Crystal structure of the uranium oxo borohydride complex U _2(μ-O)(BH _4) _6(dme) _2 (dme = 1,2-dimethoxyethane) and reformulation of U _2(μ-H) _2(BH _4) _6(dme) _2

摘要

The reaction of uranium tetrachloride, UCl _4, with sodium N,N-dimethylaminodiboranate, Na(H _3BNMe _2BH _3), in refluxing 1,2-dimethoxyethane (dme) yields small amounts of a new complex, which we formulate as (μ-oxo)hexakis(tetrahydroborato)bis(1,2- dimethoxyethane)diuranium(IV), toluene solvate, U _2(μ-O)(BH _4) _6(dme) _2·C _7H _8, 1. Most likely, the formation of BH _4 ~- groups from H _3BNMe _2BH _3 ~- occurs with the elimination of [Me _2NBH _2] _2, and the formation of the oxo group involves adventitious hydrolysis. Each uranium center in 1 adopts a fac octahedral geometry (counting the BH _4 ~- groups as occupying one coordination site); the bridging oxygen atom and the two coordinated oxygen atoms of the dme ligand occupy positions trans to the three BH _4 ~- groups. The hydrogen atom positions were located in the electron density difference maps and reveal that all three BH _4 ~- groups are bound in a κ ~3H fashion. The U?B distances to the two BH _4 ~- groups that are cis to the bridging oxygen atom are 2.574(6) and 2.584(6), whereas the U?B distance of 2.635(7) to the BH _4 ~- group that is trans to the bridging oxygen is distinctly longer. Thus, the bridging oxygen atom exerts a noticeable trans influence. The crystallographic and 1H NMR data strongly suggest that the previously reported uranium hydride complex U _2(μ-H) _2(BH _4) _6(dme) _2 should be reformulated as this oxo complex U _2(μ-O)(BH _4) _6(dme) _2.