Polycyclic aromatic hydrocarbons are of special interest due to their promising nonlinear optical and magnetic properties. A series of acene-linked zethrenes and bisphenalenyls comprising from five ...to nine benzene rings in the linker group have been computationally studied by the DFT UB3LYP/6-311++G(d,p) quantum-chemical modeling of their electronic structure, possible spin states, and exchange interactions. The zethrenes with octacene and nonacene linkers as well as bisphenalenyls comprising heptacene, octacene, and nonacene linker groups have been revealed to possess tetraradicaloid nature, which makes them promising building blocks for organic optoelectronic and spintronic devices. The results obtained open a way of constructing tetraradicaloid organic molecules characterized by the presence of two types of paramagnetic centers.
Various aspects related to the use of DFT method for the study of magnetic, geometry and energetic properties of transition metal complexes with redox-active ligands are considered. Particular ...attention is given to the correct choice of model compounds and methodology of the calculations.
In this Review we present examples of clusters, molecules, and solid‐state compounds, for which the use of σ‐aromaticity and σ‐antiaromaticity concepts is essential for understanding of chemical ...bonding. We show that the bonding patterns in these σ‐aromatic and σ‐antiaromatic compounds are similar to those of the corresponding π‐aromatic and π‐antiaromatic chemical systems, respectively. Undoubtedly, σ‐aromaticity helps us understand why the high symmetry isomers are the most stable among myriads of other potential structures. We also show that besides systems exhibiting either σ‐ or π‐aromatic features, there are species, which can possess multiple aromaticity/antiaromaticity, or conflicting aromaticity patterns. We believe that the σ‐aromaticity and σ‐antiaromaticity concepts will be helpful in rationalizing chemical bonding, structure, stability, and molecular properties of chemical species in both organic and inorganic chemistry. We hope that they will also be useful for other areas of science such as material science, catalysis, nanotechnology, and biochemistry.
Understanding chemical bonding: This Review focuses on the usefulness of the concepts of σ‐aromaticity and σ‐antiaromaticity in chemistry: from small clusters to bulky solid‐state compounds. As one of the numerous examples, the counter‐intuitive geometry of the global minimum structure of the Li2Mg2 cluster is explained using the σ‐aromaticity based on the particular delocalized bonding pattern and energy considerations (see picture).
A series of trinuclear mixed-ligand complexes of iron(II) 2,6-di(pyrazol-1-yl)pyridine moieties with linkers on the basis of 1,3,5-triazapentadiene transition metal (M = Co, Ni, Cu, Zn) bischelates ...has computationally been designed using the density functional theory TPSSh/6-311++G(d,p) calculations. The systems possessing complete (M = Co, Ni) and partial (M = Cu, Zn) two-step spin-crossover phenomenon at ferrous ions have been revealed. Two spin-state switching mechanisms determining by spin-crossover at iron centers and configurational isomerism at cobalt ion may simultaneously occur in solution of corresponding heterometallic compound (M = Co). The nature of the exchange interactions between paramagnetic metal centers is controlled by variation of a metal in bischelate linker (M = Co, Ni, Cu, Zn). Energy and magnetic characteristics of electromeric forms of the complexes with nickel and copper central metal ions (M = Co, Ni) allow to consider them as promising candidates for the design of molecular switches.
The syntheses of valence tautomeric compounds with multistep transitions using new redox-active ligands are the long-term goal of the field of bistable materials. The redox-active tetraoxolene ...ligand, 2,7-di-tert-butylpyrene-4,5,9,10-tetraone (pyreneQ–Q), is now developed to synthesize a pair of dinuclear compounds {CoL22(pyreneSq–Sq)}Co(CO)42·xCH2Cl2·2C6H5CH3 (1, x = 2, L = 1,10-phenanthroline, phen; 2, x = 1.5, L = 2,2′-bipyridine, bpy). Variable-temperature magnetic susceptibilities and single-crystal X-ray diffraction measurements indicate a partial one-step valence tautomeric transition for 1 and a rare two-step valence tautomeric transition for 2, respectively. DFT calculation results are consistent with the experimental data, revealing the correlation between thermodynamic parameters and the one-step/two-step valence tautomeric behaviors.
Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO ...complexes with access to LS–LS, LS–HS, and HS–HS states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of {FeIII(tpa)}2spiro(PF6)2 (1), {FeIII(tpa)}2Br4spiro(PF6)2 (2), and {FeIII(tpa)}2thea(PF6)2 (3) (tpa = tris(2-pyridylmethyl)amine, spiroH4 = 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-5,5′,6,6′-tetraol, Br4spiroH4 = 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol, theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the HS–HS state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed LS–HS state. In solution, all complexes undergo SCO from HS–HS at room temperature, via LS–HS to mixtures including LS–LS at 77 K, with the extent of SCO increasing in the order 1 < 2 < 3. Gas phase density functional theory calculations suggest a LS–LS ground state for all complexes, with the LS–HS and HS–HS states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro4– < Br4spiro4– < thea4–, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the LS–HS state in relation to the midpoint energy between LS–LS and HS–HS. The relative stability of the LS–HS state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 < 2 < 3. The bromo substituents of Br4spiro4– increase the ligand field strength relative to spiro4–, while the stronger ligand field provided by thea4– arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states.
A series of neutral triangular polycyclic aromatic hydrocarbons functionalized with various radical groups (dithiadiazolyl, verdazyl, nitronylnitroxyl, tert-butyl-nitroxyl and also ...(2,2,6,6-tetramethylpiperidin-1-yl)oxyl) has been computationally studied by the DFT UB3LYP/6-311++G(d,p) quantum-chemical modelling of their electronic structure and magnetic properties. The dependence of the nature and strength of the exchange interactions between paramagnetic centers on the size of the triangular core, the presence of heteroatoms in the polycyclic moiety, the type of the radical substituents and their spatial arrangement has been ascertained. The molecules with the high-spin electronic ground state possessing strong ferromagnetic exchange interactions and virtually non-interacting paramagnetic centers have been revealed, which makes them promising building blocks for organic spintronics devices.
