Synthesis, characterization, and infrared reflectance spectroelectrochemistry of organoruthenium nitrosyl porphyrins

Article abstract of DOI:10.1021/om200601g

Metal-carbon bonds are known to form during the metal-catalyzed transformations of various organic compounds such as phenylhydrazines by the heme-containing proteins cytochrome P450, hemoglobin, and myoglobin. The preparation and characterization of synth

Full text of DOI:10.1021/om200601g

Article
pubs.acs.org/Organometallics
Synthesis, Characterization, and Infrared Reflectance
Spectroelectrochemistry of Organoruthenium Nitrosyl Porphyrins
Nan Xu,* John Lilly, Douglas R. Powell, and George B. Richter-Addo*
Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019, United
States
S
* Supporting Information
ABSTRACT: Metal−carbon bonds are known to form during
the metal-catalyzed transformations of various organic
compounds such as phenylhydrazines by the heme-containing
proteins cytochrome P450, hemoglobin, and myoglobin. The
preparation and characterization of synthetic organometallic
porphyrins of the group 8 metals are thus of interest, and their properties help enlighten the general discussion of
metalloporphyrin−carbon bond chemistry. We have prepared a representative set of (por)Ru(NO)R compounds (por = T(p-
OMe)PP, T(p-CF3)PP; R = Me, Et) containing Ru−alkyl bonds trans to NO. We have determined the X-ray crystal structure of
(T(p-OMe)PP)Ru(NO)Et, which represents the first crystal structure reported for any organometallic nitrosyl porphyrin with an
alkyl ligand trans to NO; the structure reveals a significantly bent RuNO moiety at 153° in this {RuNO}6 compound. We have
characterized the redox behavior of the (T(p-OMe)PP)Ru(NO)-containing compounds by cyclic voltammetry and infrared
spectroelectrochemistry, and we have determined that the first oxidations are porphyrin-centered.
involve migration of the axial non-nitrosyl ligand to a porphyrin
N atom.
Some organometallic (por)M(NO)R compounds have been
INTRODUCTION
■
Metal−carbon bonds form during the reactions of some
substrates with the heme proteins myoglobin (Mb), hemoglo-
bin (Hb), and cytochrome P450.1−4 For example, σ-bonded
aryliron(III) complexes form when phenylhydrazines (e.g.,
PhNHNH2) react with Hb5 and Mb.6 The occurrence of
metal−carbon bonds in biology has spurred an interest in the
preparation and study of metalloporphyrin model systems of
the group 8 metals, including their redox behavior.
reported for Fe,7,10,12,32 Ru,32,33 and Os.34 Crystal structure
data for (OEP)Fe(NO)(C6H4F-p),32 (OEP)Ru(NO)(C6H4F-
p),32 and (TTP)Ru(NO)(C6H4F-p)33 revealed a cisoid
bending of the axial NO and aryl ligands from the heme
normal. Results from theoretical calculations on these
structurally characterized aryl compounds have helped explain
the inherent stabilization of the cisoid structures (i.e., deviation
from standard octahedral geometry).32,35 No (por)M(NO)-
(alkyl) derivative has been structurally characterized by X-ray
diffraction, however. Thus, we were uncertain whether the axial
ligand bending and resulting stabilization will be applicable to
A number of organometallic heme models of the group 8
metals have been synthesized and characterized.7−23 They can
generally be prepared via one or more synthetic routes: (i)
reaction of a precursor (por)metal halide (por = porphyrinato
dianion) with an alkylating agent (R−M; M = main-group
metal; R = alkyl, aryl),17,19 (ii) reaction of a metalloporphyrin
anion with an alkyl halide,24 and (iii) reaction of a
metalloporphyrin cation with an alkylating agent (R−M).25
Both five- and six-coordinate organometallic porphyrins have
been reported. The redox behavior of several of these
compounds has been studied by electrochemistry,26 although
studies on the redox behavior of organoruthenium porphyrins
have lagged behind those of other organometallic porphyrins.
Seyler and Leidner reported the cyclic voltammetric behavior of
(OEP)Ru(Ph)2 and demonstrated a Ru-to-Npor migration of an
initially Ru-bound Ph group upon oxidation (the identity of the
product was confirmed by X-ray crystallography).17 They found
that an alkyl migration from Ru to Npor also occurred when
(OEP)RuMe2 was chemically oxidized to give the [(OEP-N-
Me)RuMe]+ derivative.18 In contrast, the non-organometallic
complexes (por)Ru(NO)X (X = halide,27 pseudohalide,28−30
[H2O]+30,31) undergo electrochemical oxidations that do not
the alkyl systems as well.
There have been only a handful of electrochemical studies on
organoruthenium porphyrins reported,16−18 and there is no
report describing the electrochemical behavior of the organo-
metallic nitrosyl compounds (por)Ru(NO)R (R = alkyl, aryl).
In this article, we report the synthesis and characterization of
the (por)Ru(NO)R (R = Me, Et; por = T(p-OMe)PP, T(p-
CF3)PP) compounds, the X-ray crystal structure of the (T(p-
OMe)PP)Ru(NO)Et derivative, the redox behavior of the
compounds at a Pt-disk electrode, and the IR spectroelec-
trochemical results that help establish the identities of the redox
products.
