Aryl isocyanate, carbodiimide, and isocyanide prepared from carbon dioxide. A metathetical group-transfer tale involving a titanium-imide zwitterion

Article abstract of DOI:10.1021/ic052065e

Carbon dioxide can be readily converted quantitatively and under mild conditions into the aryl isocyanate and symmetrical carbodiimide via a metathetical reaction involving a zwitterionic titanium imide (nacnac)Ti=NAr(CH₃B(C₆F₅)₃) (nacnac^- = [ArNC(^tBu)]₂CH, Ar = 2,6- ⁱPr₂C₆H₃). The metathetical process to generate isocyanates allows also for facile formation of sterically demanding aryl isocyanide, by a deoxygenation route. Labeling studies using enriched ¹³CO₂ are also described.

Full text of DOI:10.1021/ic052065e

Inorg. Chem. 2006, 45, 487−489
Aryl Isocyanate, Carbodiimide, and Isocyanide Prepared from Carbon
Dioxide. A Metathetical Group-Transfer Tale Involving a Titanium
Zwitterion
−Imide
Uriah J. Kilgore, Falguni Basuli, John C. Huffman, and Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana UniVersity,
Bloomington, Indiana 47405
Received December 1, 2005
Scheme 1. Synthesis of Isocyanate and Carbodiimide from CO2
Utilizing a Zwitterionic Titanium Imido Complex 1a
Carbon dioxide can be readily converted quantitatively and under
mild conditions into the aryl isocyanate and symmetrical carbodi-
imide via a metathetical reaction involving a zwitterionic titanium
imide (nacnac)Ti
Ar
2,6-iPr2C6H3). The metathetical process to generate isocy-
d )
NAr(CH3B(C6F5)3) (nacnac- [ArNC(tBu)]2CH,
)
anates allows also for facile formation of sterically demanding aryl
isocyanide, by a deoxygenation route. Labeling studies using
enriched 13CO2 are also described.
Carbon dioxide is often regarded as a nonflammable and
thermodynamically inert greenhouse gas.1 As a result, there
has been considerable interest in utilizing carbon dioxide
feedstock as a reagent for a wide range of commodity
products.2 Therefore, complexes capable of both activating
and functionalizing readily available carbon dioxide into
useful chemical building blocks are attractive targets espe-
cially if such transformations can be carried out under mild
conditions.3 Employing carbon dioxide as a reagent also is
appealing from a spectroscopic standpoint because isotopi-
cally enriched forms are both affordable and commercially
available. One possible strategy is via atom or group-transfer
reactions involving electron-deficient early-transition-metal
systems with low-coordination numbers because metal-oxo
formation is predicted to be thermodynamically favorable.
a The Ti atom represents the cationic (nacnac)Ti scaffold where nacnac-
) [ArNC(tBu)]2CH and Ar ) 2,6-iPr2C6H3.
We feature in the present paper the quantitative conversion
of carbon dioxide into an isocyanate and carbodiimide via
ligand metathesis reactions mediated by the imido-
zwitterion (nacnac)TidNAr(CH3B(C6F5)3) (1; nacnac-
)
[ArNC(tBu)]2CH, Ar ) 2,6-iPr2C6H3).4 In addition, the aryl
isocyanate generated from our metathetical process can be
readily converted to an isocyanide, hence unveiling an
indirect entry of the isocyano functionality from CO2.
Labeling studies using 13CO2 have also been conducted in
order to follow the fate of both the CO and C atoms in all
of the reactions formerly mentioned.
Treatment of complex 1, prepared quantitatively
from methide abstraction using B(C6F5)3 and (nacnac)-
TidNAr(CH3) in XC6H5 (X ) F or Br; Scheme 1),4 with 1
atm of CO2 at room temperature elicited a rapid color change
from red to pale orange. Examination of the reaction mixture
* To whom correspondence should be addressed. E-mail: mindiola@
indiana.edu.
(1) (a) Leitner, W. Angew. Chem., Int. Ed. Engl. 1995, 34, 2207-2221.
(b) Braunstein, P.; Matt, D.; Nobel, D. Chem. ReV. 1988, 88, 747-
764.
