Reductive silylation of Cp?UO2(MesPDIMe) promoted by Lewis bases

Article abstract of DOI:10.1039/c5dt04776e

Functionalization of the uranyl moiety (UO₂²⁺) in Cp?UO₂(^MesPDI^Me) (1-PDI) (^MesPDI^Me = 2,6-((Mes)NCMe)₂C₅H₃N; Mes = 2,4,6-triphenylmethyl), which bears a reduced, monoanionic pyridine(diimine) ligand, is reported. Silylating reagents, R₃Si-X (R = Me, X = Cl, I, OTf, SPh; R = Ph, X = Cl), effectively add across the strong OUO bonds in the presence of the Lewis base, OPPh₃, generating products of the form (R₃SiO)₂UX₂(OPPh₃)₂ (R = Me, X = I (2-OPPh₃), Cl (3-OPPh₃), SPh (5-OPPh₃), OTf (6-OPPh₃); R = Ph, X = Cl (4-OPPh₃)). During this transformation, reduction to uranium(iv) occurs with loss of (Cp?)₂ and ^MesPDI^Me, each of which acts as a one-electron source. In the reaction, the Lewis base serves to activate the silyl halide, generating a more electrophilic silyl group, as determined by ²⁹Si NMR spectroscopy, that undergoes facile transfer to the oxo groups. Complete U-O bond scission was accomplished by treating the uranium(iv) disiloxide compounds with additional silylating reagent, forming the family (Ph₃PO)₂UX₄. All compounds were characterized by ¹H NMR, infrared, and electronic absorption spectroscopies. X-ray crystallographic characterization was used to elucidate the structures of 2-OPPh₃, 4-OPPh₃, 5-OPPh₃, and 6-OPPh₃.

Full text of DOI:10.1039/c5dt04776e

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Reductive silylation of Cp*UO2(MesPDIMe)
promoted by Lewis bases†
J. J. Kiernicki,a J. S. Harwood,a,b P. E. Fanwicka and S. C. Bart*a
Cite this: Dalton Trans., 2016, 45,
3111
Functionalization of the uranyl moiety (UO22+) in Cp*UO2(MesPDIMe) (1-PDI) (MesPDIMe = 2,6-((Mes)
NvCMe)2C5H3N; Mes = 2,4,6-triphenylmethyl), which bears a reduced, monoanionic pyridine(diimine)
ligand, is reported. Silylating reagents, R3Si-X (R = Me, X = Cl, I, OTf, SPh; R = Ph, X = Cl), effectively add
across the strong OvUvO bonds in the presence of the Lewis base, OPPh3, generating products of the
form (R3SiO)2UX2(OPPh3)2 (R = Me, X = I (2-OPPh3), Cl (3-OPPh3), SPh (5-OPPh3), OTf (6-OPPh3); R = Ph,
X = Cl (4-OPPh3)). During this transformation, reduction to uranium(IV) occurs with loss of (Cp*)2 and
MesPDIMe, each of which acts as a one-electron source. In the reaction, the Lewis base serves to activate
the silyl halide, generating a more electrophilic silyl group, as determined by 29Si NMR spectroscopy, that
undergoes facile transfer to the oxo groups. Complete U–O bond scission was accomplished by treating
the uranium(IV) disiloxide compounds with additional silylating reagent, forming the family (Ph3PO)2UX4.
Received 7th December 2015,
Accepted 14th December 2015
1
All compounds were characterized by H NMR, infrared, and electronic absorption spectroscopies. X-ray
DOI: 10.1039/c5dt04776e
crystallographic characterization was used to elucidate the structures of 2-OPPh3, 4-OPPh3, 5-OPPh3,
and 6-OPPh3.
www.rsc.org/dalton
ported the importance of potassiation for activation of the
Introduction
uranyl in situ,7,8 whereas new findings report that Lewis acids
Uranyl functionalization has long been a “Holy Grail” trans- are also effective at this.9 Alternately, Hayton has showed the
formation for the f-block elements, as this process has impor- utility of this method for uranyl silylation supported by
tant implications for environmental remediation1 and spent simpler β-ketoiminate or dibenzoylmethanate ligands using
fuel processing.2 The strong uranium–oxygen multiple bonds either an excess of Me3SiI,10 HSiPh3 coupled with B(C6F5)3,11
in the [UO2]2+ unit, which are reinforced by the inverse or Ph3SiOTf (OTf = trifluoromethanesulfonate).12 In all cases,
trans influence (ITI),3–5 are difficult to break or elaborate. An double functionalization occurs. A seminal example in this
effective route that has recently been popularized is that of field by Ephritikhine shows complete uranyl functionalization
reductive silylation, involving silyl group transfer to the oxygen is possible using a large excess of silylhalide absent ancillary
atoms, accompanied by an overall reduction of the uranium ligands.13
from 6+ to 4+. This process has proven successful with
uranyl in a variety of ligand environments, demonstrating its synthesis and characterization of a unique uranyl species
generality as a method for U–O bond cleavage. For instance, supported by monoanionic pyridine(diimine) ligand,
In a recent contribution to this field, we reported the
a
Arnold and co-workers have pioneered this idea with success- Cp*UO2(MesPDIMe) (1-PDI).14 This compound has a compli-
ful application to uranyl species ligated by large polypyrrolic cated electronic structure; while formally uranium(V), spectro-
ligands that wrap around the uranium center.6,7 Here, single- scopically this species is most consistent with having a
electron reduction is performed using potassium hexamethyl- hexavalent uranium ion. 1-PDI features a bent uranyl moiety
disilazide to generate the pentavalent, monofunctionalized (∼169°), making it particularly susceptible to functionalization
unit, [OvUOR]2+. Subsequent computational studies sup- using Me3SiI, forming the trans-siloxide uranium(IV) species,
(Me3SiO)2UI2(MesPDIMe
)
(2-PDI), by addition across the
robust uranium–oxygen multiple bonds. This silylation is
accompanied by a two-electron reduction of the metal, with the
reducing equivalents originating from Cp*-anion as well as the
radical on the MesPDIMe ligand, as evidenced by X-ray crystallo-
graphy. Additional equivalents of Me3SiI resulted in complete
aH.C. Brown Laboratory, Department of Chemistry, Purdue University,
West Lafayette, IN 47907, USA. E-mail: sbart@purdue.edu
bPurdue Interdepartmental NMR Facility, Purdue University, West Lafayette,
IN 47907, USA
†Electronic supplementary information (ESI) available: Spectroscopic and crys-
tallographic details. CCDC 1436226–1436229. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c5dt04776e
cleavage of the uranium oxygen bonds, generating tetravalent
15
UI4(1,4-dioxane)2
when the reaction was performed in
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1,4-dioxane solvent. This overall transformation was significant, of the uranyl starting material. Thus, the identity of the off-
as the uranyl functionalization and bond cleavage were per- white solid was proposed to be the uranium(IV) disiloxide,
formed at ambient temperature with stoichiometric quantities (Me3SiO)2UI2(OPPh3)2 (2-OPPh3).
of silylating reagent, in contrast to previous systems.