Received: July 7, 2011
Published: January 17, 2012
© 2012 American Chemical Society
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alkyl derivative (TTP)Ru(NO)Me, which is consistent with the
better overall electron donating ability of alkyl groups
compared to aryl groups. In any event, the presence of strongly
σ donating alkyl groups trans to NO results in low νNOs for this
class of (T(p-X)PP)Ru(NO)R compounds.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Prior to this work, only
three organoruthenium nitrosyl porphyrins had been reported,
namely the alkyl compound (TTP)Ru(NO)Me33 and the aryl
compounds (TTP)Ru(NO)(C6H4F-p)33 and (OEP)Ru(NO)-
(C6H4F-p).32 Reactions of the precursor (T(p-X)PP)Ru(NO)-
Cl27 compounds with AlR3 or RMgBr (R = Me, Et) reagents
resulted, after appropriate workup, in the generation of the
target (T(p-X)PP)Ru(NO)R (R = Me, Et) derivatives in
moderate to low isolated yields. We found that laboratory
lighting decomposed these organometallic products (e.g.,
∼10% decomposition over 10 h, as judged by NMR
spectroscopy); hence, we utilized reduced laboratory lighting
in our preparations and purifications.
The 1H NMR spectra of the (T(p-X)PP)Ru(NO)R
compounds reveal upfield signals due to the alkyl ligands
1
(Table 2). The methyl H signals in both (T(p-X)PP)Ru-
Table 2. 1H NMR Spectral Data (in ppm) for the Axial Alkyl
Ligands in (por)Ru(NO)R and (por)Ru(R)2 Complexes in
CDCl3
δ(Ru−
CH3)
δ(Ru−
δ(Ru−
compd
CH2CH3)
CH2CH3)
When the Grignard reagent MeMgBr is used in place of
AlMe3 in the alkylation reaction, the (T(p-OMe)PP)Ru(NO)-
Me product is generated in a lower isolated yield. A similar
lower isolated yield for (T(p-OMe)PP)Ru(NO)Et was
obtained using EtMgBr. In general, we find that large excesses
of the Grignard reagents and longer reaction times result in
lower yields of the desired products (not shown), presumably
due to follow-up reactions of the (T(p-OMe)PP)Ru(NO)R
products with excess reagents to yield the trans-dialkyl
products. Such reactions with excess Grignard reagents have
been reported previously by us for the reaction of (TTP)Ru-
(NO)Cl with MeMgBr; the target (TTP)Ru(NO)Me and the
dialkyl (TTP)Ru(Me)2 byproduct proved very difficult to
separate, due to their similar solubilities in organic solvents.33
IR and 1H NMR Spectroscopy. IR nitrosyl stretching
frequencies of the organoruthenium nitrosyl complexes
reported to date and selected chloro precursors are given in
Table 1. The νNOs of the (por)Ru(NO)R compounds are much
(T(p-OMe)PP)Ru(NO)Me
(T(p-OMe)PP)Ru(NO)Et
(T(p-CF3)PP)Ru(NO)Me
(T(p-CF3)PP)Ru(NO)Et
(T(p-CF3)PP)Ru(Me)2
−6.72 (s)
−6.71 (s)
−2.71 (s)
−6.00 (q)
−5.96 (q)
−1.90 (q)
−4.19 (t)
−4.17 (t)
−4.04 (t)
a
(T(p-CF3)PP)Ru(Et)2
Identified by 1NMR spectroscopy as a minor product during the
preparation of the nitrosyl compound (T(p-CF3)PP)Ru(NO)Et.
a
(NO)Me compounds described in this work, and that of
(TTP)Ru(NO)Me,33 appear as singlets at −6.7 ppm in CDCl3.
This large upfield shift of the −CH3 signals is due to the
presence of the NO ligand and the deshielding effect of the
porphyrin macrocycle: cf. the −CH3 signals of the dialkyl
compounds (T(p-CF3)PP)Ru(Me)2 (−2.71 ppm), (TTP)Ru-
(Me)2 (−2.74 ppm in CDCl3),25,33 (OEP)Ru(Me)2 (−3.51
ppm in THF-d8)25 and the alkyl/aryl compound (OEP)Ru-
(Ph)Me (−2.74 ppm in C6D6).25 The axial ethyl Ru−CH2CH3
1H resonances in the (T(p-X)PP)Ru(NO)Et compounds reveal
an interesting feature attributed to the presence of the trans
NO ligand. The axial methylene −CH2CH3 resonances in the
(T(p-X)PP)Ru(NO)Et compounds are ∼1.8 ppm upfield from
those of the −CH2CH3 methyl resonances (Table 2). However,
the reverse trend holds for the non-nitrosyl diethyl compound
(T(p-CF3)PP)Ru(CH2CH3)2 (CH2 at −1.90 ppm and CH3 at
−4.04 ppm; this work) and the previously reported diethyl
compounds (OEP)Ru(CH2CH3)2 (CH2 at −2.74 ppm and
CH3 at −4.54),23 (TTP)Ru(CH2CH3)2 (CH2 at −1.97 ppm
and CH3 at −4.08 ppm).24
Crystal Structure of (T(p-OMe)PP)Ru(NO)Et. We
reported the X-ray crystal structures of the aryl compounds
(por)Ru(NO)(C6H4F-p) (por = TTP, OEP).32,33 It proved
nontrivial to obtain a suitable crystal of any alkyl derivative for
an X-ray diffraction study, due presumably to their light
sensitivity and high reactivity of nitrosyl alkyl derivatives with
trace acid in halogenated solvents, in which they were soluble.
We were finally able, after a multiyear effort, to obtain a suitable
crystal of (T(p-OMe)PP)Ru(NO)Et for an X-ray diffraction
study from the slow anaerobic evaporation (in the dark) of a
CH2Cl2/hexane (2/1) solution of the complex in the presence
of a trace amount of benzene.
