The selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)] (dippe = 1,2-bis(di-iso-propylphosphino)ethane) as pre-catalyst is described. The required hydrogen gas is either directly employed or in situ generated upon alcoholysis of KBH₄ with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl and terminal alkynes were readily hydrogenated to yield E-alkenes in good to excellent isolated yields. The reaction proceeds at 60 °C for directly employed hydrogen or at 60-90 °C with in situ generated hydrogen and catalyst loadings of 0.5 -2 mol%. The implemented protocol tolerates a variety of electron donating and electron withdrawing functional groups including halides, phenols, nitriles, unprotected amines and heterocycles. The reaction can be upscaled to the gram scale. Mechanistic investigations including deuterium labelling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism showing that initially formed Z-isomer undergoes fast isomerization to afford the thermodynamically more stable E-isomer.

E-Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H2 - Directly Employed or in situ Generated, Farrar-Tobar, R. A., Weber, S.; Csendes, Z.; Ammaturo, A.; Fleissner, S.; Hoffmann, H.; Veiros, L. F.; Kirchner, K. ACS Catal. 2022, 12, 2253-2260.
DOI: 10.1021/acscatal.1c06022

Several Mn(I) complexes were applied as catalysts for the homogeneous hydrogenation of ketones. The most active pre-catalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The reaction proceeds at room temperature under base-free conditions with catalyst loading of 3 mol% and a hydrogen pressure of 10 bar. A temperature dependent selectivity for the reduction of α,β-unsaturated carbonyls was observed. At room temperature, the carbonyl group was selectively hydrogenated while the C=C bond stayed intact. At 60 °C fully saturated systems were obtained. A plausible mechanism based on DFT calculations which involves an inner-sphere hydride transfer is proposed.

Manganese-Catalyzed Hydrogenation of Ketones under Mild and Base-free Conditions, Weber, S.; Brünig, J.; Veiros, L. F.; Kirchner, K. Organometallics 2021, 40, 1388-1394.
DOI: 10.1021/acs.organomet.1c00161

An efficient additive-free manganese-catalyzed hydrogenation of alkenes to alkanes with molecular hydrogen is described. This reaction is atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. The most efficient pre-catalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate which undergoes rapid hydrogenolysis to form the active 16e Mn(I) hydride catalyst [Mn(dippe)(CO)₂(H)]. A range of mono- and disubstituted alkenes were efficiently converted into alkanes in good to excellent yields. The hydrogenation of 1-alkenes and 1,1-disubstituted alkenes proceeds at 25 ^oC, while 1,2-disubstituted alkenes require a reaction temperature of 60^oC. In all cases, a catalyst loading of 2 mol % and a hydrogen pressure of 50 bar was applied. A mechanism based on DFT calculations is presented which is supported by preliminary experimental studies.

Rethinking Old Concepts - Hydrogenation of Alkenes Catalyzed by Bench-Stable Alkyl Mn(I) Complexes, Weber, S.; Stöger, B.; Veiros, L. F.; Kirchner, K. ACS Catal. 2019, 9, 9715-9720.
DOI: 10.1021/acscatal.9b03963

An efficient additive-free manganese-catalyzed hydrogenation of nitriles to primary amines with molecular hydrogen is described. The pre-catalyst, a well-defined bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dpre)(CO)₃(CH₃)] (dpre = 1,2-bis(di-n-propylphosphino)ethane), undergoes CO migratory insertion into the manganese-alkyl bond to form acyl complexes which upon hydrogenolysis yields the active coordinatively unsaturated Mn(I) hydride catalyst [Mn(dpre)(CO)₂(H)]. A range of aromatic and aliphatic nitriles were efficiently and selectively converted into primary amines in good to excellent yields. The hydrogenation of nitriles proceeds at 100 ^oC with a catalyst loading of 2 mol % and a hydrogen pressure of 50 bar. Mechanistic insights are provided by means of DFT calculations.

Old Concepts, New Application: Additive-free Hydrogenation of Nitriles Catalyzed by a Bench-Stable Alkyl Mn(I) Complex, Weber, S.; Veiros, L. F.; Kirchner, K. Adv. Synth. Catal. 2019, 361, 5412-5420.
DOI: 10.1002/adsc.201901040

An efficient manganese-catalyzed dimerization of terminal alkynes to afford 1,3-enynes is described. This reaction is atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. The pre-catalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate which undergoes rapid C-H bond cleavage of the alkyne forming an active Mn(I) acetylide catalyst [Mn(dippe)(CO)₂(C≡CPh)(η²-HC≡CPh)] together with liberated butanal. A range of aromatic and aliphatic terminal alkynes were efficiently and selectively converted into head-to-head Z-1,3-enynes and head-to-tail gem-1,3-enynes, respectively, in good to excellent yields. Moreover, cross-coupling of aromatic and aliphatic alkynes yields selectively head-to-tail gem-1,3-enynes. In all cases, the reactions were performed at 70 ^°C with a catalyst loading of 1-2 mol %. A mechanism based on DFT calculations is presented.

