A four coordinate parent imide via a titanium nitridyl

Article abstract of DOI:10.1039/c1cc14574f

Treatment of d¹ [(nacnac)TiCl(Ntol₂)] with NaN ₃ results in NaCl formation and N₂ ejection to yield the first four coordinate, parent imide [(nacnac)TiNH(Ntol₂)] (nacnac^-[ArNC(CH₃)]₂CH, Ar = 2,6-iPr ₂C₆H₃, tol = 4-CH₃C ₆H₄).

Full text of DOI:10.1039/c1cc14574f

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A four coordinate parent imide via a titanium nitridylwz
Ba L. Tran, Marlena P. Washington, Danielle A. Henckel, Xinfeng Gao, Hyunsoo Park,
Maren Pink and Daniel J. Mindiola*
Received 26th July 2011, Accepted 1st September 2011
DOI: 10.1039/c1cc14574f
Treatment of d1 [(nacnac)TiCl(Ntol2)] with NaN3 results in
NaCl formation and N2 ejection to yield the first four coordinate,
that treatment of a similar Ti(III) precursor with NaN3 should
incorporate a reactive and unprecendented nitridyl moiety of
titanium [(nacnac)Ti(Nꢁ)(Ntol2)] via the entropy driven loss of
nitrogen. Accordingly, transmetallation of [(nacnac)-
TiCl2(THF)]11 with LiNtol2 in toluene readily forms the
titanium precursor [(nacnac)TiCl(Ntol2)] (1) in 60% yield as
dark green colored crystals after recrystallization from diethyl
Q
parent imide [(nacnac)Ti NH(Ntol2)] (nacnac
CH, Ar = 2,6-iPr2C6H3, tol = 4-CH3C6H4).
ꢀQ
[ArNC(CH3)]2-
Parent imides of titanium, [LnTiQNH], are rare entities which
is rather surprising given their presumed involvement in
industrially important processes such as the fabrication of
thin metal nitride films (via MOCVD methods).1,2 Likewise,
parent imides are intermediates in the Chatt-cycle involved in
the generation of NH3 from N2, protons and electrons,3 hence
playing an important role enroute to molecular metal nitrides.
To date, the only example of a terminal parent imide of
titanium (or a group 4 transition metal) is the complex
[(Ph3PO)2TiQNH(Cl)2] reported by Winter and co-workers.4
Even though the imide functionality is now a common ligand
in titanium chemistry,5 the parent imide is not because of the
lack of suitable reagents to incorporate the NH group.5 Unlike
metal nitrenes, which are often supported by electron rich
metal centers,6 we present in this work the metal nitridyl: an
electron deficient nitride monoradical ligand supported by an
ether at ꢀ37 1C (Scheme 1).12 Replacing a Clꢀ with Ntol2
ꢀ
ligand provides steric protection at the metal center in a
tetrahedral environment and also restricts salt elimination at
only one Clꢀ ligand given the poor solubility of NaN3 in most
common organic solvents. Complex 1 shows characteristic
features of a d1 species by the method of Evans (meff = 1.89 mB,
25 1C, C6D6), and by an isotropic X-band EPR spectrum
collected at room temperature (giso = 1.95, W = 45 G,
toluene).12 A solid state structure of 1y provides the connectivity
of this radical and reveals a pseudo-tetrahedral complex
reminiscent to the related structure of [(nacnac)VCl(Ntol2)].10
The molecular structure of 1 also displays a Ti atom that deviates
0.6381(9) A above the imaginary plane (N1,N2,N3) defined by
the anilide and nacnac nitrogens (Fig. 1). The above and below
positions of the tolyl fragments, with respect to the aforemen-
tioned plane, forces the flanking aryl groups of the nacnac ligand
to be locked thereby providing a sterically protected pocket at the
Ti(III) center.
