nickel-mediated N-N bond formation and N2O liberationvianitrogen oxyanion reduction

Article abstract of DOI:10.1039/d1sc02846d

The syntheses of (DIM)Ni(NO₃)₂and (DIM)Ni(NO₂)₂, where DIM is a 1,4-diazadiene bidentate donor, are reported to enable testing of bis boryl reduced N-heterocycles for their ability to carry out stepwise deoxygenation of coordinated nitrate and nitrite, forming O(Bpin)₂. Single deoxygenation of (DIM)Ni(NO₂)₂yields the tetrahedral complex (DIM)Ni(NO)(ONO), with a linear nitrosyl and κ¹-ONO. Further deoxygenation of (DIM)Ni(NO)(ONO) results in the formation of dimeric [(DIM)Ni(NO)]₂, where the dimer is linked through a Ni-Ni bond. The lost reduced nitrogen byproduct is shown to be N₂O, indicating N-N bond formation in the course of the reaction. Isotopic labelling studies establish that the N-N bond of N₂O is formed in a bimetallic Ni₂intermediate and that the two nitrogen atoms of (DIM)Ni(NO)(ONO) become symmetry equivalent prior to N-N bond formation. The [(DIM)Ni(NO)]₂dimer is susceptible to oxidation by AgX (X = NO₃^?, NO₂^?, and OTf^?) as well as nitric oxide, the latter of which undergoes nitric oxide disproportionation to yield N₂O and (DIM)Ni(NO)(ONO). We show that the first step in the deoxygenation of (DIM)Ni(NO)(ONO) to liberate N₂O is outer sphere electron transfer, providing insight into the organic reductants employed for deoxygenation. Lastly, we show that at elevated temperatures, deoxygenation is accompanied by loss of DIM to form either pyrazine or bipyridine bridged polymers, with retention of a BpinO^?bridging ligand.

Full text of DOI:10.1039/d1sc02846d

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Nickel-mediated N–N bond formation and N2O
liberation via nitrogen oxyanion reduction†
Cite this: Chem. Sci., 2021, 12, 10664
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have been paid for by the Royal Society
of Chemistry
Daniel M. Beagan, Alyssa C. Cabelof, Maren Pink, Veronica Carta, Xinfeng Gao
*
and Kenneth G. Caulton
The syntheses of (DIM)Ni(NO3)2 and (DIM)Ni(NO2)2, where DIM is a 1,4-diazadiene bidentate donor, are
reported to enable testing of bis boryl reduced N-heterocycles for their ability to carry out stepwise
deoxygenation of coordinated nitrate and nitrite, forming O(Bpin)2. Single deoxygenation of (DIM)
Ni(NO2)2 yields the tetrahedral complex (DIM)Ni(NO)(ONO), with a linear nitrosyl and k1-ONO. Further
deoxygenation of (DIM)Ni(NO)(ONO) results in the formation of dimeric [(DIM)Ni(NO)]2, where the dimer
is linked through a Ni–Ni bond. The lost reduced nitrogen byproduct is shown to be N2O, indicating
N–N bond formation in the course of the reaction. Isotopic labelling studies establish that the N–N bond
of N2O is formed in a bimetallic Ni2 intermediate and that the two nitrogen atoms of (DIM)Ni(NO)(ONO)
become symmetry equivalent prior to N–N bond formation. The [(DIM)Ni(NO)]2 dimer is susceptible to
oxidation by AgX (X ¼ NO3ꢀ, NO2ꢀ, and OTfꢀ) as well as nitric oxide, the latter of which undergoes nitric
oxide disproportionation to yield N2O and (DIM)Ni(NO)(ONO). We show that the first step in the
deoxygenation of (DIM)Ni(NO)(ONO) to liberate N2O is outer sphere electron transfer, providing insight
into the organic reductants employed for deoxygenation. Lastly, we show that at elevated temperatures,
deoxygenation is accompanied by loss of DIM to form either pyrazine or bipyridine bridged polymers,
with retention of a BpinOꢀ bridging ligand.
