Nitrogen Fixation by Manganese Complexes - Waiting for the Rush?

Manganese is currently experiencing a great deal of attention in homogeneous catalysis as a sustainable alternative to platinum group metals due to its abundance, affordable price and low toxicity. While homogeneous nitrogen fixation employing well-defined transition metal complexes has been an important part of coordination chemistry, manganese derivatives have been only sporadically used in this research area. In this contribution, the authors systematically cover manganese organometallic chemistry related to N2 activation spanning almost 60 years, identify apparent pitfalls and outline encouraging perspectives for its future development.

Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation.

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(Ph2PCH2NHC)Mn(CO)3Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000. Mechanistic investigation combining experimental and computational studies allowed to rationalize this beneficial effect as an enhanced stabilization of reaction intermediates including anionic hydride species fac-[(Ph2PC(Ph)NHC)Mn(CO)3H]- playing a crucial role in the hydrogenation process. These results highlight the interest of such carbon bridge substitution strategy being rarely employed in the design of chemically non-innocent ligands.

Substituents' Effect on the Photophysics of Trinuclear Copper(I) and Silver(I) Pyrazolate-Phosphine Cages.

A series of structurally similar trinuclear macrocyclic copper(I) and silver(I) pyrazolate complexes bearing various short-bite diphosphine R2PCH(R')PR2 ligands are reported. Upon diphosphine coordination, the planar geometry of the initial complexes undergoes bending along the line between two metal atoms coordinated to the phosphorus moieties. The complexes based on dcpm ligands (R = cyclohexyl, R' = H, Ph) do not exhibit dynamic behavior in solution at room temperature on the 31P NMR time scale as it was previously observed for similar trinuclear copper complexes bearing the dppm (R = Ph, R' = H) scaffold. All copper(I) complexes exhibit thermally activated delayed fluorescence (TADF) behavior in the solid state. Importantly, the use of aliphatic substituents on the phosphorus atoms instead of aromatic ones leads to an almost double increase in the quantum efficiency (ΦPL) of photoluminescence by eliminating nonradiative decay from the 3LCPh states of the dppm aromatic rings. The higher donating ability of the substituents in the pyrazolate ligand (CF3 vs CH3) lowers the energy of the metal-centered excited state, allowing for a significant metal impact on the T1 state. Finally, the Ag(I) complex displays an emission efficiency of approximately 14%, being the highest among known trinuclear silver(I) pyrazolate homometallic derivatives.

Phosphine-NHC-Phosphonium Ylide Pincer Ligand: Complexation with Pd(II) and Unconventional P-Coordination of the Ylide Moiety.

An efficient synthesis of two pincer preligands [Ph2PCH(R)ImCH2CH2CH2PPh3]X2 (R = H, X = OTf; R = Ph, X = BF4) was developed. Subsequent reactions with PdCl2 and an excess of Cs2CO3 led to the formation of highly stable cationic ortho-metalated Pd(II) complexes [(P,C,C,C)Pd]X exhibiting phosphine, NHC, phosphonium ylide, and σ-aryl donor extremities. The protonation of one of the latter complexes with R = H affords the Pd(II) complex [(P,C,C)Pd(MeCN)](OTf)2 bearing an unprecedented nonsymmetrical NHC core pincer scaffold with a 5,6-chelating framework. The overall donor properties of this phosphine-NHC-phosphonium ylide ligand were estimated using the experimental νCN stretching frequency in the corresponding [(P,C,C)Pd(CNtBu](OTf)2 derivative and were shown to be competitive with the related bis(NHC)-phosphonium ylide and phenoxy-NHC-phosphonium ylide pincers. The presence of a phenyl substituent in the bridge between phosphine and NHC moieties in the ortho-metalated complex [(P,C,C,C)Pd](BF4) makes possible the deprotonation of this position using LDA to provide a persistent zwitterionic complex [(P,C,C,C)Pd] featuring a rare P-coordinated phosphonium ylide moiety in addition to a conventional C-coordinated one. The comparison of the 31P and 13C NMR data for these C- and P-bound phosphonium ylide fragments within the same molecule was performed for the first time, and the bonding situation in both cases was studied in detail by QTAIM and ELF topological analyses.

