Activation of Small Molecules by the Metal-Amido Bond of Rhodium(III) and Iridium(III) (η5-C5Me5)M-Aminopyridinate Complexes

Article abstract of DOI:10.1021/acs.inorgchem.7b02283

We report the synthesis and structural characterization of five-coordinate complexes of rhodium and iridium of the type [(η⁵-C₅Me₅)M(N^N)]⁺ (3-M⁺), where N^N represents the aminopyridinate ligand derived from 2-NH(Ph)-6-(Xyl)C₅H₃N (Xyl = 2,6-Me₂C₆H₃). The two complexes were isolated as salts of the BAr_F anion (BAr_F = B[3,5-(CF₃)₂C₆H₃]₄). The M-N_amido bond of complexes 3-M⁺ readily activated CO, C₂H₄, and H₂. Thus, compounds 3-M⁺ reacted with CO under ambient conditions, but whereas for 3-Rh⁺, CO migratory insertion was fast, yielding a carbamoyl carbonyl species, 4-Rh⁺, the stronger Ir-N_amido bond of complex 3-Ir⁺ caused the reaction to stop at the CO coordination stage. In contrast, 3-Ir⁺ reacted reversibly with C₂H₄, forming adduct 5-Ir⁺, which subsequently rearranged irreversibly to [Ir](H)(=C(Me)N(Ph)-) complex 6-Ir⁺, which contains an N-stabilized carbene ligand. Computational studies supported a migratory insertion mechanism, giving first a β-stabilized linear alkyl unit, [Ir]CH₂CH₂N(Ph)-, followed by a multistep rearrangement that led to the final product 6-Ir⁺. Both β- and α-H eliminations, as well as their microscopic reverse migratory insertion reactions, were implicated in the alkyl-to-hydride-carbene reorganization. The analogous reaction of 3-Rh⁺ with C₂H₄ originated a complex mixture of products from which only a branched alkyl [Rh]C(H)(Me)N(Ph)- (5-Rh⁺) could be isolated, featuring a β-agostic methyl interaction. Reactions of 3-M⁺ with H₂ promoted a catalytic isomerization of the Ap ligand from classical κ²-N,N′ binding to κ-N plus η³-pseudoallyl coordination mode.

Full text of DOI:10.1021/acs.inorgchem.7b02283

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Activation of Small Molecules by the Metal−Amido Bond of
Rhodium(III) and Iridium(III) (η5‑C5Me5)M-Aminopyridinate
Complexes
́
Ana Zamorano, Nuria Rendon,* Joaquín Lopez-Serrano, Eleuterio Alvarez, and Ernesto Carmona
́
́
Instituto de Investigaciones Químicas (IIQ), Departamento de Química Inorgan
(ORFEO-CINQA), Universidad de Sevilla and Consejo Superior de Investigaciones Científicas (CSIC), Avenida Amer
41092 Sevilla, Spain
́
ica and Centro de Innovacion
́
en Química Avanzada
ico Vespucio 49,
́
S
* Supporting Information
ABSTRACT: We report the synthesis and structural characterization of five-
coordinate complexes of rhodium and iridium of the type [(η5-C5Me5)
M(N^N)]+ (3-M+), where N^N represents the aminopyridinate ligand
derived from 2-NH(Ph)-6-(Xyl)C5H3N (Xyl = 2,6-Me2C6H3). The two
complexes were isolated as salts of the BArF anion (BArF = B[3,5-(CF3)2C6H3]4).
The M−Namido bond of complexes 3-M+ readily activated CO, C2H4, and H2.
Thus, compounds 3-M+ reacted with CO under ambient conditions, but
whereas for 3-Rh+, CO migratory insertion was fast, yielding a carbamoyl
carbonyl species, 4-Rh+, the stronger Ir−Namido bond of complex 3-Ir+ caused
the reaction to stop at the CO coordination stage. In contrast, 3-Ir+ reacted
reversibly with C2H4, forming adduct 5-Ir+, which subsequently rearranged
irreversibly to [Ir](H)(C(Me)N(Ph)−) complex 6-Ir+, which contains an
N-stabilized carbene ligand. Computational studies supported a migratory
insertion mechanism, giving first a β-stabilized linear alkyl unit, [Ir]CH2CH2N(Ph)−, followed by a multistep rearrangement that
led to the final product 6-Ir+. Both β- and α-H eliminations, as well as their microscopic reverse migratory insertion reactions,
were implicated in the alkyl-to-hydride−carbene reorganization. The analogous reaction of 3-Rh+ with C2H4 originated a
complex mixture of products from which only a branched alkyl [Rh]C(H)(Me)N(Ph)− (5-Rh+) could be isolated, featuring a
β-agostic methyl interaction. Reactions of 3-M+ with H2 promoted a catalytic isomerization of the Ap ligand from classical
κ2-N,N′ binding to κ-N plus η3-pseudoallyl coordination mode.
(L), forming readily the corresponding six-coordinate, 18-electron
adducts [(η5-C5Me5)M(N^N)(L)]+ (L = NH3, NCMe, CNXyl,
and others). Intriguingly, none of the compounds investigated
reacted with C2H4 even under rather forcing conditions,
hampering the observation of products originating from
migratory insertion of the alkene into the M−Namido bond.12,13
It appeared plausible that failure to detect C2H4 reactivity was
due to steric reasons, in other words, that the required side-on
metal approach of the CC bond was impeded by steric
interactions with the C5Me5 and Ap ligands, in particular with the
aryl substituents of the amido nitrogen atom. In accordance with
this assumption, it was found that complexes of rhodium and
iridium alike exhibited a significant decrease in the rate of
H2-catalyzed isomerization of the coordinated pyridylamido
ligand from the common κ2-N,N′ binding to an unconventional
κ-N,η3-benzylic binding mode (Figure 1b), when the amido
nitrogen substituent was changed from 2,6-Me2C6H3 to 2,6-
INTRODUCTION
■
In recent years, we have explored the chemistry of cationic
[(η5-C5Me5)MIII] complexes of rhodium and iridium stabilized
by coordination to bulky, formally monoanionic bidentate
ligands (L^X), encompassing cyclometalated phosphines and,
especially in the context of this work, aminopyridinates.1,2
The latter are also called pyridylamido ligands and are repre-
sented onward in a simplified manner as Ap or as N^N
(see Figure 1).3 Compounds built around [(η5-C5Me5)M] rhodium
and iridium frameworks have arisen considerable interest because
they are valuable molecules finding countless applications in catal-
ysis,4−7 bioorganometallic studies,8,9 and other areas of research.10
Similar to previously reported amido complexes of late
transition metals,11 the amido nitrogen atom of metal-bound
aminopyridinate groups can function as a π donor (Figure 1a),
partially offsetting the electronic unsaturation of positively charged
metal centers and allowing stabilization of five-coordinate
complexes of the type [(η5-C5Me5)M(N^N)]+. As represented
in Figure 1, bulky aryl substituents on both the amido and six
positions of the pyridine termini may provide desirable steric
protection to the low-coordinate metal center.
i
Pr2 C6H3.
Considering the above information, we deemed of interest
studying the chemistry of related [(η5-C5Me5)M(N^N)]+
The previously reported complexes1,2 behaved as markedly
reactive Lewis acids upon treatment with a variety of Lewis bases
Received: September 6, 2017
© XXXX American Chemical Society
A
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Figure 1. (a) General representation for five-coordinate complexes of [(η5-C5Me5)MIII]+ units (M = Rh, Ir) and aryl-substituted aminopyridinate
ligands. (b) Previously reported H2-catalyzed isomerization of the Ap ligand of complexes of type a. (c) Aminopyridinate precursor, 1, utilized in this
work.
complexes, 3-Rh+ and 3-Ir+, in which the aminopyridinate ligand
is the conjugated base of phenyl[6-(2,6-dimethylphenyl)pyridine-
2-yl]amine (compound 1 in Figure 1c). Relative to previously
assayed Ap ligands, little if any relevant changes are introduced in
the overall electronic properties of the N^N chelating unit,
whereas replacing the amido nitrogen 2,6-R2C6H3 (R = Me, Pri)
substituent by the substituent-devoid phenyl group is expected to
significantly mitigate steric hindrance in the vicinity of the short
M−Namido bond of complexes 3-M+, facilitating ligand coordi-
nation.
In this contribution, we report studies on the reactivity of
complexes 3-M+ toward C2H4 that proceeds under mild condi-
tions, yielding products stemming from migratory insertion of
the olefin into the M−Namido bond.12,13 Computational studies
supporting a mechanistic path entailing irreversible C−N bond
formation, along with reversible α- and β-H eliminations and
their microscopic reverse migratory insertion reactions, are also
incorporated.14 Furthermore, for the investigated reaction of
3-Ir+ with C2H4, formation of the experimentally observed
hydride−carbene product 6-Ir+ was computed to be morefavorable
than generation of the alternative hydride−olefin isomer.
