David K. Geiger

Luminescent platinum complexes: tuning and using the excited state

Abstract The focus of recent research on square planar Pt(II) diimine dithiolate complexes has been to understand molecular factors that influence their excited state properties and to develop diad and triad systems based on them for use in light-driven reactions. Regarding the former, two series of Pt(diimine)(dithiolate) complexes have been synthesized and studied. All of the compounds display solvatochromic absorption bands and solution luminescence attributable to metal/dithiolate-to-diimine charge transfer excited states of the same orbital parentage. The excited-state energies can be tuned by approximately 1 eV through ligand variation. Excited-state redox potentials have been estimated for all of the complexes from spectroscopic and electrochemical data, and electron transfer quenching rate constants show the expected driving force dependence. Analogous Au(III) systems have been synthesized and characterized including molecular structure determinations of a cationic diimine dithiolate system and a neutral C-deprotonated-2-phenylpyridine derivative. Striking differences exist in the electronic structures of these Au(III) complexes from those of the Pt(II) systems, underscoring the key role of the metal in the excited state structure of the latter. The creation of diads and triads is being undertaken with ligand bridges capable of connecting the Pt(diimine)(dithiolate) moiety with other metal centers. Toward that end, complexes of dipyridocatecholate (dpcat) have been synthesized and characterized. These complexes may serve as models for the linking of chromophore and quencher components of a possible photosynthetic system. The dpcat complexes have been characterized by absorption and steady-state emission spectroscopies. Luminescence and redox properties of these and a related system containing a tetrapyridophenazine (tppz) bridge are described.

Mixed aggregates of phthalocyanines and porphyrins bearing oppositely charged substituents

Abstract A number of metalloporphyrins bearing charged, peripheral substituents have been shown to associate with metallophthalocyanines with oppositely charged substituents. Jobs plot analyses reveal that the aggregates contain equal numbers of phthalocyanines and porphyrins. The complexation is accompanied by significant changes in the near ultraviolet and visible spectra of the individual components. These spectral changes have been used to estimate equilibrium constants for the formation of the associated complexes. The nature of the interaction is discussed.

Susceptibility and Mössbauer study of coupled admixed‐spin state in [Fe(OEP) (3‐ClPy)]+ dimers

Susceptibility measurements of [Fe(OEP) (3‐ClPy)] ⋅ ClO4(OEP=octaethylporphyrinate) in an external field of 1 T show that its magnetic moment varies from 3.36 to 4.58 β over a temperature range of 6–300 K. The data were analyzed with the ‘‘Maltempo model’’ but could be fitted only when an antiferromagnetic exchange interaction between two neighboring iron atoms within the dimer structure was assumed. The analysis shows that the ground state quartet S=3/2 (81%) is mixed with the nearby sextet S=5/2 (19%) through spin‐orbit coupling with an unusually small coupling constant, ζ=150 cm−1. The exchange interaction within the dimer is estimated to be in the range of J=−0.4 to −0.6 cm−1. Mossbauer spectra of a polycrystalline sample of this complex were recorded in a range of temperatures (1.54–128 K) and applied magnetic fields (0–6 T). The analysis of Mossbauer spectra was in excellent agreement with that of susceptibility data. The analysis favors J=−0.6 cm−1. The values of chemical shift, δ=0.36 mm/s (with r...

Mössbauer and susceptibility study of inter‐heme spin coupling in [Fe (OEP) (2‐MeHIm)]+ dimers

Mossbauer spectra of polycrystalline [Fe(OEP)(2‐MeHIm)]ClO4 ⋅ CHCl3 (OEP=octaethylporphyrinate; 2‐meHIm=2‐methylimidazole) were recorded over a range of temperatures (1.54 to 195 K) and applied magnetic fields (0–6 T). Magnetic susceptibility was measured at both 0.2 and 1 T in the temperature range 6–300 K. Mossbauer parameters δ=0.40 mm/s and ΔE=1.39 mm/s at 4.2 K, and the high temperature susceptibility Neff=6 at 300 K indicated high spin iron, but no simple spin Hamiltonian would reproduce the rapid decrease of Neff below 50 K. We interpret the data by assuming a zero field splitting D=12 cm−1 and invoking an antiferromagnetic exchange interaction J=−0.8 cm−1 between the five‐coordinate iron atoms of closely spaced face‐to‐face heme pairs in the crystal. Slow spin relaxation at low temperature permits observation of level crossing and confirms the interpretation. It also permits the detection, through low field Mossbauer measurements, of a small rhombic distortion with E=0.024 cm−1. Mossbauer spectra ...

