4,5-Di(2-pyridyl)-1,2,3-triazolate: The elusive member of a family of bridging ligands that facilitate strong metal-metal interactions

Article abstract of DOI:10.1039/b801077c

The new click-adduct 4,5-di(2-pyridyl)-1,2,3-triazole acts as a doubly-chelating anionic bridging ligand that forms dinuclear ruthenium(ii) complexes which exhibit strong metal-metal interactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b801077c

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4,5-Di(2-pyridyl)-1,2,3-triazolate: the elusive member of a family of bridging
ligands that facilitate strong metal–metal interactions†
Chris Richardson,a Christopher M. Fitchett,b F. Richard Keeneb and Peter J. Steel*a
Received 21st January 2008, Accepted 10th March 2008
First published as an Advance Article on the web 27th March 2008
DOI: 10.1039/b801077c
The new click-adduct 4,5-di(2-pyridyl)-1,2,3-triazole acts as a
doubly-chelating anionic bridging ligand that forms dinuclear
ruthenium(II) complexes which exhibit strong metal–metal
interactions.
corresponding values for the imidazolates (5) and (6), which
also exhibited large Kc values.8 Missing from this series was
the key nitrogen-containing analogue (7), which, despite various
attempts at synthesis, proved to be elusive.9 Pleasingly, we have
now succeeded in preparing this compound and now describe
the synthesis and properties of its mononuclear and dinuclear
ruthenium(II) complexes.
The new ligand (8) was prepared (Scheme 1) by a “click”
reaction between di(2-pyridyl)acetylene and azidotrimethylsilane,
with concomitant loss of the TMS group.‡ This possesses an acidic
triazole moiety and two basic pyridine rings. Fig. 2 shows the
X-ray crystal structure§ of (8) which crystallises as a dihydrate
with the ligand in a zwitterionic form. Thus, the triazole ring is
deprotonated and negatively charged with the associated proton
Bridging nitrogen-containing heterocyclic ligands are well known
to facilitate interactions between metal atoms through the p-
system of the ligand.1–3 Within this context, ruthenium complexes
of doubly-chelating bridging ligands have been particularly well
studied.2 For example, 2,3-di(2-pyridyl)pyrazine (1) and 4,6-di(2-
2+
pyridyl)pyrimidine (2) both chelate to two Ru(bpy)2 (bpy =
2,2ꢀ-bipyridine) centres and facilitate relatively weak metal–metal
interactions as measured by the electrochemically-determined
comproportionation constants (Kc) shown in Fig. 1.2,4 Some years
ago5 we reported the corresponding values for the related ligands
(3) and (4), which exhibited extraordinary amplifications of the
metal–metal interactions. Our initial work suggested that the
magnitude of these interactions was dependent on the particular
stereoisomer (meso or rac)6 of the dinuclear complex, but this was
later shown to be due to specific associative interactions with the
counteranions.7 Various factors such as the metal–metal distance,
the degree of conjugation between the metal centres and the
electronic properties of the ligand and metals have been forwarded
to explain the magnitude of the communication between the metal
centres,1,2 but there still remains considerable uncertainty as to
what really mediates these effects.
Scheme 1 Syntheses of ligands and complexes.
Fig. 1 Bridging doubly-chelating ligands and approximate compropor-
4+
tionation constants for their {Ru(bpy)2}2 complexes.
To test whether the presence of the (O or S) heteroatom was
responsible for this amplification we subsequently reported the
aDepartment of Chemistry, College of Science, University of Canterbury,
Christchurch, 8140, New Zealand. E-mail: peter.steel@canterbury.ac.nz;
Fax: +64-3-3642110; Tel: +64-3-3642432
bSchool of Pharmacy & Molecular Sciences, James Cook University,
Townsville, Queensland, 4811, Australia
† CCDC reference numbers 673126–373128. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b801077c
Fig. 2 X-Ray structure of the dihydrate of ligand (8). Only one component
of the disordered H is shown. Hydrogen bonds are shown as dashed lines.
