Disordered ground state in the spin-orbit coupled $J_{\rm eff}= 1/2$ cobalt-based metal-organic framework magnet with orthogonal spin dimers

We present the magnetic properties of a strongly spin-orbit coupled quantum dimer magnet based on Co$^{2+}$. The metal-organic framework compound Co$_2$(BDC)$_2$(DPTTZ)$_2$$\cdot$DMF features Co$^{2+}$ dimers arranged nearly orthogonal to each other, similar to the Shastry-Sutherland lattice. Our assessment based on the magnetization and heat capacity experiments reveals that the magnetic properties at low temperatures can be described by an effective $J_{\rm eff} = 1/2$ Kramers doublet and the ground state is a singlet with a tiny spin gap. Although the magnetic susceptibility could be analyzed in terms of the interacting dimer model with an isotropic intradimer coupling $J_0/k_{\rm B} \simeq 7.6$ K, this model fails to reproduce the shape of magnetization isotherm and heat capacity data. A model of isolated spin dimers with the anisotropic exchange couplings $J_{xy} \simeq 3.5$ K and $J_{z} \simeq 11$ K provides an adequate description to the magnetic susceptibility, magnetization isotherm, and heat capacity data at low temperatures. Interestingly, no field-induced quantum phase phase is detected down to 100~mK around the critical field of gap closing, suggesting the absence of Bose-Einstein condensation of triplons and establishing isolated dimers with a negligible interdimer coupling.