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 [(POCsp3 OP)NiX] are stable for X=OSiMe3 , OMes (Mes=1,3,5-Me3 C6 H2), 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-[{κ(P),κ(C),κ(C')-(iPr2 POCH2 CHCH2 )}Ni{η(2),κ(C),κ(C')-(iPr2 P(O)CCPh)}]. Likewise, attempts to prepare [(POCsp3 OP)NiF] gave instead the zwitterionic trinuclear species [{(η(3) -allyl)Ni}2-{μ,κ(P),κ(O)-(iPr2 PO)4 Ni}]. Characterization of these two complexes provides concrete examples of decomposition processes that can dismantle POCsp3 OP-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.

This report describes the synthesis and characterization of new pincer-type complexes of divalent and trivalent nickel. Refluxing toluene mixtures of NiBr2(CH3CN) n and the unsymmetrical pincer-type ligands Ph-POCHNimine-R gave (Ph-POCNimine-R)Ni(II)Br in good yields (Ph-POCHNimine-R = 1-(Ph2PO),3-(CHNR)C6H4; R = Bn, 1; Ph, 2; t-Bu, 3; Cy, 4). The C–H nickelation step involved in these syntheses is facilitated in the presence of NEt3. Treating compounds 1–4 with Br2, I2, N-bromosuccinimide (NBS) and N-chlorosuccinimide appears to bring about the desired one-electron oxidation of the Ni(II) center in these complexes, but the putative trivalent species decompose over time and could not be isolated. One of the decomposition products obtained in the Br2 reaction was identified as the zwitterionic compound [Br3Ni(II){κO-(E)-N-(3-((diphenylphosphoryl)oxy)benzylidene)benzenaminium}], 5. To identify the factors that are important for isolation of stable trivalent derivatives, we prepared the divalent precursors (i-Pr-POCNimine-Ph)NiX (X= Br, 6, and NCS, 9) and studied their reactions with various oxidants. Whereas treatment of 6 with Br2 or NBS gave the target 17-electron complex (i-Pr-POCNimine-Ph)Ni(III)Br2 (7), no mixed-ligand trivalent complexes of the type (i-Pr-POCNimine-Ph)Ni(III)Br(X) could be isolated from reactions of (i-Pr-POCNimine-Ph)Ni(NCS) (9) with NBS or of 6 with N-chlorosuccinimide. The latter reaction gave a complex mixture from which was isolated a paramagnetic, trinuclear compound (8) composed of octahedral Ni(II) units featuring μ2-Cl and μ2,κO,κN-succinimide fragments. The solid state structures of all new complexes 2-9 have been elucidated by X-ray crystallography.

This contribution describes the reactivities of CO2 , CO, O2 , and ArNC with the pincer-type complexes [(κ(P) ,κ(C) ,κ(P') -POC sp 3OP)NiX] (POC sp 3OP=(R2 POCH2 )2 CH; R=iPr; X=OSiMe3 , NArH; Ar=2,6-iPr2 C6 H3 ). Reaction of the amido derivative with CO2 and CO leads to a simple insertion into the NiN bond to give stable carbamate and carbamoyl derivatives, respectively, the pincer ligand backbone remaining intact in both cases. In contrast, the analogous reactions with the siloxide derivative produced kinetically labile insertion products that either revert to the starting material (in the case of CO2 ) or react further to give the mixed-valent, dinickel species [(POC sp 3OP)Ni(II) {μ,κ(O) ,κ(P) ,κ(P') -OCOCH(CH2 CH2 OPR2 )2 }Ni(0) (CO)2 ]. The zero-valent center in the latter compound is ligated by a new ligand arising from transformation of the POC sp 3OP ligand backbone. The carbonylation and carboxylation of the siloxido derivative also produced minor quantities of a side-product identified as the trinickel species, [{(η(3) -allyl)Ni(μ(O) ,κ(P) -R2 PO)2 }2 Ni], arising from total dismantling of the POC sp 3OP ligand. Similar reactivities were observed with isonitrile, ArNC: reaction with the siloxido derivative resulted in a complex sequence of steps involving initial insertion, a 1,3-hydrogen shift, and an Arbuzov rearrangement to give [Ni(CNAr)4 ] and a methacrylamide based on fragments of the POC sp 3OP ligand. Oxygenation of the amido and siloxido derivatives led to the phosphinate derivative, [(POC sp 3OP)Ni(OP(O)R2 )], arising from oxidative transformation of the original ligand frame; the reaction with the Ni-NHAr derivative also gave ArHNP(O)R2 through a complex NP bond-forming reaction.

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.

CompoundNA3bound inside the active site of the enzyme.