Formation and ethene substrate reactions of iridium(II) porphyrin metal-centered dπ radicals

Article abstract of DOI:10.1039/b104301n

Monomeric iridium(II) porphyrin complexes of tetrakis-(2,4,6-trialkylphenyl)porphyrin ligands that are generated by photolysis of the Ir-Me derivatives are found to have the d_xy2 d_z22 d_xz,yz3 ground electron configuration

Full text of DOI:10.1039/b104301n

Formation and ethene substrate reactions of iridium(II) porphyrin
metal-centered dp radicals
Huili Zhai, Andrew Bunn and Bradford Wayland*
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA.
E-mail: wayland@a.chem.upenn.edu
Received (in Cambridge, UK) 16th May 2001, Accepted 1st June 2001
First published as an Advance Article on the web 27th June 2001
Monomeric iridium(II) porphyrin complexes of tetrakis-
(2,4,6-trialkylphenyl)porphyrin ligands that are generated
by photolysis of the Ir–Me derivatives are found to have the
2
22
3
dxy dz dxz,yz ground electron configuration which differs
from the dxy dxz,yz4 dz 1 configuration observed for the Co(II
)
2
2
and Rh(II) analogs; reactions of these Ir(II) species with
ethene reflect both the metallo-radical reactivity and the
varying steric demands for the series of porphyrin ligands.
Monomeric iridium(II) complexes1 are unusual because of the
dominant characteristic for these d7 species to form diamagnetic
IrII–IrII bonded dimers.2 All of the currently reported iridium(II
)
porphyrin complexes are diamagnetic dimers,3,4 and the range
of thermodynamically favorable substrate reactions is limited
by the dimer homolytic dissociation energy. This article reports
on the generation of iridium(II) complexes with sterically
demanding porphyrin ligands along with studies relevant to the
electronic structure and ethene substrate reactions.
Tetramesitylporphyrin (TMP) and derivatives where the
mesityl methyl groups in TMP are replaced by ethyl (TTEPP)
and isopropyl (TTiPP) provide a series of ligands that require
interporphyrin distances too large to support metal–metal
bonding.5,6 Photolysis (l > 350 nm) of (octaethylporphy-
rinato)iridium methyl [(OEP)Ir–Me) gives selective Ir–Me
bond homolysis and near quantitative formation of the IrII–IrII
bonded dimer {[(OEP)Ir]2}.4 Photolysis of (por)Ir-CH3 (por =
TMP, TTEPP, TTiPP) provides an approach to generate
iridium(II) porphyrin derivatives that are sterically unable to
dimerize by IrII–IrII bonding [(eqn. (1)].
Fig. 1 1H NMR spectra illustrating the pyrrole resonance (296 K): (a)
(TTiPP)RhII, dpyr +17.5; (b) (TTiPP)IrII, dpyr 220.9.
paramagnetic shift with T21 for (TTiPP)RhII is indicative of
simple Curie paramagnetic behavior associated with a single
contributing state, but curvature of the plot for (TTiPP)IrII
suggests that several states are thermally populated. The
deviation from linearity in the shift vs. T21 as the temperature is
lowered is in the direction of larger upfield contact shift which
clearly indicates that the ground configuration has an unpaired
electron in the dp (dxz,yz) orbitals. Repeated attempts to
determine EPR parameters for (TTiPP)IrII in toluene glass
(20–100 K) did not result in an observed EPR spectrum. The
presence of one or more excited states with energy close to that
of the ground state may hamper observation of EPR spectra by
producing rapid electron spin relaxation.
hv
II
_
,
(por)Ir Me
(por)Ir + ◊ Me
(1)
l > 350 nm
Photolysis of (por)Ir–Me complexes in C6D6 results in the
direct observation of (TTiPP)IrII 1 and (TTEPP)IrII 2† and
evidence for the formation of (TMP)IrII 3 through the
characteristic reaction with ethene to form (TMP)Ir–CH2CH2–
Ir(TMP)[eqn. (2)].†
2(TMP)IrII + CH2NCH2 ?(TMP)Ir–CH2CH2–Ir(TMP) (2)
Differences in steric demands of (TMP)IrII, (TTEPP)IrII and
(TTiPP)IrII are clearly manifested by the different products that
The paramagnetism of (TTiPP)IrII 1 produces shifts and
broadening for all of the 1H NMR resonances in 1† and the high
field position for the pyrrole hydrogens [d1 (pyr) 220.9 (296
K)] is particularly significant (Fig. 1). The ground electron
configuration for both Co(II) and Rh(II) porphyrin complexes is
2
4
21 7
1
known to be dxy dxz,yz dz and downfield pyrrole H NMR
shifts8 are observed for these metalloporphyrins which is
opposite in sign to that for (TTiPP)IrII 1. Upfield pyrrole
porphyrin contact shifts are associated with spin density in the
porphyrin p orbitals.9 Low spin (d5) Fe(III) porphyrin com-
plexes such as [(TPP)Fe(Im)2]Cl (Im = imidazole)10 have the
2
3
1
dxy dxz,yz ground configuration and upfield pyrrole H NMR
shift positions comparable to that for 1.10 The pyrrole proton
contact shifts for 1 clearly indicate that (TTiPP)IrII has a dxy
2
22
2
dz dxz,yz3 electron configuration which contrasts with the dxy
dxz,yz dz configuration observed for the d7 metalloporphyin
complexes of cobalt(II) and rhodium(II).
4
21
Plots of the porphyrin pyrrole shifts vs. T21 for (TTiPP)IrII
and (TTiPP)RhII are shown in Fig. 2. Linear dependence of the
Fig. 2 Plots of the pyrrole 1H NMR contact shifts in toluene-d8 vs. T21
[(TTiPP)RhII (-); (TTiPP)IrII (5)].
1294
Chem. Commun., 2001, 1294–1295
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104301n

