Synthesis and characterisation of homoleptic 2,9-diaryl-1,10-phenanthroline copper(i) complexes: Influencing selectivity in photoredox-catalysed atom-transfer radical addition reactions

Article abstract of DOI:10.1039/c8dt04116d

This report details the synthesis and characterisation of eight homoleptic bis(2,9-diaryl-1,10-phenanthroline)copper(i) complexes, seven of which are previously unreported {aryl = p-CF₃C₆H₄, p-FC₆H₄,

Full text of DOI:10.1039/c8dt04116d

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Synthesis and characterisation of homoleptic 2,9-
diaryl-1,10-phenanthroline copper(I) complexes:
influencing selectivity in photoredox-catalysed
atom-transfer radical addition reactions†
Cite this: DOI: 10.1039/c8dt04116d
b
Thomas P. Nicholls,a Chiara Caporale, b Massimiliano Massi,
a
Michael G. Gardiner a and Alex C. Bissember
*
This report details the synthesis and characterisation of eight homoleptic bis(2,9-diaryl-1,10-phenanthro-
line)copper(I) complexes, seven of which are previously unreported {aryl = p-CF3C6H4, p-FC6H4, m,p-
(OMe)2C6H3, o,p-(OMe)2C6H3, p-OMe-m,m-Me2C6H2, p-OMe-m,m-(t-Bu)2C6H2, 9,9-dimethyl-9H-
fluoren-2-yl, 4-(9H-carbazol-9-yl)phenyl)}. Where possible the solid state, photophysical and electro-
chemical properties of these complexes were studied. In order to obtain insights into the influence of the
intrinsic features of these copper(I) complexes on their reactivity in visible light-mediated photoredox cat-
alysis, their capacity to promote a known atom-transfer radical addition process was evaluated. This
specific transformation was identified as a suitable model system as it is reported to proceed via a mecha-
nism consistent with the inner-sphere reactivity enabled by coordinatively unsaturated phenanthroline-
based copper(I) species.
Received 15th October 2018,
Accepted 13th February 2019
DOI: 10.1039/c8dt04116d
rsc.li/dalton
these challenges, examples of synthetic reactions employing
phenanthroline-based visible light-mediated copper(I) photo-
Introduction
In comparison to ubiquitous photoredox catalysts featuring redox catalysts are not as well established as those utilising
second- or third-row transition metals, such as [Ru(bpy)3]Cl2 coordinatively saturated, octahedral ruthenium and iridium
(2), copper-based photoredox catalysts, including [Cu(dap)2]Cl systems.4–6
(1·Cl), have received much less attention in photoredox cataly-
We were interested in preparing a series of novel homolep-
sis (Fig. 1).1,2 The distinctive feature of copper(I) complexes is tic phenanthroline containing copper(I) complexes featuring
their preference for the metal centre to adopt a tetrahedral sterically- and electronically-varied 2,9-diaryl-1,10-phenanthro-
coordination geometry. However, upon photoexcitation to the line ligands in order to improve our understanding of the
metal-to-ligand-charge transfer (MLCT) manifold, the tempor- influence of substituent effects on the photoredox capacity.7
ary formation of a copper(II) ion favours distortion of the metal With this in mind, we evaluated the viability of these novel
centre from tetrahedral to square planar coordination geome- homoleptic (in addition to known homoleptic and heterolep-
tries.3 In addition to reducing the coordinative stability of tic) copper(I) photoredox catalysts in a notable atom-transfer
these complexes and potentially favouring the coordination of radical addition (ATRA) reaction that was recently reported
a solvent molecule or counteranion, this structural reorganis- (Scheme 1).5f We specifically selected this model system as the
ation is known to offer facile non-radiative pathways from the
excited states via enhancement of the non-radiative decay con-
stant knr, with consequent reduction of the excited state life-
time and quantum yield values.1b Perhaps, in part, because of
aSchool of Natural Sciences – Chemistry, University of Tasmania, Hobart, Tasmania,
7001, Australia. E-mail: alex.bissember@utas.edu.au
bDepartment of Chemistry and Nanochemistry Research Institute, Curtin University,
Bentley, Western Australia, 6102, Australia
†Electronic supplementary information (ESI) available. CCDC 1872365–1872368.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8dt04116d
Fig. 1 Examples of established photoredox catalysts.
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Experimental
Materials and methods
Anhydrous CH2Cl2 was distilled over CaH2, under N2, and
stored over molecular sieves, under N2, prior to use. PhMe and
CH3CN were purchased from Chem-Supply and passed
through columns on an Innovative Technologies Solvent
Purification system. 2,9-Dichloro-1,10-phenanthroline (4),8
[Cu(dap)2]Cl (1·Cl),5a [Cu(dmp)2]Cl9 (dmp = 2,9-dimethyl-1,10-
phenanthroline), heteroleptic complexes 12a–d,10 and com-
pounds 10 and 11,5f were prepared via literature procedures.
[Cu(CH3CN)4]PF6 and CF3SO2Cl were purchased from Sigma-
Aldrich and used as received. Unless otherwise noted, all other
starting materials were either purchased from commercial
sources and used as received or prepared according to reported
procedures. NMR experiments were performed either on a
Bruker Avance III NMR spectrometer operating at 400 MHz
(1H), 100 MHz (13C), or 376 MHz (19F) or on a Bruker Avance
III NMR spectrometer operating at 600 MHz (1H), 150 MHz
(13C), or 576 MHz (19F). The deuterated solvent used was
CDCl3, CD3CN or DMSO-d6. Our custom photoreactor was con-
structed using a strip of blue LEDs purchased from aliexpress.
com. This photoreactor operated at 24 W (12 V, 2 A) to provide
light predominantly at 467 nm.
Scheme 1 Divergent reactivity provided by [Cu(dap)2]Cl (1·Cl) in ATRA
transformations identified by Reiser and co-workers.5f
mechanism responsible for this transformation is consistent
with the unusual inner-sphere reactivity provided by coordina-
tively unsaturated copper(I) photoredox catalysts, such as
[Cu(dap)2]Cl (1·Cl).5f,k,l,n,o Consequently, we anticipated that
this intriguing ATRA process, which has only been mediated
by photoactive copper(I) species,5f,k,l would allow us to
indirectly study the influence of the structural features and
other fundamental properties of these copper(I) catalysts on
controlling the reaction outcome.
Herein, we report the synthesis, characterisation, and
photocatalytic studies of the homoleptic bis(2,9-diaryl-1,10-
phenanthroline)copper(I) complexes 3a–h, seven of which are
previously unreported (Fig. 2). Due to the presence of the
π-conjugated coordinating ligands, all these complexes can be
excited to MLCT excited states, as evidenced from the UV-Vis
absorption spectra. The solid state structures of three of these
complexes were determined by X-ray crystal structure analyses.
Each of the complexes 1·PF6, 3a, 3f and 3g display distorted
tetrahedral coordination geometries about the metal centres
and overall structure of the complex ions and the extent of this
distortion has been quantified by examination of flattening,
rocking and wagging distortions. The steric bulk of 2,9-diaryl
substituents play a significant role in the extent of the distor-
tion in the complexes in the ground state. These homoleptic
copper(I) species, and various known heteroleptic copper(I)
complexes, were evaluated in order to determine their capacity
to promote ATRA processes of the type illustrated in Scheme 1.
The relative ratios of trifluoromethylchlorosulfonation and
trifluoromethylchlorination products in these photoredox-cata-
lysed reactions were generally consistent with the varying
steric bulk of the copper complex employed. However, elec-
tronic effects were also found to be important.
