Bis(aryl)acenaphthenequinonediimine substituent effect on the properties and coordination environment of ligands and their bis-chelate AgI complexes

Article abstract of DOI:10.1002/ejic.201300828

The reaction of a series of Ar-BIAN ligands (Ar = o-Ph, m-Cl, m-CF ₃, p-Cl, p-MeO, and p-Me-C₆H₄) with AgNO ₃ results in the formation of the corresponding bis-chelate complexes. Substituent groups on the aryl rings seem to offer tunability of the electronic properties of the diimines, also affecting the metal coordination environment through the mono-coordination or chelation of the nitrate anion as promoted by the substituents p-MeO, p-Me, and m-CF₃. The enhanced absorption capacity of the complexes in the visible region in combination with the well-known redox activity of the ligands and their ability to potentially accommodate a fifth coordinated ligand on the metal center opens a route for the preparation of new silver species of high propensity for catalytic, photocatalytic, and biological applications. Six new Ag^I-bis(BIAN) complexes along with their ligands were synthesized and studied. Substituent groups offer tunability of the electronic properties and structural environment of the compounds. The versatility on the metal coordination sphere opens the way for the design of photoactive species with a variety of applications. Copyright

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DOI:10.1002/ejic.201300828
Bis(aryl)acenaphthenequinonediimine Substituent Effect on
the Properties and Coordination Environment of Ligands
and Their Bis-Chelate AgI Complexes
Panagiotis A. Papanikolaou,*[a] Maria Gdaniec,*[b]
Barbara Wicher,[b] Pericles D. Akrivos,[a] and
Nikolai V. Tkachenko*[c]
Keywords: N ligands / Silver / Structure elucidation / Electronic properties / Electrochemistry
The reaction of a series of Ar–BIAN ligands (Ar = o-Ph, m-Cl,
m-CF3, p-Cl, p-MeO, and p-Me–C6H4) with AgNO3 results
in the formation of the corresponding bis-chelate complexes.
Substituent groups on the aryl rings seem to offer tunability
of the electronic properties of the diimines, also affecting the
metal coordination environment through the mono-coordina-
tion or chelation of the nitrate anion as promoted by the sub-
stituents p-MeO, p-Me, and m-CF3. The enhanced absorp-
tion capacity of the complexes in the visible region in combi-
nation with the well-known redox activity of the ligands and
their ability to potentially accommodate a fifth coordinated
ligand on the metal center opens a route for the preparation
of new silver species of high propensity for catalytic, photo-
catalytic, and biological applications.
synthesized and studied for a variety of applications includ-
ing contrast agents,[4] chemical and biological sensors,[5]
and conductive coatings.[6]
Introduction
Silver(I) compounds are promising candidates in a series
of important applications, and, in contrast to their typically
isoelectronic CuI compounds, they adopt a variety of coor-
dination geometries. Despite this, only 4.7% of the struc-
tures presented until recently bear five donor atoms con-
nected to the metal core.[1a] Furthermore, this soft metal is
kinetically labile, which results in disproportionation, poly-
merization, and ligand-scrambling reactions. On the other
hand, versatility in both local and overall structures is a
prerequisite for the formation of hybrid materials, and both
π–π interligand[1b] and intermetallic interactions, especially
in d10 systems,[2] are factors controlling molecular aggrega-
tion. It is also well known that AgI has the ability to self-
assemble into two- and three-dimensional coordination net-
works characterized by tunable optical and electronic prop-
erties.[3] Silver-containing nanomaterials have also been
Coordination compounds of silver with nitrogen donors
have been utilized in the formation of highly organized hel-
icate molecules and mixed Ag–Au organometallic com-
pounds with diimine ligands and have been proposed to
act as optical fiber sensors for organic compound vapor
detection by color change upon absorption of organic mo-
lecules.[7] Among the polydentate polypyridyl ligands, α-di-
imines have been studied in particular because of their abil-
ity to form very stable five-membered chelate rings with al-
most all metals.[8] Several of their coordination compounds
have been shown to possess interesting properties related to
their electron distribution, which makes them candidates
for catalysts of redox, photochemical, and biological reac-
tions.[9] In the same context, bis(aryl)acenaphthenequinone
diimines (Ar–BIANs) are known for some time, and their
coordination chemistry has been shown to lead to com-
pounds with interesting features such as catalytic and pho-
tophysical properties.[10] These ligands possess a rigid cen-
tral backbone, while the different electronic effects of the
substituents of the pendant aryl groups may affect the elec-
tron-donor and -acceptor ability of the diimine re-
gion.[10d,11] The scarcity of information on silver(I) Ar–
BIAN complexes prompted us to undertake the present
study, in which a series of such ligands is synthesized and
their interaction with AgNO3 is studied. To the best of our
knowledge there is only one report on AgI–bis(BIAN) com-
plexes[12] bearing a distorted tetrahedral structure with a
non-interacting BF4– counteranion. In this work, we report
[a] Aristotle University of Thessaloniki, Department of Chemistry,
Laboratory of Inorganic Chemistry,
P.O.B. 135, GR-541 24 Thessaloniki, Greece
E-mail: panpapan@chem.auth.gr
Homepage: http://www.chem.auth.gr/index.php
[b] Adam Mickiewicz University, Faculty of Chemistry,
60 780 Poznan´, Poland
E-mail: magdan@amu.edu.pl
Homepage: http://www.chemia.amu.edu.pl/main/staff/
Gdaniec.htm
[c] Department of Chemistry and Bioengineering, Tampere
University of Technology,
33101 Tampere, Finland
E-mail: nikolai.tkachenko@tut.fi
Homepage: http://kemia.me.tut.fi/nick
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300828.
