Sigma bond metathesis with pentamethylcyclopentadienyl ligands in sterically crowded (C5Me5)3M complexes

Article abstract of DOI:10.1039/c002654a

To further explore the reactivity of the (C₅Me₅) ^- ligand in the sterically crowded (C₅Me₅) ₃M complexes, reactions with PhEEPh (E = S, Se, Te) have been examined. With M = La, Pr, Nd, Sm, and Y, PhSSPh reacts to form the expected reduction products, [(C₅Me₅)₂M(SPh)] ₂, but the major organic byproduct is not the sterically induced reduction product, (C₅Me₅)₂. Instead, the sigma bond metathesis product, C₅Me₅SPh, is the major byproduct. In contrast, reactions with (C₅Me₅) ₃Ce and (C₅Me₅)₃U gave a mixture of C₅Me₅SPh and (C₅Me₅)₂ as byproducts. PhSSPh reactions with the lanthanide nitrile adducts, (C ₅Me₅)₃Ln(NCCMe₃)₂ (Ln = La, Ce) and (C₅Me₅)₃Nd(NCCMe₃), formed [(C₅Me₅)₂Ln(SPh)(NCCMe ₃)]₂ and only C₅Me₅SPh as the byproduct. PhSeSePh reactions paralleled the PhSSPh results, but reactions of PhTeTePh with (C₅Me₅)₃La, (C₅Me ₅)₃Sm, and (C₅Me₅) ₃La(NCCMe₃)₂ gave only (C₅Me ₅)₂ as a byproduct. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c002654a

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www.rsc.org/dalton | Dalton Transactions
Sigma bond metathesis with pentamethylcyclopentadienyl ligands in sterically
crowded (C5Me5)3M complexes†
Thomas J. Mueller, Joseph W. Ziller and William J. Evans*
Received 9th February 2010, Accepted 13th April 2010
First published as an Advance Article on the web 13th May 2010
DOI: 10.1039/c002654a
To further explore the reactivity of the (C5Me5)- ligand in the sterically crowded (C5Me5)3M complexes,
reactions with PhEEPh (E = S, Se, Te) have been examined. With M = La, Pr, Nd, Sm, and Y, PhSSPh
reacts to form the expected reduction products, [(C5Me5)2M(SPh)]2, but the major organic byproduct is
not the sterically induced reduction product, (C5Me5)2. Instead, the sigma bond metathesis product,
C5Me5SPh, is the major byproduct. In contrast, reactions with (C5Me5)3Ce and (C5Me5)3U gave a
mixture of C5Me5SPh and (C5Me5)2 as byproducts. PhSSPh reactions with the lanthanide nitrile
adducts, (C5Me5)3Ln(NCCMe3)2 (Ln = La, Ce) and (C5Me5)3Nd(NCCMe3), formed
[(C5Me5)2Ln(SPh)(NCCMe3)]2 and only C5Me5SPh as the byproduct. PhSeSePh reactions paralleled
the PhSSPh results, but reactions of PhTeTePh with (C5Me5)3La, (C5Me5)3Sm, and
(C5Me5)3La(NCCMe3)2 gave only (C5Me5)2 as a byproduct.
A third type of reaction involving displacement of a penta-
Introduction
5
hapto (h -C5Me5)- ligand by a ligand of lower hapticity, e.g.
5
[N(SiMe3)2]-,5,8 is shown in eqn (4).
Since the isolation of the sterically crowded (h -C5Me5)3Sm com-
plex in 1991, three general modes of unexpected (C5Me5)- reactiv-
ity have been observed with these sterically crowded (C5Me5)3M
complexes depending on the substrate. The (C5Me5)3M complex
can act as a one electron reductant where one of the (C5Me5)-
ligands delivers an electron and forms half an equivalent of
pentamethylcyclopentadienyl dimer, (C5Me5)2,1–3 eqn (1).
1
2
(C5Me5)- → (C5Me5)2 + 1e-
(1)
(4)
One additional general reaction has been found for (C5Me5)3M
complexes, namely the formation of base adducts (C5Me5)3ML9–11
and (C5Me5)3ML2,5,11 eqn (5).
