Hammett Parameters Research Articles

Modulating the band structure of two‐dimensional black phosphorus via electronic effects of organic functional groups for enhanced hydrogen production activity

Electronic effects of organic functional groups play a fundamental role in determining the rate and/or direction of organic chemical reactions. The implementation of this concept in selective organic catalysis is achieved by tuning the electronic effects of organic functional groups to alter the corresponding reactivity. However, this approach has hardly been applied to modulate the band structure of inorganic materials. Here, we show that modulating the electronic band structure of two‐dimensional black phosphorus (BP) is possible via the electronic effects of organic functional groups covalently modified on its surface. Organic functional group can either donate or withdraw charge density from BP surface, which will alter the bonding/anti‐binding orbitals occupancy and thus shift the band‐edge positions of functionalized BP downward/upward. Therefore, the valence‐band maxima and the conduction‐band minima of functionalized BP can be continuously tuned by changing the binding group with different Hammett parameters. Finally, unexpectedly high hydrogen evolution reaction rates under visible light are achieved using functionalized BP heterojunctions as photocatalysts. This work underscores the significant role of electronic effects in chemically controlling BP’s band structure, offering greater flexibility and affordability beyond physical method limits.

Modulating the band structure of two‐dimensional black phosphorus via electronic effects of organic functional groups for enhanced hydrogen production activity

Electronic effects of organic functional groups play a fundamental role in determining the rate and/or direction of organic chemical reactions. The implementation of this concept in selective organic catalysis is achieved by tuning the electronic effects of organic functional groups to alter the corresponding reactivity. However, this approach has hardly been applied to modulate the band structure of inorganic materials. Here, we show that modulating the electronic band structure of two‐dimensional black phosphorus (BP) is possible via the electronic effects of organic functional groups covalently modified on its surface. Organic functional group can either donate or withdraw charge density from BP surface, which will alter the bonding/anti‐binding orbitals occupancy and thus shift the band‐edge positions of functionalized BP downward/upward. Therefore, the valence‐band maxima and the conduction‐band minima of functionalized BP can be continuously tuned by changing the binding group with different Hammett parameters. Finally, unexpectedly high hydrogen evolution reaction rates under visible light are achieved using functionalized BP heterojunctions as photocatalysts. This work underscores the significant role of electronic effects in chemically controlling BP’s band structure, offering greater flexibility and affordability beyond physical method limits.

Synthesis, structure, and solution chemistry of mixed-ligand oxidovanadium(IV) complexes incorporating ONO donor hydrazone ligands

Four mixed-ligand oxidovanadium(IV) complexes, [VIVO(L1-4)(phen)], 1-4, incorporating tridentate dibasic ONO donor hydrazone ligands (H2L1-4, general abbreviation H2L) derived from the condensation of 2-picolinyl hydrazide with 2-hydroxy acetophenone and its 5-substituted derivatives as primary ligands and 1,10-phenanthroline (phen) as auxiliary ligand have been synthesized in excellent yields. The complexes were characterized by elemental analyses, magnetic susceptibility measurements, and various spectral (e.g. IR, UV–Vis and EPR) studies. The molecular structure of one of the four complexes has been determined by single-crystal X-ray diffractometry. In 1, the hydrazone ligand is meridionally disposed of, coordinating through one enolic-O, one phenolic-O, and one imine-N. The other basal position is occupied by one pyridine-N of the phen moiety. The axial positions are occupied by one oxido-O atom and the other pyridine-N atom of the phen moiety. The complexes are paramagnetic with one unpaired electron (μeff ≈ 1.73 BM), suggesting no interaction with the neighboring molecules and are EPR active. The complexes display two d-d transitions, one near 725 and the other near 840 nm due to d xy → d xz , d yz and d xy → d x 2 − y 2 transitions, respectively. The other d-d transition associated with d xy → d z 2 transition was not observed as it falls in the UV region, which is masked by highly intense intra-ligand transitions. The complexes display one-electron oxidation peaks in the +0.75 to +0.89 V vs. Ag/AgCl region in the CH3CN solution that show a linear relationship with the Hammett parameter (σ) of the substituents in the aryloxy ring.

Resonance Effects from Substituents on L-Type Ligands Mediate Synthetic Control of Gold Nanocluster Frontier Orbital Energies.

