Manganese(II) Alkyl/π-Allyl complexes resistant to ligand redistribution

Article abstract of DOI:10.1021/om500101m

The reaction of [Li(THF)₄][(Mn{C(SiMe₃) ₃})₃(μ-Cl)₄(THF)] (1) with K(allyl ^TMS2) (allyl^TMS2 = 1,3-C₃H ₃(SiMe₃)₂) afforded [(η³- allyl^TMS2)Mn{C(SiMe₃)₃}{ClLi(THF)₃}] (2). Attempted sublimation of 2 yielded [(η³-allyl ^TMS2)Mn{C(SiMe₃)₃}(THF)] (3), indicating that 2 extrudes LiCl at elevated temperatures. Additionally, LiCl in 2 was displaced by reaction with PMe₃, quinuclidine, and dmap (dmap = 4-(dimethylamino)pyridine), providing [(η³-allyl ^TMS2)Mn{C(SiMe₃)₃}(L)] (L = PMe₃ (4), quinuclidine (5), dmap (6)). Treatment of PMe₃ complex 4 with BPh₃ yielded bright red [(allyl^TMS2)Mn{C(SiMe ₃)₃}] (7) accompanied by a precipitate of Ph ₃B(PMe₃). Mixed alkyl/π-allyl manganese(II) complexes 2-7 are pyrophoric red solids with a high-spin d⁵ configuration, and all were crystallographically characterized. In the solid-state structures, the allyl ligands in 2-6 adopt a syn,syn configuration, whereas in base-free 7, the allyl ligand has a syn,anti configuration. Complexes 4, 5, and 7 sublimed cleanly (5 mTorr) at 70, 90, and 50 °C, respectively. Complex 4 exhibited a particularly favorable combination of volatility and thermal stability, given that its appearance and powder X-ray diffraction pattern were unchanged after 24 h in a sealed flask at 100°C. Compounds 2-7 are the first high-spin d ⁵ mixed alkyl/allyl complexes, and 7 is the first room-temperature-stable example of a mononuclear transition-metal complex bearing only alkyl and allyl ligands.

Full text of DOI:10.1021/om500101m

Article
pubs.acs.org/Organometallics
Manganese(II) Alkyl/π-Allyl Complexes Resistant to Ligand
Redistribution
Preeti Chadha,† David J. H. Emslie,*,† and Hilary A. Jenkins‡
†Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada
‡McMaster Analytical X-Ray Diffraction Facility, Department of Chemistry and Chemical Biology, McMaster University, Hamilton,
Ontario L8S 4M1, Canada
S
* Supporting Information
ABSTRACT: The reaction of [Li(THF)4][(Mn{C-
(SiMe3)3})3(μ-Cl)4(THF)] (1) with K(allylTMS2) (allylTMS2
=
1,3-C3H3(SiMe3)2) afforded [(η3-allylTMS2)Mn{C(SiMe3)3}-
{ClLi(THF)3}] (2). Attempted sublimation of 2 yielded
[(η3-allylTMS2)Mn{C(SiMe3)3}(THF)] (3), indicating that 2
extrudes LiCl at elevated temperatures. Additionally, LiCl in 2
was displaced by reaction with PMe3, quinuclidine, and dmap
(dmap = 4-(dimethylamino)pyridine), providing [(η3-allylTMS2)Mn{C(SiMe3)3}(L)] (L = PMe3 (4), quinuclidine (5), dmap
(6)). Treatment of PMe3 complex 4 with BPh3 yielded bright red [(allylTMS2)Mn{C(SiMe3)3}] (7) accompanied by a precipitate
of Ph3B(PMe3). Mixed alkyl/π-allyl manganese(II) complexes 2−7 are pyrophoric red solids with a high-spin d5 configuration,
and all were crystallographically characterized. In the solid-state structures, the allyl ligands in 2−6 adopt a syn,syn configuration,
whereas in base-free 7, the allyl ligand has a syn,anti configuration. Complexes 4, 5, and 7 sublimed cleanly (5 mTorr) at 70, 90,
and 50 °C, respectively. Complex 4 exhibited a particularly favorable combination of volatility and thermal stability, given that its
appearance and powder X-ray diffraction pattern were unchanged after 24 h in a sealed flask at 100 °C. Compounds 2−7 are the
first high-spin d5 mixed alkyl/allyl complexes, and 7 is the first room-temperature-stable example of a mononuclear transition-
metal complex bearing only alkyl and allyl ligands.
INTRODUCTION
■
Base-free and oligomeric [Mn(CH2CMe2Ph)2]2, [Mn-
(CH2CMe3)2]4, and [Mn(CH2SiMe3)2]x were reported by
Wilkinson in 1976,1 and various Lewis base adducts of these
compounds (with monometallic or dimetallic structures) were
subsequently described.2,3 The range of well-characterized
manganese(II) dialkyl complexes was also expanded through
the use of alternative alkyl groups including CH3, CH2Ph, Cy,
CH(SiMe3)2, CMe3, and C(SiMe3)3.2,4,5 By comparison,
neutral manganese(II) diallyl complexes are limited to [(η1-
allylTMS2)2Mn(THF)2] and [(η1-allylTMS3)2Mn(tmeda)] (al-
lylTMS2 = 1,3-C3H3(SiMe3)2; allylTMS3 = 1,2,3-C3H2(SiMe3)3),6
and mixed alkyl/allyl manganese(II) complexes are unknown.