This process, which is exemplified in eqn (2), has been called
sterically induced reduction (SIR). A second type of reaction
1
involves h -like (C5Me5)- reactivity.
(2)
(5)
These reactions include insertion of CO, CO2, and CS2 into the M–
C(C5Me5) bond,1,4,5 olefin polymerization,6 ring-opening of THF,1
and hydrogenolysis,1,7 e.g. eqn (3).
Most of these reactions give single products in high yield.
However, the reactions of (C5Me5)3Sm with alkyl and aryl halides
were found to be more complicated,1 e.g. eqn (6). Byproducts
of nucleophilic displacement and radical reactions were observed
as well as (C5Me5)2, the byproduct that is characteristic of SIR
according to eqn (1).
(3)
Department of Chemistry, University of California, Irvine, California, 92697-
2025, (USA). E-mail: wevans@uci.edu; Fax: 949-824-2210
(6)
† Electronic supplementary information (ESI) available: experimental
details. CCDC reference numbers 765768–765771. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c002654a
We now report a reaction system for (C5Me5)3M complexes in
which more than one reaction pathway appears to be accessible,
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6767–6773 | 6767
©

namely the reactions with PhEEPh substrates (E = S, Se,
Te). The PhEEPh series is frequently examined in reductive f-
element chemistry12–15 since the (EPh)- products usually can be
fully characterized by X-ray crystallography and the reduction
potentials have a broad range: -1.75 V,16 -1.2 V,16 and -1.06 V17
vs. SCE for E = S, Se, and Te, respectively. Examination of the
reductive capacity of the (C5Me5)- ancillary ligand in (C5Me5)3M
complexes and the nitrile adducts (C5Me5)3M(NCCMe3)x with
PhEEPh reveals a surprising variation in reactivity as a function
of M, x, and E.
[(C5Me5)2La(TePh)]2, 1c
Following the procedure for 1a, PhTeTePh (51 mg, 0.12 mmol) was
reacted with (C5Me5)3La (67 mg, 0.12 mmol) in 5 mL of toluene.
[(C5Me5)2La(TePh)]2 was isolated as a yellow solid (44 mg, 76%
crystalline yield). 1H NMR (C6D6): d 7.88 (m, 2 H, TePh), 7.01 (m,
3 H, TePh), 2.25 (s, 30 H, C5Me5). Anal. Calcd for C52H70La2Te2:
C, 50.85; H, 5.75. Found: C, 50.99; H, 5.74. (C5Me5)2 was isolated
as the only byproduct along with unreacted PhTeTePh.
[(C5Me5)2La(SPh)(NCCMe3)]2, 2a
Following the procedure for 1a, PhSSPh (29 mg, 0.13 mmol)
was reacted with (C5Me5)3La(NCCMe3)2 (95 mg, 0.13 mmol) in
10 mL of toluene. [(C5Me5)2La(SPh)(NCCMe3)]2 was isolated as
a colorless solid (52 mg, 65% crystalline yield). 1H NMR (C6D6):
d 7.52 (m, 2 H, SPh), 7.05 (m, 3 H, SPh), 2.11 (s, 30 H, C5Me5),
0.82 (s, 9H, Me3CCN). Anal. Calcd for C62H88La2N2S2: C, 61.88;
H, 7.37; N, 2.33. Found: C, 61.83; H, 7.83; N, 2.29. C5Me5SPh was
isolated as the only observed byproduct.
Experimental
The syntheses and manipulations described below were con-
ducted under nitrogen with rigorous exclusion of air and water
using glovebox, vacuum line, and Schlenk techniques. Solvents
were dried over columns containing Q-5 and molecular sieves.