Received 25th May 2021
Accepted 13th July 2021
DOI: 10.1039/d1sc02846d
rsc.li/chemical-science
into value-added compounds that might be derived from
nitrogen oxyanions. An attractive route to nitrogen oxyanion
reduction is through deoxygenation,24–32 and the oxygen can be
captured by protons, metal electrophiles, or main group
elements known for their oxophilicity. The same is true for CO2
reduction and both carbon and nitrogen reduction would be
richer if they could form products with C–C and N–N bonds,
respectively. Designing methods to encourage such element/
element reductive coupling is a valuable pursuit.
Introduction
Reduction of CO2 is widely sought and has classically focused
on deoxygenation to C1 products,1–3 with recent advances
moving towards multi-carbon and value-added products which
preserve terrestrial use of that carbon resource.4–9 There is
a nitrogen analogue of this challenge, accomplished biologi-
cally through denitrication, which requires an N–N bond
ꢀ
formation upon reduction of NOx back to dinitrogen. It is
One attractive strategy to promote N–N bond formation is to
design complexes which pre-organize two nitrogen oxyanions at
a single metal center and explore the deoxygenation chemistry
of these systems. There are several reported examples of N–N
bond formation at a single metal complex,33–36 and most
terminal cis-hyponitrite complexes are reported with group 10
metals.37–41 The reagent bis-pinacolylboryl pyrazine (Scheme 1)
has been shown to deoxygenate two nitrates coordinated to iron
to yield a dinitrosyl iron complex.31 This reagent is electron rich
(8p electron ring) and carries two electrophilic boron atoms
making it a potent reductant with attractive kinetic reactivity,
including polar B–N bonds.
We chose a bidentate ancillary ligand with a steric prole
designed to allow for multidentate binding of nitrogen oxy-
anions if needed. These 1,4-diazadienes (DIM, Scheme 1)
ligands are established to be redox active and can assist in the
stabilization of low valent nickel complexes.42–45 Herein we show
widely recognized that the natural nitrogen cycle has been
perturbed as an unintended consequence of inexpensive
Haber–Bosch ammonia used to enhance food production.10–12
Soluble nitrate is abundant in runoff from agricultural elds,
and accumulates in poorly ushed rivers, bays and seacoasts
which ultimately leads to eutrophication and oxygen-poor
“dead zones.”13–15
The reduction of nitrogen oxidation states is biologically
accomplished by a variety of enzymes16–23 and its synthetic
conversion is essential to repurpose environmental pollutants
Indiana University, Department of Chemistry, 800 E. Kirkwood Ave., Bloomington, IN,
47401, USA. E-mail: caulton@indiana.edu
† Electronic supplementary information (ESI) available. CCDC 2083107 (DIM)
Ni(NO3)2, 2083108 (DIM)Ni(NO2)2, 2083111 (DIM)Ni(NO)(ONO), 2083110 [(DIM)
Ni(NO)]2, 2083109 (DIM)Ni(NO)(OTf), 2083112 (OBpin)2Ni2(NO)2(Pz) and
2083113 (OBpin)2Ni2(NO)2(Bpy). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d1sc02846d
10664 | Chem. Sci., 2021, 12, 10664–10672
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Fig. 1 (a) ORTEP representation of the molecular structure (50%
probabilities) of the nonhydrogen atoms of (a) (DIM)Ni(NO2)3 and (b)
(DIM)Ni(NO2)2 (right), showing selected atom labelling (unlabelled
atoms are carbon). Selected structural parameters for (DIM)Ni(NO2)3
(A˚): Ni1–O1, 2.110(4); Ni1–O2, 2.093(4); Ni1–O4, 2.050(4); Ni1–O5,
2.097(4); Ni1–N3, 2.009(5); Ni1–N4, 2.058(4); O1–N1, 1.265(6); O2–
N1, 1.273(6); O3–N1, 1.230(6); O4–N2, 1.298(6); O5–N2, 1.271(6); O6–
N2, 1.255(6); and (DIM)Ni(NO2)2 (A˚): Ni1–O1, 2.094(6); Ni1–O2,
2.091(6); Ni1–O3, 2.099(6); Ni1–O4, 2.072(6); Ni1–N3, 2.035(7); Ni1–
N4, 2.038(7); O1–N1, 1.287(9); O2–N1, 1.269(9); N2–O3, 1.257(10);
N2–O4, 1.290(11).
