A Mononuclear η2-Bonded Phosphaalkyne Nickel(0) Complex

NHC-ligated nickel(II) cyanoborate complexes and salts

A comprehensive study on the synthesis and characterization of NHC-ligated nickel(II) cyanoborates (CBs) is presented (NHC = N-heterocyclic carbene). Nickel(II) cyanoborates Ni[BH2(CN)2]2·H2O (Ib·H2O), Ni[BH(CN)3]2·0.5H2O (Ic·0.5H2O), Ni[B(CN)4]2·H2O (Id·H2O) were reacted with selected NHCs of different steric size. The reaction of the nickel cyanoborates with small to medium-sized NHCs Me2ImMe and iPr2Im (R2Im = 1,3-di-organyl-imidazolin-2-ylidene; R2ImMe = 1,3-di-organyl-4,5-dimethyl-imidazolin-2-ylidene) afforded cyanoborate salts containing the rare homoleptic fourfold NHC-ligated nickel(II) cations [Ni(NHC)4]2+ (NHC = Me2ImMe (1c-d), iPr2Im (2c-d)) and cyanoborate counter-anions. Bulkier NHCs such as Mes2Im and Dipp2Im afforded complexes trans-[Ni(NHC)2(CB)2] (trans-4b, trans-5c). For the combination of the cyanoborate anion [BH2(CN)2]- and iPr2ImMe the salt of the tris-NHC complex [Ni(iPr2ImMe)3(NC-BH2CN)][BH2(CN)2] (3b) was isolated. Salt metathesis of NHC-ligated nickel(II) halides (Ni(NHC)2X2) (X = Cl, Br) with silver(I) and alkali metal cyanoborates were used to synthesize mono- and disubstituted coordination compounds of the type cis- or trans-[Ni(NHC)2(CB)X] (cis-10c, cis-11c, trans-12b) and cis- or trans-[Ni(NHC)2(CB)2] (cis-13b, cis-14a-c, trans-14a-b, trans-15b, trans-5b). Further investigations reveal that NHC-ligated cyanoborate complexes can act as building blocks for coordination polymers, as observed for structurally characterized 1∞{trans-[Ni(Mes2Im)2(μ2-[NC-BH2-CN])2]·2Ag(μ2-[BH2(CN)2])} (trans-5b·Ag). This study demonstrates the diverse character of cyanoborates in coordination chemistry as both, non-coordinating counter-anions, and weakly to medium coordinating anions forming novel transition metal complexes and salts. It provides evidence that a proper choice of cyanoborate and a proper choice of co-ligand can lead to a rich coordination chemistry of cyanoborate anions.

Reactions of nickel boranyl compounds with pnictogen-carbon triple bonds

The catalytic conversion of unsaturated small molecules such as nitriles into reduced products is of interest for the production of fine chemicals. In this vein, metal-ligand cooperativity has been leveraged to promote such reactivity, often conferring stability to bound substrate - a balancing act that may offer activation at the cost of turnover efficiency. This report describes the reactivity of a [(diphosphine)Ni] compound with pnictogen carbon triple bonds (R-C[triple bond, length as m-dash]E; E = N, P), where the diphosphine contains two pendant borane groups. For E = N, cooperative nitrile coordination is observed to afford {Ni}₂ complexes displaying B-N interactions, whereas for E = P, B-P interactions are absent. This work additionally outlines a structure-activity relationship that uses nitrile dihydroboration as a model reaction to unveil the effect of SCS stabilization, employing [(diphosphine)Ni] where the diphosphine contains 0, 1, or 2 pendant Lewis acid groups.

Revealing the Role of the Cyaphide Ion as a Bridging Ligand in Heterometallic Complexes

The synthesis of heterometallic transition metal complexes featuring bridging cyaphide ions (C≡P⁻) is reported. These are synthesized from reactions of Au(IDipp)(CP) (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) with electron‐rich, nucleophilic transition metal reagents, affording Au(IDipp)(μ₂−C≡P)Ni(^MeIⁱPr)₂ (^MeIⁱPr=1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene) and Au(IDipp)(μ₂−C≡P)Rh(Cp*)(PMe₃). These studies reveal that, in contrast to the cyanide ion, bimetallic cyaphido complexes strongly favor a η¹ : η² coordination mode that maximizes the interaction of the second metal (Ni, Rh) with the π‐manifold of the ion (and not the phosphorus atom lone pair). End‐on bridging can be effectively unlocked by blocking the π‐manifold, as demonstrated by reaction of Au(IDipp)(μ₂−C≡P)Rh(Cp*)(PMe₃) with an electrophilic transition metal reagent, W(CO)₅(THF), which affords the heterotrimetallic compound Au(IDipp)(μ₃−C≡P)[Rh(Cp*)(PMe₃)][W(CO)₅].