result from reactions with ethene. Reaction of (TTEPP)IrII with
ethene produces a four-carbon bridged complex (TTEPP)Ir–
CH2CH2–CH2CH2–Ir(TTEP) [eqn. (3)] without any evidence
for the two-carbon ethylene bridged species observed for
(TMP)IrII [(TMP)Ir–CH2CH2–Ir(TMP)] [eqn. (1)].
This research was supported by the Department of Energy
Division of Chemical Sciences, Office of Science through grant
DE-FG02-86ER-13615.
Notes and references
† The synthesis of iridium complexs of TMP, TTEPP and TTiPP follows the
general procedures described by Ogoshi for the synthesis of (OEP)Ir
complexes: (por)IrII (por = TMP, TTEPP, TTiPP) is generated by photolysis
of (por)Ir–Me in benzene in a Rayonet photoreactor equipped with RPR-
350 nm lamps.
2(TTEPP)IrII + 2 CH2NCH2 ?
(TTEPP)Ir–CH2CH2–CH2CH2–Ir(TTEPP) (3)
Selected spectroscopic data: (TTiPP)IrII: dH(C6D6; 294 K): 8.14 (8H, br
s, m-H), 5.88 [8H, br, o-CH(CH3)2], 3.79 [4H, sept, p-CH(CH3)2], 2.08
(24H, br, p-CH(CH3)2], 1.73 [48H, br, o-CH(CH3)2], 220.89 (8H, br,
pyrrole H).
The increased steric demands of (TTEPP)Ir compared to
(TMP)Ir inhibits formation of the two-carbon bridged complex
and an ethene coupling process occurs to yield a four-carbon
bridged complex that relieves the steric congestion. The
reactions of (TMP)IrII and (TTEPP)IrII with ethene to form two-
and four-carbon bridged complexes directly parallel reactions
of the rhodium(II) derivatives.6 Further increase in the ligand
steric requirements to those of (TTiPP)IrII inhibits formation of
even a four-carbon bridged species which is also a property
observed for the rhodium(II) derivative.6 When a toluene
solution of (TTiPP)IrII is exposed to ethene the porphyrin NMR
spectrum disappears, new electronic absorption maxima appear
at 444 and 730 nm and an intense EPR signal is observed [ < g >
= 1.987(290 K); g|| = 1.96, g4 = 1.998(90 K)]. These
spectroscopic changes are indicative of a donor induced
intramolecular electron transfer from the IrII center to the
porphyrin ligand p* which forms an iridium(III) porphyrin
anion radical species.11,12 This behavior differs from (TTiPP)-
RhII which reacts with ethene to form a 1+1 complex where the
(TTEPP)IrII: dH(C6D6; 294 K): 7.62 (8H, br s, m-H), 4.08 (16H, br, o-
CH2CH3), 3.23 (8H, br, p-CH2CH3), 1.83 (12H, br, p-CH2CH3), 1.61 (24H,
br, o-CH2CH3), 221.46 (8H, br, pyrrole H).
Reaction of (TMP)Ir with ethene in benzene solution produces (TMP)Ir–
1
CH2CH2–Ir(TMP), which is identified by H NMR spectroscopy by the d
27.85 resonance characteristic of the –CH2CH2– bridge.6
Reaction of (TTEPP)Ir with ethene in benzene solution produces
(TTEPP)Ir–CH2CH2CH2CH2–Ir(TTEPP). The 1H NMR spectrum displays
two high field resonances centered at d 25.87 and 26.42 that are
characteristic of the four-carbon bridge.6
1 M. Kubota, M. Chan and K. Mann, Inorg. Chem., 1987, 26, 3261.
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J. Am. Chem. Soc., 1985, 107, 279.
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T. A. Eberspacher, J. Brauman, J. W. Bacon and L. H. Pignolet, Inorg.
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1986, 108, 7633.
6
2
unpaired electron is in a metal centered dz molecular orbital.
(TTiPP)RhII requires an excess of a strong donor like pyridine
2
in order to elevate the dz above the porphyrin p* to produce an
intramolecular electron transfer.7 A higher energy position for
the iridium d orbitals and or stronger iridium–substrate binding
compared to that of rhodium(II) and cobalt(II) is inferred by
these results.
Generation of monomeric iridium(II) porphyrins permits
study of the fundamental electronic structure of Ir(II) and by
removing the thermodynamic restrictions from IrII–IrII bonding
provides an opportunity to evaluate the full range of Ir(II
)
12 K. Kadish, D. Sazou, G. Maiya, B. Han, Y. Liu, A. Saoiabi, M. Ferhat
and R. Guilard, Inorg. Chem., 1988, 27, 2745.
substrate reactions.
Chem. Commun., 2001, 1294–1295
1295