Expanding yttrium bis(trimethylsilylamide) chemistry through the reaction chemistry of (N2)2-, (N2)3-, and (NO)2- complexes

Article abstract of DOI:10.1021/ic301778q

The reaction chemistry of the side-on bound (N₂)^2-, (N₂)^3-, and (NO)^2- complexes of the [(R ₂N)₂Y]⁺ cation (R = SiMe₃), namely, [(R₂N)₂(THF)Y]₂(μ- η²: η²-N₂), 1, [(R₂N)₂(THF)Y] ₂(μ- η²:η²-N₂)K, 2, and [(R₂N)₂(THF)Y]₂(μ- η²: η²-NO), 3, with oxidizing agents has been explored to search for other (E₂)^n-, (E = N, O), species that can be stabilized by this cation. This has led to the first examples for the [(R ₂N)₂Y]⁺ cation of two fundamental classes of [(monoanion)₂Ln]⁺ rare earth systems (Ln = Sc, Y, lanthanides), namely, oxide complexes and the tetraphenylborate salt. In addition, an unusually high yield reaction with dioxygen was found to give a peroxide complex that completes the (N₂)^2-, (NO) ^2-, (O₂)^2- series with 1 and 3. Specifically, the (μ-O)^2- oxide-bridged bimetallic complex, [(R ₂N)₂(THF)Y}₂(μ-O), 4, is obtained as a byproduct from reactions of either the (N₂)^2- complex, 1, or the (N₂)^3- complex, 2, with NO, while the oxide formed from 2 with N₂O is a polymeric species incorporating potassium, {[(R₂N)₂Y]₂(μ-O)₂K ₂(μ-C₇H₈)}_n, 5. Reaction of 1 with 1 atm of O₂ generates the (O₂)^2- bridging side-on peroxide [(R₂N)₂(THF)Y]₂(μ- η²:η²-O₂), 6. The O-O bond in 6 is cleaved by KC₈ to provide an alternative synthetic route to 5. Attempts to oxidize the (NO)^2- complex, 3, with AgBPh₄ led to the isolation of the tetraphenylborate complex, [(R₂N) ₂Y(THF)₃][BPh₄], 7, that was also synthesized from 1 and AgBPh₄. Oxidation of the (N₂)^2- complex, 1, with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, TEMPO, generates the (TEMPO)^- anion complex, (R₂N) ₂(THF)Y(η²-ONC₅H₆Me ₄), 8.

Full text of DOI:10.1021/ic301778q

Article
pubs.acs.org/IC
Expanding Yttrium Bis(trimethylsilylamide) Chemistry Through the
Reaction Chemistry of (N2)2−, (N2)3−, and (NO)2− Complexes
Joy H. Farnaby, Ming Fang, Joseph W. Ziller, and William J. Evans*
Department of Chemistry, University of California, Irvine, California 92697-2025, United States
S
* Supporting Information
ABSTRACT: The reaction chemistry of the side-on bound (N2)2−,
(N2)3−, and (NO)2− complexes of the [(R2N)2Y]+ cation (R =
SiMe3), namely, [(R2N)2(THF)Y]2(μ-η2:η2-N2), 1, [(R2N)2(THF)-
Y]2(μ-η2:η2-N2)K, 2, and [(R2N)2(THF)Y]2(μ-η2:η2-NO), 3, with
oxidizing agents has been explored to search for other (E2)n−, (E = N, O), species that can be stabilized by this cation. This
has led to the first examples for the [(R2N)2Y]+ cation of two fundamental classes of [(monoanion)2Ln]+ rare earth systems
(Ln = Sc, Y, lanthanides), namely, oxide complexes and the tetraphenylborate salt. In addition, an unusually high yield reaction
with dioxygen was found to give a peroxide complex that completes the (N2)2−, (NO)2−, (O2)2− series with 1 and 3. Specifically,
the (μ-O)2− oxide-bridged bimetallic complex, [(R2N)2(THF)Y}2(μ-O), 4, is obtained as a byproduct from reactions of either
the (N2)2− complex, 1, or the (N2)3− complex, 2, with NO, while the oxide formed from 2 with N2O is a polymeric species
incorporating potassium, {[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5. Reaction of 1 with 1 atm of O2 generates the (O2)2− bridging
side-on peroxide [(R2N)2(THF)Y]2(μ-η2:η2-O2), 6. The O−O bond in 6 is cleaved by KC8 to provide an alternative synthetic
route to 5. Attempts to oxidize the (NO)2− complex, 3, with AgBPh4 led to the isolation of the tetraphenylborate complex,
[(R2N)2Y(THF)3][BPh4], 7, that was also synthesized from 1 and AgBPh4. Oxidation of the (N2)2− complex, 1, with the radical
trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, TEMPO, generates the (TEMPO)− anion complex, (R2N)2(THF)Y-
(η2-ONC5H6Me4), 8.
INTRODUCTION
■
Y[N(SiMe3)2]3, originally reported by Bradley in 1973,1 has
proven to be an extremely important precursor in the LnA3/M
rare earth reduction system (Ln = Sc, Y, lanthanides; A = anion;
M = alkali metal) that reduces N2 and NO.2−5 Of all the
possible Ln(NR2)3 precursors (R = SiMe3), the yttrium variant
has provided the most information as yttrium has I = 1/2
nuclear spin that is useful in NMR and EPR studies.6 In
addition, this metal appears to be of the appropriate size with
(NR2)− ligands to readily yield crystalline products character-
izable by X-ray crystallography. The Y(NR2)3/KC8 reactions
have provided not only an (N2)2− complex, [(R2N)2(THF)-
Since the rare earths are so oxophilic, the oxo, peroxo, and
Y]2(μ-η2:η2-N2),2 1, but also the first examples of complexes
even the superoxo complexes might be expected to be the most
of the (N2)3− and (NO)2− radicals, [(R2N)2(THF)Y]2(μ-η2:η2-
stable of this series. It was also expected that a (NO)− complex
N2)K,3 2, and [(R2N)2(THF)Y]2(μ-η2:η2-NO),4 3, eq 1. This
would be discovered before (NO)2− since the monoanion is the
common anionic reduced form of nitric oxide. On the other hand,
bis(amide) yttrium system also provided the first EPR evidence
of Y2+ in solution as well as the first 15N NMR information on
it is unknown, in contrast to rare earth metallocene chemistry, if a
rare earth (N2)2− and (N2H2)2− complexes.5
monoanionic (E2)− ligand can electrostatically stabilize bimetallic
species with a Ln2(μ-η2:η2-E2) core when (NR2)− is the ancillary
ligand.
