The Pincer Platform Beyond Classical Coordination Patterns

Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations

Enhancing catalytic activity through synergic effects is a current challenge in homogeneous catalysis. In addition to the well-established metal-metal and metal-ligand cooperation, we showcase here an example of self-activation by the substrate in controlling the catalytic activity of the two-coordinate iron complex [Fe(2,6-Xyl₂C₆H₃)₂] (1, Xyl = 2,6-Me₂C₆H₃). This behavior was observed for aryl acetylenes in their regioselective cyclotrimerization to 1,2,4-(aryl)-benzenes. Two kinetically distinct regimes are observed dependent upon the substrate-to-catalyst ratio ([RC≡CH]₀/[1]₀), referred to as the low ([RC≡CH]₀/[1]₀ < 40) and high ([RC≡CH]₀/[1]₀ > 40) regimes. Both showed sigmoidal kinetic response, with positive Hill indices of 1.85 and 3.62, respectively, and nonlinear Lineweaver-Burk replots with an upward curvature, which supports positive substrate cooperativity. Moreover, two alkyne molecules participate in the low regime, whereas up to four are involved in the high regime. The second-order rate dependence on 1 indicates that binuclear complexes are the catalytically competent species in both regimes, with that in the high one being 6 times faster than that involved in the low one. Moreover, Eyring plot analyses revealed two different catalytic cycles, with a rate-determining step more endergonic in the low regime than in the high one, but with a more ordered transition state in the high regime than in the low one.

A hampered oxidative addition of pre-coordinated pincer ligands can favour alternative pathways of activation

Pre-coordination to a transition metal by the terminal donor groups of a tri-dentate ligand is a common strategy to stabilise elusive groups, to achieve unprecedented bond activation and to develop novel modes of metal-ligand-cooperation for catalysis. In the current manuscript, we demonstrate that the oxidative addition of a central E-H-bond after pre-coordination to the metal centre is disfavoured for metals with d¹⁰ electron configuration. For exemplary pincer ligands and metals with d¹⁰ electron configuration, quantum chemical calculations suggest a second barrier, which is associated with the rearrangement of the saw-horse structure, obtained after oxidative addition, to the expected square planar geometry for the resulting d⁸ electron configuration. In the case of PBP-type ligands with a central L₂BH₂-group (L = R₃P) the reaction with Pt⁰ precursors proceeds via an alternative pathway of activation, which involves the backside attack of a nucleophile to the boron atom, which facilitates the nucleophilic attack of the Pt⁰ centre and formation of a boryl complex (LBH₂). As the corresponding reaction with a Pt^II precursor leads to B-H- instead of B-L-activation and formation of complex 2 with a L₂BH donor, our results show that ligand-stabilized borylenes (L₂BH) can in principle be converted to boryls (LBH₂) via boronium salts (L₂BH₂⁺).

Theory-guided development of homogeneous catalysts for the reduction of CO2 to formate, formaldehyde, and methanol derivatives

The stepwise catalytic reduction of carbon dioxide (CO₂) to formic acid, formaldehyde, and methanol opens non-fossil pathways to important platform chemicals. The present article aims at identifying molecular control parameters to steer the selectivity to the three distinct reduction levels using organometallic catalysts of earth-abundant first-row metals. A linear scaling relationship was developed to map the intrinsic reactivity of 3d transition metal pincer complexes to their activity and selectivity in CO₂ hydrosilylation. The hydride affinity of the catalysts was used as a descriptor to predict activity/selectivity trends in a composite volcano picture, and the outstanding properties of cobalt complexes bearing bis(phosphino)triazine PNP-type pincer ligands to reach the three reduction levels selectively under different reaction conditions could thus be rationalized. The implications of the composite volcano picture were successfully experimentally validated with selected catalysts, and the challenging intermediate level of formaldehyde could be accessed in over 80% yield with the cobalt complex 6. The results underpin the potential of tandem computational-experimental approaches to propel catalyst design for CO₂-based chemical transformations.

