Emergence of a New [NNN] Pincer Ligand via Si?H Bond Activation and β-Hydride Abstraction at Tetravalent Cerium

Article abstract of DOI:10.1002/chem.202000625

The cerium(IV) pyrazolate complexes [Ce(Me₂pz)₄]₂ and [Ce(Me₂pz)₄(thf)] initiate β-hydride abstraction of the bis(dimethylsilyl)amido moiety, to afford a heteroleptic cerium(IV) species containing a dimethylpyrazolyl-substituted silylamido ligand, namely [Ce(Me₂pz)₃(bpsa)] (bpsa=bis((3,5-dimethylpyrazol-1-yl)dimethylsilyl)amido; Me₂pz =3,5-dimethylpyrazolato), along with some cerium(III) species. Remarkably, the nucleophilic attack of the pyrazolyl at the silicon atom and concomitant Si?H-bond cleavage is restricted to the tetravalent cerium oxidation state and appears to proceed via the formation of a transient cerium(IV) hydride, which engages in immediate redox chemistry. When [Ce(Me₂pz)₄]₂ is treated with [Li{N(SiMe₃)₂}], that is, in the absence of the SiH functionality, any redox chemistry did not occur. Instead, the ceric ate complex [LiCe₂(Me₂pz)₉] and the stable mixed-ligand ceric species [Ce(Me₂pz)₂{N(SiMe₃)₂}₂] were obtained.

Full text of DOI:10.1002/chem.202000625

Accepted Article
Accepted Article
Title: Emergence of a New [NNN] Pincer Ligand via Si-H-Bond
Activation and b-Hydride Abstraction at Tetravalent Cerium
Authors: Daniel Werner, Uwe Bayer, Dorothea Schädle, and Reiner
Anwander
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To be cited as: Chem. Eur. J. 10.1002/chem.202000625
Link to VoR: https://doi.org/10.1002/chem.202000625
01/2020

10.1002/chem.202000625
Chemistry - A European Journal
Emergence of a New [NNN] Pincer Ligand via Si–H-Bond Activation
and β-Hydride Abstraction at Tetravalent Cerium
Daniel Werner,a Uwe Bayer,a Dorothea Schädle,a and Reiner Anwander*a
a)
Prof. Dr. R. Anwander, Institut für Anorganische Chemie, University of Tübingen (EKUT) Auf der Morgenstelle 18, 72076, Germany,
E-mail: reiner.anwander@uni-tuebingen.de
5
Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Abstract: The cerium(IV) pyrazolate complexes [Ce(Me2pz)4]2 and [Ce(Me2pz)4(thf)] initiate b-hydride abstraction of the bis(dimethylsilyl)amido moiety,
to afford heteroleptic cerium(IV) species containing a dimethylpyrazolyl-substituted silylamido ligand, namely [Ce(Me2pz)3(bpsa)] (bpsa = bis((3,5-
dimethylpyrazol-1-yl)dimethylsilyl)amido)), along with other cerium(III) species. Remarkably, the nucleophilic attack of the pyrazolyl at the silicon atom
and concomitant Si–H-bond cleavage is restricted to the tetravalent cerium oxidation state and appears to proceed via the formation of a transient
cerium(IV) hydride, which engages in immediate redox chemistry. When [Ce(Me2pz)4]2 is treated with [Li{N(SiMe3)2}], that is, in the absence of the
SiH functionality, any redox chemistry did not occur. Instead, the ceric ate complex [LiCe2(Me2pz)9] and the stable mixed-ligand ceric species
[Ce(Me2pz)2{N(SiMe3)2}2] were obtained.
10
15
Introduction
Å).[12] Currently, interest has turned toward mono- and multi-
50 dentate ligand systems complimentarily,[13] where utilization of
chelating moieties (Figure 1, e.g., E type; oxidation state +4 is
indicated by a red sphere) has undergone a significant revival
since their debut in cerium chemistry nearly 20 years ago.[14] Not
only do they prevent the cerium(IV) ion from ligand/redox
55 disproportionation, but the chelate/scaffold routinely leaves an
open pocket for an additional reactive ligand.[14a,b,e,15] Accordingly,
the isolation of cerium(IV) complexes featuring terminal Ce=O
moieties,[14d] and even terminal azido ligands could be
accomplished.[14c] Yet, despite the advancement of these
60 polydentate ligands, implementation of simpler monoanionic
[NNN] pincer ligands (such as B), has remained an
underexplored area of ceric chemistry.
Tridentate [NNN] ligand scaffolds play a key role for exploring the
reactivity and applications of f-block complexes.[1] For example,
20 neutral terpyridines (Figure 1, A, and its derivatives; oxidation
state +3 is indicated by a blue sphere) are effective antennae for
enhancing the luminescence and subsequent chemical detection
properties of rare-earth-metal (Ln) complexes.[1b,2] Monoanionic
[NNN] pincer ligands (Figure 1, B) are successfully utilized for
25 the design of discrete Ln(III) precatalysts for polymerization
reactions.[1c,3,4] Further, tripodal tris(R,R’-pyrazolyl)borato
scorpionate ligands (Figure 1, C),[5] provide sufficient steric
protection to enable the isolation of highly reactive Ln
complexes[6] and to study and scrutinize their chemistry.[7]
30 Consequentially, the development of new [NNN] derivatives and
pincers is a continuous and important aspect of rare-earth-metal
and indeed inorganic chemistry.[8] A recent notable addition to
this ligand class is the tri-anionic [N(SiMe2Ndmp)2] ligand (Figure
1 , D, dmp = 2,6-di-mes-C6H3), observed in [Ln{N(SiMe2Ndmp)2}]
35 (Ln = Ce, Pr),[9] which was obtained by Si–H bond activation, and
concomitant formation of dihydrogen gas.
Our recent advances in cerium chemistry have also targeted
new ligand sets to increase the synthetic value of
65 cerium(IV),[11g,16] with the primary goal to investigate beyond
unidirectional redox chemistry. Referring to this, the pyrazolato
ligand class has excelled,[16] with two homoleptic cerium(IV)
complexes for both 3,5-dimethylpyrazolato (Me2pz, as
[Ce(Me2pz)4]2 (1)) and 3,5-di-tert-butylpyrazolato (tBu2pz, as
70 [Ce(tBu2pz)4].[16a] Both complexes were obtained through
H
R
R
application of
a
silylamine elimination protocol utilizing
R
B
homoleptic cerium(IV) bis(dimethylsilyl)amide [Ce{N(SiHMe2)2}4]
as a versatile precursor.[17,18] However, the synthesis of 1
appeared to involve a secondary reaction path reducing the yield
75 of 1 and forming apparent trivalent products.[16a] Considering
compound 1 is stable across a variety of different solvents (THF,
toluene, [D6]benzene, and n-hexane),[16a] and resistant to thermal
decomposition (up to 120 °C in toluene), it may be hypothesized
that the second reaction path is related to the
80 bis(dimethylsilyl)amine co-product. Such reactivity is surprising,
and warrants investigation, especially when considering the
relevance of this silylamido ligand and other ligands with non-
innocent Si–H moieties, as starting materials.[19]
N
N
N
N
N
N
N
N
N
N
N
N
R
R
Ln
R
Ln
Ln
A (neutral)
B (monoanionic)
C (monoanionic)
R'
R'
Ce
N
R'
Si
Si
N
Si
Si
N
N
N
N
N
N
R"
R"
N
R"
N
N
Ce
Ln
N
pmd
dmp
D (trianionic)
E (trianionic)
F (monoanionic, current work)
Herein, we examined the ability of HN(SiHMe2)2 and
85 [N(SiHMe2)2]− to initiate redox chemistry with dimeric 1 and
monomeric [Ce(Me2pz)4(thf)] (2a). The new [NNN] pincer ligand
[N{SiMe2(Me2pz)}2]− is proposed to form via β-hydride
abstraction and a transient ceric hydride. It features a new
member of the class of anionic [NNN] pincers which contrasts
90 the tripodal nature of classical [NNN] scorpionates (B).[5] The
significance of tetravalent cerium, and the Si−H bond, was
supported by comparative reactions with both cerium(III)
pyrazolate complexes and the [N(SiMe3)2] ligand.
Figure 1. Top and bottom left: examples of [NNN] ligands (R = alkyl, aryl; dmp
= 2,6-di-mes-phenyl) used in trivalent rare-earth metal chemistry. Bottom
40 middle: ligands which serve to encapsulate the cerium(IV) ion (e.g. R′ = alkyl,
TMS; R″ = Et, aryl), and the new monoanionic [NNN] ligand presented in this
study (right).
In the realms of tetravalent cerium, its ability to behave as a
strong one-electron oxidant[10] has attracted considerable
45 attention.[11] The choice of an appropriate ligand system is
delicate since the organic framework has to adopt to both of the
cerium(III/IV) redox couple and, in particular, to the smaller ionic
radius of cerium(IV) (Ce3+(CN 6), 1.01 Å vs. Ce4+(CN 6), 0.87
1
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10.1002/chem.202000625
Chemistry - A European Journal
Results and Discussion
was treated with half an equivalent of HN(SiHMe2)2 in
[D6]benzene. The analysis by NMR spectroscopy indicated a
slow redox process. The formation of multiple trivalent cerium
species was evident after two days at ambient temperatures, and
45 complete conversion after four months afforded a light red
solution with 3 as the only diamagnetic species (3%, Figure S2).
During the reaction progress, the following trends were
observed: the gradual decrease of the Si–H septet (4.72 ppm)
and SiMe doublet (0.11 ppm), the broadening of the Me2pz
50 singlet (5.85 ppm), and the formation of a variety of new
resonances assignable to the new bpsa ligand framework, the
most notable being the formation of the SiMe2 singlet at
0.13 ppm. Although the reaction between 1 and HN(SiHMe2)2 is
slow at ambient temperatures, its conversion could be improved
55 by heating the reaction mixture to 105 °C (Figure S3) and it
appeared complete within 16 hours under optimized conditions.
After cooling, colorless block crystals formed and analysis by X-
ray crystallography revealed the formation of trivalent co-product
[CeIII4(Me2pz)12(Me2pzH)2] (2c, Figure S4), as a direct result of a
60 redox process (vide infra). The overall structure of 2c is similar
to polymeric [La(Me2pz)3]∞,[23] but the putative polymeric chain is
capped by two terminating Me2pzH ligands to form the
tetrametallic species 2c. Such an oligomer formation is
interesting considering that the homoleptic species forms a
65 tetrametallic [Ce4(Me2pz)12] cluster.[23] Nevertheless, it is evident
that at ambient temperature release of free amine is presumably
not the direct cause of the initial rapid degradation observed
during the synthesis of 1, and it is more likely that the amido
ligand engages into Si–H bond activation in the form of transient
70 heteroleptic complexes [Ce(Me2pz)x{N(SiHMe2)2}4-x] (x = 2 (4a)
or x = 3 (4b), cf., Scheme 3).