In efforts to expand the scope of small molecule activation
with the Y[N(SiMe3)2]3/M system, reactions of 1−3 with
oxidizing agents have been explored. Among the targets of these
reactions were the (E2)n− ions (O2)2−, (O2)−, and (NO)−. These
known anions were conspicuous by their absence in the series of
Ln2(μ-η2:η2-E2) planar, side-on bound, reduced diatomic
complexes, 1−3, where (E2)n− = (N2)2−, (N2)3−, and (NO)2−,
respectively. Since the (N2)3− ligand has proven to be especially
effective in coupling paramagnetic lanthanide ions to form single
molecule magnets,7 expansion in this area is desirable.
Although rare earth complexes are highly reactive with oxygen-
containing species, these reactions are often complicated and can
produce mixtures of inseparable products including the insoluble
oxides. This has also been the case in this study of the reactions of
1−3 with oxidizing agents and only reactions that have provided
Received: August 13, 2012
© XXXX American Chemical Society
A
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Article
[(R2N)2(THF)Y]2(μ-O), 4. Crystals of 4 were obtained by recrys-
tallization of a hexane solution at −35 °C from the reaction of
[(R2N)2(THF)Y]2(μ-η2:η2-N2)K, 2, with 1.5 equiv of NO or from the
reaction of [(R2N)2(THF)Y]2(μ-η2:η2-N2), 1, with 2 equiv of NO. 1H
NMR (500 MHz, benzene-d6): δ 0.40 (s, 72H, N(SiMe3)2). 13C{1H}
NMR (126 MHz, benzene-d6): δ 6.8 (s, N(SiMe3)2). Numerous
attempts to prepare 4 from 1 with oxidants such as N2O, O2, pyridine
N-oxide, trimethylamine N-oxide, 1,2-epoxybutane, Ag2O, and K2O, in
reactions of 2 with N2O, in the reaction of 3 with 1,2-epoxybutane,
and in reactions of 7 (see below) with Ag2O, pyridine N-oxide, and
Na2O2 were unsuccessful.
{[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5. Crystals of 5 were obtained from
cooling a toluene solution to −35 °C of the reaction of [(R2N)2-
(THF)Y]2(μ-η2:η2-N2)K, 2, with N2O. 1H NMR (500 MHz, benzene-d6):
δ 0.41 (N(SiMe3)2). 13C{1H} NMR (126 MHz, benzene-d6): δ 6.8
(s, N(SiMe3)2). Complex 6 (below) provided a preparative route.
A solution of 6 (83 mg, 0.08 mmol) in THF (3 mL) was added to
a suspension of KC8 (22 mg, 0.16 mmol) in tetrahydrofuran (THF,
2 mL) at room temperature and stirred vigorously for 15 min. The
colorless solution was centrifuged and filtered to remove graphite, and
the solvent was removed in vacuo to yield 94 mg of a white solid from
which complex 5 (43 mg, 25%) was isolated by recrystallization from
toluene at −35 °C. IR: 2947s, 2877m, 1240s, 1185w, 1054m, 1005s,
937w, 870m, 828s, 769w, 749w, 694w, 662m, 599w, 549s, 485m cm−1.
Anal. Calcd for 5(−0.5 toluene) C27.5H76K2N4O2Si8Y2: C, 33.85; H,
7.85; N, 5.74. Found: C, 33.57; H, 7.98; N, 5.09.
reproducible, crystallographically characterizable products are
reported in detail. Although only one of the three (E2)n− targets
has been obtained in this study, the reactions of 1−3 have
provided the first yttrium bis(trimethylsilylamide) examples of
some common classes of compounds important for the
development of any type of bis(anionic ligand) rare earth complex.
Specifically, structural data on the first examples of oxides
and tetraphenylborate salts of the [(R2N)2Y]+ cation are reported.
The importance of these classes can be seen in metallocene
chemistry. For example, the identification of [(C5Me5)2Ln]2(μ-O)
complexes that are commonly observed byproducts of many
reactions provides a good gauge of the reactivity of the rare earth
metallocene complex and of availability of oxygen contaminants in
the reaction system.8 Previously, the only reported bimetallic oxide
of a [(R2N)2Ln]+ cation is the samarium complex [(R2N)2(THF)-
Sm]2(μ-O).9 The metallocene tetraphenylborates, [(C5Me5)2Ln]-
[(μ-Ph)2BPh2] have proven to be an important class through their
utility as precursors to (C5Me5)2LnX10 compounds (X = alkyl,
pseudohalide) and to the mixed metallocene complexes
(C5Me5)n(C5Me4H)3‑nLn.11 Analogous complexes in these
common classes have not been identified for the [(R2N)2Y]+
system until now.
There is also precedent for (TEMPO)− anion and peroxide
rare earth complexes, but specific [(R2N)2Y]+ derivatives were
not known. Complexes of the (TEMPO)− anion with rare
earths were known only with samarium, [(η1-ONC5H6Me4)2Sm-
(μ-η1:η2-ONC5H6Me4)]2 and [(C5Me5)(η2-ONC5H6Me4)-
SmCl]2.12 That study found the (TEMPO)− ligand to be com-
parable in steric bulk to large alkoxide ligands. Crystallo-
graphically characterized bis(amide) rare earth peroxide
complexes {[(Me3Si)2N]2Ln(L)}2(μ-η2:η2-O2) (Ln = La, L =
OPPh3;13 Ln = Yb, L = THF14) are known, but the lanthanum
complex was made from H2O2 and the ytterbium complex from
an Yb2+ precursor. The peroxide synthesis reported here in-
volving an Y3+ complex of (NN)2− and O2 is more unusual
and expands the reduced dianion chemistry of [(R2N)2Y]+ in
terms of both reactivity and composition.
