Reduction of copper(II) complexes of tripodal ligands by nitric oxide and trinitrosation of the ligands

Article abstract of DOI:10.1021/ja102279c

The copper(II) centers in two copper(II) complexes of tripodal amine ligands, in acetonitrile solvent, upon exposure to nitric oxide have been found to produce a thermally unstable [Cu^II-NO] intermediate followed by the reduction of copper(II)

Full text of DOI:10.1021/ja102279c

Published on Web 05/21/2010
Reduction of Copper(II) Complexes of Tripodal Ligands by Nitric Oxide and
Trinitrosation of the Ligands
Moushumi Sarma, Apurba Kalita, Pankaj Kumar, Amardeep Singh, and Biplab Mondal*
Department of Chemistry, Indian Institute of Technology-Guwahati, Guwahati, Assam 781039, India
Received March 19, 2010; E-mail: biplab@iitg.ernet.in
Nitric oxide plays key roles in mammalian biology, such as in
vascular regulation, neurotransmission, and immunocytotoxicity,
and some of these activities are attributed to the formation of
nitrosyl complexes of metalloproteins.1,2 Hence, the interaction of
nitric oxide with metal centers has long been of interest to chemists
and biochemists.3 The reduction of Cu(II) centers to Cu(I) in some
proteins, such as cytochrome c oxidase and laccase, upon exposure
to nitric oxide has also been known for a long time.4 In cytochrome
c oxidase, the NO reduction of Cu(II) to Cu(I) is believed to play
a role in regulating the electron transport activity of this protein.4a,b,5
Cu(II) is also known to facilitate the nitrosation of various thiolates,
and this reduction has been found to correlate with formation of
S-nitroso bovine serum albumin and S-nitroso glutathione.6 These
observations have been used to suggest a mechanism for the
formation of RSNO compounds in blood.7 Although, the autore-
duction of ferriheme proteins such as methemoglobin and ferricy-
tochrome c (CytIII) by nitric oxide has been studied extensively,3,4
the Cu(II) reduction has not been studied to the same extent.3
plexes were recorded in acetonitrile solvent at 77 K. Both of the
complexes exhibited characteristic four-line axial spectra (see the
SI).8
Upon exposure to nitric oxide gas, deep-blue solutions of 1 and
2 in dry, degassed acetonitrile produced thermally unstable
intermediates with shifts of λmax to 640 and 605 nm, respectively
(see the SI and Figure 2).
Figure 2. (a) UV-vis spectroscopic monitoring of the formation of a
[CuII-NO] intermediate and its gradual decomposition to CuI species in
the case of complex 2. (b) Time scan plot at 605 nm in the case of complex
2 at 298 K.
In this context, here we report two examples where tripodal
ligands coordinated to Cu(II) undergo nitrosation to the corre-
sponding N-nitroso amines in the process of reduction of Cu(II) to
Cu(I) by nitric oxide.
EPR studies of the frozen solutions (77 K) of the intermediates
revealed that these are EPR-silent.9 Hence, it is logical to believe
that in both the cases, [CuII-NO] intermediates were formed
(Scheme 1). These intermediates gradually decomposed to afford
colorless solutions following first-order kinetics, and the spectral
changes were monitored by UV-vis spectrophotometry (Figure 2).
The rate constants at 298 K for the decomposition of the
intermediates were found to be 5.64 × 10-2 and 6.45 × 10-3 s-1
for complexes 1 and 2, respectively. Though both of the complexes
in acetonitrile solvent showed characteristic axial EPR spectra, the
colorless solutions were observed to be EPR-silent (see the SI).
All of these results are consistent with the reduction of Cu(II) to
Cu(I). In the present case, both complexes presumably form an
unstable Cu(II)-nitrosyl intermediate prior to the reduction of
Cu(II) to Cu(I).
The two Cu(II) complexes 1 and 2 were synthesized using the
tripodal tetraamine ligands L1 and L2 [L1 ) tris(2-isopropylami-
noethyl)amine; L2 ) tris(2-ethylaminoethyl)amine], respectively,
as their perchlorate salts [Figure 1; also see the Supporting
Information (SI)]. The single-crystal X-ray structure of 1 revealed
that Cu(II) is surrounded by five nitrogen donor atoms (four from
L1 and one from coordinated acetonitrile solvent) in a distorted
trigonal bipyramidal geometry (see the SI). The structural index
parameter, τ, was found to be ∼0.6. The three terminal nitrogen
atoms of L1 occupy the equatorial positions, whereas the central
nitrogen of L1 and the nitrogen from coordinated acetonitrile occupy
the axial positions. Complexes 1 and 2 in acetonitrile solvent exhibit
broad d-d bands at λmax (ε/ M-1 cm-1) ) 826 nm (340) and 615
nm (110) (shoulder) for 1 and 620 nm (200) and 820 nm (150)
(shoulder) for 2, along with relatively strong intraligand absorptions
in the UV region (see the SI).
Scheme 1
Figure 1
The complexes show magnetic moments corresponding to one
unpaired electron (µobs ) 1.56 and 1.64µB for 1 and 2, respectively).
The electron paramagnetic resonance (EPR) spectra of the com-
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7846 J. AM. CHEM. SOC. 2010, 132, 7846–7847
10.1021/ja102279c  2010 American Chemical Society

