An investigation of the Excitation Wavelength-Dependent Dynamic Changes in the Mechanism of Detection of Picric Acid using Pyrene-Based Donor-Acceptor Systems.

Picric acid (PA) is an important industrial feedstock and hence the release of industrial effluents without proper remediation results in its buildup in soil and water bodies. The adverse effects of PA accumulation in living beings necessitate the development of efficient methods for its detection and quantification. Herein, we describe pyrene-based fluorescent sensors for PA, where pyrene is appended with electron-withdrawing groups, malononitrile, and 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile (DCDHF). These molecules displayed the typical emission of pyrene monomers, as well as a broad red-shifted emission resulting from an intramolecular charge transfer (ICT) in the excited state. Both the emissions displayed a turn-off response to PA with high selectivity and sensitivity and the lowest limit of detection was estimated as 27 nM. To prove the feasibility of on-site detection, test paper strips were prepared, which could detect PA up to 4.58 picograms. Using a combination of experimental and theoretical studies the mechanism of the detection was identified as primary/secondary inner filter effect, oxidative photoinduced electron transfer, or a combination of both depending on the excitation wavelength. Interestingly, the contribution of each of these mechanisms to the total quenching process varied with a change in the excitation wavelength.

Pentacycloundecanylidene and pentacycloundecanone - hyperconjugatively stabilized carbene and ketone.

Pentacycloundecanylidene was spectroscopically identified during the photolysis of the corresponding aziridine and its aerial oxidation results in the corresponding ketone. Here we report the role of hyperconjugative interactions in stabilizing the singlet pentacycloundecanylidene and its corresponding ketone pentacycloundecanone. The pentacycloundecanylidene possesses a singlet ground state with two possible geometrical isomers based on the orientation of the carbene bridge (U1 and U2). The energy difference between U1 and U2 is minimal (0.9 kcal mol-1) and the triplet state is 5.6 kcal mol-1 energetically higher than the more stable singlet state U1. The proximal C-C bonds of the carbene bridge in the singlet state are significantly elongated as compared to the distal C-C bonds. The bending of the carbene bridge in the triplet state and the carbonyl group in the ketone are minimal as compared to the parent pentacycloundecane. The molecular orbital (MO), natural bond orbital (NBO) and energy decomposition analysis (EDA) show that both Cieplak-type hyperconjugation viz. donation of electrons from the proximal C-C σ bonds to the empty p-orbital on the carbene centre and Felkin-Anh type hyperconjugation viz. donation of the lone pair of carbene carbon to the distal C-C σ* orbitals exist in carbene systems. The bending of the carbene bridge in singlet carbene is to enhance the Cieplak-type hyperconjugative interaction. The ketone is also stabilized by Cieplak-type hyperconjugative interaction but to a lower extent as compared to the singlet carbene. The reactivity study suggests that the singlet pentacycloundecanylidene is ambiphilic in nature.

Beryllium carbonyl Be(CO)n (n = 1-4) complex: a p-orbital analogy of Dewar-Chatt-Duncanson model.

Transition metal-carbonyl bonds are rationalized by M ← CO σ donation and M → CO π back donation where the d orbital of the transition metal is involved. This bonding model provided by Dewar, Chatt and Duncanson (DCD) has rationalized many transition metal-ligand bonds. The involvement of p orbital in such a DCD model can be intriguing. Alkaline earth metals with ns2np0 configuration may appear suitable as ns0np2 excitation has been recognized in many complexes. Herein, a theoretical study is presented for the Be(CO)n (n = 1-4) complex to verify this assumption. Detailed electronic structure analyses confirmed the involvement of the p orbital of beryllium in M → CO π back donation, thereby supporting the hypothesis. EDA-NOCV results reveal that the π-back donation from the central Be atom to CO ligands significantly predominates over the σ donation from the ligands for both Be(CO)3 and Be(CO)4. Our calculations reveal that Be(CO)4 is the highest carbonyl that may be experimentally detected.

A Neutral Planar Four-Membered Si2B2 2π-Aromatic Ring.

