Metal-to-Metal Distance Modulation by Ligand Design: A Case Study of Structure-Property Correlation in Planar Chiral Cyclophanyl Metal Complexes

Multinuclear metal complexes have seen tremendous progress in synthetic advances, their versatile structural features, and emerging applications. Here, we conceptualize Metal-to-Metal distance modulation in cyclophanyl metal complexes by bridging ligand design employing the co-facially stacked cyclophanyl-derived pseudo-geminal, -ortho, -meta, and -para constitutional isomers grafted with N-, O-, and P- containing chelates that allow the installation of diverse (hetero)metallic moieties in a distance-defined and spatially-oriented relation to one another. Metal-to-Metal distance modulation and innate transannular “through-space” π–π electronic interactions via the co-facially stacked benzene rings in cyclophanyl-derived complexes as well as their specific stereochemical structural features (element of planar chirality) are crucial factors that contribute to the tuning of structure-property relationships, which stand at the very center from the perspective of cooperative effects in catalysis as well as emerging material applications.     

Oxidative Addition to Gold(I) by Photoredox Catalysis: Straightforward Access to Diverse (C,N)‐Cyclometalated Gold(III) Complexes

Herein, we report the oxidative addition of aryldiazonium salts to ligand‐supported gold(I) complexes under visible light photoredox conditions. This method provides experimental evidence for the involvement of such a process in dual gold/photoredox‐catalyzed reactions and delivers well‐defined (C,N)‐cyclometalated gold(III) species. The remarkably mild reaction conditions and the ability to widely vary the ancillary ligand make this method a potentially powerful synthetic tool to access diverse gold(III) complexes for systematic studies into their properties and reactivity. Initial studies show that these species can undergo chloride abstraction to afford Lewis acidic dicationic gold(III) species.     

Heterobimetallic Gold/Ruthenium Complexes Synthesized via Post-functionalization and Applied in Dual Photoredox Gold Catalysis

The synthesis of heterobimetallic Au^I/Ru^II complexes of the general formula syn- and anti-[{AuCl}( L1 ∩ L2 ){Ru(bpy)₂}][PF₆]₂ is reported. The ditopic bridging ligand L1 ∩ L2 refers to a P,N hybrid ligand composed of phosphine and bipyridine substructures, which was obtained via a post-functionalization strategy based on Diels-Alder reaction between a phosphole and a maleimide moiety. It was found that the stereochemistry at the phosphorus atom of the resulting 7-phosphanorbornene backbone can be controlled by executing the metal coordination and the cycloaddition reaction in a different order. All precursors, as well as the mono- and multimetallic complexes, were isolated and fully characterized by various spectroscopic methods such as NMR, IR, and UV-vis spectroscopy as well as cyclic voltammetry. Photophysical measurements show efficient phosphorescence for the investigated monometallic complex anti-[( L1 ∩ L2 ){Ru(bpy)₂}][PF₆]₂ and the bimetallic analogue syn-[{AuCl}( L1 ∩ L2 ){Ru(bpy)₂}][PF₆]₂, thus indicating a small influence of the {AuCl} fragment on the photoluminescence properties. The heterobimetallic Au^I/Ru^II complexes syn- and anti-[{AuCl}( L1 ∩ L2 ){Ru(bpy)₂}][PF₆]₂ are both active catalysts in the P-arylation of aryldiazonium salts promoted by visible light with H-phosphonate affording arylphosphonates in yields of up to 91 %. Both dinuclear complexes outperform their monometallic counterparts.     

Selectivity Effects in Bimetallic Catalysis

An emerging area of homogeneous catalysis is the use of catalysts featuring two closely associated metal sites. This approach complements the traditional focus on single‐site catalysts and makes available new parameters with which to optimize catalytic behavior. Single‐site catalysts are optimized through changing 1) the identity of the metal, and 2) the steric and electronic properties of the ligands. Bimetallic catalysts introduce new optimization parameters such as 3) catalyst nuclearity (mononuclear vs. binuclear), and 4) bimetallic pairing (relative compatibility of two metal sites). In order to harness these new optimization parameters in developing systems, it is necessary to first understand the origin of bimetallic selectivity effects that already have been documented. This Concept article highlights bimetallic effects on the chemo‐, regio‐, and stereoselectivity of catalytic transformations, using selected case studies from the recent literature as illustrative examples.     

