Photoluminescence properties, molecular structures, and theoretical study of heteroleptic silver(I) complexes containing diphosphine ligands

Article abstract of DOI:10.1021/ic300333c

The homoleptic complex [Ag(L)₂]PF₆ (1) and heteroleptic complexes [Ag(L)(L_Me)]BF₄ (2) and [Ag(L)(L_Et)]BF₄ (3) [L = 1,2-bis(diphenylphosphino) benzene, L_Me = 1,2-bis[bis(2-methylp

Full text of DOI:10.1021/ic300333c

Article
pubs.acs.org/IC
Photoluminescence Properties, Molecular Structures, and Theoretical
Study of Heteroleptic Silver(I) Complexes Containing Diphosphine
Ligands
Satoshi Igawa,†,‡ Masashi Hashimoto,†,‡ Isao Kawata,†,§ Mikio Hoshino,† and Masahisa Osawa*,†
†Luminescent Materials Laboratory, RIKEN (The Institute of Physical & Chemical Research), Hirosawa 2-1, Wako-shi 351-0198,
Japan
‡Device Technology Development Headquarters and §Analysis Technology Center, Canon Incorporated, Ohta-ku, Tokyo 146-8501,
Japan
S
* Supporting Information
ABSTRACT: The homoleptic complex [Ag(L)2]PF6 (1) and
heteroleptic complexes [Ag(L)(LMe)]BF4 (2) and [Ag(L)-
(LEt)]BF4 (3) [L = 1,2-bis(diphenylphosphino)benzene, LMe
1,2-bis[bis(2-methylphenyl)phosphino]benzene, and LEt
=
=
1,2-bis[bis(2-ethylphenyl)phosphino]benzene] were synthe-
sized and characterized. X-ray crystallography demonstrated
that 1−3 possess tetrahedral structures. Photophysical studies
and time-dependent density functional theory calculations of
1−3 revealed that alkyl substituents at the ortho positions of
peripheral phenyl groups in the diphosphine ligands have a
significant influence on the energy and intensity of
phosphorescence of the complex in solution at room
temperature. The results can be interpreted in terms of the geometric preferences of each complex in the ground and
excited states. The homoleptic complex 1 exhibits weak orange phosphorescence in solution arising from its flat structure in the
triplet state, while heteroleptic complexes 2 and 3 show strong green phosphorescence from triplet states with tetrahedral
structure. Larger interligand steric interactions in 2 and 3 caused by their bulkier ligands probably inhibit geometric relaxation
within the excited-state lifetimes, leading to higher energy phosphorescence than that observed for 1. NMR experiments revealed
that 2 and 3 in solution possess structures that are much more immobilized than that of 1; fluxional motion is completely
suppressed in 2 and 3. Accordingly, conformational changes of 2 and 3 are expected to be suppressed by the alkyl substituents
not only in the ground state but also in excited states. Consequently, nonradiative decay of the excited states of 2 and 3 occurs
less efficiently than in 1. As a result, the quantum yields of phosphorescence for 2 and 3 are 6 times larger than that for the
homoleptic complex 1.
to-ligand charge-transfer (MLCT) character of the excited state.
While the d10 ground state possesses D2d symmetry, the MLCT
excited state favors D2 symmetry. The change in the structure
of the triplet state results in both a red shift of λmax and a
decrease in Φp, with λmax = 670 nm and Φp = 0.05 in 2-MeTHF
at ambient temperature. This phenomenon can be explained by
the rigidochromic effect,4,5 as is frequently seen in some
copper(I) complexes.5
According to the extensive chemistry of tetrahedral copper(I)
complexes with two diimine ligands,6 the incorporation of
bulky substituents into diimine ligands on the side of the metal
center is an effective means to prevent structural relaxation and
maintain a good phosphorescence performance even in
solution at room temperature. For instance, sterically congested
alkyl substituents are generally introduced at the 2 and 9
INTRODUCTION
■
A number of d10 coinage metal complexes that exhibit
interesting luminescence properties have been synthesized.1
With regard to silver complexes, most reported to date are
multinuclear complexes that have argentophilic bonding, an
analogue of aurophilic bonding known as “closed-shell (d10−
d10) interactions”.2 Luminous mononuclear silver complexes,
however, had rarely been reported until recently.3 In previous
papers, we reported luminescence from the tetrahedral silver
complex [Ag(dppb)2 ]PF6 [1; dppb
= 1,2-bis-
(diphenylphosphino)benzene (L)].3a This complex exhibits
an efficient phosphorescence performance in frozen 2-
methyltetrahydrofuran (2-MeTHF) at 77 K: the maximum
wavelength of phosphorescence λmax = 441 nm, and the
phosphorescence quantum yield Φp = 0.88. However, this
complex shows weakened phosphorescence in solution at room
temperature, resulting from structural distortion in the excited
state from tetrahedral to planar geometry because of the metal-
Received: February 14, 2012
Published: April 30, 2012
© 2012 American Chemical Society
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temperature and 77 K using a Hitachi U-3010 spectrophotometer and
a Hitachi F-7000 spectrofluorometer, respectively. The light-intensity
distribution of a xenon lamp incorporated in the spectrofluorometer
was corrected using Rhodamine B in ethylene glycol. The output of
the photomultiplier was calibrated in the wavelength range of 300−
850 nm using a secondary standard lamp. Absorption spectra were
measured at 77 K using a specially constructed low-temperature quartz
Dewar with four optical windows. The absorption coefficient of the
complexes in 2-MeTHF at 77 K was calculated on the assumption that
the density of 2-MeTHF was 1.06 g/mL. Laser photolysis studies were
carried out using a Nd:YAG laser (Sure Light 400, Hoya Continuum
Ltd.) equipped with second-, third-, and fourth-harmonic generators.