In fact, although alkyl and allyl ligands play a central role in
organometallic chemistry, complexes containing both alkyl and
allyl ligands are uncommon throughout the transition series7−17
(see Figure 1 for examples of open-shell mixed alkyl/allyl
complexes and transition-metal compounds bearing only alkyl
and allyl ligands), likely due in large part to the propensity of
mixed hydrocarbyl complexes to engage in ligand redistribution
reactions. Such reactions have been noted to be particularly
facile for manganese(II) complexes with a high-spin d5
configuration as a result of the weak and polar bonds that
result from an absence of ligand field stabilization energy.
These same factors have also been cited to explain why many
high-spin organomanganese(II) complexes have more in
common with main-group analogues than transition-metal
Figure 1. (a) Open-shell alkyl/allyl complexes (R = Me, CH2SiMe3).
(b) Transition-metal compounds bearing only alkyl and allyl ligands
i
(R′ = Me, Et, nPr, Pr, nBu, nPent). Analogues of the copper(III)
complex with substituents at various positions on the allyl ligand are
also known.
complexes,6,18,19 to the extent that manganese(II) has been
branded the “black sheep of the organometallic family”.20
In the broad context of developing new precursors for CVD
and ALD of manganese-containing films,21 we were interested
in exploring the stability and volatility of bis-hydrocarbyl
manganese(II) complexes in which α-hydrogen abstraction and
β-hydrogen elimination are prevented or disfavored. Mono-
meric [Mn{C(SiMe3)3}2]5 was particularly attractive in this
Received: January 27, 2014
Published: March 7, 2014
© 2014 American Chemical Society
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regard, but in our hands the reported synthesis afforded mostly
[Li(THF)4][(Mn{C(SiMe3)3})3(μ-Cl)4(THF)] (1)22 with less
than a 10% yield of [Mn{C(SiMe3)3}2], even when the reaction
time was extended to 1 week. Given that allyl ligands are
typically resistant to hydrogen abstraction/elimination reac-
tions, we turned our attention to the synthesis of mixed
C(SiMe3)3/allylTMS2 complexes using 1 as the starting material.
We reasoned that a single extremely bulky C(SiMe3)3 ligand
may be sufficient to impart a high level of thermal stability and
that the decreased steric bulk of the target complexes relative to
[Mn{C(SiMe3)3}2] may allow their synthesis in high yield.
Hetero-hydrocarbyl complexes of this type are also of interest
due to their potential to exhibit unique volatility and reactivity
characteristics relative to homo-hydrocarbyl complexes, espe-
cially when the hydrocarbyl ligands are sterically and/or
electronically dissimilar. Herein we describe the synthesis of
the first high-spin d5 mixed alkyl/π-allyl complexes: those of
manganese(II).
Compounds 2−6 were characterized by X-ray crystallog-
raphy (the structure of 2 is only suitable to establish
connectivity),25 and bulk sample purity was confirmed by
powder X-ray diffraction and elemental analysis. Room-
temperature Evans magnetic measurements26 on 2−6 yielded
magnetic moments between 5.63 and 6.13 μB ( 0.3 μB),
consistent with high-spin manganese(II), and 1H NMR spectra
consisted of two or three very broad peaks between 4 and 50
ppm. The X-ray crystal structures for 3−6 are shown in Figure
2 (see Figure S1 in the Supporting Information for the
RESULTS AND DISCUSSION
■
The reaction of [Li(THF)4][(Mn{C(SiMe3)3})3(μ-
Cl)4(THF)] (1)22 with K(allylTMS2
)
(allylTMS2 = 1,3-
23,24
C3H3(SiMe3)2) produced crude [(η3-allylTMS2)Mn{C-
(SiMe3)3}{ClLi(THF)3}] (2) as a deep red paramagnetic oil
in 65% yield (Scheme 1), and crystallization from hexanes at
Scheme 1. Synthesis of Mixed Alkyl/Allyl Manganese
Complexes 2−6
Figure 2. X-ray crystal structures of [(η3-allylTMS2)Mn{C(SiMe3)3}-
(L)] (L = THF (3), PMe3 (4), quinuclidine (5), dmap (6)).
Hydrogen atoms are omitted for clarity, and ellipsoids are given at the
50% probability level.25,28
structure of 2, and Table 1 for selected bond lengths and angles
in 3−6). In all cases the allyl ligand has a syn,syn arrangement of
the trimethylsilyl substituents, with the central C−H bond
oriented toward the neutral donor, rather than the C(SiMe3)3
ligand. The Mn−C(1) and Mn−C(3) bond lengths are very
similar in compounds 4−6 (for each compound, Δ(Mn−C1/3
)
≤ 0.05 Å), consistent with η3 coordination. In comparison,
Mn−C(1) is shorter in 3 than in 4−6, and Mn−C(3) is longer
(Δ(Mn−C1/3) = 0.23 Å), indicative of some distortion in the
direction of η2 coordination.
