Probing 'spin-forbidden' oxygen-atom transfer: Gas-phase reactions of chromium-porphyrin complexes

Article abstract of DOI:10.1021/ja9103638

Oxygen-atom transfer reactions of metalloporphyrin species play an important role in biochemical and synthetic oxidation reactions. An emerging theme in this chemistry is that spin-state changes can play important roles, and a 'two-state' reactivity model

Full text of DOI:10.1021/ja9103638

Published on Web 03/10/2010
Probing ‘Spin-Forbidden’ Oxygen-Atom Transfer: Gas-Phase
Reactions of Chromium-Porphyrin Complexes
Maria Elisa Crestoni,*,† Simonetta Fornarini,† Francesco Lanucara,†
Jeffrey J. Warren,‡ and James M. Mayer*,‡
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente AttiVe, UniVersita`
di Roma “La Sapienza”, P.le A. Moro 5, I-00185 Roma, Italy; and Department of Chemistry,
UniVersity of Washington, Box 351700, Seattle, Washington 98107-1700
Received December 8, 2009; E-mail: mariaelisa.crestoni@uniroma1.it; mayer@chem.washington.edu
Abstract: Oxygen-atom transfer reactions of metalloporphyrin species play an important role in biochemical
and synthetic oxidation reactions. An emerging theme in this chemistry is that spin-state changes can play
important roles, and a ‘two-state’ reactivity model has been extensively applied especially in iron porphyrin
systems. Herein we explore the gas-phase oxygen-atom transfer chemistry of meso-tetrakis(pentafluo-
rophenyl)porphyrin (TPFPP) chromium complexes, as well as some other tetradentate macrocyclic ligands.
Electrospray ionization in concert with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry
has been used to characterize and observe reactivity of the ionic species [(TPFPP)CrIII]+ (1) and
[(TPFPP)CrVO]+ (2). These are attractive systems to examine the effects of spin-state change on oxygen-
atom transfer because the d1 CrV species are doublets, while the CrIII complexes have quartet ground
states with high-lying doublet excited states. In the gas phase, [(TPFPP)CrIII]+ forms adducts with a variety
of neutral donors, but O-atom transfer is only observed for NO2. Pyridine N-oxide adducts of 1 do yield 2
upon collision-induced dissociation (CID), but the ethylene oxide, DMSO, and TEMPO analogues do not.
[(TPFPP)CrVO]+ is shown by its reactivity and by CID experiments to be a terminal metal-oxo with a
single, vacant coordination site. It also displays limited reaction chemistry, being deoxygenated only by
the very potent reductant P(OMe)3. In general, [(TPFPP)CrVO]+ species are much less reactive than the
Fe and Mn analogues. Thermochemical analysis of the reactions points toward the involvement of spin
issues in the lower observed reactivity of the chromium complexes.
1. Introduction
models, because this enzyme superfamily is primarily respon-
sible for processing xenobiotic substances in ViVo. In addition,
the enzymes and models accomplish very interesting reaction
chemistry, including the hydroxylation of alkanes and epoxi-
dation of olefins. The mechanisms of these reactions have
therefore been of much interest, and have been the subject of
some controversy.5
For all these reasons, there have been many computational
studies of metalloporphyrin oxidation reactions and their mech-
anisms. Shaik, Schwarz and their co-workers have proposed that
multiple spin states play an important role in these reactions,
with many of them involving what they have called “two-state
reactivity (TSR).”6,7 They state, “A thermal reaction which
involves spin crossover along the reaction coordinate from
The role of spin states in reactions of transition metal
complexes is being increasingly discussed, specifically how
reactions are affected when a change in spin state occurs
between reactants and products.1 This fundamental question
appears to be particularly important in the oxidative chemistry
of metalloporphyrin-containing enzymes and complexes. Heme-
containing monooxygenase enzymes, such as cytochromes P450,
nitric oxide synthases, and peroxidases, mediate myriad trans-
formations of biological importance.2-4 In particular, the
reaction chemistry of the cytochromes P450 has been intensively
studied, both in enzymatic systems and using small molecule
† Universita` di Roma “La Sapienza”.
‡ University of Washington.
(5) (a) Groves, J. T.; McClusky, G. A.; White, R. E.; Coon, M. J. Biochem.
Biophys. Res. Commun. 1978, 81, 154–160. (b) Auclair, K.; Hu, Z.;
Little, D. M.; Ortiz de Montellano, P. R.; Groves, J. T. J. Am. Chem.
Soc. 2002, 124, 6020–6027. (c) Newcomb, M.; Toy, P. H. Acc. Chem.
Res. 2000, 33, 449–455. (d) Newcomb, M.; Shen, R.; Choi, S.-Y.;
Toy, P. T.; Hollenberg, P. F.; Vaz, A. D. N.; Coon, M. J. J. Am. Chem.
Soc. 2000, 122, 2677–2686. (e) Wang, Q.; Sheng, X.; Horner, J. H.;
Newcomb, M. J. Am. Chem. Soc. 2009, 131, 10629–10636.
(6) (a) Meunier, B.; de Visser, S. P.; Shaik, S. Chem. ReV. 2004, 104,
3947–3980. (b) Shaik, S.; Kumar, D.; de Visser, S. P.; Altun, A.; Thiel,
W. Chem. ReV. 2005, 105, 2279–2328. (c) Shaik, S.; Hirao, H.; Kumar,
D. Acc. Chem. Res. 2007, 40, 532–542. (d) Shaik, S.; Cohen, S.; Wang,
Y.; Chen, H.; Kumar, D.; Thiel, W. Chem. ReV. 2010, 110, 949–1017.
(7) Schro¨der, D.; Shaik, S.; Schwarz, H. Acc. Chem. Res. 2000, 33, 139–
145.
(1) (a) Harvey, J. N. Chem. Phys. Phys. Chem. 2007, 9, 331–343. (b)
Poli, R.; Harvey, J. N. Chem. Soc. ReV. 2003, 32, 1–8.
(2) (a) Ortiz de Montellano, P. R. Cytochrome P-450: Structure, Mech-
anism and Biochemistry, 3rd ed.; Plenum Press: New York, 2005. (b)
Sono, M.; Roach, M. P.; Coulter, E. D; Dawson, J. H. Chem. ReV.
1996, 96, 2841–2888. (c) Denisov, I. G.; Makris, T. M.; Sligar, S. G;
Schlichting, I. Chem. ReV. 2005, 105, 2253–2278. (d) Ortiz de
Montellano, P. R.; De Voss, J. J. Nat. Prod. Rep. 2002, 19, 477–493.
(3) (a) Griffith, O. W.; Stuehr, D. J. Annu. ReV. Physiol. 1995, 57, 707–
734. (b) Woodward, J. J.; Chang, M. M.; Martin, N. I.; Marletta, M. A.
J. Am. Chem. Soc. 2009, 131, 297–305.
(4) (a) Dunford, H. B. Heme Peroxidases Wiley: New York, 1999. (b)
Davies, M. J.; Hakwins, C. L.; Pattison, D. I.; Rees, M. D. Antioxid.
Redox Signaling 2008, 10, 1199–1234.
