Case Study of N-iPr versus N-Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC)2]

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)₂] (NHC=ⁱPr₂Im^Me 1^Me , Mes₂Im 2 ) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni₂(ⁱPr₂Im^Me)₄(μ-(η² : η²)-COD)] B / [Ni(ⁱPr₂Im^Me)₂(η⁴-COD)] B’ or [Ni(Mes₂Im)₂] 2 , respectively, with alkynes afforded complexes [Ni(NHC)₂(η²-alkyne)] (NHC=ⁱPr₂Im^Me: alkyne=MeC≡CMe 3 , H₇C₃C≡CC₃H₇ 4 , PhC≡CPh 5 , MeOOCC≡CCOOMe 6 , Me₃SiC≡CSiMe₃ 7 , PhC≡CMe 8 , HC≡CC₃H₇ 9 , HC≡CPh 10 , HC≡C(p-Tol) 11 , HC≡C(4-^tBu-C₆H₄) 12 , HC≡CCOOMe 13 ; NHC=Mes₂Im: alkyne=MeC≡CMe 14 , MeOOCC≡CCOOMe 15 , PhC≡CMe 16 , HC≡C(4-^tBu-C₆H₄) 17 , HC≡CCOOMe 18 ). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p-Tol) and HC≡C(4-^tBu-C₆H₄), 11 and 12 , which were formed by addition of a C−H bond of one of the NHC N-ⁱPr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2-butyne, 4-octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1-pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1^Me is not a good catalyst. The reaction of 2 with 2-butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)₂]. DFT calculations reveal that the differences between 1^{M e} and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2 , and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N-alkyl substituted NHC, to enhanced Ni-alkyne backbonding due to a smaller C_NHC−Ni−C_NHC bite angle, and to less steric repulsion of the smaller NHC ⁱPr₂Im^Me.     

[Ni(iPr2Im)2Br2]: A Convenient Entry into NHC Nickel ­Chemistry

The reaction of the NHC iPr₂Im [NHC=N‐heterocyclic carbene, iPr₂Im = 1, 3‐bis(isopropyl)imidazolin‐2‐ylidene] with freshly prepared NiBr₂ in thf or dme results in the formation of the air stable nickel(II) complex trans‐[Ni(iPr₂Im)₂Br₂] ( 2 ). Complex 2 was structurally characterized. Thermal analysis (DTA/TG) reveals a very high decomposition temperature of 298 °C. Reduction of 2 with sodium or C₈K in the presence of the olefins COD (cyclooctadiene) or COE (cyclooctene) affords the highly reactive compounds [Ni₂(iPr₂Im)₄(COD)] ( 1 ) and [Ni(iPr₂Im)₂(COE)] ( 4 ). Alkylation of 2 with organolithiums leads to the formation of trans‐[Ni(iPr₂Im)₂(R)₂] [R = Me ( 5 ), CH₂SiMe₃ ( 6 )], whereas the reaction of 2 with LiCp* [Cp* = (η⁵‐C₅(CH₃)₅)] at 80 °C causes the loss of one NHC ligand and affords [(η⁵‐C₅(CH₃)₅)Ni(iPr₂Im)Br] ( 7 ).     

Nickelocene chemistry : III. Reactions of Bromo(η5-cyclopentadienyl)triphenylphosphinenickel with phosphorus,arsenic and sulphur ylids

The phosphorus ylids Ph₃PCHR (R = Me, Et, Prⁿ, Prⁱ, Bu_n, Cl, and OMe), and the ylids Ph₃AsCH₂, Me₂SCH₂, and Me₂S(O)CH₂ react with [Ni(η⁵-C₅H₅)Br(PPh₃)] at room temperature to give the complexes [Ni(Ph₃PCHR)(η⁵-C₅H₅(PPh₃)] Br, [Ni(Ph₃AsCH₂)(η⁵-C₅H₅)(PPh₃)]Br, [Ni(Me₂SCH₂)(η⁵-C₅H₅)(PPh₃)]Br and [Ni{Me₂S(O)CH₂} (η⁵-C₅H₅)(PPh₃)]Br, respectively. These are readily converted into the corresponding hexafluorophosphate salts on reaction with ammonium hexafluorophosphate. Under more forcing conditions the stabilised ylid Ph₃PCHCOPh gives a product believed to be the complex [Ni(Ph₃PCHCOPh)₂(η⁵-C₅H₅)]Br, isolated and characterised as its PF₆^? salt.     