−30 °C afforded bright red crystals of 2. Attempted
sublimation of 2 at 65 °C and 5 mTorr deposited a bright
red oil on the cold finger, and crystallization from pentane at
−30 °C provided red crystals of [(η3-allylTMS2)Mn{C-
(SiMe3)3}(THF)] (3; vide infra), indicating that 2 extrudes
LiCl at elevated temperatures. Additionally, LiCl in 2 was
displaced by reaction with PMe3, quinuclidine, and dmap
(dmap = 4-(dimethylamino)pyridine), yielding [(η3-allylTMS2)-
Mn{C(SiMe3)3}(L)] (L = PMe3 (4), quinuclidine (5), dmap
(6)) (Scheme 1). Complexes 4−6 were isolated in good yields
as pyrophoric red powders and are the first neutral π-allyl
complexes of manganese(II). Previously reported open-shell
mixed alkyl/allyl complexes are limited to [CpFeMe(η3-
allyl)],12 [CpVMe(η3-allyl)(PMe3)], and [MoR(η3-allyl)-
(dppe)2] (R = Me, CH2SiMe3)16 (Figure 1), and none of
these compounds have been structurally characterized.
The centroid of the allyl ligand, Mn, C(4), and E (E = Cl, O,
P, N) lie approximately in a plane, and the C(1)−Mn−C(3)
plane lies perpendicular to the C(4)−Mn−E plane. Complexes
2−6 are therefore tetrahedral 13-electron analogues of known
square-planar [(η3-allyl)M(alkyl)(L)] (M = Ni, Pd).10 The
Mn−C(4) distances in 3−6 range from 2.14 to 2.19 Å, while
the Mn−Callyl distances are longer at 2.31−2.40 Å for 4−6 and
2.27−2.50 Å for 3. The Mn−E distances are 2.170(3) Å (E = O
in 3), 2.617(1) Å (E = P in 4), 2.258(4) and 2.270(4) Å (E =
3
2
Nsp in 5), and 2.163(4) Å (E = Nsp in 6). The shorter Mn−N
distance in 6 vs that in 5 indicates that dmap binds more
3
effectively than quinuclidine, and the increasing Mn−Esp
distances in the order 3 < 5 < 4 mirror the trend in the
covalent radii.27 The C(4)−Mn−E (E = O, P, N) angles in 3
and 6 are 119−122°, in comparison with 126 and 129−131° in
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a
Table 1. Bond Lengths (Å) and Angles (deg) for Compounds 3−7
3
4
5
6
7
Mn−C(1)
Mn−C(2)
Mn−C(3)
Mn−C(4)
2.268(4)
2.321(4)
2.496(4)
2.144(4)
2.170(3)
1.420(5)
1.384(5)
118.7(1)
103.2
2.342(3)
2.327(3)
2.349(3)
2.164(3)
2.617(1)
1.403(4)
1.404(4)
125.90(7)
103.4
2.355(4), 2.355(4)
2.356(4), 2.358(4)
2.365(5), 2.390(4)
2.187(4), 2.181(5)
2.270(4), 2.258(4)
1.403(6), 1.409(6)
1.410(6), 1.413(6)
130.5(2), 128.9(2)
104.8, 104.5
2.343(5)
2.312(4)
2.397(5)
2.167(5)
2.163(4)
1.412(6)
1.395(6)
122.4(2)
102.7
2.174(1)
2.300(1)
2.510(1)
2.092(1)
Mn−E
C(1)−C(2)
C(3)−C(4)
C(4)−Mn−E
Cent−Mn−E
Cent−Mn−C(4)
1.435(2)
1.383(2)
137.4
130.7
124.7, 126.6
134.8
166.3
Only one of the two independent molecules of 3 in the unit cell is free from disorder; bond lengths and angles are provided for this molecule.25,28
a
4 and 5, respectively. This increase is presumably steric in
origin and is accompanied by a decrease in the allylcentroid−Mn−
C(4) angle: from 135 to 137° in 3 and 6 to 131° in 4 and 125−
127° in 5.
Scheme 2. Synthesis of Base-Free 7 by PMe3 Abstraction
from 4
a
Complexes 4 and 5 sublimed (5 mTorr) cleanly at 70 and 90
°C, respectively. In contrast, 6 sublimed at 115 °C (5 mTorr)
with accompanying decomposition, and pure samples were only
obtained by crystallization. Complex 4 exhibited a particularly
favorable combination of volatility and thermal stability, given
that its appearance and powder X-ray diffraction pattern were
unchanged after 24 h in a sealed flask under argon at 100 °C.
Simultaneous TGA/DSC (10 °C / min) at 760 Torr revealed a
clean mass loss step of 14 wt% from 50 to 130 °C, consistent
with PMe3 dissociation (calcd. 14 wt%). A subsequent mass
loss step suggests volatilization of the PMe3-free complex, but it
is accompanied by thermal decomposition just below 200 °C.
In contrast, TGA performed at 0.5 Torr (10 °C/min) led to
much more rapid mass loss between 80 and 125 °C, with
competing PMe3 loss and bulk sample volatilization suggested
by DSC analysis. At 225 °C, the residual mass was 2%,
indicative of volatilization of the majority of the sample.29
The high thermal stability of compounds 2−6 is remarkable
given (a) their high-spin d5 configuration, which typically
results in a susceptibility toward ligand redistribution,2,14 and
(b) the steric accessibility of the products of ligand
redistribution (homoleptic bis-hydrocarbyl manganese com-
plexes), as evidenced by [Mn{C(SiMe3)3}2],5 [Li(THF)4][Mn-
(allylTMS2)3],30 and K2[Mn(allylTMS2)4].6 Several features are
likely responsible for the high thermal stability of 2−6: a
favorable level of steric hindrance, the absence of α- and β-
hydrogen atoms in the alkyl ligand, and the η3 coordination
mode of the allyl ligand, which may help to prevent ligand
transfer via μ-allyl intermediates.
a
Compound 7 is depicted with a syn,anti configuration of the allyl
ligand in order to match the X-ray crystal structure. However, we
cannot exclude the possibility that the bulk sample contains a mixture
of the syn,syn and syn,anti isomers.
