Reductive Dimerization of Nitric Oxide to trans-Hyponitrite in the Coordination Sphere of a Dinuclear Ruthenium Complex

Article abstract of DOI:10.1021/om030618s

During the reaction of the coordinatively unsaturated compound [Ru ₂(CO)₄(μ-H)(μ-PBu₂ ^t)-(μ-dppm)] (1; dppm = Ph₂PCH₂PPh ₂) with nitric oxide, the latter dimerizes and yields the unusual toms-hyponitrite complex [Ru₂(CO)₄(μ-H)(μ-PBu ₂^t)(μ-dppm)(μ-η²-ONNO)] (2). The molecular structure of 2 has been determined by X-ray diffraction and is discussed together with the results of DFT calculations. The compound stands out by having only one of the two hyponitrite NO units bonded to Ru. The second one is not connected to Ru but in the solid state exhibits a weak intramolecular bond to the carbon atom of a neighboring carbonyl group.

Full text of DOI:10.1021/om030618s

Organometallics 2004, 23, 1269-1273
1269
Red u ctive Dim er iza tion of Nitr ic Oxid e to
tr a n s-Hyp on itr ite in th e Coor d in a tion Sp h er e of a
Din u clea r Ru th en iu m Com p lex
Hans-Christian Bo¨ttcher* and Marion Graf
Institute of Inorganic Chemistry, Martin Luther University Halle-Wittenberg,
Kurt-Mothes-Strasse 2, D-06120 Halle/ Saale, Germany
Kurt Mereiter† and Karl Kirchner‡
Institute of Chemical Technologies and Analytics and Institute of Applied Synthetic Chemistry,
Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
Received October 1, 2003
During the reaction of the coordinatively unsaturated compound [Ru2(CO)4(µ-H)(µ-PBut )-
2
(µ-dppm)] (1; dppm ) Ph2PCH2PPh2) with nitric oxide, the latter dimerizes and yields the
unusual trans-hyponitrite complex [Ru2(CO)4(µ-H)(µ-PBut )(µ-dppm)(µ-η2-ONNO)] (2). The
2
molecular structure of 2 has been determined by X-ray diffraction and is discussed together
with the results of DFT calculations. The compound stands out by having only one of the
two hyponitrite NO units bonded to Ru. The second one is not connected to Ru but in the
solid state exhibits a weak intramolecular bond to the carbon atom of a neighboring carbonyl
group.
were dried over sodium-benzophenone ketyl or molecular
In tr od u ction
sieves and were distilled under argon prior to use. [Ru2(CO)4-
The interest in the chemistry of nitric oxide has
intensified in recent years due to its importance in
biological systems.1 Since some of its chemistry in this
light is mediated by transition metals, we are investi-
gating interactions of NO with the coordinatively un-
(µ-H)(µ-PBut )(µ-dppm)]4 was prepared according to the re-
2
ported procedure, and nitric oxide was purchased from Aldrich.
Photochemical reactions were carried out with a Heraeus TQ
150 high-pressure mercury lamp in a 250 mL photoreactor.
IR spectra were recorded as KBr pellets or CH2Cl2 solutions
on a Mattson 5000 FTIR spectrometer. NMR spectra were
obtained on Varian Unity 400 MHz or Varian Gemini 200 MHz
equipment. Chemical shifts are given in ppm from SiMe4 (1H)
or 85% H3PO4 (31P{1H}). Microanalyses (C, H, N) were
performed by the University of Halle microanalytical labora-
tory.
saturated complexes [M2(CO)3L(µ-H)(µ-PBut )(µ-dppm)]
2
(M ) Fe, Ru; L ) CO, phosphines; dppm ) Ph2PCH2-
PPh2).2 Recently we reported the synthesis and the
X-ray crystal structure of [Ru2(CO)4(µ-NO)(µ-PBut )(µ-
2
dppm)] (3), a complex obtained by deprotonation of [Ru2-
(CO)4(µ-H)(µ-NO)(µ-PBut )(µ-dppm)][BF4] with strong
2
bases.3 An efficient synthesis of 3 directly by the
[Ru 2(CO)4(µ-H)(µ-P Bu t2)(µ-d p p m )(µ-η2-ONNO)] (2). A
solution of [Ru2(CO)4(µ-H)(µ-PBut )(µ-dppm)] (845 mg, 1 mmol)
reaction of [Ru2(CO)4(µ-H)(µ-PBut )(µ-dppm)] (1) with
2
2
in toluene (25 mL) was cooled to -60 °C, and a slow stream of
nitric oxide was bubbled through the solution for about 10 min.
