Photo-Triggered Chiroptical Switching of Platinum Complexes Bearing Azobenzene Moieties

Abstract

L-Alanine-derived optically active trans-platinum complexes bearing azobenzene moieties, L-trans-Pt[P(Ph) 2 -1,4-C 6 H 4 -CONH-C*H(CH 3 )-CONH-1,4- (L-2a-5c) were synthesized by the ligand exchange reaction of trans-Pt(PPh 3 ) 2 (CCPh) 2 and cis-Pt(cod)(CCPh) 2 , respectively, with L-P(Ph) 2 -1,4-C 6 H 4 -CONH-C*H(CH 2 Ph)-CONH-1,4-C 6 H 4 -NN-Ph. The trans-azobenzene moieties of the complexes isomerized to the cis form upon UV-light irradiation and returned to the trans form upon visible-light irradiation. L-2a-5t displayed weak circular dichroism (CD) signals irrespective of the trans and cis forms of the azobenzene moieties. On the other hand, L-2a-5c with trans-azobenzene moieties displayed strong CD signals, whose intensity decreased upon UV-light irradiation, accompanied by an increase in CD signals based on cis-azobenzene moieties. DFT calculations of the complexes indicated that intramolecular hydrogen bonds form between the amide moieties of two ligands in the cis-platinum complexes, resulting in retention of conformation. A noncovalent interaction plot analysis also indicated that the π−π interaction between the benzene rings of the ligands also contributes to conformational maintenance. The D-alanine-derived counterparts, D-2a-5t and D-2a-5c, exhibited mirror-image CD spectroscopic patterns in comparison to those of L-2a-5t and L-2a-5c. Solution-state IR absorption spectroscopic peaks of CO stretching and N−H bending vibrations of L-2a-5c were observed broadly at ca. 10 cm −1 lower and higher than those of P(Ph) 2 -1,4-C 6 H 4 -CONH-C*H(CH 3 )-CONH-1,4-C 6 H 4 -NN-Ph (L-2a), respectively, thus supporting the formation of intramolecular hydrogen bonding at the amide groups.

Platinum-Mediated Reversible Cross-linking/Decross-linking of Polyacetylenes Substituted with Phosphine Ligands: Catalytic Activity for Hydrosilylation

“…As described above, monomeric 2 o and 2 p predominantly gave cis -complexes by reactions with Pt­(cod)­Cl 2 . This result agrees with the kinetically favorable formation of cis -Pt complexes relative to trans -Pt complexes in ligand-exchange reactions, although trans -complexes are thermodynamically stable. − Next, poly­( 2 o ) was reacted with 1/2 equiv of Pt­(cod)­Cl 2 in CH 2 Cl 2 at 25 °C for 17 h to form 2 P–Pt bonds, as illustrated in Scheme . The reaction mixture was homogeneous throughout the set time, and the product polymer, poly­( 3 o ), was isolated by precipitation with ether.…”

Synthesis of Platinum-Containing Conjugated Polymers Bearing Chiral Phosphine Ligands. Study of Geometries and Intermolecular Interactions Leading to Aggregation

“…39,40 We have recently reported photo-triggered chiroptical switching of Pt complexes bearing azobenzene moieties, in which intramolecular HB between optically active amino acid moieties plays an important role in conformation fixation, leading to intense chirality. 41 We have also synthesized Pt-containing conjugated polymers containing amino-acid moieties to observe the formation of chiral higher-order structures responsive to the surrounding polarity. 42 In a fashion similar to the dehydrochlorination polymerization described above, Pt(PR 3 ) 2 (CC−Ar) 2 complexes are commonly synthesized by the dehydrochlorination reaction of Pt(PR 3 ) 2 Cl 2 with H−CC−Ar.…”

“…One method is substitution of chiral groups on phosphines. − Amino acids are promising chiral sources due to their ready availability and ease of functionalization. Amino-acid-derived amide groups form hydrogen bonds (HB), leading to supramolecular gels. , Metal complexes bearing amino-acid moieties are expected to be biomimetic materials in a fashion similar to metalloenzymes and metalloproteins like hemoglobin. , We have recently reported photo-triggered chiroptical switching of Pt complexes bearing azobenzene moieties, in which intramolecular HB between optically active amino acid moieties plays an important role in conformation fixation, leading to intense chirality . We have also synthesized Pt-containing conjugated polymers containing amino-acid moieties to observe the formation of chiral higher-order structures responsive to the surrounding polarity …”

Sintering of Platinum-Containing Conjugated Polymers: Gas Adsorption and Catalysis of the Formed Pt–Carbon Composites

“…We have reported a series of Pt-containing complexes and poly­(aryleneethynylene)­s, some of which adopt chirally regulated secondary structures. − Manners and co-workers synthesized metal nanoparticles by sintering poly­(arylene­ethynylene)­s with Pt complex moieties in the main chain tethering ferrocene moieties . Thus, Pt-containing π-conjugated polymers are expected to serve as precursors of Pt–carbon composites.…”