Organometallic Reactivity of [Silver(I)(Pyridine-Containing Ligand)] Complexes Relevant to Catalysis

Article abstract of DOI:10.1002/ejic.201500771

Silver(I) complexes of pyiridine-containing macrocyclic ligands (Pc-L) have already been demonstrated as active catalysts for some domino and multicomponent reactions. Here, we report new chiral [Ag^I(Pc-L)] cationic complexes that have been synthesized and fully characterized, including structural determination by single-crystal X-ray diffraction. The complexes show a rich coordination chemistry, demonstrating both the σ-philic (alcohol and nitrile coordination) and the π-philic (alkyne coordination) nature of silver. The η² coordination mode of the naphthyl pendant arm of the ligands on silver has been observed in solution by NMR spectroscopic experiments. 2D NMR spectroscopy revealed the presence of positive cross peaks resulting from rotational processes and the rate of rotation was measured by using 2D exchange spectroscopy (EXSY).

Full text of DOI:10.1002/ejic.201500771

FULL PAPER
DOI:10.1002/ejic.201500771
Organometallic Reactivity of [Silver(I)(Pyridine-
Containing Ligand)] Complexes Relevant to Catalysis
Tommaso Pedrazzini,[a] Paolo Pirovano,[a][‡] Monica Dell’Acqua,[b]
Fabio Ragaini,[a] Pasquale Illiano,[a] Piero Macchi,[c]
Giorgio Abbiati,*[b] and Alessandro Caselli*[a]
Keywords: Silver / N ligands / Macrocyclic ligands / Structure elucidation / Pyridine-containing ligands
Silver(I) complexes of pyiridine-containing macrocyclic li-
gands (Pc-L) have already been demonstrated as active cata-
lysts for some domino and multicomponent reactions. Here,
we report new chiral [AgI(Pc-L*)] cationic complexes that
have been synthesized and fully characterized, including
structural determination by single-crystal X-ray diffraction.
and the π-philic (alkyne coordination) nature of silver. The
η2 coordination mode of the naphthyl pendant arm of the li-
gands on silver has been observed in solution by NMR spec-
troscopic experiments. 2D NMR spectroscopy revealed the
presence of positive cross peaks resulting from rotational
processes and the rate of rotation was measured by using 2D
The complexes show a rich coordination chemistry, demon- exchange spectroscopy (EXSY).
strating both the σ-philic (alcohol and nitrile coordination)
chlorate and acetone,[5] “synthetic” macrocycles have com-
Introduction
peted with the “natural” ones.[6] Macrocyclic chemistry has
focused on the synthesis of species inspired by the naturally
occurring systems, which mainly contain nitrogen donor
atoms.[7] In general, polyaza macrocycles form extremely
stable complexes with transition metals of the later transi-
tion series compared with the polyoxa macrocycles.[8]
Cyclam (1,4,8,11-tetraazacyclotetradecane) and related un-
saturated and homologous systems have been the most
studied examples.[9]
Macrocyclic ligands have attracted widespread attention,
especially because of two of their unique properties:[1] the
macrocyclic and the ring size effects. The first was initially
described in 1969 in studies on the CuII complexes of some
tetraaza macrocycles.[2] The macrocyclic complexes benefit
from extra stability with respect to the common chelate ef-
fect.[3] Compared with the acyclic ones, macrocyclic ligands
also have the advantage of being conformationally pre-or-
ganized and, as a consequence, they are able to discriminate
among closely related metal ions based on the metal ion
radius (ring size effect).[4] Some nitrogen-containing macro-
cyclic molecules are also naturally occurring species that
play a vital role in biological system, such as porphrins,
corrins, and chlorins. Since the report by N. F. Curtis in
1960, in which the first macrocyclic complex was synthe-
sized by the reaction of tris-ethylenediamine nickel(II) per-
The introduction of a pyridine moiety into the skeleton
of a tetraaza-macrocycle ligand has a strong influence on
both the complexation kinetics and the thermodynamic
properties by increasing the conformational rigidity and by
changing its basicity.[10] In past years, our attention has
turned to the development of synthetic pathways that have
allowed us to obtain a new class of tetraaza-macrocyclic
ligands containing pyridine.[11] Our efforts were rewarded
in 2008 when we published the synthesis of some pyridine-
based 12-membered tetraaza-macrocyclic ligands, named
pyridine-containing ligands (Pc-L).[12] The synthetic meth-
odology employed is simple and allows us to differently
[a] Università degli Studi di Milano and ISTM-CNR,
Via Golgi 19, 20133 Milano, Italy
E-mail: alessandro.caselli@unimi.it
http://users.unimi.it/acaselli/
[b] Dipartimento di Scienze Farmaceutiche, Sezione di Chimica
Generale e Organica “A. Marchesini”, Università degli Studi di functionalize the macrocyclic framework to increase the
Milano,
molecular diversity and create different stereocenters on the
Via Venezian 21, 20133 Milano, Italy
backbone.[13] The copper(I) complexes of these Pc-L* li-
E-mail: giorgio.abbiati@unimi.it
http://users.unimi.it/istchimorg/giorgio.htm
[c] Department of Chemistry & Biochemistry, University of Bern,
Freiestrasse 3, 3012 Bern, Switzerland
gands have been fully characterized and successfully em-
ployed as catalysts in the Henry reaction[14] and in the enan-
tioselective cyclopropanation of alkenes.[15] Compared with
the more extensively studied copper and gold catalysis, re-
ports on the catalytic activity of silver complexes are rela-
tively sparse.[16] We have recently demonstrated that the sil-
[‡] Present Address: School of Chemistry, The University of
Dublin, Trinity College,
College Green, Dublin 2, Ireland
Supporting Information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500771.
