Models of molecular photocatalysts for water oxidation: Strategies for conjugating the Ru(bda) fragment (bda = 2,2′-bipyridine-6,6′-dicarboxylate) to porphyrin photosensitizers

Article abstract of DOI:10.1016/j.ica.2020.120143

Model dyads, in which the Ru(bda) water oxidation catalyst (WOC) is connected to a porphyrin, were prepared following two different modular strategies: i) the direct linkage approach, in which porphyrins bearing peripheral pyridyl rings are bound to the {

Full text of DOI:10.1016/j.ica.2020.120143

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Research paper
Models of molecular photocatalysts for water oxidation: strategies for conju‐
gating the Ru(bda) fragment (bda = 2,2'-bipyridine-6,6'-dicarboxylate) to por‐
phyrin photosensitizers
A. Vidal, F. Adamo, E. Iengo, E. Alessio
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DOI:
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https://doi.org/10.1016/j.ica.2020.120143
ICA 120143
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Please cite this article as: A. Vidal, F. Adamo, E. Iengo, E. Alessio, Models of molecular photocatalysts for water
oxidation: strategies for conjugating the Ru(bda) fragment (bda = 2,2'-bipyridine-6,6'-dicarboxylate) to porphyrin
photosensitizers, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.120143
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Models of molecular photocatalysts for water oxidation: strategies for conjugating the
Ru(bda) fragment (bda = 2,2'-bipyridine-6,6'-dicarboxylate) to porphyrin photosensitizers.
A. Vidal, F. Adamo, E. Iengo, E. Alessio*
Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri 1,
34127 Trieste, Italy.
Abstract
Model dyads, in which the Ru(bda) water oxidation catalyst (WOC) is connected to a porphyrin,
were prepared following two different modular strategies: i) the direct linkage approach, in which
porphyrins bearing peripheral pyridyl rings are bound to the {Ru(bda)} fragment as axial ligands,
and ii) the metal-containing ligand approach, in which symmetrical trans-[Ru(bda)(L)2] or
unsymmetrical trans-[Ru(bda)(L)(pic)] compounds (L = bifunctional pyridyl linker), are axially
bound to metalloporphyrins. Both synthetic strategies are modular and thus rather flexible in nature.
As proofs of principle, the following Ru(bda) – porphyrin dyads are described: trans-[Ru(bda)(4′-
MPyP)2] (2, 4′-MPyP = 5-(4′-pyridyl)-10,15,20-phenylporphyrin), and the sandwich compounds
[trans-Ru(bda)(4,4′-bpy)2{Ru(CO)(TPP)}2] (5) and [trans-Ru(bda)(4,4′-bpy)2(6Zn)]2 (7), where
6Zn is the porphyrin metallacycle [trans,cis,cis-RuCl2(CO)2(Zn·4′-cisDPyP)]2 (4′-cisDPyP = 5,10-
(4′-pyridyl)-15,20-(phenyl)-porphyrin.
Keywords
Ruthenium; bda; Water oxidation catalyst (WOC); Photocatalysis; Dyad; Porphyrin
1.
Introduction
Artificial photosynthesis is concerned with apparently simple but very challenging reactions: water
splitting to produce O2 and H2, and CO2 reduction to produce carbon-neutral fuels.
In the past few decades molecular catalysts for artificial photosynthesis have been the focus of
worldwide investigations. Even though they have not yet afforded reliable devices, and might turn
out unpractical in this regard, their easy-to-identify and tunable stereo-electronic structures (i.e.
active sites) are nevertheless of paramount value for mechanistic understanding [1].
Within artificial photosynthesis, water oxidation is a particularly demanding reaction because –
besides its high thermodynamic potential (E0 ca. 1.23 V) – has a high kinetic barrier [2]. Thus, it
requires efficient catalysts for accelerating the coupled transfer of four protons and four electrons.
Many examples of molecular water oxidation catalysts (WOCs), based on redox-active transition
metals capable of generating high-valent metal-oxo intermediates, have been described in the
literature [1 - 5]. The most promising and widely investigated WOCs are based on ruthenium
1

coordination complexes [1, 6 - 11] or iridium organometallic compounds [12], but also earth-
abundant metals such as Co, Fe, and Cu are being investigated [1, 13].
Depending on the WOC nature, two alternative pathways for oxygen formation are commonly
accepted: the WNA (water nucleophilic attack) and the I2M (interaction between two metal-oxo
intermediates) mechanism [8]. Currently, ruthenium complexes of the type trans-[Ru(bda)(L)2],
with bda = 2,2'-bipyridine-6,6'-dicarboxylate and L = an aromatic N-donor ligand, are among the
most efficient molecular WOCs [1,6]. Starting from the parent compound trans-[Ru(bda)(pic)2]
(pic = 4-picoline) first reported by Sun and coworkers in 2009 [14], structure-activity relationships
studies [15-27], mainly involving the nature of the axial ligands, led to complexes of this family
capable of achieving record TOF > 1000 s–1 and TON > 100.000 [1]. They typically use Ce4+ as
sacrificial oxidant and follow the bimolecular I2M mechanism that involves radical coupling of two
ruthenium-oxo species [27]. Dissociation of the axial ligands, leading to loss of catalytic activity, is
a limit for these catalysts. Increased stability was achieved by Würthner and co-workers by
inclusion of the Ru(bda) fragment into the trinuclear macrocycle [Ru(bda)bpb]3, where bpb is the
ditopic linker 1,4-bis(pyrid-3-il)benzene. Remarkably, this species catalyzes water oxidation with
excellent efficiency (TON = 7400 and TOF = 150 s–1) via the WNA pathway [28].
In view of sustainable energy production, light-driven catalytic water oxidation is obviously more
appealing than the chemical driven version, since sunlight is an inexhaustible energy source [1, 29-
31]. In homogeneous solution, the light-driven process – in addition to a WOC – requires the
presence of a photosensitizer (PS, e.g. [Ru(bpy)3]2+) capable of accessing to a long-lived excited
2–
state upon visible light illumination (PS*), and a sacrificial electron acceptor (SEA, e.g. S2O8 ).
The excited photosensitizer, PS*, is oxidatively quenced by the SEA to PS+, which is in turn
reduced by the WOC; the oxidized WOC+ will eventually recover electrons from water, generating
O2. The catalytic cycles of a typical three-component system are shown in Scheme 1.
2

