Ambiphilic Ligand Research Articles

Pnictogen Bonding at the Core of a Carbene-Stiborane-Gold Complex: Impact on Structure and Reactivity.

Our interest in the design of ambiphilic ligands and their coordination to gold has led us to synthesize an indazol-3-ylidene gold chloride complex functionalized at the 4-position of the indazole backbone by a stibine functionality. The antimony center of this new complex cleanly reacts with o-chloranil to afford the corresponding stiborane derivative. Structural analysis indicates that the stiborane coordination environment is best described as a distorted square pyramid whose open face is oriented toward the gold center, allowing for the formation of a long donor-acceptor, or pnictogen, Au → Sb bonding interaction. The presence of this interaction, which has been probed computationally, is also manifested in the enhanced catalytic activity of this complex in the cyclization of N-propargyl-4-fluorobenzamide.

Combining geometric constraint and redox non-innocence within an ambiphilic PBiP pincer ligand.

The synthesis of the first pincer ligand featuring a strictly T-shaped group 15 element and its coordination behaviour towards transition metals is described. The platform is itself derived from a trianionic redox non-innocent NNN scaffold. In addition to providing a rigid coordination environment to constrain a Bi centre in a T-shaped geometry to manipulate its frontier molecular orbital constitution, the NNN chelate displays highly covalent bonding towards the geometrically constrained Bi centre. The formation of intriguing ambiphilic Bi-M bonding interactions is demonstrated upon formation of a pincer complex as well as a multimetallic cluster. All compounds are comprehensively characterised by spectroscopic methods including X-ray Absorption Near Edge Structure (XANES) spectroscopy and complemented by DFT calculations.

A catalytic collaboration: pairing transition metals and Lewis acids for applications in organic synthesis.

The use of metal catalysts to accelerate an organic transformation has proven indispensable for access to structural motifs having applications across medicinal, polymer, materials chemistry, and more. Most catalytic approaches have cast transition metals in the "leading role"; these players mediate important reactions such as C-C cross coupling and the hydrogenation of unsaturated bonds. These catalysts may require collaboration, featuring Lewis acidic or basic additives to promote a desired reaction outcome. Lewis acids can serve to accelerate reactions by way of substrate stabilization and/or activation, and as such, are valuable in optimizing catalytic transformations. A burgeoning area of chemical research which unifies these concepts has thus sought to develop transition metal complexes having ambiphilic (containing a Lewis basic and acidic unit) ligands. This approach takes advantage of metal-ligand cooperativity to increase the efficiency of a given chemical transformation, leveraging intramolecular interactions between a transition metal and an adjacent secondary ligand site. While this has shown significant potential to facilitate challenging and important transformations, there remains unexplored depth for creativity and future advancement. This Frontier highlights inter- and intramolecular combinations of transition metals and Lewis acids that together, provide a collaborative platform for chemical synthesis.

Reactivity of ruthenium complexes towards organogallium reagents: Gallium as a Z-type ligand or as a gallate counterion

The reaction of RuHCl(PPh3)3 with GaMe3 gives rise to the arene complex [(η6-C6H6)Ru(PPh3)(PPh2-o-C6H4-GaClMe], 1, with the Ga atom making part of an in situ generated ambiphilic phosphinogallyl ligand in a five-membered ruthenagallacycle ring with a tetracoordinate gallium. In the presence of excess GaMe3, 1 forms complex [(η6-C6H6)Ru(PPh3)(PPh2-o-C6H4-GaMe][GaMe3Cl], 2 also bearing a phosphinogallyl ligand. Crystals suitable for single-crystal X-ray diffraction were obtained of complex 2′, [(η6-C6H6)Ru(PPh3)(PPh2-o-C6H4-GaMe][GaMe2Cl2], showing an ion pair with two Ga atoms in different coordination environments: the first with a coordination number of three makes part of a five-membered ruthenagallacyle ring, while the second Ga atom is a gallate anion. In both complexes 1 and 2, the Ga atom binds to Ru as a σ-acceptor Z-type ligand. DFT calculations are in good agreement with the experimental single crystal X-ray diffraction data and provide Ru-Ga Wiberg bond indexes of 0.38 and 0.50, for 1 and 2 respectively. In contrast, treatment of RuHCl(PPh3)3 with GaMeCl2 and of RuCl2(PPh3)3 with GaMe3 gives rise to gallate species [(η6-C6H6)Ru(PPh3)2H][GaMeCl3], 3, and [(η6-C6H6)Ru(PPh3)2Me][GaMe2Cl2], 4, respectively.