The ligands of metal nanoclusters can be used to control their properties and reactivity, but a framework guiding their use remains elusive. Hammett studies of Au8(PPh3)72+ and Au9(PPh3)83+ nanoclusters with para- and meta-methyl and -methoxy groups indicate that resonance effects, not inductive effects, yield quantitative shifts of the HOMO-LUMO transitions involving orbitals local to the cluster core. Individual ligand exchanges reveal that these shifts are caused by only four of seven ligands, inconsistent with inductive effects. Quantum chemical calculations predict no trend in Au atom charges with respect to Hammett parameter but do predict bond length trends expected for a resonance structure that includes the Au atoms. Computed orbitals show contributions from specific para-OMe oxygen lone pairs to the HOMO, indicating delocalization from the core to specific ligands. These results suggest that resonance structures could be drawn including Au and ligands, guiding efforts to modulate nanocluster electronic structure and energy transfer.

Understanding Ligand Effects on Bielectronic Transitions: Chemo‐ and Electroreduction of Rhodium Bis(Diphosphine) Complexes to Low Oxidation States

AbstractRhodium complexes in the −I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two‐electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x=P‐substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh−I d10‐complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/−I) reduction potentials are normally ordered, leading to monoelectronic stepwise transitions, or inverted, giving bielectronic events. Reductionist approaches based on Hammett parameters or the P−Rh‐P bite angles provide only partial correlations with the redox potentials. However, we identified the C−O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two‐electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.

Understanding Ligand Effects on Bielectronic Transitions: Chemo- and Electroreduction of Rhodium Bis(Diphosphine) Complexes to Low Oxidation States.

Rhodium complexes in the -I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two-electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x=P-substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh-I d10-complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/-I) reduction potentials are normally ordered, leading to monoelectronic stepwise transitions, or inverted, giving bielectronic events. Reductionist approaches based on Hammett parameters or the P-Rh-P bite angles provide only partial correlations with the redox potentials. However, we identified the C-O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two-electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.

Evaluation of hemilabile Pd(II) NNS and NNSe non-symmetric pincers in Suzuki–Miyaura cross-coupling: Unexpected selective nitrile hydration of 4-bromonitrobenzene in mild conditions

In this work, four non-symmetric NNS and NNSe Pd(II) pincer complexes (1–4) supported by tridentate iminophosphorane ligands bearing both the indole heterocycle and a varying chalcogenoether pending arm [XR = SPh (1), SePh (2), SMe (3), SeMe (4)], were tested in the Suzuki–Miyaura cross coupling reaction between phenyl boronic acid and aryl bromides. The modest catalytic activity of complex 3 under conventional heating (110 °C), was boosted under microwave irradiation, leading to excellent yields of coupling under remarkably mild conditions: 5 min of reaction time at 70 °C under aerobic conditions. The complexes supported by NNS ligands proved to be more robust and superior catalysts than those supported by NNSe ligands, which tended to decompose under catalytic conditions. Among the four complexes, pincer 3 also demonstrated the best catalytic performance and was tested in the coupling of a variety of p-substituted bromobenzenes. Notably, the catalytic performance of complex 3 was unaffected by the mercury drop test, and with most substrates in the series, the percentage of conversion reflected the aryl bromides’ Hammett parameter. However, little to no coupling occurred with p-bromobenzonitrile and the reaction with p-nitrobenzene resulted in only limited conversion yields. In fact, when activated using microwave irradiation, complex 3 unexpectedly catalyzed the nitrile hydration of p-bromobenzonitrile, yielding p-bromobenzamide under remarkably mild Suzuki–Miyaura coupling conditions.

Controlling excited-state dynamics via protonation of naphthalene-based azo dyes.

Azo dyes are a class of photoactive dyes that constitute a major focus of chemical research due to their applications in numerous industrial functions. This work explores the impact of protonation on the photophysics of four naphthalene-based azo dyes. The pKa value of the dyes increases proportionally with decreasing Hammett parameter of p-phenyl substituents from 8.1 (R = -H, σ = 0) to 10.6 (R = -NMe2, σ = -0.83) in acetonitrile. Protonation of the dyes shuts down the steady-state photoisomerization observed in the unprotonated moieties. Fluorescence measurements reveal a lower quantum yield with more electron-donating p-phenyl substituents, with overall lower fluorescence quantum yields than the unprotonated dyes. Transient absorption spectroscopy reveals four excited-state lifetimes (<1 ps, ∼3 ps, ∼13 ps, and ∼200 ps) exhibiting faster excited-state dynamics than observed in the unprotonated forms (for 1-3: 0.7-1.5 ps, ∼3-4 ps, 20-40 ps, 20-300 min; for 4: 0.7 ps, 4.8 ps, 17.8 ps, 40 ps, 8 min). Time-dependent density functional theory (TDDFT) elucidates the reason for the loss of isomerization in the protonated dyes, revealing a significant change in the lowest excited state potential energy nature and landscape upon protonation. Protonation impedes relaxation along the typical rotational and inversion isomerization axes, locking the dyes into a trans-configuration that rapidly decays back to the ground state.

