Metallostars containing {Ru(bpy)3} motifs

Article abstract of DOI:10.1016/S0020-1693(99)00483-1

Two new ligands designed to act as the core for metallostars based upon multiple bpy (bpy = 2,2'-obipyridine) metal-binding domains have been prepared. The first ligand 6 consists of a 1,3,5-triazine bearing three bpy metal-binding domains and was prepared inter alia using Stille methodology. All attempts to form complexes of 6 were unsuccessful. In contrast, a non-planar core compound based upon a tetraphenylmethane moiety bearing four bpy domains, also prepared using Stille couplings, was shown to form a tetraruthenametallostar complex containing four {Ru(bpy)₃} motifs. Each of the {Ru(bpy)₃} motifs is chiral, possessing Δ or Λ chirality and detailed NMR studies indicate that the complex is formed with little or no diastereoselectivity leading to a mixture of diastereomers and a fuzzy stereochemistry. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00483-1

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 300–302 (2000) 158–168
Metallostars containing {Ru(bpy)3} motifs
Edwin C. Constable *1, Oliver Eich, Catherine E. Housecroft *2, David C. Rees
Institute of Inorganic Chemistry, Uni6ersity of Basel, Spitalstrasse 51, CH 4056 Basel, Switzerland
Received 23 August 1999; accepted 24 September 1999
Abstract
Two new ligands designed to act as the core for metallostars based upon multiple bpy (bpy=2,2%-bipyridine) metal-binding
domains have been prepared. The first ligand 6 consists of a 1,3,5-triazine bearing three bpy metal-binding domains and was
prepared inter alia using Stille methodology. All attempts to form complexes of 6 were unsuccessful. In contrast, a non-planar
core compound based upon a tetraphenylmethane moiety bearing four bpy domains, also prepared using Stille couplings, was
shown to form a tetraruthenametallostar complex containing four {Ru(bpy)3} motifs. Each of the {Ru(bpy)3} motifs is chiral,
possessing D or L chirality and detailed NMR studies indicate that the complex is formed with little or no diastereoselectivity
leading to a mixture of diastereomers and a fuzzy stereochemistry. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Oligopyridines; Metallostars; Ruthenium complexes; Bipyridine complexes
ture based upon cores and generations to describe these
compounds. We have previously reported metallostars
with metal ions at the centre [4–9] together with a
series based upon organometallic derivatives with or-
ganic cores [10–13]. In this article, we report the syn-
thesis of three- and four-armed cores bearing
2,2%-bipyridine (bpy) metal-binding motifs and of a
metallostar in which each arm bears a {Ru(bpy)3}
motif.
1. Introduction
Dendrimers and metallodendrimers are often utilised
as polyfunctional compounds or as precursors to highly
functional species prepared in subsequent decoration
steps [1]. Although dendrimers are aesthetically pleasing
and are being successfully applied in fields as diverse as
catalysis, biomolecule recognition and polymer synthe-
sis [2], they also have a number of inherent disadvan-
tages. Progressive steric congestion at the surface means
that fractal growth through infinite generations is not
possible and the De Gennes packing model relates the
point at which defects start to appear in the topology
and length of the dendron [3]. Furthermore, defects
within individual generations are rapidly promulgated
and lead to polydispersivity in contrast to the ideal
monodispersed dendrimer structure. These difficulties
led us and others to consider the synthesis of multinu-
clear compounds in which the growth of multiple arms
from a central core occurs in a linear rather than a
branching manner. We have termed these compounds
metallostars. Metallostars possess many similarities to
dendrimers and it is convenient to adopt the nomencla-
2. Experimental
Infrared spectra were recorded on a Mattson Genesis
Fourier-transform spectrophotometer with samples in
1
compressed KBr discs. H NMR spectra were recorded
on a Bruker AM 250 MHz spectrometer. Time of flight
(MALDI) spectra were recorded using a PerSeptive
Biosystems Voyager-RP Biospectrometry Workstation.
Electrochemical measurements were performed with an
Eco Chemie Autolab PGSTAT 20 system using plat-
inum or glassy carbon working and auxiliary electrodes
with an Ag ꢀ AgCl electrode as reference using purified
acetonitrile as solvent and 0.1 M [nBu4N][BF4] as sup-
porting electrolyte; ferrocene was added at the end of
each experiment as an internal reference. The com-
pounds tetra(4-bromophenyl)methane [14], 6-bromo-
1 *Corresponding author. Tel.: +41-61-267 1001; fax: +41-61-267
1015; e-mail: constable@ubaclu.unibas.ch
2 *Corresponding author.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 4 8 3 - 1

E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
159
2,2%-bipyridine [15], [Pd(PPh3)4] [16] and [Ru(bpy)2Cl2]
[17] were prepared by the literature methods. The com-
plex [Ru(bpy-d8)2Cl2] was prepared as for [Ru(bpy)2Cl2]
but using bpy-d8.