Reactivity of [Ce(Me2pz)4]2 (1) toward HN(SiHMe2)2. As
reported previously, the protonolysis of [Ce{N(SiHMe2)2}4] with
four equivalents of Me2pzH affords either dimeric [Ce(Me2pz)4]2
(1) from toluene (Scheme 1),[16a] or monomeric [Ce(Me2pz)4(thf)]
(2a) in the presence of THF (Scheme 1).[20] Monitoring the
reaction of [Ce{N(SiHMe2)2}4] with four equiv. 3,5-
dimethylpyrazole in [D6]benzene by 1H NMR spectroscopy,
5
10 however, indicated multiple paramagnetic CeIII and diamagnetic
CeIV species.[16a] Although these unidentifiable species continued
to form over 16 h, the processes dramatically decelerated over
several days. The 1H NMR spectrum of this reaction mixture was
complicated (Figure S1), but crystallization from n-hexane gave
15 red block crystals of the heteroleptic cerium(IV) pyrazolate
complex
[Ce(Me2pz)3(bpsa)]
(3)
bearing
a
bis(3,5-
dimethylpyrazol-1-yl-dimethylsilyl)amido (bpsa) ligand (Scheme
2). Complex 3 was obtained in low yield, however, it could be
repeatedly isolated from the synthesis of either 1 or 2a at ambient
1
20 temperatures. Furthermore, the H NMR signals attributed to 3
could be identified in the reaction mixture of [Ce{N(SiHMe2)2}4]
and Me2pzH (Figure S1), indicating 3 is a co-product of the
occurring redox processes.
N
N
N
N
N
N
N
N
N
N
N
N
1
Ce
/
Ce
2
N
N
n = 4
toluene
30 sec
N
N
cf. ref. [20]
- 4 HN(SiHMe2)2
[Ce(Me2pz)4]2 (1)
Both 4a and 4b are formed during the protonolysis reactions,
but also in minor amounts during the disfavored reverse
protonolysis reaction (HN(SiHMe2)2: pKa,THF= 22.6;[24] pyrazole:
toluene
HN(SiHMe2)2
n < 4
Si
Si
[D6]benzene
16 h, 70 °C
- n HN(SiHMe2)2
N
N
N
N
75 pKa,DMSO
=
19.8).[25] Thus, when the ratio of Me2pzH to
[Ce{N(SiHMe2)2}4]
N
+
Ce
[Ce{N(SiHMe2)}4] was changed to 5:1, quantitative formation of
[Ce(Me2pz)4(Me2pzH)] (2b) and four equiv. of HN(SiHMe2)2 was
indicated by 1H NMR spectroscopic analysis in [D6]benzene (see
Figure S5). In contrast, when [Ce{N(SiHMe2)2}4] was treated with
80 two or three equivalents of Me2pzH, decomposition occurred
within minutes (see Figure S6).
n Me2pzH
N
N
N
N
N
N
[Ce(Me2pz)3(bpsa)] (3)
+ other CeIII species
n = 4
THF/cyclohexane
or
THF
HN(SiHMe2)2
n > 4
The redox-sensitive behavior of the silylamido ligand
[N(SiHMe2)2] has been observed previously in rare-earth metal
chemistry, namely in trivalentàdivalent transitions. There,
85 treatment of [EuIII{N(SiMe3)2}3] with an excess of HN(SiHMe2)2,
led to a mixed-valent EuII/EuIII species,[26] however, the oxidized
organic co-products remained unidentified. Moreover, Si–H bond
activation of [N(SiHMe2)2] devoid of any metal-based redox
reaction has been found when [Ln{N(SiHMe2)2}3(thf)2] (Ln = La,
90 Ce, Pr) were treated with H2Ndmp (e.g. dmp = 2,6-Mes-C6H3,
see Figure 1, D). As a result, homoleptic complexes bearing a
trianionic [N{SiMe2(Ndmp)}2] ligand were obtained along with
concomitant formation of hydrogen gas.[9] In the latter reaction,
the Si–H bond activation is driven by a powerful nucleophilic
95 ligand, and only occurred upon displacement of coordinating thf
from the initial adduct [Ln{N(SiHMe2)2}3(thf)2]. In a recent study,
a cerium(III) complex bearing a carbon-based analogue to
[N(SiHMe2)2], namely [Ce{C(SiHMe2)3}3], was shown to exhibit
pronounced Ce…H–Si b-agostic interactions at low temperatures
100 in solution.[27] Upon treatment of [Ce{C(SiHMe2)3}3] with B(C6F5)3,
β-hydride abstraction occurred to afford borohydrido ligand
[HB(C6F5)3] and disilacyclobutane as the co-product,
complementing previous similar findings.[19e,28]
L
toluene/cyclohexane
30 sec
- 4 HN(SiHMe2)2
N
N
N
N
Ce
N
N
N
N
[Ce(Me2pz)4(L)]
(2a, L = THF; 2b, L = Me2pzH)
25
30
Scheme 1. Synthesis of [Ce(Me2pz)4]2 (1),[14a] [Ce(Me2pz)4(thf)] (2a),[20]
[Ce(Me2pz)4(Me2pzH)] (2b), and cerium complex 3, coordinated with the [NNN]
pincer ligand.
The formation of 3 supported our initial hypothesis that the
N(SiHMe2)2 moiety engages in the decomposition process,[16a]
and the bpsa ligand formation involves Si−H-bond activation. It
should also be noted that when the diisopropylamide complex,
[Ce(NiPr2)4],[21] was treated with four equivalents of Me2pzH in
35 toluene, the reaction mixture changed color to light red within
seconds of Me2pzH addition, and crystallization from n-hexane
gave only crystals of the previously reported trivalent complex
[Ce(Me2pz)3(Me2pzH)]2,[22] in moderate yield (34%).
To further investigate into the parameters of Si–H-bond
40 activation, several experiments were performed. First, isolated 1
2
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10.1002/chem.202000625
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Cerium(IV) is a powerful Lewis acid, and although structural
elucidation of [Ce{N(SiHMe2)2}4] did indicate a degree of Ce…H–
Si b-agostic interactions,[17] the homoleptic complex is stable.
Considering that Me2pz is an effective nucleophile at cerium(IV)
to form pyrazol-functionalized multidentate ligands,[16] we initially
assumed Me2pz-based nucleophilic chemistry with HN(SiHMe2)2,
with the ceric center acting only as a spectator. However, Si−H
bond activation did not occur when the cerous
[Ce{N(SiHMe2)2}3(thf)2] was reacted with Me2pzH (Scheme 2,
amounts, leaving cerium(IV) in large excess and available for
55 reaction with the highly reactive, transient hydrogen radicals.
Thus, the reaction sequence is proposed to occur in the following
manner: activation of the Si–H moiety via b-agostic interactions
with the Ce(IV) metal, pyrazolato attack at the silicon atom with
concomitant Si−H-bond cleavage and β-hydride abstraction,
60 cerium(IV) hydride reduction and hydrogen radical formation,
finally hydrogen radical-induced reduction of another cerium(IV)
center. This reaction sequence continues until 1 is exhausted.
5
10 Figure S7), even after heating to 60 °C for several days. It is even
more remarkable that upon treatment of this reaction mixture with
[Li{N(SiHMe2)2}] in [D8]toluene at ambient temperatures for
SiMe2
N
N
several
weeks,
crystals
of
heteroleptic
complex
SiMe2
R
N
[Li2(thf)2Ce(Me2pz)2{N(SiHMe2)2}3] (5) were obtained. Crucially,
15 complex 5 is stable (under inert atmospheres) despite clear
Ce…H–Si b-agostic interactions and the presence of Me2pz
(solution 1H NMR spectra: Figures S8/9). Thus, the capability of
the Me2pz ligand to initiate Si–H-bond activation appears
exclusively to those bound to the cerium(IV) ion.
(ii)
(i)
rapid
Ce
(Me2pz)x
R = H or
Me2pz
H
[N(SiHMe2)2]2-x
Me2
Si
Me2
Si
SiMe2
N
N
H
SiMe2
R
N
R
N
20
N
Ce
(Me2pz)x
Ce
N
3 Me2pzH
[D8]THF/C6D6
(Me2pz)x
[N(SiHMe2)2]2-x
[(Me2HSi)2N]2-x
“4a/4b“
rt/ 3 d or 60 °C/ 2 d
1. evaporation
+ H
(iii)
intermediate species,
“[Ce(Me2pz)3(thf)]”
[Ce{N(SiHMe2)2}3(thf)2]
not isolable
- 3 HN(SiHMe3)2
not isolated
“NMR“
(iv)
2 "CeIV
"
Me2
Si
R = H
SiMe2H
"CeIV“ + 2 "CeIII
+
ligand scrambling
"
H / Ce(IV)
redox
chemistry
H
H
thf
N
Li
N
2. [Li{N(SiHMe2)2}]
[D8]toluene/n-hexane
Me2Si
(v)
R = Me2pz
N
Ce
N
N
N
Me2HSi
[Ce(Me2pz)3(bpsa)] (3)
+
trivalent species
H
N
Li
Si
Me2
(e.g., [Ce4(Me2pz)12(Me2pzH)2] (2c)
SiHMe2
thf
[Li2(thf)2Ce(Me2pz)2{N(SiHMe2)2}3] (5)
+ other trivalent species
65 Scheme 3. Likely formation of pincer complex
3
in the mixture
[Ce{N(SiHMe2)2}4]/HN(SiHMe2)2 with [Ce(Me2pz)4]2 (1) as the main product.
Non-isolable heteroleptic complexes [Ce(Me2pz)(Me2pz)x{N(SiHMe2)2}3-x
]
Scheme 2. Successive treatment of [Ce{N(SiHMe2)2}3(thf)2] with Me2pzH and
[Li{N(SiHMe2)2}] yielding [Li2(thf)2Ce(Me2pz)2{N(SiHMe2)2}3] (5) along with
other cerous species.
(x = 2 (4a) or x = 1 (4b)) engage in cerium(IV) mediated Si–H-bond activation
and the generation of transient hydrogen radicals H∙. The latter reduce 1 to
70 “[Ce(Me2pz)3(Me2pzH)]”-type complexes (e.g., [Ce4(Me2pz)12(Me2pzH)2], 2c).
Note, this is not a catalytic cycle.
25
In a previous study we found that the reaction between 1 and
Considering that Ce3+/Ce4+ products form relatively slowly,
these complicated side reactions during the syntheses of 1 and
2a could be minimized by limiting the formation of 4a and 4b.
75 Under optimized conditions crystals of [Ce{N(SiHMe2)2}4] are
added to a stirring, concentrated solution of Me2pzH (in slight
excess), and after addition, the solution is immediately dried
under vacuum, to afford red crystals of 1 (toluene) or 2a (THF) in
high yields.
benzophenone affords Me2pz-functionalised diphenylmethoxy
ligand
moieties
via
a
cascade
of benzophenone
association/dissociation and nucleophilic attack by the Me2pz co-
ligands, which is facilitated by weakening of the ketone C=O
30 double bond upon coordination to cerium(IV).[16b] The present
ligand transformation may bear some similarity, being triggered
by the nucleophilic attack of the “activated” Si−H functionality by
a pyrazolato ligand. The formation of the bpsa ligand is proposed
to proceed via β-hydride abstraction, most likely traversing a
80 Reactions between [Ce(Me2pz)4]2 (1) and lithium silylamides.
During the syntheses of either 1 and 2a, we observed that lithium
impurities such as Li(Me2pz) or lithium amide from the initial
synthesis of [Ce{N(SiHMe2)2}4]) can lead to the ate complex
[LiCe2(Me2pz)9] (6). Complex 6 can be directly accessed by
85 treating [Ce(Me2pz)4(Me2pzH)] (2b) with an equimolar amount of
[Li{N(SiHMe2)2}] in [D8]THF. Under inert conditions ceric 6 is
thermally stable upon storage of the reaction mixture for several
days at ambient temperatures, and any Si–H-bond activation of
the formed HN(SiHMe2)2 was not observable. Alternatively, 6
90 could be obtained when 1 was treated with half an equivalent of
[Li(Me2pz)] (Scheme 4). As 6 was less soluble than both 1 and
2a in non-coordinating solvents, it was isolated by fractional
crystallization and characterized by single-crystal X-ray
35 cerium(IV) hydride species (Scheme
3 (i)). The putative
cerium(IV) hydride presumably undergoes immediate reduction,
forming a cerium(III) species, and a hydrogen radical (Scheme 3
(ii)), which reduces other present cerium(IV) species (Scheme 3
(iii)), likely in a non-selective manner.[29] Along with tandem
40 ligand scrambling, the Si–H-bond activation occurs twice
(Scheme 3 (iv)), ultimately giving complex 3 and other cerium(III)
species (Scheme 3 (v)). Such an overall highly complicated
mechanistic scenario is supported by the following observations.
