Phosphine Evaluation on a New Series of Heteroleptic Copper(I) Photocatalysts with dpa Ligand [Cu(dpa)(P,P)]BF4

Article abstract of DOI:10.1002/ejic.202100634

Five new heteroleptic copper(I) complexes (C1-5) of the type [Cu(dpa)(P,P)]BF₄ based on dipyridylamine (dpa) as N,N ligand and commercial diphosphines as P,P ancillary ligands have been synthesised through a simple methodology with high yields. All complexes were thoroughly characterised by spectroscopic and spectrometric techniques, as well by theoretical calculations. These showed Metal to Ligand Charge Transfer (MLCT) absorptions in the 300–370 nm region, and emission in the 450–520 nm region with quantum yields and lifetimes that depend on the nature of the P,P ligand. The photocatalytic performance of copper(I) complexes C1-5 was evaluated for their use as photoredox catalysts in ATRA reactions, decarboxylative coupling and an Appel-type reaction. The use of readily available dpa as N,N ligand constitutes an attractive alternative to the well-established phenanthroline ligands typically used in photocatalysis.

Full text of DOI:10.1002/ejic.202100634

European Journal of Inorganic Chemistry
Accepted Article
Title: Phosphine evaluation on a new series of heteroleptic copper(I)
photocatalysts with dpa ligand [Cu(dpa)(P,P)]BF4
Authors: Marco A. Henriquez, Sebastian Engl, Pablo Jaque, Ivan A.
Gonzalez, Mirco Natali, Oliver Reiser, and Alan R. Cabrera
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100634
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01/2020

10.1002/ejic.202100634
European Journal of Inorganic Chemistry
FULL PAPER
Phosphine evaluation on a new series of heteroleptic copper(I)
photocatalysts with dpa ligand [Cu(dpa)(P,P)]BF4
Marco A. Henriquez,[a],[b] Sebastian Engl, [b] Dr. Pablo Jaque,[c] Dr. Ivan A. Gonzalez, [d] Prof. Dr. Mirco
Natali, [e] Prof. Dr. Oliver Reiser,* [b] and Dr. Alan R. Cabrera* [a]
[a]
[b]
[c]
[d]
[e]
Departamento de Química Inorgánica, Facultad de Química y de Farmacia, Pontificia Universidad Católica de Chile, Vicuña Mackenna
4860, Macul, Santiago, Chile. E-mail: arcabrer@uc.cl
Institut für Organische Chemie, Universität Regensburg, Universitätsstrasse 31, Regensburg, Germany, Postal code 93053. E-mail:
oliver.reiser@chemie.uni-regensburg.de
Departamento de Química Orgánica y Fisicoquímica, Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Sergio
Livingstone 1007, 8380492, Santiago, Chile.
Laboratorio de Química Aplicada, Instituto de Investigación y Postgrado, Facultad de Ciencias de la Salud, Universidad Central de
Chile, Lord Cochrane 418, Santiago, Chile.
Department of Chemical, Pharmaceutical and Agricultural Sciences (DOCPAS), University of Ferrara, Centro Interuniversitario per la
Conversione Chimica dell’Energia Solare (SOLARCHEM), sez di Ferrara, Via L Borsari 46, 44121 Ferrara, Italy.
Abstract: Five new heteroleptic copper(I) complexes (C1-5) of the type [Cu(dpa)(P,P)]BF4 based on dipyridylamine
(dpa) as N,N ligand and commercial diphosphines as P,P ancillary ligands have been synthesised through a simple
methodology with high yields. All complexes were thoroughly characterised by spectroscopic and spectrometric
techniques, as well by theoretical calculations. These showed Metal to Ligand Charge Transfer (MLCT) absorptions
in the 300–370 nm region, and emission in the 450–520 nm region with quantum yields and lifetimes that depend
on the nature of the P,P ligand. The photocatalytic performance of copper(I) complexes C1-5 was evaluated for
their use as photoredox catalysts in ATRA reactions, decarboxylative coupling and an Appel-type reaction. The
use of readily available dpa as N,N ligand constitutes an attractive alternative to the well-established phenanthroline
ligands typically used in photocatalysis.
1. Introduction
One of the long-standing goals in organic chemistry has been the search for new synthetic routes that are
sustainable, avoid toxic reagents, and exploit renewable energy sources in order to meet the requirements of green
chemistry.[1] Within this context, a promising alternative to the thermal initiation of reactions has been the use of
visible light as a mild and selective source for activating organic molecules.[2] However, most of the organic
substrates only absorb in the ultraviolet (UV) region, and for this reason, photosensitizers and photoredox catalysts
capable of absorbing energy in the visible region have been developed to trigger the desired reaction on given
organic substrates. Ruthenium and iridium complexes have become the most popular photocatalysts due to their
strong absorbance in the visible region, long excited state lifetime and high oxidation and reduction potentials that
allow efficient energy or electron transfer to various organic substrates.[3] However, these metals are rare and
expensive,[4] inspiring scientists to replace them with more abundant and environmentally friendly alternatives.[5]
With this in mind, the use of copper complexes in photocatalysis has been recently developed by several groups,
and especially, homoleptic copper complexes have been widely used for atom transfer radical additions (ATRA),
allowing the difunctionalization of double bonds with various functionalities.[6]
1
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Among the homoleptic Cu(I) complexes used, [Cu(dap)2]Cl (dap = 2,9-(p-anisyl)-1,10-phenanthroline)[7] has
been arguably the most versatile, complying with the three key features of a suitable photocatalyst, i.e. absorbing
in the visible range (437 nm), possessing a strong excited state reduction potential (–1.43 V vs. SCE), and a
sufficiently long lifetime (270 ns in CH2Cl2) in the excited state to enable efficient bimolecular processes.[8]
Nevertheless, the synthesis of the dap ligand requires several steps[8g-i] making readily available alternative
desirable.[9]
Heteroleptic copper complexes have turned into a promising alternative due to their facile synthesis by ligand
exchange with many commercially available bidentate P,P ligands.[10] Furthermore, their typical bulky structures
provide substantial stability to the metal centre at the excited state level by preventing exciplex formation.[4, 11]
Collins and coworkers have reported a library of heteroleptic complexes combining various bidentate N,N- and P,P-
ligands.[12] In this way, effective photocatalysts both for single electron transfer (SET) and energy transfer (ET)
processes could be identified for several photocatalytic reactions.[10a, 12-13] Likewise, other groups have reported
several heteroleptic complexes capable of achieving C-C, C-Cl, C-S type couplings or exchange of functional
groups. [14] Typically, the N,N-ligands used are based on the phenanthroline or bipyrdine core, representative
examples are shown in Figure 1.
