New photoluminescent Cu1N4 chromophores. Stabilisation of copper(1) by unconjugated diimines

Article abstract of DOI:10.1039/a909224b

Two new photoluminescent homoleptic distorted tetrahedral Cu¹N₄ chromophores with Cu^II/I potentials of 0.66-0.81 V vs. SCE are isolated by using two unconjugated diimines (2:1 condensates of R₁R₂C=O and ethylenediamine; R₁ = Ph, R₂ = H; R₁ = R₂ = Ph) as ligands. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a909224b

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New photoluminescent CuIN4 chromophores. Stabilisation of
copper(I) by unconjugated diimines
Shubhamoy Chowdhury,a Goutam K. Patra,a Michael G. B. Drew,b Nitin Chattopadhyayc and
Dipankar Datta*a
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India. E-mail: icdd@mahendra.iacs.res.in
b Department of Chemistry, University of Reading, Whiteknights, Reading, UK RG6 6AD
c Department of Chemistry, Jadavpur University, Calcutta 700 032, India
Received 19th November 1999, Accepted 16th December 1999
Two new photoluminescent homoleptic distorted tetra-
hedral CuIN4 chromophores with CuII/I potentials of 0.66–
0.81 V vs. SCE are isolated by using two unconjugated
diimines (2:1 condensates of R R C᎐O and ethylene-
᎐
2
diamine; R1 ؍ Ph, R2 ؍ H; R1 ؍ R21؍ Ph) as ligands.
CuIN4 chromophores which exhibit photoluminescence in
solution are known only in copper() complexes of substituted
1,10-phenanthrolines (1,10-phen).1,2 Herein we report new
photoluminescent CuIN4 chromophores isolated with unconju-
gated diimines L1 and L2.
The ligands L1 and L2 are synthesised by condensing
ethylenediamine (en) with benzaldehyde and benzophenone
respectively in anhydrous methanol.† Their copper() com-
plexes [Cu(L1)2]ClO4 (1) and [Cu(L2)2]ClO4 (2) are prepared by
reacting the ligands with [Cu(CH3CN)4]ClO4 in anhydrous
methanol under N2 atmosphere.‡ Complexes 1 and 2 are
respectively yellow and brick-red in colour. The green copper()
complexes obtained by reacting the ligands with Cu(ClO4)2ؒ
6H2O in 2:1 molar proportions in anhydrous ethanol are read-
ily hydrolysed to yield violet bis-en complexes of copper() and
hence we have not been able to characterise them. Complexes 1
and 2 are stable in air in the solid state for 2–3 weeks. However,
their solution stability in air depends on the solvent; while in
CH2Cl2 these are stable for at least 24 h, in CH3OH 1 is stable
for about 2 h and 2 is stable for about 6 h. On standing in air, a
methanol solution of 1 or 2 slowly turns violet through green.
The violet colour arises due to the formation of the bis-en
complex of copper().
Fig. 1 The structure of the cation in 1 with ellipsoids at 30%
probability. In one of the cations (see text), Cu1–N1 2.058(5), Cu1–
N4 2.094(6) Å and τ (see text) 80.8(2)Њ and in the other cation, Cu2–N5
2.082(5), Cu2–N8 2.060(6) Å and τ 65.4(2)Њ.
The structure of the cation in 1 as determined by X-ray crys-
tallography§ is shown in Fig. 1. There are two cations each with
C2 symmetry in the asymmetric unit but they have very similar
structures. The metal coordination spheres are distorted tetra-
hedra with each metal bonded to four N atoms from two
separate ligands. In a cation, the two Cu–N bond lengths for
a particular ligand differ by 0.022–0.036 Å. In 1, the average
Cu–N bond length is 2.073 Å with an average dihedral angle
(τ) of 73.1Њ between the two CuN2 coordination planes (for D2d
symmetry, this angle τ is 90Њ). The average bite angle of the
ligand in 1 is 85.3Њ. Interestingly, the two phenyl rings of a
particular ligand in a cation span in the same direction. The
structure of the cation in 2 is shown in Fig. 2. The metal
coordination sphere is more distorted than in 1 presumably due
to more steric crowding. The Cu–N distances in 2 range from
2.014(4) to 2.183(4) Å with τ of 63.9(2)Њ. The average bite angle
of the ligand in 2 is 85.3Њ.
