First Molecular Superconductor with the Tris(Oxalato)Aluminate Anion, ′′-(BEDT-TTF)4(H3O)Al(C2O4)3C6H5Br, and Isostructural Tris(Oxalato)Cobaltate and Tris(Oxalato)Ruthenate Radical Cation Salts

Peter Day’s research group reported the first molecular superconductor containing paramagnetic metal ions in 1995, β”-(BEDT-TTF)4(H3O)Fe(C2O4)3·C6H5CN. Subsequent research has produced a multitude of BEDT-TTF-tris(oxalato)metallate salts with a variety of structures and properties, including 32 superconductors to date. We present here the synthesis, crystal structure, and conducting properties of the newest additions to the Day series including the first superconductor incorporating the diamagnetic tris(oxalato)aluminate anion, β”-(BEDT-TTF)4(H3O)Al(C2O4)3·C6H5Br, which has a superconducting Tc of ~2.5 K. β”-(BEDT-TTF)4(H3O)Co(C2O4)3·C6H5Br represents the first example of a β” phase for the tris(oxalato)cobaltate anion, but this salt does not show superconductivity.


Introduction
The first paramagnetic superconductor, β"-(BEDT-TTF) 4 (H 3 O)Fe(C 2 O 4 ) 3 ·C 6 H 5 CN, was discovered in 1995 by the group of Professor Peter Day at the Royal Institution of Great Britain [1]. The ability of tris(oxalato)metallate(III) anions, M(C 2 O 4 ) 3 3− , to bridge through oxalate ions with monocations or metal(II) ions and form 2D sheets opened the door to a huge variety of structures and properties in radical cation salts with BEDT-TTF [2]. This family of salts includes not only paramagnetic superconductors, but also a ferromagnetic metal [3], antiferromagnetic semiconductor [4], and proton conductor [5,6].