electron deficient metal center.7–9 The nitridyl moiety,
:
LnMQN: 2 LnMRNꢁ, is an exceedingly rare functional
group in part because of its reactivity at the electron deficient
N-atom as well as accessibility from suitable precursors. Entry
to a terminal nitride radical ligand has been seldom reported,
involving either an outer-sphere one-electron oxidation of a
nitride anion,7–9 or oꢀne9-electron reduction of a N-atom source
Treatment of 1 with a slight excess of freshly purified
12
NaN3 gradually results in a color change from green to red
with concurrent formation of a diagmagnetic material along
with other side-products.12 Workup of the reaction mixture
and subsequent recrystallization of the orange precipitate
allows for identification of this new diamagnetic species in
30% yield.12 By 1H NMR spectroscopy, only one methyl
environment for the b-carbon of nacnac is observed, while
two iPr and tolyl environments are clearly identified for the
locked ‘‘up and down’’ groups on the aryl and Ntol2 moeities
consistent with an intact (nacnac)Ti(IV)(Ntol2) skeleton having
overall Cs symmetry in solution. Unfortunately, the third
ligand (presumably a NH resonance or N based ligand) was
such as an azide, N3
.
We recently reported the unsaturated vanadium nitrides,
[(nacnac)VRN(N[tol]R0)] (tol = 4-MeC6H4; R0 = tol or
2,4,6-Me3C6H2), by treatment of the V(III) precursor
[(nacnac)VCl(N[tol]R0)] with NaN3, followed by N2 extrusion
from the metastable azide [(nacnac)V(N3)(N[tol]R0)].10 Being
one electron shy of [(nacnac)VCl(N[tol]R0)], it was envisioned
Indiana University, 800 E. Kirkwood Avenue, Bloomington,
IndianaUSA. E-mail: mindiola@indiana.edu; Fax: +1 812 8558300;
Tel: +1 812 855 2399
w This article is part of the ChemComm ‘Emerging Investigators 2012’
themed issue.
z Electronic supplementary information (ESI) available: Complete
synthetic details and spectroscopic characterization, including NMR
spectroscopic data for complexes 1 and 2. The molecular structure of
compounds 1 and 2 have been deposited in the CCDC as 829540 and
829541. See DOI: 10.1039/c1cc14574f.
Scheme 1 Synthesis of complexes 1 and 2 from salt metathesis.
Chem. Commun., 2012, 48, 1529–1531 1529
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Fig. 1 Molecular structures of 1 (right) and 2 (left) shown with
H-atoms (except for N4, complex 2) have been excluded for clarity.
Selected bond lengths (A) and angles (deg) for 1: Ti–N(1), 2.037(4);
Ti–N(2), 1.9995(4); Ti–N(3), 1.9577(4); Ti–Cl(1), 2.3234(3);
N(1)–Ti–N(3), 124.67(19); N(1)–Ti–N(2), 90.99(17); N(1)–Ti–Cl(1),
105.82(19); N(3)–Ti–Cl(1), 111.77(19). For 2: Ti–N(4), 1.688(4);
Ti–N(2), 2.019(3); Ti–N(3), 2.016(4); N(4)–H, 0.880; Ti–N(1),
2.031(3); N(1)–Ti–N(2), 94.06(5); N(2)–Ti–N(3), 114.51(5);
N(3)–Ti–N(4), 112.27(6); N(4)–Ti–N(1), 109.06(6); N(4)–Ti–N(2),
108.56(6); N(3)–Ti–N(1), 116.85(6).
not easily identified possibly due to coupling to quadrupolar
14N. This feature is not uncommon, since Winter4 and
Schrock,13 could not locate such resonance for the few
reported titanium and vanadium complexes having a terminal
parent imide ligand. Since our data provided inconclusive
evidence for the parent imide ligand (or third ligand), we
prepared the 15N (from 98% enriched Na15N3 at one terminal
Fig. 2 (left) Expanded 15N NMR spectrum of 15N isotopmer of 2
showing 15N–1H coupling (left). Expanded FT-IR (Nujol) spectrum of
2-15N (right). Bottom figure is the HSQC NMR spectrum of 2-15
N
15
nitrogen) isotopomer [(nacnac)TiQ NH(Ntol2)] (2)-15N
1
(
15N (vertical), H (horizontal).
