A monomeric nickel-dioxygen adduct derived from a nickel(I) complex and O2

Article abstract of DOI:10.1021/ic049876n

The nickel(I) complex [PhTt^Ad]Ni(CO) (PhTt^Ad, phenyltris((1-adamantylthio)methyl)borate) reacts with O₂ generating a 1:1 species identified as a side-on dioxygen adduct based on its spectroscopic properties as supported by

Full text of DOI:10.1021/ic049876n

Inorg. Chem. 2004, 43, 3324−3326
A Monomeric Nickel−Dioxygen Adduct Derived from a Nickel(I) Complex
and O
2
Koyu Fujita,† Ralph Schenker,‡ Weiwei Gu,§ Thomas C. Brunold,‡ Stephen P. Cramer,§ and
,†
Charles G. Riordan*
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716,
Department of Chemistry, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706, and
Department of Applied Science, UniVersity of CaliforniasDaVis, DaVis, California 95616
Received January 30, 2004
The nickel(I) complex [PhTtAd]Ni(CO) (PhTtAd, phenyltris((1-ada-
complex [PhTttBu]Ni(CO) forms the purple dimer [(PhTttBu)-
Ni]2(µ-O)2, upon low-temperature exposure to O2.5,6 Analo-
gous bis-µ-oxo Ni2O2 cores are accessible from Ni(II) precur-
sors using H2O2 as the oxidant.7 To explore the mechanism
by which the bis-µ-oxo dimer is generated from O2 and to
prepare intermediates that are more reactive toward exog-
enous substrates, we sought to redesign the [PhTttBu] ligand
so as to access a Ni(I) complex that would sterically preclude
bis-µ-oxo dimer formation.8 Herein, we report on the
successful pursuit of this strategy which led to the discovery
of a side-on dioxygen complex of nickel, shown to be an
intermediate to bis-µ-oxo dimer formation.
mantylthio)methyl)borate) reacts with O generating a 1:1 species
2
identified as a side-on dioxygen adduct based on its spectroscopic
properties as supported by DFT computational results and by its
reactivity. The Ni EXAFS data are fit to a S O coordination
3
2
environment with short Ni−O distances, 1.85 Å. The brown complex
displays a rhombic EPR signal with g values of 2.24, 2.19, 2.01.
DFT and INDO/S-CI computations replicate the EXAFS and EPR
features and suggest that 2 is a side-on [NiO ]+ complex with
2
geometric and electronic properties that are best rationalized in
terms of a highly covalent Ni(II)−superoxo description. [PhTtAd]-
Ni(O ) oxidizes PPh3 to OPPh3, NOto NO -, and [PhTttBu]Ni(CO)
Exposure of toluene or THF solutions of [PhTtAd]Ni(CO),
1,8 to dioxygen produced a thermally sensitive, brown
intermediate [PhTtAd]Ni(O2), 2, that formed with kobs ) 8.4(6)
× 10-3 s-1 (toluene, -70 °C), Scheme 1. The composition
of 2 was deduced by its unique spectroscopic features9 and
supported by its reactivity. The optical spectrum of 2 in THF
exhibits bands at λmax (ꢀ [M-1 cm-1]) 310 (5900), 386 (2900),
450 (2500), and 845 (350) nm. In the UV region the spectrum
is strikingly similar to those of Co10 and Cu11,12 side-on
dioxygen complexes. In contrast, [(PhTttBu)Ni]2(µ-O)2 dis-
2
3
to the nonsymmetric [PhTtAd]Ni(µ-O)2Ni[PhTttBu] dimer.
