Assembly of Cu(I) alkynyl complexes: From Cluster to Infinite Cluster

signaling.1-9 As the alkynyl group can serve as a good σ-donor and weak π-acceptor ligand, as well as function as a good π donor through pπ–dπ ...
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Assembly of Cu(I) alkynyl complexes: From Cluster to Infinite Cluster-Based Framework Yunpeng Xie, Wen Jun-Bo, Changwang Pan, Guangxiong Duan, Lan-Yun Li, and Xing Lu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00803 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Crystal Growth & Design

Assembly of Cu(I) alkynyl complexes: From Cluster to Infinite Cluster-Based Framework Yun-Peng Xie,* Jun-Bo Wen, Chang-Wang Pan, Guang-Xiong Duan, Lan-Yun Li, Xing Lu*

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China.

Abstract Variation of the reaction conditions with Cu powder, Cu(NO3)2 and tBuC≡CH as starting

materials

afforded

three

[Cu14(tBuC≡C)10(NO3)4(MeOH)2]∙H2O

new (1),

Cu(I)

alkynyl

complexes,

Cu15(tBuC≡C)14(NO3)

namely, (2)

and

[Cu14(tBuC≡C)8(NO3)2(CN)4]n (3). Single-crystal X-ray analysis revealed that clusters 1 and 2 consist of Cu14 and Cu15 cores co-stabilized by strong by σ- and π-bonded tertbutylethynides and nitrates. Complex 3 displays an intriguing 3D cyano-bridged Cu(I) alkynyl cluster-based framework, in which the existence of CN- is assumed to have originated from the Cu(I)-mediated C–C cleavage reaction of acetonitrile under mild solvothermal conditions.

Introduction Recent growing interest in the study of coinage metal (CuI, AgI, and AuI) alkynyl cluster complexes stems from their structural diversity and their photophysical properties, consequent to promising applications in optoelectronics and luminescence signaling.1-9 As the alkynyl group can serve as a good σ-donor and weak π-acceptor ligand, as well as function as a good π donor through pπ–dπ overlap with its bonded metal atom, a large variety of Ag(I) and Au(I) alkynyl cluster complexes have been reported in the recent literature.10-30 However, only several Cu(I) alkynyl clusters have 1

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been reported, presumably due to the unstable nature of Cu(I)ion through air oxidation.31-44 Furthermore, the poor solubility of Cu(I) alkynyl complexes makes the isolation of discrete clusters more challenging. The effective strategies for the synthesis of Cu(I) alkynyl clusters have been sought.32-44 For example, Tasker et al. synthesized a series of [Cux+y(hfac)x(C≡CR)y] (hfac = hexafluoropentanedionate; R = n-propyl, nbutyl, n-pentyl, n-hexyl) through the reaction of cuprous oxide, hfacH and alkynes in n-hexane.32-35 Recently, Mak et al. proposed a facile synthesis strategy, where Cu(II) salts and Cu powder precursors were employed to react with alkynes construct a series of high-nuclearity copper(I) alkynyl clusters, such as Cu24, Cu33, Cu48, Cu62, and Cu92.41,43 On the other hand, the chemistry of cyano-bridged coordination polymers has attracted a great deal of attention due to their potential applications in the field of electrical conductivity, molecule-based magnets, luminescent materials.45-61 Recent research showed metal–cyanide coordination polymers can be produced through the solvothermal synthesis method and without introducing cyanometalates as the source.62-66 The cyano anions were generated in situ from C–C bond cleavage reaction of organonitrile in the presence of metal ions. There are a few reported d10 metal ioncatalyzed C–C cleavage reactions of acetonitrile under relatively mild conditions.67-70 Some new strategies for the cleavage of C–C bonds in acetonitrile by transition-metal complexes still need to be developed urgently. Motivated by the above new paradigm in assembly of Cu(I)-alkynyl complexes, we synthesized

two

air-stable

high-nuclearity

Cu(I)

alkynyl

clusters,

[Cu14(tBuC≡C)10(NO3)4(MeOH)2]∙H2O (1) and Cu15(tBuC≡C)14(NO3) (2), and a 3D cyano-bridged Cu(I) alkynyl cluster-based framework, [Cu14(tBuC≡C)8(NO3)2(CN)4]n (3).