Tuning the redox profile of the 6,6'-biazulenic platform through functionalization along its molecular axis.

The E1/2 potential associated with reduction of the linearly-functionalized 6,6'-biazulenic scaffold is accurately correlated to the combined σp Hammett parameters of the substituents over >600 mV range. X-ray crystallographic analysis of the 2,2'-dichloro-substituted derivative revealed unexpectedly short C-Cl bond distances, along with other metric changes, suggesting a non-trivial cycloheptafulvalene-like structural contribution.

Sigma Hole Potentials as Tools: Quantifying and Partitioning Substituent Effects.

Empirical substituent constants, such as the Hammett parameters, have found important utility in organic and other areas of chemistry. They are useful both in predicting the impact of substitutions on chemical processes and in rationalizing after-the-fact observations on chemical bonding and reactivity. We assess the impact of substitutions on monoiodinated benzene rings and find that the modifications that substituents induce on the electrostatic potentials at the sigma hole on the terminal I center correlate strongly with established trends of common substituents. As an alternative to the experimental procedures involved in obtaining empirically based substituent constants, the computationally determined constants based on induced electrostatic potentials offer a model for quantifying the influence of mono- and polyatomic, neutral, and ionic substituents on their compounds. A partitioning scheme is proposed that allows us to discretely separate σ and π contributions to generate quantitative measures of substituent effects.

Sterically attenuated electronic communication in cobalt complexes of meridional isoquinoline-derived ligands for applications in electrocatalysis.

The ability to synthetically tune the ligand frameworks of redox-active molecules is of critical importance to the economy of solar fuels because manipulating their redox properties can afford control over the operating potentials of sustained electrocatalytic or photoelectrocatalytic processes. The electronic and steric properties of 2,2':6',2″-terpyridine (Terpy) ligand frameworks can be tuned by functional group substitution on ligand backbones, and these correlate strongly to their Hammett parameters. The synthesis of a new series of tridentate meridional ligands of 2,4,6-trisubstituted pyridines that engineers the ability to finely tune the redox potentials of cobalt complexes to more positive potentials than that of their Terpy analogs is achieved by aryl-functionalizing at the four-position and by including isoquinoline at the two- and six-positions of pyridine (Aryl-DiQ). Their cobalt complex syntheses, their electronic properties, and their catalytic activity for carbon dioxide (CO2) reduction are reported and compared to their Terpy analogs. The cobalt derivatives generally experience a positive shift in their redox features relative to the Terpy-based analogs, covering a complementary potential range. Although those evaluated fail to produce any quantifiable products for the reduction of CO2 and suffer from long-term instability, these results suggest possible alternate strategies for stabilizing these compounds during catalysis. We speculate that lower equilibrium association constants to the cobalt center are intrinsic to these ligands, which originate from a steric interaction between protons on the pyridine and isoquinoline moieties. Nevertheless, the new Aryl-DiQ ligand framework has been engineered to selectively tune homoleptic cobalt complexes' redox potentials.

Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes.

A key challenge to the effective utilization of solar energy is to promote efficient photoinduced charge transfer, specifically avoiding unproductive, circuitous electron-transfer pathways and optimizing the kinetics of charge separation and recombination. We hypothesize that one way to address this challenge is to develop a fundamental understanding of how to initiate and control directional photoinduced charge transfer, particularly for earth-abundant first-row transition-metal coordination complexes, which typically suffer from relatively short excited-state lifetimes. Here, we report a series of functionalized heteroleptic copper(I)bis(phenanthroline) complexes, which have allowed us to investigate the directionality of intramolecular photoinduced metal-to-ligand charge transfer (MLCT) as a function of the substituent Hammett parameter. Ultrafast transient absorption suggests a complicated interplay of MLCT localization and solvent interaction with the Cu(II) center of the MLCT state. This work provides a set of design principles for directional charge transfer in earth-abundant complexes and can be used to efficiently design pathways for connecting the molecular modules to catalysts or electrodes and integration into systems for light-driven catalysis.