Table 1. IR Nitrosyl Stretching Frequencies of
Organoruthenium Nitrosyl Porphyrins and Their Precursors
νNO (KBr,
νNO (CH2Cl2,
−1
compd
cm )
cm−1)
ref
(T(p-OMe)PP)Ru(NO)Cl
(T(p-CF3)PP)Ru(NO)Cl
(OEP)Ru(NO)Cl
1844
1847
1829
1735
1851
1855
1842
1742
27
27
27
(T(p-OMe)PP)Ru(NO)Me
this
work
(T(p-OMe)PP)Ru(NO)Et
(T(p-CF3)PP)Ru(NO)Me
(T(p-CF3)PP)Ru(NO)Et
1724
1735
1735
1723
1748
1735
this
work
this
work
this
work
(TTP)Ru(NO)Me
1743
1773
1759
33
33
32
(TTP)Ru(NO)(C6H4F-p)
(OEP)Ru(NO)(C6H4F-p)
lower than those of their chloro precursors. For example, the
νNO for (T(p-CF3)PP)Ru(NO)Me as a KBr pellet is at 1735
cm−1, which is 112 cm−1 lower than that of (T(p-CF3)PP)Ru-
(NO)Cl at 1847 cm−1, and this reflects the strong σ donor
property of the methyl ligand. In addition, although there is no
clear trend with the νNO values of the Me vs the Et compounds
as KBr pellets, it is evident from the data that the (T(p-
The molecular structure of (T(p-OMe)PP)Ru(NO)Et is
shown in Figure 1a. The axial NO and ethyl ligands are
disordered across the porphyrin plane, and the disordered
components were best modeled in a 55/45 ratio. The relative
positions of the axial ligands with respect to the porphyrin
plane of the major disordered component are shown in Figure
X)PP)Ru(NO)Me compounds in CH2Cl2 have higher νNO
s
than the (T(p-X)PP)Ru(NO)Et analogues. We also note that
the νNO of the previously reported aryl compound (TTP)Ru-
(NO)(C6H4F-p) at 1773 cm−1 is 30 cm−1 higher than that of its
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Table 3. Selected Bond Lengths (Å) and Angles (deg) for
ab
,
(T(p-OMe)PP)Ru(NO)Et
N5−O1
Ru1−N5
1.139(6) [1.145(7)]
1.825(5) [1.827(5)]
O1−N5−Ru1
N5−Ru1−C49
153.4(5) [153(1)]
165.1(3) [166.8(3)]
Ru1−C49 2.117(7) [2.116(7)]
C49−C50 1.525(9) [1.523(10)]
Ru1−C49−C50 118.9(7) [122.2(9)]
a
Data for the second component of the disordered axial groups are
b
given in brackets. The Ru atom is apically displaced by 0.18 Å [0.23
Å] from the 24-atom porphyrin plane toward the NO ligand.
our knowledge, this is the first reported crystal structure of a
nitrosyl alkyl porphyrin for any metal.
The Ru−N−O angle in this formally {RuNO}6 compound is
bent with an angle of 153.4(5)° for the major component
(153(1)° for the minor disordered component). Such a
bending of metal−NO bonds in {MNO}6 compounds has
been noted previously for (TTP)Ru(NO)(C6H4F-p) (∠RuNO
= 152°),33 (OEP)Ru(NO)(C6H4F-p), (∠RuNO = 154.9(3)
°),32 and (OEP)Fe(NO)(C6H4F-p) (∠FeNO = 157.4(2)°).32
We determined from theoretical calculations that the nitrosyl
bending in these {MNO}6 compounds was intrinsic and was
made energetically favorable by an associated tilting of the
nitrosyl N atoms from the porphyrin normal,32 similar to that
determined computationally by Ghosh and Bocian for carbonyl
hemes.36 Indeed, Ghosh has determined, from DFT calcu-
lations for the model (porphine)Fe(NO)(Ph) compound, that
2
the metal dz orbital is “tied up” in a bonding interaction with
the axial Ph group and that the metal dxz orbital is engaged in a
favorable three-center bent π bond with the NO π* orbital.35
The nitrosyl N atom of (T(p-OMe)PP)Ru(NO)Et is tilted
from the normal to the porphyrin 24-atom plane by 11.9°
(major) and 8.9° (minor) (α; Table 4). The NO group is
further tilted by 27°, for both disordered components, from the
Ru−N(O) vector in the direction away from the porphyrin
normal (β; Table 4). The axial ethyl C49 atom is also tilted in
the general direction of the NO tilt (γ; Table 4). This results in
nonlinear axial C−Ru−N moieties displaying angles of ∼166°.
Data for the related axial ligand tilting in the (por)M(NO)R
compounds reported to date are collected in Table 4.
We note that most of the six-coordinate {MNO}6
compounds that have been structurally characterized by
single-crystal X-ray diffraction (e.g., (OEP)Ru(NO)Cl27)
display linear M−N−O geometries with no significant axial
nitrosyl N atom tilting.13,37 However, the axial ligand tilting
observed in the (por)Ru(NO)R class of compounds is not
limited to six-coordinate nitrosyl compounds; such tilting has
been reported for some five-coordinate and six-coordinate
compounds such as (OEP)Ru(Np) (12.7°; Np = neopentyl),
and (TPP)Os(CH2Si(CH3)3)2 (18.5, 21.8°).38 Such tilting in
the latter class of diamagnetic d4 compounds has been
attributed, on the basis of the results of DFT calculations, to
the strong trans influence of the alkyl ligands that destabilize a
linear axial structure.39,40
Figure 1. (a) Molecular structure of (T(p-OMe)PP)Ru(NO)Et.
Hydrogen atoms have been omitted for clarity. Only one of the two
disordered components is shown. (b) View of the porphyrin core
showing the major disordered component of the axial ligands, with the
ethyl group facing the viewer. (c) View of the porphyrin core showing
the minor disordered component of the axial ligands, with the NO
group facing the viewer.
