Table Of ContentTable of Contents
Synthesis of Ruthenium(II) Complexes of Aromatic Chelating
Heterocycles: Towards the Design of Luminescent
Compounds
R. Krause .........................
Platinum Amine Coordination Compounds as Anti-Tumor
Drugs. Molecular Aspects of the Mechanism of Action
J. Reedijk, A. M, J. Fichtinger-Schepman,
A. T. van Oosterom, P. van de Putte ............ 53
The Chemistry of Chelating Agents in Medical Sciences
R. A. Bulman ....................... 91
Author Index Volumes 1-67 ................. 341
Synthesis of Ruthenium(II) Complexes of
Aromatic Chelating :selcycoreteH
Towards the Design of Luminescent sdnuopmoC
Ronaid A. Krause
Department of Chemistry, University of Connecticut, Storrs, CT, 06268, USA
The majority of ruthenium(II) compounds which have been observed to be luminescent are com-
plexes of aromatic heterocyclic chelating ligands. Methods of synthesizing such complexes are
reviewed and recommendations made (where considered possible) as to the best procedures.
Models for the emitting excited state are reviewed, particularly with regard to the effects of
molecular structure on excited state behavior. Data are tabulated for known emitting complexes.
While a wide variety of luminescent ruthenium(II) complexes cannot be designed at present, some
guidelines are beginning to emerge.
In the future we look for continued development of the excited state model. Further structure-
emission efficiency correlations and correlation with solvent properties are seen as being important.
Abbreviations Used .................................. 3
L Introduction ...................................... 7
II. Synthesis ........................................ 8
A. General Synthetic Considerations ......................... 8
B. Tris-Chelates, Ru(AB)? +2 andBis-Chelates,Ru(ABC)2 +2 . .......... 9
1. General Conclusions .............................. 11
C. Ru(AB)2X2 and Ru(AB)2L2 +n Type Compounds ................ 11
1. Ru(AB)2C12 Complexes ............................ 11
2. Other Intermediates, Ru(AB)2Y2 +n ...................... 21
a) Nitrite Complexes ............................. 31
b) Ru(AB)z(CO)X' and Related Complexes ................ 31
3. Ru(AB)2(L)2 +n Complexes; GeneralConclusions .............. 14
D, Monochelates, Ru(AB)L4 +n and Ru(ABC)L3 +n ................ 14
E. Mixed Chelates .................................. 16
F. Potential Precursors ................................ 71
G. Photochemical Syntheses ............................. 18
Structure and Bonding 67
© Springer-Verlag Berlin Heidelberg 1987
2 R.A.Krause
III. Ruthenium(II) Emission 19
A. Luminescence-Introduction ............................ 19
1. Solvent Effects ................................. 25
2. Other Deactivating Modes ......... , ................. 27
B. Summary of Emission Results ........................... 27
1. Introduction .................................. 27
2. Emission from Complexes of Bpy and Derivatives ............... 28
3. Emission from Complexes of Phen and Derivatives .............. 34
4. Emission from Complexes of Polyazaheterocyclic Ligands ........... 37
5. Emission from Complexes of Sulfur-Nitrogen Heterocyclic Ligands ...... 40
6. Emission from Complexes of Quinoline Derived Ligands ........... 41
7. Emission from Complexes of Tridentate Ligands ................ 43
C, Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
IV. References 46
Synthesis of Ruthenium(II) Complexes of Aromatic Chelating Heterocycles
Abbreviations Used
In many cases ligands are abbreviated by giving a substituent and its location followed by
the ligand abbreviation (e.g., 4,4'-Me-bpy would be 4,4'-dimethyl-2,2'-bipyridyl). In
tables collecting complexes of one ligand type only the substituent is given. Ligand
structures appear in Fig. 1.
Azpy structure 34 Htf trifluoromethane sulfonic acid
bpy 2,2'-bipyridyl i-bq structure 51
bpym structure 23 MC metal centered
bpyz structure 24 Me methyl
bq structure 42 MLCT metal-ligand charge transfer
bt structure 37 NAz structure 35
BzImH structure 6 nbd norbornadiene
cod 1,5-cyclooctadiene NPP structure 44
CT charge transfer ox oxalate
daf structure 45 ph phenyl
dinapy structure 32 phen 1,10-phenanthroline
DMCH structure 50 pq structure 43
DMF N,N-dimethylformamide py pyridine
DMSO dimethylsulfoxide pynapy structure 31
dpp structure 11 TAP structure 30
dpt structure 48 tf trifluoromethane sulfonate anion
dqp structure 49 tro structure 47
en 1,2-diaminoethane trpy 2,2',6',2"-tripyridyl
Hdpa structure 33 tsite structure 46
Absorbance (energy and molar e value (E)) and emission spectral data are collected in
the following tables.