9
4336 J. AM. CHEM. SOC. 2010, 132, 4336–4343
10.1021/ja9103638  2010 American Chemical Society

Gas-Phase Reactions of Chromium-Porphyrin Complexes
A R T I C L E S
reactants to products needs to be described in terms of two-
state reactivity if product formation arises from an interplay of
spin inversion and the respective barrier heights on both spin
surfaces.”7 Perhaps the clearest experimental evidence for the
TSR model comes from the gas-phase reactions of FeO+ or
OCrO+ studied by Schwarz and co-workers.8 The interpretation
of the experiments and computations for the iron-porphyrin
species in oxidation reactions is complicated, however, by the
multiplicity of accessible spin states. The ‘Compound I’
intermediate, two oxidation levels above FeIII, is most simply
described as an FeIV-oxo (ferryl) species coupled to a porphyrin
radical cation and can have closely spaced doublet, quartet, and
sextet states.9 The FeIII state can similarly have accessible
doublet, quartet, and sextet states.6 Experimental and compu-
tational studies also indicate that spin-state effects play important
roles in the reactions of oxomanganese porphyrin complexes,10
other metal-oxo complexes,11 and in oxygen atom abstractions
by M(OCtBu3)3 [M ) V, Nb(PMe3), and Ta].12
species with π-acid ligands).18,19 In octahedral symmetry, the
4
ground state for CrIII is A2g and the lowest lying excited state
20
2
3
is E. Since both states have a t2g configuration, the energy
of the 2E state is much less dependent in the nature of the ligands
than most ligand-field excited states and is at ∼13,000 cm-1
20
3+
(37 kcal mol-1) for Cr(NH3)6
.
When X and XO are closed
shell (singlet) species, oxygen-atom transfer from doublet ground
state CrVO to a quartet ground state CrIII is thus formally spin
forbidden. Herein we report our investigations of the gas-phase
reactivity of ligated chromium(III) and chromium(V)–oxo
complexes with a variety of oxygen atom donors and acceptors,
using electrospray ionization (ESI) coupled to mass spectrometry
(MS).
Earlier studies based on Fourier transform ion cyclotron
resonance (FT-ICR) mass spectrometry combined with ESI have
provided valuable information on functional models of the
Compound I intermediates of monooxygenase enzymes, namely
the [(TPFPP)•+FeIVO]+ and [(TPFPP)MnVO]+ (TPFPP ) meso-
tetrakis(pentafluorophenyl)porphyrinato dianion) complexes.21
These species have been obtained as naked ions and their
reactivity has been investigated in an environment lacking any
proximal or distal ligand, solvent or counterion. In this way it
could be ascertained that the [(TPFPP)MnVO]+ complex is
responsible for catalytic activity, while a strong trans axial ligand
(an additional oxygen) can switch off the reactivity, an outcome
consistent with barriers due to TSR effects.21c By gas-phase
synthesis using ozone as oxidant, the naked core of Compound
I, [(PPIX)·+FeIVO]+ (PPIX ) protoporphyrin IX dianion) has
been obtained for the first time.22 The dilute gas phase has
ensured that these species have a lifetime long enough to reveal
both structural details and elementary steps of the catalytic
activity. High-valent manganese(V)-oxo complexes of non-
heme ligands have also been examined in the gas phase by
Plattner, showing that ESI-tandem MS techniques are a con-
venient approach for the study of the coordination chemistry
of highly reactive solution-phase species.23 On these premises
an attempt to answer the spin-forbidden question for oxo-
chromium complexes was undertaken.
The chromium(V)-oxo/chromium(III) redox couple could
provide a more direct test of the importance of spin-state changes
on oxygen-atom transfer reactions because there is one pre-
dominant spin state present at each oxidation level in such a
system (eq 1). CrV has a d1 configuration and therefore is an
obligate doublet state.13 A number of CrV oxo species have
LnCrV(O) + X a LnCrIII(OX)
(1)
doublet state
quartet state
been prepared, including with porphyrin, salen, and other
ligands.14 Some of these compounds undergo oxygen-atom
transfer reactions such as epoxidation, as found by Groves,15
Kochi,16 and others.17 CrIII coordination complexes essentially
always have quartet ground states (except for organometallic
(8) (a) Schro¨der, D.; Fiedler, A.; Ryan, M. F.; Schwarz, H. J. Phys. Chem.
1994, 98, 68–70. (b) Fiedler, A.; Kretzschmar, I.; Schro¨der, D.; Shaik,
S. J. Am. Chem. Soc. 1996, 118, 9941–9952.
(9) Lai, W.; Chen, H.; Shaik, S. J. Phys. Chem. B. 2009, 113, 7912–
7917.
(10) Compare (a) De Angelis, F.; Jin, N.; Car, R.; Groves, J. T. Inorg.
Chem. 2006, 45, 4268–4276. (b) Song, W. J.; Seo, M. S.; DeBeer
George, S.; Ohta, T.; Song, R.; Kang, M.-J.; Tosha, T.; Kitagawa, T.;
Solomon, E. I.; Nam, W. J. Am. Chem. Soc. 2007, 129, 1268–1277.
(11) Compare (a) Strassner, T.; Houk, K. N. Org. Lett. 1999, 1, 419–421.
(b) Cavallo, L.; Jacobsen, H. Eur. J. Inorg. Chem. 2003, 892–902.
(c) Abashkin, Y. G.; Burt, S. K. Org. Lett. 2004, 6, 59–62. (d)
Reference 23.
2. Experimental Section
2.1. Materials. 5,10,15,20-Tetrakis(pentafluorophenyl)porphy-
rinato chromium(III) chloride, (TPFPP)CrIIICl (1-Cl) was purchased
from Frontier Scientific (Logan, UT, U.S.A.). 5,10,15,20-meso-
Tetraphenylporphyrin (TPP, Strem) was purified by the method of
Adler.24 (TPP)CrIIICl was prepared from TPP and anhydrous CrIICl2
(Strem) according to the literature.25 3,3′-5,5′-Tetra-tert-butyl-salen,
(12) Veige, A. S.; Slaughter, L. M.; Lobkovsky, E. B.; Wolczanski, P. T.;
Matsunaga, N.; Decker, S. A.; Cundari, T. R. Inorg. Chem. 2003, 42,
6204–6224.
(13) (a) CrV is not nearly as oxidizing as FeV so CrIV/ligand radical cation
states are very high in energy and do not need to be considered for
these species (especially for the electron-deficient fluorinated porphyrin
used here). See, for example, refs 13b and 15-17. (b) Penner-Hahn,
J. E.; Benfatto, M.; Hedman, B.; Takahashi, T.; Doniach, S.; Groves,
J. T.; Hodgson, K. O. Inorg. Chem. 1986, 25, 2255–2259.
(14) Compare refs 15, 16, 17, and Rong, C.; Anson, F. C. Inorg. Chem.
1994, 33, 1064–1070.
(18) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Butterworth-Heinemann: Oxford, 1997; pp 1027-1031.
(19) Balch, A. L.; Latos-Grayski, L.; Noll, B. C.; Olmstead, M. M.;
Zovinka, E. P. Inorg. Chem. 1992, 31, 1148–1151.
(20) Figgis, B. N.; Hitchman, M. A. Ligand Field Theory and Its
Applications; Wiley-VCH: New York, 2000; especially pp 136, 207,
and 220 which show that E(2E) ≈ 20B for an octahedral d3 complex
and that B for Cr(NH3)3+ ) 657 cm-1
.