The Dichotomy of Mn-H Bond Cleavage and Kinetic Hydricity of Tricarbonyl Manganese Hydride Complexes.

Acid-base characteristics (acidity, pKa, and hydricity, ΔG°H- or kH-) of metal hydride complexes could be a helpful value for forecasting their activity in various catalytic reactions. Polarity of the M-H bond may change radically at the stage of formation of a non-covalent adduct with an acidic/basic partner. This stage is responsible for subsequent hydrogen ion (hydride or proton) transfer. Here, the reaction of tricarbonyl manganese hydrides mer,trans-[L2Mn(CO)3H] (1; L = P(OPh)3, 2; L = PPh3) and fac-[(L-L')Mn(CO)3H] (3, L-L' = Ph2PCH2PPh2 (dppm); 4, L-L' = Ph2PCH2-NHC) with organic bases and Lewis acid (B(C6F5)3) was explored by spectroscopic (IR, NMR) methods to find the conditions for the Mn-H bond repolarization. Complex 1, bearing phosphite ligands, features acidic properties (pKa 21.3) but can serve also as a hydride donor (ΔG≠298K = 19.8 kcal/mol). Complex 3 with pronounced hydride character can be deprotonated with KHMDS at the CH2-bridge position in THF and at the Mn-H position in MeCN. The kinetic hydricity of manganese complexes 1-4 increases in the order mer,trans-[(P(OPh)3)2Mn(CO)3H] (1) < mer,trans-[(PPh3)2Mn(CO)3H] (2) ≈ fac-[(dppm)Mn(CO)3H] (3) < fac-[(Ph2PCH2NHC)Mn(CO)3H] (4), corresponding to the gain of the phosphorus ligand electron-donor properties.

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N2 Ligand in Manganese Dinitrogen Complex [Cp(CO)2 Mn(N2 )].

We have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO)2 Mn(N2 )] (1, Cp=η5 -cyclopentadienyl, C5 H5 ) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N2 )MnCOPh]Li (3) that is stable only below -40 °C. Full characterization of 3 (including single crystal X-ray diffraction) was performed. This complex decomposes quickly above -20 °C with N2 loss to give a phenylate complex [Cp(CO)2 MnPh]Li (2). The latter compound was erroneously formulated as an anionic diazenido compound [Cp(CO)2 MnN(Ph)=N]Li in earlier reports, ruling out the claimed and so-far unique behavior of the N2 ligand in 1. DFT calculations were run to explore both the hypothesized and the experimentally verified reactivity of 1 with PhLi and are fully consistent with our results. Direct attack of a nucleophile on metal-coordinated N2 remains to be demonstrated.

Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes.

The coordination chemistry of the N-heterocyclic carbene ligand IMes(NMe2)2, derived from the well-known IMes ligand by substitution of the carbenic heterocycle with two dimethylamino groups, was investigated with d6 [Mn(I), Fe(II)], d8 [Rh(I)], and d10 [Cu(I)] transition-metal centers. The redox behavior of the resulting organometallic complexes was studied through a combined experimental/theoretical study, involving electrochemistry, EPR spectroscopy, and DFT calculations. While the complexes [CuCl(IMes(NMe2)2)], [RhCl(COD)(IMes(NMe2)2)], and [FeCp(CO)2 (IMes(NMe2)2)](BF4) exhibit two oxidation waves, the first oxidation wave is fully reversible but only for the first complex the second oxidation wave is reversible. The mono-oxidation event for these complexes occurs on the NHC ligand, with a spin density mainly located on the diaminoethylene NHC-backbone, and has a dramatic effect on the donating properties of the NHC ligand. Conversely, as the Mn(I) center in the complex [MnCp(CO)2 ((IMes(NMe2)2)] is easily oxidizable, the latter complex is first oxidized on the metal center to form the corresponding cationic Mn(II) complex, and the NHC ligand is oxidized in a second reversible oxidation wave.