Synthesis of ligands
General procedure 1. Ligands 5a,c–g were synthesised by
modifying conditions previously reported by Sauvage and co-
workers.11 These reactions were unoptimised. A solution of
Na2CO3 (18 equiv.) in H2O was added to a magnetically stirred
solution of 2,9-dichloro-1,10-phenanthroline (4) (1 equiv.) and
the organoborane (4 equiv.) in PhMe maintained at ambient
temperature under N2. [Pd(PPh3)4] (10%) was added and the
ensuing mixture was heated at 95 °C. After 16 h, the mixture
was cooled to ambient temperature, the phases separated, and
the aqueous phase was extracted with CH2Cl2 (2 × 20 mL). The
combined organic extracts were then dried (Na2SO4), filtered,
and concentrated under reduced pressure. The ensuing
residue was purified by flash column chromatography on silica
gel.
General procedure 2. Ligands 5b and 5h were synthesised by
modifying conditions previously reported by Stille and co-
workers.12 These reactions were unoptimised. [Pd(PPh3)4] (5%)
was added to a magnetically stirred solution of the organostan-
nane (4 equiv.) and 2,9-dichloro-1,10-phenanthroline (4)
(1 equiv.) in PhMe maintained at ambient temperature under
N2. The ensuing mixture was heated to 110 °C. After 20 h, the
mixture was cooled to ambient temperature and concentrated
under reduced pressure. The ensuing residue was purified by
flash column chromatography on silica gel.
2,9-Di(4-(trifluoromethyl)phenyl)-1,10-phenanthroline (5a).
Reaction with 4-(trifluoromethyl)phenylboronic acid (610 mg,
3.21 mmol) following General procedure 1 and purified by
flash column chromatography on silica gel (0–20% EtOAc/
hexanes to provide compound 5a as a colourless amorphous
1
Fig. 2 Homoleptic copper(I) complexes synthesised in this study.
solid (182 mg, 48% yield). H NMR (400 MHz, CDCl3): δ 8.57
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(d, J = 8.5 Hz, 4H), 8.39 (d, J = 8.5 Hz, 2H), 8.20 (d, J = 8.5 Hz, acid pinacol ester13 (556 mg, 1.61 mmol) following General
2H), 7.90–7.86 (complex m, 6H) ppm. 13C NMR (100 MHz, procedure 1 and purified by flash column chromatography on
CDCl3): δ 155.6, 146.3, 142.8, 137.5, 131.3 (q, J = 32.4 Hz), silica gel (EtOAc/hexanes 0–30%) to provide compound 5f as a
128.6, 128.1, 126.7, 126.0 (q, J = 3.8 Hz), 124.4 (q, J = 272 Hz), colourless amorphous solid (84 mg, 34% yield). This com-
120.5 ppm. IR (NaCl): 1327, 1163, 1113, 1087, 1013, 847, 797, pound is known.14 1H NMR (400 MHz, CDCl3): δ 8.29 (d, J =
733, 694 cm−1. EI-MS: m/z (M+) 468, 399.
2,9-Di(4-fluorophenyl)-1,10-phenanthroline (5b). Reaction 3.78 (s, 6H), 1.58 (s, 36H) ppm.
8.2 Hz, 2H), 8.13 (s, 4H), 8.03 (d, J = 8.6 Hz, 2H), 7.79 (s, 2H),
with tributyl(4-fluorophenyl)stannane (1.64 g, 3.21 mmol) fol-
2,9-Bis(9,9-dimethyl-9H-fluoren-2-yl)-1,10-phenanthroline (5g).
lowing General procedure 2 and purified by flash chromato- Reaction with (9,9-dimethyl-9H-fluoren-2-yl)boronic acid
graphy on silica gel (0–20% EtOAc/hexanes) to afford com- (500 mg, 2.10 mmol) following General procedure 1 and puri-
pound 5b (29 mg, 10% yield) as a colourless amorphous solid. fied by flash column chromatography on silica gel (0–10%
1H NMR (400 MHz, CDCl3): δ 8.46 (dd, J = 8.8, 5.5 Hz, 4H), EtOAc/hexanes) to afford compound 5g (52 mg, 17% yield) as a
8.33 (d, J = 8.4 Hz, 2H), 8.11 (d, J = 8.4 Hz, 2H), 7.81 (s, 2H), pale yellow amorphous solid. 1H NMR (400 MHz, CDCl3): δ
7.30 (m, 4H) ppm. 13C NMR (150 MHz, CDCl3): δ 164.8, 163.1, 8.71 (d, J = 1.2 Hz, 2H), 8.43 (dd, J = 7.9, 1.6 Hz, 2H), 8.35 (d, J
155.8, 146.0, 137.1, 129.5 (d, J = 8.4 Hz), 127.7, 126.0, 119.8, = 8.4 Hz, 2H), 8.24 (d, J = 8.4 Hz, 2H), 7.95 (d, J = 8.0 Hz, 2H),
115.8 (d, J = 21.7 Hz) ppm. 19F NMR (565 MHz, CDCl3): δ 7.88–7.85 (m 2H), 7.82 (s, 2H), 7.57–7.54 (m, 2H), 7.46–7.39
−112.3 (s, 2F) ppm. IR (NaCl): 1487, 1221, 1157, 932, 907, 838, (complex m, 4H), 1.72 (s, 12H) ppm. 13C NMR (100 MHz,
722 cm−1. ESI-MS: m/z (M + H+) 369.
CDCl3): δ 157.3, 154.6, 154.4, 146.1, 140.8, 138.9, 138.8, 137.1,
2,9-Di(3,4-dimethoxyphenyl)-1,10-phenanthroline
(5c). 128.0, 127.8, 127.2, 127.1, 126.1, 122.8, 122.2, 120.6, 120.5,
Reaction with 3,4-dimethoxyphenylboronic acid (293 mg, 120.4, 47.3, 27.5 ppm. IR (NaCl): 1684, 1653, 1584, 1559, 1504,
1.61 mmol) following General procedure 1 and purified by 1447, 1364, 1283, 1225, 1211, 1099, 854, 833, 820, 781, 760,
flash column chromatography (20–50% EtOAc/hexanes, then 739 cm−1. ESI-MS: m/z (M + H+) 565.
1% MeOH/CH2Cl2) to provide compound 5c as a colourless
2,9-Bis(4-(9H-carbazol-9-yl)phenyl)-1,10-phenanthroline
(5h).
amorphous solid (100 mg, 55% yield). 1H NMR (400 MHz, Reaction with 9-(4-(tributylstannyl)phenyl)-9H-carbazole15
CDCl3): δ 8.27–8.23 (complex m, 4H), 8.07 (d, J = 8.4 Hz, 2H), (610 mg, 1.15 mmol) following General procedure 2 and puri-
7.94 (d, J = 8.4 Hz, 2H), 7.74 (s, 2H), 7.01 (d, J = 8.4 Hz, 2H), fied by flash column chromatography on silica gel (hexanes
4.13 (s, 6H), 3.99 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ then CH2Cl2) to afford compound 5h (101 mg, 53% yield) as a
156.3, 150.8, 149.5, 145.9, 136.9, 132.7, 127.7, 125.7, 120.6, pale yellow amorphous solid. 1H NMR (400 MHz, CDCl3): δ
119.5, 111.5, 111.2, 56.4, 56.1 ppm. IR (NaCl): 1593, 1520, 8.72 (d, J = 8.4 Hz, 4H), 8.45 (d, J = 8.4 Hz, 2H), 8.29 (d, J = 8.4
1489, 1456, 1414, 1273, 1221, 1173, 1157, 1138, 1024, 851, 795, Hz, 2H), 8.18 (d, J = 7.8 Hz, 4H), 7.91 (s, 2H), 7.84 (d, J = 8.4
768, 735, cm−1. EI-MS: m/z (M+) 452, 437, 421, 405, 377.
