Heterobimetallic reductive cross-coupling of benzonitrile with carbon dioxide, pyridine, and benzophenone

Article abstract of DOI:10.1021/ic051320s

Described herein are heterobimetallic radical cross-coupling reactions between the benzonitrile adduct of the molybdenum(III) complex Mo(N[t-Bu]Ar)₃ (Ar = 3,5-C₆H₃Me₂) and titanium(III) complexes with carbon dio

Full text of DOI:10.1021/ic051320s

Inorg. Chem. 2005, 44, 7319−7321
Heterobimetallic Reductive Cross-Coupling of Benzonitrile with Carbon
Dioxide, Pyridine, and Benzophenone
Arjun Mendiratta and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Room 2-227, Cambridge, Massachusetts 02139
Received August 3, 2005
Described herein are heterobimetallic radical cross-coupling reac-
tions between the benzonitrile adduct of the molybdenum(III)
M ) Ti, Ar ) Ph, 1b; M ) Mo, Ar ) 3,5-Me2C6H3, 2) are
no exception because they mediate a variety of reductive
couplings,8,9 wherein frequently long-lived radical intermedi-
ates can be observed spectroscopically and/or intercepted by
added traps. For example, ketyl radical 1-OCPh2 (in
equilibrium with its dimer) accepts [H•] from n-Bu3SnH to
form diphenylmethoxide complex 1-OCHPh2.9,10 Similarly,
while 2 reacts with PhCN to form the expected product of
reductive dimerization,8 the long-lived intermediate 2-NCPh
is readily intercepted with 0.5 equiv of PhSSPh to furnish
(Ar[t-Bu]N)3Mo-NdC(Ph)SPh.11 Now we report that the
combination of 1 and 2 is uniquely capable of carrying out
pinacol-type cross-couplings of benzonitrile with CO2, benzo-
phenone, and pyridine. The successful use of CO2 as a
substrate is noteworthy as an example of its bimetallic
reductive coupling to give a product other than oxalate.12
The remarkable CO2 incorporation reaction highlighted
herein is evocative of (though likely mechanistically distinct
from) reverse radical decarboxylation13,14 because it is a
radical CO2 uptake process transpiring with C-C bond
formation.
complex Mo(N[t-Bu]Ar)3 (Ar
complexes with carbon dioxide, pyridine, and benzophenone. The
titanium(III) system employed was either Ti(N[t-Bu]Ar)3 (Ar 3,5-
) 3,5-C6H3Me2) and titanium(III)
)
C6H3Me2) or Ti(N[t-Bu]Ph)3. Crystal structure studies are described
for the Mo/PhCN/CO2/Ti coupled system and for an analogue of
the Mo/PhCN/Ph2CO/Ti coupled system in which PhCN is replaced
with 2,6-Me2C6H3CN. In the case of the couplings involving pyridine
and benzophenone,
dearomatization, with the new C
C
−
C
bond formation takes place with
C bond being formed between
−
the nitrile carbon of PhCN and the para carbon of pyridine or one
of the benzophenone phenyl groups. Of the radical metal complex/
substrate adducts invoked in this work, that between titanium(III)
and CO2 is the only one not directly observable. In all cases, the
selective cross-coupling reactions are interpreted as arising by
heterodimerization of titanium(III) substrate complexes (substrate
)
CO2, py, or Ph2CO) with the persistent molybdenum−PhCN
radical adduct. All of the heterobimetallic coupling products are
diamagnetic, and the metal ions Ti and Mo in them both are
Treatment of a purple, ethereal solution of 2-NCPh at
-100 °C with emerald green 1a (1.0 equiv), followed rapidly
by CO2 (1.1 equiv, introduced via syringe), was found to
elicit a color change to cherry red upon mixing. After
workup, the new diamagnetic compound (Ar[t-Bu]N)3Mo-
NdC(Ph)C(O)O-Ti(N[t-Bu]Ar)3 (3a) was isolated as dark
red crystals in 60% yield (see Scheme 1).15 We note that
assigned to the formal 4
+ oxidation state.
Electropositive metals can effect the reductive coupling
of carbonyl compounds to form metal diolates as in the
pinacol coupling.1 This reaction was expanded in scope to
include nitrile homocoupling through the use of niobium or
tantalum halides as reductants,2 and a variety of low-valent
early-metal systems are active for the pinacol-type reductive
coupling of organic nitriles.1,3-7 The trivalent three-coordinate
complexes M(N[t-Bu]Ar)3 (M ) Ti, Ar ) 3,5-Me2C6H3, 1a;
(7) Young, C. G.; Philipp, C. C.; White, P. S.; Templeton, J. L. Inorg.
Chem. 1995, 34, 6412-6414.
(8) Tsai, Y. C.; Stephens, F. H.; Meyer, K.; Mendiratta, A.; Gheorghiu,
M. D.; Cummins, C. C. Organometallics 2003, 22, 2902-2913.
(9) Agapie, T.; Diaconescu, P. L.; Mindiola, D. J.; Cummins, C. C.
Organometallics 2002, 21, 1329-1340.
(10) Covert, K. J.; Wolczanski, P. T.; Hill, S. A.; Krusic, P. J. Inorg. Chem.
1992, 31, 66-78.
(11) Mendiratta, A.; Cummins, C. C.; Kryatova, O. P.; Rybak-Akimova,
E. V.; McDonough, J. E.; Hoff, C. D. Inorg. Chem. 2003, 42, 8621-
8623.
* To whom correspondence should be addressed. E-mail:
ccummins@mit.edu. Tel: (617) 253-5332. Fax: (617) 258-5700.
(1) Wirth, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 61-63.
(2) Finn, P. A.; King, M. S.; Kilty, P. A.; McCarley, R. E. J. Am. Chem.
Soc. 1975, 97, 220-221.
(3) Cotton, F. A.; Hall, W. T. Inorg. Chem. 1978, 17, 3525-3528.
(4) Cotton, F. A.; Hall, W. T. J. Am. Chem. Soc. 1979, 101, 5094-5095.
(5) Deboer, E. J. M.; Teuben, J. H. J. Organomet. Chem. 1978, 153, 53-
57.
(12) Evans, W. J.; Seibel, C. A.; Ziller, J. W. Inorg. Chem. 1998, 37, 770-
776.
(13) Barton, D. H. R.; Crich, D.; Motherwell, W. B. Tetrahedron 1985,
41, 3901-3924.
(14) Barton, D. H. R.; Crich, D.; Motherwell, W. B. Chem. Commun. 1983,
939-941.
(6) Duchateau, R.; Williams, A. J.; Gambarotta, S.; Chiang, M. Y. Inorg.
Chem. 1991, 30, 4863-4866.
10.1021/ic051320s CCC: $30.25
Published on Web 09/14/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 21, 2005 7319

