Rhodium and iridium complexes with α-diketimine ligands: Oxidative addition of H2 and O2

Article abstract of DOI:10.1002/ejic.201100135

The treatment of [M(μ-Cl)(coe)₂]₂ (M = Rh, Ir; coe = cyclooctene) with the α-diketimines ArN=C(Me)C(Me)=NAr (Ar = Ph, Xy; Xy = 2,6-Me₂C₆H₃) in thf, followed by the addition of CNtBu, gave the α-diketimine complexes [M(Cl){ArN=C(Me)C(Me)= NAr} (CNtBu)] (1: M = Rh, Ar = Ph; 2: M = Rh, Ar = Xy; 3: M = Ir, Ar = Xy). The reaction of 3 with dihydrogen resulted in the formation of the iridium dihydrido complex [Ir(Cl)(H)₂{XyN=C(Me)C(Me)=NXy}(CNtBu)] (4). The complexes 2 and 3 reacted with O₂ or ¹⁸O₂ to yield the peroxido complexes [M(Cl)(O₂){XyN=C(Me)C(Me)=NXy}(CNtBu)] (5a: M = Rh; 6a: M = Ir) and [M(Cl)(¹⁸O₂){XyN=C(Me)C(Me)=NXy} (CNtBu)] (5b: M = Rh; 6b: M = Ir), respectively. All of the complexes were characterized by NMR and IR spectroscopy. In addition, complexes 3 and 5a were characterized by X-ray crystallography. Unique rhodium(I) and iridium(I) α-diketimine compounds were synthesized starting from [M(μ-Cl)(coe) ₂]₂ (M = Rh, Ir, coe = cyclooctene). Treatment with H ₂ or O₂ ledto the formation of an Ir^III dihydrido complex as well as Rh^III and Ir^III peroxido complexes. The reactions with H₂ and O₂ are reversible.

Full text of DOI:10.1002/ejic.201100135

FULL PAPER
DOI: 10.1002/ejic.201100135
Rhodium and Iridium Complexes with α-Diketimine Ligands:
Oxidative Addition of H2 and O2
Anna Penner[a] and Thomas Braun*[a]
Dedicated to Prof. Dr. Hansgeorg Schnöckel on the occasion of his 70th birthday
Keywords: Rhodium / Iridium / Diimines / Hydrido complexes / Peroxido ligands
The treatment of [M(μ-Cl)(coe)2]2 (M = Rh, Ir; coe = cyclooct-
ene) with the α-diketimines ArN=C(Me)C(Me)=NAr (Ar =
Ph, Xy; Xy = 2,6-Me2C6H3) in thf, followed by the addition
of CNtBu, gave the α-diketimine complexes [M(Cl){ArN=C-
(Me)C(Me)=NAr} (CNtBu)] (1: M = Rh, Ar = Ph; 2: M = Rh,
Ar = Xy; 3: M = Ir, Ar = Xy). The reaction of 3 with dihydro-
gen resulted in the formation of the iridium dihydrido com-
plex [Ir(Cl)(H)2{XyN=C(Me)C(Me)=NXy}(CNtBu)] (4). The
complexes 2 and 3 reacted with O2 or 18O2 to yield the perox-
ido complexes [M(Cl)(O2){XyN=C(Me)C(Me)=NXy}(CNtBu)]
(5a: M = Rh; 6a: M = Ir) and [M(Cl)(18O2){XyN=C(Me)-
C(Me)=NXy}(CNtBu)] (5b: M = Rh; 6b: M = Ir), respectively.
All of the complexes were characterized by NMR and IR
spectroscopy. In addition, complexes 3 and 5a were charac-
terized by X-ray crystallography.
tain tetradentate tris(2-pyridylmethyl)amine (tpa) ligands
have been described by Gal and co-workers.[13] Exposure of
the complexes [M(tpa)(C2H4)][BPh4] (M = Rh, Ir) to air in
the solid state led to a peroxygenation to give 3-metalla-1,2-
dioxolanes.[13] The cationic peroxido complexes [Ir(O2)-
(phen)(cod)][X] (X = Cl, I, SCN, phen = phenanthroline)
were also described in the literature.[14] The pincer peroxido
complex [Rh(Cl)(O2){2,6-(C{Me}=NiPr)2C5H3N}] reacted
with HCl(aq.) to give H2O2 and [Rh(Cl)3{2,6-(C{Me}=
NiPr)2C5H3N}].[15] Dioxygen reacted with a RhI dimethyl-
bipyridine species to produce the η2-peroxido complex
[Rh(Cl)(O2)(dmbpy)(dmso)] (dmbpy = 4,4Ј-dimethyl-2,2Ј-
bipyridine, dmso = dimethyl sulfoxide).[16] The complex
[Rh(Cl)(O2)(tbbpy)(CNtBu)] (tbbpy = 4,4Ј-di-tert-butyl-
2,2Ј-bipyridine) oxygenates PEt3 to give phosphane oxide
and trans-[Rh(Cl)(CNtBu)(PEt3)2].[17]
α-Diimine ligands are known to stabilize organometallic
complexes.[18] However, RhI and IrI diimine species are
fairly rare and most of them have an ionic[19,20] or a di-
meric[21] structure.[22–24] Haynes et al. analyzed the oxidat-
ive addition of MeI to [RhI{ArN=C(Me)C(Me)=
NAr}(CO)] (Ar = 2-iPrC6H4).[22] Westcott et al. prepared
the complex [Rh(η6-catBcat){ArN=C(Me)C(Me)=NAr}]
(cat = 1,2-O2C6H4, Ar = 2,6-iPr2C6H3) by addition of
B2cat3 to [Rh(acac){ArN=C(Me)C(Me)=NAr}] (acac =
acetylacetonate).[21a] Rhodium and iridium diimine com-
plexes have been applied as active catalysts for the hydro-
amination of 2-(2-phenylethynyl)aniline to N-(2-methyl-
vinyl)-2-phenylindole.[23a] The synthesis of cationic RhI car-
bonyl diimine complexes has also been reported.[20,25] The
synthesis of the cationic iridium diimine complex
Introduction
Transition metal peroxido complexes are of particular
interest when they are formed from molecular oxygen be-
cause of the push for “green” oxidations or oxygenations.[1]
They can play a key role in these processes.[1–3] RhIII and
IrIII peroxido complexes have been reported in the litera-
ture, some of which reversibly bind dioxygen.[4,5] We dem-
onstrated that peroxido complexes can be intermediates in
the rhodium-mediated formation of peroxides, but they are
also useful starting compounds for the formation of rho-
dium perborates and perboronates.[6] The peroxido complex
trans-[Rh(4-C5F4N)(O2)(CNtBu)(PEt3)2] can be applied in
the oxygenation of pinacolborane.[7] Tejel et al. found that
1,5-cyclooctadiene (cod) can be oxygenated to yield 4-cyclo-
octenone in the coordination sphere of rhodium.[8] The con-
version proceeds presumably via an intermediate peroxido
species.[8] However, metal-bound phosphanes are prone to
oxygenation reactions and, therefore, the corresponding
peroxido complexes are often less suitable as oxygenation
catalysts.[6,9,10] The synthesis and characterization of the
rhodium and iridium peroxido compounds without any
phosphane ligands is hence highly desirable. Only a limited
number of peroxido compounds that do not include any
phosphanes as additional ligands have been reported, which
include the complexes with N-heterocyclic carbene li-
gands.[11,12] Cationic rhodium peroxido complexes that con-
[a] Humboldt-Universität zu Berlin, Department of Chemistry,
Brook-Taylor-Str. 2, 12489 Berlin, Germany
Fax: +49-30-2093-6939
E-mail: thomas.braun@chemie.hu-berlin.de
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[Ir(Cl)(Cp*){ArN=C(Me)C(Me)=NAr}][Cl] (Cp* = penta- bits two strong absorption bands at 2100 and 2067 cm–1.