The generation of dihydrogen is detected (Figure S2) but it is not
45 capable of reducing complex 1 at ambient temperatures,
indicating that a transient hydrogen radical reduces other
cerium(IV) species. Further to this, complex 3 emerged as the
only remaining tetravalent species, suggesting that a degree of
ligand scrambling occurred within the system, with a preference
50 for bpsa coordination to the smaller cerium(IV) ion. As formation
crystallography. Complex
6 revealed the same structural
95 arrangement in solution at ambient temperature as determined
by 1H and 13C{1H} NMR spectroscopy (see Figures S10/11).
of the “active” heteroleptic species [Ce(Me2pz)x{N(SiHMe2)2}4-x
]
(x = 2 (4a) or x = 3 (4b)) can only occur through a disfavored
reverse protonolysis reaction, it would form slowly and in minute
3
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10.1002/chem.202000625
Chemistry - A European Journal
Scheme 4. Reactions of [Ce(Me2pz)4]2 (1) with lithium reagents to afford [LiCe2(Me2pz)9] (6) and [Ce(Me2pz)2{N(SiMe3)2}2] (7).
As formation of 6 appeared favorable, we envisioned that
treating 1 with [Li{N(SiHMe2)2}] could provide an alternative path
cerium pyrazolates.[16a] Further, when in-situ generated
“Li[CeIII(Me2pz)4]” was treated with 1,4-benzoquinone in
THF/toluene solutions, as a means to access 1 through redox
55 chemistry, only intractable product mixtures were observed.
Fortuitous crystal formation from this reaction mixture revealed
5
to
form
the
transient
heteroleptic
complexes
“[Ce(Me2pz)x{N(SiHMe2)2}4-x]” (x = 2 (4a), x = 3 (4b)) through
ligand redistribution. When 1 was treated with [Li{N(SiHMe2)2}] in
[D6]benzene, monitoring the reaction mixture by 1H NMR
spectroscopy indicated the formation of 6, and two different
the
formation
of
the
octametallic
complex
[{Li2Ce2(Me2pz)6(thf)2}2(pzhq)2] (8) containing two 2-(3,5-
dimethylpyrazol-1-yl)1,4-hydroquinolato (pzhq) ligands (Scheme
60 5, Figure S18). Thus, in the absence of the sterically demanding
N(SiMe3)2 ligands, the bq/hq and CeIII/CeIV redox couple and the
double pyrazolato-driven carbonyl attack dominate the reaction
sequence.
10 N(SiHMe2)2 environments (SiH septets at 6.08 and 6.14 ppm and
SiMe2 doublets at 0.28 and 0.34 ppm, Figures S12/1-3). These
resonances are not attributable to [Li{N(SiHMe2)2}], which occur
at 4.66 and 0.22 ppm, and are likely candidates for a heteroleptic
CeIV species ligated by N(SiHMe2)2 (i.e. 4a or 4b), respectively.
15 This is supported by the SiH shift observed in heteroleptic
[Ce(L)(N{SiHMe2}2)]
(L = tailored
tris(hydroxylaminato),
dH (SiHMe2): 6.71 and 0.89 ppm)[15c] and in homoleptic
[Ce(N{SiHMe2}2)4] (dH (SiHMe2): 6.01 and 0.34 ppm).[17]
Furthermore, the resonances for HN(SiHMe2)2 were observed
20 (4.72 and 0.11 ppm) along with the SiMe2 resonance attributable
to the bpsa ligand (singlet at 0.13 ppm). After two days at
ambient temperatures, the reaction mixture had lightened in
color, and the 1H NMR spectrum indicated the formation of
multiple CeIII/CeIV species (Figure S13).
[Ce{N(SiMe3)2}3]
+
[Li{N(SiMe3)2}]
1
O
/
2
2 Me2pzH
C6D6
2 Me2pzH
n-hexane, 10 min
- 2 HN(SiMe3)2
O
1
7
- 2 HN(SiMe3)2
- Li2(C6H4O2)
"Li[Ce(Me2pz)2{N(SiMe3)2}2]“
2 Me2pzH
not isolated
toluene, 10 min
"Li[Ce(Me2pz)4]"
- 2 HN(SiMe3)2
thf
25
To determine if such a decomposition occurs in the absence
of the Si–H functionality, complex 1 was treated with lithium
bis(trimethylsilyl)amide [Li{N(SiMe3)2}], in [D6]benzene. While 1H
NMR spectroscopic analysis indicated a ligand redistribution
process, any decomposition to paramagnetic species or
not isolated
thf
N
O
N
1
L
O
– 35 °C,
THF
/
2
Li
L
Li
O
O
L
Ce
L = Me2pz
Ce
L
L
Ce
30 formation of HN(SiMe3)2 was not observed. Instead, the following
complexes were identified: the Li/Ce bimetallic complex 6,
unreacted Li{N(SiMe3)2}, and Li(Me2pz), as well as the
heteroleptic CeIV species [Ce(Me2pz)2{N(SiMe3)2}2] (7),
containing two N(SiMe3)2 and two Me2pz ligands (Scheme 4). In
35 stark contrast to the putative [N(SiHMe2)2] analogue 4a,
compound 7 is thermally stable in [D6]benzene over several days
in the presence of the corresponding silylamine (Figures S14-16).
The formation of 7 points to the likely intermediate formation of
L
L
L
L
Ce
L
L
O
N
O
L
Li
Li
N
thf
thf
[{Li2Ce2(Me2pz)6(thf)2}2(pzhq)2] (8)
+ other trivalent species
65
Scheme 5. Distinct reactivity of in-situ generated ate complexes
“Li[CeIII(Me2pz)2{N(SiMe3)2}2]” and “Li[CeIII(Me2pz)4]” toward 1,4-benzoquinone.
heteroleptic 4a when homoleptic
1
is treated with
40 [Li{N(SiHMe2)2}]. Furthermore, the stability of 7, which features a
more sterically demanding silylamido ligand, further emphasizes
that it is solely the Si–H functionality which initiates the observed
redox chemistry.[18b]
Heteroleptic [Ce(Me2pz)2{N(SiMe3)2}2] (7) also proved to be
an excellent precursor to quantitatively access 1 via treatment
70 with two equivalents of Me2pzH (Scheme 5, Figure S17), even
with an excess of 7. More importantly, transsilylamination[31] of 7
with HN(SiHMe2)2 was probed as a means to access putative
Monomeric ceric amide complex [Ce(Me2pz)2{N(SiMe3)2}2]
45 (7). Complex 7 is a rare example of a ceric bis(amido) complex[30]
and can be accessed directly when 1,4-benzoquinone is added
to an in situ generated solution of “Li[CeIII(Me2pz)2{N(SiMe3)2}2]”,
followed by immediate filtration (Scheme 5). It is surprising that
1,4-benzoquinone is an effective oxidant for the isolation of
50 complex 7, considering our recent observations with the
benzoquinone/hydroquinolate redox couple with tetravalent
heteroleptic
species
“[Ce(Me2pz)2{N(SiHMe2)2}2]”
(4a).
Unfortunately, the envisaged protonolysis did not occur at
75 ambient temperature, but was indicated at temperatures > 60 °C
along with decomposition (Figure S19).
Spectroscopic and crystallographic characterization.
Complexes 2a, 2b, 3, and 7 display monomeric structures in the
4
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10.1002/chem.202000625
Chemistry - A European Journal
solid state. Compound [Ce(Me2pz)4(thf)] (2a) was previously
reported to crystallize from n-hexane giving red block crystals
with half a molecule within the asymmetric unit (monoclinic space
group P21/m; isostructural with the uranium analogue
[U(Me2pz)4(thf)][32]).[20] From THF, even larger block-like crystals
were obtained (2a*), crystallizing with three molecules in the
asymmetric unit (triclinic space group P-1). The av. Ce–N bond
lengths of 2.38 Å are comparable to those of the terminal Me2pz
50 region. However, the third CH resonance (d, Figure 2) indicates
that one equatorial pyrazolato ligand is interacting with the
nitrogen bound H atom of the pyrazole ligand (a, Figure 2). Such
interactions between pyrazole-H and pyrazolato ligands are well
established in rare-earth metal solid-state chemistry,[35] but
55 weren’t observed on the time scale of NMR spectroscopy.
Furthermore, it should be noted that in hafnium complex
[Hf(Me2pz)4(Me2pzH)] such pz…H–pz interactions were observed
crystallographically and that the 1H NMR spectra at ambient
temperature are almost identical to 2b, with a broadened NH
60 resonance at 13.0 ppm, and one resonance assignable to both
Me2pz and Me2pzH (at 2.12 and 5.89 ppm).[36] Unfortunately,
additional low-temperature NMR investigations such as 13C NMR
spectroscopy were unsuccessful. The tetravalent oxidation state
of 2b was difficult to confirm by X-ray crystallography, due to the
65 disorder, but surprisingly also by magnetic susceptibility
measurements. Upon analysis of 2b, a large ∆H was observed
between the two reference peaks giving a µeff of 2.43 B.M. (a
value typically higher than those of cerium(III) complexes).[37]
Such a value is possibly caused by the hydrogen exchange
70 between the pyrazole and the pyrazolato ligands. Nevertheless,
the color of the complex, and the ability to form 2b by treating 1
with one equivalent of Me2pzH, suggests that 2b is indeed
tetravalent.
5
1
ligands observed in dimeric 1 (av. Ce–N: 2.36 Å). The H NMR
10 spectrum of 2a at ambient temperature shows one set of signals
for the pyrazolato ligand, and magnetic susceptibility
measurements (determined by the Evans method)[33] were
concordant with other cerium(IV) complexes.[34]
The Me2pzH-ligated monomeric complex 2b adopts the
15 same structural motif as 2a, with three Me2pz ligands on the
meridional plane and the remaining pyrazole/pyrazolato ligands
disordered 50:50 over both axial positions. Unfortunately, the
structural solution was not straightforward. The compound
crystallized with significant molecular disorder, with the entire
20 molecule split over two positions (see Figure S20). When 2b was
analyzed by 1H NMR spectroscopy, only one ligand environment
was observed as revealed by sharp CH3 and CH resonances,
and a broadened NH signal, shifted to higher field (9.03 ppm)
compared to free Me2pzH (12.25 ppm, Figures S21/22). This
25 indicates a high proton mobility among the pyrazolato/pyrazole
ligands in solution (Figure 2, green spectrum).
