Azadipyrromethene Dye Derivatives in Coordination Chemistry: the Structure–Property Relationship in Homoleptic Metal(II) Complexes

As a chromophore closely related to dipyrromethene (DPM), the azadipyrromethene (ADPM) family has attracted much interest in the life sciences and optoelectronic fields. A high-yielding microwave-assisted synthesis is reported for new homoleptic complexes of cobalt­(II), nickel­(II), copper­(II) and zinc­(II) based on the tetrakis­(<i>p</i>-methoxyphenyl)­azadipyrromethene ligand <b>1b</b>. These complexes are compared with other homoleptic complexes of the same metal­(II) series based on the tetraphenylazadipyrromethene <b>1a</b> and also with related BF<sub>2</sub><sup>+</sup> chelates (Aza-BODIPYs <b>6a</b> and <b>6b</b>) for a better understanding of trends arising from substitution of the chelate and/or the electron-donating effect of the <i>p</i>-methoxy substituents. The electrochemical behavior of the new compounds <b>2b</b>, <b>3b</b>, and <b>5b</b> in dichloromethane revealed two pseudoreversible reductions (<b>2b</b>, −1.09 and −1.25 V vs SCE; <b>3b</b>, −1.05 and −1.29 V; <b>5b</b>, −1.13 and −1.25 V) followed by a third irreversible process (<b>2b</b>, −1.78 V; <b>3b</b>, −1.80 V; <b>5b</b>, −1.77 V) along with two pseudoreversible oxidations (<b>2b</b>, 0.55 and 0.80 V; <b>3b</b>, 0.56 and 0.80 V; <b>5b</b>, 0.55 and 0.80 V) followed by two closely spaced irreversible processes (<b>2b</b>, 1.21 and 1.27 V; <b>3b</b>, 1.21 and 1.28 V; <b>5b</b>, 1.22 and 1.25 V). On its side, copper­(II) homoleptic complex <b>4b</b> revealed only one pseudoreversible reduction at −0.59 V followed by three irreversible processes at −0.95, −1.54, and −1.74 V, respectively. The oxidation behavior of this complex exhibited two pseudoreversible processes (0.55 and 0.82 V) and two irreversible processes (1.19 and 1.25 V). The redox processes are assigned and discussed in relation to their photophysical properties. X-ray structures for <b>1b</b> and related copper­(II) complex <b>2b</b> are also discussed.