Key to notes:
c corrected emission DMSO dimethylsulfoxide
x at 7.6 K EMPA 5 : 5 : 2 diethyl ether : 3-methylpen-
RT room temperature tane : ethanol
1 absorbance at low temperature eth ethanol
glass porous Vycor glass
gly 2 : 1 ethyleneglycol : water
Solvent MeC1 dichloromethane
ac acetone MeOH methanol
alc 4 : i ethanol : methanol MF N-methylformamide
aq aqueous solution nitrile acetonitrile or other nitrile
aq-eth 1 : 1 water : ethanol pc propylene carbonate
DMF N,N-dimethylformamide S solid
4 q ? R.A.Krause
~ t
HCN s
NH HN
1
2 3
C Xx3
H H
5 6
H H
N~/ HN
7 8 9
o o-
o b c
0 O-
~ C F 3 3F C~'xCF 3
d e
1I 21
R = )a( ~)3HC(HC
(b) cyclohexyl
)c( CH2--~
2OC R 2OC R
(d) lyhthpan-3~
)e( nilaced
31 (f)
.giF 1
Synthesis of Ruthenium(II) Complexes of Aromatic Chelating Heterocycles 5
I/. 15 16
17 18
H|9 20 H21
N N.q
N N
2~
22
25 25 27
N~N
,/x} //4
Fig. 1
(continued) 28 29 3O
6 R.A.Krause
ca;<
2H 2H
31 32
33 3/, 35
:!6 37 113
qx
~ J
:!9 /,0
)252O aON
&2 /,3
2H
1,5 &7
Fig. 1 (continued)
Synthesis of Ruthenium(II) Complexes of Aromatic Chelating Heterocycles 7
/,8 /,9
2H 2H
50 15
H 3HC
52 53
2C(tP31C -)4H
.giF 1 (continued) Ptl Pt2
I. Introduction
The purpose of this article is twofold: to examine how a particular group of ruthenium(II)
complexes may be synthesized and to consider emission properties of these compounds
as a function of molecular structure. It is our hope that ligand features important to
efficient emission in this class of compounds will be somewhat clarified, and that guide-
lines for the synthesis of specific complexes will be laid out. While this review is not
meant to be exhaustive we have attempted to include the major developments in synthe-
sis and emission studies up through the end of 1985.
In the first section, we have summarized the principle synthetic methods which have
been used, and have included a number of under utilized precursors which may be
developed into generally useful materials. In the second section we consider models
which have been developed for the emitting state, and how molecular structure appears
to influence emission behavior. In short, what ligand features lead to an efficient emitter.
This section may raise more questions than answers; however, taken together this review
should facilitate determining those molecular features which might be desirable for a
particular photochemical result and selecting the optimum preparative route for obtain-
ing the desired compounds.
8 R.A.Krause
II. Synthesis
A. General Synthetic Considerations
Synthesis can be considered to consist of three separate components:
)1 the reaction forming the complex
2) isolation
3) purification
While the latter two are not within the scope of this review, it will be helpful to briefly
treat some general aspects of each of these three components here.
In conducting the reaction, the ligand and ruthenium source material should be at
least somewhat soluble in the reaction solvent. The properties of the ligand can be
important in solvent choice; solubility and stability under the employed temperature
conditions may be deciding factors. An easily oxidized ligand may preclude the use of any
source material containing ruthenium with an oxidation state greater than two, and
readily reduced ligands may preclude the use of added reductants. Extensive tar forma-
tion is an indicator of unsuitable reaction conditions.
Isolation, once the reaction is over, usually utilizes solubility properties. Uncharged
complexes are frequently insoluble in water. Rarely are coordination compounds soluble
in diethylether or hydrocarbons. Thus, the addition of water, ether, or hydrocarbons (if
miscible with the reaction medium) precipitates the product and many other components
of the solution.
When the product is a cation a common practice is to add water (in which chloride
salts are generally soluble) followed by a large anion. The most commonly used of these
today are perchlorate and fluorophosphate, both of which have disadvantages. While
perchlorates generally crystallize nicely, they have a troubling tendency to explode.
Consequently, perchlorates are usually isolated only on a milligram scale without scrap-
ing or dry grinding of the compounds. Even in solution this anion can oxidize some
ruthenium(II) compounds. Fluorophosphates generally precipitate well from aqueous
solutions, but the anion tends to decompose over a period of time and may etch sample
bottles. Fluoroborates do not precipitate complexes as well, at least in our hands, and
suffer the same drawback.
