Switching the Electron-Donating Ability of Phosphines through Proton-Responsive Imidazolin-2-ylidenamino Substituents

Stimuli-responsive ancillary ligands are valuable tools to control the activity and selectivity of transition-metal catalysts. The synthesis and characterization of a series of metal complexes containing phosphines with proton-responsive imidazolin-2-ylidenamino substituents are reported. Determination of the ligand-donor properties revealed that protonation of each substituent increases the Tolman electronic parameter (TEP) of the phosphine by 22 cm⁻¹, hence allowing for switching of the electron-donor power of phosphine 2 within an unprecedented range (ΔTEP=43.4 cm⁻¹).     

On the Edge of the Known: Extremely Electron-Rich (Di)Carboranyl Phosphines

The syntheses of the first B9-connected carboranylphosphines (B9-Phos) featuring two carboranyl moieties as well as access to B9-Phos ligands with bulky electron-donating substituents, previously deemed unattainable, is reported. The electrochemical properties of the B9-Phos ligands were investigated, revealing the ability of the mesityl derivatives to form stabilized phosphoniumyl radical cations. The B9-Phos ligands display an extremely electron-releasing character surpassing that of alkyl phosphines and commonly used N-heterocyclic carbenes. This is demonstrated by their very small Tolman electronic parameters (TEPs) as well as extremely low P−Se coupling constants. Cone angles and buried volumes attest to the high steric demand exerted by the (di)carboranyl phosphines. The dicarboranyl phosphine Au^I complexes show superior catalytic performance in the hydroamination of alkynes compared to the monocarboranyl phosphine analogs.     

The Reaction of Triphenylmethyl-substituted Primary and Secondary Phosphines with Phosgene

The nature of the products from the reaction of TrtPH₂ ( 1 ) with an equimolar amount of phosgene strongly depends on the solvent. The initial intermediate 2 was isolated from toluene, but lost CO in dichloromethane, and HCl in diethyl ether, yielding TrtP(H)Cl ( 3 ), and (TrtPCO)₂ ( 4 b ), respectively. TrtP(H)Cl ( 3 ) was found to be a halophosphine of amazing stability. Treatment of 3 with excess phosgene led to partial substitution of the P-bonded proton for C(:O)Cl with formation of 5 , which did not eliminate CO to give TrtPCl₂. Substitution of chlorine in TrtP(H)Cl ( 3 ) for fluorine or bromine furnished the halophosphines, 6 and 7 . Minute quantities of the diphosphene 8 were formed upon treatment of 3 with NEt₃ or DBU. The course of the reaction between the secondary phosphines Trt(R)PH and phosgene depends on the group R. If R was t-Bu, formation of a mixture of Trt(t-Bu)PCl ( 10 ), and t-BuPCl₂ (resulting from partial cleavage of the P–C-bond) was observed. Although in the case of R = Ph, the intermediate 12 could be isolated, at elevated temperature HCl was eliminated from 12 , giving Trt(Ph)PCl ( 13 ). The diphosphine (TrtPH)₂ ( 14 ) is inert towards HCl-free phosgene. In the presence of HCl the P–P-bond in 14 was cleaved, and upon chlorination of the resulting TrtPH₂ ( 1 ) by phosgene, TrtP(H)Cl ( 3 ) was obtained as the only phosphorus-containing product.     

Control of Chromophore Symmetry by Positional Isomerism of Peripheral Substituents

The first example of the control of porphyrinoid chromophore symmetry based on the positional isomerism of peripheral substituents has been achieved by preparing tetraazaporphyrins (TAPs) with C_4h, D_2h, C_2v, and C_s symmetry due to the relative arrangement of peripheral tert‐butylamino and cyano groups as push and pull substituents, respectively. The four structural isomers were successfully isolated and characterized by ¹H NMR spectroscopy and X‐ray crystallography. The band morphology in the Q‐band region varies depending on the molecular symmetry due to the significant perturbation introduced into the chromophore by the push and pull substituents. The C_4h and C_2v isomers exhibit a single Q band, whereas the Q bands of the D_2h and C_s isomers show a marked splitting. The magnetic circular dichroism spectra indicate that the push–pull TAPs retain the properties of the 16‐membered 18π‐electron perimeter generally observed for porphyrinoids. Theoretical calculations have demonstrated that the perturbation introduced by the substituents lowers the D_4h symmetry of the parent TAP π‐conjugated system, and this results in significant spectral changes. A novel approach to the fine‐tuning of the spectral properties of porphyrinoids based on changes in the chromophore symmetry is described.     