Synthese,Struktur und Photochemie von Olefiniridium(I)‐Komplexen mit Acetylacetonatoliganden

Synthesis, Structure, and Photochemical Behavior of Olefine Iridium(I) Complexes with Acetylacetonato Ligands The bis(ethene) complex [Ir(κ²‐acac)(C₂H₄)₂] ( 1 ) reacts with tertiary phosphanes to give the monosubstitution products [Ir(κ²‐acac)(C₂H₄)(PR₃)] ( 2 – 5 ). While 2 (R = iPr) is inert toward PiPr₃, the reaction of 2 with diphenylacetylene affords the π‐alkyne complex [Ir(κ²‐acac)(C₂Ph₂)(PiPr₃)] ( 6 ). Treatment of [IrCl(C₂H₄)₄] with C‐functionalized acetylacetonates yields the compounds [Ir(κ²‐acacR^1,2)(C₂H₄)₂] ( 8 , 9 ), which react with PiPr₃ to give [Ir(κ²‐acacR^1,2)(C₂H₄)(PiPr₃)] ( 10 , 11 ) by displacement of one ethene ligand. UV irradiation of 5 (PR₃ = iPr₂PCH₂CO₂Me) and 11 (R² = (CH₂)₃CO₂Me) leads, after addition of PiPr₃, to the formation of the hydrido(vinyl)iridium(III) complexes 7 and 12 . The reaction of 2 with the ethene derivatives CH₂=CHR (R = CN, OC(O)Me, C(O)Me) affords the compounds [Ir(κ²‐acac)(CH₂=CHR)(PiPr₃)] ( 13 – 15 ), which on photolysis in the presence of PiPr₃ also undergo an intramolecular C–H activation. In contrast, the analogous complexes [Ir(κ²‐acac)(olefin)(PiPr₃)] (olefin = (E)‐C₂H₂(CO₂Me)₂ 16 , (Z)‐C₂H₂(CO₂Me)₂ 17 ) are photochemically inert.     

A Diazabutadiene stabilized Nickel(0) Cyclooctadiene Complex: Synthesis,Characterization and the Reaction with Diphenylacetylene

The reaction of [Ni(COD)₂] with one equivalent of DAB^Mes (DAB^Mes = (2,4,6‐Me₃C₆H₂)N=C(Me)‐C(Me)=N(2,4,6‐Me₃C₆H₂)) affords a mixture of the compound [Ni(DAB^Mes)₂] ( 2 ) and starting material [Ni(COD)₂]. The crystallographically characterized, diamagnetic complex 2 can be obtained in a stoichiometric reaction of [Ni(COD)₂] and two equivalents of DAB^Mes. This reaction can be accelerated by addition of 1‐chloro‐fluorobenzene or methyl iodide. In the presence of 1‐chloro‐fluorobenzene, [Ni(DAB^Mes)(COD)] ( 3 ) is available via reaction of [Ni(COD)₂] and one equivalent of DAB^Mes. The crystallographically characterized complex 3 reacts with diphenylacetylene to afford [Ni(DAB^Mes)(Ph‐C≡C‐Ph)] ( 4 ). A long‐wavelength absorption band in the UV‐Vis spectrum of this compound has to be assigned to a mixed MLCT/LL′CT transition, as quantum chemical calculations reveal.     

A Nickel Complex Containing a Pyramidalized,Ambiphilic Pincer Germylene Ligand

Reaction of N-heterocyclic carbene (NHC)-stabilized PGeP-type germylene Ge{o-(PiPr₂)C₆H₄}₂⋅^MeIiPr ( 1 ) (^MeIiPr=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with Ni(cod)₂ gave pincer germylene complex Ni[Ge{o-(PiPr₂)C₆H₄}₂](^MeIiPr) ( 2 ), in which the Ge center of 2 is significantly pyramidalized. Theoretical calculation on 2 predicted the ambiphilicity of the germanium center, which was confirmed by reactivity studies. Thus, complex 2 reacted with both Lewis base ^MeIMe (^MeIMe=1,3,4,5-tetramethylimidazol-2-ylidene) and Lewis acid BH₃⋅SMe₂ at the germanium center to afford the adducts Ni[Ge{o-(PiPr₂)C₆H₄}₂⋅^MeIMe](^MeIiPr) ( 3 ) and Ni[Ge{o-(PiPr₂)C₆H₄}₂⋅BH₃](^MeIiPr) ( 4 ), respectively. Furthermore, the former was slowly converted to dinuclear complex Ni₂[Ge{o-(PiPr₂)C₆H₄}₂]₂(^MeIMe)₂ ( 5 ) at room temperature. Complex 5 can be regarded as a dimer of the ^MeIMe analog of 2 with a Ni-Ge-Ge-Ni linkage.     

