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Monolayer formation and aggregation of nickel(II) complexes coordinated with salen substituted by non-linear alkyl side chains
www.elsevier.nl/locate/ica Inorganica Chimica Acta 298 (2000) 90 – 93
Note
Monolayer formation and aggregation of nickel(II) complexes coordinated with salen substituted by non-linear alkyl side chains Hiroyuki Abe a, Kazuo Miyamura b,* a
Course of Applied Chemistry, Graduate School of Engineering, The Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo, Tokyo 113 -8656, Japan b Department of Chemistry, Faculty of Science, Science Uni6ersity of Tokyo, 1 -3 Kagurazaka, Shinjuku, Tokyo 162 -8601, Japan Received 27 April 1999; accepted 9 August 1999
Abstract In order to investigate the effect of a-methylene groups of alkyl-substituted [Ni(salen)] on the aggregating nature, the monolayers of nickel(II) complexes coordinated with N,N%-disalicylideneethylenediamine (salen) substituted by propyl, 1-methylpropyl and 1,1-dimethylpropyl side chains have been prepared at a water – air interface using the Langmuir – Blodgett (LB) method. The p-A isotherm measurement revealed the formation of two different types of monolayers, condensed phase and expanded phase. When the hydrogen atoms of a-methylene groups were totally substituted by the methyl groups, the transition pressure of the expanded phase to the condensed phase decreased considerably from 5.4 to 1.0 mN m − 1, and the critical area of the expanded phase increased by ca. 20%. This observation suggests the presence of CH-p interaction between the a-methylene group and the p-conjugated system in the monolayer state as has been found in the crystalline state. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Scanning tunneling microscope; Monolayer; Salen; Aggregation structure; Nickel(II) complexes; CH-p interaction
1. Introduction Exploitation of metal complexes that function as molecular devices is becoming the major concern of coordination chemists. It has been widely accepted that the control of the molecular arrangement is essentially important to attain desired function of the molecular devices. The use of specific molecular interactions to control the molecular arrangement opened up a new field of chemistry, the supramolecular chemistry [1–3]. The use of interactions between long alkyl chains, however, has a long history, and has been well investigated in the field of colloid chemistry. Langmuir –Blodgett (LB) film is one of the most important achievements and has been intensively studied because
* Corresponding author. Tel.: + 81-3-3260 4271; fax: + 81-3-3235 2214. E-mail address: [email protected] (K. Miyamura)
of their applications in the areas of nanotechnology making use of their monolayer layout of molecules [4–6]. In a previous paper, we have reported the formation of monolayer with the nickel(II) complexes of alkyl substituted disalicylideneethylenediamine, usually abbreviated as salen [7]. The complexes were also found to exist in the form of dimers in the crystalline state by X-ray crystallographic analysis [8]. In the dimer, the p-conjugated systems of the salen moieties were stacked and the CH-p interactions between CH of alkyl side chains and p-conjugated system were present. The monolayer images of the complexes, observed by the scanning tunneling microscopy (STM) measurement [9], revealed that the complex molecules were also in the form of dimers in the monolayer state. Moreover, the dimeric structures were completely in accord with those in the crystalline state. This fact suggests that the CH-p interactions are also present in the dimers in the monolayer.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 3 9 7 - 7
H. Abe, K. Miyamura / Inorganica Chimica Acta 298 (2000) 90–93
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In order to investigate the effect of CH-p interactions in the monolayer state, the hydrogen(s) of a-methylenes involved in the CH-p interaction being substituted by methyl group(s) had been synthesised and their aggregating behaviour investigated.
cm − 1 (328 nm). Anal. Found: C, 65.75; H, 6.96; N, 6.22. Calc. for C24H30N2NiO2: C, 65.93; H, 6.92; N, 6.41%.
