Reactivity of a tantalum-Lithium alkylidene supported by an anionic triazacyclononane ligand [3]

Full text of DOI:10.1021/ja016143i

8424
J. Am. Chem. Soc. 2001, 123, 8424-8425
able.8 The remaining metal-ligand bond lengths are changed only
slightly from those observed in 1.
Reactivity of a Tantalum-Lithium Alkylidene
Supported by an Anionic Triazacyclononane Ligand
To further investigate its alkylidene-like reactivity, we exposed
1 to carbon monoxide (1 atm) at ambient temperature in toluene.
The solution immediately changed from yellow to orange and,
based on NMR data, a quantitative conversion to a new
compound, 3, was achieved (Scheme 1). The pale orange product
was isolated as an orange oil that solidifies readily upon drying
under vacuum. NMR spectra of this product are qualitatively very
similar to that of 1, except that the CH(alkylidene) resonance shifts
significantly on formation of the new product (from 4.205 ppm
(1H) and 57.4 ppm (13C) in 1 to 5.517 ppm (1H) and 93.5 ppm
(13C) in 3). Additionally, the 7Li NMR resonance shows a
noteworthy change (from 2.986 ppm (1) to 1.874 ppm (3)). Based
on these data, we postulated an insertion of CO at the alkylidene
functionality to form a ketene complex.9 X-ray quality crystals
of 3 were obtained by slowly cooling a saturated octane solution,
and the resultant structure (Scheme 1) confirms the connectivity
in this complex.10 Indeed, the CO has inserted into the Ta-
alkylidene bond to form a ketene ligand, which is coordinated
η2-(C,O) to Ta and η1-O to Li. The Ta-C (2.13 Å) and Ta-O
(2.07 Å) bond distances for 3 parallel a related η2-Zr ketene (Zr-C
) 2.17 Å, Zr-O ) 2.03 Å) and a related η2-Ta acyl complex
(Ta-C ) 2.07 Å, Ta-O ) 2.11 Å),11 while the Li-O interaction
(1.82 Å) is similar to that observed in carboxylates and related
ligands.12 Again, the bridging structure of the iPr2-tacn- is retained,
with virtually no change in the remaining metal-ligand bond
lengths.
Our second goal in these studies has been the formation of
novel heterobimetallic transition metal species, and the alkylidene
1 appeared to be an ideal precursor: we envisioned exchange of
lithium for a transition metal, leading to a bimetallic species with
two metals in close proximity and linked by single-atom bridges.
Accordingly, reaction of 1 with an excess of [RhCl(COD)]2 (COD
) 1,5-cyclooctadiene) in toluene effected the salt metathesis. After
removal of LiCl, the product (4) crystallized from Et2O at low
temperatures. The 1H NMR spectrum of this compound is broad
and quite complex at all temperatures between -80 and 100 °C.
Nevertheless, a combination of elemental analysis and 2-D NMR
techniques at 90 °C suggests formulation of the product as (Me3-
SiCH2)(iPr2-tacn)(Me3SiCHd)Ta(dNAr)Rh-(COD).
Joseph A. R. Schmidt and John Arnold*
Department of Chemistry, UniVersity of California, Berkeley
and Chemical Sciences DiVision, Lawrence Berkeley National
Laboratory, Berkeley, California 94720-1460
ReceiVed May 4, 2001
ReVised Manuscript ReceiVed July 11, 2001
The development of new ancillary ligands to support stoichio-
metric and catalytic reactivity at transition metal and lanthanide
centers is currently an intensively studied area of chemistry. We,1
and others,2 recently described the synthesis and some coordina-
tion chemistry of an unusual anionic triazacyclononane ligand,
iPr2-tacn-. Our interest in this general class of new ligands stems
partially from their formal relationship to the ubiquitous Cp
moiety, i.e., R2tacn- may function as a tridentate, six-electron
donor. In addition, partial ligand dissociation can potentially free
coordination sites for further reactivity, and the ease of introducing
alternate substituents at the amino nitrogens allows for tuning of
steric properties.
