Metallic and Mott Insulating Spin‐Frustrated Antiferromagnetic States in Ionic Fullerene Complexes with a Two‐Dimensional Hexagonal C₆₀^.− Packing Motif

Abstract

(MDABCO(+))(C60(·-))(TPC) (1), in which MDABCO(+) is N-methyldiazabicyclooctanium, TPC is triptycene, and both have threefold symmetry, is a rare example of a fullerene-based quasi-2D metal and contains closely packed hexagonal fullerene layers with interfullerene center-to-center distances of 10.07 Å at 300 K. Evidence for the metallic nature of 1 was obtained by optical and microwave conductivity measurements on single crystals. The metal is characterized by a nontypical Drude response and relatively large optical mass (m*/m0 =6.7). The latter indicates a narrow-band nature, which is consistent with the calculated bandwidth of 0.10-0.15 eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200-230 K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200 K due to ordering of the fullerene and cationic sublattices. New layered complex (MQ(+))(C60(·-))(TPC) (2) with a hexagonal arrangement of C60(·-) was obtained by increasing the interfullerene distance with the bulkier N-methylquinuclidinium cations (MQ(+)) having threefold symmetry. The structure of 2 is characterized by increased interfullerene center-to-center distances in the layers (10.124, 10.155, and 10.177 Å at 250 K). Unit-cell doubling parallel to the 2D layer (along the b axis) was observed at low temperatures. In contrast to metallic 1, 2 exhibits a nonmetallic spin-frustrated state with an antiferromagnetic interaction of spins (the Weiss temperature is -27 K) and no magnetic ordering down to 1.9 K. It was supposed that the expanded interfullerene distances in the triangular arrangement decrease the bandwidth and suppress metallic conductivity in 2, and thus a Mott-Hubbard insulating state with antiferromagnetically frustrated spins results.

Design of Spin-Frustrated Monomer-Type C60•− Mott Insulator

“…Therefore, this relaxed Mott criterion is effective as long as the splitting of t 1u orbitals by Jahn-Teller distortion is not large enough, and forms one LUMO band. The upper limit of U/W requires W to be 0.27 eV, by considering the lowest value of U eff (calculated value-0.68 eV), which is about 2-3 times the calculated W value (0.10-0.15 eV for (TPC 0 )(MDABCO + ) (C 60 •− )) by the AM1 method [12]. A preliminary DFT calculation for this CT solid indicates a total W of about 0.48 eV at 160 K [12], which is still smaller than the estimated U eff and is not able to account for the metallic nature.…”

“…The upper limit of U/W requires W to be 0.27 eV, by considering the lowest value of U eff (calculated value-0.68 eV), which is about 2-3 times the calculated W value (0.10-0.15 eV for (TPC 0 )(MDABCO + ) (C 60 •− )) by the AM1 method [12]. A preliminary DFT calculation for this CT solid indicates a total W of about 0.48 eV at 160 K [12], which is still smaller than the estimated U eff and is not able to account for the metallic nature. Since the calculated W values in these systems are not reliable at present, only the ratio of overlap integrals or transfer interactions will be discussed in the following.…”

“…It should be emphasized that "localization" or "itinerancy" of spins in C 60 solids is strongly associated with the rotational disorder and ordering of C 60 The donor molecules that can reduce fullerene moieties are sparse because of the weak electron-accepting ability of fullerene moiety [140]. Simple use of very strong donor species, such as alkali metal has not led to 2D triangular or hexagonal packing of C 60 • [12,146]. Higher than threefold symmetry would be satisfactory.…”

“…C60 σ-dimer (9) [37], C60 π-dimer (10) [38], (C60 − )n polymer in MC60 (M = K, Rb, and Cs) (11) [39,40], linear polymer of (C60 3− )n trianion in Na2RbC60 (12) [41], polymeric layer of (C60 4− )n tetraanion in Li4C60 (13) [42], zigzag polymer of (C60 2+ )n dication in (C60 2+ )(AsF6 − )2 (14) [43], polymeric layer of (C60 4− )n tetraanion in Na4C60 (15) [44].…”

“…C 60 σ-dimer (9) [37], C 60 π-dimer (10) [38], (C 60 − ) n polymer in MC 60 (M = K, Rb, and Cs) (11) [39,40], linear polymer of (C 60 3− ) n trianion in Na 2 RbC 60 (12) [41], polymeric layer of (C 60 4− ) n tetraanion in Li 4 C 60 (13) [42], zigzag polymer of (C 60 2+ ) n dication in (C 60 2+ )(AsF 6 − ) 2 (14) [43], polymeric layer of (C 60 4− ) n tetraanion in Na 4 C 60 (15) [44]. 2 Cu 2 (CN) 3 are: (1) it has a nearly equilateral triangular lattice (t /t = 1.09) with very strong electron correlation (U/W = 0.93), (2) the QSL state is experimentally confirmed down to 20 mK [36,45,46], (3) the anisotropic SC state resides directly next to the QSL state without passing through the spin ordered AF state under pressure [47][48][49], (4) the transition from the QSL state to the metallic state shows positive pressure dependence, indicating that the residual spin entropy is in the QSL state [47][48][49][50], and (5) 13 C NMR measurements under hydrostatic pressure on the salt of ET with 13 C enriched at the central C=C bond ( 13 C-ET) indicate d-wave SC symmetry [51].…”