Axial ligational properties of macrocyclic cobalt complexes

Polyhedron Vol.

~

Pergamon

PII:S0277-5387(97)00296-9

17, No. 8, pp. 1355 1361, 1998 @ 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0277 5387/98 $19.00+0.00

Axial ligational properties of macrocyclic cobalt complexes M. Radhakrishna Reddy, K. Hussain Reddy* and K. Mohana Raju Department of Chemistry, Sri Krishnadevaraya University, Anantapur 515 003, India (Received 20 February 1997; accepted 27 June 1997) Abstract--Macrocyclic cobalt complexes have been prepared by template synthesis using acetylacetone and different aromatic diamines viz. 1,2-diaminobenzene and 3,4-diaminotoluene for the first time. These complexes have been alkylated using CH3I and/or C2HsBr in the presence of pyridine and/or imidazole to obtain corresponding macrocyclic organocobalt complexes. The parent cobalt complexes and their alkyl derivatives have been characterized by elemental analyses, conductivity data, magnetic susceptibility measurements, electronic, infrared and ~H N M R spectral data. Various ligand field parameters have been calculated. Electrochemical behaviour of these complexes has been studied by cyclic voltammetry. Axial ligation properties of parent cobalt complexes are supported by syntheses, electronic spectra and cyclic voltammetric studies. © 1998 Elsevier Science Ltd. All rights reserved

Keywords: macrocyclic cobalt complexes; ascial ligation; template synthesis; electronic spectra; cyclic voltammetry.

The cobaloximes [ 1] prepared using dimethylglyoxime ligand have been used as models for many B12 reactions. Subsequently cobalt complexes of SALEN [2,3] ACACEN [4] and oxime-schiff base ligands [5] have been derived as models for vitamin B~2.There has been considerable interest in the chemistry of macrocyclic metal complexes [6] which are considered as better model compounds for metalloporphyrins and metallocorrins for their intrinsic structural properties. Spontaneous self-assembly reactions have been considered as vehicles for reliable and economical preparation of macrocyclic complexes. Although macrocyclic nickel(II) and copper(lI) complexes derived using substituted acetylacetone and unsubstituted aromatic diamines have been synthesized and characterized to demonstrate the advantages of template synthesis by Jaeger [7 11] and Chipperfield [12], the corresponding macrocyclic cobalt complexes are not investigated. Recently, we have reported [13-15] macrocyclic nickel(II) and copper(II) complexes of acetylacetone buckled with different diamines. In the light of lacuna identified and in continuation of our earlier studies herein we report the synthesis, characterization, spectral studies and electrochemistry of macrocyclic cobalt complexes (I) and (VI) derived * A u t h o r to w h o m c o r r e s p o n d e n c e s h o u l d be a d d r e s s e d .

using acetylacetone and different diamines viz. 1,2diaminobenzene and 3,4-diaminotoluene. Studies on corresponding alkylated derivatives are also carried out in support of axial ligational properties.

EXPERIMENTAL All the solvents used were of AR grade. Cobalt(II) chloride hexahydrate (Qualigens) : 1,2-diaminobenzene and 3,4-diaminotoluene (Fluka AG) ; and iodomethane (Merck), bromoethane (Merck), pyridine (Qualigens) and Imidazole (spectrochem) were used in the present study.

Synthes& of complexes I and VI A 250 cm 3 Erlenmeyer flask was charged with spinbar, cobalt(II) chloride hexahydrate (2.298 g, 0.0097 mol) aromatic diamine (0.0194 mol), dry butanol (30 cm 3) and acetylacetone (2.0 cm 3, 0.0194 mol) added by syringe. The flask was set upon a hot plate stirrer and equipped with a reflux condenser. The mixture was brought to a brisk reflux and continued for 8 h. The flask was removed from the hot source and allowed to cool. Then methanol (15 cm 3) was added and the mixture was cooled on ice-salt bath for at

1355

1356

M. R. Reddy et al.

least 15 min. Then an oily layer was separated out. The oily layer was treated with water and stirred vigorously to obtain black coloured substance. It was collected, washed with hot water, then with cold methanol and dried in t~'acuo.

anol and ether and then dried in vacuo. The yield, colour and melting points for all the complexes are given in Table 1.

