Synthesis, structure, and photophysical characterization of blue-green luminescent zinc complexes containing 2-iminophenanthropyrrolyl ligands

New 2-iminophenanthro[9,10-c]pyrrole ligand precursors containing phenyl or 2,6-diisopropylphenyl groups at the imine nitrogen substituent, 2-arylformiminophenanthro[9,10-c]pyrroles (aryl = phenyl IIa, 2,6-diisopropylphenyl IIb) were synthesized and deprotonated in situ with NaH, originating solutions of the corresponding sodium salts (IVa, IVb). The reaction of these salts with zinc chloride gave the homoleptic bis-ligand Zn(II) complexes [Zn(kappa(2)N,N'-2-arylformiminophenanthro[9,10-c]pyrrolyl)(2)] (aryl = phenyl 2a, 2,6-diisopropylphenyl 2b). The new ligand precursors and complexes were characterized by NMR, elemental analysis, UV/vis spectroscopy, and X-ray crystallography, when possible. The photophysical characterization was carried out using steady-state and picosecond time-resolved luminescence techniques in solution. The influence of the pi-extended conjugation of the condensed phenanthro group on the deprotonated iminopyrrolyl ligands coordinated to Zn(2+) greatly enhances fluorescence quantum yields of the complexes (2a, 2b) in relation to those of their ligand precursors (IIa, IIb). Complex 2a shows emission in the green spectral region (lambda(max) = 494 nm), presenting the highest fluorescence quantum yield (phi(f) = 8.8%). In the case of the complex 2b (phi(f) = 3.9%), the bulkiness of the 2,6-diisopropyl substituents of the arylimino group highly restricts the aryl ring rotation toward coplanarity with the ligand framework, inducing a shift in the emission to the blue region (lambda(max) = 459 nm). The values of the radiative (k(f)) and radiationless rate constants (k(nr)) show that the fluorescence quantum yield enhancement in the complexes results from a 50-fold increase in k(f) values, indicating much more allowed pi-pi* transitions in complexes 2a and 2b than those occurring in the ligand precursors IIa and IIb, with an essentially n-pi* character. These assignments were confirmed by density-functional theory (DFT) and time-dependent DFT (TD-DFT) molecular orbital calculations. Simple 2-aryliminopyrrole ligand precursors (Ia, Ib) and their Zn(II) complexes (1a, 1b) were also prepared to compare their photophysical properties with those of the corresponding 2-aryliminophenanthro[9,10-c]pyrrolyl compounds.

Synthesis and structural studies of amido, hydrazido and imido zirconium(IV) complexes incorporating a diamido/diamine cyclam-based ligand

The preparation, characterisation and structural analysis of a series of zirconium(IV) complexes that incorporate the diamido/diamine macrocyclic ligand Bn2Cyclam (Bn2Cyclam = 1,8-dibenzyl-1,4,8,11-tetraazacyclotetradecane) are described. The reaction of one or two equivalents of the appropriate LiNHR reagents with [(Bn2Cyclam)ZrCl2] give the corresponding amido-chloride [(Bn2Cyclam)ZrCl(NHR)] (R = tBu, (2,6-iPr)Ph) or bis(amido) [(Bn2Cyclam)Zr(NHR)2] (R = iPr, tBu, (2,6-Me)Ph) complexes, respectively. Treatment of [(Bn2Cyclam)ZrCl(NH(2,6-iPr)Ph)] with one equiv. of MgClMe gives the base-free, monomeric imido complex [(Bn2Cyclam)Zr(N(2,6-iPr)Ph)]. The reaction of the tBu analog with MgClMe generates a dimeric bridging imido species [{(Bn2Cyclam)Zr}2(mu-NR)2], which can also be obtained by thermal decomposition of [(Bn2Cyclam)Zr(NHtBu)2] in toluene. The bis(hydrazido) complex [(Bn2Cyclam)Zr(NHNPh2)2] was obtained by reaction of [(Bn2Cyclam)ZrCl2] and two equiv. of LiNHNPh2. A hydrazido-chloride compound [(Bn2Cyclam)ZrCl(N(Ph)NBu(Ph))] was generated in a one-pot reaction between [(Bn2Cyclam)ZrCl2], LiBu and azobenzene. DFT calculations on [(Bn2Cyclam)ZrXY] complexes indicate that the coordination geometry adopted by these species is dictated by the steric bulk of the ligands X and Y, varying between six-coordinate prismatic and four-coordinate tetrahedral.