Crystal structures of cobalt(II), nickel(II) and zinc(II) dichloro complexes bearing 2-pyridyl-substituted nitronyl nitroxide (NIT2py)

www.elsevier.com/locate/ica Inorganica Chimica Acta 325 (2001) 187– 192

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Crystal structures of cobalt(II), nickel(II) and zinc(II) dichloro complexes bearing 2-pyridyl-substituted nitronyl nitroxide (NIT2py) Youhei Yamamoto, Takafumi Yoshida, Takayoshi Suzuki, Sumio Kaizaki * Department of Chemistry, Graduate School of Science, Osaka Uni6ersity, Toyonaka 560 -0043, Japan Received 18 May 2001; accepted 14 August 2001

Abstract The X-ray analysis of Co(II), Ni(II) and Zn(II) dichloro complexes containing 2-(2-pyridyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide (NIT2py) demonstrated that the Co(II) complex containing two dichloromethane solvate molecules, [CoCl2(NIT2py)2]·2CH2Cl2 (1), crystallizes in a space group P21/c isomorphous to the corresponding Mn(II), Ni(II) (2) and Cu(II) complexes, and that the molecular structure of the complex having Ci symmetry in 1 is isostructural to the above Mn(II), Ni(II) and Cu(II) complexes. A new Ni(II) complex having only one dichloromethane solvate molecule, [NiCl2(NIT2py)2]·CH2Cl2 (2%), could be crystallized from a mixture of dichloromethane, methanol and n-heptane. The (OC-6 -12 )-[trans(Cl)-trans(py)] geometrical structure of the Ni(II) complex in 2% is identical to that in 2, but the puckered six-membered chelate rings formed via pyridyl-N and nitroxide-O atoms of NIT2py take the different conformations; i.e. the molecular symmetry is C2 (racemate) with the l, l or d, d conformations in 2% and Ci (meso) with the d, l conformations in 2. In the case of Zn(II), a reaction of ZnCl2 with NIT2py afforded purple crystals consisting of octahedral bis(NIT2py) and tetrahedral mono(NIT2py) complexes with hydration: [ZnCl2(NIT2py)2]·2[ZnCl2(NIT2py)]·2/3H2O (3). The geometrical structure of the octahedral Zn(II) complex in 3 was found to be (OC-6 -22 )-[cis(Cl)-trans(py)]. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nitronyl nitroxide radical; Crystal structures; Conformational isomer; Octahedral and tetrahedral complexes

1. Introduction In the last two decades, there have been a large number of investigations on the magneto/structural chemistry of transition metal complexes containing nitroxide radicals [1]. In particular, the complexes of nitronyl or imino nitroxides substituted by N-heteroaromatic groups have been extensively studied [2– 10], because the auxiliary heteroaromatic-N donor forces the nitroxide-O atom to bind to a poor electrophilic metal center by the chelate effect [3]. The most typical ligand of the N-heteroaromatic-substituted nitronyl nitroxides is 2-(2-pyridyl)-4,4,5,5-tetra* Corresponding author. Tel./fax: + 81-6-6850 5408. E-mail address: [email protected] (S. Kaizaki).

methylimidazolin-1-oxyl 3-oxide (NIT2py), which was firstly utilized by Richardson and Krellick for complexation with transition metal ions [2]. In 1993, Luneau et al. reported the Mn(II) and Ni(II) complexes containing NIT2py, and determined the crystal and molecular structures of the mononuclear dichlorobis(NIT2py) complexes, [MIICl2(NIT2py)2]·2CH2Cl2 (MII = Mn, Ni), where NIT2py coordinates to a metal(II) center via pyridyl-N and nitroxide-O atoms to form a six-membered chelate ring [3]. The analogous Cu(II) complex, [CuCl2(NIT2py)2]·2CH2Cl2, has recently been investigated by X-ray analysis [4]. In contrast, for the corresponding Co(II) dichloro complex, [CoCl2(NIT2py)2], the temperature-dependent magnetic susceptibility has been studied [5] assuming that it is isomorphous and isostructural to the other metal(II)

