Three-coordinate and four-coordinate cobalt hydride complexes that react with dinitrogen

Article abstract of DOI:10.1021/ja902812y

(Figure Presented) We report the formation and N₂ reactivity of novel cobalt hydride complexes supported by bulky β-diketiminate ligands (L). Addition of KHBEt₃ to LCoCl gives [LCo(μ-H)]₂ (1) or K₂[LCoH]₂ (2), depending on the amount of borohydride used. Compound 2 is the first example of a crystallographically characterized hydride complex in which a transition metal is three-coordinate. Both 1 and 2 react with N₂ at room temperature to give dinuclear N₂ complexes with loss of H₂.

Full text of DOI:10.1021/ja902812y

Published on Web 07/21/2009
Three-Coordinate and Four-Coordinate Cobalt Hydride Complexes That React
with Dinitrogen
Keying Ding, William W. Brennessel, and Patrick L. Holland*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14618
Received April 8, 2009; E-mail: holland@chem.rochester.edu
Complexes of a transition metal (defined here as a metal with a
partially filled d shell) having only three bonds to the metal are
interesting because the orbital energies, spin states, and reaction
pathways can be different than those in traditional complexes.1
Isolating three-coordinate complexes invariably depends on the use
of extremely bulky supporting ligands, which protect the metal
center through steric effects. However, this strategy might seem to
be incompatible with three-coordinate hydride complexes, because
H- is the smallest possible ligand. Accordingly, no three-coordinate
hydride complexes are known.2 New kinds of hydride complexes
are desired because of the many roles for hydrides in organometallic
chemistry and catalysis.3
Using iron complexes, we recently utilized the very bulky
bidentate ꢀ-diketiminate ligand L [L ) 2,2,6,6-tetramethyl-3,5-
bis(2,4-diisopropylphenylimido)hept-4-yl] to enable the isolation
of three-coordinate complexes in which one of the three ligands is
small (e.g., halide, CH3).4 Here we report the use of L to stabilize
the first crystallographically characterized three-coordinate hydride
complex of any transition metal. We also report that this cobalt
hydride complex and another related complex react with N2 at room
temperature and atmospheric pressure through the bimetallic
reductive elimination of H2.
Figure 1. Syntheses and structures (using 50% thermal ellipsoids) of
unsaturated cobalt hydride complexes. Carbon-bound hydrogen atoms have
been omitted for clarity. The cobalt-bound hydrogen atoms were refined
Figure 1 shows the syntheses and structures of dimeric cobalt(II)
hydride (1) and cobalt(I) hydride (2) complexes, which are formed
by addition of different amounts of KHBEt3 to toluene solutions
of the three-coordinate cobalt(II) complex LCoCl under Ar.5
Addition of 1 equiv of KHBEt3 to LCoCl gives [LCo(µ-H)]2 (1) in
72% yield. Although we were not able to completely free 1 from
impurities (see the Supporting Information), several forms of
characterization were possible. The X-ray crystal structure of 1
reveals that it has two hydride ligands (located in the difference
Fourier map) that bridge diketiminate-bound cobalt centers. The
metal coordination and geometry closely resemble those for the
independently for position, with dependent isotropic thermal parameters.
Selected bond lengths and bond angles follow; because compound 1 has
three crystallographically independent dimers, ranges are listed here (for
details, see the Supporting Information). For 1: Co-Co, 2.472-2.481 Å;
Co-N, 1.962-1.993 Å; N-Co-N, 95.4-96.5°. For 2: Co1-Co2, 5.7386(7)
Å; Co1-H1, 1.84(2) Å; Co1-N11, 1.9162(13) Å; Co1-N21, 1.9164(12)
Å; N11-Co1-N21, 96.73(6)°; N11-Co1-H1, 128.2(7)°; N21-Co1-H1,
134.1(7)°; Co2-H2, 1.78(2) Å; Co2-N14, 1.9106(13) Å; Co2-N24,
1.9121(13) Å; N14-Co2-N24, 96.59(6)°; N14-Co2-H2, 133.5(7)°;
N24-Co2-H2, 129.4(7)°.
