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Ru-Zn Heteropolynuclear Complexes Containing a Dinucleating Bridging Ligand:
Synthesis, Structure and Isomerism.
Lorenzo Mognon,a Jordi Benet-Buchholz,a S. M. Wahidur Rahaman,a Carles Boa,* and
Antoni Llobeta,b,*

a Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans 16,
43007 Tarragona, Spain
b Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del Vallès,
08193 Barcelona, Spain

1

Abstract
Mononuclear complexes in- an out-[Ru(Cl)(trpy)(Hbpp)]+ (in-0, out-0; Hbpp is 2,2'(1H-pyrazole-3,5-diyl)dipyridine and trpy is 2,2’:6’,2”-terpyridine) are used as starting
materials for the preparation of Ru-Zn hetero dinuclear out-{[Ru(Cl)(trpy)][ZnCl2](µbpp)} (out-2) and heterotrinuclear in,in- and out,out-{[Ru(Cl)(trpy)]2(µ-[Zn(bpp)2])}2+
(in-3, out-3) constitutional isomers. Further the substitution of the Cl ligand from the
former complexes leads to Ru-aqua out,out-{[Ru(trpy)(H2O)]2(µ-[Zn(bpp)2])}4+ (out-4)
and the oxo-bridged Ru-O-Ru complex in,in-{[RuIII(trpy)]2(µ-[Zn(bpp)2(H2O)]µ-(O)}4+
(in-5). All complexes are thoroughly characterized by the usual analytic techniques as
well as by spectroscopy by means of UV-vis, MS and when diamagnetic by NMR. CV
and DPV are used to extract electrochemical information and monocrystal X-ray
diffraction to characterize complex out-2, in-3, out-3 and in-5 in the solid state.
Complex out-3 photochemically isomerizes towards in-3, as can be observed by NMR
spectroscopy and rationalized by Amsterdam Density Functional calculations.

Keywords: synthesis, coordination complexes, constitutional isomerization, transition
metal complexes, light induced isomerization

2

Introduction
Polynuclear transition metal complexes are a very interesting class of complexes whose
properties can be tuned by the type of the bridging ligands bonded to the transition
metals. The bridging ligand thus dictates the degree of electronic communication
between two or more transition metals. In addition the structure of the ligand can
generate a particular spatial disposition of the metal centers that can favor a cooperative
effect for a given reaction.1 Taking into account the relatively large number of transition
metals and the huge variety of bridging ligands, an immense number of complexes can
be potentially obtained with a large diversity of properties.

Polynuclear transition metal complexes have found interesting application in fields
where cooperation between metal centers is important such as: magneto-chemistry,2
optical properties,3 reactivity,1d catalysis4,5 etc. Nature also benefits from metal
cooperation in a number of metalloproteins whose functions range from small molecule
activation (N2, O2, and H2)6 to hydrolysis.6d, 7 Low-molecular-weight models have been
proven crucial in some cases for the understanding of the spectroscopic and structural
properties of the active site of these proteins.7g, 8

A fundamental characteristic of coordination complexes is the multiple different type of
isomers that they can display. Regarding Ru complexes containing the Hbpp ligand
(Hbpp is 2,2'-(1H-pyrazole-3,5-diyl)dipyridine) we have previously described examples
where Ru-dmso linkage isomerism takes place induced by a change in oxidation state.9
We have also described the presence of axial chirality due to the through space
supramolecular interaction and different ligand space arrangement in Ru-Hbpp
complexes containing the MeCN ligand.10 In addition we have also described
constitutional isomerism in mononuclear Ru-Hbpp complexes containing the tridentate
meridional 2,2’:6’,2”-terpyridine (trpy) ligand and Cl or H2O ligands in the case of inan out-[Ru(Cl)(trpy)(Hbpp)]+ (in-0, out-0) and in- an out-[Ru(trpy)(Hbpp)(H2O)]2+ (in1, out-1).11

The non-symmetric nature of the Hbpp ligand allows to easily prepare mononuclear
complexes and use the latter as starting materials for the preparation of the

3

corresponding multinuclear complexes. On the other hand symmetric ligands generally
lead to homodinuclear complexes and sometimes to even coordination oligomers or
polymers12 especially with first row transition metal complexes.
Here on we present the synthesis and reactivity of new hetero di and trinuclear
complexes containing the bpp- ligand, using the corresponding mononuclear Ru-Hbpp
complexes, in-0 and out-0, as synthetic intermediates.

Experimental section
Preparations. The starting complexes out-[Ru(Cl)(Hbpp)(trpy)](PF6), out-0 and in[Ru(Cl)(Hbpp)(trpy)](PF6), in-0 and their respective aqua complexes out[Ru(Hbpp)(trpy)(H2O)](ClO4)2, out-1 and in-[Ru(Hbpp)(trpy)(H2O)](ClO4)2, in-1 were
prepared as described in the literature.11a