Figure 1. Heteroleptic Cu(I) photocatalyst.
In this work, we evaluate dipyridylamine (dpa) as a modular and readily available N,N-ligand in heteroleptic
complexes with five commercial phosphines (POP, XantPhos, dpbz, dppf, S-BINAP) as potential photocatalysts,
combining electrochemical and photophysical characterisation, density functional theory (DFT) together with time-
dependent (TD-DFT) calculations, and applications in photoredox catalysis.
2. Results and Discussion
2.1. Synthesis and structural characterisation
Five new heteroleptic copper(I) complexes of the type [Cu(dpa)(P,P)]BF4 were synthesised in two steps following
15]
methodologies reported in the literature (Scheme 1):[8c,
Equimolar amounts of dpa[16] ligand and
[Cu(CH3CN)4]BF4 were stirred in CH2Cl2 under N2 atmosphere at 20 °C, followed by addition of the P,P ancillary
2
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10.1002/ejic.202100634
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ligand. The resulting complexes were isolated by crystallization from CH2Cl2/toluene mixtures in 85–95 % yield,
showing high air and thermal stability both in the solid-state as well as in solution for several days.
Scheme 1. Synthetic route to obtain heteroleptic Cu(I) complexes C1-5.
NMR characterisation of C1-5 shows the successful coordination of the ligands to the metal centre as judged
by the chemical shift in the 1H spectra of N-H (close to 9.0 ppm) in all complexes compared to that of the dpa free
ligand (close to 8.0 ppm). The phosphine signals in the 31P NMR of all complexes exhibits a chemical shift compared
to the free ligands and are observed in the range of 0 ppm to -14 ppm (see the ESI for the detailed characterisation).
The molecular structures of the complexes C1-5 were obtained by XRD (Figure 2, Table 1). These complexes
exhibit pseudo-tetrahedral coordination around the copper(I) mononuclear centre, surrounded by one dpa ligand
and one bidentate phosphine ligand. The dpa ligands have small bite angles, in the range of 91°–94°, whereas the
bidentate P,P ligands display larger bite angles, in the range of 100°–115°. The larger bite angle observed for
XantPhos (C2) than for POP (C1) ligand can be attributed to the higher rigidity of the former due to the methylated
cycloalkane. In general, the difference between the bite angles of the N,N, and P,P ligands induce the distortion
observed to the tetrahedral geometry.
Figure 2. ORTEP plot of complexes C1-5, with partial numeration scheme. Hydrogen atoms and counter ion were
removed for clarity. Thermal ellipsoids were drawn with 30 % probability
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Table 1. Selected X-Ray analysis data for C1-5.
C1 C2 C3 C4
Bond Angle [deg]
92.24 92.03 94.04 91.82
C5
N1–Cu–N3
P1–Cu–P2
92.40
110.58 115.77 90.93 100.10 110.76
Dihedral Angle [deg]
N1–Cu–N3 / P1–Cu–P2 89.47
88.69 80.57 82.50
78.38
Bond Length [Å]
Cu–N1
Cu–N3
Cu–P1
Cu–P2
2.039
2.069
2.270
2.261
2.051 2.018 2.059
2.068 2.025 2.065
2.254 2.249 2.265
2.267 2.249 2.299
2.061
2.061
2.279
2.279
Based on the principal planes between the coordination atoms (N1–Cu–N3/ P1–Cu–P2), dihedral angles were
calculated. From the point of view of the P,P ligand, the value is lower for the dppf ligand (C5) and larger for the
POP ligand (C1), where the latter is close to the value expected for perpendicular planes. This suggests that the
distortion from the tetrahedral geometry likely reflects the rigidity of the P,P ligand, where all values are similar to
those reported in the literature for analogue complexes of the type [Cu(N,N)(P,P)]+.[17] Regarding the bond
distances, these are very similar among each complex. Small distances are observed between the nitrogen and
the copper atoms (ca 2.05 Å), while longer lengths are seen for the Cu-P bond (in the range 2.24 Å – 2.29 Å), in
agreement with values already reported.[18]
2.2. Electrochemical, photophysical and TD-DFT characterisation
The electrochemical properties of the complexes C1-5 were measured by cyclic voltammetry (CV) in CH2Cl2 as
solvent (Table 2, Figure S30). Complexes C1, C2, and C4 exhibit one quasi-reversible oxidation process with a
half-wave potential at ca 0.60 V vs. SCE, attributable to the Cu(I)/Cu(II) transition. For C3, this process occurs at
less positive potentials (0.293 V vs. SCE) and is followed by an additional irreversible wave. The lower value for
the Cu(I)/Cu(II) oxidation can be related to the reduced electron-withdrawing character of the diphosphine dpbz
ligand as a consequence of the reduced bite angle imposed by the steric constrain of the ligand (cf. Table 1). In
the case of C5, a reversible oxidation process is observed at E1/2 = 0.88 V vs. SCE, followed by an intense and
irreversible wave with a peak potential at 1.57 V vs. SCE. As observed for analogous Cu(I) complexes featuring a
dppf ligand,[19] the first process can be assigned to oxidation of the ferrocenyl moiety. In contrast, the irreversible
process at more positive potentials can be attributed to the oxidation of the copper metal centre, reflecting the
electrostatic repulsion from the charged ferrocenyl moiety.[19] A comparison of the oxidation potentials of C1-4 with
those measured for the complexes reported by Hu and co-workers featuring a phenanthroline-based N,N ligand[14e]
shows that the Cu(I) oxidation occurs at less positive values. This can be explained considering the influence of
the dpa ligand being apparently less electron donating than polypyridine ligands.
