Synthesis, crystal structures and magnetic properties of copper(II) complexes with nitronyl nitroxide

Polyhedron 18 (1999) 781–785

Synthesis, crystal structures and magnetic properties of copper(II) complexes with nitronyl nitroxide a a a a a, a Lei Zhang , Su-Qi Li , Bai-Wang Sun , Dai-Zheng Liao , Zong-Hui Jiang *, Shi-Ping Yan , Geng-Lin Wang a , Xin-Kan Yao b , Hong-Gen Wang b a

Department of Chemistry, Nankai University, Tianjin, 300071, PR China b Central Laboratory, Nankai University, Tianjin, 300071, PR China Received 13 May 1998; accepted 23 September 1998

Abstract Two novel adducts of formula [Cu(pfpr) 2 (NITmNO 2 )] 2 (I) and Cu(hfac) 2 (NITmNO 2 ) (II), where pfpr5pentafluoropropionate, hfac5hexafluoroacetylacetonate and NITmNO 2 52-(3-nitrophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, have been synthesized. Compound (I) is the first example of copper (II)–nitroxide complexes bridged by pfpr. Its structure consists of binuclear units with the tetrakis(pentafluoropropionate) bridging two copper ions. Each copper ion has a distorted trigonal–bipyramidal coordination. Compound (II) is a mononuclear complex with a five-coordinated distorted square–pyramidal environment. The variable temperature magnetic susceptibilities show that there are antiferromagnetic interactions between copper ions and radicals for compound (I), and compound (II) is diamagnetic.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Nitronyl nitroxide; Copper complex; Magnetism

1. Introduction Preparing molecular magnetic materials, especially organometallic materials, is of current interest. A number of compounds carrying metal ions directly bound to stable organic radicals, as building blocks, have been successfully synthesized in the last decades [1–3]. The nitronyl nitroxides NITR52-R-4,4,5,5-tetramethylimidazoline-1-oxyl-3oxide (Fig. 1), with R5methyl, ethyl, propyl, phenyl, etc., have been proved to be extremely versatile magnetic ligands, capable of forming one-, two- and tri-dimensional magnetic systems with different transition-metal ions [4]. Since nitroxides are weak Lewis bases [5], they mainly bind to acid metal centers with electron-withdrawing ligands such as pentafluoropropionate (pfpr) [6,7] and hexafluoroacetylacetonate (hfac) [8]. Pfpr and hfac as coligands have influence on the coordination geometry of the metal complexes with NITR. Transition metal–nitroxide complexes bridged by pfpr are very rare [6,9]. However, mononuclear, di- or trinuclear complexes bridged by NITR were formed for M(hfac) 2 with NITR [10,11]. On the other hand, in the reported complexes bridging by pfpr, bridging ligands contain other molecules, such as water

and reduced amidino-oxide derivatives of NITR [6,9]. However, no carboxylate–nitroxide copper compound has been reported to date. It was therefore of interest to further explore the influence of coligand on the coordination geometry and magnetic properties. In order to extend our knowledge concerning the rich chemistry of such systems, we have now synthesized two compounds, [Cu(pfpr) 2 (NITmNO 2 )] 2 (I) and Cu(hfac) 2 (NITmNO 2 ) (II). We wish to report herein the crystal structures and magnetic properties of two compounds. To our knowledge, [Cu(pfpr) 2 (NITmNO 2 )] 2 is the first example of a copper(II)–nitroxide complex bridged by pfpr.

*Corresponding author. 0277-5387 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0277-5387( 98 )00356-8

Fig. 1. Sketch of radical.

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L. Zhang et al. / Polyhedron 18 (1999) 781 – 785

2. Experimental

2.1. Materials and instruments Cu(pfpr) 2 ?2H 2 O, NITmNO 2 and Cu(hfac) 2 ?2H 2 O were prepared as previously described [5,12–14]. Other reagents were of analytical grade. The pertinent elemental analytical data for carbon, hydrogen and nitrogen were obtained using a Model 240 Perkin-Elmer analyser. The metal contents were determined by EDTA titration. The IR spectra were recorded on an IR-408 spectrophotometer with KBr disks, and the measurements of the electronic spectra were performed using an Hitachi-240 spectrophotometer. Variable-temperature magnetic susceptibilities were measured on a Model CF-1 vibrating sample magnetometer. Dimagnetic corrections were made with Pascal’s constants for all the constituent atoms and the magnetic moments were calculated via the equation meff 52.828 ( xm T )1 / 2 .

