Coinage metal metallacycles involving a fluorinated 3,5-diarylpyrazolate

Article abstract of DOI:10.1039/d0nj03744c

Copper(i) and silver(i) pyrazolate complexes {[3,5-(3,5-(CF₃)₂Ph)₂Pz]Cu}₃and {[3,5-(3,5-(CF₃)₂Ph)₂Pz]Ag}₃have been synthesized using the fluorinated 3,5-(diaryl)pyrazole 3,5-(3,5-(CF₃)₂Ph)₂PzH and copper(i) oxide and silver(i) oxide, respectively. The gold(i) analog was obtained from a reaction between Au(THT)Cl and [3,5-(3,5-(CF₃)₂Ph)₂Pz]H/NaH. The X-ray crystal structures show that the coinage metal complexes {[3,5-(3,5-(CF₃)₂Ph)₂Pz]M}₃(M = Cu, Ag, Au) are trinuclear in the solid state. They feature distorted nine-membered M₃N₆metallacyclic cores. The M-N distances follow Cu < Au < Ag, which is the trend expected from covalent radii of the corresponding coinage metal ions. The 3,5-(3,5-(CF₃)₂Ph)₂PzH forms hydrogen bonded trimers in the solid state that are further organized by π-stacking between aryl rings. Solid samples of {[3,5-(3,5-(CF₃)₂Ph)₂Pz]M}₃display blue photoluminescence. The copper complex {[3,5-(3,5-(CF₃)₂Ph)₂Pz]Cu}₃is an excellent catalyst for mediating azide-alkyne cycloaddition chemistry.

Full text of DOI:10.1039/d0nj03744c

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Coinage metal metallacycles involving a
fluorinated 3,5-diarylpyrazolate†
Cite this:DOI: 10.1039/d0nj03744c
Jaspreet S. Lakhi,‡ Monika R. Patterson‡ and H. V. Rasika Dias
*
Copper(I) and silver(I) pyrazolate complexes {[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3 and {[3,5-(3,5-(CF3)2Ph)2Pz]Ag}3
have been synthesized using the fluorinated 3,5-(diaryl)pyrazole 3,5-(3,5-(CF3)2Ph)2PzH and copper(I)
oxide and silver(I) oxide, respectively. The gold(I) analog was obtained from a reaction between Au(THT)Cl
and [3,5-(3,5-(CF3)2Ph)2Pz]H/NaH. The X-ray crystal structures show that the coinage metal complexes
{[3,5-(3,5-(CF3)2Ph)2Pz]M}3 (M = Cu, Ag, Au) are trinuclear in the solid state. They feature distorted nine-
membered M3N6 metallacyclic cores. The M–N distances follow Cu o Au o Ag, which is the trend
expected from covalent radii of the corresponding coinage metal ions. The 3,5-(3,5-(CF3)2Ph)2PzH forms
hydrogen bonded trimers in the solid state that are further organized by p-stacking between aryl rings.
Solid samples of {[3,5-(3,5-(CF3)2Ph)2Pz]M}3 display blue photoluminescence. The copper complex
{[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3 is an excellent catalyst for mediating azide–alkyne cycloaddition chemistry.
