General synthesis of (salen)ruthenium(III) complexes via N...N coupling of (salen)ruthenium(VI) nitrides

Article abstract of DOI:10.1021/ic800263n

Reaction of [Ru^VI(N)(L¹)(MeOH)]⁺ (L ¹ = N,N′-bis(salicylidene)-o-cyclohexylenediamine dianion) with excess pyridine in CH₃CN produces [Ru^III(L ¹)(py)₂]⁺ and N₂. The proposed mechanism involves initial equilibrium formation of [Ru^VI(N)(L ¹)(py)]⁺, which undergoes rapid N...N coupling to produce [(py)(L¹)Ru^IIIN≡N-Ru^III(L ¹)(py)]²⁺; this is followed by pyridine substituion to give the final product. This ligand-induced N...N coupling of Ru ^VI≡N is utilized in the preparation of a series of new ruthenium(III) salen complexes, [Ru^III(L)(X)₂] ^± (L = salen ligand; X = H₂O, 1-Melm, py, Me ₂SO, PhNH₂, ^tBuNH₂, Cl^- or CN^-). The structures of [Ru^III(L¹)(NH ₂Ph)₂](PF₆) (6), K[Ru^III(L ¹)(CN)₂] (9), [Ru^III(L²)(NCCH ₃)₂][Au^I(CN)₂] (11) (L² = N,N′-bis(salicylidene)-o-phenylenediamine dianion) and [N ⁿBu₄][Ru^III(L³)Cl₂] (12) (L³ = N,N′-bis(salicylidene)ethylenediamine dianion) have been determined by X-ray crystallography.

Full text of DOI:10.1021/ic800263n

Inorg. Chem. 2008, 47, 5936-5944
General Synthesis of (Salen)ruthenium(III) Complexes via N · · · N
Coupling of (Salen)ruthenium(VI) Nitrides
Wai-Lun Man,† Hoi-Ki Kwong,† William W. Y. Lam,† Jing Xiang,† Tsz-Wing Wong,† Wing-Hong Lam,†
Wing-Tak Wong,‡ Shie-Ming Peng,§ and Tai-Chu Lau*,†
Contribution from the Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat
Chee AVenue, Kowloon Tong, Hong Kong, China, Department of Chemistry, UniVersity of Hong
Kong, Pokfulam Road, Hong Kong, China, and Department of Chemistry, National Taiwan
UniVersity, Taipei 106, Taiwan
Received February 13, 2008
Reaction of [RuVI(N)(L1)(MeOH)]+ (L1 ) N,N′-bis(salicylidene)-o-cyclohexylenediamine dianion) with excess pyridine
in CH3CN produces [RuIII(L1)(py)2]+ and N2. The proposed mechanism involves initial equilibrium formation of
[RuVI(N)(L1)(py)]+, which undergoes rapid N · · · N coupling to produce [(py)(L1)RuIIINtN-RuIII(L1)(py)]2+; this is
followed by pyridine substituion to give the final product. This ligand-induced N · · · N coupling of RuVItN is utilized
(
in the preparation of a series of new ruthenium(III) salen complexes, [RuIII(L)(X)2] (L ) salen ligand; X ) H2O,
t
1-MeIm, py, Me2SO, PhNH2, BuNH2, Cl- or CN-). The structures of [RuIII(L1)(NH2Ph)2](PF6) (6), K[RuIII(L1)(CN)2]
(9), [RuIII(L2)(NCCH3)2][AuI(CN)2] (11) (L2 ) N,N′-bis(salicylidene)-o-phenylenediamine dianion) and [NnBu4][RuIII(L3)Cl2]
(12) (L3 ) N,N′-bis(salicylidene)ethylenediamine dianion) have been determined by X-ray crystallography.
Introduction
various organic transformations such as cyclopropanation,8–11
epoxidation,12–15 aziridination,16–18 sulfimidation,19–21 and
Diels-Alder reactions22,23 (Scheme 1).
These catalysts have the general formula [Ru(salen)(X)-
(Y)]n+, where X or Y is a nitrosyl,12,23–33 carbonyl,16–21,34,35
phosphine,8,9,15,22,36–40 or pyridine group.10,11 However, there
Metal complexes bearing salen-type ligands have played
important roles in the development of coordination chemistry
for over half a century. In particular, manganese salen
complexes have been widely used as catalysts for alkene
epoxidation.1–7 Recently, a number of ruthenium salen
complexes have also been found to be active catalysts in
(10) Miller, J. A.; Hennessy, E. J.; Marshall, W. J.; Scialdone, M. A.;
Nguyen, S. T. J. Org. Chem. 2003, 68, 7884–7886.
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* To whom correspondence should be addressed: Email: bhtclau@
cityu.edu.hk.
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Chem.sEur. J. 2001, 7, 3776–3782.
†
City University of Hong Kong.
University of Hong Kong.
National Taiwan University.
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96, 117–122.
‡
§
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Catal. A. 1996, 110, 33–40.
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5936 Inorganic Chemistry, Vol. 47, No. 13, 2008
10.1021/ic800263n CCC: $40.75  2008 American Chemical Society
Published on Web 05/30/2008

Synthesis of (Salen)ruthenium(III) Complexes
[RuVI(N)(L1)(MeOH)]+ + 2X f [RuIII(L1)(X)2]+ +
Scheme 1. Examples of Ruthenium Salen Complexes as Active
Catalysts in Organic Synthesis
1
MeOH + N2 (5)
2
In this work, we make use of this N · · ·N coupling reaction
to synthesize a series of new ruthenium(III) complexes
bearing various salen ligands that are potential catalysts for
organic transformations. The structures of the salen ligands
used in this work are shown in Figure 1.
Experimental Section
Materials. [RuVI(N)(L1)(MeOH)](PF6) (1),41 [NnBu4][RuVI(N)-
Cl4],42 and [RuIII(L1)(OH2)2](PF6)43 (2) were prepared by literature
procedures. The Schiff base ligands, H2L1, H2L2, and H2L3 (L1, L2
and L3 ) N,N′-bis(salicylidene)-o-cyclohexylenediamine, N,N′-
bis(salicylidene)-o-phenylenediamine, and N,N′-bis(salicylidene-
)ethylenediamine dianion respectively) were synthesized by con-
densation of salicylaldehyde with the corresponding diamines in
refluxing ethanol. nBu4NPF6 (Aldrich) was recrystallized three times
from boiling ethanol and dried in vacuo at 120 °C for one day.
