Synthesis, structure, and reactivity of a mononuclear organozinc hydride complex: Facile insertion of CO2 into a Zn-H bond and CO 2-promoted displacement of siloxide ligands

Article abstract of DOI:10.1021/ja2035706

Tris(2-pyridylthio)methane, [Tptm]H, has been employed to synthesize the mononuclear alkyl zinc hydride complex, [κ³-Tptm]ZnH, which has been structurally characterized by X-ray diffraction. [κ³- Tptm]ZnH provides access to a variety of other [Tptm]ZnX derivatives. For example, [κ³-Tptm]ZnH reacts with (i) R₃SiOH (R = Me, Ph) to give [κ⁴-Tptm]ZnOSiR₃, (ii) Me ₃SiX (X = Cl, Br, I) to give [κ⁴-Tptm]ZnX, and (iii) CO₂ to give the formate complex, [κ⁴-Tptm]ZnO ₂CH. The bis(trimethylsilyl)amide complex [κ³-Tptm] ZnN(SiMe₃)₂ also reacts with CO₂, but the product obtained is the isocyanate complex, [κ⁴-Tptm]ZnNCO. The formation of [κ⁴-Tptm]ZnNCO is proposed to involve initial insertion of CO₂ into the Zn-N(SiMe₃)₂ bond, followed by migration of a trimethylsilyl group from nitrogen to oxygen to generate [κ⁴-Tptm]ZnOSiMe₃ and Me₃SiNCO, which subsequently undergo CO₂-promoted metathesis to give [κ⁴-Tptm]ZnNCO and (Me₃SiO)₂CO.

Full text of DOI:10.1021/ja2035706

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pubs.acs.org/JACS
Synthesis, Structure, and Reactivity of a Mononuclear Organozinc
Hydride Complex: Facile Insertion of CO2 into a ZnꢀH Bond and
CO2-Promoted Displacement of Siloxide Ligands
Wesley Sattler and Gerard Parkin*
Department of Chemistry, Columbia University, New York, New York 10027, United States
S Supporting Information
b
Scheme 1
ABSTRACT: Tris(2-pyridylthio)methane, [Tptm]H, has
been employed to synthesize the mononuclear alkyl zinc
hydride complex, [k3-Tptm]ZnH, which has been structu-
rally characterized by X-ray diffraction. [k3-Tptm]ZnH
provides access to a variety of other [Tptm]ZnX derivatives.
For example, [k3-Tptm]ZnH reacts with (i) R3SiOH (R =
Me, Ph) to give [k4-Tptm]ZnOSiR3, (ii) Me3SiX (X = Cl,
Br, I) to give [k4-Tptm]ZnX, and (iii) CO2 to give the
formate complex, [k4-Tptm]ZnO2CH. The bis(trimethylsilyl)-
amide complex [k3-Tptm]ZnN(SiMe3)2 also reacts with
CO2, but the product obtained is the isocyanate complex,
[k4-Tptm]ZnNCO. The formation of [k4-Tptm]ZnNCO
Scheme 2
is proposed to involve initial insertion of CO2 into the
ZnꢀN(SiMe3)2 bond, followed by migration of a tri-
methylsilyl group from nitrogen to oxygen to generate
[k4-Tptm]ZnOSiMe3 and Me3SiNCO, which subsequently
undergo CO2-promoted metathesis to give [k4-Tptm]-
ZnNCO and (Me3SiO)2CO.
inc hydride species are of considerable interest in view of their
Z
use in organic transformations1 and their role in the Cu/ZnO-
catalyzed synthesis of methanol from a mixture of CO, CO2, and
H2.2 Well-defined mononuclear zinc complexes that feature term-
inal hydride ligands are, however, rare,3ꢀ5 due to the propensity of
the hydride ligand to bridge two zinc centers.6 Furthermore,
structurally characterized mononuclear compounds that feature
both alkyl and hydride ligands are unknown.7ꢀ9 Here, we describe
the synthesis, structural characterization, and reactivity of a mono-
nuclear alkyl zinc hydride complex, including the facile insertion of
CO2 into the ZnꢀH bond. In addition, we also describe the ability
of CO2 to promote the displacement of siloxide ligands.