Direct Access to Palladium(II) Complexes Based on Anionic C,C,C-Phosphonium Ylide Core Pincer Ligand.

The reaction of readily available imidazolium-phosphonium salt [MesIm(CH2)3PPh3](OTf)2 with PdCl2 in the presence of an excess of Cs2CO3 afforded selectively in one step the cationic Pd(II) complex [(C,C,C)Pd(NCMe)](OTf) exhibiting an LX2-type NHC-ylide-aryl C,C,C-pincer ligand via formal triple C-H bond activation. The replacement of labile MeCN in the latter by CNtBu and CO fragments allowed to estimate the overall electronic properties of this phosphonium ylide core pincer scaffold incorporating three different carbon-based donor ends by IR spectroscopy, cyclic voltammetry, and molecular orbital analysis, revealing its significantly higher electron-rich character compared to the structurally close NHC core pincer system with two phosphonium ylide extremities. The pincer complex [(C,C,C)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis. The treatment of isostructural cationic complexes [(C,C,C)Pd(NCMe)](OTf) and [(C,C,C)Pd(CO)](OTf) with (allyl)MgBr and nBuLi led to the formation of zwitterionic phosphonium organopalladates [(C,C,C)PdBr] and [(C,C,C)Pd(COnBu)], respectively.

Experimental and Theoretical Insights into the Electronic Properties of Anionic N-Heterocyclic Dicarbenes through the Rational Synthesis of Their Transition Metal Complexes.

The lithiation of the NHC ligand backbone in Cp(CO)2Mn(IMes) followed by transmetalation on the C4 carbenic position with Cp(CO)2FeI led to the heterobimetallic complex Cp(CO)2Mn(µ-dIMes)Fe(CO)2Cp bearing the anionic ditopic imidazol-2,4-diylidene dIMes ligand. Subsequent treatment of the later with TfOH induced a selective decoordination of the [Cp(CO)2Mn] fragment to form the cationic abnormal NHC complex [Cp(CO)2Fe(aIMes)](OTf), which was further derivatized to the bis(iron) dIMes complex [Cp(CO)2Fe(µ-dIMes)Fe(CO)2Cp](OTf) by reaction with tAmOK and Cp(CO)2FeI. The effect of the metalation at the C4 or C2 position on the imidazole ring on the electronic donation properties of the associated C2 and C4 carbenic centers in the dIMes ligand was quantified through systematic experimental and theoretical studies of IMes, aIMes, and dIMes complexes. The evaluation of the catalytic activity of the series of cationic Fe(II) complexes based on IMes, aIMes, and dIMes ligands in a benchmark ketone hydrosilylation showed the superiority of the bimetallic derivative.

NHC Core Phosphonium Ylide-based Palladium(II) Pincer Complexes: The Second Ylide Extremity Makes the Difference.

The coordinating properties of N-heterocyclic carbene (NHC) (A), phenolate (B), and phosphonium ylide (C) moieties were investigated systematically through the preparation of a family of NHC, phosphonium ylide-based pincer ligands, where the third donor extremity can be either an NHC, a phenolate, or a phosphonium ylide. The overall donor character of such ligands [NHC(AaBbCc)] (a + b + c = 2) was analyzed by comparison of the molecular orbitals (energy and shape), oxidation potentials (Epox), and IR νCO and νCN stretching frequencies of their isostructural pincer Pd(II) complexes [NHC(AaBbCc)PdL][OTf] (L = NCCH3, CO, or CNtBu). The three categories of pincer complexes based on phosphonium ylides were easily obtained by acidic treatment of their highly stable ortho-metalated Pd(II) precursors prepared in a single step from readily available N-phosphonio-substituted imidazolium salts. Analysis of IR data indicated that NHC and phenolate ligands have a similar donor character but which remains lower than that of the phosphonium ylide. The impact on catalytic performance of the incorporation of a second strongly donating phosphonium ylide into the ligand architecture was illustrated in the Pd-catalyzed allylation of aldehydes.

