On the electronic structure of nitro-substituted bipyridines and their platinum complexes

Article abstract of DOI:10.1039/c1dt11456e

We report the preparation and electrochemical studies of a systematic series of mono- and di-nitro-substituted 2,2′-bipyridine (bipy) compounds [x-NO₂-bipy (x = 3,4) and x,x′-(NO₂) ₂-bipy (x,x′ = 3, 4, 5)] and their complexes with platinum(ii), [Pt(x-NO₂-bipy)Cl₂] and [Pt(x,x′-(NO ₂)₂-bipy)Cl₂]. The effect of the number and substitution pattern of the nitro groups on the low-lying acceptor molecular orbitals (involved in charge transfer transitions) is probed by in situ UV/Vis/NIR and EPR spectroelectrochemical methods, supported by DFT calculations. The LUMOs of x-NO₂-bipy (x = 3-5) are largely localised on the NO₂-pyridyl moiety; this is also true of their {PtCl ₂} complexes but with a small but significant shift of electron density from the nitro groups. The LUMOs of x,x′-(NO₂) ₂-bipy with x = 3 and 5 are delocalised over both NO ₂-pyridyl rings, but for 4,4′-(NO₂)₂-bipy is localised on a single NO₂-pyridyl ring. In all cases the LUMO of the [Pt(x,x′-(NO₂)₂-bipy)Cl₂] complexes is delocalised over both nitro-pyridyl rings. For all complexes, the 4(4′) derivatives allows greatest overlap with metal valence orbitals in the LUMO. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt11456e

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PAPER
On the electronic structure of nitro-substituted bipyridines and their platinum
complexes†
Paul R. Murray,*a Stephen Crawford,a Alice Dawson,a Alexander Delf,a Calum Findlay,a Lorna Jack,a
Eric J. L. McInnes,b Salma Al-Musharafi,a Gary S. Nichol,a Iain Oswalda and Lesley J. Yellowlees*a
Received 2nd August 2011, Accepted 29th September 2011
DOI: 10.1039/c1dt11456e
We report the preparation and electrochemical studies of a systematic series of mono- and
di-nitro-substituted 2,2¢-bipyridine (bipy) compounds [x-NO2-bipy (x = 3,4) and x,x¢-(NO2)2-bipy (x,x¢ =
3, 4, 5)] and their complexes with platinum(II), [Pt(x-NO2-bipy)Cl2] and [Pt(x,x¢-(NO2)2-bipy)Cl2]. The
effect of the number and substitution pattern of the nitro groups on the low-lying acceptor molecular
orbitals (involved in charge transfer transitions) is probed by in situ UV/Vis/NIR and EPR
spectroelectrochemical methods, supported by DFT calculations. The LUMOs of x-NO2-bipy (x = 3–5)
are largely localised on the NO2-pyridyl moiety; this is also true of their {PtCl2} complexes but with a
small but significant shift of electron density from the nitro groups. The LUMOs of x,x¢-(NO2)2-bipy
with x = 3 and 5 are delocalised over both NO2-pyridyl rings, but for 4,4¢-(NO2)2-bipy is localised on a
single NO2-pyridyl ring. In all cases the LUMO of the [Pt(x,x¢-(NO2)2-bipy)Cl2] complexes is
delocalised over both nitro-pyridyl rings. For all complexes, the 4(4¢) derivatives allows greatest overlap
with metal valence orbitals in the LUMO.
bipy)Cl2] has a low-lying near degenerate pair of LUMOs delo-
calised over both rings.14 Since such effects could have significant
Introduction
Reported applications for charge transfer complexes of 2,2¢-
bipyridine (bipy) include chemodosimetry,1 hydrogen production
by artificial photosynthesis,2,3 and dye sensitised solar cells.4–9
We have been probing the electronic structure, and the nature
of the important charge transfer excited states, in such species
by electrochemical techniques coupled with UV/Vis and EPR
spectroscopies, focusing in particular on the family of square
planar platinum(II) complexes.10–12 Recently, we have investigated
the effects of strongly electron withdrawing groups on the
bipyridyl, for example, in 4-NO2-bipy.13 We showed that the
LUMOs of 4-NO2-bipy and its [Pt(4-NO2-bipy)Cl2] complex, are
largely localised on the nitro-bearing half of the bipyridyl group.
Moreover, for the complex there is significantly less admixture of
Pt valence orbitals to the LUMO compared to the [Pt(bipy)Cl2]
parent complex. Previously, we have shown that [Pt(4,4¢-(NO2)2-
effects on relaxation mechanisms of the charge transfer excited
states, we now extend this study to other nitro-bearing bipyridyls.
