Nitrene Metathesis and Catalytic Nitrene Transfer Promoted by Niobium Bis(imido) Complexes

Article abstract of DOI:10.1021/jacs.5b11287

We report a metathesis reaction in which a nitrene fragment from an isocyanide ligand is exchanged with a nitrene fragment of an imido ligand in a series of niobium bis(imido) complexes. One of these bis(imido) complexes also promotes nitrene transfer to catalytically generate asymmetric dialkylcarbodiimides from azides and isocyanides in a process involving the Nb(V)/Nb(III) redox couple.

Full text of DOI:10.1021/jacs.5b11287

Communication
pubs.acs.org/JACS
Nitrene Metathesis and Catalytic Nitrene Transfer Promoted by
Niobium Bis(imido) Complexes
Benjamin M. Kriegel, Robert G. Bergman,* and John Arnold*
Department of Chemistry, University of California, Berkeley, California 94720, United States
S
* Supporting Information
Scheme 1
ABSTRACT: We report a metathesis reaction in which a
nitrene fragment from an isocyanide ligand is exchanged
with a nitrene fragment of an imido ligand in a series of
niobium bis(imido) complexes. One of these bis(imido)
complexes also promotes nitrene transfer to catalytically
generate asymmetric dialkylcarbodiimides from azides and
isocyanides in a process involving the Nb(V)/Nb(III)
redox couple.
heating to 60 °C for 2.5 h, a yellow solution of 2·XylNC in
toluene underwent a color change to give an orange solution
from which bright-orange crystals were isolated. NMR
spectroscopic and crystallographic data indicated clean con-
version to 5a, in which the nitrene fragments from one tert-
butylimido group and the coordinated isonitrile were exchanged
to generate a complex containing a thermodynamically favored
aryl(imido) ligand with coordinated tBuNC (Scheme 2). The X-
ince the initial discovery of C−H activations and cyclo-
1
Sadditions across terminal zirconium imido π bonds in 1988,
a number of nitrene transfer processes involving reactions across
metal−nitrogen multiple bonds, especially those in high-valent
group 4 complexes, have been explored.2,3 Many of these systems
have been shown to undergo cycloadditions with a variety of
unsaturated substrates.2−4 This reactivity has been developed
into useful catalytic nitrene transfer processes, including
hydroaminations2,5 and carboaminations6 as well as imine7 and
azide8 metatheses. In contrast to group 4 systems, relatively few
studies have focused on reactions across imido groups in group
39 and group 5 systems.10−12
Scheme 2
In our efforts to design group 5 systems that are reactive across
metal−nitrogen multiple bonds, we have been investigating
niobium and tantalum imido complexes supported by β-
diketiminate (BDI) ligands.13 While we have found that the
imido group behaves purely as an ancillary ligand for a variety of
low- and high-valent mono(imido) derivatives, we hypothesized
that the introduction of a second imido moiety could enhance the
reactivity of the imido groups. This “π-loading” effect results
from interactions of multiple ligand-based p orbitals with the
same metal-based d orbital, causing a net decrease in bonding
character of the HOMO.12,13b,14 We recently reported a
synthetic route to a series of four-coordinate niobium bis(imido)
complexes.11 Herein we report the novel stoichiometric and
catalytic reactivity of these niobium bis(imido) complexes with
alkyl and aryl isonitriles.
ray crystal structure of 5a (Figure 1, left) showed a geometry
intermediate between trigonal-bipyramidal and square-pyrami-
dal (τ = 0.41);15 a similar geometry was observed for 2·XylNC (τ
= 0.47; see the SI). Notably, clean conversion of 2·XylNC to 5a
was also observed when solid 2·XylNC was heated to 80 °C for
8 h. While reactions of early transition metal imido complexes
with isocyanides across imido groups have been documented in
the literature, these reactions typically lead to either coupling of
multiple equivalents of isocyanide after addition,4c,e formation of
η2-carbodiimide complexes,4a or production of carbodiimide.4g,16
Niobium bis(imido) complexes 2−4 were prepared by
reaction of the Nb(III) precursor 1 with azides RN3 (R = 2,6-
Dipp, tBu, TMS) (Scheme 1). Synthetic procedures for 2−4 and
full characterization data for compound 3 are reported in the
Supporting Information (SI).
