Heteroleptic copper(I) complexes incorporating sterically demanding diazabutadiene ligands (DABs). Synthesis, spectroscopic characterization and solid state structural analysis

Journal Pre-Proof Heteroleptic copper(I) complexes incorporating sterically demanding diazabutadiene ligands (DABs). Synthesis, spectroscopic characterization and solid state structural analysis Anastasios Peppas, Evanthia Papadaki, Gregor Schnakenburg, Victoria Magrioti, Athanassios I. Philippopoulos PII: DOI: Reference:

S0277-5387(19)30511-X https://doi.org/10.1016/j.poly.2019.07.033 POLY 14083

To appear in:

Polyhedron

Received Date: Revised Date: Accepted Date:

24 May 2019 22 July 2019 23 July 2019

Please cite this article as: A. Peppas, E. Papadaki, G. Schnakenburg, V. Magrioti, A.I. Philippopoulos, Heteroleptic copper(I) complexes incorporating sterically demanding diazabutadiene ligands (DABs). Synthesis, spectroscopic characterization and solid state structural analysis, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly. 2019.07.033

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JOURNAL PRE-PROOF Heteroleptic

copper(I)

complexes

incorporating

sterically

demanding

diazabutadiene ligands (DABs). Synthesis, spectroscopic characterization and solid state structural analysis

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Anastasios Peppasa, Evanthia Papadakib, Gregor Schnakenburgc, Victoria Magriotib*,

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a

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and Athanassios I. Philippopoulosa*

Laboratory of Inorganic Chemistry, Department of Chemistry, National and

Kapodistrian University of Athens, Panepistimiopolis Zografou 15771, Athens,

Laboratory of Organic Chemistry, Department of Chemistry, National and

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b

E-

Greece

Kapodistrian University of Athens, Panepistimiopolis Zografou 15771, Athens,

c

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Greece

Institut für Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn,

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Gerhard-Domagk-Straße 1, D-53121 Bonn, Germany

* Corresponding author: Prof. A. I. Philippopoulos. Tel.: +30-210-7274697; Fax: +30-210-7274782; e-mail: [email protected]

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JOURNAL PRE-PROOF

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Graphical Abstract Synopsis

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The synthesis and characterization of new copper(I) complexes of the general formula [Cu(RDABdipp)(LCOOMe)](PF6) is described. These are rare examples of heteroleptic

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copper(I) complexes comprising the 1,4-diaza-1,3-butadiene ligand (RDABdipp, R = H, Me; dipp = 2,6-diisopropylphenyl) and an ancillary α-diimine ligand (LCOOMe =

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methyl-2-(pyridin-2-yl)-quinoline-4-carboxylate). Abstract

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The synthesis, isolation and structural characterization of the novel heteroleptic

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copper(I) complexes [Cu(RDABdipp)(LCOOMe)](PF6) (R = H (3), R = Me (4)) incorporating the sterically demanding 1,4-diaza-1,3-butadienes RDABdipp (R = H, Me; dipp = 2,6-diisopropylphenyl) is reported. 3 and 4 were isolated in high yields and spectroscopically characterized. The synthetic strategy includes the in situ reaction of the bis-acetonitrile intermediates [Cu(RDABdipp)(NCMe)2](PF6), (R = H (1), R = Me (2)) with the required amounts of the methyl-2-(pyridin-2-yl)-quinoline4-carboxylate (LCOOMe) that was used as the ancillary ligand. 3 and 4 are rare 2

JOURNAL PRE-PROOF examples of heteroleptic copper(I) complexes comprising RDABdipp and an ancillary -diimine ligand, while 3 is the first crystallographically characterized example of this class of compounds. As a side product during the formation of 3 and 4, the homoleptic complex [Cu(LCOOMe)2](PF6) (5) is detected, that has been independently

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synthesized, for comparison. All the products were characterized by elemental

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analysis, FTIR, UVVis and NMR spectroscopy. The molecular structures of

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2·CH2Cl2, 3 and 5·CHCl3 were elucidated by single-crystal X-ray diffraction.



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Highlights

Four new copper(I)-DAB complexes have been prepared and fully

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characterized

Synthetic protocol for the isolation of copper(I) heteroleptic complexes



3 and 4 are rare examples of heteroleptic copper(I) complexes with 1,4-diaza1,3-butadienes

Structural characterization of cationic copper(I) complexes

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

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Keywords: 1,4-diaza-1,3-butadienes, methyl-2-(pyridin-2-yl)-quinoline-4-carboxylate,

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sterically demanding ligands, copper(Ι) complexes, heteroleptic complexes.

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JOURNAL PRE-PROOF

Introduction Undoubtedly 1,4-diaza-1,3-butadienes (DABs) constitute a special class of compounds due to their unique properties, such as electron donor and acceptor

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properties along with their ability to act in a variety of coordination modes towards a

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number of metal centers [1]. The coordination chemistry of various DAB containing

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metal complexes has been widely studied, taking also into consideration their potency

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to stabilize metal ions in low oxidation states [2] and the fact that they participate in various metal catalyzed organic transformations [36].

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During the last decades, research interest towards copper(I) complexes bearing DAB ligands still remains unfailing, as is clearly demonstrated by the pioneering

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work of Dieck and coworkers on the isolation and characterization of some homoleptic and heteroleptic copper(I) complexes using ligands with π-acceptor

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abilities [710].

More recently, various research groups have pointed out that copper(I)-DAB

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complexes may be useful for the catalytic preparation of a number of organic molecules. Among them, Diez-Gonzalez’s et al. focused on the preparation of

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triazoles, following the aspects of the so called “click” chemistry [11,12]. Anga et al.

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[13] and Liu et al. [14] also used copper(I)-DAB complexes as catalysts for the synthesis of benzo[b]furans and triarylamines respectively. In addition the coordination

behavior

of

the

hybrid

ligands

1,4‐bis(2‐methylchalcogenophenyl)‐2,3‐dimethyl‐1,4‐diaza‐1,3‐diene (chalcogens = O, S or Se) has been examined, demonstrating the effect of different chalcogen on the structure of the obtained complex [15]. On the other hand, a recent report by Marchetti et al. is focusing on the preparation and evaluation of the anticancer activity 4

JOURNAL PRE-PROOF of α-diimine homologues of cisplatin, where -diimines stand for [HCN(R)]2 (R = C6H11, 4-C6H10OH; 4-C6H4Me, etc.) [16]. Intriguied by these reports, in this study we wanted to further explore the fascinating synthetic chemistry of Cu(I)-DAB complexes. For this reason a systematic

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literature search was undertaken, revealing suprisingly that there is no report of a

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heteroleptic copper(I) complex bearing one DAB and one organic -diimine ligand.

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Moreover a Cambridge Crystallographic Database search showed that the number of

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well described and structurally characterized copper(I) complexes with DABs are quite rare in the literature. Therefore, from the synthetic point of view, isolation of

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heteroleptic copper(I) complexes with DABs is a challenge [2]. This fact prompted us to investigate the synthesis of new heteroleptic copper(I) complexes incorporating the

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sterically demanding RDABdipp ligand (R = H, Me; dipp = 2,6-diisopropylphenyl) and the corresponding LCOOMe that stands for methyl-2-(pyridin-2-yl)-quinoline-4-

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carboxylate, respectively (Scheme 1). Functionalization of the DAB ligand in the CC backbone was undertaken in order to also examine the coordination behaviour of

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the DAB ligand upon this type of modification. The bulky RDABdipp ligand was selectively chosen, to permit isolation of the

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target heteroleptic compounds, that is generally prevented, owing to the known

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lability of copper(I) complexes [1719]. In fact, previous reports by Diez-Gonzalez’s et al. [12] clearly demonstared that the acetonirile adducts of the type [Cu(DABR)(NCMe)2](BF4), could only be isolated when R = dipp. Attempts to isolate heteroleptic [Cu(DABR')(NCMe)2](BF4) complexes where R' ≠ dipp (R' = 2,4,6trimethylphenyl [Cu(DABR')2](BF4)

(Mes),

etc.),

complexes.

resulted In

any

the case,

corresponding the

relative

homoleptic homoleptic

[Cu(DABdipp)2](BF4) complex cannot be formed, due to steric reasons [12]. This 5

JOURNAL PRE-PROOF bidentate ligand can be easily prepared and in a high yield synthetic procedure. In addition the chemistry of the LCOOMe incoming ligand has been well developed and recently employed by our group in transfer hydrogenation reactions of polarized

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unsaturated substrates involving a number of ruthenium(II) complexes [20].

