A structurally characterized nitrous oxide complex of vanadium

Article abstract of DOI:10.1021/ja110798w

Nitrous oxide (N₂O), a widespread greenhouse gas, is a thermodynamically potent and environmentally green oxidant that is an attractive target for activation by metal centers. However, N₂O remains underutilized owing to its high kine

Full text of DOI:10.1021/ja110798w

COMMUNICATION
pubs.acs.org/JACS
A Structurally Characterized Nitrous Oxide Complex of Vanadium
Nicholas A. Piro,† Michael F. Lichterman,† W. Hill Harman,†,§ and Christopher J. Chang*,†,‡,§
†Department of Chemistry and ‡Howard Hughes Medical Institute, University of California, Berkeley, California 94720, United States
§Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
S Supporting Information
b
Scheme 1. Synthesis of 1 and Its Reaction with N2O To
Form 2
ABSTRACT: Nitrous oxide (N2O), a widespread green-
house gas, is a thermodynamically potent and environmen-
tally green oxidant that is an attractive target for activation
by metal centers. However, N2O remains underutilized
owing to its high kinetic stability, and the poor ligand
properties of this molecule have made well-characterized
metal-N2O complexes a rarity. We now report a vana-
dium-pyrrolide system that reversibly binds N2O at room
temperature and provide the first single-crystal X-ray struc-
ture of such a complex. Further characterization by vibra-
tional spectroscopy and DFT calculations strongly favor
assignment as a linear, N-bound metal-N2O complex.
reasoned that early-metal analogues would afford more electron-
deficient centers that might stabilize metal-N2O adducts. To this
end, we installed the trispyrrolide ligand [tpa ]
Mes 3- onto vanadium-
(III) by first treating H3tpaMes with 3.05 equiv of n-butyllithium in
Et2O and then adding a solution of the resulting Li3(tpaMes) salt to a
thawing, stirring suspension of VCl3(THF)3 in Et2O (Scheme 1).
This treatment results in a color change to orange-yellow, and after
the mixture is warmed to room temperature and stirred for several
hours, the LiCl is removed by successive extractions with Et2O/n-
pentane mixtures.16 This procedure affords the THF adduct of
(tpaMes)V as an orange powder in 70% yield. A single-crystal X-ray
diffraction study reveals a trigonal bipyramidal coordination environ-
ment for (tpaMes)V(THF) (1), where THF is bound in the apical
pocket supported by the three mesityl arms of the ligand (Supporting
Information, Figure S1). At room temperature, complex 1 shows no
itrous oxide (N2O) is a major greenhouse gas that is 300
times more powerful than CO2 on a per molecule basis1 and
N
remains the number one emission contributing to ozone deple-
tion in the 21st century.2 This molecule is an appealing oxidant
owing to its thermodynamic potency and environmentally
friendly nature, as oxygen atom transfer (OAT) releases N2 as
the sole byproduct; however, efforts to utilize N2O as an oxygen-
transfer oxidant are hindered by its high kinetic stability.3 Select
transition and f-block metal complexes have been shown to
activate N2O toward OAT4-10 or to insert N2O into metal-
ligand bonds11 and, in one case, to cleave the N-N bond.12 But
while phosphine-activated N2O adducts have been captured with
various Lewis acids,13 well-defined metal-N2O complexes re-
main exceedingly rare owing to the poor ligand characteristics of
this small molecule, including its low dipole moment and weak σ-
donor and π-acceptor properties.3b Indeed, since Armor and
Taube's initial report of the first metal-N2O complex,14 to our
knowledge only one other discrete metal-N2O compound has
been reported,15 and neither species has been characterized by
single-crystal X-ray diffraction. We now report a three-fold sym-
metric vanadium-pyrrolide platform that reversibly binds N2O
at room temperature and the single-crystal X-ray structure of
this well-defined vanadium-N2O complex. Further studies sup-
port that N2O binds this vanadium system in a linear, N-bound
fashion.