During this time the color of the solution changed from deep
violet to orange. The mixture was warmed to room tempera-
ture by stirring for 30 min. The initial yellow-orange solution
converted to a pale yellow suspension. The yellow solid was
collected on a glass frit, washed with hexane (3 × 10 mL), and
dried under vacuum. 2 was recrystallized from CH2Cl2/hexane
(1:6). Yield: 787 mg (87%). Mp: 150-152 °C dec. Anal. Calcd
for C37H41N2O6P3Ru2: C, 49.12; H, 4.57; N, 3.10. Found: C,
50.23; H, 4.51; N, 2.86. IR (KBr): ν(CO) 2028 s, 1973 vs, 1965
NO under various conditions could not be accomplished.
In the course of a more detailed study on the reactivity
of 1 toward nitric oxide, we are now able to report the
synthesis and characterization of a diruthenium com-
plex containing a trans-hyponitrite ligand in an unprec-
edented coordination mode. The ligand is formed by
reductive coupling of two NO molecules in the protective
coordination sphere of a diruthenium cluster core.
Exp er im en ta l Section
All synthetic operations were performed under a dry argon
atmosphere using conventional Schlenk techniques. Solvents
1
vs, 1742 s. IR (CH2Cl2): ν(CO) 2038 s, 1978 vs cm-1. H NMR
(CDCl3): δ 7.55-6.97 (m, 20H, PC6H5), 2.56 (m, 1H, PCH2P),
3
* To whom correspondence should be addressed. E-mail: boettcher@
chemie.uni-halle.de.
2.18 (m, 1H, PCH2P), 1.48 (d, 18H, J PH ) 13.8 Hz, PC4H9),
2
-12.30 (m, 1H, µ-H). 31P{1H} NMR (CDCl3): 247.8 (dd, J PP
† Institute of Chemical Technologies and Analytics.
2
2
) 177.4 Hz, J PP ) 162.6 Hz, µ-PBut ), 46.0 (dd, J PP ) 162.6
‡ Institute of Applied Synthetic Chemistry.
2
2
2
2
Hz, J PP ) 72.1 Hz, PC6H5), 35.6 (dd, J PP ) 72.1 Hz, J PP
177.9 Hz, PC6H5).
)
(1) Richter-Addo, G. B.; Legzdins, P.; Burstyn, J . Chem. Rev. 2002,
102, 857.
(2) (a) Bo¨ttcher, H.-C.; Graf, M.; Wagner, C. Proceedings of the
XXXVth International Conference on Coordination Chemistry (ICCC
35); University of Heidelberg: Heidelberg, Germany, J uly 21-26, 2002;
p 551. (b) Bo¨ttcher, H.-C.; Schmidt, H.; Tobisch, S.; Wagner, C. Z.
Anorg. Allg. Chem. 2003, 629, 686.
Com p u ta tion a l Deta ils. The DFT calculations were per-
formed using the Gaussian98 software package on the Silicon
(3) Bo¨ttcher, H.-C.; Graf, M.; Merzweiler, K.; Wagner, C. J . Orga-
nomet. Chem. 2001, 628, 144.
(4) Bo¨ttcher, H.-C.; Merzweiler, K.; Bruhn, C. Z. Anorg. Allg. Chem.
1999, 625, 586.
10.1021/om030618s CCC: $27.50 © 2004 American Chemical Society
Publication on Web 01/22/2004

1270 Organometallics, Vol. 23, No. 6, 2004
Bo¨ttcher et al.
Ta ble 1. Deta ils for th e Cr ysta l Str u ctu r e
Deter m in a tion of Com p lex 2
Sch em e 1
compd
formula
2‚solva
C37H41N2O6P3Ru2
(without solvent)
904.77
fw
cryst size, mm
cryst syst
space group
a, Å
b, Å
c, Å
0.72 × 0.60 × 0.58
monoclinic
P21/c (No. 14)
19.3385(9)
17.3924(8)
26.0465(12)
100.329(1)
8618.6(7)
and polarization effects, for crystal decay, and for absorption
were applied to the data. The structure was solved by direct
methods using the program SHELXS97.9 Structure refinement
on F2 was carried out with the program SHELXL97.10 All non-
hydrogen atoms were refined anisotropically. The hydride
hydrogen was refined with a SADI distance restraint.10 All
other hydrogen atoms were inserted in idealized positions and
were refined riding with the atoms to which they were bonded.