Heterometallic Complexes with Gold(I) Metalloligands: Self‐Assembly of Helical Dimers Stabilized by Weak Intermolecular Interactions and Solvophobic Effects

New gold(I) alkynyl metalloligands bpylC?CAuL, bpyl′C?CAuPPh₃, and PPN[Au(C?Cbpyl′)₂] (bpyl or bpyl′=2,2′‐bipyridin‐5‐yl or ?4‐yl, respectively; L=PMe_xPh_3?x (x=1–3), P(C₆H₃Me₂‐3,5)₃, PCy₃, XyNC) have been synthesized. Ligands bpylC?CH and metalloligands bpylC?CAuL (L=PPh₃, PMePh₂, PCy₃, CNXy) react with MX₂ (M=Fe, Zn, X=ClO₄; M=Co, X=BF₄) to give complexes [M(bpylC?CZ)₃]X₂ (Z=H or AuL). In most cases, these complexes are mixtures of fac and mer isomers in a statistical distribution, in both CH₂Cl₂ and MeCN. However, for L=PPh₃, the fac isomer is dominant in MeCN. NMR and ESI‐MS studies, together with the crystal structure of [Co(bpylC?CAuPPh₃)₃](BF₄)₂, suggest that this solvent dependence is originated by the formation of helical dimers between two fac complexes in MeCN. These dimers are stabilized by solvophobic effects and multiple intermolecular interactions. Complex [Fe(Ph₃PAuC?CbpdiylC?CAuPPh₃)₃](ClO₄)₂ (bpdiyl=2,2′‐bipyridin‐5,5′‐diyl) was obtained by reaction of three diauro diethynylbipyridines and Fe(ClO₄)₂.     

Rh(I) Complexes in Catalysis: A Five-Year Trend

Rhodium is one of the most used metals in catalysis both in laboratory reactions and industrial processes. Despite the extensive exploration on “classical” ligands carried out during the past decades in the field of rhodium-catalyzed reactions, such as phosphines, and other common types of ligands including N-heterocyclic carbenes, ferrocenes, cyclopentadienyl anion and pentamethylcyclopentadienyl derivatives, etc., there is still lively research activity on this topic, with considerable efforts being made toward the synthesis of new preformed rhodium catalysts that can be both efficient and selective. Although the “golden age” of homogeneous catalysis might seem over, there is still plenty of room for improvement, especially from the point of view of a more sustainable chemistry. In this review, temporally restricted to the analysis of literature during the past five years (2015–2020), the latest findings and trends in the synthesis and applications of Rh(I) complexes to catalysis will be presented. From the analysis of the most recent literature, it seems clear that rhodium-catalyzed processes still represent a stimulating challenge for the metalloorganic chemist that is far from being over.     

One-Pot Tandem Michael Addition/Enantioselective Conia-Ene Cyclization Mediated by Chiral Iron(III)/Silver(I) Cooperative Catalysis

The first one-pot tandem Michael addition/enantioselective Conia-ene cyclization of N-protected prop-2-yn-1-amines with 2-methylene-3-oxoalkanoates promoted by chiral iron(III)/silver(I) cooperative catalysts has been developed. Alkyl 4-methylenepyrrolidine-3-acyl-3-carboxylates, which can be transformed into β-proline derivatives, are obtained in high yield with high enantioselectivity.     

One‐Pot Tandem Michael Addition/Enantioselective Conia‐Ene Cyclization Mediated by Chiral Iron(III)/Silver(I) Cooperative Catalysis

The first one‐pot tandem Michael addition/enantioselective Conia‐ene cyclization of N‐protected prop‐2‐yn‐1‐amines with 2‐methylene‐3‐oxoalkanoates promoted by chiral iron(III)/silver(I) cooperative catalysts has been developed. Alkyl 4‐methylenepyrrolidine‐3‐acyl‐3‐carboxylates, which can be transformed into β‐proline derivatives, are obtained in high yield with high enantioselectivity.     

Dinuclear Au(I), Au(II) and Au(III) Complexes with (CF2)n Chains: Insights into The Role of Aurophilic Interactions in the Au(I) Oxidation

New dinuclear Au(I), Au(II) and Au(III) complexes containing (CF₂)_n bridging chains were obtained. Metallomacrocycles [Au₂{μ-(CF₂)₄}{μ-diphosphine}] show an uncommon figure-eight structure, the helicity inversion barrier of which is influenced by aurophilic interactions and steric constraints imposed by the diphosphine. Halogenation of LAu(CF₂)₄AuL (L=PPh₃, PMe₃, (dppf)_1/2, (binap)_1/2) gave [Au(II)]₂ species, some of which display unprecedented folded structures with Au−Au bonds. Aurophilic interactions facilitate this oxidation process by preorganizing the starting [Au(I)]₂ complexes and lowering its redox potential. The obtained [Au(II)]₂ complexes undergo thermal or photochemical elimination of R₃PAuX to give Au(III) perfluorinated auracycles. Evidence of a radical mechanism for these decomposition reactions was obtained.     

