Neutral and Anionic Monomeric Zirconium Imides Prepared via Selective C=N Bond Cleavage of a Multidentate and Sterically Demanding β-Diketiminato Ligand

Article abstract of DOI:10.1002/asia.201900451

A sterically encumbering multidentate β-diketiminato ligand, ^tBuL2 (^tBuL2=[ArNC(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂NMe₂]^?, Ar=2,6-iPr₂C₆H₃), is reported in this study along with its coordination chemistry to zirconium(IV). Using the lithio salt of this ligand, Li(^tBuL2) (4), the zirconium(IV) precursor (^tBuL2)ZrCl₃ (6) could be readily prepared in 85 % yield and structurally characterized. Reduction of 6 with 2 equiv of KC₈ resulted in formation of the terminal and mononuclear zirconium imide-chloride [C(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂NMe₂]Zr(=NAr)(Cl) (7) as the result of reductive C=N cleavage of the imino fragment in the multidentate ligand ^tBuL2 by an elusive Zr^II species (^tBuL2)ZrCl (A). The azabutadienyl ligand in 7 can be further reduced by 2 e^? with KC₈ to afford the anionic imide [K(THF)₂]{[CH(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂N(Me)CH₂]Zr=NAr} (8-2THF) in 42 % isolated yield. Complex 8-2THF results from the oxidative addition of an amine C?H bond followed by migration to the vinylic group of the formal [C(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂NMe₂]^? ligand in 7. All halides in 6 can be replaced with azides to afford (^tBuL2)Zr(N₃)₃ (9) which was structurally characterized, and reduction with two equiv of KC₈ also results in C=N bond cleavage of ^tBuL2 to form [C(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂NMe₂]Zr(=NAr)(N₃) (10), instead of the expected azide disproportionation to N^3? and N₂. Solid-state single crystal structural studies confirm the formation of mononuclear and terminal zirconium imido groups in 7, 8-Et₂O, and 10 with Zr=NAr distances being 1.8776(10), 1.9505(15), and 1.881(3) ?, respectively.

Full text of DOI:10.1002/asia.201900451

DOI: 10.1002/asia.201900451
Full Paper
Neutral and Anionic Monomeric Zirconium Imides Prepared via
Selective C=N Bond Cleavage of a Multidentate and Sterically
Demanding b-Diketiminato Ligand
Takashi Kurogi,[a] Jiaxiang Chu,[b] Yaofeng Chen,*[b] and Daniel J. Mindiola*[a]
Abstract: A sterically encumbering multidentate b-diketimi-
nato ligand, tBuL2 (tBuL2=[ArNC(tBu)CHC(tBu)NCH2CH2N(Me)-
CH2CH2NMe2]À, Ar=2,6-iPr2C6H3), is reported in this study
along with its coordination chemistry to zirconium(IV). Using
the lithio salt of this ligand, Li(tBuL2) (4), the zirconium(IV)
precursor (tBuL2)ZrCl3 (6) could be readily prepared in 85%
yield and structurally characterized. Reduction of 6 with
2 equiv of KC8 resulted in formation of the terminal and
Zr=NAr} (8-2THF) in 42% isolated yield. Complex 8-2THF re-
sults from the oxidative addition of an amine CÀH bond fol-
lowed by migration to the vinylic group of the formal
[C(tBu)CHC(tBu)NCH2CH2N(Me)CH2CH2NMe2]À ligand in 7. All
halides in
6 can be replaced with azides to afford
(
tBuL2)Zr(N3)3 (9) which was structurally characterized, and re-
duction with two equiv of KC8 also results in C=N bond
cleavage of tBuL2 to form [C(tBu)CHC(tBu)NCH2CH2N-
(Me)CH2CH2NMe2]Zr(=NAr)(N3) (10), instead of the expected
azide disproportionation to N3À and N2. Solid-state single
crystal structural studies confirm the formation of mononu-
clear and terminal zirconium imido groups in 7, 8-Et2O, and
10 with Zr=NAr distances being 1.8776(10), 1.9505(15), and
1.881(3) ꢀ, respectively.
mononuclear
zirconium
imide-chloride
[C(tBu)CHC(t-
Bu)NCH2CH2N(Me)CH2CH2NMe2]Zr(=NAr)(Cl) (7) as the result
of reductive C=N cleavage of the imino fragment in the mul-
tidentate ligand tBuL2 by an elusive ZrII species (tBuL2)ZrCl (A).
The azabutadienyl ligand in 7 can be further reduced
by 2 eÀ with KC8 to afford the anionic imide
[K(THF)2]{[CH(tBu)CHC(tBu)NCH2CH2N(Me)CH2CH2N(Me)CH2]-
Introduction
Whereas the ubiquitous b-diketiminato ligand has popularized
the coordination sphere for early-transition metals,[1] multiden-
tate[2–7] and macrocyclic[8] b-diketiminato scaffolds are now set-
ting the stage as unique ligands that allow for the stabilization
of the reactive metal fragments containing metal-ligand multi-
ple bonds. Specifically, this ligand class has been demonstrated
to support highly polarized, and terminal imido species[3] with
even highly electropositive such as Sc3+; an ion which falls
along the demarcation line between transition metal and rare-
earth. To this end, the Chen group has reported two multiden-
tate ligands possessing the b-diketiminato scaffold, namely the
Scheme 1. Multidentate ligands having the b-diketiminato motif, where Ar
represents iPr2C6H3.
cartoons depicted in Scheme 1. Part of the reasoning for as-
sembling these ligand scaffolds has been to coordinatively sat-
urate the electropositive metal ion, block aggregation,[9] and
prevent ligand disproportionation reactions[10] while maintain-
ing a low-charge to allow for dianionic ligand substitution re-
actions, namely a highly nucleophilic moiety such as an imide.
Only in certain exceptions,[11] there is evidence for irreversible
degradation of the b-diketiminato ligand.[12] The use of pend-
ant amino groups on these multidentate ligands stems from
the fact that scandium imides[3] can be generated or trapped
with donor ligands such as pyridine or pyridine derivatives
(e.g. DMAP) as exemplified by the work of Mindiola,[13] Cui,[14]
Piers,[15] and more recently, Mountford.[16]
tridentate MeL1
(
MeL1=[ArNC(Me)CHC(Me)NCH2CH2N(Me)2]À,
Ar=2,6-iPr2C6H3)2 and tetradentate MeL2
(
MeL2=[ArNC(-
Me)CHC(Me)NCH2CH2N(Me)CH2CH2NMe2]À, Ar=2,6-iPr2C6H3)3,4
[a] Dr. T. Kurogi, Prof. D. J. Mindiola
Department of Chemistry
University of Pennsylvania
231 South 34th Street, Philadelphia, PA 19104 (USA)
E-mail: mindiola@sas.upenn.edu
[b] Dr. J. Chu, Prof. Y. Chen
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (P. R. China)
E-mail: yaofchen@mail.sioc.ac.cn
In this study, we wish to report the synthesis of a multiden-
tate variant to the original systems MeL1 and MeL2,2,4 namely a
ligand tBuL2 having tBu groups at the b-carbon positions of the
[NCCCN] skeleton in the b-diketiminato framework (Scheme 1).
In addition to obtaining and structurally characterizing a non-
solvento salt Li(tBuL2), we also showcase how this multidentate
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/asia.201900451.