The synthesis of the first inorganic four-membered neutral 2π-aromatic compound 2 is reported. This unique ring has been synthesized from a simple and straightforward reaction of amidinato-silylene with dichlorophenylborane, followed by the reduction with KC8 in THF. Compound 2 has been fully characterized by single-crystal X-ray diffraction (SC-XRD), NMR spectroscopy, and mass spectrometry. The computational calculations reveal that the Si2B2 ring is a π-delocalized system resulting from the interaction of pπ orbital of B and Si-N σ* orbitals having pseudo π symmetry. Compound 2 is the first known example of a neutral planar inorganic analogue of cyclobutenyl dication.

Dative quadruple bonds between d10 transition metals and beryllium in BeM(PMe3 )2 and BeM(CO)2 (M = Ni, Pd, and Pt) complexes: Transition metal fragments as six-electron donor and two-electron acceptor.

The structure, chemical bonding, and reactivity of neutral 16 valence electrons (VE) transition metal complexes of beryllium, BeM(PMe3 )2 (1M-Be) and BeM(CO)2 (2M-Be, M = Ni, Pd, and Pt) were studied. The molecular orbital and EDA-NOCV analysis suggest dative quadruple bonds between the transition metal and beryllium, viz., one Be→M σ bond, one Be←M σ bond, and two Be←M π bonds. The strength of these bonding interactions varies based on the ligands coordinated to the transition metal. The Be←M σ bond is stronger than the Be→M σ bond when the ligand is PMe3, whereas the reverse order is observed when the ligand is CO. This is attributed to the higher π acceptor strength of CO as compared to PMe3 . Since these complexes have M-Be dative quadruple bonds, the beryllium center is susceptible to ambiphilic reactivity, as indicated by high proton and hydride affinity values.

σ versus π-radical: Tuning the electronic nature of neutral carbon (I) compounds with three non-bonding electrons.

The bonding and reactivity of the hypo-coordinated compounds with one, two, and four non-bonding electrons namely, carbon-centered free radical, carbenes, and carbones were well earlier established. Here, we report stability, bonding and reactivity of compounds RCL, where R is one-electron donor group (R = ï£¿CH3 (a), CHO (b), and NO2 (c)) and L is two-electron donor ligand (L = cAAC (1), CO (2), NHC (3) and PMe3 (4)), having three non-bonding electrons. The ground states of molecules exist in a doublet with a lone pair of electrons and an unpaired electron at the central carbon atom (C1). The spin hops over from π- to σ-type orbitals is observed as the π-acceptor strength of the donor ligand increases. The replacement of the methyl group by CHO and NO2 indicate that the cAAC and CHO substituted compounds gives a σ-radical except in compound 2c. These molecules show very high proton affinity and exothermic reaction energy for the hydrogen atom addition indicating dual reactivity namely, radical and lone pair reactivity.

Four-Membered Bimetallic Carbon Complex [(M(η5 -C5 (CH3 )5 ))2 (µ-NPh)(µ-C)] (M=Fe, Ru, Os) - An Ambiphilic Carbene with Lone Pair and σ-Hole Reactivity.

Singlet carbenes are extensively studied compounds capable of electrophilic, nucleophilic or ambiphilic behaviour. The ambiphilic reactivity of singlet carbenes has been conventionally observed in orthogonal planes. Here, we report a detailed bonding and reactivity study of a homobimetallic carbon complex [(MCp*)2 (µ-NPh)(µ-C)] (1M, M=Fe, Ru, Os) that shows ambiphilicity in the same direction. The structure of this complex can be considered as two fused three-membered M-C-M and M-N-M rings. The bonding analysis suggests that these 17 valence electron homobimetallic complexes have one formal M-M bond with a bridging carbene centre featuring a high-lying spn -hybridised lone pair. Accordingly, the carbene centre shows high proton affinity and act as a good 2e- donor to Lewis acids and transition metal fragment. Apart from the transition metal non-bonding electrons, the π-framework of M-C-M and M-N-M arms can be best described as 3c-2e- bonds. The two transition metals in the four-membered skeleton generate many low-lying, virtual orbitals. These low-lying virtual orbitals induce electron excitation from the spn -hybrid orbital in presence of H- and other 2e- donor ligands such as PMe3 , NHC and CO. Hence, the spn -hybrid lone pair orbital shows σ-hole reactivity in presence of Lewis bases.

