Revealing the Influence of Diverse Secondary Metal Cations on Redox-Active Palladium Complexes

Incorporation of redox-inactive metals into redox-active complexes and catalysts attracts attention for engendering new reactivity modes, but this strategy has not been extensively investigated beyond the first-row of the transition metals. Here, the isolation and characterization of the first series of heterobimetallic complexes of palladium with mono-, di-, and tri-valent redox-inactive metal ions are reported. A Reinhoudt-type heteroditopic ligand with a salen-derived [N₂,O₂] binding site for Pd and a crown-ether-derived [O₆] site has been used to prepare isolable adducts of the Lewis acidic redox-inactive metal ions ( M ⁿ⁺). Comprehensive data from single-crystal X-ray diffraction analysis reveal distinctive trends in the structural properties of the heterobimetallic species, including an uncommon dependence of the Pd⋅⋅⋅ M distance on Lewis acidity. The reorganization energy associated with reduction of the heterobimetallic species is strongly modulated by Lewis acidity, with the slowest heterogeneous electron transfer kinetics associated with the strongest incorporated Lewis acids. This hitherto unexplored reorganization energy penalty for electron transfer contrasts with prior thermodynamic studies, revealing that kinetic parameters should be considered in studies of reactivity involving heterobimetallic species.     

Structural and Oxidation State Alternatives in Platinum and Palladium Complexes of a Redox-Active Amidinato Ligand

Reaction of [Pt(DMSO)₂Cl₂] or [Pd(MeCN)₂Cl₂] with the electron-rich LH=N,N’-bis(4-dimethylaminophenyl)ethanimidamide yielded mononuclear [PtL₂] ( 1 ) but dinuclear [Pd₂L₄] ( 2 ), a paddle-wheel complex. The neutral compounds were characterized through experiments (crystal structures, electrochemistry, UV-vis-NIR spectroscopy, magnetic resonance) and TD-DFT calculations as metal(II) species with noninnocent ligands L⁻. The reversibly accessible cations [PtL₂]⁺ and [Pd₂L₄]⁺ were also studied, the latter as [Pd₂L₄][B{3,5-(CF₃)₂C₆H₃}₄] single crystals. Experimental and computational investigations were directed at the elucidation of the electronic structures, establishing the correct oxidation states within the alternatives [Pt^II(L⁻)₂] or [Pt^.(L )₂], [Pt^II(L^0.5−)₂]⁺ or [Pt^III(L⁻)₂]⁺, [(Pd^II)₂(μ-L⁻)₄] or [(Pd^1.5)₂(μ-L^0.75−)₄], and [(Pd^2.5)₂(μ-L⁻)₄]⁺ or [(Pd^II)₂(μ-L^0.75−)₄]⁺. In each case, the first alternative was shown to be most appropriate. Remarkable results include the preference of platinum for mononuclear planar [PtL₂] with an N-Pt-N bite angle of 62.8(2)° in contrast to [Pd₂L₄], and the dimetal (Pd₂⁴⁺→Pd₂⁵⁺) instead of ligand (L⁻→L ) oxidation of the dinuclear palladium compound.     

Platinum Oxoboryl Complexes as Substrates for the Formation of 1:1, 1:2, and 2:1 Lewis Acid–Base Adducts and 1,2‐Dipolar Additions

The oxoboryl complex trans‐[(Cy₃P)₂BrPt(B?O)] ( 2 ) reacts with the Group 13 Lewis acids EBr₃ (E=Al, Ga, In) to form the 1:1 Lewis acid–base adducts trans‐[(Cy₃P)₂BrPt(B?OEBr₃)] ( 6 – 8 ). This reactivity can be extended by using two equivalents of the respective Lewis acid EBr₃ (E=Al, Ga) to form the 2:1 Lewis acid–base adducts trans‐[(Cy₃P)₂(Br₃Al‐Br)Pt(B?OAlBr₃)] ( 18 ) and trans‐[(Cy₃P)₂(Br₃Ga‐Br)Pt(B?OGaBr₃)] ( 15 ). Another reactivity pattern was demonstrated by coordinating two oxoboryl complexes 2 to InBr₃, forming the 1:2 Lewis acid–base adduct trans‐[{(Cy₃P)₂BrPt(B?O)}₂InBr₃] ( 20 ). It was also possible to functionalize the B?O triple bond itself. Trimethylsilylisothiocyanate reacts with 2 in a 1,2‐dipolar addition to form the boryl complex trans‐[(Cy₃P)₂BrPt{B(NCS)(OSiMe₃)}] ( 27 ).     