methylcyclopentadienyl, Ar = 4-MeC6H4) has been de- In comparison, the IR spectrum for the [Ir(Cp*){κ2-(C,N)-
scribed by Tilset et al.[19] The cyclopentadienyl (Cp) diimine tBuN=C=NtBu}(CNtBu)] complex shows two absorption
and bipyridine complexes of Rh and Ir have been thor- bands at 2110 and 2065 cm–1 for the terminal isocyanide,[32]
oughly investigated because of their role in proton photore- whereas the [Rh(Cl)(tbbpy)(CNtBu)] complex exhibits two
duction processes,[26] CO2 reduction,[27] or NADPH/ strong absorption bands at 2089 and 2056 cm–1.[17] The oc-
NADP+ redox chemistry.[28]
currence of two absorption bands in the IR spectra of com-
This report describes the synthesis, structural characteri- parable monoisonitrile complexes has been documented in
zation, and reactivity of a new class of square-planar rhodi- the literature.[33,34]
um(I) and iridium(I) complexes with α-diketimine ligands,
and their reactivity towards dihydrogen and dioxygen.
The 1H NMR spectrum of 1 in [D8]thf shows two singlets
at δ = 1.37 and 0.23 ppm, which were assigned to the methyl
groups on the backbone of the α-diketimine ligand. Similar
chemical shifts have been found for [Rh(I){ArN=C(Me)-
C(Me)=NAr}(CO)] (Ar = 2,4,6-Me3C6H2) (δ = 1.64 and
1.04 ppm in CD2Cl2).[22] A singlet at δ = 1.15 ppm was ob-
served for the tBu group of the coordinated isocyanide. The
1H NMR spectrum for 2 revealed resonances for the methyl
groups of the α-diketimine backbone at δ = 0.62 and
–0.54 ppm. The corresponding chemical shifts for
[Ir(Cl){XyN=C(Me)C(Me)=NXy}(CNtBu)] (3) are found
at –0.09 and –2.41 ppm, respectively. The 13C NMR spec-
trum for 1 shows two doublets at δ = 169.78 and
Results and Discussion
The treatment of [M(μ-Cl)(coe)2]2 (M = Rh, Ir; coe =
cyclooctene) with 1 equiv. of α-diketimine in thf, followed
by the addition of tert-butyl isocyanide, afforded the α-di-
ketimine complexes [Rh(Cl){ArN=C(Me)C(Me)=NAr}-
(CNtBu)] (1: Ar
C(Me)=NAr}(CNtBu)] (2: Ar
=
phenyl), [Rh(Cl){ArN=C(Me)-
2,6-Me2C6H3), and
=
[Ir(Cl){ArN=C(Me)C(Me)=NAr}(CNtBu)] (3: Ar = 2,6-
Me2C6H3) (Scheme 1). The isocyanide was added slowly to
the reaction mixture in order to avoid the irreversible for-
mation of the metal(I) salts [M(CNtBu)4][Cl] (M = Rh,
Ir).[29] It should be noted that Haynes et al. reported that
the synthesis of [Rh(I){PhN=C(Me)C(Me)=NPh}(CO)]
from the dimer [Rh(μ-I)(CO)2]2 is not possible.[22] The IR
spectrum of the solution indicated the presence of [Rh(I)2-
(CO)2]– with [Rh{PhN=C(Me)C(Me)=NPh}2]+ as the
counterion.[22] We did not observe any C–H bond activation
at one of the methyl groups in the backbone of the diaza-
butadiene as was found for [Ir(Cl)(Cp*){κ2-(C,N)-
157.41 ppm for the RhN=C(CH3) carbon atoms (2JRh,C
=
2
3 Hz, JRh,C = 2 Hz) and two doublets at δ = 21.06 and
19.89 ppm for the RhN=C(CH3) carbon atoms (3JRh,C
=
1 Hz). It should be noted that Perutz et al. determined a
3
coupling of JRh,C = 0.9 Hz to rhodium for the quaternary
C atom of the tBu group in the complex [Rh(Cp)-
(CNtBu)2].[34a]
Black crystals of 3 were obtained from a solution of n-
hexane/thf (19 mL/1 mL) at 243 K. The molecular structure
was determined by X-ray diffraction analysis at low tem-
perature (Figure 1). The selected bond lengths and angles
are summarized in Table 1. The iridium atom exhibits a
square-planar coordination sphere. The Ir1–N1 bond
length of 2.053(2) Å for the α-diketimine ligand in the trans
position to the isocyanide is comparable to that of the cor-
responding iridium–nitrogen bond length in [Ir(ArF)-
ArN(H)=C(CH2)C(Me)=NAr}][Cl] (Ar
=
2,4,6-Me3-
C6H2).[19a]
(Br0.6,Cl1.4)(tbbpy)(CNXy)] [ArF
=
3,5-(CF3)2C6H3,
2.046(3) Å][35] or in [Ir(Cl)(Cp*){κ2-(C,N)-CH2C(NXy)-
C(Me)NXy}] [2.081(2) Å].[19a] The Ir1–N2 bond in 3
[1.966(2) Å] is shorter. This is possibly because of a push-
pull interaction that leads to the transfer of electron density
from the π-donor chlorido ligand to the α-diketimine li-
gand, which can act as a π-acceptor.[20d,21a,36] The diimine
Scheme 1. The synthesis of the α-diketimine complexes 1, 2, and 3;
Xy = 2,6-Me2C6H3.
