Molecular dynamics and ligand exchange in magnesium complexes: Evidence for both dissociative and associative ligand exchange

Article abstract of DOI:10.1002/anie.201209542

Making a swap: The titled studies provide substantial evidence for the kinetically labile Mg²⁺ ion reacting through both a dissociative (see scheme; thf=tetrahydrofuran) and an associative interchange mechanism. The findings are particularly relevant to the reactions involving [LMgnBu(thf)] and rac-lactide in ring-opening polymerization reactions.

Full text of DOI:10.1002/anie.201209542

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Angewandte
Communications
DOI: 10.1002/anie.201209542
Ligand Effects
Molecular Dynamics and Ligand Exchange in Magnesium Complexes:
Evidence for both Dissociative and Associative Ligand Exchange**
Malcolm H. Chisholm,* Kittisak Choojun, Albert S. Chow, and Gideon Fraenkel
Dedicated to Sir John Meurig Thomas FRS on the occasion of his 80th birthday
The chemistry of Grignard reagents is among of the most
mature in organometallic chemistry, and it continues to
inspire interest and reveal new results as seen in the develop-
ment of turbo-Grignard reagents and the alkali-metal activa-
The initiation step involved b-hydrogen-atom transfer with
the formation of the ring-opened lactide bound to the Mg
center as an alkoxide and the elimination of 1-butene.[14]
During the subsequent enchainment of rac-lactide it was
noted that while atactic polylactide (PLA) was formed in the
noncoordinating solvents, toluene and dichloromethane, the
addition of THF favored the formation of heterotactic PLA
with isi/sis tetrads. Indeed, in neat THF solution, the Pr value
for isi/sis tetrads was greater than 95%, which implicated the
presence of THF during the ring-opening event. Earlier work
had also noted the influence of THF on the stereo-outcomes
in the ROP of rac-LA, and while structures of complexes
determined by single-crystal X-ray crystallography are
insightful with respect to ground-state and thermally persis-
tent species they are of less relevance in mechanistic
considerations of reactivity. Consequently, we examined the
mechanism of ligand exchange in complexes of the form
[LMgnBu(L’)] where L is one of the chelating anionic ligands
shown in B and L’ is thf, 2-methyltetrahydrofuran (2-
MeTHF), pyridine (py), and 4-dimethylaminopyridine
(DMAP). We report herein a summary of results as deter-
mined from variable-temperature NMR spectroscopy.
The title complexes were prepared by the addition of the
donor ligand L’ (L’ = thf, 2-MeTHF, py, or DMAP) to the
hexane-soluble product formed in the reaction between LH
and MgnBu2. The preparation and structure of the thf
complexes[14] have been previously described as has that of
the pyridine adduct.[15]
ꢀ
tion of C H bonds by the Strathclyde school of s-block
element chemistry.[1–7] The reactivity of the magnesium alkyl
group has long been known to be highly solvent dependent as,
indeed, is found for organolithium and organozinc
reagents.[8–12] In part this arises from the kinetic lability of
these ions and the extensive equilibria, typified by the so-
called Schlenk equilibrium, present in polar coordinating
solvents.[13] We recently reported the synthesis of a series of
magnesium alkyls of the form [LMgR(thf)] where L repre-
sents one of the chelating anionic ligands shown in A and B.[14]
The molecular structure of the b-diiminato-supported n-butyl
complex is also shown (Figure 1).
Figure 1. Representations of the chelating pyrromethene and b-diimi-
nate ligands and the molecular structure of [LMgnBu(thf)].
The molecular structures of [LMgnBu(thf)] and
[LMgnBu(py)] can be described as pseudotetrahedral coor-
dination about Mg2+ where the pyridine or thf is bound more
The purpose of this earlier study was aimed at the taming
of the kinetically labile Mg2+ center and the suppression of the
Schlenk equilibrium with the ultimate hope of developing the
chemistry of the MgR moiety within a well-defined pocket
and coordination environment. During this study the ring-
opening polymerization (ROP) of lactide was studied
employing the n-butylmagnesium complexes as initiators.
ꢀ
ꢀ
weakly, as evidenced by its longer Mg N or Mg O bond
distances. That is to say, the structure is distorted toward the
trigonal-planar geometry involving the unsolvated [LMgnBu]
molecular fragment.[14,15]
The exchange between the thf-containing complex and
free THF has been studied in [D8]toluene and CD2Cl2 by both
1H and 13C{1H} NMR spectroscopy in the temperature range
between ꢀ1008C and 1008C ([D8]toluene). The added con-
centrations of THF have been from 0.2 to 2.5 equivalents and
the rates of exchange have been simulated by a detailed line-
shape analysis. Experimental and simulated 13C{1H} NMR
spectra are shown in Figure 2 and kinetic data are given in
Table 1.