Both perchlorate and fluorophosphate salts tend to show organic solubility (e.g.,
acetone, acetonitrile, occasionally methylene chloride) and are thus useful for further
purification and solution measurements. Other salts can generally be prepared by
metathesis. Dissolving a complex fluorophosphate salt in acetone followed by addition of
a soluble quaternary chloride usually precipitates the insoluble chloride as a tar or glass,
soluble in water. A similar result may sometimes be achieved by stirring a fluorophos-
phate salt with silver nitrate in water, precipitating sparingly soluble AgPF6.
Purification of complexes is most easily achieved by recrystallization. Most workers
have used the mixed solvent approach. The compound is dissolved in one solvent and a
second solvent, in which the complex is insoluble but which is miscible with the first, is
added. Success depends on a number of factors, one being how badly the product is
contaminated. Several repetitions may be necessary. While chromatography has been
used in a number of instances, we generally prefer recrystallization as being less time
consuming.
Synthesis of Ruthenium(II) Complexes of Aromatic Chelating Heterocycles 9
It is extremely useful to have available a technique for rapidly monitoring reaction
progress or assessing sample purity. An ideal technique for these purposes is thin layer
chromatography (TLC). The method is fast, very sensitive, readily set up, and inexpen-
sive. Uncharged complexes can generally be developed on silica gel plates with a variety
of solvents; recently we have had good success with ethylacetate.
Cationic complexes in the past were nearly impossible to develop on silica plates.
Meyer's group )1 found cations could be developed on alumina plates using a benzene-
acetonitrile developer. Anderson and Seddon )2 also used alumina plates with ethanol-
water developer. However, the best general developers for cationic complexes, which we
have presently found, are tetraethylammonium bromide in ethanol )3 or tetraethylam-
monium perchlorate in acetonitrile .)4 These are used with the readily available silica gel
TLC plates.
B. Tris-Chelates, Ru(AB)3 +2 and Bis-Chelates, Ru(ABC)2 +2
A number of different methods have been employed during the past 05 years for the
synthesis of tris-bidentate or bis-tridentate homochelated ruthenium(II) complexes. An
examination of the various methods may lead the reader to a method best suited for a
particular situation.
The first preparation of Ru(bpy)3 +2 by Burstall )5 involved fusion of 31CuR in bpy,
followed by extraction of excess ligand and crystallization of the product from water.
Morgan and Burstall employed the same method for Ru(trpy)2 .)6+2 This method is
wasteful of ligand; Cook et al. )7 commented on this fact and adopted other procedures.
The fusion technique has not been extensively utilized.
Palmer and Piper )a adopted a technique originally rejected by Burstall, involving long
reflux times (72 h) of RuCI3 • 3 H20 and excess ligand in 95% ethanol. High yields of
pure material can be obtained. A number of workers have used the method with other
ligands ,7 .)11-9 Synthesis of the 2-picolylamine complex )21 reportedly required only 21 h
reflux; it is possible that excess ligand served as the reducing agent giving a faster
reaction. The picolylamine ligand in Ru(bpy)2(AB) +2 can be oxidized to the imine )31
leaving some question as to the exact nature of the reported compound. However, the
infrared spectrum did not indicate the presence of an imine linkage.
Somewhat shorter reaction times (ca. 30 h) have been found using ethanol-water
mixtures14,15). This of course requires that the ligand be sufficiently soluble in the aque-
ous medium. While most polypyridyls are insoluble in water, sufficient solubility may
exist at the boiling point. Liu et al. )61 performed a partial asymmetric synthesis of
Ru(bpy)3 +2 from K2RuCls(H20) in water containing d-tartrate. Only 2 h heating was
required; the reducing agent most likely was the tartrate. Much shorter reaction times are
required when NaH2PO2 is employed as a reductant. Dwyer and co-workers )81,71 used
K4Ru2Cll00 in weakly acidic aqueous solution containing the ligand; addition of sodium
hypophosphite yielded tris-chelates after a few minutes refluxing. The same procedure
has been employed with K2RuC15(H20) MT 19) or 31CuR • 3 H2 '02O ,)12 as source mate-
rials. Broomhead and Young )22 recommended oven drying of commercial 31CuR • nH20
to achieve consistent results in the synthesis of Ru(bpy)3Cl2 through this route. Com-
plexes of tridentate ligands have also been obtained in this fashion .)42,32