In particular, we prepare a systematic series of 3–5 mono- and
3,3¢–5,5¢ di-substituted nitro-bipyridyls and their Pt(II) complexes,
and probe their electronic character using combined spectroscopic
and electrochemical techniques. Although there are a number of
reports of transition metal complexes of 4-NO2-bipy and 4,4¢-
NO2-bipy ligands,15–27 as these isomers are easiest to prepare, there
are few reports with 3, 3¢, 5 and 5¢ nitro-bipyridyls.28–30
Results and discussion
Synthesis
All ligands were synthesised either according to literature methods
or modifications thereof. Platinum(II) complexes [Pt(L)Cl2] (L =
nitro-derivatised bipy) were prepared simply and in good yield by
reflux with K2[PtCl4] in water.
aEaStCHEM, School of Chemistry, University of Edinburgh, King’s
Buildings, West Mains Road, Edinburgh, UK. E-mail: p.r.murray@
ed.ac.uk, l.j.yellowlees@ed.ac.uk; Fax: +44-(0)131-650-6453; Tel: +44-
(0)131-650-4759
Crystal structures
bThe EPSRC UK National Electron Paramagnetic Resonance Service at
The University of Manchester, School of Chemistry, Manchester, M13 9PL,
UK. E-mail: eric.mcinnes@manchester.ac.uk; Fax: +44(0)161-2755-6106;
Tel: +44-(0)161-275-4669
Crystal structures were obtained for the 5,5¢-(NO2)2-bipy and
3-NO2-bipy free ligands, and for the complex [Pt(3,3¢-(NO2)2-
bipy)Cl2] (see Experimental). The structures of 5,5¢-(NO2)2-bipy
and 3-NO2-bipy (Figures S1 and S2, ESI†) hold few surprises, with
the pyridyl rings lying trans with respect to each other (for 5,5¢-
(NO2)2-bipy this is a consequence of crystallographic inversion
symmetry), and with similar metric parameters to those reported
† Electronic supplementary information (ESI) available: Structures of 5,5¢-
(NO2)2-bpy and 3-NO2-bpy; solvent dependence of redox potentials for
5,5¢-(NO2)2-bpy and its {PtCl2} complex; UV/Vis/NIR data. CCDC
reference numbers 660844 [5,5¢-(NO2)2-bpy], 838414 and 838415. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt11456e
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for 3,3¢- and 4,4¢-nitro-substituted derivatives.31,32 5,5¢-(NO2)2-bipy
is planar, while in 3-NO2-bipy the two pyridyl rings are twisted
with a torsion angle of 28.8◦, much smaller than in 3,3¢-(NO2)2-
bipy (48.9◦),32 reflecting the presence of a single NO2 group. Dark
red crystals of [Pt(3,3¢-(NO2)2-bipy)Cl2] were grown by diffusion
of diethyl ether into a N,N¢-dimethylformamide (DMF) solution.
The steric demands of the 3,3¢-substituents force a significant
twisting of the two pyridyl rings (Fig. 1). Although the torsion
angle of 17.28(4)◦ is smaller than reported for other [Pt(3,3¢-
X2-bipy)Cl2] derivatives (25.5, 26.5 and 30.0◦ for X = CO2Me,33
CO2H,34 and CH2OH,34 respectively), the two nitro groups are
forced to lie coplanar with shortest inter-nitro group N . . . O
1.1–1.4 V less negative than for bipy itself. These redox potentials
are very similar to those observed for x-NO2-py, which implies
that the LUMO of x-NO2-py is not significantly perturbed by
substitution with a second py group. Surprisingly, addition of a
second nitro group in x,x¢-(NO2)2-bipy induces a further shift to
more postive potentials of only 0.1–0.2 V.
[Pt(x-NO2-bipy)Cl2] and [Pt(x,x¢-(NO2)2-bipy)Cl2]: In all cases
coordination to the {PtCl2} fragment shifts E 1 to much less neg-
2
ative values. For all complexes except [Pt(3,3¢-(NO2)2-bipy)Cl2],
the shift is 0.4–0.5 V compared to the free ligand, with the values
for the di-substituted complexes being 0.1–0.2 V more positive
than the mono-substituted equivalents (mirroring the free ligand
behaviour). The outlier is [Pt(3,3¢-(NO2)2-bipy)Cl2] for which the
first reduction potential is remarkably positive at +0.01 V. This is
despite the fact that the free ligand is the hardest to reduce among
the isomers. The complex reduces at a potential that is 0.8 V more
positive than the free ligand, and 0.4 V more positive than the
mono-substituted complex [Pt(3-NO2-bipy)Cl2]. We believe this
to be due to interaction between the two nitro groups forced by
the steric constraints of the 3,3¢-substitution (Fig. 1).
˚
distances of 2.65 A.
