Synthesis and Reactions of 3d Metal Complexes with the Bulky Alkoxide Ligand [OCtBu2Ph]

Article abstract of DOI:10.1021/acs.inorgchem.5b00795

Treatment of NiCl₂(dme) and NiBr₂(dme) (dme = dimethoxyethane) with 2 equiv of LiOR (OR = OC^tBu₂Ph) forms the distorted trigonal planar complexes [NiLiX(OR)₂(THF)₂] (THF = tetrahydrofuran)

Full text of DOI:10.1021/acs.inorgchem.5b00795

Article
pubs.acs.org/IC
Synthesis and Reactions of 3d Metal Complexes with the Bulky
Alkoxide Ligand [OCtBu2Ph]
James A. Bellow,†,¶ Maryam Yousif,†,¶ Dong Fang,†,‡ Eric G. Kratz,† G. Andres Cisneros,*,†
́
and Stanislav Groysman*,†
†Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States
‡Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
S
* Supporting Information
ABSTRACT: Treatment of NiCl2(dme) and NiBr2(dme)
(dme = dimethoxyethane) with 2 equiv of LiOR (OR =
OCtBu2Ph) forms the distorted trigonal planar complexes
[NiLiX(OR)2(THF)2] (THF = tetrahydrofuran) 5 (X = Cl)
and 6 (X = Br). The reaction of CuX2 (X = Cl, Br) with 2
equiv of LiOR affords the Cu(I) product Cu4(OR)4 (7). The
same product can be obtained using the Cu(I) starting
material CuCl. NMR studies indicated that the reduction of
Cu(II) to Cu(I) is accompanied by the oxidation of the
alkoxide RO− to form the alkoxy radical RO•, which
subsequently forms tert-butyl phenyl ketone by β-scission.
Treatment of compounds 1−4 ([M2Li2Cl2(OR)4], M = Cr−
Co) with thallium hexafluorophosphate allowed the isolation
of the distorted tetrahedral complexes of the form M(OR)2(THF)2 for M = Mn (8), Fe (9), and Co (10). Cyclic voltammetry
performed on compounds 8−10 demonstrated irreversible oxidations for all complexes, with the iron complex 9 being the most
reducing. Complex 9 shows a reactivity toward PhIO and Ph3SbS to form the corresponding dinuclear iron(III) complexes
Fe2(O)(OR)4(THF)2 (11) and Fe2(S)(OR)4(THF)2 (12), respectively. X-ray structural studies were performed, showing that
the Fe−O−Fe angle for complex 11 is 176.4(1)° and that the Fe−S−Fe angle for complex 12 is 164.83(3)°.
alkoxides (and related aryloxides) are capable of supporting
Cu(III) complexes.
INTRODUCTION
■
Alkoxides and related phenoxides and siloxides are commonly
used as ancillary ligands in coordination chemistry and
catalysis.1−4 However, the majority of alkoxide complexes
involve early transition metals, as alkoxides are π-donors and
bind stronger to the more electron-deficient metals.5 Well-
defined mid- and late-transition metal complexes of alkoxides
are relatively rare due to the destabilizing interaction of the
filled d orbitals on the metal with the p orbitals on the
alkoxides.1 The ensuing basicity of the alkoxide lone pairs leads
to the formation of polymetallic clusters via bridging.1 The
proclivity of alkoxides to form clusters can be overcome by two
different approaches: (1) making the alkoxide sterically
hindering,6−9 or (2) decreasing the nucleophilicity of the
alkoxide lone pairs.10 Demonstrating the first approach, Power
and co-workers reported middle and late transition metal
complexes stabilized by alkoxides featuring three large groups at
the central carbon (OCR3, R = Bu, Cy, Ph). Pursuing the
second approach, Doerrer and co-workers have recently
described late transition metal complexes ligated by perfluori-
nated alkoxides.10 They have demonstrated that fluorination
diminishes π-basicity, prevents bridging, and leads to the
formation of stable monomeric low-coordinate late (Ni, Cu)
metal centers. They also demonstrated that fluorinated
We seek low-coordinate mononuclear complexes that, upon
reaction with an organic azide, will form reactive nitrene
functionality. We surmised that alkoxide ligation at a middle-to-
late 3d metal would lead to highly reactive metal−nitrene as (i)
alkoxides are weak σ-donors, which should render metal−
nitrene electron-deficient and (ii) at the same time, alkoxides
are capable of π-donation, which is expected to interfere with π-
bonding from the nitrene functionality. As we are interested in
π-basic alkoxides, we decided to pursue a nonfluorinated bulky
alkoxide ligand. Our ligand of choice was [OCtBu2Ph].11 The
steric bulk of [OCtBu2Ph] is comparable to the bulkiest
alkoxide, [OCtBu3], previously utilized by Wolczanski and co-
workers7 and Power and co-workers.8 By replacing one of the
tBu groups by a Ph group we hoped to improve its packing and
crystallinity. Furthermore, the phenyl group is tunable, allowing
the ability to add additional steric bulk if needed. In our
previous publications, we reported that [OCtBu2Ph] (OR
thereafter) reacts with MCl2 precursors (M = Cr, Mn, Fe, and
Co) to form novel alkoxide clusters of [M2Li2Cl2(OR)4]
composition featuring rare seesaw geometry at transition metal
8
t
Received: December 11, 2014
Published: June 4, 2015
© 2015 American Chemical Society
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Scheme 1. Synthesis of the Alkoxide Complexes 1−12 Reported in This Paper
centers (1−4, Scheme 1 below).11a For Fe, we have also
demonstrated that [Fe2Li2Cl2(OR)4] can be transformed into a
mononuclear complex Fe(OR)2(THF)2 (THF = tetrahydrofur-
an), which displays intriguing reactivity with an alkyl
(adamantyl, Ad) azide, enabling its one-electron reductive
coupling to form the metal-ligated hexazene species
[AdNNNNNNAd]2−.11b Following these initial results, we
became interested in the synthesis of other middle-to-late
transition metal bis(alkoxide) complexes, specifically, those of
Cr, Mn, Co, and Ni. Furthermore, the intriguing reactivity of
the Fe(OR)2(THF)2 complex with azides prompted us to
investigate the reactivity of Fe(OR)2(THF)2 and other alkoxide
complexes in our study in related atom- and group-transfer
reactions. Herein we report the coordination chemistry of
[OCtBu2Ph] with 3d metals (M = Cr − Cu) and the reactions
of the resulting metal complexes with [O]- and [S]-atom
transfer reagents.