Late‐Transition‐Metal Complexes as Tunable Lewis Bases

Syntheses of the first heteroleptic N‐heterocyclic carbene (NHC)–phosphane platinum(0) complexes and formation of the corresponding Lewis acid–base adducts with aluminum chloride is reported. The influence of N‐heterocyclic carbenes on tuning the Lewis basic properties of the metal complexes was judged from spectroscopic, structural, and computational data. Conclusive experimental evidence for the enhanced Lewis basicity of NHC‐containing complexes was provided by a transfer reaction.     

Correlated Coordination and Redox Activity of a Hemilabile Noninnocent Ligand in Nickel Complexes

The compound [Ni(Q_M)₂], Q_M=4,6‐di‐tert‐butyl‐N‐(2‐methylthiomethylphenyl)‐o‐iminobenzoquinone, is a singlet diradical species with approximately planar configuration at the tetracoordinate metal atom and without any Ni?S bonding interaction. One‐electron oxidation results in additional twofold Ni?S coordination (d_Ni?S≈2.38 Å) to produce a complex cation of [Ni(Q_M)₂](PF₆) with hexacoordinate Ni^II and two distinctly different mer‐configurated tridentate ligands. The O,O′‐trans arrangement in the neutral precursor is changed to an O,O′‐cis configuration in the cation. The EPR signal of [Ni(Q_M)₂](PF₆) has a very large g anisotropy and the magnetic measurements indicate an S=3/2 state. The dication was structurally characterized as [Ni(Q_M)₂](ClO₄)₂ to exhibit a similar NiN₂O₂S₂ framework as the monocation. However, the two tridentate (O,N,S) ligands are now equivalent according to the formulation [Ni^II(Q_M⁰)₂]²⁺. Cyclic voltammetry reflects the qualitative structure change on the first, but not on the second oxidation of [Ni(Q_M)₂], and spectroelectrochemistry reveals a pronounced dependence of the 800–900 nm absorption on the solvent and counterion. Reduction of the neutral form occurs in an electrochemically reversible step to yield an anion with an intense near‐infrared absorption at 1345 nm (ε=10400 M ^?1 cm^?1) and a conventional g factor splitting for a largely metal‐based spin (S=1/2), suggesting a [(Q_M ^{. ?})Ni^II(Q_M^2?)]^? configuration with a tetracoordinate metal atom with antiferromagnetic Ni^II–(Q_M ^{. ?}) interactions and symmetry‐allowed ligand‐to‐ligand intervalence charge transfer (LLIVCT). Calculations are used to understand the Ni?S binding activity as induced by remote electron transfer at the iminobenzoquinone redox system.     

Photoactive Nickel Complexes in Cross-Coupling Catalysis

Transition metal catalyzed cross-coupling reactions are important in chemical synthesis for the formation of C−C and C-heteroatom bonds. Suitable catalysts are frequently based on palladium or nickel, and lately the cheaper and more abundant first-row transition metal element has been much in focus. The combination of nickel catalysis with photoredox chemistry has opened new synthetic possibilities, and in some cases electronically excited states of nickel complexes play a key role. This is a remarkable finding, because photo-excited metal complexes are underexplored in the context of organic bond-forming reactions, and because the photophysics and the photochemistry of first-row transition metal complexes are underdeveloped in comparison with their precious metal-based congeners. Consequently, there is much potential for innovation at the interface of synthetic-organic and physical-inorganic chemistry. This Minireview highlights recent key findings in light-driven nickel catalysis and identifies essential concepts for the exploitation of photoactive nickel complexes in organic synthesis.     

