Reactivity of dioxygen and nitric oxide with manganese complexes containing hexadentate ligands

J. inorg, nucl ('hem. Vol. 4.2, pp. 683~-,87 Pergamon Press Lid.. 1980. Printed in Greal Britain

REACTIVITY OF DIOXYGEN AND NITRIC OXIDE WITH MANGANESE COMPLEXES CONTAINING HEXADENTATE LIGANDS W. M. COLEMAN Naval Bioscience Laboratory. Naval Supply Center, Oakland, CA 94625, U.S.A. and L. T. TAYLOR* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U.S.A.

(Received 25 May 1979: receivedfor publication 10 September 1979) Abstract--Manganese complexes employing hexadentate ligands derived from salicylaldehyde and pyridine-2carboxaldehyde condensed with various linear tetramines have been prepared. These high spin manganese(lI) and (111) materials have been characterized by elemental analyses, IR-visible-UV spectra, magnetic susceptibility measurements and electron spin resonance spectra. The reactivity of these complexes with dioxygen and nitric oxide is described. Neutral pyridine-2-carboxaldehyde-derivedligands do not promote reactivity with O2 and NO, whereas, salicylaldehyde-derivedligands do. In the latter case substituents on the aromatic moiety demonstrated it dramatic effect on reactivity. INTRODUCTION Manganese complexes employing linear pentadentate ligands and their dioxygen reactivity have been actively studied over the last few years[l]. For example, five coordinate high spin, yellow manganese(ll) complexes of ZSALRDPT, I, have been prepared and characterized. Each complex rapidly reacted with 02 in solution and one derivative (Z=3-OCH3, R = CH3) reacted in the solid state. The extent and rate of 02 uptake was observed to be a function of the aromatic(Z) and amine(R) sub-

stituents. Employing rigorously anhydrous conditions two oxygenated complexes could be isolated[2] [Mn(5NO2SALRDPT)]202 (R=H, CH3). Extension to the 3nitro derivative gave an analogous material. Extension of this study to pyridine-2-carboxaldehyde related ligand:s revealed a complete absence of dioxygen reactivity[3]. Other workers have noted somewhat similar ligandrelated reactivity with manganese [4-7]. Investigations dealing with manganese complexes which incorporate a hexadentate ligand are indeed sparse.

~ O H

Z

Z~

H O ~

C"-N--(CH2)3--1~ ~(CH2)3-N-'C" V\z R

~"-"N- [CH2],,--~- [ C H ~ N : ~ " ~ H

W 2 3 3

X 2 2 3

H W 2 "~

X 2 2

Y 2 3

Y 2 3 3

H

N

Z H,5-NO~ H,5-NO~ H,5-NO~

NAME ZSAL 1,4,7,10 ZSAL 1,5,8.12 ZSAL 1,5,8.13

H

H

Z NCS , I NCS

NAME (PY 1,4,7,10) (PY 1,5,8,12) III 683

Z H

IH

684

W. M. COLEMAN and L. T. TAYLOR

Manganese complexes possessing ligands derived from substituted salicylaldehydes, !I, and pyridine-2-carboxaldehyde, III, have been synthesized. We wish to describe here the properties of these materials and their reactivity with nitric oxide and dioxygen.

precipitation was prevented by maintaining a low heat setting. After formation of the manganese(lI) complex excess NH4NCS was added as a solid which was allowed to dissolve. The N2 was then removed and dry air was bubbled through the solution. The initial red-orange solution changed to a brownish-green color followed by precipitation of a brownish material which was isolated via filtration, washed with methanol and dried in vacuo over CaCl2. Additional manganese(lit) complexes were prepared in a similar fashion. Physical measurements. IR absorption spectra in the region 5000--650cm -~ were obtained employing Nujol mulls with a Beckman IR-SA recording spectrophotometer. Magnetic susceptibility data were obtained at room temperature by the Faraday method. Diamagnetic corrections were made using Pascal's constants. UV-visible spectra were obtained with a Varian Cary 219 spectrophotometer. Elemental analyses were performed by the Microchemical Analysis Laboratory, Department of Chemistry, University of California, Berkeley, Calif. ESR spectra were obtained on a Varian Spectrometer.