COMMUNICATION
Scheme 1. Synthesis of 3a and 3b (R ) t-Bu)
Figure 1. Solid-state structure of 3b with ellipsoids drawn at 50%
probability. Selected interatomic distances (Å) and angles (deg): average
Mo1-Namide, 1.962(3); Mo1-N4, 1.759(3); N4-C41, 1.350(5); C41-C101,
1.438(6); C101-O102, 1.220(5); O10-Ti1, 1.855(3); average Ti1-Namide
,
1.938(3); Mo1-N4-C41, 169.3(3); Ti1-O10-C101, 136.8(3).
Chart 1
the 13C-labeled variant, 3a-13C, is conveniently prepared from
13CO2 and exhibits a signature 13C NMR signal at 170.7 ppm
for the labeled carbon. Also, a weak absorption in the IR at
1635 cm-1 (found at 1564 cm-1 for 3a-13C) is assigned to
ν
CO. While an X-ray diffraction study of 3a confirmed the
proposed connectivity, end-over-end disorder prevented
the determination of reliable metrical parameters. Fortu-
nately, substitution of Ti(N[t-Bu]Ph)3 (1b) for 1a permitted
the synthesis of (Ar[t-Bu]N)3Mo-NdC(Ph)C(O)O-Ti(N[t-
Bu]Ph)3 (3b), for which a single-crystal X-ray diffraction
study was accomplished without difficulty.
enolate 1a-OC(dCH2)NPhMe (1.847(3) Å),9 and while the
carboxylate residue interacts with Ti in a monodentate
fashion, still the Ti1-O10-C101 angle is an acute 136.8(3)°.
The Ti and Mo centers both experience a pseudotetrahedral
coordination environment.
We have obtained two other structures related to that of 3
by using 2-NCPh as a radical trap. Dark green 4 (Chart 1)
is the product of radical heterocombination between the
reversibly formed pyridine adduct 1a-py17 and 2-NCPh,
while green 5 stems from coupling of 2-NCPh with one of
the para carbons of titanium benzophenone ketyl radical
1-OCPh2.18 As was the case for 3a,15 both 4 and 5 are
formed in essentially quantitative yield, with the lower
isolated yields being due to inefficiency in the crystallization
of these lipophilic substances. Dearomatization similar to that
The molecular structure of 3b, shown in Figure 1, clearly
features the freshly created, CO2-derived, carboxyiminato
unit that spans the Mo/Ti centers. The carboxyiminato moiety
is both essentially planar and orthogonal to the plane of its
phenyl substituent. The newly formed C41-C101 bond
distance is 1.487(5) Å, while the N4-C41 distance is
1.350(5) Å. Both the latter value and the short Mo1-N4
distance of 1.759(3) Å are similar to corresponding param-
eters observed previously for molybdenum(IV) ketiminato
complexes.11,16 Multiple Mo1-N4 bonding is consonant with
the nearly linear (169.9(3)°) Mo1-N4-C41 linkage. The
Ti1-O10 distance (1.855(3) Å) is quite similar to the
corresponding parameter reported for titanium trisanilide
(15) The yield of 3a was essentially quantitative as assessed from proton
NMR spectra of crude reaction mixtures. The isolated yield of only
60% is due to losses incurred in the purification by crystallization of
this lipophilic substance.
(16) Sceats, E. L.; Figueroa, J. S.; Cummins, C. C.; Loening, N. M.; Van
der Wel, P.; Griffin, R. G. Polyhedron 2004, 23, 2751-2768.
(17) Figure S1 in the Supporting Information is the electronic absorption
spectrum of 1a in the presence of varying amounts of pyridine.
(18) Compound 5′, with 2,6-Me2C6H3CN in place of PhCN, has also been
synthesized and structurally characterized. See the Supporting Infor-
mation.
7320 Inorganic Chemistry, Vol. 44, No. 21, 2005