0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 6 3 6 - 3

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analogues, but the crystal structure has not been reported so far. The single-crystal X-ray structure analysis of the Co(II) complex, [CoCl2(NIT2py)2]·2CH2Cl2 (1), would be helpful in understanding the magnetic properties. Furthermore, we will also describe the crystal structures of Ni(II) and Zn(II) dichloro complexes containing NIT2py. In the course of our study on the Ni(II)NIT2py complexes [6], on the other hand, we found another type of crystals with only one dichloromethane solvate, [NiCl2(NIT2py)2]·CH2Cl2 (2%), which is different from those ([NiCl2(NIT2py)2]· 2CH2Cl2, 2) reported by Luneau et al. [3]. It is interesting to compare the crystal and molecular structures of 2% and 2 from the viewpoint of the magnetic properties. Since Zn(II) complexes tend to crystallize isomorphously to analogous Mn(II), Co(II) or Ni(II) complexes, they are often used to estimate the intra- and intermolecular magnetic interactions between two radical moieties [7,8]. However, there has been no report concerning the Zn(II) complexes corresponding to the dichloro NIT2py Co(II) and Ni(II) complexes.

2. Experimental

NIT2py (1.4 mmol) in a closed vessel, affording purple plate crystals. Yield: 60%. Anal. Found: C, 42.64; H,4.78; N, 12.48%. Calc. for C48H65.33Cl6N12O8.67Zn3: C, 42.45; H, 4.85; N, 12.38%.

2.2. Crystal structure determination The X-ray intensities were measured on a Rigaku automated four-circle diffractometer AFC-5R or AFC7R (23 °C, graphite-monochromated Mo Ka radiation (u=0.71073 A, ), the 2q–… scan mode, 2qmax =60°, three standard reflections monitored every 150 reflections with no serious decomposition). Final lattice parameters were determined by least-squares treatment using setting angles of 25 reflections in the range of 28B 2q B30°. The intensities were corrected for Lp factors and for absorption by means of the numerical integration method [12a]. The structures were solved by the direct method using SHELXS86 program [12b], and refined by SHELXL97 program [12c] (on F 2, full-matrix, complex neutral-atom scattering factors, anisotropic thermal parameters for all non-H atoms, riding model for all H atoms). All calculations were carried out using a TEXSAN software package [12d]. Crystallographic data are summarized in Table 1.

2.1. Preparation of complexes 3. Results and discussion The nitroxide, NIT2py, was prepared by the literature method [11].

2.1.1. [CoCl2(NIT2py)2] ·2CH2Cl2 (1) This complex was prepared by a modified literature method [5] to obtain single crystals suitable for X-ray analysis. An ethanol solution (3 cm3) of CoCl2·6H2O (1.0 mmol) was layered on a dichloromethane solution (3 cm3) of NIT2py (2.0 mmol), and allowed to stand in a refrigerator for 3 days to give purple prismatic crystals of 1. Yield: 75%. Anal. (after vacuum-drying to remove the solvated dichloromethane molecules completely) Found: C, 48.03; H, 5.39; N, 14.04%. Calc. for C24H32Cl2CoN6O4: C, 48.17; H, 5.39; N, 14.04%. 2.1.2. [NiCl2(NIT2py)2] ·CH2Cl2 (2 %) Heptane (5 cm3) was added to a mixture of NIT2py (2.0 mmol) in dichloromethane (30 cm3) and NiCl2· 6H2O (1.0 mmol) in methanol (5 cm3). The mixture was allowed to stand for 1 day at room temperature, affording dark green crystals of [NiCl2(NIT2py)2]·CH2Cl2. Yield: 56%. Anal. Found: C, 43.86; H, 4.97; N, 12.19%. Calc. for C25H34Cl4N6NiO4: C, 43.96; H, 5.02; N, 12.30%. 2.1.3. [ZnCl2(NIT2py)2] ·2[ZnCl2(NIT2py)] ·2 /3H2O (3) Diethyl ether vapor was diffused to an acetonitrile solution (3 cm3) containing ZnCl2 (0.70 mmol) and

