Titanium and Zirconium Amido Complexes Ligated by 2,2′ -Di(3-methylindolyl)methanes: Synthesis, Characterization, and Ethylene Polymerization Activity

Article abstract of DOI:10.1021/ic0347236

2,2′-Di(3-methylindolyl)methanes (L₂H₂) are introduced as dianionic, bidentate ligands of reduced π-donating ability. Four complexes of the type L₂Ti(NEt₂)₂ and L₂Zr(NEt₂)_2<

Full text of DOI:10.1021/ic0347236

Inorg. Chem. 2003, 42, 6592−6594
Titanium and Zirconium Amido Complexes Ligated by
2,2′-Di(3-methylindolyl)methanes: Synthesis, Characterization, and
Ethylene Polymerization Activity
Mark R. Mason,* Bassam N. Fneich, and Kristin Kirschbaum
Department of Chemistry, MS 602, UniVersity of Toledo, Toledo, Ohio 43606
Received June 24, 2003
2,2′-Di(3-methylindolyl)methanes (L H ) are introduced as dianionic,
coordination and M-N-M bridging, and reduced NfM
2
2
π-donation. These properties are attributable to the high
electronegativity of nitrogen, significantly enhanced by
extensive delocalization of the nitrogen lone pair into the
aromatic π system and onto the indolyl carbons. Delocal-
ization reduces the availability of the nitrogen lone pair for
π-donation to the metal and also limits M-N-M bridging
capability.
Surprisingly, 2,2′-diindolylmethanes and related 2,2′,2′′-
triindolylmethanes are unexplored as ligands for main group
and transition elements, aside from our recent work on these
readily synthesized compounds.1 We recently demonstrated
bidentate coordination of 1a-c to aluminum, boron, and
silicon,1 and showed that tri(3-methylindol-2-yl)methane is
an effective framework for the construction of the very bulky
and π-acidic phosphine 2.6,7
bidentate ligands of reduced π-donating ability. Four complexes
of the type L Ti(NEt2)2 and L Zr(NEt2)2(THF) have been synthesized
2
2
and characterized by elemental analysis, NMR (1H, 13C) spectros-
copy, and X-ray crystallography. Structural data confirm the reduced
π-donating ability of the η1-indolyl moiety compared to that of
diethylamido. Preliminary catalytic activities of these group 4
complexes for the polymerization of ethylene are reported.
Numerous 2,2′-diindolylmethanes are reported in the
literature that could serve as bidentate or, for those with an
additional ligating substituent, tridentate ligands for transition
and main group elements.1 Deprotonated 2,2′-diindolyl-
methanes can be regarded as dianionic analogues of scor-
pionate bis(pyrazolyl)borate ligands,2 although with greater
steric requirement. In addition, 2,2′-diindolylmethanes are
appealing ligand candidates since pyrrolyl,3-5 indolyl,1,6-8
and carbazolyl9 groups are now recognized to impart
significantly different electronic and steric properties to
complexes of main group and transition metals than those
offered by traditional amido ligands. Specific advantages of
indolyl ligands include strong electron-withdrawing ability
when bound in an η1-N mode, reduced tendency for η5
Here we introduce deprotonated 2,2′-di(3-methylindolyl)-
arylmethanes 1a and 1b as dianionic, bidentate ligands of
reduced π-donating ability for applications to the coordina-
tion chemistry of transition metals. Representative diethyl-
amido complexes of titanium and zirconium are reported,
along with their catalytic activities for the polymerization
of ethylene.
* To whom correspondence should be addressed. E-mail: mmason5@
uoft02.utoledo.edu.
(1) For an overview, see: Mason, M. R. Chemtracts 2003, 16, 272.
(2) Trofimenko, S. Chem. ReV. 1993, 93, 943.
(3) Moloy, K. G.; Petersen, J. L. J. Am. Chem. Soc. 1995, 117, 7696.
(4) (a) Shi, Y.; Hall, C.; Ciszewski, J. T.; Cao, C.; Odom, A. L. Chem.
Commun. 2003, 586. (b) Li, Y.; Turnas, A.; Ciszewski, J. T.; Odom,
A. L. Inorg. Chem. 2002, 41, 6298. (c) Harris, S. A.; Ciszewski, J.
T.; Odom, A. L. Inorg. Chem. 2001, 40, 1987.
(5) For examples, see: (a) Korobkov, I.; Gambarotta, S.; Yap, G. P. A.
Organometallics 2001, 20, 2552. (b) Ganesan, M.; Lalonde, M. P.;
Gambarotta, S.; Yap, G. P. A. Organometallics 2001, 20, 2443. (c)
Dube, T.; Gambarotta, S.; Yap, G. Organometallics 2000, 19, 121.
(d) Dube, T.; Gambarotta, S.; Yap, G. P. A. Organometallics 2000,
19, 817. (e) Dube, T.; Gambarotta, S.; Yap, G. P. A. Organometallics
2000, 19, 115.
Reactions of Ti(NEt2)410 with 1a11,12 and 1b12 in refluxing
toluene, followed by concentration of the reaction solutions
and cooling at -20 °C overnight, yielded brick red crystals
1
of 3a and 3b, respectively (eq 1). The H and 13C NMR
(6) Barnard, T. S.; Mason, M. R. Organometallics 2001, 20, 206.
(7) Barnard, T. S.; Mason, M. R. Inorg. Chem. 2001, 40, 5001.
(8) Tanski, J. M.; Parkin, G. Inorg. Chem. 2003, 42, 264.
(9) Riley, P. N.; Fanwick, P. E.; Rothwell, I. P. J. Chem. Soc., Dalton
Trans. 2001, 181.
(10) (a) Bradley, D. C.; Thomas, I. M. J. Chem. Soc. 1960, 3857. (b)
Steinborn, D.; Wagner, I.; Taube, R. Synthesis 1989, 4, 304.
(11) Dostal, V. Chem. Listy 1938, 32, 13; Chem. Abstr. 1938, 32, 5399.
(12) Mason, M. R.; Barnard, T. S.; Segla, M. F.; Xie, B.; Kirschbaum, K.
J. Chem. Crystallogr. 2003, 33, 531.
6592 Inorganic Chemistry, Vol. 42, No. 21, 2003
10.1021/ic0347236 CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/25/2003

