Niobaziridine hydrides

Article abstract of DOI:10.1021/om100522p

Presented herein are synthetic, structural, and reactivity studies delineating the characteristics of the niobaziridine hydride functional group as it pertains to the stabilization of trisanilide niobium complexes of the type Nb(N[R]Ar)₃ (1^R, Ar = 3,5-Me₂C ₆H₃). Utilization of the N-isopropyl anilide ligand, N[i-Pr]Ar, results in the niobaziridine hydride dimer [Nb(H)(η²- Me₂C=NAr)(N[i-Pr]Ar)₂]₂ ([2^i-Pr-H] ₂). Dimer [2^i-Pr-H]₂ is thermally unstable at room temperature and decomposes via ortho-metalation and i-Pr radical ejection to a species containing a Nb-Nb bond. The ligand variant N[Np]Ar (Np = neopentyl) provides the room-temperature-stable niobaziridine hydride monomer Nb(H)(η²-t-Bu(H)C=NAr)(N[Np]Ar)₂ (2^Np-H). Thermal decomposition of 2^Np-H at elevated temperature (75 °C) provides the neopentyl imido complex Nb(NNp)(Ar)(N[Np]Ar)₂ (5 ^Np). H/D isotopic labeling studies provide evidence for reversible β-H elimination interconverting 2^Np-H and its trisanilide tautomer [Nb(N[Np]Ar)₃] (1^Np), with the latter thereby implicated as an intermediate during the 2^Np-H → 5^Np conversion. Reactivity studies between 2^Np-H and certain small-molecule substrates confirm that the niobaziridine hydride group can effectively mask a reactive d² Nb(III) trisanilide center. However, 2^Np-H exhibits insertion chemistry when treated with a variety of unsaturated organic substrates, thus demonstrating a pronounced tendency to additionally function as a Lewis acidic, early transition metal hydride species. A general mechanism accounting for the divergent reactivity of 2^Np-H is proposed. Niobaziridine hydride complexes derived from the amido ligands N[CH₂Ad]Ar, N[Cy]Ar, and NCy₂ (Ad = 1-adamantyl, Cy = cyclohexyl) are also presented, and their thermal behavior and reaction chemistry are compared with those of 2^Np-H. In addition, the radical anion of 2^Np-H is reported and compared with the neutral d ¹ molybdaziridine hydride complex Mo(H)(η²-Me ₂C=NAr)(N[i-Pr]Ar)₂ (3^i-Pr-H), to which it is isoelectronic.

Full text of DOI:10.1021/om100522p

Organometallics 2010, 29, 5215–5229 5215
DOI: 10.1021/om100522p
Niobaziridine Hydrides†
Joshua S. Figueroa, Nicholas A. Piro, Daniel J. Mindiola, Michael G. Fickes, and
Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, Room 6-435, 77 Massachusetts Avenue,
Cambridge, Massachusetts 02139-4307, United States
Received May 27, 2010
Presented herein are synthetic, structural, and reactivity studies delineating the characteristics of the
niobaziridine hydride functional group as it pertains to the stabilization of trisanilide niobium complexes of
the type Nb(N[R]Ar)3 (1R, Ar = 3,5-Me2C6H3). Utilization of the N-isopropyl anilide ligand, N[i-Pr]Ar,
results in the niobaziridine hydride dimer [Nb(H)(η2-Me2CdNAr)(N[i-Pr]Ar)2]2 ([2i-Pr-H]2). Dimer [2i-Pr
-
H]2 is thermally unstable at room temperature and decomposes via ortho-metalation and i-Pr radical
ejection to a species containing a Nb-Nb bond. The ligand variant N[Np]Ar (Np = neopentyl) provides
the room-temperature-stable niobaziridine hydride monomer Nb(H)(η2-t-Bu(H)CdNAr)(N[Np]Ar)2
(2Np-H). Thermal decomposition of 2Np-H at elevated temperature (75 °C) provides the neopentyl imido
complex Nb(NNp)(Ar)(N[Np]Ar)2 (5Np). H/D isotopic labeling studies provide evidence for reversible
β-H elimination interconverting 2Np-H and its trisanilide tautomer [Nb(N[Np]Ar)3] (1Np), with the latter
thereby implicated as an intermediate during the 2Np-H f 5Np conversion. Reactivity studies between 2Np
-
H and certain small-molecule substrates confirm that the niobaziridine hydride group can effectively mask
a reactive d2 Nb(III) trisanilide center. However, 2Np-H exhibits insertion chemistry when treated with a
variety of unsaturated organic substrates, thus demonstrating a pronounced tendency to additionally
function as a Lewis acidic, early transition metal hydride species. A general mechanism accounting for the
divergent reactivity of 2Np-H is proposed. Niobaziridine hydride complexes derived from the amido
ligands N[CH2Ad]Ar, N[Cy]Ar, and NCy2 (Ad = 1-adamantyl, Cy = cyclohexyl) are also presented,
and their thermal behavior and reaction chemistry are compared with those of 2Np-H. In addition, the
radical anion of 2Np-H is reported and compared with the neutral d1 molybdaziridine hydride complex
Mo(H)(η2-Me2CdNAr)(N[i-Pr]Ar)2 (3i-Pr-H), to which it is isoelectronic.
Introduction
profiles for small molecule substrates such as N2,4-7 P4,3,8,9
and CO2.10 To date, isolable examples of three-coordinate
M(N[R]Ar)3 complexes are known only for M = Ti,11-13
V,14-17 Cr,18,19 and Mo;1 also, a uranium analogue has been
isolated as its THF adduct.20 While the reactivity of trivalent
M(N[R]Ar)3 complexes of different periodic groups depends
Our interest in the chemistry of early metal trisanilide
complexes, as exemplified by d3 Mo(N[t-Bu]Ar)3,1-3 stems
from their demonstrated ability to mediate novel activation
† Part of the Dietmar Seyferth Festschrift.
*To whom correspondence should be addressed. E-mail: ccummins@
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7321.
mit.edu.
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Cummins, C. C.; Kryatov, S. V.; Rybak-Akimova, E. V.; Capps, K. B.;
Hoff, C. D.; Haar, C. M.; Nolan, S. P. J. Am. Chem. Soc. 2001, 123,
7271–7286.
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(5) Laplaza, C. E.; Johnson, A. R.; Cummins, C. C. J. Am. Chem.
Soc. 1996, 118, 709–710.
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Wilcox, D. E. J. Am. Chem. Soc. 1995, 117, 2110–2111.
(12) Peters, J. C.; Johnson, A. R.; Odom, A. L.; Wanandi, P. W.;
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(14) Ruppa, K. B. P.; Desmangles, N.; Gambarotta, S.; Yap, G.;
Rheingold, A. L. Inorg. Chem. 1997, 36, 1194–1197.
ꢀ
ꢀ
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ꢀ
ꢀ
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€
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(9) Stephens, F. H.; Johnson, M. J. A.; Cummins, C. C.; Kryatova,
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r
2010 American Chemical Society
Published on Web 10/15/2010
pubs.acs.org/Organometallics

5216 Organometallics, Vol. 29, No. 21, 2010
Figueroa et al.
Scheme 1
Unlike prototypical Mo(N[t-Bu]Ar)3, Nb(N[Np]Ar)3 does
not exist as an isolable or kinetically persistent three-coordi-
nate monomer. Our ability to employ the Nb(N[Np]Ar)3
fragment for element activation chemistry stems from the
finding that it can be effectively masked as a tautomeric nio-
baziridine hydride species, namely, Nb(H)(η2-t-Bu(H)Cd
NAr)(N[Np]Ar)2 (2Np-H, Scheme 1).25,34
Despite its cyclometalated nature, niobaziridine hydride
Np-H serves as a source of three-coordinate 1Np by virtue of
2
reversible β-H elimination of the N[Np]Ar ligand. In general,
tautomerization via reversible cyclometalation has emerged
as a powerful and effective method for the stabilization of
highly reactive, low-valent metal centers otherwise unavail-
able in free form. The recent work of Chirik,35,36 and earlier
studies by Bercaw,37,38 has highlighted this fact in the context
of cyclopentadienyl, alkyl, and aryl complexes of the early
transition metals.
For three-coordinate M(N[R]Ar)3 species, a similar mask-
ing strategy was reported by us with respect to the d1 molyb-
daziridine hydride complex Mo(H)(η2-Me2CdNAr)(N[i-
Pr]Ar)2 (3i-Pr-H, Scheme 1).39 The latter functions as a synthon
for three-coordinate Mo(N[i-Pr]Ar)3, which is a less encum-
bered analogue of Mo(N[t-Bu]Ar)3,1 and accordingly effects
N2 cleavage40,41 among other small-molecule activation
processes.39,42,43 The enthalpy of reaction to open the molyb-
daziridine ring of 3i-Pr-H was measured as 4.5 ( 0.3 kcal/mol
by solution-phase calorimetry using thermodynamic paths
related to nitrile binding studies.44 Most interestingly however,
greatly on the dn count, reactivity differences displayed by
metals of the same group are governed by a less intuitive set of
properties.21-24 This point is highlighted by Mo(N[t-Bu]Ar)3,
which readily effects the reductive cleavage of the dinitrogen
molecule,4-7 while its Cr analogue, Cr(N[t-Bu]Ar)3, is unreac-
tive toward N2 under similar conditions.18 Accordingly, the
discovery of new paradigms in small-molecule reactivity stem-
ming from M(N[R]Ar)3 complexes has fueled our quest to
expand the library of known variants throughout the early
transition elements.
3
i-Pr-H functions exclusively as a source of Mo(N[i-Pr]Ar)3
In the last several years we have reported on the chemistry
accessible to the d2 niobium trisanilide fragment Nb(N[Np]Ar)3
(1Np, Np = neopentyl, Scheme 1), with specific attention being
paid to its ability to activate the elemental forms of nitrogen
and phosphorus.25-29 These studies have resulted in a
new phosphaalkyne synthesis27 featuring metathetical CtP
triple-bond formation in a manner reminiscent of alkyne
metathesis by Schrock-type alkylidynes.30 In addition, we
have developed an analogous organonitrile synthesis where-
by the nitrogen atom of the new newly formed NtC triple
bond finds its origin in molecular nitrogen (N2).29 Most
recently, the Nb(N[Np]Ar)3 platform has allowed for P2
transfer reactivity under mild solution-phase conditions,
thus opening the door for utilization of this reactive diatomic
unit in conventional synthesis.31-33
and does not display reaction chemistry indicative of its metal-
hydride formulation. This exclusivity is particularly impor-
tant with respect to reactions with unsaturated organic sub-
strates. Accordingly, ketones,39 nitriles,42 terminal alkynes,43
and isocyanides44 have all been shown to elicit C-H reductive
elimination44 in 3i-Pr-H, while insertion chemistry into its
Mo-H bond has not been observed. Thus, the metallaziridine
hydride functionality serves as an exceptionally reliable mask
for a reactive three-coordinate trisanilide species in the case of
molybdenum.
As communicated previously,34 the reactivity of niobazir-
idine hydride 2Np-H with unsaturated organic functionality
contrasts sharply with that of its molybdaziridine hydride
analogue. While effectively serving as a source of 1Np in
certain reactions, the Nb-H unit in 2Np-H has also been
susceptible to insertion chemistry. Seeking to understand the
(21) Kuiper, D. S.; Wolczanski, P. T.; Lobkovsky, E. B.; Cundari,
T. R. Inorg. Chem. 2008, 47, 10542–10553.
(22) Kuiper, D. S.; Douthwaite, R. E.; Mayol, A.-R.; Wolczanski,
P. T.; Lobkovsky, E. B.; Cundari, T. R.; Lam, O. P.; Meyer, K. Inorg.
Chem. 2008, 47, 7139–7153.
(34) Figueroa, J. S.; Cummins, C. C. J. Am. Chem. Soc. 2003, 125,
4020–4021.
(35) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Organometallics 2003,
22, 2797–2805.
(23) Kuiper, D. S.; Wolczanski, P. T.; Lobkovsky, E. B.; Cundari,
T. R. Inorg. Chem. 2008, 47, 10542–10553.
(24) Hirsekorn, K. F.; Hulley, E. B; Wolczanski, P. T.; Cundari, T. R.
J. Am. Chem. Soc. 2008, 130, 1183–1196.
(36) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003,
125, 2241–2251.
(37) Parkin, G.; Bunel, E.; Burger, B. J.; Trimmer, M. S.; Van Asselt,
(25) Figueroa, J. S.; Cummins, C. C. Dalton Trans. 2006, 2161–2168.
(26) Figueroa, J. S.; Cummins, C. C. Angew. Chem., Int. Ed. 2004, 43,
984–988.
(27) Figueroa, J. S.; Cummins, C. C. J. Am. Chem. Soc. 2004, 126,
13916–13917.
A.; Bercaw, J. E. J. Mol. Catal. 1987, 41, 21–39.
(38) Van Asselt, A.; Trimmer, M. S.; Henling, L. M.; Bercaw, J. E.
J. Am. Chem. Soc. 1988, 110, 8254–8255.
(39) Tsai, Y.-C.; Johnson, M. J. A.; Mindiola, D. J.; Cummins, C. C.
J. Am. Chem. Soc. 1999, 121, 10426–10427.
(28) Figueroa, J. S.; Cummins, C. C. Angew. Chem., Int. Ed. 2005, 44,
4592–4596.
(29) Figueroa, J. S.; Piro, N. A.; Clough, C. R.; Cummins, C. C.
(40) Cherry, J.-P. F.; Stephens, F. H.; Johnson, M. J. A.; Diaconescu,
P. L.; Cummins, C. C. Inorg. Chem. 2001, 40, 6860–6862.
(41) Tsai, Y. C.; Cummins, C. C. Inorg. Chim. Acta 2003, 345, 63–69.
(42) Tsai, Y.-C.; Stephens, F. H.; Meyer, K.;Mendiratta, A.; Gheorghiu,
M. D.; Cummins, C. C. Organometallics 2003, 22, 2902–2913.
(43) Tsai, Y.-C.; Diaconescu, P. L.; Cummins, C. C. Organometallics
2000, 19, 5260–5262.
J. Am. Chem. Soc. 2006, 128, 940–950.
(30) Schrock, R. R. Chem. Rev. 2002, 102, 145–180.
(31) Piro, N. A.; Figueroa, J. S.; McKellar, J. T. Science 2006, 313,
1276–1279.
(32) Piro, N. A.; Cummins, C. C. Inorg. Chem. 2007, 46, 7387–7393.
(33) Piro, N. A.; Cummins, C. C. J. Am. Chem. Soc. 2008, 130, 9524–
9535.
(44) Stephens, F. H.; Figueroa, J. S.; Cummins, C. C.; Kryatova,
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C. D. Organometallics 2004, 23, 3126–3138.

