Cobalt Porphyrin-Thiazyl Radical Coordination Polymers: Toward Metal-Organic Electronics

Herein we delineate an unusual one-dimensional coordination polymer (CP), 3, prepared from S = 1/2 Co(TPP), 1 (TPP = 5,10,15,20-tetraphenylporphyrin dianion), and S = 1/2 4-(4'-pyridyl)-1,2,3,5-dithiadiazolyl (py-DTDA) radical, 2. The atypically long S-S distance for CP 3 (2.12 Å) reflects fractional electron transfer from the formally Co(II) ion into the antibonding π-SOMO of the metal-bound py-DTDA bridging ligand. The bonding in solid CP 3 involves noninteger redox states in a resonance hybrid repeat unit best formulated as [Co(TPP)]^0.5+ hemication (Co^2.5+) bound to a dithiadiazolide hemianion (py-DTDA^0.5-). DFT calculations confirm the metal to ligand charge transfer (MLCT) character of the low-lying electronic states (641, 732, and 735 nm) observed for CP 3 and show that oligomer chains of length ≥14 repeat units tend toward a band structure with a limiting band gap energy of 0.669(6) eV. In dichloromethane, the reaction between radicals 1 and 2 involves coordination of the Co(II) ion by a py-DTDA ring sulfur atom, orbitally favored spin-pairing, and the formation of the thermodynamically favored diamagnetic five-coordinate S-bound adduct, Co(TPP)(S-py-DTDA), 3a. Polymerization and crystallization of 3a affords diamagnetic CP 3. Dissolution of CP 3 in DMSO favors Co-S bond heterolysis, yielding the diamagnetic six-coordinate purple N-bound Co^III(TPP)(N-py-DTDA^-)(O═SMe₂) complex (λ_max, 436 nm). However, monomerization of CP 3 in dry 1,2-dichloroethane affords bright green diamagnetic Co^III(TPP)(N-py-DTDA^-), 3b, with multiple MLCT bands in the 800-1100 nm NIR region and a red-shifted Soret band (λ_max, 443 nm). Implications for the use of CP 3 in electronic devices are discussed based on its density of states.