s-Block Metal Dibenzoazepinate Complexes: Evidence for Mg–Alkene Encapsulation

Abstract

The dibenzo[b,f ]azepinate (DBAP) complexes (DBAP)Li•(THF) 3 , (DBAP) 2 Mg•(THF) 2 , and (DBAP) 2 Ca•(THF) 3 could be isolated as highly air-sensitive compounds in yields of 93%, 72%, and 48%, respectively. Crystal structures of these THF adducts reveal monomeric complexes in which the degree of ring puckering depends on the nature of the metal. The most extreme deviation from planarity is found for the most covalent bound metal, Mg, but in all cases no interaction between the metal and the azepine CC bond is observed. The THF-free complex [(DBAP) 2 Mg] 2 , which could be obtained in 77% yield, crystallizes as an unusual dimer with three bridging and one terminal DBAP ligand. The bridging DBAP ligands are highly bent and span a cavity in which a Mg 2+ ion is bound through three alkene−Mg interactions with an average Mg•••C distance of 2.794(3) Å. Theoretical calculations support these contacts. A combination of AIM and NPA analyses shows polarization of the alkene π-electron density toward the metal (vertical polarization) but also demonstrates a polarization of electron density toward the C atom closest to Mg (horizontal polarization). Such metal−alkene interactions and implicit CC bond polarization are key features in main group metal catalyzed alkene conversions.

Facile Benzene Reduction by a Ca2+/AlI Lewis Acid/Base Combination

“…It is of interest to note that hitherto reported s-block metal···alkene interactions are asymmetric, showing different metal-C distances. [17] Thec oordination sphere of the significantly larger Ca 2+ ion is completed by aC 6 H 5 F···Ca 2+ interaction of 2.373(3) which is at the shorter end of reported (intramolecular) Ca 2+ ···F-C distances that range from 2.418-2.785 (average:2 .567 ;s ee Table S3). However,asmall disorder component in the crystal structure (see the Supporting Information) makes it difficult to discuss its relevance.…”

“…[9] Since the C-Al-C angle is also unusually small (77.2(1)8 8 vs.1 17.4(1)8 8 in (BDI)AlMe 2 ), [13] the long AlÀCb onds are likely caused by ring strain and/or steric repulsion with the bulky BDI ligand. [17] Thec oordination sphere of the significantly larger Ca 2+ ion is completed by aC 6 19 FNMR spectra are as expected, an 27 Al NMR signal could not be detected. It is clear from the CÀ Cbond distances in the C 6 H 6 2À ring that both negative charges are located at the opposing flagpole carbon centers C1 and C4.…”

Low Valent Magnesium Chemistry with a Super Bulky β‐Diketiminate Ligand

“…[20] Similar to the latter complex it can be described as a( DIPeP BDI)Mg-norbornadiene anion interacting with a( DIPeP BDI)Mg + cation by Mg···alkene interactions.T he latter Mg···alkene distances (Figure 1c)a re much longer than the MgÀCd istances to the boatsf lagpole positions but extremely short compared to those in other complexes. [21] Whereas the 1 HNMR spectrum of ( DIPP BDI)Ca(C 6 H 6 )Al( DIPP BDI) + gave for C 6 H 6 2À an olefinic resonance (4.64 ppm, 4H)a nd an aliphatic resonance (2.93 ppm, 2H), the current Mg complex displays one singlet in an intermediate region (3.76 ppm, 6H), indicating rapid isomerization of the Mg-norbornadieneu nit which even at À80 8 8Ci st oo fast for decoalescence of aliphatic and olefinic signals.T he isomerization process may take place through aflat C 6 H 6 2À anion as observed in the centrosymmetric THF adduct ( DIPeP BDI) 2 Mg 2 (C 6 H 6 )·(THF) 2 which was simply obtained by addition of THF (Figure 1d). Although the C 6 H 6 ring is perfectly flat (maximum deviation from the leastsquare plane:0.018(2) ), the CÀCbond distances are typical for C 6 H 6 2À with localized charges at the para-positions.T he latter THF complex easily loses its THF ligands upon storage at room temperature and also in solution the THF ligands are not bound to Mg:i ts 1 HNMR spectrum in C 6 D 6 shows chemical shifts for free THF.I tcould be obtained in af air crystalline yield of 69 %a nd as the THF is simply removed under vacuum, this is the preferred synthetic route to ( DIPeP BDI) 2 Mg 2 (C 6 H 6 ).…”

Coordination of arenes and phosphines by charge separated alkaline earth cations

“…More generally, Ae-π interactions have long been recognised to wield a significant influence over the structures and aggregation states adopted by a wide variety of organometallic, amido or phenoxy group 2 derivatives. [13][14][15][16][17][18][19][20][21] In all such instances, however, the arene or π-substituent is a component of a more complex anion and benefits from the entropic advantage afforded by its intramolecular disposition. Despite their widespread biological significance, however, definitive structural characterisation of intermolecular interactions between Mg or Ca and neutral arenes has proven elusive, such that until very recently (vide infra) the only structurally authenticated examples of molecular species to feature intermolecular Ae-π interactions in the solid state were provided by Hanusa's bis(trimethylsilyl)butadiyne adduct of decamethylcalcocene (III) 22 and various serendipitously crystallised benzene and toluene solvates of barium derivatives.…”