WOC+
O2 + 4H+
2-
CH3
4SO4
2+
PS*
PS
N
Ru
N
N
N
N
N
N
N
N
O
O
Ru
N
O
O
CH3
PS+
2-
2S2O8
x4
2H2O
WOC
Scheme 1. Typical catalytic cycles in a three component molecular system for the photocatalytic
oxidation of water, with trans-[Ru(bda)(pic)2] as WOC, [Ru(bpy)3]2+ as photosensitizer (PS) and
Na2S2O8 as sacrificial electron acceptor.
As an alternative to the three-component system of Scheme 1, molecular assemblies (dyads) in
which the photosensitizer is covalently bound to a Ru or Ir WOC have been described by several
groups. Such systems would have the advantage that the electron transfer between the catalyst and
the photosensitizer can be intra- rather than intermolecular, and thus it is not limited by diffusional
encounter (Scheme 2). In most cases the photosensitizer in the dyad was a Ru(II)-polypyridyl
complex of the type [Ru(bpy)3]2+ [32-35], but also chromophores such as perylene-bisimides [36]
and porphyrins [37, 38] have been employed. In a more practical construct, oxidation of PS* can
performed by a photoanode rather than by a sacrificial electron acceptor [37-39].
e-
e-
WOC
e-
PS*
SEA-
O2 + 4H+
x4
SEA
2H2O
Scheme 2. A schematic dyad for photocatalytic water oxidation
In this work we focus on the preparation and characterization of model dyads in which the Ru(bda)
WOC is connected to a porphyrin. We selected porphyrins as photosensitizers because of their good
light-harvesting properties in the visible region and because their excited states and redox properties
can be tuned through metalation and/or peripheral functionalization. In addition, as we have shown
in the past, they can be used for the construction of metal-mediated supramolecular assemblies [40].
3

The model dyads were prepared following two different modular strategies: i) the direct linkage
approach, in which porphyrins bearing peripheral pyridyl rings are bound to the {Ru(bda)}
fragment as axial ligands, and ii) the metal-containing ligand approach, in which symmetrical trans-
[Ru(bda)(L)2] or unsymmetrical trans-[Ru(bda)(L)(pic)] compounds (L = bifunctional pyridyl
linker), are axially bound to metalloporphyrins.
Both synthetic strategies are modular and thus rather flexible in nature. As proofs of principle, the
following Ru(bda) – porphyrin dyads are described: trans-[Ru(bda)(4′-MPyP)2] (2, 4′-MPyP = 5-
(4′-pyridyl)-10,15,20-phenylporphyrin), and the sandwich compounds [trans-Ru(bda)(4,4′-
bpy)2{Ru(CO)(TPP)}2] (5) and [trans-Ru(bda)(4,4′-bpy)2(6Zn)]2 (7), where 6Zn is the porphyrin
metallacycle [trans,cis,cis-RuCl2(CO)2(Zn·4′-cisDPyP)]2 (4′-cisDPyP = 5,10-(4′-pyridyl)-15,20-
(phenyl)-porphyrin.
2.
Results and Discussion
The well-known precursor trans-[Ru(bda)(dmso-S)2] (1), obtained by treatment of cis-
[RuCl2(dmso)4] with bda in the presence of a base [33, 41], was used for the preparation of trans-
[Ru(bda)(L)2] and trans-[Ru(bda)(L)(L′)] compounds, where L and L′ are pyridyl ligands. An
improved preparation of bda with respect to that reported in the literature [42] is described in the
supporting information. Here we notice that in the 13C NMR spectrum of bda, the resonance of
C4,4′ (139.40 ppm) falls at much higher frequencies compared to C5,5′ (124.55 ppm) and C3,3′
(125.71 ppm) (supporting information). This feature is maintained also in the {Ru(bda)} fragment
and – through the HSQC spectra – allowed us to assign unambiguously the 1H NMR triplet of the
H4,4′ protons also when it overlaps with other resonances.
2.1. The “porphyrin as ligand” approach
Treatment of trans-[Ru(bda)(dmso-S)2] (1) with a slight excess of 4′-MPyP (5-(4′-pyridyl)-
10,15,20-(phenyl)-porphyrin) in a MeOH/CHCl3 mixture (microwave reactor), where both
compounds are soluble (Scheme 3), afforded – after chromatographic purification – the bis-
porphyrin complex trans-[Ru(bda)(4′-MPyP)2] (2) in moderately good yield (ca. 50%).
4

NH
N
N
HN
O
N
S
N
4
3
N
N
CH3OH/CHCl3 55:45
M.W. 100°C, 3h
5
O
N
N
N
N
O
O
2.5
HN
NH
O
Ru
S
Ru
O
O
O
O
N
6
2
5
3
O
2

o
m
1
4'MPyP
NH
N
p
N
HN
Scheme 3. Preparation of trans-[Ru(bda)(4′-MPyP)2] (2) from trans-[Ru(bda)(dmso-S)2] (1).
The 1H NMR spectrum is fully consistent with the structure of 2. Upon coordination the doublets of
the pyridyl protons are significantly shifted to lower frequencies since they fall in the shielding cone
of bda (Figure 1). Resonances were unambiguously assigned through 2D COSY and HSQC spectra
(supporting information). The most upfield of the three bda resonances is the triplet of the 4,4′
protons.
5