It was found that the structure is built up from two crystal-
lographically independent Ru complexes and that it contains
large channels filled with considerable amounts of disordered
solvent molecules. Three CH2Cl2 molecules with approximately
half-occupied positions were seen in difference Fourier maps,
whereas the rest of the solvent was diffuse and would have to
be modeled by over 20 additional peaks with concomitant
critical parameter correlations (estimated solvent content two
CH2Cl2 molecules and one C2H5OH molecule per formula unit).
Therefore, the contributions of all solvent molecules to the
structure factors were squeezed with the program PLATON11
followed by the final least-squares refinement of the structural
backbone. Despite the crystallographically good accuracy of
the structure, there are perceptible consequences of solvent
disorder. Thus, both crystallographically independent Ru com-
plexes possess regionally synchronous displacement anisotro-
pies in different directions, which are attributed to local solvent
influence.
â, deg
V, Å3
Z
F
T, K
8
calcd, g cm-3
1.395
100(2)
0.854
multiscan
2.44-30.05
126 453
24 898 (Rint ) 0.0289)
21 814
µ(Mo KR), mm-1
abs cor
θ range for data collecn (deg)
no. of rflns measd
no. of unique rflns
no. of rflns, I > 2σ(I)
no. of params
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
residual electron density peaks (e/Å3)
907
0.0288, 0.0709
0.0337, 0.0732
+0.939, -0.813
a
Compound is a disordered solvate containing proven CH2Cl2
and likely ethanol in unknown quantities. The solvent content is
not contained in the chemical formula and derived quantities.
Graphics Origin 2000 of the Vienna University of Technology.5
The geometry and energy of the model complex were optimized
at the B3LYP level6 with the Stuttgart/Dresden ECP (SDD)
basis set7 to describe the electrons of the ruthenium atom. For
all other atoms the 6-31g** basis set was employed.8
A
vibrational analysis was performed to confirm that the struc-
ture has no imaginary frequency. The geometry was optimized
without constraints (C1 symmetry).
X-r a y Str u ctu r e Deter m in a tion . Crystals of 2 in the form
of a solvate, 2‚solv, were obtained by slow diffusion of ethanol
into a dichloromethane solution at room temperature. Due to
a high solvent content the crystals are stable only in contact
with mother liquor or at low temperatures. Crystal data and
experimental details are given in Table 1. X-ray data were
collected on a Bruker Smart CCD area detector diffractometer
(graphite-monochromated Mo KR radiation, λ ) 0.71073 Å, 0.3°
ω-scan frames covering a complete sphere of the reciprocal
space, Bruker Kryoflex cooling unit). Corrections for Lorentz
Resu lts a n d Discu ssion
Recently we found that the reaction of [Fe2(CO)4(µ-
H)(µ-PBut )(µ-dppm)] with NO in toluene at -60 °C
2
yields a variety of products with [Fe2(µ-CO)(CO)4(µ-H)-
(µ-PBut )(µ-dppm)] as the only characterized main
2
product. Presumably, the formation of CO by side reac-
tions occurred, and subsequent addition to the coordi-
natively unsaturated species afforded the electron-
precise diiron complex.2 In contrast to this, the analog-
ous reaction of [Ru2(CO)4(µ-H)(µ-PBut )(µ-dppm)] (1)
(5) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98, revision A.7; Gaussian, Inc.: Pittsburgh, PA,
1998.
(6) (a) Becke, A. D. J . Chem. Phys. 1993, 98, 5648. (b) Miehlich, B.;
Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (c)
Lee, C.; Yang, W.; Parr, G. Phys. Rev. B 1988, 37, 785.
(7) (a) Haeusermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Mol. Phys.
1993, 78, 1211. (b) Kuechle, W.; Dolg, M.; Stoll, H.; Preuss, H. J . Chem.
Phys. 1994, 100, 7535. (c) Leininger, T.; Nicklass, A.; Stoll, H.; Dolg,
M.; Schwerdtfeger, P. J . Chem. Phys. 1996, 105, 1052.
