A well-defined model system for the chromium-catalyzed selective oligomerization of ethylene

Article abstract of DOI:10.1039/c3dt00109a

The chromium(i) dinitrogen complex [(i-Pr₂Ph) ₂nacnacCr]₂(μ-η²:η²- N₂) catalyzes the selective trimerization of ethylene to 1-hexene at ambient pressure and temperature, and in the absence of any cocatalyst. After the conversion of the substrate, the catalyst cleanly converts to another chromium(i) species, namely [(i-Pr₂Ph)₂nacnacCr] ₂(μ-η²:η²-C₂H ₄), which is not catalytically active. Binuclear metallacycles containing Cr(ii) have been prepared as candidates for catalytically active intermediates; however they are not kinetically competent to explain the catalysis. Turning thus to mononuclear metallacycles featuring Cr(iii), a chromacyclopentane, chromacyclopentene and chromacyclopentadiene have been prepared as models of catalytic intermediates. Of these, the latter also catalyzes the trimerization of ethylene. These results support the proposal that selective ethylene oligomerization catalysis involves an interplay between Cr(i) ethylene complexes and mononuclear Cr(iii) metallacycles.

Full text of DOI:10.1039/c3dt00109a

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A well-defined model system for the chromium-
catalyzed selective oligomerization of ethylene†
Cite this: DOI: 10.1039/c3dt00109a
Wesley H. Monillas, John F. Young, Glenn P. A. Yap and Klaus H. Theopold*
The chromium(I) dinitrogen complex [(i-Pr2Ph)2nacnacCr]2(μ–η2:η2-N2) catalyzes the selective trimerization
of ethylene to 1-hexene at ambient pressure and temperature, and in the absence of any cocatalyst.
After the conversion of the substrate, the catalyst cleanly converts to another chromium(I) species,
namely [(i-Pr2Ph)2nacnacCr]2(μ–η2:η2-C2H4), which is not catalytically active. Binuclear metallacycles con-
taining Cr(II) have been prepared as candidates for catalytically active intermediates; however they are
not kinetically competent to explain the catalysis. Turning thus to mononuclear metallacycles featuring
Cr(III), a chromacyclopentane, chromacyclopentene and chromacyclopentadiene have been prepared as
models of catalytic intermediates. Of these, the latter also catalyzes the trimerization of ethylene. These
results support the proposal that selective ethylene oligomerization catalysis involves an interplay
between Cr(I) ethylene complexes and mononuclear Cr(III) metallacycles.
Received 11th January 2013,
Accepted 6th March 2013
DOI: 10.1039/c3dt00109a
www.rsc.org/dalton
Introduction
Results and discussion
Catalysis
α-Olefins are valuable precursors for a wide range of chemical
processes.1 Owing to the variety of their applications, selective
syntheses, e.g. of 1-hexene or 1-octene, are inherently prefer-
able to the statistical (Schulz–Flory) mixtures resulting from
the widely practiced ethylene oligomerization reactions based
on the Cossee mechanism.2 Originally discovered in the
‘60s,3,4 there has been a recent flurry of interest in the selective
trimerization of ethylene to 1-hexene, predominantly catalyzed
by chromium complexes.5–40 While the differential reactivity of
metallacycles of different ring sizes is widely held to be the
cause of this unusual selectivity, many questions about the
mechanistic details remain; among these are the formal
oxidation states and charges of the intermediates, the role of
mononuclear versus binuclear catalysts, and the exact nature of
the product forming step(s). Most relevant catalysts require
activation by cocatalysts (e.g. MAO), which complicate spectro-
scopic detection or isolation of intermediates.41 There exists
thus a need for well-defined compounds that catalyze ethylene
trimerization in isolation. This contribution describes such a
system.
Some time ago we reported the synthesis and characterization
of several compounds featuring chromium in the rare formal
oxidation state +I;42,43 the most reactive of these is the
side-on dinitrogen complex [(i-Pr2Ph)2nacnacCr]2(μ–η2:η2-N2) (1,
(i-Pr2Ph)2nacnac = 2,4-pentane-N,N′-bis(2,6-diisopropylphenyl)-
diketiminate).44 1 reacts cleanly with ethylene to eventually
yield [(i-Pr2Ph)2nacnacCr]2(μ–η2:η2-C2H4) (2) as the sole organo-
metallic product;43 however, before the latter is formed all
available ethylene is oligomerized to 1-hexene and smaller
amounts of 1-butene. Thus, structurally characterized 1 is a
catalyst – or a catalyst precursor – for the selective trimerization
of ethylene, which does not require a cocatalyst (see Scheme 1).
Monitoring (by 1H NMR) the reaction of 1 with an
excess of ethylene at ambient temperature in a sealed NMR
tube revealed the gradual replacement of the broad and
Department of Chemistry and Biochemistry, Center for Catalytic Science and
Technology, University of Delaware, Newark, Delaware 19716, USA.
E-mail: theopold@udel.edu; Fax: +1 3028316335; Tel: +1 3028311546
†CCDC 698322 and 918023–918028. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3dt00109a
Scheme 1 Chromium catalyzed trimerization of ethylene.
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Table 1 Ethylene trimerization catalyzed by 1
C2H4
pressure
(psi)
Turnover
(mmol C6/
mmol cat/h)
Catalyst
(mmol)
Solvent
(mL)
Temperature
(°C)
Runa
1
2
3
4
5
6
7
0.031
0.031
0.031
0.031
0.093
0.031
0.031
10
10
10
10
30
10
10
15
40
50
60
215
15
23
23
23
23
23
40
40
0.76
5.7
11.4
26.9
172.5
1.2
40
8.4b
Scheme 2 Mechanisms for selective ethylene oligomerization.
Catalyst (M)
Rate (M h−1
0.00064
0.00234
0.00322
0.00485
0.00810
)
8
1
9
10
11
0.0016
0.0031
0.0041
0.0062
0.0093
15
15
15
15
15
23
23
23
23
23
Attempts to isolate the spectroscopically observed inter-
mediate formed during the catalytic reaction – e.g. by cooling
solutions under ethylene – were not successful, nor were we
able to trap this species by the addition of potential ligands
(e.g. pyridine) to catalytically active solutions. However, borrow-
ing an experiment used by Mashima et al.,46 we found that
addition of a methylene chloride solution of bromine (Br2) to a
reaction in progress produced [(i-Pr2Ph)2nacnacCr(μ-Br)]2 and
1,4-dibromobutane (identified by retention time in the GC and
by GCMS). The formation of the latter at a minimum suggests
that the intermediate results from coupling of two ethylene
units; it is most likely a metallacycle of some type.
These observations are consistent with the commonly
suggested mechanism for the selective trimerization of ethyl-
ene by transition metal catalysts (see Scheme 2). According to
this proposal,47 coordination of two molecules of ethylene to
each chromium(I), followed by reductive coupling, forms a
mononuclear metallacyclopentane containing Cr(III). The latter
is a candidate for the yet unidentified species spectroscopically
observed during turnover. Insertion of another ethylene into
the relatively stable chromacyclopentane yields the homo-
logous chromacycloheptane. Due to its greater conformational
flexibility, β-hydrogen elimination and reductive elimination of
the resulting alkenyl hydride are kinetically favored over
further ring expansion, thus explaining the selectivity for
1-hexene.
The binuclear nature of both 1 and 2 raised the question
whether a bimetallic mechanism might be operative. Among
the various lines of evidence arguing against such a pathway
we mention but two at this juncture. First, the lack of reactivity
of 2 with added ethylene would seem to be an obstacle to such
a mechanism. Furthermore, we have shown that binuclear
chromium alkyl/aryl hydrides, such as might be generated by
such a mechanism, are resistant to reductive elimination,42,48
militating against such a transformation as the product
forming step of the catalytic cycle. However, a concerted
1–5 hydrogen shift has been discussed as an alternative chain
transfer step.
a Description of typical method can be found in the Experimental section.
b Turnover for reactions at 40 °C calculated before catalyst poisoning.
isotropically shifted resonances of 1 by those of a yet unidenti-
fied paramagnetic compound (the catalyst resting state), while
ethylene was being converted to 1-hexene. Only during the
final stages of the reaction, when the ethylene concentration
became very small, did the characteristic resonances of 2
appear. Interestingly, isolated 2 did not react with ethylene;
thus it is not an intermediate in the catalytic reaction.
