John D. Petersen

Electrochemical and photophysical properties of mono- and bimetallic ruthenium(II) complexes

Mono- and bimetallic complexes of ruthenium(II) containing 1,10-phenanthroline, and the bidentate, bridging ligands 2,3-bis(2′-pyridyl)pyrazine (dpp) or 2,3-bis(2′-pyridyl)quinoxaline (dpq) have been prepared and characterized. All of the mono- and bimetallic complexes, RuL2BL2+ and L2RuBL- RuL′24+ (L,L′=bpy, phen; BL=dpp, dpq), emit in acetonitrile at room temperature and have excited- state lifetimes in the 20-300 ns range. The lowest energy absorption feature has been assigned as a metal-to-ligand charge-transfer (MLCT) band localized exclusively on BL (i.e. Ru(II) dπ→+ BLpπ *). Electrochemically, the potential of the Ru(III)/ Ru(II) couple reflects the average ligand environment around the ruthenium center. There are numerous reductions that occur with each complex that are specific to the individual ligands with the reductions for the BLs (dpp and dpq) occurring at less negative potentials than for the L (bpy or phen) ligands. The positions of the absorption, emission and redox couples in these complexes are all consistent with a localized MLCT excited state and little metal- metal interaction in the isovalent Ru(II)/Ru(II) bimetallic complexes. The variety of metal/ligand combinations gives an assorted range of excited-state donor energies and excited-state redox couples. Oxidation of the excited-state species in general parallels the donor energy of the emissive state. Reduction of the excited-state species depends almost exclusively on the energy of the individual π* LUMO in the ground state. The wide variety of excited-state donor energies and redox couples results in a series of complexes that have great potential for studying excited-state energy- and electron- transfer processes.

Intramolecular energy transfer reactions in polymetallic complexes

Abstract A series of polymetallic complexes are being developed for use in converting solar radiation to usable chemical potential energy. The system is made up of three components: 1) A highly-absorbing (antenna) metal center which absorbs visible light but is photochemically unreactive; 2) A second metal center which undergoes a useful chemical reaction from a non-spectroscopic excited state; and 3) A bridging ligand which couples the two metal fragments and facilitates intramolecular energy transfer from the antenna to the reactive fragment. This paper will focus on optimizing the three components of the system.

Synthesis, characterization and photochemistry of some monometallic and bimetallic 2,2′-bipyrimidine complexes of chromium and tungsten carbonyls

Abstract Synthesis, electronic absorption spectra, 13C NMR and photochemistry are reported for the complexes M(CO)4bpym (M = Cr or W) and [W(CO)4]2bpym. The electronic absorption spectra indicate, for these complexes, that the lowest lying metal-to-ligand (L) charge transfer (MLCT) excited state is lower in energy than the ligand field (LF) excited states. The 13C NMR spectra showed that the chemical shifts of C(5) and C(6) for the M-bpym complexes move downfield with respect to that of the free ligand, bpym, while C(4) moves upfield upon complexation. Small, wavelength-dependent quantum yields for loss of CO were obtained upon irradiation. These quantum yields were an order of magnitude larger for the Cr-bpym complex than for the W complexes (Φ = 2.4 x 10−2 quanta/min for Cr-bpym, 2.5 x 10−3 quanta/min for W-bpym and 1.1 x 10−3 quanta/min for W-bpym-W, λirr = 366 nm).

Tris(2,2′-bipyrimidine)M (M = Fe(II), Co(II), Ni(II)) perchlorate complexes. Spectroscopic properties for precursor complexes in the preparation of polymetallic systems

The synthesis, electronic spectrum, magnetic susceptibility and electrochemistry of the d8 metal complex [Ni(bpym)3] (ClO4)2 (bpym = 2,2′-bipyrimidine) are reported here. Additionally, for the d6, d7, d8 series of M(bpym)32+ species, M = Fe(II), Co(II), Ni(II), electronic spectral assignments are made by comparison with the analogous M(bpy)32+ (bpy = 2,2′-bipyridine) complexes. The relative position of the LUMOs and the relative amount of dM → πL* backbonding in M(bpy)32+ vs. M(bpym)32+ is obtained from the frequency of the MLCT absorption for the series of M(bpym)32+ complexes. Cyclic voltammetry of Ni(bpym)32+ in acetonitrile shows the reduction potential (E12(3+/2+)) is +0.4 V more positive than for Ni(bpy)32+. A similar trend was also observed for the tris(2,2′-bipyrimidine)Fe(II) and -Co(II) complexes.