COMMUNICATION
observed in the structure of Gomberg’s dimer19 is a feature
and alkyne substrates,22,23 dating to seminal work by Hoberg
on such nickel(0)-induced C-C couplings.24-26 Mindful of
carbon management as an important societal concern,27 we
might now consider pursuit of radical carboxylation as a
potential alternative reaction pathway for fixation of the key
atmospheric CO2 molecule.
of interest in the reactions leading to 4 and 5, reactions
9
distinct from the CO2/PhCN coupling in that the 1a-OCPh2
and 1a-py17 adducts are spectroscopically observable.
Proposed intermediate 1a-CO2 (implied in Scheme 1) has
so far eluded direct detection despite substantial effort
expended in this regard; the interception in solution of
putative 1a-CO2 by traps other than 2-NCPh has also been
investigated (see the Supporting Information).
Acknowledgment. The authors thank Joshua S. Figueroa
for crystallographic assistance and the National Science
Foundation for financial support (Grant CHE-0316823 and
a predoctoral fellowship to A.M.).
The reductive cross-couplings described herein proceed
with both Ti and Mo being converted from the 3+ to the
4+ oxidation state and are attractively interpreted in terms
of trapping of radical titanium(III) substrate complexes by
the persistent 2-NCPh radical. While 2-NCPh is not an
aggressive radical20 and quite likely thermodynamically
incapable of directly attacking stable systems such as the
CO2 molecule, its persistence in high concentration in
solution makes it an ideal radical trap. The three examples
of selective radical cross-coupling presented herein illustrate
the principle termed “the persistent radical effect”.21 Remark-
able is the fact that products 3-5 incorporate four different
reactant molecules added to solution in sequence with zero
byproducts.
Supporting Information Available: Complete synthetic pro-
cedures and characterization data for all new compounds, details
of the attempted observation and trapping of 1a-CO2, and tables
of crystallographic data in CIF format for 3b and 5′. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC051320S
(22) Yin, X. L.; Moss, J. R. Coord. Chem. ReV. 1999, 181, 27-59.
(23) Pandey, K. K. Coord. Chem. ReV. 1995, 140, 37-114.
(24) Hoberg, H.; Peres, Y.; Kruger, C.; Tsay, Y. H. Angew. Chem., Int.
Ed. Engl. 1987, 26, 771-773.
(25) Hoberg, H.; Schaefer, D.; Burkhart, G.; Kruger, C.; Romao, M. J. J.
Organomet. Chem. 1984, 266, 203-224.
The fixation of carbon dioxide by metal-mediated coupling
with unsaturated organic molecules has focused on alkene
(26) Burkhart, G.; Hoberg, H. Angew. Chem., Int. Ed. Engl. 1982, 21, 76-
76.
(27) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E.
J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.;
Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.;
Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H.
H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, K.; Nicholas,
K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.;
Schmidt, L. D.; Sen, A.; Somorjai, G. A.; Stair, P. C.; Stults, B. R.;
Tumas, W. Chem. ReV. 2001, 101, 953-996.
(19) Bochkarev, L. N.; Molosnova, N. E.; Zakharov, L. N.; Fukin, G. K.;
Yanovsky, A. I.; Struchkov, Y. T. Acta Crystallogr. C 1995, 51, 489-
491.
(20) 2a-NCPh does not even abstract the H atom from HSnBu3; see
Scheme 1 of ref 8.
(21) Fischer, H. Chem. ReV. 2001, 101, 3581-3610.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7321