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1
ver(I) complexes of achiral Pc-L ligands are effective cata- 2 have been fully characterized by H and 13C NMR spec-
lysts in the regiospecific synthesis of 1-alkoxy-isochromenes troscopy, ESI-MS, and IR and UV/Vis spectroscopy.
under mild reaction conditions.[17] The same silver(I) cata-
All attempts to synthesize a complex with chloride as the
lysts have shown a good catalytic activity in the microwave- counterion met with failure. In fact, when we conducted the
enhanced synthesis of propargylamines starting from alde- reaction between ligand 1a and silver chloride, we did not
hydes, terminal alkynes, and amines (A3-coupling).[18]
observe the dissolution of the silver salt, and when we tried
The great advantage of the [AgI(Pc-L)] complexes with an anion-exchange reaction between complex 2a1 and tetra-
respect to simple silver salts, is their enhanced stability and butylammonium chloride, silver chloride precipitated im-
the ease of handling. In fact, they can be stored indefinitely mediately from the solution. This fact suggested that silver
in open air/light conditions at room temperature without complexes 2 could be quite sensitive to any free halide
any appreciable decomposition. However, they have the ten- source present in the environment. In particular, when using
dency to adsorb an additional ligand, for example, a water chloroform, care must be taken to avoid the presence of
molecule from moisture, which can sometimes modify and/ HCl in the solvent.
or reduce their catalytic behavior. Although from our pre-
Even though all silver complexes 2 can be used in an air
vious studies we were able to propose some mechanistic atmosphere without any observable decomposition, they
hypotheses, the structure of the silver catalysts was not yet are quite sensitive to atmospheric moisture and tend to
completely elucidated. In this paper, we report the study of form monoaquo species, as demonstrated by single-crystal
the organometallic reactivity of those silver(I) complexes X-ray structure for complex 2a1 (Figure 1). Interestingly,
with substrates relevant to the catalytic reactions studied, there are two molecules in the asymmetric unit, showing
that is, alkynes and alcohols and with other neutral ligands, significantly different conformations and coordination
such as CO, water, and acetonitrile. In some cases, the modes of the ligand. The ideal geometry of the complex is
structures of the reaction products were unambiguously de- a square pyramid, with a water molecule occupying the ver-
termined by X-ray analysis of single crystals.
tex and the four N atoms at the base. However, the base is
irregular because the coordination of the N–Ts groups to
Ag is weaker than that of the pyridine and the N–benzyl
groups (see bond lengths in caption of Figure 1). In ad-
dition, the O atoms of the Ts groups are very close to the
coordination sphere (Ag···O distances are in the range 3.2–
3.9 Å). One of the two independent molecules of 2a1 has a
distorted conformation of one N–Ts group, which is due to
the hydrogen-bond linkage with one co-crystallized water
molecule. This produces a conformational change that
brings one O atom, O(23), into a position with a rather
short distance to Ag(2) (see caption of Figure 1). The ideal
square pyramid is substantially distorted and the coordina-
tion at the Ag is extended to six atoms. This demonstrates
the rather unstable configuration of the complex, which is
easily subjected to severe deformations induced even by a
weak intermolecular perturbation. The stereochemistry of
the complex in solution is very likely even more flexible.
We evaluated the effect of the counterion on the NMR
spectra of silver complexes 2, choosing 2a1–3 as case studies
Results and Discussion
The silver(I) complexes, 2, of Pc-Ls, 1, the syntheses of
which have been previously reported,[13] have been obtained
in almost quantitative yields from different sources; respec-
tively, silver tetrafluoroborate (X1), silver triflate (X2), and
silver bis-triflimidate (X3) (Scheme 1).
1
and, as expected, we observed the same signals in the H,
13C, and 15N NMR spectra without any dependence on the
coordinating features of the counterion. These complexes
showed an apparent Cs symmetry of the structure in solu-
tion, with two signals for each couple of equivalent methyl-
Scheme 1. Synthesis of [AgI(Pc-L)] complexes 2.
Each silver(I) salt was added at room temperature to a ene groups. The 1H-19F heteronuclear Overhauser effect
solution of the ligand 1 in DCE (DCE = 1,2-dichloro- spectroscopy (HOESY) experiment conduced in CDCl3
ethane) and the reaction mixture was stirred for one hour, showed that the tetrafluoroborate anion in complex 2a1 has
keeping the flask in the dark and under a protective atmo- proximity interactions, mainly with the pyridine ring of the
sphere until isolation of the final product. Silver complexes ligand (Figure S1 in the Supporting Information).
