Synthesis and characterisation of neutral mononuclear cuprous complexes based on dipyrrin derivatives and phosphine mixed-ligands

Article abstract of DOI:10.1039/c2dt30618b

Heteroleptic neutral mononuclear cuprous complexes with dipyrrin derivatives and phosphine mixed-ligands including 1,3,7,9-tetramethyldipyrrin (1), 5-phenyl-1,3,7,9-tetramethyldipyrrin (2), 2,8-dibromo-1,3,7,9- tetramethyldipyrrin (3), 1,9-dichloro-5-phen

Full text of DOI:10.1039/c2dt30618b

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PAPER
Synthesis and characterisation of neutral mononuclear cuprous complexes
based on dipyrrin derivatives and phosphine mixed-ligands†‡
Xiaohui Liu,a,b Hongmei Nan,*c Wei Sun,a,d Qikai Zhang,d Mingjian Zhan,a Luyi Zou,a Zhiyuan Xie,a
Xiao Li,a Canzhong Lud and Yanxiang Cheng*a
Received 19th March 2012, Accepted 29th May 2012
DOI: 10.1039/c2dt30618b
Heteroleptic neutral mononuclear cuprous complexes with dipyrrin derivatives and phosphine mixed-
ligands including 1,3,7,9-tetramethyldipyrrin (1), 5-phenyl-1,3,7,9-tetramethyldipyrrin (2), 2,8-dibromo-
1,3,7,9-tetramethyldipyrrin (3), 1,9-dichloro-5-phenyldipyrrin (4), 1,9-dibromo-5-phenyldipyrrin (5),
5-pentafluorophenyl-1,3,7,9-tetramethyldipyrrin (6) and 1,5,9-triphenyldipyrrin (7) have been synthesized
and fully characterized. The central Cu(I) atoms of these complexes in general formulas of Cu(1–6)-
(PPh3)2 (1a–6a) and Cu(1–6)(DPEphos) (1b–6b) [DPEphos = bis(2-diphenylphosphinophenyl)ether] all
exhibit a pseudo-tetrahedral geometry, while complex Cu(7)(PPh3) (7a) is tricoordinated in a pyramidal
conformation due to the large steric hindrance of ligand 7. The oxidation potentials assigned to oxidations
of Cu(I)–Cu(II) are extraordinarily low in the range of 0.36–1.02 V vs. Ag/AgCl compared with traditional
[Cu(phen)(PP)]+ analogues. Their emission maxima range from 495 to 595 nm in dichloromethane at
room temperature with quantum yields of 0.05–4.03% and lifetimes on the order of nanoseconds. Unlike
the characteristic MLCT emission in cationic Cu(I) complexes, the emissions are assigned to the dipyrrin-
centered intraligand charge transition (ILCT) based on the fact that the increased conjugation within the
dipyrrinato anion leads to a weaker metal–ligand interaction, thus preventing the mixing of π orbitals of
ligand and 3d orbitals of Cu(I) atom. This conclusion is also supported by electrochemical data and
theoretical calculations.
light-emitting electrochemical cells (LECs) fabricated by solu-
Introduction
tion processing,3,4 their inherent defects from the counteranion
Cuprous complexes as more cost-effective emitters have received
more attention since their similar phosphorescent characteristics
to the noble metal complexes at room temperature were
observed.1 Relative to the Cu(I)-bisphenanthrolines of low
quantum yield,2 mixed-ligand Cu(I) complexes containing phos-
phines have shown improved luminescence properties and practi-
cal potential in the application of electroluminescent devices and
sensors.3 Although these cationic complexes are of benefit to
would cause some problems in practical applications, such as
unsuitability for sublimation and vapor deposition processing.3
Therefore, an increasing number of neutral cuprous complexes
have been developed using non-traditional 1,10-phenanthroline
ligands, which achieved some better performances in their
photophysical properties and applications.5 Recently, Eisenberg’s
group reported a series of neutral Cu(I) complexes based on sp3
N atom negative ligands, which exhibit characteristic phosphor-
escent emission from the mixed intraligand and metal-to-ligand
charge transfer transition (ILCT and MLCT).6 An interesting
property of the neutral Cu(I) complexes as oxygen sensors have
also been observed in our group with good reversibility through
energy and electron transfer mechanisms.7
BODIPY (boron difluoride dipyrromethene) and it derivatives
are widely used as molecular probes and label dyes in environ-
mental chemistry and biochemistry due to their strong molar
absorbance and sharp emission with a high quantum yield.8 The
corresponding ligand, dipyrrin as a mono-anionic chelate, is very
suitable to prepare metal complexes, especially for low oxidation
state metal ions, such as Cu(I). Furthermore, its aromatic struc-
ture from a large conjugated backbone is a favorable factor to
obtain luminescent complexes. In fact, plenty of dipyrrinato
metal complexes have been reported.8b Among them, some
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P.R. China. E-mail: yanxiang@ciac.jl.cn;
Fax: +86-431-85262572; Tel: +86-431-85262106
bThe Graduate School, Chinese Academy of Sciences, Beijing 100039,
P.R. China
cThe First Affiliated Hospital, Changchun University of Chinese
Medicine, Changchun 130021, P.R. China
dState Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou 350002, P.R. China
†Dedicated to Professor Dr Max Herberhold on the occasion of his 75th
birthday.
‡Electronic supplementary information (ESI) available: Absorption and
emission spectra Fig. S1–S15, and cyclic voltammogram Fig. S16–S22.
CCDC reference numbers 871234–871241. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2dt30618b
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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exhibit fluorescent emission with lifetime of nanoseconds, such
as Zn(II)9 and group 13 Al(III),10 Ga(III) and In(III) dipyrrinato
complexes.10,11 Recent examples of noble-metal Ru(II),12 Ir(III)13
and Pt(II)14 complexes, in contrast, display the expected phos-
phorescence with lifetime of microseconds. These results offer a
good chance to explore the properties of neutral dipyrrinato
cuprous complexes. Herein, we report the synthesis and charac-
terisation of heteroleptic neutral cuprous complexes based on the
dipyrrinato anion and auxiliary phosphine ligands. The dipyrrins
with different substituent patterns including halogen atoms
were chosen for investigating the photophysical properties of
their Cu(I) complexes as a function of the effects of electronic
structure and steric hindrance.
solved by direct methods and refined on F2 using full matrix
least-squares methods (SHELXS97 and SHELXL97 pro-
grams).17 For complexes 4a, 5a and 7a the data sets was cor-
rected using the program SQUEEZE. All non-hydrogen atoms
were refined anistropically. The positions of the hydrogen atoms
attached to carbon atoms were fixed at their ideal positions.
Table 1 summarizes crystallographic data for the dipyrrinato
cuprous complexes reported here.
