Zirconium complexes of a tacn-derived amido ligand and ring-opening to form a new diamido-amino pincer

Article abstract of DOI:10.1039/b005675h

The tribenzyl complex (Pr(i)₂tacn)Zr(CH₂Ph)₃ eliminates toluene upon heating with concomitant ring-opening of the anionic ancillary macrocycle to yield [CH₂=CHNCH₂- CH₂N(Pr(i))CH₂CH₂NPr(i)]Zr(CH₂Ph)₂; the latter features a new class of dianionic pincer ligand.

Full text of DOI:10.1039/b005675h

Zirconium complexes of a tacn-derived amido ligand and ring-opening to form
a new diamido-amino pincer
Garth R. Giesbrecht, Alex Shafir and John Arnold*
Department of Chemistry, University of California, Berkeley, and the Chemical Sciences Division, Lawrence Berkeley
National Laboratory, Berkeley, CA, 94720-1460. E-mail: arnold@socrates.berkeley.edu
Received (in Cambridge, UK) 13th July 2000, Accepted 19th September 2000
First published as an Advance Article on the web
The tribenzyl complex (Pri2tacn)Zr(CH2Ph)3 eliminates
toluene upon heating with concomitant ring-opening of the
anionic ancillary macrocycle to yield [CH2NCHNCH2-
CH2N(Pri)CH2CH2NPri]Zr(CH2Ph)2; the latter features a
new class of dianionic pincer ligand.
(Pri2tacn)ZrCl3 1 as an analytically-pure pale yellow solid in
low yield. Alternately, 1 can be synthesized via a two-step
process in nearly 90% overall yield: reaction of (Pri2tacn)H with
Zr(NEt2)4 quantitatively generates (Pri2tacn)Zr(NEt2)3 2 as a
yellow oil which is sufficiently clean to be used without further
purification. The latter reacts smoothly with excess Me3SiCl in
hot toluene to give 1, which precipitates in pure form.
Compound 1 is soluble in methylene chloride and only
sparingly soluble in solvents such as THF, diethyl ether or
toluene.
Non-metallocene complexes of Group IV metals have recently
been the subject of renewed interest due to the realization that
new ligand environments might result in novel patterns of
reactivity.1–3 For example, Group IV metal complexes bearing
bi-4–8 or tridentate9–15 diamide ligands have attracted attention
as potential alternatives to the classical homogeneous Ziegler–
Natta polymerization catalysts that are mostly based on bis- or
monocyclopentadienyl ligands. In contrast, tridentate mono-
amide ligands have received scant attention, an exception being
the tridentate, acyclic monoanion, [(Me2NCH2SiMe2)2N]2, for
which Ti, Zr and Hf derivatives have been described.16
(Pri2tacn)H reacts with Zr(CH2Ph)4 in diethyl ether to form
the tribenzyl species (Pri2tacn)Zr(CH2Ph)3 3 in good yield.
Compound 3 may also be synthesized from the trichloride 1 and
3 equiv. of benzyl Grignard, although the former reaction is
1
substantially cleaner. The H NMR spectrum of 3 shows a
doublet for the diastereotopic isopropyl resonances and the
methylene protons of the triazacyclononane ring appear as
broad singlets. Also, the two isopropyl methyl groups give rise
to a single peak in the {1H}13C NMR spectrum (18.9 ppm) in
contrast to the two distinct singlets observed for 1 (19.5 and 18.1
ppm). The six benzylic protons are equivalent on the NMR time
scale, resulting in a singlet in the 1H NMR spectrum (2.71 ppm)
and a broad resonance in the {1H}13C NMR spectrum
(76.1 ppm).
Compound 3 is monomeric in the solid-state (see Fig. 1)†
with the triazacyclononane ring attached facially to the metal
through all three nitrogen atoms. The zirconium center is best
described as distorted octahedral, with the N–Zr–Ctrans angles
ranging from 152.06(8) to 159.46(7)°; additionally, the three C–
Zr–C angles vary from 91.10(8) to 96.33(9)°. Two of the benzyl
We17 and others18 recently described the use of the lithium
salt of diisopropyltriazacyclonone as a synthon in the develop-
ment of new constrained-geometry ligand systems.17,18 By
utilizing a ligand system in which the nitrogen donors are part
of a macrocycle, we reasoned that metal complexes would
necessarily bind the azamacrocycle facially, in closer analogy to
cyclopentadienyl complexes. Here we report the use of this
fragment as an anionic, tridentate diamino-amido ancillary
ligand for the stabilization of zirconium complexes and the
conversion of this ligand into a dianionic acyclic variant.
As shown in Scheme 1, {(Pri2tacn)Li}2 reacts with
ZrCl4(THF)4 in THF to yield the mono-ligand complex,
Fig. 1 ORTEP view of (Pri2tacn)Zr(CH2Ph)3 3 drawn with 50% probability
ellipsoids. Selected bond distances (Å) and angles (°): Zr1–N1 2.073(2),
Zr1–N2 2.604(2), Zr1–N3 2.479(2), Zr1–C1 2.347(2), Zr1–C8 2.326(2),
Zr1–C15 2.303(2); N1–Zr1–N2 70.00(7), N1–Zr1–N3 73.04(7), N1–Zr1–
C1 152.06(8), N1–Zr1–C8 88.12(8), N1–Zr1–C15 115.99(8), N2–Zr1–N3
73.01(6), N2–Zr1–C1 82.50(7), N2–Zr1–C8 108.67(8), N2–Zr1–C15
155.46(7), N3–Zr1–C1 104.15(8), N3–Zr1–C8 159.46(7), N3–Zr1–C15
85.79(7), C1–Zr1–C8 96.33(9), C1–Zr1–C15 91.10(8), C8–Zr1–C15
95.54(8), Zr1–C1–C2 116.3(2), Zr1–C8–C9 113.7(2), Zr1–C15–C16
133.5(2).
Scheme 1 i, BuLi, Et2O, 278 °C; ii, ZrCl4(THF)2, THF, rt; iii, Zr(NEt2)4,
Et2O, 278 °C; iv, xs Me3SiCl, toluene, 90 °C; v, Zr(CH2Ph)4, Et2O,
278 °C; vi, 3 BzMgCl, Et2O, 278 °C; vii, 90 °C, benzene, 24 h.
DOI: 10.1039/b005675h
Chem. Commun., 2000, 2135–2136
This journal is © The Royal Society of Chemistry 2000
2135

