5 research outputs found
Do 12-Membered Cycloalkane Rings Only Exist As One Conformation in the Solid-State? A Detailed Solid-State Analysis Involving Polymorphs of <i>N,N</i>′‑Biscyclododecyl Pyromellitic Diimide
Conformational flexibility in molecules plays a key role
in many
chemical and biological processes. It is a common belief that the
larger the cycloalkane the more flexible it will be, and the more
conformations it will adopt. While theoretical studies have shown
that cyclododecane has many possible conformations, they have also
consistently shown that one conformation is slightly more stable.
In this work, we examine the effect of substitution and crystal packing
on the conformation of singly substituted cyclododecane rings. This
has been done by exploiting polymorphism in an attempt to induce new
conformations in a specific molecule, as well as by examining structures
reported in the Cambridge Structural Database (CSD). To this end,
three polymorphs of <i>N,N</i>′-biscyclododecyl pyromellitic
diimide (PMDI-12) have been identified and their structures elucidated.
To rationalize the differences between the various polymorphs, molecule···molecule
interaction energies have been calculated using atom–atom potential
methods. Though the conformation of the PMDI-12 molecules as a whole
may differ, examination of the conformation of the 12-membered ring
indicates that it is conformationally identical in all three polymorphs.
Examination of 20 other organic and organometallic structures containing
this group in the CSD, indicates that they have the same conformation
(only one possible exception in the 34 rings examined in this work),
which suggests that the 12-membered ring adopts a single conformation
([3333] with <i>D</i><sub>2</sub> symmetry) in the solid-state
that is relatively unaffected by crystal packing
Do 12-Membered Cycloalkane Rings Only Exist As One Conformation in the Solid-State? A Detailed Solid-State Analysis Involving Polymorphs of <i>N,N</i>′‑Biscyclododecyl Pyromellitic Diimide
Conformational flexibility in molecules plays a key role
in many
chemical and biological processes. It is a common belief that the
larger the cycloalkane the more flexible it will be, and the more
conformations it will adopt. While theoretical studies have shown
that cyclododecane has many possible conformations, they have also
consistently shown that one conformation is slightly more stable.
In this work, we examine the effect of substitution and crystal packing
on the conformation of singly substituted cyclododecane rings. This
has been done by exploiting polymorphism in an attempt to induce new
conformations in a specific molecule, as well as by examining structures
reported in the Cambridge Structural Database (CSD). To this end,
three polymorphs of <i>N,N</i>′-biscyclododecyl pyromellitic
diimide (PMDI-12) have been identified and their structures elucidated.
To rationalize the differences between the various polymorphs, molecule···molecule
interaction energies have been calculated using atom–atom potential
methods. Though the conformation of the PMDI-12 molecules as a whole
may differ, examination of the conformation of the 12-membered ring
indicates that it is conformationally identical in all three polymorphs.
Examination of 20 other organic and organometallic structures containing
this group in the CSD, indicates that they have the same conformation
(only one possible exception in the 34 rings examined in this work),
which suggests that the 12-membered ring adopts a single conformation
([3333] with <i>D</i><sub>2</sub> symmetry) in the solid-state
that is relatively unaffected by crystal packing
Do 12-Membered Cycloalkane Rings Only Exist As One Conformation in the Solid-State? A Detailed Solid-State Analysis Involving Polymorphs of <i>N,N</i>′‑Biscyclododecyl Pyromellitic Diimide
Conformational flexibility in molecules plays a key role
in many
chemical and biological processes. It is a common belief that the
larger the cycloalkane the more flexible it will be, and the more
conformations it will adopt. While theoretical studies have shown
that cyclododecane has many possible conformations, they have also
consistently shown that one conformation is slightly more stable.
In this work, we examine the effect of substitution and crystal packing
on the conformation of singly substituted cyclododecane rings. This
has been done by exploiting polymorphism in an attempt to induce new
conformations in a specific molecule, as well as by examining structures
reported in the Cambridge Structural Database (CSD). To this end,
three polymorphs of <i>N,N</i>′-biscyclododecyl pyromellitic
diimide (PMDI-12) have been identified and their structures elucidated.
To rationalize the differences between the various polymorphs, molecule···molecule
interaction energies have been calculated using atom–atom potential
methods. Though the conformation of the PMDI-12 molecules as a whole
may differ, examination of the conformation of the 12-membered ring
indicates that it is conformationally identical in all three polymorphs.