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Table 2. Summary of CV measurements, TD-DFT and excited state reduction potentials data of C1-5.
a
E1/2
[V]
[CuI]*/[Cu]II c
[V]
E
[eV]
P,P bite angle
l
ƒ
Complex
C1
[deg]
[nm] (Character)
4.28
3.46
290 (LC+LLCT)
358 (MLCT, Cuàdpa)
0.0793
0.0526
0.630
110.58
115.77
90.93
-1.75
-1.77
-1.48
4.37
3.50
284 (LLCT)
354 (MLCT, Cuàdpa)
0.0789
0.0509
C2
0.690
0.293
4.24
293 (LC)
378 (MLCT, Cuàdpa)
0.1611
0.0456
C3
0.785 b 3.28
4.35
3.56
285 (LLCT)
348 (MLCT, Cuàdpa)
0.0912
0.0528
C4
C5
0.550
100.10
110.76
-1.63
-1.88
2.99 415 (MLCT, dpaCuàP,P) 0.0308
4.36
3.58
2.41
284 (LC+LLCT)
354 (MLCT, Cuàdpa)
513 (d-dp*)
0.0798
0.0233
0.0005
0.881
1.575 b
a Cyclic voltammograms profiles recorded in anhydrous CH2Cl2 solution of C1-5 (1 mM) with 0.1 M TBAPF6 as supporting electrolyte at a scan
rate of 0.1 V/s. Three-electrode cell configuration (Pt disc working electrode, saturated Ag/AgCl reference electrode and Pt wire counter
electrode). E1/2 values referred to SCE. b Irreversible wave, peak potential given. E = Electronic transition energy, l = wavelength, ƒ = oscillator
strength. c Excited state reduction potentials were estimated via Rehm-Weller equation (see ESI).
The UV-Visible normalized spectra of the five heteroleptic Cu(I) complexes in CH3CN as solvent are depicted
in Figure 3. All the complexes showed intense absorption bands in the 200–300 nm region, plausibly attributable
to spin-allowed π-π* ligand centred transitions. Additionally, weak and energetically lowest absorption bands
between 300–370 nm are assignable to spin- allowed d(Cu)-π*(N,N) MLCT transitions involving the dpa ligand. Similar
spectral patterns were observed in copper(I) complexes of the type [Cu(NHC)(dpa)]+ reported by Costa’s group.[16]
Furthermore, C5 exhibits a weak broader absorption band with a maximum at 440 nm, which can be assigned to
Laporte forbidden d-d transitions involving the iron(II) centre of the dppf ligand (see ESI for more detail).
Figure 3. Normalized UV-Visible spectra for C1-5 in CH3CN at room temperature.
The vertical excited-states properties computed using the TD-DFT framework unveil the origin and the
underlying electronic transitions in the complexes C1-5. The electronic transitions for C1, provided by TD-DFT
calculations, are depicted in Figure 4 as a representative example, and the data for complexes C1-5 are
summarized in Table 2 and Table S1. Overall, it can be noted that the highest energy band is mainly dominated by
the ligand-centred (LC) π-π* transition of the dpa ligand mixed, to a minor extent, with a ligand-to-ligand charge
5
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transfer (LLCT) transition involving the dpa and P,P moieties. The weak band at lower energy is mainly
characterised by metal-to-ligand charge transfer (MLCT) transition from Cu(I) to dpa ligand (cf. Table 2). However,
one exception to this general behaviour is found, specifically, in C4. In this case, an additional MLCT band at lower
energy is observed, where its character involves a charge transfer from the dpa-Cu(I) fragment to the P,P ligand
(See Figure S31).
Figure 4. Electronic transitions character for C1, obtained by TD-DFT calculations.
The photoluminescence properties of all complexes were studied in degassed CH2Cl2 solution at room
temperature and 77 K in a 4/1 ethanol/methanol glassy matrix (Table 3, Figure 5). All complexes C1-5 exhibit
emission in CH2Cl2 at room temperature (Figure 5a). Quantum yields of 3.1 % and 1.9 % were measured for C1
and C2, respectively, whereas quantum yields <0.1 % were recorded for the remaining complexes (see Table 3).
