Oxidative Addition of the N–H Bond of Ammonia to Iridium Bis(phosphane) Complexes: A Combined Experimental and Theoretical Study

Heavy metalla vinyl-cations show metal-Lewis acid cooperativity in reaction with small molecules (NH3, N2H4, H2O, H2)

Halide abstraction from tetrylidene complexes [TbbE(Br)IrH(PMe₃)₃] [E = Ge (1), Sn (2)] and [Ar*E(Cl)IrH(PMe₃)₃] gives the salts [TbbEIrH(PMe₃)₃][BAr^F ₄] [E = Ge (3), Sn (4)] and [Ar*EIrH(PMe₃)₃][BAr^F ₄] [E = Ge (3'), E = Sn (4')] (Tbb = 2,6-[CH(SiMe₃)₂]₂-4-(t-Bu)C₆H₂, Ar* = 2,6-Trip₂C₆H₃, Trip = 2,4,6-triisopropylphenyl). Bonding analysis suggests their most suitable description as metalla-tetrela vinyl cations with an Ir[double bond, length as m-dash]E double bond and a near linear coordination at the Ge/Sn atoms. Cationic complexes 3 and 4 oxidatively add NH₃, N₂H₄, H₂O, HCl, and H₂ selectively to give: [TbbGe(NH₂)IrH₂(PMe₃)₃][BAr^F ₄] (5), [TbbE(NHNH₂)IrH₂(PMe₃)₃][BAr^F ₄] [E = Ge (7), Sn (8)], [TbbE(OH)IrH₂(PMe₃)₃][BAr^F ₄] [E = Ge (9), Sn (10)], [TbbE(Cl)IrH₂(PMe₃)₃][BAr^F ₄] [E = Ge (11a), Sn (12a)], [TbbGe(H)IrH₂(PMe₃)₃][BAr^F ₄] (13), [TbbSn(μ-H₃)Ir(PMe₃)₃][BAr^F ₄] (14), and [TbbSn(H)IrH₂(PMe₃)₃][BAr^F ₄] (15). 14 isomerizes to give 15via an 1,2-H shift reaction. Hydride addition to cation 3 gives a mixture of products [TbbGeHIrH(PMe₃)₃] (16) and [TbbGeIrH₂(PMe₃)₃] (17) and a reversible 1,2-H shift between 16 and 17 was studied. In the tin case 4 the dihydride [TbbSnIrH₂(PMe₃)₃] (18) was isolated exclusively. The PMe₃ and PEt₃ derivatives, 18 and [TbbSnIrH₂(PEt₃)₃] (19), respectively, could also be synthesized in reaction of [TbbSnH₂]^- with the respective chloride [(R₃P) _n IrCl] (R = Me, n = 4; R = Et, n = 3). Reaction of complex 19 with CO gives the substitution product [TbbSnIrH₂(CO)(PEt₃)₂] (20). Further reaction with CO results in hydrogen transfer from the iridium to the tin atom to give [TbbSnH₂Ir(CO)₂(PEt₃)₂] (21). The reversibility of this ligand induced reductive elimination transferring 20 to 21 is shown.

Activation of Ammonia by a Carbene-Stabilized Dithiolene Zwitterion

A carbene-stabilized dithiolene zwitterion (3) activates ammonia, affording 4^• and 5, through both single-electron transfer (SET) and hydrogen atom transfer (HAT). Reaction products were characterized spectroscopically and by single-crystal X-ray diffraction. The mechanism of the formation of 4^• and 5 was probed by experimental and computational methods. This discovery is the first example of metal-free ammonia activation via HAT.

Thermoneutral N-H Bond Activation of Ammonia by a Geometrically Constrained Phosphine

A geometrically constrained phosphine bearing a tridentate NNS pincer ligand is reported. The effect of the geometric constraint on the electronic structure was probed by theoretical calculations and derivatization reactions. Reactions with N−H bonds result in formation of cooperative addition products. The thermochemistry of these transformations is strongly dependent on the substrate, with ammonia activation being thermoneutral. This represents the first example of a molecular compound that reversibly activates ammonia via N−H bond scission in solution upon mild heating.

Ambiphilic boryl groups in a neutral Ni(ii) complex: a new activation mode of H2

The concept of metal–ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel–methyl complex 1 with bis(catecholato)diboron (B₂Cat₂) allows for the synthesis of nickel(ii) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H₂ is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(ii)/Ni(0)/Ni(ii) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing the formation of aminoboranes.

A bis(boryl)nickel complex promotes the facile and reversible activation of H₂ through a cooperative mechanism that involves the metal and both boryl moieties in a concerted five-center process.