Aqueous Route to Stable Luminescent Tetranuclear Copper(I

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Aqueous Route to Stable Luminescent Tetranuclear Copper(I) Dithiophosphonate Clusters Michael N. Pillay,† Jian-Hong Liao,‡ C. W. Liu,‡ and Werner E. van Zyl*,† †

School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban 4000, South Africa Department of Chemistry, National Dong Hwa University, Hualian 97401, Taiwan, Republic of China

‡

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S Supporting Information *

ABSTRACT: A series of seven luminescent copper(I) dithiophosphonate (dtp) clusters of the type Cu4L4 (L = S2PR(OR′)−) were formed from CuCl2·2H2O precursor in aqueous medium under ambient conditions. The dtp ligand serves the dual function of acting as a sacrificial reducing agent and cluster core stabilizer. The new clusters were characterized by 1H and 31P NMR and ESI-MS, and the single-crystal X-ray structures for two representative clusters [Cu4{(S2P(1,4-C6H4OMe)(OR′)}4] (R′ = OCH(CH3)2; CH2C6H5) were determined. The redox reaction yielded clusters in satisfactory yield, and all exhibited luminescence with λemmax in the range 519−534 nm and half-lives in the range 10−14 μs.

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INTRODUCTION Chalcogen-based clusters of the group 11 coinage metals (Cu, Ag, Au) have a diverse structural chemistry and a rich photochemistry.1−3 Luminescent copper(I) complexes, in particular, have seen a recent upsurge in research focus due to the observed thermally activated delayed fluorescence (TADF) phenomenon,4 and by comparison, copper has the additional advantage of earth-abundancy and lower cost. Forming copper clusters in a more cost-effective way (i.e., using readily available starting materials) coupled with a green synthesis methodology (i.e., in water solvent) should improve ease of accessibility to such materials for further studies. The coordination chemistry of Cu(I) with bidentate sulfur systems produces multinuclear clusters containing Cu4 (A),5−7 Cu6 (B),8 and Cu8 (C)9 cores (Chart 1) with the ligands displaying trimetallic triconnective (A and B) and tetrametallic tetra-connective (C) coordination modes. However, apart from early structural investigation into the topology of A with the related and symmetrical dithiophosphate ligand, attention to improved preparation methods and investigations into their physical properties have been neglected. Only a few

examples of copper(I) clusters stabilized by S−P−S fragments are known, the first example of a Cu4L4 (L = dithiophosphate) was reported by Lawton and coworkers in 1972.10 Clusters of related ligand families contain reports on diselenophosphates,11 dithiocarbamates,12 and diselenocarbamates.13 Recently, dichalcogen-stabilized copper hydride clusters for use in energy storage and conversion14 have become an exciting avenue of research due to an increasing number of highnuclearity copper hydride clusters (Cu20, Cu32) as potential hydrogen storage systems, mainly stabilized by dithiophosphate (S2P(OR)2−) ligands.15,16 By comparison, copper(I) dithiophosphonate (S2PR(OR′)−) analogues are rare,17 with only one reported18 tetranuclear cluster, [Cu(OMe)Fc(μS)(μ3-S)]4 (Fc = ferrocenyl). The synthetic protocol for reported copper(I) clusters relies mainly on air-sensitive Cu(I) precursor material and organic solvent. Here, we report on an alternative synthesis strategy by forming a series of tetranuclear Cu(I) clusters of the type Cu4L4 performed in aqueous medium and starting with readily available CuCl2·2H2O precursor. The optical properties for all new clusters were also determined. This study is a continuation of our work on dithiophosphonate and related ligands of the group 11 metals.19−23

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Chart 1. Common Coordination Motifs for Cu4, Cu6, and Cu8 Cu(I) Clusters with Bidentate Ligands

RESULTS AND DISCUSSION Synthesis and Solution Characterization. Conventional methods for the preparation of copper(I) clusters rely on a Cu(I) precursor and the use of organic solvents. In this respect, [Cu(CH3CN)4]PF6 is a commonly used precursor in Received: March 19, 2019

