2 as convenient entry into iron-modified pincer complexes: Bimetallic η6,κ1-POCOP-pincer iron iridium compounds

Article abstract of DOI:10.1039/c1nj20399a

The synthesis of [(1,2,4-tBu₃C₅H₂)FeI] ₂, [Cp′FeI]₂ (1), and its reaction chemistry are described. Complex 1 shows remarkable stability toward ligand redistribution in solution and gas-phase due to kinetic stabilization with respect to formation. In coordinating solvents the dimer is homolytically cleaved to form paramagnetic mono-solvate 16 valence-electron (VE) adducts [Cp′FeI(L)]. These solvent adducts are labile, and 1 is re-established on exposure to dynamic vacuum. Complex 1 was demonstrated to be a suitable synthon for [Cp′Fe] ⁺ transfer to Ir pincer complexes and successful η⁶- coordination of this fragment to the arene ring of κ¹- metallated POCOP-iridium pincer complexes [(Cp′Fe)(POCOP)Ir(H)(I)] ([5]⁺) and [(Cp′Fe)(POCOP)Ir(C₂H₄)] ([7]⁺) (POCOP = C₆H₃-2,6-(OP^tBu ₂)₂) was accomplished. In these heterometallic organometallic complexes, the π- and σ-electrons of the POCOP-pincer phenyl anion are involved in η⁶- and κ¹- coordination to iron(ii) and iridium(iii) and iridium(i), respectively. [Cp′Fe]⁺ coordination alters the charge distribution in the (POCOP)Ir complex by inducing significant charge transfer from the POCOP aryl π-system to the [Cp′Fe]⁺ fragment, and therefore yielding an electron-poor and more electrophilic iridium center. Density functional theory (DFT) calculations provide a detailed understanding of this effect. Preliminary catalytic studies show that [7]⁺ is an active catalyst in transfer dehydrogenation between cyclooctane (COA) and tert-butyl ethylene (TBE).

Full text of DOI:10.1039/c1nj20399a

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PAPER
[Cp0FeI]2 as convenient entry into iron-modified pincer complexes:
bimetallic g6,j1-POCOP-pincer iron iridium compoundsw
Marc D. Walter*a and Peter S. Whiteb
Received (in Victoria, Australia) 8th May 2011, Accepted 13th June 2011
DOI: 10.1039/c1nj20399a
The synthesis of [(1,2,4-tBu3C5H2)FeI]2, [Cp0FeI]2 (1), and its reaction chemistry are described.
Complex 1 shows remarkable stability toward ligand redistribution in solution and gas-phase due
to kinetic stabilization with respect to Cp02Fe formation. In coordinating solvents the dimer is
homolytically cleaved to form paramagnetic mono-solvate 16 valence-electron (VE) adducts
[Cp0FeI(L)]. These solvent adducts are labile, and 1 is re-established on exposure to dynamic
vacuum. Complex 1 was demonstrated to be a suitable synthon for [Cp0Fe]+ transfer to
Ir pincer complexes and successful Z6-coordination of this fragment to the arene ring
of k1-metallated POCOP–iridium pincer complexes [(Cp0Fe)(POCOP)Ir(H)(I)] ([5]+) and
[(Cp0Fe)(POCOP)Ir(C2H4)] ([7]+) (POCOP = C6H3-2,6-(OPtBu2)2) was accomplished.
In these heterometallic organometallic complexes, the p- and s-electrons of the POCOP-pincer
phenyl anion are involved in Z6- and k1-coordination to iron(II) and iridium(III) and iridium(I),
respectively. [Cp0Fe]+ coordination alters the charge distribution in the (POCOP)Ir complex by
inducing significant charge transfer from the POCOP aryl p-system to the [Cp0Fe]+ fragment, and
therefore yielding an electron-poor and more electrophilic iridium center. Density functional
theory (DFT) calculations provide a detailed understanding of this effect. Preliminary catalytic
studies show that [7]+ is an active catalyst in transfer dehydrogenation between cyclooctane
(COA) and tert-butyl ethylene (TBE).
systems potentially useful candidates for molecular electronics
and redox-switchable catalyst systems.14–16
Introduction
Initially regarded as nothing more than laboratory curiosities
The synthesis of bimetallic pincer systems is an interesting
metal pincer complexes have been used in a wide range of
development combining exciting aspects of both areas in one
applications over the last decade. Today some of the most
system. In these bimetallic species the aromatic backbone
active catalysts for organic transformations such as CH-bond
of the pincer systems has been modified by suitable metal
activation and functionalization are based on the pincer ligand
complex fragments and two different approaches have been
architecture.1–5 They have also gained biological significance
reported: (a) synthesis of neutral metallocene-based PCP
ligands17–19 and (b) post-modification of the aromatic phenyl
as potential therapeutics or pharmaceutical agents and dendrimeric
and supported materials.6–10
ring with a highly arenophilic cationic [(C5R5)Ru]+ fragment
to yield cationic bimetallic pincer complexes.20–22 In the
Another very active area of chemical research is metal–
metal cooperation and communication. During catalysis these
former case, it was shown that ferrocene-based neutral PCP
effects can facilitate chemical transformations by lowering the
Ir complexes are superior transfer dehydrogenation catalysts
when compared to the unmodified PCP systems,19 while in the
activation barrier and open reaction pathways that are difficult
to access with mononuclear catalysts.11–13 In addition electronic
latter case an increased electrophilicity of the corresponding
Pt/Pd center is induced on [CpRu]+ coordination.19 Encouraged
communication between metal centers make multi-metallic
by these reports we were interested in preparing pincer systems
which are modified by a 12VE [CpFe]+ fragment. These
a Institut fur Anorganische und Analytische Chemie,
¨
Technische Universitat Braunschweig, Hagenring 30,
¨
fragments are generally accessible as masked synthons with
sterically demanding ligands, e.g. as 18VE [(C5Me5)Fe(NCMe)3]+
cations prepared photochemically from [(C5Me5)Fe(CO)3]+ in
acetonitrile (MeCN),23 whereas the Cp-analogue [(C5H5)Fe-
(NCMe)3]+ is unstable above ꢀ40 1C.24,25 The synthesis of
these species generally requires multi-step procedures and the
photochemical exchange of strongly coordinating CO against
38106 Braunschweig, Germany. E-mail: mwalter@tu-bs.de;
Fax: +49 531-391-5309; Tel: +49 531-391-5312
b Department of Chemistry, University of North Carolina at Chapel
Hill, Chapel Hill, North Carolina 27599-3290, USA
w Electronic supplementary information (ESI) available. CCDC
reference numbers 824916–824919. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1nj20399a
c
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more labile solvent (MeCN) ligands. One of the reasons for the
rather tedious synthesis is the thermodynamic stability of the
corresponding ferrocene molecule, (C5H5)2Fe, which is one of
the iconic complexes in organometallic chemistry. In contrast
to the rich and well-established [(C5Me5)RuX]n half-sandwich
chemistry,26–32 the corresponding iron chemistry has not been
explored extensively. However, sterically demanding alkyl-
cyclopentadienyl ligands are instrumental in the isolation of
high-spin mono(cyclopentadienyl) iron(II) complexes and their
kinetic stability results from the effective blocking of the
dismutation pathway. Sitzmann and co-workers reported the
synthesis of [(nCp)FeBr]2 (5Cp = C5(CHMe2)5, 4Cp =
C5H(CHMe2)4 and Cp0
= 1,2,4-C5H2(CMe3)3) without
additional donor ligands.33,34 These species can be isolated
as stable compounds and react with substituted phenolates
to yield monomeric and dimeric complexes depending on
the steric bulk of the phenolate ligand.34 More recently
s/p-rearrangements of aryl ligands connected to the Cp0Fe-
fragment have been reported,35–37 whereas the related
5CpFe(C5H3-2,6-(iPr)2) complex exhibits single-molecule
magnet behavior.38
Fig. 1 ORTEP diagram of 1-I (50% probability ellipsoids). Hydrogen
atoms have been omitted for clarity. Selected bond distances (A):
Cp(cent)–Fe(ave) 1.93, Fe1–I1 2.6748(5), Fe1–I2 2.7088(5), Fe2–I1
2.7120(5), Fe2–I2 2.6781(5), Fe1ꢂ ꢂ ꢂFe2 3.5263(8).
requires good leaving properties of the corresponding anion.