Lewis Acid-Assisted C(sp3)-C(sp3) Reductive Elimination at Gold.

The phosphine-borane iPr2P(o-C6H4)BFxyl2 (Fxyl = 3,5-(F3C)2C6H3) 1-Fxyl was found to promote the reductive elimination of ethane from [AuMe2(μ-Cl)]2. Nuclear magnetic resonance monitoring revealed the intermediate formation of the (1-Fxyl)AuMe2Cl complex. Density functional theory calculations identified a zwitterionic path as the lowest energy profile, with an overall activation barrier more than 10 kcal/mol lower than without borane assistance. The Lewis acid moiety first abstracts the chloride to generate a zwitterionic Au(III) complex, which then readily undergoes C(sp3)-C(sp3) coupling. The chloride is finally transferred back from boron to gold. The electronic features of this Lewis-assisted reductive elimination at gold have been deciphered by intrinsic bond orbital analyses. Sufficient Lewis acidity of boron is required for the ambiphilic ligand to trigger the C(sp3)-C(sp3) coupling, as shown by complementary studies with two other phosphine-boranes, and the addition of chlorides slows down the reductive elimination of ethane.

A comparison of the coordination behaviour of R2PCH2BMe2 (R = Me vs. Ph) ambiphilic ligands with late transition metals.

A new synthesis that avoids the use of Me2PH is reported for (Me2PCH2BMe2)2, and this method was extended to the synthesis of (Ph2PCH2BMe2)2. The ligand precursor (Me2PCH2BMe2)2 did not react with [{M(μ-Cl)(cod)}2] (cod = 1,5-cyclooctadiene; M = Ir and Rh) or [PtCl2(cod)] at room temperature. However, after 12-48 hours at 65-70 °C, these reactions afforded (a) [Ir(cod)(μ-Cl)(Me2PCH2BMe2)] (1), (b) an equilibrium mixture of (Me2PCH2BMe2)2, [{Rh(μ-Cl)(cod)}2] and [Rh(cod)(μ-Cl)(Me2PCH2BMe2)] (2), and (c) cis-[Pt(μ-Cl)2(Me2PCH2BMe2)2] (3), respectively. By contrast, reactions between the phenyl-substituted analogue, (Ph2PCH2BMe2)2, and [{M(μ-Cl)(cod)}2] (cod = 1,5-cyclooctadiene; M = Ir and Rh) proceeded over the course of 1 hour at 20 °C to generate [M(cod)(μ-Cl)(Ph2PCH2BMe2)] (M = Ir (4) and Rh (5)), indicative of room temperature (Ph2PCH2BMe2)2 dissociation. Room temperature reactions of (Ph2PCH2BMe2)2 with [{Rh(μ-Cl)(coe)2}2] (coe = cyclooctene) using a 1 : 1 or 3 : 1 stoichiometry also afforded [{Rh(coe)(μ-Cl)(Ph2PCH2BMe2)}2] (6) or [RhCl(Ph2PCH2BMe2)3] (7), respectively, where the latter is a borane-appended analogue of Wilkinson's catalyst, and reactions of (Ph2PCH2BMe2)2 with [PtX2(cod)] (X = Cl or Me) yielded cis-[Pt(μ-Cl)2(Ph2PCH2BMe2)2] (8) and cis-[PtMe2(Ph2PCH2BMe2)2] (9). Compounds 1-9, (Me2PCH2BMe2)2 and (Ph2PCH2BMe2)2 were crystallographically characterized. In compounds 1-5 and 8, each chloride co-ligand is coordinated by the borane of an R2PCH2BMe2 ligand. Additionally, in the solid state structure of 6, each bridging chloride ligand interacts weakly with a pendent borane, and in 7, the chloride ligand is tightly coordinated to the borane of one Ph2PCH2BMe2 ligand and weakly coordinated to the borane of a second Ph2PCH2BMe2 ligand. By contrast, both boranes in 9 (and one of the three boranes in 7) are non-coordinated.

