Exploiting Strong {Cr^III–Dy^III} Ferromagnetic Exchange Coupling to Quench Quantum Tunneling of Magnetization in a Novel {Cr^III₂Dy^III₃} Single-Molecule Magnet

The {3d-4f} pentanuclear complexes with the formula [Cr(2)(III)Ln(3)(III)(PhCO2)(7)(OH)(6) ((PrO)-Pr-i)(NO3)(H2O)(3)] ((PrO)-Pr-i = isopropoxide) (where Ln = Dy (1), Gd (2)) have been synthesized and characterized using magnetic and theoretical studies. The metal core of complexes 1 and 2 has a trigonal bipyramidal arrangement with three Ln(III) ions in the triangular plane and two Cr-III ions occupying the axial positions. These ions are held together by six mu(3)-OH bridges and seven carboxylate bridges. The dc magnetic susceptibility data reveal ferromagnetic interactions presiding between Cr-III and Ln(III) ions in 1 and 2. The fitting of the susceptibility curve employing the DFT calculated J (vide infra), yield J(Gd)(-Gd)(III)(III) = +0.008 cm(-1), J(Cr)(-Gd)(III)(III) = +0.27 cm(-1), and J(Cr)(-Cr)(III)(III) = -0.007 cm(-1) for 2. The dynamic (ac) magnetic susceptibility studies on 1 indicate slow relaxation of magnetization with a U-eff value of 30.9 K (21.4 cm(-1)) and Tau(0) = 4.09 x 10(-10) s. These extracted parameters are among the highest for any reported {(CrDyIII)-Dy-III} complexes. DFT and ab initio CASSCF/RASSI-SO/SINGLE_ANISO/POLY_ANISO calculations were carried out to estimate the exchange interactions and their role in quenching the quantum tunneling of magnetization (QTM) behavior. Ab initio calculations on the Dy-III ions reveal three asymmetric Dy-III centers with the estimated single ion barrier in the range of 78-184 cm(-1), however, with a large QTM probability. Despite a triangular {Dy-3} motif, the g(zz) axes do not align in the triangular plane as observed in the {(CrDy6III)-Dy-III} single-molecule toroids (SMT) reported earlier by us. This is essentially due to the presence/absence of the (PrO-)-Pr-i/carboxylate group that alters the charge distribution around the Dy-III ion and hence the orientation of the corresponding g(zz) axis. The combination of DFT and ab initio CASSCF calculations yield J(Dy)(-Dy)(III)(III) = +0.012 cm(-1), J(Cr)(-Dy)(III)(III) = +1.20 cm(-1), and J(Cr)(-Cr)(III)(III) = -0.95 cm(-1) for 1. The mechanism of magnetization relaxation developed for the {(Cr2Dy3III)-Dy-III} cluster reveals that the relatively strong ferromagnetic Cr-III-Dy-III exchange interaction reduces the ground state QTM significantly yielding a U-eff of 38.6 cm(-1), which agrees with the experimental value. Thus, our study iterates the importance of Cr-III ion to enhance the exchange coupling in the {3d-4f} family of clusters.