We previously utilized the tris(3-tert-butylpyrazolyl)hydro-
t
borato ligand to synthesize [TpBu ]ZnH, the first structurally
authenticated monomeric zinc hydride complex,3 in which the
t
multidentate nature of the [TpBu ] ligand allows isolation of a
compound that features a terminal hydride moiety. Using a similar
approach, we have now employed a multidentate alkyl ligand derived
from tris(2-pyridylthio)methane, [Tptm]H,10 to permit isolation of
a monomeric alkyl hydride complex, namely [k3-Tptm]ZnH.
The key starting material for the synthesis of [k3-Tptm]ZnH
is the bis(trimethylsilyl)amide derivative, [k3-Tptm]ZnN(SiMe3)2,11
that is obtained via the reaction of [Tptm]H with Zn[N-
(SiMe3)2]2, as illustrated in Scheme 1. Subsequent treatment
of [k3-Tptm]ZnN(SiMe3)2 with Me3SiOH yields the siloxide
Figure 1. Molecular structure of [k3-Tptm]ZnH.
12,13
[k4-Tptm]ZnOSiMe3
that reacts with PhSiH3 to give the
hydride complex, [k3-Tptm]ZnH (Scheme 2). The molecular
structure of [k3-Tptm]ZnH has been determined by X-ray
Received: April 18, 2011
Published: June 06, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja2035706 J. Am. Chem. Soc. 2011, 133, 9708–9711
|

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Scheme 3
Figure 3. Molecular structure of [k4-Tptm]ZnNCO.
Scheme 4
Figure 2. Molecular structure of [k4-Tptm]ZnO2CH.
[k4-Tptm]ZnX derivatives upon treatment with Me3SiX (X = Cl,
Br, I, N3, NCO, OAc), as illustrated in Scheme 3.
diffraction (Figure 1), which demonstrates that the compound
exists as a discrete mononuclear complex in which the [Tptm]
ligand coordinates in a k3-manner. The ZnꢀH moiety of [k3-
Tptm]ZnH is characterized by a ZnꢀH bond length of 1.51(3)
Å,14 a singlet at δ 5.60 in the 1H NMR spectrum, and an absorption
at 1729 cmꢀ1 in the IR spectrum, which shifts to 1242 cmꢀ1 for the
zinc deuteride isotopologue [k3-Tptm]ZnD [νH/νD = 1.39]. The
zinc methyl counterpart, [k3-Tptm]ZnMe, has also been synthe-
sized via the reaction of [Tptm]H with Me2Zn (Scheme1) and has
a distorted tetrahedral geometry similar to that of [k3-Tptm]ZnH.
For example, the CꢀZnꢀH [132(1)°] and CꢀZnꢀCH3
[135.8(1)° and 134.1(1)° for two different crystalline forms] bond
angles are distinctly greater than the tetrahedral value.