NMR solvents were dried over sodium-potassium alloy, degassed,
and vacuum transferred prior to use. PhSSPh, PhSeSePh, and
PhTeTePh were purchased from Aldrich and sublimed prior
to use. The (C5Me5)3M complexes (M = La,18 Ce,1 Pr,1 Nd,19
Sm,20 Y, 21 and U6), (C5Me5)3M(NCCMe3)2 (M = La,5 Ce11),
and (C5Me5)3M(NCCMe3) (M = Nd,11 U11) were prepared as
[(C5Me5)2La(SPh)(NCCMe3)]2 was also produced in quantita-
tive yield from the reaction of [(C5Me5)2La(SPh)]2, 1b, (13 mg,
0.013 mmol) with Me3CCN (3 mL, 0.027 mmol) in C6D6.
previously described. H and 13C NMR spectra were obtained
1
[(C5Me5)2La(SePh)(NCCMe3)]2, 2b
on a Bruker DRX500 MHz spectrometer at 25 ◦C. IR samples
were prepared as KBr pellets and the spectra were obtained on a
Varian 1000 FT-IR system. Elemental analyses were performed on
a Perkin Elmer 2400 Series II CHNS analyzer. Mass spectrometry
analysis was performed on a Thermo Trace MS+ GCMS.
Following the procedure for 1a, PhSeSePh (33 mg, 0.11 mmol)
was reacted with (C5Me5)3La(NCCMe3)2 (75 mg, 0.11 mmol) in
10 mL of toluene. [(C5Me5)2La(SePh)(NCCMe3)]2 was isolated as
a pale yellow solid (39 mg, 57% crystalline yield). Pale yellow X-ray
quality crystals were grown from a concentrated toluene solution
at -35 ◦C and identified by X-ray diffraction. 1H NMR (C6D6): d
7.81 (m, 2 H, SePh), 6.99 (m, 3 H, SePh), 2.16 (s, 30 H, C5Me5),
0.80 (s, 9H, Me3CCN). Anal. Calcd for C62H88La2N2Se2: C, 57.41;
H, 6.84; N, 2.16. Found: C, 57.20; H, 6.52; N, 2.02. C5Me5SePh
was isolated as the only byproduct.
[(C5Me5)2La(SPh)]2, 1a
PhSSPh (10 mg, 0.046 mmol) was added to a stirred solution
of (C5Me5)3La (25 mg, 0.046 mmol) in 5 mL of toluene. After
stirring for 12 h, the solvent was removed under reduced pressure
to yield a tacky yellow solid which was washed with hexane to yield
the previously characterized [(C5Me5)2La(SPh)]213e in quantitative
yield. The hexane wash was dried under reduced pressure to
yield the previously characterized byproducts C5Me5SPh22 and
[(C5Me5)2La(TePh)(NCCMe3)]2, 2c
Following the procedure for 1a, PhTeTePh (44 mg, 0.11 mmol)
was reacted with (C5Me5)3La(NCCMe3)2 (76 mg, 0.11 mmol) in
10 mL of toluene. [(C5Me5)2La(TePh)(NCCMe3)]2 was isolated
23
1
(C5Me5)2 which were identified by mass spectrometry and H
NMR spectroscopy (17 : 1 ratio).
1
as a pale yellow solid (27 mg, 19% crystalline yield). H NMR
(C6D6): d 8.15 (m, 2 H, TePh), 7.00 (m, 3 H, TePh), 2.24 (s, 30 H,
C5Me5), 0.74 (s, 9H, Me3CCN). (C5Me5)2 was isolated as the only
byproduct along with unreacted PhTeTePh.
[(C5Me5)2La(SePh)]2, 1b
Following the procedure for 1a, PhSeSePh (57 mg, 0.18 mmol)
was reacted with (C5Me5)3La (98 mg, 0.18 mmol) in 10 mL of
toluene. [(C5Me5)2La(SePh)]2 was isolated as a pale yellow solid
(78 mg, 77% crystalline yield). Pale yellow X-ray quality crystals
[(C5Me5)2Ce(SPh)]2, 3
◦
were grown from a concentrated toluene solution at -35 C and
Following the procedure for 1a, PhSSPh (24 mg, 0.11 mmol)
was reacted with (C5Me5)3Ce (52 mg, 0.095 mmol) in 8 mL of
toluene. [(C5Me5)2Ce(SPh)]2 was isolated as a dark purple solid
(35 mg, 65% crystalline yield). Purple X-ray quality crystals
the structure was determined (see below). 1H NMR (C6D6): d 7.53
(m, 2 H, SePh), 7.09 (m, 2 H, SePh), 7.01 (m, 1 H, SePh), 2.21 (s, 30
H, C5Me5). 13C NMR (C6D6): d 134.9 (SePh), 129.5 (SePh), 125.3
(SePh), 123.1 (C5Me5), 12.6 (C5Me5). IR: 3053w, 2960 m, 2905 s,
2856 s, 2726w, 2214w, 1575 s, 1529w, 1471 s, 1434 s, 1380 m, 1068
m, 1020 m, 999w, 803w, 733 s, 691 s, 667 m, 611w, 570w cm-1.