We recently showed that alkylation of (iPr2-tacn)Ta(dNAr)-
Cl2 (Ar ) 2,6-iPr2C6H3) with Me3SiCH2Li leads to a unique
tantalum-lithium bridging alkyl, (Me3SiCH2)2(ArNd)Ta(µ-iPr2-
tacn)(µ-CH2SiMe3)Li, that yields the corresponding bridging
alkylidene on thermolysis.3 The unusual nature of the hetero-
bimetallic bridge in this latter compound, coupled with the
postulated involvement of heterobimetallic alkylidenes4 in a
number of important catalytic reactions, led us to study some of
its reaction chemistry. We probed two classes of reactivity: first,
alkylidene-like transformations5 at the tantalum center in an effort
to gauge the effect of the lithium ion; second, use of these
complexes as precursors to a new class of transition metal
heterobimetallic species. As detailed below, we describe two
examples for each reactivity mode.
The alkylidene, (Me3SiCH2)(ArNd)Ta(µ-CHSiMe3)(µ-η1:η3-
iPr2-tacn)Li (1), reacted cleanly with 1 equiv of benzophenone
(see Scheme 1), liberating Me3SiCHdCPh2 and forming the oxo
compound, [(Me3SiCH2)(ArNd)Ta(µ-O)(µ-η1:η3-iPr2-tacn)Li]2
(2). NMR spectra of 2 were qualitatively similar to 1 (with the
notable absence of alkylidene resonances), the elemental analysis
was consistent with an oxo formulation, and 7Li NMR data, along
with a simple flame test, indicated retention of the lithium ion.
To unambiguously determine the solid-state structure, we turned
to X-ray crystallography and grew crystals of 2 by slow
evaporation of a concentrated benzene solution.6 As shown in
Scheme 1, the compound is dimeric, with each oxo ligand
spanning one Li and two Ta atoms. The dimer is held together
by the commonly observed Ta2O2 metallocycle, with normal
Ta-O bond lengths (1.97 and 2.06 Å).7 The Ta-N(imido) bond
length (1.81 Å), though longer than in 1, is otherwise unremark-
Recrystallization of 4 from Et2O yielded high-quality crystals,
the X-ray structure of which shows a Ta-Rh bimetallic compound
in which the alkylidene and imido groups bridge the metal
(7) (a) Abbenhuis, H. C. L.; Feiken, N.; Grove, D. M.; Jastrzebski, J. T.
B. H.; Kooijman, H.; van der Sluis, P.; Smeets, W. J. J.; Spek, A. L.; van
Koten, G. J. Am. Chem. Soc. 1992, 114, 9773. (b) Preuss, F.; Lambing, G.;
Muller-Becker, S. Z. Anorg. Allg. Chem. 1994, 620, 1812.
(8) (a) Royo, P.; Sanchez-Nieves, J.; Pellinghelli, M. A.; Tiripicchio, A. J.
Organomet. Chem. 1998, 563, 15. (b) Chao, Y.-W.; Wexler, P. A.; Wigley,
D. E. Inorg. Chem. 1990, 29, 4592. (c) Williams, D. N.; Mitchell, J. P.; Poole,
A. D.; Siemeling, U.; Clegg, W.; Hockless, D. C. R.; O’Neil, P. A. J. Chem.
Soc., Dalton Trans. 1992, 739.
(9) (a) Neithamer, D. R.; LaPointe, R. E.; Wheeler, R. A.; Richeson, D.
S.; van Duyne, G. D.; Wolczanski, P. T. J. Am. Chem. Soc. 1989, 111, 9056.
(b) Fermin, M. C.; Hneihen, A. S.; Maas, J. J.; Bruno, J. W. Organometallics
1993, 12, 1845. (c) Antinolo, A.; Otero, A.; Fajardo, M.; Gil-Sanz, R.; Herranz,
M. J.; Lopez-Mardomingo, C.; Martin, A.; Gomez-Sal, P. J. Organomet. Chem.
1997, 533, 87.