However, as long as the ethylene concentration was main-
tained constant and significant, catalysis continued unabated
and the activity showed no appreciable diminution over the
course of two days at ambient temperature.
Table 1 lists the results of catalytic runs under various con-
ditions of temperature, pressure and catalyst concentration.
The data indicate that the rate of hexene production is first
order in catalyst concentration ([1], see runs 1 and 8–11) and
second order in ethylene pressure (runs 1–5). While non-
integer orders in ethylene are fairly common for this type of
reaction,7 a log(TOF)/log(PC H ) plot exhibited a slope of
2
4
2.05(9). We note that the resulting rate law does not per se
require a termolecular turnover-limiting step as part of the
catalytic cycle.45
1-Hexene was the only C6 product detected under these
conditions. Lower pressure and higher temperature favored the
formation of 1-butene. For example, the C6/C4 selectivity at
23 °C increased from 4 at 15 psi to 15 at 50 psi. Only traces of
octene were detected by GC at long reaction times (>48 h).
Notably, there was no polyethylene formed in any of these reac-
tions. Elevated temperatures resulted in slightly increased rates
of hexene formation; however, they also led to catalyst deactiva-
tion via formation of 2. We realize, of course, that the activities
reported here are minute, and pale in comparison with com-
mercially used catalysts. However, our point is that this system
is well-characterized – cleanly progressing from one structurally
characterized compound to another – and does not require any
Synthesis of potential bimetallic intermediates
co-catalyst or activator. As such it lends itself to mechanistic Crucial intermediates in a bimetallic reaction mechanism
scrutiny and the results of such an examination may be of would be dinuclear metallacycles in which carbon chains of
general relevance to selective catalytic oligomerizations.
various lengths (C4, C6, C8) bridge a dichromium fragment.
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Scheme 3 Synthesis and ethylene reactivity of binuclear metallacycles.
Fig. 1 The molecular structure of [(i-Pr2Ph)2nacnacCr]2(μ-CH2)2(CH2)2] (3);
hydrogen atoms and isopropyl groups have been omitted for clarity. Selected
interatomic distances [Å] and angles [°]: Cr1–Cr2, 2.5450(15); Cr1–N1, 2.101(6);
Cr1–N2, 2.067(5); Cr1–C1, 2.259(7), Cr1–C4, 2.164(8); Cr2–N3, 2.062(5); Cr2–N4,
2.011(6); Cr2–C1, 2.158(8); Cr2–C4, 2.288(7), C1–C2; 1.516(11), C2–C3,
1.367(11); C3–C4, 1.483(11); N1–Cr1–N2, 89.1(2); N3–Cr2–N4, 89.4(2); C1–Cr1–
C4, 75.2(3), C1–Cr2–C4, 74.7(3); Cr1–C1–Cr2, 70.4(2); Cr1–C4–Cr2, 69.7(2).
Accordingly, we set out to prepare compounds of this type.49–51
Alkylation of [(i-Pr2Ph)2nacnacCr(μ-I)]2 with 1,ω-dilithioalkanes
indeed yielded such compounds, after some optimization (see
Scheme 3).
The required dilithioalkyl reagents were synthesized using a
procedure reported by Negishi.52 For example, Li(CH2)4Li was
prepared by lithium/halide exchange between 1,4-diiodo-
butane and 4.2 equivalents of t-butyl lithium. Although the
slight excess of t-butyl lithium ensured full conversion, upon
quenching with diethyl ether solvent at room temperature,
lithium ethoxide (LiOEt) and ethylene are formed.53 The LiOEt
side product yielded an undesirable chromium ethoxide
complex that was observed crystallographically in our initial
attempts to alkylate [(i-Pr2Ph)2nacnacCr(μ-I)]2 with Li(CH2)4Li.
Therefore, the lithiation procedure was modified to eliminate
the formation of lithium ethoxide. The lithium/iodide
exchange reaction was performed at −78 °C and the diethyl
ether solvent was removed under vacuum while the solution
warmed from −78 °C to 0 °C. The crude product was extracted
with pentane and filtered under nitrogen to remove any traces
of LiOEt.54 Removal of the pentane solvent under vacuum
yielded a gummy white precipitate which was dissolved and
stored at −30 °C in Et2O as a clear yellow solution. The use of
the dilithioalkyls generated by this procedure did not produce
any chromium complexes containing ethoxide moieties.
However, several attempts to synthesize 1,6-dilithiohexane by
this route were unsuccessful, presumably due to formation of
cyclohexane or polymerization;55 accordingly the synthesis of a
bimetallacyclooctane complex was eventually abandoned.
Fig. 2 The molecular structure of [(i-Pr2Ph)2nacnacCr]2(μ-CH2)2(CH2)3] (4);
hydrogen atoms and isopropyl groups have been omitted for clarity. Selected
interatomic distances [Å] and angles [°]: Cr1–Cr2, 2.5567(8)(15); Cr1–N1, 2.122(2);
Cr1–N2, 2.075(2); Cr1–C1, 2.266(3), Cr1–C5, 2.194(3); Cr2–N3, 2.091(3); Cr2–N4,
2.138(2); Cr2–C1, 2.172(3); Cr2–C5, 2.258(3), C1–C2; 1.500(4), C2–C3, 1.434(6);
C3–C4, 1.436(6); C4–C5, 1.553(5); N1–Cr1–N2, 89.42(8); N3–Cr2–N4, 89.14(8);
C1–Cr1–C5, 83.78(10), C1–Cr2–C5, 84.47(11); Cr1–C1–Cr2, 70.27(8); Cr1–C5–
Cr2, 70.04(8).
56
Reaction of the [(i-Pr2Ph)2nacnacCr(μ-Cl)]2 with Li(CH2)4-
Li at a variety of temperatures (−78 °C to RT) was always
incomplete, giving mixtures of the starting material and the precursor with 1,5-dilithiopentane (Li(CH2)5Li) at room temp-
desired bimetallacycle. It was then found that addition of erature. The structure of 4, as determined by X-ray diffraction,
Li(CH2)4Li to iodide [(i-Pr2Ph)2nacnacCr(μ-I)]2 at room temp- is depicted in Fig. 2.
erature resulted in complete conversion to the desired
The C4 bimetallacycle 3 crystallized in the monoclinic space
product, namely [(i-Pr2Ph)2nacnacCr]2(μ-CH2)2(CH2)2] (3). The group P21 with a co-crystallized molecule of hexamethyldi-
molecular structure of 3 was determined by X-ray diffraction siloxane (solvent of recrystallization). C5 bimetallacycle 4 crystal-
ˉ
and the result is shown in Fig. 1. The homologous bimetalla- lized in the triclinic space group P1 with a co-crystallized
cycloheptane [(i-Pr2Ph)2nacnacCr]2(μ-CH2)2(CH2)3] (4) was syn- molecule of pentane. The molecular structures of 3 and 4 are
thesized in analogous fashion via alkylation of the iodide very similar, except for the incrementally larger ring size of 4.
