On the remarkable structural diversity and kinetic lability of Tp C*CaX complexes (X = NSi2Me6, OC 6H4-p-Me, TpC*) where Tp C* = tris[3-(2-methoxy-1,1-dimethylethyl)pyrazolyl]hyd

Article abstract of DOI:10.1039/b818375a

The preparation, structures and solution NMR properties of the title compounds are reported and, remarkably, the compounds (Tp^C*) ₂M, where M = Ca and Mg, exist as salts in the solid state and undergo facile Tp^C* ligand ex

Full text of DOI:10.1039/b818375a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
On the remarkable structural diversity and kinetic lability of TpC*CaX
complexes (X = NSi2Me6, OC6H4-p-Me, TpC*) where
TpC* = tris[3-(2-methoxy-1,1-dimethylethyl)pyrazolyl]hydroboratew
Malcolm H. Chisholm,* Judith C. Gallucci, Gul-sah Yaman and Tanya Young
¨
Received (in Berkeley, CA, USA) 17th October 2008, Accepted 29th January 2009
First published as an Advance Article on the web 25th February 2009
DOI: 10.1039/b818375a
The preparation, structures and solution NMR properties of the
title compounds are reported and, remarkably, the compounds
(TpC*)2M, where M = Ca and Mg, exist as salts in the solid
state and undergo facile TpC* ligand exchange in solution.
as a potential tetradentate, pentadentate or hexadentate ligand
due to the ether appendages.7,8
Ring-opening polymerization of cyclic esters derived from
renewable resources can be brought about by both metal
coordinate catalysis and organic catalysis leading to polyesters
that find numerous commercial applications.1 These polymers
can be viewed as green polymers being both derived from
renewable resources and biodegradable. In many instances
they also are biocompatible and find uses in drug delivery,
sutures, stints, pins and tissue matrices. Examples of these
polymers include polylactide, PLA, polyglycolide (PG) and
polycaprolactone (PC) along with their copolymers. Metal
coordinate catalysis can lead to trace quantities of the metal
residing in the polymer, as is the case for polyethylenes
synthesized from chromium or titanium metal catalysts. Thus
catalysts containing biologically benign metals are desirable
for all polymers employed in medical applications, and in
the storage of food and liquids for human consumption. In
our studies aimed at single-site catalyst development with
initiators of the form LMOR, we have found2,3 that the
reactivity follows the order M = Ca 4 Mg 4 Zn, where
R = alkyl and L = a sterically demanding ligand such as the
b-diketoiminate CH(CMeN-2,6-Pri2-C6H4)2, popularized by
Coates and co-workers4,5 in connection with zinc, or the bulky
tris(3-tert-butylpyrazolyl)hydroborate, commonly regarded as
a ‘‘tetrahedral enforcer.’’6 Despite the high initial reactivity of
the calcium complexes of the type LMOR toward the ROP of
cyclic esters, their chemical persistence proved problematic
and the kinetic lability of the metals, which also followed the
order Ca 4 Mg 4 Zn, meant that deleterious side reactions,
involving hydrolysis or ligand scrambling due to the
Schlenk equilibrium, led to catalyst death.3 In an attempt to
suppress these side reactions we have synthesized the TpC*
ligand, TpC* = tris[3-(2-methoxy-1,1-dimethylethyl)pyrazolyl]-
hydroborate, which, as shown in the drawing below, is similar
to the tris(3-tert-butylpyrazolyl)hydroborate in its steric
demand as a k3-N donor but has the advantage of behaving
The ability of the TpC* ligand to act in this manner toward
M+ ions has been noted previously.7,8 Herein we describe its
versatility as a ligand in calcium complexes of the form
TpC*CaX, where X = NSi2Me6, OC6H4-p-Me and TpC*,
together with a comparison with the related magnesium
compound (TpC*)2Mg. The latter provides an interesting
and rare comparison of the relative kinetic labilities of these
M2+ ions in a common coordination environment.
