Synthesis and magnetic properties of cerium macrocyclic complexes with tetramethyldibenzotetraaza[14]annulene, tmtaaH2

Abstract

3 and tmtaaH 2 , the macrocyclic ligand 6,8,15,17-tetramethyldibenzotetraaza[14]annulene, depending on the stoichiometry, solvent and temperature. The crystal structure of Ce(tmtaa) 2 is isostructural with Zr(tmtaa) 2 , however magnetic susceptibility measurements in the range 5-300 K show that Ce(tmtaa) 2 is not diamagnetic, but is a temperature-independent paramagnet (TIP), similar to Ce(cot) 2 , cerocene. 2 , and phthalacyanines, pcH 2 . Although all of these ligands form stable dianions, their electronic structure and therefore the electronic properties of their complexes are different. The tmtaa 2-is a 24 π-electron system (4nπ, n= 6) that is not Hückel aromatic. The negative charge is not delocalized over its entire framework, but is localized on the four nitrogen atoms, that carry a total negative charge of -0.98 e, and the two β-carbon atoms in the imidinate ring that carry a -0.24 e charge, NC α (Me)C β (H)C α (Me)N. 6,7 The tmaa 2-is therefore not planar but is saddle shaped with idealized C 2v symmetry. 8 The cot 2-(4nπ+2, n=2) and porph 2-(4nπ+2, n=6) are aromatic and the negative charge is delocalized over the planar rings. Pr, Nd, Sm, Gd, Tb, Er and Yb. 30 In this paper the synthesis and physical properties of Ce(tmtaa) 2 and related complexes are described. Results and Discussion Synthesis and StructureThe synthesis of the complexes involves the reaction of tmtaaH 2 with Ce[N(SiMe 3 ) 2 ] 3 in various molar ratios. Proton transfer reactions of Ce[N(SiMe 3 ) 2 ] 3 , the pK a of (Me 3 Si)NH in thf is 26, 31 depend critically on the solvent, concentration of the reactants, and temperature. These details are given in the Experimental Section, which are reproducable when close attention to detail is observed, and shown schematically in Scheme 1. Solutions of complexes Ce(tmtaa)(tmtaaH) (1) and Ce 2 tmtaa 3 (2) are extremely sensitive to trace amounts of air, which result in formation of Ce(tmtaa) 2 (3), which is stable to air and moisture. Deliberate oxidation of either 1 or 2 is therefore an excellent synthetic route to 3.Ce(tmtaa)(tmtaaH) (1) not when Ln = Ce. Crystals of Ce(tmtaa)(tmtaaH) grown from toluene, were twinned, the tmtaa and tmtaaH fragments are disordered, and therefore the X-ray diffraction pattern could not be resolved. However, the 2:1 stoichiometry of the complex is confirmed. Ce(tmtaa)(tmtaaH) (1) decomposes slowly to Ce(tmtaa) 2 (3) and tmtaaH 2 (ratio 1:1) in C 6 D 6 at 65 °C over 14 days. Close inspection of the decomposition process reveales that after one day small amounts of Ce 2 (tmtaa) 3 (2) and tmtaaH 2 are detected in the 1 H NMR spectrum. Over the time priod of two weeks the resonances due to 2 increase then decrease as 3 is formed and the resonances due to tmtaaH 2 continue to increase until a steady state is reached. The decomposition does not eliminate hydrogen since the latter is not observed in the 1 H NMR spectrum and the addition of dehydroanthracene does not yield anthracene. It seems reasonable to suggest that the initial decomposition step of 1 is the format...

The Electronic Structures of Co and Ni Tetraazaannulenes

“…Although often regarded as planar, the metal-centered tetramethyldibenzo-tetraazaannulene (TMTAA) macrocyclic complex molecules are, in fact, saddle shaped, 33 as indicated in the schematic structures shown in Figure 1, because the negative charge of TMTAA 2-is localized on the nitrogen atoms rather than the entire framework. The saddle-shaped structure of the free molecule may be readily altered upon adsorption onto a solid metal surface.…”

Covalency in CeIV and UIV Halide and N‐Heterocyclic Carbene Bonds

“…[17][18][19][20][21][22][23][24][25][26][27] Scott and co-workers reported the oxidation of the triamidoamine complex [CeA C H T U N G T R E N N U N G (NN') 3 …”

“…There are only a small number of reported Ce IV amide complexes, some of which are in fact Ce III with ligand-centred radicals. [17][18][19][20][21][22][23][24][25][26][27] Scott and co-workers reported the oxidation of the triamidoamine complex [18] Lappert and co-workers Abstract: Oxidative halogenation with trityl chloride provides convenient access to Ce IV and U IV chloroamides…”

Spectroscopic and theoretical optical properties of indoleninyl‐substituted dibenzotetraaza[14]annulenes

“…[ 7,8 ] The geometry of DBTAA is more flexible compared to the porphyrin, where the structure possesses a saddle‐shaped conformation with the introduction of a methyl group at the 1 and 3 positions of the propanediiminato moiety. [ 9,10 ] Besides, the DBTAA is easily synthesized by a simple [2 + 2] Schiff base condensation reaction between a 1, 3‐dicarbonyl and o ‐phenylene diamine reactants. [ 11 ] However, changing the substituent at the meso ‐position of the DBTAA molecule is quite challenging due to the limited synthetic possibilities.…”