The synthesis, characterization, and catalytic activity of nonclassical iron(II) polyhydride complexes containing tridentate PNP pincer-type ligands is described. These compounds of the general formula [Fe(PNP)(H)(η-H)] exhibit remarkable reactivity toward terminal alkynes. They efficiently promote the catalytic dimerization of aryl acetylenes giving the corresponding conjugated 1,3-enynes in excellent yields with low catalyst loadings. When the reaction is carried out in the presence of pinacolborane, vinyl boronates are obtained. Both reactions take place under mild conditions and are highly chemo-, regio-, and stereoselective with up to 99% Z-selectivity.
Reactions of (Bn 2 Cyclam)ZrCl 2 (1) (where Bn 2 Cyclam = trans-N,N 0 (PhCH 2 ) 2 Cyclam) with appropriate alkylating reagents produced (Bn 2 Cyclam)ZrR 2 (R = Me (2), CH 2 Ph (3), n Bu (4), CCPh (5)). Activation of the ortho C-H bond of the two macrocycle benzyl substituents in complexes 2, 3, and 4 was thermally induced, leading to the formation of a bis(ortho-metalated) complex, ((C 6 H 4 CH 2 ) 2 Cyclam)Zr ( 6). This reaction proceeds along with RH elimination and converts the original dianionic tetracoordinated cyclam in a tetraanionic hexacoordinated ligand where two new Zr-C Ph bonds complete the metal sphere. Treatment of 6 with one equivalent of HCtCPh led to ((C 6 H 4 CH 2 )BnCyclam)Zr(CCPh) ( 7) via protonation of one Zr-C Ph bond by phenylacethylene. Further reaction of 7 with an excess of HCtCPh led to 5. The reactions of 2 and 3 with B(C 6 F 5 ) 3 are strongly solvent dependent. Solvent-stabilized cationic species of formula [(Bn 2 Cyclam)ZrR(Solv)]-[RB(C 6 F 5 ) 3 ] were obtained from reactions of 2 in CD 2 Cl 2 (10) or d 8 -THF ( 12) and from 3 in d 8 -THF (9). The reactions of 3 in CD 2 Cl 2 or d 8 -toluene gave [(Bn 2 Cyclam)Zr(η 2 -CH 2 Ph)][ PhCH 2 B(C 6 F 5 ) 3 ] (8). Finally, the reaction of 2 in d 8 -toluene led to [(Bn 2 Cyclam)Zr(C 6 F 5 ){CH 2 B(C 6 F 5 ) 2 }] (14); the precursor of the latter is the zwitterionic [(Bn 2 Cyclam)Zr(CH 3 ){CH 3 B(C 6 F 5 ) 3 }] (11), which then undergoes methane elimination and C 6 F 5 migration from boron to zirconium. The mechanism of this reaction was studied by DFT and revealed that (i) methane elimination is assisted by one alphaagostic C-H bond and (ii) migration of the C 6 F 5 ring is supported by one bridging fluorine bond between the zirconium and one of the C 6 F 5 rings that remains bonded to boron.
Novel Bn(2)Cyclam-based zirconium complexes of the type (Bn(2)Cyclam)Zr(X)(X') (3, X = X' = OPh; 4, X = X' = SPh; 5, X = Cl, X' = O(i)Pr) were synthesized in good yields via metathesis routes involving the reaction of the dichloro precursor (Bn(2)Cyclam)Zr(Cl)(2) and the appropriate lithium salts. The molecular structures of compounds 3, 4 and 5, as determined by X-ray crystallographic studies, all confirmed the effective chelation of the Bn(2)Cyclam ligand in a κ(4)-N(2)N(2)' fashion, with the hexa-coordinated Zr center adopting a trigonal prismatic geometry. Complexes 3-5 as well as the diisopropoxide derivative (Bn(2)Cyclam)Zr(O(i)Pr)(2) (2) were all found to initiate the ring-opening polymerization (ROP) of rac-lactide in a controlled manner, as deduced from SEC data and linear correlations between molecular weight numbers (M(n)) and monomer conversion as the ROP proceeds. While initiator 2 polymerizes rac-lactide to afford, as expected, an O(i)Pr-ester-end PLA, the ROP of rac-lactide by species 3 or 4 affords an unusual cyclam-end group PLA, as deduced from MALDI-TOF data. The bonding and the electronic structures of the dialkoxides 2 and 3 were assessed by DFT and their possible influence on the polymerization mechanism is discussed.
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