3,5-Dimethylpyrrole-2-carbaldehyde. The compound was
synthesized using a similar method to that reported in the litera-
ture.18 Freshly distilled POCl3 (10.25 ml; 0.11 mol) was added
dropwise to anhydrous DMF (8.5 ml; 0.11 mol) at 0 °C. The
resulting solution was stirred for 15 min after the ice-bath was
removed. Dry 1,2-dichloroethane (20 ml) was then added, and
the solution was cooled to 0–5 °C. 2,4-Dimethylpyrrole (9.51 g,
0.10 mol) dissolved in dry 1,2-dichloroethane (20 ml) was
added. Following the addition, the yellow mixture was heated to
reflux for 15 min before being cooled to room temperature.
Aqueous sodium acetate (45 g in 100 ml water) was then added
(slowly at first then as rapidly as possible). The mixture was
again refluxed for 15 min with vigorous stirring. The layers were
separated, and the aqueous layer was washed three times with
dichloromethane. The combined organic layer was washed three
times with saturated brine, dried over Na2SO4, and evaporated
under vacuum to give an oily liquid. The crude product was
purified via chromatography on Al2O3 with petroleum ether to
Experimental
General
Starting materials were purchased from Sigma-Aldrich, Acros or
Fluka and used without further purification. Solvents were
freshly distilled over appropriate drying reagents under argon
atmosphere. All experiments were carried out under a dry argon
atmosphere using standard Schlenk techniques unless otherwise
stated.
Characterization. 1H and 13C{1H} NMR with TMS as
internal reference, 19F{1H} NMR with fluorobenzene as external
reference, and 31P{1H} NMR spectra with 85% H3PO4 as exter-
nal reference were recorded on a Bruker Avance 300 or
600 MHz spectrometer at room temperature. Elemental analyses
for C, H and N were performed with a BioRad elemental analy-
sis system. Absorption spectra were recorded on a Perkin-Elmer
Lambda35 UV/Vis Spectrometer. Photoluminescence spectra
were recorded on a Perkin L350 Spectrofluorophotometer. The
PL quantum yields were determined by the integrating sphere
with 409 nm excitation of a HeCd laser. Luminescence lifetimes
were measured with a Lecroy Wave Runner 6100 digital oscillo-
scope (1 GHz) using a tunable laser (pulse width = 4 ns, gate =
50 ns) for the excitation (Continuum Sunlite OPO). The samples
for all the steady optical spectra were in sealed quartz cuvettes
under nitrogen atmosphere. Cyclic voltammetry was performed
using the Chi660b electrochemical analyzer with a three-
electrode cell in dry dichloromethane solution under an argon
atmosphere. A glassy carbon working electrode, a Pt auxiliary
electrode and a Ag–AgCl reference electrode were employed.
The supporting electrolyte was 0.1 M tetrabutylammonium per-
chlorate (Bu4NClO4). Ferrocene was used as a standard to cali-
brate the system. The scan rate was 50 mV s−1. DFT calculations
were performed using the GAUSSIAN 03W software package
using a spin-restricted formalism at the B3LYP level.15 The basis
sets 6-311++G** were used for H, C, N, O, F, P and Cl atoms,
and Lanl2DZ basis sets for Br and Cu atoms. The HOMO and
LUMO energies were determined using XRD geometries to
approximate the ground state.
1
yield light yellow crystals (10.2 g, 82%). H NMR (600 MHz,
CDCl3) δ 10.27 (s, 1H), 9.44 (s, 1H), 5.83 (s, 1H), 2.30 (s, 3H),
2.29 (s, 3H); 13C NMR (151 MHz, CDCl3) δ 175.89, 138.74,
134.86, 128.75, 112.01, 13.13, 10.58.
(3,5-Dimethylpyrrol-2-yl)(phenyl)methanone. 2,4-Dimethyl-
pyrrole (8.10 g; 0.085 mol) in dry ether (150 ml) was added
dropwise to a solution of ethylmagnesium bromide in ether
(0.094 mmol, 1.1 eq.) so as to cause slight reflux. The solution
was refluxed for an additional 30 min. A solution of benzoyl
chloride (9.87 ml, 0.085 mol) in dry ether (30 ml) was dropped
to the reaction solution. The orange suspension was heated to
reflux for 2 h, and poured into saturated aqueous ammonium
chloride. The precipitate was dissolved in CH2Cl2 and washed
with water (150 ml × 3), dried over Na2SO4, and evaporated to
give a red oil. The oil was purified via chromatography on a
silica gel flash column with 4 : 1 petroleum ether–EtOAc as
eluent to yield an orange solid. Recrystallization from petroleum
ether gave the product as white crystals (13.30 g, 79%). 1H
NMR (600 MHz, CDCl3) δ 9.73 (s, 1H), 7.64–7.60 (m, 2H),
7.51–7.45 (m, 1H), 7.42 (t, J = 7.4 Hz, 2H), 5.84 (d, J = 2.6 Hz,
1H), 2.28 (s, 3H), 1.90 (s, 3H); 13C NMR (151 MHz, CDCl3)
δ 185.79, 140.28, 136.10, 130.82, 128.23, 128.20, 127.84,
112.97, 14.03, 13.11.
5-Phenyldipyrromethane. The compound was synthesized
according to the literature.19 1H NMR (300 MHz, CDCl3) δ 7.88
(br, 2H), 7.32–7.16 (m, 5H), 6.70–6.66 (m, 2H), 6.17–6.12 (m,
2H), 5.76–5.70 (m, 2H), 5.48 (s, 1H).
Crystal structure determinations. Single-crystal X-ray
measurements were carried out on a Bruker SMART APEX
CCD diffractometer equipped with a graphite monochromator
and Mo-Kα radiation (λ = 0.71073 Å). Absorption corrections
were applied using the SADABS program.16 The structures were
1,3,7,9-Tetramethyldipyrrin (1). Phosphorus oxychloride
(4.59 g, 30 mmol) was slowly added to a degassed solution of
2,4-dimethylpyrrole (2.85 g, 30 mmol) and 3,5-dimethylpyrrole-
Dalton Trans.
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2-carbaldehyde (3.69 g, 30 mmol) in n-hexane (30 ml) and
CH2Cl2 (30 ml) at 0 °C with stirring over 15 min. A red precipi-
tate formed was removed by filtration and washed with cold
hexane. The precipitate was resolved in dichloromethane (50 ml)
and washed with aqueous Na2CO3, water (100 ml × 3), dried
over Na2SO4. The resulting solution was concentrated and then
purified by flash column chromatography [inactivating Al2O3;
hexane (containing 1% triethylamine)] to yield an orange crystal-
1
line solid (5.46 g, 91.0%). H NMR (300 MHz, CDCl3) δ 7.08
(br, 1H), 6.69 (s, 1H), 5.95 (s, 2H), 2.33 (s, 6H), 2.22 (s, 6H).
13C NMR (151 MHz, CDCl3) δ 152.84, 138.50, 137.56, 117.17,
116.58, 16.16, 11.39.
5-Phenyl-1,3,7,9-tetramethyldipyrrin (2). The same procedure
as the preparation of 1 was used with (3,5-dimethylpyrrol-2-yl)-
(phenyl)methanone as the starting material. Yield 6.51 g, 78.6%.