60°C in DMF (15 ml) for 5 h after which acetone (75
ml) was added and the mixture left at −18°C overnight
to precipitate the salt 3 (0.25 g, 68%). Anal. Found: C,
60.2; H, 6.4; N, 11.8. Calc. for C12H15N2OCl: C, 60.4;
H, 6.3; N, 11.7%. IR (KBr cm−1): 2222 w.
2.1. N-{3-(4-Bromophenyl)-3-oxopropyl}-
N,N-dimethylammonium chloride (1)
2.4. 6-(4-Cyanophenyl)-2,2%-bipyridine (4)
N,N-Dimethylammonium chloride (1.20 g, 15 mmol),
paraformaldehyde (3.52 g, 59 mmol) and 4-bromoace-
tophenone (2.78 g, 14 mmol) were heated to reflux in
EtOH (40 ml) for 4 h after which an additional quan-
tity of paraformaldehyde (0.71 g, 0.012 mol) was added
and heating continued for a further 4 h. The clear
yellow solution so obtained was concentrated in vacuo
to 10 ml volume and treated with acetone (120 ml) to
give a white suspension. This was cooled to −18°C for
12 h after which the Mannich salt 1 was collected by
filtration (2.79 g, 68%). Anal. Found: C, 45.32; H, 5.08;
N, 4.83. Calc. for C11H15NBrOCl: C, 45.15; H, 5.17; N,
4.79%. Mass spectrum (+ve FAB) m/z: 293 (with
isotopomers) {HM}+, 256 (with isotopomers {M–Cl}+.
IR (KBr cm−1): 2957 m, 2938 m, 2910 m, 1681 s, 1601
s, 1476 m, 1464 m, 1411 m, 1383 m, 1297 m, 1019 m,
951 m, 823 m, 782 m.
2.4.1. Method 1
A mixture of 2-(2-pyridyl)-2-oxoethylpyridinium io-
dide (1.37 g, 4.2 mmol) and [NH4][OAc] (3.24 g, 42.1
mmol) in EtOH (50 ml) was refluxed for 10 min after
which 3 (1.00 g, 4.2 mmol) was added and heating
continued for a further 6 h. Addition of water to the
reaction mixture followed by overnight cooling resulted
in the precipitation of a small amount of 4 as a white
solid (0.043 g, 4%).
2.4.2. Method 2
A mixture of 2 (1.1 g, 4.8 mmol) and CuCN (0.43 g,
5.2 mmol) in DMF (2 cm3) was heated at 145°C for 24
h. The reaction mixture was then poured into an excess
of concentrated aqueous potassium cyanide solution
and allowed to stand overnight. The resulting precipi-
tate was filtered off, dried and extracted with toluene.
The organic extract was then evaporated to dryness in
vacuo and then recrystallised from MeOH to give 4
(0.70 g, 57%). M.p. 135–136°C. Anal. Found: C, 78.9;
H, 3.9; N, 16.4. Calc. for C17H11N3: C, 79.4; H, 4.3; N,
16.3%. Mass spectrum (TOF; 1,8,9-trihydroxyan-
thracene matrix): m/z 257 {M}+, 231 {M–CN}+. IR
(KBr cm−1): 2926 m, 2222 m, 1725 s, 1581 s, 1561 s,
1452 s, 1429 s, 1282 s, 1138 m, 1071 m, 821 m, 773 s,
2.2. 6-(4-Bromophenyl)-2,2%-bipyridine (2)
A mixture of [NH4][OAc] (4.20 g, 55 mmol) and
2-(2-pyridyl)-2-oxoethylpyridinium iodide (1.41 g, 4.32
mmol) was heated to reflux for 10 min in EtOH (10 ml)
under dinitrogen. After this period 1 (1.26 g, 4.32
mmol) was added and the mixture refluxed for a further
4.5 h. The suspension was then cooled and the beige
precipitate of 2 collected by filtration. Recrystallisation
from EtOH gave 2 as an off-white solid (0.47 g, 92%).
M.p. 89.2°C. Anal. Found: C, 61.9; H, 4.2; N, 8.8. Calc.
for C16H11N2Br: C, 61.8; H, 3.8; N, 9.0%. Mass spec-
trum (+ve FAB) m/z: 310, 312 {M}+. IR (KBr
cm−1): 1582 m, 1560 m, 1430 m, 1008 m, 771 s, 745 m,
1
744 s, 548 m. H NMR (CDCl3): l 8.70 (1H, d, H6A),
8.58 (1H, d, H3A), 8.39 (1H, d, H3B), 8.10 (2H, d, Ho),
7.85 (2H dd, H4A,4B), 7.75 (1H, d, H5B), 7.71 (2H, d,
Hm), 7.32 (1H, ddd, H5A).
2.5. 6-(4-Tri-n-butylstannylphenyl)-2,2%-bipyridine (5)
1
510 m. H NMR (CDCl3): l 8.70 (1H, d, H6A), 8.58
(1H, d, H3A), 8.39 (1H, d, H3B), 8.02 (2H, d, Ho), 7.85
(2H, dd, H4A,4B), 7.70 (1H, d, H5B), 7.62 (2H, d, Hm),
7.32 (1H, ddd, H5A). 13C NMR (CDCl3): l 156.1, 155.8,
155.2, 149.1, 138.2, 137.8, 136.8, 131.8, 128.4, 123.8,
123.4, 121.2, 119.9, 119.6.