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on a set of six compounds of formula Ag(L)2(NO3) (L = any water residues, while slight heating helped the dissol-
Ar–BIAN) in some of which AgI manifests its structural ution of the reacting materials. In the solid state, all of the
versatility through accommodation of nitrate as the fifth presented coordination compounds appear as dark-orange
ligand. This diversity in coordination behavior can only be or red powders and maintain their color in dichlorometh-
attributed to the aryl ring substituents of the diimines, ane solution.
which seem to enable tuning of the electronic properties of
the ligands and their coordination compounds. This feature
could be informative on the design of materials for catalytic
Spectroscopic Information
and photochemical applications, as the recently reported
AgI compound bearing a η2-C2H4 molecule on a five-coor-
All ligand spectra demonstrate a set of two strong ab-
dinate structure.[13] The present work constitutes the first sorptions in the region 1669–1617 cm–1 assigned to the
step of a synthetic effort already in progress in our lab tar- stretching vibration of the imine bonds, although such an
geting the preparation of photostable silver species having assignment cannot be unambiguous since the stretching fre-
an enhanced light-absorption capacity and bearing a redox- quencies of C=C bonds also appear in the same region.[17]
active organic molecule such as BIAN as well as a fifth, The absence of any absorption in the region 1730–
easily displaced ligand of catalytic or biological interest 1700 cm–1 confirms the purity of the compounds by ruling
such as CO,[13] alkenes,[14a–14d] or biomolecules.[14e]
out the presence of the 1:1 condensation byproduct of the
diketone.[18] Very small coordination shifts are observed in
all the studied complexes,[19] indicating a relatively weak
metal–ligand interaction. The imine bond absorption ap-
pears again as a set of peaks in the 1667–1617 cm–1 region
with a small variation in the pattern relative to the free li-
gands because of the effect of the coordination on the elec-
tron distribution of the chelating region.
Results and Discussion
Synthesis
The synthesis of the ligands (Figure 1) was essentially the
one proposed by Ragaini[15a] with slight modifications al-
ready reported,[15b] which help to avoid the extremely dry
conditions demanded by the use of the anhydrous ZnCl2.
In some cases like o,oЈ-iPr2C6H3–BIAN, the synthesis can
be achieved even by a direct and simple condensa-
tion,[15a,16c] while in their pioneering work van Asselt et
al.[16d] clearly indicate that the use of anhydrous zinc chlor-
ide and heating to reflux under an inert atmosphere are
essential. Similar difficulties were observed in our case and
were bypassed through a special treatment of ZnCl2.[15b]
The compounds prepared in this way were compared to
samples synthesized by the literature methods[15a,16] and
proved to be identical. Isolation yields were also compar-
able with the reported ones.[15a,16d]
The study of the behavior of the nitrate anion is interest-
ing, as this ion may be present as counteranion, mono-coor-
–
dinated or chelated ligand. Free NO3 ideally adopts a
planar D3h symmetry where the IR-active vibrations are the
low intensity bendings a2ЈЈ and eЈ and the strong stretching
ν3 (eЈ) appearing around 1350 cm–1. Lowering of symmetry
through coordination renders all vibrations IR-active,[20]
and a split band stemming from the coupling of the ν3 with
the bending eЈ[21] becomes apparent in the region 1700–
1800 cm–1. Coordination of the nitrate group in [Cu(dien)(-
NO3)](NO3) has been related to the appearance of an over-
tone band in the region 1700–1800 cm–1 with a splitting
characteristic of the coordination mode.[22,23] In this spec-
trum a band at 1320 cm–1 has been attributed to a mono-
dentate coordinated nitrato ligand, and a peak at 1360 cm–1
was assigned to a free nitrate.
In the IR spectra of the complexes the absorption peak
of the nitrate appears in two distinct forms (Figure 2). For
Ag(1)2NO3, Ag(2)2NO3, and Ag(4)2NO3, it consists of a
strong clear band located in the region 1339–1314 cm–1 with
a form characteristic of a non-interacting planar nitrate
group preserving its D3h symmetry,[24] while in complexes
Ag(3)2NO3, Ag(5)2NO3, and Ag(6)2NO3, the same band
appears as a sharp peak at a rather higher frequency
(1384 cm–1).[25] In none of the cases was the 1700–
1800 cm–1 overtone observed. Crystallographic studies re-
vealed that in Ag(5)2NO3 and Ag(6)2NO3 the metal ion
adopts a square pyramidal geometry with an axially coordi-
nated nitrate, and the same is expected for Ag(3)2NO3 be-
cause of the similarity in the absorption pattern of the ni-
trate. A tentative conclusion, therefore, is that Ag(1)2NO3,
Figure 1. Outline of the synthetic route to the ligands.
