A dinuclear extension to constrained heteroleptic Cu(i) systems

Article abstract of DOI:10.1039/c1dt10275c

This article reports the synthesis and optical properties of three dinuclear, cationic copper complexes [Cu₂(μ-dppm) ₂(μ-L)](NO₃)₂ (dppm diphenyldiphosphinomethane, L: L_A 3,6-bis(2-pyridyl)-4,5-diphen

Full text of DOI:10.1039/c1dt10275c

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PAPER
A dinuclear extension to constrained heteroleptic Cu(I) systems†
´
Bele´n Gil, Gareth A. Cooke, Deanne Nolan, Gearo´id M. O Ma´ille, Sunil Varughese, Longsheng Wang and
Sylvia M. Draper*
Received 18th February 2011, Accepted 13th May 2011
DOI: 10.1039/c1dt10275c
This article reports the synthesis and optical properties of three dinuclear, cationic copper complexes
[Cu2(m-dppm)2(m-L)](NO3)2 (dppm diphenyldiphosphinomethane, L: LA 3,6-bis(2-pyridyl)-4,5-
diphenyl-pyridazine, LB 3,6-bis(2-pyridyl)-4,5-di(4-pyridyl)-pyridazine and LC 3,6-bis(2-pyridyl)-8,9-
diazafluoranthene). These were formed on the reaction of [Cu(m-dppm)(NO3)]2 with a series of
N-donor (bppn) ligands L. The single crystal X-ray structures of [Cu2(m-dppm)2(m-L)](NO3)2·CH2Cl2
were determined and revealed that in both, the two copper atoms are held by three bridging ligands,
two dppm ligands and one bppn ligand acting as a tetradentate bridge. The absorption spectra of the
complexes present a MLCT [Cu →p*(NŸN)] band in the l 370–425 nm region. These new complexes
exhibit red-orange MLCT-based emission in the solid-state with lifetimes in the microsecond range. In
oxygen-free dichloromethane solution, the complex [Cu2(m-dppm)2(m-LC)]2+ has a long lifetime of
22.8 ms. The long emission lifetimes are attributed to a rigid conformation that precludes the possible
distortion of the copper in the excited state.
ligands generated by Diels–Alder cycloaddition have come to the
Introduction
fore in Ru(II) chemistry.7–10
Although few in number, recent publications have begun to
One development in copper chemistry has been the inclusion of
expose the potential of Cu(I) complexes as an alternative to Ru(II)
stabilizing phosphine ligands, offering a significant improvement
in low-cost solar-conversion devices.1–3 The rich photochemistry
on the photophysical properties. McMillin and co-workers have
of Ru(II),4 Pt(II),5 and Ir(III)6 complexes is undermined by the
reliance on expensive, rare metals, which in some cases exhibit
demonstrated that bulky bidentate phosphines (such as POP =
bis[2-(biphenylphosphino)phenyl]ether) inhibit the formation of
an undesirable level of toxicity. The ease of preparation of Cu(I)
quenching exciplexes, providing unusually long emission lifetimes
complexes, their ability to absorb light in the visible region, intense
and rather good quantum yields, typically in the green spectral
window.11,12
luminescence and low cost has captured the interest of researchers
in the field. Cu(I), however, undergoes a conformational change
on oxidation to Cu(II), thus decreasing device efficiencies. Strate-
gies to overcome the resultant low quantum yields and short
luminescence lifetimes of [Cu(NŸN)2]+ complexes require further
development.1,3
The electronic nature, bulk and rigidity of the diimine ligand all
play an important role in determining the photophysical properties
of Cu(I) complexes.3 Traditionally bipyridyl or phenanthroline-
based systems have been used but there is scope to rethink
and redesign ligand systems. A synthetic protocol which can be
extended to multiple systems is very attractive and new aromatic
Another possibility is to increase the number of metal centers
or to seek cooperative interactions between metal centers in
a diimine-based system.13 Only a few examples of binuclear
systems exist and these use ligands such as bipyrimidine14,15
or 2,5-bis(2-pyridyl)pyrazine.16 One downside is that the overall
rigidity is reduced, e.g. in a transoid disposition. Rigid systems
are needed to prevent distortion and/or exciplet-quenching of
the excited state and thus enhance the optical properties. 2,5-
Bis(2-pyridyl)tetrazines, can favour cisoid conformation and as
precursors via inverse electron demand Diels–Alder to pyri-
dazines offer a synthetic procedure for the inclusion of new
chromophores.17,18
School of Chemistry, Trinity College Dublin, Dublin, 2, Ireland.
E-mail: smdraper@tcd.ie; Fax: +353 1 671 2826; Tel: +353 1 896 2026
Results and discussion
† Electronic supplementary information (ESI) available: Further crystal
structures of [Cu(m-dppm)2LB]2+ and [Cu(m-dppm)2LA]2+ (Fig. S1, S2), 1H
NMR spectrum (Fig. S3) and 1H–1H correlation NMR spectrum (Fig.
S4) of [Cu(m-dppm)2LC]2+, cyclic voltammogram of [Cu(m-dppm)2LA]2+
(Fig. S5), Comparative photophysical data of the ligands bppn L (Table
S1). CCDC reference numbers 768169, 768170 and 824325. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10275c
Synthesis
Trans-stilbene and 1,4-bis(4-pyridyl)ethylene were each reacted
with 3,4-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) in toluene by
heating to reflux overnight in a sealed tube. On the loss
of the characteristic pink colour of bptz, the products were
8320 | Dalton Trans., 2011, 40, 8320–8327
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Scheme 1 Synthetic route towards [Cu2(m-dppm)2(m-L)](NO3)2.
