(i) Dilacunary arsenotungstate [As₂W₁₉O₆₇(H₂O)]^14-, as a precursor at both acidic and basic pH medium: In this work the role of polyoxometalates as ligands to control the symmetry around electronically different Ln(III) ions, leading to slow relaxation of magnetization behavior, has been investigated. For this, five di-nuclear rare earth metal complexes having molecular formula Cs_xK_15-x[(AsW₉O₃₃)₃Ln₂(H₂O)₇W₄O₉]⋅yH₂O using with different Ln(III) [Ln=Dy(1), Tb(2), Er(3), Ho(4), Yb(5), y=24-34] have been synthesized using dilacunary [As₂W₁₉O₆₇(H₂O)]^14- as a precursor under acidic pH medium. The detailed magnetic studies reveal slow relaxation of magnetization behavior for both oblate (Dy) and prolate (Yb) metal centers due to high axial ground states with minimum transverse anisotropy for Dy-POM and easy plane anisotropy for Yb-POM. Finally, this work highlights the rare example of single-molecule magnet behavior for prolate and oblate Ln(III) ions with the same polyoxometalates ligands.
Further, to increase the axial ligand field strength around Ln(III) metal centers, two polyoxometalates (POMs) ligated tetranuclear rare earth metal complexes having the molecular formula [Cs_xK_24-x{Ln₄(H₂O)₈(α-AsW₉O₃₃)₄}]⋅yH₂O {Ln =Tb(III)(1), y = 21, Dy(III)(2), y = 28} were synthesized at basic pH medium. At higher pH, the isomerization of [As₂W₁₉O₆₇(H₂O)]^14- to the [α-AsW₉O₃₃]^9- building unit observed which results similar symmetries as bis-(phthalocyaninato) lanthanide complexes. The magnetic investigations disclosed the slow relaxation of magnetization under zero dc field for both complexes 1 and 2. Further, ab initio calculations highlight high axial ground states and minimum transverse anisotropy for both complexes, which is responsible for zero-field SMM behavior.

(ii) Mixed heteroatom template Ln-POMs: In this work, we have used dual heteroatom (HOA) templates to induce diverse building blocks, which result in Ln-inserted dual-HOA template heteropolyoxotungustes. The study reports two Dy(III)-substituted polyoxotungstate two Dy(III)-substituted polyoxotungstate [H₂N(CH₃)₂]₇Na₇[Dy₂(H₂O)₇(W₄O₉)(HPSeW₁₅O₅₄)(α-SeW₉O₃₃)₂]⋅31H₂O(1) and [H₂N(CH₃)₂]₁₄K₂Na₁₈{[Dy₂(H₂O)₁₀W₁₄O₅₉]₂[α-SeW₉O₃₃]₄[HPSeW₁₅O₅₄]}₂⋅44H₂O(2) using a dual (PIII-SeIV) heteroatom-template. The most striking structural feature of 1 & 2 is the coexistence of tri-vacant Keggin [α-Se^IVW₉O₃₃] ^8- and Dawson [HP^IIISe^IVW₁₅O₅₄]^12- building units holding both P^III–Se^IV-heteroatoms and Dy(III) metal centers in high symmetries (C_2v and D_2d). The magnetic studies disclosed the presence of large magnetic anisotropy and slow relaxation of magnetization behavior for both the complexes 1&2. Further, the ab-initio calculation confirms the ground and first excited energy states contain pure highest mj states, responsible for the observed single-molecule magnet (SMM) behavior in the studied complexes. This is the first example of a mixed heteroatom based Dy(III)-substituted polyoxotungstate with both trimeric keggin [α-Se^IVW₉O₃₃]^8- and Dawson [HP^IIISe^IVW₁₅O₅₄]^12- building units showing SMM behavior.

(iii) Organic-inorganic hybrid Ln (III)-POMs: In this work, we have introduced a strong chemical bond around Ln(III) metal centers to further enhance their SMM behavior. Herein, we synthesized two organic-inorganic hybrid Dy(III)-substituted selenotungustate [H₂N(CH₃)₄]K₈Na₂[(α-SeW₉O₃₃)₄Dy₄(H₂O)₄W₆(H₂tart)₄O₁₂].27H₂O(1) and K₂₈Na₄{[W₃Dy₂O₁₆(α-SeW₉O₃₃)₂]₂[W₂Dy₂O₁₆(SeO₃)(H₂tart)(β-Se₂W₁₄O₅₂)]₂[(WO₂)(H₂tart)]₂}.60H₂O(2). Complex 1 was found to be stabilized by four flexible H2tart ligands, which induced the aggregation of four trilacunary selenotungustate (ST) building units and Dy(III) ions. In complex 2, H2tart binds to W(VI) centers, and it represents a rare example of hybrid complexes where both Keggin and Dawson building units are present. The magnetic studies of these complexes show the presence of large magnetic anisotropy and slow relaxation of magnetization behavior. Complex 1 exhibits a high effective energy barrier in the absence of an external magnetic field, while complex 2 shows filed-induced slow relaxation of magnetization behavior. The ab-initio calculation of complex 1 reveals high axial ground states with minimum transverse anisotropy due to local symmetry C_2v & D_2d around the Dy(III) metal centers. This helps in the stabilization of m_J = ±15/2, resulting in higher energy splitting of the ground states. .