Binuclear manganese(III, IV) complexes

Binudear Manganese(III,IV) Complexes Rafael Us6n*, Victor Riera and Marian 0 Laguna Department of Inorganic Chemistry, University of Zaragoza, Zaragoza, Spain (Received June 26th, 1975)

Summary Six new compounds of the type [Mn2 (O)2(A--A)4](CIO4)3 " 2 H20 and

and 2,2'obiquinoline. Positive results were obtained only with the first four ligands. Thus, in addition to confirming the preparation of the compounds described by Nyholm and Turco, we have prepared six new Mn . . . . complexes, which are the persulphates and perchlorates of the three new cationic complexes [(A-A)2Mn(O)2Mn(A-A)2] 3+, where (A--A) -- 1,10-phenanthroline, 1,10-phenanthroline N-oxide and 2,2'-bipyridine N-oxide. Some of the properties of these complexes have been studied and compared with those of the corresponding complexes with (A-A) = 2,2'bipyridine. --

[Mn2(O)2 (A--A)4] (S2Os) l,s " nH20 where (A-A) = 1,10-phenanthroline, 1,10-phenanthroline N-oxide or 2,2'-bipyridine N-oxide, and n = 2 or 3, have been prepared by oxidising aqueous manganese(ll) sulphate solutions in the presence of the chelate ligand (A-A). The u.v. and i.r. spectra of these Complexes, as well as their magnetic behavior between 65 and 300 K have been studied and compared with those of the previously reported complexes of the same stoichiometry with (A--A) = 2,2'bipyridine.

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Results and Discussion

a) Preparation of persulpbates

Introduction Nyholm and Turco reported in a short note O) that upon oxidising aqueous solutions of manganese (II) sulphate with potassium persulphate in the presence of 2,2'-bipyridine they obtained greenish-black crystals, which contained manganese in an average oxidation state of 3.4, and whose magnetic moment was 1.7 B.M. (at 25°). They suggested that the compound was the persulphate of the cation O

"13+

bipy),Mnn.'/o~Mn'V(bipyh ] and prepared thc corresponding pcrchloratc by mctathcsis with sodium perchloratc. Attempts to prepare analogous compounds with 2,2'bipyridine N,N'-dioxide as a ligand(2) wcre not successful. It has recently been reported(3) that an attempt to grow crystalsof tns(2,2 -blpyndme)manganesc(llI) perchlo atc yielded a compound, the cation of which appears to be identicalwith that in the complex described by Nyholm and Turco. It contains Mn(III,IV), and itscrystalstructure confirms the formula of the cationiccomplcx previously proposed. In view of these factswe decided to study the feasibility of preparing similarcomplexes by using differentchelating ligands.After repeating the reaction with 2,2'-bipyridinc, wc studied the process with 1,10-phenanthrolinc,1,10phcnanthrolinc N-oxide, 2,2'-bipyridincN-oxide, pirydinc N-oxide, 4-bcnzyloxipyridineN-oxide, trimcthylamine N-oxide, triphcnylphosphinc oxide, triphcnylarsincoxide, 1,2 bis(diphcnylphosphinc oxide)ethanc, 8-oxiquinolinc, ethylene diaminc, N,N,N',N~-tctramcthylethylcncdiaminc •

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* To whom all correspondence should be directed. .

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Aqueous solutions of manganese(ll) sulphate were in every case oxidised with an excess of potassium persulphate in the presence of the chelating ligand, a molar ratio Mn: (A--A)= 1:3 being used. Although the process appears to be quite straightforward, it is most important for the successful preparation of each complex to follow the given working conditions closely. Special difficulties arose with the ligand 1,10-phenanthroline, the oxidation being retarded by a yellow insoluble precipitate, which formed at the beginning of the reaction, so that a change of colour could only be observed after ca. 25 min.