2 were easily obtained as white crystals in yields up to 91%
The UV/Vis spectra of complexes 2a1 and 2a2 in chloro-
by precipitation, concentrating the solvent, and adding n- form showed the absence of absorption bands at wavelength
hexane. Complexes 2a1–3 and 2c1,2 have been already de- higher than 290 nm, which is consistent with the observed
scribed,[17,18] and experimental details for all others com- absence of color and the d10 electronic configuration for the
plexes can be found in the Experimental Section and in the metal – no d–d transitions are allowed (Figure S2). The
Supporting Information. All the obtained silver complexes shape of the UV/Vis spectra of ligand 1a does not vary
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Figure 1. The structure of 2a1 with a molecule of water coordinated to the metal center. Two independent molecules are present in the
asymmetric unit and both are shown here (with similar orientation). H atoms of the aryl groups are omitted for clarity. Atomic ellipsoids
are drawn at 30% probability level. Principal bond lengths [Å]: Ag(1)–O(101) 2.258(4); Ag(1)–N(3) 2.549(3); Ag(1)–N(9) 2.785(4); Ag(1)–
N(6) 2.384(4); Ag(1)–N(12) 2.448(3); Ag(2)–O(201) 2.394(3); Ag(2)–N(23) 2.830(4); Ag(2)–N(32) 2.608(3); Ag(2)–O(23) 2.847(4); Ag(2)–
N(29) 2.387(4); Ag(2)–N(10) 2.407(3). Note that O(24) is involved in a rather strong intermolecular hydrogen bond with a co-crystallized
water molecule (not shown in the figure).
as a result of complexation; however, the intensities of the triplet (J = 7.1 Hz) as a result of coupling with vicinal pro-
absorptions are slightly different. For this reason, the bands ton Hh (which in the free ligand resonates at δ = 7.43 ppm
recorded in the near-UV region can be assigned to ligand- compared with δ = 8.00 ppm in the complex).[19] This is
centered transitions.
due to the η2 coordination mode of the naphthyl group on
As previously reported for copper(I) complexes,[13] the silver(I), a feature that can also be observed by 13C NMR
effect of the metal complexation to the ligand was clearly spectroscopic studies in CDCl3. The shift to low frequencies
1
disclosed by H and 13C NMR spectroscopy. The partial of the naphthyl carbon i, which is involved in the η2 bond
desymmetrization of the ligand backbone is more evident with silver, from δ = 124.0 ppm in the free ligand to δ =
in the complexes with a pendant naphthyl group on the 104.7 ppm in the complex, is associated with a small fre-
amine nitrogen (2c–e). For example, the 1H NMR spectrum quency shift for carbon h from δ = 126.4 ppm in 1d to δ =
of complex 2d1 in CDCl3 displays a very low degree of sym- 125.1 ppm in 2d1. This kind of unsymmetrical shift of the
metry: the proton directly bound to carbon i, Hi, shifts to signals of the carbon atoms involved in η2 coordination
higher frequencies (δ = 9.42 ppm compared with δ = bonds is common for strongly distorted alkenes coordinated
8.16 ppm for the free ligand 1d; Figure 2). Moreover, in- to a metal atom.[20] In the complex 2d1, carbon i is held in
stead of being the expected doublet, it appears as a pseudo- close proximity to the silver atom by steric requirements
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Figure 2. 1H NMR spectrum of complex 2d1 with highlighted carbon atoms, h and i, involved in η2 coordination of the naphthyl on
silver(I). Inset: Expansion of the signal corresponding to proton i as a triplet (J = 7.1 Hz). In the box, the 13C-1H heteronuclear single
quantum correlation (HSQC) NMR spectrum of complex 2d1.
and, as a consequence, carbon h is less strongly coordi- which by 2D NMR spectroscopic techniques have been as-
nated. The resonance signal of carbon i can barely be lo- signed as those pointing away from the silver ion (Fig-
cated in the 13C NMR spectrum when using the attached ures S4 and S5). Unfortunately, we were not able to obtain
proton test pulse sequence, which is probably due to cou- single crystals of 2d, but all the spectroscopic evidence we
pling with 107Ag and 109Ag nuclei.
have collected points to a structure very close to that ob-
The presence of a strong interaction between the naphth- served for the already reported analogous copper(I) com-
yl moiety and the silver atom allowed us to locate the reso- plex of ligand 1d.[12,13]
nance frequency of the 109Ag atom by heteronuclear mul-
Interestingly, some signals were found to have positive
tiple bond correlation (HMBC) and HSQC techniques; the cross peaks in the 2D NOESY experiments. On the basis of
resonance was found at δ = 536 ppm (referenced to AgNO3 the attribution of chemical shift, it is evident, for example,
1 m in CDCl3 at δ = 41.52 ppm). The main interactions of that H2 must exchange with H10Ј (responsible for the reso-
silver are seen with Hi and Hh, but other protons of the nances at δ = 5.11 and 3.04 ppm, respectively) and that H2Ј
macrocyclic skeleton also correlate with the metal center must exchange with H10 (responsible for the resonances at
(Figure S3). Proton-decoupled experiments allowed us to δ = 3.64 and 4.40 ppm, respectively). The observed ex-
estimate the coupling constant between Hi and 109Ag as change has been attributed to the flipping of the naphthyl
J2
= 6.0 Hz, which is close to the typical value of Jortho moiety from one side to the other, with a complete inversion
Ag-H
for aromatic rings and justifies the appearance of Hi as a of the molecule (Scheme 2). To confirm the occurrence of
pseudo-triplet in the H NMR spectrum. The 15N NMR this dynamic process, 1H NMR exchange spectroscopy
1
spectrum shows a marked shift from δ = 313 ppm to δ = (EXSY) of complex 2d1 in CDCl3 was performed by vary-
265 ppm for the pyridine nitrogen atom, N12 (see Exp. Sect. ing the mixing times (τm = 5, 40, 60, 90, 250 ms; Figure 3).
