Ligand Frameworks for Transition-Metal Complexes That Model Metalloenzyme Active

Advances in the field of biomimetic inorganic chemistry require the design of sophisticated ligand frameworks that reflect the amazing complexity of metalloenzyme active sites. For instance, most active sites feature extensive hydrogen-bonding interactions between ligands bound to the metal center (the “first” coordination sphere) and nearby units in the outer (or “second”) sphere. Since these interactions modify the structural and electronic properties of the active sites, a number of inorganic chemists have sought to design ligands that permit outer-sphere functional groups to interact with first-sphere donors. This dissertation describes our contribution to these broader efforts to model the second coordination sphere. To date, our efforts have centered on the two classes of ligands based on second-sphere amide groups. The first set consists of 2,6-pyridinedicarboxamides with pendant pyridine or pyrimidine groups. Compared to the pincer ligands, the tripodal ligands posed a significantly greater synthetic challenge. We have succeeded in preparing a series of target ligands consisting of one, two, or three second-sphere heterocycles. My work has suggested that the second coordination sphere hydrogen bond interaction can be performed in our synthetic model. In addition, metalloenzymes with homobinuclear and heterobinuclear active sites play a central role in the chemistry of life. We have generated ligand scaffolds that support homo- and heterobimetallic complexes of relevance to metalloenzyme active sites. Firstly, the synthesis and coordination chemistry of a new asymmetric ligand designed to support nickel based heterobimetallic structures with relevance to bioinorganic chemistry is described. Additionally, we report the synthesis and coordination chemistry of ‘non-innocent’ pentadentate ligands intended to provide multiple sites for ligand-based oxidation and reduction. This ‘non-innocent’ ligand series contains a central diarylamido donor that serves as electron donor, in addition to ‘hard’ donor ligands (oxygen atoms), electron acceptor units, and ‘soft’ donor ligands. The resulting homobimetallic complexes (M = Co, Cu, and Zn) were characterized with X-ray crystallography and electrochemical methods. In addition, our studies found that the dicobalt(II) complex is a stable and efficient electrocatalyst for both H2 generation and H2O oxidation processes (i.e., water splitting)

From Static to Dynamic: Electron Density of HOMO at Biaryl Linkage Controls the Mechanism of Hole Delocalization

In order to extend the physical length of hole delocalization in a molecular wire, chromophores of increasing size are often desired. However, the effect of size on the efficacy and mechanism of hole delocalization remains elusive. Here, we employ a model set of biaryls to show that with increasing chromophore size, the mechanism of steady-state hole distribution switches from static delocalization in biaryls with smaller chromophores to dynamic hopping, as exemplified in the largest system, tBuHBC2 (i.e., “superbiphenyl”), which displays a vanishingly small electronic coupling. This important finding is analyzed with the aid of Hückel molecular orbital and Marcus–Hush theories. Our findings will enable the rational design of the novel molecular wires with length-invariant redox/optical properties suitable for long-range charge transfer

Bimetallic Complexes Supported by a Redox-Active Ligand with Fused Pincer-Type Coordination Sites