Addition of 2,6-dimethylphenyl isocyanide (XylNC) to a
solution of 2 resulted in slight lightening of the color of the
1
orange solution; H NMR spectroscopic and crystallographic
analysis of the yellow crystals obtained from this reaction were
Received: October 28, 2015
consistent with isocyanide adduct 2·XylNC (see the SI). Upon
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b11287
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 3
Figure 1. Molecular structures of (left) 5a and (right) 8 determined by
X-ray diffraction. H atoms and aryl iPr groups have been omitted and n-
alkyl groups have been truncated for clarity; thermal ellipsoids are
displayed at the 50% probability level.
tBuCN, hampering the conversion of 7 to 4·tBuNC. This
isomerization did not occur in the absence of the niobium
complex, indicating that it likely was metal-catalyzed; limited
examples of this metal-catalyzed process exist in the literature.17
While reactions with 3 were not explored in as much detail
because of a lack of selectivity between reactions across TMS and
tBu imido groups leading to mixtures of products, 3 did react with
a large excess of tBuNC upon heating to 120 °C for 15 h to give
2·tBuNC. In this case, isomerization of tBuNC to tBuCN was also
observed, halting the reaction to give 2·tBuNC after ∼60%
conversion.
To our knowledge, the conversion of 2 to 5 is the first example in
which the nitrene fragments are simply exchanged to generate a
new imido group and a new isocyanide. Hence, this represents a
new method for both incorporation of imido moieties into metal
complexes and integration of various substituents into isonitriles.
Although it still contained a basic tert-butylimido group, 5a was
much less reactive toward nitrene metathesis than 2; addition of a
second equivalent of XylNC to 5a resulted in the establishment
DFT calculations provide support for the proposed mecha-
nistic pathway shown in Figure 2. Coordinated isocyanide first
t
of an equilibrium between BuNC and XylNC adducts, but
heating to 120 °C for 8 h was required to exchange the remaining
t
tert-butylimido group, generating 1 equiv of free BuNC and a
new niobium compound identified as BDINb(NXyl)2CNtBu (6)
by 1H and 13C NMR spectroscopy.
Compound 2 reacted with 4-methoxy-substituted aryl
isocyanides to give compounds 5b and 5c, as shown in Scheme
2. Formation of 5b proceeded much more slowly than formation
of 5a; addition of 4-methoxy-2,6-dimethylphenyl isocyanide
(MeOXylNC) to a solution of 2 again resulted in the immediate
formation of the adduct 2·MeOXylNC, but heating to 60 °C for
16 h was needed to observe 95% conversion to 5b by 1H NMR
spectroscopy. In contrast, 2·XylNC underwent 95% conversion
to 5a after heating to 60 °C for 2 h. This observation can be
rationalized by the fact that the donating 4-methoxy substituent
increases the electron density at the isocyanide carbon, making it
a less electrophilic target for nucleophilic attack by an imido
moiety. Conversely, the formation of 5c proceeded significantly
faster than the formation of either 5a or 5b (>95% conversion
within 0.5 h at 60 °C), indicating that (a) the rate-determining
transition state is relatively sterically encumbered compared with
the starting isocyanide adduct and (b) reducing the steric profile
of the isocyanide by eliminating the o-methyl substituents has a
more pronounced effect on the reaction rate than adding a
donating p-methoxy substituent.
