Maxim P. Aarnts

Excited states of metal-metal bonded diimine complexes vary from extremely long lived to very reactive with formation of radicals or zwitterions

Abstract The metal-metal bonded complexes Re(ML n )(CO) 3 ( α -diimine) [ML n = Mn(CO) 5 , Re(CO) 5 , SnPh 3 , etc.] have a lowest 3 σπ ∗ excited state, in which σ represents the ReM bonding orbital and π ∗ the lowest empty orbital of the α-diimine. This excited state, which can be occupied via the Re→α-diimine MLCT states, is normally reactive, giving rise to homolysis of the ReM σ bond. Only the complex Re(SnPh 3 )(CO) 3 (bpy) was found to be photostable and much longer lived in its 3 σπ ∗ state than related complexes having a lowest 3 MLCT state of comparable energy. This was ascribed to the strength of the ReSn bond and to the weak distortion of this complex in its 3 σπ ∗ state with respect to the ground state. In the complexes Ru(E)(E′)(CO) 2 (α-diimine) the axial ligands E and E′ can vary strongly and so do their excited state properties. Of special interest are the compounds in which both E and E′ are SnP 3 ligands, strongly bonded to Ru by a high-lying σ orbital. Because of the strength of these bonds and the strong mixing of the σ(SnRuSn) orbital with the π ∗ (α- diimine ) orbital, the structural effects of the σ → π ∗ transition are small. This is refl in very small shifts of the CO-stretching vibrations with respect to the ground state and in a very long lifetime of the 3 σπ ∗ state ( τ = 264 μ s for Ru(SnPh 3 ) 2 (CO) 2 (iPr-DAB) in a 2-MeTHF glass at 77 K). Similarly, irradiation of a cluster Os 3 (CO) 10 (α-diimine) in an apolar solvent gives rise to homolysis of an OsOs bond with formation of a biradical, which was detected with EPR and time-resolved absorption spectroscopy ( τ = 5–110 ns). In a coordinating solvent such as CH 3 CN, the homolysis reaction is accompanied by the coordination of a solvent molecule, which induces an intramolecular electron transfer. A zwitterion is then formed, which has a lifetime of seconds in CH 3 CN. The biradical can also be transformed into such a zwitterion if a Lewis base is added to the apolar solvent. In this article the mechanistic details of these reactions are presented and discussed.

Electrochemistry of different types of photoreactive ruthenium(II) dicarbonyl α-diimine complexes

Abstract The photochemical reactivity, photophysical properties and redox behavior of the complexes trans , cis -[Ru(X)(X′)(CO) 2 (α-diimine)] and their derivatives are strongly dependent on the complex geometry, the nature and electronic properties of the α-diimine ligand and, most importantly, on the axial ligands X and X′ (alkyl, halide, phosphine, donor solvent, etc.). This paper deals mainly with comparison of reduction pathways for several different types of the trans , cis -[Ru(X)(X′)(CO) 2 (α-diimine)] complexes, also presenting some new results in this field. An equally important goal has been the comparison and discussion of the photo- and redox reactivity of these complexes from the viewpoint of the frontier orbitals involved and character of the RuX/X′ bonding.

Photochemical bond homolysis in a novel series of metal-metal bonded complexes Ru(E) (E′) (CO)2(iPr-DAB)

Photochemistry of the complexes trans,cis-Ru(E) (E′) (CO)2(iPr-DAB) (ECl, SnPh3, PbPh3, Mn(CO)5, Re(CO)5, Me;E′ (depending on E) = SnPh3, PbPh3, GePh3, Mn(CO)5, Re(CO)5) was found to be strongly dependent on the combination and characters of the axial ligands E and E′. Except for Ru(Cl) (SnPh3) (CO)2(iPr-DAB) and Ru(Cl) (PbPh3) (CO)2(iPr-DAB) which are nearly unreactive, one of the Ru-E/E′ bonds is split homolytically upon irradiation into the lowest-energy absorption band of the complex. For Ru(SnPh3)2(CO)2 (iPr-DAB), this reaction occurs from a thermally equilibrated 3σπ∗ excited state with a rate constant of 2.3 × 105s−1 and a temperature-dependent quantum yield (En = 1450 cm−1). The unselective Ru&.zsbdnd; (40%) and Ru-Sn (40%) bond homolysis of Ru(SnPh3) (GePh3) (CO)2-(iPr-DAB) follows the same mechanism. On the other hand, bond homolysis is much faster (⪢ 108s−1) for complexes which contain RuMe, RuMn or RuRe bonds. Bond homolysis in these species is highly selective, since only RuMe, RuMn and RuRe bond splitting was observed for Ru(Me) (SnPh3) (CO)2(iPr-DAB), Ru(SnPh3) (Mn(CO)5)(CO)2(iPr-DAB), respectively. The photoproduced [Ru(E) (CO)2(iPr-DAB)] radicals were detected by time resolved UV-Vis spectroscopy on a timescale 10 ns-100 μs. The [Ru(SnPh3) (CO)2(iPr-DAB)] radical was also characterised by EPR in the form of its adduct with PPh3. Depending on the solvent used, they either dimerise or abstract a chlorine atom from the solvent to produce Ru(Cl) (E) (CO)2(iPr-DAB).

