Raman spectroscopy of three average-valence dicopper cryptates: Evidence for copper-copper bonding

Comparison of the X-ray structures of two pairs of dicopper(I) and dicopper(l.5) cryptates provides evidence (over 0.5 Angstrom, contraction of the internuclear distance in one pair) for copper-copper bond formation. Although resonance Raman spectroscopy fails to identify a pure copper-copper stretching mode, because of extensive mixing of several low-frequency stretching and bending modes, isotopic substitution experiments indicate significant Cu-Cu character in a band around 250 cm(-1). A normal coordinate analysis broadly supports the spectroscopic findings. X-ray crystal structures are presented for the dicopper(1.5) complex Cu(2)imBT(ClO4)(3), C18H30C13Cu2N8O12, a = 9.140(4) Angstrom, c = 30.35(3) Angstrom, rhombohedral, R32, Z = 3, and the dicopper(I) Cu(2)imbistrpn(ClO4)(2), C24H42Cl2Cu2N8O8, a = 9.279(2) Angstrom, c = 32.846(9) Angstrom, rhombohedral, R32, Z = 3.