Mild N-O Bond Cleavage Reactions of a Pyramidalized Nitrosyl Ligand Bridging a Dimolybdenum Center

Article abstract of DOI:10.1021/acs.inorgchem.5b02292

Complex [Mo₂Cp₂(μ-PCy₂)(μ-NO)(NO)₂] (1) was prepared by reacting [Mo₂Cp₂(μ-H)(μ-PCy₂)(CO)₄] with 2 equiv of [NO]BF₄ and then treating the resulting product

Full text of DOI:10.1021/acs.inorgchem.5b02292

Communication
pubs.acs.org/IC
Mild N−O Bond Cleavage Reactions of a Pyramidalized Nitrosyl
Ligand Bridging a Dimolybdenum Center
M. Angeles Alvarez, M. Esther García, Daniel García-Vivo, Miguel A. Ruiz,* and Adrian Toyos
́ ́
́ ́
Departamento de Química Organica e Inorganica/IUQOEM, Universidad de Oviedo, E-33071 Oviedo, Spain
*
S Supporting Information
explore in more detail its structure and reactivity. As shown
below, compound 1 displays a bridging nitrosyl ligand with
significant pyramidalization at the N atom, a circumstance that
seems to be related to an unusual chemical behavior involving,
inter alia, easy activation and cleavage of its N−O bond under
mild conditions, with this, in turn, providing access to complexes
having new ligands or new coordination modes of nitrosyl-
derived ligands.
ABSTRACT: Complex [Mo Cp (μ-PCy )(μ-NO)-
2
2
2
(
NO) ] (1) was prepared by reacting [Mo Cp (μ-H)(μ-
2
2
2
PCy )(CO) ] with 2 equiv of [NO]BF and then treating
2
4
4
the resulting product [Mo Cp (μ-PCy )(CO) (NO) ]-
2
2
2
2
2
(
BF ) with NaNO at 323 K, and it was shown to display
4 2
a bridging nitrosyl ligand with significant pyramidalization
at the N atom, a circumstance related to an unusual
behavior concerning degradation of the bridging nitrosyl.
Compound 1 can be conveniently prepared via a two-step
Indeed, complex 1 reacts with HBF ·OEt to give the
4
2
procedure involving the reaction of complex [Mo Cp (μ-H)(μ-
2
2
1
2
8
nitroxyl-bridged derivative [Mo Cp (μ-PCy )(μ-κ :η -
2
2
2
PCy )(CO) ] with 2 equiv of [NO]BF in CH Cl , in the
2 4 4 2 2
HNO)(NO) ](BF ), is reduced by Zn(Hg) in the
2
4
presence of Na CO to remove the hydride ligand. This gives the
2 3
presence of trace H O to give the amido complex
2
cationic dinitrosyl [Mo Cp (μ-PCy )(CO) (NO) ](BF ) (2),
2 2 2 2 2 4
[
Mo Cp (μ-PCy )(μ-NH )(NO) ], and reacts with excess
2 2 2 2 2
which is then decarbonylated upon reaction with NaNO in
2
P(OPh) to give the phosphoraniminato-bridged deriva-
3
warm tetrahydrofuran (THF; 323 K), to render the trinitrosyl 1
tive [Mo Cp (μ-PCy ){μ-NP(OPh) }(NO) ].
9
2
2
2
3
2
with 63% overall yield (Scheme 1).
Scheme 1
he chemistry of metal nitrosyl complexes is a subject of
T
interest not only because of the great versatility of the nitric
1
oxide (NO) molecule as a ligand but also because the latter
molecule has relevant activity in living organisms associated with
1
,2
its interaction with different metal centers, while at the same
time being a major atmospheric pollutant requiring catalytic
abatement, a process also relying on the interaction of NO with
1
,3,4
metal atoms.
The latter has been much studied over the last
decades, mostly on heterogeneous systems that typically catalyze
the reduction (with CO, NH , hydrocarbons, etc.) or
3
decomposition of NO. Yet, the quest for more efficient, cheaper,
4
and durable catalysts continues. In this context, finding new
ways of activation and cleavage of the strong N−O bond of the
NO molecule when bound to metal centers remains a target
worthy of attention.
Recently, we isolated and characterized spectroscopically the
1
trinitrosyl complex [Mo Cp (μ-PCy )(μ-NO)(NO) ] (1), a
Dark-blue crystals of 1· / CH Cl were grown from CH Cl
2
2
2
2
2
2
2
2
2
side product (9%) formed in the reaction of the unsaturated
solutions of the complex. As anticipated from spectroscopic data,
the molecule of 1 is built from two MoCp(NO) moieties
arranged in a transoid disposition, connected through a single
5
compound [Mo Cp (μ-CH Ph)(μ-PCy )(CO) ] with NO.