Anal. Calcd for C52H70La2Se2: C, 55.23; H, 6.24. Found: C,
55.34; H, 6.24. The byproducts C5Me5SePh24 and (C5Me5)2 were
identified by mass spectrometry and 1H NMR spectroscopy (22 : 1
ratio).
◦
were grown from a concentrated toluene solution at -35 C and
structurally characterized. 1H NMR (C6D6): d 5.26 (s, 30 H,
C5Me5). The phenyl resonances were not observed. Anal. Calcd
for C52H70Ce2S2: C, 60.08; H, 6.79. Found: C, 59.72; H, 6.83.
The byproducts C5Me5SPh and (C5Me5)2 were identified by mass
1
spectrometry and H NMR spectroscopy (2 : 1 ratio) along with
unreacted PhSSPh.
6768 | Dalton Trans., 2010, 39, 6767–6773
This journal is
The Royal Society of Chemistry 2010
©

[(C5Me5)2Ce(SPh)(NCCMe3)]2, 4
[(C5Me5)2Sm(TePh)]2, 8c
Following the procedure for 1a, PhSSPh (36 mg, 0.16 mmol)
was reacted with (C5Me5)3Ce(NCCMe3)2 (116 mg, 0.16 mmol)
in 10 mL of toluene. [(C5Me5)2Ce(SPh)(NCCMe3)]2 was isolated
as a bright orange solid (59 mg, 60% crystalline yield). 1H NMR
(C6D6): d 4.56 (s, 30 H, C5Me5), (1.50 s, 2 H, SPh), -0.04 (s, 3
H, SPh), -4.50 (s, 9H, Me3CCN). Anal. Calcd for C62H88Ce2N2S2:
C, 61.76; H, 7.36; N, 2.32. Found: C, 61.74; H, 7.74; N, 2.31.
C5Me5SPh was isolated as the only observed byproduct.
Following the procedure for 1a, PhTeTePh (10 mg, 0.024 mmol)
was reacted with (C5Me5)3Sm (13 mg, 0.023 mmol) in 2 mL of
toluene. Quantitative conversion to the previously characterized
[(C5Me5)2Sm(TePh)]2 was observed by H NMR spectroscopy.
(C5Me5)2 was isolated as the only byproduct along with unreacted
PhTeTePh.
15a
1
[(C5Me5)2Y(SPh)]2, 9
Following the procedure for 1a, PhSSPh (12 mg, 0.055 mmol)
was reacted with (C5Me5)3Y (27 mg, 0.054 mmol) in 5 mL
of methylcyclohexane. Quantitative conversion to the previously
[(C5Me5)2Pr(SPh)]2, 5
Following the procedure for 1a, PhSSPh (14 mg, 0.064 mmol)
was reacted with (C5Me5)3Pr (33 mg, 0.060 mmol) in 5 mL of
toluene. [(C5Me5)2Pr(SPh)]2 was isolated as a bright yellow solid
(55 mg, 79% crystalline yield). 1H NMR (C6D6): d 11.49 (s, 30 H,
C5Me5). The phenyl resonances were not observed. Anal. Calcd
for C52H70Pr2S2: C, 59.99; H, 6.78. Found: C, 60.11; H, 7.41.
The byproducts C5Me5SPh and (C5Me5)2 were identified by mass
spectrometry and 1H NMR spectroscopy (13 : 1 ratio).
characterized [(C5Me5)2Y(SPh)]2 was observed by 1H NMR
13e
spectroscopy. The byproducts C5Me5SPh and (C5Me5)2 were
identified by mass spectrometry and 1H NMR spectroscopy (13 : 1
ratio).