2 (0.30 g, 0.97 mmol) was dissolved in THF (40 ml)
under dinitrogen and cooled to −100°C (liquid dini-
trogen/MeOH) after which n-butyllithium (0.73 ml, 1.6
M solution in hexane, 1.17 mmol) was added dropwise
to the stirred solution which turned deep crimson-red.
The mixture was stirred at −100°C for 40 min after
which n-Bu3SnCl (0.48 g, 0.40 ml, 1.46 mmol) was
added over 3 min. The solution became green–blue and
was stirred for 75 min and then allowed to warm to
−10°C when H2O (5 ml) was added. The THF was
removed in vacuo and the residue extracted into
CH2Cl2 (2×150 ml). The extracts were dried (Na2SO4)
and evaporated to give a pale yellow oil which was
2.3. N-{3-(4-Cyanophenyl)-3-oxopropyl}-
N,N-dimethylammonium chloride (3)
N,N-Dimethylammonium chloride (0.39 g, 4.8
mmol), paraformaldehyde (0.65 g, 22.2 mmol) and 4-
cyanoacetophenone (0.69 g, 4.8 mmol) were heated to

160
E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
purified by column chromatography (SiO2, 4:1 hex-
anes:EtOAc) to give 5 as an oil (92%, 0.47 g).
2.7. 6-Tri-n-butylstannyl-2,2%-bipyridine (7)
Mass spectrum (TOF, 1,8,9-trihydroxyanthracene
matrix): m/z 520 {M}+. IR (KBr cm−1): 2959 m, 1579
w, 1428 w, 1260 w, 1074 w, 777 w, 726 w, 691 w, 655
w, 587 w, 433 vs. 1H NMR (CDCl3): l 8.68 d, H6A), 8.63
(1H, d, H3A), 8.38 (1H, d, H3B), 8.11 (2H, d, H2C), 7.87
(2H, m, H4A, 4B), 7.78 (1H, d, H5B), 7.63 (2H, d, H3C),
7.31 (1H, dd, H5A), 1.57 (6H, m, SnCH2), 1.37 (6H, m,
SnCH2CH2), 1.12 (6H, m, CH2CH3), 0.92 (9H, m, CH3).
13C NMR (CDCl3): l 156.7, 156.5, 155.7, 149.1, 143.5,
139.0, 137.6, 136.9, 136.8, 126.3, 123.7, 121.3, 120.3,
119.3, 29.2, 27.5, 13.8, 9.7.
Freshly distilled (Na, Ph2CO) diethyl ether (4 ml) was
placed in a flame-dried apparatus under an atmosphere
of argon and cooled to −100°C and a solution (1.6 M)
n
of BuLi in hexanes (1.06 cm3, 1.70 mmol). A solution
of 6-bromo-2,2%-bipyridine (0.040 g, 1.70 mmol) in
freshly distilled diethyl ether (4 ml) was immediately
added to give a deep red solution which was stirred for
5 min. After this period, (n-Bu3)SnCl (0.46 ml, 1.70
mmol) was added and the mixture stirred whilst the
temperature was allowed to rise to 0°C. The resultant
yellow suspension was treated with H2O to give a clear
solution which was extracted with diethyl ether (100 ml).
The organic phase was separated and dried over MgSO4
and then purified by chromatography over SiO2 (Et2O/
hexanes mobile phase) to give 6-tri-n-butylstannyl-2,2%-
bipyridine (7) as a green oil (0.498 g, 65.9%) that was not
2.6. 2,4,6-Tris(4-(2,2%-bipyridin-6-yl)phenyl)-
1,3,5-triazine (6)
2.6.1. Method 1
1
A solution of 4 (0.5 g, 1.9 mmol) in PhMe (3 ml) was
added slowly over 3 h to ClSO3H (0.38 ml, 5.83 mmol)
maintained at −6°C (ice-salt bath). The reaction mix-
ture changed through yellow and orange finally forming
a deep viscous red suspension. The reaction flask was
removed from the ice bath and cooled to 3°C overnight
after which H2O was cautiously added and the resulting
orange–red product mixture was extracted with CH2Cl2.
The organic layer was washed with H2O (3×10 ml),
dried (MgSO4) and then evaporated to dryness in vacuo.