the three-coordinate intermediate [MLiCl(OR)2(THF)2],
which was directly observed only for Fe.11a In contrast, the
reaction of NiCl2(dme) (dme = dimethoxyethane) with 2 equiv
of LiOR followed by recrystallization from hexanes forms stable
orange [NiCl(OR)2Li(THF)2] (5, 65% yield). Similarly, the
reaction of NiBr2(dme) with 2 equiv of LiOR forms red-orange
[NiBr(OR)2Li(THF)2] (6, 63% yield). Complexes 5 and 6
demonstrate solution magnetic moments of 3.0 0.2 and 3.1
0.2 μB, respectively, indicating two unpaired electrons
(expected spin-only moment is 2.8 μB). The structure of 6 is
given in Figure 1, and the structure of isomorphous 5 is given in
RESULTS AND DISCUSSION
■
Reactivity of MX2 (M = Cr − Ni) with LiOR: Formation
of Seesaw Clusters [M2Li2Cl2(OR)4] Versus Heterodinu-
clear [NiLiX(OR)(THF)2] Complexes. Scheme 1 describes the
formation of various [M(OR)2] complexes obtained in this
study. Four distinctly different structural types were observed:
(i) [M2Li2Cl2(OR)4] clusters in which the seesaw transition
metal centers are ligated by two [OR] and two Cl ligands,11a
(ii) heterodinuclear [MLiX(OR)2(THF)2] complexes display-
ing trigonal transition metal centers with two [OR] and one X
ligands, (iii) mononuclear [M(OR)2(THF)2] complexes, and
(iv) tetranuclear Cu4(OR)4 complex.
Figure 1. Structure of [NiLiBr(OR)2(THF)2] (6), 50% ellipsoids. H
atoms were omitted for clarity. Dashed lines indicate largely ionic
interactions with lithium.
the Supporting Information. Both compounds display distorted
trigonal planar geometry at the Ni centers (sum of angles at Ni
360.0°). Selected structural data for 5 and 6 are presented in
Table 1. For comparison, we included the structural data for
one of the seesaw clusters (3).
As previously described, treatment of CrCl2, MnCl2, FeCl2,
and CoCl2 with 2 equiv of LiOR in THF, followed by
recrystallization from hexanes, affords [M2Li2Cl2(OR)4]
clusters.11a The reactions were postulated to proceed through
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Table 1. Selected Structural Data for the Compounds Reported in This Paper
complex
M−ORterminal (Å)
M−ORbridging (Å)
OR−M−OR (deg)
THF−M−THF (deg)
M−X−M (deg)
M = X (or M−X−M) (Å)
a
3
1.91(1)
174.0(2)
c
5
1.748(1)
1.747(1)
1.871(1)
1.872(1)
147.67(6)
c
6
148.24(5)
d
b
7
1.85(1)
165(4)
8
1.899(1)
1.838(1)
1.8491(1)
1.82(1)
130.0(1)
138.7(1)
131.1(1)
108.6(1)
86
83
88
9
10
11
12
176.4(1)
1.8031(3)
b
a
a
1.80(1)
117(4)
164.83(3)
2.186(4)
a
b
c
d
Average of two bonds/angles. Average of four bonds/angles. The angle between terminal and bridging alkoxides. Average of eight bonds.
Reactivity of Cu(II) and Cu(I) Precursors with LiOR:
Most remarkably, all of the previous Cu4(OR)4 compounds
were obtained from copper(I) precursors, while in our case it is
also obtained from various copper(II) starting materials. It is
possible that at the first step, an analogous (to [NiLiBr-
(OR)2(THF)2] or [FeLiCl(OR)2(THF)2]) product [CuLiX-
(OR)2(THF)2] is formed (see below for its calculated
structure). We also note that calculations (see below) indicate
that its dimerization to form [Cu2Li2X2(OR)4] is unlikely. The
copper center in [CuLiX(OR)2(THF)2] then undergoes
reduction, and the alkoxide constitutes the only possible
reducing agent in such transformation. Upon oxidation, it is
expected to form an [RO] radical. The cyclic voltammetry
(CV) of LiOR demonstrates an irreversible oxidation at the
Formation of Cu4(OR)4 Tetramer. We also investigated the
reactivity of copper(II) precursors with LiOR. Treatment of an
orange solution of CuBr2 in THF with 2 equiv of LiOR in ether
forms immediately a dark red solution. The red color, however,
instantaneously changes to amber, light blue, and eventually to
colorless. Solvent removal, followed by recrystallization of the
product from hexanes at −35 °C, leads to the formation of the
colorless Cu(I) cluster [Cu4(OR)4] (7) isolated in 42% yield.
The reaction of CuCl2 with LiOR under identical conditions
forms the same product in 46% yield. [Cu4(OR)4] (7) can be
also obtained from a copper(I) precursor, CuCl, in 51% yield.
We characterized compound 7 by 1H and 13C NMR
spectroscopy, X-ray crystallography, and elemental analysis.