3.1. The cobalt(II) complex, [CoCl2(NIT2py)2] ·2CH2Cl2 (1) Single crystals of the Co(II) complex suitable for X-ray analysis were deposited from a mixture of an ethanolic solution of CoCl2·6H2O and a dichloromethane solution of NIT2py as bis(dichloromethane) solvate, 1. The X-ray analysis has revealed that the crystal of 1 is isomorphous to those of [MCl2(NIT2py)2]·2CH2Cl2 (M= Mn, Ni [3] and Cu [4]); the complex exists as a discrete molecule in the crystal without any close contact to neighboring molecules. The molecular structure of the complex having (OC-6 12 )-[trans(Cl)-trans(py)] geometry and crystallographically imposed Ci symmetry is shown in Fig. 1. There are no remarkable differences in the structural parameters from those of the Mn(II) and Ni(II) complexes. The CoCl(1), CoO(1) and CoN(3) bond lengths are 2.4274(8), 2.080(2) and 2.151(2) A, , respectively, being in the ranges between the corresponding MnL and NiL (L = Cl, O or N) bond lengths, as expected from the ionic radii of Mn2 + , Co2 + and Ni2 + [13]. The coordinated and uncoordinated NO bond lengths of the nitronyl nitroxide moiety are 1.277(3) and 1.291(3) A, , respectively. The CoO(1)N(1) bond angle (117.1(1)°) and the CoO(1)N(1)C(1) torsion angle (– 41.7(3)°) as well as the dihedral angle between the

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Table 1 Crystallographic data Compounds

1

2%

3

Empirical formula Formula weight Crystal system a (A, ) b (A, ) c (A, ) h (°) i (°) k (°) U (A, ) Space group Z v(Mo Ka) (mm−1) Reflections/parameters ratio R1 [F 2: F 2\2|(F 2)] wR2 (F 2: all data)

C26H36Cl6CoN6O4 768.26 monoclinic 8.937(3) 14.066(2) 13.804(2) 90 103.04(2) 90 1690.4(6) P21/c (no. 14) 2 1.023 4892/201 0.046 0.144

C25H34Cl4N6NiO4 683.09 triclinic 10.854(3) 12.434(4) 13.489(4) 63.14(2) 73.65(2) 71.16(2) 1517.2(7) P1( (no. 2) 2 1.033 8844/369 0.042 0.113

C48H65.33Cl6N12O8.67Zn3 1357.99 orthorhombic 16.489(4) 14.394(4) 25.097(5) 90 90 90 5956(2) Pbcn (no. 60) 4 1.528 8699/365 0.046 0.132

nitronyl nitroxide mean plane (defined by O(1), N(1), C(1), N(2) and O(2)) and the pyridyl plane (28.64(8)°), are close to those of the corresponding Mn(II) complex [3].

3.2. Conformational isomers of the nickel(II) complex; [NiCl2(NIT2py)2] ·2CH2Cl2 (2) and [NiCl2(NIT2py)2] ·CH2Cl2 (2 %) Although the crystal structure and magnetic properties of [NiCl2(NIT2py)2]·2CH2Cl2 (2) have already been reported [3], we have obtained other shaped crystals of the Ni(II) complex with a different number of the solvated dichloromethane molecule, [NiCl2(NIT2py)2]· CH2Cl2 (2%). The X-ray analysis of 2% has revealed that the complex is the same geometrical isomer of (OC-6 12 )-[trans(Cl)-trans(py)] as in 2, but a different conformational isomer (Fig. 2). In both crystals of 2 and 2%, the NIT2py coordinates to a Ni(II) center to form a puckered six-membered chelate ring. The complex in 2 has a crystallographically imposed Ci symmetry, so that the tetramethylethylene groups of two NIT2py are located above and below the equatorial NiN2O2 coordination plane. On the other hand, the complex in 2% has a pseudo-C2 symmetry, where the pseudo-C2 axis coincides with the axial Cl(1)NiCl(2) bonds. The conformation of the six-membered NIT2py chelate ring around the Ni(II) ion is a kind of envelope, where the planes (defined by Ni, N(3), C(8) and C(1); by Ni, N(23), C(28) and C(21)) are nearly planar. Taking into consideration the torsion angles of N(1)C(1) C(8)N(3) and N(21)C(21)C(28)N(23) for NIT2py, two chelates around a Ni(II) ion take the l or d conformation according to the definition for the chelate ring chirality by a pair of skew lines consisting of the

N(3)C(8) and C(1)N(1) or N(23)C(28) and C(21)N(21) bonds for the NIT2py [6a]; the crystals of 2% being the racemate consisting of the l,l and d,d conformations in contrast to 2 being the meso form with the l,d conformation. Thus, each tetramethylethylene group of NIT2py exists on the Cl(1) side of the equatorial NiN2O2 plane. The conformational isomerism in the crystals of 2 and 2% seems to be related to the crystal packing of the complex or result from the difference in intermolecular interaction inferred from the different number of solvated molecules included in each crystal. However, there are no notable short intermolecular contacts in both crystals of 2 and 2%, the complex existing as a discrete molecule.