out-{[Ru(Cl)(trpy)][ZnCl2](µ-bpp)}·H2O, out-2·H2O. A 100 mg (0.132 mmol)
sample of out-[Ru(Cl)(Hbpp)(trpy)](PF6) was dissolved in 40 mL of methanol, and then
19.0 mL of triethylamine (0.136 mmol) was slowly added to the solution under
magnetic stirring and heating at 50 ºC. Afterwards, 90 mg of ZnCl2 (0.660 mmol)
previously dissolved in 10 mL of methanol was added, and the solution was kept
stirring for 1 h at 50 ºC. A black solid is then filtered from the solution, and washed
with methanol (3 x 10 mL) and ether (3 x 10 mL). Yield: 89 mg (92.8%). Anal. Calcd
for C28H20Cl3N7RuZn·H2O: C, 45.12; H, 2.98; N, 13.15. Found: C, 44.90; H, 2.66; N,
12.96. 1H NMR (400MHz, d6-DMSO): δ = 9.92 (d, 3J16-17 = 5.5 Hz, H16), 8.57 (d, 3J7-8
= 3J9-8 = 8.3 Hz, H7, H9), 8.48 (d, 3J4-3 =3J12-13 = 7.9 Hz, H4,H12), 8.30 (d, 3J19-18 = 7.6
Hz, H19), 8.19 (dd, 3J18-19 = 7.6 Hz, 3J18-17 = 7.4 Hz, H18), 8.11 (d, 3J28-27 = 4.5 Hz,
H28), 8.03 (dd, 3J26-25 = 8.0 Hz, 3J26-27 = 7.9 Hz, H26), 7.97 (t, 3J8-7 = 3J8-9 = 8.3 Hz,
H8), 7.85 (dd, 3J3-4 = 3J13-12 = 7.9 Hz, 3J3-2 =3J13-14 = 7.8 Hz, H3, H13), 7.79 (d, 3J25-26 =
8.0 Hz, H25), 7.74 (s, H22), 7.72 (dd, 3J17-18 = 7.4 Hz, 3J17-16 = 5.5 Hz, H17), 7.56 (d,
3

J1-2 = 3J15-14 = 5.2 Hz, H1, H15), 7.37 (dd, 3J27-26 = 7.92, 3J27-28 = 4.5 Hz, H27), 7.29

(dd, 3J2-3 = 3J14-13 = 7.8 Hz, 3J2-1 = 3J14-15 = 5.2 Hz, H2, H14). NMR is assigned
following the labels in Figure 1. MALDI-MS (CH2Cl2): m/z = 727.0 ([M]+). Cyclic
Voltammetry (CH2Cl2, TBAH): E1/2 = 0.543 V vs. SSCE (∆E = 76 mV).

4

out,out-{[Ru(Cl)(trpy)]2(µ-[Zn(bpp)2])}(PF6)2·H2O, out-3·H2O. A 80 mg (0.110
mmol) sample of out-{[Ru(Cl)(trpy)][ZnCl2](µ-bpp)} was dissolved in the minimum
amount of DMF, then a few drops of a saturated aqueous KPF6 solution were added.
Afterwards water was added until a precipitate appeared. The mixture was cooled in
fridge for 2 hours, and then the solid filtered on a frit and washed with water (3 x 10
mL) and ether (3 x 10 mL). Yield: 75 mg (88.7%). Anal. Calcd for
C56H40Cl2F12N14P2Ru2Zn·H2O: C, 43.24; H, 2.72; N, 12.61. Found: C, 43.33; H, 2.69;
N, 12.47. 1H NMR (400MHz, d6-acetone): δ = 9.82 (d, 3J16-17 = 3J41-40 = 5.1 Hz, H16,
H41), 8.65 (d, 3J19-18 = 3J38-39 = 7.7 Hz, H19, H38), 8.40 (dd, 3J18-17 =3J39-40 = 7.7 Hz,
3

J18-19 = 3J39-38 = 7.7 Hz, H18, H39), 8.36 (dd, 3J26-25 = 3J31-32 = 7.9 Hz, 3J26-27 = 3J31-30 =

7.9 Hz, H26, H31), 8.23 (d, 3J28-27 = 3J29-30 = 4.9 Hz, H28, H29), 8.16 (d, 3J7-8 = 3J50-49 =
8.0 Hz, H7, H50), 8.15 (d, 3J4-3 = 3J53-54 = 7.8 Hz, H4, H53), 8.11 (d, 3J12-13 = 3J45-44 =
7.9 Hz, H12, H45), 8.10 (d, 3J25-26 = 3J32-31 = 7.9 Hz, H25, H32), 8.07 (s, H22, H35),
7.79 (dd, 3J17-18 = 3J40-39 = 7.7 Hz, 3J17-16 = 3J40-41 = 5.1 Hz, H17, H40), 7.77 (d, 3J9-8 =
3

J48-49 = 7.6 Hz, H9, H48), 7.74 (dd, 3J13-12 = 3J44-45 = 7.9 Hz, 3J13-14 = 3J44-43 = 7.8 Hz,

H13, H44), 7.64 (dd, 3J27-26 = 3J30-31 = 7.9 Hz, 3J27-28 = 3J30-29 = 4.9 Hz, H27, H30), 7.57
(d, 3J1-2 = 3J56-55 = 5.3 Hz, H1, H56), 7.43 (dd, 3J2-3 = 3J55-54 = 7.8 Hz, 3J2-1 = 3J55-56 = 5.3
Hz, H2, H55), 7.38 (dd, 3J3-4 = 3J54-53 = 7.8 Hz, 3J3-2 = 3J54-55 = 7.8 Hz, H3, H54), 7.29
(d, 3J15-14 = 3J42-43 = 4.7 Hz, H15, H42), 7.21 (dd, 3J8-7 = 3J49-50 = 8.0 Hz, 3J8-9 = 3J49-48 =
7.6 Hz, H8, H49), 7.20 (dd, 3J14-13 = 3J43-44 = 7.8 Hz, 3J14-15 = 3J43-42 = 4.7 Hz, H14,
H43). NMR is assigned following the labels in Figure 2. MALDI-MS (CH2Cl2): m/z =
592.2 ([Ru(Cl)(Hbpp)(trpy)]+). Cyclic Voltammetry (CH2Cl2, TBAH): E1/2 = 0.669 V
vs. SSCE (∆E = 185 mV).