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Both feature terminal methylene groups that bridge the two hexene and ∼14% butene. At higher temperatures more
chromium atoms; in that sense they may be described as a 1-butene is observed, presumably due to shift from ring expan-
dichromabicyclo[1.1.2]hexane (3) and dichromabicyclo[1.1.3]- sion to β-hydrogen elimination. Even after two weeks at room
heptane (4). Extreme electron deficiency – discounting metal– temperature, the amount of 1-hexene formed did not reach a
metal bonding, the individual chromium atoms feature single turnover (100% yield). At an ethylene pressure of 250 psi
10-valence electron configurations – drive the formation of (17 atm) larger amounts of 1-hexene were generated; however,
3-center/2-electron M–C–M bonds. This structural feature has higher molecular weight oligomers (C10–C20) and an insoluble
ample precedent in Cr(II) chemistry; for example, we had white precipitate (presumed to be polyethylene) were also
earlier prepared compounds of the type [Cp*Cr(μ-R)]2, which observed. To summarize, while the reaction of 3 with ethylene
show strong similarities in gross structure, metal–metal dis- yielded 1-hexene and 1-butene, their rate of formation was
tances, and magnetism.57 The local coordination environ- much slower than during the catalysis with 1. The reaction
ments around the Cr atoms in 3 and 4 deviate from the also showed different selectivity and a more varied product
preferred square planar geometry for four coordinate Cr(II) profile. Based on these observations alone, it seems unlikely
complexes.58 The Cr2C2-cores of the molecules adopt a butter- that 3 is an intermediate in the catalytic oligomerization.
fly shape and the relevant Cr–C distances exhibit a short/long However, the formation of 1-hexene from 3 is intriguing and
pattern, rendering this fragment somewhat unsymmetric. The merits some further investigation.
non-coordinated carbon atoms (i.e., C(2), and C(3) of 3 and
A catalytic reaction of dimetallacycloheptane 4 with ethyl-
C(2), C(3), and C(4) of 4) exhibit positional disorder; thus their ene would presumably first produce the odd numbered
C–C single bonds appear unrealistically short.59 The Cr–Cr dis- α-olefins 1-pentene and possibly 1-heptene, which would be
tances in 3 and 4 are 2.5451(15) and 2.5567(8) Å, respectively, followed by production of even numbered α-olefins. GC/MS
and are therefore within the range of weak Cr–Cr bonding. analysis of the volatile products of a reaction of 4 with ethylene
Both compounds have identical effective magnetic moments (1 atm) showed that the only odd-numbered α-olefin produced
of 2.6(1)μB/Cr at room temperature, which is considerably was pentene (<1.0% yield), while no heptene was detected.
lower than expected of a typical high-spin Cr(II) ions (d4, S = 2). However, significant amounts of 1-butene and 1-hexene were
These values are reminiscent of similar bridging Cr(II) alkyls formed (see Table 2). This suggested that the bimetallacycles
and consistent with either antiferromagnetic coupling,60 decompose – inter alia – via β-hydrogen elimination, but that
metal–metal bonding, or a superposition of both.61
they do not undergo further ring expansion by insertion of
ethylene. The formation of even-numbered olefins suggests
that the decomposition of the bimetallacycles generates a
Reactivity with ethylene
It is difficult to imagine how any bimetallic mechanism for the catalyst for the oligomerization of ethylene. The nature of this
oligomerization of ethylene catalysed by 1 could circumvent 3 species was addressed next.
as an intermediate, and 4 is surely representative of the kind
Both 3 and 4 decompose slowly at ambient temperature in
of bimetallacycle involved in such a putative mechanism. solution, and this process becomes faster at elevated temp-
Accordingly, their reactivity with ethylene and kinetic com- eratures. 1H-NMR decomposition studies in C6D6 at 65 °C
petency vis-à-vis the catalysis described above is of interest. showed that 3 and 4 completely decomposed within two hours
1
Preliminary H-NMR spectroscopic studies of the reaction of 3 and that several paramagnetic products were formed in this
with 1 atm of ethylene in C6D6 indicated the presence of reaction. One of the organometallic products observed in the
α-olefin(s) as well as the starting material 3 after 1 day at room 1H-NMR during the thermolysis of 3 was the known hydride
temperature. GC/MS analysis confirmed the presence of complex [(i-Pr2Ph)2nacnacCr(μ-H)]2 (5).42 We reasoned that one
1-hexene and 1-butene. However, quantitation of the products β-hydrogen elimination of 3 would form an intermediate of
from this reaction revealed that only 11% of 1-hexene and 4% the type [(i-Pr2Ph)2nacnacCr]2(μ-H)(μ-CH2CH2CHvCH2); the
of 1-butene had been generated (Table 2) based on the starting latter might have a choice between suffering another β-hydro-
material 3. A similar reaction at 40 °C generated only ∼6% gen elimination to form 5 and eliminating butadiene, or
Table 2 Reaction of bimetallacycles 3 and 4 with ethylene
Yield (%)
Entry
Dimer
Temp (°C)
Time (h)
Pressure (psi)
C4
C6
C8
1
2
3
3
3
3
3
4
4
23
23
40
23
23
23
24
336
24
24
24
15
15
15
250
15
15
4.43
11.2
36.2
5.6
63.1
7.55
15.9
0
8.62
0
5.21
0
0
16.55
14.32
7.57
2.10
4.35
4
5a
6a
72
a % yield of C5 was less than 1%. C7 was not detected.
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1.378(4) Å) are identical to the formal single bond distance
(C2–C2A, 1.379(5)), indicating a modest transfer of electron
density to the diene, which is consistent with other reported
Cr-(η4-1,3-butadiene) complexes.62–65 However, the relative
Cr–C distances still favour a description as a π-complex rather
than as the valence tautomeric metallacyclopentene.66 The
room temperature effective magnetic moment of 6, μeff(295) =
3.8(1)μB, is consistent with a quartet (S = 3/2) ground state.
The chromium in 6 has a formal oxidation state of +I, giving it
a d5 electronic configuration; 6 apparently adopts an inter-
mediate spin state.
Scheme 4 Decomposition of 3 and formation of butadiene complex 5.
We note that 6 also provides a model for the binding of two
molecules of ethylene to Cr(I); it may be considered an
analogue of the proposed LnCr(C2H4)2 intermediate of ethylene
trimerization proceeding by a mononuclear metallacycle mech-
anism (see Scheme 1). The notion that 3 decomposes to form
a butadiene complex was confirmed, since the 1H-NMR
spectrum of 6 matched the unassigned paramagnetic reso-
nances observed in the decomposition of 3. To summarize, 3
decomposes to equal parts of 5 and 6. This result implies that
there are multiple pathways to generate Cr(I) fragments, which
may initiate oligomerization catalysis. The observation of
butene during thermolysis of 3 is also consistent with the
detection of pentene during the reaction of 4 with ethylene,
and it suggests that decomposition of these bimetallacycles
can lead to catalytically active Cr(I) fragments.
1
Finally, in a separate H-NMR experiment in THF-d8, 6 was
Fig. 3 The molecular structure of (i-Pr2Ph)2nacnacCr(η4-C4H6)(THF) (6); hydro-
exposed to 1 atm of ethylene, whereupon it produced 1-hexene
gen atoms have been omitted for clarity. Selected interatomic distances [Å] and and 1-butene (confirmed by GC/MS analysis). Mechanistically,
angles [°]: Cr1–N1, 2.0991(17); Cr1–C1, 2.202(3); Cr1–C2, 2.167(2); Cr1–O1,
dissociation of butadiene followed by the binding of ethylene
would lead to the formation of 1-hexene and then regenerate a
Cr(I) fragment that can either continue in trimerization or
2.222)(3); C1–C2, 1.378(4); C2–C2A, 1.379(5); N1–Cr1–N1A, 88.54(9); N1–Cr1–
C1, 96.46(9); N1–Cr1–C2, 103.64(9); N1–Cr1–O1, 92.8(3).
rebind butadiene. These results suggest that labile monomeric
undergoing reductive elimination of 1-butene and yielding two
Cr(I) fragments. However, analysis by H-NMR and GC/MS did
Cr(I) compounds can selectively oligomerize ethylene and it
provides some support for a monomeric metallacycle mechan-
ism for ethylene trimerization. This notion will be further
explored in the next section.
1
not show any butadiene; only butene was detected by GC/MS.
Since no free butadiene was found, despite the formation of 5,
it was hypothesized that another paramagnetic compound
formed during the decomposition reaction might be a buta-
diene complex. In separate 1H-NMR experiments in C6D6 or
d8-THF, 5 was exposed to 1 atm of butadiene and left to react
overnight at room temperature. 5 completely disappeared and
resonances for butene and a paramagnetic compound that
exactly matched the unassigned 1H-NMR resonances noted
during the decomposition of 3 were observed (Scheme 4).
