Insertion, protonolysis and photolysis reactivity of
a thorium monoalkyl amidinate complex†
Nicholas S. Settineri
ab
and John Arnold
*
ab
The reactivity of the thorium monoalkyl complex Th(CH
2
SiMe
3
)(BIMA)
3
[1, BIMA ¼ MeC(N
i
Pr)
2
] with various
small molecules is described. While steric congestion prohibits the insertion of N,N
0
-
diisopropylcarbodiimide into the Th–C bond in 1, the first thorium tetrakis(amidinate) complex,
Th(BIMA)
4
(2), is synthesized via an alternative salt metathesis route. Insertion of p-tolyl azide leads to the
triazenido complex Th[(p-tolyl)NNN(CH
2
SiMe
3
)-k
2
N
1,2
](BIMA)
3
(3), which then undergoes thermal
decomposition to the amido species Th[(p-tolyl)N(SiMe
3
)](BIMA)
3
(4). The reaction of 1 with 2,6-
dimethylphenylisocyanide results in the thorium iminoacyl complex Th[h
2
-(C]N)-2,6-Me
2
-
C
6
H
3
(CH
2
SiMe
3
)](BIMA)
3
(5), while the reaction with isoelectronic CO leads to the products Th[OC(]
CH
2
)SiMe
3
](BIMA)
3
(6) and Th[OC(N
i
Pr)C(CH
2
SiMe
3
)(C(Me)N(
i
Pr))O-k
2
O,O
0
](BIMA)
2
(7), the latter being the
result of CO coupling and insertion into an amidinate ligand. Protonolysis is achieved with several
substrates, producing amido (9), aryloxide (10), phosphido (11a,b), acetylide (12), and cationic (13)
complexes. Ligand exchange with 9-borabicyclo[3.3.1]nonane (9-BBN) results in formation of the
thorium borohydride complex (BIMA)
3
Th(m-H)
2
[B(C
8
H
14
)] (14). Complex 1 also reacts under photolytic
conditions to eliminate SiMe
4
and produce Th(BIMA)
2
(BIMA*)[15, BIMA* ¼ (
i
Pr)NC(CH
2
)N(
i
Pr)], featuring
a rare example of a dianionic amidinate ligand. Complexes 2, 3, 5, 6, 11a, and 12–15 were characterized
by
1
H and
13
C{
1
H} NMR spectroscopy, FTIR, EA, melting point and X-ray crystallography. All other
complexes were identified by one or more of these spectroscopic techniques.
Introduction
The synthesis and reactivity of metal alkyl complexes have
been a focus for organometallic chemists for decades due to
their fundamental interest and relevance to catalytic and
industrial processes. Studies of f-block alkyl complexes have
been perfo rmed to a lesser extent, particularly those contain-
ing actinide metal centers. Actinide complexes display diver-
gent coordination chem istry com pared to the re st of the
periodic table, and their largesizecombinedwiththeacces-
sibility of v arious oxidation states makes them particularly
interesting for struct ural, electronic and reactivity studies.
1–6
Much of t he focus concerning organoactinide investigat ions
has been devot ed to uranium, due to its redo x capabilities.
4
Fewer studies have inv olved t horiu m, despite a fundamental
question regarding whet her thorium acts more as an actin ide
or group IV metal.
7,8
Marks p ioneered much of the wo rk
regarding thorium organometallic species b earing carbocyclic
ancillary ligands (C
5
R
5
),
9–15
and many groups have continued
to exp lore the reactiv ity of these systems.
16–20
Others have
turned to non-carbocyclic ligands, in an attempt to inv estig ate
how complex stability and reactivity is affected by modifyi ng
the steric and electronic properties of the ancillary ligand
framework.
21–28
Having used a variety of non-carbocyclic ligand
frameworks to stabilize and the explore the reactivity of both
transition metal and actinide complexes,
29–34
our group
endeavored to expand these studies to thorium alkyl species.
We recently reported on the synthesis of a thorium monoalkyl
species utilizing a tris-amidinate ancillary framework, sp ec if-
ically Th(CH
2
SiMe
3
)(BIMA)
3
(where BIMA ¼ MeC(N
i
Pr)
2
)(1),
and its ability to insert chalcogen atoms t o generate rare
thorium ch alcoge nolat e complexes.
35
In addition to the chal-
cogen insertion reactivity, the increased electrophilicity of t he
metal center with respect to analogous Cp-based systems led
to the rare C–HactivationoftrimethylamineN-oxide.Inspired
by this result, we soug ht to investigate how t he unique prop-
erties of this system would impact the reactivity of 1 with
a variety of small molecules. Here we report on the reactivity of
1 with organic azides, isocyanide, CO, nitrile, 9-BBN, and
various protic substrates, as well as the stability o f 1 under
photolytic conditions.
a
Department of Chemistry, University of California, Berkeley, California 94720, USA.