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Scheme 1. Structures of the ligands including a numbering scheme of the hydrogen

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atoms of LCOOMe.

The information obtained from the present study could provide us with the required knowledge so as to examine the possible preparation of new Cu(I)-DAB

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complexes with suitable bidentate ancillary ligands functionalized with –COOH groups in their periphery. These could be further applied as efficient copper(I)

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sensitizers for solar cells applications (dye sensitized solar cells) [21]. In this report we describe at first the synthesis and spectroscopic characterization

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of two complexes of the general formula [Cu( RDABdipp)(NCMe)2](PF6 ), (R = H (1),

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R = Me (2)), that act as Cu(I)-DAB precursors, for the preparation of the heteroleptic complexes of the structure [Cu(RDABdipp)(LCOOMe)](PF6), (R = H (3), R = Me (4)). The preparation of the homoleptic complex [Cu(LCOOMe)2](PF6) (5) is reported as well, since it is observed as a side product during the formation of 3 and 4. Also, the solid state structures of 2·CH2Cl2, 3 and 5·CHCl3 are presented. 2. Materials and methods 2.1. Reagents and equipments 6

JOURNAL PRE-PROOF All manipulations (unless otherwise noted) were carried out under an argon atmosphere using standard Schlenk techniques. Dichloromethane was pre-dried and distilled over P2O5, diethyl ether and pentane were pre-dried over CaCl2 and distilled over sodium wire, while methanol was dried over 3Å molecular sieves. The organic

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ligand N,N'-bis(2,6-diisopropylphenyl)-1,4-diazabutadiene abbreviated as DABdipp

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was prepared according to the literature [22]. 2-(Pyridin-2-yl)-quinoline-4-carboxylic

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acid (LCOOH), the precursor for the synthesis of the relevant ester LCOOMe, was

was

used

for

the

synthesis

of

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prepared as described in the literature [23]. A modification of the published procedure LCOOMe [24,25].

The

starting

material

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[Cu(NCMe)4](PF6) [26] and the [Cu(DABdipp)(NCMe)2](BF4) compound [12] were also prepared following the bibliographic reports. Infrared spectra were measured on a

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Shimadzu IR Affinity-1 spectrometer as potassium bromide pellets in the spectral range of 4000–400 cm−1. The following abbreviations were used for the intensities of

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the IR absorption bands: vs = very strong, s = strong, m = medium, w = weak, br = broad. Elemental analyses were obtained from the Microanalysis Center of the Institut 13

C{1H} NMR spectra were

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für Anorganische Chemie Universität Bonn. 1H and

recorded at 298 K on a Varian 200 MHz spectrometer, while the two-dimensional

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NMR spectra (1H1H COSY, 1H13C HSQC and 1H13C HMBC) of 3 and 4 were

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taken on a BRUKER Prodigy 500 MHz spectrometer, in CDCl3 or (CD3)2CO. J values are given in Hz. The following abbreviations were used for the signal multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad. Absorption spectra were recorded with a CARY 3E UV/Vis spectrometer. Melting or decomposition points were determined using a SANYO GALLENKAMP Variable heater apparatus and are uncorrected. The samples were sealed in capillary tubes and heated slowly until the compounds melted or decomposed.

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JOURNAL PRE-PROOF 2.2. Synthetic procedure and spectral data 2.2.1.

Synthesis

of

N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-

butadiene (MeDABdipp) Into a round-bottom flask equipped with a magnetic stirring bar, 2,3-butanedione

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(2.5 mL, 28.4 mmol), 2,6-diisopropylaniline (10.1 g, 57 mmol), methanol (46 mL)

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and 2 mL of formic acid 98% were added. The mixture was heated at 50 °C for 16 h.

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The precipitated product was filtered off, subsequently rinsed with cold methanol and

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dried under reduced pressure to give a yellow powder. Yield: 94% (10.80 g). IR (KBr, ṽ in cm1): 3062 (m, (CH arom)), 2957 (m, CHaliph)), 1637 (m, (C=N)), 1630 763 (s,

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(s, (C=N)), 1466 (s(CH)), 1438 (s,  (CH)), 793 (s, (CH)),

(CH)). 1H NMR (CDCl3, 200 MHz, 298K) δ (ppm): 1.26 (d, J = 6 Hz, 12H,

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CHMe2), 1.28 (d, J = 6 Hz, 12H, CHMe2), 2.17 (s, 6H, MeC=N), 2.80 (sept, 3J = 6 Hz, 4H, CHMe2), 7.147.29 (m, 6H, CHarom).

C NMR (CDCl3, 50 MHz, 298K) δ

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(ppm): 16.57 (MeC=N), 22.70 (CHMe2), 22.98 (CHMe2), 28.48 (CHMe2), 122.96 (CH, Carom), 123.74 (CH, Carom), 135.00 (C, Carom), 146.12 (i-CaromN), 168.10 (C=N).

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ESI-MS (MeOH): m/z (100 %) = 405.54 [M+H] + (calc. 405.319).

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2.2.2. Synthesis of methyl-2-(pyridin-2-yl)-quinoline-4-carboxylate (LCOOMe)

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In a round bottom flask, 2-(pyridin-2-yl)-quinoline-4-carboxylic acid (350 mg, 1.4

mmol) was dispersed in MeOH (10 mL). To this solution, 2.5 mL of conc. H2SO4 was added portion wise, affording a slightly yellow homogenous solution which was refluxed overnight. The mixture was allowed to cool at ambient temperature, distilled H2O (10 mL) was added and the organic solvent was rotary evaporated. The pH of the mixture was adjusted to 9.5 (30% w/w, NaOH(aq)), leading to the precipitation of a colorless solid, that was filtered off and dried in vacuo. Yield: 90% (330 mg). 8

JOURNAL PRE-PROOF 2.2.3. Synthesis of [Cu(DABdipp)(NCMe)2](PF6) (1) A Schlenk tube was charged with [Cu(NCMe)4](PF6) (30 mg, 0.08 mmol) and an equimolar amount of DABdipp (30 mg) and was degassed in vacuo for about 5 min. Dry CH2Cl2 (7 mL) was then added and the resulting dark brown solution was stirred

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for 1 h at ambient temperature. The solvent was pumped down and the residue was

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washed with dry pentane (2 × 5 mL). A yellowbrown solid was obtained which was

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dried under reduced pressure. Yield: 92% (50 mg). IR (KBr, ṽ in cm1): 3067 (m,

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CHarom)), 2965 (s, CHaliph)), 2873 (m, CHaliph)), 2314 (vw, CN)), 2280 (vw,CN)), 1635 (m, (C=N)), 1467 (s, δ (CH)), 1442 (s, δ (CH)), 842 (vs,

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(PF)), 558 (s, 4 (PF)). The 1H NMR spectrum of 1 recorded in CDCl3, is in

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accord to that reported in the published procedure (vide infra) for the [Cu(DABdipp)(NCMe)2](BF4) compound [12].