We hypothesized that this functionalization occurred quite
readily due to several factors, including (1) the activated bent
uranyl moiety, (2) electrophilicity of the –SiMe3 group in
Me3SiI, and (3) the ligands acting as an electron source. While
little variation of the ligands was possible in this context, we
sought a method by which to probe the effects of tuning the
ð2Þ
electrophilicity of silyl substituents to determine its influence
on uranyl functionalization. A demonstration of this has been
reported, showing that R3Si-X reagents react with a variety of
Lewis bases to generate salts (eqn (1)).16 This reaction is an
equilibrium that can be fine tuned through three simple modi-
Confirmation of the identity of paramagnetic 2-OPPh3 was
achieved by X-ray diffraction analysis of single crystals obtained
from a dilute toluene/n-pentane (1 : 1) solution at ambient temp-
erature (Table 1). Data refinement revealed the predicted octa-
hedral uranium bis(iodide) (U1–I1 = 3.1222(7) Å) with both
trans-trimethylsiloxide (U1–O1 = 2.082(6) Å) (O1–U1–I1 = 88.26
(19)°) and trans-OPPh3 (U1–O2 = 2.333(6) Å) ligands (Fig. 1).
Structurally, 2-OPPh3 is reminiscent of UI2(O(CH2)4I)2(OPPh3)2,
which was isolated as the THF-ring opened product from
UI4(MeCN)4.18
fications: (1) altering the basicity of the Lewis base, (2) chan-
ging the electronics of R-substituents, or (3) modifying the
leaving group ability of X. We hypothesized that a more electro-
philic silylium ion would facilitate uranyl functionalization.
Herein, we report our studies aimed at expanding the scope of
reagents useful for uranyl functionalization by facilitating
reductive silylation using Lewis bases.
K1
B: þ R3Si-X Ð ½B-SiR3ꢁ½Xꢁ
ð1Þ
K
ꢀ1
We hypothesized the significant rate enhancement for the
reductive silylation of 1-PDI that occurs upon addition of the
Lewis base was due to formation of [Ph3POSiMe3][I] as had
been previously identified. Thus, a more basic phosphine
Results and discussion
Lewis base assisted reductive silylation
oxide should favour Si–I bond activation with a higher Keq
.
Therefore, we screened a variety of substituted phosphine
oxides for the same silylation reaction of 1-PDI with Me3SiI
(2 equiv.) and monitored disappearance of the intense blue
colour (λmax = 612 nm) of the starting material. The results
from these experiments show the reaction proceeds faster
when a more electron donating Lewis base is used, in the
order OP(NMe2)3 > OP(p-Tol)3 > OPPh3 > no base (Fig. 2).
In order to study the relative electrophilicities of the Lewis
acid–base pairs, [R3POSiMe3][I], they were independently syn-
thesized by the addition of the Me3Si-I to a toluene solution of
R3PvO. In each case, a white powder immediately precipitated
An initial reaction to test our hypothesis was performed by
adding the Lewis base, OPPh3, to the previously reported reac-
tion of 1-PDI and two equivalents of Me3SiI (eqn (2)). Without
the Lewis base, the reaction gradually proceeds over 24 hours,
and 2-PDI is isolated as a red solid. With the Lewis base,
however, it was observed that the reaction went to completion
more quickly (∼4 hours) and led to the isolation of an off-
white powder following work-up. Phosphine oxides, even in
large excess, were not found to react with 1-PDI.
1H NMR spectroscopic analysis of the product revealed a
paramagnetically shifted and broadened spectrum with four
resonances ranging from −16.58 to 55.83 ppm. The furthest
downfield resonance is an intense singlet (18H) reminiscent of
that observed by Ephritikhine and co-workers. They reported
that both the addition of Me3SiC5R5 (R
= H, Me) to
Table 1 Structural parameters of 2-OPPh3, 4-OPPh3, 5-OPPh3, and
UO2I2(THF)3 or [UO2Cl2(THF)2]2 and the reaction of UX4 with
NaOSiMe3 gave downfield signals (reported range: 50–60 ppm)
hypothesized to arise from the –OSi(CH3)3 group of a uranium(IV)
siloxide.17 Analysis of the off-white solid by infrared spec-
6-OPPh3
Bond (Å) or
angle (°)
2-OPPh3
X = I
4-OPPh3
X = Cl
5-OPPh3
X = SPh
6-OPPh3
X = OTf
troscopy confirmed the presence of both –SiMe3 and OPPh3 U–X
3.1222(7)
—
2.082(6)
—
2.333(6)
1.500(7)
180.00
167.4(5)
2.677(2)
2.669(2)
2.112(5)
—
2.285(6)
1.527(6)
180.00
171.0(3)
2.7812(19)
—
2.113(5)
—
2.348(5)
1.503(5)
94.10(9)
162.8(4)
2.430(5)
2.429(5)
2.053(6)
2.066(6)
2.339(4)
1.506(4)
141.67(17)
178.1(3)
173.9(3)
U–X
functional groups with respective absorptions at δSiCH = 839
3
U–O(SiR3)
(asym) and νPvO = 1070 cm−1. Additionally, analysis of organ-
ics extruded from the reaction mixture showed both an equi- U–O(PPh3)
valent of [MesPDIMe 0
and one-half equivalent of (C5Me5)2,
analogous to that previously observed in the conversion of
]
O–P
X–U–X
U–O–Si
1-PDI to (Me3SiO)UI3(THF)3, suggesting a two electron reduction U–O–Si
3112 | Dalton Trans., 2016, 45, 3111–3119
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Fig. 1 Molecular structures of 2-OPPh3, 4-OPPh3, 5-OPPh3, and 6-OPPh3, left to right, shown at 30% probability ellipsoids. Hydrogen atoms and
co-crystallized solvent molecules have been omitted for clarity.
