Group IV complexes of a tetradentate amine mono(phenolate) ligand: A second side-arm donor stabilizes cationic species

Article abstract of DOI:10.1021/ic051400w

An amine mono(phenolate) ligand bearing two side-arm donors led to octahedral trialkoxo and trialkyl group IV metal complexes, in which one of the donors was unbound, and to exceptionally stable cationic complexes in which the two side-arm donors were tightly bound.

Full text of DOI:10.1021/ic051400w

Inorg. Chem. 2005, 44, 8188−8190
Group IV Complexes of a Tetradentate Amine Mono(phenolate) Ligand:
a Second Side-Arm Donor Stabilizes Cationic Species
Stanislav Groysman, Ekaterina Sergeeva, Israel Goldberg, and Moshe Kol*
School of Chemistry, Raymond and BeVerly Sackler Faculty of Exact Sciences,
Tel AViV UniVersity, Tel AViV 69978, Israel
Received August 17, 2005
An amine mono(phenolate) ligand bearing two side-arm donors
led to octahedral trialkoxo and trialkyl group IV metal complexes,
in which one of the donors was unbound, and to exceptionally
stable cationic complexes in which the two side-arm donors were
tightly bound.
Figure 1. Tetradentate amine phenolate ligand family.
Scheme 1. Synthesis of the Amine Mono(phenolate) Ligand
Precursor
Currently, there is considerable interest in early-transition-
metal complexes of the chelating tetradentate amine pheno-
late ligands.1-5 These ligands bind strongly to those oxophilic
metals, leading to well-defined geometries at the metal
centers and allowing a precise control of the complex
structure and activity. Thus far, this family included two
subclasses: the trianionic amine tris(phenolate) ligands1,2 and
the dianionic amine bis(phenolate) ligands,3-5 possessing an
additional “side-arm” donor. Conceptually, an additional
subclass of this family is feasible: the monoanionic amine
mono(phenolate) ligands that should possess two neutral side-
arm donors to be tetradentate (Figure 1). Among other mani-
festations, this second side-arm donor may stabilize cationic
MIV metal centers. To our knowledge, no early-transition-
metal complexes of such ligands have been previously
reported.6,7 Herein, we describe the synthesis and structure
of neutral and cationic group IV metal complexes of an amine
mono(phenolate) ligand bearing two side-arm donors. The
latter complexes exhibit remarkable stability.
Aiming at complexes of the LigMX3 and LigMX2+ types,
we chose a ligand featuring methoxy side-arm donors in view
of the strong binding of this donor4c and bulky tert-butyl-
phenolate substituents to minimize the formation of unwanted
products.4d The ligand precursor (LigH) was prepared by a
Mannich condensation between 2,4-di-tert-butylphenol, bis-
(2-methoxyethyl)amine, and formaldehyde (Scheme 1). This
method may lead to a broad range of amine mono(phenolate)
ligand precursors.
* To whom correspondence should be addressed. E-mail: moshekol@
post.tau.ac.il. Tel: +972 3 6407392. Fax: +972 3 6409293.
(1) (a) Kol, M.; Shamis, M.; Goldberg, I.; Goldschmidt, Z.; Alfi, S.; Hayut-
Salant, E. Inorg. Chem. Commun. 2001, 4, 177. (b) Groysman, S.;
Segal, S.; Shamis, M.; Goldberg, I.; Kol, M.; Goldschmidt, Z.; Hayut-
Salant, E. J. Chem. Soc., Dalton Trans. 2002, 3425. (c) Groysman,
S.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Inorg. Chem. 2005, 44,
5073.
(2) (a) Bull, S. D.; Davidson, M. G.; Johnson, A. L.; Robinson, D. E. E.;
Mahon, M. F. Chem. Commun. 2003, 1750. (b) Ugrinova, V.; Ellis,
G. A.; Brown, S. N. Chem. Commun. 2004, 468. (c) Kim, Y.; Kapoor,
P. N.; Verkade, J. G. Inorg. Chem. 2002, 41, 4834. (d) Michalczyk,
L.; de Gala, S.; Bruno, J. W. Organometallics 2001, 20, 5547.
(3) Hinshaw, C. J.; Peng, G.; Singh, R.; Spence, J. T.; Enemark, J. H.;
Bruck, M.; Kristofzski, J.; Merbs, S. L.; Ortega, R. B.; Wexler, P. A.
Inorg. Chem. 1989, 28, 4483.
(4) (a) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Weitman, H.; Goldschmidt,
Z. Chem. Commun. 2000, 379. (b) Tshuva, E. Y.; Goldberg, I.; Kol,
M.; Goldschmidt, Z. Organometallics 2001, 20, 3017. (c) Tshuva, E.
