The air tolerant precatalyst, [Rh­(L)­(NBD)]Cl ([1]­Cl) [L = κ3-( i Pr2PCH2CH2)2NH, NBD = norbornadiene], mediates the selective synthesis of N-methylpolyaminoborane, (H2BNMeH) n , by dehydropolymerization of H3B·NMeH2. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh­(L)­H3 (3), forms, which sits as an outer-sphere adduct 3·H 3 BNMeH 2 as the resting state. At the end of catalysis, dormant Rh­(L)­H2Cl (2) is formed. Reaction of 2 with H3B·NMeH2 returns 3, alongside the proposed formation of boronium [H2B­(NMeH2)2]­Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative “PNHP” ligand templates dehydrogenation, releasing H2BNMeH (ΔG ‡ calc = 19.6 kcal mol–1). H2BNMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which 3 is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading (M n ∼71 000 g mol–1; Đ, of ∼ 1.6). The molecular weight can be controlled using [H2B­(NMe2H)2]Cl as a chain transfer agent, M n = 37 900–78 100 g mol–1. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of H3B·NMeH2. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % [1]­Cl) show a processing window that allows for melt extrusion of polyaminoborane strands, as well as hot pressing, drop casting, and electrospray deposition. By variation of conditions in the latter, smooth or porous microstructured films or spherical polyaminoboranes beads (∼100 nm) result.

A multidisciplinary experiment for advanced undergraduate students has been developed in the context of extractive metallurgy. The experiment serves as a model of an important modern industrial process that combines aspects of organic/inorganic synthesis and analysis. Students are tasked to prepare a salicylaldoxime ligand and samples of the corresponding copper and nickel complexes, before performing test extractions and UV−vis spectroscopic analysis. The oxime ligand demonstrates a clear preference for extraction of Cu 2+ in the presence of Ni 2+ from aqueous solution under the conditions described. It is also possible to demonstrate that the ligand can be recovered and reused. The experiment has successfully been employed in a final year project-based laboratory course involving small groups of students.

Rhodium-alkene complexes of the pincer ligand κ 3 -C 5 H 3 N-2,6-(OP i Pr 2 ) 2 (PONOP- i Pr) have been prepared and structurally characterized: [Rh(PONOP- i Pr)(η 2 -alkene)][BAr F 4 ] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ]. Only one of these, alkene = COD, undergoes a reaction with H 2 (1 bar), to form [Rh(PONOP- i Pr)(η 2 -COE)][BAr F 4 ] (COE = cyclooctene), while the others show no significant reactivity. This COE complex does not undergo further hydrogenation. This difference in reactivity between COD and the other alkenes is proposed to be due to intramolecular alkene-assisted reductive elimination in the COD complex, in which the η 2 -bound diene can engage in bonding with its additional alkene unit. H/D exchange experiments on the ethene complex show that reductive elimination from a reversibly formed alkyl hydride intermediate is likely rate-limiting and with a high barrier. The proposed final product of alkene hydrogenation would be the dihydrogen complex [Rh(PONOP- i Pr)(η 2 -H 2 )][BAr F 4 ], which has been independently synthesized and undergoes exchange with free H 2 on the NMR time scale, as well as with D 2 to form free HD. When the H 2 addition to [Rh(PONOP- i Pr)(η 2 -ethene)][BAr F 4 ] is interrogated using p H 2 at higher pressure (3 bar), this produces the dihydrogen complex as a transient product, for which enhancements in the 1 H NMR signal for the bound H 2 ligand, as well as that for free H 2 , are observed. This is a unique example of the partially negative line-shape effect, with the enhanced signals that are observed for the dihydrogen complex being explained by the exchange processes already noted.

The dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir( i Pr-PNHP)} fragment [ i Pr-PNHP = κ3-(CH2CH2P i Pr2)2NH] is reported. Neutral Ir( i Pr-PNHP)H3 or Ir( i Pr-PNHP)H2Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F2C6H4 solution. In contrast, addition of [NMeH3][BArF 4] (ArF = 3,5-(CF3)2C6H3) to Ir( i Pr-PNHP)H3 immediately starts catalysis, suggesting that a cationic catalytic manifold operates. Consistent with this, independently synthesized cationic precatalysts are active (tested between 0.5 and 2.0 mol % loading) producing poly(N-methylaminoborane) with M n ∼ 40,000 g/mol, Đ ∼1.5, i.e., dihydrogen/dihydride, [Ir( i Pr-PNHP)(H)2(H2)][BArF 4]; σ-amine-borane [Ir( i Pr-PNHP)(H)2(H3B·NMe3)][BArF 4]; and [Ir( i Pr-PNHP)(H)2(NMeH2)][BArF 4]. Density functional theory (DFT) calculations probe hydride exchange processes in two of these complexes and also show that the barrier to amine-borane dehydrogenation is lower (22.5 kcal/mol) for the cationic system compared with the neutral system (24.3 kcal/mol). The calculations show that the dehydrogenation proceeds via an inner-sphere process without metal–ligand cooperativity, and this is supported experimentally by N–Me substituted [Ir( i Pr-PNMeP)(H)2(H3B·NMe3)][BArF 4] being an active catalyst. Key to the lower barrier calculated for the cationic system is the outer-sphere coordination of an additional H3B·NMeH2 with the N–H group of the ligand. Experimentally, kinetic studies indicate a complex reaction manifold that shows pronounced deceleratory temporal profiles. As supported by speciation and DFT studies, a key observation is that deprotonation of [Ir( i Pr-NHP)(H)2(H2)][BArF 4], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH2 (via B–N bond cleavage), results in the formation of essentially inactive Ir( i Pr-PNHP)H3, with a coproduct of [NMeH3]+/[H2B(NMeH2)2]+. While reprotonation of Ir( i Pr-PNHP)H3 results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH3]+/[H2B(NMeH2)2]+/catalyst in insoluble polyaminoborane. The role of [NMeH3]+/[H2B(NMeH2)]+ as chain control agents is also noted.

The reactivity of the Ir(I) PONOP pincer complex [Ir( i Pr-PONOP)(η 2 -propene)][BAr F 4 ], 6, [ i Pr-PONOP = 2,6-( i Pr 2 PO) 2 C 6 H 3 N, Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ] was studied in solution and the solid state, both experimentally, using molecular density functional theory (DFT) and periodic-DFT computational methods, as well as in situ single-crystal to single-crystal (SC-SC) techniques. Complex 6 is synthesized in solution from sequential addition of H 2 and propene, and then the application of vacuum, to [Ir( i Pr-PONOP)(η 2 -COD)][BAr F 4 ], 1, a reaction manifold that proceeds via the Ir(III, 2, and the Ir(III) dihydride propene complex [Ir( i Pr-PONOP)(η 2 -propene)H 2 ][BAr F 4 ], 7, respectively. In solution (CD 2 Cl 2 ) 6 undergoes rapid reaction with H 2 to form dihydride 7 and then a slow (3 d) onward reaction to give dihydrogen/dihydride 2 and propane. DFT calculations on the molecular cation in solution support this slow, but productive, reaction, with a calculated barrier to rate-limiting propene migratory insertion of 24.8 kcal/mol. In the solid state single-crystals of 6 also form complex 7 on addition of H 2 in an SC-SC reaction, but unlike in solution the onward reaction (i.e., insertion) does not occur, as confirmed by labeling studies using D 2 . The solid-state structure of 7 reveals that, on addition of H 2 to 6, the PONOP ligand moves by 90°within a cavity of [BAr F 4 ] − anions rather than the alkene moving. Periodic DFT calculations support the higher barrier to insertion in the solid state (ΔG ‡ = 26.0 kcal/mol), demonstrating that the single-crystal environment gates onward reactivity compared to solution. H 2 addition to 6 to form 7 is reversible in both solution and the solid state, but in the latter crystallinity is lost. A rare example of a sigma amine-borane pincer complex, [Ir( i Pr-PONOP)H 2 (η 1 -H 3 B•NMe 3 )][BAr F 4 ], 5, is also reported as part of these studies.