(15) (a) Groves, J. T.; Kruper, W. J., Jr. J. Am. Chem. Soc. 1979, 101,
7613–7615. (b) Groves, J. T.; Haushalter, R. C. J. Chem. Soc., Chem.
Commun. 1981, 1165–1166. (c) Groves, J. T.; Kruper, W. J.;
Haushalter, R. C.; Butler, W. M. Inorg. Chem. 1982, 21, 1363–1368.
(16) (a) Samsel, E. G.; Srinivasan, K.; Kochi, J. K. J. Am. Chem. Soc.
1985, 107, 7606–7617. (b) Srinivasan, K.; Kochi, J. K. Inorg. Chem.
1985, 24, 4671–4679.
(21) (a) Crestoni, M. E.; Fornarini, S. Inorg. Chem. 2005, 44, 5379–5387.
(b) Chiavarino, B.; Cipollini, R.; Crestoni, M. E.; Fornarini, S.;
Lanucara, F.; Lapi, A. J. Am. Chem. Soc. 2008, 130, 3208–3217. (c)
Crestoni, M. E.; Fornarini, S.; Lanucara, F. Chem.sEur. J. 2009, 15,
7863–7866.
(22) Crestoni, M. E.; Fornarini, S. Inorg. Chem. 2007, 46, 9018–9020.
(23) (a) Feichtinger, D.; Plattner, D. A. J. Chem. Soc., Perkin Trans. 2
2000, 1023–1028. (b) Feichtinger, D.; Plattner, D. A. Chem.sEur. J.
2001, 7, 591–599. (c) Plattner, D. A. Int. J. Mass Spectrom. 2001,
207, 125–144.
(17) Compare: (a) Traylor, T. G.; Miksztal, A. R. J. Am. Chem. Soc. 1989,
111, 7443–7448. (b) Garrison, J. M.; Bruice, T. C. J. Am. Chem. Soc.
1989, 111, 191–198. (c) Creager, S. E.; Murray, R. W. Inorg. Chem.
1985, 24, 3824–3828. (d) Venkataramanan, S. N.; Premsingh, S.;
Rajagopal, S.; Pitchumani, K. J. Org. Chem. 2003, 68, 7460–7470.
(24) Adler, A. D.; Longo, F. R.; Finavells, J. D.; Goldmacher, J. A.;
Korsakoff, L. J. Org. Chem. 1967, 32, 476–476.
9
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A R T I C L E S
Crestoni et al.
Scheme 1. Generation of (TPFPP)CrV(O)+ (2)
and salphen were prepared by condensation of the appropriate
diamine and benzaldehyde (both from Sigma), and recrystallized
from ethanol (J. T. Baker).26 (salen)CrIIICl Complexes were
synthesized from CrIICl2 according to known procedures.27 The
corresponding triflate salts were prepared by addition of stoichio-
metric AgOTf (Strem) and filtration. Iodosylbenzene (C6H5IO) was
prepared according to the literature procedure28 and stored at -20
°C. Pyridine-N-oxide (pyO) was synthesized by reaction of pyridine
with a stoichiometric amount of m-chloroperbenzoic acid (m-CPBA)
in MeOH at room temperature. All the solvents were analytical
grade. H218O (95%), and all other chemicals were research grade
products obtained from commercial sources (Sigma-Aldrich; Mathe-
son Gas Products Inc.) and used as received.
2.2. Instrumental. Experiments were performed either with a
Bruker BioApex 4.7 T Fourier transform ion cyclotron resonance
(FT-ICR) mass spectrometer, or with a triple quadrupole linear ion
trap mass spectrometer (Applied Biosystems 2000 Q TRAP). The
first instrument is equipped with an Apollo I electrospray ionization
(ESI) source. Ions are led into a cylindrical “infinity” cell where
neutrals may be admitted by way of either pulsed valves or leak
valves. The second device consists of an ESI source combined with
atandemmassspectrometer(QHQ,quadrupole-hexapole-quadrupole
configuration).
sponding to ion 1 or 2, as desired, was isolated and allowed to
react with the selected neutral. Pseudo-first-order rate constants were
obtained from the semilogarithmic decrease of the reactant ion
abundance over time at each selected pressure and were divided
by the concentration of the neutral species to attain second-order
rate constants (kexp). The rate constants were also expressed as
percentages (efficiencies) of the collision rate constant (kcoll
)
calculated by the parametrized trajectory theory.30 Whereas the
reproducibility of kexp, reported as the average of usually three
experiments run at different neutral pressures, was good, the
estimated error in their absolute values ((30%) is mainly due to
uncertainty in pressure measurements.
2.5. Collision-Induced Dissociation (CID) Experiments. For
CID performed in FT-ICR, the mass-selected reagent ions were
accelerated in the presence of argon gas pulsed into the cell (peak
pressure ca. 4 × 10-6 mbar) for 1 s. In ESI-MS/MS effected in the
Q TRAP, the ions of interest were isolated using Q1, accelerated
at variable collision energy (CE) using N2 as a collision gas
(normally at 2 × 10-4 mbar) in the hexapole H and detected together
with their ionic fragments in Q2. The energy dependence of the
dissociation process has been analyzed yielding the phenomenologi-
cal appearance energies (Eapp) for the fragmentation processes under
study.
2.3. Generation of Oxochromium(TPFPP)+ [(TPFPP)CrV-
(O)]+ Ions. Two routes were followed in order to generate the
[(TPFPP)CrV(O)]+ ions of interest. In the first one, a 10 µM solution
of 1-Cl in methanol was submitted to ESI, and the so formed
[(TPFPP)CrIII]+ ions (1) were exposed to a stream of NO2 in the
ionic path driving the ions into the FT-ICR cell. This procedure
leads to incorporation of an O-atom into 1, presumably resulting
in formation of [(TPFPP)CrV(O)]+ (2). In the second method,
iodosylbenzene (C6H5IO, 0.044 mg, 2 × 10-4 mol) was added to
1-Cl (10 µM) in methanol (2 mL) at room temperature, yielding a
red colored solution, stable for several hours at room temperature.
When submitted to ESI, this solution delivered the same species
described as [(TPFPP)CrV(O)]+ (2). In both cases, the mass spectra
showed two prominent species, both characterized by the same
characteristic isotopic pattern. The first one is assigned to ion 1,
[(TPFPP)CrIII]+, centered at m/z ) 1024, while the second one is
centered at m/z ) 1040, consistent with ion 2 [(TPFPP)CrV(O)]+.
2.4. Reactions of [(TPFPP)CrV(O)]+. The solutions were
continuously sprayed at a 2 µL min-1 flow rate controlled by a
syringe pump. A countercurrent flow of heated dry gas (nitrogen
at 120 °C) was used to desolvate the ions. After an accumulation
interval of 0.6 s in a rf-only hexapole, the ion population was pulsed
into the ICR cell placed within a 4.7 T superconducting magnet.
In the cell, maintained at room temperature (ca. 300 K), the neutral
reagent was leaked at a constant pressure in the range of 0.5-10
× 10-8 mbar. The pressure readings, obtained from a cold-cathode
ionization gauge (IKR Pfeiffer Balzers S.pA., Milan, Italy), were
3. Results
We have primarily focused on the tetrakis(pentafluorophe-
nyl)porphyrin (TPFPP) complexes of chromium because this
ligand has proven very amenable to ESI-FT-ICR studies with
the related iron21a,b and manganese21c complexes. Studies with
other porphyrin and related tetradentate ligands are described
after the TPFPP results.