Coordination and Activation of the BF Molecule

No CO : Fluoroborylene (BF), isoelectronic with CO and N₂, can be trapped by a transition metal. The structurally characterized complex [{CpRu(CO)₂}₂(μ‐BF)] contains an unsupported bridging BF ligand, which is unprecedented in the structural chemistry of CO. With AlCl₃ , metal‐bound CO coordinates through the O atom without bond rupture, while the more polar and less π‐bonded BF ligand is heterolytically cleaved (see picture).

     

Polyboramines for Hydrogen Release: Polymers Containing Lewis Pairs in their Backbone

The one‐step polycondensation of diamines and diboranes triggered by the in situ deprotonation of the diammonium salts and concomitant reduction of bisboronic acids leads to the assembly of polymer chains through multiple Lewis pairing in their backbone. These new polyboramines are dihydrogen reservoirs that can be used for the hydrogenation of imines and carbonyl compounds. They also display a unique dihydrogen thermal release profile that is a direct consequence of the insertion of the amine–borane linkages in the polymeric backbone.     

Beryllium Phosphine Complexes: Synthesis,Properties, and Reactivity of (PMe3)2BeCl2 and (Ph2PC3H6PPh2)BeCl2

The directed synthesis, spectroscopic properties, and reactivity of bis(trimethylphosphine) beryllium dichloride ( 1 ) and bis(diphenylphosphino)propane beryllium dichloride ( 2 ) are reported, including the crystal structure of (PMe₃)₂BeCl₂ ( 1 ). These four‐coordinate beryllium compounds can be alkylated with n‐butyllithium (ⁿBuLi) to give three‐coordinate (Ph₂PC₃H₆PPh₂)BeⁿBu₂ ( 3 ) and (PMe₃)BeⁿBu₂ ( 4 ). PMe₃ can be removed from (PMe₃)BeⁿBu₂ ( 4 ) in vacuo to yield [ⁿBu₂Be]₂ ( 5 ). For the first time, the presence of [ⁿBu₂Be]₂ as a dimer in solution, which has been postulated for decades, could be observed spectroscopically. This novel, ether‐free pathway provides access to beryllium dialkyl compounds that have never been in contact with oxygen‐atom‐containing reagents or solvents. This “freeness from oxygen” is crucial for semiconductor applications where oxygen is often unwanted and must be avoided at all costs.     

Stereo‐ and Temporally Controlled Coordination Polymerization Triggered by Alternating Addition of a Lewis Acid and Base

Significant progress has been made with regard to temporally controlled radical and ring‐opening polymerizations, for example, by means of chemical reagents, light, and voltage, whereas quantitative switch coordination polymerization is still challenging. Herein, we report the temporally and stereocontrolled 3,4‐polymerization of isoprene through allosterically regulating the active metal center by alternating addition of Lewis basic pyridine to “poison” the Lewis acidic active metal species through acid–base interactions and Lewis acidic AlⁱBu₃ to release the original active species through pyridine abstraction. This process is quick, quantitative, and can be repeated multiple times while maintaining high 3,4‐selectivity. Moreover, this strategy is also effective for the switch copolymerization of isoprene and styrene with dual 3,4‐ and syndiotactic selectivity. Tuning the switch cycles and intervals enables the isolation of various copolymers with different distributions of 3,4‐polyisoprene and syndiotactic polystyrene sequences.     

