The series of complexes [Ru(bpy)(3-n)(btz)(n)][PF(6)](2) (bpy = 2,2'-bipyridyl, btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, 2n = 1, 3n = 2, 4n = 3) have been prepared and characterised, and the photophysical and electronic effects imparted by the btz ligand were investigated. Complexes 2 and 3 exhibit MLCT absorption bands at 425 and 446 nm respectively showing a progressive blue-shift in the absorption on increasing the btz ligand content when compared to [Ru(bpy)(3)][Cl](2) (1). Complex 4 exhibits a heavily blue-shifted absorption spectrum with respect to those of 1-3, indicating that the LUMO of the latter are bpy-centred with little or no btz contribution whereas that of 4 is necessarily btz-centred. DFT calculations on analogous complexes 1'-4' (in which the benzyl substituents are replaced by methyl) show that the HOMO-LUMO gap increases by 0.3 eV from 1'-3' through destabilisation of the LUMO with respect to the HOMO. The HOMO-LUMO gap of 4' increases by 0.98 eV compared to that of 3' due to significant destabilisation of the LUMO. Examination of TDDFT data show that the S(1) states of 1'-3' are (1)MLCT in character whereas that of 4' is (1)MC. The optimisation of the T(1) state of 4' leads to the elongation of two mutually trans Ru-N bonds to yield [Ru(κ(2)-btz)(κ(1)-btz)(2)](2+), confirming the (3)MC character. Thus, replacement of bpy by btz leads to a fundamental change in the ordering of excited states such that the nature of the lowest energy excited state changes from MLCT in nature to MC.

We report the unprecedented observation and unequivocal crystallographic characterization of the meta-stable ligand loss intermediate solvento complex trans-[Ru(bpy)(κ2-btz)(κ1-btz)(NCMe)]2+ (1 a) that contains a monodentate chelate ligand. This and analogous complexes can be observed during the photolysis reactions of a family of complexes of the form [Ru()(btz)2]2+ (1 a–d: btz=1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl; =a) 2,2′-bipyridyl (bpy), b) 4,4′-dimethyl-2,2′-bipyridyl (dmbpy), c) 4,4′-dimethoxy-2,2′-bipyridyl (dmeobpy), d) 1,10-phenanthroline (phen)). In acetonitrile solutions, 1 a–d eventually convert to the bis-solvento complexes trans-[Ru()(btz)(NCMe)2]2+ (3 a–d) along with one equivalent of free btz, in a process in which the remaining coordinated bidentate ligands undergo a new rearrangement such that they become coplanar. X-ray crystal structure of 3 a and 3 d confirmed the co-planar arrangement of the and btz ligands and the trans coordination of two solvent molecules. These conversions proceed via the observed intermediate complexes 2 a–d, which are formed quantitatively from 1 a–d in a matter of minutes and to which they slowly revert back on being left to stand in the dark over several days. The remarkably long lifetime of the intermediate complexes (>12 h at 40 °C) allowed the isolation of 2 a in the solid state, and the complex to be crystallographically characterized. Similarly to the structures adopted by complexes 3 a and d, the bpy and κ2-btz ligands in 2 a coordinate in a square-planar fashion with the second monodentate btz ligand coordinated trans to an acetonitrile ligand.

The photophysics and photochemistry of the complex [Ru-(bpy) 3 ] 2+ (bpy = 2,2'-bipyridyl) and its many N^N trischelate analogues have been the subject of enormous interest over the past four decades. [1] The interest stems primarily from their attractive photophysical and electrochemical properties, with potential applications in light harvesting, solar energy conversion, and artificial photosynthesis. One drawback however of [Ru(N^N) 3 ] 2+ type complexes can be photochemical ligand loss or isomerization reactions. [2] Whilst ordinarily an inconvenient side-reaction, such ligand loss pathways have been exploited, for example, in the development of photodynamic anticancer agents. [3] In these examples, ligand loss is often sterically promoted through inclusion of substituents adjacent to the N-donor atoms of the ligand that is ejected. [4] The dominant features in the visible region of their optical absorption spectra are the characteristic metal-to-ligand charge-transfer (MLCT) bands. Absorption at wavelengths in these bands involves excitation of a metal d-orbitalcentered electron to vacant p* orbitals on the N^N chelate ligands. Rapid intersystem crossing (ISC) then converts these initially formed 1 MLCT states into 3 MLCT states. It is these latter triplet states that are primarily responsible for phosphorescent emission exhibited by these complexes. In themselves, these 3 MLCT states are inert toward ligand loss and isomerization reactions. Instead, it is higher-lying metalcentered (MC) states, characteristic of population of the metal-ligand antibonding ds* orbitals, that are associated for ligand dechelation and ligand dissociation pathways and quenching of luminescent emission. Population of 3 MC states can be effected by absorption of high-energy light in the UV or two-photon absorption. [5] If close enough in energy, however, 3 MC states can undergo efficient thermal population from 3 MLCT states.Population of 3 MC states in [Ru(N^N) 3 ] 2+ type complexes was long thought to result in dechelation of one of the N^N ligands to form a coordinatively unsaturated species of the form [Ru(k 2 -N^N) 2 (k 1 -N^N)] 2+ , which is subsequently trapped by a solvent molecule. Isomerism or dissociation of the monodentate N^N ligand may then occur. More recently, computational calculations by Alary et al. suggest that the initially formed species as a result of 3 MC state population in [Ru(bpy) 3 ] 2+ and related complexes is the four-coordinate [Ru(k 2 -bpy)(k 1 -bpy) 2 ] 2+ in which two ligands dechelate through elongation of two mutually trans RuÀN bonds. [6] Tachiyashiki and co-workers [7] reported HPLC and electrospray mass spectrometry detection of the species [Ru-(bpy) 2 (3,3'-dmbpy)(NCMe)] 2+ (3,3'-dmbpy = 3,3'-dimethyl-2,2'-bipyridyl). Signals for the methyl groups were also observed by 1 H NMR spectroscopy that are suggestive of the formation of this intermediate in which the 3,3'-dmbpy ligand is coordinated in a monodentate fashion. Here, dechelation is presumably facilitated by the steric repulsion ...