Results and Discussion The reaction of Cu powder, Cu(NO3)2 and tBuC≡CH in methanol afforded red crystals of [Cu14(tBuC≡C)10(NO3)4(MeOH)2]∙H2O (1). Single-crystal X-ray analysis 2

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reveals that complex 1 is a tetradecanuclear Cu(I) alkynyl cluster, whose structure is depicted in Figure 1. Complex 1 can be described as consisting of three Cu(I) ions sandwiched

by

two

Cu(I)

alkynyl

clusters,

[Cu5(tBuC≡C)6]-

and

[Cu6(tBuC≡C)4(NO3)2(MeOH)2] (Figure 2). In the anionic [Cu5(tBuC≡C)6]- cluster, the five ethynide groups are each bound to two copper(I) atoms to produce the Cu5 ring as shown in Figure 2a. The other ethynide group consolidates the Cu5 ring via Cu(I)–C interactions. On the other hand, the hexanuclear [Cu6(tBuC≡C)4(NO3)2(MeOH)2] cluster is composed of six copper atoms bridged by four ethynide ligands and two nitrates, in which three copper atoms are further bound by two methanol molecules. The central three Cu(I) ions attaches to two outer [Cu5(tBuC≡C)6]- and [Cu6(tBuC≡C)4(NO3)2(MeOH)2] clusters through the two nitrate, Cu(I)–C and Cu(I)···Cu(I) interactions. Of the ten peripheral tBuC≡C- ligands, eight adopt a µ3 and two a µ4 bridging mode to coordinate to copper atoms. The Cu(I)−C bond distances of 1.874−2.382 Å are in accord with the values observed in [tBuC≡C]24.36 The Cu(I)···Cu(I) contacts fall in the range 2.471−2.797 Å, as seen in many other Cu(I)clusters.32-44 Additionally, the crystal structure also contains a water molecule in the unit cell.

Figure 1. Perspective view of Cu14(tBuC≡C)10(NO3)4 in 1. All hydrogen atoms and methanol 3

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molecules are omitted for clarity. The carbon atoms of the C≡C are represented by black spheres and the C≡C triple bonds are shown as thick black rods. The Cu(I)–C bonds are indicated by broken lines, and the central three copper ions are represented by yellow spheres. Color code: Cu, orange; N, blue; O, red; C (alkynyl), black; C, gray.

Figure 2.

The Cu(I) alkynyl clusters [Cu5(tBuC≡C)6]- and [Cu6(tBuC≡C)4(NO3)2] in 1. Color code:

Cu (shell), orange; N, blue; O, red; C (alkynyl), black; C, gray.

Notably, the Cu14 cluster in 1 is unlike to previously reported tetradecanuclear copper(I) ethynide cluster, [Cu14(tBuC≡C)10(CH3COO)4],44 whose skeleton can be viewed as a distorted cube and two Cu2 folding lines and two triangles which have two edge-shared Cu atoms. The synthesis procedure used to obtain Cu15(tBuC≡C)14(NO3) (2) is similar to that used for 1, except that anhydrous MgSO4 was added to the reaction mixture. Complex 2 comprises a [Cu10(tBuC≡C)10] cluster shell consolidated by one nitrate anion, which encapsulates an inner [Cu5(tBuC≡C)4]+ cluster (Figure 3). The structure of the Cu5 unit is consolidated by four tBuC≡C- ligands (Figure 4a). On the other hand, the Cu5 unit is further attached to an outer [Cu10(tBuC≡C)10] shell through the four tBuC≡C- ligands, which adopt µ3 and µ4 ligation modes to coordinate to copper atoms from the Cu5 core and Cu10 shell. The combination of gold/silver atoms with alkynyl ligands can generate different surface motifs, which are classified as V-shaped, linear and L-shaped staple structures. L-shaped tBuC≡C–Cu-C≡CtBu is found in the [Cu10(tBuC≡C)10] cluster. Note that in each of the tBuC≡C–Cu-C≡CtBu motifs, the two tBuC≡C- ligands are in an 4

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approximately perpendicular arrangement to form an L-shaped motif (Figure 4c). Five tBuC≡C–Cu-C≡CtBu

staples are bridged by five copper atoms to afford a

[Cu10(tBuC≡C)10] shell (Figure 4b). Ten tBuC≡C- ligands, nine adopt the µ2-η1,η1 ligation mode and the remaining one uses the µ3-η1,η1,η1 mode. The Cu(I)···Cu(I) distances in complex 2 are in the range of 2.416–2.778 Å. The Cu(I)−C bond lengths range from 1.896 to 2.576 Å.

