Complexation and Versatile Reactivity of a Highly Lewis Acidic Cationic Mg Complex with Alkynes and Phosphines

Abstract

[(BDI)Mg+][B(C6F5)4−] (1; BDI=CH[C(CH3)NDipp]2; Dipp=2,6‐diisopropylphenyl) was prepared by reaction of (BDI)MgnPr with [Ph3C+][B(C6F5)4−]. Addition of 3‐hexyne gave [(BDI)Mg+⋅(EtC≡CEt)][B(C6F5)4−]. Single‐crystal X‐ray analysis, NMR investigations, Raman spectra, and DFT calculations indicate a significant Mg‐alkyne interaction. Addition of the terminal alkynes PhC≡CH or Me3SiC≡CH led to alkyne deprotonation by the BDI ligand to give [(BDI‐H)Mg+(C≡CPh)]2⋅2 [B(C6F5)4−] (2, 70 %) and [(BDI‐H)Mg+(C≡CSiMe3)]2⋅2 [B(C6F5)4−] (3, 63 %). Addition of internal alkynes PhC≡CPh or PhC≡CMe led to [4+2] cycloadditions with the BDI ligand to give {Mg+C(Ph)=C(Ph)C[C(Me)=NDipp]2}2⋅ 2 [B(C6F5)4−] (4, 53 %) and {Mg+C(Ph)=C(Me)C[C(Me)=NDipp]2}2⋅2 [B(C6F5)4−] (5, 73 %), in which the Mg center is N,N,C‐chelated. The (BDI)Mg+ cation can be viewed as an intramolecular frustrated Lewis pair (FLP) with a Lewis acidic site (Mg) and a Lewis (or Brønsted) basic site (BDI). Reaction of [(BDI)Mg+][B(C6F5)4−] (1) with a range of phosphines varying in bulk and donor strength generated [(BDI)Mg+⋅PPh3][B(C6F5)4−] (6), [(BDI)Mg+⋅PCy3][B(C6F5)4−] (7), and [(BDI)Mg+⋅ PtBu3][B(C6F5)4−] (8). The bulkier phosphine PMes3 (Mes=mesityl) did not show any interaction. Combinations of [(BDI)Mg+][B(C6F5)4−] and phosphines did not result in addition to the triple bond in 3‐hexyne, but during the screening process it was discovered that the cationic magnesium complex catalyzes the hydrophosphination of PhC≡CH with HPPh2, for which an FLP‐type mechanism is tentatively proposed.

Phosphinoborane interception at magnesium by borane-assisted phosphine-borane dehydrogenation

“…25 Although examples of similar BDI γ-C-B bond formation are limited to a recent report by Jones and co-workers, 39 the formation of 5 is clearly also reminiscent of the well documented reactivity of [(BDI)Ae] + and [(BDI)AlMe] + cations towards other unsaturated small molecules such as alkynes or CO 2 . 29,40,41 Consistent with the formulation of the tripodal anion as a bis-imine unit, the N1-C2 (1.2950(19) Å) and N2-C4 (1.293(2) Å) bond lengths of compound 5 are significantly shorter than those observed in unperturbed BDI anions. The geometry about C3 is also effectively tetrahedral, with X-C3-X bond angles in the range of 110.5-119.4°.…”

Backbone Reactivity of Lithium β‐Diketiminate (NacNac) Complexes with CO2, tBuNCO and iPrNCO

“…The presence of the central oxygen anions could be through adventitious hydrolysis,t hought he reactioni sr eproducible with yields of around6 8%,o rb yf ixation of molecular oxygen, as has been observed in many other complexes. [14,35] Subsequent experimentsh ave shown that reaction of compound 1 with molecular oxygen via ad rying tube did not produce 3,s uggesting that adventitious moisture is the source of the oxygen atom. The definite point is that such systemsa re able to undergo double small molecule fixation within the same system.…”

“…Two distinct types of C−O bonds reside within the structure; namely those of the secondary amide functionality (C 30 −O 1 length of 1.226(2) Å and C 64 −O 2 length of 1.228(2) Å) and those at the centre of the (LiO) 2 core (C 49 −O 4 and C 15 −O 3 bond lengths of 1.357(2) and 1.358(2) Å, respectively). The presence of the central oxygen anions could be through adventitious hydrolysis, though the reaction is reproducible with yields of around 68 %, or by fixation of molecular oxygen, as has been observed in many other complexes . Subsequent experiments have shown that reaction of compound 1 with molecular oxygen via a drying tube did not produce 3 , suggesting that adventitious moisture is the source of the oxygen atom.…”

“…The magnesium complexes exhibit remarkably diverse reactivity, spanning cycloaddition reactions, deprotonation reactions and addition reactions of alkynes to the backbone NacNac γ‐carbon atom. They can also display interesting coordination modes, for example, η 3 coordination to toluene and benzene . Aluminium NacNac species have also been shown to engage in the fixation of small molecules such as white phosphorus and sulfur, and can react with elemental oxygen to form alkyl(alkoxy)aluminium complexes, or aluminium oxide species…”

“…They can also display interesting coordination modes, for example, h 3 coordination to toluene and benzene. [13,14] Aluminium NacNacs pecies have also been shown to engagei nt he fixation of smallm olecules such as white phosphorusa nd sulfur, [10] and can react with elementalo xygen to form alkyl(alkoxy)aluminium complexes, or aluminium oxide species. [15] In many of these examples, the NacNac ligand is utilised as ap rotective scaffold upon which the reactive centre of the complex rests.…”