The preparation of the silver complexes does not involve Ag(2)2NO3, and Ag(4)2NO3 can be formulated as [Ag(L)2]-
any deterioration and can be achieved under atmospheric NO3, although until now there has been lack of crystallo-
conditions. The solvents used were not pretreated to remove graphic data due to the failure to obtain suitable single crys-
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tals of the complexes. The absence of the overtone for the teractions between neighboring molecules leading to the
other three complexes may be assigned to a relatively weak breakdown of the equivalence of the carbon atoms through
–
metal–NO3 interaction.
a disruption of the local symmetry.
Scheme 1.
In the NMR spectra recorded in CDCl3, the typical reso-
nances expected for the proton and carbon atoms of the
Ar–BIAN ligands are observed. In particular, the methyl
protons and carbon atoms for the p-Me and p-MeO ligands
do not shift upon coordination. Very small shifts are ob-
served for the characteristic carbon atoms of the ligands,
that is, those of the imino and the aryl group directly at-
tached to the nitrogen. In the 13C NMR spectra of Ag(1)2-
NO3, Ag(5)2NO3, and Ag(6)2NO3, a single line is observed
for each chemically equivalent set of atoms, indicating that
equilibration occurs in solution, and therefore steric crowd-
ing or π–π intra- or intermolecular interactions are not im-
portant in the determination of the overall solution struc-
ture. The 13C NMR spectra of Ag(2)2NO3 and Ag(4)2NO3
could not be recorded because of inadequate solubility.
Electrochemical Studies
The redox behavior of the Ar–BIAN ligands and their
AgI complexes was investigated by differential pulse vol-
tammetry in dichloromethane containing 0.1 m tetrabu-
tylammonium tetrafluoroborate as supporting electrolyte.
The redox potentials obtained (Table S1) reveal some com-
mon characteristics presented below. Under the same exper-
imental conditions, the ferrocene/ferrocenyl oxidation po-
tential was observed at +0.67 V. For the ligands there are
two thermodynamically reversible reduction processes with
half-wave potentials in the regions –1.07 to –1.27 and –1.69
Figure 2. Comparison of the IR spectra of the free BIAN (5) (–)
and its respective silver(I) complex (--) (top) and (4) (–) and its
silver(I) complex (--) (bottom), revealing the different shape of the
nitrate absorption peak for the two complexes. Arrows indicate the
–
band related to NO3 .
According to previous studies[16c,26] symmetric Ar–BIAN to –1.88 V except for 4, where only the lower-potential cath-
molecules exist in solution only in their (E,E) form referring odic current is observed, the second process probably oc-
to the orientation of the aryl rings, while two intercon- curring beyond the applied voltage window. The peak sepa-
verting isomers denoted as syn- and anti- may also occur ration for each of these signals accounts for electrochemical
for asymmetric aryl ring structures (Scheme 1). The last reversibility while the peak heights in most cases do not
holds in our case for compounds 1, 2, and 3. Indeed for 1 agree with complete chemical reversibility because of a ra-
1
and 3 we have noted some extra signals in the H NMR pid radical decomposition. For 5 and 6, an irreversible re-
and 13C NMR spectra,[15b] which, in a first step, have been duction process appears also near –1.00 V, while for 1 this
assigned to isomerization of the diimines, but, besides that, process seems to be quasi-reversible. For the latter com-
it is known that the presence of the two isomers in solution pound, an additional reversible cathodic signal appears at
does not alter the reactivity of the BIAN ligands.[26,27] Since –1.45 V. The peak heights corresponding to the processes at
the respective spectrum of Ag(1)2NO3 reveals the expected –1.00 and –1.45 V for 1, 5, and 6 are quite small relative to
number of signals for a symmetric coordinated BIAN mole- the rest of the signals and could be assigned to a small
cule, the extra signals in the free ligand can be attributed to concentration of the electroactive species involved. Such a
the (Z,E) form. On the other hand, the 13C NMR spectrum situation could be correlated with a chemical-reaction–elec-
of Ag(3)2NO3 also reveals a number of extra signals which tron-transfer (CE) mechanism or reduction of chemisorbed
could be attributed either to the aforementioned hindered species on the Pt electrode or even further with traces of
rotation of the phenyl rings or to stacking electrostatic in- the (Z,E) isomer that might exist in solution.
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For free diimines 1, 4, 5, and 6, there are also two distinct als (FMOs) of the diimines, the electrochemical band gaps
oxidation signals with E1/2 values in the region +1.19 to of these materials can also be estimated. According to Fig-
+1.55 and +1.64 to +1.98 V. The first anodic signal appears ure 3 a good connection seems to exist between the energy
to be quasi-reversible, which suggests the chemical instabil- gap and the electron-donating ability of the substituents if
ity of the oxidized species. The second oxidation signal ap- conjugation effects are considered.
pears either as a quasi-reversible or an irreversible process.
In the case of 2 and 3, a somehow more complex oxidation
pattern is observed: for the former a quasi-reversible signal
consisting of two successive oxidations at +1.65 and
+1.75 V appears, while for the latter a quasi-reversible an-
odic signal appears at +1.7 V followed by a questionable
second oxidation at +1.9 V.