◦
˚
Table 1 Selected bond lengths [A] and angles [ ] for [Cu2(m-dppm)2(m-
L)](NO3)2·(CH2Cl2) (L = LA, LB)
purified by column chromatography to give good yields of
the respective bright yellow hydropyridazines (3,6-bis(2-pyridyl)-
4,5-diphenyl-hydropyridazine, 75%; 3,6-bis(2-pyridyl)-4,5-bis(4-
pyridyl)-hydropyridazine, 92%). Oxidation by blowing nitrous
gases19 through a dichloromethane solution of the compounds
at 0 ◦C gave the bispyridylpyridazine (bppn) ligands (LA 74%,
LB 50%, see Scheme 1). The lower yield observed for LB may be
due to the more electron-withdrawing 4-pyridyl substituents on
the hydropyridazine ring rendering oxidation more difficult. The
synthesis of LC has been reported20 as has the synthesis of ligand
LA, albeit under different conditions.21
[Cu2(m-dppm)2(LA)]2+
[Cu2(m-dppm)2(LB)]2+
Cu–Npz
Cu–Npy
Cu–P
P–Cu–P
N–Cu–N
P–Cu–N
(NCuN)–(PCuP)
2.014(6), 2.079(6)
2.122(7), 2.124(7)
2.232(2)–2.266(3)
124.65(9), 130.44(9)
76.3(3), 77.7(3)
105.8(2)–118.6(1)
87.72, 89.40
2.040(3), 2.077(4)
2.150(3), 2.114(4)
2.228(1)–2.270(1)
124.21(5), 130.57(5)
76.0(1), 76.9(1)
104.96(9)–118.70(9)
88.36, 89.24
2
Treatment of a colourless solution of [Cu(m-dppm)(NO3-k O)]2
Suitable, orange, block-shaped crystals were obtained by slow
diffusion of hexane into a dry, dichloromethane solution of the
respective complexes. [Cu2(m-dppm)2(m-L)](NO3)2 (L = LA, LB)
crystallize in the space groups P21/n and P21/c, respectively. For
both complexes, one cation and two nitrate anions are found in the
asymmetric unit together with one molecule of dichloromethane.
The structures of the cations are shown in Fig. 1 for [Cu2(m-
dppm)2(m-LA)]+ and Fig. S1 for [Cu2(m-dppm)2(m-LB)]+.† Selected
bonds and angles are presented in Table 1. The anions and solvent
molecules do not take part in any significant intermolecular
interactions in the crystal packing.
In each case, the two copper atoms are held in close proximity
by three bridging ligands: two dppm bonded through phosphorus
and one bppn L molecule. The Cu–Cu separation ([Cu2(m-
dppm)2(m-L)](NO3)2 LA 3.406 A, LB 3.390 A) is significantly
longer than the sum of the van der Waals radii indicating
the absence of a Cu–Cu interaction.22 Looking through the
Cu ◊ ◊ ◊ Cu axis, the conformations of the two PCuCuP bridges
in each complex are approximately “eclipsed” (PCuCuP torsion
in dichloromethane at room temperature with an equivalent
amount of bppn (L = LA, LB, LC) affords orange solutions. After
stirring for 1 h, the resulting mixtures were filtered through
Celite and concentrated (~2 mL). The addition of Et2O (~20 mL)
precipitates [Cu2(m-dppm)2(m-L)](NO3)2 as an orange solid (81%–
97%). Using a 1 : 2 molar ratio of metal precursor to L results in
the same products and unreacted L.
Characterisation
All the complexes are air stable as solids and in solution and were
1
fully spectroscopically characterised (1H and 13C{ H} NMR and
mass spectra) and their purity verified by elemental analysis. In
addition, the structures of complexes [Cu2(m-dppm)2(m-L)](NO3)2
(L = LA, LB) were determined by single crystal X-ray diffraction.‡
˚
˚
‡ Crystallographic data. Single crystals were carefully chosen after they
were viewed through a microscope supported by a rotatable polarizing
stage. The crystals were glued to a thin glass fibre using NIH immersion
oil and mounted on a diffractometer equipped with an APEX CCD
area detector. The data were collected either at room temperature or
at 173 K. The intensity data were processed using Bruker’s suite of
data processing programs (SAINT), and absorption corrections were
applied using SADABS. The structure solution of all the complexes
was carried out by direct methods, and refinements were performed
by full-matrix least-squares on F2 using the SHELXTL-PLUS suite of
programs. All the non-hydrogen atoms were refined anisotropically, and
the hydrogen atoms were fixed using respective HFIX options and were
refined isotropically. The solvent molecules present were highly disordered
and were difficult to model. The corresponding electron density was
removed from the reflection data and was refined using the SQUEEZE
option in PLATON and the corresponding sqf file has been appended to
the CIF. Intermolecular interactions were computed using the PLATON
program. Crystallographic data for [Cu2(dppm)2(LA)](NO3)2·CH2Cl2 and
[Cu2(dppm)2(LB)](NO3)2·CH2Cl2 have been deposited at the Cambridge
Crystallographic Data Centre, CCDC numbers 768169 and 768170. Crys-
tal data for [Cu2(l-dppm)2(l-LA)2](NO3)2.CH2Cl2 C77H64Cl2Cu2N6O6P4,
Mr = 1491.20, monoclinic, a = 14.673(3), b = 19.071(4), c = 26.536(6)
A, b = 96.795(4) , U = 7374(3) A , T = 301(2) K, space group P21/n, Z =
4, 40 382 reflections measured, 12 885 unique (Rint = 0.1040), which were
used in all calculations. The final R1 (all data) was 0.1471 and wR(F2) (all
data) was 0.2649. Crystal data for [Cu2(l-dppm)2(l-LB)2](NO3)2.CH2Cl2
C75H62Cl2Cu2N8O6P4, Mr = 1493.19, monoclinic, a = 14.570(4), b =
19.092(5), c = 26.482(8) A, b = 96.897(7) , U = 7313(4) A , T = 298(2)
K, space group P21/c, Z = 4, 62 142 reflections measured, 10 613 unique
(Rint = 0.1526) which were used in all calculations. The final R1 (all data)
was 0.0781 and wR(F2) (all data) was 0.1283.