1
distinguishable H NMR signals and displays a μeff of 2.89(7) μB
(Evans's method)17 that is consistent with an S = 1 ground state,
where the two unpaired electrons are expected to reside in the
nonbonding, degenerate dxz and dyz orbitals in the C3 field. The
observation that THF binds to the metal center in 1is noteworthy as
it suggests a high degree of Lewis acidity at vanadium; in contrast,
trigonal monopyramidal vanadium complexes of the type V[(N-
(R)CH2CH2)3N], bearing more electron-donating trisamidoamine
ligands, do not appreciably coordinate THF.6a
Addition of 1 atm of N2O to an Et2O solution of 1 triggers a
prompt color change from the orange color of (tpaMes)V(THF) to
yellow, along with precipitation of a yellow powder. This solid is
isolated by filtration, and analysis by IR spectroscopy reveals a strong
absorption at 2289 cm-1, indicative of an NNO stretch (Figure 1).
The new species is NMR silent, and measurement of its suscept-
ibility by SQUID magnetometry confirmed that an S = 1 ground
state is maintained. Accordingly, we assign this complex as the
monometallic N2O adduct (tpaMes)V(N2O) (2, Scheme 1).
Our strategy for promoting N2O binding to a single metal site
involves creating a Lewis acidic center with π-backbonding
ability but only mild reducing character. In this context, we have
been exploring complexes bearing pyrrolide ligands, which have
relatively low basicity and poor π-donating abilities as compared
to alkyl and amido congeners. Previous work from our laboratory
described the activation of N2O using Fe(II) centers supported
by substituted tris(pyrrolylmethyl)amine (tpa) ligands,6e and we
Received: December 1, 2010
Published: February 2, 2011
r
2011 American Chemical Society
2108
dx.doi.org/10.1021/ja110798w J. Am. Chem. Soc. 2011, 133, 2108–2111
|

Journal of the American Chemical Society
COMMUNICATION
In initial attempts to preparatively synthesize the vanadium-
N2O complex 2, we noticed that application of vacuum to
solutions of 2 returned the orange color of 1. To further probe
the potential reversibility of N2O binding to the (tpaMes)V
platform, we turned to in situ solution IR spectroscopy measure-
ments. Upon introduction of N2O to a toluene solution of 1, we
observed a distinct IR absorption at 2295 cm-1; when vacuum
was applied to this solution, this absorption, along with the
absorption for dissolved N2O, diminished. Reintroduction of
N2O regenerated the peak associated with the formation of 2,
and reapplication of vacuum again caused this absorption to
vanish (Supporting Information, Figure S2). In contrast, when
solid samples of 2 were subjected to vacuum for ca. 18 h, the N2O
absorption at 2289 cm-1 remained prominent. These experi-
ments establish that N2O binding to the (tpaMes)V system in
solution is reversible at room temperature.
To provide additional support for the assignment of 2 as a
metal-N2O complex, we conducted a more detailed vibrational
analysis. Specifically, wesynthesized theisotopicallylabeled15N2O
complex (tpaMes)V(15N2O) (2-15N) in a manner similar to that
used for the unlabeled species and acquired IR spectra on solid
samples of both 2 and 2-15N. Upon 15N incorporation, the ν3
vibration of the N2O ligand in 2 shifts from 2289 to 2217 cm-1
(Figure 1). Comparison of the two sets of data also allowed us to
assign the ν1 vibration of the NNO oscillator to an absorption at
1315 cm-1 in 2 that shifts to 1297 cm-1 for 2-15N (Supporting
Information, Figure S3). The magnitudes of the isotopic shifts
establish that the higher energy band is predominantly N-N in
character, whereas the lower energy band is largely N-O in
character. The energiesoftheN2O modesfor thesecomplexes and
for free N2O are collected inTable 1. Interestingly, weobserve that
both the ν1 and ν3 vibrations of N2O shift to higher energies upon
metal binding, which is counter to expectations that π-back-
bonding might reduce the energy of at least one of these modes,
as seen for Taube’s [(H3N)5Ru(N2O)]2þ complex.14 However,
N2O adsorbed onto zeolites is reported to raise the energy of both
modes when the molecule binds through its nitrogen atom.18 In
addition, the ν2 N2O bending mode was also identified from the
labeling study, and its energies are listed in Table 1.