Bimetallic Gold(I)/Chiral N,N′‐Dioxide Nickel(II) Asymmetric Relay Catalysis: Chemo‐ and Enantioselective Synthesis of Spiroketals and Spiroaminals

A highly efficient asymmetric cascade reaction between keto esters and alkynyl alcohols and amides is reported. The success of the reaction was attributed to the combination of chiral Lewis acid N,N′‐dioxide nickel(II) catalysis with achiral π‐acid gold(I) catalysis working as an asymmetric relay catalytic system. The corresponding spiroketals and spiroaminals were synthesized in up to 99 % yield, 19:1 d.r., and more than 99 % ee under mild reaction conditions. Control experiments suggest that the N,N′‐dioxide ligand was essential for the formation of the spiro products.     

Metallomicellar Catalysis: Hydrolysis of Phosphodiester with Cu(II) and Zn(II) Complexes in Micellar Solution

Abstract

The effects of two metal complexes of 2,2′‐dipyridylamine (bpya) ligand, [(bpya)Cu]Cl₂ and [(bpya)Zn]Cl₂, in promoting the hydrolysis of bis(4‐nitrophenyl) phosphate (BNPP) have been kinetically investigated in Brij35 micellar solution and at 298 K, pH ranging from 6.41 to 8.6. In neutral micellar solution at 298 K, pH 7.02, the rate constants for the catalytic hydrolysis of BNPP by [(bpya)Cu]Cl₂ and [(bpya)Zn]Cl₂ are 1.2 × 10⁶ times and 1.5 × 10⁵ times higher than those for the spontaneous hydrolysis, respectively. Kinetic studies show that the active species in the catalytic hydrolysis of BNPP is the aquo‐hydroxy form, and the relative kinetic and thermodynamic parameters indicate that the mechanism of the reaction involves intramolecular nucleophilic attack on the metal center‐bound diester.     

Acetylene as a Dicarbene Equivalent for Gold(I) Catalysis: Total Synthesis of Waitziacuminone in One Step

The gold(I)‐catalyzed reaction of acetylene gas with alkenes leads to (Z,Z)‐1,4‐disubstituted 1,3‐butadienes and biscyclopropanes depending on the donor ligand on gold(I). Acetylene was generated in situ from calcium carbide and water in a user‐friendly procedure. Reaction of acetylene with 1,5‐dienes gives rise stereoselectively to tricyclo[5.1.0.0^2,4]octanes. This novel double cyclopropanation has been applied to the one step total synthesis of the natural product waitziacuminone from acetylene and geranyl acetone.     

Oxidation of sec‐alcohols with Ru(II)‐bearing microgel star polymer catalysts via hydrogen transfer reaction: Unique microgel‐core catalysis

Oxidation of sec‐alcohols was investigated with ruthenium‐bearing microgel core star polymer catalysts [Ru(II)‐Star]. The star polymer catalysts were directly prepared via RuCl₂(PPh₃)₃‐catalyzed living radical polymerization of methyl methacrylate (MMA), followed by the arm‐linking reaction with ethylene glycol dimethacrylate ( 1 ) in the presence of diphenylphosphinostyrene ( 2 ). The Ru(II)‐Star efficiently and homogeneously catalyzed the oxidation of 1‐phenylethanol ( S1 ) to give a corresponding ketone (acetophenone) in higher yield (92%) than the analogs of polymer‐supported ruthenium complexes. Importantly, the star catalyst afforded high recycling efficiency in the oxidation. They held catalytic activity against three times catalysis even though they were recovered under air‐exposure, whereas the conventional RuCl₂(PPh₃)₃ lost the activity for same recycling procedure due to the deactivation by oxygen. The stability of the star catalysts during the recycle experiment was confirmed by detailed spectroscopic characterization. The star polymers also catalyzed oxidation for a wide range of sec‐alcohols with aromatic and aliphatic groups. The substrate affinity was different from that with RuCl₂(PPh₃)₃, suggesting the unique selectivity caused by the specific structure. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

Ferrocene based chiral binuclear η6‐benzene‐Ru(II)‐phosphinite complexes: Synthesis,characterization and catalytic activity in asymmetric reduction of ketones

In the present study, a series of chiral C₂‐symmetric ferrocenyl based binuclear η⁶‐benzene‐Ru(II) complexes bearing diphenylphosphinite and diisopropylphosphinite moieties have been synthesised. The new binuclear η⁶‐benzene‐Ru(II)‐phosphinite complexes were characterised based on nuclear magnetic resonance (¹H, ¹³C, ³¹P–NMR), FT‐IR spectroscopy and elemental analysis. Then, these complexes have been screened as catalytic precursors in the transfer hydrogenation of acetophenone with 2‐propanol as both the hydrogen source and solvent in the presence of KOH. The corresponding optically active secondary alcohols were obtained in excellent conversion rates between 96 and 99% and moderate to good enantioselectivities (up to 78% ee). The complex 5 was the most efficient catalyst among the four new complexes investigated herein.     

