Andrew Kerridge

Molecular Thin Films: A New Type of Magnetic Switch

The magnetic coupling of flexible metal phthalocyanine (MPc) thin films can be modified depending on the polymorphic form adopted by the crystals. A simple annealing procedure can switch the couplings from antiferromagnetic to ferromagnetic (MnPc) or paramagnetic (CuPc), opening up avenues for spintronic applications. Density functional and perturbation theories rationalize these trends and attribute the coupling mechanism to indirect exchange.

Is cerocene really a Ce(III) compound? All-electron spin-orbit coupled CASPT2 calculations on M(eta(8)-C8H8)2 (M = Th, Pa, Ce).

Spin-orbit free CASPT2 wave functions and energies are presented for the ground and 31 excited states of three f element sandwich molecules; thorocene (ThCOT(2)), protactinocene (PaCOT(2)), and cerocene (CeCOT(2)). Ground-state metal-ring centroid distances are optimized at this level and show excellent agreement with experiment. The effects of spin-orbit coupling are included and are found to be negligible for the ground states of ThCOT(2) and CeCOT(2), for which comparison of the electronic excitation energies is made with experimental data. For PaCOT(2), spin-orbit coupling is found to alter significantly the energies and nature of the ground and low-lying excited states, and good agreement is obtained with previous computational data. The ground state of CeCOT(2) is found to be strongly multiconfigurational, though not in the same way as previously reported. The relationship of this result to previous computational and experimental data is discussed, as is the most appropriate way to view the electronic structure of CeCOT(2). It is concluded that the occupations of the natural orbitals produce a more reliable description of the CeCOT(2) ground state than does the configurational admixture.

f-Orbital covalency in the actinocenes (An = Th-Cm) : multiconfigurational studies and topological analysis

The CASSCF methodology is used to calculate the ground state electron densities of a series of seven actinocenes, AnCOT2 (An = Th–Cm, COT = η8-C8H8). The multiconfigurational character of these complexes is found to be substantial and topological analysis of the electron density via the QTAIM approach is therefore chosen in order to investigate the electronic structure in more detail. Topological analysis reveals increased values of the electron density at the An–C bond critical point for An = Pa–Pu, suggesting enhanced covalent character in metal–ligand bonding for these complexes. In order to investigate the origins of this covalency, integrated one- and two-electron properties are evaluated. A trend for increased electronic charge, spin density and electron localisation on the An centre as one traverses the actinide series is found. The difference between atomic number and the electron localisation index is considered and found to correlate well with the expected oxidation state in these complexes, with a tendency towards trivalent character for the later actinides. Total and orbitally resolved delocalisation indices are evaluated, and increased electron delocalisation is found for the complexes containing Pa–Pu centres. It is shown that, while 5f contributions to covalency in these complexes are smaller in magnitude than 6d contributions, the variation in covalency is almost entirely accounted for by the variation in the 5f contribution.

Are the ground states of the later actinocenes multiconfigurational? : all-electron spin-orbit coupled CASPT2 calculations on An(eta(8)-C8H8)(2) (An = Th, U, Pu, Cm)

Spin-orbit free and spin-orbit coupled CASPT2 wave functions and energies are presented for the ground and low-lying excited states of four actinide element sandwich molecules; thorocene (ThCOT(2)), uranocene (UCOT(2)), plutonocene (PuCOT(2)), and curocene (CmCOT(2)). Spin-orbit coupling is found to make little difference to the equilibrium geometry of uranocene and plutonocene but has a significant effect on the energy spectrum of all the systems considered here other than thorocene. In all cases, however, the spin-orbit free ground states make the dominant contribution to their spin-orbit coupled counterparts. Following work presented in J. Phys. Chem. A 2009, 113, 2896, the variation in the multiconfigurational character of the ground-state wave functions as the 5f series is crossed is quantified via the occupation of the a(u)/b(1u) (e(2u)) metal-ring bonding and antibonding natural orbitals. The ground state of plutonocene is found to be nondegenerate with |M(J)| = 0, in agreement with its temperature-independent paramagnetism.

The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes

Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal–ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle.

Structure-dependent exchange in the organic magnets Cu(II)Pc and Mn(II)Pc

We study exchange couplings in the organic magnets copper(II) phthalocyanine [Cu(II)Pc] and manganese(II) phthalocyanine [Mn(II)Pc] by a combination of Greens function perturbation theory and ab initio density-functional theory (DFT). Based on the indirect exchange model, our perturbation-theory calculation of Cu(II)Pc qualitatively agrees with the experimental observations. DFT calculations performed on Cu(II)Pc dimer show a very good quantitative agreement with exchange couplings that our theoretical group extracts by using a global fitting for the magnetization measurements to a spin-1/2 Bonner-Fisher model. These two methods give us remarkably consistent trends for the exchange couplings in Cu(II)Pc when changing the stacking angles. The situation is more complex for Mn(II)Pc owing to the competition between superexchange and indirect exchange.