The X-ray data for heteroleptic complex 3 were solved and
75 refined in the monoclinic space group C2/c, where half of the
molecule resided within the asymmetric unit (Figure 3, top). The
bpsa ligand has a large coordination bite angle across the 6-
coordinate cerium center (N3-Ce1-N3ʹ, 146.23(9)°. Considering
the unique nature of the bpsa ligand, it was difficult to finding a
80 suitable literature comparison, however, the coordination of
bis(2-methoxyethyl)ether
in
tetravalent
[Ce{OCH(CF3)2}4(diglyme)] (diglyme = bis(2-methoxyethyl)ether)
appeared a suitable candidate. There, the diglyme ligand binds
across the equatorial position with two OCH(CF3)2 ligands, giving
85 a diglyme coordination angle of 124.26(16)°. This angle is
considerably smaller compared to the one of bpsa in 3 and is
likely due to the more compact nature of diglyme and the 7-
coordinate cerium center.[38] The Ce-N(amido) bond length of the
[NNN] pincer of 2.350(3) Å is shorter than the Ce–N(pz) bond
90 lengths (av. 2.393 Å) and the pincer-pyrazolato distances of av.
2.598 Å . The structural arrangement of 3 is retained in solution,
with signal sets in the 1H NMR spectrum showing two C(4)-H
environments in a 2:3 ratio, and three methyl group resonances
in a 3:1:1 ratio, with the methyl group at the 3-position of the
95 Me2pz substituents on the bpsa ligand resonating significantly
shifted downfield than those in 5-position (Figure S27). The
tetravalent oxidation state was further confirmed by the Evans
method, which gave a μeff of approximately zero.
Figure 2. 1H NMR spectra ([D8]toluene, 500 MHz) of [Ce(Me2pz)4(Me2pzH)]
30 (2b) at 26 °C (top, green), −80 °C (middle, red), and −100 °C (bottom, blue).
To identify any distinct pyrazolato/pyrazole environments in
2b, variable temperature 1H NMR spectroscopic experiments
were performed (Figures 2 and S23). When analyzed at −80 °C,
35 the resonances had broadened significantly (Figure 2, red).
Upon further cooling to −100 °C, the resonances decoalesced to
identify four different ligand C(4)−H environments (Figure 2,
blue), integrating (from left to right) in a ratio of 2:1:1:1. In order
to assist with peak assignment, the thf analogue, 2a, was also
40 analyzed by 1H NMR spectroscopy at −100 °C, showing two
pyrazolato C(4)−H resonances in a ratio of 3:1, corresponding to
magnetically distinct equatorial (6.34 ppm) and axial (5.91 ppm)
pyrazolato ligands (Figures S23-26). Using these assignments
as a basis for peak assignment in 2b, the far downfield shifted
45 C(4)−H resonance (e, Figure 2) corresponds to the Me2pzH
ligand, which also has two different methyl environments for the
3- and 5-positions (at 2.10 and 0.99 ppm). In accord with the 1H
NMR spectrum of 2a, the axial Me2pz ligand (c, Figure 2), and
two of the equatorial ligands (b, Figure 2) resonate in a similar
The X-ray data for heteroleptic 7 were solved in the
100 monoclinic space group C2/c with half the molecule within the
asymmetric unit (Figure 3, bottom). Ceric complex 7 adopts a
tetrahedral geometry reminiscent of that in CeIV complex
[Ce(OtBu)2{N(SiMe3)2}2] (Ce–N 2.260(7) Å),[39] and the amido
bond lengths of 2.2172(17) Å compare well with the latter. The
105 1H NMR spectrum of 7 supports the same structure in solution,
with signal sets for Me2pz and N(SiMe3)2 in a 1:1 ratio (Figures
S15/16). The tetravalent oxidation state was further corroborated
by the magnetic susceptibility measurements, determined by the
Evans method.
5
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10.1002/chem.202000625
Chemistry - A European Journal
the two remaining ones bridge between all three metal atoms in
35 a μ3-η2(N,Nʹ):2κ(N):3κ(Nʹ) fashion (Figure 4, bottom). The 1H
NMR spectrum at ambient temperature revealed distinct signal
sets for each pyrazolato environment (Figures S10/11). The four
terminal Me2pz ligands resonate at 6.00 ppm, while the signals
of the bridging ones are located at 5.76, 5.73 and 4.75 ppm with
40 an integral ratio of 1:2:2. Such a difference in chemical shift was
also observed when dimeric 1 was analyzed at −80 °C, which led
to decoalescence of the bridging and terminal Me2pz ligand
resonances (5.04 and 6.00 ppm).[16b] Magnetic susceptibility
measurements determined by the Evans method further
45 confirmed that
6 is diamagnetic, giving a μeff value of
approximately zero.
5
Figure 3. Crystal structures of monomeric cerium(IV) species
[Ce(Me2pz)3(bpsa)] (3, top) and [Ce(Me2pz)2{N(SiMe3)2}2] (7, bottom).
Ellipsoids shown at 50% probability, hydrogen atoms omitted for clarity.
Selected bond lengths (Å) and angles (°) for 3: Ce–N(1) 2.350(3), Ce–N3
2.598(2), Ce–N4 2.395(2), Ce–N5 2.372(2), Ce–N6 2.411(2); N3–Ce–N3ʹ
10 146.23(9), N1–Ce1–N3 73.11(4), N1–Ce1–C12 180.0(2). For 7: Ce–N(1)
2.330(3), Ce–N2 2.396(2), Ce–N3 2.217(2) ; C3–Ce1–C3ʹ 118.71(6), N3–
Ce1–N3 128.20(9), N3–Ce1–N1/2(centroid) 110.70(4).
The X-ray diffraction data of lithium-incorporated cerous ate
15 complex [Li2(thf)2Ce(Me2pz)2{N(SiHMe2)2}3] (5, monoclinic space
group P21/c) allowed for the manual assignment of the hydrogen
atoms of the Si–H groups and hence, the elucidation of Ce…H–
Si b-agostic interactions of the tilted amido ligand (Figure 4, top).
Accordingly, the shortest Ce…H and Ce…Si distances of 2.67(2)-
20 2.79(3) Å and 3.2370(7)-3.2648(7) Å, respectively, are similar to
those observed in the heteroleptic cerium(III) dimethylsilylamide
Figure 4. Crystal structures of [Li2(thf)2Ce(Me2pz)2{N(SiHMe2)2}3] (5, top), and
[LiCe2(Me2pz)9] (6, bottom, cf; Scheme 4). Ellipsoids are shown at 50%
50 probability. Hydrogen atoms and lattice solvent removed for clarity. Selected
bond lengths (Å) for 5: Ce1−N1: 2.392(2), Ce1−N2: 2.516(2), Ce1−N3:
2.524(3), Ce1−N4: 2.508(2), Ce1−N5: 2.926(2), Ce1−N6: 2.573(3), Ce1…H1:
2.79(3), Ce1…H4: 2.74(3), Ce1…H6: 2.67(2), Li1−N3: 2.163(4), Li1−N5:
2.155(4), Li1−N7: 2.142(4), Li2−N2: 2.159(4), Li2−N5: 2.176(4), Li2−N7:
55 2.104(7) Li1−O2: 1.987(4), Li2−O1: 1.964(4). 6: Ce−N1: 2.297(2), Ce−N2:
2.388(2), Ce−N3: 2.533(3), Ce−N4: 2.385(3), Ce−N5: 2.336(3), Ce−N6:
2.523(3), Ce1ʹ−N6: 2.503(2), Ce1ʹ−N7: 2.633(2), Ce1−N8: 2.489(2), Li1−N7:
2.062(4), Li1−N9: 1.971(3).
bis(pentamethylcyclopentadienyl)
complex
[Cp*2Ce{N(SiHMe2)2}] (2.65(2)-2.77(2) Å).[40] Each lithium atom
is encapsulated by a terminal thf ligand, and a bridging silylamido,
25 and both are coordinated by two pyrazolato ligands in κ1(N΄)
fashion.
The heterobimetallic ceric complex [LiCe2(Me2pz)9] (6)
crystallized in the monoclinic space group C2/c. The crystal
structure features four different Me2pz environments. Four
30 pyrazolato ligands coordinate in a typical η2(N,Nʹ) terminal
fashion to cerium, and cap the edges of the complex. Three
Me2pz ligands coordinate in an μ-1κ(N)-2κ(Nʹ) fashion, bridging
between either cerium atoms, or lithium and cerium atoms, while
Comparisons of selected spectroscopic features. Selected
60 spectroscopic characterization data for complexes 1-3, 6, and 7
are presented in Table 1 (data for [Ce(Me2pz)4]2 (1) and 2a
6
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10.1002/chem.202000625
Chemistry - A European Journal
1
obtained as previously reported).[16b,20] The H NMR spectra for
complexes 1, 2a, and 2b all show only one Me2pz environment
at ambient temperatures. At lower temperatures, decoalescence
of the resonances can be observed in line with the respective
solid-state structures. In general, the resonances for the
terminal/equatorial Me2pz ligands occur at lower fields than those
of the bridging/axial ligands. This is also reflected in the LiCe2 ate
complex 6, where the Me2pz ligand bridges between all three
metal centers to resonate at a comparatively higher field. The IR
For further comparison, the Me2pzH analogue 2b gave a
considerably lower value of 4100 mol−1 cm−1. Such a difference
highlights the significant effects of the coordinating co-ligands on
25 such LàM charge transfer complexes in rare-earth metal
chemistry, with the bpsa complex showing the highest
absorptivity of 20000 mol−1 cm−1.
5
As mentioned above, the magnetic susceptibility for complexes
1-7 were determined by the Evans method,[25] and except
30 complex 2b, each species gave a value consistent with other
CeIV complexes. It appears that ligand exchange processes (or
NH hydrogen exchange in 2b) result in an increased ΔHz
between the two reference peaks in the NMR spectrum,
consequentially giving larger magnetic susceptibility values
35 when analyzed by this method. In contrast, more inflexible ligand
arrangements (e.g. those in 6, or 3) give values of approximately
zero. Thus, this spectroscopic method might be not suitable to
determine the tetravalent oxidation state for systems with a high
degree of solution based ligand dynamics.
10 spectra show the Me2pz ꢀꢁ(CN) absorption at 1521-1516 cm−1 for
complexes 1-3, 6, and 7, with an additional band at 1538 cm−1 in
2b, corresponding to the Me2pzH ligand. Such spectroscopic
features are consistent with other rare-earth metal pyrazolate
complexes.[22] The UV/Vis spectra of all compounds as depicted
15 in Figures S28-32 revealed molar absorptivities consistent with
ligand to metal charge transfers in other CeIV systems,[18a,21,41]
e.g., homoleptic [Ce{N=C(NMe2)2}4]2 (17155 and 12078
mol−1 cm−1),[21] or [Ce{N(SiMe3)2}3Cl] (ε = 2520 mol−1 cm−1),[42]
Notably, the mono-solvated [Ce(Me2pz)4(thf)] (2a) complex
20 exhibits
a significantly larger molar absorptivity of 15000
Mol−1 cm−1 compared with [Ce(Me2pz)4]2 (1, 7900 mol−1 cm−1).
40
Table 1. Spectroscopic data for cerium(IV) complexes under study
Compound
Yield (%)
1H NMR* (ppm) r.t.
5.84[19]
1H NMR* (ppm) vt
UV Vis (λmax.)
ε (mol−1 cm−1
7900
)
B.M. (μeff
1.22
1.05
2.43
~0
)
[Ce(Me2pz)4]2 (1)[16b]
[Ce(Me2pz)4(thf)] (2a)[20]
[Ce(Me2pz)4(Me2pzH)] (2b)
[Ce(Me2pz)3(bpsa)] (3)
[LiCe2(Me2pz)9] (6)
70,[19] (95)
5.04/6.00[14b]a
420
418
409
422
423
430
96
6.13
5.91/6.43b
15000
4100
>99
5.93
4.65/5.77/5.91/6.32b
~8
5.59/6.00
4.75/5.73/5.75/6.00
6.21
-
-
-
20000
10400
3800
41
~0
[Ce(Me2pz)2{N(SiMe3)2}2] (7)
27 (crystal yield)
1.02
a)
Performed at −80 °C. b) performed at −100 °C. * 1H NMR resonance of the pyrazolato methine.
