Synthesis, spectroscopic investigation, and theoretical study of the ruthenium diazide complex [Ru(tpy)(PPh3)(N3)2]

Article abstract of DOI:10.1515/znb-2000-0603

The reaction of [Ru(tpy)(PPh₃)(Cl)₂] (tpy = 2,2′:6′,2″-terpyridine) with sodium azide in acetone/water afforded [Ru(tPy)(PPh₃)(N₃)₂]. The new Ru-diazide complex was fully characterized by elemental an

Full text of DOI:10.1515/znb-2000-0603

Synthesis, Spectroscopic Investigation, and Theoretical Study of the
Ruthenium Diazide Complex [Ru(tpy)(PPh3)(N3)2]
Won K. Seoka, Sun B. Yima, Heung N. Leea, and Thomas M. Klapötkeb
a Department of Chemistry, Dongguk University,
26 Pil-Dong, Chung-Ku, Seoul 100-715, Korea
b Department of Chemistry, University of Munich (LMU),
Butenandtstr. 5-13 (D), D-81377, Munich, Germany
Reprint requests to Prof. Dr. T. M. Klapötke. E-mail: tmk@cup.uni-muenchen.de
Z. Naturforsch. 55 b, 462^-66 (2000); received March 10, 2000
Ruthenium, Diazide, ZINDO/1 Calculations
The reaction of [Ru(tpy)(PPh3)(Cl)2] (tpy = 2,2':6',2"-terpyridine) with sodium azide in
acetone/water afforded [Ru(tpy)(PPh3)(N3)2]. The new Ru-diazide complex was fully char-
acterized by elemental analysis, IR, Raman, and multinuclear NMR spectroscopy. The struc-
tures, energies and vibrational frequencies of the ruthenium diazide isomers, cis- and trans-
[Ru(tpy)(PMe3)(N3)2] (Me=CH0, were calculated at the semi-empirical ZINDO/1 level of
theory.
1. Introduction
spectroscopic investigation of the ruthenium diazide
complex. The semi-empirical calculations of theo-
retical models of ruthenium diazide isomers, cis-
and rra«5,-[Ru(tpy)(PMe3)(N3)2] (Me = CH3), were
performed at the ZINDO/1 level of theory using the
VSTO-3G* basis set.
The six-electron reduction of nitrite to ammonia
catalyzed by the nitrite reductase enzyme is an im-
portant step in the overall reduction of nitrate to
ammonia in green plants [1]. Attempts to model
the active site of the enzyme via a synthetically ac-
cessible way have been reported by many research
groups [2]. Certain features of the reactivity have
been reported: R u-N 02+ is reduced to Ru-NH32+
quantitatively or Ru-NH32+ is rapidly and quanti-
tatively oxidized to give R u-N 02+ [3], The related
complexes with various nitrogen-containing ligands
have been known as models of intermediates in the
multiple electron transfer of nitrogen fixation chem-
istry. It is well known that the metal complex con-
taining the nitride ligand is a precursor in the nitrite
reduction reaction. From the point of coordination
chemistry, the strong trans effect of the nitride lig-
and plays an important role in determining stereo-
chemistry and, presumably, substitution rates [4],
2. Results and Discussion
The preparations of six-coordinate ruthenium
and osmium complexes have been reported by
many research groups [9]. The syntheses reported
here represent an alternative procedure for obtain-
ing similar complexes. The mono-chloro complex
[Ru(tpy)(PPh3)2(C1)](PF6) (tpy = 2,2':6',2"-terpyr-
idine) prepared by the reaction of the free ligand
PPh3 with [Ru(tpy)(PPh3)Cl2] was useful in obtain-
ing ligand PPh3 with [Ru(tpy)(PPh3)Cl2] was use-
ful in obtaining the mono-azide complex [Ru(tpy)-
(PPh3)2(N3)](PF6) [6, 9, 10]. We found that the
ruthenium diazide complex was obtained by the
reaction of [Ru(tpy)(PPh3)2(Cl)](PF6) with excess
NaN3 albeit in low yield. However, the reaction of
fra«s-[Ru(tpy)(PPh3)2(Cl)2] with 2.6 molar equiva-
lent of NaN3 in acetone/water solution provided the
desired product with higher yield (eq. ( 1)).