The α-diketimine complexes 1–3 are intensely colored ligand might also be considered to be a redox non-innocent
species. This is indicative of d(metal)Ǟπ*(diimine) charge ligand.[20d,36] It would then be bound as a π-radical
transfer transitions that are commonly encountered in other monoanion and the C–N distances would be fairly long
diimine metal complexes.[19a,30] Thus, complex 1 is dark whereas the C–C bond would be relatively short.[36a,36b] An
brown, complex 2 is dark blue, whereas the Ir chlorido com- antiferromagnetic coupling between an IrII radical and a
plex 3 is dark green. The complexes 1–3 are stable in water, ligand radical would result in a diamagnetic ground state.
although it has been shown that a coordinated isocyanide The ab initio calculations for [Ir(Cp)(HNCHCHNH)] sup-
at rhodium can react with water by nucleophilic ad- ported an ene-1,2-diamido-iridium(III) entity description
dition.[31] The solid state IR spectra for the rhodium com- with C–N bond lengths of 1.353 Å and a C–C distance of
pounds 1 and 2 both show one strong and one medium 1.362 Å.[20d,36d] However, the C=N bond lengths in the α-
absorption band for the CNtBu ligand at 2112 cm–1 (1), diimine ligand in 3 are 1.311(4) and 1.325(3) Å and the sep-
2107 cm–1 (2) and 2066 cm–1 (1), 2071 cm–1 (2), respectively. aration for the C1–C2 bond length is 1.447(4) Å. The irid-
The solid state IR spectrum for the iridium complex 3 exhi- ium–carbon bond length of 1.897(3) Å for the isocyanide
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Rhodium and Iridium Complexes with α-Diketimine Ligands
group in 3 is shorter than that of the corresponding Ir–C
bond
length
in
[Ir(ArF)(Br0.6,Cl1.4)(tbbpy)(CNXy)]
[2.001(5) Å][35] or in [Ir{κ2-(C,C)-CH2C(Me)2-o-C6H4}-
{C(CO2Et)=CH(CO2Et)}(tbbpy)(CNXy)] [1.962(4) Å].[37]
The C21–N3 bond length [1.157(4) Å] was comparable to
the bond lengths found for other complexes with isocyanide
ligands, such as [Ir(Cp*)(μ-H)(μ-dmpm)(CNtBu)Ir(Cp*)-
(H)][OTf]2 [dmpm
= bis(dimethylphosphanyl)methane;
1.16(2) Å].[38] Compound 3 displays a bent C–N–CMe3
linkage with a C21–N3–C22 bond angle of 166.1(3)°, which
is comparable with that observed for the latter complex
[175(1)°].
Scheme 2. The reactivity of the Rh and Ir diketimine compounds
2 and 3 towards H2 and O2.
brations. The strong background fluorescence made the ac-
quisition of the Raman data (Nd:YAG 1064 nm) imposs-
ible. The 1H NMR spectrum for 4 shows six signals between
2.85 and 1.38 ppm for the four nonequivalent methyl
groups in the aryl moiety and the two methyl groups on the
α-diketimine backbone. The singlet at δ = 0.75 ppm was
attributed to the tBu group of the metal-bound isocyanide.
The presence of two hydrides at the iridium center was re-
vealed by the signals at –17.20 and –19.73 ppm. The cou-
pling constant of J = 6.9 Hz between the two hydrides is
consistent with a mutually cis orientation for these two li-
gands.[39] In comparison, the cis dihydrido complex
[Ir(bpin)(H)2(tbbpy)(coe)] (bpin = 4,4,5,5-tetramethyl-1,3,2-
dioxaborolanyl) has a coupling constant of J = 5.3 Hz be-
tween the two hydrido ligands.[40]
Figure 1. An ORTEP diagram of 3. The ellipsoids are drawn at the
50% probability level.
Table 1. Selected bond lengths [Å] and angles [°] for
[Ir(Cl){XyN=C(Me)C(Me)=NXy}(CNtBu)] (3).
The 1H,13C-HMBC spectrum shows two cross peaks,
which have a similar intensity, for the coupling of the hy-
dride resonances with the resonances of the metal-bound
carbon of the isocyanide ligand at –17.20/121 and –19.73/
121 ppm (Figure 2). This could indicate a cis-orientation for
both of the metal-bound hydrogens to the isocyanide group.
Ir1–N1
Ir1–N2
Ir1–Cl1
Ir1–C21
C21–N3
Cl1–Ir1–N1
Cl1–Ir1–N2 170.17(7)
N1–Ir1–N2
2.053(2)
1.966(2)
2.3142(7)
1.897(3)
1.157(4)
94.19(7)
N1–C1
N2–C2
C1–C2
N3–C22
1.311(4)
1.325(3)
1.447(4)
1.453(4)
C21–Ir1–N1 169.98(12)
C21–Ir1–N2 95.91(11)
C21–N3–C22 166.1(3)
77.18(10)
93.23(9)
Cl1–Ir1–C21
In order to elucidate the reactivity of 1, 2, and 3 towards
oxidative addition reactions,[16] we investigated the com-
plexes reactivity towards dihydrogen. The rhodium com-
pounds 1 and 2 did not react with dihydrogen under ambi-
ent conditions in contrast to the rhodium bipyridine com-
plex [Rh(Cl)(tbbpy)(CNtBu)].[17] However, on treatment of
a solution of [Ir(Cl){XyN=C(Me)C(Me)=NXy}(CNtBu)]
(3) with H2, the dihydrido complex [Ir(Cl)(H)2{XyN=C-
(Me)C(Me)=NXy}(CNtBu)] (4) was formed (Scheme 2). In
solution 4 was not very stable and was readily reconverted
to 3 and H2 on bubbling argon through the solution. After
1
23 h the H NMR spectrum of a solution of 4 in [D8]thf
indicated that complex 3 was present in 44%.
