Novel phenanthroline ligands and their kinetically locked copper(I) complexes with unexpected photophysical properties

The new, sterically encumbered phenanthroline ligands 1a,b, both characterized by the presence of bulky aryl substituents (3,5-di-tert-butyl-4-methoxyphenyl, 2,4,6-trimethylphenyl) in the 2,9-position, were prepared along with their homoleptic Cu(1a,b)2]+ and heteroleptic complexes Cu(1a,b)(phen)]+ (phen = parent 1,10-phenanthroline). Due to the pronounced steric shielding, particularly effective in ligand 1a, the formation of the homoleptic complex Cu(1a)2]+ becomes very slow (5 days). Once formed, the homoleptic complexes Cu(1a,b)2]+ do not exchange ligands even with phen added in excess because they are kinetically locked due to the large tert-butylphenyl substituents at the phenanthroline unit. The electronic absorption spectra of the homoleptic complexes Cu(1a)2]+ and Cu(1b)2]+ evidence a strongly different ground state geometry of the two compounds, the former being substantially more distorted. This trend is also observed in the excited-state geometry, as derived by emission spectra and lifetimes in CH2Cl2 solution. The less distorted Cu(1b)2]+, compared to Cu(1a)2]+, is characterized by a 15- and over 100-fold stronger emission at 298 and 77 K, respectively. Noticeably, the excited-state lifetime of Cu(1a)2]+ in solution is unaffected by the presence of molecular oxygen and only slightly shortened in nucleophilic solvents. This unusual behavior supports the idea of a complex characterized by a "locked" coordination environment.