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ligand can grip onto ZrIV by making a mononuclear trichlorido
precursor, the latter which can be reduced with 2eÀ or 4eÀ to
make mononuclear Zr imides in neutral or anionic forms. Akin
to the chemistry found for the prototypical b-diketiminato
ligand,[17] we now report a bulkier ligand tBuL2 that serves as an
intramolecular imide transfer source when reducing ZrIV to ZrII.
We demonstrate how this new tetradentate and monoanionic
ligand can be transformed to either a multidentate azabuta-
dienyl by 2eÀ reduction or an unprecedented multidentate tri-
anionic ligands by virtue of 4eÀ reduction chemistry. Lastly, we
show how the replacement of chlorides for azides does not
impede the cleavage of the C=N bond when a similar ZrIV pre-
cursor is reduced by two electrons.
sis of the nitrile precursor [Me2NCH2CH2N(Me)CH2CN] followed
by reduction with LiAlH4 to produce 1.
Using compound 1 as our starting point one can condense
it with 3,3-dimethyl-2-butanone tBuC(O)CH3 in the presence of
a catalytic amount of p-TsOH·H2O (ca. 0.1 equiv) to afford the
imine [Me2NCH2CH2N(Me)CH2CH2N=C(tBu)CH3] (2) (Scheme 2).
Akin to the protocol used for the preparation of Budzelaar’s
[ArNHC(tBu)CHC(tBu)NAr] (Ar=2,6-iPr2C6H3),[20] compound
was first deprotonated via slow addition of LinBu. The reaction
most likely traverses through the salt Li
2
{Me2NCH2CH2N(CH3)CH2CH2NC(tBu)CH2} (A), which is then
quenched with the chloroimine tBuC(NAr)Cl[20] (Scheme 2).
However, unlike the reported preparation of [ArNHC(tBu)CHC(t-
Bu)NAr],[20] deprotonation of the imine methyl in 2 does not
require the need of TMEDA encapsulated LiCH3 or LinBu to
form a more powerful base. After quenching of A with tBuC(-
NAr)Cl, an oily residue and a yellow solid could be separated
Results and Discussion
Following the procedure reported in the literature,[18] the N’-(2-
1
by work-up of the reaction. The H NMR spectrum (Figure S4)
aminoethyl)-N’,N,N-trimethylethan-1,2-diamine
[Me2NCH2CH2N(Me)CH2CH2NH2] (1) was prepared accordingly
by LiAlH4 reduction of the nitrile precursor
precursor
of the oil revealed formation of [ArNHC(tBu)CHC(t-
Bu)NCH2CH2N(Me)CH2CH2NMe2], defined as H(tBuL2) (3), but ac-
companied with formation of at least two other major side-
products which we could not identify (Scheme 2). Attempts to
purify 3 from the crude reaction mixture were hampered
given the presence of several species in the mixture. Examina-
tion of the yellow solid by NMR spectroscopy (vide supra) re-
vealed formation of the lithio salt Li{ArNC(tBu)CHC(t-
Bu)NCH2CH2N(Me)CH2CH2NMe2} defined as Li(tBuL2) (4), which
was formed from the deprotonation of H(tBuL2) (3). As a result,
we resorted to deprotonating 3 without purification to obtain
the precipitate 4, isolated as a yellow powder in 48% yield in
three-steps from 2. Gratifyingly, deprotonation of the crude 3
still allows isolation of pure 4 thus leaving behind the other
neutral side products formed from the reaction of A with the
[Me2NCH2CH2N(Me)CH2CN] as documented by Glaser and co-
workers (Scheme 2). This procedure results in greater yields of
1 as opposed to an alternative process where the N-(2-bro-
moethyl)phthalimide is treated with N,N,N’-trimethylethylene-
diamine followed by reduction of the product N-(2-bromoe-
thyl)phthalimide with N2H4·H2O[19] (Scheme 2). Although both
protocols to form 1 involve a two-step process stemming from
commercially available N,N,N’-trimethylethylenediamine, the re-
action between N-(2-bromoethyl)phthalimide and N,N,N’-trime-
thylethylenediamine results in lower yields of phthalimide pro-
tected precursor N-(2-N,N,N’-trimethylethylenediamine-ethyl)-
phthalimide[17] (Scheme 2). Consequently, we prefer the synthe-
Scheme 2. Synthetic entry to the lithio salt 4 of the b-diketiminato ligand tBuL2 starting from the trisamino precursor 1, which can be prepared by two sepa-
rate reactions stemming from N,N,N’-trimethylethylenediamine.
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electrophile tBuC(NAr)Cl. After separation and washing of the
salt 4, the pentane filtrate can be concentrated which resulted
in the deposit of a small amount of colorless crystals of the tri-
azatetraene [ArNC(tBu)NC(CH2)C(tBu)CHCHC(tBu)NAr] (5). We
propose 5 to be a side-product that likely results from the de-
protonation and degradation of the ethylene group in 2 as
well as the addition of the tBuC(NAr)+ carbonium to the nucle-
ophilic nitrogen in a species such as A (Scheme 2). Fig-
ure 1(bottom) depicts a solid-state structural diagram of 5, a
species we propose to be one of the side-products of the reac-
tion between A and the chloroimine tBuC(NAr)Cl. Since com-
pound 5 is formed in a very low yield and is not relevant to
our subsequent studies reported herein no further characteri-
zation of this side-product nor the other organic material spec-
troscopically observed in solution was pursued.
The NMR spectra of 4 display asymmetric features for the b-
diketiminato scaffold in the tBuL2 ligand, such as two inequiva-
1
lent tBu groups at 1.34 and 1.39 ppm in the H NMR spectrum.
The 7Li NMR spectrum confirmed the presence of Li+ at
1.73 ppm. In addition to spectroscopic data and to conclusively
establish the connectivity of the ligand precursor 4, an XRD (X-
ray diffraction) study on a single crystal grown from pentane
at À358C was performed. As shown on the top of Figure 1,
the solid-state structure of 4 reveals a lithium ion encapsulated
by tetradentate ligand. Distortion of the lithium from an ideal-
ized tetrahedral geometry to trigonal pyramid is evident given
the planarity of N1, N3 and N4 and small deviation of the Li
atom from such an imaginary plane (N1-N3-N4), 0.235 ꢀ (t4 =
0.65).[21]
Having the lithio salt 4 in hand, we conducted a transmetal-
lation reaction with ZrCl4(THF)2 in toluene at room tempera-
ture. After mixing the two reagents and subsequent work-up
of the mixture, the zirconium trichlorido complex (tBuL2)ZrCl3
(6) was isolated in 85% yield as a pale yellow solid (Scheme 3).
Figure 1. Solid-state structures of compounds 4 (top) and 5 (bottom) show-
ing thermal ellipsoids at the 50% probability level.
Scheme 3. Synthesis of complex 6 with the lithio salt 4 and subsequent reductions with two equiv of KC8 to form 7 and 8. Plausible intermediates leading to
7 and 8 are shown in brackets.
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Figure 2. Solid-state structure of compound 6 (left) with H-atoms omitted
for clarity with the exception of H2, and the first coordination sphere at ZrIV
(right) showing thermal ellipsoids at the 50% probability level.