The Ru−N(O) bond length in (T(p-OMe)PP)Ru(NO)Et is
1.825(5) Å (cf. 1.827(5) Å for the minor component), and is
longer than that for the related (OEP)Ru(NO)(C6H4F-p) at
1.807(3) Å which was described as being very long for a
(por)Ru−NO bond. Clearly, the presence of the strong σ-
donor alkyl group trans to NO is responsible for this
lengthening. The Ru−C(ethyl) bond length is also long at
2.117(7) Å [2.116(7) Å], which is at the upper end of the Ru−
C(sp3-alkyl) (2.05−2.12 Å) and Ru−C(sp2-aryl) (2.00−2.11 Å)
1b, and those of the second component are shown in Figure 1c.
Selected bond lengths and angles are given in Table 3.
The porphyrin core is saddled, and the Ru−Npor bond
lengths in (T(p-OMe)PP)Ru(NO)Et are in the 2.043(2)−
2.092(2) Å range. The Ru atom is displaced by 0.18 Å (major
component) and 0.23 Å (minor component) from the 24-atom
mean porphyrin plane toward the nitrosyl ligand. To the best of
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Table 4. Axial Ligand Tilts (deg) in Six-Coordinate Organometallic Nitrosyl Porphyrins (por)M(NO)R
a
a
compd
α
β
γ
ref
(T(p-OMe)PP)Ru(NO)Et
(TTP)Ru(NO)(C6H4F-p)
(OEP)Ru(NO)(C6H4F-p)
(OEP)Fe(NO)(C6H4F-p)
11.9 [8.9]
12.0
27.0 [27.0]
27.8
4.8 [5.9]
4.3
this work
27
28
28
10.8
25.1
3.5
9.2
22.6
3.1
a
Tilts of the N and C atoms from the normal to the porphyrin 24-atom plane.
bond length ranges for porphyrin complexes (Table 5). In
comparison, the Ru−C(sp2-carbene) bond lengths are
significantly shorter, as shown in Table 5.
Table 5. Ru−C Bond Lengths in Organoruthenium (Non-
Carbonyl) Porphyrin Compounds
compd
(OEP)Ru(C6H5)
Ru−C (Å)
ref
Aryl
2.005(7)
21
41
14
42
33
32
(OEP)Ru(Ph)2
2.093(2), 2.098(2)
1.999(4)
[(OEP-N-Ph)Ru(C6H5)]BF4
(TMP)Ru(Ph)2
2.076(9), 2.062(8)
2.095(6)
(TTP)Ru(NO)(C6H4F-p)
(OEP)Ru(NO)(C6H4F-p)
Figure 2. Cyclic voltammograms of 1 mM (T(p-OMe)PP)Ru(NO)-
Me in CH2Cl2 containing 0.1 M NBu4PF6 at a scan rate of 0.2 V/s at
room temperature. Potentials are referenced to the Ag/AgCl couple.
2.111(3)
Alkyl
(OEP)Ru(Np)
2.069(7), 2.12(1)
2.053(4)
19
42
19
dication [(T(p-OMe)PP)Ru(NO)Me]2+, respectively (site of
oxidation not indicated; see later). In addition, these two
oxidations are chemically reversible with cathodic-to-anodic
peak current ratios (ipc/ipa) of ∼1.0. Furthermore, plots of ipa vs
(scan rate)1/2 for both the first and second oxidations show
linear relationships, indicating that these two oxidations are
diffusion-controlled. The (T(p-OMe)PP)Ru(NO)Me com-
pound undergoes an electrochemically reversible reduction at
−1.40 V at a scan rate of 0.2 V/s with a small daughter peak at
−0.60 V. The redox couple at −1.40 V becomes less chemically
reversible at lower scan rates, indicating that a slow structural
change occurs (e.g., ligand loss) upon reduction.
The cyclic voltammogram of (T(p-OMe)PP)Ru(NO)Et
(Figure 3) is more complicated than that of (T(p-OMe)PP)-
Ru(NO)Me. When the potential scan was reversed just after
the first oxidation, a well-defined reversible first redox couple
(E1°′) was observed centered at +0.74 V (Figure 3a; ipc/ipa = 1),
indicating chemical reversibility for this first oxidation.
A full cyclic voltammogram, however, reveals what appears to
be a partially reversible first oxidation couple at +0.74 V, a
second apparently irreversible process at ∼+1.25 V, and a third
oxidation process that approaches the solvent system limit
(Figure 3b). The lower chemical reversibility of the first
oxidation in the full scan is likely due to the reaction of the
unstable second oxidation product with the analyte or the
solvent system.
The (T(p-OMe)PP)Ru(NO)Et compound undergoes a
reversible reduction centered at −1.47 V with a small return
peak at −0.68 V. The first oxidation potentials of the (T(p-
OMe)PP)Ru(NO)R compounds at E1°′ = +0.77 (R = Me) and
E1°′ = +0.74 V (R = Et) are significantly more negative than
(OEP)Ru(CH2C(Me)2Ph)
[(OEP)Ru(Np)]2(μ-Li)2
(T(p-OMe)PP)Ru(NO)Et
2.100(3)
2.117(7), 2.116(7)
this work
Carbene
[(TTP)Ru(CPh2)(CH3OH)]
1.845(3)
1.868(3)
1.877(8)
1.829(9)
43
44
44
45
[(TTP)Ru[C(3-C6H4CF3)2](py)
[(TTP)Ru[C(COPh)2](py)
(TPP)Ru[C(CO2Et)2](MeOH)
Finally, we note that the axial RuNO and RuEt orientations
impart bond asymmetry to the equatorial Ru−N4(por) core, as
shown in Figure 1b,c. The asymmetry of the Ru−N4(por) core
for the minor component (Figure 1c) reproduces the
asymmetry of the porphyrin core in (OEP)Fe(NO)(C6H4F-
p);32 namely, the Ru−Npor distances in the direction of the
RuNO tilt are significantly longer than the Ru−Npor distances
that are directed away from the RuNO tilt. Such a clear
asymmetry was not, however, observed for the major
disordered component.