2. Experimental
Complex 3 was obtained from the 5-(1,1-dimethylpropyl)salicylaldehyde (2.30 g, 12 mmol) using a procedure similar to 1. Yield: 1.23 g (44.3%). 1H NMR (400 MHz, CDCl3): d 0.64 (t, J=7.6 Hz, 6H), 1.20 (s, 12H), 1.55 (q, J= 7.3 Hz, 4H), 3.38 (s, 4H), 6.94 (s, 2H), 6.99 (d, J= 8.8 Hz, 2H), 7.20 (d, J= 8.8 Hz, 2H), 7.52 (s, 2H). UV–Vis spectra (1.3× 10 − 4 mol dm − 3, CHCl3): omax (lmax)= 163 dm3 mol − 1 cm − 1 (553 nm), 6736 dm3 mol − 1 cm − 1 (419 nm), 9049 dm3 mol − 1 cm − 1 (344 nm), 8831 dm3 mol − 1 cm − 1 (329 nm). Anal. Found: C, 67.40; H, 7.52; N, 5.94%. Calc. for C26H34N2NiO2: C, 67.12; H, 7.37; N, 6.02%.
5-Alkylsalicylaldehydes (alkyl= propyl, 1-methylpropyl, 1,1-dimethylpropyl) were synthesised by the modified Reimer–Tiemann reaction [10] from the corresponding p-alkylphenols supplied from Tokyo Kasei Co. All other reagents used were commercially available and were of reagent grade. The compounds synthesised had been analysed by means of elemental analysis, 1 H NMR, IR and UV – Vis absorption spectra.
2.1. Synthesis of [N,N%-bis(5 -propylsalicylidene) ethylenediaminato]nickel(II) (1)
2.3. Synthesis of [N,N%-bis(5 -(1,1 -dimethylpropyl) salicylidene)ethylenediaminato]nickel(II) (3)
2.4. IR spectra Nickel acetate tetrahydrate (1.49 g, 6 mmol) and 1,2-diaminoethane (0.24 g, 6 mmol) were added to 100 cm3 of ethanol in a 200 cm3 flask with stirring. After the mixture had become a clear blue solution, 5-propylsalicylaldehyde (1.97 g, 12 mmol) was added dropwise to the solution. The reaction mixture was then refluxed for 3 h. The red– brown precipitates formed were filtered and recrystallised from chloroform – ethanol mixed solvents. Yield: 1.41 g (57.5%). 1H NMR (400 MHz, CDCl3): d 0.90 (t, J =7.3 Hz, 6H), 1.50 – 1.59 (m, 4H), 2.39 (t, J= 7.7 Hz, 4H), 3.42 (s, 4H), 6.60 (s, 2H), 6.88 (d, J=8.8 Hz, 2H), 6.99 (d, J = 8.8 Hz, 2H), 7.27 (s, 2H). UV–Vis spectra (1.3×10 − 4 mol dm − 3, CHCl3): omax (lmax)=168 dm3 mol − 1 cm − 1 (555 nm), 6929 dm3 mol − 1 cm − 1 (421 nm), 8857 dm3 mol − 1 cm − 1 (346 nm), 8610 dm3 mol − 1 cm − 1 (329 nm). Anal. Found: C, 64.83; H, 6.57; N, 6.92. Calc. for C22H26N2NiO2: C, 64.58; H, 6.40; N, 6.85%.
2.2. Synthesis of [N,N%-bis(5 -(1 -methylpropyl)salicylidene)ethylenediaminato]nickel(II) (2) Complex 2 was obtained from the 5-(1-methylpropyl)salicylaldehyde (2.14 g, 12 mmol) using a procedure similar to 1. The product is a mixture of enantiomers and diastereomers, but no further procedures have been performed to separate them into components. Yield: 1.38 g (52.5%). 1H NMR (400 MHz, CDCl3): d 0.79 (t, J =7.2 Hz, 6H), 1.16 (d, J =7.1 Hz, 6H), 1.45–1.54 (m, 4H), 2.39 – 2.48 (m, 2H), 3.40 (s, 4H), 6.81 (s, 2H), 6.97 (d, J =8.8 Hz, 2H), 7.06 (d, J =8.8 Hz, 2H), 7.46 (s, 2H). UV – Vis spectra (1.3 × 10 − 4 mol dm − 3, CHCl3): omax (lmax) = 166 dm3 mol − 1 cm − 1 (550 nm), 6700 dm3 mol − 1 cm − 1 (421 nm), 8752 dm3 mol − 1 cm − 1 (348 nm), 8556 dm3 mol − 1
IR spectra have been obtained using KBr pellet method. All three complexes synthesised exhibited the following absorptions. nas(CCH3) at 2960 cm − 1 (3.38 mm), nas(CH2) at 2925 cm − 1, nas(CN) at 1620 cm − 1.
