Transition Metal as Template: Reversing the Synthesis Logic in the Preparation of Pincer Complexes

The conventional synthetic approach to transition metal pincer complexes calls for the preparation of the tridentate pincer (pro)ligand first, with subsequent introduction of the transition metal center as the last step. This work demonstrates that the alternative synthetic logic, where the central main group element is introduced last, can be applicable to a number of PEP pincer complexes (E=B, Al, Si, P) derived from phosphinophenols and phosphinopyrroles. This approach obviates the need to isolate well-behaved propincer precursors, and instead relies on the formation of phosphine-metal adducts first, whose nature determines the stoichiometry of the needed main group reagent to complete the synthesis.

Iridium Complexes of a Bis(N-pyrrolyl)boryl/Bis(phosphine) PBP Pincer Ligand

This work reports the synthesis of a bis(pyrrolylphosphino)phenyl borane (PBP)Ph (2) and its incorporation of Ir by metal insertion into B-Ph to afford the dipyrrolylboryl/bis(phosphine) pincer complex (PBP)Ir(Ph)Cl (3). Hydrogenolysis of 3 afforded (PBP)Ir(H)Cl (4). Compound 4 was converted into (PBP)IrCl2 (5a) via reaction with N-chlorosuccinimide, and exposure of 5a to CO produced (PBP)IrCl2(CO) (6a). Compounds 5a and 6a were converted into their analogs (PBP)IrI2 (5b) and (PBP)IrI2(CO) (6b) via metathesis with Me3SiI, respectively. Treatment of either 3 or 4 with Li[HAl(OtBu)3] under H2 resulted in the formation of (PBP)IrH4 (7), with traces of 4 as a persistent impurity. Attempts to access 7 via the reaction of 4 with NaBH4 in isopropanol led to the loss of boron from the pincer and isolation of L2IrH5 (8, L = 2-diisopropylphosphinopyrrole). Compounds 4, 7, and 8 were examined as catalysts for alkane transfer dehydrogenation but displayed only the modest activity. Solid-state structures of 6b and 7 were established by X-ray crystallography.

C-H Activation of Pyridines by Boryl Pincer Complexes: Elucidation of Boryl-Directed C-H Oxidative Addition to Ir and Discovery of Transition Metal-Assisted Reductive Elimination from Boron at Rh

Experimental and theoretical techniques were used to investigate the mechanism of pyridine C-H activation by diarylboryl/bis(phosphine) PBP pincer complexes of Ir. The critical intermediate (PBP)IrCO (4) contains a three-coordinate, Ir-bound boron that retains Lewis acidity in the perpendicular direction. Coordination of pyridine to this boron center in 4 leads to fast insertion of Ir into the 2-CH bond of pyridine, providing a different topology of direction than the conventional directed C-H activation where both the directing group coordination and C-H activation happen at the same metal center. Beyond this critical sequence, the system possesses significant complexity in terms of possible isomers and pathways, which have been thoroughly explored. Kinetic and thermodynamic preferences for the activation of differently substituted pyridines were also investigated. In experimental work, the key intermediate 4 is accessed via elimination of benzene from a phenyl/hydride containing precursor (PBPhP)IrHCO (3). Density functional theory (DFT) investigations of the mechanism of benzene loss from 3 revealed the possibility of a genuinely new type of mechanism, whereby the Ph-H bond is made in a concerted process that is best described as C-H reductive elimination from boron, assisted by the transition metal (TMARE). For Ir, this pathway was predicted to be competitive with the more conventional pathways involving C-H reductive elimination from Ir, but still higher in energy barrier. However, for the Rh analog 3-Rh, TMARE was calculated to be the preferred pathway for benzene loss and this prediction was experimentally corroborated through the study of reaction rates and the kinetic isotope effect.

Preferred Electric Field Mechanism for Frustrated Lewis Pair Reactivity

This study employs computational methods to investigate the mechanism of H2 activation by frustrated Lewis pair (FLP) species, including both intermolecular and intramolecular nitrothane/borane FLP systems. Previous studies have proposed two qualitative reactivity mechanism models to explain the facile cleavage of H2 by FLPs. The findings of this study support the electric field mechanism as the favorable pathway for H2 cleavage. Utilizing frontier molecular orbital theory and energy decomposition analysis, the study explores the electronic structure and nature of the reactions under an external electric field (EEF). Analysis using the activation strain model highlights the significant influence of geometrical deformation energies of FLPs on the activation barriers of H2 activation reactions. Computational results suggest that H2 activation by FLP molecules follows the electric field mechanism, indicating the potential of the FLP/EEF combination as an effective activator for inert molecules.

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⁺).