Synthesis ofalk)'l derivatives

The elemental analyses were performed by the H E R A E U S (Mikro standard 8304071) Carbon and Hydrogen Analyzer. The magnetic susceptibility measurements were made using a vibrating sample magnetometer (VSM) operating at a field strength of 2 8 KG. The electrical conductivity measurements were made in dimethylformamide ( D M F ) (ca 10 -3 M) at r o o m temperature (27 + 2°C) using a Systronic 303 direct reading conductivity bridge. The electronic spectra for all complexes in the U V ~ i s region (1801100 nm) were recorded with Schimadzu UV-160A spectrophotometer. The I R spectra were recorded in the ranges 4 0 0 0 4 0 0 c m - ~ (in KBr) and 450 50 cm (in polyethylene) using Bruker IFS 66V F T - I R Spectrophotometer. ~H N M R spectra were recorded on an

In a Schlenk tube, a saturated methanolic solution of macrocyclic cobalt complex (I) and (VI) (0.0042 mol) was taken and stirred under nitrogen atmosphere. Sodium hydroxide (0.0042 moles) and pyridine/imidazole (0.0021 mol) were added to the contents of the tube. The stirred suspension was then cooled to - 5 ' C and stirred for 15 min. Then solid NaBH4 (0.0042 mol) followed by alkylating agent (CH3I or C2HsBr, 0.0021 mol) were added. The solution was gradually warmed to 20~C and stirred for a further 30 min. The solution was evaporated to half the volume and stirred with 20 cm 3 of water and filtered. It was collected washed with aqueous meth-

Physical measurements

Table 1. Colour, partial elemental analyses and conductivities of the cobalt complexes Molar conductance (cm2 Ohm- i

Found (Calc.) %

Colour Molecular Formula

(M.P.. 'C)

C

H

N

mol i)

[Co(C22H22N4)]

Blackish brown (250 D)

65.98 (65.85)

5.56 (5.49)

14.02 (13.97)

4.1

[CH3Co(C22H22N4)CsHsN]

Black ( > 300)

67.49 (67.88)

6.01 (6.06)

11.61 (11.31)

6.1

11I

[CH3Co(C22H22N4)C3H4N2]

Black ( > 300)

64.40 (64.47)

5.92 (5.99)

11.41 (11.57)

8.3

IV

[C2HsCo(C22H22N4)CsHsN]

Blackish brown (289 D)

68.98 (68.36)

6.30 (6.29)

11.12 (11.00)

2.0

[C2HsCo(C22H22N4)C3H4N2]

Brown (> 300)

65.05 (65.07)

6.11 (6.23)

11.15 (11.24)

4.2

Complex

1

V1

[Co(C24H26N4]

Blackish brown (193 D)

67.20 (67.15)

6.03 (6.06)

13.03 (13.06)

2.3

VII

[CH3Co(C24H26N4)C~HsN]

Blackish brown ( > 300)

68.76 (68.97)

6.49 (6.51)

10.58 (10.71)

12.0

VII1

[CH3Co(C24H26N4)C3H4N2]

Brown (292 D)

65.59 (65.64)

6.52 (6.45)

10.95 (10.93)

8.1

IX

[C2HsCo(C24H26N4)CsHsN ] ( > 300)

Brown 69.18 (69.27)

69.18 (69.27)

6.66 (6.70)

10.44 (10.43)

16.0

[C2HsCo(C24H26N4)C3H4N2]

Brown (>300)

66.19 (66.17)

6.61 (6.65)

10.61 (10.65)