Received 26th June 2020,
Accepted 30th July 2020
DOI: 10.1039/d0nj03744c
rsc.li/njc
An area of research activity in our laboratory concerns
the chemistry of copper, silver and gold complexes of highly
Introduction
Pyrazolate complexes of monovalent coinage metals (Cu(I), fluorinated pyrazolates.7,32–44 Several years ago, we reported a
Ag(I), Au(I)) represent an important class of coordination com- convenient synthetic route to copper(I) and silver(I) adducts
pounds whose significance spans multiple areas of chemistry, {[3,5-(CF3)2Pz]Cu}3 (2) and {[3,5-(CF3)2Pz]Ag}3 (3) using [3,5-
including acid/base and host/guest chemistry, metallophilic (CF3)2Pz]H and the corresponding metal(I) oxides (Fig. 1).7,45
d10–d10 interactions, supramolecular assemblies, M–M bonded These trinuclear d10 pyrazolates and the related {[3,5-(CF3)2Pz]Au}3
excimers and exciplexes, luminescence,1–16 and catalysis.13,17–20 (4) show remarkable photophysical properties and donor–acceptor
Trinuclear structures are the most common motif among chemistry.7,32,34,36,42,46,47 For example, the copper complex {[3,5-
homoleptic coinage metal pyrazolates while tetranuclear, hexa- (CF3)2Pz]Cu}3 exhibits bright phosphorescent emissions both in
nuclear and polymeric complexes are also known to a lesser the solid state and in frozen solutions under UV excitation that
degree.1,6,20–24 For example, a search of the Cambridge Structural can be easily fine- and coarse-tuned to multiple visible colors by
Database (CSD)25 for 3,5-diphenylpyrazolate ligand based copper, varying the solvent, copper adduct concentration, temperature,
silver and gold complexes shows that {[3,5-(Ph)2Pz]M}3 (Fig. 1, 1, and the excitation wavelength.32,34 These coinage metal complexes
M = Cu, Ag, Au; X = H),26,27 {[4-Cl-3,5-(Ph)2Pz]M}3 (Fig. 1, 1, M = 2–4 serve as p-acids, and form adducts with p-bases like benzene,
Cu, Ag, Au; X = Cl),24,28,29 {[4-Br-3,5-(Ph)2Pz]M}3 (Fig. 1, 1, M = Ag, toluene, mesitylene and naphthalene as well as with C60 forming
Au; X = Br),30 {[4-I-3,5-(Ph)2Pz]M}3 (Fig. 1, 1, M = Ag, Au; X = I),29 diverse supramolecular aggregates.38,39,42,46,47 The silver complex
29
and {[4-Me-3,5-(Ph)2Pz]Ag}3 are known and feature trinuclear {[3,5-(CF3)2Pz]Ag}3 also serves as an excellent sensor for the detec-
structures. In addition, a few tetramers {[3,5-(Ph)2Pz]Cu}4,20 {[4-Cl- tion of volatile aromatic hydrocarbons such as benzene and
3,5-(Ph)2Pz]Cu}4,28 {[4-(tBuCO2)-3,5-(Ph)2Pz]Ag}4,31 and hexamers toluene.36 In addition, the trinuclear {[3,5-(CF3)2Pz]Cu}3 complex
31
{[3,5-(Ph)2Pz]Au}6,27 and {[4-(tBuCO2)-3,5-(Ph)2Pz]Ag}6 resulting readily reacts with alkynes and CO forming dinuclear and
from the same or similar pyrazolate ligands have also been tetranuclear species.18,41,48 It is also an excellent catalyst for
observed, depending on the method of synthesis and crystallization. azide–alkyne click-chemistry.18 Various other groups have also
investigated the interesting chemistry of {[3,5-(CF3)2Pz]M}3 (2–4,
M = Cu, Ag, Au).1,2,10,15,49–51
Department of Chemistry and Biochemistry, University of Texas at Arlington,
Arlington, Texas 76019, USA. E-mail: dias@uta.edu;
Structurally authenticated coinage metal pyrazolates such as
{[3,5-(CF3)2Pz]M}3 with fluoro alkyl substituents are noticeably
less common,25 and fluorinated aryl groups are barely explored
compared to those featuring hydrocarbon substituents (e.g.,
Me, i-Pr, t-Bu, Ph) on the pyrazolate ligand backbone. Considering
current interest in metal pyrazolates and in particular, the attractive
Web: https://www.uta.edu/chemistry/faculty/rasika_dias.php
† Electronic supplementary information (ESI) available: X-ray crystallographic
data, additional figures and details, spectroscopic data, summary of bond
distances and angles. CCDC 2012029–2012032. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/d0nj03744c
‡ Contributed equally to this work.
