Synthesis, characterization and photophysical properties of [OsO2(mes)2(NC)Ru(bpy)2(CN)]

Article abstract of DOI:10.1039/a607514b

A novel heterometallic complex comprising of a d² OsO₂(mes)₂ and a d⁶ Ru(bpy)₂(CN)₂ fragment is prepared and characterized by X-ray analysis; its photophysical properties are measured.

Full text of DOI:10.1039/a607514b

View Article Online / Journal Homepage / Table of Contents for this issue
Synthesis, characterization and photophysical properties of
[OsO2(mes)2(NC)Ru(bpy)2(CN)]
Jack Y. K. Cheng, Kung-Kai Cheung and Chi-Ming Che*
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
A novel heterometallic complex comprising of a d2
OsO2(mes)2 and a d6 Ru(bpy)2(CN)2 fragment is prepared
and characterized by X-ray analysis; its photophysical
properties are measured.
distorted square-pyramidal geometry with an average Os–O
distance of 1.700 Å and O–Os–O angle of 144.9(2)°. These are
comparable to values of 1.720 Å and 147.5(3)° in the related
complex [{OsO2(mes)2}2(pyz)]4 which has a square-pyramidal
geometry at the Os atom. Although the measured N–C–Ru
angle of 177.7(4)° is close to 180°, the Os–N–C angle of
160.5(4)° reveals that the Ru–C·N–Os array is not quite linear.
The N–C and Ru–C distances for the bridging cyanide are both
0.04 Å shorter than that of the terminal cyanide. The IR data are
in accord with the X-ray structure with n(C·N) stretches at
2088.5 and 2072.8 cm21, at higher frequencies relative to
[Ru(bpy)2(CN)2] [n(C·N) 2067.4, 2052.9 cm21]. The n(OsNO)
stretches are at 893.1 and 884.6 cm21, which are comparable to
the related value of 900 cm21 for [{OsO2(mes)2}2(m-pyz)].
Complex 1 is soluble in common organic solvents such as
dichloromethane and methanol. Similarly to the reported
The design of new heterometallic complexes for photoinduced
atom transfer reactions has received current interest. One of our
approaches is to link a low-valent metal–polypyridine to a high-
valent metal–oxo fragment via self-assembly reactions.1 The
former would serve as a good light absorber while the latter
functions as an oxygen atom transfer agent.
hν
Excited
O*
[OsO2(mes)2L] complexes (L
= 1,10-phenanthroline or
M′
N
M
N,N,NA,NA-tetramethylethylenediamine),5 the equilibrium reac-
tion [eqn. (1)] exists in solution and dissociation is suppressed
by addition of an excess of [OsO2(mes)2].
N
[OsO2(mes)2(NC)Ru(bpy)2(CN)] " [OsO2(mes)2] +
In this context, the cyanide ion, which is known to form stable
complexes with metal ions in both high and low oxidation states
would be a good bridging ligand.2 Described herein are the
structure and photophysical properties of the first heteronuclear
RuIIOsVI [OsO2(mes)2(NC)Ru(bpy)2(CN)] 1 formed by reac-
tion of [OsO2(mes)2] (mes = mesityl)3 with [Ru(bpy)2(CN)2]
(bpy = 2,2A-bipyridine).2
Upon slow evaporation of a solution of [OsO2(mes)2] and
[Ru(bpy)2(CN)2] in dichloromethane–methanol at 0 °C, brown
needle crystals† of 1 were obtained (yield 95%) and the
structure was characterized by X-ray analysis.‡ Fig. 1 shows a
perspective view of the molecule. The osmium atom adopts a
[Ru(bpy)2(CN)2] (1)
Fig. 2 shows the absorption spectra of solutions of [Ru-
(bpy)2(CN)2] in the presence of various concentrations of
[OsO2(mes)2]. The MLCT band of [Ru(bpy)2(CN)2] shifts to a
higher energy (461 to 436 nm) upon addition of [OsO2(mes)2]
(0 to 10-fold excess). A similar experiment with [Ru(bpy)3]Cl2
has also been done but its MLCT band does not change even in
the presence of tenfold excess of [OsO2(mes)2]. Thus the
progressive blue shift is evidence for the formation of the
dimeric species 1 in solution.6
0.5
(a)
(b)
C(39)
C(17)
O(1)
Os
C(14)
C(11)
C(18)
C(38)
C(13)
C(15)
C(40)
N(6)
C(37)
N(2)
Ru
C(12)
C(10)
C(2)
C(7)
x = 10
C(20)
C(36)
C(16)
N(1)
C(19)
C(35)
C(34)
O(2)
C(1)
C(6)
6.7
C(3)
C(30)
N(5)
C(31)
N(4)
C(33)
N(3)
C(4)
C(8)
C(29)
C(32)
C(26)
3.3
C(21)
C(9)
C(5)
1
0
C(25)
C(22)
C(23)
C(28)
C(27)
C(24)
Fig. 1 A perspective view of 1 (thermal ellipsoids at 40% probability level).
Selected bond lengths (Å) and angles (°): Os–O(1) 1.704(3), Os–O(2)
1.696(3), Os–N(1) 2.235(4), Os–C(1) 2.063(5), Os–C(10) 2.108(4), Ru–
N(3) 2.064(4), Ru–N(4) 2.113(4), Ru–C(19) 1.972(4), Ru–C(20), 1.992(4),
N(1)–C(19) 1.143(6), N(2)–C(20) 1.145(6); O(1)–Os–O(2) 144.9(2),
O(1)–Os–N(1) 88.5(1), O(1)–Os–C(1) 110.4(2), O(1)–Os–C(10) 91.5(2),
N(1)–Os–C(1) 87.1(2), N(1)–Os–C(10) 173.4(2), C(1)–Os(1)–C(10)
99.1(2), C(19)–Ru–C(20) 89.5(2), Os–N(1)–C(19) 160.5(4), Ru–
C(19)–N(1) 177.7(4), Ru–C(20)–N(2) 178.6(4).
0.0
400
500
600
700
λ / nm
400
500
600
700
Fig. 2 UV–VIS absorption spectra of (a) [Ru(bpy)2(CN)2] (2.0 3 1025
mol dm23) and (b) [Ru(bpy)3]Cl2 (2.0 3 1025 mol dm23) at various
concentrations of [OsO2(mes)2]. Ratio of the OsO2(mes)2 :Ru
complexes = 1:x.
Chem. Commun., 1997
501