Scheme 1 Previously reported nitrate reduction and motivation for
utilizing Ni compounds (top), and summary of new reactivity presented
(bottom).
Single deoxygenation of (DIM)Ni(NO2)2
Reaction of (DIM)Ni(NO2)2 with equimolar (Bpin)2Pz occurs to
completion within 12 h at room temperature in THF with a color
change from brown to green (Scheme 2). Analysis by H NMR
stepwise deoxygenation of oxyanions coordinated to nickel,
where the location of nickel late in the 3d series disfavors two
coordinated nitrosyls in comparison to Fe or Co (Scheme 1), and
instead deoxygenation leads to N–N bond formation and
nitrous oxide liberation.
1
spectroscopy shows complete consumption of the boryl reagent,
formation of equimolar pyrazine and (Bpin)2O and a diamag-
netic metal complex with the expected 4 : 6 : 6 : 12 intensity 1H
NMR resonances of a single DIM-containing product. Workup
by pentane wash to remove organic materials yields a green
solid, and the molecular structure (Fig. 2) conrms the presence
of the singly deoxygenated species (DIM)Ni(NO)(ONO), assigned
as {Ni(NO)}10 in the Enemark–Feltham notation. The Ni1–N3–
O1 angle is linear (175.5ꢁ), and the complex has a strong nitrosyl
stretching frequency at 1785 cmꢀ1, similar to other LNi(NO)(X)
complexes.46–49 The nitrito linkage in (DIM)Ni(NO)(ONO)
contrasts the N-bound nitrite linkage in (Me2-ortho-phenan-
throline)Ni(NO)(NO2).49
Results and discussion
Nickel nitrate and nitrite complexes
A precursor complex, (DIM)NiBr2, is synthesized from (DME)
NiBr2 (DME ¼ dimethoxyethane) and DIM in a 1 : 1 mole ratio
1
in DCM and workup affords a red/brown solid. H NMR spec-
troscopy (Fig. S1†) of (DIM)NiBr2 reveals a paramagnetic species
with resonances ranging from ꢀ22 to 36 ppm and is fully
consistent with a C2v symmetric product. Salt metathesis reac-
tions of (DIM)NiBr2 with AgNO3 or AgNO2 in acetonitrile at
room temperature quickly form the bis-nitrate and bis-nitrite
complexes respectively with concurrent precipitation of AgBr
(Scheme 2). The bis-nitrate and bis-nitrite complexes are also
paramagnetic and twofold symmetric as judged by NMR spec-
troscopy (Fig. S3 and S5†) and their molecular structures (Fig. 1)
exhibit bidentate binding of both nitrate and nitrite creating an
octahedral coordination environment about the nickel metal
centers.
Deoxygenation of (DIM)Ni(NO)(ONO)
The tetrahedral nickel(0) (DIM)Ni(NO)(ONO) complex reacts
with (Bpin)2Pz (1 : 1.5 mole ratio) at room temperature in THF.
Monitoring the reaction by 1H NMR spectroscopy shows the
Fig. 2 ORTEP representation of the molecular structure (50% prob-
Scheme 2 Synthesis of (DIM)Ni(NOx)2 complexes, as well as the abilities) of the nonhydrogen atoms of (DIM)Ni(NO)(ONO) showing
synthesis of (DIM)Ni(NO)(ONO) from (DIM)Ni(NO2)2. The bis-nitrate selected atom labelling (unlabelled atoms are carbon). Selected
and bis-nitrite complexes are O-bound and bidentate in the solid- structural parameters: Ni1–N3, 1.637(2); Ni1–O2, 1.9952(19); Ni1–N2,
state molecular structures (Fig. 1), we have drawn them as mono- 2.0075(19); Ni1–N1, 2.0204(19); O1–N3, 1.176(3), O2–N4, 1.280(3);
dentate here for simplicity.
