A series of heterometallic [Ln(III)xCu(II)y] complexes, [Gd2Cu2]n (1), [Gd4Cu8] (2), [Ln9Cu8] (Ln=Gd, 3⋅Gd; Ln=Dy, 3⋅Dy), were successfully synthesized by a one-pot route at room temperature with three kinds of in situ carbonyl-related reactions: Cannizzaro reaction, aldol reaction, and oxidation. This strategy led to dysprosium analogues that behaved as single-molecule magnets (SMMs) and gadolinium analogues that showed significant magnetocaloric effect (MCE). In this study a numerical DFT approach is proposed by using pseudopotentials to calculate the exchange coupling constants in three polynuclear [GdxCuy ] complexes; with these values exact diagonalization or quantum Monte Carlo simulations have been performed to calculate the variation of the magnetic entropy involved in the MCE. For the [Dy9Cu8 ] complexes, local magnetic properties of the Dy(III) centers have been determined by using the CASSCF+RASSI method.