Acetonitrile (Aldrich) for electrochemistry was distilled over
calcium hydride. All other chemicals were of reagent grade and
used without further purification. All manipulations were performed
without precaution to exclude air or moisture unless otherwise
stated.
are no general methods for the preparation of these catalysts;
each complex has to be prepared by a different method, as
shown in eqs 1-4.
salen2-
[Ru(NO)Cl3]
8 [Ru(salen)(NO)Cl]
(1)
DMF, 110 ° C
Physical Measurements. IR spectra were obtained as KBr discs
using a Nicolet 360 FTIR spectrophotometer. 1H NMR spectra were
recorded on a Varian (300 MHz) FT NMR spectrometer. The
chemical shifts (ppm) were reported with reference to tetrameth-
ylsilane (TMS). UV-vis spectra were recorded with a PerkinElmer
Lamda 19 spectrophotometer in 1 cm quartz cuvettes. Elemental
analysis was performed using an Elementar Vario EL Analyzer.
Magnetic measurements were performed at room temperature using
a Sherwood magnetic balance (Mark II). Conductivity measure-
ments were done with a Cole-Parmer 01481-61 conductivity meter.
Electrospray ionization mass spectrometry (ESI-MS) were per-
formed with a PE-SCIEX API 300 triple quadruple mass spec-
trometer. Cyclic voltammetry measurements were performed with
a PAR model 273 potentiostat using a glassy carbon working
electrode, a Ag/AgNO3 (0.1 M in CH3CN) reference electrode, a
platinum wire counter electrode with ferrocene (FeCp2) as the
internal standard.
salen2-
[Ru(PPh3)3Cl2]
8 [Ru(salen)(PPh3)Cl] (2)
refluxing MeOH
CO
[Ru(salen)(PPh3)2]
8 [Ru(salen)(CO)]
(3)
CH2Cl2
t
3,5- Bu4salchda2-
[Ru(p - cymene)Cl2]2
8 [Ru(3,5-
LDA, py, THF
tBu4salchda)(py)2] (4)
We have reported that the ruthenium(VI) nitrido species,
[RuVI(N)(L1)(MeOH)]+ (1), can be readily prepared from
[NnBu4][RuVI(N)Cl4]. This species undergoes facile ligand-
induced N· · · N coupling as represented by eq 5 (X is a
neutral ligand in this case).41
Preparations. [RuIII(L1)(1-MeIm)2](PF6) (3). A green solution
of 1 (120 mg, 0.20 mmol) in 1-methylimidazole (2 mL) was stirred
for 1 day. Slow addition of diethyl ether (50 mL) produced a green
precipitate, which was filtered and then air-dried. Yield: (82%). IR
(KBr, cm-1): ν(CdN) 1593; ν(P-F) 845. Anal. Calcd for
C28H32N6O2PF6Ru: C, 46.03; H, 4.41; N, 11.50. Found: C, 45.87;
H, 4.56; N, 11.37. UV-vis (CH3CN): λmax [nm] (ε
[mol-1dm3cm-1]) 680 (4955), 497 (1940), 388 (17 540), 343sh
(12 890), 234sh (44 590), 214 (49 270). ESI-MS: m/z ) 586 (M+)
µeff ) 1.96 µB.
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6052.
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Chem. 2002, 41, 3728–3739.
[RuIII(L1)(py)2](PF6) (4). The green solid was prepared by a
procedure similar to that for 3 using pyridine. Yield: (76%). IR
(KBr, cm-1): ν(CdN) 1597; ν(P-F) 845. Anal. Calcd for
(32) Works, C. F.; Ford, P. C. J. Am. Chem. Soc. 2000, 122, 7592–7593.
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Wong, W. T. J. Am. Chem. Soc. 2004, 126, 478–479.
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Lau, T. C. Organometallics 2008, 27, 324–326.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5937

Man et al.
Figure 1. Structures of salen ligands.
Table 1. Crystal Data and Structure Refinement Details for 6, 9, 11, and 12
6
9
11
12
formula
C32H34F6N4O2PRu
C22H22KN4O3Ru
C26H20AuN6O2Ru
C32H50Cl2N3O2Ru
Mr
752.67
530.61
746.52
680.72
cryst dimensions/mm
0.45 × 0.08 × 0.06
0.19 × 0.15 × 0.09
0.25 × 0.10 × 0.04
0.27 × 0.21 × 0.18
cryst syst
space group
a/Å
tetragonal
monoclinic
orthorhombic
monoclinic
P42/n
21.166(3)
P21/c
12.581(1)
Pbca
13.2193(4)
P21/c
11.817(3)
b/Å
21.166(3)
16.369(2)
17.3094(5)
13.209(3)
c/Å
14.369(3)
12.031(1)
21.5531(6)
22.033(4)
ꢀ, deg
V/ Å3
Z
90
118.53(1)
2176.8(4)
4
1.619
1076
90
4931.7(2)
8
2.011
2856
97.762(19)
3407.6(14)
4
1.327
1428
6437.3(19)
8
1.553
3064
F
calcd, mg m-3
F(000)
no. of reflns. collected
no. of obsd reflns (I > 2σ(I))
final R indices, I > 2σ(I) Ra
GOF
7318
4167
4891
3343
4310
3325
4456
2628
R ) 0.0762, wR ) 0.2047
1.043
394
R ) 0.0329, wR ) 0.0674
1.016
281
R ) 0.0783, wR ) 0.1041
1.245
327
R ) 0.0572, wR ) 0.1574
1.030
314
no. of parameters
C30H30N4O2PF6Ru: C, 49.73; H, 4.17; N, 7.73. Found: C, 49.60;
H, 4.19; N, 7.64. UV-vis (CH3CN): λmax [nm] (ε [mol-1dm3cm-1])
723 (4650), 373 (16 490), 235 (43 650), 215 (41 510). ESI-MS:
m/z ) 580 (M+). µeff ) 1.90 µB.
