Cu(II)-Mediated Intramolecular Carbene Cation Radical Formation:  Relevance to Unimolecular Metal−Ligand Radical Intermediates

Abstract

We report the syntheses of the photochemically labile 9-diazo-4,5-diazafluorene (1) framework and the corresponding Cu(9-diazo-4,5-diazafluorene)(2)(NO(3))(2) compound (2). The X-ray structure of 2 reveals a 6-coordinate, tetragonal geometry with one nitrogen donor of an asymmetrically chelated diazafluorene in the equatorial position and the other defining the weak Jahn-Teller axis. The nitrate counterions bind in a monodentate fashion in the equatorial plane to complete the coordination sphere. Extended Hückel calculations reveal that the unusual solid-state structure derives from the enlarged bite angle of the fluorene skeleton and steric interactions between the adjacent hydrogen atoms in the higher energy (0.45 eV) symmetrically coordinated state. This is in contrast to Cu(py)(4)(NO(3))(2) which is 1.3 eV more stable with the nitrate counterions bound along the Jahn-Teller axis. Electron paramagnetic resonance (EPR) studies in solution reveal that the nitrates dissociate to yield 6-coordinate CuN(2)X(2)N(2)' structures with either a bound chloride ion (g(x) = 2.10, g(y) = 2.04, g(z) = 2.23, A(z) = 177 x 10(-4) cm(-1)) or a mixture of counterion and solvent (g(x)(a) = 2.05, g(y)(a) = 2.06, g(z)(a) = 2.29, A(z)(a) = 170 x 10(-4) cm(-1); g(x)(b) = 2.07, g(y)(b) = 2.08, g(z)(b) = 2.34, A(z)(b) = 155 x 10(-4) cm(-1)). Photolyses of 1 and 2 indicate loss of N(2) and formation of either carbene ([D/hc] = 0.408 cm(-1), [E/hc] = 0.0292 cm(-1)) or Cu(I)-L(*)(+) (S = (1)/(2), g = 2.0019) intermediates, which are identified by EPR, UV-vis, and time-dependent density functional theory methods. The results illustrate the important role redox active transition metals play in determining the nature of fundamental metal-ligand radical intermediates.

Mechanism of UVA-dependent DNA Damage Induced by An Antitumor Drug Dacarbazine in Relation to its Photogenotoxicity

“…7). Photodegraded product of DTIC, Diazo-IC reduces Cu(II) to generate Cu(I) (29,30). Cu(I) can catalyze the release of the diazonium group from the aromatic ring of Diazo-IC to generate carbene and aryl radical (31).…”

Magnetic Properties of Poly(propylene imine)–Copper Dendromesogenic Complexes: An EPR Study

“…[43] The OÀ NÀO angle in the nitrate counterion is 1208 and the NÀO bond distance is 1.292 . [44] So, when two oxygen atoms from the NO 3 À counterion are coordinated in an axial position to Cu II dendrimeric complexes, the Cu-Cu distance in the dimer will be 6.918 (see Scheme 3). Blue complex 1 (x = 1.9) exhibits a hexagonal columnar mesophase ( Figure 5 a), and the possibility of regular-packing Cu II dendrimeric complexes along the columns (Figure 5 a) may be indicated by the presence of additional reflections in the X-ray Guinier diffractogram of their mesophase.…”

Magnetic properties after irradiation of tetrakis[4-(α-diazobenzyl)pyridine]copper(II) in a frozen solution

“…The observed defect in the copper complex fu24 9 (NO3) 2 might be formed by a reaction of a carbene center with solvent ethanol chemically [15] and/or by a copper ion-mediated electron transfer [16]. Usually, carbenes in this type of metal complexes in frozen solutions tend to be sensitive to the temperature when compared with free ones: in EPR experiments, carbene disappeared at temperatures higher than ca.…”