COMMUNICATION
Table 1. Selected Distances (Å) and Angles (deg) for 3a,b and 4a,b
M-Nindolyl
M-Namido
Nindolyl-M-Nindolyl
3a
3b
4a
4b
1.977(1), 1.987(2)
1.968(3), 1.994(3)
2.198(2), 2.218(2)
2.185(3), 2.203(3)
1.865(1), 1.867(2)
1.862(3), 1.863(3)
2.013(2), 2.019(2)
2.014(4), 2.030(3)
97.79(6)
96.32(14)
82.95(7)
83.39(12)
The average Ti-Nindolyl distances (Table 1) for 3a (1.982
Å) and 3b (1.981 Å) fall slightly short of the range of Ti-N
distances (2.00-2.10 Å) observed for pyrrolyl4,13,14 and
carbazolyl9 complexes of titanium. The average Ti-Namido
distances for 3a (1.866 Å) and 3b (1.863 Å) are significantly
shorter than the Ti-Nindolyl distances as a result of greater
NfM π-donation for the amido ligands than for the 2,2′-
di(3-methylindolyl)methanes.
Figure 1. ORTEP drawing of 3a. Thermal ellipsoids are drawn at the
40% probability level. Hydrogen atoms are omitted for clarity.
Reactions of Zr(NEt2)410 with 1a and 1b in refluxing THF,
followed by removal of volatiles in vacuo and recrystalli-
zation of the remaining residue from hot toluene, yielded
nearly colorless crystals of toluene solvates of 4a and 4b,
respectively (eq 2). In contrast to the apparent four-coordinate
Figure 2. ORTEP drawing of 3b. Thermal ellipsoids are drawn at the
40% probability level. Hydrogen atoms are omitted for clarity.
titanium in 3a-b, the zirconium atoms in 4a and 4b are
spectra of 3a and 3b exhibit all expected ligand resonances
and indicate that both indolyl moieties of the 2,2′-di(3-
methylindolyl)methane are equivalent. Indolyl NH reso-
1
each five-coordinate with a coordinated THF based on H
and 13C NMR chemical shifts. The OCH2 resonances of the
THF are broadened considerably in the 13C NMR spectrum,
suggesting the occurrence of a dynamic process on the NMR
1
nances are absent in the H NMR spectra. Two sets of
methylene and methyl resonances are observed in the 1H and
13C NMR spectra for each complex due to the chemically
inequivalent diethylamido groups.
1
time scale. The H NMR spectrum exhibits a sharp quartet
and triplet for one set of the chemically equivalent diethyl-
amido groups. The second diethylamido group exhibits a
broad resonance for the methylene protons and a broadened
triplet for the methyl protons, again suggesting a dynamic
process in solution. The methylene resonances of both
diethylamido groups of 4a and 4b are broad in the 13C NMR
spectrum, whereas those for 3a and 3b are relatively sharp.
The molecular structures of 4a and 4b were confirmed
by X-ray crystallography (Figures 3, 4). Each structure
consists of a five-coordinate zirconium ligated by two
inequivalent diethylamido groups, a chelating bidentate 2,2′-
di(3-methylindolyl)methane, and a molecule of THF. The
2,2′-di(3-methylindolyl)methanes are folded to a greater
extent in 4a and 4b than in 3a and 3b, resulting in greatly
reduced Nindolyl-Zr-Nindolyl angles of 82.95(7)° and 83.39(12)°,
respectively (Table 1). The average Zr-Nindolyl distances for
4a (2.208 Å) and 4b (2.194 Å) are in the range observed for
The molecular structures of 3a and 3b were further
confirmed by X-ray crystallography (Figures 1, 2). Each
structure consists of an approximately tetrahedral titanium
ligated by two inequivalent diethylamido groups and a
chelating, bidentate 2,2′-di(3-methylindolyl)methane. Each
deprotonated indolyl ring of the latter is η1-coordinated to
titanium. The orientation of the 2,2′-di(3-methylindolyl)-
methane aryl group is reminiscent of the coordination mode
of scorpionate bis(pyrazolyl)borates,2 in that the aryl group
is poised for coordination to the metal.
(13) Bynum, R. V.; Hunter, W. E.; Rogers, R. D.; Atwood, J. L. Inorg.
Chem. 1980, 19, 2368.
(14) Novak, A.; Blake, A. J.; Wilson, C.; Love, J. B. Chem. Commun.
2002, 2796.
Inorganic Chemistry, Vol. 42, No. 21, 2003 6593