EXPERIMENTAL Materials. Salicylaldehyde, pyridine-2-carboxaldehyde and 1,4,7,10-tetraazadecane were obtained from Aldrich Chemical Co. 5-Nitrosalicylaldehyde was purchased from Eastman Organic Chemical Co. 1,5,8,12-tetraazadodecane and 1,5,9,13-tetraazatridecane were obtained from Strem Chemicals, Inc. Mn(OAc)2.4H20 and MnI2'4H20 were obtained from Alpha Inorganics. Nitric oxide was provided by Matheson Gas Products. All other materials were reagent grade or equivalent. Preparation of manganese(H) complexes. Mn(II)(ZSAL 1,4,7,10t. To a stirring solution of the appropriate salicylaldehyde (0.02mole) in 30ml of isopropyl alcohol, was added 1,4,7,10tetraazadecane (0.01 mole) dissolved in 30 ml of isopropyl alcohol. The resulting yellow solution was brought to reflux under N2 and held there for 0.5 hr. After removing the heat 0.02 mole of KOH dissolved 10 ml of O2-free methanol was added. To this red solution was then added 0.01 mole of Mn(OAc)2.4H20 dissolved in 20 ml of O2-free methanol. Depending on the complex, precipitation occurred within 0.5-2 hr after Mn addition was complete. The complex was isolated under a N2 blanket, washed with O2-free methanol and dried in vacuo over CaCI2. Mn(II)(ZSAL 1,5,8,12) and Mn(II)(ZSAL 1,5,9,13) were prepared in a similar manner using the appropriate tetraamine. Mn(II)(Py 1,4,7,10)(NCS)2. To a stirring solution of pyridine-2carboxaldehyde (0.02mole) in 30ml of isopropyl alcohol was added 1,4,7,10-tetraazadecane (0.01 mole) in 30ml isopropyl alcohol. The resulting yellow solution was refluxed for 0.5 hr after which the solution, still under N2, was cooled to room temperature. To this pre-formed ligand solution was added dropwise Mn(OAc)2.4H20 (0.01 mole) dissolved in 30ml of Orfree methanol whereupon an orange solution resulted. Stirring under N2 was continued for - I h r after which an excess of solid NH4NCS (-0.05 mole) was added. Precipitation of a bright yellow material occurred within 12hr under continuous stirring which was isolated, washed and dried as previously described. The analogous iodide complex Mn(II)(Py 1,4,7,10)!2 was prepared and isolated in the manner outlined above with the exception that Mn12'4H20 was substituted for the acetate salt. Mn(ll)(Py 1,5,8,12)(NCS)2could be prepared via the above method utilizing 1,5,8,12-tetraazadodecane. Mn(III)(ZSAL 1,4,7,12)(NCS). The procedure for the preparation of the comparable Mn(lI) complex was followed except that

RESULTS AND DISCUSSION Manganese complexes of the formula Mn(II) (ZSALAM) and Mn(III)(ZSALAM)(NCS) where ZSALAM represents the 2:1 aldehyde condensation product employing either 1,4,7,10-tetraazadecane, 1,5,8,12-tetraazadodecane or 1,5,9,13-tetraazatridecane and Z=H, 5-NO2 have been synthesized (Table 1). The Mn(II) complexes range in color from bright yellow to orange while the Mn(III) complexes are very dark greenbrown. Manganese(II) complexes of the formula Mn(PY 1,4,7,10)X2 and Mn(PY 1,5,8,12)X2 have also been synthesized. They are also bright yellow in color. Manganese(H) complexes The manganese salicylaldehyde complexes are insoluble in water but soluble in methanol, chloroform and dimethylformamide. Since some of the Mn(II) complexes were air sensitive, IR spectra were obtained without delay. Table 2 lists some pertinent IR data. The C=N stretching frequency of the Schiff base linkage falls between 1630-1625 cm i indicative of imine coordination[8]. With one exception, the IR spectra also show a single sharp N - H stretch between 32603200cm -1 diagnostic of a coordinated secondary amine. The sole exception to this observation is Mn(II)(SAL 1,5,9,13).2CH3OH which exhibits a doublet in this region.