for numbering scheme). Notably, the sp3 nitrogen atom, N6, The integrated cross peak intensities and diagonal peak in-
bonded to the asymmetric carbon is almost not affected by tensities determined from EXSY experiments (Table S1 in
coordination of the silver atom (δ = 39 ppm in the free li- the Supporting Information) were used to determine the
1
gand; δ = 40 ppm in the complex). From H-19F HOESY exchange rate for the process.[21] At 299.9 K, the pseudo
experiments conducted in CDCl3, it is possible to determine first-order rate constant, k, was determined to be
–
the position of the BF4 anion, which is located on the op- 0.20 (Ϯ0.02) s–1. It should be pointed out that the calcu-
2
10
posite side to the naphthyl moiety and shows interactions lated kxy values for exchange of CH2 and CH2 protons
with the methyl protons (H14), the pyridine protons (Hq), in complex 2d1 showed only little variation with different
and some of the protons of the macrocyclic backbone, τm, as expected since J cross peaks arising from scalar cou-
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pling make a negligible contribution to the intensities of the
exchange cross peaks.[21d]
Scheme 2. Schematic view of the possible rotational process occur-
ring in complex 2d1.
All silver complexes of ligands 1c–1e are characterized
by the coordination of the naphthyl pendant arm to the
metal center, although from NMR spectroscopic data it is
evident that in the case of ligand 1e, the presence of the
isopropyl substituents on the macrocyclic skeleton weakens
the η2 bond as a result of steric crowding. Having explored
the coordination behavior of the naphthyl double bond to
the silver(I) atom, we started to look at other coordinating
ligands. By exposing a dichloroethane/hexane solution of
2c1 to acetonitrile vapor, we obtained the [(2c)(CH3CN)]- Figure 3. H NMR NOESY (400 MHz) spectrum for the CH2 re-
1
gion of 2d1 in CDCl3 at T = 299.9 K. Mixing time (τm) was 250 ms.
Exchange cross peaks marked with black arrows. Negative cross
peaks arise from NOE interactions.
BF4 complex, which precipitated upon concentration of the
solution. The complex [(2c)(CH3CN)]BF4 was recovered by
1
filtration and characterized by NMR spectroscopy. The H
NMR spectrum in CDCl3 shows the presence of exactly one
equivalent of CH3CN per silver atom, as demonstrated by
the presence of a single CH3 signal of acetonitrile, located ular square pyramid with strong coordination to the
at δ = 2.08 ppm, shifted by only a small amount from its CH3CN molecule and again weaker coordination to N–Ts
position in the uncoordinated molecule (δ = 2.10 ppm). The groups.
X-ray crystallographic structure of this complex confirmed
In this complex, no special intermolecular interaction
the tendency of these species to bind an additional ligand with the anion or with the clathrated solvent is distorting
and showed that the metal is located in a very low sym- the molecular geometry; therefore, the SO2 groups of Ts are
metry site (Figure 4). The structure of 2c1 is closer to a reg- not induced to coordinate the metal (the Ag···O distances
Figure 4. Structure of [(2c)(CH3CN)]BF4 with a molecule of CH3CN coordinated to the metal center. H atoms of aryl groups are omitted
for clarity. Atomic ellipsoids drawn at 30% probability level. Principal bond lengths [Å]: Ag(1)–N(5) 2.194(5); Ag(1)–N(7) 2.378(5);
Ag(1)–N(1) 2.424(5); Ag(1)–N(4) 2.624(5); Ag(1)–N(10) 2.758(5).
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are rather long at 3.6–3.8 Å). Figure 4 reports the main was not detected, despite the presence of a 13CO atmo-
bond lengths. sphere in the sample tube and, consequently, of some
The coordination of acetonitrile to the silver(I) com- carbon monoxide in the CDCl3 solution. This can be justi-
plexes in solution was monitored by IR spectroscopy. After fied by the existence of a fast equilibrium on the NMR
the addition of two equivalents of CH3CN to a DCE solu- spectroscopic timescale between dissolved CO and metal-
tion of 2c1, four absorption bands were observed between coordinated CO. As a consequence, the observed signal
2400–2200 cm–1 (Figure S6). The bands at 2256 and could be due to the averaged environment of the CO mol-
2294 cm–1 correspond to the vibration modes of free aceto- ecules.[26] This is in contrast with what was observed for the
nitrile, specifically the CϵN triple bond stretching for the analogous copper complexes, which, under the same condi-
former and a combination of C–H bending and C–C tions, exhibit two different signals for CO,[13] thus suggest-
stretching for the latter. The two new bands at 2279 and ing a stronger coordination mode.
2308 cm–1 are related to the coordinated nitrile. Their shift
Concerning the catalytic applications we previously re-
to higher energies is a common occurrence in metal–aceto- ported for these compounds,[17,18] the interactions with alk-
nitrile complexes because the lone pair of the nitrogen lies ynes are those of greatest interest. We found that 2c1 reacted
in an orbital with partial antibonding character. As a conse- with a terminal alkyne, such as phenylacetylene, to give sil-
quence, the electron donation to the metal increases the ver phenylacetylide 4 and the protonated ligand 3, which
strength of the CϵN bond, together with the force constant partially precipitated as the tetrafluoroborate salt. This be-
of its vibrational mode.[22]
havior suggests that in the absence of a proton scavenger,
The silver complexes 2 are also capable of coordinating phenylacetylene reacts with 2c1 to give the corresponding
carbon monoxide, albeit in a weak manner. The IR spec- acetylide complex (I) and HBF4, which is able to undermine
trum of a dichloromethane solution of 2c1 exposed for 2 h the silver phenylacetylide, causing the decomplexation and
to a 1 bar CO atmosphere displayed an absorption band at the protonation of the ligand 1c (Scheme 3, path A). Con-
2156 cm–1 (Figure S7), which is located at a higher fre- versely, when an internal alkyne such as trimethyl(p-tolyl-
quency than the band of free CO (2143 cm–1). This indi- ethynyl)silane was used, η2 coordination occurred to give
cates that the CO adduct of 2c1 is a non-classical metal the complex 5, without any undesired decomposition reac-
carbonyl,[23] meaning that back-donation from the metal to tion (Scheme 3, path B).