Regiochemistry Control by Bipyridine Substituents in the Deprotonation of ReI and MoII N-Alkylimidazole Complexes

Compounds containing N-alkylimidazoles (N-RIm) and 4,4′-disubstituted 2,2′-bipyridines (4,4′-R′₂bipy) coordinated to cationic {Mo(η³-C₄H₇)(CO)₂} and {Re(CO)₃} fragments undergo deprotonation of the C2-H group of the N-RIm ligands in their reactions with KN(SiMe₃)₂. The resulting internal nucleophile adds either to one pyridyl ring, which becomes dearomatized and can undergo ring opening in the subsequent reaction with excess MeOTf, or to the metal center, yielding imidazol-2-yl complexes, which in turn add HOTf or MeOTf, affording N-heterocyclic carbene complexes. Which pathway is followed is dictated by the metal and the nature of the imidazole (R) and bipyridine (R′) substituents. For Re^I compounds, addition to pyridine is found with R′=tBu and OMe, whereas for R=Me and R′=NMe₂, imidazolyl formation is preferred. Coordination of 4,7-Cl₂-1,10-phenanthroline to Mo^II favors C−C coupling, in contrast to the analogous parent bipy or phenanthroline complexes, for which formation of the imidazol-2-yl complexes had been found. DFT calculations showed the theoretically expected products in each case, and following their predictions new types of products were obtained experimentally.     

Phosphines catalyzed nucleophilic addition of azoles to allenes: synthesis of allylazoles and indolizines

Triphenylphosphine was used as nucleophilic catalyst for umpolung addition of azoles to electron-deficient allenes. This strategy offers a simple and efficient method for functional allylation of azoles under neutral conditions and affords heterocyclic substituted Michael olefins. Furthermore, this catalytic methodology has been extended to addition-cyclization reactions between electron-deficient allenes or alkynes and pyrrole-2-carboxaldehyde in the presence of catalytic amount of tributylphosphine. In such conditions, substituted indolizine-7-carboxylates are easily obtained.     

A Theoretical and Experimental Study of the Effects of Silyl Substituents in Enantioselective Reactions Catalyzed by Diphenylprolinol Silyl Ether

The effect of silyl substituents in diphenylprolinol silyl ether catalysts was investigated. Mechanistically, reactions catalyzed by diphenylprolinol silyl ether can be categorized into three types: two that involve an iminium ion intermediate, such as for the Michael‐type reaction (type A) and the cycloaddition reaction (type B), and one that proceeds via an enamine intermediate (type C). In the Michael‐type reaction via iminium ions (type A), excellent enantioselectivity is realized when the catalyst with a bulky silyl moiety is employed, in which efficient shielding of a diastereotopic face of the iminium ion is directed by the bulky silyl moiety. In the cycloaddition reaction of iminium ions (type B) and reactions via enamines (type C), excellent enantioselectivity is obtained even when the silyl group is less bulky and, in this case, too much bulk reduces the reaction rate. In other cases, the yield increases when diphenylprolinol silyl ethers with bulky substituents are employed, presumably by suppressing side reactions between the nucleophilic catalyst and the reagent. The conformational behaviors of the iminium and enamine species have been determined by theoretical calculations. These data explain the effect of the bulkiness of the silyl substituent on the enantioselectivity and reactivity of the catalysts.     