2,9-Di(2,4-dimethoxyphenyl)-1,10-phenanthroline
Hz, 4H), 7.57 (d, J = 8.2 Hz, 4H), 7.47 (t, J = 7.6 Hz, 4H), 7.33 (t,
(5d). J = 7.4 Hz, 4H) ppm. 13C NMR (100 MHz, CDCl3): δ 156.3,
Reaction with 2,4-dimethoxyphenylboronic acid (585 mg, 146.5, 140.9, 139.0, 138.8, 137.4, 129.4, 128.3, 127.5, 126.4,
3.21 mmol) following General procedure 1 and purified by 126.2, 123.7, 120.5, 120.3, 120.2, 110.1 ppm. IR (NaCl):
flash column chromatography (30–50% EtOAc/hexanes to 1684, 1653, 1559, 1489, 1451, 1362, 750, 723 cm−1. ESI-MS: m/z
provide compound 5d as a colourless amorphous solid (M + H+) 663.
1
(113 mg, 31% yield). H NMR (400 MHz, CDCl3): δ 8.41 (d, J =
8.5 Hz, 2H), 8.26 (d, J = 8.4 Hz, 2H), 8.20 (d, J = 8.4 Hz, 2H),
Preparation of complexes
7.74 (s, 2H), 7.76 (dd, J = 8.5, 2.3 Hz 2H), 6.62 (d, J = 2.3 Hz
General procedure 3. These compounds were synthesised by
2H), 3.914 (s, 6H), 3.911 (s, 6H) ppm. 13C NMR (100 MHz, modifying conditions previously reported by Reiser.16
CDCl3): δ 161.8, 158.9, 155.8, 146.0, 135.4, 133.4, 127.1, 125.6, [Cu(CH3CN)4]PF6 (1 equiv.) and the phenanthroline ligand
124.4, 122.6, 105.6, 99.0, 55.8, 55.5 ppm. ESI-MS: m/z (M + H+) (2 equiv.) were dissolved in CH2Cl2. The ensuing magnetically
453.
stirred dark purple solution was maintained at ambient tem-
2,9-Di(4-methoxy-3,5-dimethylphenyl)-1,10-phenanthroline (5e). perature under N2. After 2 h, hexanes was added and the preci-
Reaction with 4-methoxy-3,5-dimethylphenylboronic acid pitated complex was collected by filtration.
(410 mg, 3.21 mmol) following General procedure 1 and puri-
fied by flash column chromatography (1% NEt3/CH2Cl2) to copper(I) hexafluorophosphate (3a). Reaction with 2,9-di(4-tri-
provide compound 5e as a colourless amorphous solid (99 mg, fluoromethyl)phenyl-1,10-phenanthroline (182 mg,
Bis(2,9-di(4-(trifluoromethyl)phenyl)-1,10-phenanthroline)
1
38% yield). H NMR (400 MHz, CDCl3): δ 8.28 (d, J = 8.6 Hz, 0.389 mmol) following General procedure 3. Precipitation with
2H), 8.22 (s, 4H), 8.12 (d, J = 8.12, 2H), 7.76 (s, 2H), 3.83 (s, hexanes afforded complex 3a as a dark red solid (218 mg, 97%
6H), 2.48 (s, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 158.7, yield). 1H NMR (400 MHz, DMSO-d6): δ 8.86 (d, J = 8.3 Hz, 4H),
156.3, 145.8, 136.9, 134.6, 131.1, 128.3, 127.7, 125.8, 119.6, 8.27 (s, 4H), 8.18 (d, J = 8.3 Hz, 4H), 7.66 (d, J = 8.0 Hz, 8H),
59.9, 16.5 ppm. IR (NaCl): 1479, 1321, 1229, 1190, 1165, 1074, 6.92 (d, J = 8.1 Hz, 8H) ppm. 13C NMR (150 MHz, DMSO-d6): δ
1013, 841, 634 cm−1. EI-MS: m/z (M+) 448, 433, 418, 403, 388.
154.5, 142.6, 142.0, 138.3, 128.6, 128.5, 126.8, 124.7, 123.88,
2,9-Di(4-methoxy-3,5-di-tert-butylphenyl)-1,10-phenanthroline 123.86, 123.4 (q, J = 273.3 Hz) ppm. ESI-MS: m/z 999. HRESIMS
(5f). Reaction with 4-methoxy-3,5-di-tert-butylphenylboronic [M+] Found: 999.1390. C52H28CuN4F12 requires 999.1413.
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Bis(2,9-di(4-fluorophenyl)-1,10-phenanthroline)copper(I) hexa- 125.84, 125.78, 64.6, 35.5, 31.3 ppm. HRESIMS [M+] Found:
fluorophosphate (3b). Reaction with 2,9-di(4-fluorophenyl)-1,10- 1295.7327. C84H104CuN4O4 requires 1295.7348.
phenanthroline (26 mg, 0.071 mmol) following General pro-
Bis(2,9-bis(9,9-dimethyl-9H-fluoren-2-yl)-1,10-phenanthroline)
cedure 3. Precipitation with hexanes afforded complex 3b as a copper(I) hexafluorophosphate (3g). Reaction with 2,9-bis(9,9-
1
dark red solid (32 mg, 97% yield). H NMR (400 MHz, DMSO- dimethyl-9H-fluoren-2-yl)-1,10-phenanthroline
(52
mg,
d6): δ 8.65 (d, J = 8.4 Hz, 4H), 8.15 (s, 4H), 7.98 (d, J = 8.3 Hz, 0.087 mmol) following General procedure 3. Precipitation
4H), 7.51 (m, 8H), 6.34 (t, J = 8.7 Hz, 8H) ppm. 13C NMR with hexanes afforded complex 3g as a dark red solid (49 mg,
(150 MHz, DMSO-d6): δ 161.7, 155.5, 143.2, 138.4, 138.4, 135.5 95% yield). 1H NMR (400 MHz, DMSO-d6): δ 8.55 (d, J = 8.4 Hz,
(d, J = 2.6 Hz), 130.3 (d, J = 8.6 Hz), 128.6, 126.9, 125.2, 114.4 4H), 8.07 (d, J = 8.4 Hz, 4H), 7.96 (s, 4H), 7.89 (s, 4H),
(d, J = 21.8 Hz) ppm. HRESIMS [M+] Found: 799.1522. 7.57–7.53 (m, 4H), 7.45(d, J = 7.9 Hz, 4H), 7.43–7.38 (m, 4H),
C48H28CuN4F4 requires 799.1541.
7.35–7.27 (complex m, 8H), 6.92 (d, J = 7.8 Hz, 4H), 0.93 (s,
Bis(2,9-di(3,4-dimethoxyphenyl)-1,10-phenanthroline)copper(I) 24H) ppm. 13C NMR (150 MHz, DMSO-d6): δ 156.2, 153.3,
hexafluorophosphate (3c). Reaction with 2,9-di(3,4-dimethoxy- 152.4, 143.3, 139.3, 137.6, 137.3, 137.2, 128.1, 127.9, 127.2,
phenyl)-1,10-phenanthroline (100 mg, 0.222 mmol) following 127.1, 126.1, 125.3, 122.6, 121.7, 120.3, 118.7, 46.1, 26.1 ppm.
General procedure 3. Precipitation with hexanes afforded ESI-MS: m/z 1191. HRESIMS [M+] Found: 1191.4412.
