Tanya K. Todorova

The Ru-Hbpp water oxidation catalyst.

A thorough characterization of the Ru-Hbpp (in,in-{[Ru(II)(trpy)(H(2)O)](2)(mu-bpp)}(3+) (trpy is 2,2:6,2-terpyridine, bpp is bis(2-pyridyl)-3,5-pyrazolate)) water oxidation catalyst has been carried out employing structural (single crystal X-ray), spectroscopic (UV-vis and NMR), kinetic, and electrochemical (cyclic voltammetry) analyses. The latter reveals the existence of five different oxidation states generated by sequential oxidation of an initial II,II state to an ultimate, formal IV,IV oxidation state. Each of these oxidation states has been characterized by UV-vis spectroscopy, and their relative stabilities are reported. The electron transfer kinetics for individual one-electron oxidation steps have been measured by means of stopped flow techniques at temperatures ranging from 10 to 40 degrees C and associated second-order rate constants and activation parameters (DeltaH() and DeltaS()) have been determined. Room-temperature rate constants for substitution of aqua ligands by MeCN as a function of oxidation state have been determined using UV-vis spectroscopy. Complete kinetic analysis has been carried out for the addition of 4 equiv of oxidant (Ce(IV)) to the initial Ru-Hbpp catalyst in its II,II oxidation state. Subsequent to reaching the formal oxidation state IV,IV, an intermediate species is formed prior to oxygen evolution. Intermediate formation and oxygen evolution are both much slower than the preceding ET processes, and both are first order with regard to the catalyst; rate constants and activation parameters are reported for these steps. Theoretical modeling at density functional and multireference second-order perturbation theory levels provides a microscopic mechanism for key steps in intermediate formation and oxygen evolution that are consistent with experimental kinetic data and also oxygen labeling experiments, monitored via mass spectrometry (MS), that unambiguously establish that oxygen-oxygen bond formation proceeds intramolecularly. Finally, the Ru-Hbpp complex has also been studied under catalytic conditions as a function of time by means of manometric measurements and MS, and potential deactivation pathways are discussed.

The cis‐[RuII(bpy)2(H2O)2]2+ Water‐Oxidation Catalyst Revisited

The only operating mechanism in the oxidation of water to dioxygen catalyzed by the mononuclear cis-[RuII(bpy)2(H2O)2]2+ complex when treated with excess CeIV was unambiguously established. Theoretical calculations together with 18O-labeling experiments (see plot) revealed that it is the nucleophilic attack of water on a Ru=O group.

On the Analysis of the Cr−Cr Multiple Bond in Several Classes of Dichromium Compounds

Since the discovery of a formal quintuple bond in ArCrCrAr (CrCr = 1.835 A) by Power and co-workers in 2005, many efforts have been dedicated to isolating dichromium species featuring quintuple-bond character. In the present study we investigate the electronic configuration of several, recently synthesized dichromium species with ligands using nitrogen to coordinate the metal centers. The bimetallic bond distances of Powers compound and Cr(2)-diazadiene (1) (CrCr = 1.803 A) are compared to those found for Cr(2)(mu-eta(2)-ArNC(R)NAr)(2) (2) (CrCr = 1.746 A; R = H, Ar = 2,6-Et(2)C(6)H(3)), Cr(2)(mu-eta(2)-Ar(Xyl)NC(H)NAr(Xyl))(3) (3) (CrCr = 1.740(reduced)/1.817(neutral) A; Ar(Xyl)= 2,6-C(6)H(3)-(CH(3))(2)), Cr(2)(mu-eta(2)-TippPyNMes)(2) (4) (CrCr = 1.749 A; TippPyNMes = 6-(2,4,6-triisopropylphenyl)pyridin-2-yl (2,4,6-trimethylphenyl)amide), and Cr(2)(mu-eta(2)-DippNC(NMe(2))N-Dipp)(2) (5) (CrCr = 1.729 A, Dipp = 2,6-i-Pr(2)C(6)H(3)). We show that the correlation between the CrCr bond length and the effective bond order (EBO) is strongly affected by the nature of the ligand, as well as by the steric hindrance due to the ligand structure (e.g., the nature of the coordinating nitrogen). A linear correlation between the EBO and CrCr bond distance is established within the same group of ligands. As a result, the CrCr species based on the amidinate, aminopyridinate, and guanidinate ligands have bond patterns similar to the ArCrCrAr compound. Unlike these latter species, the dichromium diazadiene complex is characterized by a different bonding pattern involving Cr-Npi interactions, resulting in a lower bond order associated with the short metal-metal bond distance. In this case the short CrCr distance is most probably the result of the constraints imposed by the diazadiene ligand, implying a Cr(2)N(4) core with a closer CrCr interaction.