Fig. 1. Perspective view (50% probability level) of the complex in (OC-6 -12 )-[CoCl2(NIT2py)2]·2CH2Cl2 (1). Hydrogen atoms are omitted for clarity.

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indicate that there are two inequivalent NIT2py moieties. The largest difference is the dihedral angle between the nitronyl nitroxide mean plane and the pyridyl plane: 31.03(8) versus 21.71(9)°, which is important to elucidate the magnetic properties. The difference in torsion angle of NiO(1)N(1)C(1) and NiO(21) N(21)C(21) is small, but still apparent. The conformational isomerism found in this study would be important to evaluate the magnetic properties of these complexes having two radical ligands in a molecule. It is plausible that the conformational isomers give a different intra- and intermolecular magnetic interaction between two nitroxide moieties. However, temperaturedependent magnetic susceptibility of 2% is apparently identical to that of 2 [3]. As mentioned above, both complexes in 2 and 2% exist as discrete molecules; the intermolecular interaction being negligible. Therefore, it is postulated that the intramolecular NIT2pyNIT2py interaction is also negligible in both conformational isomers of the Ni(II) complex, since the magnetic susceptibility data of 2 was successfully analyzed by taking into account no intramolecular NIT2pyNIT2py interaction [3]. Fig. 2. (a) Perspective view (50% probability level) of the complex in (OC-6 -12 )-[NiCl2(NIT2py)2]·CH2Cl2 (2%). Hydrogen atoms are omitted for clarity. (b) Schematic representation of the conformational isomers for (OC-6 -12 )-[NiCl2(NIT2py)2] in crystals 2 and 2%.

The average NiCl, NiN and (the coordinated and uncoordinated) NO bond lengths in 2% (Table 2) are comparable to those in 2, while the NiO bond in 2% (average 2.073 A, ) is slightly longer than that in 2 (2.064 A, ), which is consistent with the smaller NNiO bite angle of NIT2py in 2% (average 85.43°) than that in 2 (86.1°). The structural parameters of the complex in 2%

3.3. The zinc(II) complex, [ZnCl2(NIT2py)2] ·2[ZnCl2(NIT2py)] ·2 /3H2O (3) Since the analogous imino nitroxide, 2-(2-pyridyl)4,4,5,5-tetramethylimidazolin-1-oxyl (IM2py) with ZnCl2 has formed a complex isostructural to those of Mn(II), Co(II) and Ni(II) [7a], it is interesting to prepare the corresponding Zn(II) NIT2py complex. With such a Zn(II) complex having crystal structure isomorphous to the Mn(II), Co(II), Ni(II) and Cu(II) complexes, it is expected that the intramolecular

Table 2 Selected structural parameters (l (A, ), ƒ (°)) for (OC-6–12 )-[NiCl2(NIT2py)2]·CH2Cl2 (2%) NiCl(1) NiCl(2) O(1)N(1) O(21)N(21) O(1)NiN(3) Cl(1)NiCl(2) NiO(1)N(1) NiO(1)N(1)C(1)

2.404(1) 2.385(1) 1.292(2) 1.294(2) 85.45(7) 178.75(3) 115.0(1) 45.7(3)

NiO(1) NiO(21) O(2)N(2) O(22)N(22) O(21) NiN(23) O(1)NiO(21) NiO(21) N(21) NiO(21) N(21)C(21)

Plane[O(1),N(1),C(1),N(2),O(2)] vs. plane[C(7),C(8),C(9),C(10),C(11),C(12)] Plane[O(21),N(21),C(21),N(22),O(22)] vs. plane[C(27),C(28),C(29),C(30),C(31),C(32)]

31.03(8) 21.71(9)

2.072(2) 2.074(2) 1.271(3) 1.271(3) 84.51(7) 176.51(7) 115.2(1) 41.1(3)

NiN(3) NiN(23)

2.097(2) 2.076(2)

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Table 3 Selected structural parameters (l (A, ), ƒ (°)) for [ZnCl2(NIT2py)2]·2[ZnCl2(NIT2py)]·2/3H2O (3)

(OC-6 -22) -[ZnCl2(NIT2py)2] Zn(1)Cl(1) O(1)N(1)

2.353(1) 1.279(4)

Zn(1)O(1) O(2)N(2)

2.228(3) 1.270(4)

O(1)Zn(1)N(3) O(1)Zn(1)O(1%) Zn(1)O(1)N(1)

81.2(1) 81.6(2) 114.2(2)