2
with nitric oxide resulted in the clean formation of the
unprecedented complex [Ru2(CO)4(µ-H)(µ-PBut )(µ-dppm)-
2
(µ-η2-ONNO)] (2) in high yield (Scheme 1). Unfortu-
nately, no intermediate products could be detected spec-
troscopically, and therefore the mechanism of the for-
mation of 2 is currently unknown. It seems to be plaus-
ible that at low temperatures gaseous nitric oxide di-
merizes to N2O2, which is subsequently reduced by the
ruthenium(I) species, yielding the product 2 with the
metals in the formal oxidation state +II. Complex 2 is
stable to air in the solid state but decomposes slowly in
solutions exposed to air. Moreover, within several days
a slow decomposition by exposure to light is observed
in the solid state as well as in solution. Under UV
(8) (a) McClean, A. D.; Chandler, G. S. J . Chem. Phys. 1980, 72,
5639. (b) Krishnan, R.; Binkley, J . S.; Seeger, R.; Pople, J . A. J . Chem.
Phys. 1980, 72, 650. (c) Wachters, A. J . H. J . Chem. Phys. 1970, 52,
1033. (d) Hay, P. J . J . Chem. Phys. 1977, 66, 4377. (e) Raghavachari,
K.; Trucks, G. W. J . Chem. Phys. 1989, 91, 1062. (f) Binning, R. C.;
Curtiss, L. A. J . Comput. Chem. 1995, 103, 6104. (g) McGrath, M. P.;
Radom, L. J . Chem. Phys. 1991, 94, 511.
(9) Sheldrick, G. M. SHELXS97: Program for the Solution of Crystal
Structures; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(10) Sheldrick, G. M. SHELXL97: Program for Crystal Structure
Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(11) Spek, A. L. PLATON: A Multipurpose Crystallographic Tool;
University of Utrecht: Utrecht, The Netherlands, 2003.

Dimerization of Nitric Oxide to trans-Hyponitrite
Organometallics, Vol. 23, No. 6, 2004 1271
tion. The coordination to the dimetal core is accom-
plished “side-on” by only one NO fragment, and both
ruthenium atoms can be considered as being in the
formal oxidation state +II.
Although the knowledge regarding the structural,
mechanistic, and derivate chemistry of hyponitrites has
improved significantly in recent years with particular
emphasis on simple inorganic salts (e.g. cis-Na2N2O2
and trans-Na2N2O2‚5H2O) and salts with organic amines
or esters of hyponitric acid,13 the knowledge of transi-
tion-metal hyponitrites is still in its infancy, as there
are so far only four examples of structurally character-
ized transition-metal hyponitrite complexes. Two of
these concern [Pt(η2-O2N2)(PPh3)2]14 and [Ni(η2-O2N2)-
(dppf)]13 (dppf ) 1,1’-bis(diphenylphosphanyl)ferrocene),
which contain chelating bidentate cis-hyponitrite groups
that are exclusively O-bonded to the metal atoms. In
two earlier crystallographic studies the cis-hyponitrite
complex [(H3N)5CoN(O)NOCo(NH3)5]4+ as the mixed
nitrate/bromide and the trans-hyponitrite compound
[{(ON)2Co(µ-ONO)Co(NO)2}2(µ-ONNO)], a remarkable
neutral tetranuclear Co complex, have been described.15,16
In the dinuclear Co complex the two metal atoms are
asymmetrically bridged through the cis-hyponitrite di-
anion via N and O in the 1,3-positions (O-N-N-O bond
lengths of 1.29, 1.25, and 1.32 Å). Only in the tetra-
nuclear Co complex does the bonding situation of the
trans-hyponitrite resemble in part that of our novel
ruthenium complex 2, because the centrosymmetric
hyponitrite bridges two pairs of adjacent Co atoms via
its two N-O entities, thus forming an essentially planar
Co-O-N(Co)-N(Co)-O-Co arrangement with O-N-
N-O bond lengths of 1.316, 1.265, and 1.316 Å. In
comparison with all these examples, the ruthenium
complex 2 stands out by its highly asymmetric coordi-
nation with only one N-O group bonded to the two Ru
atoms, whereas the second N-O group is a spectator.