2.5. Thermal analysis Thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements were performed under a nitrogen atmosphere using Seiko Instruments Inc. TG/DTA 320 and DSC 220C, respectively. The samples were heated at a constant rate (10°C min − 1) from 20 to 450°C.
2.6. Surface pressure–area (p-A) isotherms Chloroform solution of the complex (1.3 mmol dm − 3, 0.06 cm3) was used to form a monolayer at the water–air interface of 938 (14× 67) cm2. The distilled water was used as the subphase. The p-A isotherms were obtained by means of the LB method using the surface pressure meter type AP of Kyowa Kaimenkagaku, at a constant compression rate of 7 cm2 min − 1 and a subphase temperature of 20°C.
3. Results and discussion The synthesized complexes 1, 2 and 3 differ in the number of substituted a-methyl groups in the alkyl side chains. The alkyl side chains of 1 are linear, while those of 2 and 3 are non-linear, since the hydrogen atoms of a-methylenes of 1 are eventually substituted by methyl groups.
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The fact that the UV – Vis spectra of 1, 2 and 3 were almost identical indicated that the electronic states of these three complexes were not altered by the substitution of a-hydrogens with methyl group(s). Fig. 1 shows the result of TG and DSC measurements. The TG curves show that complexes 1, 2 and 3 thermally decompose at 402.3, 398.4 and 390.6°C, respectively. The DSC curve of 1 exhibits two endothermic peaks at 177.7 and 353.7°C, while those of 2 and 3 show only one endothermic peak at 297.4 and 323.5°C, respectively. It was confirmed by the observation with the polarising microscope that the transition at 290– 360°C corresponded to the melting into the isotropic phase. The low transition temperature observed for 2 is probably due to the mixing of enantiomers and diastereomers, which usually lowers the melting point. The phase transition of 1 at 177.7°C could not be assigned using the polarising microscope but the similar phase transition observed for the hexyl-substituted analogue was assigned as the transition to smectic A phase. Such a phase transition is characteristic of the complexes substituted with linear alkyl side chains, and is usually assigned to partial melting of alkyl side chains. The crystal structures of 2 and 3 seems to differ from that of 1, but the powder X-ray diffraction
Fig. 2. p-A isotherm curves of complexes 1, 2 and 3.
Fig. 3. Estimated aggregation structure of the complex molecules (a) in the expanded phase and (b) in the condensed phase.
Fig. 1. Results of thermogravimetry and differential scanning calorimetry measurements of complexes 1, 2 and 3.
patterns were too complicated to elucidate their difference. The p-A isotherms of 1, 2 and 3 taken at 20°C, given in Fig. 2 clearly showed that two phases were present in the monolayer state of these complexes [9]. One is the expanded phase A, in which the molecules are supposed to lie on the surface as illustrated in Fig. 3(a), and the other is the condensed phase B, in which the molecules should be standing perpendicular to the surface as in Fig. 3(b). The maximum area per molecule of a phase, namely the critical area, in the expanded phase of 2 (0.72 nm2) was similar to that of 1 (0.70 nm2), while that of 3 (0.86 nm2) was about 20% larger than that of 1. The transition pressure of 3 (1.0 mN m − 1) was also largely different from those of 1
H. Abe, K. Miyamura / Inorganica Chimica Acta 298 (2000) 90–93
(5.4 mN m − 1) and 2 (5.9 mN m − 1), and was much smaller. These facts suggest that the expanded phase of 3 is loosely packed and less stable than those of 1 and 2. Although there are bulky a-methyl groups in 2 as well as in 3, the critical area of the expanded phase of 2 was not so large compared with 1. The fact that the large difference is caused by the full substitution of a-hydrogens both in critical area and the transition pressure implies the presence of similar interactions in 1 and 2, i.e. the p– p and the CH-p interactions. The alkyl substituted [Ni(salen)] was reported to form dimers with p–p and CH-p interactions in the crystalline phase [8]. The CH-p interactions were present between the a- and g-methylenes of alkyl groups and the p-conjugated system of the [Ni(salen)] moieties. The presence of such interactions in the monolayer state of alkyl substituted [Ni(salen)] had also been confirmed by the observation with STM [9]. In the case of 3, the hydrogens of the a-methylenes are fully substituted by the methyl groups, and thus there should be no CH-p interactions. This should be the reason for the instability of the expanded phase of 3 compared with those of 1 and 2.