2.2. Synthesis of the compounds 2.2.1. [ Cu( pfpr)2 ( NITmNO2 )]2 A 42.6-mg (0.10 mmol) amount of Cu(pfpr) 2 ?2H 2 O was dissolved in 10 mL of hot n-heptane, then 30.6 mg (0.11 mmol) of NITmNO 2 were added. The solution was stirred for 30 min and allowed to cool down. Blue crystals were collected by filtration after one week and were dried under vacuum. The compound analyzed satisfactorily for [Cu(pfpr) 2 (NITmNO 2 )] 2 . Yield, 72%. Anal. calc. for C 19 F 10 H 16 N 3 O 8 Cu: C, 34.2; H, 2.4; N, 6.3; Cu, 9.5. Found: C, 34.0; H, 2.5; N, 6.3; Cu, 9.5. 2.2.2. Cu( hfac)2 ( NITmNO2 ) The procedure used in the synthesis of the compound (II) was similar to that described above, however, Cu(hfac) 2 ?2H 2 O was used in place of Cu(pfpr) 2 ?2H 2 O. Dark red crystals were obtained at a yield of 63%. Anal. calc. for C 23 F 12 H 18 N 3 O 8 Cu: C, 36.5; H, 2.4; N, 5.6; Cu, 8.4. Found: C, 36.4; H, 2.5; N, 5.4; Cu, 8.4.

Table 1 Crystallographic data and experimental parameters [Cu(pfpr) 2 NITmNO 2 ] 2 (I) and Cu(hfac) 2 NITmNO 2 (II) Formula Molecular weight (g mol 21 ) Crystal system Space group ˚ a (A) ˚ b (A) ˚ c (A) a (8) b (8) g (8) ˚ 3) V (A Z Dcalc (g cm 23 ) Crystal size (mm) m (mm 21 ) Data limits (8) Unique reflections Observed data Parameters R Rw

C 38 H 32 Cu 2 F 20 N 6 O 16 1335.76 Monolinic C2 / c 25.512(5) 11.701(2) 19.926(4) 90 120.08(3) 90 5147(5) 4 1.724 0.230.330.3 0.9664 2862 1411 140 0.17 a 0.18 a

for

C 23 H 18 CuF 12 N 3 O 8 755.94 Triclinic P1¯ 11.167(4) 12.019(4) 12.142(7) 62.89(4) 82.34(4) 86.98(3) 1438(1) 2 1.746 0.230.430.4 0.8851 4#2u #46 3740 2325 412 0.069 b 0.070 b

a

w51 /(s 2 (F )10.0001F 2 ). The values of R and R w are slightly large due to the large thermal motion of the -C 2 F 5 group of pfpr at room temperature. b w51 for all of the observed reflections.

refinement by the full-matrix least-squares method with isotropic thermal parameters for non-hydrogen atoms converged with factors of R50.17 and R w 50.18, respectively. The highest peak in the last difference Fourier ˚ 23 . synthesis was about 1.08 e?A Solution of the structure of compound (II) was performed as described for compound (I). The final refinement by the full-matrix least-squares method with anisotropic thermal parameters for non-hydrogen atoms converged with factors of R50.069 and R w 50.070, respectively. The highest peak in the last difference Fourier ˚ 23 . synthesis corresponded to 0.64 e?A

3. Results and discussion

2.3. X-ray crystal structure determination Well-shaped crystals of compounds (I) and (II) were suitable for x-ray analysis. The data for both compounds were collected at room temperature with an Enraf-Nonius CAD4 diffractometer, by the v 22u scan technique, equipped with graphite-monochromated MoKa ( l5 ˚ radiation. Data were corrected for Lp factors 0.71073 A) and empirical absorption. Crystallographic data are given in Table 1. The structure of compound (I) was solved using direct methods. The Patterson map revealed the position of the copper atom. The other non-hydrogen atoms were found by successive Fourier syntheses. Anisotropic thermal parameters were introduced only for copper ions. The final