Cl(1)Zn(1)Cl(1%) N(3)Zn(1)N(3%) Zn(1)O(1)N(1)C(1)

99.89(6) 158.9(2) 46.7(5)

(T-4) -[ZnCl2(NIT2py)] Zn(2)Cl(21) Zn(2)O(21) O(21)N(21)

2.195(2) 2.031(3) 1.304(4)

Zn(2)Cl(22) Zn(2)N(23) O(22)N(22)

2.189(2) 2.080(3) 1.266(4)

O(21)Zn(2)N(23) Zn(2)O(21)N(21)

89.0(1) 113.3(2)

Cl(21)Zn(2)Cl(22) Zn(2)O(21)N(21)C(21)

128.08(6) 46.2(5)

NIT2pyNIT2py magnetic interaction could be estimated experimentally. However, a reaction of ZnCl2 with two equivalent amounts of NIT2py in a mixture of ethanol and dichloromethane did not give any crystal. A similar reaction in acetonitrile following diffusion of diethyl ether vapor afforded purple plate crystals (3) with the empirical formula of Zn3Cl6(NIT2py)4·2/

Zn(1)N(3)

2.200(3)

3H2O. The single-crystal X-ray analysis of 3 revealed that the crystal consists of a pseudo-tetrahedral fourcoordinate T-4 complex, [ZnCl2(NIT2py)], and an octahedral six-coordinate OC-6 one, [ZnCl2(NIT2py)2], in a 2:1 ratio. Even though an excess amount of NIT2py was used in the reaction, the same crystals were obtained exclusively. The NIT2py chelates via pyridyl-N and nitroxide-O atoms to a Zn(II) center in both the T-4 and OC-6 complexes. The structural parameters (Table 3) concerning NIT2py moieties are not different as compared to the other NIT2py complexes [3,4]. The molecular structure of the T-4-[ZnCl2(NIT2py)] (Fig. 3(a)) is very similar to that of the analogous 6-methyl2-pyridyl-substituted nitronyl nitroxide (NITmepy) complex, [ZnCl2(NITmepy)] [7b], except for a slightly shorter ZnN(3) bond, resulting from the steric effect of the methyl group at the o-position of the NITmepy pyridyl group. On the other hand, the molecular structure of the OC-6 complex is an (OC-6 -22 )-[cis(Cl)trans(py)] isomer, obviously different from those of the (OC-6 -12 )-[MCl2(NIT2py)2] (M = Mn, Co, Ni and Cu). This coordination geometry was found for the IM2py complexes, [MCl2(IM2py)2] (M= Mn, Co, Ni and Zn) [7a]. Two NIT2py chelates with the D absolute configuration take the l(lel) conformation or vice versa as previously defined [6a]. The reasons behind the exclusive crystallization of compound 3 and the selective formation of the OC-6 -22 isomer in the case of Zn(II) complex are not clear at present.

4. Concluding remarks

Fig. 3. Perspective views (50% probability level) of (a) (T-4 )[ZnCl2(NIT2py)] and (b) (OC-6 -22 )-[ZnCl2(NIT2py)2] in [ZnCl2(NIT2py)2]·2[ZnCl2(NIT2py)]·2/3H2O (3). Hydrogen atoms are omitted for clarity.

The structure of Co(II) complex 1 is the same as the presumed (OC-6 -12 ) one, whereas the analogous octahedral Zn(II) complex is found to be a different (OC-6 22 ) geometrical isomer, which is crystallized together with the T-4 mono(NIT2py) complex. Furthermore, a new dichlorobis(NIT2py) Ni(II) complex has been found to have a different number of solvate molecules

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and to take a different type of conformational isomer from the previously reported one, but to give a similar magnetic property due to negligible intra- and intermolecular NIT2pyNIT2py interactions. Even if a complex was presumed to be isomorphous and isostructural to the reported analogues with different transition metal ions, the crystal structure analysis would be necessary to examine the magnetic properties on the basis of the crystal and molecular structures.

5. Supplementary material Crystallographic data, tables of atomic coordinates and thermal parameters, and full lists of bond lengths and angles have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 168169– 168171. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: + 44-1223-336033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).

[3] [4] [5] [6]

[7]

[8] [9]

[10]

Acknowledgements We gratefully acknowledge support of this research by a Grant-in-Aid for Scientific Research (No. 10304056) from the Ministry of Education, Science and Culture, Japan.

[11]

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