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

Herein we delineate an unusual one-dimensional coordination polymer (CP), <b>3</b>, prepared from <i>S</i> = 1/2 Co­(TPP), <b>1</b> (TPP = 5,10,15,20-tetraphenylporphyrin dianion), and <i>S</i> = 1/2 4-(4′-pyridyl)-1,2,3,5-dithiadiazolyl (py-DTDA) radical, <b>2</b>. The atypically long S–S distance for CP <b>3</b> (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 <b>3</b> involves noninteger redox states in a resonance hybrid repeat unit best formulated as [Co­(TPP)]<sup>0.5+</sup> hemication (Co<sup>2.5+</sup>) bound to a dithiadiazolide hemianion (py-DTDA<sup>0.5–</sup>). 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 <b>3</b> 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 <b>1</b> and <b>2</b> 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)­(<i>S</i>-py-DTDA), <b>3a</b>. Polymerization and crystallization of <b>3a</b> affords diamagnetic CP <b>3</b>. Dissolution of CP <b>3</b> in DMSO favors Co–S bond heterolysis, yielding the diamagnetic six-coordinate purple N-bound Co<sup>III</sup>(TPP)­(<i>N</i>-py-DTDA<sup>–</sup>)­(OSMe<sub>2</sub>) complex (λ<sub>max</sub>, 436 nm). However, monomerization of CP <b>3</b> in dry 1,2-dichloroethane affords bright green diamagnetic Co<sup>III</sup>(TPP)­(<i>N</i>-py-DTDA<sup>–</sup>), <b>3b</b>, with multiple MLCT bands in the 800–1100 nm NIR region and a red-shifted Soret band (λ<sub>max</sub>, 443 nm). Implications for the use of CP <b>3</b> in electronic devices are discussed based on its density of states.