Purification by flash column chromatography (SiO2, 7:3
40–60° petroleum ether: EtOAc) removed the organic
impurities leaving 5 as a yellow band at the top of the
column which was eluted with the highly polar solvent
mixture MeCN:saturated KNO3:H2O (14:2:1). The yel-
low fraction was collected, extracted with CH2Cl2 and
the extract washed with H2O, dried (MgSO4) and evap-
orated to dryness to give 6 (0.015 g, 3%).
further purified. H NMR (CDCl3): l 8.65 (1H, ddd,
J=6.0 Hz, H6A), 8.53 (1H, d, J=7.8 Hz, H3A), 8.25
5
(1H, dd, J=7.8, 1.2 Hz, H3B, subspectrum with JSn–H
8.0 Hz), 7.80 (1H, td, J=7.8, 1.2 Hz, H4A), 7.63 (1H, t,
J=7.8 Hz, H4B, subspectrum with 4JSn–H 14.2 Hz), 7.40
3
(1H, dd, J=7.8, 1.2 Hz, H5B, subspectrum with JSn–H
15.5 Hz), 7.63 (1H, ddd, J=7.8, 6.0, 1.2 Hz, H5A), 1.61
(6H, m, SnCH2), 1.36 (6H, m, SnCH2CH2), 1.56 (6H, m,
CH2CH3), 0.89 (9H, t, CH3). Mass spectrum (MALDI-
TOF): m/z 447 {M}+, 390 {M–C4H9}+. IR (KBr
cm−1): 2956 m, 1544 m, 1420 s, 770 s.
2.8. Tetrakis(4-{6-[2,2-bipyridyl]phenyl)methane (8)
A mixture of 7 (2.56 g, 5.75 mmol), tetra(4-bromo-
phenyl)methane (0.827 g, 1.30 mmol), [Pd(PPh3)4] (0.120
g, 0.104 mmol) in DMF (5 ml) was heated to 90°C and
stirred for 110 h to give a dark-coloured reaction
mixture. Chromatographic purification (SiO2, CH2Cl2/
acetone mobile phase) followed by recrystallisa-
tion from CH2Cl2/EtOH gave tetrakis(4-{6-[2,2-
bipyridyl]}phenyl)methane (8) as white crystals (0.376 g,
30.8%). M.p.\300°C. Anal. Found: C, 82.9; H, 4.8; N,
2.6.2. Method 2
5 (0.30 g, 0.58 mmol), [Pd(PPh3)4] (0.013 g, 0.012
mmol) and 2,4,6-trichloro-1,3,5-triazine (0.027 g, 0.15
mmol) were dissolved in toluene freshly distilled over
sodium wire (10 ml) when a rapid reaction ensued and
precipitation of 6 began after 2 h. The reaction was left
overnight and then diethyl ether (20 ml) was added and
the product filtered off (0.10 g, 87%). Anal. Found: C,
79.6; H, 4.4; N, 16.7. Calc. for C51H33N9: C, 79.4; H, 4.3;
N, 16.3%. Mass spectrum (+ve FAB): m/z 771 {M}+.
IR (KBr cm−1): 2923 m, 1648 s, 1619 m, 1581 s, 1561
m, 1454 m, 1430 s, 774 s. 1H NMR (CDCl3) l 8.79 (2H,
d, H2C), 8.72 (1H, d, H6A), 8.67 (1H, d, H3A), 8.46 (1H,
d, H3B), 8.36 (2H, d, H3C), 7.95 (1H, m, H4B), 7.90 (1H,
m, H4B), 7.34 (1H, m, H5B), 7.17 (1H, m, H5A). 13C
NMR (CDCl3): l 152.3, 149.6, 143.3, 139.3, 137.4,
136.6, 132.5, 128.7, 127.9, 125.0, 124.7, 124.4, 124.0,
1
11.5. Calc. for C65H44N8: C, 83.3; H, 4.7; N, 12.0%. H
NMR (CDCl3): l 8.68 (4H, ddd, J=6.0 Hz, H6A), 8.62
(4H, d, J=7.8 Hz, H3A), 8.37 (1H, dd, J=7.8, 1.2 Hz,
H3B), 8.12 (8H, d, J=8.8 Hz, Ho), 7.89 (4H, t, J=7.8
Hz, H4B), 7.81 (1H, td, J=7.8, 1.2 Hz, H4A), 7.78 (4H,
dd, J=7.8, 1.2 Hz, H5B), 7.56 (8H, d, Hm), 7.31 (4H,
ddd, J=7.8, 6.0, 1.2 Hz, H5A). 13C NMR (CDCl3): l
156.3 (4C, C2A/2B/6B), 156.0 (4C, C2A/2B/6B), 155.7 (4C,
C2A/2B/6B), 149.0 (4C, C6A), 147.4 (4C, Cipso), 137.7 (4C,
C4B), 137.2 (4C, Cp), 136.8 (4C, C4A), 131.5 (8C, Co),
126.3 (8C, Cm), 123.7 (4C, C3A), 121.3 (4C, C5A), 120.2
(4C, C3B/5B), 119.3 (4C, C3B/5B), 64.8 (1C, Ccentre). UV–
Vis (CH2Cl2): umax (m/dm3 mol−1 cm−1) 272 (8700), 292
(7360), 311 (5820) nm. IR (KBr cm−1): 3052 m, 1581 m,
121.8, 118.4. UV (CDCl3): umax (m/dm3 mol−1 cm−1
)
259 (33 800), 326.4 nm (105 700).