Nickel-dioxygen intermediates have been invoked in a
number of stoichiometric and catalytic reactions. For ex-
ample, nickel(II) azamacrocycles activate O2 yielding prod-
ucts resulting from aromatic1 and aliphatic2 (ligand-based)
C-H activation. Burrows has deployed related complexes
as oxidative probes of the structure of proteins and DNA,
albeit with more reactive oxidants, i.e., H2O2 and KHSO5.3
While metal-bound reduced dioxygen species have been
suggested as the active reagents for these transformations,
thorough characterization of the corresponding intermediates
is generally lacking.4 Recently, we reported that the Ni(I)
(4) A noteworthy exception is the side-on Ni(II)-peroxo complex,
(RNC)2Ni(O2), accessed via the Ni(0)/Ni(II) couple: Otsuka, S.;
Nakamura, A.; Tatsuno, Y. J. Am. Chem. Soc. 1969, 91, 6994-6999.
(5) [PhTttBu], phenyltris((tert-butylthio)methyl)borate. Mandimutsira, B.
S.; Yamarik, J. L.; Brunold, T. C.; Gu, W.; Cramer, S. P.; Riordan,
C. G. J. Am. Chem. Soc. 2001, 123, 9194-9195.
(6) Schenker, R.; Mandimutsira, B. S.; Riordan, C. G.; Brunold, T. C. J.
Am. Chem. Soc. 2002, 124, 13842-13855.
* Author to whom correspondence should be addressed. E-mail:
riordan@udel.edu.
(7) (a) Hikichi, S.; Yoshizawa, M.; Sasakura, Y.; Akita, M.; Moro-oka,
Y. J. Am. Chem. Soc. 1998, 120, 10567-10568. (b) Itoh, S.; Bandoh,
H.; Nagatomo, S.; Kitagawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 1999,
121, 8945-8946. (c) Shiren, K.; Ogo, S.; Fujinami, S.; Hayashi, H.;
Suzuki, M.; Uehara, A.; Watanabe, Y.; Moro-oka, Y. J. Am. Chem.
Soc. 2000, 122, 254-262. (d) Hikichi, S.; Yoshizawa, M.; Sasakura,
Y.; Komatsuzaki, H.; Moro-oka, Y.; Akita, M. Chem. Eur. J. 2001,
7, 5011-5028. (e) Itoh, S.; Bandoh, H.; Nakagawa, M.; Nagatomo,
S.; Kitagawa, T.; Karlin, K. D.; Fukuzumi, S. J. Am. Chem. Soc. 2001,
123, 11168-11178.
† University of Delaware.
‡ University of WisconsinsMadison.
§ University of CaliforniasDavis.
(1) (a) Kimura, E.; Sakonaka, A.; Machida, R. J. Am. Chem. Soc. 1982,
104, 4255-4257. (b) Kimura, E.; Machida, R. J. Chem. Soc., Chem.
Commun. 1984, 499-500. (c) Kushi, Y.; Machida, R.; Kimura, E. J.
Chem. Soc., Chem. Commun. 1985, 216-218.
(2) (a) Chen, D.; Martell, A. E. J. Am. Chem. Soc. 1990, 112, 9411-
9412. (b) Cheng, C.-C.; Gulia, J.; Rokita, S. E.; Burrows, C. J. J.
Mol. Catal. A. 1996, 113, 379-391. Berkessel, A.; Bats, J. W.;
Schwarz, C. Angew. Chem., Int. Ed. Engl. 1990, 29, 106-108.
(3) (a) Muller, J. G.; Hickerson, R. P.; Perez, R. J.; Burrows, C. J. J. Am.
Chem. Soc. 1997, 119, 1501-1506. (b) Burrows, C. J.; Perez, R. J.;
Muller, J. G.; Rokita, S. E. Pure Appl. Chem. 1998, 70, 275-278.
(8) [PhTtAd], phenyltris((1-adamantylthio)methyl)borate. Fujita, K.; Rhei-
ngold, A. L.; Riordan, C. G. J. Chem. Soc., Dalton Trans. 2003, 2004-
2008.
(9) See Supporting Information for full spectroscopic and computational
details.