(which contains B 50% of unlabeled 2). The 15N{1H} NMR
spectrum of 2-15N evinced a sharp resonance at 430 ppm with
a 1JNH value of 64 Hz (Fig. 2).14 Moreover, the solid state IR
(Nujol) spectrum of 2 displayed a sharp stretch at 3367 cmꢀ1
while 2-15N showed two stretches for 50 : 50 mixture of 2 and
2-15N with the labeled compound red shifted to 3359 cmꢀ1
(Fig. 2) in accord with the value predicted by the harmonic
oscillator (3359 cmꢀ1). Careful analysis of the 1H NMR
spectrum of 2-15N revealed a doublet at 4.16 ppm with a
Formation of complex 2 from 1 and NaN3 most likely
involves salt elimination to form a titanium radical azide
complex [(nacnac)Ti(N3)(Ntol2)] (A), which undergoes N2
extrusion to form a transient nitridyl [(nacnac)Ti(Nꢁ)(Ntol2)]
(B) (Scheme 2). Although nitridyl radicals of this type are not
observable, their formation is known to accompany ligand
radical ejection, oligomerization, or H atom abstraction
processes.16 In our case, species B appears to abstract an H atom.
To probe the origin of the H atom, we conducted a similar
reaction of 1 and NaN3 or Na15N3 in THF-d8. In the case of
unlabeled azide, formation of 2 is the only detectable species,
whereas formation of [(nacnac)TiQND(Ntol2)] is not evident
by a combination of IR and 2H NMR spectroscopies; therefore,
implying that solvent is not the source of hydrogen to form 2.
Reacting 1 with Na15N3 in THF-d8 also revealed formation of
2-15N without evidence for D-incorporation from the solvent
1
1JHN value of 64 and a broad resonance, arising from H-14N
coupling, sandwich between the doublet due to the presence of
unlabeled 2 (Fig. 2). Applying an 15N/1H HSQC experiment,
we could correlate the imido TiQNH functionality to the
broad resonance at 4.16 ppm (Fig. 2).
Although the IR and 15N NMR spectroscopic data suggested
2 to contain a parent imide motif, we resorted to X-ray structural
analysis of a small crystal using a synchrotron radiation source to
conclusively establish the connectivity in 2.z A perspective view
of the solid state structure of 2 is shown in Fig. 1, along with
selected bond lengths and angles, revealing a pseudo-tetrahedral
complex whereby the nacnac and Ntol2 basal ligands sterically
protect the terminal parent imido group. Both the iPr groups of
the aryl moeities of nacnac as well as the tol groups, with respect
to the (N1, N2, N3) plane, are in the upright position. The short
TiQN distance of 1.688(4) A for the parent imide ligands lends
support to a multiple bond, comparable to Winter’s imide
(1.627(8) A)4 but possibly having triple bond character analogous
to the VRN distance of the isoelectronic nitride
[(nacnac)VRN(N[tol]2,4,6-Me3C6H2)] (1.573(2) A),10 as well
as the TiRN distance of the zwitterionic titanium nitride borane
adduct reported by Lancaster (1.671(3) A).15
15
as we clearly detected a doublet at 4.20 ppm for the TiQ NH
moiety (vide supra). Based on this information, we propose
H-atom abstraction to derive from the nacnac, Ntol2 ligands,
or even adventitious impurities present in the reaction mixture.
Scheme 2 Proposed mechanism to formation of complex 2 via azide
(A) and nitridyl (B) intermediates.
c
1530 Chem. Commun., 2012, 48, 1529–1531
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The moderate yield of 2 and the spectroscopic observation of
other side products by 1H NMR spectroscopy of the crude
reaction mixture imply H-atom abstraction to be non-selective.
However, we cannot refute the possibility of some (albeit
minor) H-atom abstraction from the solvent medium taking
place or a large KIE resulting from deuteration of the solvent.
