Synthesis , Characterization , and Photochemical Properties of a New SquareMn ( I )-Ru ( II ) Complex Using Pyrazine as Bridge Ligand

e photochemical properties of the complexes cis,fac-[Ru(phen)2(pz)2-Mn(CO)3Br]2 4+ (I), cis-[Ru(phen)2(pz)2] 2+ (II), and fac-Mn(CO)3(pz)2Br (III) where phen is phenanthroline and pz is pyrazine in acetonitrile solution are reported. e three complexes were characterized using H NMR, UV-vis and FTIR spectroscopy and electrochemical (cyclic voltammetry and spectroelectrochemical) techniques. e complexes show intense absorption in the visible region assigned to the population of MLCT excited states. e absorption spectrum of I is the sum of the spectra of the mononuclear species II and III, and the two oxidation potentials at +1.10 and +1.56V versus Ag/AGCl observed in I are ascribed to the different coordination environments of metal centers. e photolysis in the acetonitrile solution resulted in the pz dissociation to give the monoacetonitrile complexes for I, II, and III, respectively.


Introduction
e investigation of spectroscopic, electrochemical, and photochemical properties of manganese compounds has attracted much attention due to the potential application of these compounds in the development of the supramolecular system which may work photochemically for clean energy sources in renewable solar fuels [1,2].Most of current research is being focused on the covalent coupling of a photoactive Ru(II) polypyridinic complex to a high-valence mono-and/or multinuclear -oxo bridged manganese complexes.Herein, we wish to report a novel, square pyrazine donor-acceptor complex, composed of a triscarbonyl manganese complex linked to two ruthenium(II) phenanthroline complexes via two pyrazine bridge ligands.Our approach, introducing three good  acceptor (CO) and one good  donor (Br − ) ligand, was chosen because of the abilities of these ligands to accept and donate electronic density to metal centers and in this way to stabilize the high oxidation states that the manganese center may acquire during the photoinduced electron transfer reaction.For this reason, in this work the photochemical stability of the complexes was studied.

Experimental
2.1.General.All synthesis and electrochemical and spectroscopic experiments were carried out under puri�ed N 2 atmosphere, using Schlenk techniques.RuCl 3 ⋅xH 2 O, 1,10 ′ -phenanthroline (phen), pyrazine (pz), and lithium chloride were from Aldrich; tetrabutylammonium hexa�uorophosphate (TBAPF 6 ) and bromide pentacarbonyl manganese from Strem.HPLC grade acetonitrile and dichloromethane were distilled prior to use.e solutions were carefully handled in the dark before the experiments were  [5], and [Mn(CO) 3 (pz) 2 Br] [6] were prepared by the literature routes.FTIR spectra were measured in CaF 2 windows in CH 2 Cl 2 solution on a Bomem-Michelson 102 spectrometer in the 4000-1000 cm −1 region.UV-visible spectra were recorded on an HP-8453 A (Diode array) spectrophotometer.NMR spectra were recorded using a Bruker DRX400 spectrometer.All chemical shis () are given in ppm units with reference to the hydrogen signal of the methyl group of tetramethylsilane (TMS) as internal standard.Monochromatic irradiations at 350 nm and 420 nm were generated using an RMR-600 model Rayonet Photochemical reactor using RMR-3500 and RMR-4200 lamps, respectively.e continuous photolysis experiments were followed by UV-vis.Time-resolved optical spectra were obtained using a laser �ash-photolysis apparatus containing a Continuum Q-switched Nd:YAG laser (Continuum, Santa Clara, CA) with excitation provided by the third harmonic at  355 nm.Cyclic voltammetry was performed using an  Autolab Type III potentiostat.Voltammograms were obtained in CH 3 CN (1 mM TBPF 6 ) at 22 ∘ C in a lightprotected voltammetric cell with a platinum cylinder disc for both the working and the auxiliary electrodes.A silver wire coated with silver chloride was used as reference electrode, connected to the bulk of the solution by a Luggin capillary �lled with the same solvent and electrolyte.Solutions were deoxygenated with a stream of N 2 and maintained under a positive pressure of N 2 during the measurements.e concentration of the complexes was kept always at 1 mM.

Results and Discussion
e square complex was synthesized in a manner similar to a previously published procedure [1], starting from cis-[Ru(phen) 2 (pz) 2 ] 2+ and considering the complex Mn(CO) 5 Br as a ligand (Scheme 1).e 1 H NMR spectral data for the ligands and complexes in CD 3 CN are listed in Table 1 using the numbering scheme as represented for complex II as follow.e signals of the complex I were assigned by comparison and analysis of the precursor Ru(phen) 2 Cl 2 and the free pyrazine.In the complex I, the phenanthroline protons appeared as eight  square complex, all the assignments were done using the same numbering �gure as complex II, Figure 1.