(10) Crystal data for 3: C33H64N4Si2TaLiO, M ) 776.95, orthorhombic,
space group Pbca (no. 61), a ) 17.1859(3) Å, b ) 19.2859(3) Å, c ) 24.3778-
(4) Å, V ) 8079.9(2) Å3, Z ) 8, Dcalc ) 1.277 g cm-3, µ(Mo KR) ) 28.03
cm-1, F(000) ) 3216.00, T ) 166 K; 3179 independent reflections; R ) 0.025,
Rw ) 0.028, Rall ) 0.060.
(11) (a) Fryzuk, M. D.; Duval, P. B.; Mao, S. S. S. H.; Rettig, S. J.;
Zaworotko, M. J.; MacGillivray, L. R. J. Am. Chem. Soc. 1999, 121, 1707.
(b) Meyer, T. Y.; Garner, L. R.; Baenziger, N. C.; Messerle, L. Inorg. Chem.
1990, 29, 4045.
(12) (a) Hagadorn, J. R.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc.
1998, 120, 13531. (b) Ball, S. C.; Cragg-Hine, I.; Davidson, M. G.; Davies,
R. P.; Edwards, A. J.; Lopez-Solera, I.; Raithby, P. R.; Snaith, R. Angew.
Chem., Int. Ed. Engl. 1995, 34, 921.
(1) (a) Giesbrecht, G. R.; Shafir, A.; Arnold, J. Chem. Commun. 2000,
2135. (b) Giesbrecht, G. R.; Gebauer, A.; Shafir, A.; Arnold, J. J. Chem.
Soc., Dalton Trans. 2000, 4018.
(2) (a) Qian, B.; Henling, L. M.; Peters, J. C. Organometallics 2000, 19,
2805. (b) Fletcher, J. S.; Male, N. A. H.; Wilson, P. J.; Rees, L. H.; Mountford,
P.; Schro¨der, M. J. Chem. Soc., Dalton Trans. 2000, 4130.
(3) Schmidt, J. A. R.; Chmura, S. A.; Arnold, J. Organometallics 2001,
20, 1062.
(4) Wheatley, N.; Kalck, P. Chem. ReV. 1999, 99, 3379.
(5) (a) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98. (b) Nugent, W. A.;
Mayer, J. M. Metal-Ligand Multiple Bonds; John Wiley & Sons: New York,
1988.
(6) Crystal data for 2: C34H60N4OLiSiTa, M ) 756.85, triclinic, space group
P1h (no. 2), a ) 11.5447(2) Å, b ) 13.0903(1) Å, c ) 13.6676(2) Å, R )
63.756(1)°, â ) 84.930(1)°, γ ) 77.214(1)°, V ) 1806.54(5) Å3, Z ) 2, Dcalc
) 1.391 g cm-3, µ(Mo KR) ) 31.01 cm-1, F(000) ) 780.00, T ) 168 K;
5159 independent reflections; R ) 0.021, Rw ) 0.070, Rall ) 0.026.
10.1021/ja016143i CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/02/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8425
Scheme 1
centers.13 The pseudotetrahedral coordination of the tantalum is
completed by a monodentate tacn- ligand and the remaining alkyl
group, while the Rh(I), still bound to COD, resides in its preferred
square-planar environment.14 Although somewhat unexpected, this
species represents an additional structural motif by which we can
form bimetallic complexes from the alkylidene 1. In 4, the Ta-
N(imido) bond distance has lengthened to 1.87 Å, a value
indicative of double-bond character. The Rh-N(imido) bond
length (2.12 Å) is within the range of values observed for dative
rhodium-nitrogen bonds.15 This is the first example of a
monodentate coordination of tacn- to a transition metal and the
bond length (Ta-N ) 1.95 Å) is well within the established range
for tantalum amides.8 As the tantalum remains electronically
unsaturated in 4, the monodentate coordination is likely due to
steric interactions within the molecule.