out,out-{[Ru(trpy)(H2O)]2(µ-[Zn(bpp)2])}(ClO4)4,·(CH3)2CO, out-4·(CH3)2CO. A 50
mg (0.033 mmol) sample of out,out-{[Ru(Cl)(trpy)]2(µ-[Zn(bpp)2])}(PF6)2 were placed
in a 5 mL round bottom flask together with a 13.3 mg (0.064 mmol) of AgClO4. The
solids were dissolved with 10 mL of a 1:3 water/acetone solution (v/v) and refluxed for
1.5 h in the dark. The solution was then cooled in an ice bath and filtered over Celite to
remove precipitated AgCl. A few drops of a saturated aqueous NaClO4 solution was
added to the filtrate, and the volume reduced under vacuum, keeping the temperature
below 30 °C, until a precipitate appeared. The precipitation was completed by
immersion in an ice bath for 30 minutes and the solid obtained filtered through a frit,
washed with cold water (3 x 10 mL) and ether (3 x 10 mL), and dried in a vacuum.
5

Yield: 49 mg (92.9%). Anal. Calcd for C56H44Cl4N14O18Ru2Zn·(CH3)2CO: C, 42.47; H,
3.02; N, 11.75. Found: C, 42.16; H, 3.16; N, 11.65. MALDI-MS (H2O): m/z = 556.2
([Ru(bpp)(trpy)]+), 598.6 ([Ru(bpp)(trpy)(OH)(Na)]+).

in,in-{[Ru(Cl)(trpy)]2(µ-[Zn(bpp)2])}(PF6)2, in-3. ROUTE A. A 100 mg (0.132
mmol) sample of in-[Ru(Cl)(Hbpp)(trpy)](PF6) was dissolved in 40 mL of methanol,
and then 19.0 mL of triethylamine (0.136 mmol) was slowly added to the solution under
magnetic stirring and heating at 50 ºC. Afterwards, 90 mg of ZnCl2 (0.660 mmol)
previously dissolved in 10 mL of methanol was added, and the solution was kept
stirring for 1 h at 50 ºC. The solvent was evaporated and the red solid dissolved in
acetone. Some drops of a saturated KPF6 aqueous solution and water were added and
the volume reduced until the formation of a precipitate. The mixture was then left in a
fridge overnight to complete precipitation, then filtered and washed with water (3 x 10
mL) and ether (3 x 10 mL). Yield: 87 mg (85.7%). Anal. Calcd for
C56H40Cl2F12N14P2Ru2Zn·2.8(KPF6): C, 32.52; H, 1.95; N, 9.48. Found: C, 32.99; H,
2.01; N, 9.15. Cyclic Voltammetry (CH2Cl2, TBAH): E1/2 = 0.665 V vs. SSCE (∆E = 68
mV), E1/2 = 0.765 V vs SSCE (∆E = 66 mV). 1H NMR (400 MHz, d6-acetone): δ = 8.69
(d, 3J7-8 = 3J50-49 = 7.9 Hz, H7, H50), 8.67 (d, 3J9-8 = 3J48-49 = 8.0 Hz, H9, H48), 8.57 (d,
3

J12-13 = 3J45-44 = 5.0 Hz, H12, H45), 8.53 (d, 3J28-27 = 3J29-30 = 7.9 Hz, H28, H29), 8.40

(d, 3J4-3 = 3J53-54 = 7.9 Hz, H4, H53), 8.18 (m, H15, H42), 8.17 (d, 3J19-18 = 3J38-39 = 8.4
Hz, H19, H38), 8.14 (dd, 3J8-9 = 3J49-48 = 8.0 Hz, 3J8-7 = 3J49-50 = 7.9 Hz, H8, H49), 8.09
(m, H14, H43), 8.00 (d, 3J25-26 = 3J32-31 = 6.9 Hz, H25, H32), 7.99 (s, H22, H35), 7.94
(dd, 3J27-28 = 3J30-29 = 7.9 Hz, 3J27-26 = 3J30-31 = 7.0 Hz, H27, H30), 7.77 (d, 3J1-2 = 3J56-55
= 5.7 Hz, H1, H56), 7.65 (dd, 3J3-4 = 3J54-53 = 7.9 Hz, 3J3-2 = 3J54-55 = 7.7 Hz, H3, H54),
7.60 (dd, 3J18-19 = 3J39-38 = 8.4, 3J18-17 = 3J39-40 = 7.5 Hz, H18, H39), 7.59 (dd, 3J26-27 =
3

J31-30 = 7.0 Hz, 3J26-25 = 3J31-32 = 6.9 Hz, H26, H31), 7.42 (dd, 3J13-14 = 3J44-43 = 6.9 Hz,

3

J13-12 = 3J44-45 = 5.0 Hz, H13, H44), 7.10 (d, 3J16-17 = 3J41-40 = 6.4 Hz, H16, H41), 6.76

(dd, 3J17-18 = 3J40-39 = 7.5 Hz, 3J17-16 = 3J40-41 = 6.4 Hz, H17, H40), 6.42 (dd, 3J2-3 = 3J55-54
= 7.7 Hz, 3J2-1 = 3J55-56 = 5.7 Hz, H2, H55). NMR is assigned following the labels in
Figure 2. MALDI-MS (CH2Cl2): m/z = 1391.2 ([M-PF6]+).
ROUTE B. A 150 mg (0.098 mmol) sample of out,out-{[Ru(Cl)(trpy)]2(µ[Zn(bpp)2])}(PF6)2 was dissolved in 130 mL of acetone and refluxed in presence of a
100 W lamp for 72 hours. Then a few drops of a saturated KPF6 aqueous solution and
6

