Strong reducing agents containing dimolybdenum Mo24+ units and their oxidized cations with Mo25+/6+ cores stabilized by bicyclic guanidinate anions with a seven-membered ring

Abstract

The syntheses of two analogues of the bicyclic guanidinate ligand hpp (hpp = the anion of the guanidine-type compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine) which contain two fused rings are reported. Each compound contains one seven-membered ring while the other is either a five (Htbd) or a six (Htbu) membered ring. In THF/Bu₄NPF₆, the dimolybdenum compounds Mo₂(tbd)₄ and Mo₂(tbu)₄ are easily oxidized and they have signals in the differential pulse voltammograms at −1.059 and −1.009 V (vs. Ag/AgCl), respectively and for the Mo₂^5+/6+ couples and in the same order −0.242 and −0.312 V for the Mo₂^6+/5+ couples. The two compounds produce the corresponding Mo₂(bicyclic guanidinate)₄Cl compounds immediately upon dissolution in CH₂Cl₂ and these easily form species with Mo₂⁶⁺ cores. In Mo₂(tbd)₄Cl there are two crystallographically independent molecules with Mo–Mo distances of 2.1711(7) and 2.1690(7) Å. The distance between metal atoms increases to 2.206(1) Å upon oxidation to Mo₂(tbd)₄Cl₂ which has a triply bonded Mo₂⁶⁺ core. For the diamagnetic compound Mo₂(tbu)₄ this distance is 2.0677(9) Å and it increases to 2.133(2) Å upon reduction of the bond order from 4 to 3.5 in the paramagnetic compound Mo₂(tbu)₄Cl.