The 1H NMR spectrum of 7 contains one kind of OR
resonance, consistent with the approximate D2d symmetry of
the structure. The X-ray structure of compound 7 is given in
Figure 2. Compound 7 is a tetramer of [CuOR] units featuring
+
relatively accessible potential of 0.51 V versus FeCp2/FeCp2
(Figure 3). We note that tertiary alkoxide is not commonly
Figure 3. CV of LiOR in THF, (0.1 M [NBu4](PF6), 25 °C, platinum
working electrode, 100 mV/s scan rate).
observed as a reductant.1 Furthermore, we are unaware of any
previous examples in which Cu(II) is reduced by an alkoxide.
We also note that the inability of [OR] to support even Cu(II)
centers stands in sharp contrast to the behavior of fluorinated
alkoxides capable of supporting Cu(III).10
We followed the reaction of CuBr2 with 2 equiv of LiOR by
UV−vis spectroscopy and NMR spectroscopy. The addition of
0.5 mL of a 2.86 M LiOR solution into an orange 0.286 mM
solution of CuBr2 (2.5 mL in THF) immediately leads to a
color change to colorless. A UV−vis scan performed ca. 10 s
after the addition revealed the final product (Supporting
Information, Figure S22). We tried to perform the reaction at 0
or at −78 °C. Still, the reaction was too fast at 0 °C, and no
reaction took place at −78 °C. However, batch-wise addition of
the solution of LiOR to the solution of CuBr2 (Figure 4) at
room temperature (RT) provided clear evidence for the
existence of an intermediate. The red trace in Figure 4
Figure 2. Structure of [Cu4(OR)4] (7), 50% ellipsoids. H atoms and
THF solvent were omitted for clarity.
nearly linear Cu(I) centers each ligated by two alkoxides. Four
Cu(I) centers form a plane with Cu···Cu distances ranging
from 2.64 to 2.72 Å. The alkoxides coordinate below and above
the Cu4 plane, leading to the overall “butterfly” structure. Such
geometry can be also described in terms of two Cu2(OR)3
planes (O4Cu4Cu3O1O3 and O2Cu1Cu2O1O3) intersecting
at 115° angle. Several related compounds exist in the literature,
namely, Cu4(OtBu)4, Cu4(OAr)4 (Ar = C6H3Ph2), and
Cu4(OCPh3)4.1,12 Of the above, Cu4(OtBu)4 and Cu4(OAr)4
are planar, while Cu4(OCPh3)4 has a similar (to compound 7)
butterfly molecular geometry, with a wider angle (141°)
between the Cu2(OR)3 planes.
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Figure 4. UV−vis spectra following the batch-wise addition of LiOR to the solution of CuBr2. (inset) The spectrum of the Cu4(OR)4.
Figure 5. (A) 1H NMR spectrum of an aliquot of the crude reaction mixture displayed at 6.8 to 8.8 ppm. (B) 1H NMR spectrum of Cu4(OR)4 in the
1
1
same region. (C) H NMR spectrum of tert-butyl phenyl ketone (2,2-dimethyl-1-phenylpropan-1-one). (D) H NMR spectrum of HOR.
represents the UV−vis spectrum of the 0.287 mM solution of
CuBr2 in THF (2.5 mL) that is dominated by two signals at 415
and 355 nm. The addition of 0.1 mL of a 2.87 mM solution of
LiOR (0.4 equiv) leads to the decline in the intensity of the
absorption peaks attributed to CuBr2 and an appearance of a
new signal at 660 nm. The spectrum of the final product of the
reaction (Cu4(OR)4, see the black spectrum in the inset of
Figure 4) is nearly featureless. Thus, the peak at 660 nm
belongs to the short-lived intermediate of the reaction. The
addition of another batch of LiOR (purple trace, next 0.4
equiv) causes further increase in the intensity of the signal at
660 nm, which is accompanied by a decrease in the intensity of
the signals at 415 and 355 nm. The next addition (orange
trace) leads to a further decrease in the spectrum of the starting
material, while the signal attributed to the intermediate persists.
Further additions of the alkoxide (green and black traces, 2
equiv total of LiOR) lead to the disappearance of the 660 nm
signal.
To gain further insight into the reaction mechanism, we also
analyzed the reaction by means of 1H NMR spectroscopy. UV−
vis spectroscopy indicates that the reaction is complete within
several minutes. The reaction of CuBr2 with 2 equiv of LiOR in
tetrahydrofuran in the presence of internal standard trimethox-
ybenzene was allowed to stir at RT for 30 min, after which an
1
aliquot was taken, and its H NMR spectrum was collected in
C6D6. Figure 5A demonstrates the aromatic region of the
spectrum; the full spectrum is given in Figure S6 in the
Supporting Information. For comparison, Figure 5 also
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Scheme 2. Proposed Pathway to the Formation of [Cu4(OR)4], tert-Butyl Phenyl Ketone, and HOR
demonstrates NMR spectra of clean Cu4(OR)4 (B), tert-butyl
phenyl ketone (C), and HOR (D) presented in the same
region. Full spectra are given in the Supporting Information.
Spectrum A demonstrates that the crude reaction mixture
contains Cu4(OR)4. In addition, resonances attributed to tert-
butyl phenyl ketone and HOR are observed as well. Using the
internal standard (trimethoxybenzene), we quantified the
overall amount of the “[OR]” or the OR derivatives detected
(quantification was done based on the tert-butyl resonance).
We observe formation of 0.012 mmol of Cu4(OR)4 (0.048
mmol of “Cu(OR)”), 0.026 mmol of tert-butyl phenyl ketone,
and 0.022 mmol of HOR. Given the amount of LiOR used
(0.156 mmol), we account for ∼62% of [OR]; it is consistent
with observed yield of Cu4(OR)4 (60%). Furthermore, these
data also suggest that for each equivalent of Cu(OR), we are
forming ∼0.5 equiv of tert-butyl phenyl ketone and HOR each.