water were added and the volume reduced until the formation of a precipitate. The
mixture was left in a fridge overnight to complete precipitation, then filtered and
washed with water (3 x 10 mL) and ether (3 x 10 mL).
Yield: 128 mg (85.3 %).
in,in-{[RuIII(trpy)]2(µ-[Zn(bpp)2(H2O)]µ-(O)}(ClO4)]4, in-5. A 50 mg (0.033 mmol)
µ
µ
sample of in,in-{[Ru(Cl)(trpy)]2(µ-[Zn(bpp)2])}(PF6)2 were placed in a 5 mL round
bottom flask together with a 13.3 mg (0.064 mmol) of AgClO4. The solids were
dissolved with 10 mL of a 1:3 water/acetone solution (v/v) and refluxed for 1.5 h in the
dark. The solution was then cooled in an ice bath and filtered over Celite to remove
precipitated AgCl. A few drops of a saturated aqueous NaClO4 solution was added to
the filtrate, and the volume reduced under vacuum, keeping the temperature below 30
°C, until a precipitate appeared. The precipitation was completed by immersion in an ice
bath for 30 minutes and the solid obtained filtered through a frit, washed with cold
water (3 x 10 mL) and ether (3 x 10 mL), and dried in a vacuum. Yield: 11 mg (19.5
%). Anal. Calcd for C56H42Cl4N14O18Ru2Zn·2(LiClO4)·(CH3)2CO: C, 37.71; H, 2.57; N,
10.43. Found: C, 37.98; H, 2.40; N, 10.64. MALDI-MS (CH2Cl2): m/z = 656.2
([Ru(bpp)(trpy)](ClO4)+). Cyclic Voltammetry (H2O, pH = 1.0 CF3SO3H): E1/2 = 0.310
V vs SSCE (∆E = 93 mV), E1/2 = 0.730 V vs SSCE (∆E = 81 mV); (H2O, pH = 7.0
phosphate buffer): E1/2 = 0.152 V vs SSCE (∆E = 204 mV), E1/2 = 0.780 V vs SSCE
(∆E = 101 mV).

X-ray

Crystal

Structure

Determination.

Black

crystals

of

out-

{[Ru(Cl)(trpy)][ZnCl2](µ-bpp)}, out-2, were obtained by slow diffusion of acetone into
a dimethyl formamide solution of the complex. Brown crystals of out,out{[Ru(Cl)(trpy)]2(µ-[Zn(bpp)2])}(PF6)2, out-3, were obtained by slow diffusion of ether
in a methanolic solution of the complex. Dark red crystals of in,in-{[Ru(Cl)(trpy)]2(µ[Zn(bpp)2])}(PF6)2, in-3, could be obtained by slow diffusion of ether in a methanolic
solution of the complex. Green crystals of in,in-{[RuIII(trpy)]2µ-[Zn(bpp)2(H2O)]µ(O)}(ClO4)]4, in-5, were obtained by slow diffusion of ether in acetone solution of the
complex. The measured crystals were prepared under inert conditions immersed in
perfluoropolyether as protecting oil for manipulation.
Data collection: Crystal structure determination for out-2 was carried out using a Apex
DUO Kappa 4-axis goniometer equipped with an APPEX 2 4K CCD area detector, a
7

Microfocus Source E025 IuS using MoKα radiation, Quazar MX multilayer Optics as
monochromator and a Oxford Cryosystems low temperature device Cryostream 700
plus (T = -173 °C). Full-sphere data collection was used with ω and ϕ scans. Programs
used: Data collection APEX-2,13 data reduction Bruker Saint14 V/.60A and absorption
correction SADABS.1516 Crystal structure determinations for out-3, in-3 and in-5 were
carried out using a Bruker-Nonius diffractometer equipped with an APPEX 2 4K CCD
area detector, a FR591 rotating anode with MoKα radiation, Montel mirrors as
monochromator and an Oxford Cryosystems low temperature device Cryostream 700
plus (T = -173 °C).
Structure Solution and Refinement: Crystal structure solution was achieved using
direct methods as implemented in SHELXTL17 and visualized using the program XP.
Missing atoms were subsequently located from difference Fourier synthesis and added
to the atom list. Least-squares refinement on F2 using all measured intensities was
carried out using the program SHELXTL. All non hydrogen atoms were refined
including anisotropic displacement parameters.
Comments to the structures: out-2: The asymmetric unit contains two independent
molecules of the bimetallic complex, three DMF molecules and three water molecules.
One of the DMF molecules and one of the water molecules are disordered in two
positions (ratio respectively: 75:25 and 50:50). Due to the disorder of the water
molecules the positions of the corresponding hydrogen atoms could not be clearly
localized and these were added to approximate positions to satisfy the crystal
stoichiometry. This disorder is thus responsible for the B alerts that appear in the
checkcif file. out-3: The asymmetric unit contains two independent molecules of the
metallic complex, four PF6 anions and 4.6 water molecules which are highly disordered
distributed in 7 positions (1:1:0.6:0.6:0.6:0.4:0.4). As in the previous case hydrogen
atoms could not be clearly localized and were added at approximate. in-3: The
asymmetric unit contains one molecules of the metallic complex, two PF6 anions, one
acetone molecule and one methanol molecule. One of the PF6 anions is disordered in
two orientations (ratio: 80:20). The methanol molecule is disordered in three positions
(ratio: 62.5:25:12.5). in-5: The asymmetric unit contains one molecule of the metallic
complex, four ClO4 anions and one and half nonmetal coordinated water molecules.
Three of the ClO4 anions are disordered in two orientations (ratios: 60:40; 36:74;

8

81:19). The one and half nonmetal coordinated water molecules are disordered in three
positions (ratio: 65:35:50).

