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Synthesis, structures and photophysical properties of four binuclear Cu(I) complexes of 1H-imidazo[4,5-f][1,10]phenanthroline
Accepted Manuscript Synthesis, Structures and Photophysical Properties of Four Binuclear Cu(I) Complexes of 1H-imidazo[4,5-f][1,10]phenanthroline Yan-Wen Niu, Xia Liu, Ling -Zhao, Ya-Meng Guo, Wen-Xin Li, Miao-Miao Ma, Xiu-Ling Li PII: DOI: Reference:
S0277-5387(18)30652-1 https://doi.org/10.1016/j.poly.2018.10.016 POLY 13490
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
5 August 2018 22 September 2018 3 October 2018
Please cite this article as: Y-W. Niu, X. Liu, L. -Zhao, Y-M. Guo, W-X. Li, M-M. Ma, X-L. Li, Synthesis, Structures and Photophysical Properties of Four Binuclear Cu(I) Complexes of 1H-imidazo[4,5-f][1,10]phenanthroline, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.10.016
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Synthesis,
Structures
and
Photophysical
Properties of Four Binuclear Cu(I) Complexes of 1H-imidazo[4,5-f][1,10]phenanthroline Yan-Wen Niu, Xia Liu, Ling-Zhao, Ya-Meng Guo, Wen-Xin Li, Miao-Miao Ma, Xiu-Ling Li* School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou, Jiangsu 221116, China ABSTRACT.
Reaction
of
2-(2-pyridyl)imidazo[4,5-f][1,10]-phenanthroline
(pipH),
[Cu(MeCN)4]ClO4 and diphosphine ligand bis[(2-diphenylphosphino)phenyl] ether (POP) or 9,9dimethyl-4,5-bis(diphenylphosphino)-9H-xanthene (xantphos) in a 1:2:2 ratio generated four binuclear copper(I) complexes, [Cu2(pipH)(PP)2](ClO4)2 (PP = POP, 1a; xantphos, 2a) and [Cu2(pip)(PP)2]ClO4 (PP = POP, 1b; xantphos, 2b) in neutral and strong alkaline media, respectively. Complexes 1b and 2b containing deprotonated pipH show obvious blue shift and stronger emission compared to complexes 1a and 2a with neutral pipH due to the stronger binding ability, the higher rigidity and conjugate degree resulting from anionic pip ligand and less vibrational quenching due to the loss of NH bond. Keywords: Copper(I) complexes, 2-(2-pyridyl)imidazo[4,5-f][1,10]-phenanthroline, crystal structure, UV-vis absorption, luminescence 1. INTRODUCTION
1
Cu(I)-diimine-phosphine complexes have aroused widespread attention over the past nearly forty years since McMillin’s group reported the first example [Cu(PPh3)2(phen)]BF4 (PPh3 = triphenylphosphine; phen = 1,10-phenanthroline) in 1979 [1]. Such noble-metal-free phosphorescent systems possess a series of advantages such as relatively low cost and environment benignancy, tunable phosphorescent emission bands and long excited state lifetimes [237], however, some unfavorable factors such as the limited thermal and photochemical stability [5] and low emission efficiency restrict the utility of Cu(I)-diimine-phosphine complexes, particularly in solution [5, 10, 38]. One of the common used strategies is using chelating diphosphine ligands instead of mono- phosphine ligands to effectively suppress the decomplexation of Cu(I)-diimine-phosphine complexes in solution and improve the stability of such systems [37, 1014], while adopting the bulky, rigid diphosphine ligands [6, 7, 11, 13, 38] and strengthening the conjugate degree and rigidity of the imine ligands to improve the lightharvesting ability and light emission efficiency of Cu(I)-diimine-diphosphine complexes [11, 12]. The ligands 1H-imidazo[4,5-f][1,10]phenanthroline derivatives have been proved to possess good conjugacy, good light-harvesting ability and good chelating coordination ability [10, 38, 39]. A series of luminescent ionic mononuclear, binuclear and trinuclear Cu(I)-1H-imidazo[4,5f][1,10]phenanthroline-diphosphine complexes have been reported by us and other groups [10, 38, 39]. On the other hand, 2-(2-pyridyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pipH) (see Chart 1) possess two chelating N,N- coordinating sites, and it is expected to form binuclear Cu(I) complexes
easily
together
diphenylphosphino)phenyl]
with
ether
chelating
(POP)
or
diphosphine
ligands
such
as
bis[(2-
9,9-dimethyl-4,5-bis(diphenylphosphino)-9H-
xanthene (xantphos) (see Chart 1). Furthermore, the coordination of the chelating N,N- site involving one N atom from imidazole ring and the other N atom from pyridine ring will result in
2
a great enhancement of the rigidity of pipH and a consequent enhancement of the luminescent properties for Cu(I) complexes. Deprotonated pipH will lead to anionic pip form with larger conjugate degree and better light-harvesting ability, and the Cu(I)-pip complexes will have different emission wavelength and higher emission efficiency relative to Cu(I)-pipH complexes [9, 39].
Chart 1. Ligands used in this article Herein, four binuclear copper(I) mixed-ligand complexes, [Cu2(pipH)(PP)2](ClO4)2 (PP = POP, 1a; xantphos, 2a) and [Cu2(pip)(PP)2]ClO4 (PP = POP, 1b; xantphos, 2b) were synthesized in neutral and strong alkaline media, respectively, from the reaction of pipH, [Cu(MeCN)4]ClO4 and diphosphine ligand POP or xantphos in a 1:2:2 ratio. The imidazo[4,5-f][1,10]-phenanthroline ligand exists as neutral form pipH in complexes 1a and 2a and anionic form pip in complexes 1b and 2b, respectively. The crystal structure, electronic absorption and emission spectra will be discussed. 2. EXPERIMENTAL SECTION 2.1. Materials The reagents 2-pyridinecarboxaldehyde (98%), 5,6-diamino-1,10-phenanthroline (dap, 98%), POP (98%), xantphos (98%) were purchased J&K Scientific LTD. and the solvents dichloromethane (DCM) and hexane in analytical purity level were purchased from Sinopharm Group CO. LTD.. All reagents were used as received without further purification. The Cu(I) salt [Cu(CH3CN)4]ClO4 was prepared according to the published methods [40]. The complexes were
3
synthesized under argon atmosphere with standard Schlenk techniques. All the samples were dried using infrared dry technique before they were used for elemental analyses. 2.2. Instruments Infrared (IR) spectra were recorded with KBr pellets on a Bruker Optics TENSOR 27 FT-IR spectrophotometer. 1H and 31P NMR spectra were obtained from the solutions in DMSO-d6 using a Bruker-400 spectrometer with Me4Si as an internal standard and 85% H3PO4 as an external standard, respectively. UV-vis steady-state absorption spectra were determined using a Purkinje General TU-1901 UV-vis spectrophotometer. Elemental analyses (C, H, and N) were determined on a Perkin-Elmer model 240C elemental analyzer. Electrospray ionization mass spectra (ESIMS) analyses were carried out on a Bruker-micro-TOFQ-MS analyzer using a DCM/methanol mixture for the mobile phase. Steady-state emission spectra were obtained with a Hitachi F4600 fluorescence spectrophotometer. 2.3. X-Ray crystallography Single crystals growing from DCM/hexane mixed solvents were picked from the mother solutions and wrapped in colorless nail polish and transferred to a Bruker Smart APEX II diffractometer fitted with a CCD-type area detector. The data were collected using graphitemonochromatized Mo-Kα radiation (λ = 0.71073 Å) at 150 K controlled by a stream of cold N2. The CrystalClear software package 2009, Bruker SAINT was used for data reduction [41, 42]. The proper absorption corrections were performed using SADABS supplied by Bruker [43]. In all cases the structures were solved by direct methods using the SHELXL–97 program package [44] and refined by the full-matrix least-squares method on F2 data with SHELXL-2014 [45]. The non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of NH group of imidazole rings in complexes 1a and 2a were located by means of difference electron density syntheses and were refined freely or with isotropic thermal parameters riding on
