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).     

Metal–Metal Interactions in Heterobimetallic Complexes with Dinucleating Redox‐Active Ligands

The tuning of metal–metal interactions in multinuclear assemblies is a challenge. Selective P coordination of a redox‐active PNO ligand to Au^I followed by homoleptic metalation of the NO pocket with Ni^II affords a unique trinuclear Au–Ni–Au complex. This species features two antiferromagnetically coupled ligand‐centered radicals and a double intramolecular d⁸–d¹⁰ interaction, as supported by spectroscopic, single‐crystal X‐ray diffraction, and computational data. A corresponding cationic dinuclear Au–Ni analogue with a stronger d⁸–d¹⁰ interaction is also reported. Although both heterobimetallic structures display rich electrochemistry, only the trinuclear Au–Ni–Au complex facilitates electrocatalytic C?X bond activation of alkyl halides in its doubly reduced state. Hence, the presence of a redox‐active ligand framework, an available coordination site at gold, and the nature of the nickel–gold interaction appear to be essential for this reactivity.     

Two‐Coordinate Magnesium(I) Dimers Stabilized by Super Bulky Amido Ligands

A variety of very bulky amido magnesium iodide complexes, LMgI(solvent)_0/1 and [LMg(μ‐I)(solvent)_0/1]₂ (L=‐N(Ar)(SiR₃); Ar=C₆H₂{C(H)Ph₂}₂R′‐2,6,4; R=Me, Prⁱ, Ph, or OBu^t; R′=Prⁱ or Me) have been prepared by three synthetic routes. Structurally characterized examples of these materials include the first unsolvated amido magnesium halide complexes, such as [LMg(μ‐I)]₂ (R=Me, R′=Prⁱ). Reductions of several such complexes with KC₈ in the absence of coordinating solvents have afforded the first two‐coordinate magnesium(I) dimers, LMg?MgL (R=Me, Prⁱ or Ph; R′=Prⁱ, or Me), in low to good yields. Reductions of two of the precursor complexes in the presence of THF have given the related THF adduct complexes, L(THF)Mg?Mg(THF)L (R=Me; R′=Prⁱ) and LMg?Mg(THF)L (R=Prⁱ; R′=Me) in trace yields. The X‐ray crystal structures of all magnesium(I) complexes were obtained. DFT calculations on the unsolvated examples reveal their Mg?Mg bonds to be covalent and of high s‐character, while Ph???Mg bonding interactions in the compounds were found to be weak at best.     

Saturated Heavier Group 14 Compounds as ‐Electron‐Acceptor (Z‐Type) Ligands

This review article describes the chemistry of transition‐metal complexes containing heavier group 14 elements (Si, Ge, and Sn) as the σ‐electron‐acceptor (Z‐type) ligands and discusses the characteristics of bonds between the transition metal and Z‐type ligand. Moreover, we review the iridium hydride mediated cleavage of E–X bonds (E=Si, Ge; X=F, Cl), where the key intermediates are pentacoordinate silicon or germanium compounds bearing a dative M→E bond.     

Revealing the Bonding Pattern from the Molecular Electron Density Using Single Exponential Decay Detector: An Orbital‐Free Alternative to the Electron Localization Function

We introduce a new tool (single exponential decay detector: SEDD) to extract information about bonding and localization in atoms, molecules, or molecular assemblies. The practical evaluation of SEDD does not require any explicit information about the orbitals. The only quantity needed is the electron density (calculated or experimental) and its derivatives up to the second order.     

Well‐Defined Models for the Elusive Dinuclear Intermediates of the Pauson–Khand Reaction

The mechanism of the Pauson–Khand reaction has attracted significant interest due to the unusual dinuclear nature of the Co₂(CO)_x active site. Experimental and computational data have indicated that the intermediates following the initial Co₂(CO)₆(alkyne) complex are thermodynamically unstable and do not build up in appreciable concentrations during the course of the reaction. As a consequence, the key steps that control the scope of viable substrates and various aspects of selectivity have remained largely uncharacterized. Herein, a direct experimental investigation of the dinuclear metallacycle‐forming step of the Pauson–Khand reaction is reported. These studies capitalize on well‐defined d⁹–d⁹ dinickel complexes supported by a naphthyridine–diimine (NDI) pincer ligand as functional surrogates of Co₂(CO)₈.     

Silyl‐Phosphino‐Carbene Complexes of Uranium(IV)

Unprecedented silyl‐phosphino‐carbene complexes of uranium(IV) are presented, where before all covalent actinide–carbon double bonds were stabilised by phosphorus(V) substituents or restricted to matrix isolation experiments. Conversion of [U(BIPM^TMS)(Cl)(μ‐Cl)₂Li(THF)₂] ( 1 , BIPM^TMS=C(PPh₂NSiMe₃)₂) into [U(BIPM^TMS)(Cl){CH(Ph)(SiMe₃)}] ( 2 ), and addition of [Li{CH(SiMe₃)(PPh₂)}(THF)]/Me₂NCH₂CH₂NMe₂ (TMEDA) gave [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(μ‐Cl)Li(TMEDA)(μ‐TMEDA)_0.5]₂ ( 3 ) by α‐hydrogen abstraction. Addition of 2,2,2‐cryptand or two equivalents of 4‐N,N‐dimethylaminopyridine (DMAP) to 3 gave [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(Cl)][Li(2,2,2‐cryptand)] ( 4 ) or [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(DMAP)₂] ( 5 ). The characterisation data for 3 – 5 suggest that whilst there is evidence for 3‐centre P?C?U π‐bonding character, the U=C double bond component is dominant in each case. These U=C bonds are the closest to a true uranium alkylidene yet outside of matrix isolation experiments.     

f‐Element–Metal Bonding and the Use of the Bond Polarity To Build Molecular Intermetalloids

Metal–metal bonding in heterobimetallic complexes is of fundamental interest due to its implications to both bonding theory and new reactivities. In this Concept, structurally authenticated molecular compounds with direct bonds between rare‐earth metals or actinoids and transition or main group metals are summarized. Special attention is given to the use of bond polarity as a tool for designing molecular intermetalloids incorporating rare‐earth atoms and transition metals.     

Bridging the Gap between Coordination and Cluster Compounds: Unusual Bonding Modes for Zinc

Inner virtue : The reaction of [Mo⁰(GaCp*)₆] with ZnMe₂ yields the novel multinuclear zinc system [MoZn₁₂Me₉Cp*₃] which geometrically resembles a classical Wade–Mingos cluster (with an interstitial molybdenum atom, see picture). In reality, its electronic structure features little direct Zn? Zn bonding, relying instead on Mo? Zn three‐center, two‐electron bonds spanning the body diagonals of the approximately icosahedral framework.

     

Structural and Photophysical Study on Heterobimetallic Complexes with d8–d10 Interactions Supported by Carborane Ligands: Theoretical Analysis of the Emissive Behaviour

Heterobimetallic complexes of formula [M{(PPh₂)₂C₂B₉H₁₀}(S₂C₂B₁₀H₁₀)M′(PPh₃)] (M=Pd, Pt; M′=Au, Ag, Cu) and [Ni{(PPh₂)₂C₂B₉H₁₀}(S₂C₂B₁₀H₁₀)Au(PPh₃)] were obtained from the reaction of [M{(PPh₂)₂C₂B₁₀H₁₀}(S₂C₂B₁₀H₁₀)] (M=Pd, Pt) with [M′(PPh₃)]⁺ (M′=Au, Ag, Cu) or by one‐pot synthesis from [(SH)₂C₂B₁₀H₁₀], (PPh₂)₂C₂B₁₀H₁₀, NiCl₂ ? 6 H₂O, and [Au(PPh₃)]⁺. They display d⁸–d¹⁰ intermetallic interactions and emit red light in the solid state at 77 K. Theoretical studies on [M{(PPh₂)₂C₂B₉H₁₀}(S₂C₂B₁₀H₁₀)Au(PPh₃)] (M=Pd, Pt, Ni) attribute the luminescence to ligand (thiolate, L)‐to‐“P₂‐M‐S₂” (ML′) charge‐transfer (LML′CT) transitions for M=Pt and to metal (M)‐to‐“P₂‐M‐S₂” (ML′) charge‐transfer (MML′CT) transitions for M=Ni, Pd.     

Catalytic Electron‐Transfer Oxygenation of Substrates with Water as an Oxygen Source Using Manganese Porphyrins

Manganese(V)–oxo–porphyrins are produced by the electron‐transfer oxidation of manganese–porphyrins with tris(2,2′‐bipyridine)ruthenium(III) ([Ru(bpy)₃]³⁺; 2 equiv) in acetonitrile (CH₃CN) containing water. The rate constants of the electron‐transfer oxidation of manganese–porphyrins have been determined and evaluated in light of the Marcus theory of electron transfer. Addition of [Ru(bpy)₃]³⁺ to a solution of olefins (styrene and cyclohexene) in CH₃CN containing water in the presence of a catalytic amount of manganese–porphyrins afforded epoxides, diols, and aldehydes efficiently. Epoxides were converted to the corresponding diols by hydrolysis, and were further oxidized to the corresponding aldehydes. The turnover numbers vary significantly depending on the type of manganese–porphyrin used owing to the difference in their oxidation potentials and the steric bulkiness of the ligand. Ethylbenzene was also oxidized to 1‐phenylethanol using manganese–porphyrins as electron‐transfer catalysts. The oxygen source in the substrate oxygenation was confirmed to be water by using ¹⁸O‐labeled water. The rate constant of the reaction of the manganese(V)–oxo species with cyclohexene was determined directly under single‐turnover conditions by monitoring the increase in absorbance attributable to the manganese(III) species produced in the reaction with cyclohexene. It has been shown that the rate‐determining step in the catalytic electron‐transfer oxygenation of cyclohexene is electron transfer from [Ru(bpy)₃]³⁺ to the manganese–porphyrins.     

Metal–metal bonding in biscycloheptatrienyl dimetal compounds of the second‐row transition metals

Density functional theory has been used to examine the dimetallocene‐like dicycloheptatrienyl dimetal compounds of the second‐row transition metals (C₇H₇)₂M₂ (M = Ru, Tc, Mo, Nb, Zr). The lowest energy (C₇H₇)₂Mo₂ structure is a coaxial structure with terminal η⁷? C₇H₇ rings, whereas the lowest energy (C₇H₇)₂M₂ structures (M = Ru, Tc, Nb, Zr) are perpendicular structures with bridging η⁴,η⁴? C₇H₇ rings except for the perpendicular (η⁴,η³? C₇H₇)₂Ru₂ structure. The metal–metal bond orders in the (C₇H₇)₂M₂ structures (M = Ru, Tc, Mo, Nb), as determined by analysis of their frontier molecular orbitals, suggest preferred 16‐ rather than 18‐electron configurations for the central metal atoms. Thus, in the coaxial (η⁷? C₇H₇)₂M₂ structures the formal bond orders are two for M = Tc and three for M = Mo. For the perpendicular structures both (η⁴,η³? C₇H₇)₂Ru₂ and (η⁴,η⁴? C₇H₇)₂Tc₂ have 16‐electron configurations with metal–metal single bonds owing to the different modes of bonding of the bridging C₇H₇ rings in the two structures. For the (C₇H₇)₂Zr₂ system the perpendicular structure has a formal Zr?Zr double bond and the coaxial structure has a very long (～3.5 Å) Zr? Zr bond indicating only 12‐ to 14‐electron configurations for the zirconium atoms.     

The Quest for PdII–PdII Interactions: Structural and Spectroscopic Studies and Ab Initio Calculations on Dinuclear [Pd2(CN)4(μ‐diphosphane)2] Complexes

Structural and spectroscopic properties of and theoretical investigations on dinuclear [Pd₂(CN)₄(P–P)₂] (P–P=bis(dicyclohexylphosphanyl)methane ( 1 ), bis(dimethylphosphanyl)methane ( 2 )) and mononuclear trans‐[Pd(CN)₂(PCy₃)₂] ( 3 ) complexes are described. Xray structural analyses reveal Pd???Pd distances of 3.0432(7) and 3.307(4) Å in 1 and 2 , respectively. The absorption bands at λ>270 nm in 1 and 2 have 4d →5p_σ electronic‐transition character. Calculations at the CIS level indicate that the two low‐lying dipole‐allowed electronic transition bands in model complex [Pd₂(CN)₄(μ‐H₂PCH₂PH₂)₂] at 303 and 289 nm are due to combinations of many orbital transitions. The calculated interaction‐energy curve for the skewed dimer [{trans‐[Pd(CN)₂(PH₃)₂]}₂] is attractive at the MP2 level and implies the existence of a weak Pd^II–Pd^II interaction.     

Investigation of substituent effects in aerogen‐bonding interaction between ZO3 (Z=Kr,Xe) and nitrogen bases

The substituent effects in aerogen bond interactions between ZO₃ (Z = Kr, Xe) and different nitrogen bases are studied at the MP2/aug‐cc‐pVTZ level of theory. The nitrogen bases include the sp bases NCH, NCF, NCCl, NCBr, NCCN, NCOH, NCCH₃ and the sp³ bases NH₃, NH₂F, NH₂Cl, NH₂Br, NH₂CN, NH₂OH, and NH₂CH₃. The nature of aerogen bonds in these complexes is analyzed by means of molecular electrostatic potential, electron localization function, quantum theory atoms in molecules, noncovalent interaction index, and natural bond orbital analyses. The interaction energy (E_int) ranges from ?4.59 to ?9.65 kcal/mol in the O₃Z···NCX complexes and from ?5.30 to ?13.57 kcal/mol in the O₃Z···NH₂X ones. The dominant charge‐transfer interaction in these complexes occurs across the aerogen bond from the nitrogen lone‐pair (n_N) of the Lewis base to the σ*_Z‐O antibonding orbital of the ZO₃. Besides, the formation of aerogen bond tends to decrease the ⁸³Kr or ¹³¹Xe chemical shielding values in these complexes. © 2016 Wiley Periodicals, Inc.     

Unprecedented Efficient Structure Controlled Phosphorescence of Silver(I) Clusters Stabilized by Carba‐closo‐dodecaboranylethynyl Ligands

{Ag₂(12‐C≡C‐closo‐1‐CB₁₁H₁₁)}_n and selected pyridine ligands have been used for the synthesis of photostable Ag^I clusters that, with one exception, exhibit for Ag^I compounds unusual room‐temperature phosphorescence. Extraordinarily intense phosphorescence was observed for a distorted pentagonal bipyramidal Ag^I₇ cluster that shows an unprecedented quantum yield of Φ=0.76 for Ag^I clusters. The luminescence properties correlate with the structures of the central Ag^I_n motifs as shown by comparison of the emission properties of the clusters with different numbers of Ag^I ions, different charges, and electronically different pyridine ligands.     

Detailed Investigation of the Structural,Thermal, and Electronic Properties of Gold Isocyanide Complexes with Mechano‐Triggered Single‐Crystal‐to‐Single‐Crystal Phase Transitions

Mechano‐induced phase transitions in organic crystalline materials, which can alter their properties, have received much attention. However, most mechano‐responsive molecular crystals exhibit crystal‐to‐amorphous phase transitions, and the intermolecular interaction patterns in the daughter phase are difficult to characterize. We have investigated phenyl(phenylisocyanide)gold(I) ( 1 ) and phenyl(3,5‐dimethylphenylisocyanide)gold(I) ( 2 ) complexes, which exhibit a mechano‐triggered single‐crystal‐to‐single‐crystal phase transition. Previous reports of complexes 1 and 2 have focused on the relationships between the crystalline structures and photoluminescence properties; in this work we have focused on other aspects. The face index measurements of complexes 1 and 2 before and after the mechano‐induced phase transitions have indicated that they undergo non‐epitaxial phase transitions without a rigorous orientational relationship between the mother and daughter phases. Differential scanning calorimetry analyses revealed the phase transition of complex 1 to be enthalpically driven by the formation of new aurophilic interactions. In contrast, the phase transition of complex 2 was found to be entropically driven, with the closure of an empty void in the mother phase. Scanning electron microscopy observation showed that the degree of the charging effect of both complexes 1 and 2 was changed by the phase transitions, which suggests that the formation of the aurophilic interactions affords more effective conductive pathways. Moreover, flash‐photolysis time‐resolved microwave conductivity measurements revealed that complex 1 increased in conductivity after the phase change, whereas the conductivity of complex 2 decreased. These contrasting results were explained by the different patterns in the aurophilic interactions. Finally, an intriguing disappearing polymorphism of complex 2 has been reported, in which a polymorph form could not be obtained again after some period of time, even with repeated trials. The present studies provide us with a variety of hitherto unknown insights into mechano‐responsive molecular crystals, which help us to understand the phase transition behaviors upon mechanical stimulation and establish rational design principles.     

Intramolecularly Stabilised Group 10 Metal Stannyl and Stannylene Complexes: Multi‐pathway Synthesis and Observation of Platinum‐to‐Tin Alkyl Transfer

Reaction of [PdClMe(P^N)₂] with SnCl₂ followed by Cl‐abstraction leads to apparent Pd?C bond activation, resulting in methylstannylene species trans‐[PdCl{(P^N)₂SnClMe}][BF₄] (P^N=diaryl phosphino‐N‐heterocycle). In contrast, reaction of Pt analogues with SnCl₂ leads to Pt?Cl bond activation, resulting in methylplatinum species trans‐[PtMe{(P^N)₂SnCl₂}][BF₄]. Over time, they isomerise to methylstannylene species, indicating that both kinetic and thermodynamic products can be isolated for Pt, whereas for Pd only methylstannylene complexes are isolated. Oxidative addition of RSnCl₃ (R=Me, Bu, Ph) to M⁰ precursors (M=Pd or Pt) in the presence of P^N ligands results in diphosphinostannylene pincer complexes trans‐[MCl{(P^N)₂SnCl(R)}][SnCl₄R], which are structurally similar to the products from SnCl₂ insertion. This showed that addition of RSnCl₃ to M⁰ results in formal Sn?Cl bond oxidative addition. A probable pathway of activation of the tin reagents and formation of different products is proposed and the relevancy of the findings for Pd and Pt catalysed processes that use SnCl₂ as a co‐catalyst is discussed.     

d–d Dative Bonding Between Iron and the Alkaline‐Earth Metals Calcium,Strontium, and Barium

Double deprotonation of the diamine 1,1′‐(tBuCH₂NH)‐ferrocene ( 1 ‐H₂) by alkaline‐earth (Ae) or Eu^II metal reagents gave the complexes 1 ‐Ae (Ae=Mg, Ca, Sr, Ba) and 1 ‐Eu. 1 ‐Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe???Mg distance in 1 ‐Mg is too long for a bonding interaction, but short Fe???Ae distances in 1 ‐Ca, 1 ‐Sr, and 1 ‐Ba clearly support intramolecular Fe???Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related ¹H NMR chemical‐shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1 ‐Ae complexes shows that the heavier species 1 ‐Ca, 1 ‐Sr, and 1 ‐Ba possess genuine Fe→Ae bonds which involve vacant d‐orbitals of the alkaline‐earth atoms and partially filled d‐orbitals on Fe. In 1 ‐Mg, a weak Fe→Mg donation into vacant p‐orbitals of the Mg atom is observed.     

Nickel(I) Monomers and Dimers with Cyclopentadienyl and Indenyl Ligands

The reaction of (μ‐Cl)₂Ni₂(NHC)₂ (NHC=1,3‐bis(2,6‐diisopropylphenyl)‐1,3‐dihydro‐2H‐imidazol‐2‐ylidene (IPr) or 1,3‐bis(2,6‐diisopropylphenyl)imidazolidin‐2‐ylidene (SIPr)) with either one equivalent of sodium cyclopentadienyl (NaCp) or lithium indenyl (LiInd) results in the formation of diamagnetic NHC supported Ni^I dimers of the form (μ‐Cp)(μ‐Cl)Ni₂(NHC)₂ (NHC=IPr ( 1 a ) or SIPr ( 1 b ); Cp=C₅H₅) or (μ‐Ind)(μ‐Cl)Ni₂(NHC)₂ (NHC=IPr ( 2 a ) or SIPr ( 2 b ); Ind=C₇H₉), which contain bridging Cp and indenyl ligands. The corresponding reaction between two equivalents of NaCp or LiInd and (μ‐Cl)₂Ni₂(NHC)₂ (NHC=IPr or SIPr) generates unusual 17 valence electron Ni^I monomers of the form (η⁵‐Cp)Ni(NHC) (NHC=IPr ( 3 a ) or SIPr ( 3 b )) or (η⁵‐Ind)Ni(NHC) (NHC=IPr ( 4 a ) or SIPr ( 4 b )), which have nonlinear geometries. A combination of DFT calculations and NBO analysis suggests that the Ni^I monomers are more strongly stabilized by the Cp ligand than by the indenyl ligand, which is consistent with experimental results. These calculations also show that the monomers have a lone unpaired‐single‐electron in their valence shell, which is the reason for the nonlinear structures. At room temperature the Cp bridged dimer (μ‐Cp)(μ‐Cl)Ni₂(NHC)₂ undergoes homolytic cleavage of the Ni?Ni bond and is in equilibrium with (η⁵‐Cp)Ni(NHC) and (μ‐Cl)₂Ni₂(NHC)₂. There is no evidence that this equilibrium occurs for (μ‐Ind)(μ‐Cl)Ni₂(NHC)₂. DFT calculations suggest that a thermally accessible triplet state facilitates the homolytic dissociation of the Cp bridged dimers, whereas for bridging indenyl species this excited triplet state is significantly higher in energy. In stoichiometric reactions, the Ni^I monomers (η⁵‐Cp)Ni(NHC) or (η⁵‐Ind)Ni(NHC) undergo both oxidative and reductive processes with mild reagents. Furthermore, they are rare examples of active Ni^I precatalysts for the Suzuki–Miyaura reaction. Complexes 1 a , 2 b , 3 a , 4 a and 4 b have been characterized by X‐ray crystallography.