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Acknowledgements This work was partly supported by the grant-in-aid for scientific research No. 08405054 from the Ministry of Education, Science, Sports and Culture, and the Izumi Science and Technology Foundation. References [1] S.A. Hudson, P.M. Maitlis, Chem. Rev. 93 (1993) 861. [2] N. Miyajima, Kagaku Kogyo 46 (1995) 654. [3] K. Ohta, M. Moriya, M. Ikejima, H. Hasebe, N. Kobayashi, I. Yamamoto, Bull. Chem. Soc. Jpn. 70 (1997) 1199. [4] J.D. Swalen, D.L. Allara, J.D. Andrade, E.A. Chandross, S. Garoff, J. Israelachvili, T.J. McCarthy, R. Murray, R.F. Pease, J.F. Rabolt, K.J. Wynne, H. Yu, Langmuir 3 (1987) 932. [5] S. Tone, M. Kunitake, M. Uchida, T. Kunitake, T. Kajiyama, Bull. Chem. Soc. Jpn. 66 (1993) 960. [6] K. Iimura, N. Suzuki, T. Kato, Bull. Chem. Soc. Jpn. 69 (1996) 1201. [7] K. Ohta, Y. Morizumi, T. Fujimoto, I. Yamamoto, K. Miyamura, Y. Gohshi, Mol. Cryst. Liq. Cryst. 214 (1992) 161. [8] K. Miyamura, A. Mihara, T. Fujii, Y. Gohshi, Y. Ishii, J. Am. Chem. Soc. 117 (1995) 2377. [9] T. Fujii, K. Miyamura, Bull. Chem. Soc. Jpn., accepted for publication. [10] I. Fells, E.A. Moelwyn-Hughes, J. Chem. Soc. (1959) 398.
Note
Monolayer formation and aggregation of nickel(II) complexes coordinated with salen substituted by non-linear alkyl side chains Hiroyuki Abe a, Kazuo Miyamura b,* a
Course of Applied Chemistry, Graduate School of Engineering, The Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo, Tokyo 113 -8656, Japan b Department of Chemistry, Faculty of Science, Science Uni6ersity of Tokyo, 1 -3 Kagurazaka, Shinjuku, Tokyo 162 -8601, Japan Received 27 April 1999; accepted 9 August 1999
Abstract In order to investigate the effect of a-methylene groups of alkyl-substituted [Ni(salen)] on the aggregating nature, the monolayers of nickel(II) complexes coordinated with N,N%-disalicylideneethylenediamine (salen) substituted by propyl, 1-methylpropyl and 1,1-dimethylpropyl side chains have been prepared at a water – air interface using the Langmuir – Blodgett (LB) method. The p-A isotherm measurement revealed the formation of two different types of monolayers, condensed phase and expanded phase. When the hydrogen atoms of a-methylene groups were totally substituted by the methyl groups, the transition pressure of the expanded phase to the condensed phase decreased considerably from 5.4 to 1.0 mN m − 1, and the critical area of the expanded phase increased by ca. 20%. This observation suggests the presence of CH-p interaction between the a-methylene group and the p-conjugated system in the monolayer state as has been found in the crystalline state. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Scanning tunneling microscope; Monolayer; Salen; Aggregation structure; Nickel(II) complexes; CH-p interaction
1. Introduction Exploitation of metal complexes that function as molecular devices is becoming the major concern of coordination chemists. It has been widely accepted that the control of the molecular arrangement is essentially important to attain desired function of the molecular devices. The use of specific molecular interactions to control the molecular arrangement opened up a new field of chemistry, the supramolecular chemistry [1–3]. The use of interactions between long alkyl chains, however, has a long history, and has been well investigated in the field of colloid chemistry. Langmuir –Blodgett (LB) film is one of the most important achievements and has been intensively studied because
* Corresponding author. Tel.: + 81-3-3260 4271; fax: + 81-3-3235 2214. E-mail address: [email protected] (K. Miyamura)