Combined with the UV−vis data presented in Figure 4, the
detection and quantification of various reaction products
provides a key to the understanding of the reaction mechanism
(Scheme 2). We propose that [CuLiBr(OR)2(THF)2] forms
first, as observed or proposed for Cr−Ni. Next, the Cu−OR
bond undergoes a homolytic bond cleavage to form “CuI(OR)”,
which tetramerizes to form Cu4(OR)4. The remaining alkoxy
radical undergoes β-scission, which has been previously
observed for alkoxy radicals.13 The β-scission produces tert-
butyl phenyl ketone and tert-butyl radical, which may
decompose to give isobutylene and hydrogen atom that
couples with the alkoxy radical to give HOR. The formation
of HOR is also possible via the H atom abstraction by the
alkoxy radical from the solvent.13 We did not observe
isobutylene: the reaction takes place in THF/ether mixture,
and the solvent/volatiles needed to be removed prior to
dissolving the reaction mixture in C6D6.
complexes with TlPF6 in THF affords colorless Mn-
(OR)2(THF)2 (8, 60%), colorless Fe(OR)2(THF)2 (9),11b
and violet Co(OR)2(THF)2 (10, 49%). We note that
compounds 8−10 are related to the previously described
M(OR′)2(THF)2 (OR′ = OCPh3, M = Mn, Fe, Co)8e,f and the
more recently synthesized Mn(OAr)2(THF)2 complexes.14
Surprisingly, no reaction with TlPF6 was observed for the
heterodinuclear Ni complexes 5 and 6. Workup of the reaction
mixture in both cases led to the reisolation of the starting
materials. Treatment of the Cr-containing seesaw cluster 1 with
TlPF6 failed to produce the corresponding Cr(OR)2(THF)2
species.
Compounds 8−10 were characterized by IR spectroscopy,
1H NMR spectroscopy, solution magnetic measurements, UV−
vis spectroscopy, CV, and X-ray crystallography. Compounds
8−10 demonstrate μeff values of 5.5 0.3 μB for Mn (8), 4.7
0.3 μB for Fe (9), and 3.6
0.3 μB for Co (10) at RT
(measured by Evans method using average of three measure-
ments). The corresponding calculated spin-only values for the
high-spin Mn(II), Fe(II), and Co(II) centers are 5.9, 4.9, and
3.9 μB, respectively. Thus, the observed magnetic moments of
distorted tetrahedral complexes are close to spin-only magnetic
moments. We note that two-coordinate bis(aryloxide) Fe and
Co complexes demonstrate significantly higher μeff values due
to a considerable orbital contribution.15,16 The IR spectra of 8−
10 are given in Supporting Information, Figure S12. As
anticipated, compounds 8−10 give rise to almost identical IR
spectra with prominent C−O stretches around 1100 cm−1.
UV−vis spectra are given in the Supporting Information.
Colorless 8 and 9 are nearly featureless in the observed region.
Pale violet 10 (Co(OR)2(THF)2) gives rise to several weak
transitions in the visible region. The most significant peak at
734 nm exhibits an extinction coefficient of 175 M−1 cm−1.
CVs of compounds 8−10 are given in Figure 6. CVs were
obtained in THF, using [NBu4](PF6) as the electrolyte, and are
reported versus ferrocene/ferrocenium couple. Only irrever-
sible oxidations are observed, peaking at 0.45 V for
Fe(OR)2(THF)2 (onset at ∼0 V), 1.27 V for Co(OR)2(THF)2
(onset at ∼0.5 V), and 1.43 V for Mn(OR)2(THF)2 (onset at
Synthesis and Characterization of Mononuclear Bis-
(alkoxide) complexes M(OR)2(THF)2. At the next step, we
turned to the synthesis of neutral mononuclear species lacking
complexed LiX. We postulated that complexed halide anions
can be abstracted by the addition of Tl+, which is expected to
form insoluble TlX. Treatment of [M2Li2X2(OR)4] (2−4)
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are summarized in Table 1. Similarly to the previously
described 9, compounds 8 and 10 are distorted tetrahedral
complexes featuring wide RO−M−OR angles (130° for 8, 131°
for 10), and narrow THF−M−THF angles (86° for 8, 88° for
10). Related distorted tetrahedral Mn(OAr)2(THF)2 and
Fe(OCPh3)2(THF)2 were previously reported, both exhibiting
RO−M−OR angles of 140°.14,8f
Atom-Transfer Chemistry of Complexes 8−10. The
electrochemical studies indicate that Fe(OR)2(THF)2 (9) is
more reducing than Mn(OR)2(THF)2 and Co(OR)2(THF)2
complexes 8 and 10. The reactivity of 8−10 in group-transfer
chemistry correlates with these observations. We have
previously demonstrated that 9 reductively couples adamantyl
azide to form Fe(III)-bound hexazene.11b In contrast, no
reaction with adamantyl azide was observed for the Mn and Co
complexes 8 and 10. Similarly, 9 undergoes a reaction with an
oxo-transfer reagent (PhIO) and a sulfur-atom transfer reagent
(Ph3SbS)18 to furnish Fe2(O)(OR)4(THF)2 (11) and Fe2(S)-
(OR)4(THF)2 (12), respectively (Scheme 1). Treatment of 8
and 10 with the oxo- and sulfur atom-transfer reagents under
similar reaction conditions failed to produce the oxidized
products.
