Phosphane tuning in heteroleptic [Cu(N^N)(P^P)]+ complexes for light-emitting electrochemical cells

Article abstract of DOI:10.1039/c8dt03827a

The synthesis and characterization of five [Cu(P^P)(N^N)][PF₆] complexes in which P^P = 2,7-bis(tert-butyl)-4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (^tBu₂xantphos) or the chiral 4,5-bis(mesitylphenylphosphino)-9,9-dimethylxanthene (xantphosMes₂) and N^N = 2,2′-bipyridine (bpy), 6-methyl-2,2′-bipyridine (6-Mebpy) or 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me₂bpy) are reported. Single crystal structures of four of the compounds confirm that the copper(i) centre is in a distorted tetrahedral environment. In [Cu(xantphosMes₂)(6-Mebpy)][PF₆], the 6-Mebpy unit is disordered over two equally populated orientations and this disorder parallels a combination of two dynamic processes which we propose for [Cu(xantphosMes₂)(N^N)]⁺ cations in solution. Density functional theory (DFT) calculations reveal that the energy difference between the two conformers observed in the solid-state structure of [Cu(xantphosMes₂)(6-Mebpy)][PF₆] differ in energy by only 0.28 kcal mol^?1. Upon excitation into the MLCT region (λ_exc = 365 nm), the [Cu(P^P)(N^N)][PF₆] compounds are yellow to orange emitters. Increasing the number of Me groups in the bpy unit shifts the emission to higher energies, and moves the Cu⁺/Cu²⁺ oxidation to higher potentials. Photoluminescence quantum yields (PLQYs) of the compounds are low in solution, but in the solid state PLQYs of up to 59% (for [Cu(^tBu₂xantphos)(6,6′-Me₂bpy)]⁺) are observed. Increased excited-state lifetimes at low temperature are consistent with the complexes exhibiting thermally activated delayed fluorescence (TADF). This is supported by the small energy difference calculated between the lowest-energy singlet and triplet excited states (0.17-0.25 eV). The compounds were tested in simple bilayer light-emitting electrochemical cells (LECs). The optoelectronic performances of complexes containing xantphosMes₂ were generally lower with respect to those with ^tBu₂xantphos, which led to bright and efficient devices. The best performing LECs were obtained for the complex [Cu(^tBu₂xantphos)(6,6′-Me₂bpy)][PF₆] due to the increased steric hindrance at the N^N ligand, resulting in higher PLQY.

Full text of DOI:10.1039/c8dt03827a

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Phosphane tuning in heteroleptic [Cu(N^N)(P^P)]+
complexes for light-emitting electrochemical
cells†
Cite this: Dalton Trans., 2019, 48,
446
Fabian Brunner, a Azin Babaei,b Antonio Pertegás,b
a
José M. Junquera-Hernández, b Alessandro Prescimone,
Edwin C. Constable, a Henk J. Bolink, b Michele Sessolo, *b Enrique Ortí
*
b
a
and Catherine E. Housecroft
*
The synthesis and characterization of five [Cu(P^P)(N^N)][PF6] complexes in which P^P = 2,7-bis(tert-
butyl)-4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (tBu2xantphos) or the chiral 4,5-bis(mesityl-
phenylphosphino)-9,9-dimethylxanthene (xantphosMes2) and N^N = 2,2’-bipyridine (bpy), 6-methyl-2,2’-
bipyridine (6-Mebpy) or 6,6’-dimethyl-2,2’-bipyridine (6,6’-Me2bpy) are reported. Single crystal structures
of four of the compounds confirm that the copper(I) centre is in a distorted tetrahedral environment. In
[Cu(xantphosMes2)(6-Mebpy)][PF6], the 6-Mebpy unit is disordered over two equally populated orien-
tations and this disorder parallels a combination of two dynamic processes which we propose for
[Cu(xantphosMes2)(N^N)]+ cations in solution. Density functional theory (DFT) calculations reveal that the
energy difference between the two conformers observed in the solid-state structure of
[Cu(xantphosMes2)(6-Mebpy)][PF6] differ in energy by only 0.28 kcal mol−1. Upon excitation into the MLCT
region (λexc = 365 nm), the [Cu(P^P)(N^N)][PF6] compounds are yellow to orange emitters. Increasing the
number of Me groups in the bpy unit shifts the emission to higher energies, and moves the Cu+/Cu2+ oxi-
dation to higher potentials. Photoluminescence quantum yields (PLQYs) of the compounds are low in solu-
tion, but in the solid state PLQYs of up to 59% (for [Cu(tBu2xantphos)(6,6’-Me2bpy)]+) are observed.
Increased excited-state lifetimes at low temperature are consistent with the complexes exhibiting thermally
activated delayed fluorescence (TADF). This is supported by the small energy difference calculated between
the lowest-energy singlet and triplet excited states (0.17–0.25 eV). The compounds were tested in simple
bilayer light-emitting electrochemical cells (LECs). The optoelectronic performances of complexes contain-
Received 21st September 2018,
Accepted 8th November 2018
t
ing xantphosMes2 were generally lower with respect to those with Bu2xantphos, which led to bright and
efficient devices. The best performing LECs were obtained for the complex [Cu(tBu2xantphos)(6,6’-
DOI: 10.1039/c8dt03827a
rsc.li/dalton
Me2bpy)][PF6] due to the increased steric hindrance at the N^N ligand, resulting in higher PLQY.
(LECs) and interest in these devices has grown tremendously
in the last few years.1–4 OLEDs are now well established and
Introduction
Solid-state lighting technologies include organic-light emitting are widely employed in display applications. LECs
diodes (OLEDs) and light-emitting electrochemical cells feature many of the advantages of OLEDs including direct
electron-to-photon conversion and the possibility of fabrication
employing flexible surfaces and thin-film processing.
Additionally, the simple device architecture of LECs and the
aDepartment of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel,
Switzerland. E-mail: catherine.housecroft@unibas.ch
use of air-stable electrode materials might reduce the manu-
bInstituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Valencia,
facturing cost of electroluminescent devices and widen their
Spain. E-mail: enrique.orti@uv.es, michele.sessolo@uv.es
field of applications.5–7 LECs incorporating ionic transition-
†Electronic supplementary information (ESI) available: Synthetic experimental
metal complexes (iTMCs) based on iridium (and to a lesser
extent ruthenium) have been the focus of intense investi-
gations and show good performances in terms of colour tun-
ability, brightness and device lifetime.8–14 However, the
limited availability of iridium and ruthenium in the Earth’s
crust motivated the search for alternative emissive materials.
details. Fig. S1, S2 and S6–S10: IR spectra of ligands and complexes. Fig. S3 and
S11–S14: ORTEP-style plots of crystal structures. Fig. S4, S5 and S15–S22:
additional NMR figures. Fig. S23 and S24: Solution absorption and emission
spectra. Table S1: Selected structural parameters calculated at the B3LYP-D3/
(def2svp + def2tzvp) level. CCDC 1844060–1844063 and 1860879. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt03827a
446 | Dalton Trans., 2019, 48, 446–460
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
LECs employing copper(I) iTMCs show promising character- of substituents in the 4,4′-positions of bpy in [Cu(xantphos)
istics. McMillin and co-workers first reported the photolumi- (4,4′-R2bpy)][BF4] on their photo- and electroluminescent pro-
nescent properties of copper(I) complexes containing 2,2′- perties and found a direct correlation between the σ-Hammett
bipyridine (bpy) or 1,10-phenanthroline (phen) and PPh3 or parameters of the R groups in 4,4′-R2bpy and the device
chelating bis(phosphane) ligands,15,16 and the applications of efficiency.27 Most attention has focused on the structural and
[Cu(N^N)(P^P]+ compounds (N^N = diimine, P^P = bis(phos- electronic modification of the N^N ligand in [Cu(P^P)(N^N)]+
phane) in LECs followed.9 It has been shown that in many to tune the emission properties of the complexes and their
[Cu(N^N)(P^P]+ compounds, thermally activated delayed fluo- LEC behaviour.9,24,28–35
rescence (TADF) is a key feature in determining their photo-
For the P^P ligand in [Cu(P^P)(N^N)]+ complexes, the com-
luminescent properties.17–22 In this mechanism, the energy mercially available POP and xantphos are common choices,
difference between the lowest-energy singlet and triplet excited although there is some interest in complexes incorporating
states is sufficiently small to allow repopulation of the singlet 1,2-bis(diphenylphosphino)benzene36 and 9,9-dimethyl-4,5-
from the triplet state at room temperature. This enables the bis(di-tert-butylphosphino)xanthene.37 In the latter case, it has
indirect harvesting of fluorescence from the triplet state, been observed that the photoluminescence (PL) of [Cu(P^P)
which boosts device performance.23,24
(N^N)]+ species is significantly reduced when the phenyl
[Cu(P^P)(N^N)]+ complexes in which P^P is bis(2-(diphenyl- groups in xantphos (Scheme 1) are replaced by tert-butyl sub-
phosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)- stituents, attributed to vibrational quenching effects.37 Thus,
9,9-dimethylxanthene (xantphos) (Scheme 1) and N^N is a bpy for good photophysical properties it appears crucial to retain
or phen ligand have been studied systematically in an effort to aryl substituents on the phosphorus atoms in ligands related
enhance their photoluminescent properties and device per- to xantphos or POP.
