Coordination of NO2- ligand to Cu(I) ion in an O,O-bidentate fashion that evolves NO gas upon protonation: A model reaction relevant to the denitrification process

Article abstract of DOI:10.1039/b927513d

[(2,6-(Ph₂P(o-C₆H₄)CHN)₂C ₅H₃N)₂Cu₂](BF₄) ₂ (2) has been prepared by treating 2,6-(Ph₂P(o-C ₆H₄)CHN)₂C₅H₃N (1) with [Cu(NCMe)₄]BF₄. Reaction of 2 and [Ph ₃PNPPh₃]NO₂ produces (2,6-(Ph ₂P(o-C₆H₄)CHN)₂C₅H ₃N)Cu(NO₂) (3), with the nitrite ligand in a unique η²-O,O coordination mode. Protonation of 3 releases NO gas, which mimics the reactivity of the Type 2 Cu-NiRs. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b927513d

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ꢀ
Coordination of NO2 ligand to Cu(I) ion in an O,O-bidentate fashion
that evolves NO gas upon protonation: a model reaction relevant to the
denitrification processw
Chi-Shian Chen and Wen-Yann Yeh*
Received 5th January 2010, Accepted 12th March 2010
First published as an Advance Article on the web 30th March 2010
DOI: 10.1039/b927513d
[(2,6-(Ph2P(o-C6H4)CHQN)2C5H3N)2Cu2](BF4)2 (2) has been
prepared by treating 2,6-(Ph2P(o-C6H4)CHQN)2C5H3N (1)
with [Cu(NCMe)4]BF4. Reaction of 2 and [Ph3PNPPh3]NO2
produces (2,6-(Ph2P(o-C6H4)CHQN)2C5H3N)Cu(NO2) (3),
with the nitrite ligand in a unique Z2-O,O coordination mode.
Protonation of 3 releases NO gas, which mimics the reactivity of
the Type 2 Cu-NiRs.
Denitrification forms an important step in the global nitrogen
cycle.1 Copper-containing nitrite reductases (Cu-NiRs)
catalyze the reduction of nitrite to nitric oxide (NO), a key
step in denitrification.1–3 They are found in a wide variety of
denitrifying bacteria and fungi.2 The steps in the mechanism of
ꢀ
nitrite reduction by Cu-NiRs involve the binding of NO2 to
the oxidized enzyme, reduction and dehydration of bound
intermediates, followed by the release of the product NO to
reform the oxidized enzyme.1a The nature of NO2 substrate
ꢀ
Scheme 1
binding in CuNiR has been the focus of a number of
biochemical, spectroscopic, and crystallographic studies.
These studies have been in general agreement that nitrite binds
to the Type 2 Cu ion via oxygen in an asymmetric bidentate
at d 9.77 for the imine protons with JP–H = 5 Hz, and the
31P{1H} NMR spectrum presents a sharp singlet at d ꢀ12.9.
Treatment of 1 with equimolar [Cu(NCMe)4]BF4 in dichloro-
methane at ambient temperature produced an orange–red
solution, from which air-stable, red crystals of the dimetallic
complex [(2,6-(Ph2P(o-C6H4)CHQN)2C5H3N)2Cu2](BF4)2 (2)
were obtained in 70% yield by adding diethyl ether. The ESI
ꢀ
fashion.4,5 Several mononuclear CuII–NO2 complexes with
biomimetic ancillary ligands have been described to have an
Z2-O,O coordination mode,4–9 while a few known mono-
nuclear CuI–NO2 complexes contain an Z1-N bonding
ꢀ
feature.10–13 In contrast, the preparation and chemistry of
Cu(I)–nitrite complexes bearing an O,O-binding mode remain
unexplored. Herein we present the synthesis of a new bis-
(phosphinoimino)pyridine molecule and its coordination to
Cu(I) ions, leading to a novel nitrite reduction reaction.
