Density functional theory and complete active space self-consistent field computations are applied to elucidate the singlet diradical character of square planar, diamagnetic nickel complexes that contain two bidentate ligands derived from o-catecholates, o-phenylenediamines, o-benzodithiolates, o-aminophenolates, and o-aminothiophenolates. In the density functional framework, the singlet diradical character is discussed within the broken symmetry formalism. The singlet-triplet energy gaps, the energy gained from symmetry breaking, the spin distribution in the lowest triplet state, and the form of the magnetic orbitals are applied as indicators for the singlet diradical character. Moreover, a new index for the diradical character is proposed that is based on symmetry breaking. All symmetry breaking criteria show that the complexes obtained from o-catecholates and o-benzodithiolates have the largest and the smallest singlet diradical character, respectively. The singlet diradical character should be intermediate for the complexes derived from o-phenylenediamines, o-aminophenolates, and o-aminothiophenolates. The diradical character of all complexes suggests the presence of Ni(II) central atoms. This is also indicated by the d-populations computed by means of the natural population analysis.

Resonance Raman (RR) spectroscopy has been employed to study coordinated phenoxyl radicals (M = Ga, Sc, Fe) which were electrochemically generated in solution by using 1,4,7-triazacyclononane-based ligands containing one, two, or three p-methoxy or p-tert-butyl N-substituted phenolates, i.e., 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (3Lbut), 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane (3Lmet), 1,4-bis(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-7-ethyl-1,4,7-triazacyclononane (2Lmet), and 1-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-4,7-dimethyl-1,4,7-triazacyclononane (1Lmet). A selective enhancement of the vibrational modes of the phenoxyl chromophores is achieved upon excitation in resonance with the π → π* transition at ca. 410 nm. The interpretation of the spectra was supported by quantum chemical (density functional theory) calculations which facilitate the vibrational assignment for the coordinated phenoxyl radicals and provide the framework for correlations between the RR spectra and the structural and electronic properties of the radicals. For the uncoordinated phenoxyl radicals the geometry optimization yields a semiquinone character which increases from the unsubstituted to the p-methyl- and the p-methoxy-substituted radical. This tendency is indicated by a steady upshift of the ν8a mode which predominantly contains the Cortho−Cmeta stretching coordinate, thereby reflecting strengthening of this bond. The calculated normal-mode frequencies for these radicals are in a good agreement with the experimental data constituting a sound foundation for extending the vibrational analysis to the 2,6-di-tert-butyl-4-methoxyphenoxyl which is the building block of the macrocyclic ligands 3Lmet, 2Lmet, and 1Lmet. The metal-coordinated radical complexes reveal a similar band pattern as the free radicals with the modes ν8a and ν7a (CO stretching) dominating the RR spectra. These two modes are sensitive spectral indicators for the structural and electronic properties of the coordinated phenoxyl radicals. A systematic investigation of complexes containing different ligands and metal ions reveals that two parameters control the semiquinone character of the phenoxyls:  (i) an electron-donating substituent in the para position which can accept spin density from the ring and (ii) an electron-accepting metal ion capable of withdrawing excess electron density, introduced by additional electron-donating substituents in ortho positions. It appears that both effects, which are reflected by (i) the frequency of the mode ν8a and (ii) the frequency difference of the modes ν8a and ν7a, balance an optimum electron density distribution in the phenoxyl radical. Along similar lines, it has been possible to interpret the RR spectral changes between the Fe monoradical, [Fe(3Lmet)]+•, and diradical, [Fe(3Lmet)]2+••, complexes. Both the parent as well as the radical complexes of Fe exhibit a phenolate-to-iron charge transfer band >500 nm. Excitation in resonance with this transition yiel...