the antibonding CO orbital is scarce or null. This behavior
The complex 5 was fully characterized by NMR spec-
is not unexpected for a d10 closed-shell ion.[24] Brief expo- troscopy. Evidence of the complexation, upon addition of
sure to alternating vacuum and nitrogen flushing resulted 3 equiv.[27] of trimethyl(p-tolylethynyl)silane to 2c1, was
in the disappearance of the CO band from the IR spectrum. gathered from the H, 13C, and 29Si spectra (Table 1). The
1
We prepared the analogous CO complex of 2c2 in deuter- differences in chemical shift between the signals of free ver-
ated chloroform with isotopically enriched carbon mon- sus coordinated alkyne are fairly small: δ = –17.08 versus
oxide to easily detect the signal of the species [(2c)(13CO)]- –17.29 ppm for the 29Si signal, δ = 2.36 versus 2.33 ppm for
OTf. The CO resonance was found at δ = 177.3 ppm (Fig- the 1H signal of the methyl group, and δ = 91.3/105.5 versus
ure S8), which is shifted to a lower frequency than non- 93.2/105.4 ppm for the two Csp carbon atoms (see the Sup-
coordinated CO (δ = 181.3 ppm).[25] Nevertheless, the latter porting Information for details). These small differences are
Scheme 3. Reaction of 2c1 with phenylacetylene (path A) and trimethyl(p-tolylethynyl)silane (path B).
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likely due to the presence of an excess of uncoordinated Table 1. Chemical shifts for some selected signals in CDCl3 and the
numbering scheme adopted.
alkyne in equilibrium with the complex, and the remoteness
of the methyl groups from the coordination center. On the
other hand, the effects on the ligand are quite evident, with
the signal of Hi down-field shifted to δ = 7.93 ppm, (a more
significant shift than those observed with any other spe-
cies), and the signals of H5 and H5Ј diverging to δ = 3.09
and 1.91 ppm, respectively (see Table 1 for numbering
scheme).
The down-field shift of the Hi signal of complex 2c1,
towards the position observed in the free ligand 1c, is a
common feature for complexes 2c–e, characterized by the
presence of a naphthyl pendant able to coordinate to the
silver as an additional ligand. Upon interaction with an ex-
ternal nucleophile, the displacement of the η2-coordinated
naphthyl group from the metal is responsible of a geometri-
cal rearrangement of the ligand, causing the difference in
chemical shifts. Smaller chemical shifts were observed for
the corresponding 13C signals, as well as for other protons
(Table 1). The entity of these shifts varies with the nature
of the external ligand, and could in principle be taken as a
measure of the strength of coordination of weak ligands,
which are in equilibrium with the free ligand in solution.
The larger the fraction of complex that is bound to an aux-
iliary ligand, the larger the shift value. The η2 alkyne coor-
dinated complex 5 shows the biggest effects, with Hi at δ =
Compound
Hi
Ci
H2
H4
H5/H5Ј H13
1c
2c
8.13 125.1 4.30
9.16 111.8 4.87/
3.59
3.07
3.54/
2.73
2.34
2.85/
2.25
3.92
4.36
2c(L), L =
CO
8.94 114.4 4.93/
3.59
8.71 117.5 4.99/
3.57
3.49/
2.63
3.45/
2.59
3.45/
2.59
2.88/
2.17
2.89/
2.16
3.09/
1.91
4.36
4.38
4.31
CH3CN
TolCϵCSiMe3 7.93 124.9 4.93/
3.59
Alcohols have been demonstrated to be suitable reagents
7.93 ppm, which is even lower than that of the free ligand in the domino synthesis of isochromenes catalyzed by silver
1c; this probably depends on the different conformations complexes 2b,[17] but in some cases we observed a quite pro-
that are accessible to the naphthyl moiety in the free ligand longed induction time. Thus, we were intrigued to study the
to relieve steric crowding.
coordination of 2-propanol to silver complex 2b1. We have
Figure 5. Structure of [(2b)(iPrOH)]BF4 with a molecule of 2-propanol coordinated to the metal center. H atoms of aryl groups are
omitted for clarity. Atomic ellipsoids drawn at 30% probability level. Principal bond lengths [Å]: Ag(1)–N(3) 2.674(5); Ag(1)–N(6)
2.435(7); Ag(1)–N(9) 2.424(5); Ag(1)–N(12) 2.661(6); Ag(1)–O(1S) 2.283(8).
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chosen a secondary alcohol, as they have been used with
Experimental Section
success as nucleophiles in the above-mentioned reaction.
We were, in fact, confident that their coordination to sil-
ver(I) could protect the metal complex from adsorbing at-
mospheric moisture, especially in complexes 2a,b in which
the ligands do not possess the pendant naphthyl moiety.
The presence of an alcoholic ligand can slow down the reac-
tion without killing the catalytic activity. The silver complex
2b1 is only slightly soluble in 2-propanol and after two
hours of heating at reflux a white solid was collected and
characterized by NMR spectroscopic experiments as [(2b)-
(iPrOH)]BF4 (see the Supporting Information). Crystals
suitable for X-ray crystallography were grown from a satu-
rated solution of 2b1 in propanol by layering n-hexane (Fig-
ure 5).