Hβ
Ho
Hm+p
H3,5
H2,6
Hm+p
Hβ
Ho
H2,6
H5 H3
H3,5
H4
9.1
9.0
8.9
8.8
8.7
8.6
8.5
8.4
δ (ppm)
8.3
8.2
8.1
8.0
7.9
7.8
7.7
7.6
Figure 1. 1H NMR spectra (aromatic region) of 4′-MPyP (top) and trans-[Ru(bda)(4′-MPyP)2] (2,
bottom). See Scheme 3 for labeling scheme. The arrows show the remarkable upfield shift of the
pyridyl proton resonances upon coordination.
The absorption maxima in the electronic absorption spectrum of 2 (both the Soret and Q bands) are
very similar to those of the free porphyrin (supporting information). Quite remarkably, unlike the
similar trans-[Ru(bda)(L)2] species (L = pyridyl ligand, see below), compound 2 is stable in
chloroform solution and no spontaneous formation of Ru(III) species occurs. Treatment of 2 with an
excess of zinc acetate in a CHCl3/MeOH mixture afforded the corresponding zincated adduct trans-
[Ru(bda)(Zn·4′-MPyP)2] (2Zn) in good yield. The reaction was monitored through UV-vis
spectroscopy, since the increased symmetry due to the insertion of zinc leads to the typical drop of
the number of Q bands from four to two (supporting information). Regretfully 2Zn turned out to be
much less soluble in chloroform than its parent compound, the peaks in the 1H NMR spectrum are
rather broad at room temperature, and those of bda in particular are broadened beyond detection.
These findings are consistent with the occurrence of relatively weak and labile intermolecular
interactions between Zn (axial binding) and the carboxylate oxygens of bda.
2.2. The “metal-containing ligand” approach
In this approach the {Ru(bda)} fragment is embedded either into the ditopic linker trans-
[Ru(bda)(L)2], or the mono-topic linker trans-[Ru(bda)(L)(pic)], where L is a bifunctional pyridyl
ligand, either symmetrical such as 4,4′-bipyridine (4,4′-bpy) or unsymmetrical such as 4-
pyridinecarboxylic acid (nicotinic acid). The binding of Ru(bda) linkers to metal centers may lead
6

to the formation of discrete or polymeric species (MOFs), depending on the nature and coordination
environment of the metal ions. A few examples of symmetrical Ru(bda) linkers, such as trans-
[Ru(bda)(4,4′-bpy)2], have been described already [17], but their reactivity with metal centers was
not pursued.
Consistent with the published procedure [17], treatment of 1 with a large excess of 4,4′-bpy
afforded – after chromatographic purification – the linear linker trans-[Ru(bda)(4,4′-bpy)2] (3) in
reasonably good yield (65%). We report here the full assignment of the proton NMR resonances of
compound 3 (Figure 2), thus far missing in the literature (2D NMR spectra in the supporting
information) [43]. Consistent with their asymmetric coordination, the two equivalent axial ligands
show a set of four equally intense doublets. The resonance of the protons adjacent to the bound N
atom is most affected by the shielding cone of bda and falls at 7.88 ppm, i.e. it is shifted to lower
frequency compared to the free ligand (8.72 ppm). Protons H4,4′ and H3,3′ of bda give two partially
overlapping multiplets centered at 7.94 ppm; however, in the HSQC spectrum (supporting
information) such resonance (4H) is correlated to two well-resolved 13C resonances (C3,3′ at 125.67
ppm and C4,4′ at 132.21 ppm).
Ha
N
d
c
Hd
Hc
b
a
H4
Hb
N
3
4
5
O
N
N
O
H3
Ru
N
O
O
H5
N
8.85 8.80 8.75 8.70 8.65 8.60 8.55 8.50 8.45 8.40 8.35 8.30 8.25 8.20 8.15 8.10 8.05 8.00 7.95 7.90 7.85 7.80 7.75 7.70 7.65 7.60
δ (ppm)
Figure 2. 1H NMR spectrum (500 MHz, DMSO-d6 + ascorbic acid) of trans-[Ru(bda)(4,4′-bpy)2]
(3). See the insert for labeling scheme.
With a similar synthetic procedure we prepared the corresponding pyrazine (pz) derivative trans-
[Ru(bda)(pz)2] (4), that was not reported before in the literature (Scheme 4).
7

N
N
O
N
S
N
CH3OH
N
O
N
N
O
O
20 eq.
+
Ru
O
O
Ru
N
O
O
MW 100°C, 2h
O
O
N
4
S
pz
1
N
Scheme 4. Preparation of trans-[Ru(bda)(pz)2] (4) from trans-[Ru(bda)(dmso-S)2] (1).
Compound 4, which is well soluble in MeOH and DMSO, was fully characterized by NMR
spectroscopy and mass spectrometry. Consistent with terminal binding, the two axial pyrazine
ligands give two equally intense doublets in the 1H NMR spectrum (Figure 3).
Hb
Ha
N
b
a
H4
H3
N
3
4
5
N
N
O
O
Ru
N
O
O
H5
N
8.90 8.85 8.80 8.75 8.70 8.65 8.60 8.55 8.50 8.45 8.40 8.35 8.30 8.25 8.20 8.15 8.10 8.05 8.00 7.95 7.90 7.85 7.80 7.75 7.70
δ (ppm)
Figure 3. 1H NMR spectrum (500 MHz, DMSO-d6 + ascorbic acid) of trans-[Ru(bda)(pz)2] (4). See
the insert for labeling scheme. The singlet at 8.67 ppm derives from some free pz.
Both 3 and 4 can be considered as linear metal-containing linkers. In fact, treatment of 3 with two
equivalents of [Ru(CO)(TPP)] (TPP = meso-tetraphenylporphyrin) afforded quantitatively the bis-
porphyrin sandwich adduct [trans-Ru(bda)(4,4′-bpy)2{Ru(CO)(TPP)}2] (5), in which the Ru(bda)
linker is axially bound to two {Ru(CO)(TPP)} units through the 4,4′-bpy ligands (Scheme 5).
8

