Archive | 2019
Expansion of Low- and Mid-Valent Organometallic Uranium Chemistry
Abstract
A series of uranium benzyl compounds supported by two\nhydrotris(3,5-dimethylpyrazolyl) borate (Tp*) ligands has been synthesized and\ncharacterized. In addition to the\npreviously reported Tp*2U(CH2Ph) (2-Bn), examinations of both steric (tert-butyl, iso-propyl)\nand electronic (methoxy, picolyl) changes on the aromatic ring led to the\nformula 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\nof benzyl compounds with azidotrimethylsilane results in the formation of a\nneutral, monomeric U(III) compound, Tp*2U(N3) (3-N3), and substituted benzyltrimethylsilane.\nWhile there was no observed change in reactivity among the benzyl compounds and\nMe3SiN3, treatment of these compounds with\ntriphenylphosphine oxide saw unique carbon-carbon coupling occur for three of\nthe substituted benzyl compounds. With a single equivalent of OPPh3,\nthe following products were isolated: Tp*2U[OP(C6H5)(C6H5CH2C6H5)]\n(4-Ph), Tp*2U[OP(C6H5)(C6H5-p-iPrC6H4)]\n(4-iPr), Tp*2U[OP(C6H5)(C6H5-p-tBuC6H4)]\n(4-tBu),\nTp*2U[OP(C6H5)(C6H5-m-OCH3C6H4)]\n(4-OMe). A family of uranium(IV) imido\ncomplexes of the form Tp*2U(NR) (R = benzyl (7-Bn), para-tolyl (7-Tol), para-methoxyphenyl (7-OMe),\n2,6-diethylphenyl (7-detp),\n2,6-diisopropylphenyl (7-dipp)) have\nbeen generated by bibenzyl extrusion from 2-Bn.\nWhen 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\nisothiocyanates, they readily undergo [2π+2π]-cycloaddition to generate\nκ2-ureato and κ2-thioureato derivatives, respectively.\nUse of phenylisoselenocyanate with 7-Tol\nand 7-Ph generates a rare κ2-selenoureato\ncomplex. Treating 7-Tol and 7-OMe with benzonitrile or\n4-cyanopryidine results in unusual products of multiple bond metathesis, namely\nκ1-amidinate U(IV) complexes. \n\nA family of\ndinuclear bis(Tp*) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) uranium\ncompounds with conjugated organic linkers was synthesized to explore possible\nelectronic communication between uranium ions. Trivalent diuranium phenyl alkynyl compounds, Tp*2UCC(1,3-C6H4)CCUTp*2\n(14-meta)\nor Tp*2UCC(1,4-C6H4)CCUTp*2 (14-para),\nand tetravalent diuranium phenylimido compounds, Tp*2U(N-1,3-C6H4-N)UTp*2\n(15-meta)\nand Tp*2U(N-1,4-C6H4-N)UTp*2 (15-para),\nwere generated from trivalent Tp*2UCH2Ph. All compounds\nwere fully characterized both spectroscopically and structurally. The\nelectronic structures of all derivatives were interrogated using magnetic\nmeasurements, electrochemistry, and were the subject of computational analyses.\nAll of this data combined established that little electronic communication\nexists between the uranium centers in these trivalent and tetravalent diuranium\nmolecules.\n\nUranium mono(imido) species have been prepared via oxidation of Cp*U(MesPDIMe)(THF)\n(16-Cp*) and [CpPU(MesPDIMe)]2\n(16-CpP) (Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienide;\nCpP = 1-(7,7-dimethylbenzyl)cyclopentadienide;\nMesPDIMe = 2,6-((Mes)N=CMe)2C5H3N,\nMes = 2,4,6-trimethylphenyl) with organoazides. \nTreating either with N3DIPP formed uranium(IV) mono(imido)\ncomplexes, CpPU(NDIPP)(MesPDIMe) (17-CpP) and Cp*U(NDIPP)(MesPDIMe)\n(17-Cp*), featuring reduced [MesPDIMe]1-.\nAddition of electron-donating 1-azidoadamantane (N3Ad) to 16-Cp* generated a dimeric product, [Cp*U(NAd)(MesHPDIMe)]2\n(18), from radical coupling at the para-pyridine position of the\npyridine(diimine) ligand and H-atom abstraction, formed through a monomeric\nintermediate that was observed in solution but could not be isolated. To\nsupport this, Cp*U(tBu-MesPDIMe)(THF)\n(16-tBu),\nwhich has a tert-butyl group\nprotecting the para-position, was\nalso treated with N3Ad,\nand the monomeric product, Cp*U(NAd)(tBu-MesPDIMe)\n(17-tBu),\nwas isolated. All isolated complexes were analyzed spectroscopically and\nstructurally, and dynamic solution behavior was examined using electronic\nabsorption spectroscopy.