Divergent reaction pathways of tris(oxazolinyl)borato zinc and magnesium silyl compounds

Article abstract of DOI:10.1039/c2cc36953b

Synthesis and reactivity of monomeric magnesium and zinc silyl compounds To^MM-Si(SiHMe₂)₃ and To^MM- Si(SiMe₃)₃ are described (To^M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate).

Full text of DOI:10.1039/c2cc36953b

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Divergent reaction pathways of tris(oxazolinyl)borato
zinc and magnesium silyl compounds†‡
Cite this: Chem. Commun., 2013,
49, 4334
Debabrata Mukherjee, Nicole L. Lampland, KaKing Yan, James F. Dunne,
Arkady Ellern and Aaron D. Sadow*
Received 25th September 2012,
Accepted 16th November 2012
DOI: 10.1039/c2cc36953b
www.rsc.org/chemcomm
Synthesis and reactivity of monomeric magnesium and zinc silyl
compounds ToMM–Si(SiHMe2)3 and ToMM–Si(SiMe3)3 are described
(ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate). The magnesium
compounds react slowly with water and air, while the
zinc compounds are inert. With CO2, ToMMg–Si(SiHMe2)3 provides
ToMMgO2CSi(SiHMe2)3 through CO2 insertion, whereas ToMZn–
Si(SiHMe2)3 affords ToMZnOCHO.
Scheme 1 Synthesis of zinc silyls and magnesium silyls.
Zinc and magnesium alkyls are among the earliest and most
frequently used organometallic compounds, whereas related metal
silyls have received less attention. Magnesium and zinc-catalyzed observed in the 1H NMR spectra of compounds 1–4 (as well as all
hydrosilylations1 and dehydrogenative silylations2 involve mixtures the ToMMX compounds reported here). This pattern is consistent
of organosilanes and these main group organometallics. However, with time-averaged C3v-symmetry for the complexes and suggests
metal silyl species are generally not postulated intermediates in tridentate coordination of ToM. Downfield SiH chemical shifts of 1
most proposed catalytic cycles for such transformations, and (4.64 ppm) and 2 (4.71 ppm), high silicon–hydrogen coupling
studies of main-group metal silyl compounds have mostly focused constants (1JSiH = 172.2 and 169.7 Hz respectively), and infrared
on structural characterization and applications in transmetala- bands assigned to the nSiH of 1 (2064 cmꢀ1) and 2 (2054 cmꢀ1) are
tion.3,4 Less is known about their reactivity in steps that might consistent with terminal, 2-center-2-electron bonded Si–H moieties.
be involved in catalysis. Metal silyl compounds, supported by In addition, the infrared spectra provide support for tridentate ToM
tris(oxazolinyl)borate ancillaries employed in our catalytic investi- coordination by the single nCN band for each compound (1 and 3:
gations, may provide further insight into catalysis, possible side 1591 cmꢀ1; 2 and 4: 1582 cmꢀ1).
reactions, and the fundamental reactivity of these compounds.
Single crystal X-ray analyses of 1 (Fig. 1) and 3 further
The targeted silyl complexes ToMZnSi(SiHMe2)3 (1), ToMMg- support the spectroscopically assigned structures.7,8 Terminal
Si(SiHMe2)3 (2), ToMZnSi(SiMe3)3 (3) and ToMMgSi(SiMe3)3 (4) b-SiH’s in 1 are located in the difference Fourier map, but the
(ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) are efficiently Si(SiHMe2)3 is disordered. In the model, the three SiHMe2
prepared by salt metathesis reactions. ToMZnCl5 or ToMMgBr groups are oriented with the SiH’s pointing away from the zinc
react with the appropriate potassium silyl reagent, KSi(SiHMe2)3 center and are related by pseudo-C3 rotations. The Zn1–Si1
or KSi(SiMe3)3,6 in benzene at ambient temperature (Scheme 1). interatomic distance is slightly shorter in 1 (2.4028(5) Å) than in
Compounds 1, 2 and 4 are formed quantitatively within 30 min 3 (2.427(1) Å), and these distances are on the long side
at ambient temperature, whereas the preparation of compound 3 compared to other zinc silyls. For example, the ZnꢀSi distances
requires longer time (2 h). One set of oxazoline resonances was in Zn(Si(SiMe3)3)2,4 ((Me3Si)3SiZnCl(thf))2,9 and (Ph(Me3Si)2-
SiZnCl(thf))2 are similar (2.35 ꢁ 0.01 Å).9 Sterically hindered
groups give longer Zn–Si distances; for example, the distances
in Zn(Si(SiMe3)2Si(SiMe3)3)2 are B2.405 ꢁ 0.002 Å.10
Department of Chemistry, Iowa State University, Ames, IA 50011, USA.
