Reductive Elimination of Diphosphine from a Thorium-NHC-Bis(phosphido) Complex

Article abstract of DOI:10.1021/acs.organomet.7b00301

The synthesis, characterization, and reductive elimination reactivity of a bis(NHC)borate-supported thorium bis(phosphido) complex (2^Mes) is described. Treating 2^Mes with 2,2′-bipyridine leads to the reductive elimination of dimesityldiphosphine (4) and the formation of the previously reported NHC-thorium-bpy complex (3). The kinetics of the bpy-induced reductive elimination were studied by ³¹P NMR and suggest the presence of an intermediate. Treatment with alternative oxidants also leads to diphosphine elimination, but the corresponding thorium species have not been isolated cleanly. Additional primary (2^Ph) and secondary (2^PPh2) Th-bis(phosphido) complexes were synthesized but do not demonstrate the same facile reductive elimination as 2^Mes.

Full text of DOI:10.1021/acs.organomet.7b00301

Communication
pubs.acs.org/Organometallics
Reductive Elimination of Diphosphine from a Thorium−NHC−
Bis(phosphido) Complex
Mary E. Garner and John Arnold*
Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: The synthesis, characterization, and reductive
elimination reactivity of a bis(NHC)borate-supported thorium
bis(phosphido) complex (2Mes) is described. Treating 2Mes with
2,2′-bipyridine leads to the reductive elimination of dimesi-
tyldiphosphine (4) and the formation of the previously
reported NHC−thorium−bpy complex (3). The kinetics of
the bpy-induced reductive elimination were studied by 31P
NMR and suggest the presence of an intermediate. Treatment
with alternative oxidants also leads to diphosphine elimination, but the corresponding thorium species have not been isolated
cleanly. Additional primary (2Ph) and secondary (2PPh2) Th−bis(phosphido) complexes were synthesized but do not
demonstrate the same facile reductive elimination as 2Mes
.
xidative addition and reductive elimination in organo-
Ometallic chemistry and catalysis are fundamental steps for
bond-making and -breaking reactions.1,2 Mid to late transition
metals are generally preferred for such transformations over
early d- and f-block elements because they can accommodate
changes in electron density through oxidation state changes at
the metal. Yet, by using noninnocent supporting ligands as
electron reservoirs, Heyduk and co-workers were able to extend
this type of redox chemistry to early transition metals with
formal d0 electron counts.3,4 In these systems, the ability of the
supporting ligand to convert between its oxidized and reduced
forms obviated the need for oxidation state changes at the
metal center during the redox processes. Integrating non-
innocent molecules into the organoactinide regime has been a
useful endeavor as well.5−20 In fact, a major advance for metals
that are redox-active but have unstable reduced species came
from the work of Bart and co-workers involving reductive
elimination from a uranium(IV) benzyl complex.12,15 While
some uranium complexes perform metal-based oxidative
addition and reductive elimination,21 Bart and co-workers
demonstrated how providing noninnocent molecules to
attenuate electron richness at the metal center enabled cleaner,
better-controlled redox transformations.
complex (3) with potent reducing power. Utilizing its stored
electrons, this complex mediates reductive transformations such
as nitrene transfer from p-tolyl azide, coupling of carbonylated
substrates, and well-defined C−N bond cleavage of alkyl
isocyanides.16,17
Perhaps even more challenging than facilitating redox
reactivity in these ways is promoting noninnocent reactivity
from a complex that lacks both a redox-active supporting ligand
and a redox-active metal center. Here we describe the reductive
elimination of diphosphine from an NHC−Th−bis-
(phosphido) complex (2Mes). Instead of relying on oxidation
state changes at the metal center (“classical”), the reductive
elimination involves redox changes entirely at the −HP(Mes)
ligands and bpy (“ligand-based”) (Scheme 1).
Scheme 1. Classical (Top) and Ligand-Based (Bottom)
Reductive Elimination and Oxidative Addition Reactions
Unlike uranium, metal-based redox chemistry is impractical
for most thorium compounds due to the very negative ThIV/III
and ThIII/II reduction potentials (−3.9 and −4.9 V vs NHE,
respectively).22−24 Therefore, our group16,17 and
others6−9,19,20,25 have been exploring ligand-based ways to
promote oxidative and reductive chemistry from thorium
complexes. For example, we have shown that incorporating a
reduced, noninnocent 2,2′-bipyridine (bpy) ligand into the
Special Issue: Organometallic Actinide and Lanthanide Chemistry
Received: April 19, 2017
coordination sphere of a bis(NHC)borate-supported (BcMes
)
thorium compound affords a well-defined NHC−thorium−bpy
© XXXX American Chemical Society
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Communication
Treatment of the bis(iodo) complex Th(BcMes)2I2 (1) with 2
equiv of KHP(Mes) (Mes = 2,4,6-trimethylphenyl) affords the
bright orange bis(phosphido) complex Th(BcMes)2[HP(Mes)]2
(2Mes) in 68% yield (Scheme 2).