◦
3
˚
˚
◦
3
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˚
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congestion, and is achieved by the rotation of both arene
rings.
Both structures exhibit weak intramolecular p ◊ ◊ ◊ p interactions
˚
˚
(range LA 3.42–3.72 A, LB 3.43–3.76 A) between the phenyl rings
on the same dppm ligand and between the phenyl rings of the dppm
and both 2-pyridine rings of the bppn L (see Fig. S2†). These inter-
actions play an important role in stabilising the structure and prob-
ably contribute to the optical properties observed in the solid state.
The presence of these binuclear structures in solution is
confirmed by 1H NMR spectroscopy (CDCl3, room temperature).
The methylene protons of the dppm (CH2P2) resonate as two
unresolved multiplets. These relate to the (ABX2) spin system of
the two sets of chemically inequivalent protons CHaHbP ([Cu2(m-
dppm)2(m-L)]2+: LA d 3.94, 3.51 ppm; LB d 3.99, 3.55 ppm;
LC d 3.98, 3.60 ppm) and are typical of binuclear complexes
containing an eight-membered “Cu2(m-dppm)2” moiety.23 The
aromatic protons of the dppm ligands appear as two different
sets of signals (see the NMR spectra for ([Cu2(m-dppm)2(m-LC))]2+
in Fig. S3 and S4†) due to the different positions (equatorial and
axial positions) of the phenyl rings in the eight-membered cycle
(see Fig. 1b). The resonances of the two coordinated pyridine
rings of the bppn ligands L are chemically equivalent consistent
with the proposed symmetrical dichelate bridging system of L and
in agreement with the solid-state molecular structure.
1
Room temperature 31P{ H} NMR spectra exhibit a single
resonance as expected for the phosphorus atoms in each molecule
([Cu2(m-dppm)2(m-L)]2+: LA d -8.11 ppm, LB d -6.50 ppm, LC
d
-6.35 ppm). These resonances are shifted to a lower field compared
to the free ligand (-21.32 ppm) in a region similar to that of the
precursor complex [Cu(m-dppm)(NO3)]2 (31P{ H} -9.31 ppm).25
1
Fig. 1 (a) View of the structure of [Cu2(m-dppm)2LA]2+ with the hydrogen
atoms omitted and only the ipso carbons of the phenyl rings shown.
Ellipsoids are drawn at the 50% probability level. (b) The boat-chair
conformation of the [Cu(m-dppm)]2 ring.
ESI mass spectra gave parent peaks corresponding to m/z
[Cu2(dppm)2L]2+, which matched the simulated isotopic distri-
bution patterns and confirmed the formation of the dinuclear
dications.
angles range from 1.47◦ to 4.81◦). Together they form an eight-
membered Cu(m-dppm)2Cu ring in a boat-chair conformation
(see Fig. 1b). A similar feature has been observed in other
compounds such as [Cu2(m-dppm)2(m-PPDMe)]2+ (PPDMe 3,6-
bis(3,5-dimethylpyrazol-1-yl)pyridazine).23
In each complex, the copper atoms have a distorted tetrahedral
arrangement determined by two phosphorus atoms (of two dppm
ligands) and two nitrogen atoms (one from the pyridazine, one
from a pyridine of L). The N–Cu–N angles are acute [range
76.0(1)◦–77.7(3)◦] due to the small chelating bite angle typical
of bppn ligands.24 The P–Cu–P units are non-linear [range
124.21(5)◦–130.57(5)◦], resulting in a folding of the Cu2P4 core
along the Cu ◊ ◊ ◊ Cu axis in an analogous disposition to that of the
starting material [Cu(m-dppm)(NO3)]2.25
The Cu–P and Cu–N bond lengths (see Table 1) are in the typical
range of four coordinate copper(I) complexes. More notable is
that the bis-(2-pyridyl)pyridazine fragment of the ligand L is
not completely planar, as is shown by the dihedral angles of
the pyridine rings with respect to the pyridazine one (torsion
angles [Cu2(m-dppm)2(m-L)](NO3)2 LA 29.86◦, 23.09◦; LB 29.60◦,
22.53◦). Also the aryl substituents are twisted significantly out-
of-the-plane of the pyridazine ring, almost to perpendicular
(torsion angles [Cu2(m-dppm)2(m-L)](NO3)2 LA 70.71◦, 78.00◦;
LB 72.86◦, 77.37◦). Ring twisting is necessary to relieve steric
Photophysical and electrochemical properties
The absorption spectra of the ligands bppn L and of the complexes
[Cu2(m-dppm)2(m-L)]2+ were obtained in CH2Cl2 and the results are
shown in Table 2. The spectra of the free ligands LA and LB are
similar, with two absorption bands lmax 266–268 and 325–329 nm,
attributed to the transitions centred on the bispyridylpyridazine.
In the case of LC, there is a lower energy band (370 nm) due
to the fluoranthene. The absorption spectra of the complexes
are similar to those of the ligands but in addition they have a
weak, broad, low-energy shoulder in the l 370–425 nm region
(Table 2), which is responsible for the orange colour of the
complexes (Fig. 2). This low-energy band is attributed to MLCT
transitions involving the N,N- chelate ligand and the Cu(I) ion
(perhaps influenced by the phosphine ligand).26,27 In agreement
with this MLCT assignment, the low-energy absorption of the
[Cu2(m-dppm)2(m-LC)]2+ complex is slightly red shifted compared
to the others, as the diazafluoranthene-containing ligand stabilizes
the p* orbitals (Fig. 2).