(H 3 O)M(C 2 O 4 ) 3 ·G (M-G = Al-PhBr, Co-PhBr, Ru-PhCl, Ru-PhF) are isostructural with the previously reported Day series β"-(BEDT-TTF) 4 [(A)M(C 2 O 4 ) 3 ]·G.
Ru-PhBr is also isostructural, though the crystals were very thin and not suitable for a publishable X-ray dataset. They crystallise in the monoclinic space group C2/c. The asymmetric unit contains two crystallographically independent BEDT-TTF molecules, half an M(C 2 O 4 ) 3− molecule, half a guest halobenzene molecule, and half a H 3 O + molecule ( Table 1). The long-range structure consists of ordered alternating layers of BEDT-TTF donor molecules and M(C 2 O 4 ) 3− anions ( Figure 1). The two crystallographically independent donor BEDT-TTF molecules form two-dimensional stacks along the a/b crystallographic axis in a β" arrangement ( Figure 2). A number of predominately side-to-side sulphur-sulphur interactions below the sum of the van der Waals radii are present ( Table 2). The estimated charge on BEDT-TTF cations can be calculated via the method of Guionneau et al. [50] from the central C=C and C-S bond lengths of the TTF core and results in a charge of approximately +0.5 for each BEDT-TTF molecule, as expected (Table 3).         The anion layer consists of a honeycomb arrangement of M(C 2 O 4 ) 3− , perpendicular to the long axis of the BEDT-TTF molecules, resulting in a hexagonal cavity that is occupied by the guest halobenzene molecule. Each anion layer contains a single enantiomer of the tris(oxalato)metallate ion with adjacent layers containing the alternate enantiomer, which gives an overall racemic lattice. The hexagonal cavity and the orientation of the guest halobenzene molecule within it are shown in Figure 3 and Table 4. Distances a, b, h, and w represent the dimensions of the hexagonal cavity. The latter two are the height and width of the cavity, respectively, and δ is the angle of the benzene ring plane relative to the plane of the hexagonal cavity (measured as the least-squares plane of the three metal atoms making up three corners of the hexagon). For M = Rh, we see a reduction in height (h) of the hexagonal cavity going from G = PhCl to the smaller PhF, accompanied by a reduction in the length of the b axis of the unit cell. For salt Al-PhBr, we observed a T c of~2.5 K (Figure 4), which is similar to previously published salts of β"-(BEDT- When applying a magnetic field along the c* axis, the critical field of the superconductivity at 0.7 K is about 0.2 T. This is comparable to other salts in the Day series, for example: the Fe-DMF salt has a T c of 2.0 K, and H c2 in a perpendicular field is~0.1 T [51]. Higher T c salts in the Day series have higher H c2 values (2-5 T) [52], and these quasi-2D superconductors are strongly anisotropic [53]. The other salts reported in this paper are isostructural. The two crystallographically independent BEDT-TTF molecules are shown in different colours. Hydrogens have been removed for clarity. The a axis is shown in red, the b axis in green, and the c axis in blue.
The anion layer consists of a honeycomb arrangement of M(C2O4) 3− , perpendicular to the long axis of the BEDT-TTF molecules, resulting in a hexagonal cavity that is occupied by the guest halobenzene molecule. Each anion layer contains a single enantiomer of the tris(oxalato)metallate ion with adjacent layers containing the alternate enantiomer, which gives an overall racemic lattice. The hexagonal cavity and the orientation of the guest halobenzene molecule within it are shown in Figure 3 and Table 4. Distances a, b, h, and w represent the dimensions of the hexagonal cavity. The latter two are the height and width of the cavity, respectively, and δ is the angle of the benzene ring plane relative to the plane of the hexagonal cavity (measured as the least-squares plane of the three metal atoms making up three corners of the hexagon). For M = Rh, we see a reduction in height (h) of the hexagonal cavity going from G = PhCl to the smaller PhF, accompanied by a reduction in the length of the b axis of the unit cell. For salt Al-PhBr, we observed a Tc of ~2.5 K (Figure 4), which is similar to previously published salts of β″-(BEDT-TTF)4[(H3O)M(C2O4)3].G, where G = bromobenzene. When applying a magnetic field along the c* axis, the critical field of the superconductivity at 0.7 K is about 0.2 T. This is comparable to other salts in the Day series, for example: the Fe-DMF salt has a Tc of 2.0 K, and Hc2 in a perpendicular field is ~0.1 T [51]. Higher Tc salts in the Day series have higher Hc2 values (2-5 T) [52], and these quasi-2D superconductors are strongly anisotropic [53].   Table 4. Honeycomb cavity measurements in the anion layer (see Figure 3) of β"-(BEDT-TTF) 4    The Al 3+ ion of tris(oxalato)aluminate is smaller than previous examples, where M = Fe [13,17], Ga [28], Rh [30], and Ru [29] (Tc = ~3.8, ~3.0, ~2.9, ~2.8 K, respectively, for G = bromobenzene), and the Tc is smaller for M = Al at ~2.5 K. A comparison of the b axis length of these bromobenzene salts at room temperature showed that the M = Fe salt has the longest at 20.0546(15) Å and also the highest ~3.  [28] and Ru [29] salts cannot be made owing to A = Kx(H3O)1-x rather than H3O for these salts. Salts with M = Cr [23] and Mn [17] have been reported with Tcs of 1.5 K and 2.0 K, respectively, but crystal structures are not published for the comparison of the b axes. Our crystals of Co-PhBr did not show superconductivity ( Figure 5), with the b axis of this salt being much shorter than all other PhBr salts at 19.7508(5) Å . The Al 3+ ion of tris(oxalato)aluminate is smaller than previous examples, where M = Fe [13,17], Ga [28], Rh [30], and Ru [29] (T c =~3.8,~3.0,~2.9,~2.8 K, respectively, for G = bromobenzene), and the T c is smaller for M = Al at~2.5 K. A comparison of the b axis length of these bromobenzene salts at room temperature showed that the M = Fe salt has the longest at 20.0546(15) Å and also the highest~3.8 K [13,17]; M = Rh has an intermediate b axis of 20.0458(4) Å and a T c of~2.9 K [30]; while M = Al has the shortest b axis of 19.9472(4) Å and the lowest T c at~2.5 K. A direct comparison with the M = Ga [28] and Ru [29] salts cannot be made owing to A = K x (H 3 O) 1-x rather than H 3 O for these salts. Salts with M = Cr [23] and Mn [17] have been reported with T c s of 1.5 K and 2.0 K, respectively, but crystal structures are not published for the comparison of the b axes. Our crystals of Co-PhBr did not show superconductivity ( Figure 5 (Figure 1). This is confirmed by the similar Tcs that are observed for isostructural salts with the same A and G, but which differ only in the presence of paramagnetic Fe 3+ (S = 5/2) or non-magnetic Ga 3+ [53,54]. A much more marked effect on the value of Tc is observed when changing the guest molecule, G. Changing M and G leads to a change in the length of the unit cell dimensions. A correlation between the b axis length and superconducting Tc has been observed through structural analysis [31]. The effect of chemical pressure through changing G and M is mainly attributed to the guest molecule, G, which is oriented with the R-group oriented in the b direction ( Figure 3). The longest molecules, benzonitrile and nitrobenzene, have the highest Tcs observed in the family, and the relationship between Tc and the guest molecule size can be observed in the series of salts with halobenzene guest molecules [31]. Only the higher Tc salts in this family show insulating behaviour just above Tc owing to charge disproportionation in these salts [55][56][57][58]. Figure 6 shows the resistivity of β″-(BEDT-TTF)4(H3O)Ru(C2O4)3·G, where G = PhBr, PhCl, or PhF. Ru-PhBr for A = Kx(H3O)1-x has previously been studied by Prokhorova et al. [29] with a sample-dependent Tc in the range 2.8-6.3 K. Resistivity measurements on our crystals of β″-(BEDT-TTF)4(H3O)Ru(C2O4)3.PhBr gave a Tc of 2.8 K, which was as expected based on the b axis length [31]. Upon reducing the size of G from PhBr to PhCl or PhF, no superconductivity was observed. Both Ru-PhCl and Ru-PhF showed semiconducting behaviour ( Figure 6). Both Ru-PhCl and Ru-PhF had shorter b axis lengths compared to the Ru-PhBr salt. However, the b axis lengths in semiconducting Ru-PhCl and Ru-PhF were longer than that in superconducting Al-PhBr (Table 3). This indicates that other factors, such as the shape and the electric dipole of the guest molecule, may have minor influences even though the b axis length predominantly affects the electronic state, including the Tc [31].  (Figure 1). This is confirmed by the similar T c s that are observed for isostructural salts with the same A and G, but which differ only in the presence of paramagnetic Fe 3+ (S = 5/2) or non-magnetic Ga 3+ [53,54]. A much more marked effect on the value of T c is observed when changing the guest molecule, G. Changing M and G leads to a change in the length of the unit cell dimensions. A correlation between the b axis length and superconducting T c has been observed through structural analysis [31]. The effect of chemical pressure through changing G and M is mainly attributed to the guest molecule, G, which is oriented with the R-group oriented in the b direction ( Figure 3). The longest molecules, benzonitrile and nitrobenzene, have the highest T c s observed in the family, and the relationship between T c and the guest molecule size can be observed in the series of salts with halobenzene guest molecules [31]. Only the higher T c salts in this family show insulating behaviour just above T c owing to charge disproportionation in these salts [55][56][57][58]. Figure 6 shows the resistivity of β"-(BEDT-TTF) 4 (H 3 O)Ru(C 2 O 4 ) 3 ·G, where G = PhBr, PhCl, or PhF. Ru-PhBr for A = K x (H 3 O) 1-x has previously been studied by Prokhorova et al. [29] with a sample-dependent T c in the range 2.8-6.3 K. Resistivity measurements on our crystals of β"-(BEDT-TTF) 4 (H 3 O)Ru(C 2 O 4 ) 3 .PhBr gave a T c of 2.8 K, which was as expected based on the b axis length [31]. Upon reducing the size of G from PhBr to PhCl or PhF, no superconductivity was observed. Both Ru-PhCl and Ru-PhF showed semiconducting behaviour ( Figure 6). Both Ru-PhCl and Ru-PhF had shorter b axis lengths compared to the Ru-PhBr salt. However, the b axis lengths in semiconducting Ru-PhCl and Ru-PhF were longer than that in superconducting Al-PhBr (Table 3). This indicates that other factors, such as the shape and the electric dipole of the guest molecule, may have minor influences even though the b axis length predominantly affects the electronic state, including the T c [31]. Magnetochemistry 2021, 7, x FOR PEER REVIEW 8 of 12