6
iron(II) complex [LFe(µ-H)]2 and a dimeric ꢀ-diketiminate nick-
el(II) hydride complex recently reported by Limberg.7 The cobalt
atoms in 1 are separated by 2.476(5) Å, which is intermediate
between the metal-metal distances in the iron [2.624(2) Å] and
nickel [2.3939(6) Å] analogues. The presence of the hydride ligands
in 1 was confirmed through the reaction of a toluene solution of 1
with 2 equiv of cyclohexene to give the cobalt(II) cyclohexyl
product LCo(C6H11), which results from 1,2-addition of the cobalt
hydride to the C-C double bond.
Reaction of LCoCl with 2 equiv of KHBEt3 under Ar gave
compound 2, with presumed loss of H2, in 70% yield. Compound
2 is very unusual. Its X-ray crystal structure reveals two nearly
parallel three-coordinate cobalt units related by a pseudoinversion
center (Figure 2). The high quality of the crystallographic data
enabled refinement of the positions of the hydrogen atoms, giving
Co-H distances of 1.81(3) Å. The cobalt geometry is trigonal-
planar, and each H atom lies near the pseudo-C2-axis of the
ꢀ-diketiminate ligand on the same cobalt atom (N-Co-H angles
Figure 2. Side view of the core of 2, showing the relative orientation of
the two CoN2H planes.
ranging from 128 to 134°). The two H atoms are separated by
3.02(3) Å and the two cobalt atoms by 5.7386(7) Å, showing that
there is no direct connection between the cobalt-bound atoms.
Instead, the halves of the molecule are held together by K+ ions
that form cation-π interactions with the aryl rings of the
ꢀ-diketiminate ligands. The potassium ions are also close to the
hydride ligands [K-H distances of 2.60(2) and 2.67(2) Å].
The three-coordinate cobalt ions in complex 2 are high-spin
cobalt(I), as shown by the paramagnetically shifted 1H NMR
spectrum and the solution magnetic moment, µeff ) 5.6(2) µB per
9
10804 J. AM. CHEM. SOC. 2009, 131, 10804–10805
10.1021/ja902812y CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
dimeric molecule of 2. Complex 2 is soluble and gives similar 1H
NMR spectra in cyclohexane, benzene, and THF, suggesting that
the alkali metal ions remain bound in solution. The freezing-point
depression of a solution of 2 in naphthalene indicated a molecular
weight of 1140 ( 200 (3σ), supporting the dimeric formulation in
K2[LCo(µ-D)]2 (2-D) with N2 for 2 days. H2 and D2 were present
in the headspace, and no HD was detected. This result indicates
that (a) 2 does not dissociate in solution at room temperature and
(b) the elimination of H2 from 2 is an intramolecular process. The
analogous reaction of 1 and 1-D with N2 gave a significant amount
of HD, probably through the pre-equilibration of the isotopologues
of 1 through monomer-dimer equilibrium.
In conclusion, we have crystallographically characterized the first
three-coordinate transition-metal hydride complex, K2[LCo(µ-H)]2
(2). The hydride ligands in this species are labile and are eliminated
as H2 upon the addition of N2. It is surprising that 2 readily reacts
with N2 but does not react at room temperature with THF or arenes,
suggesting that the approach of additional donors to cobalt is
sterically restricted by the potassium-bound ꢀ-diketiminate ligands.
1
solution. Seven ꢀ-diketiminate resonances are observed in its H
NMR spectrum in C6D6 from 200-353 K (the hydrides are not
observed because of fast relaxation), showing that the ꢀ-diketiminate
ligands of 2 have averaged C2V symmetry in solution. Therefore,
there is a low-energy mechanism that enables the molecule to reach
an arrangement in which the CoN2H units are transiently coplanar
without dissociation of the molecule into halves.