Table 1. Analytical data Complex

%C

%H

%N ~eff

C

F

C

48.28

47.89

4.47

Mn(SAL 1,5,8,12)*I.SCH30H

58.38

58.58

Mn(5-NO2SAL 1,5,8,12)

50.29

50.35

Mn(SAL 1,5,9,13)I2CH3OH

58.42

58.07

Mn(5-NO2SAL 1,3,9,13)

51.20

31.60

Mn(P¥ k,4,7,10)(NCS) 2

48.47

48.19

Mn(PY 1,4,7,10)(1) 2

34.14

Mn(PY 1,5,8,12)(NC8) 2

50.46

Mn(SAL 1,4,7,10)NCS-I/2n20

53.16

Mn(5-NO2SAL 1,4,7,1O)NCS,I/2H20

44.69

Mn(SAL 1,5,8,12)NCS

55.97

M~(SAL 1,5, ,13)NCS

56.80

Mn(5-NO2SAL 1,4,7,10)

C

B.M.

4.44

16.90

16.60

5.84

7.09

7.08

11.58

11.17

6.17

4.99

5.07

16.00

16.41

5.96

7.45

7.17

10.91

10.47

6.06

5.24

5.27

15.58

15.61

5.86

4.89

4.92

22.61

22.32

5.96

34.40

3.83

4.02

13.27

13.14

5.85

50.42

3.40

5.45

21.40

21.42

5.84

52.86

5.32

5.21

14.77

14.42

4.94

44.59

4.12

3.66

17.38

17.23

5.06

56.03

5.72

5.72

14.19

13.95

4.70

56.77

5.97

5.96

13.80

13.79

--

685

Reactivityof dioxygenand nitric oxide with manganesecomplexes Table 2. IR data~

Mn(5-NO2SAL 1,4,7,10)

3210 sh

1630

--

Mn(SAL 1,5,8,12).1.5 CH30H

3230

1625

--

Mn(5-NO2SAL 1,5,8,12)

3260 sh

1630

--

Mn(SAL 1,5,9,13).2CH30H

3250,3200

1625

--

MnCS-NO2SAL 1,5,9,13)

3200 sh

1630

--

Mn(PY 1,4,7,10)(NCS) 2

3220

1610

2040

Mn(PY 1,4,7,10)I 2

3200 sh

1600

--

1.5.8,12)(NCS)2

3200 sh

1600

2040

Mn(SAL 1,4,7.10)NCS.I/2 H20

3300 br

1610

2045

Mn(5-NO2SAL 1,4,7,10)NCS.I/2 H20

3250

1630

2050

Mn(SAL 1,5,8,12)NCS

3190 br

1620

2040

Mn(SAL 1,5,9,13)NCS

3300 br

1625

2045

Mn(PY

a

cm

-1

The second peak (3250 cm-') could possibly arise from a molecule of non-bonded CH3OH or a slightly different environment for the two secondary nitrogen donors in this complex. Since all of the Mn(tI) complexes show absorption in the region expected for coordinated secondary nitrogen it seems reasonable to conclude that all six donors are coordinated thereby forming an octahedral manganese complex. The ESR spectra of all Mn(II) complexes were consistent with other known high spin Mn(II) complexes[9]. All spectra were taken at room temperature in DMF/toluene solutions and had signals at g values of 4.2 and 2.0. Six-line spectra occurred in all cases indicative of coupling of the unpaired electron with the Mn(II) nucleus[14]. The coupling constants were -90gauss which is again consistent with a high spin Mn(II) ion. The absence of '4N splitting would suggest that there is little or no electron delocalization from the metal center onto the O2N4 or N6 ligand donors. If any delocalization were occurring, then a '4N coupling constant of 30-35 gauss would be expected to be observed[10]. Magnetic susceptibility measurements (Table 1) also indicate the high spin nature of each complex. The Mn(II) complexes derived from PY have IR spectra very similar to that of their SAL counterparts. There is, however, a sharp peak at 1570 cm ' present in the PY complexes that is absent in the SAL series. This peak is assigned to a ring vibration of the pyridine ring. The expected other 3 ring vibrations are masked in the spectra and not readily discerned. The Mn(II)(PYAM)X2 complexes have ESR spectra consistent with high spin Mn(II) which is confirmed by solid state magnetic susceptibility data (Table 1). The Mn(II)(PYAM)X2 complexes possess solubility in a variety of alcohols, DMF, DMSO, pyridine, etc. These solutions are stable in the air for several weeks. This is in marked contrast to the behavior of the comparable Mn(II)(SALAM) complexes which are extremely air sensitive when in solution.