Synthesis and Characterization of Substituted Phosphasilenes and its Rare Homologue Stibasilene >Si=Sb.

Herein we report the reduction of R-EX2 (E=P, Sb) with two equivalents of KC8 in the presence of silylene (LSiR; L=PhC(NtBu)2 ) to give Trip-P=SiL(C6 H4 PPh2 ) (1), Ter Ph-P=(tBu)SiL (2) and Ter Ph-Sb=(tBu)SiL (3). The last (3) belongs to a new class of heavier analogues of Schiff bases (>C=N-), containing a formal >Si=Sb- double bond. The theoretical calculations suggest that lone pairs on the dicoordinated group-15 centers are stabilized by hyperconjugative interactions resulting in pseudo-Si-P/Si-Sb multiple bonds which are highly reactive as indicated by the high first and second proton affinities.

Organometallic Allene [(µ-C)(Fe(CO)4 )2 ]: Bridging Carbon Showing Transformation from Classical Electron-Sharing Bonding to Double σ-Donor and Double π-Acceptor Ligation.

Allenes (R2 C=C=CR2 ) have been traditionally perceived to feature localized orthogonal π-bonds between the carbon centres. We have carried out quantum-mechanical studies of the organometallic allenes envisioned by the isolobal replacement of the terminal CH2 groups by the d8 Fe(CO)4 fragment. Our studies have identified two organometallic allenes viz. D2d symmetric [(µ-C)(Fe(CO)4 )2 ] (2) and D3 symmetric [(µ-C)(Fe(CO)4 )2 ] (3) with trigonal bipyramidal coordination at the Fe atoms. Compound 2 features the bridging carbon atom in an equatorial position with respect to the ligands on the TM centre, while 3 features the central carbon atom in an axial position. The bis-pseudoallylic anionic delocalisation proposed in the C2-C1-C3 spine of organic allene is retained in the organometallic allene 2, and is transformed to a typical three-centre bis-allylic anionic delocalisation in the organometallic allene 3. The topological analysis of electron density also indicates a bis-allylic anionic type delocalisation in the organometallic allenes. The quantitative bonding analysis using the EDA-NOCV method suggests a transition from classical electron-sharing bonding between the central carbon atom and the terminal groups in 1 to donor-acceptor bonding in 3. Meanwhile, both electron-sharing and donor-acceptor bonding models are found to be probable heuristic bonding representations in the organometallic allene 2.

Substituent-Controlled Photophysical Responses in Dihydropyridine Derivatives and Their Application in the Detection of Volatile Organic Contaminants.

In the ever-expanding realm of organic fluorophores, structurally simple and synthetically straightforward molecules with unique photophysical properties have received special attention. Among these, 1,4-dihydropyridine (DHP) is an important scaffold that permits fine-tuning of their photophysical properties through substituents on the periphery. Herein, we describe a series of solid-emissive N-substituted 2,6-dimethyl-4-methylene-1,4-dihydropyridine derivatives appended with electron-withdrawing substituents (dicyanomethylene or 2-dicyanomethylene-3-cyano-2,5-dihydrofuran) at the C-4 position and alkyl or alkylaryl groups on the DHP nitrogen. Electronic and steric tuning exerted by these substituents resulted in interesting photophysical properties such as negative solvatochromism, solidstate, and aggregation-induced emission (AIE). Theoretical calculations were carried out to explain the solvatochromic properties. Insight into the AIE properties was obtained through variable-temperature nuclear magnetic resonance and viscosity- and temperature-dependent emission studies. The variations in molecular packing in the crystal lattice with changes in the N-substituents contributed to the tuning of solid state emission properties. Detection of aromatic volatile organic compounds (VOCs) was achieved using the aggregates of the DHP derivatives. Among the VOCs, p-xylene elicited a significant enhancement in emission, allowing its detection at submicromolar levels.

Compounds with Alternating Single and Double Bonds of Antimony and Silicon; Isoelectronic to Ethane-1,2-diimine.

