A nitrido salt reagent of titanium

Article abstract of DOI:10.1021/ja504020t

Deprotonation of the parent titanium imido (^tBunacnac)Ti= NH(Ntolyl₂) (^tBunacnac^- = [ArNC ^tBu]₂CH; Ar = 2,6-ⁱPr₂C ₆H₃) with KCH₂Ph forms a rare example of a molecular titanium nitride as a dimer, {[K][(^tBunacnac)Ti= N(Ntolyl₂)]}₂. From the parent imido or nitride salt, the corresponding aluminylimido-etherate adduct, (^tBunacnac)Ti= N[AlMe₂(OEt₂)](Ntolyl₂), can be isolated and structurally characterized. The parent imido is also a source for the related borylimido, (^tBunacnac)Ti=NBEt₂(Ntolyl₂).

Full text of DOI:10.1021/ja504020t

Communication
pubs.acs.org/JACS
A Nitrido Salt Reagent of Titanium
Rick Thompson,† Chun-Hsing Chen,‡ Maren Pink,‡ Gang Wu,§ and Daniel J. Mindiola*,†
†Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States
‡Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States
§Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
Scheme 1. Synthetic Protocol for the Parent Imido 2 and
Nitride 3
ABSTRACT: Deprotonation of the parent titanium imido
(
t B u nacnac)TiNH(Ntolyl2 )
( =
t B u nacnac−
[ArNCtBu]2CH; Ar = 2,6-iPr2C6H3) with KCH2Ph forms
a rare example of a molecular titanium nitride as a dimer,
{[K][(tBunacnac)TiN(Ntolyl2)]}2. From the parent
imido or nitride salt, the corresponding aluminylimido−
etherate adduct, (tBunacnac)TiN[AlMe2(OEt2)]-
(Ntolyl2), can be isolated and structurally characterized.
The parent imido is also a source for the related
borylimido, (tBunacnac)TiNBEt2(Ntolyl2).
ecause of the large difference in electronegativity and
Bconsequentially the polarization of the metal−nitrogen
bond, titanium complexes possessing a terminal nitride ligand
remain elusive.1 To date, the only documented examples of
mononuclear titanium nitrides are those reported by Lancaster
and co-workers, in which the nitride functionality is protected
by a borane group,2−4 rendering them somewhat inert to
further reactivity. We recently reported a rare example of a four-
coordinate parent titanium imido complex, (Menacnac)Ti
NH(Ntolyl2) (Menacnac− = [ArNCMe]2CH; Ar =
2,6-iPr2C6H3);5 however, all attempts to deprotonate the
imido ligand resulted instead in deprotonation of the β-methyl
backbone of the β-diketiminate ligand. This transformation has
precedent and is attributed to the ligand’s propensity to
tautomerize to the tethered bisanilide species.6−9 For this
reason, we focused our attention toward preparing an
analogous parent titanium imido having the more sterically
encumbering and robust tert-butyl-substituted ligand tBunacnac
titanium−nitrogen distance increased from 2.016(2) Å in the
Menacnac analogue to 2.071(1) Å in 1. Following a
protocol similar to that for the parent imido
(Menacnac)TiNH(Ntolyl2),5 treatment of 1 with a slight
excess of NaN3 in THF resulted in a gradual color change from
olive green to yellow-orange.14 Workup of the reaction mixture
and recrystallization of the yellow precipitate resulted in
isolation of the terminal parent imido (tBunacnac)TiNH-
(Ntolyl2) (2) in 45% yield (Scheme 1).14 1H NMR
spectroscopic data revealed a Cs-symmetric molecule with one
tBu environment, as well as two locked iPr and tolyl groups on
the N-aryl and Ntolyl2 moieties, respectively. A broad feature at
1
3.86 ppm in the H NMR spectrum is consistent with the
terminal NH resonance, and preparation of the isotopomer
15
tBunacnac)Ti NH(Ntolyl2) (2-15N) using 98%-enriched
(
tBunacnac− = [ArNCtBu]2CH; Ar = 2,6-iPr2C6H3).10,11 In this
(
Na15N3 resolved nearly 50% of the broad feature into a doublet
work, we showcase spectroscopic and structural evidence for
the first molecular titanium nitride salt and its reactivity in the
form of a terminal nitride. We also demonstrate that in addition
to being a precursor to the nitride ligand, the parent imido is a
convenient template for rare imido scaffolds.
