Catalysis with Non-Precious Transition Metal Complexes

The development of sustainable, more efficient and selective organic synthesis is one of the fundamental research goals in chemistry. In this respect, catalysis is a key technology, since a large amount of chemical and pharmaceutical products on a laboratory and on an industrial scale are made by catalysts. However, most catalytic reactions today rely on precious metals such as ruthenium, rhodium, iridium, or palladium.

Accordingly, to develop greener, safer, and more cost-effective chemical processes it is highly important to replace precious metal catalysts by earth abundant non-precious (base) metal catalysts. Our research aims at the discovery, development, and implementation of new catalytic methodologies for opening the door to the sustainable production of pharmaceuticals and fine chemicals. Important are new catalytic approaches to efficient C-H, C-C, and C-heteroatom bond-forming reactions with well-defined non-precious metal catalysts with emphasis on Cr, Mn, Fe, Co, Ni, and Mo-based systems.

We intend to resolve the essential structural features of well-defined molecular homogenous catalysts in solution and mechanistic details of their function. This will include the development of new processes for the selective and sustainable transformation of abundant small molecules such as H2, O2, or CO2 into high-value chemical feedstocks and energy resources. Since alcohols are accessible from indigestible biomass (lignocellulose), the development of novel catalytic reactions in which alcohols are converted into important classes of fine chemicals is a central topic of sustainable synthesis and thus plays a central role in our research. Based upon a deeper understanding and upon new insights into relationships between molecular structures and functional principles of catalysts, we hope to gain access to a rational design of new base metal catalyst.

Manganese Alkyl Carbonyl Chemistry

Hydrogenation of Ketones

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

Hydrogenation of Alkenes

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 60oC. 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

Hydrogenation of Nitriles

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

Dimerization and Cross Coupling of Terminal Alkynes

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

Dehydrogenative Silylation of Alkenes

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

Hydroboration of Terminal Alkenes and trans-1,2-Diboration of Terminal Alkynes

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

Base Metal Pincer Chemistry

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(PCPNMe-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 CrINO+ rather than CrIINO 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(iPrPCNR)Cl] (R = NMe, CH2) are obtained by metalation of the benzene-pyridine based pincer ligand iPrPCNR (R = NMe, CH2) with [Ni(dme)Cl2] (dme = 1,2-dimethoxyethane). These nickel species afforded the respective borohydride and ethyl complexes [Ni(iPrPCNR)L] (L = BH4, Et) upon treatment with NaBH4 and EtMgBr (or Et2Mg), respectively. Reacting [Ni(iPrPCNR)(κ2-BH4)] with CO2 gave the formate complexes [Ni(iPrPCNR)(OCHO)]. Treatment of [Ni(iPrPCNR)Cl] with the nitrosyl salt NOBF4 led to the formation of unusual cationic square-pyramidal nickel nitrosyl pincer complexes [Ni(iPrPCNR)(NO)Cl]+ featuring a strongly bent NO ligand (Ni-N-O ∠ 128.5o). These systems can be described as {NiNO}10 according to the Enemark-Feltham convention. Despite of the bent nature of the NO ligand the νNO 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

Head of Research Group

Karl Kirchner

Prof. Karl Kirchner

karl.kirchner@tuwien.ac.at

Getreidemarkt 9/163
BC02A08
1060 Vienna