Synthesis of Mixed Silylene–Carbene Chelate Ligands from N‐Heterocyclic Silylcarbenes Mediated by Nickel

The Ni^II‐mediated tautomerization of the N‐heterocyclic hydrosilylcarbene L²Si(H)(CH₂)NHC 1 , where L²=CH(C?CH₂)(CMe)(NAr)₂, Ar=2,6‐iPr₂C₆H₃; NHC=3,4,5‐trimethylimidazol‐2‐yliden‐6‐yl, leads to the first N‐heterocyclic silylene (NHSi)–carbene (NHC) chelate ligand in the dibromo nickel(II) complex [L¹Si:(CH₂)(NHC)NiBr₂] 2 (L¹=CH(MeC?NAr)₂). Reduction of 2 with KC₈ in the presence of PMe₃ as an auxiliary ligand afforded, depending on the reaction time, the N‐heterocyclic silyl–NHC bromo Ni^II complex [L²Si(CH₂)NHCNiBr(PMe₃)] 3 and the unique Ni⁰ complex [η²(Si‐H){L²Si(H)(CH₂)NHC}Ni(PMe₃)₂] 4 featuring an agostic Si? H→Ni bonding interaction. When 1,2‐bis(dimethylphosphino)ethane (DMPE) was employed as an exogenous ligand, the first NHSi–NHC chelate‐ligand‐stabilized Ni⁰ complex [L¹Si:(CH₂)NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni⁰ complex 6 , [L¹Si:(CH₂)NHCNi(CO)₂], is easily accessible by the reduction of 2 with K(BHEt₃) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada–Corriu‐type cross‐coupling reactions.     

Silylene–Nickel Promoted Cleavage of B−O Bonds: From Catechol Borane to the Hydroborylene Ligand

The first 16 valence electron [bis(NHC)](silylene)Ni⁰ complex 1 , [(^TMSL)ClSi:→Ni(NHC)₂], bearing the acyclic amido‐chlorosilylene (^TMSL)ClSi: (^TMSL=N(SiMe₃)Dipp; Dipp=2,6‐Prⁱ₂C₆H₄) and two NHC ligands (N‐heterocyclic carbene=:C[(Prⁱ)NC(Me)]₂) was synthesized in high yield and structurally characterized. Compound 1 is capable of facile dihydrogen activation under ambient conditions to give the corresponding HSi‐NiH complex 2 . Most notably, 1 reacts with catechol borane to afford the unprecedented hydroborylene‐coordinated (chloro)(silyl)nickel(II) complex 3 , {[cat(^TMSL)Si](Cl)Ni←:BH(NHC)₂}, via the cleavage of two B−O bonds and simultaneous formation of two Si−O bonds. The mechanism for the formation of 3 was rationalized by means of DFT calculations, which highlight the powerful synergistic effects of the Si:→Ni moiety in the breaking of incredibly strong B−O bonds.     

Stabilization of a Silaaldehyde by its η2 Coordination to Tungsten

Treatment of pyridine‐stabilized silylene complexes [(η⁵‐C₅Me₄R)(CO)₂(H)W?SiH(py)(Tsi)] (R=Me, Et; py=pyridine; Tsi=C(SiMe₃)₃) with an N‐heterocyclic carbene ^MeIⁱPr (1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene) caused deprotonation to afford anionic silylene complexes [(η⁵‐C₅Me₄R)(CO)₂W?SiH(Tsi)][H^MeIⁱPr] (R=Me ( 1‐Me ); R=Et ( 1‐Et )). Subsequent oxidation of 1‐Me and 1‐Et with pyridine‐N‐oxide (1 equiv) gave anionic η²‐silaaldehydetungsten complexes [(η⁵‐C₅Me₄R)(CO)₂W{η²‐O?SiH(Tsi)}][H^MeIⁱPr] (R=Me ( 2‐Me ); R=Et ( 2‐Et )). The formation of an unprecedented W‐Si‐O three‐membered ring was confirmed by X‐ray crystal structure analysis.     