To our surprise, however, the solid-state structure of a single
crystal grown from Et2O at À358C revealed an eight-coordinate
ZrIV complex whereby the tBuL2 not only binds through the
four nitrogen sites but with coordination also occurring at the
g-C of the [NCCCN] part in the tBuL2 ligand (Figure 2). The ZrÀC
distance of 2.612(4) ꢀ is comparable to those of eight-coordi-
nated zirconium alkyl complexes.[17a,22] In contrast to 4, the
contracted C-N and elongated CÀC bond distances (CÀN:
1.305(5) ꢀ, 1.337(5) ꢀ; CÀC: 1.440(6) ꢀ, 1.418(6) ꢀ) in the
[NCCCN] skeleton exhibits bond alternation to represent a bi-
s(imino)alkyl character of the tBuL2 ligand. As a result of this ad-
ditional binding site, the [Zr(NCCCN)] ring in 6 distorts to a
boat-like conformation with the metal ion and g-C taking up
the bow and stern, and with the three chloride ligands occupy-
ing a pseudo mer-configuration. The first coordination sphere
around the zirconium center, shown in the right side of
Figure 2, represents a distorted trigonal dodecahedral geome-
try.[23] In addition to the structural data in solid-state, the
13C{1H} NMR spectrum of 6 also showed the g-C in the [NCCCN]
framework at 78.1 ppm, which is significantly high field-shifted
compared to that of the lithio salt 4 (91.9 ppm).
Figure 3. Solid-state structures of compounds 7 (top) and 8-2Et2O (bottom)
showing thermal ellipsoids at the 50% probability level. H-atoms and co-
crystallized THF have been omitted for clarity.
ers[10b,11,12,25] have observed similar C=N bond reductive cleav-
age reactions involving the b-diketiminato scaffold and despite
having pendant amino groups, the tBuL2 ligand still succumbs
to reductive cleavage of the imino group. Formation of 7 most
likely traverses via a transient ZrII intermediate (tBuL2)ZrCl (B)
(most likely not a concerted step and involving side-on coordi-
nation of the imine group to Zr), which readily splits the C=N
bond of the ligand. It was found that another minor product
was formed in the mixture when a slight excess of reductant
was used. Optimization of the reaction conditions revealed
that the additional material was being generated by the two-
electron reduction of 7. Accordingly, treating an isolated
sample of 7 with two equiv of KC8 in THF at À358C resulted in
formation of the other ZrIV species 8-2THF (observed only in
traces using the original conditions) which could then be iso-
lated in 42% yield as a yellow solid (Scheme 3). Although the
1H NMR spectrum showed the aryl imido group, the entire
spectrum is quite broad and uninformative. To establish the
connectivity of this new species, single crystals were grown as
an Et2O adduct, instead of the original THF adduct 8-2THF,
from a concentrated solution in Et2O at À358C. Figure 3
bottom depicts the molecular structure of the ate-complex
Reduction of 6 with two equiv of KC8 in THF resulted in the
formation of the distorted octahedral zirconium imide-chloride
[C(tBu)CHC(tBu)NCH2CH2N(Me)CH2CH2NMe2]Zr(=NAr)(Cl)
(7)
isolated in 70% yield as a reddish orange solid. Salient spectro-
scopic features for 7 include a highly downfield 13C{1H} NMR
spectroscopic resonance of the a-C in [ZrCCCN] at 248.3 ppm
as well as the g-C in the [ZrCCCN] at 187.0 ppm, which are
comparable to those of reported azabutadienyl complexes.[11,17]
An XRD study of a single crystal of 7 grown from THF/pen-
tane at À358C reveals mononuclear and coordinatively saturat-
ed ZrIV complex with
a terminal imido moiety (Zr=N,
1.8776(10) ꢀ) (Figure 3 top and Table 1). This Zr=N bond length
is comparable to those of other terminal zirconium imides re-
ported in the literature (1.8185(17)–1.913(2) ꢀ).[17a,24] As the
result of C=N bond reductive cleavage of tBuL2, an azabuta-
dienyl ligand chelates to the metal center, with the two pend-
ant amino residues remaining intact. The imido ligand is trans
to the dimethyl amino group (N1-Zr1-N4: 171.91(4)8), whereas
the remaining chlorido ligand resides transoid to the azabuta-
dienyl nitrogen (Cl1-Zr1-N2: 164.64(3)8). Our group[17] and oth-
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quired whether replacement of all chlorido ligands
for azides would circumvent the reductive C=N
bond cleavage pathway for a more intuitively sim-
pler N2 extrusion process via 2eÀ disproportionation
Table 1. Metric parameters in solid-state structures of compounds 6–10. Bond distan-
ces (ꢀ) and angles (8) around the Zr center.
6
7
8-2Et2O
9
10
(X=Cl)
(X=Cl)
(X=C29)
(X=N3)
(X=N3)
À
of N3 to N2 and N3À. We anticipated that reduction
Zr-N1
Zr-N2
Zr-N3
Zr-N4
Zr-C1
Zr-X
(X=Cl, N3, C29)
2.325(3)
2.176(4)
2.522(3)
2.677(4)
–
1.8776(10) 1.9506(15) 2.320(3)
2.3613(10) 2.0986(15) 2.171(3)
2.3773(11) 2.4495(15) 2.486(3)
1.881(3)
2.349(3)
2.378(3)
2.609(3)
2.321(3)
2.186(3)
of the azide would expel N2 which would require
minimal reorganization energy as opposed to reduc-
tive C=N splitting via a more complex ligand rear-
rangement of (tBuL2), thus providing entry to a hypo-
thetical neutral zirconium nitride (tBuL2)ZrꢀN (E)
(Scheme 4). Accordingly, treatment of complex 6
with an excess of NaN3 in THF over 18 hours resulted
in smooth formation of the tris-azido complex
2.6314(11)
2.3151(12) 2.2760(17)
–
2.609(3)
–
2.4753(11) 2.5044(4) 2.3791(18) 2.133(3)
2.5008(10)
2.5608(11)
1.785
2.172(3)
2.216(3)
1.781
Zr out of [NC3N] plane
Zr-C2
N1-Zr-N2
N1-Zr-N3
N1-Zr-N4
N2-Zr-N3
N2-Zr-N4
N3-Zr-N4
X-Zr-X
–
–
–
–
–
–
2.612(4)
2.629(3)
(
tBuL2)Zr(N3)3 (9) in 88% yield as a translucent solid
77.09(13) 94.74(4)
128.15(12) 99.61(4)
141.64(13) 171.91(4)
68.95(12) 71.45(4)
136.60(12) 84.20(4)
69.60(12) 72.43(4)
84.22(4)
101.29(4)
152.61(4)
–
117.87(6)
116.36(6)
–
76.86(10) 95.56(11)
128.33(9) 96.94(11)
141.01(9) 169.22(11)
70.62(10) 71.22(10)
138.93(10) 82.27(9)
71.07(9)
84.83(11)
102.01(11)
149.90(12)
–
(Scheme 4). Although the NMR spectral data of 9
display almost identical features which were ob-
served in the trichlorido complex 6 such as the g-C
position of [NCCCN] at 77.2 ppm, the IR spectrum re-
vealed a diagnostic n(N3) stretch at 2099 cmÀ1 as a
very strong stretch similar to reported zirconium
azides[27] and other early-transition metal tris-azido
complexes.[28] An XRD of 9 confirmed the presence
68.61(5)
–
–
–
72.35(10)
–
–
(X=Cl, N3, C29)
X-Zr-C1 (X=Cl, N3, C29)
X-Zr-N1
(X=Cl, N3, C29)
113.71(3)
98.79(3)
95.26(6)
99.96(6)
114.54(11)
101.72(12)
75.14(9)
83.33(8)
132.24(9)
75.07(11)
84.83(11)
135.02(11)
of three azido ligands bound to ZrIV (Zr-Nazide
,
2.133(3)-2.216(3) ꢀ) and with an overall similar trigo-
nal dodecahedral geometry[23] to the chloride precur-
sor 6 (Figure 4, left and Table 1).