(70%). IR (KBr, cm-1): ν(CdN) 1597. Anal. Calcd for
C36H56N3O2Cl2Ru·CH2Cl2: C, 54.48; H, 7.19; N, 5.08. Found: C,
54.54; H, 6.91; N, 4.93. UV/vis (CH3CN): λmax [nm] (ε
[mol-1dm3cm-1]) 645 (5120), 514 (2395), 415 (21 930), 346
(11 850), 292 (8110), 238 (47 600), 221 (47 360). ESI-MS: m/z )
494 (M-). µeff ) 2.06 µB.
[RuIII(L1)(Me2SO)2](PF6) (5). The green solid was prepared by
a procedure similar to that for 3 using Me2SO. Yield: (66%). IR
(KBr, cm-1): ν(CdN) 1599; ν(SdO) 1017; ν(P-F) 840. Anal.
Calcd for C24H32N2O4S2PF6Ru: C, 39.89; H, 4.46; N, 3.88. Found:
C, 40.10; H, 4.41; N, 4.10. UV-vis (CH3CN): λmax [nm] (ε
[mol-1dm3cm-1]) 734 (5015), 481 (1660), 332 (16 960), 232
(43 780). ESI-MS: m/z ) 578 (M+). µeff ) 2.07 µB.
K[RuIII(L1)(CN)2]·H2O (9). KCN (130 mg, 2 mmol) was added
to an orange solution of 1 (123 mg, 0.2 mmol) in methanol (50
mL), and the mixture was stirred for 3 h at room temperature. Slow
evaporation of the resulting green solution afforded dark green
single crystals suitable for X-ray crystallography. Yield: 52%. IR
(KBr, cm-1): ν(CtN) 2090; ν(CdN) 1598. Anal. Calcd for
C22H22N4O3KRu: C, 49.80; H, 4.18; N, 10.56. Found: C, 49.95; H,
4.33; N, 10.37. ESI-MS: m/z ) 474 (M-). µeff ) 1.97 µB.
[RuVI(N)(L2)Cl] (10a). 2,6-dimethylpyridine (0.2 mL) was added
dropwise to a solution of [NnBu4][RuVI(N)Cl4] (0.2 g, 0.4 mmol)
and H2L2 (0.13 g, 0.4 mmol) in CH2Cl2 (20 mL). The deep-brown
solution was stirred at room temperature for 0.5 h. The resulting
brown solid was collected, washed with CH2Cl2, diethyl ether and
then air-dried. Yield: (80%). 1H NMR (300 MHz, CD3OD): δ7.18
(t, 2H), 7.48 (d, 2H), 7.74 (m, 2H), 7.85 (m, 2H), 8.01 (d, 2H),
[RuIII(L1)(NH2Ph)2](PF6) (6). Aniline (0.32 mL, 3.52 mmol)
was added to a solution of 2 (100 mg, 0.16 mmol) in ethanol (15
mL). The green solution was refluxed for 1.5 h and then
concentrated to ca. 1 mL. Slow addition of diethyl ether gave a
green precipitate, which was recrystallized from acetone/diethyl
ether. Yield: 51%. Single crystals suitable for X-ray crystallography
were obtained by slow diffusion of diethyl ether into an acetone
solution of the compound. IR (KBr, cm-1): ν(N-H) 3311, 3278;
ν(CdN) 1602; ν(P-F) 835. Anal. Calcd for C32H34N4O2PF6Ru:
C, 51.06; H, 4.55; N, 7.44. Found: C, 51.23; H, 4.62; N, 7.21.
UV-vis (CH3CN): λmax [nm] (ε [mol-1dm3cm-1]) 705 (4880), 492
(1870), 363 (14 530), 346 (14 150), 233 (47 960), 202 (56 570).
ESI-MS: m/z ) 608 (M+). µeff ) 1.86 µB.
14
8.49 (m, 2H), 9.89 (s, 2H). IR (KBr, cm-1): ν(Rut N) 1036;
15
ν(Rut N) 1006. Anal. Calcd for C20H14N3O2ClRu: C, 51.67; H,
t
[RuIII(L1)(NH2 Bu)2](PF6) (7). The green solid was prepared by
3.01; N, 9.04. Found: C, 51.41; H, 3.12; N, 8.89. ESI-MS: m/z )
a procedure similar to that for 6 using tert-butylamine (0.34 mL,
3.25 mmol). Yield: (40%). IR (KBr, cm-1): ν(N-H) 3300, 3244;
ν(C)N) 1600; ν(P-F) 849. Anal. Calcd. for C28H42N4O2PF6Ru:
C, 47.19; H, 5.94; N, 7.86. Found: C, 47.15; H, 5.75; N, 7.81.
UV-vis (CH3CN): λmax [nm] (ε [mol-1dm3cm-1]) 670 (5920), 497
(2510), 387 (20 100), 366 (17 260), 234 (42 970), 217 (44 440) ESI-
MS: m/z ) 568 (M+). µeff ) 2.02 µB.
430 (M+ - Cl). Conductivity in CH3OH: Λ) 53 Ω-1 cm2 mol-1
.
[RuVI(N)(L2)(MeOH)](ClO4) (10b). 10a (0.186 g, 0.4 mmol)
was dissolved in methanol (30 mL) and silver p-toluenesulfonate
(AgOTs) (0.112 g, 0.4 mmol) was added. The mixture was stirred
at room temperature for 0.5 h and then filtered to remove AgCl.
