Utility of the 1,3,4,6,7,8-Hexahydro-2<i>H</i>-pyrimido[1,2-<i>a</i>]pyrimidinato Ligand, (hpp)⁻, in Stabilizing Uranium Metallocene Mono-Alkyl and “Tuck-in” Complexes

Monoalkyl uranium chemistry has been probed by reacting the metallocene chloride complex (C5Me5)2(hpp)UCl, 1, (hpp)− = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato, with alkyl lithium reagents. Complex 1 reacts with LiMe, LiCCPh, LiPh, and LiEt to generate (C5Me5)2(hpp)UMe, 2, (C5Me5)2(hpp)U(CCPh), 3, (C5Me5)2(hpp)UPh, 4, and (C5Me5)2(hpp)UEt, 5, respectively. Complexes 2−5 react with CuI to form the iodide complex (C5Me5)2(hpp)UI, 6, and methane, phenylacetylene, benzene, and ethane, respectively. Attempts to make a neopentyl analogue of 2−5 from the reaction of 1 with neopentyllithium yielded the “tuck-in” complex (C5Me5)(η5:η1-C5Me4CH2)(hpp)U, 7. Complex 7 can also be synthesized by heating 5 to 70 °C in a reaction that forms ethane as a byproduct.