Bipy, both in free and coordinated form, tends to un-
dergo two sequential one-electron redox processes. For example,
[Pt(bipy)Cl2] is reduced at -1.06 and -1.79 (Table 1), the separation
of 0.7 V being typical of a spin-pairing process of the two electrons
in the p* LUMO. We previously reported that [Pt(4,4¢-(NO2)2-
bipy)Cl2] undergoes four consecutive one-electron reductions, with
the first two being separated by only 0.18 V in DMF solution.14
We proposed that this was due to a near-degenerate pair of p*
orbitals being sequentially singly occupied.
The mono and di-substituted species here also have multiple
redox activity. The second reductions of x-NO2-bipy are 0.8–
1.0 V more negative potential than the first, and hence have well
separated SLUMOs (second LUMOs). For x,x¢-(NO2)2-bipy the
second reductions are only 0.1–0.3 V more negative. For [Pt(x-
NO2-bipy)Cl2] the gap is 0.4–0.6 V. For [Pt(x,x¢-(NO2)2-bipy)Cl2]
there is a much bigger difference between the two processes for
x,x¢ = 3 (0.4 V) than for x,x¢ = 4 or 5 (0.19 and 0.14 V, respectively),
because of the anomalously low first reduction potential.
The electrochemical response of 5,5¢-(NO2)2-bipy (Fig. 2) and
[Pt(5,5¢-(NO2)2-bipy)Cl2] are markedly solvent dependent (Tables
S1 and S2, ESI). Systematic studies showed a good correlation
(Fig. 3) with the solvent Guttmann acceptor number35 (a.n.)
which is a measure of the solvent Lewis acidity. For example,
for 5,5¢-(NO2)2-bipy the two one-electron reductions are observed
sequentially in low a.n. solvents (e.g. THF, ethyl acetate; Fig. 2a)
whereas in solvents with an a.n. of 19 or higher (e.g. DMSO,
DCM) a single two-electron process is observed (Fig. 2b). Fig. 3
shows that the second reduction potential is effected considerably
more than the first; hence it is the interaction of the solvent with
Fig. 1 Molecular structure of [Pt(3,3¢-(NO2)2-bipy)Cl2] viewed perpen-
dicular to the {PtCl2} plane (left) and bisecting the Cl–Pt–Cl angle (right)
highlighting the steric clash of the two nitro groups. Scheme: Pt(grey), Cl
(green), N (blue) and O (red); H omitted for clarity.
Electrochemistry
Cyclic voltammetry (CV) of the ligands in 0.1 M [nBu4N][BF4]
DMF solution reveals one or more one-electron reduction pro-
cesses in each case [Table 1, including data for x-NO2-pyridines
(py) for comparison].
x-NO2-bipy and x,x¢-(NO2)2-bipy: The mono-substituted x-NO2-
bipy free-ligands undergo reduction at potentials which are a huge
Table 1 Reduction potentialsa from cyclic voltammetry in 0.1
M
[nBu4N][BF4] DMF solution
Compound
E1/V
E2/V
E1 - E2/V
3-NO2-py
-0.94
-0.74
-2.05b
-0.98
-0.72
-0.84
-0.79
-0.64
-0.61
-1.06
-0.44
-0.34
-0.40
0.01
—
—
—
—
—
—
—
—
—
0.31
0.15
0.09
0.73
0.44
0.62
0.54
0.39
0.19
0.14
4-NO2-py13
bipy
3-NO2-bipy
-1.88b
-1.74b
-1.63b
-1.10
-0.79
-0.70
-1.79
-0.88
-0.96
-0.94
-0.40
-0.45
-0.32
4-NO2-bipy13
5-NO2-bipy
3,3¢-(NO2)2-bipy
4,4¢-(NO2)2-bipy
5,5¢-(NO2)2-bipy
[Pt(bipy)Cl2]12
[Pt(3-NO2-bipy)Cl2]
[Pt(4-NO2-bipy)Cl2]13
[Pt(5-NO2-bipy)Cl2]
[Pt(3,3¢-(NO2)2-bipy)Cl2]
[Pt(4,4¢-(NO2)2-bipy)Cl2]14
[Pt(5,5¢-(NO2)2-bipy)Cl2]
-0.26
-0.18
a Potentials vs. Ag/AgCl; b irreversible process, cathodic peak quoted.
Fig. 2 Cyclic voltammetry of 5,5¢-(NO2)2-bipy vs. Ag/AgCl in 0.1 M
[nBu4N][BF4] solution in (a) THF, (b) DMF and (c) DMSO at 293 K.