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.9b00783 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of Clusters 1−7

nonaqueous solvent systems; however, it is susceptible to degradation in the presence of water and requires inert conditions for complexation.24 The use of dry conditions and costly starting materials can have a restrictive effect on the availability of these clusters for further studies. We developed a synthetic protocol for the preparation of clusters via a selfredox reaction, carried out in aqueous solutions with an inexpensive and commercially available Lawesson’s reagent and cupric chloride precursor, shown in Scheme 1. Addition of CuCl2·2H2O to a solution of NH4[S2PR(OR′)] in a 1:2 molar ratio results in the formation of a short-lived turbid brown solution which rapidly becomes clear with the simultaneous formation of a pale yellow precipitate. The separation of the cluster and the disulfide byproduct can be achieved with relative ease due to the differential solubilities, and filtration affords the cluster in satisfactory yields. The identity of the disulfide was confirmed by NMR spectroscopy and corresponds to previously reported chemical shifts.25 We previously reported the redox activity of the dithiophosphonate ligand, in which [S2PR(OR′)]− salts could readily be oxidized with a mild oxidizing agent such as I2, leading to disulfide products of the type PR(OR′)(S)SSPR(OR′)(S) (R = anisolyl or ferrocenyl; R′ = alkyl).23 Copper(II) complexes have been applied to the oxidation of thiols to disulfides for more than a century.26 Investigations into the self-redox reaction of sulfur-based Cu(II) complexes have been reported for related symmetric ligands but has not been exploited for the preparation and isolation of clusters.27 The mechanism for the redox process was thought to be an electron transfer from the ligand to metal center resulting in the formation of a Cu(I) species and a corresponding sulfur radical with the recombination of two radical species resulting in a disulfide.28 However, extensive electron paramagnetic resonance (EPR) studies expressed a different reaction sequence, found no evidence of radical formation, and suggested the reaction proceeds via an associative mechanism between two Cu(II) complexes, as shown in Chart 2.29

The self-redox reaction accordingly occurs via direct contact between the metal centers and superexchange between copper atoms via bridging ligands.27 The reduction of Cu(II) therefore occurs with the simultaneous oxidation of the ligand to form a disulfide. The intermediate which involves the association of two square planar complexes is governed mainly by steric contributions of the ligand.27 Investigations into the complex interactions between sulfur and copper atoms in dinuclear mixed valent systems have provided some insight into the mechanism of the reduction.28b The reaction proceeds via the initial formation of a Cu(II) complex, and to the best of our knowledge, no square planar homoleptic Cu(II) dithiophosphonate ligands of the type CuL2 have been reported. The reduction is rapid, and detection of the Cu(II) intermediate remains a challenge. Against this background, we assign the dark colored species, initially seen on first mixing, to a Cu(II) intermediate which is not stable for any prolonged period. One major difference between dithiophosphonates and its symmetrical counterparts (i.e., dithiophosphates) is the reduction in symmetry. This could account for the decrease in stability of the Cu(II) intermediate and as described could in fact aid the synthetic protocol. Through judicious choice of the reaction between the starting primary or secondary alcohol and Lawesson’s reagent LR (or the more soluble phenetole derived LR) in forming the parent dithiophosphonate, significant diversity can be introduced to suit the ligand design for a particular application. In this study, a variety of alkyl substituents (linear, branched, unsaturated, and aromatic groups) are utilized to illustrate the versatility of this synthetic procedure. All the clusters are moderately stable once isolated with high solubility in polar chlorinated solvents. 1H and 31P NMR and ESI mass spectroscopy confirmed the Cu4L4 structural characteristics of the clusters. A single set of resonances in the 1H spectra was observed for each of the clusters, suggesting an identical magnetic environment around each of the four ligands and confirming the rigidity of the cluster in solution. A common feature of complexes containing the asymmetric dithiophosphonate motif is the possibility of isomers that could form in solution, leading to different structural configurations. The decoupled 31P NMR for all clusters showed a single peak in the region of 100 ppm, shown in Table 1. The diverse group of ligands prepared allows for a comparison of the coupling between H and P nuclei. Three JP−H coupling constants can be distinguished for cluster 1, shown in Figure 1a. The coupling is

Chart 2. Associative Pathway for the Self-Redox Reaction

B

DOI: 10.1021/acs.inorgchem.9b00783 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1.

