Reversible N-N coupling of NO ligands on dinuclear ruthenium complexes and subsequent N2O evolution: Relevance to nitric oxide reductase

Article abstract of DOI:10.1021/ja0763504

Unprecedented N-N coupling of two nitrosyl ligands on dinuclear complexes was discovered, which is proposed as a critical step in the bacterial NO reductase (NOR). The N-N coupled complex (TpRu)₂(μ-Cl)(μ-pz){μ-κ²-N(=O)-N(=O)} (2a) (Tp = BH(pyrazol-1-yl)₃) was prepared from the reaction of TpRuCl₂(NO) (1) with pyrazole in the presence of Et₃N. The curious N-N bond distance was X-ray crystallographically determined to be 1.861(3) A for the N-N coupled analogue with a 4-methylpyrazolato bridge, and was confirmed by DFT calculation. The N-N bond of 2a was cleaved by oxidation to give [{TpRu(NO)}₂(μ-Cl)(μ-pz)](BF₄)₂ (4a·(BF₄)₂), which on reduction reformed 2a. It is interesting to note that the N-N coupled complex 2a was also transformed into the oxo-bridged dinuclear species 5a with the evolution of N₂O. These findings would provide valuable information regarding the mechanism of NO reduction to N₂O by NOR. Copyright

Full text of DOI:10.1021/ja0763504

Published on Web 10/31/2007
Reversible N-N Coupling of NO Ligands on Dinuclear Ruthenium Complexes
and Subsequent N2O Evolution: Relevance to Nitric Oxide Reductase
Yasuhiro Arikawa, Taiki Asayama, Yusuke Moriguchi, Shoko Agari, and Masayoshi Onishi*
Department of Applied Chemistry, Faculty of Engineering, Nagasaki UniVersity, Bunkyo-machi 1-14, Nagasaki
852-8521, Japan
Received August 23, 2007; E-mail: onishi@nagasaki-u.ac.jp
Scheme 1
Transition metal-mediated transformations of nitric oxide (NO)
have been biologically and environmentally attractive research fields
in these decades. One biologically important subject is denitrifi-
cation which constitutes one of the main key processes of the global
biogeochemical nitrogen cycle, and in particular, bacterial NO
reductase (NOR) has been described to catalyze the two-electron
reduction of two NO molecules to nitrous oxide (N2O) at bimetallic
active sites (close-arranged heme/non-heme diiron active centers).1
We have examined chemical reactivities of nitrosylruthenium
supported by hydrotris(pyrazolyl)borate (Tp),2 and accidentally
found N-N coupling of the two nitrosyl ligands on dinuclear
rutheniums. It is proposed that such N-N coupling would be a
key step in the NOR catalytic process. In a NO molecule, the cis-
NO dimeric molecule with weak N‚‚‚N interaction is present in its
low-temperature solid state.3 Lippard and Karlin have succeeded
in the syntheses of closely related dinitrosyl diiron model com-
plexes, but neither of them exhibits similar N-N coupling.4
Furthermore, in our system, it was revealed that oxidation of this
N-N coupled species brought about cleavage of the N-N bond to
give cis-dinitrosyl dinuclear form, and interestingly also that on
reduction this cis-dinitrosyl complex cleanly reformed the N-N
coupled species.
Treatment of TpRuCl2(NO) (1: {RuNO}6) with an equimolecular
amount of pyrazole in the presence of excess Et3N in refluxing
CH2Cl2 gave rise to the dinuclear complex (TpRu)2(µ-Cl)(µ-pz)
{µ-κ2-N(dO)-N(dO)} (2a: {RuNO}7) in 29% yield (Scheme
1). The use of 4-bromo or 4-methyl pyrazole afforded the
corresponding dinuclear complexes (2b, Br-pz (25%); 2c, Me-
pz (ca. 15%)), while the use of 3,5- dimethylpyrazole did not. The
IR spectra of 2 show much lower frequencies of ν(NO) bands (1605
(2a), 1608 (2b), and 1604 cm-1 (2c)) than that of 1 (1893 cm-1).
Their FAB-MS spectra show parent molecular ion signals with
[2-(NO)]+ fragments, in support of their formulation. The 1H NMR
spectra of 2a exhibit three sets of Tp-pyrazolyl groups and two
resonances (1:2) of bridging-pyrazolyl protons, indicating its Cs
symmetry.
moiety is confirmed by the torsion angle O1-N1-N2-O2 ) 4.7-
(2)°.
The aforementioned reaction condition has been optimized, while
the reaction mechanism is still unclear.7 The use of 3 equiv of
pyrazole led us to isolate [TpRuCl(pzH)(NO)]Cl (3a: {RuNO}6)
(ca. 52% yield).8 Taking account of the formal neutral N(dO)-
N(dO) bridge in 2, 2e- reduction occurred during the formation
of 2. This reductive formation was probably associated with the
presence of the excess Et3N, which is essential to the preparation
of 2.9
To characterize the unusual N(dO)-N(dO) bridge, chemical
oxidation of 2a was carried out. Complex 2a was allowed to react
with AgBF4 in CH3CN, affording [{TpRu(NO)}2(µ-Cl)(µ-pz)](BF4)2
(4a‚(BF4)2: {RuNO}6) in 56% yield (Scheme 2). Complex 4a‚
(BF4)2 was also obtained from oxidation reaction of 2a with [Cp2-
Fe]BF4. The 1H NMR spectrum of 4a‚(BF4)2 indicates the retention
of the Cs symmetry. In the IR spectrum, a characteristic ν(NO)
The structures of 2b and 2c were confirmed by single-crystal
X-ray structural analyses, and the ORTEP view of 2c is shown in
Figure 1. Two TpRu fragments of 2c are bridged by one chloride,
one Me-pz, and the interesting part κ2-N,N-N(dO)-N(dO) with
a Ru-Ru distance of 3.5570(2) Å. The N-N distance (1.861(3)
Å) is significantly shorter than that of cis-NO dimer (2.18 Å) in
the low-temperature solid state,3 but much longer than that of typical