The IR spectrum for 4 exhibited a strong and broad ab-
sorption band at 2140 cm–1, which was assigned to the iso-
cyanide vibration. Absorption bands that could be assigned
to the Ir(H)2 moiety were not found. Possibly these bands
1
were overlapped by the iridium isocyanide stretching vi- Figure 2. The H,13C-HMBC spectrum of 4 in C6D6.
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In the 13C NMR domain the signal for the quaternary [Rh(Cl)(O2)(tbbpy)(CNtBu)][17] and one at 2161 cm–1 was
carbon atom attached to the iridium (δ = 121 ppm) indi- found for the tetrafluoropyridyl complex trans-[Rh(4-
cated a rather electron-poor metal center.[41] It should be C5F4N)(O2)(CNtBu)(PEt3)].[6a] The absorption band at
noted that the cationic complex [{Ir(H)(μ-pz) (CNtBu)2}2- 2096 cm–1 was assigned to the isocyanide stretching vi-
(μ-Cl)][OTf] (pz = pyrazolate) shows one broad signal in bration for [Ir(CH2SiMe3)(Me)(SiMe3)(tbbpy)(CNtBu)]
the 13C NMR spectrum at δ = 108.3 ppm, which was as- (the isocyanide is located in the trans position to the SiMe3
signed to the isocyanide entity at the IrIII center, the latter ligand).[37] It should be noted that the phosphane-pincer
was also electron poor.[42] The iridaphosphirane complex RhI compound, [Rh{2,6-C6HMe2(CH2PtBu2)2}(η1-N2)],
[Ir(Cp*){κ2-(C,P)-P(Mes*)C=NXy}(CNXy)] (Mes*
2,4,6-(tBu)3C6H2) shows a signal in the 13C NMR spectrum [Rh(Cl)(IMes)(PPh3)2] [IMes
=
and the (N-heterocyclic carbene)rhodium(I) species, cis-
1,3-bis(2,4,6-trimeth-
=
at δ = 141.5 ppm for the isocyanide carbon.[43] In addition, ylphenyl)imidazol-2-ylidene], reacted with dioxygen to give
the molecular structure of 4 in the solid state was deter- the oxygenated rhodium derivatives, but with no concomi-
mined by X-ray crystallography. Complex 4 crystallizes in tant change in the oxidation state of the transition metal.[11]
the orthorhombic crystal system in the chiral space group The conversion of 2 and 3 into the dioxygen complexes 5a
P212121. Although the data were of poor quality, the analy- and 6a, respectively, was also accompanied by the appear-
sis confirmed the configuration at the iridium. The dihyd- ance of a strong band in the IR spectra at 884 and 843 cm–1,
rido complex 4 adopts a structure with the chlorido ligand respectively. The band shifted to 833 and 796 cm–1 for the
in the cis position to the isocyanide.
18O labeled isotopologues 5b and 6b, respectively, and were
The exposure of a benzene solution of the dark blue assigned unequivocally to the O–O stretching vibration of
compound 2 to molecular oxygen led to the precipitation the η2-peroxido ligands (Figures 3 and 4).[9,44]
of the brown peroxido complex [Rh(Cl)(O2){XyN=C-
(Me)C(Me)=NXy}(CNtBu)] (5a) within 3 h (Scheme 2).
The treatment of 2 with 18O2 gave the isotopologue,
[Rh(Cl)(18O2){XyN=C(Me)C(Me)=NXy}(CNtBu)] (5b).
Compounds 5a and 5b are only soluble in polar solvents
such as thf or 1,2-difluorobenzene. The dioxygen is not
bound very strongly to the rhodium and it dissociates after
prolonged treatment under vacuum or when argon is
1
bubbled through a solution of 5a in thf to give 2. The H
NMR spectrum of a freshly prepared solution of 5a in 1,2-
difluorobenzene indicated that 2 was present in 56%; after
20 h only complex 2 was detected. In the solid state com-
plex 5a extrudes O2 overnight. It has been shown that in
solution the complexes trans-[Rh(X)(CNXy)(PPh3)2] (X =
Cl, Br, SC6F5, C2Ph)[5a] bind molecular oxygen reversibly,
Figure 3. Part of the IR spectra for 5a (_____) and 5b (-----) (ATR,
whereas the complex [Rh(Cl)(tbbpy)(CNtBu)][17] binds O2
diamond).
irreversibly. The formation of [Ir(Cl)(O2){XyN=C(Me)-
C(Me)=NXy}(CNtBu)] (6a) and [Ir(Cl)(18O2){XyN=C-
(Me)C(Me)=NXy}(CNtBu)] (6b) from 3 proceeded in the
solid state under an atmosphere of O2 and 18O2, respec-
tively. The treatment of a solution of 3 in thf with O2 also
gave a certain amount of 6a according to the IR spectra.
However, the conversion was less selective and the other
products produced in the reaction could not be charac-
terized. The iridium peroxido complex 6a is stable in the
solid state for one week, whereas the 1H NMR spectrum of
a solution of 6a in thf showed that 3 was present in 10%
after 4 h. For 5a and 6a we assumed the configurations that
are depicted in Scheme 2. The former configuration was
confirmed by X-ray crystallography (see below).
The complexes 5a, 5b, 6a, and 6b were characterized by
their spectroscopic data. The IR spectrum of the rhodium
peroxido complex 5a exhibits an absorption band at
2208 cm–1 and the iridium peroxido complex 6a has one
absorption band at 2178 cm–1, which can be assigned to the
Figure 4. Part of the IR spectra for 6a (_____) and 6b (-----) (ATR,
diamond).
