New Self-Activating Organochromium Catalyst Precursor for Selective Ethylene Trimerization.

Abstract

Two catalytic systems based on anionic ligands with the NPN structural motif, bearing tri-and pentavalent phosphorus, respectively, have been compared vis-a-vis their ability to selectively trimerize ethylene. In the case of the trivalent phosphorus ligands, reaction of the Cr(II) catalyst precursor [(t-Bu)NPN(t-Bu)] 2 Cr (1) with 2 equiv of MeLi afforded the Cr(III) species [(t-Bu)NP(Me)N(t-Bu)] 2 CrLi(OEt 2 ) (2). The same reaction with 3 equiv of MeLi yielded instead the Cr(II) species {[(t-Bu)NP(Me)N(t-Bu)]Cr(μ-Me)} 2 {Li(THF)} 2 (3). The P atoms of both 2 and 3 have been methylated. Activation of 2 and 3 with MAO produced an S-F distribution of oligomers. Conversely, activation of 3 with EtAlCl 2 exclusively afforded 1-hexene together with a small amount of polymer as a byproduct. Treatment of 2 with EtAlCl 2 did not yield an active catalyst. The reaction of CrCl 2 (THF) 2 with [(t-Bu)NP(Ph) 2 N(t-Bu)] À Li + , containing pentavalent phosphorus, afforded the Cr(II) derivative [(t-Bu)NP(Ph) 2 N(t-Bu)]Cr(μ-Cl) 2 Li(THF) 2 (4). Its alkylation with EtLi gave the ethyl-bridged dimer {[(t-Bu)NP(Ph) 2 N(t-Bu)]Cr(μ-Et)} 2 (5), which, upon thermolysis, afforded [(t-Bu)NP(Ph) 2 N(t-Bu)] 2 Cr (6). The structures of 5 and 3 are closely related, having in common two bridging alkyls and the same metal oxidation state but differing with respect to the P atom oxidation state and the presence/absence of alkali-metal cations. The catalytic behavior instead is remarkably different. Complex 5 is active as a self-activating selective ethylene trimerization catalyst, while complex 3 requires activation.

The half-sandwich 18- and 16-electron arene ruthenium iminophosphonamide complexes

“…Among transition metal complexes with κ 2 -N,N-heteroallylic ligands, the iminophosphonamides bearing a coordinated R 2 P(NR′) 2 − anion (NPN) are studied fragmentarily; there have been less than a hundred of molecular structures of NPN complexes established to date, that is in sharp contrast to more than a thousand of transition metal amidinate structures, according to the Cambridge Structural Database (CSD). The IV group metals, [1][2][3][4][5][6] chromium, 7-10 nickel [11][12][13][14][15] and copper [16][17][18][19][20][21][22] iminophosphonamides are the most studied, which is due to their catalytic application in cyclopropanation, 16,21,23 olefin oligomerization [7][8][9] and polymerization. 2,6,12,15,24,25 A few platinum metal group iminophosphonamides have been reported for palladium 24 and ruthenium 26 before 2009, when we started systematic studies of these complexes.…”

The Effect of β‐Hydrogen Atoms on Iron Speciation in Cross‐Couplings with Simple Iron Salts and Alkyl Grignard Reagents

“…[7] Both the methylene and methyl carbon atoms are slightly asymmetric with respect to the two metal centers (Table 1). As econd structure with ethyl ligands bridging Cr centers,[ ((t-Bu)NP-(Ph) 2 N(t-Bu))Cr(m-Et)] 2 , [9] however shows amoderate degree of asymmetry in the methylene carbon atoms with respect to the metal centers,w hile the methyl carbon atoms are highly asymmetric to the extent that one Cr···C(methyl) distance for each metal is only approximately 0.5 greater than the sum of the covalent radii of the individual atoms. [10] Thefinal two reports show acation, {[(SIPr)Ag] 2 (m-Et)} + ,that has ahighly symmetric ethyl bridge [11] and an anion, {[Et 2 Zn] 2 (m-Et)} À , whose disordered metal centers prevented detailed discussion of bond lengths and angles.…”

“…[3b, 13] In fact, the combination of simple ferric salts,alkyl Grignard reagents and NMP co-solvent has been utilized broadly in pharmaceutical research. [14] While recent studies demonstrated that the addition of NMP to reactions with MeMgBr leads to the formation of at rimethyliron(II) ferrate complex with NMP ligated to amagnesium dication, [Mg(NMP) 6 ][FeMe 3 ] 2 ,itwas 2.133(7), 2.403 (7) 3.095 (7), 3.412(9) 114.5(2), 118.8(3) Cr-Cr [7] 2.204(16), 2.252(10)3 .21, 3.25 117.7 (13), 117.7(13) Cr-Cr [9] 2.143 (2) (15), 135.0(2) Ag-Ag [10] 2.248 (6)…”

The Effect of β‐Hydrogen Atoms on Iron Speciation in Cross‐Couplings with Simple Iron Salts and Alkyl Grignard Reagents

“…Both the methylene and methyl carbon atoms are slightly asymmetric with respect to the two metal centers (Table ). A second structure with ethyl ligands bridging Cr centers, [(( t ‐Bu)NP(Ph) 2 N( t ‐Bu))Cr(μ‐Et)] 2 , however shows a moderate degree of asymmetry in the methylene carbon atoms with respect to the metal centers, while the methyl carbon atoms are highly asymmetric to the extent that one Cr⋅⋅⋅C(methyl) distance for each metal is only approximately 0.5 Å greater than the sum of the covalent radii of the individual atoms . The final two reports show a cation, {[(SIPr)Ag] 2 (μ‐Et)} + , that has a highly symmetric ethyl bridge and an anion, {[Et 2 Zn] 2 (μ‐Et)} − , whose disordered metal centers prevented detailed discussion of bond lengths and angles …”