1
centered at 3.86 ppm with a JNH value of 64 Hz. An INEPT
15N{1H} NMR spectrum of 2-15N evinced a sharp resonance at
421 ppm with an identical JNH value.14 Lastly, the solid-state
1
IR (Nujol) spectrum of 2 displayed a sharp stretch at 3397
cm−1, while 2-15N showed a red-shifted stretch at 3390 cm−1, in
good agreement with the difference predicted by the harmonic
oscillator model (Δ = 8 cm−1).14 X-ray diffraction studies on
yellow crystals of 2 further corroborated our assessment of this
species as mononuclear and thus possessing a rare example of a
terminal NH ligand (Figure 1).15 In the solid-state structure of
2, the imido Ti−N distance [Ti1−N4 = 1.6917(15) Å] is
The compound (tBunacnac)TiCl(Ntolyl2) (1) was prepared
12
in 74% yield via transmetalation of (tBunacnac)TiCl2 with
LiNtolyl2 (Scheme 1).13,14 Complex 1 displays characteristic
features of a d1 species (Evans method: μeff = 1.82μB, 25 °C,
C6D6), while the room-temperature X-band EPR spectrum
shows features similar to those of the analogue (Menacnac)-
TiCl(Ntolyl2)5 (giso = 1.96, W = 5 G, toluene). A solid-state
structure also confirmed the formation of a mononuclear, four-
coordinate Ti(III) species (Figure 1). Likely as a result of the
increased steric bulk of the tBunacnac ligand, the Ntolyl2
Received: April 24, 2014
© XXXX American Chemical Society
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Communication
Figure 1. Solid-state structures of complexes 1−3 displaying thermal ellipsoids at the 50% probability level. All hydrogens (with exception of the
imido H in 2) and solvent molecules have been omitted for clarity. The isopropyl groups on 3 have also been omitted for clarity.
slightly longer than that in the related Menacnac species
[1.6885(14) Å]. Additionally, the N−H proton was located in
the difference map and refined isotropically. Interestingly,
oxidation of the Ti center in 1 to the d0 species 2 resulted in
substantial twisting of the anilide ligand to an up−down
conformation that allows for optimal π donation by the Ntolyl2
lone pair to the metal center. This was also corroborated by a
0.09 Å contraction of the Ti1−N3 distance in 2 compared with
1.
To assess whether the solvent is the H-atom source in the
formation of 2, the synthesis of the isotopomer (tBunacnac)-
TiND(Ntolyl2) (2-d1) in THF-d8 was attempted. The IR
spectrum of the resulting product revealed no sign of an N−D
stretching frequency. Disappointingly, traditional H-atom
Figure 2. Solid-state 15N NMR spectrum of 3 (black) and simulated
spectra of 3 (red) and 2 (blue) overlaid.
sources such as 1,4-cyclohexadiene and 9,10-dihydroanthracene
did not lead to improved yields of 2, suggesting the source of
this atom to be ligand-based (hence resulting in a yield of less
OAr].13,17 The observed 15N chemical shift tensor parameters
than 50%). Therefore, we speculate at this time that the source
are characteristic of a terminal metal−ligand triple bond, as
previously noted in terminally bound molybdenum nitride,
phosphide, and carbide compounds.16 For example, the 15N
chemical shift tensor for 15NMo(N[tBu]Ar′)3 (Ar′ = 3,5-
of the H atom must derive from degradation of the tBunacnac or
Ntolyl2 ancillaries.