3324 Inorganic Chemistry, Vol. 43, No. 11, 2004
10.1021/ic049876n CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/23/2004

COMMUNICATION
Scheme 1
Figure 1. Fourier transform of EXAFS data (solid black line) and
simulation (dashed blue line) for 2 (k, 2-16.5 Å-1). Simulation: 3 S at
2.26 Å (σ2, 0.03) and 2 O at 1.85 Å (0.03).
is limited, the data are fit much better assuming two O
ligands rather than one, consistent with a side-on coordination
of the dioxygen ligand. Side-on vs end-on O2 coordination
could in principle be distinguished spectroscopically via
vibrational analysis of the 16O18O isotopomer.10,11,16 However,
resonance Raman spectroscopic experiments on 2 have failed
to identify oxygen-isotope sensitive bands, presumably due
to the low intensity of the optical features associated with
the [NiO2]+ unit.
plays intense features at 410 nm (ꢀ ) 6000 M-1 cm-1 per
Ni) and 565 nm (8000), the latter ascribed to a OfNi charge
transfer (CT) transition.6 The electrospray ionization mass
spectrum of 2 reveals a positive ion cluster at m/z of 705.
The isotope distribution pattern agrees with that calculated
for a species of empirical formula [PhTtAd]Ni(O)+. Further-
more, the signal shifts to higher mass by two units for
samples of 2 derived from 18O2. The isotope-sensitive ion
signals confirm that oxygen is a constituent of the complex,
although there is no indication of the parent ion in the mass
spectrum.
To further corroborate the structural assignment of 2, a
computational analysis of hypothetical side-on and end-on
O2 complexes (possessing a suitably truncated derivative of
the PhTtAd ligand)9 was undertaken. The two models were
energy minimized by DFT using the B3LYP hybrid func-
tional,17 and for the optimized structures the g values were
obtained from semiempirical INDO/S-CI computations as
implemented in ORCA.9,18 These calculations yielded prin-
cipal g values of 2.29, 2.22, and 2.09 for the side-on O2
complex and 2.44, 2.33, and 2.21 for the end-on complex.
While the former values are consistent with the EPR data,
the latter are considerably too high, arguing against the
presence of an end-on bound O2 moiety in 2. In further
support of a side-on O2 description of 2, calculated 17O
hyperfine coupling constants for this model indicate very
little unpaired spin density on the O2 ligand, which concurs
nicely with the observation that the EPR spectrum of 2
prepared with 17O2 shows no resolvable coupling.9 The side-
on structure is square pyramidal, high-spin Ni(II) with two
sulfurs and two oxygens in the basal plane and the remaining
sulfur in the apical position (Figure 2a, left). The calculated
average Ni-S and Ni-O bond distances of 2.33 and 1.87
Å, respectively, are in good agreement with those determined
by EXAFS spectroscopy, while the computed O-O bond
length of 1.38 Å appears characteristic of a peroxo spe-
1
2 displays paramagnetically shifted H NMR spectral
1
resonances and an S ) /2 EPR spectrum at 4.2 K.9 The
rhombic signal with g values of 2.24, 2.19, and 2.01 supports
1
2
the monomeric formulation and is indicative of a (dz )
electron configuration typically observed for Ni(III) species.13
The Ni coordination sphere has been characterized by Ni K
edge X-ray absorption spectroscopy (XAS). A preedge 1s
f 3d transition occurs at 8331.4 eV, a value 1 eV greater
than that for the Ni(I) precursor, 1, but 1 eV lower than
observed in the Ni(III)2 dimer, [(PhTttBu)Ni]2(µ-O)2, thus
suggesting a Ni(II) oxidation state.5 The Ni EXAFS data
corroborate the monomeric nature of 2, Figure 1. Specifically,
there is no indication of a short M- - -M vector as observed
for [(PhTttBu)Ni]2(µ-O)2 and deemed diagnostic of bis-µ-oxo
rhombs.14 The Ni ligation consists of three sulfur atoms at
Ni-Sav, 2.26 Å, and two oxygen atoms at Ni-Oav, 1.85 Å.
The latter value is in the range found by X-ray diffraction
for M-O distances in side-on O2 complexes.10,12,15 Although
the accuracy of coordination number assignment by EXAFS
(10) Egan, J. W., Jr.; Haggerty, B. S.; Rheingold, A. L.; Sendlinger, S. C.;
Theopold, K. H. J. Am. Chem. Soc. 1990, 112, 2445-2446.
