Brenda D. Rossenaar

Alkyl-dependent photochemistry of Mn(R)(CO)3(R'DAB) (R=Me,Bz; R'=iPr,pTol): Homolysis of the Mn-R bond for R=Bz and release of CO for R=Me

The spectroscopic properties and photochemistry of the complexes Mn(R)(CO)3(R′-DAB) (R=Me, Bz; R′=iPr, pTol) are reported. The UV-photoelectron spectrum of Mn(Me)(CO)3(iPr-DAB) shows that the σ(MnMe) orbital has a higher ionization potential than the Dπ(Mn) orbitals, which is of importance for the photochemical behavior of this complex. The R group bound to the metal determines the photochemistry of these complexes. The Me complexes lose CO upon irradiation into their MLCT bands. The CO-loss products react with Lewis bases, the final product being cis(CO,CO),trans(Me,L)-Mn(Me)(L)(CO)2(R′-DAB). The structure of one of these products, viz. cis,trans-Mn(Me)(P(OMe)3)(CO)2(iPr-DAB)(C14H28N2O5PMn) has been determined by a single-crystal X-ray diffraction study (T=200 K). The crystal is monoclinic, space group P21/n with unit cell dimensions a=15.435(3), b=15.200(3), c=18.192(4) A, V=3915(2) A3, Z=8. The structure refinement converged to R=0.077 for 2829 observed reflections (total number of parameters: 319). Transient absorption spectroscopy shows that an equatorial CO ligand is lost upon excitation and that the final product is formed via different cis,cis-isomers. For R = Bz, visible excitation leads to efficient homolysis (Φ= 0.4 for R′ =iPr) of the MnBz bond, resulting in the formation of radicals which are characterized by ESR spectroscopy. The different behavior of the Bz and Me complexes is attributed to a difference in relative energies of two reactive excited states. For R=Me, the complexes lose CO from the lowest MLCT state; for R=Bz they undergo homolysis of the MnBz bond from the lowest σπ∗ state.

Properties and dynamics of the σ(M′-Re)π∗ excited state of photoreactive dinuclear LnM′-Re(CO)3(α-diimine) (LnM′ = Ph3Sn, (CO)5Mn, (CO)5Re; α-diimine = bpy′, iPr-PyCa, iPr-DAB) complexes studied by time-resolved emission and absorption spectroscopies

Abstract The photophysics and photochemistry of the metal-metal bonded complexes LnM′Re(CO)3(α-diimine) (LnM′ = Ph3Sn, (CO)5Re, (CO)5Mn; α-diimine = bpy′, iPr-PyCa, iPr-DAB) have been studied. According to the time-resolved emission (80K) and adsorption (room temperature) spectra, the lowest excited state has a 3 σ( M′ - Re )π ∗ character. It is a bound state, which can only be populated by surface crossing from optically excited MLCT states. Homolysis of the metal-metal bond from the σπ ∗ state is promoted by nucleophilic and chlorinated solvents. Exceptional in this respect is the complex Ph3SnRe(CO)3(bpy′), which is nearly photostable in non-chlorinated solvents. The lifetime of the 3 σπ ∗ state decrease in the order α-diimine = bpy′>iPr-PyCa>iPr-DAB>pAn-DAB. This trends is mainly determined by the energy gap law. The LnM′ dependence is more complicated because of an additional deactivating effect of an excited state distortion which depends on LnM′. At 80 K, the lifetime is determined by the weak coupling to the ground state; at room temperature by dissociation of M′-Re (with the exception of SnRe).

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.

SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF RE(R)(CO)3(ALPHA -DIIMINE) (R=ALKYL ; ALPHA -DIIMINE=R'-PYCA, R'-DAB) COMPLEXES. CRYSTAL STRUCTURE OF RE(ME) (CO)3(IPR-DAB)

Abstract The synthesis, structure and spectroscopic properties of several complexes of the type Re(R)(CO)3(α-diimine) (R  alkyl) are reported. The structure of Re(Me)(CO)3(iPr-DAB) has been determined by a single-crystal X-ray diffraction study. The molecule has a distorted octahedral geometry, with the three carbonyls in a fac-geometry. The complexes possess strong absorption bands in the visible region which are assigned to MLCT transitions with the aid of resonance Raman spectroscopy; the Raman spectra do not provide any evidence for a σ(Re — R) → π∗ (α-diimine) transition within this absorption band. The UV-vis spectrum of Re(Bz)(CO)3(iPr-DAB) (Bz = benzyl) shows a distinct shoulder at the low-energy side of the visible band, which is attributed to the interaction between the dπ(Re)-orbitals and the π-orbitals of the benzyl group. The UV-photoelectron spectrum of Re(Me)(CO)3(iPr-DAB) shows that σ(Re — Me) has a higher ionization potential than the dπ(Re)-orbitals, a result that is of importance for the interpretation of the photochemical behaviour of this complex.

Character and picosecond dynamics of the excited states of [Re(Br)(CO)3(α-diimine)] complexes as a function of the α-diimine ligand: 4,4′-di-methyl-2,2′-bipyridine, N,N-di-p-tolyl-1,4-diaza-butadiene and N,N-di-p-anisyl-1,4-diaza-butadiene

Abstract Excited state behavior of the [Re(Br)(CO) 3 (α-diimine)] complexes, α-diimine=4,4′-di-methyl-2,2′-bipyridine (bpy′), N,N -di- p -tolyl1,4-diaza-butadiene ( p -Tol-DAB) and N-N -di- p -anisyl-1,4-diaza-butadiene ( p -An-DAB), was investigated by means of time resolved visible absorption spectroscopy studied over the time range 50 ps-205 ns following the laser pulse excitation at either 355 or 532 nm. Transient spectra obtained were compared with the absorption spectra of the [Re(Br)(CO) 3 (α-dimmine)] − anions produced by a (spectro)-electrochemical reduction. The lowest excited state of [Re(Br)(CO) 3 (bpy′)] was identified as a relatively long-lived (about 60 ns), strongly emissive Re → bpy′ 3 MLCT state. Both [Re(Br)(CO) 3 (DAB)] complexes have a very short-lived (150 ps for p -Tol-DAB and 250 ps for p -An-DAB) lowest excited state of (partly) Br − → DAB ligand-to-ligand charge transfer character that is populated by the 532 nm excitation. High energy excitation at 355 nm populates both the MLCT and π → π ∗ intraligand excited states of [Re(Br)(CO) 3 (DAB)].

Photochemistry of ReR(CO)3(Pri-dab)(R = Me, Et, Bn; dab = 1,4-diazabuta-1,3-diene): homolysis of the Re–R bond, its dependence on R and evidence for the reactive σbπ* state from transient absorption spectra

From quantum yield and nanosecond flash photolysis data and UV–PE spectra it is concluded that metal-to-ligand charge transfer (MLCT) excitation of the complexes ReR(CO)3(Pri-dab)(R = Me, Et, Bn) only leads to efficient homolysis of the metal–alkyl bond if R = Et or Bn, when the reactive σbπ* state is close in energy to the MLCT states.