46540-AC3
A Protecting Group Strategy for Inorganic Functional Group Synthesis
We are developing methods for the use of the diphenyl(4-pyridyl)methyl group as a platform for atom- and group-delivery. The utility of this group stems from the ease with which homolysis of the C–X bond in molecules of the form Ph2(4-py)C–X (4-py = 4-pyridyl; X = single atom or molecular fragment) may be effected. Additionally, the simplicity of preparing molecules of the form Ph2(4-py)C–X ensures a wide range of synthetic possibilities to explore. Our investigations involve appending Ph2(4-py)C–X as a coordinated ligand in reactive molecular uranium complexes and exploring their chemistry with the intent of determining the conditions necessary to cleave the C–X bond. The reaction of (THF)U(N[t-Bu]Ar)3 (1-THF, Ar = 3,5-Me2C6H3) with the organoazide Ph2(4-py)CN3 (2) does not proceed cleanly to (Ph2(4-py)CN)U(N[t-Bu]Ar)3 (3). Rather, analysis of the reaction mixture indicates the formation of multiple products, of which one being the uranium(IV) pyridyl complex (Ph2C(HC=CH)2N)U(N[t-Bu]Ar)3 (4). Independent synthesis of 4 was achieved by treating 1-THF with the Gomberg dimer analogue (4-diphenylmethylenedihydropyridinyl)(4-pyridyl)diphenylmethane (5, Eq. 1). The solid state structure of 4 was determined via X-ray crystallographic analysis and features a tris(anilide)-supported uranium(IV) center (average U–Namido 2.20 Å) with a U–Npyridyl distance of 2.342(3) Å. The C–N and C–C interatomic distances in the substituted pyridyl ligand alternate between lengths typical of single and double bonds, reflecting reduction of the pyridyl ring by the uranium center. The solution infrared spectrum of 4 reveals a band at 1631 cm-1 assigned to an olefinic C–C stretching mode. In the course of determining what products result from the reaction of 1-THF and 2, the uranium(VI) azide complex (N3)U(N[t-Bu]Ar)3 (1-N3) was prepared by treating IU(N[t-Bu]Ar)3 (1-I) with NaN3 (Eq. 2). Complex 1-N3 exists as a trimer in the solid state, with a total of nine anilide ligands supporting a [U3(µ-1,3-N3)3] core. The solution infrared spectrum of 1-N3 reveals an intense band at 2084 cm-1, with bands of medium intensity at 2114 cm-1, and 2130 cm-1. Surprisingly, 1-N3 does not appear to be component of the product mixture obtained upon reaction of 1-THF with 2. It is clear that reaction pathways that subvert formation of the imido complex 3 from 1-THF and 2 exist. These pathways arise in part due the ability of 2 to engage the uranium center of 1-THF through the pyridyl nitrogen, rather that through the azide group. This issue was circumvented by appending a protecting group to the pyridyl nitrogen of 1. The reaction of 1 with BArF3 (ArF = C6F5) provides the borane adduct Ph2(ArF3B–NC5H4)CN3 (2•BArF3) in 85% yield, effectively protecting the pyridyl nitrogen while leaving the azide functionality unperturbed. Gratifyingly, 2•BArF3 reacts cleanly with 1-THF to afford the pentavalent imido complex (Ph2(ArF3B–NC5H4)CN)U(N[tBu]Ar)3 (3•BArF3) as a brown powder (Eq. 3). The chemistry of 3•BArF3 is now being explored, specifically directed at removal of the borane protecting group to yield 3. Upon successful deprotection of the pyridyl nitrogen, scission of the C–Nimido bond in 3 will be pursued with goal of generating the terminal uranium(VI) nitride complex NU(N[t-Bu]Ar)3 (1-N). Additionally, chemical reduction of 3•BArF3 is also being explored as means by which 1-N may be produced. Another approach by which we are testing the utility of the diphenyl(4-pyridyl)methyl platform as an ligand delivery reagent is through the use of the alkoxide derivative K[OC(4-py)Ph2] (7). The reaction of 7 with U(N[t-Bu]Ar)3I affords (Ph2(4-py)CO)U(N[t-Bu]Ar)3 (8) as an amber solid. We are now pursuing the development of synthetic methods to effect scission of the C–O bond in 8 to reveal the terminal uranium(V) oxide complex OU(N[t-Bu]Ar)3 (9).