Hexadentate macrocyclic ligands containing a 1,4,7-triazacyclononane backbone and three N-bound pendent-arm phenolates form extremely stable neutral complexes with Fe III Cl 3 . The octahedral complexes [Fe III L] undergo three reversible one-electron oxidation processes to yield the mono-and dications, [FeL] and [FeL] 2 , which are stable in solution for hours, whereas the trications, [FeL] 3 , are only stable in solution on the time scale of a cyclic voltammetric experiment. These oxidations are shown to be ligand-rather than metal-centered. Three coordinated phenoxyl radicals are formed successively as shown conclusively by Mössbauer spectroscopy. The neutral, mono-, di-, and tricationic species each contain an octahedral, high-spin ferric ion (S Fe 5 2), which is intramolecularly, antiferromagnetically coupled to the spin (S 1 2) of the bound phenoxyl ligands to yield an S t 2 ground state for the monocation, and an S t 3 2 ground state for the dications as shown by EPR spectroscopy. The vibrations of the coordinated phenolate are observed by resonance Raman (RR) spectroscopy by excitation in resonance with the phenolate-to-iron charge-transfer (CT) transition above 500 nm or, alternatively, of the coordinated phenoxyl by excitation in resonance with the intraligand p 3p* transition at about 410 nm. Use of 18 O isotopomers selectively labeled at the phenolic oxygen allowed the identification of the C À O stretching and Fe À O stretching and bending modes. It is shown that the substitution pattern of phenolates and phenoxyls in their respective ortho and para positions and the charge of the complexes have a pronounced influence on the vibrational modes observed.

The photolysis of Fe(CO)3(η4-s-cis-1,3-butadiene) (1) and Fe(CO)4(η2-1,3-butadiene) (2), formerly studied in low-temperature matrixes, is reexamined in cyclohexane solution at ambient temperature using time-resolved IR spectroscopy in the ν(CO) region. Flash photolysis of 2 (λexc = 308 nm) generates Fe(CO)3(η4-s-trans-1,3-butadiene) (5) as a transient product, which then rearranges to form the classical η4-s-cis-1,3-butadiene complex 1. Species 5, previously addressed as the coordinately unsaturated Fe(CO)3(η2-1,3-butadiene) (3), is also photogenerated from 1, in this case along with the very short-lived CO loss fragment Fe(CO)2(η4-1,3-butadiene) (τ < 4 μs under CO atmosphere). It decays by temperature-dependent first-order kinetics (τ = 13 ms at 25 °C; ΔH ⧧ = 17.3 kcal·mol-1) with nearly complete recovery of 1. According to density functional calculations at the BP86 level of theory, 5 resides in a distinct energy minimum, 20.3 kcal·mol-1 above 1 and separated from it by a barrier of 15.0 kcal·mol-1. Its computed structure involves a diene dihedral angle of 129°. Species 3 (with a diene dihedral angle of −150.1°), by contrast, is predicted to exist in a rather flat minimum, which makes it too short-lived for detection with our instrumentation. Flash photolysis of Fe(CO)5 generates the very short-lived (<1 μs) doubly unsaturated Fe(CO)3(solv) species in addition to the familiar Fe(CO)4(solv) fragment (τ = 10−15 μs), Fe2(CO)9 being the ultimate product in the absence of potential trapping agents other than CO. Deliberate contamination of the system with water gives rise to the formation of Fe(CO)4(H2O) as a longer lived transient (ca. 1 ms). In the presence of 1,3-butadiene, both 2 and 5 appear almost instantaneously. The latter decays, again in the millisecond time range, with formation of 1, thus providing clear evidence of a single-photon route from Fe(CO)5 to 1 in addition to the established two-photon sequence via the monosubstituted complex 2.

