The niobaziridinee-hydride functional group: Synthesis and divergent reactivity

Article abstract of DOI:10.1021/ja028446y

A multistep synthetic strategy enables the isolation of the niobaziridine-hydride complex Nb(H)(η²-^tBu(H)C=NAr)(N[Np]Ar)₂ (1, Np = neopentyl, Ar = 3,5-C₆H₃Me₂), which functions as a reactive synthon for its tautomer, the three-coordinate, trisamide species Nb(N[Np]Ar)₃ (2). Treatment of 1 with various small molecules has demonstrated its capacity to effect two-electron reduction chemistry. Most noteworthy is the reaction between 1 and elemental phosphorus (P₄), providing in high yield the bridging diphosphide complex (μ₂:η²,η²-P₂)[Nb(N[Np]Ar)₃]₂. However, unsaturated organic functionality including nitriles and aldehydes can insert into the Nb-H bond of 1, leaving the niobaziridine ring intact, thus demonstrating that dual pathways of reactivity are available to the niobaziridine-hydride functional group. Copyright

Full text of DOI:10.1021/ja028446y

Published on Web 03/14/2003
The Niobaziridine-Hydride Functional Group: Synthesis and Divergent
Reactivity
Joshua S. Figueroa and Christopher C. Cummins*
Massachusetts Institute of Technology, Room 2-227, 77 Massachusetts AVenue, Cambridge, Massachusetts 02139
Received September 6, 2002; E-mail: ccummins@mit.edu
Interest in the chemistry of three-coordinate trivalent niobium
complexes supported by hard, anionic ligands has arisen from
reports of their ability to mediate remarkable reactions including
C-H1 and C-N2 bond cleavages and atom transfer reactions.3
However, the high inherent reactivity of these complexes has made
their isolation a challenging endeavor, with methodologies for
stabilizing them having only recently become available.1-4 Previ-
ously, we described the ability of the N-isopropylanilide ligand
(N[iPr]Ar, Ar ) 3,5-Me2C6H3) to mask reactive, three-coordinate
molybdenum trisamide complexes in a tautomeric molybdaziridine-
hydride form via reversible â-H elimination.5 Fascinated by an
intriguing report of dinitrogen activation by a niobium trisamide
complex,6 chemistry we deemed explicable in terms of a nioba-
Figure 1. ORTEP diagram of 1 with 30% probability ellipsoids.
ziridine-hydride intermediate, we adopted the latter as a synthetic
target. Utilization of our N-isopropylanilide ligand toward this goal
resulted at best in a borane adduct of the desired target.7 Now we
show that a new ligand variant, namely, the corresponding
N-neopentyl version, permits isolation of the desired niobaziridine-
hydride Nb(H)(η2-tBu(H)CdNAr)(N[Np]Ar)2 (1, Np ) neopentyl),
which serves as a reactive synthon for the trivalent niobium
trisamide Nb(N[Np]Ar)3 (2).
A multistep synthetic strategy provided the Nb(V) diiodide
complex Nb(I)2(N[Np]Ar)3 (3) as an orange powder after diphe-
nylacetylene deprotection of the corresponding η2-alkyne complex
with elemental iodine (25% yield over four steps starting from
NbCl3(DME); see Supporting Information). Treatment of a thawing
THF solution of 3 with 1.25 equiv of Mg(THF)3(anthracene) leads
to the formation of diamagnetic 1 in 45-50% yield as orange blocks
after recrystallization.8 The 1H NMR spectrum of 1, consistent with
its solid-state structure (Figure 1), indicates C1 symmetry due to
the formation of a stereogenic center at the carbon atom of the
niobaziridine ring. A broad resonance observed at 9.24 ppm is
assigned to the Nb-H moiety,9 as is a stretch of moderate intensity
at 1701 cm-1 in the infrared spectrum of 1.
Whereas putative 2 was not observed directly, it was hoped that
1 would act as a functional equivalent of 2 when treated with
appropriate substrates. Depicted in Scheme 1 are examples of 2e
reductions effected by complex 1, providing in high yields the
crystalline species 5-10. Worthy of mention is the deoxygenation
of triphenylphosphine oxide by 1 leading to the oxo complex 5,
testifying to the ability of 1 to serve as a source of the potent two-
electron reductant 2. In addition, 1 also proves to be a convienent
route to the four-coordinate, terminal chalcogenide complexes10 6-8
when treated with an appropriate chalcogen atom source. Complex
1 can engage in incomplete atom transfer reactions as well.11 Thus,
12a
upon addition of the terminal phosphide complex PMo(N[iPr]Ar)3
to C6D6 solutions of 1, quantitative formation of the diamagnetic,
hetero-dinuclear bridging phosphide complex (Ar[Np]N)3Nb(µ-P)-
Mo(N[iPr]Ar)3 (9) is observed and further demonstrates the ability
of 1 to reduce a diverse host of substrates.
In an additional display of reductive potency, 1 reacts with white
phosphorus (P4) to afford the dark green, bridging diphosphide
complex (µ2:η2,η2-P2)[Nb(N[Np]Ar)3]2 (10) quantitatively as as-
sayed by 1H and 31P NMR. The reaction can be viewed as a 2e per
Nb reduction of the P4 tetrahedron and is reminiscent of the mild
activation of P4 reported for the Mo(III) trisamide system.12
Formation of complex 10 from 1 and P4 is selective inasmuch as
it is the only new product obtained when either an excess or