several months at room temperature under an atmosphere of
dry nitrogen. Solutions of 2Mes are stable at room temperature
for 1 week and can be heated to 60 °C with no signs of
decomposition. Unlike the case for the metallocene-supported
thorium−bis(phosphido) complex,26 heating solutions of 2Mes
above 60 °C fails to form an isolable phosphinidene (PR2−)
but instead generates 1 equiv of free H2P(Mes) and an
unidentified phosphorus-containing product. Lewis bases such
Scheme 2. Synthesis of Th(BcMes)2[HP(Mes)]2 (2Mes
)
as DMAP, pyridine, and THF do not form adducts28 with 2Mes
,
t
and BuNC does not react cleanly to form a phosphaazaal-
lene.27
Bart and co-workers have shown that treating homoleptic
uranium(IV) benzyl complexes with noninnocent moieties
circumvents the formation of unstable uranium(II) species
during the elimination of bibenzyl.12,15 Curious as to how these
types of outer-sphere moieties would affect our redox-inactive
thorium system, we turned our attention to the interaction of
2Mes and bpy. Although addition of 1 equiv of bpy to a C6D6
solution of 2Mes causes a dramatic color change from bright red-
The 1H NMR spectrum of 2Mes in C6D6 supports the
assignment of a C2-symmetric bis(phosphido) complex. The set
of peaks corresponding to the two bis(NHC)borate ligands and
those attributable to the two −PH(Mes) ligands are present in
a 1:1 ratio. Notably, the two 31P nuclei in 2Mes are magnetically
nonequivalent and as a result the atoms comprising the H−P
1
orange to brown within seconds, the H and 31P NMR spectra
taken after 5 min were unchanged. However, after several hours
at room temperature, the reaction mixture became dark green
and resonances corresponding to the previously described
direduced Th−NHC−bpy complex16 3 were clearly visible in
the 1H NMR spectrum (Figure S3 in the Supporting
Information), along with signals attributable to both isomers
of the oxidatively coupled dimesityldiphosphine product 4 in
the 31P NMR spectrum (Figure S4 in the Supporting
Information).29 The transformation is clean and proceeds to
completion with mild heating (50 °C); only trace amounts of
free H2P(Mes) side product are detected.
While the synthesis of direduced Th−bpy complexes is
precedented, this is a rare example where the reduced bpy
ligand is installed without the use of a strong reduc-
tant.7,9,16,18,25 This overall two-electron redox transformation
(Scheme 3) mimics that of reductive elimination by mid to late
1
bond exhibit virtual coupling (J = 238.0, 16.1 Hz) in the H
NMR (+4.16 ppm) and 31P NMR (+21.7 ppm) spectra
(Figures S5 and S11 in the Supporting Information). These
resonances collapse to singlets upon selective 31P and 1H
decoupling (Figures S20 and S14 in the Supporting
Information, respectively).
Red-orange crystals of 2Mes deposit from a vapor diffusion of
hexanes into a concentrated benzene solution after storage for
48 h at room temperature. Single-crystal X-ray diffraction
studies confirm that 2Mes is a rare example of a primary
bis(phosphido)−thorium complex.26,27 The compound main-
tains C2 symmetry in the solid state and crystallizes in the
monoclinic space group P21/n (Figure 1). A 2-fold rotation axis
runs through the thorium center bisecting the two −HP(Mes)
ligands.
Scheme 3. Reductive Elimination Reactivity of 2Mes on
Treatment with bpy
Similar to the Th−bis(phosphido) complexes previously
reported by Walensky and co-workers,26,27 2Mes is highly
sensitive to air and moisture but can be stored as a solid for
transition metals, but instead of occurring at the metal, the
redox process involves only the ligands (−HP(Mes) and bpy).
Performing a control reaction between the potassium
phosphide salt [KHP(Mes)] and bpy in benzene does not
afford a measurable amount of diphopshine 4. Instead, the
major product is free H2P(Mes), likely arising from H atom
abstraction from the solvent.
Alternative outer-sphere redox mediators also induce the
reductive elimination of 4. Treatment of 2Mes with 1 equiv of
iodine leads to the controlled formation of 4 along with the
known thorium−bis(iodo) complex 1.16 In contrast, the
reaction of iodine with KHPMes does not proceed cleanly to
a single product but instead leads to a mixture of 4 and other
unidentified phosphorus-containing species. Interaction of 2Mes
with other oxidants such as p-tolyl azide, white phosphorus,
Figure 1. Molecular structure of 2Mes (thermal ellipsoids drawn at the
50% probability level). Hydrogen atoms, excluding those bound to
phosphorus, are omitted for clarity. Selected bond distances (Å) and
angles (deg): Th1−P1, 2.855(5); Th1−P2, 2.938(6); P1−Th1−P2,
137.01(2).