E-mail: arnold@berke ley.edu
b
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,
California, 94720, USA
† Electronic supplementary information (ESI) available: Experimental details,
NMR spectra, and X-ray crystallographic tables. CCDC 1811106–1811116. For
ESI an d crystallographic data in CIF or other electronic format see DOI:
10.1039/c7sc05328b
Cite this: Chem. Sci.,2018,9,2831
Received 16th December 2017
Accepted 9th February 2018
DOI: 10.1039/c7sc05328b
rsc.li/chemical-science
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Results and discussion
Having previously established the ability of 1 to undergo chal-
cogen atom insertion and generate unique thorium chalcoge-
nolates,
35
we next sought to examine potential insertion
chemistry of this monoalkyl system with various small mole-
cules. Evans has described the insertion of carbodiimides and
organic azides into one alkyl moiety of Cp
*
2
ThMe
2
.
16
With this in
mind, we targeted the synthesis of a tetrakis(amidinate)
complex through the reaction of 1 with N,N
0
-diisopropylcarbo-
diimide (Scheme 1). However, despite forcing conditions (100
C), carbodiimide insertion into the Th–C bond was not
observed, most likely a result of steric saturation around the
metal center. Regardless, the alternative salt metathesis route
using one equiv. of ThCl
4
(DME)
2
(ref. 36) and 4.05 equiv. of
Li(BIMA)(THF)
37
and heating to 90
C for 5 d afforded the
desired homoleptic complex Th(BIMA)
4
(2) as colourless crys-
tals in 66% yield (Scheme 1). The
1
H NMR spectrum of 2 reects
averaged C
4
-symmetry in solution, with one set of peaks
observed for the equivalent amidinate ligands. The molecular
structure of 2, determined by single-crystal X-ray diffraction
studies, shows a pseudo-tetrahedral geometry of the amidinate
ligands around the thorium center (Fig. 1). The Th–N
amid
bond
lengths vary between 2.49 and 2.62
˚
A, a noticeably larger range
than that seen in 1 (2.49–2.54
˚
A), indicating the signicant
steric congestion imposed by the isopropyl groups of the ami-
dinate ligands. The structural parameters of 2 combined with
the reaction conditions necessary to form 2 support the notion
that insertion of carbodiimide into the Th–C bond was
hindered by steric crowding, and indicate that related insertion
reactions might be subject to this constraint. Due to the steric
protection afforded by the tetrakis(amidinate) framework, and
inspired by the work of Evans regarding low-valent thorium
chemistry,
38
we attempted the reduction of 2 with KC
8
in the
presence of 18-crown-6; nevertheless, no colour change was
observed and only starting material was isolated.
Reaction of 1 with an equivalent of p-tolyl azide resulted in
insertion to form the triazenido complex Th[(p-tolyl)
NNN(CH
2
SiMe
3
)-k
2
N
1,2
](BIMA)
3
(3) in 83% yield (Scheme 2).
The
1
H NMR spectrum displayed the diagnostic downeld shi
of the methylene resonance which was also observed to result
from chalcogen insertion.
35
In the present case, we observed
a shi from d 0.08 in 1 to d 3.99 in 3, alongside shied ami-
dinate resonances and the appearance of resonances attribut-
able to the p-tolyl group. X-ray diffraction studies revealed
a k
2
N
1,2
coordination mode of the triazenido moiety (Fig. 2),
similar to that observed by Evans in the thorium metallocene
system.
16
The metrical parameters relating to the N
3
fragment
show the effect of delocalization, with bond lengths of 1.314(6)
and 1.285(6)
˚
A for N(7)–N(8) and N(8)–N(9), respectively,
adifference of only 0.03
˚
A. In contrast, Evans' system that
utilized adamantyl azide exhibited a more localized bonding for
the analogous nitrogens, with bond lengths of 1.360(3) and
1.243(3)
˚
A, a difference of 0.12
˚
A.
16
The Th(1)–N(7) distance of
2.474(4)
˚
Ais0.1
˚
A longer than that seen in the thorium met-
allocene system, while the Th(1)–N(8) distance of 2.597(4)
˚
Ais
Scheme 1 Attempted insertion and salt metathesis routes to tetraki-
s(amidinate) species.
Fig. 1 Molecular structure of 2 (thermal ellipsoids drawn at the 50%
probability level). Hydrogen atoms omitted for clarity.
Scheme 2 Synthesis of 3 and thermal decomposition to 4.
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the same in both. This indicates that the Th(1)–N(7) interaction
is best described as anionic, whereas Th(1)–N(8) is more dative.