不同取代基巯基偶氮苯甲酸TiO2光催化降解的密度泛函理论研究

以TiO2为光催化剂,对含六种不同取代基的巯基偶氮苯甲酸进行了光催化降解实验,并采用量子化学密度泛函理论B3LYP/6-31G**计算了巯基偶氮苯甲酸的电子结构,研究了巯基偶氮苯甲酸中推、拉电子取代基对其光催化降解活性的影响.结果表明,拉电子基(-COOH或-SO3H)的引入使巯基偶氮苯甲酸的偶极矩增大,绝对电负性减小,最高占有轨道能量升高,HOMO-LUMO能隙和C-N键的键级降低,从而提高了巯基偶氮苯甲酸的光催化降解活性,而分子中推电子基团的影响则相反.     

Effect of Substituents in Mussel-inspired Surface Primers on their Oxidation and Priming Efficiency

Marine mussels contain an abundant catechol moiety, 3,4-dihydroxyphenylalanine (DOPA), in their interfacial foot proteins. DOPA contributes to both surface adhesion and bridging between the surface and overhead proteins (surface priming) by taking advantage of the unique redox properties of catechol. Inspired by the mussel surface priming mechanism, herein we synthesized a series of DOPA-mimetic analogs – a bifunctional group molecule, consisting of a catechol group and an acrylic group at the opposite ends. The surface primers with differently substituted (−COOH, −CH₃) alkyl chains in the middle spacer were synthesized. Time-dependent oxidation and redox potentials of the surface primers were studied in an oxidizing environment to gain a better understanding of the mussel‘s redox chemistry. The thickness and degree of priming of the surface primers on silicon-based substrates were analyzed by ellipsometry and UV/Vis absorption spectroscopy. The post-reactivity of the acrylic groups of the primed layer was first visualized through a reaction with an acrylic group-reactive dye.     

Synthesis of triarylphosphines having para -SH and -SMe groups: Preparation of their complexes and formation of a monolayer on a gold surface

The phosphines P(C₆H₄-4-SR)₃ (R = H, Me, 2-C₅H₉O) and (C₆H₄-4-SR)₂PCH₂CH₂P(C₆H₄-4-SR)₂ (R = H, Me) have been synthesized. The phosphines with -SMe groups can be prepared by reaction of 4-BrC₆H₄SMe with either BuLi or magnesium (to generate the corresponding Grignard compound) followed by reaction with PCl₃ or Cl₂PCH₂CH₂PCl₂, respectively. The methyl group can be eliminated by reaction with sodium in liquid NH₃. Other methods of protection/deprotection of the thiol group failed to afford the desired compounds. Reaction of 4-BrC₆H₄SH with dihydropyrane afforded the protected thiol 4-BrC₆H₄S-2-C₅H₉O from which the corresponding phosphine was successfully synthesized. However, attempts to remove the tetrahydropyranyl group by reaction with AgNO₃-HCl, gave an insoluble polymer as product. Reaction of P(C₆H₄SR)₃ (R = H, Me) with Ni(CO)₄ affords the corresponding mono phosphine complex quantitatively. The complex with the unprotected thiol group can be absorbed on a gold surface and the corresponding ν_CO bands were detected by grazing angle Fourier transform infrared reflection absorption spectroscopy (grazing angle FTIR-RAS). Reaction of Rh(acac)(CO)₂ with P(C₆H₄SR)₃ (R = Me) affords the complex Rh(acac)(CO)(P(C₆H₄SR)₃) (R = Me), but if R = H a polymer insoluble in any solvent was obtained. The same occurs in the case of PtCl₂(CO)(DMSO). Apparently, once P(C₆H₄SH)₃ is coordinated to a metal not in the zero oxidation state, oxidation of the thiol group to disulphide becomes very easy even in a dinitrogen atmosphere.     