X-ray-quality crystals of 7 were obtained from a highly
concentrated pentane solution at −25 °C, confirming a
monomeric structure (Figure 3; Table 1). Unexpectedly, the
trimethylsilyl groups of the allylTMS2 ligand now adopt a syn,anti
rather than a syn,syn arrangement, implying isomerization via an
η1-allyl structure. We were not able to determine conclusively
whether the bulk sample of 7 consists only of the syn,anti
isomer or a mixture of the syn,anti and syn,syn isomers due to
the paramagnetism and low melting point of 7, which
prevented packing and sealing of a sample in a capillary for
PXRD. However, observation of a single melting point at 41 °C
in the DSC of 7 is strongly suggestive of a single isomer,
presumably the syn,anti isomer. The C(SiMe3)3 anion in the
structure of 7 is positioned approximately trans to the allylTMS2
ligand, with a C(4)−Mn−allylcentroid angle of 166°. In
comparison with 2−6, the Mn−C(4) distance in 7 is shorter
(2.092(1) vs 2.164(3) Å) and the allyl bonding mode is
significantly less symmetrical, tending in the direction of η2
coordination (Mn−C(1) = 2.174(1) Å, Mn−C(2) = 2.300(1)
Å, Mn−C(3) = 2.510(1) Å; C(1)−C(2) = 1.435(2) Å; C(2)−
C(3) = 1.383(2) Å).
Treatment of PMe3 complex 4 (1H NMR δ 12 and 41 ppm)
with BPh3 yielded bright red [(allylTMS2)Mn{C(SiMe3)3}] (7;
vide infra) accompanied by a precipitate of Ph3B(PMe3)
(Scheme 2). Pure 7 was obtained by filtration in pentane
followed by sublimation at 50 °C (5 mTorr) and showed no
sign of decomposition after 1 week in C6D6 at room
temperature.31 Complex 7 has five unpaired electrons by
Complexes containing only alkyl and allyl ligands are limited
to 16-electron [(η3-allyl)CuIIIR2],32 [(η3-allyl)3WIV(alkyl)] (R =
33
Me, Et, Pr, Pr, Bu, Pent), and [{OsIII(CH2CMe3)2(η3-
allyl)}2]34 (Figure 1). Only the last complex, which features an
OsOs triple bond, is stable at room temperature; the
copper(III) complexes were detected by direct injection NMR
spectroscopy at −100 °C, and the tungsten(IV) complexes
decomposed between −78 and 0 °C, depending on the nature
of the R group. Compound 7 is therefore the only room-
temperature-stable example of a monometallic transition-metal
complex that is ligated solely by alkyl and allyl ligands.
n
i
n
n
1
Evans magnetic measurement, displays a distinctive H NMR
spectrum with broad signals at 22 and 69 ppm, and reacted
cleanly with PMe3 and dmap to form 4 and 6, respectively.
Samples of 7 solidified after days to weeks at −25 °C and
melted at 41 °C (determined by DSC). The low melting point
and sublimation temperature of 7 are suggestive of a
monomeric reaction product or a product that readily
dissociates to monomeric units.
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an η1-allyl coordination mode36 yielded a structure (7″) that is
25 kJ mol−1 higher in Gibbs free energy than 7 at 298 K.
SUMMARY AND CONCLUSIONS
■
The first alkyl/allyl complexes with a high-spin d5 configuration
have been synthesized: 13-electron [(allylTMS2)Mn{C-
(SiMe3)3}(L)] (L = ClLi(THF)3 (2), THF (3), PMe3 (4),
quinuclidine (5), dmap (6)) and 11-electron [(allylTMS2)Mn-
(TSI)] (7). Compound 7 is a rare example of a mononuclear
transition-metal complex bearing only alkyl and allyl ligands,
and it is the only room-temperature-stable example. Given that
high-spin d5 complexes are noted for their susceptibility to
ligand redistribution6,18 and homo-hydrocarbyl manganese(II)
complexes bearing C(SiMe3)3 and allylTMS2 ligands are
known,5,6,30 the thermal stability of 2−7 is remarkable. In
fact, it suggests that bulky acyclic hydrocarbyl ligands may be
suitable to stabilize a broad range of hetero-hydrocarbyl
complexes that are free from multidentate or anionic
supporting ligands. Complexes of this type have rarely been
reported37 but may be attractive as precursors with unique
stability, volatility, and reactivity profiles for organometallic
synthesis, chemical vapor deposition (CVD), and atomic layer
deposition (ALD).