basicity), the 29Si resonance shifts upfield accordingly. The
strongly donating HMPA results in the greatest upfield shift of
the salt adducts at 26.57 ppm. Not surprisingly, only a
minimal shift is observed between the phenyl and p-tolyl ana-
logues (Δ = 1.42 ppm). This 29Si NMR trend supports that
observed by Bassindale and Stout where salts of the form
[(Rpyr)SiMe3][OTf] (Rpyr
[R3POSiMe3][OTf] (R
=
substituted pyridines) and
=
Ph, NMe2) displayed increasingly
upfield 29Si NMR chemical shifts with increasing Lewis basi-
city.16 Spialter and co-workers noted via Hammett plots of 29Si
NMR chemical shifts constructed for (XC6H4)SiY3 (Y = F, H,
OC2H5, CH3, Cl) that compounds with electron-donating silyl
substituents (e.g. (XC6H4)SiH3 and (XC6H4)Si(CH3)3) showed
upfield shifts.19 Therefore, the trend in the upfield shift in the
29Si resonance of the [R3POSiMe3][I] salts can be attributed to
increased electrophilicity of the silicon atom as R becomes
more electron donating.
With a general understanding of the influence of Lewis
basicity of the phosphine oxide on Me3SiI activation, we
sought to extend this to more mild silylating reagents. This
would be advantageous, as previous attempts at silylation of
1-PDI with other silanes resulted in either (a) no reaction
(Me3Si-SPh, Ph3Si-Cl, iPr3Si-Cl) or (b) intractable uranium
products (Me3Si-Cl, Me3Si-OTf). Thus, the reactions of these
silanes with 1-PDI were re-examined in the presence of
triphenylphosphine oxide. While HMPA is the most basic
phosphine oxide, it often afforded greasy uranium products,
which are more difficult to isolate.
Fig. 2 Competition studies (toluene, 25 °C) showing the disappearance
(λ = 612 nm) of 1-PDI during the reductive silylation reaction with
various phosphine oxide derivatives.
and was quantitatively collected by vacuum filtration. For each
salt, the environment of the silicon atom was assessed via 29Si
NMR spectroscopy (CD2Cl2, 25 °C, Fig. 3). It was observed that
with increasing electron donation from the Lewis base (higher
In the presence of OPPh3, reductive silylation of 1-PDI with
Me3SiCl was found to proceed to completion over four hours,
affording (Me3SiO)2UCl2(OPPh3)2 (3-OPPh3), analogous to
2-OPPh3. As is typical, the presence of (Cp*)2 and neutral
[
MesPDIMe]0 were detected by 1H NMR spectroscopy, as well as a
resonance attributed to a –OSiMe3 substituent (δ = 49.40 ppm).
The presence of phosphine oxide ligands was confirmed by
IR (νPvO = 1081 cm−1) and 1H NMR spectroscopies, which
showed three upfield resonances (−16.76, 2.72, 4.38 ppm),
similar to 2-OPPh3.
Fig. 3 29Si NMR spectroscopic data (CD2Cl2, 25 °C), in ppm, showing
the upfield trend in shifts of the silicon that track with relative electro-
philicity. Me3SiI is included as a reference value.
With isolation of 3-OPPh3, multiple attempts under a
variety of conditions to isolate the pyridine(diimine) adduct
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i
were made to no avail, despite full consumption of Me3SiCl. 1-PDI with Pr3Si-Cl, no uranyl activation or consumption was
The addition of OPPh3 to the reaction mixture induces the observed in the presence of two equivalents of OPPh3. Despite
equilibrium described in eqn (1), but unlike for Me3Si-I, the the potential ability of the electron donating isopropyl substi-
inferiority of chloride as a leaving group results in only weak tuents to better stabilize silylium nature in the cation, the
activation. Keq was determined to be 3.8 0.3 (CD2Cl2, 25 °C) equilibrium in eqn (1) is likely barred by the inability to
1
by H NMR studies (Table S1†). In the conversion of alcohols perform a nucleophilic attack at Si. This also explains the lack
to chlorides using catalytic DMSO, Snyder suggests the reac- of reactivity between 1-PDI and iPr3Si-Cl, while its smaller
tion between DMSO and Me3Si-Cl forms [Me3SiOSMe2][Cl] counterpart, Me3Si-Cl, reacts readily.
with complete consumption of each reactant (Keq ≫ 1).20 In
that case, the increased electrophilicity of the Si in
[Me3SiOSMe2][Cl] facilitates nucleophilic attack by an alcohol.
Although a weaker activation of silylhalides by OPPh3 is
noted in our system as compared to the more basic DMSO, it
ð3Þ
is enough to engage the equilibrium displayed (eqn (1)) facili-
tating the reactivity of Me3SiCl with 1-PDI to afford 3-OPPh3 in
high yields and under mild conditions in short reaction times.
The reactivity of 1-PDI was investigated with Ph3SiCl, which
To investigate the generality of the Lewis base assisted
is typically difficult to activate due to its poor electron-donat- uranyl reductive silylation with R3Si-X type reagents, variation
ing R-substituents that could potentially inhibit the equili- of the leaving group, X, was also examined. Reaction of 1-PDI
brium to form [B-SiPh3][Cl]. Stirring a toluene solution of with two equivalents of Me3SiSPh in the presence of two
1-PDI with two equivalents of Ph3SiCl over seven days did not equivalents of OPPh3 resulted in the gradual conversion to
result in any consumption of the silylhalide. Repeating the (Me3SiO)2U(SPh)2(OPPh3)2 (5-OPPh3), after ∼2 days. Character-
1
reaction with two equivalents of OPPh3 resulted in gradual ization of the yellow powder by H NMR spectroscopy revealed
consumption of Ph3SiCl with full conversion to (Ph3SiO)2- seven paramagnetically shifted resonances ranging from
UCl2(OPPh3)2 (4-OPPh3) over ca. five days. Following work-up, −17.95 to 51.93 ppm. The –OSiMe3 protons are furthest down-
1
analysis of the off-white powder, 4-OPPh3, by H NMR spectro- field, while again, all equatorial based ligands are shifted
scopic analysis revealed two sets of three resonances corres- upfield with the ortho protons of the –SPh (−9.72 ppm) and
ponding to the phenyl substituents of both –OSiPh3 (furthest –OPPh3 (−17.95 ppm) affected to the greatest extent.
downfield) and OPPh3 (furthest upfield).
