Highly luminescent neutral cis-dicyano osmium(II) complexes

Article abstract of DOI:10.1039/a700051k

Two highly luminescent neutral cis-dicyanoosmium(II) complexes are prepared by photoinduced oxygen atom transfer reactions of trans-[OsO₂(CN)₂(dpphen)] with PPh₃ and Me₂SO; their MLCT excited states are emissive

Full text of DOI:10.1039/a700051k

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Highly luminescent neutral cis-dicyano osmium(ii) complexes
Jack Y. K. Cheng, Kung-Kai Cheung and Chi-Ming Che*
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
Two highly luminescent neutral cis-dicyanoosmium(II) com-
plexes are prepared by photoinduced oxygen atom transfer
reactions of trans-[OsO2(CN)2(dpphen)] with PPh3 and
Me2SO; their MLCT excited states are emissive with long
lifetimes (0.20–1.99 ms) and high quantum yields [(40–1.8)
31022] and the emission maxima show pronounced
solvatochromism.
coordination geometry of the Os atom is slightly distorted
octahedral with the cyanide ligands cis to each other. The Os–C
bond of 2.00(1) Å is comparable to that of 2.023 Å in [AsPh4]2–
[Os(PPh3)2(CN)4]·2MeCN·2H2O.6 The two Me2SO ligands are
S-bonded to the Os atom. They are trans to each other with
S–Os–S angle of 174.1(2)°. The Os–S distance of 2.293(3) Å is
similar to the related distances of 2.31(2) Å in [Os(S7)(PMe3)3]7
and 2.261 Å in [Ru(Me2SO)3Cl3]2 (S-coordinated).8 To our
knowledge, no X-ray structure of an OsII–Me2SO complex has
been reported in the literature. [Os(Me2SO)3Cl3]9 has been
proposed to be O- and S-coordinated while the structure of
[Os(bpy)2(MeSO)2]2+ is unknown.3b
Complexes 2 and 3 are luminescent in the solid state and in
solution with emission maxima ranging from 710 to 564 nm.
The spectral data are listed in Table 1. The two complexes
display a strong absorption band in the UV region (lmax ca. 300
nm, e > 105 dm3 mol21 cm21) and a moderate intense
absorption band at the visible region (lmax ca. 440 nm, e > 103
dm3 mol21 cm21) with a tailing at 500–600 nm, the latter is
ascribed to the Os dp–p* MLCT transition.10 Both the
absorption and emission spectra are strongly affected by the
solvent and both the emission lifetime and quantum yield
decrease as the solvent polarity decreases.
Luminescent metal complexes with high emission quantum
yields and which could function as molecular building blocks
for polynuclear metal complexes find useful applications in
supramolecular chemistry and photocatalysis.1 However, exam-
ples of these complexes are sparse in literature. In this context,
we were attracted to the wide applications of [RuL2(CN)2]2
(L = 2,2A-bipyridine or 1,10-phenanthroline) as building blocks
for polynuclear cyano-bridged metal complexes and the striking
2+
luminescent properties of [OsIILx(P–P)3 2 x
]
(P–P = diphos-
phine, x = 1 or 2).3 Herein is described the formation of highly