The absorption band at 497 cm–1 for 5a shifted to
isocyanide stretching vibration at the RhIII or IrIII metal 480 cm–1 for 5b and was tentatively assigned to the RhO
center. In comparison, an absorption band at 2195 cm–1 moiety. Analogously, the absorption band at 503 cm–1 for
was found for the rhodium peroxido complex 6a and 479 cm–1 for 6b was assigned to the IrO moiety.
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Rhodium and Iridium Complexes with α-Diketimine Ligands
The 1H NMR spectrum for 5a in [D6]benzene shows four identical
to
those
found
in
the
complexes
signals between 2.86 and 2.16 ppm for the four nonequiva- [Rh(Cl)(O2)(tbbpy)(CNtBu)] [1.931(12) Å][17] and trans-
lent methyl groups and two signals at 1.42 and 1.36 ppm [Rh(SC6F5)(O2)(CNXy)(PPh3)2] [1.939(4) Å].[5a] The oxy-
for the two methyl groups on the α-diketimine backbone. gen–oxygen bond length in 5a [1.448(14) Å] is comparable
The singlet at δ = 0.56 ppm for 5a was attributed to the to those found for the complexes [Rh(Cl)(O2)(tbbpy)-
1
tBu group of the metal-bound isocyanide. The H NMR (CNtBu)] [1.406(9) Å],[17] trans-[Rh(SC6F5)(O2)(CNXy)-
spectrum for 6a in 1,2-difluorobenzene revealed six signals (PPh3)2] [1.436(3) Å],[5a] trans-[Rh(4-C5F4N)(O2)(CNtBu)-
for the methyl groups of the diimine ligand. The signal at δ (PEt3)] [1.452(3) Å],[6a] and other rhodium η2-peroxido
= 0.76 ppm was attributed to the tBu group of the metal- complexes.[4e,45]
bound isocyanide for 6a.
In order to obtain more information about the geometry
and bond lengths in complex 5a, its structure was deter-
Conclusions
mined by X-ray diffraction analysis at 180 K (Figure 5).
In conclusion, we have described the synthesis of new α-
Suitable crystals for 5a were obtained from a thf solution
diketimine RhI and IrI compounds. The reactivity studies
led to the isolation of an iridium dihydrido complex 4, as
of the complex at 243 K. Selected bond lengths and angles
are summarized in Table 2. If the oxygen is treated as a
well as the formation of RhIII and IrIII peroxido com-
pounds 5a and 6a. The reactions with molecular oxygen are
reversible, which might hamper their use in oxidation or
oxygenation reactions. In comparison, the complex
[Rh(Cl)(tbbpy)(CNtBu)][17] binds dioxygen irreversibly, but
its peroxido complex is insoluble. However, preliminary
studies revealed that PEt3 was oxygenated by 6a to give the
phosphane oxide. Trace amounts of 9-fluorenone and hy-
drogen peroxide were detected after the treatment of 6a
with 9-fluorenol.
bidentate ligand then the η2-peroxido complex 5a adopts
the distorted octahedral structure with the peroxido ligand
in the trans position to the chlorido and α-diketimine li-
gand. Alternatively, if the dioxygen is treated as though it
occupies a single coordination site, then the structure is ap-
proximately trigonal-bipyramidal. The rhodium–nitrogen
bond lengths of the α-diketimine ligand to the metal in 5a
[2.056(13) and 1.970(14) Å] are comparable to those of the
rhodium–nitrogen bond lengths in [Rh(Cl)(O2)(tbbpy)-
(CNtBu)] [2.072(8) and 2.051(9) Å][17] and in the cation
[Rh{κ2-(N,P)-N(iPr)=CH-o-C6H4PPh2}2(O2)][BPh4], where
the Rh–N bond is located in the trans position to one of
the oxygen atoms [2.111(7) Å].[5b] The rhodium–carbon
bond of the isocyanide group in 5a [1.877(17) Å] is nearly
Experimental Section
General Methods and Instrumentation: The synthetic work was car-
ried out with a Schlenk line or an argon-filled glove box where the
oxygen levels were below 10 ppm. All of the solvents were purified
and dried by conventional methods and were distilled under an
argon atmosphere before use. 16O2 and CNtBu were obtained from
Aldrich. tert-Butyl isocyanide was distilled before use. 18O2 was
purchased from CAMPRO Scientific. The diphenyl- and di-ortho-
xylyl-α-diketimines,[22,46] as well as the complexes [Rh(μ-Cl)(coe)2]
2 and [Ir(μ-Cl)(coe)2]2,[47] were prepared according to the literature.
The NMR spectra were recorded with a Bruker AV 400 NMR or
1
with a Bruker DPX 300 spectrometer at 25 °C. The H/13C NMR
chemical shifts were referenced to residual C6D5H at 7.15/
128.00 ppm or to [D8]thf at 1.73/25.72 ppm. The 13C NMR experi-
ments were recorded with a delay time of 10s. The microanalyses
were obtained with a Euro EA HEKAtech elemental analyzer. The
infrared spectra were recorded with a Bruker Vector 22 spectrome-
ter that was equipped with an ATR unit (diamond).
Figure 5. An ORTEP diagram of 5a. The ellipsoids are drawn at
the 50% probability level.
[Rh(Cl){ArN=C(Me)C(Me)=NAr}(CNtBu)] (Ar = Phenyl) (1): A
solution of the diphenyl-α-diketimine (128 mg, 0.541 mmol) in thf
(8 mL) was added dropwise to a solution of [Rh(μ-Cl)(coe)2]2
(194 mg, 0.270 mmol) in thf (13 mL). A color change from orange
to dark red was observed. The reaction mixture was stirred for
3.5 h at room temperature and then tert-butyl isocyanide (61 μL,
0.541 mmol) was added. The reaction mixture was then stirred for
12 h at room temperature and the volatiles were removed in vacuo.
The residue was washed twice with a mixture of n-hexane (10 mL)
and thf (1 mL) and then twice with n-hexane (10 mL). The dark
brown product was dried under vacuum; yield 200 mg (81%).