The O-N-N-O bond lengths of the two independent
molecules in 2‚solv average 1.356(2), 1.264(2), and
1.317(2) Å (for the individual values see Table 2) and
thus exhibit a clear-cut elongation for the N-O moiety
chelating Ru in comparison with its noncoordinated
N-O counterpart. The first two bond lengths (long O-N
and N-N) also compare well with recent accurately
determined trans-hyponitrites, e.g. with Na2N2O2‚5H2O,
where O-N-N-O bond lengths within a centrosym-
metric and Na-O-bonded trans-hyponitrite were found
to be 1.362(1), 1.256(2), and 1.362(2) Å.13b In comparison
with all other known solid-state structures of hyponi-
trites the free termination of the ONNO group in the
Ru complex is unusual and should be prone to further
interactions, because the terminal oxygen O(5) is cer-
tainly distinctly charged. In fact, there is a long-range
interaction to the carbon C(1) of the next-neighboring
F igu r e 1. Perspective view of the dimetal complex [Ru2-
(CO)4(µ-H)(µ-PBut )(µ-dppm)(µ-η2-ONNO)] (2) showing 40%
2
thermal displacement ellipsoids (only one of two indepen-
dent molecules in the unit cell for 2 is shown). For selected
bond lengths and angles see Table 2.
irradiation compound 2 reacts at room temperature in
THF within 4 h quantitatively back to 1 as the sole pro-
duct. In contrast, an analogous photochemical treatment
of 2 in CH2Cl2 as the solvent yields a mixture of com-
pounds containing [Ru2(CO)4(µ-Cl)(µ-PBut )(µ-dppm)]12
2
as the solely characterized product. The starting com-
plex 1 could not be detected under these conditions.
Yellow crystals of 2‚solv were obtained by slow
diffusion of ethanol into a CH2Cl2 solution, and the
molecular structure could be elucidated by single-crystal
X-ray structure analysis. Consequently, compound 2
represents a diruthenium complex containing a N2O2
ligand in an unprecedented coordination mode. The
complex is diamagnetic, and its 31P{1H} NMR spectra
show the chemical inequivalence of the two phosphorus
nuclei of the bridging dppm ligand. They give rise to
two signals (dd) with corresponding couplings to the P
nucleus of the phosphido bridge (dd). The 1H NMR
spectroscopic data of 2 include a characteristic reso-
nance at -12.30 ppm as a multiplet attributable to a
bridging hydride ligand. The infrared spectrum of 2
(KBr) exhibits three absorption bands in the region
characteristic of terminal carbonyl ligands. Further-
more, one band at 1742 cm-1 is indicative of a carbonyl
group in a bridging position.
The result of the structural identification of 2 by X-ray
crystallography is depicted in Figure 1. The molecule
consists of a dinuclear metal core bridged by a hydride,
a phosphido group, a dppm moiety, and an unusual
N2O2 ligand. The coordination spheres of the two Ru
are completed by four carbonyl ligands, and therefore
by electron counting 2 exhibits 34 cluster valence
electrons. The most remarkable feature of the molecule
is clearly the N2O2 ligand. This group is a 4-electron
hyponitrite ligand, (ONNO)2-, in the trans configura-
(13) (a) Arulsamy, N.; Bohle, D. S.; Imonigie, J . A.; Levine, S. Angew.
Chem., Int. Ed. 2002, 41, 2371 and references therein. (b) Arulsamy,
N.; Bohle, D. S.; Imonigie, J . A.; Sagan, E. S. Inorg. Chem. 1999, 38,
2716.
(14) Bhaduri, S.; J ohnson, B. F. G.; Pickard, A.; Raithby, P. R.;
Sheldrick, G. M.; Zuccaro, C. I. J . Chem. Soc., Chem. Commun. 1977,
354.
(15) Hoskins, B. F.; Whillans, F. D.; Dale, D. H.; Hodgkin, D. C. J .
Chem. Soc., Chem. Commun. 1969, 69.
(16) Bau, R.; Sabherwal, I. H.; Burg, A. B. J . Am. Chem. Soc. 1971,
93, 4926.
(12) Bo¨ttcher, H.-C.; Graf, M.; Merzweiler, K.; Wagner, C. Z. Anorg.
Allg. Chem. 2000, 626, 597.
(17) Feldmann, C.; J ansen, M. Z. Anorg. Allg. Chem. 1997, 623,
1803.

1272 Organometallics, Vol. 23, No. 6, 2004
Bo¨ttcher et al.
