The interplay of metal and supporting ligand in labile coordination to pincer complexes of Ag(i)

Article abstract of DOI:10.1039/c2dt12112c

The bis(imino)pyridine scaffold provides support for the synthesis and characterization of unique Ag(i) pincer complexes [{ArNCPh}₂(NPh)] Ag⁺(OTf)^- (Ar = 2,5-^tBu₂C ₆H₃3; 2,6-ⁱPr₂C₆H ₃4). The bonding interactions between the cation-anion and between the bis(imino)pyridine ligand and the Ag centre are presented. Coordination of pyridine, toluene, 2-butyne and cyclooctene to the Ag centre led to the isolation and crystallographic characterization of labile transient adduct species. Bonding analysis of the adducts revealed conventional ligand-Ag coordination and important unconventional electron donation from the ligand to a π*-orbital of the bis(imino)pyridine group. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt12112c

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Volume 41 | Number 16 | 28 April 2012 | Pages 4701–5072
ISSN 1477-9226
COVER ARTICLE
Richeson et al.
The interplay of metal and supporting ligand in labile coordination to pincer
complexes of Ag(I)

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PAPER
The interplay of metal and supporting ligand in labile coordination to pincer
complexes of Ag(I)†
Titel Jurca, Sarah Ouanounou, Serge I. Gorelsky, Ilia Korobkov and Darrin S. Richeson*
Received 4th November 2011, Accepted 4th January 2012
DOI: 10.1039/c2dt12112c
The bis(imino)pyridine scaffold provides support for the synthesis and characterization of unique Ag(I)
pincer complexes [{ArNvCPh}2(NPh)]Ag+(OTf)− (Ar = 2,5-tBu2C6H3 3; 2,6-iPr2C6H3 4). The bonding
interactions between the cation–anion and between the bis(imino)pyridine ligand and the Ag centre are
presented. Coordination of pyridine, toluene, 2-butyne and cyclooctene to the Ag centre led to the
isolation and crystallographic characterization of labile transient adduct species. Bonding analysis of the
adducts revealed conventional ligand–Ag coordination and important unconventional electron donation
from the ligand to a π*-orbital of the bis(imino)pyridine group.
adduct formation. In particular, the coordination of a toluene
Introduction
molecule to the [{(2,5-tBu2C6H3)NvCPh}2(NC5H3)]Ag+ cation
Lability and reactivity are fundamental objectives in chemistry
that are in opposition to isolation and characterization of stable
species. The capture and analysis of weakly bonded complexes
has important implications on reactivity and can challenge and
enhance concepts of bonding and stability. Because such species
are fragile and vulnerable to transformation only some character-
ization tools may be applicable to their analysis, in contrast to
the examination of unreactive, stable compounds. Computations
can enhance the characterization and provide a springboard for
defining more subtle features of intermolecular interactions that
go beyond conventional electron transfer–donor bonds.
With the goal of discovering unprecedented bonding features
and concomitant reactivity, we were attracted to the bis(imino)
pyridine scaffold; a neutral ligand that presents three electron
pairs in a well-defined, planar, pincer array. The modular stereo-
electronic features and the relative ease of synthesis of these
ligands has inspired their application to a range of transition
metal ions.1 To avoid reactivity of the imino methyl groups we
replaced these moieties with phenyl groups and tuned the steric
demands of the N-aryl substituents by employing a range of sub-
stituted aryl groups.2,3 For example, the new ligands 1 and 2
was examined and the interaction was shown to involve an inter-
esting combination of conventional electron donation from the
toluene π-orbital to Ag with an unusual donation to an essen-
tially bis(imino)pyridine-based π*-orbital. This unconventional
bonding interaction points to the important role of the bis(imino)
pyridine ligand not only in stabilizing the pincer geometry of
Ag+ but in adduct formation.
The increasing profile of the complexes of the coinage metals
in catalysis and medicine encourage the exploration of ligand
environments that promote interesting and pertinent metal
coordination geometries for potential precatalysis species.4 For
example, a general feature of reactivity and transformations that
are promoted by metal complexes is the role of σ- and/or
π-Lewis acidity used for substrate activation. Herein we report
the vital and unanticipated role of a supporting ligand in the
labile coordination of a variety of pertinent ligands to an Ag
centre.