Actinide covalency measured by pulsed electron paramagnetic resonance spectroscopy

Our knowledge of actinide chemical bonds lags far behind our understanding of the bonding regimes of any other series of elements. This is a major issue given the technological as well as fundamental importance of f-block elements. Some key chemical differences between actinides and lanthanides-and between different actinides-can be ascribed to minor differences in covalency, that is, the degree to which electrons are shared between the f-block element and coordinated ligands. Yet there are almost no direct measures of such covalency for actinides. Here we report the first pulsed electron paramagnetic resonance spectra of actinide compounds. We apply the hyperfine sublevel correlation technique to quantify the electron-spin density at ligand nuclei (via the weak hyperfine interactions) in molecular thorium(III) and uranium(III) species and therefore the extent of covalency. Such information will be important in developing our understanding of the chemical bonding, and therefore the reactivity, of actinides.

U−Oyl stretching vibrations as a quantitative measure of the equatorial bond covalency in uranyl complexes:a quantum-chemical investigation

The molecular structures of a series of uranyl (UO2(2+)) complexes in which the uranium center is equatorially coordinated by a first-row species are calculated at the density functional theory level and binding energies deduced. The resulting electronic structures are investigated using a variety of density-based analysis techniques in order to quantify the degree of covalency in the equatorial bonds. It is shown that a consideration of the properties of both the one-electron and electron-pair densities is required to understand and rationalize the variation in axial bonding effected by equatorial complexation. Strong correlations are found between density-based measures of the covalency and equatorial binding energies, implying a stabilizing effect due to covalent interaction, and it is proposed that uranyl U-Oyl stretching vibrational frequencies can serve as an experimental probe of equatorial covalency.

Yttrium Complexes of Arsine, Arsenide, and Arsinidene Ligands

Deprotonation of the yttrium–arsine complex [Cp′3Y{As(H)2Mes}] (1) (Cp′=η5-C5H4Me, Mes=mesityl) by nBuLi produces the μ-arsenide complex [{Cp′2Y[μ-As(H)Mes]}3] (2). Deprotonation of the As–H bonds in 2 by nBuLi produces [Li(thf)4]2[{Cp′2Y(μ3-AsMes)}3Li], [Li(thf)4]2[3], in which the dianion 3 contains the first example of an arsinidene ligand in rare-earth metal chemistry. The molecular structures of the arsine, arsenide, and arsinidene complexes are described, and the yttrium–arsenic bonding is analyzed by density functional theory.

Dithio- and Diselenophosphinate Thorium(IV) and Uranium(IV) Complexes: Molecular and Electronic Structures, Spectroscopy, and Transmetalation Reactivity

We report a comparison of the molecular and electronic structures of dithio- and diselenophosphinate, (E2PR2)(1-) (E = S, Se; R = (i)Pr, (t)Bu), with thorium(IV) and uranium(IV) complexes. For the thorium dithiophosphinate complexes, reaction of ThCl4(DME)2 with 4 equiv of KS2PR2 (R = (i)Pr, (t)Bu) produced the homoleptic complexes, Th(S2P(i)Pr2)4 (1S-Th-(i)Pr) and Th(S2P(t)Bu2)4 (2S-Th-(t)Bu). The diselenophosphinate complexes were synthesized in a similar manner using KSe2PR2 to produce Th(Se2P(i)Pr2)4 (1Se-Th-(i)Pr) and Th(Se2P(t)Bu2)4 (2Se-Th-(t)Bu). U(S2P(i)Pr2)4, 1S-U-(i)Pr, could be made directly from UCl4 and 4 equiv of KS2P(i)Pr2. With (Se2P(i)Pr2)(1-), using UCl4 and 3 or 4 equiv of KSe2P(i)Pr2 yielded the monochloride product U(Se2P(i)Pr2)3Cl (3Se-U(iPr)-Cl), but using UI4(1,4-dioxane)2 produced the homoleptic U(Se2P(i)Pr2)4 (1Se-U-(i)Pr). Similarly, the reaction of UCl4 with 4 equiv of KS2P(t)Bu2 yielded U(S2P(t)Bu2)4 (2S-U-(t)Bu), whereas the reaction with KSe2P(t)Bu2 resulted in the formation of U(Se2P(t)Bu2)3Cl (4Se-U(tBu)-Cl). Using UI4(1,4-dioxane)2 and 4 equiv of KSe2P(t)Bu2 with UCl4 in acetonitrile yielded U(Se2P(t)Bu2)4 (2Se-U-(t)Bu). Transmetalation reactions were investigated with complex 2Se-U-(t)Bu and various CuX (X = Br, I) salts to yield U(Se2P(t)Bu2)3X (6Se-U(tBu)-Br and 7Se-U(tBu)-I) and 0.25 equiv of [Cu(Se2P(t)Bu2)]4 (8Se-Cu-(t)Bu). Additionally, 2Se-U-(t)Bu underwent transmetalation reactions with Hg2F2 and ZnCl2 to yield U(Se2P(t)Bu2)3F (6) and U(Se2P(t)Bu2)3Cl (4Se-U(tBu)-Cl), respectively. The molecular structures were analyzed using (1)H, (13)C, (31)P, and (77)Se NMR and IR spectroscopy and structurally characterized using X-ray crystallography. Using the QTAIM approach, the electronic structure of all homoleptic complexes was probed, showing slightly more covalent bonding character in actinide-selenium bonds over actinide-sulfur bonds.