2
2
2
2
2
Compound 1 is a 34-electron complex for which a single
metal−metal bond should be formulated and is devoid of any
particularly chromogenic ligand; therefore, it is expected to
display a color in the red-to-yellow range, as is the case of the
metal−metal bond [2.8935(3) Å], and bridging PCy
and
2
nitrosyl ligands (Figure 1). The latter groups define a somewhat
puckered Mo PN central skeleton (P−Mo−Mo−N ca. 164°) so
2
6
the terminal nitrosyls are not strictly antiparalell. However, the
salient feature in this structure concerns the bridging nitrosyl
ligand, which exhibits significant pyramidalization at the N atom
isoelectronic dinitrosyl complexes [W Cp (μ-PPh ) (NO) ]
2
2
2 2
2
7
and related molecules. Surprisingly, however, compound 1 is
dark blue both in solution and in the solid state. We wondered
whether such an unexpected color originated from an
unanticipated structural feature and whether this circumstance,
in turn, could lead to unexpected chemical properties. Thus, we
sought a high-yield route to this trinitrosyl complex, so we could
(
average Mo−Mo−N−O ca. 163.5°), instead of the expected
Received: October 5, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02292
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
the cation of a nitroxyl ligand bridging two metal atoms in an
1
2
alkenyl-like, κ :η fashion (Figure 2), resulting in a substantial
Figure 1. ORTEP diagram (30% probability) of compound 1 (left) with
1
H atoms and Cy groups (except their C atoms) omitted and its
projection along the Mo−Mo bond (right). Selected bond lengths (Å)
and angles (deg): Mo1−Mo2 = 2.8935(3); Mo1−N1 = 1.784(2);
Mo1−N3 = 2.031(2); Mo2−N2 = 1.798(2); Mo2−N3 = 2.018(2); N−
O = 1.227(3). Mo1−Mo2−N2 = 88.9(1); Mo2−Mo1−N1 = 105.4(1).
Figure 2. ORTEP diagram (30% probability) of the cation in compound
1
3
with most H atoms and Cy groups (except their C atoms) omitted.
Selected bond lengths (Å) and angles (deg): Mo1−Mo2 = 2.9995(5);
Mo1−N3 = 2.007(4); Mo2−N3 = 2.193(5); Mo2−O3 = 2.092(4);
N3−O3 = 1.348(6).
trigonal-planar environment. A search at the Cambridge
Structural Database revealed that only a few other complexes
have been found previously with such a distortion (five examples
10
weakening of the N−O bond, as judged from the corresponding
length of 1.348(6) Å, significantly longer than the values of ca.
with average M−M−N−O < 165°), but no attention seems to
have been paid to it.
1
.20 Å found in mononuclear complexes bearing this ligand N-
To rule out crystal forces as a possible origin of the distorted
geometry of 1, we carried out density functional theory (DFT)
14
bound to a single metal atom. We note that no nitroxyl-bridged
complexes have been characterized previously.
11
calculations on the isolated molecule and found a geometry
very similar to the one in the crystal (Mo−Mo−N−O = 170.6°
for the bridging NO). Interestingly, a structure with a bridging
nitrosyl forced into a planar environment around the N atom
Surprisingly, the electron-rich complex 1 is easily reduced in a
number of ways, all of them involving the cleavage of the N−O
bond of the bridging nitrosyl. For instance, reaction with Zn(Hg)
proceeds smoothly at 293 K in THF to give the amido derivative
(
1F) was computed to have a nearly flat Mo PN central skeleton
9
2
[
Mo Cp (μ-PCy )(μ-NH )(NO) ] (4). Although 4 is isoelec-
2
2
2
2
2
and to be some 12 kJ/mol less stable (see the Supporting
Information). Because the structures of several isoelectronic
tronic with 1, it displays a bright-yellow color (rather than blue),
so a significant structural difference between these two molecules
must exist. Indeed, an X-ray study revealed that the molecule of 4
is made up from two transoid MoCp(NO) fragments bridged by
PCy and NH ligands that now define an almost planar Mo PN
derivatives of 1 to be discussed below display flat Mo PN central
2
skeletons, we conclude that the structural distortion in 1 has an
electronic (rather than steric) origin. Analysis of the atomic
charges indicates that the distorted structure concentrates a
slightly higher negative charge at the pyramidalized N atom
compared to the undistorted structure 1F. Thus, we might view
the nitrosyl distortion of 1 as one dissipating some electron
density from a relatively electron-rich dimetal center. Taken to its
extreme, such a distortion should end up with a N atom bearing a
lone electron pair and effectively contributing with two less
electrons to the dimetal center. This sort of ambivalence, well-
2
2
2
central core (P−Mo−Mo−N = 177.3°; Figure S1). As a result,
the terminal nitrosyls are now antiparallel, and the intermetallic
length is a bit shorter [2.8654(8) Å].