Materials and Methods
Bromobenzene, chlorobenzene, fluorobenzene, ethanol, and 18-crown-6 were purchased from Sigma Aldrich and used as received. BEDT-TTF was purchased from Sigma Aldrich (Gillingham, Dorset, UK) and recrystallised from chloroform.
Al-PhBr: One-hundred milligrams of ammonium tris(oxalato)aluminate and 200 mg of 18-crown-6 ether were dissolved in 10 mL 1,2,4-trichlorobenzene, 10 mL bromobenzene, and 2 mL ethanol. The solution was then filtered into the cathodic side of the H-cell, while 20 mg of BEDT-TTF was added to the anodic side of the H-cell. The level of solvent was allowed to equilibrate in the cell, and a platinum electrode was added to each side. A constant current of 0.8 µ A was applied across the H-cell which gave small black crystals of Al-PhBr which were collected after 28 days.
Co-PhBr: One-hundred milligrams of ammonium tris(oxalato)cobaltate and 200 mg of 18-crown-6 ether were dissolved in 10 mL 1,2,4-trichlorobenzene, 10 mL bromobenzene, and 2 mL ethanol. Ten milligrams of BEDT-TTF were added to the anodic side of the H-cell. A constant current of 0.6 µ A was applied across the H-cell which gave tiny black crystals of Co-PhBr which were collected after 14 days.
Ru-PhF: One-hundred milligrams of ammonium tris(oxalato)ruthenate and 200 mg of 18-crown-6 ether were dissolved in 10 mL 1,2,4-trichlorobenzene, 10 mL fluorobenzene, and 2 mL ethanol. Ten milligrams of BEDT-TTF were added to the anodic side of the Hcell. A constant current of 1.0 µ A was applied across the H-cell which gave black block crystals of Ru-PhF which were collected after 28 days.
Ru-PhCl: One-hundred milligrams of ammonium tris(oxalato)ruthenate and 200 mg of 18-crown-6 ether were dissolved in 10 mL 1,2,4-trichlorobenzene, 10 mL chlorobenzene, and 2 mL ethanol. Ten milligrams of BEDT-TTF were added to the anodic side of the Hcell. A constant current of 1.0 µ A was applied across the H-cell which gave black block crystals of Ru-PhCl which were collected after 28 days.

