Exchange Coupling in an Isostructural Series of Face-Sharing Bioctahedral Complexes [LM^II(μ-X)₃M^IIL]BPh₄ M = Mn, Fe, Co, Ni, Zn; X = Cl, Br; L = 1,4,7-Trimethyl-1,4,7-triazacyclononane

The reaction of the divalent metal halides ZnCl₂, ZnBr₂, MnCl₂·CH₃CN, MnBr₂, FeCl·4CH₃CN, CoCl₂4CH₃CN, CoBr₂, NiCl₂·6H₂O, and NiBr₂, respectively, with the macrocycle 1,4,7-trimethyl-1,4,7-triazacyclononane (L) (1:1) in anhydrous acetonitrile, acetone, chloroform, or ethanol affords upon additon of NaBPh₄ the isomorphous series of complexes [LM^II(μ-X)₃M^IIL]BPh₄: 1, M = Zn, X = Cl; 2, Zn, Br; 3, Mn, Cl; 4, Mn, Br; 5, Fe, Cl: 6, Co, Cl; 7, Co, Br; 8, Ni, Cl; 9, Ni, Br. Six of these complexes have been structurally characterized by single-Crystal X-ray crystallography; they crystallize in the triclinic space group P1̄ (No. 2) with Z = 4. Crystal data are as follows 1, a = 16.654(1), b = 17.042(1), c = 17.684(1) Å, α = 97.30(1). β= 93.58(1), γ = 117.46(1)°; 3, a = 16.632(8), b = 17.012(8), c = 17.855(5) Å, α = 97.16(3), β= 93.37(3), γ = 117.24(3)°; 5, a = 16.658(3), b = 17.064(3), c = 17.741(4) Å, α = 97.32(3), β= 93.47(3), γ = 117.36(3)°; 6, a = 16.640(3), b = 17.040(3), c = 17.686(4) Å, α = 97.39(3), β= 93.58(3), γ = 117.39(3)°; 8, a = 16.608(3). b = 16.995(3), c = 17.555(3) Å, α = 97.36(1), β= 93.52(1), γ = 117.52(1)°; 9, a = 16.680(3), b = 17.016(2), c = 17.715(3)Å, α = 96.99(1). β= 93.70(1), γ = 117.42(1)°, All complexes consist of a dinuclear, face-sharing bioctahedral monocation with three μ₂-Cl or μ₂-Br bridging ligands and two LM fragments and well-separated tetraphenylborate anions (1:1). The cations cocrystallize in two different forms: an enantiomeric form with (λλλ) (or (δδδ)) conformation at both LM fragments and a meso form with an (λλλ) conformation at one LM fragment and (δδδ) at the other (ratio 1:1). From temperature-dependent magnetic susceptibility measurements (2-293 K) it was established that the spins of the unpaired electrons in 3 (d⁵d⁵ high spin), 4 (d⁵d⁵), 6 (d⁷d⁷ high spin), 7 (d⁷d⁷), 8 (d⁸d⁸), and 9 (d⁸d⁸) are intramolecularly, weakly antiferromagnetically coupled in each case, Surprisingly, the spins order ferromagnetically in 5 (d⁶d⁶ high spin). This is in contrast to the previously reported complex [(thf)₃Fe^II(μ-Cl)₃Fe^II(thf) ₃][SnCl₅(thf)] (thf = tetrahydrofuran)⁵ for which a new analysis of the temperature-dependence of the magnetic susceptibility and of field-dependent Mössbauer spectra establish a weak intramolecular antiferromagnetic coupling. The origin of this difference is analyzed.