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2.2.4. Synthesis of [Cu(MeDABdipp)(NCMe)2](PF6) (2) [Cu(NCMe)4](PF6) (55 mg, 0.14 mmol) and an equivalent amount of the organic Me

DABdipp (60 mg, 0.14 mmol) were mixed in a Schlenk tube and the mixture

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ligand

was subsequently degassed in vacuo for about 5 min. CH2Cl2 (10 mL) was then added

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and the resulting dark brown solution was stirred for 1 h at ambient temperature. The

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solvent was evaporated to dryness and the yellowbrown solid obtained, was washed with pentane (3 × 7 mL) and dried under vacuum for 10 h. Yield: 93% (95 mg). M.p.: 169 °C (dec). Anal. Calcd. for (2)×2.0(H2O), C32H50N4O2PF6Cu: C, 52.30; H, 6.91; N, 7.62. Found: C, 52.16; H, 6.72; N, 7.56%. IR (KBr, ṽ in cm1): 3064 (m, CHarom), 2965 (s, CHaliph)), 2872 (m, CHaliph)), 2312 (vw, CN)), 2276 (vw, CN)), 1643 (m, (C=N)), 1466 (s, δ (CH)), 1441 (s, δ (CH)), 842 (vs, (PF)), 558 (s, 4 (PF)). UVVis (ε, M1cm1): λmax (CHCl3) = 277 (2510), 303 9

JOURNAL PRE-PROOF (3900). 1H NMR (CDCl3, 200 MHz, 298 K) δ (ppm): 1.25 (d, 3J = 6 Hz, 24H, CHMe2), 2.15 (s, 6H, MeCN), 2.26 (s, 6H, MeCN), 2.78 (sept, 3J = 6 Hz, 4H, CHMe2), 7.30 (br.m, 6H, CHarom).

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C{1H} NMR (CDCl3, 50 MHz, 298 K) δ (ppm):

19.00 (MeCN), 23.64 (MeCN), 23.95 (CHMe2), 28.39 (CHMe2), 124.24 (m-

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2.2.5. Synthesis of [Cu(DABdipp)(LCOOMe)](PF6) (3)

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Carom), 126.67 (p-Carom), 137.59 (o-Carom), 142.67 (i-Carom), 169.21 (MeCN).

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Under argon atmosphere a Schlenk tube was charged with [Cu(NCMe)4](PF6) (59 mg, 0.16 mmol) and 60 mg (0.16 mmol) of DABdipp and the solids were dried in vacuo

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for 5 min. CH2Cl2 (15 mL) was added and the resulting yellowbrown solution was stirred at ambient temperature for 15 min and for an additional 15 min at 0 oC. At this

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temperature, a solution of LCOOMe (37 mg, 0.14 mmol) in CH2Cl2 (15 mL) was added dropwise to the solution above, resulting in a crimsonred solution that was warmed at

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ambient temperature over a period of 15 min. The solvent was evaporated and the residue was dried for about 8 h under reduced pressure. The remaining solid was

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thoroughly washed with diethyl ether (5 × 5 mL) and was dried in vacuo. Yield: 85% (100 mg). M.p.: 224226 °C(dec). Anal. Calcd. for C42H50N4O2PF6Cu: C, 60.23; H,

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5.97; N, 6.39. Found: C, 60.01; H, 6.05; N, 6.34%. IR (KBr, ṽ in cm1): 3066 (w,

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CHarom)), 2963 (s, CHaliph)), 2871 (m, CHaliph)), 1728 (vs, as (C=O)), 1613 (w,  (C=N)), 1462 (s, δ (CH)), 1437 (s, δ (CH)), 1260 (vs, s (CO)), 844 (vs, (PF)), 558 (s, 4 (PF)). UVVis (ε, M1cm1): λmax (CHCl3) = 257 (24520), 276 (18760), 339 (12000), 535 (3820); λmax (Me2CO) = 335 (10880), 530 (4133). 1H NMR (CDCl3, 200 MHz, 298 K) δ (ppm): 0.99 (br. d, 3J = 6 Hz, 24H, CHMe2), 3.15 (sept, 3

J = 6 Hz, 4H, CHMe2), 4.11 (s, 3H, OMe), 7.067.15 (m, 6H, CHarom), 7.73 (t, 3J = 6

Hz, 1H, H5′), 7.83 (t, 3J = 8 Hz, 1H, H8), 8.00 (t, 3J = 8 Hz, 1H, H7), 8.17 (t, 3J = 6 10

JOURNAL PRE-PROOF Hz, 1H, H4′), 8.24 (d, 3J = 8 Hz, 2H, H6), 8.49 (d, 3J = 8 Hz, 1H, H3′), 8.58 (d, 3J = 6 Hz, 1H, H6′), 8.67 (s, 2H, HCN), 8.75 (s, 1Η, H3), 8.90 (d, 3J = 8 Hz, 1H, H9). 13

C{1H} NMR (CDCl3, 50 MHz, 298 K) δ (ppm): 24.21 (CHMe2), 28.33 (CHMe2),

53.61 (OMe), 120.17 (C3), 123.78 (C3′), 123.99 (m-Carom), 126.36 (C9), 126.85 (C5),

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127.15 (p-Carom), 127.78 (C5′), 129.08 (C6), 130.63 (C8), 132.00 (C7), 137.66 (C2),

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(C4), 151.50 (C2′), 161.93 (CN), 165.37 (COOMe).

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139.18 (o-Carom), 139.71 (C4′), 145.33 (i-Carom), 146.13 (C10), 148.98 (C6′), 151.14

2.2.6. Synthesis of [Cu(MeDABdipp)(LCOOMe)](PF6) (4)

Me

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A Schlenk tube was charged with [Cu(NCMe)4](PF6) (59 mg, 0.16 mmol) and DABdipp (64 mg, 0.16 mmol) and the solids were dried in vacuo for 5 min. CH2Cl2

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(15 mL) was added and the resulting brown solution was stirred at ambient temperature for 15 min and for an additional 15 min at 0 oC. A solution of LCOOMe (30

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mg, 0.11 mmol) in CH2Cl2 (15 mL) was then added drop wise leading to a crimsonred solution that was stirred at room temperature for 5 min and then the solvent was

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removed under vacuum. The obtained solid was washed successively with diethyl ether (3 × 5 mL) and was dried under reduced pressure. Yield: 75% (74 mg). M.p.:

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208210 °C. Anal. Calcd. for C42H50N4O3PF6Cu: C, 58.16; H, 5.81; N, 6.46. Found:

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C, 57.98; H, 5.80; N, 6.33%. IR (KBr, ṽ in cm1 ): 3065 (w, CHarom)), 2964 (s, (CHaliph)), 2871 (m, CHaliph)), 1730 (vs, as (C=O)), 1628 (w,  (C=N)), 1463 (s, δ (CH)), 1437 (s, δ (CH)), 1258 (vs, s (CO)), 842 (vs, (PF)), 558 (vs, 4 (PF)). UVVis (ε, M1cm1): λmax (CHCl3) = 257 (26210), 276 (20310), 341 (12310), 530 (4610); λmax (Me2CO) = 335 (14680), 521(5700). 1H NMR (CDCl3, 200 MHz, 298 K) δ (ppm): 1.19 (d, 3J = 6 Hz, 24H, CHMe2), 2.41 (s, 6H, MeCN), 3.10 (sept, 3J = 6 Hz, 4H, CHMe2), 4.09 (s, 3H, OMe), 7.08 (br.m, 6H, CHarom), 7.73 (t, 3J = 8 Hz, 1H, 11