General Experimental Details: All of the reactions that involved the
use of reagents sensitive to oxygen or moisture were carried out
under an inert atmosphere. The syntheses of the silver complexes
were carried out under a nitrogen atmosphere by employing stan-
dard Schlenk techniques. All chemicals and solvents were commer-
cially available and were used after distillation or treatment with
drying agents. 1H NMR spectroscopic analyses were performed
with 300 or 400 MHz spectrometers at room temperature. Deuter-
ated solvents were passed over basic alumina, degassed, and stored
under nitrogen over molecular sieves. Samples were degassed prior
to acquisition with three freeze–pump–thaw cycles. The coupling
constants (J) are expressed in Hertz (Hz), and the chemical shifts
(δ) in ppm. 13C NMR spectroscopic analyses were performed with
the same instruments at 75.5 and 100 MHz, and an attached proton
test (APT) sequence was used to distinguish the methine and
methyl carbon signals from those arising from methylene and qua-
ternary carbon atoms. All 13C NMR spectra were recorded with
complete proton decoupling. The 1H NMR signals of the ligand
described in the following have been attributed by correlation spec-
troscopy (COSY) and nuclear Overhauser effect spectroscopy
(NOESY) techniques. Assignments of the resonance in 13C NMR
spectra were made using the APT pulse sequence and heteronuclear
single quantum correlation (HSQC) and heteronuclear multiple
bond correlation (HMBC) techniques. The 15N and 109Ag NMR
signals of the compounds described have been attributed by the
HMBC technique and were referenced to external NH3 at δ =
0.00 ppm and 1 m AgNO3 in CDCl3 at δ = 41.52 ppm, respectively.
The chemical shifts of the 11B and 19F NMR signals of the com-
pounds described have been attributed employing BF3·Et2O and
CFCl3, respectively, as references. 2D NOESY or EXSY experi-
ments were recorded by using the noesygpph pulse program and
were recorded at τm = 5, 40, 60, 90, 250, and 900 ms and with a
recycle delay d1 = 2 s, adjusting the temperature at T = 299.88 K
(calibrated). Low resolution MS spectra were recorded with instru-
ments equipped with electron ionization (EI), ESI/ion trap (using
a syringe pump device to directly inject sample solutions), or fast
atom bombardment (FAB) (for Pc-L and metal complexes) sources.
The values are expressed as mass–charge ratios and the relative
intensities of the most significant peaks are shown in brackets. High
resolution MS spectra were recorded with an instrument equipped
with an electrospray source and an ion cyclotron resonance–Fou-
rier transform mass spectroscopy (ICRFTMS) analyzer. UV/Vis
spectra of the ligands and their silver complexes were recorded in
CHCl3. Elemental analyses were recorded in the analytical
laboratories of Università degli Studi di Milano. Data collections
for the crystal structure determinations were carried out using an
Agilent SuperNova Mo microsource, Al-filtered[28] and working at
50 kV and 0.8 mA. All crystal structures were solved by direct
methods using SHELXS97 and refined with SHELX97,[29] within
the wings suite of programs.[30] H atoms were rigidly modeled on
the riding C or N atoms. Optical rotations were measured on a
Perkin–Elmer instruments model 343 plus. [α]D values are given in
10–1 degcm2 g–1. The ligands 1a–e[12–14] and the silver(I) complexes
2a1–3 and 2c1,2 were synthesized as previously reported.[17,18]
The structure of 2b1 is also a square pyramid with coor-
dination of 2-propoanol in the apical position, as for 2c1
and 2a1, the Ag–O coordination is the shortest bond. The
two N–tosyl groups are symmetrically coordinated and the
O atoms of the SO2 groups are quite distant from the metal
(in the range 3.4–3.9 Å). No solvent molecule is clathratred
in this crystal; therefore, the intermolecular interactions are
weak and do not perturb the molecular geometry too much,
demonstrating that the square pyramid is the preferred
stereochemistry. The apical ligand, however, is somewhat
distorted from the ideal site, as a result of a medium–weak
intramolecular hydrogen bond occurring with O1, which
moves O1S away from the vertical axis of the pyramid, as
visible from Figure 5.
Conclusion
The silver(I) complexes with pyridine-containing ligands,
[AgI(Pc-L)], have already been demonstrated as active cata-
lysts for some domino and multicomponent reactions, such
as the synthesis of 1-alkoxyisochromenes[17] and the A3-
coupling.[18] We have reported here their rich coordination
chemistry with special emphasis on their behavior with rea-
gents commonly employed in the above-mentioned reac-
tions. Both the π-philic (alkyne coordination) and the σ-
philic (alcohol and nitrile coordination) nature of silver(I)
has been clearly demonstrated. The weak and reversible co-
ordination of CO to the silver atom has been monitored
experimentally to show that the interaction has mainly σ-
donor character from the ligand to the metal, as expected.
The hygroscopic nature of the complexes has been high-
lighted, also demonstrated by a structural determination of
a water-containing complex that was obtained after expo-
sure to atmospheric moisture (non-distilled solvents).
Moreover, the silver complexes of the chiral ligands 1b and
1d–e have been prepared and fully characterized. All these
data shed some light on the in-depth understanding of the
catalytic reactions under study. Current efforts in our labo-
ratory are devoted to finding new interesting reactivities of
the silver complexes to scrutinize new potential nucleo-
philes for the silver(I) catalyzed synthesis of isochromenes,
and to explore the capability of the chiral complexes to in-
duce stereoselectivity in these transformations.