In an attempt to prepare a calcium compound of the form
TpC*CaX, where X = an amide or alkoxide, metathetic
reactions were carried out involving TpC*CaI, which exists
as a salt [TpC*Ca]+Iꢀ, and each of LiNMe2, LiN(SiMe3)2,
LiOBut, NaOBut, KOBut and KOSiMe3 in toluene by
employing standard procedures for the manipulation of air-
sensitive materials. In each case a reaction occurred but
did not yield the desired product; the group 1 metal tris-
pyrazolylhydroborate MTpC* was formed. Similar kinetic
lability was observed in ligand exchange reactions involving
tris(trimethylsilyl)cyclopentadienyl calcium iodide by Hanusa
and Harvey.9 The compound TpC*CaNSi2Me6 was, however,
formed in the reaction between Ca(NSi2Me6)2 and TlTpC*
in hydrocarbon solvents, and when this reaction was carried
out in pentane the compound TpC*CaNSi2Me6, I, was isolated
in Z 80% by its precipitation as a white microcrystalline solid.
In aromatic hydrocarbons the similar solubilities of I and
TlNSi2Me6 make their separation difficult and the preferential
vacuum sublimation of Tl(NSi2Me6) leads to recovery of I in
much lower yields.7
Compound I reacts with alcohols and phenols to give
TpC*CaX compounds, and where X = p-CH3C6H4O com-
pound II is formed in 490% isolated yield. The latter
compound and its related alkoxides are, indeed, good pre-
cursor complexes for the ring-opening polymerization of
lactides, caprolactone and trimethylene carbonate and this
topic is the subject of an ongoing study. Quite remarkably,
compound II is chemically persistent in air for several hours as
a white crystalline solid and is tolerant of undried hydro-
carbon solvents. This contrasts with other LCaOR compounds
of the type noted before.
Department of Chemistry, The Ohio State University,
100 West 18th Avenue, Columbus, OH 43210, USA.
E-mail: Chisholm@chemistry.ohio-state.edu; Fax: +1 614 292 0368;
Tel: +1 614 292 7216
w Electronic supplementary information (ESI) available: Experimental
and NMR data. CCDC 705806–705809. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b818375a
ꢁc
This journal is The Royal Society of Chemistry 2009
1828 | Chem. Commun., 2009, 1828–1830

View Article Online
Compound I slowly reacts in hydrocarbon solvents at
room temperature and more rapidly (o1 h) at 80 1C to give
Ca[(NSi2Me6)2]2 and (TpC*)2Ca, III. The latter compound
preferentially crystallizes from hydrocarbon solvents as a white
crystalline solid.3 The seemingly straightforward synthesis of III
from the metathetic reaction involving TpC*CaI and TlTpC* in
toluene was not successful. Initially a yellow material formed
which was sparingly soluble in hydrocarbons but with time
yellow TlI (insoluble in hydrocarbons) was precipitated.
Tentatively, we suggest that the initial compound is a salt,
perhaps of the form [TpC*Ca]+[ITlTpC*]ꢀ, and that this
metastability thwarts the straightforward, direct synthesis of
(TpC*)2Ca.
Rather interestingly, reactions involving either MgMe2 or
MgBun and TlTpC* (2 equiv.) lead directly to the compound
2
(TpC*)2Mg, IV, along with Tl(m) and when 1 equiv. of TlTpC*
is employed, compound IV and an equivalent of MgR2 are
formed (R = Me or Bu).4
Fig. 2 An ORTEP drawing of TpC*CaOC6H4-p-Me, II, with thermal
ellipsoids drawn at the 30% probability level. The hydrogen atoms are
omitted for clarity. Selected bond distances (A) to Ca: N(2) 2.513(2);
N(4) 2.526(2); N(6) 2.497(2); O(1) 2.526(1); O(2) 2.446(1); O(3)
2.498(1); O(4) 2.177(1).
The molecular structure of I in the solid state is shown in
Fig. 1.z The Ca2+ ion is five-coordinate and, most interest-
ingly, the TpC* ligand is ligated by only two of the pyrazolyl
ligands. Undoubtedly the steric demands of the bulky
NiSi2Me6 ligand impede the TpC* ligand from being hexa-
dentate and it is interesting that the Ca2+ ion elects to employ
the coordination of the two N,O-chelates in preference to say
three N donors supplemented by one ether donor, cf. the
molecular structure of TlTpC*.8 By 1H and 13C{1H} NMR
spectroscopy in toluene-d8 all the arms of the TpC* ligand are
equivalent even at ꢀ80 1C and NOE experiments confirm the
time-averaged proximity of the SiMe3 and OMe protons. We
conclude that this molecule is fluxional on the NMR time-
scale as was seen for a related Na[B(pz*)4] compound, where
pz* = 3-(2-methoxyl-1,1-dimethylethylpyrazolyl).8
The molecular structure of compound II is shown in Fig. 2.z
The Ca2+ is seven-coordinate with the TpC* being hexa-
dentate, k3-N3,O3. The H NMR spectrum is not surprisingly
quite simple and as expected.
