A New Phenanthroline Ligand and the Spontaneous Resolution of its Homoleptic Copper(I) Complex

Abstract

During the synthesis of 1,10‐phenanthroline derivatives sterically hindered in the 2 and/or 9 positions, serendipity led to the discovery of a new substitution pattern on phenanthroline with good and reproducible yields. When toluene was used as a solvent and the aryl‐lithio reagents generated with tBuLi, after a first substitution at the 2 position, a benzyl substituent was regioselectively introduced at the 7 position. It is proposed that the benzyllithium reagent is formed by reaction with the solvent and that the steric hindrance on the intermediates controls the nucleophilic addition. Among the complexes prepared (Pd(II), Cu(I)) with these new ligands, a chiral copper(I) complex exhibits spontaneous resolution in the solid state.

Employing Long-Range Inductive Effects to Modulate Metal-to-Ligand Charge Transfer Photoluminescence in Homoleptic Cu(I) Complexes

“…The flattened geometry that accompanies this PJT distortion exposes the transiently formed Cu­(II) core to a fifth coordination site, which is susceptible to forming exciplexes with Lewis basic solvents, thereby quenching the excited state rapidly upon its formation. − Preventing deactivation via the PJT distortion can be accomplished by sterically limiting the geometric reorganization occurring in the MLCT excited state. One strategy that has proven quite effective and leads to substantially enhanced excited-state lifetimes and blueshifted PL maxima incorporates linear or branched alkyl chains into the 2,9-positions of the phenanthroline architecture. − Simultaneously methylating the 3,8-positions provides additional steric reinforcement to the adjacent bulky 2,9-alkyl substituents, , leading to significant MLCT lifetime and PL quantum yield enhancements. , The presence of multiple alkyl substituents on the phenanthroline core renders these ligands to be more electron rich, thereby shifting their one-electron reduction processes to more reducing potentials. This inductive effect is also transmitted to the Cu­(II/I) couple, shifting the corresponding oxidation process to more negative potentials.…”

Next Generation Cuprous Phenanthroline MLCT Photosensitizer Featuring Cyclohexyl Substituents

“…The PJT distortions characteristic in the MLCT excited states of Cu­(I) bis (diimines) can be markedly reduced through structural engineering by incorporating bulky 2,9-alkyl substituents on the phenanthroline ligands, thereby extending the excited state lifetimes. ,,,,− McMillin and co-workers more recently discovered that 3,8-methylation of phenanthroline further extends the Cu­(I) MLCT excited state lifetime through cooperative steric enhancement of the 2,9-substituents, resulting in less excited state distortion and more complete shielding of the copper center from the surrounding environment . Our laboratory has previously leveraged branched alkyl substituents paired with McMillin’s 3,8-methylation strategy resulting in microsecond Cu­(I) MLCT lifetimes and high quantum yield photoluminescence at RT. − A depiction of how the PJT distortion is restricted using our structural design is presented in Figure b, which directly compares a traditional Cu­(I) MLCT photosensitizer [Cu­(dmp) 2 ]­PF 6 (dmp = 2,9-dimethyl-1,10-phenanthroline) to the one conceived in our laboratory, [Cu­(dsbtmp) 2 ]­PF 6 (dsbtmp = 2,9-di­( sec -butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline).…”

Remote Steric Control of the Tetrahedral Coordination Geometry around Heteroleptic Copper(I) Bis(Diimine) Complexes

“…The synthesis of the phenanthroline ligand L1 , shown in Scheme 1 , bearing two very large aryl groups (sometimes called super-mesityl) at positions 2 and 9, was described earlier [ 34 ]. The heteroleptic complexes were synthesized in a two-step, one-pot synthesis utilizing the HETPHEN process developed by Schmittel and co-workers [ 34 ].…”

“…The synthesis of the phenanthroline ligand L1 , shown in Scheme 1 , bearing two very large aryl groups (sometimes called super-mesityl) at positions 2 and 9, was described earlier [ 34 ]. The heteroleptic complexes were synthesized in a two-step, one-pot synthesis utilizing the HETPHEN process developed by Schmittel and co-workers [ 34 ]. One equivalent of the copper(I) source was added to the sterically hindered phenanthroline dissolved in distilled dichloromethane (the steric hindrance prevents the formation of the bis-homoleptic complex) followed by one equivalent of the less-hindered diimine ligand.…”

“…All reagents and solvents were purchased from commercial sources and were used as received. The diaryl phenanthroline ligand was prepared following a previously published procedure [ 34 ]. The MnO 2 was from Fisher Scientific, Illkirch-Graffenstaden, France (Honeywell Fluka-1890).…”