Monitoring of the Pre-Equilibrium Step in the Alkyne Hydration Reaction Catalyzed by Au(III) Complexes: A Computational Study Based on Experimental Evidences

The coordination ability of the [(ppy)Au(IPr)]²⁺ fragment [ppy = 2-phenylpyridine, IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] towards different anionic and neutral X ligands (X = Cl⁻, BF₄⁻, OTf⁻, H₂O, 2-butyne, 3-hexyne) commonly involved in the crucial pre-equilibrium step of the alkyne hydration reaction is computationally investigated to shed light on unexpected experimental observations on its catalytic activity. Experiment reveals that BF₄⁻ and OTf⁻ have very similar coordination ability towards [(ppy)Au(IPr)]²⁺ and slightly less than water, whereas the alkyne complex could not be observed in solution at least at the NMR sensitivity. Due to the steric hindrance/dispersion interaction balance between X and IPr, the [(ppy)Au(IPr)]²⁺ fragment is computationally found to be much less selective than a model [(ppy)Au(NHC)]²⁺ (NHC = 1,3-dimethylimidazol-2-ylidene) fragment towards the different ligands, in particular OTf⁻ and BF₄⁻, in agreement with experiment. Effect of the ancillary ligand substitution demonstrates that the coordination ability of Au(III) is quantitatively strongly affected by the nature of the ligands (even more than the net charge of the complex) and that all the investigated gold fragments coordinate to alkynes more strongly than H₂O. Remarkably, a stabilization of the water-coordinating species with respect to the alkyne-coordinating one can only be achieved within a microsolvation model, which reconciles theory with experiment. All the results reported here suggest that both the Au(III) fragment coordination ability and its proper computational modelling in the experimental conditions are fundamental issues for the design of efficient catalysts.     

Solvent‐assisted formation of ruthenium(II)/copper(I) complexes containing thiourea derivatives: Synthesis,crystal structure,density functional theory,enzyme mimetics and in vitro biological perspectives

N ,N ‐[(diethylamino)(thiocarbonyl)]‐substituted benzamidine ligands have been synthesized from the reaction of N ,N ‐[(diethylamino)(thiocarbonyl)]benzimidoyl chloride with functionalized amines such as 2‐aminophenol and 2‐picolylamine. The reaction of N ,N ‐[(diethylamino)(thiocarbonyl)]‐2‐hydroxyphenylbenzamidine ( H ₂ L ₁ ) with ruthenium(II) precursor [RuHCl(CO)(PPh₃)₃] afforded complex 1 of the type [Ru(L₁)(CO)(PPh₃)₂] in which the ligand coordinated in tridentate ONS mode. The reaction of H ₂ L ₁ with copper precursor [Cu(CH₃COO)(PPh₃)₂] induced C═N bond cleavage of the ligand and afforded complex 3 of the type [Cu(1,1‐DT)(Cl)(PPh₃)₂] (1,1‐DT = 1,1‐diethylthiourea) in which the ligand coordinated in a monodentate fashion. The ligand N ,N ‐[(diethylamino)(thiocarbonyl)]‐2‐picolylbenzamidine ( HL ₂ ) reacted with ruthenium(II) and copper(I) precursors to form complex 2 of the type [Ru(1,1‐DT)(Cl₂)(CO)(PPh₃)₂] and complex 3 , respectively, in which the ligand underwent C═N cleavage and coordinated in a monodentate fashion via C═S group. In complexes 1 and 2 , the two triphenylphosphine co‐ligands coordinated in trans position whereas, in complex 3 , the two triphenylphosphine co‐ligands coordinated in cis position. All the compounds were characterized using infrared, UV–visible, (¹H, ¹³C, ³¹P) NMR, ESI‐MS and elemental analyses. The molecular structures of ligand H ₂ L ₁ and complexes 1 – 3 were determined using X‐ray crystallography, which confirmed the coordination mode of the ligands with metals. The crystal structure of complexes 1 and 2 revealed a distorted octahedral geometry around the ruthenium ion and the structure of complex 3 indicated a tetrahedral geometry around the copper ion. With the X‐ray structures, density functional theory computations were carried out to determine the electronic structure of the compounds. The interactions of complexes 1 – 3 with calf thymus DNA and bovine serum albumin protein were investigated using UV–visible and fluorescence spectroscopic and viscometric methods. Catecholase‐ and phosphatase‐like activities promoted by complexes 1 – 3 under physiological conditions have been studied. In vitro anticancer activities have been demonstrated by MTT assay, acridine orange/ethidium bromide and diamidino‐2‐phenylindole staining against various cancerous cell lines.