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A cationic iron(III) complex was active for the polymerization of various epoxides, whereas the analogous neutral iron(II) complex was inactive. Cyclohexene oxide polymerization could be "switched off" upon in situ reduction of the iron(III) catalyst and "switched on" upon in situ oxidation, which is orthogonal to what was observed previously for lactide polymerization. Conducting copolymerization reactions in the presence of both monomers resulted in block copolymers whose identity can be controlled by the oxidation state of the catalyst: selective lactide polymerization was observed in the iron(II) oxidation state and selective epoxide polymerization was observed in the iron(III) oxidation state. Evidence for the formation of block copolymers was obtained from solubility differences, GPC, and DOSY-NMR studies.

It has previously been demonstrated that complexes of the form (iPrPNP)­Fe­(H)­(CNR) (iPrPNP = N­(CH2CH2P­(iPr)2)2 –, R = 2,6-dimethylphenyl or 4-methoxyphenyl), which contain a pincer ligand capable of metal–ligand cooperation (MLC), are active for CO2 hydrogenation. Herein, the synthesis and catalytic activity of a second-generation of precatalysts containing a tertiary amine ligand, which cannot participate in MLC, are presented. Specifically, the complexes (iPrPNMeP)­Fe­(H)­(HBH3)­(CNR) (iPrPNMeP = MeN­(CH2CH2P­(iPr)2)2, R = 2,6-dimethylphenyl (2a), tert-butyl (2b), or adamantyl (2c)) have been prepared and crystallographically characterized. These complexes are precatalysts for both formic acid dehydrogenation and CO2 hydrogenation to formate, and give improved activity compared to first-generation systems with isonitrile ligands. The second-generation systems 2a–c, however, give inferior activity compared to the related carbonyl complexes (iPrPNP)­Fe­(H)­(CO) and (iPrPNMeP)­Fe­(H)­(HBH3)­(CO), which have been previously reported. This study demonstrates that a ligand which can participate in MLC is not universally advantageous for promoting the hydrogenation and dehydrogenation reactions studied in this work and provides guidance for the rational design of improved catalysts for reactions relevant to energy storage.

Surface functionalization with organic electron donors (OEDs) is an effective doping strategy for 2D materials, which can achieve doping levels beyond those possible with conventional electric field gating. While the effectiveness of surface functionalization has been demonstrated in many 2D systems, the doping efficiencies of OEDs have largely been unmeasured, which is in stark contrast to their precision syntheses and tailored redox potentials. Here, using monolayer MoS2 as a model system and an organic reductant based on 4,4′‐bipyridine (DMAP‐OED) as a strong organic dopant, it is established that the doping efficiency of DMAP‐OED to MoS2 is in the range of 0.63 to 1.26 electrons per molecule. The highest doping levels to date are also achieved in monolayer MoS2 by surface functionalization and demonstrate that DMAP‐OED is a stronger dopant than benzyl viologen, which is the previous best OED dopant. The measured range of the doping efficiency is in good agreement with the values predicted from first‐principles calculations. This work provides a basis for the rational design of OEDs for high‐level doping of 2D materials.

The iron complex (iPrPNMeP)Fe(H)2(CO) (iPrPNMeP=CH3N(CH2CH2PiPr2)2), which features a pincer ligand with a tertiary amine, can give up to 100,000 turnovers for additive‐free formic acid dehydrogenation (FADH). This is two orders of magnitude higher than any previously reported base metal system. Mechanistic studies reveal the catalytic reaction pathway and provide guidance for the development of improved catalytic systems for additive‐free FADH.

