Further Study of Very Close Nonbonded Cu(I)-Cu(I) Contacts. Molecular Structure of a New Compound and Density Functional Theory Calculations.

Abstract

A new compound containing a Cu(I)-Cu(I) unit with a short internuclear distance, 2.453(1) Å, is reported. The question of whether such a short distance justifies the postulation of a metal-metal bond is addressed using density functional theory (DFT). The new compound is Cu(2)(hpp)(2), where hpp(-) (C(7)N(3)H(12)) is the anion derived from 1,3,4,6,7,8-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) Å, b = 12.418(4) Å, c = 8.689(2) Å, and beta = 93.76(2) degrees. The density functional calculations show that the close approach of the copper atoms is predictable without involving 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 Å observed vs 2.38 Å calculated) but, again, does not imply the formation of Cu-Cu bonds. Very short Cu(I) to Cu(I) distances can be attributed to a combination of strong Cu-N bonding and very short (ca. 2.2 Å) bite distances for the ligands.