In the voltammograms of the complexes, either the fur-
thest anodic or cathodic peaks correspond well with the
ones observed in the related ligands, suggesting that in the
previous step dissociation of one or both ligands has oc-
curred, or the respective redox processes occur in a region
of the ligand not affected by the presence of the metal ion
as, for example, the naphthyl part. The peaks observed
around +1.0 and +1.5 V are most probably related to suc-
Figure 3. Least-squares fit of the relation between the electrochemi-
cessive oxidations of the coordinated ligands. The peaks cal band gap, EgDPV, and the Hammett (solid line, ♦) and Brown
(dashed line, Δ) constants for the free ligands.
close to +0.5 V observed for most of the complexes may
correspond to the oxidation of the electron-rich diimine
part of the ligand by the formation of highly oxidized AgII
species further stabilized by the extended conjugation of the
X-ray Crystal Structure
BIAN molecule, although this seems ambiguous consider-
ing the low potential values. Finally, reductive signals ap-
Coordination numbers higher than four are not uncom-
pearing close to –0.3 V for complexes Ag(2)2NO3, Ag(4)2- mon for silver, and there exist several examples of such co-
NO3, and Ag(5)2NO3 could represent the formation of ordination geometries realized for silver complexes with
either Ag0 or an anionic compound where the metal retains heterocyclic nitrogen donors, mainly polypyridines. These
its monovalent state and the BIAN ligands are in their an- compounds are usually dimeric in nature, that is, [Ag2Ln]2+
ionic form; this is well documented.[28a–28c]
and the chromophores formed are of the formula AgN5,[29]
The evaluation of the energy of the frontier MOs although higher nuclearities have been reported.[30] Of spe-
(Table S1) provides further insight into the effect of the sub- cial interest in this type of compounds is the nature of the
stitution pattern of the diimines. For the silver systems, counteranions that participate in the determination of the
there is ambiguity concerning the assignment of the first overall structure through their interaction with the cationic
reduction potential due to the existence of several peak cur- complexes or solvent molecules. When silver nitrate is used
rents of low intensity, so only the EHOMO is calculated. The as a starting material, the geometric preferences are ex-
impact of the substitution can be expressed through the panded due to the ability of the anion to participate in hy-
Hammet (σp,m) and Brown (σ+p,m) constants, the latter of drogen bonding to sites of the complex or solvent molecules
which is improved for conjugation effects.[28d,28e] Since o- and, under certain circumstances, to coordinate to the silver
groups do not afford reliable data for steric reasons[28d] center either as a chelate[31] or in a monodentate fashion.[32]
compound 1 and its respective complex were not consid- The structure of Ag(5)2(NO3) proved to be dichlorometh-
ered. By fitting the data reported in Figures S23 and S24, ane-solvated and presents an example of rectangular coor-
it becomes obvious that there is a better correlation of the dination geometry around AgI defined by the four nitrogen
EHOMO of the free ligands with the Brown constants, while atoms of the two p-MeO–C6H4–BIAN ligands (Figure 4).
ELUMO correlates better with the respective Hammett con- The binding of the ligands is slightly asymmetrical with
stants, which reveals that the aryl substituents act on the Ag–N distances of 2.4158(19) and 2.4516(18) Å for one and
HOMO mainly through the π-system and on the LUMO 2.4113(18) and 2.4227(17) Å for the other ligand. These dis-
through inductive effects. The energies of the HOMOs (and tances are significantly longer than the values of 2.329(2) Å
LUMOs) of the ligands increase with the donor strength of reported earlier[12] for [Ag(o,oЈ,p-CH3–C6H2–BIAN)2]BF4,
the substituents, as a result of an increase in the electronic where the metal center showed distorted tetrahedral coordi-
ox(I)
repulsion. This reflects on a less positive E1/2
(Table S1) nation geometry. In Ag(5)2(NO3), the silver ion is displaced
although a broad generalization is not attempted on the from the mean plane defined by the four nitrogen atoms by
–
basis of the discussed data only. For the silver complexes 0.470 Å toward the NO3 anion, which forms two short
(Figure S25), the correlation of EHOMO with the substituent Ag–O contacts of 2.612(2) and 2.771(2) Å with the metal
constants appears to be really poor, and no trends can be center. Thus an alternative description of the AgI environ-
observed. From the energies of the frontier molecular orbit- ment is a rectangular pyramid in which the four N atoms
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form a basal plane and the chelating nitrate anion is in the
apical position. Moreover, the nitrate anion also interacts
with the p-MeO–C6H4–BIAN ligands in the same complex
molecule through three weak intramolecular C–H···O
hydrogen bonds with H···O distances in the range 2.58–
2.66 Å. Perhaps this explains why the pending phenyl sub-
stituents of the p-MeO–C6H4–BIAN ligands, which face
each other in the molecule, are not parallel but form di-
hedral angles of 9.3 and 15.3°.
Figure 5. X-ray structure of [Ag(6)2(NO3)]. Hydrogen atoms have
been omitted for clarity. The displacement ellipsoids are shown at
the 50% probability level. Selected bond lengths (Å) and angles (°)
are: Ag1–N11 2.526(3), Ag1–N12 2.405(3), Ag1–N21 2.428(3),
Ag1–N22 2.472(3), Ag1–O33 2.394(3), C11–C21 1.516(5), C12–
C22 1.512(5); N11–Ag1–N22 159.06(10), N21–Ag1–N12
134.19(10), N11–Ag1–N21 68.77(10), N12–Ag1–N22 69.11(10).
coordination geometries in AgI–bis(BIAN) complexes. In
case of the non-coordinating BF4 anion, the AgN4 chromo-
phore shows distorted tetrahedral coordination and sym-
metrical binding of the BIAN ligands. In the case of the
weakly interacting nitrate anion, as in Ag(5)2(NO3), the
AgN4 chromophore changes its geometry toward rectangu-
lar and a small degree of asymmetry is observed for the
ligand binding. In turn, in Ag(6)2(NO3), where the nitrate
anion forms a short Ag–O bond, the AgN4O chromophore
adopts a geometry intermediate between rectangular py-
ramidal and trigonal bipyramidal and the asymmetry of the
BIAN binding becomes much more pronounced.