1H NMR (300 MHz, CDCl3) δ 13.21 (br, 1H), 7.42–7.13 (m,
5H), 5.88 (s, 2H), 2.34 (s, 6H), 1.29 (s, 6H). 13C NMR
(151 MHz, CDCl3) δ 151.53, 140.40, 138.80, 138.07, 136.41,
129.24, 128.56, 128.14, 119.55, 16.03, 14.46.
2,8-Dibromo-1,3,7,9-tetramethyldipyrrin (3). Bromine (1.92 g,
12 mmol) in 20 ml chloroform was added dropwise to a solution
of 1,3,7,9-tetramethyldipyrrin (1) (1.0 g, 5 mmol) in 15 ml
chloroform over 10 min. The solution was stirred at room temp-
erature for 3 h, and then washed with saturated aqueous
Na2S2O3 (100 ml × 3) and brine (100 ml × 3). The organic layer
was dried over Na2SO4, evaporated under vacuum. The residue
was purified by flash chromatography on Al2O3 with petroleum
ether containing 1% triethylamine as eluent to yield 3 (1.59 g,
1
89%). H NMR (600 MHz, CDCl3) δ 11.04 (br, 1H), 6.66 (s,
1H), 2.33 (s, 6H), 2.15 (s, 6H); 13C NMR (151 MHz, CDCl3)
δ 150.59, 136.18, 136.02, 117.01, 107.90, 14.89, 10.97.
1,9-Dichloro-5-phenyldipyrrin (4). A solution of N-chloro-
succinimide (NCS) (1.34 g, 10 mmol) in 20 ml THF was added
to a solution of 5-phenyldipyrromethane (1.11 g, 5 mmol) in
50 ml dry THF at −78 °C. The reaction was stirred at −78 °C for
2 h and recovered to room temperature stirring 2 h. A solution of
5 mmol of DDQ in THF was added, and stirring was continued
overnight. The resulting mixture was dissolved in dichloro-
methane (50 ml) and washed with water (100 ml × 3). The
organic layer was dried over Na2SO4, concentrated and purified
by flash chromatography on Al2O3 with petroleum ether to yield
1
4 (0.88 g, 61%). H NMR (300 MHz, CDCl3) δ 12.43 (br, 1H),
7.52–7.40 (m, 5H), 6.52 (d, J = 4.2 Hz, 2H), 6.25 (d, J =
4.2 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 141.78, 139.92,
138.45, 135.48, 130.79, 130.19, 129.32, 127.82, 116.93.
1,9-Dibromo-5-phenyldipyrrin (5). Ligand 5 was synthesized
as for the preparation of 4 but with NBS as the starting material.
1
Yield 1.30 g, 61%. H NMR (600 MHz, CDCl3) δ 12.47 (br,
1H), 7.50–7.38 (m, 5H), 6.46 (d, J = 4.2 Hz, 2H), 6.32 (d, J =
4.2 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 140.35, 139.39,
135.43, 130.74, 130.19, 129.53, 129.35, 127.85, 120.43.
5-Pentafluorophenyl-1,3,7,9-tetramethyldipyrrin
(6). TFA
(0.07 ml, 1 mmol) was added to a solution of 2,4-dimethyl-
pyrrole (1.13 ml, 11 mmol) and pentafluorobenzaldehyde
(0.62 ml, 5 mmol) in 30 ml absolute dichloromethane, and the solu-
tion was stirred at room temperature for 15 min. 1.36 g (6 mmol)
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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1
Complex 2a. 0.66 g (76%). Mp 137 °C; H NMR (300 MHz,
CDCl3): δ 7.46–7.28 (m, 35H), 5.91 (s, 2H), 1.83 (s, 6H), 1.27
(s, 6H); 13C NMR (75 MHz, CDCl3): δ 156.84, 143.30, 133.94,
133.83, 129.86, 129.58, 129.21, 128.62, 128.57, 128.57, 128.32,
127.62, 120.28; 31P NMR (242 MHz, CDCl3): δ −1.16. Anal.
Calc. for C55H49CuN2P2: C, 76.50; H, 5.72; N, 3.24. Found: C,
76.03; H, 5.40; N, 2.70%.
DDQ was added and then the mixture was stirred for 2 h and
filtered. The filtrate was washed with brine (100 ml × 2), dried
(Na2SO4), and concentrated under vacuum. The crude product
was purified by flash column chromatography [Al2O3; hexane
(containing 1% triethylamine)] to yield 6 (0.97 g, 53%) as a red
1
solid: H NMR (600 MHz, CDCl3) δ 13.15 (br, 1H), 5.92 (s,
2H), 2.33 (s, 6H), 1.52 (s, 6H); 13C NMR (151 MHz, CDCl3)
δ 153.23, 145.41, 143.77, 142.36, 140.67, 138.88, 138.29,
137.20, 135.88, 120.58, 119.41, 112.42, 16.08, 13.71; 19F NMR
(282 MHz, CDCl3): δ −32.60 (dd, J = 22.9, 8.6 Hz, 2F), −45.36
(t, J = 20.8 Hz, 1F), −53.54 (dt, J = 22.8, 8.8 Hz, 2F).
1
Complex 2b. 0.69 g (79%). Mp 159 °C; H NMR (300 MHz,
CDCl3): δ 7.48–6.77 (m, 33H), 5.84 (s, 2H), 1.66 (s, 6H), 1.19
(s, 6H); 13C NMR (150 HMz, CDCl3): δ 155.30, 144.40,
142.25, 141.94, 136.54, 134.75, 134.09, 133.92, 130.40, 130.02,
129.20, 128.54, 127.86, 126.96, 123.99, 120.30, 119.40, 18.93,
15.88; 31P NMR (242 MHz, CDCl3): δ −16.6. Anal. Calc. for
C55H47CuN2OP2: C, 75.28; H, 5.40; N, 3.19. Found: C, 74.30;
H, 5.34; N, 2.92%.
1,5,9-Triphenyldipyrrin (7). Compound 7 was synthesized
according to the literature.20 2-Phenylpyrrole (1.43 g, 10 mmol),
benzoyl chloride (2.0 g, 7.4 mmol) and 1,2-dichloroethane
(37 ml) were heated to reflux for 45 h, during which time the
solution gradually acquired a purple hue. The reaction mixture
was diluted with CH2Cl2 (50 ml) and washed with H2O (50 ml),
1 M HCl (50 ml), 1 M NaOH (50 ml), and brine (50 ml),
respectively. The organic portions were dried (Na2SO4), filtered
and concentrated. The crude product was purified via flash
chromatography on alumina using 30% CH2Cl2/hexanes as
eluent to yield 7 as a dark purple solid (1.94 g, 49% yield);
1H NMR (300 MHz, CDCl3) δ 13.94 (br, 1H), 7.98 (d, J =
7.5 Hz, 4H), 7.62–7.44 (m, 11H), 6.89 (d, J = 4.2 Hz, 2H), 6.76
(d, J = 4.5 Hz, 2H); 13C NMR (151 MHz, CDCl3) δ 154.13,
142.02, 139.69, 137.17, 133.24, 133.00, 130.91, 129.95, 129.58,
129.02, 127.75, 126.20, 115.62.