1
Fig. 3 An ORTEP drawing of [TpC*Ca]+[TpC*]ꢀ (III) with thermal
ellipsoids drawn at the 30% probability level. The hydrogen atoms are
omitted for clarity.
Compounds III and IV are structurally similarz and reveal
their salt-like nature [TpC*M]+[TpC*]ꢀ, where M = Ca and
Mg. The molecular structure of III is shown in Fig. 3z and a
comparison of selected structural parameters for the
[TpC*M]+ cation is given in Table 1. The M–O distances are
notably somewhat smaller than the M–N distances and this
difference is greater for M = Ca than for M = Mg. We
propose that this subtle difference is largely a result of the fact
that the larger Ca2+ ion with an ionic radius of B1.0 A is
more ideally suited for the TpC* ligand, as is the Na+ ion
Table 1 Selected bond distances (A) and angles (1) for the [TpC*M]+
ions in the (TpC*)2M complexes (M = Mg and Ca)
Distance/A
Mg Ca
Angle/1
A
B
A
B
C
Mg
Ca
79.0(2)
75.6(2)
M
M
M
M
M
M
N2 2.219(3) 2.436(5) N2
N4 2.187(3) 2.423(5) N2
N6 2.173(3) 2.434(5) N2
O1 2.157(3) 2.380(4) N2
O2 2.163(3) 2.369(4) O1
O3 2.150(3) 2.346(4) O3
M
M
M
M
M
M
N6
O1
87.3(1)
76.9(1)
O2 120.7(1) 126.5(2)
O3 150.5(1) 131.0(2)
N4 146.7(1) 131.9(2)
Fig. 1 An ORTEP drawing of TpC*CaN(SiMe3)2, I, with thermal
ellipsoids drawn at the 50% probability level. The hydrogen atoms are
omitted for clarity. Selected bond distances (A) to Ca: N(7) 2.310(1);
N(2) 2.455(1); N(6) 2.461(1); O(1) 2.370(1); O(3) 2.376(1).
O1
87.4(1)
90.9(2)
ꢁc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 1828–1830 | 1829

View Article Online
employing HC(CMeNC6H4-Pri2-2,6)2Ca(NSi2Me6).13,14 We
propose that it will, indeed, be possible to develop LCaOR
catalysts for the ROP of polyoxygenates given the appropriate
L and reaction conditions, and note the recent success reported
by Darensbourg and co-workers.15
We thank the Department of Energy, Office of Basic
Science, Chemistry Division for financial support of this work.
Notes and references
z Crystal data for TpC*CaN(SiMe3)2, I, C30H58BCaN7O3Si2,
M = 671.90, monoclinic, a = 13.1496(5), b = 15.5703(8), c =
18.9206(8) A, b = 99.351(2)1, U = 3822.4(3) A3, T = 95 K, space
group P21/c (no. 14), z = 4, 42 864 reflections measured, 10 864 unique
(Rint = 0.051), which were used in all calculations. Final R values are
R1 = 0.063 and wR2 = 0.112 (on all data).
Fig. 4 Stacked VT 13C{1H}-NMR spin saturation transfer spectra of
compound IV in toluene-d8. (See ESIw for details.)
Crystal data for TpC*Ca(OC6H4-p-Me), II, C31H47BCaN6O4ꢂ
toluene, M = 710.77, monoclinic, a = 13.390(1), b = 15.831(1),
c = 19.583(2) A, b = 109.058(3)1, U = 3923.6(6) A3, T = 150 K,
space group P21/n (no. 14), z = 4, 63 707 reflections measured, 6918
unique (Rint = 0.039), which were used in all calculations. Final R
values are R1 = 0.055 and wR2 = 0.103 (on all data).
relative to the Li+ ion in the related MTpC* compounds.8
Given the similar ionic radii10 of Li+ and Mg2+ it is thus
noteworthy that the Li–O distances are B0.3 A longer
than their Mg–O counterparts. This can most reasonably be
correlated with the greater effective charge on the Mg2+ ion
and its greater oxophilicity.
