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New BEDT-TTF salts with transition-metal containing anions
SV TIHI TII[ Im TnL_ Synthetic Metals 70 (1995) 767-770
ELSEVIER
New BEDT-TTF Salts with Transition-Metal Containing Anions P. Day, A.W. Graham, C.J. Kepert and M. Kurmoo The Royal Institution of Great Britain, 21 Albemarle Street, l ~ n d o n W1X 4BS, UK Abstract A wide variety of B E D T - T F F (bis(ethylenedithio)-tetrathiafulvalene) salts have been synthesised with anionic 3d, 4d and 5d metal complexes. Both mono- and polynuclear complexes have been employed. Most new phases are semiconducting.
1. I N T R O D U C T I O N Although many salts of BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) have been prepared in the last few years in the search for new superconducting phases, a desire to find new structure types continues to drive synthetic efforts. A major goal of our own work in this direction has been to incorporate anionic metal complexes into the salts, with two objectives. The first is to introduce localized, though possibly interacting, magnetic moments to observe to what extent they may interact with the conduction electrons of the organic units. Thus, conducting salts containing tetrahalogenocuprate0I) ions have already been reported and their physical properties extensively studied [ 1-4]. A second goal has been to utilize oligomeric and cluster complexes to broaden the range of anion shapes and dimensions so as to induce novel packing motifs within the BEDT-TTF arrays [5]. This paper presents a brief overview of our work in progress, with a summary of the structures and properties found. More detail about two of the new phases will be found in ref [6]. 2. E X P E R I M E N T A L All the p h a s e s d e s c r i b e d in this paper w e r e electrocrystallized by conventional methods in H-shaped cells
Figure 1. The crystal structure of (BEDTTFF)4KFe(C204)3C6HsCN.
0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02644-E
with Pt electrodes. BEDT-TTF was synthesised following the method of Larsen and Lenoir [7] and the transition metal complexes in the form o f N(n-C4H9)4 + and K + salts by literature methods [8]. In the latter case the salts were dissolved in organic solvents in the presence of 18-crown-6. Solvents used for crystal growth included CH2C12, CH3CN, C6HsCN and tetrahydrofuran. Conductivity was measured by 4-probe a.c. or 2-probe d.c. methods, contact being made to the crystals with Pt paint. Reflectivity spectra were measured from 400-4300 cm -1 by a Perkin-Elmer 1710 spectrometer and a Spectra-tech microscope. Magnetic susceptibility was determined using a Quantum Design MPMS 7 SQUID magnetometer.
3. M O N O N U C L E A R P A R A M A G N E T I C ANIONS
3.1 Tris-oxalato-complexes The anions M(C204)33- (M = Cr, Fe, Co) present an interesting sequence with which to form BEDT-TTF salts. To date crystals have been obtained using M = Fe, Co in the form of their Na, K and NH4 salts. The crystal structure of one such phase has been determined, and is shown in Fig. 1. The electrical properties of a range of phases are summarized in Table 1. All are semiconductors with quite similar activation energies.
768
P. Day et al. / Synthetic Metals 70 (1995) 767-770
The K, Fe member of the series is shown by its crystal structure to have the formula (B E D T TI'F)4KFe(C204)3C6HsCN, the crystal having been grown from C6H5CN. The structure presents the layer arrangement customary in B E D T - T F F salts, with two-dimensional arrays of organic cations formed from side-by-side stacks interleaved with layers containing only the tris-oxalato-anion, K ÷ and solvent of crystallization. The BEDT-TI'F stacks consist of well defined tetrads of molecules, within which the terminal (CH2) 2 moeities are ordered so that the conformations of adjacent molecules alternate. The K + is coordinated on one side by the O of two C2042- and on the other by the N of C6H5CN, the latter being disordered between two orientations (Fig. 1). Given the tetramerization of the B EDT-rlWF stacks it is not surprising that the compound is semiconducting, the activation energy being identical along all three crystalline axes (Fig. 2). Furthermore the magnetic susceptibility o f a polycrystalline sample measured in a field of 100 G obeys the Curie-Weiss law down to 5K with a small Weiss constant (-0.4K) indicating modest antiferromagnetic interaction between the Fe(C20,1)33-, and an effective spin close to the expected S = 5/2, As in earlier cases where a large m o m e n t has been placed on the anion in a BEDTTI'F salt [9], any contribution from the organic cation sublattice is obscured by that of the 3d ion. Magnetic measurements on the Co analogue of the Fe salt may reveal the susceptibility of the B E D T - T F F since the transition metal anion is then diamagnetic. Nevertheless it is noteworthy that the semiconducting activation energy of the K, Co derivative is very similar to that of the K, Fe one. 108
"
~'
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~
i ....