(11) Fujisawa, K.; Tanaka, M.; Moro-oka, Y.; Kitajima, N. J. Am. Chem.
Soc. 1994, 116, 12079-12080.
(12) Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.; Holland, P. L.;
Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108-2109.
(13) Spectral characteristics of [PhTttBu]Ni(O2): UV-vis (toluene) 310 nm,
390 nm, 450 nm; EPR (toluene, 20 K) g values 2.24, 2.20, 2.02.
(14) Que, L.; Tolman, W. B. Angew. Chem., Int. Ed. 2002, 41, 1114-
1137.
(15) (a) Kitajima, N.; Komatsuzaki, H.; Hikichi, S.; Osawa, M.; Moro-
oka, Y. J. Am. Chem. Soc. 1994, 116, 11596-11597. (b) Cramer, C.
J.; Tolman, W. B.; Theopold, K. H.; Rheingold, A. L. Proc. Natl.
Acad. Sci. U.S.A. 2003, 100, 3635-3640.
(16) (a) Kurtz, D. M.; Shriver, D. F.; Klotz, I. M. J. Am. Chem. Soc. 1976,
98, 5033-5035. (b) Pate, J. E.; Cruse, R. W.; Karlin, K. D.; Solomon,
E. I. J. Am. Chem. Soc. 1987, 109, 2624-2630. (c) Chaudhuri, P.;
Hess, M.; Weyhermu¨ller, T.; Wieghardt, K. Angew. Chem., Int. Ed.
1999, 38, 1095-1098.
(17) The minimized energy for the side-on complex is 10.0 kcal/mol lower
than that for the end-on complex.
(18) Neese, F.; Max Planck Institute for Radiation Chemistry, Mu¨lheim/
Ruhr, Germany; neese@mpi-muelheim.mpg.de. For information re-
garding the accuracy of g values computed by the INDO/S-CI method,
see: (a) Neese, F. Curr. Opin. Chem. Biol. 2003, 7, 125-135. (b)
Neese, F.; Solomon, E. I. Inorg. Chem. 1998, 37, 6568-6582. (c)
Neese, F.; Zaleski, J. M.; Loeb-Zaleski, K. E.; Solomon, E. I. J. Am.
Chem. Soc. 2000, 122, 11703-11724.
Inorganic Chemistry, Vol. 43, No. 11, 2004 3325

COMMUNICATION
Preliminary exploration indicates the reactivity of 2 to be
rich, Scheme 1. Unlike [(PhTttBu)Ni]2(µ-O)2, 2 transfers an
equivalent of oxygen to PPh3, generating OPPh3 quantita-
tively; 18O labeling confirms O2 as the source. The electro-
philic character of 2 is reminiscent of reactivity observed
for a Ni(II)-peroxo adduct.22 2 oxidizes nitric oxide to
nitrate forming [PhTtAd]Ni(NO3) (the nitrite complex,
[PhTtAd]Ni(NO2) is a minor (15%) product; 1H NMR spectral
quantitation and comparison with authentic samples). More
interestingly, 2 reacts with the smaller Ni(I) precursor,
[PhTttBu]Ni(CO), forming the corresponding purple bis-µ-
oxo dimer, 3 (85% yield), with different borato ligands on
each Ni.23 Also, oxygenation of [PhTttBu]Ni(CO) proceeds
initially to an intermediate with optical and EPR spectral
parameters similar those observed for 2.13 The short-lived
[PhTttBu]Ni(O2) decays, yielding [(PhTttBu)Ni]2(µ-O)2. These
observations indicate that 2 and the related side-on O2
complex, [PhTttBu]Ni(O2), are bona fide intermediates in bis-
µ-oxo Ni(III)2 dimer formation.
Figure 2. (a) Geometric structure (left) and total unpaired spin density
distribution (right). Unpaired spin densities on individual atoms are indicated.
