Pyridine diamide complexes of early transition metals
Creators: name: Guérin, Frédéric
Publisher: name: University of British Columbia
Year: 2009

description: Titanium complexes bearing a pyridine-diamide ligand [2,6-(RNCH2)2NC5H3]2' (R = 2,6-diisopropylphenyl (BDPP); R = 2,6-dimethylphenyl (BDMP)) have been synthesized. The dichloride complexes [2,6-(RNCH2)2NC5H3]TiCl2 are prepared in high yield from {2,6- [(Me3Si)RNCH2J2NC5H3} and TiCl4 via the elimination of 2 equiv of ClSiMe3. Monoalkyl and bis(alkyl) complexes are prepared from [2,6-(RNCH2)2NC5H3]TiCl2 and various Grignard reagents. Reduction of the dichloride precursors (BDPP)TiCl2 and (BDMP)TiCl2 with excess 1% Na/Hg amalgam in the presence of &gt;2 equiv of internal (PhC=CPh, EtC=CEt, PrC^CPr) or terminal (HC=CSiMe3, PhC^CH) alkynes yields metallacyclopentadiene derivatives in good yield No cyclotrimerization of alkyne is observed. Ligand activation is observed in certain cases for complexes bearing the BDMP ligand. In a similar way, the reaction of {2,6-[(Me3Si)RNCH2]2NC5H3} with TaCl5 yields the complex, w^r-(BDPP)TaCl„ and two equiv of ClSiMe3. The reduction of the trichloride complex with excess Na/Hg in the presence of alkynes yields the pseudo 5-coordinate Ta(III) derivatives, (BDPP)Ta(rr-RC=CR')Cl (R = R' = Pr, Et, Ph; R = Ph, R' = H). The 4-octyne compound reacts with LiC^CR to give the acetylide octyne derivatives (BDPP)Ta(r|2-PrC=CPr)(C=CR) (R = Ph, Bu, SiMe3, o-Me3SiC6H4). The phenylacetylide complex reacts with phenylacetylene to give the metallacyclic derivative (BDPP)Ta[(r|2-PhC=C)PrC=CPrHC=CPh]. Similarly, (BDPP)Ta(r|2-PrC=CPr)(C=CBu) and (BDPP)Ta(Ti2-PrCsCPr)(C=CSiMe3) react with HC^CBu and HC=CSiMe3 to give the expected metallacycles. (BDPP)Ta(ri2-PrC^CPr)(C&lt;:Bu) reacts with HC^CPh to give (BDPP)Ta[(r|2-BuC=C)PrC=CPrHC=CPh] only, establishing that the starting acetylide is retained in the final product. Zirconium complexes bearing a pyridine-diamide ligand [2,6-(RNCH2)2-NC5H3]2' (R = 2,6-diisopropylphenyl (BDPP); R = 2,6-diethylphenyl (BDEP); R = 2,6-dimethylphenyl (BDMP); R = Pr (iPAP); R = Cy (CyAP); R = 2,4-dimethyl-3-pentylamine (LiAP); R = 'Bu (tBAP)) have been synthesized. The mixed amide complexes [2,6-(RNCH2&gt;2- NC5H3]Zr(NMe2)2 are prepared in high yield from [2,6-(RHNCH2)2-NC5H3] and Zr(NMe2 ) 4. The mixed amides react with excess ClSiMe3 to afford the dichlorides [2,6-(RNCH2)2- NC5H3]ZrCl2 in nearly quantitative yield. Dimethyl complexes are prepared from [2,6- (RNCH2)2-NC5H3]ZrCl2 and 2 equiv of MeMgBr. The frontier orbitals of a model of the fragment (BDEP)Zr are very similar the those of Cp2Zr. A similar protocol has been used to prepare [2,6-(RNCH2)2NC5H3]ZrX2 (X = NMe2 , Cl, Me; R = 2-PhC6H4 (BPhP); 2-'PrC6H4 (BMPP); 2-'BuC6H4 (BMBP), 2-'Pr-6-MeC6H3 (MPPP)). NMR spectroscopy has been used to identify rotameric isomers derived from restricted rotation about the N-Ci p s o bond of the ligand. The aryl groups in (BPhP)ZrX2 complexes freely rotate at all temperatures (-8CTC to +80"C) while (BMPP)ZrX2 (X = NMe2 , C l , Me) derivatives adopt meso and rac rotamers at low temperatures. In contrast, (BMBP)ZrX2 and (MPPP)ZrX2 (X = NMe2 , Cl, Me) compounds are locked at all temperatures. (BMBP)ZrCl2 is isolated as a single isomer, likely the meso rotamer, while (MPPP)ZrCl2 is a near statistical mixture of me so and rac isomers.