The molecular structures of 1-I and 2 are shown in Fig. 1 and 2,
respectively, and selected bond distances and angles are given
in the figure captions. The Fe–Cp centroid distance in
1-I of 1.93 A is significantly larger than for hexa-tert-butyl-
ferrocene, Cp02Fe (2), (1.71 A), despite the fact that latter is
exposed to significantly enhanced steric strain (Table 1).39
The steric requirements of Cp0 are intermediate between 4Cp
We have recently set out to investigate the reactivity of the
related [Cp0FeI]2 molecule as a synthon to attractive target
molecules similar to the extensive [(C5Me5)RuX]n half-sandwich
chemistry. Herein, we report on our initial studies on Cp0Fe-
modified POCOP–Ir-pincer complexes (POCOP = C6H3-2,6-
(OPtBu2)2) using [Cp0FeI]2 as our starting material of choice.
5
and Cp, which has been recently demonstrated by reactivity
studies on various lanthanide complexes.40 Interestingly, for
the most sterically demanding Cp-ligand in this series, 5Cp,
ferrocene formation was not observed, instead the stable 5Cpꢁ
radical was isolated on reaction of Na5Cp with FeCl2.41 The
enhanced steric demand of Cp0 vs. 4Cp is also reflected in
an increased rotation barrier of the Cp0 ligands in 2 of
DGz = 14.2 kcal molꢀ1 (see ESIw for details, Fig. S1 and S2)
compared to 13.6 kcal molꢀ1 for [(4Cp)2Fe]42 or 13.1 kcal molꢀ1
for [(1,3-(Me3C)2C5H3)2Fe].43 It is noteworthy that the
Results and discussion
Synthesis and properties of [Cp0FeI]2
The synthesis of [Cp0FeI]2 (1-I) is readily accomplished by
reaction of FeI2(thf)2 and NaCp0 in THF, and crystallization
from pentane at ꢀ30 1C gives red crystals of the base-free
complex in good yield. Complex 1-I contains high-spin iron(II)
centers with four unpaired electrons as determined by solution
magnetic susceptibility studies yielding a magnetic moment
(per iron center at 290 K) of 5.3(2) mB (cf. [Cp0FeBr]2 (1-Br):
34
5.1 mB ). In an attempt to prepare other [Cp0FeX]2 derivatives
using Fe(OTf)2 or Fe(OAc)2 as starting materials we isolated
the sterically very encumbered hexa-tert-butylferrocene,
Cp02Fe (2) (Scheme 1, and see Experimental section for details)
which illustrates that ferrocene formation is indeed feasible
despite the steric bulk imposed by the Cp0 ligand, but it
Fig. 2 ORTEP diagram of 2 (50% probability ellipsoids). There are
two independent molecules in the asymmetric unit, but only one of
them is shown. Hydrogen atoms have been omitted for clarity.
Selected bond distances (A) and angles (1): Cp(cent)–Fe(ave) 1.71,
Cp(cent)–Fe1–Cp(cent) 175.
Scheme 1
c
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Table 1 X-Ray crystal structure determination of 1-I, 2, [5]+, 6 and [7]+
Compound reference
1-I
2
[5]+
6
[7]+
Chemical formula
Formula mass
Crystal system
a/A
b/A
c/A
a/1
C34H58Fe2I2
416.15
C34H58Fe
522.65
C64H71BCl2F20FeIIrO2P2
1770.81
Monoclinic
17.3392(2)
12.8333(2)
30.5957(4)
90.00
97.5170(10)
90.00
6749.62(16)
100(2)
P21/c
C22H39IIrO2P2
716.57
Triclinic
C73H84BF24FeIrO2P2
1770.20
Monoclinic
19.7885(3)
21.0378(3)
20.1495(3)
90.00
115.7078(9)
90.00
7558.07(19)
100(2)
P21/c
Monoclinic
13.9554(2)
18.5992(3)
15.2199(3)
90.00
113.8950(10)
90.00
3611.87(11)
100(2)
P21/c
Monoclinic
20.4092(4)
11.1680(2)
27.5120(5)
90.00
96.0860(10)
90.00
6235.5(2)
100(2)
P21/n
8.2850(2)
12.3186(2)
13.3510(2)
100.7810(10)
95.6730(10)
104.3080(10)
1281.87(4)
100(2)
b/1
g/1
Unit cell volume/A3
Temperature/K
Space group
%
P1
2
No. of formula units per unit cell, Z
Radiation type
4
8
4
4
CuKa
19.989
36 147
6510
0.0380
0.0296
0.0718
0.0328
0.0734
CuKa
3.989
30 254
10 999
0.0617
0.0435
0.1004
0.0617
0.1083
MoKa
2.867
85 961
20 659
0.0332
0.0254
0.0555
0.0333
0.0581
MoKa
6.552
30 081
7717
0.0302
0.0320
0.0654
0.0377
0.0683
CuKa
6.212
41 439
14 174
0.0659
0.0530
0.1197
0.0618
0.1238
Absorption coefficient, m/mmꢀ1
No. of reflections measured
No. of independent reflections
Rint
Final R1 values (I 4 2s(I))
Final wR(F2) values (I 4 2s(I))
Final R1 values (all data)
Final wR(F2) values (all data)
Scheme 2
exchange of the CMe3 against SiMe3 groups on the Cp-system
reduces the steric demand significantly as reflected in a barrier
of DGz = 11.0 kcal molꢀ1 for [(1,2,4-(Me3Si)3C5H2)2Fe].44
Besides electronic contributions the reduced steric demand
and therefore decreased kinetic stabilization have hampered
the development of a low-coordinate iron half-sandwich
chemistry with this ligand system.45
(VT) NMR studies of 1-I may indicate a monomer–dimer
equilibrium as reflected in a non-linearity of the d vs. Tꢀ1 plots
especially at higher temperatures (Fig. 3; the same plots in a
variety of solvents with different dielectric constants to probe
the monomer–dimer hypothesis are given in ESIw, Fig. S3–S5).
To address the possibility of monomer–dimer equilibria in
solution and potentially gas-phase the corresponding bromide
and chloride bridged species (1-Br and 1-Cl) were synthesized
(see Experimental section for details) and used in cross-over
experiments.z
Solution state and gas phase behavior
The behavior of 1-I in various solvents is rather curious and
will be discussed below (Scheme 2). The red crystals of 1-I are
readily soluble in a variety of organic solvents: red solutions
are obtained in aliphatic and aromatic hydrocarbons, yellow
solutions in coordinating solvents such as ethers and acetone,
and purple solutions in acetonitrile. Variable temperature
Mixing 1-I and 1-Br in a 1 : 1 ratio in the solid state, melting
them to form a homogeneous solid and conducting an
z One should note that all halide-bridged complexes 1-Cl, 1-Br and 1-I
have similar chemical shifts suggesting that these molecules are
isostructural in solution.
c
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When a pentane solution of 1-I is exposed to CO (1 atm) the
dimer is cleaved irreversibly to form the orange CO-adduct
[Cp0Fe(CO)2I] (3) which shows only low solubility in hydrocarbon
and benzene solvents, but has good solubility in CH2Cl2 and
C6H5Cl (see Experimental section for details).
However, complex 1-I dissolves in strongly coordinating
acetonitrile to form a deep purple solution which is a mixture
of a diamagnetic 6-coordinate [Cp0Fe(NCMe)3]+[I]ꢀ and
paramagnetic 5-coordinate [Cp0FeI(NCMe)] species in an
approximate 8 : 1 ratio as determined by 1H NMR spectro-
scopy. This observation is reminiscent of reports on
[PhB(CH2PPh2)3]FeCl in MeCN solutions for which
a
5-coordinate species tentatively assigned as [PhB(CH2PPh2)3]-
Fe(Cl)(NCMe) was also observed as minor component
besides the diamagnetic {[PhB(CH2PPh2)3]Fe(NCMe)3}+{Cl}ꢀ
complex.46 Anion exchange from Iꢀ to a weakly coordinating
[PF6]ꢀ anion yields the expected diamagnetic complex
[Cp0Fe(NCMe)3][PF6] (4) as deep blue-purple blocks which
are stable in solid state at room temperature for prolonged
period of time, but in CD2Cl2 solution 4 slowly decomposes at
ambient temperature. Similarly to the previously reported
[(C5R5)Fe(NCMe)3]+ (R = H, Me)23,24 rapid exchange between
free and bound MeCN ligands is observed for 4 on the NMR
time scale. At room temperature all 1H NMR resonances of 4
in CD2Cl2 solutions are significantly broadened suggesting a
rapid loss of MeCN and exchange with the CD2Cl2 solvent on
the NMR time scale. On cooling the 1H NMR resonances
sharpen significantly consistent with slow chemical exchange.