I ....
~ ....
~
I ....
I '~'
//plate
_>,10 6
i~]0' ,-¢
B EDT-TI'F)4 104
~i~
3
3.5
i!~
KFe(C204)
3 • C6H5C
~,llll~l~l~.l:,~jij.,~l,
4 4.5 5 5.5 6 6 5 1000 / Temperature (K -l)
~
7
Figure 2. Temperature dependence of resistance of (BEDTTI'F)4KFe(CeO4)3C6H5CN along the three crystal axes. 3.2 4(1 a n d 5d ions Apart from the ReO4 salts reported some years ago [10] and which in any case have d o electron configuration, there do not appear to have been any studies of BEDT-T'FF salts with anions containing monomeric 4d or 5d elements. W e have prepared five new phases of this type and their structures and properties are being studied. In this note we present a brief overview. As a preface, however, we draw attention to the perrhenate phase 13-(BEDT-'l~FF)3(ReO4)2 whose structure is illustrated in Fig. 3 [11]. It is semiconducting due to the trimcrization of the B E D T - T r F stacks, with a temperaturedependent activation energy E A of 0.06 eV (300K) to 0.08eV (70K).
An example of a 4d ] electron configuration is (BEDTTTF)[MoOCIz-(H20)] whose structure and properties are described in more detail in ref [6]. Suffice to say here that it is a rare instance of a BEDT-TFF monocafion, the molecules forming well-defined edge to edge chains. As expected it is a semiconductor. A significant feature of the magnetic behaviour is the marked decrease in the )~T versus T plot below 10K, indicating onset of antiferromagnetic exchange between the Mo. The Curie-Weiss law is obeyed (C = 0.314 EMU K m o l l , 0 = -0.23K). The value of C is consistent with S = 1/2, g = 1.85, as is the fit of the 5K magnetization isotherm to a Brillouin function. It is a matter for surprise that no significant contribution to the magnetism is observed from the BEDT-TI'F cation sublattice and EPR measurements are in progress to clarify this matter. The other monomeric Mo containing anion studied, MoC163, yields only twinned crystals of poor quality, whose resistance is too high to measure, Crystalline samples of two B EDT-TI'F salts ofmonomeric Re-containing anions (ReCI62- and Re(NCS)62-) have been isolated, in the latter case in two morphologies (plates and needles). Structure determination is still in progress but the phases are semiconducting and the infrared reflectivity shows no sign of a Drude edge. 4. P O L Y N U C L E A R D I A M A G N E T I C A N I O N S Numerous BEDT-TI'F salts have been reported with polyoxometallate anions, but very few in which the anions contain metal-metal bonds. The compound (BEDTq"I'F)4(Mo6Cl14)THF has been reported to be semiconducting [12] but we now find that (BEDT-TFF)a(Mo6C114)2CH2C12 is metallic down to 50K, where it exhibits a metal-insulator transition. This behaviour is confirmed by the infrared reflectivity. Such aradical change in electrical behaviour by changing solvent of crystallization is quite unusual and band structure calculations are in progress to understand the effect of such a minor perturbation of the crystal structure. The structure of the new phase will be reported fully later. Multiple metal-metal bonds are a well known feature of the chemistry of Re, and two new examples of BEDT-TFF salts with dimeric Re-containing anions have been isolated. The phase (BEDT-'VFF)3[Re2(NCS)10]'CH2CI 2, described in more detail in ref [6], was prepared in order to see whether intermolecular interactions between non-bonded S atoms, whichis such a common feature of BEDT-'Iq'F salts in general, could be extended to interaction between the organic cations and the S in the anion. In fact such interactions are present and may indeed account for the fact that there is no continuous network of S...S contacts between the B E D T - T F F molecules, thus rendering the phase semiconducting. The related ion Re-2Cls 2- also forms a BEDT-TTF salt whose structure has not been determined so far because the crystals are twinned. However the unit cell is monoclinic (a=7.36, b=12.85, c=9.56A, 13=108"). In general, twinning is very prevalent in all the phases described in this paper, suggesting a c o m m o n structural origin of the effect.