Figure 2. DFT-calculated energy profile for nitrene metathesis.
adds across an imido moiety in I-1 to give η2-carbodiimide
intermediate I-2, which has precedent in the literature.4a,g,16 The
lowest-energy pathway found for the following step involves
passage through the nearly linear η3-carbodiimide transition state
T-2 to give I-3, which then proceeds to give the thermodynami-
cally favored product I-4 by the reverse of the first step. While the
highest-energy transition state T-2 is 25 kcal/mol higher in
energy than intermediate I-2, it is not unreasonable that this
process could proceed readily at 60 °C. In fact, neither
intermediate I-2 nor I-3 was observed experimentally when the
reaction was followed by 1H NMR spectroscopy, indicating that
they are likely higher in energy than the calculated values relative
to I-1. Diisopropylphenyl substituents were modeled as phenyl
substituents in the calculations, so this discrepancy is not
surprising, as I-2 and I-3 are much more prone to steric clashing
with BDI aryl groups than I-1 since the imido and isocyanide
substituents of I-1 are nearly 180° from the metal center.
Calculated reaction pathways for analogues incorporating 4-
While no evidence of nitrene metathesis was observed from
reactions of alkyl-substituted isocyanides with 2 (see below), the
mixed aryl/alkyl bis(imido) complex 4 reacted with alkyl
t
isocyanides to generate BuNC and exchange the alkyl
substituent into the imido position. The reaction with cyclohexyl
isocyanide (CyNC) was particularly clean, and compound 7 was
isolated after heating of a solution of 4 in refluxing toluene in the
presence of excess CyNC for 15 h (Scheme 3). Heating of
isolated 7 to 120 °C in the presence of excess tBuNC resulted in
1
conversion to 4·tBuNC, which was identified by H NMR
spectroscopy and prepared independently from coordination of
tBuNC to 4. Interestingly, during the course of this reaction, a
t
significant amount of BuNC was isomerized to unreactive
B
DOI: 10.1021/jacs.5b11287
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
MeOPh and 2,6-Xyl isocyanide substituents are shown in Figures
S10 and S11, respectively, and are consistent with the
experimentally observed influence of the electron-donating
ability and steric bulk of the isocyanide aryl substituent on the
reaction rate (see above).
starting material, thus rendering the formation of carbodiimide
catalytic. In fact, within seconds at room temperature, addition of
excess tBuN3 to a solution of 2·BnNC resulted in a color change
from yellow-orange to orange, consistent with conversion to the
four-coordinate bis(imido) complex 2. Conversion to 2 and
1
Reaction of 2 with primary alkyl isocyanides AkNC (Ak = nBu,
Bn) resulted in color changes to red-brown solutions. In both
cases, conversion to a single major metal-containing product was
observed by 1H NMR spectroscopy within 12 h for Ak = nBu or
within 20 min for Ak = Bn. In contrast to reactions with aryl
isocyanides, both reactions proceeded at room temperature
production of BnNCNtBu were confirmed by H NMR
spectroscopy and GC−MS. Similarly, generation of 2 and the
t
corresponding carbodiimides was observed after excess BuN3
was added to 2·nBuNC or 2·CyNC, although the reaction with
2·nBuNC only proceeded within 12 h at room temperature and
the reaction with 2·CyNC required heating at 60 °C for 15 h to
go to completion. No formation of carbodiimide was observed
t
without any generation of BuNC; instead, in each case
generation of 1 equiv of the organic product AkNCNtBu
with BuNC even at temperatures up to 120 °C. Catalytic
t
was observed.
reactions were carried out in J. Young valve NMR tubes using
n
Remarkably, compound 2 reacted with 6 equiv of BuNC to
mixtures of C6D6 and tBuN3 as the solvent, and the conversion to
release 1 equiv of nBuNCNtBu and give compound 8 as a
carbodiimide was followed by H NMR spectroscopy; higher
1
red-orange microcrystalline material in 53% yield (Scheme 4).
turnover numbers were observed when the catalytic reactions
were carried out in the presence of a large excess (∼200 equiv) of
tBuN3. Up to 17 turnovers (6% catalyst loading) were observed
for Ak = Cy (20 h at 80 °C) before complete catalyst degradation,
while lower turnover numbers (5−8) were observed for Ak = Bn
and Ak = nBu.