1
Complex 3a. 0.79 g (84%). Mp 146 °C; H NMR (300 MHz,
CDCl3): δ 7.32–7.14 (m, 30H), 6.97 (s, 1H), 2.24 (s, 6H), 1.74
(s, 6H); 13C NMR (150 MHz, CDCl3): δ 155.14, 138.30,
135.29, 135.19, 135.10, 133.86, 133.71, 129.44, 128.63, 128.58,
123.41, 106.66, 17.01, 11.41; 31P NMR (242 MHz, CDCl3):
δ 0.19. Anal. Calc. for C49H43Br2CuN2P2: C, 62.27; H, 4.59; N,
2.96. Found: C, 62.10; H, 4.40; N, 2.57%.
1
Complex 3b. 0.89 g (93%). Mp 147 °C; H NMR (300 MHz,
CDCl3): δ 7.20–7.03 (m, 26H), 6.82 (d, J = 8.0 Hz, 2H), 6.68 (s,
1H) , 2.15 (s, 6H), 1.63 (s, 6H); 13C NMR (150 MHz, CDCl3):
δ 158.56, 153.30, 135.14, 134.04, 133.90, 133.77, 133.71,
130.67, 128.73, 127.89, 127.58, 124.33, 123.36, 119.66, 106.06,
17.33, 11.36; 31P NMR (242 MHz, CDCl3): δ −17.0. Anal.
Calc. for C49H41Br2CuN2OP2: C, 61.36; H, 4.31; N, 2.92.
Found: C, 61.27; H, 4.18; N, 2.45%.
Synthesis of complexes
Complex 1a. A degassed mixture of 1 (0.20 g, 1.0 mmol) and
NaH (0.12 g, 5.0 mmol) were stirred in 10 ml newly evaporated
THF for 2 h in flask A. CuI (0.19 g, 1.0 mmol) and triphenyl-
phosphine (0.53 g, 2.0 mmol) were degassed and stirred in
10 ml THF for 2 h in flask B. The clear solution of sodic ligand
in flask A after standing was transferred into the mixture of flask
B. The reaction mass was stirred for 2 h, then the solvent was
evaporated under vacuum to dryness. The solid residue
was extracted with 10 ml absolute dichloromethane. The extract
was filtered and transferred to an argon protected flask. 10 ml
Methanol was layered above the resulting solution to afford
1
Complex 4a. 0.71 g (81%). Mp 175 °C; H NMR (300 MHz,
CDCl3) δ 7.50–7.28 (m, 35H), 6.45 (d, J = 4.2 Hz, 2H), 6.12 (d,
J = 3.9 Hz, 2H), 3.49 (d, J = 5.1 Hz, 3H); 13C NMR (150 MHz,
CDCl3): δ 159.06, 146.13, 146.02, 140.34, 139.90, 135.79,
134.73, 134.77, 133.92, 133.04, 131.29, 130.41, 128.44, 128.40,
127.66, 124.09, 119.90, 116.64; 31P NMR (242 MHz, CDCl3):
δ 0.47. Anal. Calc. for C51H39Cl2CuN2P2·CH3OH: C, 68.76; H,
4.77; N, 3.08. Found: C, 67.83; H, 4.39; N, 2.55%.
1
1
Complex 4b. 0.76 g (85%). Mp 139 °C; H NMR (300 MHz,
yellow crystals of complex 1a 0.69 g (87%). Mp 119 °C; H
CDCl3): δ 7.40–6.66 (m, 33H), 6.30 (d, J = 4.1 Hz, 2H), 6.03
(d, J = 4.1 Hz, 2H); 13C NMR (75 MHz, CDCl3): δ 158.46,
145.63, 145.12, 139.48, 138.26, 134.00, 133.94, 133.89, 133.50,
131.99, 130.60, 130.44, 128.55, 128.29, 127.79, 127.45, 126.62,
123.93, 119.42, 115.14; 31P NMR (121 MHz, CDCl3): δ −14.9.
Anal. Calc. for C51H37Cl2CuN2OP2: C, 68.81; H, 4.19; N, 3.15.
Found: C, 67.53; H, 4.35; N, 2.48%.
NMR (300 MHz, CDCl3): δ 7.44–7.31 (m, 30H), 6.99 (s, 1H),
5.96 (s, 2H), 2.29 (s, 12H); 13C NMR (150 MHz, CDCl3):
δ 157.68, 141.07, 136.78, 135.36, 135.27, 133.91, 133.80,
129.34, 128.57, 128.52, 122.52, 116.63, 18.21, 11.75; 31P NMR
(242 MHz, CDCl3): δ −0.18; Anal. Calc. for C49H45CuN2P2:
C, 74.74; H, 5.76; N, 3.56. Found: C, 73.60; H, 5.95; N, 3.06%.
The following complexes were synthesized using the same
procedure as the preparation of complex 1a.
1
Complex 5a. 0.85 g (88%). Mp 147 °C; H NMR (300 MHz,
1
Complex 1b. 0.73 g (91%). Mp 175 °C; H NMR (300 MHz,
CDCl3): δ 7.48–7.28 (m, 35H), 6.41 (d, J = 4.2 Hz, 2H), 6.26
(d, J = 4.2 Hz, 2H), 3.49 (d, J = 5.1 Hz, 3H); 13C NMR
(150 MHz, CDCl3): δ 146.14, 140.48, 138.81, 137.18, 134.86,
134.55, 134.35, 133.30, 132.43, 131.11, 129.85, 128.89, 128.77,
128.58, 127.42, 119.73; 31P NMR (242 MHz, CDCl3):
δ −0.005; Anal. Calc. for C51H39Br2CuN2P2·CH3OH: C, 62.63;
H, 4.35. N, 2.81. Found: C, 62.62; H, 4.34; N, 2.44%.
CDCl3): δ 7.45–6.97 (m, 28H), 6.70 (s, 1H), 5.85 (s, 2H), 2.17
(s, 6H), 1.55 (s, 6H); 13C NMR (150 MHz, CDCl3): δ 158.99,
156.04, 140.13, 136.80, 135.11, 134.11, 130.58, 128.60, 128.24,
127.94, 124.29, 122.71, 119.91, 116.43, 18.88, 12.04; 31P NMR
(242 MHz, CDCl3): δ −17.6; Anal. Calc. for C49H43CuN2OP2:
C, 73.44; H, 5.41; N, 3.50. Found: C, 72.65; H, 5.38; N, 2.87%.
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1
Complex 5b. 0.82 g (84%). Mp 150 °C; H NMR (300 MHz,
CDCl3): δ 7.37–7.00 (m, 31H), 6.79–6.78 (m, 2H), 6.22 (d, J =
4.1 Hz, 2H), 6.12 (d, J = 4.1 Hz, 2H); 13C NMR (75 MHz,
CDCl3): δ 140.58, 139.81, 135.75, 134.73, 134.57, 134.46,
134.36, 134.19, 132.53, 130.86, 128.13, 127.02, 124.31, 119.84,
119.37; 31P NMR (121 MHz, CDCl3): δ −17.91. Anal. Calc. for
C51H37Br2CuN2OP2: C, 62.56; H, 3.81; N, 2.86. Found: C,
61.38; H, 4.12; N, 2.19%.
