The oxidation and subsequent co-ordination chemistries of 4′,4″-diphenyl-2,2′:6′,2″:6″,2?- quaterpyridine

Article abstract of DOI:10.1039/b203087j

Selective oxidation of 4′,4″-diphenyl-2,2′:6′,2″:6″,2?- quaterpyridine leads to two new ligands which, when complexed to a trivalent lanthanide centre, yield either approximately cubic or dodecahedral complexes.

Full text of DOI:10.1039/b203087j

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The oxidation and subsequent co-ordination chemistries of
4Ј,4Љ-diphenyl-2,2Ј:6Ј,2Љ:6Љ,2ٞ-quaterpyridine
Angelo J. Amoroso,*a Miles W. Burrows,a Thomas Gelbrich,b Robert Haigha and
Michael B. Hursthouseb
a Department of Chemistry, Cardiff University, PO Box 912, Cardiff, UK CF10 3TB.
E-mail: amorosoaj@cf.ac.uk
b Department of Chemistry, The University of Southampton, Southampton, UK
Received 27th March 2002, Accepted 7th May 2002
First published as an Advance Article on the web 15th May 2002
Selective oxidation of 4Ј,4Љ-diphenyl-2,2Ј:6Ј,2Љ:6Љ,2ٞ-quater-
pyridine leads to two new ligands which, when complexed
to a trivalent lanthanide centre, yield either approximately
cubic or dodecahedral complexes.
Attempts to oxidise this species to the tetra-oxide species
using the standard procedure of 30% H2O2 and acetic acid gave
varying ratios of starting material and bis-oxide. Even with
extended reaction times and additional hydrogen peroxide, the
reaction did not proceed to completion and thus stronger oxid-
ising conditions were required. In order to avoid the use of
hazardous 90% H2O2 or FOH,9 the urea–hydrogen peroxide
adduct and trifluoroacetic anhydride were used10 to generate
peroxytrifluoroacetic acid which resulted in the full N-oxidation
of the quaterpyridine. In keeping with the oxidation of ter-
pyridine,11 the addition of two equivalents of MCPBA to the
quaterpyridine gave the N,Nٞ-bis-oxide in almost quantitative
yield. However, in contrast to terpyridine, addition of one or
three equivalents of MCPBA yielded largely the bis-oxide or
starting material.
The use of polypyridine ligands to complex a variety of metals
has been explored in depth by several research groups.1 While
the terpyridine unit has been used with great success in areas
such as molecular electronics2 and lanthanide luminescence,3
longer chain ligands such as quater-, quinque- and sexi-
pyridine have been found to give an interesting array of helical
molecules with interesting structural and electronic properties.4
In addition, the co-ordination chemistry of 2,2Ј:6Ј,2Љ:6Љ,2ٞ:6ٞ,
2Јٞ:6Јٞ,2Љٞ-sexipyridine has been expanded to lanthanide
metals and was observed to form mononuclear monohelical
complexes with the europium ion.5
In contrast, we have been interested in the oxidative adap-
tation of such ligands to form the poly-N-oxide ligands.6 Such
ligands should have a greater affinity for lanthanide metals than
their analogous parent pyridyl systems, due to the harder oxy-
gen donor atom. Also, it has been noted by several groups that
the inclusion of the N-oxide fragment within a europium com-
plex often results in a higher quantum yield in the photo-
chemistry of the resulting complex, relative to the unoxidised
parent species.7
Both the bis-oxide and tetra-oxide quaterpyridines (L2 and
L1, respectively)† co-ordinate to a variety of metal centres. The
addition of two equivalents of ligand to an aqueous solution of
lanthanide perchlorate (Eu, Gd and Tb) results in the precipita-
tion of the desired complex [ML2](ClO4)3.‡
While such species are also of interest due to their potential
use in the preparation of functionalised oligopyridines, a par-
ticular feature of interest to us is the change in the ligand’s
stereochemical preferences which occur upon ligand oxidation.6
We have observed the greater flexibility of the terpyridine
trisoxide in its abilty to facially co-ordinate a metal centre and
were naturally interested in the change in the co-ordinating
properties of longer oligomers. To this end we focussed upon
the oxidation of readily synthesised 4Ј,4Љ-diphenyl-2,2Ј:6Ј,2Љ:6Љ,
2ٞ-quaterpyridine.8
Crystals of [Gd(L1)2][ClO4]3, 1, and [Tb(L2)2][ClO4]3ؒ
2CH3CN, 2, were obtained and their structures were deter-
mined by X-ray crystallography (see Fig. 1).§ Both complexes
consist of an eight co-ordinate lanthanide centre. While 1 has
Fig. 1 Crystal structures of (a) [Gd(4Ј,4Љ-diphenyl-2,2Ј:6Ј,2Љ:6Љ,2ٞ-quaterpyridine-1,1Ј,1Љ,1ٞ-O4)2][ClO4]3 1 (phenyl groups omitted for clarity) and
(b) [Tb(4Ј,4Љ-diphenyl-2,2Ј:6Ј,2Љ:6Љ,2ٞ-quaterpyridine-1,1ٞ-O2)2][ClO4]3 2 (anions omitted for clarity). Selected bond lengths (Å) for 1: Gd(1)–O(1)
2.354(4), Gd(1)–O(2) 2.389(4), Gd(1)–O(3) 2.412(4), Gd(1)–O(4) 2.365(4); for 2: Tb(1)–O(1) 2.282(5), Tb(1)–O(2) 2.253(5), Tb(1)–O(1*) 2.301(5),
Tb(1)–O(2*) 2.290(5), Tb(1)–N(2) 2.544(6), Tb(1)–N(3) 2.566(5), Tb(1)–N(2*) 2.555(5), Tb(1)–N(3*) 2.489(6).
