Facilitating Access to the Most Easily Ionized Molecule: an Improved Synthesis of the Key Intermediate, W2(hpp)4Cl2, and Related Compounds

Publisher Website
Google Scholar
Abstract
A far superior synthesis is reported for W2(hpp)4Cl2, a key intermediate in the synthesis of the most easily ionized closed-shell molecule W2(hpp)4 (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). At 200 °C, the one-pot reaction of the air-stable and commercially available compounds W(CO)6 and Hhpp in o-dichlorobenzene produces W2(hpp)4Cl2 in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)2(CO)4 or W2(μ-CO)2(μ-hpp)2(η2-hpp)2, which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W2(hpp)4Cl2 and W2(hpp)4. Two additional derivatives, W2(hpp)4X2 where X is PF6- or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W2(μ-CO)2(μ-hpp)2(η2-hpp)2 and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W2(hpp)4(TFPB)2 in THF shows two reduction processes with an E1/2 of −0.97 V for the first and −1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W2(hpp)42+ units in W2(hpp)4X2 compounds show that the metal−metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W−W distances. The low-energy gas-phase ionizations of W2(hpp)4 are also reported and discussed.