Spin structures and properties of paramagnetic nickel(II) complexes with liquid crystalline β-diketone ligands

ELSEVIER

Synthetic

Metals

85 (1997)

1669-1670

Spin structures and properties of paramagnetic nickel(H) complexes with liquid crystalline P-diketone ligands Institute of Materials

G. Piao, K. Akagi, and H. Shirakawa Science, University of Tsukuba, Tsukuba, Ibaraki

305 Japan

Abstract We synthesized a series of nickel(B) complexes with liquid crystalline P-diketone as coordination ligands. These classified into symmetric and asymmetric ones in terms of coordination structure. It is of keen interest that in addition to common to /3-diketone type Ni (II) complexes, the present complexes gave rise to fine structures in ESR spectra temperature. This enabled us to evaluate values of g-tensors and parameters for the fine structure associated with a zero leading to the fact that the structures of these complexes are of rhombic symmetry, a distorted tetrahedral one. Keywords: Electron spin resonance; Magneticproperties;

Liquid

crystallinephase

complexes are high spin state even at room field splitting,

transitions

1. Introduction Metal complexes with liquid crystalline (LC) ligands are called metallomesogens. They have versatile potentialities to be used for colorful display devices, anisotropic polymerization catalysts, and advanced materials exhibiting peculiar physicochemical properties such as mixed-valence states. Since delectron configurations of transition metal complexes are sensitive to ligand field environments, spin structures and properties also crucially depend on geometrical structures. The primary concern in the present study is to elucidate how spin structures of metal complexes are affected upon coordinations of bulky LC ligands. Here we have synthesized two kinds of Ni complexes by coordinating symmetrical and asymmetrical P-diketone LC groups, as illustrated in Fig. 1. The complexes of type 1 with Ri = n-C&Irs, n-&I&,, which are abbreviated as Ni(asym-I&, and that of type 2 with Ra= n-C&I= abbreviated as Ni(sym-L& We now report the spin structures and properties of paramagnetic nickel(R) complexes with liquid crystalline P-diketone ligands. Behaviors of phase transitions of these complexes were also examined, especially from a view point of ligand structure, through measurements of differential scanning calorimeter and polarizing optical microscope. 2. Experimental 2.1. Preparation

of diketone and complexes

[l-(p-n-alkylbiphenyl)-3-(phenyl)propane]-l,3-dione (asymIA), [1,3-di(p-n-alkylbiphenyl) propane]-1,3-dione (sym-In), and Ni complexes with these ligands were prepared with the methods reported by Sadashiva et al.[l] and Giroud et a1.[2]. The corn lexes obtained are as follows. NI*Pasym-L&-Dark green precipitate [from 2-Butanone], yield 0.28 g, 23 %; UV-VIS(THF): h,,, (log a), 290 nm (5.26), 370 nm (5.36), 628 nm (1.03); IR(KI): 1.591 cm-r v(CzO) and 1528 cm’r v(CzC); Anal. Found (Calcd. For CsHH04Ni), C, 78.19 (78.55); H, 6.56 (6.59) %. Ni(asym-L&-Yellow-green precipitate [from 2-Butanone], 0379-6779197/$17.00 PII SO379-6779(96)04543-2

0 1997

Elsevier

Science

S.A. All rights

reserved

2 M=Ni(m;

Fig. 1.

RI = n-Can,

n-CuHzs;

Rz = n-Cdfzs

Structures of Ni(Il) complexes

yield 0.16 g, 25.5 %; UV-VIS(THF): h,,, (log a), 278 nm (4.24), 360 nm (4.84), 636 nm (0.79); IR(KI): 1593 cm’r v(C10) and 1528 cm-r v(C:C); Anal. Found (Calcd. For C&TaOdNi), C, 78.73 (79.75); H, 7.87 (7.91) %. Ni(sym-Llh - Deep yellow-green precipitate [from 2Butanone], yield 0.41 g, 26 %; UV-VIS(THF): h,, (log .a), 278 nm (4.24), 360 nm (4.84), 636 nm (0.79); IR(KI): 1591 cm-i v(C***O) and 1531 cm“ v(C***C); Anal. Found (Calcd. For CrzH04Ni), C, 81.5 (82.6); HT9.1 (9.1) %. 2.2

Phase studies and ESR measurements

Phase transition behaviors of the complexes were measured by means of a Linkam THMS 600 hot stage and controller in conjunction with a Nikon polarizing optical microscope. Thermal’ transition characteristics were determined by using a PerkinElmer differential scanning calorimeter (DSC 7) at a rate lO”C/min under argon atmosphere.