Y.; Groysman, S.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organo-
metallics 2002, 21, 662. (d) Tshuva, E. Y.; Goldberg, I.; Kol, M.;
Goldschmidt, Z. Inorg. Chem. 2001, 40, 4263. (e) Groysman, S.;
Goldberg, I.; Kol, M.; Genizi, E.; Goldschmidt, Z. Organometallics
2003, 22, 3013. (f) Groysman, S.; Goldberg, I.; Kol, M.; Genizi, E.;
Goldschmidt, Z. Organometallics 2004, 23, 1880.
(5) (a) Wolff, F.; Lorber, C.; Choukroun, R.; Donnadieu, B. Inorg. Chem.
2003, 42, 7839. (b) Cai, C.-X.; Amgoune, A.; Lehmann, C. W.;
Carpentier, J.-F. Chem. Commun. 2004, 330. (c) Alcazar-Roman, L.
M. B.; O’Keefe, J.; Hillmyer, M. A.; Tolman, W. B. Dalton Trans.
2003, 3082. (d) Lehtonen, A.; Sillanpa¨a¨, R. Inorg. Chem. 2004, 43,
6501. (e) Boyd, C. L.; Toupance, T.; Tyrrell, B. R.; Dubberley, S. T.;
Ward, B.; Wilson, C. R.; Cowley, A. R. Mountford, P. Organometallics
2005, 24, 309.
The group IV metal alkoxides, Ti(OiPr)4 and Zr(OtBu)4,
reacted with LigH at room temperature (RT) to give the
corresponding complexes LigTi(OiPr)3 (1) and LigZr(OtBu)3
(2) in high yields. Both complexes feature a single amine
phenolate ligand and three alkoxo groups, according to the
relative peak integration. The two OMe donors and the three
(6) A late-transition-metal (Cu) complex of a similar ligand has been
recently reported: Inoue, Y.; Matyjaszewski, K. Macromolecules 2004,
37, 4014.
(7) An amine mono(phenolate) ligand, carrying a triamino macrocycle
was reported. Lawrence, S. C.; Ward, B. W.; Dubberley, S. R.; Kozak,
C. M.; Mountford, P. Chem. Commun. 2003, 2880.
8188 Inorganic Chemistry, Vol. 44, No. 23, 2005
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© 2005 American Chemical Society
Published on Web 10/18/2005

COMMUNICATION
Scheme 2. Proposed Equilibration Processes for the Alkoxide
Complexes 1 and 2
alkoxo groups are each identical in the spectra of 1 and 2.
Variable-temperature NMR experiments indicated that both
1 and 2 undergo a dynamic process equilibrating the side-
arm donors. For 1, a single set of two triplets for the
[NCH2CH2O] protons of the two side arms and a sharp
singlet for the benzylmethylene protons at RT signify an
averaged Cs-symmetric complex on the NMR time scale. For
2, this equivalence is apparent only at elevated temperatures.
Cooling down both complexes to ca. 230-240 K results in
Figure 2. Crystal structure of 2. ORTEP representation, 50% probability.
Selected bond distances (Å) and angles (deg): Zr1-O2, 1.928(2); Zr1-
O3, 2.403(2); Zr1-O5, 1.984(2); Zr1-O6, 1.936(2); Zr1-O7, 1.928(2);
Zr1-N8, 2.500(2); O2-Zr1-O5, 156.26(7); O3-Zr1-O7, 161.51(7); N8-
Zr1-O6, 162.46(7); O2-Zr1-O3, 81.28(6); O5-Zr1-O7, 9.05(7).
1
more complex H NMR spectra, displaying, among others,
apparent decomposition was observed in a period of several
days to several weeks). The cationic alkoxo complexes
possess a rigid C1 symmetry at RT, consistent with coordina-
tion of both donors to the metal. The structure of these
unusual cationic complexes was confirmed by an X-ray
structure determination (for 4, see Figure 3).11 As anticipated,
both side arms are bound to the metal center [Zr-O3, 2.383-
(2) Å; Zr-O4, 2.268(2) Å], completing the octahedral
geometry at Zr. Overall, the cationic complex presents
slightly more compressed bond distances for all donor atoms
in comparison to the neutral species (2): For example, the
PhO-Zr bond is 2.080(2) Å in 2 and 1.973(2) Å in 4. In 4,
the “second” methoxy group is trans to the phenolate oxygen,
having replaced the most weakly bound tert-butoxide group.
In this disposition, all of the neutral donors are trans to the
anionic (σ and π) donors (alkoxo and phenolate oxygens).