4
the of the parent nitrogen atoms, while other hydrogen atoms were positioned geometrically and refined with isotropic thermal parameters riding on those of the parent atoms. The contribution of the solvents in complexes 1a and 2a to the structure factors was removed using procedure SQUEEZE of program PLATON and the final refinement proceeded basing on the modified structure factors in the usual fashion [46]. 2.4. Preparation of the complexes 2.4.1. 2-(2-pyridyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pipH) The ligand 2-(2-pyridyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pipH) was prepared by the following procedure. The mixture of 2-pyridinecarboxaldehyde (160.5 mg, 1.5 mmol) and dap (315.1 mg, 1.5 mmol) in ethanol (20 mL) was refluxed for 18 h and then the mixture was cooled to room temperature. After filtration, the yellowish-brown precipitate was washed by cold ethanol and dried in vacuo. Yield: 61.2% (272.6 mg). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S1): 14.40 (s, 1H, NH), 9.23 (dd, J = 8.2 Hz, J = 1.8 Hz, 1H), 9.05 (ddd, J = 4.4 Hz, J = 3.0 Hz, J = 1.8 Hz, 2H), 8.95 (dd, J = 8.0 Hz, J = 2.0 Hz, 1H), 8.80 (ddd, J = 4.8 Hz, J = 1.6 Hz, J = 0.9 Hz, 1H), 8.44 (d, J = 8.0 Hz, 1H), 8.07 (td, J = 8.0 Hz, J = 1.8 Hz, 1H), 7.84 (ddd, J = 8.2 Hz, J = 4.2 Hz, J = 2.4 Hz, 2H), 7.57 (ddd, J = 7.4 Hz, J = 4.8 Hz, J = 1.2 Hz, 1H). 2.4.2. [Cu2(pipH)(POP)2](ClO4)2 (1a) [Cu(CH3CN)4]ClO4 (16.3 mg, 0.050 mmol) was added to a mixture of pipH (7.4 mg, 0.025 mmol) and POP (26.9 mg, 0.050 mmol) in DCM under a stream of dry argon by using Schlenk techniques and a vacuum-line system at room temperature. A lemon-yellow solution was obtained quickly and then stirred for 2 h at room temperature. After filtration through the absorbent cotton, layering n-hexane dropwise onto the dichloromethane filtrate carefully produced the target product as yellow crystals a few days later. The sample was obtained in an
5
84.4% yield (35.8 mg) after being dried using infrared dry technique. Anal. Calcd for C90H67Cl2Cu2N5O10P4 (1a) (1697.307): C 63.64, H 3.98, N 4.12%. Found: C 63.55, H 3.95, N 4.12%. ESI-MS (m/z) (see Figure S2): 1500.35 [Cu2(pip)(POP)2]+ (calcd 1500.28); 898.195 [Cu(pipH)(POP)]+ (calcd 898.192); 601.093 [Cu(POP)]+ (calcd 601.091). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S3): 15.25 (s, 1H, NH), 9.06 (d, J = 8.0 Hz, 2H), 8.94 (s, 1H), 8.84 (s, 0.5 H), 8.78 (d, J = 8.0 Hz, 1H), 8.47 (s, 1H), 8.37 (d, J = 8.0 Hz, 1H), 8.24 (s, 1H), 8.03 (s, 1H), 7.92 (s, 0.5 H), 7.70 (s, 1H), 7.487.01 (m, 40H), 6.846.67 (m, 16H).
31
P NMR (400
MHz, DMSO-d6, , ppm): 11.34, 13.30, 19.47. Characteristic IR spectrum (KBr, cm1): 1094s (ClO4). 2.4.3. [Cu2(pipH)(xantphos)2](ClO4)2 (2a) This complex was synthesized and dried by the similar synthetic procedure as that of 1a except for using xantphos instead of POP. Color: yellow. Yield: 57.6% (25.6 mg). Anal. Calcd for C96H75Cl2Cu2N5O10P4 (2a) (1777.263): C 64.82, H 4.25, N 3.94%. Found: C 64.66, H 4.30, N 3.89%. ESI-MS (m/z) (see Figure S4): 1580.43 [Cu2(pip)(xantphos)2]+ (calcd 1580.34); 938.228 [Cu(pipH)(xantphos)]+ (calcd 938.223); 641.131 [Cu(xantphos)]+ (calcd 641.122). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S5): 15.38 (s, 1H, NH), 9.087.63 (m, 14H), 7.406.91 (m, 38H), 6.756.21 (m, 10H), 1.961.53 (m, 12H).
31
P NMR (400 MHz, DMSO-d6, , ppm):
12.44, 12.82, 14.17, 18.48. Characteristic IR spectrum (KBr, cm1): 1096vs (ClO4). 2.4.4. [Cu2(pip)(POP)2]ClO4 (1b) [Cu(CH3CN)4]ClO4 (16.3 mg, 0.050 mmol) was added to a mixture of pipH (7.4 mg, 0.025 mmol) and POP (26.9 mg, 0.050 mmol) in DCM under a stream of dry argon by using Schlenk techniques and a vacuum-line system at room temperature. Then the NaOH (10.0 mg, 0.250 mmol) suspension in about 1.00 mL methanol was added. A yellow solution was obtained
6
quickly and then stirred for 2 h at room temperature. Layering n-hexane onto the DCM filtrate carefully gave the product as yellow crystals a few weeks later. The sample was obtained in 80.1% yield (32.0 mg) after being dried using infrared dry technique. Anal. Calcd for C90H66ClCu2N5O6P4 (1b) (1597.244): C 67.62, H 4.16, N 4.38%. Found: C 67.50, H 4.12, N 4.33%. ESI-MS (m/z) (see Figure S6): 1500.26 [Cu2(pip)(POP)2]+ (calcd 1500.28); 898.195 [Cu(pip)(POP)] + H+ (calcd 898.192). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S7): 9.00 (d, J = 8.0 Hz, 1H), 8.73 (d, J = 8.4 Hz, 1H), 8.64 (t, J = 5.2 Hz, 2H), 8.38 (d, J = 7.2 Hz, 1H), 8.32 (d, J = 4.4 Hz, 1H), 7.92 (t, J = 7.8 Hz, 1H), 7.74 (dd, J = 8.4 Hz, J = 4.8 Hz, 1H), 7.467.03 (m, 36H), 6.956.83 (m, 18H), 6.706.6.62 (m, 4H). 31P NMR (400 MHz, DMSO-d6,
, ppm): 11.84, 15.25. Characteristic IR spectrum (KBr, cm1): 1097s (ClO4). 2.4.5. [Cu2(pip)(xantphos)2]ClO4 (2b) This complex was synthesized and dried by the similar synthetic procedure as that of 1b except for using xantphos instead of POP. Color: yellow. Yield: 50.2% (21.1 mg). Anal. Calcd for C96H74ClCu2N5O6P4 (2b) (1677.307): C 68.68, H 4.45, N 4.17%. Found: C 68.49, H 4.50, N 4.17%. ESI-MS (m/z) (see Figure S8): 1580.31 [Cu2(pip)(xantphos)2]+ (calcd 1580.34); 938.2235 [Cu(pip)(xantphos)] + H+ (calcd 938.2233). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S9). 9.01 (d, J = 7.2 Hz, 1H), 8.67 (d, J = 4.8 Hz, 1H), 8.51 (d, J = 8.0 Hz, 1H), 8.40 (d, J = 4.4 Hz, 1H), 8.02 (t, J = 8.2 Hz, 1H), 7.91 (dd, J = 7.8 Hz, J = 1.0 Hz, 2H), 7.73 (dd, J = 8.4 Hz, J = 4.8 Hz, 2H), 7.65 (dd, J = 7.8 Hz, J = 1.0 Hz, 3H), 7.45 (t, J = 6.8 Hz, 1H), 7.347.20 (m, 10H), 7.167.08 (m, 12H), 7.03 (t, J = 7.2 Hz, 4H), 6.986.88 (m, 10H), 6.836.79 (m, 4H), 6.696.64 (m, 4H), 6.536.49 (m, 2H), 6.316.29 (m, 2H), 5.98 (dd, J = 8.0 Hz, J = 4.8 Hz, 1H), 1.98 (s, 3H), 1.81 (s, 3H), 1.72 (s, 3H), 1.52 (s, 3H).