Computational details: All calculations were carried out by using the Amsterdam
Density Functional (ADF v2013.01) package.18 DFT-based methods employed the
generalized-gradient-approximation (GGA) level, with the Becke exchange19 and the
Perdew correlation20 functionals (BP86). A triple-ξ plus polarization Slater basis set was
used on all atoms. Relativistic corrections were introduced by scalar-relativistic zeroorder regular approximation (ZORA).21 Full geometry optimizations were performed
without constraints, and also including Grimme’s DFT-D empirical dispersion energy
corrections.22

Results and Discussion
Synthesis and structure. The non-symmetric dinucleating nature of Hbpp ligand23
allows for the easy preparation of heteronuclear complexes given the different reactivity
of the two available sites. Initially mononuclear complexes can be prepared leaving the
second site ready for a second transition metal to coordinate. In the present report we
have used the mononuclear isomeric complexes in- and out-[RuCl(Hbpp)(trpy)]+, in-0
and out-011a as starting materials following the synthetic strategy described in Scheme
1.
Reaction of out-[RuCl(Hbpp)(trpy)]+, out-0, with excess of ZnCl2 in presence of
triethylamine, that acts as a base, in methanol at 50 ºC for 1 hour produces the complex
out-{[Ru(Cl)(trpy)][ZnCl2](µ-bpp)}, out-2, almost quantitatively.
out-[RuCl(Hbpp)(trpy)]+ + NEt3 + ZnCl2 ->
out-0

out-{[RuCl(trpy)][ZnCl2](µ-bpp)} + HNEt3+

(1)

out-2

Complex out-2 precipitates readily from the methanolic solution and is insoluble in
water and in most organic solvents except in DCM, DMSO and DMF. Dissolved in the

9

latter the complex has a strong tendency to dimerize generating a heterotrinuclear
complex, according to,

2 out-{[RuCl(trpy)][ZnCl2](µ-bpp)} ->
out-2

out,out-{[RuCl(trpy)]2(µ-[Zn(bpp)2])}2+ + ZnCl42- (2)
out-3

where the Zn metal is now bonded to two bpp- ligands in an tetrahedral fashion and acts
as bridged for the two [RuCl(trpy)] moieties. Addition of Ag(I) to out-3 generates the
corresponding Ru diaqua complex,
out,out-{[RuCl(trpy)]2(µ-[Zn(bpp)2])}2+ + 2 Ag+

->

out-3
out,out-{[Ru(trpy)(H2O)]2(µ-[Zn(bpp)2])}4+ + 2 AgCl (3)
out-4

It is interesting to realize here that the yield for this reaction is nearly quantitative (93
%) in sharp contrast to the yield obtained for the formation the aqua complex out-1
from out-0 of 53%, under comparable conditions. This is a consequence of the potential
coordination of Ag to the available coordination position of the bpp- ligand in out-0 that
can drive the systems to additional undesired pathways thus lowering the yield. In the
present case the Zn metal avoids the interaction of silver with the bpp- ligands and thus
is responsible for this large enhancement of the yield.
The parallel chemistry with the in isomer in-[RuCl(Hbpp)(trpy)]+, in-0, is radically
different from the previously described reactions. In this case the addition of ZnCl2
generates directly the heterotrinuclear complex,
2 in-[RuCl(Hbpp)(trpy)]+ + 2 NEt3 + 2 ZnCl2 ->
in-0
in,in-{[RuCl(trpy)]2(µ-[Zn(bpp)2])}2+ + ZnCl42- + 2 HNEt3+ (4)
in-3

10

presumably due to the solubility of the corresponding in-2 dinuclear isomer that could
not be isolated. Addition of Ag(I) to in-3 generates the corresponding diqua complex
in-4, that again could not be isolated because it has a strong tendency to generate the
oxo-bridged derivative, in-5,
in,in-{[RuII(trpy)(H2O)]2(µ-[Zn(bpp)2])}4+ ->
in-4
in,in-{[RuIII(trpy)]2(µ-[Zn(bpp)2(H2O)]µ-(O)}4+ + 2H+ + 2e- (5)
in-5

This is a consequence of the ligand architecture that in the case of the in-4 complex
places the two Ru-aqua groups very close to one another and favors the intramolecular
[Ru-OH2 --- H2O-Ru] coupling, to generate the highly stable oxo bridge group, Ru-ORu concomitant with the oxidation of Ru(II) to Ru(III) doubly bridging the Ru centers.
The formation of this group is also most likely responsible for the five coordination of
the Zn complex with an additional aqua group. Given the absence of crystal field
stabilization energy for a d10 ion, the new structural demands imposed by the oxobridging group forces a distortion of the N4 coordination by the bpp- ligands favoring a
pentacoordinate geometry with an additional Ru-O bond from the aquo ligand. See
below for further details of the coordination geometry of the Zn metal ion.

Monocrystals for complexes out-2, out-3, in-3 and in-5 were obtained and their crystal
structures were solved by means of x-ray diffraction analysis. Their Ortep view is
presented in Figure 1, and all the crystallographic parameters can be found in the SI as
cif files. For all these complexes the coordination around the low spin d6 Ru(II) or d5
Ru(III) is a distorted octahedral and very similar to previously reported related Ru
complexes.10a, 11b, 24 In sharp contrast, while the bonding distances for the Zn(II) are also
in agreement with previously published related complexes,25 their geometries are
radically different from one another. This is a consequence of the absence of Crystal
Field Stabilization Energy (CFSE) for a d10 Zn(II) ion where the Zn metal center
geometry is strongly dependent on the spatial effects exerted by the coordinating
bridging bpp- ligand, that are in turn dependent on their geometrical arrangement around
the Ru metal center.

11

For the out-2 heterodinuclear complex, the Zn center is a coordinated by two Cl and
two N-atoms from the bpp- ligands in a distorted tetrahedral fashion. The dihedral angle
between the planes generated by the Cl2Zn1Cl3 atoms and the N6Zn1N7 is 86.4o, close
to the 90º expected for an ideal tetrahedral geometry. For the out-3 heterotrinuclear
complex, the geometry around the Zn metal center is again a distorted tetrahedron
although now the coordination atoms are two N atoms from a pyridyl-pyrazolyl moiety
from two different bpp- ligands. The dihedral angle between the two NZnN (N6Zn1N7
and N8Zn1N9) planes is now 78.2o. It is worth noting here that the trpy an bpp- moieties
bonded to different Ru centers, are situated in a nearly parallel manner although they are
too far for a potential π- π stacking interaction.