b) Preparation of percblorates Here, no general scheme can be given. In every case we attempted to oxidise the manganese(lI) sulphate with persulphate or periodate in the presence of the ligand (A-A), NaC104 being added afterwards. This method, however, was only successful with (A--A) as 2,2'-bipyridine N-oxide and persulphate the oxidant; and with (A-A) as 1,10phenanthroline and NalO4 as oxidant. When (A--A) was 1,10-phenanthroline N-oxide and 2,2'-bipyridine, yellow precipitates were formed initially, which hindered progress of the reaction. These compounds were therefore prepared in virtually quantitative yield by metathesis from the corresponding persulphates with sodium perchlorate. This method can also be used for (A-A) = 2,2'-bipyridine N-oxide and 1,10-phenanthroline. The analytical results are given in Table 1. For the perchlorates the average oxidation number was closer to 3.5 than in the case of the persulphates. The conductivities, magnetic behavior between 65 and 300 K, i.r. and u.v. spectra of all the complexes have been

Table 1. Analytical results for binuclear manganese complexes

Complex

Mn2(O)2(bipy)4(S2Os)l.S • 3H20 Mn2(O)2(bipy)4(CIO4)3 • 2H20 Mn2(O)2(phen)4(S2Os)1.s • 3H20 Mn2(O)2(phen)4(CIO4)3 • 2H20 Mn2(O)2(phen 0)4($208)1.S. 2H20 Mn2(O)2(phen 0)4(C104) 3 • 2 H 2 0

Mn2(O)2(bipy)4(S208) l.S • 3H20 Mn2(O)g(bipy 0)4(C104)3 • 2H20

Found (Calcd.) % Mn S 10.38 (9.91) 10.32 (9.98) 8.89 (9.12) 8.79 (9.18) 8.71 (8.78) 8.62 (8.71) 9.43 (9.37) 9.66 a) (9.43) 9.52b)

CI

9.13 (8.67) 9.43 (9.67) 8.27 (7.98) 8.66 (8.88) 7.78 (7.69) 8.31 (8.43) 8.22 (8.20) 8.89 (9.13) 8.94

C 43.47 (43.33) 43.62 (43.64) 46.86 (47.84) 48.29 (48.00) 45.72 (46.09) 45.60 (45.72) 40.31 (40.96) 41.25 (41.24) 40.98

H 3.19 (3.27) 2.94 (3.29) 3.46 (3.18) 2.75 (2.83) 2.70 (2.90) 2.64 (2.88) 3.00 (3.09) 2.94 (3.11) 2.92

N 10.10 (10.12) 10.11 (10.18) 9.07 (9.30) 9.25 (9.36) 8.94 (8.96) 8.87 (8.89) 9.23 (9.55) 9.42 (9.62) 9.39

Average Oxidation number 3.42 (3.50) 3.49 (3.50) 3.45 (3.50) 3.53 (3.50) 3.42 (3.50) 3.48 (3.50) 3.47 (3.50) 3.51 (3.50) 3.48

a) Oxidation with NaCIO4 + K2S208. b) From 7 + NaCIO4.

recorded. All complexes behaved analogously, which is in good accord with the supposition that they contain cations, [Mn2(O)2(A-A)4]3÷.

c) Conductivities All the complexes are stable at room temperature, even when exposed for several days to moist air and daylight, but they decompose in solution. The decomposition of aqueous solutions of the persulphate and perchlorates of 2,2'-bipyridine and 1,10-phenanthroline is, however, slow enough to allow the determination of their conductivities (cf. Experimental). The conductivities of the four compounds are similar; and because of decomposition the resulting A M values range between those expected for 2 : 1 and 3 : 1 electrolytes(4' s).

d) Magnetic susceptibilities The magnetic moments of all eight compounds, measured between 65 and 300 K are given in Table 2. The eight compounds behave as though they were magnetically concentrated. Their magnetic susceptibilities do not obey a Curie-Weiss law, and their effective magnetic moments are well below the value (4.42 B.M.) expected for the case in which no magnetic interaction between the manganese ions takes place. Not only the small effective magnetic moments b u t also their temperature dependence supports the existence of • • 73+ 4 + exchange interactions between Mn and Mn , which are likely to be transmitted by the oxygen bridge, since a direct metal-metal interaction is unlikely because of the probable large Mn-Mn distance (e.g. 2•716 fit in [Mn2(Oi~bipy2](CIO4)s(a).

