C–H nickellation of phenol-derived phosphinites: regioselectivity and structures of cyclonickellated complexes

Abstract

This report describes the results of a study on the ortho-C–H nickellation of the aryl phosphinites i-Pr₂P(OAr) derived from the following four groups of substituted phenols: 3-R-C₆H₄OH (R = F (b), Me (c), MeO (d), Cl (e)); 3,5-R₂-C₆H₃OH (R = F (f), Me (g), Cl (h), OMe (i)); 2-R-C₆H₄OH (R = Me (j), Ph(k)); and 2,6-R₂-C₆H₃OH (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^P,κ^C-(i-Pr)₂POAr}Ni(μ-Br)]₂ (1b–1f, 1j, and 1k) or the monomeric acetonitrile adduct {κ^P,κ^C-ArOP(i-Pr)₂}Ni(Br)(NCMe) (1i-NCMe). Analysis of C–H nickellation regioselectivity with 3-R-C₆H₄OH 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 sp³ (R = Me (j)) or sp² (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-R₂-C₆H₃O)P(i-Pr)₂. Tests with the parent dimer derived from i-Pr₂P(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.