Au → M bonds promote catalytic alkyne hydrofunctionalisation.

The impact of metalloligands on Au catalysed alkyne hydrofunctionalisation is studied. Ambiphilic PMP-type ligands (M = CuI, AgI, ZnII) stabilise Au → M bonds including unprecedented AuI → ZnII interactions. The Lewis acidity of Au increases in the order CuI < AgI < ZnII promoting catalytic cycloisomerisation of propargylamide 14. Au/Zn complex 8 is an excellent catalyst for alkyne hydroamination.

Implementing the Ambiphilicity of an Organotellurenyl Cation for the Synthesis of a Platinum(II)‐Based Carboxylate‐Bridged Heterobimetallic Complex: Structure and Bonding Analysis

AbstractThe metathesis reaction of aryltellurium chloride, ppyTeCl [ppy=2‐(2’‐pyridyl)phenyl] (5) with AgClO4 afforded Intramolecular Chalcogen Bonding (IChB)‐stabilized organotellurenyl cation [ppyTe] ⋅ ClO4 ([6] ⋅ ClO4). The reaction of [6] ⋅ ClO4 with K2PtCl4 in acetic acid resulted in the formation of platinum(II)‐based carboxylate‐bridged heterobimetallic complex [Pt(ppyTe)(μ‐OOCMe)Cl2], 7 containing an organotellurenyl cation as ligand. This is the first time a Pt(II) complex of an organotellurenyl cation donor‐acceptor ligand has been reported. Complex 7 was characterized by multi‐nuclear NMR spectroscopy (1H, 13C, and 125Te), FT‐IR spectroscopy, CHN analysis, and single‐crystal X‐ray diffraction studies. The single‐crystal X‐ray diffraction studies and DFT analysis infer that the Te center in 7 behaves as an ambiphilic ligand, accepting electrons from the N‐atom of the ppy moiety and the O‐atom of the carboxylate groups while simultaneously donating electrons to the Pt(II) center. The 2D‐fingerprint plots derived from the Hirshfeld surface analysis reveal multiple intermolecular interactions in the packing diagrams of complex 7. The natural charges accumulated on the atoms in complex 7 were calculated by Natural Population Analysis (NPA). In addition, the HOMO‐LUMO energy gap in 7 was calculated.

Rational design of nickel–borane complexes for methane activation and functionalization

For the methane activation reaction, the mono-phosphine ligand supported Ni–B complex is more active than the traditional ambiphilic ligand supported one.

How Strong is a Reverse Dative Bond? Compliance Constants as Unique Bond Strength Descriptors.

The concepts of bond strength and bond order are at the very heart of chemistry. Nevertheless, the study of local mechanical bond strengths in large polyatomic molecules is a nontrivial task. In a recent publication, Peters and Lehnert et al. [Inorg. Chem. 2020, 59 (20), 14967-14982] found a quite strong (reverse) dative Fe···B bond for a synthesized low-spin {FeNO}10 complex supported by an ambiphilic trisphosphineborane ligand. Their conclusion was, among other criteria, based on a surprisingly high Fe-B force constant of 1.56 mdyn/Å. Nevertheless, this high value might be based on numerical ambiguities of (rigid) real-space force constants, a flaw, that may affect many other combined spectroscopic and electronic structure studies.