The crystal structure of compound (I) displays the typical copper carboxylate dimer core, as shown in Fig. 2. The geometry around each copper atom has a distorted trigonal–bipyramidal coordination, with four oxygens of four pfpr ligands and a nitroxyl oxygen atom at a bonding ˚ The normal coordination pattern for distance of 1.931 A. copper (II) carboxylates is square pyramidal; the pattern of trigonal–bipyramid is very rare [15]. Each pfpr ligand is bidentately coordinated through an axial and an equatorial bound oxygen atom to two copper atoms. Two equatorial planes of two trigonal–bipyramid make an angle of 88.9(7)8, so the axes of two trigonal–bipyramidal cores are approximately orthogonal, and the axis of each trigonal– bipyramid is in the equatorial plane of another trigonal–

L. Zhang et al. / Polyhedron 18 (1999) 781 – 785

783

Fig. 3. ORTEP view of Cu(hfac) 2 (NITmNO 2 ) (II) with atom numbering.

Fig. 2. ORTEP view of [Cu(pfpr) 2 NITmNO 2 ] 2 (I) with atom numbering.

˚ bipyramid. The Cu(1)–Cu(1a) bond distance is 3.139 A. ˚ The shortest intermolecular -NO distance is 6.826 A. Selected bond lengths and angles are shown in Table 2 . However, compound(II) is a mononuclear complex and has a five-coordinated distorted square–pyramidal environment with four oxygens of two hfac molecules and one nitroxyl oxygen of NITmNO 2 . The asymmetric unit is sketched in Fig. 3. The nitroxyl oxygen atom occupies the equatorial position. The Cu–O bond distances range from ˚ The axial Cu(1)–O(5) bond distance is 1.904 to 2.239 A. larger than the equatorial ones. The bond distance in the ˚ than that coordinated N(1)–O(5) group is longer by 0.03 A of the uncoordinated one. The N(1)–O(5) group is more favourable to coordination compared to N(2)–O(6). It may be because of this that the -NO 2 substituent offers steric hindrance, as found in compound (I). The copper(II) ion ˚ above the average plane of the equatorial lies 0.1606 A oxygen atoms. Selected bond lengths and angles are reported in Table 3. The NITmNO 2 ligand has the usual

shape [16,17]. The five-atom fragment O(5)–N(1)–C(1)– N(2)–O(6) is found to be very nearly planar and makes an angle of 32.868 with the phenyl plane. A five-membered ring C(1)–N(1)–C(3)–C(2)–N(2) twisted out as a result of the probable staggered conformation of the methyl groups. In the IR region of the spectrum, the N–O stretching vibration for free NITmNO 2 shifted to lower frequencies by about 20 cm 21 for both compounds. This indicates that the N–O group of NITmNO 2 is coordinated, which is consistent with the results of X-ray structure analysis. The UV–Vis spectra of the two complexes in n-heptane are similar, and the absorptions can be attributed to p – p * of the benzene ring at 270 nm, to p – p * of the ONCNO conjugate group of the radical at 358 nm [11].

3.1. Magnetic properties The observed magnetic moment at room temperature is 3.29 B.M. for (I), lower than the spin-only value of 4.89 B.M., implying the operation of an antiferromagnetic spinexchange interaction. In order to quantitatively study the magnetic interaction between copper (II) ions and radicals, variable temperature (2–300 K) magnetic susceptibility was measured. The data are shown in Fig. 4. From the magnetic point of view, the interactions between copper

Table 2 ˚ and angles (8) for (I) Selected bond lengths (A) Cu(1)–O(3) Cu(1)–O(8) Cu(1)–O(6) Cu(1)–O(7a) Cu(1)–O(5a) N(1)–O(3) N(2)–O(4)

1.931(13) 1.958(21) 1.924(25) 2.206(20) 2.164(18) 1.340(30) 1.285(26)

O(3)–Cu(1)–O(6) O(3)–Cu(1)–O(5a) O(6)–Cu(1)–O(5a) O(6)–Cu(1)–O(8) O(8)–Cu(1)–O(7a) C(4)–O(8)–Cu(1)

87.8(8) 117.6(7) 90.7(8) 179.2(8) 87.8(8) 116.4(19)

O(3)–Cu(1)–O(8) O(3)–Cu(1)–O(7a) O(6)–Cu(1)–O(7a) O(8)–Cu(1)–O(5a) C(1)–O(6)–Cu(1) N(1)–O(3)–Cu(1)

93.0(7) 114.3(7) 91.8(9) 89.0(8) 116.8(17) 123.7(12)