Conclusions
Experimental
General methods, instrumentation, and starting materials: All
manipulations were performed using glovebox (MBraun 200B; <0.1 ppm
O2, <0.1 ppm H2O) or Schlenk techniques under an atmosphere of purified
argon gas, in oven dried glassware. Solvents THF, n-hexane, and toluene
were purified by Grubbs columns (MBraun SPS, solvent purification
system), while cyclohexane, [D8]THF, [D8]toluene and [D6]benzene were
dried over NaK alloy and degassed. All solvents were stored inside a
glovebox. Me2pzH was purchased from Sigma Aldrich and used as
received. Anhydrous CeCl3 was purchased from ABRC chemicals and was
activated by Soxhlet extraction with THF, giving [CeCl3(thf)1.05].
[Li{N(SiHMe2)2}] was synthesized according to published procedures.[43]
Tetravalent cerium complex [Ce{N(SiHMe2)2}4] revealed
competing reactivities toward dimethylpyrazole. Although, the
protonolysis reaction gives the desired [Ce(Me2pz)4]2 (or
[Ce(Me2pz)4(thf)]) in an efficient manner, an alternative path
involves Si–H-bond activation and hydride abstraction at
cerium(IV), followed by rapid reduction to cerium(III) and a
hydrogen radical. The latter complicated reaction sequence
accomplishes
the
[NNN]
dipyrazolyl
pincer
ligated
[Ce(Me2pz)3(bpsa)] as a minor product, involving heteroleptic
species of type [Ce(Me2pz)x{N(SiHMe2)2}4-x]” (x = 2, 3) as
intermediates. In the absence of the Si–H functionality such pincer
co-products were not observed. In accordance, treatment of
[Ce(Me2pz)4]2 with [Li{N(SiMe3)2}] afforded two isolable ceric
complexes, bimetallic [LiCe2(Me2pz)9] and heteroleptic
[Ce(Me2pz)2{N(SiMe3)2}2]. Crucially, the observed hydride
abstraction at the dimethylsilylamido moiety only occurred in
cerium(IV) systems and, for comparison, not in reactions with the
weaker Lewis acidic cerium(III) compounds. Such findings
highlight an additional synthetic scope of cerium(IV) in chemical
transformations, and provide a potential pathway for obtaining
cerium(IV) hydride species. Future work will involve determining
a direct high-yielding protocol for the bpsa ligand, since our initial
attempt of the noncerium(IV) mediated reaction of
[Li{N(SiHMe2)2}] and [Li(Me2pz)] did not lead to the desired ligand
framework (Figure S33), further highlighting the importance of
tetravalent cerium.
[Ce{N(SiHMe2)2}3(thf)2] was synthesized by treatment of [CeCl3(thf)1.05
]
with three equivalents of [Li{N(SiHMe2)2}] in n-hexane and was purified by
filtration, evaporation to dryness and crystallization from n-hexane. 1,4-
Benzoquinone was purchased from Sigma-Aldrich and sublimed before
use. [Ce{N(SiHMe2)2}4] was synthesized according to the literature,[17b,18b]
involving oxidation of the ate complex [Li(thf)Ce{N(SiHMe2)2}4] with 1,4-
benzoquinone in toluene. The NMR spectra of air- and moisture-sensitive
compounds in [D6]benzene or [D8]THF were recorded with J.Young valved
NMR tubes. Unless specified otherwise, analyses were performed at 26 C
with either a Bruker-Avance II 400 (1H: 400 MHz, 13C: 101 MHz), a Bruker-
Avance II 500 (1H: 500 MHz; 13C: 126 MHz) or a Bruker DRX-250 (1H: 250
MHz, 13C 63 MHz) spectrometer. Magnetic susceptibilities were
determined by the Evans method.[33] Infrared spectra were recorded on a
Nicolet 6700 FTIR spectrometer (ṽ = 4000-600 cm−1) by using a DRIFT
chamber with dry KBr/sample mixtures and KBr windows. Elemental
analyses (C, H, N) were performed on the bulk sample (unless specified
otherwise), with an Elementar Vario Micro cube by Mr. W. Bock (EKUT).
Unless specified otherwise, reported yields are given for compounds after
7
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10.1002/chem.202000625
Chemistry - A European Journal
a
satisfactory microanalysis was obtained from the bulk sample.
Method b, 1H NMR-scale experiment: [Ce{N(SiHMe2)2}4] (15.1 mg, 0.026
mmol) and Me2pzH (11.0 mg, 0.11 mmol), were combined in [D6]benzene
at ambient temperature indicating complete conversion to
[Ce(Me2pz)4(Me2pzH)] and HN(SiHMe2)2. 1H NMR ([D6]benzene, 400 MHz,
300 K): δ = 0.12 (d, 48 H, Si(CH3)2), 2.20 (s, 30 H, CH3), 4.72 (sextet, 6 H,
SiH, lower integration than expected), 6.09 (s, 5 H, CH), 9.00 ppm (s, 1 H,
NH).
[Ce(Me2pz)4]2 (1) was initially synthesized by a literature procedure.[16a]
.
Note that dimeric 1 could be isolated with varied degrees of toluene
solvation, namely either [Ce(Me2pz)4]2·1/2PhMe or [Ce(Me2pz)4]2·1/4PhMe.
The complex used was treated as
[Ce(Me2pz)4]·1/8PhMe) for stoichiometric calculations.
a monomeric species (e.g.
Products obtained from protonolysis reactions:
Ce(Me2pz)3(bpsa)] (3):
Method a, heating reaction mixture in [D6]benzene: [Ce{N(SiHMe2)2}4]
(23.8 mg, 0.036 mmol), and Me2pzH (13.0 mg, 0.014 mmol), were
combined and shaken in [D6]benzene. After 30 min, the reaction mixture
was heated in a J. Young NMR tube at 70 °C for 16 h giving a light red
solution (refer to Figure S27). The contents were transferred into the
glovebox and evaporated to dryness (in vacuo). n-Hexane was added, and
the sample was filtrated and stored at −35°C, giving light red plate crystals
of [Ce(Me2pz)3(bpsa)] (approximate yield: 2 mg, ~8%), along with a
colorless precipitate. The crystals were separated by hand picking.
Optimized synthesis of [Ce(Me2pz)4]2 (1): [Ce{N(SiHMe2)2}4] (120 mg,
0.18 mmol), were intermittently added (over a 30 second period), to a
concentrated, stirring, toluene solution of Me2pzH (70.0 mg, 0.71 mmol).
After addition, the sample was immediately evaporated to dryness leaving
a red microcrystalline powder of 1 (92.7 mg, 96%). The spectroscopic data
were consistent with those previously published.[16a]
[Ce(Me2pz)4(thf)] (2a):
Method a, in THF: [Ce{N(SiHMe2)2}4] (99.6 mg, 0.15 mmol) was dissolved
in THF, and added to a THF solution of Me2pzH (57.0 mg, 0.62 mmol) at
ambient temperature. The reagents were shaken for 30 seconds before
immediate exposure to vacuum. Upon concentration, filtration, and storage
at −35 C, gave large red block crystals of [Ce(Me2pz)4(thf)] (2a*, 39.0 mg,
42%, crystals indicated three molecules of 2a within the asymmetric unit).
1H NMR ([D6]benzene, 250 MHz, 300 K): δ = 6.13 ppm (s, 4 H, CH), 3.31
(m, 4 H, α-thf), 2.34 (s, 24 H, CH3), 0.94 (m, 4 H, β-thf); 13C{1H} NMR
([D6]benzene, 63 MHz, 300 K): δ = 144.6 ppm (s, Me2pz-C(4)), 112.6 (s,
Me2pz-C(3,5)), 70.3 (s, α-thf-CH2(1,4)), 25.2 (s, β-thf-CH2(2,3)), 13.5 (s,
CH3); DRIFT: ꢀꢁ = 3099 (w), 3024 (w), 3013 (s), 2942 (s), 2878 (s), 2859
(m), 1517 (vs), 1474 (w), 1470 (w), 1455 (w), 1444 (m), 1432 (vs), 1417
(s), 1364 (m), 1315 (w), 1301 (w), 1106 (w), 1050 (w), 1028 (w), 1006 (m),
Elemental analysis calcd. (%) for C29H47CeN11Si2 (746.06 g mol−1
,
performed on two handpicked crystals, dried under vacuum prior to
analysis): C 46.69, H 6.35, N 20.65; Found: C 47.27, H 5.56, N 20.42; χmol
= −2.39 x 10−4 cm3 mol−1, μeff = −0.07 B.M (ΔHz: −3, 3.0 x 10−3 mol L−1).
Method b, from a THF solution: From the attempted synthesis of 2a (see
above), the reaction mixture was allowed to stand at ambient temperature
for several weeks. Upon concentration and n-hexane layering, storage at
−35 °C gave light red crystals of 3. amongst a colorless microcrystalline
powder. 1H NMR ([D6]benzene, 400 MHz, 300 K, trace solvent peaks not
included): δ = 6.00 (br s, 3 H, Me2pz-CH), 5.95 (br s, 2 H, Me2pzSiMe2-
CH), 2.82 (br s, 6 H, Me2pzSiMe-CH3), 2.10 (br s, 6 H, Me2pzSiMe2-CH3),
2.03 (s, 18 H, Me2pz-CH3), 0.13 (s, 12 H, Si(CH3)2) ppm. UV(n-hexane):
λmax = 422 (ε = 20000 mol−1 cm−1).
959 (w), 921 (w), 871 (m), 806 (w), 781 (w), 728 (w) cm-1; χmol
=
4.64x10−4 cm3 mol−1, μeff = 1.05 B.M (ΔHz: 1.2 Hz, 0.033 mol L−1);
UV(toluene): λmax = 418 (ε = 15000 mol−1 cm−1), 316 nm (ε = 8400
mol−1 cm−1).
Protonolysis reaction using [Ce(NiPr2)4] and Me2pzH: [Ce(NiPr2)4]
(153.5 mg, 0.28 mmol), was dissolved in toluene (1 mL), and a solution of
Me2pzH (110.0 mg, 1.1 mmol), was added under vigorous stirring. The
flask was shaken and within seconds the color changed from dark blue to
light red. Crystallization from n-hexane gave colorless single crystals of
trivalent complex [Ce(Me2pz)3(Me2pzH)]2. The unit cell was in accord with
the literature data.[18] Crystal yield ~50 mg (~34%); Elemental analysis
calcd. (%) for C40H58Ce2N16 (1042.25 g mol−1): C 46.05, H 5.60, N 21.48;
found: C 46.14, H 5.54, N 20.99. It should be noted that adding [Ce(NiPr2)4]
to a solution of Me2pzH also did not give 1 and reduction ensued in a
similar manner as above.
[20]
Method
b,
in
THF
and
cyclohexane:
see
ref.