Electrochemical studies of the ligands showed no oxidation
processes in the potential ranges (0 V–2 V vs. SCE). The ligand
reduction of the three complexes and the reductions of the free lig-
ands were reversible and lowered upon complexation (see Table 2).
This is consistent with s-bonding of the ligand to the metal
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Table 2 Absorption (dichloromethane, 5 ¥ 10-5 M) and electrochemical [acetonitrile (or dimethylformamide in the case of LC due to its poor solubility),
1 ¥ 10-3 M] data for LA, LB, LC and the corresponding copper complexes. Electrochemical measurements were recorded at room temperature, using a
glassy carbon working electrode, a Pt wire counter electrode and a SCE reference electrode, using a scan rate of 100 mV/s. Potentials are quoted vs.
Fc/Fc+, which was used as an internal standard
l/nm (e/103 M-1cm-1)
E1/2ox/V
E1/2red/V
LA
266 (16.9), 325 (1.2)
268 (26.9), 329 (0.9)
271 (28.2), 308 (17.8), 326 (13.2), 370 (13.4)
260 (71.6), 328 (21.6), 380 (10.2)
274 (76.7), 331 (21.2), 373 (11.7)
275 (72.5), 326 (36.0), 374 (32.8), 426 (16.1)
—
—
—
-1.22, -1.94a, -2.32b
-1.22a, -2.01a
LB
LC
- 1.24, -2.01
[Cu2(m-dppm)2LA]2+
[Cu2(m-dppm)2LB]2+
[Cu2(m-dppm)2LC]2+
1.17b
1.07b
1.15b
-1.34, -1.87
-1.33, -1.71b, -1.93, -2.14a
-1.14, -1.67, -1.97
a quasi-reversible; b irreversible process, peak potentials quoted.
indicates that the Cu(II) species rapidly decompose on forming,
as revealed by the disappearance of the oxidation peak under
repetitive scans (Fig. S5†). The intensity and the broad character
of the oxidations imply that multiple electron-transfer is occurring,
1
which must originate from the presence of two metal centres. H
NMR investigations of the electrochemically oxidised solutions
did not shed any light on the identity of the decomposition
products.
All the ligands and their respective copper complexes were
weakly luminescent in the solid state, in solution at room
temperature and in frozen solution at 77 K. Photoluminescence
spectra and lifetime data are summarised in Table 3 (complexes)
and Table S1† (ligands). For the copper systems, all the spectra are
broad without vibronic progression, suggesting that the emissive
excited states have charge transfer character. In solid state, the
ligands bppn L present high energy emissions (~ 350–490 nm) with
excited state lifetimes in the nanosecond range. In the case of Lc,
a lower energy band is also observed (~ 600 nm). Considering the
longer lifetime of this band, this emission could have some triplet
Fig. 2 The UV-vis spectra in dichloromethane of LC (broken line) and
[Cu2(m-dppm)2LC]2+ (solid line) where the MLCT is clearly observed.
centre and the positive charge of the metal complex, which render
the ligands more electron-accepting on complexation. The first
reduction potential for [Cu2(m-dppm)2LC]2+ reflects the greater
electron-accepting nature of the aromatic platform in Lc, but
subsequent reductions appear to be affected by the pyridyl or
phenyl substituents. A broad irreversible oxidation band was
observed for each complex that was assigned to the Cu(I)/Cu(II)
redox couple on comparison with similar data reported in the
literature (see Fig. S5† and Table 2).28–30 Such oxidations occur
at high potentials because of the need to radically alter the
complex geometry for Cu(II) and are indicative of ligand rigidity.
The irreversible character of the oxidation process in all cases
3
state character (e.g. from a pp* excited state localised on the p-
conjugated fluoranthene chromophore), or could be intra-ligand
charge transfer in nature (Table S1†).
In the solid state at room temperature, the compounds exhibit
a red-orange luminescence (see Fig. 3), with the bands detected
in the range between l 587 and 676 nm ascribable to radiative
decay from the MLCT states.1,31 In the case of [Cu2(m-dppm)2(m-
LA)]2+ (lem 635 nm) and [Cu2(m-dppm)2(m-LB)]2+ (lem 642 nm),
the emissions are similar, but for [Cu2(m-dppm)2(m-LC)]2+ (lem
676 nm), the emission is red shifted due to the presence of the
diazafluoranthene, which increases the degree of p-conjugation
Table 3 Comparative Photophysical data of complexes ([Cu2(m-dppm)2(m-L)]2+
T/K
lem [nm]
f
t/ms
[Cu2(m-dppm)2(m-LA)]2+
[Cu2(m-dppm)2(m-LB)]2+
[Cu2(m-dppm)2(m-LC)]2+
solid
298
77
298
77
298
77
298
77
298
77
298
77
587sh, 635, 660sh (lexc 500)
670 (lexc 495)
670 br (lexc 400)
643, 664max(lexc 500)
642, 666sh (lexc 490)
681 (lexc 500)
675 br (lexc 450)
678 (lexc 450)
676 (lexc 550)
645sh, 695 (lexc 495)
675max br (lexc 450)
685 (lexc 450)
1.0 (100%) (lexc 460 - lem 630)
5.2 (74%), 0.7 (26%) (lexc 460 - lem 670)
1.1 (89%), 1.3 (11%) (lexc 460 - lem 630)
CH2Cl2
solid
0.013
0.009
0.021
0.3 (100%) (lexc 460 - lem 640)
4.6 (73%), 0.6 (27%) (lexc 460 - lem 680)
2.3 (87%), 0.1 (13%) (lexc 460 - lem 675)
CH2Cl2
solid
0.6 (100%) (lexc 460 - lem 675)
4.7 (72%), 0.7 (28%) (lexc 460 - lem 695)
22.8 (85%), 1.0 (15%) (lexc 460 - lem 670)
CH2Cl2
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Fig. 3 Excitation (broken line) and emission (solid line) spectra of
[Cu2(m-dppm)2(m-LA)](NO3)2 in solid state at room temperature (black)
and at 77 K (grey).