Figure 4. X-ray structure of [Ag(5)2(NO3)]·CH2Cl2. Hydrogen
atoms and the solvent molecule have been omitted for clarity. The
displacement ellipsoids are shown at the 50% probability level. Se-
lected bond lengths (Å) and angles (°) are: Ag1–N11 2.4516(18),
Ag1–N12 2.4113(18), Ag1–N21 2.4158(19), Ag1–N22 2.4227(17),
Ag1–O23 2.771(2), Ag1–O33 2.612(2), C11–C21 1.522(3), C12–
C22 1.518(3); N11–Ag1–N22 154.45(6), N21–Ag1–N12 160.47(6),
N11–Ag1–N21 70.66(6), N12–Ag1–N22 70.27(6).
This diversity of AgI coordination geometries has been
observed in some other silver nitrate diimine complexes. In
(nitrato)bis[5-(pyridin-2-yl)pyrazine-2-carbonitrile]silver(I),
a rectangular pyramidal metal environment was observed
The coordination of AgI in Ag(6)2(NO3) is somewhat dif- with a Ag–ONO2 distance of 2.547 Å and Ag–N bonds be-
ferent from that in Ag(5)2(NO3), as the unidentate nitrate tween 2.301 and 2.580 Å.[32c] Oxygen coordination to the
anion forms a much shorter Ag–O bond of 2.394(3) Å (Fig- oxo anion is also observed in the case of some trifluorome-
ure 5). The binding of p-Me–BIAN ligands is more asym- thylsulfonates such as bis[μ2-4-cyano-3,6-bis(2-pyridyl)-
metrical with Ag–N distances of 2.428(3) and 2.526(3) Å pyridazine-N,NЈ,NЈЈ,NЈЈЈ]bis(trifluoromethanesulfonato–O)-
for one and 2.405(3), and 2.472(3) Å for the other ligand. disilver(I) and bis[μ2-4-(4-bromophenyl)-3,6-bis(2-pyridyl)-
According to the τ index[33] of 0.40 (τ = 0 for rectangular pyridazine-N,NЈ,NЈЈ,NЈЈЈ](trifluoromethanesulfonato–O)di-
pyramid and τ = 1 for trigonal bipyramid), this coordina- silver(I)trifluoromethanesulfonate, where silver adopts a
tion geometry is best characterized as intermediate between rectangular pyramidal geometry with Ag–O distances of
rectangular pyramidal and trigonal bipyramidal. Like in the 2.626 and 2.684 Å, respectively.[34]
previous case, the nitrate anion interacts with the p-Me–
There are also known examples of AgN4O chromo-
C6H4–BIAN ligands; however, it forms only two weak in- phores whose environment is approximately trigonal bi-
tramolecular C–H···O hydrogen bonds with H···O distances pyramidal. Two reports on (nitrato)bis(4,5-diazafluoren-9-
of 2.52 and 2.68 Å.
one)silver have the nitrate in an equatorial position with a
The two crystal structures reported here, Ag(5)2(NO3) Ag–O distance of 2.418 Å, axial Ag–N bonds of 2.599 and
and Ag(6)2(NO3), and the structure of [Ag(o,oЈ,p-CH3– 2.626 Å, and equatorial Ag–N bonds of 2.349 and
C6H2–BIAN)2]BF4 reported earlier represent three different 2.426 Å[32a] or a Ag–O distance of 2.381 Å, axial Ag–N
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bonds of 2.567 Å and 2.643 Å, and equatorial Ag–N bonds
of 2.344 and 2.385 Å.[32b]
Electronic Absorption Spectra
The spectra of the ligands in dichloromethane consist of
intense absorptions in the ultraviolet region and a broad
chromophore band extending to the region of 500 nm (Fig-
ure S29). The absorption assignments have been given else-
where.[15b] In the respective complexes, the absorption in the
UV (Figure 6) is as in the free ligands assigned to intrali-
gand transitions (IL) of the π-system. The chromophoric
transitions observed in the region above 400 nm are mainly
of intraligand character with a participation of metal center
orbitals in the form of L(π)ǞAg(dp) (LMCT) transitions,
which appear as a shoulder on the low-energy side of the
main visible absorption.
Figure 7. Least-squares curve of the optical band gap, Egopt, esti-
mated by the absorption onsets with the Hammett (solid line, ♦)
and Brown (dashed line, Δ) constants for the free ligands.