General Procedure for the Synthesis of Silver Complexes 2b,d,e: The
silver salt and all silver-containing solutions were kept in the dark
until the final isolation of the product. Ligand 1 was dissolved in
1,2-dichloroethane (≈ 2ϫ10–2 m), the silver salt (weighed under a
nitrogen atmosphere) was added, and the mixture was stirred for
1 h, then filtered to remove any traces of unreacted solid. The sol-
vent was concentrated to half volume, then n-hexane was added.
Eur. J. Inorg. Chem. 2015, 5089–5098
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The mixture was evaporated to dryness and n-hexane was added
again, yielding a well-dispersed white powder. Finally, the product
was recovered by filtration in open air.
NMR (40 MHz, CDCl3): δ = 265 (N12), 101 and 97 (N3 and N9),
40 (N6) ppm. 109Ag NMR (19 MHz, CDCl3): δ = 536 ppm.
C37H40AgBF4N4O4S2 (863.54): calcd. C 51.46, H 4.67, N 6.49;
found C 51.15, H 4.57, N 6.21.
2e1-(4 S,8 S,13 R): 1e-(4 S,8 S,13 R) (MW = 753.03; 0.143 g;
0.190 mmol), AgBF4 (MW
= 194.67; 0.037 g; 0.190 mmol),
C2H4Cl2 (7 mL), and n-hexane (≈ 14 mL) were used as described
1
above. Yield: 0.0943 g (MW = 947.70) 52%. H NMR (300 MHz,
CDCl3): δ = 8.69 (d, J = 8.4 Hz, 1 H, Hi), 8.08–7.86 (m, 6 H, ArH),
7.81 (d, J = 7.6 Hz, 1 H, ArH), 7.72 (pst, J = 7.5 Hz, 1 H, Hh),
7.59–7.44 (m, 8 H, ArH), 7.37 (d, J = 7.5 Hz, 1 H, ArH), 6.02 (q,
J = 6.5 Hz, 1 H, CH13), 5.38 (d, J = 17.5 Hz, 1 H, CH2 ), 4.93 (d,
2
J = 14.0 Hz, 1 H, CH210), 4.59 (pst, J = 12.1 Hz, 1 H, CH2), 4.24
(d, J = 17.5 Hz, 1 H, CH22Ј), 4.17 (m, 1 H, CH), 4.07 (d, J =
15Ј
14.0 Hz, 1 H, CH210Ј), 2.50 (s, 3 H, CH315), 2.47 (s, 3 H, CH3
)
overlapping with 2.49 (m, 1 H, CH2), 2.30–2.20 (m, 2 H, CH16 and
CH), 2.04 (d, 3 H, J = 6.5 Hz, CH314), 1.62 (m, 2 H, CH2), 1.03
(m, 1 H, CH16Ј), 0.88 (m, 3 H, CH317), 0.46 (d, 3 H, J = 5.6 Hz,
CH318), –0.01 (d, 3 H, J = 5.0 Hz, CH317Ј), –0.32 (d, 3 H, J =
5.6 Hz, CH318Ј) ppm. 13C NMR (75 MHz, CDCl3): δ = 155.8 (C),
152.6 (C), 145.8 (C), 145.5 (C), 141.3 (CH), 135.3 (C), 134.52 (C),
134.48 (C), 134.4 (C), 132.5 (C), 130.8 (CH), 130.6 (CH), 129.8
(CH), 128.8 (CH), 128.4 (CH), 127.2 (CH), 126.8 (ChH), 126.5
(CH), 126.0 (CH), 125.3 (CH), 125.0 (CH), 124.2 (CH), 122.7
(CiH), 63.6 (CH), 59.1 (CH), 57.1 (C10H2), 54.4 (C13H), 54.1
(CH2), 52.5 (CH2), 49.1 (C2H2), 30.4 (CH), 28.4 (C16ЈH), 23.9
(C14H3), 22.7 (C18H3), 21.8 (C15H3 and C15ЈH3), 21.1 (C17H3), 21.0
(C18ЈH3), 17.7 (C17ЈH3) ppm. 19F NMR (282 MHz, CDCl3): δ =
–153.13 (10BF4), –153.19 (11BF4) ppm. 15N NMR (40 MHz,
CDCl3): δ = 270 (N12), 45 (N6) ppm; N3 and N9 were not detected.
C43H52AgBF4N4O4S2 (947.70): calcd. C 54.50, H 5.53, N 5.91;
found C 54.35, H 5.90, N 5.89.
