Synthesis and Structural Investigations of [Mn₃O₄(phen)₄(H₂O)₂](NO₃)₄·2.5H₂O:  A Water-Bound Complex Obtained by Cerium(IV) Oxidation

Abstract

The trinuclear manganese complex [Mn(3)O(4)(phen)(4)(H(2)O)(2)](NO(3))(4).2.5H(2)O, 1 (where, phen = 1,10-phenanthroline), has been synthesized by the Ce(IV) oxidation of a concentrated solution of manganese(II) acetate and phen in 1.6 N nitric acid. The complex crystallizes in the triclinic space group P&onemacr; with a = 10.700(2) Å, b = 12.643(3) Å, c = 20.509(4) Å, alpha = 78.37(3) degrees, beta = 83.12(3) degrees, gamma = 82.50(3) degrees, and Z = 2. The structure was solved by direct methods and refined by least-squares techniques to the conventional R (R(w)) factors of 0.055 (0.076) based on 4609 unique reflections with F(o) >/= 6.0sigma(F(o)). The structure of the cation consists of an oxo-bridged Mn(3)O(4)(4+) core, with the geometry of the manganese atoms being octahedral. The coordination polyhedron of one of the manganese atoms (Mn(1)) consists of two &mgr; oxo ligands and two pairs of nitrogen atoms of two phen moieties, whereas that of each of the remaining two manganese atoms consists of three &mgr;-oxo ligands, two nitrogen atoms of a phen moiety, and the oxygen atom of a water molecule. The complex represents the second example for water coordination to manganese(IV) centers in complexes with a Mn(3)O(4)(4+) core. Optical spectra in ligand buffer (pH 4.5) reveal complete conversion of the complex into a Mn(III)Mn(IV) species. The observed room-temperature (298 K) magnetic moment of 3.75 &mgr;(B) indicates the presence of strong antiferromagnetic coupling in the complex.

Mechanistic studies on the oxidation of glyoxylic and pyruvic acids by a {Mn3O4}4+ core in aqueous media

“…The trinuclear Mn oxidant is involved in a pH-and buffer-dependent (phen/phenH + ) transformation to the dinuclear mixed-valent species [41] upon dissolution in water, particularly at pH ≥ 4.5 where free ligand phenanthroline acts as reducing agent [9]. This conversion, as noted here, is only less than 5% within 10 h when the media pH was kept within ca.…”

“…The complex salt hydrate [Mn 3 (μ-O) 4 (phen) 4 (H 2 O) 2 ]-(NO 3 ) 4 ·2.5H 2 O was prepared according to a known procedure [9] that involves oxidation of Mn II -acetate by Ce 4+ in ca. 1.6 mol dm −3 HNO 3 containing 1,10-phenanthroline, and the purity of the product was checked by elemental analysis (found for C 48 H 41 N 12 O 20.5 : C 43.9, H 3.35, N,12.9 (%).…”

“…Although it is difficult to explain the higher reactivity of the reducing acids (HA) than their anions with 1 or 2, stabilization of hydrogenbonded species formed through the oxo-bridges of 1 or 2 with the reducing acids (HA instead of A − ) that makes close proximity of the redox agents, may be a reason. It has been shown [9] that the coordinated water molecules in 1 participate in hydrogen bonding with lattice water molecules and nitrate anions. …”

“…1), could be a putative one that could be synthesized from aqueous medium [9]. It has some important features common to the WOC of PSII: [10][11][12] oxo-bridged Mn ions in high oxidation states (oxo-bridged manganese clusters containing N-and O-donor ligands are sometimes called "molecular bricks" for the WOC in PSII) [1,2], two sets of Mn· · ·Mn contacts, ca.…”

Kinetics and mechanism of the oxidation of hydroxylamine by a {Mn3O4}4+ core in aqueous acidic media

“…1.6 M HNO 3 in the presence of phen. 9 In spite of preparing compound 1 from highly acidic media, no protonated oxo-bridge is found in its solid-state structure. 9 The trinuclear Mn(IV) oxidant 1 is a weak acid in aqueous solution (pK 1 = 4.00 (± 0.15) in H 2 O and 4.70 (± 0.20) …”