Hammett Σp Parameters Research Articles

Emission Switching in the Near-Infrared by Reversible Trans-Cis Photoisomerization of Styrylbenzene-Conjugated Osmium Terpyridine Complexes.

A new array of homoleptic osmium(II) complexes based on styrylbenzene-conjugated terpyridine ligands (tpy-pvp-X) were synthesized and their photophysical, electrochemical, and photoisomerization behaviors thoroughly investigated in this work. Both electron-donating and -withdrawing substituents were incorporated onto a tpy-pvp-X (X = H, Me, Cl, NO2, and Ph) moiety to tune the optical properties and also the rate of photoisomerization behaviors in the complexes. All complexes display strong spin-allowed singlet metal-to-ligand charge-transfer bands in the visible (495-506 nm) and weak singlet ground state to triplet metal-to-ligand charge-transfer (3MLCT) broad bands within the 600-700 nm range. The complexes also exhibit strong phosphorescence emission from their 3MLCT state in the near-infrared domain (737-752 nm) at room temperature with excited-state lifetimes spanning between 107 and 165 ns. Two styrylbenzene units promote reversible trans-trans to trans-cis/cis-cis isomerization induced by light. The rate constants and quantum yields of photoisomerization were found to vary linearly with the Hammett σp parameters of the substituents. The rate and quantum yields were also found to decrease with increasing polarity of the solvents. Considerable modulation of the optical behavior along with luminescence switching in the complexes has been achieved upon photoisomerization. Moreover, the optical outputs as a function of two photonic stimuli inputs were used to demonstrate the binary function of a two-input IMPLICATION logic gate. In conjunction with the experimental study, computational investigations were also carried out in all three conformations of the complexes (trans-trans, trans-cis, and cis-cis) to have a perception of their electronic structures and for correct assignment of their absorption and emission spectral bands.

Photonic materials derived from the [closo-B10H10]2−anion: tuning photophysical properties in [closo-B10H8-1-X-10-(4-Y-NC5H5)]−

Substitution of the parent [closo-B10H9-1-NC5H5]−either at the B(10) position or at the pyridine ring tunes absorption and emission energy.

Effect of Ligands on the Lewis Acidity of the Metal and the Binding of N‐Bases to Iridium Pincer Complexes

The coordination of pyridine (Py) and benzonitrile (PhCN) to benzene based pincer hydridochlorides [(PCP)IrH(Cl)] (1), [(POCOP)IrH(Cl)] (2), [{EtO(O)C‐POCOP}IrH(Cl)] (3), and [(PCN)IrH(Cl)] (4) was studied spectroscopically and computationally to deduce the ligand influence. The variable temperature NMR (1H, 31P, 15N) and UV/Visible spectroscopic measurements revealed preferential coordination of these N‐donor ligands in the apical position of 1–4 and gave the formation enthalpies for the hexacoordinate complexes, which follow the order: 1 ≈ 4 > 3 > 2. This order nicely agrees with the order of Lewis acidity obtained for these complexes in DFT calculations. The orbital and electron density distribution analysis were performed at the DFT/M06 theory level for 1–4 and a series of p‐substituted PCP‐based hydridochlorides. The increasing Lewis acidity of iridium [as a maximum energy, VS,max, of molecular electrostatic potential (MEP)] correlates with decreasing basicity of Cl‐ligand (as MEP minimum, VS,min) as well as with Hammett σp parameters of p‐substituents. Importantly, these properties are conserved upon conversion into the corresponding dihydrides, as shown on the example of 1 and 4.

Synthesis, Photophysics, and Switchable Luminescence Properties of a New Class of Ruthenium(II)-Terpyridine Complexes Containing Photoisomerizable Styrylbenzene Units.

We report here the synthesis and structural characterization of a new class of homoleptic terpyridine complexes of Ru(II) containing styrylbenzene moieties to improve room-temperature luminescence properties. Solid-state structure determination of 2 was done through single-crystal X-ray diffraction. Tuning of photophysical properties was done by incorporating both electron-donating and electron-withdrawing substituents in the ligand. The complexes exhibit strong emission having lifetimes in the range of 10.0–158.5 ns, dependent on the substituent and the solvent. Good correlations were also observed between Hammett σp parameters with the lifetimes of the complexes. Styrylbenzene moieties in the complexes induce trans–trans to trans–cis isomerization accompanied by huge alteration of their spectral profiles upon treating with UV light. Reversal of trans–cis to trans–trans forms was also achieved on interacting with visible light. Change from trans–trans to the corresponding trans–cis form leads to emission quenching, whereas trans–cis to the corresponding trans–trans form leads to restoration of emission. In essence, “on–off” and “off–on” photoswitching of luminescence was observed. Calculations involving density functional theory (DFT) and time-dependent-DFT methods were performed to understand the electronic structures as well as for appropriate assignment of the absorption and emission bands.

