Supramolecular Stereocontrol of Octahedral Metal-Centered Chirality. Ligand Modulation

Chiral control of spin-crossover dynamics in Fe(II) complexes

Iron-based spin-crossover complexes hold tremendous promise as multifunctional switches in molecular devices. However, real-world technological applications require the excited high-spin state to be kinetically stable-a feature that has been achieved only at cryogenic temperatures. Here we demonstrate high-spin-state trapping by controlling the chiral configuration of the prototypical iron(II)tris(4,4'-dimethyl-2,2'-bipyridine) in solution, associated for stereocontrol with the enantiopure Δ- or Λ-enantiomer of tris(3,4,5,6-tetrachlorobenzene-1,2-diolato-κ²O¹,O²)phosphorus(V) (P(O₂C₆Cl₄)₃^- or TRISPHAT) anions. We characterize the high-spin-state relaxation using broadband ultrafast circular dichroism spectroscopy in the deep ultraviolet in combination with transient absorption and anisotropy measurements. We find that the high-spin-state decay is accompanied by ultrafast changes of its optical activity, reflecting the coupling to a symmetry-breaking torsional twisting mode, contrary to the commonly assumed picture. The diastereoselective ion pairing suppresses the vibrational population of the identified reaction coordinate, thereby achieving a fourfold increase of the high-spin-state lifetime. More generally, our results motivate the synthetic control of the torsional modes of iron(II) complexes as a complementary route to manipulate their spin-crossover dynamics.

Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes ‐ Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late‐stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron‐donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near‐infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG‐functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red‐shifted absorptions (Δλ_abs up to 25 nm) and emissions (Δλ_em up to 73 nm, Φ_PL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in ¹H NMR spectroscopy (218 K), N−CH₂ protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol⁻¹. This phenomenon is due to planar chiral conformations (S_p and R_p configurations), induced by the geometry of the alkyl (n‐propyl) side‐chains next to the acetylenic substituents. Ion pairing studies with Δ‐TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.

Asymmetric induction in quadruple-stranded europium(III) helicates and circularly polarized luminescence

Chiral supramolecular lanthanide-helicates are regarded as promising chiroptical materials due to their combination of ground and excited state chirality and special luminescence property from Ln3+, named as circularly polarized luminescence...

Chiroptical detection of a model ruthenium dye in water by circularly polarized-electrochemiluminescence

We demonstrate the possibility to detect selectively the two single enantiomers of a model [Ru(bpy)₃]²⁺-based dye by circularly polarized-electrochemiluminescence (CP-ECL).

Sugar derived hexacoordinated phosphates: Chiral anionic auxiliaries with general asymmetric efficiency

Mannose-derived hexacoordinated phosphate anions, prepared in as few as two steps from methyl-alpha-D-mannopyranoside and tris(dimethylamino)phosphine, are chiral anionic auxiliaries with broad asymmetric efficiency. These chemically robust anions are effective NMR chiral solvating agents and efficient asymmetry-inducers, able to control the configuration of conformationally labile C2-symmetric monomethinium cations (organic, helical, charge 1+, P or M enantiomers) and D3-symmetric iron(II) trisdiimine complexes (metallo-organic, octahedral, charge 2+, Delta or Lambda enantiomers). Diastereomeric control with often unmatched selective levels was achieved with the help of mannopyranoside backbone of the anions and the substituents on the [1,3]dioxane ring that play a key role in the recognition process, something not obvious at first sight.

Experimental and quantum-chemical studies of 1H, 13C and 15N NMR coordination shifts in Pd(II) and Pt(II) chloride complexes with methyl and phenyl derivatives of 2,2'-bipyridine and 1,10-phenanthroline

1H, 13C and 15N NMR studies of platinide(II) (M=Pd, Pt) chloride complexes with methyl and phenyl derivatives of 2,2'-bipyridine and 1,10-phenanthroline [LL=4,4'-dimethyl-2,2'-bipyridine (dmbpy); 4,4'-diphenyl-2,2'-bipyridine (dpbpy); 4,7-dimethyl-1,10-phenanthroline (dmphen); 4,7-diphenyl-1,10-phenanthroline (dpphen)] having a general [M(LL)Cl2] formula were performed and the respective chemical shifts (delta1H, delta13C, delta15N) reported. 1H high-frequency coordination shifts (Delta1Hcoord=delta1Hcomplex-delta1Hligand) were discussed in relation to the changes of diamagnetic contribution in the relevant 1H shielding constants. The comparison to literature data for similar [M(LL)(XX)], [M(LL)X2] and [M(LL)XY] coordination or organometallic compounds containing various auxiliary ligands revealed a large dependence of delta1H parameters on inductive and anisotropic effects. 15N low-frequency coordination shifts (Delta15Ncoord=delta 15Ncomplex-delta15Nligand) of ca 88-96 ppm for M=Pd and ca 103-111 ppm for M=Pt were attributed to both the decrease of the absolute value of paramagnetic contribution and the increase of the diamagnetic term in the expression for 15N shielding constants. The absolute magnitude of Delta15Ncoord parameter increased by ca 15 ppm upon Pd(II)-->Pt(II) transition and by ca 6-7 ppm following dmbpy-->dmphen or dpbpy-->dpphen ligand replacement; variations between analogous complexes containing methyl and phenyl ligands (dmbpy vs dpbpy; dmphen vs dpphen) did not exceed+/-1.5 ppm. Experimental 1H, 13C, 15N NMR chemical shifts were compared to those quantum-chemically calculated by B3LYP/LanL2DZ+6-31G**//B3LYP/LanL2DZ+6-31G*, both in vacuo and in DMSO or DMF solution.