The difference between 3 and Evans' metallocene system is
likely a combination of both steric and electronic effects.
Insertion with tert-butyl azide was also achieved; however,
a mixture of two species was always observed, with the major
product slowly converting to the minor product in solution until
a ratio of 4 : 1 was established. Heating did not alter this ratio,
although elevated temperatures (100
C) induced decomposi-
tion of the products. We postulate that the two products are
both triazenido complexes that differ only in their coordination
mode, with the k
2
N
1,2
and k
2
N
1,3
species present in solution.
Exposing 3 to elevated temperatures in solution to see if
a similar change in coordination mode would occur brought
about a different result; 3 undergoes clean thermal decompo-
sition to a new complex, with signi cantly shied p-tolyl
aromatic resonances and, most notably, no methylene reso-
nance. Complete conversion was achieved within 48 h while
heating at 70
C. This new product was stable to further heating.
Close inspection of the
1
H NMR spectrum revealed a resonance
attributable to ethylene. With this information we envisioned
that 3 was losing diazomethane (N
2
CH
2
), which then decom-
posed to dinitrogen and ethylene,
39
resulting in the thorium
amido species Th[(p-tolyl)N(SiMe
3
)](BIMA)
3
(4, Scheme 2).
Ethylene formation may be the result of singlet methylene
40
generation and coupling upon N
2
CH
2
decomposition. In order
to try and trap the transient singlet methylene, similar heating
experiments were conducted in the presence of 2-butyne and
1,1-diphenylethylene and monitored by
1
H NMR spectroscopy.
Although it was difficult to unambiguously identify the trapped
products (1,2-dimethylcyclopropene and 1,1-diphenylcyclopro-
pane, respectively), the formation of ethylene was not seen with
either trapping reagent, and a singlet at d 1.13 was observed in
the 1,1-diphenylethylene experiment, which we tentatively
assign to 1,1-diphenylcyclopropane.
41
We were able to conrm
the identity of 4 as the thorium amido complex by X-ray
diffraction studies (Fig. 2). To the best of our knowledge, this
is the rst example of clean thermal decomposition of an acti-
nide triazenido complex to the corresponding amido species.
Bart and co-workers have observed thermal instability in certain
uranium triazenido species, which has led to intractable
mixtures of products.
42
However, the thermal decomposition of
L
tBu
Fe(h
2
-HNNNAd) (where L
tBu
¼ tert-butyl substituted-N,N
0
-
diaryl-b-diketiminate, aryl ¼ 2,6-
i
Pr
2
-C
6
H
3
) to the corresponding
primary amido species L
tBu
FeNHAd has been observed.
43
This
was rationalized based on the instability of free H
2
NNNR
compounds with respect to loss of dinitrogen. The Th(1)–N(7)
bond distance of 2.399(2)
˚
Ain4 is very close to the Th–N bond
distance of 2.389(2)
˚
A observed by Walter and co-workers in [h
5
-
1,2,4-(Me
3
C)
3
C
5
H
2
]
2
Th(Cl)-[N(p-tolyl)SiH
2
Ph], which has
a similar silyl amide environment.
44
The nitrogen atom of the
amido exhibits a trigonal planar geometry (S: z 360
), also
consistent with Walter's complex. A series of NMR scale exper-
iments revealed the thermal decomposition to be concentration
dependent, with higher concentrations of 3 leading to the
generation of the silyl amine (p-tolyl)NH(SiMe
3
) (as determined
by
1
H NMR spectroscopy) and a mixture of unknown species
(see Fig. S7 in ESI†). The identity of these products, along with
mechanistic studies regarding the formation of 4 from 3,is
currently under investigation.
Although achieved with both transition metal
45–48
and
uranium
49–52
species, isocyanide insertion into thorium alkyl
bonds to form the corresponding h
2
-iminoacyl complexes has
not yet been reported. While sterics precluded the insertion of
N,N
0
-diisopropylcarbodiimide, isocyanide insertion was real-
ized with one equivalent of 2,6-dimethylphenylisocyanide and
moderate heating, resulting in Th[h
2
-(C]N)-2,6-Me
2
-C
6
H
3
-
(CH
2
SiMe
3
)](BIMA)
3
(5) as a colourless, crystalline solid in 81%
yield (Scheme 3). To the best of our knowledge this is the rst
thorium h
2
-iminoacyl complex. Monitoring the reaction by
1
H
NMR spectroscopy conrms that this insertion proceeds slowly
at room temperature (>95% conversion aer 96 h), presumably
due to the steric clash between the xylyl moiety and amidinate
ligands. As expected, the diagnostic downeld shi of the
methylene singlet to d 2.85 indicated successful isocyanide
insertion, along with new methyl and aromatic resonances
Fig. 2 Molecular structures of 3 (top) and 4 (bottom) (thermal ellip-
soids drawn at the 50% probability level). Hydrogen atoms omitted for
clarity.