The 6,6-dicyanopentafulvene core: a template for the design of electron-acceptor compounds.

The electron-accepting ability of 6,6-dicyanopentafulvenes (DCFs) can be varied extensively through substitution on the five-membered ring. The reduction potentials for a set of 2,3,4,5-tetraphenyl-substituted DCFs, with varying substituents at the para-position of the phenyl rings, strongly correlate with their Hammett σp-parameters. By combining cyclic voltammetry with DFT calculations ((U)B3LYP/6-311+G(d)), using the conductor-like polarizable continuum model (CPCM) for implicit solvation, the absolute reduction potentials of a set of twenty DCFs were reproduced with a mean absolute deviation of 0.10 eV and a maximum deviation of 0.19 eV. Our experimentally investigated DCFs have reduction potentials within 3.67-4.41 eV, however, the computations reveal that DCFs with experimental reduction potentials as high as 5.3 eV could be achieved, higher than that of F4-TCNQ (5.02 eV). Thus, the DCF core is a template that allows variation in the reduction potentials by about 1.6 eV.

Homoleptic nickel(II) complexes of redox-tunable pincer-type ligands.

Different synthetic methods have been developed to prepare eight new redox-active pincer-type ligands, H(X,Y), that have pyrazol-1-yl flanking donors attached to an ortho-position of each ring of a diarylamine anchor and that have different groups, X and Y, at the para-aryl positions. Together with four previously known H(X,Y) ligands, a series of 12 Ni(X,Y)2 complexes were prepared in high yields by a simple one-pot reaction. Six of the 12 derivatives were characterized by single-crystal X-ray diffraction, which showed tetragonally distorted hexacoordinate nickel(II) centers. The nickel(II) complexes exhibit two quasi-reversible one-electron oxidation waves in their cyclic voltammograms, with half-wave potentials that varied over a remarkable 700 mV range with the average of the Hammett σ(p) parameters of the para-aryl X, Y groups. The one- and two-electron oxidized derivatives [Ni(Me,Me)2](BF4)n (n = 1, 2) were prepared synthetically, were characterized by X-band EPR, electronic spectroscopy, and single-crystal X-ray diffraction (for n = 2), and were studied computationally by DFT methods. The dioxidized complex, [Ni(Me,Me)2](BF4)2, is an S = 2 species, with nickel(II) bound to two ligand radicals. The mono-oxidized complex [Ni(Me,Me)2](BF4), prepared by comproportionation, is best described as nickel(II) with one ligand centered radical. Neither the mono- nor the dioxidized derivative shows any substantial electronic coupling between the metal and their bound ligand radicals because of the orthogonal nature of their magnetic orbitals. On the other hand, weak electronic communication occurs between ligands in the mono-oxidized complex as evident from the intervalence charge transfer (IVCT) transition found in the near-IR absorption spectrum. Band shape analysis of the IVCT transition allowed comparisons of the strength of the electronic interaction with that in the related, previously known, Robin-Day class II mixed valence complex, [Ga(Me,Me)2](2+).

Analysis of the Palladium-Catalyzed (Aromatic)C–H Bond Metalation–Deprotonation Mechanism Spanning the Entire Spectrum of Arenes

A comprehensive understanding of the C-H bond cleavage step by the concerted metalation-deprotonation (CMD) pathway is important in further development of cross-coupling reactions using different catalysts. Distortion-interaction analysis of the C-H bond cleavage over a wide range of (hetero)aromatics has been performed in an attempt to quantify the various contributions to the CMD transition state (TS). The (hetero)aromatics evaluated were divided in different categories to allow an easier understanding of their reactivity and to quantify activation characteristics of different arene substituents. The CMD pathway to the C-H bond cleavage for different classes of arenes is also presented, including the formation of pre-CMD intermediates and the analysis of bonding interactions in TS structures. The effects of remote C2 substituents on the reactivity of thiophenes were evaluated computationally and were corroborated experimentally with competition studies. We show that nucleophilicity of thiophenes, evaluated by Hammett σ(p) parameters, correlates with each of the distortion-interaction parameters. In the final part of this manuscript, we set the initial equations that can assist in the development of predictive guidelines for the functionalization of C-H bonds catalyzed by transition metal catalysts.

