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Sci.Rep_Fukushima Univ.,Na65 (2001)

Ruthenium-mediated Intramolecular Conversion of Coordinated Oxime toTerminal Nitrosyl

Dai OoYAMA* and Ayako HARAt

Faculty of Education,Fukushima University,Kanayagawa,Fukushima 960-1296,Japan

(Received April 10,2001)

ABSTRACT

The novel ruthenium(Il) complex possessing a bidentate oximato ligand is prepared。 The oxidation of the compound has been studied by cyclic voltammetry and contro11ed-poten-tial electrolysis. Based on the electrochemical and solution IR measurements,it is found that coordinated oxime undergoes a C-N bond cleavage accompanied by an oxidation of the Ru metal site to give a coordinated nitrosyl.

Keytoords:Ruthenium;0ximatoComplex;Electrochemistry; Nitrosyl

1.INTRODUCTION

Transition metal nitrosyl complexes have received much attention for many years in connection with playing a significant reje ct nitric oxide in several biologically important process' A number of nitrosyl complexes have been synthesized sofar using various methods such as reactions of coordinated ligands (e.g,nitrite-nitrosyl conversion,oxidations of coor-

dinated amines,and protonationof aminate-type ligands),external sources of NO(e.g. gaseous NO,nitrosonium salts,inorganic or organic nitroso compounds,nitrite sources,and hydrox-

ylamine), etc.2-7 It is very important to explore the new synthetic methods of nitrosyl complexes due to not only interests in coordination chemistry but also elucidation of NO evolution mechanism zn m?o. We describe here the synthesis of cis-[Ru(bpy)2(N,N'-pao)]+(bpy=2,2'-bipyridine, pao-deprotonated tom of pyridine-2-aldoxime), and the electro- chemical- and chemical oxidation of the complex. We further report that the coordinated oxime may have a potential as a useful compound for the nitrosylation. To best of our knowledge, this method, the nitrosyl complex formation accompanied by oxidation of a coordinated oxime,is the first example known in respect with the synthetic method for nitrosyl complex preparation.

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10 Ooyama and Hara: Ruthenium-mediated conversion reaction

2.EXPERIMENTAL

2.1 Chemicals and InstrumentsAcetonitrile for electrochemical experiments was distilled over calcium hydride under

nitrogen prior to use. Other chemicals were obtained as reagent grade. ct's-[Ru(bpy)2(0H2)2]- (PF6)2 was prepared from RuC13 according to the literature methods。8 Elemental analyses were performed at the Chemical Materials Center of the Institute for Molecular Science. IR spectra were recorded on a Shimadzu FTIR-8100 spectrometer using KBr pellets (solid) or KRS-5 demountable cells (CH3CN solution). 'H and'3C NMR spectra were obtained on a JEOL EX270 spectrometer. Electronic absorption spectra were acquired on a Shimadzu UV240 spectre-photometer. Cyclic voltammetry were measured with a three-electrode system and a Hokuto

Denko HAB-151 potentiostat equipped te a Riken Denshi Co..F-35C X-Y recorder. The working(0 -1.6mm)and counter(wire)electrodes were platinum,and Ag/AgN03(0_01M)was used as a reference electrode_ These electrodes were purchased from Bioanalytical Systems Co. All potentials are reported in volts vs SCE(saturated sodium chloride ca]cruel electrode) in CH3CN containing0.1 M TBA P (tetra-n-butylammonium perchlorate)as supporting electro- lyte. The Ell2 value of ferrocene used as a standard is 0。40V vs SCE in CH3CN under our conditions. All experiments were performed under a nitrogen atmosphere at a scan rate of 0.1V s-'. The controlled-potential electrolyses were performed with a HokutoDenkoHA-501 potentiostat,and the electricity consumed in the electrolyses was measured with a Hokuto Denko HF-201 oculomotor.

2。2 Synthesis of the complexThe preparation of the present compound was carried out by the following procedures.

To an aqueous solution of as-[Ru(bpy)2(0H2)2](PF6)2(100mg/20cm3)in a single-necked round-bottom flask was added Hpao(20mg,1.3eq.),and then the solution PH was adjusted to 11.6by adding several drops of aqueous NaOH sohltion. The solution was stirred at room temperature for 24h,during which the solution color changed from dark brown to red. This resulted in a red suspension,from which the red solid was separated,washed with co]d water, and dried in 11acuo. Yield:60mg (80%). Ana1.Calcd for C26H2,N60PF6Ru,0.5H20:N,12_21; C,45.36;H,3.23%. Found:N,12_22;C;45.42;H,2.79%. 'H NMR(?,CD3CN):6.73(1H),7.22- 847(20H). '3C NMR (δ,CD3CN):118.78,119.37,119.85,123.66 124.02,124。47,126.88,127.78, 128.16,136.51,137.01,137.37,137.77,139.66,150.28,151.50,152.02,152.63,152.99,157.32,157.82, 157.89,158.52(bpy and py),162.28(-CH=NO). IR(cm-',KBr):1246(vN0), Electronic spectrum (λma:K/nm(g/M-'cm-'),CH3CN):444(1.15X104),288(6.15X104),254(2_23X104),242(2.70X10). CV(vs SCE,CH3CN):? a=0.80V.

