Malonate-Containing Manganese(III) Complexes:  Synthesis, Crystal Structure, and Magnetic Properties of AsPh₄[Mn(mal)₂(H₂O)₂]

The novel manganese(III) complexes PPh₄[Mn(mal)₂(H₂O)₂] (1) and AsPh₄[Mn(mal)₂(H₂O)₂] (2) (PPh₄⁺ = tetraphenylphosphonium cation, AsPh₄⁺ = tetraphenylarsonium cation, and H₂mal = malonic acid) have been prepared, and the structure of 2 was determined by X-ray diffraction analysis. 2 is a mononuclear complex whose structure is made up of trans-diaquabis(malonato)manganate(III) units and tetraphenylarsonium cations. Two crystallographically independent manganese(III) ions (Mn(1) and Mn(2)) occur in 2 that exhibit elongated octahedral surroundings with four oxygen atoms from two bidentate malonate groups in equatorial positions (Mn(1)−O = 1.923(6) and 1.9328(6) Å and Mn(2)−O = 1.894(6) and 1.925(6) Å) and two trans-coordinated water molecules in the axial sites (Mn(1)−Ow = 2.245(6) Å and Mn(2)−Ow = 2.268(6) Å). The [Mn(mal)₂(H₂O)₂]^- units are linked through hydrogen bonds involving the free malonate−oxygen atoms and the coordinated water molecules to yield a quasi-square-type anionic layer growing in the ab plane. The shortest intralayer metal−metal separations are 7.1557(7) and 7.1526(7) Å (through the edges of the square). The anionic sheets are separated from each other by layers of AsPh₄⁺ where sextuple- and double-phenyl embraces occur. The magnetic behavior of 1 and 2 in the temperature range 1.9−290 K reveals the occurrence of weak intralayer ferromagnetic interactions (J = +0.081(1) (1) and +0.072(2) cm^-1 (2)). These values are compared to those of the weak antiferromagnetic coupling [J = −0.19(1) cm^-1], which is observed in the chain compound K₂[Mn(mal)₂(MeOH)₂][Mn(mal)₂] (3), where the exchange pathway involves the carboxyate−malonate bridge in the anti−syn conformation. The structure of 3 was reported elsewhere. Theoretical calculations on fragment models of 2 and 3 were performed to analyze and substantiate both the nature and magnitude of the magnetic couplings observed.