In hopes of determining if this new paramagnetic com-
pound was indeed a butadiene complex, 1 was exposed to 1
atm of butadiene in THF; this reaction indeed yielded a mono-
meric Cr butadiene adduct, namely (i-Pr2Ph)2nacnacCr(η4-
C4H6)(THF) (6). The molecular structure of 6 was determined
by X-ray diffraction; its molecular structure is shown in Fig. 3.
6 exhibits square pyramidal geometry and contains a cis-η4-
bonded butadiene molecule and a coordinated molecule of
THF in the apical position (the basal plane is defined by the
nacnac ligand and the butadiene). The bond distances of the
butadiene’s erstwhile double bonds (C1–C2 and C1A–C2A,
Synthesis of mononuclear model compounds
As the binuclear hypothesis based on Cr(I) olefin complexes
and Cr(II) bimetallacycles became discredited (see above), a
premium was placed on preparing possible intermediates of a
mononuclear mechanism, which would presumably involve
Cr(III) metallacycles. Notably, we tried to devise synthetic
routes to a mononuclear chromacyclobutane, which might be
a credible candidate for the active intermediate observed
during catalysis. A severe obstacle to the synthesis of e.g.
(i-Pr2Ph)2nacnacCr(CH2)4 was presented by our finding that
the (i-Pr2Ph)2nacnacCr-fragment does not readily support the
preparation of trivalent chromium dialkyls of the type
(i-Pr2Ph)2nacnacCrR2, whereas the corresponding methyl-
substituted nacnac ligand affords ready access to compounds
of the composition (Me2Ph)2nacnacCrR2 (R
= Me, Bn,
CH2SiMe3).67 Indeed, whereas treatment of (Me2Ph)2nacnac-
CrIIICl2(THF)2 with lithium alkyls produces Cr(III) dialkyls,
similar treatment of (i-Pr2Ph)2nacnacCrIIICl2(THF)2 or
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[(i-Pr2Ph)2nacnacCrIIICl(μ-Cl)]2 with such reagents invariably proposed to constitute the catalytic cycle (Scheme 1), while
leads to reduction and formation of Cr(II) alkyls of the type employing the same ancillary ligand as the actual catalytic
(i-Pr2Ph)2nacnacCrR (R = CH2SiMe3) or [(i-Pr2Ph)2nacnacCr- system. First up, we wondered about an analogue of the mono-
(μ-R)]2 (R = Me).68 In view of this dichotomy, we chose to nuclear ethylene complex (i-Pr2Ph)2nacnacCr(C2H4), which
initially attempt the synthesis (Me2Ph)2nacnacCr(CH2)4. may enjoy a fleeting existence after the elimination/substi-
However, as detailed elsewhere, reaction of (Me2Ph)2nacnac- tution of the α-olefin product. In the presence of excess ethyl-
CrIIICl2(THF)2 with Li(CH2)4Li actually yielded the bimetalla- ene, it would be expected to form (i-Pr2Ph)2nacnacCr(C2H4)2 to
cycle [(Me2Ph)2nacnacCr]2(μ-CH2)2(CH2)2] (an analogue of 3), begin another turnover, while lack of substrate presumably
presumably due to the greater reducing power of the dilithio leads to irreversible formation of 2. We hypothesized that a
reagent. To overcome this problem, we decided to make the sterically more demanding π-acceptor might stabilize such a
chromium(III) ion in the halide precursor more electron rich 1 : 1 complex, and indeed the reaction of 1 with one equivalent
by substituting it with stronger σ-donors. Gratifyingly, this was of diphenylacetylene produced the mononuclear alkyne
met with success. Treatment of (Me2Ph)2nacnacCrIIICl2(py)2 adduct (i-Pr2Ph)2nacnacCr(η2-C2Ph2) (8). The molecular struc-
with Li(CH2)4Li afforded the mononuclear chromacyclobutane ture of 8 as determined by X-ray diffraction is shown in Fig. 5.
(Me2Ph)2nacnacCr(CH2)4(py) (7). The molecular structure of 7 The diphenylacetylene fragment is bound nearly perpendicu-
was determined by X-ray diffraction, and the result is shown in larly to the plane of the ligand. Both C1 and C2 were found to
Fig. 4.
be located at similar distances from the chromium at distances
7 is a rare example of a mononuclear chromacyclopentane. of 1.957(4) Å and 1.963(4) Å, respectively, with a Cr-centroid
It differs from the putative catalytic intermediate only by virtue distance of 1.846(6) Å. The C–C distance of the diphenylacetyl-
of the coordinated pyridine. However, the latter apparently ene was found to have been elongated by 0.115 Å from free
makes all the difference. 7 does not catalyze the trimerization diphenylacetylene to a distance of 1.320(5) Å.69 The latter dis-
of ethylene, nor does it decompose readily upon heating. We tance is the strikingly similar to the CvC double bond dis-
believe that the strongly coordinated pyridine ligand prevents tance in stilbene, which measures 1.321(5) Å.70 The
binding and insertion of ethylene as well as decomposition by diphenylacetylene fragment in 8 is no longer linear but fea-
β-elimination. Attempts to remove the pyridine by adding tures an average C–C–Cipso angle of 137.3(4)°. This value is
various Lewis acids (AlMe3, B(C6F5)3, BF3) did not yield tract- within
able products, nor did these additives facilitate reactions of 7 characterized chromium η2-diphenylacetylene complexes,
with ethylene. In summary, 7 may be a suggestive structural which possess C–C–Cipso angles in the range of
the
range
of
other
structurally
a
model for a catalytic intermediate, but the strong coordination 136.3–155.9°.71 These structural changes in the diphenylacety-
by pyridine blocks further reactivity (such as binding and lene fragment, and the fact that 8 has an effective magnetic
insertion of ethylene or even β-hydrogen elimination). Its ancil- moment of 3.9(1)μB, suggests that the chromium center of 8 is
lary ligand also differs from the one which gives rise to cata- in the +III oxidation state, with corresponding twofold
lysis. Accordingly, 7 does not by itself provide convincing evidence reduction of the diphenylacetylene ligand, resulting in a more
for the catalytic activity of mononuclear metallacycles.
appropriate description as a metalla-cyclopropene.
We then turned our attention to the preparation of com-
If 8 is to be a model of a 1 : 1 ethylene complex, it might be
pounds that might serve as models for some of the species expected to rapidly react with additional ethylene. We already
Fig. 4 The molecular structure of (Me2Ph)2nacnacCr(CH2)4(py) (7); hydrogen
atoms, except those on the alkyl chain have been removed for clarity. Selected
interatomic distances [Å] and angles [°]: Cr–C1 2.076(5), Cr–C4 2.120(5), Cr–N1
2.048(4), Cr–N2 2.126(4), Cr–N3, 2.138(4), C1–Cr–C4 83.2(2), N1–Cr–N2,
88.99(16).
Fig. 5 The molecular structure of (i-Pr2Ph)2nacnacCr(η2-C2Ph2) (8); hydrogens
have been removed for clarity. Selected interatomic distances [Å] and angles [°]:
Cr1–N1, 1.980(2); Cr1–C7, 1.944(4); Cr1–C8, 1.970(4); C7–C8, 1.307(5); N1–Cr1–
N1A, 90.78(14); N1–Cr1–C7, 130.52(8); N1–Cr1–C8, 132.17(8).
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Fig. 6 The molecular structure of (i-Pr2Ph)2nacnacCr(CPh)2(CH2)2 (9); hydrogen
atoms, except for those on the alkyl chain, have been removed for clarity.
Selected interatomic distances [Å] and angles [°]: Cr–N1, 2.137(2); Cr–N2, 2.052(2);
Cr–O1, 2.1260(18); Cr–C1, 2.091(3); Cr–C4, 2.051(3); C1–C2, 1.526(4); C2–
C3, 1.508(4); C3–C4, 1.354(4); N1–Cr–N2, 88.87(10); N1–Cr–O1, 86.15(8); N2–
Cr–C1, 93.35(11); C1–Cr–O1, 85.56(10); C4–Cr–N1, 120.82(10); C4–Cr–N2,
107.19(10); C4–Cr–O1, 88.30(9); C4–Cr–C1, 80.11(11).
