Reactivity of diazoalkanes with tantalum(v) complexes of a tridentate amido-bis (phenol ate) ligand

Article abstract of DOI:10.1021/om900701n

While a dimethyltantalum(V) complex of the [ONO^cat]^3- ligand([ONO^cat]H₃ = N,N-bis(3,5-di-tert-butyl2-phenol) amine) reacted with diazoalkane to afford products of insertion into the Ta-Me bond, the dichlorotanta

Full text of DOI:10.1021/om900701n

Organometallics 2009, 28, 6629–6631 6629
DOI: 10.1021/om900701n
Reactivity of Diazoalkanes with Tantalum(V) Complexes of a Tridentate
Amido-Bis(phenolate) Ligand
Ryan A. Zarkesh and Alan F. Heyduk*
Department of Chemistry, University of California, Irvine, California 92697
Received August 7, 2009
Summary: While a dimethyltantalum(V) complex of the
[ONO
cat 3- ligand ([ONOcat]H3 = N,N-bis(3,5-di-tert-butyl-
have been able to realize halogen oxidative addition4 and
carbon-carbon reductive elimination5 reactions at Zr(IV)
centers, despite formal d0 electron counts. More recently, we
developed the chemistry of a redox-active tris(amide) ligand
with tantalum(V) and showed that the ligand could act as a
two-electron reservoir, enabling the transfer of a nitrene
group from an organic azide to the tantalum center to form
a new tantalum imide;6 however, sterics seemed to preclude
an analogous reaction to form the tantalum alkylidene from
the corresponding diazoalkane.7 To reduce the steric de-
mands of the redox-active ligand, we turned to the tridentate
]
2-phenol)amine) reacted with diazoalkane to afford products
of insertion into the Ta-Me bond, the dichlorotantalum(V)
complex [ONOcat]TaCl2(OEt2) reacted with 2 equiv of
Ph2CN2 to form the ketazine adduct [ONOcat]TaCl2(η2(N,N0)-
Ph2CdNNdCPh2). When Ph2CN2 was added to the dichlo-
ride in neat styrene, catalytic carbene transfer was observed to
form the corresponding cyclopropane
Interactions of diazoalkanes with transition-metal com-
plexes lead to varied reactivity, depending on the electronic
properties of the diazoalkane and the formal charge on the
metal center.1 Addition of diazoalkanes to electron-rich
transition-metal complexes has proven to be a useful strategy
for the generation of transition-metal carbene function-
alities.2 Studies carried out on the reaction of diazoalkanes
with various copper(I) complexes revealed N2 expulsion
from the diazoalkane to generate a putative copper carbene
that then transfers the carbene fragment to unsaturated
carbon-carbon double bonds, generating cyclopropanes.3
Given the importance of metal valence electrons in stabi-
lizing the carbene and alkylidene resonance structures shown
in eq 1, we hypothesized that a redox-active ligand might
allow us to generate reactive metal-carbene functionalities
for formally d0 transition metal complexes. By incorporating
a redox-active ligand in a d0 metal coordination sphere, we
[ONO ]
cat 3- ligand platform ([ONOcat]H3 = N,N-bis(3,5-di-
tert-butyl-2-phenol)amine),8 which also displays two-elec-
tron reactivity on coordination to tantalum(V).9
The tantalum(V) dimethyl complex [ONOcat]TaMe2 (1)
reacted with diazoalkane to afford the product of 1,1-inser-
tion into the tantalum-methyl bond, as has been observed
for other d0 metal alkyls.10 Hence, the addition of 1 equiv of
Ph2CN2 to 1 resulted in the formation of [ONOcat]TaMe-
{η2(N,N0)-NMeNdCPh2} (2) in nearly quantitative yields,
as shown in Scheme 1. Compound 2 was characterized
by NMR and IR spectroscopy. Equal-intensity singlet reso-
1
nances at 2.78 and 1.30 ppm in the H NMR spectrum are
indicative of N-CH3 and Ta-CH3 groups, respectively. The
IR spectrum shows a diagnostic stretch for the hydrazonato
ligand at 1164 cm-1. When 2 equiv of Ph2CN2 was added
to 1, the double-insertion product [ONOcat]Ta{η2(N,N0)-
NMeNdCPh2}2 (3) was obtained as a dark red microcrystal-
*To whom correspondence should be addressed. E-mail: aheyduk@
uci.edu.
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integrating as six protons was consistent with two N-CH3
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r
2009 American Chemical Society
Published on Web 11/10/2009
pubs.acs.org/Organometallics