Cyclic Voltammetry and Infrared Spectroelectro-
chemistry. No electrochemical properties of any organo-
ruthenium nitrosyl porphyrins have been reported previously.
The redox behavior of (T(p-OMe)PP)Ru(NO)Me and (T(p-
OMe)PP)Ru(NO)Et in CH2Cl2 was examined by cyclic
voltammetry. These compounds are more soluble, and
exhibited greater thermal stability, than their (T(p-CF3)PP)-
Ru-containing analogues. The cyclic voltammograms of (T(p-
OMe)PP)Ru(NO)Me are shown in Figure 2. The (T(p-
OMe)PP)Ru(NO)Me compound undergoes two reversible
oxidations at E1°′ = +0.77 V and E2°′ = +1.30 V vs Ag/AgCl, to
generate the monocation [(T(p-OMe)PP)Ru(NO)Me]+ and
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Figure 3. Cyclic voltammograms of 1 mM (T(p-OMe)PP)Ru(NO)Et
in CH2Cl2 containing 0.1 M NBu4PF6 at a scan rate of 0.2 V/s at room
temperature: (a) cyclic voltammogram showing only the first
oxidation; (b) cyclic voltammogram scanned from +0.30 to +1.50 V;
(c) cyclic voltammogram scanned from −0.40 to −1.70 V. Potentials
are referenced to the Ag/AgCl couple.
Figure 4. Difference IR spectra showing the products from the first
oxidation of (a) (T(p-OMe)PP)Ru(NO)Me and (b) (T(p-OMe)-
PP)Ru(NO)Et in CH2Cl2 containing 0.1 M NBu4PF6, with the
potentials held at 0.86 V vs Ag/AgCl.
that reported for the (T(p-OMe)PP)Ru(NO)Cl compound at
E1°′ = +0.93 V,27 consistent with the presence of the strongly
electron donating alkyl ligands.
Ru(CO) compounds were performed; similar new bands at
∼1600 cm−1 were observed upon the first oxidations of these
compounds (data not shown). In addition, a 1600 cm−1 band in
the IR spectrum of the (TPP)FeCl compound shifts to a band
at 1584 cm−1 upon one-electron oxidation of the compound;47
this shift was assigned to absorption changes of phenyl modes
after the one-electron porphyrin-centered oxidation. Thus,
combined with the relatively small shift in νNO, we conclude
that the new band at 1599 cm−1 in Figure 4a is most likely due
to the phenyl mode absorption changes of the porphyrin
macrocycle upon the first oxidation.
The IR spectroelectrochemistry results for the first oxidation
of (T(p-OMe)PP)Ru(NO)Et are similar to those for (T(p-
OMe)PP)Ru(NO)Me (Figure 4b). The relatively small ΔνNO
value of 56 cm−1 is likewise attributed to a porphyrin-based first
oxidation for (T(p-OMe)PP)Ru(NO)Et. We note that other
Ru nitrosyl porphyrins such as (T(p-OMe)PP)Ru(NO)Cl27
and (OEP)Ru(NO)(OEt)29 also undergo porphyrin-based first
oxidations.
The reductions of (T(p-OMe)PP)Ru(NO)Me and (T(p-
OMe)PP)Ru(NO)Et were also examined by IR spectroelec-
trochemistry. As shown in Figure 5a, the reduction of (T(p-
OMe)PP)Ru(NO)Me results in loss of the initial νNO band at
1742 cm−1; however, no new νNO band was observed in the
spectral window available to us. The reduction of (T(p-
OMe)PP)Ru(NO)Et shows a similar disappearance of the
initial νNO band at ∼1723 cm−1 (Figure 5b). We attribute the
disappearance of the band at 1608 cm−1 to a change in
porphyrin ring absorptions during the reduction process.
Kaim and co-workers31 reported the electrochemical and
spectroelectrochemical properties of the [(por)Ru(NO)(L)]+
compounds (L = pyridine and substituted pyridines), and they
showed that the reductions of these compounds shifted the νNO
bands by ∼300 cm−1 to lower energy. Their DFT calculations
supported their conclusions that the reductions were largely
NO-centered. If such NO-centered reductions occurred for our
(por)Ru(NO)R compounds, the expected new bands in the
∼1440 cm−1 region would fall outside our spectroelectrochem-
ical spectral window. Due to the low chemical reversibility of
the reductions for the (por)Ru(NO)R compounds at lower
Further, the difference of 0.53 V in potentials between the
first (E1°′) and second (E2°′) oxidations for (T(p-OMe)PP)-
Ru(NO)Me is larger than that for the (T(p-OMe)PP)Ru(NO)
Cl compound (E2°′ − E1°′ = 0.42 V); the difference is due
primarily to the much lower first oxidation potential of the
methyl derivative (both compounds have similar E2°′ values).
