A bis-NHC-CAAC dimer derived dicationic diradical.

The isolation of carbon-centered diradicals is always challenging due to synthetic difficulties and their limited stability. Herein we report the synthesis of a trans-1,4-cyclohexylene bridged bis-NHC-CAAC dimer derived thermally stable dicationic diradical. The diradical character of this compound was confirmed by EPR spectroscopy. The variable temperature EPR study suggests the singlet state to be marginally more stable than the triplet state (2J = -5.5 cm-1 (ΔE ST = 0.065 kJ mol-1)). The presence of the trans-1,4-cyclohexylene bridge is instrumental for the successful isolation of this dicationic diradical. Notably, in the case of ethylene or propylene bridged bis-NHC-CAAC dimers, the corresponding dicationic diradicals are transient and rearrange to hydrogen abstracted products.

N,N'-Ethylene-Bridged Bis-2-Aryl-Pyrrolinium Cations to E-Diaminoalkenes: Non-Identical Stepwise Reversible Double-Redox Coupled Bond Activation Reactions.

This work presents a stepwise reversible two-electron transfer induced hydrogen shift leading to the conversion of a bis-pyrrolinium cation to an E-diaminoalkene and vice versa. Remarkably, the forward and the reverse reaction, which are both reversible, follow two completely different reaction pathways. Establishing such unprecedented property in this type of processes was possible by developing a novel synthetic route towards the starting dication. All intermediates involved in both the forward and the backward reactions were comprehensively characterized by a combination of spectroscopic, crystallographic, electrochemical, spectroelectrochemical, and theoretical methods. The presented synthetic route opens up new possibilities for the generation of multi-pyrrolinium cation scaffold-based organic redox systems, which constitute decidedly sought-after molecules in contemporary chemistry.

Structural Diversity in Supramolecular Organization of Anionic Phosphate Monoesters: Role of Cations.

Syntheses and structures of anionic arylphosphate monoesters [ArOP(O)2(OH)]- (Ar = 2,6-CHPh2-4-R-C6H2; R = Me/Et/iPr/tBu) with different counter cations are reported. The counter cations were varied systematically: imidazolium cation, 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N'-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, 4,4'-bipyridinium dication, and magnesium(II) dication. The objective was to examine if the supramolecular structure of anionic arylphosphate monoesters could be modulated by varying the cation. It was found that an eight-membered P2O4H2-hydrogen-bonded dimeric motif involving intermolecular H-bonding between the [P(O)(OH)] unit of the anionic phosphate monoester along with the counter cation is formed with 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N'-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, and magnesium(II) dication; both discrete and polymeric H-bonded structures are observed. In the case of imidazolium cations and 1,4-diazabicyclo[2.2.2]octan-1-ium cation, the formation of one-dimensional polymers (single lane/double lane) was observed. On the other hand, two types of phosphate motifs, intermolecular H-bonded dimer and an open-form, were observed in the case of 4,4'-bipyridinium dication.

Influence of N-Substitution on the Formation and Oxidation of NHC-CAAC-Derived Triazaalkenes.

We have studied the effect of N-substitution on the course of the reaction of imidazolium triflate. The reaction of N-heterocyclic carbene with N-tBu-substituted pyrrolinium triflate afforded 2-(pyrrolidin-2-yl)-imidazolium triflate, 3R. Treatment of 3R with potassium bis(trimethylsilyl)amide (KHMDS) leads to either the dealkylation product 4 or the deprotonation product, triazaalkene 5, depending on the N-substitution at the imidazolium moiety. Density functional studies using the B3LYP/TZVP setup have been employed to explore various pathways for the dealkylation reaction and the calculated energies support the dealkylation by a large energy margin compared to the deprotonatation process. Theoretical calculations revealed that dealkylation reaction is thermodynamically more favorable than deprotonation. The triazaalkene 5 could be oxidized by AgOTf to the corresponding radical cation 6 and dication 7 in-situ. While 6 and 7 could not be isolated, the formation of the former is inferred by electron paramagnetic resonance spectroscopy and its abstraction of a H-atom to afford 3Me. Similarly, the formation of the dication 7 is inferred by its ready elimination of isobutylene affording 8.

Direct access to 2-aryl substituted pyrrolinium salts for carbon centre based radicals without pyrrolidine-2-ylidene alias cyclic(alkyl)(amino)carbene (CAAC) as a precursor.

The synthesis of organic radicals is challenging due to their inherent instability. In recent years, cyclic(alkyl)(amino)carbene (CAAC)-derived 2-substituted pyrrolinium salts have been used as synthons for the synthesis of isolable carbon-based radicals. Herein, we report a direct, easy and convenient method for the synthesis of 2-aryl substituted pyrrolinium salts without using CAAC as a precursor. These cations can be reduced to the corresponding radicals. The influence of the aryl substituent at the C-2 position on radical stabilization and dimerization has been investigated. Because of the large scope of our strategy (capability to modulate different substituents at all the C- and N-centres of the pyrrolinium salts), it has the merit to be an extremely effective and productive route for generating carbon-based radicals whose stability as well as reactivity can be varied.