Figure 3. Perspective view of Cu15(tBuC≡C)14(NO3) (2). All hydrogen atoms and are omitted for clarity. The carbon atoms of the C≡C are represented by black spheres and the C≡C triple bonds are shown as thick black rods. The Cu(I)–C bonds are indicated by broken lines. Color code: Cu (shell), orange; Cu (core), yellow; N, blue; O, red; C (alkynyl), black; C, gray.

Figure 4.

(a) and (b) The Cu(I) alkyny cluster core [Cu5(tBuC≡C)4] and shell [Cu10(tBuC≡C)10] 5

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in 2. (c) L-shaped tBuC≡C–Cu-C≡CtBu staple. Color code: Cu (shell), orange; Cu (core), yellow; N, blue; O, red; C (alkynyl), black; C, gray.

The solvothermal reaction of Cu powder, Cu(NO3)2 and

tBuC≡CH

in

methanol/acetonitrile afforded colorless crystals of [Cu14(tBuC≡C)8(NO3)2(CN)4]n (3). Single-crystal X-ray diffraction analysis reveals that complex 3 is a 3D network based on the [Cu14(tBuC≡C)8]6+ cluster as the subunit and CN- as the linker, in which CNanion is generated from Cu(I)-mediated C–C bond cleavage of acetonitrile under solvothermal conditions. It crystallizes in the monoclinic space group P21/c. The asymmetric unit of 3 consists of a Cu7 cluster, four tBuC≡C- ligands, one capping NO3anion, and two CN- bridges. Notably, the C and N atoms of the CN- ligands were processed into two disordered parts in the ratio of 0.5 : 0.5. Of the four independent tert-butylethynide anions, the two ethynide group (C1≡C2 and C21≡C22) is capped by a butterfly-shaped Cu4 basket in the μ4-η1,η1,η2,η2 coordination mode, and such Cu4 baskets are bridged by the remaining two ethynide groups (C9≡C10 and C15≡C16) to afford a (tBuC≡C)4⊃Cu8 aggregate (Figure 5a), with Cu(I)−C bond lengths ranging from 1.916 to 2.645 Å. Two Cu8 coalesce by sharing two vertexes to produce a [Cu14(tBuC≡C)8]6+ cluster with Cu(I)···Cu(I) in the range of 2.468–2.782 Å, and such cluster

is

consolidated

by

NO3-

two

anions

(Figure

5b).

Each

[Cu14(tBuC≡C)8(NO3)2(CN)4] subunit cluster linked six neighboring clusters through CN- anions, affording a three dimensional coordination network (Figures 6 and 7). The bond lengths of the cyano anions in 3 ranging from 1.123 to 1.192 Å, as well as the bond lengths of Cu–N/Cu–C in the range of 1.891 to 1.947 Å, are comparable to the reported data.67-70 The cyano anions were also confirmed by the IR data. The band of 2148 cm-1 corresponds to the stretching vibrations of the cyano anions. On the basis of the experimental results, a reasonable formation mechanism of complex 3 has been proposed (Scheme 1). Copper(I) ions from [tBuC≡CCu]n or CuNO3 are used to catalyze the C–C bond cleavage of acetonitrile in situ under mild solvothermal conditions and then [tBuC≡CCu]n and CuNO3 capture the resulting CN- to form a 3D cyano-bridged multinuclear Cu(I) alkynyl network. 6

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Figure 5. (a) The view of the (tBuC≡C)4⊃Cu8 aggregate and the [Cu14(tBuC≡C)8(NO3)2(CN)4] subunit in 3. All hydrogen atoms and are omitted for clarity. The carbon atoms of the C≡C are represented by black spheres and the C≡C triple bonds are shown as thick black rods. The Cu(I)–C bonds are indicated by broken lines. Color code: Cu, orange; N, blue; O, red; C, gray.