202 | Dalton Trans., 2012, 41, 201–207
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Table 2 Isotropic g-values and hyperfine coupling constants (a/10-4 cm-1)
and line-widths (DHpp/G) from fluid solution EPR spectra
Compound
[3-NO2-py]-
g
DHpp a(195Pt) a(14N)
a(1H)
2.0048 0.45
—
1 ¥ 8.60 1 ¥ 4.29
1 ¥ 2.28 1 ¥ 3.01
1 ¥ 1.22
1 ¥ 1.14
[4-NO2-py]-
2.0055 0.30
2.0053 0.60
—
—
1 ¥ 7.89 2 ¥ 2.85
1 ¥ 2.27 2 ¥ 0.42
1 ¥ 7.54 1 ¥ 4.59
1 ¥ 2.20 1 ¥ 3.00
1 ¥ 1.21
1 ¥ 6.74 2 ¥ 2.43
1 ¥ 2.81 1 ¥ 0.38
1 ¥ 6.64 1 ¥ 3.74
1 ¥ 2.20
[3-NO2-bipy]-
[4-NO2-bipy]-
[5-NO2-bipy]-
2.0055 0.50
2.0031 1.10
—
—
Fig.
3
E1 (+) and E2 (¥) vs. Guttman acceptor number for
5,5¢-(NO2)2-bipy (solid line) and [Pt(5,5¢-(NO2)2-bipy)Cl2] (dashed).
1 ¥ 2.29
1 ¥ 1.03
the dianion that is dominating the effect. The effect is lesser, but
still apparent, for [Pt(5,5¢-(NO2)2-bipy)Cl2] (Fig. 3) reflecting the
greater delocalisation of the charge on complexation to {PtCl2}.
[3,3¢-(NO2)2-bipy]-
[4,4¢-(NO2)2-bipy]-
2.0037 0.50
2.0055 0.55
—
—
2 ¥ 3.50 2 ¥ 1.87
2 ¥ 1.29
1 ¥ 6.34 1 ¥ 3.04
1 ¥ 2.53 1 ¥ 2.80
1 ¥ 0.19
UV/Vis/NIR Spectroelectrochemistry
[4,4¢-(15NO2)2-bipy]-
[5,5¢-(NO2)2-bipy]-
2.0053 0.50
2.0041 0.30
—
—
1 ¥ 8.89 1 ¥ 3.04
1 ¥ 2.53 1 ¥ 2.80
2 ¥ 1.54 2 ¥ 1.54
2 ¥ 0.62
In situ UV/Vis/NIR spectroelectrochemical studies were un-
dertaken for those processes that were reversible on the bulk
electrolysis timescale. Full spectroscopic data are given in Table
S3,† and we summarise the important points here.
2 ¥ 0.35
[Pt(4-NO2-bipy)Cl2]-
[Pt(5-NO2-bipy)Cl2]-
2.0036 1.40 33.2
2.0095 1.00 18.8
1 ¥ 4.21 1 ¥ 2.90
1 ¥ 3.09 1 ¥ 1.40
—
4 ¥ 1.1
—
None of the free ligands in their uncharged form have any peaks
below 30 kcm-1. On one-electron reduction of x-NO2-bpy to [x-
NO2-bipy]-, two intense peaks appear at 25–30 and 15–20 kcm-1.
This behaviour is similar to that of [x-NO2-py]-. This implies
that the electronic structure is similar, i.e. the SOMO is largely
localised on the NO2-bearing ring, also consistent with the redox
data (see above). This distinctive pattern is also seen for the di-
substituted [4,4¢-(NO2)2-bipy]-, i.e. the conjugation between the
rings is largely broken. However, the x,x¢ = 3 and 5 isomers also
have peaks at 8–10 kcm-1 which are characteristic of reduced
bipyridyls i.e. with delocalisation over both rings. These distinct
differences in the electronic structures of the [x,x¢-(NO2)2-bipy]-
family are confirmed by EPR (see below).
On complexation of the uncharged ligands to {PtCl2} a metal-
to-ligand charge transfer transition is observed at 20–25 kcm-1.
The trend in the MLCT energies of [Pt(x,x¢-(NO2)2-bipy)Cl2] (21.4,
23.0, 22.4 kcm-1 for x = 3, 4 and 5, respectively) is consistent
with that of the reduction potentials. All the mono-reduced [Pt(x-
NO2-bipy)Cl2]- and [Pt(x,x¢-(NO2)2-bipy)Cl2]- complexes have
the low-energy bands at 7–11 kcm-1, characteristic of reduced
bipyridyls. This includes [Pt(4,4¢-(NO2)2-bipy)Cl2]- which suggests
that there is a pronounced change in the electronic structure of
[4,4¢-(NO2)2-bipy]- on coordination, from localised on a single
ring to delocalised over both, presumably related to the forced
conformation.
