Formation of N2O from a nickel nitrosyl: Isolation of the cis -[N2O2]2- intermediate

Article abstract of DOI:10.1021/ja304204q

Addition of 2,2′-bipyridine (bipy) to [Ni(NO)(bipy)][PF₆] (1) results in formation of a rare five-coordinate nickel nitrosyl [Ni(NO)(bipy)₂][PF₆] (2). This complex exhibits a bent NO^- ligand in the solid state. On standing in acetonitrile, 2 furnishes the NO coupled product, [Ni(κ²-O₂N ₂)(bipy)] (8) in moderate yield. Subsequent addition of 2 equiv of acetylacetone (H(acac)) to 8 results in formation of [Ni(acac) ₂(bipy)], N₂O, and H₂O. Preliminary mechanistic studies suggest that the N-N bond is formed via a bimetallic coupling reaction of two NO^- ligands.

Full text of DOI:10.1021/ja304204q

Communication
pubs.acs.org/JACS
Formation of N2O from a Nickel Nitrosyl: Isolation of the cis-[N2O2]2−
Intermediate
Ashley M. Wright, Guang Wu, and Trevor W. Hayton*
Department of Chemistry and Biochemistry, University of CaliforniaSanta Barbara, Santa Barbara, California 93106, United States
S
* Supporting Information
Treatment of [Ni(NO)(CH3NO2)3][PF6]37 with 0.95 equiv
ABSTRACT: Addition of 2,2′-bipyridine (bipy) to [Ni-
(NO)(bipy)][PF6] (1) results in formation of a rare five-
coordinate nickel nitrosyl [Ni(NO)(bipy)2][PF6] (2).
This complex exhibits a bent NO− ligand in the solid
state. On standing in acetonitrile, 2 furnishes the NO
coupled product, [Ni(κ2-O2N2)(bipy)] (8) in moderate
yield. Subsequent addition of 2 equiv of acetylacetone
(H(acac)) to 8 results in formation of [Ni(acac)2(bipy)],
N2O, and H2O. Preliminary mechanistic studies suggest
that the N−N bond is formed via a bimetallic coupling
reaction of two NO− ligands.
of 2,2-bipyridine (bipy) in CH2Cl2 results in formation of the
three-coordinate nickel nitrosyl [Ni(NO)(bipy)][PF6] (1) in
good yield. Complex 1 exhibits a νNO value of 1869 cm−1 in
CH2Cl2. For comparison, the related cationic three-coordinate
nickel nitrosyl, [(dtbpe)Ni(NO)][BArF ], exhibits a much
4
lower νNO value of 1836 cm−1 (solid state).39 The connectivity
of 1 was confirmed by X-ray crystallography (Figure S1). The
nickel center in 1 is ligated by a single bipy ligand and one NO
ligand in a trigonal planar geometry. The metrical parameters of
the Ni−N−O moiety [Ni1−N1 = 1.617(3) Å, N1−O1
=1.152(4) Å, Ni1−N1−O1 = 176.2(3)°] are consistent with
previously characterized three-coordinate {NiNO}10 spe-
cies.39,40
he abatement of NOx in industrial and automotive exhaust
T
has immense environmental and societal relevance.1−5
The current catalysts for transportation applications consist of
nanoparticulate Pt and Rh on a ceramic support,3 and there has
been an intense effort to understand how NO reduction occurs
in these systems.6−11 There is also interest in replacing the
expensive noble metals currently in use with more sustainable,
earth abundant elements.3,6,12 Additionally, there is concern
that current technologies will not be able to meet future NOx
emission standards,3 providing further justification for
fundamental research on NO reduction. During catalysis in
these systems, it is thought that the first step in NO reduction
occurs through an initial NO(g) dissociation step,3,10,11 but
more recent studies point to the possible role of a hyponitrite-
like (NO)2 dimer in the catalytic cycle.6,8,13,14
Scheme 1
A number of homogeneous systems that can reduce NO have
also been studied.1,15−20 Hyponitrite is a postulated inter-
mediate in many of these reactions, but its direct detection has
often eluded researchers because of its reactive nature.21−26 For
example, Trogler and co-workers studied a Pd2+ catalyst that
reduces NO to N2O in acidic media.27,28 During the catalytic
cycle, [PdCl3(NO)]2− is proposed to undergo electrophilic
attack by NO• in the critical N−N bond forming step.
However, the [N2O2]2− intermediate could not be de-
tected.27,28 More recently, Onishi and co-workers developed a
Ru bimetallic complex that effects NO reduction.22,29 In this
example, the hyponitrite intermediate was isolated and
structurally characterized, making it one of only a few well-
defined hyponitrite complexes.25,26,30−35 Herein, we report the
isolation of a nickel nitrosyl, [Ni(NO)(bipy)2][PF6], which
forms N2O on standing via a cis-[N2O2]2− intermediate. To our
knowledge, this is the first well-defined example of an Ni-
mediated NO reduction.36−38
Further reaction of 1 with 1 equiv of bipy in CH2Cl2 affords a
deep red solution (Scheme 1). Storage at −25 °C results in
precipitation of the five-coordinate nickel nitrosyl complex,
[Ni(NO)(bipy)2][PF6] (2), in 93% yield. Alternatively, treat-
ment of 1 with 2,9-dimethylphenanthroline (Me2phen)
furnishes [Ni(NO)(bipy)(Me2phen)][PF6] (3) in 77% yield.