geometries, with the sum of the three angles subtended by
nitrogen approaching 360°. The metal center displays two short
zirconium–nitrogen bonds (Zr1–N1 = 2.131(5), Zr1–N3 =
2.043(5) Å) and one longer bond (Zr1–N2 = 2.410(5) Å)
consistent with a diamido-amino ligand. The benzyl groups are
unremarkable (Zr–C–Cbenzyl = 109.2(4), 114.7(4)°), although
1
they are inequivalent by H and 13C NMR spectroscopy. This
implies that the structure observed in the solid state is
maintained in solution, and the benzyl groups are not averaged
by some fluxional process as is observed for tribenzyl 3.
In order to test the generality of this reaction, (Pri2tacn)ZrCl3
1 was reacted with three equiv. of RLi (R = Me, CH2SiMe3,
Ph). The NMR spectra of these products also indicate the
formation of [CH2NCHNCH2CH2N(Pri)CH2CH2NPri]ZrR2
with the elimination of the corresponding alkane. A related
ligand decomposition has been observed for the tantalum
species [N3N]TaMe2 ([N3N] = [N(CH2CH2NSiR3)3]32, R =
Me20, Et21). Thermolysis of the deuterated complex (Pri2-
tacn)Zr(CD2C6D5)3 affords C7D7H, ruling out a-hydrogen
abstraction and a benzylidene intermediate. A more detailed
study of the kinetics and mechanism of this ring-opening
reaction will be reported shortly, along with other examples of
the synthetic utility of (Pri2tacn2).
Fig.
2
ORTEP view of [CH2NCHNCH2CH2N(Pri)CH2CH2N-
Pri]Zr(CH2Ph)2 4 drawn with 50% probability ellipsoids. Selected bond
distances (Å) and angles (°): Zr1–N1 2.131(5), Zr1–N2 2.410(5), Zr1–N3
2.043(5), Zr1–C1 2.289(6), Zr1–C8 2.295(6), C15–C16 1.317(9); N1–Zr1–
N2 71.3(2), N1–Zr1–N3 142.3(2), N1–Zr1–C1 100.5(2), N1–Zr1–C8
94.3(2), N2–Zr1–N3 71.5(4), N2–Zr1–C1 124.3(2), N2–Zr1–C8 125.1(2),
N3–Zr1–C1 105.3(2), N3–Zr1–C8 101.9(2), C1–Zr1–C8 110.2(2), Zr1–
C1–C8 114.7(2), Zr1–C8–C9 109.2(4).
This work was supported by the Department of Energy
contract no. DE-AC03-76SF00098.
Notes and references
† Crystal data: for 3: C33H47N3Zr, M = 576.97, trigonal, a = 39.6517(3),
b = 39.6517(3), c = 9.7469(1) Å, V = 13 271.5(2) Å3, T = 173 K, space
group R-3(h) (no. 148), Z = 18, m(Mo-Ka) = 0.4 mm21, 20082 reflections
measured, 5097 unique (Rint = 0.035), Dcalc = 1.30 g cm23, R = 0.024, Rw
= 0.028. For 4: C26H39N3Zr, M = 484.83, monoclinic, a = 8.881(5), b =
25.936(2), c = 11.7707(7) Å, b = 112.017(1)°, V = 2513.5(6) Å3, T = 164
K, space group P21/c (no. 14), Z = 4, m(Mo-Ka) = 0.45 mm21, 11 204
reflections measured, 4314 unique (Rint = 0.082), Dcalc = 1.28 g cm23, R
= 0.043, Rw = 0.054. CCDC 182/1783. See http://www.rsc.org/suppdata/
cc/b0/b005675h/ for crystallographic files in .cif format.
ligands are positioned with their phenyl rings pointing down-
wards with respect to the tacn ring, while the third has its phenyl
ring pointing upwards, being disposed almost parallel to the