We note that reversible first oxidations were also observed
previously for the [(por)Ru(NO)(H2O)]+ cations (por =
TPP,30,31 TTP,28 OEP31). However, and as noted in the
Introduction, the oxidation of the six-coordinate (OEP)RuPh2
compound is irreversible and involves a Ru-to-Npor migration of
the Ph group;17 in contrast, the first oxidation of the five-
coordinate (OEP)Ru(THF)Ph compound does not lead to the
migration of the Ph group.17
Infrared Spectroelectrochemistry. To investigate the
site(s) of redox events in the six-coordinate (T(p-OMe)PP)-
Ru(NO)R (R = Me, Et) compounds, we performed fiber-optic
infrared spectroelectrochemistry measurements of these com-
pounds at room temperature while holding the electrode
potential just beyond what was needed to achieve the first
oxidation or reduction.
The difference IR spectra reveal the changes that occur
(using the IR spectrum of the respective neutral (T(p-
OMe)PP)Ru(NO)R compound as the background) in the
spectra between 1550 and 2000 cm−1. The difference IR
spectrum for the first oxidation of (T(p-OMe)PP)Ru(NO)Me
is shown in Figure 4a, and shows the consumption of the νNO
band of the starting material at 1742 cm−1 and the formation of
a new compound displaying a new νNO band at 1794 cm−1. The
relatively small shift in νNO (ΔνNO = 52 cm−1) is indicative of a
porphyrin-centered first oxidation. The characteristic IR marker
bands for oxidized tetraarylporphyrins (π cation radicals) are in
the 1270−1295 cm−1 range.46 Unfortunately, strong absorp-
tions of the solvent system in this region do not allow for the
identification of these marker bands. A new band at 1599 cm−1
was also observed in the difference IR spectrum (Figure 4a). In
order to determine if this band was associated with the oxidized
porphyrin macrocycle, IR spectroelectrochemical experiments
using the non-nitrosyl H2T(p-OMe)PP and (T(p-OMe)PP)-
831
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Article
Instrumentation. Electrochemical measurements were performed
using a BAS CV-50W instrument (Bioanalytical Systems, West
Lafayette, IN) as described previously.27,29 The solutions for all
electrochemical experiments were deaerated by bubbling prepurified
nitrogen through the solution for 10 min before each set of
measurements, and then a nitrogen atmosphere was maintained
during the measurements.
Infrared spectra for the synthetic work were performed on a Bio-
Rad FT-155 FTIR spectrometer. For the spectroelectrochemical
experiments, the infrared spectra were recorded using a Bruker Vector
22 FTIR spectrometer equipped with a mid-IR fiber-optic dip probe
and liquid nitrogen cooled MCT detector (Remspec Corporation,
Sturbridge, MA). Proton NMR spectra were obtained on a Varian 300
MHz spectrometer and the signals referenced to the residual signal of
the solvent employed (CDCl3 at 7.26 ppm). All coupling constants are
in Hz.
Preparation of (T(p-X)PP)Ru(NO)R Compounds (X = OMe,
CF3; R = Me, Et). Method I. All compounds were synthesized at
room temperature under reduced laboratory lighting. The following
reaction is representative.
Figure 5. Difference IR spectra showing the results of the first
reduction of (a) (T(p-OMe)PP)Ru(NO)Me and (b) (T(p-OMe)-
PP)Ru(NO)Et in CH2Cl2 containing 0.1 M NBu4PF6, with the
potentials held at −1.65 and −1.63 V vs Ag/AgCl, respectively.
(T(p-OMe)PP)Ru(NO)Me. To a toluene solution (20 mL) of (T(p-
OMe)PP)Ru(NO)Cl (0.050 g, 0.056 mmol) was added AlMe3 (0.11
mL, 2.0 M in toluene, 0.22 mmol). The mixture was stirred under
reduced lighting for 30 min, during which time the color changed from
brownish green to dark green. The progress of the reaction was
monitored by IR spectroscopy; however, we found that removal of the
toluene solvent from the aliquots and redissolution of the product in
CH2Cl2 gave more defined IR spectra. We also monitored the
reactions by thin-layer chromatography using a 1/1 benzene/hexane
mixture. After the reaction was complete, the solvent was removed in
vacuo, the residue was dissolved in benzene (10 mL), and the solution
was transferred to the top of a neutral alumina column (1 × 15 cm)
prepared in hexane. Elution with a benzene/hexane (1/1) mixture
under nitrogen yielded a green band. The green band was collected
and taken to dryness in vacuo. The residue was dissolved in CHCl3/
hexane (5 mL, 4/1), and slow evaporation of the solvent mixture
under an inert atmosphere gave microcrystals of the product (0.023 g,
47% isolated yield). IR (CH2Cl2, cm−1): νNO 1742. IR (KBr, cm−1):
νNO 1735; also 1606 m, 1528 m, 1510 m, 1286 m, 1243 s, 1174 s, 1070
scan rates, it is more likely that eventual NO dissociation occurs
from these electron-rich complexes under our spectroelec-
trochemical conditions. This is consistent with the presence of
a long Ru−N(O) bond in the X-ray crystal structure of (T(p-
OMe)PP)Ru(NO)Et that might be expected to favor NO
dissociation.
In summary, we have prepared and determined the redox
behavior of a representative set of (por)Ru(NO)R compounds
that differ in tetraarylporphyrin and alkyl substitution. We show
by fiber-optic infrared spectroelectrochemistry that the first
oxidations of these compounds are porphyrin-based. In the
neutral precursors, we demonstrate a trans influence of the NO
ligand on the 1H NMR spectroscopic shifts of the alkyl groups,
an influence that results in a marked upfield shift of the proton
signals for alkyl H atoms close to Ru. In addition, we have
obtained the first X-ray crystal structure of an organometallic
nitrosyl porphyrin containing an alkyl ligand. The structure
reveals a cisoid arrangement of the trans ligands and a
significant bending of the axial ligands away from the porphyrin
normal.