Regarding the in-3 complex, the constitutional isomer of out-3, the geometry presented
by Zn is now closer to a distorted octahedral geometry, where due to the new spatial
arrangement of the Ru-Cl-Hbpp moiety, their Cl atoms weakly interact with the Zn
metal (dZn – Cl = 3.06 and 3.04 Å respectively). In this case the NZnN (N6Zn1N7 and
N8Zn1N9) dihedral angle is 85.9º. Finally for the in-5 complex, that contains a Ru-ORu group, the Zn metal displays penta-coordination with an additional Zn-OH2 group in
a distorted square pyramidal type of geometry. Here the NZnN (N6Zn1N7 and
N8Zn1N9) dihedral angle is 47.9º, which is the smallest of all the complexes described
in the present work due to the special arrangement forced by the additional oxo-bridging
group between the two Ru(III) centers. It is also worth mentioning here that the oxygen
atom of the Zn-OH2 groups is hydrogen bonded to a H56, of one of the trpy ligands
(C56H56O2 angle 146.6o; C56O2, 3.43 Å; H56O2, 2.60 Å).

Spectroscopic proprieties. The optical properties of the Ru-Cl complexes presented in
this work were evaluated by means of UV-vis spectroscopy in DCM and are displayed
in Figure 3. All complexes present strong ligand based π-π* bands below 340 nm and
lower intensity MLCT dπ-π* transitions around 400 and 700 nm together with their
vibronic components.26 TDFT has been used to corroborate the nature of these transition
and the results are shown in the supporting information (Figure S16).
The spectra of all the complexes are very similar since the chromophores are identical.
As expected, the presence in the system of a d0 metal, such as Zn(II), doesn’t
significantly modify the overall optical properties in the visible region. A slight
12

difference can be found from in- an out-3 on energies and extinction coefficients that
might be due to the slightly geometrical distortions found in the Ru metal centers.
Similar behavior is observed for the in complexes (Fig 2, right), where the interaction
between the chlorido ligands and the Zinc atom seems to produce a small shift of the
dπ-π* MLCT band to higher energy.
The in-5 complex was investigated in aqueous solution. At pH 1.0, it presents the
typical bands of the Ru-O-Ru transition at 610 and 680 nm as can be observed in Figure
4. These two bands are red-shifted by about 50 nm at pH 7.0 which is indicative of the
acid base properties of the oxo-bridge as had been previously observed for related oxobridged-Ru dimers.27
Fluorescence experiments have been carried out for the complexes out-2, out-3 and in3, and for their mononuclear analogues out-0 and in-0 for comparison purposes. All
these spectra are presented as insets in Figure 3. From the figure it can be clearly seen
that the emission is affected by the nature of the monodentate ligand but is practically
not affected by the nature of the isomer.

Redox proprieties. The redox properties of the Ru-Cl complexes presented in this work
were analyzed by means of cyclic voltammetry in DCM containing 0.1 M TBAH
[(nBu4N)(PF6)] as supporting electrolyte, and are displayed in Figure 5. All redox
potentials reported in this work are referred to the SSCE electrode and all CV were run
at a scan rate of 50 mV/s. Glassy carbon electrodes were used as working electrodes and
platinum wire as auxiliary electrodes.
The dinuclear complex out-2 exhibits a chemically reversible and electochemically
quasi-reversible wave at E1/2 = 0.54 V (∆Ea,p = 76 mV), assigned to the Ru(III)/Ru(II)
couple, which is 80 mV lower than out-0 (E1/2 = 0.62 V) under identical conditions.
Thus the replacement of the pyrazolic-Hbpp proton by the ZnCl2 group produces an
enhancement of the electron density on the Ru metal center. The trinuclear complex
out-3 shows two waves in very close proximity at E1/2 = 0.64 V (∆E = 120 mV) and E1/2
= 0.695 V (∆E = 150 mV), assigned to the Ru2(III,II)/Ru2(II/II) and
Ru2(III,III)/Ru2(III/II) couples respectively. The close proximity of the two couples
suggests a weak electronic coupling between the two Ru centers through the {Zn(bpp)2}
bridging unit, as expected.10a, 28 Comparing the redox potential for the mononuclear out0 with regard to out-3 a decrease of electron density over the Ru metal centers is now
observed as suggested by the anodic shift of the III/II redox potentials.
13

In the case of in-3, two quasi-reversible waves at E1/2 = 0.66 V (∆E = 80 mV) and E1/2 =
0.76 V (∆E = 80 mV) assigned to the Ru2(III,II)/Ru2(II/II) and Ru2(III,III)/Ru2(III/II)
couples respectively, indicate again a weak coupling but now the two waves are
cathodically shifted by 80-180 mV with regard to in-0 (E1/2 = 0.84 V). This effect can
be associated with the Zn-Cl contacts (3.04 and 3.06 A) that each {Ru-Cl} entity has in
the in isomer and that do not exist in the corresponding out isomer. These Zn-Cl
contacts that generate the difference in redox potentials are also most likely responsible
for the difference in the weak electronic coupling through the two Ru metals. NIR
experiments for the mixed valence III,II in-3 complex were carried out in order to
further characterized the electronic coupling. Unfortunately the IVCT band is so weak29
that we were not able to properly observe it (see Figure S14-15) even working at the
highest possible concentration.