Synthesis and properties of a fifteen-coordinate complex: The thorium aminodiboranate [Th(H3BNMe2BH3)4]

The concept of coordination number is extremely useful and is widely employed to describe the local chemical environments of atoms. Originally defined by Alfred Werner in 1893, the coordination number is closely related to many other important properties such as atomic radius, molecular and electronic structure, and chemical reactivity. An important modification of Werner s original concept was the recognition that, for certain ligands such as ethylene, two linked atoms jointly occupy a single coordination site. This modified definition is widely used to describe both transitionmetal (d-block) and inner-transition-metal (f-block) complexes. An interesting question is: what is the largest possible coordination number? This question has recently been considered theoretically, and the 15-coordinate ion PbHe15 2+ has been predicted to be a bound species. The complexes tetrakis(cyclopentadienyl)uranium [UCp4] and its thorium analogue [ThCp4] are each connected to 20 atoms, [10] but the Werner coordination number of 12 (counting p bonds as occupying one site) is widely acknowledged to be more appropriate to describe the metal–ligand bonding in these compounds. Very high Werner coordination numbers are seen for metal complexes of the borohydride anion BH4 , which can coordinate to a single metal through as many as three hydrogen atoms. From an electronic perspective, each B-H-M interaction involves a separate electron pair, 14] and each BH-M interaction can be considered as a separate bond. Accordingly, [Zr(BH4)4], [15–17] [Hf(BH4)4], [15,16, 18] [Np(BH4)4], [19] and [Pu(BH4)4], [19] all have coordination numbers of 12, and [Th(BH4)4], [15, 16] [Pa(BH4)4], [19] and [U(BH4)4], [20] all of which are polymers in the solid state, have coordination numbers of 14. Some derivatives of these compounds also have high coordination numbers, such as the 14-coordinate tetrahydrofuran complex [U(BH4)4(thf)2]. [21] No complex of any kind, however, has been definitively shown to adopt a Werner coordination number higher than 14. Herein, we report the synthesis, single-crystal X-ray and neutron diffraction studies, and DFT investigations of the first 15-coordinate complex. DFT calculations suggest that this complex may adopt a 16-coordinate structure in the gas phase. This compound extends our recent studies of a new class of chelating borohydride ligands, that is, the aminodiboranates, some of which form highly volatile complexes that are useful as precursors for the chemical vapor deposition of thin films. Reaction of ThCl4 with four equivalents of sodium N,Ndimethylaminodiboranate, Na(H3BNMe2BH3), in tetrahydrofuran produced [Th(H3BNMe2BH3)4] (1), which could be isolated as colorless prisms by crystallization from diethyl ether. The IR spectrum of 1 contains strong bands at 2420 cm 1 that arise from terminal B–H stretches, and at 2264 and 2208 cm 1 that arise from bridging B-H···Th stretches. For comparison, [Th(BH4)4] contains a strong terminal B–H band at 2530 cm 1 and bridging B-H-M bands at 2270, 2200, and 2100 cm . The H NMR spectrum of 1 (C6D6 at 20 8C) contains peaks at d = 2.11 ppm (s, NMe2) and d = 4.23 ppm (br 1:1:1:1 q, JBH = 90 Hz, BH3); the terminal and bridging B–H units thus exchange rapidly on the NMR time scale. The B NMR spectrum consists of a binomial quartet at d = 2.75 ppm, which arises from coupling of the B nuclei with the three rapidly exchanging H nuclei (JHB = 90 Hz). For comparison, the B spectrum of [Th(BH4)4] consists of a quintet at d = 8.0 ppm (JBH = 86.5 Hz). Single-crystal X-ray and neutron diffraction studies of 1 reveal that it is monomeric with four chelating aminodiboranate ligands. The eight boron atoms describe a distorted D2d dodecahedral structure, in which boron atoms B1, B2, B2A, and B1A describe one planar trapezoidal array, and atoms B3, B4, B5, and B6 describe the other (Figure 1). The B2-Th1B2A and B4-Th1-B6 angles between wingtip boron atoms are almost linear at 172.61(12)8 and 171.85(13)8, respectively. Interestingly, seven of the eight Th···B distances (those for boron atoms B1–B5) range from 2.882(3) to 2.949(3) , but [*] S. R. Daly, Prof. G. S. Girolami School of Chemical Sciences University of Illinois at Urbana-Champaign 600 South Matthews Avenue, Urbana, IL 61801 (USA) Fax: (+ 1)217-244-3186 E-mail: girolami@scs.illinois.edu

Systematic truncation of the virtual space in multiconfigurational perturbation theory

A method is suggested which allows truncation of the virtual space in Cholesky decomposition-based multiconfigurational perturbation theory (CD-CASPT2) calculations with systematic improvability of the results. The method is based on a modified version of the frozen natural orbital (FNO) approach used in coupled cluster theory. The idea is to exploit the near-linear dependence among the eigenvectors of the virtual-virtual block of the second-order Moller-Plesset density matrix. It is shown that FNO-CASPT2 recovers more than 95% of the full CD-CASPT2 correlation energy while requiring only a fraction of the total virtual space, especially when large atomic orbital basis sets are in use. Tests on various properties commonly investigated with CASPT2 demonstrate the reliability of the approach and the associated reduction in computational cost and storage demand of the calculations.

The ligand-based quintuple bond-shortening concept and some of its limitations

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a Cr-Cr distance of 1.7056 (12)u2005Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4)u2005Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr(I) dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.