Article
Organometallics, Vol. 29, No. 21, 2010 5217
Scheme 2
reactivity profile of 2Np-H, with the aim of directing its
reactivity toward that of the powerful two-electron reduc-
tant 1Np, we have studied in greater detail the reaction
chemistry of this system with a diverse host of substrates.
Accordingly, in this contribution we present a more com-
plete picture of the chemistry and solution behavior of 2Np-H
and several additional derivatives containing the niobazir-
idine hydride functionality.
Figure 1. Molecular structure of [(μ-NAr)Nb(N[t-Bu]Ar)2]2.
Selected bond distances (A) and angles (deg): Nb(1)-Nb(1A) =
2.7594(13); Nb(1)-N(1)=2.041(6); Nb(1)-N(2)=2.022(5); Nb1-
N(3) = 1.977(6); Nb(1)-Nb(3A) = 2.023(6); Nb(1)-N(3)-
Nb(1A)=87.2(2); N(3)-Nb(1)-N(3A)=92.8(2); N(1)-Nb(1)-
N(3)=114.7(2).
˚
Results and Discussion
Niobaziridine Hydride Formation with the β-H-Containing
N[i-Pr]Ar Ligand. That a niobaziridine hydride complex could
serve as a source of a reactive Nb-trisamide [Nb(NR2)3] frag-
ment was first suggested to us by an intriguing report from
Berno and Gambarotta.45 Their treatment of NbCl4(THF)2
with three equivalents of the β-H-containing amide LiNCy2
provided the niobaziridine chloride complex Nb(Cl)(η2-
C5H10CdNCy)(NCy2)2. The latter provided the bridging dini-
trogen complex (μ2-N2)[Nb(NCy2)3]2 upon treatment with Na-
[HBEt3] in the presence of N2. Interestingly, the intermediacy of
a niobaziridine hydride species was not proposed in this report.
However, it is conceivable that H- for Cl- exchange provides
the niobaziridine hydride complex Nb(H)(η2-C5H10CdNCy)-
(NCy2)2, which is responsible for the observed N2 chemistry by
serving as a source of Nb(NCy2)3.
Scheme 3
and Gambarotta50-53 in their investigations of low-valent Nb
complexes supported by monoanionic aryloxide and amido
ligands, respectively.
Inspired by this study, we set out to utilize the N[i-Pr]Ar
ligand39 for niobaziridine hydride formation. Previously, we
had reported that treatment of Nb(N[i-Pr]Ar)3Cl2 with AgOTf
and 5 equiv of LiBH4 afforded the niobaziridine borohydride
species Nb(BH4)(η2-Me2CdNAr)(N[i-Pr]Ar)2 (2i-Pr-BH4,
Scheme 2).46 The latter is best viewed as a BH3-trapped version
of the desired target Nb(H)(η2-Me2CdNAr)(N[i-Pr]Ar)2
(2i-Pr-H).
For radical ejection, it is pertinent that this ligand degradation
pathway is also the primary mode of decomposition for N[t-
Bu]Ar-ligated Nb systems. Accordingly, our attempts to prepare
the three-coordinate complex Nb(N[t-Bu]Ar)3 by chemical re-
duction of the Nb(IV) monochloride complex Nb(Cl)[N[t-
Bu]Ar)3 have invariably resulted in the formation of the bridging
bis-imido dimer [(μ-NAr)Nb(N[t-Bu]Ar)2]2 (Scheme3,Figure1).
Formation of [(μ-NAr)Nb(N[t-Bu]Ar)2]2 is proposed to occur
by a multistep process in which putative [Nb(N[t-Bu]Ar)3] is
formed upon reduction and elicits the ejection of tert-butyl
radical,54 concomitant with the formation of the NbIV imido
species [Nb(NAr)(N[t-Bu]Ar)2]. The latter is thought to rapidly
dimerize with Nb-Nb bond formation to the observed product.
Furthermore, the relatively large size of the remaining t-Bu
substituents in [(μ-NAr)Nb(N[t-Bu]Ar)2]2 may presumably
Unfortunately, borane abstraction from 2i-Pr-BH4 does
not provide a straightforward route to 2i-Pr-H. Instead, treat-
ment of 2i-Pr-BH4 with the Lewis base quinuclidine (quin) led
to the forest green complex 4i-Pr (Scheme 2), which has been
formulated on the basis of crystallographic characteriza-
tion.47 Complex 4i-Pr is remarkable in that it is the product
of not only Nb-Nb bond formation and ortho-metalation
with formal H atom loss but also of i-Pr radical ejection
from the N[i-Pr]Ar ligand. Notably, the two former ligand
degradation modes have been observed by both Rothwell48,49
(50) Tayebani, M.; Kasani, A.; Feghali, K.; Gambarotta, S.; Bensimon,
C. Chem. Commun. 1997, 2001–2002.
(51) Tayebani, M.; Feghali, K.; Gambarotta, S.; Yap, G. Organo-
(45) Berno, P.; Gambarotta, S. Organometallics 1995, 14, 2159–2161.
(46) Mindiola, D. J.; Cummins, C. C. Organometallics 2001, 20,
3626–3628.
(47) Mindiola, D. J. Ph.D. Thesis, Department of Chemistry,
Massachusetts Institute of Technology, Cambridge, MA, 2000.
(48) Steffey, B. D.; Chesnut, R. W.; Kerschner, J. L.; Pellechia, P. J.;
Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1989, 111, 378–380.
(49) Chesnut, R. W.; Jacob, G. G.; Yu, J. S.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1991, 10, 321–328.
metallics 1998, 17, 4282–4290.
(52) Tayebani, M.; Gambarotta, S.; Yap, G. Organometallic 1998, 17,
3639–3641.
(53) Tayebani, M.; Feghali, K.; Gambarotta, S.; Bensimon, C.
Organometallics 1997, 16, 5084–5088.
(54) For thermally induced t-Bu radical ejection stemming from d1
(E)Mo(N[t-Bu]Ar)3 complexes (E = O, S, Se, Te), see: Johnson, A. R.;
Davis, W. M.; Cummins, C. C.; Serron, S.; Nolan, S. P.; Musaev, D. G.;
Morokuma, K. J. Am. Chem. Soc. 1998, 120, 2071–2085.