Scheme 4
The catalytic reactions likely proceeded via the mechanism
outlined in Scheme 5. Notably, buildup of an intermediate was
Scheme 5
The reaction to give 8 involved the formation of four new C−C
bonds and a C−N bond to generate a heavily substituted κ2-N,N-
pyrrolediamido ligand. The solid-state structure (Figure 1, right)
shows a distorted square-pyramidal geometry (τ = 0.1) with the
imido moiety in the apical position. A plausible reaction
mechanism for the formation of 8 is shown in Scheme S1.
While related couplings of isocyanides by reduced metal centers
have been observed,18 to our knowledge this is the first example
of the formation of a pyrrolediamido transition metal complex.
A related reaction occurred between 2 and 3 equiv of benzyl
isocyanide (BnNC) to give 9, which precipitated from the
reaction mixture within minutes as pale-yellow needles (Scheme
4). In 9, a benzyl group was transferred from one equivalent of
isocyanide to another to give a C-bound terminal cyanide ligand
and an η2-iminoacyl ligand.
Several pieces of experimental evidence support that these
reactions proceeded through Nb(III) intermediates. First, the
same products 8 and 9 were observed from the reactions of
Nb(III) complex 1 with either 5 equiv of nBuNC or 2 equiv of
BnNC. Second, addition of 1 equiv of BnNC to 2 to give 2·
BnNC followed by addition of 3 equiv of XylNC resulted in an
immediate color change to green-blue, and the 1H NMR
spectrum was consistent with the formation of the reported
Nb(III) complex BDINb(NtBu)(CNXyl)3.12c
observed by 1H NMR spectroscopy under the catalytic
conditions. The intermediate was especially prominent for Ak
= nBu; in fact, when a solution containing 2·nBuNC, 4 additional
n
t
equiv of BuNC, and 20 equiv of BuN3 was left at room
temperature for 16 h, complete conversion of 2·nBuNC to this
new species was observed. The product of this reaction was
isolated and identified by X-ray crystallography and NMR
spectroscopy as the terminal (tert-butylazido)niobium complex
10. This represents the first characterized example of a terminal
(organoazido)niobium complex, but sparse examples of other
terminal (organoazido)metal species exist in the literature.19 In
one notable example,19c detailed mechanistic studies showed that
a related (phenylazido)tantalum complex is an intermediate
species in nitrene transfer from an azide to a low-valent tantalum
center and that nitrene transfer to generate an imido group and
release N2 goes through a four-center intermediate analogous to
C. Similarly, isolated 10 slowly converts to 2·nBuNC and N2 in
solution and is also a competent catalyst for carbodiimide
formation.
On the basis of these results, it seemed possible that the
Nb(III) intermediate generated in these reactions could be
trapped by an azide to regenerate the four-coordinate bis(imido)
While a handful of mid and late transition metal catalysts for
this transformation have been reported,20 this represents only
C
DOI: 10.1021/jacs.5b11287
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
2001, 1392. (e) Blake, A. J.; Collier, P. E.; Gade, L. H.; McPartlin, M.;
Mountford, P.; Schubart, M.; Scowen, I. J. Chem. Commun. 1997, 1555.
(f) Li, Y.; Shi, Y.; Odom, A. L. J. Am. Chem. Soc. 2004, 126, 1794.
(g) Dunn, S. C.; Hazari, N.; Cowley, A. R.; Green, J. C.; Mountford, P.
Organometallics 2006, 25, 1755.
(5) (a) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc.
1992, 114, 1708. (b) Odom, A. L.; McDaniel, T. J. Acc. Chem. Res. 2015,
48, 2822.
(6) (a) Basuli, F.; Aneetha, H.; Huffman, J. C.; Mindiola, D. J. J. Am.
Chem. Soc. 2005, 127, 17992. (b) Ruck, R. T.; Zuckerman, R. L.; Krska,
S. W.; Bergman, R. G. Angew. Chem., Int. Ed. 2004, 43, 5372.
(7) Zuckerman, R. L.; Krska, S. W.; Bergman, R. G. J. Am. Chem. Soc.