The metal nitride complex containing a metal-
nitrogen triple bond can be formed through chem-
ical activation and photolysis of the metal azide
complex [5]. Like NCS and NCO, the azide anion
is often used as a ligand and its geometry can be lin-
ear or non-linear. Recent results on preparation and
structural characterization of metal azides [6, 7] and
diazides [8] prompt us to report the synthesis and
[Ru(tpy)(PPh,)Cl2] + 2 NaN,
L°;' >
[Ru(tpy)(PPh3)(N3)2] + 2NaCl ( 1)
0932-0776/00/0600-0462 $ 06.00 © 2000 Verlag der Zeitschrift für Naturforschung. Tübingen ■www.znaturforsch.com
K
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W. K. Seok et al. • The Ruthenium Diazide Com plex [Ru(tpy)(PPh3)(N 3 )2]
463
1329
Fig. 1. Raman spectrum of
[Ru(tpy)(PMe3)(N3)2].
Raman Shift / cm
Fig. 2. 'H-NMR spectum of
[Ru(tpy)(PMe3)(N3)2] in the
region of 6.0~ 10.0 ppm.
2.1. Spectroscopy
for the aromatic protons of the PPh^ ligand
rings.
The new diazide complex of ruthenium was char-
acterized by elemental analysis and spectroscopic
methods. A broad MLCT band at A max = 550 nm was
observed in the UV-vis spectrum. The most intense
and characteristic absorption peaks for the asym-
metric in plane and out ofplane diazide stretching
modes were found at 2032 and 2014 cm -1 in the
FT-IR spectrum.
It should be noted that the chemical shift of one of
the 6 or 6" protons of the tpy ligand in complexes of
the type [(tpy)(PPh3)2Run-L]+/2+ is affected by the
nature of the Ligand L and is usually observed far
downfield as a doublet of doublets [6]. This obser-
vation can be explained by the molecular structure,
which shows that both 6 and 6" protons of the tpy
ligand ring are out of the ring currents of the tpy
and PPI13 rings. We observed that the chemical shift
of these particular protons in the [Ru(tpy)(PPh3)2-
(N3)]+ complex are slightly shifted from 8.73 to
8.80 ppm in the [Ru(tpy)(PPh3)(N3)2] complex. Be-
cause most of the protons for the tpy ligand are
effectively shielded by in plane phenyl groups in
The Raman spectrum of the ruthenium diazide
complex shown in Fig. 1 displays the characteristic
symmetric azide stretching mode as a very strong
peak at 1329 cm -1 .
The characterization of the complex was corrob-
orated with the use of proton, nitrogen, and phos-
phorus NMR spectroscopy. The 'H-NM R spec- the PPh3 ring systems, they are observed in a re-
trum (Fig. 2) showed well resolved peaks in
the region between 7.5 and 8.9 ppm for the
tpy ligand ring and between 6.9 and 7.3 ppm
gion upheld from that proton presumably located
in a deshielding region. The 'H NMR data sug-
gest the trans geometry. The 31P NMR spectrum of
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464
W. K. Seok et al. • The Ruthenium Diazide Com plex [Ru(tpy)(PPh3)(N 3 )2]
Table 1. Semi-empirically calculated structural parameters and total energies for m7/25-[Ru(tpy)(PMe3)(N3)2] (la) and
c/j,-[Ru(tpy)(PMe3)(N3)2] (lb) obtained using ZINDO/1 with a VSTO-3G* basis set.
Complex E [kcal/mol] NIMAX (N3, asym) [cm-1] (Int.) J(Ru-Na) [A] d(Na-Nb) [Ä] d(Nb-Nc) [A] Z(NaNbNc) [°]
la
lb
-15317
-15306
0
0
2094(6) ip
1.24
1.19
174.7
1.97
2078(10) oop
2090(10) ip
2074(6) oop
2032, 2014
1.24a
1.24b
1.94a
1.96b
173.8a
170.2b
1.19“
1.18b
Expl.c
a frans-position with respect to the PMe3group;bc/s-position with respect to the PMe3group;c [(tpy)Ru(PPh3)(N3)2];
d scaled with the empirical factor of 0.62.
the complex shows a singlet resonance at 32.3 ppm
for the coordinated PPh3 group, which suggests a
trans complex [6, 11]. It is shifted to slightly lower
field than that of ruthenium mono-azide molecule
[Ru(tpy)(PPh3 )2(N3)]+ (26.5 ppm).