EXPERIMENTAL SECTION
■
General Details. An argon-filled MBraun UNIlab glovebox was
employed for the manipulation and storage of all oxygen- and
moisture-sensitive compounds, and air-sensitive preparative reactions
were performed on a double-manifold high-vacuum line using standard
techniques. Compounds 2−7 are exceptionally air-sensitive, and most
are pyrophoric. Residual oxygen and moisture was removed from the
argon stream by passage through an Oxisorb-W scrubber from
Matheson Gas Products. A VWR Clinical 200 Large Capacity
Centrifuge (with 28° fixed-angle rotors that hold 12 × 15 mL or 6
× 50 mL tubes) in combination with VWR high-performance
polypropylene conical centrifuge tubes was used when required
(inside the glovebox). Vacuum was measured using a Varian Model
531 Thermocouple Gauge Tube with a Model 801 Controller.
Anhydrous diethyl ether was purchased from Aldrich and then dried
over sodium benzophenone ketyl. Hexanes, pentanes, and THF were
dried and distilled at atmospheric pressure from sodium benzophe-
none ketyl; sodium was used to dry toluene. Unless otherwise noted,
all anhydrous solvents were stored over an appropriate drying agent
prior to use (OEt2, THF, THF-d8, toluene, toluene-d8, and C6D6 over
Na/Ph2CO; pentane and hexanes over Na/Ph2CO/tetraglyme).
Anhydrous MnCl2, CoCp2, BPh3, KOtBu, CHBr3, SiMe3Cl, MeLi
(1.6 M in Et2O), nBuLi (1.6 M in hexane), sBuLi (1.4 M in
cyclohexane), 3-(trimethylsilyl)propene (abbreviated as H[allylTMS]),
PMe3, quinuclidine, and 4-dimethylaminopyridine (dmap) were
purchased from Aldrich or Strem Chemicals. K[B(C6F5)4] was
purchased from Boulder Scientific. [NBu4][B(C6F5)4] was prepared
via a slight modification of the original literature procedure38 (using
K[B(C6F5)4] in place of Li(OEt2)x[B(C6F5)4]) and dried thoroughly
before use.
Figure 3. (top left) X-ray crystal structure of [(syn,anti-allylTMS2)Mn-
{C(SiMe3)3}] (7). Hydrogen atoms are omitted, and ellipsoids are
given at the 50% probability level. (top right) Calculated structure of 7.
(bottom left) Calculated structure of [(syn,syn-allylTMS2)Mn{C-
(SiMe3)3}] (7ss). (bottom right) Calculated structure of PMe3
complex 4.
DFT calculations were carried out on 4 and 7 (ADF 2013.1,
PBE, D3-BJ, TZ2P all-electron, unrestricted, five unpaired
electrons, ZORA), and the geometries show reasonable
agreement with the X-ray crystal structures (Table S1,
Supporting Information). Consistent with the slippage toward
a more η2-coordinated structure in 7, the sum of the calculated
C−C−Si, C−C−H, and Si−C−H angles around C(1) and
C(3) in 7 are 351 and 358°, respectively, in comparison with
355° around both C(1) and C(3) in compound 4. Calculations
on [(allylTMS2)Mn{C(SiMe3)3}] with a syn,syn arrangement of
the trimethylsilyl groups yielded a structure (7ss; Figure 3) with
an asymmetric allyl bonding mode similar to that of 7 but a
Gibbs free energy that is 13 kJ mol−1 higher at 298 K,
corresponding to an equilibrium constant approaching 200 in
favor of 7. The favorability of a strongly asymmetric allyl
bonding mode in 7 (calculated Δ(Mn−C1/3) = 0.33 Å) may
derive in part from close approach between the SiMe3 groups of
the allyl and alkyl ligands, given that the structures for
hypothetical [(syn,anti-allylTMS2)MnR] (R = C(SiH3)3, Me)
feature an allyl ligand that is more symmetrically coordinated
(Δ(Mn−C1/3) = 0.10−0.12 Å).35 In addition, hypothetical
[(syn,syn-allylTMS2)MnH] and [(allyl)MnH] were previously
calculated to be Cs symmetric.6 However, restraining the Mn−
C(1) and Mn−C(3) bonds in 7 to be equal and allowing them
to refine freely as a single variable led to an η3-allyl structure
(7′; Mn−C(1) = Mn−C(3) = 2.30 Å) that is only 2 kJ mol−1
higher in Gibbs free energy at 298 K. The very small ΔG value
between 7 and 7′ suggests that 7 will be fluxional, with facile
access to η2- and η3-allyl bonding modes. In contrast, enforcing
Diethyl ether was removed from a commercial MeLi solution, and
the resulting powder was stored in the glovebox. CHBr3 was extracted
three to four times with water, dried over CaCl2 followed by activated
molecular sieves, and then distilled prior to use.39 H[allylTMS] and
SiMe3Cl were dried over CaH2 and distilled before use. Quinuclidine
and dmap were sublimed, and BPh3 was recrystallized from diethyl
ether and dried in vacuo before use. HC(SiMe3)3,40 LiC(SiMe3)3·
2THF,41 and Mn[C(SiMe3)3]2 (in low yield)5 were prepared as
42
23
previously reported. H[allylTMS2
]
and K[allylTMS2
]
were prepared
following modifications of the literature procedures (see the
Supporting Information).
