DABCO molecule in the M2(C8H4O4)2·C6H12N2 (M = Co, Ni, Cu, Zn) metal-organic frameworks

The versatility of DABCO as a reagent in organic synthesis: a review

DABCO (1,4-diazabicyclo[2.2.2]octane) has garnered a lot of interest for numerous organic transformations since it is a low-cost, environmentally friendly, reactive, manageable, non-toxic and basic organocatalyst with a high degree of selectivity. Moreover, DABCO functions as a nucleophile as well as a base in a variety of processes for the synthesis of a wide array of molecules, including carbocyclic and heterocyclic compounds. Protection and deprotection of functional groups and the formation of carbon-carbon bonds are also catalyzed by DABCO. The reagent also finds applications in the synthesis of functional groups like isothiocyanate, amide and ester. Application of DABCO in cycloaddition, coupling, aromatic nucleophilic substitution, ring-opening, oxidation and rearrangement reactions is also noteworthy. This is a state of the art review that has encompassed a variety of processes for the synthesis of organic frameworks using DABCO.

Properties of Aliphatic Ligand-Based Metal-Organic Frameworks

Ligands with a purely aliphatic backbone are receiving rising attention in the chemistry of coordination polymers and metal-organic frameworks. Such unique features inherent to the aliphatic bridges as increased conformational freedom, non-polarizable core, and low light absorption provide rare and valuable properties for their derived MOFs. Applications of such compounds in stimuli-responsive materials, gas, and vapor adsorbents with high and unusual selectivity, light-emitting, and optical materials have extensively emerged in recent years. These properties, as well as other specific features of aliphatic-based metal-organic frameworks are summarized and analyzed in this short critical review. Advanced characterization techniques, which have been applied in the reported works to obtain important data on the crystal and molecular structures, dynamics, and functionalities, are also reviewed within a general discussion. In total, 132 references are included.

A Computational Approach to Nontraditional Intrinsic Luminescence: Vibrationally Resolved Absorption and Fluorescence Spectra of DABCO

Recently, so-called "nontraditional intrinsic luminescence" has been reported in several macromolecular systems. Although DABCO (1,4-diazabicyclo[2.2.2]octane) is the first system in which the effect was observed, a thorough analysis of the optical properties of the molecule, which would reveal the origin of this mysterious effect, is still pending. We perform an advanced post-Hartree-Fock treatment of the low-lying electronic states of this molecule, which need to be described with care because of their pronounced Rydberg character. We take a deeper look into the low-lying electronic transitions of DABCO targeting the explanation of the complex vibronic structures of its absorption and fluorescence spectra. Two electronic states, the ¹E'(n₊3p_xy) and ¹A₂^″(n₊3p_z) states, contribute to the absorption spectrum in the 39000-46000 cm^-1 spectral range. We also reveal the spectroscopic signature of the ¹A₂^″(n₊3p_z) state. The analyses of the contributions of individual vibrational normal modes allowed the identification of those giving rise to the complex vibronic structures of the spectra. Fluorescence emission arises from the vibronic coupling of the one-photon forbidden transition between the ¹A₁^'(n₊3s) state and the electronic ground state. The spectrum, which can be interpreted in terms of populating a few vibrational normal modes, is shifted toward visible wavelengths mostly due to the forced interaction of the lone pair electrons of the two nitrogen atoms. Our work on DABCO may help to rationalize the luminescence of more complex systems containing tertiary amine groups.

Chirality and Relativistic Effects in Os3(CO)12

The energy and structural parameters were obtained for all forms of the carbonyl complex of osmium Os3(CO)12 with D3h and D3 symmetries using density functional theory (DFT) methods. The calculations took into account various levels of relativistic effects, including those associated with nonconservation of spatial parity. It was shown that the ground state of Os3(CO)12 corresponds to the D3 symmetry and thus may be characterized either as left-twisted (D3S) or right-twisted (D3R). The D3S↔D3R transitions occur through the D3h transition state with an activation barrier of ~10-14 kJ/mol. Parity violation energy difference (PVED) between D3S and D3R states equals to ~5 × 10-10 kJ/mol. An unusual three-center exchange interaction was found inside the {Os3} fragment. It was found that the cooperative effects of the mutual influence of osmium atoms suppress the chirality of the electron system in the cluster.

Understanding structural flexibility of the paddle-wheel Zn-SBU motif in MOFs: influence of pillar ligands

Structural parameters and energies of secondary building units (SBUs) in Zn2(C8H4O4)2·C6H12N2 have been examined with DFT and CCSD(T) methods. The SBU structure exhibits flexibility due to close energies of formation of its D4h, D4, and D2d forms. The potential energy surfaces (PESs) calculated for these systems testify that the presence of pillar ligands with negatively charged nitrogens (NCH or DABCO) can lead to almost barrier-free transitions between structural SBU forms. The mobility regions of the SBU motif have been determined.

Overview of Low-Temperature Heat Capacity Data for Zn2(C8H4O4)2.C6H12N2 and the Salam Hypothesis

The review presents the progress in the analysis of low-temperature heat capacity of the metal-organic framework Zn2(C8H4O4)2.C6H12N2 (Zn-DMOF). In Zn-DMOF, left-twisted D3(S) and right-twisted D3(R) DABCO molecules (C6H12N2) can transform into each other by tunneling to form a racemate. Termination of tunneling leads to a phase transition in the subsystem of twisted molecules. It is suggested that Zn-DMOF may be considered a model system to study the mechanisms of phase transitions belonging to the same type as hypothetical Salam phase transitions.

Parity violating energy difference for mirror conformers of DABCO linker between two M2+ cations (M = Zn, Cd, and Hg)

We have studied the potential energy surface of [ M 2 D A B C O ] 4 + cations (M = Zn, Cd, and Hg), which are considered as a model for the DABCO linker in metal-organic frameworks, a new prospective class of materials. Relativistic four-component and two-component calculations of parity violating energy difference (PVED) for twisted isomers of [ M 2 D A B C O ] 4 + cations have been performed. The right-twisted conformers of [ M 2 D A B C O ] 4 + are more stable than the left-twisted ones. The increase in PVED with the nuclear charge of the transition metal atom M (Z _M ) is discussed.