The planar tridentate ligand 2,6-diacetylpyridine dioxime (H2dapd) forms complexes with Mn(II) having stoichiometries that are determined by the nature of the counterion. The monomeric bis chelate complex Mn(H2dapd)2X2 forms with outer-sphere counterions X = Br- (3) and ClO4 - (2). However, with Cl- a discrete oligomeric complex, [Mn(H2dapd)Cl2]n, probably dimeric (n = 2) or tetrameric (n = 4), forms in nonprotic solvents like DMF, while only a monomeric Mn(II) complex forms in protic solvents. This is in equilibrium with a linear-chain polymer n = ∞ (1) in the solid state. Compound 1 crystallizes in the monoclinic space group Cc [Z = 4, a = 17.843 (3) Å, b = 10.422 (2) Å, c = 6.809 (1) Å, β = 106.49 (0)°]. Mn coordinates to form pentagonal bipyramids Mn(N3Cl2)Cl2 in which each chloride occupies axial and equatorial sites on adjacent monomer units in the helical chains. Variable-temperature magnetic susceptibility indicates weak ferromagnetic coupling in 1 (μeff = 8.40 μB at 4.2 K and 6.02 μB at 294 K), with an effective spin-exchange temperature of T0 = 5 K (J = 0.2 cm-1), using a one-dimensional linear-chain classical model. This contrasts with the monomeric complex 2, which exhibits a temperature-independent moment 5.94-5.81 μB between 294 and 4.2 K, indicating no intermolecular coupling. The oligomeric complex of 1 in DMF also exhibits ferromagnetic coupling, as seen by the EPR-active paramagnetic ground state. Possible structures are discussed. The weaker ionic bonding between Mn(II) and Br- and ClO4 - vs Cl- accounts for the formation of the bis chelate complexes 2 and 3. The capacity for Cl- to spontaneously assemble an oligomeric Mn(II) cluster in solution is compared with the special requirement for Cl- for assembly of the polynuclear manganese complex responsible for photosynthetic water oxidation.
|Number of pages||6|
|Publication status||Published - 1989|
ASJC Scopus subject areas
- Inorganic Chemistry