[(R2N)2(THF)Y]2(μ-η2:η2-O2), 6. A pale blue, frozen, N2-saturated
solution of 1 (220 mg, 0.22 mmol) in THF (7 mL) was exposed to O2
(1 atm) and stirred at ambient temperature for 40 min. On thawing
there was an immediate color change to bright yellow. This color
persisted for 2 min and then the solution became colorless. The
solution was centrifuged and filtered to exclude a white precipitate,
and the solvent was removed in vacuo to yield 6 as a white powder
(185 mg, 84%). Crystals suitable for X-ray diffraction were grown
1
from toluene at −35 °C. H NMR (500 MHz, benzene-d6): δ 0.41
(s, 72H, N(SiMe3)2). 13C{1H} NMR (126 MHz, benzene-d6): δ 6.1
(s, N(SiMe3)2). IR: 2952s, 2897m, 1595m, 1413w, 1248s, 974s, 867sh,
831s, 774m, 753m, 670m, 609m, 477m cm−1. Raman (16O2): 767 cm−1;
(18O2): 730 cm−1 (calcd 723 cm−1). Anal. Calcd for 6(−0.5 THF)
C30H84N4O3.5Si8Y2: C, 37.57; H, 8.77; N, 5.85. Found: C, 37.14; H,
9.36; N, 5.41. Complexes 1 and 6 have been observed to undergo partial
desolvation under vacuum.2
[(R2N)2(THF)3Y][BPh4], 7. A cold (−35 °C) solution of 1 (40 mg,
0.04 mmol) in toluene (20 mL) was added to a pre-cooled, light-
protected vial containing AgBPh4 (34 mg, 0.08 mmol) and stirred at
room temperature for 48 h. The mixture was centrifuged and filtered
to remove dark solids, the solvent was removed in vacuo from the pale
yellow solution, and the product was washed with hexane to yield 7 as
a white powder (35 mg, 45%). Crystals suitable for X-ray diffraction
EXPERIMENTAL SECTION
■
All syntheses and manipulations described below were conducted
under nitrogen or argon with rigorous exclusion of air and water using
glovebox, Schlenk, and vacuum line techniques. Solvents were dried
over columns containing Q-5 and molecular sieves. Benzene-d6 and
THF-d8 were dried over sodium-potassium alloy, degassed using three
freeze−pump−thaw cycles, and vacuum transferred before use.
Potassium and sodium were purchased from Aldrich, washed with
hexanes, and scraped to provide fresh surfaces before use. NO gas was
purchased from Aldrich (98.5%) and passed through two U-shaped
glass columns connected in series and cooled to −78 °C. N2O gas
(99%) and 18O2 gas (99%) were purchased from Aldrich and used as
received. Compressed O2 gas was purchased from Airgas and was
passed through a U-shaped glass column cooled to −78 °C prior to
use. (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) was purchased
from Aldrich and sublimed in vacuo prior to use. The complexes
[(R2N)2(THF)Y]2(μ-η2:η2-N2), 1,2 [(R2N)2(THF)Y]2(μ-η2:η2-N2)K,
1
were grown from a THF solution at −35 °C. H NMR (500 MHz,
THF-d8): δ 0.19 (s, 36H, N(SiMe3)2), 6.73 (br m, 4H, B(p-C6H5)4),
6.87 (br m, 8H, B(m-C6H5)4), 7.28 (br s, 8H, B(o-C6H5)4). 13C{1H}
NMR (126 MHz, THF-d8): δ 6.0 (N(SiMe3)2), 122.1 (BPh4),
125.9(BPh4), 129.8 (BPh4), 137.3 (BPh4). IR: 3056m, 3039m,
2955s, 2899m, 1945w, 1884w, 1820w, 1764w, 1580m, 1480m,
1458w, 1427m, 1340w, 1250s, 1183w, 1136w, 1069m, 1032w, 998m,
959s, 900s, 844s, 774m, 734s, 706s, 669m, 613s cm−1. Anal. Calcd for
7(−THF) C44H72BN2O2Si4Y: C, 60.53; H, 8.31; N, 3.21. Found: C,
60.41; H, 8.13; N, 2.95.
(R2N)2(THF)Y(η2-ONC5H6Me4), 8. Addition of an orange solution
of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (25 mg, 0.16 mmol) in
toluene (2 mL) dropwise to a stirred pale blue solution of 1 (81 mg,
0.08 mmol) in toluene (5 mL) caused the immediate formation of a
colorless solution. The volume of the solution was reduced to about
2 mL, and 8·THF (48 mg, 83%) was isolated as colorless crystals at
−35 °C. Crystals suitable for X-ray diffraction were grown from
another toluene solution at −35 °C. 1H NMR (500 MHz, benzene-d6):
δ 0.43 (s, 36H, N(SiMe3)2), 1.20 (s, 12H, ONC5H6Me4), 1.35
(m, 14H, ONC5H6Me4, THF), 3.71 (m, 8H, THF). 13C{1H} NMR
(126 MHz, benzene-d6): δ 6.7 (s, N(SiMe3)2), 17.6 (ONC5H6Me4),
2,3 [(R2N)2(THF)Y]2(μ-η2:η2-NO), 3,4 AgBPh4,15 and KC8 were
16
synthesized according to the literature. H and 13C{1H} NMR spectra
1
were obtained on a Bruker CRYO 500 MHz spectrometer at 25 °C.