Lifetimes of 8.6, 7.9, and 20.8 μs were measured for C1, C2, and C4, respectively, while for the other complexes,
the lifetimes were below the instrumental resolution (<10 ns). The nature of the emissive state can be discussed in
relation to the luminescence spectra measured at 77 K (Figure 5b). Complexes C1, C2, C3, and C5 show an
emission which undergoes a substantial blue-shift when recorded in the glassy matrix at 77 K. This can be attributed
to the rigidochromic effect exerted by the solid-state matrix and is characteristic of excited states of charge transfer
nature. Accordingly, the corresponding luminescence can be attributed to phosphorescence from a triplet MLCT
involving a formal Cu(II) centre and a reduced dpa ligand. However, the appreciably long lifetime observed in C1-
3 at 77 K (hundreds μs, Table 3) strongly suggests that contribution of a triplet LLCT state cannot be completely
ruled out.[20] This attribution agrees with the trend in the energy of the luminescence maxima that reflects the
observed trend in the oxidation potential of the Cu(I) centre. Furthermore, these considerations well comply with
the quantum yield and lifetime data measured at room temperature for complexes C1 and C2 and the
corresponding radiative constants (~3x104 s-1), whose values are in the order of those found for similar copper(I)
complexes.[21] On the other hand, both reduced quantum yields and lifetimes for complexes C3 and C5 seem to be
characteristic of these types of complexes and thus correlate with the nature of the P,P ligand. For C3, the small
P–Cu–P angle (~90°) and smaller dihedral angle (see Table 1) very likely promote efficient flattening distortion
within the excited state, thus improving non-radiative deactivation routes. On the other hand, in C5, the ferrocenyl
moiety within the dppf ligand is expected to quench the triplet MLCT excited state via energy transfer.[19] This
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quenching process maintains its intrinsic efficiency in the glassy matrix at 77 K, thus explaining the short lifetime
measured for C5 also under these latter conditions.
Table 3. Summary of emission properties of C1-5.
CH2Cl2
77 K
λ [nm] Φ [%]
λ [nm]
t [μs]
8.6
t [ms]
0.46
0.41
1.6
C1
C2
C3
C4
C5
520
504
590
567
449
3.1 a
1.9 a
453
455
7.9
0.08 b
0.09 b
0.02 c
-
507
d
20.8
500,539,581,634
380, 399, 420
15.1
d
d
-
-
a using fluoresceine in 0.1 M NaOH (Φ = 0.94) as the standard. b using Ru(bpy)32+ in H2O (air-equilibrated, Φ = 0.028) as the standard. c using
quinine sulfate in 0.05 M H2SO4 (Φ = 0.53) as the standard. d below the detection limit of the laser flash photolysis (ca 10 ns).
Figure 5. Emission spectra of C1-5 in a) N2-purged CH2Cl2 at room temperature and b) 4/1 ethanol/methanol
glassy matrix at 77 K
The emission of complex C4 at 77 K occurs at similar energies as observed at room temperature. It displays
a structured profile with three distinct maxima suggesting a different character of the emitting excited state with
respect to the other Cu(I) complexes of the series. Interestingly, this phosphorescence is similar to that found for a
+
related Cu(BINAP)2 complex[22] and can be assigned to a triplet LC involving the binaphthyl group of the P,P ligand.
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The observed long lifetime (15.1 ms), characteristic of spin-forbidden deactivation from LC excited states, is entirely
consistent with this attribution. An emitting state of LC character even in CH2Cl2 at room temperature well explains
the small quantum yield experimentally measured. The latter, combined with the lifetime of 20.8 μs, indeed provides
a radiative constant of 43 s-1, which is considerably smaller than that calculated for the triplet MLCT state in
complexes C1 and C2 (see above) and thus consistent with the different nature of the excited state in C4
responsible for the observed luminescence.
2.3. Photocatalytic studies
To study the photocatalytic activity of the synthesised complexes, the ATRA reaction between styrene (1) and CBr4
(2) was chosen as the model reaction (Table 4). Importantly, the desired product 3 is not obtained by omitting the
catalyst (Table 4, entry 1). Established Cu-photocatalysts like [Cu(dap)2]Cl or [Cu(cup)(rac-BINAP)]PF6 are capable
of delivering the 3 in good to excellent yields (Table 4, entries 2-3). Hence, we subjected the complexes C1-5 to
the reaction (entries 4-8), which revealed that C4 indeed a competent catalyst in the title reaction, giving rise to 3
in excellent yield of 99 % (Table 4, entry 7). Notably, this reaction can also be scaled to gram-quantities of product
(Scheme 2, A) showcasing the viability of the catalyst for preparative purposes. In line with the observed
electrochemical and photophysical properties (Table 2 and 3), the improved activity of complex C4 is associated
with S-BINAP as the P,P ancillary ligand which confers rigidity to the complex. As a consequence, a long excited-
state lifetime arises, which seems to be pivotal in favouring enhanced photocatalytic activity via bimolecular routes.
No asymmetric induction in 3 was observed, suggesting that the introduction of the CBr3 group into the product
does not benefit from the chiral nature of catalyst C4.
