Photodissociation of Free Metalloporphyrin Dimer Multianions.

We have used action photofragmentation spectroscopy in the visible spectral range (410 to 650 nm) to investigate the optical properties of different monomeric and dimeric M(II)-meso-tetra-(4-sulfonatophenyl)-porphyrin (with M = Pd(II), Cu(II), Zn(II)) multianions isolated in the gas phase without solvent. In particular, we report the position of the Q-bands (S0 → S1 transitions) as a function of charge state, counterions, oligomerization, and dimer structure type. The results for the monomers (charge states = 4- and 3-, sodiated and protonated) are in good agreement with TDDFT calculations and condensed-phase spectra. For both homo and heterometallic dimers, photofragmentation spectra were recorded for two charge states, 5- and 3-, corresponding to coplanar and cofacial structure types, respectively. The fragmentation patterns observed for the dimers depend significantly on charge state, with fragmentation into monomers being dominant for the 5- species, while the 3- charge state predominantly fragments by SO2 loss. The monomer → dimer Q-band spectral shifts observed in the gas phase were compared with the optical properties of porphyrin aggregates in solution.

Metal Complexes of a Redox-Active [1]Phosphaferrocenophane: Structures, Electrochemistry and Redox-Induced Catalysis.

The synthesis and characterisation of several metal complexes of a redox-active, mesityl(Mes)-substituted [1]phosphaferrocenophane, FcPMes (1), are reported. Cyclic voltammetry studies on the bimetallic complexes [M(κ1 P-1)(cod)Cl] (M=Rh: 2; M=Ir: 4), [Rh(κ1 P-1)2 (CO)Cl] (3) and [AuCl(κ1 P-1)] (5), in conjunction with DFT calculations, provided indications for a good electronic communication between the metal atoms. To confirm that the ferrocenophane unit might be able to electrochemically influence the reactivity of the coordinated transition metal, the rhodium complex 2 was employed as stimuli-responsive catalyst in the hydrosilylation of terminal alkynes. All reactions were greatly accelerated with in situ generated 2+ as a catalyst as compared to 2. Even more importantly, a markedly different selectivity was observed. Both factors were attributed to different mechanisms operating for 2 and 2+ (alternative Chalk-Harrod and Chalk-Harrod mechanism, respectively). DFT calculations revealed relatively large differences for the activation barriers for 2 and 2+ in the reductive elimination step of the classical Chalk-Harrod mechanism. Thus, the key to the understanding is a cooperative "oxidatively induced reductive elimination" step, which facilitates both a higher activity and a markedly different selectivity.

Quaternionic formulation of the two-component Kohn-Sham equations and efficient exploitation of point group symmetry.

The quaternionic formulation of the time-reversal invariant quasirelativistic Kohn-Sham equations with exact Hartree-Fock exchange leads to hypercomplex one-component equations with half of the dimension compared to the original two-component problem. The combination of the quaternionic equations with point group symmetry exploitation for D2h and its subgroups by construction of corepresentations leads to quaternionic, complex, or real algorithms depending on the structure of the point group. In this work, the quaternionic approach with point group symmetry exploitation of the relativistic four-component Dirac-Hartree-Fock theory by Saue and Jensen [J. Chem. Phys. 111, 6211 (1999)] will be adopted to the quasirelativistic two-component Kohn-Sham scheme for closed-shell systems. The implementation in the program system TURBOMOLE is applied to the large lead cluster Pb56 as a test case.

Tris(3,5-dimethylpyrazolyl)methane-based heterobimetallic complexes that contain Zn - and Cd - transition-metal bonds: synthesis, structures, and quantum chemical calculations.

Reactions of the tris(3,5-dimethylpyrazolyl)methanide amido complexes [M'{C(3,5-Me2 pz)3 }{N(SiMe3 )2 }] (M'=Mg (1 a), Zn (1 b), Cd (1 c); 3,5-Me2 pz=3,5-dimethylpyrazolyl) with two equivalents of the acidic Group 6 cyclopentadienyl (Cp) tricarbonyl hydrides [MCp(CO)3 H] (M=Cr (2 a), Mo (2 b)) gave different types of heterobimetallic complex. In each case, two reactions took place, namely the conversion of the tris(3,5-dimethylpyrazolyl)methanide ligand (Tpmd*) into the -methane derivative (Tpm*) and the reaction of the acidic hydride M = H bond with the M' = N(SiMe3 )2 moiety. The latter produces HN(SiMe3 )2 as a byproduct. The Group 2 representatives [Mg(Tpm*){MCp(CO)3 }2 (thf)] (3 a/b) form isocarbonyl bridges between the magnesium and chromium/molybdenum centres, whereas direct metal-metal bonds are formed in the case of the ions [Zn(Tpm*){MCp(CO)3 }](+) (4 a/b; [MCp(CO)3 ](-) as the counteranion) and [Cd(Tpm*){MCp(CO)3 }(thf)](+) (5 a/b; [Cd{MCp(CO)3 }3 ](-) as the counteranion). Complexes 4 a and 5 a/b are the first complexes that contain Zn - Cr, Cd - Cr, and Cd - Mo bonds (bond lengths 251.6, 269.8, and 278.9 pm, respectively). Quantum chemical calculations on 4 a/b* (and also on 5 a/b*) provide evidence for an interaction between the metal atoms.