Scheme 1. Reactions of the Low-Coordinate Cobalt Hydride
Complexes with Dinitrogen (R ) Isopropyl)
Acknowledgment. Funding was provided by the National
Institutes of Health (GM-065313). We thank Richard Eisenberg
for the use of a gas chromatograph for the H2 measurements.
Supporting Information Available: Synthetic, spectroscopic, ki-
netic, and crystallographic (CIF) data. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685. (b) Alvarez, S.
Coord. Chem. ReV. 1999, 193-195, 13.
(2) Putative three-coordinate complexes [Pt(PtBu3)2H]+ were not crystallized
and probably have a weak fourth ligand. See: (a) Goel, R.; Srivastava,
R. C. J. Organomet. Chem. 1981, 204, C13. (b) Goel, R.; Srivastava, R. C.
Can. J. Chem. 1983, 61, 1352. (c) Butts, M. D.; Scott, B. L.; Kubas, G. J.
J. Am. Chem. Soc. 1996, 118, 11831.
(3) Recent AdVances in Hydride Chemistry; Peruzzini, M., Poli, R., Eds.;
Elsevier Science Ltd.: Amsterdam, 2001.
(4) Holland, P. L. Acc. Chem. Res. 2008, 41, 905.
(5) (a) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte,
R. J. J. Am. Chem. Soc. 2002, 124, 14416. (b) Ding, K.; Pierpont, A. W.;
Brennessel, W. W.; Lukat-Rodgers, G.; Rodgers, K. R.; Cundari, T. R.;
Bill, E.; Holland, P. L. J. Am. Chem. Soc. 2009, 131, 9471.
(6) (a) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2003,
125, 15752. (b) Yu, Y.; Sadique, A. R.; Smith, J. M.; Dugan, T. R.; Cowley,
R. E.; Brennessel, W. W.; Flaschenriem, C. J.; Bill, E.; Cundari, T. R.;
Holland, P. L. J. Am. Chem. Soc. 2008, 130, 6624.
(7) Pfirrmann, S.; Limberg, C.; Ziemer, B. Dalton Trans. 2008, 6689.
(8) The analogous iron hydride complex loses H2 only upon photolysis or the
addition of stronger ligands (see ref 6b).
(9) (a) MacKay, B. A.; Fryzuk, M. D. Chem. ReV. 2004, 104, 385. (b)
Gambarotta, S.; Scott, J. Angew. Chem., Int. Ed. 2004, 43, 5298.
(10) (a) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc. 1998,
120, 11024. (b) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati,
A.; Mason, S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960. (c)
Akagi, F.; Matsuo, T.; Kawaguchi, H. Angew. Chem., Int. Ed. 2007, 46,
8778.
(11) (a) Aresta, M.; Giannoccaro, P.; Rossi, M.; Sacco, A. Inorg. Chim. Acta
1971, 5, 203. (b) Brintzinger, H.; Bercaw, J. E. J. Am. Chem. Soc. 1971,
93, 2045. (c) Brintzinger, H.; Marvich, R. H. J. Am. Chem. Soc. 1971, 93,
2046. (d) de Wolf, J. M.; Blaauw, R.; Meetsma, A.; Teuben, J. H.; Gyepes,
R.; Varga, V.; Mach, K.; Veldman, N.; Spek, A. L. Organometallics 1996,
15, 4977. (e) Chirik, P. J.; Henling, L. M.; Bercaw, J. E. Organometallics
2001, 20, 534. (f) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2003, 125, 2241. (g) Hanna, T. E.; Bernskoetter, W. H.; Bouwkamp,
M. W.; Lobkovsky, E.; Chirik, P. J. Organometallics 2007, 26, 2431. (h)
Avenier, P.; Taoufik, M.; Lesage, A.; Solans-Monfort, X.; Baudouin, A.;
de Mallmann, A.; Veyre, L.; Basset, J. M.; Eisenstein, O.; Emsley, L.;
Quadrelli, E. A. Science 2007, 317, 1056. (i) Scott, J.; Vidyaratne, I.;
Korobkov, I.; Gambarotta, S.; Budzelaar, P. H. M. Inorg. Chem. 2008, 47,
896. (j) Pfirrmann, S.; Limberg, C.; Herwig, C.; Sto¨sser, R.; Ziemer, B.
