Jennifer Meyer

Infrared multiple-photon dissociation (IRMPD) spectra in the 800–1750 cm–1 range of a set of pseudo-octahedrally coordinated manganese complexes {[Mn(dgpy)2][PF6]x}n+ (x = 0, 2, 3; n = 1, 2, 3; dgpy = 2,6-diguanidylpyridine) isolated in an ion trap are presented. The complexes contain two neutral tridendate dgpy ligands and in some cases [PF6]− ions form ion clusters with the positive manganese species, exhibiting different manganese oxidation states within the same ligand environment. While IR spectra assigned to Mn(II, III, IV) species show similar vibrational patterns, confirming largely metal-centered redox processes, a clearly different spectrum was observed for the in situ generated monocation [Mn(dgpy)2]+, raising the question of whether this complex contains a MnI central ion or a reduced ligand (dgpy)•–. Comparison of the spectroscopic data to the results of density functional theory (DFT) calculations using the CAM-B3LYP functional assigns its structure to a [MnII(dgpy)(dgpy)•– ]+ configuration. This “non-innocent” electronic behavior of the dgpy ligand in the gas phase is elucidated by electronic structure computations and attributed to the asymmetric, localized charge distribution of the extra electron on the pyridine moiety of only one of the dgpy ligands. The results demonstrate the novel perspectives offered by mass spectrometrically controlled gas phase spectroscopy for the generation, isolation, and characterization of unusual charge states of ligand-stabilized metal complexes.

Transition-metal complexes with earth-abundant metal centers hold significant potential for light-induced processes. Pseudo-octahedral vanadium(III) and vanadium(II) complexes with d² and d³ electron configurations are promising candidates, yet their redox and excited-state properties are only insufficiently understood. Here, we combine quantum chemistry and experimental data (structure, cyclic voltammetry, spectroelectrochemistry, electron-transfer dissociation, collision-induced dissociation, ultraviolet photodissociation mass spectrometry, and ultrafast transient absorption spectroscopy) to elucidate redox and photostability as well as excited-state lifetimes of the vanadium(III/II) redox couple cisfac-[V(dgpy)₂]^3+/2+ (dgpy = 2,6-diguanidylpyridine). Ground-state splitting, mixing of spin-flip excited states with metal-to-ligand charge transfer states, and multiphonon relaxation are identified as the key challenges for excited-state lifetimes in these d²/d³ systems.

Bimolecular nucleophilic substitution (S _N 2) and base-induced elimination (E2) reactions play crucial roles in chemistry. Here, we report a combined study of ion-molecule crossed-beam 3D velocity map imaging experiments and dynamics simulations on an accurate 39-dimensional potential energy surface for the Cl ⁻ + (CH ₃ ) ₃ CI reaction, allowing the most rigorous investigation of the atomistic dynamics. Good agreement between experimental and theoretical product angular and energy distributions is achieved, revealing that predominantly direct E2 reactions produce the majority of highly excited neutral products and slow ion product distributions. Moreover, we uncover a direct “flip-over” retention mechanism for the S _N 2 reaction, where substitution occurs via the direct flipping of the tert-butyl group, leading to retention of the tetrahedral carbon center. This mechanism manifests as a predominant forward-scattering feature in the product angular distribution and provides an important perspective on substitution dynamics in organic reactions.

Energy and angle differential cross sections show a competition between spin changing and conserving pathways for CO ₂ activation by transition metal ions in the gas phase. The intersystem crossing acts as bottleneck leading to indirect dynamics.

Photoactive and luminescent chromium(III) complexes are typically coordinated by aromatic N-heterocyclic chelate ligands such as polypyridines. Photoluminescence quantum yields and lifetimes can achieve up to 30% and several milliseconds, respectively. The effect of aliphatic N-donor ligands (guanidines) on the nonradiative decay of emissive doublet states remained, however, underexplored. Here, we describe the preparation, structure, redox chemistry, gas phase stability, and photophysical properties of the complex [Cr(dgpy)₂][PF₆]₃ with pyridine and guanidine donors (dgpy = 2,6-diguanidylpyridine). Variable-temperature luminescence spectroscopy, near-infrared overtone spectroscopy as well as quantum chemical calculations identified the major thermally activated and nonactivated decay pathways and inform on future design criteria for photoactive chromium(III) complexes.

Dynamics and kinetics of model reactions between Zr⁺ and CO₂ investigated by experiment and theory reveal a competition between spin conserving and changing reaction pathways, underscoring spin as leading influence in chemical reactivity.

The kinetics of Ta⁺ + CH₄ and related reactions TaC_nH_m⁺ + CH₄ (n = 2-4, m = n, 2n, 3n) are measured from 300-600 K using a selected-ion flow tube apparatus. Complicated kinetics are analyzed through a novel bootstrapping methodology, and rate constants for 38 unimolecular, bimolecular, and ternary processes are reported at each of the four temperatures. As has been well-established, Ta⁺ efficiently dehydrogenates methane through a non-spin-conserved process. Sequential chemistry leads to the dehydrogenation of up to four methane molecules per tantalum center through the competing processes of TaC_nH_m⁺ + CH₄ → TaC_n+1H_m+2⁺ + H₂ (dehydrogenation) and TaC_n+1H_m+4⁺ (association). Supported by density functional theory calculations, the distinct mechanisms and product structures of the sequential reactions are derived. The activation energy for oxidative insertion of Ta into a C-H bond is well-predicted by a simple heuristic: whether or not the reactant tantalum atom possesses unbound valence electrons of opposite spin. TaCH₂⁺ is predicted to have a small activation energy for oxidative insertion but can only proceed to dehydrogenation of methane via carbon-carbon bond formation, enabled by three separate intersystem crossing events. The product is determined to be the tantalapropene dihydride cation, not the more intuitive tantalapropane cation, via comparison of measured and calculated thermal dissociation rates. The TaC₂H₄⁺ tantalapropene dihydride has a prohibitive barrier to oxidative insertion. It proceeds instead through a ring-opening insertion of the entire tantalapropene moiety into a C-H bond via σ-bond metathesis; the unbroken metallacycle bond acts as a tether, preventing the activated products from separating and allowing for further isomerization, leading to dehydrogenation. This and subsequent dehydrogenation processes occur without carbon-carbon bond formation; no evidence of a tantalabutane or larger metallacycle is found.

Methane is facilely activated by gas phase tantalum cations via a spin-forbidden process leading to statistical product energy disposal.