Guest-induced magnetic exchange in paramagnetic [M 2 L 4 ] 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 ^II ₂ L ₄ ] 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 ReBr ₆ ^2- switches on antiferomagnetic exchange between the Cu and Re ions, leading to an S = 1/2 ground state for the non-covalent complex [ReBr ₆ ^2- ⊂Cu ^II ₂ L ₄ ] ²⁺ . Comparison of this complex to a "control" palladium-cage host-guest complex, [ReBr ₆ ^2- ⊂Pd ^II ₂ L ₄ ] ²⁺ , shows that the encapsulated ReBr ₆ ^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 _{x ² - y ²} )|Br _sp |Re(d _yz ) 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.