Redox-switched oxidation of dihydrogen using a non-innocent ligand

Article abstract of DOI:10.1021/ja076801k

Organometallic complexes containing non-innocent ligands of the type Cp^*Ir(^tBA^FPh)(1), where H₂^tBA^FPh is 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol, were found to activate H₂ in a redox-switchable manner. The 16e- complex 1 was inert with respect to H₂, CO, as well as conventional basic substrates until oxidation. Oxidation of 16^-electron 1 with 1 equiv of Ag⁺ resulted in ligand-centered oxidation affording salts of [1]⁺, which were characterized by crystallographically, EPR, and elemental analyses. [1]⁺ was reduced to 1 in the presence of H₂ and the sterically hindered base, 2,6-(^tBu)₂C₅H₃N, via a pathway that is first-order in both metal and dihydrogen. Compound [1]⁺ forms adducts with MeCN, which inhibits catalysis. The catalytic oxidation of H₂ was established by electrochemical methods to be associated with the monocation. Copyright

Full text of DOI:10.1021/ja076801k

Published on Web 12/29/2007
Redox-Switched Oxidation of Dihydrogen Using a Non-Innocent Ligand
Mark R. Ringenberg, Swarna Latha Kokatam, Zachariah M. Heiden, and Thomas B. Rauchfuss*
Department of Chemistry, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801
Received September 8, 2007; E-mail: rauchfuz@uiuc.edu
Non-innocent ligands have long attracted attention because of
their unusual redox properties and their apparent, often deceptive,
ability to stabilize metals in unusual oxidation states.1 Copper
quinoid complexes have been demonstrated as excellent oxidation
catalysts2 and nickel dithiolenes binding alkenes in a redox-
switchable manner.3 C-C coupling with complexes of diimine and
aminophenolate ligands has been reported more recently.4 In this
report, we describe experiments demonstrating a role for non-
innocent ligands in the activation of dihydrogen.
The present work focuses on complexes of the type (η5-C5R5)-
MX2 where R ) H, Me. Prototypical complexes of this type are
Cp*M(E2C6H4) (Cp* ) C5Me5-, E ) O, S, and M ) Rh, Ir).5,6
Such species display low Lewis acidity despite their 16e config-
uration, consistent with the π-donor properties of dioxalene and
dithiolene ligands. We posed the following question: could their
Figure 1. Cyclic voltammogram of ∼10-3 M CH2Cl2 solutions of 1 (100
mV/s, 0.1 M Bu4NPF6; E1/2 ) 0.050, 0.355 V vs Fc/Fc+).
reactivity toward dihydrogen be “switched on” by oxidation of the
complex, especially when the oxidation is ligand-localized?
In considering the oxidation of Cp*M(E2C6H4), two problems
become apparent: (i) dithiolenes and dioxalenes are relatively
ineffective at stabilizing cationic derivatives as reflected in their
high redox potentials;6 and (ii) due to the lack of steric protection,
oxidation would be expected to induce aggregation, thereby
precluding Lewis acidity.7 Guided by these considerations, we
examined complexes of the electronically related ligand tBAFPh2-
t
(where H2 BAFPh is 2-(2-trifluoromethyl)anilino-4,6-di-tert-bu-
tylphenol) that has been popularized by Wieghardt et al.8,9
t
Complexes derived from H2 BAFPh enjoy excellent solubility,
withstand one- and two-electron oxidations, and the resulting
oxidized products resist dimerization.
The Cp*Ir(tBAFPh) (1) was synthesized from [Cp*IrCl2]2 and
t
H2 BAFPh in the presence of 2 equiv of base. This intensely colored
1
species exhibits conventional H and 19F NMR spectra, and its
optical properties are unaffected by changes in concentration and
solvents. X-ray crystallographic analysis confirms that 1 is mon-
omeric and contains the dianionic ligand (tBAFPh2-) as indicated
Figure 2. UV-vis spectrum of 1.0 × 10-4 M [1]PF6 in a CH2Cl2 solution
with 1.5 equiv of TBP and 0.33 atm H2 (17.35 equiv). Each trace required
2.5 s with 75 s delay between traces. Formation of 1 is indicated by the
growth of the peak at 460 nm.
by the ring C-C distances that are nearly equidistant at 1.40 (
0.01 Å. The long C(11)-O(1) and C(16)-N(1) bond distances
in 1 at 1.34(1) and 1.33(1) Å also support the description of
tBAFPh2- (Supporting Information).9 Complex 1 displays no affinity
for donor ligands such as CO, MeCN, much less H2.
Cyclic voltammetric measurements indicate that 1 undergoes
In the presence of the non-coordinating base 2,6-(t-Bu)2C5H3N