6630 Organometallics, Vol. 28, No. 23, 2009
Zarkesh and Heyduk
Scheme 1
Figure 1. ORTEP drawing of [ONOcat]TaCl2(η2-Ph2CdNNd
CPh2) (5). Ellipsoids are shown at the 50% probability level.
A toluene molecule has been omitted for clarity.
affording the free ketazine and [ONOcat]TaCl2(py) (4b, L =py),
which is unreactive toward additional diazoalkane. Alter-
natively, excess Ph2CN2 also displaced the ketazine in 5, allow-
ing for catalytic turnover. In a typical experiment, addition of
30 equiv of Ph2CN2 to a benzene solution of 5 resulted in effer-
vescence, indicative of the release of N2 from the diazoalkane.
To circumvent the insertion of diazoalkane into the
Ta-CH3 bond, reactivity studies were conducted between
various diazoalkanes and the dihalide complex [ONOcat]-
TaCl2(OEt2) (4a). The addition of 2 equiv of Ph2CN2 to a
dark red solution of 4a resulted in the liberation of 1 equiv of
N2 gas and the formation of a green solution from which
[ONOcat]TaCl2(η2-Ph2CdNNdCPh2) (5) was isolated as
a green solid in 76% recrystallized yield (Scheme 1). Sub-
stoichiometric ratios of Ph2CN2 to 4a afforded mixtures
of 5 and unreacted 4a with no evidence for an intermediate
species. Complex 5 also was prepared by the addition of
1 equiv of Ph2CdNNdCPh2 to a solution of 4a.
1
Monitoring reaction mixtures by H NMR spectroscopy re-
vealed complete consumption of Ph2CN2 under these condi-
tions; however, the reaction is strongly inhibited by the ketazine
product. The addition of excess ketazine to the reaction effec-
tively stopped catalytic turnover. Similarly, reactions carried out
with more than a 30:1 ratio of Ph2CN2 to 5 stalled without
complete consumption of the diazoalkane. Preliminary kinetics
experiments suggest a complicated rate law for the conversion of
Ph2CN2 to Ph2CdNNdCPh2; however, the coupling reaction
appears to be general, as reactions carried out with PhHCN2,
diazofluorenone, (Me3Si)HCN2, and ethyl diazoacetate resulted
in similar reaction times and quantitative conversions to the
corresponding ketazine products.13
The reaction to form a ketazine from 2 equiv of diazoalkane
can be considered a carbene transfer from one diazoalkane to
another with concomitant loss of 1 equiv of N2. To test this
hypothesis, 5 was treated with excess Ph2CN2 in neat styrene as
the solvent. As shown in the Supporting Information, after
6 h at 300 K, complete consumption of the diazoalkane was
accompanied by 78% conversion to 1,1,2-triphenylcyclopro-
pane and only 22% to Ph2CdNNdCPh2. Cooling the reaction
mixture to 273 K effectively stopped all consumption of
Ph2CN2, while heating itn to 338 K resulted in a decreased yield
of the cyclopropane product and a corresponding increase in
ketazine yield. Reactions carried out in mixtures of styrene and
benzene also resulted in decreased yields of the cyclopropane
and increased yields of ketazine. Results similar to those for
the cyclopropanation of styrene were obtained for diazofluore-
none and (Me3Si)HCN2; however, all attempts to effect carbene
transfer to less reactive olefins such as cyclohexene, 1-vinyl-
cyclohexene, and R-methylstyrene failed, resulting only in the
formation of the corresponding ketazine.
X-ray diffraction studies on a single crystal of 5 showed a
tantalum(V) metal coordinated to a reduced [ONOcat 3-
]
ligand, with the ketazine ligand acting as a neutral donor
ligand. As shown in Figure 1, 5 adopts a pseudo-pentagonal-
bipyramidal structure with axial chloride ligands and the
ketazine and [ONOcat 3-
ligands occupying the equatorial
]
plane. Bond distances within the [ONOcat]Ta framework are
consistent with the fully reduced or catecholate form of the
ligand.9 Specifically, short Ta-O and Ta-N bond distances
˚
of 1.94 and 2.02 A, respectively, are consistent with tanta-
lum-phenoxide and tantalum-anilide interactions. Simi-
˚
˚
larly, long C-O (1.36 A) and C-N (1.41 A) distances and
˚
aromatic C-C distances (1.40 A average) argue against
oxidation of the ligand to the semiquinonate ([ONOsq• 2-
] )
or quinonate ([ONOq]-) oxidation states. The Ta-N dis-
tance for the ketazine ligand is long at 2.33 A, consistent with
˚
a dative NfTa interaction, and the N-N distance is length-
ened only slightly from that of the free ketazine.11 The CdN
˚
distance of 1.30 A is consistent with a formal double bond, as is
the CdN stretch at 1525 cm-1 in the IR spectrum and the imine
carbon resonance at 172.5 ppm in the 13C NMR spectrum.
Complex 5 acts as a precatalyst for the formation of ketazine
from 2 equiv of diazoalkane.12 The coordinated ketazine in 5
could be displaced by strong donor ligands such as pyridine,
(13) (a) Wiebe, R.; Gaddy, V. L. J. Am. Chem. Soc. 1932, 54, 3628–
3641. (b) Beringer, F. M.; Farr, J. A.; Sands, S. J. Am. Chem. Soc. 1953, 75,
3984–3987. (c) Park, S. B.; Hishiyama, H.; Itoh, Y.; Itoh, K. J. Chem. Soc.,
Chem. Commun. 1994, 1315–1316. (d) Curtius, T.; Lang, H. J. Prakt.
Chem. 1891, 44, 544–568.
(11) Wolstenholme, D. J.; Cameron, T. S. J. Phys. Chem. A 2006, 110,
9870–9878.
(12) Bertani, R.; Biaziolo, M.; Darini, K.; Michelin, R. A.; Mozzon,
M.; Fabiano, V.; Zanotto, L. J. Organomet. Chem. 2002, 642, 32–39.