N-N single bond (ca. 1.42 Å).5 To verify this N-N bond, DFT
calculations starting from the X-ray structural data were performed,
and the HOMO is revealed to bear N-N bonding character.
Moreover, X-ray structural data of the N(dO)-N(dO) moiety
indicate the N-O bond distances of 1.197(3) and 1.193(3) Å,
excluding known hyponitrite form.6 The Ru-N-O angles are
136.1(2) and 137.4(2)°. The planarity within the N(dO)-N(dO)
Figure 1. Molecular structures of 2c (top) and 4a (bottom), H-atoms are
not shown. Toluene solvent molecule and BF4 counterions in the structures
of 2c and 4a‚(BF4)2, respectively, are omitted.
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J. AM. CHEM. SOC. 2007, 129, 14160-14161
10.1021/ja0763504 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
complexes, which is proposed as a critical step in the NOR catalytic
cycle.1 It is remarkable that this N-N bond is easily cleaved by
oxidation and regenerated again by reduction. Interestingly, trans-
formation of the N-N coupled complexes into the oxo-bridged
dinuclear complexes with the evolution of N2O was observed. This
observation would provide significant information regarding the
mechanism of NO reduction to N2O by NOR. Studies to find the
optimized reaction conditions for generation of the oxo-bridged
complexes and evolution of the N2O gas and also to complete the
NO reduction cycle like NOR are currently underway.
Figure 2. Molecular structure of 5b, H-atoms are not shown. CHCl3 solvent
molecules in the structure of 5b are omitted.
Acknowledgment. This work was supported by Grant-in-Aid
for Scientific Research on Priority Areas (No. 19028053, “Chem-
istry of Concerto Catalysis”) and Young Scientists (B) (No.
18750050) from the Ministry of Education, Culture, Sports, Science
and Technology, Japan. We thank Prof. K. Tatsumi and Dr. Y.
Ohki (Nagoya University) and also Prof. R. E. Cramer (Hawaii
University) for their useful discussions on the X-ray analyses of
2b and 2c.
Scheme 2
Supporting Information Available: Full experimental and spec-
troscopic details for all new compounds, ORTEP drawing of 2b,
depiction of HOMO for the calculated complex 2c, cyclic voltammetry
diagram of 2a, and X-ray structural data for complexes 2b, 2c, 4a, and
5b (PDF and CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
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(7) The reaction is complicated, and several other uncharacterized complexes
were also produced.
(8) Complex 3a was contaminated by trace amounts of inseparable impurities.
(9) Whitten, D. G. Acc. Chem. Res. 1980, 13, 83-90.
(10) Two-electron redox between 2a and 4a was confirmed by their peak
separation between oxidation and reduction waves in the cyclic voltam-
mogram and also by their controlled potential coulometry.
(11) In the protonation reaction of 2a, it seems that protonated species on the
bridging oxygen atom of 5a would be formed, and then removal of the
proton would occur in the step of chromatographic separation to release
5a.
- +
FAB-MS spectra exhibit the signal [4a + BF4
] at m/z 878.1,
and moreover the structure of 4a‚(BF4)2 was X-ray crystallographi-
cally confirmed.
The crystal structure of 4a‚(BF4)2 (Figure 1) verified the presence
of two {TpRu(NO)} units bridged by chloride and pyrazolate,
accompanied by two BF4-. Bonding interaction between two NO
nitrogen atoms is no longer observable as exemplified by N‚‚‚N
separation of 3.006(8) Å. The N-O bond distances are 1.169(9)
and 1.131(8) Å, and expansion of the Ru-N-O angles (169.7(6)
and 165.4(6)°) compared with those of 2b and 2c is evident.
Comparison of 4a‚(BF4)2 with 2b and 2c clearly shows the
shortening of the Ru-NO bond lengths.
Oxidation of 2a induced cleavage of the N-N bond. The
reversibility of this N-N bonding was supported by the cyclic
voltammogram of 2a, which is featured by a reversible two-electron
redox couple at 0.389 V (E1/2 vs Ag/AgCl).10 Actually, reductive
treatment of 4a‚(BF4)2 with Zn cleanly reformed complex 2a (70%
yield).
Isolation of 2 led us to inquire as to whether N2O can be produced
from 2 and then the corresponding oxo-bridged dinuclear complex
can be formed similarly to the proposed mechanism for the NOR
catalytic cycle.1 To check this possibility, we preliminarily examined
the reaction conditions and found that complex 2a was transformed
into the oxo-bridged dinuclear complex (TpRu)2(µ-Cl)(µ-pz)(µ-O)
(5a) (21% yield) besides 4a‚(BF4)2 (43% yield), by treatment with
HBF4‚OEt2 in CH2Cl2 (Scheme 2).11 Concomitantly, the evolution
of N2O was gas-chromatographically detected.12 Unfortunately, the
yields of complex 5a and the evolved N2O gas were relatively low
in these reaction conditions. Complex 5a was characterized by
spectral data (NMR, IR, and EI-MS) and elemental analysis, and
the structure was confirmed by the X-ray diffraction observation
of the bromo derivative 5b (Figure 2).
(12) N2O was detected in 25% yield on the basis of the separated complex 5a;
1
that is, about
/
mole of N2O gas per 1 mole of 5a.
4
In conclusion, we have communicated an example of unprec-
edented N-N coupling of the two nitrosyl ligands on dinuclear
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