The remarkable chemistry of mononuclear complexes featuring tridentate, meridionally chelating “pincer” ligands has stimulated the development of ligand frameworks containing multiple pincer sites. Here, the coordination chemistry of a novel pentadentate ligand (LN3O2) that provides two closely spaced NNO pincer-type compartments fused together at a central diarylamido unit is described. The trianionic LN3O2 chelate supports homobimetallic structures in which each M(II) ion (M = Co, Cu, Zn) is bound in a meridional fashion by the bridging diarylamido N atom and O,N-donors of the salicyaldimine arms. The metal centers are also coordinated by a mono- or bidentate auxiliary ligand (Laux), resulting in complexes with the general form [M2(LN3O2)(Laux)2]+ (where Laux = 1-methyl-benzimidazole (1MeBI), 2,2′-bipyridine (bpy), 4,4′-dibromo-2,2′-bipyridine (bpyBr2), or (S)-2-(4-isopropyl-4,5-dihydrooxazolyl)pyridine (S-iPrOxPy)). The fused nature of the NNO pincer sites results in short metal–metal distances ranging from 2.70 Å for [Co2(LN3O2) (bpy)2]+ to 3.28 Å for [Zn2(LN3O2) (bpy)2]+, as revealed by X-ray crystallography. The complexes possess C2 symmetry due to the twisting of the aryl rings of the μ-NAr2 core; spectroscopic studies indicate that chiral Laux ligands, such as S-iPrOxPy, are capable of controlling the helical sense of the LN3O2 scaffold. Since the four- or five-coordinate M(II) centers are linked solely by the amido moiety, each features an open coordination site in the intermetallic region, allowing for the possibility of metal–metal cooperativity in small-molecule activation. Indeed, the dicobalt(II) complex [Co2(LN3O2) (bpyBr2)2]+ reacts with O2 to yield a dicobalt(III) species with a μ-1,2-peroxo ligand. The bpy-containing complexes exhibit rich electrochemical properties due to multiple metal- and ligand-based redox events across a wide (3.0 V) potential window. Using electron paramagnetic resonance (EPR) spectroscopy and density functional theory (DFT), it was determined that one-electron oxidation of [Co2(LN3O2) (bpy)2]+ results in formation of a S = 1/2 species with a LN3O2-based radical coupled to low-spin Co(II) centers

Synthesis of Homo- and Heterobimetallic Ni\u3csup\u3eII\u3c/sup\u3e–M\u3csup\u3eII\u3c/sup\u3e (M = Fe, Co, Ni, Zn) Complexes Based on an Unsymmetric Ligand Framework: Structures, Spectroscopic Features, and Redox Properties

Several homo- and heterobimetallic NiII–MII complexes (MII = Fe, Co, Ni, Zn) supported by an unsymmetric polydentate ligand (L13−) are reported (L13− is the trianion of 2-[bis(2-hydroxy-3,5-tert-butylphenyl)aminomethyl]-4-methyl-6-[(2-pyridylmethyl)iminomethyl]phenol). The L13− chelate provides two distinct coordination environments: a planar tridentate {N2O} site (A) and a tetradentate {NO3} site (B). Reaction of L13− with equimolar amounts of NiII and MII salts provides bimetallic complexes in which the NiII ion exclusively occupies the tetragonal A-site and the MII ion is found in the tripodal B-site. X-ray crystal structures revealed that the two metal centers are bridged by the central phenolate donor of L13− and an anionic X-ligand, where X = μ-1,1-acetate, hydroxide, or methoxide. The metal ions are separated by 3.0–3.1 Å in the MAMBX structures, where MA and MB indicate the ion located in the A and B sites, respectively, and X represents the second bridging ligand. Analysis of magnetic data and UV–Vis–NIR spectra indicate that, in all cases, the two metal ions adopt high-spin states in solution. The NiAII centers undergo one-electron reduction at −1.17 V vs. SCE, while the NiII and CoII ions in the phenolate-rich B-site are reduced at lower potentials. Significantly, the NiAII center possesses three open or labile coordination sites in a meridional geometry, which are generally occupied by solvent-derived ligands in the crystal structures. The NiMBX complexes serve as structural mimics of heterometallic Ni-containing sites in biology, such as the C-cluster of carbon monoxide dehydrogenase (CODH)

Unraveling the Coulombic Forces in Electronically Decoupled Bichromophoric Systems during Two Successive Electron Transfers