6.3

Macrocyclic cobalt complexes AMX-400 MHz high resolution N M R instrument in DMSO-d6 solvent at room temperature. The cyclic voltammetry was performed with a BAS model CV27 controller and a conventional three electrode, Ag/AgC1 reference electrode, glassy carbon working electrode and platinum counter electrode. Nitrogen was used as a purge gas and all solutions were 0.1 M concentration in TBACIO4. RESULTS AND DISCUSSION Attempts to isolate free ligands using acetylacetone and aromatic diamines were, however, unsuccessful. Hence the corresponding macrocyclic complexes were prepared by using the template method. All the complexes are insoluble in water but readily soluble in methanol dimethylformamide and dimethylsulfoxide. The conductivity measurements show their non-electrolytic nature and the molecular weights of cobalt complexes are consistent with monomers. Maynetic susceptibility and electronic spectra The magnetic moment values of parent macrocyclic cobalt complexes are found to be in the 2.1-2.7 BM range as expected for low spin cobalt(II) complexes [16]. All alkyl derivatives (II, III, IV, V, VII, VIII, IX and X) are found to be diamagnetic. In the electronic spectra of macrocyclic cobalt complexes (I and VI) (in DMF), two bands are observed in 16,103 16,530 (v2) and 22,371-22,936 cm t (v3) assigned to 4Tig(F ) ~ 4A2g(F ) and 4Tlg(F ) ~ 4TLg(P ) transitions respectively in favour of an octahedral geometry [17]. The vt values are calculated using the following equation, vl = 5 D q - 7 . 5 B + I / 2 ( 2 2 5 B 2 + 1 0 0 D q

2

1357

The interelectronic repulsion (B) and 10 Dq for all cobalt complexes are found to be in the range of 916-940 and 8074-8359, respectively. The ratio of observed v2 to calculated v~ lie between 2.22 and 2.36 as required for an octahedral cobalt complex [18]. The values of Dq, B, B35 and v2/v3 are given in Table 2. The electronic spectral data suggests an octahedral geometry for the present cobalt complexes facilitated by axial coordination of solvent (DMF) molecules. Electronic spectral data of alkyl complexes are given in Table 3. The most intense bands in the highest energy region have been assigned [19] to = -~ ~* transition of equatorial ligand. The lowest energy band found in 21,000-25,000 cm-~ has been assigned to C o - - C charge transfer transition. This band shifts to longer wavelength with increasing electron donating ability of alkyl groups and axial bases. IR spectra In the IR spectra of the complexes, the stretching and deformation vibrations of NH2 are absent suggesting the formation of parent macrocyclic cobalt complexes (I) and (VI). Strong bands appearing in the range 1614-1634cm ~are assigned [20-23] to the stretching vibration of coordinated and conjugated v ( C = N ) group present in all the complexes. Bands characteristic of acetylacetone moiety are observed in the 145(~1456 [v~,~(C--CH3)] and 1372-1394 cm [Vsym(C--CH3) ] regions in the IR spectra of all complexes. Characteristic bands are observed in the far IR spectra of complexes. Bands in the 432-466 cm region are assigned to v(Co--N) vibrations [24-27] suggesting the participation of conjugated (and equatorial) nitrogen in coordination. In organocobalt complexes bands observed in 312-341 and 235-269 cm -~ regions are tentatively assigned [19] to C o - - C and C o - - N (axial base) vibration, respectively.

+ 180 Dq B)12 H N M R spectra

CH3

.3% 7.3 H3c/ ~

H3c,

/ ~

/oH3

H3C

~

oH3

~cH3

%c 1. II. Ill. I V. V.

