DFT Studies on the Magnetic Exchange Across the Cyanide Bridge

Abstract

Exchange coupling across the cyanide bridge in a series of novel cyanometalate complexes with Cu II -NC-M III (M ) Cr and low-spin Mn,Fe) fragments has been studied using the broken-symmetry DFT approach and an empirical model, which allows us to relate the exchange coupling constant with σ-, π-, and π*-type spin densities of the CN -bridging ligand. Ferromagnetic exchange is found to be dominated by π-delocalization via the CN -π pathway, whereas spin polarization with participation of σ orbitals (in examples, where the d z 2 orbital of M III is empty) and π* orbitals of CN -yields negative spin occupations in these orbitals, and reduces the Cu II -M III exchange coupling constant. When the d z 2 orbital of M III is singly occupied, an additional positive spin density appears in the σ(CN) orbital and leads to an increase of the ferromagnetic Cu-NC-M exchange constant. For low-spin [M III (CN) 6 ] 3-complexes, the d z 2 orbital occupancy results in high-spin metastable excited states, and this offers interesting aspects for applications in the area of molecular photomagnetism. The DFT values of the exchange coupling parameters resulting from different occupations of the t 2g orbitals of low-spin (t 2g 5 ) Fe III are used to discuss the effect of spin-orbit coupling on the isotropic and anisotropic exchange coupling in linear Cu-NC-Fe pairs.

A series of chiral cyanide-bridged Fe–Cu complexes: Synthesis, crystal structures and magnetic properties

“…Therefore, the weakly ferromagnetic coupling is common, and can be observed in almost all the reported cyanide-bridged low spin Fe(III)( t 2g 5 e g 0 )-Cu(II)(t 2g 6 e g 3 ) examples. This result can be easily interpreted by the orthogonality of the t 2g magnetic orbitals of Fe(III) ion with the e g one of Cu(II) ion, which have been confirmed by detailed theoretical calculation [33,34]. However, the bending of the Cu-N≡C angle and the torsion angle (φ) defined as the rotation of the x and z axes for Cu(II) compared with the xz plane for Fe(III) are also important for tuning the coupling nature and strength between the cyanide-bridged Fe(III)-Cu(II) [35], since they can remove the strict orthogonality of the t 2g and e g orbital on iron(III) and copper(II) ions.…”

Bispidine Coordination Chemistry

“…These studies were based on melamine-derived mono-, bis-, and tris-macrocyclic copper(II) and nickel(II) complexes and their host-guest compounds with cyanometalates (341)(342)(343)(344)(345). Magnetic exchange through the cyano bridge was shown to lead to multivarious effects and based on a thorough interpretation, there is the possibility to tune the magnetic properties of heterooligonuclear complexes (345,346). A thorough theoretical study indicates that s, as well as p-MO effects, are of importance, and these can be varied through the geometry of the cyano bridges and the electronic ground states of the interacting metal centers (346).…”

“…Magnetic exchange through the cyano bridge was shown to lead to multivarious effects and based on a thorough interpretation, there is the possibility to tune the magnetic properties of heterooligonuclear complexes (345,346). A thorough theoretical study indicates that s, as well as p-MO effects, are of importance, and these can be varied through the geometry of the cyano bridges and the electronic ground states of the interacting metal centers (346). The relatively well-understood steric and electronic properties of bispidine coordination compounds, in particular the interesting variation of substrate (CN) binding to the trans-to-N3 and trans-to-N7 site, as well as the variation of the binding strength through substitution of the bispidine backbone, makes bispidine complexes interesting building blocks for the synthesis of heterooligonuclear cyanometalates.…”

Interpretation and Prediction of Properties of Transition Metal Coordination Compounds

“…A model for the magnetic exchange through cyanide bridges has been developed and used to analyze spin delocalization and spin polarization across the CN -bridge of cyanometalates with Cu II -NC-M III fragments [153], and a ligand-field-based quantum-chemical model has subsequently been created [154] and used to analyze a series of trinuclear cyanometalates [155]. Recent studies with Cu II -X-Cu II systems (X ¼ halogenide [156], CN À [157]), supported by a DFT-based analysis, have demonstrated the important role of the type of bonding involving the Cu II centers, the variation of the Cu II -X-Cu II geometry, and the nature of the terminal ligands.…”