Fig. 7 The molecular structure of (i-Pr2Ph)2nacnacCr(η2-C4Me4)(THF) (10);
hydrogen atoms have been removed for clarity. Selected interatomic distances
[Å] and angles [°]: Cr–N1, 2.019(2); Cr–N2, 2.126(2); Cr–O1, 2.1102(19); Cr–
C1–2.081(3); Cr–C4, 2.007(3); C1–C2, 1.359(4); C2–C3, 1.485(4); C3–C4, 1.347(4);
N1–Cr–N2, N1–Cr–C1, 96.42(10); N2–Cr–O1, 87.05(8); C4–Cr–N1, 98.97(10);
C4–Cr–N2, 105.98(10); C4–Cr–O1, 97.10(10); C4–Cr–C1, 82.56(12).
alkynes. Accordingly, exposure of a THF solution of 1 to
have a model of sorts for the ensuing bis(ethylene) complex, in 2-butyne resulted in the rapid formation of a white precipitate.
Permitting the reaction to continue yielded a large amount of
the form of butadiene complex 6, but the covalent linkage of
the two olefins in butadiene prevents it from modelling the the white precipitate and effected a slow color change of the
subsequent reductive coupling step. On this score, 8 delivers. solution from brown to green-grown. 13C NMR spectroscopy
proved that the white precipitate was hexamethylbenzene, i.e.
Monitoring the addition of ethylene to a THF-d8 solution of 8
by H NMR spectroscopy showed formation of a new paramag- the product of a catalytic cyclotrimerization of the alkyne.72,73
netic chromium compound, but no formation of hexene or Removal of the organic product by filtration and standard
1
work-up of the filtrate afforded the five-membered metallacycle
any other olefin. Isolation and determination of the structure
of the new compound (see Fig. 6) showed it to be the five- (i-Pr2Ph)2nacnacCr(η2-C4Me4)(THF) (10). The molecular struc-
membered metallacycle (i-Pr2Ph)2nacnacCr(CPh)2(CH2)2 (9), ture of 10 has been determined by X-ray diffraction, and is
shown in Fig. 7.
which can the thought of as resulting from the insertion of
ethylene into one of the Cr–C bonds of the metallacyclo-
The compound is a metallacyclopentadiene (or ‘metallole’),
propene 8, or alternatively as the product of a reductive which presumably results from reductive coupling of two
alkynes bound to chromium. The compound possesses square
coupling of a coordinated alkyne and alkene. 9 adopts a
square pyramidal geometry with a THF molecule occupying pyramidal geometry with one terminus of the butadienediyl
one of the basal sites. The C3–C4 distance of 1.354(4) Å is con- ligand in the axial position. As expected, the coordinated
C4Me4-fragment is planar and exhibits clear bond length alter-
sistent with a CvC double bond. The C1–C2 distance (1.526(4)
Å) and C2–C3 distance (1.508(4) Å) on the other hand are nations, as C1–C2 and C3–C4 were found to be 1.359(4) Å and
characteristic of C–C-single bonds, consistent with the descrip- 1.347(4) Å, respectively, while C2–C3 was found to be 1.485(4)
Å. The effective magnetic moment of 10 is 3.8(1)μB, consistent
atoms on C1 and C2 can be located in the difference map and with chromium in the +III oxidation state. Compound 10 is
tion of 9 as a metallacyclopentene. In addition, the hydrogen
their positions refined.
the resting state of the cyclotrimerization catalyst, as its
exposure to 2-butyne reinitiates the catalysis and yields more
Although the THF molecule coordinated to chromium in 9
might be expected to be substitutionally labile, the compound hexamethylbenzene.
does not react further with additional ethylene. In particular,
it does not catalyse the oligomerization of ethylene in THF (as
More to the point, 10 is another mononuclear chromacycle,
which (unlike 9) is not encumbered by excessive steric hin-
1 does). We note the possibility that the large steric demand of drance, and the coordinated THF of which is apparently
the phenyl substituents is responsible for this reluctance, subject to displacement by additional substrate. Therefore it
might be expected to react with ethylene as well. Gratifyingly
effectively blocking the coordination of additional ethylene.
On the other hand, 9 is already the second mononuclear chro- then, 10 was indeed found to be a catalyst for the selective
macycle (besides 7) that does not produce any higher olefins oligomerization of ethylene. Exposure of a THF-d8 solution of
10 to ethylene yielded 1-hexene, ethylene complex 2, and
under ethylene. Fortunately, the situation is about to improve.
Diphenylacetylene is a rather ‘large’ stand-in for ethylene by an unidentified organic compound (presumably 1,2,3,4-
any reckoning, which in turn led us to exploring smaller tetramethylhexatriene).
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were obtained by the University of Delaware Mass Spectrometry
Facility. Elemental analyses were performed by Desert
Conclusions
We have shown that
a
well-characterized chromium(I) Analytics. Room-temperature molar magnetic susceptibilities
dinitrogen complex (i.e., 1) catalyzes the selective trimerization (χm) in the solid state were determined using a Johnson
of ethylene to 1-hexene in homogeneous THF solution, albeit Matthey magnetic susceptibility balance. They were corrected
slowly. To our knowledge, this is the only example of a catalyst for diamagnetism using Pascal constants and converted into
for the trimerization of ethylene that unambiguously starts effective magnetic moments (μeff).
and finishes in the formal oxidation state Cr(I).74,75 Since the
GC/MS measurements for oligomerizations were taken on a
catalyst functions in the absence of any cocatalysts, which HP G1800A GCD. To determine the response factor for the GC
often obscure assignments of structure and oxidation 1 mmol of 1-pentene, 1-hexene, 1-heptene, 1-octene, and
state,76,77 this system presents an excellent chance to address nonane were placed in a 50 mL volumetric flask and then
fundamental questions regarding a commercially important filled to 50 mL with THF. The flask was shaken and 1 μL was
and mechanistically intriguing catalytic reaction. For example, injected into the GC, the average area for three runs was calcu-
we have prepared relevant binuclear Cr(II) metallacycles and lated and the ratio of the GC/MS response to the α-olefin to
shown that they are not intermediates in the catalysis. By extra- nonane was calculated and plotted versus carbon number. The
polation, and absent any unambiguous evidence for a role of slope of the linear trendline allowed for the calculation of the
binuclear sites in this type of catalysis, we suggest that the RF for the 1-hexene; the RF for nonane was arbitrarily set to
alternative mononuclear Cr(III) metallacycles are most likely 1.0. The raw integrals for the 1-hexene analyses were divided
intermediates in selective oligomerization catalysis. While the by the RF to obtain the corrected integrals.
independent synthesis of the most likely intermediate in our
system – i.e., (i-Pr2Ph)2nacnacCr(CH2)4(THF) – proved elusive, Typical oligomerization procedure for reactions <90 psi
we have prepared several closely related analogues of it. Of
A thick walled reactor was heated to 250 °C overnight to
these, two proved inactive, but for cogent reasons. However,
remove all traces of moisture and fitted with a cap consisting
the mononuclear chromacyclopentadiene (i-Pr2Ph)2nacnacCr-
of an inlet port, gauge, and connections for vacuum and ethyl-
ene. The reactor was evacuated and backfilled with ethylene
three times. The reactor was charged with 1 atm of ethylene.
(η2-C4Me4)(THF) (10) reacted with ethylene to catalytically
produce 1-hexene and enter the same reaction channel as 1.