1
Complex 6a. 0.70 g (74%). Mp 132 °C; H NMR (300 MHz,
CDCl3): δ 7.44–7.34 (m, 30H), 5.95 (s, 2H), 1.82 (s, 6H), 1.52
(s, 6H); 13C NMR (75 MHz, CDCl3): δ 158.77, 140.77, 135.34,
135.26, 135.16, 133.88, 133.77, 129.40, 128.64, 128.59, 121.33,
18.74, 15.10; 19F NMR (282 MHz, CDCl3) δ −33.08 (dd, J =
24.2, 8.2 Hz, 2F), −48.40 (t, J = 20.8 Hz, 1F), −55.40 (td, J =
23.1, 8.0 Hz, 2F); 31P NMR (121 MHz, CDCl3): δ 0.87. Anal.
Calc. for C55H44CuF5N2P2: C, 69.28; H, 4.65; N, 2.94. Found:
C, 67.95; H, 4.63; N, 2.32%.
1
Complex 6b. 0.80 g (83%). Mp 131 °C; H NMR (300 MHz,
CDCl3): δ 7.20–6.89 (m, 28H), 5.84 (s, 2H), 1.53 (s, 6H), 1.47
(s, 6H); 13C NMR (75 MHz, CDCl3): δ 158.68, 157.16, 147.03,
139.98, 136.72, 135.69, 134.88, 134.55, 134.34, 134.24, 130.84,
128.99, 128.26, 125.18, 124.54, 121.77, 119.72, 19.16, 15.63;
19F NMR (282 MHz, CDCl3) δ −32.75 (dd, J = 23.3, 8.7 Hz,
2F), −47.03 (t, J = 21.0 Hz, 1F), −54.50 (t, J = 18.5, 8.0 Hz,
2F); 31P NMR (121 MHz, CDCl3): δ −16.24. Anal. Calc. for
C55H42CuF5N2OP2·CH2Cl2: C, 63.91; H, 4.21; N, 2.66. Found:
C, 63.23; H, 4.31; N, 2.13%.
1
Complex 7a. 0.52 g (75%). Mp 105 °C; H NMR (300 MHz,
CDCl3): δ 7.83–7.79 (m, 4H), 7.62–7.59 (m, 2H), 7.45–7.42 (m,
2H), 7.29–7.24 (m, 6H), 7.16–7.11 (m, 6H), 6.95–6.81 (m,
10H), 6.78 (d, J = 4.2 Hz, 2H), 6.71 (d, J = 4.1 Hz, 2H), 3.48
(q, J = 7.0 Hz, 4H), 1.21 (t, J = 7.0 Hz, 6H); 13C NMR
(75 MHz, CDCl3): δ 133.86, 133.67, 132.72, 131.50, 129.74,
128.68, 128.55, 128.27, 127.99, 127.66, 127.27, 116.57; 31P
Scheme 1 The synthesis route of ligands.
NMR (121 MHz, CDCl3):
C45H34CuN2P·CH3CH2OCH2CH3: C, 76.29; H, 5.75; N, 3.63.
Found: C, 75.93; H, 5.39; N, 3.74%.
δ −15.72. Anal. Calc. for
Ligand 3 was obtained by the bromination from compound
1 using bromine in higher yield than using N-bromosuccinimide
(NBS). Dipyrrins 4 and 5 were isolated via the DDQ oxidation
of their corresponding dipyrromethane precursor prepared by the
bromination of 5-phenyldipyrrin using NCS or NBS. It is inter-
esting that the regioselective halogenations of dipyrromethane
only take place at 1,9-positions when using NCS and NBS. In
contrast, the bromination first take place at 2,8-positions if using
BODIPY and bromine as the starting materials according to
recently reported results.21 Another aim of introducing halogen
atoms is to investigate the influence on the electronic structures
of the Cu(I) complexes.
Results and discussion
Synthesis of ligands
All of the dipyrrins employed in this work are symmetrical struc-
tures containing substituents at the 1,9-positions. A major con-
sideration is to inhibit the geometry torsion through the steric
hindrance effect of these groups, because the change between
tetrahedral and square-planar geometries in Cu(I) complexes
always take place from the ground to excited states as a result of
MLCT.1 The basic dipyrrins were synthesized according to the
literature as previously reported.8b For example, ligands 1 and 2
were prepared by the condensation of 2,4-dimethylpyrrole and
the corresponding aldehyde or ketone in presence of phosphorus
oxychloride (POCl3) in good yields, while 6 was prepared via
oxidation after the reactions of 2-substituted pyrrole and benzoyl
chloride (Scheme 1). Attempts to synthesize 2 via one-pot con-
densation of 2,4-dimethylpyrrole and the aldehyde or acyl chlor-
ide in the presence of acid just gave low yields (27% and 18%).
Synthesis of complexes
The chemical structures of the complexes are shown in
Scheme 2. All ligands were first deprotonated using NaH in THF
to give the sodium salts of dipyrrin. After standing, the solution
of the sodic ligand was added to the mixture of cuprous iodide
and phosphine aged in advance so as to easily obtain the neutral
dipyrrinato cuprous complexes via the elimination of NaI. The
use of stronger bases such as n-butyllithium, however, causes
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Fig. 1 The molecular structure of 1b. The intramolecular edge-to-face
π–π interactions between dipyrrin and phenyl rings are indicated (C19–
N2 3.437 Å, C45–N2 3.487 Å, C25–N1 3.523 Å). Hydrogen atoms
have been omitted for clarity.
The 31P resonance of complex 7a is at δ = −15.73 ppm, which is
at higher field relative to the free PPh3 ligand (δ = −4.60 ppm),
while the 31P signals of other complexes with PPh3 as the ancil-
lary ligand (δ = −1.18–0.47 ppm) are shifted down field. The
31P signals of complex 1b–6b (δ = −14.9–17.9 ppm) are near to
those of the free DPEphos ligand (δ = −16.40 ppm), which are
the similar to those observed in the literatures.5f,6a
Crystal structural investigations
Single-crystal X-ray diffraction study of eight complexes was
carried out to further confirm the structures of these molecules.
The molecular structures of complexes 1b, 2b, 3a, 4a, 5a, 6a,
6b and 7a are shown in Fig. 1–8. Selected bond distances and
angles of complexes are reported and compared in Table 2.
Except for complex 7a, which is tricoordinated in pyramidal
coordination geometry owing to serious steric hindrance, the
other complexes all display a pseudo-tetrahedral coordination
environment. The a series complexes always tend towards an
ideal tetrahedron due to the symmetrical structural ligand and
monodentate PPh3. In contrast, the b series often adopt the more
distorted tetrahedral geometries due to the restricted bite angle of
the bidentate ligand of DPEphos.5f,7,22 The dihedral angles (φ)
between the N–Cmeso–N and P–Cu–P planes for the complexes
are in the normal range of 85.4–89.0°.