The excitation light used for emission lifetime measurements was the
third harmonic (355 nm). The duration and energy of the laser pulse
were 5 ns and 30 mJ/pulse, respectively. The system used to monitor
the decay of emission has been reported elsewhere.8 Emission
quantum yields in solution at room temperature and 77 K were
determined with an absolute photoluminescence (PL) quantum yield
measurement system (C-9920-02G, Hamamatsu).9 Electrochemical
measurements were obtained with an electrochemical analyzer (Als/
CHI, model 600A) equipped with a single-compartment cell under an
argon atmosphere. A glassy carbon electrode and coiled platinum wire
were used as the working and counter electrodes, respectively. The
reference electrode was Ag/AgNO3 in CH3CN, and the scan rate was
100 mV/s. Sample solutions were prepared by dissolving a sample (1
mM) in an electrolyte solution of [(Bu)4N][PF6] in CH3CN (0.1 M, 8
mL). The reference electrode was calibrated before measurements
using an external ferrocene standard.
Crystal Structure Determination. The crystallographic data and
the results of structure refinements are summarized in Table S1 in the
Supporting Information (SI). In the reduction of data, Lorentz and
polarization corrections and empirical absorption corrections were
made. The structures were solved by direct methods (SIR2004).10 All
non-hydrogen atoms were refined with anisotropic thermal parame-
ters. Hydrogen atoms were fixed at calculated positions. The X-ray
crystallographic data of 1 have been reported previously.3a CCDC
reference numbers are 756865 for 1, 865271 for 2, and 865272 for 3.
Computational Details. Optimization of the geometries of the
singlet ground (S0) and lowest triplet excited (T1) states in
tetrahydrofuran (THF; dielectric constant ε = 7.4257) was computed
using density functional theory (DFT)11 with Becke three-parameter
exchange with the Perdew−Wang 1991 correlation (B3PW91).12
Calculations of excitation energies and oscillator strengths in THF for
the above optimized structures were performed by time-dependent
(TD)-DFT with B3PW91. The LANL2DZ effective core potentials
and valence basis set13 were used to describe the valence electrons of
silver. Phosphorus, carbon, and hydrogen were described with 6-
31+G*, 6-31G*, and 6-31G basis sets, respectively. Solvent effects
were described by exploiting the integral equation formalism14 version
of the polarizable continuum model,15 as implemented in Gaussian0316
and Gaussian09.17
positions of phenanthroline (or the 6 and 6′ positions of 2,2′-
bipyridine) to attain high Φp, as shown in Chart 1a.
Chart 1. Ligands and Silver Complexes
In line with this approach used for copper(I) diimine
complexes, we have prepared diphosphine ligands in the form
of dppb derivatives with substituents [Me (LMe), Et (LEt), and
iPr (LiPr)] at the ortho positions of four peripheral phenyl
groups to afford rigid environments around the metal center in
their silver(I) complexes (Chart 1b). Heteroleptic silver(I)
complexes [Ag(L)(LMe)]BF4 (2) and [Ag(L)(LEt)]BF4 (3)
were readily synthesized using dppb derivatives (Chart 1c). The
steric effects of bulky substituents at the ortho positions have
been briefly described in a previous paper on three-coordinate
7
copper(I) complexes with LMe
.
1H and 31P{1H} NMR
experiments confirmed that heteroleptic silver(I) complexes 2
and 3 were stable and did not show ligands scrambling in
solution.
The photophysical properties of the dppb derivatives 2 and 3
are discussed on the basis of molecular orbital (MO)
calculations, and the ability of the dppb derivatives to prevent
flattening distortion through steric protection is described in
detail.
EXPERIMENTAL SECTION
■
Chemicals. 1,2-Bis(dichlorophosphino)benzene and magnesium
were obtained from Wako Pure Chemical Industries, Ltd. (2-
Methylphenyl)magnesium bromide was purchased from Sigma-
Aldrich. Silver(I) tetrafluoroborate, silver(I) hexafluorophosphate, (2-
bromoethyl)benzene, and (2-isopropylphenyl)benzene were obtained
from TCI Co., Ltd. Iodine and magnesium were purchased from
Kanto Chemical Co., Inc.
A compact orbital representation for the electronic transition
density matrix18 was calculated to interpret the qualitative nature of
̂
the T1 state. The transition density matrix is given by Tia = ⟨ϕi|T|ϕa⟩,
where ϕi and ϕa are occupied and virtual canonical MOs, respectively.
The “natural transition orbitals” (NTOs) were obtained by orbital
transformation followed by a singular value decomposition of the
transition density matrix. In the NTO representation, the electronic
transitions can be expressed by one single “electron−hole” pair with an
associated eigenvalue of essentially 1, even transitions that are highly
mixed in the canonical MO basis set. This procedure gives us a simple
orbital interpretation of “what got excited to where”.19 All quantum-
chemical calculations were carried out on a PC cluster system
(Fujitsu). NTO calculations were performed using a laboratory-made
code.
Synthesis of Compounds. 1,2-Bis[bis(2-methylphenyl)-
phosphino]benzene (LMe). A THF solution of (2-methylphenyl)
magnesium bromide (1 M, 25 mL, 25 mmol) was added dropwise to a
solution of 1,2-bis(dichlorophosphino)benzene (1.0 g, 3.57 mol) in
THF (20 mL) at 0 °C. The reaction mixture was stirred for 4 h at
General Information. All synthetic reactions were carried out
under an atmosphere of argon unless otherwise indicated. 1H, 13C, and
31P NMR spectra were recorded on a Bruker AVANCE-500
1
spectrometer. H and 13C chemical shifts were referenced to residual
solvent peaks. 31P chemical shifts were referenced to external 85%
phosphoric acid (δ = 0 ppm). Electrospray ionization mass
spectrometry (ESI-MS) spectra were recorded on a ThermoQuest
Finnigan LCQ Duo mass spectrometer. Elemental analyses (C and H)
were carried out with a Vario EL CHNOS elemental analyzer from
Elementar Analytical, and quantitative analyses of phosphorus for LMe
,
LEt, and LiPr were performed at the Advanced Technology Support
Division of RIKEN Advanced Science Institute. For emission studies,
dissolved oxygen was removed by repeated freeze−pump−thaw cycles.