Imidazole-based ambiphilic ligands for anion sensing, metallation and photophysical properties

Imidazole-based ambiphilic ligands for anion sensing, metallation and photophysical properties

Phosphine–Borane Ligands Induce Chemoselective Activation and Catalytic Coupling of Acyl Chlorides at Palladium

Acyl chlorides are highly reactive and widely used substrates in catalytic cross-coupling reactions, but so far, site selectivity over other functional groups has remained an issue. In this work, Pd complexes deriving from the phosphine-boranes [i-Pr2P(o-C6H4)](2)BFXyl and i-Pr2P(o-C6H4)-BFXyl(2) (Fxyl = 3,5-(F3C)(2)C6H3) were found to preferentially activate acyl chlorides over C-I, C-Br, C-Cl, C-OTf, and C-OTs bonds. The system is amenable to catalysis (Stille and Negishi couplings), providing a simple and efficient means to forge C(=O)-C bonds in a site-selective manner and to readily access functionalized ketones. To gain insight into the role and influence of the ambiphilic ligands, key Pd complexes have been authenticated and the reaction profiles have been analyzed by density functional theory (DFT) calculations.

Rational design of the nickel-borane complex for efficient hydrogenation of styrene.

The Ni-B complex 1BCF with a facilely accessible monophosphine (Pt Bu3 ) unit was theoretically designed, which was found to be more active than that with an ambiphilic ligand for hydrogenation of styrene. Substituting Pt Bu3 with a stronger electron donating ligand N-heterocyclic carbene largely improves the activity of the Ni-B complex.

N-Heterocyclic silylenes as ambiphilic activators and ligands.

This frontiers article highlights recent developments of the use of N-heterocyclic silylenes (NHSis), the higher homologues of the Arduengo-carbenes, as ambiphilic activators and ligands in organometallic chemistry and provides a comparison of five-membered ring NHSi ligands with ubiquitous N-heterocyclic carbene (NHC) and phosphine ligands. The frontier orbital region of NHSis differs considerably from that of NHCs which results in different ligation properties. The donor properties of NHSis are closer to those of phosphines than to those of NHCs. NHSis reveal a much stronger tendency to act as bridging ligands between two metal centres than NHCs or phosphines and NHSi insertion reactions into metal-ligand bonds are more facile to achieve compared to similar insertion reactions of NHCs. These interesting properties clearly distinguish NHSi ligands from their NHC or phosphine counterparts and should provide novel reactivities in basic organometallic chemistry and catalysis.

Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes.

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}8-10 complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)]+/0/-. Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ ligand throughout with strong Fe-NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe-B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)]- complex, an example of a ls-{FeNO}10 species, features a true reverse dative Fe → B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}8 system, with two additional electrons "stored" on site in an Fe-B single bond. The outlier in this series is the ls-{FeNO}9 complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe-NO bond. These data are further contextualized by comparison with a related N2 complex, [Fe(TPB)(N2)]-, which is a key intermediate in Fe(TPB)-catalyzed N2 fixation. Our present study finds that the Fe → B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.

Tunable carbocation-based redox active ambiphilic ligands: synthesis, coordination and characterization.

The synthesis of novel redox active ambiphilic ligands L1-L3 and their coordination chemistry to first-row late transition metal halides (M = Co and Ni) is reported. The heterocyclic carbocation scaffolds act as Lewis acid moieties while the pyridine anchor acts as the coordinating Lewis base. The high synthetic tunability of this ligand scaffold allows for control of its rigidity and electronic properties. Anion exchange and coordination of a chloride anion to the metal center was observed resulting in the formation of [MCl3]- metallate. Upon coordination to the pyridine anchor, the metallate centers adopt a canonical tetrahedral geometry, resulting in an overall neutral complex best described as a zwitterionic metallate trichloride bound to a cationic ligand. Characterization techniques including single crystal X-ray diffraction, cyclic voltammetry, and UV-Vis absorption spectroscopy were employed to better understand the structural and chemical properties of the ligands and metal complexes. A possible weak interaction between one of the chlorides and the carbenium moiety in the ligand is observed in crystals of both of the Co(ii) and Ni(ii) complexes with ligand L1. Density functional theory (DFT) calculations support that this electrostatic interaction for complexes 2a and 2b exists only in the solid state.