F igu r e 2. Optimized geometries for trans-[Ru2(CO)4(µ-H)(µ-PH2)(µ-H2PCH2PH2)(µ-η2-ONNO)] (2a ) and cis-[Ru2(CO)4(µ-
H)(µ-PH2)(µ-H2PCH2PH2)(µ-η2-ONNO)] (2b) calculated at the B3LYP (Ru, sdd; C, H, N, O, P, 6-31g**) level of theory.
Ta ble 2. Obser ved P a r a m eter s a n d Com p u ta tion a l
Resu lts (in Å a n d d eg) for Com p lex 2 a n d th e
Mod el Com p ou n d s 2a a n d 2b
DFT calculations were carried out to determine the
structure and the relative stability of the two model
complexes trans-[Ru2(CO)4(µ-H)(µ-PH2)(µ-H2PCH2PH2)-
(µ-η2-ONNO)] (2a ) and the corresponding isomer cis-
[Ru2(CO)4(µ-H)(µ-PH2)(µ-H2PCH2PH2)(µ-η2-ONNO)] (2b).
In addition, the possibility of a hypothetic ONON ligand
instead of the hyponitrite ONNO has also been inves-
tigated on the basis of the model complex [Ru2(CO)4(µ-
H)(µ-PH2)(µ-H2PCH2PH2)(µ-η2-ONON)] (2c). According
to the calculations, the geometry of 2a is in very good
agreement with the X-ray structural data of 2, despite
the absence of substituents in the model complex. Table
2 compares salient geometric data of the two indepen-
dent trans-hyponitrite complexes in 2‚solv found by the
X-ray crystal structure analysis with the computational
results for 2a and 2b. The optimized geometries for 2a
and 2b are shown in Figure 2. The largest deviation is
observed for the intramolecular distance C(1)‚‚‚O(5).
Therefore, in the model complex 2a a bonding contact
may be envisaged, while for the real structure of 2 we
would interpret this as a “noticeable bonding interac-
tion”, additionally by consideration of a space-filling
model of the critical part Ru2(CO)2(µ-H)(µ-ONNO) of the
complex. (For the second independent molecule in the
unit cell of 2 the contact C(1)′-O(5)′ ) 2.244(4) Å was
found, and normally this is still longer than a bonding
distance.) It is worth noting that the isomeric cis
complex 2b, which has not been observed in the solid
state, lacks a C‚‚‚O interaction and is 8.8 kcal/mol less
stable than the trans isomer 2a . This barrier may be
even lower in solution. However, DFT computational
methods are not well set for taking into account solute-
solvent interactions, especially if large organometallic
molecules such as 2 are involved. Such calculations are
beyond our present computational abilities. The latter
result may suggest that perhaps in solution a fast
equilibrium between these two isomers is present, being
in line with the solution IR data, where indeed only
absorptions for terminal carbonyls were observed. On
the other hand, for the hypothetical complex 2c (with
an ONON ligand), despite several attempts, we were
unable to locate a stationary point, suggesting that 2c
is a stable entity.
2‚solv (obsd)
complex 1
complex 2
2a
2b
Ru(1)-Ru(2)
Ru(1)-N(2)
Ru(1)-C(1)
Ru(1)-C(2)
Ru(1)-H(1)
Ru(2)-O(6)
Ru(2)-C(3)
Ru(2)-C(4)
Ru(2)-H(1)
N(2)-O(6)
2.8479(2)
2.060(2)
1.944(2)
1.883(2)
1.743(9)
2.125(2)
1.909(2)
1.874(2)
1.743(9)
1.357(2)
1.261(2)
1.313(3)
2.062(3)
115.5(2)
112.5(2)
158.9(2)
2.8373(2)
2.062(2)
1.920(3)
1.894(3)
1.743(9)
2.123(1)
1.909(2)
1.862(2)
1.743(9)
1.355(2)
1.267(2)
1.321(2)
2.244(4)
114.9(2)
112.4(2)
164.7(3)
2.891
2.062
2.022
1.901
1.819
2.174
1.928
1.879
1.777
1.336
1.256
1.351
1.573
118.3
111.8
140.8
2.889
2.118
1.918
1.907
1.779
2.099
1.919
1.900
1.781
1.384
1.317
1.251
N(1)-N(2)
N(1)-O(5)
C(1)‚‚‚O(5)
N(1)-N(2)-O(6)
N(2)-N(1)-O(5)
Ru(1)-C(1)-O(1)
118.3
117.4
178.5
CO group. This interaction is geometrically indicated
by the short distances C(1)‚‚‚O(5) of 2.062(3) Å in
complex 1 and 2.244(4) Å in complex 2. Although these
distances exceed C-O single bonds by more than 0.6
Å, they are on the other hand too short for being merely
van der Waals contacts. Further evidence for this comes