It should also be noted that the 19F and 31P{1H} NMR
resonances remain sharp at all temperatures ruling out any
involvement of the [PF6]ꢀ anion.
Fig. 3 Chemical shift (d) vs. Tꢀ1 plot for the 1H NMR resonances of
[Cp0FeI]2 (1-I) in methylcyclohexane-d14 from ꢀ100 to +90 1C.
Reactivity towards (POCOP)Ir pincer complexes
Fig. 4 UV-vis spectra of 1-Cl, 1-I and a mixture of 1-Cl and 1-I in
methylcyclohexane.
With complex 4 in hand an initial attempt was undertaken to
coordinate this fragment to (POCOP)Ir(H)(Cl), POCOP =
C6H3-2,6-(OPtBu2)2,47 in analogy to [(C5Me5)Ru(NCMe)3]+.
Unfortunately, no coordination was observed in the CH2Cl2
solvent, instead slow degradation of 4 occurred over time
at ambient temperature. The reactivity difference may be
explained with the significantly increased arenophilicity of
the cationic [(C5Me5)Ru]+ system29 and a substantial steric
bulk of the [Cp0Fe]+ fragment.
EI-MS experiment, showed m/z ratios corresponding to the
molecular ions of 1-I ([M+] = 832), 1-Br ([M+] = 738)
and the mixed-halide bridged species [(Cp0Fe)2(m-Br)(m-I)]
([M+] = 784). To investigate the solution behavior, the
UV-Vis spectra of the 1-I, [Cp0FeCl]2 (1-Cl) and a mixture of
1-I and 1-Cl were recorded in methylcyclohexane solution
(Fig. 4, see ESIw for details, Fig. S6). Fig. 4 shows that the
UV-vis spectrum of the mixture is not a simple superposition
of the individual spectra of 1-I and 1-Cl which is consistent
with the formation of a mixed-halide species and also consis-
tent with the variable temperature NMR behavior.
However, Brookhart and co-workers have recently demon-
strated that the cationic [(POCOP)Ir(H)(acetone)]+ is a potent
catalyst for the reduction of alkyl halides, ketones and hydro-
dechlorination of metal chlorides and cleavage of alkyl ethers
with triethylsilane.48–51 Therefore, compound 1-I was treated
Previous reports stated that 1-Br and the related [4CpFeBr]2
and [5CpFeBr]2 complexes do not undergo a spin-state change
when dissolved in coordinating solvents.33,34 We can confirm
this observation for 1-I in ether and acetone solvents. In both
cases yellow, paramagnetic mono-solvent adducts (Evans’
solution moment: meff(292 K) = 5.3(2) mB (S = 2)) are formed.
This behavior is rather remarkable considering the large excess
of coordinating solvent relative to complex 1-I. When the
solvent is removed the dimeric structure of 1-I is re-established,
and repeated attempts to grow crystals of these labile solvent
adducts failed.
with 2 equiv. of [(POCOP)Ir(H)(acetone)][B(C6F5)4] in
CH2Cl2 at ambient temperature to form dark red solutions
from which [(Cp0Fe)(POCOP)Ir(H)(I)][B(C6F5)4] (5) was
crystallized as the endo isomer ([5]+-endo) (Fig. 5). The
endo-configuration is also collaborated by NOE NMR
experiments. The hydride resonance (d ꢀ38.30) shows apparent
communication with the tert-butyl groups on the substituted
Cp-ring (d 1.43, 1.28) and the tert-butyl groups (d 1.69) on the
POCOP-pincer systems.
This reaction clearly demonstrates that
a controlled
modification of the POCOP fragment with [Cp0Fe]+ is indeed
c
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Fig. 5 ORTEP diagram of [5]+-endo (50% probability ellipsoids). Hydrogen atoms have been omitted for clarity, except H100 which was located
in the Fourier difference map and refined isotropically. Selected bond distances (A) and angles (1): Fe1–Cp(cent) 1.70, Fe1–C6(cent) 1.64,
Ir1–I1 2.68792(15), Ir1–P1 2.3059(5), Ir1–P2 2.3029(5), Ir1–C25 1.9972(19), Cp(cent)–Fe1–C6(cent) 173.
feasible. However, the complex is sterically congested and a
rotation barrier of DGz = 9.8 kcal molꢀ1 was determined
by VT 31P{1H} NMR studies (Tc = 240 K; 31P{1H} NMR
The same downfield shift on isomerization from the endo to
exo isomer was observed for the neutral ferrocene-derived
Rh/Ir–PCP-pincer systems, but in these cases the exo isomer
was shown to be thermodynamically favored.17,19 However,
complexes [5]+-endo and [5]+-exo reach an equilibrium
mixture of a ca. 6.5 : 1 ratio in CD2Cl2 after 2 days at 298 K
which corresponds to a slight thermodynamic preference of
DG E 1.1 kcal molꢀ1. No degradation has been observed in
CD2Cl2 solution at ambient temperature under an argon
atmosphere for prolonged periods of time (47 days). Hence,
in contrast to previous reports the endo isomer is the more
stable configuration for [5]+.
2
(161.9 MHz, 190 K): AB system, dd, JPP = 333 Hz). This
barrier is lower than for 2. For comparison the molecular
structure of (POCOP)Ir(H)(I) (6) has also been determined
(see ESIw for details, Fig. S7).
Interestingly, when crystals of [5]+-endo are dissolved in
CD2Cl2 only one species is observed. However, steric hindrance
in [5]+ leads to isomerization between endo and exo isomers in
solution (Scheme 3). Isomerization between endo and exo
isomers has been previously reported for ferrocene-derived
Rh/Ir–PCP-pincer systems. In these cases the authors
suggested a reversible C–H activation process or phosphine
dissociation and recombination.17,19 Probably the most
interesting spectroscopic feature of the exo complex ([5]+-exo)
is the significantly downfield shifted 31P{1H} NMR resonance
(d 200.5) when compared to [(POCOP)Ir(H)(acetone)]+
(d 174.6), (POCOP)Ir(H)(I) (d 180.2) and [5]+-endo (d 190.6).
This reaction was investigated in more detail with NMR
spectroscopy. A crude reaction mixture of 1-I and [(POCOP)-
Ir(H)(acetone)]+ in CD2Cl2 shows [5]+-endo and [5]+-exo
isomers in an initial ratio of ca. 2 : 1 which slowly equilibrates
to a ratio of ca. 6.5 : 1 as described above. This observation
suggests that at the initial site preference of the [Cp0Fe]+
fragment differentiating the two inequivalent pincer-faces is
Scheme 3
c
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1846 New J. Chem., 2011, 35, 1842–1854

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Scheme 4
less pronounced, but the kinetic preference for the formation
of the [5]+-endo isomer persists. Interestingly, a different
product distribution is observed when the reaction between
1-I and [(POCOP)Ir(H)(acetone)]+ is carried out in C6D5Br.
Under these conditions the major product is (POCOP)Ir(H)(I)
(6) (ca. 70%) and [5]+-endo/exo (ca. 30%) as determined by
31P{1H} NMR spectroscopy. The remaining [Cp0Fe]+ fragment
is presumably trapped as the cationic [Cp0Fe(Z6-C6D5Br)]+
complex. This observation may be rationalized by an iodide-
transfer from the monomeric [Cp0FeI] to the cationic
[(POCOP)Ir(H)]+ fragment which probably occurs similarly
to the reaction between [(POCOP)Ir(H)]+ and Et3SiH52 via an
[(POCOP)Ir(H)(k1-I-FeCp0)]+ intermediate. The resulting
[Cp0Fe]+ and (POCOP)Ir(H)(I) fragments have to recombine
in the proper orientation to form [5]+, and solvent trapping of
the [Cp0Fe]+ fragment is competitive. We also noted that a
pure sample of [5]+-endo dissolved in C6D5Br shows the
formation of [5]+-exo besides small amounts of 6 after 12 h
at ambient temperature. Considering the large excess of
C6D5Br this observation suggests that the process of dissociation
is slow and does not require a complete dissociation of the
[Cp0Fe]+ and (POCOP)Ir(H)(I) fragments. However, after 4
days in C6D5Br at 65 1C nearly complete conversion of [5]+-
endo/exo to 6 was observed by 31P{1H} NMR spectroscopy.