and thus stabilizes the p* of the ligand, with concurrent conse-
3
quences for MLCT emissive states. These red-orange emissions
for heteroleptic copper compounds are quite unusual (bipyridine
and phenanthroline complexes usually emit in the green region)
and only a few examples have been found in the literature.16,32 They
could be related to the dinuclear nature of these systems.
When cooling the solid samples (T = 77 K), the emissions
undergo a small red-shift and are less intense ([Cu2(m-dppm)2(m-
LA)]2+ 298 K lem 635 nm, 77 K lem 670 nm; [Cu2(m-dppm)2(m-
LB)]2+ 298 K lem 642 nm, 77 K lem 681 nm; [Cu2(m-dppm)2(m-
LC)]2+ 298 K lem 676 nm, 77 K lem 695 nm). As proposed by
McMillin, this relatively unusual phenomenon may be attributed
to the presence of a thermal equilibrium between two emissive
states, 1MLCT and 3MLCT, where 3MLCT dominates at low
temperature, shifting the emission maximum to lower energy (see
Fig. 3).1 For CH2Cl2 solutions, the luminescence is quenched
almost completely. However, when the samples are thoroughly
purged with oxygen through several freeze-thaw-pump cycles
under low pressure, a weak red-orange emission is observed in
all the complexes (see Fig. 4), which is slightly red-shifted with
respect to the solid state for [Cu2(m-dppm)2(m-L)]2+ (L = LA, LB)
([Cu2(m-dppm)2(m-LA)]2+ solid lem 635 nm, CH2Cl2 lem 670 nm;
[Cu2(dppm)2(m-LB)]2+ solid lem 642 nm, CH2Cl2 lem 675 nm).
No changes within the concentration (range 10-5 M–10-3 M)
were observed. The solution emission quantum yields determined
according to the method described by Demas33 (Table 2) are poor
and in the range 0.009 to 0.021, similar to those observed for
analogous copper complexes.31,34–36
The complexes show long lifetimes in the microsecond range in
the solid state and in solution at room temperature. In solid state at
room temperature these systems present a monoexponential decay,
however, in solution and in solid state at low temperature, the decay
has two components, best modelled by a biexponential function.
Although biexponential decay is unusual for copper complexes
it is not without precedence.37,38 The excited state lifetimes of the
ligands (Table S1†) exhibit similar exponential-type behaviour. For
[Cu2(m-dppm)2(m-L)]2+ (L = LA, LB), the lifetimes are very similar
and in both cases longer in solution than in the solid state ([Cu2(m-
dppm)2(m-LA)]2+ solid 1.0 ms, CH2Cl2 1.1 ms; [Cu2(m-dppm)2(m-
Fig. 4 Normalized emission spectrum of [Cu2(m-dppm)2(m-LC)](NO3)2
in degassed dichloromethane solution at room temperature (a) and its
emission decay with fitting (b).
LB)]2+ solid 0.3 ms, CH2Cl2 2.3 ms). The presence of the weak
p ◊ ◊ ◊ p interactions observed in crystalline samples could increase
the non-radiative relaxation paths and thus explain the shorter
lifetimes in the solid state. The excited state lifetime of [Cu2(m-
dppm)2(m-LC)]2+ as a solid is shorter than that observed for the
others (0.6 ms), here p ◊ ◊ ◊ p interactions involving the fluoranthene
moieties would be expected to be stronger than those observed for
[Cu2(m-dppm)2(m-L)]2+ (L = LA, LB) and could contribute to the
deactivation of the excited state. However, in oxygen-free solution,
this complex [Cu2(m-dppm)2(m-LC)]2+ has a longer lifetime of
22.8 ms. As far as we know, this value is the longest found for a sys-
tem of the type [Cu(NŸN)(PŸP)]+, being longer than that previously
described by Armaroli for [Cu(POP)(dmdp-phen)]+ (2,9-dimethyl-
4,7-diphenylphenanthroline) under the same conditions (17.3 ms
in dichloromethane at room temperature).12 It should be noted
however that the neutral amidophosphine complexes of copper
[RPN]Cu(L)2 reported by Miller and co-workers in some cases
present longer lifetimes (16–150 ms).39 Although neutral complexes
are highly desirable, some examples of OLED devices with charged
species as active materials have been reported.40 In all cases the
consideration of emission lifetimes is important; guest phosphors
ideally exhibit a phosphorescence lifetime in the region of 5–50
ms at room temperature.41 The desirable solid-state photophysics
of the simple systems reported here can be attributed to the rigid
conformation of the eight-membered ring, which precludes the
possible distortion of the copper in the excited state and the
formation of quenching exciplexes.