In addition to its role in methanol synthesis,2 the abundance of
CO2 has stimulated efforts to discover other synthetic methods
that employ CO2 as a C1 building block,19 as illustrated by the
metal-catalyzed hydrogenation to formic acid and formates,20 and
the formation of polycarbonates by copolymerization with
epoxides.21 The reactivity of CO2 towards metal compounds
has, therefore, been the focus of much attention.19,22 For
example, the insertion of CO2 into MꢀNR2 bonds to give
carbamato derivatives has been widely studied.23 In contrast,
however, there are few reports concerned with the reactivity of
CO2 towards bis(trimethylsilyl)amido complexes.24 It is, there-
fore, notable that [k3-Tptm]ZnN(SiMe3)2 reacts with CO2 to
give the isocyanate complex, [k4-Tptm]ZnNCO, which has been
structurally characterized by X-ray diffraction (Figure 3). Thus,
rather than undergoing only a simple insertion reaction of the
type that is observed for LnMNR2,23 the reaction is accompanied
by deoxygenation of CO2, a transformation that is made possible
by the formation of strong SiꢀO bonds.25,26 The formation of
the isocyanate complex [k4-Tptm]ZnNCO also provides a con-
trast to the reaction of Zn[N(SiMe3)(Ad)]2 (Ad = adamantyl)
with CO2, which gives AdNdCdNAd and unidentified trimethyl-
siloxide species.27
[k3-Tptm]ZnH serves as a precursor to a variety of other
[Tptm]ZnX derivatives, as illustrated in Scheme 3. For example,
the zinc hydride bond of [k3-Tptm]ZnH is (i) protolytically
cleaved by R3SiOH (R = Me, Ph) to give [k4-Tptm]ZnOSiR3
and (ii) undergoes metathesis with Me3SiX (X = Cl, Br, I) to
give [k4-Tptm]ZnX.15,16 Of most note, however, is the fact that
[k3-Tptm]ZnH reacts rapidly with CO2 to give the formate com-
plex, [k4-Tptm]ZnO2CH (Scheme 3 and Figure 2), which may
also be obtained from the reaction of [k3-Tptm]ZnH with
HCO2H.17 Such reactivity is of particular interest in view of
the fact that formate species are proposed intermediates in the
ZnO- and Cu/ZnO-catalyzed synthesis of methanol.2 The
insertion of CO2 into zinc hydride bonds is not, however, well
precedented, and the only other examples involving monomeric
zinc hydride complexes pertain to [TpR]ZnH derivatives.3,4d,18
Both the formate and siloxide derivatives, [k4-Tptm]ZnO2CH
and [k4-Tptm]ZnOSiR3, can be converted to a variety of other
The generation of [k4-Tptm]ZnNCO is necessarily a multi-
step process, of which the initial sequence is proposed to involve
insertion of CO2 into the ZnꢀN(SiMe3)2 bond to give [Tptm]-
Zn[O2CN(SiMe3)2], which subsequently converts to the tri-
methylsiloxide derivative [k4-Tptm]ZnOSiMe3 and Me3SiNCO
(Scheme 4). In support of this proposal, both [k4-Tptm]ZnO-
SiMe3 and Me3SiNCO are observed by 1H NMR spectroscopy
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during the course of the reaction. Interestingly, however,
although the simplest rationalization for the formation of
[k4-Tptm]ZnNCO from [k4-Tptm]ZnOSiMe3 and Me3SiNCO
involves direct metathesis, additional experiments suggest that
this is not the operative mechanism. Specifically, while an inde-
pendent experiment indicates that [k4-Tptm]ZnNCO is formed
upon treatment of [k4-Tptm]ZnOSiMe3 with Me3SiNCO, the
reaction is extremely slow by comparison to the formation of
[Tptm]ZnNCO upon treatment of [k3-Tptm]ZnN(SiMe3)2 with
CO2. The formation of the isocyanate complex [Tptm]ZnNCO
is, however, accelerated if CO2 is added to a mixture of
[k4-Tptm]ZnOSiMe3 and Me3SiNCO. On this basis, it is pro-
posed that the formation of [k4-Tptm]ZnNCO is promoted
by insertion of CO2 into the ZnꢀOSiMe3 bond to give
the carbonate derivative, [k4-Tptm]ZnO2COSiMe3, that is
more susceptible towards metathesis with Me3SiNCO than is
[k4-Tptm]ZnOSiMe3.
Evidence for facile reversible insertion of CO2 into the Znꢀ
OSiMe3 bond is provided by variable-temperature 1H and
13C{1H} NMR spectroscopic studies that allow for observation of
[k4-Tptm]ZnO2COSiMe3 at temperatures e5 °C, as illustrated by
a signal at 157.9 ppm in the 13C{1H} NMR spectrum attributable to
the carbonate moiety. In addition, the reaction between [k3-Tptm]-
ZnN(SiMe3)2 and CO2 is accompanied by formation of the
carbonate (Me3SiO)2CO, an observation that is in accord with
Me3SiNCO undergoing metathesis with [k4-Tptm]ZnO2CO-
SiMe3 rather than with [k4-Tptm]ZnOSiMe3. Two factors that
may contribute to the more facile cleavage of the ZnꢀOC-
(O)OSiMe3 bond than the ZnꢀOSiMe3 bond are (i) insertion
of the CO2 group displaces the OSiMe3 group from the metal
center, thereby reducing steric interactions, and (ii) the presence of
the C(O) group allows for a six-membered transition state.