Absorption
Properties. e absorption spectra of complexes I, II, and III in CH 3 CN solution are shown in Figure 2. e absorption maximum of 380 nm ( max = 1400 mol −1 L cm −1 ) for III appears as a shoulder on the strong - * absorption bands of the ligands, while the complex II exhibits the intense and broad MLCT absorption ( max = 397 nm;  max = 9200 mol −1 L cm −1 ) typical of Ru II polypyridine complexes [8].e absorption spectrum of I ( máx = 397 nm;  máx = 11230 mol −1 L cm −1 ) is consistent with the superposition of bands characteristics of the corresponding mononuclear complexes.e data for complex I show that the energy of the charge transfer band Ru II → L (394 e 442 nm) remains unchanged upon the introduction of Mn ion into the Ru complex, whereas it is slightly shied to shorter wavelength compared to the tris-(phenanthroline) complexes (422 and 446 nm) [9], which is characteristic of ligands serving as better -acceptors ligands than phen.e FTIR spectrum of III, shown in Figure 3, exhibitsthree intense v(CO) absorptions at 2041, 1953, and 1932 cm −1 consistent with the facial arrangement of the three COs in the coordination sphere.For complex I, the v(CO) stretching frequency appears as weak and broad bands around 2032 and 1940 cm −1 , and the two lower energy bands are overlapped suggesting the attachment of Mn(CO) 3 unit into the Ru II complex.2. Figure 4 shows a cyclic voltammogram (scan rate 100 mVs −1 ) for a 1 mM solution of the complexes I, II, and III over the range 0-1.8 V (versus AG/AGCl) in acetonitrile (TBPF 6 1 mM).

Electrochemistry. e voltammetric data are summarized in Table
Complex II exhibits a redox couple at  1/2 = 1.52 V ( ox = 1.56 V and  red = 1.49V versus Ag/AgCl) of Ru II/III which is more positive than those found to [Ru(phen) 3 ] 2+ [10].e complex III, on the other hand, displays a shoulder at 0.80 V corresponding to Mn I/II oxidation followed by an oxidation peak at 1.10 V attributed to the oxidation of Mn II to Mn III , which is paired with a nonreversible reductive wave at 0.90 due to Mn III/II reduction.

Spectroelectrochemistry.
Insights into the bonding characteristics of the Ru(II) complexes for complexes I and II were obtained by spectroelectrochemical experiments.For the Ru(II) complexes, a constant potential 1.5 V (determined from cyclic voltammetry) was applied and the extent of oxidation to Ru(III) was monitored by UV-vis spectroscopy (Figure 5).e spectrum shows the disappearance of the broad absorption band at 400 nm.Aer 30 min of oxidative electrolysis, the spectral changes were completed.e oxidative spectroelectrochemistry at 1.5 V leads to disappearance of the MLCT band indicating that oxidation was a Ru II/III process which is irreversible.
3.4.Transient Absorption Spectra. Figure 6 shows the excited state absorption spectrum for complex I in CH 3 CN solution aer excitation with an 8 ns pulse at 355 nm irradiation.ere is a bleach of absorption band at 400 nm and new structured absorption with maxima centered at 350 nm and 600 nm consistent with formation of an MLCT (Ru → phen) excited state [11].
3.5.Photochemistry.e complexes are stable in deaerated solutions in the absence of light.When solutions of complex II were subject to continuous photolysis, the resulting optical spectral changes were consistent with the substitution of only one pyrazine molecule by a solvent molecule (1) and ( 2) For example, Figure 7 illustrates the spectral changes seen when an acetonitrile solution of complex I (0.22 mM) was irradiated at  irr = 420 nm,  0 = 1.27 × 10 −8 einstein s −1 .e spectra show a progressive depletion of the characteristic absorption band at 400 nm concomitant with formation of two broad shoulders at 385 and 422 nm, in accordance with the formation of complex Ru(phen) 2 (CH 3 CN) 2 [12].For complexes I and II, exhaustive photolysis leads to the same �nal stable spectrum assigned to the monosolvated complex.
3.6.Photoinduced Electron Transfer Reactions.Figure 8 shows the UV-vis spectral changes of the thermal reaction (MV •+ → MV 2+ ) of a solution containing complex III and MV 2+ (methyl viologen) in pure water immediately aer 10 s continuous irradiation at 355 nm light.Before irradiation, the absorption spectrum of the mixture shows the characteristic absorption of starting complex ( max = 380 nm).A broad absorption band with maximum near 605 nm and a peak at 394 nm appeared just aer irradiation.ese new absorptions match the methyl viologen radical (MV •+ ) absorption spectrum [13].
e photoinduced electron transfer reactions for complexes I and II did not occur, since in the UV-vis spectra was not observed any MV •+ characteristic band even aer exhaustive photolysis.ese results show that the intermolecular photoinduced electron transfer reaction is activated in water only in certain conditions.e presence of Ru(II) unit inhibits the MV 2+ reduction.

F 1 : 1 H
NMR spectra in CD 3 CN of the aromatic region to the complex II (a) and complex I (b).

F 6 :S 1 :
Transient (upper)  and experimental (low) absorption spectra of complex I in acetonitrile.Synthesis of square complex I.

F 8 :
Spectral changes accompanying the consume of reduced methyl viologen (MV •+ ), from the thermal reaction MV •+ → MV 2+ in H 2 O, (black line-spectrum of the mixture: complex III and MV 2+ before irradiation).
T 1: 1 H NMR spectral data of complexes I and II in CD 3 CN.
[7]Ru(II) to Mn(I) (Figure2, Table1), the linewidth of all signals were broadened due to the presence of the bromide ion coordinated at the manganese center.eabsence of new signals in the whole spectrum, as expected to a triangle complex, suggests the formation of the square complex, which was further con�rmed by signals integration[7].In the T 2: Electrochemistry properties of complexes I, II, and III in CH 3 CN.