Figure 6. Each [Cu14(tBuC≡C)8(NO3)2(CN)4] subunit cluster linked six neighboring clusters through CN- anions. The carbon and nitrogen atoms of the C≡N are represented by sky blue spheres and the C≡N triple bonds are shown as sky blue rods. Color code: Cu, orange; yellow; N, blue; O, 7

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red; C, gray.

Figure 7. The 3D cyano-bridged multinuclear Cu(I) alkynyl network. All peripheral tBuC≡C- and NO3- groups are omitted for clarity.

Scheme 1. The proposed formation mechanism of complex 3 containing the solvothermal in situ generation of CN- from C–C bond cleavage reaction of acetonitrile in the presence of Cu(I) ions.

The syntheses and structural characterizations of 1-3 indicate that the nitrate is directly involved in the formation of Cu(I) alkynyl complexes, and their structures are affected by variation of the reaction condition and the solvents employed. The 8

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anaerobic condition is not obligatory during the syntheses. However, anhydrous condition played a role in the formation of Cu(I) alkynyl cluster. The synthesis procedures used to obtain 1 and 2 are studied under hydrous and anhydrous conditions, respectively. This is demonstrated by the reactions giving complex 2 using anhydrous MgSO4 as a desiccant. The crystal structure of 2 does not contains any water molecule. However, in absent of MgSO4, the crystal structure of 1 contains a water molecule. It is remarkable that, in the presence of acetonitrile, the CN- ion is originated from the Cu(I)-mediated C–C cleavage reaction of acetonitrile under mild solvothermal conditions, then CN- ion functions as bridging ligand for the generation of a 3D network based on Cu(I) alkynyl clusters. Our attempts to prepare complex 3 with cyanometalates (CuCN) instead of Cu(NO3)2•3H2O and Cu as reactants, were unsuccessful and always yielded intractable solids.

Photophysical Properties. The UV−vis absorption data of complexes 1-3 are presented in Figures S1 and S2. In a dichloromethane solution, cluster 1 displays absorption band at ca. 452 nm, whereas cluster 2 exhibits two peaks centered at 466 and 521 nm. The absorptions in the UV and visible regions should originate from π–π* transition of tBuC≡C- and ligand-to-metal charge transfer, respectively. Complex 3 possesses two absorption envelopes at 330 and 374 nm. The peak at 330 nm can be assigned to internal ligand transitions (π–π*, n–π*). In addition, complex 3 shows one weak absorption in the region 374 nm which can be tentatively assigned to M–L or L–M charge transfer. The d–d absorption band is not observed due to the d10 electronic configuration of CuI ion. Upon excitation under UV light of λ = 527 nm at room temperature, the emission spectrum of 1 in dichloromethane at room temperature displays band centered at 621 nm. Under 563 nm excitation, the fluorescence emission spectrum of complex 2 in dichloromethane solution is determined. In the emission spectrum of 2, two main emission peaks at 573 and 617 nm are displayed. Their NIR emission bands could be tentatively assigned to a ligand-to-metal charge transfer (LMCT) transition associated with the cluster-centered (CC) transition which is modified by cuprophilic interactions, 9

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which has been assigned for polynuclear Cu15, Cu16, Cu20, Cu17, and Cu18 clusters.36-44 Complex 3 is non-emissive in solution at ambient temperature. Compound 3 do not display the luminescent property.

Figure 8. Fluorescence emission and excitation spectra of complex 1 in dichloromethane at room temperature.

Figure 9. Fluorescence emission and excitation spectra of complex 2 in dichloromethane at room temperature.