1
by H and 13C NMR spectroscopy revealed quantitative
(2) (a) Arakawa, H.; et al. Chem. ReV. 2001, 101, 953-996. (b) Gibson,
D. H. Chem. ReV. 1996, 96, 2063-2095. (c) Dell’Amico, D. B.;
Calderazzo, F.; Labella, L.; Marchetti, F.; Pampaloni, G. Chem. ReV.
2003, 103, 3857-3898.
(3) (a) Byrne, C. M.; Allen, S. D.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 2004, 126, 11404-11405. (b) Sita, L. R.; Babcock,
J. R.; Xi, R. J. Am. Chem. Soc. 1996, 118, 10912-10913. (c)
Wannagat, U.; Kuckertz, H.; Kruger, C.; Pump, J. Z. Anorg. Allg.
Chem. 1964, 333, 54-61. (d) Takimoto, M.; Mori, M. J. Am. Chem.
Soc. 2001, 123, 2895-2896. (e) Takimoto, M.; Shimizu, K.; Mori,
M. Org. Lett. 2001, 3, 3345-3347. (f) Saito, S.; Nakagawa, S.;
Koizumi, T.; Hirayama, K.; Yamamoto, Y. J. Org. Chem. 1999, 64,
3975-3978. (g) Louie, J.; Gibby, J. E.; Farnworth, M. V.; Tekavec,
T. N. J. Am. Chem. Soc. 2004, 126, 8590-8590.
formation of the isocyanate OCNAr [also determined by
electrospray ionization (EIMS) and IR] along with the
titanium-oxo complex (nacnac)Ti(OB(C6F5)3)(CH3) (2;
Scheme 1).5 Single-crystal X-ray analysis of complex 2
disclosed this species to be a monomer with coordination of
(4) Basuli, F.; Clark, R. L.; Bailey, B. C.; Brown, D.; Huffman, J. C.;
Mindiola, D. J. Chem. Commun. 2005, 2250-2252.
(5) See the Supporting Information for complete experimental and relevant
crystallographic details.
10.1021/ic052065e CCC: $33.50
Published on Web 12/23/2005
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 2, 2006 487

COMMUNICATION
Figure 2. Molecular structure of complex 3 depicting thermal ellipsoids
i
at the 50% probability level. The hydrogen atoms, Pr groups on the aryl
rings, and solvent molecules have been excluded for clarity. Selected
metrical parameters: Ti1-N39, 1.702(2) Å; Ti1-N2, 2.076(2) Å; Ti1-
N6, 2.090(2) Å; Ti1-O52, 2.094(7) Å; Ti1-O54, 2.128(7) Å; C53-C55,
1.488(4) Å; Ti1-N39-C40, 175.3(9)°; N2-Ti1-N6, 92.07(8)°; O52-
Ti1-O55, 61.88(6)°; O52-C53-O54, 117.0(2)°.
Figure 1. Molecular structure of complex 2 depicting thermal ellipsoids
at the 50% probability level. The hydrogen atoms and solvent have been
excluded for clarity. Selected metrical parameters: Ti1-N2, 1.969(2) Å;
Ti1-N6, 2.004(8) Å; Ti1-O39, 1.732(5) Å; Ti1-C73, 2.091(3) Å; Ti1-
O39-B40, 172.3(4)°; N2-Ti1-N6, 98.82(8)°.
imidodicarboxylates.9 The role of the borane is critical
because it provides a latent low-coordinate titanium-imide
species capable of activating CO2. Such a charge-separated
species also prevents the TidO species from dimerizing via
Ti-O-Ti linkages.8-11 More importantly, the ionic nature
of complex 2 renders this complex insoluble in C6H5X (X
) F, Br, Cl), which facilitates separation from the neutral
organic product (OCNAr).