5218 Organometallics, Vol. 29, No. 21, 2010
Figueroa et al.
revealed that it exists as a centrosymmetric, dihydrido-
bridged dimer in the solid state (Figure 2). This dimeric
nature of 2i-Pr-H is particularly surprising given that the
monomeric solid-state structure of the d1 molybdaziridine
hydride 3i-Pr-H has been unequivocally established by both
X-ray and neutron diffraction.39 Evidently the steric proper-
ties of the N[i-Pr]Ar ligand are insufficient to preclude
dimerization of niobaziridine hydride complexes derived
from it. Therefore, the contrasting solid-state structures
point strikingly to the differences in dn configuration be-
tween 2i-Pr-H and 3i-Pr-H, with the d0 nature of 2i-Pr-H
rendering it much more Lewis acidic than its Mo counterpart
(vide infra).
Critical to the isolation of 2i-Pr-H is the low-temperature
workup that must be employed. Once isolated, 2i-Pr-H is
thermally unstable in solution and decomposes to dimer 4i-Pr
over the course of ca. 2 h at room temperature (Scheme 4).
While the instability of 2i-Pr-H prevented an accurate deter-
mination of its degree of aggregation in solution, the forma-
tion of 4i-Pr points strongly to the possibility that the dimeric
structure as observed in the solid state may be preserved.
Furthermore, formation of a reactive d2 [Nb(N[i-Pr]Ar)3]
fragment via reversible β-H elimination from one or both
niobaziridine hydride units in dimeric 2i-Pr-H may account
for the observed ligand degradation processes leading
Figure 2. Molecular structure of [2i-Pr-H]2. Selected bond dis-
tances (A): Nb(1)-H(1)=1.802(6); Nb(1)-H(1A)=1.987(6);
Nb(1)-Nb(1A) = 3.301(8); Nb(1)-N(1) = 1.948(3); Nb(1)-
C(37)=2.248(4); Nb(1)-N(2)=2.013(3); N(1)-C(37)=1.408(5);
N(2)-C(27) = 1.482(5).
˚
Scheme 4
to 4i-Pr
.
Monomeric Niobaziridine Hydride Formation with the β-H-
Containing N[Np]Ar Ligand. Evidence for Reversible β-H
Elimination in Solution. To combat the limitations of the N[i-
Pr]Ar ligand, we reasoned that a thermally robust niobaziridine
hydride might be obtained if a ligand resistant to radical ejection
was employed. Therefore, the β-H-containing anilido ligand
N[Np]Ar was selected due to the greater thermodynamic barrier
to neopentyl radical formation than that for either the isopropyl
or tert-butyl radical.59 In addition, it was thought that three
neopentyl substituents would likely impart the necessary steric
protection to engender the formation of a monomeric niobazir-
idine hydride. This postulate was confirmed with the successful
synthesis34 of the kinetically persistent niobaziridine hydride
2
Np-H via reduction of NbX2(N[Np]Ar)3 precursors (X = I34
or OSO2CF325,27) with Mg(THF)3(anthracene). Furthermore,
X-ray diffraction has revealed that 2Np-H indeed exists as a
discrete monomer in the solid state (Figure 3).34
Diamagnetic niobaziridine hydride 2Np-H retains its in-
tegrity as an orange crystalline solid for months when stored
at low temperature (-35 °C). However, when dissolved in
C6D6, 2Np-H is observed to decay slowly and irreversibly at
room temperature (t1/2 ≈ 4.5 d) to its neopentylimido isomer,
Nb(NNp)(Ar)(N[Np]Ar)2 (5Np, Scheme 5). Importantly,
radical ligand degradation processes are not observed when
utilizing the N[Np]Ar ligand with Nb. Therefore, the forma-
tion of 5Np can be rationalized by a sequence involving
(i) reversible β-H elimination from 2Np-H leading to the reac-
tive three-coordinate species 1Np, followed by (ii) two-electron
C-N oxidative addition of an intact N[Np]Ar ligand by
the latter (Scheme 5). While the observation of facile C-N
bond rupture implies that d2 1Np is accessible in solution,
direct spectroscopic evidence for such a species has not been
obtained during the 2Np-H to 5Np conversion.
account for a retardation of additional ligand activation process,
as compared to the N[i-Pr]Ar system of 4i-Pr
.
In the N[i-Pr]Ar-ligated Nb system, complex 4i-Pr is also the
major product obtained when the Nb(V) dihalides Nb(Cl)2-
55
56
(N[i-Pr]Ar)3 and Nb(I)2(N[i-Pr]Ar)3 are combined with
strong reductants such as Na/Hg and KC8.57 Contrastingly,
treatment of Nb(I)2(N[i-Pr]Ar)3 with the relatively mild
reducing agent Mg(THF)3(anthracene),58 followed by a
rapid, low-temperature workup, successfully provides nio-
baziridine hydride 2i-Pr-H as a dark orange solid (Scheme 4).
Consistent with its formulation, diamagnetic 2i-Pr-H displays
1
a Cs-symmetric pattern of N[i-Pr]Ar resonances in its H
NMR spectrum (C6D6). However, crystallographic analysis
(55) Mindiola, D. J.; Meyer, K.; Cherry, J.-P. F.; Baker, T. A.;
Cummins, C. C. Organometallics 2000, 19, 1622–1624.
(56) Experimental details are provided in the Supporting
Information.
(59) Heats of formation (ΔHo , kcal/mol) for the following radicals
f
(57) Schwindt, M. A.; Lejon, T.; Hegedus, L. S. Organometallics
1990, 9, 2814–2819.
(58) Freeman, P. K.; Hutchinson, L. L. J. Org. Chem. 1983, 48, 879.
scale as tert-C4H9 (7.6 ( 1.2) < iso-C3H7 (18.2 ( 1.5) < neo-C5H11 (30.0
( 1.0) . CRC Handbook of Chemistry and Physics, 67th ed.; CRC Press:
Boca Raton, FL, 1987.

Article
Organometallics, Vol. 29, No. 21, 2010 5219
formation of reactive, coordinatively unsaturated metal
fragments.61-66 Furthermore, the formation of imido 5Np
is interpreted to result from a slower C-N bond cleavage
process, which serves to irreversibly trap 1Np in the absence
of a C-H activation event (k2, Scheme 5).67,68
Whereas the isotopic labeling results strongly suggest that
three-coordinate 1Np interchanges 2Np-H and 5Np, it is at best a
fleeting species. Due to its reactive nature, 1Np is likely stabilized
by agostic C-H69,70 and/or π-arene interactions71,72 from the
N[Np]Ar ligand and never present in free form. Indeed, evi-
dence for π-arene interactions in d2 group 5 M(N[R]Ar)3
complexes can be garnered from the molecular structure of
V(N[1-Ad]Ar)3,14 in which a single anilido ligand binds in an
η3-(N-Cipso-Cortho) fashion. Thus, we suggest that the latter
structural feature, as observed for V(N[1-Ad]Ar)3, can be con-
sidered as a snapshot of the C-N cleavage process involving the
reactive 1Np fragment.
Imido 5Np is stable for extended periods at elevated
temperature, thus rendering it the thermodynamic product
relative to niobaziridine hydride 2Np-H. This notion has been
substantiated by DFT calculations performed on the hypothe-
tical niobaziridine hydride Nb(H)(η2-Me(H)CdNPh)(NH2)2
(2m-H), which is calculated to lie ca. 29 kcal/mol higher in
energy than its ethyl imido isomer Nb(NEt)(Ph)(NH2)2 (5m).
This remarkable discrepancy highlights the kinetic formation of
the niobaziridine hydride functionality from 1Np via C-H acti-
vation, when C-N oxidative addition is clearly energetically
preferred. Furthermore, the marked ability of putative low-
valent Nb species to activate C-N bonds in organoamide
ligands has been documented by Gambarotta for several NR2
derivatives.50,52,53 Accordingly, such rampant C-N activation
capability further punctuates the ability of the niobaziridine
hydride unit, as derived from the N[Np]Ar ligand, to impart
kinetic stabilization to the reactive 1Np fragment in solution.
Divergent Reactivity of Niobaziridine Hydride 2Np-H with
Small-Molecule Substrates. Atom Abstraction and Com-
plexation vs Hydride Transfer. The prolonged stability of
Figure 3. Molecular structure of 2Np-H.
Scheme 5
2
Np-H in solution has allowed for a detailed survey of its reac-
tivity toward small-molecule substrates. We have shown34 that
2
Np-H readily generates the terminal chalcogenido73 species
(E)Nb(N[Np]Ar)3 (1Np-E, E = O, S, Se, Te), when treated
with suitable chalcogen-atom sources. In addition, treatment of
In order to further probe for the formation of 1Np in solution,
the decay of 2Np-H and its selectively labeled niobaziridine deu-
teride isotopomer, Nb(D)(η2-t-Bu(D)CdNAr)(N[Np-d2]Ar)2
(2Np-D; Np-d2 = CD2(t-Bu)),60 were measured as a function of
time. For both 2Np-H and 2Np-D, clean, reproducible first-order
kinetics were obtained at 75 °C (C6D6), resulting in a system
characterized by a significant observed inverse isotope effect
(kH/kD = 0.41(5)).61,62 Such an observed isotope effect is
consistent with the rapid interconversion of 2Np-H and three-
2
Np-H with mesitylnitrile (MesCN, Mes = 2,4,6-Me3C6H2)
leads to the exclusive formation of the η2-nitrile complex
(η2-MesCN)Nb(N[Np]Ar)3 (1Np-NCMes).34 Accordingly,
the formation of complexes 1Np-E and 1Np-NCMes, which
contain three intact N[Np]Ar ligands, demonstrates that the
niobaziridine hydride functionality of 2Np-H is indeed capable
of reversible β-H elimination upon reaction with incoming
substrates.
coordinate 1Np via reversible β-H elimination (k1/k-1
,
Scheme 5) and finds analogy with alkane elimination studies
from LnM(H)(R) complexes, which likewise result in the
Of these latter reactions, the clean and irreversible forma-
tion of the terminal oxo complex (O)Nb(N[Np]Ar)3 (1Np-O)
(66) Parkin, G.; Bercaw, J. E. Organometallics 1989, 8, 1172–1179.
(67) Bullock, R. M.; Headford, C. E. L.; Hennessy, K. M.; Kegley,
S. E.; Norton, J. R. J. Am. Chem. Soc. 1989, 111, 3897–3908.
(68) Gould, G. L.; Heinekey, D. M. J. Am. Chem. Soc. 1989, 111,
5502–5504.
(69) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250,
395–408.
(70) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem.
(60) Proton incorporation into the methylene-d2 positions in 2Np-D is
not observed by 1H NMR spectroscopy (C6D6), thereby ruling out a
complicated, Nb-mediated exchange process between all C-H bonds of
the N[Np]Ar ligand framework.
(61) Jones, W. D. Acc. Chem. Res. 2003, 36, 140–146.
(62) Churchill, D. G.; Janak, K. E.; Wittenberg, J. S.; Parkin, G.
J. Am. Chem. Soc. 2003, 125, 1403–1420.
(63) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104,
352–354.
(64) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J. Am. Chem.
Soc. 1986, 108, 1537–1550.
(65) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108,
7332–7346.
1988, 36, 1–124.
(71) Belt, S. T.; Dong, L.; Duckett, S. B.; Jones, W. D.; Partridge,
M. G.; Perutz, R. N. Chem. Commun. 1991, 266–269.
(72) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L.
J. Am. Chem. Soc. 2001, 123, 12724–12725.
(73) Parkin, G. Prog. Inorg. Chem. 1998, 47, 1–165.

5220 Organometallics, Vol. 29, No. 21, 2010
Figueroa et al.
Scheme 6
Table 1. OPPh3 Concentration (M) and Observed Rate Constants
(kobs, s-1) for the Deoxygenation of OPPh3 by 2Np-H (C6D6,
20 °C) under Pseudo-First-Order Conditions
with tert-butylnitrile (t-BuCN) results in the formation of the
ketimido complex Nb(NdC(H)t-Bu)(η2-t-Bu(H)CdNAr)(N-
[Np]Ar)2 (2Np-NC(H)t-Bu, Scheme 6, top), which features
an unperturbed niobaziridine ring as determined by X-ray
crystallography.34 Complex 2Np-NC(H)t-Bu can be viewed as
[OPPh3]
equivalents
kobs
2.7(1) ꢀ 10-4
5.9(2) ꢀ 10-4
5.5(3) ꢀ 10-4
6.0(3) ꢀ 10-4
5.4(4) ꢀ 10-4
the product of t-BuCN insertion78 into the Nb-H bond of 2Np
-
0.15
0.23
0.30
0.38
0.45
10
15
20
25
30
H and provides a striking contrast to the behavior observed with
MesCN. Furthermore, insertion into the Nb-H unit of 2Np-H is
not limited specifically to t-BuCN. As depicted in Scheme 6, a
diverse array of organic substrates behave similarly,79 thus
revealing that insertion chemistry is pronounced for the nioba-
ziridine hydride functionality in 2Np-H.
by treatment of 2Np-H with triphenylphosphine oxide
(OPPh3) is particularly noteworthy.34 Well-characterized
examples of the deoxygenation of phosphine oxides by
transition metal complexes are rare,74 with the reports of
Mayer75 and Wolczanski76,77 serving as precedent. In both
the latter cases, reactive low-valent early transition metal
complexes were utilized for the cleavage of the phosphoryl
PdO bond, which lends further credence to the notion that
Notably, the ability to insert an organic functionality appears
to be generally characteristic of the niobaziridine hydride group.
For the N[i-Pr]Ar-containing system, we have previously re-
ported the synthesis of the niobaziridine alkoxide complex
Nb(OC(H)Ph2)(η2-Me2CdNAr)(N[i-Pr]Ar)2 (2i-Pr-OC(H)Ph2)
from treatment of the borohydride complex 2i-Pr-BH4 with
quinuclidine in the presence of benzophenone.46 In this latter
case, the highly Lewis acidic monomer 2i-Pr-H is presumably
formed via sequential borane abstraction/benzophenone inser-
tion without competing formation of dimer 4i-Pr. Thus while
the steric demands of the N[Np]Ar ligand serve to prevent
dimerization of 2Np-H, its Nb center remains highly Lewis
acidic, serving to impart substantial nucleophilic character to
the hydride unit.
2
Np-H can function as the masked equivalent of three-
coordinate 1Np. For the deoxygenation of OPPh3 by 2Np-H,
1
preliminary measurements by H NMR spectroscopy indi-
cate that the system adheres to smooth pseudo-first-order
kinetics in C6D6 before reaching a saturation point in
[OPPh3] at ca. 0.23 M (Table 1).
Despite exhibiting reversible β-H elimination as illustrated
above, 2Np-H displays a second reactivity pattern, which has
revealed its tendency to additionally function as a Lewis
acidic early metal hydride. For example, treatment of 2Np-H
Although the insertion of substrates by 2Np-H removes the
hydride moiety, the niobaziridine ring remains active for
(74) Typically the reverse reaction, namely, transfer of a terminal oxo
moiety to phosphine, is encountered; see: (a) Holm, R. H. Chem. Rev.
1987, 87, 1401–1449. (b) Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12,
571–589.
(75) Brock, S. L.; Mayer, J. M. Inorg. Chem. 1991, 30, 2138–2143.
(76) Veige, A. S.; Slaughter, L. M.; Wolczanski, P. T.; Matsunaga, N.;
Decker, S. A.; Cundari, T. R. J. Am. Chem. Soc. 2001, 123, 6419–6420.
(77) Veige, A. S.; Slaughter, L. M.; Lobkovsky, E.; Wolczanski, P. T.;
Matsunaga, N.; Decker, S. A.; Cundari, T. R. Inorg. Chem. 2003, 42,
6204–6224.
(78) For examples of organonitrile insertion into early transition
metal and lanthanide hydrides, see: (a) Churchill, M. R.; Wasserman,
H. J.; Belmonte, P. A.; Schrock, R. R. Organometallics 1982, 1, 559–561.
(b) Evans, W. J.; Meadows, J. H.; Hunter, W. E.; Atwood, J. L. J. Am. Chem.
Soc. 1984, 106, 1291–1300. (c) Alelyunas, Y. W.; Guo, Z.; LaPointe, R. E.;
Jordan, R. F. Organometallics 1993, 12, 544–553.
(79) R-H insertion of N2CPh2 into early transition metal hydride
complexes has been reported previously; see: Gambarotta, S.; Floriani,
C.; Chiesi-Villa, A.; Guastini, C. Inorg. Chem. 1983, 22, 2029–2034.