Selective Manganese-Catalyzed Dimerization and Cross Coupling of Terminal Alkynes, Weber, S.; Veiros, L. F.; Kirchner, K. ACS Catal. 2021, 11,6474-6483.
DOI: 10.1021/acscatal.1c01137

We report on an additive-free Mn(I)-catalyzed dehydrogenative silylation of terminal alkenes. The most active pre-catalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate which undergoes rapid Si-H bond cleavage of the silane HSiR₃ forming the active 16e^- Mn(I) silyl catalyst [Mn(dippe)(CO)₂(SiR₃)] together with liberated butanal. A broad variety of aromatic and aliphatic alkenes was efficiently and selectively converted into E-vinylsilanes and allylsilanes, respectively, at room temperature. Mechanistic insights are provided based on experimental data and DFT calculations revealing that two parallel reaction pathways are operative: an acceptorless reaction pathway involving dihydrogen release and a pathway requiring an alkene as sacrificial hydrogen acceptor.

Manganese-Catalyzed Dehydrogenative Silylation of Alkenes Following two Parallel Pathways, Weber, S.; Glavic, M.; Stöger, B.; Pittenauer, E.; Veiros, L. F.; Kirchner, K. J. Am. Chem. Soc. 2021, 143, 17825-17832.
DOI: 10.1021/jacs.1c09175

A Mn(I)-catalyzed hydroboration of terminal alkenes and a 1,2-diboration of terminal alkynes with pinacolborane (HBPin) is described. In the case of alkenes anti-Markovnikov hydroboration takes place, while in the case of alkynes the reaction proceeds with excellent trans-1,2-selectivity. The most active pre-catalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate which undergoes B-H bond cleavage of HBPin (in the case of alkenes) and rapid C-H bond cleavage (in the case of alkynes) forming the active Mn(I) boryl and acetylide catalysts [Mn(dippe)(CO)₂(BPin)] and [Mn(dippe)(CO)₂(C≡CR)], respectively. A broad variety of aromatic and aliphatic alkenes and alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations revealing that an acceptorless reaction is operating involving dihydrogen release.

Hydroboration of Terminal Alkenes and trans-1,2-Diboration of Terminal Alkynes Catalyzed by a Mn(I) Alkyl Complex, Weber, S.; Zobernig, D. P.; Stöger, B.; Veiros, L. F.; Kirchner, K. Angew. Chem., Int. Ed. 2021, 60, 24488-24492.
DOI: 10.1002/anie.202110736

Non-classical iron(II) polyhydride complexes supported by PNP-pincer ligands were found to be very active catalysts for the Z-selective hydroboration and the head-to-head dimerization of terminal alkynes. Iron(II) (bis)acetylide complexes are identified as catalytically active species in both transformations which, however, proceed via different reaction pathways. These species as well as intermediates relevant for the elucidation of selectivity could be trapped and fully characterized spectroscopically. X-ray structures of key intermediates are presented. Detailed DFT-calculations provide a deeper insight into the activation of the pre-catalysts and comprehensive catalytic cycles are elucidated. The proposed mechanisms provide a consistent picture of the different reaction pathways and fully explain the stereoselective steps of these transformations.

Efficient Z-Selective Semihydrogenation of Internal Alkynes Catalyzed by Cationic Iron(II) Hydride Complexes, Gorgas, N.; Brünig, J.; Stöger, B.; Veiros, L. F.; Kirchner, K. J. Am. Chem. Soc. 2019, 141, 17452-17458.
DOI: 10.1021/jacs.9b09907

Stable, Yet Highly Reactive Non-classical Polyhydride Iron Pincer Complexes - Z-Selective Dimerization and Hydroboration of Terminal Alkynes, Gorgas, N.; Alves, L. G.; Stöger, B.; Martins, A. M.; Veiros, L. F.; Kirchner, K. J. Am. Chem. Soc. 2017, 139, 8130-8133.
DOI: 10.1021/jacs.7b05051

The synthesis, characterization and catalytic activity of low-spin {CoNO}⁸ pincer complexes of the type [Co(PCP)(NO)(H)] is described. These compounds are obtained either by reacting [Co(PCP)(κ²-BH₄)] with NO and Et₃N or, alternatively, by reacting [Co(PCP)(NO)]⁺ with boranes, such as NH₃⸱BH₃ in solution. The five-coordinate, diamagnetic Co(III) complex [Co(PCP^NMe-iPr)(NO)(H)] was found to be the active species in the hydroboration of alkenes with anti-Markovnikov selectivity. A range of aromatic and aliphatic alkenes were efficiently converted with pinacolborane (HBpin) under mild conditions in good to excellent yield. Mechanistic insight into the catalytic reaction is provided by means of isotope labelling, NMR spectroscopy and APCI/ESI-MS as well as DFT calculations.