We next confirmed that 2 is a well-defined metal-N2O
complex by single-crystal X-ray analysis. Single crystals of 2
suitable for X-ray diffraction were grown under an atmosphere of
N2O by vapor diffusion of diisopropyl ether into a toluene
solution of 2. The structure of the (tpaMes)V(N2O) complex 2
confirms that the NNO ligand coordinates in a linear fashion
within the apical pocket of the (tpaMes)V platform (Figure 2a).
Lehnert and Bottomley both refer to the linear coordination
mode of the NNO ligand as a signature of an N-bound isomer.19
We also assign the coordination mode of N2O as N-bound to the
metal on the basis of the observed interatomic distances and
supporting DFT calculations (vide infra). Moreover, as the IR
data do not reflect marked activation of the N2O molecule, the
N-N linkage is expected to be the shorter of the two interatomic
Figure 1. IR spectra of 1, 2, and 2-15N reveal the prominent NNO
stretch of 2 that shifts upon 15N-labeling (an asterisk indicates the
position of residual benzene).
Table 1. Comparison of Experimental and Calculated Struc-
tures and Vibrational Energiesa for 2 and Free N2O
2 (exptl)
2 (calcd)
N2O (exptl)
r(V-N)/Å
r(N-N)/Å
r(N-O)/Å
—(V-N-N)/ꢀ
ν1/cm-1
ν3/cm-1
ν2/cm-1
2.139(1)
2.14
—
1.119(2)
1.13
1.128
1.187(2)
1.18
1.184
176.7(1)
179.8
—
1315 (1297)b
2289 (2217)b
551 (534)b
1313 (1297)b
2335 (2261)b
536 (520)b
1286 (1266)b,c
2224 (2156)b,c
589 (572)b,c
a All vibrational energies output from calculations have been scaled by a
factor of 0.96. b Values in parentheses are those for the 15N2O iso-
topomer c Values are taken from ref 21.
Figure 2. (a) Single-crystal X-ray structure of 2 with thermal ellipsoids plotted at 50% and hydrogen atoms omitted for clarity. Selected bond lengths
(Å) and angles (ꢀ): V1-N11, 2.1389(10); N11-N12, 1.1191(16); N12-O13, 1.1869(17); V1-N4, 2.1165(10); V1-N11-N12, 176.73(11).
(b) One of the two degenerate SOMOs calculated for 2, plotted at an isovalue of 0.02. (c) A d-orbital splitting diagram for the three-fold symmetric
(tpaMes)V platform, showing the half-filled, π-backbonding dxz/dyz orbitals in the C3 field.
2109
dx.doi.org/10.1021/ja110798w |J. Am. Chem. Soc. 2011, 133, 2108–2111

Journal of the American Chemical Society
COMMUNICATION
distances, as observed in free N2O. The two other reported
examples of well-characterized monometallic N2O complexes,
both on ruthenium, have been assigned as N-bound isomers.15,19
The N-bound nature of the ligand and the steric protection of
the ligand “pickets” are likely contributing factors in preventing
vanadium(V)-oxo formation.
support. Computational facilities used in this study were funded
in part by the National Science Foundation grant CHE-0840505.
We thank Joe Zadrozny and Prof. Jeff Long for the acquisition of
SQUID data, and Profs. F. Dean Toste and Robert G. Bergman
for use of their groups’ in situ IR spectrophotometer.
As a further probe into the binding mode of N2O in complex 2,
we carried out DFT calculations using the crystallographically
determined structure as a basis for geometry optimization.20 When
the N-bound isomer was utilized as a starting point, the structure
converged on a geometry that agreed well with the input (Table 1).
However, when the binding of N2O to the vanadium center was
switched to O-bound in silico, the N2O ligand took on a significant
bend at the oxygen atom (144ꢀ), and the bond lengths converged
on a structure in which the N-O and N-N distances again
resemble those in free N2O and are not consistent with the observed
crystallographic data (Supporting Information, Table S2).