1670

G. Pmo et al. /Synthetic

Room-temperature ESR spectra were measured on solid samples using a JESTE200 ESR spectrometer. Throughout the spectroscopic measurements we adopted the following conditions: microwave frequency 9.23444 - 9.22749 GHz(X band), central magnetic field 3210 G, swift range *150 G, magnetic modulation frequency 100 kHz, width 10 G, magnetic swift time 240 seconds, time constant 0.03 second, amplitude 160 and accumulation 5. The sample tubes were sealed under dynamic pumping, except for Ni(asym-L&, and stored in room temperature.

Metals

85 (1997)

1669-I

670

because the Ni(acac)a exists in the associated form, in contrast with the present complexes.

4-+-t-i-Hi-

tz e

3. Results and discussion Behaviors of phase transitions of the present complexes were examined by DSC and polarizing optical microscope. Neither Ni(sym-In)2 nor Ni(asym-In)2 exhibited mesophase, implying that geometrical structures of these Ni complexes are not suitable for generation of LC phase in spite of coordinations of LC ligands. Fig. 2 shows ESR spectra of these complexes, indicating that they are paramagnetic in spite of even-numbered electron systems. Ni(lI) ion has 3d8 electron configuration. It forms both non-planar tetrahedral structure and planar four-coordinated tetragonal one, depending on weak and strong ligand field

Da

Td

High spin ( S=l ) Low spin ( S=O) Fig. 3 Electronic configurationsand spin states of Ni(II) complexeswith Tdand&h symmetries Through analysesof ESR spectra of Fig. 2, we evaluated values of g-tensorsand parametersfor the fine structures associatedwith the zero field splitting using the method of Wassermanet a1.[3]. The resultsare summarizedin Table 1. Parametersof D and E are fine structure(zero-field splitting) constants, as shown in the following equation of spin Hamiltonian, Hs=p(gxH,S,+gyH,S,+g~H,S,)+D[S,‘-1/3S(S+1)]t E (S,’ -S,‘) +,4,&Z, +A,S,Z,+A,&Z, + Q’[Z:-1/3Z(Z+1)]+Q”(Z?+ZYZ)-g,vP&Z*Z wheregi is g-value,D andE are fine structureconstants, Ai is hyperfinesplittingparameter,andQ’ andQ” are quadrupole couplingconstants. Table 1 ESR datafor Ni(II) complexes Complexes

Fig.2 ESR spectra of Ni(II) complexes with zero- splitting. The solid line : Ni(asym-I& ; the broken line : Ni(asym-Lra)a ; the dotted line: Ni(sym-L&. interactions, respectively. As schematically shown in Fig. 3, the former and the latter interactions give rise to high-spin and lowspin states, respectively. The high-spin state is open shell in which two t2 orbitals are half-occupied, whereas the low-spin state is closed shell. The present Ni(II) complexes can be classified into the high-spin state with total spin multiplicity of 1 ( S = 1 ). Therefore the ESR signals observed are due to the transitions of the triple states (Am, = 1 ). However, their nonplanar structures with high-spin states are not suitable for generation of LC mesophase, which accounts well of the results of DSC and polarizing optical microscopes. Nevertheless, the effect of coordination of bulky LC ligands can be seen in the ESR fine structure of zero-field splitting observed at room temperature. In fact, the Ni(II) complex with ordinary p-diketone ligands, e.g., Ni(acac)z (acac: acetylacetonato) has been reported to show fine structure only in host lattices at 4.2K. This is

1 D 1 (G)

1 E 1 (G)

sxx

&Y

&z

Ni(asym-L&

65.6

10.1

2.046

2.041

2.043

Ni(asym-Lh

65.6

10.2

2.046

2.041

2.043

66.0

10.0

2.045

2.040

2.042

Ni(sym-Lh I

L

I

Acknowledgments This work was supportedby Grant-in-Aids for Scientific Researchfrom Ministry of Education, Culture and Scienceof Japan,and from JapanSociety for the PromotionScience,and The MitsubishiFoundation. References

PI

PI

B. K. Sadashiva,P. R. RaoandB. S. Srikanta,Mol. Cryst. 168 (1989)103. A,- M.‘Giroud and J.‘Billard, Mol. Cryst. Liq. Cryst., 97

Lia. Cwt.,

3983) 287.

131 E. Wasserman,L. C. Snyder and W.A. Yager, J. Chem. Phys., 41 (1964)1763.