The mentioned reactivity adds to the observation of CO migratory
insertion into the Rh−Namido bond of 3-Rh+ and to the diverse
and unusual H−H, C−H, and N−H bond activation encountered
in the reactions of H2 with complexes 3-M+, although the latter
activations are predictable on the basis of previous work.1,2
by a pronounced color change from the original yellow or reddish
to almost black or dark brown for Ir and Rh, respectively.
As noted previously,1,2 the dark color of these complexes is
typical of compounds of this kind in which the M−Namido
functionality has multiple-bond character because of σ and π
donation from the anionic nitrogen atom.10,15
The four complexes 2-M and 3-M+ (M = Rh, Ir) were fully
characterized by microanalysis, IR, and multinuclear 1D and
2D NMR spectroscopy (see the Experimental Section and
Supporting Information). In addition, neutral compounds 2-M,
as well as 3-Ir+, were structurally authenticated by single-crystal
X-ray diffraction (XRD) studies. Figure 2 depicts the molecular
structure of 3-Ir+, while those of the chloride derivatives 2-M
(Ir and Rh) are collected in Figures S35−S36. Metrical
parameters have normal values, although it is worth remarking
that the Ir−Namido bond length of this complex (Ir1−N2 in
Figure 2) at 1.972(6) Å is significantly shorter than the corre-
sponding bond intheneutral chloride precursor 2-Ir [2.092(2) Å],
in agreement with the proposed π-donor nature of the amido
nitrogen function of 3-Ir+. Besides, the Ir1−N2 bond is also
significantly shorter than the dative covalent Ir1−N1 bond to the
pyridine moiety [2.109(6) Å].
Reactions of Complexes 3-M+ with CO and C2H4. Similar
to related complexes of diversely substituted Ap ligands,1,2 the
rhodium and iridium derivatives 3-M+ exhibited distinct behavior
toward CO (Scheme 2). Thus, the room temperature reaction of
3-Ir+ afforded the carbonyl adduct 4-Ir+, characterized by an
IR absorption at 2048 cm−1 due to stretching of the iridium-
bound C−O bond. Under the reaction conditions (Scheme 2),
migratory insertion of CO into the Ir−Namido bond did not take
place. In contrast, an analogous CO adduct was not detected for
rhodium because nucleophilic attack of the Rh−Namido bond to
the coordinated carbonyl occurred very rapidly with formation of
a five-membered chelating carbamoylpyridine group that became
stabilized by coordination of a second molecule of CO, giving the
isolated complex 4-Rh+. The incorporated CO units of this
derivative gave rise to IR bands at 2077 cm−1 (Rh−CO) and
1687 cm−1 [Rh−C(O)N], along with corresponding 13C NMR
resonances with 187.1 and 189.3 ppm (1JCRh = 76 and 30 Hz),
respectively. Although not unprecedented, the insertion of CO
RESULTS AND DISCUSSION
■
Cationic Rhodium and Iridium [(η5-C5Me5)M(Ap)]+
Complexes. Synthesis of the amine ligand precursor ApH
(compound 1; Figure 1c) was performed as reported previously
for analogous, differently substituted aminopyridines.1b Synthetic
and characterization details can be found in Scheme S1.
The neutral chloride complexes [(η5-C5Me5)M(Cl)(Ap)] (2-Ir
and 2-Rh) were obtained from the corresponding [(η5-C5Me5)-
MCl2]2 dimers and the lithium amide, LiAp. As shown in Scheme 1,
the chloride ligand of compounds 2-M was readily extruded by
the action of NaBArF (BArF = B[3,5-(CF3)2C6H3]4) to give the
five-coordinate amidopyridine complexes [(η5-C5Me5)-
M(N^N)]+ (3-Ir+ and 3-Rh+). The reactions were accompanied
B
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Scheme 1. Synthesis of the Cationic Complexes 3-M+
As was already recalled, the reported [(η5-C5Me5)M(N^N)]+
iridium and rhodium complexes analogous to 3-M+ (Figure 1b)
were unreactive against C2H4 (1 bar, room temperatureto 130°C).
Alkenes react readily with M−H and M−C bonds through
migratory insertion, a fundamental reaction in organotransition
metal chemistry, key to a large number of stoichiometric and
catalytic transformations.10 In marked contrast, the analogous
reaction of an olefin with the M−N bond of an isolated
metal−amido complex to generate a new C−N bond is a less
well-established transformation, with the first examples having
been reported in recent years.12,13
When an ethylene atmosphere (1 bar) was introduced in a
Young NMR tube containing a CD2Cl2 solution of 3-Ir+, an
immediate color change from almost black to orange took
place, hinting at the formation of the desired C2H4 adduct
[(η5-C5Me5)Ir(N^N)(C2H4)]+ (5-Ir+), with the structure shown
in Scheme 3. Although C2H4 coordination was reversible, such
that removal of the ethylene atmosphere under vacuum restored
the original black color of complex 3-Ir+, upon standing at room
temperature under C2H4 over a period of 24 h, adduct 5-Ir+
rearranged irreversibly to the hydride complex 6-Ir+. As depicted
in Scheme 3, coordination of the iridium(III) center of this
species is completed by a chelating carbene−pyridine ligand
resulting from C−N coupling between a molecule of C2H4 and
the amido nitrogen atom of the original pyridylamido ligand.
The adduct 5-Ir+ could only be studied by solution NMR
spectroscopy because of the facility of C2H4 dissociation, whereas
the stable complex 6-Ir+ was isolated as a crystalline solid and
fully characterized by microanalytical, spectroscopic (IR and
NMR), and X-ray data. No 1H NMR resonances were recorded
at room temperature for the coordinated ethylene molecule of
5-Ir+, but at −40 °C, a characteristic AA′BB′ multiplet was
observed centered at 4.02 ppm, with δA nearly equal to δB. In the
13C{1H} NMR spectrum registered also at −40 °C, the corre-
sponding signal appeared at 61.7 ppm. A comparison with the
δ value found for free C2H4 of 123 ppm reveals a Δδ shift to low
frequency of around 61 ppm. This shift is smaller than that
recorded for the related [(η5-C5Me5)Rh(P^C)(C2H4)]+ species
−
Figure 2. X-ray structure of complex 3-Ir+ (30% ellipsoids, anion BArF
,
and hydrogen atoms are omitted for clarity). Selected bond lengths (Å)
and angles (deg): Ir1−N1 2.109(6), Ir1−N2 1.972(6), Ir1−C22
2.158(7), Ir1−C23 2.176(7), Ir1−C20 2.133(8), Ir1−C24 2.194(7),
Ir1−C21 2.155(7); N2−Ir1−N1 64.3(2), C1−N1−Ir1 93.2(4),
C1−N2−Ir1 98.2(4), N1−C1−N2 104.3(6).
into a late-transition-metal−amide bond is a rather uncommon
reaction.2,16 It seems probable that the stronger third-row
metal−Namido bond, relative to the Rh−Namido bond, retards
migratory insertion of CO, preventing observation of the
analogous iridium−carboxamide linkage. In this regard, it is
worth remarking that migratory insertion of CO into the
M−CH3 bond of the two different systems, fac-[M(CH3)-
(I)3(CO)2]− and [(η5-C5Me5)M(Me)Cl(CO)], is 5 or 6 orders
of magnitude faster on rhodium than on iridium.17
The carbonyl derivatives 4-M+ were also characterized by
single-crystal XRD studies. Figure 3 contains ORTEP
perspective views of the molecules of the two compounds.
Some relevant bond distances and angles are also given. Besides
the noticeable, although expected, increase in the length of the
Ir−Namido bond of 4-Ir+, now devoid of π components, relative to
3-Ir+ [2.089(6) vs 1.972(6) Å], metrical parameters have normal
values.
18
i
of cyclometalated PPr2 Xyl (ca. δ 47 and Δδ 76 ppm),
a
Scheme 2. Synthesis of the Carbonyl Complexes 4-Ir+ and 4-Rh+
a
Reactions were performed at room temperature under 1 bar of CO.
C
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Figure 3. X-ray structure of complexes 4-Ir+ and 4-Rh+ (30% ellipsoids, anion BArF−, and hydrogen atoms were omitted for clarity). Selected bond
lengths (Å) and angles (deg) for 4-Ir+: Ir1−N1 2.137(7), Ir1−C21 2.163(6), Ir1−N2 2.089(6), Ir1−C22 2.232(7), Ir1−C30 1.874(8),
Ir1−C23 2.259(8), O1−C30 1.145(9), Ir1−C24 2.216(10), Ir1−C20 2.181(7); C30−Ir1−N1 94.2(3), C1−N1−Ir1 94.3(5), C30−Ir1−N2 94.5(3),
C1−N2−Ir1 96.3(5), N2−Ir1−N1 62.0(2), N1−C1−N2 107.1(8). Selected bond lengths (Å) and angles (deg) for 4-Rh+: Rh1−N2 2.152(3),
Rh1−C21 1.908(5), O2−C21 1.115(6), Rh1−C20 2.026(4), O1−C20 1.200(5), Rh1−C22 2.235(5), Rh1−C23 2.251(5), Rh1−C24 2.157(4), Rh1−C25
2.262(4), Rh1−C26 2.333(5); C21−Rh1−N2 94.55(17), C21−Rh1−C20 88.9(2), C20−Rh1−N2 79.13(15), N1−C20−Rh1 111.9(3),
C1−N2−Rh1 112.5(3), N2−C1−N1 115.0(4).