O3–N4, 1.228(3); N4–O2–Ni1, 118.25(16); O1–N3–Ni1, 175.5(2).
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formation of boryl ether and pyrazine, as well as the depletion of opposing the dipoles in each (DIM)Ni(NO) monomer and
signals for (DIM)Ni(NO)(ONO). Crystalline precipitate forms benets from opposed NOd+ to DIMdꢀ attraction between
during the course of the reaction. Executing this reaction in monomer units. Because the dimeric product is insoluble in
CD3CN shows complete consumption of starting materials in a wide range of solvents of polarity up to acetonitrile and
1
less than four hours at room temperature. All DIM-containing dichloromethane, a H NMR spectrum could not be obtained.
product is insoluble based on the lack of any ligand reso- The solid state IR spectrum (Fig. S15†) shows a strong NO
nances, but the pyrazine and boryl ether byproducts are detec- absorption at 1664 cmꢀ1, a low value consistent with greater
ted by 1H NMR spectroscopy (Fig. S19†) in the correct back donation to the nitrosyl than in (DIM)Ni(NO)(ONO). This
stoichiometric ratio in the colorless solution. This conrms that can be attributed to the increased donor ability of the reduced
no free ligand is formed upon deoxygenation, nor any degra- DIM ligand as well as better orbital overlap.
dation products containing the DIM ligand. The reaction
The reaction of (DIM)Ni(NO)(ONO) with equimolar (Bpin)2Pz
executed in CD2Cl2 shows the same result, including no shows complete consumption of borylated reagent along with
detectable 1H NMR signals of the product nickel complex. This incomplete consumption of nickel starting material. In order to
solvent gives slightly larger crystals, which were used for the establish the correct stoichiometry, (DIM)Ni(NO)(ONO) was
better of two single crystal structure determinations. The reacted with excess (3 : 1 mole ratio) (Bpin)2Pz in the presence
molecular structure (Fig. 3) as determined by SCXRD reveals the of naphthalene as an internal standard. From this experiment,
product to be [(DIM)Ni(NO)]2, a dimer with a Ni–Ni bond.
pyrazine is produced in a 1 : 1.5 mole ratio with respect to the
[(DIM)Ni(NO)]2 contains a crystallographic inversion center (DIM)Ni(NO)(ONO) starting material, indicating a 2 : 3 stoichi-
˚
at the Ni–Ni midpoint and the Ni–Ni bond length (2.6103(10) A) ometry of nickel to borylating reagent. Labelling studies are
is short compared to other Ni–Ni bonds with no bridging employed here for characterization, and later for mechanistic
ligands.50–53 There are no other examples of two Ni–NO units insight. The nitrosyl stretch shis to 1625 cmꢀ1 (Fig. S16†) for
linked solely by a Ni–Ni bond. The Ni lies only 0.16 A out of the isotopically labelled [(DIM)Ni(15NO)]2, which is consistent with
˚
three-nitrogen atom plane, and the Ni0/N1 distance is longer by the expected reduced mass calculation. The reduced nitrogen
˚
1.24 A, indicating that the NO is not bridging. Analysis of the containing byproduct upon deoxygenation of (DIM)
bond lengths in the DIM backbone (N–C–C–N) (Table S1†) Ni(NO)(ONO) was conrmed to be N2O via vacuum transfer of
reveals
a
long-short-long pattern (vs. the comparison the headspace volatiles to a gas IR cell, which clearly show the P
compounds in Table S1†) consistent with increased electron and R bands of N2O at 2224 cmꢀ1 (Fig. S30†). All of this allows
density in the di-imine moiety. The C–C bond in particular is us to write a balanced reaction for the formation of [(DIM)
˚
quite short (1.436 A) compared to the other (DIM)Ni complexes, Ni(NO)]2 with concomitant loss of N2O (Scheme 3).