DOI: 10.1039/b203087j
J. Chem. Soc., Dalton Trans., 2002, 2415–2416
This journal is © The Royal Society of Chemistry 2002
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in colour, light yellow after 10 minutes and a fine white suspension was
an almost cubic structure similar to that of La(bipyridyl-N,NЈ-
bis-oxide)4
3ϩ 12 the ligand in 2 is more constricted in its binding
formed after 15 minutes. Stirring was continued at room temperature
for 48 hours. The solvent was removed under reduced pressure and
acetone (60 ml) was added to the remaining solid. After stirring at room
temperature for 30 minutes the solution was left at Ϫ10 ЊC for 2 hours
and the white precipitate formed was collected by filtration to yield L2
(1.02 g, 96%). δH (400 MHz; CDCl3) 9.23 [2H, d, J 1.5, H3Ј], 8.75 [2H, d,
J 1.5, H5Ј], 8.35 [2H, dd, J 1.95, 6.1, H3/6], 8.30 [2H, d, J 6.37, H3/6], 7.79
,
mode and gives a less symmetrical configuration, of approxi-
mately dodecahedral geometry. The Gd–O bond lengths [av.
2.380(4) Å] are similar to those observed for europium com-
plexes of pyridine-N-oxides13 but are slightly shorter than those
observed for the La(bipyridyl-N,NЈ-bis-oxide)4 species (2.506
Å). In comparison, the terbium species exhibits slightly shorter
Tb–O bonds [av. 2.282(5) Å] but the remaining Eu–N bonds are
much longer (av. 2.539 Å). This lowering of symmetry from
cubic to dodecahedral (C2v) on going from complexes 1 to 2 is
probably due to two factors and is expected to be due to the
steric preferences of the ligand and not the metal. First, due to
the presence of the central bipyridyl donors in L2, these donors
will have a smaller bite angle [N2–Tb–N3 63.90(19), NЈ–Tb–
N3Ј 64.3(2)Њ] than if these are the flexible, oxide donors [e.g.
O3–Gd–O3Ј 71.90(19), O2–Gd–OЈ 71.83(17)Њ]. This results in
the two central donors being pulled together as shown in Fig.
2a. Secondly, while the tetra-oxide is a more flexible donor, and
[2H, d, J 7.09 Hz, Ho], 7.48–7.39 [8H, m, H4
p], 7.26 [2H, m, H5];
ϩ
m ϩ
IR(KBr)/cmϪ1 1610 (w), 1587(s), 1546(s), 1485(s), 1438(s), 1381(s),
1277(s), 1255(w), 1223(s), 1157(w), 1074(w), 1040(s), 880.3(s), 856(s),
754(s), 723(w), 690(s); FAB-MS (NOBA matrix) m/z 495 (M ϩ H,
100%), 479 (M ϩ H Ϫ O, 30%), 463 (M ϩ H Ϫ 2O, 15%).
‡ General procedure for the synthesis of metal complexes. The metal
perchlorate salt (0.5 mol equiv.) was dissolved in the minimum amount
of ethanol and added to a suspension of ligand in hot ethanol (3 ml).
A precipitate was formed almost immediately which was collected
by filtration. Yield 70–88%. Recrystallisation of all complexes was by
vapour diffusion of diethyl ether into a CH3CN solution.