Scheme 3. Reaction of Cation 3-Ir+ with Ethylene
suggesting reduced π-back-bonding to ethylene in the present
complex.
subsequent chemical changes must be invoked to account for the
chemical constitution of the resulting product 7-Ir+. As specified
in Scheme 4, this complex incorporates two molecules of PMe3
and in addition contains a β-functionalized linear alkyl
Ir−CH2CH2Namido(Ph)−, which is formally derived from a direct
N−C coupling reaction between the amido terminus of the
Ap ligand and a molecule of C2H4. In other words, beyond
accomplishing migratory insertion reactivity between the hydride
and carbene units of 6-Ir+, PMe3 induces a branched-to-linear
isomerization of the resulting secondary alkyl ligand, IrCH-
(Me)N(Ph)−, and decoordination of its pyridine end. As discussed
in the following paragraphs dealing with computational studies,
the reactivity represented in Schemes 3 and 4 entails competitive
α- and β-H elimination reactions and their microscopic reverse
migratory insertions of carbene and olefin functionalities into
Ir−H bonds. Monitoring of the reaction between 6-Ir+ and PMe3
by 31P{1H} NMR spectroscopy at low temperature (from −80
to +20 °C) did not provide additional useful information.
The immediate formation of a complex mixture of products
occurred at low temperatures, and at 0 °C, this mixture slowly
converted into the final complex 7-Ir+, which became the only
observable species after 15 h at room temperature.
The existence of hydride and carbene ligands in the
formulation proposed for the isomeric complex 6-Ir+ is strongly
backed by spectroscopic data. The Ir−H bond gives rise to an IR
band at 2096 cm−1 because of its stretching vibration and to a
1
shielded H NMR resonance at −16.42 ppm that appears as a
broad singlet. In turn, the 13C nucleus of the carbene ligand
resonates at 235.9 ppm. Figure 4 shows the solid-state structure
of the molecules of this complex, corroborating the formulation
anticipated in Scheme 3 on the basis of spectroscopic data.
The Ir1−C20 bond to the carbene carbon atom has a length of
1.911(12) Å, very similar to the distances found for other cationic
and neutral carbene complexes of iridium(III) reported by our
group.18−21 The Ir1−N1 bond to the pyridine ring has a distance
of 2.113(8) Å, which is also comparable to the Ir−Npyridine
distances found in related complexes described in this paper or
previously.1,2
By analogy with previously reported studies, the α-H elimina-
tion that leads to complex 6-Ir+ is expected to be reversible.18−23
Indeed, observation of the hydride resonance of 6-Ir+ as a broad
singlet (vide supra) may be suggestive of fast equilibration of this
complex with undetectable concentrations of the cyclic
alkylpyridine solvent species 6-Ir·CD2Cl2+ (Scheme 4), resulting
from fast migratory insertion of the carbene ligand of 6-Ir+ into
the Ir−H bond, promoted by coordination of CD2Cl2.19a
Reaction of 6-Ir+ with an excess of PMe3 triggered the anticipated
1,2-H shift from iridium to the carbene carbon atom, although
Although the NMR properties of 7-Ir+ permitted unambig-
uous structural identification, additional support was sought
through X-ray crystallography. The 31P{1H} NMR spectrum
contains the expected singlet with a chemical shift of −45.4 ppm.
1
In the H NMR spectrum, the two equivalent α-alkyl protons,
IrCH2CH2−, appear as a multiplet centered at 1.47 ppm, as a
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Figure 4. X-ray structure of complex 6-Ir+ (30% ellipsoids, hydrogen
atoms, and anion BArF− were omitted for clarity). Selected bond lengths
(Å) and angles (deg): Ir1−C20 1.911(12), Ir1−C23 2.340(11), Ir1−N1
2.113(8), Ir1−C24 2.280(13), Ir1−H(1Ir) 1.6070, Ir1−C25 2.161(12),
N2−C20 1.345(14), Ir1−C26 2.226(10), Ir1−C22 2.284(11);
C20−Ir1−N1 78.4(4), C20−N2−C1 115.5(10), C20−Ir1−H(1Ir)
89.1, N1−C1−N2 116.2(10), N1−Ir1−H(1Ir) 87.0, N2−C20−Ir1
118.8(8), C1−N1−Ir1 111.1(7).
Figure 5. X-ray structure of complex 7-Ir+ (30% ellipsoids, hydrogen
atoms, and anion BArF− were omitted for clarity). Selected bond lengths
(Å) and angles (deg): Ir1−C21 2.140(4), Ir1−C23 2.246(5), Ir1−P1
2.2854(13), Ir1−C24 2.278(4), Ir1−P2 2.2734(12), Ir1−C25 2.281(4),
C20−C21 1.534(6), Ir1−C26 2.236(5), Ir1−C22 2.267(5); P1−Ir1−
P2 96.22(5), Ir1−C21−C20 117.6(3), P1−Ir1−C21 83.23(13),
C21−C20−N2 112.0(4), C21−Ir1−P2 89.86(13).
The 1H NMR spectrum of this mixture (Figure S23) contained
discernible η5-C5Me5 resonances between 1.2 and 1.6 ppm for at
least four rhodium compounds. One of these species originated,
in addition, a significantly shielded doublet resonance at
−0.21 ppm (3JHH ∼ 6 Hz). Keeping in mind the results already
described for the analogous iridium system and by similarity with
NMR data reported in the literature for the Rh···CH3 δ-agostic
bond found in the cationic rhodium(III) [(η5-C5Me5)Rh(H)-
(PMeXyl2)]+ complex (−0.02 ppm),24 it can be proposed that
one of the components of the said mixture contains a chelating
secondary alkylpyridine ligand in which the RhC(H)MeCH2N−
unit is further engaged in a β-agostic methyl interaction
(structure 5-Rh+ in Figure 6). In one of the many crystallization
attempts effected, a few single crystals were obtained, and while
no full NMR characterization of the pure isolated complex could
be accomplished, its molecular structure was disclosed by a
single-crystal XRD study. The ORTEP view of the molecules of
5-Rh+ is shown on the right-hand-side of Figure 6 and provides
unequivocal proof for a C−N coupling reaction alike that
described for the analogous iridium complex. As can be seen,
the complex contained a five-membered C^N metallacycle
consequence of coupling to the two β-hydrogen atoms and the
two 31P nuclei. Likewise, the IrCH2CH2− resonance is a multiplet,
albeit substantially deshielded (3.67 ppm). Corresponding
13C{1H} resonances appear at −7.1 and +56.0 ppm, with two-
and three-bond coupling to the 31P nuclei of 7 and 5 Hz, respectively.
Figure 5 presents an ORTEP perspective view of the molecular
structure of complex 7-Ir+. As for related complexes, coor-
dination of the C5Me5 ring is fairly symmetrical, with Ir−C
distances spanning the rather narrow range 2.236(5)−2.281(4) Å.
The two Ir−P bonds have similar distances and are indistinguish-
able within experimental error [2.2734(12) and 2.2854(13) Å],
and the σ Ir−C bond to C21 has a length of 2.140(4) Å.
Similar to the iridium cation 3-Ir+, the rhodium analogue
3-Rh+ reacted instantly with ethylene. Rather disappointingly, as
discussed below, a very complex mixture of compounds was
observed even at low temperature (−80 °C), considerably
limiting the synthetic and mechanistic utility of this reaction.
Nevertheless, some useful information could be collected and
will be briefly discussed. Mixing the rhodium complex 3-Rh+
and C2H4 at 0 °C caused immediate consumption of the
metal reagent and generation of a complex manifold of products.
Scheme 4. Reaction of Complex 6-Ir+ with PMe3
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Figure 6. X-ray structure of complex 5-Rh+ (30% ellipsoids, hydrogen atoms, and anion BArF− were omitted for clarity). Selected bond lengths (Å) and
angles (deg): Rh1−C30 2.009(7), Rh1−C31 2.362(7), Rh1−C21 2.153(9), Rh1−H(C31) 1.9373, Rh1−C22 2.123(8), Rh1−N2 2.153(5), Rh1−C23
2.179(6), N1−C30 1.431(8), Rh1−C24 2.259(6), Rh1−C20 2.188(7); C30−Rh1−N2 78.2(3), C7−N1−C30 118.3(5), C30−Rh1−H(C31) 63.9,
N1−C7−N2 114.5(5), N2−Rh1−H31C 88.5, N1−C30−Rh1 111.5(4), C7−N2−Rh1 112.3(4), C31−H31C−Rh1 103.15.
consisting of secondary alkyl and pyridine ligands, with the
rhodium(III) center being additionally decorated by a β-agostic
interaction, helping to achieve an 18-electron count. Despite the
uncertainties in determining the positions of the hydrogen atoms
by XRD, the metric parameters for the RhCH(CH3)− linkage
are in agreement with the existence in the solid state of a
Rh···CH3 β-agostic interaction. The Rh1−C31distanceisrelatively
short at 2.362(7) Å, and both the Rh1−H(C31) distance of
1.937 Å and the Rh1−H−C31 angle of 103.15° are well in the
1.8−2.3 Å and 90−140° ranges commonly found, for such
bonds.25 Although the limited experimental evidence obtained
for the reaction of 3-Rh+ and C2H4 prevents a proper discussion
of these results, it should be noted that observation of the agostic
structure for 5-Rh+ is in line with common knowledge that
agostic bonds are favored for cationic complexes of first- and
second-row metals, whereas their third-row counterparts prefer
the isomeric hydride formulation.25,26 Computational data
discussed next are also in accordance with these observations.
