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Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of well-established and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.

A phosphinine-borane adduct of a Me3Si-functionalized phosphinine and the Lewis acid B(C6F5)3 has been synthesized and characterized crystallographically for the first time. The reaction strongly depends on the nature of the substituents in the α-position of the phosphorus heterocycle. In contrast, the reaction of B2H6 with various substituted phosphinines leads to an equilibrium between the starting materials and the phosphinine–borane adducts that is determined by the Lewis basicity of the phosphinine. The novel phosphinine borane adduct (6-B(C6F5)3) shows rapid and facile insertion and [4+2] cycloaddition reactivity towards phenylacetylene. A hitherto unknown dihydro-1-phosphabarrelene is formed with styrene. The reaction with an ester provides a new, facile and selective route to 1-R-phosphininium salts. These salts then undergo a [4+2] cycloaddition in the presence of Me3Si−C≡CH and styrene to cleanly form unprecedented derivatives of 1-R-phosphabarrelenium salts.

While Mn-catalyzed (de)hydrogenation of carbonyl derivatives have been well established, the reactivity of Mn hydrides with olefins remains very rare. Herein we report the first Mn(I) pincer complex that effectively promotes site-controlled transposition of olefins. This new reactivity is shown to emerge once the N-H functionality within Mn/NH bifunctional complex is suppressed by alkylation. While detrimental for carbonyl (de)hydrogenation, such a masking of the cooperative NH functionality allows for the highly efficient conversion of a wide range of allylarenes to higher-value 1-propenybenzenes in near-quantitative yield with excellent stereoselectivities. The reactivity towards a single positional isomerization was also retained for longer-chain alkenes resulting in the highly regioselective formation of 2-alkenes, which are less thermodynamically stable compared to other possible isomerization products. The detailed mechanistic analysis of reaction between activated Mn catalyst and olefins points to catalysis operating via a metal alkyl mechanism - one of the three conventional transposition mechanisms previously unknown in Mn complexes.

Activation of homogeneous catalysts is an important step in ensuring efficient operation of any catalytic system as a whole. For the majority of pincer catalysts, the activation step leans heavily on the metal ligand cooperative chemistry that allows these complexes to react with small molecule substrates and engage in catalytic transformations. While the majority of such catalysts require a single activation event to become cooperative, herein we report an exception to this trend. Specifically, we demonstrate that a Ru-PN3P aminopyridine pincer catalyst, which lacks conventional reactivity with hydrogen upon typical one-fold activation, can exhibit this reactivity when a sequential two-step activation is performed. The resulting anionic complexes readily activate molecular hydrogen and react further with CO2 showing the previously unknown reactivity that is critical for CO2 hydrogenation catalysts. While active in CO2 hydrogenation, Ru-PN3Ps are significantly more efficient in hydrogenation of bicarbonates – a likely consequence of the chemistry of these pincers requiring formation of anionic complexes for hydrogen activation.

A diglyme solution of Na[cyclo-P5] (1) reacts with alkynes and isolobal nitriles and phosphaalkynes to afford the otherwise elusive (aza)phospholide anions 2 a–c, 4 a,b, and 6. The reaction of Na[cyclo-P5] with alkynes and nitriles was studied by means of DFT methods, which suggested a concerted mechanism for the formation of 2 a and 4 b. The anions 2 a–c, 4 a,b, and 6 coordinate in an η5-fashion towards FeII to give the sandwich (aza)phosphametallocenes 3 a–c, 5 a,b and 7 in moderate to good yields. The new compounds were characterized by means of multinuclear NMR spectroscopy, single-crystal X-ray diffraction and cyclic voltammetry.

The reaction of 2,4,6-triaryl-λ3-phosphinine-Cr(CO)3-π-complexes with [Rh(COD)2]BF4 leads to unusual diamagnetic Rh0-dimers, which contain two phosphinine-π-complexes acting as a bridging 2e−-ligand towards the Rh2(CO)2 core. These compounds represent a missing coordination mode for the aromatic 6-membered phosphorus heterocycle.