Structural parameters of 5-OPPh3 were obtained by X-ray
Structural confirmation of 4-OPPh3 was achieved by the diffraction analysis of single crystals that precipitated from a
analysis of single crystals obtained from a concentrated THF benzene solution at room temperature (Table 1, Fig. 1). Refine-
solution (−35 °C) by X-ray diffraction (Table 1, Fig. 1). Refine- ment of the data revealed an octahedral uranium trans-bis(tri-
ment of the data revealed an octahedral uranium dichloride methylsiloxide) (U1–O1 = 2.113(5) Å; 174.2(3)°) complex with
(U1–Cl1 = 2.677(2); U1–Cl2 = 2.669(2) Å; Cl1–U1–Cl2 = 180.00°) cis-phenylthiolate (94.10(9)°) and cis-triphenylphosphine oxide
species with both trans-triphenylsiloxide and trans-OPPh3 (U1–O2 = 2.348(5) Å; 86.9(3)°) ligands. The U–S distances
(U1–O2 2.285(6) Å) ligands. The triphenylsiloxide–uranium (2.7812(19) Å) are similar to those reported for the terminal
distances in 4-OPPh3 (2.112(5) Å) are significantly longer –SPh ligands in [U(SPh)2(μ2-SPh)2(MeCN)2]2 (2.813(2) Å)22
than the only other crystallographically reported uranium bis reported by Neu and co-workers and the U1–S1–C10 bond
(triphenylsiloxide) complexes. Hayton and co-workers syn- angle (108.0(3)°) is similar to other cis-thiolate compounds
thesized two uranium(V) complexes, U(OSiPh3)2(dbm)2(OTf) including Cp2*UðSCH3Þ2 (108.1(5)°).23
(dbm = OC(Ph)CHC(Ph)O) (2.005(2), 2.018(2) Å) and [U(OSi-
Ph3)2(Aracnac)2][OTf] (Aracnac
ArNC(Ph)CHC(Ph)O; Ar
The formation of 5-OPPh3 via Lewis base promoted reduc-
tive silylation of 1-PDI illustrates that halide leaving groups are
=
=
3,5-tBu2C6H3) (2.044(2) Å), by reductive silylation of the parent not a requirement for the equilibrium in eqn (1) to be present.
uranyl species using two equivalents of Ph3Si-OTf.12
Hayton and co-workers have used highly reactive R3Si-OTf (R =
With the ability of OPPh3 to assist the reaction between the Ph, Me) to achieve reductive silylation of their systems.24 The
electron deficient silane, Ph3Si-Cl, and 1-PDI, we sought to reactivity of 1-PDI with Me3Si-OTf was investigated to continue
determine whether a sterically bulky silane would prevent the silane screen and to serve as a basis for comparison. Treat-
Lewis base coordination and therefore stop reactivity. Piers ing a toluene solution of 1-PDI and two equivalents of OPPh3
and co-workers observed that Lewis acid activation (B(C6F5)3) with two equivalents of Me3Si-OTf results in rapid quenching
of tertiary silanes for catalytic hydrosilylation of ketones did of the blue colour of 1-PDI and formation of an off-white
not proceed when iPr3SiH was employed because the LUMO of powder following workup. Investigation of the product by
the iPr3Si–H–B(C6F5)3 adduct lies along the Si–H–B axis (acces- 1H NMR spectroscopy revealed a spectrum nearly identical to
sible lobe located trans to the Si–H bond) and is sterically inac- 2-OPPh3 with four paramagnetically broadened resonances
cessible to nucleophilic attack.21 When the system was applied with the furthest downfield (57.02 ppm) assigned to equivalent
to uranyl reductive silylation, Hayton and co-workers found Me3SiO– protons and the most upfield resonance
iPr3Si-H was also unreactive.11 Analogously, upon reacting (−16.34 ppm) assigned to the ortho protons of equatorially
3114 | Dalton Trans., 2016, 45, 3111–3119
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bound OPPh3 ligands. Confirmation of a –OTf was confirmed reported here despite the commonality of 31P NMR characteriz-
by a resonance at −111.20 ppm in the 19F NMR spectrum, ation of uranium(IV) complexes.27 We attribute this absence
1
leading to the assignment of the powder as (Me3SiO)2- to extreme signal broadening, as the H NMR signals observed
U(OTf)2(OPPh3)2 (6-OPPh3).
are all display very large peak widths at half-height. Evans and
Interestingly, repeating the reaction absent OPPh3 results in coworkers have recently explored the utility of 29Si NMR
intractable products with full consumption of Me3Si-OTf, con- resonances of uranium complexes as they relate to uranium oxi-
sistent with its highly reactive nature. In aromatic solvents, dation state.28 Unfortunately, 29Si NMR resonances were
Me3SiOTf is known to react completely with OPPh3 to form undetectable for complexes 2-OPPh3–6-OPPh3, though it is
[Ph3POSiMe3][OTf]. The formation of this salt likely tempers unclear whether this is a result of paramagnetic broadening or
the silane reactivity allowing for a more controlled attack of if a potential signal is being obstructed by the inherent glass
2+
the silyl group by the UO2 moiety. Thus, the Lewis base resonance.
adducts of silyl-halides/psuedohalides can be employed to
modulate the reactivity of less electrophilic (Me3Si-Cl, Me3Si- be correlated to the silyl reagent activation by OPPh3 as indi-
SPh) or highly reactive (Me3Si-OTf) silanes (Table 1, Fig. 1). cated by the equilbrium constant. In the case of Me3SiX (X = I,
The ease of the observed reductive silylation chemistry can
Once again, single crystal X-ray crystallography was used to OTf), complete Si–X dissociation was noted with isolation of
assess the structural environment around uranium. Refine- the ion pair salts possible, and uranyl reductive silylation pro-
ment of the data obtained from crystals grown from a concen- ceeds readily, in less than 4 hours. For Me3SiCl and Me3SiSPh,
trated THF/n-pentane (1 : 1) solution revealed
a
seven respective equilibrium constants of 3.8 0.3 and 1.3 0.1 were
coordinate uranium trans-trimethylsiloxide (176.85(17)°) measured. In these cases, reductive silylation proceeds over a
complex with the equatorial plane comprised of two trifluoro- longer scale, requiring reaction times of 4 and 46.5 hours,
methanesulfonate ligands, two THF molecules (U1–O10 = respectively. For Ph3SiCl, a weak equilibrium constant is sus-
2.480(5); U1–O11 = 2.461(4) Å), and a single OPPh3 (U1–O3 = pected (Keq ≪ 1), and is consistent with the 115 h reaction
2.339(4) Å). The U–OSiMe3 distances (U1–O1 = 2.066(6); U1–O2 time required for completion of the reaction.
