Multireference Description of Nickel-Aryl Homolytic Bond Dissociation Processes in Photoredox Catalysis

Multireference electronic structure calculations consistent with known experimental data have elucidated a novel mechanism for photo-triggered Ni(II)–C homolytic bond dissociation in Ni 2,2′-bipyridine (bpy) photoredox catalysts. Previously, a thermally assisted dissociation from the lowest energy triplet ligand field excited state was proposed and supported by density functional theory (DFT) calculations that reveal a barrier of ∼30 kcal mol⁻¹. In contrast, multireference ab initio calculations suggest that this process is disfavored, with barrier heights of ∼70 kcal mol⁻¹, and highlight important ligand noninnocent and multiconfigurational contributions to excited state relaxation and bond dissociation processes that are not captured with DFT. In the multireference description, photo-triggered Ni(II)–C homolytic bond dissociation occurs via initial population of a singlet Ni(II)-to-bpy metal-to-ligand charge transfer (¹MLCT) excited state, followed by intersystem crossing and aryl-to-Ni(III) charge transfer, overall a formal two-electron transfer process driven by a single photon. This results in repulsive triplet excited states from which spontaneous homolytic bond dissociation can occur, effectively competing with relaxation to the lowest energy nondissociative triplet Ni(II) ligand field excited state. These findings guide important electronic structure considerations for the experimental and computational elucidation of the mechanisms of ground and excited state cross-coupling catalysis mediated by Ni heteroaromatic complexes

Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes

Iron porphyrin carbenes (IPCs) are important reaction intermediates in engineered carbene transferase enzymes and homogeneous catalysis. However, discrepancies between theory and experiment complicate the understanding of IPC electronic structure. In the literature, this has been framed as whether the ground state is an open- vs closed-shell singlet (OSS vs CSS). Here we investigate the structurally dependent ground and excited spin-state energetics of a free carbene and its IPC analogs with variable trans axial ligands. In particular, for IPCs, multireference ab initio wave function methods are more consistent with experiment and predict a mixed singlet ground state that is dominated by the CSS (Fe(II) ← {:C(X)Y}0) configuration (i.e., electrophilic carbene) but that also has a small, non-negligible contribution from an Fe(III)–{C(X)Y}−• configuration (hole in d(xz), i.e., radical carbene). In the multireference approach, the “OSS-like” excited states are metal-to-ligand charge transfer (MLCT) in nature and are energetically well above the CSS-dominated ground state. The first, lowest energy of these “OSS-like” excited states is predicted to be heavily weighted toward the Fe(III)–{C(X)Y}−• (hole in d(yz)) configuration. As expected from exchange considerations, this state falls energetically above a triplet of the same configuration. Furthermore, potential energy surfaces (PESs) along the IPC Fe–C(carbene) bond elongation exhibit increasingly strong mixings between CSS/OSS characters, with the Fe(III)–{C(X)Y}−• configuration (hole in d(xz)) growing in weight in the ground state during bond elongation. The relative degree of electrophilic/radical carbene character along this structurally relevant PES can potentially play a role in reactivity and selectivity patterns in catalysis. Future studies on IPC reaction coordinates should evaluate contributions from ground and excited state multireference character

Quantifying Entatic States in Photophysical Processes: Applications to Copper Photosensitizers

The entatic or rack-induced state is a core concept in bioinorganic chemistry. In its simplest form, it is present when a protein scaffold places a transition metal ion and its first coordination sphere into an energized geometric and electronic structure that differs significantly from that of the relaxed form. This energized complex can exhibit special properties. Under this purview, however, entatic states are hardly unique to bioinorganic chemistry, and their effects can be found throughout a variety of important chemistries and materials science applications. Despite this broad influence, there are only a few examples where entatic effects have been quantified. Here we extend the entatic concept more generally to photophysical processes by developing a combined experimental and computational methodology to quantify entatic states across an entire class of functional molecules, e.g., Cu-based photosensitizers. These metal complexes have a broad range of applications, including solar electricity generation, solar fuels synthesis, organic light emitting diodes (OLEDs), and photoredox catalysis. As a direct consequence of quantifying entatic states, this methodology allows the disentanglement of steric and electronic contributions to excited state dynamics. Thus, before embarking on the syntheses of new Cu-based photosensitizers, the correlations described herein can be used as an estimate of entatic and electronic contributions and thus guide ligand design and the development of next-generation transition metal complexes with improved or tailored excited state dynamics. Lastly, entatic energies in some Cu photosensitizers are the largest yet quantified and are found here to approach 20 kcal/mol relative to the conformationally flexible [Cu(phen)₂]⁺. These energetics are significant relative to typical chemical driving forces and barriers, highlighting the utility in extending entatic state descriptors to new classes of molecules and materials with interesting functional properties involving the coupling between electron and vibrational dynamics

