Proton-induced Lewis acidity of unsaturated iridium amides

Article abstract of DOI:10.1021/ja0653831

Oxidation of Cp*Ir((rac-TsDPEN)H (DPEN = H₂NCHPhCHPhNTs) with Cp₂FePF₆ or Ph₃CPF₆ in MeCN solution generates [Cp*Ir(TsDPEN)(NCMe)]PF₆ ([1H(NCMe)]PF₆) together with H₂ and Ph₃CH, respectively. Labeling studies revealed that the Ir-H was abstracted. The formation of a transient electrophilic species is implicated by the formation of a cyclometalated derivative. The labile species [1H(NCMe)]⁺ was also obtained by protonation of the diamido derivative Cp*Ir(TsDPEN-H) (1) in MeCN solution (BAr^F₄^- = B(C₆H₃-3,5-(CF₃)₂)₄^-). The unsaturated, naked cation [1H]BAr^F₄ can be prepared by protonation of 1 with H(OEt₂)₂BAr^F₄ in CH₂Cl₂ solution or by thermal elimination of MeCN from [1H(NCMe)]⁺. Crystallographic analysis confirms the structure of this 16e cation in [1H]BAr^F₄. The formally unsaturated species 1 and [1H]BAr^F₄ have strongly contrasting Lewis acidities, with the cation binding PPh₃, CO, and NH₃. 1 does not measurably bind these same ligands. [1H]BAr^F₄ is reactive toward H₂, at least in the absence of inhibiting donor ligands such as MeCN. [1H]BAr^F₄ (CH₂Cl₂ solutions) catalyzes the addition of H₂ to 1 by proton transfer from an apparent dihydrogen complex. This work demonstrates that the protonation activates the Lewis acidity of unsaturated Ir(III) amides, giving rise to novel organometallic Lewis acids. Copyright