= 2.053(6) Å) are shorter than 2-OPPh3 and 5-OPPh3, but are
Vibrational spectroscopic measurements of 2-OPPh3–6-
similar to (Me3SiO)2UI2(2,2′-bipyridyl)2 (2.084(4) Å).10 The pair OPPh3 all display little variation in the extent of reduction of the
of η1-OTf ligands displays long uranium–oxygen contacts phosphorus–oxygen double bond of the OPPh3 ligands, with
(U1–O4 = 2.430(5); U1–O7 = 2.429(5) Å), but are not dissimilar values ranging from 1066–1082 cm−1 (Table 2). Their magnitude
to other reported uranium(IV) trifluoromethanesulfonates is on the order of those reported for other uranium complexes
including (COT)U(OTf)2(OPPh3)2 (2.382(4), 2.433(4) Å)25 and bearing OPR3 ligands, regardless of oxidation state.29
((tBuO)3SiO)3U(OTf)(THF)2 (2.421(13) Å).26 Crystallization from
Electronic absorption spectroscopic data was acquired for
THF caused displacement of one OPPh3 by two THF the entire series of compounds between 300–2100 nm, and
molecules; however, integration of resonances by 1H NMR each displays similar features (Fig. 4). All spectra display
spectroscopy clearly indicated two OPPh3 ligands per uranium sharp, weakly intense (ε = 5–70 M−1 cm−1) f–f transitions
in analogy to 2-OPPh3–5-OPPh3.
throughout the near-infrared region and into the visible
As in the reductive silylation of 1-PDI, formation of region, characteristic of the uranium(IV) oxidation state.30
2-OPPh3–6-OPPh3 proceeds by the reduction of uranium(VI) to Light yellow 5-OPPh3 displays the lowest energy absorbance
uranium(IV), with electrons provided from both [MesPDIMe 1−
]
(λmax = 373 nm), which tails into the visible region. The other
and Cp*-anion. The resulting six-coordinate uranium(IV) com- four complexes are colourless with the three halide com-
plexes show similar NMR spectroscopic characteristics, with pounds displaying strong ultraviolet features, while 6-OPPh3
the trans-siloxide protons appearing as the most downfield does not absorb in the experimental window (Table 2).
shifted resonances, independent of the substituents (Me, Ph).
Cleavage of oxo-derived –OSiR3
Likewise, ligands situated in the equatorial plane always
1
display resonances in the H NMR spectrum shifted upfield of With the variety of examples that have now been developed to
their diamagnetic reference value regardless if they are due to achieve the oxo-silylation of UO22+, relatively few examples
the neutral OPPh3 or an anionic X-ligand (Table 2). Additionally, exist of the complete cleavage of the oxo derived –OSiR3 substi-
no 31P NMR signals were detected for any of the compounds tuent on reasonable scales to produce viable materials.12,31,32
Table 2 1H NMR, infrared, and electronic absorption spectroscopic data for 2-OPPh3, 3-OPPh3, 4-OPPh3, 5-OPPh3, and 6-OPPh3
SiMe3
(ppm)
o-OPPh3
(ppm)
m-OPPh3
(ppm)
p-OPPh3
(ppm)
PvO stretch
λmax
(nm)
Complex
(cm−1
)
(Me3SiO)2UI2(OPPh3)2 (2-OPPh3)
(Me3SiO)2UCl2(OPPh3)2 (3-OPPh3)
(Ph3SiO)2UCl2(OPPh3)2 (4-OPPh3)
(Me3SiO)2U(SPh)2(OPPh3)2 (5-OPPh3)
(Me3SiO)2U(OTf)2(OPPh3)2 (6-OPPh3)
55.83
49.40
—
51.93
56.89
−16.58
−16.76
−14.15
−17.95
−16.89
3.14
2.72
3.59
2.42
3.17
4.59
4.38
4.72
4.14
4.63
1070
1081
1080
1082
1066
362
328
325
373
—
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Dalton Transactions
respectively. The reactions to cleave the uranium–siloxide
bond did not proceed when Me3Si-Cl was employed, indicat-
ing the necessity for a strong silylating reagent for this
transformation.
Finally, the trifluoromethanesulfonate species, 6-OPPh3,
was converted to the tetrakis(trifluoromethanesulfonate) com-
pound, (Ph3PO)2U(OTf)4 (7-OTf), previously described by
Ephritikhine and coworkers (Scheme 1).25 Treating a colour-
less toluene solution of 6-OPPh3 with two equivalents of
Me3Si-OTf results in an immediate colour change to mint
green with expulsion of Me3Si-O-SiMe3 (1H NMR spectroscopy,
C6D6). Investigation of the uranium species revealed both res-
onances assignable to bound OPPh3 (1H, δ = 10.66 (p), 11.89
(m), 33.36 (o) ppm) and –OTf (19F, δ = −102.65).
The formation of 7-I and 7-OTf from oxo-derived siloxide
ligands proceeds cleanly, in high yields (72–98%), with
minimal work-up. We surmise this general procedure should
be broadly applicable to generic uranium(IV) complexes con-
taining siloxide ligands for the production of useful homoleptic
tetra(halide) starting materials.
Fig. 4 Electronic absorption spectra for 2-OPPh3, 3-OPPh3, 4-OPPh3,
5-OPPh3, and 6-OPPh3. Data were collected from 300–2100 nm in THF
at ambient temperature. Solvent overtones from 1670–1760 nm have
been omitted for clarity.