Results and discussion
afforded
a
series of low-valent,
In(I)
complexes
The first reported Ag complexes of the bis(imino)pyridine ligand
frame were prepared by the reaction of 1 and 2 with Ag
(O3SCF3) (eqn (1)), which provided 3 and 4 as bright yellow
crystalline solids.3a The 1H and 13C NMR spectra of these
materials were consistent with the proposed formulations. Broad-
ened resonances and the appearance of diastereotopic methyl
groups for the iPr groups in 4 suggested hindered rotation of the
Ar–Nimine bonds. The identities and structural details of these
compounds as bis(imino)pyridine complexes [{ArNvCPh}2
(NPh)]Ag+(OTf)−] (Ar = 2,5-tBu2C6H3 3; 2,6-iPr2C6H3 4) were
established by single crystal X-ray analysis.3a These Ag(I)
species are not only unique in possessing the bis(imino)pyridine
ligand, but the constrained geometry and coplanar tridentate
[{ArNvCPh}2(NC5H3)]In+(OTf)− where the In+ accepts little
covalent donor–acceptor interaction from the ligand.3 These
investigations were extended to the synthesis of analogous Ag+
complexes and yielded unique pincer complexes with an orbital
array and geometry that allows for interesting coordination and
Centre for Catalysis Research and Innovation and Department of
Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.
E-mail: darrin@uottawa.ca; Fax: +6135625170
†Electronic supplementary information (ESI) available: Infrared data on
compounds 3, 4 and 3a–d and 4a. Combined cif for structures 3a, 4a,
3c and 3d. CCDC 849370–849373. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt12112c
This journal is © The Royal Society of Chemistry 2012
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array provided by the ligand makes these compounds unusual
pincer complexes for silver.5–7
ð1Þ
A more intimate examination of the electronic features of
compounds 3 and 4 was obtained through a density functional
theory (DFT) computational study using the B3LYP functional
and the mixed DZVP–TZVP basis set.3a The electronic inter-
action energy between the Ag[bis(imino)pyridine]+ (LAg+)
cation and the OTf− anion was determined to be similar for 3
and 4 with values of −85.7 and −84.5 kcal mol−1, respectively.
Furthermore, these compounds displayed similar cation–anion
bond orders of 0.31–0.32, which corresponded to approximately
0.18–0.19 electrons transferred from the triflate to the LAg+ frag-
ment. Two unoccupied acceptor orbitals on the LAg+ fragment
(LUFO and LUFO + 2) participated in covalent bonding with
the OTf− anion. The Ag character of these fragment orbitals
(FOs) was dominated by the empty Ag 5s orbital.
Fig. 1 The four most significant donor orbitals of the pincer ligand in
3. The energy of each orbital is provided with the label.
These computational results also provided details for the
bonding interaction between the bis(imino)pyridine ligand and
the silver centre within the LAg+ cation, and revealed that
greater than 50% of the electronic interaction energy (average
energy = −88.4 kcal mol−1) is due to covalent bonding between
the metal ion and the ligand. In both compounds the bis(imino)
pyridine ligands donate 0.4 electrons to the Ag+ ion. This
donation comes primarily from the four highest occupied orbitals
of the ligand that are shown in Fig. 1. Four orbitals on the Ag+
cation fragments accept this electron density: LUFO–LUFO +
3. The major (60%) contribution to covalent bonding is between
the LUFO of the Ag+ centre, with 100% 5s character, and the
HOFO-6 ligand orbital. The three remaining unoccupied accep-
tor orbitals on the Ag cation are of 5p character; two of these
orbitals (LUFO + 1 and LUFO + 2) lie in the ligand plane and
accept σ-type electron density from the ligand donor orbitals
HOFO and the HOFO-5/HOFO-1 combination respectively. The
remaining metal acceptor orbital, LUFO + 3, is the 5p Ag orbital
oriented perpendicular to the ligand plane and is of π-symmetry
and accepts very little electron density from the bis(imino)pyri-
dine ligand.
microanalysis led to results consistent with the loss of the coordi-
nating species (i.e. pyridine, toluene, 2-butyne, cyclooctene).
When 3 and 4 are recrystallized from pyridine, single crystals
of the pyridine adducts 3a and 4a were repeatedly obtained,
which were analysed by single crystal X-ray diffraction. The
results are summarized in Fig. 2 and 3 and Table 1. Both com-
pounds display LAg(pyridine)+ cations with an LAg+ fragment
in a pincer geometry of a planar tridentate bis(imino)pyridine
ligand array.