R and B absent R=CH3; B = Py R=CH3; B = Im R= C2H5; B= Py R:C2H5; B=Im

V]. R and B absen~ VII. R=CH3; B~ Py VIll. R=CH3; B=lm IX. R=C2Hs;IB= Py X. R=C2H5; 8=Irn

All complexes are characterized by high resolution (400 MHz) tH N M R spectra. These spectral data incorporating chemical shifts (6), multiplicity and the proton assignment are presented in Table 4. Broad multiplets centered between 6.6-7.1 ppm are observed in low field strength regions of spectra due to aromatic protons. As expected, methyl protons of the equatorial ligand resonate at high field strength (2.09 2.38 ppm). The spectra of methyl cobalt complexes show a singlet in 0.68~).88 ppm range, where as the spectra of the ethylcobalamins exhibit two signals corresponding to methylene and methyl protons in 1.5(~ 1.77 and 0.68~.93 ppm, respectively. The signals due to the coordinated pyridine and Imidazole in the present complexes gave broad multiplets. However, the signals due to ~ and /~ protons of the pyridine are obscured by the signals due to aromatic protons of equatorial ligand.

1358

M. R. R e d d y et al. Table 2. Electronic spectral data and ligand field parameters of non-alkylated cobalt complexes VLa

Complex I V!

Y2

(cm i) (cm 1) 7265 6995

16103 16529

Y3

(cm l)

B

22371 22936

916 940

B~5~ 10 Dq v2-vl v:/vl LFSE 0.94 0.97

8359 8074

8838 9534

2.22 2.36

23.88 23.07

"Calculated value. hB35 = B/971.

Table 3. Electronic spectral data (cm ~)ofalkyl derivatives Complex

g--,u* (equatorial)

Co--C (Charge trans~r)

II

32895 28986

22523

III

32787 28986

22989

IV

33222 28818

21598

V

32787 28818

22624

VII

33112 28571

22371

VII1

33003 28653

24938

IX

32680 28901

21834

X

33003 28818

22573

Electrochemical studies

The C V d a t a o f all the complexes are given in T a b l e 5. The cyclic v o l t a m m o g r a m s o f unalkylated complexes (I) a n d (VI) have cathodic peaks in the range - 0 . 7 1 to - 0 . 7 9 V, c o r r e s p o n d i n g to Co"I/Co H reduction couple which is irreversible. These n o n alkylated complexes have reversible one electron [Co~[ ~ Co ~] couple with E~/2 in the range - 1 . 1 2 to 1.17 V. The E~/2 values o f (I) m a y be c o m p a r e d with interest. Complex VI has macrocyclic ligand containing two methyl groups. It is reduced at more negative potentials in c o m p a r i s o n with complex I. This suggests t h a t the complexes with less electron d o n a t ing ligands have less negative Ej/2 values t h a n those o f ligands c o n t a i n i n g electron d o n a t i n g groups. In general, CV o f alkylcobalt complexes show two

one electron reduction c o r r e s p o n d i n g to Co Hx~ Co [[ a n d a further Co n ~ Co ~reduction. Except for II, III, IV a n d V complexes, all other alkyl cobalt complexes show reversible one electron reduction. The complexes of If, III, IV a n d V shows an irreversible reduction at Epc in the range -- 1.00 to 1.38 V which c o r r e s p o n d s to the C o " -~ Co ~couple. It is interesting to c o m p a r e the half wave potentials of present complexes c o n t a i n i n g different axial bases. It is observed t h a t the imidazole c o o r d i n a t e d complexes are reduced at less negative potentials [29]. The second reduction wave (cathodic peak) which is irreversible is present in the CV of all alkylated complexes t h a t c o r r e s p o n d to Coil/Co ~ reduction couple. Inspection of the d a t a reveals t h a t the nature of alkyl groups have p r o f o u n d effect [30] o n the first

Macrocyclic cobalt complexes

1359

Table 4. ~H N M R Spectral data of alkyl cobalt complexes Chemical Complex Shift (5) 11

III

1V

V

VII

VIII

IX

X

6.6-6.93 7.1 8.25 4.32 2.38 0.87 6.6-6.93 7.4-8.23 4.21 2.37 0.86 6.5-6.93 7.1 7.98 4.22 2.38 1.77 0.88 6.6-9.4 7.1-8.24 4.66 2.23 1.56 0.86 6.94-7.0 7.18 8.32 4.23 2.39-2.45 2.51 0.88 6.92-7.00 7.2 8.1 4.03 2.15 2.38 2.54 0.68 6.96-7.11 7.21 8.28 4.16 2.21-2.32 2.50 1.65 0.72 6.9 7.1 7.1 8.1 4.51 2.09-2.38 2.56 1.77 0.93