(C5Me5)2U(SPh)2, 10
Following the procedure for 1a, PhSSPh (30 mg, 0.14 mmol)
was reacted with (C5Me5)3U (44 mg, 0.068 mmol) in 5 mL of
toluene. Quantitative conversion to the previously characterized
[(C5Me5)2Nd(SPh)]2, 6
(C5Me5)2U(SPh)2 was observed by 1H NMR spectroscopy. The
25
Following the procedure for 1a, PhSSPh (20 mg, 0.096 mmol)
was reacted with (C5Me5)3Nd (51 mg, 0.092 mmol) in 5 mL of
toluene. [(C5Me5)2Nd(SPh)]2 was isolated as a pale green solid
(45 mg, 87% crystalline yield). 1H NMR (C6D6): d 11.70 (s, 30 H,
C5Me5). The phenyl resonances were not observed. Anal. Calcd
for C52H70Nd2S2: C, 59.61; H, 6.73. Found: C, 59.07; H, 6.57.
The byproducts C5Me5SPh and (C5Me5)2 were identified by mass
spectrometry and 1H NMR spectroscopy (32 : 1 ratio).
byproducts C5Me5SPh and (C5Me5)2 were identified by mass
1
spectrometry and H NMR spectroscopy (1 : 4 ratio) along with
unreacted PhSSPh.
10 from (C5Me5)3U(NCCMe3) with PhSSPh
Following the procedure for 1a, PhSSPh (36 mg, 0.16 mmol) was
reacted with (C5Me5)3U(NCCMe3) (60 mg, 0.083 mmol) in 5 mL
of toluene. Quantitative conversion to the previously characterized
[(C5Me5)2Nd(SPh)(NCCMe3)]2, 7
(C5Me5)2U(SPh)2 was observed by 1H NMR spectroscopy. The
25
Following the procedure for 1a, PhSSPh (16 mg, 0.073 mmol) was
reacted with (C5Me5)3Nd(NCCMe3) (46 mg, 0.073 mmol) in 5 mL
of toluene. [(C5Me5)2Nd(SPh)(NCCMe3)]2 was isolated as a blue
green solid (31 mg, 70% crystalline yield). 1H NMR (C6D6): d 9.36
(s, 30 H, C5Me5), -8.80 (s, 9H, Me3CCN). The phenyl resonances
were not observed. Anal. Calcd for C62H88Nd2N2S2: C, 61.34; H,
7.31; N, 2.31. Found: C, 61.01; H, 7.23; N, 2.23. C5Me5SPh was
isolated as the only byproduct.
byproducts C5Me5SPh and (C5Me5)2 were identified by mass
spectrometry and H NMR spectroscopy (1 : 3 ratio) along with
unreacted PhSSPh.
1
C5Me5· with PhSSPh
(C5Me5)2 (12 mg, 0.044 mmol) and PhSSPh (10 mg, 0.046 mmol)
were dissolved in 1 mL of C6D6 and transferred to a J-Young tube
which was subsequently evacuated to the vapor pressure of the
solvent and heated to 90 ◦C. After 12 h, the solution turned bright
yellow. The 1H NMR spectrum showed the complete conversion of
(C5Me5)2 to C5Me5H and tetramethylfulvene along with unreacted
PhSSPh. No evidence was observed for C5Me5SPh.
[(C5Me5)2Sm(SPh)]2, 8a
Following the procedure for 1a, PhSSPh (17 mg, 0.078 mmol)
was reacted with (C5Me5)3Sm (42 mg, 0.076 mmol) in 5 mL of
toluene. Quantitative conversion to the previously characterized
15a
1
[(C5Me5)2Sm(SPh)]2 was observed by H NMR spectroscopy.
The byproducts C5Me5SPh and (C5Me5)2 were identified by mass
spectrometry and 1H NMR spectroscopy (15 : 1 ratio).
X-ray data collection, structure determination, and refinement for
1b, 2b, 3, and 5
Complexes [(C5Me5)2La(SePh)]2, 1b, Fig. 1, [(C5Me5)2Ce(SPh)]2,
3, and [(C5Me5)2Pr(SPh)]2, 5, are isomorphous and analogous to
those of the previously characterized [(C5Me5)2M(SPh)]2 (M =
La,15e Sm,15a Y15e) and display no unusual bond distances or angles.