2 ) 2 ], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-containing complexes were isolated and characterized by NMR and IR spectroscopies, as well as X-ray diffraction. Comparisons to previously published analogues ligated by iPr PN H P and iPr PN Me P [CH 3 N(CH 2 CH 2 P i Pr 2 ) 2 ] illustrate that there are large changes in the coordination chemistry that occur when the nitrogen substituent of the pincer ligand is altered. For example, ruthenium hydrides supported by the iPr PN Ph P ligand always form the syn isomer (where syn/anti refer to the relative orientation of the group on nitrogen and the hydride ligand on ruthenium), whereas complexes supported by iPr PN H P form the anti isomer and complexes supported by iPr PN Me P form a mixture of syn and anti isomers. We evaluated the impact of the nitrogen substituent of the pincer ligand in catalysis by comparing a series of iPr PN R P (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO 2 ) hydrogenation to formate. The iPr PN Ph P-ligated species is the most active for formic acid dehydrogenation, and mechanistic studies suggest that this is likely because there are kinetic advantages for catalysts that operate via the syn isomer. In CO 2 hydrogenation, the iPr PN Ph P-ligated species is again the most active under our optimal conditions, and we report some of the highest turnover frequencies for homogeneous catalysts. Experimental and theoretical insights into the turnover-limiting step of catalysis provide a basis for the observed trends in catalytic activity. Additionally, the stability of our complexes enabled us to detect a previously unobserved autocatalytic effect involving the base that is added to drive the reaction. Overall, by modifying the nitrogen substituent on the MACHO ligand, we have developed highly active catalysts for formic acid dehydrogenation and CO 2 hydrogenation and also provided a framework for future catalyst development.

The iron pincer complex (iPrPNP)­Fe­(H)­(CO) (1, iPrPNP– = N­(CH2CH2PiPr2)2 –) is an active (pre)­catalyst for many hydrogenation and dehydrogenation reactions. This is in part because 1 can reversibly add H2 across the iron-amide bond to form (iPrPNHP)­Fe­(H)2(CO) (2, iPrPNHP = HN­(CH2CH2PiPr2)2). However, rapid decomposition limits the catalytic performance of 1 and related complexes. We explored the pathways through which catalytic intermediates related to 1 and 2 undergo decomposition. This involved characterizing the unstable and previously unobserved complexes [(iPrPNHP)­Fe­(H)­(CO)­(L)]+ (5-L; L = THF or N2) and [(iPrPNHP)­Fe­(H)­(H2)­(CO)]+ (8), which are proposed as intermediates when 1 and 2 are used as catalysts. Compound 8 was synthesized through the reaction of (iPrPNHP)­Fe­(H)­(CO)­(PF6) (6) with H2, and the solid-state structure was established using both X-ray and neutron diffraction. As part of our studies on understanding the reactivity of 5-L, we determined the thermodynamic hydricity of 2, which is valuable for predicting its reactivity as a hydride donor. Further, it is shown that species such as 5-L decompose to the same inactive species observed in catalysis using 1 and 2, and theoretical calculations suggest that this likely occurs via a bimolecular pathway. To provide support for this hypothesis, we isolated the dimeric species [{(iPrPNHP)­Fe­(H)­(CO)}2{μ-CN}]+ (11) and [{(iPrPNHP)­Fe­(H)­(CO)}2{μ-OC­(H)­O}]+ (12), which show that catalytic intermediates ligated by iPrPNHP can form dimeric species. Our results provide general strategies for improving catalysis using 1 and 2, and we used this information to rationally increase the performance of 1 in formic acid dehydrogenation.

A pair of manganese complexes containing MACHO-type pincer ligands bearing a secondary amine, [HN­{CH2CH2(P i Pr2)}2]­MnH­(CO)2, which can participate in pathways involving metal–ligand cooperation (MLC), and a tertiary amine, [MeN­{CH2CH2(P i Pr2)}2]­MnH­(CO)2, which cannot participate in pathways involving MLC, are compared for the hydrogenation of CO2 to formate in the presence of a base. Lewis acid cocatalysts are crucial for increasing the activity of both catalysts, with [MeN­{CH2CH2(P i Pr2)}2]­MnH­(CO)2 reaching TONs of up to 18,300 and yields of up to 73% in the presence of lithium triflate. This productivity is far greater than for the MLC capable secondary amine MACHO-supported manganese catalyst. Preliminary mechanistic experiments indicate that CO2 insertion into the Mn–H of each catalyst affords a stable manganese formate complex. In situ NMR spectroscopy and comparative catalytic experiments are consistent with the intermediacy of these manganese formate complexes in the catalytic cycle, likely representing the catalyst resting states. Our findings suggest that the tertiary amine ligated system gives greater productivity due to a combination of longer catalyst lifetime and greater enhancement from Lewis acid additives.