Angewandte
Chemie
DOI: 10.1002/anie.200804218
Carbon Dioxide Fixation
Insertion of Carbon Dioxide into Main-Group Complexes: Formation
of the [N(CO2)3]3À Ligand**
Diane A. Dickie, Marie V. Parkes, and Richard A. Kemp*
Under mild conditions CO2 is found to react with [Sr{N-
(PPh2)2}2](THF)3 (1) to oxidatively cleave PPh2 groups from
N, resulting in the previously unknown ligand [N(CO2)3]3À
coordinated to a Sr6 backbone. Overall, the Sr6 framework
chemically fixates twelve molar equivalents of CO2, half by
the formation of two [N(CO2)3]3À ligands and the other half by
CO2 insertion to form the unprecedented phosphanyl-sub-
stituted carbamate O2CN(PPh2)2.
Scheme 1. Production of isocyanates and/or carbodiimides using diva-
The reaction chemistry of carbon dioxide to form valuable
chemicals has garnered significant attention for several
lent main-group metal bis(silylamides) (M=Ge, Sn).[3c]
decades. With interests extending from the economic desire
to convert a “useless” C1 source into higher-valued products
to the desire to mitigate global warming, CO2 fixation studies
span an enormous range.[1] More specifically, we[2] and
others[3] have previously studied the reaction chemistry of
CO2 with main-group metal amides to yield organic isocya-
nates and carbodiimides as products. Previous reactions of
these main-group complexes with CO2 have utilized silyl-
containing amido groups as ligands. It has been postulated
that the major driving force to produce the organic products is
cleavage of the ligand and oxidation of the P atom to yield a
polyphosphazene product. Interestingly, the conditions
required in these reactions to scramble the N(PPh2)2 ligand
are typically quite forcing (e.g., boiling toluene for 5–60 h).[8]
Recently, Roesky reported the preparation and structural
characterization of the heavier Group 2 elements (Sr, Ba)
containing this N(PPh2)2 ligand.[9] Concurrent with Roeskyꢀs
work, in our own laboratory we had prepared and charac-
terized the lighter Group 2 element (Mg, Ca) complexes of
the same ligand,[10] both of which were essentially isostruc-
tural to the Sr salt prepared by Roesky.[9]
Based on this background we examined the reaction of
CO2 with these compounds, including the highly ionic
complex 1. The reaction of 1 with CO2 is shown in
Scheme 2. Bubbling an excess of dry CO2 at room temper-
ature through a solution of 1 in THF led to a rapid reaction, as
evidenced by the complete disappearance of the signal in the
31P NMR spectrum of 1 in less than 15 min. 31P NMR
spectroscopic analysis of the product solution indicated a
mixture of P-containing compounds. Easily identifiable by
their 31P NMR chemical shifts were 2, a compound prepared
previously by Nꢁth using a direct synthesis,[11] and trace
amounts of N(PPh2)3[12] and HN(PPh2)2.[11] It was thus obvious
to us that the expected reaction analogous to Scheme 1 had
not occurred, or at least not cleanly.
À
the migration of the silyl group to form a strong Si O bond
subsequent to CO2 insertion into the metal–nitrogen bond.[3c]
As part of our efforts in this area we have been interested
in expanding the different types of amido ligands that can be
utilized in reactions analogous to that shown in Scheme 1.
More specifically, we focused on replacing the SiMe3 groups
with PR2 substituents to prepare either phosphanyl-substi-
tuted isocyanates or bis(phosphanyl)carbodiimides. These
phosphorus-containing organic products are relatively rare
compounds that are of interest in a variety of reactions,
including novel heterocycle preparations, or as possible cross-
linking agents or biologically active compounds.[4] Despite
significant use in transition-metal[5] and now lanthanide
chemistry,[6] the N(PPh2)2 fragment is relatively unexplored
as a ligand in main-group chemistry.[7] Part of the reason for
this may be the tendency for the N(PPh2)2 ligand to undergo
oxidative scrambling reactions in many cases, resulting in
[*] Dr. D. A. Dickie, M. V. Parkes, Prof. R. A. Kemp
Department of Chemistry and Chemical Biology
University of New Mexico, Albuquerque, NM 87131 (USA)
E-mail: rakemp@unm.edu
[**] We thank the Natural Science and Engineering Research Council of
Canada (Post-Doctoral Fellowship to D.D.), the National Science
Foundation (Grants CHE-0213165 and CHE-0443580) and the
Sandia LDRD Program (Grants 105932 and 113486) for funding this
work. Sandia is a multiprogram laboratory operated by Sandia
Corporation, a Lockheed Martin Company, for the United States
Department of Energy under Contract No. DE-AC04-94AL85000. We
also thank Dr. E. N. Duesler and Prof. A. L. Rheingold for aid in the
X-ray structure analysis.
Scheme 2. Synthesis of compounds 2 and 3.
Angew. Chem. Int. Ed. 2008, 47, 9955 –9957
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9955