31
P NMR (400 MHz, DMSO-d6, , ppm):
12.50, 15.21. Characteristic IR spectrum (KBr, cm1): 1096s (ClO4).
7
3. RESULTS AND DISCUSSION 3.1. Syntheses 3.1.1. Syntheses of the ligand pipH Wang’s group and Eseola’s group synthesized pipH through refluxing the mixture of phenanthroline-5,6-dione, ammonium acetate and 2-pyridinecarboxaldehyde with glacial acetic acid as solvent and catalyst, following dilution with water, the adjustment of the pH value using concentrated aqueous ammonia [47, 48]. Here, pipH was prepared by a modified and simpler procedure from refluxing the mixture of dap and 2-pyridinecarboxaldehyde in ethanol at lower temperature without any catalyst. The 1H NMR of pipH in DMSO-d6 shows some difference from that in CDCl3 [48]. The most obvious difference is that the signal of H from –NH group is observed at 14.40 ppm in DMSO-d6 at lower field compared with that 11.60 ppm in CDCl3 [ 48]. This is probably related to the possible intermolecular hydrogen bonds because the oxygen atom of DMSO can be a good proton acceptor. 3.1.2. Syntheses of Copper(I) Complexes The copper(I) complexes were synthesized according to the method reported previously [10, 18, 38, 49, 50]. Reaction of [Cu(MeCN)4]ClO4, pipH and the chelating diphosphine ligands, POP or xantphos under neutral or alkaline conditions in a 2:1:2 ratio followed by crystallization from CH2Cl2/hexane system gave four binuclear Cu(I) complexes in good yields (Scheme 1).
8
Scheme 1. The synthetic routes used to synthesize the Cu(I) complexes. 3.2. X-ray Structure Analysis
Figure 1. ORTEP drawing of the cation [Cu2(pipH)(POP)2]2+ of complex 1a with the atom labelling scheme, showing 30% thermal ellipsoids. Most hydrogen atoms and some phenyl rings of POP are omitted for clarity.
9
Figure 2. ORTEP drawing of the cation [Cu2(pipH)(xantphos)2]2+ of complex 2a with the atom labelling scheme, showing 30% thermal ellipsoids. Most hydrogen atoms and some phenyl rings of xantphos are omitted for clarity.
Figure 3. ORTEP drawing of the cation [Cu2(pip)(xantphos)2]+ of complex 2b with the atom labelling scheme, showing 30% thermal ellipsoids. All the hydrogen atoms and some phenyl rings of xantphos are omitted for clarity.
10
Table 1. Selected bond lengths (Å) and angles (º) for complexes 1a, 2a and 2b3CH2Cl2 1a Cu1N1
2.055(2)
Cu1N2
2.066(2)
Cu1P1
2.248(1)
Cu1P2
2.233(1)
Cu2N4
2.115(2)
Cu2N5
2.109(2)
Cu2P3
2.279(1)
Cu2P4
2.247(1)
C13N3
1.360(4)
C13N4
1.328(3)
N1Cu1N2
81.13(9)
N1Cu1P1
116.01(8)
N1Cu1P2
122.36(7)
N2Cu1P1
106.14(7)
N2Cu1P2
110.46(7)
P1Cu1P2
113.98(3)
N4Cu2N5
79.98(9)
N4Cu2P3
106.17(7)
N4Cu2P4
122.32(7)
N5Cu2P3
113.30(8)
N5Cu2P4
119.34(7)
P3Cu2P4
111.88(3)
2a Cu1N1
2.058(3)
Cu1N2
2.089(3)
Cu1P1
2.274(1)
Cu1P2
2.230(1)
Cu2N4
2.062(3)
Cu2N5
2.152(3)
Cu2P3
2.233(1)
Cu2P4
2.326(1)
C13N3
1.345(5)
C13N4
1.340(5)
N1Cu1N2
80.05(13)
N1Cu1P1
105.07(10)
N1Cu1P2
124.92(9)
N2Cu1P1
105.29(10)
N2Cu1P2
118.22(9)
P1Cu1P2
116.72(4)
N4Cu2N5
79.87(12)
N4Cu2P3
124.92(10)
N4Cu2P4
108.14(10)
N5Cu2P3
125.20(9)
N5Cu2P4
97.02(9)
P3Cu2P4
114.60(4)
2b3CH2Cl2 Cu1N1
2.070(3)
Cu1N2
2.074(3)
Cu1P1
2.252(1)
Cu1P2
2.250(1)
Cu2N4
2.032(2)
Cu2N5
2.140(3)
Cu2P3
2.260(1)
Cu2P4
2.287(1)
C13N3
1.346(4)
C13N4
1.355(4)
N1Cu1N2
81.04(10)
N1Cu1P1
116.77(8)
N1Cu1P2
112.94(8)
N2Cu1P1
102.34(8)
N2Cu1P2
122.59(7)
P1Cu1P2
116.38(3)
N4Cu2N5
80.56(10)
N4Cu2P3
122.91(7)
N4Cu2P4
110.00(7)
N5Cu2P3
113.77(7)
N5Cu2P4
109.69(7)
P3Cu2P4
114.79(3)
Single crystals of complexes 1a, 2a and 2b were determined at 150 K. The crystallographic data and selected refinement details are listed in Table S1 (see Appendix B). Selected bond lengths and angles for complexes 1a, 2a and 2b are presented in Table 1. ORTEP drawings of the cations for complexes 1a, 2a and 2b are depicted in Figures 13, respectively. All the three complexes exhibit binuclear structures. The 2-(2-Pyridyl)imidazo[4,5-f][1,10]-phenanthroline binds to two Cu(PP) units in the bridging coordination modes with the neutral form in complexes
11
1a and 2a and an deprotonated one in complex 2b. As in most reported Cu(I)-imine-diphosphine complexes, all the Cu(I) centres display distorted tetrahedral coordination geometries with two N atoms from the N,N- phenanthroline chelating sites or from N,N- 2-(2-pyridyl)imidazole sites and two P atoms from the chelating diphosphine ligands [37, 1014, 38, 49, 50]. The CuN distances (2.032(2)2.152(2) Å) and CuP (2.230(1)2.326(1) Å) distances are comparable to those reported values [3, 5, 10, 38, 50]. The CuN distances are affected by the following factors, the position of N atom, the form of imidazole ring and the diphosphine ligands. The Cu2N4 bond lengths decrease from 2.062(3) Å in complex 2a to 2.032(2) Å in complex 2b with a 0.030 Å difference, indicating the stronger binding ability of N atom from anionic imidazole ring than that from neutral imidazole ring. The dihedral angles (dhas) between the pyridine ring (C14C18, N5) and the mean plane of imidazo[4,5-f][1,10]phenanthroline (C1C13, N1N4) decrease from 21.7 in complex 1a, 17.4 in complex 2a to 6.2 in complex 2b, indicating the gradual enhancement of the coplanarity and conjugate degree of pipH or pip ligand from complexes 1a, 2a to complex 2b (see Figure 4). The rapid decrease of dhas from complex 2a to complex 2b is suggested to arise from the increasing resonance structure and conjugate degree among the whole pip plane because of the deprotonation of the imidazole ring [10].
