Magnetic behavior and ground spin states for coordination {L·[MII(Hal)2]3}3− assemblies (Hal = Cl or I) of radical trianion hexacyanohexaazatriphenylenes (L) with three coordinated high-spin FeII (S = 2) or CoII (S = 3/2) centers

A series of trianion assemblies of hexaazatriphenylenehexacarbonitrile {HAT(CN)6} and hexaazatrinaphthylenehexacarbonitrile {HATNA(CN)6} with three Fe(ii) or Co(ii) ions: {cryptand(K+)}3·{HATNA(CN)6·(FeIII2)3}3−·2C6H4Cl2 (1), {cryptand(K+)}3·{HATNA(CN)6·(CoIII2)3}3−·2C6H4Cl2 (2), and (CV+)3·{HAT(CN)6·(CoIICl2)3}3−·0.5(CVCl)·2.5C6H4Cl2 (3) are synthesized (CVCl = crystal violet). Salt 1 has a χMT value of 9.80 emu K mol−1 at 300 K, indicating a contribution of three high-spin FeII (S = 2) and one S = 1/2 of HATNA(CN)6·3−. The χMT value increases with cooling up to 12.92 emu K mol−1 at 28 K, providing a positive Weiss temperature of +20 K. Such behavior is described using a strong antiferromagnetic coupling between S = 2 and S = 1/2 with J1 = −82.1 cm−1 and a weaker FeII–FeII antiferromagnetic coupling with J2 = −7.0 cm−1. As a result, the spins of three Fe(ii) ions (S = 2) align parallel to each other forming a high-spin S = 11/2 system. Density functional theory (DFT) calculations support a high-spin state of CoII (S = 3/2) for 2 and 3. However, the χMT value of 2 and 3 is 2.25 emu K mol−1 at 300 K, which is smaller than 6 emu K mol−1 calculated for the system with three independent S = 3/2 and one S = 1/2 spins. In contrast to 1, the χMT values decrease with cooling to 0.13–0.36 emu K mol−1 at 1.9 K, indicating that spins of cobalt atoms align antiparallel to each other. Data fitting using PHI software for the model consisting of three high-spin Co(ii) ions and an S = 1/2 radical ligand shows very large CoII–L·3− coupling for 2 and 3 with J1 values of −442 and −349 cm−1. The CoII–CoII coupling via the ligand (J2) is also large, being −100 and −84 cm−1, respectively, which is more than 10 times larger than that of 1. One of the reasons for the J2 increase may be the shortening of the Co–N(L) bonds in 3 and 2 to 2.02(2) and 1.993(12) Å. DFT calculations support the population of the quartet state for the Co3 system, whereas the high-spin decet (S = 9/2) state is positioned higher by 680 cm−1 and is not populated at 300 K. This is explained by the large CoII–CoII coupling. Thus, a balance between J1 and J2 couplings provides parallel or antiparallel alignment of the FeII and CoII spins, leading to high- or low-spin ground states of {L·[MII(Hal)2]3}3−.