Synthesis and Catalytic Oligomerization Activity of Chromium Catalysts of Ligand Systems with Switchable Connectivity

Nucleophilic attack of in situ generated bis­(diphenylphosphino)­methane
(DPPM–) anion at CO2, CyNCO, t-BuNCO, 2,6-(i-Pr)2PhNCO, and
2,4,6-(Me3)­PhNCO resulted in the formation of the novel
anionic ligands {[(Ph2P)2CHCO2]­Li­(THF)2}2 (1), {[(Ph2P)2CCNH­(R)­O]­Li­(OEt)2}2 (R = Cy (2), R = t-Bu (3)), [Ph2PCHP­(Ph2)­CN­(2,6-i-Pr2C6H3)­O]­Li­(OEt2)2 (4), and {[(Ph2P)2CCNH­(2,4,6-Me3C6H2)­O]­Li]n (5), respectively. Ligand 4, however,
showed a connectivity resulting from a nonclassical type of attack
where the P atom acted as a nucleophilc center, thus affording a mixed-valent
P­(III)/P­(V) species. Instead, the closely similar 5 showed
a classical type of connectivity. The reaction of the in situ generated
DPPM– anion with 1 and 0.5 equiv of CrCl3(THF)3 gave the chelated chromium complexes [HC­(PPh2)2]­Cr­[(μ-Cl)2Li­(THF)2]2 (6) and [HC­(PPh2)2]2Cr­(μ-Cl)2Li­(THF)2]·1.5THF
(7), respectively. The reaction of ligand 1 with CrCl2(THF)2 afforded the dimeric [{[(Ph2P)2C­(H)­CO2]2}­Cr­(THF)]2 (8), whereas the reaction of 3 with
CrCl3(THF)3 resulted in the octahedral complex
[(Ph2P)2C­(H)­CN­(t-Bu)­O]­CrCl2(THF)2·0.5THF·0.5­(toluene) (9). The complexation of ligand 4 with
CrCl3(THF)3 switched the connectivity to classical
form and afforded the octahedral chromium complex [(Ph2P)­C­(H)­CN­(2,6-i-Pr2C6H3)­O]­CrCl2(THF)2·1.5THF (10). In contrast, the reaction of the classical ligand 5 with CrCl3(THF)3 resulted in [(Ph2P)­C­(H)P­(Ph2)­CN­(2,4,6-Me3C6H2)­O]­Cr­(THF)2Cl2 (11) with a nonclassical type of connectivity. Reaction of 11 with DEAC switched the connectivity back to a classical
type, affording {(EtCl2Al)­[(Ph2P)2C­(H)­CN­(2,4,6-Me3C6H2)­OAlEt2]­(μ-Cl)­Cr}2(μ-Cl)2·(toluene)
(12). The catalytic behavior of all of these complexes
has been assessed under different oligomerization conditions, and
it was found that the modification of the DPPM framework with cumulenes
considerably enhances their catalytic performance in comparison to
catalysts 6 and 7. In any event, a Schultz–Flory
distribution of oligomers was obtained. However, the in situ catalytic
testing of ligands 2–4 using Cr­(acac)3 as metal precursor and DMAO as cocatalyst, in methylcyclohexane,
switched the catalytic behavior to selective formation of 1-hexene
and 1-octene (no higher liquid oligomers) along with a significant
amount of narrowly dispersed, low-molecular-weight polyethylene wax.
Interestingly, the precatalyst 12 showed self-activating
trimerization capability with moderate activity.