Density Functional Theory and X-ray Analysis of the Structural Variability in η⁵,η⁵,η¹-Tris(ring) Rare Earth/Actinide Tetramethylpyrrolyl Complexes, (C₅Me₅)₂M(NC₄Me₄)

Abstract

(C5Me5)2M(NC4Me4) complexes (M = Y, Sm, Ce, U) were synthesized to act as structural models for the (η(5)-C5Me5)2M(η(1)-C5Me5) intermediate postulated to give pseudoalkyl reactivity to sterically crowded (C5Me5)3M complexes. This synthesis was accomplished through reaction of the tetraphenylborate complexes, [(C5Me5)2M][(μ-Ph)2BPh2], with potassium tetramethylpyrrolyl, KNC4Me4. X-ray crystallographic studies on the resulting (C5Me5)2M(NC4Me4) complexes showed that, although the two (C5Me5)(-) rings bind to the metal with η(5) coordination and tetramethylpyrrolyl has a primary η(1) coordination, the complexes are not symmetrical in the solid state, and disparate M-N-C(ring) angles within a complex orient a (NC4Me4)(-) ring carbon and methyl carbon near the metal in a pseudo-η(3) binding mode. Moreover, these (C5Me5)2M(NC4Me4) complexes display unexpectedly large structural variations not only between metals but also between crystals grown from the same mother liquor. Large variations are observed in the M-N-C(ring) angles that lead to close metal ring carbon distances [105.6(1)-115.7(2)°] as well as in the M-N-(NC4Me4 ring centroid) angles (152.2-167.3°). The synthesis and structure of 4d, 4f, and 5f metal examples are described, and the results are compared to predictions from the density functional theory. The reasons for the variable structures displayed by the (C5Me5)2M(NC4Me4) complexes are discussed.

Pentamethylcyclopentadienyl‐Supported Cerocene(III) Complexes

“…Whereas in 2a the C–H resonance is observed at +21.32 ppm, meaning at lower field compared to the methyl groups, the Ce ··· SiH β‐agostic interactions in 2b cause a strong shift to higher field of the 1 H NMR signal ( δ = –28.63 ppm), much like in previously reported [Ce(N i Pr 2 ) 4 Li(thf)] ( δ = –27.27 ppm)[24e] and [Ce{N(SiHMe 2 ) 2 } 4 Li(thf)] ( δ = –17.70 ppm). [24c] These observations coincide with those made by Evans for the tetramethylpyrrolyl complex [Cp* 2 Ce(NC 4 Me 4 )], for which in the solid state the protons of one β‐positioned pyrrolyl methyl group approach the cerium center. The 1 H NMR spectrum of complex [Cp* 2 Ce(NC 4 Me 4 )] showed a strong high‐field shift to –31.03 ppm of the β‐positioned methyl groups, whereas the more distant γ‐positioned methyl groups resonate at δ = 0.57 ppm.…”

“…Notably, complexes Cp* 2 Ln[E(SiMe 3 ) 2 ] (Ln = La, Nd, Sm, Lu; E = N, CH) proved to be excellent precatalysts for ethylene polymerization and hydroamination/cyclization reactions . The first structurally characterized amide derivative [Cp* 2 CeR] (R = tertramethylpyrrolyl) was reported by Evans et al only in 2013 …”

Lanthanides and actinides: Annual survey of their organometallic chemistry covering the year 2013

“…This synthesis was accomplished through reaction of the tetraphenylborate complexes, [Cp * 2 Ln][(-Ph) 2 BPh 2 ], with tetramethylpyrrolylpotassium, KNC 4 Me 4 , according to Scheme 90 [65].…”

“…The synthesis and structure of 4d, 4f, and 5f metal examples were also described, and the results were compared to predictions from the density functional theory. In addition, the reasons for the variable structures displayed by the Cp * 2 Ln(NC 4 Me 4 ) complexes were discussed [65]. The influence of the torsion angle in 3, 3 - The temperature-independent value of n f of 0.17 showed that the valence of ytterbium in the neutral adduct is multiconfigurational, in reasonable agreement with a CASSCF calculation that yielded a n f value of 0.27; that is, the two configurations in the wave function are f 13 (* 1 ) 1 and f 14 (* 1 ) 0 in a ratio of 0.27:0.73, respectively, and the open-shell singlet lies 0.28 eV below the triplet state (n f accounts for f-hole occupancy; that is, n f = 1 when the configuration is f 13 and n f = 0 when the configuration is f 14 ).…”

“…The uranium(III) tetramethylpyrrolyl complex Cp * 2 U(NC 4 Me 4 ) has been synthesized according to Scheme 212 (green crystals, 82% yield) and structurally characterized by X-ray diffraction [65].…”