We present an approach for preparing chain-type unsaturated molecules with low oxidation state Si(I) and Sb(I) supported by amidinato ligands that exploit to generate heavy analogues of ethane 1,2-diimine. The reduction of antimony dihalide (R-SbCl2) with KC8 in the presence of silylene chloride afforded L─(Cl)Si═Sb─Tip (1) and L(Cl)Si═Sb─TerPh (2), respectively. Compounds 1 and 2 further undergo reduction with KC8 to produce Tip─Sb═LSi─LSi═Sb─Tip (3) and TerPh─Sb═LSi─LSi═Sb─TerPh (4). The solid-state structures and DFT studies show that all compounds have σ-type lone pairs at each Sb atom. It forms a strong pseudo-π-bond with Si. The pseudo-π-bond is formed by the hyperconjugative donation of the π-type lone pair at Sb to the Si-N σ* MO. The quantum mechanical studies indicate that compounds 3 and 4 has delocalized pseudo-π-MOs arising from hyperconjugative interactions. Hence, 1 and 2 can be considered as isoelectronic to imine, while 3 and 4 are isoelectronic to ethane-1,2-diimine. The proton affinity studies indicate that the pseudo-π-bond resulting from the hyperconjugative interaction is more reactive than the σ-type lone pair.

A Neutral Borylene and its Conversion to a Radical by Selective Hydrogen Transfer.

A successful selective reduction of X2B-Tip (Tip = 1,3,5-iPr3-C6H2, X = I, Br) with KC8 and Mg metal, respectively, in the presence of a hybrid ligand (C6H4(PPh2)LSi) leads to a stable low-valent five-membered ring as a boryl radical [C6H4(PPh2)LSiBTip][Br] (1) and neutral borylene [C6H4(PPh2)LSiBTip] (2). Compound 2 reacts with 1,4-cyclohexadiene, resulting in hydrogen abstraction to afford the radical [C6H4(PPh2)LSiB(H)Tip] (3). Quantum chemical studies reveal that compound 1 is a B-centered radical, and compound 2 is a phosphane and silylene stabilized neutral borylene in a trigonal planar environment, whereas compound 3 is an amidinate-centered radical. Although compounds 1 and 2 are stabilized by hyperconjugation and π-conjugation, they display high H-abstraction energy and basicity, respectively.

Inorganometallic allenes [(Mn(η5-C5H5)(CO)2)2(µ-E)] (E = Si-Pb): bis-allylic anionic delocalisation similar to organometallic allene but differential σ-donation and π-backdonation.

The chemistry of heavy group-14 tetrel atoms is known to diverge from that of the lighter congener carbon. Here, we report the structure and bonding in inorganometallic allenes [(MnCp(CO)2)2(µ-E)] (2E, E = Si-Pb; Cp = η5-C5H5). These inorganometallic allenes are structurally similar to the lighter organometallic analog [(MnCp(CO)2)2(µ-C)] (2C). The bonding analysis of these compounds at the M06/def2-TZVPP//BP86/def2-SVP level of theory identifies a linear Mn-E-Mn spine with delocalised, mutually orthogonal π-systems across this back-bone. This results in a bis-allylic anionic bonding scenario. However, the strength of the Mn-E bonding is found to be weaker in these inorganometallic allenes. The energy decomposition analysis at the BP86/TZ2P//BP86/def2-SVP level of theory further reveals that the bonding in these compounds cannot be represented by one unique heuristic bonding model, but multiple bonding models. For all 2E (E = C-Pb), the Dewar-Chatt-Duncanson bonding model is one of the best bonding representations, where the central tetrel atom acts as a 4e- σ-donor and 4e- π-acceptor. The bonding analysis indicates that the carbon atom in the organometallic allene acts as a better π-acceptor than σ-donor, while the heavier tetrel atoms in the inorganometallic allenes are better σ-donors than π-acceptors. The npz-orbital is found to be a better σ-donor than the valence ns-orbital. However, when the bonding representation is changed to a traditional electron-sharing model, the contribution from the ns-orbital was found to be the largest in comparison to the interaction from the remaining three valence np-orbitals. It can be suggested that the ns-orbitals contribute more towards chemical bonding when participating via an electron-sharing interaction than a donor-acceptor interaction.

