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Mono- and binuclear complexes of iron(II) and cobalt(II) with deprotonated trans-indigo anions. Effect of cationic surrounding on structure and magnetic properties.
Mono- and binuclear complexes of FeII and CoII with deprotonated trans-indigo were obtained. Reduction of indigo by metallic iron in the presence of crystal violet (CV) yields (CV+)[(FeCl2)HIndigo]−⋅C6H4Cl2 (1). The reaction of FeI2 and CoI2 with one equivalent of decamethylchromocene (Cp∗2Cr) and one or half equivalent of indigo yields mononuclear (Cp∗2Cr+)[(CoI2)HIndigo]−⋅C6H4Cl2 (2), (Cp∗2Cr+)[(FeI2)HIndigo]−⋅ xC6H4Cl2, where x = 1 (3) or 0.5 (4), and binuclear (Cp∗2Cr+)2[(CoI2)2Indigo]2−⋅4C6H4Cl2 (7) complexes. Interaction of indigo with CoI2 in the presence of TBAI yields complex (TBA+)2[(CoI2)2Indigo]2− (5), while its reduction by potassium graphite with Cryptand[2.2.2] and CoI2 affords complex {(K+)Cryptand[2.2.2]}2[(CoI2)2Indigo]2−⋅2C6H4Cl2 (6). As a result, coordination units of indigo were obtained in different cationic surroundings allowing us to study how these cations affect magnetic exchange in the binuclear complexes. The most intense absorption band of indigo (665 nm) is red-shifted to 706–740 nm for mononuclear and 860–878 nm for binuclear complexes. The lowest energy bands observed at 1060 and 1376–1400 nm are primarily attributed to metal-to-ligand charge transfer. Short N–Co (1.944(4) Å) and O–Co bonds (1.951(5) Å) are observed in 5 with the TBA+ cations but these bonds are elongated to 1.968(3) Å and 1.970(4) Å, respectively, in 6 with the {(K+)Cryptand[2.2.2]} cations which are inserted between the CoI2 fragments. Ferromagnetic coupling is found in the {[(FeCl2)HIndigo]−}2 dimers of 1, providing parallel alignment of the FeII (S = 2) spins within the dimers at low temperatures. Weak magnetic coupling between FeII (S = 2) and CrIII (S = 3/2) spins is found in 3 that is explained by large spatial separation between magnetic centers. Magnetic exchange values between two high-spin CoII ions (S = 3/2) in the [(CoI2)2Indigo]2− dianions (5 and 6) depend on bond lengths. Shorter Co–N and Co–O bonds in 5 provide substantial antiferromagnetic coupling between the CoII spins (J = −7.40 cm−1), and as a result, these dianions transfer to a diamagnetic state at low temperatures. Longer Co–N(O) bonds in 6 weaken coupling, which can be described as averaged interaction with Jinter = −0.33 cm−1. As a result, variation of cationic surrounding affects intramolecular coupling between the CoII spins through the diamagnetic Indigo2− dianions.