Stereoselective Synthesis of Fluoroalkenoates and Fluorinated Isoxazolidinones: N‐Substituents Governing the Dual Reactivity of Nitrones

α‐Fluoroalkenoates and 4‐fluoro‐5‐isoxazolidinones are of vast interest due to their potential biological applications. We now demonstrate the syntheses of (E)‐α‐fluoroalkenoates and 4‐fluoro‐5‐isoxazolidinones by the reactions between nitrones and α‐fluoro‐α‐bromoacetate. By altering N‐substituents in nitrones, (E)‐α‐fluoroalkenoates and 4‐fluoro‐5‐isoxazolidinones can be achieved, respectively, with high chemo‐ and stereoselectivities. Experimental and computational studies have been conducted to elucidate the reaction mechanisms. Linear free energy relationship studies further revealed that the N‐substituent effects are primarily of electronic origin.     

Synergistic Effects of Lewis Bases and Substituents on the Electronic Structure and Reactivity of Boryl Radicals

Boryl radicals have the potential for the development of new molecular entities and for application in new radical reactions. However, the effects of the substituents and coordinating Lewis bases on the reactivity of boryl radicals are not fully understood. By using first‐principles methods, we investigated the spin‐density distribution and reactivity of a series of boryl radicals with various substituents and Lewis bases. The substituents, along with the Lewis bases, only affect the radical reactivity when an unpaired electron is in the boron p_z orbital, that is, for three‐coordinate radicals. We found evidence of synergistic effects between the substituents and the Lewis bases that can substantially broaden the tunability of the reactivity of the boryl radicals. Among Lewis bases, pyridine and imidazol‐2‐ylidene show a similar capacity for stabilization by delocalizing the spin density. Electron‐donating substituents, such as nitrogen, more efficiently stabilize boryl radicals than oxygen and carbon atoms. The reactivity of a boryl radical is always boron based, irrespective of the spin density on boron.     

取代基对手性唑噁唑硼烷催化还原苯基乙基酮对映体选择性影响的理论研究

用AM1方法和过渡态理论研究了取代基对含侧链手性噁唑硼烷催化还原苯 基乙杂程逖≡裥缘挠跋欤扑憬峁砻鳎杂逞≡裥匀【鲇?R)和(S)两个对映体? 叫蟹从Φ乃俾食Ｊ龋盋(4)和C(5)位上的取代基呈顺式时，所得对映体以(R) 型为主．对映选择性的高低与噁唑硼烷环上不同位置原子连接的取代基基团的大? ∮泄兀扑憬峁胧笛樽芙岢龅慕崧巯辔呛希? $601 $AThe influence of substituents on the enantioselectivity in asymmetric branched-oxazaborolidine-catalyzed reductions of phenyl- ethyl ketone has been studied by using AMI MO method and transition state theory. The results show that the enantioselectivity is determined by the ratio of the rate constants of two parallel reactions conducting to spatial configurations (R) and (S). When the substituents connecting with C(4) and C(5) are in cis-form the reduction products of phenyl-ethyl ketone in the reactions are mainly of configuration ( R). In asymmetric boron-catalyzed reduction reactions the higher or lower magnitude for enantioselectivity and reaction barrier are governed by various substituents connecting with atoms of oxazaborolidine ring. The computational results are in consistence with experiments.     

A label-free colorimetric detection of lead ions by controlling the ligand shells of gold nanoparticles

We have developed a simple, colorimetric and label-free gold nanoparticle (Au NP)-based probe for the detection of Pb²⁺ ions in aqueous solution, operating on the principle that Pb²⁺ ions change the ligand shell of thiosulfate (S₂O₃²⁻)-passivated Au NPs. Au NPs reacted with S₂O₃²⁻ ions in solution to form Au⁺·S₂O₃²⁻ ligand shells on the Au NP surfaces, thereby inhibiting the access of 4-mercaptobutanol (4-MB). Surface-assisted laser desorption/ionization time-of-flight ionization mass spectrometry (SALDI-TOF MS) and inductively coupled plasma mass spectrometry (ICP-MS) measurements revealed that PbAu alloys formed on the surfaces of the Au NPs in the presence of Pb²⁺ ions; these alloys weakened the stability of the Au⁺·S₂O₃²⁻ ligand shells, enhancing the access of 4-MB to the Au NP surfaces and, therefore, inducing their aggregation. As a result, the surface plasmon resonance (SPR) absorption of the Au NPs red-shifted and broadened, allowing quantitation of the Pb²⁺ ions in the aqueous solution. This 4-MB/S₂O₃²⁻-Au NP probe is highly sensitive (linear detection range: 0.5-10 nM) and selective (by at least 100-fold over other metal ions) toward Pb²⁺ ions. This cost-effective sensing system allows the rapid and simple determination of the concentrations of Pb²⁺ ions in real samples (in this case, river water, Montana soil and urine samples).     