the fact that the hydride derivative adopts k3-coordination while the
bulky triphenylsiloxide derivative adopts k4-coordination, it is evident
that steric factors do not dictate the differences in coordination
mode. A consideration of [Tptm]ZnX derivatives in which X is
monoatomic (X = F, Cl, Br, I, and H), however, indicates that the
preference for k4-coordination correlates well with the electro-
negativity of X, a trend which suggests that k4-coordination
becomes more favored with increasing charge on the zinc center.34
In summary, the mononuclear alkyl zinc hydride complex,
[k3-Tptm]ZnH, may be synthesized via treatment of [k4-Tptm]-
ZnOSiMe3 with PhSiH3. [k3-Tptm]ZnH exhibits a variety of
different reaction pathways, including insertion of CO2 to give
the formate complex, [k4-Tptm]ZnO2CH. The bis(trimethylsilyl)-
amide complex [k3-Tptm]ZnN(SiMe3)2 also reacts with
CO2, but the product obtained is the isocyanate complex,
[k4-Tptm]ZnNCO, that results from a multistep sequence of
which the initial steps are insertion of CO2 into the ZnꢀN-
(SiMe3)2 bond followed by rearrangement to [k4-Tptm]ZnO-
SiMe3 and Me3SiNCO. An important discovery, however, is that
the final metathesis step to give [k4-Tptm]ZnNCO is promoted
by CO2, an observation which indicates that the carbonate
complex [k4-Tptm]ZnO2COSiMe3 is more susceptible towards
metathesis than is the siloxide derivative, [k4-Tptm]ZnOSiMe3.
As such, this finding has ramifications with respect to inducing
reactivity of other siloxide compounds.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, compu-
b
tational data, crystallographic data (CIF), and complete ref 19c.
This material is available free of charge via the Internet at http://
pubs.acs.org.
The ability of CO2 to promote the overall displacement of a
siloxide ligand has implications with respect to the fact that siloxides
find frequent use as ancillary ligands.28 Thus, while the insertion of
CO2 into MꢀOSiR3 bonds has been little investigated,29,30 it is
evident from the above studies that the presence of CO2 could
provide a general means to enhance reactivity of compounds with
MꢀOSiR3 bonds;31 correspondingly, it suggests that the presence
of CO2 could be detrimental for situations in which the siloxide
ligand is intended to play the role of a spectator.
Finally, it is pertinent to comment on the fact that the [Tptm]ZnX
complexes described herein belong to two structural classes that
differ according to whether the [Tptm] ligand binds in a k4 or k3
manner. Specifically, k4-coordination is observed in the solid state for
[k4-Tptm]ZnI, [k4-Tptm]ZnNCO, [k4-Tptm]ZnN3, [k4-Tptm]-
ZnOSiR3 (R = Me, Ph), [k4-Tptm]ZnO2CH, and [k4-Tptm]-
ZnO2CMe,32 while k3-coordination is observed for [k3-Tptm]-
ZnH and [k3-Tptm]ZnMe. Low-temperature 1H NMR spectro-
scopic studies suggest that these coordination modes are also
preserved in solution. Specifically, although all [Tptm]ZnX com-
plexes exhibit three chemically equivalent pyridyl groups at
room temperature, a 2:1 pattern emerges for [k3-Tptm]ZnH,
[k3-Tptm]ZnMe, and [k3-Tptm]ZnN(SiMe3)2 at ca. ꢀ10 °C,
consistent with k3-coordination. In contrast, the low-temperature
1H NMR spectra of [k4 -Tptm]ZnX [X = Cl, Br, I, NCO, N3, OSiR3
(R = Me, Ph), O2CMe, O2CH] exhibit chemically equivalent
pyridyl groups indicative of k4-coordination.33 This observation is
in accord with density functional theory calculations which predict
that the preference for k4- versus k3-coordination in [Tptm]ZnX
for monodentate X ligands increases in the sequence Me < N-
(SiMe3)2 < H < I < OSiMe3 < Br < Cl < N3 < NCO < F. In view of
’ AUTHOR INFORMATION
Corresponding Author
parkin@columbia.edu
’ ACKNOWLEDGMENT
We thank the National Science Foundation (CHE-0749674)
for support of this research and for acquisition of an NMR
spectrometer (CHE-0840451).
