Simple routes to bulky silyl-substituted acetylide ligands and examples of V(iii), Fe(ii), and Mn(ii) complexes

Article abstract of DOI:10.1039/c1cc14758g

Synthesis of substituted phenylacetylide ligands 2,6-bis(trimethylsilyl) phenylacetylene (H1) and 2-(triphenylsilyl)phenylacteylene (H2) is reported. Ligand 1 supports tetrahedral complexes of V(iii), Fe(ii), and Mn(ii) (3-5). Complexes 3-5 are high-spin

Full text of DOI:10.1039/c1cc14758g

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Simple routes to bulky silyl-substituted acetylide ligands and examples of
V(III), Fe(II), and Mn(II) complexesw
Gereon M. Yee, Kristin Kowolik, Shuhei Manabe, James C. Fettinger and Louise A. Berben*
Received 2nd August 2011, Accepted 10th September 2011
DOI: 10.1039/c1cc14758g
Synthesis of substituted phenylacetylide ligands 2,6-
bis(trimethylsilyl)phenylacetylene (H1) and 2-(triphenylsilyl)-
phenylacteylene (H2) is reported. Ligand 1 supports tetrahedral
complexes of V(III), Fe(II), and Mn(II) (3–5). Complexes 3–5 are
high-spin and redox active.
ZnII which are four-coordinate due to an electronic preference
at the metal center (rather than a steric preference).7 We
demonstrate that bulky acetylide ligand 1 prevents formation
of six-coordinate complexes and supports coordinatively
unsaturated, tetrahedral and high-spin complexes of V(III),
Fe(II), and Mn(II).
The acetylide ligand 1 was obtained in two steps from
readily available phenylacetylene (Scheme 1). In the first step,
2-trimethylsilylphenylacetylene was synthesized in 95% yield
based on a reported procedure.8 In the second step, incorpora-
tion of the second TMS substituent to yield TMS-protected 1
was achieved by subjecting 2-trimethylsilylphenylacetylene to
a closely analogous procedure employed for its formation: two
equivalents of nBuLi, followed by two equivalents of both
KOtBu and then TMSCl were added to 2-trimethylsilylphe-
nylacetylene. Base hydrolysis of the unpurified reaction
mixture yielded a mixture of the three deprotected isomers
of 1 which were separated to give 1 in 49% yield (details are
given in the ESIw).
Synthesis of 2-(triphenylsilyl)phenylacetylene (2) was
achieved by heating a mixture of 2-bromo(phenylacetylene),
sodium metal and triphenylsilylchloride at reflux in toluene for
16 h.9 Upon workup, TMS-protected 2 was obtained in 50%
yield. Compound 2 could be obtained after removal of the
TMS protecting group by reaction with KOH in ethanol
solution (90% yield).
The acetylide ligand has enjoyed significant use in transition
metal chemistry. A notable example of its utility includes the
ability of the unsaturated p-system to mediate electronic
communication between metal centers up to extended
distances as in, for example, [C6F5(PPh3)2Pt]2(CRC)n,1 and
CpFe(CRC)2FeCp.2 Acetylide is a strongly donating ligand
and it is well-known that strongly donating and tunable
ligands such as phosphines and N-heterocyclic carbenes
feature prominently in transition metal synthesis and catalysis.
However, unlike phosphine and N-heterocyclic carbene
ligands, facile routes to substituted acetylide ligands are not
readily available. It has been established that the more
strongly donating N-heterocyclic carbene ligand offers
enhanced stability, and hence improved catalytic activity over
analogous phosphine complexes in some instances.3 We are
exploring whether the more donating and anionic acetylide
ligand can also serve as a tunable and highly donating
ancillary ligand.
Herein we present routes to modify the electronic and steric
properties of acetylenes by 2-or 2,6-substitution of the readily
available phenylacetylene architecture by using silyl substituents.
Transition metal complexes of moderately bulky acetylide
ligands have previously been reported; use of t-butylsilox-
substituted acetylide ligands enabled detailed electronic
characterization of the octahedral and trigonally distorted,
six-coordinate Ta, Zr, and Hf complexes.4 Homoleptic complexes
of –CRCH and –CRCSiMe3, for the first row transition
metals from vanadium across to cobalt are also hexacoordinate.5,6
In all of these examples the acetylide ligands are not bulky
enough to prevent formation of octahedral complexes. The
notable exceptions to this rule are complexes of NiII, CuII, and
Transition metal complexes of 1 were prepared by salt
metathesis reactions between the appropriate metal halide salt;
VCl3Á3THF suspended in benzene, or FeCl2, or MnI2 suspended
in THF and the lithium salt of 1. Following reaction for 24 h,
workup afforded [LiÁ4THF][(2,6-(Me3Si)2PhCRC)4V(III)] (3) as
Department of Chemistry, University of California, Davis, CA, 95616,
USA. E-mail: laberben@ucdavis.edu; Fax: (+1) 530 752 8995;
Tel: (+1) 530 752 8475
w Electronic supplementary information (ESI) available: Experimental
details, X-ray tables and electrochemical data. See DOI: 10.1039/
c1cc14758g
Scheme 1
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dark blue needle-shaped crystals in 42% yield, [LiÁTHF]2-
[(2,6-(Me3Si)2PhCRC)4Fe(II)] (4) as yellow block-shaped crystals
in 34% yield, or [LiÁTHF]2[(2,6-(Me3Si)2PhCRC)4Mn(II)] (5)