The structure presented in Figure 6 for complex 5-Rh+ may be
viewed as a model for the intermediate preceding β-H elimina-
tion to furnish an isomeric hydride−alkene product. DFT calcu-
lations revealed that while an energy barrier of only 7.8 kcal mol−1
Figure 7. Calculated energy profiles for β-H (solid line) and α-H
(dotted line) elimination from the appropriate conformer of 5-Rh+.
Optimized geometries of 5-Rh+ and the transition state for β-H
elimination. Zero-point-corrected potential energies calculated in
dichloromethane relative to the ethylene adduct.
needs be overcome (Figure 7), the β-H elimination reaction is
thermodynamically uphill by 5.1 kcal mol−1. Moreover, in a
different conformation, 5′-Rh+, which is just 2.2 kcal mol−1 above
the observed structure (Figure 7), α-H elimination to yield a
hydride−alkylidene complex analogous to 6-Ir+, thereby
containing an N-stabilized carbene ligand, could occur with an
affordable energy barrier of 6.4 kcal mol−1. Notwithstanding, the
purported hydride−carbene rhodium complex would be
thermodynamically unstable in comparison with the isolated
complex 5-Rh+ and also with the conformation 5′-Rh+.
Computational Studies on the Reaction of 3-Ir+ with
C2H4. The nature of the carbene ligand of product 6-Ir+, which
besides the N-heteroatom features a methyl substituent at the
carbene carbon, denotes that 6-Ir+ forms by α-H elimination
from a chelating pyridine−alkyl ligand like intermediate
6-Ir·ClCD2Cl+ of Scheme 4 (vide supra). Hence, we initially
envisioned a reaction route implicating a highly reactive cationic
iridium ethylidene group, [Ir]C(H)Me, with a structure akin
to A in Figure 8, formally arising from an iridium-promoted
tautomerization of coordinated C2H4 in 5-Ir+. The said proto-
tropism could be followed by ethylidene migratory insertion
into the Ir−Namido bond, yielding an unsaturated branched alkyl
(B in Figure 8), which would convert into the product by α-H
elimination.
Figure 8. Possible intermediates for an ethylidene route to complex 6-Ir+.
A direct 1,2-shift within the coordinated ethylene molecule
could account for the ethylene-to-ethylidene tautomerism,
similar to well-established metal-promoted acetylene-to-vinyl-
idene rearrangements.27 While the C2H4 and C2H2 tautomeriza-
tions are strongly endothermic (ca. 80 and 45 kcal mol−1 for the
former21a and latter,27 respectively), the relative energies of the
corresponding π-hydrocarbon and carbene isomers change
dramatically in the coordination sphere of transition metals.
In many reported examples, the driving force for the isomer-
ization is largely associated with the strong electronic interaction
F
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Scheme 5. Key Intermediates in the Proposed Migratory Insertion Leading to Complex 6-Ir+
Figure 9. Energy profile including the initial steps of the migratory insertion path. Zero-point-corrected potential energies calculated in dichloromethane
relative to the ethylene adduct.
between the metal and carbene-type ligand.21a,27c Indeed, our
computational studies evince that the ethylidene intermediate A
(Figures 8 and S45 and S46) is only 12.5 kcal mol−1 less stable
than its olefin isomer 5-Ir+ and, moreover, that the consequent
N−C bond formation resulting from a nucleophilic attack of the
Ir−Namido bond to the IrC(H)Me one needs to overcome a
barrier of just 12.1 kcal mol−1. Nevertheless, all attempts to
model the direct 1,2-H shift within the Ir−C2H4 linkage of 5-Ir+
gave unrealistically high energy barriers, which justified ruling out
this path.
As a variant to the direct 1,2-H shift, we analyzed a shuttling
function28−30 of the Ir−Namido unit, abstracting first an olefinic
hydrogen atom to yield an iridium vinyl C (Figures 8 and S46)
and then protonating the β-carbon atom of the ligated vinyl to
produce A. Once again, whereas structure C is energetically
feasible, lying in energy 10.5 kcal mol−1 above 5-Ir+, conversion
of the latter into the former requires one to surmount a barrier of
Figure 10. DFT-calculated energy profiles for α-H (solid line) and β-H
ca. 32.6 kcal mol−1, which is at odds with the experimental
(dotted line) elimination from the appropriate conformer, B or B′, and
conditions. Aside from that, this barrier is significantly higher
than the topmost step in the migratory insertion mechanism
discussed next.
optimized geometries of relevant species. Zero-point-corrected
potential energies calculated in dichloromethane relative to 5-Ir+.
Scheme 5 summarizes the mechanistic route that implies
migratory insertion of ethylene into the Ir−Namido bond of
complex 5-Ir+. Figures 9 and 10 display relevant information on
key steps of the calculated energy profile. The migratory
insertion is relatively facile, and an energy barrier of ΔE⧧ =
22.1 kcal mol−1 needs be surpassed to afford intermediate D
(Scheme 5 and Figure 9), which is in actuality isoenergetic with
the initial (pyridylamido)ethylene complex 5-Ir+. The proposed
intermediate exhibits a six-membered chelating moiety consist-
ing of pyridine and alkyl ends and becomes stabilized by the
formation of a dative covalent bond between iridium and the
former Namido function, now converted into an NH amino one
(Ir−Namino distance of 2.27 Å vs 2.15 Å for Ir−Npyridine).
Elongation of the Ir−Namino distance of D to 3.00 Å results in a
conformation, D′ (Figures 9 and S47; ΔE° = 2.9 kcal mol−1),
from which β-H elimination can take place easily (ΔE⧧
=
10.3 kcal mol−1). As was already anticipated, the formation of 6-Ir+
from the resulting species E (Figure 9; ΔE° = −2.2 kcal mol−1)
requires a change of the coordinated face of the prochiral olefin.
If this process is assisted by the amido nitrogen functionality, the
barrier for Ir−C to Ir−N slippage required to allow olefin
G
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Article
dissociation to yield F (Figure 9) turned out to be rate-limiting
(ΔE⧧ = 27.4 kcal mol−1 from E), but it is still more accessible
than the hydrogen abstraction from C2H4 discussed previously.
As expected, rotation of the NaCHCH2 bond and
recoordination of the olefin through the other face to give the
hydride-olefin species G, i.e., stereoisomer of E, are fast, and the
latter step is also very exothermic (ΔE° = −26.3 kcal mol−1 from F).
From this point, formation of 6-Ir+ requires hydride insertion
and α-elimination from B, both steps having accessible energy
barriers. Notice that α-H elimination from B appears faster than
β-H elimination from B′ and, more significantly, that the energy
return from the former process to produce the carbene product
6-Ir+ is larger than that associated with β-H elimination from B′
to generate the unobserved hydride−olefin tautomer G. It is
worth recalling that although β-H elimination is usually faster
than α-H elimination when the two reactions can compete,
examples are known where the opposite is true. Specifically, in
sterically congested systems in which the M−Cα−Cβ−H atoms
cannot readily adopt the required syn approximately coplanar
arrangement,31 the formation of α-agostic bonds and α-H
elimination may become faster than the corresponding processes
for β-hydrogen atoms.21,23,32
aminopyridinate ligand between classical κ2-N,N-bidentate
binding and an unprecedented κ-N,η3-pseudoallyl coordination,
as depicted in Scheme 6 (left part) for 3-Ir+ and 8-Ir+.
The isomerization implies reversible formation and cleavage
of H−H, C−H, and N−H bonds. Experimental and computa-
tional studies1,2 support the mechanistic pathway presented in
Scheme S2.
The treatment of 3-Ir+ with an excess of H2 yielded the known
binuclear hydride {[(η5-C5Me5)Ir]2(μ-H)3}+,33 whereas catalytic
amounts of H2 (ca. 2 mol %) promoted isomerization of the Ap
ligand of 3-Ir+ to the η3-benzylic−pyridine coordination found in
the new complex 8-Ir+ (Scheme 6). The prototropic rearrange-
ment resulted in a 3-Ir+−8-Ir+ equilibrium mixture of ca. 1:3,
with a half-life, t1/2, of about 1.2 h (20 °C, CD2Cl2). Under similar
experimental conditions and a H2 concentration of 1.6 mol %, the
analogous complex in which the amido nitrogen aryl substituent
was 2,6-Pri2C6H3 instead of C6H5 equilibrated with t1/2 = 9.1 h.