View Article Online
Complex 1 in the solid state is non-emissive at 298 and 77 K.
In solution, it gives an emission at the same energy as that of
[Ru(bpy)2(CN)2] measured under similar conditions. However,
the lifetime and intensity of the emission are much reduced.
Fig. 3 shows f0/f and t0/t vs. [quencher] plots for [Ru-
(bpy)2(CN)2] and [Ru(bpy)3]Cl2 with [OsO2(mes)2] as
quencher (f and t are emission intensity and decay lifetime,
respectively). It is interesting that the decay time Stern–Volmer
quenching constant, KSVt {deduced from the slope of the plot of
t0/t vs. concentration of [OsO2(mes)2]} for both complexes are
similar (1038 and 1098 dm3 mol21). The value of the Stern–
quenching product is [OsO2(mes)2]*. If intramolecular electron
transfer proceeds, the quenching product is either the reduced or
oxidized species of [OsO2(mes)2]. In any case, the energetic or
reactive site shifts from ruthenium to the Os–oxo moiety. The
rich oxidation chemistry and photochemistry of high-valent
osmium–oxo complexes are well documented in literature.9
Thus formation of an activated five-coordinate osmium–oxo
complex through intramolecular electron- and/or energy-trans-
fer would provide an entry to a new class of metal–oxo
photooxidants with good light absorbing properties.
We acknowledge support from the University of Hong Kong,
the Croucher Foundation, and the Hong Kong Research Grants
Council.
f
Volmer quenching constant, KSV {deduced from the slope of
the plot of f0/f vs. concentration of [OsO2(mes)2]} for
t
[Ru(bpy)3]Cl2 (1842 dm3 mol21) is slightly larger than the KSV
value, presumably this is due to the overlap of the absorption
spectrum of [OsO2(mes)2] with the excitation spectrum of
[Ru(bpy)2(CN)2]. Interestingly, the Stern–Volmer plots for
[Ru(bpy)2(CN)2] are not linear and exhibits upward curvature
characteristic of a system showing both static and dynamic
quenching.7 The static quenching is assigned to ground-state
complex formation of [OsO2(mes)2(NC)Ru(bpy)2(CN)] from
[OsO2(mes)2] and [Ru(bpy)2(CN)2] in solution. This hete-
rometallic complex in the solid state is non-emissive suggesting
that the long-lived emissive MLCT state of the Ru(bpy)2(CN)2
chromophore is completely quenched by the OsO2(mes)2 unit
Footnotes
† A mixture of [OsO2(mes)2] (60 mg, 0.13 mmol) and [Ru(bpy)2(CN)2] (50
mg, 0.11 mmol) in dichloromethane–methanol (1:1, 10 ml) was stirred for
1 h. The solvent was slowly evaporated to dryness in a water bath (25 °C)
to give a brown microcrystalline solid which was washed with diethyl ether
(3
3 10 ml). This was recrystallized by slow evaporation of a
dichloromethane–methanol mixture at 0 °C to afford brown needle crystals;
yield 97 mg (95%). 1H NMR (CD3OD): d (bpy) 9.61 (d, 2), 8.52 (d, 2), 8.46
(d, 2), 8.09 (t, 2), 7.94 (t, 2), 7.58 (m, 4), 7.29 (t, 2), (mes) 6.92 (s, 4), 2.43,
(s, 6), 2.32 (s, 12). IR n(C·N) 2088.5s, 2072.8s; n(OsNO) 893.1s, 884.6s,
849.1m. Anal. Calc. for C40H38N6O2OsRu: C, 51.88; N, 9.08; H, 4.14.
Found: C, 50.91; N, 8.90; H, 3.98%.
t
via the bridging cyano group. Because the KSV values for
‡ Crystal data: [C40H38N6O2OsRu·2CH3OH]; Mr = 990.133, monoclinic,
space group P21/c (no. 14), a = 17.421(3), b = 10.194(3), c = 23.259(3)
Å, b = 95.91(2)°, U = 4108(1) Å3, Z = 4, Dc = 1.601 g cm23, m(Mo-
Ka) = 35.03 cm21, F(000) = 1968, T = 301 K. A brown crystal of
dimensions 0.25 3 0.20 3 0.30 mm was used for data collection at 28 °C