formances. We have investigated the influence of introducing
Here, we report on the effects of modifying the xantphos
alkyl and phenyl substituents into the 6- and 6,6′-positions of ligand and investigate the influence on both the structural and
bpy in [Cu(P^P)(bpy)]+ complexes, and demonstrated improved electronic properties. It has previously been reported that in
emission and device performance with LECs containing [Cu(POP)(N^N)]+ and [Cu(xantphos)(N^N)]+ complexes, the
[Cu(xantphos)(Mebpy)][PF6], [Cu(xantphos)(Etbpy)][PF6] and HOMO is mainly centred on copper with small contributions
[Cu(POP)(Etbpy)][PF6] having lifetimes longer than 15, 40 and from phosphorus, while the LUMO is localized on the N^N
80 h, respectively.25,26 Weber et al. have studied the influence ligand.24,26 Thus, structural modifications made to the P^P
ligand will have little if any effect on the energy level of the
HOMO and any structural tuning should lead primarily to
changes in steric effects. We have introduced tert-butyl groups
into the 2,7-positions of xantphos to give tBu2xantphos
t
(Scheme 1); Bu2xantphos has previously been used in combi-
nation with palladium(II) in vinylarene hydroamination cataly-
sis.38 The introduction of peripheral tert-butyl groups was
expected to additionally result in a larger spatial separation of
complex ions in the active layer in a LEC and therefore have an
influence on the electroluminescent properties. The other P^P
ligands investigated in this work were 4,5-bis(dimesityl-
phosphino)-9,9-dimethylxanthene (xantphosMes4) and 4,5-bis
(mesitylphenylphosphino)-9,9-dimethylxanthene (xantphosMes2),
in which the PPh2 groups in xantphos were replaced by either
PMes2 or PPhMes units (Scheme 1), thereby tuning the Tolman
cone angle39 of the phosphane. The N^N ligands chosen for the
investigation were bpy, 6-Mebpy and 6,6′-Me2bpy (Scheme 1). The
photophysical properties of [Cu(xantphos)(N^N)][PF6] (N^N =
bpy, 6-Mebpy and 6,6′-Me2bpy) have been previously
reported,24–26 giving a benchmark series for the present
investigation.
Experimental
Details of general instruments are given in the ESI.†
tBu2xantphos38 and [Cu(MeCN)4][PF6]40 were prepared accord-
ing to literature procedures. Other chemicals were purchased
Scheme 1 Structures of the P^P and N^N ligands.
from Sigma Aldrich, Apollo Scientific, Angene or Fluorochem
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 446–460 | 447

View Article Online
Paper
Dalton Transactions
and were used as received. Details of the syntheses and (R1 all data = 0.0362), wR2 = 0.0851 (wR2 all data = 0.0870),
characterizations of 6-Mebpy, xantphosMes4, chloro(mesityl) GOF = 1.0356. CCDC 1844062.†
phenylphosphane, xantphosMes2, [Cu(tBu2xantphos)(bpy)][PF6],
[Cu(xantphosMes2)(6-Mebpy)][PF6]
[Cu(tBu2xantphos)(6-Mebpy)][PF6],
[Cu(tBu2xantphos)(6,6′-
Me2bpy)][PF6],
[Cu(xantphosMes2)(6-Mebpy)][PF6] are given in the ESI.†
[Cu(xantphosMes2)(bpy)][PF6]
and C56H54CuF6N2OP3, M = 1041.51, yellow block, monoclinic,
space group C2/c, a = 38.1932(19), b = 12.0648(6), c =
26.5270(13) Å, β = 127.0427(19)°, U = 9756.6(9) Å3, Z = 8, Dc =
1.418 Mg m−3, μ(Cu-Kα) = 2.117 mm−1, T = 123 K. Total 31 926
Crystallography
Data were collected on a Bruker Kappa Apex2 diffractometer reflections, 8749 unique, Rint = 0.033. Refinement of 7701
with data reduction, solution and refinement using the pro- reflections (685 parameters) with I > 2σ(I) converged at final
grams APEX41 and CRYSTALS.42 Structural analysis was carried R1 = 0.0467 (R1 all data = 0.0531), wR2 = 0.0586 (wR2 all data =
out using Mercury v. 3.7.43,44 In [Cu(tBu2xantphos)(6-Mebpy)] 0.0623), GOF = 0.9991. CCDC 1844061.†
[PF6]·1.5CH2Cl2·0.5H2O, one CH2Cl2 molecule was refined and
Computational details
SQUEEZE45 was used to treat part of the solvent region; for-
mulae and numbers were modified in the cif to keep this A set of density functional theory (DFT) calculations were per-
t
result into account. In [Cu(xantphosMes2)(6-Mebpy)][PF6], the formed for the [Cu(P^P)(N^N)]+ cations (P^P = Bu2xantphos
6-Mebpy is orientationally disordered over two orientations and xantphosMes2; N^N = bpy, 6′-Mebpy, and 6,6′-Me2bpy)
and the relevant aromatic rings were refined as rigid bodies.
using the A.03 revision of Gaussian 16.46 The Becke’s three-
parameter B3LYP exchange–correlation functional47,48 was
used in all the calculations. The “double-ζ” quality def2svp
xantphosMes2
C45H44OP2, M = 662.79, colourless plate, monoclinic, space basis set was employed for C, H, P, N and O atoms, whereas
group P21/n, a = 10.6380(9), b = 15.0841(13), c = 23.545(2) Å, β = the “triple-ζ” quality def2tzvp basis set was used for the Cu
102.585(3)°, U = 3687.4(5) Å3, Z = 4, Dc = 1.194 Mg m−3
,
atom.49,50 Intramolecular non-covalent interactions are
μ(Cu-Kα) = 1.317 mm−1, T = 123 K. Total 40 993 reflections, expected to play a relevant role in determining the molecular
6810 unique, Rint = 0.035. Refinement of 6264 reflections geometry of the studied complexes owing to the presence of
(433 parameters) with I> 2σ(I) converged at final R1 = 0.0336 the bulky xantphos-derived ligands. To get a better description
(R1 all data = 0.0365), wR2 = 0.0776 (wR2 all data = 0.0794), of those interactions, the D3 Grimme’s dispersion term with
GOF = 0.9775. CCDC 1860879.†
Becke–Johnson damping was added to the B3LYP functional
(B3LYP-D3).51,52 The geometries of all the complexes in both
their singlet ground electronic state (S0) and their lowest-
[Cu(tBu2xantphos)(bpy)][PF6]·0.5Et2O
C59H61CuF6N2O1.5P3, M = 1092.60, yellow block, triclinic, space energy triplet excited state (T1) were optimized without impos-
ˉ
group P1, a = 12.0991(12), b = 13.3253(13), c = 18.6750(18) Å, ing any symmetry restriction. For T1 the spin unrestricted
α = 91.353(3), β = 90.939(3), γ = 115.475(2)°, U = 2716.2(5) Å3, UB3LYP approximation was used with a spin multiplicity of
Z = 2, Dc = 1.336 Mg m−3, μ(Cu-Kα) = 1.932 mm−1, T = 123 K. three. The lowest-lying excited states of each complex, both
Total 35 686 reflections, 9826 unique, Rint = 0.029. Refinement singlets and triplets, were computed at the minimum-energy
of 9540 reflections (664 parameters) with I > 2σ(I) converged at geometry optimized for S0 using the time-dependent DFT
final R1 = 0.0416 (R1 all data = 0.0423), wR2 = 0.1012 (wR2 all (TD-DFT) approach.53–55 All the calculations were performed in
data = 0.1016), gof = 0.9707. CCDC 1844060.†
the presence of the solvent (CH2Cl2). Solvent effects were con-
sidered within the self-consistent reaction field (SCRF) theory
using the polarized continuum model (PCM) approach.56–58
[Cu(tBu2xantphos)(6-Mebpy)][PF6]·1.5CH2Cl2·0.5H2O
C59.5H62Cl3CuF6N2O1.5P3, M = 1205.97, yellow block, triclinic,
Device preparation and characterization
ˉ
space group P1, a = 10.7080(9), b = 13.4475(12), c = 22.167(2) Å,
α = 73.142(6), β = 79.483(6), γ = 86.606(6)°, U = 3003.4(5) Å3, Z = LECs were prepared on top of patterned indium tin oxide (ITO,
2, Dc = 1.33 Mg m−3, μ(Cu-Kα) = 2.997 mm−1, T = 123 K. Total 15 Ω sq−1) coated glass substrates previously cleaned by chemi-
35 561 reflections, 10 903 unique, Rint = 0.082. Refinement of cal and UV-ozone methods. Prior to the deposition of the emit-
7619 reflections (700 parameters) with I > 2σ(I) converged at ting layer, 80 nm thick films of poly-(3,4-ethylenedioxythio-
final R1 = 0.1178 (R1 all data = 0.1490), wR2 = 0.1151 (wR2 all phene):poly(styrenesulfonate) (PEDOT:PSS) (CLEVIOS™ P VP
data = 0.1470), GOF = 1.0317. CCDC 1844063.†
AI 4083, Heraeus) were coated in order to flatten the ITO elec-
trode and to increase its work function. The emitting layer
(100 nm thick) was prepared by spin-coating a dichloro-
[Cu(xantphosMes2)(bpy)][PF6]
C55H52CuF6N2OP3, M = 1027.49, yellow block, monoclinic, methane solution of the emitting compound with the addition
space group C2/c, a = 38.341(2), b = 11.8342(7), c = 26.5208(14) of the ionic liquid 1-ethyl-3-methylimidazolium hexafluoro-
Å, β = 126.4524(19)°, U = 9679.2(10) Å3, Z = 8, Dc = 1.410 phosphate [Emim][PF6] (>98.5%, Sigma-Aldrich), in
a
Mg m−3, μ(Cu-Kα) = 2.126 mm−1, T = 123 K. Total 29 726 reflec- 4 : 1 molar ratio. The devices were then transferred to an inert
tions, 8726 unique, Rint = 0.027. Refinement of 7795 reflections atmosphere glovebox (<0.1 ppm O2 and H2O), where the alu-
(613 parameters) with I > 2σ(I) converged at final R1 = 0.0320 minium cathode (100 nm) was thermally deposited in high
448 | Dalton Trans., 2019, 48, 446–460
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
vacuum using an Edwards Auto500 chamber integrated in the
glovebox. The thickness of all films was determined with an
Ambios XP-1 profilometer. The active area of the devices was
6.5 mm2. LECs were not encapsulated and were characterized
inside the glovebox at room temperature. The device lifetime
was measured by applying a pulsed current and monitoring
the voltage and luminance versus time by a True Colour Sensor
MAZeT (MTCSiCT Sensor) with a Botest OLT OLED Lifetime-
Test System. The electroluminescence (EL) spectra were
measured using an Avantes AvaSpec-2048 Fiber Optic
Spectrometer during device lifetime measurement.