Article
Organometallics, Vol. 29, No. 21, 2010 5221
Scheme 8
Figure 4. Molecular structure of 6Np-t-Bu. Selected bond distances
˚
(A) and angles (deg): Nb(1)-N(4) = 1.759(2), Nb(1)-N(3) =
2.017, C(37)-C(41) = 1.560(4), Nb(1)-N(4)-C(41) = 128.6(2).
Scheme 7
benzonitrile (PhCN). Thus treatment of C6D6 solutions of
Np-H with PhCN at room temperature rapidly generates the
2
ketimido complex 2Np-NC(H)Ph as an observable inter-
mediate by H NMR spectroscopy.88 The latter smoothly
1
additional chemistry. Accordingly, the ketimido complex
decays without heating to the tethered imido complex 6Np-Ph
over the course of ca. 4 h (Scheme 8), thereby indicating that
the thermal stability of 2Np-NC(H)t-Bu results in part from
its sterically encumbering ketimido t-Bu substituent.
2
Np-NC(H)t-Bu cleanly undergoes ring expansion to its
tethered imido80 isomer 6Np-t-Bu when heated at 120 °C in
toluene solution (Scheme 7). This complex, with its imido
tethered in a five-membered ring, displays an unusually small
Nb-N-C angle of 128.6(2)° at the imido nitrogen. Similarly
small metrics are found for the Nb-N-Nb angles of
129-131° in the six-membered rings of [(Ar[iPr]N)2Nbd
N]3,55 while an angle of 151° has been reported for the
Nb-N-C angle (η5-C5H4)(CH2)3NNbCl2PMe3.81 The for-
mation of complex 6Np-t-Bu (Figure 4) likely arises via
intramolecular nucleophilic attack by the niobaziridine car-
bon on the ketimido ligand in 2Np-NC(H)t-Bu. Notably,
early transition metal metallaziridine complexes are well
known to elicit nucleophilic C-C bond formation with
unsaturated substrates.82-87 An analogous ring expansion
process is also observed in the reaction between 2Np-H and
It is worthy of mention that the niobaziridine rings in com-
plexes 2Np-NC(H)t-Bu and 2Np-NC(H)Ph preferentially attack
at the ketimide sp2 carbon, despite the fact that the ketimide
ligand also features an accessible β-H atom. While β-H abstrac-
tion by the niobaziridine ring can conceivably furnish the corres-
ponding η2-nitrile isomers of 2Np-NC(H)t-Bu and 2Np-NC(H)Ph
(i.e., (η2-t-BuCN)Nb(N[Np]Ar)3 and (η2-PhCN)Nb(N-
[Np]Ar)3), this pathway is evidently disfavored. The same
is true of complexes 2Np-OCH2Ph, 2Np-C(H)C(H)t-Bu, and 2Np
-
NC(H)NMe2 (Scheme 6), which all retain their integrity for
extended periods at elevated temperatures (120 °C, 3 d, C7D8).
Again, similar behavior was observed with the niobaziridine
alkoxide complex 2i-Pr-OC(H)Ph2, which did not interconvert
thermally with the independently prepared η2-benzophenone
complex (η2-Ph2CO)Nb(N[i-Pr]Ar)3 (1i-Pr-OCPh2) via β-H
abstraction.46 Furthermore, for the N[Np]Ar system, it is notable
that ketimide 2Np-NC(H)NMe2 does not undergo ring expan-
sion when heated. This circumstance for 2Np-NC(H)NMe2 is
likely due to electronic saturation of the sp2 ketimido carbon by
its pendant amino group.89
(80) Gountchev, T. I.; Tilley, T. D. J. Am. Chem. Soc. 1997, 119,
12831–12841.
(81) Antonelli, D. M.; Greem, M. L. H.; Mountford, P. J. Organo-
met. Chem. 1992, 438, C4.
(82) Roskamp, E. J.; Pedersen, S. F. J. Am. Chem. Soc. 1987, 109,
6551–6553.
(83) Buchwald, S. L.; Lucas, E. A.; Davis, W. M. J. Am. Chem. Soc.
1989, 111, 397–398.
(84) Buchwald, S. L.; Watson, B. T.; Wannamaker, M. W.; Dewan,
J. C. J. Am. Chem. Soc. 1989, 111, 4486–4494.
(85) Gately, D. A.; Norton, J. R.; Goodson, P. A. J. Am. Chem. Soc.
1995, 117, 986–996.
(86) Gately, D. A.; Norton, J. R. J. Am. Chem. Soc. 1996, 118, 3479–
3489.
(87) Tunge, J. A.; Czerwinski, C. J.; Gately, D. A.; Norton, J. R.
(88) Complex 2Np-NC(H)Ph is characterized by a well-resolved,
diagnostic aldehydic-type proton resonance centered at δ 9.57 ppm,
which is observed to decay over time.
(89) Spectroscopic evidence for this notion is provided from the1H NMR
spectrum of 2Np-NC(H)NMe2, where the amino-methyl groups appear as
distinct singlets at room temperature, thus indicating hindered rotation.
Organometallics 2001, 20, 254–260.

5222 Organometallics, Vol. 29, No. 21, 2010
Figueroa et al.
Scheme 9
Figure 5. Molecular structure of complex 7. Selected bond dis-
˚
tances (A) and angles (deg): Nb(1)-O(1) = 1.956(2); Nb(1)-
N(3)=1.762(3); Nb(1)-N(2)=1.996(3); C(41)-C(42)=1.321(5);
Nb(1)-N(3)-C(31) = 174.3(3).
Scheme 10
The fact that MesCN exclusively forms the η2-nitrile
complex 1Np-NCMes and that β-H abstraction is not ob-
served for complexes 2Np-NC(H)R (R = t-Bu, Ph, NMe2)
suggests that dual pathways of reactivity are accessible to
niobaziridine hydride 2Np-H depending on the nature of the
substrate. This notion is further substantiated by the reactiv-
ity observed between 2Np-H and ortho-tolylnitrile (o-tolCN).
Remarkably, treatment of 2Np-H with one equivalent of
o-tolCN at room temperature leads rapidly to the formation
of both the ketimide 2Np-NC(H)o-tol and the η2-nitrile complex
1
1
Np-NC-o-tol in a 1:1.5 ratio, as determined by H NMR
spectroscopy (Scheme 9). Over the course of 5 h, 2Np-NC-
(H)o-tol converts to imido 6Np-o-tol while maintaining an
overall (2Np-NC-o-tol þ 6Np-o-tol):1Np-NC-o-tol ratio of
eneamido-imido complexes 7 and 8, respectively. Crystal-
lographic characterization of 7 and 8 (Figures 5 and 6,
respectively) revealed that a neopentylidene group is incor-
porated into the newly formed olefinic fragments, demon-
strating that both CO and i-PrNC induce the cleavage of the
N-CH(t-Bu) linkage of the N[Np]Ar ligand. Furthermore, in
both 7 and 8, an H atom is delivered to the terminal carbon
atom of the incoming substrate. Therefore, we suggest that
the formation of complexes 7 and 8 arises from rapid
insertion of CO or i-PrNC into the Nb-H unit in 2Np-H,
concomitant with the formation of an incipient formyl91-94
or iminoformyl95,96 ligand, respectively. The latter subsequently
1.5:1. Extended heating of the resultant 6Np-o-tol þ 1Np
-
NC-o-tol mixture did not affect the relative product distribu-
tion (120 °C, ca. 1 week), thus indicating distinct pathways of
formation90 for 1Np-NC-o-tol and 2Np-NC(H)o-tol from
2
Np-H. To this end, it is clear that the ortho methyl groups
of the substituted benzonitriles MesCN and o-tolCN play a
critical role in dictating the behavior of 2Np-H. As elaborated
upon below, we believe such steric attributes serve to block
hydride transfer from 2Np-H, thereby allowing complexation
pathways to dominate.
An additional illustration of the early metal hydride
character of 2Np-H is obtained from its reactivity toward
both carbon monoxide (CO) and isopropylisocyanide
(i-PrNC). As shown in Scheme 10, treatment of 2Np-H with
carbon monoxide (CO) or isopropylisocyanide (i-PrNC)
results in the clean formation of the enolate-imido and
(91) Wolczanski, P. T.; Bercaw, J. E. Acc. Chem. Res. 1980, 13, 121–
127.
(92) Fagan, P. J.; Moloy, K. G.; Marks, T. J. J. Am. Chem. Soc. 1981,
103, 6959–6962.
(93) Gladysz, J. A. Adv. Organomet. Chem. 1982, 20, 1–38.
(94) Reinartz, S.; Brookhart, M.; Templeton, J. L. Organometallics
2002, 21, 247–249, and references therein.
(95) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059–1079.
(96) Clark, J. R.; Fanwick, P. E.; Rothwell, I. P. Organometallics
1996, 15, 3232–3237.
(90) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd
ed.; McGraw-Hill: New York, 2002; pp 58-61.