Schiff base condensation14 of 2,6-diaminopyridine and
2 equiv. of o-(diphenylphosphino)benzaldehyde in refluxing
benzene gave the expected potentially pentadentate molecule
2,6-(Ph2P(o-C6H4)CHQN)2C5H3N (1; 80%) as an air-stable,
yellow solid after crystallization from dichloromethane–
n-hexane (Scheme 1). Compound 1 contains two phosphino-
mass spectrum displays the ion peak at m/z 1519 corresponding
ꢀ +
to the [M ꢀ BF4
]
fragment. The ORTEP drawing for the
cation of 2 (Fig. 1) shows a dimeric [CuI2ꢁ12] structure where
each copper(I) ion is bonded to the phosphine-imine groups
from one molecule of 1 and the phosphine-pyridine groups
from the other.z Most interestingly, the two polydentate
ligands wind the Cu(I) ions in a double-helical fashion. The
two Cu(I) ions are separated by 4.15 A. Coordination about
each Cu(I) ion can be described as distorted tetrahedral, with
the bond angles ranging from 96.2(2)1 to 114.74(6)1 for Cu1
and from 96.3(1)1 to 117.03(6)1 for Cu2. Coordination of
the imine N atom causes slight lengthening of the CHQN
bond distance, such that the N3–C25 (1.287(7) A) and
N6–C68 (1.296(7) A) lengths are ca. 0.02 A longer than the
uncoordinated N4–C62 (1.269(7) A) and N1–C19 (1.267(8) A)
lengths. The 31P{1H} NMR spectrum at ꢀ80 1C presents two
broad signals at d 3.7 and d ꢀ18.3 which collapse at ꢀ50 1C
and merge as a singlet at d ꢀ7.5 above ꢀ30 1C (Fig. 2).
imino groups connected to
a pyridine moiety at the
1
2,6-positions. The H NMR spectrum shows a doublet signal
Department of Chemistry and Center for Nanoscience and
Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804,
Taiwan. E-mail: wenyann@mail.nsysu.edu.tw; Fax: +886-7-5253908;
Tel: +886-7-5252000 Ext. 3927
w Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic data and X-ray crystal files in CIF format
for 2 and 3. CCDC 759228 and 759229. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b927513d
1
Consistently, the H NMR spectrum shows two broad imine
proton resonances at d 8.36 and d 7.69 at ꢀ80 1C, while only
one resonance at d 7.98 is recorded with the spectrum taken at
ꢂc
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Fig. 3 Molecular structure of 3 with 30% probability ellipsoids. The
Fig. 1 Molecular structure for the cationic part of 2 with 30%
probability ellipsoids. The hydrogen atoms have been omitted for
clarity.
hydrogen atoms have been omitted for clarity.
suitable for an X-ray diffraction study were grown from
dichloromethane–diethyl ether at ambient temperature. The
ORTEP diagram for 3 (Fig. 3) shows the pseudotetrahedral
Cu1 ion is linked to two phosphine groups and one nitrite
species.y The Cu–P distances are about equal (Cu1–P1 =
2.262(1) A and Cu1–P2 = 2.260(1) A), and the P1–Cu1–P2
angle (129.86(4)1) is obtuse. The Cu(P–P) macrocycle
generates a small void within the complex. The most striking
feature of 3 is the nitrite ligand which binds the Cu(I) ion in an
asymmetric bindentate fashion (Cu1–O1 = 2.197(4) A and
Cu1–O2 = 2.131(4) A) with the bite angle O1–Cu1–O2 =
50.3(2)1. The Cu1 atom is located slightly away from the
O1–N1–O2 plane by 0.11 A, and the dihedral angle between
the O1–N1–O2 plane and the P1–Cu1–P2 plane is 831. On the
covalent model the O,O-coordinated nitrite is a 4e ligand,
giving rise to an 18-electron Cu(I) complex. Though the C2v
bidentate Z2-O,O isomer is calculated to be the most stable
species for CuNO2,15,16 examples of nitrite O,O-coordinated
to the copper(I) ion are very rare. In the only other analogue,
(PPh3)2Cu(NO2),17 the nitrite ligand is coordinated to Cu(I)
atom symmetrically with Cu–O = 2.191(4) A, while the
O–N–O angle (113.26(2)1) and the N–O distance (1.245(5) A)
are comparable with the free nitrate ion (114.9(5)1 and
1.240(3) A).18 In contrast, the N–O bonds of 3 (N1–O1 =
1.011(6) A and N1–O2 = 1.032(6) A) are unequal and
remarkably shorter, and the O1–N1–O2 angle (128.7(6)1)
becomes flatter. This X-ray structural feature lends support
for a weak interaction between the Cu1 and N1 atoms, which
are separated by 2.40 A, whereas the Cuꢁ ꢁ ꢁN distance is 2.61 A
in (PPh3)2Cu(NO2).
Fig. 2 Variable-temperature 31P{1H} NMR spectra of complex 2 in
CD2Cl2.
Scheme 2
20 1C. This suggests a dynamic exchange between the coordi-
nated and the noncoordinated imine groups in solution
(Scheme 2), and an approximate value of DGz = 8.8 kcal molꢀ1
can be estimated for this process.