The ligand array and geometry of cationic complexes 3 and 4
provide an interesting set of compounds to investigate the nature
of interactions with common Lewis base ligands. The reactivity
of the open face of compounds 3 and 4 with σ and π ligands was
probed by the reactions shown in Scheme 1. The addition of pyr-
idine, toluene, 2-butyne or cyclooctene to the LAg+ centres of 3
or 4 led to weak coordination of these species to the open site of
the cation species. In fact, the LAg+–donor interactions produce
labile species that could be captured and examined by single
crystal X-ray analysis but these adducts did not exhibit distinct
NMR signals, indicating that the coordinating species is released
when compounds 3a–d and 4a were dissolved in solution. Fur-
thermore, preparation of samples of compounds 3a–d and 4a for
Scheme 1
4766 | Dalton Trans., 2012, 41, 4765–4771
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Table 1 Selected bond lengths (Å) for 3, 4, 3a–d and 4a
Bis(imino)pyridine
ligand
Coordinated ligand
Triflate
Ag–
Ag–
Ag–
Ag–
Ag–
Ag–
Ag–
Npy
Nimine
OTf
N(4)
Carene
Calkyne
Colefin
33 2.270(9) 2.523(7) 2.331(8)
2.526(6) 2.638(6)
43 2.347(6) 2.492(6)
2.638(2) 3.04
3a 2.255(2) 2.537(3)
2.556(2) 2.66
2.187(3)
2.212(2)
4a 2.298(2) 2.559(2)
2.556(3) 6.94
2.447(4)
2.464(4)
3b3 2.287(2) 2.581(3)
2.596(2)
Fig. 2 Structure of compound 3a with the hydrogen atoms and triflate
counterion omitted for clarity.
2.373(3)
2.369(3)
3c 2.276(2) 2.703(2) 2.95
2.622(4) 17.02
2.418(5)
2.360(6)
3d 2.296(4) 2.503(4)
Fig. 3 Structure of compound 4a with the hydrogen atoms and triflate
counterion omitted for clarity.
The bonding parameters within the [bis(imino)pyridine]Ag+
fragments of 3a and 4a are very similar to the parent species.
The coordinated pyridine fragment lies in the open face of the
LAg+ moiety to yield a distorted square planar metal centre
within the limits due to the restrictions of the bis(imino)pyridine
ligand. Interestingly, the plane of the coordinated pyridine
ligands in these two compounds is nearly co-planar/coincides
with that of the bis(imino)pyridine ligands. In the case of 3a, the
angle between the planes defined by the pyridyl group of the bis
(imino)pyridine ligand and the coordinated pyridine ligand was
10.2(1)° with an analogous angle for 4a of 25.9(1)°. This dispo-
sition appears to be a result of the steric pocket provided by the
NAr′ groups. In both compounds, the triflate anion is more
removed than in the parent compounds. The shortest Ag–triflate
contact for 3a was Ag(1)–O(2) at >3.0 Å and in 4a it was Ag
(1)–O(1) at ≈2.7 Å. The lability of the coordinated pyridine in
compounds 3a and 4a was demonstrated during efforts to obtain
elemental analysis, whereby loss of pyridine adduct was evident
even in samples subjected to minimal sample preparation/hand-
ling prior to combustion.
Fig. 4 The structure of compound 3b, with the hydrogen atoms and
triflate counterion omitted for clarity.
6.3 Å and the silver centre bonds symmetrically to the m- and
p-carbons of the toluene molecule. The distance between the Ag
centre and the mean aromatic plane of 2.279 Å and Kochi’s geo-
metric criteria gave a hapticity for 3b of η = 2.02.9 Again, while
the formation of crystals of 3b was reproducible, attempts to
obtain microanalysis of this species was hindered by the loss of
toluene during sample isolation and transport.
Similarly, the alkyne adduct of 3 was obtained when this
silver complex was crystallized in excess 2-butyne. The results
of a single crystal X-ray analysis of this new compound 3c are
presented in summary form in Fig. 5 and Table 1. As with the
pyridine and toluene adducts, 3a and 3b, the alkyne complex,
3c, displayed a silver centre coordinated by the tridentate pincer
ligand. Overall the Ag–N distances in the LAg+ fragment (Ag–
Npy and Ag–Nimine) are similar to those observed for 3 and
3a–b. Similarly, the Ag–OTf distance is increased relative to the
starting material 3. The 2-butyne is coordinated symmetrically to
the Ag centre with Ag–C distances of 2.373(3) and 2.369(3) Å.
This coordination only very slightly lengthens the CuC bond
distance with the observed value of 1.186(4) Å vs. the reported
value of 1.1160 Å.10 In addition, the alkyne moiety exhibits only
a minor deviation from linearity with a C(49)–C(50)–C(51)
angle of 169.14(3)°. Overall, these features suggest little, if any,
There is a growing literature reporting the formation of stable
crystalline Ag–arene π-complexes with the arene bonded in the
η2 or η3 mode.8 As previously reported, when 3 was crystallized
from toluene, crystals of 3b were isolated that demonstrate the
Lewis acidity of the LAg+ pincer cation (Fig. 4).3a The coordi-
nation of toluene has displaced the OTf− anion to a distance >
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Fig. 5 Structure of compound 3c, with the hydrogen atoms and triflate
Fig. 6 Structure of compound 3d, with hydrogen atoms and triflate
counterion omitted for clarity.
counterion omitted for clarity.
back donation from the metal centre to the 2-butyne. The alkyne
is coordinated along the same vector as was observed for the
pyridine coordination in 3a and, like that compound, the C(50)–
Ag(1)–C(49) plane approaches a co-planar orientation with that
of the bis(imino)pyridine ligand with an observed angle between
pyridine ring and alkyne plane of 31.8(5)°.