—
2 ¥ 1.1
—
[Pt(3,3¢-(NO2)2-bipy)Cl2]- 2.0037 1.40 20.1
[Pt(4,4¢-(NO2)2-bipy)Cl2]- 2.0018 13.0 37.4
[Pt(5,5¢-(NO2)2-bipy)Cl2]- 1.9983 1.00 22.3
—
—
the same electrogeneration potentials as in the UV/Vis/NIR
experiments. All compounds were EPR silent prior to application
of a negative potential, with the exception of [Pt(3,3¢-(NO2)2-
bipy)Cl2] which displayed a weak signal. This is due to its
very positive reduction potential (see above). The signal grew
in intensity on application of a negative potential, and could
be removed entirely at +0.5 V. All resolved hyperfine coupling
constants from isotropic (fluid solution) spectra are summarised
in Table 2 (including data for x-NO2-py for comparison). In the
discussion below we focus on the 14N and 195Pt coupling.
[x-NO2-bipy]-: for each isomer the dominant hyperfine coupling
is to the nitro group 14N nucleus, of magnitude ca. 7 ¥ 10-4 cm-1.
The coupling constants are similar to values observed for [x-NO2-
py]-, supporting their similar electronic structure.
[x,x¢-(NO2)2-bipy]- (Fig. 4): for x,x¢ = 3 and 5, the dominant NO2-
¯
coupling is now to two equivalent 14N nuclei, with values (3.5 and
1.5 ¥ 10-4 cm-1, respectively) reduced by at least a factor of two from
those of [x-NO2-bipy]-. This is consistent with the SOMO now
being delocalised over both pyridyl rings. In contrast, for x,x¢ =
4 two inequivalent single 14N nulei are observed with couplings
6.3 and 2.5 ¥ 10-4 cm-1. Spectra of the isotopically labelled [4,4¢-
(15NO2)2-bipy]- confirm the larger splitting to be to the nitro group
and the smaller to a pyridyl-N (Fig. 4c; a(15 N) = 8.9, a(14 N) = 2.5 ¥
10-4 cm-1). These coupling constants are similar to those for [4-
NO2-bipy]-.13 Hence, the SOMO of [4,4¢-(15NO2)2-bipy]- is largely
localised on only one of the nitro-bearing rings, not delocalised
over the whole molecule, consistent with the UV/Vis data (see
above).
A number of the transitions in the spectra of 5-NO2-bipy and
5,5¢-(NO2)2-bipy and their complexes showed vibronic structure
with spacing of ca. 1400 cm-1. This can be attributed to the NO2
symmetric stretch; IR spectra of 5,5¢-(NO2)2-bipy (KBr disc) show
a strong band at ca. 1600 cm-1.
EPR Spectroelectrochemistry
The reduction products of the free-ligands and complexes were
also studied by in situ EPR spectroelectrochemical methods, using
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The difference in behaviour of [x,x¢-(NO2)2-bipy]- for x,x¢ = 3
or 5 (SOMO delocalised over both rings) and x,x¢ = 4 (SOMO
largely localised on one ring) may be due to a enhanced preference
for a coplanar conformation due to increased conjugation for the
former (NO2 groups ortho or para to the bridgehead C2 position)
vs. the latter (NO2 groups meta to C2).
[Pt(x-NO2-bipy)Cl2]-: For x = 4, coupling is observed to two
single 14N (4.2 and 3.1 ¥ 10-4 cm-1), as for the free-ligand radical-
anion. We previously showed by 15N labelling that the smaller of
these couplings is to the nitro-N, in contrast to the free ligand.13
Hence there is a significant redistribution of the spin density from
the nitro group to the pyridyl ring on coordination to a metal
ion. In addition, a 33.2 ¥ 10-4 cm-1 coupling to 195Pt (33% natural
abundance, I = 1/2) is resolved. For x = 5, the only hyperfine
structure resolved is to 195Pt: the coupling constant of 19 ¥ 10-4
cm-1 is much smaller than for x = 4, showing a much lower spin
density at Pt. The x = 3 radical anion proved unstable on the bulk
electrosynthesis timescale, even at -40 ◦C.
[Pt(x,x¢-(NO2)2-bipy)Cl2]-: 195Pt hyperfine is observed for all
three complexes, with values of 20.1, 37.414 and 22.3 ¥ 10-4 cm-1
for x,x¢ = 3, 4 and 5, respectively. Coupling to ligand nuclei is only
observed for x,x¢ = 3 (1.1 ¥ 10-4 cm-1 to four 14N; this is presumably
a coincidentally similar coupling to the two nitro and the two ring
nitrogens). We previously noted that the x,x¢ = 4 isomer has a near-
degenerate pair of SOMOs and that there was a huge narrowing
of the EPR linewidth from mono- to di-anion;14 we speculated
this to be due to an electron hopping mechanism (between the
near degenerate orbitals) in the monoanion which is quenched on
occupation of the second orbital. The well resolved spectrum for
x,x¢ = 3 is then consistent with the much larger separation between
the first and second reduction potentials than for x,x¢ = 4. The
spectrum of the x,x¢ = 4 dianion14 is consistent with delocalisation
over both rings.