the borane to be on the TidO motif (Figure 1).5 Structure
parameters consistent with this formulation for 2 include a
short TidO bond [1.732(5) Å] confined in a highly distorted
tetrahedral geometry. The perfluorinated aryl groups on the
boron are twisted in a propellerlike fashion, and the B atom
deviates from the plane defined by the three ipso carbons
(∼0.545 Å), lending further support for Lewis acid-base
adduct formation in 2.6,7
Methide abstraction in complex 1 is critical inasmuch as
the neutral complex (nacnac)TidNAr(CH3)4 reacts cleanly
with CO2 to afford the acetate (nacnac)TidNAr(η2-O2CCH3)
(3), a product originating from CO2 insertion into the
Ti-CH3 bond. Connectivity of 3 was established by a
Generation of 2 suggests that ion-paired reorganization
processes are occurring in the reaction and that formation
of a titanium-oxo dimer is forbidden via blockage by the
Lewis acid.6 We propose that production of OCNAr and 2
proceeds by means of displacement of the labile borate ligand
[CH3B(C6F5)3]- by CO2 and subsequent [2 + 2] cycloaddi-
tion to afford a hypothetical carbamate complex [(nacnac)-
Ti(OCONAr)]+,8 which then undergoes cycloreversion to
form the strong TidO bond (Scheme 1).8-11 Unlike other
imido systems, excess CO2 does not appear to insert into
the putative carbamate to afford six-membered-ring aryl
1
combination in H and 13C NMR spectra in addition to the
single-crystal solid-state molecular structure. Figure 2 depicts
the molecular structure of 3, clearly revealing insertion of
CO2 into the Ti-CH3 bond to afford an η2-acetate ligand.12
When the reaction was carried out with a stoichiometric
amount of CO2 (1/2 equiv at 25 °C, >48 h), we observed the
formation of 2 along with carbodiimide ArNCNAr.5 The
occurrence of the carbodiimide is proposed to proceed via a
[2 + 2] cycloaddition of OCNAr with 1 and cycloreversion
to extrude the organic product and 2 (Scheme 1).13-15 Such
a result indicates that the formation of 2 is slow vis-a`-vis
(6) (a) Sa´nchez-Nieves, J.; Frutos, L. M.; Royo, P.; Castan˜o, O.;
Herdtweck, E. Organometallics 2005, 24, 2004-2007. (b) Vidovic,
D.; Moore, J A.; Jones, J. N.; Cowley, A. H. J. Am. Chem. Soc. 2005,
127, 4566-4567.
(7) Crystal data for 2‚1/2Et2O: C56H61BF15N2O2Ti, triclinic, space group
P1h, a ) 12.7063(18) Å, b ) 13.5595(18) Å, c ) 16.805(2) Å, R )
105.689(4)°, â ) 100.502(4)°, γ ) 99.263(4)°, Z ) 2, µ(Mo KR) )
0.256 mm-1, V ) 2672.4(6) Å3, Dc ) 1.414 mg/mm3, GOF on F 2
)
(11) (a) Basuli, F.; Bailey, B. C.; Tomaszewski, J.; Huffman, J. C.;
Mindiola, D. J. J. Am. Chem. Soc. 2003, 125, 6052-6053. (b) Basuli,
F.; Bailey, B. C.; Watson, L. A.; Tomaszewski, J.; Huffman, J. C.;
Mindiola, D. J. Organometallics 2005, 24, 1886-1906.
(12) Crystal data 3‚C6H14: C55H87N2O3Ti, monoclinic, space group P2(1)/
c, a ) 13.1392(16) Å, b ) 18.319(2) Å, c ) 21.385(3) Å, â )
94.109(3)°, Z ) 4, µ(Mo Kσ) ) 0.209 mm-1, V ) 5133.9(11) Å3, Dc
) 1.128 mg/mm3, GOF on F 2 ) 0.723, R1 ) 5.05% and wR2 )
10.05% (F 2, all data). Out of a total of 55 343 reflections collected,
11 890 were unique and 4688 were observed (Rint ) 15.47%) with I
> 2σ(I) (yellow prism, 0.25 × 0.05 × 0.05 mm, 27.57° g Θ g 2.04°).
In addition to the molecule of interest, there is a region of disordered
solvent that could not be resolved. The latter was modeled as a series
of partial occupancy carbon atoms. All nonsolvent hydrogen atoms
were located in subsequent Fourier maps and included as isotropic
contributors in the final cycles of refinement. Solvent hydrogen atoms
were ignored.
0.911, R1 ) 5.12% and wR2 ) 12.15% (F 2, all data). Out of a total
of 23 577 reflections collected, 12 264 were unique and 8056 were
observed (Rint ) 7.48%) with I > 2σ(I) (yellow needle, 0.30 × 0.12
× 0.06 mm, 27.43° g Θ g 3.80°). A disordered diethyl ether is present
in the cell.