This compound can be compared to both trigonal planar and
tetrahedrally coordinated Ag–alkyne complexes. The Ag–Calkyne
distances in 3c are longer than those observed in the trigonal
planar hexyne adduct of a triazapentadienyl Ag complex with
Ag–C distances of 2.233 and 2.241 Å11 as well as those in Ag
(hfac)(Me3SiCCSiMe3) (hfac−v(CF3COCHCOCF3)−) with Ag–
Calkyne distances of 2.255(4) and 2.267(4) Å.12 A trispyrazolyl
borate complex of Ag(I) is coordinated by both acetylene (Ag–
Calkyne = 2.293(4) Å) and phenylacetylene (Ag–Calkyne = 2.263
(5), 2.407(5) Å) to yield a tetrahedral species.13
The combination of our experimental observations on the
nature of compounds 3a–d and the analysis of the crystallo-
graphic features of these compounds indicated that they exhibit
weak interactions between the LAg+ cation and the ligands. In
order to quantify the degree and details for the interactions
between the [bis(imino)pyridine]Ag+ cation and the coordinated
moieties we examined the interaction of these fragments using
computations with the resulting data summarized in Table 2. Fre-
quency analysis on the optimized structures confirmed that they
were minima and provided the data for the calculation of free
energy of adduct formation (ΔGf) and sum of electronic and
zero-point energies, ΔE, according to eqn (2).
ð2Þ
We explored the coordination of alkenes to the LAg+ fragment
by crystallizing compound 3 in excess cyclooctene. Single
crystal X-ray analysis revealed this species to be compound 3d,
[LAg+(cyclooctene)][OTf−], with the results presented in Fig. 6
and Table 1. The triflate anion in 3d is well separated from the
cation with the shortest contact, Ag1–O3, greater than 17 Å.
Similar to compounds 3 and 3a–c, the bis(imino)pyridine
ligand provides a planar pincer coordination with a typical Ag
(1)–N(2) distance (2.296(4) Å) and only slightly asymmetrically
to the Nimine sites of the ligand. The Ag–Colefin distances (2.418
(5), 2.360(6) Å) are slightly longer than those of the four coordi-
nate species [HB(3,5-(CF3)2Pz)3]Ag-(H2CCH2) (Ag–C = 2.294
(7) and 2.307(7) Å).13 The cyclooctene adduct, 3d, exhibited a
CvC distance (C(48)–C(49) 1.361(8) Å) slightly elongated
from the free cyclooctene value of 1.22(4) Å.14 Like the pyridine
and the alkyne adducts, the olefin displays a rather small angle
with the bis(imino)pyridine ligand plane; the angle between the
pyridine ring and olefin–Ag fragment is 27.8 (0.4)°.
The LAg+–pyridine interactions in compound 4a were exam-
ined with a DFT computational study using the B3LYP func-
tional and the mixed DZVP/TZVP basis set. Full optimization of
this compound yielded a structure in accord with the cation in 4a
including the features of the Ag–pyridine bonding. Our analysis
indicated that the electronic interaction energy between the two
fragments was −23.9 kcal mol−1 with donation of only 0.11
electrons from the N-based lone pair of the pyridine (e.g. 5.5%
of the population of the HOFO). This corresponded to a bond
order of 0.25 between the fragments. The two LAg+ acceptor
orbitals, LUFO and LUFO + 2 (Fig. 7), receive nearly equal
components of this electron donation (2.3 and 3.9% respect-
ively). In terms of Ag contribution, both of these orbitals consist
mainly of the empty Ag 5s orbital with the LUFO being 25% 5s
and 6% 5p and the LUFO + 2 being 53% 5s and 7% 5p. Signifi-
cantly, almost 70% of the LUFO is actually based on the bis
(imino)pyridine ligand. Furthermore, even the LUFO + 2 is
about 40% bis(imino)pyridine in character. Although the
Table 2 Summary of computational results on compounds 4a, 3a–d
Ligand, L′
Eint (kcal mol−1
)
Bond order
Charge donation (electrons)
ΔGf (kcal mol−1
)
ΔE (kcal mol−1
)
4a
Pyridine
Toluene
2-Butyne
Cyclooctene
−23.9
−12.9
−17.1
−17.6
0.25
0.44
0.53
0.62
0.11
0.19
0.28
0.26
−6.2
1.2
1.6
−18.9
−9.4
3b3a
3c
−9.6
3d
1.2
−11.0
4768 | Dalton Trans., 2012, 41, 4765–4771
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toluene is a key component in the adduct formation. The nearly
zero value of the ΔGf is, once again, consistent with the labile
nature of this interaction.