Coulombic forces are vital in modulating the electron transfer dynamics in both synthetic and biological polychromophoric assemblies, yet quantitative studies of the impact of such forces are rare, as it is difficult to disentangle electrostatic forces from simple electronic coupling. To address this problem, the impact of Coulombic interactions in the successive removal of two electrons from a model set of spirobifluorenes, where the interchromophoric electronic coupling is nonexistent, is quantitatively assessed. By systematically varying the separation of the bifluorene moieties using model compounds, ion pairing, and solvation, these interactions, with energies up to about 0.4 V, are absent at distances greater than about 9 Å. These findings can be (quantitatively) applied for the design of polychromophoric assemblies, whereby the redox properties of donors and/or acceptors can be tuned by judicious positioning of the charged groups to control the electron‐transfer dynamics

Energy Gap between the Poly-\u3cem\u3ep\u3c/em\u3e-phenylene Bridge and Donor Groups Controls the Hole Delocalization in Donor–Bridge–Donor Wires

Poly-p-phenylene wires are critically important as charge-transfer materials in photovoltaics. A comparative analysis of a series of poly-p-phenylene (RPPn) wires, capped with isoalkyl (iAPPn), alkoxy (ROPPn), and dialkylamino (R2NPPn) groups, shows unexpected evolution of oxidation potentials, i.e., decrease (−260 mV) for iAPPn, while increase for ROPPn (+100 mV) and R2NPPn (+350 mV) with increasing number of p-phenylenes. Moreover, redox/optical properties and DFT calculations of R2NPPn/R2NPPn+• further show that the symmetric bell-shaped hole distribution distorts and shifts toward one end of the molecule with only 4 p-phenylenes in R2NPPn+•, while shifting of the hole occurs with 6 and 8 p-phenylenes in ROPPn+• and iAPPn+•, respectively. Availability of accurate experimental data on highly electron-rich dialkylamino-capped R2NPPn together with ROPPn and iAPPn allowed us to demonstrate, using our recently developed Marcus-based multistate model (MSM), that an increase of oxidation potentials in R2NPPn arises due to an interplay between the electronic coupling (Hab) and energy difference between the end-capped groups and bridging phenylenes (Δε). A comparison of the three series of RPPn with varied Δε further demonstrates that decrease/increase/no change in oxidation energies of RPPn can be predicted based on the energy gap Δε and coupling Hab, i.e., decrease if Δε \u3c Hab (i.e., iAPPn), increase if Δε \u3e Hab (i.e., R2NPPn), and minimal change if Δε ≈ Hab (i.e., ROPPn). MSM also reproduces the switching of the nature of electronic transition in higher homologues of R2NPPn+• (n ≥ 4). These findings will aid in the development of improved models for charge-transfer dynamics in donor–bridge–acceptor systems

The Electronic Properties of Ni(PNN) Pincer Complexes Modulate Activity in Catalytic Hydrodehalogenation Reactions

Three chloronickel(II) complexes of PNN‐ pincer ligands with pyrazolyl and diphenylphosphino donors appended to different arms of diarylamido anchors were prepared and fully characterized. The three derivatives (1‐OMe, 1‐Me, 1‐CF3) differ only by the identity of the para‐aryl substituent on the pyrazolyl arm with 1‐OMe being 310 mV easier to oxidize than 1‐CF3. All three complexes are competent catalysts for hydrodehalogenation reactions of 1‐bromooctane and a variety of aryl halides in dimethylacetamide using NaBH4 as both base and hydride source. Comparative studies using diverse substrates showed that catalytic activity correlates with electron donor properties; 1‐OMe was superior to the other two. Deuterium labeling studies verified NaBD4 as the deuteride source and excluded solvent‐assisted radical pathways

Vertical vs. Adiabatic Ionization Energies in Solution and Gas-Phase: Probing Ionization-Induced Reorganization in Conformationally-Mobile Bichromophoric Actuators Using Photoelectron Spectroscopy, Electrochemistry and Theory

