Oxidation,Coordination, and Nickel‐Mediated Deconstruction of a Highly Electron‐Rich Diboron Analogue of 1,3,5‐Hexatriene

The reductive coupling of an N‐heterocyclic carbene (NHC) stabilized (dibromo)vinylborane yields a 1,2‐divinyldiborene, which, although isoelectronic to a 1,3,5‐triene, displays no extended π conjugation because of twisting of the C₂B₂C₂ chain. While this divinyldiborene coordinates to copper(I) and platinum(0) in an η²‐B₂ and η⁴‐C₂B₂ fashion, respectively, it undergoes a complex rearrangement to an η⁴‐1,3‐diborete upon complexation with nickel(0).     

Exohedral Complexation of B40, C60 and Arenes with Transition Metals: A Comparative DFT Study

Holes are inevitable in borospherenes. The surface topography of B₄₀ and its π MOs isolobal to benzene allow for better η⁷‐, η⁶‐ and η³‐ exohedral complexation with transition metal fragments than it is possible with C₆₀ and arenes. η⁷‐complexes of B₄₀ is lower in energy than the η⁶‐complexes for metal fragments such as C₅H₅Mn, C₄H₄Fe, and C₃H₃Co that have relatively diffuse frontier orbitals. The fragment C₆H₆Cr prefers η⁶‐coordination. Near‐isodesmic equations based on density functional theory computations of the transition metal complexes of B₄₀, C₆₀ and C₆H₆ support these anticipations. Transition metal complexation increases the stability of B₄₀.     

Synthesis and Characterization of Novel Lanthanocene Complexes with Dichalcogenolate o-Carboranyl Ligands

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Platinum Complexes of a Borane‐Appended Analogue of 1,1′‐Bis(diphenylphosphino)ferrocene: Flexible Borane Coordination Modes and in situ Vinylborane Formation

A bis(phosphine)borane ambiphilic ligand, [Fe(η⁵‐C₅H₄PPh₂)(η⁵‐C₅H₄PtBu{C₆H₄(BPh₂)‐ortho})] (FcPPB), in which the borane occupies a terminal position, was prepared. Reaction of FcPPB with tris(norbornene)platinum(0) provided [Pt(FcPPB)] ( 1 ) in which the arylborane is η³BCC‐coordinated. Subsequent reaction with CO and CNXyl (Xyl=2,6‐dimethylphenyl) afforded [PtL(FcPPB)] {L=CO ( 2 ) and CNXyl ( 3 )} featuring η²BC‐ and η¹B‐arylborane coordination modes, respectively. Reaction of 1 or 2 with H₂ yielded [PtH(μ‐H)(FcPPB)] in which the borane is bound to a hydride ligand on platinum. Addition of PhC₂H to [Pt(FcPPB)] afforded [Pt(C₂Ph)(μ‐H)(FcPPB)] ( 5 ), which rapidly converted to [Pt(FcPPB′)] ( 6 ; FcPPB′=[Fe(η⁵‐C₅H₄PPh₂)(η⁵‐C₅H₄PtBu{C₆H₄(BPh‐CPh=CHPh‐Z)‐ortho}]) in which the newly formed vinylborane is η³BCC‐coordinated. Unlike arylborane complex 1 , vinylborane complex 6 does not react with CO, CNXyl, H₂ or HC₂Ph at room temperature.     

Ligand Site Preference in Iron Tetracarbonyl Complexes Fe(CO)4L (L = CO,CS, N2, NO+, CN–, NC–, η2‐C2H4, η2‐C2H2, CCH2, CH2, CF2, NH3, NF3, PH3, PF3, η2‐H2)