The 2-butyne ligand interacts more strongly with the LAg+
cation of 3c than the toluene in adduct 3b as evidenced by a
greater electronic interaction energy between the LAg+ cation
and the alkyne of −17.1 kcal mol−1 as well as a higher bond
order (b.o. = 0.53) and a greater degree of charge transfer from
the alkyne to LAg+ of 0.23 electrons. The only significant inter-
action is σ in nature and is donation of 8.9% of the electronic
population from the alkyne HOFO (π bonding) to Ag. There was
no significant back donation contribution to the Ag–alkyne inter-
action (i.e. into the LUFO of the alkyne) a feature consistent
with the crystallographic data and supported by the observed
lability of this adduct. In this case the donation from the alkyne
moiety is distributed between the LUFO (3%) and LUFO + 2
(5.9%) (see Fig. 7). Once again, adduct bonding involves accep-
tor orbitals that have significant bis(imino)pyridine ligand-based
components. For example, the LUFO is only 20% Ag-centred
with 80% bis(imino)pyridine character while the LUFO + 2 is
68% on Ag 32% on the tridentate ligand. Consistent with the
previous observations, the Ag contribution is dominated by the
5s orbital (i.e. 65.4% 5s for LUFO + 2).
The degree and nature of the interaction between the LAg+
cation and olefin in 3d is similar to that observed in the alkyne
complex 3c. Specifically, the interaction energy between the
fragments was determined to be −17.6 kcal mol−1 with a bond
order of 0.62 and corresponding charge donation of 0.26 elec-
trons. In this case, the olefin contributes 10.8% of the electron
density in the HOFO (olefin π) through a σ interaction with the
two cation acceptor orbitals: the LUFO (4.7%) and LUFO + 2
(6.3%). The nature of these two orbitals was as observed for the
other adducts, with the LUFO being largely bis(imino)pyridine
in nature (71 vs. 29% Ag) and the LUFO + 2 being 66% Ag
with a substantial contribution (33%) from the supporting bis
(imino)pyridine ligand.
Fig. 7 FOs for the optimized cation fragment, [{(2,5-tBu2C6H3)
NvCPh}2(NC5H3)]Ag+. These are the FOs with the largest electron
acceptor contributions to bonding in 4a, and 3a–d, as discussed in the
text.
donation of electron density from a coordinated pyridine to the
Ag 5s orbital is as expected, the important role of the supporting
ligand in this bonding, revealed in this analysis, is unexpected.
The small value for the free energy of formation of this adduct is
in agreement with our observations that the pyridine ligand is
only weakly bound to the Ag centre.
A similar analysis of the bonding interactions between the
LAg+ cation and the toluene moiety was carried out. In order to
facilitate the computations, the LAg(toluene)+ cation in 3b was
modelled by replacing the 2,5-tBu2C6H3 groups with simple
phenyl rings. Using the B3LYP functional and the mixed DZVP/
TZVP basis set on this model yielded a structure in agreement
with the cation in 3b. The electronic interaction energy between
the [bis(imino)pyridine]Ag+ and the toluene fragments was
determined to be −12.9 kcal mol−1 with a corresponding bond
order of 0.44 and a charge transfer of 0.19 electrons from
toluene to the silver fragment. The toluene–Ag interaction is σ in
nature and involves the π-orbitals on toluene and LAg+ FOs
shown in Fig. 7. As was observed with compound 4a, the silver
fragment acceptor orbitals LUFO and LUFO + 2 are the major
recipients of electron donation from the toluene HOFO (4.4%)
and HOFO-4 (2.5%).3a Again, the LUFO + 2 is predominantly
an Ag orbital. Again, it was both interesting and unexpected that
the other acceptor orbital was the bis(imino)pyridine-based π*-
orbital. This unusual bonding points to the important role of the
bis(imino)pyridine ligand not only in stabilizing the pincer geo-
metry of Ag+ but in adduct formation. The interaction between
the supporting bis(imino)pyridine ligand and the coordinated
Conclusions
The bis(imino)pyridine ligand scaffold does more than provide a
supporting framework for unique pincer complexes of Ag. In
the complexes [{ArNvCPh}2(NPh)]Ag+(OTf)−] (Ar
=
2,5-tBu2C6H3 3; 2,6-iPr2C6H3 4), important charge transfer–
bonding interactions between the triflate anion and the ligand are
a significant component in the stability of these species. Further-
more, we have shown that the interplay of the bis(imino)pyridine
ligand with weakly coordinating ligands, ranging from pyridine
to arene, alkyne and olefin species, is a significant general
feature with impact on geometry and complex formation. In
addition to the conventional electron donation from the ligand to
the metal centre, a remarkable contribution to the bonding in
these species is an unusual donation to an essentially bis(imino)
pyridine-based π*-orbital. This unconventional bonding inter-
action involving a non-innocent role of the bis(imino)pyridine
ligand in adduct formation may have broader implications on the
application of these and other transient species to organic trans-
formations. Our continuing efforts are focused on expanding on
the reactivity of pincer Ag complexes and on elaborating our
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understanding of features that favour labile bonding and its rel-
evance to chemical transformations.