Method c, the optimized synthesis: Crystals of [Ce{N(SiHMe2)2}4] (118.0
mg, 0.18 mmol), were intermittently added (over a 30 second period), to a
concentrated, stirring, THF solution of Me2pzH (68.0 mg, 0.71 mmol) at
ambient temperature. After addition, the sample was immediately
evaporated to dryness leaving a red microcrystalline powder of 2a (100.6
mg, 96%). The 1H NMR spectrum was consistent with the data of the
sample obtained by method a. 1H NMR ([D8]toluene, 400 MHz, 173 K): δ =
6.34 (br s, 3 H, Me2pz-CHequatorial), 5.91 (br s, 1 H, Me2pz-CHaxial), 3.08 (s,
4 H, β-thf), 2.47 (br s, 18 H, Me2pz-CH3equatorial), 2.11 (br s, 6 H, Me2pz-
CH3axial), 0.36 (s, 4 H, β-thf) ppm. Elemental analysis calcd. (%) for
C24H36CeN8O (592.71 g mol−1): C 48.63, H 6.12, N 18.91; found: C 48.47,
H 5.56, N 19.16.
Investigations into Si–H bond activation:
Reaction between [Ce{N(SiHMe2)2}4] and three equivalents of
Me2pzH: [Ce{N(SiHMe2)2}4] (13.5 mg, 0.02 mmol) and Me2pzH (6.0 mg,
0.06 mmol) were combined and shaken in [D6]benzene and analyzed
immediately by 1H NMR spectroscopy. The 1H NMR spectrum indicated
decomposition after several minutes of stirring (see Figure S6).
[Ce(Me2pz)4(Me2pzH)] (2b):
Method a, with an excess of Me2pzH: [Ce{N(SiHMe2)2}4] (74.5 mg, 0.11
mmol) was dissolved in cyclohexane and added to a toluene solution of
excess Me2pzH (53.0 mg, 0.55 mmol) at ambient temperature. The
reaction mixture was concentrated and stored at −35 °C, giving dark red
crystals of [Ce(Me2pz)4(Me2pzH)] (2b, crystal yield: 17.0 mg, 25%). 1H
NMR ([D6]benzene, 400 MHz, 300 K): δ = 9.03 ppm (s, 1 H, N−H), 5.93 (s,
5H, C−H), 2.20 (s, 30 H, CH3); 13C{1H} NMR ([D6]benzene, 100 MHz, 300
K): δ = 144.6 ppm (s, Me2pz-C(4)), 110.9 (s, Me2pz-C(3,5)), 13.0 (s,
Me2pz-CH3); 1H NMR ([D8]toluene, 500 MHz, 173 K,): δ = 8.30 (br s, 1 H,
NH), 6.32 (s, 2 H, CH), 5.91 (br s, 1 H, CH), 5.77 (br s, 1 H, CH), 4.65 (br
s, 1 H, CH), 2.47 (br s, 18 H, CH3), 2.16 (br s, 3H, CH3), 2.10 (br s, 6H,
CH3), 0.89 (br s, 3 H, CH3) ppm; DRIFT: ꢀꢁ = 3348 (br m), 3105 (w), 2940
(m), 2916 (m), 2870 (m), 1566 (m), 1538 (m), 1516 (vs), 1486 (w), 1470
(s), 1456 (s), 1430 (vs), 1418 (vs), 1373 (m), 1363 (s), 1302 (m), 1275 (s),
1153 (w), 1102 (w), 1029 (w), 1016 (m), 1006 (m), 957 (w), 782 (m), 728
(m) cm−1. Elemental analysis calcd. (%) for C25H36CeN10 (616.75 g mol−1):
C 48.69, H 5.88, N 22.71; Found: C 48.44, H 5.42, N 22.72; χmol = 0.00247
cm3 mol−1, μeff = 2.43 B.M (ΔHz: 36 Hz, 0.0022 mol L−1); UV(toluene): λmax
= 409 (ε = 5100 mol−1 cm−1), 321 nm (ε = 2800 mol−1 cm−1).
Reaction between [CeIII{N(SiHMe2)2}3(thf)2] and three equivalents of
Me2pzH: [Ce{N(SiHMe2)2}3(thf)2] (72.0 mg, 0.11 mmol), and Me2pzH (30.1
mg, 0.32 mmol), were combined in an NMR tube. [D8]THF (~0.1 mL), and
0.4 mL of [D6]benzene were added, dissolving the solids. The pale-yellow
solution was analyzed by 1H NMR spectroscopy after either 5 minutes or
30 minutes, showing no differences, and exclusive formation of
[Ce(Me2pz)3] and HN(SiHMe2)2. 1H NMR ([D6]benzene, 300 K, 400 MHz):
δ = 11.39 ppm (br s, ~3 H, Me2pz-CH), 6.08 (br s, ~18 H, Me2pz-CH3), 4.58
(m, 6 H, Si−H), 0.14 (br s, 36 H, Si−CH3) ppm. After 3 d at ambient
temperature, or 2 d at 60 °C, no changes were observed by 1H NMR
spectroscopy. After evaporation of the reaction mixture to dryness,
addition of [D8]toluene, and addition of [Li{N(SiHMe2)2}] (14.0 mg, 0.10
mmol), a complex spectrum was obtained but did not change over time.
Storage of this reaction mixture at ambient temperature for several weeks,
followed by addition of n-hexane and storage at −35 °C, crystallization
from this reaction mixture gave a mixture of powder and colorless crystals.
8
This article is protected by copyright. All rights reserved.

10.1002/chem.202000625
Chemistry - A European Journal
Some crystals were handpicked and analyzed by single-crystal X-ray
crystallography giving the structure of [Li2(thf)2Ce(Me2pz)2{N(SiHMe2)2}3]
(5); approximate yield after separation (15.2 mg, 51%, from handpicked
crystals). Elemental analysis calcd. (%) for: C30H72CeLi2N7O2Si6 (885.46
g mol−1); C 40.69, H 8.20, N 11.07; found: C 40.48, H 7.44, N 11.71.
Reaction
between
[Ce(Me2pz)4]
and
[Li{N(SiHMe2)2}]:
[Ce(Me2pz)4].1/2PhMe (22.0 mg, 0.0421 mmol), and [Li{N(SiHMe2)2}] (45.0
mg, 0.03 mmol) were combined in [D6]benzene giving a deep red solution.
The initial 1H NMR spectrum indicated the following species were present
in solution: [Li(Me2pz)], [LiCe2(Me2pz)9] (6), [Ce(Me2pz)3(bpsa)] (3), and
two different species containing “Ce{N(SiHMe2)2}” moieties (see Figure
S9). After two days, the reaction mixture showed complete conversion to
multiple metal species (see Figure S13).
Reaction between [Ce(Me2pz)4] and HN(SiHMe2)2 in [D6]benzene at
ambient temperature: [Ce(Me2pz)4].1/2PhMe (1.1/2PhMe, 44.0 mg, 0.08
mmol), and HN(SiHMe2)2 (~5.0 mg, 0.04 mmol), were combined in
[D6]benzene and periodically monitored by 1H NMR spectroscopy over the
course of 4 months at ambient temperature (refer to Figure S2 for
spectrum). After four months, FeCp2 (1.0 mg, 0.01 mmol) was added to
determine the yield of 3 as approximate 3%.
Reaction
between
[Ce(Me2pz)4]
and
[Li{N(SiMe3)2}]:
[Ce(Me2pz)4].1/2PhMe (11.3 mg, 0.02 mmol) and [Li{N(SiMe3)2}] (3.5 mg,
0.02 mmol) were combined in [D6]benzene giving a deep red solution. 1H
NMR analysis indicated the presence of [Li{N(SiMe3)2}], [Li(Me2pz)],
[LiCe2(Me2pz)9] (6), and [Ce(Me2pz)2{N(SiMe3)2}2] (7) (see Figure S15).
Reaction between [Ce(Me2pz)4] and HN(SiHMe2)2 in [D8]toluene at
elevated temperatures. [Ce(Me2pz)4].1/2PhMe (1, 10.4 mg, 0.02 mmol),
FeCp2 (1.3 mg, 0.01 mmol), and HN(SiHMe2)2 (5.3 g, 0.04 mmol), were
combined in [D8]toluene and heated at 80 °C for 3 h. No change was noted
in the 1H NMR spectrum. The temperature was increased to 105 °C, and
Deliberate synthesis of [Ce(Me2pz)2{N(SiMe3)2}2] (7): [Ce{N(SiMe3)2}3]
(150.8 mg, 0.24 mmol), and [Li{N(SiMe3)2}] (40.6 mg, 0.25 mmol), were
stirred for 10 minutes in n-hexane (~2-5 mL) prior to addition of Me2pzH
(0.0467 g, 0.4858 mmol), dissolved in toluene (2 mL). The reaction was
stirred for additional 10 minutes before 1,4-benzoquinone (13.0 mg, 0.28
mmol) was added. The solution turned to dark brown/red. After one minute
of stirring the reaction mixture was filtered, concentrated to approximately
1-2 mL, and stored at −35 °C, giving large red block crystals of
[Ce(Me2pz)2{N(SiMe3)2}2] (7) (43.4 g, 27% crystal yield). 1H NMR
([D6]benzene, 300 K, 400 MHz): δ = 6.21 (s, 2 H, CH), 2.44 (s, 12 H, CH3),
0.29 (s, 36 H, Si(CH3)3) ppm; 13C{1H} NMR ([D6]benzene, 100 MHz, 300
K): δ = 145.63 (C(4)), 114.09 (C(3/5)), 13.6 (Me2pz-CH3), 3.5 (Si(CH3)3)
ppm; UV(toluene): λmax = 430 (ε = 3800 mol−1 cm−1). DRIFT: ꢀꢁ = 3103 (vw),
249 (m), 2893 (w), 1539 (vw), 1517 (s), 1471 (w), 1455 (m), 1432 (s), 1366
(w), 1311 (w), 1297 (w), 1247 (vs), 1100 (vw), 1053 (vw), 1003 (w), 935
(vs), 837 (vs), 795 (m), 775 (m), 756 (m), 725 (w), 680 (w), 654 (w), 609
after
5 min decomposition was noted by the formation of small
paramagnetic peaks within the 1H NMR spectrum. After 16 h, the reaction
was cooled and analysis by 1H NMR spectroscopy indicating complete
consumption of 1 (refer to Figure S3). Small colorless crystals were
observed within the NMR tube, analysis by X-ray crystallography revealed
the formation of [Ce4(Me2pz)12(Me2pzH)2] (2c). The crystals were insoluble
in [D8]toluene even upon heating.
Reaction between [Li{N(SiHMe2)2}] and [Li(Me2pz)]: [Li{N(SiHMe2)}]
(0.114 g, 0.82 mmol, 3 equiv.) and Me2pzH (52.6 mg, 0.54 mmol, two
equiv.) were stirred in toluene (5 mL) for 1-2 h. The reaction mixture was
dried and toluene was re-added and the sample was heated to 115 °C for
two d. The sample was dried leaving a white powder. Analysis by 1H NMR
spectroscopy indicated only slight changes in the spectrum of [Li(Me2pz)]
and [Li{N(SiHMe2)2}] (refer to Figure S33).
(s) cm−1
. χmol = , μeff = 1.02 B.M (ΔHz: 6,
4.38x10−4 cm3 mol−1
0.0614 mol L−1). Elemental analysis calcd. (%) for C22H50CeN6Si4
(651.13 g mol−1): C 40.58, H 7.74, N 12.91; found: C 40.76, H 7.64, N
12.91.