3.1. Generation of [(TPFPP)CrV(O)]+. Treatment of chro-
mium(III)-porphyrin chloride complexes (P)CrCl with oxygen
atom donors are reported to give CrIVoxo and/or CrVoxo
compounds, depending on the reaction conditions; in the
presence of limited oxidant, CrV and CrIII comproportionate to
CrIV.14,31 With the TPFPP ligand, there has been a brief
comment about the formation of (TPFPP)CrIV(O) from (TPFPP)-
CrII and O2, but no mention of the CrVoxo complex.31 We find
that treatment of (TPFPP)CrIIICl (abbreviated 1-Cl) with iodo-
sylbenzene (C6H5IO) in methanol solution gives substantial
amounts of [(TPFPP)CrV(O)]+ (2), as indicated by electrospray
ionization (ESI) mass spectrometry of these mixtures (Scheme
1). The same product is formed by the gas-phase reaction of
[(TPFPP)CrIII]+ (1) and NO2 in the ICR (Scheme 1). Other
reactions of 1 are described below.
Figure 1 shows a high-resolution ESI-FT-ICR mass spectrum
of 2 generated in solution from PhIO + [(TPFPP)CrIII]+ (1),
showing ions at m/z ) 1039.9806 (calculated value 1039.9778).
Complex 1 is also seen in such mass spectra, at m/z ) 1023.9857
(calculated value 1023.9829). If no oxidizable species are added
to the solution, 2 is remarkably inert toward side reactions, as
has been found for iron-oxo cations containing porphyrin
calibrated by using the rate constant k ) 11.7 × 10-10 cm3
molecule-1 s-1 for the reference reaction CH4•+ + CH4 f CH5
+
+
CH3• and weighted by using individual response factors.29 The ion
of interest was isolated by ejection routines as commonly performed
in FT-ICR mass spectrometry and allowed to react with the neutral
leaked by a needle valve. To avoid unintended off-resonance
excitation of the reactant ion, the whole isotopic cluster corre-
(25) (a) Summerville, D. A.; Jones, R. D.; Hoffman, B. M.; Basolo, F.
J. Am. Chem. Soc. 1977, 99, 8195–8202. (b) Garrison, J. M.; Bruice,
T. C. J. Am. Chem. Soc. 1989, 111, 191–198. (c) Traylor, T. G.;
Miksztal, A. R. J. Am. Chem. Soc. 1989, 111, 7443–7448.
(26) Pfeiffer, P.; Breith, E.; Lu¨bbe, E.; Tsumaki, T. Justus Liebigs Ann.
Chem. 1933, 503, 84–103.
(27) Coggon, P.; McPhail, A. T.; Mabbs, F. E.; Richards, A.; Thornley,
A. S. J. Chem. Soc. A 1970, 3296–3303.
(28) Saltzman, H.; Sharefkin, J. G. Organic Syntheses; Wiley: New York,
1973; Collect. Vol. 5, p 658.
(29) (a) Meot-Ner, M. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.;
Academic Press: New York, 1979; Vol. 1. (b) Bartmess, J. E.;
Gerogiadis, R. M. Vacuum 1983, 33, 149–153.
(30) Su, T.; Chesnavich, W. J. J. Chem. Phys. 1982, 76, 5183–5185.
(31) Liston, D. J.; West, B. O. Inorg. Chem. 1985, 24, 1568–1576.
9
4338 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010

Gas-Phase Reactions of Chromium-Porphyrin Complexes
A R T I C L E S
[(TPFPP)CrV(O)]+
+ γ-picoline f
2
[(TPFPP)CrV(O)(γ-picoline)]+
2-γ-picoline
(2)
[(TPFPP)CrV(O)]+
+ P(OMe)3 f
2
[(TPFPP)CrIII(OP(OMe)3)]+
2-P(OMe)3
(3)
display the release of 1 by concomitant loss of OP(OMe)3
(Figure 2). The CID spectra show no significant fragmentation
of the phosphite ligand in 2-P(OMe)3, irrespective of the origin
of 2 (either from the solution or the gas-phase synthesis). These
products imply a net transfer of oxygen from 2 to P(OMe)3.
The same fragment ion 1 is observed when the CID experiment
is performed on labeled [(TPFPP)Cr18OP(OMe)3]+, from 2-18O
+ P(OMe)3, indicating loss of 18OP(OMe)3. These results
support a CrIII-trimethylphosphate coordination structure for the
ion, [(TPFPP)CrIII(OP(OMe)3)]+. The observed reactivity with
P(OMe)3 also shows that 2 is a high-valent chromium-oxo
complex. Alternative structures that could be conceived for the
elemental composition of the sampled ion 2, for example
isomers bearing an O-atom on the porphyrin ligand, would not
account for the observed net O-transfer to P(OMe)3.
Compound 2-P(OMe)3 also undergoes a subsequent addition
process to give [(TPFPP)CrIII{OP(OMe)3}{P(OMe)3}]+. This
is evidence that the complex has a vacant sixth coordination
site. In the presence of Me2S, 2-P(OMe)3 undergoes a second
addition step, yielding what must be a six-coordinate species
with OP(OMe)3 and Me2S ligands, [(TPFPP)CrIII{OP-
(OMe)3}(Me2S)]+ (eq 4).
Figure 1. ESI (+) FT-ICR mass spectrum of a methanol solution of
(TPFPP)CrIIICl (1-Cl, 10 µM) and a 10-fold excess of iodosylbenzene,
showing [(TPFPP)CrIII]+ (1) and [(TPFPP)CrIII(O)]+ (2). Inset: enlargement
of the isotopic cluster at m/z 1040.
ligands with electron-withdrawing groups.21b When the oxida-
tion of 1-Cl with iodosylbenzene is effected in the presence of
excess H218O (95%-18O content) the ion at m/z ) 1040 is shifted
to m/z ) 1042 (2-18O). This indicates that compound 2 contains
an added oxygen atom, likely a formal CrVoxo complex (Figure
1), as discussed in further detail below.
3.2. Gas-Phase Reactions of [(TPFPP)CrV(O)]+. The nature
and reactivity of [(TPFPP)CrV(O)]+ (2), has been assayed by
FT-ICR mass spectrometry. In all cases, there is no difference
between 2 formed either in solution with PhIO or in the ICR
with NO2. When mass-selected and exposed in the ICR cell to
a stationary concentration of potential reducing agents (L), 2
proved to be quite unreactive. No change was observed on
exposure to NO, NO2, Me2S, ꢀ-pinene, pyridine or thioanisole.
This indicates that reactions with these substrates are disfavored
either by a positive free energy change,32 or by rate constants
less than kexp < 10-13 cm3 molecule-1 s-1. Reactions do occur
with γ-picoline (p-CH3C5H4N) and trimethylphosphite
(P(OMe)3) to give addition products [(TPFPP)CrOL)]+ (2-L)
and, for P(OMe)3, the bis-addition product [(TPFPP)CrOL2]+
(2-L2). These adduct ions likely owe their stabilization to the
dissipation of any excess energy released in the association
process by radiative and/or collisional cooling.33 The bimo-
lecular rate constants (kexp) and the relative efficiencies (Φ) are
summarized in Table 1. Note that the abbreviations used in this
paper indicate how the ions were made, in this case “2-L” and
“2-L2” indicating that the complexes were made from 2 and L.