1
complex 3c as a dark red solid (117 mg, 95% yield). H NMR C84H64CuN4 requires 1191.4422.
(400 MHz, DMSO-d6): δ 8.70 (d, J = 8.0 Hz, 4H), 8.20–8.05
(complex m, 8H), 7.12 (d, J = 7.5 Hz, 4H), 6.96 (s, 4H), 6.03 (d, copper(I) hexafluorophosphate (3h). Reaction with 2,9-bis(4-
J = 7.9 Hz, 4H), 3.46 (s, 12H), 3.23 (s, 12H) ppm. 13C NMR (9H-carbazol-9-yl)phenyl)-1,10-phenanthroline
(100 mg,
Bis(2,9-bis(4-(9H-carbazol-9-yl)phenyl)-1,10-phenanthroline)
(150 MHz, DMSO-d6): δ 155.7, 149.2, 147.4, 142.9, 137.2, 131.0, 0.150 mmol) following General procedure 3. Precipitation with
127.5, 125.8, 124.5, 120.2, 110.6, 109.8, 55.1, 54.5 ppm. hexanes afforded complex 3h as a dark red solid (222 mg, 97%
ESI-MS: m/z 967. HRESIMS [M+] Found: 967.2738. yield). 1H NMR (400 MHz, DMSO-d6): δ 8.89 (d, J = 8.3 Hz, 4H),
C56H48CuN4O8 requires 967.2763.
8.36 (d, J = 8.2 Hz, 4H), 8.27–8.19 (complex m, 12 H), 8.01 (d,
Bis(2,9-di(2,4-dimethoxyphenyl)-1,10-phenanthroline)copper(I) J = 8.0 Hz, 8H), 7.51 (t, J = 7.5 Hz, 8H), 7.33 (t, J = 7.3 Hz, 8H),
hexafluorophosphate (3d). Reaction with 2,9-di(2,4-dimethoxy- 7.04 (d, J = 7.9 Hz, 8H), 6.85 (d, J = 8.0 Hz, 8H) ppm. 13C NMR
phenyl)-1,10-phenanthroline (113 mg, 0.250 mmol) following (150 MHz, DMSO-d6): δ 155.5, 142.8, 139.3, 138.3, 137.4, 137.2,
General procedure 3. Precipitation with hexanes afforded 129.7, 128.3, 126.7, 126.2, 125.5, 125.0, 122.8, 120.6, 120.4,
1
complex 3d as a dark red solid (133 mg, 95% yield). H NMR 109.4 ppm. ESI-MS: m/z 1388. HRESIMS [M+] Found:
(400 MHz, DMSO-d6): δ 8.61 (d, J = 8.3 Hz, 4H), 8.09 (s, 4H), 1388.4244. C84H64CuN4 requires 1388.4270.
7.91 (d, J = 8.3 Hz, 4H), 7.08 (d, J = 8.4 Hz, 4H), 5.99 (d, J = 2.2
Photoredox-catalysed reactions
Hz, 4H), 5.40 (dd, J = 8.4, 2.2 Hz, 4H), 3.46 (s, 12H), 3.44 (s,
12H) ppm. 13C NMR (150 MHz, DMSO-d6): δ 160.6, 156.7,
General procedure 4. In a nitrogen filled glovebox, CF3SO2Cl
154.0, 142.7, 135.7, 129.8, 127.2, 127.0, 125.9, 120.4, 103.1, (33.9 mg, 200 μmol) was added to a 4 mL screw top vial (PTFE
96.8, 55.0, 54.7 ppm. HRESIMS [M + H+] Found: 967.2763. tape-lined thread) containing alkene (100 μmol), K2HPO4
C56H48CuN4O8 requires 967.2768.
(35.1 mg, 200 μmol), copper catalyst (1.0 μmol), CH3CN
Bis(2,9-di(4-methoxy-3,5-dimethylphenyl)-1,10-phenanthroline) (300 μL) and a magnetic stir bar. The vial was then capped, the
copper(I) hexafluorophosphate (3e). Reaction with 2,9-di(4- joint was wrapped with PARAFILM®, and removed from the
methoxy-3,5-dimethylphenyl)-1,10-phenanthroline (99 mg, glovebox and placed in a water bath (suspended with copper
0.219 mmol) following General procedure 3. Precipitation with wire) maintained at 45 °C that was contained within a 24 W
hexanes afforded complex 3e as a dark red solid (89 mg, 73% blue LED photoreactor (switched off) and magnetically stirred.
yield). 1H NMR (400 MHz, DMSO-d6): δ 8.75 (d, J = 8.2 Hz, 4H), The vial was then irradiated. After 24 h, the reaction mixture
8.28 (s, 4H), 8.06 (d, J = 8.2 Hz, 4H), 7.21 (s, 8H), 3.28 (s, 12H), was quenched by passing through a silica plug (contained
1.40 (s, 24H) ppm. 13C NMR (100 MHz, DMSO-d6): δ 157.0, within a Pasteur pipette; Et2O elution) and analysed by 19F
155.6, 142.7, 137.4, 133.8, 128.9, 128.7, 127.8, 126.4, 124.4, NMR spectroscopy (after the addition of PhCF3 as an internal
58.9, 14.9 ppm. HRESIMS [M+] Found: 959.3583. standard).
C60H56CuN4O4 requires 959.3592.
3-Chloro-1,1,1-trifluoro-4-methylhexane (13). 1H NMR
Bis(2,9-di(4-methoxy-3,5-di-tert-butylphenyl)-1,10-phenanthro- (600 MHz, CDCl3) δ 3.72 (m, 1H), 2.48 (m, 2H), 1.63 (m, 1H),
line)copper(I) hexafluorophosphate (3f). Reaction with 2,9-di(4- 1.48 (m, 2H), 0.98 (t, J = 7.2 Hz, 3H), 0.90 (s, J = 7.3 Hz, 3H)
methoxy-3,5-di-tert-butylphenyl)-1,10-phenanthroline (115 mg, ppm.19F NMR (576 MHz, CDCl3) δ −66.4 (t, J = 14 Hz, 3F) ppm.
0.186 mmol) following General procedure 3, however, this reac- 13C NMR (150 MHz, CDCl3) δ 125.8, 63.1, 39.8 (q, J = 31 Hz),
tion required 7 days to proceed to completion. Precipitation 35.6, 27.8, 16.4, 11.8 ppm. HRESIMS [M + H+] Found 252.0192.
with hexanes afforded known complex 3f as a dark red solid C7H12ClF3O2S requires 252.0199.
(105 mg, 78% yield).17 1H NMR (600 MHz, CDCl3): δ 8.43 (d,
1,1,1-Trifluoro-4-methylhexane-3-sulfonyl chloride (14).