Case Study of N-i Pr 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^Me1^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≡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 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^Me 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.

N-Heterocyclic silylenes as ambiphilic activators and ligands

Recent developments of the use of N-heterocyclic silylenes (NHSis), higher homologues of Arduengo-carbenes, as ambiphilic activators and ligands are highlighted and a comparison of NHSi ligands with NHC and phosphine ligands is provided.

Di-tert-butyldiphosphatetrahedrane: Catalytic Synthesis of the Elusive Phosphaalkyne Dimer

While tetrahedranes as a family are scarce, neutral heteroatomic species are all but unknown, with the only reported example being AsP₃. Herein, we describe the isolation of a neutral heteroatomic X₂Y₂ molecular tetrahedron (X, Y=p‐block elements), which also is the long‐sought‐after free phosphaalkyne dimer. Di‐tert‐butyldiphosphatetrahedrane, (tBuCP)₂, is formed from the monomer tBuCP in a nickel‐catalyzed dimerization reaction using [(NHC)Ni(CO)₃] (NHC=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene (IMes) and 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene (IPr)). Single‐crystal X‐ray structure determination of a silver(I) complex confirms the structure of (tBuCP)₂. The influence of the N‐heterocyclic carbene ligand on the catalytic reaction was investigated, and a mechanism was elucidated using a combination of synthetic and kinetic studies and quantum chemical calculations.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Uranium and thorium complexes of the phosphaethynolate ion

New tris-amidinate actinide (Th, U) complexes containing a rare O-bound terminal phosphaethynolate (OCP ^- ) ligand were synthesized and fully characterized. The cyanate (OCN ^- ) and thiocyanate (SCN ^- ) analogs were prepared for comparison and feature a preferential N-coordination to the actinide metals. The Th(amid)₃(OCP) complex reacts with Ni(COD)₂ to yield the heterobimetallic adduct (amid) ₃ Th(μ-η ¹ (O):η ² (C,P)-OCP)Ni(COD) featuring an unprecedented reduced (OCP ^- ) bent fragment bridging the two metals.

Efficient nickel mediated carbon–carbon bond cleavage of organonitriles

The reactions of the nickel complex [Ni(2)(iPr(2)Im)4(COD)] 1 with organonitriles smoothly and irreversibly proceed via intermediates with eta(2)-coordinated organonitrile ligands such as [Ni(iPr(2)Im)2(eta(2)-(CN)-PhCN)] 2 and [Ni(iPr(2)Im)2(eta(2)-(CN)-pTolCN)] 4 to yield aryl cyanide complexes of the type trans-[Ni(iPr(2)Im)2(CN)(Ar)] (Ar = Ph 3, pTol 5, 4-CF(3)C(6)H(4) 6, 2,4-(OMe)2C(6)H(3) 7, 2-C(4)H(3)O 8, 2-C(5)H(4)N 9). The compounds 3, 7, 9 and have been structurally characterized. For the conversion of 2 to 3 a free activation enthalpy DeltaG++(328 K) of 103.47 +/- 0.79 kJ mol(-1) was calculated from time dependent NMR spectroscopy. The analogous reaction of arylnitriles with electron releasing substituents or heteroaromatic organonitriles is significantly faster compared to the reaction with benzonitrile or toluonitrile. The reactions of 1 with acetonitrile or trimethylsilyl cyanide afforded [Ni(iPr(2)Im)2(CN)(Me)] 10 and structurally characterized [Ni(iPr(2)Im)2(CN)(SiMe(3))] 11. The usage of an organonitrile with a longer alkyl chain, adiponitrile, yielded [Ni(iPr(2)Im)2(eta(2)-(CN)-NCC(4)H(8)CN)] 12 as well as the C-CN activation product [Ni(iPr(2)Im)2(CN)(C(4)H(8)CN)]13 in thermal and photochemical reactions, although this pathway seems to be significantly interfered with by decomposition pathways under the formation of the dicyanide complex [Ni(iPr(2)Im)(2)(CN)(2)] 14.