Full text of DOI:10.1021/ja0653831

Published on Web 09/16/2006
Proton-Induced Lewis Acidity of Unsaturated Iridium Amides
Zachariah M. Heiden and Thomas B. Rauchfuss*
School of Chemical Sciences, UniVersity of Illinois, Urbana, Illinois 61801
Received July 26, 2006; E-mail: rauchfuz@uiuc.edu
Amine-coordinated metal hydrides have attracted ever increasing
attention as the basis of new generations of organometallic catalysts
and reagents.1 A topical class of such species are the Ikariya-
Noyori-Tani catalysts for the enantioselective transfer hydrogena-
tion of ketones and imines.2 In the catalytic cycle, 16e amido
complexes, L4M-NR2, add H2 to form the corresponding 18e amino
hydrides L4HM-NHR2, which transfer the equivalent of a proton
and hydride to polar substrates. Despite the considerable advances
in catalysis, the coordination chemistry per se of the amino-amido
platform remains lightly explored. These 16e amido species
represent a potentially rich source of unusual organometallic Lewis
acids.3 We were also intrigued by the possibility of hydrogenase-
like4 redox in these species, which are renown for their ability to
react with hydrogen heterolytically.
Figure 1. Molecular structure of the cation [Cp*Ir(H2NCHPhCHPhNTs)]-
BArF4, with thermal ellipsoids set at 50% probability level. Key bond
distances (Å): Ir-NTs, 1.984(4); Ir-NH2, 2.096(5).
A cyclic voltammetry study revealed that Cp*Ir(TsDPEN)H
(1HH) (DPEN ) H2NCHPhCHPhNTs) irreversibly oxidizes at the
relatively mild potential of -0.12 V versus Fc/Fc+ (Fc ) Cp2Fe)
in MeCN solution. Chemical oxidation required one equiv FcPF6;
additional Fc+ had no effect. Analysis of the products indicated
that the reaction proceeded efficiently according to eq 1.
to the resolved diamine, which tended to give oily, less easily
characterized products. Treatment of the 1 with H(OEt2)2BArF4 in
MeCN solution cleanly gave [1H(NCMe)]BArF4 (BArF - ) B(C6H3-
4
3,5-(CF3)2)4-).7 A similar cationic Ru complex was implicated by
Noyori and co-workers in recent work on asymmetric hydrogena-
tion.8 The MeCN ligand in [1H(NCMe)]BArF4 is labile and can be
removed by subjecting the solid to a vacuum, as signaled by a color
change from yellow to red. Similarly, protonation of 1 with
H(OEt2)2BArF4 in CH2Cl2 solution gave the salt of the unsaturated,
Cp*Ir(H NCHPhCHPhNTs)
+ FcPF6 + MeCN f
2
1H(H)
[Cp*Ir(H2NCHPhCHPhNTs)(NCMe)]PF
6 + Fc + 0.5H2
“naked” cation [1H]BArF . This red-colored salt, which was isolated
4
[1H(NCMe)]PF6
1
in analytical purity, displays the expected H NMR spectroscopic
features, such as diastereotopic NH2 centers (δ4.18, 4.40 in CD2Cl2
soln). A crystallographic study indicated that [1H]BArF4 has a nearly
planar Cp* (centroid) IrN2 core; the Ir-NTs distance is shortened
by ca. 0.2 Å relative to the value for related 18 e adducts9 (Figure
1).
(1)
Oxidation of 1H(H) with Ph3CPF6 gave [1H(NCMe)]PF6, the
coproduct being exclusively Ph3CH.5 Since 1H(H) is formed from
H2, its oxidation represents a formal oxidation of dihydrogen, the
proton residing on the untosylated amine. Iridium undergoes no
net change in oxidation state in this conversion.
Oxidation of 1H(H) is localized on the Ir-H subunit, since
treatment of the partially deuterated complex Cp*Ir(TsNCH(Ph)-
CH(Ph)ND2)H with FcPF6 yielded only H2 (not HD) and [Cp*Ir-
As expected for a highly electrophilic 16e Ir(III) species,
[1H]BArF binds a wide range of Lewis bases. MeCN and PPh3
4
rapidly gave the expected adducts [1H(NCMe)]BArF and
4
[1H(PPh3)]BArF . CO gave [1H(CO)]BArF (νCO ) 2064 cm-1
,
4
4
CH2Cl2). The ammonia complexes, [1H(NH3)]BF4 and [1H(NH3)]-
1
(TsNCH(Ph)CH(Ph)ND2)(NCMe)]PF6. H NMR spectroscopy re-
BArF , were prepared from 1 and NH4BF4 or by the addition of
4
vealed that oxidation of 1H(H) with FcPF6 is competitive with the
formation of a metallacycle, 2. This species arises from the
cyclometalation of one phenyl ring on the diamine backbone
(Scheme 1, Supporting Information including X-ray crystal-
lography). The formation of 2 points to the transient formation of
an electrophilic species capable of C-H activation. Metallacycle
formation was favored when the oxidation was conducted in CH2Cl2
(25% yield), more so than in MeCN solution (10%). Independent
experiments indicate that [1H]+ (see later) is not an intermediate
in the formation of the metallacycle. A related metalated amido-
amine of ruthenium was recently described by Ikariya.6
NH3 to cation [1H]BArF , respectively.
4
[1H(NCMe)]+ is mildly acidic with a pKa of 21 in MeCN
solution. Deprotonation of MeCN solutions of [1H(L)]+ (L )
MeCN, PPh3) with 1,1,3,3-tetramethylguanidine (TMG) gave 1 and
free ligand, an example of an SN1CB pathway. In CH2Cl2 solution,
the base-free derivative, [1H]+, is readily deprotonated by Et3N to
give the diamide 1, which is also 16e but not Lewis-acidic. Solutions
of 1 are unreactiVe toward PPh3 and MeCN, until the addition of
a Brønsted acid. Proton-exchange between 1 and the MeCN adduct
of its conjugate acid, [1H(NCMe)]BArF , is slow on the NMR time
4
scale in MeCN solution, a finding that reinforces the strong
electronic distinction between these species.
Given the promise of [1H(NCMe)]+ as a Lewis acid, we
developed alternative methods for the synthesis of a range of related
complexes (Scheme 1). Overall, the synthetic chemistry was
facilitated by working with the racemic (()-TsDPEN as opposed
Preliminary studies show that [1H]BArF4 is reactive toward H2.
Under an atmosphere of H2, a CH2Cl2 solution of [1H]BArF
4
completely converted to [Cp*2Ir2H3]+ over the course of 24 h.10
9
13048
J. AM. CHEM. SOC. 2006, 128, 13048-13049
10.1021/ja0653831 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Supporting Information Available: Methods, properties of new
This hydrogenolysis highlights a vulnerability of the IrTsDPEN
system; the corresponding diaminocyclohexane derivative hydro-
genolyzes more rapidly. MeCN inhibits this hydrogenolysis by
preventing formation of the Brønsted-acidic dihydrogen complex.
compounds, and pKa determination of [1H]BArF ; crystal structures of
4
[Cp*Ir(TsDPEN)]BArF and cyclometalated complex 2. This material
4
is available free of charge via the Internet at http://pubs.acs.org.
The high reactivity of [1H]BArF (CH2Cl2 solutions) toward H2
4
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