31

P NMR Data for Clusters

cluster

R

1 2 3 4 5 6 7

OCH3 OCH2CH3 OCH(CH3)2 OCH2CH2CH2CH3 OCH2CHCH2 OCH2C6H5 m-OCH2C6H4Br

31

P NMR (δ) 103.3 100.2 98.5 100.2 101.5 101.6 102.6

mainly proximity based with the 15 Hz coupling of methylene protons to the 31P nuclei being the largest, followed by the coupling to the ortho-aromatic protons and meta-aromatic protons at 13 and 3 Hz, respectively. The broadening of resonances for protons located on the carbon adjacent to the oxygen atom are a consequence of multiple couplings. In cluster 1, no vicinal protons are present, and a well resolved doublet is observed (Figure 1b), unlike 2 and 5, where the coupling cannot be quantified due to the combination of JP−H and JH−H of adjacent vicinal protons. Furthermore, the combination of the vicinal JH−H and JP−H couplings results in a dd for the aromatic protons. ESI mass spectrometry aided in the bulk characterization of the clusters as CH elemental analysis would only provide an empirical formula CuL. Structural Analysis. Cluster 3 crystallizes in a bodycentered lattice in a tetragonal space group, I41/a, and a molecular representation is shown in Figure 2. The asymmetric unit consists of one-quarter of the molecule. The introduction of the anisole group on the P atom creates a sterically demanding environment, thus limiting the rotation of the C− O bond of the isopropyl fragment. The resultant loss of freedom forces the isopropyl group to occupy equivalent orientations on each of the four ligands and within the crystal lattice. The metal framework reveals a core of four copper atoms arranged in a trigonal pyramidal arrangement, shown in Figure 3. The tetrahedron is stabilized by six Cu···Cu contacts, which can be further divided into two types: (i) four shorter contacts that are bridged by sulfur at 2.7572 (11) Å and (ii) two nonbridged metal contacts, measuring 2.9042 (18) Å, the latter being significantly longer than the sum of the van der Waals radius of two Cu atoms (2.8 Å). An empirical comparison of the Cu···Cu distances can be achieved with Pauling’s expression developed for internuclear distances in metal bonds.30,31 Bond orders for (i) and (ii) above were found to be 1.18 and 0.67, respectively, suggesting that the

Figure 2. Molecular representation of 3; thermal ellipsoids were drawn at 35% probability.

Figure 3. Representation of 3 showing trimetallic triconnective coordination mode of ligand and Cu(I) core with two different Cu··· Cu contacts (ligand substituents omitted for clarity).

three copper atoms act cooperatively to stabilize the fourth (i.e., two bridged contacts (ii) and one nonbridged contact

Figure 1. (a) Representation of of JP−H coupling present in dithiophosphonates. (b) 15 Hz JP−H doublet for methyl protons (blue); broadening of alkyl resonances for ethyl (red) and allyl (green). C

DOI: 10.1021/acs.inorgchem.9b00783 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (i)). The propensity for Cu(I) centers to cluster has been shown to be driven by both Cu(I) interactions and stereochemical aspects of the ligand.32 Monoanionic dithiophosphonate ligands, with bite angles at ca. 115.70°, cap each triangular face of the tetrahedron, resulting in a closed-shell d10 electronic configuration on the copper atoms. The intraligand S···S bite distance is 3.417 Å with Cu−S bond distances of 2.2917 (15) Å. Each ligand coordinates in a trimetallic, triconnective (μ2-S; t-S) coordination mode. Although the charge is delocalized across the S−P−S fragment, differential P−S bond lengths exist with the P−S1 bond (2.057 (2) Å) being relatively longer than the P−S2 bond (1.978 (2) Å). The small elongation arises as a result of the denticity with S1 bridging two Cu atoms, and as a result, loss of electron density occurs in the corresponding P−S bond. This arrangement plays a role in characteristic luminescent patterns. Cluster 6 crystallizes in the monoclinic space group C2/c, Figure 4. The overall loss of symmetry can be attributed to the

Table 2. Selected Bond Distances (Å) and Angles (deg) for 3 and 6a 3 P1−S1 P1−S2 P1−O1 P1−C4 Cu1−Cu11 Cu1−Cu12 Cu1−Cu13 a

