D.J. Stufkens

Ligand-dependent excited state behaviour of Re(I) and Ru(II) carbonyl–diimine complexes

Abstract The relations between the structure and excited state properties of Re(E)(CO) 3 ( α -diimine) and Ru(E)(E′)(CO) 2 ( α -diimine) complexes (axial ligand E, E′=halide, alkyl, benzyl, metal fragment) are unravelled and discussed in detail. For example, it is shown how the increasing π -donor strength of an axial ligand, such as a halide changes the character of the lowest excited state from MLCT to LLCT. On the other hand, the presence of a covalently bound axial ligand in the coordination sphere introduces a σ π * lowest excited state that involves an excitation of an electron from the metal–ligand σ -bonding orbital to the π * orbital of the α -diiamine. While the orbital parentage of the lowest excited state—MLCT, LLCT, σ π * or IL—is mostly determined by the axial ligand(s), its detailed properties (energy, lifetime, reactivity, decay mechanism) are dependent on both the axial and diimine ligands. Depending on the molecular structure and the medium, the excited state behaviour of these complexes ranges from a strong, long-lived emission to a very fast photochemical homolysis of a metal–ligand bond. Photochemical and photophysical properties of the complexes with different types of the lowest excited state are explored and pertinent structural effects discussed. It is shown how the excited state properties of the Re and Ru carbonyl–diimine complexes can be controlled by a judicious choice of the axial and diimine ligands and by the medium. These relations can be employed to design new functional molecular photonic materials, e.g. sensitisers, luminophores, photocatalysts, radical photoinitiators, luminescent probes or sensors.

Spectroscopy, photophysics and photochemistry of zerovalent transition metal α-diimine complexes

Formation de complexes metalliques carbonyles avec des derives de diimine et etude du mecanisme de reactions photochimiques au moyen de plusieurs techniques spectrometriques

Characterization of chromium oxide supported on Al2O3, ZrO2, TiO2, and SiO2 under dehydrated conditions

Abstract In the present investigation, Raman and IR spectroscopy were used to study the surface structures of chromium oxide supported on alumina, titania, zirconia, and silica, as a function of the loading under dehydrated conditions. It was found, that the dehydrated surface structures of chromium oxide differ strongly from those previously reported under ambient conditions, in which the surfaces are hydrated. Two species, each possessing one short terminal CrO bond, and one (or more) oligomer (s) are proposed to be present on the dehydrated alumina, titania, and zirconia surfaces. The relative concentrations of these different chromium oxide species is independent of the surface coverage. The chromium oxide species present on the dehydrated silica surface are completely different from those observed on the other three supports. The Raman and IR spectra indicate the presence of an isolated chromium oxide species possessing two short CrO bonds together with a small amount of surface species possessing a terminal CrO 3 unit, isolated or not. The gradual disappearance of the surface hydroxyl groups of all four supports upon addition of chromium oxide, as monitored by IR spectroscopy, suggests that the chromium oxide species interacts with the surface by removal of the surface hydroxyl groups.

Bonding properties of Mo(CO)4-xL(PR3)x (x = 0, 1, 2; L =diazabutadiene, pyridine-2-carbaldehyde imine and 2,2′-bipyridine). Infrared, electronic absorption, 1H, 13C and 31P NMR and resonanceraman spectra

Abstract Infrared and electronic absorption, 1 H, 13 C and 31 P NMR and resonance raman spectra of the complexes Mo(CO) 4 L, Mo(CO) 3 L(PR 3 ) and Mo(CO) 2 L(PEt 3 ) 2 (L = 1,4-diazabutadiene, pyridine-2-carbaldehyde imine and 2,2′-bipyridine; R = OCH 3 , C 6 H 5 , C 2 H 5 , C 4 H 9 , c-C 6 H 11 ) are reported. The π-backbonding between Mo and L appears to be much stronger for L = DAB than for the other two ligands. The influences of basicity and bulkiness of the PR 3 group on the bonding properties of the complexes are discussed. Several Mo to L CT bands are observed and assigned. The character of these bands and the properties of the excited states are investigated with the use of the resonance Raman effect. The absorption spectra of some of the complexes show drastic changes in low temperature alkane glasses.

Structural determination of surface rhenium oxide on various oxide supports (Al2O3, ZrO2, TiO2 and SiO2)

In the present investigation Raman and IR spectroscopy were used to determine the structure of Re2O7/Al2O3, Re2O7/ZrO2, Re2O7/TiO2 and Re2O7/SiO2 as a function of loading under ambient and in situ dehydrated conditions. Under ambient conditions, the surface rhenium oxide is hydrated and possesses the structure of the ReO4 ion in aqueous solution, independent of coverage or support type. Under dehydrated conditions, the in situ Raman and IR spectra show that only one surface rhenium oxide species is present on the silica support, whereas two surface rhenium oxide species are present on the alumina, zirconia and titania supports. The concentration ratio of the two species is a function of the coverage, and their structures are similar, possessing three terminal ReO bonds and one bridging ReO-support bond (C3v symmetry). Differences in properties between the surface rhenium oxide species were determined by TPR as a function of coverage and support type. The in situ Raman, IR and TPR measurements suggest that the bridging ReO-support bond strength decreases with increasing coverage, while the TPR data further indicate that the ReO-support bond strength decreases in the order Al2O3 ⪢ SiO2 ≈ ZrO2 ⪢ TiO2 for a given rhenium oxide loading.