Steady-state absorption and emission spectra were recorded at room
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room temperature, and then a solution of saturated aqueous NH4Cl
(100 mL) was added to the reaction mixture. The product was
extracted with CH2Cl2 (3 × 60 mL). The combined organic extracts
were washed with saturated aqueous NaCl (150 mL) and dried over
MgSO4. The drying agent was removed by filtration, and then the
solvent was removed in vacuo to give a pale-yellow oil. The residue
was purified by column chromatography on silica gel (n-hexane/
CH2Cl2, 2:1) to afford LMe (1.35 g, 75%) as a colorless solid. 1H NMR
(500 MHz, CD2Cl2, 300 K): δ 2.19 (s, 12H), 6.76 (m, 4H), 6.92 (m,
2H), 7.02 (t, 4H, J = 7.3 Hz), 7.23−7.15 (m, 8H), 7.26 (m, 2H). 13C
NMR (125 MHz, CD2Cl2, 300 K): δ 20.92 (t, J(13C−31P) = 11.0 Hz),
125.91, 128.52, 130.00, 133.61, 133.94, 135.44, 142.64 (t, J(13C−31P)
= 13.1 Hz), 142.84 (t, J(13C−31P) = 11.7 Hz). 31P{1H } NMR (202
MHz, CD2Cl2, 300 K): δ −27.10. Anal. Calcd for C34H32P2: C, 81.26;
H, 6.42; P, 12.33. Found: C, 80.99; H, 6.30; P, 12.18.
1,2-Bis[bis(2-ethylphenyl)phosphino]benzene (LEt). A solution of
(2-ethylphenyl)magnesium bromide in THF (25 mL) was prepared
from magnesium (2.2 g, 90 mmol) and (2-bromoethyl)benzene (15.9
g, 86 mmol) and then cooled to 0 °C. A solution of 1,2-
bis(dichlorophosphino)benzene (3.0 g, 10.1 mmol) in THF (20
mL) was added dropwise to the THF solution of (2-ethylphenyl)
magnesium bromide. The reaction mixture was heated under reflux for
4 h. The mixture was cooled to room temperature. A solution of
saturated aqueous NH4Cl (150 mL) was added to the reaction
mixture, and then the product was extracted with CH2Cl2 (3 × 60
mL). The combined organic extracts were washed with saturated
aqueous NaCl (150 mL) and dried over MgSO4. The drying agent was
removed by filtration, and the solvent was removed in vacuo to give a
pale-yellow oil. The residue was purified by column chromatography
on silica gel (n-hexane/CH2Cl2, 2:1) to afford LEt (2.15 g, 36%) as a
colorless solid. 1H NMR (500 MHz, CD2Cl2, 300 K): δ 0.99 (t, 12H, J
= 7.5 Hz), 2.60 (m, 8H), 6.79 (m, 4H), 6.91 (m, 2H), 7.02 (t, 4H, J =
7.3 Hz), 7.20−7.26 (m, 10H). 13C NMR (125 MHz, CD2Cl2, 300 K):
δ 14.91, 27.51 (t, J(13C−31P) = 11.3 Hz), 125.81, 128.19 (t,
J(13C−31P) = 2.6 Hz), 128.73, 129.08, 134.16, 134.43 (t, J(13C−31P)
= 3.1 Hz), 135.56 (t, J(13C−31P) = 3.3 Hz), 143.74 (t, J(13C−31P) =
12.0 Hz), 148.69 (t, J(13C−31P) = 12.8 Hz). 31P{1H } NMR (202
MHz, CD2Cl2, 300 K): δ −29.81. Anal. Calcd for C38H40P2: C, 81.69;
H, 7.22; P, 11.09. Found: C, 81.38; H, 6.89; P, 10.91.
removed in vacuo. The residue was purified by recrystallization from
THF to give colorless crystals [Ag(L)(LMe)]BF4·2THF (2·2THF).
1
Yield: 199 mg, 82%. H NMR (500 MHz, CD2Cl2, 300 K): δ 1.88 (s,
12H), 6.71 (m, 8H), 6.77 (m, 6H), 6.88 (m, 2H), 7.11 (m, 4H), 7.16−
7.26 (m, 8H), 7.28−7.44 (m, 8H), 7.49 (m, 2H), 7.54 (m, 4H), 7.60
1
(m, 2H). H NMR (500 MHz, CD2Cl2, 220 K): δ 1.60 (s, 6H), 1.76
(s, 6H), 6.60 (m, 8H), 6.66 (m, 4H), 6.79 (m, 2H), 6.81 (m, 2H), 7.03
(m, 4H), 7.18 (m, 6H), 7.21 (m, 2H), 7.24 (m, 2H), 7.30 (t, 2H, J =
7.5 Hz), 7.34 (m, 2H), 7.40 (t, 2H, J = 7.5 Hz), 7.44 (m, 2H), 7.50 (m,
2H), 7.54 (t, 2H, J = 7.5 Hz), 7.58 (m, 2H). 13C NMR (125 MHz,
CD2Cl2, 220 K): δ 21.17 (t, J(13C−31P) = 8.4 Hz), 21.36 (m), 125.91,
126.21, 127.73 (t, J(13C−31P) = 4.3 Hz), 128.30 (t, J(13C−31P) = 13.0
Hz), 128.43 129.12, 129.37 (t, J(13C−31P) = 11.6 Hz), 129.54, 129.57
(t, J(13C−31P) = 12.4 Hz), 129.79, 129.94, 130.57, 130.65, 130.87,
131.24, 131.43, 131.49, 131.55, 131.97 (t, J(13C−31P) = 13.9 Hz),
133.24 (t, J(13C−31P) = 8.8 Hz), 134.89, 137.12, 137.13 (t, J(13C−31P)
= 28.1 Hz), 137.59 (t, J(13C−31P) = 30.6 Hz), 140.05(t, J(13C−31P) =
9.6 Hz), 141.24 (t, J(13C−31P) = 9.1 Hz). 31P{1H } NMR (202 MHz,
CD2Cl2, 300 K): δ −12.76 (2dt, 1J(31P−107Ag) = 231 Hz,
1J(31P−109Ag) = 268 Hz, 2J(31P−31P) = 23 Hz), −2.86 (2dt,
1
2
1J(31P−107Ag) = 227 Hz, J(31P−109Ag) = 262 Hz, J(31P−31P) = 31
Hz). ESI-MS. Calcd for C64H56AgP4: m/z 1055.24. Found: m/z 1055.2
([M − BF4 ]). Anal. Calcd for C72H72AgBF4O2P4 (2·2THF): C,
67.15; H, 5.63. Found: C, 67.34; H, 5.58.
