Treatment of the lithium salt of t-BuN(H)PPh 2 in THF with CrCl 2 3 (THF) 2 afforded a dinuclear, [(t-BuNPPh 2 )Cr 2 (μ-t-BuNPPh 2 ) 3 ] 3 (toluene) 1.5 (1), and a tetrameric cluster, [(t-BuNPPh 2 )Cr(μ-t-BuNPPh 2 ) 2 Cr(μ-Cl)] 2 3 (toluene) 2 (5), depending on the reagents' stoichiometric ratio. Complex 1 is dimeric with a long intermetallic distance and the ligands adopting an asymmetric and distorted bridging-chelating bonding mode. Complex 5 is instead a symmetry-generated tetramer with two identical dimetallic units, each closely related in geometry to 1, linked by two bridging chlorine atoms. The reactions of 1 with alkyl aluminum activators afforded a series of divalent complexes, [{(μ-AlMe 3 )- 4), containing organo-aluminum residues. Similarly, reaction of 5 with AlMe 3 gave [{(μ-AlMe 2 )(t-BuNPPh 2 ) 2 }Cr(μ-Cl)] 2 3 (toluene) 1.9 (6), also characterized by an X-ray crystal structure. Finally, the trivalent complex [(t-BuNPPh 2 ) 3 Cr] (7) was readily prepared via reaction of the lithiated ligand with CrCl 3 (THF) 3 . Upon treatment with AlMe 3 or Et 2 AlCl, complexes 2 and [{(μ-AlEtCl 2 )(t-BuNPPh 2 )} 2 Cr] 3 (toluene) (8) were isolated and fully characterized. In turn, this indicated that reduction to the divalent state is the primary stage of the activation process of the trivalent species. The catalytic behavior of all of these complexes has been assessed in the presence and absence of cocatalyst and with different solvents. The result showed a pronounced solvent effect, allowing switching from nonselective oligomerization to selective trimerization.

Reaction of (Me(2)pz)(2)CHSiMe(2)N(H)R (R = (i)Pr or Ph) or (Me(2)pz)(2)CHSiMe(2)NMe(2) with CrCl(3)(THF)(3) or CrCl(2)(THF)(2) gave Cr{(Me(2)pz)(2)CHSiMe(2)NR(1)R(2)}Cl(3) (R(1) = H, R(2) = (i)Pr (10) or Ph (11); R(1) = R(2) = Me (15)) or Cr{(Me(2)pz)(2)CHSiMe(2)NR(1)R(2)}Cl(2)(THF) (R(1) = H, R(2) = (i)Pr (12) or Ph (13); R(1) = R(2) = Me (16)), respectively. Compounds 10 and 11 were crystallographically characterized and the magnetic behaviour of all the new compounds was evaluated using SQUID magnetometry. Reaction of CrCl(3)(THF)(3) with Li{C(Me(2)pz)(3)}(THF) gave the zwitterionic complex Cr{C(Me(2)pz)(3)}Cl(2)(THF) (17) containing an apical carbanion. Reaction of the analogous phenol-based ligand (Me(2)pz)(2)CHArOH (ArO = 2-O-3,5-C(6)H(2)(t)Bu(2)) with CrCl(3)(THF)(3) gave Cr{(Me(2)pz)(2)CHArOH}Cl(3) (19) whereas the corresponding reaction with CrCl(2)(THF)(2) unexpectedly gave the Cr(III) phenolate derivative Cr{(Me(2)pz)(2)CHArO}Cl(2)(THF) (20) which could also be prepared from CrCl(3)(THF)(3) and the sodiated ligand [Na{(Me(2)pz)(2)CHArO}(THF)](2). Reaction of the corresponding ether (Me(2)pz)(2)CHArOMe with CrCl(3)(THF)(3) or CrCl(2)(THF)(2) gave Cr{(Me(2)pz)(2)CHArOMe}Cl(3) (23) and Cr{(Me(2)pz)(2)CHArOMe}Cl(2)(THF) (24), respectively. The catalytic performance in ethylene oligomerisation/polymerisation of all of the new Cr(II) and Cr(III) complexes was evaluated. Most of the complexes showed high activity, but produced a Schultz-Flory distribution of alpha-olefins. Compound 23 had an exceptionally low alpha-value of 0.37 and showed a preference for 1-hexene and 1-octene formation. While replacing a secondary amine (10-13) for a tertiary amine (15-16) resulted in loss of catalytic activity, replacing a phenol (19) for an anisole (23) group afforded a more selective and more active catalyst. Changing from MAO to DIBAL-O as cocatalyst induced a switch in selectivity to ethylene polymerisation.

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.