[Gd(PhquaterO4)2][ClO4]3 (1), yield 76%. (Found C, 50.74; H, 2.69;
N, 7.33. C64H44Cl3N8O20Gd requires C, 50.95; H, 2.94; N, 7.43%);
IR(KBr)/cmϪ1 1607(s), 1497(m), 1447(m), 1415(s), 1236(s), 1226(s),
1217(s), 1091(vs), 870(s), 837(s), 770(s), 624(m); FAB-MS (NOBA
matrix) m/z 1309 (M Ϫ 2ClO4, 97%), 1210 (M Ϫ 3ClO4, 100%).
[Tb(PhquaterO2)2][ClO4]3, (2), yield 85%. (Found C, 53.07; H, 2.98;
N, 9.13. C68H50Cl3N10O16Tb requires C, 53.43; H, 3.30; N, 9.16%);
IR(KBr)/cmϪ1 1604(s), 1544(m), 1494(m), 1458(w), 1447(w), 1396(s),
1233(m), 1083(vs), 858(w), 811(w), 767(s), 702(w), 623(s); FAB-MS
(NOBA matrix) m/z 1347 (M Ϫ ClO4, 100%), 1247 (M Ϫ 2ClO4, 65%).
§ Crystallographic data for 1: C64H44Cl3GdN8O20, monoclinic, space
group C2/c, a = 16.783(3), b = 19.408(4), c = 19.979(4) Å, β = 93.93(3)Њ,
U = 6492(2) Å3, Z = 4, µ(Mo-Kα) = 1.226 mmϪ1, 30729 reflections
measured, 7535 observed reflections [Rint = 0.1208]. R indices (observed
data), R1 = 0.0657, wR2 = 0.1253.
¯
For 2: C68H50Cl3N10O16Tb, triclinic, space group P1, a = 15.3287(5),
b = 15.4120(6), c = 17.7298(10) Å, α = 69.9040(15), β = 65.6000(16),
γ = 66.882(2)Њ, U = 3424.9(3) Å3, Z = 2, µ(Mo-Kα) = 1.224 mmϪ1, 23857
reflections measured, 11870 unique [Rint = 0.0946]. R indices (observed
data), R1 = 0.0726, wR2 = 0.1125. CCDC reference numbers 178771
and 180509. See http://www.rsc.org/suppdata/dt/b2/b203087j/ for
crystallographic data in CIF or other electronic format.
Fig. 2 (a) The distortion from cubic to dodecahedral geometry, (b) the
relative positions of the two tetradentate donors.
can arrange itself in numerous ways (thus allowing the cubic
geometry), the bis-oxide ligand has the central bipyridyl frag-
ment. This makes the ligand tend towards a more linear form of
co-ordination and favours the dodecahedral geometry (Fig.
2b).
Further studies will investigate the comparative stability and
fluorescence properties of these complexes and their further
functionalisation into polydentate ligands with enhanced
stabilities.
1 E. C. Constable, Prog. Inorg. Chem., 1994, 42, 67.
2 J.-P. Collin, P. Laine, J.-P. Launay, J.-P. Sauvage and A. Sour,
J. Chem. Soc., Chem. Commun., 1993, 434; E. C. Constable, E. R.
Schofield, S. Encinas, N. Armaroli, F. Barigelletti, L. Figggemeier
and J. G. Vos, Chem. Commun., 1999, 869.
3 H. Takalo, V. M. Mukkala, L. Merio, J. C. Rodriguez-Ubis,
R. Sedano, O. Juanes and E. Brunet, Helv. Chim. Acta, 1997, 80,
372; A. P. de Silva, H. Q. N. Gunaratne, T. E. Rice and S. Stewart,
Chem. Commun., 1997, 1891.
4 M. D. Ward and F. Barigelletti, Coord. Chem. Rev., 2001, 216–7, 127;
E. C. Constable, S. M. Elder, M. J. Hannon, A. Martin, P. R.
Raithby and D. A. Tocher, J. Chem. Soc., Dalton Trans., 1996, 2423;
P. K. K. Ho, K. K. Cheung and C. M. Che, Chem. Commun., 1996,
1197; E. C. Constable, M. A. M. Daniels, M. G. B. Drew,
D. A. Tocher, J. V. Walker and P. D. Wood, J. Chem. Soc.,
Dalton Trans., 1993, 1947; E. C. Constable and M. D. Ward,
J. Am. Chem. Soc., 1990, 112, 1256; R. Chotalia, E. C. Constable,
M. Neuburger, D. R. Smith and M. Zehnder, J. Chem. Soc.,
Dalton Trans., 1996, 4207.