Frozen solution spectra: On freezing the solutions, well resolved
spectra are obtained for all members of the [Pt(x,x¢-(NO2)2-
bipy)Cl2]- family (Table 3; Figure S4).36 We have analysed the
195Pt hyperfine couplings using Maki’s equations37 with Rieger’s
formalism,38 as described elsewhere.11,12 The calculated admixture
of Pt valence orbitals (5d and 6p) to the SOMOs are very small
(although they dominate the EPR spectra), ranging from 0.5–4%,
with the values for x = 4 and 3 the highest and lowest, respectively.
These are all significantly smaller than the values derived for
the parent, unsubstituted complex [Pt(bipy)Cl2]- (ca. 11%),11,12
obviously due to the electron withdrawing effect of the nitro
groups. For the mono-subtituted [Pt(x-NO2-bipy)Cl2]- family,
only the x = 4 species gives a well resolved spectrum.13 Similar
analysis gives Pt valence orbital admixture to the SOMO of 7%,
intermediate between the mono- and un-substituted complexes.
Fig. 4 Experimental (left) and simulated (right) isotropic EPR spectra of
(a) [3,3¢-(NO2)2-bipy]-, (b) [4,4¢-(NO2)2-bipy]-, (c) [4,4¢-(15NO2)2-bipy]- and
(d) [5,5¢-(NO2)2-bipy]- in 0.1 M [nBu4N][BF4] DMF at 233 K, generated in
situ Egen = -1.2 V vs. Ag/AgCl.
DFT Calculations
DFT calculations were performed on the free ligands and their
complexes: selected MO coefficients for the LUMOs are in Table
4. The trends in the electronic structure can be rationalised by
considering the LUMO of x-NO2-py. Unsurprisingly, the LUMO
is dominated by the antibonding p* orbital of the nitro group
in each case, with a bonding interaction between this fragment
and the ring (carbon 2pz). The greatest delocalisation to the ring-
N position occurs for x = 4 because only this position allows a
Mono- and di-reduced forms of 4,4¢-(NO2)2-bipy give identical
spectra in fluid solution, differing only in intensity. This implies
that the second electron enters an equivalent, and near-degenerate,
orbital on the second ring. Formation of a biradical is confirmed
by observation of a half-field peak in the frozen solution spectrum
(Figure S3†).
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Table 3 Isotropic and anisotropic g-values and 195Pt hyperfine coupling constants (A/10-4 cm-1) from fluid and frozen solution EPR spectra
a
giso
g1
g2
g3
Aiso
A1
A2
A3
[Pt(4-NO2-bipy)Cl2]-
2.005
2.004
2.002
1.998
2.027
2.015
2.033
2.005
2.009
2.005
2.006
2.006
1.985
1.991
1.968
1.963
-33
-20
-37
-22
-29
-17
-32
-19
-53
-23
-48
-29
(-17)
(-20)
(-31)
(-18)
[Pt(3,3¢-(NO2)2-bipy)Cl2]-
[Pt(4,4¢-(NO2)2-bipy)Cl2]-
[Pt(5,5¢-(NO2)2-bipy)Cl2]-
a Not resolved experimentally; calculated from Aiso = (A1 + A2 + A3)/3.
Table 4 Selected DFT calculated % composition of the LUMOs of free
ligands and {PtCl2} complexes
nitro groups on the redox and spectroscopic properties of a series
of bipyridines and their complexes with {PtCl2}. We have shown
marked differences in the properties of the x,x¢-(NO2)2-bipy family
depending on substitution position, including gross changes in the
nature of the electron acceptor orbitals. For their complexes there
are more subtle variations in the spin distribution, but the reduc-
tion potential varies by ca. 0.3 V with x,x¢. The most remarkable
redox behaviour is observed for [Pt(3,3¢-(NO2)2-bipy)Cl2] which
reduces at +0.01 V vs. Ag/AgCl, whereas [Pt(3-NO2-bipy)Cl2] is
the hardest to reduce of the monosubstituted series, which must be
partly due to the steric effects of disubstitution at the 3,3¢ positions.