Manganese(lll) complexes The IR spectra of the Mn(III) complexes also show that the imine N of the C=N is coordinated. Un-

fortunately the N-H stretching frequencies in the Mn(III) complexes are broad, and do not lend themselves to easy interpretation. The complexes demonstrate a C--N stretch at ,-2045 cm ~ indicative of an N-bonded thiocyanate. This assignment for N-bonded coordination is further substantiated by the presence of a C-S stretching frequency between 840 and 800 cm '[11]. The position of these frequencies would suggest that the thiocyanate complexes are seven coordinate. However, with only IR evidence available seven coordination at this point must be considered tenuous. The Mn(III) complexes are ESR silent which is predicted for even electron systems [9]. Magnetic susceptibility measurements were made on the complexes at room temperature and these data (Table 1) support the conclusion that the Mn(III) complexes are of a high spin electron configuration. In summary these materials behave as normal manganese(Ill) complexes [12].

Dioxygen reactivity Recently we published our results on the reaction of dioxygen with a series of pentacoordinate Mn(ll) complexes[l, 2]. In this discussion we wish to compare the results published there with our new findings on hexadentate ligands. With pyridine-2-carboxaldehyde derived ligands no dioxygen reactivity was observed either with 5- or 6-coordinate ligands. In the ZSALDPT case, all of the 5 coordinate Mn(II) complexes were shown to be reactive with dioxygen. In contrast, the 5-NO2SAL derivatives of the complexes cited here are surprisingly unreactive toward dioxygen in the solid state and in solution. No change in color, visible spectrum, IR spectrum or ESR spectrum occur when the complexes are exposed to 02. Starting materials were recovered in all 5-NO2SAL cases. A number of postulates for this non-reactivity come to mind. First, the complexes appear to be octahedral, thereby leaving no available site for 02 attack which is, otherwise, available in the 5 coordinate ZSALDPT series. The X-ray structure[13] of Fe(III) (SAL 1,4,7,10)CI confirms this to some degree in that a]ll donor atoms except CI are indeed coordinated to the metal center. Secondly, as shown in the ZSALDPT series, nitro substituents render the manganese(Ill) state

686

W.M. COLEMAN and L. T. TAYLOR

-3

4,5

3Go

4~o

560 nm

s~

?60

Fig. 1. Oxygenation of [Mn(II)(SAL 1,5,9,13)] 2CH3OH, 1O-3M

in CH~OH. (1) 02 free solution; (2) Solution plus 02, 5 min; (3) Solution plus 02, 10rain; (4) Solution plus 02, 15rnin; (5) Solution plus 02, 20 min. less stable[14], therefore, oxygenation via an oxidative addition route should be less favored. Quite possibly the electronic effect maybe a more cogent argument since Mn(II)(SAL 1,4,7,10) etc. are reactive toward 02 both in the solid state and solution. Figure 1 illustrates a UV-visible spectral study of the reaction of Mn(SAL 1,5,9,13).2CH3OH and dioxygen in

methanol as a function of time. Curve 1 is the spectrum of the manganese(II) precursor prior to oxygenation of the solution. The absence of absorption in the visible region is expected. The well-defined peak at 344 nm is no doubt ligand related (e.g. intra-ligand or metal-ligand charge transfer). When dioxygen is admitted to the solution the band at 344 shifts to higher energy and appears as a shoulder of slightly lower intensity. A new broad band centered at 510 nm appears which is assignable to a d-d transition on the now formally manganese(Ill) complex. Bubbling N2 through the solution does not regenerate the starting spectrum. A similar pattern is found for the other unsubstituted salicylaldehyde complexes (Fig. 2). Oxygen uptake experiments as a function of time were carried-out on these complexes in a manner similar to that previously reported. Oxygen consumption is immediate and continues well past noJnM,= 1.0. This behavior is reminiscent of the analogous manganese(II) pentadentate ligand (ZSALDPT) species (Fig. 3). It is interesting to note that some difference in dioxygen reactivity was noted between Mn(II)(SAL 1,5,8,12) and Mn(II)(SAL 1,5,9,13). This difference in behavior will manifest itself again in the solid state. The continuous absorption of 02 no doubt, is due to oxidation of manganese as well as the coordinated tigand. The latter fact precluded isolation of a characterizable product. Changing the solvent was found to produce no significantly different oxygenation observations. As mentioned previously, the HSALAM complexes react with dioxygen in the solid state. Mn(II)(SAL !,4,7,10) is extremely reactive and attempts to isolate the pure Mn(II) complex have met with failure. Mn(II)(SAL 1,5,9,13) also reacts with dioxygen in the solid state but very slowly with the complex only gradually changing color over approximately a month duration. Mn(II)(SAL 1,5,8,12) is intermediate in its reactivity with dioxygen. Bright orange platelets of the Mn(II) complex turn silvery-brown within one week after exposure to dioxygen. This silvery-brown material has a UV-VIS spectrum in methanol very similar to that of the oxidized material in solution. It seems, therefore, that the complex is oxidiz-

2.0

1.5

NO;, NMn

IO

.5.

