Symmetrical and Unsymmetrical Compounds Having a Linear Co₃⁶⁺ Chain Ligated by a Spiral Set of Dipyridyl Anions

Four compounds of the type Co3(dpa)4XYwhere X and Y are Cl or BF4 and dpa is the anion of dipyridylaminehave been prepared and structurally characterized. For X = Y = Cl, two significantly different types of complexes are formed. In one of them, Co3(dpa)4Cl2·CH2Cl2, 1, there is only one crystallographically independent Co−Co distance (2.3178(9) Å). In the second type of crystal, a mononuclear Co(dpa)2 molecule is also found in the interstices of the crystal to give Co3(dpa)4Cl2·Co(dpa)2, 2. Compound 2 has two independent and highly unsymmetrical Co−Co separations (2.285(1) and 2.459(1) Å) corresponding to a bonding and a nonbonding interaction. Reaction of 1 with 1 or 2 equiv of AgBF4 yields Co3(dpa)4Cl(BF4)·2CH2Cl2, 3, and Co3(dpa)4(BF4)2·2CH2Cl2, 4, respectively. Similarly to 2, compound 3 has a short and a long Co−Co separation (2.277 and 2.504(2) Å, respectively). However, 4 has independent but equal Co−Co distances of 2.253[2] Å, which are similar to that of 1. We propose that since compounds of 1 and 2 differ only in the composition of the interstitial molecules (CH2Cl2 and Co(dpa)2, respectively) but the Co−Co bonding is so different, then the Co3(dpa)4Cl2 species should be considered as “stretch isomers”.