e) I.r. spectra The i.r. spectra of all the compounds are in good agreement with the proposed formulae and show absorptions due to water of crystallization, to the perchlorate and persulphate anions and to the respective ligands. The absorptions arising from the water of crystallization are in the region 3500 cm -1 and 1620 cm-*, and are assigned to the stretching v(O--H) and to the bending vibration Y~(H--OH), respectively(6). In some complexes two bands are observed in these regions. The i.r. spectra of the perchlorates exhibit absorptions at 1080 (br, vs) cm -1 and at 620 (vs) cm -1, assignable to vibrations of an ionic perchlorate group with T a symmetry (7). The spectra of the [S2Os]2-group indicate that also this group is ionic(s), the bands being observed at 1270 (br, vs) c m - l a n d 660 (br, vs) cm -1 to~gether with other bands at 1000 cm "1 and 6 0 0 - 5 5 0 c m - ' . The very rich spectra of the different ligands in the 1 6 0 0 - 7 0 0 cm -1 region are modified in the complexes (9,10, n ) The shift towards higher energies of the absorptions at 1600 cm -1 and at 850 cm -1 (phen and phen 0) or at 760 cm -1 (bipy, bipy 0), which in the free ligands are assigned to ~/(C-H), suggests coordination via the nitrogen atoms(9,12). ~Fhe spetra of the N-oxides show two 7(N--O) absorptions which in the b o u n d ligand are shifted towards lower energies. Thus free phen O shows 7 ( N - O ) bands at 1270 (s) cm -1 and 1250 cm -1, whilst in complex 5 a single band occurs at 1236 (s) cm -1, and in complex 6 bands at 1251 (m) cm -1 and 1232 (s) cm -1 are to be seen. The absorptions of the free bipy O are located at 1250 (s) and 1232 (s) cm -1 , whilst the complexes 7 and 8 exhibit a single band at 1196 (vs) cm -a and 1198 (vs) cm -l, respectively. The 5 ( N - O ) vibrations of the free ligands are also slightly modified in the complexes• This indicates coordination of the ligands through the oxygen of the N-oxide groups O2' 13, 14, 15).

Table 2. Magneticsuceptibilities of binuclear manganese complexes

Table 3. U.v.diffuse reflectance spectra and oxygen bridge frequency assignments

Complex 1

Temp. (o)

~tcorr. c.g.s.u, x 106

]2eff'/at. Mn, B.M.

293.86 269.55 231.11 78.28 71.44 65.69

1.97 2.05 2.00 4.37 4.82 5.29

1.61 1.56 1.43 1.23 1.24 1.24

298.79 268.96 230.99

2.09 2.10 2.18

1.66 1.58 1.49

299.78 268.78 231.31 77.56 67.69

5.32 5.83 6.56 19.59 21.79 22.93

2.77 2.75 2.71 2.70 2.72 2.73

298.61 269.31 230.43 77.50 70.62 67.70

5.43 5.87 6.52 17.92 19.59 21.17

2.79 2.75 2.68 2.58 2.57 2.62

295.04 269.70 231.36 79.44 71.63 66.73

3.05 3.07 3.18 5.09 5.83 6.22

2.12 2.04 1.92 1.42 1,44 1.44

292.16 268.49 230.14 77.38 70.80 65.86

2.98 3.05 3.00 4.92 5.20 5.35

2.10 2.03 1.87 1.37 1.37 1.33

294.24 269.10 230.73 77.42 70.95 65.61

2.43 2.51 2.49 3.90 4.15 4.42

1.82 1.78 1.64 1.19 1.18 1.17

292.74 268.77 230.71 77.48 71.28 66.59

2.60 2.62 2.65 3.78 3.98 4.21

1.88 1.81 1.69 1.17 1.15 1.13

70.71

The assignation of the absorptions characteristic of the oxygen bridge is a matter of special interest. The existence of the bridge can clearly be deduced from the analytical data and from the low magnetic moment of the complexes. Furthermore, since no absorption bands are observed in the region 9 0 0 - 7 5 0 cm -l, a polymeric structure of the [-Mn--O--]x type is unlikely(16' I~, 18, 19). The vibrations characteristic of a double oxygen bridge between two metal atoms have been studied by several authors(17, 18, 19), all of whom place these absorptions in the 7 0 0 - 5 5 0 cm -1 region. The absorptions due to the oxygen bridge are listed in Table 3 ; though only those of the perchlorates are given,