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Fig. 2 Molecular structure of 3,5-(3,5-(CF3)2Ph)2PzH showing the trimers
resulting from NHꢁ ꢁ ꢁN hydrogen bonding.
Fig. 1 Several coinage metal complexes involving pyrazolate ligand
support.
The analysis of the crystals obtained from CHCl3 by single
crystal X-ray diffraction show that 3,5-(3,5-(CF3)2Ph)2PzH forms
hydrogen bonded trimers in the solid state (Fig. 2) that are
further organized by p-stacking between aryl rings (with centroid
to centroid distances ranging from 3.70–3.87 Å). The trimeric form
is unexpected considering the steric demands of the bulky aryl
groups at the 3- and 5-positions, facing each other in a planar
arrangement. The aggregate accommodates that encounter by
twisting pyrazolyl planes and placing aryl groups above and below
each other (Fig. 2). The nitrogen atoms of the trimeric core forms a
distorted chair configuration with NꢁꢁꢁN distances (the distances
between donor and acceptor nitrogen atoms of the hydrogen
bond) of 2.85 Å. Solid state structures of NH-pyrazoles have indeed
attracted interest as they show diverse structures ranging from
features of the fluorinated analogs, we embarked on a project to
develop fluorinated diarylpyrazolate ligands such as [3,5-(3,5-
(CF3)2Ph)2Pz]ꢀ and investigate their chemistry. Here we report
the synthesis of 3,5-(3,5-(CF3)2Ph)2PzH, and the isolation of
coinage metal complexes {[3,5-(3,5-(CF3)2Ph)2Pz]M}3 (Fig. 1, 5–7;
M = Cu, Ag, Au), as well as their X-ray crystal structural data and
preliminary photophysical and catalytic properties.
Results and discussion
The diketone 1,3-bis(3,5-bis(trifluoromethyl)phenyl)propane- dimers, trimers, tetramers, hexamers, and polymers (catemers)
1,3-dione was synthesized from a mixture of 3,5-bis(trifluoro- and due to the tautomerism.53–55 They are also of interest because
methyl)benzoate and 3,5-bis(trifluoromethyl)acetophenone under binary group 11 metal pyrazolates (in-which N–H is replaced by
basic conditions using a modified literature method utilized in the N–M; M = Cu, Ag, Au) also show some parallels as noted earlier.
synthesis of somewhat related 1,3-bis(4-methylphenyl)propane-1,3- Among 1,3-diarylpyrazoles,25 the dimeric form appears to be the
dione.52 It was isolated as a white solid in 87% yield. The 1H NMR most common (e.g., in 4-(CF3)-3,5-(Ph)2PzH,56 4-(NC)-3,5-(Ph)2PzH,57
spectrum of 3,5-(3,5-(CF3)2PhCO)2CH2 in CDCl3 suggest the 4-(Br)-3,5-(Ph)2PzH,53 3-(4-(CF3)Ph)-5-(Ph)PzH58) while examples
presence of enol form, 1,3-bis(3,5-bis(trifluoromethyl)phenyl)- of tetramers (e.g., 3,5-(Ph)2PzH)59 and polymers (e.g., 3,5-(4-Cl-
3-hydroxyprop-2-en-1-one in solution. The pyrazole 3,5-(3,5- Ph)2PzH,60 3,5-(4-NC-Ph)2PzH,61) are also known.