The calculated vibrational frequencies for each isomer alone are
inconclusive, but when free N2O is calculated using the same
computational method, the picture becomes clearer. The differences
between the experimental and calculated values for free N2O are
nearly identical to the differences between the experimental data on 2
and those calculated for the N-bound isomer (Table S2). Therefore,
we assign these deviations to a systematic error in the calculation and
continue to favor an N-bound assignment based on all available
experimental and computational data. Finally, from the DFT calcula-
tions, we confirmed the electronic structure of 2 and visualized the
critical bonding orbitals. In the three-fold symmetric field provided by
the (tpaMes)V platform, the dxz and dyz orbitals of vanadium hold the
two unpaired electrons of 2, which backbond weakly into the
degenerate π* orbitals of the N2O ligand (Figure 2b,c). The lone
pair of N2O contributes to the bonding combination of all nitrogen
lone pairs around the metal into dz2 that falls as the HOMO-4. We
suggest that the three-fold symmetric environment provides a key
factor in stabilizing the putative d2 metal-N2O adduct, as distortions
to lower symmetries will weaken these π-accepting interactions.
In conclusion, we have presented the synthesis of a unique
vanadium complex that reversibly binds N2O at room temperature,
the single-crystal X-ray structure of this well-defined monometallic
N2O complex, and vibrational and DFT studies to support its assign-
ment as an N-bound metal-N2O adduct. Efforts to use (tpaMes)-
V(N2O) and related species as a source of activated N2O in stoich-
iometric and catalytic oxidation reactions are currently underway.
’ REFERENCES
(1) Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M.,
Averyt, K., Tignor, M. M. B., Miller, H. L., Eds. Contribution of Working
Group I to the Fourth Assessment Report of the Intergovernmental Panel on
Climate Change, 2007; Cambridge University Press: Cambridge, UK,
2007.
(2) Ravishankara, A. R.; Daniel, J. S.; Portmann, R. W. Science 2009,
326, 123.
(3) (a) Leont’ev, A.; Fomicheva, O.; Proskurnina, M.; Zefirov, N.
Russ. Chem. Rev. 2001, 70, 91. (b) Tolman, W. B. Angew. Chem., Int. Ed.
2010, 49, 1018.
(4) (a) Bottomley, F.; Brintzinger, H. H. J. Chem. Soc., Chem.
Commun. 1978, 234. (b) Bottomley, F.; Lin, I.; Mukaida, M. J. Am.
Chem. Soc. 1980, 102, 5238. (c) Bottomley, F.; Paez, D. E.; White, P. S. J.
Am. Chem. Soc. 1981, 103, 5581. (d) Bottomley, F.; Paez, D. E.; White,
P. S. J. Am. Chem. Soc. 1982, 104, 5651.
(5) (a) Whited, M. T.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130,
16476. (b) Harrold, N. D.; Waterman, R.; Hillhouse, G. L.; Cundari,
T. R. J. Am. Chem. Soc. 2009, 131, 12872.
(6) (a) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Inorg. Chem.
1994, 33, 1448. (b) Baranger, A. M.; Hanna, T. A.; Bergman, R. J. Am.
Chem. Soc. 1995, 117, 10041. (c) Dionne, M.; Jubb, J.; Jenkins, H.;
Wong, S.; Gambarotta, S. Inorg. Chem. 1996, 35, 1874. (d) Figueroa,
J. S.; Cummins, C. C. J. Am. Chem. Soc. 2003, 125, 4020. (e) Harman,
W. H.; Chang, C. J. J. Am. Chem. Soc. 2007, 129, 15128. (f) Andino, J. G.;
Kilgore, U. J.; Pink, M.; Ozarowski, A.; Krzystek, J.; Telser, J.; Baik, M.-
H.; Mindiola, D. J. Chem. Sci. 2010, 1, 351.
(7) (a) Evans, W.; Grate, J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J.
Am. Chem. Soc. 1985, 107, 405. (b) Berg, D.; Burns, C.; Andersen, R.;
Zalkin, A. Organometallics 1989, 8, 1865. (c) Smith, M., III; Matsunaga,
P.; Andersen, R. J. Am. Chem. Soc. 1993, 115, 7049. (d) Howard, W.;
Trnka, T.; Waters, M.; Parkin, G. J. Organomet. Chem. 1997, 528, 95. (e)
McNeill, K.; Bergman, R. J. Am. Chem. Soc. 1999, 121, 8260.