12
Figure 4. The perspective drawings for Cu atoms and pipH (or pip) for complexes 1a, 2a and 2b showing the coplanar conditions of pipH or pip (left: ball-and-stick mode; right: wires mode).
3.3. 31P NMR spectra.
Figure 5. 31P NMR spectra of all complexes in DMSO-d6.
The 31P NMR spectra of all complexes in DMSO-d6 are shown in Figure 5. Complexes 1a and 2a display multiple
31
P NMR signals similar to those discussed for Cu(I) complexes with
puckered xantphos or POP [10, 51, 52]. The 31P NMR spectra of binuclear and trinuclear Cu(I) complexes with xantphos and 1H-imidazo[4,5-f][1,10]phenanthroline derivatives are not simply two and three groups of single signal peaks [10], but multiple 31P NMR signals due to several possible reasons [10]. For tetrahedral metal centers, the conformers may occur when four coordination atoms are from two bidentate chelating ligands, respectively, and one chelating ligand such as POP or xantphos is puckered or easy to twist, and the other is asymmetric [51, 52]. Compared with complexes 1a and 2a, complexes 1b and 2b exhibit two groups of simple single signal peaks, and this may arise from less conformers for complexes 1b and 2b due to the increasing symmetry of pip anion, which is tentatively attributed to the resonance structure of anionic imidazole ring and the rotation of CC bond between imidazole ring and pyridine ring.
13
Compared with the 31P NMR data reported before, the signals with chemical shifts at about 11.34 and 11.84 ppm for complexes 1a and 1b, and 12.44, -12.82 and 12.50 ppm for complexes 2a and 2b are assigned to POP and xantphos around Cu1 centers [10, 38], and the left signals are assigned to POP and xantphos around Cu2 centers. 3.4. Physical Properties 3.4.1. Absorption spectra UV-vis absorption data of all complexes in dichloromethane solutions are listed in Table 2, while their electronic absorption spectra are shown in Figure 6. Compared with the absorptions of ligands pipH, POP, xantphos, the reported [Cu(diimine)(PP)]+ and [Cu2(diimine)2(PP)2]2+ analogs [3, 5, 10, 18, 38, 49, 50] and the Cu(I) complexes of pyridine-imidazole [5355], the intense high energy bands with wavelengths shorter than ca. 360 nm for complexes 1a, 2a and those shorter than ca. 330 nm for complexes 1b, 2b are suggested to arise from spin-allowed ligand-centered * transitions of coordinated pipH (pip) and diphosphine ligands. The broad, low-energy bands with the maximums at ca. 400 nm for complexes 1a and 2a and at ca. 350 nm for complexes 1b and 2b are tentatively assigned to metal-to-ligand charge transfer (d(Cu)
*(pip or pipH) (MLCT) transitions [10, 18, 38, 49, 50, 5355]. The absorption band with the lowest energy between 400450 nm in complexes 1b and 2b are tentatively assigned to the intraligand charge transfer (ILCT) transitions according to the density functional theory (DFT) calculations about Cu(I) complexes of pyridine-benzimidazole [53]. Despite the similar coordination motif, complexes 1b and 2b show blue shift MLCT absorption relative to complexes 1a and 2a, and this can be attributed to the larger HOMO-LUMO energy gap.
14
Figure 6. UV-Vis absorption spectra of all complexes in DCM at room temperature.
Table 2. Photophysical data of all complexes at room temperature. abs/nm (/M1cm1)
compd
(CH2Cl2) 1a 1b
271 (66600), 396 (10120) 264 (55520), 284 (60200), 347 (33600), 416 (8040), 440 (6880)
em/nm
em/nm
(CH2Cl2)
(solid)
633
574
616
555
2a
277 (86660), 326 (32810), 410 (9440)
630
591
2b
284 (86860), 351 (39080), 425 (9040), 444 (8120)
608
549
3.4.2. Photoluminescence (PL) All the complexes show good luminescence behavior in the crystalline state and weak luminescence in DCM solutions (2.5105 molL1) at room temperature and their corresponding emission spectra are shown in Figure 7. The corresponding emission wavelengths are listed in Table 2. All the complexes show 3MLCT emission bands with maximum emission wavelengths falling in the range of 549633 nm [3, 5, 10, 18, 38, 49, 50, 5356]. Some complexes show dual emission and the high energy emission bands are tentatively assigned to ILCT emission according to the analysis result of the above absorption spectra. Complexes 1b and 2b with
15
anionic pip ligand show obvious blue shift emission compared to complexes 1a and 2a with neutral pipH ligand, which is similar to those for the aforementioned electronic absorption spectra. On the other hand, complexes 1b and 2b show stronger emission compared to complexes 1a and 2a, which is ascribed to the higher rigidity resulting from the stronger coordination of anionic pip ligand and less vibrational quenching due to the loss of NH bond and the less counter ions in complexes 1b and 2b [53].
Figure 7. Photoluminescence of all complexes in the crystalline state (left) and in DCM solutions (right) with an excitation wavelength at 330 nm and a 395 nm filter.
4. Conclusion Four binuclear copper(I) complexes with 2-(2-pyridyl)imidazo[4,5-f][1,10]-phenanthroline or its anionic ion and the chelating diphosphine POP or xantphos as mixed ligands have been synthesized and characterized. Complexes 1b and 2b with anionic pip ligand show obvious blue shift emission and stronger emission compared to complexes 1a and 2a with neutral pipH ligand. The higher emission efficiency is ascribed to the higher rigidity resulting from the stronger coordination of anionic pip ligand, the better light harvesting ability resulting from the larger conjugate degree of deprotonated pipH and less vibrational quenching due to the loss of NH bond and the fewer counter ions.
16
Acknowledgments This work was supported financially by the National Natural Science Foundation of China (NSFC) (21271091), PAPD of Jiangsu Higher Education Institutions and Jiangsu college students' innovation and entrepreneurship training program project (201610320107X). Appendix A. Supplementary data CCDC-1860167,
CCDC-1860168
and
CCDC-1860169
contain
the
supplementary
crystallographic data for complexes 1a, 2a and 2b3CH2Cl2, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK. Fax: (+44) 1223-336033; or e-mail: [email protected]. Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. REFERENCES [1] M. T. Buckner, T. G. Matthews, F. E. Lytle, D. R. McMillin, J. Am. Chem. Soc. 101 (1979) 5846. [2] J. R. Kirchhoff, D. R. Mcmillin, W. R. Robinson, D. R. Powell, A. T. Mckenzie, S. Chen, Inorg. Chem. 24 (1985) 3928. [3] D. G. Cuttell, S. Kuang, P. E. Fanwick, D. R. McMillin, R. A. Walton, J. Am. Chem. Soc. 124 (2002) 6.
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Synthesis, Structures and Photophysical Properties of Four Binuclear Cu(I) Complexes of 1Himidazo[4,5-f][1,10]phenanthroline
Yan-Wen Niu, Xia Liu, Ling-Zhao, Ya-Meng Guo, Wen-Xin Li, Miao-Miao Ma, Xiu-Ling Li* Four binuclear Cu(I) complexes of neutral or deprotonated 2-(2-pyridyl)imidazo[4,5-f][1,10]phenanthroline (pipH) and diphosphine were synthesized. The two Cu(I) complexes with deprotonated pipH show obvious blue shift and stronger emission compare to the other two with neutral pipH.