E-mail: sadow@iastate.edu; Fax: +1 515 2940105; Tel: +1 515 2948069
† This article is part of the ChemComm ‘Emerging Investigators 2013’ themed
issue.
Metal silyls 1–4 are thermally resilient, and starting materials are
recovered after heating toluene-d8 solutions in sealed NMR tubes at
170 1C for 24 h. Furthermore, the zinc compounds 1 and 3 are inert
to reaction with O2 at pressures up to 100 psi and temperatures up
to 120 1C for 24 h, under a 450 W Hg lamp at ambient temperature,
‡ Electronic supplementary information (ESI) available: Synthesis of compounds
1–9, DOSY experiments and crystallographic data files. CCDC 902693–902696.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc36953b
c
4334 Chem. Commun., 2013, 49, 4334--4336
This journal is The Royal Society of Chemistry 2013

View Article Online
Communication
ChemComm
Surprisingly, zinc silyls 1 and 3 are also inert to water and
methanol. Only starting materials are detected after attempted
thermolysis of 1 or 3 in benzene-d6 with excess water at 80 1C
for 12 h. Compounds 1 and 3 are insoluble in pure water, and
the 1H NMR spectra of the solids recovered from water
and redissolved in benzene-d6 contain only signals assigned
to the zinc silyls. Similarly, 1 and 3 are insoluble in methanol
at room temperature. A suspension of 1 partly dissolves in
methanol-d4 after heating at 60 1C for 4 h. The 1H NMR
spectrum of the resulting solution contained silyl and oxazoline
resonances for 1 that are distinct from HSi(SiHMe2)3 and
ToMZnOMe. Additionally, the 1H NMR spectrum (acquired
in benzene-d6) of the solid recovered from methanol contained
only signals assigned to 1. This inert nature contrasts
the reactions of ToMZnR (R = H, alkyl, phenyl) and H2O
that provide a trimeric hydroxide (ToMZnOH)3.11 Addition of
12 M HCl to a benzene-d6 solution of 1, however, produces
HSi(SiHMe2)3. The magnesium silyls 2 and 4 are hydrolytically
sensitive, forming hydrosilanes HSi(SiHMe2)3 and HSi(SiMe3)3
upon treatment with water. Compounds 2 and 4 slowly react
with methanol (2: 4 h, r.t.; 4: 5 h, r.t.) to provide 5. Note
that ToMMgMe and methanol provide 5 quantitatively after
5 min in benzene.
Fig. 1 ORTEP diagram of ToMZnSi(SiHMe2)3 (1) with ellipsoids plotted at 35%
probability. One of the Si(SiHMe2)3 positions and the hydrogen atoms on ToM
and Me groups are not illustrated for clarity.
Fig. 2 ORTEP diagram of (ToMMgOMe)2 (5) with ellipsoids plotted at 35%
probability. Symmetry-related atoms are labelled with #. Hydrogen atoms and
two benzene molecules are not plotted for clarity.