The N–Cu bond lengths are obviously shorter than those of
the cationic [Cu(phen)(PP)]+ complexes (phen = phenanthroline,
PP = one bidentate phosphine ligand or two monodentate phos-
phine ligands),3 and similar to those in analogues.6 Most of the
P–Cu bond lengths are longer than those of the cationic
[Cu(phen)(PP)]+ complexes.3 The N–Cu–N bond angles for all
complexes are larger than those of the cationic Cu(I) complexes.
These data indicate that there is a stronger link in the neutral
Scheme 2 The molecular structures of the Cu(I) complexes.
side reactions and isolation problems of the complexes. The
preparation reactions were easily monitored by the quenching of
the bright green fluorescence of the sodium salt of the ligand and
the color change of the solution from orange–red to brown. All
complexes were further purified via recrystallization using the
slow diffusion of methanol into CH2Cl2 solutions to lead to met-
allic lustre crystals. The complexes in solution oxidize readily to
form Cu(II) compounds accompanied by a color change of dark
green, but the crystals are relative stable on exposure to air.
The identity of the complexes were confirmed by multinuclear
NMR spectroscopy and elemental analyses. The resonance
signals of active proton at 12 ppm for free ligands are absent in
the complexes. For other proton and 13C signals, only slight
changes were found, indicating that the electronic structures of
the dipyrrins remain relatively unperturbed upon coordination.
However, the 31P signals occur over a large shift range as a result
of the coordination effect. Only one 31P signal was observed for
all the Cu(I) complexes due to the symmetrical ligand structure.
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Fig. 2 The molecular structure of 2b. The intramolecular π–π inter-
actions between dipyrrin and phenyl rings are indicated (C25–N2
3.187 Å (face-to-face) and C55–N1 3.572 Å (edge-to-face)). Hydrogen
atoms have been omitted for clarity.
Fig. 4 The molecular structure of 4a. Hydrogen atoms have been
omitted for clarity.
Fig. 3 The molecular structure of 3a. Hydrogen atoms have been
Fig. 5 The molecular structure of 5a. Hydrogen atoms have been
omitted for clarity.
omitted for clarity.
complexes than the cationic complexes between the NN ligand
and the central Cu(I) atom. The Cpyrrole–Cmeso–Cpyrrole bond
angles of the ligand backbone in these dipyrrinato Cu(I) com-
plexes are expanded compared to those (119.66 and 121.34°) in
BODIPYs,23 which results in a clear twist of two pyrrole rings
rather than the expected coplanar conjugate system. The charac-
teristic has also been observed for other luminescent transition-
metal complexes containing an analogous ligand.14
membered metal chelating ring (CuNCCCN) are nearly 720° for
a coplanar hexagon. For complex 6b, however, the sum of
internal angles is 694.84° and Cu atom is pulled out of the dipyr-
rin plane by 0.98 Å, and meanwhile induce a smallest N–Cu–N
bond angle [88.63(11)°]. A similar case is also observed in the
complex 7a with Cu 0.55 and 0.56 Å out of dipyrrin plane
because of the steric hindrance of α-phenyls. In the b series
complexes, intramolecular π–π interactions between dipyrrin and
phenyl rings are found while DPEphos acts as the ancillary
ligand (see Fig. 1, 2 and 7). In complexes 1b and 6b, only edge-
Most of the Cu(I) atoms of the complexes are nearly coplanar
with the dipyrrin ligand. The sum internal angles of the six-
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Fig. 8 The molecular structure of 7a. Hydrogen atoms have been
omitted for clarity.
Fig. 6 The molecular structure of 6a. Hydrogen atoms have been
omitted for clarity.
Photophysical properties
UV-Vis spectra of the complexes were recorded in dichloro-
methane as shown in Fig. 9, and the resulting data are summar-
ized in Table 3. The complexes all show the similar absorption
features to those of BODIPYs and aza-BODIPY dyes including
an analogous Cu(I) complex with an azadipyrromethene
ligand.24 The absorption maxima appearing at 495 nm is
assigned to the 0–0 band of a strong S0–S1 transition within the
dipyrrinato anion, while its shoulder at short wavelength
(∼460 nm) is attributed to the 0–1 vibrational band of the same
transition.25 The weak absorption bands between 250–400 nm
are due to the S0–S2 transition of the dipyrrinato anion, which
may be contain an MLCT contribution since the absorption band
of heteroleptic [Cu(phen)(PP)]+ complexes generally occurs in
the range of 300–450 nm.3a Another maximum absorption at
∼230 nm are assigned to the π–π* absorption of phenyl groups,
while its shoulder at ∼260 nm are due to proximal phenyl group
participation in the ligand-centered transitions. The maximum of
the S0–S1 transition within the dipyrrinato anion exhibits a con-
siderable variation with the various substitution patterns. Com-
plexes 6a and 6b with strong electron withdrawing
pentafluorophenyl at meso-carbon red-shift their S0–S1 tran-
sition to 512 and 510.5 nm, respectively. The maximum absorp-
tions of 3a, 3b, 5a and 5b are located at 506, 504.5, 499 and
500 nm, respectively, which indicates that bromine substituent at
2,8-positions is more efficient than at 1,9-positions (α-positions)
in red-shifting the maximum of the S0–S1 transition. The chlor-
ine atom is somewhat weaker than bromine atom as auxochrome
in red-shifting spectra according to the maximum absorption of
4a and 4b (493 and 496 nm). An exception is the complex 7a,
for which the maximum absorption red-shifts ∼60 nm relative to
the simple substituted complexes 1a and 1b (487 and 484 nm),
but blue-shifts ∼50 nm compared to the same three-coordinate
azadipyrromethene Cu(I) complex.24c
Fig. 7 The molecular structure of 6b. The intramolecular edge-to-face
π–π interactions between dipyrrin and phenyl rings are indicated (C51–
N1 3.367 Å and C45–N2 3.483 Å). The solvent and hydrogen atoms
have been omitted for clarity.
to-face interactions exist at 3.19–3.57 Å. In 2b, there are near
parallels between dipyrrin plane and a phenyl ring with face-to-
face interaction. These differences result in the differently dis-
torted extent of the tetrahedron around Cu(I) atom, in which the
complex 6b exhibits the most distorted tetrahedral geometry due
to the presence of the interaction only at single-side.