/'J¢

0 Mn (SALI,5,8,12) • Mn" (SALI, 5, 9, 13)

~/ 35o

.bo

5oo

65o

~6o

nm

Fig. 2. Oxygenation of [Mn(II)(SAL 1,5,8,12)], 10-3M in CH3OH. (I) 0, free solution (2) Solution plus 02, 5rain; (3) Solution plus 02, 10 rain. Other traces at 5 rain intervals.

I I

I 2

I I I I I I 3 4 5 6 7 8 TIME, HOURS Fig. 3. 02 uptake vs time for various Mn(II) complexes in pyridine.

Reactivity of dioxygen and nitric oxide with manganese complexes ing to Mn(III) in the solid state. In addition, the IR spectrum of the oxidized material contains a new band at 630cm-' attributable to a Mn-O stretch [15,16]. An oxygen uptake experiment was conducted on Mn(II) (SAL 1,5,8,12) in the solid state. Reaction with dioxygen was confirmed. The complex continued to consume dioxygen well beyond the stiochiometric ratios of n ~ / n M , = 0.5 or 1.0. There was, nonetheless, a deflection in the consumption curve at no~./nM, = 1.0 which occurred between day 8 and 10. Nitric oxide reactivity

Chloroform solutions of the manganese(II)-salicylaldehyde related complexes readily react with nitric oxide to yield red solutions which when exposed to air rapidly turn green. In some cases a fluffy red material forms from the CHCI3 solution. Exposure of this red solid to air produces a green product. These results parallel those found for the ZSALDPT case [17] wherein the red and green solution corresponded to formation of a manganese(I)-nitrosyl and a manganese(III)-nitrite species respectively. In the case of Mn(II)(SAL 1,5,8,12), we were able to isolate a product characterized as Mn(III)(SAL 1,5,8,12)(NO2). The unreactivity of the 5nitro derivatives persisted in that none of these materials reacted with NO either in solution or in the solid state. The (PY 1,4,7,10) and (PY 1,5,8,12) complexes, likewise, possessed no reactivity with NO regardless of experimental conditions. Reaction of manganese(Ill) complexes with NO was studied spectrophotometrically in DMF (Fig. 4). Thd d - d transition at 546nm disappears entirely when NO is bubbled through the solution. This observation is interpreted to be a Mn(llI)/Mn(II) reduction. The reaction is

Uf2

400

500

600

700

8O0

nm

Fig. 4. Nitric oxide reaction with [Mn(III)(SAL 1,5,8,12)]NCS, 10 3 M in DMF. (1) O2 free solution; (2) Solution plus NO under N, cover for 15 min; (3) Solution No. 2 plus N2 for 15 rain.

687

irreversible under these conditions since degassing the, solution with N2 does not regenerate the original spectrum. The nitrosyl solutions exhibit a six-line pattern ESR spectrum (coupling = 89gauss) indicative of high spin manganese(II)[10]. The data described here provide further support regarding the need for charged ligands in order to promote manganese-small molecule reactivity. This result might be expected if we consider the reaction with dioxygen to be an oxidative addition. In other words, the better donating dianionic ligand should be more effective in the stabilization of a higher oxidation state. This appears to especially be the case with all nitrogen and mixed oxygen-nitrogen donors. The "charged-ligand-requirement" does not universally promote reactivity with O2 in the case of all oxygen donors. Salicylaldehydo and acetylacetonato complexes, for example, are insensitive to dioxygen[18]. A rationalization for the nitric oxide reactivity pattern is less clear. More extenswe metal-ligand orbital overlap may, in fact, accompany the better polydentate donor resulting in more effective dispersal of charge. Electrochemical experiments are in progress on these materials in order to test these hypotheses.

Acknowledgements--This research was supported in part by NIH Research Grant 21844-03.

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