JOURNAL PRE-PROOF H5′), 7.84 (t, 3J = 8 Hz, 1H, H8), 8.07 (t, 3J = 8 Hz, 1H, H4′), 8.25 (t, 3J = 8 Hz, 1H, H7), 8.34 (d, 3J = 8 Hz, 1H, H3′), 8.52 (d, 3J = 8 Hz, 1H, H6), 8.63 (s, 1H, H3), 8.83 (d, 3J = 10 Hz, 1H, H9), 8.89 (d, 3J = 4 Hz, 1H, H6′). 13C{1H} NMR (CDCl3, 50 MHz, 298 K) δ (ppm): 19.98 (MeCN), 24.30 (CHMe2), 28.38 (CHMe2), 53.49 (OMe),

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119.69 (C3), 123.08 (C3′), 124.14 (m-Carom), 125.75 (C9), 126.33 (p-Carom), 126.56

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(C5), 127.36 (C5′), 129.90 (C6), 130.94 (C8), 132.70 (C7), 136.94 (C2), 138.77 (o-

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Carom), 143.42 (i-Carom), 146.54 (C10), 149.59 (C6′), 151.15 (C2′), 151.44 (C4), 165.64

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2.2.7. Synthesis of [Cu(LCOOMe)2](PF6) (5)

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(COOMe), 171.62 (CN).

A Schlenk tube was charged with 50 mg (0.13mmol) of [Cu(NCMe)4](PF6) and

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two equivalents of LCOOMe (73 mg, 0.27 mmol). CH2Cl2 (5 mL) was added giving instantly a dark purple solution that was stirred at room temperature for about 1.5 h.

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Subsequently, the volume of the solution was concentrated under reduced pressure to some milliliters and the product was precipitated by the addition of diethyl ether (in

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excess). The resulting precipitate was washed with diethyl ether (2 × 5 mL) and vaccum dried affording complex 5 as an analytically pure purpleblack solid. Yield:

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91% (90 mg). M.p.: 240242 °C. Anal. Calcd. for (5)×1.5(H2O), C32H27N4O5.5P

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F6Cu: C, 50.30; H, 3.56; N, 3.73. Found: C, 50.21; H, 3.40; N, 7.40%. IR (KBr, ṽ in cm1): 2952 (w, CHaliph)), 2852 (w, CHaliph)), 1724 (vs, as (C=O)), 1257 (s, s (CO)), 840 (vs, (PF)), 558 (vs, 4 (PF)). UVVis (ε, M1cm1): λmax (CHCl3) = 259 (43272), 275 (36900), 284 (34252), 337 (22160), 534 (5547); λmax (Me2CO) = 335 (30280), 527 (7351); λmax (MeOH) = 334 (11040), 524 (2498); λmax (MeCN) = 255 (46815), 275 (30736), 332 (13700), 527 (340). 1H NMR ((CD3)2CO, 200 MHz, 298 K) δ (ppm): 4.17 (s, 6H,OMe), 7.56 (t, J = 8 Hz, 2H, Η7), 7.717.85 (m, 4Η, 12

JOURNAL PRE-PROOF Η4΄/Η8), 8.08 (d, 3J = 8 Hz, 2Η, H6), 8.40 (t, 3J = 8 Hz, 2Η, H5΄), 8.80 (m, 4H, Η3΄/Η9),

9.05 (d, 3J = 8 Hz, 2H, H6΄), 9.23 (s, 2H, H3). 1H NMR (CDCl3, 200 MHz, 298 K) δ (ppm): 4.19 (s, 6H, OMe), 7.48 (t, 3J = 8 Hz, 2H, Η7), 7.627.70 (m, 4H, Η4΄/Η8), 7.81 (d, 3J = 8 Hz, 2H, Η6), 8.26 (t, 3J = 8 Hz, 2H, H5΄), 8.51 (d, J = 8 Hz, 2H, H3΄), 13

C{1H}

F

8.74 (d, 3J = 8 Hz, 2H, Η9), 8.90 (d, 3J = 8 Hz, 2H, H6΄), 9.05 (s, 2H, H3).

O

NMR ((CD3)2CO, 50 MHz, 298 K) δ (ppm): 53.69, 121.29, 124.66, 126.74, 127.34,

O

128.64, 129.85, 130.79, 132.37, 138.48, 139.75, 146.91, 150.39, 152.64, 153.23,

PR

166.70.

E-

2.3. X-Ray analysis

The data collections of 2 and 5 were performed on a Bruker X8-KappaApexII

PR

diffractometer by using graphite monochromated Mo-K radiation ( = 0.7103 Å), generated by a sealed tube. The data collection of 3 was done using a Bruker D8-

AL

Venture diffractometer with Cu-K radiation ( = 1.5418 Å) generated from an Is Helios Optics source. The diffractometers were equipped with a low-temperature

R N

device (Bruker Kryoflex I, and Oxford Cryostream 800er series, both at 100(2) K). Intensities were measured by fine-slicing and-scans and corrected for background,

U

polarization and Lorentz effects [, 27].

JO

The structures were solved by intrinsic phasing methods implemented in Sheldrick’s XT program and refined anisotropically by the least-square procedure implemented in the SHELX program system [28]. Hydrogen atoms were included using the riding model on the bound carbon atoms. Crystal data, data collection and structure refinement details for complexes 23 and 5 are summarized in Table 1.

13

JOURNAL PRE-PROOF Table 1. Crystal and refinement data for 2·CH2Cl2 , 3 and 5·CHCl3

2·CH2Cl2

3

5·CHCl3

Empirical formula Molecular weight Crystal color

C33H48Cl2F6N4PCu 780.16 Clear red plate

C33H25Cl3F6N4O4PCu 856.43 Green flake

Crystal size (mm3) Temperature (K) Crystal system Space group Unit cell dimensions (Å) (Å) (Å)    V (Å3)

0.12 × 0.09 × 0.05 100 Monoclinic C2/c

C42H48F6N4O2PCu 849.35 Clear reddish orange plate 0.18 × 0.17 × 0.03 100 Monoclinic P21/n

27.5555(13) 13.0706(6) 25.0751(12) 90 122.133(2) 90 7647.8(6) 8 1.335 0.810 3.558 to 55.994 6, 3617, 17/33, 33 113982 9227 436 0.0305 1.046 0.0420

16.3683(4) 12.9992(3) 19.0369(5) 90 90.9935(10) 90 4049.96(17) 4 1.393 1.723 7.062 to 135.494 , 1915, 15/22, 22 81228 7349 514 0.0360 1.047 0.0300

15.773(3) 9.460(2) 23.228(7) 90 95.386(6) 90 3450.7(14) 4 1.649 0.990 6.818 to 51.996 18,19/10, 11/28, 28 9310 5632 472 0.0382 1.057 0.0491

0.0772 0.41/0.51

0.1261 0.85/0.78

F

O

O

PR

R N

AL

Refllections collected Data unique [I2I)] Parameters refined Rint Goodness-of-fit on F2 Final R indexess [I2I)] wR2 (all data) Largest diff. peak/hole (e Å3)

E-

calc (g cm 3) mm1) 2range (°) Index ranges

PR



0.24 × 0.12 × 0.04 296.15 Monoclinic Ia

U

0.1022 1.51/1.61

JO

3. Results and discussion 3.1. Synthesis and characterization of the diazabutadiene and LCOOMe ligands. The synthesis of the organic ligand DABdipp was performed according to the method reported in the literature [22] using methanol as a solvent rather than ethanol. The NMR spectroscopic data (1H and

13

C) were found to be identical to those of the

literature report.