The structure of [(C5Me5)2La(SePh)(NCCMe3)2]2, 2b, Fig. 2, is
similar to that of 1b with one tert-butyl nitrile coordinated to each
metal center. Detailed X-ray data information is available in the
supporting information.†
[(C5Me5)2Sm(SePh)]2, 8b
Following the procedure for 1a, PhSeSePh (14 mg, 0.045 mmol)
was reacted with (C5Me5)3Sm (25 mg, 0.045 mmol) in 5 mL of
toluene. Quantitative conversion to the previously characterized
15a
1
[(C5Me5)2Sm(SePh)]2 was observed by H NMR spectroscopy.
C5Me5SePh was isolated as the only byproduct.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6767–6773 | 6769
©

Scheme 1. Indeed, the reaction with E = Te formed (C5Me5)2
as the only observed byproduct, eqn (8).
(8)
Fig. 1 Thermal ellipsoid plot of [(C5Me5)2La(SePh)]2, 1b, drawn at the
50% probability level. Hydrogen atoms have been omitted for clarity.
Scheme 1
However, in the (C5Me5)3La reactions with PhEEPh where E = S
and Se, the major byproduct of the reaction was the corresponding
C5Me5EPh. (C5Me5)2 was observed only as a minor byproduct with
C5Me5EPh : (C5Me5)2 ratios of roughly 20 : 1, eqn (9).
(9)
The formation of C5Me5EPh in these reactions could be
1
explained by a sigma bond metathesis (SBM)26 involving an (h -
C5Me5)- ligand, Scheme 2. This type of SBM was previously
observed in the reaction of [(C5H4CMe3)2Ln(m-Me)]2 (Ln = Y,
Lu) with REER (E = S, Se; R = Ph, nBu, tBu, CH2Ph) that forms
MeER and [(C5H4CMe3)2Ln(m-ER)]2.27
Fig. 2 Thermal ellipsoid plot of [(C5Me5)2La(SePh)(NCCMe3)]2, 2b,
drawn at the 50% probability level. There are two independent molecules of
2b in the crystal structure each lying about independent inversion centers.
The second independent molecule of 2b, hydrogen atoms and benzene
solvent molecules have been omitted for clarity.
Results
The reaction of (C5Me5)3La with 1 equiv of PhEEPh yielded the
[(C5Me5)2La(EPh)]2 organometallic products (E = S, 1a; Se, 1b;
Te, 1c) expected on the basis of previous reductions of PhEEPh
with divalent lanthanide complexes, eqn (7).15a
Scheme 2
Alternatively, C5Me5EPh could form in a process in which the
C5Me5· radical, formed when a (C5Me5)- ligand gives up one
electron, e.g. eqn (1),1,2,3,21,28 is trapped by PhSSPh, a known radical
trapping reagent,29 Scheme 3. To test the possibility in Scheme 3,
a solution of (C5Me5)2 was heated in the presence of PhSSPh.
It has previously been shown that heat will crack (C5Me5)2
to produce two C5Me5· radicals.30 However, no evidence was
observed for C5Me5SPh. Only the (C5Me5)2 disproportionation
products, C5Me5H and tetramethylfulvene, were observed along
(7)
In a sterically induced reduction reaction, this would produce
(C5Me5)2 as the byproduct from the half-reactions shown in
Scheme 3
6770 | Dalton Trans., 2010, 39, 6767–6773
This journal is
The Royal Society of Chemistry 2010
©

Table 1 Percentages of the byproducts, C5Me5SPh and (C5Me5)2, formed
in the reactions of (C5Me5)3M and (C5Me5)3M(NCCMe3)x with PhSSPh
C5Me5SPh
(C5Me5)2
(C5Me5)3La
(C5Me5)3La(NCCMe3)2
(C5Me5)3Ce
(C5Me5)3Ce(NCCMe3)2
(C5Me5)3Pr
94
100
67
100
93
6
0
33
0
7
(C5Me5)3Nd
(C5Me5)3Nd(NCCMe3)
(C5Me5)3Sm
97
100
94
3
0
6
(12)
However, even with Ln = Ce, the only observed byprod-
uct was C5Me5SPh. Analogous results were obtained from
the reaction of (C5Me5)3La(NCCMe3)2 with PhSeSePh that
formed [(C5Me5)2La(SePh)(NCCMe3)]2, 2b, and C5Me5SePh as
the only observed byproduct. Although these nitrile reactions
with PhSSPh and PhSeSePh favored formation of C5Me5EPh,
the reaction of (C5Me5)3La(NCCMe3)2 with PhTeTePh formed
[(C5Me5)2La(TePh)(NCCMe3)]2, 2c, and only (C5Me5)2 as a
byproduct.