Reaction of [Ce(Me2pz)4]2 (1) with lithium reagents:
Isolation of [{Li2Ce2(Me2pz)6(thf)2}2(pzhq)2] (8) from the reaction
between 1,4-benzoquinone and “Li[Ce(Me2pz)4]”: [Ce{N(SiMe3)2}3]
(154.3 mg, 0.25 mmol), Li(N(SiMe3)2) (28.0 mg, 0.2 mmol), and Me2pzH
(76.9 mg, 0.8 mmol) were stirred in THF for 16 h. The solution was dried
in vacuo, and the resulting powder dissolved in toluene and stored at
−35 °C. 1,4-Benzoquinone (13.0 mg, 0.12 mmol), was added at −35 °C,
giving an immediate Bordeaux-colored solution. The solution was stirred
for 60 seconds before storage at −35 °C for 12 h. The solution was filtered,
concentrated in vacuo and n-hexane was added giving a colorless powder
and Bordeaux-colored crystals. X-ray structural analysis of the crystals
indicated the formation of [{Li2Ce2(Me2pz)6(thf)2}2(pzhq)2] (8).
[LiCe2(Me2pz)9] (6)
Method a, Li impurity in the initial synthesis of 1: Me2pzH (30.0 mg, 0.3
mmol)) was dissolved in toluene and added to a toluene solution of
[Ce{N(SiHMe2)2}4] (52.2 mg, 0.08 mmol, note: reaction mixture contained
an unknown amount of
a Li impurity from initial synthesis of
[Ce{N(SiHMe2)2}4]). The solution was immediately exposed to vacuum,
and after 30 seconds it was stored at −35 °C, producing dark red block
crystals of [LiCe2(Me2pz)9] (6) identified by X-ray crystallography. The
crystals were briefly dried under vacuum (18.0 mg, 40%). DRIFT: ꢀꢁ = 3100
(vw), 2951 (w), 2920 (m), 2862 (w), 1565 (vw), 1521 (vs), 1460 (m), 1434
(s), 1413 (vs), 1376 (m), 1365 (w), 1328 (w), 1309 (s), 1264 (m), 1057 (w),
1021 (m), 1012 (m), 965 (m), 821 (vw), 797 (w), 787 (w), 762 (m), 740 (vw),
729 (m), 694 (vw) cm−1. Elemental analysis calcd. (%) for C52H71Ce2LiN18
(1235.41 g mol−1): C 50.55, H 5.79, N 20.41; found: C 50.52, H 5.24, N
20.69. The crystals were further dried under vacuum before 1H NMR
analysis, removing the lattice toluene (assignments are provided in SI,
Figures S8 and S26). 1H NMR ([D6]benzene, 300 K, 250 MHz): δ = 6.00
(s, 4 H, pz-C(4)H), 5.76 (s, 1 H, pz-C(4)H), 5.73 (s, 2 H, pz-C(4)H), 4.75
(s, 2 H, pz-C(4)H), 2.65 (s, 6 H, CH3), 2.17 (s, 12 H, CH3), 1.98 (s, 24 H,
CH3), 1.95 (s, 12 H, CH3) ppm; 13C{1H} NMR ([D6]benzene, 300 K, 400
MHz): δ = 165.1, 145.0, 144.8, 143.6, 113.3, 105.8, 100.7, 14.5, 13.4,
12.81, 12.77 ppm; χmol = −2.192 x 10−4 cm3 mol−1, μeff = ~0.0 B.M (ΔHz:
−4.3, 2 x 10−3 mol L−1); UV(toluene): λmax = 423 (ε = 10400 mol−1 cm−1).
Reaction
between
[Ce(Me2pz)4]
and
[Ce{N(SiHMe2)2}4]:
[Ce(Me2pz)4]2.1/2PhMe (0.0080 g, 0.0077 mmol), and [Ce{N(SiHMe2)2}4]
(0.010 g, 0.015 mmol), were combined in C6D6 giving a deep red solution.
The initial 1H NMR spectrum indicated no reaction. After one day, the
reaction mixture showed decomposition of [Ce(Me2pz)4]2 and evolution of
H2 (see Figure S34).
Reaction between [Ce(Me2pz)4] and H2: [Ce(Me2pz)4]2.1/2PhMe (0.0084
g, 0.0081 mmol) was dissolved in C6D6 and treated with H2. The 1H NMR
spectrum indicated no reaction after one day (see Figure S35).
X-ray crystallography: Prior analysis, all compounds were submerged in
n-paratone crystallography oil, mounted on a fiber loop, and measured on
a 'Bruker APEX-II CCD' diffractometer (Mo Kα, λ = 0.71073 nm), at either
100 K (2a*, 2b, 3, 6, 7, 8) or 161 K (5). Absorption corrections were
completed using the Apex II program suite.[44] Structural solutions were
obtained by charge flipping methods or direct methods and refined using
full matrix least squares methods against F2 using SHELX2013,[45] within
the OLEX 2 graphical interface.[46] A list of the parameters are depicted in
Table 2. Notes: 2a*: ISOR command used on disordered carbon atoms of
Method
b,
protonolysis
with
[Ce(Me2pz)4(Me2pzH)]·1/2PhMe:
[Ce(Me2pz)4(Me2pzH)]·1/2PhMe (7.0 mg, 0.01 mmol) and [Li{N(SiHMe2)2}]
(2.0 mg, 0.01 mmol) were combined in [D8]THF and stored for 12 h. n-
Hexane was added, and upon storage at −35 °C, crystals of
[LiCe2(Me2pz)9] (6) formed and were identified by 1H NMR spectroscopy,
giving a spectrum in accord with the sample obtained by method a (Crystal
yield: 5.0 mg, 51%).
9
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10.1002/chem.202000625
Chemistry - A European Journal
[13]
a) M. Gregson, E. Lu, J. McMaster, W. Lewis, A. J. Blake, S. T.
Liddle, Angew. Chem. Int. Ed. Engl. 2013, 52, 13016-13019; Angew.
Chem. 2013, 125, 13254-13257; b) Y.-M. So, G.-C. Wang, Y. Li, H.
H.-Y. Sung, I. D. Williams. Z. Lin, W.-H. Leung, Angew. Chem. Int.
Ed. Engl. 2014, 53, 1626-1629; Angew. Chem. 2014, 126, 1652-
1655; c) J. R. Levin, T. Cheisson, P. J. Carroll, E. J. Schelter, Dalton
Trans. 2016, 45, 15249-15258; d) D. Schneider, N. Harmgarth, F. T.
Edelmann, R. Anwander, Chem. Eur. J. 2017, 23, 12243-12252; e)
A. V. Zabula, Y. Qiao, A. J. Kosanovich, T. Cheisson, B. C. Manor,
P. J. Carroll, O. V. Ozerov, E. J. Schelter, Chem. Eur. J. 2017, 23,
17923-17934; f) R. P. Kelly, L. Maron, R. Scopelliti, M. Mazzanti,
Angew. Chem. Int. Ed. Engl. 2017, 56, 15663-15666; Angew. Chem.
2017, 129, 15869-15872; g) J. Friedrich, C. Maichle-Mössmer, R.
Anwander, Chem. Commun. 2017, 53, 12044-12047; h) J. Friedrich,
D. Schneider, L. Bock, C. Maichle-Mössmer, R. Anwander, Inorg.
Chem. 2017, 56, 8114-8127.
a) C. Morton, N. W. Alcock, M. R. Lees, I. J. Munslow, C. J. Sanders,
P. Scott, J. Am. Chem. Soc. 1999, 121, 11255-11256; b) D. Peter,
G. Jochen, Z. Anorg. Allg. Chem. 2008, 634, 87-90; c) P. Dröse, J.
Gottfriedsen, C. G. Hrib, P. G. Jones, L. Hilfert, F. T. Edelmann, Z.
Anorg. Allg. Chem. 2011, 637, 369-373; d) P. L. Damon, G. Wu, N.
Kaltsoyannis, T. W. Hayton, J. Am. Chem. Soc. 2016, 138, 12743-
12746; e) M. K. Assefa, G. Wu, T. W. Hayton, Chem. Sci. 2017, 8,
7873-7878.
a) A. Mustapha, J. Reglinski, A. R. Kennedy, Inorg. Chim. Acta 2009,
362, 1267-1274; b) L. A. Solola, A. V. Zabula, W. L. Dorfner, B. C.
Manor, P. J. Carroll, E. J. Schelter, J. Am. Chem. Soc. 2016, 138,
6928-6931; c) L. A. Solola, A. V. Zabula, W. L. Dorfner, B. C. Manor,
P. J. Carroll, E. J. Schelter, J. Am. Chem. Soc. 2017, 139, 2435-
2442.
a) D. Werner, G. B. Deacon, P. C. Junk, R. Anwander, Dalton Trans.
2017, 46, 6265-6277; b) D. Werner, G. B. Deacon, P. C. Junk, R.
Anwander, Eur. J. Inorg. Chem. 2017, 3419-3428;
a) A. R. Crozier, A. M. Bienfait, C. Maichle-Mössmer, K. W. Törnroos,
R. Anwander, Chem. Commun. 2013, 49, 87-89; b) U. Bayer, L.
Bock, C. Maichle-Mössmer, R. Anwander, Eur. J. Inorg. Chem. 2020,
101-106.
a) U. J. Williams, D. Schneider, W. L. Dorfner, C. Maichle-Mössmer,
P. J. Carroll, R. Anwander, E. J. Schelter, Dalton Trans. 2014, 43,
16197-16206; b) D. Werner, G. B. Deacon, P. C. Junk, R. Anwander,
Chem. Eur. J. 2014, 20, 4426-4438; c) J. E. Kim, P. J. Carroll, E. J.
Schelter, Chem. Commun. 2015, 51, 15047-15050; d) J. R. Levin,
W. L. Dorfner, A. X. Dai, P. J. Carroll, E. J. Schelter, Inorg. Chem.
2016, 55, 12651-12659.
a THF ring and on NPD atom of a Me2pz ligand. 2b: Entire molecule was
disordered over two positions. Metals and axial ligands were modelled
over two positions with some atoms left isotropic, meridional ligands were
not (attempted refinement with anisotropic atoms led to an unstable
solution). ISOR command was used to restrain NPD atoms, and DFIX
command was used to model partial disorder. N−H hydrogen atom
assignment was unstable and was not included in the model. 5: Si-H
hydrogen atoms were manually assigned. 6: ISOR command used on
disordered toluene molecule within lattice. Toluene was disordered over
two positions, and on a special position, refined with PART refinement. 8:
One THF ligand, and one Me2pz ligand were disordered over several
positions. ISOR command used on NPD carbon atoms (two THF carbon
atoms remain isotropic), and the Me2pz ligand was modelled over two
positions, attempts at modelling the THF ligand led to an unstable
refinement. Highly disordered solvent within the lattice, identified as
toluene and also potentially THF were removed through application of the
SQUEEZE[37] program suite in PLATON. Structural data for complexes 2a*
(1540352), 2b (1540356), 2c (1858038) 3 (1540356), 5 (1858039), 6
(1540351), 7 (1540350), and 8 (1540355) can be obtained free of charge
[14]
[15]
from
The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif. Selected data are listed in Tables
S1 and S2 in the supporting information.
References
[16]
[17]
[1]
a) N. Marques, A. Sella, J. Takats, Chem. Rev. 2002, 102, 2137-
2160; b) A. de Bettencourt-Dias, P. S. Barber, S. Viswanathan,
Coord. Chem. Rev. 2014, 273-274, 165-200; c) M. M. Hänninen, M.
T. Zamora, P. G. Hayes, Top. Organomet. Chem. 2016, 54, 93–178.
a) A. Prasanna de Silva, H. Q. Nimal Gunaratne, T. E. Rice, Angew.