IR samples were prepared as KBr pellets on a Varian 1000 FT-IR
system. Elemental analyses were performed on a PerkinElmer Series II
2400 CHNS analyzer. Raman experiments were performed on
crystalline samples in a quartz cell sealed with a Teflon stopcock
with a Renishaw inVia confocal Raman Microscope using 532 nm laser
excitation (laser power 10%, laser focus 50% at 200 s exposure) and a
5 X objective lens.
B
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Table 1. X-ray Data Collection Parameters for [(R2N)2(THF)Y]2(μ-O), 4, {[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5,
[(R2N)2(THF)Y]2(μ-η2:η2-O2), 6, [(R2N)2(THF)3Y][BPh4], 7, and (R2N)2(THF)Y(η2-ONC5H6Me4), 8
4
5
6
7
8
empirical formula
formula weight
T(K)
C32H88N4Si8O3Y2
979.60
[C31H80K2N4O2Si8Y2]n
1021.73
C32H88N4O4Si8Y2·C7H8
1087.74
C48H80BN2O3Si4Y2
945.22
C25H62N3O2Si4Y·C7H8
730.18
143(2)
148(2)
143(2)
143(2)
103(2)
crystal system
space group
a (Å)
triclinic
triclinic
triclinic
monoclinic
P21/n
monoclinic
P21
P1
̅
P1
P1
̅
̅
10.8326(6)
11.9799(6)
12.1523(6)
60.6233(6)
76.3438(6)
89.1598(6)
1325.85(12)
1
11.7914(6)
12.0827(6)
20.3900(10)
98.8969(7)
106.6799(7)
103.1034(7)
2634.1(2)
2
11.2181(17)
12.922(2)
13.156(2)
107.0028(18)
108.8235(17)
109.3789(18)
1522.9(4)
1
21.6041(17)
12.7933(10)
21.7958(17)
90
12.2932(7)
12.8887(7)
13.2122(7)
90
b (Å)
c (Å)
α (deg)
β (deg)
118.0084(10)
90
102.6215(6)
90
γ (deg)
volume (Å3)
5318.5(7)
4
2042.80(19)
2
Z
ρcalcd (Mg/m3)
1.227
1.288
1.186
1.180
1.187
μ (mm−1
)
2.389
2.561
2.088
1.225
1.574
a
R1 [I > 2.0σ(I)]
0.0256
0.0324
0.0333
0.0437
0.0175
a
wR2 (all data)
0.0666
0.0762
0.0897
0.1075
0.0435
a
2
2
Definitions: R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
25.7 (ONC5H6Me4), 40.5 (ONC5H6Me4), 60.4 (ONC5H6Me4). IR:
2949s, 1462w, 1360w, 1251s, 1180w, 1132w, 1013sh, 974s, 941s,
887sh, 843s, 779m, 665m, 604m, 517m cm−1. Anal. Calcd for 8·THF
C29H70N3O3Si4Y: C, 49.05; H, 9.94; N, 5.92. Found: C, 49.19; H, 9.77;
N, 5.93.
X-ray Data Collection, Structure Solution, and Refinement
for 4−8. A summary of the X-ray data collection parameters is
presented in Table 1. Full information is available in the Supporting
Information.
RESULTS
■
Oxide Complexes [(R2N)2(THF)Y]2(μ-O), 4, and
{[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5 (R = SiMe3). The oxide
[(R2N)2(THF)Y]2(μ-O), 4, was initially identified as a crys-
talline byproduct, Figure 1, in the reaction of the (N2)3− complex,
product.4 The isolation of 4 was the first evidence that a bridging
oxide of the [Y(NR2)2]+ cation was accessible, analogous to the
ubiquitous [(C5Me5)2Ln]2(μ-O) byproducts in rare earth
metallocene reactions.8
After complex 4 was identified from both reaction mixtures
described above by X-ray crystallography, it was identified in
other reaction mixtures containing [(R2N)2Y]+ by its character-
istic 1H and 13C{1H} NMR shifts. However, numerous attempts
to generate 4 deliberately were unsuccessful. This is not unlike
the situation with the metallocene oxides: the compounds are
often observed by NMR spectroscopy, but direct high yield syn-
theses are either not known or non-trivial, as in the case of the
most studied system [(C5Me5)2Sm]2(μ-O).17−19 Hence, varia-
tions in the conditions of the NO reactions that initially gave 4
as a byproduct were not successful in generating high yields of 4.
In addition, 4 was not obtained in reactions of 1 with oxidants
such as N2O, O2, pyridine N-oxide, trimethylamine N-oxide,
1,2-epoxybutane, Ag2O, and K2O, in reactions of 2 with N2O, in
the reaction of 3 with 1,2-epoxybutane, and in reactions of the
cationic complex [(R2N)2(THF)3Y][BPh4], 7 (see below), with
Ag2O, pyridine N-oxide, and Na2O2. No fully characterizable new
oxide or oxygen-containing complexes were isolated from these
reactions, except in the case of the synthesis of [(R2N)2(THF)-
Y]2(μ-η2:η2-O2), 6, from the reaction of 1 with O2 (see later
section). The reaction of 1 with pyridine N-oxide gave [(R2N)2-
(C5H5N)Y]2(μ-η2:η2-N2),20 eq 3, by NMR and X-ray crystallo-
graphy, which indicated that oxygen had been delivered by the
N-oxide reagent, but the oxide product was not identified. In
contrast to the pyridine N-oxide reaction in eq 3, only THF
Figure 1. Thermal ellipsoid plot of [(R2N)2(THF)Y}2(μ-O), 4,
showing the calculated average position of the two disordered (O)2−
positions, drawn at the 30% probability level. Hydrogen atoms are
omitted for clarity.
[(R2N)2(THF)Y]2(μ-η2:η2-N2)K, 2,3 with NO that was previously
reported to form the (NO)2− complex, [(R2N)2(THF)Y]2(μ-
η2:η2-NO), 3,4 as the main product, eq 2. Crystals of 4 were also
obtained as a byproduct from the reaction of [(R2N)2(THF)-
Y]2(μ-η2:η2-N2), 1, with two equiv of NO that forms the (ON
NO)2− complex [(R2N)2Y]4(μ3-ONNO)2(THF)2 as the main
C
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substitution occurred when 1 was treated with trimethylamine
N-oxide to yield the adduct [(R2N)2(Me3NO)Y]2(μ-η2:η2-N2),20
eq 4.
like the previously reported samarium analogue, [(R2N)2(THF)-
Sm]2(μ-O).9 Although 4 crystallizes in the same space group as
its samarium analogue, the complexes are not isomorphous. The
samarium compound has a 180° Sm−O−Sm linkage, the most
compact arrangement for two [(R2N)2Ln]+ cations around a
bridging (O)2− ion, with equivalent 2.0819(2) Å Sm−O distances.