Fig. 1 Crystallographically determined structure of the (S,S)-enantio-
mer of xantphosMes2. See also Fig. S3.†
Results and discussion
Preparation and characterization of P^P ligands
consistent with one phosphorus environment in each com-
t
The P^P ligand Bu2xantphos was synthesized using the litera-
1
pound. H and 13C NMR spectra (see Experimental section in
ture procedure.38 The synthetic routes to xantphosMes4 and
xantphosMes2 were based on the strategy of Hamann and
Hartwig to prepare bidentate phosphanes with varying steric
the ESI†) were assigned by 2D methods and were in accord
with functionalization in the 4,5-positions of the xanthene
unit (Scheme 2). The 1H NMR spectra are shown in Fig. S4 and
S5.† The 1H NMR spectrum of xantphosMes2 (Fig. S5†) also
shows the presence of a subspecies in solution, present in
<10% based on integration. The chemical shifts of the low
intensity signals and the presence of diagnostic NOESY peaks
suggest the major and minor species are structurally related,
and we assign them to the (rac)- and (meso)-forms, respectively.
Based on the preference seen in the solid-state, we propose
that the dominant species is the (rac)-form. Thus, the bispho-
sphane is preorganized to give particular diastereoisomers
upon complexation with copper(I) and this indeed is the case
as discussed later.
properties.59
The syntheses
of
xantphosMes4 and
xantphosMes2 are summarized in Scheme 2. Both compounds
were isolated as white solids, but facile oxidation to the phos-
phane oxides made it difficult to obtain analytically pure
samples. The electrospray (ESI) mass spectra of xantphosMes4
and xantphosMes2 showed base peaks at m/z 747.3 and 663.5,
respectively, arising from the [M + H]+ ions. The solid-state IR
spectra of tBu2xantphos and xantphosMes2 are shown in Fig. S1
and S2.† Single crystals of xantphosMes2 were grown from an
Et2O solution of the compound by slow evaporation. Fig. S3†
shows an ORTEP-style plot of the molecule and important bond
parameters are given in the figure caption. Few chiral xantphos-
derived ligands have been reported in the literature,60,61 and the
solid-state structure of xantphosMes2 represents the (rac)-form
of xantphosMes2. It crystallizes in the monoclinic P21/n space
group with both the (R,R)- and (S,S)-enantiomers in the unit
cell; the (S,S)-enantiomer is shown in Fig. 1. The xanthene unit
deviates very slightly from planarity, in contrast to the ‘bowl’
shape that is commonly adopted (see discussion below).
Preparation and characterization of copper(I) complexes
Attempts to prepare [Cu(xantphosMes4)(bpy)][PF6], [Cu
(xantphosMes4)(6-Mebpy)][PF6] and [Cu(xantphosMes4)(6,6′-
Mebpy)][PF6] by reaction of [Cu(MeCN)4][PF6] and
xantphosMes4 followed by the corresponding 2,2′-bipyridine
ligand failed to yield the desired heteroleptic complexes. The
31P spectrum of each crude reaction mixture was dominated by
a signal for the free ligand (δ −36.2 ppm), and also exhibited
several other unassigned signals. Evidence for the formation
of [Cu(xantphosMes4)]+ came from ESI mass spectrometry with
a peak envelope at m/z 809.5. Mononuclear [Cu(P^P)]+ com-
plexes containing sterically demanding substituents attached
to the phosphorus atoms are known, for example [Cu(tBu-
xantphos-κP,O,P)][PF6]37 (tBu-xantphos = 9,9-dimethyl-4,5-bis(di-
tert-butylphosphino)xanthene) and 3-coordinate copper(I)
complexes with P^N^P pincer ligands.62 Since xantphosMes4
proved to be too sterically demanding for the formation of
[Cu(xantphosMes4)(N^N)]+ species, we turned our attention to
the use of xantphosMes2.
The 31P NMR spectra of xantphosMes4 and xantphosMes2
exhibit resonances at δ −36.2 and −25.8 ppm, respectively,
Heteroleptic [Cu(P^P)(N^N)]+ complexes with P^P
=
tBu2xantphos or xantphosMes2 were prepared using the estab-
lished procedure24 by the addition of a mixture of the
xantphos and bpy ligands to a solution of [Cu(MeCN)4][PF6] in
Scheme 2 Syntheses of xantphosMes4 and xantphosMes2. Conditions:
(i) nBuLi, dry heptane, TMEDA, reflux, 20 min; (ii) Mes2PCl, THF, 0 °C, 1 h;
(iii) MesPhPCl, THF, 0 °C, 1 h.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 446–460 | 449

View Article Online
Paper
Dalton Transactions
CH2Cl2. This procedure avoids competitive formation of homo- each complex. The tBu2xantphos-containing compounds
leptic [Cu(P^P)2]+ complexes.63 Homoleptic [Cu(N^N)2]+ com- crystallized in the triclinic space group P1, whereas
ˉ
plexes where N^N is a bpy derivative, and heteroleptic [Cu(xantphosMes2)(bpy)][PF6]
and
[Cu(xantphosMes2)(6-
[Cu(xantphos)(N^N)]+ species are typically red and yellow Mebpy)][PF6] crystallized in the monoclinic space group C2/c.
respectively. In all cases, the reaction mixture remained orange Fig. S11–S14† show ORTEP-style plots of the [Cu(tBu2xantphos)
after being stirred for 1–2 hours, suggesting incomplete con- (bpy)]+, [Cu(tBu2xantphos)(6-Mebpy)]+, [Cu(xantphosMes2)
version to the heteroleptic complex. To force full conversion, an (bpy)]+ and [Cu(xantphosMes2)(6-Mebpy)]+ cations. Selected
additional 0.2 equivalents of the bisphosphane ligand were structural parameters are given in the captions to Fig. S11–
added, resulting in the solution turning yellow. After being S14,† and important parameters are compared in Table 1. In
stirred for a further 2.5 hours, the solvent was removed and the each cation, the copper(I) centre is in a distorted tetrahedral
excess ligand was removed by washing with Et2O. This pro- geometry and the xanthene backbone adopts a ‘bowl’ (boat)
cedure yielded [Cu(tBu2xantphos)(bpy)][PF6], [Cu(tBu2xantphos) conformation, which is observed for the majority of free and
(6-Mebpy)][PF6],
[Cu(xantphosMes2)(bpy)][PF6]
[Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6], coordinated xantphos ligands. A search of the Cambridge
and
[Cu(xantphosMes2) Structural Database (CSD65 v. 5.39, Conquest43 v. 1.2, search 10
(6-Mebpy)][PF6] as yellow powders. In the reaction between August 2018) revealed 254 xantphos-containing structures,
[Cu(MeCN)4][PF6], xantphosMes2 and 6,6′-Me2bpy, the yellow with flattening of the xanthene unit usually being associated
colour corresponding to
a
heteroleptic complex was not with a κP,O,P coordination mode. The angle between the P–Cu–
observed and only a red solid, identified by NMR spectroscopy P and N–Cu–N planes (Table 1) is close to 90° in [Cu(P^P)(6-
and mass spectrometry as [Cu(6,6′-Me2bpy)2][PF6],64 could be Mebpy)][PF6] and slightly smaller in [Cu(P^P)(bpy)][PF6]. The
isolated. We suggest that the steric demands of the substituents bite angle of the P^P ligand increases slightly on going from
in 6,6′-Me2bpy combined with the two mesityl groups in tBu2xantphos to xantphosMes2 (Table 1), consistent with the
xantphosMes2
[Cu(xantphosMes2)(6,6′-Me2bpy)][PF6].
militate
against
the
formation
of increased steric effects of PPhMes versus PPh2.
On going from [Cu(P^P)(bpy)]+ to [Cu(P^P)(6-Mebpy)]+, the
1
The five heteroleptic compounds were characterized by H, introduction of the 6-methyl group in the bpy unit lowers the
13C and 31P NMR spectroscopies, elemental analysis, ESI mass symmetry of the cation. The methyl substituent can, in prin-
spectrometry and IR spectroscopy (see Fig. S6–S10†), as well as ciple, lie over the xanthene ‘bowl’ or be remote from it.26
representative single crystal structures. The ESI mass spectrum Fig. 2a shows that in [Cu(tBu2xantphos)(6-Mebpy)][PF6], the
of each complex containing tBu2xantphos exhibited a base methyl group lies over the xanthene unit. In contrast, in
peak corresponding to the [M–PF6]+ ion. The ESI mass spec- [Cu(xantphosMes2)(6-Mebpy)][PF6], the 6-Mebpy ligand is orienta-
trum of [Cu(xantphosMes2)(bpy)][PF6] showed a base peak
envelope at m/z 725.4 assigned to [Cu(xantphosMes2)]+ and a
lower intensity peak envelope at m/z 881.5 arising from
[M − PF6]+. For [Cu(xantphosMes2)(6-Mebpy)][PF6], the ESI
mass spectrum exhibited only a peak envelope for the
[Cu(xantphosMes2)]+ ion (m/z 725.4). Elemental analysis and
the NMR spectra confirmed the formation of a heteroleptic
complex. The solution NMR spectroscopic properties are dis-
cussed after the solid-state structures.
Single crystal structures of copper(I) complexes
Single crystals of [Cu(tBu2xantphos)(bpy)][PF6]·0.5Et2O,
[Cu
(xantphosMes2)(bpy)][PF6] and [Cu(xantphosMes2)(6-Mebpy)]
Fig. 2 (a) Structure of the [Cu(tBu2xantphos)(6-Mebpy)]+ cation. (b)
Structure of one conformer of [Cu(xantphosMes2)(6-Mebpy)]+ in [Cu
(xantphosMes2)(6-Mebpy)][PF6] in which the 6-Mebpy ligand is dis-
[Cu(tBu2xantphos)(6-Mebpy)][PF6]·1.5CH2Cl2·0.5H2O,
[PF6] were obtained by layering Et2O over a CH2Cl2 solution of ordered over two sites (see text). H atoms are omitted.