luminescent neutral cyanoosmium(ii) complexes which have
the combined features and properties of both [RuL2(CN)2] and
[OsIILx(P–P)3 2 x]2+. These complexes are anticipated to pro-
vide a new entry to supramolecular photochemistry.
[OsO2(CN)2(dpphen)] 1 (dpphen = 4,7-diphenyl-1,10-phen-
anthroline) was prepared from dpphen and [OsO2(C-
N)2(OH)2]22 following the reported literature method.4 As with
other dioxoosmium(vi) complexes,4,5 it has a long-lived and
Plots of 1n knr vs. Eem for complexes 2 and 3 in various
solvents are shown in Fig. 2. Except for the data of 2 in protic
solvents, a linear correlation between 1n knr and Eem values is
observed suggesting that the non-radiative decay is governed by
the energy-gap law.11 The slope of 26.8 eV21 and intercept of
emissive excited state [lifetime 1 ms; quantum yield 5 3 1024
;
l
max, em 650 nm].
Under UV irradiation, complex 1 oxidizes PPh3 and Me2SO
to give OPPh3 and Me2SO2, respectively. For the photoreaction
of 1 in neat Me2SO, removal of the solvent after photolysis gave
an orange solid, which was recrystallized from MeOH–Et2O to
give [Os(CN)2(Me2SO)2(dpphen)]·2H2O 2. Similar photo-
reaction with PPh3 gave [Os(CN)2(PPh3)2(dpphen)] 3.†
Table 1 Spectroscopic data for complexes 2 and 3
l
nm
max, em (lmax, abs)b/ Lifetime/ms (quantum
Solvent (ANa)
yield)
2
C6H6 (8.2)
629 (377)
630 (375)
635 (369)
630 (359)
616 (361)
630 (364)
625 (359)
613 (359)
597 (357)
600 (344)
594 (341)
595 (340)
575 (332)
564 (327)
710 (438)
700 (420)
681 (420)
675 (418)
673 (414)
660 (409)
672 (418)
595 (379)
645 (399)
632 (387)
639 (396)
1.12 (0.24)
1.09 (0.23)
0.92 (0.17)
0.95 (0.17)
1.45 (0.39)
1.04 (0.20)
1.10 (0.18)
1.45 (0.34)
1.99 (0.49)
1.42 (0.28)
1.43 (0.34)
1.34 (0.25)
1.62 (0.04)
1.37 (0.08)
0.33 (0.03)
0.42 (0.04)
0.48 (0.06)
0.55 (0.09)
0.57 (0.08)
1.39 (0.05)
0.62 (0.09)
3.72 (0.33)
0.79 (0.13)
1.09 (0.17)
0.94 (0.11)
Fig. 1 shows a perspective view of 2.‡ The structure features
a rare example of OsII–CN and OsII–Me2SO complexes. The
C6H5Me (8.2)
Me2CO (12.5)
dmf (16.0)
C2H4Cl2 (16.7)
Me2SO (19.3)
MeCN (19.3)
CH2Cl2 (20.4)
CHCl3 (23.1)
EtOH (37.1)
HCONH2 (39.8)
MeOH (41.3)
MeOH–H2O
H2O (54.8)
O(1′)
C(3′)
C(13′)
C(2′)
N(2)
Si(1′)
C(15′)
C(8)
N(1′)
C(14)
C(1)
3
C6H6 (8.2)
Me2CO (12.5)
Py (14.2)
C(9)
C(13)
C(7)
C(15)
C(5)
Os
N(1)
C(6)
C(10)
dmf (16.0)
C(1′)
C(4)
Me2SO (19.3)
MeCN (19.3)
CH2Cl2 (20.4)
CHCl3 (23.1)
EtOH (37.1)
HCONH2 (39.8)
MeOH (41.3)
N(2′)
C(11)
C(12)
C(3)
O(1)
S(1)
C(2)
Fig. 1 A perspective view of 2 (40% probability ellipsoids): Os–S(1)
2.293(3), Os–N(1) 2.111(8), Os–C(1) 2.00(1); S(1)–Os–S(1A) 174.1(2),
S(1)–Os–C(1) 87.7(3), S(1)–Os–N(1) 91.1(2), N(1)–Os–C(1) 94.5(4),
C(1)–Os–C(1)A 94.6(6)
a
Gutmann’s solvent acceptor number.14b Measurements at room tem-
perature. b Spin-allowed MLCT absorption maximum.
Chem. Commun., 1997
623