indicative of either a singly or doubly reduced ligand. The
Deoxygenation
of
isotopically
labelled
(DIM)
monomer (DIM)Ni(NO) is an odd-electron (radical) species Ni(15NO)(O15NO) results in the formation of (15N)2O, evidenced
which dimerizes to the crystallographically observed product. by a band at 2158 cmꢀ1 via gas IR (Fig. S30†). The deoxygenating
The DGꢁ for dissociation of the dimer into two (DIM)Ni(NO) agent employed in Scheme 3 can be replaced by the reduced
radical monomers is calculated to be 14.7 kcal molꢀ1, similar to 4,40-bipyridine analogue, (Bpin)2Bpy, which reacts completely
reported values for dissociation of an iron carbonyl dimer.54 The within 2 hours. The faster deoxygenation using (Bpin)2Bpy vs.
MALDI-TOF spectrum (Fig. S18†) of [(DIM)Ni(NO)]2 has an m/z (Bpin)2Pz is a consequence of reduced bipyridine being a more
at 816.3, consistent with the intact dimer. The trans rotational potent reductant than reduced pyrazine.55
conformation around the nickel–nickel bond is sterically
favored. This conformation has the stabilizing effect of
XPS of nickel complexes
XPS provides useful resolution of the chemical environment in
these four molecules (see Fig. S44†). The nitrogen 1s region
resolves nitrate, nitrite, nitrosyl and all the DIM nitrogen atoms,
with progressively lower NOx binding energy along that series.
The oxygen 1s intensity decreases progressively from nitrate to
nitrite to nitrosyl when integrated with respect to the nitrogen.
Compound (DIM)Ni(NO)(ONO) shows two oxygen 1s binding
energies with relative intensities of 1 : 2, indicating resolution
Fig. 3 ORTEP representation of the molecular structure (50% prob-
abilities) of the nonhydrogen atoms of centrosymmetric [(DIM)
Ni(NO)]2, showing selected atom labelling (unlabelled atoms are
carbon). Selected structural parameters: Ni1–N3, 1.960(4) A˚; Ni1–N2, Scheme 3 Synthesis of [(DIM)Ni(NO)]2 from (DIM)Ni(NO)(ONO) and
1.893(4); Ni1–N1, 1.647(3); O1–N1, 1.193(4); O1–N1–Ni1 172.5(3)ꢁ.
(Bpin)2Pz.
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of two of the three types of oxygens. The oxygen 1s binding and homogeneity of the solution. The 19F NMR spectrum
energy decreases progressively from (DIM)Ni(NO3)2 to (DIM) (Fig. S21†) is indicative of one diamagnetic product, and the 1H
Ni(NO2)2 to [(DIM)Ni(NO)]2, consistent with a lower nitrogen NMR (Fig. S21†) spectrum indicates twofold symmetry. The
oxidation state being less electron withdrawing towards the solid state nitrosyl stretch appears at 1780 cmꢀ1 (Fig. S22†)
oxygen. Although linear nitrosyl and nitrite are both formally which is shied to lower energy by 100 cmꢀ1 compared to
trivalent nitrogen, the lower binding energy of oxygen by 0.57 eV authentic 3-coordinate (bpy)Ni(NO)+ (bpy ¼ bipyridine).49 The
in [(DIM)Ni(NO)]2 compared to (DIM)Ni(NO2)2 can be attributed shi in nNO can be attributed to coordination of THF solvent to
to the increased backdonation into the nitrosyl of the dimer, give the 4-coordinate species [(DIM)Ni(NO)(THF)]+. When the
consistent with the observed low nNO stretching frequency. The reaction of [(DIM)Ni(NO)]2 and AgOTf is repeated in the non-
nitrogen 1s spectrum of [(DIM)Ni(NO)]2 resolves the nitrosyl coordinating solvent DCM, the nNO is higher by 31 cmꢀ1 in
from imine in the intensity ratio of 1 : 2. The Ni 2p spectrum the solid state (Fig. S23†) and 45 cmꢀ1 in a DCM solution
shows a lower metal binding energy for [(DIM)Ni(NO)]2 (Fig. S24†). If the same material is redissolved in THF, the IR
compared to the bis-nitrate and bis-nitrite complexes, consis- spectrum shows two nitrosyl stretches suggesting an equilib-
tent with it having a lower metal oxidation state.