This difference in the rate of almost 1 order of magnitude clearly
reflects the importance of the steric effects of the amido nitrogen
aryl substituent in the reactivity of complexes like 3-M+.
Spectroscopic data for 8-Ir+ are similar to those recorded for
related complexes.1 The NH resonance is, however, slightly
deshielded relative to the values found for analogous complexes
(6.25 vs ca. 5.94 ppm).1 It is tempting to associate this shift with
the relatively short Ir···HN contact of 2.89 Å observed in the
solid-state molecular structure of the complex (Figure 11, left).
Reactions of Iridium and Rhodium Complexes 3-M+
with H2. The title reactions proceeded much as expected on the
basis of previous studies.1,2 Briefly, we recall that catalytic
amounts of H2 promote reversible isomerization of the
Scheme 6. H2-Catalyzed Isomerization of the Aminopyridinate Ligand of 3-Ir+ and Formation of the Carbonyl Complex 9-Ir+
Figure 11. X-ray structures of complexes 8-Ir+ (left) and 8-Rh+ (right) (30% ellipsoids, hydrogen atoms, and anion BArF− were omitted for clarity).
Selected bond lengths (Å) and angles (deg) for [8-Ir]BArF: Ir1−N1 2.086(3), C10−C11 1.378(6), Ir1−C6 2.387(4), C6−C11 1.423(6), Ir1−C7
2.252(4), Ir1−C20 2.170(3), Ir1−C12 2.118(4), Ir1−C21 2.153(4), C6−C7 1.449(5), Ir1−C22 2.202(3), C7−C12 1.449(6), Ir1−C23 2.225(4),
C7−C8 1.423(6), Ir1−C24 2.187(4), C8−C9 1.355(7), Ir1−H(N2)2.891, C9−C10 1.408(6); N1−Ir1−C6 62.04(12), N1−Ir1−C12 80.11(14), C7−Ir1−
C6 36.27(13), N1−C5−C6 108.5(3), C12−Ir1−C7 38.59(16), N1−C1−N2 115.1(3). Selected bond lengths (Å) and angles (deg) for [8-Rh]BArF:
Rh1−N1 2.101(3), C10−C11 1.372(7), Rh1−C6 2.379(4), C6−C11 1.428(7), Rh1−C7 2.237(4), Rh1−C20 2.176(4), Rh1−C12 2.124(5),
Rh1−C21 2.155(4), C6−C7 1.441(6), Rh1−C22 2.191(4), C7−C12 1.437(7), Rh1−C23 2.218(4), C7−C8 1.432(7), Rh1−C24 2.174(4), C8−C9
1.350(8), Rh1−H(N2) 2.987, C9−C10 1.406(8); N1−Rh1−C6 62.43(14), N1−Rh1−C12 81.00(17), C7−Rh1−C6 36.21(16), N1−C5−C6
110.3(4), C12−Rh1−C7 38.37(19), N1−C1−N2 115.4(4).
H
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spectrometers. Spectra were referenced to external SiMe4 (δ 0 ppm)
using the residual protio solvent peaks as internal standards (1H NMR
experiments) or the characteristic resonances of the solvent nuclei
(13C NMR experiments). Spectral assignments were made by routine
one- and two-dimensional NMR experiments where appropriate.
All manipulations were performed under dry, oxygen-free dinitrogen,
following conventional Schlenk techniques. The crystal structures were
determined in a Bruker-Nonius,X8 Kappa diffractometer. Metal com-
plexes [Cp*IrCl2]2,37 [Cp*RhCl2]2,37 and NaBArF38 were prepared as
previously described. The lithium salt of the aminopyridinate ligand
employed in this work was prepared according to published
Nonetheless, recalling that hydrogen positions determined by
X-ray studies are not particularly accurate and that the Ir···HN
contact in 8-Ir+ is longer than the known Ir···HC agostic bonds,34
such a NH···Ir interaction must be very weak and most probably
meaningless in terms of electron density sharing. A close
similarity of 8-Ir+ to earlier analogues was also found in the
reaction with CO (Scheme 6) that occurred with a η3-to-η1-
benzyl coordination change and formation of the carbonyl
complex 9-Ir+. The latter exhibits ν(CO) at 2029 cm−1 (see the
̅
Experimental Section).
procedures.1b The H and 13C{1H} NMR spectral data for the BArF
1
Monitoring the progress of the reaction of 3-Ir+ with an excess
of H2 (1 bar) allowed observation of a hydride intermediate
in high spectroscopic yield (−6.32 ppm, relative intensity 1 H).
Isolation of this complex proved unattainable because in the
presence of H2 it converted quickly to {[(η5-C5Me5)Ir]2
(μ-H)3}+, whereas in the absence of this gas, it yielded, also
rapidly, the 1:3 equilibrium mixture of 3-Ir+ and 8-Ir+. No NH
resonance or other relevant NMR data could, however, be
assigned with certainty to this intermediate, which keeps us from
putting forward a definitive structural proposal.
anion (BArF = B[3,5-(CF3)2C6H3)4]) in CD2Cl2 are identical for all
1
complexes and therefore are not repeated below. H NMR: δ 7.75
(s, 8 H, o-Ar), 7.58 (s, 4 H, p-Ar). 13C{1H} NMR: δ 162.1 (q, 1JCB = 37 Hz,
ipso-Ar), 135.3 (o-Ar), 129.2 (q, 2JCF = 31 Hz, m-Ar), 124.9 (q, 1JCF
273 Hz, CF3), 117.8 (p-Ar).
=
Compound 2-Ir. A toluene solution of LiAp (1 g, 3.57 mmol, 35 mL)
at −50 °C was added to a suspension of [Cp*IrCl2]2 (1.4 g, 1.78 mmol)
in toluene at −50 °C. The resulting mixture was stirred while allowing
it to warm to room temperature and stirred further for a period of 14 h.
The solution was filtered through Celite and the solvent removed under
reduced pressure. 1H NMR analysis of the crude reaction mixture
showed only the presence of 2-Ir, which was crystallized from CH2Cl2/
Complex 3-Rh+ also reacted quickly with 1 bar of H2 at room
temperature although a complex mixture of compounds was
obtained including {[(η5-C5Me5)Rh]2(μ-Cl)3}+.35 At least two
η4-C5Me5H-containing complexes36 were detected by NMR, the
latter possibly related to those that were fully characterized for
bulkier aminopyridinate ligands.2 Likewise, the η3-benzyl
complex 8-Rh+ resulting from the H2-catalyzed isomerization
of the aminopyridinate ligand was detected in the crude reaction
mixture. The latter complex was subsequently generated in
moderate quantities (ca. 50% spectroscopic yield) using sub-
stoichiometric amounts of H2 (60 mol %). Complex 8-Rh+ was
fully characterized. Its X-ray molecular structure is presented in
Figue 11 (right-hand side) and features metrical parameters for the
metal−η3-benzyl moiety similar to those of analogous complexes.
1
hexane mixtures at −23 °C. Yield: 1.55 g (68%). H NMR (C6D6,
25 °C): δ 7.54, 7.26, 6.96 (d, t, t, 2:2:1, 3JHH ∼ 7.5 Hz, 5 CHPh), 7.06,
7.00, 6.96 (t, br d, br d, 1 H each, 3JHH ∼ 7.5 Hz, 3 CHXyl), 6.80 (t, 1 H,
3JHH = 7.9 Hz, 1 CHPyr), 6.35 (d, 1 H, 3JHH = 8.7 Hz, 1 CHPyr), 5.81 (d,
1 H, 3JHH = 7.0 Hz, 1 CHPyr), 2.66, 2.13 (s, 3 H each, 2 MeXyl), 1.16 (s,
15 H, 5 MeCp*). 13C{1H} NMR (C6D6, 25 °C): δ 171.2, 156.4 (Cq‑Pyr),
146.3 (Cq‑Ph), 140.0, 139.0, 136.7 (Cq‑Xyl), 137.3, 108.9, 106.5 (CHPyr),
129.0, 124.1, 122.1 (2:2:1, CHPh), 128.5, 128.3, 127.0 (CHXyl), 84.0
(Cq‑Cp*), 21.9, 20.8 (MeXyl), 9.1 (MeCp*). Elem anal. Calcd for
C29H32ClIrN2: C, 54.7; H, 5.1; N, 4.4. Found: C, 54.6; H, 5.1; N, 4.5.