The related complex, [(C5Me5)2Y]2(μ-O),26 has a Y−O distance
of 2.053(1) Å and a linear Y−O−Y angle.27 Large variations in
Ln−O−Ln angles have been observed in analogous bimetallic rare
earth metallocenes.8,17,18,23
The oxide ligand in 4, on the other hand, is disordered such
that it is not possible to determine from the X-ray data whether
the Y−O−Y angle is linear or whether the Y−O distances are
symmetric. The model used to describe the disorder involves
the displacement of the (O)2− atom to either side of the inver-
sion center with 0.5 site occupancy in each position (see
Supporting Information). Figure 1 is a representation of the
structure of 4 that shows the average of those two disordered
oxide positions and therefore the Y−O−Y angle appears linear.
However, the actual angle cannot be determined from the data.
The 2.257(1) Å and 2.266(2) Å Y−N(amide) distances in 4
are normal for bis(trimethylsilyl)amide yttrium complexes2,20
and comparable to the 2.322(3) and 2.333(3) Å distances in
[(R2N)2(THF)Sm]2(μ-O)9 given the 0.058 Å larger ionic
radius of samarium.28 However, the 2.367(2) Å Y−O(THF)
bond is shorter than the 2.468(3) Å samarium analogue. Hence, 4
Since oxides like 4 are usually unwanted decomposition
products and are not commonly used as precursors, it was
more important to identify 4 than to find a high yield synthesis
for it. Interestingly, the synthesis of the related samarium oxide,
[(R2N)2(THF)Sm]2(μ-O),9 also proved challenging. Although
the samarium oxide can be made from the isolable Sm2+ pre-
cursor, (R2N)2Sm(THF)2,21 with nitrosobenzene or 1,2,3,4-
di(epoxy)butane, isolation of the product proved problematic.
Neither yields nor elemental analysis were reportable since the
compound readily loses THF and decomposes when removed
from the mother liquor of the reaction.9
The reaction of 2 with N2O mentioned above did not form 4,
but gave crystals of another yttrium amide oxide, namely, the
potassium bridged species, {[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n,
5, which when recrystallized from toluene exists as a polymer
in the solid state, eq 5, Figures 2 and 3. Multi-metallic bridged
oxides containing alkali metals are well-known for the lanthanides
and actinides,22 but there are few examples either with the
[N(SiMe3)2]− ligand23,24 or with yttrium.25
Structures of [(R2N)2(THF)Y]2(μ-O), 4, and {[(R2N)2Y]2-
(μ-O)2K2(μ-C7H8)}n, 5. Complex 4 crystallizes with one THF
of solvation per yttrium such that each metal is four coordinate
Figure 2. Thermal ellipsoid plot of a dimeric unit of {[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5, drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity.
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Figure 3. Thermal ellipsoid plot of {[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5, showing the intermolecular interactions in the polymeric structure, drawn at
the 50% probability level. Hydrogen atoms are omitted for clarity.
and its samarium analogue do not match as well structurally as
many analogous compounds in series of rare earth species.
The solid-state structure of 5, Figure 2, is complicated by two
types of orthogonal bridging networks. The fundamental
formula of 5 is just [(R2N)2YO][K], that is, it is formally a
potassium “ate salt” variant of 4. In the solid state structure of 5,
two [(R2N)2YO]− moieties combine to form a bis(oxo) bridged
{[(R2N)2Y](μ-O)2[Y(NR2)2]}2− bimetallic unit; there are two
types of these in the repeat unit that generates the extended
structure as described below. Each oxide ligand has a potassium
within bonding distance, but each potassium also has additional
interactions with a nearby [N(SiMe3)2]− ligand, which is a
common feature of rare earth complexes of the bis(trimethylsilyl)
amide ligand.14,29 The coordination sphere of the potassium ion
also contains a toluene molecule that functions as a bridge between
the K2[(R2N)2Y](μ-O)2[Y(NR2)2] units. Additional long-range
potassium-amide interactions are present perpendicular to the
bridging toluene chain that lead to a two-dimensional network
(Figure 3).
The structure of 5 has additional complexity in that there are
two types of {[(R2N)2Y](μ-O)2[Y(NR2)2]}2− units, one formed
from Y1 and its symmetry equivalent Y1′ and the other
composed of Y2 and Y2′. K1 is associated with the Y2O2 unit
made of Y1 and K2 is associated with the Y2O2 unit made of Y2.
Both K ions attach to bridging toluene molecules in an η6-mode.
The unique crystallographic repeat unit has one Y1 from one-half
of one bimetallic unit and one Y2 from one-half of the other and
includes K1 and K2 and the bridged toluene in between.
Despite this complicated solid state structure, each Y atom is
simply four coordinate with distorted tetrahedral geometry
formed by two [N(SiMe3)2]− ligands and two (μ-O)2− ligands
(Table 2). The 2.319(2)−2.346(2) Å range of Y−N distances is
longer than the Y−N distances in 4 and at the longer end of the
range observed for yttrium bis(trimethylsilylamide) complexes
in the literature.24,30−32 The 2.103(2)−2.113(2) Å Y−O distances
Table 2. Selected Bond Distances (Å) and Bond Angles
(deg) for {[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5
parameter
length
parameter
angle
Y1−O1
Y1−O1′
Y2−O2
Y2−O2′
2.103(2)
2.113(2)
2.104(2)
2.109(2)
O1−Y1−O1′
N2−Y1−N1
O1−Y1−N2
O1′−Y1−N2
O1−Y1−N1
O1′−Y1−N1
81.33(6)
120.17(7)
105.79(6)
122.95(6)
119.66(6)
101.57(7)
Y1−N2
Y1−N1
Y2−N3
Y2−N4
2.337(2)
2.346(2)
2.319(2)
2.325(2)
O2−Y2−O2′
O2−Y2−N3
O2′−Y2−N3
O2−Y2−N4
O2′−Y2−N4
N3−Y2−N4
81.24(7)
109.21(7)
123.48(7)
118.42(7)
102.31(7)
117.53(7)
K1−O1
K2−O2
2.561(2)
2.504(2)
K1−C2
K1−N1
K2−C6
K2−C24
K2−N4
3.403(3)
3.373(2)
3.447(3)
3.536(3)
3.478(2)
O1−Y1−Y1′
O1′−Y1−Y1′
O2−Y2−Y2′
O2′−Y2−Y2′
40.78(4)
40.55(4)
40.68(4)
40.56(4)
in 5 are longer than the Y−O(oxo) distances in 4 as expected
since they are coordinated to three metals rather than two in 5.