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S. K.; Ho, D. M.; Orchin, M. Organometallics 1993, 12, 1714–1719. (f)
Ballivet-Tkatchenko, D.; Chermette, H.; Plasseraud, L.; Walter, O. Dalton
Trans. 2006, 5167–5175.
(10) de Castro, V. D.; de Lima, G. M.; Filgueiras, C. A. L.; Gambardella,
M. T. P. J. Mol. Struct. 2002, 609, 199–203.
(11) k3-Coordination of the [Tptm] ligand is indicated by the
1
observation of a 2:1 ratio of pyridyl groups in the H NMR spectrum
at e ꢀ10 °C.
(12) [k4-Tptm]ZnOSiPh3 may be likewise obtained by a similar
procedure employing Ph3SiOH.
(13) Trimethylsiloxide complexes of zinc are not common, and the
only structurally characterized one listed in the Cambridge Structural
0
Databaseis[TpR,R ]ZnOSiMe3: Chisholm, M. H.; Eilerts, N. W.; Huffman,
J. C.; Iyer, S. S.; Pacold, M.; Phomphrai, K. J. Am. Chem. Soc. 2000,
122, 11845–11854.
(14) For comparison, the mean ZnꢀH bond length for terminal
hydride compounds listed in the Cambridge Structural Database13 is 1.55 Å.
(15) [k4-Tptm]ZnX (X = Cl, Br) have been recently reported:
Kitano, K.; Kuwamura, N.; Tanaka, R.; Santo, R.; Nishioka, T.; Ichimura,
A.; Kinoshita, I. Chem. Commun. 2008, 1314–1316.
(16) [k4-Tptm]ZnX (X = Cl, I) can also be obtained via treatment of
[k4-Tptm]Li with ZnX2 (Supporting Information).
(17) Furthermore, [k4-Tptm]ZnO2CMe can be obtained via reac-
tion of [k3-Tptm]ZnH with MeCO2H.
(31) In this regard, we have also observed that CO2 promotes
formation of [k4-Tptm]ZnX (X = Cl, Br) upon treatment of [k4-Tptm]-
ZnOSiR3 (R = Me, Ph) with Me3SiX.
(18) For CO2 insertion reactions of multinuclear zinc hydride
complexes, see: (a) Merz, K.; Moreno, M.; L€offler, E.; Khodeir, L.
Rittermeier, A.; Fink, K.; Kotsis, K.; Muhler, M.; Driess, M.Chem. Commun.
2008, 73–75. (b) Schulz, S.; Eisenmann, T.; Schmidt, S.; Bl€aser, D.;
Westphal, U.; Boese, R. Chem. Commun. 2010, 46, 7226–7228.
(19) (a) Aresta, M.; Dibenedetto, A. Dalton Trans. 2007,
2975–2992. (b) Mikkelsen, M.; Jørgensen, M.; Krebs, F. C. Energy
Environ. Sci. 2010, 3, 43–81. (c) Arakawa, H.; et al. Chem. Rev. 2001,
101, 953–996. (d) Sakakura, T.; Choi, J.-C.; Yasuda, H. Chem. Rev. 2007,
107, 2365–2387. (e) Aresta, M.; Dibenedetto, A. Catal. Today 2004,
(32) [k4-Tptm]ZnCl also exhibits k4-coordination; see ref 15.
(33) The observation of equivalent pyridyl groups for [k4-Tptm]-
ZnO2CR (R = H, Me) implies that access to the freely rotating k1-O2CR
isomer is facile.
(34) Accordingly, DFT studies indicate that the difference in energies
between [k3-Tptm]ZnX and [k4-Tptm]ZnX (X = F, Cl, Br, I, H)
correlates well with the NBO charges on zinc (Supporting Information).
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dx.doi.org/10.1021/ja2035706 |J. Am. Chem. Soc. 2011, 133, 9708–9711