Compound 2-Rh. Following the procedure described above for 2-Ir
but using [Cp*RhCl2]2, compound 2-Rh was obtained in quantitative
spectroscopic yield and was crystallized from CH2Cl2−hexane mixtures
at −23 °C. Yield: 0.8 g (64%). 1H NMR (C6D6, 25 °C): δ 7.65, 7.28 (d, t,
1:1, 3JHH ∼ 7.5 Hz, 4 CHPh), 7.04 (m, 3 H, 1 CHPh + 2 CHXyl), 6.95 (d,
1 H, 3JHH ∼ 7.5 Hz, 1 CHXyl), 6.79, 6.33, 5.81 (m, d, d, 1 H each, 3JHH
∼
CONCLUSIONS
■
7.5 Hz, 3 CHPyr), 2.70, 2.18 (s, 3 H each, 2 MeXyl), 1.10 (s, 15 H,
5 MeCp*). 13C{1H} NMR (C6D6, 25 °C): δ 170.4, 158.1 (Cq‑Pyr), 147.9
(Cq‑Ph), 140.4, 139.2, 136.6 (Cq‑Xyl), 137.4, 108.8, 105.0 (CHPyr), 129.1,
This work demonstrated that the MNamido bond of the
pyridylamido complexes [(η5-C5Me5)M(N^N)]+ (3-M+) readily
activates CO, C2H4, and H2. Whereas CO and C2H4 brought
about migratory insertion reactivity, H2 catalyzed a prototropic
rearrangement within the Ap ligand, whereby remote transfer of
a benzylic hydrogen atom from the pyridine xylyl substituent to
the amido nitrogen atom occurred readily in a reversible fashion.
Only 3-Rh+ experienced CO migratory insertion into the
Rh−Namido bond, expanding the original four-membered
125.3, 122.2 (2:2:1, CHPh), 128.2, 127.9, 126.8 (CHXyl), 91.9 (d, 1JCRh
=
8 Hz, Cq‑Cp*), 21.9, 20.8 (MeXyl), 8.8 (MeCp*). Elem anal. Calcd for
C29H32ClRhN2: C, 63.7; H, 5.9; N, 5.1. Found: C, 63.7; H, 6.0; N, 5.2.
Compound [3-Ir]BArF. To a solution of 2-Ir (1.5 g, 2.36 mmol) in
CH2Cl2 (40 mL) was added NaBArF (2.1 g, 2.36 mmol) in CH2Cl2
(25 mL). Immediately the solution turned from orange to dark gray as a
consequence of the formation of the cationic complex. The resulting
mixture was filtered through Celite and evaporated to dryness and the
residue washed with pentane to yield quantitatively the desired product.
An analytically pure sample was obtained by crystallization from
solutions in hexane−ether mixtures at −23 °C. Yield: 2.8 g (80%). 1H
NMR (CD2Cl2, 25 °C): δ δ 7.65 (t, 1 H, 3JHH = 8.0 Hz, 1 CHPyr), 7.44
(t, 2 H, 3JHH = 7.7 Hz, 2 CHPh), 7.33 (t, 1 H, 3JHH = 7.6 Hz, 1 CHXyl),
7.19 (m, 3 H, 1 CHPh + 2 CHXyl), 7.08 (d, 2 H, 3JHH = 7.8 Hz, 2 CHPh),
6.36, 5.89 (br s, 1 H each, 2 CHPyr), 2.25 (s, 6 H, 2 MeXyl), 1.31 (s, 15 H,
5 MeCp*). 13C{1H} NMR (CD2Cl2, 25 °C): δ 178.3, 155.8 (br, Cq‑Pyr),
145.1, 119.5, 104.9 (br, CHPyr), 143.5 (br, Cq‑Ph), 136.8, 135.9 (1:2,
Cq‑Xyl), 130.5, 128.6 (1:2, CHXyl), 129.9, 128.1, 123.1 (br, 2:1:2, CHPh),
88.1 (Cq‑Cp*), 20.6 (MeXyl), 10.2 (MeCp*). HRMS (FAB). Calcd for
C29H32N2Ir ([M]+): m/z 601.2195. Found: m/z 601.2202.
Rh−N−C−N ring to a five-membered pyridine−carbamoyl
rhodacycle, whereas for iridium, the stronger M−Namido bond
decisively retarded the migratory CO insertion step, affording the
carbonyl adduct 4-Ir+ as the only observable product. In contrast,
3-Ir+ formed easily, albeit reversibly, the ethylene adduct 5-Ir+,
which rearranged spontaneously by migratory insertion, with the
end product of the reaction being the hydride N-substituted
alkylidene complex 6-Ir+. DFT studies reinforced a mechanistic
scheme encompassing irreversible C−N bond formation
followed by reversible β- and α-H elimination reactions.
EXPERIMENTAL SECTION
General Procedures. Microanalyses were performed by the
Microanalytical Service of the Instituto de Investigaciones Quimicas
Compound [3-Rh]BArF. Following the procedure described above
for [3-Ir]BArF but using 2-Rh, complex [3-Rh]BArF was obtained in
quantitative spectroscopic yield. 1H NMR (CD2Cl2, 25 °C): δ 7.44 (m,
3 H, 1 CHPyr + 2 CHPh), 7.36, 7.26 (t, d, 1:2, 3JHH ∼ 7.5 Hz, 3 CHXyl),
7.22 (m, 3 H, 3 CHPh), 6.33, 6.07 (d, 1 H each, 3JHH ∼ 7.5 Hz, 2 CHPyr),
2.32 (s, 6 H, 2 MeXyl), 1.34 (s, 15 H, 5 MeCp*). 13C{1H} NMR (CD2Cl2,
25 °C): δ 175.3, 157.5 (Cq‑Pyr), 145.6 (Cq‑Ph), 143.5, 116.5, 104.2
■
́
(Sevilla, Spain). IR spectra were obtained from a Bruker Vector 22
spectrometer. The mass spectrometry (MS) spectra were obtained at
the Mass Spectroscopy Service of the University of Sevilla. The NMR
instruments were Bruker DRX-500, DRX-400, and DPX-300
I
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(CHPyr), 137.9, 136.4 (1:2, Cq‑Xyl), 130.2, 123.2 (2:3, CHPh), 130.1,
(Cq‑Ph), 139.4, 137.7, 137.3 (Cq‑Xyl), 138.2, 124.6, 113.9 (CHPyr), 131.8,
131.4, 127.6 (2:1:2, CHPh), 130.4, 129.2 (1:2, CHXyl), 98.4 (Cq‑Cp*),
35.4 (IrCMe), 21.5, 19.7 (MeXyl), 9.8 (MeCp*). Elem anal. Calcd for
C63H48BF24IrN2: C, 50.7; H, 3.2; N, 1.9. Found: C, 50.6; H, 3.5; N, 1.7.
Compound [7-Ir]BArF. To a solution of compound [6-Ir]BArF
(0.02 g, 0.013 mmol) in CD2Cl2 (0.5 mL) in a Young NMR tube was
added an excess of PMe3 (0.05 mL, 0.5 mmol). 1H NMR analysis of the
crude product revealed quantitative conversion into complex [7-Ir]BArF
after 15 h at room temperature. Free trimethylphosphine was removed
under reduced pressure, and compound [7-Ir]BArF was crystallized
from CH2Cl2−pentane mixtures at −23 °C. Yield: 15 mg (70%).
1
128.6 (1:2, CHXyl), 95.2 (d, JCRh ∼ 8 Hz, Cq‑Cp*), 20.8 (MeXyl), 9.6
(MeCp*). HRMS (FAB). Calcd for C29H32N2Rh ([M]+): m/z 511.1621.
Found: m/z 511.1619.
Compound [4-Ir]BArF. CO(g) was bubbled through a solution of
compound [3-Ir]BArF (0.2 g, 0.14 mmol) in CH2Cl2 (5 mL) at room
temperature for 5 min. During this period of time, the color of the
solution changed from dark gray to bright orange. The resulting mixture
was stirred for 10 min, and the volatiles were then removed under
1
reduced pressure. H NMR analysis of the crude product revealed
quantitative conversion into the desired complex, which was crystallized
from CH2Cl2−pentane mixtures at −23 °C. Yield: 60%. IR (Nujol):
ν(CO) 2048 cm−1. 1H NMR (CD2Cl2, 25 °C): δ 7.59 (m, 1 H, 1 CHPyr),
3
1H NMR (CD2Cl2, −40 °C): δ 7.49, 7.31 (t, m, 2:3, JHH ∼ 7.5 Hz,
3 CHPh), 7.35, 6.50, 6.21 (t, d, d, 1 H each, 3JHH ∼ 7.5 Hz, 3 CHPyr), 7.15,
7.06 (m, 1:2, 3 CHXyl), 3.67 (m, 2 H, 1 IrCH2CH2), 2.09 (s, 6 H,
2 MeXyl), 1.62 (s, 15 H, 5 MeCp*), 1.47 (m, 2 H, 1 IrCH2CH2), 1.38 (d,
18 H, 2JHP = 9.5 Hz, 2 PMe3). 13C{1H} NMR (CD2Cl2, 25 °C): δ 158.6,
158.3 (Cq‑Pyr), 145.4 (Cq‑Ph), 142.5, 136.3, 127.4 (Cq‑Xyl), 131.0, 129.8
(2:3, CHPh), 128.1, 127.9 (1:2, CHXyl), 126.8, 113.4, 106.9 (CHPyr), 99.0
3
3
7.40 (t, 2 H, JHH = 7.7 Hz, 2 CHPh), 7.33 (t, 1 H, JHH = 7.6 Hz,
1 CHXyl), 7.19 (m, 3 H, 2 CHXyl + 1 CHPh), 7.02 (d, 2 H, 3JHH = 7.8 Hz,
3
3
2 CHPh), 6.53 (d, 1 H, JHH = 8.8 Hz, 1 CHPyr), 6.42 (d, 1 H, JHH
=
7.3 Hz, 1 CHPyr), 2.19 (s, 6 H, 2 MeXyl), 1.59 (s, 15 H, 5 MeCp*). 13C{1H}
NMR (CD2Cl2, 25 °C): δ 174.4, 157.0 (Cq‑Pyr), 168.8 (IrCO), 141.9
(Cq‑Ph), 140.8, 112.8, 107.9 (CHPyr), 137.1, 136.3, 136.2 (Cq‑Xyl), 130.3,
125.1, 123.7 (2:1:2, CHPh), 130.3, 128.7, 128.5 (CHXyl), 101.6 (Cq‑Cp*),
21.3, 20.4 (MeXyl), 9.3 (MeCp*). Elem anal. Calcd for C62H44BF24IrN2O:
C, 49.9; H, 3.0; N, 1.9. Found: C, 49.6; H, 3.0; N, 1.8.