Complex Diffuse reflectance spectra (kK) 1 2 3 4 5 6 7 8

10.5-14.3, 10,9-14.7, 11.1-14.8, 11.1-14.8, 14.3-16.7, 14.6, 12.2 sh, 15.5, 12.2sh, 15.3,

18.0 sh, 17.9 sh, 20.0 sh, 18.5sh, 17.9 sh, 18.2 sh 17.9 sh 17.9sh

Bridge frequencies (cm-') 23.3 sh 22.7 sh 22.0 sh '22.0sh 21.2 sh

686 s 652 s 580 m 690s 655m582m 680 s 648 m 583 m 692s 660sh578m 653 vs

because those of the persulphates are masked by a broad band at 670 cm -1 , together with several additional bands between 600 and 550 cm -l, which are characteristic of the anion. The three observed bands coincide perfectly with the assignations reported by Wing(19) for double oxygen bridges with a symmetry other than D2h. Thus the i.r. spectra of the complexes suggest a binuclear structure, with double oxygen bridges between the two manganese atoms.

jO U.v. spectra The absorption maxima in the vis.-u.v, regions of the spectra exhibited by the complexes were taken by diffuse reflectance and are also listed in Table 3. The spectra show great similarity; a broad band between 16,000 and 11,000 cm -1, an absorption minimum and another band with several more or less pronounced shoulders. It has not been possible to assign the bands without knowing the extinction coefficients. Notwithstanding, since the lower energy transition for the Mn (IV), 4A2~ -> "T2~ is normall~ observed (2°) at wave numbers above 18,000 cm -1, the aforementioned broad band could be assigned to the manganese(Ill). The breadth of the band along with its assymetrv may result from superposition of two absorptions, which(16, 17)should corresponding to the transitions SBlg --> SAlg and SBlg -> SB2g of the Mn 3+ ion in an octahedral field with tetragonal distortion.

Experimental The total manganese was determined by complexometry and the average oxidation state of the manganese by iodometry. A sample of the complex was reacted with an acidified KI solution and the liberated iodine was titrated with a standard thiosulphate solution (none of the ligands rea(~t with I2). The persulphate was determined gravimetrically (as BaSO4) and the calculated iodine set free by the anion was deducted from the total. The perchlorate analyses were carried out by White's method (24). The C, H, N analyses were made with a Perkin Elmer 240 microanalyzer. The conductivities were measured with a Philips PW 9501 conductimeter. The magnetic susceptibib ities, between 65 and 300 K, were measured with a balance constructed in our Department (2s). The i.r. spectra were recorded on a Beckman I R 20 A spectrophotometer (over

the range of 4 0 0 0 - 2 5 0 cm-1). U.v. spectra of the solids were recorded on a Beckman D U mod. 2 400 instrument.

Ligands 1,10-phenanthroline N-oxide was prepared b y Corey's method f21), and was identified by its melting point (181 °) (21, 22), b y the melting point of its picrate ( 2 0 3 - 2 0 5 ° ) (22) and by its i.r. spectrum (12). 2,2'-bipyridine N-oxide was prepared b y Murase's method (23), and was identified b y its melting point ( 5 3 - 5 6 0 ) (23) and i.r. spectrum (12). Other ligands were obtained from commercial sources.

