Low-valent chemistry of titanium benzamidinates leading to new Ti μ-N2, μ-O, alkyl derivatives, and the cyclometalation of TMEDA

Full text of DOI:10.1021/ja953449e

J. Am. Chem. Soc. 1996, 118, 893-894
893
Communications to the Editor
bonding mode of the bridging N2.18 The benzamidinates are
bonded in a typical manner, with the metal atom essentially in
the NCN plane. The extremely short Ti-(µ-N2) bond distances
(1.771(5) and 1.759(5) Å) and the long N-N bond length
(1.275(6) Å) are similar to values reported19,20 for the diamag-
netic species {[(Me3Si)2N]TiCl(C6H5N)2}2(µ-N2) (1.759 and
1.263 Å) and {[(Me3Si)2N]TiCl(TMEDA)}2(µ-N2) (1.762 and
1.289 Å) and are suggestive of extensive delocalization of the
Ti2(µ-N2) core compared to metallocene systems. For com-
parison, the paramagnetic metallocene complexes (Cp*2Ti)2(µ-
N2),21 [Cp2Ti(PMe3)]2(µ-N2),22 and [Cp2Ti(p-CH3C6H4)]2Ti(µ-
N2)23 show Ti-(µ-N2) bonds of 2.017, 1.921, and 1.962 Å and
N-N bond lengths of 1.160, 1.191, and 1.162 Å, respectively.
In spite of the long N-N bond in {[PhC(NSiMe3)2]2Ti}2(µ-
N2), the compound is thermally robust and is unreactive toward
H2, CO, or alkynes, even after heating to 70 °C for several days.
Carrying out the above reduction under an atmosphere of CO
resulted in the formation of highly O2-sensitive {[PhC(NSiMe3)2]2-
Ti}2(µ-O), which was isolated as red crystals from hexanes in
18% yield. No other products were isolated from the reaction
mixture, and the fate of the carbon atom remains to be
determined. Repeating the reduction in the absence of CO
yielded only intractable oils. Related reactions involving low-
valent tantalum are known to form oxo and dicarbide species;24
nonetheless, in general, C-O bond cleavage remains rare.25 Due
to the scarcity of structurally characterized (µ-oxo)dititanium-
(III) species, we carried out single-crystal X-ray analysis,17
which confirmed the five-coordinate, bridging oxo structure.
The molecule has a nearly linear Ti-O-Ti linkage of 177.2-
(2)° which is intermediate to values reported for (Cp2Ti)2(µ-
O)26 and (Me3tacn)2Ti2(NCO)4(µ-O)27 (170.9 and 180.0° re-
spectively). The short Ti-O bond (1.821(4) Å) suggests partial
double bond character and is similar to those of (Cp2Ti)2(µ-
O)26 and (Me3tacn)2Ti2(NCO)4(µ-O)27 (tacn ) triazacyclononane),
which both have bond lengths of 1.838 Å. The compound is
paramagnetic, with a µeff ) 2.4 µB, consistent with two
essentially uncoupled d1-Ti centers. This value is similar to
those for (Me3tacn)2Ti2-(NCO)4(µ-O) and (Me3tacn)2Ti2(NCS)4-
(µ-O), which display only weakly antiferromagnetic coupling
(J ) 7 cm-1 av).27
Low-Valent Chemistry of Titanium Benzamidinates
Leading to New Ti µ-N2, µ-O, Alkyl Derivatives,
and the Cyclometalation of TMEDA
John R. Hagadorn and John Arnold*
Department of Chemistry, UniVersity of California
Berkeley, California 94720-1460
ReceiVed October 13, 1995
In efforts to develop alternative ligand arrays to the ubiquitous
Cp2 fragment, whose transition metal chemistry has been widely
explored1-3 and has provided a host of important discoveries
over the years, a number of research groups have turned their
attention to nitrogen-based donors.4-7 For comparison to our
work on early transition metal porphyrin derivatives,8-10 we
recently began a study of the N4-donor environment provided
by bis-amidinate ligand systems. In particular, we were attracted
to the N,N′-bis(trimethylsilyl)benzamidinate ligand system, as
this has shown considerable promise for the stabilization of a
wide variety of early transition metal complexes in a variety of
oxidation states.11-13
Following our recent report14 on the reaction chemistry of
[PhC(NSiMe3)2]2ZrCl2, we now report related results on the
corresponding Ti system developed earlier by Roesky15 and
Gambarotta.16 Our studies have yielded a range of new Ti(III)
species with some unusual reactivity, including the cyclometa-
lation of N,N,N′,N′-tetramethylethylenediamine (TMEDA) and
the cleavage of an amidinate ligand.
