By using thermodynamic measurements and density-functional band-structure calculations, we explore the magnetic behavior of . Despite being a structural sibling of a three-dimensional frustrated system , this spin- quantum magnet shows remarkably different low-temperature behavior. Above 7 K, magnetic susceptibility of follows the spin-chain model with the antiferromagnetic intrachain coupling of K. We ascribe this quasi-one-dimensional behavior to the leading third-neighbor coupling that involves a weakly bent superexchange pathway with a short contact of 2.79 Å. Below 5 K, undergoes two consecutive magnetic transitions that may be triggered by the frustrated nature of interchain couplings. The field dependence of the magnetic transitions (phase diagram) is reported.