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corresponding to the xylyl group. X-ray diffraction studies
revealed the h
2
-coordination mode of the imine moiety, with
the N(7)–C(29) bond length of 1.299(4)
˚
A falling in the range
seen for other transition metal iminoacyl species (Fig. 3).
45–52
The Th(1)–N(7) distance of 2.469(2)
˚
A is noticeably shorter than
that typically observed for a dative Th–N bond,
53–56
while the
Th(1)–C(29) bond length of 2.529(3)
˚
A is in the range observed
for other s-bonded alkyl moieties.
14,19,57
This insertion differs
from that observed by Andersen and co-workers in their
Th(CH
2
SiMe
2
NSiMe
3
)(NR
2
)
2
(where R ¼ SiMe
3
) complex, which
undergoes insertion of tert-butyl isocyanide into the C–Si bond
of the metallacycle to produce Th[(
t
Bu)NC(CH
2
)SiMe
2
NSiMe
3
]
(NR
2
)
2
.
58
Similar reactivity was observed with CO in Andersen's
system, resulting in Th[(OC(]CH
2
)SiMe
2
NSiMe
3
)]
(NR
2
)
2
. Exposing 1 to 1 atm of CO resulted in the formation of
two products with similar solubilities in non-polar solvents,
precluding their clean isolation and characterization, despite
yields of 80% for the bulk mixture. The major species, as
identied by
1
H NMR spectroscopy, exhibited inequivalent
methylene protons that are consistent with those observed in
Th[(OC(]CH
2
)SiMe
2
NSiMe
3
)](NR
2
)
2
(see Fig. S10 in ESI†). This
inequivalency would not be seen in the
1
H NMR spectrum of the
thorium acyl species generated by simple CO insertion into the
Th–C bond; thus, we postulated that it was likely a similar
insertion into the C–Si bond of 1 occurred (Scheme 3). This
hypothesis was proven by X-ray diffraction studies, as a few X-
ray quality crystals were isolated from a very concentrated
pentane solution stored at 35
C for 3 days, conrming the
identity of the CO insertion as the enolate complex Th[OC(]
CH
2
)SiMe
3
](BIMA)
3
(6) (Fig. 4). Complex 6 crystallized with two
independent molecules in the asymmetric unit due to disorder
in the enolate moiety; thus, the metrical parameters of only the
nondisordered molecule will be discussed. The Th(1)–O(1) bond
Scheme 3 Insertion reactivity of 1.
Fig. 3 Molecular structure of 5 (thermal ellipsoids drawn at the 50%
probability level). Hydrogen atoms omitted for clarity.
Fig. 4 Molecular structures of 6 (top) and 7 (bottom) (thermal ellip-
soids drawn at the 50% probability level). Hydrogen atoms omitted for
clarity.
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length of 2.216(2)
˚
A is slightly longer than the Th–O bond length
of 2.166(2)
˚
A seen in Th(OCH
2
NMe
2
)(BIMA)
3
,
35
while the C(25)–
C(26) bond length of 1.338(5)
˚
A is consistent with a carbon–
carbon double bond, and the trigonal planar geometry of C25
(S: z 360
) indicates sp
2
hybridization. This type of reactivity
has precedent in both uranium and thorium systems,
52,58–61
and
has been explained by initial CO insertion into the M–C bond to
form the metal acyl, which then isomerizes to form a carbene-
like intermediate that can then insert into the Si–C bond
(Scheme 4). Alongside crystals of 6 were crystals of a different
product, which we have tentatively assigned to the other reso-
nances observed in the
1
H NMR spectrum of the bulk material
(see Fig. S10 in ESI†). X-ray diffraction studies revealed this
product to be Th[OC(N
i
Pr)C(CH
2
SiMe
3
)(C(Me)N(
i
Pr))O-
k
2
O,O
0
](BIMA)
2
(7), the result of reductive CO coupling and
insertion into an amidinate ligand (Fig. 4). The Th(1) –O(1) and
Th(1)–O(2) bond lengths of 2.220(2) and 2.290(2)
˚
A, respectively,
are slightly longer than that seen in 6 and Th(OCH
2
NMe
2
)-
(BIMA)
3
,
35
while the bond length of 2.764(3)
˚
A for Th(1)–N(5) is
indicative of a dative interaction.
