Thomas A. Albright

Theoretical aspects of the coordination of molecules to transition metal centers

Abstract When bound in transition metal complexes, organic molecules such as olefins, acetylenes, polyenes, cyclopolyenes and carbenes exhibit a variety of equilibrium geometries. The range of observed barriers to a conformational change as simple as rotation around the metal-ligand coordination axis is impressively large, from 0 to > 40 kcal/mole. Clearly it is electronic factors that are operative in setting the preferred geometries in these molecules and in controlling the magnitude of the conformational barriers. In this lecture several specific examples of the theory of these conformational phenomena will be presented.

The structure of trimethylenemethane-ML2 complexes and Their relationship to cycloaddition reactions

Abstract An electronic reason is offered why d 10 and d 8 trimethylenemethane-ML 2 complexes should be η 3 rather than η 4 . This was checked by molecular orbital calculations of the extended Huckel type. Both sets of complexes can be regarded as zwitterionic with the uncomplexed methylene group for the d 10 compounds anionic and cationic for the d 8 analogs. In both cases, interconversion between the three equivalent η 3 geometries does not proceed via an η 4 structure. We find that η 2 geometries serve as way-points in this fluxional rearrangement. Implications are also drawn to the mode of cycloaddition of methylenecyclopropanes to electron deficient olefins by d 10 ML n catalysis and ring-opening of the methylenecyclopropanes.

The molecular structures of pyridine-chromiumpentacarbonyl and bis(pyridine)-chromiumtetracarbonyl

Abstract The structures of pyridinechromiumpentacarbonyl, (1), and bis(pyridine)chromiumtetracarbonyl,(2) have been determined. (1) crystallizes in the space group Pbam with a = 15.289(3) A, b = 19.276(5) A and c = 7.677(6) A. (2) crystallizes in the space group P 1 with a = 7.365(2) A, b = 8.136(2) A, c = 13.491(4) A, α = 89.49(2)°, β = 88.89(2)°, and γ = 63.09(2)°. The structures refined to R w values of 0.020 and 0.034 for (1) and (2), respectively. In both cases the pyridine rings are planar and stagger the cis CrCO bonds. A comparison of the structural results from these two compounds to piperidinechromiumpentacarbonyl and Cr(CO) 6 seems to indicate that the pyridine ligand is a weaker σ-donor and stronger π-acceptor than the saturated analog, piperidine.

Diamagnetic Group 6 Tetrakis(di-tert-butylketimido)metal(IV) Complexes

The addition of 4 equiv of LiN=C-t-Bu(2) to CrCl(3), MoCl(5), and WCl(6) in diethyl ether produced the complexes M(N=C-t-Bu(2))(4) (M = Cr, Mo, W). Single-crystal X-ray diffraction studies revealed that the molecules have flattened tetrahedral geometries with virtual D(2d) symmetry in the solid state. (1)H and (13)C NMR spectra indicated that the complexes are diamagnetic, and a qualitative MO analysis showed that the orthogonal π-donor and -acceptor orbitals of the ketimide ligand cooperatively split the d(xy) and d(z2) orbitals sufficiently to allow spin pairing in the d(xy) orbital. A more sophisticated quantum-mechanical analysis of Cr(N=C-t-Bu(2))(4) using density functional/molecular mechanics methods confirmed the qualitative analysis by showing that the singlet state is 27 kcal/mol more stable than the triplet state.

Photo–Thermal Haptotropism in Cyclopentadienylcobalt Complexes of Linear Phenylenes: Intercyclobutadiene Metal Migration†

An attractive strategy for achieving solar–thermal energy conversion is to harvest sunlight in the form of activated chemical bonds through photoisomerization of a suitable molecule that can release, on demand, such stored and transportable energy by thermal reversal to its original form. When such reversible isomerizations entail significant topological alterations, they provide blueprints for eliciting further functions, for example in switches, machines, datastorage, sensors, and other devices. Because of their expanded tunability and generally favourable electronic absorption regimes, organometallic complexes are advantageous in this respect, yet have remained relatively unexplored. Among them, topologically simplest are metallohaptotropic arrays in which a single metal moiety photomigrates, thermally reversibly, to a higher-energy position along a fused p framework, without the assistance of additives. Only two such systems are known, [Mo(PMe3)3] complexes of indole and quinazoline, discovered as part of a study focusing on catalytic hydrogenations of heterocycles. We report 1) the photothermal reversibility of {CpCo} complexes of linear phenylenes by a novel mode of haptotropism, namely, h:h from one cyclobutadiene ring to another (Scheme 1); 2) the first X-ray structures of metalated linear phenylenes, illustrating the aromatization of the ligand on