Table 4. ATRA test reaction evaluation using C1-5.a
Entry
Catalyst
LEDx
455
530
455
455
455
455
455
455
Yield % b
n.r
1
2
3
4
5
6
7
8
without
Cu(dap)2Cl
99
71[9]
[Cu(cup)(rac-BINAP)]PF6
[Cu(dpa)(POP)]BF4 (C1)
[Cu(dpa)(XantPhos)]BF4 (C2)
[Cu(dpa)(dpbz)]BF4 (C3)
[Cu(dpa)(S-BINAP)]BF4 (C4)
[Cu(dpa)(dppf)]BF4 (C5)
37
15
51
99
15
a styrene (0.5 mmol, 1.0 equiv), CBr4 (0.5 mmol, 1.0 equiv), catalyst (1 mol%) in CH3CN (dry, degassed, 3 mL); Irradiation at X nm (LED) under
N2 atmosphere for 24 h at 20 °C. b 1H-NMR Yield was determined using 1,3,5-trimethoxybenzene as an internal standard.
Further utilisation of C4 in other types of photochemical reactions was investigated (Scheme 2).
Chlorosulfonylation of styrene was achieved in 93 % similar to the previous report by [Cu(dap)2]Cl (96 %).[9] Again
scale-up of the transformation is possible accessing gram-amounts of 6 with only a small decrease in reaction yield.
Remarkably, subjecting alkynes to the chlorosulfonylation reaction, the selective formation of the E isomer is
observed in good yields (7a-7c), contrasting [Cu(dap)2]Cl which produces E/Z mixtures of products. It is assumed
that Cu(N,N)(P,P)+ complexes are able to promote an additional energy transfer pathway to convert the Z product
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to the corresponding E isomer, which is in accordance to recent reports by Hu and coworkers[23] and Collins and
coworkers.[24] C4 was moderately effective in the visible-light Appel-type reaction (Scheme 2, C), converting alcohol
8 to halide 9 in 56 % yield, which nevertheless constitutes a higher yield compared to [Cu(dap)2]Cl (40 %).[9] Finally,
N-(acyloxy)phtalimides derivatives have been used as radical source in carbon-carbon bond forming processes for
several groups.[25] Indeed, C4 achieved the decarboxylative fragmentation of 10 and subsequent coupling with 11
to access Csp3-Csp coupling product 12 in 52 % yield, comparable to other heteroleptic Cu(I) photocatalysts reported
by Collins in 2018.[12]
Scheme 2. (A) ATRA test reaction on gram scale. Use of [Cu(dpa)(S-BINAP)]BF4 in different photo-redox reactions.
(B) Chlorosulfonylation reaction (C) Appel-type reaction (D) Decarboxylative coupling reaction. For more details
see ESI.
3. Conclusion
Five new heteroleptic Cu(I) complexes containing dpa N,N ligand and P,P auxiliary ligand have been successfully
synthesised, with yields between 85–95 %, and structurally characterised by NMR, FT-IR, HRMS, and XRD. All
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structural characterisations are in agreement with the formation of mononuclear heteroleptic complexes. The
electrochemical and optical characterisation have been examined, and TD-DFT calculations supported and
assigned the character of observed UV-Vis bands. C1-5 exhibited redox potentials in the range 0.293–1.575 V v/s
SCE, MLCT band centred around 300–370 nm, and excited-state lifetimes of 8.6–20.8 μs. The longer lifetime that
has been observed for C4 is pivotal towards profitable photocatalytic applications as demonstrated for five repre-
sentative transformations, rivalling other homo- and heteroleptic Cu(I)-photocatalysts with respect to versatility and
efficiency. Given the facile availability of the dipyridylamine (dpa) on multigram scale, this ligand appears to be an
attractive alternative to the more established N,N-dimine and in particular to the widely used 2,9-p-anisyl-
phenanthroline (dap) ligand.
4. Experimental Section
All reagents were purchased from commercial sources, unless otherwise specified and used as received. Based
on the methodology reported by Sekar et al.[26] we developed a new synthetic route to obtain N,N dpa ligand in
higher yields (81 %). For more details, see ESI. NMR spectra were recorded on NMR Bruker AV 400 MHz and AV
300 MHz. Chemical shifts are given in parts per million relatives to TMS (1H and 13C, δ(SiMe4) = 0). Most NMR
assignments were supported by additional 2D experiments. HRMS-ESI-MS experiments were carried out using a
Thermo Scientific Exactive Plus Orbitrap Spectrometer. FT-IR spectra were recorded on a Bruker Vector 22
Spectrophotometer using Merck GF-254 type 60 silica gel. Column chromatography was carried out using Merck
silica gel 60 (70–230 mesh). The cyclic voltammograms were recorded using a PalmSens 3 Instruments
Potentiostat, a platinum disc working electrode with an area of 0.02 cm2, a saturated Ag/AgCl reference electrode
and a platinum wire counter electrode. All electrochemical measurements were carried out in anhydrous
dichloromethane solutions of Cu(I) complexes (1 mM) with tetrabutylammonium hexafluorophosphate (TBAPF6)
(0.1 M) as the supporting electrolyte at a scan rate of 0.1 V s-1. UV-Vis absorption spectra were recorded on a
Jasco V-570 UV/Vis/NIR spectrophotometer. Photoluminescence spectra were taken on an Edinburgh Instrument
spectrofluorometer. Emission lifetimes were taken on a laser flash photolysis apparatus comprised of a Continuum
Surelite II Nd:YAG laser (excitation at 355 nm, FWHM = 6–8 ns, was provided by THG from the 1064-nm
fundamental). Light emitted by the sample was focused onto the entrance slit of a 300 mm focal length Acton
SpectraPro 2300i triple grating, flat field, and double exit monochromator equipped with a photomultiplier detector
(Hamamatsu R3896). Signals from the photomultiplier were processed by means of a TeledyneLeCroy 604Zi (400
MHz, 20 GS/s) digital oscilloscope. CH2Cl2 solutions of metal complexes C1-5 were purged using nitrogen gas for
20 minutes before steady-state and time-resolved emission measurements. Single-crystal X-ray diffraction data
were recorded on an Agilent Technologies SuperNova diffractometer with Cu Kα radiation (λ = 1.54184 Å). Crystals
were selected under mineral oil, mounted on MicroMount loops, and quench-cooled using an Oxford Cryo-system
open flow N2 cooling device. The structure was solved with the ShelXT[27] structure solution program using the dual
solution method and by using Olex2[28] as the graphical interface. The model was refined with version 2018/3 of
ShelXL[29] using Least Squares minimisation. Full details can be found in the independently deposited
crystallography information files (cif), CCDC numbers: 2098431 for C1, 2098436 for C2, 2098449 for C3, 2098451
for C4, and 2098493 for C5.