As shown is Scheme 1, reduction of [PhC(NSiMe3)2]2TiCl2
with 1% Na/Hg in toluene under N2, followed by crystallization
from hexanes, yielded blue-black crystals of the diamagnetic
complex {[PhC(NSiMe3)2]2Ti}2(µ-N2) in 18% yield. The X-ray
structure17 reveals five-coordinate titanium and the end-on
(1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987.
(2) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley Interscience: New York, 1994.
(3) Elschenbroich, C.; Salzer, A. Organometallics: A Concise Introduc-
tion, 2nd ed.; VCH: New York, 1992.
A rare example of a Ti(III) alkyl, [PhC(NSiMe3)2]2TiMe, was
prepared in 69% yield via the Na/Hg reduction of [PhC-
(NSiMe3)2]2TiMe(Cl) in THF. The paramagnetic product was
isolated as red crystals from hexamethyldisiloxane and is stable
for weeks at room temperature in a fluorescent-lit drybox.
Attempts to carry out the reduction in benzene resulted in the
(4) Jacoby, D.; Isoz, S.; Floriani, C.; Chiesi-Villa, A.; Rizzoi, C. J. Am.
Chem. Soc. 1995, 117, 2793.
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497.
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R. F. J. Am. Chem. Soc. 1993, 115, 8493.
(7) Andersen, R. A. Inorg. Chem. 1979, 18, 2928.
(8) Brand, H.; Arnold, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 95.
(9) Brand, H.; Capriotti, J. A.; Arnold, J. Organometallics 1994, 13, 4469.
(10) Brand, H.; Arnold, J. Coord. Chem. ReV. 1995, 140, 137.
(11) Edelmann, F. T. Coord. Chem. ReV. 1994, 137, 403.
(12) Dehnicke, K. Chem.-Ztg. 1990, 114, 295.
(13) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219.
(14) Hagadorn, J. R.; Arnold, J. Organometallics 1994, 13, 4670.
(15) Roesky, H. W.; Meller, B.; Noltemeyer, M.; Schmidt, H. G.; Scholz,
U.; Sheldrick, G. M. Chem. Ber. 1988, 121, 1403.
(16) Dick, D. G.; Duchateau, R.; Edema, J. H.; Gambarotta, S. Inorg.
Chem. 1993, 32, 1959.
a ) 10.1066(4) Å, b ) 16.5269(7) Å, c ) 11.4930(3) Å, â ) 115.628(1)°,
V ) 1730(1) Å, Z ) 4, dcalc ) 1.159 cm-1, µ ) 3.76 cm-1, T ) -133 °C,
no. of observations ) 2074, no. of parameters ) 351, R ) 4.06, Rw
)
4.21. Details of the structure determination are provided as supporting
information.
(18) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115.
(19) Beydoun, N.; Duchateau, R.; Gambarotta, S. J. Chem. Soc., Chem.
Commun. 1992, 244.
(20) Duchateau, R.; Gambarotta, S.; Beydoun, N.; Bensimon, C. J. Am.
Chem. Soc. 1991, 113, 8986.