AgOSO2CF3, and allowed to stir for 14 hours. During this time
the reaction gradually became opaque yellow. The mixture was
cooled to −20 °C and a bright yellow precipitate formed. The
solid was removed by filtration, washed with 5 × 2 mL hexanes,
and allowed to dry under vacuum. A bright yellow powder was
isolated in 84% yield. Large yellow cubic crystals suitable for
single-crystal X-ray analysis were grown in a saturated solution
Experimental
General methods
1
of toluene : hexanes (1 : 1) that was cooled to −20 °C. H NMR
All reactions were performed in a glovebox with a nitrogen
atmosphere, with the exception of ligand synthesis, which was
performed using standard Schlenk technique under a flow of N2.
All solvents were sparged with nitrogen and then dried by
passage through a column of activated alumina using an appar-
atus purchased from Anhydrous Engineering. Deuterated chloro-
form was dried using activated molecular sieves. All other
chemicals were purchased from Aldrich and used without further
purification. Compounds 1 and 2 were synthesized according to
literature procedure.15 NMR spectra were run on a Bruker
Avance 300 MHz spectrometer with CDCl3 as solvent and
internal standard. Elemental analyses were performed by
Midwest Microlab LLC, Indianapolis, IN.
(CDCl3, 300 MHz): δ 7.98 (t, 1H, py, p-CH), 7.79 (d, 2H, py, m-
CH), 7.42–6.95 (br m, 16H, aromatic), 2.92 (br m, 4H, iPr–CH),
1.21 (br d, 12H, CH3), 0.91 (br d, 12H, CH3). 13C NMR
(CDCl3). δ 165.6 (CvN imine), 152.3 (py, o-CvN), 144.5 (py,
m-CH), 139.3 (py, p-CH), 136.7 (Ph, m-CH), 133.2 (Ph, o-CH),
130.7 (Ar–iPr, C–iPr), 129.4 (Ar–iPr, CH), 129.3 (Ph, i-C),
128.4 (Ph, p-CH), 125.8 (Ar–iPr, C–iPr), 123.6 (Ar–iPr, CH),
28.7 (Ar–iPr,CH3), 24.6 (Ar–iPr, CH–(CH3)2), 22.6 (Ar–iPr,
CH3). A sample for elemental analysis was obtained by recrystal-
lization from toluene, resulting in a 1 : 1 toluene adduct of 4.
Calculated (%) for [C44H47AgF3N3O3S][C7H8]: C 64.15, H
5.81, N 4.40, found C 64.83, H 5.78, N 4.19.
Formation of adducts 3a–d, 4a
Synthesis of [Ag-2,6-bis{1-[(2,5-ditertbutylphenyl)imino]-benzyl}
pyridine][SO3CF3] (3)
Adducts 3a–d and 4a were prepared by dissolution of either 3 or
4 in a neat solution of the appropriate ligand species (∼15 mg of
Ag complex in 1.5 mL of solvent). The solutions were allowed
to stir for 1 hr and then filtered through a Celite plug into a small
vial. This vial was left uncovered and placed within a larger vial
with hexanes. The large vial was sealed and placed in the freezer
at −30 °C for several d. During this time some hexane diffused
into the smaller, internal vial and crystals suitable for single-
crystal X-ray crystallography formed. All of these species pro-
duced crystals that were bright yellow and these were either
plates (3a–c) or blocks (3d, 4a).
AgOSO2CF3 powder (38 mg, 0.150 mmol) was added to a clear
yellow solution of 1 (100 mg, 0.151 mmol) in 8 mL of toluene.
The reaction mixture was sealed and wrapped in aluminium foil,
due to the light sensitivity of AgOSO2CF3, and allowed to stir
for 14 hours. During this time the reaction gradually became
opaque yellow. The mixture was cooled to −20 °C and a bright
yellow precipitate formed. The solid was removed by filtration,
washed with 5 × 2 mL hexanes, and allowed to dry under
vacuum. A bright yellow powder was isolated in 69% yield.