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being shared (crystallographically disordered) between the two
pyridine nitrogens, which are hydrogen bonded to one another.
This hydrogen bond serves to hold all three rings in the same
plane, with triazole/pyridine interplanar angles of only 4.2(1)
and 4.7(1)◦. Each of the three nitrogens of the triazole ring is
hydrogen bonded to a different water hydrogen, the fourth of
which participates in an OH · · · O hydrogen bond. This leads to a
complex three-dimensional hydrogen bonded network.
Ligand (8) reacts with three equivalents of [Ru(bpy)2Cl2] in the
presence of sodium hydroxide to give the dinuclear complex (9)
in 93% yield.¶ Reaction with one equivalent of [Ru(bpy)2Cl2]
gave the mononuclear complex (10). In order to assist with the
assignments of the NMR spectra, the model ligand (11) was
prepared by reaction of 2-pyridylacetylene with benzyl azide.‡
It readily formed the mononuclear complex (12)¶. The dinuclear
complex (9) exists as two (meso and rac) diastereoisomers, which
were formed in an approximately 1 : 1 ratio and separated by
column chromotography using a method pioneered in one of our
laboratories,10 employing SP Sephadex C-25 and aqueous 0.20 M
sodium 4-toluenesulfonate solution as eluent. Distinction between
these isomers was initially based on comparison of their 1H NMR
spectra with those of the corresponding complexes of ligand (5)8
and later confirmed by X-ray crystallography.
˚
Fig. 4 X-Ray structure of the cation (12). Selected bond lengths (A):
Ru1–N3 2.038(3), Ru1–N6 2.084(3), N1–N2 1.353(5), N2–N3 1.331(5).
The X-ray crystal structures provide useful information for
the assignment and rationalisation of the H NMR spectra of
the complexes. Table 1 lists the H NMR chemical shifts and
1
1
coordination induced shifts (CIS) for the pyridine rings of (8)
and (11) in the complexes (9), (10) and (12). In all complexes the
H6 signal of the coordinated pyridine ring experiences a large
upfield shift as this proton lies over the shielding face of one of
the bpy rings. In the mononuclear complex (10) the signals for
the non-coordinated ring show only small CIS values, but the
H3 proton of the coordinated pyridine ring experiences a large
downfield shift (+1.51 ppm). We attribute this to the presence
of a C–H · · · N interaction, as was previously observed in the
corresponding complex of ligand (3).
The electronic absorption spectra and redox potentials for
the four complexes in acetonitrile solution are listed in Table 2,
along with the corresponding values for [Ru(bpy)3]2+. The model
mononuclear complex (12) exhibits a similar oxidation potential
to [Ru(bpy)3]2+, but is somewhat more difficult to reduce. This
reflects the p-excessive nature of the triazole ring and results in
In order to unambiguously confirm their structures, single
crystal X-ray structure determinations were carried out on (9rac)
and (12).§ The structure of the cation (9rac) is shown in Fig. 3. The
potential C2 symmetry of this cation is destroyed in the solid state
by a curious twisting of one side of the cation. Thus, whereas Ru1
˚
lies in the same plane as the triazole ring [deviation 0.001(7) A]
˚
Ru2 lies 0.443(7) A out of the plane. The origin of this twisting is
not clear but may result from the need to relieve a steric interaction
between the H3 protons of the pyridine rings of the bridging
ligand. Indeed the N12 pyridine ring is twisted by only 4.1(1)◦
from the plane of the triazole ring, whilst the N6 ring is twisted at
an angle of 13.9(1)◦. The Ru–N bonds to the triazolate nitrogens
are shorter than those to any pyridine nitrogens, which suggests
that this anionic ring system is a better donor than pyridine.11
Table 1 1H NMR chemical shift and CIS valuesa for (8)–(12)
H6
8.70
7.77
−0.93
7.67
H5
7.46
7.28
−0.18
7.24
H4
7.98
8.04
+0.06
8.03
H3
8.37
8.48
+0.11
8.47
8
9rac
CIS
9meso
CIS
10
−1.03
−0.22
+0.05
+0.10
7.63
7.14
7.91
9.88
CIS
−1.07
−0.33
−0.08
+1.51
H6ꢀ
H5ꢀ
H4ꢀ
H3ꢀ
˚
10
CIS
8.78
+0.08
7.36
−0.11
7.90
−0.09
8.24
−0.13
Fig. 3 X-Ray structure of the cation (9rac). Selected bond lengths (A):
Ru1–N3 2.028(7), Ru1–N12 2.094(7), Ru2–N1 2.035(7), Ru2–N6 2.079(8),
N1–N2 1.347(9), N2–N3 1.330(9).