769
P. Day et al. / Synthetic Metals 70 (1995) 767-770
Figure 3. The crystal structure of ~-(BEDT-Tl'F)3(ReO4)2.
5. CONCLUSION We have crystallized many new B EDT-TI'F salts containing both mono- and polynuclear transition metal complexes as anions. Novel effects observed include the interpenetration of anions into the cation sublanice (Re2(NCS)103-), variation from semiconducting to metallic behaviour by changing solvent of crystallization (Mo6Cl142-) and one-dimensional antiferromagnetism within the anion sublattice, brought about by H-bonding [MoOCI4(H20)]-. The large majority of the new phases are semiconductors. 6. ACKNOWLEDGMENTS We thank SERC for partial support (AWG, MK). CJK thanks the Universityof Western Australia for a Hackett Scholarship. We are grateful to Dr W. Hayes for access to the IR
reflectivity equipment, and to Prof. M. Hursthouse for the structure analysis of (BEDT-TI'F)4KFe(C204)3.C6HsCN.
Table 1 Conductivity of BEDT-TTF-M-M' (C204) 3 salts
M
M'
Morphology
EA/eV
Na K NH4 NH4 K NH4
Fe Fe Fe Fe Co Co
Hexagon Diamond Hexagon Diamond Needle Diamond
0.08 0.14 0.12 0.14 0.11 0.21
770
P. Day et al. / Synthetic Metals 70 (1995) 767-770
Table 2 Conductivity of BEDT-TFF salts of 4d and 5d anions EA/eV Mo2C193Mo6Cl142MoC163MoOClz(H20)* ReCI62Re(NCS)62 Re2C182 Re2(NCS)IO4"*
0.04 MI transition, 50K Insulator Temperature dependent 0.10-0.15 0.19 (plates); 0.22 (needles) 0.20 0.30
* See ref [6] for further details
REFERENCES 1. 2.
T. Mori, F. Sakai, G. Saito and H. Inokuchi, Chem. Lett. (1987) 927. M. Kurmoo, D. Kanazawa and P. Day, Synth. Met. 41-43 (1991) 2127.
3.
P. Day, M. Kurmoo, T. Mallala, L. Marsden, M. Allan, R.H. Friend, F.L. Pratt, W. Hayes, D. Chasseau, G. Bravic and L. Ducasse, J. Amer. Chem. Soc. 114 (1992) 10722. 4. I.R. Marsden, M.L. Allan, R.H. Friend, M. Kurmoo, D. Kanazawa, P. Day, G. Bravic, D Chasseau, L. Ducasse and W. Hayes, Phys. Rev. B, in press. 5. D. Attanasio, C. Bellitto, M. Bonamico, V. Fares and P. Imperatori, Gazz. Chim. Ital. 121 (1991) 151; S. Triki, L. Ouahab, J.F. Halet, O. Pena, J. Padiou, D. Grandjean, C. Garrigon-Lagrange and P. Delhaes, J. Chem. Soc., Dalton Trans. (1992) 1217 6. C.J. Kepert, M.R. Truter, M. Kurmoo and P. Day, These Proceedings. 7. J. Larsen and C. Lenoir, Synthesis 2 (1988) 134. 8. See 'Inorganic Synthesis', passim. 9. T. MaUah, C. Hollis, S. Bott, M. Kurmoo and P. Day, J. Chem. Soc., Dalton Trans. (1990), 859. 10. S.S.P. Parkin, E.M. Engler, V.Y. Lee, R.R. Schumaker, Mol. Cryst. Liq. Cryst. 119 (1985) 375. 11. C.J. Kepert, M.R. Truter, M. Kumloo, L. Ducasse and P. Day, to be published. 12. H.Fuchs, S.Fuchs, K.Polbom, Th. Lehnert, C.P. Heidmann, H. Mtiller, Synth. Met. 27 (1988) A271-A276.