L. Zhang et al. / Polyhedron 18 (1999) 781 – 785

784 Table 3 ˚ and angles (8) for (II) Selected bond lengths (A) Cu(1)–O(1) Cu(1)–O(2) Cu(1)–O(3) Cu(1)–O(4) Cu(1)–O(5) N(1)–O(5) N(2)–O(6)

2.239(5) 1.918(4) 1.904(4) 1.958(5) 1.959(5) 1.307(6) 1.274(7)

O(1)–Cu(1)–O(2) O(1)–Cu(1)–O(3) O(1)–Cu(1)–O(4) O(1)–Cu(1)–O(5) O(2)–Cu(1)–O(3) O(2)–Cu(1)–O(4) O(2)–Cu(1)–O(5)

ion and radical are assumed to be identical, and the complex can be schematized as shown below. j

j

J (radical) NO-Cu-Cu-ON (radical) S1

S2

S3

S4

The magnetic analysis was carried out using the magnetic equation based on the Heisenberg spin-exchange operator ˆ 22j(S ˆ 1S ˆ 2 1S ˆ 3S ˆ 4 )22JS ˆ 2S ˆ 3 , where the exchange inteH5 gral, J or j, is negative for antiferromagnetic and positive for ferromagnetic interactions. The mole susceptibility of the system (S 1 5S 2 5S 3 5S 4 51 / 2) is given in equation. N0 g 2 b 2 A xM 5 ]]] ] 1 Na kT B

SD

A 5 10 exp(2E1 /kT ) 1 2 exp(2E2 /kT ) 1 2 exp(2E3 /kT ) 1 2 exp(2E4 /kT ) B 5 5 exp(2E1 /kT ) 1 3[exp(2E2 /kT ) 1 exp(2E3 /kT ) 1 exp(2E4 /kT )] 1 exp(2E5 /kT ) 1 exp(2E6 /kT ) E1 5 2 j 2 0.5J, E2 5 j 2 0.5J, E3

89.2(2) 100.3(2) 88.2(2) 101.4(2) 170.0(3) 191.5(2) 83.5(2)

O(3)–Cu(1)–O(4) O(3)–Cu(1)–O(5) O(4)–Cu(1)–O(5) Cu(1)–O(5)–N(1)

92.1(2) 91.3(2) 169.2(3) 123.7(4)

between copper ion and radical, J is the exchange integral between two copper ions, and other symbols have their usual meanings. The magnetic susceptibility data (30–300 K) were simulated based on a standard least-squares procedure, giving J5 2124 cm 21 , j5 269 cm 21 and g52.02, Na 5120310 26 . The agreement factor was 1.07310 24 (F 5o[( xM T ) obs 2( xM T ) calc ] 2 / o( xM T ) obs ). The data below 30 K largely deviate from the value of theory, which may be caused by the intermolecular interactions and / or small noncoupled paramagnetic ions. uJu.u ju indicates that an antiferromagnetic interaction between copper (II) ions is stronger than the interaction between copper and radical. The antiferromagnetic interaction between copper ions (J5 2124 cm 21 ) is close to that of the compound [Cu(OOCCH 3 ) 2 (H 2 O)] 2 (J5 2143 cm 21 ) [18]. Measurement of the susceptibility for Cu(hfac) 2 (NITmNO 2 ) indicates that the complex is diamagnetic, presumably as a result of a strong antiferromagnetic interaction between unpaired electrons, due to a large overlap between magnetic orbitals. Similar behavior has already been observed for Cu(hfac) 2 (NITPh) and Cu(hfac) 2 (TEMPO) [19,20].

5 0.5J 1 (J 2 1 j 2 )1 / 2 , E4 5 0.5J 1 (J 2 1 j 2 )1 / 2 , E5 5 j 1 0.5J 1 (4j 2 2 2jJ 1 J 2 )1 / 2 , E6 5 j 1 0.5J 2 (4j 2 2 2jJ 1 J 2 )1 / 2 , where xM is mole susceptibility, j is the exchange integral

Acknowledgements This work was supported by the National Natural Science Foundation of China and the Foundation of a Doctoral Special Grant of Chinese University.

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Fig. 4. Temperature dependence of the magnetic susceptibilities and magnetic moment of [Cu(pfpr) 2 (NITmNO 2 )] 2 .

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