Tris(pyrazolyl)borate Complexes of the Alkaline-Earth Metals: Alkylaluminate Precursors and Schlenk-Type Rearrangements

A series of 3,5-substituted tris­(pyrazolyl)­borate (Tp^R,Me; R = Me, Ph, <i>t</i>Bu) complexes of the alkaline-earth metals (Mg, Ca, Ba) was synthesized by salt metathesis reactions. The influence of different organometallic precursors on Schlenk-type rearrangement reactions was studied, putting emphasis on the metal size and the steric encumbrance of the Tp ligands. Magnesium alkyls MgR₂ (R = AlMe₄, CH₃) react with KTp^R,Me to form the heteroleptic complexes (Tp^Me,Me)­Mg­(CH₃), (Tp^{<i>t</i>Bu,Me})­Mg­(CH₃), and (Tp^Me,Me)­Mg­(AlMe₄). The latter tetramethylaluminate complex can also be obtained by treatment of Tp^Me,MeMg­(CH₃) with an excess of trimethylaluminum. The formally six-coordinate cyclopentadienyl derivative (C₅Me₅)­Mg­(Me)­(thf)₂ is synthesized from MeMgBr and 1 equiv of K­(C₅Me₅). Equimolar reactions of the tetraethylaluminates [M­(AlEt₄)₂]_<i>n</i> of the heavier alkaline-earth metals calcium and barium with KTp^R,Me give the homoleptic complexes of Ca­(Tp^R,Ph)₂ and Ba­(Tp^R,Me)₂. Heterotrimetallic [BaK­(AlEt₄)₃]_<i>n</i> is identified as a ligand rearrangement product and can be independently obtained by adding [K­(AlEt₄)]_<i>n</i> to [Ba­(AlEt₄)₂]_<i>n</i>. Treatment of Ba­[N­(SiMe₃)₂]₂(thf)₂ with KTp^Me,Me generates the heteroleptic complex (Tp^Me,Me)­Ba­[N­(SiMe₃)₂]­(thf)₂. All complexes are fully characterized including X-ray structure analyses

Tris(pyrazolyl)borate Complexes of the Alkaline-Earth Metals: Alkylaluminate Precursors and Schlenk-Type Rearrangements

A series of 3,5-substituted tris­(pyrazolyl)­borate (Tp^R,Me; R = Me, Ph, <i>t</i>Bu) complexes of the alkaline-earth metals (Mg, Ca, Ba) was synthesized by salt metathesis reactions. The influence of different organometallic precursors on Schlenk-type rearrangement reactions was studied, putting emphasis on the metal size and the steric encumbrance of the Tp ligands. Magnesium alkyls MgR₂ (R = AlMe₄, CH₃) react with KTp^R,Me to form the heteroleptic complexes (Tp^Me,Me)­Mg­(CH₃), (Tp^{<i>t</i>Bu,Me})­Mg­(CH₃), and (Tp^Me,Me)­Mg­(AlMe₄). The latter tetramethylaluminate complex can also be obtained by treatment of Tp^Me,MeMg­(CH₃) with an excess of trimethylaluminum. The formally six-coordinate cyclopentadienyl derivative (C₅Me₅)­Mg­(Me)­(thf)₂ is synthesized from MeMgBr and 1 equiv of K­(C₅Me₅). Equimolar reactions of the tetraethylaluminates [M­(AlEt₄)₂]_<i>n</i> of the heavier alkaline-earth metals calcium and barium with KTp^R,Me give the homoleptic complexes of Ca­(Tp^R,Ph)₂ and Ba­(Tp^R,Me)₂. Heterotrimetallic [BaK­(AlEt₄)₃]_<i>n</i> is identified as a ligand rearrangement product and can be independently obtained by adding [K­(AlEt₄)]_<i>n</i> to [Ba­(AlEt₄)₂]_<i>n</i>. Treatment of Ba­[N­(SiMe₃)₂]₂(thf)₂ with KTp^Me,Me generates the heteroleptic complex (Tp^Me,Me)­Ba­[N­(SiMe₃)₂]­(thf)₂. All complexes are fully characterized including X-ray structure analyses

Synthesis of Rare-Earth-Metal Iminopyrrolyl Complexes from Alkyl Precursors: Ln→Al N‑Ancillary Ligand Transfer