The N-bound H atoms in 4 likely come from trace water
present in the solvent because the on-purpose addition of H O to
2
9
the reaction solvent increases the yield of 4. A related NO-to-
NH transformation has been previously reported for the
2
1a,12
dichromium complex [Cr Cp (μ-NO) (NO) ], but this re-
2 2 2 2
known for terminal nitrosyl ligands,
seems to be unreported
quired the use of strong hydride donors and was of poor
for bridging nitrosyls but is well established in the chemistry of
bridging phosphinidene ligands (Chart 1).
15
selectivity. More interestingly, we found that 4 also was formed
slowly as the unique organometallic product upon reaction of 1
with CO (40 atm) in toluene at 353 K, thus suggesting that even
very mild reducing agents might trigger an N−O cleavage at the
bridging nitrosyl. Prompted by this observation, we then
examined reactions with P donors and found that 1 is reactive
toward several phosphites. For instance, its reaction with a 10-
Chart 1
fold excess of P(OPh) in refluxing toluene was completed in 6 h
3
to give the phosphoraniminato-bridged derivative [Mo Cp (μ-
2
2
9
PCy ){μ-NP(OPh) }(NO) ] (5) as a major product, along
2
3
2
with smaller amounts of 4, the latter obviously arising from a side
reaction with trace water in the medium. Indeed, the on-purpose
addition of water to the solvent increased the relative amount of
4, although it did not suppress the formation of 5. Because water
alone did not react with 1 in refluxing toluene, it has to be
concluded that the phosphite acts as an oxygen acceptor in this
The chemical behavior of 1 reveals considerable nucleophil-
icity at the N atom of the bridging NO. Indeed, 1 is readily
protonated at this site upon reaction with HBF ·OEt in CH Cl
2
4
2
2
solution to give the nitroxyl-bridged derivative [Mo Cp (μ-
reaction [indeed P(O)(OPh) is present in the final reaction
2
2
3
1
2
9
PCy )(μ-κ :η -HNO)(NO) ](BF ) (3), instead of the hydrox-
mixture] to give an undetected nitrido-bridged intermediate that
would add a second phosphite molecule to build the
phosphoraniminato complex 5. To our knowledge, related
2
2
4
imido complex that would have been expected from protonation
1
a,13
at the O site.
An X-ray study of 3 confirmed the presence in
B
DOI: 10.1021/acs.inorgchem.5b02292
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
nitrosyl transformations have only been described so far in
reactions of mononuclear complexes with phosphines.
of the Universities of Oviedo and Santiago de Compostela for
acquisition of diffraction data.
1a
An X-ray study of 5 (Figure 3) confirmed the presence of a
phosphoraniminato ligand [P−N = 1.515(3) Å] symmetrically
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(
Figure 3. ORTEP diagram (30% probability) of compound 5 with H
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1
1
bond lengths (Å) and angles (deg): Mo1−Mo2 = 2.8778(4); Mo1−N3
=
=
2.131(3); Mo2−N3 = 2.104(3); N3−P2 = 1.515(3); Mo1−Mo2−N2
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2
(
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3
(
́ ́
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substituents. Recently, a mononuclear iron(IV) nitride complex
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(
1
16
not actually isolated.
(
́
z, D.
In summary, we have shown that compound 1 displays a
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N atom, likely to drain some electron density away from the
dimetal center, and this increases the basicity of this ligand at the
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(
1
̈
(
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02292.
package using the hybrid method B3LYP, along with the standard 6-
31G* basis set on all atoms except Mo, for which a valence double-ζ-
quality basis set and LANL2DZ effective core potentials were used. See
the SI for further details.
■
*
S
(
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data for compounds 1 and 3−5 (CIF)
AUTHOR INFORMATION
Corresponding Author
E-mail: mara@uniovi.es.
■
(15) (a) Ball, R. G.; Hames, B. W.; Legzdins, P.; Trotter, J. Inorg. Chem.
1
980, 19, 3626. (b) Hames, B. W.; Legzdins, P.; Oxley, C. Inorg. Chem.
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*
(
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the DGI of Spain for financial support (Project
CTQ2012-33187) and the Consejeria de Educacion
for a grant (to A.T.). We also thank the CMC of the Universidad
de Oviedo for access to computing facilities, and the X-ray units
■
́
́
de Asturias
C
DOI: 10.1021/acs.inorgchem.5b02292
Inorg. Chem. XXXX, XXX, XXX−XXX