In contrast to the lanthanide nitrile adducts, the presence of
nitrile did not significantly affect the byproduct ratio in the
analogous uranium reaction. Hence, (C5Me5)3U(NCCMe3) reacts
with PhSSPh to form (C5Me5)2U(SPh)2, 10, with a 1 : 3 ratio of
C5Me5SPh and (C5Me5)2 byproducts that is similar to the 1 : 4 ratio
observed in the reaction of (C5Me5)3U with PhSSPh.
(C5Me5)3Y
(C5Me5)3U
(C5Me5)3U(NCCMe3)
93
20
25
7
80
75
with unreacted PhSSPh. A control reaction was run by heating
an isolated sample of C5Me5SPh to 110 ◦C in toluene: no
decomposition was observed. Hence, the formation of C5Me5EPh
is most consistent with sigma bond metathesis.
The formation of (C5Me5)2 in the E = Te reaction is consistent
with SIR. However, it would be possible for C5Me5TePh to form via
SBM and then decompose to (C5Me5)2 and PhTeTePh, eqn (10).
To test this possibility, a procedure analogous to the synthesis used
to produce C5Me5SPh22 was attempted with PhTeTePh. However,
no reaction was observed between LiC5Me5 and PhTeTePh.
?
(10)
2C5Me5TePh ⎯⎯→(C5Me5 )2 + PhTeTePh
Discussion
The results show that at least two reaction pathways are possible
in reactions of (C5Me5)- ligands in sterically crowded (C5Me5)3M
complexes with PhEEPh which form [(C5Me5)2M(EPh)]2 products
in high yield. One of these is SIR, eqn (1), which should form
only (C5Me5)2 as a byproduct. This type of reaction has been
extensively demonstrated with a variety of substrates.1 Another
possible reaction pathway is sigma bond metathesis that should
form C5Me5EPh. Sigma bond metathesis is not a commonly
encountered reaction for (C5Me5)3M complexes and has only been
observed previously with H2 in the hydrogenolysis shown in eqn
(3).6 The fact that thermolysis of (C5Me5)2 in the presence of
PhEEPh does not form C5Me5EPh suggests that a third possible
reaction pathway involving a SIR process in which a C5Me5· is
trapped by PhEEPh is less likely.
Reactions with (C5Me5)3M complexes (M = Ce, Pr, Nd, Sm, Y, U)
The reactions of PhSSPh with the more sterically crowded
(C5Me5)3M complexes of the smaller metals, Pr, Nd, Sm, and Y,
also produced the expected organometallic reduction products,
[(C5Me5)2M(SPh)]2 (M = Pr, 5; Nd, 6; Sm, 8a; Y, 9) along
with the byproducts C5Me5SPh and (C5Me5)2, in this case in
an approximately 15 : 1 ratio. The percentage of each byproduct,
C5Me5SPh and (C5Me5)2, for each reaction, is shown in Table 1.
The reactions of PhSSPh with (C5Me5)3M (M = Ce, U) gave
the analogous metal-containing products, [(C5Me5)2Ce(SPh)]2, 3,
and (C5Me5)2U(SPh)2, 10, eqn (11), respectively. However, much
different, yet reproducible C5Me5SPh to (C5Me5)2 ratios were
observed: 2 : 1 for cerium and 1 : 4 for uranium, Table 1.
