First ruthenium complex of glyoxalbis(N-phenyl)osazone (L NHPhH2): Synthesis, X-ray structure, spectra, and density functional theory calculations of (LNHPhH2)Ru(PPh 3)2Cl2

Article abstract of DOI:10.1021/ic070294q

The first ruthenium complex containing the parent osazone ligand, glyoxalbis(N-phenyl)osazone (L^NHPhH₂), is reported. The complex (L^NHPhH₂)Ru(PPh₃)₂Cl ₂ (1) was characterized with mass, IR, ¹H NMR, and UV-vis spectroscopy as well as with theoretical calculations. Density functional theory calculations on the model compound (L^NHPhH₂)Ru(PMe ₃)₂Cl₂ (2) reproduce the geometrical features observed for 1 and verify that it formally contains a ruthenium(II) metal center coordinated by a neutral osazone. Subsequent bonding analyses identify π-interactions between the occupied orbitals of the metal fragment and the LUMO of the osazone, which results in transfer of approximately 0.3 electrons from the metal to the ligand. The complex 1 absorbs strongly at 405 nm, which is assigned to a ruthenium-to-ligand charge-transfer band on the basis of results of theoretical calculations. Complex 1 is also electroactive and displays a single one-electron oxidation wave at 0.39 V; coulometric oxidation gives the oxidized species [1]⁺ as a [PF₆]^- salt. Simulation of the EPR spectra of [1][PF₆], a one-electron paramagnetic species, affords g-tensor parameters g_x = 2.2649, g _y = 2.0560, and g_z = 1.9064 consistent with a ruthenium(III) description for [1]⁺, thereby confirming a metal-centered redox reaction.

Full text of DOI:10.1021/ic070294q

Inorg. Chem. 2007, 46, 5942−5948
First Ruthenium Complex of Glyoxalbis(N-phenyl)osazone (LNHPhH2):
Synthesis, X-ray Structure, Spectra, and Density Functional Theory
Calculations of (LNHPhH2)Ru(PPh3)2Cl2
Amit Saha Roy,† Heikki M. Tuononen,‡ Sankar P. Rath,§ and Prasanta Ghosh*,†
Departments of Chemistry, R. K. Mission Residential College, Narendrapur, Kolkata-103, India,
UniVersity of JyVa¨skyla¨, P.O. Box 35, FI-40014, JyVa¨skyla¨, Finland, and Indian Institute of
Technology Kanpur, Kanpur-208016, India
Received February 14, 2007
The first ruthenium complex containing the parent osazone ligand, glyoxalbis(N-phenyl)osazone (LNHPhH2), is reported.
The complex (LNHPhH2)Ru(PPh3)2Cl2 (1) was characterized with mass, IR, 1H NMR, and UV
−vis spectroscopy as
well as with theoretical calculations. Density functional theory calculations on the model compound (LNHPhH2)Ru(PMe3)2-
Cl2 (2) reproduce the geometrical features observed for 1 and verify that it formally contains a ruthenium(II) metal
center coordinated by a neutral osazone. Subsequent bonding analyses identify
π-interactions between the occupied
orbitals of the metal fragment and the LUMO of the osazone, which results in transfer of approximately 0.3 electrons
from the metal to the ligand. The complex 1 absorbs strongly at 405 nm, which is assigned to a ruthenium-to-
ligand charge-transfer band on the basis of results of theoretical calculations. Complex 1 is also electroactive and
displays a single one-electron oxidation wave at 0.39 V; coulometric oxidation gives the oxidized species [1]+ as
a [PF6]- salt. Simulation of the EPR spectra of [1][PF6], a one-electron paramagnetic species, affords g-tensor
parameters gx
)
2.2649, gy
)
2.0560, and gz
)
1.9064 consistent with a ruthenium(III) description for [1]+, thereby
confirming a metal-centered redox reaction.
1. Introduction
which facilitates their coordination to metals using two, four,
or (in rare occasions) as much as eight2 electrons. In addition,
Complexes containing a diimine fragment, LH2, coordi-
nated to a metal center are of topical interest because of their
numerous photophysical and catalytic activities.1 The lone
pairs of the nitrogen atoms and the π-electrons of the CdN
bonds allow LH2 ligands to act as efficient electron donors,
the identities of the two substituents on the NdC-CdN
backbone can also be varied, allowing for the steric and
electronic properties of the ligand to be fine-tuned. The
diimine ligand is also redox-active and readily accepts an
electron to its low-lying lowest unoccupied molecular orbital
(LUMO) thereby undergoing a one-electron reduction to
form a diimine anion radical.3
* To whom correspondence should be addressed. E-mail:
ghoshp_chem@yahoo.co.in. Phone: +91-33-2477-2205.
† R. K. Mission Residential College.
‡ University of Jyva¨skyla¨.
§ Indian Institute of Technology Kanpur.