A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes

The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base‐metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)₂]²⁺ (bpp=2,6‐di{pyrazol‐1‐yl}pyridine) can be high‐spin, low‐spin, or spin‐crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T ) in solution vs. the relevant Hammett parameter show that the low‐spin state of the complex is stabilized by electron‐withdrawing pyridyl (“X”) substituents, but also by electron‐donating pyrazolyl (“Y”) substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe‐L σ and π bonding.     

Effects of Substituents on Metastable‐State Photoacids: Design,Synthesis, and Evaluation of their Photochemical Properties

Recently, metastable‐state photoacids have been widely used to control proton transfer in numerous chemical and biological processes as well as applications with visible light. Generally, substituents have a great influence on the photochemical properties of molecules, which will further affect their applications. Yet, the effects of substituents on metastable‐state photoacids have not been studied systematically. In this work, 16 metastable‐state photoacid derivatives were designed and synthesized on the basis of substituents having a large range of σ–π electron–donor–acceptor capabilities. The effects of substituents on the color display [or maximum absorption band(s)], solubility, pK_a values, dark/photoacidity, photosensitivity, and relaxation kinetic(s) were investigated in detail. This study will be helpful for the targeted design and synthesis of promising photoacids and the application of their photocontrolled proton‐release processes in functional materials/devices.     

共轭CN与非共轭CF3吸电子取代基对PPV类衍生物分子结构及光电性质的影响

运用密度泛函DFT B3LYP／6-31G(d)方法对CN和CF3吸电子基团取代的PPV类衍生物的三聚体进行了几何构型优化,并采用含时密度泛函TD-DFT、B3LYP／6-31G(d)方法计算了其相应化合物的紫外吸收光谱．通过对CN和CF3取代的PPV类衍生物的分子几何结构、前线分子轨道能级、电子云分布规律的分析,从理论上解释了共轭CN与非共轭CF3吸电子取代基对其光谱性质影响的差异：前者使相应PPV类衍生物的吸收光谱发生红移,后者则发生蓝移．计算结果还表明用TD-DFT方法计算该体系的紫外吸收光谱值与实验数据吻合得很好;另外引入CN和CF3基团之后,使其相应的PPV衍生物的LUMO能级降低,电子亲合势增加,都是很好的电子传输材料．     

The density of surface ligands regulates the luminescence of thiolated gold nanoclusters and their metal ion response

The fascinating luminescence properties of gold nanoclusters(AuNCs) have drawn considerable research interests,and been widely harnessed for a wide range of applications.However,a fundamental understanding towards ligand density's role in the luminescence properties of these ultrasmall AuNCs remains unclear yet.In this communication,through systematic investigation of surface chemistries of glutathione-protected Au NCs(GSH-Au NCs) with diffe rent density of GSH as well as other thiolates,it is discovered that the density of surface ligands can significantly regulate the luminescence properties of AuNCs.Fluorescence lifetime spectroscopy and X-ray photoelectron spectroscopy showed that AuNCs with a higher density of electron-rich ligands facilitate their luminescence generation.Moreover,differences in the surface coverage of AuNCs can also affect their interactions with foreign species,as illustrated by significantly different fluorescence quenching capability of GSH-AuNCs with different ligand density towards Hg~(2+).This study provides new insight into the intriguing luminescence properties of metal NCs,which is hoped to stimulate further research on the design of metal NCs with strong luminescence and sensitive/specific responses for promising optoelectronic,sensing and imaging applications.