Interestingly, 1 and CO2 (60 psi) react at 120 1C to form the zinc
formate ToMZnOCHO (6) (t1/2 = 18 h, eqn (1)). The 1H NMR spectrum
of 6 contained a downfield singlet resonance attributed to the
ZnOCHO (8.76 ppm); a similar signal was reported for TptBuZnO-
CHO (TptBu = tris(3-tert-butyl-pyrazolyl)borate) (8.91 ppm).13 A peak
at 169 ppm in the 13C{1H} NMR spectrum of 6 was assigned to the
formate carbon.
or in the presence of AIBN at 60 1C. For comparison, ToMZnH and
ToMZnMe also are inert to O2, whereas ToMZnR compounds react
with O2 following the trend (R = Et o n-C3H7 o i-C3H7 o t-Bu) to
form isolable alkylperoxyzinc species.11 These data suggest that,
despite their steric bulk, the resistance of 1 and 3 to oxidation may
result from electronic effects associated with the ToM ligand. For
comparison, Zn(Si(SiMe3)3)2 and Zn(Si(SiMe3)3)2TMEDA are air sen-
sitive (the TMEDA adduct reacts slowly in air).4
ð1Þ
Magnesium silyls 2 and 4 react with 1 atm of O2 at ambient
temperature over 24 and 36 h forming mixtures of unidentified
species. Under similar reaction conditions, ToMMgMe and O2
form (ToMMg(m-OMe))2 (5) within 5 min. Compound 5 is crystal-
lographically characterized as a dimer containing tridentate tris-
(oxazolinyl)borate and bridging methoxide ligands (Fig. 2), centered
on a two-fold axis.12 The 1H NMR spectrum is consistent with
pseudo-C3v symmetry (Fig. 3).
The identity of compound 6 is further supported by its
independent preparation from ToMZnH and CO2 (1 atm)
in benzene at ambient temperature. Two new bands in the
solid-state infrared spectrum of 6 at 1629 and 1307 cmꢀ1 are
assigned to the nCO(asym) and nCO(sym) of the [Zn]OCHO moiety;
the DnCO of 322 cmꢀ1 suggests monodentate formate coordina-
tion.14 For comparison, the assigned bands for TptBuZnOCHO
are 1655 and 1290 cmꢀ1 (DnCO = 365 cmꢀ1), and the bands
for TptmZnOCHO are 1621 and 1317 cmꢀ1 (DnCO = 304 cmꢀ1
,
Tptm = tris(2-pyridylthio)methane).15
An X-ray structure determination provides additional
characterization of 6 (Fig. 3).16 The zinc–oxygen distances
associated with the formate moiety are inequivalent (Zn1–O4,
1.909(1); Zn1–O5, 2.792 Å). The first distance is equal to the sum
of covalent radii of Zn–O (1.9 Å),17 and the latter distance is
slightly shorter than the sum of van der Waal radii (2.91 Å).
For comparison, the Zn–O distance in monomeric ToMZnOtBu
is 1.835(1) Å,5 whereas the distance in TptmZnOCHO is
2.036(1) Å.15
Fig. 3 ORTEP diagram of ToMZnOCHO (6), with ellipsoids plotted at 35%
Interestingly, reactions of CO2 with isostructural 1 and 2
provide dissimilar products. CO2 (1 atm) reacts with 2 at
probability. Only the formate hydrogen atom is illustrated.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 4334--4336 4335

View Article Online
ChemComm
Communication
pathway for the formation of 6 is shown in Scheme 2, although 9 is
not detected in the reaction mixture and our current evidence does
not rule out a one step pathway. We are currently investigating this
transformation to better understand the route(s) to compound 6.
Still, the dichotomic reactivity of isostructural zinc and magne-
sium silyls and silylcarboxylates toward carbon dioxide is intriguing.
In addition, the reaction of zinc silylcarboxylate 9 with CO2 provides
a new fundamental step that could be applied in catalytic CO2
conversions.
Scheme 2 A possible pathway for the formation of ToMZnOCHO from 1 and CO2.
ambient temperature in benzene to give ToMMgO2CSi(SiHMe2)3
(7) quantitatively over 24 h (eqn (2)).
We are grateful to the NSF (CHE-0955635, CRIF-0946687,
and MRI-1040098) for financial support of this work. We thank
Andreja Bakac and Karen Goldberg for advice with carbon
dioxide reactions. A.D.S. is an Alfred P. Sloan Fellow.