(Cq‑Cp*), 56.0 (t, 3JCP = 5 Hz, IrCH2CH2), 20.7 (MeXyl), 18.1 (d, 1JCP
=
38 Hz, PMe3), 10.1 (MeCp*), −7.1 (t, 3JCP = 7 Hz, IrCH2CH2). 31P{1H}
NMR (160 MHz, CD2Cl2, 25 °C): δ −45.4.
Compound [8-Ir]BArF. A solution of complex [3-Ir]BArF (0.05 g,
0.034 mmol) in CH2Cl2 (1.25 mL) was treated with H2 (1 atm), and the
resulting mixture was stirred for 10 min at room temperature. 1H NMR
analysis of the reaction mixture revealed the formation of complex
[8-Ir]BArF together with its isomer [3-Ir]BArF in a ca. 3:1 ratio.
[8-Ir]BArF was separated by fractional crystallization from CH2Cl2−
pentane mixtures at −23 °C as bright-orange crystals. Yield: 10 mg
(22%). 1H NMR (CD2Cl2, 25 °C): δ 7.71 (br s, 1 H, 1 CHXyl), 7.69 (m,
Compound [4-Rh]BArF. Following the procedure described above
for [4-Ir]BArF but using [3-Rh]BArF, compound [4-Rh]BArF was
obtained in quantitative spectroscopic yield (the color of the solution
changed from dark brown to orange) and was crystallized from
CH2Cl2−pentane mixtures at −23 °C. Yield: 50%. IR (Nujol): ν(CO)
2077 cm−1, ν(COamide) 1687 cm−1. 1H NMR (CD2Cl2, 25 °C): δ 7.77,
7.00, 6.66 (t, d, d, 1 H each, 3JHH ∼ 7.5 Hz, 3 CHPyr), 7.63, 7.23 (m, br d,
3:1, 3JHH ∼ 7.5 Hz, 4 CHPh), 7.40 (t, 1 H, 3JHH ∼ 7.5 Hz, 1 CHXyl), 7.28
(m, 3 H, 2 CHXyl + 1 CHPh), 2.20, 2.12 (s, 3 H each, 2 MeXyl), 1.59 (s,
15 H, 5 MeCp*). 13C{1H} NMR (CD2Cl2, 25 °C): δ 189.3, 187.1 (d,
1JCRh = 30 Hz, 1JCRh = 76 Hz, RhCON and RhCO, respectively), 162.7,
158.7 (Cq‑Pyr), 141.4, 123.4, 111.2 (CHPyr), 138.7, 137.2, 137.0 (Cq‑Xyl),
136.5 (Cq‑Ph), 131.3, 131.2, 130.8, 129.3, 129.0 (CHPh), 130.6, 129.4
3
1 H, 1 CHPyr), 7.50 (t, 2 H, JHH = 7.8 Hz, 2 CHPh), 7.46 (m, 1 H, 1
CHXyl), 7.33 (t, 1 H, 3JHH = 7.5 Hz, 1 CHPh), 7.24 (d, 2 H, 3JHH = 7.8 Hz,
3
3
2 CHPh), 6.98 (d, 1 H, JHH = 8.4 Hz, 1 CHXyl), 6.93 (d, 1 H, JHH
=
3
8.7 Hz, 1 CHPyr), 6.25 (br s, 1 H, NH), 6.23 (d, 1 H, JHH = 7.7 Hz,
1 CHPyr), 3.61, 2.11 (d, 1 H each, 2JHH = 4.8 Hz, IrCH2), 2.47 (s, 3 H,
1 MeXyl), 1.58 (s, 15 H, 5 MeCp*). 13C{1H} NMR (CD2Cl2, 25 °C):
δ 154.5, 154.0 (Cq‑Pyr), 141.4, 118.8, 108.8 (CHPyr), 137.8 (Cq‑Xyl), 136.9
(Cq‑Ph), 134.0, 130.4, 129.5 (CHXyl), 130.8, 126.8, 122.8 (2:1:2, CHPh),
100.8 (IrCH2Cq), 93.9 (IrCq‑Xyl), 90.3 (Cq‑Cp*), 35.6 (IrCH2), 20.8
(MeXyl), 9.3 (MeCp*). HRMS (FAB). Calcd for C29H32N2Ir ([M]+): m/z
601.2195. Found: m/z 601.2181. Elem anal. Calcd for C61H44BF24IrN2:
C, 50.0; H, 3.0; N, 1.9. Found: C, 49.8; H, 3.1; N, 1.8.
1
(1:2, CHXyl), 109.3 (d, JCRh = 5 Hz, Cq‑Cp*), 22.3, 21.7 (MeXyl), 9.5
(MeCp*). Elem anal. Calcd for C63H44BF24N2O2Rh: C, 52.9; H, 3.1; N,
2.0. Found: C, 53.3; H, 3.2; N, 1.8. HRMS (FAB). Calcd for
C31H32N2O2Rh ([M]+): m/z 567.1519. Found: m/z 567.1503.
Compound [5-Ir]BArF. C2H4(g) was bubbled through a solution of
compound [3-Ir]BArF (0.02 g, 0.014 mmol) in CH2Cl2 (3 mL) in a
Young NMR tube for 3 min. During this period of time, the color of the
solution changed from black to orange. 1H NMR analysis of the crude
product revealed quantitative conversion into complex [5-Ir]BArF in an
admixture with free ethylene. This complex could not be isolated
because of its reversion to the starting material in the absence of C2H4
and to its evolution at room temperature to 6-Ir+. 1H NMR (CD2Cl2,
−40 °C): δ 7.44 (t, 1 H, 3JHH = 8.1 Hz, 1 CHPyr), 7.34 (t, 2 H, 3JHH = 7.
1 Hz, 2 CHPh), 7.28, 7.17 (m, 1:2, 3 CHXyl), 7.09 (t, 1 H, 3JHH = 7.4 Hz, 1
CHPh), 7.00 (d, 2 H, 3JHH = 7.9 Hz, 2 CHPh), 6.50 (d, 1 H, 3JHH = 8.9 Hz,
Compound [8-Rh]BArF. In a Young NMR tube, a solution of
complex [3-Rh]BArF (40 mg, 0.029 mmol) in CH2Cl2 (0.5 mL) was
treated with H2 (60 mol %), and after 24 h at room temperature,
1H NMR analysis of the reaction mixture revealed transformation into
complex [8-Rh]BArF in 50% spectroscopic yield. [8-Rh]BArF was
separated by fractional crystallization from CH2Cl2−pentane mixtures at
−23 °C as bright-red crystals. Yield: 16 mg (38%). IR (Nujol): ν(NH)
1
3396 cm−1. H NMR (CD2Cl2, 25 °C): δ 7.73 (br s, 1 H, 1 CHXyl
,
−
detected by NOESY experiment, under the BArF signal), 7.63, 6.90,
6.30 (t, d, d, 1 H each, 3JHH ∼ 7.5 Hz, 3 CHPyr), 7.49 (m, 3 H, 1 CHXyl
+
1 CHPyr), 6.24 (d, 1 H, 3JHH = 7.2 Hz, 1 CHPyr), 4.02 (br d, 4 H, 3JHH
=
2 CHPh), 7.31, 7.25 (t, d, 1:2, 3JHH ∼ 7.5 Hz, 3 CHPh), 6.97 (d, 1 H, 3JHH
∼ 7.5 Hz, 1 CHXyl), 6.14 (br s, 1 H, NH), 3.64, 2.54 (d, 1 H each, 2JHH
∼ 3.5 Hz, RhCH2), 2.48 (s, 3 H, 1 MeXyl), 1.51 (s, 15 H, 5 MeCp*).