Article
Organometallics, Vol. 29, No. 21, 2010 5223
induced C-H reductive elimination102-106 and access to the
Mo(N[i-Pr]Ar)3 fragment. Accordingly, it is suggested that
the complexation and atom abstraction chemistry observed
by 2Np-H is also the result of coordinatively induced C-H
reductive elimination, but is only manifest with substrates for
which insertion is inhibited. Thus for MesCN, substitution at
both ortho positions apparently serves to protect its CN unit
from hydride transfer, thereby allowing for C-H reductive
elimination and subsequent complexation to the 1Np fragment.
Contrastingly, PhCN readily undergoes insertion due to the
unprotected nature of its CN unit. For the intermediate case of
o-tolCN, a partial inhibition of hydride transfer is postulated to
be operative, where insertion vs complexation chemistry is
dictated by the spatial presentation of the single ortho-Me
group to the Nb-H unit in 2Np-H.107
Associative binding followed by C-H reductive elimination
is also likely operative in the deoxygenation of OPPh3 by 2Np-H,
since, to our knowledge, hydrogenation of phosphine oxides by
transition metal hydrides is unprecedented. This fact, coupled
with the demonstrated need for a powerful reductant to effect
phosphoryl PdO bond cleavage,75-77 further adds to the
suggestion that the reactive, d2 1Np fragment becomes available
upon the addition of OPPh3 to 2Np-H. However, OdPPh3
hydrogenation is not completely precluded by the present set
of data. Accordingly, whereas coordinatively induced reductive
elimination may account for the formation of the heavier
chalcogenido complexes 1Np-E (E = S, Se, Te), direct chalco-
gen-atom insertion into the Nb-H bond of 2Np-H, followed by
R-H abstraction by the niobaziridine ring, also cannot be ruled
out at present. Indeed, R-H abstraction of hydrochalcogenido
ligands (M-EH) has been proposed as a likely mechanistic
pathway for the formation of heavier terminal chalcogenido
functionalities (MdE; E = S, Se, Te).73
Figure 6. Molecular structure of complex 8. Selected bond dis-
˚
tances (A) and angles (deg): Nb(1)-N(3) = 1.760(3); Nb(1)-
N(4)=2.037(3); Nb(1)-N(1)=1.997(3); Nb(1)-N(2)=2.030(3);
N(4)-C(41) = 1.400(4); C(41)-C(42) = 1.336(5); Nb(1)-N(3)-
C(31) = 170.9(2).
succumbs to nucleophilic attack96,97 by the niobaziridine
carbon atom to furnish the observed products after electro-
nic rearrangement and C-N bond cleavage.
Proposed Mechanism of Reaction of Niobaziridine Hydride
2
Np-H with Small Molecules. Both the isotopic labeling
studies and reaction chemistry described above offer insight
into the elementary steps by which niobaziridine hydride
2
Np-H interacts with incoming substrates. The inverse iso-
tope effect observed for the decomposition of 2Np-H and
Np-D strongly indicates that rapid interconversion between
2
three-coordinate 1Np and 2Np-H occurs in solution. How-
ever, 1Np is exceedingly reactive and likely not present in
appreciable concentration in which to interact with sub-
strate. Therefore the direct reaction of substrates with 1Np
likely does not take place (Scheme 11, pathway A). Instead,
we suggest that substrates first associatively bind to five-
coordinate 2Np-H to form a six-coordinate substratef2Np-H
species (pathway B). This mechanism is inferred from the
finding that unsaturated substrates readily succumb to migra-
tory hydride transfer as described above (pathway C). Indeed,
insertion into the M-H bond of early metal hydrides is well
known to involve a requisite substrate binding event,98 which
precedes migratory hydride transfer. While a substratef
Additional Niobaziridine Hydride Variants. Synthesis and
Thermal Behavior of Niobaziridine Hydrides Derived from
N-Methylene-1-adamantyl (N-CH2-1Ad) and N-Cyclohexyl
(N-cyclo-C6H11) Anilides. Due to the rich chemistry dis-
played by 2Np-H, we became interested in expanding the
library of kinetically persistent niobaziridine hydrides. To this
end, we targeted for synthesis niobaziridine hydrides derived
1
from both N-methylene-1-adamantylanilide (N[CH2 Ad]Ar)
and N-cyclohexylanilide (N[Cy]Ar, Cy=cyclo-C6H11), hop-
ing that stable niobaziridine hydrides would be generally
obtainable by judicious choice of ancillary ligand. The deci-
1
2
Np-H species has not been observed spectroscopically by us
sion to employ N[CH2 Ad]Ar as a variant was based on its
structural similarity to the N[Np]Ar ligand. In addition, we
thought the relatively encumbering N[Cy]Ar ligand may
provide a room-temperature-stable niobaziridine hydride
featuring a secondary hydrocarbyl substituent, whereas the
N[i-Pr]Ar ligand failed in that regard.
to date, evidence for such intermediates has been obtained
previously for other early metal hydrides.91,99-101
Importantly, on the basis of stopped-flow kinetic measure-
ments and isotopic labeling studies, an analogous associative
mechanism has also been proposed for the reaction between
molybdaziridine hydride 3i-Pr-H and isocyanides (RNC).
In this latter case, insertion chemistry is not observed.
Instead, binding of RNC to 3i-Pr-H promotes coordinatively
Gratifyingly, Mg(THF)3(anthracene) reduction of the diio-
dide precursors 1CH Ad-I2 and 1Cy-I2 readily provided the nio-
2
1
baziridine hydrides Nb(H)(η2-1Ad(H)CdNAr)(N[CH2 Ad]-
Ar)2 (2CH Ad-H) and Nb(H)(η2-C5H10CdNAr)(N[Cy]Ar)2
2
(2Cy-H), respectively (Scheme 12). Complexes 2CH Ad-H and
2
(97) For a proposed nucleophilic attack on an incipient formyl
ligand, see: Fryzuk, M. D.; Mylvaganam, M.; Zaworotko, M. J.;
MacGillivray, L. R. Organometallics 1996, 15, 1134–1138.
(102) Yoshifuji, M.; Gell, K. I.; Schwartz, J. J. Organomet. Chem.
1978, 153, C15–C18.
(98) For early examples, see: (a) Fachentti, G.; Floriani, C. J. Organo-
€
ꢁ
ꢁ
met. Chem. 1974, 71, C5–C7. (b) Roder, A.; Thiele, K.-H.; Palyi, G.; Marko,
L. J. Organomet. Chem. 1980, 199, C31–C34.
(99) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J. E.
(103) Gell, K. I.; Schwartz, J. J. Am. Chem. Soc. 1981, 103, 2687–2695.
(104) Gell, K. I.; Schwartz, J. J. Organomet. Chem. 1978, 162, C11–C15.
(105) Jones, W. D.;Hessell, E. T. J. Am. Chem. Soc. 1992, 114, 6087–6095.
(106) Burkey, D. J.; Debad, J. D.; Legzdins, P. J. Am. Chem. Soc.
1997, 119, 1139–1140.
J. Am. Chem. Soc. 1978, 100, 2716–2724.
(100) Marsella, J. A.; Curtis, C. J.; Bercaw, J. E.; Caulton, K. G.
J. Am. Chem. Soc. 1980, 102, 7244–7246.
(101) Burger, B. J.; Santarsiero, B. D.; Trimmer, M. S.; Bercaw, J. E.
(107) The observed product ratio of 2Np-NC(H)o-tol:1Np-NC-o-tol
may be a manifestation of the rate of rotation about of the aryl unit
about the Cipso-Cnitrile bond.
J. Am. Chem. Soc. 1988, 110, 3134–3146.

5224 Organometallics, Vol. 29, No. 21, 2010
Figueroa et al.
Scheme 11
Scheme 12
Figure 7. Molecular structure of 2CH Ad-H. Selected bond dis-
2
˚
tances (A) for one of two independent molecules: Nb(1)-N(1) =
1.994(6); Nb(1)-C(17)=2.209(7); Nb(1)-N(2)=1.973(6);Nb(1)-
N(3)=1.992(6); C(17)-N(1)=1.415(8); C(11)-N(1)=1.396(9).
2Cy-H are characterized by broad 1H NMR resonances
centered at 9.34 and 9.50 ppm, respectively, which are
attributed to their Nb-H units and are comparable with that
of 2Np-H (9.24 ppm). While 2Cy-H has not been structurally
appear when 2Cy-H is heated at 65 °C, as assayed by 1H NMR
spectroscopy (C6D6). Such decomposition behavior is remi-
niscent of the i-Pr derivative 2i-Pr-H, presumably indicating
that anilido ligands featuring a secondary hydrocarbyl sub-
stituent do not effectively direct niobaziridine hydride decom-
position in a clean unimolecular fashion.
characterized in the solid state, X-ray diffraction revealed that
2
2
CH Ad-H is a monomer in the solid state and possesses overall
features similar to 2Np-H(Figure 7). Moreover, both2Cy-H and
Reinvestigation of Dicyclohexyl Amido-Derived (NCy2)
Niobaziridine Systems. To date, we have not obtained evidence
for the ability of complex 2Np-H alone to engage in dinitrogen
activation chemistry.109 Indeed, 2Np-H does not react with N2 at
room temperature or below, or at N2 pressures ranging from 1 to
2
2
CH Ad-H insert benzaldehyde to give the alkoxy products 2R-
OCH2Ph, and 2CH Ad-H will deoxygenate N2O to afford ONb-
2
1
(N[CH2 Ad]Ar)3 (1CH Ad-O).
2
With respect to their thermal behavior in solution, both
2
CH Ad-H and 2Cy-H are indeed stable at room temperature
for extended periods. When heated in C6D6, niobaziridine
4 atm.110 In addition, niobaziridine hydrides 2i-Pr-H, 2CH Ad-H,
2
2
and 2Cy-H are similarly unreactive toward N2 under ambient
conditions.
hydride 2CH Ad-H smoothly converts to its aryl-imido isomer,
2
1
as indicated by H NMR spectroscopy, thus confirming its
similarity to2Np-H (Scheme12).108 Contrastingly, cyclohexyl-
substituted 2Cy-H does not exhibit clean decomposition beha-
vior at elevated temperatures. Instead, a myriad of resonances
(109) In previous work, we employed dinitrogen anions of molybde-
num for the delivery of N2 to the 1Np fragment. That work was
independent of niobaziridine hydride 2Np-H. See ref 29.
(110) We have also investigated the treatment of 2Np-H with Lewis
basic additives in order to promote niobaziridine hydride opening and
subsequent N2 binding. However such experiments, where excesses of
[2.2.2]-diazabicyclooctane (DABCO) or PMe3 were added to solutions
of 2Np-H, failed to elicit N2 activation. In addition, neither DABCO nor
PMe3 was observed to bind to 2Np-H on the 1H NMR time scale.
(108) Under similar thermolysis conditions, niobaziridine hydride2CH Ad
2
has been qualitatively observed to decompose faster than the neopentyl
variant 2Np-H. We tentatively ascribe this relative difference to added steric
pressures in 2CH Ad-H as induced by the pendent 1-Ad substituents.
2