Table 1 Selected structural parameters of [Cu(P^P)(N^N)]+ complex cations
P–Cu–P chelating
angle/°
N–Cu–N chelating
angle/°
Angle between P–Cu–P
and N–Cu–N planes/°
Complex cation
N–C–C–N torsion angle/°
[Cu(tBu2xantphos)(bpy)]+
113.56(2)
112.70(7)
115.96(2)
115.25(3)
79.90(7)
80.1(2)
79.08(6)
78.6(1)a
79.4(1)b
84.1
89.3
86.1
89.3a
89.0b
−0.4(3)
2.5(8)
[Cu(tBu2xantphos)(6-Mebpy)]+
[Cu(xantphosMes2)(bpy)]+
[Cu(xantphosMes2)(6-Mebpy)]+
15.0(2)
−23.3(6)a
−22.5(6)b
a Values for the conformer with 6-Mebpy oriented with the 6-Me group away from the xanthene bowl as in Fig. 2b. b Values for the conformer
with 6-Mebpy oriented with the 6-Me group lying over the xanthene bowl.
450 | Dalton Trans., 2019, 48, 446–460
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
t
NOESY, HMBC and HMQC methods. The Bu substituents give
rise to a singlet at δ 1.11 ppm in [Cu(tBu2xantphos)(bpy)][PF6] and
δ 1.16 ppm in [Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6]. Fig. 4 and
S16† reveal that the two pyridine rings of bpy in
[Cu(tBu2xantphos)(bpy)][PF6] and [Cu(xantphosMes2)(bpy)][PF6]
are magnetically equivalent. Similarly, the 6,6′-Me2bpy ligand
in [Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6] is symmetric on the
NMR timescale at room temperature (Fig. S17†). We have pre-
viously detailed a solution dynamic behaviour for [Cu(xant-
phos)(N^N)]+ complexes involving inversion of the xanthene
unit (‘bowl’),26,32 and this is depicted in the first dynamic
process illustrated in Fig. 5. The scheme demonstrates that
inversion of the xanthene bowl exchanges the environments of
Fig. 3 Structure of the [Cu(xantphosMe2)(bpy)]+ cation (a) emphasizing
the relative positions of the phenyl and mesityl substituents with respect
to the bpy unit, and (b) with the bpy (blue) and one mesityl group (red)
shown in space-filling representation to emphasize their spatial
proximity.
tionally disordered over two sites, each with 50% occupancy.
Fig. 2b depicts the conformer in which the Me group is remote
from the xanthene unit; the second conformer is structurally
related to [Cu(tBu2xantphos)(6-Mebpy)]+ (Fig. 2a). The N–C–C–N
torsion angles in Table 1 demonstrate that the bpy unit is
significantly more twisted in the cations containing the
xantphosMes2 ligand than those with tBu2xantphos. This
appears to be associated with the fact that in both
[Cu(xantphosMes2)(bpy)][PF6] (Fig. 3) and [Cu(xantphosMes2)
(6-Mebpy)][PF6] one methyl group of one mesityl substituent is
directed towards the middle of the bpy domain (Fig. 3b). This
spatial proximity is characterized by CMe(Mes)⋯centroidpyridine
distances of 3.98 and 4.37 Å (HMe(Mes)⋯centroidpyridine
=
3.16 and 3.56 Å) in the [Cu(xantphosMes2)(bpy)]+ cation.
Corresponding separations in [Cu(xantphosMes2)(6-Mebpy)]+
are 4.00 and 4.38 Å (3.24 and 3.57 Å) and 3.85 and 4.39 Å (3.27
and 3.48 Å) for the two partial occupancy 6-Mebpy sites.
Solution NMR spectroscopy
The aromatic regions of the solution 1H NMR spectra of
[Cu(tBu2xantphos)(bpy)][PF6], [Cu(xantphosMes2)(bpy)][PF6] and
[Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6] are shown in Fig. 4 (see also
Fig. S15†), S16 and S17,† respectively. All signals are sharp and
well-resolved at room temperature and were assigned using COSY,
Fig. 5 Upper: Proposed dynamic processes in [Cu(xantphos)(bpy)]+
type compounds. NA and NB represent pyridine rings
For tBu2xantphos, R1
R2 Ph. For xantphosMes2. R1
R2 Ph. Lower: Crystallographically determined structure of the
[Cu(xantphosMes2)(6-Mebpy)]+ cation in which the 6-Mebpy is orienta-
tionally disordered: (a) orientation 1 of 6-Mebpy, and (b) orientation 2 of
6-Mebpy shown in two views of the cation (see text).
A
and B.
= Mes,
=
=
=
Fig. 4 Aromatic region of the 1H NMR spectrum (500 MHz, acetone-
d6) of [Cu(tBu2xantphos)(bpy)][PF6]. See Fig. S15† for the full spectrum.
See Scheme 3 for atom labelling.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 446–460 | 451

View Article Online
Paper
Dalton Transactions
the xanthphos methyl groups (green and magenta in Fig. 5)
between axial and equatorial sites, but does not render
pyridine rings A and B (represented by NA and NB) equivalent.
By invoking a second process involving movement of the
{Cu(bpy)} unit (i.e., a change in conformation of the chelate
ring, Fig. 5), NA and NB experience both sides of the xanthene
bowl and are equivalent on the NMR timescale.
On
going
from
[Cu(tBu2xantphos)(bpy)][PF6]
to
[Cu(tBu2xantphos)(6-Mebpy)][PF6], the symmetry of the cation
is lowered and phenyl rings D (see Scheme 3) split into two
sets, those proximate to the methyl group of 6-Mebpy and
those on the side of the unsubstituted pyridine ring (Fig. 2a).
Fig. 6 shows the aromatic region of the solution 1H NMR
spectrum of [Cu(tBu2xantphos)(6-Mebpy)][PF6], in which the
sets of D rings are labelled D and D′. In the NOESY spectrum
at 298 K, exchange (EXSY) peaks are observed between pairs of
signals for protons D2/D2′ and D3/D3′; the D4/D4′ EXSY peaks
appear too close to the diagonal in the NOESY spectrum to be
cleary resolved. NOESY cross peaks (no EXSY) are observed
between MeCq1 and MeCq1′ (Fig. S18†). These observations
are consistent with inversion of the chelate ring (‘copper flip’
in Fig. 5) and no inversion of the xanthene bowl.
Fig. 6 Aromatic region of the 1H NMR spectrum (500 MHz, acetone-d6)
of [Cu(tBu2xantphos)(6-Mebpy)][PF6]. See Scheme 3 for atom labelling.
the two PPhMes groups of the xantphosMes2 ligand are
mutually oriented as shown in Fig. 3a and 5. This desymme-
trizes the xanthene unit (labelled rings C and C′). In addition,
the equatorial and axial positions of the Ph and Mes substitu-
ents with respect to the chelate ring leads to chemical shift
1
differences in the H NMR spectrum for pairs of phenyl rings
(D and D′) and mesityl groups (E and E′). Fig. S19† shows the
1H NMR spectrum of [Cu(xantphosMes2)(6-Mebpy)][PF6], and
Fig. 7 and S20† show exchange peaks observed in the NOESY
spectrum. Exchange peaks between the signals for phenyl
proton D2/D2′ and D4/D4′ is consistent with the ‘copper flip’
shown in Fig. 5. This leads to equivalence of the outer rings of
the xanthene unit as confirmed by the EXSY peak between
the signals for protons C3/C3′. The EXSY peak between signals
for the xanthene methyls MeCq1 and MeCq1′ (Fig. S20†)
confirms the inversion of the xanthene bowl (Fig. 5). This
contrasts with [Cu(tBu2xantphos)(6-Mebpy)][PF6] where no
exchange (only NOESY) peaks are observed (see above and
Fig. S18†).
The single crystal structures of [Cu(xantphosMes2)
(bpy)][PF6] and [Cu(xantphosMes2)(6-Mebpy)][PF6] reveal that
Earlier, we noted an orientational disorder of the 6-Mebpy
ligand in the solid-state structure of [Cu(xantphosMes2)
Fig. 7 Part of the NOESY spectrum (500 MHz, acetone-d6) of
[Cu(xantphosMes2)(6-Mebpy)][PF6] showing exchange (EXSY) peaks
between pairs of protons C3 and C3’, D2 and D2’, and D4 and D4’. See
also Fig. S20.†
Scheme 3 Structures
of
[Cu(tBu2xantphos)(6-Mebpy)]+
and
[Cu(xantphosMes2)(6-Mebpy)]+ with labelling for NMR spectroscopic
assignments.
452 | Dalton Trans., 2019, 48, 446–460
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
Table 2 Cyclic voltammetric data for [Cu(N^N)(P^P)][PF6] complexes
referenced to internal Fc/Fc+ = 0 V; degassed HPLC grade CH2Cl2 solu-
tion with [nBu4N][PF6] as supporting electrolyte and a scan rate of
(6-Mebpy)][PF6]. The disorder was modelled with a 50% occu-
pancy of each orientation and Fig. 5a and b show the
[Cu(xantphosMes2)(6-Mebpy)]+ with the two orientations of
6-Mebpy. The structure in Fig. 5a corresponds to the top
diagram in the scheme in Fig. 5, while Fig. 5b corresponds to
the bottom diagram in the scheme. The disorder therefore par-
allels a combination of the two dynamic processes which we
propose the cation undergoes in solution.
