Formation and Reactivity of an Aluminabenzene Ligand at Pentadienyl‐Supported Rare‐Earth Metals

Abstract

The half-open rare-earth-metal aluminabenzene complexes[(1-Me-3,5-tBu 2 -C 5 H 3 Al)(m-Me)Ln(2,4-dtbp)] (Ln = Y, Lu) are accessible via as alt metathesis reaction employing Ln(AlMe 4 ) 3 and K(2,4-dtbp). Treatment of the yttrium complex with B(C 6 F 5 ) 3 and tBuCCH gives access to the pentafluorophenylalane complex [{1-(C 6 F 5 )-3,5-tBu 2 -C 5 H 3 Al}{m-C 6 F 5 }Y{2,4-dtbp}] and the mixed vinyl acetylide complex [(2,4-dtbp)Y(m-h 1 :h 3 -2,4-tBu 2 -C 5 H 4 )(m-CCtBu)-AlMe 2 ], respectively.Since the 1980s it has been known that pentadienyl ligands impart as tabilizing environment for the organometallic chemistry of alkali, alkaline-earth, transition, and rare-earth metals. [1] In comparison to the ubiquitous cyclopentadienyl congeners these "open variants" actively engage in distinct reaction paths,t he most prominent being the formation of metallabenzenes.T he feasibility of such metallacycles was initially shown for osmium by Elliot and co-workers in 1982 and later extended to the transition metals molybdenum, iridium, ruthenium, and nickel by the research groups led by Roper, Ernst, and Salzer. [2] In contrast to these aromatic transition-metal derivatives and the well-established borabenzene chemistry, [3] such ac ompound was only recently described for the Group 13 metal aluminum. In fact, the first isolation and subsequent characterization of the anionic aluminabenzene Li[1-Mes-2,6-C 5 H 3 Al] (Mes = 2,4,6-trimethylphenyl) was accomplished by Yamashita et al. in 2014 through as ophisticated reaction pathway employing asilyl-substituted diyne,DIBAL-H, and mesityllithium. [4] The same group also utilized the aforementioned aluminabenzene derivative in reactions with CpZrCl 3 ,C p*ZrCl 3 ,a nd ZrCl 4 , yielding distinct chlorido-bridged aluminabenzene-bearing complexes but at the expense of losing aromaticity in the aluminacycle to some degree. [5] Furthermore,n onbridged aluminabenzene complexes of the transition metals rhodium and iridium were published very recently by the Yamashita group as was az witterionic zirconium complex derived from an aluminacyclohexadiene and homoleptic benzyl ZrBn 4 . [6] These complexes also showed promising performances in C À Hb orylation and ethylene polymerization, respectively.Here we wish to report the formation of ad ianionic aluminabenzene ligand at rare-earth-metal centers according to an unprecedented reaction path, exploiting an alkylaluminate-mediated pentadienyl deprotonation and ring closure.A ccordingly,w hen we reacted the potassium salt of 2,4-di-tert-butylpentadiene K(2,4-dtbp) [1f] with rare-earthmetal tetramethylaluminates Ln(AlMe 4 ) 3 (Ln = Y, Lu) [7] at ambient temperature,w ew ere able to isolate the unusual "half-open" sandwich complexes [(1-Me-3,5-tBu 2 -C 5 H 3 Al)(m-Me)Ln(2,4-dtbp)] (1-Y,L n= Y) and 1-Lu (Ln = Lu) in high yields (Scheme 1). Complexes 1-Ln bear one 2,4-di-tertbutylpentadienyl ligand and one methyl-bridged 3,5-di-tertbutylaluminabenzene ligand, thus displaying another member of this rare family of metal-coordinated aluminabenzenes,...

Metallaaromatic Chemistry: History and Development

“…Similar to the procedures for preparing the η 6 -coordinated ruthenabenzenes exhibited in Schemes 25 and 26, this method also involves an insertion of an Al fragment into the "open" side of a pentadienyl ligand. 230 Gallatabenzenes were discovered before aluminabenzenes. Ashe reported the first gallatabenzene 188 by the abstraction of a proton from 187 by bases, such as t-BuLi, Ph 3 CLi, and LDA (Scheme 63).…”

Stable Actinide π Complexes of a Neutral 1,4‐Diborabenzene

“…Thus, π‐ligated heteroarene complexes of the d‐elements have been realized for a large number of aromatic heterocycles all across the periodic table including B‐based systems (BNC 2 , B 2 N 2 , BC 4 , BNC 3 , BC 5 , B 2 C 4 , BNC 4 , BC 6 ) and benzene analogs EC 5 (E=B‐Ga, Si‐Sn, N‐Sb), to name only a few. For the f‐elements, π complexation of anionic BNC 3 , BC 5 , AlC 5 , NC 4 , N 2 C 3 , C 2 P 2 , PC 4 , P 3 C 2 , PN 2 C 2 , P 4 /As 4 , and P 5 has been verified for selected lanthanide and actinide molecules. By contrast, f‐block complexes bearing neutral heteroarenes as sandwich‐type ligands are exceedingly rare, and limited to [( t Bu 3 ‐C 5 H 2 P) 2 Ho] ( I ) .…”

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

“…At room temperature the reaction affords the aluminabenzene complexes [RE(2,4-dtbp)(C 5 H 3 AlMe-1- t Bu 2 -3,5)] ( 280-RE ; RE = Y, Lu) ( Scheme 75 ). 703 However, when the same reaction is carried out at −40 °C, the pseudo -metallocenes [RE(2,4-dtbp) 2 (AlMe 4 )] ( 281-RE ; RE = Y, Lu) are obtained instead, which can undergo thermal decomposition to give aluminabenzene derivatives 280-RE . 703 …”

“… 703 However, when the same reaction is carried out at −40 °C, the pseudo -metallocenes [RE(2,4-dtbp) 2 (AlMe 4 )] ( 281-RE ; RE = Y, Lu) are obtained instead, which can undergo thermal decomposition to give aluminabenzene derivatives 280-RE . 703 …”