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.

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.

A combined experimental and theoretical study on the mechanism of the C–F bond activation of C6F6 with [Ni(NHC)2] is provided.

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.

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.

The complexes trans-[Ni(iPr 2 Im) 2 Br 2 ] 1, trans-[Ni-(iPr 2 Im Me ) 2 Br 2 ] 2, trans-[Ni(Me 2 Im) 2 I 2 ] 3, and trans-[Ni-(Me 2 Im Me ) 2 Br 2 ] 4 (R 2 Im = 1,3-diorganyl-imidazolin-2-ylidene, R 2 Im Me = 1,3-diorganyl-4,5-dimethyl-imidazolin-2-ylidene) are useful precursor for the synthesis of NHC-stabilized nickel olefin, alkyne, alkyl and cyanido complexes. [Ni(NHC) 2 (η 2 -olefin)] and [Ni(NHC) 2 (η 2 -alkyne)] can be prepared via metallic reduction in the presence of the olefin or alkyne, as exemplified by the synthesis of [Ni(Me 2 Im) 2 (η 2 -C 2 H 4 )] 5, [Ni(Me 2 Im) 2 (η 2 -C 2 Me 2 )] 6, [Ni(Me 2 Im) 2 (η 2 -COE)] 7 and [Ni(Me 2 Im Me ) 2 (η 2 -COE)] 8 (COE = [a]

The dehydrocoupling of primary phosphines with N-heterocyclic carbenes (NHCs) to yield NHC-phosphinidenes is reported. The reaction of two equivalents of the NHCs Me 2 Im (1,3dimethylimidazolin-2-ylidene), Me 4 Im (1,3,4,5-tetramethylimidazolin-2-ylidene), iPr 2 Im (1,3-di-iso-propylimidazolin-2ylidene) and Mes 2 Im (2,4,6-trimethylphenylimidazolin-2-ylidene) with PhPH 2 and MesPH 2 led to the NHC stabilized phosphinidenes (NHC)PAr: (iPr 2 Im)PPh ( 1), (Mes 2 Im)PPh ( 2), (Me 4 Im)PPh ( 3), (Mes 2 Im)PMes ( 4), (Me 2 Im)PMes ( 5), (Me 4 Im)PMes ( 6) and (iPr 2 Im)PMes ( 7). The reaction of tBuPH 2 with two equivalents of the NHCs afforded the corresponding NHC stabilized parent phosphinidenes (NHC)PH: (iPr 2 Im)PH ( 8), (Mes 2 Im)PH ( 9) and (Me 4 Im)PH (10). Reaction of 1 with oxygen and sulfur led to isolation of iPr 2 Im-P(O) 2 Ph (11) and iPr 2 Im-P(S) 2 Ph (12), whereas the reaction with elemental selenium and tellurium gave (NHC) PPh cleavage with formation of (iPr 2 Im)Se ( 13), iPr 2 ImTe (14) and different cyclo-oligophosphines. Furthermore, the complexes [{(iPr 2 Im)PPh}W(CO) 5 ] (15), [Co(CO) 2 (NO){(iPr 2 Im)PPh}] (16) and [(η 5 -C 5 Me 5 )Co(η 2 -C 2 H 4 ){(iPr 2 Im)PPh}] (17) have been prepared starting from 1 and a suitable transition metal complex precursor. The complexes 16 and 17 decompose in solution upon heating to ca. 80°C to yield the NHC complexes [Co(iPr 2 Im)(CO) 2 (NO)] and [(η 5 -C 5 Me 5 )Co(iPr 2 Im)(η 2 -C 2 H 4 )] with formation of cyclo-oligophosphines. The reaction of 1 with [Ni(COD) 2 ] afforded the diphosphene complex [Ni(iPr 2 Im) 2 (trans-PhP = PPh)] 18.

The reactivity of [Ni(iPr2Im)4(µ‐COD)] 1 (iPr2Im = 1,3‐diisopropyl‐imidazolin‐2‐ylidene, COD = 1,4‐cyclooctadiene) in Hiyama‐ and Negishi‐type cross‐coupling reactions as well as the synthesis of several novel nickel fluoroaryl alkyl complexes is reported. Hiyama coupling of 1.1 equiv. perfluoroaromatics and 1 equiv. PhSi(OR)3 (R = Me, Et) with 5 mol‐% of 1 as catalyst leads to the C–C coupling product ArF–Ph in good to fair yields. In presence of the additive NMe4F alkoxy transfer from PhSi(OR)3 to the perfluoroarene occurs to yield ArF–OR and PhSiF(OR)2. Negishi cross‐coupling between C6F6 or C7F8 (1 equiv.), diorganozinc reagents [ZnR2] (R = Me, Et) (2.1 equiv.) and 5 mol‐% 1 as the catalyst in toluene at 115 °C leads to ArF–R only in traces. However, NMR experiments revealed that nickel alkyl complexes are readily formed from the reaction of trans‐[Ni(iPr2Im)2(F)(ArF)] with [ZnR2] (R = Me, Et). In course of these investigations, a series of novel nickel alkyl complexes trans‐[Ni(iPr2Im)2(R)(ArF)] (R = Me, ArF = C6F5 2, C7F7 3, C12F9 4; R = Et, ArF = C6F5 5, C7F7 6, C12F9 7) have been synthesized in stoichiometric reactions starting from trans‐[Ni(iPr2Im)2(F)(ArF)] (ArF = C6F5, C7F7, C12F9) and [ZnR2] (R = Me, Et) in thf at –78 °C. As these nickel alkyl complexes 2–7 are stable at room temperature in solution for several days with respect to reductive elimination, their thermal stability was investigated. Heating trans‐[Ni(iPr2Im)2(Me)(C6F5)] 2 for 24 hours at 100 °C leads to 91 % unreacted complex 2 and only traces of reductive elimination product, i.e. C6F5Me, are formed. Furthermore, the nickel ethyl complex trans‐[Ni(iPr2Im)2(Et)(C6F5)] 5 is also very stable, even with respect to β‐hydride elimination. After heating this complex to 100 °C for 24 hours there is still 26 % unreacted 5 left.