Synthesis and characterization of homo- and heterodinuclear M(ii)-M′(iii) (M(ii) = Mn or Fe, M′(iii) = Fe or Co) mixed-valence supramolecular pseudo-dimers. the effect of hydrogen bonding on spin state selection of M(ii)

Reaction of H₃L¹, the Schiff base condensate of tris(2-aminoethyl)amine with three equivalents of 5-methyl-1H-pyrazole-3- carboxaldehyde, with manganese(ii)perchlorate or iron(ii)tetrafluoroborate results in the isolation of [MH₃L¹]X₂ (M = Mn and X = ClO₄ and M = Fe and X = BF₄). These complexes are high spin d⁵ and d⁶, respectively, as inferred from the long M-N bond distances obtained by single crystal X-ray diffraction for both and variable temperature magnetic susceptibility and Mössbauer spectroscopy for the iron complex. Aerobic treatment of a solution of [CoH₃L ¹]²⁺ with three equivalents of potassium hydroxide produced [CoL¹]. Homonuclear pseudo-dimers were prepared by the aerobic reaction of [FeH₃L¹](BF₄)₂ with 1.5 equivalents of potassium hydroxide to give {[FeH_1.5L ¹](BF₄)}₂ or by the metathesis reaction of [FeH₂L¹][FeHL¹](ClO₄)₂ with sodium hexafluorophosphate to give [FeH₃L¹][FeL ¹](PF₆)₂. The complexes were characterized by EA, IR, ESI-MS, variable temperature single crystal xray diffraction and Mössbauer spectroscopy. The iron(iii) atom is low spin while the iron(ii) atom is spin crossover. Heteronuclear pseudo-dimers were prepared by the 1:1 reaction of [FeH₃L¹](BF₄)₂ or [MnH₃L¹](ClO₄)₂ with [CoL ¹]. [MH₃L¹][CoL¹](X)₂ (M = Fe and X = BF₄ or M = Mn and X = ClO₄), were characterized by IR, EA, variable temperature single crystal X-ray diffraction and Mössbauer spectroscopy in the iron case. The data support a spin crossover and high spin assignment for the iron(ii) and manganese(ii), respectively. DFT calculations demonstrate that the spin state of the iron(ii) atom in {[FeH₃L¹][FeL¹]}²⁺ changes from high spin to low spin as the iron(ii)-iron(iii) distance decreases. This is supported by experimental results and is a result of hydrogen bonding interactions which cause a significant compression of the M(ii)-N _pyrazole bond distances.