of their applications in the areas of nanotechnology making use of their monolayer layout of molecules [4–6]. In a previous paper, we have reported the formation of monolayer with the nickel(II) complexes of alkyl substituted disalicylideneethylenediamine, usually abbreviated as salen [7]. The complexes were also found to exist in the form of dimers in the crystalline state by X-ray crystallographic analysis [8]. In the dimer, the p-conjugated systems of the salen moieties were stacked and the CH-p interactions between CH of alkyl side chains and p-conjugated system were present. The monolayer images of the complexes, observed by the scanning tunneling microscopy (STM) measurement [9], revealed that the complex molecules were also in the form of dimers in the monolayer state. Moreover, the dimeric structures were completely in accord with those in the crystalline state. This fact suggests that the CH-p interactions are also present in the dimers in the monolayer.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 3 9 7 - 7
H. Abe, K. Miyamura / Inorganica Chimica Acta 298 (2000) 90–93
91
In order to investigate the effect of CH-p interactions in the monolayer state, the hydrogen(s) of a-methylenes involved in the CH-p interaction being substituted by methyl group(s) had been synthesised and their aggregating behaviour investigated.
cm − 1 (328 nm). Anal. Found: C, 65.75; H, 6.96; N, 6.22. Calc. for C24H30N2NiO2: C, 65.93; H, 6.92; N, 6.41%.
2. Experimental
Complex 3 was obtained from the 5-(1,1-dimethylpropyl)salicylaldehyde (2.30 g, 12 mmol) using a procedure similar to 1. Yield: 1.23 g (44.3%). 1H NMR (400 MHz, CDCl3): d 0.64 (t, J=7.6 Hz, 6H), 1.20 (s, 12H), 1.55 (q, J= 7.3 Hz, 4H), 3.38 (s, 4H), 6.94 (s, 2H), 6.99 (d, J= 8.8 Hz, 2H), 7.20 (d, J= 8.8 Hz, 2H), 7.52 (s, 2H). UV–Vis spectra (1.3× 10 − 4 mol dm − 3, CHCl3): omax (lmax)= 163 dm3 mol − 1 cm − 1 (553 nm), 6736 dm3 mol − 1 cm − 1 (419 nm), 9049 dm3 mol − 1 cm − 1 (344 nm), 8831 dm3 mol − 1 cm − 1 (329 nm). Anal. Found: C, 67.40; H, 7.52; N, 5.94%. Calc. for C26H34N2NiO2: C, 67.12; H, 7.37; N, 6.02%.
5-Alkylsalicylaldehydes (alkyl= propyl, 1-methylpropyl, 1,1-dimethylpropyl) were synthesised by the modified Reimer–Tiemann reaction [10] from the corresponding p-alkylphenols supplied from Tokyo Kasei Co. All other reagents used were commercially available and were of reagent grade. The compounds synthesised had been analysed by means of elemental analysis, 1 H NMR, IR and UV – Vis absorption spectra.
2.1. Synthesis of [N,N%-bis(5 -propylsalicylidene) ethylenediaminato]nickel(II) (1)
2.3. Synthesis of [N,N%-bis(5 -(1,1 -dimethylpropyl) salicylidene)ethylenediaminato]nickel(II) (3)
2.4. IR spectra Nickel acetate tetrahydrate (1.49 g, 6 mmol) and 1,2-diaminoethane (0.24 g, 6 mmol) were added to 100 cm3 of ethanol in a 200 cm3 flask with stirring. After the mixture had become a clear blue solution, 5-propylsalicylaldehyde (1.97 g, 12 mmol) was added dropwise to the solution. The reaction mixture was then refluxed for 3 h. The red– brown precipitates formed were filtered and recrystallised from chloroform – ethanol mixed solvents. Yield: 1.41 g (57.5%). 1H NMR (400 MHz, CDCl3): d 0.90 (t, J =7.3 Hz, 6H), 1.50 – 1.59 (m, 4H), 2.39 (t, J= 7.7 Hz, 4H), 3.42 (s, 4H), 6.60 (s, 2H), 6.88 (d, J=8.8 Hz, 2H), 6.99 (d, J = 8.8 Hz, 2H), 7.27 (s, 2H). UV–Vis spectra (1.3×10 − 4 mol dm − 3, CHCl3): omax (lmax)=168 dm3 mol − 1 cm − 1 (555 nm), 6929 dm3 mol − 1 cm − 1 (421 nm), 8857 dm3 mol − 1 cm − 1 (346 nm), 8610 dm3 mol − 1 cm − 1 (329 nm). Anal. Found: C, 64.83; H, 6.57; N, 6.92. Calc. for C22H26N2NiO2: C, 64.58; H, 6.40; N, 6.85%.