The coordinate system is defined by the g matrix orientation as obtained
from INDO/S-CI computations. (b) Key natural orbitals of 2 as obtained
from spin-unrestricted DFT computations.
cies.19,20 However, as DFT computations on peroxo species
typically yield O-O bond lengths greater than 1.4 Å,15b this
result actually agrees well with the XAS data, also indicating
that 2 possesses substantial Ni(II)-superoxo character.
Nonetheless, the DFT-computed total unpaired spin density
for 2 is predominantly located on the Ni center (Figure 2a,
In summary, spectroscopic analysis of the thermally
sensitive brown intermediate, 2, produced from dioxygen
supports the structural assignment of a side-on dioxygen
adduct best described as a Ni(II)-superoxo complex in which
the superoxide radical is antiferromagnetically coupled to
the high-spin Ni(II) center to yield an S ) 1/2 EPR spectrum
right). These findings suggest an electronic structure in which
-
2
2
the unpaired electron in the Ni dx -y orbital and the O2
radical are strongly antiferromagnetically coupled, consistent
with the fact that the HOMO (Figure 2b) possesses nearly
2
characteristic of a dz ground state electronic configuration.
Electronic structure calculations provide strong support for
the side-on coordination mode as the calculated EPR g values
and metric parameters agree well with those determined
experimentally.9 The reactivity of 2 points to an O2 adduct
that is competent to transfer O2 to NO and an O atom to
PPh3. 2 is an intermediate in bis-µ-oxo Ni(III)2 dimer
generation provided the sulfur substituents of its Ni(I)
reaction partner are sufficiently small so as to allow for close
approach of two metals.
2
2
identical contributions from the Ni dx -y and the in-plane
O2 π* orbitals. The remaining single unpaired electron
resides primarily in the Ni dz orbital (SOMO, Figure 2b),
-
2
which explains the general features of the EPR spectrum and
the lack of sizable hyperfine broadening in samples of 2
prepared with 17O2. A similar model has been invoked to
explain the EPR properties of {FeNO}7 systems, for which
detailed studies by Solomon and co-workers revealed that
3
the observed S )
/ ground state arises from strong
2
Acknowledgment. We thank the National Science Foun-
dation (C.G.R.), the University of Wisconsin and the Sloan
Research Foundation (T.C.B.), and the Swiss National
Science Foundation (R.S.) for financial support and Dr. Frank
Neese (MPI Mu¨lheim), Profs. Brian Hoffman (Northwest-
ern), and Ed Solomon (Stanford) for useful discussions.
antiferromagnetic coupling between the Fe(III) center (S )
5/2) and NO- (S ) 1).21 Together, the spectroscopic and
computational results suggest that 2 is a side-on [NiO2]+
complex with geometric and electronic properties that are
consistent with a formal Ni(II)-superoxo description.19,20
Supporting Information Available: Synthetic procedures along
with computational and characterization data (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(19) Gubelmann, M. H.; Williams, A. F. Struct. Bonding (Berlin) 1983,
55, 1.
(20) Alternatively, the computed O-O distance of 1.29 Å for the end-on
model of 2 indicates that this isomer has predominantly Ni(II)-
superoxo character, consistent with the DFT calculated spin densities
for that model (Table 3S).
IC049876N
(21) Zhang, Y.; Pavlosky, M. A.; Brown, C. A.; Westre, T. E.; Hedman,
B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1992, 114,
9189-9191. Brown, C. A.; Pavlosky, M. A.; Westre, T. E.; Zhang,
Y.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc.
1995, 117, 715-732.
(22) Otsuka, S.; Nakamura, A.; Tatsuno, Y.; Miki, M. J. Am. Chem. Soc.
1972, 94, 3761-3767.
(23) A similar strategy yielded the LCu(µ-O)2CuL′ complexes: Aboelella,
N. W.; Lewis, E. A.; Reynolds, A. M.; Brennessel, W. W.; Cramer,
C. J.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 10660-10661.
3326 Inorganic Chemistry, Vol. 43, No. 11, 2004