The structures of 11 and 12 are given in Figure 9. Both
complexes feature distorted tetrahedral Fe(III) centers ligated
by two terminal alkoxides, one THF ligand, and a bridging oxo
(11) or sulfido (12) ligand. Thus, similarly to the previously
reported hexazene complex [Fe2III(AdNNNNNNAd)-
(OR)4],11b the Fe centers retain the bis(alkoxide) ligation
upon chemical transformation. Complex 11 displays crystallo-
graphic C2 symmetry, with only half the molecule occupying
the asymmetric unit. The Fe−O−Fe angle is 176.4(1)°, and the
Fe−(μ2-O) distance is 1.8031(3) Å. The Fe−S−Fe angle in the
non-C2-symmetric 12 is 164.8°, and the Fe−S bond distances
are 2.1821(5) and 2.1901(5) Å. While Fe2(μ2-O) compounds
are common in the literature, compound 12 is one of the few
known di-Fe(III) complexes in which the Fe centers are
bridged by a single sulfide.19 The IR spectra of compounds 11
and 12 in the 600−1700 cm−1 region are presented in Figure
10 below. The overlay of the spectra demonstrates a significant
structural similarity, with the exception of the strong peak at
795 cm−1 that is observed only for compound 11 and is
therefore assigned to the Fe−oxo stretch. The Fe−sulfido
stretch is expected to occur in the far-IR region (<500 cm−1)
and was not observed. The UV−vis spectra of 11 and 12 are
given in Supporting Information, Figures S20 and S21. The
spectrum of pale yellow-orange 11 exhibits a weak absorption
(shoulder) around 500 nm. Unlike other compounds reported
in this paper, sulfur-containing deep red 12 demonstrates
relatively intense absorptions around 497 (21 350 M−1 cm−1)
and 339 (7570 M−1 cm−1) nm.
Figure 6. CVs of compounds 8−10 in THF, (0.1 M [NBu4](PF6), 25
°C, platinum working electrode, 100 mV/s scan rate).
∼0.7 V), respectively. The lack of reversible features in the CVs
is not uncommon in the electrochemistry of complexes
supported by bulky monodentate alkoxides,9c,10a,17 indicating
that the complexes undergo rearrangement upon oxidation. No
reductions were observed up to −2 V. This electrochemical
behavior demonstrates that (i) the complexes in the present
form are unlikely to reach lower than M(II) oxidation states
and that (ii) while the oxidation of Fe(II) is relatively
accessible, it may be less feasible for the Mn and Co
compounds 8 and 10.
X-ray quality crystals of compounds 8−10 were obtained
from hexanes at −35 °C. The structure of 9 has been previously
reported,11b and the structures of compounds 8 and 10 are
presented in Figures 7 and 8. Selected structural data for 8−10
Figure 7. Structure of Mn(OR)2(THF)2 (7), 50% probability
ellipsoids. H atoms are omitted for clarity.
Density Functional Theory Calculations of the
Stability of the Seesaw “Dimers” Versus the Trigonal
Planar “Monomers”. Intrigued by the stark difference in the
reactivity between Cr−Co and Ni, Cu, we performed density
functional theory (DFT) calculations on the stability of selected
[MLiX(OR)2(THF)2] (“monomers”) and [M2Li2X2(OR)4]
(“dimers”). We calculated the energy change during the
formation of dimers as in Scheme 3 both in gas phase and in
solvent environment. The relative change in electronic energy
(ΔE0) during the reaction in gas phase for Ni (39.1 kcal/mol)
is indeed larger than the other metals (Co, 31.5 kcal/mol; Fe,
34.6 kcal/mol; Mn, 28.2 kcal/mol; Cr, 32.6 kcal/mol). When
solvation effects are included in the calculations, the dimers
Figure 8. Structure of Co(OR)2(THF)2 (9), 50% probability
ellipsoids. H atoms are omitted for clarity.
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Figure 9. Structures of 11 (right) and 12 (left), 50% probability ellipsoids. H atoms are omitted for clarity.
Figure 10. IR spectra of compounds 11 and 12 in the 600−1700 cm−1 range.
Scheme 3. A Transformation Used to Determine the Free
Energy Differences between Monomers and Dimers
kcal/mol in hexane, 10.5 kcal/mol in THF) compared to Mn
(−5.2 kcal/mol in hexane, −6.0 kcal/mol in THF).