Fig. 2 The structure of the cation in 2 with ellipsoids at 30%
probability. Selected bond lengths: Cu1–N11 2.014(4), Cu1–N14
2.183(4), Cu1–N21 2.065(4), Cu1–N24 2.077(5) Å; τ (see text) 63.9(2)Њ.
been examined by cyclic voltammetry and coulometry in CH2-
Cl2 at platinum electrodes under an N2 atmosphere. Complex 1
shows a quasireversible CuII/I couple with an E1/2 of 0.81 V vs.
SCE (saturated calomel electrode). In cyclic voltammetry, a
The electrochemical behaviour of complexes 1 and 2 has
J. Chem. Soc., Dalton Trans., 2000, 235–237
This journal is © The Royal Society of Chemistry 2000
235

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Fig. 3 Cyclic voltammograms of 1 (broken line; concentration
c = 1.03 mmol dmϪ3) and 2 (full line; c = 1.00 mmol dmϪ3) in 0.1 mol
dmϪ3 dichloromethane in tetrabutylammonium perchlorate at a plat-
inum electrode; scan rate v, 50 mV sϪ1. Under the same experimental
conditions, the ferrocene–ferrocenium couple appears at 0.46 V vs. SCE
Fig. 4 Emission spectra of 1 (lower trace) and 2 (upper trace) in
methanol at room temperature. Excitation wavelength, 350 nm.
Absorbances of both the solutions at 350 nm are 0.32.
with a ∆Ep of 125 mV at v = 50 mV sϪ1
.
Interestingly, the emission spectrum of complex 2 shows an
additional band around 550 nm in methanol albeit very weak
(Fig. 4) which is absent in dichloromethane. Both the emissions
in 2 in methanol originate from the MLCT state. The observ-
ation of such dual emission is really novel in the photophysics
of CuIN4 chromophores. The high energy emission is possibly a
fluorescence while the low energy one a phosphorescence. It
should be noted in this context that the exact nature of the
single emission band observed in bis-copper() complexes of
substituted 1,10-phens is not yet known.1,2 We are presently
investigating the factors (e.g. nature of the substituents on the
imino C atoms, solvent effect, counteranion effect etc.) which
may increase the emission yield so that we can study the photo-
physics in detail.
peak-to-peak separation (∆Ep) of 160 mV is observed at a scan
rate v of 50 mV sϪ1 (Fig. 3) and the ratio of the anodic and
cathodic peak currents (ipa/ipc) approaches unity as the scan rate
increases showing that the corresponding copper() species is
not quite stable on the cyclic voltammetric time scale. This
couple is much more reversible in complex 2 with an E1/2 of
0.66 V vs. SCE (∆Ep = 120 mV and ipa/ipc ≈ 1 at v = 50 mV sϪ1
;
Fig. 3). The CuII/I potential in a CuIN4 chromophore is believed
to increase with the π-acidity of the ligand(s) and with the
extent of tetrahedral distortion occurring in the correspond-
ing CuIIN4 chromophore.3–5 This has been demonstrated so
much in bis-copper() complexes of 1,4-diimines [specifically
2,2Ј-bipyridine,3 1,10-phen,5 2,2Ј-biquinoline6 and N-alkyl/
aryl-(2-pyridyl)methanimine7] with bulky substituents at steric
positions that unconjugated diimines have never been used
seriously to stabilise copper(). To our knowledge, in this regard
there is only one report of using an unconjugated diimine earlier
where the ligand is di-2-pyridylamine (DPA).8 [Cu(DPA)2]ClO4
has a CuI/II potential of Ϫ0.2 V vs. SCE in dimethylsulfoxide.8
Incidentally, the corresponding copper() complex of DPA,
[Cu(DPA)2](ClO4)2, is known to have a pseudotetrahedral
CuIIN moiety.9 The π-acidity of a C᎐N group is likely to be less
Acknowledgements
M. G. B. D. thanks EPSRC and the University of Reading for
funds for the Image Plate System. D. D. thanks the Department
of Science and Technology, New Delhi, India and N. C. the
Council of Scientific and Industrial Research, New Delhi, India
for financial support.
᎐
4
than that of pyridine.10 Thus when the E1/2 values of 1 and 2 are
compared with that of [Cu(DPA)2]ClO4, our results appear to
be remarkable. Further studies on the effect of the substituents
on the imino C atoms on the relative stability of the CuIN4
chromophores are ongoing.
Notes and references
† L1: 15 ml of benzaldehyde (148 mmol) and 5 ml of distilled ethylene-
diamine (74 mmol) were refluxed in 75 ml of anhydrous methanol for
6 h. Then the reaction mixture was evaporated on a water bath until the
volume reduced to ca. 20 ml to obtain a viscous liquid. It was cooled in
the refrigerator overnight to obtain a yellowish wax-like compound. It
was recrystallised from n-hexane to obtain large colourless blocks.
