P-Diketiminate complexes

Since Piers and coworkers [80] reported the first scandium P-diketiminate complexes by using category B ligands in 1999, a few scandium P-diketiminate complexes have been revealed to be highly active catalysts for the polymerization of ethylene and the intramolecular hydroamination [81,82]. 

Complexes of these ligands have not yet been developed into commonly used catalysts, although early metal p-diketiminate complexes with co-catalysts do catalyze the polymerization of ethylene. More striking, these ligands have led to the synthesis of a number of unusual low-coordinate complexes. Examples of such complexes are described later in this section. Other examples are shown in later chapters of this text. ... 

The steric, as well as electronic, properties of these ligands can be tuned by the choice of substituent at nitrogen. The renaissance of 3-diketiminate chemistry resulted from the synthesis of p-diketiminate complexes in which the substituents on nitrogen were steri-cally demanding. This steric effect led to the predominant formation of monomeric complexes and to the formation of unsaturated early and late metal complexes. The earlier p-diketiminate complexes containing smaller substituents at nitrogen tended to be stable and saturated. 

Transition metal p-diketiminate complexes are typically prepared by one of three routes. In one, these complexes are prepared by the reaction of a metal halide with an alkali metal P-diketiminate generated from the reaction of the p-diketiminate with an alkali metal base. An example of this synthesis is shown for the scandium system in Equation 4.51 In a second method, these complexes are prepared by the reaction of a transition metal complex containing a basic ligand, such as an alkyl or amido group, with the neutral p-diketimine. Two examples of this route for zirconium systems are shown in Equations 4.52 and 4.53, 3... 

It has been found that magnesium complexes supported with bidentate p-diketiminate are more active in the ROP of l- and rac-lactide than their structurally analogous zinc complexes. Faster rate of polymerization of magnesium complexes were such that an almost complete conversion (97%) occurred in 1 min at 20 °C for a p-diketiminate magnesium complex [(BDI-l)Mg(0 Pr)]2 31 (O Pr = isopropoxide) whereas the zinc analog needed 33 min for a similar conversion at the same temperature [64] (Table 3, entry 1). Similarly a higher... 

Recently, tris-p-diketiminate lanthanide complexes [LnL3 ] (X = Cl, L Ln = Pr 111, Nd 112, Sm 113 X = H, L Ln = Nd 114 X = Me, L Ln = Nd 115) (Scheme 12) displaying a high activity in producing PLAs under mild conditions via ROP of L-lactide have been reported. This reactivity may be attributed to the crowded coordination sphere around the central metal, which incidentally affords an activated Ln-N(p-diketiminate) bond. The activity depends on the central metals, and the active trend of Sm < Nd < Pr is consistent with the sequence of the ionic radii [113]. 

The second approach is a popular route to cationic lanthanide alkyl complexes, which have proven to be the important intermediates for ethylene polymerization and the stereospecific polymerization of diene [5]. Various monocationic lanthanide monoalkyl complexes have been synthesized by the alkyl abstraction/elimination reaction of lanthanide dialkyl complexes. The reaction of a bisbenzyl scandium complex supported by P-diketiminate with B(C6Fs)3 affords the cationic complex with a contact ion pair structure, in which a weak bonding between the cation and the anion exists (Figure 8.21) [77]. The reaction of an amidinate... 

Zr complexes of L21 showed catalytic activity for ethylene polymerization [68,69], in addition, Coates and coworkers discovered that the P-diketiminate Zn alkoxide bridging complexes have surprisingly high catalytic activity for cyclohexene oxide (CHO) and CO2 polymerization to produce polycarbonates under mild conditions (Scheme 27) [70],... 

CO2 as single-component catalysts. After systematic investigation of a series of P-diketiminate rare-earth metal complexes, they suggested that the center metal with larger ionic radius and less steric hinder was beneficial for the copolymerization of CHO and CO2 [73],... 

Compared to ligands L22 and L23, L24 is a bulkier P-diketiminate ligand by altering methyl with wo-propyl on A -aryl substituents on L23. L24 has been broadly used in rare-earth metal chemistry and many interesting complexes have been studied. 

The triple chloro bridging dinuclear P-diketiminate ytterbium complex [Yb(L24)Cl( u-Cl)3Yb(L24)(THF)] (89) was an unexpected product from... 

Additional catalytic investigation of p-diketiminate scandium complexes by Piers and coworkers showed that well-characterized complexes 121 and 122 with the bulky ligand L27 were highly active catalysts for intramolecular hydroamination to form nitrogen heterocycles. The catalytic reaction was monitored by determining starting material and product with NMR. Both the neutral complex 121 and the CIP complex 122 are effective catalysts (10 mol%) for the intramolecular hydroamination of 5-phenyl-4-pentyl-l-amine (R = H, R = Ph, n = 1 in Scheme 42). However, they are not active catalysts for the potential application to the intermolecular hydroamination of 1-hexyne with alkylamines [82],... 

All tris-P-diketiminato complexes 130-134 exhibited high catalytic activity for the ROP of e-caprolactone and L-lactide. a,co-dihydroxytelechelic polymers in high yield with high molar mass and moderate molar mass distributions (Mw/M = 1.38-1.89) were produced from the ROP of e-caprolactone (Scheme 8) catalyzed by 130-134.130-134 are highly active catalysts for the ROP of L-lactide to produce polylactide (Scheme 14). The electronic factor of the P-diketiminate ligand largely affects the catalytic activity of the complexes. 131 containing an... 

In this chapter we collect representative procedures for the preparation of the most popular family of /V-aryl p-diketiminate ligands, and of alkali metal and thallium salts that are especially useful precursors to transition metal complexes. We then describe the preparation of a series of useful p-diketiminato metal complexes with 3d transition metals from Sc to Zn, each of which serves as a template for further elaboration. 

A two-step procedure is employed based on the literature procedure for LMe , Pr2MnI(THF), which first generates the potassium salt KLMe Pr2 prior to its reaction with MnL.1 The preparation of the potassium salt of the p-diketimine fl Me, Pr2 was reported by Mair and coworkers from HLMe, Pr2 and KN(SiMe3)2 in relatively low yield (27%).2 Winter and coworkers have reported a related THF adduct from KH.3 Here, the same reagents in diethyl ether give a base-free potassium salt that was used to introduce LMe Pr2 into manganese and zinc complexes. 

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