The exploration of model systems exhibiting geometrical spin frustration remains a key avenue for uncovering emergent quantum phenomena. In this work, we report the synthesis and characterization of a different type of frustrated quantum magnets, $\mathrm{Li}A{\mathrm{Nd}}_2{\left({\mathrm{BO}}_3\right)}_3$ $(A=\mathrm{Ba},\mathrm{Sr})$, featuring spatially confined magnetic spin clusters. These compounds host a two-dimensional triangular network of ${\mathrm{Nd}}_6$ spin hexamers in $ab$ planes, separated along the $c$ axis by nonmagnetic $A{\mathrm{O}}_{10}$ and ${\mathrm{BO}}_3$ units. Magnetic susceptibility and specific-heat measurements reveal the realization of an effective $J_{\mathrm{eff}}=1/2$ ground state and the absence of long-range magnetic order or spin freezing down to at least 0.1 K, below which a substantial fraction of spin entropy persists. Under quasi-adiabatic demagnetization from an initial temperature of about 2 K, they exhibit a promising cooling effect, with the temperature-field profile $T\left({\mu }_{0}H\right)$ displaying a distinct minimum or shoulder near 1 T, presumably associated with the intracluster spin correlations. Upon further reducing the field to zero, additional temperature drops down to $\sim 106/117$ mK become evident, hinting at the development of intercluster spin interactions. The probable hierarchical spin interactions and the strong $T\left({\mu }_{0}H\right)$ responses characteristic of both compounds suggest that $\mathrm{Li}A{\mathrm{Nd}}_2{\left({\mathrm{BO}}_3\right)}_3$ is a promising spin-cluster system for exploring quantum spin magnetism and sub-Kelvin magnetocaloric effect.

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• Magnetic interactions
• Magnetic order
• Magnetic phase transitions
• Magnetic susceptibility
• Magnetism