COMMUNICATION
4a and 4b each show two short Zr-C distances (2.838(3)-
2.958(3) Å), two short Zr-H distances (2.56(3)-2.83(3) Å),
and two small Zr-N-C angles (108.1(2)-115.5(2)°). Com-
parison of these data to those for diethylamido complexes
of titanium and zirconium listed in the Cambridge Structural
Database strongly supports the presence of two agostic
interactions in each of the structures 3a,b and 4a,b. Huang
et al. previously assigned a Zr-C distance of 2.808(3) Å
and a Zr-H distance of 2.526(3) Å to an agostic interaction.17
Tanski and Parkin similarly assigned comparable distances
in a dimethylamido complex of zirconium to an agostic
interaction.16 NMR spectra of 3a,b and 4a,b do not support
retention of static agostic interactions in solution. Although
Figure 3. ORTEP drawing of 4a. Thermal ellipsoids are drawn at the
40% probability level. Hydrogen atoms are omitted for clarity.
1
broadened methylene resonances were observed in the H
and 13C NMR spectra of 4a and 4b, 1JCH values for the amido
methylenes are normal18 and in the range of 122-133 Hz.
As is often the case,18 IR data were inconclusive as to the
presence of agostic interactions in 3a,b and 4a,b.
Considering the recent interest in non-metallocene catalysts
for the polymerization of alkenes, we tested 3a,b and 4a,b
for catalytic activity for the polymerization of ethylene. Upon
activation with methylaluminoxane, 3a and 3b polymerized
ethylene at 25 °C in toluene with activities of 1.1 and 8.4
kg of polymer/mol of Ti/h, respectively. Activities were
slightly greater for 4a (13.0 kg of polymer/mol of Zr/h) and
4b (36.1 kg of polymer/mol of Zr/h). These activities are
approximately 2 orders of magnitude less than those obtained
for Cp2TiCl2 (2100 kg of polymer/mol of Ti/h) and Cp2ZrCl2
(3900 kg of polymer/mol of Zr/h) under the same conditions.
In conclusion, 2,2′-di(3-methylindolyl)methanes have been
demonstrated to serve as dianionic, bidentate ligands of
reduced π-donating ability for support of transition metal
complexes. The coordination chemistry of 2,2′-diindolyl-
methanes bearing additional ligating substituents will be
reported in the near future.
Figure 4. ORTEP drawing of 4b. Thermal ellipsoids are drawn at the
40% probability level. Hydrogen atoms are omitted for clarity.
representative pyrrolyl4b,13-16 and carbazolyl9 complexes of
zirconium (2.07-2.33 Å). The greater Zr-Nindolyl distances
compared to the average Zr-Namido distances (2.019 Å)
substantiate the reduced π-donating ability of η1-indolyl
ligands.
Another feature of the molecular structures of 3a,b and
4a,b is the close proximity of the metal to one methylene
carbon per diethylamido group. In the structure of 3a, for
example, two short Ti-C distances of 2.709(2) and 2.771(3)
Å are observed, whereas the other two Ti-C distances are
substantially longer at 3.107(4) and 3.044(3) Å. The short
Ti-C distances are accompanied by one short Ti-H distance
per methylene (2.56(2), 2.58(3) Å), and small Ti-N-C
angles (108.0(1)°, 111.6(1)° vs 132.3(1)°, 138.2(1)°). Ti-C
and Ti-H distances observed for 3b are comparable with
those observed for 3a. Similarly, the zirconium complexes
Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the American Chemical
Society (Grant 37172-AC3), for partial support of this
research.
Supporting Information Available: Experimental details of
the syntheses, characterization, X-ray analyses (CIF, fully labeled
ORTEPs), and polymerizations. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC0347236
(17) Huang, J.-H.; Kuo, P.-C.; Lee, G.-H.; Peng, S.-M. J. Chin. Chem.
Soc. 2000, 47, 1191.
1
(18) For discussion of JCH values for static agostic interactions (60-90
(15) Bynum, R. V.; Zhang, H.-M.; Hunter, W. E.; Atwood, J. L. Can. J.
Chem. 1986, 64, 1304.
(16) Tanski, J. M.; Parkin, G. Organometallics 2002, 21, 587.
Hz), fluxional agostic interactions, and normal C(sp3)-H bonds (120-
130 Hz), see: Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog.
Inorg. Chem. 1988, 36, 1.
6594 Inorganic Chemistry, Vol. 42, No. 21, 2003