Multiplicity" (n) d m s s s d m s s s d m s t s s d m s s s s m m s t s s m m s m s s m m s m s s s m m s t s s s

No. of Protons Assignment 8H 5H 2H 12H 3H 8H 4H 2H 12H 3H 8H 5H 2H 12H 2H 3H 8H 4H 2H 12H 2H 3H 6H 5H 2H 12H 3H 3H 6H 4H 2H 12H 3H 3H 6H 5H 2H 12H 3H 2H 3H 6H 4H 2H 12H 3H 3H 2H

Phen H Py H Methine H (CH3--CzN) H (CH3--Co) H Phen H Im H Methine H (CH3--CzN) H (CH3--Co) H Phen H Py H Methine H (CH3--CzN) H CH2 of ethyl H CH3 of ethyl H Phen H Im H Methine H (CH3--CzN) H CH2 of ethyl H CH3 of ethyl H Phen H Py H Methine H (CH3--C~N) H Ar--CH3 H (CH3--Co) H Phen H Im H Methine H (CH3--~N) H Ar--CH3 H (CH3--Co) H Phen H Py H Methine H (CH3--CzN) H Ar--CH3 H CH2 of ethyl H CH3 of ethyl H Phen H Im H Methine H (CH3--~N) H Ar--CH3 H CH2 of ethyl H CH3 of ethyl H

"d = doublet; m = multiplet ; s = singlet ; t = triplet.

r e d u c t i o n potential. F o r e x a m p l e , c o m p l e x V I I I is r e d u c e d at m o r e n e g a t i v e p o t e n t i a l in c o m p a r i s o n w i t h c o m p l e x X. T h u s axial ligation p r o p e r t i e s o f p r e s e n t m a c r o cyclic c o b a l t c o m p l e x e s are evidenced by (i) the synthesis a n d c h a r a c t e r i z a t i o n o f alkyl derivatives, (ii)

electronic a n d i n f r a r e d t r o c h e m i c a l studies.

spectra

and

(iii)

elec-

Acknowledgement~The authors thank the Department of Science and Technology, Government of India, New Delhi (SP/S1/F-07/92) for financial support. The authors also

M. R. Reddy et al.

1360

Table 5. Cyclic voltammetric data of cobalt complexes (10 3 M) in DMF containing 0.1 M TBACIO~ at 1.0 V/s at glassy carbon electrode, temp. 26"C Complex l

Gain

Redox couple

Ep,V

0.05

lll/ll

--0.71

0.10

11,'I

- 1.20

111/1I

--0.76

11/I

-1.20

EpaV

-

E .2

1.03

-

-1.04

1.12

AE

170

--

--

-1.12

160

1I

0.02

II1,'11

1II

0.01 & 0.02

111,'11

IV

0.01 & 0.02

111,'11

V

0.01

111/11 11/I

-0.50 --

--

- 1.38

Ill/11 ll/l II/I

-0.79 1.21 -1.21

-1.09 -1.12

-1.15 -1.17

0.01 & 0.02

111/1I

1.23

-0.38 . .

.

0.02

111/11 I1.'1

--0.60 -- 1.27

0.43

II1,'11 I1,'1

--0.40 - 1.28

-0.45

--0.43

50

--0.51 -1.35

0.39

--0.45

120

VI

0.05 0.10

Vll

VIII

IX

0.01

X

0.02

I1,1

11/I

III,'II

11/1

thank the RSIC, Madras and SIF, Bangalore for providing IR and ~H NMR spectral data, respectively.