24
S0277-5387(18)30652-1 https://doi.org/10.1016/j.poly.2018.10.016 POLY 13490
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
5 August 2018 22 September 2018 3 October 2018
Please cite this article as: Y-W. Niu, X. Liu, L. -Zhao, Y-M. Guo, W-X. Li, M-M. Ma, X-L. Li, Synthesis, Structures and Photophysical Properties of Four Binuclear Cu(I) Complexes of 1H-imidazo[4,5-f][1,10]phenanthroline, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.10.016
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Synthesis,
Structures
and
Photophysical
Properties of Four Binuclear Cu(I) Complexes of 1H-imidazo[4,5-f][1,10]phenanthroline Yan-Wen Niu, Xia Liu, Ling-Zhao, Ya-Meng Guo, Wen-Xin Li, Miao-Miao Ma, Xiu-Ling Li* School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou, Jiangsu 221116, China ABSTRACT.
Reaction
of
2-(2-pyridyl)imidazo[4,5-f][1,10]-phenanthroline
(pipH),
[Cu(MeCN)4]ClO4 and diphosphine ligand bis[(2-diphenylphosphino)phenyl] ether (POP) or 9,9dimethyl-4,5-bis(diphenylphosphino)-9H-xanthene (xantphos) in a 1:2:2 ratio generated four binuclear copper(I) complexes, [Cu2(pipH)(PP)2](ClO4)2 (PP = POP, 1a; xantphos, 2a) and [Cu2(pip)(PP)2]ClO4 (PP = POP, 1b; xantphos, 2b) in neutral and strong alkaline media, respectively. Complexes 1b and 2b containing deprotonated pipH show obvious blue shift and stronger emission compared to complexes 1a and 2a with neutral pipH due to the stronger binding ability, the higher rigidity and conjugate degree resulting from anionic pip ligand and less vibrational quenching due to the loss of NH bond. Keywords: Copper(I) complexes, 2-(2-pyridyl)imidazo[4,5-f][1,10]-phenanthroline, crystal structure, UV-vis absorption, luminescence 1. INTRODUCTION
1
Cu(I)-diimine-phosphine complexes have aroused widespread attention over the past nearly forty years since McMillin’s group reported the first example [Cu(PPh3)2(phen)]BF4 (PPh3 = triphenylphosphine; phen = 1,10-phenanthroline) in 1979 [1]. Such noble-metal-free phosphorescent systems possess a series of advantages such as relatively low cost and environment benignancy, tunable phosphorescent emission bands and long excited state lifetimes [237], however, some unfavorable factors such as the limited thermal and photochemical stability [5] and low emission efficiency restrict the utility of Cu(I)-diimine-phosphine complexes, particularly in solution [5, 10, 38]. One of the common used strategies is using chelating diphosphine ligands instead of mono- phosphine ligands to effectively suppress the decomplexation of Cu(I)-diimine-phosphine complexes in solution and improve the stability of such systems [37, 1014], while adopting the bulky, rigid diphosphine ligands [6, 7, 11, 13, 38] and strengthening the conjugate degree and rigidity of the imine ligands to improve the lightharvesting ability and light emission efficiency of Cu(I)-diimine-diphosphine complexes [11, 12]. The ligands 1H-imidazo[4,5-f][1,10]phenanthroline derivatives have been proved to possess good conjugacy, good light-harvesting ability and good chelating coordination ability [10, 38, 39]. A series of luminescent ionic mononuclear, binuclear and trinuclear Cu(I)-1H-imidazo[4,5f][1,10]phenanthroline-diphosphine complexes have been reported by us and other groups [10, 38, 39]. On the other hand, 2-(2-pyridyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pipH) (see Chart 1) possess two chelating N,N- coordinating sites, and it is expected to form binuclear Cu(I) complexes
easily
together
diphenylphosphino)phenyl]
with
ether
chelating
(POP)
or
diphosphine
ligands
such
as
bis[(2-
9,9-dimethyl-4,5-bis(diphenylphosphino)-9H-
xanthene (xantphos) (see Chart 1). Furthermore, the coordination of the chelating N,N- site involving one N atom from imidazole ring and the other N atom from pyridine ring will result in
2
a great enhancement of the rigidity of pipH and a consequent enhancement of the luminescent properties for Cu(I) complexes. Deprotonated pipH will lead to anionic pip form with larger conjugate degree and better light-harvesting ability, and the Cu(I)-pip complexes will have different emission wavelength and higher emission efficiency relative to Cu(I)-pipH complexes [9, 39].
Chart 1. Ligands used in this article Herein, four binuclear copper(I) mixed-ligand complexes, [Cu2(pipH)(PP)2](ClO4)2 (PP = POP, 1a; xantphos, 2a) and [Cu2(pip)(PP)2]ClO4 (PP = POP, 1b; xantphos, 2b) were synthesized in neutral and strong alkaline media, respectively, from the reaction of pipH, [Cu(MeCN)4]ClO4 and diphosphine ligand POP or xantphos in a 1:2:2 ratio. The imidazo[4,5-f][1,10]-phenanthroline ligand exists as neutral form pipH in complexes 1a and 2a and anionic form pip in complexes 1b and 2b, respectively. The crystal structure, electronic absorption and emission spectra will be discussed. 2. EXPERIMENTAL SECTION 2.1. Materials The reagents 2-pyridinecarboxaldehyde (98%), 5,6-diamino-1,10-phenanthroline (dap, 98%), POP (98%), xantphos (98%) were purchased J&K Scientific LTD. and the solvents dichloromethane (DCM) and hexane in analytical purity level were purchased from Sinopharm Group CO. LTD.. All reagents were used as received without further purification. The Cu(I) salt [Cu(CH3CN)4]ClO4 was prepared according to the published methods [40]. The complexes were
3
synthesized under argon atmosphere with standard Schlenk techniques. All the samples were dried using infrared dry technique before they were used for elemental analyses. 2.2. Instruments Infrared (IR) spectra were recorded with KBr pellets on a Bruker Optics TENSOR 27 FT-IR spectrophotometer. 1H and 31P NMR spectra were obtained from the solutions in DMSO-d6 using a Bruker-400 spectrometer with Me4Si as an internal standard and 85% H3PO4 as an external standard, respectively. UV-vis steady-state absorption spectra were determined using a Purkinje General TU-1901 UV-vis spectrophotometer. Elemental analyses (C, H, and N) were determined on a Perkin-Elmer model 240C elemental analyzer. Electrospray ionization mass spectra (ESIMS) analyses were carried out on a Bruker-micro-TOFQ-MS analyzer using a DCM/methanol mixture for the mobile phase. Steady-state emission spectra were obtained with a Hitachi F4600 fluorescence spectrophotometer. 2.3. X-Ray crystallography Single crystals growing from DCM/hexane mixed solvents were picked from the mother solutions and wrapped in colorless nail polish and transferred to a Bruker Smart APEX II diffractometer fitted with a CCD-type area detector. The data were collected using graphitemonochromatized Mo-Kα radiation (λ = 0.71073 Å) at 150 K controlled by a stream of cold N2. The CrystalClear software package 2009, Bruker SAINT was used for data reduction [41, 42]. The proper absorption corrections were performed using SADABS supplied by Bruker [43]. In all cases the structures were solved by direct methods using the SHELXL–97 program package [44] and refined by the full-matrix least-squares method on F2 data with SHELXL-2014 [45]. The non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of NH group of imidazole rings in complexes 1a and 2a were located by means of difference electron density syntheses and were refined freely or with isotropic thermal parameters riding on