O
C
N
Ru
N
N
N
N
N
N
O
C
N
CHCl3, r. T.
N
Ru
N
N
N
N
O
N
O
O
2
N
N
+
Ru
N
O
O
Ru
N
O
O
O
5
EtOH
N
N
N
Ru
N
3
N
N
C
O
Scheme 5. Preparation of [trans-Ru(bda)(4,4′-bpy)2{Ru(CO)(TPP)}2] (5).
The 1H NMR spectrum of 5 (Figure 4, and supporting information) is fully consistent with the
stoichiometry (according to integration) and geometry of this adduct.
9

Figure 4. 1H NMR spectrum (500 MHz, CDCl3) of [trans-Ru(bda)(4,4′-bpy)2{Ru(CO)(TPP)}2] (5).
See the insert for labeling scheme.
As previously observed in similar side-to-face adducts [44], by virtue of the strong and inert Ru–
N(4,4′-bpy) bonds, the compound is stable in solution and the resonances are sharp. Upon axial
coordination to ruthenium, the four doublets of each 4,4′-bpy, pairwise connected in the H-H COSY
spectrum (supporting information) are remarkably shifted to lower frequencies by the shielding
cone of Ru(TPP). In particular, the doublet of protons H3,5 in the pyridyl rings closest to the
porphyrin (d in Figure 4) is so much shifted upfield that falls under the broad resonance of residual
water. To be noted that, as evidenced by the HSQC spectrum, the 13C NMR resonances of 4,4′-bpy
are not significantly affected by the TPP shielding cone (supporting information). Another spectral
feature typical of similar sandwich compounds [44] is the splitting of the resonances of the ortho
and meta protons of TPP in two multiplets (8H each): each meso phenyl ring lays almost
perpendicular to the porphyrin plane and is in slow rotation (on the NMR time scale) about the
Cmeso–Cring bond. As a consequence, the protons facing the inside of the assembly – and thus closer
to the shielding cone of the opposite macrocycle – resonate at a lower frequency compared to those
facing outside. Consistently, each pair of multiplets correlates to the same 13C resonance in the
HSQC spectrum (supporting information).
10

In a similar fashion, compound 3 reacts with two equivalents of the 2+2 molecular square of zinc-
porphyrins [trans,cis,cis-RuCl2(CO)2(Zn·4′-cisDPyP)]2 (6Zn, 4′-cisDPyP = 5,10-(4′-pyridyl)-15,20-
(phenyl)-porphyrin) affording the corresponding sandwich adduct [trans-Ru(bda)(4,4′-bpy)2(6Zn)]2
(7, Scheme 6). The metallacycle 6Zn can be considered as a flat panel with two axial binding sites
separated by ca. 14.0 Å [45], and has been exploited by us in the past for the preparation of several
higher-order multiporphyrin 3D assemblies [40]. Also in this case, the unambiguous formation of
the sandwich adduct is evidenced by the typical pattern of the four upfield-shifted doublets of the
4,4′-bpy ligands, best seen in the H-H COSY NMR spectrum (Figure 5, and supporting
information). As observed in similar systems, due to the relative lability of the axial Zn–pyridyl
bond, the equilibrium between the molecular sandwich and the free components leads to a slight
broadening of the resonances.
Cl
CO
N
OC
N
2
m
Ru
Cl
3
p
o
6
5
N
Zn
N
N
Zn
N

N
N
N
N
Cl
N
N
N
N
N
N
Ru
Cl
d
OC
CO
c
Cl
OC
N
CO
N
b
a
Ru
Cl
N
N
3
4
CHCl3
N
Zn
N
N
Zn
N
5
N
N
N
N
N
N
O
O
N
+
N
N
O
O
O
O
N
Ru
N
Ru
N
Ru
N
O
O
O
O
O
O
7
Cl
N
N
Ru
OC
CO
Cl
Cl
CO
N
OC
N
N
N
6Zn
Ru
Cl
N
3
N
Zn
N
N
Zn
N
N
N
N
N
Cl
N
N
Ru
Cl
OC
CO
Scheme 6. Preparation of [trans-Ru(bda)(4,4′-bpy)2(6Zn)]2 (7).
Consistent with what observed by us in similar supramolecular systems [40], the electronic
absorption spectra of the side-to-face adducts 5 and 7 are the sum of those of the components, i.e. 3
+ [Ru(CO)(TPP)] for 5 [46], and 3 + 6Zn for 7 [40a, 45].
11

Figure 5. H-H COSY NMR spectrum (500 MHz, CDCl3, room temperature) of [trans-
Ru(bda)(4,4′-bpy)2(6Zn)]2 (7). See Scheme 6 for labeling scheme.
Due to the very low solubility of the bis-pyrazine derivative trans-[Ru(bda)(pz)2] (4) in chloroform
or DCM, the preparation of the corresponding sandwich compounds with either [Ru(CO)(TPP)] or
6Zn was not pursued.
Finally, we also demonstrated that it is possible to include the {Ru(bda)} fragment in a single-sided
metal-containing linker. This requires the preparation of unsymmetrical complexes of the type
trans-[Ru(bda)(L)(L′)], in which L is a bifunctional pyridyl ligand, whereas L′ is not. Specifically,
we found that the unsymmetrical complex trans-[Ru(bda)(pic)(4-pca)] (8), where 4-pca is 4-
pyridinecarboxylic acid (nicotinic acid), can be prepared either through a stepwise route, with
isolation of the intermediate trans-[Ru(bda)(pic)(dmso-S)], or through a one-pot procedure
(Scheme 7). Since both synthetic approaches require the chromatographic purification of the
product and afford comparable yields, only the faster one-pot procedure is described here.
12