Cu(II)- and Mn(III)-porphyrin-derived oligomeric multianions: structures and photoelectron spectra.

We present structures and photoelectron spectra of Mn(III) and Cu(II) meso-tetra(4-sulfonatophenyl)porphyrin (TPPS) multianions, as well as of homomolecular dimers and trimers thereof. The structural assignments are based on a combination of mass spectrometry, ion mobility measurements, and semiempirical as well as density functional theory (DFT) calculations. Depending on the type of central metal atom, two completely different dimer structural motifs are found. With a central Mn(III), the monomeric units are connected via sulfonic-acid-manganese bonds resulting in a tilted stack arrangement of porphyrin rings. With Cu(II) as the central atom, the sulfonic acid groups preferentially bind to the sodium counterions, resulting in a flat dimer structure with coplanar porphyrins. Photoelectron spectra were recorded for monomers, dimers, and trimers, each in a number of different negative charge states as determined by protonation degree (+nH). In some cases, e.g., [Cu(II)TPPS](4-), [(Mn(III)TPPS)2 + H](5-), and [(Mn(III)TPPS)3 + 3H](6-), we observe electron detachment energies close to zero, or even slightly negative. In all cases, we find a large repulsive Coulomb barrier. The observed trends in detachment energies can be interpreted in terms of a simple electrostatic model.

Q and Soret Band Photoexcitation of Isolated Palladium Porphyrin Tetraanions Leads to Delayed Emission of Nonthermal Electrons over Microsecond Time Scales.

We have used both action and photoelectron spectroscopy to study the response of isolated Pd(II) meso-tetra(4-sulfonatophenyl)porphyrin tetraanions ([PdTPPS](4-)) to electronic excitation over the 2.22-2.98 eV photon energy range. The action spectrum obtained by recording the wavelength-dependent intensity of charged decay products closely resembles the absorption spectrum of PdTPPS in aqueous solution (which shows pronounced Q and Soret absorption bands). The two main decay channels observed are sulfonate group loss and, predominantly, electron emission. To better understand the electron emission channel, we have also acquired photoelectron spectra at multiple detachment photon energies covering the range probed in action spectroscopy. Upon both Q and Soret band excitation, we find that electrons are emitted in three characteristic kinetic energy ranges. The corresponding detachment processes are identified as (delayed) tunneling emission from both excited singlet and triplet states (each of which is accessed by/after one-photon absorption) as well as resonant two-photon detachment. The first triplet state lifetime of isolated [PdTPPS](4-) is significantly longer than 10 µs, possibly on the 100 µs time scale. We estimate that more than 50% of the electron emission observed upon photoexcitation occurs by way of this triplet state.

Self-consistent treatment of spin-orbit interactions with efficient Hartree-Fock and density functional methods.

Efficient self-consistent field (SCF) schemes including both scalar relativistic effects and spin-orbit (SO) interactions at Hartree-Fock (HF) and density functional (DFT) levels are presented. SO interactions require the extension of standard procedures to two-component formalisms. Efficiency is achieved by using effective core potentials (ECPs) and by employing the resolution-of-the-identity approximation for the Coulomb part (RI-J) in pure DFT calculations as well as also for the HF-exchange part (RI-JK) in the case of HF or hybrid-DFT treatments. The procedures were implemented in the program system TURBOMOLE; efficiency is demonstrated for comparably large systems, such as Pb54. Relevance of SO effects for electronic structure and stability is illustrated by treatments of small Pb and Po clusters with and without accounting for SO effects.

Basis-set extensions for two-component spin-orbit treatments of heavy elements.

The accuracy of standard basis sets of quadruple-zeta and lower quality for the use in two-component self-consistent field procedures including spin-orbit coupling is investigated for the elements In-I and Au-At. Spin-orbit coupling leads to energetic and spatial splittings of inner shells, which are not described accurately with standard basis sets optimized for scalar relativistic calculations. This results in large errors in total atomic energies and significant errors in atomization energies of compounds containing these atoms. We show how these errors can be corrected by adding just a few steep sets of basis functions and demonstrate the quality of the resulting extended basis sets.