Investigation of the correlation between the crystal structure and the magnetic behavior of the selected complexes

S ciljem proučavanja utjecaja kristalne strukture na magnetsko ponašanje istraživano je petnaest tetranuklearnih spojeva Ni(II), trinaest kubanske strukture i dva dikubanske strukture, tri dimera Cr(III) u kojima su Cr(III) premoš´ceni dijamagnetskim mostom O–Nb(V)–O i ˇcetiri heterometalna kompleksna spoja koji se sastoje od dimera Cu(II), Ni(II), Co(II), Mn(II) u kojima su metalni centri premošćeni oksalatnim anionom, C2O2-4 , u mostu i monomera Cr(III). Fine promjene strukture postižu se vezanjem različitih liganada na pojedine metalne centre. Kod tetranuklearnih Ni(II) kompleksnih spojeva kubanske strukture vezanjem različitih alkohola kao liganada na metalni centar mijenjaju se parametri magnetskih međudjelovanja i magnetsko osnovno stanje. Parametri magnetskih međudjelovanja korelirani su sa strukturnim karakteristikama Ni..O..Ni..O četverokuta: prosječnim kutom između veza Ni–O–Ni, prosječnim kutom između O-Ni-O ravnina i prosječnim kutom između jedne O-Ni-O ravnine i Ni..Ni vektora. Kako dikubanski spojevi imaju isti strukturni motiv preko kojega se međudjelovanje prenosi ove magnetno-strukturne korelacije razmatrane su i u dikubanskim spojevima. Utvrđeno je da značajne strukturne deformacije i različito koordinacijsko okruženje iona nikla dikubanske strukture mogu narušiti magnetno-strukturnu korelaciju. Kod dimerskih kompleksnih spojeva Cr(III) dijamagnetski -O-Nb(V)-O- most prenosi snažno antiferomagnetsko međudjelovanje. Odstupanje od ispruženog kuta oko kisika u ovim spojevima rezultira slabljenjem jakosti antiferomagnetskog međudjelovanja Cr(III) iona. U istraživanim heterometalnim kompleksnim spojevima utvrđeno je da se zamjenom metalnog centra u dimeru mijenja jakost i karakter magnetskog međudjelovanja preko oksalatnog aniona. Na magnetsko međudjelovanje, koje se prenosi preko oksalatnog aniona, ne utječe jedan dominantni faktor, već je jakost i karakter međudjelovanja uvjetovana s više različitih faktora: vrsta koordiniranih liganada, odstupanje od pravilnosti koordinacijske sfere, udaljenost magnetskih centara, odstupanje od planarnosti u oksalatnom mostu.Fifteen tetranuclear Ni(II) complexes, three Cr(III) dimers in which Cr(III) are bridged by the diamagnetic O–Nb(V)–O bridge and four heterometallic complexes consisting of the Cu(II), Ni(II), Co(II), Mn(II) dimers in which metal centers are bridged by the oxalate anion, C2O4, were studied in order to investigate the influence of the crystal structure to the magnetic behavior. Mounting different ligands to the metallic center fine structural changes are achieved resulting in changing the strength of magnetic interaction due to the deferent overlap of magnetic orbitals. By knowing the link between particular structural feature and the value of the magnetic parameter we can adjust the magnetic properties of complexes. Tetranuclear Ni(II) complexes As ligands in tetranuclear Ni(II) cubane complexes different alcohols (methanol, ethanol, propanol, butanol, pentanol) were mounted. Magnetic interaction of Ni(II) ions is mediated by two oxygen bridges. Due to the fine structural changes by mounting different ligands values of the parameters of magnetic interaction and ground state are changed as well as the position of the maximum in the χ­­T(T) curve. In some of the complexes unexpected high maximum in the χ­T(T) curve, above 20 emuK/molOe is observed. Reported significantly high χ­­T(T) maxima come from the quadratic spin-spin interactions depicted in the ZFS term as a consequence of influence of the structural distortions and intermolecular interactions on the magnetic environment of Ni(II) ions. Magneto-structural correlations between the parameters of magnetic interaction and three structural features: average Ni–O–Ni angle, average angle between O-Ni-O planes and the average angle between one O-Ni-O plane and Ni..Ni vector are shown. These correlations can be viewed as different representations of structural features that influences the overlap of the orbitals and therefore to the character and the value of the magnetic superexchange interaction. As dicubane Ni(II) complexes possess the same Ni.O..Ni..O structural motif through which the exchange interaction is realized, it is expected that dicubane complexes will follow the same linear correlation. However, different coordination of Ni(II) and therefore different magnetic environment od Ni as well as greater structural distortions in dicubane complexes result that this correlation is not valid. Magnetic interaction of Cr(III) mediated by the long diamagnetic O–Nb(V)–O bridge The correlation between the crystal structure and the magnetic behavior is also analyzed in structurally more complicate situation were the magnetic interaction is mediated by the bridge consisting of more than one atoms. Therefore, magneto-structural correlations are investigated in three novel complexes with Cr(III) dimers in which interaction of Cr(III) is mediated by the diamagnetic O–Nb(V)–O bridge. Fine structural changes are achieved by mounting different ligands (bipyridine, phenanthroline, terpyridine) to the magnetic Cr(III) centers. It is found out that strong antiferromagnetic interaction, -18 K, can be mediated by the diamagnetic O–Nb(V)–O bridge. This is particularly interesting as the distance between chromium ions over the bridge is as large as 7.4Å . Structural feature that is changing in the investigated complexes are the angles between chromium, oxygen and niobium. Deviation of the linearity around oxygen in the bridge results in weakening the strength of the antiferromagnetic interaction of Cr(III) ions. Magnetic interaction of Cu(II), Ni(II), Co(II) and Mn(II) mediated by the oxalate anion Another possibility to study molecular design with desired magnetic properties is changing the metal center in the complex. By changing the metal center orbitals that contribute to the magnetic interaction are changed resulting in different strength and character of the interaction. With this motivation a series of novel heterometallic complexes consisting of Cu(II), Ni(II), Co(II), Mn(II) dimers and Cr(III) monomers was investigated. In each of the dimers metal centers are bridged by the oxalate anion which is almost planar what favors the potential magnetic interaction. Magnetic behavior of these complexes is not governed by a single, dominant, factor. The strength and the character of the magnetic interaction is influenced by different factors: kind of ligand, distortion of the metal coordinate environment, distance between metal centers, deviation of planarity in the oxalate bridge. Investigation show that in homometallic Ni(II), Co(II) and Mn(II) dimers with oxalate anion in the bridge antiferromagnetic interaction is mediated, while in Cu(II) dimers antiferromagnetic and ferromagnetic interaction can be mediated