To the concentrated filtrate excess LiClO4 (0.1 g, 0.9 mmol) was
added and the deep reddish-brown solution was cooled in ice. The
resulting brick-red microcrystalline solid was collected, washed with
methanol, and then diethyl ether. Yield: (65%). 1H NMR (300 MHz,
CD3OD): δ7.17 (t, 2H), 7.48 (d, 2H), 7.73 (m, 2H), 7.86 (m, 2H),
8.03 (d, 2H), 8.49 (m, 2H), 9.89 (s, 2H). Anal. Calcd for
[NnBu4][RuIII(L1)Cl2] (8). [NnBu4Cl] (0.55 g, 2.0 mmol) was
added to an orange solution of 1 (120 mg, 0.2 mmol) in acetone
(20 mL) and refluxed for 1 day. The green solution was then
concentrated to ca. 1 mL. Addition of diethyl ether (20 mL) gave
a green solid, which was recrystallized from CH2Cl2/Et2O. Yield:
5938 Inorganic Chemistry, Vol. 47, No. 13, 2008

Synthesis of (Salen)ruthenium(III) Complexes
Table 2. UV-vis (CH3CN) and Electrochemical Data for 3-8 and 12
a
E1/2 (Volts vs Cp2Fe+/0
)
complex
λmax (nm(ꢀ, M-1, cm-1))
RuIV/III
RuIII/II
3
680 (4955), 497 (1940), 388 (17 540), 343sh (12 890), 234sh (44 590),
214 (49 270)
0.54
-0.89
4
5
6
723 (4650), 373 (16 490), 235 (43 650), 215 (41 510)
734 (5015), 481 (1660), 332 (16 960), 232 (43 780)
705 (4880), 492 (1870), 363 (14 530), 346 (14 150), 233 (47 960), 202
(56 570)
0.69
-0.58
-0.30
-0.86
0.89b
0.58b
7
670 (5920), 497 (2510), 387 (20 100), 366 (17 260), 234 (42 970), 217
(44 440)
645 (5120), 514 (2395), 415 (21 930), 346 (11 850), 292 (8110), 238
(47 600), 221 (47 360)
647 (2360), 515 (1000), 415 (10 240), 347 (5880), 237 (24 400), 222
(25 000)
0.57
0.10
0.16
-0.90
-1.50b
-1.39b
8
12
a Glassy carbon working electrode, platinum counter electrode, Ag/AgNO3 reference electrode, 0.1 M [NnBu4]PF6 in CH3CN as supporting electrode.
Ferrocene was added as internal standard. b Irreversible.
C21H18N3O7ClRu: C, 44.96; H, 3.21; N, 7.49. Found: C, 44.72; H,
3.11; N, 7.36. ESI-MS: m/z ) 430 (M+). Conductivity in CH3OH:
Λ) 87 Ω-1cm2 mol-1
.
Caution! Perchlorate salts are potentially explosiVe. Although
we haVe not encountered any explosions so far, the amount of
perchlorate salts used should be less than 100 mg each time.
[RuIII(L2)(NCCH3)2][AuI(CN)2] (11). A mixture of 10a and
K[AuI(CN)2] in CH3OH/H2O (20 mL, 4:1 v/v) was stirred at 30
°C for 1 h. The deep-brown solution was concentrated to give a
light-brown solid, which was recrystallized by slow evaporation
of a solution in CH3CN/H2O (2:1 v/v). Yield (30%). IR (KBr,
cm-1): ν(CtN) 2140; ν(CdN) 1597. Anal. Calcd for C26H22-
N6O2AuRu: C, 41.82; H, 2.68; N, 11.26. Found: C, 42.02; H, 2.80;
N, 11.03. ESI-MS: m/z ) 498 (M+).
[NnBu4][RuIII(L3)Cl2] (12). [NnBu4][RuVI(N)Cl4] (0.2 g, 0.4
mmol) was dissolved in THF (20 mL) and H2L3 (0.108 g, 0.4 mmol)
was added. The solution was stirred at room temperature for 16 h
and then evaporated to dryness. The resulting green residue was
dissolved in CH2Cl2 and purified by column chromatography
(neutral alumina) with CH2Cl2 as eluant. Dark-green crystals were
obtained by diffusion of diethyl ether into a concentrated CH2Cl2
solution of the solid. Yield: (45%). IR (KBr, cm-1): ν(CdN) 1601.
Anal. Calcd for C32H50N3O2Cl2Ru·CH2Cl2: C, 51.77; H, 6.85; N,
5.49. Found: C, 51.92; H, 6.63; N, 5.49. ESI-MS: m/z ) 438 (M-).
Conductivity in CH3CN: Λ ) 111 Ω-1 cm2 mol-1. µeff ) 1.79 µB.
Kinetics. Kinetic experiments were done using either a Perki-
nElmer Lamda 19 or a Hewlett-Packard 8452A diode-array UV-vis
spectrophotometer. The ionic strengths of the solutions were
maintained with nBu4NPF6. The concentrations of py (4 × 10-2 to
3.0 M) were at least in 10-fold excess of that of 1 (1 × 10-5 to 5
× 10-5 M). The reaction progress was monitored by observing the
absorbance changes at 724 nm. Pseudo-first-order rate constants,
kobs, were obtained by nonlinear least-squares fits of At versus time
t according to the equation At ) A∞ + (A0 - A∞) exp(-kobst), where
A0 and A∞ are the initial and final absorbances, respectively.
X-ray Crystallography. All measurements of 6, 9, 11, and 12
were made on either a Bruker Smart 1000 CCD or a Rigaku AFC7R
diffractometer with graphite-monochromated Mo KR radiation (λ
) 0.71073 Å) in the ω-scan mode. Details of the intensity data
collection and crystal data are given in Table 1. The data were
corrected for Lorentz and polarization effects. Absorption correc-
tions by the Ψ-scan method or an approximation by interimage
scaling were applied. The structures were resolved by direct
methods44 and expanded using Fourier techniques.45 All calcula-
Figure 2. Spectral changes at 180 s intervals for the reaction of 1 (4 ×
10-5 M) with py (0.01 M) in CH3CN at 298.0 K and I ) 0.05 M (a )
initial, b ) immediately after mixing using a two-compartment cell, c
) final). Inset shows the plot of kobs vs [py] for the second step (b f
c) [slope ) (4.96 ( 0.12) × 10-1; y intercept ) (5.81 ( 0.17) × 10-2
r ) 0.9986].
;
tions were performed using the Crystal Structure or TeXsan
crystallographic software package from Molecular Structure Cor-
poration.46
Results and Discussion
Ligand-Induced N · · · N coupling of [RuVI(N)(L1)-
(MeOH)]+ (1). A solution of 1 in CH3CN (4.0 × 10-5 M)
is stable for at least 24 h at room temperature. However, a
very rapid spectral change occurs when excess pyridine (1.0
× 10-2 M) is added; this is followed by a much slower step
with isosbestic points at 323, 397, and 478 nm. The final
spectrum is consistent with the quantitative formation of
[RuIII(L1)(py)2]+ (4). The formation of 4 follows clean
pseudo-first-order kinetics; the pseudo-first-order rate con-
stant, kobs, is independent of [1] (1 × 10-5 to 5 × 10-5 M)
but depends linearly on [py]. At 298.0 K and I ) 0.05 M,
the second-order rate constant, k2, is (4.96 ( 0.12) × 10-1
M-1 s-1 (Figure 2).