Establishing the Coordination Chemistry of Antimony(V) Cations: Systematic Assessment of Ph4Sb(OTf) and Ph3Sb(OTf)2 as Lewis Acceptors

The coordination chemistry of the stiboranes Ph₄Sb(OTf) ( 1 a , OTf = OSO₂CF₃) and Ph₃Sb(OTf)₂ ( 3 ) with Lewis bases has been investigated. The significant steric encumbrance of the Sb center in 1 a precludes interaction with most ligands, but the relatively low steric demands of 4‐methylpyridine‐N‐oxide (OPyrMe) and OPMe₃ enabled the characterization of [Ph₄Sb(OPyrMe)][OTf] ( 2 a ) and [Ph₄Sb(OPMe₃)][OTf] ( 2 b ), rare examples of structurally characterized complexes of stibonium acceptors. In contrast, 3 was found to engage a variety of Lewis bases, forming stable isolable complexes of the form [Ph₃Sb(donor)₂][OTf]₂ [donor=OPMe₃ ( 6 a ), OPCy₃ ( 6 b , Cy=cyclohexyl), OPPh₃ ( 6 c ), OPyrMe ( 6 d )], [Ph₃Sb(dmap)₂(OTf)][OTf] ( 6 e , dmap=4‐(dimethylamino)pyridine) and [Ph₃Sb(donor)(OTf)][OTf] [donor=1,10‐phenanthroline ( 7 a ) or 2,2′‐bipy ( 7 b , bipy=bipyridine)]. These compounds exhibit significant structural diversity in the solid‐state, and undergo ligand exchange reactions in line with their assignment as coordination complexes. Compound 3 did not form stable complexes with phosphine donors, with reactions instead leading to redox processes yielding SbPh₃ and products of phosphine oxidation.     

N-Heterocyclic Carbene Derived 3-Azabutadiene as a π-Base in Classic and Frustrated Lewis Pair Chemistry

N-Heterocyclic carbene (NHC) derived 3-azabutadienes 1 and 2 have been prepared by a single-step reaction of the corresponding NHC with cyclohexyl isocyanide. Compound 1 features π-basic, delocalized nucleophilic sites over the 3-azabutadiene moiety, therefore allowing for coordinating with small Lewis acids, such as AlCl₃, GaCl₃, and Me₂SAuCl, to form diverse classic Lewis adducts 3 – 5 . Combination of 1 with B(C₆F₅)₃ or [Ph₃C][B(C₆F₅)₄] resulted in single-electron transfer and the obtained radical cation was detected by EPR. In addition, a frustrated Lewis pair comprised of the π-basic 1 and BPh₃ effects the splitting of the O−H bond of phenol and the N−H bond of imidazole to give 7 and 8 , respectively. An intrinsic bond orbital (IBO) analysis of the pathway leading to 8 showcases the transformation of the delocalized π-electrons of 1 to a newly formed C−H localized σ-bond.     

Synthesis of Square‐Planar Aluminum(III) Complexes

The synthesis of two four‐coordinate and square planar (SP) complexes of aluminum(III) is presented. Reaction of a phenyl‐substituted bis(imino)pyridine ligand that is reduced by two electrons, Na₂(^PhI₂P^2?), with AlCl₃ afforded five‐coordinate [(^PhI₂P^2?)Al(THF)Cl] ( 1 ). Square‐planar [(^PhI₂P^2?)AlCl] ( 2 ) was obtained by performing the same reaction in diethyl ether followed by lyphilization of 2 from benzene. The four‐coordinate geometry index for 2 , τ₄, is 0.22, where 0 would be a perfectly square‐planar molecule. The analogous aluminum hydride complex, [(^PhI₂P^2?)AlH] ( 3 ), is also square‐planar, and was characterized crystallographically and has τ₄=0.13. Both 2 and 3 are Lewis acidic and bind 2,6‐lutidine.     