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•Dissociation of trimeric Ni(II) bis-acetylacetonate into the monomers was studied.•Four possible reaction paths for this spin-forbidden rearrangement were found.•The energy preferred ...mechanism involves three consecutive changes of spin state.•Energy barriers necessary to overcome along preferred route do not exceed 33 kcal/mol.•This reaction is an example of a thermally induced multi-step CISSS phenomenon.
The possible reaction paths for the spin-forbidden dissociation of paramagnetic nickel(II) bis-acetylacetonate trimer (Ni(acac)2) have been studied based on the DFT UB3LYP/6-311++G(d,p) calculations. Two energy preferred reaction routes involve three consecutive steps resulting in spin state changes of the system: (S = 3) → (S = 2) → (S = 1) → (S = 0). The highest theoretically-estimated limits for energy barriers necessary to overcome along these routes, as estimated by the relative energies of the minimum energy crossing points (MECPs) on the seams of the intersecting potential energy surfaces of different multiplicity, do not exceed the value of 33 kcal/mol. The results obtained are in a good agreement with available experimental data. The considered dissociation – association rearrangement represents an example of a thermally induced multi-step Coordination-Induced Spin-State Switching phenomenon.
The bis‐chelate tin complex 1, based on the 4,6‐di‐tert‐butyl‐N‐(tert‐butyl)‐ortho‐aminophenol ligand and representing the first example of a redox‐isomeric compound in main‐group chemistry, is ...synthesized and characterized by X‐ray diffraction analysis. Complex 1 exists in two electromeric forms in nonpolar solvents: a diamagnetic pseudotetrahedral (AP)2SnIV (1a) and a paramagnetic tetragonal‐pyramidal structure of low‐valence tin (imSQ)2SnII (1b) (where AP and imSQ are the dianionic and radical‐anionic forms of the ligand, respectively). The reversible redox‐isomeric rearrangement between 1a and 1b is investigated in solution by means of magnetochemistry, EPR spectroscopy, UV/Vis spectroscopy, and 119Sn Mössbauer spectroscopy. This interconversion can be quenched by an addition of a strong donor ligand, such as pyridine (Py), resulting in an octahedral complex (AP)2SnIV(Py)2 (2) that does not undergo redox isomerism.
A bis‐chelate tin complex bearing a redox‐active o‐aminophenol ligand, representing the first example of a redox‐isomeric compound in main‐group chemistry, is synthesized and investigated. This complex exists in nonpolar solvents as an equilibrium mixture of two different isomers: a diamagnetic pseudotetrahedral tin(IV) compound and a paramagnetic tetragonal‐pyramidal tin(II) structure.
Valence tautomerism (VT) involves the stimulated reversible intramolecular electron transfer between a redox-active metal and ligand. Dinuclear cobalt complexes bridged by bis(dioxolene) ligands can ...undergo thermally-induced VT with access to {Co
III
-cat-cat-Co
III
}, {Co
III
-cat-SQ-Co
II
} and {Co
II
-SQ-SQ-Co
II
} states (cat
2−
= catecholate, SQ&z.rad;
−
= semiquinonate, Co
III
refers to low spin Co
III
, Co
II
refers to high spin Co
II
). The resulting potential for two-step VT interconversions offers increased functionality over mononuclear examples. In this study, the bis(dioxolene) ligand 3,3′,4,4′-tetrahydroxy-5,5′-dimethoxy-benzaldazine (thMH
4
) was paired with Me
n
tpa (tpa = tris(2-pyridylmethyl)amine,
n
= 0-3 corresponds to methylation at 6-position of the pyridine rings) to afford {Co(Me
n
tpa)}
2
(thM)(PF
6
)
2
(
1a
,
n
= 0;
2a
,
n
= 2;
3a
,
n
= 3). Structural, magnetic susceptibility and spectroscopic data show that
1a
and
3a
remain in the temperature invariant {Co
III
-cat-cat-Co
III
} and {Co
II
-SQ-SQ-Co
II
} forms in the solid state, respectively. In contrast,
2a
exhibits incomplete thermally-induced VT between these two tautomeric forms
via
the mixed {Co
III
-cat-SQ-Co
II
} tautomer. In solution, room temperature electronic absorption spectra are consistent with the assignments from the solid-state, with VT observed only for
2a
. From electrochemistry, the proximity of the two 1e
−
-processes for the thM
n
−
ligand indicates weak electronic communication between the two dioxolene units, supporting the potential for a two-step VT interconversion in thM
n
−
containing complexes. Comparison of the redox potentials of the Co and thM
n
−
processes suggests that only
2a
has these processes in sufficient proximity to afford the thermally-induced VT observed experimentally. Density functional theory calculations are consistent with the prerequisite energy ordering for a two-step transition for
2a
, and temperature invariant {Co
III
-cat-cat-Co
III
} and {Co
II
-SQ-SQ-Co
II
} states for
1a
and
3a
, respectively. This work presents the third example, and the first formally conjugated example, of a bridging bis(dioxolene) ligand that can afford two-step VT in a Co complex, suggesting new possibilities towards applications based on multistep switching.
Judicious pairing of ancillary and redox-active ligands affords thermally-induced two-step valence tautomerism for a dinuclear cobalt complex bridged by a conjugated bis(dioxolene) ligand.
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