[Rh(κ 2 -PP-DPEphos){η 2 η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ] acts as an effective precatalyst for the dehydropolymerization of H 3 B·NMeH 2 to form N -methylpolyaminoborane (H 2 BNMeH) n . Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1–1 mol %, an inverse relationship) and use of the chain-modifying agent H 2 : with M n ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H 2 evolution studies (1,2-F 2 C 6 H 4 solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh] TOTAL . Speciation studies at 10 mol % indicate the initial formation of the amino–borane bridged dimer, [Rh 2 (κ 2 -PP-DPEphos) 2 (μ-H)(μ-H 2 BN=HMe)][BAr F 4 ], followed by the crystallographically characterized amidodiboryl complex [Rh 2 ( cis -κ 2 -PP-DPEphos) 2 (σ,μ-(H 2 B) 2 NHMe)][BAr F 4 ]. Adding ∼2 equiv of NMeH 2 in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh] TOTAL is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., M n = 40 000 g/mol). Speciation studies suggest that NMeH 2 acts to form the  precatalysts [Rh(κ 2 -DPEphos)(NMeH 2 ) 2 ][BAr F 4 ] and [Rh(κ 2 -DPEphos)(H) 2 (NMeH 2 ) 2 ][BAr F 4 ], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ 2 -PP-DPEphos){η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ], which has been doped with amine. This promoting effect of added amine in situ is shown to be genera...

A detailed study of H3B·NMeH2 dehydropolymerization using the cationic precatalyst [Rh­(DPEphos)­(H2BNMe3(CH2)2 tBu)]­[BArF 4] identifies the resting state as dimeric [Rh­(DPEphos)­H2]2 and boronium [H2B­(NMeH2)2]+ as the chain-control agent. [Rh­(DPEphos)­H2]2 can be generated in situ from Rh­(DPEphos)­(benzyl) and catalyzes polyaminoborane formation (H2BNMeH) n [M n = 15 000 g mol–1]. Closely related Rh­(Xantphos)­(benzyl) operates at 0.1 mol % to give a higher molecular weight polymer [M n = 85 000 g mol–1] on the gram scale with low residual [Rh], 81 ppm. This insight offers a mechanistic template for dehydropolymerization.

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 simple Co-based catalyst system promotes the efficient and controlled dehydropolymerisation of amine–boranes on scale.

Contents 1. Introduction 2. Complexes [CpM(CO)2(η 3 -allyl)] and related compounds 2.1. Synthetic strategies 2.2. Exo ⇌ endo isomerism 2.3. 95 Mo and 183 W NMR spectroscopy 2.4. Oxidized cationic complexes -reactivity, fluxionality, structural features 2.5. Mixed nitrosyl carbonyl complexes [CpM(CO)(NO)(η 3 -allyl)] + (M = Mo, W) 3. Complexes [M(CO)2(η 3 -allyl)(α-diimine)X] (X = anionic monodentate ligand) and related compounds 3.1. Synthetic strategies 3.2. Mechanistic features, structural aspects, and dynamic behaviour 3.3. Monodentate ligands bound to M(CO)2(η 3 -allyl)(L⏜L): reactivity and intermediacy in allylic alkylations 3.4. Redox Properties and Electrocatalysis 4. Amidinato and pyrazolato complexes related to [M(CO)2(η 3 -allyl)(α-diimine)X] 4.1. Synthesis and dynamic behaviour 4.2. Coordinatively unsaturated species and their reactivity 5. Summary and outlook Acknowledgement References 2 ABSTRACTTransition metal complexes with π-allylic ligands remain an attractive topic in organometallic chemistry, given the numerous reports of a wide variety of synthetic routes, dynamic behaviour and reactivity, structural (including isomerism), spectroscopic and redox properties, and applications in organic synthesis and catalysis. Surprisingly, despite the considerable interest in the rich and varied chemistry of this family of organometallic compounds, there is no recent review. This review is focused on π-allylic representatives of low-cost Group-6 metals bearing one or more carbonyl ligand, the coordination sphere being complemented with η 5cyclopentadienyl (Section 2), chelating ligands, including redox active α-diimines and various complementary diphosphine (Section 3) and novel anionic amidinate or pyrazolate (Section 4) ligands. In Section 1 particular attention is paid to rearrangements of the π-allylic ligand, namely exo and endo isomerism, interconversion mechanisms, fluxionality, and agostic interactions. In addition, the application of multinuclear NMR spectroscopy to the elucidation of such isomerism, and the effect of the metal centre oxidation state on the bonding, dynamic behaviour and reactivity of the π-allylic ligand are described. The detailed mechanistic description of the synthetic routes and dynamic behaviour of selected representatives of αdiimine complexes in Section 2 is followed by a description of the [M(CO)2(η 3 -allyl-H,R)(αdiimine)] 0/+ fragment as a convenient scaffold for diverse monodentate ligands participating in a range of substitution, insertion, intramolecular migration and C-C coupling reactionsfrequently involving also the π-allylic ligand, such as allylic alkylation. Special attention is devoted to selected examples of redox and acid-base reactivity of the α-diimine complexes with emphasis on prospects in electrocatalysis. The amidinate (and related pyrazolate) ligands treated in Section 4 may directly replace the π-allylic ligand in some cyclopentadienyl complexes (Section 2) or the α-diimine ligand in some dicarbonyl π-allylic complexes (Section 3). The brief description...

Transient Cu–H monomers have long been invoked in the mechanisms of substrate insertion in Cu–H catalysis. Their role from Cu–H aggregates has been mostly inferred since ligands to stabilize these monomeric intermediates for systematic studies remain limited. Within the last decade, new sterically demanding N-heterocyclic carbene (NHC) ligands have led to isolable Cu–H dimers and, in some cases, spectroscopic characterization of Cu–H monomers in solution. We report an NHC ligand, IPr*R, containing para R groups of CHPh2 and CPh3 on the ligand periphery for the isolation of a Cu–H monomer for insertion of internal alkenes. This reactivity has not been reported for (NHC)­CuH complexes despite their common application in Cu–H-catalyzed hydrofunctionalization. Changing from CHPh2 to CPh3 impacts the relative concentration of Cu–H monomers, rate of alkene insertion, and reaction of a trisubstituted internal alkene. Specifically, for R = CPh3, monomeric (IPr*CPh3)­CuH was isolated and provided >95% monomer (10 mM in C6D6). In contrast, for R = CHPh2, solutions of [(IPr*CHPh2)­CuH]2 are 80% dimer and 20% (IPr*CHPh2)­CuH monomer at 25 °C based on 1H, 13C, and 1H–13C HMBC NMR spectroscopy. Quantitative 1H NMR kinetic studies on cyclopentene insertion into Cu–H complexes to form the corresponding Cu–cyclopentyl complexes demonstrate a strong dependence on the rate of insertion and concentration of the Cu–H monomer. Only (IPr*CPh3)­CuH, which has a high monomer concentration, underwent regioselective insertion of a trisubstituted internal alkene, 1-methylcyclopentene, to give (IPr*CPh3)­Cu­(2-methylcyclopentyl), which has been crystallographically characterized. We also demonstrated that (IPr*CPh3)­CuH catalyzes the hydroboration of cyclopentene and methylcyclopentene with pinacolborane.

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