14

JOURNAL PRE-PROOF For the

Me

DABdipp analogue, a modification of the published protocol was

followed [29]. Thus the reaction mixture consisting of 2,3-butanedione, 2,6diisopropylaniline and drops of formic acid was heated in methanol at 50 oC over a period of 16 hours rather than at 0 oC and subsequent overnight stirring at room

Me

DABdipp precipitates easily in this medium, since

O

without signs of decomposition.

F

temperature. As a result we managed to get the desired compound in high yield

13

C NMR

O

its solubility in methanol is considerably lower than in ethanol. 1H and

PR

spectra of MeDABdipp are included in the supporting information (Figs. S1,S2). As far as the methyl-2-(pyridin-2-yl)-quinoline-4-carboxylate (LCOOMe) ligand is

E-

concerned, this compound was first synthesized by Farah et. al. [24,25] by refluxing 2-(pyridin-2-yl)-quinoline-4-carboxylic acid (LCOOH) in benzene, with methanol and

PR

catalytic amount of concentrated sulfuric acid. In our approach the synthesis was performed without benzene, while the mixture that remained after the evaporation of

AL

the solvent, was treated with NaOH(aq) to pH 9.5 to achieve precipitation of the product. As a result, the use of toxic benzene was avoided while the need for large

R N

amounts of diethyl ether [24,25] or dichloromethane [20], in order to extract the final product, was not necessary. Finally LCOOMe was obtained analytically pure after

U

simple filtration and drying, while its 1H NMR spectrum (Fig. S3) is in accordance

JO

with that reported in the literature [20, 24]. Notably, the obtained yield has improved significantly from 82% of the published procedure [24] to that of 90%.

3.2. Synthesis and characterization of the copper(I) complexes 15 The

heteroleptic

cationic

complexes

of

the

general

formula

[Cu(RDABdipp)(NCMe)2](PF6), (R = H (1), R = Me (2)) were synthesized in a straight

15

JOURNAL PRE-PROOF forward reaction, by mixing equimolar amounts of the required ligands with [Cu(NCMe)4](PF6) in dry dichloromethane under an argon atmosphere (Scheme 2). A brown solution was instantly formed and after treatment with pentane, the expected products 1, 2 were obtained as yellow-brown solids in 92% and 93% yields,

F

respectively. Both complexes are air stable in the solid state, and the newly

PR

E-

PR

O

O

synthesized complex 2 decomposes at 169 ºC.

Scheme 2. Synthesis of the copper(I) complexes 15.

AL

The IR spectra of 12 (KBr disks) present characteristic bands at 1643, 1588 cm1 due to ν (C=N) symmetric stretching vibrations [12,13] that were significantly

R N

shifted to lower frequencies compared to those of the organic precursors. Also, the very strong bands at 842 cm1 and 558 cm1 are due to the 3 and 4, stretching and

U

deformation vibration modes of PF6 [30]. The weak band at 2280 cm1 for 1 and at

JO

2276 cm1 for 2 is due to the  (C≡N) stretching vibration mode. It should be pointed out, that both compounds display a higher energy vibration (2312 cm1 for 1 and 2314 cm1 for 2) of equal intensity to that of (C≡N), which could be assigned to a combination band between the s deformation of Me and the  (CC) stretch [31]. The 1H NMR spectrum of 1 which has been isolated as the PF6 salt is in accord to that reported for the [Cu(DABdipp)(NCMe)2](BF4) analogue (Fig. S4) [12]. The 1H NMR spectrum of 2 reveals the presence of coordinated acetonitrile as a single 16

JOURNAL PRE-PROOF resonance at  2.15 (Fig. S5). Also, the presence of the

Me

DABdipp ligand is clearly

demonstrated from the two resonance peaks at 1.25 (doublet resonance) and at

~2.722.85 (septet), attributable to the CHMe2 protons of the 2,6diisopropylphenyl moiety. The characteristic singlet resonance at 2.26 is due to the

O

F

methyl protons of the MeCN group.

The synthetic strategy followed for the preparation of the heteroleptic complexes

O

[Cu(RDABdipp)(LCOOMe)](PF6) (R = H (3), R = Me (4)) is based on the so called

PR

“HETPHEN” approach, firstly reported by Schmittel [32] and subsequently applied by Odobel, for the synthesis of a number of copper(I) heteroleptic compounds

E-

[19,33]. Potent applications of this strategy in the area of DSSC’s has been

PR

successfully developed within the last years [34, 35].

Our working protocol includes the in situ reaction of the initially formed bisacetonitrile intermediates 12 with appropriate amounts (see experimental part) of

AL

LCOOMe that serves as the ancillary ligand. Apparently, the RDABdipp ligand provides the required steric hindrance around the copper(I) metal center, allowing therefore the

R N

formation of the heteroleptic analogues, after replacement of the coordinated nitriles by the incoming ancillary ligand. The obtained complexes 3 and 4 were isolated in

U

moderate to high yields and were fully characterized. Our first attempts to synthesize

JO

3 included the in situ preparation of 1 in dichloromethane and drop wise addition of an equimolar amount of LCOOMe. The 1H NMR spectrum of the crude product that was obtained after evaporation of the solvent, revealed the presence of one more compound (Fig. S6), which was identified to be the homoleptic complex [Cu(LCOOMe)2](PF6) (5), after comparison with an authentic sample. The molar ratio of 3/5 was found to be approximately 85/15, according to integration. Upon optimization of the reaction conditions it became evident that a sub-stoichiometric amount of 17

JOURNAL PRE-PROOF LCOOMe was required in order to obtain 3 analytically pure. In fact, the stoichiometry used for the combined reagents [(Cu(NCMe)4](PF6)/DABdipp/LCOOMe was that of 1/1/0.9. The bis-acetonitrile adduct 1 that was formed, due to the excess of the DABdipp ligand and the [Cu(NCMe)4](PF6) metal precursor, could be easily removed

F

by simple washing with diethyl ether. In any case purity of the new substance was

O

supported by elemental analysis and NMR spectroscopic data (vide infra). Complex 3

O

is stable in the solid state towards oxidation by air and moisture and decomposes upon

PR

heating in the range of 224226 oC. In solution, it is quite stable in chlorinated solvents and in acetone, but it decomposes immediately in solvents with coordinating

E-

ability like MeOH, MeCN, DMSO and DMF, liberating free DABdipp ligand (visual inspection). In fact precipitation of free DABdipp takes place along with a change of

PR

colour from crimsonred to colourless.

The IR spectrum of 3 (in KBr) shows a strong absorption band at 1728 cm1

AL

which is assigned to the as (C=O) stretching vibration of the –COOMe group, indicating coordination of LCOOMe. This band is shifted to higher wave numbers as

R N

compared to that of the free ligand (1717 cm1). The symmetric s (CO) is observed as a very strong band at 1260 cm1. Moreover, the characteristic vibrations, due to the

U

δ (CH) stretchings at 1462 and 1437 cm1 can be attributed to the coordinated

JO

DABdipp ligand. The 1H and 13C{1H} NMR spectra of 3 were recorded in CDCl3 immediately after

dissolution of the complex (Figs. S7a,b). The 1H NMR spectrum shows well resolved signals in the aromatic region which were assigned to the organic ligands coordinated to the metal center, confirming formation of the desired heteroleptic complex. Assignment of these resonances was possible with the help of 1H1H NMR correlation spectroscopy (Fig. 1). The presence of the DABdipp ligand can be readily identified by 18

JOURNAL PRE-PROOF the characteristic singlet resonance of the imino protons at  8.67 that is shifted towards lower fields as compared to that of the free ligand (8.09). Observation of the CHMe2 moiety becomes evident from the typical resonance signals at 0.99 (d) and

3.15 (sept) respectively. Notably, the strong singlet resonance at  4.11 for the

O

F

methyl protons of the COOMe group is a powerful diagnostic tool to check the purity of the relevant compound. This signal is shifted to lower field in comparison to that of

O

5 ( 4.20), providing strong evidence for detection of the homoleptic complex 5.