Tuning material properties of covalent adaptable networks containing boronate‐TetraAzaADamantane bonds through systematic variation in electron density of ring substituents

AbstractAn outstanding challenge in modern society remains how to make crosslinked polymers (thermosets) more recyclable. A breakthrough solution to this challenge has been the introduction of dynamic covalent bonds in polymer networks, yielding covalent adaptable networks (CANs). Ongoing research is focused on finding new suitable dynamic covalent chemistries and on how to tune the material properties of CANs derived from these new chemistries. Here, we first compare two different dynamic boronic acid based covalent adaptable networks, namely, a conventional boronate‐diol and a novel boronate‐TetraAzaADamantane (TAAD) system. We show that incorporating boronate‐TAAD bonds in networks results in stiffer materials, as seen in a slower relaxation and higher shear and storage moduli. This offers access to more mechanically robust boronate‐based materials, compared to conventional boronate‐based gels. Next, we investigate the effect of molecular tuning via the electron density of meta‐positioned ring substituents on the macroscopic material properties for the boronate‐TAAD network. By comparing relaxation experiments on materials with different substituents, we show that the macroscopic network relaxation can be tuned through the Hammett parameter of the meta‐substituent and the activation energy of the boronate‐TAAD exchange. This enables subtle control over the (dynamic) material properties of these novel, robust boronate‐based networks.

Correlating Metal Redox Potentials to Co(III)K(I) Catalyst Performances in Carbon Dioxide and Propene Oxide Ring Opening Copolymerization

AbstractCarbon dioxide copolymerization is a front‐runner CO2 utilization strategy but its viability depends on improving the catalysis. So far, catalyst structure‐performance correlations have not been straightforward, limiting the ability to predict how to improve both catalytic activity and selectivity. Here, a simple measure of a catalyst ground‐state parameter, metal reduction potential, directly correlates with both polymerization activity and selectivity. It is applied to compare performances of 6 new heterodinuclear Co(III)K(I) catalysts for propene oxide (PO)/CO2 ring opening copolymerization (ROCOP) producing poly(propene carbonate) (PPC). The best catalyst shows an excellent turnover frequency of 389 h−1 and high PPC selectivity of &gt;99 % (50 °C, 20 bar, 0.025 mol% catalyst). As demonstration of its utility, neither DFT calculations nor ligand Hammett parameter analyses are viable predictors. It is proposed that the cobalt redox potential informs upon the active site electron density with a more electron rich cobalt centre showing better performances. The method may be widely applicable and is recommended to guide future catalyst discovery for other (co)polymerizations and carbon dioxide utilizations.

Correlating Metal Redox Potentials to Co(III)K(I) Catalyst Performances in Carbon Dioxide and Propene Oxide Ring Opening Copolymerization.

Carbon dioxide copolymerization is a front-runner CO2 utilization strategy but its viability depends on improving the catalysis. So far, catalyst structure-performance correlations have not been straightforward, limiting the ability to predict how to improve both catalytic activity and selectivity. Here, a simple measure of a catalyst ground-state parameter, metal reduction potential, directly correlates with both polymerization activity and selectivity. It is applied to compare performances of 6 new heterodinuclear Co(III)K(I) catalysts for propene oxide (PO)/CO2 ring opening copolymerization (ROCOP) producing poly(propene carbonate) (PPC). The best catalyst shows an excellent turnover frequency of 389 h-1 and high PPC selectivity of >99 % (50 °C, 20 bar, 0.025 mol% catalyst). As demonstration of its utility, neither DFT calculations nor ligand Hammett parameter analyses are viable predictors. It is proposed that the cobalt redox potential informs upon the active site electron density with a more electron rich cobalt centre showing better performances. The method may be widely applicable and is recommended to guide future catalyst discovery for other (co)polymerizations and carbon dioxide utilizations.

Elucidating the Racemization Mechanism of Aliphatic and Aromatic Amino Acids by In Silico Tools.