CH3
N
O
OH
CH3
N
O
S
CH3
N
1 eq.
1 eq.
N
O
N
N
N
O
N
O
O
Ru
O
O
Ru
S
O
O
O
O
CH3OH anhydrous, Ar
r.T. 24 h
CH3OH anhydrous, Ar
r.T. 24 h
S
1
N
N
O
O
Ru
O
S
O
O
O
OH
CH3
N
N
N
O
N
O
O
Ru
CH3OH anhydrous, Ar
r.T. 24 h
O
O
+
1
1
+
N
S
O
OH
1
8
Scheme 7. The two synthetic routes for the preparation of trans-[Ru(bda)(pic)(4-pca)] (8).
Treatment of precursor 1 with one equiv each of pic and 4-pca at room temperature afforded a dark-
brown mixture from which, after column chromatography, we obtained a pure sample of 8, in
addition to ca. equivalent amounts of symmetrical complexes trans-[Ru(bda)(pic)2] (9) – the well-
known WOC [14] – and trans-[Ru(bda)(4-pca)2] (10). The 1H NMR resonances of 8 (Figure 6)
were assigned through 2D spectra and by comparison with the spectra of 9 and 10 (supporting
information).
13

Figure 6. 1H NMR spectrum (500 MHz, CD3OD + ascorbic acid) of trans-[Ru(bda)(pic)(4-pca)]
(8). See insert for labeling scheme.
To be noted that this Ru(bda) linker – as well as the symmetrical 10 – might be a suitable axial
ligand for oxophilic metallo-porphyrins, such as tin- and aluminum-porphyrins [47], through the
free carboxylic group of nicotinic acid.
3.
Conclusions
As proofs of principle we demonstrated that the {Ru(bda)} fragment can be conjugated to
porphyrins through two different modular approaches. The obtained compounds – bearing a
chromophore and a WOC – are good models of catalysts for the photoinduced water oxidation.
First we prepared and fully characterized trans-[Ru(bda)(4′-MPyP)2] (2), demonstrating that
pyridylporphyrins are suitable axial ligand for Ru(bda). This model complex, that was also found to
be remarkably stable in chloroform solution, is quite flexible in nature: in fact, the meso phenyl
groups can be rather easily functionalized and metal ions can be inserted into the porphyrin, thus
allowing to tune the photophysical and redox parameters of the adduct. Compound 2 can be seen
also as a promising model towards the preparation of the 4+4 metallacycle [{Ru(bda)(4′-
cisDPyP)}4] (11), where 4′-cisDPyP is the 90° angular building block 5,10-(4′-pyridyl)-15,20-
(phenyl)-porphyrin (Figure 7). In principle, compound 11 – in addition to including the
14

photosensitizer – would be expected to benefit from the extra-stability of the metallacyclic structure
similarly to [Ru(bda)bpb]3 [28]. We are currently investigating this possibility.
O
O
NH
NH
N
N
NH
N
N
N
N
N
Ru
N
HN
N
HN
N
HN
O
O
N
N
N
?
O
O
N
N
N
N
Ru
O
O
O
O
O
Ru
O
Ru
N
N
O
O
O
O
N
N
N
O
O
NH
N
NH
N
N
NH
N
N
N
Ru
N
N
N
N
HN
HN
HN
O
O
11
2
Figure 7. The hypothetical 4+4 metallacycle [{Ru(bda)(4′-cisDPyP)}4] (11).
In the second approach we prepared mono- and ditopic linkers that contain the {Ru(dba)} fragment
and demonstrated that they can be axially connected to metalloporphyrins. In particular, the ditopic
linear linker trans-[Ru(bda)(4,4′-bpy)2] (3) was exploited for making the sandwich bis-porphyrin
compound [trans-Ru(bda)(4,4′-bpy)2{Ru(CO)(TPP)}2] (5) as well as the tetra-porphyrin adduct
[trans-Ru(bda)(4,4′-bpy)2(6Zn)]2 (7), where 6Zn is the flat 2+2 porphyrin metallacycle [trans,cis-
RuCl2(CO)2(Zn·4′-cisDPyP)]2 (Figure 8).
15

