Reactions of organozinc thiolates with nitrosonium ion: C -nitroso formation by possible transnitrosation

Article abstract of DOI:10.1021/om300922m

The organometallic complexes [ZnPAThEt] and [ZnPAThMes], where PATh is 2-methyl-1-[methyl-(2-pyridin-2-yl-ethyl)amino]propane-2-thiolate, were prepared and their reactions with NOBF₄ investigated. Formation of C-nitrosoethane and C-nitrosomesitylene, respectively, was established, and structural characterization of the latter by X-ray crystallography conclusively proved the dimeric nature of [MesNO]₂ in the solid state. A transnitrosation pathway for C-nitroso formation is proposed based on theoretical calculations.

Full text of DOI:10.1021/om300922m

Communication
pubs.acs.org/Organometallics
Reactions of Organozinc Thiolates with Nitrosonium Ion: C‑Nitroso
Formation by Possible Transnitrosation
Julia Kozhukh,† Juliana Fedoce Lopes,†,§ Helio F. Dos Santos,‡ and Stephen J. Lippard*,†
́
†Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
‡Nucleo de Estudos em Química Computacional, Departamento de Química-ICE, Universidade Federal de Juiz de Fora, Campus
́
Universitar
́
io, 36036-330, Juiz de Fora-MG, Brazil
S
* Supporting Information
ABSTRACT: The organometallic complexes [ZnPAThEt] and [ZnPAThMes], where PATh is 2-methyl-1-[methyl-(2-pyridin-
2-yl-ethyl)amino]propane-2-thiolate, were prepared and their reactions with NOBF4 investigated. Formation of C-nitrosoethane
and C-nitrosomesitylene, respectively, was established, and structural characterization of the latter by X-ray crystallography
conclusively proved the dimeric nature of [MesNO]2 in the solid state. A transnitrosation pathway for C-nitroso formation is
proposed based on theoretical calculations.
he importance of nitric oxide to the function of the
immune, nervous, and circulatory systems has spurred
PAThNO and APAThNO in the presence of Zn2+ (Chart 1,
e and f). Building on this work, we were interested to extend
our studies to include organometallic analogues, specifically
[ZnPAThEt] and [ZnPAThMes] (Chart 1, c and d), and in this
report we describe their chemistry with NOBF4.
T
interest in its chemistry1 and that of its organic derivatives,
including S-nitrosothiols,2 organic nitrites, N-nitrosamines, and
C-nitroso species.3 Recently, we described the reactivity of
three zinc thiolate complexes (Chart 1, a and b) toward a
Earlier reports and reviews6 describe reactions of nitro-
sonium ions with organometallic complexes of mercury,7
magnesium,8 tin,9 cadmium,10 lithium,11 thallium,12 silicon,13
aluminum,14 molybdenum,15 and chromium.16 The coordina-
tion chemistry of C-nitroso species has been explored,17 and
reactions of alkylzinc complexes with nitric oxide have been
exploited for nitrogen doping of ZnO films.18 Additionally, the
chemistry of organozinc complexes with dioxygen, alcohols, and
water has been examined,19 as well as with alkyl nitro and
nitroso species.20 A search of the literature did not uncover any
study of the chemistry of organozinc compounds with
nitrosonium ion, however, which contributed in part to the
motivation for studies undertaken here.
Chart 1. N2S-Coordinated Zn2+ Complexes (a) [ZnPAThX]
(X = Cl, OTf), (b) [ZnAPAThCl], (c) [ZnPAThEt], (d)
[ZnPAThMes], and the S-Nitrosothiols PAThNO (e) and
APAThNO (f)
Exposure of the protonated PAThH21 ligand to ZnEt2 and
ZnMes2 afforded the desired organometallic species
[ZnPAThEt] and [ZnPAThMes], respectively, in satisfactory
yields. From X-ray-quality crystals the structures in Figure S1
(Supporting Information) were determined. The bond
distances and angles in [ZnPAThEt] and [ZnPAThMes] are
comparable to those reported for [ZnPAThMe].22
Addition of [ZnPAThEt] to 1 equiv of NOBF4 resulted in
little change in the optical spectrum (Figure S2, Supporting
Information). Similarly, little optical change occurred upon
variety of reactive nitrogen and oxygen species, including NO.4
We demonstrated S-nitrosation of the ligand upon exposure of
[ZnPAThCl], [ZnPAThOTf], and [ZnAPAThCl] to NO2(g),
NOBF4, or NO(g) and air but not to anaerobic NO(g).