Syntheses, structures and spectroscopic properties of novel inorganometallic complexes Ru(E)(E′)(CO)2(iPr-DAB): (E=Cl, E′=SnPh3, PbPh3; E=Me, E′=SnPh3, PbPh3; E=SnPh3, E′=SnPh3, SnMe3, GePh3; E=PbPh3, E′=PbPh3, PbMe3, GePh3; iPr-DAB=N,N′-diisopropyl-1,4-diaza-1,3-butadiene)

Abstract This article describes the syntheses, structures and spectroscopic (IR, Raman, NMR, visible absorption) properties of novel inorganometallic complexes of the type Ru(E)(E′)(CO) 2 (iPr-DAB) in which E represents a Cl, Me, SnPh 3 or PbPh 3 group and E′=GePh 3 , SnR 3 or PbR 3 (R=Ph, Me) depending on E. The X-ray structures of Ru(Cl)(SnPh 3 )(CO) 2 (iPr-DAB) and Ru(Cl)(PbPh 3 )(CO) 2 (iPr-DAB) show that these complexes have a distorted octahedral geometry with E and E′ in axial positions. The latter complex is, to our knowledge, the first with a Ru–PbR 3 bond for which a crystal structure has been reported. Replacement of Cl by the more electron releasing ligands Me, SnR 3 or PbR 3 causes a shift of ν (CO) and ν (CN) to lower frequencies. At the same time, the strong absorption band shifts from ∼440 to 510–550 nm, with a concomitant decrease of solvatochromism. The very large decrease of solvatochromism upon replacement of Me by GePh 3 or ER 3 (E=Pb, Sn; R=Me, Ph) is tentatively explained with a delocalisation of charge over the axial ER 3 groups. Support for this explanation is provided by the 1 H NMR spectra, which show very large 4 J ( 117/119 Sn,H) and 4 J ( 207 Pb,H) coupling constants of the imine protons in the SnR 3 and PbR 3 containing complexes respectively.

Syntheses, structures and spectroscopic properties of a novel series of metalmetal bonded complexes Ru(E)(E′)(CO)2(iPrDAB): (E Br, E′ Mn(CO)5; E SnPh3, E′ Mn(CO)5, Re(CO)5, CO(CO)4; E Me, E′ Re(CO)5; E E′ Mn(CO)5, Re(CO)5; iPrDAB N,N′-diisopropyl-1,4-diaza-1,3-butadiene)

Abstract This article describes the syntheses, structures and spectroscopic (IR, Raman, NMR, visible absorption) properties of novel metalmetal bonded complexes of the type trans,cis-Ru(E)(E′)(CO)2(iPrDAB) in which E  Br, Me, SnPh3, Mn(CO)5 or Re(CO)5 and E′  Mn(CO)5, Re(CO)5 or Co(CO)4 (depending on E). The structures of Ru(SnPh3)(Mn(CO)5)(CO)2(iPrDAB) Ru(SnPh3)(Co(CO)4)(CO)2(iPrDAB) and Ru(Re(CO)5)2(CO)2(iPrDAB) were determined by single crystal X-ray diffraction. The complexes have a distorted octahedral geometry with E and E′ in axial positions. The structure of Ru(SnPh3)(Co(CO)4)(CO)2(iPrDAB) is noteworthy, since one of the carbonyl ligands of the Co(CO)4 group forms a semi-bridge with Ru. The IR and Raman spectra in the ν(CO) and νs(CN) wavenumber region are assigned. The absorption spectra show one or two charge-transfer bands in the visible region, their position and number depending on E and E′. The SnPh3-complexes show in the 1H NMR spectra a large 4 J( 117 119 Sn,H) coupling constant for the imine protons of their iPrDAB ligands, which points to a strong delocalisation of charge within these complexes.