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(1.5) cryptates provides evidence (over 0.5 Å 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(ClO(4))(3), C(18)H(30)Cl(3)Cu(2)N(8)O(12), a = 9.140(4) Å, c = 30.35(3) Å, rhombohedral, R32, Z = 3, and the dicopper(I) Cu(2)imbistrpn(ClO(4))(2), C(24)H(42)Cl(2)Cu(2)N(8)O(8), a = 9.279(2) Å, c = 32.846(9) Å, rhombohedral, R32, Z = 3.