53–56
A single bond length of
1.558(5)
˚
A is observed for C(22)–C(23), whereas imine bonds are
seen for C(23)–N(6) and C(20)–N(5) (1.266(4) and 1.282(4)
˚
A,
respectively). This type of CO coupling mimics the enediolate
formation observed with various actinide bis-alkyl systems.
59,62
Regarding the formation of 7, it seems unlikely that this
product is obtained from the interaction of a molecule of CO
with 6, due to the intact nature of the trimethylsilylmethyl alkyl
fragment. Instead, it is likely that 7 results from “trapping” of
the carbene-like intermediate, which precedes formation of 6,
by another molecule of CO to form a transient ketene,
63
and
subsequent insertion and rearrangement steps lead to 7 as the
nal product (Scheme 4). Kinetically this intermolecular
process is slower than the intramolecular attack by the carbene
intermediate on the C–Si bond, producing 6 as the major
product. Exposing a mixture of 6 and 7 to additional CO did not
change the ratio of products observed, conrming that 7 is not
generated from 6. In an attempt to avoid the formation of 7, the
slow addition of 1 eq. of CO to a stirred hexanes solution of 1
was conducted, resulting in clean formation of 6 in 64% yield.
Development of a synthetic strategy to produce 7 is currently
underway. Looking to other small molecules, insertion reac-
tivity with CO
2
and CS
2
did not proceed cleanly, yielding
intractable mixtures.
Nitrile insertion was examined by the NMR-scale reaction of
1 and benzonitrile. The reaction was sufficiently complete
(>95% conversion by
1
H NMR spectroscopy) upon heating to
100
C for 72 h, resulting in the ketimide complex Th[–N]
C(Ph)(CH
2
SiMe
3
)](BIMA)
3
(8) (Scheme 3). The methylene singlet
is seen downeld at d 2.73, along with new resonances corre-
sponding to the aromatic protons of the phenyl ring, as well as
aromatic resonances corresponding to the cyclotrimerization
product of benzonitrile, 2,4,6-triphenyl-1,3,5-triazine (see
Fig. S13 in ESI†).
64
The Lewis acid-catalysed cyclotrimerization
of benzonitrile has been previously reported for lanthanide-
imido species,
64
although the active species in this trans-
formation has not been identied.
Looking to exploit the basic nature of the alkyl moiety of 1,
protonolysis reactivity was explored with a variety of protic
substrates, the results of which are summarized in Scheme 5.
NMR-scale experiments were carried out with 1 and 2,6-diiso-
propylaniline, resulting in the primary amido species Th(NH-
2,6-
i
Pr
2
-C
6
H
3
)(BIMA)
3
(9), as well as 1 and 2,6-di-tert-butylphe-
nol, resulting in the aryloxide complex Th(O-2,6-
t
Bu
2
-C
6
H
3
)
(BIMA)
3
(10), as determined by
1
H NMR spectroscopy. While
phenol addition and subsequent elimination of SiMe
4
occurred
within 12 h at room temperature, deprotonation of the aniline
required extended reaction times at elevated temperatures
before conversion to 9 was achieved. This can be rationalized on
the basis of the higher pK
a
of the aniline (30 vs. 16.8 in
Scheme 4 Proposed pathways for the formation of 6 and 7. Scheme 5 Protonolysis reactivity of 1.
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DMSO),
65,66
as the sterics imposed by the 2,6-di- tert-butylphenol
are greater than that of 2,6-diisopropylaniline.
Similar NMR experiments were also performed with primary
and secondary phosphines, namely mesitylphosphine and
diphenylphosphine, resulting in the thorium phosphido
complexes Th(PHMes)(BIMA)
3
(11a) (Mes ¼ 2,4,6-trimethyl-
phenyl) and Th(PPh
2
)(BIMA)
3
(11b), respectively, as determined
by
1
H and
31
P NMR spectroscopy. Complex 11a exhibits
a doublet at d 45.4 in the
31
P NMR spectrum with a
1
J
P,H
coupling constant of 195 Hz; the corresponding doublet in the
1
H NMR is observed at d 3.41. In the
31
P NMR spectrum of 11b
a singlet is observed at d 89.5 corresponding to the thorium
phosphido, alongside peaks consistent with the dehy-
drocoupled product Ph
2
P–PPh
2
(d 14.9)
67
and an unidentied
phosphorus-containing species (d 106). These
31
P chemical
shis are in the range reported for other primary and secondary
thorium phosphido species.
68–70
The dehydrocoupling of phos-
phines has been reported with zirconium phosphido complexes
under similar reaction conditions.
71
Both complexes 11a and
11b required elevated temperatures and prolonged reaction
times to reach completion. We sought a more scalable strategy
to synthesize 11a without the need for harsh reaction condi-
tions, as complex 11a has the potential to form a thorium
phosphinidene via deprotonation of the phosphide ligand.