Binding H2, N2, H-, and BH3 to transition-metal sulfur sites: synthesis and properties of [RuL(PR3)(N2Me2S2)] Complexes (L=eta2-H2, H-, BH3; R=Cy, iPr)

The reactions of [Ru(N₂)(PR₃)(‘N₂Me₂S₂’)] [‘N₂Me₂S₂’=1,2‐ethanediamine‐N,N′‐dimethyl‐N,N′‐bis(2‐benzenethiolate)(2?)] [ 1 a (R=iPr), 1 b (R=Cy)] and [μ‐N₂{Ru(N₂)(PiPr₃)(‘N₂Me₂S₂’)}₂] ( 1 c ) with H₂, NaBH₄, and NBu₄BH₄, intended to reduce the N₂ ligands, led to substitution of N₂ and formation of the new complexes [Ru(H₂)(PR₃)(‘N₂Me₂S₂’)] [ 2 a (R=iPr), 2 b (R=Cy)], [Ru(BH₃)(PR₃)(‘N₂Me₂S₂’)] [ 3 a (R=iPr), 3 b (R=Cy)], and [Ru(H)(PR₃)(‘N₂Me₂S₂’)]^? [ 4 a (R=iPr), 4 b (R=Cy)]. The BH₃ and hydride complexes 3 a , 3 b , 4 a , and 4 b were obtained subsequently by rational synthesis from 1 a or 1 b and BH₃?THF or LiBEt₃H. The primary step in all reactions probably is the dissociation of N₂ from the N₂ complexes to give coordinatively unsaturated [Ru(PR₃)(‘N₂Me₂S₂’)] fragments that add H₂, BH₄^?, BH₃, or H^?. All complexes were completely characterized by elemental analysis and common spectroscopic methods. The molecular structures of [Ru(H₂)(PR₃)(‘N₂Me₂S₂’)] [ 2 a (R=iPr), 2 b (R=Cy)], [Ru(BH₃)(PiPr₃)(‘N₂Me₂S₂’)] ( 3 a ), [Li(THF)₂][Ru(H)(PiPr₃)(‘N₂Me₂S₂’)] ([Li(THF)₂]‐ 4 a ), and NBu₄[Ru(H)(PCy₃)(‘N₂Me₂S₂’)] (NBu₄‐ 4 b ) were determined by X‐ray crystal structure analysis. Measurements of the NMR relaxation time T₁ corroborated the η² bonding mode of the H₂ ligands in 2 a (T₁=35 ms) and 2 b (T₁=21 ms). The H,D coupling constants of the analogous HD complexes HD‐ 2 a (¹J(H,D)=26.0 Hz) and HD‐ 2 b (¹J(H,D)=25.9 Hz) enabled calculation of the H? D distances, which agreed with the values found by X‐ray crystal structure analysis ( 2 a : 92 pm (X‐ray) versus 98 pm (calculated), 2 b : 99 versus 98 pm). The BH₃ entities in 3 a and 3 b bind to one thiolate donor of the [Ru(PR₃)(‘N₂Me₂S₂’)] fragment and through a B‐H‐Ru bond to the Ru center. The hydride complex anions 4 a and 4 b are extremely Brønsted basic and are instantanously protonated to give the η²‐H₂ complexes 2 a and 2 b .     

Synthese und dynamisches Verhalten von [Rh2(μ-H)3H2(PiPr3)4]+. Beiträge zur Reaktivität des Tetrahydridodirhodium-Komplexes [Rh2H4(PiPr3)4]