(CF3)2Ph)2PzH was synthesized by a cyclocondensation reaction
The trinuclear {[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3 (5) was synthe-
involving 1,3-bis(3,5-bis(trifluoromethyl)phenyl)-3-hydroxyprop- sized by using copper(I) oxide and 3,5-(3,5-(CF3)2Ph)2PzH via a
2-en-1-one and hydrazine monohydrate, and isolated as a color- method similar to that reported for {[3,5-(CF3)2Pz]Cu}3.45 How-
less crystalline solid in 87% yield. The 3,5-(3,5-(CF3)2Ph)2PzH is ever, unlike the 3,5-trifluoromethyl pyrazole analog, it was
soluble in acetone and tetrahydrofuran at room temperature found that the formation of 5 did not take place when benzene
and in hot benzene and hot chloroform. It was characterized by was used as a solvent but works well when higher boiling
1
several methods including X-ray crystallography. The H NMR toluene was used. The Cu(I) oxide/pyrazole mixture in toluene
spectrum in CDCl3 includes three peaks assignable to 3,5- was refluxed under nitrogen overnight and filtered while maintain-
(CF3)2Ph protons (d 8.18 and 7.87 ppm) and CH of the pyrazole ing the solution temperature above 90 1C to remove the excess metal
(d 7.11 ppm) ring. 19F NMR spectrum displayed a singlet at d oxide. It was found that if the solution was allowed to cool below
ꢀ63.23 ppm.
90 1C, the product began to precipitate. The hot-filtered clear
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solution was allowed to slowly come to room temperature to obtain
colorless needles. 1H NMR spectrum of these crystals in CDCl3
showed an upfield shifted CH resonance of the pyrazolyl rings (d
7.00 ppm) with respect to the starting material (d 7.11 ppm),
indicating the formation of the trinuclear complex {[3,5-(3,5-
1
(CF3)2Ph)2Pz]Cu}3. The H NMR also indicated the inclusion of
some toluene molecules in the crystalline copper complex. The
trapped toluene can be removed by drying the crystals under
vacuum at 90 1C for about 4 hours. It was found that the toluene-
free crystals were much less soluble in solvents such as CDCl3.
The copper(I) complex {[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3 (5) was
re-crystallized from toluene. The X-ray crystal structure of 5 shows
that it crystallizes with 1.5 molecules of toluene in the asymmetric
unit. One of these toluene molecules and {[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3
forms extended columns as illustrated in Fig. 3, with closest
Cuꢁ ꢁ ꢁC(toluene) separations of 3.32 and 3.47 Å. These distances
are slightly longer than sum of the Bondi’s van der Waals radii
of copper and carbon 1.40 + 1.70 = 3.10 Å.63 However, more
recent work from Alveraz64 places van der Waals contact dis-
tance of copper and carbon at 4.15 Å implying noteworthy
interactions between these moieties. This tendency of {[3,5-
(3,5-(CF3)2Ph)2Pz]Cu}3 to intercalate arenes is similar to that
observed with {[3,5-(CF3)2Pz]Cu}3 (2).38 The partially occupied
(second) toluene molecule occupies the space between these
columns. Selected bond distances and angles of {[3,5-(3,5-
(CF3)2Ph)2Pz]Cu}3 are given in Table 1. It is a trinuclear species
with a somewhat twisted Cu3N6 metallacyclic core.
The silver(I) complex {[3,5-(3,5-(CF3)2Ph)2Pz]Ag}3 (6), was
synthesized using a method similar to that used for {[3,5-
(CF3)2Pz]Ag}3 using the appropriate pyrazole 3,5-(3,5-(CF3)2Ph)2PzH
and silver(I) oxide.45 Similarly to the copper analog, it was found
that the higher boiling toluene (instead of benzene) is needed for
the efficient reaction. The {[3,5-(3,5-(CF3)2Ph)2Pz]Ag}3 was isolated
Fig. 3 Top: Molecular structure of {[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3 (toluene
molecules in the crystal lattice have been omitted for clarity), and Bottom:
A view showing intercalation of toluene molecules between {[3,5-(3,5-
(CF3)2Ph)2Pz]Cu}3 (hydrogen and fluorine atoms have been omitted for
clarity).
1
as a white solid in moderate yield (51%). The H NMR spectrum
of the solid samples indicated the presence of some trapped
toluene, which was removed by drying the solids under vacuum
at 60 1C overnight. These solids were then recrystallized from
dichloromethane/hexanes (4 : 1) at ꢀ20 1C to give colorless
Table 1 Selected bond distances (Å) and angles (1) of trinuclear copper(I), silver(I) and gold(I) complexes supported by pyrazolates with 3,5-diaryl groups
on the pyrazolyl rings. M = Cu, Ag or Au
Metal pyrazolate
M–N
N–M–N
Mꢁ ꢁ ꢁM (intra-trimer)
Mꢁ ꢁ ꢁM (inter trimer)
Ref.
