A Highly Active PBP–Iridium Catalyst for the Dehydrogenation of Dimethylamine–Borane: Catalytic Performance and Mechanism

Abstract

A long‐tethered boron‐containing (P‐B‐P) pincer ligand with an aliphatic backbone was synthesized. Oxidative addition of the B−H bond in the ligand to [Ir(coe)2Cl]2 (coe=cyclooctene) afforded the (P‐B‐P)Ir(H)Cl complex. A catalyst system comprising the iridium complex and KOtBu showed excellent performance in the dehydrocoupling of N,N‐dimethylamine–borane (DMAB), releasing 1 equivalent of dihydrogen gas from a concentrated solution of DMAB at a low catalyst loading of 0.05 mol % and with an initial turnover frequency of approximately 3400 h−1. The dihydride (P‐B‐P)Ir(H)2 complex also catalyzed the dehydrogenation of DMAB and was postulated as one of the intermediates in the catalytic cycle.

Amine‐Borane Dehydrogenation and Transfer Hydrogenation Catalyzed by α‐Diimine Cobaltates

“…As proposed by Schneider and co-workersf or dehydrogenations catalyzed by aR u-amido pincer complex and by Weller and coworkers with ac ationic Rh-phosphine complex, [5c, 21] the loss of two dihydrogen molecules operates over two steps when the reactionp roceeds through B.I nt he case of C,b oth dihydrogen molecules are eliminated in the first step and ac ycloaddition gives the terminal product D.W ec annot rule out that B was also converted into D by the loss of one molecule H 2 .T he side product HB(NMe) 2 was reportedf or many catalytic DMAB hydrogenations in the literature. [5,8,9] Related mechanisms for DMAB dehydrogenation have also been described for dehydrogenationc atalysts based on zirconium and iridium. [22] Monitoring the time-dependent H 2 formation (see the Supporting Information)r evealed an induction period of approximately 1min (Supporting Information, Figure S13).…”

“…[4] Various dehydrogenation and transfer hydrogenation protocols have been developed with precious metal catalysts, and the underlying mechanismsh ave been thoroughly studied. [5] By contrast, dehydrogenations are far less advanced with the abundant and cheaper late 3d metals,d espite the recent progress with Ti,M n, Fe, Co, and Ni catalysts. [6][7][8][9][10][11][12] Althoughanumber of iron catalystsf or amine-borane dehydrogenations have been studied recently, [8] effective cobalt catalysts are scarce.…”

Synthesis of A Pincer‐IrVComplex with A Base‐Free Alumanyl Ligand and Its Application toward the Dehydrogenation of Alkanes

“…[14] TheP AlP-pincer ligand precursor 4 was synthesized by as lightly modified procedure used for the preparation of the PBP-pincer ligand precursor (Scheme 1). [14] Ther eduction of phosphinoboranetethered phenylenediamine 1 with LiAlH 4 followed by hydrolysis afforded 2.D eprotection of the phosphineborane moiety of 2 with N-acetylethylenediamine resulted in the formation of 3.T he reaction of 3 with AlH 3 •NMe 2 Et provided phosphine-tethered AlÀHp recursor 4.T he IR spectrum of 4 showed ac haracteristic absorption band at ñ = 1861 cm À1 ,w hich is assignable to an Al À Hs tretching vibration, similar to those of previously reported diaminoaluminum hydrides. [15] Thec omplexation of 4 with [Ir(cod)Cl] 2 (cod = 1,5-cyclooctadiene) in toluene at 100 8 8Cl ed to the isolation of [PAlP]Ir(H) 4 (5;S cheme 2).…”

“…[21] Complex 5 was thermally stable even in o-xylene solution at 150 8 8C, as judged by 1 Ha nd 31 P{ 1 H} NMR spectroscopy.T he solid-state structure of 5 is shown in Figure 2. All four hydride ligands around the Ir center were assigned to the residual peaks in the differential Fourier map.T he Al À Ir bond [2.3819 (14) ]i s apparently shorter than the sum of the covalent radii of the Al and Ir atoms (2.51 ), [22] and is the shortest hitherto reported bond for Ir complexes bearing an Al atom in the coordination sphere. [9b, 11d, 23] Theslightly shorter NÀAl bonds [1.812(4), 1.818(4) ]i n5 compared to those of previously reported N PA lP À Rh complexes (1.822-1.884 ) [11h] indicated pp-pp interactions between the Na nd Al atoms in 5.…”

The Pincer Platform Beyond Classical Coordination Patterns

“…[ 52 ] The pre‐ligand 4 includes a borane moiety and can react with a number of transition metal complexes via B–H‐oxidative addition to give boryl‐based pincer‐type complexes ( 5 , Scheme 4). [ 53–60 ] Depending on the employed metal precursor a sequence of B–H‐oxidative addition and H–X‐reductive elimination is observed. In either case the presence of a strongly σ‐donating boryl group facilitates a number of bond activation reaction and catalytic applications, such as the transfer‐dehydrogenation of alkanes, [ 56,58 ] the dehydrogenation of dimethylamine‐borane [ 57,61,62 ] or the hydrogenation and hydrosilylation of olefins.…”