Indian Journal of Chemistry .

Vol. 38A, September 1999, pp.92 1 -924

Notes

A molecular mechanics study on the structure of

CH300CCH2CH2Co(DH)rH20 [DH = dimethylglyoximate mono anion]

Nita A Lewis Department of Chemistry, University of Miami, Coral Gables,

Aorida 3 3 1 24, USA and

Di pankar Datta * Department of Inorganic Chemistry,

Indian Association for the Cultivation of Science, Calcutta 700 032, India

Received 24 May /999

The logK, t:.H and f)S values for the equilibrium displacement of the axial water molecule in alkylaquocobaloximes [RCo(DH)rHP; DH == dimethylglyoximate mono anion] by dimethoxyethylamine in aqueous medium are found to correlate Taft's cr* parameter mixed with Dubois' steric parameter E's' for a number of R groups except for CH2CH2COOCH3• To investigate the reason, the structure of CHPOCCH2CH2Co(DH)rHP (1) has been determined by molecu- . lar mechanics. It is found that the alkyl group in 1 has a bent confor­mation which allows H-bonding between the carbonyl oxygen atom and the equatorial oximate H atom. This possibly changes the steric and electronic effects of the alkyl group in 1. It is mentioned that the X-ray crystal structure of the cobaloxime .l is not yet known.

With the recent discovery of the replacement of the axial benzimidazole moiety in cobalamin coenzymes when bound to methionine synthase I and to methylmalonyl­CoA mutase2 , there has been a renewed interest in the study of trans influence of the axial ligands in the alkylcobalamins and their model complexes .3 The trans influence (a static phenomenon) of the axial R group in the alkylcobalarnins is manifested in many of their physi­cochemical properties. One such property is the equilib­rium displacement of the coordinated water molecule (trans to the R group) in the alkylaquocobalamins by the pendent dimethylbenzimidazole nucleotide. This dis­placement reaction, which has been studied quite ex­tensively\ is schematically represented by Eq.( l ) . Re­action ( l ) has been very ingeniously modeled by axial binding of N-donor bases to alkylaquocobaloximes [RCo(DH)2.Hp; DH == dimethylglyoximate mono an­ion] .5·7 In a thorough study, Brown and Awtrey6 em-

R I (Cf' )

OH2

NW

R I (COIII ) + H O+ I 3

N .

. . . ( 1 )

ployed dimethoxyethylamine (DEA) as the base and determined the various thennodynamic parameters of the reaction (2) -K, MI and !lS with fair amount of accu­racy for a number of R groups in aqueous medium (Table 1 ).

K RCo(DH)rHP+DEA � RCo(DH)rDEA+HP . . . (2)

Earlier we were the first to point out the relative impor­tance of the electronic and steric effects of an R group in understanding its trans influence reflected in the reac­tion (2), (ref.8) · we used Taft's a* parameter of an R group as an index of its electronic effect9• 10 and Dubois'

E's' parameter as a measure"· 12 of its bulk. For an inde­pendent assessment of our approach, the reader is re­ferred to ref. 1 3 . Here we have added two more R groups, CH3CH=CH2 and CH2CH2COOCH3.

to the earlier list (Table 1 )6 . It is found that all the alkyl groups except CH2CH2COOCH3 obey the fol lowing equations

logK = 2.857 + 0.978 (cr* + 0. 1 2 E') . . . . (3)

t:.H = -5. 103 - 2.86 1 (cr* + 0. 1 6 E') . . . (4)

f)S = -4.034 - 4.57 1 (cr* + 0.22 E') . . . (5)

Methodology

The statistical technique followed here is same as adopted earlierx. The correlation coefficients for Eqs (3), (4) and (5) are 0.972, 0.993 and 0.997 respectively. The CH2CH2COOCH3 group is identified as an exception. The E's value given in Table I for R = CH2CHPPh, CH2CHpMe, (CH2)3CN and CH2CH2Ph are estimated ones; these were not determined experimentally by

922 INDIAN 1 CHEM, SEC. A, SEPTEMBER 1 999

Table I - The various data on RCo(DH)2"HP used in the present study"

R cr' E' ,

logK MI 115

(CH)CHz)zCH -0.225 -2.00 2.4 1 0 -3.60 - 1 .07

(CH)zCH -0. 1 9 -0.48 2.520 -4. 1 9 -2.52

Ci-I3CH2 -0. 1 0 -0.08 2.784 -4.86 -3.58

C,H,CH1CHz 0.08 -0.35 2.854 -5. 1 0 -4. 1 3

CH, 0.00 0.00 2.935 -5. 1 8 -3 .96

NCCH1CH1CHz 0. 1 7 -0. 3 1 3 .025 -5 .47 -4.53

CH,DCH2CHz 0. 1 9 -0.3 1 3 .025 -5 .50 -4.62

C,H,oCH1CH1 0.3 1 -0.3 I 3.053

CH,CH=CH 0.36h -2 .07< 3.029 -5. 1 7 -3.53

CH,DOCCH2CHz 0.26 (-0.3 1 )d 3 .0 1 7 -4.99 -2.94

C"H, 0.60 -2.3 1 3 . 1 43

"For the meanings of the symbols. see text. The values of cr* and E', are taken from ref.8 unless otherwise spec i fied. The logK, 6H (in keal mol" ) and 65 (in eu) data are taken from ref.6 . hFrom ref. 9. 'From ref. I I . dSee text and Fig.2 for the exaet nature of this value.