Equilibrium geometries, bond dissociation energies and relative energies of axial and equatorial iron tetracarbonyl complexes of the general type Fe(CO)₄L (L = CO, CS, N2, NO⁺, CN^–, NC^–, η²‐C₂H₄, η²‐C₂H₂, CCH₂, CH₂, CF₂, NH₃, NF₃, PH₃, PF₃, η²‐H₂) are calculated in order to investigate whether or not the ligand site preference of these ligands correlates with the ratio of their σ‐donor/π‐acceptor capabilities. Using density functional theory and effective‐core potentials with a valence basis set of DZP quality for iron and a 6‐31G(d) all‐electron basis set for the other elements gives theoretically predicted structural parameters that are in very good agreement with previous results and available experimental data. Improved estimates for the (CO)₄Fe–L bond dissociation energies (D₀) are obtained using the CCSD(T)/II//B3LYP/II combination of theoretical methods. The strongest Fe–L bonds are found for complexes involving NO⁺, CN^–, CH₂ and CCH₂ with bond dissociation energies of 105.1, 96.5, 87.4 and 83.8 kcal mol^–1, respectively. These values decrease to 78.6, 64.3 and 64.2 kcal mol^–1, respectively, for NC^–, CF₂ and CS. The Fe(CO)₄L complexes with L = CO, η²‐C₂H₄, η²‐C₂H₂, NH₃, PH₃ and PF₃ have even smaller bond dissociation energies ranging from 45.2 to 37.3 kcal mol^–1. Finally, the smallest bond dissociation energies of 23.5, 22.9 and 18.5 kcal mol^–1, respectively are found for the ligands NF₃, N₂ and η²‐H₂. A detailed examination of the (CO)₄Fe–L bond in terms of a semi‐quantitative Dewar‐Chatt‐Duncanson (DCD) model is presented on the basis of the CDA and NBO approach. The comparison of the relative energies between axial and equatorial isomers of the various Fe(CO)₄L complexes with the σ‐donor/π‐acceptor ratio of their respective ligands L thus does not generally support the classical picture of π‐accepting ligands preferring equatorial coordination sites and σ‐donors tending to coordinate in axial positions. In particular, this is shown by iron tetracarbonyl complexes with L = η²‐C₂H₂, η²‐C₂H₄, η²‐H₂. Although these ligands are predicted by the CDA to be stronger σ‐donors than π‐acceptors, the equatorial isomers of these complexes are more stable than their axial pendants.     

Endohedral and Exohedral Metalloborospherenes: M@B40 (M=Ca,Sr) and M&B40 (M=Be,Mg)

The recent discovery of the all‐boron fullerenes or borospherenes, D_2d B₄₀^−/0, paves the way for borospherene chemistry. Here we report a density functional theory study on the viability of metalloborospherenes: endohedral M@B₄₀ (M=Ca, Sr) and exohedral M&B₄₀ (M=Be, Mg). Extensive global structural searches indicate that Ca@B₄₀ ( 1 , C_2v, ¹A₁) and Sr@B₄₀ ( 3 , D_2d, ¹A₁) possess almost perfect endohedral borospherene structures with a metal atom at the center, while Be&B₄₀ ( 5 , C_s, ¹A′) and Mg&B₄₀ ( 7 , C_s, ¹A′) favor exohedral borospherene geometries with a η⁷‐M atom face‐capping a heptagon on the waist. Metalloborospherenes provide indirect evidence for the robustness of the borospherene structural motif. The metalloborospherenes are characterized as charge‐transfer complexes (M²⁺B₄₀²⁻), where an alkaline earth metal atom donates two electrons to the B₄₀ cage. The high stability of endohedral Ca@B₄₀ ( 1 ) and Sr@B₄₀ ( 3 ) is due to the match in size between the host cage and the dopant. Bonding analyses indicate that all 122 valence electrons in the systems are delocalized as σ or π bonds, being distributed evenly on the cage surface, akin to the D_2d B₄₀ borospherene.     

Photochemische Reaktionen von Cyclopentadienylbis(ethen)rhodium mit Phenanthren,Acenaphthylen und Triphenylen,sowie ungewöhnlicher H-Austausch zwischen η2-koordinierten Phenanthren- bzw. Acenaphthylen- und η5-Cyclopentadienyl-Liganden