Attempts for cleaner formation of 2 from the reaction of 1 and
NaN3 in 1,3-cyclohexadiene or 9,10-dihydroanthracene
resulted in no improvement of the yield. Using N3SiMe3 with
1 also formed 2 in similar isolated yields (34%).12
absorption coefficient = 0.142, F(000) = 1456, y range (deg) =
0.82 ꢀ 18.54, no. of reflections collected = 85 507, no. of unique
reflections = 14 417, data/parameter ratio = 32.39, R(F) = 0.0535,
Rw(F2) = 0.1017, GOF = 1.040, largest diff. peak and hole = 0.953
and ꢀ1.002.
1 J. B. Everhart and B. S. Ault, Inorg. Chem., 1995, 34, 4379;
J. P. Dekker, P. J. van der Put, H. J. Veringa and J. Schoonman,
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A. W. Laubengayer, J. Am. Chem. Soc., 1955, 77, 5250.
Preliminary attempts to generate a titanium nitride anion by
deprototonation of complex 2-15N using bases such as KH or
KCH2Ph in THF failed to remove the proton. Instead, a new
product is produced in the reaction mixture based on multi-
2 G. W. A. Fowles and F. H. Pollard, J. Chem. Soc., 1953, 2588;
R. M. Fix, R. G. Gordon and D. M. Hoffman, J. Am. Chem. Soc.,
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B. R. Zegarski and G. S. Girolami, J. Electrochem. Soc., 1992,
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1994, 13, 1329; A. Intemann, H. Koerner and F. Koch,
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3 J. Chatt, J. R. Dilworth and R. L. Richard, Chem. Rev., 1978,
78, 589; R. R. Schrock, T. E. Glassman and M. G. Vale, J. Am.
Chem. Soc., 1991, 113, 725; R. A. Henderson, G. J. Leigh and
C. J. Pickett, J. Chem. Soc., Dalton Trans., 1989, 425;
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Chim. Acta, 1983, 71, 21; T. E. Glassman, M. G. Vale and
R. R. Schrock, Organometallics, 1991, 10, 4046; R. R. Schrock,
T. E. Glassman, M. G. Vale and M. Kol, J. Am. Chem. Soc., 1993,
1
nuclear (15N, H) NMR spectroscopy. For example, the 15N
NMR spectrum displays a new doublet at 394 ppm (1JNH
=
58 Hz) indicative of retention of the NH group. Further
analysis of the new product by 1H NMR shows the 15N–H
resonance also as a doublet at 5.78 ppm (1JHN = 58 Hz). As a
result, we propose that deprotonation of the nacnac b-methyl
group has occurred. This conclusion is corroborated by the
loss of symmetry of the nacnac ligand: a singlet at 1.56 ppm
integrated to 3H [(Me)C(NAr)CH(CH2)(NAr)] and two
different singlets at 3.12 and 2.46 ppm integrated to one
H for each resonance and assigned to the geminal methylene
115, 1760; M. R. Reithofer, R. R. Schrock and P. Muller, J. Am.
¨
group in [(Me)C(NAr)CH(CH2)(NAr)]2ꢀ 17
.
Chem. Soc., 2010, 132, 8349; D. V. Yandulov and R. R. Schrock,
J. Am. Chem. Soc., 2002, 124, 6252.
4 P. J. McKarns, G. P. A. Yap, A. L. Rheingold and C. H. Winter,
Inorg. Chem., 1996, 35, 5968.
In this work, we have demonstrated that incomplete
reduction of the N3 ligand by a d1 metal center still results
ꢀ
5 P. Mountford, Acc. Chem. Res., 2005, 38, 839; A. R. Fout,
U. J. Kilgore and D. J. Mindiola, Chem.–Eur. J., 2007, 13, 9428.
6 C. T. Saouma and J. C. Peters, Coord. Chem. Rev., 2011, 255,
920–937; J. F. Berry, Comments Inorg. Chem., 2009, 30, 28–66.
7 D. J. Mindiola, K. Meyer, J. P.-F. Cherry, T. A. Baker and
C. C. Cummins, Organometallics, 2000, 19, 1622.
8 D. J. Mindiola, PhD. Thesis, Department of Chemistry, Massachusetts
Institute of Technology, Cambridge, MA, 2000.