The chemical structure of these ions is discussed below.
[(TPFPP)CrIII(OP(OMe)3)]+
P(OMe)3
Me2S
2.
8
.
8
2-P(OMe)3
gas-phase
ICR Cell
[(TPFPP)CrIII{OP(OMe)3}(Me2S)]+
kexp ) 4.8 × 10-10 cm3 molecule-1 -1 Φ ) 39% (4)
s
The reactivity of 2-P(OMe)3 has been compared with that of
the isobaric species 1-OP(OMe)3 generated by electrospray of
in situ mixtures of 1-Cl + OP(OMe)3. Interestingly, this cation
1-OP(OMe)3 is also able to undergo addition of one molecule
of Me2S with a reaction efficiency similar to that of reaction 4.
Further analogy is found when comparing the CID behavior of
Table 1. Gas-Phase Reactivity of [(TPFPP)CrV(O)]+ Ions (2)a with
Substrates (L) in the Gas Phase
c
kexp
L
productb
Φ (efficiency, %)d
NO
NO2
Me2S
ꢀ-pinene
pyridine
γ-picoline
P(OMe3)
NR
NR
NR
NR
NR
-
-
-
-
When assayed by low-energy collision-induced dissociation
(CID), the primary adduct [(TPFPP)Cr(O)(γ-picoline)]+ yields
ion 2 and γ-picoline as dissociation products. This suggests that
γ-picoline has simply bound to 2 as a ligand to chromium,
namely that this material is a picoline-ligated CrV-oxo complex,
2-γ-picoline (eq 2). In contrast, the CID spectra of [(TPFPP)-
CrOP(OMe)3]+ (2-P(OMe)3, formed as shown in eq 3)
-
-
-
-
1.2
3.9
-
-
8.7
35
[(TPFPP)Cr(O)(γ-picoline)]+
e
[(TPFPP)Cr(O)P(OMe)3]+
a [(TPFPP)CrV(O)]+ ions were produced either in solution, by
treatment of 1-Cl with C6H5IO, or in the gas phase by reaction of 1 with
NO2. b NR ) no observed reaction. c Second-order rate constants in
units of 10-10 cm3 molecule-1 s-1. The estimated error is (30%, while
the internal consistency of the data is within (10%. d Reaction
efficiency, Φ ) kexp/kcoll × 100. Collision rate constants (kcoll) evaluated
with the parametrized trajectory theory.30 e In a consecutive second
addition step the [TPFPP)Cr(O)(P(OMe)3)2]+ ion is formed.
(32) Bouchoux, G.; Salpin, J. Y.; Leblanc, D. Int. J. Mass. Spectrom. Ion
Processes 1996, 153, 37–48.
(33) (a) Angelelli, F.; Chiavarino, B.; Crestoni, M. E.; Fornarini, S. J. Am.
Soc. Mass Spectrom. 2005, 16, 589–598. (b) Dunbar, R. C. Mass
Spectrom. ReV. 2004, 23, 127–158.
9
J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010 4339

A R T I C L E S
Crestoni et al.
properties is present in both 2-P(OMe)3 and 1-OP(OMe)3 under
these experimental conditions, and therefore that 2-P(OMe)3 and
1-OP(OMe)3 are actually the same species.
3.3. Reactions of [(TPFPP)CrIII]+ (1) with Oxygen Atom
Donors and CID Experiments. [(TPFPP)CrIII]+ (1) reacts with
a variety of small molecules to form addition products [(TPF-
PP)CrIII(L)]+ and, in some cases, [(TPFPP)CrIII(L)2]+, as sum-
marized in Table 2. Some of these ions are formed by
electrospraying an equimolar solution (5 µM) of 1-Cl and L in
MeOH, while others have been formed in the ICR by gas-phase
ion-molecule reactions. Many of the ligands are potential
oxygen atom donors, such as ethylene oxide which can be
deoxygenated to ethylene, but in only two cases was the oxo
complex 2 formed. These are the two reactions described above,
the solution reaction of 1-Cl + iodosylbenzene and the gas-
phase reaction of 1 + NO2 (Scheme 1). The latter reaction is
characterized by a notable efficiency of 22% (note d in Table
2). The potential oxygen atom donors XO are given in the Table
in increasing order of X-O bond dissociation enthalpy (BDE),
as discussed below.
To understand the nature of the ligand adducts, a number of
them were analyzed by collision-induced dissociation, CID.
Upon CID the adducts of ethylene oxide, DMSO (Me2SO),
OPPh3, and OP(OMe)3 all dissociated to give the neutral ligand
and 1. A representative CID spectrum of the mass-selected ion
1-OP(OMe)3, showing the loss of intact trimethylphosphate, is
presented in Figure 2c. In contrast, pyridine N-oxide and
γ-picoline N-oxide undergo CID to yield the oxo complex 2
and (presumably) the deoxygenated ligand. The phenomenologi-
cal appearance energies for these CID processes are 0.41 and
0.48 eV, respectively. The adduct of 1 with TEMPO (2,2′-6,6′-
tetramethylpiperidin-1-oxyl), dissociates into TEMPO+ and,
presumably, neutral (TPFPP)CrII as the only product observed.
3.4. Reactivity of Other [(L4)CrV(O)]+ and [(L4)CrIII]+
Cations. Several chromium complexes with tetradentate mac-
rocyclic ligands L4 other than TPFPP have been tested for
potential oxygen transfer reactivity using ESI-FT-ICR. These
have involved L4 ) 5,10,15,20-meso-tetraphenylporphyrin
(TPP), 3,3′-5,5′-tetra-tert-butyl-salen (salen), and N,N′-phenyle-
nebis(salicylideneimine) (salphen); see Table S1 in the Sup-
porting Information. As in the TPFPP case, addition of ligands
to [(L4)CrIII]+ results in sequential formation of the mono- and
bis-adducts, without formation of the corresponding CrVoxo
complex. The one exception is the reaction of [(tBu4salen)CrIII]+
with NO2 to give [(tBu4salen)CrV(O)]+, the product of a net
O-atom transfer. However, neither [(salphen)CrIII]+ nor [(TPP)-
CrIII]+ reacts with NO2.
Figure 2. CID spectrum of mass-selected [(TPFPP)CrIII(OP(OMe)3)]+ (m/z
) 1164) prepared in a FT-ICR mass spectrometer by ion-molecule reactions
of 2 (a) and 2-18O (b) with P(OMe)3 and by electrospray ionization of a
1-Cl + OP(OMe)3 mixture in acetonitrile (c).
the complex at m/z 1226 obtained by the gas-phase addition of
Me2S to either complex 2-P(OMe)3 or 1-OP(OMe)3 formed in
solution. In both cases, CID of these mixed adducts release
1-Me2S by loss of neutral OP(OMe)3. This is further indication
that the m/z 1226 ion is best described as a [(TPFPP)CrIII]+ ion
bound to OP(OMe)3 and Me2S axial ligands.