Photochemical Reactions of Cyclopentadienylbis(ethene)rhodium with Phenanthrene, Acenaphthylene, and Triphenylene, and Unusual H Exchange between η²-Coordinated Phenanthrene or Acenaphthylene and η⁵-Cyclopentadienyl Ligands During UV irradiation of [CpRh(C₂H₄)₂] (Cp = η⁵-C₅H₅) in hexane/ether in the presence of phenanthrene one ethene ligand is displaced by coordination of the 9,10 double bond of phenanthrene, and (η⁵-cyclopentadienyl) (η²-ethene)(η²-9,10-phenanthrene)rhodium ( 1 ) is formed. The analogous reaction in hexane in the presence of acenaphthylene occurs with formation of the complexes (η²-1,2-acenaphthylene)(η⁵-cyclopentadienyl)(²-ethene)rhodium 2 and bis(η²-1,2-acenaphthylene)(η⁵-cyclopentadienyl)rhodium 3 in which one and two ethene molecules of [CpRh(C₂H₄)₂], respectively, are substituted by η²-1,2-acenaphthylene. The irradiation of [CpRh(C₂H₄)₂] with triphenylene in hexane yields the compounds [CpRh(η⁴-1,2,3,4-triphenylene)] ( 4 ), [(CpRh)₂(μ-η³: η³-triphenylene)] ( 5 ), and [(CpRh)₃(μ₃-η²: η²: η²-triphenylene)] ( 6 ). Despite the partially very low yields the new complexes could be unequivocally characterized spectroscopically and in the case of 1 and 3 by X-ray structural analysis. The compounds 1 and 2 in solution reveal a novel dynamic behaviour; via an intramolecular C? H activation, exchange occurs between the protons of the η²-coordinated arene and the Cp ligand. The complex 4 in solution is fluxional, too.     

Reaction of η-enone and enal-platinum(0) complexes with Lewis acidic compounds

Reaction of η²-enone and enal-platinum(0) complexes Pt(CH₂CHCOR)(PPh₃)₂ (R=H, Me) with Lewis acidic compounds BX₃ (X=F, C₆F₅) afforded adducts formed by coordination of boron to oxygen of the carbonyl group. The X-ray structure determination of the adduct formed from B(C₆F₅)₃ and η²-methylvinylketone complex showed no strong interaction between Pt and carbonyl carbon. In contrast to the inability of the palladium(0) η²-enone complexes to form any Me₃Al adduct, η²-cyclohexenoneplatinum(0) complex formed an isolable adduct with Me₃Al, the structure of which was also confirmed by X-ray analysis. The NMR spectral parameters (Pt-C, Pt-P and P-P coupling constants) of these adducts were compared with those of the original η²-enone or enal-platinum(0) complexes as well as the ordinary η³-allylplatinum cation [Pt(PPh₃)₂(MeCHCHCH₂)]⁺.     

Endohedral and Exohedral Metalloborospherenes: M@B40 (M=Ca,Sr) and M&B40 (M=Be,Mg)

The recent discovery of the all‐boron fullerenes or borospherenes, D_2d B₄₀^?/0, paves the way for borospherene chemistry. Here we report a density functional theory study on the viability of metalloborospherenes: endohedral M@B₄₀ (M=Ca, Sr) and exohedral M&B₄₀ (M=Be, Mg). Extensive global structural searches indicate that Ca@B₄₀ ( 1 , C_2v, ¹A₁) and Sr@B₄₀ ( 3 , D_2d, ¹A₁) possess almost perfect endohedral borospherene structures with a metal atom at the center, while Be&B₄₀ ( 5 , C_s, ¹A′) and Mg&B₄₀ ( 7 , C_s, ¹A′) favor exohedral borospherene geometries with a η⁷‐M atom face‐capping a heptagon on the waist. Metalloborospherenes provide indirect evidence for the robustness of the borospherene structural motif. The metalloborospherenes are characterized as charge‐transfer complexes (M²⁺B₄₀^2?), where an alkaline earth metal atom donates two electrons to the B₄₀ cage. The high stability of endohedral Ca@B₄₀ ( 1 ) and Sr@B₄₀ ( 3 ) is due to the match in size between the host cage and the dopant. Bonding analyses indicate that all 122 valence electrons in the systems are delocalized as σ or π bonds, being distributed evenly on the cage surface, akin to the D_2d B₄₀ borospherene.     

Photochemische Reaktionen von Cyclopentadienylbis(ethen)rhodium mit Benzolderivaten

Photochemical Reactions of Cyclopentadienylbis(ethene)rhodium with Benzene Derivatives During UV irradiation of [CpRh(C₂H₄)₂] ( 1 ) (Cp = η⁵‐C₅H₅) in hexane in the presence of hexamethylbenzene the di‐ and trinuclear arene bridged complexes [(CpRh)₂(μ‐η³ : η³‐C₆Me₆)] ( 3 ) and [(CpRh)₃(μ₃‐η² : η² : η²‐C₆Me₆)] ( 4 ) are formed besides known [CpRh(η⁴‐C₆Me₆)] ( 2 ). It was shown by a separate experiment that 3 besides small amounts of 4 is formed by attack of photochemically from 1 arising CpRh fragments at the free double bond of the η⁴‐bonded benzene ring in 2 . Irradiation of 1 in the presence of diphenyl (C₁₂H₁₀) affords the compounds [(CpRh)₂(μ‐η³ : η³‐C₁₂H₁₀)] ( 5 ) and [(CpRh)₃(μ₃‐η² : η² : η²‐C₁₂H₁₀)] ( 6 ) as analogues of 3 and 4 , in the presence of triptycene (C₂₀H₁₄) only [(CpRh)₂(μ‐η³ : η³‐C₂₀H₁₄)] ( 7 ) is obtained; the bridging in 5 , 6 , and 7 always occurs via the same six‐membered ring of the corresponding ligand system. During the photochemical reaction of 1 in the presence of styrene (C₈H₈) substitution of the ethene ligands by the vinyl groups with formation of [CpRh(C₂H₄)(η²‐C₈H₈)] ( 8 ) and known [CpRh(η²‐C₈H₈)₂] ( 9 ) is observed exclusively. The new complexes were characterized analytically and spectroscopically, in the case of 3 also by X‐ray structure analysis.     

New ternary rare-earth metal boride carbides R15B4C14 (R=Y, Gd-Lu) containing BC2 units: Crystal and electronic structures, magnetic properties

The ternary rare-earth boride carbides R₁₅B₄C₁₄ (R=Y, Gd-Lu) were prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for 1 month. The crystal structures of Tb₁₅B₄C₁₄ and Er₁₅B₄C₁₄ were determined from single crystal X-ray diffraction data. They crystallize in a new structure type in space group P4/mnc (Tb₁₅B₄C₁₄: a=8.1251(5) Å, c=15.861(1) Å, Z=2, R₁=0.041 (wR₂=0.088) for 1023 reflections with I_o>2σ(I_o); Er₁₅B₄C₁₄: a=7.932(1) Å, c=15.685(2) Å, Z=2, R₁=0.037 (wR₂=0.094) for 1022 reflections with I_o>2σ(I_o)). The crystal structure contains discrete carbon atoms and bent CBC units in octahedra and distorted bicapped square antiprisms, respectively. In both structures the same type of disorder exists. One R atom position needs to be refined as split atom position with a ratio 9:1 indicative of a 10% substitution of the neighboring C⁴⁻ by C₂⁴⁻. The actual composition has then to be described as R₁₅B₄C_14.2. The isoelectronic substitution does not change the electron partition of R₁₅B₄C₁₄ which can be written as (R³⁺)₁₅(C⁴⁻)₆(CBC⁵⁻)₄•e⁻. The electronic structure was studied with the extended Hückel method. The investigated compounds Tb₁₅B₄C₁₄, Dy₁₅B₄C₁₄ and Er₁₅B₄C₁₄ are hard ferromagnets with Curie temperatures T_C=145, 120 and 50 K, respectively. The coercive field B_C=3.15 T for Dy₁₅B₄C₁₄ is quite remarkable.     

New members of ternary rare-earth metal boride carbides containing finite boron-carbon chains: RE25B14C26 (RE=Pr, Nd) and Nd25B12C28