−
[Ag(L)(LEt)]BF4 (3). Silver(I) tetrafluoroborate (39 mg, 0.20 mmol)
was added to a solution of LEt (110 mg, 0.20 mmol) in THF (15 mL).
The mixture was stirred and heated under reflux. After 2 h, L (89 mg,
0.20 mmol) was added, and then the mixture was again stirred and
heated under reflux for 2 h. Upon cooling, an insoluble solid was
removed from the reaction mixture by filtration, and then the solvent
was removed in vacuo. The residue was purified by recrystallization
from acetone to give colorless crystals [Ag(L)(LEt)]BF4·(CH3)2CO
1
(3·(CH3)2CO). Yield: 221 mg, 88%. H NMR (500 MHz, CD2Cl2,
300 K): δ 0.17 (t, 6H, J = 7.3 Hz), 0.38 (t, 6H, J = 7.3 Hz), 2.12 (dq,
2H, J = 15.8 and 7.8 Hz), 2.31 (m, 4H), 2.57 (dq, 2H, J = 15.8 and 7.8
Hz), 6.62 (m, 4H), 6.75 (m, 4H), 6.79 (m, 6H), 7.01 (m, 2H), 7.13−
7.18 (m, 8H), 7.23 (t, 2H, J = 7.5 Hz), 7.24−7.29 (m, 4H), 7.38 (t,
2H, J = 7.5 Hz), 7.43 (m, 4H), 7.45 (m, 2H), 7.54 (m, 2H), 7.60 (m,
2H), 7.65 (t, 2H, J = 7.5 Hz). 13C NMR (125 MHz, CD2Cl2, 300 K): δ
10.99, 11.84, 27.52 (t, J(13C−31P) = 9.6 Hz), 27.75 (m), 126.49,
127.02, 128.19, 128.28, 128.84 (t, J(13C−31P) = 4.4 Hz), 129.25 (t,
J(13C−31P) = 4.7 Hz), 130.17, 130.23 (t, J(13C−31P) = 10.5 Hz),
130.48, 130.62, 130.76 (t, J(13C−31P) = 17.3 Hz), 130.90, 131.32,
131.43 (t, J(13C−31P) = 13.2 Hz), 134.88, 131.96, 132.43, 132.82 (t,
J(13C−31P) = 8.1 Hz), 132.88 (t, J(13C−31P) = 8.7 Hz), 133.76 (t,
J(13C−31P) = 8.7 Hz), 135.80, 138.29, 138.92 (t, J(13C−31P) = 26.4
Hz), 139.17 (t, J(13C−31P) = 25.1 Hz), 146.02 (t, J(13C−31P) = 9.1
Hz), 146.90 (t, J(13C−31P) = 8.7 Hz). 31P{1H} NMR (202 MHz,
CD2Cl2, 300 K): δ −15.16 (2dt, 1J(31P−109Ag) = 267 Hz,
1J(31P−107Ag) = 233 Hz, 2J(31P−31P) = 31 Hz), −4.25 (2dt,
1,2-Bis[bis(2-isopropylphenyl)phosphino]benzene (LiPr). This
compound was prepared using the same procedure as that of LEt,
except that (2-bromoisopropyl)benzene (17.1 g, 86 mmol) was used
instead of (2-bromoethyl)benzene. Column chromatography on silica
gel (n-hexane/CH2Cl2, 2:1) gave analytically pure LiPr (805 mg, 12%)
1
as a colorless solid. H NMR (500 MHz, CD2Cl2, 300 K): δ 0.90 (d,
12H, J = 6.7 Hz), 1.03 (br s, 12H), 3.34 (br s, 4H), 6.80 (m, 4H), 6.92
(m, 2H), 7.03 (t, 4H, J = 7.3 Hz), 7.23 (m, 2H), 7.30 (m, 8H). 13C
NMR (125 MHz, CD2Cl2, 300 K): δ 23.19 (br), 23.85, 31.28 (t,
J(13C−31P) = 13.0 Hz), 125.27 (t, J(13C−31P) = 2.3 Hz), 125.72,
128.86, 129.06, 134.21, 134.44 (t, J(13C−31P) = 2.7 Hz), 134.99,
144.46 (t, J(13C−31P) = 12.3 Hz), 153.35 (t, J(13C−31P) = 12.3 Hz).
31P{1H } NMR (202 MHz, CD2Cl2, 300 K): δ −31.47. Anal. Calcd for
C42H48P2: C, 82.05; H, 7.87; P, 10.08. Found: C, 82.24; H, 7.69; P,
10.31.
1
2
1J(31P−109Ag) = 263 Hz, J(31P−107Ag) = 228 Hz, J(31P−31P) = 31
Hz). ESI-MS. Calcd for C68H64AgP4: m/z 1111.30. Found: m/z 1111.3
([M − BF4 ]). Anal. Calcd for C68H64AgBF4P4: C, 68.07; H, 5.38.
Found: C, 67.82; H, 5.21.
−
[Ag(L)2]PF6 (1). This compound was synthesized according to the
literature.3a 1H NMR (500 MHz, CD2Cl2, 300 K): δ 7.53 (m, 8H),
7.33 (m, 8H), 7.11−7.10 (m, 32H). 1H NMR (500 MHz, CD2Cl2, 220
K): δ 7.05 (br m, 32H), 7.28 (br m, 8H), 7.52 (m, 4H), 7.55 (m, 4H).