13C{1H} NMR (CD2Cl2, 25 °C): δ 155.6, 153.8 (Cq‑Pyr), 141.4, 117.5,
109.1 (CHPyr), 137.9 (Cq‑Xyl), 137.3 (Cq‑Ph), 133.5, 131.0, 129.0
8.6 Hz, IrC2H4), 2.15, 2.06 (s, 3 H each, 2 MeXyl), 1.34 (s, 15 H,
5 MeCp*). 13C{1H} NMR (CD2Cl2, −40 °C): δ 171.0, 156.6 (Cq‑Pyr),
141.7 (Cq‑Ph), 139.1, 111.4, 108.0 (CHPyr), 137.1, 136.3, 135.2 (Cq‑Xyl),
129.5, 123.4, 122.2 (2:1:2, CHPh), 129.4, 128.1, 127.5 (CHXyl), 98.5
(Cq‑Cp*), 61.7 (IrC2H4), 21.8, 20.9 (MeXyl), 8.5 (MeCp*).
Compound [6-Ir]BArF. C2H4(g) was bubbled through a solution of
compound [3-Ir]BArF (0.1 g, 0.068 mmol) in CH2Cl2 (15 mL) for
3 min. The resulting mixture was stirred for 24 h. The color changed
from orange to green, and then the volatiles were removed under
reduced pressure. Quantitative conversion into [6-Ir]BArF was
ascertained by 1H NMR, and the product was crystallized from Et2O−
pentane mixtures at −23 °C. Yield: 78 mg (75%). IR (Nujol): ν(Ir−H)
2096 cm−1. 1H NMR (CD2Cl2, 25 °C): δ 7.73−7.67 (m, 5 H, 5 CHPh),
(CHXyl), 130.8, 126.6, 122.6 (2:1:2, CHPh), 107.3 (br, RhCH2Cq‑Xyl
,
detected by HMBC experiment), 100.6 (br, RhCq‑Xyl), 96.7 (d, 1JCRh
=
1
7.3 Hz, Cq‑Cp*), 47.9 (d, JCRh = 13.8 Hz, RhCH2), 20.8 (MeXyl), 9.5
(MeCp*). HRMS (FAB). Calcd for C29H32N2Rh ([M]+): m/z 511.1621.
Found: m/z 511.1620.
Observation of a Hydride Intermediate in the Reaction of
[3-Ir]BArF with H2. In a Young NMR tube, a solution of complex
[3-Ir]BArF (0.02 g, 14 μmol) in CD2Cl2 (0.5 mL) was treated with H2
(500 mol %), and after 1−2 min at room temperature, the solution color
3
3
7.60 (t, 1 H, JHH = 8.0 Hz, 1 CHPyr), 7.34 (t, 1 H, JHH = 7.7 Hz,
1
1 CHXyl), 7.25 (m, 2 H, 2 CHXyl), 6.93 (dd, 1 H, 3JHH = 7.5 Hz, 4JHH
=
changed from dark gray to orange. H NMR analysis of the reaction
1.5 Hz, 1 CHPyr), 6.84 (dd, 1 H, 3JHH = 8.5 Hz, 4JHH = 1.5 Hz, 1 CHPyr),
2.69 (d, 3 H, 4JHH = 2.3 Hz, IrCMe), 2.10, 2.06 (s, 3 H each, 2 MeXyl),
1.65 (s, 15 H, 5 MeCp*), −16.42 (br s, 1 H, IrH). 13C{1H} NMR
(CD2Cl2, 25 °C): δ 235.9 (IrCMe), 165.9, 160.6 (Cq‑Pyr), 139.7
mixture revealed the formation of a hydride intermediate in ≥95%
spectroscopic yield. This complex could not be isolated or be completely
characterized (see the text). 1H NMR (CD2Cl2, 25 °C): δ 8.03 (t, 1 H,
3JHH = 7.9 Hz, 1 CHAr), 7.53−6.85 (10 CHAr), 2.12 (s, 6 H, 2 MeXyl),
J
DOI: 10.1021/acs.inorgchem.7b02283
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1.20 (s, 15 H, 5 MeCp*), −6.32 (s, 1 H, IrH). A resonance attributable to
a possible NH proton could not be located.
ORCID
Nuria Rendon: 0000-0003-1760-185X
́
Compound [9-Ir]BArF. CO(g) was bubbled through a solution of
compound [8-Ir]BArF (0.01 g, 6.8 μmol) in CH2Cl2 (2.5 mL) at room
temperature for 5 min. The solution changed color immediately from
orange to light yellow. The solvent was removed under reduced pressure,
Joaquín Lopez-Serrano: 0000-0003-3999-0155
́
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
1
and H NMR analysis of the reaction product showed quantitative
conversion into [9-Ir]BArF. It was washed with pentane and dried under
vacuum. IR (Nujol): ν(NH) 3356 cm−1, ν(CO) 2029 cm−1. 1H NMR
(CD2Cl2, 25 °C): δ 7.64, 6.87 (t, d, 1 H each, 3JHH ∼ 7.5 Hz, 2 CHPyr),
Notes
The authors declare no competing financial interest.
7.54, 7.42 (m, t, 2:1, 3JHH = 7.5 Hz, 3 CHPh), 7.30 (m, 3 H, 2 CHPh
1 CHXyl), 7.19 (m, 3 H, 1 CHXyl + 1 CHPyr + NH), 7.10 (d, 1 H, 3JHH
+
=
7.5 Hz, 1 CHXyl), 3.73, 2.99 (d, 1 H each, 2JHH = 11.0 Hz, IrCH2), 2.32
(s, 3 H, 1 MeXyl), 1.67 (s, 15 H, 5 MeCp*). 13C{1H} NMR (CD2Cl2,
25 °C): δ 168.1 (CO), 159.7, 155.2 (Cq‑Pyr), 144.6 (Cq‑Ph), 140.4, 118.6,
108.4 (CHPyr), 138.1, 137.9, 137.5 (Cq‑Xyl), 131.1, 128.2, 125.3 (2:1:2,
CHPh), 129.9, 129.3, 123.6 (CHXyl), 101.9 (Cq‑Cp*), 21.9 (MeXyl), 8.9
(IrCH2), 8.8 (MeCp*). Elem anal. Calcd for C62H44BF24IrN2O: C, 49.9;
H, 3.0; N, 1.9. Found: C, 49.7; H, 3.4; N, 1.6.
ACKNOWLEDGMENTS
■
Financial support (FEDER contribution) from the Spanish
Ministerio de Economía y Competitividad (Projects CTQ2016-
80814-R, CTQ2016-75193-P, and CTQ2016-81797-REDC)
and Junta de Andalucía (Grant FQM-119) is acknowledged.
The use of computational facilities of The Supercomputing
Center of Galicia (CESGA) is thankfully acknowledged.
Computational Details. DFT calculations were carried out with
the Gaussian 09 program.39 Geometry optimizations of all species were
calculated in the gas phase without restrictions using the PBE0
functional.40 All light atoms were represented with the 6-31g(d,p) basis
set, while the iridium and rhodium atoms were described by the SDD
basis set and its associated electron core potential.41 Frequency
calculations were performed on the optimized structures at the same
level of theory to characterize the stationary points, as well as for
calculation of the gas-phase enthalpies (H), entropies (S), and Gibbs
energies (G). The nature of the intermediates connected by a transition
state was determined by intrinsic reaction coordinate (IRC) calculations
or by perturbation of the transition states along the transition-state
coordinate and optimization to a minimum. Bulk solvent effects were
modeled using Truhlar’s SMD continuum solvent model,42 and
empirical dispersion corrections were added with the D3 version of
Grimme’s dispersion.43−45 Both corrections were made on the gas-
phase geometries by single-point calculations. All energies reported
throughout the text are zero-point-corrected potential energies in
dichloromethane.
DEDICATION
■
Dedicated to Prof. Peter M. Maitlis on the occasion of his 85th
birthday in recognition of his outstanding contributions to
organometallic chemistry.
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* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b02283.
Experimental procedures for the synthesis of compounds 1
and [5-Rh]BArF, proposed mechanism for theH2-catalyzed
isomerization between 3-M+ and 8-M+, NMR spectra of
all compounds, crystallographic data and details of the
structure determinations for 2-Ir, 2-Rh, [3-Ir]BArF,
[4-Ir]BArF, [4-Rh]BArF, [5-Rh]BArF, [6-Ir]BArF,
[7-Ir]BArF, [8-Ir]BArF, and [8-Rh]BArF, and additional
computational details (PDF)
Geometric coordinates of all transition states and inter-
mediates (XYZ)
Accession Codes
CCDC 1572330−1572339 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
(5) Zuccaccia, C.; Bellachioma, G.; Bortolini, G.; Bucci, A.; Savini, A.;
Macchioni, A. Transformation of a Cp*−Iridium(III) Precatalyst for
Water Oxidation when Exposed to Oxidative Stress. Chem. - Eur. J. 2014,
20, 3446−3456.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nuria@iiq.csic.es (N.R.). Fax: (+34)954460565.
K
DOI: 10.1021/acs.inorgchem.7b02283
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