With the azido ligands in place, we then subjected
complex 9 to reducing conditions akin to how com-
plex 6 was treated with KC8. Accordingly, adding
two equiv of KC8 to 9 in THF at À358C, and allowing
the mixture to stir for 2 hours at room temperature
resulted in formation of the azide-imide
[C(tBu)CHC(tBu)NCH2CH2N(Me)CH2CH2NMe2]Zr=
X-Zr-N2
(X=Cl, N3, C29)
84.15(10) 164.64(3) 136.50(6) 82.60(11)
159.71(11)
95.98(11)
78.78(10)
106.22(10)
140.51(9)
78.12(9)
82.42(9)
145.69(9)
76.80(9)
77.01(8)
78.25(8)
109.05(11)
138.00(11)
78.96(11)
79.17(11)
144.58(11)
74.63(11)
76.23(11)
77.27(11)
X-Zr-N3
(X=Cl, N3, C29)
98.98(3)
81.38(3)
76.05(6)
–
X-Zr-N4
(X=Cl, N3, C29)
[K(OEt2)2]{[CH(tBu)CHC(tBu)NCH2CH2N(Me)CH2CH2N(Me)CH2]Zr=
NAr} (8-2Et2O) (Table 1). Inspection of the structure immediate-
ly reveals that a methyl moiety of the amino group (Zr1ÀC29,
2.3791(18) ꢀ), has been activated with the hydrogen being
now shifted to the carbon of the formal azabutadienyl ligand
in 7. The bond distances in the five-membered framework
Zr[CCCN] showed a shorter C2ÀC3 bond (1.391(3) ꢀ) and
longer C1ÀC2 (1.478(2) ꢀ) and C3ÀN2 (1.388(2) ꢀ) bonds than
those of compound 7 (1.4814(18), 1.3502 (18), 1.2974(16), re-
spectively). A potassium counter cation coordinates to the
imido nitrogen (Zr1=N, 1.9506(15) ꢀ) as well as to the b-nitro-
gen at the metallated site, with two Et2O ligands completing
its coordination sphere. The formation of 8-2L (L=THF or
Et2O) from 7 and two equiv of KC8 most likely traverses via an
anionic ZrII imido species (C) or ZrIV ligand-based reduction
species (D), which has a Zr=C double bond. Lastly, abstraction
of hydrogen in a CÀH bond of a methylamino group to the
azametallacyclopentadienyl moiety in D results in the final
product 8-2L.
We recently have found that reduction of the trans-ZrIV bis
azido precursor (PN)2Zr(N3)2 (PNÀ =N-(2-iPr2P-4-methylphenyl)-
2,4,6-trimethylanilide) with KC8 promoted both azide elimina-
tion and N2 extrusion to afford an unprecedented mononu-
clear parent imide of zirconium, namely (PN)2Zr=NH.[26] We in-
Scheme 4. Synthesis of complex 9 from salt metathesis of 6 and NaN3 as
well as subsequent two electron reduction to 10. Formation of a hypotheti-
cal zirconium nitride E via reduction of 9 was not observed.
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ethereal solvents were dried by passage through two columns of
activated alumina. All bulk solvents were kept over 4 ꢀ molecular
sieves and sodium with the exception of CH2Cl2. Benzene-d6 (Cam-
bridge Isotope Laboratories) was dried and degassed over a potas-
sium mirror prior to use. Chloroform-d (Cambridge Isotope Labora-
tories) was dried and distilled over calcium hydride prior to use.
Celite and 4 ꢀ molecular sieves were activated under vacuum over-
night at 2008C. ZrCl4(THF)2,[29] KC8,[30] N1-(2-aminoethyl)-N1,N2,N2-tri-
methylethane-1,2-diamine (1),[18] and 1-chloro-2-(2,6-diisopropyl-
phenylimino)-2,2-dimethylpropane[31] were prepared according to
the reported procedures. NaN3 was dried by stirring in anhydrous
THF overnight, washed with anhydrous Et2O and pentane and
then evacuated at room temperature overnight. All other chemi-
cals were purchased from commercial sources and used as re-
ceived. H, 13C, Li and 2D (COSY, HSQC, and HMBC) NMR spectra
were recorded on a Bruker AV-II 500 MHz or AV-III 400 MHz spec-
trometer. 1H and 13C{1H} NMR chemical shifts are reported refer-
enced to the internal residual proton or carbon resonances of C6D6
(d=7.16 ppm or 128.06 ppm) or CDCl3 (d=7.26 ppm or
77.16 ppm). 7Li NMR chemical shift is reported referenced to the
external LiCl solution in D2O (d=0.00 ppm). IR samples were pre-
pared by a KBr plate method (JASCO Tablet Master) and IR spectra
were obtained on a JASCO FT/IR-4600 spectrometer. Elemental
analyses were performed by Midwest Microlab, Inc (Indiana, USA).
Figure 4. Solid-state structures of compounds 9 (left) and 10 (right) showing
thermal ellipsoids at the 50% probability level. H-atoms and co-crystallized
THF have been omitted for clarity.
1
7
NAr(N3) (10) isolated in 51% yield as a red solid (Scheme 4).
Disappointingly, we observe no evidence for the formation of
the nitride E, therefore implying that azide disproportionation
of the intermediate the ZrII (tBuL2)Zr(N3) to N2 and N3À is unfav-
ourable with respect to C=N bond cleavage of the tBuL2, at
least under our reaction conditions. Formation of 10 is based
on 1H and 13C NMR spectroscopic data, which resemble the
chlorido analogue 7 (vide supra), as well as an IR stretch (n(N3):
2019 cmÀ1). In addition, an XRD study on a single crystal of 10
confirmed the monomeric and pseudo octahedral nature of
the Zr center, the terminal imido group (Zr=N, 1.881(3) ꢀ) and
the reductive cleavage of the C=N bond on tBuL2 (Figure 4,
right and Table 1).
Synthesis of compound 2.
To
a
colorless solution of 3,3-dimehyl-2-butanone (3.12 g,
31.2 mmol) and 1 (4.28 g, 29.5 mmol) in toluene (100 mL) was
added p-toluenesulfonic acid monohydrate (500 mg, 2.63 mmol) as
a solid at room temperature. The reaction mixture was heated at
1208C under a dry nitrogen atmosphere equipped with a Dean–
Stark apparatus and a Vigreux column. During condensation, the
reaction mixture gradually changed in color to brown and water
(ca. 0.5 mL) was collected in the Dean–Stark apparatus. After reflux-
ing for 18 hours, the reaction mixture was cooled down to room
temperature. The solvent was removed under vacuum at room
temperature to yield a brown oil. From the brown oil, a colorless
oil of pure N1-(2-((3,3-dimethylbutan-2-ylidene)amino)ethyl)-
N1,N2,N2-trimethylethane-1,2-diamine 2 (4.73 g, 20.8 mmol, 71%
yield) was obtained by distillation (200 mTorr at 80–858C).