(8) (a) Blake, R. E., Jr.; Antonelli, D. M.; Henling, L. M.; Schaefer, W.
P.; Hardcastle, K. I.; Bercaw, J. E. Organometallics 1998, 17, 718-
725. (b) Boyd, C. L.; Clot, E.; Guiducci, A. E.; Mountford, P.
Organometallics 2005, 24, 2368-2385. (c) Swallow, D.; McInnes, J.
M.; Mountford, P. Dalton Trans. 1998, 2253-2260. (d) Dubberley,
S. R.; Friedrich, A.; Willman, D. A.; Mountford, P.; Radius, U.
Chem.sEur. J. 2003, 9, 3634-3654. (e) Boyd, C. L.; Toupance, T.;
Tyrrell, B. R.; Ward, B. D.; Wilson, C. R.; Cowley, A. R.; Mountford,
P. Organometallics 2005, 24, 309-330. (f) Boyd, C. L.; Clot, E.;
Guiducci, A. E.; Mountford, P. Organometallics 2005, 24, 2347-
2367. (g) Molina, P.; Alajarin, M.; Arques, A. Synthesis 1982, 596-
597. (h) Hsu, S.-H.; Chang, J.-C.; Lai, C.-L.; Hu, C.-H.; Lee, H. M.;
Lee, G.-H.; Peng, S.-M.; Huang, J.-H. Inorg. Chem. 2004, 43, 6786-
6792. (i) Royo, R.; Sa´nchez-Nieves, J. J. Organomet. Chem. 2000,
597, 61-68.
(13) Complex (nacnac)TidNAr(Cl) fails to react with CO2 under the same
conditions.
(14) (a) DeLaet, D. L.; del Rosario, R.; Fanwick, P. E.; Kubiak, C. P. J.
Am. Chem. Soc. 1987, 109, 754-758. (b) Babcock, J. R.; Sita, L. R.
J. Am. Chem. Soc. 1998, 120, 5585-5586.
(15) (a) Wang, H.; Chan, H.-S.; Xie, Z. Organometallics 2005, 24, 3772-
3779. (b) Lee, S. Y.; Bergman, R. G. J. Am. Chem. Soc. 1996, 118,
6396-6406.
(9) Guiducci, A. E.; Cowley, A. R.; Skinner, M. E. G.; Mountford, P.
Dalton Trans. 2001, 1392-1394.
(10) Blake, A. J.; McInnes, J. M.; Mountford, P.; Nikonov, G. I.; Swallow,
D.; Watkin, D. J. Dalton Trans. 1999, 379-392.
488 Inorganic Chemistry, Vol. 45, No. 2, 2006

COMMUNICATION
Scheme 2. Formation of the Isotopically Labeled 13CNAr from
O13CNAr, Which Is Prepared Easily from 1 and 13CO2
carbodiimide metathesis might be a competing reaction in
this type of process.
Isotopically labeled isocyanate O13CNAr is not only a
precursor to the 13C-labeled carbodiimide ArN13CNAr but
also a clean source to an isocyanide following the deoxy-
genation procedure described by Baldwin and co-workers.17
Based on this precedent, treatment of O13CNAr with HSiCl3
and excess NEt3 in CH2Cl2 under mild conditions affords
the isotopically enriched aryl isonitrile 13CNAr in >78%
(Scheme 2). The identities of both labeled and unlabeled
CNAr were unambiguously confirmed with a combination
Figure 3. Molecular structure of complex 4 depicting thermal ellipsoids
at the 50% probability level. The hydrogen atoms, iPr groups on the nacnac
aryl rings, and solvent molecules have been excluded for clarity. Selected
metrical parameters: Ti1-N39, 1.701(1) Å; Ti1-N2, 1.969(1) Å; Ti1-
N6, 2.013(1) Å; Ti1-C47, 2.417(3) Å; C47-B48, 1.665(9) Å; Ti1-N39-
C40, 175.33(9)°; N2-Ti1-N6, 97.67(4)°; Ti1-C47-B48, 175.4(1)°.