Yield, 10.34 g (60%); mp 49–52 ЊC. Anal. Found (calc.): C, 81.37
(81.32); H, 6.76 (6.82); N, 11.78 (11.85)%. IR ν/cmϪ1 (KBr): 1640vs
The ligands L1 and L2 are non-fluorescent in dichloro-
methane and methanol at room temperature. However, their
copper() complexes display photoluminescence upon excit-
ation at 350 nm (within the MLCT envelope) in both the
solvents at room temperature. In dichloromethane, the two
complexes show single broad structureless emission around
430 nm; the quantum yield (φ) is 4 × 10Ϫ5 (against quinine sulf-
ate 11 in 0.1 N H2SO4) for complex 1 and 2 × 10Ϫ4 for complex 2.
A change of solvent from dichloromethane to methanol does
not affect the emission maxima appreciably (430 nm for 1 and
420 nm for 2; Fig. 4) but leads to a considerable lowering of φ
(2 × 10Ϫ5 for 1 and 6 × 10Ϫ5 for 2). All the known examples of
photoluminescent CuIN4 chromophores are known to exhibit a
single emission band.1,2 Studies on the copper() complexes of
substituted 1,10-phens reveal that φ increases as the copper()
centre generated in the photoexcited state acquires a distorted
tetrahedral geometry imposed by steric hindrance of the sub-
stituents.1,2 For example, while [Cu(1,10-phen)2]ϩ does not show
any emission in CH2Cl2 at room temperature, φ of [Cu{2,9-di-
(2-methylphenyl)-1,10-phen}2]PF6 under the same condition is
15 × 10Ϫ4. We believe a similar effect is operative in our case
also, resulting in three- to five-fold enhancement of φ in 2 com-
pared to 1; the extra steric influence comes from the additional
phenyl groups in L2. This is in accord with our cyclic voltam-
metric results where the CuII/I couple in 2 is found to be more
reversible than in 1.
(C᎐N). UV/VIS λmax/nm (ε/dm3 molϪ1 cmϪ1) (CH3OH): 245 (24 500).
᎐
L2: 10.87 g of benzophenone (60 mmol) and 2 ml of distilled ethylene-
diamine (30 mmol) were refluxed in 50 ml of anhydrous methanol for
6 h. Then the reaction mixture was evaporated on a water bath until a
white semi-solid appeared. This was cooled to room temperature. The
compound was recrystallised from n-hexane as white blocks. Yield, 8.6
g (85%); mp 43–45 ЊC. Anal. Found (calc.): C, 86.48 (86.56); H, 6.19
(6.23); N, 7.27 (7.21)%. IR ν/cmϪ1 (KBr): 1625vs (C᎐N). UV/VIS λmax
/
᎐
nm (ε/dm3 molϪ1 cmϪ1) (CH3OH): 219 (7700), 252 (17 500).
‡ 1: 0.47 g (2 mmol) of the ligand was dissolved in 25 ml of anhydrous,
degassed methanol to which 0.33 g of freshly prepared [Cu(CH3-
CN)4]ClO4 (1 mmol) was added under dry N2 atmosphere. The reaction
mixture was stirred for 15 min. The silky yellow compound precipitated
was filtered, washed with n-hexane and stored in vacuo over fused
CaCl2. It was recrystallised from a dichloromethane–n-hexane mixture
as yellow needles. Yield, 0.51 g (80%). Single crystals were grown by
direct diffusion of n-hexane into a dilute dichloromethane solution of
the complex. Anal. Found (calc.): C, 60.52 (60.47); H, 5.02 (5.07); N,
8.75 (8.81); Cu, 10.07 (10.00)%. ΛM (CH3OH): 108 ΩϪ1 cm2 molϪ1 (1:1
electrolyte). IR ν/cmϪ1 (KBr): 1630vs (C᎐N); 1100vs, 630m, split (ClO4).
᎐
UV/VIS λmax/nm (ε/dm3 molϪ1 cmϪ1) (CH3OH): 218 (20 350), 252
(29 650), 311 sh (8250), 366 sh (5000).
