Unprecedented luminescence behaviour and structural characterization of a novel class of ruthenium(II) 2,2′-bipyridine complexes with orthometallated aminocarbene ligands

Article abstract of DOI:10.1039/a807281g

Novel luminescent ruthenium(II) bipyridine complexes with orthometallated aminocarbene ligands have been prepared and their photophysical properties studied.

Full text of DOI:10.1039/a807281g

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Unprecedented luminescence behaviour and structural characterization of a
novel class of ruthenium(ii) 2,2A-bipyridine complexes with orthometallated
aminocarbene ligands
Vivian Wing-Wah Yam,* Ben Wai-Kin Chu and Kung-Kai Cheung
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China.
E-mail: wwyam@hkucc.hku.hk
Novel luminescent ruthenium(II) bipyridine complexes with
orthometallated aminocarbene ligands have been prepared
and their photophysical properties studied.
the Ru(1)–C(1) bond distance [1.963(7) Å] is shorter than an
average Ru–C bond [2.105(5) Å],9e which can be ascribed to the
presence of Ru–C double bond character.9f,10 A substantial
double-bond character between the heteroatom and the carbene
carbon is noticed as the C(1)–N(5) distance [1.318(9) Å] is
reduced below that characteristic of a single bond between N
and an sp2 C, typical of Fischer type aminocarbenes (ca. 1.31
Å).10
The electronic absorption spectra of complexes 2a and 2b
show moderately intense bands in the visible region which are
tentatively assigned as MLCT transitions (Table 1). The intense
absorptions in the UV region are assigned as intraligand
The chemistry of metal s-acetylide complexes has attracted
considerable interest because of the unique properties of the
delocalizable p systems. While most studies on ruthenium
s-acetylide complexes were focused on the use of phosphine
ligands,1 corresponding studies on nitrogen donor ligands are
relatively rare.2 In view of the rich photophysical and
photochemical behaviour of ruthenium(ii) polypyridyl com-
plexes, we have started a program to design and synthesize
luminescent organometallic ruthenium(ii) s-acetylide com-
plexes containing polypyridyl ligands. In an attempt to prepare
these complexes, we obtained the novel ruthenium(ii) orthome-
tallated aminocarbene complexes instead. Here we report the
synthesis, characterization, electrochemistry, photophysical
behaviour and X-ray crystal structure of this new class of
ruthenium(ii) orthometallated aminocarbene complexes with
2,2A-bipyridyl ligands, which represents the first of its kind.
Reaction of cis-[Ru(bpy)2(Me2CO)2](OTf)2 with phenyl-
acetylene or 4-methoxyphenylacetylene in the presence of
sodium ethoxide in ethanol, followed by metathesis reaction
using NH4PF6 and subsequent recrystallization from MeCN–
Et2O, afforded cis-[Ru(bpy)2(CO)(h-CH2Ph)]+ 1a3 or cis-
[Ru(bpy)2(L)2]2+
Z
C
CH
–L
Z = H, OMe
2+
H
Cβ
Cα
(bpy)2Ru
L
X
NH2
Z
2
[Ru(bpy)2(CO)(h-CH2C6H4OMe)]+ 1b as the PF6 salt in
reasonable yield; the structure of 1b was confirmed by X-ray
diffraction studies.4 On the other hand, treatment of cis-
[Ru(bpy)2(Me2CO)2](OTf)2 with phenylacetylene in the pres-
ence of aniline in dry acetone under an inert atmosphere of
nitrogen gave a stable orthometallated aminocarbene complex,
X = H, OMe
Z = H
H2O
2+
X
OH
2+
[Ru(bpy)2NC(CH2Ph)NHC6H4]+ 2a. Similar reaction with p-
(bpy)2Ru
L
anisidine in place of aniline gave [Ru(bpy)2NC(CH2Ph)NHC6-
H3OMe]+ 2b (Scheme 1).5–7 The formulation of which were
confirmed by satisfactory elemental analyses, FABMS, 1H
NMR and 13C NMR spectroscopy† and the structure of 3b was
further established by X-ray crystallography (Fig. 1).‡
Z
H
NH
(bpy)2Ru
Ph
L
Complex 2b shows a distorted octahedral structure. The
N–Ru–N bond angles subtended by the chelating diimines are
77.0(2) and 77.6(2)°. A C(1)–Ru(1)–C(2) bond angle of
79.5(3)° subtended by the orthometallated N-arylcarbene has
also been observed. The deviation from the ideal 90° for a
regular octahedral geometry is a result of the steric requirement
of the bidentate ligands. The bond angles around C(1) are
128.7(5), 116.9(5) and 114.3(6)°, consistent with the sp2
hybridization of the carbene carbon. The bond distances of
Ru(1)–N(1) [2.060(6) Å] and Ru(1)–N(4) [2.058(6) Å] are
similar to those reported in other ruthenium(ii) polypyridyl
complexes (ca. 2.05 Å)8 but those of Ru(1)–N(2) [2.141(5) Å]
and Ru(1)–N(3) [2.120(5) Å] are longer than normal. This may
be accounted for by the strong trans effect of the carbon atoms
in the orthometallated N-arylcarbene ligand. The Ru(1)–C(2)
bond distance [2.047(6) Å] is similar to that observed in other
ruthenium(ii) complexes with s-bonded carbon ligands,9 while
–H+, –L
–H+, –L
+
X
+
(bpy)2Ru
CO
NH
(bpy)2Ru
Ph
Z
2a X = H
b X = OMe
1a Z = H
b Z = OMe
Scheme 1 Synthetic route to orthometallated ruthenium(ii) aminocarbene
complexes; L = Me2CO
Chem. Commun., 1998
2261