2000, 122, 751.
the second example of catalytic carbodiimide formation
mediated by an early transition metal complex.16 In the previous
example, the Zr center remained in its more stable Zr(IV)
oxidation state throughout the cycle, while two-electron redox
changes were mediated by a redox-active ligand. In the present
case, the energy difference between high-valent Nb(V) and low-
valent Nb(III) is small enough that reductive elimination of η2-
carbodiimide occurs readily in the presence of either an alkyl
azide or excess isocyanide. As demonstrated here and in our
previous work,13e,f a niobium center with a suitable ligand
environment can both give access to the unique reactivity
observed for early transition metal complexes and allow cycling
between Nb(III) and Nb(V) in catalytic processes.
(8) Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1995,
117, 974.
To summarize, we have discovered a novel nitrene exchange
process in which a substituent is exchanged between an
alkylimido group in a Nb(V) bis(imido) complex and an aryl
isocyanide. The reaction likely proceeds through cycloaddition
and cycloreversion involving Nb(V) η2-carbodiimide intermedi-
ates and occurs at a single metal center without elimination of
carbodiimide. In contrast, reaction with unhindered alkyl
isocyanides resulted in extrusion of carbodiimide, a process
that was rendered catalytic in the presence of excess azide.
Further exploration of the scope and mechanism of the
stoichiometric and catalytic nitrene transfer processes is ongoing.
(9) (a) Chu, J.; Lu, E.; Liu, Z.; Chen, Y.; Leng, X.; Song, H. Angew.
Chem., Int. Ed. 2011, 50, 7677. (b) Scott, J.; Basuli, F.; Fout, A. R.;
Huffman, J. C.; Mindiola, D. J. Angew. Chem., Int. Ed. 2008, 47, 8502.
(c) Chu, T.; Piers, W. E.; Dutton, J. L.; Parvez, M. Organometallics 2013,
32, 1159.
(10) (a) Anderson, L. L.; Arnold, J.; Bergman, R. G. Org. Lett. 2004, 6,
2519. (b) Blake, R. E.; Antonelli, D. M.; Henling, L. M.; Schaefer, W. P.;
Hardcastle, K. I.; Bercaw, J. E. Organometallics 1998, 17, 718.
(c) Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1993, 32, 131.
(11) Obenhuber, A. H.; Gianetti, T. L.; Berrebi, X.; Bergman, R. G.;
Arnold, J. J. Am. Chem. Soc. 2014, 136, 2994.
(12) (a) Obenhuber, A. H.; Gianetti, T. L.; Bergman, R. G.; Arnold, J.
Chem. Commun. 2015, 51, 1278. (b) La Pierre, H. S.; Arnold, J.; Toste, F.
D. Angew. Chem., Int. Ed. 2011, 50, 3900. (c) De With, J.; Horton, A. D.;
Orpen, A. G. Organometallics 1993, 12, 1493.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b11287.
■
S
(13) (a) Tomson, N. C.; Yan, A.; Arnold, J.; Bergman, R. G. J. Am.
Chem. Soc. 2008, 130, 11262. (b) Tomson, N. C.; Arnold, J.; Bergman,
R. G. Organometallics 2010, 29, 2926. (c) Tomson, N. C.; Arnold, J.;
Bergman, R. G. Organometallics 2010, 29, 5010. (d) Gianetti, T. L.;
Nocton, G.; Minasian, S. G.; Tomson, N. C.; Kilcoyne, A. L. D.;
Kozimor, S. A.; Shuh, D. K.; Tyliszczak, T.; Bergman, R. G.; Arnold, J. J.
Am. Chem. Soc. 2013, 135, 3224. (e) Gianetti, T. L.; Tomson, N. C.;
Arnold, J.; Bergman, R. G. J. Am. Chem. Soc. 2011, 133, 14904.
(f) Gianetti, T. L.; Bergman, R. G.; Arnold, J. Chem. Sci. 2014, 5, 2517.
(g) Kriegel, B. M.; Bergman, R. G.; Arnold, J. Dalton Trans. 2014, 43,
10046.