New ternary rare-earth metal boride carbides RE₂₅B₁₄C₂₆ (RE=Pr, Nd) and Nd₂₅B₁₂C₂₈ were synthesized by co-melting the elements. Nd₂₅B₁₂C₂₈ is stable up to 1440 K. RE₂₅B₁₄C₂₆ (RE=Pr, Nd) exist above 1270 K. The crystal structures were investigated by means of single-crystal X-ray diffraction. Nd₂₅B₁₂C₂₈: space group P, a=8.3209(7) Å, b=8.3231(6) Å, c=29.888(2) Å, α=83.730(9)°, β=83.294(9)°, γ=89.764(9)°. Pr₂₅B₁₄C₂₆: space group P2₁/c, a=8.4243(5) Å, b=8.4095(6) Å, c=30.828(1) Å, β=105.879(4)°, V=2100.6(2) Å³, (R1=0.048 (wR2=0.088) from 2961 reflections with I_o>2σ(I_o)); for Nd₂₅B₁₄C₂₆ space group P2₁/c, Z=2, a=8.3404(6) Å, b=8.3096(6) Å, c=30.599(2) Å, β=106.065(1)°. Their structures consist of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with cumulene-like molecules [B₂C₄]⁶⁻ and [B₃C₃]⁷⁻, nearly linear [BC₂]⁵⁻ and bent [BC₂]⁷⁻ units and isolated carbon atoms. Structural and theoretical analysis suggests the ionic formulation for RE₂₅B₁₄C₂₆: (RE³⁺)₂₅[B₂C₄]⁶⁻([B₃C₃]⁷⁻)₂([BC₂]⁵⁻)₄([BC₂]⁷⁻)₂(C⁴⁻)₄·5e⁻ and for Nd₂₅B₁₂C₂₈: (Nd³⁺)₂₅([B₂C₄]⁶⁻)₃([BC₂]⁵⁻)₄([BC₂]⁷⁻)₂(C⁴⁻)₄·7e⁻. Accordingly, extended Hückel tight-binding calculations indicate that the compounds are metallic in character.     

Chemistry of new electrophilic metal ALKYL compounds

The reaction of the neutral carborane C₂B₉H₁₃ with Cp*M(CH₃)₃ (M = Zr (a), Hf (b); Cp* = η⁵-C₅Me₅) yields [Cp(C₂B₉H₁₁)M(CH₃)]_n (3a, b). Complexes 3a, b form THF adducts Cp*(C₂B₉H₁₁)M(CH₃)(THF) 4, insert 2-butyne to yield Cp*(C₂B₉H₁₁)M{C(Me=CMe₂} 5, and undergo methane elimination upon thermolysis to yield methylene-bridged complexes [Cp*(C₂B₉H₁₁)M]₂(μ-CH₂) (6). These chemical studies, and companion structural and theoretical studies establish that 3a, b are neutral analogues of the cationic Cp₂M(R)⁺ species (1; Cp = η⁵-C₅H₅) and Cp₂M(R)(L)⁺ (2) which are believed to be active in Cp₂MX₂-based Ziegler catalysts. Despite the lower metal charge, 3–6 exhibit characteristic “electrophilic metal alkyl” properties including agostic M-H-C and M-H-B interactions, high insertion and intramolecular C-H activation reactivity, and high ethylene polymerization and propene oligomerization activity. These observations suggest that the key requirement for high insertion/polymerization activity in metallocene systems is high metal unsaturation (i.e. two empty metal-centered orbitals) rather than charge.     

Structure of the tetrakis(benzoylacetonate) compound of europium used to obtain the laser effect

The spatial symmetry group of unit cells of diethylammonium and plperidinium europium tetrakis(benzoylactonate) B₄EuHD and B₄EuHP, and also the structure of the B₄Eu^– anion in B₄EuHD, which belongs to the C₂ symmetry group and exists as the cis, cis, cis isomer, have been determined. On the basis of an analysis of the luminescence spectra and structure of the B₄Eu^– anion it has been shown that the splitting of the levels of the ground state of the europium ion formally corresponds to C₂ symmetry, but may be determined by the C₄ symmetry of the immediate oxygen environment of europium EuO₈. Both complexes give a stable laser effect at a wavelength of 613 nm, corresponding to the transition from the ⁵D₀ level to the x- or y-component of the ⁷F₂ level, split by a crystal field with C₂ or C₄ symmetry.     

Di-μ-chlorodichlorobis(η4-tetrakis-p-methoxyphenylcyclobutadiene)dipalladium: X-ray structure determination,spectroscopic properties,and reactions with nucleophiles