A few Cu(II)–nitrite complexes with Z2-O,O coordination
mode of the NO2ꢀ ligand have been characterized,4–8 but these
compounds do not support nitric oxide formation from nitrite.
The Cu(I) ions in 2 are each coordinated by two ‘soft’
phosphorus and two ‘hard’ nitrogen atoms. This feature
makes 2 susceptible to ligand substitution. Thus, reaction
ꢀ
On the other hand, the known mononuclear CuI–NO2
of
2
with two equiv. of [Ph3PNPPh3]NO2 in aceto-
complexes carrying out the denitrification reaction contain
an Z1-N bonding feature.9–12 In order to probe the viability of
complex 3 as a functional NiR model under anaerobic
conditions, a CH2Cl2 solution of 3 was treated with 2.5 equiv.
of glacial acetic acid (HOAc) at ambient temperature under
dinitrogen, resulting in an immediate color change from
yellow to brown. Analysis of the head-space gas by a GC
nitrile solvent resulted in dissociation of the imine-pyridine
ligation to produce the neutral, monomeric complex
(2,6-(Ph2P(o-C6H4)CHQN)2C5H3N)Cu(NO2) (3; 85%) as an
air-sensitive, canary yellow solid. Compound 3 presents one
single imine proton resonance at d 10.62 and one broad
phosphorus resonance at d ꢀ7.43. Single crystals found
ꢂc
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wR2 = 0.1421 (all data), GooF = 1.001, reflections measured = 6361,
Rint = 0.0449.
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Scheme 3
analyzer indicated that NO had been generated (95%). On the
basis of the mechanisms proposed in the literature,19 the
reaction likely proceeds via nitrite protonation and dehydra-
tion to afford a [CuIINO] intermediate, and subsequent release
of NO from this intermediate and coordination of acetates
would give the observed results (Scheme 3). The KBr IR
spectrum of the product 1ꢁCuII(OAc)2 is identical to that
obtained from the reaction of Cu(OAc)2 with 1.
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In summary, we have prepared a potentially pentadentate
ligand 1 and its Cu(I) complexes. The dinuclear complex 2
reveals a double-helical architecture, which is fluxional in
solution. Complex 3 contains a nitrite ligand in a unique
Z2-O,O coordination mode, protonation of which to release
NO gas is of interest within the context of the chemistry of the
biological denitrification process. Our preliminary study
provides evidence that the acetate species of 1ꢁCuII(OAc)2
can be replaced by nitrite ions to generate 1ꢁCuII(NO2)2.
Further investigation of this complex and attempts to reduce
it to reproduce the starting compound 3, which would
complete the denitrification cycle, are in progress in our
laboratory.
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This work was financially supported by the National
Science Council of Taiwan. We thank Mr Ting-Shen Kuo
(National Taiwan Normal University, Taipei) for X-ray
diffraction analysis.
Notes and references
´
15 S. Sklenak and J. Hrusak, J. Chem. Theory Comput., 2006, 2, 997.
16 L. Rodrıguez-Santiago, X. Solans-Monfort, M. Sodupe and
z Crystallographic data for 2ꢁCH2Cl2: C87H68B2Cl2Cu2F8N6P4, M =
´
V. Branchadell, Inorg. Chem., 1998, 37, 4512.
ꢀ
1692.95, triclinic, space group P1, a = 12.8700(2), b = 15.1448(2), c =
23.0511(3) A, a = 98.7830(10)1, b = 93.5300(10)1, g = 101.6710(10)1,
=
0.0899 (I > 2s(I)), wR2 = 0.2540 (all data), GooF = 1.240, reflections
measured = 52637, Rint = 0.0957.
17 This complex was obtained by treating (PPh3)2Cu(NO3) with
NaNO2, while the reaction yield was not reported and the complex
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V = 4328.59(10) A3, Z = 2, m = 0.690 mmꢀ1, T = 200 K, R1
y Crystallographic data for 3: C43H33CuN4O2P2, M = 763.21, triclinic,
space group P1, a = 9.882(2), b = 13.036(3), c = 15.533(3) A,
ꢀ
a = 81.005(5)1, b = 76.103(7)1, g = 83.054(5)1, V = 1911.3(7) A3,
Z = 2, m = 0.697 mmꢀ1, T = 200 K, R1 = 0.0545 (I > 2s(I)),
ꢂc
This journal is The Royal Society of Chemistry 2010
3100 | Chem. Commun., 2010, 46, 3098–3100