Dubois and co-workers l l . Conformational level l ing of the steric effect has been invoked to assign their E', value l2h. A survey of the experimental E', parameters shows that the steric parameters for the mono-substi­tuted ethyl groups (at � position) are levelled to ca -0.3 1 ( refs 8 , I I ) Thus, it i s apparent that the group CH?CH?COOCH takes up a conformation quite differ-

_ _ .1 ent from that assumed by the other four �-substituted e thy l groups , v i z . , CH2CH20Ph, CH 2CH20Me , (CH2),CN and CH2CHlh i n RCo(DH)2 .Hp. From Eqs (3), (4) and (5) with the knowledge of the appropri­ate logK, r1H and r1S and 0'* values (Table I ) , the E', for CH CH COOCH, is calculated as -0.80 ± 0.46, 2 2 ., - 1 . 87 ± 0. 1 6 and -2.27 ± 0.20 respectively with the av-erage being -1 .65 ± 0.32. In order to check the type of conformation assumed by this alkyl group, we decided to inves t igate the s truc ture of CH,GOCCH2CH2Co(DH)2 .Hp by means of molecular mechanics since its X-ray crystal structure is sti ll not available.

Molecular mechanics (MM) calculations were carried out with the CAChe suite of programs available from Oxford Molecular Group Inc. 14 This program starts with MM2 force field developed by All inger15 and augments

it in three ways: ( I ) extending the force field to addi­tional bond and atom types by including weak, coordi­nate and ionic bonds and atoms with hybridisations higher than sp\ (2) recognising conjugated and other aromatic systems, and (3) systematical ly applying a set of empirical rules which estimate missing force-field con­stants . The energy terms for bond stretch, bond angle, dihedral angle, improper torsion, van der Waals, elec­trostatics and hydrogen bonding interactions are included in each calculation. The covalent radius for Co used in the CAChe augmented force-field is 1 . 1 60 A. Cobalt is assigned a hardness value of 0. 1 85 and the van der Waals radius 11i employed for this metal is 2 .800 A. A block­diagonal Newton-Raphson technique was used for the optimisation process.

Results and discussion

Our MM calculations show that there are two idealised structures (Fig. I and Fig.2) having separate minima for CH,GOCCH2CH2Co(DH)2 .HP. Fig . I describes the minimum energy structure. It is found that in this struc­ture there is a H-bonding between the carbonyl 0 atom of the axial alkyl group and the equatorial H atom of the oximate fragment. In fact, the oximato H rises a bit from

NOTES 923

Fig. 1 - A "ball and stick" representation of the minimum energy structure of CH,00CCH2CH2Co(DH)rHP obtained by MM calculations showing the H-bonding between the carbonyl oxygen of the axial alkyl group and the equatorial oximate H atom. Meaning of the colours: larger red, Co; smaller red, 0; blue, N; grey, C; white, H.

the equatorial plane in the process . The other fonn of CHPOCCH2CHCo(DH)2 .HP is displayed in Fig.2 . Here the alkyl group has a straight chain conformation ; it is energetical ly higher than the H-bonded form (Fig. 1 ) by 5 .8 kcal mol· ' . Incidentally, the conforma­tion of the R group in Fig. 2 corresponds to an E', of -0.3 ph .Earlier, we have shown a very good linear rela­tion between the apical angle 8R of the minimum vol­ume cone within which an alkyl group can be enclosed and Dubois' steric parameter E>Eq.(6)Y The 8R de­pends on the nature of the conformation

E', = 5 .400 - 0.0448R . . . . (6)

of an R group. Our cone angle calculations l 2h show that the 8R for the CH2CH2COOCH3 group corresponding to the conformation adopted by it in Fig. 1 is 1 35°. Ac­cording to Eq.(6), 8R = 1 350 yields an E', value of -0.54 ± 0.40. Since this value somewhat differs from that (- 1 .65 ± 0.32) derived from Eqs (3)-(5), it seems that the H-bonding has altered the electronic effect i .e. the cr* value of the CH2CH2COOCH3 group.

Thus here we have pointed out that there is possibly a H-bonding between the keto oxygen and the equatorial oximate H in CHPOCCH2CH2Co(DH)2 .HP which affects the steric parameter (and probably the electronic parameter as wel l ) of the axial alkyl group. At present X-ray crystal structures of a number of RCo(DH)2 .L

Fig .2 - A "ba l l and s t ick" f igure o f an MM structure of CH,00CCH2CH2Co(DH)2.Hp where the alkyl group takes up a straight chain conformation. This structure is higher in energy than that described in Fig. 1 by 5.8 kcal mol· l . Note that the conformation of the alkyl group here corresponds to an E'" value of -0.3 1 . Colour code: same as in Fig. I .

compounds where L is a monodentate l igand , are known 17o '9 • Interestingly, in the X-ray crystal structures of (CH3CHPOC)2C(CH3)CH2Co(DH)2 pyridine and (CH,CH200C)2C(CH)CH2Co(DH\triphenylphosphine 'x , one of the two CH1CH100C fragments takes up a con­formation like that of the CH3CHPOC fragment in Fig. 2 and the keto oxygen of the other CH3CHPOC frag­ment turns away from equatorial oximate H atom evad­ing the possibility of a H-bonding.

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