Complex 2 reacts cleanly with benzylpotassium in toluene to
afford a poorly soluble crystalline product along with toluene
Me2C6H3) is δiso = 833 ppm, δ11 = δ22 = 1229 ppm, and δ33
=
1
1
(detected by H NMR spectroscopy). The H NMR spectrum
of this new material is characteristic of a similar Cs-symmetric
species with the two aryl rings of the Ntolyl2 group still locked
in place, consistent with formation of a nitride having the
formula “[K][(tBunacnac)Ti(N)(Ntolyl2)]”. The most salient
42 ppm. Interestingly, the 15N chemical shift tensor in
“[K][(tBunacnac)TiN(Ntolyl2)]” deviates considerably from
axial symmetry [κ = 3(δ22 − δiso)/(δ11 − δ33) = 0.64], and this
feature can be attributed to the fact that the Ti center in
“[K][(tBunacnac)TiN(Ntolyl2)]” does not possess a C3-
symmetric environment, inasmuch as the nitrido atom is
further involved in interactions with K+ cations. This latter
structural feature in “[K][(tBunacnac)TiN(Ntolyl2)]” is also
similar to that observed in the isovalent Mo−carbido having
two bridging K+, {[K][(13CMoN[tBu]Ar)3]}2, for which the
13C chemical shift tensor is also nonaxially symmetric (κ =
0.88).16c For completeness, the 15N chemical shift tensor for
15N-enriched 2 (2-15N) was also determined: δiso = 416(2)
ppm, δ11 = δ22 = 656(50), δ33 = −64(50) ppm. Once again, the
general features of this 15N chemical shift tensor for the
1
change in the H NMR spectrum is the disappearance of the
broad parent imido (TiNH) resonance centered at 3.86
1
ppm. Likewise, the doublet H NMR resonance for 2-15N was
also absent after this species was treated with benzylpotas-
sium.14 Despite several attempts, no signal was observed in the
solution-state 15N NMR spectrum, which we attribute to the
low solubility of the “[K][(tBunacnac)TiN(Ntolyl2)]” salt in
most common nonprotic organic solvents in addition to the
low NMR sensitivity of the 15N nucleus (∼0.1% as sensitive as
1H). To circumvent this limitation, we acquired solid-state 15N
NMR spectra to further characterize the Ti−N bonding in this
insoluble product.14 Gratifyingly, the 15N NMR signal for the
15
Ti NH moiety in 2-15N are analogous to those of the 13C
chemical shift tensor reported for H13CMo(N[tBu]Ar)3,16c
15
15N-enriched nitrido ligand in “[K][(tBunacnac)Ti N-
in accord with a triple-bonded ligand.
(Ntolyl2)]” is highly deshielded [δiso = 898(2) ppm] and also
exhibits an exceedingly large chemical shift anisotropy spanning
a range of over 1000 ppm [δ11 = 1353(50), δ22 = 1143(50), δ33
= 198(50) ppm; Figure 2]. The highly deshielded resonance,
albeit in the solid-state, is comparable to those of neutral
vanadium nitrides having similar scaffolds, namely,
[(Menacnac)VN(X)] [X = N(mesityl)(tolyl), Ntolyl2, and
To conclusively establish the connectivity of this salt-like
material and to ascertain the degree of aggregation in
“[K][(tBunacnac)TiN(Ntolyl2)]” we collected X-ray diffrac-
tion data on a single crystal grown rapidly from Et2O at −37
°C. Scheme 1 and Figure 1 portray two titanium nitride
fragments, [(tBunacnac)TiN(Ntolyl2)]−, bridged by two K+
ions, labeled as compound {[K][(tBunacnac)TiN(Ntolyl2)]}2
B
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Communication
(3). The most notable feature is the short titanium−nitride
distance of 1.660(4) Å, which is slightly shorter than that in the
B(C6F5)3-protected titanium nitrides reported by Lancaster and
co-workers (1.671 Å).2−4 The K2N2 core forms a perfect
diamond shape (sum of the four angles = 360°) while the K+
interacts with the π components (mostly the ipso and ortho
positions) of the aryl moieties of the β-diketiminate ligand.