Phosphine-NHC Manganese Hydrogenation Catalyst Exhibiting a Non-Classical Metal-Ligand Cooperative H2 Activation Mode.

Deprotonation of the MnI NHC-phosphine complex fac-[MnBr(CO)3 (κ2 P,C-Ph2 PCH2 NHC)] (2) under a H2 atmosphere readily gives the hydride fac-[MnH(CO)3 (κ2 P,C-Ph2 PCH2 NHC)] (3) via the intermediacy of the highly reactive 18-e NHC-phosphinomethanide complex fac-[Mn(CO)3 (κ3 P,C,C-Ph2 PCHNHC)] (6 a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16-e mangana-substituted phosphonium ylide complex fac-[Mn(CO)3 (κ2 P,C-Ph2 P=CHNHC)] (6 b) as key intermediate able to activate H2 via a non-classical mode of metal-ligand cooperation implying a formal λ5 -P-λ3 -P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre-catalysts for ketone hydrogenation in the MnI series reported to date (TON up to 6200).

Oxidative Coupling of Anionic Abnormal N-Heterocyclic Carbenes: Efficient Access to Janus-Type 4,4'-Bis(2H-imidazol-2-ylidene)s.

The oxidative coupling of anionic imidazol-4-ylidenes protected at the C2 position with [MnCp(CO)2 ] or BH3 led to the corresponding 4,4'-bis(2H-imidazol-2-ylidene) complexes or adducts, in which the two carbene moieties are connected through a single C-C bond. Subsequent acidic treatment of the later species led to the corresponding 4,4'-bis(imidazolium) salts in good yields. The overall procedure offers practical access to a novel class of Janus-type bis(NHC)s. Strikingly, the coplanarity of the two NHC rings within the mesityl derivative 4,4'-bis(IMes), favored by steric hindrance along with stabilizing intramolecular C-H⋅⋅⋅π aryl interactions, allows the alignment of the π-systems and, as a direct consequence, significant electron communication through the bis(carbene) scaffold.

Catalytic Scanning Probe Nanolithography (cSPL): Control of the AFM Parameters in Order to Achieve Sub-100-nm Spatially Resolved Epoxidation of Alkenes Grafted onto a Surface.

Scanning probe lithography (SPL) appears to be a reliable alternative to the use of masks in traditional lithography techniques as it offers the possibility of directly producing specific chemical functionalities with nanoscale spatial control. We have recently extend the range of applications of catalytic SPL (cSPL) by introducing a homogeneous catalyst immobilized on the apex of a scanning probe. Here we investigate the importance of atomic force microscopy (AFM) physical parameters (applied force, writing speed, and interline distance) on the resultant chemical activity in this cSPL methodology through the direct topographic observation of nanostructured surfaces. Indeed, an alkene-terminated self-assembled monolayer (alkene-SAM) on a silicon wafer was locally epoxidized using a scanning probe tip with a covalently grafted manganese complex bearing the 1,4,7-triazacyclononane macrocycle as the ligand. In a post-transformation process, N-octylpiperazine was covalently grafted to the surface via a selective nucleophilic ring-opening reaction. With this procedure, we could write various patterns on the surface with high spatial control. The catalytic AFM probe thus appears to be very robust because a total area close to 500 µm(2) was patterned without any noticeable loss of catalytic activity. Finally, this methodology allowed us to reach a lower lateral line resolution down to 40 nm, thus being competitive and complementary to the other nanolithographical techniques for the nanostructuration of surfaces.

Umpolung of methylenephosphonium ions in their manganese half-sandwich complexes and application to the synthesis of chiral phosphorus-containing ligand scaffolds.