Stabilized Carbenium Ions as Latent, Z‐type Ligands

Controlling the reactivity of transition metals using secondary, σ-accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o-Ph2 P(C6 H4 )Acr)AuCl]+ ([3]+ ; Acr=9-N-methylacridinium) and [(o-Ph2 P(C6 H4 )Xan)AuCl]+ ([4]+ ; Xan=9-xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [4]+ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative (7) in which the metal atom is covalently bound to the former carbocationic center. This anion-induced AuI /AuIII oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [4]+ into an X-type ligand in 7. We conclude that the carbenium moiety of this complex acts as a latent Z-type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.

Unsaturated vicinal frustrated phosphane/borane Lewis pairs as ligands in gold(i) chemistry.

The P/B FLP 6, formed by the 1,1-hydroboration of dimesitylphosphino(trimethylsilyl)acetylene with HB(C6F5)2 forms the Au(i)X complexes 9a (X: Cl) and 10a (X: NTf2), respectively. Both show marked AuB interactions. The P/B FLP isomer 8, featuring the bulky SiMe3 substituent at the carbon adjacent to boron forms gold complexes 9b and 10b, both of which show weaker AuB interactions. Complex 10a was employed in the catalytic hydroamination of a series of alkynes with p-toluidine. Complexes 9 and 10 were characterized by X-ray diffraction.

Diferrocenylmercury‐Bridged Diphosphine: A Chiral, Ambiphilic, and Redox‐Active Bidentate Ligand

A diphosphine chelate ligand with a wide and flexible bite angle, a unique stereochemical environment, and redox-active and ambiphilic character is reported. Initially generated as its HgCl2 complex by reaction of 1,2-fc(PPh2 )(SnMe3 ) (fc=ferrocenediyl) with HgCl2 in acetone, treatment with [n-Bu4 N]CN readily liberates the free chiral bidentate ligand. An intermolecular ClHg-Cl→Hgfc2 (2.9929(13) Å) interaction that is unprecedented in ambiphilic ligand chemistry is seen in the solid structure of Hg(fcPPh2 )2 ⋅HgCl2 where the bridging mercury atom acts as a σ-acceptor. Furthermore, a bis-[Rh(COD)Cl] complex is introduced, which displays relatively short Rh⋅⋅⋅Hg contacts of 3.4765(5) and 3.4013(1) Å. Wiberg indices of 0.12 are determined for these Rh⋅⋅⋅Hg interactions and an AIM analysis reveals bond paths with an electron density ρ(r) of 1.2×10-2 and 1.4×10-2 e/a03 at the bond critical points.

Diferrocenylmercury‐Bridged Diphosphine: A Chiral, Ambiphilic, and Redox‐Active Bidentate Ligand

AbstractA diphosphine chelate ligand with a wide and flexible bite angle, a unique stereochemical environment, and redox‐active and ambiphilic character is reported. Initially generated as its HgCl2 complex by reaction of 1,2‐fc(PPh2)(SnMe3) (fc=ferrocenediyl) with HgCl2 in acetone, treatment with [n‐Bu4N]CN readily liberates the free chiral bidentate ligand. An intermolecular ClHg−Cl→Hgfc2 (2.9929(13) Å) interaction that is unprecedented in ambiphilic ligand chemistry is seen in the solid structure of Hg(fcPPh2)2⋅HgCl2 where the bridging mercury atom acts as a σ‐acceptor. Furthermore, a bis‐[Rh(COD)Cl] complex is introduced, which displays relatively short Rh⋅⋅⋅Hg contacts of 3.4765(5) and 3.4013(1) Å. Wiberg indices of 0.12 are determined for these Rh⋅⋅⋅Hg interactions and an AIM analysis reveals bond paths with an electron density ρ(r) of 1.2×10−2 and 1.4×10−2 e/a03 at the bond critical points.