Interestingly, the use of NaHBEt3 as an alternative base with
2 resulted in neither 3 nor a hypothetical borane-capped nitride
salt “[Na][(tBunacnac)TiNBEt3(Ntolyl2)]” but rather
showed conversion to the borylimido complex (tBunacnac)-
TiNBEt2(Ntolyl2) (4) in addition to H2 (Scheme 2). The
Scheme 2. Convergent Synthesis of the First Boryl and
Aluminyl Imidos of Group 4 Transition Metals Using Either
Nitride 3 or the Parent Imide
Figure 3. Solid-state structures of complexes 4 and 5 depicting thermal
ellipsoids at the 50% probability level. All hydrogen atoms have been
omitted for clarity.
orange precipitate that is poorly soluble in arenes. Slow
evaporation of a concentrated Et2O solution of this complex led
to the formation of yellow-green dichroic crystals of the
aluminylimido−etherate adduct (tBunacnac)TiN-
[AlMe2(OEt2)](Ntolyl2) (5) (Scheme 2). Compound 5 is
also Cs-symmetric in solution as gauged by 1H NMR
spectroscopy, which revealed a single methyl resonance at
−0.925 ppm as well as broad Et2O resonances at 0.594 and 3.05
ppm. The 27Al NMR spectrum of 5 exhibits a very broad signal
located at ∼191 pm, confirming the Al to be four-coordinate in
solution.14
The solid-state structure of 5 (Figure 3) divulges a linear Ti−
N−Al moiety [166.34(14)°] containing a short Ti−N bond
[1.6922(19) Å], again indicative of multiple-bond character,
while the Al−N distance [1.834(2) Å] is more in accord with a
single bond as a result of coordination of a molecule of Et2O.
Furthermore, the Al−Me bond lengths [1.961(4) and 1.969(3)
Å] are not significantly elongated relative to an average Al−Me
bond, also signifying little hyperconjugation with the imido π
system. The geometry around the aluminum can be considered
as distorted tetrahedral, with the most acute angle being the
N4−Al1−O1 angle [98.48(10)] because of the longer Al−O
dative bond [1.960(2) Å]. To the best of our knowledge,
complex 5 constitutes the first example of a transition metal
having a terminally bound aluminylimido ligand. Recognizing
that the nitride salt could undergo salt elimination reactions to
afford the same product, we added a dilute solution of AlMe2Cl
in hexanes to a benzene solution of 3, which resulted in the
formation of the same poorly soluble orange product,
presumably “(tBunacnac)TiN[AlMe2](Ntolyl2)”, which upon
exposure to Et2O yielded 5, as gauged by 1H NMR
spectroscopy.
use of 2 equiv of NaHBEt3 was required in order to drive the
reaction to completion as well as the formation of precipitates.
1H NMR spectroscopy of the crude mixture showed no
evidence of salt biproducts such as NaEt, but this is not
surprising given its poor solubility in nonpolar solvents.18
1
Furthermore, ethane or ethylene were not detected by H
NMR spectroscopy, suggesting that an ethyl radical pathway is
not likely. However, the 11B NMR spectrum of the crude
mixture was more informative, displaying a resonance at −13
ppm which we tentatively assign as NaBEt4;19 the relative
instability of this anion is also shown by the presence of a
second 11B resonance at 86 ppm, which corresponds to BEt3.14
1
The H and 13C NMR spectra of 4 reveal an overall geometry
akin to that of parent imido 2. Notably, the solid-state structure
of 4 (Figure 3) depicts a tetrahedral titanium complex having a
terminal linear borylimido [TiN, 1.722(3) Å; NB,
1.428(6) Å; Ti−N−B, 168.1(3)°], in accord with a delocalized
π TiNB topology. Early transition metal borylimidos are
extremely rare,20 and complex 4 represents the first example
involving a group 4 transition metal. The formation of 4 is also
intriguing because the B−N bond results from the breaking of a
very strong B−C bond (bond dissociation energy = 107.1 kcal/
mol). We found that direct addition of BEt3 to 3 did not afford
4, suggesting that 4 does not result from thermal decom-
position of a putative [Na][(tBunacnac)TiNBEt3(Ntolyl2)]
species. This result also argues that the second equivalent of
NaHBEt3 is indispensable in the formation of 4.