E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
161
1561 s, 1454 m, 1429 s, 1320 w, 1258 w, 1157 w, 1099
w, 1038 w, 1015 m, 989 w, 814 m, 776 s, 748 m, 712 w,
618 w cm–1. Mass spectrum (MALDI-TOF): m/z 937
{M}+, 783 {M–C10H7N2}+, 705 {M-C6H4C10H7N2}+,
474 {M-2(C6H4C10H7N2)}+.
irradiation (440 W, 10 min.) for 10 min. The orange
reaction mixture was cooled and then stirred with
[NH4][PF6] (0.900 g) and sufficient H2O to give an
essentially mother liquor and an orange precipitate.
The precipitate was collected by filtration, dissolved in
the minimum amount of MeCN, purified by chro-
matography (SiO2, MeCN:saturated aqueous KNO3:
H2O, 7:1:0.5 v/v) and finally precipitated by [NH4][PF6].
Recrystallisation from acetone gave red crystals of
[8{Ru(bpy)2}4][PF6]8 (0.131 g, 65.2%). Anal. Found: C,
44.6; H, 3.4; N, 8.7. Calc. for C145H108N24F48P-
8Ru4·8H2O: C, 44.85; H, 3.4; N, 8.7%. 1H NMR
(CD3CN): See Table 1. 13C NMR (CDCl3): l 167.2 (4C,
C6B), 158.95, 158.9, 158.4, 158.2, 158.0, 157.3, 153.6
(24C, C2A,2B,2D,2E,2F,2G), 153.6, 153.0, 152.7, 152.5, 151.8
(20C, C6A,6D,6E,6F,6G), 144.5 (4C, Cipso), 139.0 (3C),
2.9. 2,2%-Bipyridine-d8
2,2%-Bipyridine (1.0 g, 6.4 mmol) was heated at 200°C
in a sealed glass tube with D2O (5 ml) and palladium-
charcoal (10%, 0.05 g) for 5 d. After this period, the
tube was cooled, opened and sufficient Me2CO added
to dissolve all organics. The mixture was filtered
through celite and concentrated in vacuo to give a
white solid shown by 1H NMR spectroscopy to be
:90% deuterated. The deuteration procedure was re-
peated to give fully deuterated 2,2%-bipyridine-d8 as
white crystals (0.83 g, 85%).
138.9, 138.8, 128.7, 137.3 (24C, C4A,4B,4D,4E,4F,4G,p
)
131.5, 128.9, 128.8, 128.7, 128.3, 128.2, 126.8, 125.9,
125.5, 125.2, 124.9, 124.6, 123.5, 123.7 (64C,
C3A,5A,3B,5B,3D,5D,3E,5E,3F,5F,3G,5G,m,o), 64.4 (1C, Ccentre).
UV–Vis (MeCN): umax 244, 290, 451 nm: IR (KBr
cm−1): 1604 w, 1466 w, 1448 m, 1426 w, 841 s, 765 m,
731 w, 558 m. Mass spectrum (MALDI-TOF): m/z
3607 {M–PF6}+, 2909 {M–Ru(bpy)2–3PF6}+.
2.10. [8{Ru(bpy)2}4][PF6]8
A
mixture of 8 (0.050 g, 0.053 mmol) and
[Ru(bpy)2Cl2] (0.114 g, 0.235 mmol) was suspended in
ethane-1,2-diol (2.5 ml) and refluxed under microwave
Table 1
1H NMR data for the ligand 8 in [8{Ru(bpy)2}4][PF6]8 and ligand 9
in [Ru(bpy)2(9)][PF6]2
3. Results and discussion
a
3.1. Ligand synthesis and characterisation
[8{Ru(bpy-d8)2}4][PF6]8
[Ru(bpy)2(9)][PF6]2
A6 (dd)
A5
7.54
7.32
8.06
8.55
7.42
8.20
8.61
6.40*
7.30*
5.90*
6.09*
7.15
6.33
7.20
7.70
7.86
7.43
8.10
8.10
7.27
7.20
7.92
8.32
8.00
7.56
8.11
8.41
7.55
7.30
8.10
8.60
7.35
8.15
8.65
6.7*
7.2*
7.0*
6.0*
6.95
6.80
7.55
8.00
7.80
7.45
8.10
8.37
7.30
7.25
7.90
8.30
8.05
7.50
8.10
8.40
3.1.1. The three-armed 1,3,5-triazine ligand
The aim of this study was to develop new cores for
use in metallodendrimers and metallostars. The initial
motif that we considered was a 2,4,6-trifunctionalised
1,3,5-triazine as a planar core with three pendant arms.
We adopted this choice for two reasons: (i) the triazine
can be prepared in a convergent sense from three
precursor molecules and (ii) the spatial arrangement of
the substituents is reasonably well-defined.
A4 (t)
A3 (d)
B5 (dd)
B4 (td)
B3 (d)
C
C
C
C
D6
D5
D4
D3
E6
E5
E4
E3
F6
F5
F4
F3
G6
G5
G4
G3
a All data for CD3CN solutions. Unambiguous assignments for the
C-ring protons have not been made (see text).