0.1
(ir
V
=
s−1
irreversible). Data for [Cu(xantphos)(N^N)][PF6] (N^N
. Processes are quasi reversible unless otherwise stated
=
bpy,
6-Mebpy, 6,6’-Me2bpy) are also included
ox
E
/V
1=2
Complex cation
(Epc − Epa/mV)
Eopxc/V
Erpead/V
Variable temperature (VT) NMR spectra were recorded for [Cu(tBu2xantphos)(bpy)]+
+0.76 (90)
+0.83 (90)
+0.85 (100)
−2.20
−2.22
−2.28
[Cu(tBu2xantphos)(6-Mebpy)]+
an acetone-d6 solution of [Cu(xantphosMes2)(6-Mebpy)][PF6].
[Cu(tBu2xantphos)(6,6′-Me2bpy)]+
The 31P NMR spectrum (Fig. S21†) shows only one signal over
[Cu(xantphosMes2)(bpy)]+
+0.80ir
the range 298–180 K. Fig. 8 shows the effect of temperature on [Cu(xantphosMes2)(6-Mebpy)]+
+0.84 (70)
+0.76 (110)
+0.85 (100)
+0.90 (150)
[Cu(xantphos)(bpy)]+ a
the alkyl region of the 1H NMR spectrum of
[Cu(xantphos)(6-Mebpy)]+ a
[Cu(xantphos)(6,6′-Me2bpy)]+ a
[Cu(xantphosMes2)(6-Mebpy)][PF6]. The collapse of the signals
for mesityl-methyl protons MeE2 and MeE2′ and the appear-
ance of four signals for these methyls below 218 K are consist-
ent with freezing out the rotation of the mesityl groups. A
similar temperature dependence is observed for the mesityl E3
protons in the aromatic region of the spectrum (Fig. S22†).
Significant shifting of the xanthene methyl protons MeCq1
and MeCq1′ (Fig. 8) and 6-Mebpy protons A6 and MeB6
(Fig. S22†) can be attributed to changes in their magnetic
environments as the mesityl groups adopt a static configur-
ation. Both 31P and 1H VT NMR spectra are consistent with the
presence of only one conformer in solution.
a Values taken from ref. 24.
When scanning beyond +1.0 V, additional oxidation waves
appear and the copper oxidation is no longer reversible.
Instead a reduction signal appears at around +0.25 V. This
indicates decomposition of the complexes at potentials higher
t
than +1.0 V. Complexes containing Bu2xantphos also show an
irreversible reduction wave at around −2.2 V (Table 2) arising
from a ligand-based process. For the complexes containing
xantphosMes2, no reduction processes were observed within
the solvent accessible window.
Electrochemistry
Along the series [Cu(tBu2xantphos)(N^N)][PF6] with N^N =
bpy to 6-Mebpy to 6,6′-Me2bpy, the copper oxidation shifts to
higher potentials (as observed for the analogous [Cu(xantphos)
(N^N)][PF6] series, Table 2 24) while the reduction moves
towards more negative potentials. This demonstrates an
increase in the HOMO–LUMO gap as the steric demand of the
bpy ligand increases. This trend was also observed for a related
series of compounds and has been rationalized using DFT
calculations.24
The electrochemical behaviour of the [Cu(N^N)(P^P)][PF6]
complexes was studied using cyclic voltammetry and the
results are summarized in Table 2 and Fig. 9. All compounds
with the exception of [Cu(xantphosMes2)(bpy)][PF6] exhibit a
quasi-reversible process in the range +0.76 to +0.85 V, which is
assigned to a copper-centred oxidation. For [Cu(xantphosMes2)
(bpy)][PF6] the oxidation at +0.80 V is irreversible. Although
the corresponding reduction peak could not be resolved, the
ox
position of this oxidation peak (E ) is similar the one
pc
observed for [Cu(tBu2xantphos)(bpy)][PF6] (+0.80 V, Fig. 9).
Fig. 8 Alkyl regions of the variable temperature 1H NMR spectra
(500 MHz, acetone-d6) of [Cu(xantphosMes2)(6-Mebpy)][PF6]. See
Scheme 3 for atom labelling.
Fig. 9 Cyclic voltamogramms of [Cu(P^P)(N^N)][PF6] compounds in
CH2Cl2 at a scan rate of 100 mV s−1 referenced to internal Fc/Fc+ = 0 V.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 446–460 | 453

View Article Online
Paper
Dalton Transactions
Theoretical calculations: geometry and molecular orbitals
four-coordinate d9 Cu(II) complexes, instead of the tetrahedral
conformation typical of d10 Cu(I) coordination complexes. This
The geometries of all the [Cu(P^P)(N^N)]+ cations in their elec- effect can be studied by following the changes in the angle
tronic ground state (S0) were optimized at the DFT B3LYP-D3/ formed by the N–Cu–N and P–Cu–P planes, which decreases in
(def2svp + def2tzvp) level in the presence of the solvent going from S0 to T1 as the molecule becomes more planar
(CH2Cl2) and without imposing any symmetry restriction (see (Table S1†). The distortion degree from the tetrahedral struc-
the Experimental section for full computational details). The ture in going from S0 to T1 is indeed limited by number of
values obtained for the most representative geometrical para- methyl groups in the 6,6′-positions of the bpy ligand, because
meters defining the Cu(I) coordination sphere are summarized substituents in these positions impede the movement of the
in Table S1.† Calculations successfully reproduce the distorted ligands towards more planar dispositions.24 In this way, the
tetrahedral structures observed experimentally around the Cu [Cu(tBu2xantphos)(bpy)]+ complex, with no substituent in the
centre for all the studied complexes. Compared with the values 6,6′-positions, shows the largest reduction (25.6°) passing from
from X-ray diffraction, the Cu–N and Cu–P bond distances and 82.8° in S0 to 57.2° in T1. The [Cu(tBu2xantphos)(6-Mebpy)]+
the P–Cu–P and N–Cu–N chelating angles are calculated with complex, including one Me group in the 6-position, undergoes
errors below 0.05 Å and 2°, respectively. The angle formed by a smaller reduction of 23.3° (from 87.9 to 64.6°), and the
the P–Cu–P and N–Cu–N planes, which can be used as an indi- [Cu(tBu2xantphos)(6,6′-Me2bpy)]+ complex, featuring Me sub-
cation of the deviation from the orthogonal disposition of the stituents in both the 6- and 6′-positions, shows a reduction of
P^P and N^N ligands, has values lying between 82 and 89°, in only 15.4° (from 86.1 to 70.7°). Thus, the presence of Me
good agreement with those observed experimentally and with groups in the 6- and 6′-positions strongly affects the degree of
those computed in previous works for similar com- geometrical relaxation of the T1 excited state, and limits its
plexes.24,26,66 The N–C–C–N torsion angles remain in a small stabilization. The energy position of the T1 state relative to S0,
range between −10 and 14° indicating that the bpy ligand is and thereby the emission properties of the complexes, there-
essentially planar in all the cations. In contrast to the experi- fore depend not only on the electron-donating or electron-
mental results (Table 1), the cations containing the withdrawing character of the substituent groups present in the
xantphosMes2 ligand do not feature significantly more twisted ligands but also on the positions where the substituents are
bpy ligands (Table S1†), suggesting that the packing forces introduced and on the structural effects they induce.
play an important role in determining the structure in the
Fig. 10 shows the evolution of the energy calculated for the
crystal. Theoretical calculations correctly reproduce the longer highest-occupied (HOMO) and lowest-unoccupied molecular
Cu–P bond distances observed for these complexes, and the
spatial proximity of the equatorial mesityl group to the bpy
unit shown in Fig. 3. The differences in the torsion angles
between theoretical and X-ray geometries are probably due to
the fact that the former are obtained for an isolated molecule
optimized in solution and do not take into account the
packing forces and intermolecular interactions acting in the
solid state.
Two geometry minima were found for [Cu(xantphosMes2)(6-
Mebpy)]+, which show a different relative orientation of the
6-Mebpy ligand and correspond to the two conformations
observed in the single-crystal structure determination (Fig. 5).
They possess close energies, the conformation with the
6-methyl group lying over the xanthene bowl being more stable
that with the Me group away from the xanthene unit by only
0.28 kcal mol−1. This is in good agreement with the occupancy
of 50% experimentally found for each conformation as dis-
cussed above.
The geometry of the first triplet excited state (T1) was also
optimized at the UB3LYP level for all the [Cu(P^P)(N^N)]+
cations, and the most significant geometry parameters are also
included in Table S1.† The molecular geometries in the T1
state significantly differ from those in the ground state S0. As
discussed below, the T1 state implies a charge transfer from a
Fig. 10 Energy diagram showing the energies calculated for the HOMO
molecular orbital that mainly involves a d orbital of the Cu
atom to a molecular orbital spreading over the bpy ligand.
and LUMO of [Cu(tBu2xantphos)(N^N)]+ and [Cu(xantphosMes2)(N^N)]+
complexes. The HOMO–LUMO energy gap is also quoted. Isovalue
Consequently, the metal atom is partially oxidized and tends
contour plots ( 0.03 a.u.) are shown for the HOMO and LUMO of
to adopt the square-planar coordination sphere expected for [Cu(tBu2xantphos)(bpy)]+.
