Ground Spin State Variability in Manganese Oxo Aggregates. Demonstration of an S = 3/2 Ground State for [Mn < inf > 3 < /inf > O < inf > 4 < /inf > (OH)(bpea) < inf > 3 < /inf > ](ClO < inf > 4 < /inf > ) < inf > 3 < /inf >

Abstract

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 P1 with a = 12.956 (1) Å,b = 12.987 (2) Å,c= 13.391 (2) Å, α = 77.20 (1)°, β = 83.34 (1)°, γ = 80.36 (1)°, V = 2158.8 (6) Å3, and Z = 2. Bond distances and angles for 1 are consistent with a MnIV2 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 = -2JŜ1·Ŝ2 (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 P21/n with a = 10.875 (4) Å,b = 26.074 (9) Å, c = 18.437 (5) Å, β = 103.80 (2)°, V= 5077.2 (6) Å3, and Z = 4. The structure of 2 consists of three Mn(IV) ions, with two (Mnb) being coupled together by two doubly-bridging oxo groups (Ob) and the third (Mna) being singly-bridged to each Mnb atom by oxo groups (Oa). Atom Mna bears a terminal OH group which is hydrogen bonded to an Ob group. This interaction appears to be in part responsible for a distortion that brings Mna out of the plane of the two Mnb and two Oa 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(Ŝ1·Ŝ2) -2J′(Ŝ1·Ŝ3 +Ŝ2·Ŝ3) (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. © 1992, American Chemical Society. All rights reserved.