J = 8.4 Hz, 4H), 8.04 (s, 4H), 7.85 (d, J = 8.3 Hz, 4H), 7.49 (s, 1H NMR (600 MHz, CDCl3) δ 4.13 (m, 1H), 2.05 (m, 2H), 1.94
8H), 3.23 (s, 12H), 0.93 (s, 72H) ppm. 13C NMR (150 MHz, (m, 1H), 1.54 (quin, J = 4.3 Hz, 2H), 0.93 (s, 3H), 0.89 (s, 3H)
CDCl3): δ 161.6, 157.2, 143.8, 143.7, 136.4, 133.2, 128.5, 126.7, ppm. 19F NMR (576 MHz, CDCl3) δ −66.2 (t, J = 10 Hz, 3F)
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ppm. 13C NMR (150 MHz, CDCl3) δ 123.6, 65.2, 40.3 (q, J = 26 steady-state emission and excitation spectra were recorded
Hz), 40.0, 25.9, 19.3, 10.6 ppm. HRESIMS [M + H+] Found using an Edinburgh FLSP980-stm spectrometer equipped with
185.0585. C7H12ClF3 requires 185.0580.
a 450 W xenon arc lamp, double excitation and emission
monochromators, a Peltier-cooled Hamamatsu R928P photo-
multiplier (185–850 nm). Emission and excitation spectra were
Crystallography
X-ray data were collected at 100 K on crystals mounted on a corrected for source intensity (lamp and grating) and emission
Hampton Scientific cryoloop on a Bruker AXS D8 Quest diffr- spectral response (detector and grating) by a calibration curve
actometer using Cu-Kα radiation (λ = 1.54178 Å) or on the MX2 supplied with the instrument. Emission and excitation spectra
beamlines of the Australian Synchrotron.18 The structures were were corrected for source intensity (lamp and grating) and
solved by direct methods with SHELXT,19 refined using full emission spectral response (detector and grating) by a cali-
matrix least squares routines against F2 with SHELXL-97 and bration curve supplied with the instrument. To record the
visualised with OLEX2.20 All non-hydrogen atoms were refined luminescence spectra at 77 K, the samples were placed in
anisotropically. All hydrogen atoms were placed in calculated quartz tubes (2 mm diameter) and inserted in a special quartz
positions and refined using a riding model with fixed C–H dis- Dewar filled with liquid nitrogen. Degassing of the dichloro-
tances of 0.95 Å (sp2 CH), 0.99 Å (CH2), 0.98 Å (CH3). The methane solutions was performed using the freeze–pump–
thermal parameters of all hydrogen atoms were estimated as thaw method. Experimental uncertainties are estimated to be
Uiso(H) = 1.2Ueq(C), except for CH3 where Uiso(H) = 1.5Ueq(C).
2 nm and 5 nm for absorption and emission peaks, respect-
The structure of 1·PF6 has been previously report at 100 K by ively. All the solvents used in the preparation of the solutions
Mayer5m and described as containing diffused solvent (two for the photophysical investigations were of spectrometric
molecules of water in the asymmetric unit, electronic contri- grade.
bution removed with SQUEEZE) when crystallised from a
Electrochemical measurements
toluene/acetonitrile solvent mixture. Our new report, also at
100 K, appears to be isomorphic (see Results and Discussion
section for crystallisation conditions), with the refinement pre-
sented in the obtuse triclinic cell setting and containing no
solvent accessible voids. We note the cell volume is 12 Å larger
in our case despite the apparent lack of solvent inclusion, with
significant improvements in data collection (Rint) and refine-
ment metrics presented (R1 and wR2). A summary of crystallo-
graphic data is provided below, and fully labelled figures are
provided in the ESI.†
Cyclic voltammetry was performed at room temperature using
a Metrohm Autolab PGSTAT101 and data acquired with
Metrohm Autolab, Nova 2.0 software. Experiments were per-
formed in dry, degassed CH3CN (0.25 mM), NBu4PF6 (0.1 M)
electrolyte, with a CH Instruments glassy carbon working elec-
trode, Pt wire counter electrode, and Ag wire quasi-reference
electrode. Ferrocene was added as an internal standard (Fc+/Fc
at 0 V) to reference experiments. The Fc+/Fc couple has a
potential of +0.40 V vs. saturated calomel electrode (SCE) in
acetonitrile (NBu4PF6 electrolyte).21
1·PF6 (CSD 1872365): C52H40CuF6N4O4P, M = 993.39, tri-
clinic, a = 13.7555(4), b = 14.1948(4), c = 14.2099(6) Å, α =
100.0947(18), β = 118.9449(11), γ = 106.6158(14)°, U = 2157.70
3
ˉ
(14) Å , T = 100 K, space group P1 (no. 2), Z = 2, 70597 reflec-
Results and discussion
Synthesis of ligands and copper(I) complexes
tions measured, 7587 unique (Rint = 0.0434), 6528 > 4σ(F), R =
0.0368 (observed), wR2 = 0.0969 (all data).
3a (CSD 1872366): C52H31CuF18N5P, M = 1186.35, monoclinic, Ligands were prepared using a modular strategy that allowed
a = 13.3534(7), b = 25.1829(12), c = 13.9516(7) Å, β = 116.8790 rapid access to a library of novel phenanthroline derivatives.
(10)°, U = 4811.5(4) Å3, T = 100 K, space group C2/c (no. 15), Z = Key dichloride 4 was synthesised on a multigram scale via a
4, 11 427 reflections measured, 2291 unique (Rint = 0.0330), previously reported 3-step sequence.22 Palladium-catalysed
2018 > 4σ(F), R = 0.0368 (observed), wR2 = 0.0838 (all data).
cross-couplings were then utilised to construct a library of
3f (CSD 1872367): C184H244Cu2F12N10O11P2, M = 3188.90, eight 2,9-diaryl-1,10-phenanthrolines 5a–h (Scheme 2). This
monoclinic, a = 43.219(9), b = 15.561(3), c = 26.670(5) Å, β = approach was adopted as this allowed rapid access to a range
102.720(3)°, U = 17496(6) Å3, T = 100 K, space group P2/c (no. 13), of ligands from key intermediate dichloride 4 and commer-
Z = 4, 156 936 reflections measured, 46 557 unique (Rint = 0.0548), cially available or readily accessible aryl nucleophiles. A
27 444 > 4σ(F), R = 0.0721 (observed), wR2 = 0.2416 (all data).
common alternative strategy involving lithium-halogen
3g (CSD 1872368): C86H67CuF6N5P, M = 1378.95, monocli- exchange of aryl halides followed by trapping with 1,10-phe-
nic, a = 24.792(3), b = 18.900(2), c = 15.4325(13) Å, β = 108.117 nanthroline and oxidation with MnO2 can deliver higher yields
(8)°, U = 6872.7(13) Å3, T = 100 K, space group C2/c (no. 15), Z for a limited number of related compounds.5m,23 However, in
= 4, 21 703 reflections measured, 6032 unique (Rint = 0.0632), our experience, product purification and the reproducibility of
4932 > 4σ(F), R = 0.0529 (observed), wR2 = 0.1432 (all data).
this chemistry upon scale-up can be challenging.
The synthesis of homoleptic complexes 3a–h was achieved
by mixing [Cu(MeCN)4]PF6 and the phenanthroline ligand in
Photophysical measurements
Absorption spectra were recorded at room temperature on a dichloromethane.11 In this way, a range of electronically- and
PerkinElmer Lambda 35 UV/Vis spectrometer. Uncorrected sterically-varied complexes was prepared. Typically, the for-
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diethyl ether and two acetonitrile solvent molecules and two
complete anion residues. The Cu–N bond lengths were typical
of related bis(2,9-diarylphenanthroline)copper(I) complexes.
For example, Cu–N bond lengths of 2.032(3)–2.112(3) Å have
been reported for [bis(2,9-diphenyl-1,10-phenanthroline)
copper(I)] tetrafluoroborate (Table 1).24
There is significant distortion from the ideal D2d symmetry
in each of the complexes 1·PF6, 3a, 3f, and 3g consistent with
most previously reported bis(phenanthroline)copper(I) specie-
s.1a,5m,25 The distortions can be defined by three key com-
ponents; flattening, rocking and wagging distortions
(Table 1).1a Flattening distortions were quantified by measur-
ing the interplanar angle between least squares planes of the
coordination ring of each ligand. For an ideal tetrahedral
metal coordination geometry, the angle would be 90°, which
compares to an angle of 0° for an ideal square planar struc-
ture. Rocking distortions were quantified by production of a
vector between the metal centre and a centroid between C5
and C6 on the phenanthroline backbone of one ligand and
measuring the obtuse angles to both nitrogen atoms of the
other ligand from this vector. In an ideal tetrahedral metal
coordination geometry these angles would be equal. The
wagging distortion is a ligand structural feature not directly
related to the metal coordination geometry and is defined as
the distance between a least squares plane of the phenanthro-
line backbone of one ligand and the nitrogen atoms of the
other ligand. In the absence of wagging these would be equal
in absolute value.