H6
H5
H4
H3
Fig. 4 shows the structure of (12), which exhibits a more regular
octahedral geometry about the ruthenium atom. Once again the
shortest Ru–N bond is to the triazole nitrogen and the planes of the
two coordinated rings of the triazole ligand are almost coplanar
[torsion angle 3.8(1)◦].
11
8.61
7.66
−0.95
7.32
7.34
+0.02
7.87
8.03
+0.16
8.12
8.13
+0.01
12
CIS
a CIS = dcomplex − dligand. In CD3CN. Values in italics.
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Table 2 Absorption maxima (nm), redox potentials (mV versus SCE) and Kc values for (9), (10) and (12) compared to [Ru(bpy)3]2+
a
b
Compound
UV-vis
Eox2
Eox1
946
946
866
Ered1
Ered2
Eox1 − Ered1
DEox
Kc
9rac
453 [22 400]
450 [21 500]
481 [10 600], 441 [11 300]
442 [10 800], 415 [10 500]
452
1306
1286
—
—
—
−1499
−1510
−1538
−1450
−1330
−1658
−1670
−1790
−1698
−1510
2445
2456
2404
2676
2590
360
340
—
—
—
1 200 000
560 000
—
9meso
10
12
1226
1260
—
—
[Ru(bpy)3]2+
a DEox = Eox2 − Eox1
.
b Comproportionation constant, Kc = exp{DEoxF/RT}, where F/RT takes the value 38.92 V−1 at 298 K.
a higher HOMO–LUMO gap, as reflected in a blue shift in the
MLCT absorption. The mononuclear complex (10), being singly-
charged, is considerably more easily oxidised than the doubly-
charged [Ru(bpy)3]2+ and is significantly more difficult to reduce.
The difference in the first redox potentials and the red-shifted
absorption indicate a smaller HOMO–LUMO gap relative to
[Ru(bpy)3]2+. The two stereoisomers of the binuclear complex
(9) each show two well-separated oxidation potentials (Fig. 5),
indicating strong metal–metal interactions, as seen in the large
Kc values. Indeed, these represent the highest Kc values of any
complexes of ligands of this type (Fig. 1). As was found to be
the case with the analogous complexes of the imidazolate (5), the
rac isomer shows stronger metal–metal interactions than the meso
isomer, when the counteranion is PF6−. The first reductions of
these isomers are at a similar potential to that of the mononuclear
analogue (10) and are probably associated with the ancillary bpy
ligands.
