We report a novel series of cobalt(iii)-polypridyl complexes, 4-6, that can selectively release diflunisal, a nonsteroidal anti-inflammatory drug, under reducing conditions. Remarkably, the 1,10-phenanthroline-bearing complex 5 displays selective potency towards hard-to-kill cancer stem cells (CSCs) (IC50 = 2.1 ± 0.1 μM) over bulk cancer (IC50 = 3.9 ± 0.2 μM) and normal cells (IC50 = 21.2 ± 1.3 μM). This complex induces CSC apoptosis by DNA damage and cyclooxygenase-2 inhibition.