Bis(phenanthroline)copper(I) complexes 6a–b are shown for
comparative purposes. Complex 6a arguably displays the least
distortion from an ideal tetrahedral coordination geometry out
of the series of 2,9-disubstituted bis(1,10-phenanthroline)
copper(I) complexes.26 The steric bulk of the t-butyl groups
and the absence of π-stacking interactions presumably force
the complex to adopt a more ideal tetrahedral geometry and
all three flattening, rocking and wagging distortions remain
minimal. Conversely, prior to this report, the copper centre
within complex 6b displayed one of the most distorted tetra-
hedral coordination geometries of the series reported
(vide infra).27 This potentially stems from enhanced π-stacking
Scheme 2 Synthesis of phenanthroline ligands 5a–h and copper(I)
complexes 3a–h. Reagents and conditions used to synthesise hetero-
cycles 5a–h: 4 equiv. organoborane, [Pd(PPh3)4], Na2CO3, 2 : 1 PhMe/
H2O, 95 °C; or 4 equiv. organostannane, [Pd(PPh3)4], PhMe, 110 °C.
mation of corresponding bis(phenanthroline)copper(I) com-
plexes 3a–h was accompanied by the rapid colour change of
the reaction mixture (colourless to dark red within 2 h). The
conspicuous exception was the synthesis of species 3f, which
proceeded to completion only after 7 days. Armaroli and co-
workers made similar observations when they prepared the
same complex.17 In total, eight homoleptic copper(I) species
were prepared by this approach, seven of which are previously
unreported complexes. The 1H and 13C NMR spectra were
obtained for each of the complexes 3a–h. These spectra are
consistent with time-averaged D2d symmetric structures in
solution in all cases indicating some fluxionality in compari-
son to the lowered symmetry seen in the solid state.
1
Comparison of the H NMR spectra of the ligands 5a–h with
the respective complexes 3a–h displays no relative signal
broadening or additional signals. This suggests that there is
no restricted rotation about the σ-bond connecting the aryl
substituents to the phenanthroline moiety in either the free
ligands 5a–h or the complexes 3a–h.
Solid state structures of complexes 1·PF6, 3a, 3f, and 3g
Vapour diffusion of diethyl ether into saturated acetonitrile interactions as a result of extended conjugation of the
solutions of complexes 1·PF6, 3a, 3f, and 3g provided dark red 1-naphthyl groups.
single crystals suitable for X-ray crystal structure analyses
Flattening distortions were prevalent in all complexes 1·PF6,
(Fig. 3). Complexes 3a, and 3g belong to the monoclinic space 3a, 3f, and 3g and likely originate from π-stacking interactions
group C2/c and complex 3f to the monoclinic space group P2/c that become available with significant flattening. Furthermore,
ˉ
while complex 1·PF6 belongs to the triclinic space group P1. it has previously been suggested that the flattening distortion
The asymmetric unit for complex 1·PF6 comprises a single derives from more efficient packing in the crystal lattice.25
CuL2 unit and an anion residue. Both complexes 3a and 3g Complexes containing the [bis(1,10-phenanthroline)copper(I)]+
have crystallographic C2 symmetry and contain acetonitrile in complex ion (7) display significant variability in the three key
the lattice. For 3a, the acetonitrile molecule is not disordered structural distortions described above depending on the
and the anion residue resides on a crystallographic inversion counter anion.28 Complex 7·ClO4 exhibits a pronounced flat-
centre, while for 3g, the acetonitrile molecule and the anion tening distortion {54.5(5)°} but no rocking or wagging distor-
residue are disordered on a general site with fixed 50% occu- tion is observed. In contrast, complex 7·CuBr2 displays only a
pancies by virtue of the Cu oxidation state of +1. The asym- modest flattening distortion {78.8(3)°} and significant rocking
metric unit for complex 3f includes one complete CuL2 unit and wagging distortions. However, when comparing complexes
and two others residing on crystallographic C2 axes, three 1·PF6 and 1·BF4 only small differences in each of the flatten-
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Fig. 3 Structural representations of copper(I) complexes 1·PF6, 3a, 3f, and 3g. Thermal ellipsoids are drawn at the 50% probability level.
Hexafluorophosphate anions, solvent molecules, hydrogen atoms and disorder have been omitted for clarity (the residue containing a complete CuL2
unit was chosen to display for complex 3f). Key bond lengths (Å) and angles (°): 1·PF6; Cu–N1 2.045(2), Cu–N2 2.030(2), Cu–N3 2.055(2), Cu–N4
2.027(2), N1–Cu–N2 83.23(8), N3–Cu–N4 83.10(9), 3a (’ denotes symmetry operator 1 − x, y, 1/2 − z); Cu–N1 2.055(3), Cu–N2 2.026(3), N1–Cu–N2
83.11(11), 3f; Cu1–N1 2.028(3), Cu1–N2 2.025(3), Cu2–N3 2.030(2), Cu2–N4 2.022(2), Cu2–N5 2.016(2), Cu2–N6 2.034(2), Cu3–N7 2.015(3), Cu3–
N8 2.006(2), N1–Cu1–N2 84.68(18), N3–Cu2–N4 84.63(9), N5–Cu2–N6 84.34(9), N7–Cu3–N8 83.73(13), 3g (’ denotes symmetry operator 1 − x, y,
3/2 − z); Cu–N1 2.000(2), Cu–N2 2.112(2), N1–Cu–N2 82.88(8).
ing, rocking and wagging distortions are observed.29 It is poss- increased compared to [bis(2,9-dimethyl-1,10-phenanthroline)
ible that the strong π-stacking interaction, which arises only copper(I)] chloride (90 ns)31 which features 2,9-dimethyl substi-
when 2,9-diaryl substituents are present, increases the effect of tuents that are incapable of participating in π-stacking. The
intramolecular interactions, such that intermolecular forces flattening distortion in complex 3a is similar to that of
do not dominate the influences on the metal coordination geo- complex 1·PF6 (Table 1). Both of these complexes feature aryl
metry and overall structure of the complex ions.
substituents with similar steric bulk (p-OMeC6H4 and
Stronger intramolecular interactions derived from p-CF3C6H4). Due to the low steric bulk, both of these substitu-
π-stacking may also explain the increased photoexcited state ents allow efficient π-stacking interactions. The flattening dis-
lifetimes observed for several [bis(2,9-diaryl-1,10-phenanthro- tortion was more pronounced in complexes 3f and 3g. Whilst
line)copper(I)]+ complexes when compared with related com- this is associated with the increased steric bulk on the 2,9-
plexes that do not feature 2,9-diaryl substituents. The excited diaryl substituents the intra- and intermolecular influences are
state lifetimes for complex 1·PF6 (270 ns)5a and [bis(2,9-diphe- complex.