Analogous results had been reported for Zn2+ complexes
containing monodentate thiolates.5 We also examined the
behavior of the independently synthesized S-nitrosothiols
Received: October 4, 2012
Published: November 12, 2012
© 2012 American Chemical Society
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Communication
exposure of [ZnPAThMes] to 1 equiv of NOBF4 (Figure S2).
The headspace of the [ZnPAThEt]/NOBF4 reaction mixture,
analyzed by EI-MS, contained gaseous NO, butane, and NO2
(Figure S3, Supporting Information) and indicated EtNO
formation. EtNO is unstable and prone to decomposition along
the pathway outlined in Scheme S1.23 The presence of these
gases suggests radical pathways for decomposition of
[ZnPAThEt] in the presence of nitrosonium ion.
Difficulties in characterizing the nongaseous products of the
[ZnPAThEt]/NOBF4 reaction were overcome by examination
of the related [ZnPAThMes]/NOBF4 system. In this case, the
C-nitrosomesitylene dimer, [MesNO]2, could be isolated
following treatment of [ZnPAThMes] with NOBF4 in
acetonitrile, quenching with water, and extraction into diethyl
ether (Figure 1), a procedure that led to the formation of X-ray
diffraction quality crystals.
recovery of [MesNO]2 from the direct nitrosation of mesitylene
with NOBF4.26 The comparable yield and shorter reaction time
for the [ZnPAThMes]/NOBF4 chemistry compared to the
direct nitrosation of mesitylene by nitrosonium ion lead us to
conclude that nitrosation is facilitated by mesitylene binding to
Zn2+.
Long-standing discussions in the literature regarding the
structural properties of C-nitrosomesitylene in the solid state27
are resolved with our determination of the [MesNO]2 crystal
structure. Historically, differences in color between C-nitroso
compounds in solution and in the solid state generated interest
in such species. The differences in color were attributed to
monomer/dimer equilibria, which drew attention to the
structural properties. In 1924, Ingold and Piggott proposed a
ring structure for [MesNO]2.28 This hypothesis was revised in
the 1930s to describe [MesNO]2 as the N−N bonded dimer
that we have crystallographically characterized in the present
work.27 Fenimore structurally characterized tribromonitroso-
benzene in 1950, and he simultaneously reported the unit cell
parameters of C-nitrosomesitylene.29 He found that tribromo-
nitrosobenzene exists as a dimer in the solid state, with the
individual monomer units bound together through their
nitrogen atoms. No interaction between the oxygen atoms of
the monomers existed. In 1956, Nakamoto and Rundle used IR
spectroscopy to conclude that nitrosomesitylene is a dimer in
the solid state.30 The structural characteristics of [MesNO]2
described in this work conclusively confirm the dimeric
structure of C-nitrosomesitylene in the solid state.
Figure 1. Thermal ellipsoid plot of the C-nitrosomesitylene dimer,
shown at 50% probability. The major component of the disordered
molecule is shown. Hydrogen atoms have been omitted for clarity.
In pursuit of the zinc-containing products of the
[ZnPAThMes]/NOBF4 reaction, we combined a stoichiomet-
ric quantity of NOBF4 with [ZnPAThMes] in a 2:1
MeCN:CH2Cl2 solvent mixture. Subsequent workup led to
the isolation of crystalline [ZnPAThCl] (Figure S5, Supporting
Information). This complex crystallized with a space group and
unit cell parameters comparable to previously reported values.31
C-Nitrosomesitylene exists as a dimer in the solid state, with
the two monomer units linked by an N−N bond (Figure 1).