We have recently demonstrated from the reductive silylation of
1-PDI that this is possible.14 Thus, we targeted a facile, high
yielding method for the formation of uranium tetra(halides) or
psuedohalides from the uranium(IV) products obtained via
Lewis base mediated reductive silylation.
Treating a toluene slurry of 2-OPPh3 with two equivalents of
Me3Si-I results in the gradual deposition of a yellow powder
identified as (Ph3PO)2UI4 (7-I) by 1H NMR spectroscopy
(Scheme 1).18 Repeating the reaction in C6D6 confirmed the
formation of Me3Si-O-SiMe3. This reaction, when performed in
THF, neither produced Me3Si-O-SiMe3 nor 7-I, but rather an
oily uranium containing compound suspected to arise from
THF ring opening as well as the organic byproduct, Me3SiO-
(CH2)4-I.
Experimental
General considerations
All air- and moisture-sensitive manipulations were performed
using standard Schlenk techniques or in an MBraun inert
atmosphere drybox with an atmosphere of purified nitrogen.
The MBraun drybox was equipped with a cold well designed
for freezing samples in liquid nitrogen as well as two −35 °C
freezers for cooling samples and crystallizations. Solvents for
sensitive manipulations were dried and deoxygenated using lit-
erature procedures with a Seca solvent purification system.33
Benzene-d6 was purchased from Cambridge Isotope Labora-
tories, dried with molecular sieves and sodium, and degassed
by three freeze–pump–thaw cycles while CDCl3 and CD2Cl2
were distilled from CaH2. Me3Si-I, Me3Si-OTf, Me3Si-SPh,
Ph3Si-Cl, iPr3Si-Cl, and OPPh3 were used as received from
Sigma-Aldrich. Me3Si-Cl and HMPA were distilled from CaH2.
Thus, a general method to convert the chloride containing
species, 3-OPPh3 and 4-OPPh3, to 7-I was also established.
While complexes of the type UI4−nXn (X = Br, Cl) exist and have
been studied, their use as a viable starting material is impracti-
cal because of the facile halide exchange results in every poss-
ible halide combination.18 Therefore, toluene slurries of each
3-OPPh3 and 4-OPPh3 were treated with four equivalents of
Me3Si-I, resulting in the slow deposition of insoluble 7-I.
1-PDI 14 and OP(p-Tol)3 were synthesized according to litera-
ture procedures.
34
1H NMR spectra were recorded on a Varian Inova 300
spectrometer operating at 299.992 MHz. All chemical shifts are
reported relative to the peak for SiMe4, using 1H (residual)
chemical shifts of the solvent as a secondary standard. The
spectra for paramagnetic molecules were obtained by using an
acquisition time of 0.5 s, thus the peak widths reported have
an error of 2 Hz. For paramagnetic molecules, the 1H NMR
data are reported with the chemical shift, followed by the peak
width at half height in Hertz, the integration value, and, where
possible, the peak assignment. 31P spectra were recorded on a
Varian Inova 300 spectrometer operating at 121.423 MHz and
chemical shifts are reported relative to 85% H3PO4. 29Si
spectra were recorded on a Bruker AV-III-500-HD spectrometer
operating at 99.38 MHz with an approximate 30-degree observe
pulse, acquisition time of 0.8 s, and a relaxation time of
1
Monitoring the reaction by H NMR spectroscopy revealed the
formation of two equivalents of Me3Si-Cl as well as both
Me3Si-O-SiMe3 and Me3Si-O-SiPh3 for 3-OPPh3 and 4-OPPh3,
Scheme 1 Cleavage of oxo derived –OSiR3 substituents.
3116 | Dalton Trans., 2016, 45, 3111–3119
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6 s. Elemental analyses were performed by Complete Analysis which (C5Me5)2 and MesPDIMe were identified by 1H NMR
Laboratories, Inc., Parsippany, NJ. Electronic absorption spectroscopy.
measurements were recorded at 294 K in sealed 1 cm quartz
cuvettes with a Jasco V-6700 spectrophotometer.
Synthesis of (Ph3SiO)2UCl2(OPPh3)2 (4-OPPh3). A 20 mL
scintillation vial was charged with (0.200 g, 0.249 mmol)
Single crystals suitable for X-ray diffraction were coated Cp*UO2(MesPDIMe) (1-PDI) and 10 mL of toluene. Triphenyl-
with poly(isobutylene) oil in a glovebox and quickly transferred phosphine oxide (0.138 g, 0.496 mmol) was added and stirred
to the goniometer head of a Rigaku Rapid II image plate for 5 min. Chlorotriphenylsilane (0.147 g, 0.499 mmol) was
diffractometer equipped with a MicroMax002+ high intensity added and stirred for 115 h resulting in a gradual colour
copper X-ray source with confocal optics and examined change from blue to purple. Volatiles were removed in vacuo.
with Cu Kα radiation (λ = 1.54184 Å). The data were collected The crude mixture was washed with n-pentane (5 × 10 mL) and
at 200(1) K.
the washings set aside. The remaining solid was washed with
Synthesis of (Me3SiO)2UI2(OPPh3)2 (2-OPPh3). A 20 mL benzene (10 mL) to afford off-white powder upon drying
scintillation vial was charged with (0.100 g, 0.125 mmol) (0.171 g, 0.145 mmol, 58%) assigned as (Ph3SiO)2UCl2(OPPh3)2
Cp*UO2(MesPDIMe) (1-PDI) and 10 mL of toluene. Triphenyl- (4-OPPh3). Single, X-ray quality crystals were obtained from a
phosphine oxide (0.070 g, 0.252 mmol) was added and stirred concentrated tetrahydrofuran solution at −35 °C. Elemental
for 5 min. Via microsyringe, Me3SiI (0.035 mL, 0.245 mmol) analysis of C72H60O4Si2P2Cl2U: Calculated, C, 61.05; H, 4.28;
was added resulting in a gradual colour change from blue to N, 0.00. Found, C, 60.99; H, 4.03; N, <0.02. 1H NMR (CDCl3,
purple to purple/red. After 4 h, volatiles were removed in vacuo. 25 °C) δ = −14.15 (32, 12H, o-TPPO-CH), 3.59 (20, 12H,
The crude mixture was washed with n-pentane (3 × 10 mL) and m-TPPO-CH), 4.72 (38, 6H, p-TPPO-CH), 11.83 (17, 6H, p-SiPh3-
the washings set aside. The remaining light red solid was CH), 12.55 (18, 12H, m-SiPh3-CH), 41.19 (23, 12H, o-SiPh3-CH).
recrystallized from dilute THF layered with pentane (−35 °C) to IR: ν(vO) = 1080 cm−1. UV-Vis: λmax = 325 nm, ε = 1396 M−1
afford off-white powder (0.126 g, 0.103 mmol, 82%) assigned cm−1. The solvent from the pentane washings was removed
as (Me3SiO)2UI2(OPPh3)2 (2-OPPh3). Single, X-ray quality in vacuo to afford a slurry from which (C5Me5)2 and MesPDIMe
crystals were obtained from
(1 : 1) solution at room temperature. Elemental analysis of
a
dilute toluene/n-pentane were identified by 1H NMR spectroscopy.