The complexes were tested for possible luminescence
upon irradiation in their visible absorption band, but none
revealed any appreciable emission in dichloromethane solu-
tion at room temperature, as expected.[35a–35c] The lack of
luminescence could be further assigned to an underlying
3IL state, from which the complexes deactivate towards
their ground state through a nonradiative path, as has been
reported for similar CuI complexes.[35d]
Theoretical Results
The electronic structure of the complexes was studied
through DFT calculations on the [Ag(4)2]+ complex cation
by utilizing the polarizable continuum model (PCM).[36]
relevant view of the optimized structure, atomic coordi-
nates, and geometric parameters are given in Table S5. This
A
Figure 6. UV/Vis absorption spectra of the Ag complexes with di-
imines 1 (--), 2 (–), and 5 (--) recorded in dichloromethane solution.
opt
The plot of Eg (Table S1) vs. the Hammet and Brown cation was selected for the calculations in order to avoid any
constants for the free ligands with different substituents discrepancies resulting from the coordination of the nitrate
(Figure 7) gives a better correlation when conjugation ef- anion in solution. The complex reveals a distorted tetrahe-
fects are taken into account, and the optical band gap dral geometry with a dihedral angle of 64° between the di-
seems to decrease with increasing donor strength of the aryl imine planes. The phenyl rings are twisted at about 67° rela-
substituents as a result of a respective increase in EHOMO
.
tive to the acenaphthene skeleton. The Ag–N bond lengths
The same trend applies in the case of the silver complexes are all predicted to be almost equal to 2.4 Å, while the C–
(Table S4 and Figure S26), although the estimated EHOMO C and C=N bond lengths of the chelating region (1.53 and
does not reveal any regularity.
1.28 Å, respectively) of the ligands fall in the range of the
For the free diimines, a connectivity seems to exist be- respective pure single and double bonds indicating the lack
tween their optical band gap and the C=N vibrational fre- of any synergistic π-back bonding from the metal to the
quency (Figure S27), revealing that their lowest energy ab- BIAN molecules as should be expected for such a metal
sorption resides in the diimine region and supports its as- core of high reducing capacity.[13]
signments as an n–π* transition. With increasing electron-
The highest occupied orbital of this molecule (Figure 8)
donor ability of the substituent, an increase in electronic is of a d–σ* nature comprising contributions from the metal
density in the C=N region occurs, which weakens the bond- center, the diimine region, and the π-system of the phenyl
ing due to electronic repulsions while it makes lone-pair rings. The lowest unoccupied MO accommodates a bonding
electrons easier to excite. Finally, there is a rather satisfying character between the carbon atoms of the diazabutadiene
correlation between the optical and the electrochemical and an antibonding character for the two imine bonds with
band gap of the uncoordinated diimines (Figure S28). Devi- small participation of the naphthyl moiety and the metal
ations in this case are related to the fact that absorption core orbitals. On the other hand, the HOMO-1 bears sig-
spectroscopy refers to vertical transitions while energetic nificant contributions from the metal ion and the π-system
estimations based on electrochemical data comprise an of the naphthyl part of the ligands, while the LUMO+1 is
overall structural equilibration of the system.
analogous to the LUMO (almost degenerate orbitals) and
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mainly locates to the diimine part of the BIAN molecules. stable AgI compounds with tunable electronic properties
Considering the fact that the lowest-lying transitions of bearing axially coordinated labile species of biological or
these molecules will mainly arise from electronic excitations catalytic interest.
between the FMOs of the system, it becomes obvious that
they could be characterized as IL/ILCT bands with a small
participation of the AgI center.
Experimental Section
Materials and Measurements: Acenaphthenequinone and the sub-
stituted anilines were p.a. quality ACROS products and were used
without any prior purification procedure. The solvents used were
of reagent grade and were not subjected to any further drying pro-
cess prior to their use. Elemental analyses for C, H, and N were
performed with a Perkin–Elmer 240B elemental analyzer. Infrared
spectra were recorded in KBr pellets with a Perkin–Elmer Spec-
trum One FTIR spectrometer having a resolution of 2 cm–1 follow-
ing the collection of 16 scans over the range 4000–360 cm–1. Elec-
tronic excitation spectra were recorded in dichloromethane in 1 cm
cuvettes with a Shimadzu UV-3600 UV/Vis/NIR spectrophotome-
ter having a resolution of 0.1 nm. Emission studies were performed
in optically diluted solutions with a Jobin–Yvon Fluorolog-3 Fluo-
1
rometer. Solution H NMR and 13C NMR spectra were measured
at 300 and 75 MHz, respectively, in CDCl3 solutions with a Varian
300 spectrometer by using tetramethylsilane Si(CH3)4 (TMS) as in-
ternal standard. Differential pulse voltammetry (DPV) was used
for the electrochemical measurements with an Ivium CompactStat
potentiostat/galvanostat with impedance analyzer. Measurements
were done in a one-compartment cell with a silver wire as a pseu-
doreference electrode and a glassy carbon auxiliary electrode. A
platinum working electrode was used for the measurements. Re-
cordings were carried out in the potential region –2000 to
+2000 mV at ambient temperature. Recrystallized tetrabutylammo-
nium tetrafluoroborate (0.1 m) was used as the electrolyte, and the
scan rate was 50 mVs–1. All scans were recorded in degassed sol-
vents and under a current of moisture-free dinitrogen over the solu-
Figure 8. FMOs of the optimized structure of [Ag(4)2]+.
tion surface.
An additional point that deserves to be emphasized is
the influence of the substituent on the phenyl rings on the
energy of the lowest energy excitations of these complexes.