Likewise, when 2 is exposed to a gaseous mixture of P(OMe)3
and OP(OMe)3, only the primary addition of the former ligand
exhibits a second addition step, whereas OP(OMe)3 displayed
the addition of a just one molecule to ion 2, to form 2-O-
P(OMe)3. This result is inconsistent with the presence of distinct
oxo and P(OMe)3 axial ligands within 2-P(OMe)3. Rather, it
implies that an O-coupling occurs upon reaction of 2 with
P(OMe)3, such that a sixth coordination site within 2-P(OMe)3
is still vacant and may further coordinate a second ligand
molecule. However, when the neutral ligand is not a good
oxygen atom acceptor, when L ) OP(OMe)3 or γ-picoline, a
six-coordinate addition complex is formed with 2 in which the
oxo group and L are distinct axial ligands. Consistently, these
complexes do not display any tendency to give a second addition
step.
As a further comparison, separately prepared 2-P(OMe)3 and
1-OP(OMe)3 were each reacted with a 70:30 mixture of
P(OMe)3 and OP(OMe)3. Once again, 2-P(OMe)3 and 1-O-
P(OMe)3 display the same reactivity. Reversible addition
reactions are observed, which attain the same equilibrium when
starting from either reactant ion (Figure 3). One may thus infer
that a sixth coordination site with indistinguishable binding
The [L4Cr(O)]+ complexes (see Supporting Information)
showed no reactivity with potential reductants including Me2S,
P(OMe)3, γ-picoline, NO, p-Me-C6H4SMe, and ꢀ-pinene. The
lack of reactions with P(OMe)3 or alkenes contrasts with
the reactivity of these oxo complexes in solution, such as the
reported epoxidations.15,16 We have also found that, in solution,
the salen complex is very reactive with phosphines, rapidly
forming CrIII products.34 A distinct effect by donor ligands such
as (OPEt3) is also reported to enhance the epoxidation rates and
yields displayed by (salen)CrV(O) complexes in solution.16a The
reaction of [(tBu4salen)CrV(O)(OPPh3)]+ probed in the gas phase
with a powerful reductant such as p-CH3C6H4SCH3 did not lead
to any enhanced reactivity, however, and this complex remains
(34) Warren, J. J.; Mayer, J. M. Unpublished results.
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4340 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010

Gas-Phase Reactions of Chromium-Porphyrin Complexes
A R T I C L E S
Figure 3. Time dependence of relative ion abundances when either 2-P(OMe)3 or 1-OP(OMe)3 ions are allowed to react with a 70:30 mixture of P(OMe)3,
and OP(OMe)3 at 7 × 10-8 mbar in the FT-ICR cell. The reactant ion is seen to undergo addition of either one P(OMe)3 or one OP(OMe)3 molecule.
Table 2. Products from [(TPFPP)CrIII]+ (1) + Ligands and Their
Table 3. Homolytic X-O Bond Dissociation Enthalpies (BDEs)a
Behavior upon CID
b
reaction
∆H° (kcal mol-1
)
synthesisa
L
product
CID reaction
Eapp
b
PhIO f PhI + O
∼26c
62.6
63.3
73.2
84.7
86.8
86d
Sc
S
PhIO
γ-picoline-O + L
2
-
-
0.48
γ-picoline - O f γ-picoline + O
pyO f py + O
1-L f 2 + γ-picoline
(90%)
NO2 f NO + O
1-L f 1 + γ-picoline-O 0.58
(10%)
C2H4O f C2H4 + O
Me2SO f Me2S + O
Me2NO• f Me2N• + O
H2NO• f H2N• + O
Ph3PO f Ph3P + O
(EtO)3PO f (EtO)3P + O
S,G
S
γ-picoline
+ L
+ L
1-L f 1 + γ-picoline
0.55
0.41
91d
py-O
1-L f 2 + py (75%)
1-L f 1 + py-O (25%) 0.38
139
148
S,G
G
G
py
NO2
C2H4O
+ L
1-L f 1 + py
0.56
d
c
2
-
-
e
a Data from ref 44 unless otherwise noted. b Derived from solution
data or calculated gas-phase heats of formation. c Estimated from data in
ref 45. d See Supporting Information.
+ L, + 2 L 1-L f 1 + L
1-L2 f 1-L + L
+ L
+ L, + 2 L 1-L f 1 + L
1-L2 f 1-L + L
+ L
-
e
-
S
G
DMSO
Me2S
1-L f 1 + L
0.43
e
-
e
-
OP(OMe)3 with the same equilibrium constants. This assignment
is similar to chromium(V)-terminal oxo complexes made in
solution with porphyrin- and salen-supporting ligands.15-17
Net oxygen-atom transfers have been observed, from NO2
to 1, and from 2 to P(OMe)3. However, 2 shows no observable
reactivity with NO, NO2, Me2S, ꢀ-pinene, or pyridine. This
chromium-oxo cation is thus much less reactive than the
analogous iron cation, [(TPFPP•+)FeIVO]+, which reacts rapidly
with all of these substrates.21a The difference in reactivity is
consistent with the generally much higher oxidizing power of
high-valent iron versus chromium compounds as observed in
solution.35
e
S,G
TEMPO
1-L f TEMPO+
+
-
[(TPFPP)CrII]
S
S,G
OPPh3
OP(OMe)3
+ L, + 2 L 1-L f 1 + L
+ L, + 2 L 1-L f 1 + L
1-L2 f 1-L + L
0.56
0.58
S,G
P(OMe)3
+ L
1-L f 1 + L
0.31
a S ) prepared in solution from 5 µM 1-Cl + 5 µM LO in MeOH,
followed by electrospray; G ) formed in the gas phase from 1 + L.
b Phenomenological appearance energies in eV. c The so-formed ion 2 is
stable toward CID in the energy range explored. d Efficiency 22%. e Not
available.
unreactive just as [(tBu4salen)CrV(O)]+ (See Table S1, Support-
ing Information).
4.1. Thermochemistry of Oxygen-Atom Transfer. The oxy-
gen atom donors/acceptors XO/X used in this study have a wide
range of homolytic bond dissociation enthalpies (BDEs, ∆H°
for XO f X + O), as shown in Table 3.
4. Discussion
The gas-phase reactivity of several porphyrin- and salen-
ligated complexes of chromium have been explored using gas-
phase ICR-MS. Most of the work has focused on the tetrak-
is(pentafluorophenyl)porphyrin (TPFPP) complexes because of
their higher reactivity. The results, particularly the reactions with
P(OMe)3 and CID studies of the products, show that complex
2 is the metal-oxo cation, [(TPFPP)CrVtO]+. For example,
the addition of P(OMe)3 to 2 gives an adduct that loses
OP(OMe)3 on collisional activation. This adduct is essentially
identical to the complex formed from [(TPFPP)CrIII]+ (1) and
OP(OMe)3; for instance, both species bind P(OMe)3 and
The reaction of NO2 with [(TPFPP)CrIII]+ (1) to give
[(TPFPP)CrVtO]+ (2) and (presumably) NO (eq 5) is a
particularly simple reaction that provides a lower limit for the
CrVtO bond strength. This reaction proceeds with a moderately
high efficiency (22% at room temperature, Table 2, footnote d)
consistent with a negative value of ∆G°.32,36 Indeed, definitely
exoergic reactions with ∆G° less than -10 kcal mol-1 and
negligible intrinsic barriers (such as proton-transfer processes)
(35) Sheldon, R. A., Ed. Metalloporphyrins in Catalytic Oxidations; Marcel-
Dekker: New York, 1994.