Compound
Ring-N 2pz
Nitro-N 2pz
Pt 5d/6pz
3-NO2-py
4-NO2-py
0.9
10.3
0.6
11.4
1.8
0.8
8.2
3.1
12.9
3.4
16.0
6.5
25.6
24.7
21.2
23.1
19.3
11.5
9.7
7.7
—
14.8
12.4
8.1
—
—
—
—
—
—
—
—
6.6
2.5
7.1
4.2
3.0
8.8
4.4
3-NO2-bpy
4-NO2-bpy13
5-NO2-bpy
3,3¢-(NO2)2-bpy
4,4¢-(NO2)2-bpy
5,5¢-(NO2)2-bpy
[Pt(bpy)Cl2]12
[Pt(3-NO2-bpy)Cl2]
[Pt(4-NO2-bpy)Cl2]13
[Pt(5-NO2-bpy)Cl2]
[Pt(3,3¢-(NO2)2-bpy)Cl2]
[Pt(4,4¢-(NO2)2-bpy)Cl2]
[Pt(5,5¢-(NO2)2-bpy)Cl2]
Experimental
4.0
11.0
6.4
7.0
4.3
3.7
Synthesis
3-NO2-py was commercially available; 5-NO2-bipy,30 and 4,4¢-
(NO2)2-bipy15 were prepared by literature methods. We have re-
ported [Pt(4-NO2-bipy)Cl2]13 and [Pt(4,4¢-(NO2)2-bipy)Cl2]14 pre-
viously.
symmetry match between the nitro group and the the b1 LUMO of
pyridine itself (Fig. 5). This argument also holds for all the species
in Table 4: the greatest contribution to the LUMO from the ring-N
2pz orbital is for the x(x¢) = 4 isomers. [Note this also explains the
trend in reduction potentials for the free ligands, Table 1.] This
could also be described in terms of conjugation effects between
the nitro group and the ring-N in the 4(4¢) substituents (para)
vs. 3(3¢) or 5(5¢) (both meta). This then allows greatest overlap
with Pt valence orbitals when coordinated, in agreement with the
parameters derived from the EPR data in Table 3. The calculations
also support the shift of electron density from nitro-N to ring-N
(as observed experimentally from the relative magnitudes of the
hyperfine coupling constants) on coordination to the Pt(II) ion.
3-nitro-2,2¢-bipyridine (3-NO2-bipy): Prepared using the method
reported for 5-NO2-bipy,30 but with 2-chloro-3-nitropyridine re-
placing 2-chloro-5-nitropyridine. The beige product precipitated
from the solution on filtering through the silica gel/celite bed
(yield 70%). Elemental analysis (%), observed (calculated for
C10H7N3O2): C, 59.25 (59.70), H, 3.53 (3.51), N, 20.55 (20.89).
1H NMR (CD3Cl, 199.972 MHz): d = 8.845 (d, J = 4.69 Hz, 1H),
8.621 (d, J = 4.69 Hz, 1H), 8.621 (d, J = 4.69 Hz, 1H), 8.094 (d,
J = 8.206 Hz, 2H), 7.486 (dd, J = 8.20, 4.69 Hz, 1H), 7.384 (dd,
J = 7.62, 5.02 Hz, 1H) ppm, Mass spectra (EI) m/z: 201 (M+).
3,3¢-dinitro-2,2¢-bipyridine (3,3¢-(NO2)2-bipy): Adapted from lit-
erature preparation.28 2-Chloro-3-nitropyridine (5 g, 0.031 mol)
◦
was heated to 100 C with copper bronze (5 g) in DMF (20 ml)
for 15 h. The mixture was cooled, diluted with water (70 ml)
and filtered. The resulting brown solid was filtered and triturated
with ammonia (3 ¥ 4 ml) before being extracted exhaustively with
boiling 1,4-dioxane. The brown, crystalline solid obtained was
recrystallised with hot filtering from boiling 1,4-dioxane. Yield
(1.448 g, 37.9%). Elemental analysis (%), observed (calculated for
C10H6N2O2): C, 48.89 (48.79), H 2.49 (2.46), N 22.76 (22.76). 1H
NMR (CD3Cl, 199.972 MHz): d = 8.889 (dd, J = 4.69, 1.56 Hz,
2H), 8.594 (dd, J = 1.56, 8.20 Hz, 2H), 7.654 (dd, J = 8.20, 4.69
Hz, 2H) ppm, Mass spectra (FAB) m/z: 247 ((M+1)+).
5,5¢-dinitro-2,2¢-bipyridine (5,5¢-(NO2)2-bipy): Adapted from lit-
erature preparation.29 2-Iodo-5-nitropyridine (14.99 g, 0.059 mol)
was dissolved in DMF and treated with copper bronze (50 g).
This was stirred and heated to reflux under nitrogen for 7h.
The hot mixture was filtered and poured into water (1000 ml).