NMR spectroscopy (1H, 11B, 31P{1H}) was performed on a Bruker
1
DRX-500 spectrometer. All H NMR spectra were referenced relative
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1
1
to SiMe4 through a resonance of the employed deuterated solvent or
measurements. H NMR (C6D6; δ): 4.5, 11.9, 34.1 ppm. H NMR
(C4D8O; δ): 11.0, 29.2 ppm. Anal. Found (calcd): C, 50.91 (50.96);
H, 10.15 (9.93). μeff = 6.08 μB. Epa = −0.16 V vs [FeCp2]0/+ in THF (ν
= 200 mV s−1).
proteo impurity of the solvent: C6D6 (δ 7.16 ppm) and THF-d8 (3.58,
1
1.73 ppm) for H NMR. 11B and 31P NMR spectra were referenced
using external standards of BF3·OEt2 (0.0 ppm) and 85% H3PO4 in
D2O (0.0 ppm), respectively.
[(allylTMS2)Mn{C(SiMe3)3}(THF)] (3). Attempted sublimation of 2
(0.080 g, 0.11 mmol) at 65 °C yielded a red oil which was washed off
the cold finger with the minimum volume of pentane and cooled to
−30 °C, yielding X-ray-quality crystals (0.050 g, 84%) over several
days. 1H NMR (C6D6; δ): 14.4, 46.0 ppm. 1H NMR (C4D8O; δ): 10.9,
34.5 ppm. Anal. Found (calcd): C, 50.81 (50.78); H, 10.62 (10.37).
μeff = 6.13 μB. Epa = −0.06 V vs [FeCp2]0/+ in THF (ν = 200 mV s−1).
[(allylTMS2)Mn{C(SiMe3)3}(PMe3)] (4). Compound 2 (2.76 g, 3.78
mmol) was dissolved in hexanes (40 mL), PMe3 (0.5 mL, 4.83 mmol)
was introduced at room temperature, and the reaction mixture was
stirred overnight. No color change was observed, but some white
solids precipitated. The solution was evacuated to dryness in vacuo to
remove any excess PMe3 and then taken into the glovebox. The
residue was redissolved in hexanes, the solution was centrifuged to
remove solids, and the red solution was evaporated to dryness. The
resulting red solid was sublimed at 70 °C (5 mTorr) for further
purification, yielding 1.6 g (77%) of pure 4. X-ray-quality crystals were
Combustion elemental analyses were performed on a Thermo
EA1112 CHNS/O analyzer. Samples for elemental analysis (typically
2−4 mg) were packed and sealed in preweighed 3 × 6 mm smooth-
wall tin capsules inside the glovebox. After removal from the glovebox,
these capsules were packed into 5 × 8 mm pressed aluminum capsules
containing approximately 10 mg of V2O5. DSC of 7 was performed on
a TA Instruments DSC 2910 using a 2.8 mg sample that was sealed
into a hermetic aluminum pan inside the glovebox. Simultaneous
TGA/DSC was performed on a dual TA Instruments Q600 SDT
within an argon-filled glovebox at a typical rate of 10 °C/min. Due to
the high air and moisture sensitivity of the samples, crucibles
(aluminum) were dried at 130 °C under vacuum prior to use.
Atmospheric pressure temperature ramps were performed using a 100
mL/min flow rate of inert gas, while reduced pressure measurements
were performed under dynamic vacuum (0.5 Torr). Cyclic
voltammetry (CV) studies were carried out using a PAR (Princeton
Applied Research) Model 283 potentiostat (using PAR PowerCV
software) in conjunction with a three-electrode cell under an argon
atmosphere. The auxiliary electrode was a platinum wire, the
pseudoreference electrode a silver wire, and the working electrode a
glassy-carbon disk (3.0 mm diameter, Bioanalytical Systems).
Solutions were 1 × 10−3 M in test compound and 0.1 M in
[NBu4][B(C6F5)4] as the supporting electrolyte in dry THF. In all
experiments, potentials were calibrated by addition of [CoCp2]; the
E1/2 value for [CoCp2]0/+ in THF/[NBu4][B(C6F5)4] was measured
1
obtained by cooling a concentrated hexanes solution to −30 °C. H
NMR (C6D6; δ): 12.3, 40.9 ppm. H NMR (C4D8O; δ): 11.0, 34.1
1
ppm. Anal. Found (calcd): C, 48.62 (48.22); H, 10.64 (10.48) . μeff
=
6.07 μB. Epa = −0.14 V vs [FeCp2]0/+ in THF (ν = 200 mV s−1).
[(allylTMS2)Mn{C(SiMe3)3}(quinuclidine)] (5). Compound 2 (0.23
g, 0.32 mmol) was dissolved in hexanes (15 mL) in a vial in the
glovebox, quinuclidine (0.37 g, 0.33 mmol) was added at room
temperature, and the reaction mixture was stirred for 2 days. Solids
were removed by centrifugation, and the clear mother liquor was
evaporated to dryness to give a sticky red solid. Sublimation at 90 °C
(5 mTorr) gave a pure sample (0.20 g, 52%; note that higher
sublimation temperatures should be avoided since complete
decomposition was observed after 3 h at 110 °C). X-ray-quality
crystals were obtained from a concentrated pentane solution at −30
°C. 1H NMR (C6D6; δ): 5.6, 16.9, 49.1 ppm. Anal. Found (calcd): C,
to be −1.33 V vs [FeCp2]0/+
.