Reaction of bis-p-methoxyphenylacetylene (An₂C₂) with Na₂[PdCl₄] gave [(η⁴-An₄C₄Pd)₂Cl₃] [Pd₂Cl₆]_0.5 which with HCl gave [(An₄C₄Pd)₂Cl₄] (6). The complexes [(An₄C₄Pd)₂Y₄] (Y = Br, I, NCS, or NCO) were obtained by metathesis. Reaction of 6 with NaN₃ gave the η³-cyclobutenyl complex Na[{(N₃)An₄C₄Pd}(N₃)Cl]. With other nucleophiles OR- (6) gave the dinuclear η³-cyclobutenyl complexes [{(RO)An₄C₄Pd}₂Cl₂] (R = H, Me, Et, Prⁱ or COCH₃); AgPF₆ in the presence of L (MeCN, Bu^tNC, Me₂NC, Me₂C₂ or py) gave [(η⁴-Ar₄C₄Pd)₂Cl₂L₂][PF₆]₂ and in the presence of bipy gave [η⁴-An₄C₄Pd(bipy)Cl]PF₆. The X-ray crystal structure of the dinuclear complex 6 showed each Pd to be η⁴-coordinated to a square planar cyclobutadiene, and bound to three chlorines (two bridging and one terminal). The far IR spectra provide a useful guide to the structures of many of these complexes, but 6 is remarkable in that the spectrum between 200 and 300 cm^?1 changes dramatically on grinding.     

Synthesis of [Ni(η-CH2C6H4R-4){PPh(CH2CH2PPh2)2}] (R = H, Me or MeO) and protonation reactions with HCl

The complexes [Ni(η²-CH₂C₆H₄R-4)(triphos)]BPh₄ {R = H, Me or MeO; triphos = PhP(CH₂CH₂PPh₂)₂} have been prepared and characterised by spectroscopy and X-ray crystallography. In all cases the coordination geometry of the nickel is best described as square-planar with an η²-benzyl ligand occupying one of the positions. The orientation of the η²-benzyl ligand is dictated by the steric restrictions imposed by the phenyl groups on the triphos ligand, so that the phenyl group on the unique secondary phosphorus and the aromatic group of the benzyl ligand (which are trans to one another) are oriented in the same direction. [Ni(η²-CH₂C₆H₄R-4)(triphos)]⁺ react with an excess of anhydrous HCl in MeCN to form [NiCl(triphos)]⁺ (characterised as the [BPh₄]⁻ salt by X-ray crystallography) and the corresponding substituted toluene. The kinetics of the reaction of all [Ni(η²-CH₂C₆H₄R-4)(triphos)]⁺ and HCl in the presence of Cl⁻ have been determined using stopped-flow spectrophotometry. All reactions exhibit a first-order dependence on the concentration of complex and a first-order dependence on the ratio [HCl]/[Cl⁻]. Varying the 4-R-substituent on the benzyl ligand shows that electron-withdrawing substituents facilitate the rate of the reaction. It is proposed that the mechanism involves initial rapid protonation at the nickel to form [NiH(η²-CH₂C₆H₄R-4)(triphos)]²⁺, followed by intramolecular proton migration from nickel to carbon to yield the products.     

η1-Allyrhenium pentacarbonyl and η3-allylrhenium tetracarbonyl; preparations, 1H N.M.R., mass and vibrational spectra

η¹-C₃H₅)Re(CO)₅ has been prepared from [Re(CO)₅]^? and photo-decarbonylated to give η³-C₃H₅)Re(Co)₄. Both allyl complexes have been characterised by ¹H n.m.r., mass spectrometry and liquid-phase infrared and Raman spectra. The vibrations of the Re(CO)₅ unit in the η¹-allyl compound can be assigned in terms of local C_4v symmetry, but such an an approximation is not valid for the η³-allyl compound which must be discussed in terms of overall C_s symmetry. The η¹- and n³-allyl internal modes are discussed in terms of C_s symmetry.     

Octahapto cyclooctatetraene rings and metal-metal multiple bonds in binuclear niobium carbonyl chemistry