Coordination mode and stability of the tetrahydroborate ligand in group 10 metal pincer complexes

The coordination mode of the BH₄^- ligand in transition metal tetrahydroborate complexes is mainly dominated by the nature of the metal centres. However, other factors can also play important roles sometimes. In order to rationalize the coordination modes and the stability of the BH₄^- ligand in group 10 metal tetrahydroborate pincer complexes, [2,6-(^tBu₂PO)₂C₆H₃]Pt(η¹-HBH₃) and [C₆H₄-o-(NCH₂P^tBu₂)₂B]M(η²-H₂BH₂) (M = Ni, Pt) were prepared and characterized. A structural comparison of [2,6-(^tBu₂PCH₂)₂C₆H₃]Ni(BH₄), [2,6-(^tBu₂PO)₂C₆H₃]M(BH₄) and [C₆H₄-o-(NCH₂P^tBu₂)₂B]M(BH₄) (M = Ni, Pd, and Pt) indicates that the M-P bond length, the P-M-P bite angle and the trans-influence of the central atom in the pincer platform also affect the coordination mode of the BH₄^- ligand. The nickel complexes tend to adopt a monodentate coordination mode while the palladium and platinum complexes can adopt either the monodentate or the bidentate mode depending on the structural features of the pincer platforms. Longer M-P bonds and smaller P-M-P bite angles favour the bidentate mode. The stability of the BH₄^- ligand is influenced by both the coordination mode and the nature of the metal centre. The BH₃ species is released more easily from complexes with less electron rich metal centres. Following the series of Ni, Pd, and Pt, complexes with the same pincer ligand more easily lose a BH₃ moiety.

Cooperative C-H activation of pyridine by PBP complexes of Rh and Ir can lead to bridging 2-pyridyls with different connectivity to the B-M unit

Pyridine and quinoline undergo selective C–H activation in the 2-position with Rh and Ir complexes of a boryl/bis(phosphine) PBP pincer ligand, resulting in a 2-pyridyl bridging the transition metal and the boron center. Examination of this reactivity with Rh and Ir complexes carrying different non-pincer ligands on the transition metal led to the realization of the possible isomerism derived from the 2-pyridyl fragment connecting either via B–N/C–M bonds or via B–C/N–M bonds. This M–C/M–N isomerism was systematically examined for four structural types. Each of these types has a defined set of ligands on Rh/Ir besides 2-pyridyl and PBP. A pair of M–C/M–N isomers for each type was computationally examined for Rh and for Ir, totaling 16 compounds. Several of these compounds were isolated or observed in solution by experimental methods, in addition to a few 2-quinolyl variants. The DFT predictions concerning the thermodynamic preference within each M–C/M–N isomeric match the experimental findings very well. In two cases where DFT predicts <2 kcal mol⁻¹ difference in free energy, both isomers were experimentally observed in solution. Analysis of the structural data, of the relevant Wiberg bond indices, and of the ETS-NOCV partitioning of the interaction of the 2-pyridyl fragment with the rest of the molecule points to the strength of the M–C(pyridyl) bond as the dominant parameter determining the relative M–C/M–N isomer favorability. This M–C bond is always stronger for the analogous Ir vs. Rh compounds, but the nature of the ligand trans to it has a significant influence, as well. DFT calculations were used to evaluate the mechanism of isomerization for one of the molecule types.

The thermodynamic preference between two isomeric products of C–H activation of pyridine, with 2-pyridyl bridging boron and iridium or rhodium, primarily depends on the M–C bond strength.

Coordination Flexibility of the Rh(PXP) Complex to NH3, CO, and C2H4 (PXP = Diphosphine-Based Pincer Ligand; X = B, Al, and Ga): Theoretical Insight

The recently synthesized rhodium-aluminum bimetallic complex Rh(PAlP) 1 (PAlP = pincer-type diphosphino-aluminyl ligand Al{[N(C₆H₄)]₂NMe}[CH₂P(ⁱPr)₂]₂) containing a unique Rh-Al direct bond exhibits coordination flexibility because Rh and Al can play the role of coordination site for the substrate. DFT calculations of NH₃, CO, and C₂H₄ adducts with 1 show that the Rh atom is favorable for all these substrate but the Al atom is as favorable as the Rh atom for NH₃ and unfavorable for CO and C₂H₄. NH₃ and CO prefer the coordination at the Rh-axial (Ax) site to the Rh-equatorial (Eq) site, but C₂H₄ prefers coordination at the Rh-Eq site to the Rh-Ax site. Consequently, two CO and C₂H₄ molecules coordinate with 1 at the Rh-Ax and Rh-Eq sites to afford trigonal bipyramidal complexes Rh(PAlP)(CO)₂ and Rh(PAlP)(C₂H₄)₂, which is consistent with the experimental observation of Rh(PAlP)(CO)₂. Energy decomposition analysis reveals that an electrostatic term plays an important role for NH₃ coordination with the Al atom of 1, because Al has a significantly large positive charge and NH₃ has a much negatively charged N atom and exhibits a considerably negative electrostatic potential at the Al position. In B and Ga analogues Rh(PBP) 2 and Rh(PGaP) 3, B and Ga atoms are not good for CO and C₂H₄ like the Al atom in 1. NH₃ adducts with 2 and 3 at the B and Ga sites are less stable than those adducts at the Rh-Ax site unlike the NH₃ adduct with 1 at the Al site. This difference in the NH₃ adduct between Rh(PAlP) and others (Rh(PBP) and Rh(PGaP)) arises from much less positive charges of B and Ga and a smaller atomic size of B than that of Al. These results indicate that the significantly large electropositive nature and appropriate atomic size of Al are responsible for the characteristic coordination flexibility of Rh(PAlP).

