Stereochemically nonrigid organometallic molecules. XXVII. Fluxional behavior of tetra(cyclopentadienyl)titanium

Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes

Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η5-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η4-pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2'-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)]+, an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H)Rh(bpy)]+. Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.

Multicomponent Mechanochemical Synthesis of Cyclopentadienyl Titanium tert-Butoxy Halides, Cp x TiX y (O t Bu)4-(x+y) (x, y = 1, 2; X = Cl, Br)

Titanium tert-butoxy halides of the formula Cp _x TiX _y (O ^t Bu)_4-(x+y) (x, y = 1, 2; X = Cl, Br) have been prepared thorough milling the reagents without solvent. In the case of the chloride derivatives, Cp₂TiCl₂ is used as a starting material; in the case of the bromides, a mixture of LiCp, TiBr₄, and Li[O ^t Bu] is used. The stoichiometric ratios of the starting materials are reflected in the major products of the reactions. Single-crystal X-ray structures are reported for Cp₂TiCl(O ^t Bu), Cp₂TiBr(O ^t Bu), and CpTiBr₂(O ^t Bu), as well as for Cp₂TiCl(O ⁱ Pr) and a redetermination of Cp₂TiCl(OMe). The tert-butoxy derivatives are notable for their nearly linear Ti-O-C angles (>170°) that reflect Ti-O π-bonding, an interpretation supported with density functional theory calculations.

Oxygen-atom transfer to a nucleophilic molybdenum complex

The reactivity of the imido-Mo(IV) species [(eta(5)-Cp)(2)Mo(N(t)Bu)], 1, towards the iodine-based oxidant PhIO was investigated and yielded the micro-oxodimolybdenum(V) dimer [{(eta(5)-Cp)(eta(1)-Cp)Mo(N(t)Bu)}(2)O], 2. X-Ray crystallography and (1)H-NMR of 2 indicate slipping of one of the eta(5)-Cp ring on each Mo center to eta(1)-Cp as the result of the coordination of the oxygen atom. DFT investigation of the OAT reaction to complex 1 revealed a transient Mo(IV)-oxo species as the most likely intermediate.

Mechanistic studies of ligand fluxionality in [M(eta(5)-Cp)(eta(1)-Cp)(L)2]n

Density functional theory has been used to provide a thorough investigation of the mechanistic factors affecting Cp ligand fluxionality in a series of organometallic complexes, [M(eta(5)-Cp)(eta(1)-Cp)(L)(2)](n), involving different metals, different oxidation states, and different ligands. Excellent agreement with experiment for the barrier heights for the 1,5-shift were obtained for the complexes [Fe(eta(5)-Cp*)(eta(1)-Cp)(CO)(2)] and [Fe(eta(5)-Cp)(eta(1)-Cp)(CO)(2)]. For the range of complexes studied, the barriers have been successfully rationalized in terms of hyperconjugation, metal-Cp bond strength, and steric effects. In addition, the eta(1)-eta(5) interconversion of the Cp binding mode is shown to be a high-energy process, consistent with experimental observations. The L substitution reactions by eta(1)-Cp are quite sensitive to the nature of the metal center and ancillary ligand. A detailed theoretical explanation of the factors involved in all of these transformations is provided.

A half-century of nonclassical organometallic chemistry: a personal perspective

A review of some developments in transition metal (nonclassical) organometallic chemistry is presented, with emphasis on areas to which the author himself has made major and/or seminal contributions: especially fluxional organometallics, fluxional metal carbonyls, and the discovery and full explanation of agostic hydrogen atoms. An effort has been made to present these topics from the perspective of today rather than in strictly chronological order, with a sufficient number of specific examples to be concrete, but by no means encyclopedic. Particular topics covered are "ring whizzers", concerted shifts of two or more CO ligands, twirling M(CO)(3) groups, and merry-go-rounds. In addition, the following topics are discussed: the unfinished business of mu(2),eta(2),eta(2)-ethene complexes and arene complexes of the lanthanides and actinides.