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4.1. Synthesis of the compounds
General synthetic procedure of complexes: dpa ligand (1.0 equiv) in CH2Cl2 was added dropwise to a solution
of [Cu(CH3CN)4]BF4 (1.0 equiv) in CH2Cl2 under nitrogen atmosphere. The reaction mixture was stirred for 30
minutes at 20 °C. After this time, a diphosphine ligand (1.0 equiv) solution in CH2Cl2 was added dropwise to the
reaction mixture and stirred for additional 30 minutes. Finally, the solvent was removed under vacuum, and the
crude product was purified by crystallization using a CH2Cl2/Toluene mixture at -20 °C.
Complex C1: Isolated as white powder in 85 % yield (853.2 mg, 0.992 mmol). 1H-NMR (400 MHz, CDCl3, 298 K):
δ/ppm = 9.0 (s,1H, N-H), 7.70 (dd, J = 5.7 Hz, 1.7 Hz, 2H ), 7.50 (ddd, J = 8.9 Hz, 7.2 Hz, 1.9 Hz, 2H), 7.36 (d, J =
8.3 Hz, 2H), 7.34–7.26 (m, 6H), 7.23 (t, J = 7.5 Hz, 8H), 7.13 (dt, J = 7.1 Hz, 5.0 Hz, 8H), 7.06–6.98 (m, 4H), 6.91–
6.85 (m, 2H), 6.48 (t, J = 6.4 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ/ppm = 157.8 (t, JC-P = 6.2 Hz),
153.8, 147.1, 138.7, 134.3, 133.2 (t, JC-P = 8.2 Hz), 131.7, 131.1 (t, JC-P = 16.0 Hz), 129.9, 128.7 (t, JC-P = 4.7 Hz),
125.0, 124.6 (t, JC-P = 13.1 Hz), 120.1, 116.8, 115.8. 19F NMR (400 MHz, CDCl3, 298 K): δ/ppm = -151.17 (s, BF4),
11B NMR (128 MHz, CDCl3, 298 K): δ/ppm = -0.64 (s, BF4). 31P(1H) (160 MHz, CDCl3, 298 K): δ/ppm = -13.37 (s,
POP). HRMS (ESI): m/z [M]+ for C46H37CuN3OP2: Calculated = 772.1758, Found = 772.1758.
Complex C2: Isolated as white powder in 90 % yield (946.3 mg, 1.05 mmol) 1H-NMR (400 MHz, CD2Cl2, 298 K):
δ/ppm = 8.85 (s, 1H, N-H), 7.65 (dd, J = 7.8 Hz, 1.4 Hz, 2H), 7.52 (ddd, J = 8.8 Hz, 7.2 Hz, 1.9 Hz, 2H), 7.42 (dd,
J = 5.6 Hz, 1.2 Hz, 2H), 7.31 (t, J = 7.3 Hz, 4H), 7.24 (m, 4H), 7.17 (m, 20H), 6.55 (m, 2H), 6.42 (m, 2H), 1.76 (s,
6H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ/ppm = 155.3 (t, JC-P= 6.3), 154.2, 147.5, 139.6, 138.5, 134.0 (t, JC-
P= 1.6), 133.64 (t, JC- P= 8.1), 132.5 (t, JC-P= 16.1), 131.55, 130.46, 129.5, 129.3 (t, JC-P= 4.7), 128.72, 127.44, 125.8,
125.64 (t, JC-P= 2.4), 121.14 (t, JC-P = 12.8), 117.53, 116.10, 28.20. 19F NMR (400 MHz, CD2Cl2, 298 K): δ/ppm = -
150.61 (s, BF4). 11B NMR (128 MHz, CD2Cl2, 298 K): δ/ppm = -0.79 (s, BF4). 31P(1H) NMR (160 MHz, CD2Cl2, 298
K): δ/ppm = -13.15 (s, XantPhos). HRMS (ESI): m/z [M]+ for C49H41CuN3OP2: Calculated = 812.2021, Found =
812.2075.
Complex C3: Isolated as white powder in 86 % yield (771.5 mg, 1.00 mmol) 1H-NMR (400 MHz, CDCl3, 298 K):
δ/ppm = 9.00 (s, 1H, N-H), 7.55 (m, 4H), 7.49 (m, 2H), 7.23 (m, 26H), 7.02 (m, 2H), 6.90 (m, 2H), 6.42 (m, 2H).