Large yellow cubic crystals suitable for single-crystal X-ray
analysis were grown by diffusion of hexanes into a saturated
CDCl3 solution with cooling to −20 °C. 1H NMR (CDCl3,
300 MHz): δ 7.89 (br t, 1 H, py, p-CH), 7.60 (br d, 2 H, py,
m-CH), 7.40–7.10 (br m, 12 H, aromatic), 6.96 (br d, 1H, aro-
matic), 6.93 (br d, 1 H, aromatic), 6.53 (br d, 2 H, aromatic),
Crystal structure determination of complexes 3a, 4a, 3c, 3d
Adducts 3a–d and 4a were isolated from neat solution of either
compound 3 or 4 in compounds a–d respectively (∼15 mg of
Ag complex in 1.5 mL of solvent). Vials were sealed and placed
in the freezer at −30 °C for several d while crystals suitable for
single-crystal X-ray crystallography formed. All crystals were
bright yellow and were either plate (3a, 3b, 3c) or block like
(3d, 4a). Crystals were mounted in inert oil and transferred to
the cold stream of the diffractometer for data collection.
1.49 (br s, 18H, Bu), 0.96 (br s, 18H, Bu). 13C NMR (CDCl3,
75 MHz). δ 165.6 (CvN imine), 152.7 (py, o-CvN), 149.9
(Ar–CH), 148.3 (Ar–CH), 138.9 (Ar, i-C), 138.1 (Ar–CH),
134.3 (Ar, i-C), 129.8 (Ar–CH), 129.3 (Ar–CH), 128.3 (Ar–
CH), 126.1 (Ar–CH), 125.0 (Ar–CH), 121.9 (Ar–tBu, C–tBu),
120.7 (Ar–tBu, C–tBu), 35.5 (Ar–tBu, C– (CH3)3), 34.2 (Ar–tBu,
C– (CH3)3), 31.2 (Ar–tBu, CH3), 30.9 (Ar–tBu, CH3). A sample
for elemental analysis was obtained by recrystallization from
toluene, resulting in a 1 : 1 toluene adduct of 3. Calculated (%)
for [C48H55AgF3N3O3S][C7H8]: C 65.34, H 6.28, N 4.16, found
C 64.92, H 5.97, N 3.90.
t
t
Crystal data C53H60AgF3N4O3S (3a). Mr = 997.98, triclinic, a
= 11.1402(4), b = 15.4794(5), c = 15.6244(6) Å, α = 98.249(3)°,
β = 110.516(2)°, γ = 91.383(2)°, V = 2489.27(15) Å3, T =
ˉ
200 K, space group P1, Z = 2, reflections collected/unique
40 097/8451 (Rint = 0.0689). The final wR2 was 0.0942 (all
data).
Crystal data C44H47AgF3N3O3S(C5H5N)(C4H6)0.35 (4a). Mr =
960.81, monoclinic, a = 14.3389(16), b = 18.947(2), c = 18.189
(2) Å, β = 98.286(5)°, V = 4890.0(10) Å3, T = 200 K, space
group P21/c, Z = 4, reflections collected/unique 99 977/12 077
(Rint = 0.0213). The final wR2 was 0.1183 (all data).
Crystal data C60H76AgF3N3O3S (3d). Mr = 1084.17, ortho-
rhombic, a = 31.0722(11), b = 9.7664(4), c = 19.8728(7) Å, V =
Synthesis of [Ag-2,6-bis{1-[(2,6-diisopropylphenyl)imino]-
benzyl}pyridine][SO3CF3] (4)
AgOSO2CF3 powder (42 mg, 0.164 mmol) was added to a clear
yellow solution of 2 (100 mg, 0.165 mmol) in 8 mL of hexanes :
toluene (5 : 1) solution. The reaction mixture was sealed,
wrapped in aluminium foil due to the light sensitivity of
4770 | Dalton Trans., 2012, 41, 4765–4771
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6030.7(4) Å3, T = 200 K, space group Pca21, Z = 4, reflections
collected/unique 91 642/7330 (Rint = 0.0584). The final wR2 was
0.1277 (all data).
Crystal data C48H55AgF3N3O3S(C4H6)1.5 (3c). Mr = 1000.01,
monoclinic, a = 32.106(3), b = 20.658(2), c = 19.954(2) Å, β =
118.949(5)°, V = 11 581(2) Å3, T = 200 K, space group C2/c,
Z = 8, reflections collected/unique 94 508/14 448 (Rint = 0.0382).
The final wR2 was 0.1600 (all data).
Rev., 2008, 108, 3379; (c) Z. Li, D. A. Capretto, R. Rahaman and C. He,
Angew. Chem., Int. Ed., 2007, 46, 5184.
5 Recent minireview of pyridine-derived tridentate pioncer ligands.
J. I. van der Vlugt and J. N. H. Reek, Angew. Chem., Int. Ed., 2009, 48,
8832.