Fig. 6 ORTEP diagram of [7]+ (50% probability ellipsoids). Hydrogen
atoms have been omitted for clarity. Selected bond distances (A) and
angles (1): Fe1–Cp(cent) 1.70, Fe1–C6(cent) 1.64, Ir–C2H4(cent) 2.09,
Ir–P1 2.2908(14), Ir–P2 2.2780(14), Ir–C51 2.034(5), C100–C101 1.392(9),
Cp(cent)–Fe1–C6(cent) 175, P2–Ir–C100–C101 25.5.
Cp0-rotation barrier of DGz(200 K) = 8.9 kcal molꢀ1. The
electron withdrawing effect of the [Cp0Fe]+ fragment is most
pronounced in a significantly downfield shifted 31P{1H} NMR
resonance at d 200.3 in [7]+ compared to d 181.9 in (POCOP)-
Ir(C2H4). As a proof of concept the transfer dehydrogenation
activity of [7]+ towards cyclooctene and tert-butylethylene
(TBE) was investigated. Complex [7]+ is insoluble in the
reaction mixture at ambient temperature, but slowly dissolves
when the solution is heated to 200 1C. After 1300 TOs no
further catalytic activity was observed accompanied by the
formation of a brown residue. 31P{1H} NMR spectroscopy of
this precipitate confirmed a complete degradation of [7]+ to
unidentified products. Thus, the reduced reactivity of [7]+
compared to the (POCOP)Ir(H)(Cl)/NaOtBu system (see ESIw
for details, Table S1)47 should be attributed to a combination
of different effects: (i) its electron-deficient nature, (ii) the
reduced thermal stability and (iii) the lower solubility of the
ionic complex [7]+.
Catalytic transfer hydrogenation activity
These initial studies clearly demonstrated the feasibility of
modifying (POCOP)Ir-pincer systems by a cationic [Cp0Fe]+
fragment. To probe the effect of this modification on a
catalytically competent species 1-I was reacted with
NaB(ArF)4 (B(ArF)4 = B[3,5-(F3C)2C6H3]4) in CH2Cl2 in
the presence of (POCOP)Ir(C2H4),47 and [(Cp0Fe)(POCOP)-
Ir(C2H4)][B(ArF)4] ([7]+) was obtained as dark red crystals in
good yield. An ORTEP diagram of [7]+ is shown in Fig. 6, and
selected bond distances and angles are given in the figure
caption.
The conformation of the Ir-bonded C2H4 ligand is nearly
coplanar with the plane defined by the PCP and the central Ir
atoms. For square-planar complexes two geometries are realized,
i.e. the in-plane (ip) and the upright (u) conformations which
are usually of similar energies, but the u-conformation shows
reduced steric strain. Therefore, the u-geometry is usually
favored in pincer-like complexes, but recently a few examples
with the unusual ip-geometry have been reported.53–55
Complex [7]+ is slightly less sterically congested compared
to [5]+-endo. VT 1H NMR studies revealed a reduced
Density functional theory (DFT) calculation
DFT calculations in combination with experimental data have
become valuable tools in organometallic chemistry. Therefore
c
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the influence of the [Cp0Fe]+ fragment on the (POCOP)Ir
pincer complexes was investigated by DFT methods. First, the
optimized geometries for [5]+-endo, 6 and [7]+ were compared
to those of [5]+-endo, 6 and [7]+ determined by single crystal
crystallography. The molecular geometries of [5]+-endo and
[7]+ optimized at the BP86/6-311G(d,p) (C, H, O, P, Fe) and
SDD (Ir, I) level of theory are shown in Fig. 7. Selected bond
angles and distances are compared in Table 2 and calculated
and experimental metric parameters are in good agreement.
Encouraged by this fact the energetics between the two
possible isomers [5]+-endo/exo were evaluated in the gas
phase. Gratifyingly the BP86 functional also reproduces the
experimentally observed preference of the [5]+-endo over the
[5]+-exo isomer. Furthermore, the calculated Gibbs free
Fig. 7 Molecular geometry of [5]+-endo and [7]+ optimized at the BP86/6-311G(d,p) (C, H, O, P, Fe) and SDD (Ir) level.
Table 2 Calculated vs. experimental bond distances (A) and angles (1)
(POCOP)Ir(C2H4) (calc) [7]+ (calc) [7]+ (exp) [6] (calc) [6] (exp) [5]+-endo (calc) [5]+-endo (exp) [5]+-exo (calc)
C1–Ir
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C1
C2–O1
C6–O2
O1–P1
O2–P2
Ir–P1
Ir–P2
C7–C8
Ir–C7/C8(cent)
Ir–I
2.058
1.413
1.402
1.401
1.402
1.401
1.413
1.385
1.387
1.719
1.718
2.320
2.318
1.412
2.113
2.031
1.430
1.421
1.422
1.422
1.422
1.431
1.373
1.374
1.732
1.736
2.329
2.341
1.405
2.132
2.034
1.405
1.413
1.414
1.412
1.392
1.415
1.372
1.380
1.681
1.674
2.278
2.291
1.393
2.093
2.031
1.411
1.400
1.403
1.403
1.400
1.411
1.394
1.394
1.710
1.710
2.327
2.327
2.007
1.393
1.388
1.396
1.397
1.391
1.396
1.393
1.392
1.659
1.661
2.300
2.301
2.010
1.427
1.423
1.424
1.424
1.422
1.427
1.375
1.373
1.752
1.746
2.340
2.338
2.016
1.427
1.421
1.424
1.423
1.421
1.426
1.374
1.373
1.752
1.747
2.303
2.306
2.016
1.427
1.421
1.424
1.423
1.421
1.426
1.374
1.373
1.752
1.747
2.342
2.339
2.791
2.705
2.751
1.703
1.648
174.3
2.688
1.698
1.641
172.9
2.749
1.700
1.652
173.9
Cp(cent)–Fe
C6(cent)–Fe
Cp(cent)–Fe–Cp(cent)
P1–Ir–C7–C8
1.697
1.649
175.4
22.4
1.695
1.644
175.0
25.5
81.4
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energy difference, DG, at 298 K between [5]+-endo and
[5]+-exo of 1.07 kcal molꢀ1 is in excellent agreement with
the experimentally determined value of ca. 1.1 kcal molꢀ1. It
might be tempting to explain the slight energetic preference of
the [5]+-endo isomer with a weak interaction between the
Ir–H and Cp0Fe+ moieties. However, there are also several
interactions between the sterically demanding tBu groups on
the POCOP pincer and on the Cp0 ligand which make the
situation more complex. To demonstrate this we used
compliance constants (or inversed relaxed force constants),56
which have been applied successfully in recent years to several
chemical problems, e.g. to quantify the strength of agostic
interactions.57 However, the relaxed force constant for the
Ir–H and Cp0Fe+ interaction is very small with 0.1 N cmꢀ1
(i.e. indicative of a very weak interaction). Furthermore,
we also identified several other interactions of the same
magnitude, e.g. between Ir–H and one tBu-group of the Cp0
ligand which is closest to the iridium hydride. In our opinion
this indicates a complex interplay of several attractive and
repulsive interactions which make the [5]+-endo isomer
slightly more stable.
As pointed out above, the ip-geometry of the Ir-bound
ethylene is unusual. While no crystallographic data for
(POCOP)Ir(C2H4) are available DFT calculations suggest that
the expected u-geometry is preferred to avoid steric repulsions
between the tBu groups and the C2H4 ligand. However, a
switch to the sterically more congested ip-geometry is induced
on [Cp0Fe]+ coordination (vide supra). Two interactions
determine the bonding between olefin ligands and transition
metal fragments: (i) donation from the ethylene ligand to the
LUMO of the metal fragment and (ii) back-donation from the
dxy or dxz orbital for the ip and u isomer (the coordination
plane is defined as xy), respectively. However, DFT calculations
allow a more quantitative analysis of the influence of the
[Cp0Fe]+ fragment on the electronic structure of the (POCOP)Ir
pincer system. Atomic net charges, natural atomic orbital charges
and populations were calculated for [5]+-endo/exo, 6, (POCOP)-
Ir(C2H4) and [7]+ and these results are shown in Table 3.
Table 3 Atomic net charges and natural atomic orbital populations
Atomic net charges
(POCOP)Ir
MPA NBO
[Cp0Fe(POCOP)]Ir+
(POCOP)Ir(C2H4)
[7]+
[6]
[5]+-endo
[5]+-exo
MPA
NBO
MPA
NBO
MPA
NBO
MPA
NBO
MPA
NBO
MPA
NBO
Ir
P1
P2
ꢀ0.218 ꢀ0.557 ꢀ0.039
ꢀ0.421
1.333
1.322
0.050
0.869
0.864
ꢀ0.677
1.525
1.516
0.143 ꢀ0.573 ꢀ0.359 ꢀ0.900 ꢀ0.416 ꢀ0.877 ꢀ0.481 ꢀ0.863
0.881
0.881
1.365
1.365
0.848
0.828
0.823
0.802
1.545
1.530
0.976
0.976
1.579
1.578
0.959
0.950
1.564
1.565
0.953
0.956
1.587
1.581
O1 ꢀ0.486 ꢀ0.777 ꢀ0.479
O2 ꢀ0.486 ꢀ0.777 ꢀ0.478
C1 ꢀ0.343 ꢀ0.142 ꢀ0.320
ꢀ0.750
ꢀ0.745
ꢀ0.138
0.225
ꢀ0.490
ꢀ0.491
ꢀ0.477
0.205
ꢀ0.795
ꢀ0.795
ꢀ0.300
0.319
ꢀ0.480 ꢀ0.779 ꢀ0.489 ꢀ0.799 ꢀ0.483 ꢀ0.770 ꢀ0.479 ꢀ0.774
ꢀ0.483 ꢀ0.775 ꢀ0.489 ꢀ0.799 ꢀ0.484 ꢀ0.772 ꢀ0.477 ꢀ0.770
ꢀ0.380 ꢀ0.251 ꢀ0.375 ꢀ0.222 ꢀ0.367 ꢀ0.225 ꢀ0.400 ꢀ0.236
C2
0.179
0.274
0.069
0.062
0.247
0.197
0.296
0.115
0.248
0.140
0.256
C3 ꢀ0.102 ꢀ0.278 ꢀ0.133
C4 ꢀ0.133 ꢀ0.204 ꢀ0.160
C5 ꢀ0.102 ꢀ0.278 ꢀ0.160
C6 ꢀ0.218 ꢀ0.557 ꢀ0.039
C7
ꢀ0.292
ꢀ0.241
ꢀ0.306
ꢀ0.421
ꢀ0.126
ꢀ0.106
ꢀ0.125
0.050
ꢀ0.287
ꢀ0.178
ꢀ0.287
ꢀ0.677
ꢀ0.440
ꢀ0.420
ꢀ0.148 ꢀ0.297 ꢀ0.114 ꢀ0.277 ꢀ0.177 ꢀ0.305 ꢀ0.165 ꢀ0.294
ꢀ0.308 ꢀ0.240 ꢀ0.120 ꢀ0.190 ꢀ0.144 ꢀ0.239 ꢀ0.140 ꢀ0.237
ꢀ0.176 ꢀ0.309 ꢀ0.114 ꢀ0.277 ꢀ0.158 ꢀ0.293 ꢀ0.184 ꢀ0.305
0.143 ꢀ0.573 ꢀ0.359 ꢀ0.900 ꢀ0.416 ꢀ0.877 ꢀ0.481 ꢀ0.863
ꢀ0.329 ꢀ0.456
ꢀ0.291
C8
ꢀ0.339
ꢀ0.337 ꢀ0.455
Natural atomic orbital contributions
(POCOP)Ir
[Cp0Fe(POCOP)]Ir+
(POCOP)Ir(C2H4)
[7]+
[6]
[5]+-exo
[5]+-exo
Ir (dxy
Ir (dxz
Ir (dyz)
Ir (dx ꢀy
Ir (dz
P1 (pz)
P2 (pz)
O1 (pz)
O2 (pz)
C1 (pz)
C2 (pz)
C3 (pz)
C4 (pz)
C5 (pz)
C6 (pz)
)
)
1.840
1.857
1.873
1.278
1.797
0.714
0.714
1.847
1.847
1.057
1.029
1.074
1.032
1.074
1.029
1.850
1.587
1.900
1.342
1.929
0.692
0.687
1.838
1.832
1.089
0.989
1.089
1.000
1.080
0.987
1.855
1.615
1.870
1.449
1.783
0.739
0.684
1.790
1.756
1.048
1.024
1.056
1.099
1.066
1.038
1.754
1.722
1.822
1.502
1.816
0.703
0.659
1.800
1.764
1.077
1.020
1.058
1.092
1.068
1.035
1.887
1.904
1.933
1.399
1.456
1.039
1.017
1.072
1.014
1.072
1.014
1.844
1.844
0.697
0.697
1.896
1.563
1.918
1.470
1.711
1.046
1.032
1.064
1.078
1.054
1.020
1.803
1.774
0.709
0.671
1.898
1.682
1.921
1.480
1.583
1.063
1.017
1.052
1.073
1.063
1.031
1.809
1.782
0.714
0.674
2
2)
2
)
c
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Krogh-Jespersen and Goldman have recently analyzed the
influence of O vs. CH2 substitution in POCOP vs. PCP iridium
pincer complexes. The O - C(aryl) p-donation leads to a
significant buildup of negative charge on the carbon atom (C1)
directly connected to Ir, whereas the substantial P - O
s-donation leads to a large positive net charge on the P atoms.
However, these opposing effects compensate each other, and
therefore the charge on iridium is nearly the same as for the
(PCP)Ir fragment.58 Furthermore, it was shown that an
increased p-electron density at the Ir-bound aryl carbon
(C1) leads to very favorable energetics for C–H and H–H
activation.59
(POCOP)Ir(C2H4). The next question to be addressed is the
influence on the C2H4 binding energy. The ethylene bond
dissociation energies (BDE) were calculated for the neutral
(POCOP)Ir(C2H4) and [7]+ to give 15.2 and 13.3 kcal molꢀ1
,
respectively, which is consistent with the arguments developed
above. In conclusion the experimentally observed reactivity
difference is most likely due to the cationic nature of [7]+
(low solubility, and reduced thermal stability) and due to the
reduced electron density at C1(pz) on [Cp0Fe]+ coordination
which makes oxidative C–H and H–H addition less favorable.y
Conclusions
In conclusion we have reported on the synthesis of [Cp0FeI]2
(1-I), its solution and gas phase behavior as well as its
reactivity with cationic and neutral (POCOP)Ir complexes.
In contrast to the [(C5Me5)Ru(NCMe)3]+ fragment
[Cp0Fe(NCMe)3]+ does not bind to the aromatic p-system
of Ir pincer complexes, and a different synthetic methodology
had to be developed. A clean [Cp0Fe]+ transfer was observed
in the reaction of [(POCOP)Ir(H)(acetone)]+ with 1-I to yield
[(Cp0Fe)(POCOP)Ir(H)(I)][B(C6F5)4] ([5]+). Two isomers of
[5]+ have been observed in solution, of which the endo-isomer
was obtained in a pure form on crystallization. The equilibrium
between endo- and exo-isomers have been established in solution
which slightly favors the endo-isomer by DG(298 K) E
1.1 kcal molꢀ1. From these studies the dissociative loss of
the [Cp0Fe]+ fragment in the presence of an aromatic solvent
was established which is most likely due to the steric strain
imposed by the [Cp0Fe]+ fragment on (POCOP)Ir(H)(I).
Alternatively, 1-I may be used to functionalize (POCOP)-
Ir(C2H4) in the presence of NaB(ArF)4 to yield [(Cp0Fe)-
(POCOP)Ir(C2H4)][B(ArF)4] ([7]+) which was also successfully
structurally characterized. Complex [7]+ is a competent
catalyst for hydrogen transfer dehydrogenation reactions,
but due to its cationic nature its catalytic activity and thermal
stability is reduced compared to the neutral (POCOP)Ir(C2H4)
system. The influence of the [Cp0Fe]+ fragment on the
electronic structure of the Ir pincer complexes was evaluated
using DFT calculations. The calculated structures are in good
agreement with the crystallographic data and reproduce the
energetic relationship between the two isomers, [5]+-endo and
[5]+-exo. Furthermore, they provide a more detailed under-
standing of the reduced catalytic activity of [7]+, which can be
rationalized by a significant reduced electron density on the Ir
Our calculations on neutral (POCOP)Ir complexes are in
good agreement with these earlier studies. The influence of the
cationic [Cp0Fe]+ fragment on the (POCOP)Ir was then
investigated. This coordination significantly alters the charge
distribution within the (POCOP)Ir pincer system by removing
electron density from the aryl p-system and thereby increasing
the positive charge on Ir in [Cp0Fe(POCOP)Ir]+. However,
the degree of charge transfer depends on the method used
for the determination of atomic net charges (Table 3). The most
pronounced effect is the significant reduction of the electron
density at the Ir dxz orbital by ca. 0.3e which is directly
conjugated with the p(pz) orbitals of the aryl system while the
electron densities on the Ir-centered dz2, dx2ꢀy2 and dyz orbitals
are increased.
The natural atomic orbital population of the Ir dxz-orbital
for [5]+ and [7]+ which is conjugated with the aryl-system was
calculated. We found an inversed trend on coordination of the
[Cp0Fe]+ fragment. For [5]+ the population is reduced by
0.34e compared to 6 consistent with the charge transfer from
the aryl p-system to the cationic [Cp0Fe]+ fragment. However,
for [7]+ this trend is reversed, and the population is increased
by 0.14e compared to (POCOP)Ir(C2H4). This can be
rationalized by the change in ethylene coordination. In the
u-conformation of 6 the Ir(dxz) is not only conjugated with
the C1(pz) orbital, but also responsible for the backbonding to
the ethylene ligand which leads to a significantly reduced
electron density in the Ir dxz orbital. However, on [Cp0Fe]+
coordination electron density is removed from the p-aryl
system which further decreases the electron density of the
C1(pz) orbital. Therefore, the electron withdrawing effect of
the [Cp0Fe]+ fragment has to compete with the backbonding
to the ethylene ligand. This competition makes the ip-geometry
more favorable, so electronic rather than steric effects induce
the observed switch from the u- to the ip-configuration in [7]+.
The backbonding to the ip-coordinated ethylene is then
mediated by the filled Ir(dxy) orbital for which the electron
density is reduced by 0.1e. However, the reduced backbonding
compared to (POCOP)Ir(C2H4) is probably due to steric
effects. This is also reflected in a slightly shorter, less activated
CQC bond distance in [7]+ of 1.405 A compared to 1.412 A in
dxz orbital, which renders the Ir center more electrophilic, but
less capable for C–H and H–H addition.
Future work will focus on the development of the reaction
chemistry of 1-I, and the electrochemistry of [5]+ and [7]+.
The possible electron-reservoir behavior60 and electronic
communication between Fe and Ir will be interesting to
investigate. These new systems might allow the development
of
a redox-active and switchable catalyst system with
y Exposure of a CD2Cl2 solution of [7]+ to an H2 atmosphere (1 atm)
results in a significant slower formation of [Cp0Fe(POCOP)Ir(H)4]+
(
31P{1H} NMR: d 196.1) compared to the neutral (POCOP)Ir(H)4
system (31P{1H} NMR: d 182.9 (I. Gottker-Schnettmann, P. S. White,
M. Brookhart, Organometallics, 2004, 23, 1766).
¨
c
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enhanced thermal stability and catalytic activity combined
with straightforward catalyst–product separation.
ꢀ7.8 (18 H, n1/2 = 400 Hz), ꢀ13.5 (9 H, n1/2 = 400 Hz). IR
(Nujol mull; CsI windows; cmꢀ1): 1400w, 1365s, 1265m,
1245s, 1238sh, 1208m, 1170m, 1100vbr.w., 1020br.w., 995m,
948w, 859vw, 822vs, 809sh, 670s, 540w, 440m, 360m, 250br.m.
Anal. calcd for C34H58Fe2I2: C, 49.06; H, 7.02. Found: C,
48.71; H, 6.99%. The E.I. mass spectrum showed a molecular
ion at m/z = 832 amu. The parent ion isotopic cluster was
simulated: (calcd %, observd %): 830 (13, 13), 831 (5, 5),
832 (100, 100), 833 (42, 44), 834 (9, 10). UV-vis (methylcyclo-
hexane, 25 1C): 371 nm (sh, e = 3324 mol cmꢀ1), 428 nm
(sh, e = 1500 mol cmꢀ1), 509 nm (sh, e = 642 mol cmꢀ1),
642 nm (e = 582 mol cmꢀ1).
Experimental
General procedures
All reactions and product manipulations were carried out
under an atmosphere of dry, oxygen free argon using standard
high-vacuum, Schlenk, or drybox techniques. Argon was
purified by passage through BASF R3-11 catalyst (Chemalog)
and 4 A molecular sieves. Dry, oxygen-free solvents were
employed throughout. NMR spectra were recorded on Bruker
DRX 500 MHz, a Bruker DRX 400 MHz, or a Bruker 400 MHz
[Cp0FeBr]2 (1-Br). A mixture of FeBr2(thf)2 (3.68 g,
10.22 mmol) and NaCp0 (2.62 g, 10.22 mmol) was suspended
in ca. 60 mL of tetrahydrofuran. The reaction mixture slowly
turned dark green-brown. After the mixture was stirred
for 24 h at ambient temperature, the tetrahydrofuran was
removed under reduced pressure, and the dark brown residue
was suspended in ca. 100 mL of pentane. The dark red-brown
solution was filtered, and the volume of the solution was
reduced to ca. 50 mL. Slow cooling to ꢀ25 1C gave two crops
of red-brown blocks (0.74 g, 1.00 mmol, 20%). M.p. 173–175 1C
(dec.). 1H NMR (C6D6, 292 K): d 48.5 (2 H, n1/2 = 1900 Hz),
ꢀ8.0 (18 H, n1/2 = 300 Hz), ꢀ13.5 (9 H, n1/2 = 300 Hz). IR
(Nujol mull; CsI windows; cmꢀ1): 1620br.w., 1365vs, 1265w,
1245vs, 1238sh, 1208m, 1170w, 1112vw, 1020br.w., 1005s,
960w, 830vs, 722vbr.w., 680s, 550m, 450m, 420vw, 370m,
260br.m. Anal. calcd for C34H58Fe2Br2: C, 55.31; H, 7.92.
Found: C, 55.21; H, 7.99%. The E.I. mass spectrum showed a
molecular ion at m/z = 738 amu. The parent ion isotopic
cluster was simulated: (calcd %, observd %): 734 (6, 5), 735(3, 3),
736(57, 58), 737(25, 25), 738(100, 100), 739(42, 42), 740(53, 52),
741(5, 5). UV-vis (methylcyclohexane, 25 1C): 300 nm
(sh, e = 6520 mol cmꢀ1), 354 nm (e = 3070 mol cmꢀ1), 409 nm
(sh, e = 1320 mol cmꢀ1), 514 nm (e = 100 mol cmꢀ1),
499 nm (sh, e = 490 mol cmꢀ1), 798 nm (sh, e = 99 mol cmꢀ1).
1-Br was previously prepared using another synthetic route.34
1
AVANCE spectrometer. H and 13C{1H} chemical shifts are
referenced relative to residual CH(D)Cl2 (d 5.32 for 1H),
C6HD5 (d 7.15 for 1H), 13CD2Cl2 (d 53.8 for 13C) and
13C6D6 (d 128.0 for 13C); 31P chemical shifts are referenced
to an external standard of H3PO4. Probe temperatures were
calibrated using ethylene glycol and methanol as previously
described.61 Due to strong 31P–31P coupling in the pincer
1
ligand, many H and 13C{1H} NMR signals appear as virtual
triplets (vt) and are reported as such with the apparent coupling
noted.
All chemical shifts are reported in d units with reference to
the residual protons of the deuterated solvents, which are
internal standards, for proton chemical shifts. The elemental
analyses were performed by the analytical facilities at the
University of California at Berkeley or by Robertson Microlit
Laboratories of Madison, NJ.
Materials
All solvents were deoxygenated and dried by passage over
columns of activated alumina.62,63 Tetrahydrofuran was dried
over sodium/benzophenone and freshly distilled prior to use.
Deuterated solvents, CD2Cl2, C6D6, C7D8, cyclohexane-d12
and methylcyclohexane-d14, were purchased from Cambridge
Laboratories, Inc., dried over sodium metal (with the exception
of CD2Cl2 which was dried over CaH2), vacuum transferred to
a Teflon sealable Schlenk flask containing 4 A molecular
sieves, and degassed via three freeze–pump–thaw cycles. IrCl3
was obtained from W.C. Heraeus GmbH and used as received.
[(POCOP)Ir(H)(acetone)][B(C6F5)4],48 (POCOP)Ir(C2H4),47
[Cp0FeCl]2 (1-Cl). A mixture of FeCl2(thf)1.5 (1.88 g,
8.0 mmol) and NaCp0 (2.05 g, 8.0 mmol) was suspended in
ca. 60 mL of tetrahydrofuran, and the solution turned light
red-brown. After stirring for 24 h at ambient temperature, the
tetrahydrofuran was removed under reduced pressure, and the
yellow-brown residue was suspended in ca. 60 mL of pentane.
The suspension was stirred for 4 h at ambient temperature.
The yellow solution was filtered and concentrated to ca. 10 mL.
On cooling to ꢀ80 1C large orange crystals were isolated
[H(OEt2)2][B(ArF)4],64 Na[B(ArF)4],65 FeI2(thf)2,66 FeCl2(thf)1.5
67
and NaCp068 were synthesized according to literature methods. All
other reagents were purchased from Aldrich, Acros, Alphar Aesar
or Strem Chemicals and used as received.
Synthesis
1
(0.3 g, 0.46 mmol, 12%). M.p. 156–158 1C (dec.). H NMR
[Cp0FeI]2 (1-I). A mixture of FeI2(thf)2 (4.54 g, 10 mmol)
and NaCp0 (2.56 g, 10 mmol) was suspended in ca. 60 mL of
tetrahydrofuran. The reaction mixture slowly turned dark
brown, and was stirred for 24 h at ambient temperature. The
tetrahydrofuran solvent was removed under reduced pressure,
and the dark brown residue was suspended in 100 mL of
pentane. The dark red solution was filtered, and the volume of
the filtrate was reduced to ca. 50 mL. Slow cooling to ꢀ25 1C
gave two crops of red-brown blocks (2.14 g, 2.57 mmol, 51%).
1H NMR (C6D6, 292 K): d 36.8 (2 H, n1/2 E 1800 Hz),
(C6D6, 292 K): d 53.5 (2 H, n1/2 = 1600 Hz), ꢀ8.5 (18 H,
n1/2 = 230 Hz), ꢀ12.6 (9 H, n1/2 = 225 Hz). IR (Nujol mull;
CsI windows; cmꢀ1): 1365s, 1260m, 1240vs, 1220sh, 1200m,
1170br.w., 1100vbr.m., 1020br.m., 1000w, 960w, 830vs, 805sh,
722vbr.w., 680s, 550vbr.w., 450m, 420vw, 370m, 280br.s.,
255s. Anal. calcd for C34H58Fe2Cl2: C, 63.08; H, 8.72. Found:
C, 62.99; H, 8.98%. The E.I. mass spectrum showed a
molecular ion at m/z = 648 amu. The parent ion isotopic
cluster was simulated: (calcd %, observd %): 646(12, 12),
647(5, 6), 648(100, 100), 649(43,42), 650(70, 70), 651(28, 28),
c
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New J. Chem., 2011, 35, 1842–1854 1851

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1
652(16, 16), 653(5, 4). UV-vis (methylcyclohexane, 25 1C):
321 nm (e = 6520 mol cmꢀ1), 369 nm (sh, e = 1018 mol cmꢀ1),
713 nm (e = 153 mol cmꢀ1), 890 nm (e = 80 mol cmꢀ1),
904 nm (e = 72 mol cmꢀ1).
(3 C, CH3). 19F NMR (CD2Cl2, 292 K): ꢀ73.9 (d, JF–P
=
=
710 Hz, PF6). 31P{1H} (CD2Cl2, 292 K): ꢀ143.1 (sept, 1JP–F
710 Hz, PF6). Anal. calcd for C23H38N3PF6Fe: C, 49.56; H,
6.87. Found: C, 49.72; H, 6.94%.
Cp02Fe (2). A mixture of Fe(OTf)2 (0.56 g, 2 mmol) and
NaCp0 (1.02 g, 4 mmol) was suspended in ca. 40 mL of
tetrahydrofuran and heated under reflux for 14 h. During this
time the solution turned dark red. The reaction mixture was
allowed to cool to ambient temperature, the tetrahydrofuran
solvent was removed under reduced pressure, and the red
residue was suspended in 30 mL of pentane. The dark red
solution was filtered, and the volume of the filtrate was
reduced to ca. 5 mL. Slow cooling to ꢀ80 1C gave two crops
of red blocks (0.65 g, 1.25 mmol, 62%). The compound
sublimed very slowly at 80–90 1C in diffusion pump vacuum.
[(Cp0Fe)(POCOP)Ir(H)(I)][B(C6F5)4]*(CH2Cl2) ([5]+-endo).
[(POCOP)Ir(H)(acetone)][B(C6F5)4] (0.532 g, 0.30 mmol) and
[Cp0FeI]2 (0.126 g, 0.15 mmol) were dissolved in CH2Cl2
(20 mL) to form a deep red solution. The solution was
concentrated and cooled to ꢀ38 1C to form deep red blocks
which also incorporated 1 equiv. of CH2Cl2 solvent (0.377 g,
0.21 mmol, 71%). [5]+-endo: 1H NMR (CD2Cl2, 292 K):
3
d 6.06 (2 H, d, JHH = 6.0 Hz, POCOP-m-CH), 5.68 (1 H,
t, 3JHH = 6.0 Hz, POCOP-p-CH), 4.43 (2 H, s, Cp0-ring-CH),
1.61 (18 H, vt, J = 8.0 Hz, POCOP-CMe3), 1.43 (18 H, s,
Cp0-ring-CMe3), 1.28 (9 H, s, Cp0-ring-CMe3), 1.19 (18 H, vt,
J = 8.0 Hz, POCOP-CMe3), ꢀ38.30 (1 H, t, 2JHP = 12.0 Hz,
Ir-H). 13C{1H} NMR (CD2Cl2, 300 K): d 149.1 (2 C, pt,
J = 4.3 Hz, POCOP-C-O), 116.2 (1 C, pt, J = 4.3 Hz,
POCOP-C-Ir), 105.7 (1 C, Cp0-ring-CCMe3), 104.6 (2 C,
Cp0-ring-CCMe3), 75.4 (1 C, POCOP-p-C), 68.9 (2 C,
Cp0-ring-CH), 66.4 (2 C, pt, J = 3.4 Hz, POCOP-m-C), 44.4
(2 C, pt, J = 8.4 Hz, POCOP-CMe3), 39.5 (2 C, pt, J =
10.8 Hz, POCOP-CMe3), 33.4 (6 C, Cp0-CMe3), 32.6 (2 C,
Cp0-CMe3), 30.9 (1 C, Cp0-CMe3), 30.3 (3 C, Cp0-CMe3), 28.3
(2 C, br.s., POCOP-CMe3), 27.9 (2 C, br.s., POCOP-CMe3).
31P{1H} NMR (CD2Cl2, 292 K): d 190.6. [5]+-exo: 5.98 (2 H,
1
M.p. 4 320 1C. H NMR (C6D6, 300 K): d 4.07 (4 H, br.s.,
ring-CH), 1.51 (36 H, s, ring-CMe3), 1.33 (18 H, s, ring-CMe3).
13C{1H} NMR (C6D6, 300 K): d 96.4 (ring-CCMe3), 64.5
(ring-CH), 34.6 (CH3), 32.8 (CH3), 31.5 (CMe3). IR (Nujol
mull; CsI windows; cmꢀ1): 3040vw, 1580vs, 1530vs, 1495w,
1462vs, 1280s, 1250m, 1200vw, 1175w, 1030m, 1005w, 960w,
938m, 868m, 832vw, 820sh, 810w, 778m, 730br.w., 672s, 662w,
572m, 562w, 445s, 422m, 412m, 305s. Anal. calcd for
C34H58Fe: C, 78.13; H, 11.11. Found: C, 78.49; H, 11.40%.
The E.I. mass spectrum showed a molecular ion at m/z =
522 amu. The parent ion isotopic cluster was simulated:
(calcd %, observd %): 520(6, 6), 521(2, 2), 522(100, 100),
523(40, 40), 524(8, 8), 525(1, 1). UV-vis (pentane, 25 1C): 310 nm
(sh, e = 2300 mol cmꢀ1), 360 nm (sh, e = 130 mol cmꢀ1),
500 nm (sh, e = 200 mol cmꢀ1). Alternatively, Fe(OAc)2 was
used for the synthesis of 2.
3
3
d, JHH = 6.0 Hz, POCOP-m-CH), 5.70 (1 H, t, JHH
=
6.0 Hz, POCOP-p-CH), 4.48 (2 H, s, Cp0-ring-CH), 1.70 (18 H,
vt, J = 8.0 Hz, POCOP-CMe3), 1.40 (18 H, s, Cp0-ring-CMe3),
1.36 (9 H, s, Cp0-ring-CMe3), 1.18 (18 H, vt, J = 8.0 Hz,
2
POCOP-CMe3), ꢀ40.32 (1 H, t, JHP = 14.4 Hz, Ir-H).