on a MAR diffractometer with a 300 mm image plate detector using
graphite-monochromated Mo-Ka radiation (l = 0.71073 Å). 7804 unique
reflections were obtained from a total of 33227 measured reflections. 5747
reflections with I > 3 s(I) were considered observed and used in the
structural analysis. All 54 non-H atoms were refined anisotropically.
Hydrogen atoms of the methanol molecules were not found but 38 H atoms
of the complex molecule at calculated positions with thermal parameters
equal to 1.3 times that of the attached C atoms were not refined.
Convergence for 487 variable parameters by least-squares refinement on F
with w = 4F02/s (F02), where s (F02) = [s (I) + (0.018 F02)2] for 5747
reflections with I > 3s(I) was reached at R = 0.035 and wR = 0.043 with
a goodness-of-fit of 1.61 (D/s)max = 0.04 for atoms of the complex
molecule. The final difference Fourier map was featureless, with maximum
positive and negative peaks of 0.66 and 1.24 eÅ23 respectively. Atomic
coordinates, bond lengths and angles, and thermal parameters have been
deposited at the Cambridge Crystallographic Data Centre (CCDC). See
Information for Authors, Issue No. 1. Any request to the CCDC for this
material should quote the full literature citation and the reference nubmer
182/350.
[Ru(bpy)3]Cl2 and [Ru(bpy)2(CN)2] are similar, we propose
that the dynamic quenching mechanism which represents the
collision of [OsO2(mes)2] and the excited ruthenium partner
through diffusion would be outer sphere in terms of electron-
and/or energy-transfer.
Previous studies showed that the Ru(bpy)2(CN)2 chromo-
phore could be easily functionalized via cyanide bridge
formation with a suitable choice of adducts [Pt(dien)2b or
Ru(NH3)52b] but Ru–CN–M (M
= Lewis acid) adduct
formation were found not to substantially modify the excited-
state properties of the Ru(bpy)2(CN)2 chromophore.2b,6 In this
work, the excited state of 1 which localizes at the ruthenium
centre should be MLCT in nature. This excited state is rapidly
quenched by intramolecular energy and/or electron transfer
pathways. If intramolecular energy transfer proceeds, the
2
2
2
8
[Ru(bpy) (CN) ], τ
7
6
5
4
3
2
1
2
2
[Ru(bpy) (CN) ], φ
2
2
[Ru(bpy) ]Cl , τ
3
2
References
[Ru(bpy) ]Cl , φ
3
2
1 P. K. K. Ho, K. K. Cheung and C. M. Che, Chem. Commun., 1996,
1197.
2 (a) C. A. Bignozzi, S. Roffia and F. Scandola, J. Am. Chem. Soc., 1985,
107, 1644; (b) C. A. Bignozzi and F. Scandola, Inorg. Chem., 1984, 23,
1540.
3 B. S. McGilligan, J. Arnold, G. Wilkinsin, B. Hussain-Bates and
M. B. Hursthouse, J. Chem. Soc., Dalton Trans., 1990, 2465.
4 W. H. Leung, J. Y. K. Cheng, T. S. M. Hun, C. M. Che, W. T. Wong and
K. K. Cheung, Organometallics, 1996, 15, 1497.
5 K. F. Chin, Y. K. Cheng, K. K. Cheung, C. X. Guo and C. M. Che,
J. Chem. Soc., Dalton Trans., 1995, 2967.
6 C. Bartocci, C. A. Bignozzi, F. Scandola, R. Rumin and P. Courtot, Inorg.
Chim. Acta, 1983, 76, L119.
7 J. N. Demas and J. W. Addington, J. Am. Chem. Soc., 1974, 96, 3663;
J. N. Demas, J. W. Addington, S. H. Peterson and E. W. Harris, J. Phys.
Chem., 1977, 81, 1039.
8 A. Vogler and J. Kisslinger, J. Am. Chem. Soc., 1982, 104, 2311.
9 V. W. W. Yam and C. M. Che, J. Chem. Soc., Dalton Trans., 1990,
3741.
0.5
0.0
1.0
1.5
2.0
4
–3
10 [OsO (mes) ] / mol dm
2
2
Fig. 3 Plots of f0/f and t0/t vs. [OsO2(mes)2] for complex 1
Chem. Commun., 1997
Received, 5th November 1996; Com. 6/07514B
502