Divergent Reactivity of an Isolable Nickelacyclobutane

Abstract Nickelacyclobutanes are mostly invoked as reactive intermediates in the reaction of nickel carbenes and olefins to yield cyclopropanes. Nevertheless, early work suggested that other decomposition routes such as β‐hydride elimination and even metathesis could be accessible. Herein, we report the isolation and characterization of a stable pentacoordinated nickelacyclobutane incorporated in a pincer complex. The coordination of different coligands to the nickelacyclobutane determines its selective decomposition along cyclopropanation, metathesis or apparent β‐hydride elimination pathways. DFT calculations shed light on the mechanism of these different pathways.

Herein we show that a pincer ligand framework incorporating a precoordinated olefin allows trapping of a transient nickel carbene to form a stable nickelacyclobutane.[25][26][27] Depending on the presence and nature of an additional ligand, the nickelacyclobutane selectively undergoes cyclopropanation, [2+2] cycloreversion, or apparent b-hydride elimination.Experiments and computations provide insight into the mechanistic pathways.
Exposing complex 2 to bis(p-tolyl)diazomethane led to release of p-fluorostyrene and formation of adduct 3, which readily releases N 2 at room temperature (also in the dark) to form nickelacyclobutane 4 and could not be isolated in bulk (Scheme 2, see SI section 2.1).The IR spectrum of a mixture of 3 and 4 showed a characteristic n(N=C) absorption band at 2044 cm À1 .Gratifyingly, crystals of 3 suitable for X-ray diffraction could also be obtained from such a mixture.The resulting crystal structure confirms the h 1 (N) binding mode of the diazo ligand and the tridentate P-h 2 (C,C)-P pincer coordination (see SI, section 4).The CÀN distance of 1.312-  4) ). [17,29,30] AX-ray crystal structure of nickelacyclobutane 4 (Figure 1) reveals a nickel(II) center that can be described as pentacoordinated with four ligands from the pincer framework (P1, P2, C7, C39) and an additional p-interaction with one of the tolyl groups (C40, C41).A transannular Ni À C38 distance of 2.577( 2) and an angle between the planes C7-C38-C39 and C7-Ni1-C39 of 27.69( 19)8 shows a high degree of puckering in the four-membered ring. [30]MR analysis of 4 in [D 8 ]toluene solution at À40 8C is consistent with the solid-state structure.The methylene group affords diastereotopic 1 H signals at d = 4.35 and 4.40 ppm.One aromatic 1 H signal is shifted upfield at 5.35 ppm and the two CH 3 groups are diasterotopic, which we attribute to pcoordination of one of the p-tolyl group.Accordingly, the 31 P NMR spectrum at À40 8C presents two doublets at 21.9 and 44.9 ppm (J P,P = 77 Hz).Upon warming to 25 8C, the 31 P NMR signals coalesce to one broad signal in the 20-40 ppm range while the methylene and CH 3 1 H signals coalesce to 4.40 ppm and 1.76 ppm, respectively.These observations are consistent with exchange of the bound and unbound p-tolyl groups on the NMR timescale, for which a Gibbs free energy of activation of 12.0 AE 0.2 kcal mol À1 at À20 8C can be estimated (see SI, section 2.2).
Upon exposure of compound 4 to CO (1 atm) as a pacceptor coligand, a new complex was formed that displays symmetrical 1 H and 31 P NMR spectra and a strong IR absorption at n(C = O) = 1984 cm À1 (see SI, section 2.3).The spectroscopic data are consistent with the trigonal bipyramidal (TBP) nickelacyclobutane structure 4-CO (Scheme 3), in which a CO ligand has displaced the p-interaction and occupies an axial position.4-CO is unstable in solution and fully converts over 16 h under CO to compound 5, which contains two CO ligands as shown by IR absorptions at 1944 and 2002 cm À1 .An X-ray crystal structure (Figure 2) identified complex 5 as a tetrahedral nickel(0) complex of a diphosphine ligand containing a cyclopropane unit that results from reductive elimination of the metallacycle. [30]The orientation of the cyclopropane ring in 5 is not that expected from direct reductive elimination: the CH 2 group instead of the Ctol 2 group points towards the nickel center, which likely reduces steric repulsion.In the absence of any indication of ring rearrangements in the nickelacyclobutane, [31][32][33] we propose transient phosphine decoordination after cyclopropane formation as the most likely isomerization pathway, but inversion of the chelate cycle cannot be excluded.
To study the influence of a s-donor coligand, the reactivity of nickelacyclobutane 4 with MeCN was explored (Scheme 3).In CD 3 CN/C 6 D 6 , 1 H NMR and 31 P NMR indicate a symmetrical species at À30 8C (see SI, section 2.4), consistent with the formation of TBP complex 4-MeCN featuring an MeCN ligand in axial position.Over 1 h at room temperature in MeCN, 4-MeCN decayed to a symmetrical complex devoid of tolyl fragments (6) with release of 1,1-di(p-tolyl)ethylene. Complex 6 was crystallographically and spectroscopically identified as a square planar nickel(II) complex bearing a P(CH)P pincer ligand and a CH 2 CN group (Figure 2). [30]epeating the experiment with a CD 2 group instead of CH 2 in nickelacyclobutane 4 yielded d 2 -1,1-di(p-tolyl)ethylene, confirming the origin of the methylene group.The formation of these products can be explained by [2+2] cycloreversion of the nickelacyclobutane to yield the alkene and the carbene pincer complex (PC carbene P)Ni(MeCN), which subsequently activates a CÀH bond from MeCN.[36][37] To our knowledge, this is the first observation of a nickelacyclobutane selectively undergoing a metathesis-like ring opening.
In non-polar solvents benzene and toluene, nickelacyclobutane 4 slowly decomposed to a new compound (7); full conversion requires more than 24 h at room temperature and can be accelerated by heating.An X-ray crystal structure revealed 7 as a dinuclear nickel(0) complex featuring a bridging dinitrogen molecule (Scheme 3, SI section 4). [30]The ligand backbone contains a new olefin formally resulting from the insertion of a di-p-tolylcarbene fragment into one of the olefinic CÀH bonds of ligand 1. Reaction of 7 with  benzonitrile yielded the monomeric complex 8 (Figure 2).While complex 7 could be seen as the result of b-hydride elimination from nickelacyclobutane 4, [11,13,14,21,22] the syn coplanar geometry required for such a concerted reaction would likely be strained.This intramolecular pathway was indeed ruled out by a crossover experiment with equal amounts of nickelacyclobutane and d 2 -nickelacyclobutane in toluene.After reaction, the two C-H positions originating from the ligand CH 2 /CD 2 group exhibited 23 % and 77 % deuterium incorporation, respectively, whereas twice 50 % would be expected for an intramolecular process.Therefore, a more intricate intermolecular mechanism is at play (see SI sections 2.5 and 5.2.2).