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1,10-phenanthroline, and acenaphthenequinone all lead to the
formation of 4, but despite our efforts no thorium-containing
products have been isolated cleanly from these reactions. Still,
our group is currently focused on utilizing this unique ligand-
based reductive elimination of diphosphine as a potential
strategy to access rare, or unknown, thorium compounds.
The kinetics of the elimination reaction by complex 2Mes on
treatment with bpy (Scheme 3) were studied by 31P NMR
spectroscopy. An initial rate investigation confirms that the
disappearance of 2Mes is first order, with no observable
intermediates. The observed rate of 4 formation displays
saturation behavior as the initial concentration of bpy is
increased from 0.01 to 0.77 M (Figure 2).
Investigating the thermodynamics of these phosphorus−
phosphorus bond-forming reactions at the DFT level shows
that the formations of 3 and 4, 5, or 6 from bpy and 2Mes, 2Ph,
or 2PPh2, respectively, are each very energetically favorable with
nearly equal ΔGrxn values (−19.1, −19.7, and −20.2 kcal/mol,
respectively). The reason for the difference in reductive
elimination reactivity between the bis(phosphido) compounds
is unclear and is currently being explored. However, crossover
experiments involving 2Mes and 2Ph expose the lability of the
Th−P bond and demonstrate that the phosphido ligands can be
readily exchanged (Figures S21−S23 in the Supporting
Information).
To summarize, we have described an NHC−Th−bis-
(phosphido) complex that reductively eliminates diphosphine
on treatment with bpy and other outer-sphere redox-active
substrates. Installing the mesityl-substituted phosphido ligand
affords a convenient model complex (2Mes) amenable to
kinetics investigations. The rate of formation of 4 shows a first-
order dependence on [2Mes] but displays saturation behavior in
[bpy], which is confirmed by flooding experiments. Computa-
tional studies reveal that changes to phosphido ligands do not
alter the thermodynamics of the elimination reaction
significantly; however, experimentally the ease and cleanliness
of the reaction are greatly affected. Performing this reductive
elimination chemistry with a redox-inactive thorium center
demonstrates how the overall redox changes occur entirely at
the −HP(Mes) ligands and bpy. Efforts to define and expand
the scope of this reductive elimination reactivity with other
NHC−thorium complexes are currently underway.
Figure 2. Variation in the observed rate of formation of 4 as a function
of the initial concentration of bpy.
Such saturation behavior points to a pre-equilibrium in the
mechanism and suggests that bpy is involved after this
equilibrium is reached (Scheme 4).30 Applying the steady-
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00301.
■
S
Scheme 4. Proposed Mechanism for the Formation of 4
NMR, FT-IR, and UV−vis spectra of new compounds,
computational details, and crystallographic data of
compounds 2Mes, 2Ph, and 2PPh2 (PDF)
Accession Codes
state approximation to the concentration of intermediate A
leads to the rate expression described by Scheme 4 and given in
eq 1 (see the Supporting Information for derivation). This
expression is consistent with the experimentally observed
dependences on [2Mes] and [bpy] and is also confirmed by bpy
flooding experiments ([bpy]0:[2Mes]0 > 10:1). As predicted
from eq 1, the formation of 4 demonstrates a zero-order
dependence on the concentration of bpy when large excesses of
bpy are used. The identity of A is unknown at this time;
however, a mono(phosphido)−thorium complex is likely.
CCDC 1517847−1517849 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.A.: arnold@berkeley.edu.
■
ORCID
Mary E. Garner: 0000-0002-4630-0661
John Arnold: 0000-0001-9671-227X
Notes
Syntheses of the additional primary bis(phosphido)−Th−
NHC complex 2Ph and the secondary bis(phosphido)−Th−
NHC complex 2PPh2 offer more information about the scope of
the elimination reaction. Unlike 2Mes, treating 2Ph with bpy does
not lead to the formation of diphenyldiphosphine (5) (Scheme
S3 in the Supporting Information), despite extended reaction
times and/or heating. In the case of the secondary bis-
(phosphido) complex 2PPh2, reductive elimination of tetraphe-
nyldiphosphine (6) can be achieved by heating to 70 °C but is
accompanied by competing thermal decomposition of 2PPh2
(Scheme S4 in the Supporting Information).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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. The authors
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(28) Lv, Y.; Kefalidis, C. E.; Zhou, J.; Maron, L.; Leng, X.; Chen, Y. J.
Am. Chem. Soc. 2013, 135, 14784−14796.
acknowledge the NSF (CHE-0840505) for supporting the
computational work conducted at the University of California,
Berkeley, Molecular Graphics and Computation Facility.
M.E.G. acknowledges the NSF-GRFP for a graduate research
fellowship (DGE 1106400). M.E.G. gratefully acknowledges
Mr. Bernard Parker and Dr. Hasan Celik for assistance with
NMR techniques, Dr. Olayinka Olatunji-ojo and Dr. Daniel
Levine for advice concerning computations, and Dr. Stephan
Hohloch and Mr. Nicholas Settineri for their valuable
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