The 2.562(2) Å and 2.504(2) K−O distances are significantly
shorter than the range of K−O distances 2.661(6)−2.945(4) Å
found in bridging alkoxide or salen potassium salts of yttrium.33−35
The K−C(toluene) distances in 5 range from 3.393(3)−
3.474(3) Å. These values are on average longer than those
observed in ate salt complexes of the lanthanides, for example,
in {K(C7H8)[Ln(tBu2pz)4]} (Ln = La, Sm, Tb, Ho, Yb, Lu),36
involving terminal toluene η6-bound to potassium, but they are
comparable to the average values of 3.275−3.59 Å found in
{K[Ln(O-2,6-i-Pr2C6H3)4]}n (Ln = La, Nd, Er), which form ex-
tended chain structures.37 A two-dimensional polymeric structure
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is also observed in {K(THF)x[Sc(CH2Ph)4]}n which has
potassium interactions with the aryl rings of the benzyl ligands,38
but the structure of 5 is unusual in that the η6-arene-bridged
polymer is formed by interaction of potassium with toluene
solvent, rather than with an arene of a ligand.
one of the least intense features of the overall spectrum, single
crystalline samples were required to obtain definitive data. The
Raman spectrum of 6 displayed a resonance at 767 cm−1 that
was absent in the spectrum of 6−18O2, which had a resonance at
730 cm−1 (calculated, 723 cm−1). In comparison, for most transi-
tion metal peroxides the range of υO−O bands is 800−930 cm−1;
metal superoxide complexes have υO−O at 1050−1200 cm−1.39,40
In cases of μ-η2:η2-O2 transition metal binding, the peroxide υO−O
has been found to shift to lower wavenumbers (υO−O < 750
cm−1).41,42 In the closely related complex, [(R2N)2(THF)Yb]2(μ-
η2:η2-O2), 9,14 which has a 1.543(4) Å O−O distance by X-ray
crystallography, a Raman resonance at 775 cm−1 was assigned
as υO−O. However, density functional theory (DFT) predicted
a shorter O−O bond of 1.519 Å with a υO−O resonance at
853 cm−1, and the 18O2 isotopomer was not synthesized. Re-
cently Lappert et al. reported a [Ce(NR2)3]2(μ-O2) complex
believed to be a peroxide rather than a superoxide, but
neither X-ray or Raman data were definitive.23
The shortest K−(NR2) contacts are the 3.373(2) Å K1−N1
and 3.403(3) Å K1−C2(Me) distances. The distances to K2 are
longer: 3.478(2) Å for K2−N4, and 3.536(3) Å for K2−C24(Me).
In comparison, the cyclometalated bis(trimethylsilyl) amide
yttrium complex, {(R2N)2Y[CH2Si(Me)2NR]}[K(C6H6)2], has
an intramolecular K−C(Me) distance of 3.000(4) Å and an
intermolecular K−C(Me) distance of 3.228(4) Å.29a,b The
benzene-free Yb analogue, {(R2N)2Yb[CH2Si(Me)2NR]}K, is
even more similar to 5 in that it has multiple K−C(Me) contacts
and a polymeric layer structure.29a,b
Peroxide Complex [(R2N)2(THF)Y]2(μ-η2:η2-O2), 6. The
reaction of [(R2N)2(THF)Y]2(μ-η2:η2-N2), 1, with oxygen was
attempted as part of this study. The reaction was initially run at
−78 °C in toluene with 0.5 equiv of O2 and produced a single
Attempts to oxidize 6 to a superoxo species with AgBPh4
were unsuccessful. Complex 6 is also stable toward excess O2,
as the reaction is run at 1 atm, and no reaction was observed
with NO (1 equiv) in contrast to 1 (see earlier section). The
reaction of 6 with KC8 (1 equiv) in cold THF and recrys-
tallization from toluene resulted in the isolation of the bridged
species {[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5, the product of
complete reduction of the O−O bond. The reaction of 6 with
KC8 (2 equiv) in THF resulted in the quantitative conversion,
1
new diamagnetic product by H and 13C{1H} NMR spectros-
copy in a 50% yield. Subsequent reactions with up to 1 atm O2
in either toluene or THF at temperatures up to ambient
temperature generated the same product in good yields. An
84% yield was found for the reaction in THF, in which 1 atm of
O2 was added to a previously frozen solution of 1 with fast
warming to room temperature. The reaction is not sensitive to
the concentration of nitrogen in solution and proceeds irrespec-
tive of whether the solvent is degassed or not. The structure of
the product was determined to be the (O2)2− peroxide species,
[(R2N)2(THF)Y]2(μ-η2:η2-O2), 6, by single crystal X-ray analysis,
Figure 4, eq 6. The peroxide unit in 6 has the same bridging
1
by H NMR spectroscopy, to a new diamagnetic species. Recrys-
tallization from toluene at −35 °C yielded 5 in 25% yield. This
improved synthetic route allowed full characterization of complex 5.
To date there is only one example of a superoxide complex of a
rare earth metal, [(TpMe2)2Sm(η2-O2)].43
Structure of [(R2N)2(THF)Y]2(μ-η2:η2-O2), 6. Complex 6
crystallized in the same space group as the ytterbium analogue,
[(R2N)2(THF)Yb]2(μ-η2:η2-O2), 9,14 but the complexes are
not isomorphous. The structures are similar, however, with
analogous Ln2(μ-η2:η2-O2) cores with trans THF ligands. The
1.535(3) Å O−O distance in 6 matches the 1.543(4) Å value in
9 and is normal for a peroxide (superoxide: 1.2−1.3 Å; peroxide:
1.4−1.5 Å).36,37 The 2.206(2) Å and 2.240(2) Å Y−O(O2) dis-
tances are slightly larger than the 2.186(2) and 2.219(2) Å values
of 9 as expected for the slightly larger yttrium. The O−Ln−O
angle of 40.38(7)° in 6 also matches the 41.00(9)° analogue in 9.
Tetraphenylborate Complex [(R2N)2(THF)3Y][BPh4], 7.