When higher concentrations of RuVI are used (>2 × 10-2
M), gas bubbles of N2 (GC/MS) can be observed upon adding
(45) DIRDIF 99, Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman,
W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-99
program system; Technical Report of the Crystallography Laboratory,
University of Nijmegen: The Netherlands, 1999.
(44) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5939

Man et al.
excess pyridine. Using 15N labeling, it is shown that both
nitrogen atoms in N2 come from RuVItN.41 These results
are consistent with the reaction scheme shown in eqs 6-9.
The observed rapid first step is probably due to eqs 6 and 7,
whereas the second step can be accounted for by eqs 8 and
9, with 8 being the rate-limiting step.
peaks due to [Ru(L)]+ (m/z ) 422.1), [Ru(N)(L)]+ (m/z )
436.2), [Ru(L)(NCCH3)]+ (m/z ) 463.2), [Ru(L)(NCCH3)2]+
(m/z ) 504.3) and [Ru(L)(py)(NCCH3)]+(m/z ) 542.3).
The coordination of py onto RuVItN activates the complex
toward N · · · N coupling or attack by nucleophiles.41,47
A
similar activating effect by ligands has been observed in the
epoxidation of alkenes by [CrV(salen)(O)]+.6,49 In the pres-
ence of ligands such as pyridine N-oxide (pyO), the
6-coordinate species [CrV(salen)(O)(pyO)]+ is formed, which
reacts much more rapidly with alkenes to give epoxide and
a (salen)CrIII product. X-ray crystallographic studies indicate
that in the 5-coordinate [CrV(salen)(O)]+, the chromium atom
is displaced 0.53 Å above the mean salen plane. However,
in the 6-coordinate [CrV(salen)(O)(pyO)]+, the chromium
atom is pulled back to 0.26 Å. On the other hand, in
6-coordinate (salen)CrIII complexes such as [Cr(salen)(OH2)2]+,
the chromium is only 0.077 Å displaced from the salen plane.
Although the CrdO bond length increases by only 0.01 Å,
the ν(CrdO) stretch decreases from 1004 cm-1 to 943 cm-1
upon ligation, indicating a weakening of the CrdO bond.
Hence, upon coordination by a good donor ligand, the
CrVdO species becomes more product-like, and this should
lower the reorganization energy prior to oxygen atom
transfer. Similarly, in [RuVI(N)(L1)(MeOH)]+ the Ru atom
is displaced 0.39 Å from the mean L1 plane,41 whereas in a
typical six-coordinate ruthenium(III) species such as
[RuIII(L1)(NH2Ph)2]+, the ruthenium atom is only 0.075 Å
displaced from the L1 plane. Although we were unable to
obtain the crystal structure or the IR spectrum of
[RuVI(N)(L1)(py)]+ due to its high reactivity, it is reasonable
to assume that upon pyridine coordination, the ruthenium
atom is pulled back toward the L1 plane and the RutN bond
is weakened.
This facile ligand-induced N · · · N coupling reaction is
utilized in the preparation of a series of new ruthenium(III)
salen complexes, as described in the next section.
Synthesis of Ruthenium(III) Salen Complexes. Treat-
ment of [RuVI(N)(L1)(MeOH)]+ (1) with various ligands
affords a series of RuIIIL1 complexes, as illustrated in Scheme
2.
The diaquo species [RuIII(L1)(OH2)2](PF6) (2), which is
readily prepared from 1 by refluxing in water/acetone,43 is
also a useful starting material for preparing various RuIIIL
complexes. 2-9 have room temperature magnetic moments
(solid sample, Gouy method) of µeff ) 1.86-2.07 µB,
consistent with their formulation as low-spin d5 ruthe-
nium(III) complexes. Their UV-vis spectra in CH3CN
(Table 2) display a broad band at 645-734 nm, which is
assigned to salen to a RuIII LMCT transition.15 There are
also intense peaks at 332-415 nm, which can be attributed
to intraligand transitions of the coordinated salen ligand. The
IR spectra of 2-9 display sharp ν(CdN) stretches at around
1600 cm-1. The IR spectrum of 5 shows a peak at 1017 cm-1,
which is absent in other RuIIIL1 complexes, and this is
assigned to ν(SdO) stretch of S-bonded Me2SO. ν(SdO)
stretch of S-bonded Ru-Me2SO complexes typically occur
VI
1
Ru N L NCCH 3 + + py h [RuVI(N)(L1)(py)]++
[
(
)
]
(
)
(
)
CH3CN (6)
2[RuVI(N)(L1)(py)] +
f [(py)(L1)RuIIIN ≡ NRuIII(L1)(py)]2+ (7)
[(py)(L1)RuIIIN ≡ NRuIII(L1)(py)]2+
+
py f [RuIII(L1)(py)2]++[RuIII(L1)(py)(N2)]+(slow) (8)
[RuIII(L1)(py)(N2)]+ + py f [RuIII(L1)(py)2]++N2 (9)
In the pyridine-activated aziridination of alkenes by 1,
there is kinetic evidence for the equilibrium described in eq
6, with K ) 15.6 M-1 at 298 K.47 The [RuVI(N)(L1)(py)]+
species undergoes rapid N · · · N coupling to produce a µ-N2
RuIII species (eq 7). Rapid N · · · N coupling of [OsV(NH3)4-
(N)]2+, generated from photolysis of [OsVI(NH3)4(N)]3+ in
the presence of an electron donor, has been observed with k
) 3.75 × 105 M-1 s-1.48 Because there should not be
significant π back bonding between N2 and RuIII, loss of
bridging or terminal N2 from RuIIIL1 is expected to be facile
(eqs 8 and 9). The y intercept of 5.81 × 10-2 s-1 in the plot
of kobs versus [py] is probably due to a minor solvolysis
pathway shown in eq 10.
[(py)(L1)RuIIIN ≡ N - RuIII(L1)(py)]2++
CH3CN f [RuIII(L1)(py)(NCCH3)]++[RuIII(L1)(py)(N2)]+
(10)
The assumption that eq 8 is the rate-limiting step is valid
if the coupling rate constant of eq 7 is similar to (or faster
than) that of [OsV(NH3)4(N)]2+ (4 × 105 M-1 s-1). On the
basis of the concentrations of 1 (4 × 10-5 M) and py (0.01
M) in the experiment, and the equilibrium constant of eq 6
(K ) 15.6 M-1), the equilibrium concentration of the adduct
[RuVI(N)(L1)(py)]+ is 6.2 × 10-6 M. If the coupling rate
constant of eq 7 is 4 × 105 M-1 s-1, then the coupling rate
in eq 7 is 1.6 × 10-5 M s-1 at 298 K, which is around two
orders of magnetude faster than the observed rate 2 × 10-7
M s-1 (which is assumed to be that of eq 8).