The racemization of biomolecules in the active site can reduce the biological activity of drugs, and the mechanism involved in this process is still not fully comprehended. The present study investigates the impact of aromaticity on racemization using advanced theoretical techniques based on density functional theory. Calculations were performed at the ωb97xd/6-311++g(d,p) level of theory. A compelling explanation for the observed aromatic stabilization via resonance is put forward, involving a carbanion intermediate. The analysis, employing Hammett's parameters, convincingly supports the presence of a negative charge within the transition state of aromatic compounds. Moreover, the combined utilization of natural bond orbital (NBO) analysis and intrinsic reaction coordinate (IRC) calculations confirms the pronounced stabilization of electron distribution within the carbanion intermediate. To enhance our understanding of the racemization process, a thorough examination of the evolution of NBO charges and Wiberg bond indices (WBIs) at all points along the IRC profile is performed. This approach offers valuable insights into the synchronicity parameters governing the racemization reactions.

Inferring the Energetics of CO2-Aniline Adduct Formation from Vibrational Spectroscopy.

Control of atmospheric CO2 is an important contemporary scientific and engineering challenge. Toward this goal, the reaction of CO2 with amines to form carbamate bonds is an established method for CO2 capture. However, controllable reversal of this reaction remains difficult and requires tuning the energetics of the carbamate bond. Through IR spectroscopy, we show that a characteristic frequency observed upon carbamate formation varies as a function of the substituent's Hammett parameter for a family of para-substituted anilines. We present computational evidence that the vibrational frequency of the adducted CO2 serves as a predictor of the energy of formation of the carbamate. Electron donating groups typically enhance the driving force of carbamate formation by transferring more charge to the adducted CO2 and thus increasing the occupancy of the antibonding orbital in the carbon-oxygen bonds. Increased occupancy of the antibonding orbital within adducted CO2 indicates a weaker bond, leading to a red-shift in the characteristic carbamate frequency. Our work serves the large field of CO2 capture research where spectroscopic observables, such as IR frequencies, are more easily obtainable and can stand in as a descriptor of driving forces.

Electronic Influences on the Dynamic Range of Photoswitchable Dithienylethene-Thiourea Organocatalysts.

Thiourea-based organocatalysts bearing a photoswitchable dithienylethene (DTE) core and a wide range of substituents were prepared and extensively tested for their ability to accelerate the Michael reaction between acetylacetone and trans-β-nitrostyrene. There is a strong correlation between the Hammett parameter of the modulating groups and catalytic activity following UV irradiation. Electron-withdrawing groups afford the largest reactivity difference between the catalysts in their ring-open form and their ring-closed isomer, with evidence for electronic coupling between the two halves in both oDTE and cDTE.

Structure–Activity Relationships for Hypervalent Iodine Electrocatalysis

AbstractThe design and optimization of novel electrocatalysts requires robust structure–activity data to correlate catalyst structure with electrochemical behavior. Aryl iodides have been gaining attention as metal-free electrocatalysts but experimental data are available for only a limited set of structures. Herein we report electrochemical data for a family of 70 aryl iodides. Half-peak potentials are utilized as proxies for reduction potentials and reveal that, despite differences in electrochemical reversibility, the potential for one-electron oxidation of 4-substituted aryl iodides to the corresponding iodanyl radicals is well-correlated with standard Hammett parameters. Additional data are presented for 3- and 2-substituted aryl iodides, including structures with potentially chelating 2-substituents that are commonly encountered in hypervalent iodine reagents. Finally, potential decomposition processes relevant to the (in)stability of iodanyl radicals are presented. We anticipate that the collected data will advance the design and application of aryl iodide electrocatalysis.

Ligand Hybridization for Electro‐reforming Waste Glycerol into Isolable Oxalate and Hydrogen

AbstractThe electro‐reforming of glycerol is an emerging technology of simultaneous hydrogen production and biomass valorization. However, its complex reaction network and limited catalyst tunability restrict the precise steering toward high selectivity. Herein, we incorporated the chelating phenanthrolines into the bulk nickel hydroxide and tuned the electronic properties by installing functional groups, yielding tunable selectivity toward formate (max 92.7 %) and oxalate (max 45.3 %) with almost linear correlation with the Hammett parameters. Further combinatory study of intermediate analysis and various spectroscopic techniques revealed the electronic effect of tailoring the valence band that balances between C−C cleavage and oxidation through the key glycolaldehyde intermediate. A two‐electrode electro‐reforming setup using the 5‐nitro‐1,10‐phenanthroline‐nickel hydroxide catalyst was further established to convert crude glycerol into pure H2 and isolable sodium oxalate with high efficiency.