ELSEVIER
New BEDT-TTF Salts with Transition-Metal Containing Anions P. Day, A.W. Graham, C.J. Kepert and M. Kurmoo The Royal Institution of Great Britain, 21 Albemarle Street, l ~ n d o n W1X 4BS, UK Abstract A wide variety of B E D T - T F F (bis(ethylenedithio)-tetrathiafulvalene) salts have been synthesised with anionic 3d, 4d and 5d metal complexes. Both mono- and polynuclear complexes have been employed. Most new phases are semiconducting.
1. I N T R O D U C T I O N Although many salts of BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) have been prepared in the last few years in the search for new superconducting phases, a desire to find new structure types continues to drive synthetic efforts. A major goal of our own work in this direction has been to incorporate anionic metal complexes into the salts, with two objectives. The first is to introduce localized, though possibly interacting, magnetic moments to observe to what extent they may interact with the conduction electrons of the organic units. Thus, conducting salts containing tetrahalogenocuprate0I) ions have already been reported and their physical properties extensively studied [ 1-4]. A second goal has been to utilize oligomeric and cluster complexes to broaden the range of anion shapes and dimensions so as to induce novel packing motifs within the BEDT-TTF arrays [5]. This paper presents a brief overview of our work in progress, with a summary of the structures and properties found. More detail about two of the new phases will be found in ref [6]. 2. E X P E R I M E N T A L All the p h a s e s d e s c r i b e d in this paper w e r e electrocrystallized by conventional methods in H-shaped cells
Figure 1. The crystal structure of (BEDTTFF)4KFe(C204)3C6HsCN.
0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02644-E
with Pt electrodes. BEDT-TTF was synthesised following the method of Larsen and Lenoir [7] and the transition metal complexes in the form o f N(n-C4H9)4 + and K + salts by literature methods [8]. In the latter case the salts were dissolved in organic solvents in the presence of 18-crown-6. Solvents used for crystal growth included CH2C12, CH3CN, C6HsCN and tetrahydrofuran. Conductivity was measured by 4-probe a.c. or 2-probe d.c. methods, contact being made to the crystals with Pt paint. Reflectivity spectra were measured from 400-4300 cm -1 by a Perkin-Elmer 1710 spectrometer and a Spectra-tech microscope. Magnetic susceptibility was determined using a Quantum Design MPMS 7 SQUID magnetometer.
3. M O N O N U C L E A R P A R A M A G N E T I C ANIONS
3.1 Tris-oxalato-complexes The anions M(C204)33- (M = Cr, Fe, Co) present an interesting sequence with which to form BEDT-TTF salts. To date crystals have been obtained using M = Fe, Co in the form of their Na, K and NH4 salts. The crystal structure of one such phase has been determined, and is shown in Fig. 1. The electrical properties of a range of phases are summarized in Table 1. All are semiconductors with quite similar activation energies.
768
P. Day et al. / Synthetic Metals 70 (1995) 767-770
The K, Fe member of the series is shown by its crystal structure to have the formula (B E D T TI'F)4KFe(C204)3C6HsCN, the crystal having been grown from C6H5CN. The structure presents the layer arrangement customary in B E D T - T F F salts, with two-dimensional arrays of organic cations formed from side-by-side stacks interleaved with layers containing only the tris-oxalato-anion, K ÷ and solvent of crystallization. The BEDT-TI'F stacks consist of well defined tetrads of molecules, within which the terminal (CH2) 2 moeities are ordered so that the conformations of adjacent molecules alternate. The K + is coordinated on one side by the O of two C2042- and on the other by the N of C6H5CN, the latter being disordered between two orientations (Fig. 1). Given the tetramerization of the B EDT-rlWF stacks it is not surprising that the compound is semiconducting, the activation energy being identical along all three crystalline axes (Fig. 2). Furthermore the magnetic susceptibility o f a polycrystalline sample measured in a field of 100 G obeys the Curie-Weiss law down to 5K with a small Weiss constant (-0.4K) indicating modest antiferromagnetic interaction between the Fe(C20,1)33-, and an effective spin close to the expected S = 5/2, As in earlier cases where a large m o m e n t has been placed on the anion in a BEDTTI'F salt [9], any contribution from the organic cation sublattice is obscured by that of the 3d ion. Magnetic measurements on the Co analogue of the Fe salt may reveal the susceptibility of the B E D T - T F F since the transition metal anion is then diamagnetic. Nevertheless it is noteworthy that the semiconducting activation energy of the K, Co derivative is very similar to that of the K, Fe one. 108
"
~'
I ....