As the form of the FMOs show, the π-system of these rings
is strongly involved in the HOMO, and so the substituent
on those rings will have an energetic impact on it. There-
fore, the energy variations of the respective absorptions
upon altering the substitution pattern will be mainly ex-
pressed through the energy changes of this MO.
Synthesis of the Ligands: In general, the preparation of the free
ligands includes pretreatment of the zinc salt with diethyl ether and
condensation of acenaphthenequinone with the respective aniline
in the presence of the zinc–Et2O adduct[37] in acetic acid followed
by decomplexation of the resulting intermediate, Zn–bis(aryl)acen-
aphthenequinone diimine complex, with potassium oxalate. Details
on the synthetic procedure and analytical data for the ligands can
be found in our previous work.[15b]
Synthesis of the Complexes: The synthesis of the AgI complexes
was carried out according to the following general procedure: An
appropriate amount of AgNO3 (0.5 mmol) dissolved in MeOH
(20 mL) was added to a solution of the Ar–BIAN ligand (25 mL,
1.0 mmol) in CH2Cl2 and stirred for 2 h at room temperature. In
some cases, slight heating was used. The formation of the respective
complex was almost immediate, as indicated by the color change
of the solution towards dark red or orange depending on the sub-
stitution of the ligand. The resulting solid was filtered, and the
filtrate was left to concentrate in air after adding Et2O (3 mL) in
order to retrieve the rest of the complex. Crystals of complexes
[Ag(5)2(NO3)] and [Ag(6)2(NO3)] suitable for X-ray analysis were
obtained by slow diffusion of diethyl ether into their CH2Cl2 solu-
tions.
Conclusions
A series of six AgI complexes bearing two redox-active
chromophore molecules along with their free Ar–BIAN li-
gands were prepared and studied by spectroscopic, electro-
chemical, and theoretical methods. The infrared spectra
corroborated by the crystal structure data indicate the coor-
dination versatility of the metal core: the coordination
number of Ag varies between four and five as affected by
the substitution pattern of the diimine. The fifth nitrato li-
gand occupies the axial position of the pyramid, for which
the strength of the coordination bond is relatively low. Fur-
ther the substituents on the aryl rings of the diimines seem
to govern, through conjugation effects, the spectroscopic
and electrochemical properties of these systems. This obser-
[Ag(1)2]NO3: Red solid, overall yield 87% (0.44 mmol, 0.50 g).
C72H48AgN5O3: calcd. C 75.92, H 4.25, N 6.15; found C 75.79, H
4.30, N 5.96. IR (KBr): 3055 (w) (=C–H), 1662 (s) (C=C), 1626 (s)
–
(C=N–), 1589 (m), 1471 (s), 1339 (s) (NO3 ), 1104 (m), 832 (s), 781
vation could open the way for the preparation of photo- (s), 758 (s), 740 (s), 697 (s) cm–1. 1H NMR (CDCl3, 300 MHz,
Eur. J. Inorg. Chem. 0000, 0–0
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25 °C): δ = 8.00 (d, J = 8 Hz, 2 H), 7.48–7.33 (m, 8 H), 7.13 (t, J placed in idealized positions and refined by using riding models.
= 8 Hz, 2 H), 7.05–6.95 (m, 8 H), 6.91 (d, J = 8 Hz, 2 H), 6.81 (d, Additional details of crystal data and structure refinement are
J = 8 Hz, 2 H) ppm. 13C{1H} NMR (CDCl3, 75 MHz, 25 °C): δ = given in Table S3. Molecular graphics were generated with ORTEP-
161.99, 146.80, 142.25, 138.18, 132.64, 131.61, 131.29, 131.06,
129.39, 128.87, 128.64, 128.60, 127.53, 127.40, 127.22, 125.18,
120.03 ppm.
3 for Windows.[40]
CCDC-914464 and -914465 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[Ag(2)2]NO3: Dark-red solid, overall yield 93% (0.47 mmol, 0.45 g).
C48H28AgCl4N5O3: calcd. C 59.28, H 2.90, N 7.20; found C 59.03,
H 2.97, N 7.13. IR (KBr): 3054 (w) (=C–H), 1669 (s) (C=C), 1630
–
Computational Details: DFT calculations on the geometry of
[Ag(4)2]+ were carried out with the Gaussian 03[41] software pack-
age containing the PCM[36] algorithm. Calculation of the vi-
brational frequencies ensured that the obtained geometry is a real
minimum. Becke’s three-parameter hybrid exchange functional[42]
and the Lee–Yang–Parr nonlocal correlation functional[43]
(B3LYP) were used in combination with the 6-31G(d)[44] basis set
for the light elements (C, H, N, Cl) and the LanL2DZ[45] basis set
for the metal core. FMOs of interest were visualized with
GaussView 4.1.
(s) (C=N–), 1586 (s), 1468 (s), 1314 (s) (NO3 ), 1106 (m), 949 (s),
823 (s), 775 (s), 758 (s), 686 (s) cm–1. 1H NMR (CDCl3, 300 MHz,
25 °C): δ = 7.96 (d, J = 8 Hz, 2 H), 7.43 (t, J = 7 Hz, 2 H), 7.43–
7.19 (m, 14 H), 7.07–7.01 (m, 4 H), 6.83 (d, J = 7 Hz, 2 H) ppm.