The oxo-bridged complex in-5 presents a completely different redox behavior. In water
at pH = 1.0 a reversible wave at E1/2 = 0.84 V (Ep,a = 0.87 V, Ep,c = 0.81 V; ∆E = 60
mV) associated with the Ru2(IV,III)/Ru2(III/III) couple is observed as can be observed
in Figure 4. In addition a chemically irreversible wave is observed at 0.25 V that is
associated with the one electron reduction of the oxo-bridged trinuclear complex to
generate Ru2(III/II). The chemical irreversibility is associated with the instability of the
Ru-O-Ru bridged in the lower oxidation state where the population of antibonding
orbitals27 favors the breaking of the bridging group generating the corresponding
mononuclear complexes.
NMR Spectroscopy. 1H-NMR spectra of the complexes described in this work were
recorded in d6-dmso or d6-acetone and are assigned in the Experimental Section and
their spectra are shown in Figure 2 and in the SI.
In solution complex out-2 presents a symmetry plane that contains the bpp- ligand and
bisects the trpy moiety, thus rendering the upper and lower pyridyl of the trpy ligand
magnetically equivalent. The deshielding effect produced by the Cl ligand at H16
allows one to identify the pyridyl system on the side of the Ru center. This information,
together with the COSY spectra and symmetry of the complex previously mentioned,
allow to unambiguously assigning all the resonances of the spectra (See Figure S1 in
SI).

14

The 1H-NMR spectra of complexes out-3 and in-3 in d6-acetone is depicted in Figure 2.
Both exhibit a C2 symmetry axis passing through the Zn atom but not coinciding with
any bond and thus in both cases the two {Ru-Hbpp-trpy} moieties are magnetically
equivalent. Symmetry, together with COSY and the deshielding effect of the chloro
ligands to H16 and H41 for the out-3 complex, allow fully identifying all resonances.
For the case of the in-3, a NOE between H14 and H29 (2.80 Å) or H28 and H42 (2.78
Å) is the key experiment that allows fully assigning the spectrum shown in Figure 2.

Reactivity. In both neutral and acidic conditions the trinuclear out-4 Ru-aqua complex
breaks apart and generates the corresponding mononuclear complex out-1, as indicated
in the following equation,
out,out-{[RuII(trpy)(H2O)]2(µ-[Zn(bpp)2])}4+ + 2 H2O
out-4

->

2 out-[RuII(Hbpp)(trpy)(H2O)]2+ + Zn(OH)2 (6)
out-1

The generation of the mononuclear complex is instantaneous once the trinuclear
complex is dissolved in aqueous solution as can be clearly observed by UV-vis and CV.

On the other hand the trinuclear out-3 complex is photosensible and under irradiation
with a tungsten lamp, cleanly and irreversibly and quantitatively isomerizes to in-3.
This isomerization process has been monitored by 1H-NMR spectroscopy and is shown
in Figure 2. This is in sharp contrast with the photostability of the synthetic precursors,
the mononuclear in- and out-1 complexes.

The irreversible isomerization of the complex out-3 towards in-3, is a consequence of
the higher thermodynamic stability of the latter over the former. In addition, even
though experimentally we could not detect any intermediate, the transformation of the
out,out complex into the in,in complex necessarily has to take place via the
corresponding in,out intermediate complex, as indicated in the following equations (bpp
and trpy ligands and overall charges have been omitted).

15

out,out-{[RuCl]2(µ-Zn)} -> in,out-{[RuCl]2(µ-Zn)} -> in,in-{[RuCl]2(µ-Zn)} (7)
out-3

in,out-3

in-3

In order to shed light into the driving force for the isomerization process, a DFT based
study was carried out and a summary of the main results is presented in Figure 6 and in
the SI. For this purpose we determined the relative stability of isomers out-3, in,out-3
and in-3. As it can be observed in Figure 6, the in,out-3 complex is about 12.7 kcal/mol
more stable than out-3 mainly due to the formation of a new Zn-Cl bond (Zn-Cl, 2.621
Å). The isomerization of the second Ru center toward the formation of in-3 is 13.8
kcal/mol more stable, now due to the two Zn-Cl contacts. This is actually a structure
that is very close the one observed experimentally through the X-ray diffraction analysis
that is represented in Figure 1. The Zn-Cl bond length obtained from the geometry
optimization process (2.920; 2.877 Å) compares fairly well with the X-ray determined
parameters (3.06; 3.04 Å) although is overestimated by approximately 0.1-0.2 Å.
Including dispersion in the calculations further decrease the Zn-Cl distance by roughly
0.05 Å.

Conclusions
We have prepared and thoroughly characterized a family of dinuclear and trinuclear ZnRu complexes containing the bbp- ligand as a bridge. These include constitutional
isomers that have been prepared from their corresponding in and out Ru mononuclear
counterparts. The different constitutional isomers give rise not only to different
solubilities but also to radically different reactivity based on the relative space
disposition of the Ru-OH2 or Ru-Cl groups. The access to through space intramolecular
Ru-aqua---aqua-Ru interactions in in-4 gives rise to the oxo-bridged complex. The oxobridge is holding strongly and rigidly together the two Ru centers and that enhances its
stability so that now the trinuclear Ru complex is stable at pH 1.0 and 7.0 whereas the
trinuclear out compelex is not. On the other hand for the Ru-Cl case the presence of
contact interactions stabilize the in isomer with regard to the corresponding out one.
The relative stability of the isomers is nicely demonstrated by the out->in
photochemcial interconversion that is further supported by DFT.
The use of labile first row transition metals ion such as Zn, Fe or Mn to react with
bridging dinucleating ligands such as Hbpp, generally produces oligomers30 or grids12a
16

or even coordination polymers.12b-d Using a substitutionally kientically inert Ru(II) ion
(at least with regard to the N-containing ligands) to coordinate first with the Hbpp
ligand with the subsequent addition of a substitutionally kinetically labile ion such as
Zn(II) precludes the potentials oligomerization process and thus allows to isolate the
dinuclear and trinuclear complexes reported in the present work. As such these
complexes can be considered as the corresponding synthetic intermediates used by first
row transition metals to generate oligomers or coordination polymers.