The ruthenium bis-acetate complex Ru(κ(2)-OAc)(2)(PPh(3))(2) reacts with HC≡CPh to afford the vinylidene-containing species Ru(κ(1)-OAc)(κ(2)-OAc)(=C=CHPh)(PPh(3))(2). An experimental study has demonstrated that this reaction occurs under very mild conditions, with significant conversion being observed at 255 K. At lower temperatures, evidence for a transient metallo-enol ester species Ru(κ(1)-OAc)(OC{Me}O-C=CHPh)(PPh(3))(2) was obtained. A comprehensive theoretical study to probe the nature of the alkyne/vinylidene tautomerisation has been undertaken using Density Functional Theory. Calculations based on a number of isomers of the model system Ru(κ(1)-OAc)(κ(2)-OAc)(=C=CHMe)(PH(3))(2) demonstrate that both the η(2)(CC) alkyne complex Ru(κ(1)-OAc)(κ(2)-OAc)(η(2)-HC≡CMe)(PH(3))(2) and the C-H agostic σ-complex Ru(κ(1)-OAc)(κ(2)-OAc)(η(2){CH}-HC≡CMe)(PH(3))(2) are minima on the potential energy surface. The lowest energy pathway for the formation of the vinylidene complex involves the intramolecular deprotonation of the σ-complex by an acetate ligand followed by reprotonation of the subsequently formed alkynyl ligand. This process is thus termed a Ligand-Assisted Proton Shuttle (LAPS). Calculations performed on the full experimental system Ru(κ(1)-OAc)(κ(2)-OAc)(=C=CHPh)(PPh(3))(2) reinforce the notion that lowest energy pathway involves the deprotonation/reprotonation of the alkyne by an acetate ligand. Inclusion of the full ligand substituents in the calculations are necessary to reproduce the experimental observation of Ru(κ(1)-OAc)(κ(2)-OAc)(=C=CHPh)(PPh(3))(2) as the thermodynamic product.

Reaction of the ruthenium bis-acetate complex cis-Ru(κ2-OAc)2(PPh3)2 with a range of terminal alkynes HCCR (R = Ph, CO2Me, C{OH}Ph2, C{OH}Me2) results in the formation of the vinylidene complexes Ru(κ2-OAc)(κ1-OAc)(CCHR)(PPh3)2. The acetate ligands in these species are fluxional and, in the case where R = Ph, are undergoing fast exchange on the NMR time scale even at 195 K. In the case of the hydroxy-substituted complex Ru(κ2-OAc)(κ1-OAc)(CCHC{OH}Ph2)(PPh3)2 a hydrogen bond exists between the OH group on the γ-position of the vinylidene and the κ1-OAc ligand. This hydrogen bond inhibits the exchange of the OAc ligands, which now becomes slow on the NMR time scale at 215 K. In addition, and in contrast to the Cl-substituted analogues, elimination of water from the hydroxy vinylidene complex does not readily occur, possibly due to the presence of this hydrogen-bonding interaction. Instead, a slow reaction to form Ru(κ2-OAc)(κ1-OAc)(CO)(PPh3)2 is observed with the concomitant formation of H2CCPh2. An analogous process with Ru(κ2-OAc)(κ1-OAc)(CCHC{OH}Me2)(PPh3)2 affords H2CCMe2 and Ru(κ2-OAc)(κ1-OAc)(CO)(PPh3)2.