C21H25ClN3Rh (457.80): calcd. C 55.10, H 5.50, N 9.18; found C
Table 2. Selected bond lengths [Å] and angles [°] for
[Rh(Cl)(O2){XyN=C(Me)C(Me)=NXy}(CNtBu)] (5a).
Rh1–O1
Rh1–O2
O1–O2
Rh1–Cl1
Rh1–C21
1.981(11)
1.959(18)
1.448(14)
2.417(4)
Rh1–N1
Rh1–N2
C21–N3
N3–C22
2.056(13)
1.970(14)
1.23(2)
1.45(2)
1.877(17)
O2–Rh1–N2 117.6(5)
N2–Rh1–C21
N1–Rh1–N2
Cl1–Rh1–N2
C21–N3–C22
O2–Rh1–O1
97.9(7)
76.91(5)
88.8(4)
168.6(16)
43.1(4)
O2–Rh1–N1
O2–Rh1–C21
Cl1–Rh1–N1
91.0(5)
86.8(6)
94.1(3)
54.98, H 5.38, N 8.99. IR (ATR): ν = 3055 (w), 2986 (w), 2934 (w),
˜
Cl1–Rh1–C21 91.0(5)
2112 (s, CϵN), 2066 (m, CϵN), 1592 (m), 1564 (m), 1485 (s), 1449
Eur. J. Inorg. Chem. 2011, 2579–2587
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2583

A. Penner, T. Braun
FULL PAPER
(m), 1388 (m), 1368 (m), 1337 (s), 1251 (m), 1205 (m), 1070 (m), benzene (10 mL) was treated with H2 for 5 min. After stirring for
989 (s), 865 (m), 825 (m), 747 (s), 739 (m), 695 (s), 621 (s), 523 (m), 12 h at room temperature under a H2 atmosphere, the brown reac-
463 (m) cm–1. 1H NMR (400 MHz, [D8]thf): δ = 7.47–7.18 (m, tion mixture was dried in vacuo for a short period of time; yield
10 H, Ph-H), 1.37 (s, 3 H, CH3), 1.16 (s, 9 H, tBu), 0.23 (s, 3 H, 72 mg (99%). C25H35ClIrN3 (605.24): calcd. C 49.61, H 5.83, N
2
CH3) ppm. 13C NMR (100 MHz, [D8]thf): δ = 169.78 (d, JRh,C
=
6.94; found C 49.40, H 5.79, N 7.24. IR (ATR): ν = 2971 (w), 2923
˜
2
3 Hz, C=N), 157.41 (d, JRh,C = 2 Hz, C=N), 160.88, 151.98, (m), 2854 (w), 2140 (s, br, CϵN), 2052 (w), 2023 (w), 1590 (w),
129.86, 128.56, 126.77, 126.34, 123.78, 123.75 (C-Ar), 57.23
1466 (m), 1442 (m), 1381 (m), 1368 (m), 1260 (m), 1225 (s), 1210
(s), 1095 (m), 1035 (m), 991 (m), 863 (m), 822 (m), 803 (m), 766
(s), 706 (w), 680 (m), 558 (w), 516 (m), 475 (m) cm–1. 1H NMR
(400 MHz, C6D6): δ = 7.08–6.88 (m, 6 H, Ar-H), 2.85, 2.84, 2.16,
2.14 (all s, 12 H, Ar-CH3), 1.40, 1.38 (s, 6 H, CH3), 0.75 (s, 9 H,
3
[C(CH3)3], 31.09 [C(CH3)3], 21.06 (d, JRh,C = 1 Hz, CH3), 19.89
3
(d, JRh,C = 1 Hz, CH3) ppm, the signal for the isocyanide carbon
was not observed.
[Rh(Cl){ArN=C(Me)C(Me)=NAr}(CNtBu)] (Ar = 2,6-Me2C6H3)
2
2
tBu), –17.20 (d, JH,H = 6.9 Hz, 1 H, IrH), –19.73 (d, JH,H
=
(2):
A solution of the di-ortho-xylyl-α-diketimine (371 mg,
6.9 Hz, 1 H, IrH) ppm. 1H NMR (300 MHz, [D8]thf): δ = 7.17–
6.96 (m, 6 H, Ar-H), 2.58, 2.53, 2.18, 2.13, 2.10, 2.09 (s, 12 H, Ar-
CH3, and 6 H, CH3), 1.06 (s, 9 H, tBu), –18.31 (d, 2JH,H = 6.9 Hz,
1.260 mmol) in thf (10 mL) was added dropwise to a solution of
[Rh(μ-Cl)(coe)2]2 (455 mg, 0.634 mmol) in thf (10 mL). A color
change from orange to raspberry red was observed. The reaction
mixture was stirred for 1 h at room temperature and then tert-butyl
isocyanide (144 μL, 1.260 mmol) was added. A color change to
dark blue was observed. The reaction mixture was stirred for 12 h
at room temperature and the volatiles were removed in vacuo. The
remaining crude product was washed four times with n-hexane
(7 mL) and dried under vacuum. A dark blue substance remained;
yield 562 mg (86%). C25H33ClN3Rh (513.91): calcd. C 58.43, H
2
1 H, IrH), –20.29 (d, JH,H = 6.9 Hz, 1 H, IrH) ppm. 13C NMR
(100 MHz, C6D6): δ = 173.99, 169.70 (C=N), 150.17, 149.26,
130.67, 130.63, 129.48, 128.96, 128.57, 126.58, 126.00 (C-Ar, three
signals are covered by the C6D5H signal), 121.24 (Ir-C), 56.07
[C(CH3)3], 30.38 [C(CH3)3], 20.71, 20.50 (CH3), 19.10, 18.98, 18.33,
18.31 (CH3-Ar) ppm.