4
the of the parent nitrogen atoms, while other hydrogen atoms were positioned geometrically and refined with isotropic thermal parameters riding on those of the parent atoms. The contribution of the solvents in complexes 1a and 2a to the structure factors was removed using procedure SQUEEZE of program PLATON and the final refinement proceeded basing on the modified structure factors in the usual fashion [46]. 2.4. Preparation of the complexes 2.4.1. 2-(2-pyridyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pipH) The ligand 2-(2-pyridyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pipH) was prepared by the following procedure. The mixture of 2-pyridinecarboxaldehyde (160.5 mg, 1.5 mmol) and dap (315.1 mg, 1.5 mmol) in ethanol (20 mL) was refluxed for 18 h and then the mixture was cooled to room temperature. After filtration, the yellowish-brown precipitate was washed by cold ethanol and dried in vacuo. Yield: 61.2% (272.6 mg). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S1): 14.40 (s, 1H, NH), 9.23 (dd, J = 8.2 Hz, J = 1.8 Hz, 1H), 9.05 (ddd, J = 4.4 Hz, J = 3.0 Hz, J = 1.8 Hz, 2H), 8.95 (dd, J = 8.0 Hz, J = 2.0 Hz, 1H), 8.80 (ddd, J = 4.8 Hz, J = 1.6 Hz, J = 0.9 Hz, 1H), 8.44 (d, J = 8.0 Hz, 1H), 8.07 (td, J = 8.0 Hz, J = 1.8 Hz, 1H), 7.84 (ddd, J = 8.2 Hz, J = 4.2 Hz, J = 2.4 Hz, 2H), 7.57 (ddd, J = 7.4 Hz, J = 4.8 Hz, J = 1.2 Hz, 1H). 2.4.2. [Cu2(pipH)(POP)2](ClO4)2 (1a) [Cu(CH3CN)4]ClO4 (16.3 mg, 0.050 mmol) was added to a mixture of pipH (7.4 mg, 0.025 mmol) and POP (26.9 mg, 0.050 mmol) in DCM under a stream of dry argon by using Schlenk techniques and a vacuum-line system at room temperature. A lemon-yellow solution was obtained quickly and then stirred for 2 h at room temperature. After filtration through the absorbent cotton, layering n-hexane dropwise onto the dichloromethane filtrate carefully produced the target product as yellow crystals a few days later. The sample was obtained in an
5
84.4% yield (35.8 mg) after being dried using infrared dry technique. Anal. Calcd for C90H67Cl2Cu2N5O10P4 (1a) (1697.307): C 63.64, H 3.98, N 4.12%. Found: C 63.55, H 3.95, N 4.12%. ESI-MS (m/z) (see Figure S2): 1500.35 [Cu2(pip)(POP)2]+ (calcd 1500.28); 898.195 [Cu(pipH)(POP)]+ (calcd 898.192); 601.093 [Cu(POP)]+ (calcd 601.091). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S3): 15.25 (s, 1H, NH), 9.06 (d, J = 8.0 Hz, 2H), 8.94 (s, 1H), 8.84 (s, 0.5 H), 8.78 (d, J = 8.0 Hz, 1H), 8.47 (s, 1H), 8.37 (d, J = 8.0 Hz, 1H), 8.24 (s, 1H), 8.03 (s, 1H), 7.92 (s, 0.5 H), 7.70 (s, 1H), 7.487.01 (m, 40H), 6.846.67 (m, 16H).
31
P NMR (400
MHz, DMSO-d6, , ppm): 11.34, 13.30, 19.47. Characteristic IR spectrum (KBr, cm1): 1094s (ClO4). 2.4.3. [Cu2(pipH)(xantphos)2](ClO4)2 (2a) This complex was synthesized and dried by the similar synthetic procedure as that of 1a except for using xantphos instead of POP. Color: yellow. Yield: 57.6% (25.6 mg). Anal. Calcd for C96H75Cl2Cu2N5O10P4 (2a) (1777.263): C 64.82, H 4.25, N 3.94%. Found: C 64.66, H 4.30, N 3.89%. ESI-MS (m/z) (see Figure S4): 1580.43 [Cu2(pip)(xantphos)2]+ (calcd 1580.34); 938.228 [Cu(pipH)(xantphos)]+ (calcd 938.223); 641.131 [Cu(xantphos)]+ (calcd 641.122). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S5): 15.38 (s, 1H, NH), 9.087.63 (m, 14H), 7.406.91 (m, 38H), 6.756.21 (m, 10H), 1.961.53 (m, 12H).
31
P NMR (400 MHz, DMSO-d6, , ppm):
12.44, 12.82, 14.17, 18.48. Characteristic IR spectrum (KBr, cm1): 1096vs (ClO4). 2.4.4. [Cu2(pip)(POP)2]ClO4 (1b) [Cu(CH3CN)4]ClO4 (16.3 mg, 0.050 mmol) was added to a mixture of pipH (7.4 mg, 0.025 mmol) and POP (26.9 mg, 0.050 mmol) in DCM under a stream of dry argon by using Schlenk techniques and a vacuum-line system at room temperature. Then the NaOH (10.0 mg, 0.250 mmol) suspension in about 1.00 mL methanol was added. A yellow solution was obtained
6
quickly and then stirred for 2 h at room temperature. Layering n-hexane onto the DCM filtrate carefully gave the product as yellow crystals a few weeks later. The sample was obtained in 80.1% yield (32.0 mg) after being dried using infrared dry technique. Anal. Calcd for C90H66ClCu2N5O6P4 (1b) (1597.244): C 67.62, H 4.16, N 4.38%. Found: C 67.50, H 4.12, N 4.33%. ESI-MS (m/z) (see Figure S6): 1500.26 [Cu2(pip)(POP)2]+ (calcd 1500.28); 898.195 [Cu(pip)(POP)] + H+ (calcd 898.192). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S7): 9.00 (d, J = 8.0 Hz, 1H), 8.73 (d, J = 8.4 Hz, 1H), 8.64 (t, J = 5.2 Hz, 2H), 8.38 (d, J = 7.2 Hz, 1H), 8.32 (d, J = 4.4 Hz, 1H), 7.92 (t, J = 7.8 Hz, 1H), 7.74 (dd, J = 8.4 Hz, J = 4.8 Hz, 1H), 7.467.03 (m, 36H), 6.956.83 (m, 18H), 6.706.6.62 (m, 4H). 31P NMR (400 MHz, DMSO-d6,
, ppm): 11.84, 15.25. Characteristic IR spectrum (KBr, cm1): 1097s (ClO4). 2.4.5. [Cu2(pip)(xantphos)2]ClO4 (2b) This complex was synthesized and dried by the similar synthetic procedure as that of 1b except for using xantphos instead of POP. Color: yellow. Yield: 50.2% (21.1 mg). Anal. Calcd for C96H74ClCu2N5O6P4 (2b) (1677.307): C 68.68, H 4.45, N 4.17%. Found: C 68.49, H 4.50, N 4.17%. ESI-MS (m/z) (see Figure S8): 1580.31 [Cu2(pip)(xantphos)2]+ (calcd 1580.34); 938.2235 [Cu(pip)(xantphos)] + H+ (calcd 938.2233). 1H NMR (400 MHz, DMSO-d6, , ppm) (see Figure S9). 9.01 (d, J = 7.2 Hz, 1H), 8.67 (d, J = 4.8 Hz, 1H), 8.51 (d, J = 8.0 Hz, 1H), 8.40 (d, J = 4.4 Hz, 1H), 8.02 (t, J = 8.2 Hz, 1H), 7.91 (dd, J = 7.8 Hz, J = 1.0 Hz, 2H), 7.73 (dd, J = 8.4 Hz, J = 4.8 Hz, 2H), 7.65 (dd, J = 7.8 Hz, J = 1.0 Hz, 3H), 7.45 (t, J = 6.8 Hz, 1H), 7.347.20 (m, 10H), 7.167.08 (m, 12H), 7.03 (t, J = 7.2 Hz, 4H), 6.986.88 (m, 10H), 6.836.79 (m, 4H), 6.696.64 (m, 4H), 6.536.49 (m, 2H), 6.316.29 (m, 2H), 5.98 (dd, J = 8.0 Hz, J = 4.8 Hz, 1H), 1.98 (s, 3H), 1.81 (s, 3H), 1.72 (s, 3H), 1.52 (s, 3H).