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Table 2 Selected bond distances (Å) and angles (°) for the dipyrrinato Cu(I) complexes
Complex
1b
2b
3a
4a
5a
6a
6b
7a
Cu–N(2)
Cu–N(1)
Cu–P(2)
Cu–P(1)
2.028(3)
2.036(3)
2.3078(12)
2.3061(11)
1.996(4)
2.003(4)
2.072(2)
2.067(2)
2.0519(17) 2.061(2)
2.0414(16) 2.059(2)
2.038(3) 2.068(3) 1.994(3) [1.994(3)]
2.050(3) 2.020(3) 1.966(3) [1.962(3)]
2.2383(15) 2.3075(8) 2.2765(6)
2.3780(16) 2.2976(9) 2.2696(6)
2.2765(10) 2.287(2) 2.3555(10) n/a
2.2882(10) 2.305(2) 2.2424(10) 2.1708(10) [2.1650(10)]
N(2)–Cu–N(1)
N(2)–Cu–P(2)
N(1)–Cu–P(2)
N(2)–Cu–P(1)
N(1)–Cu–P(1)
P(2)–Cu–P(1)
97.35(13)
110.82(9)
112.74(9)
106.58(10)
111.33(9)
116.20(4)
132.2(4)
95.17(16)
95.05(9)
92.45(7)
92.57(10)
114.41(7)
108.77(7)
105.29(7)
108.55(7)
122.95(3)
128.3(3)
92.49(14) 88.63(11) 95.43(13) [95.83(13)]
119.70(12) 106.18(7) 107.83(5)
118.33(12) 113.55(7) 105.69(5)
100.56(12) 112.36(7) 108.27(5)
103.60(12) 107.29(7) 113.85(5)
115.95(6)
129.4(5)
109.39(9) 111.28(8)
112.20(10) 104.82(8)
109.88(10) 117.03(8)
110.10(11) 123.25(8)
119.46(5) 109.89(4)
130.1(3) 128.4(3)
n/a
n/a
138.15(10) [133.58(10)]
122.37(10) [127.55(9)]
n/a
119.71(3) 124.06(2)
131.3(3)
C
pyrrole–Cmeso–Cpyrrole
128.0(2)
127.8(4) [127.7(4)]
Cu–(N–Cmeso–N)/Å
P(1)–(N–Cmeso–N)/Å
P(2)–(N–Cmeso–N)/Å
0.1068
2.1954
1.6923
0.2087
2.5547
1.0790
0.0123
2.0053
1.9639
0.1234
2.1537
1.8548
0.1199
1.8653
2.1376
0.0173
1.9727
1.9925
0.9758
0.8397
3.2299
0.5536 (0.5625)
1.9521 (1.9260)
n/a
φ(N–Cmeso–N/P–Cu–P)/°
Σmetallacycle (CuNCCCN)/° 719.56
88.5
88.0
717.51
85.4
719.39
87.1
719.50
86.7
719.64
89.0
716.87
86.0
694.83
n/a
710.33 (711.03)
As shown in Fig. 10, all complexes show a similar emission
band to BODIPY compounds with narrow Stokes’ shift, absorp-
tion and emission ‘mirror image’ relationship and a nanosecond-
order lifetime. The emission of complex 7a red-shifts to the
orange–red region with the maximum at 595 nm. The emission
patterns are not affected by the ancillary phosphine ligands with
nearly identical maxima emission wavelength between the pairs
of complexes a and b. This is different from the classical
[Cu(NN)(PP)]+ complexes often accompanying 10 nm vari-
ation.3 The perfect mirror symmetry (see ESI, Fig. S1–S12‡)
indicates that the main absorption band at ∼500 nm corresponds
to the S0–S1 transition. The quantum yields of these complexes
in CH2Cl2 were measured and are given in Table 3. The values
of the complexes are similar with either PPh3 or DPEphos as
ancillary ligand, however, they are obviously lower than those of
the BODIPY dyes due to the twisted structures of dipyrrinato
anion upon coordination (see Crystal structural investigations
section). A reasonable explanation is that the vibration of the
dipyrrin skeleton substantially increases the probability of non-
radiative transition due to geometric distortion.14 The results
described above suggest that the emissive properties of these Cu
(I) complexes fundamentally depend on the structure of the
dipyrrin ligands.
The small Stokes’ shift and nanosecond-order lifetimes in all
complexes are consistent with singlet fluorescence. Similar emis-
sive behaviors to BODIPYs also indicate that the emission of
these complexes should be from the dipyrrin-centered intraligand
charge transition (ILCT). To further verify the assignment, boron
difluoride compound (1)BF2 was synthesized by the reaction of
ligand 1 and BF3·OEt2 according to the literature.1 The absorp-
tion and emission spectra of the boron compound and free
ligand in CH2Cl2 are presented in Fig. S14 and S15 (ESI‡). The
compound exhibited maximum absorption and emission peaks at
506 and 522 nm in CH2Cl2, respectively, with minor variation
compared to the literature (505 and 516 nm in EtOH).1
Obviously, these peaks are red-shifted relative to those of the
complexes 1a and 1b. However, this is reasonable because of
better coplanarity, indicating the smaller energy difference,
Fig. 9 Absorption spectra of the complexes 1a–7a (top) and 1b–6b
(bottom) in CH2Cl2 at room temperature.
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Table 3 Photophysical properties of the complexesa
λmabasx/nm (10−3ε/M−1 cm−1
)
ΔλStokes/nm
λmemax/nm
τ/ns
Φ (%)
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
487 (44.7), 450 (21.5), 252 (30.8), 231 (47.2)
484 (49.5), 460 (27.9), 280 (16.3), 231 (55.8)
496 (42.1), 457 (33.2), 274 (26.3), 230 (56.1)
496 (45.5), 465 (26.3), 273 (27.0), 230 (58.0)
506 (46.7), 251 (34.5), 230 (53.6)
9.0
11.0
14.0
13.0
9.0
11.5
12.0
9.0
10.0
9.0
33.0
33.5
46.0
496
495
510
509
515
516
505
505
509
509
545
544
595
5.9
6.2
5.5
5.4
5.3
5.6
7.5
5.8
5.9
6.0
7.0
6.8
7.6
0.4
0.9
0.1
0.1
3.3
4.0
0.3
0.6
0.1
0.6
0.1
0.1
0.2
504.5 (46.4), 261 (24.6), 230 (58.1)
493 (48.1), 463.5 (21.5), 362 (27.7), 231 (53.8)
496 (45.0), 471 (19.7), 267 (25.8), 232 (56.1)
499 (45.4), 475.5 (22.9), 260 (27.5), 230 (51.5)
500 (46.5), 477.5 (21.2), 290 (14.0), 230 (52.1)
512 (40.5), 488 (22.4), 286 (14.5), 230 (57.4)
510.5 (48.7), 484 (24.9), 272 (22.4), 232 (59.5)
549 (53.4), 298 (43.7), 224 (48.8)
a [conc.] ≈ 5 × 10−6 M in dichloromethane.
Fig. 10 Room-temperature emission spectra for complexes 1a–7a (a) and 1b–6b (b) in degassed CH2Cl2 at ∼5 × 10−6 M.
between the two pyrrolic rings in the compound (1)BF2.23 In
addition, the free ligand even shows red-shifted emission at
507 nm in comparison with the corresponding Cu(I) complexes.
