Asymmetric Transformation of a Double-Stranded, Dicopper(I) Helicate Containing Achiral Bis(bidentate) Schiff Bases

Abstract

Reactions of the bis(bidentate) Schiff-bases N,N'-bis(6-alkyl-2-pyridylmethylene)ethane-1,2-diamine (where alkyl = H, Me, iPr) (L) with tetrakis(acetonitrile)copper(I) hexafluorophosphate and silver(I) hexafluorophosphate afforded, respectively, the double-stranded, dinuclear metal helicates [T-4-(R,R)]-(+/-)-[M2L2](PF6)2 (M = Cu, Ag). The helicates were characterized by 1H and 13C NMR spectroscopy, conductivity, microanalysis, and single-crystal X-ray structure determinations on selected compounds. Intermolecular ligand exchange and intramolecular inversion rates for the complexes were investigated by 1H NMR spectroscopy. Reversible intermolecular ligand exchange between two differently substituted helicates followed first-order kinetics. The rate constants (k) and corresponding half-lives (t(1/2)) for ligand exchange for the dicopper(I) helicates were k = (1.6-1.8) x 10(-6) s(-1) (t(1/2) = 110-120 h) in acetone-d6, k = 4.9 x 10(-6) s(-1) (t(1/2) = 40 h) in dichloromethane-d2, and k> 2 x 10(-3) s(-1) (t(1/2) < 5 min) in acetonitrile-d3. Ligand exchange for the disilver(I) helicates occurred with k > 2 x 10(-3) s(-1) (t(1/2) < 5 min). Racemization of the dicopper(I) helicate by an intramolecular mechanism was investigated by determination of the coalescence temperature for the diastereotopic isopropyl-Me groups in the appropriate complex, and DeltaG() >> 76 kJ mol(-1) was calculated for the process in acetone-d6, nitromethane-d3, and dichloromethane-d2 with DeltaG() = 75 kJ mol(-1) in acetonitrile-d3. Complete anion exchange of the hexafluorophosphate salt of a dicopper(I) helicate with the enantiomerically pure Delta-(-)-tris(catecholato)arsenate(V) ([As(cat)3]-) in the presence of Dabco gave the two diastereomers (R,R)-[Cu2L2][Delta-(-)-[As(cat)3]]2 and (S,S)-[Cu2L2][Delta-(-)-[As(cat)3]]2 in up to 54% diastereomeric excess, as determined by (1)H NMR spectroscopy. The diastereomerically and enantiomerically pure salt (R,R)-[Cu(2)L2][Delta-(-)-[As(cat)3]]2 crystallized from the solution in a typical second-order asymmetric transformation. The asymmetric transformation of the dicopper(I) helicate is the first synthesis of a diastereomerically and enantiomerically pure dicopper(I) helicate containing achiral ligands.

Binuclear Pd-methyl complexes of N,N′-{1, n}-alkanediyl-bis(pyridinyl-2-methanimine) ligands (n = 5, 8, 9, 10 and 12): Evaluation as catalysts precursors for phenylacetylene polymerization

“…The other methylene protons of the alkanediyl linker (eNCH 2 CH 2 CH 2 e) were observed around d 1.27 ppm. These assignments were similar to those reported for analogous Pd systems [11,14,17].…”

N′-[1-(2-Hydroxyphenyl)ethylidene]thiophene-2-carbohydrazide

“…For applications of Schiff base compounds, see: Casas et al (2000); Habermehl et al (2006). For related structures, see: Li & Jian (2010); Li & Meng (2010 Table 1 Hydrogen-bond geometry (Å , ).…”

“…(Casas et al, 2000). They are important intermediates which have been reported to be chiral coordination compounds with many interesting properties (Habermehl et al, 2006). As part of our search for new schiff-base compounds we synthesized the title compound (I), and its crystal structure is determined herein.…”

1-(3,4-Dimethylbenzylidene)-4-ethylthiosemicarbazide

“…For applications of Schiff base compounds, see: Casas et al (2000); Habermehl et al (2006). For the structure of 4-ethyl-1-(4-methylbenzylidene)thiosemicarbazide, see: Li & Jian (2010 Table 1 Hydrogen-bond geometry (Å , ).…”