rium of THF binding to the nickel center. Crystals were grown
via vapor diffusion of pentane into a concentrated THF solution
and the solid state molecular structure (Fig. 4) shows the
{Ni(NO)}10 complex (DIM)Ni(NO)(OTf), with triate coordinated
to nickel. However, based on spectroscopic data, the OTf is in
equilibrium when dissolved in coordinating solvents.
With this weak nucleophile, the tetrahedron is distorted to
atten the (DIM)NiN(O) plane: the angle from the DIM N–C]C–
N midpoint to the Ni center to N1 is 135.6ꢁ. Regardless of the
abundance of THF as crystallization solvent, this neutral
molecule is apparently the least soluble equilibrium
participant.
Redox reactivity of [(DIM)Ni(NO)]2
Despite the insolubility of [(DIM)Ni(NO)]2, it reacts readily
(Scheme 4) with oxidants to cleave the Ni–Ni bond. The reaction
between a slurry of [(DIM)Ni(NO)]2 with AgNO2 or AgNO3 results
in homogeneity of the solution and a color change to green in
both cases. Isolation of the green solid from the reaction with
AgNO2 reveals the product to be the previously characterized
(DIM)Ni(NO)(ONO) by 1H and 13C NMR spectroscopy along with
the diagnostic NO stretch at 1785 cmꢀ1. The corresponding
reaction of the dimer and AgNO3 results in the formation of the
{Ni(NO)}10 complex (DIM)Ni(NO)(ONO2), indicating that the one
electron oxidation product of [(DIM)Ni(NO)]2 is susceptible to
nucleophilic attack even by weak nucleophiles such as nitrate. It
is important here to control the Ag : Ni stoichiometry because
both (DIM)Ni(NO)(ONO) and (DIM)Ni(NO)(ONO2) are subject to
further oxidation by silver salts, forming the bis-nitrite or bis-
nitrate complexes respectively. The oxidations of the mixed
oxyanion complexes result in liberation of an equivalent of
nitric oxide, which was detected via gas phase IR (Fig. S34†).
There is precedence for nitric oxide liberation upon one elec-
tron oxidation of [Ni(NO)(bipy)][PF6] with either AgPF6 or [NO]
[PF6] as the oxidant.48
Nitric oxide disproportionation
With the knowledge that [(DIM)Ni(NO)]2 reacts rapidly with
silver oxidants, we were interested in its reactivity with NO gas.
A suspension of [(DIM)Ni(NO)]2 in THF was exposed to excess
NO and there was an immediate color change to a homoge-
neous brown/green solution. Workup of the solution indicates
that the nickel-containing product of this reaction is (DIM)
Ni(NO)(ONO), which is established by a variety of spectroscopic
techniques along with an independent unit cell determination
of the molecular structure via SCXRD. Vacuum transfer of the
volatiles generated during the reaction into a gas IR cell show
Even an oxidant with a more weakly nucleophilic triate
anion succeeds in this oxidative process. The reaction of [(DIM)
Ni(NO)]2 with AgOTf in THF results in a color change to green
Fig. 4 ORTEP representation of the molecular structure (50% prob-
abilities) of the nonhydrogen atoms of (DIM)Ni(NO)(OTf), showing
selected atom labeling (unlabelled atoms are carbon). Selected
structural parameters (A˚, ꢁ): Ni1–O2, 2.059(2); Ni1–N3, 1.994(2); Ni1–
N2, 2.010(2); Ni1–N1, 1.630(2); O1–N1–Ni1, 173.1(3); N1–Ni1–O2,
124.22(11).
Scheme 4 Summary of oxidative reactivity with [(DIM)Ni(NO)]2.
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