Cl
O
C
O
C
CO
O
C
OC
Ru
Cl
N
N
N
Zn
N
N
Ru
N
N
Zn
N
N
N
N
N
N
N
Cl
N
N
N
N
Ru
N
OC
CO
Cl
N
N
N
N
N
N
N
N
N
O
O
O
O
O
O
7
Ru
N
Ru
N
Ru
N
O
O
O
O
O
O
5
Cl
CO
OC
Ru
Cl
N
N
N
N
N
N
Zn
N
N
Ru
N
N
Zn
N
N
N
N
N
N
N
Cl
N
N
C
O
C
O
C
O
Ru
OC
CO
Cl
Figure 8. The side-to-face Ru(bda) – metalloporphyrin adducts [trans-Ru(bda)(4,4′-
bpy)2{Ru(CO)(TPP)}2] (5) and [trans-Ru(bda)(4,4′-bpy)2(6Zn)]2 (7).
This is an explorative work, dealing exclusively with synthetic aspects. In the near future these dyad
compounds – as well as others that are being prepared with the same modular approaches – will be
tested for their catalytic and photocatalytic activity in the water oxidation reaction.
4.
Experimental section
4.1. Materials
The Ru(II) compounds cis-[RuCl2(dmso)4] [48], trans-[Ru(bda)(dmso-S)2] (1) [33, 41], trans-
[Ru(bda)(4,4′-bpy)2] [17], the porphyrins 4′MPyP, and 4′-cisDPyP [49], [Ru(CO)(TPP)] [46], and
the 2+2 metallacycle [trans,cis-RuCl2(CO)2(Zn·4′-cisDPyP)]2 (6Zn) [40a] were synthesized and
purified as previously reported by us and by others. The improved preparation of bda is described
in the supporting information. Anhydrous MeOH and other solvents were purchased from Aldrich.
4.2. Instrumental methods
Mono- and bi-dimensional (1H-1H COSY, 1H-13C HSQC) NMR spectra were recorded at room
temperature on a Varian 400 or 500 spectrometer (1H: 400 or 500 MHz, 13C{1H}: 100.5 or 125.7
MHz). 1H and 13C{1H} chemical shifts were referenced to the peak of residual non-deuterated
solvent (δ = 7.26 and 77.16 for CDCl3, 2.50 and 39.52 for DMSO-d6). Carbon resonances were
assigned through the HSQC spectra. ESI mass spectra were collected in the positive and negative
mode on a Perkin-Elmer APII spectrometer at 5600 eV. The UV-vis spectra were obtained on an
Agilent Cary 60 spectrophotometer, using 1.0 cm path-length quartz cuvettes (3.0 mL). A CEM
Discover microwave reactor was used for the microwave-assisted reactions performed in 10 mL
16

vessels. Elemental analyses were performed in the Department of Chemistry of the University of
Bologna (Italy).
4.3. Synthesis
trans-[Ru(bda)(4′-MPyP)2] (2). A 26.0 mg amount of trans-[Ru(bda)(dmso-S)2] (1) (0.052
mmol) was partially dissolved in 4.5 mL of a degassed 55:45 mixture of anhydrous CH3OH and
CHCl3. 2.5 equiv. of 4′-MPyP (80.0 mg, 0.13 mmol) were added and the mixture was heated in a
microwave reactor at 100 °C for 3h. The solvent was removed under reduced pressure and the
residual was subjected to column chromatography on silica gel eluted with CHCl3 for the first
fraction containing the unreacted 4′-MPyP and then with CHCl3:EtOH 99:1 for the second fraction
containing the desired complex 2.Yield: 48% (39.0 mg, 0.025 mmol). Analysis for C98H64N12O4Ru
(MW = 1574,74): Calcd. C 74.75%, H 4.10%, N 10.67%; Found: C 73.96%, H 4.20%, N 10.49%.
1H NMR (500 MHz, CDCl3) δ, ppm: 8.70-8.87 (m, 16Hβ), 8.45 (d, 2H5bda), 8.37-8.40 (m, 6H:
2H3bda + 4H2,6), 8.19-8.21 (m, 12Ho), 8.00 (d, 4H3,5), 7.92 (t, 2H4bda), 7.73-7.82 (m, 18H: 12Hm +
6Hp), –2.85 (s, 4HNH). UV-vis in CHCl3 (λmax, nm, εrel = M–1 cm–1): 422 (Soret, 100), 518 (9.26),
555 (6.30), 592 (4.07), 650 (3.30).
[Ru(bda)(Zn·4′-MPyP)2] (2Zn). A 600 μL CH3OH solution of Zn(CH3COO)2·2H2O (7.5 mg,
0.034 mmol) was added to the deep purple chloroform solution (23.0 mL) of 2 (15.0 mg, 0.0095
mmol) and the system was allowed to react at room temperature in the dark for 16 hours under
magnetic stirring. The reaction was monitored by TLC and UV-Vis spectroscopy. Afterwards the
solvent was removed under reduced pressure, the solid was re-suspended in 10 mL of CH3OH and
filtered. The purple solid was washed with MeOH and Et2O and dried under vacuum. Yield 75%
(12.0 mg, 0.0071 mmol). Analysis for C98H60N12O4RuZn2 (MW = 1701.47): Calcd. C 69.18%, H
3.55%, N 9.88%; Found: C 68.46%, H 3.40%, N 9.47%. UV-vis in CHCl3 (λmax, nm, εrel = M–1 cm–
1): 427 (Soret, 100), 556 (6.93), 602 (2.90).
trans-[Ru(bda)(pz)2] (4). A 25.0 mg amount of trans-[Ru(bda)(dmso-S)2] (1) (0.050 mmol) was
partially dissolved in 5.0 mL of degassed anhydrous CH3OH. 20 equiv. of pyrazine (80.0 mg, 0.13
mmol) were added, and the mixture was heated in a microwave reactor at 100 °C for 2h. A brown
precipitate found in the vial, probably oligomeric species, was removed by filtration and the solvent
of the mother liquor was removed under reduced pressure. The obtained residue was purified by
means of flash chromatography on alumina using CHCl3:MeOH mixtures as eluent, starting from
95:5 to 90:10. The first red fraction was collected and the solvent removed obtaining 4 as a red
solid. Yield: 48% (12.0 mg, 0.024 mmol). Analysis for C20H14N6O4Ru (MW = 503,44): Calcd. C
1
47.72%, H 2.80%, N 16.69%; Found: C 47.36%, H 2.90%, N 16.49%. H NMR (500 MHz,
17