When the (NO)2− complex [(R2N)2(THF)Y]2(μ-η2:η2-NO),4
3, was initially discovered, there were no (NO)− complexes of
the rare earth metals.44 In efforts to synthesize such a complex,
the oxidation of 3 was examined. Treatment of 3 with AgBPh4
did not give an isolable (NO)− complex, but instead formed the
tetraphenylborate salt of the [Y(NR2)2]+ cation, [(R2N)2(THF)3Y]-
[BPh4], 7, Figure 5. Like the oxide 4 above, cationic salts of this
type are another basic type of rare earth complex that had not been
identified for the yttrium bis(trimethylsilylamide) system. The
isolation of 7 showed that an analogue of the metallocene cation
was available for yttrium amides.
Figure 4. Thermal ellipsoid plot of [(R2N)2(THF)Y}2(μ-η2:η2-O2), 6,
drawn at the 30% probability level. Hydrogen atoms are omitted for
clarity.
η2:η2-coordination mode as the (N2)2−, (NO)2−, and (N2)3−
complexes previously synthesized from the Y[N(SiMe3)2]3/K
reduction system.1−5 Hence, crystallographic data are now available
for all three variations of the (E2)2− unit in Y2E2 complexes where
E = N and O.
The isolation of 7 spurred attempts to find more direct
routes to this compound. Simple reactions like that of Y(NR2)3
with [HNEt3][BPh4] do not form 7 even at 70 °C for 24 h. The
(N2)2− complex, 1, does not react with [HNEt3][BPh4] and the
(N2)3− complex, 2, reacts with [HNEt3][BPh4] via reductive
disproportionation to form the doubly protonated (N2H2)2−
complex, [(R2N)2(THF)Y]2(μ-η2:η2-N2H2).5 However, the re-
action of the (NN)2− complex, 1, with 2 equiv of AgBPh4 in
Raman spectroscopy experiments were performed on 6 and
6−18O2, the analogue prepared from 18O2. Since the υO−O is
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Figure 5. Thermal ellipsoid plot of [(R2N)2(THF)3Y][BPh4], 7, drawn
at the 30% probability level. Hydrogen atoms are omitted for clarity.
toluene did yield complex 7 as an analytically pure material in a
modest yield, eq 7. The reaction was also run in THF, the only
Figure 6. Thermal ellipsoid plot of (R2N)2(THF)Y(η2-ON-
C5H6Me4), 8, drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity.
solvent in which 7 displays good solubility, but multiple products
were obtained, even when the reaction was maintained at low
temperature.
Structure of [(R2N)2(THF)3Y][BPh4], 7. Complex 7 crys-
tallizes from THF with the cations and anions separated in the
lattice beyond bonding distance. Three molecules of THF are
coordinated to yttrium to give a five coordinate metal center.
The [(R2N)2(THF)3Y]+ cation adopts a slightly distorted trigonal
bipyramidal geometry with the amide ligands in equatorial posi-
tions. The O(THF)−Y−O(THF) angles of 79.94(7)°, 84.97(7)°
and 155.79(7)° demonstrate the distortion from idealized trigonal
bipyramidal symmetry. The 2.218(2) and 2.227(2) Å Y−N(amide)
distances are similar to the 2.224(6) Å Y−N(amide) distance in
the parent complex, Y[N(SiMe3)2]3,45 which is formally three
coordinate, but has additional silylmethyl−yttrium interactions.
The Y−O(THF) distances in 7 are typical for five coordinate
yttrium.46,29c
Structure of (R2N)2(THF)Y(η2-ONC5H6Me4), 8. Complex
8 crystallizes from toluene with a THF of solvation and the
(TEMPO)− anion bound to yttrium in an η2-fashion. This
coordination is like that in [(C5Me5)(η2-ONC5H6Me4)SmCl]2.
Since the structural data on the latter complex provided con-
nectivity only, comparisons will be made with [(η1-ONC5H6-
Me4)2Sm(μ-η1:η2-ONC5H6Me4)]2, 10.12 There are relatively few
examples of TEMPO complexes of the transition metals, but the
η2-binding mode is the most commonly observed.47
The 2.274(1) and 2.269(1) Å Y−N(amide) distances and the
2.3987(9) Å Y−O(THF) distance are comparable to those
seen in complexes 1 and 4. The 1.452(1) Å O1−N3 distance in
8 is equivalent within error limits to the 1.427(8)−1.434(7) Å
N−O distances in 10 and is consistent with reduction of the
TEMPO radical, which has a 1.283(9) Å N−O distance.48
Since the (TEMPO)− ligand is bound η2 in 8 but is η1:η2 and η1
in 10, Ln−TEMPO distance comparisons are not so direct, but
the numbers seem reasonable. The Y−O1(TEMPO) distance
of 2.116(1) Å in 8 is shorter than the bridging Sm−O(TEMPO)
distances in 10, 2.295(4) and 2.354(4) Å, but longer than the
terminal Sm−O(TEMPO) distances of 2.110(5) and 2.124(5) Å,
when the 0.058 Å difference in ionic radii is considered.28 The
Ln−N(TEMPO) distances of 2.516(1) Å for Y−N3 and 2.537(6)
Å for Sm−N are comparable.
(TEMPO)− Complex, (R2N)2(THF)Y(η2-ON-C5H6Me4), 8.
Given the propensity of the [(R2N)2Y]+ system to stabilize
radicals like (N2)3− and (NO)2−, the reaction of the (N2)2−
complex, 1, with the radical (2,2,6,6-tetramethylpiperidin-1-yl)-
oxyl, TEMPO, was examined. This did not generate a new
radical product, but instead reduced TEMPO with loss of
dinitrogen to form the (TEMPO)− anion in the complex
(R2N)2(THF)Y(η2-ONC5H6Me4), 8, eq 8, Figure 6.
Complex 8 is synthesized in a straightforward manner from
the addition of 2 equiv of the TEMPO radical to a solution of
complex 1 in toluene, and can be isolated from the same sol-
vent as colorless crystals in high yield with a THF of solvation
and a molecule of either toluene or THF in the crystal lattice.
The complex has diamagnetic NMR spectra consistent with the
structure.