Communication
Organometallics, Vol. 28, No. 23, 2009 6631
While 4a and 5 both function as precatalysts for the cyclo-
propanation of an activated olefin such as styrene, less reactive
substrates failed to give the desired carbene transfer product.
Moreover, control reactions with Rothwell’s tantalum(V) com-
plex, TaCl2(OR)3 (R = 2,6-diphenylphenoxide),14 showed
reactivity analogous with that of diazoalkanes, yielding the
ketazine product in benzene or the cyclopropane product in
neat styrene.15 The modest reactivity displayed by 4a and 5,
which is paralleled by TaCl2(OR)3, suggests that the redox-
carbene species [ONOq]TaCl2(dCPh2). Oxidation of the
ONO ligand platform from the [ONOcat 3-
form to the
]
[ONOq]- form results in a stark color change from burgundy
to green,9 and during the reactions of 2a with the various
diazoalkanes studied here, no spectroscopic evidence for
the formation of a green [ONOq]Ta species was obtained.
Instead of a redox mechanism for diazoalkane activation,
coordination of the diazoalkane to 2a should enhance
the electrophilic character of the diazoalkane, enabling
nucleophilic attack by a substrate: either another diazo-
alkane or an activated olefin. That a diazoalkane oxida-
tion of 2a could be thermodynamically unfavorable is en-
tirely consistent with our previous report of PhNdNPh
reductive elimination from the dimer of [ONOq]TaCl2-
(dNPh)9 and highlights the differing reducing powers of
active [ONO ]
cat 3- ligand in 4a and 5 is not playing a key role to
enable the formation of a TadCR2 species and subsequent
carbene transfer to substrate. Instead, it is likely that the
tantalum center is acting as a simple Lewis acid to activate a
diazoalkane for nucleophilic attack according to the simple
mechanism shown in eq 2. A rate-determining bimolecular
reaction between a tantalum-diazoalkane adduct and free
diazoalkane would lead to ketazine formation with liberation
of N2. Free ketazine would strongly inhibit this bimolecular reac-
tion by shifting the pre-equilibrium away from the tantalum-
diazoalkane adduct.
the redox-active [ONOcat 3- and [NNNcat 3-
] ligand plat-
]
forms. In the tantalum chemistry of the latter ligand, the
reaction with diazoalkane gave a product that was best
described as [NNNq]TaCl2(dNNdCPh2),6 with an oxidized
redox-active ligand and a reduced hydrazido group. In effect,
the highly reducing [NNN ]
cat 3- ligand quenched the electro-
philic character of the diazoalkane, shutting it down for
further reactivity. In this way it is intriguing to consider how
strongly the electronic properties of the redox-active ligand
modulate the reactivity at a metal center, and this points
toward a strategy for tuning the metal’s reactivity with a
given substrate.
Acknowledgment. This work was supported by the
NSF-CAREER program (Grant NO. CHE-0645685).
A.F.H. is an Alfred P. Sloan Foundation Fellow and a
Camille-Dreyfus Teacher-Scholar.
This interpretation suggests that the diazoalkanes do not
oxidize the [ONO ]
cat 3- ligand to form the putative tantalum
Supporting Information Available: Text and tables giving
detailed experimental procedures and cyclopropanation yields
and a CIF file giving complete crystallographic details for 5.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(14) Chestnut, R. W.; Durfee, L. D.; Fanwick, P. E.; Rothwell, I. P.;
Folting, K.; Huffman, J. C. Polyhedron 1987, 6, 2019–2026.
(15) While TaCl5 did convert Ph2CN2 to Ph2CdNNdCPh2, it did not
afford the cyclopropanation product in styrene.