Examination of 20 other organic and organometallic structures containing
this group in the CSD, indicates that they have the same conformation
(only one possible exception in the 34 rings examined in this work),
which suggests that the 12-membered ring adopts a single conformation
([3333] with <i>D</i><sub>2</sub> symmetry) in the solid-state
that is relatively unaffected by crystal packing
Probing the Nature of the Co(III) Ion in Corrins: Comparison of Reactions of Aquacyanocobyrinic Acid Heptamethyl Ester and Aquacyano-Stable Yellow Cobyrinic Acid Hexamethyl Ester with Neutral N‑Donor Ligands
Equilibrium constants (log <i>K</i>) for substitution
of coordinated H<sub>2</sub>O in aquacyanocobyrinic acid heptamethyl
ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic
acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs),
in which oxidation of the C5 carbon of the corrin interrupts the normal
delocalized system of corrins, by neutral N-donor ligands (ammonia,
ethanolamine, 2-methoxyethylamine, <i>N</i>-methylimidazole,
and 4-methylpyridine) have been determined spectrophotometrically
as a function of temperature. Log <i>K</i> values increase
with the basicity of the ligand, but a strong compensation effect
between Δ<i>H</i> and Δ<i>S</i> values
causes a leveling effect. The aliphatic amines with a harder donor
atom produce Δ<i>H</i> values that are more negative
in their reactions with ACSYCbs than with ACCbs, while the softer,
aromatic N donors produce more negative Δ<i>H</i> values
with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and
a quantum theory of atoms in molecules analysis of the electron density)
shows that complexes of the aliphatic amines with SYCbs produce shorter
and stronger Co–N bonds with less ionic character than the
Co–N bonds of these ligands with the cobester. Conversely,
the Co–N bond to the aromatic N donors is shorter, stronger,
and somewhat less ionic in the complexes of the cobester than in those
of the SYCbs. Therefore, the distinction between the harder CoÂ(III)
in ACSYCbs and softer CoÂ(III) in ACCbs, reported previously for anionic
ligands, is maintained for neutral N-donor ligands
The Synthesis of a Corrole Analogue of Aquacobalamin (Vitamin B<sub>12a</sub>) and Its Ligand Substitution Reactions
The
synthesis of a CoÂ(III) corrole, [10-(2-[[4-(1<i>H</i>-imidazol-1-ylmethyl)Âbenzoyl]Âamino]Âphenyl)-5,15-diphenylcorrolato]ÂcobaltÂ(III),
DPTC-Co, bearing a tail motif terminating in an imidazole ligand that
coordinates CoÂ(III), is described. The corrole therefore places CoÂ(III)
in a similar environment to that in aquacobalamin (vitamin B<sub>12a</sub>, H<sub>2</sub>OCbl<sup>+</sup>) but with a different equatorial
ligand. In coordinating solvents, DPTC-Co is a mixture of five- and
six-coordinate species, with a solvent molecule occupying the axial
coordination site trans to the proximal imidazole ligand. In an 80:20
MeOH/H<sub>2</sub>O solution, allowed to age for about 1 h, the predominant
species is the six-coordinate aqua species [H<sub>2</sub>O–DPTC-Co].
It is monomeric at least up to concentrations of 60 μM. The
coordinated H<sub>2</sub>O has a p<i>K</i><sub>a</sub> =
9.76(6). Under the same conditions H<sub>2</sub>OCbl<sup>+</sup> has
a p<i>K</i><sub>a</sub> = 7.40(2). Equilibrium constants
for the substitution of coordinated H<sub>2</sub>O by exogenous ligands
are reported as log <i>K</i> values for neutral N-, P-,
and S-donor ligands, and CN<sup>–</sup>, NO<sub>2</sub><sup>–</sup>, N<sub>3</sub><sup>–</sup>, SCN<sup>–</sup>, I<sup>–</sup>, and Cys in 80:20 MeOH/H<sub>2</sub>O solution
at low ionic strength. The log <i>K</i> values for [H<sub>2</sub>O–DPTC-Co] correlate reasonably well with those for
H<sub>2</sub>OCbl<sup>+</sup>; therefore, CoÂ(III) displays a similar
behavior toward these ligands irrespective of whether the equatorial
ligand is a corrole or a corrin. Pyridine is an exception; it is poorly
coordinated by H<sub>2</sub>OCbl<sup>+</sup> because of the sterically
hindered coordination site of the corrin. With few exceptions, [H<sub>2</sub>O–DPTC-Co] has a higher affinity for neutral ligands
than H<sub>2</sub>OCbl<sup>+</sup>, but the converse is true for anionic
ligands. Density functional theory (DFT) models (BP86/TZVP) show that
the Co–ligand bonds tend to be longer in corrin than in corrole
complexes, explaining the higher affinity of the latter for neutral
ligands. It is argued that the residual charge at the metal center
(+2 in corrin, 0 in corrole) increases the affinity of H<sub>2</sub>OCbl<sup>+</sup> for anionic ligands through an electrostatic attraction.
The topological properties of the electron density in the DFT-modeled
compounds are used to explore the nature of the bonding between the
metal and the ligands