Expansion of Low- and Mid-Valent Organometallic Uranium Chemistry

2019-08-16T14:23:13Z (GMT) by Caleb J Tatebe

A series of uranium benzyl compounds supported by two hydrotris(3,5-dimethylpyrazolyl) borate (Tp*) ligands has been synthesized and characterized. In addition to the previously reported Tp*2U(CH2Ph) (2-Bn), examinations of both steric (tert-butyl, iso-propyl) and electronic (methoxy, picolyl) changes on the aromatic ring led to the formula Tp*2U(CH2Ar) (Ar = 4-tert-butylphenyl (2-tBu), 4-isopropyl (2-iPr), 2-picolyl (2-pyr), 3-methoxyphenyl (2-OMe). Treatment of the entire series of benzyl compounds with azidotrimethylsilane results in the formation of a neutral, monomeric U(III) compound, Tp*2U(N3) (3-N3), and substituted benzyltrimethylsilane. While there was no observed change in reactivity among the benzyl compounds and Me3SiN3, treatment of these compounds with triphenylphosphine oxide saw unique carbon-carbon coupling occur for three of the substituted benzyl compounds. With a single equivalent of OPPh3, the following products were isolated: Tp*2U[OP(C6H5)(C6H5CH2C6H5)] (4-Ph), Tp*2U[OP(C6H5)(C6H5-p-iPrC6H4)] (4-iPr), Tp*2U[OP(C6H5)(C6H5-p-tBuC6H4)] (4-tBu), Tp*2U[OP(C6H5)(C6H5-m-OCH3C6H4)] (4-OMe).

A family of uranium(IV) imido complexes of the form Tp*2U(NR) (R = benzyl (7-Bn), para-tolyl (7-Tol), para-methoxyphenyl (7-OMe), 2,6-diethylphenyl (7-detp), 2,6-diisopropylphenyl (7-dipp)) have been generated by bibenzyl extrusion from 2-Bn. When 7-Bn and 7-Tol, along with previously reported Tp*2U(N-Ph) (7-Ph) and Tp*2U(N-Ad) (7-Ad), are treated with isocyanates or isothiocyanates, they readily undergo [2π+2π]-cycloaddition to generate κ2-ureato and κ2-thioureato derivatives, respectively. Use of phenylisoselenocyanate with 7-Tol and 7-Ph generates a rare κ2-selenoureato complex. Treating 7-Tol and 7-OMe with benzonitrile or 4-cyanopryidine results in unusual products of multiple bond metathesis, namely κ1-amidinate U(IV) complexes.

A family of dinuclear bis(Tp*) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) uranium compounds with conjugated organic linkers was synthesized to explore possible electronic communication between uranium ions. Trivalent diuranium phenyl alkynyl compounds, Tp*2UCC(1,3-C6H4)CCUTp*2 (14-meta) or Tp*2UCC(1,4-C6H4)CCUTp*2 (14-para), and tetravalent diuranium phenylimido compounds, Tp*2U(N-1,3-C6H4-N)UTp*2 (15-meta) and Tp*2U(N-1,4-C6H4-N)UTp*2 (15-para), were generated from trivalent Tp*2UCH2Ph. All compounds were fully characterized both spectroscopically and structurally. The electronic structures of all derivatives were interrogated using magnetic measurements, electrochemistry, and were the subject of computational analyses. All of this data combined established that little electronic communication exists between the uranium centers in these trivalent and tetravalent diuranium molecules.

Uranium mono(imido) species have been prepared via oxidation of Cp*U(MesPDIMe)(THF) (16-Cp*) and [CpPU(MesPDIMe)]2 (16-CpP) (Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienide; CpP = 1-(7,7-dimethylbenzyl)cyclopentadienide; MesPDIMe = 2,6-((Mes)N=CMe)2C5H3N, Mes = 2,4,6-trimethylphenyl) with organoazides. Treating either with N3DIPP formed uranium(IV) mono(imido) complexes, CpPU(NDIPP)(MesPDIMe) (17-CpP) and Cp*U(NDIPP)(MesPDIMe) (17-Cp*), featuring reduced [MesPDIMe]1-. Addition of electron-donating 1-azidoadamantane (N3Ad) to 16-Cp* generated a dimeric product, [Cp*U(NAd)(MesHPDIMe)]2 (18), from radical coupling at the para-pyridine position of the pyridine(diimine) ligand and H-atom abstraction, formed through a monomeric intermediate that was observed in solution but could not be isolated. To support this, Cp*U(tBu-MesPDIMe)(THF) (16-tBu), which has a tert-butyl group protecting the para-position, was also treated with N3Ad, and the monomeric product, Cp*U(NAd)(tBu-MesPDIMe) (17-tBu), was isolated. All isolated complexes were analyzed spectroscopically and structurally, and dynamic solution behavior was examined using electronic absorption spectroscopy.