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Sci_Rep.Fukushima Univ.,Nl;t65(2001) 11

3.RESULTS AND DISCUSSION

3.1 Synthesis and Characterization of theOximatoComplexDespite the possibility of several coordination manners in mononuclear systems,.most

oximes bind metal ions via theN-site only,as in1(chart 1).9 It is explained that oximes react with bases and undergodeprotonation because they are weak oxygen acids. Thus,the N-

Chart 1

0(H)、、.、 l/ C=N、M

l

0-M 、、 l/ C= N

2

M/

-,

OlN

一一C

//

3

coordinationof oximeligands tea metal center is favorable to form the correspondingoximato ligands.'°'' Therefore, the product was formulated as bidentate (MN -coordination) pao complex,[Ru(bpy)2(M.N'-pao)](PF6)_ The diamagnetic complex is readily soluble in a range of common organic solvents such as acetonitrile, acetone, dichloromethane,etc. and slightly soluble in water. The bound oxime l,(NO)stretch band was observed af t246 cm-'in the IR spectra. The shift of the ,,(NO)tea higher frequency compared to the free Hpao(y(NO):985 cm-')on complex formation is in line with five membered cherate ring.'2 The electronic spectral data in CH3CN solution showed a characteristic band at 444nm. This is assigned as the metal-to-ligand charge-transfer (MLCT)transition.'2 'H NMR spectra of the complex showed a complicated resonance pattern which prevented an exact assignment,but the total number of protons calculated by the integration agree well with the numbers expected for the complex:for[Ru(bpy)2(MN -pao)]'-twobpy and py proton region of 20 protons occurs fromδ8.5to 7.2 in addition to the assignable resonance to the -CH=N moiety at (f 6.73. '3C NMR spectra measurements indicated a cts configuration: 23 resonances were detected in the polypyridyl region from 118to158ppm,where 24 resonances of [Ru(bpy)2(M,N'-pao)]+were expected;this disagreement in the number of resonance lines is due to their overlapping. In addition,the complex shows a carbon resonance af t62.28ppm. We assigned this resonance to the oxime carbon. Since single crystals suitable for X-ray structural work are not available, the molecular structure of the complex cannot yet be determined. However, elemental analyses,IR and NMR data support the present formulation and structure(Fig.1).

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12 Ooyama and Hara: Ruthenium-mediated conversion reaction

Figure 1 Proposed structure of cls-[Ru(bpy)2(NN -pao)]' cation

3.2 Redox Properties of the ComplexThe redox behaviors of the present complex were elucidated using some electrochemical

techniques in acetonitrile. The complex is electroactive and an irreversible 1-electron cyclic voltammetric response was observed ate。80V vs SCE using platinum as the working electrode,. which is within the potential region expected for Ru''/Ru' (Fig。2(a))_ This voltammogram indicates that another species is formed accompanying the 1-electron oxidation of the central ruthenium ion even in the CV timescale,while the other ruthenium-oximato complex shows

the reversible 1-electron redox process in the CV time scale.'2 When oxidative controlled- potential electrolysis was carried out at 298 Ken the identical solution,the new waves (E12- 0.48, 4 e--0.50 V) gradually developed (Fig. 2(b)); the experimental data from coulometry showed that l mot et electron/motet the complex (m-1.02)was released in this oxidation。 Although the current heights of the new species are smaller than that of theorigina1oximato complex due to unidentified side reactions,the new redox processes would be very similar to those of typical {RuNe}6-type nitrosylruthenium(II)complexes expressed by equation 1.34 The oximato moiety, therefore changes to the terminal nitrosyl by the chemical reaction accompanied by 1-electron oxidation。 Furthermore, we tried to confirm the formation of terminal nitrosyl moiety by chemical oxidation using a solution IR technique,

eq.1

RuIl-N0't _ e - RuIl-N00 e - Run-N0-

5

T

iNu」-M:a:n0l

V

0一P〇uCn

〇一P〇L-.g

Sci.Rep。Fukushima Univ_,Mt65(2001)

I 2ｵA

(a)

(b)

13

-1.0 0 1.0

E/V vs SCE

Figure 2. Cyclic voltammograms of a.s-[Ru(bpy)2(MN;-pao)]+(1 mmo1 dm-3) inTBAP(0.1mol dm-3)-CH3CN at298K.(a)before electrolysis;(b)after electrolysis(:;g-0.9)。

When the CH3CN solution of (NH4)2Ce(NO3)6as an oxidant was added te a solution of the complex in CH3CN,the initial red solution immediately changed to an orange one. It showed a characteristic IR absorption band which could be assigned to the terminal nitrosyl at 1942cm-'(Fig。3(b)),indicating the existence of the cationic nitrosyl. However,attempts to isolate the oxidation species with nitrosyl moiety have been unsuccessful.These results suggest that the ruthenium-mediated oxidation of the pao ligand gives the terminal nitrosyl by C-N bond cleavage of thatligand.though the details of the reaction mechanism is not yet clear(Fig.

