An investigation of the reactivity of a trans-bis(imido) uranium(VI) complex
For the past 150 years, the chemistry of uranium(VI) has been generally directed toward studies of the chemical behavior and unique bonding in the uranyl ion (UO22+). Interest in this area is driven in part by the technological and environmental importance of uranium oxide in nuclear fuel reprocessing and waste management. Compared to the interest and vast chemistry reported for the uranyl ion, examples of the isoelectronic imido uranium(VI) analog (-U(NR)22+) are relatively rare.
Remarkably, the synthesis and isolation of the trans-bis(imido) uranium(VI) complex (U(NR)22+) was reported in 2005 by Los Alamos researchers Trevor Hayton, James Boncella, Brian Scott, Phillip Palmer, Enrique Batista, and P. Jeffrey Hay. This discovery has allowed the opportunity to compare the valence bonding and covalency in both UO22+ and U(NR)22+ fragments. Density functional theory calculations show that while U–O and U–N bond interactions are similar, the bis(imido) ion possesses a more covalent nature. One question that arises from these calculations is how this reduced positive charge on the metal center influences reactivity of the U(NR)22+ ion.
Organic isocyanates have often been used in transition-metal chemistry to effect transformations of M=N imido functional groups. Given this precedent, the reactivity of substituted isocyanates with the U(NR)22+ ion was explored.
The addition of one equivalent of aryl isocyanate (ArNCO; Ar = C6H5, 2,4,6-Me3C6H2) to U(NtBu)2(I)2(OPPh3)2 initiates an aryl- for alkyl-imido exchange reaction in which U(NtBu)(NAr)(I)2(OPPh3)2 is synthesized. The solid-state molecular structure of U(NtBu)(NAr)(I)2(OPPh3)2 has been determined by X-ray crystallography.
This substitution reaction can also be exploited to introduce a second aryl imido ligand as shown by the reaction between two equivalents of ArNCO and U(NtBu)2(I)2(OPPh3)2 to generate U(NAr)2(I)2(OPPh3)2 in moderate yields. While there is precedence for this imido substitution in transition-metal chemistry, it is not clear why this reaction does not form a thermodynamic and kinetically stable U=O bond. Given this unexpected reactivity, density functional theory calculations are under way to elucidate a mechanism for this imido exchange.
The use of aryl isocyanates to accomplish exchange in this manner suggests that a similar synthetic strategy with other electrophilic cumulenes may lead to the isolation of unique uranium-heteroatom multiply bonded complexes. For example, the reaction between the U(NtBu)2(I)2(THF)2 (THF = tetrahydrofuran) and diphenylketene (Ph2C=C=O) was examined. This reaction yields the previously characterized mixed oxo imido species U(NtBu)(O)(I)2(THF)3 in reasonable yield, as shown in the following equation.
Investigation of hard-metal–soft-ligand interactions in the U(NR)22+ ion are also of interest. To this end, the synthesis of bis(imido) uranium(VI) chalcogenate complexes has been investigated. Recent results have shown that the coordination of anionic chalcogenate donors (-OR, -SR, -SeR, -TeR) to the U(NR)22+ ion can be achieved by metathesis reactions, as shown in the following equation.
For example, the reaction between U(NtBu)2(I)2(OPPh3)2 and two equivalents of NaSePh yielded the dichalcogenate complex U(NtBu)2(SePh)2(OPPh3)2. The solid-state molecular structure of U(NtBu)2(SePh)2(OPPh3)2 has been determined by X-ray crystallography and is shown in the figure at left. This methodology has been used to synthesize bis(imido) uranium(VI) chalcogenate complexes of the general formula U(NtBu)2(EAr)2(OPPh3)2 (E=O, Ar=2-tBuC6H4; E=S, Ar= C6H5; E=Se, Ar= C6H5; E=Te, Ar= C6H5) which have been characterized by X-ray crystallography, nuclear magnetic resonance spectroscopy, and elemental analysis. Density functional calculations are in progress with Los Alamos researcher Enrique Batista to investigate the degree of covalency in the uranium-chalcogenate bond.