Selected Experimental Data (See the Supporting Information for
Full Details)
2b1-(13 S): 1b-(13 S) (MW = 618.81; 0.246 g; 0.398 mmol), AgBF4
(MW = 194.67; 0.077 g; 0.398 mmol), C2H4Cl2 (20 mL), and n-hex-
ane (≈ 40 mL) were used as described above. Yield: 0.217 g (MW
813.48) 67%. 1H NMR (300 MHz, CDCl3): δ = 7.82 (m, 4 H, Hn),
7.68 (t, J = 7.7 Hz, 1 H, Hr), 7.48–7.35 (m, 10 H, ArH + Ho
=
+
HqЈ), 7.22 (d, J = 7.7 Hz, 2 H, Hq), 5.14 (d, J = 15.9 Hz, 1 H,
CH2 ), 4.83–4.80 (m, 2 H, CH2 and CH13), 4.01 (m, 1 H, CH2),
3.85 (d, J = 15.9 Hz, 1 H, CH22Ј), 3.73 (m, 1 H, CH2), 3.59 (d, J
= 14.9 Hz, 1 H, CH210Ј), 2.97 (m, 1 H, CH2), 2.57 (m, 1 H, CH2),
2
10
2.48 (br. s, 6 H, CH3 and CH315Ј), 2.35 (m, 1 H, CH2), 2.12 (m,
15
1 H, CH2), 1.77 (d, J = 6.0 Hz, 3 H, CH314) overlapping with 1.75–
1.68 (m, 2 H, CH2) ppm. 13C NMR (75 MHz, CDCl3): δ = 154.0
(C1), 152.6 (C11), 146.1 (C), 145.8 (C), 140.4 (CrH), 137.5 (C), 131.3
(C), 130.8 (CoH), 130.7 (CoЈH), 129.5 (CH), 129.3 (C), 128.9 (CH),
128.2 (CH), 127.9 (CnH), 125.5 (CqЈH), 124.8 (CqH), 56.7 (C2H2),
56.6 (C13H), 56.3 (C10H2), 49.0 (CH2), 48.4 (CH2), 48.0 (CH2), 46.1
(CH2), 21.8 (C15H3), 19.2 (C14H3) ppm. 19F NMR (282 MHz,
CDCl3): δ = –152.85 (10BF4), –152.90 (11BF4) ppm. MS (FAB): m/z
(%) = 725/727 (91:100) [M – BF4]+, 619 (35) [MH – AgBF4]+.
C33H38AgBF4N4O4S2 (813.48): calcd. C 48.72, H 4.71, N 6.89;
found C 48.75, H 4.51, N 6.53.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures, text, figures, and tables re-
porting full NMR spectra for all compounds along with 2D NMR
spectra for compound 2d1 and data for naphthylidene rotation rate
determination. UV and IR spectra for coordination complexes and
crystallographic details.
CCDC-1413034 [for 2a1(H2O)], 1413035 [for 2c1(CH3CN)], and
1413036 [for 2b1(iPrOH)] 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.
2d1-(13 R): 1d-(13 R) (MW = 668.87; 0.297 g; 0.444 mmol), AgBF4
(MW = 194.67; 0.089 g; 0.457 mmol), C2H4Cl2 (12 mL), and n-hex-
ane (≈ 20 mL) were used as described above. Yield: 0.375 g (MW
=
863.54) 98%. 1H NMR (400 MHz, CDCl3): δ = 9.42 (pst, J =
7.1 Hz, 1 H, Hi), 8.15 (d, J = 8.3 Hz, 1 H, ArH), 8.00 (m, 1 H,
Hh), 7.94 (d, J = 7.5 Hz, 1 H, ArH), 7.87 (d, J = 8.2 Hz, 2 H, Hn),
7.83 (pst, J = 7.8 Hz, 1 H, Hr), 7.68–7.62 (m, 3 H, ArH), 7.52 (d,
J = 8.2 Hz, 2 H, Ho), 7.38 (d, J = 8.2 Hz, 2 H, HnЈ), 7.33 (d, J =
7.9 Hz, 1 H, HqЈ), 7.29 (d, J = 8.2 Hz, 2 H, HoЈ), 7.08 (d, J =
7.6 Hz, 1 H, Hq), 5.66 (q, J = 6.7 Hz, 1 H, CH13), 5.11 (d, J =
Acknowledgments
Financial support from the Università degli Studi di Milano (Svil-
uppo Unimi Bando TRANSITION GRANT – HORIZON 2020 –
Linea A1_B Progetto “Italia per l’Europa. Cod.: 18499”) is grate-
fully acknowledged.
2
4
15.4 Hz, 1 H, CH2 ), 4.70 (m, 1 H, CH2 ), 4.40 (d, J = 13.9 Hz, 1
H, CH210), 3.64 (d, J = 15.4 Hz, 1 H, CH22Ј), 3.04 (d, J = 13.9 Hz,
1 H, CH210Ј), 2.99–2.95 (m, 2 H, CH2), 2.63 (m, 1 H, CH2), 2.54
(s, 3 H, CH315), 2.49 (m, 1 H, CH2), 2.41 (s, 3 H, CH315Ј), 2.41–
2.09 (m, 3 H, CH2), 1.65 (d, J = 6.7 Hz, 3 H, CH314) ppm. 13C
NMR (100 MHz, CDCl3): δ = 154.3 (C1), 152.6 (C11), 145.9 (Cp),
145.7 (CpЈ), 141.3 (CrH), 137.4 (C), 135.2 (C), 133.8 (CH), 132.4
(C), 131.8 (C), 130.8 (CoH), 130.3 (CoЈH), 129.7 (C), 129.6 (CH),
128.7 (CnЈH), 128.3 (CnH), 127.7 (CH), 126.30 (CH), 126.26 (CH),
125.5 (CH), 125.3 (CH), 125.1 (ChH), 104.7 (CiH)*, 56.7 (C2H2),
56.4 (C10H2), 51.8 (C13H), 49.2 (CH2), 48.9 (CH2), 48.8 (CH2), 45.5
(CH2), 21.9 (C15H3), 21.8 (C15ЈH3), 9.0 (C14H3)* ppm. 19F NMR
(282 MHz, CDCl3): δ = –153.03 (10BF4), –153.08 (11BF4) ppm. 11B
NMR (128 MHz, CDCl3): δ = –1.39 (p, JB-F = 1.1 Hz) ppm. 15N
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