Synthesis of (Me3SiO)2U(SPh)2(OPPh3)2 (5-OPPh3). A 20 mL
C42H48O4Si2P2I2U: Calculated, C, 41.12; H, 3.95; N, 0.00. scintillation vial was charged with (0.200 g, 0.249 mmol)
1
Found, C, 41.17; H, 3.95; N, <0.02. H NMR (CDCl3, 25 °C) δ = Cp*UO2(MesPDIMe) (1-PDI) and 10 mL of toluene. Triphenyl-
−16.58 (91, 12H, o-Ph-CH), 3.14 (44, 12H, m-Ph-CH), 4.59 (38, phosphine oxide (0.138 g, 0.496 mmol) was added and
6H, p-Ph-CH), 55.83 (111, 18H, Si(CH3)3). IR: δ(SiCH
=
stirred for 5 min. Trimethyl(phenylthio)silane (0.094 mL,
sym)
3
1245 cm−1; ν(PvO) = 1070 cm−1; δ(SiCH asym) = 839 cm−1. UV-Vis: 0.496 mmol) was added via microsyringe resulting in a gradual
3
λmax = 362 nm, ε = 2205 M−1 cm−1. The solvent from the colour change from blue to dark purple. After 46.5 h, volatiles
pentane washings was removed in vacuo to afford a slurry from were removed in vacuo. The crude mixture was washed with
which (C5Me5)2 and MesPDIMe were identified by 1H NMR n-pentane (5 × 10 mL) until the washing ran clear and the wash-
spectroscopy.
ings set aside. The remaining solid was recrystallized from a
Synthesis of (Me3SiO)2UCl2(OPPh3)2 (3-OPPh3). A 20 mL THF solution layered with an equal amount of n-pentane
scintillation vial was charged with (0.121 g, 0.151 mmol) (−35 °C) to afford light yellow block shaped crystals (0.080 g)
Cp*UO2(MesPDIMe) (1-PDI) and 10 mL of toluene. Triphenyl- assigned as (Me3SiO)2U(SPh)2(OPPh3)2 (5-OPPh3). Subsequent
phosphine oxide (0.084 g, 0.302 mmol) was added and stirred recrystallization of the mother liquor afforded an additional
for 5 min. Via microsyringe, chlorotrimethylsilane (0.039 mL, 0.115 g (0.195 g total, 0.161 mmol, 65%) of (Me3SiO)2-
0.307 mmol) was added resulting in a gradual colour change U(SPh)2(OPPh3)2 as yellow powder. Single, X-ray quality crystals
from blue to purple over 30 min. After 4 h, volatiles were were obtained from a concentrated benzene solution stored at
removed in vacuo. The crude mixture was washed with room temperature. Elemental analysis of C54H58O4Si2P2S2U:
n-pentane (3 × 15 mL) and the washings set aside. The remain- Calculated, C, 53.64; H, 4.84; N, 0.00. Found, C, 53.81; H, 4.79;
ing solid was recrystallized from a concentrated THF solution N, <0.02. 1H NMR (C6D6, 25 °C) δ = −17.95 (54, 12H,
layered with n-pentane (−35 °C) to afford off-white powder o-TPPO-CH), −9.72 (35, 4H, o-SPh-CH), 2.42 (27, 12H,
(0.081 g, 0.078 mmol) assigned as (Me3SiO)2UCl2(OPPh3)2 m-TPPO-CH), 4.14 (26, 6H, p-TPPO-CH), 5.07 (24, 4H, m-SPh-CH),
(3-OPPh3). Further recrystallization of the mother liquor 6.12 (25, 2H, p-SPh-CH), 51.93 (77, 18H, CH3). IR: δ(SiCH
=
sym)
3
afforded an additional 0.072 g of off-white powder (0.153 g 1243 cm−1; ν(PvO) = 1082 cm−1; δ(SiCH asym) = 835 cm−1. UV-Vis:
3
total, 0.147 mmol, 97%). Elemental analysis of C42H48O4Si2P2- λmax = 373 nm, ε = 986 M−1 cm−1. The solvent from the
Cl2U: Calculated, C, 48.31; H, 4.64; N, 0.00. Found, C, 48.46; pentane washings was removed in vacuo to afford a slurry from
1
H, 4.72; N, <0.02. H NMR (C6D6, 25 °C) δ = −16.76 (308, 12H, which (C5Me5)2 and MesPDIMe were identified by 1H NMR
o-TPPO-CH), 2.72 (251, 12H, m-TPPO-CH), 4.38 (222, 6H, spectroscopy.
p-TPPO-CH), 49.40 (561, 18H, CH3). IR: δ(SiCH sym) = 1243 cm−1
;
Synthesis of (Me3SiO)2U(OTf)2(OPPh3)2 (6-OPPh3). A 20 mL
scintillation vial was charged with (0.200 g, 0.249 mmol)
3
ν(PvO) = 1081 cm−1; δ(SiCH
= 836 cm−1. UV-Vis: λmax
=
asym)
3
328 nm, ε = 635 M−1 cm−1. The solvent from the pentane Cp*UO2(MesPDIMe) (1-PDI) and 10 mL of toluene. Triphenyl-
washings was removed in vacuo to afford a slurry from phosphine oxide (0.139 g, 0.499 mmol) was added and stirred
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for 5 min. Via microsyringe, trimethylsilyl trifluoromethane- (19, 12H, m-TPPO-CH), 33.30 (70, 12H, o-TPPO-CH). 19F NMR
sulfonate (0.090 mL, 0.497 mmol) was added resulting in a (C6D6, 25 °C) δ = −102.65 (s, 12F, CF3). IR: ν(asym
=
=
=
SO3)
gradual colour change from blue to purple over ca. 15 min. 1349 cm−1; ν(asym CF ) = 1121 cm−1; ν(PvO) = 1063 cm−1; δ(sym CF )
After 4 h, volatiles were removed in vacuo. The crude mixture 729 cm−1; δ(sym SO ) = 634 cm−1; δ(asym CF ) = 590 cm−1; δ(asym SO )
was washed with n-pentane (3 × 10 mL) and the washings set 539 cm−1
3
3
3
3
3
.