Theoretical studies on (C₈H₈)₂Nb₂(CO)_n (n = 6, 5, 4, 3, 2, 1) predict structures mainly with octahapto and tetrahapto C₈H₈ rings. In all cases, the lowest energy singlet spin state structures lie below the corresponding lowest energy triplet spin state structures. Thus the lowest energy (C₈H₈)₂Nb₂(CO)₄ structure has two η⁸-C₈H₈ rings and an unbridged Nb-Nb single bond of length ∼3.15 Å. The lowest energy (C₈H₈)₂Nb₂(CO)₂ structure has two η⁸-C₈H₈ rings but a doubly bridged NbNb triple bond of length ∼2.64 Å. The lowest energy structure of (C₈H₈)₂Nb₂(CO)₃ also has a formal NbNb triple bond of similar length (2.66 Å) but with only one of the rings fully coordinated as an octahapto η⁸-C₈H₈ ligand. The other C₈H₈ ring in this tricarbonyl has “slipped” to form a hexahapto η⁶-C₈H₈ ligand. The lowest energy structure of the monocarbonyl (C₈H₈)₂Nb₂(CO) again has two octahapto η⁸-C₈H₈ rings and an extremely short NbNb distance of 2.45 Å, suggesting a formal quadruple bond. The lowest energy structures for the carbonyl-richer species (C₈H₈)₂Nb₂(CO)_n (n = 6, 5) have one η⁸-C₈H₈ and one η⁴-C₈H₈ ring (n = 5) and two η⁴-C₈H₈ rings (n = 6). The qualitatively assigned Nb-Nb bond orders are consistent with the Wiberg bond indices obtained from the Weinhold natural bond orbital analysis. Comparison of the (C₈H₈)₂Nb₂(CO)_n (n = 6, 5, 4, 3, 2, 1) derivatives with the isovalent (C₇H₇)₂Mo₂(CO)_n is made.     

Binding,Release and Functionalization of Intact Pnictogen Tetrahedra Coordinated to Dicopper Complexes

The bridging MeCN ligand in the dicopper(I) complexes [(DPFN)Cu₂(μ,η¹ : η¹-MeCN)][X]₂ (X=weakly coordinating anion, NTf₂ ( 1 a ), FAl[OC₆F₁₀(C₆F₅)]₃ ( 1 b ), Al[OC(CF₃)₃]₄ ( 1 c )) was replaced by white phosphorus (P₄) or yellow arsenic (As₄) to yield [(DPFN)Cu₂(μ,η² : η²-E₄)][X]₂ (E=P ( 2 a – c ), As ( 3 a – c )). The molecular structures in the solid state reveal novel coordination modes for E₄ tetrahedra bonded to coinage metal ions. Experimental data and quantum chemical computations provide information concerning perturbations to the bonding in coordinated E₄ tetrahedra. Reactions with N-heterocyclic carbenes (NHCs) led to replacement of the E₄ tetrahedra with release of P₄ or As₄ and formation of [(DPFN)Cu₂(μ,η¹ : η¹-^MeNHC)][X]₂ ( 4 a,b ) or to an opening of one E−E bond leading to an unusual E₄ butterfly structural motif in [(DPFN)Cu₂(μ,η¹ : η¹-E₄^DippNHC)][X]₂ (E=P ( 5 a,b ), E=As ( 6 )). With a cyclic alkyl amino carbene (^EtCAAC), cleavage of two As−As bonds was observed to give two isomers of [(DPFN)Cu₂(μ,η² : η²-As₄^EtCAAC)][X]₂ ( 7 a,b ) with an unusual As₄-triangle+1 unit.     

Komplexkatalyse. XXXII. Synthese und Charakterisierung von η3-Allyl-, η3-Crotyl-und η1,η2-Cyclooct-4(Z)-en-1-yl-nickel(II)-bis(brenzcate-chinato)-borat und ihre Eignung als Katalysatoren für die stereospezifische Butadienpolymerisation

Complex Catalysis. XXXII. Synthesis and Characterization of η³-Allyl-, η³-Crotyl-, and η¹,η²-Cyclooct-4(Z)-en-1-yl-nickel(II)-bis(brenzcatechinato)borate and their Suitability as Catalysts for the Stereospecific Butadiene Polymerization By reaction of [(η³-C₃H₅)₂Ni], [(η³-C₄H₇)₂Ni], and [Ni(cycloocta-1,5-diene)₂] with one equivalent bis(brenzcatechinato)boric acid HB(O₂C₆H₄)₂ in ether the complexes given in the title could be synthesized in good yields. The allyl complex [η³-C₃H₅NiB(O₂C₆H₄)₂] reacts with cycloocta-1,5-diene (COD) to give a cationic complex [η³-C₃H₅Ni(COD)]B(O₂C₆H₄)₂ and catalyses the 1,4-trans-polymerization of butadiene with an activity of ca. 150 ml C₄H₆/mol Ni · h and a selectivity of 78% under standard conditions at room temperature.