A different type of new heterobimetallic complex was synthe-
sized by reaction of 1 with [FeCl2(TMEDA)]2 in tetrahydrofuran
at room temperature (Scheme 1). The 1H NMR spectrum of this
complex (5) suggested strong paramagnetism with broad reso-
nances ranging from +40 to -30 ppm; using Evans’ NMR
method,16 the compound was found to have a solution magnetic
susceptibility of µeff ) 4.2 µB, consistent with a high-spin d6 Fe-
(II). The compound was recrystallized from diethyl ether and its
empirical formula, [(Me3SiCH2)(ArNd)Ta(dCHSiMe3)(iPr2-
tacn)FeCl]‚Et2O, was confirmed by elemental analysis.
The X-ray crystal structure of 5 showed the connectivity of
this bimetallic species.17 The iron atom is in a pseudotetrahedral
environment with the bridging tacn- ligand coordinated in a
bidentate fashion (through the two amino nitrogens). The tantalum
remains in a coordination environment quite similar to 1.
Surprisingly, the anionic nitrogen of the tacn- does not appear
to interact with the iron appreciably and exhibits a nearly trigonal
planar environment (sum of angles ) 359(2)°), even though the
Ta-N(amido) bond length (1.99 Å) is typical of a Ta-N single
bond. The Ta-Cbr bond length (2.02 Å) is much shorter than the
Fe-Cbr distance (2.18 Å) and is most likely due to a greater Ta-C
bond order.
In summary, we have shown two novel reactivity pathways
derived from the tantalum-lithium alkylidene species 1. The
compound reacts with small organic molecules in a manner
reminiscent of traditional alkylidenes, although the structural
connectivity of the products is clearly dictated by the presence
of the lithium ion. In addition, 1 reacts with metal chlorides to
form new heterobimetallic compounds containing metals linked
by one-atom bridges. On the basis of electronic requirements of
the metals employed, two motifs are observed: (1) coordination
of the alkylidene and the tacn- ligands and (2) coordination of
the alkylidene and imido ligands. Our future investigations will
include synthesis of further heterobimetallic complexes, as well
as investigation of their cooperative reactivity with an eye for
catalytic reactions. Additionally, we plan to further pursue the
reactivity of 1 with other small organic molecules.
(13) Crystal data for 4: C44H86N4OSi2RhTa, M ) 1027.21, triclinic, space
group P1h (no. 2), a ) 11.0050(2) Å, b ) 13.5425(3) Å, c ) 17.5754(4) Å,
R ) 108.303(1)°, â ) 93.722(1)°, γ ) 95.154(1)°, V ) 2464.74(9) Å3, Z )
2, Dcalc ) 1.384 g cm-3, µ(Mo KR) ) 26.32 cm-1, F(000) ) 1064.00, T )
149 K; 5984 independent reflections; R ) 0.040, Rw ) 0.044, Rall ) 0.056.
(14) (a) Hostetler, M. J.; Butts, M. D.; Bergman, R. G. J. Am. Chem. Soc.
1993, 115, 2743. (b) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic
Chemistry, 5th ed.; Wiley: New York, 1988.
(15) de Bruin, B.; Donners, J. J. J. M.; de Gelder, R.; Smits, J. M. M.;
Gal, A. W. Eur. J. Inorg. Chem. 1998, 401.
(16) Evans, D. F. J. Chem. Soc. 1959, 2005.
(17) Crystal data for 5: C36H74N4ClOSi2TaFe, M ) 907.43, orthorhombic,
space group Pbca (no. 61), a ) 21.2081(2) Å, b ) 14.7430(2) Å, c ) 29.3458-
(6) Å, V ) 9175.6(2) Å3, Z ) 8, Dcalc ) 1.314 g cm-3, µ(Mo KR) ) 28.35
cm-1, F(000) ) 3760.00, T ) 153 K; 2652 independent reflections; R ) 0.033,
Rw ) 0.037, Rall ) 0.107.
Acknowledgment. The authors gratefully acknowledge the NSF for
funding and the NDSEG Fellowship Program for fellowship support
(J.A.R.S.).
Supporting Information Available: Experimental procedures, char-
acterization, and tables of coordinates, anisotropic displacement param-
eters, bond lengths, and angles for 2-5 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA016143I