To try to rationalize the observed Cu chemistry, we
performed computational analysis on the hypothetical Cu
monomer and dimer. The optimized structure of the Cu
monomer is given in Supporting Information, Figure S25, and
the calculated structure of the dimer is given in Figure 11. The
structure of the Cu monomer is similar to the structures of the
Ni and Fe monomers, whereas the optimized structure of the
Cu dimer is strikingly different. In the case of the dimer
formation with Cu, the ΔG for the gas phase reaction was
calculated to be 5.7 kcal/mol, which is similar to that found for
the Co complex. When the calculations were repeated in THF
(hexane), the free energy of reaction decreases to −1.7 (2.9)
kcal/mol. While the calculations suggest that the dimer is stable
in THF, note that the Br atoms are not bridging between the
Cu atoms in the optimized geometry (Figure 11). Furthermore,
repeating the calculations at the B97D/SDDall level of theory
did not predict a stable product. The relative free energy
change due to solvation effects (ΔΔG)23 was found to be −7.3
and −2.8 kcal/mol in THF and hexane, respectively. This is
comparable to the values for the Mn complex, which are −9.6
and −8.8 kcal/mol in THF and hexane, respectively. This
suggests that the overall reaction is stabilized by THF to a
similar extent for the Mn and Cu complexes (in contrast with
become more stable; however, this effect is smaller for the Ni
complexes than for the other metal complexes. Most of the
change in the free energy of reaction is due to the entropic term
(−TΔS), which corresponds to −27.2 and −19.6 kcal/mol for
Ni and Mn, respectively, in the gas phase. In THF (hexane),
the contribution for Ni is smaller in comparison, −26.3 (−26.9)
kcal/mol. In contrast, the entropic contribution decreases to
−29.5 (−29.0) kcal/mol for Mn in THF (hexane). As a result, a
relatively large electronic energy change and small stabilization
from the solvation effects lead to a positive ΔG for Ni (6.9
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solvents were purified using an MBRAUN solvent purification system
and stored over 3 Å molecular sieves. Compounds 5−12 were
characterized by 1H and 13C NMR spectroscopy, IR spectroscopy,
UV−vis spectroscopy, solution-state magnetic susceptibility, X-ray
crystallography, CV, and elemental analysis. NMR spectra were
recorded at the Lumigen Instrument Center (Wayne State Univ.) on a
Varian Mercury 400 MHz NMR spectrometer in C6D6 or C6D5CD3 at
RT. Chemical shifts and coupling constants (J) were reported in parts
per million (Δ) and Hertz, respectively. IR spectra were recorded on a
Shimadzu IR-Affinity1 FT-IR spectrometer as paratone oil mull
suspensions. UV−vis spectra were obtained in a Shimadzu UV-1800
spectrometer. Solution-state effective magnetic moments were
determined using the Evans method,20 and diamagnetic corrections
were calculated using the Pascal’s constants method;21 three
independent measurements were performed for each sample (see
Supporting Information for further details). Electrochemical properties
were determined using CV on a BAS Epsilon system in a nitrogen-
filled glovebox. Samples were prepared in anhydrous THF with 0.1 M
tetrabutylammonium hexafluorophosphate [NBu4][PF6] as the
supporting electrolyte. Redox potentials were determined with a
scan rate of 100 mV/s at 25 °C by using a platinum disc working
electrode (2 mm diameter), a platinum wire counter electrode, and a
nonaqueous Ag+/Ag reference electrode, and referenced to ferrocene/
ferrocenium couple. Elemental analyses were performed by Midwest
Microlab LLC.
Figure 11. Calculated structures of the for the Cu dimer. The isovalue
is 0.5, and data are plotted in the range of −0.07 < sign(λ2)ρ < 0.07
au.
the Ni complex). However, the ΔG for the reaction with Cu is
less favorable by 4.6 kcal/mol than for the Mn complexes.
While the approximations employed in the calculations may
introduce errors in the ΔG values,24 clearly the Cu dimer is
significantly less stable than those containing Cr−Co. We also
performed NBO25 and NCI26−30 analysis on the Cu dimer (see
Supporting Information for details) that suggests that most of
the interactions between the two halves of the dimer are
primarily weak van der Waals interactions.
Synthesis and Characterization of Compounds. NiLi(Cl)-
(OR)2(THF)2 (5). A 5 mL diethyl ether solution of LiOR (47.7 mg,
0.211 mmol) was added in one portion to a stirred THF solution of
NiCl2(dme) (23.2 mg, 0.105 mmol). The solution color changed from
light orange to deep orange. The reaction was stirred for 1 h, after
which the volatiles were removed in vacuo. The resulting residue was
redissolved in hexane and filtered. Recrystallization from hexane at
−35 °C afforded the product as orange crystals (46.9 mg, 65%). IR
(cm−1): 2972 (m), 2876 (m), 1487 (w), 1391 (w), 1360 (w), 1099
(s), 1072 (m), 1043 (w), 1022 (s), 891 (m), 745 (s), 708 (s), 677(w),
SUMMARY AND CONCLUSIONS
■
We have described coordination chemistry of a bulky alkoxide
ligand [OtBu2Ph] with 3d transition metals Cr−Cu. The ligand
demonstrates versatile behavior at the 3d metal centers,
forming mononuclear (M(OR)2(THF)2, M = Mn, Fe, Co)
and heterodinuclear (M2Li2Cl2(OR)4 and MLiCl-
(OR)2(THF)2) complexes. DFT calculations demonstrate
that the formation of seesaw complexes M2Li2Cl2(OR)4 is
preferred for M = Cr−Co, whereas for Ni the MLiX-
(OR)2(THF)2 species are more stable. Surprisingly,
CuII(OR)2-type complexes are not stable, decomposing to
give [CuI(OR)]4 and the organic products consistent with the
transient alkoxy radical. All complexes under investigation
demonstrate coordination of two alkoxides to a single transition
metal center. Electrochemical and reactivity studies indicate
that hard alkoxide ligation prohibits redox chemistry at the
mononuclear Mn and Co centers in M(OR)2(THF)2. The
corresponding Fe(II) complex Fe(OR)2(THF)2, however,
demonstrates rich group-transfer and atom-transfer chemistry,
enabling formation of Fe(III)-hexazene,11b Fe(III)-(μ2-oxo),
and Fe(III)-(μ2-sulfido) species. We are currently investigating
the reactivity of the Fe complex Fe(OR)2(THF)2 in other
redox transformations.
1
650 (w). μeff = 3.0 0.2 μB. H NMR (C6D6, 400 MHz) δ 14.18,
10.55, 9.74, 7.86, 0.46, 0.02, −0.44, −0.65, −2.81. λmax (ε): 676 (22),
547 (99) M−1 cm−1. Anal. Calcd for C38H62O4ClLiNi: C, 66.7; H, 9.1.
Found: C, 66.7, H, 9.0.
