Low‐Nuclearity Mn II Complexes Based on Pyrimidine‐4,6‐dicarboxylato Bridging Ligand: Crystal Structure, Ion Exchange and Magnetic Properties

• Published in: European journal of inorganic chemistry 2011-01, Vol.2011 (1), p.68-77
• Main Authors: Beobide, Garikoitz, Castillo, Oscar, Cepeda, Javier, Luque, Antonio, Pérez‐Yáñez, Sonia, Román, Pascual, Vallejo‐Sánchez, Daniel
• Format: Article
• Language: English
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201000708

The use of 1,10‐phenanthroline (phen) has allowed the polymerisation process commonly occurring in manganese(II)/pyrimidine‐4,6‐dicarboxylate (pmdc) systems to be blocked off giving the first examples of low‐nuclearity coordination compounds containing the pmdc dianion, namely, the dimeric [Mn 2 (μ‐pmdc) 2 (H 2 O) 2 (phen) 2 ] · 8H 2 O compound ( 1 ) and the tetrameric [Mn 4 (μ‐pmdc) 3 (phen) 4 (H 2 O) 4 ](X) 2 · 11H 2 O[X = BF 4 – ( 2 ), ClO 4 – ( 3 ), NO 3 – ( 4 )] complexes. X‐ray analyses show that the hexacoordinate Mn II atoms are bridged by pmdc dianions that exhibit an asymmetric κ 2 N , O :κ O ′ coordination mode in compound 1 and adopt a bis‐bidentate κ 2 N , O :κ 2 N ′, O ′ binding pattern in the tetrameric units of compounds 2 – 4 . The distorted octahedral O 3 N 3 donor set of the metal centres in 1 is completed by two nitrogen atoms from a phenanthroline ligand and one water molecule. In the linear zigzag shaped tetrameric cations of compounds 2 – 4 , the two inner manganese centres with a O 2 N 4 donor set are, in addition, coordinated to two nitrogen atoms from a phenanthroline ligand, whereas the outer ones complete their O 3 N 3 coordination environment with a bidentated phenanthroline ligand and two cis coordinated water molecules. The crystal packing of the complex units generates voids filled by hydrogen‐bonded C3‐like water chains in compound 1 , whereas compounds 2 – 4 resemble a 3D open‐framework in which the channels are occupied by the counterions and the water molecules of crystallisation. Ion exchange tests were carried out showing that compound 4 exhibits a high capacity to capture other inorganic anions by releasing nitrates. Cryomagnetic susceptibility measurements have shown the occurrence of weak antiferromagnetic interactions ( J = –0.06, –0.21, –0.26 and –0.25 cm –1 for compounds 1 – 4 , respectively). The magneto‐structural analysis indicates that the magnetic interaction taking place through the μ‐pyrimidine ring, although being weak, predominates over the longer exchange pathway involving the two carboxylate groups of the pmdc ligand.