2: 0.78 g of the ligand (2 mmol) was dissolved in 35 ml of anhydrous,
degassed methanol to which 0.33 g of freshly prepared [Cu(CH3-
CN)4]ClO4 (1 mmol) was added under dry N2 atmosphere. The reaction
236
J. Chem. Soc., Dalton Trans., 2000, 235–237

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mixture was stirred for 15 min. The red compound precipitated was
filtered, washed with n-hexane and dried in vacuo over fused CaCl2. It
was recrystallised from a dichloromethane–n-hexane mixture as brick-
red cuboids. Yield, 0.5 g (55%). Single crystals were grown by direct
diffusion of n-hexane into a dilute dichloromethane solution of the
complex. Anal. Found (calc.): C, 71.45 (71.55); H, 5.12 (5.15); N, 6.05
(5.96); Cu, 6.70 (6.76)%. ΛM (CH3OH): 94 ΩϪ1 cm2 molϪ1 (1:1 electro-
CCDC reference number 186/1769.
See http://www.rsc.org/suppdata/dt/a9/a909224b/ for crystallo-
graphic files in .cif format.
1 M. K. Eggleston, D. R. McMillin, K. S. Koenig and A. J.
Pallenberg, Inorg. Chem., 1997, 36, 172 and references therein.
2 M. T. Miller, P. K. Gantzel and T. B. Karpishin, Inorg. Chem., 1999,
38, 3414 and references therein.
3 B. R. James and R. J. P. Williams, J. Chem. Soc., 1961, 2007.
4 D. Datta and A. Chakravorty, Inorg. Chem., 1983, 22, 1085.
5 M. T. Miller, P. K. Gantzel and T. B. Karpishin, Angew. Chem., Int.
Ed., 1998, 37, 1556 and references therein.
6 Y. Jahng, J. Hazelrigg, D. Kimball, E. Riesgo, F. Wu and R. P.
Thummel, Inorg. Chem., 1997, 36, 5390.
7 D. M. Haddleton, D. J. Duncalf, D. Kukulj, M. C. Crossman,
S. G. Jackson, S. A. F. Bon, A. J. Clark and A. J. Shooter, Eur. J.
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lyte). IR ν/cmϪ1 (KBr): 1620vs (C᎐N); 1100vs, 630m, split (ClO4).
᎐
UV/VIS λmax/nm (ε/dm3 molϪ1 cmϪ1) (CH3OH): 215 (29 600), 240
(32 000) 314 sh (5700), 387 sh (3300).
§ Crystal data: C32H32ClCuN4O4 1: Mw = 635.60, monoclinic, space
group C2/c, a = 17.75(2), b = 18.316(19), c = 20.84(2) Å, β = 113.51(1)Њ,
U = 6212 Å3, Z = 8, µ = 0.831 mmϪ1, Dc = 1.359 g cmϪ3, 8306 data col-
lected, 5151 unique. C56H48ClCuN4O4 2: Mw = 939.97, monoclinic,
space group P21/c, a = 15.446(17), b = 13.270(15), c = 23.70(3) Å,
β = 91.58(1)Њ, U = 4856 Å3, Z = 4, µ = 0.555 mmϪ1, Dc = 1.286 g cmϪ3
,
13 972 data collected, 8013 unique. The data were collected using
Mo-Kα radiation using the MARresearch Image Plate System. The
crystals were positioned at 70 mm from the Image Plate. 95 frames were
measured at 2Њ intervals with a counting time of 2 min. Data analyses
were carried out with the XDS program.12 The structures were solved
using direct methods with the SHELXS-86 program.13 In 1 there were
two molecules in the asymmetric unit, both with crystallographic C2
symmetry. The perchlorate anion was in a general position, but was
disordered and two sets of oxygen tetrahedra were included each with
50% occupancy. There were no unusual features in the structure
determination of 2. Both the structures were refined on F2 using
SHELXL-93.14 Final R values: for 1 with observed data I > 2σ(I),
R1 = 0.0731, wR2 = 0.1871 and for all data R1 = 0.1736, wR2 = 0.2193;
for 2 with observed data R1 = 0.0837, wR2 = 0.1540 and for all data
R1 = 0.1515, wR2 = 0.1733.
8 A. Cinquantini, G. Opromolla and P. Zanello, J. Chem. Soc., Dalton
Trans., 1986, 1907.
9 E. M. Gouge, J. F. Geldard and E. Sinn, Inorg. Chem., 1980, 19,
3356.
10 D. Datta, J. Chem. Soc., Dalton Trans., 1991, 3161.
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12 W. Kabsch, J. Appl. Crystallogr., 1988, 21, 916.
13 G. M. Sheldrick, SHELXS-86, Program for crystal structure solu-
tion, Acta Crystallogr., Sect. A, 1990, 46, 467.
14 G. M. Sheldrick, SHELXL-93, University of Göttingen, 1993.
Communication a909224b
J. Chem. Soc., Dalton Trans., 2000, 235–237
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