5. tl-Diox o-tetrak is( l , l O-pbenantbroline N-oxide) dimanganese(lll, IV) persulpbate dibydrate, [Mn2(O)2(pben 0)4] (S20s) Ls " 2 HO. -- When K2S2Os (2.10 g,

Preparation of the complexes 1. ta-Dioxo-tetrakis(2,2'-bipyridine)dimanganese(llI, IV) persulpbate tribydrate, [Mn2(O )2(bipy)4](S2Os)LS'31-120.-K2S2Oa (3,2 g, 12 mmol) was added quickly to a magnetically stirred solution of MnSO4" H20 (1.13 g, 7 mmol) and 2,2 -blpyndme (3.28 g, 21 mmol) m 60 ml H20 at 80 . The solution gradually turned dark. After a further 10 min K2S2Os (3 g) was added, and after 5 min the resulting darkbrown crystals were filtered off (2.12 g, 55 %), washed with small portions of water, ethanol and ether and vacuum-dried over P2Os. For 10411] = 4.97 mol 1-1 at 19 °, A M = 292.0 ohm -1 cm 2 mol -~. t

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A solution of MnSO4 "IH20 (0.59 g, 3,5 mmol) and 1.10phenanthroline monohydrate (2.08 g, 10.5 mmol) in H20 (30 ml) was heated to 75 °. The addition of NalO4 (2.57 g, 12 mmol) quickly gave rise to a dark-brown solution. After a further 10 min NaIO4 (1.0 g) was added and the reaction was allowed to continue for another 5 rain. The solution was allowed to cool to room temperature, and filtered. A dark-green precipitate formed upon dropwise addition of a solution of NaCIO4 (2.5 g, 20 mmol) in H20 (20 ml). The product was purified for 10 rain b y digestion at 50 ° in a solution of NalO4 (0.5 g) in H20 (20 ml). The precipitate was then stirred in HzO for 10 min, the process being repeated with a second portion of H20. The product was finally filtered off, washed with ethanol and ether and vacuum-dried over P2Os, to give complex 4 (1.17 g, 56%). For 104 [4] = 5.43 mol 1-1 at 18 °, A M = 278.1 ohm -l cm 2 mo1-1 .

O

2. la-Dioxo-tetrakis(2,2'-bipyridine)dimanganese(llI, IV) percblorate dibydrate, [Mn2(,o)2(bipy)4](CI04)3 • 2 H20. Complex 1 (1.15 g, 1 mmol) was added to a solution of NaC104 ( 4 g , 31 mmol) in H20 (20 ml) at 20 ° and the mixture was stirred for 4 h. Although complex I did not dissolve and seemingly no change could be observed, the filtered, washed solid did not contain persulphate. Complex II: (1.19 g, 96%). For 104 [2] = 4.10 mol 1-1 at 17 °, A M = 283.8 ohm - l cm 2 mo1-1.

3. la-Dioxo-tetrakis(1, l O-pbenantbroline)dimanganese (Ill, IV) persulpbate dibydrate, [Mn2(O)2 (pben)4](S2Oa)Ls " 3 H20. -- K2S208 (1 g, 4 mmol) was added to a solution of MnSO4 • H20 (0.59 g, 3.5 mmol) and 1,10-phenanthroline monohydrate in H20 (20 ml) at 74 °. The instantaneously formed yellow precipitate (the Mn(ll)-complex with the ligand) caused the oxidation to slow down. Thus, after 25 min only a slight darkening of the solution could be observed. The addition of two 0.5 g portions of KzS2Os at 10 min intervals gave rise to a dark-brown precipitate. After a further 10 min, the mixture was poured into a sintered-glass filter funnel, set aside for 2 min, and the liquid was quickly removed with a filterpump. A maximum quantity of brown crystals can only be obtained by following these instructions closely in order to prevent the separation of white crystals, which are also formed if the mixture is allowed to cool. The yellow solid (1.23 g, 58 %) was washed with small portions of water, ethanol and ether and vacuum-dried over P2Os. For 10413] = 5.60 mol 1-t at 18 °, A M = 316.9 ohm -1 cm 2 mo1-1.

4. la-Dioxo-tetrakis(1,10-pbenantbroline)dimanganese(III, IV) percblorate dibydrate, [Mn2(pben)4 ] (Cl04)a • 2 H~O. --

8 retool) was added to a solution of MnSO4 " H20 (0.78 g, 5 mmol) and 1,10-phenanthroline N-oxide (2.72 g, 15 mmol) in H20 (150 ml) at 75 °, a yellowish-red precipitate was formed which gradually turned green. After a further 10 rain, K2S208 (2.10 g) was added to the intense-green solution, which was left to react for another 10 min at 75 °. The resulting compound was filtered off, washed successively with two portions of H20 (10 ml) at 75 °, ethanol and ether and then finally vacuum-dried to give complex 5 (1.0 g, 32 %).

