Excited State Dynamics of a Conformationally Fluxional Copper Coordination Complex.

The conversion of solar energy into chemical fuel represents a capstone goal of the 21st century and has the potential to supply terawatts of power in a globally distributed manner. However, the disparate time scales of photodriven charge separation (∼fs) and steps in chemical reactions (∼µs) represent an inherent bottleneck in solar-to-fuels technology. To address this discrepancy, we are developing earth-abundant coordination complexes that undergo light-induced conformational rearrangements such that charge separation (CS) is hastened, while charge recombination (CR) is slowed. To these ends, we report the preparation and characterization of a new series of conformationally fluxional copper coordination complexes that contain a twisted intramolecular charge transfer (TICT) fluorophore as part of their ligand scaffold. Structural and spectroscopic characterization of the Cu(I) and Cu(II) complexes formed with these ligands in their ground states establish oxidation state-dependent conformational dynamicity, while time-resolved emission and transient absorption spectroscopies define the photophysical parameters of photo-induced excited states. Building on initial reports with a related set of molecules, the improved ligand design presented here greatly simplifies the observed photophysics, effectively shutting down unwanted ligand-centered excited states previously observed. Time-dependent density functional theory (TDDFT) analyses reveal an unusual metal-to-TICT electronic transition only reported once before, and though the formation of a CS state is not observed directly through experiments, TDDFT geometry optimizations in the excited states support the formation of transient Cu(II) CS species, lending credence to the potential success of our approach. These studies establish a clear model for the excited state dynamics at play in proof-of-concept systems and clarify key design parameters for future optimizations toward achieving long-lived CS via photoinduced conformational gating.

Toward Improved Charge Separation through Conformational Control in Copper Coordination Complexes.

The continued development of solar energy as a renewable resource necessitates new approaches to sustaining photodriven charge separation (CS). We present a bioinspired approach in which photoinduced conformational rearrangements at a ligand are translated into changes in coordination geometry and environment about a bound metal ion. Taking advantage of the differential coordination properties of CuI and CuII, these dynamics aim to facilitate intramolecular electron transfer (ET) from CuI to the ligand to create a CS state. The synthesis and photophysical characterization of CuCl(dpaaR) (dpaa = dipicolylaminoacetophenone, with R = H and OMe) are presented. These ligands incorporate a fluorophore that gives rise to a twisted intramolecular charge transfer (TICT) excited state. Excited-state ligand twisting provides a tetragonal coordination geometry capable of capturing CuII when an internal ortho-OMe binding site is present. NMR, IR, electron paramagnetic resonance (EPR), and optical spectroscopies, X-ray diffraction, and electrochemical methods establish the ground-state properties of these CuI and CuII complexes. The photophysical dynamics of the CuI complexes are explored by time-resolved photoluminescence and optical transient absorption spectroscopies. Relative to control complexes lacking a TICT-active ligand, the lifetimes of CS states are enhanced ∼1000-fold. Further, the presence of the ortho-OMe substituent greatly enhances the lifetime of the TICT* state and biases the coordination environment toward CuII. The presence of CuI decreases photoinduced degradation from 14 to <2% but does not result in significant quenching via ET. Factors affecting CS in these systems are discussed, laying the groundwork for our strategy toward solar energy conversion.

Conformationally dynamic copper coordination complexes.

The interplay between oxidation state and coordination geometry dictates both kinetic and thermodynamic properties underlying electron transfer events in copper coordination complexes. An ability to stabilize both CuI and CuII oxidation states in a single conformationally dynamic chelating ligand allows access to controlled redox reactivity. We report an analysis of the conformational dynamics of CuI complexes bearing dipicolylaniline (dpaR) ligands, with ortho-aniline substituents R = H and R = OMe. Variable temperature NMR spectroscopy and electrochemical experiments suggest that in solution at room temperature, an equilibrium exists between two conformers. Two metal-centered redox events are observed which, bolstered by structural information from single crystal X-ray diffraction and solution information from EPR and NMR spectroscopies, are ascribed to the CuII/I couple in planar and tetrahedral conformations. Activation and equilibrium parameters for these structural interconversions are presented and provide entry to leveraging redox-triggered conformational dynamics at Cu.

Exploring Ligand-Centered Hydride and H-Atom Transfer.

The nickel(II) complex [ON(H)O]Ni(PPh3) ([ON(H)O]2- = bis(3,5-di-tert-butyl-2-phenoxy)amine), bearing a protonated redox-active ligand, was examined for its ability to serve as a hydrogen atom (H•) and hydride (H-) donor. Deprotonation of [ON(H)O]Ni(PPh3) afforded the square-planar anion {[ONOcat]Ni(PPh3)}1-, whereas hydrogen atom transfer from [ON(H)O]Ni(PPh3) to TEMPO• in the presence of added PPh3 afforded five-coordinate [ONO]Ni(PPh3)2 that has been structurally characterized. In solution, this five-coordinate complex exists in equilibrium with four-coordinate [ONO]Ni(PPh3), and this ligand exchange equilibrium correlates with a valence tautomerization between the redox-active ligand and the nickel center. Abstraction of a hydride from [ON(H)O]Ni(PPh3) in the presence of PPh3 afforded the octahedral complex, [ONOq]Ni(OTf)(PPh3)2, which was characterized as an S = 1, nickel(II) complex. Bond dissociation free energy (BDFE) and hydricity (ΔG°H-) measurements benchmark the thermodynamic propensity of this complex to participate in ligand-centered H• and H- transfer reactions.

