The planar tridentate ligand 2,6-diacetylpyridine dioxime (H2dapd) forms complexes with Mn(Il) having stoichiometries that are determined by the nature of the counterion. The monomeric bis chelate complex Mn(H2dapd)2X2forms with outer-sphere counterions X = Br- (3) and C104- (2). However, with Cl- a discrete oligomeric complex, [Mn(H2dapd)Cl2]„, 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)Cl2in 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 (μBat 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 5.94-5.81mbbetween 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 C104' 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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Inorganic Chemistry