Understanding, Controlling and Programming Cooperativity in Self-Assembled Polynuclear Complexes in Solution

Deviations from statistical binding, that is cooperativity, in self-assembled polynuclear complexes partly result from intermetallic interactions DeltaE(M,M), whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point-charge model, which suggests severe and counter-intuitive deviations from predictions based solely on the Coulomb law when considering the variation of DeltaE(M,M) with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty-nine homometallic and heterometallic polynuclear triple-stranded helicates obtained from the coordination of the segmental ligands L1-L11 with Zn(2+) (a spherical d-block cation) and Lu(3+) (a spherical 4f-block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance (DeltaE(1-4)(Lu,Lu)>DeltaE(1-2)(Lu,Lu)), ii) of larger magnitude when Zn2+ replaces Lu3+ (DeltaE(1-2)(Zn,Lu)>DeltaE(1-2)(Lu,Lu) and iii) attractive between two triply charged cations held at some specific distance (DeltaE(1-3)(Lu,Lu)<0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.

DFT and CASPT2 Analysis of Polymetallic Uranium Nitride and Oxide Complexes: How Theory Can Help When X-Ray Analysis Is Inadequate

Recent studies of organouranium chemistry have provided unusual pairs of similar polymetallic molecules containing (N)(3-) and (O)(2-) ligands, namely [(C(5)Me(5))U(mu-I)(2)](3)(mu(3)-N), 1, and [(C(5)Me(5))U(mu-I)(2)](3)(mu(3)-O), 2, and chair and boat conformations of [(C(5)Me(5))(2)U(mu-N)U(mu-N(3))(C(5)Me(5))(2)](4), 3. These compounds were analyzed by density functional theory and multiconfigurational quantum chemical studies to differentiate nitride versus oxide in molecules for which the crystallographic data were not definitive and to provide insight into the electronic structure and unique chemical bonding of these polymetallic compounds. Calculations were also performed on [(C(5)Me(5))(2)UN(3)(mu-N(3))](3), 4, and [(C(6)F(5))(3)BNU(N[Me]Ph)(3)], 5, for comparison with 1 and 3. On the basis of these results, the complex, [(C(5)Me(5))U(mu(3)-E)](8), 6, for which only low-quality X-ray crystallographic data are available, was analyzed to predict if E is nitride or oxide.

Structural, spectroscopic, and multiconfigurational quantum chemical investigations of the electron-rich metal-metal triple-bonded Tc(2)X(4)(PMe(3))(4) (X = Cl, Br) complexes

The compounds Tc(2)Cl(4)(PMe(3))(4) and Tc(2)Br(4)(PMe(3))(4) were formed from the reaction between (n-Bu(4)N)(2)Tc(2)X(8) (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV-visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc(2)Cl(4)(PMe(3))(4) crystallizes in the monoclinic space group C2/c [a = 17.9995(9) A, b = 9.1821(5) A, c = 17.0090(9) A, beta = 115.4530(10) degrees ] and is isostructural to M(2)Cl(4)(PMe(3))(4) (M = Re, Mo, W) and to Tc(2)Br(4)(PMe(3))(4). The metal-metal distance (2.1318(2) A) is similar to the one found in Tc(2)Br(4)(PMe(3))(4) (2.1316(5) A). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc(2)X(8)(2-) and Tc(2)X(4)(PMe(3))(4) (X = Cl, Br) indicate stronger pi bonds in the Tc(2)(4+) core than in Tc(2)(6+) core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10 000-26 000 cm(-1). Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the delta* --> sigma* transition, the difference between calculated and experimental values being 228 cm(-1) for X = Cl and 866 cm(-1) for X = Br. The next bands are attributed to delta* --> pi*, delta --> sigma*, and delta --> pi* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc(2)Br(4)(PMe(3))(4) exhibits more positive oxidation potentials than Tc(2)Cl(4)(PMe(3))(4.) This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing pi-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.

Theoretical Study of the Gas-Phase Chemiionization Reactions La + O and La + O2

The La + O and La + O 2 chemiionization reactions have been investigated with quantum chemical methods. For La + O 2(X (3)Sigma g) and La + O 2(a (1)Delta g), the chemiionization reaction La + O 2 --> LaO 2 (+) + e (-) has been shown to be endothermic and does not contribute to the experimental chemielectron spectra. For the La + O 2(X (3)Sigma g) reaction conditions, chemielectrons are produced by La + O 2 --> LaO + O, followed by La + O --> LaO (+) + e (-). This is supported by the same chemielectron band, arising from La + O --> LaO (+) + e (-), being observed from both the La + O( (3)P) and La + O 2(X (3)Sigma g) reaction conditions. For La + O 2(a (1)Delta g), a chemielectron band with higher electron kinetic energy than that obtained from La + O 2(X (3)Sigma g) is observed. This is attributed to production of O( (1)D) from the reaction La + O 2(a (1)Delta g) --> LaO + O( (1)D), followed by chemiionization via the reaction La + O( (1)D) --> LaO (+) + e (-). Potential energy curves are computed for a number of states of LaO, LaO* and LaO (+) to establish mechanisms for the observed La + O --> LaO (+) + e (-) chemiionization reactions.