Correlations and Contrasts in Homo‐ and Heteroleptic Cyclic (Alkyl)(amino)carbene‐Containing Pt0 Complexes

An improved synthetic route to homoleptic complex [Pt(CAAC^Me)₂] (CAAC=cyclic (alkyl)(amino)carbenes) and convenient routes to new heteroleptic complexes of the form [Pt(CAAC^Me)(PR₃)] are presented. Although the homoleptic complex was found to be inert to many reagents, oxidative addition and metal‐only Lewis pair (MOLP) formation was observed from one of the heteroleptic complexes. The spectroscopic, structural, and electrochemical properties of the zero‐valent complexes were explored in concert with density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) calculations. The homoleptic [Pt(CAAC)₂] and heteroleptic [Pt(CAAC)(PR₃)] complexes were found to be similar in their spectroscopic and structural properties, but their electrochemical behavior and reactivity differ greatly. The unusually strong color of the CAAC‐containing Pt⁰ complexes was investigated by TD‐DFT calculations and attributed to excitations into the LUMOs of the complexes, which are predominantly composed of bonding π interactions between Pt and the CAAC carbon atoms.     

Experimental and Computational Studies of the Molybdenum‐Flanking Arene Interaction in Quadruply Bonded Dimolybdenum Complexes with Terphenyl Ligands

To clarify the nature of the Mo?C_arene interaction in terphenyl complexes with quadruple Mo?Mo bonds, ether adducts of composition [Mo₂(Ar′)(I)(O₂CR)₂(OEt₂)] have been prepared and characterized (Ar′=Ar^Xyl₂, R=Me; Ar′=ArMes₂, R=Me; Ar′=Ar^Xyl2, R=CF₃) (Mes=mesityl; Xyl=2,6‐Me₂C₆H₃, from now on xylyl) and their reactivity toward different neutral Lewis bases investigated. PMe₃, P(OMe)₃ and PiPr₃ were chosen as P‐donors and the reactivity studies complemented with the use of the C‐donors CNXyl and CN₂C₂Me₄ (1,3,4,5‐tetramethylimidazol‐2‐ylidene). New compounds of general formula [Mo₂(Ar′)(I)(O₂CR)₂( L )] were obtained, except for the imidazol‐2‐ylidene ligand that yielded a salt‐like compound of composition [Mo₂(Ar^Xyl2)(O₂CMe)₂(CN₂C₂Me₄)₂]I. The Mo?C_arene interaction in these complexes has been analyzed with the aid of X‐ray data and computational studies. This interaction compensates the coordinative and electronic unsaturation of one of the Mo atoms in the above complexes, but it seems to be weak in terms of sharing of electron density between the Mo and C_arene atoms and appears to have no appreciable effect in the length of the Mo?Mo, Mo?X, and Mo? L bonds present in these molecules.     

First Dinuclear Copper/Gallium Complexes: Supporting Cu0 and CuI Centres by Low‐Valent Organogallium Ligands

The synthesis and structural characterisation of low‐valent dinuclear copper(I) and copper(0) complexes supported by organogallium ligands has been accomplished for the first time by the reductive coordination reaction of [GaCp*] (Cp*=pentamethylcyclopentadienyl) and [Ga(ddp)] (ddp=HC(CMeNC₆H₃‐2,6‐iPr₂)₂ 2‐diisopropylphenylamino‐4‐diisopropylphenylimino‐2‐pentene) with readily available copper(II) and copper(I) precursors. The treatment of CuBr₂ and Cu(OTf)₂ (OTf=CF₃SO₃) with [Ga(ddp)] under mild conditions resulted in elimination of [Ga(L)₂(ddp)] (L=Br, OTf) and afforded the novel gallium(I)/copper(I) compounds [{(ddp)GaCu(L)}₂] (L=Br ( 1 ), OTf ( 2 )). The single‐crystal X‐ray structure determinations of 1 and 2 reveal that these molecules are composed of {(ddp)GaCu(L)} dimeric units, with planar Cu^I? Ga^I four‐membered rings and short Cu^I???Cu^I distances, with 2 exhibiting the shortest Cu^I???Cu^I contact reported to date of 2.277(3) Å. The all‐gallium coordinated dinuclear [Cu₂(GaCp*)(μ‐GaCp*)₃Ga(OTf)₃] ( 3 ) is formed when Cu(OTf)₂ is combined with [GaCp*] instead of [Ga(ddp)]. Notably, in the course of this redox reaction Lewis acidic Ga(OTf)₃ is formed, which coordinates to one of the electron‐rich copper(0) centres. Compound 3 is suggested as the first case of a structurally characterised complex of copper(0). By changing the copper(II) to a copper(I) source, that is, [Cu(cod)₂][OTf] (cod=1,5‐cyclooctadiene), the salt [Cu₂(GaCp*)₃(μ‐GaCp*)₂][OTf]₂ ( 4 ) is formed, the cationic part of which is related to previously described isoelectronic dinuclear d¹⁰ complexes of the type [M₂(GaCp*)₅] (M=Pd, Pt).     