{[3,5-(Ph)2Pz]Cu}3
2.041(7)–2.105(7)
1.849(2)–1.867(2)
1.843(7)–1.869(7)
2.073(4)–2.106(4)
2.082(10)–2.112(11)
2.054(10)–2.132(10)
2.074(5)–2.093(5)
2.059(3)–2.092(3)
2.072(4)–2.088(5)
1.978(9)
1.997(7)–2.009(6)
1.998(6)–2.016(6)
2.003(15)–2.045(13)
1.996(3)–2.013(3)
169.2(3)–178.6(3)
173.22(9)–178.42(9)
175.2(3)–178.1(3)
171.17(17)–172.30(16)
175.6(4)–177.8(4)
170.1(4)–178.4(4)
173.6(2)–174.8(2)
169.85(14)–175.95(15)
174.2(2)–177.1(2)
179.6(3)
178.2(3)–179.2(3)
178.1(2)–179.6(3)
175.4(6)–178.0(6)
176.66(14)–177.28(14)
3.280–3.406
3.140–3.251
3.133–3.285
3.357–3.525
3.469–3.549
3.312–3.632
3.414–3.596
3.342–3.573
3.368–3.433
3.368
3.340–3.386
3.348–3.379
3.347–3.455
3.288–3.369
over 5
4.704
over 5
2.979
over 5
over 5
over 5
3.937
over 5
over 5
4.387
4.442
over 5
over 5
26
28
{[4-Cl-3,5-(Ph)2Pz]Cu}3
{[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3
{[3,5-(Ph)2Pz]Ag}3
{[4-Cl-3,5-(Ph)2Pz]Ag}3
{[4-Br-3,5-(Ph)2Pz]Ag}3
{[4-I-3,5-(Ph)2Pz]Ag}3
{[4-(Me)-3,5-(Ph)2Pz]Ag}3
{[3,5-(3,5-(CF3)2Ph)2Pz]Ag}3
{[3,5-(Ph)2Pz]Au}3
{[4-Cl-3,5-(Ph)2Pz]Au}3
{[4-Br-3,5-(Ph)2Pz]Au}3
{[4-I-3,5-(Ph)2Pz]Au}3
{[3,5-(3,5-(CF3)2Ph)2Pz]Au}3
This work
62
29
30
29
29
This work
27
29
30
29
This work
For comparison, the van der Waals contact distance of two metal atoms based on values suggested by Bondi63 and Alvarez64 are: for Cuꢁ ꢁ ꢁCu = 2.80
and 4.76 Å, Agꢁ ꢁ ꢁAg = 3.44 and 5.06 Å, and Auꢁ ꢁ ꢁAu = 3.32 and 4.64 Å, respectively.
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needle-like crystals. It is soluble in most common organic
solvents such benzene, dichloromethane, acetonitrile, toluene,
and tetrahydrofuran and in hot hexane.
The silver(I) complex {[3,5-(3,5-(CF3)2Ph)2Pz]Ag}3 (6) crystallizes
with molecules of dichloromethane the asymmetric unit. The
molecular structure of {[3,5-(3,5-(CF3)2Ph)2Pz]Ag}3 (Fig. 4) shows
that it is a trinuclear species and forms a somewhat twisted Ag3N6
core to avoid unfavorable steric interactions of 3,5-aryl groups of
the neighboring pyrazolates in 6. There are no close inter-trimer
AgꢁꢁꢁAg contacts as seen with several trinuclear silver pyrazolates
bearing alkyl substituents24,42,65 and in some analogs with
3,5-diarylated pyrazolates (Table 1). The bulky aryl groups in
{[3,5-(3,5-(CF3)2Ph)2Pz]Ag}3 likely hinder the close face-to-face
approach of neighboring trimers from above and below the trimer-
plane to form inter-trimer argentophilic contacts (Fig. 4).