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irreversible nature of the oxidation in 2a is indicative of the
instability of the Ru(iii) aminocarbene complex in which the
electron-rich methoxy substituent capable of stabilizing the
electron-deficient Ru(iii) metal center is absent, which may lead
to its decomposition.
Further spectroscopic studies to elucidate the nature of the
lowest lying excited state are in progress.
V. W.-W. Y. acknowledges financial support from the
Research Grants Council and The University of Hong Kong.
B. W.-K. C. acknowledges the receipt of a postgraduate
studentship, administered by The University of Hong Kong.
Notes and References
† 2a: Elemental analysis: Calc. for 2a (found) %: C 54.18 (54.22), H 3.85
(3.56), N 9.30 (9.23); positive FABMS: m/z 607 [M 2 PF6]+; 1H NMR (300
MHz, CD3CN, 298 K): d 4.3 (s, 2H, CH2), 6.2–8.3 (m, 25H, aromatic H),
11.2 (s, 1H, NH); 13C NMR (67.8 MHz, CD3CN, 298 K): d 52.71 (CH2),
113.98–176.58 (aromatic C), 266.01 (RuNC). 2b : Elemental analysis: Calc.
for 2b (found) (%): C 57.07 (57.24), H 4.08 (4.08), N 9.51 (9.74); 1H NMR
(300 MHz, CD3CN, 298 K): d 3.5 (s, 3H, OCH3), 4.3 (s, 2H, CH2), 5.8–8.3
(m, 24H, aromatic H), 11.2 (s, 1H, NH); 13C NMR (125.76 MHz, CD3CN,
298 K): d 53.25 (OCH3), 55.70 (CH2), 105.90–180.09 (aromatic C), 262.70
(RuNC).
Fig. 1 Perspective drawing of the complex cation 2b with atomic numbering
scheme. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids
are shown at the 50% probability levels. Selected bond lengths (Å) and
angles (°): Ru(1)–N(1) 2.060(6), Ru(1)–N(2) 2.141(5), Ru(1)–N(3)
2.120(5), Ru(1)–N(4) 2.058(6), Ru(1)–C(1) 1.963(7), Ru(1)–C(2) 2.047(6),
C(1)–C(8) 1.514(9), C(8)–C(9) 1.519(10), C(1)–N(5) 1.318(9), C(3)–N(5)
1.426(9); N(1)–Ru(1)–N(2) 77.6(2), N(3)–Ru(1)–N(4) 77.0(2), C(1)–
Ru(1)–C(2) 79.5(3), Ru(1)–C(1)–C(8) 128.7(5), C(8)–C(1)–N(5) 114.3(6),
Ru(1)–C(1)–N(5) 116.9(5).
‡ Crystal data for 2b : {[C35H30ON5Ru]+ClO42}, Mr = 737.18, triclinic,
¯
space group P1 (no. 2), a = 9.398(4), b = 12.843(4), c = 14.994(4) Å, a
= 67.74(3), b = 77.36(3), g = 71.07(3)°, V = 1574(1) Å3, Z = 2, Dc
=
1.555 g cm23, m(Mo-Ka) = 6.35 cm21, F(000) = 752, T = 301 K.
Convergence for 436 variable parameters by least-squares refinement on F
with w = 4 Fo2 / s2(Fo2), where s2(Fo2) = [s2(I) + (0.018Fo2)2] for 3911
reflections with I > 3s(I) was reached at R = 0.049 and wR = 0.066 with
a goodness-of-fit of 2.75. CCDC 182/1023.
Table 1 Photophysical data for complexes 2a and 2b
labs/nm
Complex (e/dm3 mol21 cm21
)
Medium T/K lema/nm t0/ms
1 See, for example: P. H. Dixneuf, D. Touchard, P. Haquette, N. Pirio and
L. Toupet, Organometallics, 1993, 12, 3132; Z. Atherton, C. W.