Theoretical studies on the stabilities of 1,2- and 1,3-isomers of cyclic H4A2X2 (AC, Si; XO,S,CH2): the anomeric effect at silicon

Abstract 1,2- and 1,3-isomers of dioxetanes, dithietanes, and their silicon analogues ( 1, 2 ) have been studied by Molecular Orbital methods (using both HF/3–21G and MNDO). The 1,3-isomers ( 2 ) are found to be more stable than their 1,2-counterparts ( 1 ) except for dithietane where the energy difference is negligible. The relative energies of the 1,2- and 1,3-isomers are also estimated using bond energy differences (eqn. 1) at the 3–21G level on models. These are corrected for the anomeric effect (eqn. 2) present in the 1,3-isomer and compared to the 3–21G estimate of the relative energies of 1 and 2 . The discrepancy between the two estimates is equated to the strain energy difference between the two four-membered rings. This is found to be maximum for dioxetane and disilacyclobutane. The anomeric effect at silicon (10.0 kcal mol −1 at the 3–21G level for oxygen substitution) is smaller than that at carbon (17.4 kcal mol −1 at the same level) for the same substituent.

The interaction of O2− with water

Abstract Ab initio calculations at the 4–31 + G level have been carried out for the interaction of F − , O 2 − , and Cl − with H 2 O. Computation of the binding energies gave reasonable agreement with hydration energies. The O 2 − (H 2 O) system was also investigated at the 6-31 + G ★★ level including Moller-Plesset perturbation theory to second order. The strength of O 2 − binding to H 2 O is intermediate to that of F − and Cl − with H 2 O.

Reversible intercyclobutadiene haptotropism in cyclopentadienylcobalt linear [4]phenylene

Cyclopentadienylcobalt complexes of linear [4]phenylene undergo thermally reversible photoinduced metallahaptotropism between the inner and outer cyclobutadiene ring.

Hydrogen site exchange in CpIr(PR3)H3+ complexes

Abstract Ab initio molecular orbital calculations on CpIr(PH 3 )H 3 + yield an optimized structure for the ground state which is very close to the neutron structure of a related complex. The hydrogen site exchange process is computed to occur with an activation energy of 17.0 kcal mol −1 via an η 2 -H 2 species which constitutes a pairwise exchange mechanism in agreement with the NMR lineshape analysis of [(η-C 5 R 5 )Ir(L)H 3 ]BF 4 (RH, Me; L = various phosphines). No evidence was found for an ‘open’ or ‘closed’ η 3 -H 3 isomer as a stationary point on the potential energy surface, which would correspond to a random exchange mechanism. The computed energy differences between hypothetical η 3 -H 3 geometries and the ground state were found to be prohibitively high. Kinetic isotope effect calculations performed on partially deuterated analogs are in agreement with experimental data and confirm the viability of an η 2 -H 2 species as the transition state.

A terminal, fluxional η4-benzene complex with a thermally accessible triplet state is the primary photoproduct in the intercyclobutadiene haptotropism of (CpCo)phenylenes.

Low-temperature irradiation of linear [3]- and [4]phenylene cyclopentadienylcobalt complexes generates labile, fluxional η(4)-arene complexes, in which the metal resides on the terminal ring. Warming induces a haptotropic shift to the neighboring cyclobutadiene rings, followed by the previously reported intercyclobutadiene migration. NMR scrutiny of the primary photoproduct reveals a thermally accessible 16-electron cobalt η(2)-triplet species, which, according to DFT computations, is responsible for the rapid symmetrization of the molecules along their long axes. Calculations indicate that the entire haptotropic manifold along the phenylene frame is governed by dual-state reactivity of alternating 18-electron singlets and 16-electron triplets.