454 | Dalton Trans., 2019, 48, 446–460
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
orbital (LUMO) of the five complexes. The atomic orbital com-
position of the molecular orbitals remains almost unchanged
along the series and only the contour plots computed for [Cu
(tBu2xantphos)(bpy)]+ are displayed as
a
representative
example. As reported previously for similar copper(I) com-
plexes,24,26,66 the HOMO appears mainly centred on the metal
with a small contribution from the phosphorus atoms, and
t
the LUMO spreads over the bpy ligand. The addition of Bu
groups to the xantphos moiety has a negligible effect on the
energy of the HOMO, and the [Cu(tBu2xantphos)(N^N)]+
(N^N = bpy, 6-Mebpy and 6,6′-Me2bpy) complexes have a
slightly higher HOMO energy, around −5.97 eV (Fig. 10), than
that calculated for the reference complex [Cu(xantphos)(bpy)]+
(−6.00 eV) at the same theoretical level.66 Replacement of PPh2
moieties by PPhMes groups in the xantphos ligand has a more
significant effect moving up the HOMO of [Cu(xantphosMes2)
(bpy)]+ and [Cu(xantphosMes2)(6-Mebpy)]+ to −5.81 and
−5.89 eV, respectively. There are small changes in this energy
within each series, as the structural changes introduced in the
complexes are made in regions where the HOMO is not
Fig. 11 Solution absorption spectra of [Cu(P^P)(N^N)][PF6] complexes
(CH2Cl2, 2.5 × 10−5 mol dm−3).
centered. The HOMO energies, even considering the small all cases, both the S1 and the T1 state result from the
changes described, are quite close, in good agreement with HOMO → LUMO monoexcitation, with a contribution exceed-
ox
1=2
the close E
values reported for the complexes in the ing 95%. This excitation implies an electron transfer from the
Electrochemistry section. The LUMO undergoes a small desta- Cu(P^P) moiety of the complex to the bpy ligand, supporting
bilization when a Me group (a weak electron donor) is added the MLCT character of S1 and T1. The broad absorption band
to the bpy ligand, a destabilization that becomes more pro- observed experimentally at around 380 nm therefore originates
nounced when a second group is added. The HOMO–LUMO in the S0 → S1 transition, and the calculated values, although
energy gap increases in the [Cu(tBu2xantphos)(N^N)]+ series somewhat displaced to the red, reproduce the experimentally
with the number of Me substituents of the bpy ligand in agree- observed shift of the absorption maxima to bluer wavelengths
ment with the experimental CV data (Table 2). It is therefore as the number of Me substituents of the bpy ligand increases.
expected that excited states described by HOMO → LUMO tran- This is also in good accord with the increase of the HOMO–
sitions appear at bluer wavelengths as more Me groups are LUMO gap along each series of complexes predicted above
attached to the N^N ligand.
(Fig. 10).
The powder and solution emission spectra are displayed in
Fig. 12 and S24,† respectively, and data are given in Table 3.
Photophysical properties
The solution absorption spectra of the [Cu(P^P)(N^N)][PF6] Upon excitation into the MLCT region (λexc = 365 nm), all the
complexes in CH2Cl2 are shown in Fig. 11. The intense bands compounds show an emission in the orange to yellow
below 330 nm are assigned to ligand-based n → π* and π → π* region. On going from [Cu(tBu2xantphos)(bpy)][PF6] to
transitions and vary little within a series of a certain phos- [Cu(tBu2xantphos)(6-Mebpy)][PF6] to [Cu(tBu2xantphos)(6,6′-
phane ligand. The broad absorption at around 380 nm Me2bpy)][PF6], or from [Cu(xantphosMes2)(bpy)][PF6] to
(Table 3) arises from a metal-to-ligand charge transfer (MLCT) [Cu(xantphosMes2)(6-Mebpy)][PF6], the introduction of additional
excitation. In the series [Cu(tBu2xantphos)(N^N)][PF6], the methyl groups in the bpy ligand shifts the emission to higher
MLCT absorption slightly shifts to higher energies on going energies (Table 3). Additionally, the photoluminescent
from bpy to 6-Mebpy and to 6,6′-Me2bpy (see Fig. S23†) as has quantum yield (PLQY) increases for solution and especially for
previously been observed for the analogous [Cu(xantphos) powder emission along the series (Table 3). [Cu(tBu2xantphos)
(N^N)][PF6] series.26 This behaviour is also observed in passing (bpy)][PF6] and [Cu(xantphosMes2)(bpy)][PF6], just like
from [Cu(xantphosMes2)(bpy)][PF6] to [Cu(xantphosMes2) [Cu(xantphos)(bpy)][PF6],24 exhibit low PLQY values, especially
(6-Mebpy)][PF6], and is in good agreement with the electro- in solution. This is most probably due to the accessibility of
chemical properties discussed above.
the copper centre, which leads to solvent quenching of the
To get a better understanding of the nature of the electronic excited state. Consequently, PLQY values in the powder
excited states involved in the absorption spectra, the time- samples are higher than in solution. The emission in the solid
dependent DFT (TD-DFT) approach was used to study the state is in all cases blue-shifted compared to the solution
lowest lying singlet (Sn) and triplet (Tn) excited states. Table 4 emission. This trend has also been observed for related
collects the energies and oscillator strengths ( f ) computed for complexes containing POP or xantphos.25,26
the S1 and T1 states of all the complexes, together with those
The theoretical results reproduce the trends observed in the
obtained for [Cu(xantphos)(bpy)]+ included as a reference. In experimental emission spectra. The emitting T1 state shifts to
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 446–460 | 455

View Article Online
Paper
Dalton Transactions
Table 4 Vertical excitation energies (E) calculated at the TD-DFT
B3LYP/(def2svp + def2tzvp) level for the lowest singlet (S1) and triplet
(T1) excited states of complexes [Cu(N^N)(P^P)]+ in CH2Cl2 solution.
Oscillator strengths ( f ) are given within parentheses for the S0 → S1
transition
S1
T1
E (eV)
Complex cation
E (eV nm−1) ( f )
[Cu(xantphos)(bpy)]+
2.816/440 (0.08)
2.803/442 (0.09)
2.823/439 (0.10)
2.923/424 (0.07)
2.664/465 (0.06)
2.750/451 (0.06)
2.569
2.554
2.577
2.694
2.470
2.582
[Cu(tBu2xantphos)(bpy)]+
[Cu(tBu2xantphos)(6-Mebpy)]+
[Cu(tBu2xantphos)(6,6′-Me2bpy)]+
[Cu(xantphosMes2)(bpy)]+
[Cu(xantphosMes2)(6-Mebpy)]+
Fig. 12 Normalized solid-state emission spectra of [Cu(P^P)(N^N)][PF6]
complexes (λexc = 365 nm).
higher energies as more methyl groups are added to the bpy
ligand (Table 4). The [Cu(tBu2xantphos)(6,6′-Me2bpy)]+
complex is predicted to have the higher energy T1, followed by
[Cu(tBu2xantphos)(6-Mebpy)]+
and
[Cu(xantphosMes2)(6-
Mebpy)]+, and finally by the complexes with no methyl substi-
tuent. This is in good agreement with the emission wave-
lengths observed in experimental spectra (Table 3). The broad
and mostly unstructured shape of the emission band
(Fig. S24†) also agrees with the MLCT nature predicted for the
emitting HOMO → LUMO T1 state.
As discussed above, the geometry relaxation of the emitting
T1 state leads to the flattening of the tetrahedral coordination
environment. This flattening is more hindered as the number
of methyl substituents attached to positions 6 and 6′ of the
bpy ligand is increased, and the relaxation of the T1 triplet is
impeded thus leading to higher emission energies. Inspection
of Table 3 shows that increasing the steric hindrance of the
bpy ligand is beneficial for the emissive properties. Less flat-
tening of the tetrahedral coordination environment of the
copper centre gives rise to higher emission energies and, as
456 | Dalton Trans., 2019, 48, 446–460
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
expected, longer excited state lifetimes and higher PLQYs. The pared to the solid state emission and a greatly enlarged excited
same is true for the series of [Cu(xantphos)(N^N)][PF6] state lifetime. This indicates the possibility that all complexes
(Table 3).24,26 However, from a synthetic point of view, the are TADF emitters at room temperature. The energy difference
unsuccessful attempt to [Cu(xantphosMes2)(6,6′-Me2bpy)][PF6] between the lowest energy singlet and triplet excited states has
demonstrates a limitation in the combined steric properties of been calculated to lie between 0.17 and 0.25 eV (Table 4), and
the xantphosMes2 and 6,6′-Me2bpy ligands.
is small enough to allow the occurrence of TADF processes.22
Low temperature data further support the idea that the
The photophysical properties of [Cu(tBu2xantphos)
(bpy)][PF6] and [Cu(xantphosMes2)(bpy)][PF6] are similar both position of the emission bands of the [Cu(tBu2xantphos)(N^N)]
in solution and in the solid state (Table 3). In the case of the complexes is strongly affected by the flattening of the tetra-
[Cu(P^P)(6-Mebpy)][PF6] complexes, a similar behaviour is hedral environment in the T1 state. In solution, this flattening
observed in the solid state emission. Both complexes are blue is not impeded and the difference in peak position between
shifted compared to their [Cu(P^P)(bpy)][PF6] analogues but [Cu(tBu2xantphos)(bpy)][PF6]
and
[Cu(tBu2xantphos)(6-
the emission profile is still very similar and the difference in Mebpy)][PF6] is 86 nm (0.30 eV) (Table 3). In powder, the flat-
peak position is again low (Δλ = 5 nm). It is notable that the tening is more restricted and the difference decreases to
shift difference when going from [Cu(P^P)(bpy)][PF6] to [Cu 62 nm (0.25 eV). Finally, at 77 K, where the relaxation is even
(P^P)(6-Mebpy)][PF6] complexes is larger in the complexes con- more impeded for all the complexes, the emission maxima
taining xantphosMes2 (Δλ = 42 nm) compared to tBu2xantphos range between 597 and 555 nm, in a window of just 42 nm
complexes (Δλ = 32 nm). On the other hand, the solution emis- (0.15 eV).
sion profiles differ significantly for the two [Cu(P^P)(6-
Device performances
Mebpy)][PF6] complexes (Fig. S24†). The emission maximum
of [Cu(tBu2xantphos)(6-Mebpy)] is blue shifted compared to
The series of compounds was tested in LECs using ITO/
PEDOT:PSS as the anode, an emitting layer consisting of the
[Cu(tBu2xantphos)(bpy)] (Δλ = 47 nm) as expected, but in the
case of [Cu(xantphosMes2)(6-Mebpy)][PF6] the emission maximum
is blue-shifted very little compared to [Cu(xantphosMes2)
(bpy)][PF6] (Δλ = 10 nm). Both [Cu(tBu2xantphos)(6-Mebpy)][PF6]
complex in the presence of [Emim][PF6] (4 : 1 molar ratio) and
an aluminium cathode. Devices were tested monitoring the
electroluminescence and voltage over time, and were driven
with a pulsed current (100 A m−2 average, 50% duty cycle,
1 kHz). The main device parameters obtained for the entire
samples series are reported in Table 5. The time evolution of
the voltage and luminance for the LECs are reported in
Fig. 14, together with the electroluminescence spectra. All
and [Cu(xantphosMes2)(6-Mebpy)][PF6] also show
a second
unstructured emission feature at 628 nm and 623 nm respectively,
which is not observed in the solid-state emission profile.
