Calcium, Strontium, Barium, and Ytterbium Complexes with Cyclooctatetraenyl or Cyclononatetraenyl Ligands¹

Abstract

Neutral triple-decker complexes of the heavy alkaline earth metals and ytterbium with tetraisopropylcyclopentadienide anions as terminal ligands and a cyclooctatetraene dianion as a middle deck have been synthesized from tetraisopropylcyclopentadienyl metal halide precursors and disodium cyclooctatetraenide. The pentaisopropylcyclopentadienyl analogue [{(C5iPr5)Yb}2(C8H8)] was prepared from ytterbium metal, cyclooctatetraene, and the free pentaisopropylcyclopentadienyl radical. X-ray crystal structure determinations for the barium and the calcium derivative show an almost linear arrangement of ring centers and metal atoms in both cases with metal-ring center distances of 2.33 A (Ca-Cp), 1.99/1.98 A (Ca-COT) and 2.71 A (Ba-Cp), 2.40 A (Ba-COT). The geometrical features of these molecules could be modeled quite accurately with density functional calculations. With potassium cyclononatetraenide, sparingly soluble bis(cyclononatetraenyl)barium could be prepared and characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. Cyclononatetraenyl(tetraisopropylcyclopentadienyl)barium was obtained from [(C5HiPr4)BaI(THF)2]2 and KC9H9 as a 1:1 mixture with octaisopropylbarocene. Density functional calculations predict sandwich structures with parallel rings and a 2.37 A Ba-ring distance for [Ba(C9H9)2] and a 174 bending with metal-ring distances of 2.72 A (Ba-Cp) and 2.35 A (Ba-CNT) for [(C5HiPr4)Ba(C9H9)]. All alkaline earth sandwich and triple-decker complexes mentioned above have been heated to 250 degrees C without decomposition and have been sublimed in oil pump vacuum.

Cleavage of the SnSn Multiple Bond in a Distannyne by Cyclooctatetraene: Formation of the π‐Bound Inverse Sandwich Complex [(Ar′Sn)2(μ2‐η2:η3‐cot)]

“…The 13 C NMR chemical shift of δ =97.1 ppm is shifted upfield with respect to uncoordinated cot ( δ =132.7 ppm), approaching the shift found in K 2 ‐cot ( δ =89.9 ppm),22 consistent with the increased ring current of the planar aromatic form. The corresponding cot coupling constant 1 J C‐H of 157 Hz is similar to those observed for [(Cp i Pr4 M) 2 (μ 2 ‐cot)] (M=Ca, Sr, Ba: 157, 157, and 158 Hz, respectively)19 and uncoordinated cot (155 Hz), but larger than for K 2 ‐cot (143 Hz) 22. A single resonance is found in the 119 Sn{ 1 H} NMR spectrum at δ =−85 ppm, well upfield of the range generally observed for two‐coordinate stannylenes (e.g.…”

“…We present here studies of the reaction of 1 with cyclic polyolefinic molecules, in particular cyclooctatetraene (cot), 1,5‐cyclooctadiene (cod), 1,4‐cyclohexadiene and cyclohexene, and show that the reaction with cot results in the complete cleavage of the SnSn bond to give a new inverse‐sandwich structure composed of mono aryl–stannyl units and a planar C 8 H 8 ring. These types of cot‐bridged molecules are known for f‐block14–17 and s‐block elements,18, 19 whereas transition metal analogues isolated to date display olefinic or “semi‐aromatic” cot character 20–23. The inverse sandwich coordination archetype is unknown for cot derivatives of the p‐block elements and no π‐coordinated cot 2− complex of a p‐block element has been characterized.…”

Research progress in cation-π interactions

“…1,3,5,7-Cyclooctantetraene (COT) has 8 π-electrons, but does not possess any aromaticity. Recent experimental studies revealed that COT 2− could form stable complicated complex with Ba 2+ and cyclopentadiene anion derivatives, and their structure has been resolved by X-ray crystallography [47] . By theoretical calculations, Gong et al [44] demonstrated that COT could accept two electrons from alkaline-earth metal during the complexation reaction, resulting in a planar 10-electron planar structure of COT 2− (Figure 4).…”

Diradical Character and Second Hyperpolarizability of Alkaline Earth Metal Inverse Sandwich Complexes Involving Cyclopentadienyl and Cyclooctatetraene Ligands

“…Actually, there are known to be several complexes with similar structures. Walter et al synthesized a complex with cyclooctatetraene and Ca, which has the same partial structure with 8‐Ca except for the existence of Cp ligands . Note here that the Cp ligands lead to the oxidation of Ca atoms, resulting in Ca 2+ without radical electron.…”