Attempts to detect the proposed [(py)(L1)RuIIINtNRuIII(L1)-
(py)]2+ intermediate by ESI-MS were unsuccessful. The mass
spectrum (+ve mode) of (1) (4.0 × 10-5 M) and pyridine
(0.01 M) in CH3CN collected 5 min after mixing shows only
(46) CrystalStructure, Single Crystal Structure Analysis Software, version
3.5.1; Rigaku/MSC Corporation: The Woodlands, Texas, USA,
Rigaku, Akishima, Tokyo, Japan, 2003; Watkin, D. J. Prout. C. K.,
Carruthers. J. R., Betteridge, P. W. Crystals, Chemical Crystallography
Lab: Oxford, UK, 1996; issue 10.
(47) Man, W. L.; Lam, W. W. Y.; Yiu, S. M.; Lau, T. C.; Peng, S. M.
J. Am. Chem. Soc. 2004, 126, 15336–15337.
(48) Lam, H. W.; Che, C. M.; Wong, K. Y. J. Chem. Soc., Dalton Trans.
1992, 1411–1416.
(49) Siddall, T. L.; Miyaura, N.; Huffman, J. C.; Kochi, J. K. J. Chem.
Soc., Chem. Commun. 1983, 1185–1186.
5940 Inorganic Chemistry, Vol. 47, No. 13, 2008

Synthesis of (Salen)ruthenium(III) Complexes
Scheme 2. Synthesis of RuIIIL1 Complexes via Ligand-Induced N · · ·N Coupling of the Corresponding Ruthenium(VI) Nitride
Figure 3. ESI mass spectra of 3 in the positive mode (left) and 8 in the negative mode (right) in acetone. Insets show the experimental (top) and calculated
(bottom) isotopic patterns.
at 1020-1134 cm-1, whereas that of O-bonded Ru-Me2SO
complexes occur at lower wave numbers (885-954
cm-1).50–53 This assignment is also supported by electro-
chemical studies (below). The ν(N-H) stretches for the
amines 6 and 7 occur at 3311, 3278 and 3300, 3244 cm-1
respectively. The ν(CtN) stretch at 2090 cm-1 for 9 is
similar to that of trans-[RuIII(acac)2(CN)2]- (2099 cm-1)54
and trans-[RuIII(salen)(CN)2]- (2080 cm-1).55
The ESI-MS of 2-9 in acetone exhibit a peak due to the
parent ion (Experimental Section). For example, the ESI mass
spectra of 3 (positive mode) and 8 (negative mode) (Figure
3) show one single peak that arises from the parent ion
[Ru(L1)(1-MeIm)2]+ (m/z ) 568) and [Ru(L1)Cl2]- (m/z )
492), respectively.
(50) Reisner, E.; Arion, V. B.; Guedes da Silva, M. F. C.; Lichtenecker,
R.; Eichinger, A.; Keppler, B. K.; Kukushkin, V. Y.; Pombeiro, A. J. L.
Inorg. Chem. 2004, 43, 7083–7093.
(51) Alessio, E.; Mestroni, G.; Nardin, G.; Attia, W. M.; Calligaris, M.;
Sava, G.; Zorzet, S. Inorg. Chem. 1988, 27, 4099–4106.
(52) Alessio, E.; Bolle, M.; Milani, B.; Mestroni, G.; Faleschini, P.;
Geremia, S.; Calligaris, M. Inorg. Chem. 1995, 34, 4716–4721.
(53) Alessio, E.; Milani, B.; Bolle, M.; Mestroni, G.; Faleschini, P.; Todone,
F.; Geremia, S.; Calligaris, M. Inorg. Chem. 1995, 34, 4722–4734.
(54) Yeung, W. F.; Man, W. L.; Wong, W. T.; Lau, T. C.; Gao, S. Angew.
Chem., Int. Ed. 2001, 40, 3031–3033.
(55) Yeung, W. F.; Lau, P. H.; Lau, T. C.; Wei, H. Y.; Sun, H. L.; Gao,
S.; Chen, Z. D.; Wong, W. T. Inorg. Chem. 2005, 44, 6579–6590.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5941

Man et al.
Figure 4. Cyclic voltammograms of 3 (a), 6 (b), and 7 (c) in CH3CN.
Figure 6. ORTEP diagram of [RuIII(L1)(CN)2]- anion (9), thermal ellipsoids
are drawn at 30% probability (hydrogen atoms are omitted for clarity).
Table 3. Selected Bond Distances (Angstroms) and Angles (Degrees)
of 6
Ru(1)-O(1)
2.029(5) Ru(1)-N(3)
2.027(4) Ru(1)-N(4)
1.986(5) N(1)-C(7)
1.982(6) N(2)-C(14)
91.25(17) N(1)-Ru(1)-N(2)
2.126(5)
2.141(12)
1.291(9)
1.287(9)
83.3(2)
94.3(2)
89.4(6)
94.8(2)
86.2(7)
Ru(1)-O(2)
Ru(1)-N(1)
Ru(1)-N(2)
O(1)-Ru(1)-O(2)
O(1)-Ru(1)-N(1)
O(1)-Ru(1)-N(2) 176.0(2)
O(1)-Ru(1)-N(3)
O(1)-Ru(1)-N(4)
O(2)-Ru(1)-N(1) 175.9(2)
O(2)-Ru(1)-N(2)
O(2)-Ru(1)-N(3)
O(2)-Ru(1)-N(4)
92.8(2)
N(1)-Ru(1)-N(3)
N(1)-Ru(1)-N(4)
N(2)-Ru(1)-N(3)
N(2)-Ru(1)-N(4)
N(3)-Ru(1)-N(4)
Ru(1)-N(1)-C(7)
86.6(2)
92.7(7)
176.3(5)
125.9(5)
92.6(2)
86.7(2)
89.7(6)
Ru(1)-N(2)-C(14) 126.1(5)
Table 4. Selected Bond Distances (Angstroms) and Angles (Degrees)
of 9
Ru(1)-O(1)
2.018(2) Ru(1)-C(22)
2.022(2) N(1)-C(7)
1.996(2) N(2)-C(14)
1.993(2) C(21)-N(3)
2.076(4) C(22)-N(4)
2.067(4)
1.286(4)
1.290(4)
1.157(4)
1.147(4)
83.28(10)
94.77(4)
88.55(11)
85.63(12)
91.15(12)
Figure 5. ORTEP diagram of [RuIII(L1)(NH2Ph)2]+ cation (6); thermal
ellipsoids are drawn at 30% probability (hydrogen atoms are omitted except
N(3)-H and N(4)-H for clarity).