To investigate whether the compounds showed thermally
activated delayed fluorescence (TADF) at room temperature,
low temperature emission spectra and excited state lifetimes
were recorded in frozen Me-THF at 77 K (Table 3 and Fig. 13).
All complexes show a red shift in emission of 5–40 nm com-
t
LECs based on copper(I) complexes with Bu2xantphos as the
P^P ligand show
a
fast luminescence turn-on time
(ton, defined here as the time to reach the maximum lumi-
nance), ranging from 1 minute for the complex with 6,6′-
Me2bpy to 4.5 minutes for the one with 6-Mebpy (Fig. 14b).
The maximum luminance registered for these compounds
increases with increasing substitution to the N^N ligand,
going from 20 cd m−2 for the complex with unsubstituted bpy,
to 230 cd m−2 and 370 cd m−2 for the ones containing
6-Mebpy and 6,6′-Me2bpy, respectively. This trend follows that
of the PLQY registered for the same compounds (Table 3) and
is consistent with an augmented steric hindrance of the N^N
ligand, resulting in a higher stabilization of the tetrahedral
complex geometry. The highest efficiency of 3.7 cd A−1 for the
LECs using [Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6] corresponds
to an external quantum efficiency of 1.0%, which is substan-
Table 5 Main device parameters obtained from LECs driven at
100 A m−2
ton
Lummax t1/2
Eff.
[Cu(P^P)(N^N)]+
(min) (cd m−2
)
(min) (cd A−1
)
[Cu(tBu2xantphos)(bpy)]+
1.1
4.5
20
230
370
50
5.1
53.8
4.9
0.2
2.3
3.7
0.5
[Cu(tBu2xantphos)(6-Mebpy)]+
[Cu(tBu2xantphos)(6,6′-Me2bpy)]+ 1.0
Fig. 13 Normalized emission spectra of [Cu(P^P)(N^N)][PF6] complexes
in Me-THF at 77 K (λexc = 410 nm).
[Cu(xantphosMes2)(6-Mebpy)]+
0.7
34.6
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 446–460 | 457

View Article Online
Paper
Dalton Transactions
Fig. 14 Time evolution of the (a) voltage and (b) the luminance for a series of LECs driven at an average current density of 100 A m−2. (c)
Electroluminescence spectra for the same device series with (inset) the corresponding colour coordinates in the CIE 1931 colour space.
tially lower as compared to the PLQY of the same compound the green region of the CIE 1931 colour space. The EL spec-
in thin-film (23%). This implies that even in such bright trum of [Cu(xantphosMes2)(6-Mebpy)][PF6] peaks at 582 nm, in
device, non-radiative losses dominate the recombination of agreement with the PL signal of the thin-film (Table 3).
the injected electrons and holes. Interestingly, the quantum
efficiencies for photo- and electro-luminescence of the
[Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6] are very close to the
Conclusions
unsubstituted xantphos analogue (PLQY = 22% and maximum
EQE = 1.0%),26 highlighting the dominant role of the N^N We
have
ligands on the optical and optoelectronic properties of these (tBu2xantphos)(6-Mebpy)][PF6], [Cu(tBu2xantphos)(6,6′-Me2bpy)]
compounds.
[PF6], [Cu(xantphosMes2)(bpy)][PF6] and [Cu(xantphosMes2)(6-
prepared
[Cu(tBu2xantphos)(bpy)][PF6],
[Cu
In general, the device lifetime (t1/2, time to decay to one- Mebpy)][PF6], but steric effects militate against the isolation of
half of the peak luminance) for complexes containing [Cu(xantphosMes2)(6,6′-Me2bpy)][PF6]. To prepare the latter,
tBu2xantphos was found to be low, approximately 5 minutes in the chiral xantphosMes2 was prepared and characterized, and
the cases of bpy and 6,6′-Me2bpy and above 50 minutes for the the single crystal structure reveals the presence of the (rac)-
LECs with [Cu(tBu2xantphos)(6-Mebpy)][PF6]. The low lifetime form. In solution, one dominant diastereoisomer is observed,
of the complexes with bpy and 6,6′-Me2bpy might be due to a proposed as the (rac) rather than the (meso)-form. This makes
reduced stability of the materials toward charge transport, as the bisphosphane preorganized to give particular diastereo-
seen from the corresponding LECs voltage profile which drasti- isomers when coordinated to copper(I) in [Cu(xantphosMes2)
cally increases after only few minutes of operation (Fig. 14a).
(N^N]+ complexes. Single crystal structures of four complexes
The optoelectronic performance of complexes containing were determined. In [Cu(xantphosMes2)(6-Mebpy)][PF6], the
the xantphosMes2 ligand were in general lower as compared to 6-Mebpy unit is disordered over two orientations with 50%
t
those involving Bu2xantphos. We could not observe any elec- occupancies. The disorder corresponds to a combination of
troluminescence from [Cu(xantphosMes2)(bpy)][PF6], perhaps two dynamic processes, which the [Cu(xantphosMes2)(N^N)]+
due to its low PLQY both in solution and in the solid cations undergo in solution. DFT calculations reveal that the
state. Moderate electroluminescence was measured for energy difference between the two conformers observed in the
[Cu(xantphosMes2)(6-Mebpy)][PF6], with fast turn-on (<1 minute) solid-state structure differ only by 0.28 kcal mol−1
.
and a maximum luminance of 50 cd m−2
.
The [Cu(P^P)(N^N)][PF6] compounds show a broad MLCT-
The spectral shape and position of the electroluminescence absorption around 380 nm which shifts to higher energies on
(EL, Fig. 14c) signals correlate with the PL maxima observed going from bpy to 6-Mebpy to 6,6′-Me2bpy. Upon excitation
for the complexes in solution and in the solid state. For the into the MLCT band, the [Cu(P^P)(N^N][PF6] complexes emit
tBu2xantphos-containing complexes, the EL maxima blue-shift in the yellow to orange region; additional Me groups in the
from 584 nm to 575 and 557 nm when increasing the substi- bpy ligand result in a blue-shift in the emission. The MLCT
tution at the bpy, i.e. going from bpy to 6-Mebpy and 6,6′- nature of the absorption and emission is supported by DFT
Me2bpy, respectively. As highlighted in the inset of Fig. 14c, calculations, which associate the lowest-energy S1 and T1
this shift corresponds to a colour variation from the orange to excited states to the HOMO → LUMO monoexcitation implying
458 | Dalton Trans., 2019, 48, 446–460
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
an electron transfer from the Cu(P^P) moiety to the bpy
ligand. In solution, PLQY values are low, but in the solid state,
PLQYs of 26 and 59% are observed for [Cu(xantphosMes2)(6-
Mebpy)][PF6] and [Cu(tBu2xantphos)(6,6′-Me2bpy)]+, respect-
ively, compared to benchmark values of 34 and 37% for
5 S. Tang and L. Edman, Top. Curr. Chem., 2016, 374, 40.
6 J. Wu, F. Li, Q. Zeng, C. Nie, P. C. Ooi, T. Guo, G. Shan and
Z. Su, Org. Electron., 2016, 28, 314.
7 A. Sandström and L. Edman, Energy Technol., 2015, 3, 329.
8 Light-emitting electrochemical cells: Concepts, advances and
challenges, ed. R. D. Costa, Springer, Cham, 2017.
[Cu(xantphos)(6-Mebpy)][PF6]
and
[Cu(xantphos)(6,6′-
Me2bpy)][PF6]. Increased excited state lifetimes at low tempera-
ture are consistent with the complexes being TADF emitters
9 R. D. Costa, E. Ortí, H. J. Bolink, F. Monit, G. Accorsi and
N. Armaroli, Angew. Chem., Int. Ed., 2012, 51, 8178.
and this is supported by a calculated energy difference 10 C. E. Housecroft and E. C. Constable, Coord. Chem. Rev.,
between S1 and T1 of 0.17–0.25 eV. 2017, 350, 155.
The compounds were tested in simple bilayer LECs. The 11 A. M. Bünzli, E. C. Constable, C. E. Housecroft,
optoelectronic performance of complexes containing the
xantphosMes2 ligand were generally lower than those with
tBu2xantphos, which led to bright and efficient devices. The
A. Prescimone, J. A. Zampese, G. Longo, L. Gil-Escrig,
A. Pertegás, E. Ortí and H. J. Bolink, Chem. Sci., 2015, 6,
2843.
current efficiency of the LECs follows the trend observed for 12 D. Tordera, A. M. Bünzli, A. Pertegás, J. M. Junquera-
the PLQY, increasing with increasing substitution at the bpy
ligand. In particular, luminances as high as 370 cd m−2 were
Hernández, E. C. Constable, J. A. Zampese,
C. E. Housecroft, E. Ortí and H. J. Bolink, Chem. – Eur. J.,
2013, 19, 8597.
obtained
Me2bpy)][PF6], which correspond to an efficiency of 3.7 cd A−1
These encouraging results suggest that tBu2xantphos is a
for
the
complex
[Cu(tBu2xantphos)(6,6′-
.
13 S. B. Meier, D. Tordera, A. Pertegás, C. Roldán-Carmona,
E. Ortí and H. J. Bolink, Mater. Today, 2014, 17, 217.
promising ligand to develop novel and efficient copper emit- 14 B. N. Bideh, C. Roldán-Carmona, H. Shahroosvand and
ters for LECs and OLEDs.