6. la-Dioxo-tetrak is( l, l O-pbenantbroline N-oxide )dimanganese(llI, IV) percblorate dibydrate, [Mn2(O)2(pben 0)4](Cl04)~ " 21-120. - Complex 5 (0.4 g, 0.33 mmol) was added to a solution of NaCIO4 (5.5 g) in H20 (35 ml) and magnetically stirred for 5 h at 20 °. The resulting solid was filtered off, washed with water, ethanol and ether and vacuum-dried P2Os to give complex 6 (0.49, 99%)."

Z la-Dioxo-tetrakis(2,2'-bipyridine N-oxide)dimanganese (Ili, IV) persulpbate tribydrate, [Mn2(O)2(bipy 0)4] (S20s)l.s" 3 H20. - K2S2Oa (1.15 g, 4.3 mmol) was added to a solution of MnSO4 • H20 (0.34 g, 2.04 mmol) and 2,2'-bipyridine N-oxide in H20 (20 ml) at 45 ° . The formation of green crystals can be observed after 2 min. K2S2Os (1.1 g) was added, and the mixture was left to react for another 10 rain. The green crystals (0.30 g) were filtered off and identified as a compound with an average manganese oxidation state of only 3.39 The filtrate was left standing at room temperature to precipitate green crystals, which were filtered off, washed with water, ethanol and ether and vacuum-dried over PsOs to give 0.6 g (50%) of complex 7.

8. Iz-Dioxo-tetrakis(2,2'-bipyridine N-oxide)dimanganese (Ill, IV) percblorate dibydrate, [Mn2(O)2(bipy)4](ClO4)3"2H20. - a) A solution of K2S202 (1 g, 3.7 mmol) and NaC104 (5 g,41 mmol) in H20 (20 ml) was prepared and cooled to 0 ° in order to crystallize KC104, which was filtered off. The filtrate was added drop by drop to a solution of MnSO4 "H20 (0.34 g, 2 retool) and 2,2'bipyridine N-oxide (1.15 g, 6.6 mmol) in H20 (15 ml) at 74 ° . No change could be observed during the addition, but the formation o f green crystals was observed once the solution was cooled to 65 °. After 10 min, NaCIO4 (10 g) was added, the mixture was left to react for another I 0 rain, the

crystals were filtered off, washed with water (thus, a few yellow crystals which were mixed with the green ones disappeared), washed with ethanol and ether and vacuum-dried over P2Os to give complex 8 (0.12 g, 10%). b) Complex 7 (0.1 g, 0.09 mmol) was added to a solution of NaCIO4 (3 g) in H20 (10 ml) and stirred for 5 h at 20 ° . The resulting solid was filtered off, washed with water, ethanol and ether and vacuum-dried over P2Os, to yield 8 (0.1 g, 100%).

References 1. R.S. Nyholm and A. Turco, Chem. Ind. (London), 74 (1960). 2. R. S. Nyholm and A. Turco, J. Chem. Soc., 1121 (1962). 3. P.M. Plaksin, R. C. Stoufer, M. Mathew and G. I. Palenik, J. Amer. Cbem; Soc., 94, 2121 (1972). 4. P. L. Robinson, Experimental Inorganic Cbemistry, Elsevier, 1954, p. 378. 5. R. B. Angelici, Synthesis and Technique in Inorganic Chemistry, W. B. Saunders Co., 1969, p. 18. 6. K. Nakamoto, Infrared Spectra of lnorganic and Coordination Compounds, Wiley lnterscience, New York, 1963. 7. B. J. Hataway and A. E. Underhill, J. Chem. Soc., 3091 (1961). 8. D. P. Murtha and R. A. Walton, Inorg. Nucl. Cbem. Lett., 9, 819 (1973).