13C NMR (125 MHz, CD2Cl2, 220 K): δ 128.19, 129.64, 129.90 (m),
130.54, 132.66 (m), 133.32, 139.05 (m). 31P{1H } NMR (202 MHz,
RESULTS AND DISCUSSION
■
Synthesis and Characterization of dppb Derivatives
and Heteroleptic Silver(I) Complexes. Bidentate bisphos-
phine ligands were synthesized from bis(dichlorophosphino)-
benzene and the appropriate Grignard reagents: (2-methyl-
phenyl)magnesium bromide for LMe, (2-ethylphenyl)magnesi-
um bromide for LEt, and (2-isopropylphenyl)magnesium
bromide for LiPr.20 Large alkyl substituents at the ortho
position of the peripheral phenyl groups of dppb considerably
decreased the yield of the derivative because of their steric
encumbrances; the yields were 75% for LMe, 36% for LEt, and
12% for LiPr. Heteroleptic complex 2 was prepared by heating 1
equiv of LMe and 1 equiv of AgPF6 under reflux in THF under a
1
1
CD2Cl2, 300 K): δ 0.81 (d, J(31P−109Ag) = 264 Hz, J(31P−107Ag) =
229 Hz). ESI-MS. Calcd for C60H48AgP4: m/z 999.18. Found: m/z
−
999.2 ([M − PF6 ]). Anal. Calcd for C60H48AgF6P5: C, 62.90; H, 4.22.
Found: C, 62.76; H, 4.39.
[Ag(L)(LMe)]BF4 (2). Silver(I) tetrafluoroborate (39 mg, 0.20 mmol)
was added to a solution of LMe (100 mg, 0.20 mmol) in THF (15 mL).
The mixture was stirred and heated under reflux. After 2 h, L (89 mg,
0.20 mmol) was added, and again the mixture was stirred and heated
under reflux for 2 h. Upon cooling, an insoluble solid was removed
from the reaction mixture by filtration, and then the solvent was
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dinitrogen atmosphere with stirring. After reaction for 2 h, the
solid silver salt completely dissolved and the solution became
clear, presumably because of the formation of Ag(LMe)(THF)n
(n = 1 or 2). A total of 1 equiv of L was then added, and the
solution was heated under reflux for a further 2 h. The reaction
mixture was filtered, and then the solvent was removed in vacuo
to give the crude heteroleptic complex. Recrystallization from
THF was sufficient to purify 2 by removing a small amount of
the homoleptic byproduct [Ag(L)2]BF4. Complex 3 was
prepared in a manner similar to 2, except that 3 was
recrystallized from acetone. High yields of 82% and 88%
were obtained for 2 and 3, respectively.
We were unable to synthesize [Ag(LiPr)(L)]BF4 using LiPr
under the reaction conditions described above. The isolated
product was solely a stoichiometric amount of homoleptic
[Ag(L)2]BF4, suggesting that heteroleptic [Ag(LiPr)(L)]BF4, if
produced, readily undergoes disproportionation to yield the
homoleptic complex. It is concluded that the bulkiness of the
alkyl group at the ortho position of the phenyl groups in the
dppb derivatives governs the stability of the resulting
heteroleptic complexes; the more bulky the alkyl group, the
less stable the heteroleptic complex. Furthermore, attempts to
prepare homoleptic complexes [Ag(LR)2]BF4 (R = Me, Et, and
iPr) only generate mono-diphosphine silver(I) complexes.
The crystal structures of complexes 1−3 are shown in Figure
1a−c. Selected bond lengths and angles are summarized in
Table 1. The o-methyl and o-ethyl groups are oriented in the
direction of the central metal, resulting in interligand steric
interactions between the four peripheral phenyl groups of the
other ligand (L) in solution. The coordination geometries of
1−3 are distorted from a D2d pseudotetrahedral geometry that
Table 1. Selected Bond Distances (Å) and Angles (deg) for
1−3
c
1
2
3
a
a
Ag−P1
Ag−P2
Ag−P3
Ag−P4
2.4592(6)
2.5027(6)
2.5435(6)
2.4611(6)
2.553(2)
2.4958(19)
2.501(2)
2.519(2)
2.517
2.5147(10)
2.5063(10)
2.5030(10)
2.5216(10)
2.511
average
(Ag−P)
2.492
dihedral
82.56(3)
85.25(3)
84.91(10)
81.38(4)
b
angle
a
b
P1 and P2 belong to L for 1, LMe for 2, and LEt for 3. The dihedral
c
angle between two planes, P1−Ag−P2 and P3(1a)−Ag−P4(2a). Two
molecules in the unit cell.
might be expected for a d10 ion. The dihedral angles between
the two planes defined by P1−AgI−P2 and P3(1a)−AgI−
P4(2a) are 81.38(4)−85.25(3)°. The average bond lengths
(Ag−P) in 2 and 3 are almost equal to those in 1, suggesting
that the chelating abilities of LMe and LEt are similar to that of L
.
The strong chelating abilities of the dppb derivatives were
confirmed by 31P{1H} NMR spectroscopic measurements in
CD2Cl2. It is known that Ag−P coupling in the room
temperature spectra of silver phosphine complexes is
unresolved because of the rapid exchange equilibria.21
Complexes 1−3, however, show typical Ag−P couplings even
at ambient temperature. Complex 1 shows two doublet of
doublets with J(31P−107Ag) = 229 Hz and J(31P−109Ag) = 264
Hz, and 2 and 3 exhibit two doublet of triplets (A2X2 systems).
As an example, the 31P{1H} NMR spectrum of 3 is shown in
Figure 2. Two well-resolved pairs of doublet of triplets are
Figure 2. 31P{1H} NMR spectrum of 3 (in CD2Cl2 at 300 K).
observed at −15.16 and −4.25 ppm with 2J(AX) =
2
31/1J(31P−107Ag) = 233/1J(31P−109Ag) = 267 Hz and J(AX)
= 31/1J(31P−107Ag) = 228/1J(31P−109Ag) = 263 Hz,
respectively. Because no signals from disproportionation
products were observed, the heteroleptic structures of 2 and
3 are concluded to be very stable even in solution.