Materials and Methods
Bromobenzene, chlorobenzene, fluorobenzene, ethanol, and 18-crown-6 were purchased from Sigma Aldrich and used as received. BEDT-TTF was purchased from Sigma Aldrich (Gillingham, Dorset, UK) and recrystallised from chloroform.
Al-PhBr: One-hundred milligrams of ammonium tris(oxalato)aluminate and 200 mg of 18-crown-6 ether were dissolved in 10 mL 1,2,4-trichlorobenzene, 10 mL bromobenzene, and 2 mL ethanol. The solution was then filtered into the cathodic side of the H-cell, while 20 mg of BEDT-TTF was added to the anodic side of the H-cell. The level of solvent was allowed to equilibrate in the cell, and a platinum electrode was added to each side. A constant current of 0.8 µA was applied across the H-cell which gave small black crystals of Al-PhBr which were collected after 28 days.
Co-PhBr: One-hundred milligrams of ammonium tris(oxalato)cobaltate and 200 mg of 18-crown-6 ether were dissolved in 10 mL 1,2,4-trichlorobenzene, 10 mL bromobenzene, and 2 mL ethanol. Ten milligrams of BEDT-TTF were added to the anodic side of the H-cell. A constant current of 0.6 µA was applied across the H-cell which gave tiny black crystals of Co-PhBr which were collected after 14 days.
Ru-PhF: One-hundred milligrams of ammonium tris(oxalato)ruthenate and 200 mg of 18-crown-6 ether were dissolved in 10 mL 1,2,4-trichlorobenzene, 10 mL fluorobenzene, and 2 mL ethanol. Ten milligrams of BEDT-TTF were added to the anodic side of the H-cell. A constant current of 1.0 µA was applied across the H-cell which gave black block crystals of Ru-PhF which were collected after 28 days.
Ru-PhCl: One-hundred milligrams of ammonium tris(oxalato)ruthenate and 200 mg of 18-crown-6 ether were dissolved in 10 mL 1,2,4-trichlorobenzene, 10 mL chlorobenzene, and 2 mL ethanol. Ten milligrams of BEDT-TTF were added to the anodic side of the H-cell. A constant current of 1.0 µA was applied across the H-cell which gave black block crystals of Ru-PhCl which were collected after 28 days.
A constant current of 1.0 µA was applied across the H-cell which gave thin needle crystals of Ru-PhBr which were collected after 28 days. The crystals were very thin and not suitable for a publishable X-ray dataset.

Single-Crystal X-ray Crystallography
Data were collected using a RigakuRapid II (Tokyo, Japan) imaging plate system with the MicroMax-007 HF/VariMax rotating-anode X-ray generator and confocal monochromated Mo-Kα radiation.

Conducting Properties
Out-of-plane electrical resistance was measured using the standard four-terminal AC method with the current along the c* axis. Four gold wires were attached using carbon paint on both plane surfaces of single crystals.

Conclusions
We reported the synthesis and characterization of β"-(BEDT-TTF) 4 (H 3 O) Al(C 2 O 4 ) 3 ·C 6 H 5 Br (Al-PhBr), which represents the first superconductor in the Day series to contain the tris(oxalato)aluminate anion. This salt (M = Al) is isostructural with bromobenzene salts where M = Fe, Ga, Rh, Ru, Mn, Cr. A relationship between the b axis length and superconducting T c has previously been observed in the Day series [31]. The b axis length of these bromobenzene salts at room temperature showed that the M = Fe salt had the longest b axis and also the highest T c of~3.8 K, while M = Al had the shortest b axis and the lowest T c of~2.5 K. We also reported the isostructural M = Co salt (Co-PhBr), which did not show superconductivity. The b axis of this salt was much shorter than all other bromobenzene salts. Isostructural salts Ru-PhCl and Ru-PhF were presented in which the b axes were longer than that observed in superconducting Al-PhBr, but these two ruthenium salts did not show superconductivity. This indicates that even though the b axis length predominantly affected the electronic state, including the T c , other factors may also be at work, such as the shape and the electric dipole of the guest molecules, which may have minor influences on the electronic states.