M. K. Nazeeruddin, Dalton Trans., 2016, 45, 7195.
15 M. T. Buckner and D. R. McMillin, J. Chem. Soc., Chem.
Commun., 1978, 759.
Conflicts of interest
16 R. A. Rader, D. R. McMillin, M. T. Buckner, T. G. Matthews,
D. J. Casadonte, R. K. Lengel, S. B. Whittaker,
L. M. Darmon and F. E. Lytle, J. Am. Chem. Soc., 1981, 103,
5906.
There are no conflicts to declare.
17 C. L. Linfoot, M. J. Leitl, P. Richardson, A. F. Rausch,
O. Chepelin, F. J. White, H. Yersin and N. Robertson, Inorg.
Chem., 2014, 53, 10854.
18 M. J. Leitl, V. a. Krylova, P. I. Djurovich, M. E. Thompson
and H. Yersin, J. Am. Chem. Soc., 2014, 136, 16032.
19 R. Czerwieniec and H. Yersin, Inorg. Chem., 2015, 54, 4322.
20 T. Hofbeck, U. Monkowius and H. Yersin, J. Am. Chem.
Soc., 2015, 137, 399.
21 R. Czerwieniec, M. J. Leitl, H. H. H. Homeier and
H. Yersin, Coord. Chem. Rev., 2016, 325, 2.
22 H. Yersin, R. Czerwieniec, M. Z. Shafikov and
A. F. Suleymanova, ChemPhysChem, 2017, 18, 3508.
23 J. Liu, J. Oliva, K. Tong, F. Zhao, D. Chen and Q. Pei, Sci.
Rep., 2017, 7, 1524.
Acknowledgements
Financial support from the Swiss National Science Foundation
(Grant number 162631), the University of Basel, the MINECO
of Spain (CTQ2015-71154-P, MAT2017-88821-R and Unidad de
Excelencia María de Maeztu MDM-2015-0538), the Generalitat
Valenciana (PROMETEO/2016/ 135) and European FEDER
funds (CTQ2015-71154-P) is acknowledged. Prof. Dr Oliver
S. Wenger and Dr Christopher B. Larsen (Universty of Basel)
are thanked for the use of the LP920-KS instrument from
Edinburgh Instruments for low-temperature emission and life-
time measurements. MS thanks the MINECO for his RyC con-
tract. Dr Sarah Keller is acknowledged for fruitful discussions
and synthetic assistance. Kenneth Atz and PD Dr Daniel
Häussinger are acknowledged for assistance with variable
temperature NMR measurements.
24 S. Keller, F. Brunner, J. M. Junquera-Hernández,
A. Pertegás, M.-G. La-Placa, A. Prescimone, E. C. Constable,
H. J. Bolink, E. Ortí and C. E. Housecroft, ChemPlusChem,
2018, 83, 217.
25 S. Keller, E. C. Constable, C. E. Housecroft, M. Neuburger,
A. Prescimone, G. Longo, A. Pertegás, M. Sessolo and
H. J. Bolink, Dalton Trans., 2014, 43, 16593.
26 S. Keller, A. Pertegás, G. Longo, L. Martínez, J. Cerdá,
J. M. Junquera-Hernández, A. Prescimone, E. C. Constable,
C. E. Housecroft, E. Ortí and H. J. Bolink, J. Mater. Chem.
C, 2016, 4, 3857.
Notes and references
1 M. Y. Wong and E. Zysman-Colman, Adv. Mater., 2017, 29,
1605444.
2 C. Bizzarri, E. Spuling, D. M. Knoll, D. Volz and S. Bräse,
Coord. Chem. Rev., 2018, 373, 49.
3 D. Volz, M. Wallesch, C. Fléchon, M. Danz, A. Verma,
J. M. Navarro, D. M. Zink, S. Bräse and T. Baumann, Green 27 M. D. Weber, M. Viciano-Chumillas, D. Armentano, J. Cano
Chem., 2015, 17, 1988.
and R. D. Costa, Dalton Trans., 2017, 46, 6312.
4 E. Fresta and R. D. Costa, J. Mater. Chem. C, 2017, 5, 5643.
28 E. Fresta and R. D. Costa, J. Mater. Chem. C, 2017, 5, 5643.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 446–460 | 459

View Article Online
Paper
Dalton Transactions
29 M. Heberle, S. Tschierlei, N. Rockstroh, M. Ringenberg,
W. Frey, H. Junge, M. Beller, S. Lochbrunner and
M. Karnahl, Chem. – Eur. J., 2017, 23, 312.
30 N. Armaroli, G. Accorsi, M. Holler, O. Moudam,
J.-F. Nierengarten, Z. Zhou, R. T. Wegh and R. Welter, Adv.
Mater., 2006, 18, 1313.
31 F. Brunner, L. Martínez-Sarti, S. Keller, A. Pertegás,
A. Prescimone, E. C. Constable, H. J. Bolink and
C. E. Housecroft, Dalton Trans., 2016, 45, 15180.
J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd,
E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith,
R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi,
J. M. Millam, M. Klene, C. Adamo, R. Cammi,
J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas,
J. B. Foresman and D. J. Fox, Gaussian 16, Revision A.03,
Gaussian, Inc., Wallingford CT, 2016.
47 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens.
Matter Mater. Phys., 1988, 37, 785.
32 F. Brunner, S. Graber, Y. Baumgartner, D. Häussinger,
A. Prescimone, E. C. Constable and C. E. Housecroft, 48 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
Dalton Trans., 2017, 46, 6379.
49 F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005,
33 C. Yi, S. Xu, J. Wang, F. Zhao, H. Xia and Y. Wang,
Eur. J. Inorg. Chem., 2016, 2016, 4885.
7, 3297.
50 F. Weigend, Phys. Chem. Chem. Phys., 2006, 8, 1057.
34 R. Czerwieniec, J. Yu and H. Yersin, Inorg. Chem., 2011, 50, 51 S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem.
8293. Phys., 2010, 132, 154104.
35 Q. Zhang, Q. Zhou, Y. Cheng, L. Wang, D. Ma, X. Jing and 52 S. Grimme, S. Ehrlich and L. Goerigk, J. Comput. Chem.,
F. Wang, Adv. Funct. Mater., 2006, 16, 1203. 2011, 32, 1456.
36 R. D. Costa, D. Tordera, E. Ortí, H. J. Bolink, J. Schönle, 53 M. Petersilka, U. J. Gossmann and E. K. U. Gross, Phys. Rev.
S. Graber, C. E. Housecroft, E. C. Constable and
J. A. Zampese, J. Mater. Chem., 2011, 21, 16108.
37 S. Keller, A. Prescimone, E. C. Constable and
C. E. Housecroft, Photochem. Photobiol. Sci., 2018, 17, 375.
38 A. M. Johns, N. Sakai, A. Ridder and J. F. Hartwig, J. Am.
Chem. Soc., 2006, 128, 9306.
Lett., 1996, 76, 1212.
54 C. Jamorski, M. E. Casida and D. R. Salahub, J. Chem.
Phys., 1996, 104, 5134.
55 M. E. Casida, C. Jamorski, K. C. Casida and D. R. Salahub,
J. Chem. Phys., 1998, 108, 4439.
56 J. Tomasi and M. Persico, Chem. Rev., 1994, 94, 2027.
57 C. J. Cramer and D. G. Truhlar, in Solvent Effects and
Chemical Reactivity, ed. O. Tapia and J. Bertrán, Kluwer,
1996, pp. 1–80.
58 J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005,
105, 2999.
59 B. C. Hamann and J. F. Hartwig, J. Am. Chem. Soc., 1998,
120, 3694.
39 C. A. Tolman, Chem. Rev., 1977, 77, 313.
40 G. J. Kubas, Inorg. Synth., 1979, 19, 90.
41 W. Bruker Analytical X-ray Systems, Inc., APEX2, version 2
User Manual, M86-E01078, Madison, 2006.
42 P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout
and D. J. Watkin, J. Appl. Crystallogr., 2003, 36, 1487.
43 I. J. Bruno, J. C. Cole, P. R. Edgington, M. Kessler,
C. F. Macrae, P. McCabe, J. Pearson and R. Taylor, Acta 60 J. Holz, K. Rumpel, A. Spannenberg, R. Paciello, H. Jiao
Crystallogr., Sect. B: Struct. Sci., 2002, 58, 389. and A. Börner, ACS Catal., 2017, 7, 6162.
44 C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, 61 Y. Hamada, F. Matsuura, M. Oku, K. Hatano and T. Shioiri,
P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, Tetrahdron Lett., 1997, 38, 8961.
J. van de Streek and P. A. Wood, J. Appl. Crystallogr., 2008, 62 J. I. van der Vlugt, E. A. Pidko, D. Vogt, M. Lutz and
41, 466. A. L. Spek, Inorg. Chem., 2009, 48, 7513.
45 A. L. Spek, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 63 J. Yuasa, M. Dan and T. Kawai, Dalton Trans., 2013, 42,
9. 16096.
46 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, 64 B. Bozic-Weber, V. Chaurin, E. C. Constable,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato,
A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts,
C. E. Housecroft, M. Meuwly, M. Neuburger, J. A. Rudd,
E. Schönhofer and L. Siegfried, Dalton Trans., 2012, 41,
14157.
B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, 65 C. R. Groom, I. J. Bruno, M. P. Lightfoot and S. C. Ward,
J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini,
F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson,
Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater.,
2016, 72, 171.
D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, 66 S. Keller, A. Prescimone, H. Bolink, M. Sessolo, G. Longo,
W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery Jr.,
L.
Martínez-Sarti,
J.
M.
Junquera-Hernández,
E. C. Constable, E. Ortí and C. E. Housecroft, Dalton Trans.,
2018, 47, 14263.
460 | Dalton Trans., 2019, 48, 446–460
This journal is © The Royal Society of Chemistry 2019