NiLi(Br)(OR)2(THF)2 (6). A 5 mL diethyl ether solution of LiOR
(49.1 mg, 0.217 mmol) was added in one portion to a stirred THF
solution of NiBr2(dme) (33.5 mg, 0.109 mmol). The solution color
changed to deep orange. The reaction was stirred for 1 h, after which
the volatiles were removed in vacuo. The resulting residue was
redissolved in hexane and filtered. Recrystallization from hexane at
−35 °C afforded the product as orange crystals (49.8 mg, 63%). IR
(cm−1): 2972 (w), 2876 (w), 1522 (w), 1489 (m), 1393 (m), 1366
(w), 1098 (w), 1072 (m), 1043 (s), 1022 (w), 991 (s), 889 (m), 745
(s), 708 (s), 675 (m). μeff = 3.1 0.2 μB (calcd 2.83). 1H NMR (C6D6,
400 MHz) δ 13.79, 10.09, 9.75, 6.88, −1.16, −2.49, −3.28. λmax (ε):
682 (33), 559 (91) M−1 cm−1. Anal. Calcd for C38H62O4BrLiNi: C,
62.7; H, 8.6. Found: C, 61.7, H, 8.0%. Our repeated attempts to obtain
better elemental analysis for compound 6 failed. However, compound
6 is obtained in an identical fashion to compound 5 as large orange
crystals, and its spectroscopic and magnetic data match well that of
compound 5.
Cu4(OR)4 (7) via CuCl2. A 5 mL diethyl ether solution of LiOR
(45.2 mg, 0.200 mmol) was added in one portion to a stirred THF
solution of CuCl2 (13.4 mg, 0.100 mmol). The solution color
immediately changed to deep red before darkening to brown. The
reaction was stirred for 1 h, after which the volatiles were removed in
vacuo. The resulting residue was redissolved in hexane and filtered.
Recrystallization from hexane at −35 °C afforded the product as
EXPERIMENTAL SECTION
■
General Methods and Procedures. All reactions involving air-
sensitive materials were executed in a nitrogen-filled glovebox.
Compounds 1−4 and 9, as well as the lithium salt of the ligand
(LiOR), were synthesized according to previously reported
procedures.11 Thallium hexafluorophosphate, copper(II) chloride,
copper(II) bromide, and copper(I) chloride were purchased from
Strem. Manganese(II) chloride, cobalt(II) chloride, nickel(II) chloride
dimethoxyethane, nickel(II) bromide dimethoxyethane, and triphenyl-
stibine sulfide were purchased from Aldrich. Iodosobenzene was
purchased from TCI. All materials were used as received. All solvents
were purchased from Fisher Scientific and were of HPLC grade. The
1
colorless crystals (26.0 mg, 46%). H NMR (C6D6, 400 MHz) δ 8.52
(d, J = 8.0, 4H), 7.53 (d, J = 8.0, 4H), 7.17 (t, J = 4.0, 4H), 7.06 (m,
8H), 1.39 (s, 72H). 13C NMR (C6D6, 75 MHz) δ 149.02, 130.19,
128.50, 127.47, 126.07, 125.13, 86.32, 41.61, 31.22. IR (cm−1): 3001
(w), 2951 (w), 2905 (w), 2878 (w), 1489 (w), 1389 (m), 1362 (w),
1153 (w), 1049 (s), 995 (s), 895 (w), 745 (s), 706 (s). No discernible
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Article
features in the UV−vis spectrum. Anal. Calcd for C60H92O4Cu4: C,
63.7; H, 8.2. Found: C, 63.6, H, 8.0%.
0.1 μB. Anal. Calcd for C68H108Fe2O7: C, 71.1; H, 9.5. Found: C, 71.2;
H, 9.3%.
Cu4(OR)4 (7) via CuBr2. A 5 mL diethyl ether solution of LiOR
(47.8 mg, 0.211 mmol) was added in one portion to a stirred THF of
solution of CuBr2 (23.6 mg, 0.106 mmol). The solution color rapidly
changed through red, amber, and light blue before ultimately turning
colorless. The volatiles were removed in vacuo. The resulting residue
was redissolved in hexane and filtered. Recrystallization from hexane at
Fe2(S)(OR)4(THF)2 (12). To a stirred solution of Fe(OR)2(THF)2
(86.8 mg, 0.136 mmol) in toluene, 5 mL solution of triphenylstibine
sulfide (26.7 mg, 0.0693 mmol) in toluene was added at once. The
solution color immediately changed from light brown to deep red. The
reaction was stirred for 4 h, after which the solvent was removed. The
residual was dissolved in hexanes and filtered. Crystallization from
hexanes at −35 °C produced deep red X-ray quality crystals (47.3 mg,
60% yield). IR (cm−1): 2990 (s), 2879 (s), 1481(s), 1150 (m), 1013
(m), 891 (s), 745 (m), 704 (m). λmax (ε): 339 (7570), 497 (21 350)
1
−35 °C afforded the product as colorless crystals (25.2 mg, 42%). H
NMR spectrum of the product matched that of the product obtained
via CuCl2.
M
−1 cm−1. μeff = 2.8 0.2 μB. Anal. Calcd for C68H108Fe2O6S: C, 70.1;
Cu4(OR)4 (7) via CuCl. This procedure is identical to that for the
reaction with copper(II) halides. LiOR (49.3 mg, 0.218 mmol) was
added to CuCl (21.6 mg, 0.218 mmol) to give a colorless solution.
The reaction was stirred for 1 h, after which the volatiles were removed
in vacuo. The resulting residue was redissolved in hexane and filtered.
Recrystallization from hexane at −35 °C afforded the product as
colorless crystals (31.4 mg, 51%). 1H NMR matched that of the
product obtained via CuCl2.
