GROUND SPIN STATE VARIABILITY IN MANGANESE OXO AGGREGATES - DEMONSTRATION OF AN S=3/2 GROUND-STATE FOR [MN3O4(OH)(BPEA)3](CLO4)3

Synthesis and properties of bi- and trinuclear manganese oxo complexes of the ligand N,N-bis(2-pyridylmethyl)ethylamine (bpea) are reported. The binuclear species [Mn2O2(O2CCH3)(bpea)2](ClO4)3 (1) was isolated in 41% yield from a MeOH reaction mixture that contained Mn(O2CCH3)3-2H2O, bpea, and aqueous HClO4. Compound 1 crystallizes from CHCl3/ CH3CN/n-hexane as 1.CH3CN.0.5CHCl3 in the triclinic space group P1BAR with a = 12.956 (1) angstrom, b = 12.987 (2) angstrom, c = 13.391 (2) angstrom, alpha = 77.20 (1)-degrees, beta = 83.34 (1)-degrees, gamma = 80.36 (1)-degrees, V = 2158.8 (6) angstrom3, and Z = 2. Bond distances and angles for 1 are consistent with a Mn(IV)2 oxidation state assignment. The variable temperature magnetic susceptibility data for 1 in the range 6-280 K were fit with the expression generated from the isotropic spin Hamiltonian H = -2JS1.S2 (S1 = S2 = 3/2), with J = -124 cm-1. Compound 1 was converted to [Mn3O4(OH)(bpea)3](ClO4)3 (2) in 90% yield by adding an aqueous solution of NaClO4 to a CH3CN solution of 1. Compound 2 crystallizes from CH3CN/C6H5CH3 in the monoclinic space group P2(1)/n with a = 10.875 (4) angstrom, b = 26.074 (9) angstrom, c = 18.437 (5) angstrom, beta = 103.80 (2)-degrees, V = 5077.2 (6) angstrom3, and Z = 4. The structure of 2 consists of three Mn(IV) ions, with two (Mn(b)) being coupled together by two doubly-bridging oxo groups (O(b)) and the third (Mn(a)) being singly-bridged to each Mn(b) atom by oxo groups (O(a)). Atom Mn(a) bears a terminal OH group which is hydrogen bonded to an O(b) group. This interaction appears to be in part responsible for a distortion that brings Mn(a) out of the plane of the two Mn(b) and two O(a) atoms. The EPR spectral properties of 2 are consistent with an S = 3/2 ground state. Magnetic susceptibility data in the range 6-280 K using an applied magnetic field of 5 kG were also consistent with an S = 3/2 ground state and were fit using the Hamiltonian H = -2J(S1.S2) - 2J'(S1.S3 + S2.S3) (S1 = S2 = S3 = 3/2; J = -76 cm-1, J' = -11 cm-1). Both 1 and 2 were further characterized with UV-vis, infrared, and electrochemical measurements. The relationship between 2 and two other species that contain the {Mn3O4}4+ core is discussed. The similarity between the ground spin state variability for synthetic trinuclear manganese oxo complexes and the manganese site in photosystem II is noted.