The distance between these symmetry-equivalent nitrogen
atoms is 1.3248(15) Å. This value matches the 1.32 Å average
N−N bond distance in dimeric C-nitrosoaryl compounds in the
Cambridge Structural Database (CSD)24 and is comparable to
the 1.33 Å N−N average bond distance of C-nitrosoaryl species
tabulated in reviewed work.25 The monomer units are
crystallographically related by an inversion center, located at
the center of the N−N bond. The planar N(O)N(O) moiety in
[MesNO]2 subtends an angle of 73.70° with respect to the
plane of the mesityl groups. This nearly perpendicular
orientation prevents unfavorable steric interactions of the
N(O)N(O) fragment with the methyl groups of mesitylene.
The nitrogen atom of the nitroso group has an idealized
trigonal environment, with angles of 121.44(15), 119.82(14),
and 118.71(9)° at nitrogen. The N1−C1 and N1−O1 distances
are 1.4615(12) and 1.263(3) Å, respectively, and are
comparable to values of 1.44 and 1.26 Å for the N−C and
N−O average bond lengths in C-nitrosoaryl species.24,25 The C-
nitrosomesitylene dimer exhibits whole molecule disorder in
the crystal lattice (Figure S4, Supporting Information), with the
mesitylene moiety of the minor component tilting at an angle
of 10.78° relative to the mesitylene moiety of the major
component. The minor component accounts for ∼15% of the
electron density. The atomic displacement parameters for the
atoms of the two components of the disorder were constrained
to be equivalent in order to facilitate refinement of the
structure.
A
1H NMR spectroscopic experiment revealed complete
conversion of the coordinated mesitylene anion to C-nitro-
somesitylene and ∼60% conversion to [ZnPAThCl] (Figure
S6, Supporting Information). The presence of additional
species in the reaction mixture precluded kinetic analyses and
mechanistic studies of the [ZnPAThMes]/NOBF4 reaction.
We hypothesize that the nitrosation reaction takes place as
outlined in Scheme S2, in which formation of [MesNO]2
generates an unstable [ZnPATh]+ cation that abstracts chloride
from methylene chloride. The presence of the poorly
coordinating tetrafluoroborate anion appears inadequate to
stabilize [ZnPATh]+. Similarly, previous attempts to recrystal-
lize the triflate salt [ZnPAThOTf] from methylene chloride
solutions resulted in isolation of [ZnPAThCl].32
Our prior work on the interaction of S-nitrosothiols with
Zn2+ complexes4 prompted us to consider the possibility that
C-nitroso compounds might form in the reaction of
[ZnPAThR] (R = Et, Mes) with NO+ through transient S-
nitrosothiol formation. In particular, we wondered whether
NO+ attack on [ZnPAThR] (R = Et, Mes) might initially occur
at the sulfur atom of the PATh ligand, with subsequent C-
nitroso formation following transnitrosation chemistry.
We used theory to probe aspects of the nitrosation reaction.
In particular, we were interested to determine whether the
initial site of nitrosation in the organometallic [ZnPAThR]
complexes would be the metal-bound carbon atom or at sulfur,
with C-nitroso formation occurring following transnitrosation.
Theoretical calculations were performed to examine the atomic
The C-nitrosomesitylene dimer, [MesNO]2, was isolated in
63% yield from the [ZnPAThMes]/NOBF4 mixture after 2 h of
reaction time. This yield is comparable to the reported 78%
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Communication
orbital contribution to the HOMO of [ZnPAThEt] (Figure 2
and Figures S7 and S8 in the Supporting Information).