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Seven new bis(o-iminosemiquinonato)copper(II) complexes, 1- 5, 1a, 1b, derived from differently substituted N-phenyl-2-aminophenol-based ligands, are described. Their crystal structures were determined by X-ray diffraction, and their electronic structures were established by various physical methods including electron paramagnetic resonance and variable-temperature (2-290 K) susceptibility measurements. Like complex 6, which was reported recently by us, all complexes exhibit an S t = (1)/ 2 ground state, based on the "isolated" copper(II)-spin character resulting from the dominating antiferromagnetic spin coupling between the two radicals; the ground-state electronic configuration can thus be designated as (increasing, increasing, decreasing)[R-Cu-R]. In addition, broken spin symmetry density functional solutions have been obtained. From the set of unrestricted canonical Kohn-Sham orbitals, the magnetic orbitals have been identified. The identification procedure is based on the nonvanishing overlap integrals between the space parts of orbitals occupied by electrons of opposite spin. The theoretically determined magnetic orbitals support the spin configurations suggested by the experiments. Electrochemical measurements (cyclic voltammetry and square-wave voltammetry) indicate ligand-centered redox processes. Complex 1 is found to be the best catalyst among the Cu(II) complexes for oxidation of primary alcohols with aerial oxygen as the sole oxidant to afford aldehydes under mild conditions. Thus, the function of the copper-containing enzyme Galactose Oxidase has been mimicked. Kinetic measurements in conjunction with electron paramagnetic resonance and electronic spectral studies have been used to decipher the catalytic oxidation process. A ligand-derived redox activity has been proposed as a mechanism for the aerial oxidation of primary alcohols.

Hydroxymethyl radical • CH 2 OH and its radical anion • CH 2 O -(pK a 10.7) were generated pulse-radiolytically in N 2 O-saturated aqueous solutions of methanol. The overall decay is by second-order kinetics. At pH e 8 ( • CH 2 OH, 2k ) 1.7 × 10 9 dm 3 mol -1 s -1 ), one observes ethylene glycol and formaldehyde plus methanol in a disproportionation/recombination ratio of 0.17. At pH g 12 ( • CH 2 O -) the rate constant is 2k ) 0.5 × 10 9 dm 3 mol -1 s -1 . From the pK a and the dependence of the rate of bimolecular decay on pH, a mixed-termination rate constant k( • CH 2 OH + • CH 2 O -) ) 1.2 × 10 9 dm 3 mol -1 s -1 is calculated. With increasing pH, the yield of ethylene glycol decreases while that of formaldehyde (plus methanol) shows a corresponding increase. Ethylene glycol is no longer formed at pH g 11.3. Quantum-chemical calculations indicate that • CH 2 Opossesses considerable spin density also at oxygen (mesomeric structure: -CH 2 O • ). Since in their bimolecular termination reactions the • CH 2 Oradicals can no longer disproportionate by a straightforward H atom transfer, it is concluded either that termination must occur by a C-O type recombination, giving rise to the (unstable) hemiacetal, CH 3 OCH 2 OH, or that the disproportionation reaction is water-assisted. The • CH 2 Oradical reduces N 2 O (k ) 350 dm 3 mol -1 s -1 ); this gives rise to a chain reaction at the low (compared to pulse radiolysis) dose rates of γ-radiolysis (0.02-2 Gy s -1 ).

We devise and apply a simple computational scheme for obtaining localized bonding schemes and their weights from a CASSCF wave function. These bonding schemes are close to resonance structures drawn by chemists. Firstly, a CASSCF wave function is computed. Secondly, the CASSCF computation is repeated but now the delocalized complete active space MOs are substituted by Weinhold's localized natural atomic orbitals. In this way the original CASSCF wave function is represented by a sequence of Slater determinants composed of localized natural atomic orbitals. Thus, a standard CASSCF wave function can be reinterpreted in terms of a local picture. To test the method we obtain localized bonding schemes and their weights for the ground and the pi-pi* excited state of ethylene. Moreover, bonding schemes and their weights are derived for the ground, the 1(1)B(u), and the 2(1)Ag pi-pi* excited states of trans-butadiene. The large weight bonding schemes are shown to be a qualitative indicator for the known photochemistry of butadiene. The remarkable stability of the Arduengo carbene is discussed by obtaining bonding schemes that indicate a stabilizing delocalization of the pi electrons. We illustrate that the large weight bonding schemes are in line with the observed reactivity of the Arduengo carbene.