R

E

F

E

R

E

N

C

E

S

I. Schrauzer, G. N., Acc. Chem. Res., 1968, 1, 97. 2. Costa, G., Mestroni, G. and Pellizer, G., J. Organomet. Chem., 1968, 11,333. 3. Biggotto, A., Costa, G., Mestroni, G., Pellizer, G., Puxeddu, A., Reisenhofer, E., Stefani, L. and Tauzer, G., Inorg. Chimica Acta Rev., 1970, 4, 41. 4. Costa, G., Mestroni, G., Tauzer, G. and Stefani, L., J. Organomet. Chem., 1966, 6, 181. 5. Darbien, D. H., Dahan, F., Costes, J. P., Laurent, J. P. and Cros, G., J. Chem. Soc., Dalton Trans., 1988, 219. 6. Lindoy, L.F., The Chemistry of Macrocyclic Ligand Complexes. Cambridge University Press, Cambridge (1989). 7. Jaeger, E. G., Z. Chem., 1964, 4, 437. 8. Jaeger, E. G., Z. Chem., 1967, 7, 63. 9. Jaeger, E. G., Z. Chem., 1968, 8, 30. 10. Jaeger, E. G., Z. Chem., 1968, 8, 470.

--

0.95 --0.51

-- 1.07

-0.45

120 90

.

-0.52 - -

170 - -

11. Jaeger, E. G., Z. Anorg. Allg. Chem., 1969, 364, 177. 12. Chipperfield, J. R. and Woodward, S., J. Chem. Ed., 1994, 71, 175. 13. Hussain Reddy, K., Krishnaiah, G. and Sreenivasulu, Y., Polyhedron, 1991, 10, 2785. 14. Radhakrishna Reddy, M., M o h a n a Raju, K. and Hussain Reddy, K., Ind. J. Chem., 1996, 3fiA 677. 15. Hussain Reddy, K., Radhakrishna Reddy, M. and M o h a n a Raju, K., Polyhedron, 1997, 16, 2673. 16. Cotton, F. A. and Wilkinson, G., Advanced Inorganic Chemistry, p. 772, John Wiley, New York (1980). 17, Konig, E., Structure and Bonding, p. 175, Springer, New York (1971). 18. Lever, A. B. P., Inorganic Electronic Spectroscopy, pp. 199-332, Elsevier, Amsterdam (1965). 19, Toscano, P. J. and Marzilli, L. G., Progress in Inorganic Chemistry (Ed. S. J. Lippard), 31, pp. 140 & 142. John Wiley, New York (1984). 20, Eggleston, D. S. and Jackels, S. C., Inorg. Chem., 1980, 19, 1953. 21, Coltrain, B. K. and Jackels, S. C., Inorg. Chem., 1981, 20, 2932.

Macrocyclic cobalt complexes 22. Baldwin, D. A., Peiffer, R. M., Richgott, D. W. and Rose, N. J., J. Am. Chem. Soc., 1975, 95, 515. 23. Jakels, S. C., Farmery, K., Barefield, E. K., Rose, N. J. and Busch, D. H., Inor9. Chem., 1972, 11, 2893. 24. Nakamoto, K., Infrared Spectra of lnorganic and Coordination Compounds. Wiley, New York (1970). 25. Ferraro, J. R., Low Frequency Vibrations of lnor9anic and Coordination Compounds, Plenum Press, New York (1972).

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26. Rana, V. B., Singh, D. P., Singh, P. and Teotia, M. P., Trans. Met. Chem., 1981, 6, 36. 27. Malik, W. U., Bembi, R. and Singh, R., Polyhedron, 1983, 2, 369. 28. Christodoulou, D., Kanatzidis, M. G. and Couconvanis, D., lnor9. Chem., 1990, 29, 191. 29. Hogenkamp, H. P. C. and Holmes, S., Biochem., 1970, 9, 1886. 30. Costa, G., Puxeddu, A. and Tauzher, G., Inorg. Nucl. Chem. Lett., 1968, 4, 319.