Therefore, these direct experimental data supports the above pos-
tulate. This is different from the traditional 1,10-phenanthroline
cuprous cationic complexes, in which the main emission is
usually assigned to MLCT.3,26 However, similar fluorescent
behaviors have also been found in the Pt(II) complexes with
ligands showing extended conjugation.27 The increased conju-
gation within the electron-rich dipyrrinato anion decreases the
π–π* transition energy barrier, and simultaneously weakens the
interaction between the dipyrrin ligand and Cu(I) atom, which
makes the π–π* transition more favorable. The weak interaction
prevents the effective mixing of π orbitals of dipyrrin and 3d
orbitals of Cu(I) atom due to the bigger gap of the energy level
(see Electrochemistry part), and restrains the spin-forbidden
d–π* transition.27
To further understand the photophysics properties of the
neutral dipyrrinato cuprous compounds, density functional
theory (DFT) calculation was performed using the B3LYP func-
tional with the Lanl2DZ basis set for Cu and Br, and
6-311++g** for the other atoms. The geometries for DFT calcu-
lation were from the X-ray structure results. As shown in Fig. 11,
the highest occupied molecular orbitals (HOMOs) and lowest
unoccupied molecular orbitals (LUMOs) of the dipyrrinato
cuprous complexes are all distributed on the dipyrrin π and π*
orbitals, which is consistent with the investigation in the
BODIPY system.28 No contribution from the central Cu(I) atom
illustrates that there is no MLCT transition in the absorption
spectra of these complexes and the emission should be assigned
to ILCT within the dipyrrinato anion.
Electrochemistry
The electrochemical properties of the complexes and ligands
were investigated using cyclic voltammetry and all data are sum-
marized in Table 4. Compared to the voltammetric behavior of
free ligands, a novel and lower oxidation peak is observed at
potentials ranging from 0.56 to 1.02 V and attributed to the oxi-
dation of Cu(I) to Cu(II). Facile oxidations of the complexes are
reasonable because the electron-donating effect of dipyrrinato
anion significantly increases the electron density on the central
Cu(I) atom. The oxidation potentials of the ligands only display
a small variation, but their reduction waves distinctly shift to
more cathodic values according to substituents with the poten-
tials ranging from −1.24 to −1.72 V. The reduction potentials of
1a, 1b and 2b are not found in the electrochemical window.
The redox potentials of the complexes and the free ligands are
affected by the electronic character and the substitution position
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Fig. 11 Qualitative orbital energy diagram illustrating the HOMO (transparent) and LUMO (mesh) orbitals of 1b, 2b, 3a, 4a, 5a, 6a, 6b and 7a. All
values reported in eV.
Table 4 Electrochemical data (E/V) for the complexes and their ligands in dichloromethane (scan rate 50 mV s−1
)
Ox
Red
Ox
Red
Ox
Red
E1/2
E1/2
E1/2
E1/2
E1/2
E1/2
1
2
3
4
5
6
7
0.82, 1.53
0.86, 1.46
1.01
1.33
1.35
1.02
1.02, 1.46
−1.63
−1.57
−1.34
−1.09
−1.06
−1.37
−1.11
1a
2a
3a
4a
5a
6a
7a
0.52, 0.73, 1.30
0.64, 0.99, 1.33
0.70, 0.96, 1.34
0.86, 1.21
1.02, 1.39
0.64, 0.86, 1.32
0.72, 1.34, 1.49
n/a
1b
2b
3b
4b
5b
6b
0.36, 0.75
n/a
n/a
−1.69
−1.51
−1.26
−1.72
−1.69
−1.55
−1.33
−1.24
−1.59
−1.42
0.38, 0.65, 1.46
0.59, 1.26, 1.69
0.82, 1.18, 1.71
0.90, 1.35
0.54, 1.54
of the substituents. Complexes 5a with α-bromine atoms exhibit
the most anodic Cu(I)–Cu(II) oxidation potential (1.02 V) in this
series of complexes, while its free ligand 5 also exhibit the most
anodic first oxidation potential (1.35 V) in the ligands. The ana-
logous complex 4a (0.86 V) and its free ligand 4 (1.33 V) with
α-chlorine atoms show relatively anodic oxidation waves.
However, the complexes 3a (0.70 V) and the free ligand
3 (1.01 V) with bromine atom substituents at 2,8-positions on
dipyrrin are not as anodic as their analogues with halogen substi-
tuents at 1,9-position. The increased conjugation of dipyrrin
reduces the HOMO–LUMO gap of the ligand and corresponding
complex, and thus complex 7a (2.14 V) and its free ligand
7 (2.13 V) have the lowest HOMO–LUMO gap. The cuprous
complexes with stronger electron-donor DPEphos cathodically
shift the oxidation potentials ∼0.1 V compared to those of com-
plexes with PPh3 as ancillary ligand, and meanwhile, DPEphos
shifts the reduction potentials of the dipyrrinato anion cathodi-
cally somewhat as well. In general, the b series complexes
display quasi-reversible and multiple oxidation peaks attributed
to multiple electron-transfer originating from the Cu(I) center
and phosphine ligand, implying that these complexes have
higher electrochemical stability due to the chelating effect of the
DPEphos ligand.29 Complexes 4a and 4b, with the more nega-
tive chlorine atoms on dipyrrin, all show a reversible reduction
process. Due to the decreased conjugation within the dipyrrinato
anion resulting from the steric hindrance upon coordination,
complex 7a exhibits quasi-reversible reduction while the
reduction process of ligand 7 is reversible at −1.11 V.
The relatively low oxidation potentials of Cu(I) ion indicate
that its d orbital energy levels are high. In contrast, the increased
conjugations within the dipyrrinato anion certainly bring down
the energy levels of the π orbitals of the ligands. The widened
energy level difference between the d and π orbitals is bound to
suppress the mixing of the orbitals from metal and ligand. This
result, in addition to the decreased energy levels of π* orbitals of
the ligands confirmed from the low reduction potentials, is
beneficial to the ILCT rather than MLCT transition. Similar
cases were also found in other Cu(I) complexes, in which the ILCT
emission was observed in addition to the MLCT transition.6,7
Conclusion
A series of neutral mononuclear heteroleptic cuprous complexes
based on the dipyrrinato anion have been synthesized and
characterized. Crystallographic analyses reveal the cuprous com-
plexes maintain a pseudo-tetrahedral coordination geometry
except the complex 7a with pyramidal structure due to steric hin-
drance. The studies of photophysical properties demonstrate that
the low energy absorption and the emission of these complexes
are assigned to ILCT of the dipyrrinato anion. The increased
conjugation in the dipyrrin ligand after deprotonation decreases
the energy level of π orbitals, which inhibits the effective spin–
orbital coupling, leading to fluorescence rather than the expected
phosphorescence. The experimental data and quantum chemical
calculation supported the conclusions, which have also been
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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9 I. V. Sazanovich, C. Kirmaier, E. Hindin, L. Yu, D. F. Bocian,
J. S. Lindsey and D. Holten, J. Am. Chem. Soc., 2004, 126, 2664–2665.
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verified by other metal complexes. These results imply that the
careful selection and rational design of ligands maintaining a
matched energy level with metal d orbitals are the key factors to
take full advantage of the triplet excited states in organic–metal
complexes. Theoretical calculations in advance utilizing d orbital
energy levels as a reference parameter should be a feasible route
to prepare complexes with such desired properties.
13 K. Hanson, A. Tamayo, V. V. Diev, M. T. Whited, P. I. Djurovich and
M. E. Thompson, Inorg. Chem., 2010, 49, 6077–6084.
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Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (Nos. 20874098 and 51073152) and the 973
Project (2009CB623600).
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Dalton Trans.
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