(TBP), the reaction progress was monitored by changes in optical
spectra (Figure 2). In a control experiment to establish a possible
role of 1 as the base, a 2:1 mixture of [1]PF6 and 1 (CD2Cl2 solution)
was found to be unreactive toward H2, except for the formation of
small amounts of Cp*2Ir2H3+. The closeness of the 10/+ and 1+/2+
couples indicates Kdisp ∼ 0.085 and thus raises the possibility that
small amounts of 12+ are responsible for the observed oxidation of
H2. Chronoamperometry experiments showed, however, that the
rate of H2 oxidation increased by <5% when the electrolysis was
conducted at 662 versus 514 mV (vs Ag/Ag+). The reaction was
unaffected by the addition of Hg.
The rate of reduction of [1]PF6 was first-order in both [1]PF6
and [H2] with an overall second-order rate constant of 0.57 ((0.14)
M-1 s-1. In a MeCN solution, however, the reduction of [1]PF6 by
H2 is slower, indicating that MeCN competes with binding of H2.
sequential one-electron oxidations in CH2Cl2 solution at readily
accessible potentials (Figure 1). A Cottrell plot (ip vs (scan rate)1/2
)
indicates that both steps are diffusion-controlled. On a preparative
scale, oxidation of 1 by a CH2Cl2 suspension of AgPF6 gave the
corresponding salt [1]PF6 isolated in analytical purity. [1]PF6 was
found to be EPR-active and paramagnetic with an effective magnetic
moment of 1.75 µB.
Our key finding is that a CH2Cl2 solution of [1]PF6 is reduced
by H2 (eq 1).
2 [Cp*Ir(tBAFPh)]+ + H2 f 2 Cp*Ir(tBAFPh) + 2 H+ (1)
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C O M M U N I C A T I O N S
complex. The C-O bond is significantly shortened (Figure 3); the
Ir-N and Ir-O distances are elongated consistent with the N, O
ligand being a poorer donor. The Ir-Cp*(centroid) distance
contracts slightly upon oxidation of 1 (1.788(2) vs 1.766(2) Å).
The C-C distances within the aminophenolate display increased
bond alternation, diagnostic of semiquinonate character,8,9 which
is manifested in enhanced Lewis acidity for the metal center.
In summary, ligand-based oxidation has been demonstrated to
enhance the Lewis acidity of a metal complex sufficiently to induce
a reaction with H2.14 The kinetics, stoichiometry, and crystal-
lography provide a consistent pattern that encourages further
investigations of non-innocent ligands in other aspects of organo-
metallic chemistry. We note that redox is an inextricable aspect of
the hydrogenases.15
Figure 3. Molecular structure of the cation in [1]BArF with thermal
4
ellipsoids shown at 50% probability. Key distances (Å, corresponding
distances for 1 in brackets): Ir-N1, 2.010(6) [1.963(4)]; Ir-O1 2.045(5)
[1.996(3)]; C11-O1, 1.287(8) [1.34(1)]; C12-N1, 1.343(9) [1.33(1)]; C11-
C16, 1.428(10) [1.407(5)]; C11-C12, 1.452(9) [1.416(5)]; C12-C13, 1.398-
(10) [1.396(6)]; C13-C14, 1.363(10) [1.396(6)]; C14-C15, 1.454(11)
[1.388(5)]; C15-C16, 1.371(10) [1.395(6)].
Acknowledgment. This research was supported by the Depart-
ment of Energy. We thank Scott Wilson for help with the
crystallographic analysis, and Fre´de´ric Gloaguen (CNRS-Brest) for
advice on the electrochemistry.
Supporting Information Available: Synthetic methods, electro-
chemical results, kinetics, and crystallographic analysis of 1 and
Scheme 1. Proposed H2 Oxidation Cycle
[1]BArF . This material is available free of charge via the Internet at
4
http://pubs.acs.org.
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The rate law is consistent with the intermediacy of [1(H2)]+ (Scheme
1) although it is difficult to completely exclude the involvement of
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1.13 The reaction gave 1 (80% yield) as well as some [Cp*2Ir2H3]+
and precipitated copious amounts of silver metal. In a separate
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found to uptake 3 equiv of H2 over the course of 1.5 h.
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The oxidation of H2 is attributed to the increased Lewis acidity
of [1]+ induced by ligand-centered oxidation. Complex [1]+ forms
stable adducts with acetonitrile, whereas the neutral complexes do
not. Crystallographic analysis of the naked cation in [1]BArF
4
(BArF ) B(C6H3-3,5-(CF3)2)4) revealed a quasi-pentacoordinate
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