Coordination Complexes of Ph3Sb2+ and Ph3Bi2+: Beyond Pnictonium Cations

The syntheses of salts containing ligand‐stabilized Ph₃Sb²⁺ and Ph₃Bi²⁺ dications have been realized by in situ formation of Ph₃Pn(OTf)₂ (Pn=Sb or Bi) and subsequent reaction with OPPh₃, dmap and bipy. The solid‐state structures demonstrate diversity imposed by the steric demands and nature of the ligands. The synthetic method has the potential for broad application enabling widespread development of the coordination chemistry for Pn^V acceptors.     

Organotellurium Ligands & Their Metal Complexes: Recent Developments

Abstract

The ligand chemistry of telluroethers, halotellurium ligands, and polytellurides has received good attention in the last decade. Tellurium-containing species have been used to design clusters. In the recent past the ligation of di and tri-telluroethers (including bis(4-methoxyphenyltelluro)methane) has been studied. Hybrid organotellurium ligands, N-[2-(4-methoxyphenyltelluro)propyl]phthalimid (L ¹ ), 2-(4-ethoxyphenyltelluromethyl)-tetrahydro-2H-pyran (L ² ), 2-(2-{4-ethoxyphenyl} telluroethyl)-1,3-dioxane (L ³ ), N-{2-(4-methoxyphenyltelluro)ethyl}morpholine (L ⁴ ), N-{2-(4-methoxyphenyltelluro)ethyl}-pyrrolidine (L ⁵ ), bis{2-(pyrrolidine-N-yl)ethyl}telluride (L ⁶ ), 1-(4-methoxyphenyltelluro)-2-[3-(6-methyl-2-pyridyl) propoxy]ethane (L ⁷ ), and 2-[2-(4-methoxyphenyltelluro)ethyl]thiophene (L ⁸ ) have been designed recently and studied for their complexation reactions. The (Te, N) and (N, Te, N) ligands, L ⁵ and L ⁶ , coordinate with Hg(II) through Te and N both, but the bonding with N is some what weak. The morpholine nitrogen of L ⁴ does not coordinate with Pd(II) or Pt(II) along with Te. The L ⁷ behaving as a (Te, N) ligand has formed 20-membered metallomacrocycle ring with Pt(II). Tellurated Schiff bases 4-MeOC₆H₄TeCH₂CH₂N═C(CH₃)C₆H₄-2-OH (L ⁹ ) and 2-HO-C₆H₄-(CH₃)C═NCH₂CH₂TeCH₂CH₂N═C(CH₃)C₆H₄-2-OH (L ¹⁰ ) and their reduction products 4-MeOC₆H₄TeCH₂CH₂NHCH(CH₃)C₆H₄-2-OH (L ¹¹ ) and 2-HO-C₆H₄-(CH₃)CHNHCH₂CH₂TeCH₂CH₂NHCH(CH₃)C₆H₄-2-OH (L ¹² ) respectively have been synthesized and studied for ligation behaviour. The L ⁹ on reaction with the [Ru(p-cymene)Cl₂]₂ results in [Ru(p-cymene)(4-MeOC₆H₄TeCH₂CH₂NH₂)Cl]Cl · H₂O whereas in the reaction of L ¹⁰ with [Ru(p-cymene) Cl₂]₂, p-cymene ligand is lost resulting in [RuCl(L ¹⁰ -H)]. The recent developments, particularly designing of L ¹ to L ¹² and their ligand chemistry, are reviewed in the present paper.