Crystal data for [TpC*Ca]TpC*, III, C48H80B2CaN12O6,
M = 982.94, monoclinic, a = 14.2546(2), b = 16.7217(3), c =
24.6146(5) A, b = 102.840(1)1, U = 5720.4(2) A3, T = 230 K, space
group P21/c (no. 14), z = 4, 51 480 reflections measured, 7420 unique
(Rint = 0.085), which were used in all calculations. Final R values are
R1 = 0.150 and wR2 = 0.259 (on all data).
The NMR spectra of compounds III and IV in toluene-d8
are also of particular interest in comparing the coordination
properties of the two elements as M2+ ions. The 1H NMR
spectrum for the magnesium compound [TpC*Mg]+[TpC*]ꢀ
shows two distinct sets of TpC* resonances consistent with the
bound and free ligands, whereas the calcium complex III
shows only one set of TpC* signals, even at ꢀ80 1C. By spin
magnetization transfer experiments, the 13C{1H} signals of the
compound IV in toluene-d8 show evidence of dynamic
exchange of the coordinated and free TpC* ligands, see
Fig. 4. We propose that both complexes, III and IV, exist as
tight ion pairs in toluene-d8 and that the dynamic exchange
between free and coordinated TpC* is 4100 times faster for the
larger and softer Ca2+ ion relative to the smaller, harder M2+
Crystal data for [TpC*Mg]TpC*, IV, C48H80B2MgN12O6, M =
967.17, triclinic, a = 14.3126(1), b = 14.5131(2), c = 14.9368(2) A,
a = 97.536(1)1, b = 115.884(1)1, g = 99.546(1)1, U = 2679.34(5) A3,
ꢀ
T = 150 K, space group P1 (no. 2), z = 2, 47 258 reflections measured,
9449 unique (Rint = 0.038), which were used in all calculations. Final
R values are R1 = 0.119 and wR2 = 0.289 (on all data).
1 O. Dechy-Caberet, B. Martin-Vaca and D. Bourissou, Chem. Rev.,
2004, 104, 6147.
2 M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg. Chem.,
2002, 41, 2785.
3 M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg. Chem.,
2004, 43, 6717.
4 B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt and
G. W. Coates, J. Am. Chem. Soc., 2001, 123, 3229.
5 L. R. Rieth, D. R. Moore, E. B. Labkovsky and G. W. Coates,
J. Am. Chem. Soc., 2002, 124, 15239.
6 S. Trofimenko, Scorpionates: The Coordination Chemistry of
Polypyrazolylborate Ligands, Imperial College Press, London,
1999.
7 M. H. Chisholm, J. C. Gallucci and G. Yaman, Chem. Commun.,
2006, 1872.
ion for which exchange is o10ꢀ1
s
ꢀ1. These are, at this time,
‘‘ball-park’’ estimates and are the subject of further study.
Although considerable recent attention has been given to the
coordination chemistry of the heavier group 2 elements,11,12 an
understanding of their reaction chemistry is in its infancy. What
we believe, is abundantly apparent from the data reported in
this communication, is the remarkable lability of the ligands
bound to Ca2+ ions. One might thus question the ability of ever
establishing single-site catalysis by a compound of the form
LCaOR based on the facility of ligand exchange and side
reactions such as hydrolysis. However, we believe that this
really has more to do with the nature of L and the reaction
under consideration and point to the recent success of Barrett
and Hill in hydroamination and hydrophosphination catalysis
8 M. H. Chisholm, J. C. Gallucci and G. Yaman, Inorg. Chem., 2007,
46, 8678.
9 M. J. Harvey and T. P. Hanusa, Organometallics, 2000, 19, 1556.
10 R. D. Shanon, Acta Crystallgr., Sect. A, 1976, 32, 751.
11 T. P. Hanusa, Organometallics, 2002, 21, 2559.
12 T. P. Hanusa, Polyhedron, 1990, 9, 1345.
13 M. R. Crimmin, I. Casely and M. S. Hill, J. Am. Chem. Soc., 2005,
127, 2042.
14 M. R. Crimmin, A. G. M. Barrett, M. S. Hill, P. B. Hitchcock and
A. Panayiotis, Organometallics, 2007, 26, 2953.
15 D. J. Darensbourg, W. Choi, O. Karroonnirun and N. Bhuvanesh,
Macromolecules, 2008, 41, 3493.
ꢁc
This journal is The Royal Society of Chemistry 2009
1830 | Chem. Commun., 2009, 1828–1830