8324 | Dalton Trans., 2011, 40, 8320–8327
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5.56, N 14.49. dH (400 MHz; CDCl3; Me4Si) 9.43 (s, 1H, H15), 8.67
(d, 1H, H6, J = 3.8 Hz), 8.61 (d, 1H, H6¢, J = 4.4 Hz), 8.10 (d,
1H, H3¢, J = 7.9 Hz), 7.63 (Wtd, 1H, H4¢, J = 7.8, 1.8 Hz), 7.57
(d, 2H, H8¢, J8¢,9¢ = 7.0 Hz), 7.40 (Wtd, 1H, H4, J = 7.8, 1.5 Hz),
7.30–7.17 (m, 11H, HPh), 5.82 ppm (s, 1H, H12). dC (101 MHz;
CDCl3; Me4Si) 154.18 (1C, C11), 151.56 (1C C14), 149.02 (1C
C6), 148.30 (1C C6¢), 141.85 (1C CQ), 138.85 (1C CQ), 136.08
(1C C4¢), 135.86 (1C C4¢), 135.11 (1C CQ), 129.85 (1C), 128.54
(1C C3), 128.27 (1C C5), 128.25 (1C), 127.16 (1C), 126.70 (1C),
125.47 (1C), 123.02 (1C), 122.67 (1C), 121.48 (1C), 108.15 (1C
C13), 41.54 ppm (1C C12). HRMS (CH3CN); calculated [MH]+
m/z 389.1766, found: 389.1782]+.
Conclusions
The use of extended polyaromatic pyridazines and aza fluo-
ranthenes as new N-donor ligand motifs in the formation of
dimeric Cu(I) bisphosphine complexes has resulted in three novel
sterically hindered dication systems. The N-donor ligands act
as bridging supports to a rigid eight-membered ring comprising
Cu(m-dppm)2Cu and provide a stable framework to secure the
distorted tetrahedral geometry of the Cu centres. Red-orange
MLCT emission is characteristic of these complexes and the struc-
tural features give rise to emission lifetimes in the microsecond
range similar to those associated with neutral amidophosphine
complexes.
Experimental
General Methods
Elemental analyses were carried out in a Perkin–Elmer 240-B
analyser. NMR spectra were recorded on a DPX-400 spectrometer
by using the standard references. Electrospray ionization mass
spectra were recorded on a micromass LCT electrospray mass
spectrometer. Nuclear magnetic resonance spectra were recorded
with a Brucker Avance DPX-400 MHz at the following frequen-
Synthesis of 3,6-bis(2-pyridyl)-4,5-diphenyl pyridazine (LA). 20
mL of a 6 M solution of NaNO2 was added dropwise to a
solution of concentrated HCl (12 mL). The gas formed was
then passed through a 40 mL dichloromethane solution con-
taining 0.807 g (2.093 mmol) of 3,6-bis(2-pyridyl)-4,5-diphenyl
hydropyridazine, by blowing nitrogen through the acid and into
the dichloromethane solution. When all the sodium nitrite had
been added, the dichloromethane solution was allowed to return
to room temperature. The solvent was evaporated and the reaction
mixture dissolved in water and neutralized with the addition of
10% ammonia solution. The mixture was extracted into CH2Cl2,
dried over MgSO4 and run through a column of silica (10%
methanol in ethyl acetate) recovering a white product LA (0.598
g, 74%). Anal. calcd for C26H18N4 (386.46) C 80.81, H 4.69, N
14.50. Found: C 80.92, H 4.64, N 14.26. dH (400 MHz; CD3CN;
Me4Si) 8.42 (d, 2H, H6, J = 4.5 Hz), 7.64 (m, 4H, H3,4), 7.17 (Wtd,
2H, H5, J = 6.5, 1.5 Hz), 7.05 (m, 6H, H8, H10), 6.87 ppm (m,
2H, H9). dC (101 MHz; CDCl3; Me4Si) 158.50 (2C, C11), 155.55
(2C, C2), 148.42 (2C, C6), 138.72 (2C, C12), 135.72 (2C, C4),
134.20 (2C, C7), 129.67 (4C, C8), 127.11 (4C, C9), 126.86 (2C,
C10), 124.58 (2C, C3), 122.48 ppm (2C, C5). HRMS (CH3CN);
calculated [MH]+ m/z 387.1596, found: 387.1610
1
1
cies: 400.1 MHz for H, 100.6 MHz for 13C{ H} and 162.2 for
1
31P{ H}. Chemical shifts are reported in ppm relative to external
1
1
standards (SiMe4 for 1H and 13C{ H} and 85% H3PO4 for 31P{ H})
and all the coupling constants are given in Hz to two decimal
places.
UV-vis spectra were obtained on a Shimadzu UV-2401PC UV-
vis spectrometer. Emission and excitation spectra were obtained
on a Fluorolog FL-3-11 spectrofluorimeter. A Jobin Yvon Fluoro-
Hub single photon counting controller fitted with an appropriate
wavelength Jobin Yvon NanoLED was used to measure lifetimes,
which were determined from the observed decays using DataSta-
tion v2.4. The solution emission quantum yields were measured by
the Demas and Crosby method using [Ru(bipy)3]Cl2 in degassed
water as standard (f450 nm 0.042).33 All electrochemical experiments
were perfomed with a CH Instrument Potentiostat, model 660B.
Cyclic voltammograms were measured on 1 mM solutions of
the complexes in dry, degassed acetonitrile or dimethylformamide
with tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M)
as the supporting electrolyte with a glassy carbon working elec-
trode, a Pt wire counter electrode and a saturated calomel electrode
as the reference electrode. All the potentials were referenced to
internal ferrocene added at the end of each experiment.
3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz),19 3,6 bis(2-pyridyl)-
8,9-diazafluoranthene (LC)20 and [Cu(m-dppm)(NO3)]2 were
25
prepared by literature methods. Trans-stilbene and 1,4-di(4-
pyridyl)ethylene were purchased from Aldrich.