DMSO-d6) δ, ppm: 8.76 (d, 2H5), 8.42 (d, 4Ha), 7.94-8.01 (m, 4H: 2H4 + 2H3), 7.91 (d, 4Hb). ESI-
MS (m/z): 505.0 [M+H+] (calculated 505.02), 526.9 [M+Na+] (calculated 527.00). UV-Vis in
CH3OH (λmax, nm, εrel = M–1 cm–1): 205 (100), 255 (65.09), 299 (74.53), 434 (30.19).
[trans-Ru(bda)(4,4′-bpy)2{Ru(CO)(TPP)}2] (5). This reaction was performed on a small scale
in the NMR tube exclusively. A 1.2 mg amount of trans-[Ru(bda)(4,4′-bpy)2] (3) (0.0018 mmol)
was partially dissolved in 600 μL of CDCl3 in an NMR tube. Small amounts of [Ru(CO)(TPP)]
were added to the suspension and the reaction was monitored by 1H NMR spectroscopy after each
addition. The mono-coordinated species [trans-Ru(bda)(4,4′-bpy)2{Ru(CO)(TPP)}] is characterized
by two of equally intense upfield-shifted 4,4′-bpy doublets (coupled in the H-H COSY spectrum) at
6.21 and 5.25 ppm (attributed to protons b and c, respectively, in Figure 3). The addition of
[Ru(CO)(TPP)] was continued until they were completely replaced by two new doublets at 6.10 and
5.17 ppm, attributed to the same protons in the sandwich assembly 5. 1H NMR (500 MHz, CDCl3)
δ, ppm: 8.57 (s, 16Hβ), 8.17 (m, 8Ho), 7.96 (d, 8Ho′), 7.88 (d, 2H5), 7.79 (d, 2H3), 7.61-7.68 (m,
24H: 8Hm + 8Hm′ + 8Hp), 7.47 (t, 2H4), 7.29 (4Ha), 6.10 (d, 4Hb), 3.92 (d, 4Hc), 1.57 (4Hd
overlapped by the peak of residual water).
[trans-Ru(bda)(4,4′-bpy)2(6Zn)]2 (7). This reaction was performed on a small scale in the NMR
tube exclusively. A 1.53 mg amount of [Ru(bda)(4,4′-bpy)2] (3) (0.0023 mmol) was dissolved in
600 μL of CDCl3. 1 equiv. of 6Zn (4.24 mg, 0.0023 mmol) was added to the solution in three
aliquots and the reaction was monitored by 1H NMR spectroscopy after every addition. As observed
before in similar sandwich systems [39], this is an all-or-nothing reaction and no intermediate is
observed. 1H NMR (500 MHz, CDCl3) δ 9.78 (br d, 8 H2,6), 9.11–8.70 (m, 32Hβ), 8.55 (d, 8H3,5),
8.19, (m, 8Ho′) 8.00 (d, 4H5), 7.88–7.51 (m, Ha+Ho+Hm+Hp+H4+H5), 6.79 (br d, 8Hb), 5.47 (br d,
8Hc), 2.26 (br d, 8Hd).
trans-[Ru(bda)(4-pic)(4-pca)] (8). A 200 mg amount of trans-[Ru(bda)(dmso-S)2] (5) (0.40
mmol) was partially dissolved in 40.0 mL of degassed anhydrous CH3OH. 1 equiv. of 4-pic (39 μL,
0.40 mmol) was added, and the mixture and stirred at 40 °C under an inert atmosphere for 5
minutes. Afterwards 1 equiv. of 4-pca (49.0 mg, 0.40 mmol) was added and stirred continued at r.t.
for other 24 hours. The solvent was then removed under reduced pressure and the solid was
subjected to column chromatography on silica gel eluted with CHCl3:MeOH mixtures starting from
2:1 to 1:2. Three fraction were collected, the first contained trans-[Ru(bda)(4-pic)2] (9), the second
the desired product 8, and the third contained trans-[Ru(bda)(4-pca)2] (10). After removal of the
solvent from the second fraction, the product was obtained as a red solid. Yield: 28% (62 mg, 0.11
mmol). Analysis for C24H18N4O6Ru (MW = 559,50): Calcd. C 51.52%, H 3.24%, N 10.01%;
1
Found: C 52.03%, H 3.42%, N 9.88%. H NMR ( 500 MHz, CD3OD + ascorbic acid) δ, ppm: 8.62
18

