Zirconium and hafnium complexes containing N-alkyl-substituted amine biphenolate ligands: unexpected ligand degradation and divergent complex constitutions governed by N-alkyls.

The reactivity and thermal stability of zirconium and hafnium complexes containing the N-alkyl-substituted amine biphenolate ligands of the type [RN(CH2-2-O-3,5-C6H2(tBu)2)2](2-) ([R-ONO](2-); R = tBu (1a), iPr (1b), or nPr (1c)) were investigated. The reactions of either [1a]M(OiPr)2 (M = Zr or Hf) with equimolar H2[1a] or M(OiPr)4(HOiPr) (M = Zr or Hf) with 2 equiv of H2[1a] at 25 °C in diethyl ether or 80 °C in toluene afford moderate yields of colorless crystals of M[1a](OiPr)(iPrOCH2-2-O-3,5-C6H2(tBu)2) (M = Zr (4a) or Hf (5a)). Controlled experiments revealed that the production of 4a and 5a proceeds via unexpected thermal degradation of H2[1a] that produces a highly reactive, transient ortho-quinone methide intermediate. Similar reactions employing H2[1b] and H2[1c], however, led to the formation of homoleptic bis-ligand complexes Zr[1b]2 (8b) and M[1c]2 (M = Zr (8c) or Hf (9c)) as colorless crystals. Decisive factors governing these divergent reaction pathways and complex constitutions are discussed. The X-ray structures of 4a, 5a, 8b, 8c, and 9c are presented.

Zirconium and hafnium complexes containing N-alkyl substituted amine biphenolate ligands: coordination chemistry and living ring-opening polymerization catalysis.

The coordination chemistry of zirconium and hafnium complexes containing the tridentate amine biphenolate ligands [RN(CH2-2-O-3,5-C6H2(tBu)2)2](2-) ([R-ONO](2-); R = tBu (1a), iPr (1b), nPr (1c)) featuring distinct N-alkyl substituents is described. Alcoholysis of Zr(OiPr)4(HOiPr) or Hf(OiPr)4(HOiPr) with H2[1a] in diethyl ether solutions at -35 °C generates the corresponding five-coordinate [1a]M(OiPr)2 (M = Zr (2a), Hf (3a)) in high isolated yield. Similar reactions employing H2[1b] produce six-coordinate [1b]M(OiPr)2(HOiPr) (M = Zr (2b·HOiPr), Hf (3b·HOiPr)) as an isopropanol adduct. Repetitive trituration of 2b·HOiPr and 3b·HOiPr with diethyl ether gives five-coordinate 2b and 3b, respectively. Treatment of M(OiPr)4(HOiPr) with H2[1c] under similar conditions affords six-coordinate [1c]M(OiPr)2(HOiPr) (M = Zr (2c·HOiPr), Hf (3c·HOiPr)), subsequent recrystallization of which from acetonitrile-diethyl ether solutions leads to acetonitrile adducts 2c·MeCN and 3c·MeCN. Reactivity studies of these zirconium and hafnium complexes revealed that they are all active catalysts for ring-opening polymerization of ε-caprolactone. Among them, the N-isopropyl derived complexes are most reactive. Polymerizations catalyzed by 2b, 3b and 3c·MeCN were proved to be living. The X-ray structures of 2a·HOiPr, 2a·MeCN, 2c·HOiPr, 2c·MeCN, and 3c·MeCN are presented.

Titanium complexes of tridentate aminebiphenolate ligands containing distinct N-alkyls: profound N-substituent effect on ring-opening polymerization catalysis.

The synthesis, structural characterization, and reactivity studies of titanium complexes supported by tridentate amine biphenolate ligands of the type [RN(CH(2)-2-O-3,5-C(6)H(2)(tBu)(2))(2)](2-) {[R-ONO](2-); R = tBu (1a), iPr (1b), nPr (1c)} are described. Alcoholysis of Ti(OiPr)(4) with H(2)[1a-1c] in diethyl ether solutions at 25 °C generates quantitatively the corresponding [R-ONO]Ti(OiPr)(2) (2a-2c) as a yellow crystalline solid. X-ray diffraction studies of 2b and 2c showed them to be five-coordinate, trigonal-bipyramidal species. Ring-opening polymerization of ε-caprolactone (ε-CL) catalyzed by 2b and 2c proved to be living, as evidenced by the narrow molecular weight distributions of the derived polymers and the linear dependence of number-averaged molecular weights on the monomer-to-catalyst ratios or polymerization time. Kinetic studies revealed that the polymerization rates are first-order in the concentration of ε-CL and first-order in that of 2b and 2c. The propagation rate of 2c is ca. 15 times faster than that of 2b, highlighting a profound substituent effect of primary versus secondary N-alkyls. In sharp contrast, reactions employing catalytic 2a produce either low-molecular-weight oligomers or polymers characteristic of somewhat wider molecular weight distributions, depending on the polymerization temperatures.

Group 4 complexes of a tert-butylphosphine-bridged biphenolate ligand.

The coordination chemistry of group 4 complexes supported by the tridentate, dianionic biphenolate phosphine ligand that carries a phosphorus-bound tert-butyl group, 2,2'-tert-butylphosphino-bis(4,6-di-tert-butylphenolate) ([(t)Bu-OPO](2-)), is described. Metathetical reactions of {[(t)Bu-OPO]Li(2)(DME)}(2) with 2 or 1 equiv of TiCl(4)(THF)(2) selectively produce [(t)Bu-OPO]TiCl(2)(THF) (1a) and Ti[(t)Bu-OPO](2) (2a), respectively. Protonolysis of Ti(O(i)Pr)(4) with 2 or 1 equiv of H(2)[(t)Bu-OPO] cleanly generates 2a and [(t)Bu-OPO]Ti(O(i)Pr)(2) (3a), respectively. Complex 1a can alternatively be prepared from comproportionation of 2a with 1 equiv of TiCl(4)(THF)(2). Treatment of 1a with 2 equiv of NaO(t)Bu affords [(t)Bu-OPO]Ti(O(t)Bu)(2) (4a). In contrast, reactions of {[(t)Bu-OPO]Li(2)(DME)}(2) with ZrCl(4)(THF)(2) or HfCl(4)(THF)(2), regardless of stoichiometry of the starting materials employed, selectively give bis-ligated M[(t)Bu-OPO](2) [M = Zr (2b), Hf (2c)]. Comproportionation of 2b,c with MCl(4)(THF)(2) (M = Zr, Hf) leads to the formation of [(t)Bu-OPO]MCl(2)(THF) [M = Zr (1b), Hf (1c)], which, upon being treated with 2 equiv of NaO(t)Bu, generates [(t)Bu-OPO]M(O(t)Bu)(2)(THF) (4b,c). These synthetic results are markedly different from those obtained from analogous reactions employing a biphenolate phosphine ligand bearing a phosphorus-bound phenyl group ([Ph-OPO](2-)), highlighting a profound phosphorus substituent effect on complex conformation. The alkoxide complexes 3a and 4a-c are all active initiators for catalytic ring-opening polymerization of ε-caprolactone. To assess the potential phosphorus substituent effect on catalysis, [Ph-OPO]Ti(O(i)Pr)(2) (5a) was prepared, and its reactivity was examined. Interestingly, polymers prepared from 3a are characterized by low polydispersities with molecular weights that are linearly dependent on the monomer-to-initiator ratio, thus featuring a living system. The polydispersitiy indexes of polymers prepared from 5a, however, are relatively larger, indicative of the significance of the phosphorus-bound tert-butyl group in 3a in view of discouraging the undesirable transesterification.