C O M M U N I C A T I O N S
12
It would be worth mentioning here that Cao and co-workers9
reported the reduction of a series of copper(II) dithiocarbamates
with nitric oxide in aqueous solution to form air-stable copper
nitrosyl and dinitrosyl species. Detailed kinetics studies of Cu(II)/
NO reactions are scarce.3,10 In this regard, Tran et al.4c studied
NO reduction of the copper(II) complex Cu(dmp)2(H2O)2+ (dmp
) 2,9-dimethyl-1,10-phenanthroline) in aqueous solution and
various mixed solvents.
It is interesting to note that the nitric oxide reductions of Cu(II)
ion in complexes 1 and 2 in acetonitrile were accompanied by
concomitant nitrosation of the ligands and release of the modified
nitrosoamine ligands L1′ and L2′, respectively (∼30% yield in each
case) (Scheme 1). L1′ was found to precipitate from the reaction
medium as its perchlorate salt. The formation of L1′ perchlorate
was confirmed by single-crystal X-ray structure determination
(Figure 3). The 1H NMR spectra of L1′ perchlorate and L2′ indicate
that the terminal amine nitrogens are the nitrosation sites on both
the cases. The 1446 and 1449 cm-1 bands in the FT-IR spectra of
L1′ perchlorate and L2′, respectively, are consistent with the
expected νNO of nitrosoamine.11 It is important to note that the free
ligands do not react with NO under the reaction conditions.
center, as reported in case of [CuII(DAC)]2+
.
Alternatively, the
key step could be initial NO coordination to the copper ion followed
by NO+ migration to the secondary amine.12 The observation of
the transient intermediates in the UV-vis and EPR spectra prior
to reduction supports the second possibility. However, the reason
for trinitrosation in the present case is not very clear. In comparison
with the other reported results, one could think of the trinitrosation
as a result of the combined effect of the geometry of the complexes,
the presence of electron-donor groups at the terminal amine
positions, and the difference in mechanistic pathways. However,
the presence of some other disproportionation processes facilitated
by the metal center cannot be ruled out.
In conclusion, the nitric oxide reduction of the CuII centers in
complexes 1 and 2 to Cu(I) have been found to result in concomitant
nitrosation at the nitrogen of amine coordination sites. Nitrosation
at all three secondary amine sites was observed in the case of the
present electron-rich amines L1 and L2.
Acknowledgment. The authors thank the Department of Science
and Technology, India; BRNS-YSA for financial support; and DST-
FIST for the X-ray diffraction facility.
Supporting Information Available: Synthetic processes; UV-vis,
FT-IR, 1H NMR, and 13C NMR spectra of complexes 1, 2, L1′
perchlorate, and L2′; spectroscopic monitoring of the formation of
[CuII-NO] intermediates and the reduction of CuII to CuI; and
crystallographic data and CIF files for complex 1 and L1′ perchlorate.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 3. ORTEP diagram of L1′ perchlorate.
Ford and co-workers12 reported their observations on the complex
[CuII(DAC)]2+ {DAC ) the 1,8-bis(9-anthracylmethyl) derivative
of the macrocyclic tetraamine cyclam (1,4,8,11-tetraazacyclotet-
radecane)} in methanol solution. However, the marked difference
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(RSNO) are proposed to react with metalloprophyrins.11,14
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