A 1,2,3-triazole-derived pincer-type mesoionic carbene complex of iron(II): carbonyl elimination and hydrosilylation of aromatic aldehydes via the concerted reaction with hydrosilane and a base

Iron complexes bearing 1,2,3-triazol-5-ylidene were synthesized and applied to the reaction with hydrosilane and homogeneous catalytic hydrosilylation of aromatic ketones and aldehydes. Addition of a free carbene to a solution of Fe(CO)₄Br₂ yielded an octahedral, diamagnetic and cationic iron(II) complex [Fe(1,2,3-triazolylidene)(CO)₂Br]⁺. Pyrolysis of the dicarbonyl complex eliminated the two CO ligands to form a paramagnetic four-coordinate complex. A theoretical study using DFT calculations indicated that the spin state changed from singlet to quintet during ligand elimination. Investigations of the successful hydrosilylation of acetophenone and benzaldehyde derivatives using MIC-iron(II) bromide suggested the importance of the base for efficient conversion in the catalytic process. The bromide-to-hydride exchange reaction, transmetallation, of MIC-iron(II) bromide in the presence of KO^tBu and HSi(OEt)₃ which could occur in the initial process of hydrosilylation was proposed, and supported by a theoretical study.

Experimental and Computational Studies of Ruthenium Complexes Bearing Z-Acceptor Aluminum-Based Phosphine Pincer Ligands

Reaction of [Ru(C₆H₄PPh₂)₂(Ph₂PC₆H₄AlMe(THF))H] with CO results in clean conversion to the Ru-Al heterobimetallic complex [Ru(AlMePhos)(CO)₃] (1), where AlMePhos is the novel P-Al(Me)-P pincer ligand (o-Ph₂PC₆H₄)₂AlMe. Under photolytic conditions, 1 reacts with H₂ to give [Ru(AlMePhos)(CO)₂(μ-H)H] (2) that is characterized by multinuclear NMR and IR spectroscopies. DFT calculations indicate that 2 features one terminal and one bridging hydride that are respectively anti and syn to the AlMe group. Calculations also define a mechanism for H₂ addition to 1 and predict facile hydride exchange in 2 that is also observed experimentally. Reaction of 1 with B(C₆F₅)₃ results in Me abstraction to form the ion pair [Ru(AlPhos)(CO)₃][MeB(C₆F₅)₃] (4) featuring a cationic [(o-Ph₂PC₆H₄)₂Al]⁺ ligand, [AlPhos]⁺. The Ru-Al distance in 4 (2.5334(16) Å) is significantly shorter than that in 1 (2.6578(6) Å), consistent with an enhanced Lewis acidity of the [AlPhos]⁺ ligand. This is corroborated by a blue shift in both the observed and computed ν_CO stretching frequencies upon Me abstraction. Electronic structure analyses (QTAIM and EDA-ETS) comparing 1, 4, and the previously reported [Ru(ZnPhos)(CO)₃] analogue (ZnPhos = (o-Ph₂PC₆H₄)₂Zn) indicate that the Lewis acidity of these pincer ligands increases along the series ZnPhos < AlMePhos < [AlPhos]⁺.

Nickel and Palladium Complexes of a PP(O)P Pincer Ligand Based upon a peri-Substituted Acenaphthyl Scaffold and a Secondary Phosphine Oxide

A PP(O)P pincer ligand based upon a peri-substituted acenaphthyl (Ace) scaffold and a secondary phosphine oxide, (5-Ph₂P-Ace-6-)₂P(O)H, was prepared and fully characterized including a neutron diffraction study. The reaction with [Ni(H₂O)₆]Cl₂ and PdCl₂ produced ionic metal(II) complexes [κ³-P,P',P''((5-Ph₂P-Ace-6-)₂P(OH))MCl]Cl, which upon addition of Et₃N gave rise to zwitterionic metal(II) complexes κ³-P,P',P''((5-Ph₂P-Ace-6-)₂P(O))MCl (M = Ni, Pd). The reaction with Ni(COD)₂ (COD = cyclooctadiene) provided the η³-cyclooctenyl Ni(II) complex κ³-P,P',P''((5-Ph₂P-Ace-6-)₂P(O))Ni(η³-C₈H₁₃). A detailed complementary bonding analysis of the P-H, P-O, and P-M interactions was carried out (M = Ni, Pd).