from solid-state IR spectroscopy, which shows an IR
band of 1742 cm-1 indicative of a bridging CO group,
and this can only be attributed to C(1)-O(1) with its
long C(1)‚‚‚O(5) interaction. For comparison, the CO
stretching frequencies of the “normal” terminal CO
ligands are found in the range of 2028-1965 cm-1 (see
the Experimental Section). Further aspects of this
feature are addressed in the discussion of the DFT
calculations. Another interesting point is the compara-
tively large difference in the C(1)‚‚‚O(5) distances of the
two independent Ru complexes. Looking for an explana-
tion, we found out that the environments of the two
oxygen atoms O(5) and O(5)′ are different, with O(5)′
having adjacent solvent neighbors but not O(5). It is
therefore reasonable to assume that for O(5)′ hydrogen
bond interactions to CH2Cl2 and/or ethanol are respon-
sible for this feature. Interestingly, in CH2Cl2 solution
this C(1)‚‚‚O(5) interaction should obviously be inter-
rupted, since the corresponding IR band is absent in the
spectrum and only two absorptions for terminal carbon-
yls are observed.
The bonding characteristics of the N2O2 group found
by X-ray structure analysis substantiate this arrange-
ment unambiguously as a trans-hyponitrite ligand. The
N(1)-N(2) distance shows a double-bond character and
is comparable with the N-N bonds in the cis-hyponitrite

Dimerization of Nitric Oxide to trans-Hyponitrite
Organometallics, Vol. 23, No. 6, 2004 1273
ligands of [Pt(η2-O2N2)(PPh3)2] (N(1)-N(2) ) 1.21(5) Å)14
and [Ni(η2-O2N2)(dppf)] (N(1)-N(2) ) 1.236(6) Å).13
Furthermore, the N-O distances agree well for all these
compounds and are comparable with those found for
[(H3N)5CoN(O)NOCo(NH3)5]4+ and [{(ON)2Co(µ-ONO)-
Co(NO)2}2(µ-ONNO)].15,16 The planarity within the hy-
ponitrite ligand of 2 is well documented by the torsion
angle O(5)-N(1)-N(2)-O(6) ) -176.0(2)°, and taken
together, the hyponitrite ligands in the former com-
plexes were also found to be planar.
Currently, there seems to be an increased electron
density on the noncoordinating oxygen atom of the
hyponitrite ligand in 2, manifested by the close in-
tramolecular contact to one carbonyl carbon, and con-
sequently we have to prove the reactivity of this ligand
toward electrophiles.
trans-hyponitrite compound [Ru2(CO)4(µ-H)(µ-PBut )(µ-
2
dppm)(µ-η2-ONNO)] (2) is formed in high yield. The
advantage of the synthesis consists of the use of nitric
oxide as the source for the hyponitrite formation,
whereas for other preparations of hyponitrites, e.g. for
the complexes [M(η2-O2N2)L2] (M ) Ni, Pt; L ) phos-
phines)13 diazenium diolates, RN2O2-, are necessary for
their synthesis. Since 2 exhibits a very close intramo-
lecular contact between one C atom of a carbonyl group
and the noncoordinating O atom of the hyponitrite
ligand, the latter should be a center of high nucleophil-
icity. Currently we are exploring the chemical trans-
formation of 2 to confirm such a pattern of reactivity.
Ack n ow led gm en t. We are grateful to the Kul-
tusministerium des Landes Sachsen-Anhalt for financial
support and Degussa AG for a generous loan of RuCl3‚
3H2O.
Con clu sion
We described the reaction of the coordinatively un-
saturated complex [Ru2(CO)4(µ-H)(µ-PBut )(µ-dppm)] (1)
Su p p or tin g In for m a tion Ava ila ble: Listings of atomic
coordinates, anisotropic temperature factors, and bond lengths
and angles for 2‚solv. This material is available free of charge
via the Internet at http://pubs.acs.org.
2
with nitric oxide. The reaction does not result in a
simple addition of NO, affording [Ru2(CO)4(µ-NO)(µ-
PBut )(µ-dppm)]; rather, in a clean manner a reductive
2
dimerization of NO occurs and thus the unprecedented
OM030618S