10

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Conclusion In summary, we have synthesized and structurally characterized three unprecedented Cu(I) alkynyl complexes that were fabricated with generated Cu(I) atoms through a facile comproportionation reaction. Clusters 1 and 2 exhibit Cu14 and Cu15 cores are protected by tBuC≡C- and NO3- ligands. Compound 3 provides a precedent of a wonderful 3D cyano-bridged alkynyl cluster-based framework. The cyano anions groups in 3 are all in situ generated from the Cu(I)-mediated C–C cleavage reaction of acetonitrile. Furthermore, the in situ synthesized strategy here proposes a new approach for the construction of cyano-bridged metal organic frameworks.

Experimental Materials and methods: All reagents employed are commercially available and used as received without further purification. Elemental analyses for C, H, and N were performed with a PerkinElmer 2400 CHN elemental analyzer. The Fourier transform infrared (FT-IR) spectra were recorded from KBr pellets in the range 4000–400 cm−1 on a Bruker VERTEX 70 spectrometer. The Vis-NIR experiments were carried out on a PE Lambda 750S UV-vis-NIR spectrophotometer. Fluorescent spectra were recorded on a FP-6500 fluorescence spectrometer, using 5 mm path length cuvettes. Crystal data of complexes 1-3 were collected on a Bruker D8 Quest diffractometer with Mo-Kα radiation (λ = 0.71073 Å). The structures were solved with direct method and were refined with SHELXL-2014. Synthesis of 1: CuII(NO3)2·3H2O (0.50 mmol, 0.121 g), Cu (0.063 g, 1.00 mmol) and tBuC≡CH

(0.42 mL, 2 mmol) were added to 10 mL of methanol under stirring. After 8

h, an orange-red solution was collected by filtration and stored at -4 ℃ . The red crystals of 1 were formed within 20 days. Yield: ca. 17 % (based on CuII). Elemental analysis (%) calcd for C62H98Cu14N4O15: C 36.70, H 4.87, N 2.76; found: C 36.92, H 4.63, N 2.93; Selected IR data (KBr): 2027 (C≡C). Synthesis of 2: CuII(NO3)2·3H2O (0.50 mmol, 0.121 g), Cu (0.063 g, 1.00 mmol) and tBuC≡CH

(0.42 mL, 2 mmol) were added to 10 mL of methanol under stirring. After 8 11

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h, anhydrous MgSO4 was added, and an orange-red solution was collected by filtration and stored at -4 ℃. The red crystals of 2 were formed within 3 days. Yield: ca. 25 % (based on CuII). Elemental analysis (%) calcd for C84H126Cu15NO3: C 46.90, H 5.90, N 0.65; found: C 46.71, H 6.21, N 0.85; Selected IR data (KBr): 2032 (C≡C). Synthesis of 3: CuII(NO3)2·3H2O (0.25 mmol, 0.121 g), Cu (0.032 g, 0.50 mmol) and tBuC≡CH

(0.42 mL, 2 mmol) were added to 10 mL of methanol under stirring. After 8

h, 1 mL acetonitrile was added, and an orange-red solution was collected by filtration. The above mixture was sealed in a 25 mL Teflon-lined reaction vessel and heated at 80 oC

for 48 h. After cooling to room temperature, the colorless crystals of 3 can be

obtained. Yield: ca. 15 % (based on CuII). Elemental analysis (%) calcd for C26H36Cu7N3O3: C 35.35, H 4.11, N 4.76; found: C 35.57, H 4.33, N 4.51; Selected IR data (KBr): 2148 (C≡N).

Associated Content Supporting Information Figure S1 and crystallographic data (CIF) for the new structure. This material is available free of charge via the Internet at http://pubs.acs.org. Accession Codes CCDC 1897139-1897141 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, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Author Information Corresponding Author [email protected] (Y.-P. Xie); [email protected] (X. Lu).

Notes The authors declare no competing financial interest. 12

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Acknowledgements We gratefully acknowledge financial support by National Natural Science Foundation of China (No. 21771071, 51672093 and 51472095). We thank the Analytical and Testing Center at the Huazhong University of Science and Technology for PL measurements.

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Table of Contents synopsis

Three new Cu(I) alkynyl complexes have been synthesized from the reaction of Cu powder, Cu(NO3)2 and tBuC≡CH. The figure shows the intriguing 3D cyano-bridged alkynyl cluster-based framework coupled with Cu(I)-mediated C–C bond cleavage of acetonitrile.

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