Faulkner, S. L. Ingham, A. K. Kakkar, M. S. Khan, J. Lewis, N. J. Long
and P. R. Raithby, J. Organomet. Chem., 1996, 462, 265; Y. Sun, N. J.
Taylor and A. J. Carty, J. Organomet. Chem., 1992, 423, C43; G. Jia,
J. C. Gallucci, A. L. Rheingold, B. S. Haggerty and D. W. Meek,
Organometallics, 1991, 10, 3459.
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Commun., 1985, 648; A. M. Echavarren, J. Lopez, A. Santos, A.
Romero, J. A. Hermoso and A. Vegas, Organometallics, 1991, 10,
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Romero, J. Organomet. Chem., 1992, 426, 383; T. Rappert and A.
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2a
250 (37 960), 298
(46 400), 370 (11 630),
484 (6230), 572 (6605)
MeCN
298
808
< 0.1
< 0.1
< 0.1
< 0.1
Solid
298
77
77
775
704
742
813
Solid
Glassb
MeCN
2b
250 (37 680), 298
(52 720), 370 (12 645),
482 (6850), 571 (7090)
298
Solid
Solid
Glassb
298
77
77
767
701
745
a Excitation wavelength at 580 nm. Emission maxima are corrected values.
b EtOH–MeOH (4:1, v/v).
transitions. Excitation of 2a and 2b at l > 350 nm at room
temperature produces red luminescence. It is likely that the
3
origin of the emission is MLCT in nature, arising from states
derived from either a dp(Ru) ? p*(bpy) or a dp(Ru) ?
p*(alkylidene) MLCT transition. The close similarity of the
absorption and emission characteristics of complexes 2a and 2b
suggests that the methoxy substituent on the N-aryl ring of the
aminocarbene unit has relatively little influence on the charge-
transfer transition in these complexes.
The redox properties of the complexes 2a and 2b are
investigated by cyclic voltammetry in acetonitrile using 0.1 mol
dm23 NBu4PF6 as the supporting electrolyte. Reversible to
quasi-reversible reduction couples are observed at 21.66 and
21.89 V vs. SCE for 2a and 21.65 and 21.88 V vs. SCE for 2b;
the potentials of which are relatively independent of the scan
rate with D(Epa 2 Epc) values of ca. 60–90 mV, assigned to the
successive reduction of the bipyridine ligand. The relative
insensitivity of the reduction potentials to the substituent effect
on the aminocarbene unit in 2a and 2b further confirms its
assignment as bpy-centered reduction. A quasi-reversible
oxidation couple is observed at +1.42 V vs. SCE for 2b and an
irreversible oxidation wave is noted at Epa = +1.67 V vs. SCE
for 2a, which are assigned as metal-centered oxidation. The
4 V. W. W. Yam and B. W. K. Chu, unpublished work.
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Received in Cambridge, UK, 20th July 1998; revised manuscript received
15th Setpember 1998; 8/07281G
2262
Chem. Commun., 1998