31
P NMR (400 MHz, DMSO-d6, , ppm):
12.50, 15.21. Characteristic IR spectrum (KBr, cm1): 1096s (ClO4).
7
3. RESULTS AND DISCUSSION 3.1. Syntheses 3.1.1. Syntheses of the ligand pipH Wang’s group and Eseola’s group synthesized pipH through refluxing the mixture of phenanthroline-5,6-dione, ammonium acetate and 2-pyridinecarboxaldehyde with glacial acetic acid as solvent and catalyst, following dilution with water, the adjustment of the pH value using concentrated aqueous ammonia [47, 48]. Here, pipH was prepared by a modified and simpler procedure from refluxing the mixture of dap and 2-pyridinecarboxaldehyde in ethanol at lower temperature without any catalyst. The 1H NMR of pipH in DMSO-d6 shows some difference from that in CDCl3 [48]. The most obvious difference is that the signal of H from –NH group is observed at 14.40 ppm in DMSO-d6 at lower field compared with that 11.60 ppm in CDCl3 [ 48]. This is probably related to the possible intermolecular hydrogen bonds because the oxygen atom of DMSO can be a good proton acceptor. 3.1.2. Syntheses of Copper(I) Complexes The copper(I) complexes were synthesized according to the method reported previously [10, 18, 38, 49, 50]. Reaction of [Cu(MeCN)4]ClO4, pipH and the chelating diphosphine ligands, POP or xantphos under neutral or alkaline conditions in a 2:1:2 ratio followed by crystallization from CH2Cl2/hexane system gave four binuclear Cu(I) complexes in good yields (Scheme 1).
8
Scheme 1. The synthetic routes used to synthesize the Cu(I) complexes. 3.2. X-ray Structure Analysis
Figure 1. ORTEP drawing of the cation [Cu2(pipH)(POP)2]2+ of complex 1a with the atom labelling scheme, showing 30% thermal ellipsoids. Most hydrogen atoms and some phenyl rings of POP are omitted for clarity.
9
Figure 2. ORTEP drawing of the cation [Cu2(pipH)(xantphos)2]2+ of complex 2a with the atom labelling scheme, showing 30% thermal ellipsoids. Most hydrogen atoms and some phenyl rings of xantphos are omitted for clarity.
Figure 3. ORTEP drawing of the cation [Cu2(pip)(xantphos)2]+ of complex 2b with the atom labelling scheme, showing 30% thermal ellipsoids. All the hydrogen atoms and some phenyl rings of xantphos are omitted for clarity.
10
Table 1. Selected bond lengths (Å) and angles (º) for complexes 1a, 2a and 2b3CH2Cl2 1a Cu1N1
2.055(2)
Cu1N2
2.066(2)
Cu1P1
2.248(1)
Cu1P2
2.233(1)
Cu2N4
2.115(2)
Cu2N5
2.109(2)
Cu2P3
2.279(1)
Cu2P4
2.247(1)
C13N3
1.360(4)
C13N4
1.328(3)
N1Cu1N2
81.13(9)
N1Cu1P1
116.01(8)
N1Cu1P2
122.36(7)
N2Cu1P1
106.14(7)
N2Cu1P2
110.46(7)
P1Cu1P2
113.98(3)
N4Cu2N5
79.98(9)
N4Cu2P3
106.17(7)
N4Cu2P4
122.32(7)
N5Cu2P3
113.30(8)
N5Cu2P4
119.34(7)
P3Cu2P4
111.88(3)
2a Cu1N1
2.058(3)
Cu1N2
2.089(3)
Cu1P1
2.274(1)
Cu1P2
2.230(1)
Cu2N4
2.062(3)
Cu2N5
2.152(3)
Cu2P3
2.233(1)
Cu2P4
2.326(1)
C13N3
1.345(5)
C13N4
1.340(5)
N1Cu1N2
80.05(13)
N1Cu1P1
105.07(10)
N1Cu1P2
124.92(9)
N2Cu1P1
105.29(10)
N2Cu1P2
118.22(9)
P1Cu1P2
116.72(4)
N4Cu2N5
79.87(12)
N4Cu2P3
124.92(10)
N4Cu2P4
108.14(10)
N5Cu2P3
125.20(9)
N5Cu2P4
97.02(9)
P3Cu2P4
114.60(4)
2b3CH2Cl2 Cu1N1
2.070(3)
Cu1N2
2.074(3)
Cu1P1
2.252(1)
Cu1P2
2.250(1)
Cu2N4
2.032(2)
Cu2N5
2.140(3)
Cu2P3
2.260(1)
Cu2P4
2.287(1)
C13N3
1.346(4)
C13N4
1.355(4)
N1Cu1N2
81.04(10)
N1Cu1P1
116.77(8)
N1Cu1P2
112.94(8)
N2Cu1P1
102.34(8)
N2Cu1P2
122.59(7)
P1Cu1P2
116.38(3)
N4Cu2N5
80.56(10)
N4Cu2P3
122.91(7)
N4Cu2P4
110.00(7)
N5Cu2P3
113.77(7)
N5Cu2P4
109.69(7)
P3Cu2P4
114.79(3)
Single crystals of complexes 1a, 2a and 2b were determined at 150 K. The crystallographic data and selected refinement details are listed in Table S1 (see Appendix B). Selected bond lengths and angles for complexes 1a, 2a and 2b are presented in Table 1. ORTEP drawings of the cations for complexes 1a, 2a and 2b are depicted in Figures 13, respectively. All the three complexes exhibit binuclear structures. The 2-(2-Pyridyl)imidazo[4,5-f][1,10]-phenanthroline binds to two Cu(PP) units in the bridging coordination modes with the neutral form in complexes
11
1a and 2a and an deprotonated one in complex 2b. As in most reported Cu(I)-imine-diphosphine complexes, all the Cu(I) centres display distorted tetrahedral coordination geometries with two N atoms from the N,N- phenanthroline chelating sites or from N,N- 2-(2-pyridyl)imidazole sites and two P atoms from the chelating diphosphine ligands [37, 1014, 38, 49, 50]. The CuN distances (2.032(2)2.152(2) Å) and CuP (2.230(1)2.326(1) Å) distances are comparable to those reported values [3, 5, 10, 38, 50]. The CuN distances are affected by the following factors, the position of N atom, the form of imidazole ring and the diphosphine ligands. The Cu2N4 bond lengths decrease from 2.062(3) Å in complex 2a to 2.032(2) Å in complex 2b with a 0.030 Å difference, indicating the stronger binding ability of N atom from anionic imidazole ring than that from neutral imidazole ring. The dihedral angles (dhas) between the pyridine ring (C14C18, N5) and the mean plane of imidazo[4,5-f][1,10]phenanthroline (C1C13, N1N4) decrease from 21.7 in complex 1a, 17.4 in complex 2a to 6.2 in complex 2b, indicating the gradual enhancement of the coplanarity and conjugate degree of pipH or pip ligand from complexes 1a, 2a to complex 2b (see Figure 4). The rapid decrease of dhas from complex 2a to complex 2b is suggested to arise from the increasing resonance structure and conjugate degree among the whole pip plane because of the deprotonation of the imidazole ring [10].