2.2. Synthesis of [N,N%-bis(5 -(1 -methylpropyl)salicylidene)ethylenediaminato]nickel(II) (2) Complex 2 was obtained from the 5-(1-methylpropyl)salicylaldehyde (2.14 g, 12 mmol) using a procedure similar to 1. The product is a mixture of enantiomers and diastereomers, but no further procedures have been performed to separate them into components. Yield: 1.38 g (52.5%). 1H NMR (400 MHz, CDCl3): d 0.79 (t, J =7.2 Hz, 6H), 1.16 (d, J =7.1 Hz, 6H), 1.45–1.54 (m, 4H), 2.39 – 2.48 (m, 2H), 3.40 (s, 4H), 6.81 (s, 2H), 6.97 (d, J =8.8 Hz, 2H), 7.06 (d, J =8.8 Hz, 2H), 7.46 (s, 2H). UV – Vis spectra (1.3 × 10 − 4 mol dm − 3, CHCl3): omax (lmax) = 166 dm3 mol − 1 cm − 1 (550 nm), 6700 dm3 mol − 1 cm − 1 (421 nm), 8752 dm3 mol − 1 cm − 1 (348 nm), 8556 dm3 mol − 1
IR spectra have been obtained using KBr pellet method. All three complexes synthesised exhibited the following absorptions. nas(CCH3) at 2960 cm − 1 (3.38 mm), nas(CH2) at 2925 cm − 1, nas(CN) at 1620 cm − 1.
2.5. Thermal analysis Thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements were performed under a nitrogen atmosphere using Seiko Instruments Inc. TG/DTA 320 and DSC 220C, respectively. The samples were heated at a constant rate (10°C min − 1) from 20 to 450°C.
2.6. Surface pressure–area (p-A) isotherms Chloroform solution of the complex (1.3 mmol dm − 3, 0.06 cm3) was used to form a monolayer at the water–air interface of 938 (14× 67) cm2. The distilled water was used as the subphase. The p-A isotherms were obtained by means of the LB method using the surface pressure meter type AP of Kyowa Kaimenkagaku, at a constant compression rate of 7 cm2 min − 1 and a subphase temperature of 20°C.
3. Results and discussion The synthesized complexes 1, 2 and 3 differ in the number of substituted a-methyl groups in the alkyl side chains. The alkyl side chains of 1 are linear, while those of 2 and 3 are non-linear, since the hydrogen atoms of a-methylenes of 1 are eventually substituted by methyl groups.
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The fact that the UV – Vis spectra of 1, 2 and 3 were almost identical indicated that the electronic states of these three complexes were not altered by the substitution of a-hydrogens with methyl group(s). Fig. 1 shows the result of TG and DSC measurements. The TG curves show that complexes 1, 2 and 3 thermally decompose at 402.3, 398.4 and 390.6°C, respectively. The DSC curve of 1 exhibits two endothermic peaks at 177.7 and 353.7°C, while those of 2 and 3 show only one endothermic peak at 297.4 and 323.5°C, respectively. It was confirmed by the observation with the polarising microscope that the transition at 290– 360°C corresponded to the melting into the isotropic phase. The low transition temperature observed for 2 is probably due to the mixing of enantiomers and diastereomers, which usually lowers the melting point. The phase transition of 1 at 177.7°C could not be assigned using the polarising microscope but the similar phase transition observed for the hexyl-substituted analogue was assigned as the transition to smectic A phase. Such a phase transition is characteristic of the complexes substituted with linear alkyl side chains, and is usually assigned to partial melting of alkyl side chains. The crystal structures of 2 and 3 seems to differ from that of 1, but the powder X-ray diffraction
Fig. 2. p-A isotherm curves of complexes 1, 2 and 3.