9. J.R. Ferraro, L. J. Basile and D. L. Kovanic, Inorg. Chem., 5, 391 (1966). 10. Z. Dega-Szafran, Rocz. Chem., 46, 827 (1972). 11. Z. Dega-Sza_fran,Rocz. Chem., 44, 2371 (1970). 12. A. N. Speca, N. M. Karayannis, L. L. Pytlewski, L. J. Winters and K. Kandasamy, lnorg. Chem., 12, 1221 (1973);A. N. Speca, L. L; Pytlewski and N. M. Karayannis, J. lnorg. Nucl. Chem., 36, 1227 (1974). 13. E. Contreras, V. Riera and R. Us6.'n,lnorg. Nucl. Cbem. Lett., 8, 287 (1972); Rev. Acad. Cienc. Zaragoza, XXVIII, 43 (1973). 14. M.A. Ciriano, V. Riera and R. Us,6n,unpublished observations. 15. M. Karayannis, L. L. Pytlewski and C. M. Mikulski, Coord. Chem. Re~., II, 93 (1973). 16. B.J. Trzebiatowska and W. Wojciechowski, Transition Metal Cbem. 6, 1 (1970), and references therein. 17. D. J. Hewkin and W. P. Griffith, J. Chem. Soc. (A), 474 (1966). 18. W. P. Griffith, J. Cbem. Soc. (A), 211 (1969). 19. R. M. Wing and K. P. Callahan, Inorg. Cbem., 8, 871 (1969). 20. G.C. Allen, G. A. M. EI-Sharrarwy and K. D. Warren, Inorg. NucL Chem. Lett, 725 (1969); P. C. Moews Jr., lnorg. Chem., 5, 5 (1966); B. W. Dale, J. M. Bucley and M. T. Pope, J. Cbem. Soc., (A), 301 (1969). 21. E.J. Corey, A. L. Borror and T. Foglia, J. Org. Cbem., 30, 238 (1965). 22. G.M. Maerker and F. H. Case, ). Amer. Chem. Soc., 80, 2745 (1958). 23. I. Murase, Nippon Kagaku Zassbi, 77, 682 (1956); Chem. Abstr. 52, 9100 (1958). 24. D.C. White, Microcbim. Acta, 449 (1961). 25. F. G6mez-Beltran and J. Alvarez P~rez, Rev. Acad. Cienc. Zaragoza, 22, 151 (1967). "

Effect of Coordination with the Cr(CO), Group on the Polarographic Reduction of some Alkyl and Aryl Phenyl Kentones in Dimethylformamide Alberto Ceccon*, Annamaria Romanin and Alfonso Venzo Istituto di Chimica Fisica ed Elettrochimica, Via Loredan 2, 35100 Padova, Italy (Received June 27th, 1975)

Summary The mechanism of the electrochemical reduction of (chromium tricarbonyl)-pivalophenone, -benzophenone, and -fluorenone was studied by means of polarography and electron spin resonance spectroscopy. The polarograms of the complexed ketones in anhydrous dimethylformamide consisted of two well-defined waves: the compounds are reduced to the corresponding radical anions which are further reduced to the dinegative anion. The influence of the Cr(CO)3 group on the half-wave potentials of the first and second wave is discussed. Introduction Although a considerable effort has been made to elucidate the electronic properties and the mechanism of electron dis* To whom all correspondence should be directed.

tribution in 7r-arene chromium tricarbonyl derivatives, interpretations are still controversial0 ' 2, 3) Recently, we have attempted to elucidate the effect of coordination on the reactivity of side-chain processes at and/~ carbon atoms (4). While considerable information could be obtained from kinetics, further insight on the localization and transmission of the charge can be acquired by e.s.r, and electrochemical techniques. Redox processes, in fact, which imply loss or gain of one or more electrons by the organometallic system, do not alter significantly the structure, the symmetry and the nature of the bonds of the ligands(s). Furthermore, if the electrochemical process is well defined and reversible, a correlation exists between the half-wave potential, Et/2, and the energy levels of the system (6, 7). In a very recent communication(s) we discussed the e.s.r. results for pivalophenonetricarbonylchromiumin dimethoxyethane. In the present work we describe the polaro-