Scheme 1. Preparation of complexes.
relativistic effects.10 These trends have been previously reported
by Schmidbaur11 and also by us.7b,12 The related copper(I) com-
plex, [CuLMe(Ph)]3, where LMe(Ph)H = 4-phenyl-3,5-methyl-
1-pyrazole, was structurally characterized as a trinuclear com-
plex with relatively long Cu£Cu separations of 3.494(1) and
3.671(1) ¡.13
The intramolecular M£M interactions were almost twice the
Bondi van der Waals radius, Cu: 2.80 ¡, Ag: 3.44 ¡, and Au:
3.32 ¡,14 consistent with only very weak metallophilic intra-
molecular interactions. The bond angles of N-M-N were close
to 180 deg, indicating little steric deformation of the almost
linear coordination mode. Packing diagrams for [MLEt(Ph)]
complexes are shown in Figures S2-S4. A pseudo dimeric
structure was observed for {[AgLEt(Ph)]3}2 with an Ag-Ag
distance of 2.9690(6) ¡ clearly shorter than twice the Bondi’s
van der Waals radii for Ag of 3.44 ¡.14 By comparison, the
shortest M£M distance in [AgL1(Ph)]3 is 6.0632(5) ¡.8 This
difference is presumably due to the reduced bulkiness of the
substituents, which also results in smaller dihedral angles
between the phenyl and pyrazole planes of 45.8, 48.7, and
62.0 deg in {[AgLEt(Ph)]3}2 compared to 56.5, 88.7, and 89.1
deg in [AgL1(Ph)]3. The solid-state structures of the corre-
sponding gold and copper complexes possessed intermolecular
M£M distances greater than twice the Bondi’s van der Waals
radii.14 The cyclic trinuclear structures in [CuLEt(Ph)] and
[AuLEt(Ph)] were slipped relative to one another to give an
overall stepwise structure, as was also observed for [AgL1(Ph)]3
and [AuL1(Ph)]3.8
Figure 2. Molecular structure of {[LEt(Ph)Ag]3}2(mesitylene)
showing 50% displacement ellipsoids and the atom labeling for
Ag and N atoms. The incorporated mesitylene molecule is
colored green. Hydrogen atoms are omitted for clarity.
cm¹1 for [AuLEt(Ph)]3, which is consistent with the M-N bond
lengths discussed above: Cu-N < Au-N < Ag-N.
Interestingly, when the recrystallization of {[AgLEt(Ph)]3}2
was carried out in mixed solvents of CH2Cl2 and mesitylene or
toluene, the arene-sandwiched complexes {[AgLEt(Ph)]3}2(Ar),
Ar = mesitylene and toluene were obtained (Figures 2, S7 and
S8). Packing diagrams for both complexes {[AgLEt(Ph)]3}2(Ar)
indicated slipped, stepwise structures different from the hex-
anuclear, irregular pattern of {[AgLEt(Ph)]3}2. Consequently,
Ag£Ag interactions were longer at 3.2556(7) ¡ (Ag2£Ag4) in
{[AgLEt(Ph)]3}2(mesitylene) and 3.2108(9) ¡ (Ag3£Ag4) in
{[AgLEt(Ph)]3}2(toluene). Silver(I) trinuclear pyrazolato com-
plexes with incorporated organic molecules have been previ-
ously reported,5a,5c,5d,5f-5g,5j however, these examples generally
had strongly electron deficient ligands to enhance the π-acid/
base interaction. We propose that the phenyl substituted pyrazole
ligand creates a nano-space, or nano-pocket, for the planar
aromatic. Arene-sandwiched formation was only observed for
{[AgLEt(Ph)]3}2, presumably due to appropriate dimensions of
the micro-cavity and/or acidity for this complex. These arene
guest molecules are tightly incorporated in the solid state and
could not be removed by vacuum at room temperature
(Figures S9-S10).
Photoluminescence spectra from the powdered samples of
the ligand, three coinage metal(I) complexes and arene-sand-
wiched Ag(I) complexes are shown in Figures 3, S11-S16.
Spectra were acquired at variable excitation wavelengths and at
three different temperatures (83, 173, and 298 K). Phenyl group
based emissions were observed for LEt(Ph)H (³320 nm at
280 nm excitation) and vibrational fine structure was observed
at lower temperatures without any changes in intensity
(Figure S11). Lower energy emissions at room temperature
were observed at 684.5 nm (λex 320 nm) for [CuLEt(Ph)]3, at
699 nm (λex 300 nm) for {[AgLEt(Ph)]3}2, and at 685.0 nm for
[AuLEt(Ph)]3. These lower energy emissions are attributed to
metal-based phosphorescence arising from closed shell d10-d10
The 1H- and 13C NMR spectra of the polynuclear coinage(I)
complexes were quite different from those of the ligand,
indicating that the cyclic trinuclear coordination was retained
in solution (Table S3). All electronic absorption bands measured
in both solution (cyclohexane) and the solid-state (nujol) were
very broad, mainly due to overlapping π-π* transition bands
associated with the phenyl substituents and the pyrazolyl rings
(Table S4 and Figure S5). Higher energy absorption bands due
to metal to ligand charge transfer (MLCT) were observed on
complexation in the order Cu (246.0 nm) < Au (223.5 nm) < Ag
(206.0 nm). This trend was previously observed for other series
of pyrazolato complexes.7b,8
Coordination of the ligand LEt(Ph)H to the metal caused a
¹1
shift in the C=N IR stretching vibration from 1519 cm to
¹1
³1542 cm (Table S5). The far-IR and FT-Raman spectra of
the pyrazole ligand and three complexes are shown in Figure S6.
The M-N stretching vibrations would be expected in the 550
¹1
¹1
¹1
cm to 450 cm region7,8,15 and were observed at ³483 cm
¹1
for [CuLEt(Ph)]3, ³449 cm for {[AgLEt(Ph)]3}2 and ³459
© 2020 The Chemical Society of Japan | 671
Fujisawa, Kiyoshi
