Synthesis and characterization of molybdenum oxo complexes of two tripodal ligands: reactivity studies of a functional model for molybdenum oxotransferases

Reaction of the tetradentate ligand N-(2-hydroxybenzyl)-N, N-bis(2-pyridylmethyl) amine (L - OH) with MoO2Cl2 in methanol in the presence of NaOMe and PF6- results in the formation of [MoO2(L - O)] PF6. Similarly, the reaction of N-(2-mercaptobenzyl)- N,N-bis(2-pyridylmethyl) amine (L -SH) with MoO2(acac)(2) leads to the formation of [MoO2(L - S)](+). The dioxo-molybdenum complex [MoO2(L - O)](+) reacts with phosphines in methanol to afford phosphine oxides and an air-sensitive molybdenum complex, tentatively identified as [Mo(IV) O(L - O)(OCH3)]. The latter complex is capable of reducing biological oxygen donors such as DMSO or nitrate, thereby mimicking the activity of DMSO reductase and nitrate reductase. Reaction of [MoO2(L - O)] PF6 with PPh3 in other solvents than methanol leads to the formation of the Mo(V) dimer [( L - O) OMo(mu-O) MoO(L - O)](PF6)(2). The crystal structures of [MoO2(L - O)] PF6 and the mu-oxo bridged dimer are presented

Tetraaminoperylenes: their efficient synthesis and physical properties

Trimethylsilylation of 1,8-diaminonaphthalene gave 1,8-bis(trimethylsilylamino)naphthalene (1a), which was in turn lithiated with two molar equivalents of n-butyllithium to give the tris(thf)-solvated dilithium diamide 1,8- {(Me 3SiN)Li(thf)} 2C 10H 6(thf) (2a). Metal exchange of 2a with TlCl was carried out in two steps, via the previously characterized mixed-metal amide 1-{(Me 3SiN)Li(thf) 2}-8-{(Me 3SiN)Tl}- C 10H 6, to give the dithallium diamide 1,8-{(Me 3SiN)Tl} 2C 10H 6 (3a). Thermolysis of 3a cleanly gave a 1:1 mixture of the 4,9-bis(trimethylsilylamino)peryle- nequinone-3,10-bis(trimethylsilylimine) (4a) and 1a. By this route, a whole series of silylated homologues of 4a was obtained in good yields, while the same method proved to be inefficient for the synthesis of the alkyl-substituted analogues. Compound 4a and its tert-butyldimethylsilyl derivative 4d were reduced with sodium amalgam to give, after protonation, the corresponding 3,4,9,10-tetraaminoperylenes 7a and 7d. Cyclic voltammetry showed two reversible, closely spaced reduction waves (E red1 = - 1.39, E red2 = - 1.59 V versus SCE) corresponding to this conversion. The perylenes 7a and 7d are thought to be the primary products in the reaction cascade leading to the perylene derivatives, involving the thermal demetalation of the thallium amides, possibly via Tl 11-Tl 11 intermedi- ates, first to give 7a and its analogues. The final oxidation of the tetraaminoperylenes by one molar equivalent of 3a and analogous thallium amides gave the quinoidal derivatives such as 4a and 4d, a step that could be studied by direct reaction of the isolated species. The UV/ Vis absorption spectra of the 4,9-bis(silylamino)perylenequinone-3,10-bis(silylimines) are characterized by a long-wavelength absorption band with a pronounced vibrational structure (λ max = 639 nm, lg ε = 4.53) attributed to a �* � and a �* � n absorption band at 454 nm (lg ε 4.83), along with intense absorption in the UV region. A weak red emission with a rather low quantum yield (Φ n = 0.001, λ max = 660 nm) is observed upon irradiation of a sample; the lifetime of the emission is only 66 ps. The low emission quantum yield is attributed to the *� � n transition of the amino perylene, which induces strong spin-orbit coupling, leading to a large triplet yield. The triplet state was probed by transient absorption spectroscopy and found to have a lifetime of 200 ns in air, and 1100 ns in argonflushed solution. Treatment of 4a with a stoichiometric amount of KF in methanol/water under phase-transfer conditions (with the cryptand C 222) gave an almost quantitative yield of the parent compound 4,9-diaminoperylenequinone-3,10-diimine (8). Treatment of 8 with two molar equivalents of the ruthenium complex Ru(bpy) 2(acetone) 2(PF 6) 2, generated in situ, yielded the blue dinuclear ruthenium complex (bpy) 4Ru 2{μ 2-N,N�:N�, N�-4,9-(NH 2) 2-3,10-(NH) 2 C 20H 8]}](PF 6) 4 (9), the redox properties of which were studied by cyclic voltammetry. The difference in the potentials of the two one-electron redox steps (225 mV) indicates strong coupling of the metal centers through the 4,9-diaminoperylenquinone-3,10-dimine bridging ligand and corresponds to a comproportionation constant K c of 6.3 x 10 3. The UV/Vis absorption spectrum of the mixed valent form, which is stable in air, has a characteristic intervalence charge-transfer (IVCT) band in the near infrared at 930 nm (lg ε = 3.95), from which an electronic coupling parameter J of 760 cm -1 could be estimated, placing compound 9 at the border-line between the class II and class III cases in the Robin-Day classification

High-valent Ruthenium-Manganese Complexes for Solar Energy Production.

We present progress in the development of artificial photosynthesis, as a means to harvesting and storage of solar energy. The plan is to compose molecular systems that combine known photochemistry with emerging functional model compounds. A photochemical device for solar energy conversion contains a photosensitizer, an electron acceptor system and a donor system that prevents charge recombination. Our goal is to utilize water as sacrificial electron donor, which will allow a net production of reducing equivalents, and the ultimate production of fuel. The only light-driven molecular catalyst for water oxidation exists in Photosystem II (PSII), which has a tetranuclear Mn-cluster in the active site. Here we present several Mn-compounds, that we have developed for the purpose of creating water-oxidizing catalysts. Our idea is to link Ru-tris(bipyridine) derivatives, which mimicks the function of the primary donor in PS II, with manganese complexes, mimicking the tetra-Mn cluster on the PSII donor side. We have constructed a number of heteronuclear complexes, containing a Ru-photosensitizer and various Mn-complexes. The compounds have been characterized with regards to their photophysical and photochemical properties, redox potentials and structure. The most promising compounds are capable of undergoing several electron transfers from the Mn-complex to the photosensitizer, leaving 3 to 4 oxidizing equivalents on the Mn. In the latest development, we have constructed ligands that stabilize higher oxidation states in Mn, in order to promote formation of Mn(V) which many believes is an intermediate in the water oxidation mechanism