nyl-1,10-phenanthroline)copper(I)] tetrafluoroborate (310 ns)30
which both feature 2,9-diaryl substituents are significantly that of complex 1·PF6 and these complexes also exhibit similar
The rocking distortion for complex 3a was also similar to
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Table 1 Quantification of the distortions within the coordination spheres of complexes 1·PF6, 3a, 3f, and 3g and related complexes. The flattening
distortion is defined by the interplanar angle between coordination planes for each ligand. The rocking distortion is defined by the angle between a
centroid between C5 and C6, the metal centre and the nitrogen atoms on the other ligand. The wagging distortion is defined as the distance
between a least squares plane of the phenanthroline backbone of one ligand and the nitrogen atoms of the other ligand. The π-stacking distance
refers to the distance between a ring centroid of the aryl substituent on one ligand and the least squares plane of the phenanthroline backbone of
the other
[Cu]
Flattening distortion (°) Rocking distortion (°)
Wagging distortion (Å)
π-Stacking distance (Å)
Cu–N distances (Å)
2.096(1)–2.129(1)
6a
88.92(5)
73.70 (17)
54.4(5)
136.26(4) & 138.90(3)
133.19(3) & 142.38(4)
113.57(13) & 158.59(13)
110.29(13) & 164.29(13)
139.6(2) & 139.6(2)
−1.322(2) & 1.519(2)
—
−1.076(2) & 1.7371(19)
−0.134(8) & 2.365(6)
6b
3.463(3) & 3.639(3)
2.017(4)–2.120(5)
2.044(8) & 2.054(8)
2.007(8)–2.071(5)
−0.147(8) & 2.228(7)
7·ClO4
−1.058(8) & 1.058(8)
—
139.3(2) & 139.3(2)
−1.060 (8) & 1.060(8)
7·CuBr2
1·PF6
78.8(3)
69.326(2)
123.9(2) & 149.88(16)
119.537(2) & 157.0082(9)
124.569(3) & 150.259(1)
121.5(2) & 155.22(18)
119.737(2) & 155.445(1)
137.659(8) & 137.659(8)
137.770(8) & 137.770(8)
135.883(16) & 139.374(8)
137.43(2) & 138.198(11)
137.211(8) & 137.211(8)
138.134(11) & 138.134(11)
109.364(7) & 162.497(3)
−0.560 (10) & 1.925(9)
−0.416046(13) & 1.93495(6)
−0.80242(3) & 1.61656(6)
−0.495(12) & 1.879(11)
−0.61598(2) & 1.76772(7)
−1.1938(4) & 1.1938(4)
−1.1819(3) & 1.1819(3)
−1.0427(2) & 1.3268(3)
−1.15031(18) & 1.20839(18)
−1.2199(3) & 1.2199(3)
−1.2150(3) & 1.2150(3)
−0.150878(11) & 2.00949(14)
—
3.41379(10) & 3.4001(1) 2.027(2)–2.055(2)
1·BF4
3a
3f
69.3(4)
69.081(4)
63.551(11)–66.94(3)
1.396(15)
3.3836(1)
3.5862(6)–3.8021(9)
2.052(6) & 2.067(7)
2.026(3) & 2.055(3)
2.0066(4)–2.0337(4)
3g
64.195(8)
3.5378(3)
2.0000(2) & 2.112(2)
π-stacking distances to each other (Table 1). In contrast, the with related homoleptic bis(phenanthroline)copper(I) com-
rocking distortion is increased in complex 3g. The gem- plexes that feature less bulkier ligands than phenanthroline 5f.
dimethyl groups present in complex 3g provide an increase in Such complexes typically participate in ligand substitution
the π-stacking distance, however, a more pronounced rocking reactions in the presence of even less bulky 1,10-phenanthro-
distortion is observed. The rocking distortion in complex 3f is line-based ligands according to the HETPHEN (heteroleptic
significantly reduced and a longer π-stacking distance is bisphenanthroline complexes) principle.32 Armaroli and co-
observed. Potentially, the greater steric bulk, particularly of the workers also reported extended timeframes for the synthesis of
m-(t-Bu) groups and the symmetrical distribution of the bulk complex 3f and suggested that these observations derive from
almost completely prevents the rocking distortion in 3f.
the steric bulk of the ligand, however, the solid state structure
Wagging distortions follow a similar trend with the extent was not reported in their study. Notably, the solid state struc-
of the distortion for complex 3a remaining similar to that of ture reported here suggests that the bulk of the 2,9-diaryl sub-
complex 1·PF6 and complex 3f displays more limited distor- stituents in complex 3f may be responsible for the kinetic
tion. This potentially derives from the steric parameters of the locking of the ligands within the complex. The high steric con-
respective aryl substituents. Interestingly, complex 3g features gestion in this complex 3f, discussed above, supports this.
the most significant distortion. This potentially originates
Photophysical studies
from the unsymmetrical steric bulk of the 2,9-diaryl substitu-
ents with the orientation of the gem-dimethyl group forcing a Selected photophysical data are reported in Table 2. The
more significant wagging distortion.
absorption spectra of the homoleptic copper(I) complexes,
Armaroli and co-workers previously reported complex 3f measured from diluted dichloromethane solutions, were typi-
and demonstrated its kinetic inertness to ligand dis- cally quite similar and with most complexes exhibiting strong
sociation.17 These researchers demonstrated that the complex absorption bands between 250–400 nm, which are attributed
was inert to ligand exchange even in the presence of excess to ligand centred (LC) π–π* transitions (Fig. 4 and ESI†). The
1,10-phenanthroline, over extended time frames and at reflux complexes also exhibited broad absorption bands of weaker
in dichloromethane or acetonitrile despite the lack of steric intensity between 420–600 nm. These respective bands are
bulk in unsubstituted 1,10-phenanthroline. This contrasts attributed to excitation to the MLCT manifold.1 In each case,
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Table 2 Electronic absorption and luminescence data for copper-com-
plexes 1·PF6, and 3a–h. Measurements were performed in CH2Cl2 at
298 K unless otherwise noted
Table 3 Electrochemical data for copper complexes 1·PF6 and 3a–h in
degassed CH3CN at ambient temperature. All potentials are given in
volts (V) versus the saturated calomel electrode (SCE)
Absorption
Excitation
Emission
[Cu]
E1/2 (MII/MI)
a
1
2
3
4
5
6
7
8
9
1·PF6
3a
3b
3c
3d
3e
3f
+0.62
+0.89
+0.78
+0.68
+0.51
+0.66
+0.69
+0.68
+0.79
λmax
(nm)
104 ε
λmax b deaerated
(nm)
λmax
[Cu]
(M−1 cm−1
)
λexc (nm)
(nm)
1·PF6
443
577
436
563
439
567
453
587
473
450
571
451
574
524
560
0.404
0.235
0.411
0.203
0.388
0.159
0.347
0.133
0.323
0.517
0.372
0.483
0.417
0.295
0.127
440
550sh, 716
705
—
3a
3b
3c
430
400
430
750
760
590
720
3g
3h
530, 715sh
3d
3e
430
400
580
750
750
785
(EI1=II = +0.62 V vs. SCE) (Table 3). However, there were several
notable deviations. Both complexes 3a and 3b, featuring 2,9-
2
3f
400
640
820
diarylphenanthroline ligands containing p-electron-withdraw-
ing substituents, featured higher oxidation potentials than
parent complex 1·PF6. Molecules 3c, 3e, 3f, and 3g, which
contain electron-donating aryl substituents, maintained oxi-
dation potentials similar to [Cu(dap)2]PF6 (1·PF6). In contrast,
complex 3d, which features o,p-dimethoxy-aryl groups, dis-
3g
3h
430
430
630
535, 715sh
760
560
a Measurements were performed at 77 K. b The shoulder in the emis-
sion spectrum is reported as “sh”
played a lower oxidation potential (EI1=II = +0.51 V vs. SCE),
2
while molecule 3h containing a carbazole moiety featured a
higher oxidation potential (EI1=II = +0.79 V vs. SCE).