The key finding from this work is that the exchange
process is dissociative as shown in Scheme 1. For the
dissociative THF exchange we calculate DH° = (13.4 ꢁ
0.4) kcalmolꢀ1 and DS° = (+ 6.3 ꢁ 1.6) calmolꢀ1 Kꢀ1.
[*] Prof. M. H. Chisholm, K. Choojun, Dr. A. S. Chow, Prof. G. Fraenkel
Department of Chemistry and Biochemistry
The Ohio State University
100W 18thAvenue Columbus Ohio 43210 (USA)
E-mail: chisholm@chemistry.ohio-state.edu
[**] We thank the Department of Energy, Office of Basic Sciences
Chemistry Division for financial support. K.C. thanks the Royal Thai
Government for a scholarship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209542.
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DMAP. Thus, the exchange reflects the Mg-L’ bond strengths
which are in the order of DMAP > py > thf > 2-MeTHF.
Interestingly, the rates of aryl group rotation, which can be
monitored by both 1H and 13C{1H} NMR spectroscopy, reveal
within experimental error the identical activation energies
when compared to the L’ dissociative process. The aryl group
rotation involves exchange of equal site populations and is
readily monitored by following the methine CH signals of the
2,6-diisopropylphenyl ligands of the b-diiminate (L). The
coalescence temperatures for aryl group rotation in
[D8]toluene at 500 MHz are: ꢀ408C (2-MeTHF), ꢀ158C
(thf), 358C (py), and 758C (DMAP; Figure 3). Given the
Figure 2. Variable-temperature 13C NMR (125 MHz) spectra of 0.035m
[LMgnBu(thf)] with 0.038m THF (1:1.09) in [D8]toluene. Both the
experimental (left) and simulation (right) data are shown only for the
a-carbon atoms of thf. The signal(1) for the coordinated thf is
downfield of that for the free THF.
ꢀÀ
Á
Table 1: The values of 1 tLMgnBuðTHFÞ (k1) from 13C NMR line-shape
analysis simulation at 0.035m of [LMgnBu(THF)] and different added
THF concentrations and temperatures.[a]
ꢀÀ
Á
Figure 3. Variable-temperature 1H NMR spectra of CHMe2 of the aryl
ring of [LMgnBu(py)] (0.035m) in [D8]toluene (left) and [D5]py (right).
Equiv. Free
added THF
THF
1
tLMgnBuðTHFÞ ¼ k1
223 K 233 K 243 K 253 K 263 K 273 K 300 K
0.20
0.23
0.32
0.66
0.78
1.09
1.21
1.50
2.00
2.36
0.0070 10
33
33
33
33
33
33
33
33
33
33
100
100
100
100
100
100
100
100
100
100
280
280
280
280
280
280
280
280
280
280
950
950
950
950
950
950
950
950
950
950
2000 35000
2000 35000
2000 35000
2000 35000
2000 35000
2000 35000
2000 35000
2000 35000
2000 35000
2000 35000
steric crowding at the four-coordinate metal centers in the
[LMgnBu(L’)] complexes, it is not surprising that aryl group
rotation is restricted. Upon dissociation of L’ one anticipates
a three-coordinate Mg2+ ion akin to that of the [LZnnBu]
which has little affinity to bind Lewis bases. The crystal
structures of similar three-coordinate zinc alkyl analogues
were previously reported by Parkin and co-workers in 2010.[16]
Indeed, this zinc compound, [LZnnBu], shows extremely
facile aryl group rotation which cannot be frozen out on the
NMR time scale, even at ꢀ1008C in [D8]toluene. We should
note that Mg2+ and Zn2+ have essentially identical ionic radii,
and thus steric constraints on the rotation of the aryl ligands
of the b-diiminate should be similar.[17]
0.0080 10
0.0112 10
0.0232 10
0.0273 10
0.0382 10
0.0422 10
0.0525 10
0.0700 10
0.0826 10
[a] See the Experimental Section and Supporting Information for details.
Interestingly, the coalescence behavior of the aryl group
rotation is temperature dependent in the presence of
a relatively large excesses of L’ in [D8]toluene. Thus, a limiting
coalescence is obtained when the solvent employed is
[D8]THF or [D5]pyridine for the complexes [LMgnBu(L’)]
where L’ is thf or py, respectively. Here we observed in neat
[D8]THF that the coalescence temperature is suppressed from
ꢀ158C to ꢀ758C, and for the pyridine complex in
[D5]pyridine from 358C to ꢀ158C. This suppression is quite
a dramatic and systematic change in the rate of aryl group
rotation for each complex and leads us to suggest that in the
Scheme 1. The dissociative process of the [LMgnBu(thf)] complex.