13C{1H} NMR (100 MHz, CDCl3, 298 K): δ/ppm = 153.7, 147.8, 141.6 (t, JC-P = 33.6 Hz), 137.0, 135.0, 132.82–
132.3, 131.2, 130.2, 129.9, 129.0 (t, JC-P = 4.7 Hz), 116.4, 115.8. 19F NMR (400 MHz, CDCl3, 298 K): δ/ppm = -
151.25 (s, BF4), 11B NMR (128 MHz, CDCl3, 298 K): δ/ppm = -0.67 (s, BF4). 31P(1H) NMR (160 MHz, CDCl3, 298
K): δ/ppm = -7.06 (s, dpbz). HRMS (ESI): m/z [M]+ for C40H33CuN3P2: Calculated = 680.1446, Found = 680.1490.
Complex C4: Isolated as green powder in 92 % yield (1.04 g, 1.07 mmol) 1H-NMR (400 MHz, CDCl3, 298 K): δ/ppm
= 8.75 (s,1H, N-H), 7.79 (dd, J = 5.5 Hz, 1.8 Hz, 2H), 7.62–7.48 (m, 6H), 7.45–7.40 (m, 4H), 7.35–7.27 (m, 6H),
7.26–7.18 (m, 6H), 6.91 (t, 2H, J = 7.7 Hz), 6.82–6.75 (m, 4H), 6.70 (t, 2H, J = 6.3 Hz), 6.65 (t, 2H, J = 7.5 Hz),
6.53 (d, 2H, J = 8.6 Hz), 6.49–6.44 (m, 4H). 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ/ppm = 154.8, 147.7, 139.5,
139.4, 134.4 (t, JC-P = 9.7 Hz), 133.8 (t, JC-P = 4.0 Hz), 133.1, 132.9 (t, JC-P = 8.7 Hz), 132.2–131.8, 130.5, 129,7,
129.3–129.1, 128.9, 127.9, 127.6 (t, JC-P = 5.1 Hz), 127.2, 126.8, 126.7, 126.5, 117.5, 116.5. 19F NMR (400 MHz,
CDCl3, 298 K): δ/ppm = -151.13 (s, BF4). 11B NMR (128 MHz, CDCl3, 298 K): δ/ppm = -0.70 (s, BF4). 31P(1H) NMR
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(160 MHz, CDCl3, 298 K): δ/ppm = -1.15 (s, S-BINAP). HRMS (ESI): m/z [M]+ for C54H41CuN3P2: Calculated =
856.2066, Found = 856.2133.
Complex C5: Isolated as bright red powder in 85 % yield (971.9 mg, 1.12 mmol). 1H NMR (400 MHz, CDCl3, 298
K): δ/ppm = 8.97 (s,1H, N-H), 7.73 (dd, J = 5.6 Hz, 1.8 Hz, 2H), 7.55 (t, J = 7.5 Hz, 2H), 7.40 (m, 2H), 7.38–7.2 (m,
20 H), 6.61 (t, J = 6.3 Hz), 4.52 (s, 4H), 4.27 (s, 4H). 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ/ppm = 154.5, 147.68,
139.16, 133.48, 133.33, 133.17, 133.09, 133.01, 130.16, 128.91, 128.86, 128.82, 117.22, 116.27, 76.31(t, JC-P
=
19.1 Hz), 74.24, 72.30. 19F NMR (400 MHz, CDCl3, 298 K): δ/ppm = -151.34 (s, BF4). 11B NMR (128 MHz, CDCl3,
298 K): δ/ppm = -0.96 (s, BF4). 31P(1H) NMR (160 MHz, CDCl3, 298 K): δ/ppm = -13.87 (s, dppf). HRMS (ESI): m/z
[M]+ for C44H37CuFeN3P2: Calculated = 788.1108, Found = 788.1164.
4.2. General procedures for the photocatalytic studies
ATRA test reaction: A 10.0 mL flame-dried Schlenk tube with magnetic stirring bar was charged with CBr4 (0.5
mmol), copper catalyst (1 mol%), and 3 mL of CH3CN anhydrous solvent, sealed with a screwcap and subsequently
degassed by three consecutive freeze-pump-thaw cycles. Afterward, alkene (0.5 mmol) was added under N2, and
the screwcap was replaced with a Teflon sealed inlet for a glass rod, through which irradiation with LED (455 nm)
took place from above. At the same time, the reaction mixture was magnetically stirred at 20 °C for 24 h. The
reaction was monitored by TLC. Then, the volatiles were removed, and the crude concentrated. The residue was
purified by flash chromatography on silica gel (eluent hexanes/ethyl acetate, 9:1).
Chlorosulfonylation reaction over alkene: A 10.0 mL flame-dried Schlenk tube with magnetic stirring bar was
charged with TsCl (0.5 mmol), [Cu(dpa)(S-BINAP)]BF4 (1 mol%), and 3 mL of CH3CN:CH2Cl2 (1.5 mL: 1.5 mL)
anhydrous solvent, sealed with a screwcap and subsequently degassed by three consecutive freeze-pump-thaw
cycles. Afterward, alkene (0.5 mmol) was added under N2, and the screwcap was replaced with a Teflon sealed
inlet for a glass rod, through which irradiation with LED (455 nm) took place from above. At the same time, the
reaction mixture was magnetically stirred at 20 °C for 24 h. The reaction was monitored by TLC. Then, the volatiles
were removed, and the crude concentrated. The residue was purified by flash chromatography on silica gel (eluent
hexanes/ethyl acetate, 9:1).