(d, 2H5), 8.04 (d, 2H3), 7.92 (m, 4H: 2H4 + 2Hc), 7.67 (d, 2Ha), 7.59 (d, 2Hd), 7.06 (d, 2Hb), 2.27 (s,
3CH3). ESI-MS(m/z): 582.0 [M+– H++Na+] (calculated 582.03).
5.
Acknowledgements
Financial support from the University of Trieste (FRA2018) is gratefully acknowledged. We wish
to thank BASF Italia Srl for a donation of hydrated ruthenium chloride.
6.
Supporting information
An improved preparation of bda, additional NMR, UV-vis and ESI-MS spectral data for bda, and
compounds 2, 2Zn, 3 – 5, 7 – 10.
7.
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assembled multiporphyrin cages with tunable shape Chem. Commun. 48 (2012) 5012-5015;
DOI: 10.1039/C2CC31420G; d) E. Iengo, P. Cavigli, D. Milano, P. Tecilla Metal mediated
self-assembled porphyrin metallacycles: Synthesis and multipurpose applications Inorg.
Chim. Acta 417 (2014) 59-78; DOI: 10.1016/j.ica.2014.02.018
41. F. Li, B. Zhang, X. Li, Y. Jiang, L. Chen, Y. Li, L. Sun Highly Efficient Oxidation of Water
by a Molecular Catalyst Immobilized on Carbon Nanotubes Angew. Chem. Int. Ed. 50 (2011)
12276-12279. DOI: 10.1002/anie.201105044.
42. A. Jalba, G. Levitre, H. Keipour, S. Lauzon, T. Ollevier New chiral bis(oxazolinyl)bipyridine
ligands and application in the iron catalyzed asymmetric hydrosilylation of ketones French-
Ukrainian J. Chem. 3 (2015) 44-53. DOI: 10.17721/fujcV3I2P44-53.
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43. In agreement with the literature reports, we found that compounds of this type are not very
stable in solution (2 is an exception), where they undergo spontaneous partial oxidation to
Ru(III), evidenced by the color change of the solutions from dark brown to deep green. To
avoid the broadening of the NMR resonances due to the formation of paramagnetic Ru(III)
species, NMR spectra are typically recorded after the addition of small amounts of ascorbic
acid as reducing agent.
44. a) E. Iengo, E. Zangrando, S. Mestroni, G. Fronzoni, M. Stener, E Alessio Complexed
bridging ligands: oxorhenium(V) compounds with mono-coordinated pyrazine or pyrimidine
as possible building-blocks for the construction of polynuclear architectures J. Chem. Soc.,
Dalton Trans. (2001) 1338-1346; DOI: 10.1039/b009295i; b) A. Prodi, C. Chiorboli, F.
Scandola, E. Iengo, E. Alessio, R. Dobrawa, F. Würthner Wavelength-Dependent Electron
and Energy Transfer Pathways in a Side-to-Face Ruthenium Porphyrin/Perylene Bisimide
Assembly J. Am. Chem. Soc. 127 (2005) 1454-1462; DOI: 10.1021/ja045379u; c) A. Amati,
M. Natali, M. T. Indelli, E. Iengo, F. Würthner Photoinduced Energy‐ and Electron‐Transfer
Processes in a Side‐to‐Face RuII‐Porphyrin/Perylene‐bisimide Array ChemPhysChem 20
(2019) 2195-2203. DOI: 10.1002/cphc.201900611.
45. E. Iengo, E. Zangrando, M. Bellini, E. Alessio, A. Prodi, C. Chiorboli, F. Scandola
Pyridylporphyrin metallacycles with a slipped cofacial geometry: spectroscopic, X-ray and
photophysical characterization Inorg. Chem. 44 (2005) 9752-9762. DOI: 10.1021/ic051210l.
46. A. Vidal, F. Battistin, E. Iengo, B. Milani, E. Alessio The insertion of ruthenium into
porphyrins revisited and improved: proof of concept results with a Ru(II) mono-carbonyl
compound, and the spectacular effect of propionic acid. Eur. J. Inorg. Chem. 2019, 2883-
2890. DOI: 10.1002/ejic.201900428.
47. a) P. Cavigli, G. Balducci, E. Zangrando, N. Demitri, A. Amati, M. T. Indelli, E. Iengo
Structural and photophysical characterization of a tin(IV) porphyrin–rhenium(I)(diimine)
conjugate Inorg. Chim. Acta 439 (2016) 61-68. DOI: 10.1016/j.ica.2015.10.001; b) A. Amati,
P. Cavigli, A. Kahnt, M. T. Indelli, E. Iengo Self-Assembled Ruthenium(II)Porphyrin-
Aluminium(III)Porphyrin-Fullerene Triad for Long-Lived Photoinduced Charge Separation J.
Phys. Chem. A 121 (2017) 4242−4252. DOI: 10.1021/acs.jpca.7b02973; c) M. Natali, A.
Amati, N. Demitri, E. Iengo Formation of a long-lived radical pair in a Sn(IV) porphyrin–di(l-
tyrosinato) conjugate driven by proton-coupled electron-transfer Chem. Commun. 54 (2018)
6148-6152. DOI: 10.1039/C8CC03441A; d) A. Amati, P. Cavigli, N. Demitri, M. Natali, M.
T. Indelli, E. Iengo Sn(IV) Multiporphyrin Arrays as Tunable Photoactive Systems Inorg.
Chem. 58 (2019) 4399-4411. DOI: 10.1021/acs.inorgchem.8b03542; e) M. Natali, A. Amati,
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S. Merchiori, B. Ventura, E. Iengo Photoinduced Proton-Coupled Electron Transfer in
Supramolecular SnIV Di(l-tyrosinato) Porphyrin Conjugates J. Phys. Chem. C 124 (2020)
8514–8525. DOI: 10.1021/acs.jpcc.0c00224.
48. I. Bratsos, E. Alessio Ruthenium(II)-chlorido complexes of dimethylsulfoxide Inorg. Synth.
35 (2010) 148-152. DOI: 10.1002/9780470651568.ch8.
49. E. Alessio, M. Macchi, S. L. Heath, L. G. Marzilli Ordered supramolecular porphyrin arrays
from a building block approach utilizing pyridylporphyrins and peripheral ruthenium
complexes and identification of a new type of mixed-metal building block Inorg. Chem. 36
(1997) 5614-5623. DOI: 10.1021/ic970340u.
Models of molecular photocatalysts for water oxidation: strategies for conjugating the
Ru(bda) fragment (bda = 2,2'-bipyridine-6,6'-dicarboxylate) to porphyrin photosensitizers.
A. Vidal, F. Adamo, E. Iengo, E. Alessio*
Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri 1,
34127 Trieste, Italy.
Highlights
Model dyads in which the Ru(bda) WOC is connected to a porphyrin were prepared and
characterized.
The model dyads were prepared following two different modular strategies.
In the direct linkage approach pyridylporphyrins are bound to the {Ru(bda)} fragment as axial
ligands
In the metal-containing ligand approach trans-[Ru(bda)(L)2] compounds are axially bound to
metalloporphyrins.
No statement to be done
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