1
w, 1015 s, 849 w, 809 m, 715 w, 609 w. H NMR (CDCl3, ppm): δ
8.89 (s, 8H, pyrrole-H of T(p-OMe)PP), 8.20 (d, 4H, J = 7 Hz, o-H of
T(p-OMe)PP, 8.11 (d, 4H, J = 7 Hz, o′-H of T(p-OMe)PP), 7.30 (t,
8H, J = 6 Hz, m-H of T(p-OMe)PP), 4.11 (s, 12H, OCH3 of T(p-
OMe)PP), −6.72 (s, 3H, CH3).
(T(p-CF3)PP)Ru(NO)Me. The (T(p-CF3)PP)Ru(NO)Me compound
was generated similarly (using 2-fold excess Al(Me)3) in 28% isolated
yield. Anal. Calcd for C49H27F12N5ORu·0.03CHCl3: C, 56.93; H, 2.63;
N, 6.77; Cl, 0.31. Found: C, 56.55; H, 2.98; N, 6.32; Cl, 0.28. IR
(CH2Cl2, cm−1): νNO 1748. IR (KBr, cm−1): νNO 1735; also 1616 m,
1404 m, 1324 s, 1168 m, 1129 s, 1068 m, 1013 s, 814 m, 797 m, 717 w.
1H NMR (CDCl3, ppm): δ 8.82 (s, 8H, pyrrole-H of T(p-CF3)PP),
8.41 (d, 4H, J = 7 Hz, o-H of T(p-CF3)PP), 8.33 (d, 4H, J = 7 Hz, o′-H
of T(p-CF3)PP), 8.06 (app t (overlapping d’s), 8H, J = 6, m/m′-H of
T(p-CF3)PP), −6.71 (s, 3H, CH3).
EXPERIMENTAL SECTION
■
All reactions were performed under an atmosphere of prepurified
nitrogen using standard Schlenk glassware and/or in an Innovative
Technology Labmaster 100 Drybox. Solutions for spectral studies were
also prepared under a nitrogen atmosphere. Solvents were distilled
from appropriate drying agents under nitrogen just prior to use:
CH2Cl2 (CaH2), CHCl3 (CaH2), THF (CaH2), hexane (CaH2),
benzene (Na), and toluene (Na).
(T(p-OMe)PP)Ru(NO)Et. The (T(p-OMe)PP)Ru(NO)Et com-
pound was generated similarly (using 2-fold excess Al(Et)3) in 56%
isolated yield. Anal. Calcd for C50H41N5O5Ru·0.1CHCl3: C, 66.50; H,
4.58; N, 7.74; Cl, 1.18. Found: C, 66.34; H, 4.53; N, 7.74; Cl, 1.18. IR
(CH2Cl2, cm−1): νNO 1723. IR (KBr, cm−1): νNO 1724; also 1606 m,
1527 w, 1510 m, 1349 m, 1245 s, 1174 s, 1070 w, 1015 s, 849 w, 808
m, 715 w, 609 w. 1H NMR (CDCl3, ppm): δ 8.87 (s, 8H, pyrrole-H of
T(p-OMe)PP), 8.19 (d, 4H, J = 7 Hz, o-H of T(p-OMe)PP), 8.10 (d,
4H, J = 7 Hz, o′-H of T(p-OMe)PP), 7.29 (t, 8H, m-H of T(p-
OMe)PP), 4.11 (s, 12H, OCH3 of T(p-OMe)PP), −4.19 (t, 3H, J = 8,
CH2CH3), −6.00 (q, 2H, J = 8, CH2CH3).
Chemicals. The compounds (T(p-OMe)PP)Ru(NO)Cl and (T(p-
CF3)PP)Ru(NO)Cl ((T(p-OMe)PP) = tetrakis(p-methoxyphenyl)-
porphyrinato dianion; (T(p-CF3 )PP)
= tetra(para-
trifluoromethylphenyl)porphyrinato dianion) were prepared from the
reaction of the (por)Ru(NO)(O-i-C5H11) precursors with BCl3 as
described previously.27 AlMe3 (2.0 M in toluene), AlEt3 (1.9 M in
toluene), MeMgBr (1.0 M in toluene/THF (3/1)), and EtMgBr (1.0
M in THF) were purchased from Aldrich Chemical Co. and used as
received. Ferrocene (Cp2Fe; Cp = η5-cyclopentadienyl anion, 98%)
was purchased from Aldrich Chemical Co. and sublimed prior to use.
NBu4PF6 (98%; Aldrich Chemical Co.) was recrystallized from hot
ethanol. Chloroform-d (99.8%) was obtained from Cambridge Isotope
Laboratories, purified by three freeze−pump−thaw cycles, and stored
over Linde 4 Å molecular sieves. Elemental analyses were performed
by Atlantic Microlab, Norcross, GA.
A suitable dark green prism-shaped crystal was grown by slow
evaporation of a CH2Cl2/hexane/benzene (2/1/trace) solution of
(T(p-OMe)PP)Ru(NO)Et at room temperature under an inert
atmosphere.
832
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Article
Method II. The following reaction is representative of the reactions
at room temperature.
measured in the range 1.68 < θ < 27.53° using ω oscillation frames.