ð2Þ
Notes and references
1 B. Marciniec, Hydrosilylation:
Higher temperature and greater CO2 pressure increase the
reaction rate. For example, complete conversion of 2 to 7 is achieved
in 45 min at 100 1C with 70 psi of CO2. A 13C{1H} NMR resonance
at 202.61 ppm is assigned to the silanecarboxylate MgOC(O)-
Si(SiHMe2)3. This value is similar to those of other metal
silanecarboxylates, such as monomeric Mo(QNAr)(QCHCMe2Ph)-
{O2CSi(SiMe3)3}2 (211.4 ppm, Aryl = 2,6-C6(i-C3H7)2H3)18 and
(Cp2Sc{m-O2CSi(SiMe3)3})2 (200.81 ppm).19 Unfortunately, X-ray
diffraction experiments on 7 were unsuccessful, and the IR bands
associated with silanecarboxylate were not assigned. Compound
7 was characterized as a monomeric species by DOSY experi-
ments that reveal similar diffusion constants of monomeric 2
(6.95 ꢂ 10ꢀ10 m2 sꢀ1) and 7 (6.85 ꢂ 10ꢀ10 m2 sꢀ1) (See ESI‡).
Compound 7 is thermally robust, and the starting material is
unchanged after a benzene-d6 solution is heated at 120 1C for 12 h.
During the conversion of 2 to 7, signals assigned to those two
A
comprehensive Review on
Recent Advances, Springer, Berlin, 2009; H. Mimoun, J. Y. de Saint
Laumer, L. Giannini, R. Scopelliti and C. Floriani, J. Am. Chem. Soc.,
1999, 121, 6158–6166; V. M. Mastranzo, L. Quintero, C. Anaya de
Parrodi, E. Juaristi and P. J. Walsh, Tetrahedron, 2004, 60,
1781–1789; V. Bette, A. Mortreux, D. Savoia and J.-F. Carpentier,
´
Adv. Synth. Catal., 2005, 347, 289–302; S. Gerard, Y. Pressel and
O. Riant, Tetrahedron: Asymmetry, 2005, 16, 1889–1891.
2 H. Mimoun, J. Org. Chem., 1999, 64, 2582–2589; D. Mukherjee,
R. R. Thompson, A. Ellern and A. D. Sadow, ACS Catal., 2011, 698–702;
J. F. Dunne, S. R. Neal, J. Engelkemier, A. Ellern and A. D. Sadow, J. Am.
Chem. Soc., 2011, 133, 16782–16785.
3 C. Marschner, Organometallics, 2006, 25, 2110–2125.
4 J. Arnold, T. D. Tilley, A. L. Rheingold and S. J. Geib, Inorg. Chem.,
1987, 26, 2106–2109.
5 D. Mukherjee, A. Ellern and A. D. Sadow, J. Am. Chem. Soc., 2010,
132, 7582–7583.
6 C. Marschner, Eur. J. Inorg. Chem., 1998, 221–226.
7 Crystal data for C27H47BN3O3Si4Zn (1), M = 650.22, monoclinic, P21/c,
a = 9.4488(12) Å, b = 27.824(4) Å, c = 13.6110(18) Å, b = 93.087(2)1,
V = 3573.2(8) Å3, T = 173(2) K, Z = 4, reflections: 38908 collected, 9895
independent (Rint = 0.0609), R1 = 0.0471, wR2 = 0.1285 (I > 2s(I)).
1
compounds are the only ones observed in the H NMR spectra of
reaction mixtures. In contrast, Cp2Zr(Z2-SiMe2NtBu) and CO2 react
to give the decarbonylated product [Cp2Zr(m-O-k2-O,N-OSiMe2NtBu)]2
and CO.20 The dimeric silanecarboxylate (Cp2Sc{m-O2CSi(SiMe3)3})2
is unchanged after 60 h at 95 1C.19 However, a side-product,
postulated to be Cp2ScOSi(SiMe3)3, is formed during the reaction
of Cp2ScSi(SiMe3)3THF and CO2. This species may form from
monomeric Cp2ScO2CSi(SiMe3)3 prior to dimerization.
%
8 Crystal data for C30H56BN3O3Si4Zn (3), M = 695.32, triclinic, P1, a =
10.242(4) Å, b = 11.576(4) Å, c = 17.786 Å, a = 92.073(6)1, b =
106.240(6)1, g = 103.685(6)1, V = 1955(1) Å3, T = 173(2) K, Z = 2,
reflections: 15 694 collected, 6954 independent (Rint = 0.0690), R1
0.0468, wR2 = 0.0783 (I > 2s(I)).