[Rh(Cl)(O2){ArN=C(Me)C(Me)=NAr}(CNtBu)] (Ar
=
2,6-
6.47, N 8.18; found C 58.30, H 6.44, N 8.34. IR (ATR): ν = 3015
˜
Me2C6H3) (5a): A dark blue solution of [Rh(Cl){ArN=C(Me)-
C(Me)=NAr}(CNtBu)] (2) (124 mg, 0.241 mmol) in benzene
(7 mL) was treated with oxygen for 2 min. The sample turned red
brown and a brown precipitate was observed. After 3 h at room
temperature under an O2 atmosphere, the reaction mixture was fil-
tered and the residue was treated twice with n-hexane (3 mL). The
brown residue was dried in vacuo for a short period of time; yield
(w), 2977 (w), 2929 (w), 2107 (s, CϵN), 2071 (m, CϵN), 1566 (w),
1466 (m), 1439 (m), 1368 (m), 1337 (m), 1261 (w), 1236 (m), 1207
(m), 1090 (w), 1037 (w), 994 (m), 921 (w), 869 (m), 832 (w), 765
(s), 760 (s), 708 (m), 561 (w), 524 (w), 502 (w), 458 (m), 431 (w)
1
cm–1. H NMR (300 MHz, C6D6): δ = 7.17–6.94 (m, 6 H, Ar-H),
2.43 (s, 6 H, Ar-CH3), 2.24 (s, 6 H, Ar-CH3), 0.75 (s, 9 H, tBu),
0.62 (s, 3 H, CH3), –0.54 (s, 3 H, CH3) ppm. 13C NMR (100 MHz,
124 mg (94%). IR (ATR): ν = 2980 (w), 2909 (w), 2208 (s, CϵN),
˜
2
2
C6D6): δ = 168.25 (d, JRh,C = 3 Hz, C=N), 153.71 (d, JRh,C
=
2115 (w), 1467 (m), 1437 (w), 1382 (m), 1372 (m), 1227 (m), 1190
(m), 1095 (w), 1036 (w), 999 (m), 884 (s, 16O–16O), 853 (w), 771 (s),
678 (m), 620 (w), 550 (m), 523 (m), 497 (m, Rh16O) cm–1. 1H NMR
(300 MHz, C6D6): δ = 7.02–6.87 (m, 6 H, Ar-H), 2.86, 2.58, 2.37,
2.16 (s, 12 H, Ar-CH3), 1.42, 1.36 (s, 6 H, CH3), 0.56 (s, 9 H, tBu)
ppm. 1H NMR (300 MHz, C6D6 capillary, C6H4F2): δ = 2.77, 2.61,
2.50, 2.30 (s, 12 H, Ar-CH3), 2.21 (s, 6 H, CH3), 0.86 (s, 9 H, tBu)
ppm, the ArH signals are covered by the C6H4F2 signal. Although
the compound was spectroscopically pure, satisfactory microana-
lytical data could not be obtained despite several attempts. It ap-
pears that the reversibility of the reaction with O2 hampered our
efforts.
1 Hz, C=N), 157.81, 149.46, 129.32, 129.03, 125.84, 125.45 (C-Ar,
two signals are covered by the C6D5H signal), 56.00 [C(CH3)3],
3
30.05 [C(CH3)3], 19.09 (CH3-Ar), 18.69 (d, JRh,C = 2 Hz, CH3),
3
17.95 (CH3-Ar), 17.64 (d, JRh,C = 1 Hz, CH3) ppm, the signal for
the isocyanide carbon was not detected.
[Ir(Cl){ArN=C(Me)C(Me)=NAr}(CNtBu)] (Ar
= 2,6-Me2C6H3)
(3): solution of the di-ortho-xylyl-α-diketimine (346 mg,
A
1.183 mmol) in thf (10 mL) was added dropwise to a solution of
[Ir(μ-Cl)(coe)2]2 (530 mg, 0.591 mmol) in thf (30 mL). A color
change from orange to green was observed. The reaction mixture
was stirred for 2 h at room temperature. Then a solution of tert-
butyl isocyanide (134 μL, 1.183 mmol) in thf (7 mL) was added
dropwise. The dark green reaction mixture was stirred for 12 h at
room temperature and the volatiles were removed in vacuo. The
remaining crude product was washed three times with a mixture of
n-hexane (9.5 mL) and thf (0.5 mL) and dried under vacuum. A
dark green substance remained; yield 691 mg (97%). C25H33ClIrN3
(603.22): calcd. C 49.78, H 5.51, N 6.97; found C 50.43, H 5.62, N
[Rh(Cl)(18O2){ArN=C(Me)C(Me)=NAr}(CNtBu)] (Ar
=
2,6-
Me2C6H3) (5b): A dark blue solution of 2 (45 mg, 0.088 mmol) in
benzene (2 mL) was cooled in vacuo to 77 K for 10 min. 18O2 was
then added and the solution was slowly warmed to room tempera-
ture. The color of the solution changed from dark blue to dark red
and a brown precipitate was formed. After 2 h the reaction mixture
was filtered and the residue was dried for a very brief period in
6.61. IR (ATR): ν = 3016 (w), 2978 (w), 2924 (w), 2100 (s, CϵN),
˜
vacuo; yield 26 mg (55%). IR (ATR): ν = 2980 (m), 2910 (w), 2208
˜
2067 (s, CϵN), 1591 (w), 1528 (w), 1465 (m), 1439 (m), 1391 (m),
1366 (s), 1349 (s), 1263 (m), 1240 (m), 1209 (s), 1089 (m), 1035 (w),
994 (m), 903 (w), 886 (m), 835 (w), 764 (s), 760 (s), 721 (m), 712
(m), 562 (m), 529 (m), 471 (s) cm–1. 1H NMR (300 MHz, C6D6): δ
= 7.22–7.17, 7.08–6.88 (both m, 6 H, Ar-H), 2.47 (s, 6 H, Ar-CH3),
2.14 (s, 6 H Ar-CH3), 0.87 (s, 9 H, tBu), –0.09 (s, 3 H, CH3), –2.41
(s, 3 H, CH3) ppm. 13C NMR (75 MHz, C6D6): δ = 178.42, 159.73
(C=N), 156.58, 152.52, 127.20, 126.20, 125.85 (C-Ar, three signals
are covered by the C6D5H signal), 55.94 [C(CH3)3], 30.24 [C(CH3)
3], 22.83 (CH3), 19.09 (CH3-Ar), 18.96 (CH3), 17.54 (CH3-Ar) ppm,
the signal for the isocyanide carbon was not detected.