Notes and references
‡ Selected data for ligands: 8: Yield 62%. Mp 98–103 ◦C. 1H NMR
(500 MHz, CD3CN) d: 7.46 (dd, 2 H, J = 7.5, 5.0 Hz, H5), 7.98 (t,
2 H, J = 7.5 Hz, H4), 8.37 (d, 2 H, J = 7.3 Hz, H3), 8.70 (d, 2 H,
J = 4.6 Hz, H6). 13C NMR (75 MHz, CD3CN) d: 124.75, 124.79, 139.45,
149.12. ESMS: Calc. for C12H10N5 224.0936, found 224.0945. Calc. for
C12H9N5·4H2O: C, 48.81; H, 5.80; N, 23.72. Found: C, 49.10; H, 5.63; N,
23.83%. 11: Yield 37%. Mp 113–114 ◦C. 1H NMR (500 MHz, CDCl3) d:
5.57 (s, 2 H, CH2), 7.21 (dd, 1 H, J = 6.8, 5.3 Hz, H5ꢀ), 7.40–7.34 (m, 5 H,
phenyl), 7.77 (t, 1 H, J = 7.7 Hz, H4ꢀ), 8.07 (s, 1 H, H5), 8.18 (d, 1 H,
J = 8.1, H3ꢀ), 8.52 (d, 1 H, J = 4.6 Hz, H6ꢀ). 13C NMR (75 MHz, CDCl3)
d: 54.31, 120.18, 121.81, 128.25, 128.78, 129.11, 134.27, 136.90, 148.58,
149.20, 150.12. Calc. for C14H12N4: C, 71.15; H, 5.12; N, 23.72. Found: C,
71.40; H, 5.09; N, 23.61%.
¯
§ Summary of X-ray details: 8: C12H9N5·2H2O, M 259.27, triclinic, P1,
˚
Z = 2, a = 7.4884(3), b = 9.1666(3), c = 9.8501(3) A, a = 66.450(2),
◦
3
˚
b = 84.740(2), c = 76.235(2) , V = 602.02(4) A , T = 148 K, Rint
=
0.031, wR2 (all 3468 data) = 0.122, R1 [3163 data with I>2r(I)]
0.038; 9rac: C52H40N13Ru2·3(PF6)·0.5(C4H10O)·1.5(CH3CN), M 3159.06,
¯
triclinic, P1, Z = 2, a = 13.4722(5), b = 13.6208(5), c = 19.1473(7)
◦
3
˚
˚
A, a = 73.191(2), b = 87.006(2), c = 70.094(2) , V = 3158.3(2) A ,
T = 101 K, Rint = 0.038, wR2 (all 11120 data) = 0.247, R1 [7920
data with I>2r(I)] 0.075; 12: C34H28N8Ru·2(PF6)·C3H6O·0.5H2O, M
1005.73, monoclinic, P21/c, Z = 4, a = 11.6741(3), b = 14.0245(4),
◦
3
˚
˚
c = 26.2735(8) A, b = 98.946(2) ,V = 4249.3(2) A , T = 123 K,
Rint = 0.041, wR2 (all 7693 data) = 0.161, R1 [6515 data with I>2r(I)]
0.048.
¶ Selected data for complexes: 9: Yield 93%. ESMS: Calc. for C52H40N13Ru2
350.0539, found 350.0542. Calc. for C52H40N13F18P3Ru2·H2O: C, 41.58; H,
2.82; N, 12.12. Found: C, 41.42; H, 2.90; N, 11.89%. 9rac: 1H NMR
(500 MHz, CD3CN) d: 8.50 (d, 1 H, bpyA H3), 8.48 (d, 1 H, H3), 8.45
(d, 1 H, bpyB H3), 8.42 (d, 1 H, bpyC H3), 8.23 (d, 1 H, bpyD H3), 8.15
(t, 1 H, bpyC H4), 8.04 (m, 3 H, H4 and bpyA H6 and bpyA H4), 7.97
(m, 2 H, bpyA H6 and bpyD H4), 7.85 (t, 1 H, bpyD H4), 7.77 (d, 1 H,
H6), 7.53 (t, 1 H, bpyA H5), 7.44 (m, 2 H, bpyA H5 and bpyD H6), 7.28
(t, 1 H, H5), 7.18 (t, 1 H, bpyD H5), 7.05 (t, 1 H, bpy B H5), 6.85 (d, 1 H,
H6). 9meso: 1H NMR (500 MHz, CD3CN) d: 8.54 (m, 2 H, bpyA H3 and
bpyB H3), 8.47 (d, 1 H, H3), 8.27 (m, 2 H, bpyC H3 and bpyD H3), 8.10