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E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
Scheme 1.
The target molecule was 2,4,6-tris(4-(2,2%-bipyridin-6-
spectrum was well-resolved and typical for a 6-substi-
tuted 2,2%-bipyridine. The conversion of 2 to 4 was
achieved by heating with copper(I) cyanide in DMF to
give 4 as a white solid in 57% yield. The IR spectrum of
yl)phenyl)-1,3,5-triazine (6) in which three bpy metal-
binding domains are attached to the 1,3,5-triazine
through para-phenylene spacers. The original approach
that we developed was based upon the cyclotrimerisa-
tion of 6-(4-cyanophenyl)-2,2%-bipyridine (4). This latter
compound was prepared as indicated in Scheme 1.
Initially, we prepared the Mannich salt N-{3-(4-
cyanophenyl)-3-oxopropyl}-N,N-dimethylammonium
chloride (3) from 4-cyanoacetophenone under standard
conditions. The salt was isolated in 68% yield and was
fully characterised. We used Krohnke methodology [18]
to convert 3 to 4 by reaction with 2-(2-pyridyl)-2-oxo-
ethylpyridinium iodide and ammonium acetate. Despite
investigating a range of reaction conditions, the best
yield that we achieved in this conversion was 4%, which
we considered to be unacceptable.
4 exhibits a typical w(CN) mode at 2222 cm−1
.
Acid-mediated trimerisation of nitriles is a standard
preparative method for 1,3,5-triazines [19] and the reac-
tion of 4 with chlorosulfonic acid at −5°C gave 6 in
optimised 3% yield.
A significantly better route for the preparation of 4
was subsequently adopted based upon a Stille coupling
reaction (Scheme 1). The starting point was the previ-
ously prepared bromo compound 2. Lithiation with
n-BuLi in THF followed by reaction with n-Bu3SnCl
gave the stannylated compound 6-(4-tri-n-butylstan-
nylphenyl)-2,2%-bipyridine (5) in 92% yield as a pale
yellow oil. The presence of the tin gives rise to charac-
teristic 117Sn and 119Sn satellites on both the aliphatic
and aromatic resonances in the 1H NMR spectrum.
Figs. 1 and 2 present the aromatic region of a CHCl3
solution of 5 showing the coupling to H3C of the
phenylene. Stille coupling [20] of 5 with commercially
available 2,4,6-trichloro-1,3,5-triazine in toluene in the
presence of [Pd(PPh3)4] gave the desired ligand 6 as an
extremely insoluble white solid in 87% yield. The high
yield is somewhat surprising in view of the presence of
bpy metal-binding domains in both 5 and 6 which
could coordinate to palladium. The new ligand 6 was
fully characterised (see Section 2). Ligand 6 proved to
be extremely insoluble, compatible with extensive p-
In a second approach to 4, we decided to make a
functional group interconversion in the final step to
introduce the nitrile late in the synthesis. The conver-
sion of 4-bromoacetophenone to the Mannich salt 1
was achieved under standard conditions to give 1 in
68% yield as a white solid which was fully character-
ised. The salt exhibits a characteristic carbonyl absorp-
tion at 1681 cm−1 in its IR spectrum. The reaction of
1 with ammonium acetate and the Krohnke reagent
2-(2-pyridyl)-2-oxoethylpyridinium iodide [18] in EtOH
gave 6-(4-bromophenyl)-2,2%-bipyridine (2) as a white
solid in excellent yield. The compound 2 exhibits a pair
of parent ions in its mass spectrum at m/z 310/312
1
corresponding to the two isotopomers. The H NMR

E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
163
3.1.2. The four-armed tetraphenylmethane ligand
The non-planar ligand was to be based upon a
tetrahedral tetraphenylmethane core. This latter motif
has been used by Wuest and co-workers as an extended
tetrahedral tecton for the synthesis of novel materials
and in crystal engineering [21,22]. We planned to struc-
turally develop the tetraphenylmethane core by func-
tionalisation with a metal-binding domain and our
planned ligand was 8.
In view of the success in the synthesis of 6, we hoped
to use palladium-catalysed Stille reactions for the at-
tachment of stannylated metal-binding domains to
halogenated tetraphenylmethane derivatives (Scheme
2). Commercially available tetraphenylmethane was
converted to tetrakis(4-bromophenyl)methane by reac-
tion with bromine using the literature methods [14].
Once again, the metal-binding domain of choice was
bpy which was to be introduced via the known com-
pound 6-bromo-2,2%-bipyridine. Lithiation of 6-bromo-
2,2%-bipyridine with n-BuLi proceeded smoothly at low
Fig. 1. 250 MHz 1H NMR spectrum of a CDCl3 solution of 6-(4-tri-
n-butylstannylphenyl)-2,2%-bipyridine (5) showing the assignments
and the numbering scheme adopted.
stacking in the solid state, as expected for a highly
conjugated, and most likely planar, system. We decided
to design a second ligand in which the core structure
orients the metal binding domains in such a way that
extensive solid state stacking is not possible.