Bimetallic Iridium(III) Complexes Consisting of Ir(ppy)2 Units (ppy = 2-Phenylpyridine) and Two Laterally Connected N∧N Chelates as Bridge: Synthesis, Separation, and Photophysical Properties

Reaction of the precursor complex Ir2(ppy)4Cl2 (ppy = 2-phenylpyridine) with the bridging ligand 3,8-dipyridyl-4,7-phenanthroline (L) affords, in 94% yield, the cyclometalated iridium dinuclear complex [(ppy)2Ir(mu-L)Ir(ppy)2]2+ (12+) as a mixture of three stereoisomers. This mixture consists of a meso form Delta,Lambda and a racemic form (enantiomeric pair Delta,Delta and Lambda,Lambda) in the ratio 1:1.5. Single-crystal X-ray characterization of the perchlorate salt of the meso form reveals (i) the distortion of the bridging ligand from the planarity and (ii) the location of the two iridium subunits above and below the medium plane of the bridging ligand. Ion-pair chromatography with Delta-TRISPHAT anion (TRISPHAT = tris(tetrachlorobenzenediolato)phosphate(V)) as resolving anion permits the separation of the three stereoisomers. The 1H NMR spectroscopic analysis of each fraction indicates high diastereomeric purity. Electronic circular dichroism properties and comparison with literature data establish their absolute configuration. The absorption and emission properties of the three stereoisomers show only very small variations. The anisotropic properties can be interpreted as distinct interactions of the isomers with the chiral resolving Delta-TRISPHAT anion.

Enantiodifferentiation of chiral cationic cages using trapped achiral BF4? anions as chirotopic probes

The addition of enantiopure TRISPHAT anions to chiral cationic cages of type [Co(4)(L)(6)(BF(4))](7+) leads to the enantiodifferentiation of the ligands of the racemic salts and, even more effectively, of the achiral tetrafluoroborate anion trapped inside.

Stereoisomer Discrimination in Complexes of Halogen-Substituted Difuranes and Li or Na Cations

The theoretical study of the stereoisomer discrimination of the 2:1 homo- and heterochiral complexes between chiral 5,5'-dihalogen bifuranes and lithium or sodium cations has been carried out using DFT methods. To understand the chiral effect produced by the introduction of a second bifurane molecule, the 1:1 complexes also have been calculated. All the 2:1 heterochiral complexes computed showed a nonplanar configuration around the metallic cation, but in the case of the homochiral complexes, the dibromo- and dichloro-bifurane systems around Na+ were quasi-planar. The nature of the interactions established between cations and bifurane systems has been analyzed by means of AIM and NBO, and correlations between the electron density topological parameters with the O...M distance and with the orbital interaction energy have been found. Stereodiscrimination is observed favoring the heterochiral complexes except for the Na+ complexes with chloro and bromo substituents in which the homochiral forms are more stable. Stereodiscrimination values correlate with the difference in electron density at the bond critical point and orbital interaction energy between homo- and heterochiral systems.

Supramolecular Asymmetric Induction in Dinuclear Triple-Stranded Helicates1

Supramolecular chiral induction has been observed in five dinuclear triple-stranded helicates composed entirely of achiral components. Three different chiral cations were found to be very effective at inducing high levels of enantiomeric enrichment in racemic mixtures of the helicates. The mechanism of intermolecular chiral induction in one cation-helicate pair, s-nic/K6[Ga2L(1)3], has been elucidated through circular dichroism spectroscopy, X-ray crystallography, and one and two-dimensional NMR spectroscopy (s-nic = N-methyl-s-nicotinium, H4L(1) = 1,4-bis(2',3'-dihydroxybenzamido)phenylene).

Effective chiral recognition among ions in polar media

Effective homochiral recognition occurs between the cationic [Fe(eilatin)3]2+ complex and TRISPHAT anions even in polar media such as 90% acetone-CHCl3 (de > or = 89%).

Diastereoselective formation and optical activity of an M4L6 cage complex

A chiral bridging ligand affords a single diastereoisomer of tetrahedral M4L6 cage complex in which the optical rotation of each ligand is increased by a factor of 5 on coordination.

Vibrational and electronic circular dichroism of Δ-TRISPHAT [tris(tetrachlorobenzenediolato)phosphate(V)] anion

Herein is reported an experimental and theoretical study of the circular dichroism properties of TRISPHAT (1) anion. ECD analysis of the [tetramethylammonium][delta-1] salt confirms the absolute configuration assignment obtained through X-ray crystallographic analysis of the parent cinchonidium salt. The structure, infrared, and vibrational circular dichroism (VCD) spectra derived from density functional theory (DFT) calculations are compared with experimental data.

Chiral Induction in a Ribose-Decorated Metallostar through Intrinsic and Interionic Diastereomeric Interactions

A ribose-functionalized bpy ligand has been prepared and shown to give modest diastereomeric excesses of Lambda-[FeL(3)](2+) complexes; interconversion of Delta and Lambda cations is relatively fast, and in CHCl(3), the favored complexes with Delta- or Lambda-TRISPHAT counterions are homochiral, (Delta(+)Delta(-)) or (Lambda(+)Lambda(-)). In the case of the Delta-TRISPHAT salt, a single diastereomer is observed (de > or = 96%).