Single-crystal X-ray crystallographic analyses were performed on
crystals coated in Paratone oil and mounted on a SMART APEX II
diffractometer with a 3 kW sealed-tube Mo generator and
SMART6000 CCD detector. Powder X-ray diffraction (PXRD)
experiments were performed on a Bruker D8 Advance powder
diffractometer with Cu Kα radiation (λ = 0.154 nm) operated at 40 kV
and 40 mA. Powders were packed in 0.5 mm o.d. special glass (SG;
wall thickness 0.01 mm) capillary tubes for X-ray diffraction
(purchased from Charles Supper Co.) and sealed by inverting to
submerge the open end in a pool of Apiezon H-grease within a
glovebox. Calculated powder patterns for 2−7 were generated from
the low-temperature single-crystal data and then refined using Topas
4.2 (Bruker software).
52.69 (53.55); H, 10.64 (10.54); N, 2.62 (2.40). μeff = 5.91 μB. Epa
=
−0.14 V vs [FeCp2]0/+ in THF (ν = 200 mV s−1).
[(allylTMS2)Mn{C(SiMe3)3}(dmap)] (6). Compound 2 (0.53 g, 0.72
mmol) was dissolved in hexanes (10 mL) in a vial in the glovebox, a
solution of dmap (0.89 g, 0.72 mmol) in toluene (5 mL) was added at
room temperature, and the reaction mixture was stirred overnight.
Solids were removed by centrifugation, and the clear mother liquor
was evaporated to dryness. The residue was redissolved in hexanes and
crystallized at −30 °C (0.30 g, 70%). X-ray-quality crystals were
obtained from a concentrated hexane solution at −30 °C. Compound
6 was found to sublime at 115 °C with substantial accompanying
thermal decomposition; therefore, sublimation was avoided as a
[(allylTMS2)Mn{C(SiMe3)3}{ClLi(THF)3}] (2). Anhydrous MnCl2
(2.38 g, 18.9 mmol) was ground to a fine powder and introduced in
a round-bottom flask. LiC(SiMe3)3·2THF (7.23 g, 18.9 mmol) was
weighed out in another flask. Both flasks were appended to a vacuum
line, and ∼100 mL of THF was vacuum-transferred into each flask.
The LiC(SiMe3)3·2THF solution was then added to the suspension of
MnCl2 in THF at −78 °C via cannula. The cold bath was removed
after 1 h, and the reaction mixture was stirred at room temperature
overnight (20 h), during which time the solution turned clear yellow.
The reaction mixture, now containing [Li(THF)4][Mn3{C-
(SiMe3)3}3(μ-Cl)4], was cooled to −45 °C and a solution of
K[allylTMS2] (4.24 g, 18.9 mmol; THF, 100 mL) was added dropwise
via cannula. The reaction mixture turned red during addition. The cold
bath was removed after 1 h, and the reaction mixture was stirred at
room temperature for 24 h. THF was then removed in vacuo, and 100
mL of hexanes was introduced. After the mixture was stirred for 1 h,
hexane was also removed and the oily residue was taken in the
glovebox. The residue was redissolved in a 2/1 hexane/benzene
mixture, the solution was centrifuged to remove white solids, the
supernatant was collected, and all solvents were removed to obtain a
dark red oil (9.0 g, 65%). Some of the oil was redissolved in hexanes to
give a clear red solution, which yielded X-ray-quality crystals over
several days at −30 °C (if a small amount of oil initially precipitated,
the mother liquors were decanted and returned to −30 °C for
crystallization). The oil was used in all subsequent reactions.
Crystalline samples were used for all spectroscopic and analytical
1
1
method of purification. H NMR (C6D6; δ): 11.6, 19.9, 35.0 ppm. H
NMR (C4D8O; δ): 9.7, 16.9, 28.0 ppm. Anal. Found (calcd): C, 52.18
(52.56); H, 9.80 (9.84); N, 5.05 (4.72). μeff = 5.63 μB. Epa = −0.27 V
vs [FeCp2]0/+ in THF (ν = 200 mV s−1).
[(allylTMS2)Mn{C(SiMe3)3}] (7). BPh3 (0.16 g, 6.57 mmol) was
added to a sublimed sample of 4 (0.36 g, 6.57 mmol) in 10 mL of
pentane. The reaction mixture was stirred for 2 h, during which time
white solids precipitated from solution. Solids were removed by
centrifugation and identified as Ph3B(PMe3) by NMR spectroscopy
(in C6D6; 1H δ 7.45 (d, 6H, 1JHH = 7.4 Hz, Ph-o), 7.30 (app t, 6H, 1JHH
= 7.4 Hz, Ph-m), 7.22 (t, 3H, 1JHH = 7.4 Hz, Ph-p), 0.54 (d, 9H, 1JH−P
= 9.8 Hz, PMe3) ppm; 31P δ −15.45 ppm; 11B δ −7.51 ppm). The
bright red supernatant was separated and evaporated to dryness to give
a red oil which was sublimed at 50 °C (5 mTorr) using a dry ice
cooled cold finger. The compound sublimed on the cold finger as a
viscous oil, which was quickly taken into the glovebox and removed by
washing with pentane (0.28 g, 88%). After removal of pentane in
vacuo and cooling to −25 °C in the glovebox freezer, the oil solidified
over a period of days to weeks (crystallization was immediate if a small
amount of solid 7 was added to the oil to “seed” the crystallization
process). X-ray-quality crystals were obtained from a highly
concentrated pentane solution at −25 °C. PXRD could not be
1471
dx.doi.org/10.1021/om500101m | Organometallics 2014, 33, 1467−1474

Organometallics
Article
performed due to the waxy nature of the solids. A satisfactory
elemental analysis was not obtained; we attribute this to the waxy
nature of 7, which resulted in smearing of a small amount of
compound on the walls of the tin capsules, some of which was likely
above the level at which the vials were sealed (this material would have
decomposed upon removal from the glovebox, resulting in low C and
ACKNOWLEDGMENTS
■
D.J.H.E. thanks Intel Corporation for funding through the
Semiconductor Research Corporation (SRC) and the NSERC
of Canada for a Discovery grant. We are grateful to Dr. Patricio
E. Romero of Intel Corporation for simultaneous TGA/DSC
measurements on compound 4.