as colorless block-shaped crystals in 45% yield, respectively.
A single crystal X-ray crystallography experiment revealed in
each case, that the bulky acetylide ligand had successfully
constrained the coordination number of the first row transi-
tion metal ion to four (Fig. 1 and 2, S1, S2w). As mentioned
previously, the more commonly observed [M(CRCR)n]mÀ
formulation has n = 6.5,6 We have thus far been unsuccessful
in our attempts to prepare transition metal complexes of 2
and we speculate that the great steric bulk of 2 may hinder
formation of metal–ligand bonds. New approaches are under
way to overcome the current obstacles.
Fig. 2 Solid state structure of complex 5 showing the coordination
of Li+ ions to the alkyne ligands. Complex 4 displays an identical
Li+ coordination pattern. Purple, white, light blue atoms represent
manganese, carbon and silicon atoms, respectively. Thermal ellipsoids
shown at 40% probability and H atoms omitted for clarity.
Solid state structures of 3–5 confirmed formation of tetra-
hedral complexes. Complex 3 has the most regular tetrahedral
geometry and the C–M–C bond angles are each defined by the
symmetry of the space group as 109.41. In 4 and 5 the lithium
ions are coordinated between pairs of the CRC p-bonds and
two of the C–M–C angles are pinched whereas two are wider
than 109.41 at 101.3 A. In all three of the complexes, the
CRC bond lengths are very close to an ideal CRC triple
bond; 1.220(3), 1.208(8), and 1.225(2) A, which indicates that
there is no MRC–C–R (carbene) or MQCQC–R (cummulene)
character to the M–C–C–R moiety. The CRC bond length in
free acetylene is 1.2022(2) A.10
Electrochemical measurements were performed using the
cyclic voltammetry method on all compounds in 0.1 M
Bu4NPF6 THF solution (Fig. 3, S3–S5). For each of the
complexes an irreversible oxidation wave is observed; at +0.17
and +1.11 V vs. SCE for 3, at À0.23 V for 4 and at +0.35 V
for 5. No reversible oxidation events were observed. Some-
what surprisingly, the complexes each display reduction chem-
istry including reversible electrochemical behavior in some
cases. Compound 3 can be reduced at À2.24 V and this process
is followed by another irreversible process at À2.74 V vs. SCE
(Fig. 3). In contrast, 4 and 5 both display just one pseudo-
reversible reduction event at À2.12 V (DEp = 100 mV) and
À2.11 V (DEp = 90 mV), respectively. A cyclic voltammogram
Fig. 3 Electrochemical measurements for complexes
recorded in 0.1 M Bu4NPF6 THF solution with a glassy carbon
working electrode. Scan rate 100 mVsÀ1
3 and 4
.
for the ligand, 1, was collected under same conditions and
revealed no electrochemical events (Fig. S3w). However, it is
still possible that the reduction events observed for complexes
3–5 arise from reduction of the ligands and that these events
become more favorable after the ligand is coordinated to the
positively charged metal center. At present, we have no
conclusive evidence to rule out this possibility. The presence
of multiple reduction events in 3 suggests that at least one of
them is metal-based.
Magnetic susceptibility measurements performed on complexes
3–5 confirm the expected high spin states generally associated
with tetrahedral geometries. Each of the complexes displays
temperature independent behavior from 5–300 K and the
magnetic susceptibilities of the complexes at room temperature
are 2.78, 5.08, and 6.01 mB for V(III), Fe(II), and Mn(II),
respectively. These magnetic moments imply electronic
configurations for each of the ions which are d2, high-spin d4
and high-spin d5, respectively. The high-spin state for Fe(II)
acetylide complexes is unknown. Previously reported alkynyl
complexes of Fe(II) are all low-spin because the smaller
–CRCH and –CRCSiMe3 ligands permitted an octahedral
coordination geometry.5,6
Fig. 1 Solid state structure of complexes 3 and 4. Blue, green, white,
light blue atoms represent vanadium, iron, carbon and silicon atoms,
respectively. Thermal ellipsoids shown at 40% probability and H
atoms omitted for clarity. Selected bond lengths (A) and angles
(deg) for 3, [4]: M–C, 2.023(2) [2.043(3)]; CRC, 1.218(3) [1.219(5)];
C-Ar, 1.437(3) [1.445(5)]; C–M–C, 105.5(1), 106.8(1), 116.4(1)
[103.4(2), 122.7(1)]; M–CRC, 169.8(2) [165.3(3)]; CRC-Ar,
177.3(2) [177.0(4)].
We have demonstrated that the bulky acetylide ligand 1 can
be employed to reduce the coordination number of the acetylide
complexes of V(III), Fe(II), and Mn(II) down to four and alter
the electronic structure of the transition metal ions compared
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with the –CRCH ligand. The resulting complexes have a
tetrahedral coordination geometry in each case. Future work
will focus on exploration of higher oxidation states of these
complexes and on exploiting the unsaturated metal centers
toward the activation of small molecules.
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This work was supported by University of California Davis
start-up funding and the Office of Research. We thank T. W.
Myers for assistance with magnetic susceptibility
measurements.
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