PR

Furthermore, the stability of 3 in solution was examined by recording its 1H NMR in CDCl3 (Fig. S8). Remarkably, 3 is quite stable in this solvent and upon standing at

E-

ambient temperature in the dark over 24 h, a tiny amount of 5 was determined 13

C{1H} NMR spectrum of 3 displays

PR

(approximately 1% based on integration). The

the expected resonance signals for the carbon atoms of both ligands (Fig. S7b). Assignments made using HSQC and HMBC NMR techniques (Fig. S7c, d) revealed

AL

that the two signals at  161.93 and  165.37 are due to the CN and –COOMe

JO

U

R N

groups respectively.

Figure 1. 1H1H COSY NMR spectrum of 3 in CDCl3. The synthesis of 4 was initially attempted following the same molar ratio as that reported for the synthesis of 3 (ratio of [Cu(NCMe)4](PF6)/MeDABdipp/LCOOMe is 1/1/0.9). According to the 1H NMR spectrum of the crude product (in CDCl3), the amount of 5 detected was approximately 1.52%. The same amount of 5 was also 19

JOURNAL PRE-PROOF detected by changing the ratio to that of 0.9/1/0.85. Attempts to purify 4 were unsuccessful and systematically leading to increased amounts of 5 compared to that of 4. For example, dissolution of 4 in benzene and precipitation from dry pentane afforded a 94/6 mixture of 4/5 (Fig. S9). On the other hand, washing of the crude

F

solid with a mixture of dichloromethane/diethyl ether (1/9, v/v), resulted in a

O

disappointing 22/11/67 mixture of 4/5/MeDABdipp. Finally, we succeeded to obtain 4

O

in a pure form, when 0.7 equivalents of LCOOMe were used and the optimized molar

PR

ratio proved to be that of 1/1/0.7 respectively. 1H NMR spectroscopy showed that the final product was slightly contaminated with approximately 1% of 5 (vide infra).

E-

Complex 4 is a deep red solid, stable in the air and moisture and decomposes at 208210 oC. The IR spectrum of 4 is almost identical to that of 3 as expected for

PR

compounds with similar structures. Indicative is the strong absorption band at 1730 cm1 for the as (C=O) stretching vibration of the –COOMe group, that is shifted to

AL

higher wave numbers compared to the value reported for the free ligand. This trend is followed for 3 as well. In addition, coordination of the MeDABdipp ligand to copper(I)

R N

is clearly demonstrated by the intense bands at 1463, 1437 cm1 due to the δ (CH) stretching vibrations.

U

The 1H NMR spectrum of 4 was recorded in CDCl3 (Fig. S10a) and assignment

JO

was made with the aid of a 1H1H COSY NMR spectrum (Fig. S10b). The 1H NMR spectrum of 4 displays the characteristic singlet for the methyl protons of the COOMe group at  4.09, while the doublet resonance at 1.19 and the septet at  3.10 can be attributed to the CHMe2 moiety of dipp. In the aromatic region the expected signals of both ligands are present, but not of the imino protons, as they have been substituted by the methyl groups, which can be assigned to the singlet resonance at  2.41. The latter is downfield shifted compared to that of the free ligand (2.17). 20

JOURNAL PRE-PROOF The

13

C{1H} spectrum of 4 shows the characteristic resonance signals in the

expected region (Fig. S10c). The HSQC NMR spectrum is included in Fig. 2 and the corresponding HMBC spectrum is given in the supporting information (Fig. S10d). A pronounced downfield shift (Δδ = 9.7 ppm) for the CN signal is observed in

F

comparison to that of 3. Thus, the characteristic signal at  161.93 (for 3) shifts to 

O

171.62 (for 4). On the other hand, the typical signal for the –COOMe group remains

PR

E-

PR

O

unaltered at ~165.5.

AL

Figure 2. 1H13C HSQC NMR spectrum of 4 in CDCl3

R N

On going from 3 to 4, remarkable differences in the chemical shifts of their 1H NMR spectra are observed. In particular, the doublet resonances for the H6 and H6' protons of the quinoline and pyridine rings in 3 (δ 8.24 and 8.58) are downfield

U

shifted (Δδ ~ 0.3) in complex 4 (δ 8.52 and δ 8.89 respectively). However, for both

JO

complexes the chemical shift of H9 is slightly altered changing from  8.90 (3) to  8.83 (4). Treatment of the starting material [Cu(NCMe)4](PF6) with two equivalents of LCOOMe in dry dichloromethane resulted in the formation of the homoleptic complex 5 in 91% yield (Scheme 2). This was isolated as an air stable dark purpleblack solid that decomposes upon heating at 240-242 °C. Complex 5 was characterized by

21

JOURNAL PRE-PROOF elemental analysis, FTIR, UVVis,

1

H and

13

C{1H} NMR and with X-ray

crystallography. The IR spectrum of 5 (in KBr) exhibits a strong band at 1725 cm-1 which can be attributed to the as (C=O) stretching vibration along with two strong peaks for the

F

PF6anion at 840 cm1 and 558 cm1. The 1H NMR spectrum of 5 in CDCl3 shows

O

well resolved peaks in the aromatic region for the esterified ligand LCOOMe that is

O

coordinated to copper(I). The characteristic singlet resonance for the H3 proton

PR

appears at δ 9.06 and that for the methyl protons of the –COOMe at δ 4.19. The latter resonance signal, has been used as a probe to check the purity of the heteroleptic

E-

complexes 3 and 4 as already mentioned in the main text (vide supra). Due to better solubility of 5 in acetone, the 1H NMR spectrum of the complex was also recorded in

1

PR

(CD3)2CO (Fig. S11a). Assignment of the protons of 5 was made with the help of H1H COSY NMR spectroscopy along with literature reports on similar systems

AL

(Fig. S11b) [36,37].

Finally, the 13C{1Η} NMR spectrum of 5 in (CD3)2CO presents a low intensity

R N

resonance signal at δ 166.70 due to the CO of the –COOMe group. The signal at δ 53.69 is due to the carbon of the methyl group. All other signals are as expected (Fig.

JO

U

S11c).

3.3. Electronic spectra The solution absorption spectra of 15 in chloroform are depicted in Fig. 3. The absorption spectrum of 2 shows a rather weak MLCT absorption band at 303 nm accompanied by a typical π-π* band at 277 nm. The visible spectra of 3 and 4 (in 22

JOURNAL PRE-PROOF chloroform) display a broad band centered at 535 nm (3) and 530 nm (4) that is assigned to MLCT charge transfer bands. These are red shifted, when compared to the absorption maxima of the bis-acetonitrile adducts 12 (352 nm for 1, 303 nm for 2), apparently due to coordination of the incoming ancillary ligand. In acetone, a more

F

polar solvent, these maxima, for 3 and 4, are blue shifted by 5 nm and 9 nm

O

AL

PR

E-

PR

Cu(I) compounds reported in the literature [12,19,38].