Protonolysis of [YMe₃]_<i>n</i> with 2-{(N-2,6-dialkylphenyl)­iminomethyl)}­pyrroles (alkyl = <i>i</i>Pr (L¹), Me (L²)) gave homoleptic iminopyrrolyl complexes YL¹₃ and YL²₃ as well as the complex [L²YL^2,Me]₂ containing a dianionic pyrrolaldiminato ligand, formed via methylation of the imino backbone. Treatment of the half-sandwich complex [(C₅Me₅)­YMe₂]₃ and yttrocene (C₅Me₅)₂YMe­(THF) with either 2 or 1 equiv of HL afforded the monomeric complexes (C₅Me₅)­YL₂ and (C₅Me₅)₂YL,<b> </b>respectively. The complex (C₅Me₅)­YL²₂ readily underwent Ln→Al iminopyrrolyl ligand transfer in the presence of trimethylaluminum, producing the known (C₅Me₅)­Y­(AlMe₄)₂. Salt metatheses of homoleptic Ln­(AlMe₄)₃ (Ln = Y, La) with KL gave complicated reaction mixtures from which the η⁵/η¹:κ¹ pyrrolaldiminato-bridged complex [L^1,MeLa­(AlMe₄)]₂ and bis­(tetramethylaluminate) complex L²Y­(AlMe₄)₂ could be isolated and crystallographically characterized. Moreover, the solid-state structures of YL²₃, [L²YL^2,Me]₂, (C₅Me₅)­YL¹₂, (C₅Me₅)₂YL¹, and L²AlMe₂ are presented

C–H Bond Activation and Isoprene Polymerization by Rare-Earth-Metal Tetramethylaluminate Complexes Bearing Formamidinato N‑Ancillary Ligands

The bimetallic formamidinate complexes Ln­(Form)­(AlMe₄)₂ (Ln = Y, Form (ArNCHNAr) = EtForm (Ar = 2,6-Et₂C₆H₃), MesForm (Ar = 2,4,6-Me₃C₆H₂), DippForm (Ar = 2,6-<i>i</i>Pr₂C₆H₃), <i>t</i>BuForm (Ar = 2-<i>t</i>BuC₆H₄); Ln = La, Form = DippForm, <i>t</i>BuForm) were obtained in high yield by protonolysis reactions between formamidines (FormH) and homoleptic rare-earth-metal tetramethylaluminates Ln­(AlMe₄)₃. Y­(Form)­(AlMe₄)₂ (Form = EtForm, DippForm) were also prepared by treatment of Y­(Form)­[N­(SiHMe₂)₂]₂(thf) with trimethylaluminum after the former were prepared by the protonolysis of Y­[N­(SiHMe₂)₂]₃(thf)₂ complexes with EtFormH or DippFormH. The monomeric six-coordinate complexes Ln­(Form)­(AlMe₄)₂ (Ln = Y, Form = EtForm, MesForm, DippForm, <i>t</i>BuForm; Ln = La, Form = DippForm, <i>t</i>BuForm) show similar molecular structures with distorted-octahedral geometry and bidentate (N,N′) Form and AlMe₄ ligands. The complex [La­(EtFormAlMe₃)­(AlMe₄)₂]­(C₇H₈)_1.5 from a protonolysis reaction between La­(AlMe₄)₃ and EtFormH has the EtForm ligand adopting a configuration in which one nitrogen and one aryl substituent are coordinated to the eight-coordinate lanthanum center in an η¹(N):η⁶(arene) manner. From the reaction of La­(AlMe₄)₃ with MesFormH, C–H bond activation of an <i>o</i>-methyl group of the mesityl moiety occurred, yielding [La­{η¹(N):η⁶(Ar)-Me₂CH₂FormAlMe₃}­(AlMe₃)­(AlMe₄)]­[La­(Me₂CH₂FormAlMe₃)­(AlMe₃)­(AlMe₄)]­(C₆H₁₄)_1.5 (Me₂CH₂Form = MesForm-H­(<i>o</i>-Me)), in which two linkage isomers of Me₂CH₂Form were observed. Investigations were carried out on the compounds [Ln­(Form)­(AlMe₄)₂] (Ln = Y, La; Form = EtForm, DippForm) as precatalysts activated by [Ph₃C]­[B­(C₆F₅)₄] or [PhNMe₂H]­[B­(C₆F₅)₄] in isoprene polymerization. While the lanthanum complexes showed narrower molecular weight distributions (PDI < 1.2), a stereodirecting role was evidenced for the cocatalysts (trityl borate, maximum 87% trans-1,4-selectivity; anilinium borate, maximum 82% cis-1,4-selectivity)