~
i ....
I ....
~ ....
~
I ....
I '~'
//plate
_>,10 6
i~]0' ,-¢
B EDT-TI'F)4 104
~i~
3
3.5
i!~
KFe(C204)
3 • C6H5C
~,llll~l~l~.l:,~jij.,~l,
4 4.5 5 5.5 6 6 5 1000 / Temperature (K -l)
~
7
Figure 2. Temperature dependence of resistance of (BEDTTI'F)4KFe(CeO4)3C6H5CN along the three crystal axes. 3.2 4(1 a n d 5d ions Apart from the ReO4 salts reported some years ago [10] and which in any case have d o electron configuration, there do not appear to have been any studies of BEDT-T'FF salts with anions containing monomeric 4d or 5d elements. W e have prepared five new phases of this type and their structures and properties are being studied. In this note we present a brief overview. As a preface, however, we draw attention to the perrhenate phase 13-(BEDT-'l~FF)3(ReO4)2 whose structure is illustrated in Fig. 3 [11]. It is semiconducting due to the trimcrization of the B E D T - T r F stacks, with a temperaturedependent activation energy E A of 0.06 eV (300K) to 0.08eV (70K).
An example of a 4d ] electron configuration is (BEDTTTF)[MoOCIz-(H20)] whose structure and properties are described in more detail in ref [6]. Suffice to say here that it is a rare instance of a BEDT-TFF monocafion, the molecules forming well-defined edge to edge chains. As expected it is a semiconductor. A significant feature of the magnetic behaviour is the marked decrease in the )~T versus T plot below 10K, indicating onset of antiferromagnetic exchange between the Mo. The Curie-Weiss law is obeyed (C = 0.314 EMU K m o l l , 0 = -0.23K). The value of C is consistent with S = 1/2, g = 1.85, as is the fit of the 5K magnetization isotherm to a Brillouin function. It is a matter for surprise that no significant contribution to the magnetism is observed from the BEDT-TI'F cation sublattice and EPR measurements are in progress to clarify this matter. The other monomeric Mo containing anion studied, MoC163, yields only twinned crystals of poor quality, whose resistance is too high to measure, Crystalline samples of two B EDT-TI'F salts ofmonomeric Re-containing anions (ReCI62- and Re(NCS)62-) have been isolated, in the latter case in two morphologies (plates and needles). Structure determination is still in progress but the phases are semiconducting and the infrared reflectivity shows no sign of a Drude edge. 4. P O L Y N U C L E A R D I A M A G N E T I C A N I O N S Numerous BEDT-TI'F salts have been reported with polyoxometallate anions, but very few in which the anions contain metal-metal bonds. The compound (BEDTq"I'F)4(Mo6Cl14)THF has been reported to be semiconducting [12] but we now find that (BEDT-TFF)a(Mo6C114)2CH2C12 is metallic down to 50K, where it exhibits a metal-insulator transition. This behaviour is confirmed by the infrared reflectivity. Such aradical change in electrical behaviour by changing solvent of crystallization is quite unusual and band structure calculations are in progress to understand the effect of such a minor perturbation of the crystal structure. The structure of the new phase will be reported fully later. Multiple metal-metal bonds are a well known feature of the chemistry of Re, and two new examples of BEDT-TFF salts with dimeric Re-containing anions have been isolated. The phase (BEDT-'VFF)3[Re2(NCS)10]'CH2CI 2, described in more detail in ref [6], was prepared in order to see whether intermolecular interactions between non-bonded S atoms, whichis such a common feature of BEDT-'Iq'F salts in general, could be extended to interaction between the organic cations and the S in the anion. In fact such interactions are present and may indeed account for the fact that there is no continuous network of S...S contacts between the B E D T - T F F molecules, thus rendering the phase semiconducting. The related ion Re-2Cls 2- also forms a BEDT-TTF salt whose structure has not been determined so far because the crystals are twinned. However the unit cell is monoclinic (a=7.36, b=12.85, c=9.56A, 13=108"). In general, twinning is very prevalent in all the phases described in this paper, suggesting a c o m m o n structural origin of the effect.