Conclusions
We have shown a synthetic route to the lithio salt of a multi-
dentate b-diketiminato ligand having pendant amino groups,
Li(tBuL2) (4). Using the lithio salt 4, eight-coordinated com-
plexes of zirconium with three chlorido and azido ligands have
been prepared and structurally characterized. Reduction of
these zirconium(IV) complexes 6 and 9 with KC8 resulted in for-
mation of azabutadienyl-imido complexes 7 and 10 via degra-
dation of the b-diketiminato scaffold by a low-valent zirconium
species. Further reduction of the azabutadienyl-imide 7 by KC8
formed an anionic imido species 8-2L, which has also been
structurally characterized. Syntheses of other metal complexes
and further reactivity studies of the zirconium complexes will
be forthcoming.
1H NMR (500 MHz, Benzene-d6, 300 K): d=3.36 (t, 3JHH =8 Hz, 2H,
3
3
NCH2CH2N), 2.83 (t, JHH =8 Hz, 2H, NCH2CH2N), 2.59 (t, JHH =8 Hz,
2H, NCH2CH2N), 2.41 (t, 3JHH =8 Hz, 2H, NCH2CH2N), 2.28 (s, 3H,
NMe), 2.13 (s, 6H, NMe2), 1.48 (s, 3H, N=C-Me), 1.11 ppm (s, 9H,
N=C-tBu). 1H NMR (400 MHz, Chloroform-d, 300 K): d=3.35 (t,
3
3JHH =8 Hz, 2H, NCH2CH2N), 2.64 (t, JHH =8 Hz, 2H, NCH2CH2N), 2.55
3
3
(t, JHH =8 Hz, 2H, NCH2CH2N), 2.41 (t, JHH =8 Hz, 2H, NCH2CH2N),
2.31 (s, 3H, NMe), 2.22 (s, 6H, NMe2), 1.77 (s, 3H, N=C-Me),
1.07 ppm (s, 9H, N=C-tBu). 13C{1H} NMR (126 MHz, Benzene-d6,
300 K): d=173.78 (N=C), 59.75 (NCH2CH2N), 58.50 (NCH2CH2N),
57.13 (NCH2CH2N), 50.79 (NCH2CH2N), 46.11 (N (CH3)2), 43.54 (NCH3),
40.61 (N=CÀC(CH3)3), 28.00 (N=CÀC(CH3)3), 12.75 ppm (N=C-CH3). IR
(KBr): n˜ =2965 (s), 2901 (s), 2864 (s), 2765 (s), 1654 (s), 1462 (s),
1362 (m), 1130 (m), 1029 (m), 935 (w), 846 (w), 827 (w), 790 (w),
467 (w), 412 cmÀ1 (w). Anal. Calcd. for C13H29N3: C, 68.67; H, 12.86;
N, 18.48. Found: C, 68.57; H, 12.98; N, 18.33.
Experimental Section
General Procedure.
All operations were performed in an M. Braun glove box or using
Schlenk techniques under a nitrogen atmosphere unless otherwise
stated. Anhydrous hydrocarbon solvents and CH2Cl2 were pur-
chased from Fisher Scientific. Stabilizer-free ethereal solvents (Et2O
and THF) were purchased from Alfa Aesar. All anhydrous hydrocar-
bon solvents (pentane, hexane and toluene) were purified and
dried by passage through two columns of activated alumina and
Q-5 drying agent in a Grubbs-type solvent system. CH2Cl2 and
Synthesis of compound 3.
Under a dry nitrogen atmosphere, a clear solution of 2 (2.00 g,
8.80 mmol) in THF (40 mL) was cooled at À788C. LinBu (2.5m in
hexane, 3.55 mL, 8.88 mmol) was added via a syringe into the THF
solution of 2 dropwise. After addition of LinBu, the reaction mix-
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ture was allowed to warm up to room temperature, resulting in a
color change to pale yellow. After stirred at room temperature for
2 hours, the pale yellow solution was transferred via a cannula into
1-chloro-2-(2,6-diisopropylphenylimino)-2,2-dimethylpropane
(2.47 g, 8.83 mmol) diluted in THF (30 mL) cooled at À788C, result-
ing in a color change to yellowish orange and precipitation of a
white solid (LiCl). The reaction mixture was warmed up to room
temperature. After stirred at room temperature for 1 hour, all vola-
tile materials were removed under vacuum. The orange waxy resi-
due was dissolved in hexane (50 mL) and filtered through Celite to
remove LiCl. The filtrate was concentrated to ca. 10 mL and a
yellow solid was precipitated. The precipitated yellow solid was
collected by filtration and dried under vacuum. The obtained
yellow solid was confirmed as the lithio salt 4 (1.87 g, 3.92 mmol,
45% yield) by NMR spectroscopy and X-ray crystallography. The fil-
trate part was evaporated under vacuum and the crude product 3
(Figure S4) was obtained as an orange oil (1.11 g, 2.35 mmol calcu-
lated as 3, 27% crude yield). Purification of 3 from the obtained oil
by distillation or column chromatography was failed.
1.41 (m, overlapped, 1H, NCH2), 1.39 (s, 9H, tBu), 1.34 (s, 9H, tBu),
1.20 (d, 3H, 3JHH =6 Hz, ArCHMe2), 1.16 ppm (d, 3H, 3JHH =6 Hz,
ArCHMe2). 13C{1H} NMR (101 MHz, Benzene-d6, 300 K): d=172.14
(CtBu), 171.12 (CtBu), 151.60 (Ar), 140.81 (Ar), 138.54 (Ar), 122.82
(Ar), 122.65 (Ar), 119.52 (Ar), 91.89 (NC(tBu)CHC(tBu)N), 60.68
(NCH2CH2N), 58.03 (NCH2CH2N), 51.07 (NCH2CH2N), 46.76
(NCH2CH2N), 45.66 (N(CH3)2), 44.99 (NCH3), 44.17 (C(CH3)3), 40.05
(C(CH3)3), 33.39 (C(CH3)3), 30.62 (C(CH3)3), 27.83 (Ar-CH(CH3)2), 25.98
7
(Ar-CH(CH3)2), 23.59 (Ar-CH(CH3)2), 22.21 ppm (Ar-CH(CH3)2). Li NMR
(155 MHz, Benzene-d6, 300 K): d=1.73 ppm (s). IR (KBr): n˜ =3141
(m), 3050 (s), 2953 (s), 2863 (s), 2786 (s), 2499 (s), 1561 (s), 1491 (s),
1455 (br), 1400 (br), 1361 (s), 1330 (s), 1286 (s), 1269 (m), 1255 (m),
1213 (m), 1183 (m), 1164 (m), 1141 (m), 1119 (m), 1102 (m), 1065
(m), 1038 (m), 1020 (m), 986 (w), 933 (w), 903 (w), 884 (m), 831 (m),
805 (w), 774 (s), 756 (s), 711 (s), 688 (s), 657 (w), 628 (w), 585 (w),
527 (w), 498 (w), 453 (w), 430 (w), 414 (w), 402 cmÀ1 (w). Anal.
Calcd. for C30H53N4Li: C, 75.58; H, 11.21; N, 11.75. Found: C, 75.72;
H, 11.29; N, 11.61.
Synthesis of compound 6.