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J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010 4341

A R T I C L E S
Crestoni et al.
proceed with high efficiency (>70%).32 We may then consider
that if ∆G° for the reaction of 1 with NO2 is <0, the bond
dissociation free energy (BDFE) should be higher than that of
NO2, which has a BDFE of 64 kcal mol-1 (see Supporting
Information).37 Since the entropy change for reaction 5 is likely
to be small,38 we can conclude that the more commonly
Scheme 2. Thermochemical Analysis of the Lack of Oxygen Atom
Transfer from 1-OP(OMe)3 or 1-(TEMPO)
discussed bond dissociation enthalpy (BDE) of CrVtO should
39
be greater than that of NO2, BDE ) 73 kcal mol-1
.
For
comparison, the Cr-O BDE in the gas-phase diatomic CrO+
implies that the CrVtO BDE must be less than ∼143 kcal mol-1
to account for the observed reaction of 2 with P(OMe)3. Similarly,
the lack of deoxygenation of TEMPO• by 1 suggests that
BDE(CrVtO) j 101 kcal mol-1. These thermochemical consid-
erations are consistent with the estimated BDE for CrVtO in ion
2 to be higher than 73 kcal mol-1 based on the reaction of 1 with
NO2.
40
has been measured to be 86 ( 3 kcal mol-1
.
[(TPFPP)CrIII]+
[(TPFPP)CrV(O)]+ + NO
f
+ NO2
(5)
1
2
Rough upper limits to the CrVtO bond strength can be
derived from the reduction of 2 by P(OMe)3 and by the absence
of O-atom transfer from TEMPO (2,2′-6,6′-tetramethylpiperidin-
1-oxyl) to chromium (eq 6), with the assumption that the lack
of reaction has a thermochemical rather than kinetic origin. The
analyses of these reactions are more complicated than that of
eq 5 because the ligand, OP(OMe)3 or TEMPO, is bound to
the CrIII so that the enthalpy of binding plays a role.
4.2. Effect of Spin on the Oxygen-Atom Transfer Reac-
tions. It is surprising that none of the XO or X reagents with BDEs
between NO2 and OP(OMe)3 (73.2 and ∼148 kcal mol-1, Table
3) are observed to interconvert 1 and 2. The loss of ethylene from
1·C2H4O should be only slightly less favorable than the loss of
NO from the putative 1·NO2 intermediate in the NO2 reaction.
However, not only does this process not occur, but 2 is also unable
to release an O atom to an activated olefin such as ꢀ-pinene.
Similarly, the 1·DMSO adduct does not fragment into 2 and Me2S
when activated to by CID, displaying no evidence for an O-atom
transfer from sulfur to 1. Once again the reverse reaction is not
observed either, because 2 appears to be unreactive toward gaseous
Me2S. The pyridine N-oxides py-O and γ-picoline-O are among
the strongest oxidizing agents in the set of species that were
considered (Table 3). Both pyridine N-oxide complexes 1·Opy and
1·O-γ-picoline share similar Eapp values upon CID at variable
energy, leading to either 2 (75-90%) or 1 (25-10%) in comparable
amounts. This finding of only partial oxidation of 1 by relatively
powerful oxidizing agents, stronger than NO2 which does efficiently
oxidize 1 in the gas phase, suggests that kinetic effects play an
important role in the partitioning of the O atom between 1 and the
pyridine nitrogen, and, more generally, in O-atom transfer reactions
involving 1 and 2.
As noted above, the interconversion of 1 + XO with 2 + X
is formally spin-forbidden with closed-shell (singlet) reagents
X and XO. All but two of the X/XO reagents used here are
closed-shell species, including ethylene oxide/ethylene, pyO/
py, Me2SO/Me2S, and OP(OMe)3/P(OMe)3. The two exceptions
are NO2 and TEMPO, which are both free radicals with an
unpaired electron and are therefore doublet states. Reaction of
quartet 119 with doublet NO2 can therefore give doublet 2 +
doublet NO in a spin-allowed reaction, as shown in eq 7 (the
orbitals underneath the reagents show only the unpaired
electrons). Reaction 7 formally involves transfer of a triplet
oxygen atom NO2 to CrIII. This analysis is consistent with the
measured efficiency of 22% for reaction 7.
The Cr-OP(OMe)3 binding energy is estimated to be near 13
( 5 kcal mol-1 based on the CID experiments at variable
collision energy (Table 2). The Cr-O bond strength in 1-TEM-
PO is taken as equal to the Ti-O BDE in Cp2TiCl(TEMPO),
41
25 kcal mol-1
.
This should be an upper limit because Cr is
less oxophilic than Ti and because CrIV is more oxidizing than
TiIV [these compounds are formally M4+(TEMPO-)]. In addi-
tion, the CrV sides of these reactions are likely to be entropically
favorable by ∼10 kcal mol-1 (∆S° ≈ 34 cal K-1 mol-1).42 The
reactions at the top of Scheme 2, with X ) OP(OMe)3 or
TEMPO, should be at least 10 kcal mol-1 enthalpically
unfavorable. Finally, the BDEs of OP(OMe)3 and TEMPO are
taken to be the same as those of OP(OEt)3 and Me2NO•,
respectively (Table 3). With these various values, Scheme 2
(36) (a) Meot-Ner, M. J. Am. Chem. Soc. 1982, 104, 5–10. (b) Bucher,
H.-H.; Grutzmacher, H.-F. Int, J. Mass Spectrom. Ion Processes 1991,
109, 95–104. (c) Baciocchi, E.; Bietti, M.; Chiavarino, B.; Crestoni,
M. E.; Fornarini, S. Chem.sEur. J. 2002, 8, 532–537.
(37) Afeefy, H. Y.; Liebman, J. F.; Stein, S. E. Neutral Thermochemical
Data. In NIST Chemistry WebBook, NIST Standard References
Database Number 69, Linstrom, P. J.; Mallard, W. G., Eds.; National
Institutes of Standards and Technology, Gaithersburg, MD, 20899,
http://webbook.nist.gov/chemistry/ (accessed 9 October 2009).
(38) Gas-phase A + B f C + D reactions typically have small entropy
changes. In this case, the S° values for NO2 and NO (57.37, 50.37 cal
K-1 mol-1) differ only by 7 cal K-1 mol-1. For a discussion of
entropies in hydrogen-atom transfer reactions, including cases where
∆S° * 0, see: (a) Mader, E. A.; Manner, V. W.; Markle, T. F.; Wu,
A.; Franz, J. A.; Mayer, J. M. J. Am. Chem. Soc. 2009, 131, 4335–
4345. (b) Mader, E. A.; Davidson, E. R.; Mayer, J. M. J. Am. Chem.
Soc. 2007, 129, 5153–5166.
(39) For XO f X + O, when S°(XO) ) S°(X) the gas-phase BDFE will
be smaller than the gas-phase BDE by TS°(O) ) (298 K) (38.49 cal
TEMPO can react with 1 in a similar spin-allowed fashion.43
Therefore the absence of O-atom transfer from TEMPO to
K-1 mol-1) ) 11.5 kcal mol-1
.
(40) Schro¨der, D.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1995, 34,
1973–1195.
(41) Acree, W. E., Jr.; Pilcher, G.; Ribeiro da Silva, M. D. M. C. J. Phys.
Chem. Ref. Data 2005, 34, 553–572.