Furthermore, H NMR experiments revealed that 3 has the
most rigid structure in solution. The ring protons of the two
types of phenyl groups (labeled as A and B in Figure 1a)
Figure 1. ORTEP drawings of the structures of cations in 1 (a), 2 (b),
and 3 (c). Thermal ellipsoids are drawn at the 50% probability level.
Hydrogen atoms and counteranions PF6 and BF4 are omitted for
clarity. The two types of phenyl groups (A and B) and fluxional
motion in 1 are shown in parts a and d, respectively.
1
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bonded to the phosphorus atom in 1−3 are nonequivalent
because of the weak π−π interaction between two type A
1
phenyl groups. However, the H NMR spectra of 1 in CD2Cl2
at 300 and 220 K show ring protons of A and B as a single
broad peak (see Figure S8 in the SI). This can be attributed to
rapid interconversion based on a dynamic fluxional process in 1
(Figure 1d). In the case of 2 at 220 K, the protons attached to
phenyl and tolyl groups are observed as four kinds of well-
resolved signals consistent with aryl groups, and the methyl
protons of LMe appear as two sharp singlets (see Figure S9 in
the SI). These two singlets become one broad singlet at 300 K
in CD2Cl2 (see Figure S10 in the SI). However, the four
multiplets observed for the two kinds of tolyl and phenyl
groups are still observed at 300 K (see Figure S8 in the SI),
suggesting there is no fluxional motion of 2 at either 220 or 300
K. A possible explanation for this is that a small waving motion
of the two diphosphine ligands in 2 makes the methyl protons
of LMe chemically equivalent at 300 K. In contrast, the 1H NMR
spectrum of 3 exhibits two sharp triplets [CH3(A), 0.17 ppm;
CH3(B), 0.38 ppm] corresponding to the terminal methyl
protons of LEt (Figure 3). Moreover, the methylene protons of
Figure 4. Absorption and corrected emission spectra of L (red), LMe
(purple), LEt (green), and LiPr (orange) in 2-MeTHF at 77 K; λexc
=
300 nm.
properties are summarized in Table 2. These diphosphine
ligands exhibit quite similar behavior; the absorption spectra
Table 2. Photophysical Data for Diphosphine Ligands in 2-
MeTHF
absorption λmax/nm (ε/
emission
Epa (V) vs Fc/
a
a
b
c
ligand
M−1 cm−1
)
λmax/nm
ΦPL
Fc+
L
284 (1.51 × 104)
290 (1.53 × 104)
292 (1.73 × 104)
300 (1.58 × 104)
499
500
505
500
0.68
0.75
0.70
0.69
+0.36
+0.40
+0.44
+0.38
LMe
LEt
LiPr
a
Absorption and PL peak wavelength in 2-MeTHF at 77 K; λexc = 300
b
nm. Absolute PL quantum yield in 2-MeTHF at 77 K; λexc = 300 nm.
c
Anodic peak potential in argon-saturated CH3CN at 293 K. These
waves are irreversible.
display broad, intense bands at 284−300 nm [extinction
coefficient (ε) = ∼16000 M−1 cm−1], which are characteristic of
arylphosphine compounds, as shown in Figure 4.22 These
bands are assigned to a mixed transition of l−π* and π−π*; the
former is the transition of an electron from the lone-pair orbital
on phosphorus to an empty antibonding π orbital on an o-
phenylene or phenyl-type ring, while the latter are transitions
localized on a phenyl-type or phenylene ring and those from a
phenyl-type ring to a phenylene ring. These assignments are
consistent with recent MO calculations on dppb.23 Solutions of
all of these diphosphine ligands are not emissive at room
temperature, but strong blue-green emission (λmax = 499−505
nm; Φp = 0.68−0.75) is observed in frozen 2-MeTHF at 77 K
(Figure 4). Our calculations revealed that the emitting state of
dppb is mainly the charge-transfer state from the lone pair on
the phosphorus atoms to the o-phenylene groups.9 The dppb
derivatives exhibit absorption and phosphorescence spectra
quite similar to those of dppb, suggesting that alkyl substituents
(methyl, ethyl, and isopropyl) at ortho positions in peripheral
aryl rings have a small effect on the electronic states of these
ligands.
The electrochemical properties of the diphosphine ligands
were investigated by cyclic voltammetry (CV). The oxidation
potentials of diphosphine ligands (L, LMe, LEt and LiPr) are very
similar to each other, as shown in Table 2.
Photophysical Properties of Silver(I) Complexes 1−3
in 2-MeTHF Glasses at 77 K and a Comparison with MO
Calculations. Figure 5 shows the absorption and emission
spectra of silver(I) complexes 1−3 at 77 K in 2-MeTHF.
Absorption and emission peaks, lifetimes (τT), Φp at 77 K, and
DFT calculation data (dihedral angles and T1 energy levels) are
1
Figure 3. H NMR spectrum of 3 (in CD2Cl2 at 300 K).
the ethyl substituents are split into two doublet of quartets and
one multiplet consisting of two doublet of quartets (Ha−d)
with J = 7.8 and 15.8 Hz in CD2Cl2 at 300 K. Resonance signals
(Ha, 2.12 ppm; Hb, 2.57 ppm; Hc, 2.29 ppm; Hd, 2.32 ppm)
1
were assigned using COSY H NMR spectra. These results
demonstrate that complex 3 does not exhibit fluxional motion
or rotation about the Caryl−Cethylene bonds. Similarly, the
motion of the P−Caryl bonds in 3 is obstructed in solution at
ambient temperature on the NMR time scale. The NMR
studies of 1−3 lead to the conclusion that interligand steric
interactions fix the geometry of 3 in solution.