O

respectively (Fig. S12). The UVVis spectra of 15 compare well to those of similar

R N

Figure 3. Absorption spectra of 15 in CHCl3 at 298 K. For compound 5 the absorption spectrum was also recorded in chloroform,

JO

U

acetone, methanol and acetonitrile (1×104 M) in order to compare the relative behavior of the complex in these solvents (Fig. 4). In the UVVis absorption spectrum of 5, two characteristic MLCT bands lying above 500 nm can be seen. According to literature reports obtained for Cu(I) phenanthroline complexes, three bands are observed abbreviated as band I, band II and band III [39,40]. Following this nomenclature in our complexes the intense main MLCT band may be attributed to band II, alongside with band III typically hidden within band II. The lowest energy

23

JOURNAL PRE-PROOF MLCT broad band above 600 nm can be attributed to band I which may be indicative of a distorted tetrahedral geometry. Notably, this is also supported from an X-ray crystal structure analysis performed where a strong distortion from the ideal tetrahedral is observed for complex 5 (vide infra in Fig. 7). For the structurally related

F

complexes 3 and 4 the corresponding band I almost disappeared when compared to

O

the spectrum of 5 (see Fig. 3). In the case of 3, this could be attributed to a minor

O

distortion from the ideal tetrahedral geometry which is in agreement with the results

PR

of a single-crystal analysis elucidation performed (vide infra in Fig. 6).

The intense main MLCT band at 534 nm in chloroform (ε = 5547 L mol1 cm1)

E-

blueshifts to 527 nm in acetone (ε = 7351 L mol1 cm1) and to 523 nm in methanol, with a dramatic decrease in its intensity (ε = 2498 L mol1 cm1), while in MeCN, this

PR

MLCT band almost disappears (a very weak band is present at approximately 527 nm). In fact, visual inspection shows a change in the solution’s color, from deep

AL

purple in CHCl3 to almost colorless in MeCN. We may propose that as the coordinating ability of the solvent increases and in dilute solutions, dissociation of

R N

LCOOMe takes place. This is in accord with similar Cu(I) complexes where degradation

JO

U

occurs in coordinating solvents [41].

24

PR

O

O

F

JOURNAL PRE-PROOF

Figure

4.

Absorption spectra of 5 in different solvents (1×104 M).

E-

3.4. Structural studies

PR

The solid-state structures of the copper(I) complexes 2·CH2Cl2, 3 and 5·CHCl3 were unambiguously determined by single-crystal X-ray diffraction. Suitable clear red plates of 2·CH2Cl2 and reddish-orange plates of 3 were obtained upon slow diffusion

AL

of pentane into a concentrated dichloromethane solution (2) or a chloroform solution

R N

(3) at 25 ºC. Dark green flakes of 5 were grown by diffusion of hexane into a chloroform solution of 5 at ambient temperature. 2, 3 and 5 are isostructural and crystallize in the space groups C2/c (2), P21/n (4) and Ia (5). The molecular structures

U

of 2, 3 and 5 are depicted in Figures 57 and selected bond distances and angles are

JO

listed in Table 2. The hexafluoridophosphate anions are not shown. A complete list of bond lengths and bond angles is given in the supporting information (Tables S1S6).

25

5.

Molecular

structure

of

the

complex

cation

O

Figure

O

F

JOURNAL PRE-PROOF

of

PR

[Cu(MeDABdipp)(NCMe)2](PF6)·CH2Cl2 (2·CH2Cl2). Hydrogen atoms and solvent

AL

PR

E-

molecules omitted for clarity. The ellipsoids were plotted at 50% probability.

Figure 6. Molecular structure of the complex cation of [Cu(DABdipp)(LCOOMe)](PF6)

R N

(3). Hydrogen atoms omitted for clarity. The ellipsoids were plotted at 50%

JO

U

probability

Figure 7. Molecular structure of the complex cation of [Cu(LCOOMe)2](PF6)·CHCl3 (5·CHCl3). Hydrogen atoms and solvent molecules omitted for clarity. The ellipsoids were plotted at 50% probability 26

JOURNAL PRE-PROOF All complexes posses a four-coordinated copper(I) metal center with coordination geometries varying from slightly distorted tetrahedral to that of a distorted tetrahedron. In order to quantify this distortion, the 4 geometry index introduced by Houser was employed [42]. The values of 4 range from 1.00 (ideal tetrahedral geometry) to

O

F

0.00 (ideal square-planar geometry) and can be calculated by the formula 4 = [360 – (α + β)]/141, where α and β are the largest θ angles of a four-coordinate complex.

O

Thus, the calculated τ4 value for 2 is 0.89, indicating only a minor distortion from the

PR

ideal tetrahedral. For 3 an intermediate τ4 value of 0.69 is calculated, that is an indication of severe distortion from the ideal tetrahedral geometry. Our result well

to

the

τ4

values

of

0.667

and

E-

compares

0.656

reported

for

PR

[Cu(bpy(Mes)2)(phen)](BF4) and [Cu(bpy(Mes)2 )(dmp)](BF4) (bpy(Mes) = 6,6'dimesityl-2,2'-bipyridine; dmp = 2,9-dimethyl-1,10-phenanthroline), respectively where a distorted trigonal pyramidal geometry has been proposed [43]. A “see-saw”

AL

solid-state geometry may be proposed for the homoleptic derivative 5 on the basis of

R N

the 4 geometry index [42]. The τ4 value of 0.59 indicates that its geometry deviates significantly from tetrahedral. A comparison of the obtained 4 values for 1, 2·CH2Cl2, 3 and 5·CHCl3 is

JO

U

summarized in Table 2.

27

JOURNAL PRE-PROOF Table 2. Selected bond lengths and bond angles for 1, 2·CH2Cl2, 3 and 5·CHCl3 Selected bond lengths (Å) and angles (o ) CuN1 = 2.0821(12) CuN2 = 2.0798 (12) CuN3 = 1.9424(15) CuN4 = 1.9391(14)

F

O O

PR

1

N1CuN2 = 78.82(5) N1CuN3 = 112.60(6) N1CuN4 = 117.91(5) N2CuN3 = 110.21(6) N2CuN4 = 117.88(5) N3CuN4 = 114.52(6) CuN1 = 2.0678 (15) CuN2 = 2.0580 (14) CuN3 = 1.9496 (17) CuN4 = 1.9345 (16)

τ4 = 0.88 slightly distorted tetrahedral geometry

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2·CH2Cl2

N1CuN2 = 78.58 (6) N1CuN3 = 113.61 (6) N1CuN4 = 114.25 (6) N2CuN3 = 110.98 (6) N2CuN4 = 117.97 (6) N3CuN4 = 116.01 (7) CuN1 : 2.0253 (12) CuN2 = 2.0264 (13) CuN3 = 2.0933 (12) CuN4 = 2.0144 (12)

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3

τ4 = 0.89 slightly distorted tetrahedral geometry

N1CuN2 = 81.46 (5) N1CuN3 = 124.89 (5) N1CuN4 = 133.04 (5) N2CuN3 = 110.32 (5) N2CuN4 = 129.74(5) N3CuN4= 81.03 (5) CuN1 = 2.020(6) CuN2 = 2.016(7) CuN3 = 2.023(6) CuN4 = 2.024(7) N1CuN2 = 81.5(3) N1CuN3 = 115.65 (19) N1CuN4 = 138.7 (3) N2CuN3 = 138.4 (2) N2CuN4 = 110.9 (2) N3CuN4 = 81.8 (3)

5·CHCl3

28

τ4 = 0.69 distorted tetrahedral geometry

τ4 = 0.59 see-saw geometry

JOURNAL PRE-PROOF Complex 2 crystallizes with one molecule of dichloromethane in the unit cell (Fig. 5). The geometry of the compound is almost tetrahedral and the planes defined by the diisopropylphenyl groups [N1(C5C10)] and [N2(C17C22)] are almost perpendicular to the (CuN1C1C2N2) plane. The relative dihedral angles are

F

88.05o and 85.03o respectively. The structural features of 2 (bond distances, bond

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angles) are in accordance with the values reported for the structurally related complex