To a white suspension of ZrCl4(THF)2 (633 mg, 1.68 mmol) in tolu-
ene (30 mL) was added 4 (800 mg, 1.68 mmol) as solid at room
temperature. The reaction mixture gradually changed in color from
yellow to pale yellow. After stirred at room temperature for
3 hours, all volatile materials were removed under vacuum. The
pale yellow residue was suspended in CH2Cl2 (30 mL) and filtered
through Celite to remove insoluble materials. The pale yellow fil-
trate was dried under vacuum. The residue was suspended in pen-
tane (10 mL) and the pale yellow solid was collected by filtration,
washed with pentane (5 mL) and dried under vacuum to yield
compound 6 (947 mg, 1.42 mmol, 85% yield). Single crystals suita-
ble for an XRD study were obtained from a concentrated solution
in Et2O at À358C.
Synthesis of compound 4.
Route A: To a yellowish orange solution of crude 3 (1.00 g,
2.10 mmol calculated as 3) in hexane (20 mL) was added LinBu
(2.5m in hexane, 0.85 mL, 2.13 mmol) dropwise at À358C. The re-
action mixture was warmed up to room temperature, resulting in a
small amount of a white powder. After stirred at room temperature
for 1 hour, the yellowish orange reaction mixture was filtered
through Celite to remove insoluble materials. The filtrate was dried
under vacuum. The brown residue was suspended in pentane
(10 mL) and a yellow solid was formed. The yellow solid was col-
lected by filtration and dried under vacuum to yield pure com-
pound 4 (394 mg, 826 mmol, ca. 39% yield based on crude 3). The
brown filtrate from this synthesis was cooled at À358C overnight.
A few translucent crystals were grown and confirmed as com-
pound 5 by X-ray crystallography.
3
1H NMR (400 MHz, Chloroform-d, 300 K): d=7.11 (d, JHH =6 Hz, 2H,
Ar-H), 6.95 (br, 1H, Ar-H), 5.38 (s, 1H, NC(tBu)CHC(tBu)N), 4.41 (br,
1H, NCH2), 4.13 (br, 3H, NCH2 and 2 x ArCHMe2), 3.85 (br, 1H,
NCH2), 3.34 (br, 1H, NCH2), 2.78 (s, 6H, NMe2), 2.75 (NMe), 2.61 (br,
1H, NCH2), 2.36–2.10 (br, 3H, NCH2), 1.53 (s, 9H, tBu), 1.36 (br, 3H,
ArCHMe2), 1.26 (br, 3H, ArCHMe2), 1.15 (s, 9H, tBu), 1.11 (br, 3H,
ArCHMe2), 1.00 ppm (br, 3H, ArCHMe2). 13C{1H} NMR (101 MHz,
Chloroform-d, 300 K): d=173.08 (CtBu), 166.37 (CtBu), 146.16 (Ar),
144.27 (Ar), 141.29 (Ar), 125.84 (Ar), 124.23 (Ar), 122.21 (Ar), 78.12
(NC(tBu)CHC(tBu)N), 63.38 (NCH2CH2N), 59.35 (NCH2CH2N), 58.95
(NCH2CH2N), 52.14 (N(CH3)2), 50.52 (NCH2CH2N), 49.83 (NCH3), 43.56
(C(CH3)3), 40.40 (C(CH3)3), 30.83 (C(CH3)3), 29.64 (C(CH3)3), 28.20 (Ar-
CH(CH3)2), 27.32 (Ar-CH(CH3)2), 25.98 (2 x Ar-CH(CH3)2), 24.82 (Ar-
CH(CH3)2), 23.42 (Ar-CH(CH3)2). IR (KBr): n˜ =2962 (s), 2867 (m), 1646
(w), 1558 (w), 1466 (s), 1387 (m), 1363 (m), 1326 (m), 1204 (m),
1042 (m), 933 (m), 784 (m), 761 (m), 623 (w), 517 (w), 432 cmÀ1 (w).
Anal. Calcd. for C30H53N4Cl3Zr: C, 53.99; H, 8.01; N, 8.40. Found: C,
53.79; H, 8.13; N, 8.22.
Method B: Under a dry nitrogen atmosphere, a colorless solution
of 2 (6.00 g, 26.4 mmol) in THF (80 mL) was cooled at À788C.
LinBu (2.5m in hexane, 10.8 mL, 27.0 mmol) was added via a sy-
ringe into the THF solution of 2 dropwise and the reaction mixture
was at room temperature for 2 hours. The pale yellow solution was
transferred via a cannula into 1-chloro-2-(2,6-diisopropylphenylimi-
no)-2,2-dimethylpropane (7.45 g, 26.6 mmol) in THF (50 mL) cooled
at À788C. The reaction mixture was warmed up to room tempera-
ture and stirred for 1 hour. All volatile materials were removed
under vacuum. The orange residue was dissolved in hexane
(100 mL) and filtered through Celite. THF (10 mL) was added into
the filtrate. The mixture was cooled at À788C and LinBu (2.5m in
hexane, 10.5 mL, 26.3 mmol) was added dropwise. After addition
of LinBu, the mixture was stirred at room temperature for 1 hour.
The orange mixture was filtered through Celite and the filtrate was
dried under vacuum. The yellowish orange residue was suspended
in pentane (20 mL). A yellow solid was collected by filtration,
washed with pentane (10 mLꢂ2) and dried under vacuum to give
compound 4 (6.12 g, 12.8 mmol, 48% yield from 2). Single crystals
suitable for an XRD study were obtained from a concentrated solu-
tion in pentane at À358C.
Synthesis of compound 7.
To a pale yellow solution of 6 (200 mg, 300 mmol) in THF (5 mL)
was added KC8 (86.1 mg, 637 mmol) in THF (5 mL) at À358C. The
reaction mixture was stirred at room temperature for 2 hours and
filtered through Celite to remove insoluble materials. The reddish
orange filtrate was concentrated to ca. 5 mL, layered with pentane
(5 mL) and stored at À358C to afford compound 7·0.5(THF)
(133 mg, 210 mmol, 70% yield) as an orange crystalline solid. The
amount of the residual THF molecule in the obtained solid was de-
termined by 1H NMR spectroscopy and elemental analysis. Single
1H NMR (400 MHz, Benzene-d6, 300 K): d=7.16 (overlapped with
C6D5H, 2H, Ar-H), 7.02 (t, 3JHH =8 Hz, 1H, Ar-H), 5.32 (s, 1H,
NC(tBu)CHC(tBu)N), 3.65–3.53 (m, overlapped, 3H, ArCHMe2 and
NCH2), 3.46 (m, 1H, ArCHMe2), 2.34 (m, 2H, NCH2), 1.93 (s, 6H,
NMe2), 1.95–1.90 (m, overlapped, 2H, NCH2), 1.71 (s, 3H, NMe),
1.72–1.60 (m, overlapped, 1H, NCH2), 1.45 (br, 6H, ArCHMe2), 1.47–
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crystals suitable for an XRD study were obtained from a layered so-
for an XRD study were obtained from a layered solution in THF
lution in THF with pentane at À358C.
with pentane at À358C.