(43) Huang, K.-W.; Han, J. H.; Cole, A. P.; Musgrave, C. B.; Waymouth,
R. M. J. Am. Chem. Soc. 2005, 127, 3807–3816.
(42) Conry, R. R.; Mayer, J. M. Inorg. Chem. 1990, 29, 4862–4867. The
BDE(PhI-O) of 26 kcal is estimated from the solution thermodynamic
oxygen-transfer potentials (TOP) in this reference, assuming constant
solvation effects on the XO/X couples.
(44) Conry, R. R.; Mayer, J. M. Inorg. Chem. 1990, 29, 4862–4867.
(45) The initial adduct 1-TEMPO is probably best described as CrIV-
(TEMPO-) and is probably a spin triplet. This species can then
dissociate a doublet tetramethylpiperidinyl radical to give 2.
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4342 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010

Gas-Phase Reactions of Chromium-Porphyrin Complexes
A R T I C L E S
chromium (eq 6) is likely due to the reaction being endoergic,
rather than due to a spin-based spin barrier. The thermodynamic
values given above support this conclusion: the CrtO BDE is
indicated to be only a little larger than that the N-O BDE in
NO2 (73 kcal mol-1), and the N-O BDE in TEMPO is estimated
as being significantly stronger, close to that in Me2NO·, namely
86 kcal mol-1 (Table 3). Hence, the reaction of 1 with TEMPO
islikelytobeunfavorablebasedonthermodynamicconsiderations.
The reactivity pattern of 1 and 2 discussed above appears to
be a case where the spin state change between CrIII and CrV
affects the reaction chemistry. The most striking feature of these
chromium complexes is their low reactivity in either reaction
direction. One notable exception is the facile and spin-allowed
reaction of 1 + NO2. The other exception is the reaction of 2
+ P(OMe)3 to give (TPFPP)CrIII(OP(OMe)3)+. This reaction is
so highly exoergic that it could proceed in a spin-allowed fashion
reactivity behavior of the 1/2 couple has been interpreted with
reference to the thermodynamic parameters of the reactions,
namely to the X-O bond dissociation free energies of the XO/X
neutral reaction partners. The gas-phase reaction of 1 with NO2
sets an approximate value for the CrVtO BDE of 2. The
efficiency of this reaction and the absence of the reverse reaction
(2 + NO) indicate a CrVtO BDE somewhat larger than the
O-NO BDE. In agreement with this evaluation, no O-atom
transfer is observed with TEMPO, whose N-O BDE is
definitely higher. Both NO2 and TEMPO are free radicals with
one unpaired spin, and therefore they can transfer an oxygen
atom to 1 to give 2 in a spin-allowed process.
Oxygen-atom transfer reactions of the 1/2 couple with closed-
shell XO/X neutrals, however, are formally spin forbidden. In
these cases, the O-atom transfer reactivity is found to be
abnormally low. Compound 2 does not oxidize olefins, sulfides,
or pyridines, and the corresponding epoxides, sulfoxides, and
pyridine N-oxides are not deoxygenated by 1. Thus, there are
substantial kinetic impediments to these reactions, and it seems
likely that this is due to the requirement of a spin change from
doublet 2 to quartet 1. Of the various closed-shell reagents
explored, only trimethylphosphite reacts with 2 to give a CrIII
product ([(TPFPP)CrIII(O(P(OMe)3]+ (1-OP(OMe)3), albeit with
a relatively modest efficiency of 35%. P(OMe)3 has a very high
affinity for an oxygen atom, with an estimated (MeO)3P-O
BDE of >140 kcal mol-1, so that its reaction with 2 could
2
to initially form the E excited state of the CrIII product. The
P(OMe)3 reaction is >60 kcal mol-1 downhill, using the
conservative upper limit on BDE(CrVtO) from the lack of
deoxygenation of TEMPO. This is much larger than the estimate
2
of the energy of the E state by analogy with [CrIII(NH3)6]3+
,
∼37 kcal mol-1 (as noted above). Attempts in solution to
observe emission from such a potential 2E intermediate (chemi-
luminescence), potentially generated thermally upon reaction
of the (salen)CrV(O)+ derivatives with phosphines, have unfor-
tunately not been successful.34 Consistent with the suggestion
that the P(OMe)3 reaction could proceed via a doublet CrIII
excited state, this reaction has a rate constant very similar to
that of the analogous reaction of gas-phase (TPFPP•+)FeIVO with
trimethylphosphite. The iron reaction proceeds with somewhat
higher efficiency and proceeds with full oxygen atom transfer
to give [(TPFPP)FeIII]+, while 2 forms the CrIII adduct (TPF-
PP)CrIII(OP(OMe)3)+, perhaps because of the more substitution-
inert character of CrIII complexes.
2
proceed in a spin-allowed fashion to form the E excited state
of the CrIII product. Thus, all of the data are consistent with
spin-forbidden reactions of 1 and 2 being inhibited while
thermodynamically favorable spin-allowed reactions occur with
reasonable efficiency.
As a final remark, the relative reactivity of [(TPFPP)-
Metal(O)]+ complexes in the gas phase may now be compared
with regard to the oxygen-atom transfer reaction with olefins.
While the iron complex reacts with 2-butenes with ∼1%
efficiency21a and the manganese complex with 0.8-8%
efficiency,21c depending on the configuration at the double bond,
the corresponding chromium complex is unreactive, even with
a highly activated olefin such as ꢀ-pinene. Dissecting the
thermodynamic and kinetic contribution to the observed reactiv-
ity behavior is not straightforward. Moreover the mechanism
of the alkene epoxidation by compound I and model species
involves intermediates of different nature and presents TSR and
multistate reactivity scenarios.6 A more complete analysis of
these issues will require computational examination, as well as
more experimental studies.
5. Conclusions
High-valent chromium(V)–oxo complexes have been obtained
from chromium(III) precursors both by an oxidation reaction
in solution, and by a gas-phase reaction using NO2 as an oxygen-
atom donor. Among several sampled species, the [(TPFPP)-
CrVO]+ ion (2) from the oxidation of the [(TPFPP)CrIII]+ (1)
has been the most thoroughly studied. The presence of a CrVO
unit in 2 has been probed by a variety of processes in the gas
phase, including O-atom transfer to potential reductants and the
assessment of association equilibria. These tools allow us to
establish that the complex formed in solution using PhIO and
then transferred to the gas phase by ESI, and the species obtained
by the gas-phase reaction of 1 with NO2, are actually the same
chromium(V)–oxo cation [(TPFPP)CrVO]+ (2). Gas-phase FT-
ICR mass spectrometry has allowed us to investigate the gas-
phase reactivity of these cationic Cr species.
The major purpose of this study was aimed at exploring the
role of spin state constraints on the O-atom transfer reactivity
connecting [(TPFPP)CrIII]+ (1) and [(TPFPP)CrVO]+ (2). To this
end, gas-phase reactivity data have been collected regarding both
the bimolecular kinetics for the reaction with selected gaseous
neutrals, and the fragmentation patterns observed when adducts
of 1 and selected ligands undergo CID at variable energy. The
Acknowledgment. We gratefully acknowledge support from the
Universita` di Roma “La Sapienza” and the Italian Ministero
dell’Universita` e della Ricerca to S.F. and M.E.C., and from the
U.S. National Institutes of Health (GM50422) to J.M.M.
Supporting Information Available: Derivation of hydroxy-
lamine BDE and BDFE, analysis of reactions with TEMPO,
Table S1. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA9103638
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J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010 4343