769
P. Day et al. / Synthetic Metals 70 (1995) 767-770
Figure 3. The crystal structure of ~-(BEDT-Tl'F)3(ReO4)2.
5. CONCLUSION We have crystallized many new B EDT-TI'F salts containing both mono- and polynuclear transition metal complexes as anions. Novel effects observed include the interpenetration of anions into the cation sublanice (Re2(NCS)103-), variation from semiconducting to metallic behaviour by changing solvent of crystallization (Mo6Cl142-) and one-dimensional antiferromagnetism within the anion sublattice, brought about by H-bonding [MoOCI4(H20)]-. The large majority of the new phases are semiconductors. 6. ACKNOWLEDGMENTS We thank SERC for partial support (AWG, MK). CJK thanks the Universityof Western Australia for a Hackett Scholarship. We are grateful to Dr W. Hayes for access to the IR
reflectivity equipment, and to Prof. M. Hursthouse for the structure analysis of (BEDT-TI'F)4KFe(C204)3.C6HsCN.
Table 1 Conductivity of BEDT-TTF-M-M' (C204) 3 salts
M
M'
Morphology
EA/eV
Na K NH4 NH4 K NH4
Fe Fe Fe Fe Co Co
Hexagon Diamond Hexagon Diamond Needle Diamond
0.08 0.14 0.12 0.14 0.11 0.21
770
P. Day et al. / Synthetic Metals 70 (1995) 767-770
Table 2 Conductivity of BEDT-TFF salts of 4d and 5d anions EA/eV Mo2C193Mo6Cl142MoC163MoOClz(H20)* ReCI62Re(NCS)62 Re2C182 Re2(NCS)IO4"*
0.04 MI transition, 50K Insulator Temperature dependent 0.10-0.15 0.19 (plates); 0.22 (needles) 0.20 0.30
* See ref [6] for further details
REFERENCES 1. 2.
T. Mori, F. Sakai, G. Saito and H. Inokuchi, Chem. Lett. (1987) 927. M. Kurmoo, D. Kanazawa and P. Day, Synth. Met. 41-43 (1991) 2127.
3.
P. Day, M. Kurmoo, T. Mallala, L. Marsden, M. Allan, R.H. Friend, F.L. Pratt, W. Hayes, D. Chasseau, G. Bravic and L. Ducasse, J. Amer. Chem. Soc. 114 (1992) 10722. 4. I.R. Marsden, M.L. Allan, R.H. Friend, M. Kurmoo, D. Kanazawa, P. Day, G. Bravic, D Chasseau, L. Ducasse and W. Hayes, Phys. Rev. B, in press. 5. D. Attanasio, C. Bellitto, M. Bonamico, V. Fares and P. Imperatori, Gazz. Chim. Ital. 121 (1991) 151; S. Triki, L. Ouahab, J.F. Halet, O. Pena, J. Padiou, D. Grandjean, C. Garrigon-Lagrange and P. Delhaes, J. Chem. Soc., Dalton Trans. (1992) 1217 6. C.J. Kepert, M.R. Truter, M. Kurmoo and P. Day, These Proceedings. 7. J. Larsen and C. Lenoir, Synthesis 2 (1988) 134. 8. See 'Inorganic Synthesis', passim. 9. T. MaUah, C. Hollis, S. Bott, M. Kurmoo and P. Day, J. Chem. Soc., Dalton Trans. (1990), 859. 10. S.S.P. Parkin, E.M. Engler, V.Y. Lee, R.R. Schumaker, Mol. Cryst. Liq. Cryst. 119 (1985) 375. 11. C.J. Kepert, M.R. Truter, M. Kumloo, L. Ducasse and P. Day, to be published. 12. H.Fuchs, S.Fuchs, K.Polbom, Th. Lehnert, C.P. Heidmann, H. Mtiller, Synth. Met. 27 (1988) A271-A276.