[Ag(3)2(NO3)]: Dark-orange solid, overall yield 91% (0.45 mmol,
0.50 g). C52H28AgF12N5O3: calcd. C 56.44, H 2.55, N 6.33; found
C 56.37, H 2.56, N 6.24. IR (KBr): 3059 (w) (=C–H), 1670 (m)
–
(C=C), 1632 (s) (C=N–), 1590 (s), 1440 (s), 1384 (s) (NO3 ), 1328
(s), 1180 (s), 1118 (s), 1068 (s), 953 (m), 831 (s), 801 (s), 777 (s),
1
698 (s) cm–1. H NMR (CDCl3, 300 MHz, 25 °C): δ = 7.97 (d, J =
8 Hz, 2 H), 7.46–7.36 (m, J = 7 Hz, 8 H) 7.24 (s, 2 H), 6.62 (d, J
Supporting Information (see footnote on the first page of this arti-
= 7 Hz, 2 H) ppm. 13C{1H} NMR (CDCl3, 75 MHz, 25 °C): δ = cle): Electrochemical and energetic data of the ligands (Table S1),
161.8, 151.8, 150.8, 142.4, 131.3, 130.9, 130.8, 128.4, 127.4, 124.9,
123.4, 123.3, 122.5, 122.4, 115.5, 115.4 ppm.
Hammett and Brown constants of the substituents (Table S2), elec-
trochemical (Table S3) and energetic data of the silver compounds
(Table S4), Cartesian coordinates from DFT calculations
(Table S5), crystallographic data (Table S6), IR spectra (Fig-
ures S1–S6), 1H NMR and 13C NMR spectra (Figures S7–S16),
voltammograms (Figures S17–S22), correlation diagrams (Fig-
ures S23–S28), absorption spectra (Figure S29) in dichloromethane,
crystal packing information (Figures S30 and S31).
[Ag(4)2]NO3: Dark-red solid, overall yield 85% (0.42 mmol, 0.41 g).
C48H28AgCl4N5O3: calcd. C 59.28, H 2.90, N 7.20; found C 59.10,
H 2.98, N 7.06. IR (KBr): 3047 (w) (=C–H), 1677 (s) (C=C), 1635
–
(s) (C=N–), 1589 (m), 1483 (s), 1328 (s) (NO3 ), 1246 (m), 1094 (s),
844 (s), 831 (s), 814 (s), 778 (s) cm–1. 1H NMR (CDCl3, 300 MHz,
25 °C): δ = 7.96 (d, J = 8 Hz, 2 H), 7.43 (t, J = 7 Hz, 2 H), 7.32
(d, J = 7 Hz, 4 H), 7.02 (d, J = 7 Hz, 2 H), 6.90 (d, J = 7 Hz, 2 H)
ppm.
Acknowledgments
[Ag(5)2(NO3)]: Dark-red solid, overall yield 92% (0.46 mmol,
0.44 g). C52H40AgN5O7: calcd. C 65.41, H 4.22, N 7.34; found C
65.28, H 4.30, N 7.18. IR (KBr): 3050 (w) (=C–H), 2945 (–C–H),
1654 (s) (C=C), 1621 (s) (C=N–), 1600 (s), 1586 (m), 1503 (s), 1384
P. P. acknowledges support from the Centre for International Mo-
bility (CIMO) of Finland under grant TM-11-7670. A. Efimov and
V. Chukharev at Tampere University of Technology (TUT) are
kindly acknowledged for their support and technical assistance.
–
(s) (NO3 ), 1295 (m), 1239 (s), 1031 (s), 826 (s), 778 (s) cm–1. 1H
NMR (CDCl3, 300 MHz, 25 °C): δ = 3.91 (s, 3 H, MeO), 7.98 (d,
J = 8 Hz, 2 H), 7.45 (t, J = 7 Hz, 2 H), 7.14 (d, J = 7 Hz, 2 H),
7.05 (d, J = 7 Hz, 4 H), 6.92 (d, J = 7 Hz, 4 H) ppm. 13C{1H}
NMR (CDCl3, 75 MHz, 25 °C): δ = 160.9, 157.9, 142.5, 141.9,
131.05, 130.3, 128.00, 127.5, 124.6, 121.0, 114.8, 55.5 ppm.
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[Ag(6)2(NO3)]: Dark-red solid, overall yield 91% (0.45 mmol,
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Received: July 1, 2013
Published Online: ᭿
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Job/Unit: I30828
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Date: 12-08-13 16:47:25
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FULL PAPER
Ligand Substituent Effects
P. A. Papanikolaou,* M. Gdaniec,*
B. Wicher, P. D. Akrivos,
N. V. Tkachenko* ............................ 1–11
Six new AgI–bis(BIAN) complexes along
with their ligands were synthesized and
studied. Substituent groups offer tunability
of the electronic properties and structural
environment of the compounds. The versa-
tility on the metal coordination sphere op-
ens the way for the design of photoactive
species with a variety of applications.
Bis(aryl)acenaphthenequinonediimine Sub-
stituent Effect on the Properties and Coor-
dination Environment of Ligands and
Their Bis-Chelate AgI Complexes
Keywords: N ligands / Silver / Structure
elucidation / Electronic properties / Elec-
trochemistry
Eur. J. Inorg. Chem. 0000, 0–0
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