azamacrocycle. Two of the benzyl groups possess normal Zr–
C–C bond angles (113.7(2) and 116.3(2)°) while the third
displays an angle that is larger than expected (Zr1–C15–C16 =
133.5(2)°). The disposition of the benzyl groups is similar to
that found for Cp*Ti(CH2Ph)3, in which a single anomalous Ti–
C–C angle is a consequence of a double agostic CH2–Ti
interaction.19 Owing to the broadness of the peak arising from
the benzylic methylene groups in the {1H}13C NMR spectrum,
we were unable to extract the C–H coupling constant to
determine if any additional interaction was present in our case;
it is likely that this unusually large angle is a result of packing
forces in the crystal.
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Heating (Pri2tacn)Zr(CH2Ph)3 3 for 24 h at 80 °C results in
elimination of one equiv. of toluene and complete conversion to
1
a new metal complex. The H NMR spectrum reveals a total
6 J. D. Scollard and D. H. McConville, J. Am. Chem. Soc., 1996, 118,
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lack of symmetry in solution as evidenced by the presence of
four isopropyl methyl doublets and separate multiplets for each
of the methylene protons of the ligand backbone. Particularly
diagnostic of this new species are resonances consistent with a
vinyl group; a doublet of doublets corresponding to one olefinic
proton is present at 7.92 ppm (3JH-HA = 8.4, 3JH-HB = 14.8 Hz)
with the other two protons of the vinyl unit giving rise to two
doublets farther upfield (4.12 ppm, 3JH-H’ = 8.4 Hz; 3.93 ppm,
3JH-H” = 14.8 Hz). These conclusions are supported by
{1H}13C NMR spectroscopy which shows four separate
isopropyl methyl peaks, two isopropyl methine resonances and
distinct peaks for four methylene groups of the ancillary ligand.
13C DEPT experiments allowed the vinyl resonances to be
assigned (143.1 and 79.0 ppm); the presence of a vinyl group is
visible in the IR spectrum which shows a strong absorption in
the olefinic region (ca. 1600 cm21).
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To further investigate the structure of the thermolysis
product, we turned to X-ray diffraction (Fig. 2).† The ORTEP of
4 shows that the zirconium center is coordinated by two benzyl
groups and a new tacn-derived pincer ligand in which the
original anionic, macrocyclic, diamino-amido ligand has been
transformed into a dianionic, acyclic, diamido-amino moiety.
Additionally, one of the ethylene units of the triazacyclononane
ring has been converted into a vinyl group (C15–C16). The
zirconium center is best described as distorted trigonal bipyr-
amidal, with N1 and N3 occupying the axial sites (N1–Zr1–N3
= 142.3(2)°). Both amido nitrogens in 4 display trigonal planar
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2136
Chem. Commun., 2000, 2135–2136