The resulting grey solid was filtered through a sintered funnel,
Fig. 5 Calculated LUMO of 4-NO2-py.
Conclusions
To summarise, we have undertaken a systematic study of the
influence of substitution pattern of strongly electron-withdrawing
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triturated with ammonia solution (3 ¥ 60 ml) and extracted
with boiling 1,4-dioxane. After cooling, the 1,4-dioxane extracts
were filtered and the resulting solid was recrystallised from 1,4-
dioxane. A pale yellow solid was obtained. Further product was
obtained by evaporating the filtrate from each 1,4-dioxane extract
and recrystallising the resulting solid from 1,4-dioxane. Yield
(2.360 g, 16.25%). Elemental analysis (%), observed (calculated
for C10H6N2O2): C, 48.42 (48.79), H 2.42 (2.46), N 22.43 (22.76).
1H NMR (CD3Cl, 199.972 MHz): d = 9.546 (d, J = 2.34 Hz, 2H),
8.786 (d, J = 8.59 Hz, 2H), 8.667 (dd, J = 8.52, 2.74 Hz, 2H) ppm,
Mass spectra (EI) m/z: 246 (M+).
Pt Complexes: The ligand was suspended in water and heated
to reflux with a molar equivalent of K2[PtCl4] and the pre-
cipitated product was filtered off. [Pt(5,5¢-(NO2)2-bipy)Cl2] was
recrystallised from DMSO whereas [Pt(3,3¢-(NO2)2-bipy)Cl2] was
dissolved in DMF and precipitated slowly by diffusion of Et2O.
For complexes of 5,5¢-(NO2)2-bpy the solutions required heating
for 48 h to obtain a reasonable yield; for the other ligands heating
was for 24 h.
ical studies were carried out in N2 purged 0.1 M [nBu4N][BF4]
DMF solutions. Data quoted were recorded at 0.1 V s-1 scan
rate. All values quoted are referenced to Ag/AgCl, against which
the ferrocinium/ferrocene couple is measured at +0.55 V. Bulk
electrolysis was carried out in an H-cell with a Pt basket working
electrode. Electro-generations were carried out at -40 ◦C (dry
ice/acetone bath) under a nitrogen atmosphere.
In situ UV/Vis/NIR electro-generation was performed using
an optically transparent thin layer electrode (OTTLE) cell39 in a
Perkin-Elmer Lambda 9 spectrophotometer, with spectra recorded
every ten minutes. The potential was set back to. Reversibility was
tested by re-generation of the original species.
X-band EPR data were recorded on a Bruker ER 200-D SRC
spectrometer connected to a datalink 486DX desktop PC running
EPR Acquisition System version 2.42. A variable temperature in
situ electrolysis cell was used, as described elsewhere.40 Simulations
used Bruker WINEPR Simfonia Version 1.25; g-values were
calibrated against 2,2¢-diphenyl-1-picrylhydrazyl (DPPH).
DFT calculations used Gaussian 03 (revision D.01).41 The
images were generated using Arguslab.42 The EPRIII basis set
was used for the C, N, H, O, the 6-31G* basis set for the Cl atoms
VDZ (n + 1) ECP basis sets for the Pt(II) centre.43
[Pt(3-NO2-bipy)Cl2]: Orange solid (96%). Elemental analysis
(%), observed (calculated for C10H7N3O2PtCl2): C, 26.62 (25.71),
H 1.53 (1.51), N 8.66 (8.99).
[Pt(3,3¢-(NO2)2-bipy)Cl2]: Dark red crystalline solid (85%). El-
emental analysis (%), observed (calculated for C10H6N4O4PtCl2):
C, 23.27 (23.45), H 1.53 (1.18), N 10.53 (10.94).
[Pt(4,4¢-(NO2)2-bipy)Cl2]:14 Dark green crystalline solid
(60%). Elemental analysis (%), observed (calculated for
C10H6N4O4PtCl2): C, 23.04 (23.45), H 0.91 (1.18), N 10.35 (10.94).
[Pt(5-NO2-bipy)Cl2]: Dark red/brown solid (86%). Elemental
analysis (%), observed (calculated for C10H7N3O2PtCl2): C, 31.77
(25.71), H 2.10 (1.51), N 9.57 (8.99).
Acknowledgements
We acknowledge Professor Peter Tasker for many useful discus-
sions on the subject of resonance structures and congratulate him
on his 65th birthday and retirement. We are grateful to the EPSRC
for funding the National EPR Facility.
[Pt(5,5¢-(NO2)2-bipy)Cl2]: Brown solid (71%). Elemental anal-
ysis (%), observed (calculated for C10H6N4O4PtCl2): C, 24.04
(23.45), H 1.46 (1.18), N 10.04 (10.94).
Notes and references
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