Photophysical Properties of dppb Derivatives. dppb
derivatives have two types of aromatic groups attached to
phosphorus atoms; a bridging o-phenylene group and four
peripheral phenyl groups (phenyl for L, methylphenyl for LMe,
ethylphenyl for LEt, and isopropyl for LiPr). The absorption and
phosphorescence spectra of the diphosphine ligands in 2-
MeTHF at 77 K are shown in Figure 4, and their photophysical
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[approximately the highest occupied molecular orbitals
(HOMO)] and electron [approximately the lowest unoccupied
molecular orbital (LUMO)] in T1 in the optimized S0 structure
of 3 are shown in Figure 6. The hole distribution in 3 is mainly
Figure 6. NTO pairs for the lowest triplet excited state of 3 in the
optimized S0 geometry. The generation probability is 56%.
confined to the orbitals of the σ bonds between the silver(I)
and two phosphorus (LEt) atoms and slightly extended over the
π system of LEt; the contributions from the silver, phosphorus,
and other (π) systems are 4, 22, and 74%, respectively (see
Table S2 in the SI). The electron distribution is essentially
delocalized over LEt. These results indicate that strong blue
phosphorescence mainly results from π−π* transitions (the
contribution is about 81%), which include transitions localized
on a phenyl group and a phenylene ring as well as those with
charge-transfer character from a phenyl-type ring to a
phenylene ring. The long lifetimes (2.3−2.8 ms) of 1−3 at
77 K support the assignment of the origin of phosphorescence
as π−π*. Meanwhile, the contribution from the σ−π*
transition to phosphorescence is 19%. The σ−π* transition
contains contributions from MLCT (4%) and intraligand
charge-transfer (ILCT, 15%) transitions because the σ orbital is
primarily composed of d orbitals (silver atom; dxy, dyz, dzx) and
p orbitals (phosphorus atoms) including lone-pair electrons.
Complexes 1 and 2 show similar NTO maps (see Figures S17
and S18 in the SI) and origin of phosphorescence to 3 (see
Tables S3 and S4 in the SI); the contribution from π−π* is
78% for 1 and 82% for 2.
Molecular motion in 2-MeTHF at 77 K is strongly
prohibited in both the ground and excited states because of
the high viscosity of the medium. Thus, the structures of
complexes 1−3 resemble the optimized S0 with pseudotetrahe-
dral geometry even in the T1 state. λmax is located at 441 nm for
1, 473 nm for 2, and 480 nm for 3. From Table 3, it is obvious
that λmax shifts to longer wavelength as the dihedral angle of the
optimized S0 structure of the complex decreases.
Photophysical Properties of Silver Complexes 1−3 at
Room Temperature. Figure 5 shows the absorption and
emission spectra of silver complexes 1−3 at 293 K in 2-
MeTHF. Photophysical and calculated data are summarized in
Table 4. The optimized T1 structures in THF were used for
these calculations. Complex 1 exhibits bright blue emission in
2-MeTHF glasses, whereas it shows weak orange luminescence
(λmax = 670 nm; Φp = 0.05) in degassed 2-MeTHF at room
temperature (red dashed and solid lines in Figure 5B).
Although the MLCT character responsible for the Jahn−Teller
effect is not strong (4% for 1), the large red shift of the
emission peak of 230 nm and the low Φp in 2-MeTHF at room
temperature indicate that a large change in the geometry of 1
occurs in the excited state from pseudotetrahedral to a more
Figure 5. (A) Absorption and (B) corrected emission spectra for 1
(red), 2 (purple), and 3 (green) in 2-MeTHF at 77 (dashed lines) and
293 K (solid lines); λexc = 300 nm.
summarized in Table 3. The optimized S0 structures in THF
were used for these calculations. Complexes 1−3 show
Table 3. Photophysical Data for Complexes 1−3 in 2-
MeTHF at 77 K
absorption λmax/nm
λmax/nm
a
dihedral
c
T /
1 c
a
b
complex
(ε/M−1 cm−1
)
(τ/ms)
ΦPL
angle/deg
nm
1
2
3
272 (3.77 × 104)
276 (4.01 × 104)
278 (4.46 × 104)
441 (2.3)
473 (2.5)
480 (2.8)
0.88
0.75
0.70
81
78
77
360
365
365
a
Absorption and PL peak wavelength in 2-MeTHF at 77 K; λexc = 300
b
nm. Absolute PL quantum yield in 2-MeTHF at 77 K; λexc = 300 nm.
c
Dihedral angle between two planes, P1−Ag−P2 and P3(1a)−Ag−
P4(2a), and the T1 ← S0 energy were calculated for the optimized S0
structures of 1−3.
absorption spectra typical of diphosphine ligands; broad
bands with maxima at 272−278 nm are assigned to a mixed
transition of σ → π* and π−π*. Complexes 1−3 show efficient
luminescence performance at 77 K, as listed in Table 3. Strong
blue phosphorescence from 1−3 was observed in frozen 2-
MeTHF (λmax = 441−480 nm; Φp = 0.70−0.88). Because a
change of the structure of the excited state is highly restricted in
2-MeTHF glass at 77 K because of the rigidity of the medium,
blue phosphorescence is concluded to arise from an excited
state with a tetrahedral structure.
The MO calculations determined the dihedral angles
between the two planes P1−Ag−P2 and P3(1a)−Ag−P4(2a)
for 1−3 in a THF solution to be 81, 78, and 77°, respectively,
which are smaller than those found in X-ray crystal structures.
This can be attributed to the presence of a BF4− counteranion,
trapped solvent in the crystal lattice, and weak noncovalent
interactions (π−π, hydrogen-bonding, and CH−π interactions)
in single crystals.
