Alkaline-Earth Derivatives of Diphenylphosphine–Borane

Abstract

Treatment of β-diketiminato (BDI = HC{C(CH 3 )Ndipp)} 2 where dipp = 2,6-iPr 2 C 6 H 3 ) magnesium butyl [(BDI)MgBu] (I) and calcium hexamethyldisilazide [(BDI)Ca{N(SiMe 3 ) 2 }] (II) complexes with equimolar quantities of diphenylphosphine− borane, Ph 2 PH•BH 3 , results in the formation of the respective alkaline earth (Ae) phosphidoborane derivatives [(BDI)Mg(Ph 2 PBH 3 )] 2 (6a) and [(BDI)Ca(Ph 2 PBH 3 )] (7a). Although satisfactory single crystals of 7a could not be obtained, 6a was crystallographically characterized and both compounds display similar NMR spectra. The dimeric Ae-hydride complexes [(BDI)AeH] 2 (IIIa, Ae = Mg; IIIb, Ae = Ca) react with substoichiometric quantities of Ph 2 PH•BH 3 , allowing the crystallization of the dimeric Mg and trimeric Ca phosphidoborane species [(BDI)Mg(H)(H 3 BPPh 2 )Mg(BDI)] (8) and [{(BDI)Ca} 3 (H)-(H 3 BPPh 2 ) 2 ] (9). In the absence of coordinating Lewis bases, compounds 6a, 7a, 8, and 9 display dynamic solution-state behavior (in benzene and toluene), while addition of THF furnishes the monomeric adducts [(BDI)Mg(H 3 BPPh 2 )•THF] (6b) and [(BDI)Ca-(H 3 BPPh 2 )•THF] (7b). Addition of Ph 2 PH•BH 3 to compound 6a results in BH 3 transfer to eliminate Ph 2 PH and generate the phosphinodiboronate complex [(BDI)Mg{(H 3 B) 2 PPh 2 }] 2 (10) in preference to dehydrocoupling of the phosphidoborane and phosphine−borane reagents.Article pubs.acs.org/Organometallics

Molecular Main Group Metal Hydrides

“…More recently, the dimeric calcium hydride complex, [( Dipp Nacnac)­Ca­(μ-H)] 2 ( 233 ) was shown to react with substoichiometric quantities of Ph 2 PH·BH 3 , allowing the crystallization of the trimeric Ca phosphidoborane species [{( Dipp Nacnac)­Ca} 3 (H)­{H 3 BPPh 2 } 2 ] ( 282 ) . Addition of THF to a solution of compound 282 provided the monomeric adduct [( Dipp Nacnac)­Ca­{H 3 BPPh 2 }·THF] ( 283 ) (Scheme ).…”

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

“…15,16 Following previous investigations into Ae-mediated catalytic and stoichiometric amine-borane dehydrogenation and dehydrocoupling, [17][18][19][20][21][22][23][24][25][26] we recently reported a series of β-diketiminate (BDI) supported alkaline-earth (Ae) phosphidoborane complexes, which are prepared by exposing diphenylphosphine-borane to readily accessible BDI-Ae hydride, alkyl, or amide precursors (Scheme 2a). 27 Attempts to achieve complete phosphine-borane dehydrogenation were unsuccessful, however, and the use of super-stoichiometric quantities of phosphine-borane resulted in BH 3 transfer to provide the phosphinodiboronate complex 2 and uncomplexed diphenylphosphine (Scheme 2a). The preference for these systems to undergo BH 3 transfer rather than hydrideelimination was rationalised to be a consequence of the charge-dense Lewis acidic Ae-centre in conjunction with the soft phosphorus-centred Lewis base.…”

Phosphido-bis(borane) complexes of the alkaline earth metals

“…The Mg-H1A distance of 2.059(18) Å is significantly shorter than the remaining Mg-H distances (3.00(3) and 3.19(3) Å), while the Mg (7) and 2.63(7) Å, and 2.866(8) The isolation of the separated ion triples 2b/2c allows a direct comparison between the structural features of isolated and coordinated phosphido-bis(borane) ligands. The structures of the anion and cation in these two modifications are essentially identical and so only 2b will be discussed here; the structure of 2b is shown in Figure 2, along with selected bond lengths and angles.…”

“…In contrast, the reaction between the secondary phosphine-borane {(Me3Si)2CH}(Ph)PH(BH3) (1) 4b and a suitable diorgano-alkaline earth metal complex, followed by treatment of the intermediate phosphido-borane complex with BH3•SMe2, gives the corresponding phosphido-bis(borane) complexes in good yields. Thus, the reaction between nBu2Mg and two equivalents of 1 in toluene gives the intermediate 2), P1-B2 1.941(2), P1-C1 1.843(2), P1-C8 1.829(2); 4 Sr1-O1 2.553 (7), Sr1-O2 2.513( 8), Sr1-H1A 2.68 (7), Sr1-H1B 2.74 (7), Sr1-H1C 2.63 (7), Sr1•••B1 2.866( 8), P1-B1 1.939( 9), P1-B2 1.955 (10), P1-C1 1.871(5), P1-C8 1.838 (7).…”

“…5 Nonetheless, there is increasing interest in these ligands and, in addition to their alkali metal complexes, phosphido-bis(borane) anions have recently been used for the synthesis of a series of lanthanide and uranium(III) complexes. 6 Very recently, a single example of an alkaline earth metal complex of a phosphido-bis(borane) ligand was reported by Hill, Mahon, Manners and co-workers: treatment of the phosphido-borane complex [(BDI)Mg{Ph2P(BH3)}]2 with two equivalents of Ph2PH(BH3) was found not to lead to hydride elimination as expected, but instead gave the phosphido-bis(borane) bridged dimer 7 Structural studies have established three principal phosphido-bis(borane) ligand coordination modes, each involving one or more B-H•••M contacts: chelating (I), bridging (II) and "dangling" modes (III). The coordination mode adopted is determined by the nature of the metal centre and the presence of any additional donor ligands.…”