Quantifying entatic states in photophysical processes: Applications to copper photosensitizers

The concept of an entatic or rack-induced state describes how a protein fold can place a transition metal center in a strained geometric environment. This strain can finely tune the active site's electronic structure and thus its phys. and chem. properties. Beyond bioinorg. chem., the entatic state concept has relevance to a multitude of processes in chem. and materials science. However, entatic states and energetics have only been defined and quantified for a few systems. In this work, a combined exptl. and computational approach is developed and used to extend the entatic state description to dozens of Cu-based photosensitizers, a set of mols. widely studied for solar energy conversion, org. light emitting diodes, and photoredox catalysis. Entatic energies detd. here are the largest yet detd. (~20 kcal/mol) and approach typical chem. driving forces and barriers. Furthermore, the approach outlined here provides a method to decouple steric and electronic influences on the excited state potential energy surfaces and dynamics of transition metal complexes. These insights can guide the design of transition metal complexes with tailored excited state lifetimes and properties, as well as inspire the extension of entatic concepts to catalysis and magnetism

Quantifying entatic states in photophysical processes: Applications to copper photosensitizers

The concept of an entatic or rack-induced state describes how a protein fold can place a transition metal center in a strained geometric environment. This strain can finely tune the active site's electronic structure and thus its phys. and chem. properties. Beyond bioinorg. chem., the entatic state concept has relevance to a multitude of processes in chem. and materials science. However, entatic states and energetics have only been defined and quantified for a few systems. In this work, a combined exptl. and computational approach is developed and used to extend the entatic state description to dozens of Cu-based photosensitizers, a set of mols. widely studied for solar energy conversion, org. light emitting diodes, and photoredox catalysis. Entatic energies detd. here are the largest yet detd. (~20 kcal/mol) and approach typical chem. driving forces and barriers. Furthermore, the approach outlined here provides a method to decouple steric and electronic influences on the excited state potential energy surfaces and dynamics of transition metal complexes. These insights can guide the design of transition metal complexes with tailored excited state lifetimes and properties, as well as inspire the extension of entatic concepts to catalysis and magnetism

Multiconfiguration Pair-Density Functional Theory for Chromium(IV) Molecular Qubits

Pseudo-tetrahedral organometallic complexes containing chromium(IV) and aryl ligands have been experimentally identified as promising molecular qubit candidates. Here we present a computational protocol based on multiconfiguration pair-density functional theory for computing singlet-triplet gaps and zero-field splitting (ZFS) parameters in Cr(IV) aryl complexes. Notably, complete active space second-order perturbation theory (CASPT2) and hybrid multiconfiguration pair-density functional theory (HMC-PDFT) perform better than Kohn-Sham density functional theory for singlet-triplet gaps. Despite the very small values of the ZFS parameters, all the examined multiconfigurational methods performed qualitatively well. CASPT2 and HMC-PDFT performed particularly well for predicting the trend in the ratio of the rhombic and axial ZFS parameters, |E/D|. We have also investigated the dependence and sensitivity of the calculated ZFS parameters on the active space and the procedure for geometry optimization. The methodologies outlined here will guide future prediction of ZFS parameters in molecular qubit candidates

Harvesting Water from Air with High-Capacity, Stable Furan-Based Metal–Organic Frameworks

We synthesized two isoreticular furan-based metal–organic frameworks (MOFs), MOF-LA2-1(furan) and MOF-LA2-2(furan) with rod-like secondary building units (SBUs) featuring 1D channels, as sorbents for atmospheric water harvesting (LA = long arm). These aluminum-based MOFs demonstrated a combination of high water uptake and stability, exhibiting working capacities of 0.41 and 0.48 g of water per g of MOF (under isobaric conditions of 1.70 kPa), respectively. Remarkably, both MOFs showed negligible loss in water uptake after 165 adsorption-desorption cycles. These working capacities rival those of MOF-LA2-1(pyrazole), which has a working capacity of 0.55 g of water per g of MOF. The current MOFs stand out for their high water stability as evidenced by 165 cycles of water uptake and release. MOF-LA2-2(furan) is the first aluminum MOF to employ a double \u27long arm\u27 extension strategy, confirmed through single-crystal X-ray diffraction (SCXRD). The MOFs were synthesized using a straightforward synthesis route. This study offers valuable insights into designing durable, water-stable MOFs and underscores their potential for efficient water harvesting