2.057(2) 1.978(2) 1.574(5) 1.777(6) 2.7572(11) 2.7571(11) 2.9042(15)

6 P1−S1 P1−S2 P1−O2B P1−C1 Cu1−Cu2 Cu1−Cu11 Cu2−Cu21

2.0481(18) 1.954(2) 1.468(7) 1.894(5) 2.7600(10) 2.9494(13) 2.9353(14)

Estimated standard deviation in brackets.

occupy equivalent positions in the crystal lattice, resulting in a static positional disorder, shown in Figure 5. The disorder can be related to the similarity in the size, volume, composition (C6H5OCH3 and C6H5CH2O), and geometry of the two substituents.

Figure 5. Separation of disorder present on 15; thermal ellipsoids were drawn at 35% probability, and hydrogens were omitted for clarity.

Because the crystals were stable (photograph of crystal shown in Figure 6), it can be assumed that the difference

Figure 4. Molecular structure of 6; thermal ellipsoids were drawn at 35% probability, and hydrogens were omitted for clarity.

disorder present in the ligand. The metal core is essentially isostructural to 3, but in 6, two crystallographically independent Cu(I) sites are apparent. This results in the slight variation of the Cu···Cu contacts with two bond lengths for the bridged contacts (2.7545 (10) and 2.7600 (10) Å) and the nonbridged contacts (2.9494 (13)−2.9353 (14) Å). However, the deviation is not significant, and the electronic argument presented for 3 remains the same. A slight increase in the bite angle of the S−P−S fragment of ca. 117° is observed, and this could be due to the increase in the sterically more demanding substituents, which influence the tetrahedral configuration on the P atom. The length of the P−S bonds can be described in the similar manner as 3, with bond lengths of 2.0456 (18) and 1.965 (2) Å for the P−S(1) and P−S(2), respectively. Clusters 3 and 6 have few literature precedents for structural comparison. However, Lawton described a related dithiophosphate stabilized Cu4L4 (L = S2P(OiPr)), which contained an analogous Cu···Cu core with contacts (2.74 (3) Å and 2.950 (6) Å) and P−S bond lengths 1.972 (8) Å and 2.036 (11) Å.10 Selected bond distances and angles for 3 and 6 can be found in Table 2. Interestingly, the disorder present in 6 is unique and arises from the O-benzyl and anisole groups which interchangeable

Figure 6. Image of emission from single crystal of cluster 7.

between the two conformations does not significantly impact the overall lattice energy. In the preparation of cluster 7, we attempted to remove this positional exchange by introducing a bromide substituent at the meta position of the benzyl ring. However, we were unable to obtain suitable crystals for X-ray analysis of 7, which could be a consequence of loss of the positional exchange between the two groups present in 6. The combination of benzyl and anisole substituents creates a faux symmetry between the two groups, which we suspect is responsible for improving the chances for better quality crystal growth, and attempts to break the symmetry lead to difficulty in crystal growth; this may explain why cluster structures with symmetrical dithiophosphate (or dithiocarbamate) ligands vastly exceed those of asymmetrical dithiophosphonate ligands. D

DOI: 10.1021/acs.inorgchem.9b00783 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

greatly influence the position of the emission band, as shown in Figure 7. Thus, the assigned emission results from the core and

The space behind the coordinating S−P−S fragment is sterically restrictive with certain groups being able to occupy space more efficiently. This has proven to be the dominant factor in successful crystallization and stability of other derivatives which were not of adequate quality for data collection. It should be noted that single-crystal XRD analysis of 1 and 2 revealed an intact copper core analogous to 3; however, the disorder in the organic regions was significant. Collection and refinement data for 3 and 6 can be found in Table 3. Table 3. Collection and Refinement Data for 3 and 6 cluster

3

empirical formula Mw temperature (K) size (mm) crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z ρcal (g cm−3) absorption coefficient (mm−1) F(000) σ range (°) Rint number of data/restraints/ parameters goodness of fit F2 final R indices [I > 2σ(I)] R indices (all data)