Fig. 2. 250 MHz 1H NMR spectrum of a CDCl3 solution of 6-(4-tri-n-butylstannyl)-2,2%-bipyridine (7) showing the assignments and the numbering
scheme adopted.

164
E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
Scheme 2.
temperature and subsequent reaction of the 6-lithio-
2,2%-bipyridine with n-Bu3SnCl gave the stannyl
derivative 6-tri-n-butylstannyl-2,2%-bipyridine (7) as an
oil in 66% yield. The compound was characterised by
conventional methods. Once again, the observation of
coupling to the 117Sn and 119Sn nuclei of the protons
at the stannylated pyridine ring in the H NMR spec-
trum, confirms the formation of the desired metal-
lated compound.
The Stille coupling of 6-tri-n-butylstannyl-2,2%-
bipyridine with tetrakis(4-bromophenyl)methane pro-
ceeded smoothly in DMF to give the desired ligand
tetrakis(4-{6-[2,2%-bipyridyl]}phenyl)methane (8) as a
white crystalline solid in 31% yield. The compound
exhibited a parent ion at m/z 937 in its mass spec-
trum and the high symmetry led to a simple 1H
NMR spectrum which was fully assigned on the basis
of COSY methods (Fig. 3).
In contrast, the more soluble ligand 8 reacted
smoothly with slightly more than four equivalents of
[Ru(bpy)2Cl2] in ethane-1,2-diol under microwave irra-
diation to give a deep orange solution from which the
red crystalline compound [8{Ru(bpy)2}4][PF6]8 was
isolated in 65% yield after chromatographic purifica-
tion (Scheme 3). The compound exhibited a single
spot on TLC analysis in a variety of mobile phases.
The new tetranuclear metallostar exhibited a peak in
its MALDI-TOF mass spectrum corresponding to
{[8{Ru(bpy)2}4][PF6]7}+ together with a fragmenta-
tion peak corresponding to the loss of a {Ru(bpy)2}
unit assigned to {[8{Ru(bpy)2}3][PF6]5}+. The orange
colour is typical of the {Ru(bpy)3} chromophore and
arises from an MLCT band at 451 nm. The absorp-
tion is significantly blue-shifted with respect to
[Ru(bpy)3]2+, presumably as a result of the distortion
of the idealised D3 symmetry upon introducing the
6-substituent on one of the bpy rings. A similar blue
shift is seen in [Ru(bpy)2(9)]2+ which has umax 447
nm [23,24]. The complex [8{Ru(bpy)2}4][PF6]8 is
strongly luminescent and irradiation in MeCN solu-
tion at 451 nm results in an emission at 616 nm
(compared to 615 nm for [Ru(bpy)3]2+ in MeCN at
298 K). The complex is electrochemically active and
exhibits a single, reversible (Ea−Ec=66 mV) rutheni-
um(II)/(III) process at +0.92 V (vs. Fc/Fc+). This is
shifted slightly to more positive potential with respect
to the model compound [Ru(bpy)2(9)]2+ which shows
a ruthenium(II)/(III) process at +0.85 V [23,24]. In
addition, a reversible (Ea−Ec=96 mV) ligand cen-
tred reduction is observed at −1.68 V together with
a second reduction at −1.91 V with an associated
stripping peak on the return wave. These are com-
parable to the reductions at −1.76 and −1.91 V
observed for [Ru(bpy)2(9)]2+ [23,24].
1
3.2. Coordination beha6iour of the new core ligands
After investing a considerable effort in preparing
ligand 6 we were extremely disappointed to find that
we were unable to prepare any coordination com-
plexes with it! The ligand gives no coloration with
solutions of either iron(II) or copper(I) salts and
would not react with [Ru(bpy)2Cl2] in a variety of
solvents (MeOH, EtOH, n-BuOH, MeOH-CH2Cl2).
Attempted reaction with [Ru(bpy)2Cl2] in boiling
ethane-1,2-diol under microwave irradiation resulted
in the formation of
a deep violet solution of
[Ru(bpy)2(HOCH2CH2OH)]Cl2 and an insoluble
residue of 6! Molecular modelling indicates that no
serious steric interactions beyond those in the known
complex [Ru(bpy)2(9)]2+ are anticipated in a complex
such as [6{Ru(bpy)2}3]6+. In part, the insolubility of
the ligand, which contains 10 conjugated aromatic
rings which can stack in the crystal lattice, con-
tributes to its low reactivity.

E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
165
Fig. 3. 250 MHz 1H NMR COSY spectrum of a CD3CN solution of 8 showing the assignments and the numbering scheme adopted.
Scheme 3.
3.2.1. Stereochemistry of the tetraruthenametallostar
A pseudo-octahedral trischelate complex is chiral [25]
and each of the {Ru(bpy)3} motifs in the tetra-
ruthenametallostar [8{Ru(bpy)2}4][PF6]8 represents a
chiral centre. Using the conventional nomenclature for
such centres, five diastereomers, denoted (DDDD),
(DDDL), (DDLL), (DLLL) and (LLLL) are possible.