We have also similarly examined the ligand exchange
process involving 2-MeTHF and pyridine. The exchange
involving 2-MeTHF occurs more readily while that involving
pyridine self-exchange is slower. The related complex involv-
ing L’ = DMAP has an even slower exchange with added
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[D8]THFat 3.58 ppm (1H NMR) and 67.57 ppm (13C NMR), [D5]py at
8.74 ppm (1H NMR) and 150.35 ppm (13C NMR).
presence of a significant excess of the L’ the complex is
capable of showing an exchange process involving L’ by an
interchange associative mechanism. It is easy to envisage
a five-coordinate interchange activated complex where the
Mg2+ ion is in a pseudo-trigonal-bipyramidal geometry as
shown C. Here the aryl isopropyl
Chemicals and Solutions. Deuterated toluene, THF, and pyridine
were purchased from Cambridge Isotope Laboratory, stirred over
calcium hydride overnight, distillated under nitrogen gas, and stored
over 4 ꢀ molecular sieves overnight prior use. The solutions were
made inside glove-box under nitrogen atmosphere and transferred to
a J-young NMR tube.
ligands become equivalent. We note
Line-shape Analyses. The programs were written based on the
density formulism developed by Fraenkel and Kaplan.[21] The density-
matrix equations used for 13C NMR spectrum simulations of different
temperatures is based on the two uncoupled site exchange system and
they are derived with and without explicit exchanging mechanism. All
the information is provided in supporting information.
the recent report by Mountford and
co-workers that the complex [LMg-
(BH4)(thf)] has similar restricted
rotation of the aryl groups whereas
the bis(thf) complex of calcium [LCa-
(BH4)(thf)2] shows equivalent isopro-
pyl groups.[18]
Received: November 28, 2012
Published online: February 10, 2013
If we assume that the activation
parameters for the aryl group rota-
Keywords: lactide polymerization · ligand effects · magnesium ·
molecular dynamics · NMR spectroscopy
tion in neat [D8]THF correlate with
the dissociative plus associative inter-
change processes, we can obtain the
.
activation parameters for the associative interchange as
DH° = (5.4 ꢁ 0.1) kcalmolꢀ1 and DS° = (ꢀ20.9 ꢁ 0.3) calm-
olꢀ1 Kꢀ1.
[1] R. E. Mulvey, Acc. Chem. Res. 2009, 42, 743 – 755.
[2] A. Krasovskiy, P. Knochel, Angew. Chem. 2004, 116, 3396 – 3399;
Angew. Chem. Int. Ed. 2004, 43, 3333 – 3336.
The above findings are particularly relevant to the
reactions involving [LMgnBu(thf)] and rac-lactide (LA). In
toluene and dichloromethane these reactions are extremely
fast but show no stereoselectivity, whereas in THF, the
reactions, though still rapid, are relatively slower and show
a high propensity for the formation of heterotactic PLA. In
the presence of excess THF, the LA monomer presumably has
to react by an associative process where THF is still present
within the coordination sphere of the metal center. While this
conclusion has been inferred previously,[14] the present study
provides more substantial evidence that this kinetically labile
Mg2+ ion can react through both a dissociative and an
associative interchange mechanism.
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[5] See Ref. [4].
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Experimental Section
1
NMR Spectroscopy. Variable-temperature H and 13C NMR spectra
were acquired on a Bruker DPX-500 NMR Spectrometer from 183 to
363 K, using a J-young NMR tube. The temperature of the probe was
adjusted with a standard temperature-control unit, using liquid
nitrogen at low temperature and a heating element at high temper-
ature. The probe temperature was calibrated by using methanol
standard for low temperature and 80% ethylene glycol in DMSO
standard for high temperature. The accuracy of the temperature
measurement was (ꢁ 0.5)8C.[19,20] The probe was tuned at each
temperature and the sample tubes were left in the probe for at least
15 min to achieve the thermal equilibrium before taking measure-
ments. The spectra were referenced by the residual priority impurity
of [D8]toluene at 2.08 ppm (1H NMR) and 137.86 ppm (13C NMR),
[21] J. I. Kaplan, G. Fraenkel, NMR of Chemically Exchanging
Systems, Academic Press, New York, 1980.
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