2
On the basis of the photophysical data obtained for copper
complexes 3, we did not determine the excited state redox
potentials of these complexes.34
Photoredox catalysis
In 2015, Reiser and co-workers reported a photoredox-catalysed
ATRA reaction facilitated by [Cu(dap)2]Cl (1·Cl) that provided
notable divergent reactivity in comparison to [Ru(bpy)3]Cl2 (2)
(Scheme 1).5f The authors postulated that in addition to
serving as a classical photoredox catalyst, the copper complex
coordinates reactive intermediates derived from triflyl chloride
and, thus, is also involved in key-bond forming steps leading
to the formation of the chlorosulfonylation product.
Consistent with this postulate, these researchers also observed
that increasing steric bulk, both with respect to the copper
photoredox catalyst and the olefin substrate, disfavours the
chlorosulfonylation process relative to the chlorination
pathway.5f Consequently, we aimed to utilise this intriguing
reaction to indirectly investigate the influence of various struc-
tural and other fundamental properties of copper(I) complexes
3a–f on the outcomes of such ATRA processes. In order to com-
plete a more extensive study, [Cu(dap)2]Cl (1·Cl), [Cu(dmp)2]Cl,
and known heteroleptic copper(I) complexes 8a–d were also
included in this work (Scheme 3).35
Fig. 4 Absorption spectra of selected homoleptic copper(I) complexes
performed in CH2Cl2.
these broad absorption bands fall within the visible spectrum
and shows that these complexes can be excited by irradiating
with visible light for the purpose of photocatalysis.5a
Upon excitation to the MLCT manifold, the complexes are
poorly emissive in degassed dichloromethane solutions at
room temperature (see ESI†). For some complexes, a band of
weak intensity is visible between 600 and 800 nm, which is
1
mainly ascribed to emission from singlet MLCT as a result of
thermally activated delayed fluorescence (TADF) commonly
observed for copper(I) complexes.33
First, we investigated the copper-photoredox-catalysed reac-
tion of triflyl chloride with 1-octene (9) (Scheme 3A). All of the
Electrochemical studies
The oxidation potential of the MI/MII couple was measured complexes that were screened promoted a reaction, and in the
directly by cyclic voltammetry for copper(I) complexes 3 (and majority of cases these reactions proceeded to complete
compound 1·PF6). The majority of the synthesised copper(I) conversion.36–38 [Cu(dap)2]Cl (1·Cl) provided a ∼4 : 1 ratio of
complexes maintained reversible behaviour for the MI/MII compounds 11 and 10. Interestingly, the essentially isosteric
couple similar to that of the parent [Cu(dap)2]PF6 (1·PF6) complexes 3a and 3b, which both feature ligands containing
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and 3f afforded ∼1.5 : 1 and ∼1 : 3 ratios of chlorosulfonyla-
tion/chlorination products, respectively. [Cu(dmp)2]Cl and
heteroleptic copper(I) catalysts 8a–d, which are less bulky than
[Cu(dap)2]Cl, provided these chlorosulfonylation/chlorination
products in generally much higher ratios (∼5 : 1 to ∼13 : 1).
Next, 3-methyl-1-pentene (12) was employed as a substrate
and, consistent with our expectations, employing this more
sterically hindered olefin significantly biased these transform-
ations to heavily favour the chlorination pathway (Scheme 3B).
Specifically, homoleptic copper(I) catalysts 1·Cl, 3b, 3e furn-
ished chlorination product 13 essentially exclusively, while
complexes 3a, 3c, and 3f afforded ∼1 : 7, and ∼1 : 3, ∼1 : 9
ratios of chlorosulfonylation/chlorination products, respect-
ively. Interestingly, photoredox-catalyst 3d facilitated a ∼1.5:1
ratio of compounds 14 and 13. The reasons for this are
unclear. Although [Cu(dmp)2]Cl enabled the synthesis of com-
pounds 14 and 13 in a ∼1 : 1 ratio, heteroleptic copper(I) cata-
lysts 8a–d particularly favoured the formation of chloride 13.
The ability of several members of this class of copper-based
complexes to catalyse this ATRA reaction showcases the utility of
these complexes as photoredox catalysts. The simple variation of
the steric and electronic parameters of the copper complexes has
a pronounced effect on the outcome of the reaction. This demon-
strates the potential of these complexes for catalyst screening pur-
poses in other photoredox-catalysed processes, particularly those
in which inner-sphere reactivity is invoked.
Conclusions
We have synthesised a range of previously unreported homo-
leptic 2,9-diaryl-1,10-phenanthroline copper(I) complexes
(3a–h). These complexes were spectroscopically characterised
and the solid state structures of three of these compounds
were determined by single-crystal X-ray analysis. The quantifi-
cation of the distortion away from an ideal tetrahedral coordi-
nation geometry suggests that the 2,9-diaryl substituents play a
significant role in the overall structure of the complex.
Structure 3g displayed the most significant distortion. In
addition, our results suggest that the reported kinetic locking
of the ligands within complex 3f is consistent with the high
steric congestion observed in the solid state.17 The modular
approach employed for ligand synthesis allows access to a
range of electronically- and sterically-varied complexes.
Scheme 3 (A) Outcomes of copper(I) photoredox-catalysed ATRA reac-
tions featuring 1-octene (9). (B) Outcomes of copper(I) photoredox-cat-
alysed ATRA reactions featuring sterically hindered 3-methyl-1-pentene
(12). dmp = 2,9-dimethyl-1,10-phenanthroline. a Yields determined by
19F NMR spectroscopy with the aid of a calibrated internal standard
(average of 2 experiments).
The complexes can be photoexcited with visible light to
their MLCT manifold, thus allowing photocatalysed reactions
activated by visible light. These homoleptic copper(I) species,
p-electron-withdrawing aryl substituents afforded ∼1 : 1 and and various known heteroleptic copper(I) complexes, were also
∼2 : 1 ratios of chlorosulfonylation/chlorination products, evaluated in order to determine their capacity to promote
respectively. Importantly, these data suggest that electronic ATRA processes leading to trifluoromethylchlorosulfonation
factors may also play a significant role in controlling the viabi- and trifluoromethylchlorination products. The outcomes of
lity of the chlorosulfonylation pathway in this chemistry. these photoredox-catalysed reactions were generally consistent
Bulkier catalysts 3c and 3d, which feature ligands containing with the varying steric bulk of the copper complex employed.
m,p-(OMe)2- and o,p-(OMe)2-substituted aryl moieties, furn- However, our results reveal that electronic factors may also
ished ∼3 : 1 and ∼4 : 1 ratios of compounds 11 and 10, respect- play a significant role in controlling the viability of the chloro-
ively. In contrast, the more sterically encumbered species 3e sulfonylation pathway in this chemistry.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the University of Tasmania, School
of Natural Sciences – Chemistry, ATA Scientific (T.P.N.), RACI
(T.P.N.), and the Collier Charitable Fund (A.C.B.) for financial
support and PhosAgro/UNESCO/IUPAC for a Green Chemistry
for Life Grant (A.C.B.). T.P.N. thanks the Australian
Government for a Research Training Program Scholarship. We
thank the University of Tasmania Central Science Laboratory
for providing access to NMR spectroscopy services. This
research was undertaken, in part, using the MX2 beamline at
the Australian Synchrotron, part of ANSTO, and made use of
the Australian Cancer Research Foundation (ACRF) detector.
We thank reviewers for helpful comments.
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