C(CH3)3), 1.30 (10H, m, C6H11), 1.21 (4H, q, CH2CH3, JHH
=
Computational details
8 Hz), 0.76 (6H, t, CH2CH3, JHH = 8 Hz). 13C NMR (C6D6,
298 K) d: 160.1, 139.6, 138.9, 126.6, 126.4, 125.5 (12C, Ar), 63.3
(NCH2Ar), 60.1 (2C, C6H11), 37.3 (C(CH3)3), 35.9 (C(CH3)3), 34.3
(NCH3), 31.7 (C(CH)3)3), 31.2 (C(CH)3)3), 26.1, 25.9, 24.3 (10C,
C6H11), 13.2 (CH2CH3), 0.7 (CH2CH3).
Full optimization of the geometries was performed at the DFT
level with Becke (B3) exchange functional10 and Lee–Yang–Parr
(LYP) correlation functional11 and the standard 6–31G* basis
set. Vibrational frequencies obtained at the same level were
used to verify the nature of all stationary points found during
optimization. The chemical shielding calculations were performed
with the gauge-including atomic orbitals (GIAO) approach at
the B3LYP/6–31G** level of theory. The calculated shieldings
s were transformed to chemical shifts in the TMS scale. All the
results presented in this paper were evaluated by using Gaussian
03 program.12
[Zn(L2)2] (2). To a solution of L2-H (1.32 g, 3.98 mmol) in
toluene (20 mL) ZnEt2 (2.00 mL, 2.00 mmol) was added dropwise.
The mixture was stirred at room temperature for 5 h after which
time volatiles were removed in vacuo to yield a white powder.
10 mL of hexanes were added and the suspension was stirred
for 2 h. The white solid was collected by filtration and dried in
vacuum. Yield: 1.29 g, (1.77 mmol; 89%). Crystals suitable for
X-ray crystallography were grown from toluene at 5 ◦C. Anal.
Calc. (found) for C44H72N2O2Zn (726.41): C 72.75 (72.52), H 9.99
X-Ray crystallography
X-ray diffraction data were collected using a KUMA KM4
CCD (w scan technique) diffractometer equipped with an Oxford
Cryosystem-Cryostream cooler.13 The space groups were deter-
mined from systematic absences and subsequent least-squares
refinement. Lorentz and polarization corrections were applied.
The structures were solved by direct methods and refined by
full-matrix least squares on F2 using SHELXTL Package.14 Non
hydrogen atoms were refined with anisotropic thermal parameters.
Hydrogen atom positions were calculated and added to the
structure factor calculations, but were not refined.
1
(9.94), N 1.96 (1.89%). H NMR for major 2a form (C6D6, 298
K) d: 7.72 (2H, s, ArH), 7.09 (2H, s, ArH), 4.16 (2H, d, NCH2Ar,
JHH = 13 Hz), 3.47 (2H, d, NCH2Ar, JHH = 13 Hz), 3.20 (2H, m,
C6H11), 2.48 (6H, s, NCH3), 1.74 (18H, s, C(CH3)3), 1.66 (10H, m,
C6H11), 1.59 (18H, s, C(CH3)3), 1.05 (10H, m, C6H11). 13C NMR
(C6D6, 298 K) d: 164.3, 138.4, 135.5, 125.9, 124.4, 120.5 (12C, Ar),
65.3 (NCH2Ar), 61.9 (2C, C6H11), 36.8 (C(CH3)3), 35.5 (C(CH3)3),
33.8 (NCH3), 32.3 (C(CH)3)3), 30.2 (C(CH)3)3), 26.9, 26.1, 24.4
(10C, C6H11). Selected resonances for 2b form (C6D6, 298 K) d:
7.06 (2H, s, ArH), 4.22 (2H, d, NCH2Ar, JHH = 13 Hz), 3.62 (2H,
d, NCH2Ar, JHH = 13 Hz), 2.43 (6H, s, NCH3), 1.87 (18H, s,
C(CH3)3), 1.78 (18H, s, C(CH3)3). Selected resonances for 2c form
Results and discussion
(C6D6, 298 K) d: 7.03 (2H, s, ArH), 4.68 (2H, d, NCH2Ar, JHH
=
Syntheses
13 Hz), 3.77 (2H, d, NCH2Ar, JHH = 13 Hz), 2.23 (6H, s, NCH3),
1.82 (18H, s, C(CH3)3), 1.73 (18H, s, C(CH3)3).
