10 research outputs found
A halogen bond route to shorten the ultrashort sextuple bonds in Cr-2 and Mo-2
Sextuple bonded group 6 diatomics Cr-2 and Mo-2 possess ultrashort metal-metal bonds. Yet their bond dissociation energy is very low. The destabilising nature of sigma-bonds is responsible for this. Selective extraction of these sigma-electrons via a sigma-hole on a halogen bond donor shortens and strengthens the metal-metal bond. This study constitutes a hitherto unexplored application of halogen bonding and an example for the true violation of bond order-bond strength relation
Halogen bond shortens and strengthens the bridge bond of 1.1.1]propellane and the open form of 2.2.2]propellane
Detailed electronic structural analysis of 1.1.1]propellane and the open form of 2.2.2]propellane, especially their highest occupied molecular orbital (HOMO), shows the existence of significant electronic congestion at their bridge bond. The HOMO of 1.1.1]propellane is a spread-out orbital of its inverted tetrahedral bridgehead atoms. The HOMO of the open form of 2.2.2]propellane is an anti-bonding combination of its bridgehead atoms due to the stabilizing through-bond interaction. This unique spatial disposition of the HOMO enables a high electron density at the bridgehead atoms. Herein, we utilize the electron scavenging power of halogen bond donors to extract a fraction of destabilizing electrons from the bridge bond with the aim to alleviate its electronic congestion, which results in shortening and strengthening of the bridge bond with a reduction in the bond order. This result answers the seminal question raised by K. B. Wiberg in 1983, how can one have a relatively strong bond' without much bonding character
Contrasting Behavior of the Z Bonds in X–Z···Y Weak Interactions: Z = Main Group Elements Versus the Transition Metals
In
contrast to the increasing family of weak intermolecular interactions
in main-group compounds (X–Z···Y, Z = main-group
elements), an analysis of the Cambridge Structural Database indicates
that electron-saturated (18-electron) transition-metal complexes show
reluctance toward weak M bond formation (X–M···Y,
M = transition metal). In particular, weak M bonds involving electron-saturated
(18-electron) complexes of transition metals with partially filled
d-orbitals are not found. We propose that the nature of valence electron
density distribution in transition-metal complexes is the primary
reason for this reluctance. A survey of the interaction of selected
electron-saturated transition-metal complexes with electron-rich molecules
(Y) demonstrates the following: shielding the possible σ-hole
on the metal center by the core electron density in 3d series, and
enhanced electronegativity and relativistic effects in 4d and 5d series,
hinders the formation of the M bond. A balance in all the destabilizing
effects has been found in the 4d series due to its moderate polarizability
and primogenic repulsion from inner core d-electrons. A changeover
in the donor–acceptor nature of the metal center toward different
types of incoming molecules is also unveiled here. The present study
confirms the possibility of M bond as a new supramolecular force in
designing the crystal structures of electron-saturated transition-metal
complexes by invoking extreme ligand conditions
Continuum in the X-Z-Y Weak Bonds: Z= Main Group Elements
The Continuum in the variation of the X-Z bond length change from blue-shifting to red-shifting through zero-shifting in the X-Z---Y complex is inevitable. This has been analyzed by ab-initio molecular orbital calculations using Z= Hydrogen, Halogens, Chalcogens, and Pnicogens as prototypical examples. Our analysis revealed that, the competition between negative hyperconjugation within the donor (X-Z) molecule and Charge Transfer (CT) from the acceptor (Y) molecule is the primary reason for the X-Z bond length change. Here, we report that, the proper tuning of X-and Y-group for a particular Z-can change the blue-shifting nature of X-Z bond to zero-shifting and further to red-shifting. This observation led to the proposal of a continuum in the variation of the X-Z bond length during the formation of X-Z---Y complex. The varying number of orbitals and electrons available around the Z-atom differentiates various classes of weak interactions and leads to interactions dramatically different from the H-Bond. Our explanations based on the model of anti-bonding orbitals can be transferred from one class of weak interactions to another. We further take the idea of continuum to the nature of chemical bonding in general. (C) 2015 Wiley Periodicals, Inc
Negative hyperconjugation and red-, blue- or zero-shift in X-Z center dot center dot center dot Y complexes
A generalized explanation is provided for the existence of the red-and blue-shifting nature of X-Z bonds (Z = H, halogens, chalcogens, pnicogens, etc.) in X-Z center dot center dot center dot Y complexes based on computational studies on a selected set of weakly bonded complexes and analysis of existing literature data. The additional electrons and orbitals available on Z in comparison to H make for dramatic differences between the H-bond and the rest of the Z-bonds. The nature of the X-group and its influence on the X-Z bond length in the parent X-Z molecule largely controls the change in the X-Z bond length on X-Z center dot center dot center dot Y bond formation; the Y-group usually influences only the magnitude of the effects controlled by X. The major factors which control the X-Z bond length change are: (a) negative hyperconjugative donation of electron density from X-group to X-Z sigma* antibonding molecular orbital (ABMO) in the parent X-Z, (b) induced negative hyperconjugation from the lone pair of electrons on Z to the antibonding orbitals of the X-group, and (c) charge transfer (CT) from the Y-group to the X-Z sigma* orbital. The exchange repulsion from the Y-group that shifts partial electron density at the X-Z sigma* ABMO back to X leads to blue-shifting and the CT from the Y-group to the sigma* ABMO of X-Z leads to red-shifting. The balance between these two opposing forces decides red-, zero- or blue-shifting. A continuum of behaviour of X-Z bond length variation is inevitable in X-Z center dot center dot center dot Y complexes