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27 agree with similar values of ca. 27.5 eV21 (slope) and
28–30 (intercept) of complexes [OsX4L] and [OsX2L2] (X ≠
CO) in which cases the non-radiative decay is dominated by the
ligand-based C–C vibrational stretch modes (ca. 1350 cm21).12
Complex 2 shows derivation from the linear correlation in
protic solvents. Presumably, a change of dominant acceptor
vibration for non-radiative decay is attributed to the hydrogen-
bonding interaction between the solvent and Me2SO.2c
low-lying d–d states being close in energy.3b Complexes 2 and
3 were found to be stable even after photolysis in acetonitrile for
2 days. Presumably, the OsII?CN p-back bonding increases the
d–d state energy relative to the MLCT emitting state.13
We acknowledge support from the University of Hong Kong,
the Croucher Foundation, and the Hong Kong Research Grants
Council.
For both complexes, the plots of absorption and emission
energy versus Gutmann’s solvent acceptor number, AN, are
shown in Fig. 3. Good linear relationship is observed in each
case. The absorption and emission spectra shift to higher energy
with increasing AN. This implies the donor–acceptor inter-
action between the solvent and the complexes. The donor–
acceptor interaction results in decrease of the s-donating ability
of the cyanide ligand (and/or Me2SO ligand).
Most osmium(ii) polypyridine complexes are known to be
stable towards photosubstitution reaction. However, it has been
noted that the [Os(bpy)2(Me2SO)2]2+ complex is photo-
chemically unstable and this has been ascribed to its MLCT and
Footnotes
† 2 1H NMR (CDCl3) d 10.01 (d, 2 H), 8.06 (s, 2 H), 7.84 (d, 2 H), 7.67–7.59
(m, 10 H), 3.39 (s, 12 H, OSMe2). UV–VIS (MeCN) lmax/nm (e dm3 mol21
cm21): 269 (34721), 287 (32589), 359 (10457), 405 (sh) (6498), 450 (sh)
(1427). (H2O): 257 (18400), 287 (33098), 327 (13814), 385 (sh) (5697). 3
1H NMR (CDCl3) d 8.34 (d, 2H), 8.17 (d, 2H), 7.62–7.58 (m, 4H),
7.50–7.41 (m, 18H), 7.17–7.04 (m, 18H), 6.72 (dd, 2H). UV–VIS (MeCN)
l
max/nm (e/dm3 mol21 cm21): 282 (36590), 410 (7454), 440 (6445), 515
(sh) (1185).
‡ Crystal data: [OsS2O2N4C30H28·2H2O], Mr = 766.93, monoclinic, space
group C2/c (no. 15), a = 19.180(5), b = 19.902(5), c = 9.471(3) Å, b =
118.18(3)°, U = 3186(1) Å3, Z = 4, Dc = 1.598 g cm23, m = 41.69 cm21
,
F(000) = 1520, T = 301 K. An orange–brown crystal of dimensions 0.15
3 0.10 3 0.25 mm, was used for data collection on a Rigaku AFC7R
diffractometer with graphite-monochromated Mo-Ka radiation (l
=
0.71073 Å). 2905 unique reflections measured, 2117 of which [with I >
3s(I)] were considered observed. The structure was solved by the Patterson
15.0
2, non-protic solvent
3, non-protic solvent
2, protic solvent
method and refined by full-matrix least-squares. Convergence for 186
variable parameters by least-squares refinement on F with w = 4Fo2/s(Fo
)
2
14.5
2
2
where s (Fo2) = [s (I) + (0.014Fo2)2] for 2117 unique reflections was
reached at R = 0.040, Rw = 0.057, G.O.F. = 1.89. The final difference
Fourier map was featureless, with maximum positive and negative peaks of
1.38 and 20.81 e Å23 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 number 182/370.
3, protic solvent
EtOH
14.0
MeOH
HCONH
2
HCONH
k
13.5
13.0
12.5
12.0
2
MeOH
H O
2
EtOH
MeOH–H O
2
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Fig. 2 A plot of 1n knr vs. Eem for complexes 2 and 3
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Fig. 3 Plots of nabs (circles) and nem (squares) vs. Gutmann’s acceptor
number (AN) for complexes 2 and 3
Received, 2nd January 1997; Com. 7/00051K
624
Chem. Commun., 1997