On the Stability of a POC_sp3OP‐Type Pincer Ligand in Nickel(II) Complexes

Abstract

We describe the results of a study on the stabilities of pincer‐type nickel complexes relevant to catalytic hydroalkoxylation and hydroamination of olefins, CC and CX couplings, and fluorination of alkyl halides. Complexes [(POCsp3OP)NiX] are stable for X=OSiMe3, OMes (Mes=1,3,5‐Me3C6H2), NPh2, and CCH, whereas the O(tBu) and N(SiMe3)2 derivatives decompose readily. The phenylacetylide derivative transforms gradually into the zero‐valent species cis‐[{κP,κC,κC′‐(iPr2POCH2CHCH2)}Ni{η2,κC,κC′‐(iPr2P(O)CCPh)}]. Likewise, attempts to prepare [(POCsp3OP)NiF] gave instead the zwitterionic trinuclear species [{(η3‐allyl)Ni}2‐{μ,κP,κO‐(iPr2PO)4Ni}]. Characterization of these two complexes provides concrete examples of decomposition processes that can dismantle POCsp3OP‐type pincer ligands by facile CO bond rupture. These results serve as a cautionary tale for the inherent structural fragility of pincer systems bearing phosphinite donor moieties, and provide guidelines on how to design more robust analogues.

Small Molecule Activation by POCOP‐Nickel Complexes

“…The identity of Ni‐C(O)NHAr could be inferred from the following spectral features. A 31 P signal downfield of the corresponding signal for the precursor (185 versus 176 ppm) was consistent with the replacement of the NiN bond by a NiC bond,11 as were the 13 C signal at approximately 209 ppm for Ni‐ C (O)N and the IR absorption at 1638 cm −1 for ν CO 21. In addition to the anticipated resonances for the POCOP ligand, the 1 H NMR spectrum also showed a significantly downfield‐shifted (6.62 ppm in Ni‐C(O)NHAr versus approximately 0.6 ppm in Ni‐NHAr ) and slightly broadened N‐ H singlet and the corresponding aromatic signals for a molecule of ArNH 2 .…”

“…The stabilities of the other (minor) products of CO 2 insertion did not appear to depend on P , but their isolation was circumvented by the gradual decomposition of the product represented by the 31 P singlet at 124 ppm, a species that decayed gradually. In the absence of informative spectral clues, we have not succeeded in identifying the minor product represented by the 31 P signal at approximately 176 ppm,20 but the product represented by the minor signal at 124 ppm could be assigned to a zwitterionic trinuclear species that was obtained previously from a different route (Scheme ) and characterized fully 11. It is unclear how this compound forms.…”

“…We have previously reported the synthesis of the siloxide derivative [(POCOP)Ni(OSiMe 3 )], Ni‐OSiMe 3 ,11 and a similar approach was used to prepare the amide derivative [(POCOP)Ni(NHAr)], Ni‐NHAr , by treating the bromo precursor7a with excess NaNHAr (Scheme ). Ni‐NHAr is extremely air‐ and moisture‐sensitive, both as a solid and in solution, and has a limited thermal stability in solution (∼2 h at 90 °C); by comparison, Ni‐OSiMe 3 is less air‐sensitive and has a greater thermal stability (∼5 h at 100 °C).…”

“…This mechanistic postulate does not account for the formation of the phosphamide 4 from Ni‐NHAr . However, other mechanistic schemes can be envisaged for the formation of 4 if we recall that [(POCOP)NiX] is susceptible to spontaneous CO bond cleavage reactions that can give different Ni‐bound allylic fragments and X‐P(O)R 2 products arising from Michaelis–Arbuzov‐like rearrangements (Scheme , X=PhCC) 11. It occurred to us that such a rearrangement might provide a pathway to 4 in the absence of molecular oxygen.…”

“…On the other hand, complexes of conformationally flexible pincer ligands are also more prone to so‐called “non‐innocent” behavior 10. For instance, the complexes [(POCOP)NiX] featuring X ligands such as fluoride (a potential π‐donor, hard ligand) or phenylacetylide (a potential acceptor, soft ligand) generate highly labile species that undergo unprecedented transformations and rearrangements not available to their POCOP analogues (Scheme ) 11…”