Due to the inherent difficulties in achieving a defined and exclusive formation of multicomponent assemblies against entropic predisposition, we present the rational assembly of a heteroleptic [PdLL] coordination cage achieved through the geometric complementarity of two carefully designed ligands, L and L. With Pd(II) cations as rigid nodes, the pure distinctly angular components readily form homoleptic cages, a [PdL] strained helical assembly and a [PdL] box-like structure, both of which were characterized by X-ray analysis. Combined, however, the two ligands could be used to cleanly assemble a cis-[PdLL] cage with a bent architecture. The same self-sorted product was also obtained by a quantitative cage-to-cage transformation upon mixing of the two homoleptic cages revealing the [PdLL] assembly as the thermodynamic minimum. The structure of the heteroleptic cage was examined by ESI-MS, COSY, DOSY, and NOESY methods, the latter of which pointed toward a cis-conformation of ligands in the assembly. Indeed, DFT calculations revealed that the angular ligands and strict Pd(II) geometry strongly favor the cis-[PdLL] species. The robust nature of the cis-[PdLL] cage allowed us to probe the accessibility of its cavity, which could be utilized for shape recognition toward stereoisomeric guests. The ability to directly combine two different backbones in a controlled manner provides a powerful strategy for increasing complexity in the family of [PdL] cages and opens up possibilities of introducing multiple functionalities into a single self-assembled architecture.
A new aspherical scattering factor formalism was implemented in SHELXL. It relies on Gaussian functions and can optionally complement the independent atom model to take into account the deformation of electron-density distribution due to chemical bonding and lone pairs. The automated atom-type assignment was derived from the invariom formalism.
Single-crystal X-ray diffraction (XRD) is often considered the gold standard in analytical chemistry, as it allows element identification as well as determination of atom connectivity and the solid-state structure of completely unknown samples. Element assignment is based on the number of electrons of an atom, so that a distinction of neighboring heavier elements in the periodic table by XRD is often difficult. A computationally efficient procedure for aspherical-atom least-squares refinement of conventional diffraction data of organometallic compounds is proposed. The iterative procedure is conceptually similar to Hirshfeld-atom refinement (Acta Crystallogr. Sect. A- 2008, 64, 383-393; IUCrJ. 2014, 1,61-79), but it relies on tabulated invariom scattering factors (Acta Crystallogr. Sect. B- 2013, 69, 91-104) and the Hansen/Coppens multipole model; disordered structures can be handled as well. Five linear-coordinate 3d metal complexes, for which the wrong element is found if standard independent-atom model scattering factors are relied upon, are studied, and it is shown that only aspherical-atom scattering factors allow a reliable assignment. The influence of anomalous dispersion in identifying the correct element is investigated and discussed.
Our new model of electron density
augmented by point charges (aug-PROmol) provides an estimation of
electrostatic interaction energies including penetration effects (ChemPhysChem
2016, 17, 2455–2460).
In this paper we prove that it can be applied using sources of point
charges other than those from direct restrained fitting to electrostatic
potential (RESP). We used a newly established databank of tabulated
invariom point charges and a widely known semiempirical method. Both
sources perform equivalently to the basic aug-PROmol method as well
as to reference energies at the DFT-SAPT/aug-cc-pVTZ level of theory.
This is possible due to the universal character of the penetration
model included in the aug-PROmol. Aug-PROmol may become a basis for
development of new nonbonded terms in force fields or a high success
rate scoring function.
α,α-Disubstituted α-amino acids are central to biotechnological and biomedical chemical processes for their own sake and as substructures of biologically active molecules for diverse biomedical applications. Structurally, these compounds contain a quaternary stereocenter, which is particularly challenging for stereoselective synthesis. The pyridoxal-5'-phosphate (PLP)-dependent L-serine hydroxymethyltransferase from Streptococcus thermophilus (SHMT(Sth); EC 2.1.2.1) was engineered to achieve the stereoselective synthesis of a broad structural variety of α,α-dialkyl-α-amino acids. This was accomplished by the formation of quaternary stereocenters through aldol addition of the amino acids D-Ala and D-Ser to a wide acceptor scope catalyzed by the minimalist SHMT(Sth) Y55T variant overcoming the limitation of the native enzyme for Gly. The SHMT(Sth) Y55T variant tolerates aromatic and aliphatic aldehydes as well as hydroxy- and nitrogen-containing aldehydes as acceptors.
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