Fig. 3. Estimated aggregation structure of the complex molecules (a) in the expanded phase and (b) in the condensed phase.
Fig. 1. Results of thermogravimetry and differential scanning calorimetry measurements of complexes 1, 2 and 3.
patterns were too complicated to elucidate their difference. The p-A isotherms of 1, 2 and 3 taken at 20°C, given in Fig. 2 clearly showed that two phases were present in the monolayer state of these complexes [9]. One is the expanded phase A, in which the molecules are supposed to lie on the surface as illustrated in Fig. 3(a), and the other is the condensed phase B, in which the molecules should be standing perpendicular to the surface as in Fig. 3(b). The maximum area per molecule of a phase, namely the critical area, in the expanded phase of 2 (0.72 nm2) was similar to that of 1 (0.70 nm2), while that of 3 (0.86 nm2) was about 20% larger than that of 1. The transition pressure of 3 (1.0 mN m − 1) was also largely different from those of 1
H. Abe, K. Miyamura / Inorganica Chimica Acta 298 (2000) 90–93
(5.4 mN m − 1) and 2 (5.9 mN m − 1), and was much smaller. These facts suggest that the expanded phase of 3 is loosely packed and less stable than those of 1 and 2. Although there are bulky a-methyl groups in 2 as well as in 3, the critical area of the expanded phase of 2 was not so large compared with 1. The fact that the large difference is caused by the full substitution of a-hydrogens both in critical area and the transition pressure implies the presence of similar interactions in 1 and 2, i.e. the p– p and the CH-p interactions. The alkyl substituted [Ni(salen)] was reported to form dimers with p–p and CH-p interactions in the crystalline phase [8]. The CH-p interactions were present between the a- and g-methylenes of alkyl groups and the p-conjugated system of the [Ni(salen)] moieties. The presence of such interactions in the monolayer state of alkyl substituted [Ni(salen)] had also been confirmed by the observation with STM [9]. In the case of 3, the hydrogens of the a-methylenes are fully substituted by the methyl groups, and thus there should be no CH-p interactions. This should be the reason for the instability of the expanded phase of 3 compared with those of 1 and 2.
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Acknowledgements This work was partly supported by the grant-in-aid for scientific research No. 08405054 from the Ministry of Education, Science, Sports and Culture, and the Izumi Science and Technology Foundation. References [1] S.A. Hudson, P.M. Maitlis, Chem. Rev. 93 (1993) 861. [2] N. Miyajima, Kagaku Kogyo 46 (1995) 654. [3] K. Ohta, M. Moriya, M. Ikejima, H. Hasebe, N. Kobayashi, I. Yamamoto, Bull. Chem. Soc. Jpn. 70 (1997) 1199. [4] J.D. Swalen, D.L. Allara, J.D. Andrade, E.A. Chandross, S. Garoff, J. Israelachvili, T.J. McCarthy, R. Murray, R.F. Pease, J.F. Rabolt, K.J. Wynne, H. Yu, Langmuir 3 (1987) 932. [5] S. Tone, M. Kunitake, M. Uchida, T. Kunitake, T. Kajiyama, Bull. Chem. Soc. Jpn. 66 (1993) 960. [6] K. Iimura, N. Suzuki, T. Kato, Bull. Chem. Soc. Jpn. 69 (1996) 1201. [7] K. Ohta, Y. Morizumi, T. Fujimoto, I. Yamamoto, K. Miyamura, Y. Gohshi, Mol. Cryst. Liq. Cryst. 214 (1992) 161. [8] K. Miyamura, A. Mihara, T. Fujii, Y. Gohshi, Y. Ishii, J. Am. Chem. Soc. 117 (1995) 2377. [9] T. Fujii, K. Miyamura, Bull. Chem. Soc. Jpn., accepted for publication. [10] I. Fells, E.A. Moelwyn-Hughes, J. Chem. Soc. (1959) 398.