6 Anionic and neutral pincer ligands of Ag(I) and Au(I) are reported but
they do not display pincer geometries. For Ag(I) examples see:
(a) A. Fraix, M. Lutz, A. L. Spek, R. J. M. Klein Gebbink, G. van Koten,
J.-Y. Salaün and P.-A. Jaffrè, Dalton Trans., 2010, 39, 2942; (b) J. I. van
der Vlugt, M. A. Siegler, M. Janssen, D. Vogt and A. L. Spek, Organo-
metallics, 2009, 28, 7025; (c) J. C. DeMott, F. Basuli, U. J. Kilgore,
B. M. Foxman, J. C. Huffman, O. V. Ozerov and D. J. Mindiola, Inorg.
Chem., 2007, 46, 6271; (d) F. Camso, M. Camalli, H. Rimml and
L. M. Venanzi, Inorg. Chem., 1995, 34, 673; For examples of Au(I) see:
(e) M. Contel, M. Stol, M. A. Casado, G. P. M. van Klink, D. D. Ellis, A.
L. Spek and G. van Koten, Organometallics, 2002, 21, 4556;
(f) M. Contel, D. Nobel, A. L. Spek and G. van Koten, Organometallics,
2000, 19, 3288; (g) S.-J. Shieh, X. Hong, S.-M. Peng and C.-M. Che,
J. Chem. Soc., Dalton Trans., 1994, 3067.
7 Two bis(pyrazolyl)pyridine Ag complexes with pincer geometries are
reported. Y. Zhou, W. Chen and D. Wang, Dalton Trans., 2008, 1444.
8 P. Pérez-Galán, N. Delpont, E. Herrero-Gómez, F. Maseras and
A. M. Echavarren, Chem.–Eur. J., 2010, 16, 5324 and references therein.
9 A. V. Vasilyev, S. V. Lindeman and J. K. Kochi, Chem. Commun., 2001,
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Computational details
DFT calculations have been performed using the Gaussian 03
package.16 Wave function stability calculations were performed
to confirm that the calculated wave functions corresponded to
the electronic ground state. The structures of all species were
optimized using the B3LYP exchange-correlation (XC) func-
tional with the mixed basis set (DZVP on Ag and TZVP on all
other atoms). Tight SCF convergence criteria (10−8 a.u.) were
used for all calculations. Harmonic frequency calculations with
the analytic evaluation of force gradients were used to determine
the nature of the stationary points. The analysis of the molecular
orbital (MO) compositions in terms of occupied and unoccupied
orbitals of the fragment species (HOFOs and LUFOs, respect-
ively), the construction of the MO diagram and Mayer bond
orders were calculated using the AOMix program.17,18 Atomic
charges and Wiberg bond orders in the natural atomic orbital
basis were evaluated by using the natural population analysis.19
Frequency calculations of the optimized structures provided
zero-point corrections and confirmed that the structures represent
energy minima.
10 NIST Computational Chemistry Comparison and Benchmark Database
NIST Standard Reference Database Number 101 Release 15b, August
2011, Editor: Russell D. Johnson III, http://cccbdb.nist.gov/.
11 H. V. R. Dias, J. A. Flores, J. Wu and P. Kroll, J. Am. Chem. Soc., 2009,
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12 W. J. Evans, D. G. Giarikos, D. Josell and J. W. Ziller, Inorg. Chem.,
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13 H. V. R. Dias, Z. Wang and W. Jin, Inorg. Chem., 1997, 36, 6205.
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15 T. Jurca, K. Dawson, I. Mallov, T. Burchell, G. P. A. Yap and
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16 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
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H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov,
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H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro,
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R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
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G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
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Notes and references
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3 (a) T. Jurca, S. I. Gorelsky, I. Korobkov and D. S. Richeson, Dalton
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S. I. Gorelsky and D. S. Richeson, Inorg. Chem., 2010, 49, 10635;
(c) T. Jurca, K. Dawson, I. Mallov, T. Burchell, G. P. A. Yap and
D. S. Richeson, Dalton Trans., 2010, 39, 1266; (d) T. Jurca, J. Lummis,
T. J. Burchell, S. I. Gorlesky and D. S. Richeson, J. Am. Chem. Soc.,
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18 S. I. Gorelsky and A. B. P. Lever, J. Organomet. Chem., 2001, 635, 187–
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19 A. E. Reed, R. B. Weinstock and F. Weinhold, J. Chem. Phys., 1985, 83,
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4 Selected recent examples: (a) P. Belmont and E. Parker, Eur. J. Org.
Chem., 2009, 6075; (b) M. Mar Díaz-Requejo and P. J. Pérez, Chem.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 4765–4771 | 4771