While compound 3 is partially soluble and stable in both
Et2O and DME for several hours, the presence of THF rapidly
decomposes this nitride to a dark-orange compound of much
1
lower symmetry based on H NMR. Similarly, attempts to
encapsulate the K+ of 3 by forming the discrete salt [K(2,2,2-
Cryptand)][(tBunacnac)TiN(Ntolyl2)] (2,2,2-Cryptand =
1,10-diaza-4,7,13,16,21,24-hexaoxabicyclo[8,8,8]hexacosane)
failed under various conditions, instead resulting in what we
propose to be intramolecular cross-metathesis rearrangements
involving the β-diketiminate scaffold and the nitride (among
other products). We have observed similar metathesis trans-
formations of nacnac with nitride, alkylidyne, and alkylidene
ligands.13,17,21
Intrigued by the ability of 2 to function as a nitride synthon,
we reacted this species with AlMe3 to readily eliminate methane
1
gas (observed by H NMR spectroscopy), forming a dark-
C
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Journal of the American Chemical Society
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(15) McKarns, P. J.; Yap, G. P. A.; Rheingold, A. L.; Winter, C. H.
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In conclusion, we have shown that as a result of a
modification of the nacnac ligand, deprotonation of a parent
titanium imido leads to the formation of the first molecular
titanium nitride salt and that this species serves as a source of a
terminal nitride. Likewise, we confirmed that the parent imido
2 may be used as a complementary nitride synthon, taking
advantage of the vulnerable N−H bond.
(16) (a) Sceats, E. L.; Figueroa, J. S.; Cummins, C. C.; Loening, N.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, spectral data, and crystallographic
information (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(18) Whitmore, F. C.; Zook, H. D. J. Am. Chem. Soc. 1942, 64, 1783.
(19) Noth, H.; Vahrenkamp, H. Chem. Ber. 1966, 99, 1049.
̈
AUTHOR INFORMATION
Corresponding Author
mindiola@sas.upenn.edu
(20) (a) Danopoulos, A. A.; Redshaw, C.; Vaniche, A.; Wilkinson, G.;
Hussain-Bates, B.; Hursthouse, M. D. Polyhedron 1993, 12, 1061.
(b) Fryzuk, M. D.; MacKay, B. A.; Johnson, S. A.; Patrick, B. O. Angew.
Chem., Int. Ed. 2002, 41, 3709. (c) Weber, K.; Korn, K.; Schorm, A.;
Kipke, J.; Lemke, M.; Khvorost, A.; Harms, K.; Sundermeyer, J. Z.
Anorg. Allg. Chem. 2003, 629, 744.
■
Notes
The authors declare no competing financial interest.
(21) Basuli, F.; Bailey, B. C.; Tomaszewski, J.; Huffman, J. C.;
Mindiola, D. J. J. Am. Chem. Soc. 2003, 125, 6052.
ACKNOWLEDGMENTS
■
This work was supported by the Chemical Sciences, Geo-
sciences, and Biosciences Division, Office of Basic Energy
Sciences, Office of Science, U.S. Department of Energy (DE-
FG02-07ER15893). G.W. thanks the NSERC of Canada for
financial support. We also thank Dr. Ba L. Tran and Keith
Searles for insightful discussions.
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D
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