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=i Pr2 ImMe 1Me , Mes2 Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni2 (i Pr2 ImMe )4 (µ-(η2 : η2 )-COD)] B/ [Ni(i Pr2 ImMe )2 (η4 -COD)] B' or [Ni(Mes2 Im)2 ] 2, respectively, with alkynes afforded complexes [Ni(NHC)2 (η2 -alkyne)] (NHC=i Pr2 ImMe : alkyne=MeC≡CMe 3, H7 C3 C≡CC3 H7 4, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me3 SiC≡CSiMe3 7, PhC≡CMe 8, HC≡CC3 H7 9, HC≡CPh 10, HC≡C(p-Tol) 11, HC≡C(4-t Bu-C6 H4 ) 12, HC≡CCOOMe 13; NHC=Mes2 Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4-t Bu-C6 H4 ) 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-t Bu-C6 H4 ), 11 and 12, which were formed by addition of a C-H bond of one of the NHC N-i Pr 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 1M 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 CNHC -Ni-CNHC bite angle, and to less steric repulsion of the smaller NHC i Pr2 ImMe .

[Ni(NHC)2 ] as a Scaffold for Structurally Characterized trans [H-Ni-PR2 ] and trans [R2 P-Ni-PR2 ] Complexes.

The addition of PPh2 H, PPhMeH, PPhH2 , P(para-Tol)H2 , PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2 ] (NHC=IiPr2 , IMe4 , IEt2 Me2 ) 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 [R2 P-Ni-PR2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4 >IEt2 Me2 >IiPr2 ) and phosphines are employed. P-P activation of the diphosphines R2 P-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 (Ph2 P-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.

Selective Photocatalytic C-F Borylation of Polyfluoroarenes by Rh/Ni Dual Catalysis Providing Valuable Fluorinated Arylboronate Esters.

A highly selective and general photocatalytic C-F borylation protocol that employs a rhodium biphenyl complex as a triplet sensitizer and the nickel catalyst [Ni(IMes)2] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the C-F bond activation and defluoroborylation process is reported. This tandem catalyst system operates with visible (blue, 400 nm) light and achieves borylation of a wide range of fluoroarenes with B2pin2 at room temperature in excellent yields and with high selectivity. Direct irradiation of the intermediary C-F bond oxidative addition product trans-[NiF(ArF)(IMes)2] leads to very fast decomposition when B2pin2 is present. This destructive pathway can be bypassed by indirect excitation of the triplet states of the nickel(II) complex via the photoexcited rhodium biphenyl complex. Mechanistic studies suggest that the exceptionally long-lived triplet excited state of the Rh biphenyl complex used as the photosensitizer allows for efficient triplet energy transfer to trans-[NiF(ArF)(IMes)2], which leads to dissociation of one of the NHC ligands. This contrasts with the majority of current photocatalytic transformations, which employ transition metals as excited state single electron transfer agents. We have previously reported that C(arene)-F bond activation with [Ni(IMes)2] is facile at room temperature, but that the transmetalation step with B2pin2 is associated with a high energy barrier. Thus, this triplet energy transfer ultimately leads to a greatly enhanced rate constant for the transmetalation step and thus for the whole borylation process. While addition of a fluoride source such as CsF enhances the yield, it is not absolutely required. We attribute this yield-enhancing effect to (i) formation of an anionic adduct of B2pin2, i.e., FB2pin2-, as an efficient, much more nucleophilic {Bpin-} transfer reagent for the borylation/transmetalation process, and/or (ii) trapping of the Lewis acidic side product FBpin by formation of [F2Bpin]- to avoid the formation of a significant amount of NHC-FBpin and consequently decomposition of {Ni(NHC)2} species in the reaction mixture.

Isolation of a Reactive Cyclopropane Intermediate via a Unique Catalyst-Free Spontaneous Cyclopropanation Step at 0 °C.

A remarkably mild, catalyst-free cyclopropanation reaction is confirmed by low-temperature isolation of a fused bicyclic cyclopropane intermediate. This intermediate is one of two that were isolated and fully characterised in a three-step temperature-controllable reaction cascade.

Preparing (Multi)Fluoroarenes as Building Blocks for Synthesis: Nickel-Catalyzed Borylation of Polyfluoroarenes via C-F Bond Cleavage.