Lewis Acid Stabilized Diatomic Molecules of Group 14: A Computational Study on [(CO)4Fe]2E2 (E = C, Si, Ge, Sn, Pb).

A Lewis base and acid combination has been effectively employed to stabilize and isolate the low-valent group 14 compounds. We report DFT studies on stabilizing low-valent group 14 diatomics as adducts of Lewis acids employing transition metal carbonyl fragment iron tetracarbonyl [Fe(CO)4] as Lewis acid. Computational studies on [(CO)4Fe]2E2, E = C, Si, Ge, Sn, and Pb, predict five plausible isomers on its potential energy surface: linear (E2_L), bent (E2_B), three-membered (E2_T), dibridged (E2_D), and four-membered (E2_F). For the carbon analogue, the lowest energy configuration is linear and has a typical cumulenic structure, while silicon and germanium analogues favor three-membered cyclic isomers. Four-membered cyclic isomers are the most stable for tin and lead analogues.

A Carbene-Stabilized Boryl-Phosphinidene.

Herein, the synthesis and characterization of the carbene-stabilized boryl phosphinidenes 1-3 are reported. Compounds 1-3 are obtained by reacting Me-cAAC=PK (Me2 -cAAC=dimethyl cyclic(alkyl)(amino)carbene) and dihaloaryl borane in toluene. All three compounds were characterized by X-ray crystallography. Quantum mechanical studies indicated that these compounds have two lone pairs on the P center viz., an σ-type lone pair and a "hidden" π-type lone pair. Hence, these compounds can act as double Lewis bases, and the basicity of the π-type lone pair is higher than the σ-type lone pair.

Insertion Reaction of Me3SiN3 with Bis(germylene).

Herein, we describe the redox reaction of bis(germylene) PhC(NtBu)2Ge-Ge(NtBu)2CPh with different equivalents of Me3SiN3 affording two distinct products. The reaction of Me3SiN3 with bis-germylene in a 1:1 molar ratio results in compound 1 at -78 °C; however, treatment of bis-germylene with a 2.1 equiv of Me3SiN3 at room temperature results in compound 2. The formation of 1 and 2 can be rationalized by two successive 3 + 1 cycloadditions of Me3SiN3 with the germanium center of bis(germylene) and N2 elimination. All of the compounds are well-characterized by various spectroscopic techniques and single-crystal X-ray structural analyses. Density functional theory (DFT) calculations suggest that compound 2 has a dicoordinated nitrogen atom, which is stabilized by hyperconjugative interactions, resulting in pseudo-germylimine moiety. However, the dicoordinated nitrogen atom shows high basicity as indicated by proton affinity values. These are rare examples of isolated products that show insertion as well as simultaneous redox properties of bis(germylene).

First-Row Transition Metal Complexes of a Phosphine-Silylene-Based Hybrid Ligand.

We have prepared two new silylene-phosphine-based hybrid ligands Si{N(R)C6H4(PPh2)}{PhC(NtBu)2} [R = TMS {trimethylsilyl} (1) and TBDMS {tert-butyldimethylsilyl} (2)], which possess two donor sites. Furthermore, the treatment of the bidentate ligand 1 with base metal halides {FeBr2, CoBr2, NiCl2·dme [nickel chloride(II) ethylene glycol dimethyl ether]} and 2 with NiBr2·dme [nickel bromide(II) ethylene glycol dimethyl ether] afforded four-coordinate six-membered metal complexes 3-6, respectively, which feature coordination from both Si(II) and P(III) sites. Subsequently, complexes 3 [(FeBr2)Si{N(SiMe3)C6H4(PPh2)}{PhC(NtBu)2}], 4 [(CoBr2)Si{N(SiMe3)C6H4(PPh2)}{PhC(NtBu)2}], 5 [(NiCl2)Si{N(SiMe3)C6H4(PPh2)}{PhC(NtBu)2}], and 6 [(NiBr2)Si{N(SitBuMe2)C6H4(PPh2)}{PhC(NtBu)2}] are studied for their redox and magnetic properties with the help of UV-vis spectroscopy, cyclic voltammetry, SQUID magnetometry, and theoretical calculations. Complexes 3-6 were found to display a paramagnetic behavior. All the compounds are well established by single-crystal X-ray diffraction studies.