Half-sandwich manganese methylenephosphonium complexes [Cp(CO)2Mn(η(2)-R2P=C(H)Ph)]BF4 were obtained in high yield through a straightforward reaction sequence involving a classical Fischer-type manganese complex and a secondary phosphine as key starting materials. The addition of various nucleophiles (Nu) to these species took place regioselectively at the double-bonded carbon center of the coordinated methylenephosphonium ligand R2P(+)=C(H)Ph to produce the corresponding chiral phosphine complexes [Cp(CO)2Mn(κ(1)-R2P-C(H)(Ph)Nu)], from which the phosphines were ultimately recovered as free entities upon simple irradiation with visible light. The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer-type phosphine-NHC-phosphine ligand architectures.

A direct, modular, and efficient construction of the P-C-P structural motif through coupling of manganese carbyne complexes with phosphines.

Easily available carbyne complexes of manganese were used as a source of carbyne fragments in an unconventional synthesis of backbone-substituted diphosphinomethanes and cyclic P-ylides upon coupling with secondary or tertiary phosphines, respectively, followed by demetalation under mild conditions.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation.

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Fac-to-mer isomerization triggers hydride transfer from Mn(I) complex fac-[(dppm)Mn(CO)3H].

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex fac-[(dppm)Mn(CO)3H] and Lewis acids (LA = Ph3C+, B(C6F5)3) involving the formation of so-far elusive meridional hydride species mer-[(dppm)Mn(CO)3H⋯LA] and unusual dearomatization of the Ph3C+ cation upon hydride transfer.

Carbenes and phosphonium ylides: a fruitful association in coordination chemistry.

Among a plethora of σ-donor ligands available, carbon-centered ones have become essential, in particular with the emergence of N-heterocyclic carbenes (NHCs), positioning themselves as credible alternatives to traditional nitrogen- and phosphorus-based systems. Phosphonium ylides representing another class of neutral η1-bonded carbon ligands have also been shown to act as effective Lewis bases. Considering the intrinsic features of the carbene and phosphonium ylide ligands, similar in terms of electronic properties, but different in terms of bonding mode, the design of hybrid systems combining these two types of carbon functionalities appeared to be a natural and exciting challenge. This Perspective comprehensively covers the chemistry of such ligand architectures from synthesis and fundamental aspects to catalytic applications.

Half-sandwich manganese complexes Cp(CO)2Mn(NHC) as redox-active organometallic fragments.

Oxidation of the half-sandwich MnI complexes Cp(CO)2Mn(NHC) bearing dialkyl-, arylalkyl- and diarylsubstituted N-heterocyclic carbene ligands (NHC = IMe, IMeMes, IMes) affords the corresponding stable MnII radical cations [Cp(CO)2Mn(NHC)](BF4) isolated in 92-95% yield. Systematic X-ray diffraction studies of the series of MnI and MnII NHC complexes revealed the expected characteristic structural changes upon oxidation, namely the elongation of the Mn-CO and Mn-NHC bonds as well as the diminution of the OC-Mn-CO angle. ESR spectra of [Cp(CO)2Mn(IMes)](BF4) in frozen solution (CH2Cl2/toluene 1 : 1, 70 K) allowed the identification of two conformers for this complex and their structural assignment using DFT calculations. The stability of these NHC complexes in both metal oxidation states, moderate oxidation potentials and the ease of detection of MnII species by a variety of spectroscopic techniques (UV-Vis, IR, paramagnetic 1H NMR, and ESR) make these compounds promising objects for applications as redox-active organometallic fragments.

Bis[diphenylphosphino]methane and its bridge-substituted analogues as chemically non-innocent ligands for H2 activation.

Deprotonation of fac-[(κ2P,P-Ph2PCH(R)PPh2)Mn(CO)3Br] (R = H, Me, Ph) produces the corresponding diphosphinomethanide derivatives fac-[(κ3P,C,P-Ph2PC(R)PPh2)Mn(CO)3], which are prone to activate H2 to form the hydride complexes fac-[(κ2P,P-Ph2PCH(R)PPh2)Mn(CO)3H]. The substitution of the dppm bridge improves dramatically the reaction efficiency and this was rationalized by DFT calculations.