1
of H and 13C NMR, IR, and CIMS spectroscopies.5,18
In conclusion, we have shown that a zwitterion-imido
complex of titanium can readily convert CO2, under mild
conditions, into the corresponding aryl isocyanate and
symmetrical carbodiimide. The role of the borane is critical
in these reactions, and separation of the organic products is
facile given the low solubility of the TidO material. It has
been shown that 13C isotopic labeling of these two heterocu-
mulenes is readily accessible given the availability of 13CO2.19
We have also demonstrated that the isotopically enriched
isocyanate can be a precursor to the labeled isocyanide
CNAr, a building block in both organic and organometallic
chemistry. We are currently exploring ways to convert oxo
species such as 2 into reactive systems such as 1 or 3 in
order to make the metathetical C and CO transfer process
from CO2 cyclic or catalytic.
the consumption of CO2 by 1. To test our hypothesis, we
treated 1 with OCNAr in C6H5Br, which consequently
yielded 2 along with the carbodiimide in quantitative yield
based on 1H NMR (25 °C, 48 h; Scheme 1). As was observed
with the isocyanate formation from 1 (vide supra), separation
of the organic product (ArNCNAr) was facilitated by the
low solubility of 2 in BrC6H5. Interestingly, this result is in
a stark contrast to isocyanate cycloaddition reactions with
titanium and zirconium imides supported by tetraaza mac-
rocyclic ligands, which takes place at the (N, C) site of the
isocyanate to form N,N-disubstituted ureates (RNCONR′2-).10
It is likely that in our system the steric hindrance at the
titanium center disfavors the N,N bound isomer from
forming. Isotopic labeling studies using 13CO2 confirmed
exclusive CO and C atom transfer to generate the corre-
sponding isotopomers O13CNAr and ArN13CNAr, as sup-
ported by 13C NMR, IR, chemical ionization MS (CIMS),
and EIMS spectroscopies.5
Attempts to prepare asymmetrical carbodiimides of the
type ArNCNAr′ via this route were not clean.14 For instance,
treatment of 1 with CO2, followed by addition of the tolyl
imide zwitterion analogue (nacnac)TidNtol(CH3B(C6F5)3)
(4; Figure 3)5,16 (tol ) 4-CH3C6H4), resulted in the formation
of the asymmetrical carbodiimide ArNCNtol, concomitant
with the generation of the oxo 2 and other side products.5,
Arguably, sterics in 1 appear to play a key role in the clean
formation of both OCNAr and ArNCNAr, since CdN
Acknowledgment. For financial support of this research,
we thank Indiana UniversitysBloomington, the Camille and
Henry Dreyfus Foundation, the Alfred P. Sloan Foundation
(Fellowship to D.J.M.), and the National Science Foundation
(Grant CHE-0348941 and PECASE award to D.J.M.). The
authors thank Prof. John D. Protasiewicz for insightful
discussions.
Supporting Information Available: Complete X-ray data for
structures (CIF), synthesis and characterization of 2-4, and
preparations for all 13C-labeled and unlabeled isocyanates, carbo-
diimides, and isocyanides. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC052065E
(16) Crystal data for 4: C61H63BF15N3Ti, triclinic, space group P1h, a )
12.0226(7) Å, b ) 13.9555(8) Å, c ) 18.399(1) Å, R ) 89.910(2)°,
â ) 83.022(2)°, γ ) 66.659(1)°, Z ) 2, µ(Mo KR) ) 0.245 mm-1
,
(17) Baldwin, J. E.; Bottaro, J. C.; Roirdan, P. D.; Derome, A. E. J. Chem.
Soc., Chem. Commun. 1982, 942-943.
V ) 2809.7(3) Å3, Dc ) 1.397 mg/mm3, GOF on F 2 ) 0.941, R1 )
3.76% and wR2 ) 8.70% (F 2, all data). Out of a total of 77 094
reflections collected, 16 420 were unique and 11 475 were observed
(Rint ) 6.06%) with I > 2σ(I) (orange cleaved fragment, 0.30 × 0.30
× 0.30 mm, 30.04° g Θ g 2.12°).
(18) Kamer, P. C. J.; Nolte, R. J. M.; Drenth, W. J. Am. Chem. Soc. 1988,
110, 6818-6825.
(19) Dean, D. C.; Wallace, M. A.; Marks, T. M.; Melillo, D. G. Tetrahedron
Lett. 1997, 38, 919-922.
Inorganic Chemistry, Vol. 45, No. 2, 2006 489