The synthesis of the trinuclear pyrazolate complex of gold(I),
{[3,5-(3,5-(CF3)2Ph)2Pz]Au}3 (7) involved the reaction between
[3,5-(3,5-(CF3)2Ph)2Pz]Na (prepared from NaH and 3,5-(3,5-
(CF3)2Ph)2PzH) and Au(THT)Cl in THF. The product {[3,5-(3,5-
(CF3)2Ph)2Pz]Au}3 was found to be light sensitive and generates
the starting pyrazole ligand back upon prolonged exposure to
light (B2 days). The molecular structure of the gold(I) complex
{[3,5-(3,5-(CF3)2Ph)2Pz]Au}3 (7) is illustrated in Fig. 5. It is also a
trinuclear species similar to the lighter coinage metal adduct
analogs 5 and 6. It has a relatively less twisted (more planar)
M3N6 core. For example, there are three, six, and five nitrogen
or metal atoms with deviations over 0.2 Å (largest 0.26, 0.34 and
0.29 Å) from mean-plane of M3N6 core of the Au, Ag, and Cu
adducts 7–5, respectively. Table 1 summarizes selected bond
distances and angles for {[3,5-(3,5-(CF3)2Ph)2Pz]Au}3 in addition
Fig. 5 Molecular structure of {[3,5-(3,5-(CF3)2Ph)2Pz]Au}3 (7).
to other related systems. The M–N distances of 5–7 follow the
trend expected from covalent radii of the corresponding coinage
metal ions,7,66 with longest and shortest M–N distances observed
in the silver(I) and copper(I) complexes, respectively.
Photophysical properties
All three metal pyrazolates 5–7 exhibited blue luminescence
upon exposure to UV radiation. The copper and silver analogs
exhibited luminescence only at lower temperatures (e.g., 77 K),
while the gold analog display luminescence both at room tem-
perature and lower temperatures. For example at 77 K, {[3,5-(3,5-
(CF3)2Ph)2Pz]Cu}3 (5) displayed a light blue luminescence under
UV radiation with an excitation maximum at 315 nm and emission
maximum at 460 nm, with a Stokes Shift of B8200 cmꢀ1 (Fig. 6).
The gold(I) complex, {[3,5-(3,5-(CF3)2Ph)2Pz]Au}3 emitted bright
blue light under UV light with an excitation maximum at
310 nm and emission maximum at 460 nm, with a Stokes
shift of B8500 cmꢀ1. (Fig. 6). As noted above, these {[3,5-(3,5-
(CF3)2Ph)2Pz]M}3 complexes do not show close inter-trimer
metallophilic interactions in the solid state. The somewhat similar
blue emissions observed in these three coinage metal complexes
{[3,5-(3,5-(CF3)2Ph)2Pz]M}3 supported by 3,5-diaryl substituted
pyrazolates (featuring extended aromatic systems) are likely
a result of pyrazole ligand centered luminescence58 that are
sensitized via internal heavy atom effect. We have observed
similar emissions in {[3-(CF3),5-(Ph)Pz]Cu}3.34
Catalysis
Copper catalyzed synthesis of 1,2,3-triazoles via the cycloaddition
of azides to triple bonds of alkynes is a very important process
involving copper and alkynes.67–72 The standard catalytic system
uses copper(II) salts such as copper sulfate pentahydrate in the
presence of a reducing agent, such as sodium ascorbate.68
Recently, we and others reported that {m-[3,5-(CF3)2Pz]Cu}3 is an
excellent, stand-alone catalyst for the cycloaddition of azides to
terminal alkynes.18,19 In an attempt to extend this work to other
Fig. 4 Molecular structure of {[3,5-(3,5-(CF3)2Ph)2Pz]Ag}3 (dichloro-
methane molecules in the crystal lattice have been omitted for clarity).
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