Supporting Information content. Additional spectroscopic, electrochemical and DFT
data. X-ray crystallographic data in CIF format (CCDC # 1014664-1014667). This
material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments
Research supported by the Spanish Ministerio de Economia y Competitividad
(MINECO) through projects CTQ2011-29054-C02-02, CTQ-2013-49075-R, PRIPIBIN-2011-1278, and by the Generalitat de Catalunya (2009SGR-00462 and
2014SGR-915), and the ICIQ Foundation. L.M thanks ICIQ foundation for a PhD grant.

17

Scheme 1. Synthetic pathway.

2+
Reflux, 1.5 h

NH N N
N
Ru
N
OH2

h

out-[Ru(Hbpp)(trpy)(H2O)]2+
out-1

N
NH N N
Ru
N
H2O

Reflux, 1.5 h

NH N N
N
Ru
N
Cl

N

N

N

N

NH N N
N
Ru
N
Cl

H2O/Acetone 1:3

2+

+

+
N

N

H2O/Acetone 1:3

N

N

out-[Ru(Cl)(Hbpp)(trpy)]+
out-0

in-[Ru(Hbpp)(trpy)(H2O)]2+
in-1

in-[Ru(Cl)(Hbpp)(trpy)]+
in-0

NEt3 , ZnCl2 , MeOH
50 ºC, 1h

N
N

Cl
Zn
Cl

N

N

N
N

Ru

NEt3 ,ZnCl2 , MeOH

Cl

50 ºC, 1h
DMF, RT

N

out-{[Ru(Cl)(trpy)][ZnCl2]( -(bpp))}
out-2
2+
N

N
N
Ru

Cl

N
N

N

N

Zn

N

N

N

N

2+
N

N

N

N

N

Zn

N

Ru

Cl

N

N

out,out-{[Ru(Cl)(trpy)]2( -[Zn(bpp)2])}2+
out-3

N

N

N

in,in-{[Ru(Cl)(trpy)]2( -[Zn(bpp)2])}2+
in-3

H2O/Acetone 1:3

AgClO4

N
N

Cl
Ru

Acetone
Reflux

N
N

N

N

h

Cl

Ru

H2O/Acetone 1:3
Reflux, 1.5 h

AgClO4

Reflux, 1.5 h

4+
4+

N

N
N

N

N
N
H2O

Ru

N
N
N

N

Zn
N
N

Ru

N
Ru

N
N

N

N

N

Ru

O

N

OH2

N
OH2

N
N

N
N

N

out,out-{[Ru(trpy)(H2O)]2( -[Zn(bpp)2])}4+
out-4

N
N

Zn
N

in,in-{[Ru(trpy)]2( -[Zn(bpp)2(H2O)][ -O])}4+
in-5

.
.
.

18

Figure 1. X-Ray structure Ortep plots (probabilites at 50%) and labelling scheme for
(A) out-2, (B) out-3, (C) in-3 and (D) in-5.

A

B

C

D

19

Figure 2. Top, 1H-NMR spectra (400 MHz, 298 K, d6-acetone) following the
photochemical (100 W lamp) isomerization reaction from out-3 to in-3. Bottom, time
profile based on the integration of resonances H16 for out-3 and H12+H28 for in-3.

100

Isomer %

80

60

out-3
in-3
40

20

0
0

5

10

15

t (h)

20

20

25

Figure 3. UV-vis and (inset) emission spectra in DCM of out-0, out-2 and out-3 (left)
and in-0, in-3 and out-3 (right).

50000

20000

20000

-1
10000

20000

40000

Intensity (CPS)

-1

ε (M cm )

30000

30000

30000

-1

-1

50000

in-0
in-3
out-3

40000

Intensity (CPS)

40000

ε (M cm )

40000

50000

out-0
out-2
out-3

30000

20000

10000

0
0

650
650

700

750

800

0

750

850

0
300

400

500

600

700

800

300

400

500

λ (nm)

600

λ (nm)

Figure 4. UV-vis at pH 1.0 and 7.0, and (inset) CV at pH 1.0 (E vs SCEE) of in-5.
100000

12

10

in-5 pH 1
in-5 pH 7

80000

8

I (µ A)

60000

-1

ε (M cm )

6

4

-1

2

0

40000
-2

-4
0.0

20000

0.2

0.4

0.6

0.8

1.0

1.2

1.4

E (V)

0
200

300

400

500

λ (nm)

21

800

λ (nm)

10000

λ (nm)

10000

700

850

600

700

800

700

800

Figure 5. Cyclic voltammograms (E vs SCEE) in DCM containing 0.1 M TBAH
[(nBu4N)(PF6)] of: left, of out-0, out-2 and out-3 and right, cyclic voltammograms of
in-0 and in-3 in DCM containing 0.1 M TBAH.

2.0

1.5

1.5

1.0

1.0

I (µ A)

I (µ A)

0.5
0.0

0.0

-0.5

out-0
out-2
out-3

-1.0
-1.5

-0.5

in-0
in-3
-1.0

-2.0
-0.2

0.5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-0.2

0.0

E (V)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

E (V)

Figure 6. Energy diagram for optimized structures of three isomers of 3, based on DFT.

∆E
kcal/mol
26.5
out,out-3

13.8
in,out-3

0.0
in,in-3
Zn-Cl, 2.92

22

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For Table of Contents Only

Table of Content Synopsis:
The coordination chemistry of new dinuclear Ru-Zn and self-assembled heterotrinuclear Ru2-Zn complexes that contain the Hbpp ligand have been thoroughly
characterized by means of spectroscopic and electrochemical techniques. Additionally,
a light induced linkage isomerization reaction has been shown to occur for the Ru-Cl
heterotrinculear complexes.

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