Ionization-induced structural and conformational reorganization in various π-stacked dimers and covalently linked bichromophores is relevant to many processes in biological systems and functional materials. In this work, we examine the role of structural, conformational, and solvent reorganization in a set of conformationally mobile bichromophoric donors, using a combination of gas-phase photoelectron spectroscopy, solution-phase electrochemistry, and density functional theory (DFT) calculations. Photoelectron spectral analysis yields both adiabatic and vertical ionization energies (AIE/VIE), which are compared with measured (adiabatic) solution-phase oxidation potentials (Eox). Importantly, we find a strong correlation of Eox with AIE, but not VIE, reflecting variations in the attendant structural/conformational reorganization upon ionization. A careful comparison of the experimental data with the DFT calculations allowed us to probe the extent of charge stabilization in the gas phase and solution and to parse the reorganizational energy into its various components. This study highlights the importance of a synergistic approach of experiment and theory to study ionization-induced structural and conformational reorganization

Spreading Electron Density Thin: Increasing the Chromophore Size in Polyaromatic Wires Decreases Interchromophoric Electronic Coupling

The development of novel polychromophoric materials using extended polycyclic aromatic hydrocarbons as a single large chromophore holds promise for long-range charge-transfer applications in photovoltaic devices and molecular electronics. However, it is not well-understood how the interchromophoric electronic coupling varies with the chromophore size in linearly connected molecular wires. Here, we show with the aid of electrochemistry, electronic spectroscopy, density functional theory calculations, and theoretical modeling that as the number of aromatic moieties in a single chromophore increases, the interchromophoric electronic coupling decreases and may reach negligible values if the chromophore is sufficiently large. The origin of this initially surprising result becomes clear when one considers this problem with the aid of Hückel molecular orbital theory, as at the polymeric limit energies of the molecular orbitals cluster to form bands and thus the energy spacing between orbitals, and thereby the electronic coupling must decrease with the chromophore expansion

Structural, Spectroscopic, and Electrochemical Properties of Nonheme Fe(II)-Hydroquinonate Complexes: Synthetic Models of Hydroquinone Dioxygenases

Using the tris(3,5-diphenylpyrazol-1-yl)borate (Ph2Tp) supporting ligand, a series of mono- and dinuclear ferrous complexes containing hydroquinonate (HQate) ligands have been prepared and structurally characterized with X-ray crystallography. The monoiron(II) complexes serve as faithful mimics of the substrate-bound form of hydroquinone dioxygenases (HQDOs) – a family of nonheme Fe enzymes that catalyze the oxidative cleavage of 1,4-dihydroxybenzene units. Reflecting the variety of HQDO substrates, the synthetic complexes feature both mono- and bidentate HQate ligands. The bidentate HQates cleanly provide five-coordinate, high-spin Fe(II) complexes with the general formula [Fe(Ph2Tp)(HLX)] (1X), where HLX is a HQate(1-) ligand substituted at the 2-position with a benzimidazolyl (1A), acetyl (1B and 1C), or methoxy (1D) group. In contrast, the monodentate ligand 2,6-dimethylhydroquinone (H2LF) exhibited a greater tendency to bridge between two Fe(II) centers, resulting in formation of [Fe2(Ph2Tp)2(μ-LF)(MeCN)]·[2F(MeCN)]. However, addition of one equivalent of “free” pyrazole (Ph2pz) ligand provided the mononuclear complex, [Fe(Ph2Tp)(HLF)(Ph2pz)]·[1F(Ph2pz)], which is stabilized by an intramolecular hydrogen bond between the HLF and Ph2pz donors. Complex 1F(Ph2pz) represents the first crystallographically-characterized example of a monoiron complex bound to an untethered HQate ligand. The geometric and electronic structures of the Fe/HQate complexes were further probed with spectroscopic (UV-vis absorption, 1H NMR) and electrochemical methods. Cyclic voltammograms of complexes in the 1X series revealed an Fe-based oxidation between 0 and −300 mV (vs. Fc+/0), in addition to irreversible oxidation(s) of the HQate ligand at higher potentials. The one-electron oxidized species (1Xoxox) were examined with UV-vis absorption and electron paramagnetic resonance (EPR) spectroscopies