12
Figure 4. The perspective drawings for Cu atoms and pipH (or pip) for complexes 1a, 2a and 2b showing the coplanar conditions of pipH or pip (left: ball-and-stick mode; right: wires mode).
3.3. 31P NMR spectra.
Figure 5. 31P NMR spectra of all complexes in DMSO-d6.
The 31P NMR spectra of all complexes in DMSO-d6 are shown in Figure 5. Complexes 1a and 2a display multiple
31
P NMR signals similar to those discussed for Cu(I) complexes with
puckered xantphos or POP [10, 51, 52]. The 31P NMR spectra of binuclear and trinuclear Cu(I) complexes with xantphos and 1H-imidazo[4,5-f][1,10]phenanthroline derivatives are not simply two and three groups of single signal peaks [10], but multiple 31P NMR signals due to several possible reasons [10]. For tetrahedral metal centers, the conformers may occur when four coordination atoms are from two bidentate chelating ligands, respectively, and one chelating ligand such as POP or xantphos is puckered or easy to twist, and the other is asymmetric [51, 52]. Compared with complexes 1a and 2a, complexes 1b and 2b exhibit two groups of simple single signal peaks, and this may arise from less conformers for complexes 1b and 2b due to the increasing symmetry of pip anion, which is tentatively attributed to the resonance structure of anionic imidazole ring and the rotation of CC bond between imidazole ring and pyridine ring.
13
Compared with the 31P NMR data reported before, the signals with chemical shifts at about 11.34 and 11.84 ppm for complexes 1a and 1b, and 12.44, -12.82 and 12.50 ppm for complexes 2a and 2b are assigned to POP and xantphos around Cu1 centers [10, 38], and the left signals are assigned to POP and xantphos around Cu2 centers. 3.4. Physical Properties 3.4.1. Absorption spectra UV-vis absorption data of all complexes in dichloromethane solutions are listed in Table 2, while their electronic absorption spectra are shown in Figure 6. Compared with the absorptions of ligands pipH, POP, xantphos, the reported [Cu(diimine)(PP)]+ and [Cu2(diimine)2(PP)2]2+ analogs [3, 5, 10, 18, 38, 49, 50] and the Cu(I) complexes of pyridine-imidazole [5355], the intense high energy bands with wavelengths shorter than ca. 360 nm for complexes 1a, 2a and those shorter than ca. 330 nm for complexes 1b, 2b are suggested to arise from spin-allowed ligand-centered * transitions of coordinated pipH (pip) and diphosphine ligands. The broad, low-energy bands with the maximums at ca. 400 nm for complexes 1a and 2a and at ca. 350 nm for complexes 1b and 2b are tentatively assigned to metal-to-ligand charge transfer (d(Cu)
*(pip or pipH) (MLCT) transitions [10, 18, 38, 49, 50, 5355]. The absorption band with the lowest energy between 400450 nm in complexes 1b and 2b are tentatively assigned to the intraligand charge transfer (ILCT) transitions according to the density functional theory (DFT) calculations about Cu(I) complexes of pyridine-benzimidazole [53]. Despite the similar coordination motif, complexes 1b and 2b show blue shift MLCT absorption relative to complexes 1a and 2a, and this can be attributed to the larger HOMO-LUMO energy gap.
14
Figure 6. UV-Vis absorption spectra of all complexes in DCM at room temperature.
Table 2. Photophysical data of all complexes at room temperature. abs/nm (/M1cm1)
compd
(CH2Cl2) 1a 1b
271 (66600), 396 (10120) 264 (55520), 284 (60200), 347 (33600), 416 (8040), 440 (6880)
em/nm
em/nm
(CH2Cl2)
(solid)
633
574
616
555
2a
277 (86660), 326 (32810), 410 (9440)
630
591
2b
284 (86860), 351 (39080), 425 (9040), 444 (8120)
608
549
3.4.2. Photoluminescence (PL) All the complexes show good luminescence behavior in the crystalline state and weak luminescence in DCM solutions (2.5105 molL1) at room temperature and their corresponding emission spectra are shown in Figure 7. The corresponding emission wavelengths are listed in Table 2. All the complexes show 3MLCT emission bands with maximum emission wavelengths falling in the range of 549633 nm [3, 5, 10, 18, 38, 49, 50, 5356]. Some complexes show dual emission and the high energy emission bands are tentatively assigned to ILCT emission according to the analysis result of the above absorption spectra. Complexes 1b and 2b with
15
anionic pip ligand show obvious blue shift emission compared to complexes 1a and 2a with neutral pipH ligand, which is similar to those for the aforementioned electronic absorption spectra. On the other hand, complexes 1b and 2b show stronger emission compared to complexes 1a and 2a, which is ascribed to the higher rigidity resulting from the stronger coordination of anionic pip ligand and less vibrational quenching due to the loss of NH bond and the less counter ions in complexes 1b and 2b [53].
Figure 7. Photoluminescence of all complexes in the crystalline state (left) and in DCM solutions (right) with an excitation wavelength at 330 nm and a 395 nm filter.
4. Conclusion Four binuclear copper(I) complexes with 2-(2-pyridyl)imidazo[4,5-f][1,10]-phenanthroline or its anionic ion and the chelating diphosphine POP or xantphos as mixed ligands have been synthesized and characterized. Complexes 1b and 2b with anionic pip ligand show obvious blue shift emission and stronger emission compared to complexes 1a and 2a with neutral pipH ligand. The higher emission efficiency is ascribed to the higher rigidity resulting from the stronger coordination of anionic pip ligand, the better light harvesting ability resulting from the larger conjugate degree of deprotonated pipH and less vibrational quenching due to the loss of NH bond and the fewer counter ions.
16
Acknowledgments This work was supported financially by the National Natural Science Foundation of China (NSFC) (21271091), PAPD of Jiangsu Higher Education Institutions and Jiangsu college students' innovation and entrepreneurship training program project (201610320107X). Appendix A. Supplementary data CCDC-1860167,
CCDC-1860168
and
CCDC-1860169
contain
the
supplementary
crystallographic data for complexes 1a, 2a and 2b3CH2Cl2, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK. Fax: (+44) 1223-336033; or e-mail: [email protected]. Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. REFERENCES [1] M. T. Buckner, T. G. Matthews, F. E. Lytle, D. R. McMillin, J. Am. Chem. Soc. 101 (1979) 5846. [2] J. R. Kirchhoff, D. R. Mcmillin, W. R. Robinson, D. R. Powell, A. T. Mckenzie, S. Chen, Inorg. Chem. 24 (1985) 3928. [3] D. G. Cuttell, S. Kuang, P. E. Fanwick, D. R. McMillin, R. A. Walton, J. Am. Chem. Soc. 124 (2002) 6.
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23
Synthesis, Structures and Photophysical Properties of Four Binuclear Cu(I) Complexes of 1Himidazo[4,5-f][1,10]phenanthroline
Yan-Wen Niu, Xia Liu, Ling-Zhao, Ya-Meng Guo, Wen-Xin Li, Miao-Miao Ma, Xiu-Ling Li* Four binuclear Cu(I) complexes of neutral or deprotonated 2-(2-pyridyl)imidazo[4,5-f][1,10]phenanthroline (pipH) and diphosphine were synthesized. The two Cu(I) complexes with deprotonated pipH show obvious blue shift and stronger emission compare to the other two with neutral pipH.
24