6

C40H56Cu4O8P4S8 1299.37 296 0.22 × 0.21 × 0.12 tetragonal I41/a 20.955(3) 20.955(3) 12.350(2) 90.00 90.00 90.00 5422.8(15) 4 1.592 2.019

C56H56Cu4O8P4S8 1491.52 296 0.42 × 0.35 × 0.21 monoclinic C2/c 29.275(7) 14.725(3) 20.737(5) 90 134.715(10) 90 6352(3) 4 1.560 1.735

2656.0 3.82−50.22 0.0492 2416/0/145

3040.0 3.39−50 0.0473 5601/596/497

1.042 R1 = 0.0471, wR2 = 0.1298 R1 = 0.0796, wR2 = 0.1531

1.030 R1 = 0.0462, wR2 = 0.1174 R1 = 0.0775, wR2 = 0.1367

Figure 7. Room temperature emission spectra for 1−7.

the primary coordination sphere with a lifetime dependent on the ability of the ligand to stabilize the excited state. The radiative lifetime of the emissive states proved to be single exponentials with an average of 11.98 μs. The duration of the lifetime and the relatively large Stokes shift indicates phosphorescence with decay from a triplet excited state. Tetranuclear clusters of the configuration Cu4X4L4 (X = halide, L = neutral ligand) have been well-studied,34−38 and recent examples display mechanochromism39 and thermochromism.40,41 In the present study, the emission band position remains constant over a wide temperature range (297−77 K), indicating no thermochromatic behavior. The reduction in temperature does, however, cause an increase in intensity and narrowing of the emission band, which supports a reduction in energy loss due to molecular vibrations and in increase in the radiative pathway.

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CONCLUSION A series of new neutral tetranuclear copper(I) dithiophosphonate clusters of the type Cu4L4 were produced in aqueous medium. The synthesis route avoids the need for Cu(I) precursors, and the sacrificial oxidation of the ligand provides a cost-effective alternative to expensive reagents. Structural analysis confirmed the structure of the tetranuclear Cu(I) core to be a tetrahedron and the ligand substituents play a key role in the success of crystallization. An interesting case of positional disorder was noted for 6. The luminescent properties of the clusters were investigated for the first time, and the emission profiles were very similar in all cases with only a variance in the lifetime of the emission. The current set of ligands does not influence the position of the emission band but does affect the stabilization of the excited state.

Luminescence. The luminescent characteristics of the coinage metals were extensively reported and reviewed by Yam.33 We report the first luminescence data for a series of tetranuclear copper(I) dithiophosphonate clusters. The solidstate photophysical data are summarized in Table 4. A yellowgreen emission color was noted for all clusters (Figure 6). The broad, indistinguishable characteristics of the emission bands are consistent with a MLCT with a contribution from the Cu···Cu interactions (3d → 3d transitions). Variation in the substitution on the dithiophosphonate ligand does not Table 4. Photophysical Data for Clusters 1−7

1 2 3 4 5 6 7

state (T/K)

λex (nm)

λem (nm)

τ (μs)

298 298 298 77 298 298 298 298

364 370 376 380 365 368 367 364

520 519 524 534 522 525 523 523

10.99 12.29 12.23

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EXPERIMENTAL SECTION

All commercially available chemicals were reagent grade and used without further purification. Anisole, phenetole, phosphorus-pentasulfide, hexafluorophosphoric acid, copper(I) oxide, and copper(II) chloride dihydrate were obtained from Sigma-Aldrich. Ammonia gas was purchased from Afrox (South Africa). Ligands17 and [Cu(CH3CN)4]PF624 were prepared according to literature methods. 1H