(d, 1 H, bpyA H6), 8.06 (t, 1 H, bpyA H4), 8.03 (m, 2 H, bpyB H4 and
H4), 7.88 (m, 2 H, bpyC H4 and bpyD H4), 7.84 (d, 1 H, bpyB H6), 7.67
(d, 1 H, H6), 7.52 (m, 2 H, bpyC H6 and bpyD H6), 7.48 (t, 1 H, bpyA
H5), 7.37 (t, 1 H, bpyB H5), 7.24 (t, 1 H, H5), 7.10 (m, 2 H, bpyC H5 and
bpyD H5). 10: Yield 65%. 1H NMR (500 MHz, CD3CN) d: 9.88 (d, 1 H,
H3), 8.78 (d, 1 H, H6ꢀ), 8.54 (d, 1 H, bpyA H3), 8.52 (d, 1 H, bpyB H3),
8.49 (d, 1 H, bpyC H3), 8.47 (d, 1 H, bpyD H3), 8.24 (d, 1 H, H3ꢀ), 8.07 (t,
1 H, bpyA H4), 8.02 (m, 2 H, bpyB H4 and bpyC H4), 8.01 (t, 1 H, bpyD
H4), 7.98 (d, 1 H, bpyA H6), 7.97 (d, 1 H, bpyC H6), 7.91 (t, 1 H, H4),
7.90 (t, 1 H, H4ꢀ), 7.87 (d, 1 H, bpyD H6), 7.78 (d, 1 H, bpyB H6), 7.63 (d,
1 H, H6), 7.44 (t, 2 H, bpyA H5), 7.43 (t, 1 H, bpyB H5), 7.40 (t, 1 H, bpyC
H5), 7.36 (t, 1 H, bpyD H5 and H5ꢀ), 7.14 (t, 1 H, H5). ESMS Calc. for
C32H24N9Ru 636.1198, found 636.1215. Calc. for C32H24N9F6PRu·2H2O:
C, 47.06; H, 3.46; N, 15.44. Found: C, 47.23; H, 3.28; N, 14.81. 12: Yield
70%. 1H NMR (500 MHz, CD3CN) d: 8.69 (s, 1 H, H5), 8.57 (d, 2 H, bpyA
and bpyB H3), 8.52 (d, 1 H, bpyC H3), 8.49 (d, 1H, bpyD H3), 8.13 (m,
4 H, bpyA, bpyB and bpyC H4, H3), 8.07 (t, 1 H, bpyD H4), 8.03 (t, 1 H,
H4), 7.93 (m, 2 H, bpyA and bpy B H6), 7.87 (d, 1 H, bpyC H6), 7.81 (d,
1 H, bpyD H6), 7.66 (d, 1 H, H6), 7.51 (t, 1 H, bpyA H5), 7.48 (t, 1 H,
bpyB H6), 7.47 (t, 1 H, bpyC H6), 7.44 (m, 3 H, m- and p-phenyl), 7.40
(t, 1 H, bpyD H6), 7.34 (t, 1 H, H5), 7.22 (d, 2 H, o-phenyl), 5.59 (s, 2 H,
CH2). ESMS Calc. for C34H28N8Ru 325.0740, found 325.0759. Calc. for
Fig. 5 Cyclic voltammogram of (9meso).
In conclusion, we have synthesised the new bridging ligand
(8), which exists in the solid state in a zwitterionic form. We
believe that this ligand will find use as a versitile synthon for the
formation of metal complexes of varying nuclearity. Furthermore,
it represents a new addition to the family of ligands that form
dinuclear ruthenium(II) complexes which exhibit strong metal–
metal interactions, and is the first such example that is both anionic
and has a heteroatom between the two chelating nitrogens.
We thank the Australian Research Council, the Royal Society of
New Zealand Marsden Fund and the University of Canterbury for
financial support of this work. PJS also thanks the RSNZ for the
award of a James Cook Research Fellowship. CMF thanks James
Cook University for the award of a post-doctoral fellowship.
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