The data were corrected for absorption by the semiempirical from
equivalents method. The data were merged to form a set of 9874
independent data with R(int) = 0.0508 and a coverage of 100.0%. The
monoclinic space group P21/n was determined by systematic absences
and statistical tests and verified by subsequent refinement. The
structure was solved by direct methods and using the SHELXTL
system and refined by full-matrix least-squares methods on F2. The
axial ligands, NO and ethyl, and hence the metal were disordered and
modeled in two orientations. The occupancies of the disordered atoms
refined to 0.554(15) and 0.446(15) for the unprimed and primed
atoms, respectively. Restraints on the positional and displacement
parameters of the disordered atoms were required. Also, the atoms in
the axial groups directly bonded to the metals were constrained to
have the same displacement parameters. Non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen atom
displacement parameters were set to 1.2× (1.5× for methyl) the
isotropic equivalent displacement parameters of the bonded atoms. A
total of 605 parameters were refined against 16 restraints and 9874
data to give wR2(F2) = 0.1178 and S = 1.056 for weights of w = 1/
[σ2(F2) + (0.0640P)2 + 4.4000P], where P = [Fo2 + 2Fc2]/3. The final
R1(F) value was 0.0442 for the 9145 observed (F > 4σ(F)) data. The
largest shift/su was 0.001 in the final refinement cycle. Thermal
ellipsoids for Figure 1 are drawn at the 35% probability level. CCDC
862081 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
(T(p-OMe)PP)Ru(NO)Me. To a THF solution (20 mL) of (T(p-
OMe)PP)Ru(NO)Cl (0.080 g, 0.090 mmol) was added MeMgBr
(0.16 mL, 1.4 M in toluene/THF (3/1), 0.22 mmol). The mixture was
stirred under reduced lighting for 1 h, and all solvent was removed in
vacuo. The resulting solid was dissolved in a minimum amount of
benzene (7 mL) and filtered through a neutral alumina column (1 ×
15 cm) with benzene as eluent. The green fraction was collected and
taken to dryness in vacuo. The residue was dissolved in a CHCl3/
hexane (5 mL, 4/1), and slow evaporation of the solvent mixture
under an inert atmosphere gave (T(p-OMe)PP)Ru(NO)Me (0.018 g,
0.020 mmol, 23% isolated yield).
(T(p-CF3)PP)Ru(NO)Me. The (T(p-CF3)PP)Ru(NO)Me compound
was generated similarly (2-fold excess using MeMgBr, 5 h reaction
time) in 18% isolated yield.
(T(p-CF3)PP)Ru(Me)2. The (T(p-CF3)PP)Ru(Me)2 compound was
generated similarly (using 9-fold excess MeMgBr, 1 h reaction time).
1H NMR (CDCl3, ppm): δ 8.25 (s, 8H, pyrrole-H of T(p-CF3)PP),
8.19 (d, 8H, J = 8.4 Hz, o-H of T(p-CF3)PP), 7.98 (d, 8H, J = 8 Hz, m-
H of T(p-CF3)PP), −2.71 (s, 6H, CH3).
(T(p-OMe)PP)Ru(NO)Et. The (T(p-OMe)PP)Ru(NO)Et com-
pound was generated similarly (using EtMgBr, 1.0 M in THF, 2-fold
excess, 80 min reaction time) in 26% isolated yield.
(T(p-CF3)PP)Ru(NO)Et. The (T(p-CF3)PP)Ru(NO)Et compound
was generated similarly (using EtMgBr, 1.0 M in THF, 2-fold excess, 1
h reaction time) in 17% isolated yield. IR (CH2Cl2, cm−1): νNO 1735.
IR (KBr, cm−1): νNO 1735; also 1617 m, 1405 m, 1324 s, 1169 s, 1128
1
s, 1069 s, 1013 s, 858 m, 814 m, 716 w, 684 w. H NMR (CDCl3,
ppm): δ 8.81 (s, 8H, pyrrole-H of T(p-CF3)PP), 8.42 (d, 4H, J = 8 Hz,
o-H of T(p-CF3)PP), 8.33 (d, 4H, J = 8 Hz, o′-H of T(p-CF3)PP), 8.06
(app t (overlapping d’s), 8H, J = 7, m/m′-H of T(p-CF3)PP), −4.17 (t,
3H, J = 8, CH2CH3), −5.96 (q, 2H, J = 8, CH2CH3).
Solid-State Structural Determination of (T(p-OMe)PP)Ru-
(NO)Et. Details of the crystal data and refinement are given in Table 6.
ASSOCIATED CONTENT
■
S
* Supporting Information
A CIF file giving crystallographic data for (T(p-OMe)PP)Ru-
(NO)Et. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Table 6. Crystal Data and Structure Refinement
■
a
Corresponding Author
*Tel: (405) 325-6401. E-mail: xunan@ou.edu (N.X.);
grichteraddo@ou.edu (G.B.R.-A.).
compd
(T(p-OMe)PP)Ru(NO)Et
C50H41N5O5Ru·CH2Cl2 (977.87)
103(2)
formula (fw)
T (K)
cryst syst
monoclinic
ACKNOWLEDGMENTS
space group
a (Å), α (deg)
b (Å), β (deg)
c (Å), γ (deg)
V (Å3), Z/Z′
D(calcd), g/cm3
P21/n
■
14.5153(15), 90
18.666(2), 95.930(2)
15.9805(17), 90
4306.6(8), 4/1
1.508
We are grateful to the National Science Foundation (CHE-
0911537 to G.B.R.-A.) for funding for this work. We thank
Professor Brian R. James (Canada) for very helpful discussions
regarding ruthenium porphyrin chemistry, Dr. Li Chen for
assistance with some of the earlier synthetic work and for
helpful discussions, and Dr. Masood Khan for assistance with
the X-ray diffraction data collection.
−1
abs coeff, mm
0.545
F(000)
2008
cryst size (mm)
0.36 × 0.14 × 0.12
1.68−27.53
51 341
θ range for data collecn (deg)
no. of rflns collected
no. of indep rflns
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