The formation of some (C5Me5)2 byproduct in the E = S and Se
reactions is consistent only with a SIR reaction. The implications
of the formation of (C5Me5)2 in the E = Te reactions are less clear
with respect to SIR since C5Me5TePh is not known. It is possible
that C5Me5TePh may have been produced by SBM followed by
a decomposition to (C5Me5)2 and PhTeTePh, eqn (10). Since
C5Me5TePh could not be generated from the reaction of LiC5Me5
with PhTeTePh, it was not possible to test this decomposition.
The exclusive formation of (C5Me5)2 in reductions of PhTeTePh
may be related to the fact that PhTeTePh is more easily reduced
(-1.06 V vs. SCE)17 than PhSeSePh (-1.2 V vs. SCE)16 and
PhSSPh (-1.75 V vs. SCE).16 It is conceivable that with this easily
reduced substrate, SIR is the predominant reaction channel. With
PhSeSePh and PhSSPh, sigma bond metathesis may have the
lowest activation energy.
(11)
Reactions with (C5Me5)3Ln(NCCMe3)x
To further assess the effects of coordination environment on this
system, reactions of the nitrile adducts (C5Me5)3Ln(NCCMe3)x
(Ln = La, x = 2; Ce, x = 2; Nd, x = 1) with PhSSPh were
examined. These yielded nitrile adducts of the organometallic
reduction products found above, [(C5Me5)2Ln(SPh)(NCCMe3)]2,
2a, 4, and 7, respectively, eqn (12).
To examine if the ratio of (C5Me5)2 SIR byproducts to
C5Me5EPh SBM products was influenced by the reduction
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6767–6773 | 6771
©

Table 2 Percentages of the byproducts, C5Me5EPh and (C5Me5)2, formed
substrates, more than one reaction pathway is accessible with
these sterically crowded complexes. In previous studies, one single
type of byproduct is normally observed. These studies also show
that sigma bond metathesis may be much more prevalent for
(C5Me5)3M complexes than previously thought. The PhTeTePh
reactions suggest that SIR will be favored when the substrate
is easily reduced. The (C5Me5)3Ln(NCCMe3)x reactions suggest
that adding ligands to (C5Me5)3Ln complexes will favor SBM
over SIR. The reaction of (C5Me5)3Ce with PhSSPh indicates
that cerium can have a reaction profile different from other
(C5Me5)3Ln complexes, a feature previously not observed in
(C5Me5)3Ln reactivity. The implications of these conclusions on
other (C5Me5)3M reactions are under study.
in the reactions of (C5Me5)3La and (C5Me5)3Sm with PhEEPh
(C5Me5)3La
(C5Me5)3Sm
E
C5Me5EPh
(C5Me5)2
E
C5Me5EPh
(C5Me5)2
S
Se
Te
94
96
0
6
4
100
S
Se
Te
94
100
0
6
0
100
potentials of the substrates and the reducing capacity of the
(C5Me5)3M complexes, the reactions of (C5Me5)3La with PhSSPh
and PhSeSePh were compared to reactions with the stronger
reducing agent,1 (C5Me5)3Sm. (C5Me5)3Sm was selected as it has
previously been shown to reduce cyclooctatetraene (-1.86 V vs.
SCE)31 in a SIR process.7 As shown in Table 2, the greater
reducing power of (C5Me5)3Sm made little difference in the ratios
of byproducts observed in these reactions and the more easily
reduced PhSeSePh has a higher ratio of C5Me5EPh to (C5Me5)2
than that observed in the PhSSPh reactions. This higher ratio of
C5Me5SePh to (C5Me5)2 is consistent with a SBM mechanism in
which the weaker Se–Se bond in PhSeSePh is easier to break.
The reactions of the lanthanide nitrile adducts,
(C5Me5)3Ln(NCCMe3)x, with PhSSPh that yield C5Me5SPh
as the only byproduct, with no evidence for (C5Me5)2, are also
consistent with a SBM reaction pathway. The nitrile adducts are
more sterically crowded and have Ln–C(C5Me5) bonds that are
Acknowledgements
We thank the National Science Foundation for support of this
research and Dr Michael K. Takase for assistance with X-ray
crystallography.
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