Two σ- and two π-dative quadruple bonds between the s-block element and transition metal in [BeM(CO)4; M = Fe - Os].

We report the chemical bonding and reactivity of the first example of neutral 18 valence electron transition metal complexes of beryllium, [BeM(CO)4; M = Fe - Os], in trigonal bipyramidal coordination geometry, where the bonding between the transition metal and the s-block element beryllium (M-Be) can be best described by dative quadruple bonds. In contrast to the conventional multiple bonding pattern, the quadruple bonds comprise two σ-bonds and two π-bonds, viz., one Be → M σ-bond, one M → Be σ-bond, and two M → Be π-bonds. Since the M-Be quadruple bonds are described by dative interactions, the Be centre shows ambiphilic character as indicated by the high proton and hydride affinity values.

Stabilization of group 14 elements E = C, Si, Ge by hetero-bileptic ligands cAAC, MCOn with push-pull mechanism.

The stability and bonding of a series of hetero-diatomic molecules with general formula (cAAC)EM(CO)n , where cAAC = cyclic alkyl(amino) carbene; E = group 14 elements (C, Si, and Ge); M = transition metal (Ni, Fe, and Cr) have been studied by quantum chemical calculations using density functional theory (DFT) and energy decomposition analysis-natural orbital chemical valence (EDA-NOCV). The equilibrium geometries were calculated at the BP86/def2-TZVPP level of theory. The tri-coordinated group 14 complex (1a, 4a, and 7a) in which one of the CO groups is migrated to the central group 14 element from adjacent metal is theoretically found to be more stable when the central atom (E) is carbon. On the other hand, the two-coordinate group 14 element containing metal-complexes (2, 5, 8, 3, 6, and 9) are found to be more stable with their corresponding heavier analogues. The electronic structures of all the molecules have been analyzed by molecular orbital, topological analysis of electron density and natural bond orbital (NBO) analysis at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. The nature of the cAACE and EM bonds has been studied by EDA-NOCV calculations at BP86-D3(BJ)/TZ2P level of theory. The EDA analysis suggests that the bonding of cAACC(CO) can be best represented by electron sharing σ and π interactions, whereas, C(CO)M(CO)n-1 by dative σ and π interactions. On the other hand, EDA-NOCV calculations suggests both dative σ and π interactions for cAACE and EM(CO)n bonds of the corresponding Si and Ge analogues having stronger σ- and relatively weaker π-bonds. The topological analysis of electron density supports the closed-shell interaction for the Si and Ge complexes and open-shell interaction for the carbon complexes. The calculated proton affinity and hydride affinity values corroborated well with the present bonding description. This class of complexes might act as efficient future catalysts for different organic transformations due to the presence of electron rich group 14 element and metal carbonyl.

Stabilization of Reactive Nitrene by Silylenes without Using a Reducing Metal.

Herein, we report the stabilization of nitrene reagents as the source of a nitrogen atom to synthesize nitrogen-incorporated R1 LSi-N←SiLR2 (1) [L=PhC(NtBu)2 ; R1 =NTMS2 , R2 =NTMS]. Compound 1 is synthesized by reacting LSi(I)-SiI L with 3.1 equivalents of Me3 SiN3 at low temperature to afford a triene-like structural framework. Whereas the reaction of the LSi(I)-SiI L with 2.1 equivalents of Me3 SiN3 at room temperature produced silaimine 2 with a central four-membered Si2 N2 ring which is accompanied by a silylene LSi and a cleaved silylene fragment. 1 further reacts with AgOTf at room temperature to yield compound 3 which shows coordination of nitrene to silver with the triflate salt. The compounds 1 and 2 were fully characterized by NMR, mass spectrometry, and X-ray crystallographic analysis. The quantum mechanical calculations reveal that compounds 1 and 2 have dicoordinated monovalent N atoms having two active lone pairs of electrons. These lone pairs are stabilized by hyperconjugative interactions.