Article
Organometallics, Vol. 29, No. 21, 2010 5225
Scheme 13
Scheme 14
Figure 8. Molecular structure of 2Cy -H. Selected bond distances
2
˚
(A): Nb(1)-H(1) = 1.818(10); Nb(1)-N(1) = 1.959(2); Nb(1)-
C(111)=2.231(3); Nb(1)-N(2)=1.986(2); Nb(1)-N(3)=1.986(2);
N(1)-C(111) = 1.425(3); N(1)-C(121) = 1.467(3); Nb(1)
3 3 3
H(221)=2.492; Nb(1) H(311)=2.441.
3 3 3
pressures of the encumbering Cy substituents.45 On the basis
of the structures of both dimer [2i-Pr-H]2 and monomer 2Np-H
however, we suggest that the HCyfNb contacts in 2Cy -H and
2
2Cy -Cl result from the presence of highly Lewis acidic Nb
2
centers, which agostic interactions serve to stabilize. To this
end 2Cy -H, like 2Np-H, has been found to function as a Lewis
2
acidic hydride complex as demonstrated by the facile inser-
tion of benzaldehyde into its Nb-H bond (Scheme 14, 2Cy
2
-
OCH2Ph).111
Most interestingly, niobaziridine hydride 2Cy -H is
stable for extended periods in common organic solvents,
but does not form the bridging dinitrogen complex (μ2-
N2)[Nb(NCy2)3]2 when exposed to N2. Furthermore, to
date, we have not identified suitable conditions under
2
which 2Cy -H will elicit N2 activation chemistry. Thus we
2
This lack of N2 reactivity is surprising given the ability of the
NCy2-ligated niobaziridine chloride complex of Berno and
Gambarotta to activate N2 upon the addition of Na[HBEt3].45
Notably, a niobaziridine chloride complex of our N[Np]Ar
system is readily available from the reaction between 2Np-H
and CHCl3 or CH2Cl2 (2Np-Cl, Scheme 13). However, treat-
ment of 2Np-Cl with Li[HBEt3] in toluene in the presence
of N2, conditions which closely mimic those employed for
the NCy2/Nb system, results exclusively in the regeneration
of 2Np-H.
contend that the electronic and steric differences attendant
in dicyclohexylamido vs N-hydrocarbylanilido ligation do
not affect the ability of the niobaziridine hydride func-
tionality to interact with dinitrogen under ambient condi-
tions. Accordingly, the factors governing the ability of
2Cy -Cl to activate N2 in the presence of NaHBEt3, as reported
2
by Berno and Gambarotta, remain unclear.
Niobaziridine Hydride Radical Anion and Comparison of
Nioba- and Molybdaziridine Hydride Complexes. Despite
their shared characteristics, the fact that insertion chemis-
try is prominent for niobaziridine hydride 2Np-H but non-
existent for molybdaziridine hydride 3i-Pr-H is particularly
striking. Furthermore, the ability of molybdaziridine hy-
dride 3i-Pr-H to cleave dinitrogen, whereas niobaziridine
hydrides 2R-H do not react with N2, highlights another
major dichotomy between these Mo- and Nb-based sys-
tems. While the greater electronegativity of Mo relative to
Nb may impede the hydride in 3i-Pr-H from serving as a
nucleophile,112 such an argument falters given that several
Therefore, on the basis of the possibility that the nioba-
ziridine hydride Nb(H)(cyclo-C5H10CdNCy)(NCy2)2 (2Cy -H)
2
is an intermediate en route to N2 complexation in the NCy2-
ligated system, we sought its isolation in pure form in order
to compare its behavior with that of our anilido-derived
complexes.
A multistep synthetic strategy provided niobaziridine hydride
2Cy -H, thus confirming that it is indeed amenable to isolation
2
(Scheme 14). As revealed by X-ray diffraction (Figure 8), the
complement of cyclohexyl units provides adequate protection to
engender a monomeric niobaziridine hydride in the solid state.
However, the Nb center in 2Cy -H is found to possess intramo-
2
(111) Complex 2Cy -OCH2Ph has been formulated on the basis of its
2
1H NMR spectroscopic signatures, which indicate three distinct
cyclohexyl environments and OCH2Ph ligand resonances similar to
those in complex 2Np-OCH2Ph. For details, see the Supporting
Information.
lecular H-C agostic interactions69,70 from two intact NCy2
ligands (2.466 A av). Close HCyfNb contacts are also observed
˚
in the solid-state structure of Nb(Cl)(cyclo-C5H10CdNCy)-
(NCy2)2 (2Cy -Cl), but were attributed to arise from steric
2
(112) Labinger, J. A.; Bercaw, J. E. Organometallics 1988, 7, 926–928.

5226 Organometallics, Vol. 29, No. 21, 2010
Figueroa et al.
Figure 9. X-band EPR spectrum of K[2Np-H] in THF at 20 °C.
Scheme 15
Figure 10. Molecular structure of Na(THF)3[2Np-H]. Selected
˚
bond distances (A): Nb(1)-H(1) = 1.76(4); Nb(1)-N(1) =
2.113(3); Nb(1)-C(17) = 2.187(4); Nb(1)-N(2) = 2.092(3);
Nb(1)-N(3) = 2.022(3); C(17)-N(1) = 1.434(4); C(11)-N(1) =
1.400(5); H(1)-Na(1) = 2.36(4); N(1)-Na(1) = 2.474(3).
temperature induced a color change to brown and the
formation of an intractable mixture of products (ca. 1 h),
as indicated by both 1H NMR and EPR spectroscopic
measurements.
mononuclear Mo-hydride species are known to insert un-
saturated substrates.113-117 Therefore, we were intrigued at
the possibility that the differing d-electron count between
complexes 3i-Pr-H and 2Np-H (Mo(V), d1 vs Nb(V), d0) may
ultimately be responsible for modulating the disparate
reactivity of these metallaziridine hydride complexes. To
test this hypothesis, we sought the radical anion of 2Np-H as
Fortuitously however, during a Na/Hg reduction of the
bis-triflate complex 2Np-(OTf)2, we obtained a small amount
of a dark green, n-pentane-insoluble material121 that proved
amenable to crystallization from THF. X-ray diffraction
established the identity of this material as the [Na(THF)3]
salt of the [2Np-H]- radical anion (Figure 10), thereby
confirming the latter’s accessibility via chemical reduction.
Furthermore, dissolution of crystalline [Na(THF)3][2Np-H]
in THF followed by EPR spectroscopic analysis gave rise to
the identical spectral signature obtained upon reduction of
an isoelectronic surrogate to molybdaziridine hydride 3i-Pr
-
H on the basis that it may likewise be resistant to insertion
chemistry.
Synthetic access to a radical anion appeared feasible based
on cyclic voltammetry measurements on 2Np-H, which re-
vealed a clean and reversible reduction wave centered at
-2.7 V (vs Fc/Fcþ, 0.2 M nBu4NPF6/THF).118 Accordingly,
addition of KC857 to 2Np-H in THF rapidly induced a color
change from orange to forest green. Analysis of this solution
by EPR spectroscopy revealed a 10-line spectral pattern (ca.
1000 G, Figure 9), indicative of a paramagnetic, formally
NbIV species (93Nb, I = 9/2 100%).55,119,120 Whereas we
attribute this spectroscopic signature to the radical species
K[2Np-H] (Scheme 15), its isolation as generated from KC8
proved difficult. Indeed, while the green color of presumed
K[2Np-H] persists at -35 °C in THF, warming to room
2
Np-H with KC8. Unfortunately, repeated attempts to isolate
[Na(THF)3][2Np-H] from the 2Np-(OTf)2/Na/Hg reaction
system led to inconsistent results. However, due to the
EPR spectral similarities between [Na(THF)3][2Np-H] and
presumed K[2Np-H], we felt confident that in situ generation
of the latter would serve adequately for synthetic investiga-
tions.
While stable in solution at -35 °C, K[2Np-H] does not
react with N2, as indicated by 1H NMR spectroscopy. This
finding contrasts with the behavior of d1 molybdaziridine
hydride 3i-Pr-H, which effects N2 cleavage at low temperature
in solution en route to the bridging nitrido species (μ-N)-
[Mo(N[i-Pr]Ar)3]2.39,41 Addition of Lewis basic additives,
which are known to promote and accelerate the overall N2
cleavage reaction by 3i-Pr-H,41 had no effect on the integrity
of K[2Np-H]. Furthermore, treatment of K[2Np-H] with
PhCN rapidly generated the ketimido species 2Np-NC(H)Ph
(113) Kubas, G. J.; Wasserman, H. J.; Ryan, R. R. Organometallics
1985, 4, 2012–2021.
(114) Ito, T.; Igarashi, T. Organometallics 1987, 6, 199–201.
(115) Liang, F. P.; Jacobsen, H.; Schmalle, H. W.; Fox, T.; Berke, H.
Organometallics 2000, 19, 1950–1962.
(116) Liang, F.; Schmalle, H. W.; Fox, T.; Berke, H. Organometallics
2003, 22, 3382–3393.
(117) Liang, F.; Schmalle, H. W.; Berke, H. Inorg. Chem. 2004, 43,
993–999.
(118) Such a negative reduction potential is within the range attain-
able by strong chemical reductants; see: Connelly, N. G.; Geiger, W. E.
Chem. Rev. 1996, 96, 877–910.
1
according to H NMR analysis, presumably generating K0
as a byproduct. Notably, molybdaziridine hydride 3i-Pr-H is
competent for both the coupling and CtN cleavage of
(119) Cotton, F. A.; Lu, J. Inorg. Chem. 1995, 34, 2639–2644.
(120) Sanchez, C.; Vivien, D.; Livage, J.; Sala-Pala, J.; Viard, B.;
Guerchais, J.-E. J. Chem. Soc., Dalton Trans. 1981, 64–68.
(121) The major product of this reduction experiment was niobazir-
idine hydride 2Np-H.