Reversible addition of ethylene to a pincer-based boryl-iridium unit with the formation of a bridging ethylidene

This report examines reactions of a series of Ir complexes supported by the diarylboryl/bis(phosphine) PBP pincer ligand with ethylene: (PBP)IrH₄ (1), (PBP)IrH₂(CO) (2), and (PBP)Ir(CO)₂ (3). The outcomes of these reactions differ from those typical for Ir complexes supported by other pincer ligands and do not give rise to simple ethylene adducts or products of insertion of Ir into the C–H bond of ethylene. Instead, the elements of ethylene are incorporated into the molecules to result in B–C bonds. In the case of 2 and 3, ethylene addition results in the formation of B/Ir bridging ethylidene complexes 5 and 6. For 6, the addition of ethylene (and the analogous addition of 1-hexene) is shown to be partially reversible. Addition of ethylene to 2 and 3 is remarkable because they are saturated at Ir and yet the net outcome is such that ethylene binds without replacing any ligands already present. A mechanistic inquiry suggests that dissociation of CO from 3 or 6 is necessary in order for the addition or loss of ethylene to proceed.

(PBP)Ir pincer complexes containing a boryl-iridium linkage reversibly bind ethylene as an ethylidene bridging B and Ir.

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

Recent development in catalytic application of transition metal complexes having an M-E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized. Main group metal and metalloid supporting ligands furnish unusual electronic and steric environments and molecular functions to transition metals, which are not easily available with standard organic supporting ligands such as phosphines and amines. These characteristics often realize remarkable catalytic activity, unique product selectivity, and new molecular transformations. This perspective demonstrates the promising utility of main group metal and metalloid compounds as a new class of supporting ligands for transition metal catalysts in synthetic chemistry.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Catalytic oxidations by dehydrogenation of alkanes, alcohols and amines with defined (non)-noble metal pincer complexes

The present review highlights the latest developments in the field of transition metal-catalysed oxidations, in particular C–C–, C–O– and C–N-bond dehydrogenations.

Palladium(ii) complexes supported by PBP and POCOP pincer ligands: a comparison of their structure, properties and catalytic activity

A Pd(ii) chloride complex supported by a Yamashita-Nozaki PBP pincer ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdCl (1a), was synthesized. The structure, properties and catalytic activity of complex 1a were compared with those of the corresponding POCOP pincer complex [C6H3-2,6-(OPtBu2)2]PdCl (2a). It was found that the Pd centre in complex 1a is more electron rich and easier to be oxidized than that in complex 2a; complex 1a is a much better catalyst for Suzuki-Miyaura cross-coupling reactions than complex 2a. Starting from complexes 1a and 2a, two series of Pd(ii) pincer complexes bearing a SH, BH4, N[combining low line]CS, N[combining low line]CSe or N3 covalent ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdY (Y = SH, 1b; BH4; 1c; N[combining low line]CS, 1d; N[combining low line]CSe, 1e; and N3, 1f) and [C6H3-2,6-(OPtBu2)2]PdY (Y = SH, 2b; BH4, 2c; N[combining low line]CS, 2d; N[combining low line]CSe, 2e; and N3, 2f), were synthesized and fully characterized. Single crystal X-ray diffraction analysis indicated that the Pd centre is less tightly chelated in PBP pincer complexes. The strong σ-donor ability of the PBP pincer ligand has little influence on the structure of the covalent ligand possessing both σ-donor and π-acceptor properties. However, the stretching vibrational frequencies of N[combining low line]CS, N[combining low line]CSe and N3 ligands and the coordination mode of the BH4 ligand are significantly different in these two types of palladium pincer complexes.

Rhodium-Catalyzed C-H Activation Enabled by an Indium Metalloligand

Rhodium complexes with an indium metalloligand were successfully synthesized by utilizing a pyridine-tethered cyclopentadienyl ligand as a support for an In-Rh bond. The indium metalloligand dramatically changes the electronic and redox properties of the rhodium metal, thereby enabling catalysis of sp² C-H bond activation.

Synthesis, structure, and reactivity of pincer-type iridium complexes having gallyl- and indyl-metalloligands utilizing 2,5-bis(6-phosphino-2-pyridyl)pyrrolide as a new scaffold for metal-metal bonds

The synthesis and structural analyses of pincer-type iridium complexes having gallyl- and indyl-metalloligands were achieved utilizing 2,5-bis(6-phosphino-2-pyridyl)pyrrolide as a new scaffold for metal-metal bonds. A BH3-coordinated PInP-Ir dihydride complex was also developed as an equivalent to an iridium dihydride complex, which could be a useful catalyst for synthetic reactions.