(17) Crystal data for {[PhC(NSiMe3)2]2Ti}2(µ-N2): Pna21, a ) 25.2377-
(14) Å, b ) 14.1803(8) Å, c ) 19.4904(11) Å, V ) 6975(1) Å, Z ) 4, dcalc
) 1.122 cm-1, µ ) 4.04 cm-1, T ) -110 °C, no. of observations ) 4166,
no. of parameters ) 647, R ) 4.48, Rw ) 4.86. For {[PhC(NSiMe3)2]2-
Ti}2(µ-O): P21/n, a ) 15.753(1) Å, b ) 27.0170(3) Å, c ) 18.9861(3) Å,
â ) 90.388(1)°, V ) 8080(1) Å, Z ) 4, dcalc ) 1.100 cm-1, µ ) 3.56
cm-1, T ) -100 °C, no. of observations ) 7344, no. of parameters ) 974,
R ) 6.83, Rw ) 8.06%. For [PhC(NSiMe3)2]2TiMe: P21/n, a ) 10.4828-
(2) Å, b ) 32.0722(4) Å, c ) 11.5537(2) Å, â ) 116.0329(1)°, V ) 3490-
(1) Å, Z ) 4, dcalc ) 1.123 cm-1, µ ) 4.03 cm-1, T ) -124 °C, no. of
observations ) 3222, no. of parameters ) 521, R ) 3.99, Rw ) 4.37. For
[PhC(NSiMe3)2]Ti[η2-Me3SiNC(H)Ph][η3-CH2N(Me)CH2CH2N(Me)2]: P21,
(21) Sanner, R. D.; Duggan, D. M.; McKenzie, T. C.; Marsh, R. E.;
Bercaw, J. E. J. Am. Chem. Soc. 1976, 98, 8358.
(22) Berry, D. H.; Procopio, L. J.; Carroll, P. J. Organometallics 1988,
7, 570.
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1979, 170, 39.
(24) LaPointe, R. E.; Wolczanski, P. T.; Mitchell, J. F. J. Am. Chem.
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0002-7863/96/1518-0893$12.00/0 © 1996 American Chemical Society

894 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Communications to the Editor
Scheme 1a
the X-ray crystal structure shows a seven-coordinate Ti bonded
to an amidinate ligand, an η2-imine, and a tridentate metalated
TMEDA ligand. The long C-N (imine) bond (1.446(8) Å)
implies32 substantial metallaaziridine character and formulation
of the product as Ti(IV) as opposed to Ti(II). The Ti-C(29)
bond (2.130(8) Å) of the η3-TMEDA ligand is typical for Ti-
(IV) alkyls; this interaction is no doubt responsible for the
shortness of the Ti-N(5) bond (2.220(5) Å), which is consider-
ably shorter than the 2.351(5) Å found between Ti and N(4).
The room temperature 1H-NMR spectrum is complex, show-
ing a multitude of broad resonances, apparently due to hindered
rotation and/or the presence of diastereomers. On warming to
90 °C, however, the spectrum sharpens to give a pattern
consistent with the solid-state structure, with three singlets for
the inequivalent-SiMe3 groups and a singlet for the imine
hydrogen at 5.49 ppm.33 The complexity of the room temper-
ature spectrum is independent of concentration or the addition
of TMEDA, consistent with an intramolecular rearrangement.34
C-N oxidative cleavage of an amidinate is precedented14 by
our recent discovery of a similar reaction in the reduction of
[PhC(NSiMe3)2]2ZrCl2; however, the Ti case differs in that it
apparently involves bimolecular reactivity (perhaps via a
bridging amidinate), in which a N(SiMe3) moiety is transferred35
from one Ti center to another. We speculate that the coordi-
natively unsaturated η2-iminoacyl complex thus formed reacts
with TMEDA, resulting in cyclometalation and reduction of the
iminoacyl to the imine final product.36 Although we are
unaware of other cases involving TMEDA, this unusual cyclo-
metalation bears an analogy to the Ru-mediated metalation of
DMPE first reported in 1965 by Chatt.37-39 Further synthetic
and mechanistic studies are currently in progress.
a Phenyl groups (except for the ipso carbon on the η2-imine ligand)
and methyl groups on Si are omitted from the ORTEP diagrams for
clarity.
isolation of low yields of [PhC(NSiMe3)2]2TiMe2, possibly due
to disproportionation. X-ray structure analysis17 revealed a
monomeric, five-coordinate titanium, as shown in Scheme 1.