To the extent that these results may prove general, they
In the nitrogen dry box the requisite amount of catalyst was
strongly support the notion that chromium catalysts for selec-
dissolved in 10 mL THF; nonane was added as the standard. A
tive ethylene oligomerization alternate between mononuclear
syringe containing the aforementioned mixture was removed
Cr(I) ethylene complexes and Cr(III) metallacycles. Our results
from the dry-box and the solution mixture was injected into
also show that neutral compounds are competent catalysts for
the reactor. The reactor pressure was raised to the necessary
the selective oligomerization.78 However, the activity of this
pressure and the reaction mixture was stirred. At various times
catalyst is rather low by comparison, and we surmise that
throughout the oligomerization an aliquot was removed from
cationic, more electrophilic intermediates are the mainstays of
the reactor and injected in the GC.
more active ones.
Typical oligomerization procedure for reaction at 215 psi
In the dry-box, 90 mg (93 mmol) of catalyst was added to a
Experimental
300 mL Parr reactor followed by 90 mL THF and the nonane
General considerations
standard. Pentane was also added as an additional check. The
reactor assembly was brought of the dry-box and the connec-
tion line was purged with ethylene for 2 minutes. The reaction
was opened to ethylene and the pressure was regulated to
215 psi and stirred for certain time periods. When the set time
was reached the ethylene pressure was released, the solution
is exposed to air and an aliquot injected into the GC. This
procedure was repeated for each time frame.
All manipulations of compounds were carried out using stan-
dard Schlenk, high vacuum line, and glovebox techniques
under an atmosphere of N2. Solvents were purchased from
Fisher Scientific, degassed, and dried by passing through acti-
vated alumina. THF-d8 and C6D6 were purchased from
Cambridge Isotopes Laboratory and stored under vacuum over
Na/K alloy. All other reagents were purchased from Aldrich
or Acros and dried using standard procedures as needed.
[(i-Pr2Ph)2nacnacCr]2(μ–η2:η2-N2) (1), [(i-Pr2Ph)2nacnacCr]2-
(μ–η2:η2-C2H4) (2), and [(i-Pr2Ph)2nacnacCr(μ-X)]2 (X = Cl, I)
were prepared by previously published procedures.43
Typical oligomerization procedure for the reactions of 3 and 4
with ethylene
In a nitrogen-filled dry box the bimetallacycle was dissolved in
1H NMR spectra were taken on a Bruker DRX-400 spectro- 10 mL THF and nonane was added via a micropipette to a
meter and were referenced to the residual protons of the glass ampoule. The ampule was removed from the nitrogen
solvent. FTIR spectra were taken on Mattson Alpha Centauri or dry box and placed onto a high vacuum line. After three
Mattson Genesis Series spectrometers. UV/vis spectra were freeze–pump–thaw cycles 1 atm of ethylene was backfilled into
recorded using a HP 8453 spectrophotometer. Mass spectra the vessel. After 24 h the reaction mixture was exposed to air,
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filtered with celite, and then 1 μL was injected onto the Preparation of μ-pentamethylene) [2,4-pentane-N,N′-bis(2,6-
GC/MS.
diisopropylphenyl)ketiminato] dichromium(II),
[(i-Pr2Ph)2nacnacCr]2[(μ-CH2)2(CH2)3] (4)
Procedure for the reaction done at 250 psi
To
a
solution of [(i-Pr2Ph)2nacnacCr(μ-I)]2 (0.200 g,
In a nitrogen-filled dry-box, 180 mg (0.154 mmol) of 3 was
added to a 300 mL Parr reactor followed by 30 mL THF and the
nonane standard. The reactor assembly was removed out of
the dry-box and the connection line was purged with ethylene
for 2 minutes. The reactor was opened to ethylene and the
pressure was regulated to 250 psi (17 atm) and the solution
stirred for 24 h. After 24 h the ethylene pressure was released
and reaction mixture was exposed to air, filtered with celite,
and then 1 μL was injected onto the GC/MS.
0.168 mmol) in 70 mL pentane, 0.480 M Et2O solution of
Li(CH2)5Li (0.385 mL, 0.185 mmol) was added dropwise. The
solution rapidly changed color from green to brown. The solu-
tion was stirred at room temperature for 3 hours. The solvent
was removed under vacuum leaving a crude brown powder.
Pentane was then added to the crude reaction mixture and
cooled to −30 °C for 1 hour. The mixture was filtered and the
pentane solvent was removed under a vacuum leaving a crude
brown powder. This was recrystallized from hexamethyldisil-
oxane by slow cooling to −30 °C, yielding X-ray quality crystals
(0.093 g, 47% yield). 1H-NMR (400 MHz, C6D6): 10.81 (br), 9.56
(br), 7.03 (br), 4.36 (br), 3.07 (br), 1.85, 1.52, 1.32, 0.22
((CH3)3Si)2O) ppm. IR (KBr): 3057 (w), 2959 (s), 2925 (m), 2867
(m), 1525 (s), 1434 (s), 1311 (s), 1254 (s), 1171 (m), 1098 (m),
Preparation of 1,4-dilithiobutane Li(CH2)4Li
10 mL of diethyl ether was added to a Schlenk flask in a nitro-
gen-filled glove box. The flask was then cooled to −78 °C using
a dry ice/acetone bath. 4.2 equivalents of 1.7 M t-BuLi in
pentane (11.95 mL, 20.3 mmol) were added to the cold ether
solution and allowed to stir for 15 minutes. One equivalent of
1,4-diiodobutane (1.50 g, 4.84 mmol) was then added via
syringe. The reaction was allowed to stir for 1 h at −78 °C.
Afterwards, the diethyl ether was removed under vacuum at
−78 °C. The reaction was warmed over 1 hour to 0 °C, while
maintaining a dynamic vacuum. The resulting white/yellow
product was kept under vacuum for an additional hour at 0 °C.
The precipitate was then extracted, in an inert atmosphere,
with 100 mL of pentane in 50 mL portions. The gummy white
precipitate was then dissolved in dry ethyl ether giving a trans-
lucent yellow solution which was stored at −30 °C.
1056 (m), 1042 (m), 932 (w), 846 (m), 793 (s), 760 (m) cm−1
.
UV/Vis (toluene): λmax (ε) = 477 (125 M−1 cm−1) nm. μeff = 2.6
(1)μB/Cr (294 K). Mp: 95 °C dec. Mass spectrum m/z (%): 469
(100%) (C29H41N2Cr+). Anal. Calcd for C65H110N4OSi2Cr2: C,
70.72; H, 9.46; N, 4.78. Found: C, 69.66; H 8.42; N, 4.75. The
compound decomposes slowly at room temperature.
Preparation of [2,4-pentane-N,N′-bis(2,6-diisopropylphenyl)-
ketiminato](η4-cis-butadiene)(tetrahydrofuran) chromium(I),
(i-Pr2Ph)2nacnacCr(η4-C4H6)(THF) (6)
1 (0.400 g 0.336 mmol) (0.400 g 0.336 mmol) was placed in
ampoule with a stir bar and 50 mL THF and attached to a
vacuum manifold. Three freeze–pump–thaw cycles were per-
formed, followed by back filling with 1 atm of 1,3-butadiene.
The solution was stirred for 6 hours at room temperature
resulting in a color change from brown to dark red. Butadiene
and solvent were removed by vacuum transfer, and the
ampoule was then sealed and brought back into the dry-box.
The crude solid was recrystallized from pentane. X-ray quality
crystals were grown by slow cooling of a solution in pentane to
−30 °C (100% conversion by 1H-NMR). 1H-NMR (400 MHz,
C6D6): 11.35 (br), 9.0 (br), 7.82 (br), 3.44 (br), 1.63 (s), 1.42 (m),
1.35 (s), 0.87 (s), −0.75 (br), −4.29 (br) ppm. IR 3054 (w), 2959
(s), 2867 (s), 1518 (s), 1405 (s), 1359 (s), 1316 (s), 1260 (m),
1227 (m), 1174 (m), 1100 (m), 1020 (m), 932 (m), 867 (m), 790
(s), 758 (s), 437 (w) cm−1. UV/Vis (pentane): λmax (ε) = 475 (716
M−1 cm−1) nm. μeff = 3.8(1)μB (294 K). Mp: 108 °C dec. Mass
spectrum m/z (%): 596.1 (12%) (C37H55N2OCr+), 524.2 (100%)
(C33H47N2Cr+).
