FURTHER STUDY OF VERY CLOSE NONBONDED CU-I-CU-I CONTACTS - MOLECULAR-STRUCTURE OF A NEW COMPOUND AND DENSITY-FUNCTIONAL THEORY CALCULATIONS

A new compound containing a Cu-I-Cu-I unit with a short internuclear d istance, 2.453(1) Angstrom, is reported. The question of whether such a short distance justifies the postulation of a metal-metal bond is ad dressed using density functional theory (DFT). The new compound is Cu- 2(hpp)(2), where hpp(-) (C7N3H12) is the anion derived from ,3,4,6,7,8 -hexahydro-2H-pyrimido[1,2-a]pyrimidine. Cu-2(hpp)(2) crystallizes in the monoclinic space group P2(1)/c with Z = 2 and cell dimensions a = 7.320(2) Angstrom, b = 12.418(4) Angstrom, c = 8.689(2) Angstrom, and beta = 93.76(2)degrees. The density functional calculations show that the close approach of the copper atoms is predictable without involvin g any significant amount of covalent bonding. A DFT calculation on the Cu-3[(p-tol)N-5(p-tol)](3) molecule also foretells the observed short Cu-Cu separation (2.35 Angstrom observed vs 2.38 Angstrom calculated) but, again, does not imply the formation of Cu-Cu bonds. Very short C u-I to Cu-I distances can be attributed to a combination of strong Cu- N bonding and very short (ca. 2.2 Angstrom) bite distances for the lig ands.