Chlorosulfonylation reaction over alkyne: A 10.0 mL flame-dried Schlenk tube with magnetic stirring bar was
charged with TsCl derivative (0.5 mmol), [Cu(dpa)(S-BINAP)]BF4 (2.5 mol%), Na2CO3 (1.5 equiv), and 3 mL of
CH3CN:CH2Cl2 (1.5 mL: 1.5 mL) anhydrous solvent, sealed with a screwcap and subsequently degassed by three
consecutive freeze-pump-thaw cycles. Afterward, alkyne (0.5 mmol) was added under N2, and the screwcap was
replaced with a Teflon sealed inlet for a glass rod, through which irradiation with LED (455 nm) took place from
above. At the same time, the reaction mixture was magnetically stirred at 20 °C for 24 h. The reaction was monitored
by TLC. Then, the volatiles were removed, and the crude concentrated. The residue was purified by flash
chromatography on silica gel (eluent hexanes/ethyl acetate, 5:1).
Appel-type reaction: A 10.0 mL flame-dried Schlenk tube with magnetic stirring bar was charged with
tetrabromomethane (2) (331.6 mg, 1.0 mmol, 2.0 equiv), sodium bromide (102.9 mg, 1.0 mmol, 2.0 equiv),
[Cu(dpa)(S-BINAP)]BF4 (2.0 mol%) and dissolved in anhydrous DMF (3.0 mL, 0.17 M), sealed with a screw-cap
12
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and subsequently degassed by three consecutive freeze- pump-thaw cycles. Afterwards, 1-phenylethan-1-ol (8)
(61.1 mg, 0.5 mmol, 1.0 equiv) was added under N2, and the screwcap was replaced with a Teflon sealed inlet for
a glass rod, through which irradiation with LED (455 nm) took place from above. At the same time, the reaction
mixture was magnetically stirred at 20 °C for 24 h. The reaction was monitored by TLC. Afterwards, the reaction
mixture was quenched by addition of water (10 mL) and diethyl ether (10 mL). The layers were separated, and the
aqueous phase was extracted with diethyl ether (2x 10 mL). The combined organic layers were washed with
saturated Na2S2O3 solution (10 mL), brine (10 mL), dried over anhydrous Na2SO4 and concentrated in vacuum.
The residue was purified by flash column chromatography on silica gel (hexanes).
Decarboxylative coupling reaction: A 10.0 mL flame-dried Schlenk tube with magnetic stirring bar was charged
with 1,3- dioxoisoindolin-2-yl cyclohexanecarboxylate[12, 30] (10) (136.6 mg, 0.5 mmol, 1.0 equiv), Hantzsch ester
(190.0 mg, 0.75 mmol, 1.5 equiv), [Cu(dpa)(S-BINAP)]BF4 (2.0 mol%) and dissolved in anhydrous THF (1.5 mL,
0.33 M), sealed with a screw-cap and subsequently degassed by three consecutive freeze-pump-thaw cycles.
Afterwards, diisopropylethylamine (129.3 mg, 1.0 mmol, 2.0 equiv) (bromoethynyl)benzene[12, 31] (12) (135.8 mg,
0.75 mmol, 1.5 equiv) were added under N2, and the screwcap was replaced with a Teflon sealed inlet for a glass
rod, through which irradiation with LED (455 nm) took place from above. At the same time, the reaction mixture
was magnetically stirred at 20 °C for 24 h. The reaction was monitored by TLC. Afterwards, Then, the volatiles
were removed, and the crude concentrated. The residue was purified by flash chromatography on silica gel using
as eluent hexanes.
4.3. Computational details
Fully relaxed geometry for all complexes in their ground states was carried out using the Grimme’s dispersion
corrected (D3)[32] hybrid B3LYP[33] functional (i.e., B3LYP-D3) combined with Def2SVPP double-z basis set. The
implicit solvent effect was also taken into account in this procedure by the integral equation formalism of the
polarizable continuum model (IEFPCM)[34] using acetonitrile as solvent. All structures were confirmed as minima
through the harmonic vibrational frequency calculations. The (IEFPCM)-B3LYP-D3/Def2SVPP equilibrium
structure has been used to compute vertical excited-states properties within the time-dependent density functional
theory (TD-DFT) formalism at the level of theory. For this purpose, a non-equilibrium protocol for solvation was
employed to compute vertical excitation energies and oscillator strengths of electronic transitions. All calculations
have been performed using Gaussian 09 suite of programs.[35]
Acknowledgements
We gratefully acknowledge the financial support from FONDECYT grant 1210661, Fundamental research project
DIPOG 3913-406-81, FONDEQUIP program EQM 160042, EQM 120021, EQM 130021, EQM 180024. M.
Henriquez acknowledges Ph.D. CONICYT fellowship 21180705 and iPUR program. M. N. gratefully acknowledges
the University of Ferrara for funding (FAR2020). I.G.P acknowledges ONR grant N62909-18-1-2180 and
FONDECYT 11180185. S. Engl thanks the Studienstiftung des Deutschen Volkes and the Elite Network of Bavaria.
13
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10.1002/ejic.202100634
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: Chlorosulfonylation • Copper(I) • Dipyridylamine • Heteroleptic • Photocatalysis
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European Journal of Inorganic Chemistry
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