The Use of NMR Chemical Shifts to Predict Reaction Pathways: Methanol Formation from Oxazolidinones

AbstractThe reduction of various N‐substituted oxazolidin‐2‐ones with LiAlH4 was investigated to open a pathway for methanol formation. By 15N and 13C NMR analysis and correlation with Hammett σp parameters appropriate substituents can be found which direct the reduction into the desired pathway. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Synthesis, spectroscopy and redox properties of mononuclear manganese(II) and manganese(IV) complexes with N-(aryl)-pyridine-2-aldimine (L) and its amide derivatives. X-ray structural characterization of [Mn(MeL)2(NCS)2] (MeL = N-(4-methylphenyl)-pyridine-2-aldimine)

A series of mononuclear MnII and MnIV complexes of general formulae [MnL2(NCS)2] (1a–1d) and [Mn(L′)2(NCS)2] (2a–2c) have been prepared where L are Schiff bases obtained by the condensation of pyridine-2-aldehyde with para-alkyl-substituted aniline, and L′ are the corresponding amide ligands. The room temperature magnetic susceptibility data of (1a–1d) indicate that MnII is in a high spin state. The cyclic voltammograms of (1a–1d) exhibit a one-electron quasi-reversible MnII→MnIII oxidation. A linear correlation has been found when E0[MnIII/MnII] is plotted against Hammett σp parameters. X-ray crystallographic data of (1b) shows that the central MnII ion adopts a distorted octahedral geometry with six different Mn–N distances. Upon oxidation of MnII complexes (1b–1d) by H2O2, the corresponding MnIV complexes (2a–2c) were obtained, and the Schiff base ligands were oxidized to the corresponding amides. The lowest energy LMCT bands of these MnIV complexes correlate linearly with Hammett σp parameters. The redox behavior of the MnIV complexes has been investigated by cyclic voltammetry. E.p.r. spectra of the MnII and MnIV complexes are also reported.

Synthesis and Electronic Properties of Alkynylated Phenothiazines

AbstractAlkynylated phenothiazines 1, 2, 5, 7, and 12−15 can be synthesized in moderate to excellent yields by sequences of aldehyde−alkyne transformations and/or Sonogashira cross‐coupling reactions from suitable phenothiazine aldehydes or bromides. The electronic properties of (hetero)aryl ethynyl‐substituted N‐methyl‐phenothiazines (UV/Vis absorption, fluorescence, redox potentials) strongly correlate with Hammett σP parameters and indicate that remote substituents transmit their electronic information through π‐electron delocalization and the σ‐framework. Phenothiazinyl dyads (7g, 12, 13) and triads (14) reveal different degrees of intramolecular electronic coupling as demonstrated by cyclic voltammetry, qualifying them as good model candidates for redox‐active molecular wires. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Rank correlation of AM1 and PM3 derived molecular electrostatic potentials (RACEL) with Hammett σp-parameters

Spearman non-parametric correlation coefficients obtained by comparison of the rank order of Hammett σp-parameters and the value of AM1 and PM3 derived molecular electrostatic potentials calculated for a range of para-substituted benzoic acids and amines show a chemically significant three-dimensional spatial distribution; the correlation is best for PM3, is improved for carboxylic acids by the inclusion of a solvent water molecule and is substantially decreased if point-charge derived electrostatic potentials are employed.

Nitrogen-15 nuclear magnetic resonance spectroscopy and electronic effects in dinitrogen complexes of molybdenum and tungsten with trans nitrile and diphosphine ligands

Nitrogen-15 chemical shifts and oxidation potentials are reported for the complexes trans-[M(15N2)(NCR)(P–P)2](A)[M = Mo or W; R = Me, Prn, or 4-XC6H4(X = H, MeO, MeCO, or F); P–P = Ph2PCH2CH2PPh2 or Et2PCH2CH2PEt2]. Linear correlations are observed for values of δ(15N), E½ox, and Hammett σp parameters for the X substituent of the benzonitrile. The shielding of both nitrogens in the 15N2 ligand decreases with increase of (σ+π) electron-releasing properties of the trans ligand on replacement of one N2 in the complexes trans-[M(15N2)2(P–P)2](B) by NCR. The shielding of (only) the ligating nitrogen of 15N2 in (A) increases with increase of the σ-donor ability of the trans nitrile ligand as the R group is varied.