Guest-induced magnetic exchange in paramagnetic [M₂L₄]⁴⁺ coordination cages

Paramagnetic complexes that possess magnetically switchable properties show promise in a number of applications. A significantly underdeveloped approach is the use of metallocages, whose magnetic properties can be modulated through host-guest chemistry. Here we show such an example that utilises a simple [Cu^II₂L₄]⁴⁺ lantern complex. Magnetic susceptibility and magnetisation data shows an absence of exchange in the presence of the diamagnetic guest triflate. However, replacement of the bound triflate by ReBr₆^2- switches on antiferomagnetic exchange between the Cu and Re ions, leading to an S = 1/2 ground state for the non-covalent complex [ReBr₆^2-⊂Cu^II₂L₄]²⁺. Comparison of this complex to a "control" palladium-cage host-guest complex, [ReBr₆^2-⊂Pd^II₂L₄]²⁺, shows that the encapsulated ReBr₆^2- anions retain the same magnetic anisotropy as in the free salt. Theoretically calculated spin-Hamiltonian parameters are in close agreement with experiment. Spin density analysis shows the mode of interaction between the Cu^II and Re^IV centres is through the Re-Br⋯Cu pathway, primarily mediated through the Cu(d)|Br_sp|Re(d) interaction. This is further supported by overlap integral calculations between singly occupied molecular orbitals (SOMOs) of the paramagnetic ions and natural bonding orbitals analysis where considerable donor-to-acceptor interactions are observed between hybrid 4s4p orbitals of the Br ions and the empty 4s and 4p orbitals of the Cu ions.