Photochemistry of Cu—Olefin complexes: a flash photochemical investigation of the reactivity of Cu(ethylene)+ and Cu(cis,cis-1,5-cyclooctadiene)+2

Abstract The UV-photochemistries of Cu(ethylene) + , Cu(C 2 H 4 ) + , and Cu( cis,cis -1,5-cyclooctadiene) + 2 , Cu( c,c -COD) + 2 , have been investigated by continuous and flash photolysis. Photolyses of Cu(C 2 H 4 ) + in ethylene saturated methanol produce hexane and formaldehyde. This process and the photoinduced photoisomerization of Cu( c,c COD) + 2 to Cu( c,c -COD)( c,t -COD) + , c,t -COD = cis,trans -1,5-cyclooctadiene, proceed through several intermediates with lifetimes in a nanosecond-second time domain and spectra with absorption bands in the 400–600 nm range. These intermediates have been assigned as σ-bound copper-alkyl species. The mechanisms for product formation are discussed.

The effect of mixed phthalocyanine-porphyrin in aggregation on the photoreduction of manganese porphyrins

Abstract The ultraviolet photochemistry of mixed porphyrin···phthalocyanine aggregates containing ( meso - tetramethylpyridylporphyrinato)manganese(III) chloride has been investigated. Photolysis results in the photoreduction of the manganese center in the presence of a radical scavenger. Aggregation has a pronounced effect on the quantum yield of the process. The results are explained in terms of a charge transfer between the component macrocycles. The stability of the photoproduct is markedly influenced by the presence of phthalocyanines as evidenced by the rate of reoxidation of the photoproduct by oxygen.

Reductive addition of OH radicals to macrocyclic complexes

Abstract The reactions of OH radicals with complexes of the macrocycle Me 2 -3,4,5-pyo-[14]trieneN 4 and Cu(II), Co(II), Co(III) have been investigated by pulse radiolysis. The addition of the OH radical to the macrocycle leads to metal-ligand radical intermediates which achieve stability by reduction of the metal center. The mechanism of such reactions is discussed in terms of the redox potentials of the complexes and known mechanisms of addition to organic aromatic compounds.

5-Bromo-2-(thio­phen-2-yl)-1-(thio­phen-2-ylmeth­yl)-1H-benzimidazole

There are two independent molecules in the asymmetric unit of the title compound, C16H11BrN2S2. In the crystal, weak C—H⋯N hydrogen bonds and C—H⋯thiophene ring interactions link the molecules into chains along [100]. The structure exhibits disorder of the 2-thiophen-2-yl substituent of one of the symmetry-unique molecules with a major:minor component ratio of 0.914 (3):0.086 (3).

2-(Thio-phen-2-yl)-1-(thio-phen-2-ylmeth-yl)-1H-benzimidazole.

In the title compound, C16H12N2S2, the thiophene groups are rotationally disordered over two sets of sites, by approximately 180°, with occupancy ratios of 0.916 (2):0.084 (2) and 0.903 (2):0.097 (2). The major components of the thiophene and methylene substituted thiophene rings are canted by 24.06 (12) and 85.07 (10)°, respectively, from the benzimidazole ring system plane and the dihedral angle between the major component thiophene ring planes is 84.90 (14)°. In the crystal, there is a weak C—H⋯N hydrogen bond which links molecules into chains.

Structural characterization of two benzene-1,2-di-amine complexes of zinc chloride: a mol-ecular compound and a co-crystal salt.

The structures of two zinc complexes containing bidentate benzene-1,2-diamine ligands are reported. (Benzene-1,2-diamine-κ2 N,N′)dichloroidozinc displays a distorted tetrahedral geometry. The 1:1 co-crystal salt trans-diaquabis(4,5-dimethylbenzene-1,2-diamine- κ2 N,N′)zinc chloride 4,5-dimethylbenzene-1,2-diamine exhibits a tetragonally distorted octahedral zinc coordination sphere.