aside. The remaining solid was recrystallized from a saturated
THF solution layered with n-pentane (−35 °C) to afford off-
Formation of 7-I and 7-OTf via NMR experiments
white powder with a slight hint of green (0.151 g) assigned as A J-Young NMR tube was charged with (R3SiO)2UX2(OPPh3)2
(Me3SiO)2U(OTf)2(OPPh3)2 (6-OPPh3). Further recrystallization and 0.75 mL C6D6. To the slurry, Me3SiI (for X = I, 2 eq.; X =
of the mother liquor afforded an additional 0.062 g (0.213 g Cl, 4 eq.) or Me3Si-OTf (for X = OTf) was added via micro-
total, 0.168 mmol, 79%). Single, X-ray quality crystals were syringe. The slurry in the tube was shaken for fifteen minutes
obtained from a concentrated tetrahydrofuran/n-pentane (1 : 1) and the 1H NMR spectrum obtained.
solution at −35 °C. Elemental analysis of C44H48O10Si2P2S2F6U:
Calculated, C, 41.57; H, 3.81; N, 0.00. Found, C, 41.43; H, 3.53;
N, <0.02. 1H NMR (C6D6, 25 °C) δ = −16.34 (40, 12H, Conclusions
o-TPPO-CH), 2.64 (25, 12H, m-TPPO-CH), 3.87 (25, 6H,
In summary, the studies presented demonstrate that uranyl
functionalization, which has typically been a challenging
p-TPPO-CH), 57.05 (45, 18H, CH3). 19F NMR (C6D6, 25 °C) δ =
−111.20 (s, 6F, CF3). IR: ν(SO
= 1333 cm−1; δ(SiCH
=
sym)
asym)
3
3
transformation due to the high uranium–oxygen multiple
bond strength, can be facilitated by Lewis bases. The substi-
tuted phosphine oxides used here are commercially available
and operationally simple to use, serving to activate less reactive
silylating reagents towards nucleophilic attack by uranyl oxo
ligands, thus broadening the scope of the family of reagents
that can be used. This is evidenced by the fact that Me3Si-SPh,
Me3SiCl, and Ph3SiCl, which are effective in our Lewis base
promoted system, have not been demonstrated previously to
be successful for stoichiometric uranyl functionalization. In
some cases, we have also demonstrated that phosphine oxides
can mediate highly reactive silanes, thus directing their reactiv-
ity toward the desired goal. During the course of the silylation
reaction, reduction of the central uranium from 6+ to 4+
occurs, with both the MesPDIMe and Cp* ligands serving as
electron sources. Once addition across the strongly bonded
OvUvO unit has been accomplished, the weaker U–O single
bonds can now be easily cleaved with the stoichiometric
addition of Me3SiI.
1241 cm−1; ν(CF
= 1236 cm−1; ν(CF
= 1124 cm−1
;
=
sym)
asym)
3
3
ν(PvO) = 1066 cm−1; ν(SO
= 1017 cm−1; δ(SiCH
sym)
asym)
3
3
864 cm−1; δ(CF sym) = 748 cm−1; δ(SO sym) = 636 cm−1; δ(CF
asym)
3
3
3
= 584 cm−1; δ(SO
= 539 cm−1. The solvent from the
asym)
3
pentane washings was removed in vacuo to afford a slurry from
which (C5Me5)2 and MesPDIMe were identified by 1H NMR
spectroscopy.
Lewis base competition experiments by UV-Vis
From stock toluene solutions, Me3SiI and Lewis base (1.25 ×
10−7 moles each) were added to a 3 mL screw-top cuvette
equipped with an injectable septum. The mixture was diluted
with toluene to 3.00 mL and mixed for 30 min. At the spectro-
meter, 1-PDI (6.25 × 10−8 moles, toluene stock solution) was
injected into the cuvette. The cuvette was shaken violently for
3 s prior to data collection.
General synthesis of (Ph3PO)2UI4 (7-I) from
(R3SiO)2UX2(OPPh3)2
From 2-OPPh3: a 20 mL scintillation vial was charged with
2-OPPh3 (0.100 g, 0.082 mmol) and 5 mL of toluene. To the
stirring slurry, Me3SiI (0.023 mL, 0.162 mmol) was added via
microsyringe resulting in a gradual colour change to a yellow
suspension from off-white (note: for X = Cl, four equivalents of
Me3SiI were used). After 4 h, the volatiles were removed
in vacuo. The resulting solid was washed with n-pentane to
afford yellow powder (0.102 g, 0.078 mmol, 96%) identified
as (Ph3PO)2UI4. Yields from 3-OPPh3, 72%; 4-OPPh3, 98%;
5-OPPh3, 74%.
Acknowledgements
The authors acknowledge support from the Division of Chemi-
cal Sciences, Geosciences, and Biosciences, Office of Basic
Energy Sciences, Heavy Elements Chemistry Program of the
U.S. Department of Energy through Grant DE-AC0212ER16328
(SCB).
Synthesis of (Ph3PO)2U(OTf)4 (7-OTf). A 20 mL scintillation
vial was changed with 6-OPPh3 (0.100 g, 0.079 mmol) and
Notes and references
5
mL of toluene. While stirring, Me3SiI (0.029 mL,
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colour change from colourless to mint green. After 30 min,
volatiles were removed in vacuo. The resulting solid was
washed with n-pentane to afford light green powder (0.098 g,
0.070 mmol, 90%) assigned as (Ph3PO)2U(OTf)4. 1H NMR
(C6D6, 25 °C) δ = 10.69 (t, J = 5.4, 6H, p-TPPO-CH), 11.91
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3118 | Dalton Trans., 2016, 45, 3111–3119
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