The N-[methyl(2-hydroxy-3,5-di-tert-butylphenyl)]-N-methyl-N-
cyclohexylamine (L2-H) ligand was readily prepared by Mannich
condensation from N-cyclohexylmethylamine, paraformaldehyde,
and 2,4-di-tert-butylphenol in refluxing methanol as described
earlier.9
[(g2-L2)Zn(l-OBn)]2 (3). To a solution of 1 (1.10 g, 1.29 mmol)
in toluene (50 mL) BnOH (0.27 mL, 2.60 mmol) was added
dropwise and the reaction mixture was stirred for 1 h. After
3 days a white solid precipitated. It was collected by filtration and
washed with hexanes and dried in vacuo. Yield 0.90 g, (0.90 mmol;
69%). Anal. Calc. (found) for C58H86N2O4Zn2 (1006.09): C 69.24
Treatment of L2-H with one or two equivalents of ZnEt2 in
2
toluene at room temperature gave molecular zinc complexes [(m,h -
2
1
L2)ZnEt]2 (1) and [Zn(h -L2)2] (2), respectively, in 74–89% yield as
(69.25), H 8.62 (8.63), N 2.78 (2.60%). H NMR (C6D6, 298 K)
shown in Scheme 1. Compound 1, due to the presence of ethyl
substituent at each Zn centre further reacts with benzyl alcohol
(BnOH) in toluene to give dimeric [(h -L2)Zn(m-OBn)]2 (3) in 69%
d: 7.75 (2H, s, ArH), 7.07 (10H, m, Ph), 6.91 (2H, s, ArH), 4.77
(4H, CH2Ph), 3.79 (2H, d, NCH2Ar, JHH = 12 Hz), 3.52 (2H,
d, NCH2Ar, JHH = 12 Hz), 2.33 (2H, m, C6H11), 2.12 (6H, s,
NCH3), 2.03 (18H, s, C(CH3)3), 1.68 (10H, m, C6H11), 1.55 (18H, s,
C(CH3)3), 1.13 (10H, m, C6H11). 13C NMR partial (C6D6, 298 K) d:
164.7, 137.0, 144.9, 137.8, 135.4, 127.8, 127.5, 126.4, 125.6, 124.6
(Ar + Ph), 70.1 (CH2Ph), 64.9 (NCH2Ar), 64.2 (2C, C6H11), 36.7
(C(CH3)3), 36.2 (C(CH3)3), 34.2 (NCH3), 32.5 (C(CH)3)3), 30.4
(C(CH)3)3), 26.4, 26.0, (C6H11).
2
yield (Scheme 1).
All compounds are soluble in polar organic solvents and in
toluene and are insoluble in hexanes. Compounds 1 and 3 are
moisture sensitive. Interestingly, 2 is stable in air for several hours
and according to 1H NMR spectroscopy only slightly decomposes
after a few days at room temperature in C6D6 in an NMR tube. The
complexes were formulated on the basis of NMR spectroscopy,
elemental analysis and, in case of 2, by X-ray crystallography.
The 1H NMR spectra for 1–3 were informative as to formation
of dimeric or monomeric species and diagnostic as to stability
of the compounds in solution. The methylene protons of the
PhCH2N linker in each complex are diastereotopic giving at room
temperature doublets in contrast to the singlet resonance in the
Polymerization
Representative procedure. The monomer L-LA was placed in a
Schlenk flask and zinc complex in CH2Cl2 was added. The reaction
was stirred at the desired temperature for the prescribed time. Next,
at certain time intervals about 1 mL aliquots were removed for
1
1
determination of the conversion using H NMR. After reaction
free L2-H ligand. The H NMR spectra of 1 and 3 support the
was completed it was quenched with methanol, the solution was
concentrated in vacuum and the polymer was precipitated with an
excess of cold methanol. Filtration and drying in vacuum yielded
a white polymer.
existence of only one dimeric species in solution. This is evidenced
by the equivalence of the phenyl resonances as well as the presence
of two weakly coupled doublets for the diastereotopic PhCH2N
protons.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 6556–6562 | 6557
©