Syntheses and thermal reactivities of tetradentate metalloenediynes of Cu(II) and Zn(II).
The syntheses and Bergman cyclization temperatures of disubstituted tetradentate enediyne ligands based on a dibenzylethylenediamine backbone are reported relative to the corresponding Cu(II) and Zn(II) analogues. For these compounds, the R-groups dimethylamine (dma), pyridine (py), quinoline (quin), and 3-oxypyridine (pyO) have been symmetrically and asymmetrically incorporated at the alkyne termini positions directly (0:0) or via a methylene spacer (1:0, 0:1, 1:1). Electron paramagnetic resonance (EPR) reveals that all Cu(II) complexes are monomeric with near axial symmetry and g-values (g(x) approximately 2.04, g(y) approximately 2.09 g(z) approximately 2.25) representative of tetragonal Cu(II) geometries. The hyperfine splitting parameter A(z) values are approximately 170 x 10(-)(4) cm(-)(1), which is consistent with distorted 4-coordinate, or weakly 6-coordinate, structures. In contrast, solution conductivity measurements show that Zn(II) complexes with rigid py or quin ligands (e.g., py-py 0:0, py-quin 0:0) behave as 1:4 electrolytes indicative of dimeric, bridging enediyne structures. Consequently, these Zn(II) complexes have very high Bergman cyclization temperatures (>290 degrees C), while their less rigid, 1:1 analogues (<185 degrees C) and monomeric Cu(II) counterparts (110-136 degrees C) have markedly lower cyclization temperatures. The results underscore the important consequences metal center structure plays in influencing Bergman cyclization temperatures of metalloenediynes.
Chandra
Tilak
T
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA.
Allred
Russell A
RA
Kraft
Brian J
BJ
Berreau
Lisa M
LM
Zaleski
Jeffrey M
JM
D002152:Calorimetry, Differential Scanning; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003300:Copper / Q000737:chemistry*; D003500:Cyclization; D004578:Electron Spin Resonance Spectroscopy; D007202:Indicators and Reagents; D008024:Ligands; D009942:Organometallic Compounds / Q000138:chemical synthesis*; D015032:Zinc / Q000737:chemistry*
D007202:Indicators and Reagents; D008024:Ligands; D009942:Organometallic Compounds; D003300:Copper; D015032:Zinc