A New Paradigm in Pincer Iridium Chemistry: PCN Complexes for (De)Hydrogenation Catalysis and Beyond

Abstract

The discovery and development of organometallic catalysts is of paramount importance in modern organic synthesis, among which the ligand scaffolds play a crucial role in controlling the activity and selectivity. Over the past several decades, d 8 transitionmetal complexes of pincer ligands have been developed extensively thanks to their easy structural modification, versatile reactivities, and high stability. One paradigm is the bis(phosphine)-based pincer iridium complexes PCP-Ir, which are highly active for alkane dehydrogenation, partly due to their high thermostability. However, except for alkane dehydrogenation and related transformations, few applications of pincer iridium catalysis have been seen in organic synthesis. This mainly arises from the low functional-group compatibility and poor substrate scope and the limited catalytic chemistry that invariably involves Ir(I/III) redox processes initiated by oxidative addition of substrates to 14-electron (PCP)Ir fragments (the proposed catalytically active intermediates). In this Account, we describe our endeavor on the development of a new family of PCN-Ir complexes with initial intention on creating more efficient alkane dehydrogenation catalysts. The replacement of a soft, σdonor phosphine arm in the PCP ligands by a harder, π-acceptor N-heteroarene (pyridine or oxazoline) not only provides an additional platform to modify the structural properties but also offers new modes of bond activation and novel reactivities and catalysis. One uniqueness of the PCN-Ir system lies in the formation, via ortho-C(sp 2 )−H cyclometalation of the pyridine unit in the PCN Py ligand, of the neutral monohydride (PCC)Ir III HL (L = neutral ligand), which catalyzes positional and stereoselective 1alkene-to-(E)-2-alkene isomerization. Moreover, the PCN-Ir catalysts effect ethanol dehydrogenation without decarbonylation, allowing for transfer hydrogenation of unactivated alkenes and trans-selective semihydrogenation of internal alkynes with userfriendly ethanol as the H-donor. Another feature originates from the ability of the pentacoordinate hydrido chloride complex (PCN)Ir III HCl to undergo reversible solvent-coordination-induced-ionization (SCII), furnishing a cationic monohydride [(PCN)Ir III H(Sol)] + Cl − bearing an uncoordinated Cl anion that effects selective hydrometalation of internal alkynes over the corresponding (Z)-alkenes; the resulting (PCN)Ir III (vinyl)Cl complex undergoes amine-assisted formal alcoholysis involving the protonation of the Cl anion by the activated Ir III -bound EtOH, again via the SCII pathway. Together these elementary reactions lay the foundation for cis-selective semihydrogenation of alkynes with EtOH. Further, the design of the oxazoline-containing chiral complexes (PCN Oxa )Ir III HCl enables asymmetric transfer hydrogenation of alkenes/ketones with ethanol. The efficient catalytic αalkylation of unactivated esters/amides with alcohols is another case showing the benefit that the PCN-Ir catalyst can offer. These examples illustrate the prof...

Breaking Conventional Site Selectivity in C–H Bond Activation: Selective sp3 versus sp2 Silylation by a Pincer-Based Pocket

“…By comparison, the pyridine-containing PCN complex ( t Bu PCN)­IrHCl ( 4g ) gave low conversion (30%) and poor site selectivity in favor of the sp 2 silylation ( 2a : 3a = 2.5:1) (entry 10). The PCP-Ru complex ( 5 ) was ineffective for the silylation of the unactivated C­(sp 3 )–H bonds (entry 11), although it was known to catalyze the intramolecular silylation of primary C­(sp 3 )–H bonds adjacent to heteroatoms…”

PCP Pincer Complexes of Titanium in the +3 and +4 Oxidation States

“…1 H NMR (400 MHz, C 6 D 6 /THF): δ 7.46 (br m, 2H, phenyl-H m ), 6.89 (t, 2 J HH = 2.59 Hz, 1H, phenyl-H p ), 3.36 (d, 2 J HP = 2.39 Hz, 4H, −CH 2 ), 1.43 (d, 3 J HP = 9.94 Hz, 36H, −CH 3 ). 7 Li{ 1 H} NMR (400 MHz, THF): δ 2.66 (s).…”

PCP Complexes of Titanium in the +3 and +4 Oxidation State

“…¹H NMR (400 MHz, C₆D₆/THF): δ 7.46 (br m, 2H, phenyl-Hₘ), 6.89 (t, 2 JHH = 2.59 Hz, 1H, phenyl-Hₚ), 3.36 (d, ²JHP = 2.39 Hz, 4H, -CH₂), 1.43 (d, ³JHP = 9.94 Hz, 36H, -CH₃). 7 Li{ 1 H} NMR (400 MHz, THF): δ 2.66 (s).…”

“…[2][3][4] Pincer complexes have been applied extensively in catalysis, particularly for hydrogenation/dehydrogenation reactions. [5][6][7] They have also been applied in the area of small molecule activation, notably in recent work by Nishibayashi using Molybdenum pincer complexes for record-breaking dinitrogen reduction. 8 The bulk of research carried out with pincer complexes has focused on the late transition metals.…”

“…The [BAr F 4]− anion, hydrogen atoms and methyl groups on the tert-butyl substituents of the phosphines are omitted for clarity. Selected bond lengths (Å) and angles (°): Ti(1)-Cl(1) 2.2591(7), Ti(1)-Cl(2) 2.4758(7), Ti(1)-P(1) 2.6357(7), Ti(1)-P(2) 2.6199(7), Ti(1)-C(1) 2.173(2), Ti(1)-Ti(2) 4.8972(6), Ti(2)-Cl(2) 2.4378(7), Ti(2)-Cl(3) 2.2719(7), Ti(2)-P(3) 2.5924(7), Ti(2)-P(4) 2.6374(7), Ti(2)-C(25) 2.187(2); Cl(1)-Ti(1)-C(1) 128.19(7), P(1)-Ti(1)-P(2) 151.03(2), Cl(1)-Ti(1)-Cl(2) 105.77(3), P(1)-Ti(1)-Cl(2) 95.56(2), P(2)-Ti(1)-Cl(2) 100.51(2), C(1)-Ti(1)-Cl(2) 126.02(7), Ti(1)-Cl(2)-Ti(2) 170.62(3), Cl(3)-Ti(2)-C(25) 147.76(7), P(3)-Ti(2)-P(4) 146.80(2), Cl(2)-Ti(2)-Cl(3) 104.82(3), Cl(2)-Ti(2)-P(3) 101.05(2), Cl(2)-Ti(2)-P(4) 106.10(2), Cl(2)-Ti(2)-C(25) 107.10(7) The two titanium centres both adopt a square-based pyramid geometry, although one, Ti(1), is more distorted (τ = 0.067 and 0.016, Ti(1) and Ti(2) respectively). The PCP ligands are almost perpendicular to one another, demonstrated by the angles between the C(1)-Ti(1)-Cl(2) and C(25)-Ti(2)-Cl(2) planes: 79.58°.…”