The reaction of the heteroscorpionate lithium salts [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd = N,N‘-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with 1 equiv of RMgCl proceeds to give very high yields of
the neutral heteroscorpionate alkyl magnesium complexes [Mg(R)(NNN)] (NNN = tbpamd, R = C3H5
1,
t
Bu 2, CH2SiMe3
3; NNN = pbpamd, R = C3H5
4,
t
Bu 5, CH2SiMe3
6). On heating toluene solutions
of complexes 1
−
3, 5, and 6, a ligand redistribution process occurs to give the corresponding 6-coordinated
sandwich complexes [Mg(tbpamd)2] (7) and [Mg(pbpamd)2] (8). Interestingly, the allyl derivative 4 can
be easily transformed to 8 at room temperature. In addition, the cationic sandwich complex [Mg(tbpamdH)2]Cl2 (9) [tbpamdH = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidine] was obtained from
7 by means of a protonation process. Finally, alkyl-containing complexes 1
−
6 can act as highly effective
single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides
over a wide range of temperatures. ε-Caprolactone is polymerized within seconds to give high molecular
weight polymers with narrow polydispersities. Lactide afforded PLA materials with medium molecular
weights and polydispersities as narrow as M
w/M
n = 1.05. Additionally, polymerization of l-lactide occurred
without racemization in the propagation process and offered highly crystalline, isotactic poly(l-lactides)
with high melting temperatures (T
m = 160 °C). Polymer end-group analysis shows that the polymerization
process is initiated by alkyl transfer to the monomer.
Scorpionates represent one of the most versatile types of tridentate ligand that can coordinate to a wide variety of elements, e.g. from early to late transition metals, and the coordination chemistry of these systems has developed greatly in recent years. This Perspective gives an account of studies on the following aspects: (1) the preparative methods for a new class of heteroscorpionate [RR'C(pz)2] ligand derived from bis(pyrazol-1-yl)methane and (2) the description of metal complexes containing these ligands, examples of which incorporate a range of different metals from the Periodic Table.
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