f‐Element Half‐Sandwich Complexes: A Tetrasilylcyclobutadienyl–Uranium(IV)–Tris(tetrahydroborate) Anion Pianostool Complex

Abstract

Despite there being numerous examples of f‐element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C5–8), cyclobutadienyl (C4) complexes remain exceedingly rare. Here, we report that reaction of [Li2{C4(SiMe3)4}(THF)2] (1) with [U(BH4)3(THF)2] (2) gives the pianostool complex [U{C4(SiMe3)4}(BH4)3][Li(THF)4] (3), where use of a borohydride and preformed C4‐unit circumvents difficulties in product isolation and closing a C4‐ring at uranium. Complex 3 is an unprecedented example of an f‐element half‐sandwich cyclobutadienyl complex, and it is only the second example of an actinide‐cyclobutadienyl complex, the other being an inverse‐sandwich. The U−C distances are short (av. 2.513 Å), reflecting the formal 2− charge of the C4‐unit, and the SiMe3 groups are displaced from the C4‐plane, which we propose maximises U−C4 orbital overlap. DFT calculations identify two quasi‐degenerate U−C4 π‐bonds utilising the ψ2 and ψ3 molecular orbitals of the C4‐unit, but the potential δ‐bond using the ψ4 orbital is vacant.

Heteroleptic actinocenes: a thorium(iv)–cyclobutadienyl–cyclooctatetraenyl–di-potassium-cyclooctatetraenyl complex

“…We previously reported that reaction of [Li 2 {C 4 (SiMe 3 ) 4 }(-THF) 2 ] 12 with UCl 4 led to reduction of uranium(IV) to intractable mixtures of uranium(III)-containing products. 8 However, thorium(IV) has a far greater reduction potential (E q Th(IV) / Th(III) À3.7 V) than that of uranium(IV) (E q U(IV) / U(III) À0.6 V). 13 Nevertheless, addition of a solution of [K 2 {C 4 (SiMe 3 ) 4 }] 14 to [ThCl 4 (THF) 3.5 ] 15 resulted in oxidation of dianionic {C 4 (SiMe 3 ) 4 } 2À to the neutral cyclobutadiene {C 4 (SiMe 3 ) 4 }, 16 as conrmed by multinuclear NMR spectroscopy, and deposition of an insoluble dark grey precipitate presumed to be KCl and colloidal thorium.…”

“…Indeed, although transition metal cyclobutadienyl chemistry was established in the 1960s, 5 the rst f-elementcyclobutadienyl complex, an inverted sandwich tetraphenylcyclobutadienyl diuranium(IV) species, was reported in 2013. 6,7 Very recently, the uranium(IV)-cyclobutadienyl half sandwich pianostool complex [U{C 4 (SiMe 3 ) 4 }(BH 4 ) 3 ][Li(THF) 4 ] (A) was reported, 8 and shortly aer that four uranium(IV)-cyclobutadienyl complexes [U{C 4 (SiMe 3 ) 4 }(BH 4 ) 3 ][Na(12-crown-4)], 9 [U{C 4 -(SiMe 3 ) 4 }(BH 4 ) 2 (m-BH 4 ){K(THF) 2 }] 2 , 9 [U(BH 4 ){C 4 (SiMe 3 ) 4 }{k 3 -C 4 H(SiMe 3 ) 3 -k-(CH 2 SiMe 2 )}][Na( t BuOMe) 3.6 (THF) 0.4 ], 9 and [{U(C 4 [SiMe 3 ] 4 )(m-I) 2 } 3 {m 3 -O}][Mg(THF) 6 ] were disclosed. 10 These six uranium-cyclobutadienyl complexes constitute all actinidecyclobutadienyl chemistry to date, in stark contrast to the plethora of reported f-element h n -carbocyclic C 5-9 complexes.…”

f‐Element Half‐Sandwich Complexes: A Tetrasilylcyclobutadienyl–Uranium(IV)–Tris(tetrahydroborate) Anion Pianostool Complex

“…To conclude, [27] we have prepared and characterised the first example of an f-element half-sandwich cyclobutadienyl complex, and indeed it is only the second example of an actinide-cyclobutadienyl complex. Whereas synthetic endeavours with UCl 4 have thus far been intractable,u sing the halide-free uranium(III)-tris(tetrahydroborate), itself an infrequently used class of complex in actinide chemistry, proved decisively fruitful.…”

Bergmann Medal: R. T. Raines / Bergmann Young Investigator Award: C. Schnepel / Bessel Award: S. T. Liddle

“…Liddle's research interests include f‐block–ligand multiple bonding, metal–metal bonds, small‐molecule activation and catalysis, and single‐molecule magnetism. Among his most recent contributions to Angewandte Chemie are a Guest Editorial on lanthanide and actinide chemistry and a Communication in which he introduced a tetrasilylcyclobutadienyl–uranium half‐sandwich complex …”