(s, CϵN), 2118 (w), 1467 (m), 1437 (w), 1382 (m), 1372 (m), 1227
(m), 1190 (m), 1095 (w), 1036 (w), 999 (m), 853 (w), 833 (s, 18O–
18O), 771 (s), 678 (w), 560 (w), 520 (m), 480 (m, Rh18O) cm–1.
[Ir(Cl)(O2){ArN=C(Me)C(Me)=NAr}(CNtBu)]
(Ar
=
2,6-
Me2C6H3) (6a): A dark green solid of [Ir(Cl){ArN=C(Me)-
C(Me)=NAr}(CNtBu)] (3) (344 mg, 0.570 mmol) was treated with
oxygen for 3 min. After 3.5 h the sample turned dark brown. After
21 h at room temperature and under an O2 atmosphere, the solid
was washed three times with a mixture of n-hexane (10 mL) and
benzene (3 mL) and then twice with n-hexane (5 mL). The brown
[Ir(Cl)(H)2{ArN=C(Me)C(Me)=NAr}(CNtBu)]
Me2C6H3) (4): A dark green solution of 3 (72 mg, 0.119 mmol) in
(Ar
=
2,6- residue was dried for a very brief period in vacuo; yield 326 mg
(90%). IR (ATR): ν = 2980 (w), 2918 (w), 2178 (s, CϵN), 2052
˜
2584
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2579–2587

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(w), 1628 (w), 1588 (w), 1466 (m), 1384 (m), 1369 (m), 1334 (m),
1260 (w), 1230 (s), 1200 (s), 1095 (w), 1036 (w), 991 (m), 922 (w),
843 (s, 16O–16O), 769 (s), 682 (s), 564 (m), 554 (m), 524 (m), 503 (s,
Ir16O), 464 (m) cm–1. 1H NMR (300 MHz, C6D6 capillary,
C6H4F2): δ = 2.57, 2.45, 2.17, 2.14, 2.00, 1.95 (s, 12 H, Ar-CH3,
and 6 H, CH3), 0.76 (s, 9 H, tBu) ppm, the ArH signals are covered
by the C6H4F2 signal. Although the compound was spectroscopi-
cally pure, satisfactory microanalytical data could not be obtained
despite several attempts. It appears that the reversibility of the reac-
tion with O2 hampered our efforts.
[2]
[Ir(Cl)(18O2){ArN=C(Me)C(Me)=NAr}(CNtBu)] (Ar
=
2,6-
Me2C6H3) (6b): [Ir(Cl){ArN=C(Me)C(Me)=NAr}(CNtBu)] (3)
(44 mg, 0.073 mmol), a dark green solid, was cooled to 77 K for
10 min in vacuo. Then 18O2 was added and the mixture was slowly
warmed to room temperature. After 24 h the color of the solid
turned from dark green to brown. IR (ATR): ν = 2982 (w), 2918
˜
(w), 2870 (w), 2180 (s, CϵN), 2052 (w), 1585 (w), 1466 (m), 1384
(m), 1370 (m), 1334 (m), 1259 (w), 1230 (s), 1201 (s), 1096 (w),
1035 (w), 991 (m), 922 (w), 796 (s, 18O–18O), 769 (s), 684 (s), 560
(m), 554 (m), 522 (m), 479 (s, Ir18O), 466 (m) cm–1.
Structure Determination: The black crystals for 3 were obtained at
room temperature from a solution of complex 3 in n-hexane/thf
(9.5 mL/0.5 mL). The diffraction data were collected with a Stoe
IPDS 2θ diffractometer for a fragment with the dimensions of
0.32ϫ0.16ϫ0.14 mm3. C25H33ClIrN3, M = 603.19, monoclinic,
space group P21/c, a = 14.9260(6) Å, b = 13.0619(5) Å, c =
14.0412(6) Å, V = 2429.45(17) Å3, ρcalcd. = 1.649 gcm–3, T =
100(2) K, Z = 4, μ(Mo-Kα) = 5.622 mm–1, 42966 reflections mea-
sured, 6550 unique (Rint = 0.0968). Final R1, wR2 values on all
data: 0.0378, 0.0810. R1, wR2 values for 5978 reflections with
I0 Ͼ2σ(I0) = 0.0341, 0.0790; residual electron density +3.258/
–4.189 eÅ–3. The dark red crystals for 5a were obtained under an
O2 atmosphere at 243 K from a solution of the complex in thf. The
diffraction data were collected with a Stoe IPDS diffractometer for
[3]
a
fragment with the dimensions of 0.08ϫ0.08ϫ0.08 mm3.
C25H33ClN3O2Rh, 545.90, orthorhombic, space group
M
=
P212121, a = 10.810(2) Å, b = 14.110(2) Å, c = 17.025(5) Å, V =
2596.9(10) Å3, ρcalcd. = 1.396 gcm–3, T = 180(2) K, Z = 4, μ(Mo-
Kα) = 0.786 mm–1, 21515 reflections measured, 2535 unique (Rint
= 0.2286), Mo-Kα radiation (λ = 0.71073 Å). Final R1, wR2 values
on all data: 0.1097, 0.1426. R1, wR2 values for 1303 reflections with
I0 Ͼ2σ(I0) = 0.0614, 0.01226; residual electron density +0.659/
–1.034 eÅ–3. Both of the structures were solved by direct methods
(SHELXS-97) and were refined with the full-matrix least-squares
method on F2 (SHELX-97).[48] The hydrogen atoms were placed at
the calculated positions and were refined by using a riding model.
[4]
CCDC-811453 (for 5a) and -811454 (for 3) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
We would like to acknowledge the Deutsche Forschungsgemein-
schaft (DFG) for financial support. A. P. is a fellow of Berlin Inter-
national Graduate School of Natural Sciences and Engineering
(BIG-NSE). We are grateful to Prof. Dr. S. Troyanov for help with
the crystallography and to M. Chilleck and B. Horn for their exper-
imental assistance.
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2585

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Received: February 8, 2011
Published Online: April 21, 2011
Eur. J. Inorg. Chem. 2011, 2579–2587
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2587