1H NMR (400 MHz, Benzene-d6, 300 K): d=7.25 (d, 3JHH =7 Hz, 2H,
1H NMR (400 MHz, Benzene-d6, 300 K): d=7.16 (overlapped with
C6D5H, 2H, Ar-H), 7.03 (t, 3JHH =7 Hz, 1H, Ar-H), 5.53 (s, 1H,
NC(tBu)CHC(tBu)N), 3.98 (br, 1H ArCHMe2), 3.70 (m, 1H, NCH2), 3.48
(br t, 1H, NCH2), 3.11 (br t, 1H, NCH2), 2.63 (br t, 1H, NCH2), 2.33
(br, 1H ArCHMe2), 2.30–2.25 (overlapped, m, 2H, NCH2), 2.28 (s, 6H,
NMe2), 2.08 (NMe), 1.61 (overlapped, m, 1H, NCH2), 1.62 (br, 3H,
ArCHMe2), 1.52 (br, 3H, ArCHMe2), 1.33 (s, 9H, tBu), 1.30–1.28 (over-
lapped, m, 1H, NCH2), 1.16 (br, 3H, ArCHMe2), 0.97 ppm (br, 3H,
ArCHMe2). 13C{1H} NMR (101 MHz, Benzene-d6/THF, 300 K): d=
174.80 (CtBu), 166.62 (CtBu), 145.71 (Ar), 142.63 (Ar), 140.62 (Ar),
126.49 (Ar), 124.88 (Ar), 123.20 (Ar), 77.21 (NC(tBu)CHC(tBu)N),
65.25 (NCH2CH2N), 59.01 (NCH2CH2N), 58.24 (NCH2CH2N), 49.94
(N(CH3)2), 45.39 (NCH2CH2N), 43.67 (NCH3), 43.67 (C(CH3)3), 40.17
(C(CH3)3), 31.19 (C(CH3)3), 29.56 (C(CH3)3), 28.41 (Ar-CH(CH3)2), 27.68
(Ar-CH(CH3)2), 25.74 (Ar-CH(CH3)2), 25.39 (Ar-CH(CH3)2), 25.05 (Ar-
CH(CH3)2), 23.51 ppm (Ar-CH(CH3)2). IR (KBr): n˜ =3059 (m), 3025 (s),
2968 (s), 2926 (s), 2870 (s), 2099 (s), 1543 (br), 1466 (s), 1392 (s),
1349 (m), 1329 (m), 1285 (m), 1256 (w), 1237 (w), 1203 (m), 1176
(m), 1164 (m), 1141 (m), 1129 (m), 1102 (m), 1078 (m), 1063 (m),
1049 (m), 1029 (m), 1008 (m), 961 (m), 935 (m), 911 (w), 892 (w),
847 (w), 805 (w), 785 (s), 770 (w), 738 (w), 696 (w), 624 (w), 601 (w),
590 (w), 576 (w), 506 (w), 494 cmÀ1 (w). Anal. Calcd. for C30H53N13Zr:
C, 52.44; H, 7.78; N, 26.50. Found: C, 52.56; H, 7.91; N, 26.21.
3
Ar-H), 6.96 (t, JHH =7 Hz, 1H, Ar-H), 6.92 (s, 1H, ZrC(tBu)CHC(tBu)N),
4.73 (br, 2H, ArCHMe2), 3.58 (br, 2H, 0.5 THF), 3.52–3.47 (m, 1H,
NCH2), 3.56–3.21 (m, overlapped, 2H, NCH2), 2.96 (br t, 1H, NCH2),
2.49 (br s, 6H, NMe2), 2.46 (s, 3H, NMe), 2.11 (br t, 1H, NCH2), 1.64
3
3
(br, 2H, 0.5 THF), 1.56 (d, JHH =7 Hz, 6H, ArCHMe2), 1.49 (d, JHH
=
6 Hz, 6H, ArCHMe2), 1.39 (s, 9H, tBu), 1.43–1.39 (m, overlapped, 1H,
NCH2), 1.22 (br t, 2H, NCH2), 1.10 ppm (s, 9H, tBu). 13C{1H} NMR
(101 MHz, Benzene-d6/THF, 300 K): d=248.29 (ZrCtBu), 187.04
(NCtBu), 173.06 (Ar), 152.87 (Ar), 144.28 (Ar), 129.82 (Ar), 125.98
(Ar), 122.06 (Ar), 117.22 (C(tBu)CHC(tBu)N), 59.24 (NCH2CH2N), 56.94
(NCH2CH2N), 52.31 (NCH2CH2N), 48.29 (NCH2CH2N), 47.46 (N(CH3)2),
43.53 (NCH3), 40.82 (C(CH3)3), 40.34 (C(CH3)3), 31.17 (C(CH3)3), 30.93
(C(CH3)3), 26.98 (Ar-CH(CH3)2), 25.90 (Ar-CH(CH3)2), 25.11 (Ar-
CH(CH3)2), 24.60 ppm (Ar-CH(CH3)2). IR (KBr): n˜ =2956 (s), 2864 (s),
1625 (w), 1582 (w), 1540 (w), 1464 (s), 1420 (s), 1398 (w), 1340 (w),
1324 (s), 1284 (s), 1237 (m), 1204 (m), 1127 (m), 1101 (m), 1077 (m),
1065 (m), 1025 (m), 1006 (m), 984 (m), 949 (m), 930 (m), 893 (m),
861 (m), 802 (m), 785 (s), 761 (s), 684 (w), 632 (w), 550 (w),
517 cmÀ1 (w). Anal. Calcd. for C32H57N4O0.5ClZr: C, 60.77; H, 9.08; N,
8.86. Found: C, 60.87; H, 9.29; N, 8.75.
Synthesis of compound 8-2THF.
To an orange solution of 7·0.5(THF) (100 mg, 158 mmol) in THF
(10 mL) was added KC8 (48.1 mg, 356 mmol) in THF (5 mL) at
À358C. The reaction mixture was stirred at room temperature for
2 hours and filtered through Celite to remove insoluble materials.
The dark brown filtrate was concentrated to ca. 1 mL, layered with
pentane (5 mL) and stored at À358C to afford compound 8-2THF
(49.3 mg, 826 mmol, 42% yield) as yellow crystals. Single crystals
suitable for an XRD study were obtained from a concentrated solu-
tion in Et2O at À358C as an Et2O adduct 8-2Et2O.
Synthesis of compound 10.
To a white suspension of 9 (300 mg, 437 mmol) in THF (10 mL) was
added KC8 (65.1 mg, 482 mmol) in THF (5 mL) at À358C. The reac-
tion mixture was stirred at room temperature for 2 hours and fil-
tered through Celite to remove insoluble materials. The reddish
orange filtrate was concentrated to ca. 5 mL, layered with pentane
(5 mL) and stored at À358C to afford compound 10·THF (151 mg,
224 mmol, 51% yield) as an orange crystalline solid. The amount of
the residual THF molecule in the obtained solid was determined
by 1H NMR spectroscopy and elemental analysis. Single crystals
suitable for an XRD study were obtained from a concentrated solu-
tion in THF at À358C.
1H NMR (400 MHz, Benzene-d6, 300 K): d=7.09 (d, 3JHH =6 Hz, 2H,
3
Ar-H), 6.69 (t, JHH =6 Hz, 1H, Ar-H), 4.82 (br, 1H, C(tBu)CHC(tBu)N),
4.30–3.70 (br, overlapped, 5H, ArCHMe2 and NCH2), 3.56 (br, THF),
3.31 (br, 1H, NCH2), 2.56 (br, 1H, NCH2), 2.25–1.90 (br, overlapped,
4H, NMe and NCH2), 1.84 (br, 3H, NMe), 1.50 (br, 6H, ArCHMe2),
1.40 (br, THF), 1.35 (br, 6H, ArCHMe2), 1.31 (br s, 18H, tBu), 1.30–
1.20 (overlapped, 4H, NCH2 and ZrCH2N), 0.56 ppm (br, 1H, ZrCH).
