A Soft Grip: Magnesium Complexes with a Phosphine‐Modified Phosphonium Diylidic Lewis Base

Abstract

Simple strategies to obtain magnesium complexes with the soft chelating diylidic ligand [Ph PCHPPh (fluorenylidene)] (dppmflu ) were developed to evaluate the influence of the hard acid (cation) and soft base (anion) mismatch on the stability and reactivity of the formed derivatives. Deprotonation of the precursor Ph PCH PPh (flu) (dppmfluH) by an alkylmagnesium derivative or magnesium amide provided access to [{Mg(dppmflu)(μ-nBu)} ], [Mg(dppmflu){N(SiMe ) }], and [{Mg(dppmflu)(μ-Me)} ], which were used as starting materials for further investigations. The reaction of [{Mg(dppmflu)(μ-nBu)} ] with PhSiH in the presence of THF allowed isolation of the magnesium hydride complex [{Mg(dppmflu)(μ-H)(thf)} ] without a stabilizing nitrogen donor ligand. Prolonged heating enforced ligand redistribution and [{Mg(dppmflu)(μ-H)(thf)} ] was converted to [Mg(dppmflu) ] and MgH . The homoleptic derivative [Mg(dppmflu) ], in which the magnesium center is in a very soft ligand environment, can open a THF molecule by frustrated Lewis pair reactivity to give [{Mg(dppmflu)(μ-OC H dppmflu)} ].

Frustrated Lewis Pair Chemistry with Magnesium Lewis Acids

“…Langer et al reported very recently an Mg complex with an intramolecular coordinating phosphine group that acts as a masked FLP (Scheme ) . Due to a hard/soft mismatch, the P–Mg bond is weak and can be easily cleaved by THF coordination.…”

Self‐Assembly of Magnesium Hydride Clusters Driven by Chameleon‐Type Ligands

“…Multinuclear magnesium hydride complexes [1][2][3][4][5][6][7][8][9][10] are being increasingly studied as molecular model systems for the hydrogen storage material [MgH 2 ] 1 .O ther interest in such complexes stems from their use as potent hydride sources in synthesis and their recognition as key intermediates in early main-group metal catalysis. [11] Starting from am agnesium alkyl or amide complex, typical synthetic routes involve metathesis with silane hydride reagents.Other routes include cyclohexadiene [12] or alane [13] induced hydrogenation of an Mg I complex or b-hydride elimination of am agnesium amidoborane species.…”

“…[9] While hard chelating Lewis bases are obviously preferred for the stabilization of such oligonuclear entities,s ome magnesium hydride clusters feature unusual soft-hard ligand-Mg contacts,s uch as IPr···Mg or P···Mg contacts in Mg 4 H 6 (IPr) 2 [N(SiMe 3 ) 2 ] 2 and Mg 4 H 4 [N(DIPP)PPh 2 ], respectively.W erecently reported on the isolation and structure of [MgH(dppmflu)(THF)] 2 using al igand exclusively based on soft donors. [10] Inspired by the efficient bonding of soft ligands to hard Mg 2+ centers,w ei ntroduce an alternative concept for the kinetic stabilization of magnesium hydride clusters.R ather than using as terically encumbered, rigid ligand with ap redetermined, strongly binding nitrogen donor set, we reasoned that the use of flexible donors,s uch as hybrid ligands containing ac ombination of hard and soft donor atoms in close proximity,s hould be able to stabilize ag rowing magnesium hydride fragment by adapting its coordination mode.W hereas rigid and bulky DIPPnacnac ligands may stabilize magnesium hydride clusters through prevention of ligand exchange by steric shielding,use of asofter chameleontype ligand system with abroad variety of coordination modes guarantees efficient bonding to the large diversity of Mg 2+ centers at any stage of ag rowing magnesium hydride aggregate,t hus preventing precipitation of [MgH 2 ] 1 from the highly dynamic reaction mixture.…”

“…Thes ignals are broadened and no interpretable HÀHc oupling pattern can be recognized. Thec rystal structure of complex 5 bears little resemblance to the brucite-type clusters 3 and 4.S imilar to Mg 4 H 4 [N(DIPP)PPh 2 ] [10] aring structure is formed. [8] To elucidate the effect of the slightly larger bite angle of PN5 (relative to PN7) on the structure of magnesium hydride complexes,t he previously described experiments were repeated with [Mg(PN5)(n-Bu)] 2 (1).…”

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While magnesium hydride complexes are generally stabilized by hard, bulky N-donor ligands,s ofter ligands with abroad variety of coordination modes are shown to efficiently adapt themselves to the large variety of Mg 2+ centers in ag rowing magnesium hydride cluster.AP, N-chelating ligand is introduced that displays coordination modes between that of enamide,aza-allyl, and phosphinomethanide.Slight changes in the ligand bite angle have dramatic consequences for the structure type.The hitherto largest neutral magnesium hydride clusters are isolated either in an onanuclear sheet-structure (brucite-type) or adodecanuclear ring structure.Multinuclear magnesium hydride complexes [1][2][3][4][5][6][7][8][9][10] are being increasingly studied as molecular model systems for the hydrogen storage material [MgH 2 ] 1 .O ther interest in such complexes stems from their use as potent hydride sources in synthesis and their recognition as key intermediates in early main-group metal catalysis. [11] Starting from am agnesium alkyl or amide complex, typical synthetic routes involve metathesis with silane hydride reagents.Other routes include cyclohexadiene [12] or alane [13] induced hydrogenation of an Mg I complex or b-hydride elimination of am agnesium amidoborane species.

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