Synthesis and Dynamic Behaviour of [Rh₂(μ-H)₃H₂(PiPr₃)₄]⁺. Contributions to the Reactivity of the Tetrahydridodirhodium Complex [Rh₂H₄(PiPr₃)₄] An improved synthesis of [Rh₂H₄(PiPr₃)₄] ( 2 ) from [Rh(η³-C₃H₅)(PiPr₃)₂] ( 1 ) or [Rh(η³-CH₂C₆H₅)(PiPr₃)₂] ( 3 ) and H₂ is described. Compound 2 reacts with CO or CH₃OH to give trans-[RhH(CO)(PiPr₃)₂] ( 4 ) and with ethene/acetone to yield a mixture of 4 and trans-[RhCH₃(CO)(PiPr₃)₂] ( 5 ). The carbonyl(methyl) complex 5 has also been prepared from trans-[RhCl(CO)(PiPr₃)₂] ( 6 ) and CH₃MgI. Whereas the reaction of 2 with two parts of CF₃CO₂H leads to [RhH₂(η²-O₂CCF₃) · (PiPr₃)₂] ( 8 ), treatment of 2 with one equivalent of CF₃CO₂H in presence of NH₄PF₆ gives the dinuclear compound [Rh₂H₅(PiPr₃)₄]PF₆ ( 9a ). The reactions of 2 with HBF₄ and [NO]BF₄ afford the complexes [Rh₂H₅(PiPr₃)₄]BF₄ ( 9b ) and trans-[RhF(NO)(PiPr₃)₂]BF₄ ( 11 ), respectively. In solution, the cation [Rh₂(μ-H)₃H₂(PiPr₃)₄]⁺ of the compounds 9a and 9b undergoes an intramolecular rearrangement in which the bridging hydrido and the phosphane ligands are involved.     

Cycloaddition Chemistry of a Silylene-Nickel Complex toward Organic π-Systems: From Reversibility to C−H Activation

The versatile cycloaddition chemistry of the Si−Ni multiple bond in the acyclic (amido)(chloro)silylene→Ni⁰ complex 1 , [(^TMSL)ClSi→Ni(NHC)₂] (^TMSL=N(SiMe₃)Dipp; Dipp=2,6-iPr₂C₆H₄; NHC=C[(iPr)NC(Me)]₂), toward unsaturated organic substrates is reported, which is both reminiscent of and expanding on the reactivity patterns of classical Fischer and Schrock carbene–metal complexes. Thus, 1:1 reaction of 1 with aldehydes, imines, alkynes, and even alkenes proceed to yield [2+2] cycloaddition products, leading to a range of four-membered metallasilacycles. This cycloaddition is in fact reversible for ethylene, whereas addition of an excess of this olefin leads to quantitative sp²-CH bond activation, via a 1-nickela-4-silacyclohexane intermediate. These results have been supported by DFT calculations giving insights into key mechanistic aspects.     

Synthesis of a “Masked” Terminal Zinc Sulfide and Its Reactivity with Brønsted and Lewis Acids

The “masked” terminal Zn sulfide, [K(2.2.2‐cryptand)][^MeLZn(S)] ( 2 ) (^MeL={(2,6‐ⁱPr₂C₆H₃)NC(Me)}₂CH), was isolated via reaction of [^MeLZnSCPh₃] ( 1 ) with 2.3 equivalents of KC₈ in THF, in the presence of 2.2.2‐cryptand, at ?78 °C. Complex 2 reacts readily with PhCCH and N₂O to form [K(2.2.2‐cryptand)][^MeLZn(SH)(CCPh)] ( 4 ) and [K(2.2.2‐cryptand)][^MeLZn(SNNO)] ( 5 ), respectively, displaying both Brønsted and Lewis basicity. In addition, the electronic structure of 2 was examined computationally and compared with the previously reported Ni congener, [K(2.2.2‐cryptand)][^tBuLNi(S)] (^tBuL={(2,6‐ⁱPr₂C₆H₃)NC(^tBu)}₂CH).     

Heterobimetallic Nickel(II) Complexes of Ferrocenyldithiophosphonates. Molecular Structures of [{FcP(OR)S2}2Ni] [Fc = Fe(η5‐C5H4)(η5‐C5H5), R = Et,Pri, Bus,Bui]

The reaction of 2,4‐diferrocenyl‐1,3‐dithiadiphosphetane 2,4‐disulfide [FcPS(μ‐S)]₂ [Fc = Fe(η⁵‐C₅H₄)(η⁵‐C₅H₅)] with alcohols ROH gave the corresponding ferrocenyldithiophosphonic acids [FcPS(OR)(SH)], which were treated in situ with Ni(CH₃COO)₂·4H₂O in acetic acid to yield the square‐planar heterobimetallic trinuclear complexes [{FcP(OR)S₂}₂Ni] (R = Me ( 1 ), Et ( 2 ), Prⁱ ( 3 ), Bu^s ( 4 ) and Buⁱ ( 5 )). Compounds 1‐5 were characterized by elemental analysis, MS, NMR (¹H, ¹³C and ³¹P), IR spectroscopy, and 2‐5 also by X‐ray crystallography. Cyclovoltammetric studies on the heterobimetallic nickel(II) complexes 1‐5 showed irreversible reduction to unstable nickel(I) complexes and an irreversible two‐electron oxidation of the sulfur‐containing nickel fragments, followed by a reversible one‐electron oxidation of the two ferrocenyl groups.     