The support of Cardiff University and the EPSRC is grate-
fully acknowledged. We would also like to thank the EPSRC
National Mass Spectrometry Service.
Notes and references
† 4Ј,4Љ-Diphenyl-2,2Ј:6Ј,2Љ:6Љ,2ٞ-quaterpyridine-1,1,1Љ,1ٞ-tetra-oxide,
(L1). Trifluoroacetic anhydride (8.4 g, 40 mmol) in acetonitrile (10 ml)
was added dropwise over 10 min at 0 ЊC to a stirring suspension of
urea–hydrogen peroxide (4.7 g, 50 mmol) in acetonitrile (25 ml). Stir-
ring was continued at 0 ЊC for 30 minutes after which time a suspension
of 4Ј,4Љ-diphenyl-2,2Ј:6Ј,2Љ:6Љ,2ٞ-quaterpyridine (1 g, 2.16 mmol) in
acetonitrile was added dropwise. The suspension was allowed to rise to
room temperature then refluxed at 50 ЊC for 48 hours. The reaction
mixture was evacuated at reduced pressure to yield a yellow oil which
was washed with distilled water (3 × 50 ml). The remaining oil was
refluxed in ethanol (50 ml) with stirring (1 h) and allowed to cool. The
white precipitate formed was collected by filtration to yield L1 (0.4 g,
35%). δH (400 MHz; CDCl3) 8.29 [2H, m, H6], 8.05 [2H, d, J 2.9, H3Ј],
7.9 [2H, d, J 2.7, H5Ј], 7.69 [2H, m, H3], 7.69 [4H, dd, J 1.4 Hz, 7.1, Ho],
5 E. C. Constable, R. Chotalia and D. A. Tocher, J. Chem. Soc., Chem.
Commun., 1992, 771.
6 A J. Amoroso, M. W. Burrows, A. A. Dickinson, C. Jones, D. J.
Willock and W. T. Wong, J. Chem. Soc., Dalton Trans., 2001, 225.
7 L. Prodi, M. Maestri, V. Balzani, J.-M. Lehn and C. Roth, Chem.
Phys. Lett., 1991, 180, 45; J.-M. Lehn and C. Roth, Helv. Chim.
Acta, 1991, 74, 572; C. de Mello Donegá, S. A. Junior and G. A. de
Sá, Chem. Commun., 1996, 1199.
8 E. C. Constable, P. Haverson, D. R. Smith and L. A. Whall,
Tetrahedron, 1994, 50, 7799.
9 S. Rozen and S. Dayan, Angew. Chem., Int. Ed., 1999, 38, 3472.
10 R. Ballini, C. Marcantoni and M. Petrini, Tetrahedron Lett., 1992,
33, 4835.
11 R. P. Thummel and Y. Jahng, J. Org. Chem., 1985, 50, 3635.
12 A. R. Al-Karaghouli, R. O. Day and J. S. Wood, Inorg. Chem., 1978,
17, 3702.
13 For example: C. O. Paul-Roth, J.-M. Lehn, J. Guilhem and
C. Pascard, Helv. Chim. Acta, 1995, 78, 1895; P. Gawryszewska,
L. Jerzykiewicz, M. Pietraszkiewicz, L. Legendziewicz and J. P.
Riehl, Inorg. Chem., 2000, 39, 5365.
7.39–7.34 [8H, m, H4
p], 7.26 [2H, m, H5]; IR(KBr)/cmϪ1 1635(m),
ϩ
m
1493(m), 1423(s), 1360(mϩ), 1261(vs), 1230(sh), 1096(s), 1021(s), 875(m),
801(s), 766(s), 697(m), 597(m); MS m/z 527 (M ϩ H, 75%), 511 (M ϩ H
Ϫ O, 22%), 496 (M ϩ 2H Ϫ 2O, 19%), 465 (M ϩ 3H Ϫ 4O, 11%).
4Ј,4Љ-Diphenyl-2,2Ј:6Ј,2Љ:6Љ,2ٞ-quaterpyridine-1,1ٞ-bis-oxide, (L2).
60% MCPBA (1.25 g, 4.33 mmol) was added to a stirring solution of
4Ј,4Љ-diphenyl-2,2Ј:6Ј,2Љ:6Љ,2ٞ-quaterpyridine (1 g, 2.16 mmol) in
dichloromethane (30 ml). The solution immediately turned light orange
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