Inorganic Chemistry, Vol.47, No.11, 5006-5021, 2008
High-spin Mn-4 and Mn-10 molecules: Large spin changes with structure in mixed-valence (Mn4Mn6III)-Mn-II clusters with azide and alkoxide-based ligands
The use has been explored of both azide (N-3(-) and alkoxide-containing groups such as the anions of 2-(hydroxymethyl)pyridine (hmpH), 2,6-pyridinedimethanol (pdmH(2)), 1,1,1-tris(hydroxymethyl)ethane (thmeH(3)) and triethanolamine (teaH(3)) in Mn cluster chemistry. The 1:11:1 reactions of hmpH, NaN3 and NEt3 with Mn(ClO4)(2) center dot 6H(2)O or Mn(NO3)(2) center dot H2O in MeCN/MeOH afford [(Mn4Mn6O4)-Mn-II-O-III(N-3)(4)(hMP)(12)](X)2 [X = ClO4- (1), N-3(-) (2)]. The [Mn-10(mu(4)-O)(4)(mu(3)-N-3)(4)](14+) core of the cation has a tetra-face-capped octahedral topology, with a central Mn(III)6 octahedron, whose eight faces are bridged by four mu(3)-N-3(-) and four mu(4)-O2- ions, the latter also bridging to four extrinsic Mn-II atoms. The core has T-d symmetry, but the complete [(Mn4Mn6O4)-Mn-II-O-III(N-3)(4)(hMP)(12)](2+) Cation has rare T symmetry, which is crystallographically imposed. A similar reaction of Mn(ClO4)(2) center dot 6H(2)O with one equiv each of NaN3, thmeH(3), pdmH(2), and NEt3 in MeCN/MeOH led to [(Mn4Mn6O2)-Mn-II-O-III(N-3)(6)(pdmH)(4)(thme)(4)] (3). Complex 3 is at the same oxidation level as 1/2 but its core is structurally different, consisting of two edge-fused [(Mn12Mn4III)-Mn-II(mu(4)-O)(14+) octahedra. Replacement of thmeH3 with teaH3 in this reaction gave instead [(Mn2Mn2III)-Mn-II(N-3)(4)(pdmH)(2)(teaH)(2)] (4), containing a planar Mn-4 rhombus. Variable-temperature, solid-state dc and ac magnetization studies were carried out on 1-4 in the 5.0-300 K range. Complexes 1 and 2 are completely ferromagnetically coupled with a resulting S = 22 ground state, one of the highest yet reported. Fits of dc magnetization vs field (h) and temperature (T) data by matrix diagonalization gave S = 22, g.= 2.00, and D approximate to 0.0 cm(-1) ( D is the axial zero-field splitting parameter). In contrast, the data for 3 revealed dominant antiferromagnetic interactions and a resulting S = 0 ground state. Complex 4 contains weakly ferromagnetically coupled Mn atoms, leading to an S = 9 ground-state and low-lying excited states, and exhibits out-of-phase ac susceptibility signals characteristic of a single-molecule magnet. Theoretical values of the exchange constants in 1 obtained with density functional theory and ZILSH calculations were in good agreement with experimental values. The combined work demonstrates the synthetic usefulness of alcohol-based chelates and azido ligands when used together, and the synthesis in the present work of two "isomeric" (Mn6MnII4)-Mn-III cores that differ in spin by a remarkable 22 units.