Modulation of the nuclearity of molecular Mg(ii)-phosphates: solid-state structural change involving coordinating solvents.

Herein, we report the synthesis and molecular structures of various magnesium(ii)-phosphate monoesters. By using a bulky aryl substituted phosphate monoester, ArOPO3H2 (Ar = 2,6-(CHPh2)2-4-tBu-C6H2), we have reproducibly assembled mono-, di-, tetra- (cage and ring), hexa-, and polynuclear magnesium(ii)-phosphate monoesters. Interestingly, the hexanuclear magnesium(ii)-phosphate monoester encapsulates an open-cage dodecanuclear water cluster.

Neutral and anionic phosphate-diesters as molecular templates for the encapsulation of a water dimer.

The study of water clusters in a confined state has received considerable attention. The simplest among such assemblies of water molecules is the water dimer. Stabilizing this unit at ambient conditions in an appropriate host-template, however, has remained a challenge. Herein, we report 2,6-(diphenylmethyl)-4-iso-propyl-phenyl substituted phosphate diesters (both in their neutral and anionic forms) as molecular templates for hosting the water dimer. The robustness of the water dimer within the neutral phosphate-diester ensemble is remarkable. Even when treated with triethylamine the water dimer is not disrupted, while exclusively the P(O)(OH) unit is deprotonated.

Reactivity enhancement of a diphosphene by reversible N-heterocyclic carbene coordination.

Diphosphene TerMesP = PTerMes (1; TerMes = 2,6-Mes2C6H3; Mes = 2,4,6-Me3C6H2) and NHCMe42 (NHCMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) exist in an equilibrium mixture with the NHCMe4 -coordinated diphosphene 3. While uncoordinated 1 is inert to hydrolysis, the NHC adduct 3 readily undergoes hydrolysis to afford a phosphino-substituted phosphine oxide with the liberation of NHCMe4 . On this basis, conditions suitable for the catalytic use of NHCMe4 were identified. Similarly, while the hydrogenation of free diphosphene 1 with H3N·BH3 is very slow, 3 reacts instantaneously with H3N·BH3 at room temperature to afford a dihydrodiphosphane.

"Abnormal" Addition of NHC to a Conjugate Acid of CAAC: Formation of N-Alkyl-Substituted CAAC.

The addition reactions of N-heterocyclic carbenes (NHCs) are mostly known to occur through the carbenic centre (C2), which leads to a "normal" adduct. Herein, we report the "abnormal" addition of NHCDip 1 (1,3-(2,6-iPr2 C6 H3 )-imidazole-2-ylidene) to a conjugate acid of cyclic (alkyl)(amino)carbene 2 (CAACiPr =1-iPr-3,3,5,5-Me4 -pyrrolinium triflate). Mechanistic study revealed that this reaction proceeded through the in situ formation of 1,3-(2,6-iPr2 C6 H3 )-imidazolium cation 4 and N-iPr-substituted CAAC 5 followed by the oxidative addition of compound 5 across the C4-H bond (alias backbone C-H) of compound 4. The in situ formation of compound 5 was also proven by the oxidative addition of it to the N-H group of iPrNH2 . DFT calculations also supported the mechanistic findings. A different methodology for the in situ generation of compound 5 by using TMPLi is also described.

Stepwise Reversible Oxidation of N-Peralkyl-Substituted NHC-CAAC Derived Triazaalkenes: Isolation of Radical Cations and Dications.

Herein, the isolation and characterization of N-peralkyl-substituted NHC-CAAC derived triazaalkenes in three oxidation states, neutral, radical cation, and dication, are reported. Cyclic voltammetry has shown the reversible electronic coupling between the first and second oxidation to be ΔE1/2 = 0.50 V. As a proof-of-principle, to demonstrate the electron-rich nature of the triazaalkene, it was shown that it can be used as an electron donor in the reduction of an aryldiazonium salt to the corresponding arene.

Decanuclear Ln10 Wheels and Vertex-Shared Spirocyclic Ln5 Cores: Synthesis, Structure, SMM Behavior, and MCE Properties.