Mn(OR)2(THF)2 (8). A 5.0 mL THF solution containing 27.8 mg
(0.0795 mmol) of thallium(I) hexafluorophosphate was added
dropwise to a 5.0 mL THF solution containing 42.6 mg (0.0397
mmol) of the dimeric manganese bis(alkoxide) complex
[Mn2Li2Cl2(OR)4] (2). Immediately a white precipitate formed. The
reaction was stirred for 1 h, after which the volatiles were removed in
vacuo. Hexanes (∼2.0 mL) were added to the remaining white cake,
and subsequent filtration gave a pale yellow solution. Recrystallization
from hexane at −35 °C afforded the product as white crystals (30.4
mg, 60%). IR (cm−1): 2988 (m), 2963 (m), 2880 (m), 1377 (w), 1356
(w), 1107 (s), 1067 (m), 1026 (m), 872 (m), 745 (s), 708 (s), 642
(w). μeff = 5.5 0.3 μB (calc. 5.9). No discernible features in the UV−
vis spectrum. Anal. Calcd for C38H62O4Mn: C, 71.6; H, 9.8. Found: C,
71.3, H, 9.7%.
H, 9.3. Found: C, 70.3; H, 9.5%.
Computational Details. All geometry optimizations were
performed at the B3LYP/SDDall level with the Gaussian0920 software
package as in our previous study.11a In all cases the Bu substituents
t
were replaced by Me groups for calculation efficiency. As shown in our
previous work,11a the structure of the metal and its coordination
sphere is minimally affected by this substitution. The correction to the
Gibbs free energy (G) was calculated using the harmonic
approximation using a scaling factor of 0.9806.21 Since both Ni−Cl
(5) and Ni−Br (6) show identical chemistry, both structures were
considered for the calculations as a consistency check. No difference
was observed between the complexes with the two different halides. In
the case of the Mn and Ni complexes, solvation was modeled with the
self-consistent reaction field (SCRF) method using the SMD solvation
approach.22 Two solvents were modeled: THF (ε = 7.4257) and
hexane (ε = 1.8819). All optimized structures were confirmed by
frequency calculations to have no imaginary frequencies. The relative
stability between dimer and monomer structures was calculated by
determining the Gibbs free energy change as described in Scheme 2.
The NCI surfaces were calculated by the NCIPLOT26,27 program.
NCI plots the reduced density gradient versus the product of the sign
of the second eigenvalue (λ2) of the electron-density Hessian matrix
and the electron density. The peaks at low electron density
characterize the noncovalent interactions.
Co(OR)2(THF)2 (10). A 5.0 mL THF solution containing 28.1 mg
(0.0806 mmol) of thallium(I) hexafluorophosphate was added
dropwise to a 5.0 mL THF solution containing 43.5 mg (0.0403
mmol) of the dimeric cobalt bis(alkoxide) complex [Co2Li2Cl2(OR)4]
(4). Immediately a white precipitate formed, and the reaction color
changed from blue to violet. The reaction was stirred for 1 h, after
which the volatiles were removed in vacuo. Hexanes (∼2.0 mL) were
added to the remaining white cake, and subsequent filtration gave a
violet solution. Recrystallization from hexane at −35 °C afforded the
product as violet crystalline rods (25.3 mg, 49%). IR (cm−1): 2988
(w), 2967 (w), 2895 (w), 2876 (w), 1489 (w), 1381 (w), 1360 (w),
ASSOCIATED CONTENT
* Supporting Information
■
S
Evans method formula and procedure, NMR, IR, and UV−vis
spectra, X-ray data in cif format, optimized Cu monomer, NCI
plot for the Fe dimer complex and for the hypothetical Cu
dimer, optimized geometries for all monomers and dimers, and
complete ref 22. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.inorgchem.5b00795.
1
1090 (m), 1063 (m), 1018 (s), 868 (m), 745 (s), 708 (s). H NMR
(C6D6, 400 MHz) δ 42.81, 24.95, 17.09, 13.05, 9.59, 9.39, 5.34. μeff
=
3.6 0.3 μB (calc. 3.87). λmax (ε): 734 (176), 578 (64), 472 (40), 386
(98) M−1 cm−1. Anal. Calcd for C38H62O4Co: C, 71.1; H, 9.7. Found:
C, 70.8, H, 9.6%.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: groysman@chem.wayne.edu. (S.G.)
*E-mail: andres@chem.wayne.edu. (A.C.)
■
Reaction of Ni(OR)2BrLi(THF)2 with TlPF6. The nickel complex 6
(80.7 mg, 0.11 mmol) and TlPF6 (38.7 mg, 0.11 mmol) were dissolved
in separate THF solutions. The thallium hexafluorophosphate solution
was added dropwise to the nickel solution, leading to the formation of
a white precipitate. The reaction was stirred for 1 h, after which the
volatiles were removed in vacuo, and the resulting residue was taken
up in hexane. After the insoluble precipitate was removed by filtration,
the solution was concentrated before crystallization at −35 °C was
attempted. Orange crystals were isolated, but they were confirmed to
be the starting complex 6 (48.8 mg, 60% recovery).
Fe2(O)(OR)4(THF)2 (11). To a stirred solution of Fe(OR)2(THF)2
(60.2 mg, 0.0943 mmol), 5 mL solution of PhIO (13.0 mg, 0.0591
mmol) in toluene was added at once. The reaction changed color
slowly from light brown to orange brown. The reaction was stirred for
4 h, after which the solvent was removed. The residual was dissolved in
hexanes and filtered. Crystallization from concentrated hexanes
produced orange crystals at −35 °C overnight (13.0 mg, 24% yield).
IR (cm−1): 2970 (m), 2878 (m), 1483 (s), 1387 (s), 1053 (m), 1009
(m), 864 (s), 795 (m), 750 (m), 706 (m). λmax (ε): 734 (176), 578
(64), 472 (40), 386 (98) M−1 cm−1. No discernible features in the
UV−vis spectrum, except for the shoulder around 480 nm. μeff = 2.8
Author Contributions
¶These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge Wayne State Univ. for the start-up funding
and American Chemical Society Petroleum Research Fund for
the current support of this project (54178-DNI3). We
gratefully acknowledge computing time from Wayne State
C&IT.
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