Search

The synthesis of the first terminal mono‐boryl complexes of nickel, which are not stabilized by a pincer ligand, is reported. The reaction of the nickel bis‐boryl complex cis‐[Ni(iPr2ImMe)2(Bcat)2] 1 (cat=1,2‐O2C6H4) with the small donor ligand PMe3 led to a complete ligand exchange at nickel with reductive elimination of B2cat2 and formation of the bis‐NHC adduct [B2cat2 ⋅ (iPr2ImMe)2] 3 and [Ni(PMe3)4] 2 as the metal‐containing species. Electrophilic attack of MeI on complex 1 or ligand dismutation of 1 with trans‐[Ni(iPr2ImMe)2Br2] led to loss of only one boryl ligand of 1 and afforded the nickel mono‐boryl complexes trans‐[Ni(iPr2ImMe)2(Bcat)Br] 4 a and trans‐[Ni(iPr2ImMe)2(Bcat)I] 4 b. [ABSTRACT FROM AUTHOR]

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)2] (NHC=iPr2ImMe1Me, Mes2Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni2(iPr2ImMe)4(μ‐(η2 : η2)‐COD)] B/ [Ni(iPr2ImMe)2(η4‐COD)] B' or [Ni(Mes2Im)2] 2, respectively, with alkynes afforded complexes [Ni(NHC)2(η2‐alkyne)] (NHC=iPr2ImMe: alkyne=MeC≡CMe 3, H7C3C≡CC3H74, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me3SiC≡CSiMe37, PhC≡CMe 8, HC≡CC3H79, 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 1Me 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 1Me 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 CNHC−Ni−CNHC bite angle, and to less steric repulsion of the smaller NHC iPr2ImMe. [ABSTRACT FROM AUTHOR]

The addition of PPh2H, PPhMeH, PPhH2, P(para‐Tol)H2, PMesH2 and PH3 to the two‐coordinate Ni0 N‐heterocyclic carbene species [Ni(NHC)2] (NHC=IiPr2, IMe4, IEt2Me2) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H−Ni−PR2] or novel trans [R2P−Ni−PR2] geometries. The bis‐phosphido complexes are more accessible when smaller NHCs (IMe4>IEt2Me2>IiPr2) and phosphines are employed. P−P activation of the diphosphines R2P−PR2 (R2=Ph2, PhMe) provides an alternative route to some of the [Ni(NHC)2(PR2)2] complexes. DFT calculations capture these trends with P−H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni−P bond. P−P bond activation from [Ni(NHC)2(Ph2P−PPh2)] adducts proceeds with computed barriers below 10 kcal mol−1. The ability of the [Ni(NHC)2] moiety to afford isolable terminal phosphido products reflects the stability of the Ni−NHC bond that prevents ligand dissociation and onward reaction. [ABSTRACT FROM AUTHOR]

Fluoride abstraction from different types of transition metal fluoride complexes [LnMF] (M=Ti, Ni, Cu) by the Lewis acid tris(pentafluoroethyl)difluorophosphorane (C2F5)3PF2 to yield cationic transition metal complexes with the tris(pentafluoroethyl)trifluorophosphate counterion (FAP anion, [(C2F5)3PF3]−) is reported. (C2F5)3PF2 reacted with trans‐[Ni(iPr2Im)2(ArF)F] (iPr2Im=1,3‐diisopropylimidazolin‐2‐ylidene; ArF=C6F5, 1 a; 4‐CF3‐C6F4, 1 b; 4‐C6F5‐C6F4, 1 c) through fluoride transfer to form the complex salts trans‐[Ni(iPr2Im)2(solv)(ArF)]FAP (2 a‐c[solv]; solv=Et2O, CH2Cl2, THF) depending on the reaction medium. In the presence of stronger Lewis bases such as carbenes or PPh3, solvent coordination was suppressed and the complexes trans‐[Ni(iPr2Im)2(PPh3)(C6F5)]FAP (trans‐2 a[PPh3]) and cis‐[Ni(iPr2Im)2(Dipp2Im)(C6F5)]FAP (cis‐2 a[Dipp2Im]) (Dipp2Im=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were isolated. Fluoride abstraction from [(Dipp2Im)CuF] (3) in CH2Cl2 or 1,2‐difluorobenzene led to the isolation of [{(Dipp2Im)Cu}2]2+2 FAP− (4). Subsequent reaction of 4 with PPh3 and different carbenes resulted in the complexes [(Dipp2Im)Cu(LB)]FAP (5 a–e, LB=Lewis base). In the presence of C6Me6, fluoride transfer afforded [(Dipp2Im)Cu(C6Me6)]FAP (5 f), which serves as a source of [(Dipp2Im)Cu)]+. Fluoride abstraction of [Cp2TiF2] (7) resulted in the formation of dinuclear [FCp2Ti(μ‐F)TiCp2F]FAP (8) (Cp=η5‐C5H5) with one terminal fluoride ligand at each titanium atom and an additional bridging fluoride ligand. [ABSTRACT FROM AUTHOR]