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1 [12]. In the crystal lattice, pairs of 2 are stabilized by characteristic intermolecular

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CH∙∙∙π interactions (Fig. S13a) between the coordinated acetonitrile molecules and the phenyl rings of the dipp substituents [(C30H30)∙∙∙centroid (ring C(5)) and

E-

(C32AH32A)∙∙∙centroid (ring C(17))] at separations of 2.609 Å and 2.763 Å

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respectively [44]. In addition, the two planes derived by the coordinated MeCN molecules are almost perpendicular, as demonstrated by the characteristic dihedral angle of 83o. The subunits (dimers) are further stabilized by non-classical CH∙∙∙F

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hydrogen bonding interactions (Fig. S13b). The fluoride atoms of the PF6 counter anion have close contacts to the phenyl ring of dipp (CH∙∙∙F) and to the methyl

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groups of dipp, DAB and acetonitrile (less than 2.900 Å) [45]. Complex 3 displays a distorted tetrahedral geometry owing to the high steric

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hindrance implied by the DAB ligand (Fig. 6). The dihedral angle between the planes

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derived by the two five membered chelated rings (CuN3C17C18N4) and (CuN1C5C12N2) is 81.70o, deviating only slightly from the tetrahedral geometry. Notably, the diisopropylphenyl groups (C19C24) and (C31C36) are no more perpendicular to the plane derived by the five membered chelated ring (CuN3C17C18N4), as clearly demonstrated by the relative dihedral angles of 69.46o and 61.99o respectively. The torsion of the diisopropylphenyl groups is rather a result of the coordination of the ancillary ligand. A significant shortening of the 29

JOURNAL PRE-PROOF CuN(4) bond distance (2.0144 Å) is observed for 3 when compared to 1 (CuN1/ CuN2 mean bond length of 2.0810 Å) while the CuN(3) bond length has been slightly elongated (2.0933 Å). The relevant CuN(1) and CuN(2) bond distances of 2.0253(12) Å and 2.0264(13) Å are in the range expected for complexes with similar

F

structures [46,47].

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The lattice of 3 consists of four molecules that are stabilized by intermolecular

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CH∙∙∙π interactions. Typical CH∙∙∙π interactions include the Me group of dipp and

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the centroids of the phenyl group (dipp substituent) of an adjacent molecule (distance between (C39H39A)∙∙∙centroid (C31C36) of 2.946 Å) (Fig. S14). Also, the

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aromatic hydrogen atoms H33, H34 of the same phenyl group (C31C36) show

(distances

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CH∙∙∙π interactions with the Ncontaining ring of the quinoline moiety (N1C5) between (C34H34)∙∙∙centroid

(N1C5)

and

(C33H33)∙∙∙centroid

(N1C5) of 3.269 Å and 3.358 Å). Finally, the aromatic hydrogen H8, of the

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quinoline group, participates in two intermolecular CH∙∙∙π interactions including the

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ring centroids of the pyridine moiety (N2C16) and the dipp group (C19C24) of an adjacent molecule, at a distance of 3.118 Å and 3.071 Å respectively [43]. Compound 5 crystallized with one molecule of chloroform in the crystal lattice

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(Fig. 7). The dihedral angle between the planes derived by the two five membered

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chelated rings (CuN1C5C12N2) and (CuN3C21C28N4) is 63.41o, deviating strongly from the tetrahedral geometry. This structural feature is in accord with the calculated τ4 index (vide supra). The pyridine ring close to the quinoline ring deviates from planarity (angle between the least square planes of rings containing atoms N1–N2 = 9.93° and N3–N4 = 14.57o). The CuN bond distances, compare well

30

JOURNAL PRE-PROOF with the values observed in other Cu(I) complexes containing biquinoline or bipyridine ligands [46,47]. In the lattice of 5 two molecules participate in a double ππ stacking interaction where the rings of symmetry related molecules are involved (Fig. S15a). The distance

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between centroids (N1C5)∙∙∙(C22C25) and (N3C21)∙∙∙(C6C9) is 3.690 Å and

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3.713 Å, which is within the expected range of a typical sliced stacking interaction

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[44]. The structure is further stabilized by CH∙∙∙π bonding interactions (Fig. S15a),

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between the esterified methyl group and the centroid of the adjacent pyridine moiety (distances of (C27AH27A)∙∙∙centroid (ring N(4)) and (C11BH11B)∙∙∙centroid (ring

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N(2)) is 2.943 Å and 2.900 Å) [48]. Finally, one-dimensional chains are formed (view

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along the b axis Fig. S15b) that are further interconnected by characteristic CH∙∙∙F contacts (C42H42A∙∙∙F4 = 2.860 Å, C32H32B∙∙∙F4 = 2.453 Å, C13H13C∙∙∙F4 =

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2.864 Å, C19H19∙∙∙F2 = 2.617 Å).

4. Conclusions

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In conclusion, we have successfully prepared two novel heteroleptic copper(I) complexes, bearing a bulky diazabutadiene ligand (DABdipp or bifunctional

DABdipp) and the

ligand

(LCOOMe).

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methyl-2-(pyridin-2-yl)-quinoline-4-carboxylate

Me

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Starting materials for the synthesis of the new heteroleptic complexes 3, 4 were the copper(I) bis-acetonitrile adducts 1 and 2, which were prepared in situ and were further used for reaction with the ancillary ligand LCOOMe. Complex 2 has also been independently synthesized and characterized spectroscopically, including also an Xray crystal structure analysis. The newly synthesized compounds 3 and 4 are rare examples of copper(I) complexes incorporating both DAB and an ancillary bidentate ligand. The molecular structure of 3 was defined by single-crystal X-ray diffraction 31

JOURNAL PRE-PROOF revealing a distorted tetrahedral environment, as clearly demonstrated by the calculated τ4 value of 0.69. The heteroleptic complexes 3, 4 are stable in the solid state under air and moisture. The homoleptic complex 5 was selectively prepared in excellent yield and was structurally characterized revealing a highly distorted

F

tetrahedral geometry with a τ4 value of 0.59.

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Following this strategy the synthesis of other structurally related copper(I)

O

complexes is underway. Emphasis is given on the preparation of DAB containing

PR

copper(I) complexes with substituted 2-(2΄-Pyridyl)-quinoline ligands incorporating – COOH functional groups, which is a prerequisite for efficient grafting of a sensitizer

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to the titania surface. These materials could be further evaluated as new low cost copper(I) sensitizers into the DSSC’s (dye sensitized solar cells) technology. Work

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Acknowledgements

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towards this goal is in progress.

A.I. Philippopoulos would like to thank Prof. Dr. A. C. Filippou of the Chemistry

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Department of the University of Bonn for the elemental analyses and the twodimensional NMR measurements (HMBC and HMQC). We thank Mrs. Charlotte

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Rödde from the X-ray Department of the Institute für Anorganische Chemie, University of Bonn. Ph.D candidate, A. Peppas would like to thank the University of

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Bonn for financial support within the programme entitled: “Forschungsstipendium aus Mitteln des Sonderforschungsbereichs (SFB) 813 “Chemie an Spinzentren”. This research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT), under the HFRI PhD Fellowship grant (GA. no. 2055 /14475 & 2286 / 14477 for the Ph.D candidates A. Peppas and E. Papadaki).

32

JOURNAL PRE-PROOF Appendix A. Supplementary data CCDC

1915751, 1915752 and 1915753 contains the supplementary

crystallographic data for 5·CHCl3, 3, 2·CH2Cl2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge

F

Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)

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1223-336-033; or e-mail: [email protected].

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Supplementary data include: spectroscopic characterization of the complexes 15

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along with the crystallographic data for 2·CH2Cl2, 3 and 5·CHCl3.

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