9 M. Nanjo, T. Oda and K. Mochida, J. Organomet. Chem., 2003, 672,
100–108.
10 W. Gaderbauer, I. Balatoni, H. Wagner, J. Baumgartner and
C. Marschner, Dalton Trans., 2010, 39, 1598–1603.
11 D. Mukherjee, A. Ellern and A. D. Sadow, J. Am. Chem. Soc., 2012,
134, 13018–13026.
=
Additional evidence for Si–C bond formation is provided by
the reaction of 7 and MeOH, which gives compound 5 and
HO2CSi(SiHMe2)3 (8). Compounds 7 and 8 are characterized by 12 Crystal data for C56H76B2Mg2N6O8 (5), M = 1031.47, monoclinic, C2/
c, a = 9.500(2) Å, b = 26.648(6) Å, c = 22.844(5) Å, b = 97.235(3)1, V =
their independent preparation: 8 is synthesized by adapting a
5753(2) Å3, T = 173(2) K, Z = 4, reflections: 29 698 collected, 7356
literature procedure for HO2CSi(SiMe3)3.21 Compound 7 is
independent (Rint = 0.0326), R1 = 0.0403, wR2 = 0.1001 (I > 2s(I)).
independently prepared from ToMMgMe and 8. The identical 13 A. Looney, R. Han, I. B. Gorrell, M. Cornebise, K. Yoon, G. Parkin
and A. L. Rheingold, Organometallics, 1995, 14, 274–288.
14 G. B. Deacon and R. J. Phillips, Coord. Chem. Rev., 1980, 33, 227–250.
spectroscopic properties of the two species support the assign-
ment of 7 as a silanecarboxylate.
15 W. Sattler and G. Parkin, J. Am. Chem. Soc., 2011, 133, 9708–9711.
ToMZnO2CSi(SiHMe2)3 (9) is a possible intermediate in the 16 Crystal data for C22H30BN3O5Zn (6), M = 492.67, monoclinic, P21/c,
formation of 6 from 2. Therefore, the zinc silanecarboxylate ToMZ-
a = 11.1030(5) Å, b = 13.3614(5) Å, c = 16.2115(7) Å, b = 95.795(1)1, V =
2392.7(2) Å3, T = 173(2) K, Z = 4, reflections: 24 027 collected, 5942
nO2CSi(SiHMe2)3 was prepared from ToMZnEt and 8. The 13C{1H}
independent (Rint = 0.0369), R1 = 0.0307, wR2 = 0.0699 (I > 2s(I)).
NMR spectrum of 9 contained a resonance at 193.38 ppm assigned 17 B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves, J. Echeverria,
to the carboxylate. The diffusion constant 6.43 ꢂ 10ꢀ10 m2 sꢀ1
E. Cremades, F. Barragan and S. Alvarez, Dalton Trans., 2008, 2832–2838.
18 R. R. Schrock, Z. J. Tonzetich, A. G. Lichtscheidl, P. Mu¨ller and
obtained from a DOSY experiment in benzene-d6 indicated that 9 is
F. J. Schattenmann, Organometallics, 2008, 27, 3986–3995.
monomeric in solution. Thermolysis of 9 at 120 1C for 12 h under a 19 B. K. Campion, R. H. Heyn and T. D. Tilley, Inorg. Chem., 1990, 29,
4355–4356.
N2 atmosphere returns starting material unchanged. However,
thermal treatment of 9 under 60 psi of CO2 at 120 1C affords the
20 L. J. Procopio, P. J. Carroll and D. H. Berry, Organometallics, 1993,
12, 3087–3093.
formate 6 (t1/2 = 6 h). Thus, a plausible and kinetically competent 21 A. G. Brook and L. Yau, J. Organomet. Chem., 1984, 271, 9–14.
c
4336 Chem. Commun., 2013, 49, 4334--4336
This journal is The Royal Society of Chemistry 2013