indian journal of chemistry vol. 38a, september...
TRANSCRIPT
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.
References
Drennan C L, Huang S, Drummond 1 T, Matthews R G & Ludwig M L, Science, 266 ( 1 994) J 669.
2 Mancia F, Keep N H, Nakagawa A, Leadlay P F, McSweeney S, Rasmussen B, Bosecke P, Diat 0 & Evans P R, Structure, 4 (1996)
339.
3 Garr C D, Sirovatka 1 & Finke R G, J Am chem Soc, 1 1 8 (1996)
11142; Brown K L, Zhao D, Cheng S & Zhao X, lnorg Chern, 36
(1997) 1 764; Brasch N E. MUlier F. Zahl A & Eldik R v, lnorg
Chem, 36 ( 1 997) 4891; Cini R, Moore S 1 & Marzilli L G, lnorg
Chem, 37 (1998) 6890.
4 Brown K L & Wu G, lllorg Chem, 23 (1994) 4 1 22 and refer-ences thcrei n.
5 Ewen 1 A & Darensbourg D, J Am chem Soc, 98 ( 1 976) 4317.
6 Brown K L & Awtrey A W, 1110rg Chem, 17 (1978) I I I . 7 Garlatti R D, Taruzer G & Costa G, lllorg chem Acta, 70 (1983)
83; 71 (1983) 9.
8 Datta D & Sharma G T, J chell! Soc Dalton TrailS, ( 1 989) 1 1 5.
924 INDIAN J CHEM, SEC. A, SEPTEMBER ) 999
9 R W Taft, Steric effects in organic chemistry, edited by M S Newman (Wiley, New York) 1 956, ch . 1 3.
1 0 Datta D, J phys Org Chem, 4 ( 1 99 1 ) 96. I I MacPhee J A, Panaye A & Dubois J E, Tetrahedron, 34 ( 1 978)
3553. 1 2 (a) Datta D & Shanna G T, J chem Research (S), ( 1 987) 422; (b)
Datta D & Majumdar D, J phys Org Chem, 4 ( 1 99 1 ) 6 1 1 . 1 3 R L Sweany, Comprehensive organometallic chemistry II, ed
ited by E W Abel, F G A Stone & G Wilkinson (Pergamon, Oxford), vol. 8 ( 1 985), 1?' 1 .
1 4 Oxford Molecular Group lnc, P.O. Box 4003, Beaverton, Oregon 97076, USA.
1 5 Allinger N L, J Am chem Soc, 99 ( 1977) 8 1 27; Burkert U & Allinger N L, Molecular mechanics (American Chemical Society, Washington, DC), 1 982.
1 6 Bondi A, J phys Chem, 63 ( 1 994) 44 1 .
1 7 Bresciani-Pahor N , Forcolin M, Marzilli L G , Randaccio L , Summers M F & Toscano P J, Coord Chem Rev, 63 ( 1 985) I .
1 8 Randaccio L, Bresciani-Pahor N , Orbell J D, Calligaris M , Summers M F, Snyder B, Toscano P J & Marzilli L G, Organometallics, 4 ( 1 985) 469 and references therein.
1 9 Randaccio L & Geremia S, Organometallics, 1 6 ( 1997) 4951 and references therein.