Nucleophilic Reactivity at a ═CH Arm of a Lutidine-Based CNC/Rh System: Unusual Alkyne and CO2 Activation

Reaction of an amido pincer complex [(CNC)*Rh(CO)] (1) (CNC* is the deprotonated form of CNC) with carbon dioxide gave a neutral complex [(CNC-CO₂)^Mes*Rh(CO)] (2), which is the result of a C-C bond-forming reaction between the deprotonated arm of the CNC* ligand and CO₂. The molecular structure of 2 showed a zwitterionic complex, where the CO₂ moiety is covalently connected to the former ═CH arm of the CNC* pincer ligand. The unusual structure of 1 allowed us to explore the reactivity of the CO₂ moiety with selected primary amines RNH₂ (benzylamine and ammonia), which afforded cationic complexes [(CNC)^MesRh(CO)][HRNC(O)O] (R = Bz (3), H (4)). Compounds 3 and 4 are the result of a C-N coupling between the incoming amine and the CO₂ fragment covalently connected to the pincer ligand in 2, a process that involves protonation of the "CH-CO₂" fragment in 2 from the respective amines. Once revealed the nucleophilic character of the ═CH fragment in 1, we explored its reactivity with alkynes, a study that enlightened a novel reactivity trend in alkyne activation. Reaction of 1 with terminal alkynes RC≡CH (R = Ph, 2-py, 4-C₆H₄-CF₃) yielded neutral complexes [(CNC-CH═CHR)^Mes*Rh(CO)] (R = Ph (5), 2-py (6), 4-C₆H₄-CF₃ (7)) in good yields. Deuterium labeling experiments with PhC≡CD confirmed that complex 5 is the product of a formal insertion of the alkyne into the C(sp²)-H bond of the deprotonated arm in 1. This structural proposal was further confirmed by the X-ray molecular structure of phenyl complex 5, which showed the alkyne covalently linked to the pincer ligand. Besides, this novel transformation was analyzed by DFT methods and showed a metal-ligand cooperative mechanism, based on the initial electrophilic attack of the alkyne to the ═CH arm of the CNC^Mes* ligand (making a new C-C bond) followed by the action of a protic base (HN(SiMe₃)₂), which is able to perform a proton rearrangement that leads to the final product 5.

Spin-state crossover in photo-catalyzed nitrile dihydroboration via Mn-thiolate cooperation

The role of a phosphine-free SNS-pincer ligand in metal–ligand cooperative hydroboration catalysis was investigated. The bifunctional thiolate donor and spin-state change to high-spin Mn are crucial to accessing low-energy activation barriers.

Chemically robust and readily available quinoline-based PNN iron complexes: application in C-H borylation of arenes

Iron catalysts have been used for over a century to produce ammonia industrially. However, the use of iron catalysts generally remained quite limited until relatively recently, when the abundance and low toxicity of iron spurred the development of a variety of iron catalysts. Despite the fact that iron catalysts are being developed as alternatives to precious metal catalysts, their reactivities and stabilities are quite different because of their unique electronic structures. In this context, our group previously developed a new family of quinoline-based PNN pincer-type ligands for low- to mid-valent iron catalysts. These chemically robust PNN ligands provide air- and moisture-tolerant iron complexes, which exhibit excellent catalytic performances in the C-H borylation of arenes. This feature article summarises our recent work on PNN iron complexes, including their conception and design, as well as related reports on iron pincer complexes and iron-catalysed C-H borylation reactions.

Synthesis and Characterization of Cobalt NCN Pincer Complexes

A series of cobalt complexes, stabilized by a monoanionic tridentate NCN pincer ligand, was synthetized and characterized. Preparation of the paramagnetic 15 VE complex [Co(NCNCH2-Et)Br] (1) was accomplished by transmetalation of Li[2,6-(Et2NCH2)2C6H3] with CoBr2 in THF. Treatment of this air-sensitive compound with NO gas resulted in the formation of the diamagnetic Co(III) species [Co(NCNCH2-Et)(NO)Br] (2) as confirmed by X-ray diffraction. This complex features a strongly bent NO ligand (Co-N-O∠135.0°). The νNO is observed at 1609 cm-1 which is typical for a bent metal-N-O arrangement. Coordinatively unsaturated 1 could further be treated with pyridine, isocyanides, phosphines and CO to form five-coordinate 17 VE complexes. Oxidation of 1 with CuBr2 led to the formation of the Co(III) complex [Co(NCNCH2-Et)Br2]. Treatment of [Co(NCNCH2-Et)Br2] with TlBF4 as halide scavenger in acetonitrile led to the formation of the cationic octahedral complex [Co(NCNCH2-Et)(MeCN)3](BF4)2. A combination of X-ray crystallography, IR-, NMR- and EPR-spectroscopy as well as DFT/CAS-SCF calculations were used to characterize all compounds.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.