4). Taking into account the experimental condition,the generated nitrosyl species may have a pyridine or an acetonitrile molecule as coligand. The formation of such complexes,however, was ruled out based on the disagreement of some data between the generated species and pyridine-or acetonitrile-containing complexes(Tablet).'4 The data in Table l suggest that the ligand which has better electron donor ability than that of pyridine and acetonitrile coordinates to the ruthenium ion.

It is known that freeoximes can be easily converted into the nitroso compounds by Cl2,'5 but some oxime ligands coordinated to platinum retain those moieties in the same condi- tions_'6,'7 On the other hand,it has been reported that nitrosylation of ruthenium centers by oximes were observed when RuC13 was treated with some oxime derivatives.'8,'9 The results of the present work may explain that theoximatocomplex arising from the metal-oxime bond formation is a key intermediate in the above nitrosylation of RuC13. The isolation and characterization of the formed nitrosyl compound,and elucidation of the nitrosylation mecha- nism are now in progress.

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14 Ooyama and Hara: Ruthenium-mediated conversion reaction

(a)

(b)

L pyccH

CNC 1970 0.56

22002000 1900 1800 1700

wavenumber/cm-1

Figure 3. Solution IR spectra of as-[Ru(bpy)2(MN -pao)]-' in nitrosyl region.(a)before oxidation;(b)after oxidation(in CH3CN).

0l

Ru- N=C::: e 一 CIVbond cleavage Ru- N≡0+

Figure 4, Reaction scheme of the conversion from coordinated oxime to terminal nitrosyl

Table l IR and Electrochemical Data for Related Nitrosylruthenium Complexes

This work L=

IRa/cm-' 1942 1953

E 0.48 053

-0.50 -0.37 -0.35

a In CH3CN. b vs.SCE in CH3CN at298K. C Ref.14

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Sci_Rep.Fukushima Univ.,№65 (2001) 15

4. SUMMARY

We conclude that the electrochemical and solution IR spectroscopic evidences presented here indicate that the oxidation of oximato complex can play an important role for the intramolecular nitrosylation process. It is to note that theoximatocomplex has added a new dimension in nitrosyl synthetic chemistry. Sofar,the synthetic method of nitrosyl complex by intramolecular NO affording has not been reported,the studies on this field would develop further from the viewpoint of synthetic chemistry of nitrosyl complexes.

ACKNOWLEDGEMENT

This work was supported by the Joint Studies Program (1999-2000)of the Institute for Molecular Science.

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l6 Ooyama and Hara: Ruthenium-mediated conversion reaction

REFERENCES AND NOTES

†Present address Sibamiya Elementary School,Asaka,Koriyama963-0111,Japan。1 J.Lancaster,in''Nitric Oxide:Principles and Applications'Academic Press,San Diego

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Chemistry''Pergamon Press,Oxford(1987),Vol_2,pp.99-159.4 A.R.Butler,C.Glidewe11,and M.-H.Li,Ad,v Iriorg Chem.,32,335(1988).5 D.M_P.Mingos and D.J.Sherman,Adv.fnorg.Chern.,34,293(1989).6 F.Bottomley,in “Reactions of Coordinated Ligands''Plenum Press New York (1989),

Vol.2,pp.115-222.7 G.B.Richter-Addoand P.Legzdins,in ''Metal Nitrosyls,''Oxford University Press,New

York (1992).8 E.C.Johnson,B.P.Su11ivan,D.J.Salmon,.S.A_Adeyemi,and T.J.Meyer,.fnorg.Chem_,

17,2211(1978).9 S.Ganguly,S.Karmakar,C.K.Pa1,and A.Chakravorty,1norg.Chem.,38,5984(1999). 10 V.Y.Kukushkin;D.Tudela,and A.J.L.Pombeiro,Coord.Chem.Re11,l56,333(1996). 11 V.Y.Kukushkin and A.J.L.Pombeiro,Coord Chem.RelJ.,181,147(1999).12 P.K.Das and B.K_Ghosh,Polyhedron,13,2665(1994).13 J_H.Enemark and R.D.Feltham,Coord Chem.Rev.,13,339(1974).

Mononitrosyl complexes are conveniently described an{MN0}n,where n is the number ofthe d-electron on the metal atom when the NO group is formally bound as,NO+.

14 R.W.Ca1lahan and T.J.Meyer,fnorg.Chem.,16,574(1977).15 M.W.Barnes and J.M_Patterson,.J Org.Chem.,41,733(1976)and references therein_ 16 V.Y.Kukushkin,V.K.Belsky,V.E.Konova1ov,E.A.Aleksandrova,and E.Y.Pankova,

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kosyan,inorg_Chem.,31,3836(1992).18 Y.N.Kukushkiri,M.V.Bavina,and A.V.Zinchenko;Zh Obstch.Khi'm_,66,1572(1996). 19 Y.N.Kukushkin,M_V.Bavina,and A.V.Zinchenko,Zh.0bstch.Khim.,66,1591(1996).