Ru(1)-O(2)
Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-C(21)
The cyclic voltammograms of 3-8 in CH3CN (Table 2)
exhibit two reversible couples, which are assigned to RuIV/III
and RuIII/II couples, except for 5 and 6 where the oxidation
waves are irreversible (Figure 4). As expected, compounds
with anionic axial ligands (8, 12) have lower RuIV/III and
RuIII/II potentials. For a compound with neutral axial ligands,
the RuIII/II potential increases with the π-accepting ability of
the ligand. 5 has the highest RuIII/II potential, consistent with
evidence from IR that the Me2SO ligands are S-bonded;
S-bonded Me2SO is a strong π-acceptor ligand that would
stabilize RuII, whereas the O-bonded ligand would have little
π-accepting ability.56
The molecular structures of 6 (Figure 5) and 9 (Figure 6)
have been determined by single-crystal X-ray diffraction
method. The crystal data and structural refinement details
are given in Table 1. Selected bond distances and angles
are listed in Table 3 and 4. 6 and 9 adopt a distorted
octahedral geometry; the ruthenium center is bonded to the
two oxygen atoms and the two nitrogen atoms of the L1
ligand in the equatorial plane. They have similar Ru-O bond
distances (2.027(4) and 2.029(4) Å for 6, 2.018(2) and
2.022(2) Å for 9) and Ru-N bond distances (1.986(5) and
1.982(6) Å for 6, (1.996(2) and 1.993(2) Å for 9). 6 is the
first crystal structure of a ruthenium(III) complex of PhNH2.
Its Ru-Naniline bond distances of 2.126(5) and 2.141(12) Å
are slightly shorter than that of the ruthenium(II) species
[RuII(NH2Ph){PhNC(H)NPh}(Me2SO)2Cl] (2.1907(19) Å).57
O(1)-Ru(1)-O(2)
O(1)-Ru(1)-N(1)
O(1)-Ru(1)-N(2)
O(1)-Ru(1)-C(21)
O(1)-Ru(1)-C(22)
O(2)-Ru(1)-N(1)
O(2)-Ru(1)-N(2)
O(2)-Ru(1)-C(21)
O(2)-Ru(1)-C(22)
91.82(8)
92.67(9)
175.56(9)
92.93(11) N(2)-Ru(1)-C(21)
90.56(11) N(2)-Ru(1)-C(22)
174.90(9)
92.31(9)
87.40(11) Ru(1)-N(2)-C(14)
N(1)-Ru(1)-N(2)
N(1)-Ru(1)-C(21)
N(1)-Ru(1)-C(22)
C(21)-Ru(1)-C(22) 175.07(13)
Ru(1)-N(1)-C(7)
124.9(2)
124.8(2)
89.00(11)
In 9, the Ru-Ccyanide bond distances (2.067(4) and 2.075(4)
Å) are similar to that in (NnBu4)[RuIII(salen)(CN)2] (2.04(2)
and 2.09(2) Å).55
We have previously used [Au(CN)2]- as a bridging ligand
for the construction of coordination polymers.58,59 Our initial
goal in this study was to use [Au(CN)2]- as a ligand to induce
N · · · N coupling, which may result in the formation of a
polynuclear species. However, treatment of [RuVI(N)(L2)Cl]
with K[Au(CN)2] in CH3OH/H2O did not result in the
coordination of [Au(CN)2]- onto ruthenium, instead
[RuIII(L2)(NCCH3)2][Au(CN)2] (11) was isolated after re-
(56) Wishart, J. F.; Taube, H.; Breslauer, K. J.; Isied, S. S. Inorg. Chem.
1984, 23, 2997–3001.
(57) Clark, T.; Cochrane, J.; Colson, S. F.; Malik, K. Z.; Robinson, S. D.;
Steed, J. W. Polyhedron 2001, 20, 1875–1880.
(58) Yeung, W. F.; Wong, W. T.; Zuo, J. L.; Lau, T. C. J. Chem. Soc.,
Dalton Trans. 2000, 629–631.
(59) Shek, I. P. Y.; Wong, W. Y.; Lau, T. C. New J. Chem. 2000, 24,
733–734.
(60) Demerseman, B.; Renaud, J. L.; Toupet, L.; Hubert, C.; Bruneau, C.
Eur. J. Inorg. Chem. 2006, 7, 1371–1380.
5942 Inorganic Chemistry, Vol. 47, No. 13, 2008

Synthesis of (Salen)ruthenium(III) Complexes
Figure 8. Cyclic voltammogram of 12 in CH3CN.
Figure 7. ORTEP diagram of [RuIII(L2)(NCCH3)2]+ cation (11), thermal
ellipsoids are drawn at 30% probability (hydrogen atoms are omitted for
clarity).
Table 5. Selected Bond Distances (Angstroms) and Angles (Degrees)
of 11
Ru(1)-O(1)
2.011(6) Ru(1)-N(4)
2.002(6) N(1)-C(7)
1.992(8) N(2)-C(14)
2.016(8) N(3)-C(21)
2.041(9) N(4)-C(22)
2.040(9)
1.305(13)
1.297(12)
1.145(13)
1.111(15)
82.4(3)
88.0(4)
89.8(4)
92.8(4)
90.4(4)
Ru(1)-O(2)
Ru(1)-N(1)
Ru(1)-N(2)
Figure 9. ORTEP diagram of [RuIII(L3)Cl2]- anion (12), thermal ellipsoids
are drawn at 30% probability (hydrogen atoms are omitted for clarity).