Angew. Chem., Int. Ed. 2009, 48, 3357.
(12) Schrock’s catalytic N2 reduction can be initiated by a molybdenum hydride
species, indicating that this hydride can somehow form an N2 complex.
See: Yandulov, D. V.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103.
(13) A number of metal-dihydrogen complexes react with N2 to generate N2
complexes. Selected examples: (a) Chaudret, B.; Devillers, J.; Poilblanc,
R. Organometallics 1985, 4, 1727. (b) Hills, A.; Hughes, D. L.; Jimenez-
Tenorio, M.; Leigh, G. J. J. Organomet. Chem. 1990, 391, C41. (c) Hills,
A.; Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. J.; Rowley, A. T.
J. Chem. Soc., Dalton Trans. 1993, 3041. (d) Gilbertson, J. D.; Szymczak,
N. K.; Tyler, D. R. J. Am. Chem. Soc. 2005, 127, 10184.
Solutions of 1 and 2 in aromatic and hydrocarbon solvents show
no signs of decomposition by 1H NMR when heated to 100 °C for
several days under an Ar atmosphere. On the other hand, exposure
of room-temperature solutions of each compound to an atmosphere
of purified N2 in pentane, diethyl ether, or toluene leads to the
growth of resonances in the 1H NMR spectrum that are characteristic
of the analogous bimetallic dinitrogen complexes (Scheme 1). These
cobalt dinitrogen complexes have been characterized separately.5b
The reaction of 1 with N2 was complete in less than 1 h, giving an
88% spectroscopic yield of LCoNNCoL. H2, the other product of
the reaction, was detected by gas chromatography in 80% yield.
The reaction of 2 with N2 over several hours gave K2[LCoNNCoL]
in 90% spectroscopic and isolated yield. In the latter reaction, H2
was detected in 83% yield. The reactions in Scheme 1 occur at
roughly the same rate in the dark as in ambient light.8
The reactions to produce H2 are formal reductive eliminations:
the dicobalt(II) complex 1 leads to the dicobalt(I) dinitrogen
complex LCoNNCoL, and the dicobalt(I) complex 2 gives the
formally dicobalt(0) dinitrogen complex K2[LCoNNCoL]. In the
latter complex, the N-N distance is 1.22 Å, indicating that
the dinitrogen ligand is best described as [NdN]2-. Thus, in this
picture, the electrons from the reductive elimination of H2 end up
in the π* orbital of the bound N2. This is interesting in the context
of N2 binding and activation.9 Although H2 reductive elimination
from metastable hydride complexes has been used previously as a
route to dinitrogen complexes,10-13 there is only one literature
example of N2 binding directly from a crystallographically verified
hydride complex.10c This H2-N2 exchange is of interest in the
context of catalytic N2 reduction because the formation of N2
complexes in this way avoids the use of harsh reducing agents.14
We explored the mechanism of H2 reductive elimination by
treating a mixture of the isotopologues K2[LCo(µ-H)]2 (2) and
(14) See: Fryzuk, M. D. Acc. Chem. Res. 2009, 42, 127. The necessity of strong
reducing agents has prevented catalysis in some systems that perform
stoichiometric N2 reduction cycles. See: Curley, J. J.; Sceats, E. L.;
Cummins, C. C. J. Am. Chem. Soc. 2006, 128, 14036.
JA902812Y
9
J. AM. CHEM. SOC. VOL. 131, NO. 31, 2009 10805