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6105. (b) Clarke, M. J. Coord. Chem. ReV. 2003, 236, 207. (c) Fox,
M. A. Synthetic Application of Photocatalytic oxidation and reduction
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In contrast to the extremely versatile nature of LH2 ligands
in coordination complexes, the diimine fragment in osazones,
LNHRH2, behaves chemically differently and, consequently,
the coordination chemistry of this ligand is more limited in
scope: so far only a few transition metal complexes of
(2) Keijsper, J.; van Koten, G.; Vrleze, K.; Zoutberg, M.; Stam, C. H.
Organometallics 1985, 4, 1306.
5942 Inorganic Chemistry, Vol. 46, No. 15, 2007
10.1021/ic070294q CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/27/2007

A Ru Complex with Glyoxalbis(N-phenyl)osazone
osazones have been reported in literature.4 There are two
main reasons which have hindered the use of osazone as a
coordinating ligand in metal complexes: (i) Osazones of the
type LNHRH2 contain a reactive dN-NH-R fragment which
readily undergoes chemical transformations.4 (ii) In the
presence of protons and oxidizing agents, osazones are easily
oxidized to osotriazoles.5 Thus, all of the metal complexes
of osazone currently reported in the literature contain only
ligands of the type LNHRR2, i.e., in which the hydrogen atoms
in the diimine fragment have been replaced with bulkier
organic groups. So far there has not been a single report of
a coordination compound containing the parent osazone
ligand, LNHRH2, in which hydrogen atoms at both amino
nitrogen and diimine carbon centers are retained. Because
of their diimine fragments, osazones hold potential as useful
ligands in metal complexes functioning as luminophores,
probes, photosensitizers, and photoinitiators of radical reac-
tions.1,2 For this reason the development of osazone ligand
chemistry represents a valuable objective.
Herein we respond to the above challenge by reporting
the synthesis, X-ray crystal structure, and spectral features
of (LNHPhH2)Ru(PPh3)2Cl2 (1), the first transition metal
complex containing the phenyl-substituted parent osazone
ligand LNHPhH2. The molecular structure of complex 1 makes
it a very interesting system also from a chemical point of
view as ruthenium-diimine complexes are known to display
tunable photochemical properties which may be varied out
by independent replacement of the auxiliary ligands or by
careful fine-tuning of the electronic structure of the diimine
fragment.6 Hence, the effect of the two NHR groups in the
coordinated diimine fragment of 1 on the photophysical,
redox, and bonding properties of the ligand are characterized
using a combination of spectroscopic and theoretical meth-
ods. The bonding features of the complex are also discussed
on the basis of its determined X-ray crystal structure, and a
theoretical, molecular orbital based rationalization to the
observed binding of the ligand fragment in 1 is given.
2. Experimental Section
2.1. Synthesis. 2.1.1. Ru(PPh3)2Cl2. Ru(PPh3)2Cl2 was prepared
by an appropriate literature procedure.7
2.1.2. Glyoxalbis(N-phenyl)osazone. To a 40% aqueous solution
of glyoxal (5 mmol) was added phenylhydrazine (10 mmol) with
stirring. Immediately a yellow solid was formed. To the solid was
added methanol (15 mL), and the yellow suspension was stirred
for 0.5 h at 20 °C. The suspension was filtered and dried in air.
Yield: 1100 mg (91% with respect to glyoxal). Mass spectrum
1
(EI): m/z 238. H NMR (CDCl3, δ): 12.16 (s, 2H, NH), 7.63 (s,
2H, NdCH), 7.55 (d, 2H, Ph), 7.38-6.88 (m, 6H, Ph), 6.63 (d,
2H, Ph). Anal. Calcd for C14H14N4: C, 70.58; H, 5.80; N, 23.50.
Found: C, 70.46; H, 5.64; N, 23.10. IR (KBr): ν ) 3305, 3292
(s) 1598 (vs), 1567 (vs), 1505 (vs) 1486 (vs), 1253 (vs), 1121 (vs),
752 (vs), 692 (vs), 513 (s) cm-1
.
2.1.3. (LNHPhH2)Ru(PPh3)2Cl2. To a hot solution of glyoxalbis-
(N-phenyl)osazone ligand (0.4 mmol) in absolute ethanol (30 mL)
was added Ru(PPh3)2Cl2 (0.1 mmol), and the reaction mixture was
refluxed for 45 min (78 °C). A red crystalline solid separated out.
The mixture was cooled to 20 °C and filtered, and the residue was
dried in air. Yield: 85 mg (91% with respect to ruthenium). Mass
spectrum (ESI, positive ion, CH2Cl2): m/z 899.61, {1 - Cl}+. 1H
NMR (CDCl3, δ): 8.83, 8.68 (s, 2H, NH), 8.3 (s, 1H, NdCH),
5.95 (d, 4H, Ph), 7.71-6.78 (m, 37H, PPh3, Ph, and NdCH). Anal.
Calcd for C50H44Cl2N4P2Ru: C, 64.24; H, 4.74; N, 5.99. Found:
C, 63.96; H, 4.54; N, 5.90. IR (KBr): ν ) 3225, 3214(m) 1595
(m), 1491 (s), 1433 (vs), 1093 (s), 695 (vs), 519 (vs) cm-1
.
2.2. X-ray Crystallographic Data Collection and Refinement
of the Structures. Single crystals for X-ray structure determination
were grown by diffusion of n-hexane to the dark red dichloro-
methane solution of 1. Single-crystal X-ray data were collected at
-173 °C on a Bruker SMART APEX CCD diffractometer using
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The
linear absorption coefficients, scattering factors for the atoms, and
the anomalous dispersion corrections were taken from International
Tables for X-ray Crystallography. The data integration and reduc-
tion were processed with SAINT software.8 An absorption correc-
tion was applied.9 The structure was solved by direct methods using
SHELXS-97 and was refined on F2 by full-matrix least-squares
technique using the SHELXL-97 program package.10 Non-hydrogen
atoms were refined anisotropically. In the refinement, hydrogen
atoms were treated as riding atoms using SHELXL default
parameters. Crystallographic data of compound 1 are listed in Table
1.
(3) See, for example: (a) Cloke, F. G. N.; Dalby, C. I.; Henderson, M.
J.; Hitchcock, P. B.; Kennard, C. H. L.; Lamb, R. N.; Raston, C. L.
J. Chem. Soc., Chem. Commun. 1990, 1394. (b) Cloke, F. G. N.;
Hanson, G. R.; Henderson, M. J.; Hitchcock, P. B.; Kennard, C. H.
L.; Raston, C. L. J. Chem. Soc., Chem. Commun. 1989, 1002. (c) Pott,
T.; Jutzi, P.; Neumann, B.; Stammler, H. G. Organometallics 2001,
20, 1965. (d) Baker, R. J.; Farley, R. D.; Jones, C.; Mills, D. P.; Kloth,
M.; Murphy, D. M. Chem.sEur. J. 2005, 11, 2972. (e) Ghosh, P.;
Bill, E.; Weyhermuller, T.; Neese, F.; Wieghardt, K. J. Am. Chem.
Soc. 2003, 125, 1293. (f) Gardiner, M. G.; Hanson, G. R.; Henderson,
M. J.; Lee, F. C.; Raston, L. C. Inorg. Chem. 1994, 33, 2456. (g)
Corvaja, C.; Pasimeni, L. Chem. Phys. Lett. 1976, 39, 261. (h) Kaupp,
M.; Stoll, H.; Preuss, H.; Kaim, W.; Stahl, T.; van Koten, G.; Wissing,
E.; Smeets, W. J. J.; Spek, A. L. J. Am. Chem. Soc. 1991, 113, 5606.
(i) Richter, S.; Daul, C.; Zelewsky, A. Inorg. Chem. 1976, 15, 943.
(4) See, for example: (a) Caglioti, L.; Cattalini, L.; Ghedini, M.; Gasparrini
F.; Vigato, P. A. J. Chem. Soc., Dalton Trans. 1972, 514. (b) Caglioti,
L.; Cattalini, L.; Gasparrini F.; Ghedini, M.; Paolucci, G.; Vigato, P.
A. Inorg. Chim. Acta 1973, 538. (c) Maresca, L.; Natile, G.; Cattalini
L. J. Chem. Soc., Dalton Trans. 1975, 1601. (d) Maresca, L.; Natile,
G.; Calligaris, M.; Delise, P.; Randaccio, L. J. Chem. Soc., Dalton
Trans. 1976, 2386. (e) Bolger, J. A.; Ferguson, G.; James, J. P.; Long,
C.; McArdle, P.; Vos, J. G. J. Chem. Soc., Dalton Trans. 1993, 1577.
(f) Ammendola, P.; Ciajolo, M. R.; Panunzi, A.; Tuzi, A. J.
Organomet. Chem. 1983, 254, 389. (g) Bavoso, A.; Funicello, M.;
Morelli, G.; Pavone, V. Acta Crystallogr. 1984, C40, 2035. (h) Mail,
R. E.; Garralda, M. A.; Herna´ndez, R.; Ibarlucea, L.; Pinilla, E.; Torres,
M. R.; Zarandona, M. Eur. J. Inorg. Chem. 2005, 1671. (i) Bikrani,
M.; Mail, R. E.; Garralda, M. A.; Ibarlucea, L.; Pinilla, E.; Torres,
M. R. J. Organomet. Chem. 2000, 601, 311.
2.3. Computational Details. Density functional theory (DFT)
calculations were performed for the model system (LNHPhH2)Ru-
(PMe3)2Cl2 (2) and its hydrogen-substituted analogue, as well as
the one-electron oxidized [2]+ and reduced [2]- species. The
calculations utilized a combination of the hybrid PBE1PBE ex-
(7) Stephenson, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1996, 28,
945.
(8) SAINT+, 6.02 ed.; Bruker AXS: Madison, WI, 1999.
(9) Sheldrick, G. M. SADABS 2.0. Program for Empirical Absorption
Correction of Area Detector Data; University of Go¨ttingen: Go¨ttingen,
Germany, 2000.
(5) Hassan, E. K.; Pawel, C.; Christian, S. Carbohydr. Res. 1992, 224,
327.
(6) See, for example: (a) van Slageren, J.; Hartl, F.; Stufkens, D. J.;
Martino, D. M.; van Willigen, H. Coord. Chem. ReV. 2000, 208, 309.
(b) van Slageren, J.; Stufkens, D. J. Inorg. Chem. 2001, 40, 277.
(10) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 46, No. 15, 2007 5943

Roy et al.
Table 1. Crystallographic Data for (LNHPhH2)Ru(PPh3)2Cl2 (1)
formula
fw
space group P21/n
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
C50H44Cl2N4P2Ru T, K
934.80
calcd, g cm-3
reflcns collcd/2θmax
100(2)
1.454
25 940
F
14.4605(10)
unique reflcns/I > 2σ(I) 9296
21.1321(14)
14.7606(10)
108.7730(10)
4270.6(5)
4
no. of params
532
λ, Å/µ(Mo KR), cm-1
0.710 73/0.690
0.0444/0.929
0.0893
R1 /GOFb
a
wR2c (I > 2σ(I))
resid density, e Å-3
1.009
a Observation criterion: I > 2σ(I). R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b GOF )
2
2
2
2
2
[Σ[w(Fo - Fc )2]/(n - p)]1/2 c wR2 ) [Σ[w(Fo - Fc )2]/Σ[w(Fo )2]]1/2
,
.
2
2
2
where w ) 1/σ2(Fo ) + (aP)2 + bP (P ) (Fo + 2Fc )/3).
change correlation functional11 with the Ahlrichs’ triple-ú valence
basis set augmented by polarization functions (def2-TZVP);12 for
ruthenium, the corresponding ECP basis set was used. Vibrational
analyses were conducted to ensure that all optimized structures
correspond to true minima in the potential energy hypersurface.
The full optimized geometry of (LNHPhH2)Ru(PMe3)2Cl2 is given
in the Supporting Information in mol2 format. Geometry optimiza-
tions and frequency calculations were performed with the Turbo-
mole 5.9 program package,13a whereas ADF 2006.01b13b was used
in fragment molecular orbital analyses (PBE1PBE/TZ2P level of
theory with scalar relativistic ZORA treatment of all electrons).
Figure 1. Single-crystal X-ray structure of 1. (Hydrogen atoms are omitted
for clarity.)
Table 2. Selected Bond Distances (Å) and Angles (deg) of 1
Ru-N(2)
2.023(3)
2.3830(8)
2.4379(8)
1.374(3)
1.408(4)
1.349(3)
1.400(4)
1.382(5)
1.369(5)
1.425(4)
1.381(4)
1.375(5)
1.383(4)
Ru-N(3)
2.026(3)
2.3636(9)
2.4563(8)
1.308(4)
1.316(4)
1.410(4)
1.362(5)
1.368(5)
1.395(4)
1.390(4)
1.381(4)
1.388(4)
Ru-P(1)
Ru-P(2)
Ru-Cl(1)
Ru-Cl(2)
N(1)-N(2)
C(7)-C(8)
N(3)-N(4)
C(1)-C(2)
C(3)-C(4)
C(5)-C(6)
N(1)-C(1)
C10-C(11)
C(12)-C(13)
C(9)-C(14)
N(2)-C(7)
C(8)-N(3)
N(4)-C(9)
C(2)-C(3)
C(4)-C(5)
C(6)-C(1)
C(9)-C(10)
C(11)-C(12)
C(13)-C(14)
3. Results and Discussion
3.1. Characterization and Molecular Structure. The
synthesis of the ligand LNHPhH2 has been reported previ-
ously,14 but it was prepared by a modified procedure (see
above). The compound 1 was synthesized in high yield by
reacting the LNHPhH2 ligand with Ru(PPh3)2Cl2 in boiling
N(2)-N(1)-C(1)
C(7)-N(2)-N(1)
119.9(2)
121.1(3)
C(8)-N(3)-N(4)
N(3)-N(4)-C(9)
124.1(3)
130.3(3)
1
ethanol under air. The H NMR spectrum of 1 in CDCl3
confirmed the presence of both NH protons which appear
as two singlets at δ ) 8.83 and 8.68 ppm. For comparison,
the NH protons of a free LNHPhH2 ligand resonate at δ )
12.16 ppm in CDCl3 but appear at δ ) 7.65 ppm in DMSO-
d6.14 Some of the aromatic protons in 1 are shielded by the
diamagnetic ring current of the phenyl rings of the two PPh3
ligands and, therefore, resonate at higher field. The trend in
NH resonance frequency between the complex and the free
ligand is consistent with the binding of the metal ion to the
adjacent imine nitrogen atoms in the former species. The
effect of complex formation is also clearly visible in the IR
spectrum: the N-H stretching vibrations of the free ligand
appear at 3305 and 3295 cm-1 while those of the complex
1 are observed at slightly lower frequencies, 3225 and 3214
cm-1.
are summarized in Table 2. The Ru-P(1), Ru-P(2), and
Ru-Cl distances in 1 correspond to the reported Ru(II)-
P15 and Ru(II)-Cl16 bond lengths in complexes with trans-
Ru(PPh3)2 geometry. The diimine CC bond in 1 is consid-
erably shorter than a typical single bond between two sp2-
hybridized carbon atoms, 1.47 Å, but not significantly
different from CC bonds in closely related transition metal
complexes with osazone-type ligands, 1.46 ( 0.03 Å
(average value from 34 crystal structures found in the
Cambridge Structural Database v5.28). The two NC bonds
in 1 are only slightly elongated with respect to ideal
nitrogen-carbon double bond length, 1.28 Å, and, thus, are
typical for metal coordinated diimine fragments.
As already mentioned in the Introduction, it is a well-
known fact that the diimine chromophore is electronically
flexible and can readily accept excess electron density. There
are two common mechanisms which operate in diimine metal
complexes: (i) The CC bonding LUMO of the diimine can
abstract an electron from the metal thereby forming an
The complex 1 crystallizes in a P21/n space group. The
molecular structure of 1 with the atomic numbering scheme
is depicted in Figure 1, and the relevant bond parameters
(11) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865. (b) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett.
1997, 78, 1396. (c) Perdew, J. P.; Ernzerhof, M.; Burke, K. J. Chem.
Phys. 1996, 105, 9982. (d) Ernzerhof, M.; Scuseria, G. E. J. Chem.
Phys. 1999, 110, 5029.
(12) All basis sets were used as they are referenced in the Turbomole 5.9
internal basis set library. See: ftp://ftp.chemie.uni-karlsruhe.de/pub/
for the explicit basis set listings.
(15) (a) Ghosh, P.; Chakravorty, A. Inorg. Chem. 1997, 36, 64. (b) Menon,
M.; Pramanik, A.; Chattopadhyay, S.; Bag, N.; Chakravorty, A. Inorg.
Chem. 1995, 34, 1361. (c) Pramanik, A.; Bag, N.; Chakravorty, A. J.
Chem. Soc., Dalton Trans. 1992, 97. (d) Sahajpal, A; Robinson, S.
D.; Mazid, M. A.; Motevalli, M.; Hursthouse, M. B. J. Chem. Soc.,
Dalton Trans. 1990, 2119.
(13) (a) Turbomole 5.9: Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel,
C. Chem. Phys. Lett. 1989, 162, 165. (b) ADF2006.01b, SCM;
Theoretical Chemistry, Vrije Universiteit: Amsterdam, The Nether-
lands, 2006; http://www.scm.com.
(16) (a) Bag, N.; Choudhury, S. B.; Pramanik, A.; Lahiri, G. K.;
Chakravorty, A. Inorg. Chem. 1990, 29, 1361. (b) Bag, N.; Choudhury,
S. B.; Lahiri, G. K.; Chakravorty, A. J. Chem. Soc., Chem. Commun.
1990, 1626. (c) Ghosh, P.; Bag, N.; Chakravorty, A. Organometallics
1996, 15, 3042.
(14) Gallucci, R. R. J. Chem. Eng. Data 1982, 27, 217.
5944 Inorganic Chemistry, Vol. 46, No. 15, 2007

A Ru Complex with Glyoxalbis(N-phenyl)osazone
anionic radical ligand.3 (ii) If the orbital energies of the ligand
and the metal fragment match, the formally unoccupied
LUMO of the diimine fragment can contribute to MOs with
ligand-metal π-bonding character, which in essence leads
to transfer of electron density to atoms in the diimine
backbone.17,18 It can sometimes be difficult to differentiate
which one of the two mechanisms operates in each particular
case as they both give rise to structures with nearly identical
geometrical parameters: the resulting complexes generally
have diimine CC and CN bond lengths which are close to
1.40 and 1.33 Å, i.e., between that of single and double bond
values.3,17
Considering the molecular structure established for com-
plex 1, the geometrical parameters of the osazone ligand are
very close what one would expect if the complex contained
a monoanionic diimine radical.3 On the other hand, some
related metal-diimine complexes with trans-σ-bonded ligands
display similar bonding pattern in their diimine fragments
solely due to orbital interactions.17a,b The significant differ-
ence between the two possibilities with respect to complex
1 is that the former mechanism leads to a structure which
has internal singlet diradical character (a monoradical ligand
coordinated to a Ru(III) center), whereas the latter one gives
a complex with a pure singlet ground state. Hence, to
establish unequivocally whether the geometrical features
observed for the diimine ligand in 1 could be explained with
either one of the above two mechanism, the bonding in 1
was inspected in detail using density functional theory (DFT)
calculations and the fragment molecular orbital (FMO)
method. We note that EPR spectroscopy cannot be used to
identify between the two alternatives as no signal can be
detected for molecules with singlet diradical ground states.
3.2. Molecular Orbital Analysis. To better understand
the electronic structure and bonding properties of complex
1, theoretical calculations were conducted for a model system
(LNHPhH2) Ru(PMe3)2Cl2 (2) in which the two triphenylphos-
phine groups are replaced with computationally much less
demanding trimethylphosphines. The optimized bond pa-
rameters of the model complex 2 are in excellent agreement
with results from crystal structure determination of 1: the
calculated values of some key parameters are Ru-N ) 2.01
Å, NdC ) 1.31 Å, C-C ) 1.41 Å, N-N ) 1.34 Å,
-NRuN ) 78.8°, -RuNC ) 115.1°, and -NCC ) 115.5°.
Hence, the chosen level of theory accurately reproduces the
bonding features observed for the diimine backbone of the
osazone ligand in 1.
Figure 2. (a) Important σ-interactions between the ligand LNH2H2 and
the metal fragment Ru(PH3)2Cl2 in 1. (b) Occupied π MOs of the free
LNH2H2 ligand.
simplify the MO figures in the orbital interaction diagram,
we limit the discussion to the analysis of the fully unsub-
stituted system, i.e., (LNH2H2)Ru(PH3)2Cl2; results obtained
for complex 2 are virtually identical at both qualitative and
quantitative levels to the ones presented in below.
The fragment MO analysis revealed that the σ-symmetric
interactions between the ligand and the metal fragment take
place via ligand MOs 11a and 11b, which correspond to the
two nitrogen lone pairs; the metal fragment participates in
σ-type bonding primarily via three orbitals which are linear
combinations of ruthenium d orbitals with either p orbitals
of phosphorus or chlorine. The resulting MOs in the complex
are pictured in Figure 2a. Such interactions are expected
considering that the osazone ligand acts as a four-electron
donor in the current case. The σ-bonding framework of the
complex does not, however, reveal any interactions which
would explain the observed geometrical features as electron
density is transferred to the metal fragment from orbitals
which are primarily lone pair in character. Hence, the
π-system of the complex was analyzed in detail.
The free LNH2H2 ligand has four occupied MOs predomi-
nantly π-like in character which correspond to eight electrons,
four in two lone pairs and four in two CdN double bonds
(see Figure 2b). As the two terminal amino centers are sp3
hybridized in LNH2H2 (and in LNRHH2), the nonplanarity of
the ligand system does not allow full delocalization of
π-electrons. However, some conjugation throughout the
entire N-NdC-CdN-N backbone takes place as evi-
denced by the composition of the four orbitals in Figure 2b.
When the LNH2H2 ligand chelates a metal, its π-orbitals
interact with the orbitals of the metal fragment. A FMO
analysis conducted for the π-framework reveals that this
results in a net transfer of electron density from the metal to
the ligand via back-bonding interactions. The orbital which
functions as the primary electron acceptor is the LUMO of
the osazone ligand (orbital 12b), which interacts with the
occupied orbitals 20b and 21b of the metal fragment: the
percentage contribution of the ligand LUMO to the occupied
MOs 31b (HOMO-2) and 32b (HOMO-4) in the complex is
5.3 and 10.6%, respectively. Hence, the formally empty CC
bonding LUMO of the ligand accepts approximately 0.32
The bonding in complex 2 was analyzed by using the FMO
approach which investigates the orbital interactions between
the osazone ligand and the metal fragment in detail. To
(17) See, for example: (a) Aarnts, M. P.; Wilms, M. P.; Peelen, K.; Fraanje,
J.; Goubitz, K.; Hartl, F.; Stufkens, D. J.; Baerends, E. J.; Vicek, A.
Inorg. Chem. 1996, 35, 5468. (b) Aarnts, M. P.; Hartl, F.; Peelen, K.;
Stufjens, D. J.; Amatore, C.; Verpeaux, J.-N. Organometallics 1997,
16, 4686. (c) Makedonas, C.; Mitsopolou, C. A. Eur. J. Inorg. Chem.
2007, 110.
(18) The interaction of diimine fragment with different metals has been
analyzed in detail. See, for example: (a) Mealli, C.; Ienco, A.; Anillo,
A.; Garc´ıa-Granda; Obeso-Rosete, R. Inorg. Chem. 1997, 36, 3724.
(b) Anillo, A.; Garcia-Granda, S.; Obeso-Rosete, R.; Galindo, A.;
Ienco, A.; Mealli, C. Inorg. Chim. Acta 2003, 350, 557.
Inorganic Chemistry, Vol. 46, No. 15, 2007 5945

Roy et al.
Figure 4. X-Band EPR spectrum of [1]+ in frozen CH2Cl2 at -243 °C
(frequency, 9.43 GHz; power, 2.5 × 10-6 mW; modulation amplitude,
8.0 G).
Figure 3. Important π-interactions between the ligand LNH2H2 and the
metal fragment Ru(PH3)2Cl2 in 1.
electronssclearly a nonnegligible numbersfrom the metal,
which leads to the observed bond parameters in 1. A pictorial
representation of the above is shown in Figure 3, which
includes an orbital correlation diagram of the important
π-interactions.
The FMO analysis greatly enforces the interpretation that
the complex 1 formally contains a neutral LNHPhH2 ligand
and a Ru(II) center (low-spin d6, S ) 0). This view is backed
up by SCF stability calculations conducted for 2 which show
that the optimized solution obtained for the singlet ground
state has no instabilities characteristic of an internal diradical
character. In addition, the RuN bond lengths in the complex
1 show values which are atypical for strong bonding
interactions involving a Ru(III) center and an anionic ligand
but instead very characteristic for single bonds between a
divalent Ru atom and a neutral donor ligand.
3.3. Electro- and Spectroelectrochemistry. Compound
1 is electroactive: its cyclic voltammogram in CH2Cl2 (0.20
M [N(n-Bu)4]PF6 supporting electrolyte, glassy carbon
working electrode, scan rate 100 mV s-1) at 20 °C displays
a single reversible one-electron oxidation wave at 0.391 V
referenced vs the ferrocenium/ferrocene couple (Fc+/Fc). The
one electron oxidation process observed in the cyclic
voltammogram appears to be ruthenium-centered, and the
coulometrically generated (at -20 °C) oxidized species1[PF6]
was characterized by EPR and UV-vis spectroscopy (see
below). No reduction wave typical for similar Ru-diimine
complexes17a,b was observed up to -1.8 V (minimum
potential allowed by the experimental setup), but one electron
reduction at a more negative potential could result in
formation of a diimine anion radical. This hypothesis is
backed up by theoretical calculations, which indicate that
complex 1 readily accepts an electron as the anion is found
to be extremely stable species: [2]- is computationally
approximately 115 kJ mol-1 lower in energy than the neutral
complex. Another interesting feature revealed by the calcula-
tions is the predicted electron distribution in [2]-: as the
Figure 5. Spectral changes observed during coulometric oxidation of 1
to [1]+ in CH2Cl2 (0.20 M [N(n-Bu)4]PF6) at -25 °C.
Table 3. Experimental UV-Vis Data for Complexes 1 and [1][PF6]
complex
λ
max (nm) (ꢀ (104 M-1 cm-1))
1
534 (0.23), 406 (2.06), 318 (0.57), 272 (3.06), 237 (3.27)
816 (0.16), 690 (0.23), 457 (1.50), 299 (2.50), 237 (3.30)
[1][PF6]
LUMO of the osazone ligand has pπ orbital contributions
also from the two amino nitrogen atoms (see orbital 12b in
Figure 3), the unpaired electron in [2]- becomes delocalized
throughout the entire NNCCNN fragment, which could
function to increase the stability of the paramagnetic radical.
The X-band EPR spectrum of [1][PF6] in CH2Cl2 solution
at -243 °C yields a single broad resonance depicted in Figure
4, which confirms the paramagnetic nature of [1]+ (S ) 1/2).
From an approximate simulation of the frozen solution
spectra, the following g-tensor parameters were deduced: gx
) 2.2649; gy ) 2.0650; gz ) 1.9064; giso ) 2.0241. These
parameters are consistent with a metal-centered oxidation,
in good agreement with results of DFT calculations which
showed that the highest occupied molecular orbital (HOMO)
of 2 contains a significant contribution from ruthenium d
orbitals.
The electronic spectra of both 1 and its electrochemically
generated monocation [1][PF6] measured at -25 °C are
shown in Figure 5; relevant spectral data are summarized in
Table 3. The brown solution of [1][PF6] is stable only at
low temperature and in an argon blanket atmosphere. The
UV-vis spectrum monitored during the spectroelectrochemi-
5946 Inorganic Chemistry, Vol. 46, No. 15, 2007

A Ru Complex with Glyoxalbis(N-phenyl)osazone
Table 4. Calculated Excitation Energies and Assignments of Low-Lying Electronic Transitions of Complexes 2 and [2]+
complex
symmetry
energy (nm)a
significant contributions (>10%)
dominant type
2
a1
a2
b2
392 (0.099)
346 (0.027)
566 (0.025)
HOMO-3 f LUMO (93%)
HOMO-5 f LUMO (98%)
HOMO f LUMO (62%)
HOMO-1 f LUMO (24%)
HOMO-2 f LUMO (13%)
HOMO-2 f LUMO (65%)
HOMO-1 f LUMO+6 (21%)
HOMO-1 f LUMO+6 (67%)
HOMO-2 f LUMO (19%)
SOMO â f LUMO â (68%)
SOMO-2 â f LUMO â (30%)
SOMO-4 â f LUMO â (51%)
SOMO-3 â f LUMO â (27%)
SOMO-5 â f LUMO â (20%)
SOMO-9 â f LUMO+1 â (37%)
SOMO â f LUMO+1 â (24%)
SOMO-1 R f LUMO+1 R (20%)
SOMO-2 â f LUMO+1 â (15%)
SOMO-9 â f LUMO â (57%)
SOMO-2 â f LUMO+1 â (18%)
dRu + pCl f p*L
πL f π*L
dRu + pCl + πL f π*L
dRu + pCl f π*L
dRu + pCl f π*L
dRu + pCl f π*L
dRu + pCl f pCl + dRu
dRu + pCl f pCl + dRu
dRu + pCl f π*L
πL f dRu + πCl
b3
b4
a2
b3
390 (0.395)
380 (0.189)
877 (0.002)
686 (0.002)
[2]+
dRu + pCl f dRu + pCl
d
Ru f dRu + pCl
dRu + pCl f dRu + pCl
dRu + πL f dRu + pCl
b6
423 (0.045)
418 (0.076)
pCl f π*L
πL f π*L
πL f π*L
dRu + pCl f π*L
b7
p
Cl f dRu + pCl
dRu + pCl f π*L
a Oscillator strengths are given in parentheses.
cal oxidation shows a clean isosbestic conversion. The parent
complex 1 displays three absorbance maxima at 405, 272,
and 237 nm in CH2Cl2. Upon oxidation of 1, the strong band
at around 400 nm is replaced by two bands of the paramag-
netic cation at 450 and 690 nm. No emission bands were
observed in the fluorescence spectrum measured for complex
1, which indicates that the parent osazone ligand holds
limited possibilities to be used in complexes with highly
tunable photochemical properties; it will be of interest to
study whether this property can be affected via selective
substitution of the ligand in an analogical fashion as found
for some structurally similar Ru-osazone complexes.4b
To better understand the optical spectrum measured for 1
and [1][PF6], theoretical calculations were carried out for 2
and [2]+ at the TD-DFT level of theory; the results are
collected in Table 4. Computational analysis of the absorption
properties of 2 finds three high-intensity transitions between
380 and 390 nm all of which involve electron transfer from
high-lying MOs having significant metal d orbital character
to the ligand based unoccupied orbitals, primarily LUMO.
Such transitions are very typical for a variety of Ru-diimine
complexes and are, thus, assigned to the experimental band
at around 400 nm.4b,6,17a,b,19 The lower intensity band with
an experimental band maximum at around 535 nm corre-
sponds nicely with the calculated transition at 565 nm with
62% HOMO f LUMO character.
4): this result corresponds reasonably well with the birth of
a low-intensity band with an absorbance maximum at 816
nm to the UV-vis spectrum of 1 upon oxidation. The results
of DFT calculations for [2]+ suggest that the second low-
intensity band in the experimental spectrum of [1][PF6] most
likely has its origin in metal-centered excitations: both the
calculated transition energy as well as the oscillator strength
match with the properties of the transition observed in the
experimental spectrum at 690 nm. Ligand-centered and
ligand-to-metal type excitations with higher oscillator strengths
are also found at around 420 nm (two transitions), which
are in reasonable agreement with the most intense band
observed in the high-energy side of the visible spectrum of
[1][PF6] at around 460 nm.
4. Conclusion
This study represents a significant addition to the coor-
dination chemistry of simple organic ligands which contain
a diimine chromophore by reporting the first ruthenium(II)
complex of glyoxalbis(N-phenyl)osazone, (LNHPhH2)Ru(PPh3)2-
Cl2 (1), in which two chlorine ions and triphenylphosphine
groups act as coligands. The photophysical, redox, and
bonding properties of the complex were characterized using
a combination of spectroscopic and theoretical methods. The
results demonstrate that (LNHPhH2)Ru(PPh3)2Cl2 has electronic
properties similar to those of its known diimine counterparts.
In particular, complex 1 shows an UV-vis absorption
spectrum with a high-intensity metal-to-ligand charge-transfer
band typical for analogous Ru-diimine complexes; complex
1 is also electroactive and displays a single one-electron
oxidation wave at 0.39 V. DFT-based bonding analyses
showed that (LNHPhH2)Ru(PPh3)2Cl2 exhibits moderately
strong π-interactions which transfer approximately 0.32
electrons from the occupied orbitals of the ruthenium(II)
fragment to the formally unoccupied LUMO of the osazone
ligand. The encouraging results obtained in this study indicate
that the hitherto unexplored coordination chemistry of
osazone with 3d and 5d transition metals is a sought-after
DFT calculations for the bare cation [2]+ indicate that the
removal of electron from the HOMO of 2 does not lead to
significant changes in geometry: the only notable difference
is the elongation of Ru-N bonds by 0.07 Å to 2.08 Å as
electron density is removed from an orbital with significant
Ru d orbital character. The calculations show that the
transition with primarily SOMO f LUMO character should
be of low intensity and observed at the near-IR portion of
the electromagnetic spectrum, close to 880 nm (see Table
(19) Adams, H.; Alsindi, W. Z; Davies, G. M.; Duriska, M. B.; Easun, T.
L.; Fentony, H. E.; Herrera, J. M.; George, M. W.; Ronayne, K. L.;
Sun, X. Z.; Towrie, M.; Ward, M. D. Dalton Trans. 2006, 39.
Inorganic Chemistry, Vol. 46, No. 15, 2007 5947

Roy et al.
objective. In addition, considering the facile synthesis of 1,
it will be of interest to examine the effect on different
substituents to its electronic properties in more detail. The
DFT predicted stability for the anionic metal complex [1]-
containing a paramagnetic osazone radical should also be
verified experimentally.
We are grateful to MPI-fur Bioanorganische Chemie, Muel-
heim, Germany, for spectroelectrochemical measurements.
H.M.T thanks the Academy of Finland for financial support.
Supporting Information Available: X-ray crystallographic files
for complex 1 as well as optimized coordinates for compound 2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was supported by the
University Grants Commission (UGC), New Delhi, India.
IC070294Q
5948 Inorganic Chemistry, Vol. 46, No. 15, 2007