Ru(1)-N(3)
O(1)-Ru(1)-O(2)
O(1)-Ru(1)-N(1)
90.4(3)
93.8(3)
N(1)-Ru(1)-N(2)
N(1)-Ru(1)-N(3)
N(1)-Ru(1)-N(4)
N(2)-Ru(1)-N(3)
N(2)-Ru(1)-N(4)
N(3)-Ru(1)-N(4)
Table 6. Selected Bond Distances (Angstroms) and Angles (Degrees)
of 12
O(1)-Ru(1)-N(2) 175.9(3)
O(1)-Ru(1)-N(3)
O(1)-Ru(1)-N(4)
88.6(3)
88.1(4)
Ru(1)-O(1)
2.029(6) Ru(1)-Cl(1)
2.040(6) Ru(1)-Cl(2)
1.984(7) N(1)-C(3)
1.992(8) N(2)-C(16)
2.386(2)
2.378(2)
1.283(11)
1.270(12)
89.18(18)
83.8(4)
88.2(2)
91.7(2)
90.4(2)
88.8(2)
Ru(1)-O(2)
O(2)-Ru(1)-N(1) 175.4(3)
175.9(4)
Ru(1)-N(1)
Ru(1)-N(2)
O(2)-Ru(1)-N(2)
O(2)-Ru(1)-N(3)
O(2)-Ru(1)-N(4)
93.5(3)
90.2(4)
92.2(4)
Ru(1)-N(3)-C(21) 177.1(9)
Ru(1)-N(4)-C(23) 169.6(11)
O(1)-Ru(1)-O(2)
O(1)-Ru(1)-N(1)
O(1)-Ru(1)-N(2)
O(1)-Ru(1)-Cl(1)
O(1)-Ru(1)-Cl(2)
O(2)-Ru(1)-N(1)
O(2)-Ru(1)-N(2)
O(2)-Ru(1)-Cl(1)
91.9(2)
92.2(3)
175.9(3)
89.66(19) N(1)-Ru(1)-Cl(2)
91.12(19) N(2)-Ru(1)-Cl(1)
O(2)-Ru(1)-Cl(2)
N(1)-Ru(1)-N(2)
N(1)-Ru(1)-Cl(1)
crystallization from CH3CN/H2O. It shows a peak at 2140
cm-1 in the IR, which is assigned to the ν(CtN) stretch of
[Au(CN)2]-. The [Au(CN)2]- functions only as a counterion,
in fact [RuIII(L2)(NCCH3)2]PF6 can be obtained by refluxing
[RuVI(N)(L2)(CH3OH)](ClO4) in CH3CN. In both compounds
the ν(CtN) stretch of the coordinated CH3CN is too weak
to be observed. The X-ray crystal structure of 11 is shown
in Figure 7, and the details of the crystal data and selected
bond data are listed in Tables 1 and 5, respectively. The two
CH3CN ligands are trans with a N(3)-Ru-N(4) angle of
175.9(4)°. The two Ru-Nacetonitrile bond distances are identical
(2.040(9) and 2.041 (9) Å), and are comparable to that of
[RuII(Cp*)(Ph2POMe)(NCCH3)2] (2.067(2) and 2.076(2) Å)60
and mer-[RuIII(NCCH3)3Cl3] (2.014(7)-2.065(7) Å).61
Attempts to prepare a ruthenium(VI) nitrido complex of
the parent salen ligand by a reaction of [NnBu4][RuVI(N)Cl4]
with H2L3 was unsuccessful; instead the dark-green complex
[NnBu4][RuIII(L3)Cl2] (12) was obtained. 12 has a room-
temperature magnetic moment of µeff ) 1.79 µB (solid
sample, Gouy method) consistent with its formulation as a
low-spin d5 RuIII complex. The ESI mass spectrum (-ve
mode) of 12 in acetone shows a single peak at m/z ) 438,
which is assigned to the parent ion [RuIII(L3)Cl2]-. The cyclic
voltammogram of 12 (Figure 8) in CH3CN shows a reversible
oxidation wave at E1/2 ) +0.16 V (vs Cp2Fe+/0) and an
irreversible reduction wave at Epa ) -1.39 V (vs Cp2Fe+/0),
which are assigned to RuIV/III and RuIII/II couples, respectively.
175.8(3)
92.2(3)
N(2)-Ru(1)-Cl(2)
Cl(1)-Ru(1)-Cl(2) 179.22(10)
90.83(18)
The potentials are comparable to that of 8 (Table 2). The
irreversibility of the RuIII/II couple may be due to chloride
dissociation in the RuII state.
Single crystals suitable for X-ray crystallography were
obtained by slow diffusion of diethyl ether into a CH2Cl2
solution of 12. The molecular structure of the anion is shown
in Figure 9. Crystal data, structural refinement details, and
selected bond distances and angles are given in Tables 1
and6.ThetwochlorineatomsaretranswithaCl(1)-Ru-Cl(2)
angle of 179.22(10)°. The two Ru-Cl bond distances are
identical (2.386(2) and 2.378(2) Å) and are comparable to
those in trans-PPh4[Ru(acac)2Cl2] (2.355(2) and 2.362(1)
Å).62
Conclusions
Ruthenium(VI) nitrido complexes bearing salen-type ligands
readily undergo facile ligand-induced N · · ·N coupling reac-
(61) Appelbaum, L.; Heinrichs, C.; Demtschuk, J.; Michman, M.; Oron,
M.; Schafer, H. J.; Schumann, H. J. Organomet. Chem. 1999, 592,
240–250.
(62) Hasegawa, T.; Lau, T. C.; Taube, H.; Schaefer, W. P. Inorg. Chem.
1991, 30, 2921–2928.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5943

Man et al.
tions, which provide a general synthetic pathway for a variety
of (salen)ruthenium(III) complexes. Our studies should
greatly increase the scope for the application of ruthenium
salen complexes as catalysts for various organic transforma-
tions. The catalytic properties of some of these complexes
will be investigated.
(CityU 2/06C, CityU 101404) and the City University of
Hong Kong (7001799).
Supporting Information Available: UV-vis spectra of 3-9
and the ESI-MS of reaction of (1) and py in CH3CN taken in 5
min. This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. The work described in this article was
supported by the Research Grants Council of Hong Kong
IC800263N
5944 Inorganic Chemistry, Vol. 47, No. 13, 2008