Enhancing single molecule magnet parameters. Synthesis, crystal structures and magnetic properties of mixed-valent Mn4 SMMs

Abstract

A novel synthetic route is reported for the synthesis of three dicubane complexes [Mn^II₂Mn^III₂(bdea)₂(bdeaH)₂(O₂CCMe₃)₄] (1), [Mn^II₂Mn^III₂(bdea)₂(bdeaH)₂(O₂CPh)₄] (2), and [Mn^II₂Mn^III₂(teaH)₂(teaH₂)₂(O₂CPh)₂](O₂CPh)₂·0.7MeCN·0.3EtOH (3), obtained from the reaction of N-butyldiethanolamine (bdeaH₂, for 1 and 2) or triethanolamine (teaH₃, for 3) with Mn(OAc)₂·4H₂O as manganese source and [M₃O(O₂CR)₆](O₂CR) (R = CMe₃ for 1, and Ph for 2 and 3 and M: Fe or Mn) compounds as carboxylate source. Complex 1 crystallises in the monoclinic space group P2₁/c (a = 13.2125(7) Å, b = 10.9602(6) Å, c = 22.2329(11) Å, β = 95.053(1)°, V = 3235.9(3) ų, with Z = 2). Complex 2 crystallises in the monoclinic space group P2₁/n (a = 16.6894(14) Å, b = 8.7360(7) Å, c = 21.3378(18) Å, β = 92.949(2)°, V = 3106.9(4) ų, with Z = 2). Complex 3 crystallises in the monoclinic space group C2/c (a = 30.242(3) Å, b = 8.6710(6) Å, c = 26.618(3) Å, β = 119.915(9)°, V = 6050.0(10) ų, with Z = 4). The X-ray crystallographic structure analyses revealed that 1–3 are essentially isostructural, each consisting of a planar mixed-valent “butterfly” core lying about an inversion centre and comprising two outer “wingtip” heptacoordinate Mn^II ions and two inner hexacoordinate Mn^III. Magnetic properties have been studied for all three complexes using combined ac and dc measurements, revealing single-molecule magnet (SMM) behaviour for 1–3, with an S = 9 spin ground state. A detailed analysis of the slow relaxation of the magnetisation has been performed to characterise their SMM behaviour.