Proposed mechanism for the formation of 8, experimental
procedures, analytical data, crystal data, DFT methods and
results, and NMR data (PDF)
CIF files for 2·XylNC, 3, 5a, 5c, 7, 8, 9, and 10 (ZIP)
AUTHOR INFORMATION
■
Corresponding Authors
*arnold@berkeley.edu
*rbergman@berkeley.edu
(14) (a) Cundari, T. R. Organometallics 1994, 13, 2987. (b) O’Reilly,
M. E.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2012,
134, 11185.
Notes
(15) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G.
C. J. Chem. Soc., Dalton Trans. 1984, 1349.
The authors declare no competing financial interest.
(16) Nguyen, A. I.; Zarkesh, R. a.; Lacy, D. C.; Thorson, M. K.;
Heyduk, A. F. Chem. Sci. 2011, 2, 166.
(17) (a) Jones, W. D.; Kosar, W. P. Organometallics 1986, 5, 1823.
(b) Millich, F. Chem. Rev. 1972, 72, 101.
(18) Boyarskiy, V. P.; Bokach, N. A.; Luzyanin, K. V.; Kukushkin, V. Y.
Chem. Rev. 2015, 115, 2698.
(19) (a) Fickes, M. G.; Davis, W. M.; Cummins, C. C. J. Am. Chem. Soc.
1995, 117, 6384. (b) Proulx, G.; Bergman, R. G. J. Am. Chem. Soc. 1995,
117, 6382. (c) Proulx, G.; Bergman, R. G. Organometallics 1996, 15, 684.
(20) (a) Wiese, S.; Aguila, M. J. B.; Kogut, E.; Warren, T. H.
Organometallics 2013, 32, 2300. (b) Laskowski, C. A.; Hillhouse, G. L.
Organometallics 2009, 28, 6114. (c) Saegusa, T.; Ito, Y.; Shimizu, T. J.
Org. Chem. 1970, 35, 3995. (d) Cowley, R. E.; Golder, M. R.; Eckert, N.
A.; Al-Afyouni, M. H.; Holland, P. L. Organometallics 2013, 32, 5289.
(e) Zhang, Z.; Li, Z.; Fu, B.; Zhang, Z. Chem. Commun. 2015, 51, 16312.
(f) Yousif, M.; Tjapkes, D. J.; Lord, R. L.; Groysman, S. Organometallics
2015, 34, 5119.
ACKNOWLEDGMENTS
■
We thank the NSF (Grant CHE-1465188) for financial support,
Prof. C. Camp, L. N. Grant, A. I. Nguyen, T. D. Lohrey, J. A.
Ziegler, Dr. S. Hohloch, and Dr. T. L. Gianetti for helpful
discussions, Dr. K. Durkin and Dr. O. A. Olatunji-Ojo for
assistance with DFT calculations, and Dr. A. G. DiPasquale for
assistance with difficult X-ray structures. We also thank the NIH
(Grant S10-RR027172) for financial support of our X-ray
crystallographic facility.
REFERENCES
■
(1) (a) Cummins, C. C.; Baxter, S. M.; Wolczanski, P. T. J. Am. Chem.
Soc. 1988, 110, 8731. (b) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J.
Am. Chem. Soc. 1988, 110, 8729.
(2) Duncan, A. P.; Bergman, R. G. Chem. Rec. 2002, 2, 431.
(3) Hazari, N.; Mountford, P. Acc. Chem. Res. 2005, 38, 839.
(4) (a) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. Organometallics
1993, 12, 3705. (b) Lee, S. Y.; Bergman, R. G. Tetrahedron 1995, 51,
4255. (c) Bashall, A.; Collier, P. E.; Gade, L. H.; McPartlin, M.;
Mountford, P.; Pugh, S. M.; Radojevic, S.; Schubart, M.; Scowen, I. J.;
Trosch, D. J. M. Organometallics 2000, 19, 4784. (d) Guiducci, A. E.;
̈
Cowley, A. R.; Skinner, M. E. G.; Mountford, P. J. Chem. Soc. Dalt. Trans.
D
DOI: 10.1021/jacs.5b11287
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX