Springloaded porphyrin NHC hybrid rhodium(iii) complexes: Carbene dissociation and oxidation catalysis

Article abstract of DOI:10.1039/c4cc00497c

Porphyrin rhodium(iii) complexes accommodate one or two NHC ligands in the apical position, which leads to severe porphyrin distortion and dearomatization. The strain in the bis(carbene) complex induces facile carbene dissociation and the formation of a catalytically active site for alcohol oxidation.

Full text of DOI:10.1039/c4cc00497c

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Springloaded porphyrin NHC hybrid rhodium(III)
complexes: carbene dissociation and oxidation
catalysis†
Cite this: Chem. Commun., 2014,
50, 3488
Received 20th January 2014,
Accepted 5th February 2014
Juan Olguın, Helge Muller-Bunz and Martin Albrecht*
´
¨
DOI: 10.1039/c4cc00497c
www.rsc.org/chemcomm
Porphyrin rhodium(III) complexes accommodate one or two NHC
ligands in the apical position, which leads to severe porphyrin
distortion and dearomatization. The strain in the bis(carbene)
complex induces facile carbene dissociation and the formation of
a catalytically active site for alcohol oxidation.
The utilization of N-heterocyclic carbenes (NHCs) as ligands
has modified the properties of transition metals substantially.1
Advantageous effects have been, in parts, attributed to the
strong metal–carbon bond in NHC complexes,2 which originate
from a relatively high covalent contribution to the overall
bonding.3 As a consequence, NHCs have been combined with
a range of other privileged ligand classes in attempts to foster
synergies, including for example bipyridine-type scaffolds,
phosphine NHC hybrids, and pincer platforms.4 The combi-
nation of NHC ligands with porphyrins is remarkably under-
explored,5 despite the beneficial features of porphyrins such as
their photochemical activity.6 Porphyrins offer the unique
added benefit that coordination of a ligand L to a metallo-
porphyrin will enable its trans effect to be fully exploited, as any
reaction will involve the distal site trans to L. Consideration of
the strong trans effect of NHCs prompted us to investigate
hybrid NHC porphyrin complexes and to evaluate structural
and catalytic implications arising from NHC bonding to a
rhodium porphyrin unit.
Scheme 1 Synthesis and ORTEP plots of complexes 1 and 2.
the characteristic deshielding of the imidazole protons induced
by the porphyrin ring current (DdH up to 6.5 ppm).8
Complex 2 was prepared by thermal decarboxylation of N,N0-
dimethylimidazolium carboxylate9 in the presence of [Rh(TTP)Cl]
(Scheme 1). The carbene resonance appeared as a broad signal at
dC 143.15 ppm ( JRhC not resolved), some 40 ppm higher field than
1
in typical rhodium(III) NHC complexes.10 The H NMR spectrum
revealed two singlets at diagnostically high field, dH 4.67 and
ꢀ0.46 ppm in a 1 : 3 ratio for the CNHC–H and N–CH3 protons,
respectively. The ring current effects are smaller than those
observed for 1 and suggest a lower aromaticity8 of the por-
phyrin skeleton in 2. Such a model is further supported by the
resonances due to the ortho phenyl protons, which appeared as
a broad resonance at 8.05 ppm upon NHC coordination (Fig. S1
and S2, ESI†), while in 1 they retain AX multiplicity.†
This broadening is temperature-dependent11 and suggests
either inversion of the TPP puckering or rotation about the
For comparison, two [Rh(TPP)(L)Cl] complexes 1 and 2 were
prepared which feature N-methylimidazole (1) and N,N0-
dimethylimidazolylidene (2) as axial ligands, respectively
(Scheme 1, TPP = meso-tetraphenyl porphyrin). Complex 1 was
synthesised from [Rh(TPP)Cl]7 and an excess (10 equiv.) of
1-methylimidazole. Coordination of one imidazole ligand was
unambiguously confirmed by the pertinent integral ratios and
C
meso–CPh bond due to an out-of-plane bending of the meso
School of Chemistry & Chemical Biology, University College Dublin, Belfield,
Dublin 4, Ireland. E-mail: martin.albrecht@ucd.ie; Fax: +353 17162501;
Tel: +353 17162504
substituents.
The molecular structure of complexes 1 and 2 indicates a
staggered orientation of the apical heterocyclic ligand with
respect to TPP, and its ortho substituents point towards the
meso carbons, indicating negligible Rh–L p-bond interaction.12
† Electronic supplementary information (ESI) available: Synthetic, catalytic, and
crystallographic details. CCDC 958621–958623. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c4cc00497c
3488 | Chem. Commun., 2014, 50, 3488--3490
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imidazolium carboxylate was extended from 3 to 18 h. Using
a stoichiometric 2 : 1 ligand/metal ratio afforded 3 with several
side products which could not be removed even by column
chromatography. The 1H NMR spectrum of 3 showed the
diagnostic upfield shifts of the NHC signals, though the ring
current and hence the aromaticity of the porphyrin ring are
slightly less pronounced than in the monocarbene analogue 2.
Likewise, the doublet of the carbene carbon is less shielded
(dC 153.15 ppm, JRh–C = 35 Hz).
X-ray crystallography confirmed a substantial distortion of
the TPP ligand. For example, the meso carbons deviate from the
porphyrin mean plane by 0.533–0.639 Å (Fig. S4, ESI†), signifi-
cantly more than even in 2, and the pyrrole rings are substan-
tially twisted out of plane (S = 11.81). The two NHCs are
mutually perpendicular. Despite the strong Rh–CNHC bonds,
indicated inter alia by the substantial porphyrin deformation,
the Rh–CNHC distances are significantly larger (2.141(2) and
2.124(2) Å) than in the less distorted complex 2 (2.051(2) Å) and
indicate a high trans influence of the NHCs.
Fig. 1 Distortion of the porphyrin macrocycle and out of plane deviation
of meso phenyl groups in complexes 1, 2, and 3 (ytrans and ycis are averaged
dihedral angles between pyrrole rings in mutual cis and trans positions,
respectively).
These rhodium(III) porphyrin complexes were used as
catalyst precursors for alcohol oxidation (Table 1).†14 With
The macrocycle is essentially undistorted and planar in 1 benzyl alcohol as a model substrate, the monocarbene complex
(Fig. 1). In contrast, complex 2 features a strongly ruffled 2 showed a significantly higher activity than the imidazole-
porphyrin and the octahedral distortion parameter,13 S = containing analogue 1 (72% vs. 19%, entries 2 and 3). The
19.821, is significantly larger than in 1 (S = 7.21). The rhodium imidazole had an inhibiting effect, as complex 1 was less active
center resides 0.08 Å out of the porphyrin plane (towards the than the simple [Rh(TPP)Cl] precursor salt (45%, entry 1). The
NHC ligand) and the meso-carbons deviate from this plane by bis-NHC complex 3 shows initial activity that is comparable to
0.204–0.439 Å.† Such distortion reduces aromaticity, in good 2, though it remains active over a longer time period and
agreement with structural analysis from solution NMR data.
achieves essentially quantitative conversions (entry 4). The high
Even though the methyl substituents of the NHC ligand activity of the bis(carbene) complex 3 is remarkable when
induce significant distortion of the TPP macrocycle, the Rh– considering that the rhodium center is coordinatively saturated
carbene bond length is identical to the Rh–imidazole bond and suggests the substitution of (at least) one carbene ligand
length in 1, Rh–CNHC 2.051(2) Å vs. Rh–NMeIm 2.058(2) Å. The with a substrate alcohol or alkoxide in order to initiate
similar distances and the loss of aromaticity of the TPP ligand b-hydrogen elimination.15 Indeed, a substoichiometric experi-
indicate a remarkably strong Rh–C bond, which is also demon- ment using a 3 : 1 ratio of alcohol and complex 3 revealed the
strated by the 0.1 Å longer Rh–Cl bond in complex 2 than in 1 quantitative formation of the monocarbene complex 2.† As a
(2.4290(4) Å vs. 2.3302(6) Å). Moreover, attempts have been direct implication, the notion of carbenes as robust spectator
unsuccessful thus far to coordinate 1,2,4,5-tetramethylimidazole, ligands requires caution.2 Carbene dissociation16 is presumably
a ligand that is isosteric to the carbene in 2 but featuring N- rather
than C-bonding. Presumably the apical Rh–N bond is not strong
Table
1
Catalytic aerobic oxidation of benzylalcohol by rhodium(III)
enough to compensate for the steric deformation and associated
reduction of aromaticity of the porphyrin while the Rh–CNHC
bond strength counterbalances this loss and yields complex 2.
The cationic bis-NHC complex 3 was isolated as a by-product
during the formation of 2 and was separated by column
chromatography (Scheme 2). The yield of 3 was markedly
increased when the reaction time of Rh(TPP)Cl with the
porphyrin complexesa
NMR yields (%)
1 h
Entry
Cat.
24 h
1
2
3
4
[Rh(TPP)Cl]
20
2
38
43
30
55
45
19
72
92
58
[Rh(TPP)(MeIm)Cl] 1
[Rh(TPP)(NHC)Cl] 2
[Rh(TPP)(NHC)2]Cl 3
[Rh(TPP)(NHC)(PF6)]
[Rh(TPP)(NHC)2]Cl 3
5
6b
495
a
General conditions: benzyl alcohol (0.5 mmol), KOtBu (0.05 mmol)
and catalyst precursor (0.025 mmol) were stirred in dichlorobenzene
(2.5 mL) at 150 1C for 24 h; yields determined by 1H NMR spectroscopy,
b
Scheme 2 Synthesis of bis(NHC) complex 3.
average of at least two runs. Irradiation using a xenon lamp.
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We thank Johnson Matthey for a generous loan of rhodium.
This work was financially supported by the Irish Research
Council, the ERC, and Science Foundation Ireland.
Notes and references
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3490 | Chem. Commun., 2014, 50, 3488--3490
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