With few examples of thorium phosphinidenes available, this
would provide valuable information regarding metal–ligand
multiple bonding between thorium and the heavier pnic-
togens.
72–74
Salt metathesis between the previously reported
ThCl(BIMA)
3
(ref. 35) and KPHMes
70
provided a more direct
route to 11a as bright yellow crystals in 58% yield. The
13
C{
1
H}
NMR spectrum of 11a exhibits rare through-space coupling of
the phosphorus atom and the isopropyl methyl carbons of the
BIMA ligand (
TS
J
P,C
¼ 2.1 Hz), as well as the ortho-methyls of the
mesitylene ring (
TS
J
P,C
¼ 9.6 Hz).
75–78
This coupling is supported
by the X-ray crystal structure of 11a, which displays a close
proximity of the phosphorus atom to one of the BIMA isopropyl
methyls (3.746(4) and 3.779(4)
˚
A) and mesityl methyls (3.062(3)
and 3.070(3)
˚
A), along with signicant pyramidalization at P
(S: z 311
) in the two independent molecules found in the
asymmetric unit (Fig. 5). The orientation of the phosphorus
lone pair toward these carbon atoms facilitates this spin–spin
interaction. This is the rst crystallographically characterized
example of a thorium monophosphido species bearing
a primary phosphide ligand; to date, only a handful of primary
bis(phosphido)-thorium species have been isolated and char-
acterized.
70,72,79
The Th –P bond lengths seen in the two mole-
cules in the asymmetric unit (3.0497(8) and 3.0404(8)
˚
A) are
0.15
˚
A longer than those observed in the previously reported
bis(phosphido)-thorium complexes. Attempts to deprotonate
11a to form the corresponding thorium phosphinidene have
thus far proven unsuccessful.
The reaction of 1 with p-tolylacetylene proceeds cleanly,
providing the thorium acetylide complex Th(C^C-p-
tolyl)(BIMA)
3
(12) as colourless crystals in 95% yield. This new
alkynyl species may serve as a useful starting material for future
chemistry, as other thorium acetylide complexes have been
shown to be active catalysts for the linear oligomerization of
terminal alkynes.
80
The
1
H NMR spectrum displays a C
3
-
symmetric amidinate environment along with resonances
attributable to the p-tolyl group, while the
13
C{
1
H} NMR spec-
trum features a downeld resonance of d 189.2 corresponding
to the thorium-bound carbon atom of the alkyne, consistent
with other thorium and group IV acetylides.
81–84
The IR spec-
trum exhibits a characteristic signal at 2061 cm
1
assigned to
the C^C stretch. X-ray diffraction studies revealed a near-linear
Th–C^C bond angle of 175.7(2)
and bond lengths of 2.542(2)
and 1.219(3)
˚
A for Th(1)–C(25) and C(25)–C(26), respectively
(Fig. 6). The Th–C bond length in 12 is 0.05
˚
A longer than that
of the few other thorium acetylide species to have been char-
acterized crystallographically, namely [(L)Th(C^CSiMe
3
)
2
] and
[(L)Th(C^CSi
i
Pr
3
)
2
] (where L ¼ trans-calix[2]benzene[2]pyrro-
lide), and Th(Bc
Mes
)
2
(C^C-p-tolyl)
2
(where Bc
Mes
¼ mesityl-
substituted bis(NHC)borate, NHC ¼ N-heterocyclic carbene),
which were only recently reported.
27,75
A cationic species was targeted as a potential precursor to
a Th(
III) amidinate complex, as Evans has shown that reduction
of a mixed cyclopentadienyl amidinate thorium cation, namely
{(C
5
Me
5
)
2
[
i
PrNC(Me)N
i
Pr]Th}{BPh
3
Me}, can be achieved with
KC
8
.
85
Treatment of 1 with [Et
3
NH][BPh
4
]
86
in THF led to the
isolation of [Th(THF)(BIMA)
3
][BPh
4
](13) as colourless crystals in
81% yield. The NH
3
and SiMe
4
byproducts were easily removed
under vacuum upon workup, and X-ray diffraction studies show
a well separated ion pair with one THF molecule coordinated to
the thorium center and another co-crystallized in the lattice
(Fig. 7). The Th–N
amid
bond lengths are noticeably shorter (2.45–
2.48
˚
A) than that of 1 or 2, likely due to reduced steric congestion
and higher electrophilicity of the metal center. The Th(1)–O(1)
distance of 2.504(2)
˚
A is in the range observed for other THF-
bonded thorium complexes.