A study of the electronic properties of M(CO)4DAB (M = Cr, Mo, W; DAB = diazabutadiene). I. electronic `absorption, resonance raman, infrared, 13C- and 15N-NMR spectra

During the reaction of M(CO)5THF with DAB an intermediate M(CO)5DAB was formed, In which DAB is coordinated as a monodentate ligand. The complexes M(CO)4DAB posess structured absorption bands in the visible which were assigned, mainly on the basis of resonance Raman results, to different CT transitions from the metal to DAB. The resonance Raman spectra showed that orbitals of both DAB and the cis carbonyls are involved in the first excited states. A very pronounced enhancement of Raman intensity was found in polar solvents for a polarized band at about 215 cm−1, which could therefore be assigned to the νs(M-N). The character of this mode was investigated by 15N enrichment of DAB. 1H, 13C and 15NMR spectra show a large electron shift from DAB to the metal in polar solvents

Ring size effects of some 13C, 31P and 195Pt NMR parameters in the series {Ph2P(CH2)nPh2}Pt(CH3)2

Abstract The 31P, 13C, and 195Pt NMR spectra of the compounds {Ph2P(CH2)nPPh2}Pt(CH2)2 were studied and compared with the spectra of the acyclic analogue, cis-(Ph2PCH3)2Pt(CH3)2. Comparison with structural data showed that irregularities in the NMR parameters can be explained on the basis of the existence of angle strain for four-membered but not for five-membered rings.

1,4-Diazabutadiene olefin complexes of zerovalent palladium: preparation and characterization

Abstract A number of mixed olefin diazabutadiene (RN: CR′CR′:NR; DAB) complexes of zero-valent palladium [(olefin)Pd(DAB)] have been prepared by the simultaneous reactions of an activated olefin and a DAB with an appropriate source of Pd(O). The complexes have been characterized by 1 H- 13 C NMR, I.R. and Raman spectroscopy. The physical and chemical properties of the complexes are dependent upon the olefin and DAB ligands employed. Complexes where olefin = dimethylfumarate(dmf) or diethylfumarate (def) and DAB = tBuN:CHCH:NtBu (DAB H tBu ) readily undergo ligand displacement reactions with PPh 3 and tBuNC (leading to displacement of the DAB), oxidative addition of the olefin) yielding a dinuclear Pd(II) complex. ([(methylallyl)(chloride)Pd] 2 tBuN: CHCH:NtBu) and react with excess olefin yielding a dinuclear Pd(O) complesx ([(olefin) 2 Pd 2 ]tBuN: CHCH:NtBu).

Detection and assignment of different electronic transitions within the first CT band of M(CO)4L (M = Cr, Mo, W; L 1,4-diazabutadiene (DAB) and pyridine-2-carbaldehydeirnine (PyCa)) with the use of resonance raman and MCD spectra

Abstract The magnetic circular dichroism (MCD) together with the electronic absorption spectra of Mo(CO) 4)-pyridine-2-carbaldehyde-(N-phenyi -imine revealed that the first Mo to L CT band consists of different electronic transitions which could be asssigned with the help of resonance Raman spectra. The excited states of M(CO) 4-2 L(PR 3 x ) complexes (M = Cr, Mo, W; x= 0, 1; L = 1,4-diazabutadiene and pyridine-2-carbaldehyde-(R 1 )-imine, R 1 = i-prop, phenyl; R - phenyl and n-butyl are investigated with the resonance Ratnan effect and interpreted with the help of electronic absorption spectra and CNDO/S calculations. The resonance Raman spectra gave detailed information about the properties of the excited states of the various complexes.

Cumulated double bond systems as ligands

Abstract Each of the compounds [MCl(Pr3)2(ArylNSO)] (M = RhI, IrI; R = i-Pr, Cy: Aryl = C6H5, 4-MeC6H4, 4-ClC6H4, 2,4,6-Me3C6H2 appears to exist as two isomers both in the solid state and in solution. The molecular and single crystal structure of one of the isomers of [RhCl(P-i-Pr)3)2(4-Me6H4NSO)] shows that the N-sulfinylaniline ligand is in the cis-configuration and coordinated to the rhodium atom via the sulfur-atom. The ligand lies in a plane which includes the rhodium atom and is in agreement with the Rh-S distance of 2.10 A. IR results of the compounds (solid and solutions), 21P NMR data and 15N NMR of a 15N labelled compound, which yielded a 103Rh15N coupling constant of 15.5 Hz, show that in the second isomer the N-sulfinylaniline ligand is probably bonded to the metal atom via the π-NS bond. The ratio of the metal-π-NS bonded isomer and the metal-S bonded isomer decreases in the order Aryl = 4-ClC6H4 > C6H5 > 4-MeC6H4; R = i-Pr > Cy and M = Rh > Ir. The interconversion of the two isomers is intramolecular and becomes observable on the 31P NMR time scale at about 40° C for M = Rh. In the case of [Ir(P-i-Pr3)2(4-MeC6H4NSO)], cyclometallation of the sul- finylaniline is observed via the ortho-carbon atom, whereas cyclometallation via P-i-Pr3 is observed when the ortho-positions are blocked by methyl groups, e.g. when L = 2,4,6-Me3C6H2NSO.