A cozy carbon wrap for osmium

In high-coordination-number metal complexes, the coordinating atoms on the ligand are most often heteroatoms, such as nitrogen, oxygen, or phosphorus. Carbon is typically not involved. A team including Congqing Zhu, Caixia Yang, Xin Lu, Gang Liu, and Haiping Xia of Xiamen University has turned that situation completely around by preparing an unprecedented pentadentate chelating ligand in which all five metal-binding atoms are carbons—a new record for planar carbon coordination (Sci. Adv. 2016, DOI: 10.1126/sciadv.1601031). The metallacycles that are formed feature osmium encircled by a 12-carbon loop. The researchers suggest this new ligand, which they have named carbolong, could become a standard building block in coordination chemistry. Computational analysis indicates that the conjugated ligand could be viewed as a polycyclic aromatic hydrocarbon, but one that is distinctive from any observed before. The team has further shown that the osmium carbolongs absorb UV to near-infrared light. That makes the complexes promising candidates for photothermal therapy, in which localized heating caused by absorption of laser light destroys cancer cells. In collaboration with the U.S. National Institutes of Health, the Xiamen researchers have observed tumor regression in mice treated with an osmium carbolong.