Ancillary Ligand Effects in Zirconium(IV) Aminoborollide and Nitrogen Chelated Pt(II) Complexes

Citation

Wong-Foy, Antek Golangco (2002) Ancillary Ligand Effects in Zirconium(IV) Aminoborollide and Nitrogen Chelated Pt(II) Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6xyc-e660. https://resolver.caltech.edu/CaltechTHESIS:04102012-133622013

Abstract

The preparation of new chloro derivatives of pentamethylcyclopentadienyl aminoborollide complexes of Zr are described. Treatment of the dianions 1.9-1.12 with Cp*ZrCl_3 yields Cp*{η^5-C_4H_4BN(Si(CH_3)_3)_2}ZrCl•LiCl (1.13), Cp*{η^5-C_4H_4BNC(CH_3)_3Si(CH_3)_3}ZrCl•Li•CI (1.14). |Cp*{η^5-C_4H_4BN(Et)CH_2CH_2Net_2}ZrCl_2Li]_2 (1.15), and Cp*{η^5-2,5-Ph_2C_4H_2BNMe_2}ZrCl•LiCl (1.16). The electronic spectra of these complexes were measured and compared to the parent complex Cp*{η^5-C_4H_4BN(CHMe_2)_2}ZrCl•LiCl (1.17a) in THF solvent. In general, as the substituents directly bonded to nitrogen increase in size, a blue shift of the low energy, aminoborollide to zirconium charge transfer band (LMCT), occurs. λ_(max) decreases in the order 1.16 > 1.17a > 1.14 > 1.15 ~ 1.13. The anionic portions of complexes 1.13-1.16 have also been structurally characterized by x-ray crystallography. Although the changes are very small, in general a lengthened B-N bond correlates linearly with the observed blue shift of the LMCT band.

Studies directed towards the development of a Pt(II)-catalyzcd oxidation of ethylene to ethylene glycol based on the Shilov system for alkane functionalization is described. The first step is the activation of ethylene towards nucleophilic attack by water to generate a Pt(II) β-hydroxyalkyl intermediate that is oxidized in a second step to the Pt(IV) β-hydroxyalkyl. Reductive elimination via an S_N2-type mechanism at the α-C of the Pt(IV) β-hydroxyalkyl liberates the oxidized product leaving the reduced Pt(II) center to bind another equivalent of olefin. The first system examined was the methyl ethylene complex [(tmeda)PtMe(η^2-C_2H_4)][SbF_6,], 2.2. Nucleophilic attack at the bound ethylene was not observed: instead displacement of ethylene occurred. The bound ethylene in the neutral complexes cis-Cl_2PtL(η^2-C_2H_4), (L =PPh_3 (2.23), AsPh_3 (2.24), Me_2SO (2.25)) and trans-Cl_2Pt(η^2-C_2CH_4)(C_5H_5N). 2.26 arc susceptible towards attack by OH-. Under catalytic conditions (excess ethylene and H_2O_2) decomposition of 2.23, 2.24, and 2.25 was observed. In 2.26, 1 turnover was observed before decomposition occurred. The hound ethylene in the complex [(tmeda)PtCl(η^2-C_2H_4)][ClO_4] is activated towards nucleophilic attack by water and OH-, allowing the isolation of the Pt(II) β-hydroxyalkyl. This is rapidly oxidized to the Pt(IV) β-hydroxyalkyl by hydrogen peroxide. In the presence of HCl, it undergoes reductive elimination to yield 2-chlorocthanol and (tmeda)PtCl_2. Unfortunately, this system also showed no catalytic activity.

The dicationic complexes (|(ArN=C(Me)-C(Me)=NAr)Pt(solv)_2 |X_2. (Λr = 2.6-(CH_3)_2C_6,H_3: 3.5a: solv = CH_3CN, X = CF_3SO_3-, BF_4-, SbF_6-; 3.5b: solv = (CH_3)_2CO, X = BF_4-, SbF_6-,) and 3.6 [(CyN=C(H)-C(H)=NCy)Pt(CH_3CN)_2]X_2, (Cy = C_6H_(11) , X= OTf, BF_4-, Pf_6-, SbF_6-) were synthesized from the corresponding Pt dichlorides with 2 equiv. of AgX. The reaction of 3.5a with 1-phenylpyrazole, 2-phenylpyridine, 2-vinylpyridine, and 2-(2-thienyl)pyridine in acetone affords the cyclometalation products 3.11-3.14 via intramolecular C-H activation of an sp^2 C-H bond of the unsaturated sidegroup. Pyridines with saturated groups at the 2-position do not undergo a similar cyclomctalation reaction. 3.6 undergoes cyclometalation of one of the cyclohexyl groups, an example of sp^3 C-H bond activation. The later reaction proceeds only partway to completion, implying that an equilibrium has been reached: in the case where X = OTf, the equilibrium favors the starting dication. Furthermore, the intramolecular C-H activation occurs in trifluoroethanol but not in acetone under comparable conditions in contrast to the reactions of 3.5a with the substituted pyridines.

The diaqua complexe [(ArN=C(Me)-C(Me)=NAr)Pt(H_2O)_2]X_2, 4.3. (Λr = 2.6-(CH_3)_2C_6H_3; X = OTf^-, BF_4^-) decompose in aqueous solution to yield a red-orange precipitate. Spectroscopic characterization of the precipitate by ^1H, IR, and conductivity measurements is consistent with C_(2v), symmetric structure containing hydroxo groups. Confirmation of the dicationic, dinuclear Pt(II) complex, 4.4, where the two Pt centers are bridged by two OH groups was revealed by x-ray crystallography.

Thesis Files