Metal alkyl cationic species are known to be highly
unstable, often yielding poorly defined mixtures upon
isolation attempts. In addition, weakly coordinating coun-
teranions may lead to the formation of dinuclear species.12
One possible role of an “extra” donor is the stabilization of
mononuclear metal alkyl cationic compounds. The reaction
of LigH with titanium tetrabenzyl was not successful, leading
to a complex mixture of products. On the other hand, the
reaction of LigH with zirconium tetrabenzyl formed the
two different peaks for the OMe groups and an AB system
for the benzylmethylene protons. ∆Gq values of 50 and 53
kJ mol-1 for 1 and 2, respectively, were extracted from the
1
coalescence temperatures of the -OCH3 peaks in the H
NMR spectra.8 The process that equilibrates the side-arm
donors can be envisioned as “hopping” of the metal between
the two donors (Scheme 2) accompanied by a “flip” of the
phenolate ring, which explains the equivalence of the
benzylmethylene protons at higher temperatures. This process
probably proceeds via a pentacoordinate intermediate (“in-
tramolecular dissociative” mechanism), in which the scram-
bling of the alkoxo groups takes place.
X-ray structure determinations were carried out for both
1 and 2 (for the structure of Zr complex 2, see Figure
2).9 In both structures, in agreement with the spectroscopic
data, the second side arm is unbound, resulting in an
octahedral geometry at the metal center. The coordination
mode of the ligand is fac, imposing a fac coordination of
the three alkoxo groups. The bond distances of Zr-OtBu
are different [1.984(2) Å vs 1.928(2) and 1.936(2) Å],
probably because of the different trans influence of the OPh
function versus the neutral amine and methoxy donors,
respectively. The structure of the Ti complex 1 is analogous
to the structure of the Zr complex 2.
Reports of cationic early-transition-metal alkoxo species
are scarce,10 even though such species may be of great
importance because they should exhibit higher electrophi-
licity than the analogous uncharged species. We anticipated
that the “extra” donor in the ligand framework will bind to
the metal center upon abstraction of one of the three alkoxo
groups, therefore stabilizing the cationic bis(alkoxo) species.
Upon reaction of the tris(alkoxo) complexes 1 and 2 with
[PhNMe2H][B(C6F5)4], a clean formation of monocationic
complexes (whose counteranion is tetrakis(pentafluorophe-
nyl)borate, 3 and 4, respectively) was observed. Compounds
3 and 4 are highly soluble in common organic solvents such
as diethyl ether, benzene, and toluene. These compounds
were found to be exceptionally stable in solution at RT (no
(8) ∆Gq ) 19.14Tc[9.97 + log(Tc/δν)]. See: Gu¨nther, H. NMR
spectroscopysan Introduction; Wiley: New York, 1980.
(9) Crystal data for 2: C33H63NO6Zr, M ) 661.06, monoclinic, C2/c, a
) 27.186(2) Å, b ) 13.295(1) Å, c ) 20.779(2) Å, â ) 90.692(5)°,
V ) 7509.8(9) Å3, Z ) 8, Dc ) 1.169 g cm-1, T ) 110 K, 8663
unique reflns, R1 ) 0.046, wR2 ) 0.1064 for 6249 reflns with I >
2σ(I).
(10) (a) Jordan, R. F.; Taylor, D. F.; Baenziger, N. C. Organometallics
1990, 9, 1546. (b) Collins, S.; Koene, B. E.; Ramachandran, R.; Taylor,
N. J. Organometallics 1991, 10, 2092. (c) Stoebenau, E. J., III; Jordan,
R. F. J. Am. Chem. Soc. 2003, 125, 3222.
(11) Crystal data for 4: C53H54BF20NO5Zr, M ) 1267.00, monoclinic, P21/
n, a ) 19.9790(5) Å, b ) 10.9770(4) Å, c ) 26.8330(7) Å, â )
109.905(2)°, V ) 5533.2(3) Å3, Z ) 4, Dc ) 1.521 g cm-1, T ) 110
K, 13 315 unique reflns, R1 ) 0.0592, wR2 ) 0.1447 for 7896 reflns
with I > 2σ(I).