14N NMR spectroscopy was also used to char-
acterize the prepared diazide complex. From earlier
l4N NMR data for coordinated azides [12, 13], indi-
vidually well resolved resonances of Na, Nb, and Nc
(connectivity: Ru-Na-Nb-Nc) have been assigned in
the 14N NMR spectrum of the complex. Each res-
onance at -337, -136, and -225 ppm corresponds
to azide coordinated to Ru in the [Ru(tpy)(PPh3)2-
(Nß)]+ complex [6], Although the three N atoms of
the tpy ligand system in the [Ru(tpy)(PPh3)2 ( ^ 3 )2]
complex could not be observed, the three nitrogen
chemical shifts for the diazide ligands coordinated
to the ruthenium atom in the complex were obtained
at -335, -132, -278 ppm. It should be noted that the
relative line widths in the l4N NMR spectra for the
Ru-azide complex are related to the local symmetry
around the nitrogen atom [14- 16].
2.2. Semi-empirical calculations
The structure and total energy of the cis and
trans isomers of the ruthenium diazide complex
containing PMe3 instead of PPI13 were calcu-
lated at the semi-empirical ZINDO/1 level of the-
ory. These cis- and trans- [(tpy)Ru(PMe3)(N3)2]
molecules have been used instead of the correspond-
Fig. 3. Structure of a formula unit of (a) rä-[Ru(tpy)-
ing [(tpy)Ru(PPh3)(N3)2] complexes, because the
(PMe3)(N3)2] and (b) rratts-[Ru(tpy)(PMe3)(N3)2] based
trimethylphosphine ligand is electronically similar
on semi-empirical calculations with HyperChem. Grey
circles denote carbon atoms.
and closely related to the triphenylphosphine lig-
and. Reducing the number of parameters can also
save cpu time.
namically slightly favored over the cis isomer
[AE(cis-trans) = 11 kcal mol-1], as is shown in
Table 1.
The isomers are very close in their total
energy with the trans form being thermody-
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W. K. Seok et al. • The Ruthenium Diazide Com plex [Ru(tpy)(PPh3)(N 3 )2]
Raman spectrum was obtained on a Perkin Elmer Sys-
tem R 2000 at 25 °C. Elemental analyses were performed
by the analytical laboratory at Basic Science Institute of
Korea using a Carlo Erba E A 1180.
Because the calculations were performed for iso-
lated molecules and ions in the gas phase, the sta-
bilities in the condensed phase might be reversed
for cis and trans isomers due to cation-anion inter-
actions, neutral molecule-ion interactions, solvent
interventions, and crystal packing effects [17].
For the both cis and trans isomers the stationary
points were characterized as true minima (within
the ZINDO/1 model) by frequency analyses (in all
cases NIMAG = 0; NIMAG, number of imaginary
frequencies) [18]. The assignments of the experi-
mentally observed IR frequencies of the complex
[Ru(tpy)(PPh3)2(N3)2] were done in accord with
those of the previously reported trans-[Ru(tpy)~
(PPh3)2(N3)]+ complex [6]. The asymmetric N3
stretching mode, which is very strong in the IR spec-
trum, was also calculated (Table 1). More important
than the numbers, however, is the fact that trans- and
ci\y-[(tpy)Ru(PMe3)(N3)2] are expected to show two
absorption peaks for the asymmetrical N3 stretch-
ing mode. These two bands were calculated to be
of comparable intensity and to appear very close
in wavenumbers. For the asymmetric N3 stretching
mode of the trans- and c/s-[Ru(tpy)(PPh3)(N3)2]
complex, one absorption was observed [6].