11.55 10.53 12.58 13.64 E

DOI: 10.1021/acs.inorgchem.9b00783 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and 31P{1H} spectra were recorded on a Bruker Advance 400 MHz spectrometer at 298 K. NMR data are expressed in parts per million (ppm). 1H spectra are referenced internally to residual proton impurity in the deuterated solvents (CDCl3 in all cases). 31P NMR spectra chemical shifts were referenced relative to an 85% H3PO4 in D2O external standard solution. Data are reported as resonance position (δH), multiplicity, assignment, and relative integral intensity. Melting points were determined with an Electrothermal 9100 melting point apparatus. Luminescence spectra were recorded on a PerkinElmer LS-55 spectrometer equipped with a front surface accessory. Variable temperature emission spectra were recorded on a Cary Eclipse B10 fluorescence spectrophotometer. Phosphorescence lifetime measurements were determined from the exponential regression of the intensity values against gate time.33 Synthesis of [Cu4{S2P(1,4-C6H4OEt)(OCH3)}4] (1): A Schlenk flask was charged with (4-C6H4OEtP(S)S)2 (700 mg, 1.16 mmol) and placed under vacuum for 10 min. The solid was heated to 80 °C before the addition of CH3OH (110 mg, 3.43 mmol). The temperature was maintained for 15 min, until a clear residue was observed. After cooling the residue in an ice bath, anhydrous NH3 gas was bubbled through the residue, resulting the formation of a white precipitate. The excess ammonia was removed in vacuo and the salt immediately dissolved in ETOH:H2O (1:1) solution. A concentrated aqueous solution of CuCl2•2H2O (280 mg, 1.64 mmol) was added dropwise, resulting in a short-lived brown color which immediately dissipates to yield a yellow precipitate. The precipitate is collected by filtration and washed with water (2 × 5 mL) followed by ETOH (2 × 5 mL). The crude product was recrystallized from dichloromethane to yield a pale yellow powder. Yield = 352 mg (70%). Melting point 156 °C. 1H NMR (400 MHz, CDCl3): δ (ppm), J (Hz) 1.42 (t, CH3, 3H, JH−H = 6.98), 3.86 (d, POCH3, 3H, JP−H = 14.85), 4.07 (q, CH2, 2H, JH−H = 6.99), 6.93 (dd, ArCH, 2H, JH−H = 8.72, JP−H = 3.16), 7.97 (dd, ArCH, 2H, JH−H = 8.66, JP−H = 13.58) 31P NMR (400 MHz, CDCl3): δ (ppm) 103.32. ESI-MS (m/z) (calcd) 1264.6251 (1264.71) for ([Cu{(S2P(1,4-C6H4OEt)(OCH3)}]4 + Na+). Synthesis of [Cu4{S2P(1,4-C6H4OEt)(OCH2CH3)}4] (2): Yield = 312 mg (83%) Melting Point 158−159 °C. 1H NMR (400 MHz, CDCl3): δ (ppm), J(Hz) 1.36 (t, CH3, 3H, JH−H = 7.02), 1.42 (t, CH3, 3H, JH−H = 7.02), 4.06 (q, OCH2, 2H, JH−H = 6.98), 4.26 (m, POCH2, 2H), 6.92 (dd, ArCH, 2H, JH−H = 8.60, JP−H = 3.44), 7.97 (dd, ArCH, 2H, JH−H = 8.36, JP−H = 13.41) 31P NMR (400 MHz, CDCl3): δ (ppm) 100.20. ESI-MS (m/z) (calcd) 1322.7241(1322.77) for ([Cu{(S2P(1,4-C6H4OEt)(OCH2CH3)}]4 + Na+). Synthesis of [Cu4{S2P(1,4-C6H4OMe)(OCH(CH3)2)}4] (3): Yield = 298 mg (76%). Melting point 160−161 °C. 1H NMR (400 MHz, CDCl3): δ (ppm), J(Hz) 1.27 (d, CH3, 6H, JH−H = 6.04), 3.76 (s, OCH3, 3H), 5.10(m, 1H), 6.82 (dd, ArCH, 2H, JH−H = 8.74, JP−H = 3.02), 7.90(dd, 2H, JH−H = 8.60, JP−H = 13.40 Hz) 31P NMR (400 MHz, CDCl3): δ (ppm) 31P NMR 98.5. ESI-MS (m/z) (calcd) 1320.6203 (1320.78) for ([Cu{(S2P(1,4-C6H4OMe)(OCH2CH3)}]4 + Na+). Synthesis of [Cu4{S2P(1,4-C6H4OEt)(OCH2CH2CH2CH3)}4] (4):Yield = 320 mg (78%). Melting point 160.2−160.4 °C. 1H NMR (400 MHz, CDCl3): δ (ppm), J(Hz) 0.92 (t, CH3, 3H, JH−H = 7.38), 1.42 (t, CH3, 3H, JH−H = 7.02), 1.44 (m, CH2,2H), 1.705(p, CH2, 2H, JH−H = 6.97), 4.07 (q, OCH2, 2H, JH−H = 6.99), 4.21 (b, OCH2, 2H) 6.91(dd, ArCH, 2H, JH−H = 8.75, JP−H = 3.14), 7.96 (dd, ArCH, 2H, JH−H = 8.60, JP−H = 13.49) 31P NMR (400 MHz, CDCl3): δ (ppm) 100.15. ESI-MS (m/z) (calcd) 1432.9615 (1432.90) for ([Cu{(S2P(1,4-C6H4OEt)(OCH2CH3)}]4 + Na+). Synthesis of [Cu4{S2P(1,4-C6H4OEt)(OCH2CHCH2)}4] (5): Yield = 254 mg (65%). Melting point: 159 °C. 1H NMR (400 MHz, CDCl3): δ (ppm), J(Hz) 1.42 (t, CH3, 3H, JH−H = 6.98), 4.06 (q, OCH2, 2H, JH−H = 7.03), 4.71 (m, POCH2, 2H), 5.21 (d,, 1H, JH−H = 8.60), 5.35 (dd,, 1H, JH−H = 17.16, JP−H = 1.40), 5.99(m,CH,1H) 6.92(dd, ArCH, 2H, JH−H = 8.78, JP−H = 3.26), 7.98 (dd, ArCH, 2H, JH−H = 8.70, JP−H = 13.71) 31P NMR (400 MHz, CDCl3): δ (ppm) 101.53. ESI-MS 1366.7764 (1366.7773) (m/z) (calcd) for ([Cu{(S2P(1,4-C6H4OEt)(OCH2CHCH2)}]4 + Na+).

