Binuclear Copper(I) Complex Constructed by two End-to-End, µ-1,3-Azide bridges, [Cu2(PPh3)4(µ-N3)2]

Introduction

The azide group (N₃⁻) is a versatile bridging agent. It can bond to metal centers in several ways, as a terminal ligand via one nitrogen donor, as a bridge in the μ-1,1 (end-on, EO) and μ³-1,1,1 modes via one nitrogen donor, and in the μ-1,3 (end-to-end, EE) and μ³-1,1,3 via both of the peripheral nitrogen donor atoms¹. Azide bridged metal complexes have been received much attention because of their remarkable structural diversity and rich magnetic behavior². Its bonding in transition metal centres has generated varieties of magnetic materials. Non-transition metal complexes of N₃⁻ are less developed compared to that of transition metal complexes³.

In the past few years we have studied complexation of multi N-donor ligands⁴⁻⁶. In continuation of our previous studies, in this work the preparation of copper(I) complex, [Cu₂(PPh₃)₄(μ-N₃)₂] (1), with mixed-ligand, azide and triphenylphosphine, was described. This complex was characterized by elemental analysis, FT-IR, Raman, UV-Vis, ¹H NMR and ¹³C NMR spectroscopy, molar conductivity, and X-ray crystallography.

Material And Methods

General methods

All starting chemicals and solvents were reagent or analytical grade and used as received. [Cu(PPh₃)₃N₃]was prepared accordingly to the literature⁷. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker BRX-100 AVANCE instrument operating at 100 MHz; chemical shifts are given in parts per million, with values in reference to an internal standard of TMS. Raman spectrum was obtained using a Nicolet Model 910 Fourier-transform spectrometer. The infrared spectrum of a KBr pellet was recorded in the range 400–4000 cm⁻¹ using a FT-IR 8400-Shimadzu spectrometer. The electronic spectrum was recorded in DMF with a Shimadzu model 2550 UV/Vis spectrophotometer (190–900 nm). The molar conductance value of 1.0 × 10⁻³ M DMF solutions of the complex was measured with a Metrohm 712 conductometer. The carbon, hydrogen and nitrogen contents were determined in a Thermo Finnigan Flash Elemental Analyzer 1112 EA. Melting point was determined using a Barnsted Electrothermal 9200 electrically heated apparatus.

Synthesis of [Cu₂(PPh₃)₄(μ-N₃)₂], 1.

A hot ethanolic solution of isatine-3-thiosemicarbazone (1 mmol, 0.23 g ) was added with stirring to a hot solution of [Cu(PPh₃)₃N₃](2 mmol, 0.18 g) in CH₂Cl₂ (30 mL). The reaction mixture was stirred at 70 °C for 3 h and filtered. Colorless crystals suitable for X-ray diffraction were obtained from the solution after standing for 7 days. Yield (0.78 g) 62 %; m.p.: 213 °C. Anal. Calcd for C₇₂H₆₀Cu₂N₆P₄ (%): C, 68.62; H, 4.80; N, 6.67. Found: C, 68.11; H, 4.75; N, 6.69. IR (KBr, cm⁻¹): 3055 vw (ν CH_ar), 2052 s (ν_a NNN), 1435 w (ν C=C), 1311 w (ν_s NNN), 1095 w (ν P−C), 748 m (γ CH), 628 w (δ NNN), 509 m (ν Cu−N). Raman (cm⁻¹): 1424 s (ν C=C), 1308 w (ν_s NNN), 1020 m (ν P−C), 771 m (γ CH), 612 w (δ NNN), 493 m (ν Cu−N), 311 m (ν Cu−P). ¹H NMR (100 MHz, [D₆]-DMSO): 7−7.5 (m, 60 H) ppm. ¹³C NMR (100 MHz, [D₆]-DMSO): 128.6, 129.6, 132.3, 133.2 ppm. UV-Vis (DMF, λ_max (nm)/ε): 444/1.1× 10⁴ (n→π^*), 362/1.2 × 10⁴ (n→π^*), 256/3.2 × 10⁴ (π→π^*). Molar conductivity (1 × 10⁻³ mol L⁻¹, Ethanol): 9.837 Ω⁻¹ cm² mol⁻¹.

Crystal structure determination and refinement

The data collection for 1 was carried with a Bruker APEX-II CCD diffractometer, using graphite-monochromatized MoKα (λ = 0.71073 Å) radiation at 296 K. The data were integrated with SAINT⁸ and corrected for Lorentz and polarization effects. Absorption was corrected for using SADABS⁹. The structure was solved by Patterson methods, implemented in SHELXS-97¹⁰. Refinement by full-matrix least-squares methods based on F² values against all reflections has been performed by SHELXL-97¹⁰, including anisotropic displacement parameters for all non-H atoms. The position of hydrogen atoms belonging to the carbon atoms Csp² were geometrically optimized applying the riding model [Csp²–H, 0.93 Å; U_iso(H) = 1.2 U_eq(C)]. Calculations concerning the molecular geometry, the verification of space group and the analysis of hydrogen bonds were performed with PLATON¹¹. Table 1 contains crystallographic data and details of the data collection and structure refinement. Selected bond lengths (Å) and angles (°) and dimensions of the hydrogen bonds (Å and °) for complex are listed in table 2 and table 3, respectively.

Table 1: Crystal data and structure refinement for 1.