Article
Organometallics, Vol. 29, No. 21, 2010 5227
PhCN without displaying insertion reactivity.39,42 Thus for
N2 and PhCN, the [2Np-H]- fragment behaves similarly to
neutral 2Np-H, rather than d1 3i-Pr-H, despite the presence of
added electron density.
2.0 h to avoid decomposition of 2iPr-H to the forest green
dimeric species 4iPr. Dark orange single crystals of 2iPr-H were
obtained from a saturated Et2O solution stored at -35 °C for 1 d
in approximately 30% yield (0.897 mmol). Complex 2iPr-H was
found to completely decompose over the course of 1 h to forest
green 4iPr when heated at 65 °C and over the course of 4 h when
left standing at room temperature (C6D6). Data for 2iPr-H: 1H
NMR (300 MHz, C6D6, 23 °C): δ 9.23 (br s, 1H, Nb-H), 7.23 (s,
2H, o-Ar, aziridine), 7.15 (s, 1H, p-Ar, aziridine), 6.70 (s, 4H,
o-Ar amido), 6.59 (s, 2H, p-Ar amido), 3.68 (septet, 2H, J = 4 Hz,
CH(CH3)2), 2.37 (s, 6H, Ar-CH3 aziridine), 2.07 (s, 12H, Ar-
CH3 amido), 2.02 (s, 6H, NdC(CH3)2 aziridine), 1.00 (d, 6H,
J = 4 Hz, CH(CH3)2 amido), 0.96 (d, 6H, J = 4 Hz, CH(CH3)2
amido). The thermal instability of 1iPr-H precluded a satisfac-
Conclusions
The niobaziridine hydride functional group has been char-
acterized for several ligand variants. In doing so, the divergent
reactivity of these complexes insertion reactions vs retro-
;
cyclometalation has been explored. We have proposed two
;
possible mechanisms for the observed reactivity: rapid rever-
sible β-hydride elimination to interconvert three-coordinate,
d2 1Np with 2R-H or competing coordinatively induced re-
ductive elimination and insertion from an intermediate
1
tory combustion analysis. Data for 4iPr: H NMR (300 MHz,
C6D6, 23 °C): δ 7.28 (s, p-Ar), 6.71 (s, o-Ar), 6.69 (s, o-Ar), 6.55
(s, p-Ar), 6.23 (s, p-Ar), 4.09 (br, CH(CH3)2), 3.57(br, CH(CH3) 2),
2.67 (s, ArCH3), 2.45 (s, ArCH3), 2.26 (s, ArCH3), 2.18 (s,
ArCH3), 1.21 (br, CH(CH3)), 0.99 (d, CH(CH3)), 0.94 (d, CH-
(CH3)). 13C{1H} NMR (75.0 MHz, C6D6, 23 °C): δ 168.6, 152.9,
139.7, 138.2, 137.9, 137.8, 128.9, 128.7, 128.0, 127.9, 125.8,
125.3, 118.7, 117.8, 66.27, 53.41, 52.44, 34.78, 27.64, 24.67,
23.84, 23.08, 22.41, 21.99, 21.76, 15.95, 14.64. IR (CaF2, Et2O):
2960 (m), 2933 (m), 2873 (m), 1998 (w), 1811 (w), 1596 (s),
1583 (s), 1375 (w), 1295 (w) 1170 (w) cm-1. Anal. Calcd for
C63H89N6Nb2: C, 67.79; H, 8.04; N, 7.53. Found: C, 67.64; H,
7.94; N, 7.42. Mp: 152-154 °C.
[Substrate 2R-H] complex. The latter reactivity is consistent
3
with the high degree of Lewis acidity that these complexes
display. One aspect of this Lewis acidity is revealed in the
decomposition pathways of 2R-H and the need for sufficient
steric protectiontorender them kinetically stable for isolation.
Despite efforts to probe the possible role of niobaziridine
hydrides in N2 activation chemistry, to date they have not
been shown to be capable of N2 binding with either d0 or d1
electron configurations. In any case, a rich chemistry of the
niobaziridine hydride functional group has been elucidated,
but by no means has this area been exhaustively explored.
Synthesis of the Insertion Products 2Np-NC(H)NMe2, 2Np
-
NC(H)dC(H)t-Bu, and 2Np-N(H)NCPh2. For each reaction,
0.100 g (0.150 mmol) of 2Np-H was dissolved in 1.5 mL of
C6D6. To this solution was added 1.05 equiv of the correspond-
ing reagent (Me2NCN, t-BuCCH, and Ph2CN2, respectively)
dissolved in 0.5 mL of C6D6. The reactions were monitored by
1H NMR ca. 30 min after mixing and were observed to be
complete. Total reaction time for each was approximately 1.5 h. The
reaction mixtures were evaporated to dryness, extracted with
n-pentane, and filtered through Celite. For 2Np-C(H)dC(H)t-Bu:
Orange crystals from Et2O (-35 °C, 3 d). Yield: 0.080 g, 0.107
mmol, 71% in two crops. 1H NMR (300 MHz, C6D6, 23 °C): δ 8.19
(d, 1H, J = 18 Hz, C(H)dC(H)t-Bu), 7.31 (s, 2H, o-Ar), 6.98 (s,
2H, o-Ar), 6.62 (s, 1H, p-Ar), 6.58 (s, 1H, p-Ar), 6.55 (s, 1H, p-Ar),
6.41 (s, 2H, o-Ar), 6.36 (d, 1H, J = 18 Hz, C(H)dC(H)t-Bu), 4.26
(d, 1H, J = 14 Hz, N-CH2), 4.16 (d, 1H, J = 14 Hz, N-CH2), 2.87
(d, 1H, J = 14 Hz, N-CH2), 2.75 (d, 1H, J = 14 Hz, N-CH2), 2.60
(s, 1H, NdC(H)t-Bu aziridine), 2.37 (s, 6H, Ar-CH3), 2.26 (s, 6H,
Ar-CH3),2.15(s,6H,Ar-CH3),1.39(s, 9H, t-Bu), 1.26 (s, 9H, t-Bu),
0.83 (s, 9H, t-Bu), 0.45 (s, 9H, t-Bu). Anal. Calcd for C45H70N3Nb:
C, 72.45; H, 9.46; N, 5.63. Found: C, 71.93; H, 9.28; N, 5.89.
Isolation procedures and characterization data for 2Np-NC(H)-
NMe2 and 2Np-N(H)NCPh2 are provided in the Supporting
Information.
Experimental Section
General Procedures. All manipulations were carried out at
room temperature, under an atmosphere of purified dinitrogen,
using a Vacuum Atmospheres glovebox or Schlenk techniques.
All solvents were obtained anhydrous and oxygen-free accord-
ing to standard purification procedures. Benzene-d6 (C6D6) and
toluene-d8 (C7D8) were degassed and stored in the glovebox over
˚
4 A molecular sieves for at least 3 d prior to use. Celite 435 (EM
˚
Science) and 4 A molecular sieves (Aldrich) were dried under
vacuum at 250 °C overnight and stored under dinitrogen.
Benzaldehyde and tert-butylnitrile were purchased from Aldrich
and distilled from CaH2 prior to use. 2,4,6-Trimethylbenzo-
nitrile (MesCN, Aldrich) was recrystallized from n-hexane
and dried in vacuo prior to use. Nitrous oxide (N2O) was
obtained from BOC Gases and passed through a 1 ft column
of P2O5 before being introduced to the reaction vessel via a
Schlenk line. The compounds Mg(THF)3(anthracene),58 mer,
cis-NbCl3(THF)2(PhCCPh),122 and Ph2CN2
were prepared
123
according to literature procedures. Complexes 2Np-H, 5Np, 2Np
-
NCMes, 2Np-NC(H)t-Bu, and 2Np-OBz were prepared as pre-
viously reported unless otherwise stated.34 Complexes 1R-I2 and
1R-PhCCPh were prepared according to the methods outlined
for the 1Np system, and these procedures are detailed in the
Supporting Information. For details concerning niobium com-
plexes of the N(t-Bu)Ar ligand system, please see the Ph.D.
Thesis of Michael G. Fickes.123 All other reagents were obtained
from commercial sources and used as received or purified
according to standard procedures. All glassware was oven-dried
at a temperature of 170 °C prior to use.
Synthesis of the Cyclic Imido Complex Nb(dNC(H)t-BuC-
(H)t-BuNAr)(N[Np]Ar)2 (6Np-t-Bu). A toluene solution of com-
plex 2Np-NC(H)t-Bu (0.150 g, 0.200 mmol, 3 mL) was heated at
120 °C for 1.5 h. A gradual color change from blood red to
bright yellow was observed over this time. The toluene solvent
was removed under reduced pressure, and the resulting yellow
residue was extracted with n-pentane and filtered through
Celite. The solvent was removed again, and the crude solid
obtained was dissolved in approximately 0.5 mL of Et2O.
Yellow plates of 6a-t-Bu were obtained from this solution upon
standing at -35 °C for 3 d. Yield: 0.097 g, 0.130 mmol, 65% in
Synthesis of the Niobaziridine Hydride Complex Nb(H)(η2-
Me2CdNAr)(N[i-Pr]Ar)2 (2iPr-H) and (μ-NAr)(μ-N[i-Pr]C6H2-
Me2)Nb2(N[i-Pr]Ar)4 (4iPr). Complex 2iPr-H was prepared ana-
logously to complex 2Np-H by employing 2.5 g (2.99 mmol) of
Nb(I)2(N[i-Pr]Ar)3 (2iPr-I2). It was found that the total reaction
time including workup should be carried out on the order of
1
two crops. H NMR (400 MHz, C6D6, 23 °C): δ 7.02 (s, 2H,
o-Ar), 6.95 (s, 2H, o-Ar), 6.56 (s, 1H, p-Ar), 6.49 (s, 1H, p-Ar),
6.35 (s, 1H, p-Ar), 6.05 (s, 2H, o-Ar), 5.29 (s, 1H, backbone),
4.68 (d, 1H, J = 13.4 Hz, N-CH2), 4.29 (d, 1H, J = 13.4 Hz,
N-CH2), 4.14 (s, 1H, backbone), 3.94 (d, 1H, J=14 Hz, N-CH2),
3.38 (d, 1H, J = 14 Hz, N-CH2), 2.18 (s, 6H, Ar-CH3), 2.14 (s,
6H, Ar-CH3), 1.99 (s, 6H, Ar-CH3), 1.26 (s, 9H, t-Bu), 1.14
(s, 9H, t-Bu), 0.99 (s, 9H, t-Bu), 0.84 (s, 9H, t-Bu). Anal. Calcd
(122) Hartung, J. B.; Pedersen, S. F. Organometallics 1990, 9, 1414–
1417.
(123) Fickes, M. G. Ph.D. Thesis, Department of Chemistry,
Massachusetts Institute of Technology, Cambridge, MA, 1998.