Synthesis of [Cu4{S2P(1,4-C6H4OMe)(OCH2C6H5)}4] (6): Yield = 345 mg (77%). Melting point 152−153 °C 1H NMR (400 MHz, CDCl3): δ (ppm), J(Hz) 3.85 (s, OCH3, 3H), 5.29 (s,2H), 6.95 (dd, ArCH, 2H, JH−H = 8.78, J P−H = 3.14), 7.35 (d, ArCH, 2H, JH−H = 7.56 Hz), 7.44 (d, ArCH, 2H, JH−H = 7.00 Hz), 8.04 (dd, ArCH, 2H, JH−H = 8.64, JP−H = 13.65) 31P NMR (400 MHz, CDCl3): δ (ppm) 101.6. ESI-MS (m/z) (calcd) 1512.6606 (1512.78) for ([Cu{(S2P(1,4-C6H4OMe)(OCH2C6H5)}]4 + Na+). Synthesis of [Cu4{S2P(1,4-C6H4OEt)(OCH2C6H4Br)}4] (7): Yield = 357 mg (67%). Melting point 155−156 °C 1H NMR (400 MHz, CDCl3): δ (ppm), J(Hz) 1.42 (t, CH3, 3H, J H−H = 7.00), 4.07 (q, OCH2, 2H, JH−H = 6.98), 5.21 (d, POCH2, JP−H = 8.12 Hz), 7.19 (t, m-ArH, 1H, JH−H = 7.80), 7.56 (s, o-ArH, 1H), 7.42 (d, p-ArH, 1H, JH−H = 7.84), 7.33 (d, o-ArH, 1H, JH−H = 7.72 (dd, ArCH, 2H, J H−H = 8.76, JP−H = 3.24), 7.98 (dd, ArCH, 2H, J H−H = 8.66, J P−H = 13.79) 31P NMR (400 MHz, CDCl3): δ (ppm)102.60.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00783. Crystallographic details and NMR and mass spectra (PDF) Accession Codes

CCDC 1900695−1900696 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

C. W. Liu: 0000-0003-0801-6499 Werner E. van Zyl: 0000-0002-2012-8584 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.N.P and W.E.v.Z. acknowledge the National Research Foundation of South Africa (NRF) and the Eskom TESP program for financial assistance.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00783 Inorg. Chem. XXXX, XXX, XXX−XXX