Metal-Ion Influence on Ligand-Centered Hydrogen-Atom Transfer.

The ligand-centered hydrogen-atom-transfer (HAT) reactivity was examined for a family of group 10 metal complexes containing a tridentate pincer ligand derived from bis(2-mercapto-p-tolyl)amine, [SNS]H3. Six new metal complexes of palladium and platinum were synthesized with the [SNS] ligand platform in different redox and protonation states to complete the group 10 series previously reported with nickel. The HAT reactivity was examined for this family of nickel, palladium, and platinum complexes to determine the impact of a metal ion on the ligand-centered reactivity. Thermodynamic measurements revealed that N-H bond dissociation free energies increased by approximately 10 kcal mol-1 along the series Ni < Pd < Pt driven by changes to both the redox potential and pKa of the ligand. Kinetic analyses for all three metal complexes suggest that the barrier to the HAT reactivity is primarily entropic rather than enthalpic for this system.

Hydrogen-Atom Noninnocence of a Tridentate [SNS] Pincer Ligand.

Double deprotonation of bis(2-mercapto-4-methylphenyl)amine ([SNS]H3) followed by addition to NiCl2(PR3)2 in air-free conditions afforded [SN(H)S]Ni(PR3) (1a, R = Cy; 1b, R = Ph) complexes, characterized as diamagnetic, square-planar nickel(II) complexes. When the same reaction was conducted with 3 equiv of KH, the diamagnetic anions K{[SNS]Ni(PR3)} were obtained (K[2a], R = Cy; K[2b], R = Ph). In the presence of air, the reaction proceeds with a concomitant one-electron oxidation. When R = Cy, a square-planar, S = 1/2 complex, [SNS]Ni(PCy3) (3a), was isolated. When R = Ph, the bimetallic complex {[SNS]Ni(PPh3)}2 ({3b}2) was obtained. This bimetallic species is diamagnetic; however, in solution it dissociates to give S = 1/2 monomers analogous to 3a. Complexes 1-3 represent a hydrogen-atom-transfer series. The bond dissociation free energies (BDFEs) for 1a and 1b were calculated to be 63.9 ± 0.1 and 62.4 ± 0.2 kcal mol-1, respectively, using the corresponding p Ka and E°' values. Consistent with these BDFE values, TEMPO• reacted with 1a and 1b, resulting in the abstraction of a hydrogen atom to afford 3a and 3b, respectively.

A Selenium-Containing Diarylamido Pincer Ligand: Synthesis and Coordination Chemistry with Group 10 Metals.

The synthesis of new bifunctional organoselenium diarylamine compounds RN(4-Me-2-SeMe-C6H3)2 (R = Me: 1; R = tert-butoxycarbonyl (Boc): 2; R = H: 3-H) via aryllithium chemistry is disclosed. Compound 1 serves as a Se,Se-bidentate neutral ligand toward Pd(II), forming the coordination complex {PdCl2[MeN(4-Me-2-SeMe-C6H3)2-κ(2)Se)]} (1-Pd) in reaction with [PdCl2(COD)] (COD = 1,5-cyclooctadiene), while the protio ligand 3-H forms tridentate pincer complexes [MCl(N(4-Me-2-SeMe-C6H3)2)] (M = Pd: 3-Pd; M = Pt: 3-Pt) with [MCl2(COD)] (M = Pd, Pt) in the presence of triethylamine. Complex 1-Pd does not undergo N-C cleavage at high temperature, unlike related alkylphosphine-bearing complexes. All compounds have been characterized by multinuclear ((1)H, (13)C, (77)Se) NMR spectroscopy, and crystal structures of 1, 1-Pd, 3-Pd, and 3-Pt are reported. Additionally, density functional theory calculations have been performed on the pincer complexes to contrast them with well-known analogues containing phosphine donor groups.

Crystal structure of 1-bromo-2-(phenyl-selen-yl)benzene.

In the title compound, C12H9BrSe, the Se atom exhibits a bent geometry, with a C-Se-C bond angle of 99.19 (6)°. The ortho Se and Br atoms are slightly displaced from opposite faces of the mean plane of the benzene ring [by 0.129 (2) and 0.052 (2) Å, respectively]. The planes of the benzene and phenyl rings form a dihedral angle of 72.69 (5)°. In the crystal, π-stacking inter-actions between inversion-related phenyl rings are observed, with a centroid-centroid distance of 3.630 (1) Å.