A study on the reactivity of the N‐heterocyclic silylene Dipp2NHSi (1,3‐bis(diisopropylphenyl)‐1,3‐diaza‐2‐silacyclopent‐4‐en‐2‐yliden) with the transition metal complexes [Ni(CO)4], [M(CO)6] (M=Cr, Mo, W), [Mn(CO)5(Br)] and [(η5‐C5H5)Fe(CO)2(I)] is reported. We demonstrate that N‐heterocyclic silylenes, the higher homologues of the now ubiquitous NHC ligands, show a remarkably different behavior in coordination chemistry compared to NHC ligands. Calculations on the electronic features of these ligands revealed significant differences in the frontier orbital region which lead to some peculiarities of the coordination chemistry of silylenes, as demonstrated by the synthesis of the dinuclear, NHSi‐bridged complex [{Ni(CO)2(μ‐Dipp2NHSi)}2] (2), complexes [M(CO)5(Dipp2NHSi)] (M=Cr 3, Mo 4, W 5), [Mn(CO)3(Dipp2NHSi)2(Br)] (9) and [(η5‐C5H5)Fe(CO)2(Dipp2NHSi‐I)] (10). DFT calculations on several model systems [Ni(L)], [Ni(CO)3(L)], and [W(CO)5(L)] (L=NHC, NHSi) reveal that carbenes are typically the much better donor ligands with a larger intrinsic strength of the metal–ligand bond. The decrease going from the carbene to the silylene ligand is mainly caused by favorable electrostatic contributions for the NHC ligand to the total bond strength, whereas the orbital interactions were often found to be higher for the silylene complexes. Furthermore, we have demonstrated that the contribution of σ‐ and π‐interaction depends significantly on the system under investigation. The σ‐interaction is often much weaker for the NHSi ligand compared to NHC but, interestingly, the π‐interaction prevails for many NHSi complexes. For the carbonyl complexes, the NHSi ligand is the better σ‐donor ligand, and contributions of π‐symmetry play only a minor role for the NHC and NHSi co‐ligands. [ABSTRACT FROM AUTHOR]

We herein demonstrate that the amide base generated in situ from CsF and N(TMS)3 catalyzes the deprotonative coupling reactions of terminal alkynes with polyfluoroarenes, wherein mono‐ and dialkynylations occur efficiently for penta‐ and hexafluorobenzenes, respectively. Tetraalkynylated products could also be synthesized from dialkynylated compounds. [ABSTRACT FROM AUTHOR]

C−F and C−H bond activation reactions of polyfluoroaromatics at the cyclic (alkyl)(amino)carbene (cAAC) cAACmethyl ( 1) are reported. Studies on the C−F bond activation using the cAAC-stabilized nickel(0) complex [Ni(cAACmethyl)2] ( 2) have shown that 2 does not react with fluorinated arenes. However, these investigations led to the observation of C−F bond cleavage of perfluorinated arenes by the carbene ligand cAACmethyl ( 1) itself. The reaction of 1 with C6F6, C6F5−C6F5, C6F5−CF3, and C5F5N afforded the insertion products of cAAC into one of the C−F bonds of the substrate, that is, the C−F bond activation products (cAACmethyl)F(Arf) (Arf=C6F5 4 a, C6F4−C6F5 4 b, C6F4−CF3 4 c, C5F4N 4 d). These products decompose readily upon heating to 80 °C within a few hours in solution with formation of ionic iminium salts [(cAACmethyl)(Arf)][X] 6 a- d or neutral alkenyl perfluoroaryl imine compounds 7 a- d. The compounds (cAACmethyl)F(Arf) 4 a- d readily transfer fluoride, which has been exemplified by the fluoride transfer of all compounds using BF3 etherate as fluoride acceptor. Fluoride transfer has also been achieved starting from (cAACmethyl)F(C6F4-CF3) ( 4 c) or (cAACmethyl)F(C5F4N) ( 4 d) to other selected substrates such as trimethylchlorosilane, benzoyl chloride and tosyl chloride. Instead of C−F bond activation, insertion of the cAAC into the C−H bond was observed if 1 was treated with the partially fluorinated arenes C6F5H, 1,2,4,5-C6F4H2, 1,3,5-C6F3H3, and 1,3-C6F2H4. The compounds (cAACmethyl)H(Arf) (Arf=C6F5 12 e, 2,3,5,6-C6F4H 12 f, 2,4,6-C6F3H2 12 g and 2,6-C6F2H3 12 h) have been isolated in good yields and have been characterized including X-ray analysis. Fluorobenzene C6FH5 (p Ka ≈37), the least C−H acidic fluoroarene used in this study, does not react. In order to investigate the scope and limitations of this type of cAAC C−H bond activation reaction, cAACmethyl ( 1) was treated with several other reagents of different C−H acidity such as imidazolium salts, imidazoles, esters, and trimethylphosphine. These investigations led to the isolation and characterization of the compounds [(cAACmethyl)H(R2ImMe2)]X ( 13 a,b), (cAACmethyl)H(ImR2) ( 14 a- c), (cAACmethyl)H(CH(COOCH3)2) ( 15 b) and (cAACmethyl)H(CH2-PMe2) ( 16). Deprotonation of [(cAACmethyl)H(Me2ImMe2)][BF4] ( 13 a) at the cAAC carbon atom using KHMDS as a base led to isolation and structural characterization of the cAACmethyl-NHC heterodimer ( 17). [ABSTRACT FROM AUTHOR]