NTO analyses were performed to clarify the origin of
phosphorescence in the complexes. Maps of the hole
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Table 4. Photophysical Data for Complexes 1−3 in 2-MeTHF at 293 K
a
a
b
d
c
c
e
f
complex
absorption λmax/nm (ε/M−1 cm−1
)
λmax/nm (τ/μs)
ΦPL
dihedral angle/deg (Δθ /deg)
T1/nm
E1/2/V (ΔEp/mV)
1
2
3
270 (3.63 × 104)
274 (4.58 × 104)
274 (4.01 × 104)
670 (22)
527 (10)
509 (15)
0.05
0.33
0.26
30 (51)
65 (13)
70 (7)
612
596
526
+0.48 (80)
+0.70 (80)
+0.72 (130)
a
b
Absorption and PL peak wavelength in 2-MeTHF at 77 K; λexc = 300 nm. Absolute PL quantum yield in 2-MeTHF at 77 K; λexc = 300 nm.
c
Dihedral angle between two planes, P1−Ag−P2 and P3(1a)−Ag−P4(2a), and T1 ← S0 energy were calculated for the optimized T1 structures of
d
e
1−3. Difference in dihedral angles between optimized S0 and T1 structures. Half-wave potential (vs Fc/Fc+) recorded in argon-saturated CH3CN.
f
Peak-to-peak separation.
flattened structure, which is accompanied by energetic
relaxation. The calculated dihedral angle (30°) and T1 energy
(612 nm) of 1 in the optimized T1 structure also suggest that
the structure of the excited state is flatter; the angular difference
from the optimized S0 structure is as large as 51° (Table 4).
Complexes 2 and 3 exhibit intense green phosphorescence
(Φp = 0.33 and 0.26) in a 2-MeTHF solution at 293 K (purple
lines for 2 and green for 3 in Figure 5B). The phosphorescence
peaks are located at longer wavelengths than those at 77 K.
However, the shifts are smaller: 54 nm for 2 and 29 nm for 3.
Therefore, the excited states of 2 and 3 undergo some changes
in geometry at room temperature, resulting in a small energetic
relaxation of the phosphorescent state.
The emitting state in solution at 293 K is considered to have
the optimized T1 structure. The MO calculations revealed that
the calculated dihedral angles between two planes, P1−Ag−P2
and P3(1a)−Ag−P4(2a), in the optimized T1 structures are 65°
for 2 and 70° for 3. These angles are at most 13° smaller than
those in the optimized S0 structures. The interligand steric
interactions caused by alkyl substituents at the ortho positions
in LMe and LEt probably prevent adoption of a flattened
geometry.
silver and four phosphorus atoms, and the distribution on the π
system appreciably decreases compared to that of 3 in the
optimized S0 structure, as seen in Figure 6. The electron
distribution is largely confined to the o-phenylene group. Thus,
the contribution of π−π* in phosphorescence decreases from
81% to 47%, and that of σ−π* increases to 53%; attributable
fractions of MLCT and ILCT are 10% and 43%, respectively
(see Table S5 in the SI). It is noteworthy that a small distortion
(difference of the dihedral angle is 7°) between the optimized
S0 and T1 structures in the excited state of 3 considerably alters
the origin of phosphorescence. The distortion increases the
hole energy levels by reducing the symmetry (from D2d), which
removes the degeneracy of the HOMO energy levels. If the
hole energy levels increase, the contribution of π orbitals in
diphosphine ligands to the hole decreases, so the π−π*
character of phosphorescence is reduced.
When the contribution of the d orbitals in the silver atom
and p orbitals in phosphorus atoms to the hole increases, the
σ−π* character of phosphorescence increases. Complex 2
possesses NTO maps similar to those of 3 (see Figure S19 in
the SI), while details of the origin of phosphorescence are
shown in Table S6 in the SI. The contributions of σ−π* and
π−π* transitions are 52% and 48%, respectively, for 2.
A single reversible wave was observed for silver complexes
1−3 by CV at E1/2 = +0.48, +0.70, and +0.72V vs Fc/Fc+ in
CH3CN, respectively (Table 4). Silver(I) in 1 is oxidized more
readily than that in 2 and 3. This result is consistent with the
fact that interligand steric interactions stabilize the tetrahedral
silver(I) versus flattened silver(II) complexes in 2 and 3 in
solution at 293 K. A similar correlation between the degree of
steric effects and the one-electron-oxidation potential was
reported for tetrahedral copper(I) diimine complexes.24
NTO analysis based on the optimized T1 structure in THF
showed that transitions can be described as single-electron
pairs, which reproduce over 90% of the transition density. The
maps from NTO analysis based on the optimized T1 structure
of 3 in THF are shown in Figure 7 as an example. It is clear that
the hole distribution of 3 mainly lies on σ orbitals composed of
CONCLUSIONS
■
The synthesis and photophysics of heteroleptic tetrahedral
silver(I) complexes 2 and 3 containing diphosphine ligands
have been reported. 1H and 31P{1H} NMR experiments
confirmed that the stability of heteroleptic silver(I) complexes
2 and 3 in solution and the alkyl substituents at the ortho
positions of phenyl groups bonded to phosphorus atoms in
diphosphine ligands induce a rigid environment around the
silver center in 2 and 3. Interligand steric interactions between
the alkyl groups inhibit geometric relaxation of the excited state,
leading to intense green phosphorescence from 2 and 3 in
solution even at ambient temperature. The quantum yields of
phosphorescence for 2 and 3 at 293 K are 0.33 and 0.26, which
are ca. 6 times as large as that of the homoleptic complex 1. On
the basis of theoretical studies, the origin of green
phosphorescence from the complexes 1−3 is ascribed to a
mixture of σ−π* and π−π* transitions.
We believe that the introduction of o-alkyl substituents onto
the phenyl groups of dppb ligands is an effective way to induce
interligand steric interactions in metal complexes. Interligand
steric interactions are expected to improve the luminescence of
metal complexes in both solid crystals and solution.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data for complexes 2 and 3 in CIF
format, X-ray crystallographic data of complexes 1−3, NMR
spectra and cyclic voltammograms of ligands and complexes 1−
Figure 7. NTO pairs for the lowest triplet excited state of 3 in the
optimized T1 geometry. The generation probability is 90%.
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3, NTO pairs for the T1 state, compositions of the hole and
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structures of 1−3. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: osawa@postman.riken.jp.
■
Notes
The authors declare no competing financial interest.
(7) Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.;
Osawa, M. J. Am. Chem. Soc. 2011, 133, 10348−10351.
ACKNOWLEDGMENTS
The authors thank Dr. Daisuke Hashizume for technical
assistance with X-ray structural analysis.
■
(8) Hoshino, M.; Sonoki, H.; Miyazaki, Y.; Iimura, Y.; Yamamoto, K.
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