Synthesis, Characterization, and Catalytic Reactivity of {CoNO}⁸ PCP Pincer Complexes”, Pecak, J.; Eder, W.; Stöger, B.; Realista, S.; Martinho, P. N.; Calhorda, M. J.; Linert, W.; Kirchner, K. Organometallics 2020, 39, 2594-2601.
DOI: 10.1021/acs.organomet.0c00167

Synthesis, Characterization, and Catalytic Reactivity of {CoNO}⁸ PCP Pincer Complexes, Pecak, J.; Eder, W.; Stöger, B.; Realista, S.; Martinho, P. N.; Calhorda, M. J.; Linert, W.; Kirchner, K. Organometallics 2020, 39, 2594-2601. 
DOI: 10.1021/acs.organomet.0c00167

The reaction of [Cr(CO)₆] with the ligand precursor PO(C-Br)OP-tBu (1a) was investigated. When a suspension of [Cr(CO)₆] and 1a in toluene was transferred into a sealed microwave glass vial and stirred for 3 h at 180 ^oC the square-planar Cr(II) complex [Cr(POCOP-tBu)Br] (2a) was obtained. Treatment of 2a with 1 equiv of LiBH₄ in THF led to the formation of the borohydride complex [Cr(POCOP-tBu)(κ²-BH₄)] (3). Exposure of a toluene solution of 3 to NO gas (1 bar) at room temperature affords the Cr(I) complex [Cr(POCOP-tBu)(NO)(κ²-BH₄)] (4). Alternatively, 4 was also obtained by reacting [Cr(POCOP-tBu)(NO)(Br)] (5) with LiBH₄. Based on magnetic and EPR measurements as well as DFT calculations, compounds 4 and 5 adopt a low-spin d⁵ configuration and feature a nearly linear bound NO ligand suggesting Cr^INO⁺ rather than Cr^IINO^• character. The reaction of 2a with 1 equiv of LiCH₂SiMe₃ in toluene afforded the square planar alkyl complex [Cr(POCOP-tBu)(CH₂SiMe₃)] (6) in 57% yield. This compound is catalytically active for the hydrosilylation of ketones at room temperature with a catalyst loading of 0.5 mol%. X-ray structures of all complexes are presented.

Cr(II) and Cr(I) PCP Pincer Complexes: Synthesis, Structure, and Catalytic Reactivity, Himmelbauer, D.; Stöger, B.; Pignitter, M.; Veiros, L. F.; Kirchner, K. Organometallics 2019, 38, 4669-4678.
DOI: 10.1021/acs.organomet.9b00651

Non-symmetrical nickel PCN pincer complexes [Ni(^iPrPCN^R)Cl] (R = NMe, CH₂) are obtained by metalation of the benzene-pyridine based pincer ligand ^iPrPCN^R (R = NMe, CH₂) with [Ni(dme)Cl₂] (dme = 1,2-dimethoxyethane). These nickel species afforded the respective borohydride and ethyl complexes [Ni(^iPrPCN^R)L] (L = BH₄, Et) upon treatment with NaBH₄ and EtMgBr (or Et₂Mg), respectively. Reacting [Ni(^iPrPCN^R)(κ²-BH₄)] with CO₂ gave the formate complexes [Ni(^iPrPCN^R)(OCHO)]. Treatment of [Ni(^iPrPCN^R)Cl] with the nitrosyl salt NOBF₄ led to the formation of unusual cationic square-pyramidal nickel nitrosyl pincer complexes [Ni(^iPrPCN^R)(NO)Cl]⁺ featuring a strongly bent NO ligand (Ni-N-O ∠ 128.5^o). These systems can be described as {NiNO}¹⁰ according to the Enemark-Feltham convention. Despite of the bent nature of the NO ligand the ν_NO is observed at 1849 cm^-1 which is more typical for a linear Ni-N-O arrangement. The structure of these complexes is rationalized by means of DFT calculations. The molecular structures of representative complexes are presented.

Non-Symmetric Benzene-Pyridine-Based Nickel Pincer Complexes featuring Borohydride, Formate, Ethyl and Nitrosyl Ligands, Himmelbauer, D.; Schratzberger, H.; Käfer, M. G.; Stöger, B.; Veiros, L. F.; Kirchner, K. Organometallics 2021, 40, 3331-3340.
DOI: 10.1021/acs.organomet.1c00441