87–90
The second equivalent of THF
can be removed under high vacuum. The
1
H NMR spectrum of
dried 13 in CDCl
3
displays equivalent amidinate resonances, the
Fig. 5 Molecular structure of 11a (thermal ellipsoids drawn at the 50%
probability level). Non-phosphorus-bound hydrogen atoms omitted
for clarity.
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aromatic peaks of the BPh
4
anion, and one set of resonances
corresponding to coordinated THF. The complex displays
appreciable stability in CDCl
3
, but begins to decompose aer
24 h at room temperature. Attempted reduction of 13 with KC
8
in THF led to the isolation of 2. No colour change was observed
throughout the reaction. Increasing the sterics of the R groups
on the amidinate nitrogens may help stabilize a tris-amidinate
Th(
III) complex, and work is currently ongoing to test this
hypothesis. Attempts to utilize H
2
as a protic substrate and form
a thorium hydride complex were not successful.
Complex 1 undergoes ligand exchange with one equivalent
of 9-borabicyclo[3.3.1]nonane (9-BBN), affording (BIMA)
3
Th(m-
H)
2
[B(C
8
H
14
)] (14) alongside one equivalent of (C
8
H
14
)B(CH
2
-
SiMe
3
), as determined by
11
B{
1
H} NMR spectroscopy (resonance
observed at d 84.3 in C
6
D
6
).
91
Complex 14 was isolated in 66%
crystalline yield aer workup (Scheme 6). The thorium boro-
hydride complex exhibits equivalent amidinate ligands in
solution according to
1
H NMR spectroscopy. Additionally,
broad m-H resonances and several multiplets corresponding to
the C
8
H
14
fragment are observed. The
11
B{
1
H} NMR spectrum
exhibits a single resonance at d 4.90, which is in the range
typically observed for boron hydrides.
92
FTIR spectroscopy
reveals a broad B–H stretch centered at 2021 cm
1
. X-ray
diffraction studies show an eight-coordinate thorium center
bearing bridging hydrides bound to the 9-BBN moiety (Fig. 8).
The hydrides were located in the Fourier difference map and
rened isotropically. The Th(1)–B(1) distance of 2.952(9)
˚
Ais
signicantly longer than typically observed with other thorium
complexes containing bridging borohydrides (2.49(6)–2.670(2)
˚
A),
93–97
but is within the range observed by Girolami and co-
workers in the complexes [Th(H
3
BNMe
2
BH
3
)
4
] and [Th(H
3
-
BNMe
2
BH
3
)
2
(BH
4
)
2
], which exhibit Th–B distances between
2.848(9) to 3.193(5)
˚
A.
97
Complex 14 is surprisingly stable both
under photolytic and elevated temperature conditions, with no
decomposition or elimination of H
2
observed.
Storage of 1 at room temperature under dry nitrogen for
several weeks led to slight discolouration of the crystalline solid.
While the
1
H NMR spectrum of this material showed very little
change from that of pure 1, we decided to investigate the
stability of 1 under photolytic conditions. The UV-Vis spectrum
of 1 features an absorption with l
max
¼ 295 nm, likely a ligand-
based p–p* transition (see Fig. S31 in ESI†). Irradiation of 1
with UV-light centered at 253 nm in C
6
D
6
and monitoring by
1
H
NMR spectroscopy showed elimination of SiMe
4
alongside the
Fig. 6 Molecular structure of 12 (thermal ellipsoids drawn at the 50%
probability level). Hydrogen atoms omitted for clarity.
Fig. 7 Molecular structure of 13 (thermal ellipsoids drawn at the 50%
probability level). Hydrogen atoms and THF solvent molecule omitted
for clarity.
Scheme 6 Ligand exchange and photolytic reactivity of 1.
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production of a single new product, which displayed inequiva-
lent amidinate ligands. Reaction times ranged from 24 h to 10
days, depending on the concentration of 1 in the sample
(0.02–0.45 M), with higher concentrations taking longer.
Optimization of the reaction conditions, by use of a quartz
reaction vessel, cyclohexane-d
12
and a xenon arc lamp, resulted
in signicantly reduced reaction times (2 h for 0.03 M
solution). Removal of SiMe
4
under vacuum and crystallization
from toluene afforded Th(BIMA)
2
(BIMA*)(15) in 53% yield
[BIMA* ¼ (
i
Pr)NC(CH
2
)N(
i
Pr)]. Heating a solution of 1 at 100
C
for 24 h and monitoring by
1
H NMR spectroscopy showed no
decomposition of 1 or production of 15, eliminating the possi-
bility that conversion of 1 to 15 was thermally-induced.