Of these, the (DDDD) and (LLLL) pair are related as
enantiomers, as are the (DDDL) and (LLLD) pair. The
remaining diastereomer, (DDLL), is an achiral meso
form with S4 symmetry. The question was, whether any
diastereoselectivity is observed or whether a product
with fuzzy stereochemistry consisting of a mixture of
the possible diastereomers had been obtained. Chro-

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E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
matography indicated the presence of a single species,
although the various diastereomers might be expected
to exhibit very similar characteristics.
The 13C NMR spectrum of [8{Ru(bpy)2}4][PF6]8 ex-
hibits a total of 35 resonances, as expected for a single
diastereomer with magnetically equivalent (or indistin-
guishable) arms, although the signals tentatively as-
signed to the phenylene spacer were broadened and
showed signs of additional, very poorly resolved, split-
ting. A single resonance at l 64.4 is observed for the
central carbon atom. This argues for diastereoselective
formation of
enantiomers).
a single diastereomer (or pair of
In order to simplify the initial analysis of the 1H
NMR spectrum, we prepared the analogous compound
1
[8{Ru(bpy-d8)2}4][PF6]8 from [Ru(bpy-d8)2Cl2]. The H
NMR spectrum of this complex will only exhibit reso-
nances due to the ligand 8. As far as the protons
associated with the various bpy metal-binding domains
are concerned, the spectrum is consistent with a single
solution species, in accord with the 13C NMR data.
Table 1 presents the 1H NMR data for ligand 8 in
[8{Ru(bpy-d8)2}4][PF6]8 and for comparison includes
Fig. 4. A view of a single {Ru(bpy)3} unit in [8{Ru(bpy)2}4][PF6]8
showing how the stacking of the C and D rings (Fig. 5) leads to the
magnetic non-equivalence of the phenylene protons. All protons
except those attached to the phenylene ring have been removed for
clarity.
Fig. 5. 600 MHz 1H NMR spectrum of a CD3CN solution of [8{Ru(bpy)2}4][PF6]8 showing the assignments and the numbering scheme adopted.

E.C. Constable et al. / Inorganica Chimica Acta 300–302 (2000) 158–168
167
data for the mononuclear model compound
[Ru(bpy)2(9)]2+; all assignments were made on the
basis of COSY spectroscopy. It is remarkable how
closely the two sets of data correlate with each other —
the only significant differences arise with the phenylene
ring C. Firstly, it is also apparent that the four protons
of the phenylene ring appear as four separate reso-
nances. We have commented upon this phenomenon in
earlier discussions of the spectrum of [Ru(bpy)2(9)]2+
salts. Ring C is stacked and approximately coplanar
with the bpy ring D (Fig. 4). The consequence is that
the two ortho and the two meta protons of ring C are
no longer magnetically equivalent. One ortho and one
meta proton lie on the ‘outside’ of the bpy ring, whilst
the other two are on the ‘inside’. Initial assignments of
the phenylene protons were made on the basis of COSY
spectra; the l 6.40 and 7.30 resonances were strongly
coupled and showed only very weak cross peaks to the
l 5.90 and 6.09 peaks, which were also strongly coupled
to one another. However, inspection of the spectrum at
various fields (250, 300 and 600 MHz) reveals that each
of the signals assigned to the phenylene ring protons is
a cluster of three or four overlapping resonances. We
have been unable to assign the protons unambiguously.
Even at 600 MHz we have been unable to resolve the
various sub-spectra, but this observation provides ex-
tremely strong evidence for a random assembly of D
Fig. 6. Modelled structure of [8{Ru(bpy)2}4][PF6]8. The homochiral
complex is shown, although the heterochiral systems do not exhibit
any significant interactions between the {Ru(bpy)3} motifs.
sufficiently remote from each other that they behave as
essentially independent sub-units and are to all intents
and purposes isochronous. Only the phenylene spacer
groups are sensitive to the asymmetric environment and
act as spectators for the fuzziness.
and
L
units to give
a
complex with fuzzy
stereochemistry.
The 600 MHz 1H NMR spectrum is presented in Fig.
5 showing the final assignments made on the basis of
COSY experiments and illustrating the multiple subspec-
tra observed for the phenylene protons. The final as-
signments are compared with those for in Table 1.
Overall, the similarity of the spectra of the two com-
pounds is remarkable.
Acknowledgements
We gratefully acknowledge the Schweizerischer Na-
tionalfonds zur Fo¨rderung der wissenschaftlichen
Forschung and the University of Basel for financial
support.
Molecular modelling studies (Fig. 6) indicate that the
tetraruthenastar compound possesses a diameter of
,
about 25 A. The overall structure does not obviously
vary greatly as the relative stereochemistry at the metal
centres is varied and in no diastereomer are there
serious steric interactions between the {Ru(bpy)3}
motifs.
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