Dinuclear and tetranuclear gold-nitrogen complexes. Solvent influences on oxidation and nuclearity of gold guanidinate derivatives.

The sodium salt of the Hhpp ligand, Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, a 6,6 bicyclic, guanidine system, reacts with (THT)AuCl (THT = tetrahydrothiophene) in THF or CH2Cl2 to form the Au(II) complex, [Au2(hpp)2Cl2]. The Au(II) complex forms either by oxidation with solvents such as CH2Cl2 or disproportionation of Au(I) with concomitant Au(0) formation. The reaction in ethanol gives the colorless tetranuclear Au(I) complex, [Au4(hpp)4]. The tbo ligand, Htbo = 1,4,6-triazabicyclo[3.3.0]oct-4-ene a bicyclic 5,5 guanidine system, behaved differently from the hpp ligand, and only the colorless tetranuclear gold complex, [Au4(tbo)4], formed in THF, CH2Cl2, or ethanol. The X-ray structure of the oxidized species, [Au2(hpp)2Cl2], shows a Au(II)-Au(II) distance of 2.4752(9) A, the shortest gold-gold bond reported prior to the characterization here of the [(PhCOO)6Au4(hpp)2Ag2], Au(II)-Au(II) = 2.4473(19) A. A preliminary description of the formation of this material, obtained by reacting [Au2(hpp)2Cl2] with Ag(OOCPh), is included in this paper. The four Au(I) atoms in the tetranuclear complexes are arranged in a parallelogram with Au-Au distances ranging from 2.8975(5)-2.9392(6) A in [Au4(hpp)4] and 3.1139(12)-3.2220(13) A in [Au4(tbo)4]. density functional theory (DFT) and MP2 calculations on [Au2(hpp)2Cl2] find that the highest occupied molecular orbital (HOMO) is predominately hpp and chlorine-based with some Au-Au delta* character. The lowest unoccupied molecular orbital (LUMO) has metal-to-ligand (M-L) and metal-to-metal (M-M) sigma* character (approximately 50% hpp/chlorine, and 50% gold). DFT calculations on [Au4(hpp)4] show that the HOMO and HOMO-1 are each a mixture of metal-metal antibonding character and metal-ligand antibonding character and that the LUMO is predominately metal based s character (85% Au and 15% hpp).