This report describes the results of a study on the ortho-C-H nickellation of the aryl phosphinites i-PrP(OAr) derived from the following four groups of substituted phenols: 3-R-CHOH (R = F (b), Me (c), MeO (d), Cl (e)); 3,5-R-CHOH (R = F (f), Me (g), Cl (h), OMe (i)); 2-R-CHOH (R = Me (j), Ph(k)); and 2,6-R-CHOH (R = Me (l), Ph (m)). No nickellation was observed with the phosphinites derived from the 3,5-disubstituted phenols g and h, and the 2,6-disubstituted phenols l and m; in all other cases nickellation occurred at an ortho-C-H to generate either the Br-bridged dimers [{κ,κ-(i-Pr)POAr}Ni(μ-Br)] (1b-1f, 1j, and 1k) or the monomeric acetonitrile adduct {κ,κ-ArOP(i-Pr)}Ni(Br)(NCMe) (1i-NCMe). Analysis of C-H nickellation regioselectivity with 3-R-CHOH pointed to the importance of substituent sterics, not electronics: nickellation occurred at the least hindered position either exclusively (for R = Me (c), MeO(d), and Cl (e)) or predominantly (for R = F (b); 6 : 1). This conclusion is also consistent with the observation that C-H nickellation is possible with the 3,5-disubstituted aryl phosphinites bearing F and OMe, but not with the more bulky substituents Me or Cl. For the 2-substituted aryl phosphinites, C-H nickellation occurs at the unsubstituted ortho-C-H and not on the R substituent, regardless of whether the alternative C-H moiety of the substituent is sp (R = Me (j)) or sp (R = Ph (k)). The system thus reveals a strong preference for formation of 5-membered metallacycles. Consistent with this reactivity, no nickellation occurs with (2,6-R-CHO)P(i-Pr). Tests with the parent dimer derived from i-PrP(OPh) showed that conversion to the monomeric acetonitrile adduct is highly favored, going to completion with only a small excess of MeCN. All new cyclonickellated complexes reported in this study were fully characterized, including by single crystal X-ray diffraction studies. The solid state structures of the dimers 1b and 1d showed an unexpected feature: two halves of the dimers displayed non-coplanar conformations that place the two Ni(ii) centers at shortened distances from each other (2.94-3.16 Å). Geometry optimization studies using DFT have shown that such non-coplanar conformations stabilize the complex, implying that the "bending" observed in these complexes is not caused by packing forces. Indeed, it appears that the occurrence of coplanar conformations in the solid state structures of these dimers is a simple consequence of packing forces rather than an intrinsic property of the compound.

The present report describes the results of a combined experimental and computational study on the mechanism of aryl phosphinite cyclonickelation. The reaction of ArOP­(i-Pr)2 with [(i-PrCN)­NiBr2] n proceeds more readily in acetonitrile relative to toluene; this is because the greater nucleophilicity of acetonitrile toward Ni stabilizes a more reactive acetonitrile adduct bearing one phosphinite ligand (as opposed to two). A sufficiently strong external base such as Et3N serves to quench the HBr generated at the nickelation step, thus allowing isolation of the cyclonickelated species. However, nickelation tests conducted in the absence of external base revealed that D/H scrambling occurs at the ortho positions of C6D5OP­(i-Pr)2, implying that the cyclonickelation proceeds independently of the base. Thus, the main role of the external base is to prevent protonation of the Ni–aryl moiety formed via C–H nickelation. Tests have also shown that nickelation rates are affected by the quantity of the base used: the presence of more than 1 equiv of Et3N generates the less reactive biphosphinite complex trans-{PhOP­(i-Pr)2}2NiBr2, thus inhibiting the desired C–H nickelation. Experimental studies have shown that nickelation is faster with aryl phosphinites bearing electron-donating substituents (Hammett slope ρ ≈ −4) and the proton transfer is rate limiting (KIE ≈ 11). The activation parameters were found to be ΔH ⧧ = 17.7 kcal mol–1 and ΔS ⧧ = −27.1 cal mol–1 K–1. DFT analyses have provided support for these findings and suggest that aryl phosphinite C–H nickelation proceeds via an ion-pair-assisted deprotonation.

This report describes the results of a study on the C–H nickelation of phosphinites derived from variously substituted 1- and 2-naphthols, as well as the C–P functionalization of the Ni-naphthyl moiety arising from the C–H cyclonickelation. Refluxing 4-X-1-naphthyl phosphinites (X = H, 1a; MeO, 1b; Cl, 1c) with {(i-PrCN)­NiBr2} n and Et3N in acetonitrile gave the nickelacyclic complexes {(κP,κC-4-X-1-OP­(i-Pr)2-naphth-2-yl)­Ni­(μ-Br)}2, 2a–c, resulting from cyclonickelation at the C2–H, whereas cyclonickelation of the 2-naphthyl phosphinite analogue 1e under the same conditions occurred at C3–H. Placing a Me substituent at the C3 position of a 2-naphthyl phosphinite (1f) led to a very sluggish nickelation at the C1–H position, whereas 2-ethyl-1-naphthyl phosphinite (1d) failed to nickelate at C8–H. H/D scrambling tests conducted on the deuterated analogue of 1a (1a-d 7 ) confirmed that nickelation occurs exclusively at C2. Similar tests conducted on deuterated analogues of alkyl-substituted 1- and 2-naphthyl phosphinites showed that no nickelation takes place at Csp3 –H sites of the alkyl substituents. In contrast, very facile C–H nickelation was observed with 2-allyl-1-naphthyl phosphinite 1g to give a product featuring a π-allyl-Ni moiety. A series of tests have shown that the nickelation of substrates 1a, 1e, and 1f can be accelerated dramatically at 120–160 °C. On the other hand, conducting the high temperature reaction of 1a in the absence of Et3N resulted in an unanticipated and interesting C–P functionalization of the C2–H site, thus generating a i-Pr2P-substituted bidentate phosphine-phosphinite. A similar tandem C–H nickelation/C–P­(O) functionalization was also observed at the C8–H position of substrate 1d. The mechanisms of these functionalization reactions have been probed and outlined.

The alkylation of N-(2-pyridyl)imidazole by the Mn methylene-phosphonium complex [Cp(CO)Mn(η-P,C-PhP[double bond, length as m-dash]C(H)Ph)]BF offers a straightforward route to a tridentate pro-ligand featuring an NHC core and phosphine/pyridine arms. The ability of this PĈN ligand to coordinate in a pincer mode was recognized in Rh, Rh, and Ni complexes.