1
H values). H NMR (C6D6; δ): 22.4, 68.9 ppm. Anal. Found (calcd):
C, 46.70 (48.35); H, 10.05 (10.25). μeff = 5.97 μB.
DFT Calculations. All structures were fully optimized with the
ADF DFT package (SCM, version 2013.01).43 Calculations were
conducted using the zero-order regular approximation (ZORA)44 for
relativistic effects and 1996 Perdew−Burke−Ernzerhof exchange and
correlation for the GGA part of the density functional (PBE),45
combined with Grimme’s DFT-D3-BJ dispersion correction.46 All
calculations on manganese(II) complexes were unrestricted with five
unpaired electrons. Preliminary geometry optimizations were con-
ducted with frozen cores corresponding to the configuration of the
preceding noble gas (core = medium) using a double-ζ basis set with
one polarization function (DZP), an integration value of 5, and default
convergence criteria. These structures were further refined using an all-
electron TZ2P basis set (the size and quality of ADF basis sets
increases in the order SZ < DZ < DZP < TZP < TZ2P < QZ4P), an
integration value of 7, and a convergence gradient of 0.0001.
The structure of 7, in which the Mn−C(1) and Mn−C(3) distances
are set to have a single freely refining value, was calculated starting
from the unrestrained structure of 7. This was achieved by (a)
geometry reoptimization using the RESTRAINT keyword to set both
Mn−C distances to 2.35 Å followed by (b) geometry optimization of
the resulting structure using internal coordinates with a single variable
for both the Mn−C(1) and Mn−C(3) distances in the z matrix; the
starting value for this variable was defined as 2.35 Å using the
GEOVAR keyword.
Analytical frequency calculations showed (a) no imaginary
frequencies in the cases of 4, 7, 7ss, and [(syn,anti-allylTMS2)Mn{C-
(SiH3)3}], consistent with converged minima, and (b) one imaginary
frequency at −12 cm−1 in the case of [(syn,anti-allylTMS2)MnMe]. This
imaginary frequency is associated with a slight rotation of the
trimethylsilyl groups of the allyl group and not with changes involving
the core of the molecule; therefore, the structure of [(allylTMS2)-
MnMe] can still be considered to have converged to a minumum.
Analytical frequency calculations were also used to obtain free energy
(G) values using the equation provided in the ADF manual (G = total
bonding energy + total nuclear internal energy + pV − (T × total
entropy), where T = 298 K and pV = nRT = 0.592 kcal mol−1).
Visualization of the computational results was performed using the
ADF-GUI (SCM) or Discovery Studio Visualizer (Accelrys).
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text, tables, figures, and CIF files giving modified syntheses for
H[allylTMS2] and K[allylTMS2], Evans magnetic measurement
1
details, the X-ray structure of 2, H NMR spectra of 2−7,
experimental and calculated PXRDs for 2−6, crystallographic
data for 2−7, and additional computational details. This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC 952943−952948 also contain the CIF files
for 2−7, respectively.
AUTHOR INFORMATION
■
Corresponding Author
*D.J.H.E.: tel, 905-525-9140; fax, 905-522-2509; e-mail,
emslied@mcmaster.ca.
Notes
The authors declare no competing financial interest.
1472
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Organometallics
Article
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́
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(28) In all X-ray and calculated structures, C(1) is the terminal
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(29) The residual mass of 2% in the TGA at 0.5 Torr may be due to
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comparison of the 1H NMR spectrum of 7 with that of an
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(36) A value of 2.90 Å was used for Mn−C(2), since the
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lylTMS2)2(THF)2], and [Mn(allylTMS2)2(κ2-tmeda)] are between 2.88
and 2.99 Å.15,16
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of a MnSixOy diffusion barrier at the interface between copper
interconnect wiring and silicon- and oxygen-rich dielectric materials.
Additionally, ultrathin manganese metal, MnOx, and MnNx layers
deposited directly on a low-k dielectric such as SiCOH (“organosilicate
glass”) by chemical vapor deposition (CVD) or plasma-enhanced
CVD (PECVD) are reported to be highly effective copper diffusion
barriers. These manganese-containing barrier layers have been
proposed as replacements for more conventional Ta/TaN, Ti/TiN,
W/TiN, Ru/TaN, or Mo/WN barrier layers in future microprocessors.
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C 2013, 117, 160 and references therein. (b) Kurokawa, A.; Sutou, Y.;
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N.; Barbarin, Y.; Croes, K.; Siew, Y. K.; Van Elshocht, S.; Tokei, Z.;
Vancoille, E. ECS Solid State Lett. 2013, 2, P25. (e) Au, Y.; Lin, Y. B.;
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