Preparation of (μ-tetramethylene) [2,4-pentane-N,N′-bis(2,6-
diisopropylphenyl)ketiminato] dichromium(II),
[(i-Pr2Ph)2nacnacCr]2[(μ-CH2)2(CH2)2] (3)
To
a
solution of [(i-Pr2Ph)2nacnacCr(μ-I)]2 (0.200 g,
0.168 mmol) in 70 mL pentane, 0.480 M Et2O solution of Li-
(CH2)4Li (0.385 mL, 0.185 mmol) was added dropwise. The solu-
tion rapidly changed color from green to brown. The solution
was stirred at room temperature for 3 hours. The solvent
was removed under vacuum, leaving a crude brown powder.
Pentane was then added to the crude reaction mixture and
cooled to −30 °C for 1 hour. The mixture was filtered and the
pentane solvent was removed under a vacuum leaving a crude
brown powder. This was recrystallized from hexamethyldisil-
oxane by slow cooling to −30 °C, yielding X-ray quality crystals
(0.095 g, 49% yield). 1H-NMR (400 MHz, C6D6): 11.25 (br),
10.09 (br), 7.46 (br), 4.43 (br), 3.14 (br), 1.79 (br), 1.49 (br), 1.34
(br), 1.14 (br), 0.22 ((CH3)3Si)2O) ppm. IR (KBr): 3057 (w), 2959
(s), 2927 (s), 2867 (m), 1525 (s), 1434 (s), 1310 (s), 1253 (s),
1170 (m), 1097 (w), 1055 (m), 930 (w), 845 (m), 793 (m), 758
Preparation of (2,6-Me2Ph)2nacnacCr(CH2)4(py) (7)
(m) cm−1. UV/Vis (pentane): λmax (ε) = 455 (1326 M−1 cm−1
)
(2,6-Me2Ph)2nacnacCrCl2(py) (0.200 g, 0.394 mmol) was
nm. μeff = 2.6(1)μB/Cr (294 K). Mp: 100 °C dec. Mass spectrum partially dissolved in 150 mL pentane giving a dark brown
m/z (%): 469 (100%) (C29H41N2Cr+), 938 (28%) (C58H82N4Cr2 ). solution. The solution was chilled to −30 °C and 1,4-dilithio-
+
Anal. Calcd for C64H108N4OSi2Cr2: C, 69.77; H, 9.14; N, 5.04. butane (0.39 M, 0.394 mmol) was added. The color deepened
Found: C, 69.55; H, 8.68; N, 4.78. The compound decomposes as the reaction stirred for 3 hours while warming up to room
slowly at room temperature.
temperature. The solvent was removed under a reduced
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1
pressure. The crude solid was extracted with pentane and the 60% yield). H NMR (400 MHZ, THF-d8): 29.78 (br), 22.84 (br),
extract filtered, concentrated, and cooled to −30 °C to yield 3 14.86 (br), 11.01 (br), 7.70 (br), 4.73 (br), −6.66 (br), −13.01
(0.082 g, 42%). 1H NMR (400 MHz, C6D6): 12.2 (s), 9.80 (b), (br) ppm. IR (KBr): 3057 (w), 3016 (w), 2960 (s), 2926 (s), 2866
6.99 (s), 4.80 (s), 3.59 (b), 2.11 (s), 1.52 (s), 1.40 (s), 1.19 (s), (m), 1522 (s), 1490 (m), 1472 (m), 1465 (m), 1457 (s), 1437 (s),
0.91 (s), 0.28 (s) ppm. IR (KBr): 2961 (m), 2923 (m), 2855 (w), 1419 (m), 1384 (s), 1374 (s), 1339 (w), 1313 (m), 1256 (m), 1172
1624 (2), 1547 (m), 1524 (m), 1540 (m), 1392 (s), 1261 (s), 1183 (w), 1101 (w), 1025 (w), 933 (w), 795 (m), 756 (m), 698 (m), 668
(m), 1095 (m), 1022 (m), 800 (s), 765 (m), 555 (w) cm−1. UV/Vis (w) cm−1. UV/vis (pentane): λmax (ε) = 370 (4200 M−1 cm−1),
(pentane): λmax (ε) = 422 (1665 M−1 cm−1) nm, 482 (416 M−1 454 (1195 M−1 cm−1) nm. μeff (293 K) = 3.4(1)μB. Mp: 192 °C
cm−1) nm, 668 (142 M−1 cm−1) nm. μeff = 3.9(1)μB (294 K). Mp (dec). Mass spectrum m/z (%): 469.2 (51) [M+ − C16H14
−
= 92–95 °C (dec).
C4H8O]. Anal. Calc. for C59H75N2CrO: C, 80.50; H, 8.59; N,
3.18. Found: C, 78.81; H, 7.86; N, 3.18.
Preparation of (i-Pr2Ph)2nacnacCr(η2-C2Ph2) (8)
(LiPrCr)2(μ2–η2:η2-N2) (1) (0.200 g, 0.206 mmol) was dissolved
Preparation of (i-Pr2Ph)2nacnacCr(η2-C4Me4)(THF) (10)
in 20 mL THF and 0.074 mg (0.416 mol) of diphenylacetylene (LiPrCr)2(μ2–η2:η2-N2) (1) (0.200 g, 0.206 mmol) was dissolved
was added and the solution was stirred for 4 hours. The THF in 20 mL THF and 2-butyne (0.07 mL, 0.850 mmol) was added
was removed and the product was extracted with pentane. Con- and the solution was stirred 2 hours. The THF was removed
centration and cooling to −30 °C yielded brown crystals of 8 and the product was extracted with pentane. Concentration
1
(0.197 g, 74% yield). H NMR (400 MHZ, THF-d8): 143.6 (br), and cooling to −30 °C yielded green crystals of 5, which always
39.9 (br), 20.4 (br), 17.1 (br), 10.0 (br), 6.64 (br), 2.80 (br), contained some residual hexamethylbenzene (0.080 g, ∼30%
1
−4.55 (br) ppm. IR (KBr): 3056 (w), 2961 (s), 2926 (m), 2867 yield). H NMR (400 MHZ, THF-d8): 11.00 (br), 9.13 (br), 8.44
(m), 1589 (w), 1526 (s), 1489 (m), 1462 (m), 1436 (m), 1383 (s), (br), 7.46 (br), 6.92 (br), 5.46 (br), 5.11 (br), 4.48 (br), 3.16 (br)
1373 (s), 1315 (s), 1257 (m), 1174 (m), 1103 (m), 1056 (m), 1024 ppm. IR (KBr): 2962 (s), 2925 (s), 2866 (m), 1653 (w), 1539 (m),
(m), 933 (w), 856 (w), 796 (m), 759 (m), 697 (m) cm−1. UV/vis 1524 (s), 1464 (s), 1436 (s), 1385 (s), 1368 (m), 1317 (m), 1260
(pentane): λmax (ε) = 390 (4657 M−1 cm−1) nm. μeff (293 K) = 3.9 (m), 1172 (m), 1099 (s), 1057 (m), 1024 (s), 935 (w), 797 (s), 761
(1)μB. Mp: 211 °C (dec). Mass spectrum m/z (%): 469.3 (70) (w) cm−1. UV/vis (pentane): λmax (ε) = 380 (4077 M−1 cm−1),
[M+ − C2Ph2], 178.1 (100) [C2Ph2]. Anal. Calcd for C43H51N2Cr: 748 (317 M−1 cm−1) nm. μeff (293 K) = 3.8(1)μB. Mp: 73 °C
C, 79.71; H, 7.94; N, 4.33. Found: C, 79.41; H, 7.65; N, 4.37.
(dec). Mass spectrum m/z (%): 577.3 (69) [M+ − C4H8O]. Anal.
Calc. for C41H61CrN2O: C, 75.77; H, 9.46; N, 4.31. Found: C,
73.58; H, 9.36; N, 4.28.