(12) For example, see: Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000,
100, 1391.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8189

COMMUNICATION
Scheme 3. Synthesis and Proposed Structure of the Monocationic
Dialkylzirconium Complex in 6 and 7
decomposition was observed after a period of 4 days for 7
in C6D6 at RT). We attribute this remarkable stability to the
binding of the “extra” donor to the metal.15
The structure of the dibenzyl cationic complex deserves
some consideration. The overall structure of the cation is
consistent with a rigid Cs-symmetric complex, in which the
methylene protons of the two identical benzyl groups are
diastereotopic, appearing as an AB spin system. Namely, the
mirror plane reflects, rather than bisects, the two benzyl
groups. This symmetry is not consistent with the “octahedral”
Cs-coordination mode, typical of the amine bis(phenolate)
ligands.4 An alternative structure features a “tripodal” binding
of the tetradentate ligand, with the two mirror-plane-reflected
benzyl groups occupying the Cs-symmetric cavity above the
three oxygen donors. An analogous structure was previously
reported for the “[N3N]TaR2”-type complexes (see Scheme
3).16 Thus, the cationic dialkoxo complexes and the cationic
dialkyl complexes exhibit a different binding mode. The
origin of this difference may lie in the presence of two strong
(σ and π) donor ligands (alkoxo groups) in 3 and 4 versus
two σ donors only (benzyl ligands) in 6 and 7.
In summary, we introduced the first member of a new class
of tetradentate amine phenolate ligands to the early-transition-
metal chemistry realm. This ligand led, on the one hand, to
flexible neutral complexes, in which one of the side-arm
donors is not bound to the metal, and, on the other hand, to
exceptionally stable cationic alkoxo and alkyl complexes,
in which the two side-arm donors bind to the metal. The
straightforward synthetic pathway and the well-defined
coordination modes hint on the rich chemistry that these
ligands may sustain.
Figure 3. Crystal structure of 4. ORTEP representation, 40% probability.
Selected bond distances (Å) and angles (deg): Zr1-O2, 1.973(2);
Zr1-O3, 2.383(2); Zr1-O4, 2.268(2); Zr1-O5, 1.909(2); Zr1-O6, 1.919-
(2); Zr1-N7, 2.413(3); O2-Zr1-O4, 152.45(9); O5-Zr1-O3, 163.23-
(10); O6-Zr1-N7, 154.04(9); O2-Zr1-O3, 86.60(9).
desired LigZr(Bn)3 complex 5, quantitatively, whose spectral
data supported a structure analogous to those of complexes
1 and 2. Upon the addition of a benzene solution of the strong
Lewis acid B(C6F5)3 to a solution of 5, a yellow homoge-
1
neous solution formed immediately. According to H, 13C,
and 19F NMR spectroscopies, formation of a single species
(6) took place. A 2:1 ratio of the benzyl (phenyl) peaks in 6
implies that the activation of one of the benzyl groups took
place. The methylene protons at 3.40 ppm and the broad
signal for the carbon atom at ca. 32 ppm are consistent with
a Ph-CH2-B fragment of a counteranion. Moreover, the
19F NMR absorptions for the meta and para fluorines
(-163.75 and -166.73 ppm, respectively) indicate that the
counteranion does not coordinate to the metal.4c,13 Thus, we
suggest that 6 is a well-separated ion pair (Scheme 3). We
propose that the “extra” donor binds to the metal in this
electron-deficient cationic species.
Acknowledgment. We thank the Israel Science Founda-
tion for financial support.
Supporting Information Available: Experimental procedures,
spectroscopic, and crystallographic details and CIF files. This ma-
terial is available free of charge via the Internet at http://pubs.acs.org.
IC051400W
To further validate the structure of the cationic dibenzyl
complex, we reacted the tribenzyl complex 5 with a different
activator, [PhNMe2H][B(C6F5)4]. Because the activation
mechanism by this compound relies on protonation of one
of the benzyl groups to form toluene, the formation of a well-
separated ion pair should be unequivocally anticipated in this
case. This reaction led to the formation of compound 7
(Scheme 3) and was accompanied by toluene liberation. The
1H NMR spectra corresponding to the cationic species of
both 6 and 7 are very similar.14 Most significantly, the cation
is stable in solution for a prolonged period of time (no
(13) (a) Horton, A. D.; de With, J. Organometallics 1997, 16, 5424. (b)
Pellecchia, C.; Immirzi, A.; Grassi, A.; Zambelli, A. Organometallics
1993, 12, 4473.
(14) For example, the methoxy groups give rise to a single peak at 2.95
ppm (7) and 2.97 ppm (6), and the remaining benzyl groups give rise
to an AB system at 2.12 and 2.02 ppm in 7 and 2.22 and 2.09 ppm in
6. The tBu signals and the ligand phenyl signals are essentially identical
in these complexes.
(15) The aniline could be removed almost completely by pentane washings
without affecting other signals in the 1H NMR spectrum; this further
supports the binding of both side-arm donors in this electron-deficient
complex.
(16) Freundlich, J. S.; Schrock, R. R.; Davis, W. M. J. Am. Chem. Soc.
1994, 118, 3643.
8190 Inorganic Chemistry, Vol. 44, No. 23, 2005