Empirical formula	C72H60Cu2N6P4
Formula weight (g mol−1)	1260.22
Temperature (K)	296
Crystal system	Monoclinic
Space group	P21/c
Unit cell dimensions (Å, °)
A	23.4833(18)
B	13.6611(10)
C	19.9645(15)
Β	106.522(1)
Volume (Å3), Z	6140.3(8), 4
Calculated density (g cm−3)	1.363
Absorption coefficient (mm−1)	0.85
F(000)	2608
Crystal size (mm3)	0.10 × 0.10 × 0.05
θ range for data collection (°)	1.7–27.5
h, k, l ranges	−26:30, −17:16, −25:21
Reflections collected	33960
Independent reflections	13812
Rint	0.026
Data / restraints / parameters	13812 / 0 / 752
Goodness-of-fit on F2	1.07
Final R indices [I > 2σ(I)]	R1 = 0.039, wR2 = 0.1012
R indices (all data)	R1 = 0.0620, wR2 = 0.123
Largest diff. peak and hole (e.Å−3)	0.47 and −0.60

Table 2: Selected bond length (Å) and angles (°) for 1 with estimated standard deviations in parentheses.

Distances
Cu1–N1	2.115(2)			Cu2–N3	2.101(2)
Cu1–N4	2.114(2)			Cu2–N6	2.103(2)
Cu1–P1	2.2721(8)			Cu2–P3	2.2458(7)
Cu1–P2	2.2614(7)			Cu2–P4	2.2600(7)
N1–N2	1.186(3)			N2–N3	1.174(3)
N4–N5	1.171(3)			N5–N6	1.185(3)
Angles
N1–Cu1–N4		98.61(10)	N3–Cu2–N6		99.12(9)
N1–Cu1–P1		103.08(8)	N3–Cu2–P3		114.22(7)
P1−Cu1−P2		120.88(3)	P3−Cu2−P4		121.69(3)
P2–Cu1–N4		108.31(7)	P4–Cu2–N6		107.75(8)
N1–N2–N3		177.0(3)	N4–N5–N6		177.6(3)

Table 3: Dimensions of the hydrogen bonds (Å and °) in 1

D–H···A	d(D–H)	d(H···A)	<(DHA)	d(D···A)	Symmetry code
C29–H29A···C4	0.930	2.862	140.0	3.624(4)	1− x, − 0.5 + y, 1.5 − z
C30–H30A···C11	0.930	2.857	155.9	3.725(5)	1− x, − 0.5 + y, 1.5 − z
C40–H40A···C57	0.930	2.839	143.9	3.633(4)	x, 1 + y, z
C46–H40A···C41	0.930	2.888	154.1	3.747(5)	x, 1.5 − y, 0.5 + z
C51–H51A···N3	0.929	2.576	148.8	3.405(4)	2− x, 1 − y, 2 − z
C63–H63A···C58	0.929	2.746	140.6	3.514(4)	x, 0.5 − y, 0.5 + z

 

Results and Discussion

Similar structure to 1 was reported previously¹². In this paper we used new synthetic rout and different precursors for synthesis of 1 and extensive spectroscopic methods for characterization. Also the data collection of 1 was carried in different temperature respect to the previous report.

Presence of N₃‾ in the structure of 1 affects IR and Raman spectra in three regions including 2052 and 1311 cm⁻¹ for asymmetric and symmetric NNN stretches and 628 cm⁻¹ for NNN bending¹³. Information about the low-frequency vibrations of the metal-ligand bonds can be obtained by Raman spectroscopy¹⁴. The Cu−N and Cu−P stretching vibrations have been assigned to the bands at 493 and 311 cm⁻¹, respectively, consistent with the values reported in the literature¹⁵. UV–Vis spectrum of 1 in DMF exhibited a broad absorption with the λ_max at 444 and 362 nm for n→π^* transitions and 256 nm for π→π^* transition.

Description of the crystal structure

The crystal structure of 1 was determined by X-ray single-crystal diffraction. The molecular graphics were drawn with ORTEP-III¹⁶ and Mercury¹⁷.

Figure 1. The ORTEP-III diagram of the molecular structure of 1. The ellipsoids are drawn at the 50 % probability level. The hydrogen atoms were omitted for clarity. Click here to View figure

In the crystal structure of 1 (figure 1), each copper atom has a distorted tetrahedral coordination environment. Two sites are occupied by nitrogen atoms of the two azide groups with the Cu–N bond lengths in the range of 2.114(3)–2.115(2) Å. The two other sites are occupied by two phosphorus atoms of the two triphenylphosphine groups with the Cu–P bond lengths in the range of 2.2614(6)–2.2720(8) Å. If the distortion in 1 is disregarded, with a C₂ axis and two perpendicular C₂ axes to C₂ axis it has D₂ symmetry (figure 2).

Figure 2. Showing the D2 symmetry. All C−H bonds were omitted for clarity. Each CuN2P2 unit is shown as an tetrahedron. Click here to View figure

 

In the crystal network of 1 (figure 3), there are intermolecular C–H···N and C–H···C hydrogen bonds (table 3). The nitrogen atoms in hydrogen bonds act as proton acceptors whereas the carbon atoms participate in hydrogen bonding as both proton donors and acceptors. In addition to these hydrogen bonds there are π−π stacking¹⁸ between parallel aromatic rings belonging to adjacent complexes.

Figure 3. Crystal structure of complex 1, showing the hydrogen bonded chain formation in bc plane. Only the hydrogen atoms involved in hydrogen bonding are shown. Click here to View figure

 

Supplementary material

CCDC 917542 for [Cu₂(PPh₃)₄(μ-N₃)₂] (1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Acknowledgement

We are grateful to Payame Noor University of I. R. Iran for financial support.

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