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Chemie Zuschriften 26724 www.angewandte.de(1.7 kcal mol À1 ) to yield three-coordinated nickel carbene 11, which is more stable by 3 kcal mol À1 .From structure 10, TS3 (18.3 kcal mol À1 ) yields nickelacyclobutane 4 via the Chauvin mechanism [39] with an overall free energy gain of 30.3 kcal mol À1 from diazoadduct 3.For comparison, a non-carbene pathway for nickelacyclobutane formation via nucleophilic attack of the diazoalkane on the alkene backbone [17] was predicted to have a total energy barrier of 54.6 kcal mol À1 with respect to diazoadduct 3 and is therefore implausible (see SI section 5.2.1).
The cyclopropanation pathway (blue) starts with the coordination of CO yielding TBP complex 4-CO that undergoes a concerted reductive elimination (TS4) with a barrier of 24.5 kcal mol À1 .The resulting tricoordinate nickel complex 12 shows no nickel-cyclopropane interaction and accepts another CO ligand to form tetracoordinated complex 13 with an exergonicity of 8.1 kcal mol À1 .Isomerization to the less sterically encumbered cyclopropane complex 5 is favored by 16.2 kcal mol À1 .For comparison, the metathesis pathway from 4-CO was computed.The corresponding TS is associated with a similar energy barrier of 24.4 kcal mol À1 and leads to a more energetic (PC carbene P)Ni(CO) product in comparison with the cyclopropane complex (5) by 19.8 kcal mol À1 .Hence, cyclopropanation is predicted to be thermodynamically favored in the presence of an excess of CO, but metathesis might become competitive with limited CO reactant (see SI, sections 2.2 and 5.2.4).
For the metathesis pathway in MeCN (purple), the formation of TBP nickelacyclobutane compound 4-MeCN is nearly ergoneutral (+ 1.1 kcal mol À1 ), consistent with coordination occurring in the presence of a large excess of MeCN.The subsequent transition state for cycloreversion (TS5) yields a (PC carbene P)Ni(MeCN) complex ( 14) and 1,1-di(ptolyl)ethylene with a barrier of 19.9 kcal mol À1 .Afterwards, C-H activation to produce complex 6 provides an overall driving force of À13 kcal mol À1 for this pathway.For comparison, the transition state for cyclopropane formation from 4-MeCN is significantly higher in energy (28.5 kcal mol À1 ) than for cycloreversion (20.3 kcal mol À1 ), disfavoring this pathway in accord with experimental observations (see SI, section 5.2.3).
Additionally, cyclopropanation and cycloreversion routes from nickelacyclobutane 4 were computed in the absence of exogenous ligands.Cyclopropanation is endergonic by 25.2 kcal mol À1 and associated with a barrier of 30.5 kcal mol À1 , and therefore inaccessible at room temperature (see SI, section 5.2.3).In contrast, calculations associated cycloreversion from nickelacyclobutane (4) with a barrier of 24.4 kcal mol À1 , but the product (PC carbene P)Ni(1,1-di(p-tolyl)ethylene) is thermodynamically disfavored by 13.5 kcal mol À1 .
In summary, we presented the synthesis and characterization of a nickelacyclobutane via a nickel carbene pathway.The cycle is stabilized by a diphosphine pincer ligand that favors an unusual pentacoordinated geometry.Selective

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Zuschriften decomposition to a cyclopropane derivative can be induced by introducing a p-acceptor coligand such as CO in axial position, whereas selective [2+2] cycloreversion/metathesis is observed with a s-donor coligand such as MeCN.Additionally, an olefin product that would be consistent with a bhydride elimination/reductive elimination sequence from the nickelacyclobutane in the absence of coligands was shown to instead originate from an intermolecular pathway.The accessibility of these different pathways owes to the ability of the pincer scaffold to accommodate both Ni 0 and Ni II species and to sample different coordination modes.Our findings underline the influence of the coordination environment of nickelacyclobutane intermediates on their selective reactivity and suggests that controlled Ni-catalyzed olefin metathesis may be accessible.This possibility is currently under investigation in our laboratory.

Figure 3 .
Figure 3. Gibbs free energy profiles for the formation of nickelacyclobutane 4 and further reactivity with CO and MeCN computed at B3LYP-GD3BJ/def2TZVP//B3LYP-GD3BJ/6-31(d,p) level of theory.In blue: cyclopropanation pathway, in purple: metathesis pathway.Dashed lines connect intermediates between which no transition state was optimized.