(8) (a) Matsunaga, P.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem.
Soc. 1993, 115, 2075. (b) Koo, K.; Hillhouse, G.; Rheingold, A.
Organometallics 1995, 14, 456.
(9) (a) Bleeke, J.; Behm, R. J. Am. Chem. Soc. 1997, 119, 8503. (b)
Kaplan, A.; Bergman, R. Organometallics 1998, 17, 5072. (c) Lee, J.; Pink,
M.; Tomaszewski, J.; Fan, H.; Caulton, K. J. Am. Chem. Soc. 2007, 129,
8706. (d) Bar-Nahum, I.; Gupta, A.; Huber, S.; Ertem, M.; Cramer, C.;
Tolman, W. J. Am. Chem. Soc. 2009, 131, 2812.
(10) (a) Yamamoto, A.; Kitazume, S.; Pu, L.; Ikeda, S. J. Am. Chem.
Soc. 1971, 93, 371. (b) Groves, J.; Roman, J. J. Am. Chem. Soc. 1995, 117,
5594.
(11) (a) Vaughan, G.; Rupert, P.; Hillhouse, G. J. Am. Chem. Soc.
1987, 109, 5538. (b) Vaughan, G. A.; Sofield, C. D.; Hillhouse, G. L.;
Rheingold, A. L. J. Am. Chem. Soc. 1989, 111, 5491. (c) Demir, S.;
Montalvo, E.; Ziller, J. W.; Meyer, G.; Evans, W. J. Organometallics 2010,
29, 6608.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, details
b
on DFT calculations, supporting figures, and complete ref 20;
X-ray structure information in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(12) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.;
Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999.
Corresponding Author
chrischang@berkeley.edu
(13) (a) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am. Chem. Soc.
2009, 131, 9918. (b) Neu, R. C.; Otten, E.; Stephan, D. W. Angew. Chem.,
Int. Ed. 2009, 48, 9709. (c) Neu, R. C.; Otten, E.; Lough, A. J.; Stephan,
D. W. Chem. Sci. 2011, 2, 170.
(14) Armor, J. N.; Taube, H. J. Am. Chem. Soc. 1969, 91, 6874.
(15) Pamplin, C.; Ma, E.; Safari, N.; Rettig, S.; James, B. J. Am. Chem.
Soc. 2001, 123, 8596.
(16) Incomplete removal of LiCl can result in isolation of the red
complex [Li(solv)n][(tpaMes)VCl], which has been identified by X-ray
diffraction.
’ ACKNOWLEDGMENT
This work was supported by DOE/LBNL Grant 403801 and
the Packard Foundation. C.J.C. is an Investigator with the
Howard Hughes Medical Institute. We thank the Miller Institute
for Basic Research (N.A.P.) and Arkema (W.H.H.) for fellowship
2110
dx.doi.org/10.1021/ja110798w |J. Am. Chem. Soc. 2011, 133, 2108–2111

Journal of the American Chemical Society
COMMUNICATION
(17) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Sur, S. K. J. Magn.
Reson. 1989, 82, 169.
(18) Forster, H.; Remmert, M. J. Mol. Struct. 1988, 174, 357.
(19) (a) Paulat, F.; Kuschel, T.; Nather, C.; Praneeth, V.; Sander, O.;
Lehnert, N. Inorg. Chem. 2004, 43, 6979. (b) Bottomley, F.; Brooks,
W. V. F. Inorg. Chem. 1977, 16, 501.
(20) Frisch, M. J.; et al. Gaussian 09, Revision B.1; Gaussian, Inc.:
Wallingford, CT, 2009.
(21) Begun, G. M.; Fletcher, W. J. Chem. Phys. 1958, 28, 414.
2111
dx.doi.org/10.1021/ja110798w |J. Am. Chem. Soc. 2011, 133, 2108–2111