Both complexes have been fully characterized (see the
Supporting Information).
As revealed by X-ray crystallography, both 2 and 3 consist of
a cationic nickel center coordinated by two dipyridyl ligands
and one NO ligand (Figure 1). The metrical parameters of the
Ni−N−O moiety in 2 and 3 are similar; however, they contrast
Received: May 1, 2012
Published: June 7, 2012
© 2012 American Chemical Society
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Communication
Scheme 2
Figure 1. ORTEP drawing of the cationic portion of [Ni(NO)-
(bipy)2][PF6] (2·1.5CH2Cl2) (left) and [Ni(NO)(bipy)(Me2phen)]-
[PF6] (3·CH2Cl2) (right), shown with 50% thermal ellipsoids.
Hydrogen atoms, PF6 anions, and CH2Cl2 solvate molecules have
been omitted for clarity.
significantly with those of 1. For example, the Ni1−N1 [2,
1.762(3) Å; 3, 1.766(6) Å] and N1−O1 [2, 1.207(3) Å; 3,
1.196(9) Å] bond lengths are both elongated in comparison to
those seen in 1. The Ni1−N1−O1 angle is also small in both 2
and 3 [2, 129.5(2)°; 3, 128.2(5)°]. The changes in metrical
parameters upon addition of a second dipyridyl ligand are
consistent with a reduction of the nitrosyl moiety to NO−.40
This is also evidenced by the significantly lower νNO value of 2
[1567 cm−1] and 3 [1585 cm−1], in comparison to 1.40,41
Notably, the coordination environment around the Ni center
changes subtly between 2 and 3. The geometry of 2 is
intermediate between trigonal bipyramidal and square
pyramidal (τ5 = 0.49), while 3 is closer to square pyramidal
(τ5 = 0.18) where N4 of the Me2phen ligand forms the apex of
the square pyramid.42 In addition, one of the methyl groups of
the Me2phen ligand partially blocks the open coordination site
of the Ni center in 3. There are only a few Ni nitrosyls that
feature a bent NO ligand.16,43,44 For example, (Cp)Ni(NHC)-
(NO) (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-yli-
dene) exhibits similar Ni−NO metrical parameters to those
yield].47 Interestingly, monitoring the addition of H(acac) to 2
in CD3CN by H NMR spectroscopy reveals the formation of
1
[Ni(bipy)3][PF6]2 (4) and [Ni(acac)2(bipy)]48 (7) at short
reaction times (1 h). On standing, the resonances associated
with these two complexes disappear and those associated with
complex 6 grow in intensity, suggesting there is ligand
redistribution between 4 and 7 to form 6. This was confirmed
by reaction of independently prepared samples (see the SI).
The formation of N2O from NO can occur by either HNO
disproportionation or hyponitrite formation,2,4 and we
endeavored to determine which pathway was operative.
Fortuitously, dissolution of 2 in acetonitrile results in
precipitation of the NO-coupled product, [Ni(κ2-O2N2)(bipy)]
(8), after standing for 1 h, suggesting that N2O formation from
2 proceeds via a hyponitrite intermediate. Complex 8 can be
isolated as a diamagnetic, orange crystalline solid in moderate
but consistent yields (30−50%). Its solid-state IR spectrum
(Nujol mull) contains stretches at 937 and 1265 cm−1,
consistent with presence of a [N2O2]2− ligand.25,49 In the
solid-state, complex 8 exhibits a square planar Ni center
coordinated by one bipy ligand and one cis-[N2O2]2− ligand
(Figure 2). The cis-[N2O2]2− ligand is symmetrically coordi-
nated to the Ni center through two oxygen atoms [Ni1−O1 =
1.800(1) Å, Ni1−O2 = 1.800(1) Å].25 The O1−N1 and O2−
N2 bond distances are 1.384(3) and 1.391(3) Å, respectively,
while the N1−N2 bond length [1.240(3) Å] is consistent with
a double bond. Overall, these parameters support the presence
of a dianionic hyponitrite ligand, [N2O2]2−, in 8.31,50 For
seen in 2 and 3,40 while [BseMe]Ni(PPh3)(NO) (BseMe
=
bis(mercaptoimidazolyl)borate) features a larger Ni−N−O
angle of 149.6(1)°.43,44
Complex 2 is unstable in CH2Cl2 solutions at room
temperature under an N2 atmosphere. Monitoring this
decomposition by IR spectroscopy over 40 min reveals growth
of an IR stretch assignable to N2O [2228 cm−1].45 Its presence
is also supported by analysis of the head space by gas
chromatography (see the Supporting Information (SI)).
1
Monitoring the transformation of 2 for 24 h by H NMR
spectroscopy in CD2Cl2 reveals the formation of several new
products, including free bipy, [Ni(bipy)3][PF6]2 (4),46 and
[(bipy)Ni(NO)(Cl)] (5). The identities of 4 and 5 were
corroborated by comparison against the authentic materials
(see the SI). In contrast to the instability of 2, complex 3 is
stable under the same conditions over 40 min by IR
spectroscopy. Moreover, only minor amounts of decomposition
1
are observed by H NMR spectroscopy after 15 h in CD2Cl2.
The formation of complex 5, likely formed by chloride
abstraction from CD2Cl2, can be circumvented by addition of a
proton source to the reaction mixture. Thus, addition of 1 equiv
of H(acac) to 2 in CH2Cl2 results in the formation of N2O,
H2O and [Ni(acac)(bipy)2][PF6] (6) (Scheme 2). Complex 6
can be isolated from CH2Cl2/hexanes in 74% yield. The
generation of N2O in the reaction was confirmed by solution IR
spectroscopy and gas chromatography (56% yield when the
reaction is performed in pyridine). H2O is also generated, as
detected by 1H NMR spectroscopy [δ = 1.54 ppm, 86%
Figure 2. ORTEP drawing of [Ni(κ2-O2N2)(bipy)] (8) shown with
50% thermal ellipsoids. Hydrogen atoms have been omitted for clarity.
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comparison, (PPh3)2Pt(cis-O2N2) exhibits similar O−N
[1.39(5) and 1.32(3) Å] and NN [1.21(5) Å] bond
lengths.34 Similarly, (dppf)Ni(cis-O2N2) (dppf =1,1′-bis-
(diphenylphosphanyl)ferrocene) exhibits O−N bond lengths
of 1.400(6) and 1.385(5) Å, and an NN bond length of
1.236(6) Å.25,33 In contrast, (TpRu)2(μ-Cl)(μ-pz){μ−κ2-N,N-
N2O2}, which features a neutral N2O2 ligand, exhibits a much
longer N−N bond length [1.861(3) Å] and shorter N−O bond
distances [1.197(3) and 1.193(3) Å].22
Complex 8 is very sparingly soluble in CH2Cl2, MeCN, and
DMF. Once in solution it slowly decomposes, forming an
intractable mixture over the course of several days. In contrast,
8 rapidly reacts with 2 equiv of H(acac) in CD2Cl2 to cleanly
generate 7 and N2O (Scheme 2), demonstrating its
intermediacy in the formation of N2O from 2.
Thus, an overall picture of N2O formation emerges in which
two molecules of complex 2 rearrange to generate 8 and 4.
Complex 8 then rapidly reacts with H(acac) to induces N−O
bond cleavage with concomitant N2O generation. Finally,
ligand rearrangement occurs between [Ni(bipy)3][PF6]2 (4)
and [Ni(acac)2(bipy)] (7), generating complex 6 as the final
product (Scheme 2).
Figure 3. Space filling diagrams of the cationic part of complexes 2
(left) and 3 (right) rendered using Pov-ray.
from [N2O2]2−. Previously, Bohle and co-workers predicted
that N2O evolution occurs spontaneously with concomitant
formation of a Lewis basic “MO” intermediate.24,33 In
contrast, Cenini and co-workers reported that N2O release
occurs upon acidification of (PPh3)2Pt(O2N2).35 Other systems
also require protonation of [N2O2]2− to induce N2O
release.22,29,31,35 Ultimately, understanding the chemical behav-
ior of the [N2O2]2− ligand will allow more accurate predictions
of the mechanisms of both heterogeneous and enzymatic
systems.
ASSOCIATED CONTENT
* Supporting Information
■
Several questions remain concerning the critical N−N bond
forming step that results in hyponitrite formation. This step
likely follows one of three pathways: (A) coupling of two NO−
ligands via a bimetallic intermediate;19 (B) coupling of a NO−
ligand with NO gas (in this mechanism, rupture of the Ni−NO
bond in 2 or 3 generates the required NO•);34,51 or (C)
generation of HNO by protonation of the nitrosyl ligand in 2.
HNO then disproportionates via a metal-free dimerization.52
We can rule out pathway C, as addition of H(acac) to 3 does
not result in any reaction. We also disfavor pathway B. For this
mechanism to be operative, NO• has to be generated in situ by
homolytic cleavage of a Ni−NO bond. Spontaneous NO
dissociation from an {MNO}10 complex is extremely rare.17
Additionally, we expect that Ni−NO bond cleavage would also
occur in 3, given the similarity of the electronic structures of 2
and 3, according to IR and UV−vis spectroscopies; yet 3 is
stable in solution. Nonetheless, to test the plausibility of this
mechanism, we monitored the reactions of 2 and 3 with added
NO• directly. When NO• gas is added to CH2Cl2 solutions of
2, a rapid reaction does occur. N2O is detected by solution IR
S
Experimental procedures, crystallographic details (as CIF files)
and spectral data for 1−8. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
hayton@chem.ucsb.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the University of California, Santa Barbara, for
financial support of this work. We also thank Prof. Peter Ford,
Dr. Julie Heinecke, and Agustin Pierri for help with gas
chromatography measurements.
REFERENCES
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