Synthesis of a “Masked” Terminal Zinc Sulfide and Its Reactivity with Brønsted and Lewis Acids

The “masked” terminal Zn sulfide, [K(2.2.2-cryptand)][^MeLZn(S)] ( 2 ) (^MeL={(2,6-ⁱPr₂C₆H₃)NC(Me)}₂CH), was isolated via reaction of [^MeLZnSCPh₃] ( 1 ) with 2.3 equivalents of KC₈ in THF, in the presence of 2.2.2-cryptand, at −78 °C. Complex 2 reacts readily with PhCCH and N₂O to form [K(2.2.2-cryptand)][^MeLZn(SH)(CCPh)] ( 4 ) and [K(2.2.2-cryptand)][^MeLZn(SNNO)] ( 5 ), respectively, displaying both Brønsted and Lewis basicity. In addition, the electronic structure of 2 was examined computationally and compared with the previously reported Ni congener, [K(2.2.2-cryptand)][^tBuLNi(S)] (^tBuL={(2,6-ⁱPr₂C₆H₃)NC(^tBu)}₂CH).     

A Mononuclear η2‐Bonded Phosphaalkyne Nickel(0) Complex

The synthesis and characterization of the new complex [Ni(Im^iPr)₂(η²‐P≡C‐tBu)] ( 1 ) is reported. Compound 1 represents the first structurally characterized example of a mononuclear nickel(0) complex with a side on coordinated phophaalkyne ligand.     

Der Dihydridoiridium(III)‐Komplex [IrH2Cl(PiPr3)2] als Molekülbaustein für unsymmetrische Rhodium–Iridiumund Iridium–Iridium‐Zweikernverbindungen

The Dihydridoiridium(III) Complex [IrH₂Cl(P i Pr₃)₂] as a Molecular Building Block for Unsymmetrical Binuclear Rhodium–Iridium and Iridium–Iridium Compounds The title compound [IrH₂Cl(PiPr₃)₂] ( 3 ) reacts with the chloro‐bridged dimers [RhCl(PiPr₃)₂]₂ ( 1 ) and [IrCl(C₈H₁₄)(PiPr₃)]₂ ( 5 ) by cleavage of the Cl‐bridges to give the unsymmetrical binuclear complexes 4 and 6 with Rh(μ‐Cl)₂Ir and Ir(μ‐Cl)₂Ir as the central building block. The reactions of 3 with the bis(cyclooctene) and (1,5‐cyclooctadiene) compounds [MCl(C₈H₁₄)₂]₂ ( 7 , 8 ) and [MCl(η⁴‐C₈H₁₂)]₂ ( 9 , 10 ) (M = Rh, Ir) occur analogously and afford the rhodium(I)‐iridium(III) and iridium(I)‐iridium(III) complexes 11 – 14 in 70–80% yield. Treatment of [(η⁴‐C₈H₁₂)M(μ‐Cl)₂IrH₂(PiPr₃)₂] ( 13 , 14 ) with phenylacetylene leads to the formation of the substitution products [(η⁴‐C₈H₁₂)M(μ‐Cl)₂IrH(C≡CPh)(PiPr₃)₂] ( 15 , 16 ) without changing the central molecular core. Similarly, the compound [(η⁴‐C₈H₁₂)Rh(μ‐Br)₂IrH(C≡CPh)(PiPr₃)₂] ( 18 ) has been prepared; it was characterized by X‐ray crystallography.     