The Y-TEMPO distances in 8 are also comparable to the
2.172(2) Å Y−O and 2.437(2) Å Y−N distances observed in
the hydroxylaminato complex {Cp2Y[ON(C2H4-o-py)2]}.49 In
contrast, [C(CN)2NO]− dicyanonitrosomethanide lanthanide
complexes, that also have η2-ON binding, show variable binding
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dependent on the steric constraints of the complex: in some
cases the Ln-O and Ln-N bonds are the same length and in
other cases they are asymmetric like those in 8.50
CONCLUSION
■
The reaction chemistry of (N2)2−, (N2)3−, and (NO)2− com-
plexes of [(R2N)2Y]+ has led to the first examples of oxide,
peroxide, and tetraphenylborate complexes of this yttrium
bis(amide) cation. The high yield conversion of the Y2(μ-η2:η2-
N2) complex, 1, to the Y2(μ-η2:η2-O2) complex, 6, is another
example of a Ln2(μ-η2:η2-E2) complex reacting with a diatomic
molecule, E′2, to make an Ln2(μ-η2:η2-E′2) product and E2.
This was previously observed in the conversion of the (N2)3−
complex 2 to the (NO)2− complex, 3. These reactions are
unusual in that a well-protected, diatomic, reduced dianionic
ligand is replaced by another diatomic molecule, which enters
the reaction as the free diatomic species.
DISCUSSION
■
Although oxide and tetraphenylborate complexes of the
[(R2N)2Y]+ cation have been identified crystallographically in
this study, their direct synthesis has proven challenging. This is
consistent with the fact that Y[N(SiMe3)2]3 has been known
since 19731 and these common types of bis(ligand) rare earth
complexes are only now being identified. It is surprising that
the [(R2N)2Y]+ moiety that has been so successful in stabilizing
unusual anions like (N2)3− and (NO)2− has not previously
provided simple oxide and tetraphenylborate complexes with
the common (O)2− and (BPh4)− ligands.
ASSOCIATED CONTENT
* Supporting Information
■
S
The formation of the oxides [(R2N)2(THF)Y]2(μ-O), 4, and
{[(R2N)2Y]2(μ-O)2K2(μ-C7H8)}n, 5, from NO and N2O is
consistent with the fact that these nitrogen oxides can be oxide
delivery agents. Isolation of 4 now allows it to be identified by
NMR spectroscopy when it is formed as a byproduct. The
synthesis of [(R2N)2(THF)3Y][BPh4], 7, in eq 7 does show
that Ag+ can oxidize the (N2)2− complex, but the reaction is not
synthetically useful and precludes using 7 as an alternative
synthetic precursor in bis(amide) yttrium chemistry. The oxida-
tion of 1 by TEMPO similarly provides a new heteroleptic
amide complex, (R2N)2(THF)Y(η2-ONC5H6Me4), 8. The yield
of that reaction is sufficient for 8 to be used as a precursor into
mixed ligand yttrium chemistry.
Crystallographic data collection, structure solution, and refine-
ment (PDF) and X-ray diffraction details of compounds 8, 5, 7,
4, and 6 (CIF, CCDC No. 883505−883508 and 894283). This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wevans@uci.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Neither [(R2N)2(THF)Y]2(μ-η2:η2-N2), 1, nor [(R2N)2-
(THF)Y]2(μ-η2:η2-O2), 6, reacted with AgBPh4 to form Y2E2
complexes containing an (E2)− monoanion. It will be interest-
ing to see if a monoanion can form a bridge that connects two
[(R2N)2Y]+ cations. Recently it has been found that (CO)− and
(CO2)− will bridge Y(NR2)3 units,51 but even simple [(R2N)2-
Ln−X−Ln(NR2)2]+ moieties are unknown when X = monoanion
and Ln = Y. In contrast, monoanionic bridges are common in rare
earth metallocene chemistry.52
■
We thank the U.S. National Science Foundation (CHE-1010002)
for support of this research, Dr. Michael K. Takase and Dr. Ryan
A. Zarkesh for assistance with X-ray crystallography, and
Dr. Wytze E. van der Veer and Megan E. Fieser for assistance
with Raman spectroscopy.
REFERENCES
■
(1) Bradley, D. C.; Ghotra, J. S.; Hart, A. S. J. Chem. Soc., Dalton
Trans. 1973, 1021.
The direct formation of [(R2N)2(THF)Y]2(μ-η2:η2-O2), 6,
from 1 and O2 in eq 6 at ambient temperatures as well as the
stability of 6 toward excess O2 and NO is very unusual in rare
earth chemistry. Syntheses of rare earth peroxides such as
{[(L)nCe]2(μ-η2:η2-O2)2}m− [ (L)n = (CO3)3, m = 8 or EDTA],
m = 4,53 [Ln4(μ3-η2:η2:η2-O2)2(Cl)8(py)10] (Ln = Sm, Eu,
Gd),54 {[Ln(Cl)(L)]2(μ-η2:η2-O2)]}[ClO4]2 [Ln = Yb, Lu; L =
2,14-dimethyl-3,6,10,13,19-pentaazabicyclo[13.3.1]nonadeca-
1(19),2,13,15,17-pentaene],55 Ln2(NR2)4(O2)(OPPh3)2 [Ln =
La, Pr, Sm, Eu, Lu],13 and [Yb(η5-{[C5H4(cyclo-C5H9)]2(THF)2-
Yb]2(μ-η2:η2-O2)56 generally require low temperature and con-
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adventitiously or in low yield. The synthesis of [(R2N)2(THF)-
Yb]2(μ-η2:η2-O2), which can be isolated in a good yield, is an
exception, but the reaction is done at −60 °C and from Yb2+.
The isolation of [(R2N)2(THF)Y]2(μ-η2:η2-O2), 6, specifi-
cally is valuable for comparison with 1−3. When 1−3 were
initially isolated, there was some concern that the diatomic ligand
between the two metals was dioxygen since it is difficult to
differentiate N from O by X-ray crystallography in such complexes.
Isolation of 6 with a 1.535(3) Å O−O distance and a υO−O
767 cm−1 Raman resonance clearly excludes the presence of
peroxide in complexes 1−3.
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