3.1. Preparation of [Ru(tpy)(PPhj)(N3)?]
First method: Previously prepared [Ru(tpy)(PPh3 )Cl2]
(100 mg, 0.15 mmol) [9c)] and NaN3 (25 mg, 0.39 mmol)
dissolved in 10 ml of acetone and 1 ml of distilled water
were heated under reflux under a stream of Ar for 2 h. Dur-
ing this period, a precipitate developed under a deep pur-
ple solution. The hot reaction mixture was cooled down
in the refrigerator to r. t. overnight. The precipitate was
filtered off in air and washed with cold acetone and wa-
ter. The deep purple-colored product was vacuum-dried.
Yield: 61 mg (60%).
Second method: The same procedure as for the prepa-
ration of [Ru(tpy)(PPh3 )2 (N3 )](PF6 ) was applied [6], ex-
cept using 0.10 mmol of [Ru(tpy)(PPh3)2(Cl)](PF6) and
0.43 mmol of sodium azide. Yield: 20 mg (30%).
Elemental analysis for RUPC3 3 H2 6 N9 (680.67): Anal.
Calcd C 58.23, H 3.85, N 18.52. Found: C 58.19, H 3.95,
N 18.56%.
IR (c m -1): 3054 (w), 2032 (vs), 2014 (vs), 1600 (m),
1482 (m), 1446 (s), 1386 (s), 1280 (w), 1094 (w), 771
(s), 747 (m), 700 (s), 527 (s), 516 (s), 500 (m). - Raman
(1064 nm, 200 mW, 500 scans, cm -1 , (Int)): 3060 (4),
2040(1), 1596 (12), 1484(3), 1463 (5), 1329 (17), 1280
(5), 1178 (4), 1038 (8), 1007 (12), 725 (2), 646 (4) 526
(4). - 'H NMR ( 6 (multiplicity)): 8.80 (d), 8.26 (d), 8.13
(d), 7.91 (t), 7.69 (t), 7.57 (t), 7.32 - 6.88 (m). - 31P NMR
(6): 32.3. 14N NMR ( 6 ) : -335, -132, -278.
3. Experimental
RuCb, 2,2':6',2"-terpyridine(tpy), NaN3, and NH4 PF6
were purchased from Aldrich Chemical Co. and
used without further purification. Triphenylphosphine
(Aldrich Chemical Co.) was recrystallized twice from
ethanol [19]. Dimethylsulfoxide- ^ 6 (99.9 atom% D), and
dichloromethane-di (99.6% D) (Aldrich Chemical Co)
were used as received. Acetonitrile was purified by distil-
lation over P4 O 10 under an Ar atmosphere. Distilled water
was used after passing through a Nanopure™ (Bamstead)
water system.
3.2. Semi-empirical calculations
All semi-empirical calculations were performed with
the program package HyperChem (version 4.0 and 5.0) at
the ZINDO/1 level of theory using a VSTO-3G* basis set
[20, 21], The ZINDO/1 is a variation of INDO extended
to transition metals, which is equivalent to the most re-
cent version of the INDO/1 method that differs from the
original by using constant orbital exponents. The model
that is within the unrestricted Hartree-Fock formalism is
characterized by inclusion of one-center exchange terms
as well as an accurate description of integrals involving
3d atomic orbitals.
Routine UV-visible spectra were recorded on
a
Hewlett-Packard 8452A Diode Array spectrophotometer.
FT-IR spectra were obtained on a Bomen MB 100 FT-IR
spectrophotometer either on nujol mulls or in solutions us-
ing NaCl plates. All 'H and31P NMR data were obtained
with a Varian Gemini-200 FT-NMR spectrometer using
(CD^bSO as solvent at 25 °C. The chemical shift param-
eters are presented in parts per million ( 6 ) downfield from
internal reference tetramethylsilane (TMS) while the MP
chemical shifts were referenced to external 85% H3 PO4 .
The 14N NMR spectrum was recorded in a 10 mm NMR
tube using CD2CI2 as solvent at -2 0 °C using a Brucker
SY 200 spectrometer operating at -14.462 MHz and was
referenced to external CH3 NO2 in CD2 CI2 solution. The
Acknowledgements
Financial support of this research by the Korea Sci-
ence and Engineering Foundation, MOE, the DFG and
the Fonds der Chemischen Industrie is gratefully acknow-
ledged.
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W. K. Seok et al. • The Ruthenium Diazide Com plex [Ru(tpy)(PPh3 )(N 3 )2]
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