Dynamical Origin of Rebound versus Dissociation Selectivity during Fe-Oxo-Mediated C–H Functionalization Reactions
The mechanism of catalytic C–H
functionalization of alkanes
by Fe-oxo complexes is often suggested to involve a hydrogen atom
transfer (HAT) step with the formation of a radical-pair intermediate
followed by diverging pathways for radical rebound, dissociation,
or desaturation. Recently, we showed that in some Fe-oxo reactions,
the radical pair is a nonstatistical-type intermediate and dynamic
effects control rebound versus dissociation pathway selectivity. However,
the effect of the solvent cage on the stability and lifetime of the
radical-pair intermediate has never been analyzed. Moreover, because
of the extreme complexity of motion that occurs during dynamics trajectories,
the underlying physical origin of pathway selectivity has not yet
been determined. For the reaction between [(TQA_Cl)FeIVO]+ and cyclohexane, here, we report explicit solvent
trajectories and machine learning analysis on transition-state sampled
features (e.g., vibrational, velocity, and geometric) that identified
the transferring hydrogen atom kinetic energy as the most important
factor controlling rebound versus nonrebound dynamics trajectories,
which provides an explanation for our previously proposed dynamic
matching effect in fast rebound trajectories that bypass the radical-pair
intermediate. Manual control of the reaction trajectories confirmed
the importance of this feature and provides a mechanism to enhance
or diminish selectivity for the rebound pathway. This led to a general
catalyst design principle and proof-of-principle catalyst design that
showcases how to control rebound versus dissociation reaction pathway
selectivity
Experimental and theoretical study of intramolecular O center dot center dot center dot O interaction in structurally rigid beta-keto carboxylic esters
Here we report the crystal structures of quinolone carboxylate and bisethoxycarbonylvinylaniline derivates containing an O center dot center dot center dot O distance shorter than the sum of their van der Waals radii, which can be ascribed to their steric demand per se, which provide unequivocal evidence of intramolecular 1,5-closed shell type interaction. Theoretical studies including Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) analysis are employed to characterize the nature of the closed shell O center dot center dot center dot O interaction. We found that the lone pair electrons on the interacting oxygens undergo stabilization due to negative hyperconjugation and maintains the otherwise repulsive O center dot center dot center dot O close contact
Dynamical Origin of Rebound versus Dissociation Selectivity during Fe-Oxo-Mediated C–H Functionalization Reactions
The mechanism of catalytic C–H
functionalization of alkanes
by Fe-oxo complexes is often suggested to involve a hydrogen atom
transfer (HAT) step with the formation of a radical-pair intermediate
followed by diverging pathways for radical rebound, dissociation,
or desaturation. Recently, we showed that in some Fe-oxo reactions,
the radical pair is a nonstatistical-type intermediate and dynamic
effects control rebound versus dissociation pathway selectivity. However,
the effect of the solvent cage on the stability and lifetime of the
radical-pair intermediate has never been analyzed. Moreover, because
of the extreme complexity of motion that occurs during dynamics trajectories,
the underlying physical origin of pathway selectivity has not yet
been determined. For the reaction between [(TQA_Cl)FeIVO]+ and cyclohexane, here, we report explicit solvent
trajectories and machine learning analysis on transition-state sampled
features (e.g., vibrational, velocity, and geometric) that identified
the transferring hydrogen atom kinetic energy as the most important
factor controlling rebound versus nonrebound dynamics trajectories,
which provides an explanation for our previously proposed dynamic
matching effect in fast rebound trajectories that bypass the radical-pair
intermediate. Manual control of the reaction trajectories confirmed
the importance of this feature and provides a mechanism to enhance
or diminish selectivity for the rebound pathway. This led to a general
catalyst design principle and proof-of-principle catalyst design that
showcases how to control rebound versus dissociation reaction pathway
selectivity
Designing M-bond (X-M···Y, M = transition metal): σ-hole and radial density distribution
Intramolecular Heteroatom and Styryl Diels–Alder Reactions, Asymmetric Cycloadditions of Chiral 3‑Phenylallyl Maleic Esters
Polycyclic aryl naphthalene and tetralin dihydro arylnaphthalene
lactone lignans possess anticancer and antibiotic activity. Related
furo[3,4-c]pyranones, typified by the sequester-terpenoid
isobolivianine, show similar antiproliferative bioactivity. Efficient
syntheses of compounds featuring these polycyclic cores have proven
challenging due to low yields and poor stereoselectivity. We report
the synthesis of chiral cinnamyl but-2-enanoates and 3,3-diphenylallyl-but-2-enoates
1 as new Diels–Alder substrates. These compounds undergo [4
+ 2]-cycloadditions to give furo[3,4-c]pyranones
2 in good yield (70%) and diastereoselectivity (7:1), together with
naphthyl 3 and dihydronaphthyl tetralins 4 as minor products. Molecular
structures and stereochemistries of the major products were verified
using X-ray diffraction. Density functional theory calculations revealed
that the cycloaddition process involves a bispericyclic/ambimodal
process where there is a single transition state that leads to both
intramolecular styryl Diels–Alder (ISDA) 3, 4 and intramolecular
hetero Diels–Alder (IHDA) cycloadducts 2. With the elevated
temperature conditions after cycloaddition, the resulting ISDA cycloadduct
either undergoes [3,3]-sigmatropic rearrangement to the more stable
major IHDA product or aromatization leading to the phenyltetralin