Synthesis of 3,6-bis(2-pyridyl)-4,5-bis(4-pyridyl)-hydropyrida-
zine. This compound was prepared as a bright yellow solid
(0.618 g, 75%) similarly to compound LA starting from 3,4-bis(2-
pyridyl)-1,2,4,5-tetrazine (0.500 g, 2.116 mmol) and 1,4-bis(4-
pyridyl)-ethylene (0.410 g, 2.251 mmol) and purified by column
chromatography on silica (20% methanol in diethyl ether). Anal.
calcd for C24H18N6 (390.45) C 79.54, H 5.01, N 15.46. Found: C
80.01, H 5.11, N 15.72. dH (400 MHz; CD3CN; Me4Si) 9.50 (s, 1H,
H16), 8.72 (d, 1H, H6, J = 5.0 Hz), 8.61 (d, 1H, H6¢, J = 6.0 Hz),
8.50 (d, 2H, H9/9¢, J = 6.5 Hz), 8.41 (d, 2H, H9/9¢, J = 6.0 Hz), 8.13
(d, 1H, H3¢, J = 8.0 Hz), 7.66 (Wtd, 1H, H4¢, J = 7.8, 1.5 Hz), 7.55
Synthesis of 3,6-bis(2-pyridyl)-4,5-diphenyl hydropyridazine.
3,4-bis(2-pyridyl)-1,2,4,5-tetrazine (0.600 g, 2.540 mmol) and
trans-stilbene (0.469 g, 2.600 mmol) were refluxed in 20 mL of
toluene for 24 h. At this stage, the colour of the solution had
changed from dark pink to bright yellow. The solvent was removed
under vacuum, and the reaction mixture was purified by running it
through a column of silica (10% diethyl ether in dichloromethane)
obtaining as a pure yellow solid (0.906 g, 92%). Anal. calcd for
C26H20N4 (388.47): C 80.39, H 5.19, N 14.42. Found: C 79.99, H
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(Wtd, 1H, H4, J = 7.5, 1.5 Hz), 7.46 (d, 2H, H8/8¢, J = 6.0 Hz), 7.36
(d, 1H, H3, J = 6.0 Hz), 7.32 (d, 1H, H5, J = 2.0 Hz), 7.22 (m, 1H,
H5¢, J = 2.0 Hz), 7.06 (d, 2H, H8/8¢, J = 6.0 Hz), 5.86 ppm (s, 1H,
H12). dC (101 MHz; CDCl3; Me4Si) 153.39 (1C, C2¢), 150.79 (1C,
C2), 150.09 (1C, C9), 150.02 (1C, C9¢), 150.00 (1C, C6), 148.42
(1C, C6¢), 146.30 (1C, C7), 140.67 (1C, C10), 138.74 (2C, C13),
136.48 (2C, C4), 136.21 (1C, C4¢), 125.64 (1C, C3), 124.14 (1C,
C5), 123.90 (1C, C8), 123.27 (1C, C5¢), 123.09 (1C, C8¢), 121.45
(1C, C3¢), 102.52 (1C, C11), 39.48 (1C, C7¢). HRMS (CH3CN);
calculated for C24H19N6: [MH]+ m/z 391.1671, found: 391.1667.
(67 mg, 97%) according to the general procedure given for
[Cu2(m-dppm)2(m-LA)](NO3)2 starting from 3,6-bis(2-pyridyl)-
4,5-bis(4-pyridyl)pyridazine (LB) (19 mg, 0.049 mmol) and
[Cu(m-dppm)(NO3)]2 (50 mg, 0.049 mmol). Anal. calcd for
C74H60N8Cu2P4O6·2CH2Cl2 (1578.18) C 57.84, H 4.09, N 7.10.
Found C 57.37, H 3.89, N 7.00 dH (400 MHz; CDCl3; Me4Si) 8.70
(4H, s br, H9), 8.02 (2H, d, J = 4.2, H6), 7.76 (4H, d, J = 3.8,
H8), 7.45 (10H, m br, H4 and Ho), 7.36 (2H, d, J = 8.3, H3),
7.19 (4H, t, J = 7.8, Hp), 7.19 (8H, t, J = 7.5, Hm), 7.10 (4H, t,
J = 7.1, Hp¢), 6.90 (18H, m br, H5, Ho¢ and Hm¢), 3.99 (2H, m
br, P–CH2–P), 3.55 (2H, m br, P–CH2–P). dP (162 MHz; CDCl3;
85% H3PO4) -6.50 ppm. dC (101 MHz; CDCl3; Me4Si) 151.94 (2C,
C10), 150.61 (4C, C9), 149.37 (2C, C6), 148.38 (2C, C2), 132.87
(8C, br, Co), 132.47 (2C, C7), 131.54 (8C, br, Co¢), 130.75 (4C,
Cp), 130.19 (2C, C4), 129.69 (4C, Cp¢), 129.03 (8C, Cm), 128.67
(8C, Cm¢), 128.56 (2C, C11),128.32 (2C, C3),126.49 (2C, C5),
124.89 (4C, C8), 26.24 ppm (2C, br, PCH2P) (Missing Ci). HRMS
(CH3CN); calculated: [Cu2(m-dppm)2(LB)]2+ m/z: 641.1211, found:
641.1202.
Synthesis of 3,6-bis(2-pyridyl)-4,5-bis(4-pyridyl)-pyridazine (LB).