deficiency of P4 is used. The molecular structure of 10 (Figure 2)
shows the P2 unit to be disposed in a side-on and butterfly
conformation relative to the Nb-Nb vector (Nb-P-P-Nb dihedral
angle ) 135.61(5)°). The Nb-Nb distance of 4.2052(8) Å obviates
the existence of a M-M bond and the observed diamagnetism of
10 substantiates the formulation of the P2 unit as a (4-) ligand, a
rarely documented functionality in early transition metal chemis-
try.13 Furthermore, while N2 is known to bridge two Nb(NCy2)3
(Cy ) cyclohexyl) fragments in a linear fashion,6 the P2 bridge in
complex 10 provides a striking contrast that is reminiscent of the
structural chemistry of elemental nitrogen and phosphorus.14
With the ability of 1 to act as a source of 2 established, reactions
with organic substrates were also probed. Treatment of 1 with 1.0
In an effort to observe reversible â-H elimination, complex 1
1
was studied by variable temperature H NMR. The spectrum of 1
remained static up to 80 °C, at which temperature a new Cs
1
symmetric product appeared. H and 13C NMR data indicate the
product of thermolysis to be the neopentylimido complex, Nb-
(NNp)(Ar)(N[Np]Ar)2 (4), formally the product of C-N bond
oxidative addition by putative intermediate 2, or alternatively the
product of 1,2-aryl migration (N f Nb) via intermediate 2. It is
noteworthy that although 2 is also the likely precursor to 1, crude
reaction mixtures assayed during the synthesis of 1 do not indicate
any formation of 4. However, C6D6 solutions of pure 1 at room
temperature do become enriched with 4 over time (t1/2 ) ca. 4.5
d), pointing to a slow conversion of 1 to 2 in solution. Furthermore,
thermal rearrangement of 1 proceeds quantitatively to 4, which
retains its integrity at elevated temperatures. Thus, we hypothesize
that 1 is a kinetic product while isomer 4 is the thermodynamic
product.
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J. AM. CHEM. SOC. 2003, 125, 4020-4021
10.1021/ja028446y CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
goal of further exploiting the ability of 1 to serve as a powerful
two-electron reductant, are currently underway.
Acknowledgment. We thank the National Science Foundation
(CHE-9988806) for financial support. The authors also thank P. J.
Chirik for stimulating our interest in the N-neopentyl ligand. J.S.F.
thanks F. H. Stephens for a generous gift of PMo(N[iPr]Ar)3 and
W. M. Davis and W. Bu for assistance with X-ray crystallography.
Supporting Information Available: Full experimental and spec-
troscopic details for all new compounds and X-ray structural data for
complexes 1 and 5-12 (PDF and CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 2. ORTEP diagram of 10 with 30% probability ellipsoids.
References
equiv of mesitylnitrile (MesCN, Mes ) 2,4,6-Me3C6H2) leads
exclusively to the η2-nitrile complex (η2-MesCN)Nb(N[Np]Ar)3
(11), again demonstrating the accessibility of three-coordinate 2.
Crystallographic characterization confirmed the formulation of 11
as an η2-nitrile complex possessing an elongated nitrile N-C bond
of 1.258(4) Å, consistent with significant π-back-donation from a
reducing metal center.15
In contrast, when 1 is treated with stoichiometric tert-butyl nitrile
(tBuCN), sole formation of the nitrile insertion product 12 is
obtained in which the niobaziridine ring remains intact. Likewise,
treatment of 1 with benzaldehyde (PhCHO) leads exclusively to
the benzyloxide complex 13, highlighting the divergent reactivity
accessible to the niobaziridine-hydride functional group. Indeed,
insertion chemistry of this type has been encountered previously
upon borane abstraction of our BH3 adduct in the presence of
benzophenone.7 Extended heating of complexes 12 and 13 did not
produce η2-bound tautomers analogous to 11,16 an observation
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are available to 1 depending on the substrate employed.
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tion and subsequent small molecule activation chemistry,6 with the
present work confirming this hypothesis. However, and interest-
ingly, no reaction of 1 with dinitrogen has yet been observed under
ambient conditions. Whereas the molybdaziridine-hydride func-
tionality has been shown conclusively to resist insertion into its
Mo-H bond,5 it is now shown for niobium that insertion pathways
are accessible upon addition of certain unsaturated substrates.
Investigations into the factors governing this dichotomy, with the
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Craig, P. C.; Pregosin, P. S.; Ruegger, H. Magn. Reson. Chem. 1993, 31,
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(13) (a) Scherer, O. J. Acc. Chem. Res. 1999, 32, 751. (b) Fermin, M. C.; Ho,
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(16) Thermolysis of 12 does proceed cleanly to a new cyclized product which
will be the focus of an upcoming publication. Furthermore, complex 11
does not rearrange to an analogue of 12 when heated (C6D6, 100 °C, 3
d).
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