The reaction of a Schiff base ligand (LH3) with lanthanide salts, pivalic acid and triethylamine in 1:1:1:3 and 4:5:8:20 stoichiometric ratios results in the formation of decanuclear Ln10 (Ln = Dy (1), Tb (2), and Gd (3)) and pentanuclear Ln5 complexes (Ln = Gd (4), Tb (5), and Dy (6)), respectively. The formation of Ln10 and Ln5 complexes are fully governed by the stoichiometry of the reagents used. Detailed magnetic studies on these complexes (1-6) have been carried out. Complex 1 shows a SMM behavior with an effective energy barrier for the reversal of the magnetization (Ueff) = 16.12(8) K and relaxation time (τo) = 3.3×10(-5) s under 4000 Oe direct current (dc) field. Complex 6 shows the frequency dependent maxima in the out-of-phase signal under zero dc field, without achieving maxima above 2 K. Complexes 3 and 4 show a large magnetocaloric effect with the following characteristic values: -ΔSm = 26.6 J kg(-1) K(-1) at T = 2.2 K for 3 and -ΔSm = 27.1 J kg(-1) K(-1) at T = 2.4 K for 4, both for an applied field change of 7 T.

A hexaicosametallic copper(II) phosphonate.

Structure and characterization of [Cu26{2,3,5,6-(Me)4C6H-CH2-PO3}18(µ2-OH)4(µ3-OH)6(µ4-Cl)6(µ-OH2)2(OH2)2(MeCN)4]·6MeCN·15H2O (1) is reported. Complex 1 is the largest discrete molecular homometallic transition metal phosphonate assembly. Remarkably, this gigantic molecular phosphonate has been prepared at room temperature using a normal solution synthetic method.

Metalation studies of 3- and 4-pyridyloxycyclophosphazenes: metallamacrocycles to coordination polymers.

Pyridyloxy cyclophosphazenes, spiro-N3P3(O2C12H8)(O-C5H4N-3)4 (L1), dispiro-N3P3(O2C12H8)2(O-C5H4N-3)2 (L2) and dispiro-N3P3(O2C12H8)2(O-C5H4N-4)2 (L3) were prepared and characterized. The crystal structure of L1 was determined which showed that the cyclophosphazene ring is planar. The reaction of L1 with anhydrous copper(II) chloride or silver nitrate afforded L2M4 metallamacrocycles [{L1}2{ClCu(µ-Cl)2CuCl}]2·8CHCl3 (3) and [{L1}2{(DMF)0.5(NO3)0.5Ag}2{Ag(µ2-(O)NO2)(H2O)}2]{µ-[NO3]}·CH3OH (4) respectively. The molecular structure of 3 reveals that two CuCl2 dimers are bridged to each other in a complex metallamacrocycle that involves four metal ions and two ligands. The molecular structure of 4 is similar to that of 3 except for the presence of weak argentophilic interactions between the two Ag(I) centers. Also, a nitrate anion, which is trapped in the centre of the tetrameric assembly, assists in holding two Ag(I) ions together and 4 serves as a single source precursor for silver nanoparticles. The reaction L1 with ZnCl2 and Cd(NO3)2·4H2O afforded the 1D-coordination polymers [{L1}{ZnCl2}2]n·2nCH3OH·2nH2O (5) and [{L1}2{Cd(CH3OH)2(NO3)2}2{Cd(NO3)2(H2O)}]n·3nCH3OH·3nH2O (6) respectively. While in 5 the 20-membered macrocycles are inter-connected directly, in 6 they are bridged by a Cd(II) connector. The reaction of L2 with CoCl2 afforded the 1D-coordination polymer [{L2}2{CoCl2}]n·2nCHCl3 (7) which also contains interconnected 20-membered metallamacrocycles. On the other hand, the reaction of L2 with CuCl2 afforded [{L2}2{ClCu(µ-Cl)}2]n·2nCHCl3·nH2O (8) which is a 1D-coordination polymer that contains {ClCu(µ-Cl)2CuCl} units. The reaction of L3 with CuCl2 and Cd(NO3)2·4H2O afforded [{L3}2{CuCl2}]n·nDMF (9) and [{L3}2{Cd(NO3)2}]n·nCHCl3·nCH2Cl2·nH2O (10) respectively. While 9 is a 2D-coordination polymer containing 48-membered metallamacrocycles, 10 is a 1D-coordination polymer containing interconnected 24-membered metallamacrocycles which possess spirocyclic cadmium ions as nodes.

Pyridyloxy cyclophosphazenes and carbophosphazenes: inorganic ring-supported coordination platforms.

This review deals with the utility of cyclophosphazenes and carbophosphazenes as supports for the construction of multi-site coordination platforms. The rich nucleophilic substitution chemistry of the chlorocyclophosphazenes and the analogous carbocyclophosphazenes can be utilized to replace chlorine atoms from these inorganic rings with coordinating side arms. This leads to the assembly of interesting compounds that have the capability to bind to multiple transition metal ions. Using this strategy several coordination ligands have been constructed. After a brief introduction to such ligands, this review deals with pyridyloxy cyclophosphazenes and carbophosphazenes. These ligands, in addition to possessing multiple coordinating arms, are also considered to be structurally flexible systems. This is because the pyridyl substituents are connected to the inorganic ring skeleton through flexible oxygen spacer atoms. The coordination chemistry of these pyridyloxy systems is discussed particularly in light of the work that has emanated from our laboratories in India.