Poor solubility and fluxional behavior of 8-2THF in solution pre-
vent us from acquiring the 13C NMR and VT-NMR spectra. IR (KBr):
n˜ =2954 (s), 2865 (s), 2807 (w), 2700 (w), 1637 (br), 1579 (s), 1525
(w), 1459 (s), 1407 (s), 1354 (s), 1343 (s), 1282 (s), 1259 (w), 1237
(w), 1209 (w), 1172 (w), 1136 (w), 1109 (w), 1068 (w), 1050 (w), 1016
(w), 969 (w), 916 (m), 846 (w), 777 (w), 752 (m), 712 (w), 637 (w),
602 (w), 573 (w), 512 (w), 458 (w), 446 cmÀ1 (w). Anal. Calcd. for
C38H69N4O2KZr: C, 61.32; H, 9.34; N, 7.53. Found: C, 60.91; H, 9.14;
N, 7.61. The elemental analysis data shows a low carbon value
probably due to incomplete combustion.
1H NMR (400 MHz, Benzene-d6, 300 K): d=7.24 (d, 3JHH =7 Hz, 2H,
3
Ar-H), 6.93 (t, JHH =7 Hz, 1H, Ar-H), 6.88 (s, 1H, ZrC(tBu)CHC(tBu)N),
4.52 (br, 2H, ArCHMe2), 3.58 (br, 4H, THF), 3.48–3.44 (m, 1H, NCH2),
3.33–3.12 (m, overlapped, 2H, NCH2), 2.47 (br t, 1H, NCH2), 2.28 (br
s, 6H, NMe2), 2.19 (s, 3H, NMe), 2.01 (br t, 1H, NCH2), 1.60 (br, 2H,
0.5 THF), 1.65–1.60 (overlapped, 1H, NCH2), 1.53 (br, 6H, ArCHMe2),
1.42 (br, 6H, ArCHMe2), 1.31 (s, 9H, tBu), 1.18 (br d, 2H, NCH2),
1.10 ppm (s, 9H, tBu). 13C{1H} NMR (101 MHz, Benzene-d6/THF,
300 K): d=152.89 (Ar), 143.70 (Ar), 130.10 (Ar), 121.68 (Ar), 116.83
(C(tBu)CHC(tBu)N), 61.61 (NCH2CH2N), 59.30 (NCH2CH2N), 57.30
(NCH2CH2N), 52.24 (NCH2CH2N), 48.19 (N(CH3)2), 46.27 (NCH3), 41.35
(C(CH3)3), 41.08 (C(CH3)3), 31.01 (C(CH3)3), 29.81 (C(CH3)3), 27.33 (Ar-
CH(CH3)2), 25.76 (Ar-CH(CH3)2), 24.52 (Ar-CH(CH3)2), 24.44 ppm (Ar-
CH(CH3)2). Two resonances of the [C(tBu)] groups could not be lo-
cated due to poor solubility of the compound. IR (KBr): n˜ =3040
(m), 2956 (s), 2861 (s), 2091 (s), 1581 (m), 1459 (s), 1417 (s), 1363
(s), 1334 (s), 1282 (s), 1258 (m), 1204 (w), 1175 (w), 1128 (w), 1105
(w), 1084, 1065 (s), 1022 (w), 1000 (w), 965 (w), 944 (m), 930 (w),
893 (w), 857 (w), 781 (s), 748 (s), 718 (w), 656 (w), 601 (w), 551 (w),
512 (w), 481 (w), 456 cmÀ1 (w). Anal. Calcd. for C34H61N7OZr: C,
60.49; H, 9.11; N, 14.52. Found: C, 60.31; H, 9.19; N, 14.66.
Synthesis of compound 9.
To a pale yellow solution of 6 (160 mg, 240 mmol) in THF (10 mL)
was added NaN3 (84.1 mg, 1.29 mmol) in THF (5 mL) at room tem-
perature. The reaction mixture was stirred at room temperature for
18 hours, resulting in a color change from pale yellow to colorless.
The reaction mixture was filtered through Celite. The colorless fil-
trate was concentrated to ca. 1 mL, layered with pentane (5 mL)
and stored at À358C to afford compound 9 (146 mg, 212 mmol,
88% yield) as a translucent crystalline solid. Single crystals suitable
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Crystallographic studies.
Crystallographic data (CCDC 1898832–1898838) are summarized in
Table S1–S7. Suitable crystals for X-ray analyses were placed on the
end of a Cryoloop coated in NVH oil. The X-ray intensity data col-
lection was carried out on a Bruker APEXII CCD area detector for 4,
5, 6, 7, and 8-2Et2O and a Bruker D8QUEST CMOS area detector for
9 and 10 using graphite-monochromated MoKa radiation (l=
0.71073 ꢀ) at 100(2) K. Preliminary indexing was performed from a
series of twenty-four for 4, 5, 6, 7, and 8-2Et2O, or thirty-six for 9
and 10, 0.58 rotation frames with exposures of 10 seconds. Rota-
tion frames were integrated using SAINT,[32] producing a listing of
non-averaged F2 and s(F2) values. The intensity data were correct-
ed for Lorentz and polarization effects and for absorption using
SADABS.[33] The initial structure was determined by the direct
method on SHELXS.[34] The further structure determination was per-
formed by Fourier transform method and refined by least squares
method on SHELXL.[34] All reflections were used during refinement.
Non-hydrogen atoms were refined anisotropically and hydrogen
atoms were refined using riding models. For 4, the amino-ethylene
group was disordered over two positions. For 8-2Et2O, one of the
iPr groups was disordered over two positions. One of the solvent
molecules on K+ was disordered as a mixture of Et2O and THF in a
74:26 ratio, refined by FVAR constants. The thermal ellipsoids were
fixed by SHELXL restraints. For 10, the co-crystallized THF molecule
was disordered over two positions. The thermal ellipsoids were
fixed by SHELXL restraints. These results were checked using the
IUCR’s CheckCIF routine. For 4 and 10, the absolute structures
could not be determined in chiral space groups and the alerts in
the output are related to their Flack parameters. The other alerts in
the output are related to the disordered groups and crystal sol-
vents. CCDC 1898832–1898838 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
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Acknowledgements
For funding, we thank the University of Pennsylvania and the
Chemical Sciences, Geosciences, and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S. Depart-
ment of Energy (DEFG02-07ER15893), and the National Natural
Science Foundation of China (No. 21732007). D.J.M. acknowl-
edges some financial support for a Chinese Academy of Scien-
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Conflict of interest
The authors declare no conflict of interest.
Keywords: azides · imides · reduction · zirconium
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Manuscript received: April 11, 2019
Revised manuscript received: May 13, 2019
Version of record online: && &&, 0000
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FULL PAPER
Takashi Kurogi, Jiaxiang Chu,
Yaofeng Chen,* Daniel J. Mindiola*
&& – &&
Neutral and Anionic Monomeric
Zirconium Imides Prepared via
Selective C=N Bond Cleavage of a
Multidentate and Sterically
Demanding b-Diketiminato Ligand
A sterically encumbering multidentate
b-diketiminato ligand, tBuL2, is reported
in this study along with its coordination
chemistry to zirconium(IV). Using the
lithio salt of this ligand, Li(tBuL2), the zir-
conium(IV) chloride and azide could be
prepared and structurally characterized.
Reduction of the tBuL2 zirconium(IV)
complexes by KC8 resulted in formation
of mononuclear neutral and anionic zir-
conium imides via reductive cleavage of
the imino group in the tBuL2 ligand.
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