Preparation of (i-Pr2Ph)2nacnacCr(CPh)2(CH2)2 (9)
(i-Pr2Ph)2nacnacCr(η2-C2Ph2) (8) (0.100 g, 0.154 mmol) was dis-
solved in 20 mL, excess ethylene was added and the solution
Crystallographic structure determinations
was stirred for up to 24 hours. The THF was removed and the A summary of crystal data collection and refinement para-
product was extracted with pentane. Concentration and meters for compounds 3, 4, 6, 7, 8, 9 and 10 can be found in
cooling to −30 °C yielded light brown crystals of 9 (0.140 g, Table 3. Suitable crystals were selected, mounted with viscous
Table 3 Crystallographic data and refinement details for 3, 4, 6, 7, 8, 9 and 10a
3
4
6
7
8
9
10
Formula
Fw
Space group
Color
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
68H108Cr2N4OSi2
C68H104Cr2N4
1081.55
P1
C37H55CrN2O
595.83
Pnma
C31.88H42.50CrN3
519.69
P21/n
Green
11.960(6)
14.191(7)
16.977(8)
90
97.191(5)
90
2859(2)
4, 1
1.207
0.424
120(2)
4952/337
1.081
C43H51CrN2
647.86
Pnma
Orange
21.020(4)
18.707(3)
19.091(4)
90
90
90
7507(2)
8, 2
1.146
C59H75CrN2O
880.21
P21/c
Brown
13.834(4)
17.440(5)
20.034(5)
90
104.087(5)
90
4688(2)
4, 1
1.247
0.288
120(2)
8258/500
1.054
C41H61CrN2O
649.92
P1
1157.76
P21
Brown
13.404(5)
14.089(5)
18.924(7)
90
109.268(7)
90
3374(2)
2, 1
1.140
0.400
120(2)
11 746/732
1.025
ˉ
ˉ
Brown
Red
Green
13.187(4)
13.567(4)
18.405(6)
88.750(5)
74.798(4)
88.291(4)
3175.8(17)
2, 1
1.131
0.383
120(2)
13 753/679
1.059
16.702(3)
20.310(4)
9.768(2)
90
90
90
3313.5(11)
4, 0.5
1.194
8.924(3)
11.555(4)
19.287(7)
106.796(6)
96.988(6)
100.378(7)
1840.8(12)
2, 1
1.173
0.343
120(2)
6485/420
1.049
γ (°)
V (Å3)
Z, Z′
D(calcd) (g cm−3
)
)
μ(MoKα) (mm−1
Temp, K
0.376
0.335
120(2)
4153/322
1.049
5.36
13.33
180(2)
6835/474
1.036
5.56
15.24
Data/params
GOF on F2
R(F) (%)
7.42
12.77
0.03(3)
5.84
16.20
—
8.41
18.15
—
6.09
16.82
—
5.27
14.18
—
Rw(F2) (%)
Flack param
—
—
a Quantity minimized: Rw(F2) = Σ[w(Fo2 − Fc2)2]/Σ[(wFo2)2]1/2
.
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oil and cooled to the data collection temperature. Data were
collected on a Bruker APEX CCD diffractometer using graphite
monochromated Mo-Kα radiation (λ = 0.71073). Unit cell para-
meters were obtained from 60 data frames, 0.3° ω, from three
different sections of the Ewald sphere. No symmetry higher
than triclinic was observed for 4 and 10. The systematic
absences in the data and the unit cell parameters were consist-
ent to P21 and P21/m for 3, and Pnma and Pn21a (Pna21) for 6
and 8, and, uniquely, P21/n for 7, and P21/c for 9. The centro-
3 R. M. Manyik, W. E. Walker and T. P. Wilson, Union Carbide
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symmetric space group options for 4 and 10 (P1), and 6 and 8
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(Pnma), and the noncentrosymmetric space group for 3 (P21)
yielded chemically reasonable and computationally stable
results of refinement. The data-sets were treated with SADABS
absorption corrections based on redundant multiscan data.79
The structures were solved using direct methods and refined
with full-matrix, least-squares procedures on F2. A close inspec-
tion of the packing diagram did not reveal any overlooked sym-
metry for 3 and the Flack parameter80 refined to 0.03(3) 10 B. H. Muller, N. Peulecke, A. Spannenberg, U. Rosenthal,
indicating that the true hand of the data was determined.
M. H. Al-Hazmi, R. Schmidt, A. Wohl and W. Muller,
Compound 4 consistently deposits as small, weakly-diffracting
Organometallics, 2012, 31, 3695–3699.
crystals resulting in lower-than-ideal 2θ range. Two symmetry 11 W. Muller, A. Wohl, S. Peitz, N. Peulecke, B. R. Aluri,
unique but chemically identical compound molecules were
B. H. Mueller, D. Heller, U. Rosenthal, M. H. Al-Hazmi and
found in the asymmetric unit of 8. The metallacyclic carbon
F. M. Mosa, ChemCatChem, 2010, 2, 1130–1142.
atoms and the isopropyl moieties displayed unresolvable dis- 12 S. Peitz, B. R. Aluri, N. Peulecke, B. H. Muller, A. Wohl,
order causing a larger than usual Ueq range for carbon atoms
and associated hydrogen atoms. Compound 6 was located dis-
W. Muller, M. H. Al-Hazmi, F. M. Mosa and U. Rosenthal,
Chem.–Eur. J., 2010, 16, 7670–7676.
ordered at a mirror plane. Solvent molecules were found 13 S. Peitz, N. Peulecke, B. R. Aluri, S. Hansen, B. H. Muller,
cocrystallized in the asymmetric units of 3 (hexamethyldisil-
oxane), 4 (pentane), 7 (pentane) and 9 (pentane). Disordered
pentane molecules of crystallization in 7 with net occupancy of
A. Spannenberg, U. Rosenthal, M. H. Al-Hazmi,
F. M. Mosa, A. Wohl and W. Muller, Eur. J. Inorg. Chem.,
2010, 1167–1171.
1.5 per unit-cell, in two void spaces located away from the com- 14 S. Peitz, N. Peulecke, B. R. Aluri, B. H. Muller,
pound molecule, and two molecules of co-crystallized pentane
per compound molecule in 9 were treated as diffused con-
tributions.81 All non-hydrogen atoms were refined with
A. Spannenberg, U. Rosenthal, M. H. Al-Hazmi,
F. M. Mosa, A. Wohl and W. Muller, Organometallics, 2010,
29, 5263–5268.
anisotropic displacement parameters. Hydrogen atoms on 15 S. Peitz, N. Peulecke, B. R. Aluri, B. H. Muller,
metallacycles were located from the difference map, treated
A. Spannenberg, U. Rosenthal, M. H. Al-Hazmi, F. M. Mosa,
with chemically sensible restraints, and refined with a riding
A. Wohl and W. Muller, Chem.–Eur. J., 2010, 16, 12127–12132.
model. All other hydrogen atoms were treated as idealized con- 16 S. Peitz, N. Peulecke, B. H. Muller, A. Spannenberg,
tributions with 1.2–1.5Ueq of the associated carbon atom.
Atomic scattering factors are contained in the SHELXTL 6.12
program library.
H. J. Drexler, U. Rosenthal, M. H. Al-Hazmi, K. E. Al-Eidan,
A. Wohl and W. Muueller, Organometallics, 2011, 30, 2364–
2370.
17 A. Wohl, W. Muller, S. Peitz, N. Peulecke, B. R. Aluri,
B. H. Muller, D. Heller, U. Rosenthal, M. H. Al-Hazmi and
F. M. Mosa, Chem.–Eur. J., 2010, 16, 7833–7842.
18 S. Licciulli, I. Thapa, K. Albahily, I. Korobkov,
S. Gambarotta, R. Duchateau, R. Chevalier and K. Schuhen,
Angew. Chem., Int. Ed., 2010, 49, 9225–9228.
19 I. Thapa, S. Gambarotta, I. Korobkov, R. Duchateau,
S. V. Kulangara and R. Chevalier, Organometallics, 2010, 29,
4080–4089.
Acknowledgements
This work was supported by the U.S. National Science
Foundation, through grants CHE-0616375 and CHE-0911081
to KHT.
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