Irradiation of 1 results in C–H activation of a methyl group
on an amidinate ligand by the –CH
2
SiMe
3
moiety, eliminating
SiMe
4
and reducing the activated amidinate to a dianionic
ligand (Scheme 6). Heterolytic bond cleavage of the Th–C bond
resulting in a –CH
2
SiMe
3
anion, which then attacks the methyl
backbone of an amidinate ligand, is a possible explanation for
the C–H activation and subsequent ligand reduction observed.
This is a rare example of an amidinate dianion,
64
and the rst
generated under photolytic conditions. The
1
H NMR spectrum
of 15 exhibits a set of equivalent amidinate resonances, along-
side the resonances attributable to the amidinate dianion,
specically a 2H septet at d 4.24, a 2H singlet at d 3.44, and
a 12H doublet at d 1.53. The 6H singlet corresponding to the
methyl groups of the monoanionic amidinates also appears at
d 1.53 (see Fig. S29 in ESI†). In the
13
C{
1
H} NMR spectrum the
terminal methylene carbon resonance is observed noticeably
downeld at d 53.3, shied by 40 ppm from the amidinate
methyls seen at d 12.2, but further up eld than that typically
seen with alkenes. The
1
J
C–H
coupling constant of 157.6 Hz (as
measured from the
13
C satellites observed in the
1
H NMR
spectrum) is consistent with sp
2
hybridization and similar to
that observed for ethylene.
98
The ipso-carbon of the amidinate
dianion is shied upeld to d 154.2 from d 172.8 as observed for
the monoanionic amidinates. X-ray diffraction studies revealed
a dimeric structure where the thorium centers are bridged by
the methylene carbon of the dianionic amidinate ligand (Fig. 9).
Complex 15 crystallizes in P
1 with the asymmetric unit con-
taining only the monomer unit; the dimer is generated through
inversion symmetry. The Th(1)–C(21) bond distance of 2.749(3)
˚
A is signicantly longer than the Th–C bond observed in 1
(2.557(3)
˚
A), but shorter than the long Th–s-alkyl bond distance
of 2.875(9)
˚
A observed by Liddle and co-workers in [Th{N(CH
2
-
CH
2
NSiMe
2
t
Bu)
2
(CH
2
CH
2
NSiMe
t
Bu-m-CH
2
)}]
2
.
90
The C(20)–
C(21) bond distance of 1.438(4) is longer by 0.1
˚
A than typically
observed for C–C double bonds, but this lengthening may be
a result of delocalized electron density involved in the Th–C
contact, reminiscent of a three-center two-electron bond. This is
also manifested in the lack of planarity seen in the –N(CH
2
)CN–
unit of the amidinate dianion.
Conclusions
The amidinate-supported thorium monoalkyl complex 1
exhibits a variety of reactivity with small molecules, including
insertion, protonolysis and photolysis. The insertion of p-tolyl
azide lead to the thorium triazenido complex 3 which
undergoes clean thermal decomposition at low concentrations
to the corresponding amido complex 4, through the loss of the
unstable “N
2
CH
2
” fragment. This is the rst example of an
actinide complex undergoing this rare transformation. Inser-
tion of xylyl isocyanide results in the rst crystallographically
characterized thorium iminoacyl complex. This insertion reac-
tivity differs from that observed with CO, which instead results
in the corresponding enolate species 6 upon CO insertion and
rearrangement, as well as the unique double CO insertion and
amidinate cleavage product 7. The utility of the alkyl moiety as
an internal base was demonstrated with a variety of protic
Fig. 8 Molecular structure of 14 (thermal ellipsoids drawn at the 50%
probability level). Non-hydride hydrogen atoms omitted for clarity.
Fig. 9 Molecular structure of 15 (thermal ellipsoids drawn at the 50%
probability level).
2838
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substrates, with the thorium phosphido complex 11a generated
via both protonolysis and salt metathesis routes, the latter
providing a more scalable option for the synthesis of 11a. The
photolytic elimination of SiMe
4
concomitant with the reduction
of an amidinate ligand to form complex 15 is unprecedented
reactivity with amidinate-supported metal complexes, and
a rare example of a complex bearing a dianionic amidinate
ligand. Mechanistic and reactivity investigations of several of
the complexes reported are currently ongoing.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences Heavy Element Chemistry
Program of the U.S. Department of Energy (DOE) at LBNL under
Contract No. DE-AC02-05CH11231. We would like to acknowl-
edge the NIH shared instrumentation grant S10-RR027172 for
use in X-ray diffraction studies. N. S. S. acknowledges the
Department of Energy Nuclear Energy University Programs for
a graduate research fellowship. N. S. S. gratefully acknowledges
Ms M. Garner, Mr M. Boreen and Mr T. Lohrey for their valuable
discussions, and Mr P. Smith for assistance with the xenon arc
lamp experiment.
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