The Ti-CH3 bond length (2.120(5) Å) is considerably shorter
than related parameters in (η7-C7H7)(DMPE)TiEt28 and Cp*2-
TiCH2C(CH3)329 (2.211 and 2.231 Å, respectively), presumably
due to a lower coordination number in the former. The methyl
hydrogens were located and refined isotropically, revealing a
close contact between Ti and H(3) at a distance of 2.51(5) Å.
In addition, the acute Ti-C(1)-H(3) angle (104(3)°), compared
to the other Ti-C-H angles (113(3) and 112(3)°), lends
additional support to the existence of an agostic interaction with
the unsaturated metal center.30
Unprecedented reactivity leading to the formation of two
products was discovered when [PhC(NSiMe3)2]2TiCl2 was
reduced by 1% Na/Hg in the presence of TMEDA in toluene.
Following extraction into hexanes and cooling to -40 °C,
yellow crystals of [PhC(NSiMe3)2]2TiNSiMe3 were isolated in
24% yield. The identity of the imido species follows from a
combination of analytical data (MS, combustion analysis) and
NMR spectroscopy.31 Further concentration of the mother
liquor and cooling yielded red-black crystals of a second
product, [PhC(NSiMe3)2]Ti[η2-Me3SiNC(H)Ph][η3-CH2N(Me)-
CH2CH2N(Me)2], in 42% yield. As illustrated in Scheme 1,
Acknowledgment. We thank Prof. R. A. Anderson for helpful
discussions, the Petroleum Research Fund, administered by the ACS,
for funding, and the Sloan Foundation for the award of a fellowship to
J.A.
Supporting Information Available: Full experimental details and
characterization data for all new compounds; details of structure
determinations, including tables of crystal and data collection param-
eters, temperature factor expressions, ORTEP representations, and
positional parameters (48 pages). This material is contained in many
libraries on microfiche, immediately follows this article in the microfilm
version of the journal, can be ordered from the ACS, and can be
downloaded from the Internet; see any current masthead page for
ordering information and Internet access instructions.
JA953449E
(32) Durfee, L. D.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1990,
9, 75.
(33) See supporting information for details.
(34) Further studies to elucidate the solution structure are in progress.
(35) Leung, W. H.; Wu, M. C.; Lau, T. C.; Wong, W. T. Inorg. Chem.
1995, 34, 4271.
(36) For related Ti-mediated reduction of iminoacyls to imines, see ref
32.
(28) Davies, C. E.; Gardiner, I. M.; Green, J. C.; Green, M. L. H.;
Grebenik, P. D.; Mtetwa, V. S. B.; Prout, K. J. Chem. Soc., Dalton Trans.
1985, 669.
(29) Luinstra, G. A.; ten Cate, L. C.; Heeres, H. J.; Pattiasina, J. W.;
Meetsma, A.; Teuben, J. H. Organometallics 1991, 10, 3227.
(30) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem.
1988, 36, 1.
(37) Chatt, J.; Davidson, J. M. J. Chem. Soc. 1965, 843.
(38) Cotton, F. A.; Hunter, D. L.; Frenz, B. A. Inorg. Chim. Acta 1975,
15, 155.
(31) We formulate the imido to be monomeric by analogy with [PhC-
(NSiMe3)2]2TiNCMe3, which has been structurally characterized. Hagadorn,
J. R.; Arnold, J., manuscript in preparation.
(39) For deprotonation reactions involving TMEDA, see: Harder, S.;
Lutz, M. Organometallics 1994, 13, 5173. Kohler, F. H.; Hertkorn, N.;
Blumel, J. Chem. Ber. 1987, 120, 2081.