This compound was prepared as an off-white product LB
(0.308 g, 50%) similarly to compound LA, starting from NaNO2
(20 mL, 6 M), HCl (12 mL) and 3,6,-bis(2-pyridyl)-4,5-bis(4-
pyridyl)-1,4-dihydropyridazine (0.618 g, 1.587 mmol). It was
purified by column chromatography on silica (50% ether, 50%
methanol). Anal. calcd for C24H16N6 (388.43) C 74.21, H 4.15, N
21.64. Found: C 73.83, H 4.60, N 21.88. dH (400 MHz; CDCl3;
Me4Si) 8.34 (d, 2H, H9, J = 5.5 Hz), 8.30 (d, 2H, H6, J = 4.5 Hz),
8.01 (d, 2H, H3, J = 7.6 Hz), 7.81 (Wtd, 2H, H4, J = 7.5, 1.5 Hz),
7.23 (m, 2H, H5), 6.85 ppm (d, 2H, H8, J = 6.0 Hz). dC (101 MHz;
CDCl3; Me4Si) 157.03 (4C, C10), 154.18 (2C, C2), 148.65 (4C,
C9), 148.31 (2C, C6), 142.59 (2C, C7), 136.28 (2C, C4), 136.10
(2C, C11), 124.49 (2C, C3), 124.10 (2C, C8), 123.19 ppm (2C,
C5). HRMS (CH3CN); calculated: [MH]+ m/z 387.1504, found:
387.1515
Synthesis of [Cu2(l-dppm)2(l-LC)](NO3)2. This complex
[Cu2(m-dppm)2(m-LC)](NO3)2 (55 mg, 81%) was prepared as a
deep orange solid similarly to [Cu2(m-dppm)2(m-LA)](NO3)2 start-
ing from 3,6-bis(2-pyridyl)-8,9-diazafluoranthene (LC) (35 mg,
0.049 mmol) and [Cu(m-dppm)(NO3)]2 (50 mg, 0.049 mmol). The
product was filtered, washed with cold MeOH and dried. Anal.
calcd for C74H58N6Cu2P4O6·(1378.30) C 64.94, H 4.24, N 6.10.
Found C 64.69, H 4.61, N 5.89. dH (400 MHz; CDCl3; Me4Si)
9.16 (2H, d, J = 8.0, H6), 8.87 (2H, d, J = 7.3, H8/H10), 8.87
(2H, d, J = 8.3, H8/H10), 8.34 (2H, d, J = 8.4, H3), 8.23 (2H, t,
J = 7.8, H5), 8.08 (2H, d, J = 7.8, H9), 7.43 (12H, m br, Ho and
HP), 7.34 (2H, dd; J = 7.0, 5.0; H4), 7.29 (8H, t, J = 7.3, Hm),
6.89 (4H, t, J = 7.5, Hp¢), 6.74 (8H, s br, Ho¢), 6.62 (8H, t, J =
7.5, Hm¢), 3.98 (2H, m br, P–CH2–P), 3.60 ppm (2H, m br, P–
Synthesis of [Cu2(l-dppm)2(l-LA)](NO3)2. 3,6-bis(2-pyridyl)-
4,5-diphenyl pyridazine (LA) (19 mg, 0.049 mmol) was added to a
colourless solution of [Cu(m-dppm)(NO3)]2 (50 mg, 0.049 mmol)
in DCM (10 mL) changing colour immediately to orange. The
resulting mixture was stirred for 1 h, filtered through Celite and
concentrated to 2 mL. The addition of Et2O (20 mL) affords the
precipitation of [Cu2(m-dppm)2(m-LA)](NO3)2 as an orange solid
(62 mg, 90%), which was filtered, washed with cold MeOH and
dried. Anal. calcd for C76H62N6Cu2P4O6·1.5 CH2Cl2 (1533.74) C
60.68, H 4.69, N 5.48. Found C 60.17, H 4.14, N 5.49. dH (400
MHz; CDCl3; Me4Si) 8.09 (2H, d, J = 3.6, H6), 7.60 (4H, d, J =
6.0, H8), 7.43 (18H, m br, H3, H4, H9, H10 and Ho), 7.29 (4H,
t, J = 6.0, Hp), 7.22 (8H, t, J = 6.8, Hm, dppm), 7.00 (2H, t, J
= 6.1, H5), 6.93 (4H, t, J = 7.5, Hp¢), 6.81 (16H, m br, Ho¢ and
Hm¢), 3.94 (2H, m br, P–CH2–P), 3.51 ppm (2H, m br, P–CH2–
P). dP (162 MHz; CDCl3; 85% H3PO4) -8.11 ppm. dC (101 MHz;
CDCl3; Me4Si) 152.95 (2C, C11), 149.34 (2C, C2), 148.79 (2C, C6),
143.93 (2C, C12), 137.02 (2C, C4), 133.32 (2C, C7), 132.28 (8C,
br, Co), 131.33 (8C, br, Co¢), 130.18 (4C, Cp), 129.68 (4C, C8),
129.40 (8C, Cp¢), 129.14 (2C, C10), 128.75 (8C, Cm), 128.59 (4C,
C9), 128.38 (2C, C3), 128.17 (8C, Cm¢), 126.05 (2C, C5), 25.94
ppm (2C, br, PCH2P) (Missing Ci). HRMS (CH3CN); calculated:
[Cu2(m-dppm)2(LA)]2+ m/z: 640.1259, found: 640.1237.
1
CH2–P). dP (162 MHz; CDCl3; 85% H3PO4) -6.35 ppm. 13C{ H}
not obtained due to poor solubility. HRMS (CH3CN); calculated:
[Cu2(m-dppm)2(LC)]2+ m/z: 626.1102, found: 626.1081.
Acknowledgements
We wish to acknowledge the technical contribution of Dr John
O’Brien, Dr Manuel Ruether and Dr Martin Feeney. We acknowl-
edge the funding provided by Science Foundation Ireland-SFI-
08RFPCHE1465 (DN), 05PICAI819 (GC, SV, GOM) and EU-
FP6-MTKD014472 (BG, LW).
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8326 | Dalton Trans., 2011, 40, 8320–8327
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©