5228 Organometallics, Vol. 29, No. 21, 2010
Figueroa et al.
for C44H69N4Nb: C, 70.75; H, 9.31; N, 7.50. Found: C, 69.94; H,
9.24; N, 7.30.
and 1.66 (m, 36H, 2Ad) ppm. 13C{1H} NMR (C6D6, 20 °C,
125.8 MHz): δ 154.8 (ipso-Ar), 138.9 (o-Ar), 126.0 (m-Ar), 122.8
(p-Ar), 76.2 (N-CH2), 42.1, 38.1 (NCH2-C), 37.9, 29.6, 21.9
(Ar-CH3) ppm.
Reaction of 1a-H with CO. Synthesis of the Enolate Imido
Complex Nb(OC(H)dC(H)t-Bu)(NAr)(N[Np]Ar)2 (7). An ex-
cess of CO(g) was introduced to an Et2O solution of 2Np-H (0.100 g,
0.150 mmol, 3 mL), eliciting a rapid color change from orange to
pale brown. The reaction mixture was allowed to stir for 1 h, at
which point all volatile materials were removed in vacuo. The
resulting pale yellow residue was extracted with n-pentane and
filtered through Celite. The solvent was removed again, and the
crude solid obtained was dissolved in approximately 0.5 mL of
Et2O. Yellow plates of 7 were obtained from this solution upon
standing at -35 °C for several days. Yield: 0.046 g, 0.067 mmol,
45%. 1H NMR (400 MHz, C6D6, 23°C): δ7.29 (d, 1H, J=6.6Hz,
enolate), 6.98 (s, 2H, p-Ar amido), 6.70 (s, 4H, o-Ar amido), 6.52
(s, 1H, p-Ar imido), 6.36 (s, 1H, o-Ar, imido), 4.45 (d, 1H,
J = 6.6 Hz, enolate), 4.45 (d, 2H, J = 13 Hz, N-CH2), 3.83 (d,
2H, J = 13 Hz, N-CH2), 2.11 (s, 6H, Ar-CH3 amido), 2.10 (s, 12H,
Ar-CH3 imido), 1.47 (s, 9H, t-Bu enolate), 0.89 (s, 18H, t-Bu amido).
13C{1H} NMR (75.0 MHz, C6D6, 23 °C): δ 150.2 (aryl ipso amido),
149.2 (aryl ipso imido), 139.6 (o-Ar amido), 138.5 (o-Ar imido),
126.6 (p-Ar imido), 126.0 (p-Ar amido), 123.3 (m-Ar imido), 120.8
(m-Ar amido), 117.6 (enolate), 111.8 (enolate), 72.5 (N-CH2), 35.6
(C(CH3)3 anilide), 32.2 (C(CH3)3 enolate), 31.8 (C(CH3)3 enolate),
28.7 (C(CH3)3 amido), 21.9 (Ar-CH3 amido), 21.6 (Ar-CH3 imido).
FT-IR (KBr windows, C6D6 solution): 2953, 2903, 1634, 1584, 1476,
1213, 1080, 686 cm-1. Anal. Calcd for C40H60N3ONb: C, 69.44; H,
8.74; N, 6.07. Found: C, 68.34; H, 8.56; N, 5.93.
Synthesis of the Niobaziridine Hydride Complex Nb(H)(η2-
C5H10CdNAr)(N[Cy]Ar)2 (2Cy-H). Complex 2Cy-H was prepared
analogously to complex 2Np-H employing 2.00 g (2.10 mmol) of
1
Cy-I2 and 0.970 g (2.32 mmol, 1.10 equiv) of Mg(THF)3-
(anthracene). Crude 2Cy-H was extracted from MgI2 and an-
thracene with n-pentane. A filtration using thawing n-pentane
was performed to remove residual anthracene. The product was
crystallized from a mixture of Et2O and (Me3Si)2O over several
days at -35 °C to afford 1Cy-H as an orange microcrystalline
1
solid (550 mg, 0.78 mmol, 38% yield). H NMR (500 MHz,
C6D6, 23 °C): δ 9.50 (br s, 1H, Nb-H), 7.29 (s, 2H, p-Ar), 6.83
(s, 4H, o-Ar), 6.64 (s, 2H, o-Ar), 6.58 (s, 1H, p-Ar), 3.1-3.3 (m,
4H, Cy), 2.4 (m, 2H, Cy), 2.36 (s, 6H, Ar-CH3), 2.12 (s, 12H,
Ar-CH3), 1.8 (m, 6H, Cy), 1.4-1.6 (m, 5H, Cy), 1.15-1.4
(m, 4H, Cy), 0.85-1.2 (m, 9H, Cy), 0.65 (m, 2H, Cy) ppm.
Benzaldehyde Insertion. Synthesis of (PhCH2O)Nb(η2-
C5H10CdNAr)(N[Cy]Ar)2 (2Cy-OCH2Ph). At 22 °C benzalde-
hyde (5.0 μL, 0.050 mmol, 1.05 equiv) was added to an Et2O
solution (2 mL) of orange (H)Nb(η2-C6H10dNAr)(NCyAr)2
(33 mg, 0.047 mmol). The solution immediately lightened
slightly to yellow. After 30 min the solvent was removed in
1
vacuo to reveal a yellow powder. H NMR (C6D6, 20 °C, 500
MHz): δ 7.34 (d, J=8 Hz, 2H, o-Ph), 7.26 (t, J=6 Hz, 2H, m-Ph),
7.22 (s, 2H, p-Ar), 7.16 (t, 1H, p-Ph), 6.75 (s, 4H, o-Ar), 6.66
(s, 2H, o-Ar), 6.60 (s, 1H, p-Ar), 5.65 (s, 2H, Ph-CH2), 3.85 (m,
2H, Cy), 3.15 (m, 2H, Cy), 2.36 (s, 6H, Ar-CH3), 2.14 (s, 12H,
Ar-CH3), 1.7-2.0 (m, 11H, Cy), 1.57 (m, 4H, Cy), 1.25-1.45 (m,
5H, Cy), 1.0-1.2 (m, 6H), 0.6-0.8 (m, 2H, Cy) ppm. 13C{1H}
NMR (C6D6, 20 °C, 125.8 MHz): δ 151.1, 147.1, 143.1, 139.0,
138.2, 138.0, 128.9, 127.8, 127.6, 127.5, 122.6, 117.7, 76.0 (ArN-C),
74.5(ArNdC), 64.7 (PhCH2O), 36.0, 35.5, 35.0, 27.8, 27.3, 26.9,
26.7, 26.1, 22.3 (Ar-CH3), 21.8 (Ar-CH3) ppm.
Synthesis of the Niobaziridine Hydride Complex Nb(H)(η2-Ad-
(H)CdNAr)(N[CH2-1-Ad]Ar)2 (2CH Ad-H). Complex 2CH Ad-H
was prepared analogously to complex 2Np-H employing 1.00 g
2
2
(0.868 mmol) of 1CH Ad-I2 and 0.454 g (1.085 mmol, 1.25 equiv) of
2
Mg(THF)3(anthracene).34 No special handling or attention to reac-
tion time is required. Crude 2CH Ad-H was extracted from MgI2 and
2
anthracene with Et2O due to its low solubility in n-pentane. The
cold-filtration step was performed using Et2O; however some
1
anthracene remained in the sample as assayed by H NMR. To
Synthesis of ONb(NCy2)3. Solid LiNCy2 (7.0 g, 59.3 mmol,
3.04 equiv) was added to a thawing Et2O slurry (100 mL) of
ONbCl3(THF)2 (7.0 g, 19.5 mmol, 1 equiv). The color of the
mixture changed from white to purple and then brown over the
course of the addition and the following 2.5 h, over which time it
was stirred while warming to 23 °C. After this time, the mixture
was filtered through Celite and then dried in vacuo. The residue
was extracted with a pentane/toluene mixture and filtered once
more. Precipitation from 30 mL of pentane at -35 °C afforded
the desired product as an off-white powder (4.95 g, 40% yield).
Crystallization from toluene/Et2O affords a bright white pow-
der. 1H NMR (C6D6, 20 °C, 500 MHz): δ 3.01 (m, 6H, N-CH),
1.89 (m, 12H, Cy), 1.79 (dd, 24H, Cy), 1.57 (d, 6H, Cy), 1.32 (q,
12H, Cy), 1.18 (t, 6H, Cy) ppm. 13C NMR (C6D6, 20 °C, 125.8
MHz): δ 60.1 (NC), 37.2, 27.7, 26.4 ppm. Anal. Calcd for
C36H66N3ONb: C, 66.54; H, 10.24; N, 6.47. Found: C, 66.74;
H, 10.02; N, 6.48.
Synthesis of (TfO)2Nb(NCy2)3. To a ca. -50 °C yellow-brown
solution of ONb(NCy2)3 (2.9 g, 4.46 mmol) in Et2O (80 mL) was
added a -35 °C solution of Tf2O (1.25 g, 4.43 mmol, 0.98 mmol)
in Et2O (10 mL). Upon addition, the solution turned bright
yellow concomitant with the formation of a yellow precipitate.
After stirring for 30 min, the solids were filtered off and washed
with pentane to afford the desired product (3.6 g, 3.86 mmol,
87% yield). 1H NMR (C6D6, 20 °C, 500 MHz): δ 4.86 (m, 6H),
2.24 (m, 12H), 1.79 (m, 12H), 1.58 (m, 30H), 0.97 (m, 6H) ppm.
13C NMR (C6D6, 20 °C, 125.8 MHz): δ 65.6 (N-CH), 36.2 (Cy),
27.7(Cy), 26.2 (Cy) ppm. Anal. Calcd for C38H66N3O6F6S2Nb:
C, 48.97; H, 7.14; N, 4.51. Found: C, 48.66; H, 6.87; N, 4.30.
Synthesis of Nb(H)(η2-C5H10CdNCy)(NCy2)2 (2Cy2-H). To a
thawing THF solution of (TfO)2Nb(NCy2)3 (250 mg, 0.27 mmol)
was added magnesium anthracene tris(tetrahydrofuran) (125 mg,
0.30 mmol, 1.1 equiv) solid in small portions. After the addition,
the mixture was allowed to stir for ∼45 min prior to removal of
remove the residual anthracene, crude 2CH Ad-H was dissolved in
2
ca. 6 mL of Et2O and left to stand overnight at -35 °C, whereupon
the residual anthracene crystallized out. Concentration of the
mother liquor to approximately 4 mL, followed by storage at
-35 °C, afforded pure 2CH Ad-H as a bright orange microcrystalline
2
solid. Orange single crystals of 2CH Ad-H were obtained from a
2
dilute Et2O solution left to stand at -35 °C for several weeks. Yield:
0.428 g, 0.477 mmol, 55%. 1H NMR (300 MHz, C6D6, 23 °C): δ
9.34 (br s, 1H, Nb-H), 7.11 (s, 2H, o-Ar), 7.03 (s, 2H, o-Ar), 6.62 (s,
1H, p-Ar), 6.57 (s, 1H, p-Ar), 6.54 (s, 1H, p-Ar), 6.14 (s, 2H, o-Ar),
4.30 (s, 2H, N-CH2), 2.70 (d, 1H, J = 13 Hz, N-CH2), 2.62 (d, 1H,
J = 13 Hz, N-CH2), 2.33 (s, 6H, Ar-CH3), 2.30 (s, 1H, NdC(H)-
Ad), 2.15 (s, 6H, Ar-CH3), 2.14 (s, 6H, Ar-CH3), 1.90-1.66 (m,
1-Ad), 1.47-1.32 (m, 1-Ad), 1.04-0.96 (m, 1-Ad). 13C{1H} NMR
(75.0 MHz, C6D6, 23 °C): δ 156.3 (aryl ipso), 145.5 (aryl ipso), 141.3
(p-Ar), 140.4 (p-Ar), 138.6 (p-Ar), 128.8 (aryl ipso), 127.4 (m-Ar),
125.79 (m-Ar), 122.8 (m-Ar), 117.4 (o-Ar), 116.8 (o-Ar), 115.5
(o-Ar), 82.7 (NdC(H)Ad), 68.7 (N-CH2), 66.3 (N-CH2), 57.8
(Ad), 44.8 (Ad), 42.3 (Ad), 41.6 (Ad), 38.3 (Ad), 38.2 (Ad), 37.6
(Ad), 37.4 (Ad), 37.3 (Ad), 36.3 (Ad), 34.7 (Ad), 30.0 (Ad), 29.4
(Ad), 29.3 (Ad), 23.1 (Ar-CH3), 22.3 (Ar-CH3), 22.0 (Ar-CH3),
21.9 (Ad), 15.9 (Ad), 14.6 (Ad).
Synthesis of (O)Nb(N[CH2-1Ad]Ar)3 (1CH Ad-O). To a 25 mL
2
Teflon-stoppered Schlenk tube was added a THF solution
(7 mL) of (H)Nb(η2-1Ad(H)CdNAr)(N[CH2-1Ad]Ar)2 (125 mg,
0.14 mmol). The head space was evacuated, then backfilled with
1 atm of N2O at 22 °C (∼1 mmol, ∼10 equiv). The solution was
allowed to stir for 18 h prior to removal of all volatiles in vacuo.
The residue was taken up in pentane/ethyl ether, and the desired
product was precipitated as a white powder (that produces
1
yellow solutions) at -35 °C in several crops. H NMR (C6D6,
20 °C, 500 MHz): δ 6.56 (s, 3H, p-Ar), 6.49 (s, 6H, o-Ar), 4.33
(s, 6H, N-CH2), 2.11 (s, 18H, Ar-CH3), 1.95 (s, 9H, 1Ad), 1.69

Article
Organometallics, Vol. 29, No. 21, 2010 5229
solvent and a thawing pentane extraction of the residue with
filtration through chilled Celite to remove anthracene and
salts. The filtrate was dried in vacuo to give a nice orange
powder, which could be recrystallized from Et2O/hexamethyl-
Reduction of Nb(H)(η2-t-Bu(H)CdNAr)(N[Np]Ar)2. To a 3:1
Et2O/THF (5 mL) solution of 2Np-H (100 mg, 0.15 mmol) was
added a slurry of KC8 (22 mg, 0.16 mmol, 1.1 equiv) in Et2O at
-35 °C. An immediate color change to emerald green ensued. An
aliquot of this reaction was analyzed by EPR spectroscopy to reveal
a 10-line pattern indicative of d1 Nb(IV). EPR (X-band, THF,
293 K): giso=1.97, Aiso(93Nb)=110 G. On one occasion crystals of
green [(THF)3Na][Nb(H)(η2-t-Bu(H)CdNAr)(N[Np]Ar)2] were
obtained from the reaction mixture of 1Np-OTf2 and 1% Na/Hg
following removal of 2Np-H by pentane extraction and crystal-
lization of the green byproduct from Et2O at -35 °C.
1
disiloxane in good yield (ca. 70%). H NMR (C6D6, 20 °C,
500 MHz): δ 9.0 (br s, NbH, 1H), 2.88 (m, 2H), 2.69 (pseudo-t,
1H), 2.46 (m, 2H), 1.0-2.1 (m, 60H) ppm. 13C NMR (C6D6,
20 °C, 125.8 MHz): δ 73.4 (NdC), 63.8 (N-CH), 53.7 (N-CH),
40.5, 38.9, 38.3, 37.3, 28.6, 27.45, 27.35, 27.3, 27.2, 26.5,
26.4 ppm.
Benzaldehyde Insertion into Nb(H)(η2-C5H10CdNCy)(NCy2)2
(2Cy2-OCH2Ph). Synthesis of(PhCH2O)Nb(η2-C5H10CdNCy)-
(NCy2)2. At -35 °C, an Et2O solution (2 mL) of benzaldehyde
(22 mg, 0.21 mmol, 1.0 equiv) was added to an Et2O solution
(4 mL) of orange Nb(H)(η2-C5H10CdNCy)(NCy2)2 (130 mg,
0.21 mmol). The solution immediately lightened slightly to yellow.
After 30 min the solvent was removed in vacuo to reveal a yellow
powder. 1H NMR (C6D6, 20 °C, 300 MHz): δ 7.78 (d, J = 7 Hz,
2H, o-Ph), 7.27 (t, J = 7 Hz, 2H, m-Ph), 7.13 (t, 1H, p-Ph), 5.73 (s,
2H, Ph-CH2), 3.71 (m, 1H), 3.2 (m, 4H), 2.69 (m, 2H), 2.45 (m,
2H), 2.15 (m, 2H), 1.0-2.0 (m, 54H) ppm.
Acknowledgment. We gratefully thank the USA Na-
tional Science Foundation for financial support (CHE-
719157) and for a predoctoral fellowship to N.A.P.
Supporting Information Available: Full experimental and
spectroscopic details for all new compounds, results of crystal-
lographic structure determinations, computational and electro-
chemical studies and kinetics data (PDF and CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.