9 M. G. Fickes, PhD. Thesis, Department of Chemistry, Massachusetts
Institute of Technology, Cambridge, MA, 1998.
10 B. L. Tran, M. Pink, X. Gao, H. Park and D. J. Mindiola, J. Am.
Chem. Soc., 2010, 132, 1458.
11 F. Basuli, B. C. Bailey, J. Tomaszewski, J. C. Huffman and
D. J. Mindiola, J. Am. Chem. Soc., 2003, 125, 6052.
12 See supporting information.
in N2 extrusion with presumed formation of an electron
deficient titanium nitridyl, thereby providing entry to a rare
example of a group 4 parent imide. Rather than undergoing
4ꢀ
ligand radical ejection or N–N coupling to form an N2
ligand, H-atom abstraction (not from solvent) appears to be
the dominant pathway in formation the N–H bond. Current
effort in our group is geared toward utilizing the t-butyl
version of the nacnac scaffold, [(tBu)C(NAr)CH(tBu)(NAr)]ꢀ,
to circumvent the problem of backbone deprotonation.
We thank Indiana University, Bloomington, the Chemical
Sciences, Geosciences and Biosciences Division, Office of Basic
Energy Science, Office of Science, US Department of Energy
(DE-FG02-07ER15893) for financial support of this research.
We also thank ChemMatCARS Sector (NSF/DOE CHE-
0535644) and Advance Photo Source (DOE, Office of Science,
Office of Basic Energy Sciences under DE-AC02-06CH11357).
D.A.H. thanks the Harry G. Day and the Women in Chemistry
summer scholarships at Indiana University.
13 C. C. Cummins, R. R. Schrock and W. M. Davis, Inorg. Chem.,
1994, 33, 1448.
14 This value compares well a terminal imido complex of Ta(V).
J. S. Freundlich, R. R. Schrock, C. C. Cummins and
W. M. Davis, J. Am. Chem. Soc., 1994, 116, 6476; For a another
example see: G. Parkin, A. v. Asselt, D. J. Leahy, L. Whinnery,
N. G. Hua, R. W. Quan, L. M. Henling, W. P. Schaefer,
B. D. Santarsiero and J. E. Bercaw, Inorg. Chem., 1992, 31, 82.
15 Mononuclear titanium nitrides are extremely rare: A.-M. Fuller,
W. Clegg, R. W. Harrington, D. L. Hughes and S. J. Lancaster,
Chem. Commun., 2008, 5776; A. J. Mountford, S. J. Lancaster and
S. J. Coles, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
2007, C63, m401.
16 D. J. Mindiola, K. Meyer, J.-P. F. Cherry, T. A. Baker and
C. C. Cummins, Organometallics, 2000, 19, 1622.
17 D. Adhikari, F. Basuli, J. H. Orlando, X. Gao, J. C. Huffman,
M. Pink and D. J. Mindiola, Organometallics, 2009, 28, 4115;
F. Basuli, J. C. Huffman and D. J. Mindiola, Inorg. Chem., 2003,
42, 8003.
Notes and references
y X-ray data for 1: monoclinic, P21/c, T = 150(2)K a = 10.6177(10),
b = 33.391(3), c = 11.5326(11), b = 107.582(2)1, Z = 4, V = 3897.7(6)
A3, absorption coefficient = 0.321, F(000) = 1492, y range (deg) =
0.1.95–27.60, no. of reflections collected = 39483, no. of unique
reflections = 9003, data/parameter ratio = 18.83, R(F) = 0.0439,
Rw(F2) = 0.0985, GOF = 1.034, largest diff. peak and hole = 0.362
and ꢀ0.329.
z X-ray data for 2: monoclinic, P21/c, T = 120(2)K, a = 11.(9), b =
28.874(2), c = 12.3682(9), b = 115.276(2)1, Z = 4, V = 3806.5(5) A3,
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1529–1531 1531