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental methods; thermal ellipsoid plots, crystallographic
tables, and an analysis of structural properties for [ZnPAThEt]
and [ZnPAThMes]; optical spectra of [ZnPAThEt] and
[ZnPAThMes] reactions with NOBF4; EI-MS data for the
analysis of the headspace of the [ZnPAThEt]/NOBF4 reaction;
a depiction of thermal ellipsoid plots at 50% probability for
[MesNO]2, showing whole molecule disorder; crystallographic
tables for [MesNO]2; a thermal ellipsoid plot of [ZnPAThCl];
1H NMR spectra for the spectroscopic yield experiment for the
[ZnPAThMes]/NOBF4 reaction; a discussion of the formation
and decomposition of EtNO; a proposed reaction pathway for
the nitrosation of [ZnPAThMes]; a summary of computational
results; a comparison of the HOMOs for the S,R and R,S
configurations of [ZnPAThEt]; tables comparing experimen-
tally and computationally determined structural parameters; a
comparison of the effects of solvation on charge distribution
and the composition of the HOMO of [ZnPAThEt]; an
intrinsic reaction coordinate describing the interaction of
[ZnPAThEt] with NO+; and XYZ matrices for all computa-
tionally determined species. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 2. Highest occupied molecular orbital for [ZnPAThEt],
calculated in the gas phase and visualized at an electron density
isovalue of 0.06.
Comparison of natural atomic charges for [ZnPAThEt]
reveals that partial negative charge density accumulates at both
the sulfur and coordinated carbon atoms (Table S8). The
contribution of the carbon atom to the HOMO increases with
increasing solvent polarity (Figure S8), whereas the sulfur atom
consistently makes a significant contribution to the HOMO,
regardless of solvation. It is possible that C-nitroso formation
might occur via transnitrosation following initial NO+ attack on
the sulfur atom of [ZnPAThR] (R = Et, Mes) to form an S-
nitrosothiol, which then transfers NO+ to a coordinated carbon
to generate the C-nitroso products (Scheme 1). Such a reaction
pathway is supported by transition state and intrinsic reaction
coordinate (IRC) theoretical calculations (Figure S9). A
transition state for the attack of NO+ at the sulfur atom of
[ZnPAThEt] was characterized by theoretical methods, and
IRC analysis revealed that the formation of zinc-coordinated S-
nitrosothiol constitutes a feasible reaction pathway. Further
experimental studies are required to investigate this possibility.
To summarize, exposure of [ZnPAThEt] to NOBF4 led to
formation of EtNO. When the analogous NOBF4 reaction was
performed with [ZnPAThMes], we were able to to trap C-
nitrosomesitylene as a stable product, which was isolated and
characterized as a dimer in the solid state. When the
[ZnPAThMes]/NOBF4 reaction was performed in a 2:1
mixture of CH3CN:CH2Cl2, the zinc-containing product
isolated was [ZnPAThCl]. This result suggests that chloride
abstraction may be effected by a transiently formed [ZnPATh]+
cation. Theoretical calculations were performed to analyze the
character of the [ZnPAThEt] HOMO. The results of these
calculations suggest possible attack by NO+ on [ZnPAThR] (R
= Et, Mes) at sulfur, with C-nitroso formation by subsequent
transnitrosation.
AUTHOR INFORMATION
Present Address
■
§
́
́
Departamento de Fısica e Quımica, Instituto de Cien
̂
cias
Exatas, Universidade Federal de Itajuba-
Itajuba, MG, Brazil.
Notes
́
UNIFEI, 37500-903
́
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Dr. Yogesh Surendranath for technical assistance
with EI-MS studies, Dr. Peter Muller for assistance with X-ray
■
̈
crystallography, and Dr. Joel Rosenthal and Mr. Justin Wilson
for valuable advice and discussion. This work was supported by
National Science Foundation Grant CHE0907905 (S.J.L.), the
CAPES-BRAZIL postdoctoral fellowship (process 4470-08-7)
(J.F.L.), the CNPq-BRAZIL and FAPEMIG-BRAZIL grants
(H.F.D.S.), and National Institute of Health Grant
1S10RR013886-01, awarded to the MIT Department of
Chemistry Instrumentation Facility.
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Scheme 1. Possible Reaction Pathway for C-Nitroso Formation via Inter- or Intramolecular Transnitrosation in the Reaction of
[ZnPAThR] (R = Et, Mes) with NO+
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