Effects of alternation in some quasi‐one‐dimensional magnetic materials

Exchange coupling in Cu(II) and Mn(III) compounds with unusual structures is discussed. {[Cu(bipyrimidine)(OH)(H2O)] (ClO4)}n has an alternatingly bridged structure with alternating ferromagnetic (+167.6 cm−1 through the hydroxo bridge) and antiferromagnetic (−79.8 cm−1 through the bipyrimidine bridge) interactions. Copper(II) phthalate monohydrate has alternating next‐nearest‐neighbor exchange with J=−12.3 cm−1 and α=0.06. This is the first member of this class. The compound K2[Mn(III) (salicylate)2][Mn(III) (salicylate)2]{CH3OH]2 has manganese ions in two environments alternating along the chain. A modified model for the chain is presented, and exchange coupling is found to be small since magnetic orbitals are not linked by the bridging ligand.

Tris(2,2′-bipyrimidine)cobalt(III, II, I). A cobalt polyazine electrochemical system with large storage capabilities

Abstract The preparation of [Co(bpym) 3 ](ClO 4 ) 2 where bpym = 2,2′-bipyrimidine, is described. This complex does not undergo the same Co(II) to Co(III) oxidation with H 2 O 2 as the 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) analogs. Comparison of the electrochemistry of the bpym, bpy, and phen complexes shows that the Co(II)/Co(III) oxidation is 0.7 volts more positive for the bpym complex. This large difference in potential, along with an increase in the Co(III)/Co(II) to Co(II)/Co(I) separation for the bpym complex, are discussed in terms of potential for use in a storage battery.

Excited-state energy- and electron-transfer reactions in multimetal systems

The ability of scientists to convert light into stored chemical potential energy has increased over the past ten years.1 A critical component of this development has been the sophistication of the synthesis of multi metal systems.1~2 Conceptually, the design of individual, but coupled, components in supramolecular systems with specific properties such as light absorption, light emission, chemical reaction, charge storage and conductivity are now possible through elaborate synthetic schemes.2 However, little is known about the coupling of these components. Extensive electronic coupling may perturb the desired characteristics of the mononuclear fragments. On the other hand, minimal coupling may preclude energyor electron-transfer processes within the lifetime of the excited state of the molecular ensemble. Our goal in this paper is to outline some of the design constraints present in supramolecular systems in terms of the coupling of metal centers in excited-state, energyand electrontransfer processes. Metal azine systems have been used extensively in the design of multimetal complexes. 13 One reason for this extensive use in the longlived excited state of complexes such as Ru(bpy)

Intramolecular Energy and Electron Transfer in Polymetallic Complexes

+, where bpy = 2,2’bipyridine. Another reason is that polyazaaromatic ligands form stable, chelating, bridging ligands. The azines used as bridging and terminal ligands in this work are pictured in Figure 1.

Preparation and electrochemistry of biimidazolate complexes of ruthenium(II)

The use of polydentate, bridging ligands has enabled the preparation and characterization of mono-, bi-, tri-, and tetrametallic systems. Experimental measurements such as absorption and emission spectroscopy, and electrochemistry have indicated that the exact structure of the ligand determines the amount of communication between the various metal centers in the polymetallic complexes. This lack of electronic communication with some bridging ligands allows these systems to have a lifetime long enough to undergo emission at room temperature. These polymetallic systems supply a wide range of excited-state donor energies and redox potentials that give additional flexibility for energy- and electron-transfer processes. The ability to prepare supramolecular systems with multi-function and controllable molecular properties is the future for these molecules.

The design and preparation of transition metal triad systems for excited-state charge separation

The ruthenium(III) complex, Ru(NH3)4(bimH2)3+ (bimH2 = 2,2′-biimidazole) has been prepared and characterized. The complex displays a ligand-to- metal charge-transfer (LMCT) transition at 640 nm. Reduction of the complex at a pH < 6, results in the formation of Ru(NH3)4(bimH2)2+. This Ru(Il) complex has a metal-to-ligand charge-transfer (MLCT) transition at 407 nm. At pH= 6, the Ru(III) to Ru(II) reduction is accompanied by loss of a proton from the coordinated bimH2 ligand.