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A detailed study of H3B·NMeH2 dehydropolymerization using the cationic precatalyst [Rh(DPEphos)(H2BNMe3(CH2)2tBu)][BArF4] 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 [Mn = 15 000 g mol–1]. Closely related Rh(Xantphos)(benzyl) operates at 0.1 mol % to give a higher molecular weight polymer [Mn = 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-(iPr2PCH2CH2)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·H3BNMeH2 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 (Mn ∼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, Mn = 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(ii)-based amine-borane dehydropolymerisation catalyst system is reported that operates at low loadings, to selectively give (H2BNMeH)n polymer on scale, with catalyst control over Mn, narrow dispersities and low residual metal content.

Transition 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 5 -cyclopentadienyl (Section 2), chelating ligands, including redox-active α-diimines and various complementary diphosphines (Section 3), and novel anionic amidinate or pyrazolate ligands (Section 4). 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 α-diimine complexes in Section 2 is followed by a description of the [M(CO) 2 (η 3 -allyl-H,R)(α-diimine)] 0/+ fragment as a convenient scaffold for diverse monodentate ligands participating in a range of substitution, insertion, intramolecular migration and C–C coupling reactions – frequently 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 of their synthetic routes is complemented by intriguing examples of fluxionality and characteristic reactivity encountered for these unusual four-electron donor ligands.

A simple Co(ii)-based amine-borane dehydropolymerisation catalyst system is reported that operates at low loadings, to selectively give (H2BNMeH)n polymer on scale, with catalyst control over Mn, narrow dispersities and low residual metal content.

[Rh(κ2-PP-DPEphos){η2η2-H2B(NMe3)(CH2)2tBu}][BArF4] acts as an effective precatalyst for the dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane (H2BNMeH)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 H2: with Mn ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H2 evolution studies (1,2-F2C6H4 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, [Rh2(κ2-PP-DPEphos)2(μ-H)(μ-H2BN=HMe)][BArF4], followed by the crystallographically characterized amidodiboryl complex [Rh2(cis-κ2-PP-DPEphos)2(σ,μ-(H2B)2NHMe)][BArF4]. Adding ∼2 equiv of NMeH2 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., Mn = 40 000 g/mol). Speciation studies suggest that NMeH2 acts to form the precatalysts [Rh(κ2-DPEphos)(NMeH2)2][BArF4] and [Rh(κ2-DPEphos)(H)2(NMeH2)2][BArF4], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ2-PP-DPEphos){η2-H2B(NMe3)(CH2)2tBu}][BArF4], which has been doped with amine. This promoting effect of added amine in situ is shown to be general in other cationic Rh-based systems, and possible mechanistic scenarios are discussed.

The complex [Re(CO)3(bpy)Cl] (bpy=2,2′-bipyridine) has been known for several decades as the catalyst precursor for both electrochemical and photochemical reduction of CO2, currently being outperformed by the cheaper and Earth-abundant manganese congeners. On the other hand, the catalytic potential of the related group-6 tetracarbonyl α-diimine complexes has only recently been discovered, showing particularly strong dependence on the cathodic material initially revealed by cyclic voltammetry and IR spectro-electrochemistry. This chapter tackles in some detail these general aspects, including fundamental experimental techniques used to investigate the cathodic behaviour and the electronic structures of group-6 [M(CO)4(α-diimine)] and related Re complexes as well as their reduced forms.

The complex [Re(CO)3(bpy)Cl] (bpy=2,2′-bipyridine) has been known for several decades as the catalyst precursor for both electrochemical and photochemical reduction of CO2, currently being outperformed by the cheaper and Earth-abundant manganese congeners. On the other hand, the catalytic potential of the related group-6 tetracarbonyl α-diimine complexes has only recently been discovered, showing particularly strong dependence on the cathodic material initially revealed by cyclic voltammetry and IR spectro-electrochemistry. This chapter tackles in some detail these general aspects, including fundamental experimental techniques used to investigate the cathodic behaviour and the electronic structures of group-6 [M(CO)4(α-diimine)] and related Re complexes as well as their reduced forms.