Chem. Int. Ed. Engl. 1996, 35, 2116-2118; Angew. Chem. 1995, 18,
1889-1891; b) H.-R. Murner, E. Chassat, R. P. Thummel, J.-C. G.
Bunzli, J. Chem. Soc., Dalton Trans. 2000, 2809-2816; c) R.
Shunmugam, G. N. Tew, J. Am. Chem. Soc. 2005, 127, 13567-
13572; d) R. Shunmugam, G. N. Tew, Chem. Eur. J. 2008, 14, 5409-
5412; e) S. Sarkar, A. Mondal, A. K. Tiwari, R. Shunmugam, Chem.
Commun. 2012, 48, 4223-4225; f) G. H. Dennison, M. R. Johnston,
Chem. Eur. J. 2015, 21, 6328-6338.
[2]
[18]
[19]
a) S. N. König, G. Gerstberger, C. Schädle, C. Maichle-Mössmer, E.
Herdtweck, R. Anwander, Main Group Met. Chem. 2013, 36, 169-
180; b) S.-C. Rosca, T. Roisnel, V. Dorcet, J.-F. Carpentier, Y.
Sarazin, Organometallics 2014, 33, 5630-5642; c) S. N. König, D.
Schneider, C. Maichle-Mössmer, B. M. Day, R. A. Layfield, R.
Anwander, Eur. J. Inorg. Chem. 2014, 4302-4309; d) A. M. Bienfait,
B. M. Wolf, K. W. Törnroos, R. Anwander, Organometallics 2015,
34, 5734-5744; e) D. Mukherjee, S. Shirase, T. P. Spaniol, K.
Mashima, J. Okuda, Chem. Commun. 2016, 52, 13155-13158; f) H.
Osseili, D. Mukherjee, T. P. Spaniol, J. Okuda, Chem. Eur. J. 2017,
23, 14292-14298.
[3]
[4]
J. Huang, Z. Liu, D. Cui and X. Liu, ChemCatChem 2018, 10, 42–
61.
For examples, see: a) M. Zimmermann, K. W. Törnroos, R. M.
Waymouth, R. Anwander, Organometallics 2008, 27, 4310-4317.L.
Wang, D. Liu, D. Cui, Organometallics 2012, 31, 6014-6021.; H. Liu,
J. He, Z. Liu, Z. Lin, G. Du, S. Zhang, X. Li, Macromolecules 2013,
46, 3257-3265.; W. Rong, D. Liu, H. Zuo, Y. Pan, Z. Jian, S. Li, D.
Cui, Organometallics 2013, 32, 1166-1175.; P. Zhang, H. Liao, H.
Wang, X. Li, F. Yang, S. Zhang Organometallics, 2017, 36, 2446-
2451.
[20]
U. Bayer, D. Werner, C. Maichle-Mössmer, R. Anwander, Angew.
Chem. Int. Ed. 2020, 59, 5830-5836; Angew. Chem. 2020, 132,
5879-5885.
D. Schneider, T. Spallek, C. Maichle-Mössmer, K. W. Törnroos, R.
Anwander, Chem. Commun. 2014, 50, 14763-14766.
G. B. Deacon, R. Harika, P. C. Junk, B. W. Skelton, D. Werner, A.
H. White, Eur. J. Inorg. Chem. 2014, 2412-2419.
D. Werner, U. Bayer, N. E. Rad, P. C. Junk, G. B. Deacon, R.
Anwander, Dalton Trans. 2018, 47, 5952-5955.
J. Eppinger, M. Spiegler, W. Hieringer, W. A. Herrmann, R.
Anwander, J. Am. Chem. Soc. 2000, 122, 3080-3096.
[5]
[6]
a) S. Trofimenko, J. Am. Chem. Soc. 1966, 88, 1842-1844; b) S.
Trofimenko, in Scorpionates, Imper. Coll. Press, 1999.
a) J. Cheng, K. Saliu, G. Y. Kiel, M. J. Ferguson, R. McDonald, J.
Takats, Angew. Chem. Int. Ed. Engl. 2008, 47, 4910-4913; Angew.
Chem. 2008, 120, 4988-4991; b) R. Litlabø, M. Zimmermann, K.
Saliu, J. Takats, K. W. Törnroos, R. Anwander, Angew. Chem. Int.
Ed. Engl. 2008, 47, 9560-9564; Angew. Chem. 2008, 120, 9702–
9706; c) M. Zimmermann, R. Litlabø, K. W. Törnroos, R. Anwander,
Organometallics 2009, 28, 6646-6649; d) D. Schädle, C. Maichle-
Mössmer, C. Schädle, R. Anwander, Chem. Eur. J. 2015, 21, 662-
670.
[21]
[22]
[23]
[24]
[25]
[26]
F. G. Bordwell, Acc. Chem. Res. 1988, 21, 456-463.
A. M. Bienfait, C. Schädle, C. Maichle-Mössmer, K. W. Törnroos, R.
Anwander, Dalton Trans. 2014, 43, 17324-17332.
[7]
[8]
a) N. Marques, A. Sella, J. Takats, Chem. Rev. 2002, 102, 2137-
2160; b) K. C. Armstrong, S. Hohloch, T. D. Lohrey, R. A. Zarkesh,
J. Arnold, M. R. Anstey, Dalton Trans. 2016, 45, 18653-18660; c)
H.-D. Amberger, F. T. Edelmann, J. Gottfriedsen, R. Herbst-Irmer,
S. Jank, U. Kilimann, M. Noltemeyer, H. Reddmann, M. Schäfer,
Inorg. Chem. 2009, 48, 760-772.
J. L. Brown, M. B. Jones, A. J. Gaunt, B. L. Scott, C. E. MacBeth, J.
C. Gordon, Inorg. Chem. 2015, 54, 4064-4075.
D. Bubrin, M. Niemeyer, Inorg. Chem. 2014, 53, 1269-1271.
a) T.-L. Ho, Synthesis 1973, 347-354; b) V. Sridharan, J. C.
Menéndez, Chem. Rev. 2010, 110, 3805-3849.
a) L. J. Nugent, R. D. Baybarz, J. L. Burnett, J. L. Ryan, J. Inorg.
Nucl. Chem. 1971, 33, 2503-2530; b) N. A. Piro, J. R. Robinson, P.
J. Walsh, E. J. Schelter, Coord. Chem. Rev. 2014, 260, 21-36; c)
Y.-M. So, W.-H. Leung, Coord. Chem. Rev. 2017, 340, 172-197; d)
R. Anwander, M. Dolg, F. T. Edelmann, Chem. Soc. Rev. 2017, 46,
6697-6709; e) Y. Qiao, E. J. Schelter, Acc. Chem. Res. 2018, 51,
2926-2936.
[27]
A. Pindwal, S. Patnaik, W. C. Everett, A. Ellern, T. L. Windus, A. D.
Sadow, Angew. Chem. Int. Ed. Engl. 2017, 56, 628-631; Angew.
Chem. 2017, 129, 643-646.
K. Yan, G. Schöndorff, B. M. Upton, A. Ellern, T. L. Windus, A. D.
Sadow, Organometallics 2013, 32, 1300-1316.
Such hydride-based abstractions and concomitant reductions are
known to occur in other metal systems, for example reduction of PtIV
to PtII: N. A. Smythe, K. A. Grice, B. S. Williams, K. I. Goldberg,
Organometallics 2009, 28, 277-288.
a) M. P. Coles, P. B. Hitchcock, A. V. Khvostov, M. F. Lappert, Z. Li
and A. V. Protchenko, Dalton Trans., 2010, 39, 6780-6788; P. L.
Arnold, Z. R. Turner, N. Kaltsoyannis, P. Pelekanaki, R. M.
Bellabarba and R. P. Tooze, Chem. Eur. J., 2010, 16, 9623-9629.
I. Nagl, W. Scherer, M. Tafipolsky, R. Anwander, Eur. J. Inorg.
Chem. 1999, 1405-1407.
[28]
[29]
[9]
[10]
[11]
[30]
[31]
[32]
S. A. Kozimor, B. M. Bartlett, J. D. Rinehart, J. R. Long, J. Am. Chem.
[12]
R. D. Shannon, Acta Cryst. 1976, A32, 155-169.
Soc. 2007, 129, 10672-10674.
10
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Chemistry - A European Journal
[33]
[34]
[35]
a) D. F. Evans, J. Chem. Soc. 1959, 2003-2005; b) E. M. Schubert,
J. Chem. Educ. 1992, 69, 62.
J. R. Levin, W. L. Dorfner, P. J. Carroll, E. J. Schelter, Chem. Sci.
2015, 6, 6925-6934.
[40]
[41]
[42]
D. Schneider, R. Anwander, Eur. J. Inorg. Chem. 2017, 1180-1188.
A. Vogler, H. Kunkely, Inorg. Chim. Acta 2006, 359, 4130-4138.
U. J. Williams, P. J. Carroll, E. J. Schelter, Inorg. Chem. 2014, 53,
6338-6345.
a) C. Meermann, K. Ohno, K. W. Törnroos, K. Mashima, R.
Anwander, Eur. J. Inorg. Chem. 2009, 76-85.
W. Kabsch, J. Appl. Cryst. 1993, 26, 795-800.
a) G. M. Sheldrick, Acta Cryst. 2015, C71, 3-8; b) G. M. Sheldrick,
Acta Crystallogr. A 2008, A64, 112.
a) L. J. Bourhis, O. V. Dolomanov, R. J. Gildea, J. A. K. Howard, H.
Puschmann, Acta Cryst. 2015, A71, 59-75; b) O. V. Dolomanov, L.
J. Bourhis, R. J. Gildea, J. A. K. Howard, H. Puschmann, J. Appl.
Cryst. 2009, 42, 339-341.
G. B. Deacon, C. M. Forsyth, A. Gitlits, R. Harika, P. C. Junk, B. W.
Skelton, A. H. White, Angew. Chem. Int. Ed. 2002, 41, 3249-3251;
Angew. Chem. 2002, 114, 3383-3385.
E. Sebe, I. A. Guzei, M. J. Heeg, L. M. Liable-Sands, A. L. Rheingold,
C. H. Winter, Eur. J. Inorg. Chem. 2005, 3955-3961.
W. J. Evans, M. A. Hozbor, J. Organomet. Chem. 1987, 326, 299-
306.
S. Daniele, L. G. Hubert-Pfalzgraf, M. Perrin, Polyhedron 2002, 21,
1985-1990.
A. England, Ph.D Thesis, Massachusetts Institute of Technology,
[43]
[44]
[45]
[36]
[37]
[38]
[39]
[46]
Cambridge, MA 1995.
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10.1002/chem.202000625
Chemistry - A European Journal
Entry for the Table of Contents
5
FULL PAPER
Putative heteroleptic ceric species
[Ce(Me2pz)x{N(SiHMe2)2}4-x] promote
the formation of a new monoanionic
dipyrazolylamido pincer ligand, while
both tetravalent
[Ce(Me2pz)2{N(SiMe3)2}2] and trivalent
[Li2(thf)2Ce(Me2pz)2{N(SiHMe2)2}3] are
isolable and stable.
Daniel Werner, Dorothea Schädle, and
Reiner Anwander *
Si
Si
N
N
N
Page No. – Page No.
N
N
CeIV
Emergence of a New [NNN] Pincer
Ligand via Si–H-Bond Activation and
β-Hydride Abstraction at Tetravalent
Cerium
N
N
N
N
N
N
12
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