The [Ni(IMes)2]-catalyzed transformation of fluoroarenes into arylboronic acid pinacol esters via C-F bond activation and transmetalation with bis(pinacolato)diboron (B2pin2) is reported. Various partially fluorinated arenes with different degrees of fluorination were converted into their corresponding boronate esters.

Regioselective Catalytic and Stepwise Routes to Bulky, Functional-Group-Appended, and Luminescent 1,2-Azaborinines.

The regioselective syntheses of 1,2-azaborinines is achieved using an unsymmetrical iminoborane through both catalytic and stepwise modular routes. The 1,2-azaborinine ring can be selectively functionalized in the 4- and/or 6-position through control of the stepwise reaction sequence, allowing access to vinyl-functionalized and redox-active, luminescent, donor-functionalized 1,2-azaborinines. The electrochemistry and photochemistry of a tetraarylamine-substituted 1,2-azaborinine are studied. Cyclic voltammetry of this compound, relative to a non-B,N-substituted reference molecule, showed an additional oxidation wave assigned to the oxidation of the azaborinine ring, while emission spectroscopy indicated that the azaborinine was significantly more fluorescent than the reference.

NHC Nickel-Catalyzed Suzuki-Miyaura Cross-Coupling Reactions of Aryl Boronate Esters with Perfluorobenzenes.

An efficient Suzuki-Miyaura cross-coupling reaction of perfluorinated arenes with aryl boronate esters using NHC nickel complexes as catalysts is described. The efficiencies of different boronate esters (p-tolyl-Beg, p-tolyl-Bneop, p-tolyl-Bpin, p-tolyl-Bcat) and the corresponding boronic acid (p-tolyl-B(OH)2) in this type of cross-coupling reaction were evaluated (eg, ethyleneglycolato; neop, neopentylglycolato; pin, pinacolato; cat, catecholato). Aryl-Beg was shown to be the most reactive boronate ester among those studied. The use of CsF as an additive is essential for an efficient reaction of hexafluorobenzene with aryl neopentylglycolboronates.

Coligand role in the NHC nickel catalyzed C-F bond activation: investigations on the insertion of bis(NHC) nickel into the C-F bond of hexafluorobenzene.

The reaction of [Ni(Mes2Im)2] (1) (Mes2Im = 1,3-dimesityl-imidazolin-2-ylidene) with polyfluorinated arenes as well as mechanistic investigations concerning the insertion of 1 and [Ni(iPr2Im)2] (1ipr) (iPr2Im = 1,3-diisopropyl-imidazolin-2-ylidene) into the C-F bond of C6F6 is reported. The reaction of 1 with different fluoroaromatics leads to formation of the nickel fluoroaryl fluoride complexes trans-[Ni(Mes2Im)2(F)(ArF)] (ArF = 4-CF3-C6F4 2, C6F5 3, 2,3,5,6-C6F4N 4, 2,3,5,6-C6F4H 5, 2,3,5-C6F3H2 6, 3,5-C6F2H3 7) in fair to good yields with the exception of the formation of the pentafluorophenyl complex 3 (less than 20%). Radical species and other diamagnetic side products were detected for the reaction of 1 with C6F6, in line with a radical pathway for the C-F bond activation step using 1. The difluoride complex trans-[Ni(Mes2Im)2(F)2] (9), the bis(aryl) complex trans-[Ni(Mes2Im)2(C6F5)2] (15), the structurally characterized nickel(i) complex trans-[NiI(Mes2Im)2(C6F5)] (11) and the metal radical trans-[NiI(Mes2Im)2(F)] (12) were identified. Complex 11, and related [NiI(Mes2Im)2(2,3,5,6-C6F4H)] (13) and [NiI(Mes2Im)2(2,3,5-C6F3H2)] (14), were synthesized independently by reaction of trans-[Ni(Mes2Im)2(F)(ArF)] with PhSiH3. Simple electron transfer from 1 to C6F6 was excluded, as the redox potentials of the reaction partners do not match and [Ni(Mes2Im)2]+, which was prepared independently, was not detected. DFT calculations were performed on the insertion of [Ni(iPr2Im)2] (1ipr) and [Ni(Mes2Im)2] (1) into the C-F bond of C6F6. For 1ipr, concerted and NHC-assisted pathways were identified as having the lowest kinetic barriers, whereas for 1, a radical mechanism with fluoride abstraction and an NHC-assisted pathway are both associated with almost the same kinetic barrier.