On the Stability of a POCsp3OP‐Type Pincer Ligand in Nickel(II) Complexes

We describe the results of a study on the stabilities of pincer‐type nickel complexes relevant to catalytic hydroalkoxylation and hydroamination of olefins, C? C and C? X couplings, and fluorination of alkyl halides. Complexes [(POC_sp3OP)NiX] are stable for X=OSiMe₃, OMes (Mes=1,3,5‐Me₃C₆H₂), NPh₂, and CC? H, whereas the O(tBu) and N(SiMe₃)₂ derivatives decompose readily. The phenylacetylide derivative transforms gradually into the zero‐valent species cis‐[{κ^P,κ^C,κ^C′‐(iPr₂POCH₂CHCH₂)}Ni{η²,κ^C,κ^C′‐(iPr₂P(O)CCPh)}]. Likewise, attempts to prepare [(POC_sp3OP)NiF] gave instead the zwitterionic trinuclear species [{(η³‐allyl)Ni}₂‐{μ,κ^P,κ^O‐(iPr₂PO)₄Ni}]. Characterization of these two complexes provides concrete examples of decomposition processes that can dismantle POC_sp3OP‐type pincer ligands by facile C? O bond rupture. These results serve as a cautionary tale for the inherent structural fragility of pincer systems bearing phosphinite donor moieties, and provide guidelines on how to design more robust analogues.     

Adduct Formation,B−H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1 ) with unsaturated and saturated NHCs as well as CAAC^Me. Mono‐NHC adducts of the type HBcat?NHC (NHC=nPr₂Im, iPr₂Im, iPr₂Im^Me, and Dipp₂Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC^Me with HBcat yielded the B?H activated product CAAC^Me(H)Bcat via insertion of the carbine‐carbon atom into the B?H bond. The saturated NHC Dipp₂SIm reacted in a 2:2 ratio yielding an NHC ring‐expanded product at room temperature forming a six‐membered ?B?C=N?C=C?N? ring via C?N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene‐carbon atom.     

Ein‐ und zweikernige Rhodiumkomplexe mit Arsino(phosphino)methanen in unterschiedlichen Koordinationsformen

Mono‐ and Dinuclear Rhodium Complexes with Arsino(phosphino)methanes in Different Coordination Modes The cyclooctadiene complex [Rh(η⁴‐C₈H₁₂)(κ²‐tBu₂AsCH₂PiPr₂)](PF₆) ( 1a ) reacts with CO and CNtBu to give the substitution products [Rh(L)₂(κ²‐tBu₂AsCH₂PiPr₂)](PF₆) ( 2 , 3 ). From 1a and Na(acac) in the presence of CO the neutral compound [Rh(κ²‐acac)(CO)(κ‐P‐tBu₂AsCH₂PiPr₂)] ( 4 ) is formed. The reactions of 1a , the corresponding B(Ar_F)₄‐salt 1b and [Rh(η⁴‐C₈H₁₂)(κ²‐iPr₂AsCH₂PiPr₂)](PF₆) ( 5 ) with acetonitrile under a H₂ atmosphere affords the complexes [Rh(CH₃CN)₂(κ²‐R₂AsCH₂PiPr₂)]X ( 6a , 6b , 7 ), of which 6a (R = tBu; X = PF₆) gives upon treatment with Na(acac‐f₆) the bis(chelate) compound [Rh(κ²‐acac‐f₆)(κ²‐tBu₂AsCH₂PiPr₂)] ( 8 ). From 8 and CH₃I a mixture of two stereoisomers of composition [Rh(CH₃)I(κ²‐acac‐f₆)(κ²‐tBu₂AsCH₂PiPr₂)] ( 9/10 ) is generated by oxidative addition, and the molecular structure of the racemate 9 has been determined. The reactions of 1a and 5 with CO in the presence of NaCl leads to the formation of the “A‐frame” complexes [Rh₂(CO)₂(μ‐Cl)(μ‐R₂AsCH₂PiPr₂)₂](PF₆) ( 11 , 12 ), which have been characterized crystallographically. From 11 and 12 the dinuclear substitution products [Rh₂(CO)₂(μ‐X)(μ‐R₂AsCH₂PiPr₂)₂](PF₆) ( 13 ‐ 16 ) are obtained by replacing the bridging chloride for bromide, hydride or hydroxide, respectively. While 12 (R = iPr) reacts with NaI to give the related “A‐frame” complex 18 , treatment of 11 (R = tBu) with NaI yields the mononuclear chelate compound [RhI(CO)(κ²‐tBu₂AsCH₂PiPr₂)] ( 20 ). The reaction of 20 with CH₃I affords the acetyl complex [RhI₂{C(O)CH₃}(κ²‐tBu₂AsCH₂PiPr₂)] ( 21 ) with five‐coordinate rhodium atom.