31P{1H} NMR (CD2Cl2, 292 K): d 200.5. [B(C6F5)4]ꢀ: 19F
NMR (CD2Cl2, 292 K): ꢀ133.7 (‘‘s’’, 8 F, o-CF), ꢀ164.3 (8 F,
t, 3JFF = 19 Hz, m-CF), ꢀ168.1 (4 F, t, 3JFF = 19 Hz, p-CF).
Anal. calcd for C63H69BF20IO2P2Ir*CH2Cl2: C, 43.41; H,
4.04. Found: C, 43.12; H, 3.92%.
[Cp0FeI(CO)2] (3). [Cp0FeI]2 (0.21 g, 0.25 mmol) was
dissolved in ca. 15 mL of pentane to form a dark red solution.
Exposure of this solution to CO (1 atm) resulted in the
formation of an orange crystalline precipitate which was only
moderately soluble in benzene and pentane. Yield: 0.19 g
(0.40 mmol, 80%). 1H NMR (C6D5Br, 292 K): d 5.15 (2 H, s,
ring-CH), 1.46 (27 H, br.s., ring-CMe3). 13C{1H} NMR
(C6D5Br, 292 K): d 218.9 (2 C, CO), 111.3 (2 C, ring-CCMe3),
110.8 (1 C, ring-CCMe3), 91.5 (2 C, ring-CH), 36.3 (6 C, CH3),
35.6 (2 C, CMe3), 34.8 (1 C, CMe3), 34.2 (3 C, CH3). Anal.
calcd for C19H29FeO2I: C, 48.33; H, 6.19. Found: C, 48.05;
H, 6.25%.
(POCOP)Ir(H)(I) (6). [(POCOP)Ir(H)(acetone)][B(C6F5)4]
(0.268 g, 0.15 mmol) and [Cp0FeI]2 (0.064 g, 0.075 mmol)
were dissolved in acetone (10 mL), and an orange-red
precipitate was formed immediately which was isolated and
re-crystallized from CH2Cl2 at ꢀ38 1C to give red crystals
(0.09 g, 0.125 mmol, 84%). 1H NMR (CD2Cl2, 292 K): d 6.88
(1 H, t, 3JHH = 7.8 Hz, POCOP-p-CH), 6.57 (2 H, d, 3JHH
=
7.8 Hz, POCOP-m-CH), 1.36 (36 H, m, POCOP-CMe3),
2
[Cp0Fe(NCCH3)3][PF6] (4). A mixture of [Cp0FeI]2 (0.21 g,
0.25 mmol) and KPF6 (0.092 g, 0.5 mmol) was dissolved in ca.
10 mL of MeCN to form a deep blue-purple solution. The
reaction mixture was stirred at ambient temperature for 12 h
and the solvent was removed under dynamic vacuum. The
blue-purple residue was extracted with CH2Cl2 and filtered.
Large purple crystals were isolated by slow diffusion of
pentane into a concentrated CH2Cl2 solution at ꢀ25 1C. Yield:
ꢀ41.93 (1 H, t, JHP = 11.6 Hz, Ir-H). 13C{1H} NMR
(CD2Cl2, 292 K): d 166.8 (2 C, pt, J = 6.8 Hz, POCOP-C-
O), 127.2 (1 C, br.s., POCOP-p-C), 125.7 (1 C, pt, J = 7.8 Hz,
POCOP-C-Ir), 104.7 (2 C, vt POCOP-m-C), 43.2 (2 C, pt,
J = 12.3 Hz, POCOP-CMe3), 39.7 (2 C, pt, J = 10.2 Hz,
POCOP-CMe3), 27.8 (2 C, br.s., POCOP-CMe3), 25.8 (2 C,
br.s., POCOP-CMe3). 31P{1H} NMR (CD2Cl2, 292 K): d
180.2. Anal. calcd for C22H40IO2P2Ir: C, 36.82; H, 5.62.
Found: C, 36.70; H, 5.45%.
1
0.2 g (0.36 mmol, 71%). H NMR (CD2Cl2, 292 K): d 4.03
(2 H, br.s., ring-CH), 2.44 (9 H, br.s., CH3CN), 1.39 (18 H,
br.s., ring-CMe3), 1.31 (9 H, br.s., ring-CMe3). 13C{1H} NMR
(CD2Cl2, 250 K): d 131.1 (3 C, NRC-Me), 89.5 (2 C,
ring-CCMe3), 85.2 (1 C, ring-CCMe3), 76.3 (2 C, ring-CH),
32.8 (6 C, CH3), 32.3 (2 C, CMe3), 30.9 (1 C, CMe3), 30.2
[(Cp0Fe)(POCOP)Ir(C2H4)][B(ArF)4] ([7]+). In a Schlenk
tube [Cp0FeI]2 (0.126 g, 0.15 mmol) and NaB(ArF)4 (0.266 g,
0.30 mmol) were dissolved in CH2Cl2 (10 mL) at 0 1C
and stirred for 10 min. To this solution [(POCOP)Ir(C2H4)]
c
1852 New J. Chem., 2011, 35, 1842–1854
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(0.185 g, 0.30 mmol) was added and the reaction mixture was
allowed to warm to room temperature and stirred for an
additional 20 min. The red solution was filtered and concen-
trated. A layer of pentane was added and dark red crystals
were grown at ꢀ30 1C by slow pentane diffusion. Yield: 0.32 g
Electronic population analysis was carried out with the
NBO module incorporated into Gaussian 09.74 Geometrical
parameters were reported within an accuracy of 10ꢀ3 A and
10ꢀ1 degrees.
1
(0.18 mmol, 60%). H NMR (CD2Cl2, 292 K): d 7.72 (8 H, s,
Acknowledgements
B(ArF)4-CH), 7.56 (4 H, s, B(ArF)4-CH), 5.84 (2 H, d, 3JHH
=
3
We thank the Alexander-von Humboldt Foundation for a
Feodor-Lynen Fellowship (MDW), Prof. Maurice Brookhart
for providing financial support (through NSF grant
CHE-0615704) and laboratory facilities (MDW) during the
6.0 Hz, POCOP-m-CH), 5.63 (1 H, t, JHH = 6.0 Hz,
POCOP-p-CH), 4.31 (2 H, s, Cp0-ring-CH), 3.14 (4 H, s,
C2H4), 1.61 (18 H, vt, J = 6.5 Hz, POCOP-CMe3), 1.38
(18 H, s, Cp0-ring-CMe3), 1.27 (9 H, s, Cp0-ring-CMe3), 1.04
(18 H, vt, J = 6.5 Hz, POCOP-CMe3). 31P{1H} NMR
(CD2Cl2, 292 K): d 200.3. Anal. calcd for C73H80O2F24P2B-
FeIr: C, 49.64; H, 4.57. Found: C, 49.33; H, 4.35%.
initial period of this research program, Dr. Jorg Grunenberg
¨
for helpful discussions. Furthermore, MDW gratefully
acknowledges the current financial support by Deutsche
Forschungsgemeinschaft (DFG) through the Emmy-Noether
program (WA 2513/2-1) and W.C. Heraeus for a generous gift
of IrCl3.
Scrambling experiment between 1-Br and 1-I
A 1 : 1 mixture of [Cp0FeBr]2 and [Cp0FeI]2 was molten
together in a melting point capillary, and this mixture was
subsequently subject to an EI-MS study. The dominant peaks
were due to the monomeric species [Cp0FeI] and [Cp0FeBr].
However, the ion isotopic cluster corresponding to the hetero-
leptic species [(Cp0Fe)2(m-Br)(m-I)] was also observed. E.I.
mass spectrum showed a molecular ion at m/z = 784 amu.
The parent ion isotopic cluster was simulated: (calcd %,
observd %): 782(12, 12), 783(5, 4), 784(100, 100), 785(43,
42), 786(96, 96), 787(39, 39), 788(9, 8), 789(2, 2).
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has been reported yet. The space group of Cp02Fe determined at
293 K is different from the one acquired in this study. Cell
dimensions and space group at 293 K: orthorhombic, Pna21,
a = 17.0954(10) A, b = 21.0918(16) A, c = 17.9288(12) A,
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1854 New J. Chem., 2011, 35, 1842–1854