Thermodynamics of the binding of nitrogenous ligands to β-trifluoromethylcobinamide and β-cyanomethylcobinamide

ELSEVIER

Inorganica Chimica Acta 279 (1998) 178-185

Thermodynamics of the binding of nitrogenous ligands to I -trifluoromethylcobinamide and 13-cyanomethylcobinamide Mohamed S.A. Hamza a, Kenneth L. Brown b., Depurlment of Chemisto', Facully of Science. Ain Shams Universio" Abbassia, Cairo. E~ypt t' Deparlmenl of Chemistry, Ohio Uaiversily. Athens, OH 45701 USA Received 17 July 1997: received in revised form 7 October 1997: accepted 26 November 1997

Abmrxt.t p-trifluommethylcobinamide was ~ from I~-trifluoromethylcobalamin using the trifluoromethanesulfonic acid method for cleavage of the phosphcxgester bond, The equilibrium constanL~, K~..for the coordination of I~-trifluommethyloobinamide (I~-CF~Cbi) and I~-cyanomethykobinamide (I~-NCCH.,,Cbi) by imidazole, pyddine, bmmoethylamine and azide have been determined spectrophotometrically as a function of temperature. The values of K~.at 2 5 ~ for the cooMination of I~-CF~Cbi by imidazole, pyridine, bromoethylamine and azide were found to be 700+20. 108+8, 35+3 and 12.0+0.8 M =* respectively. The values of !(i. at 25*C for coordination of 13-NCCH2Cbi by imidazole, pyridine, bromoethylamine and azide were 300 ± 10, 52 + 3, 6.2 +0.3, I I + I, respectively. The values of (AHt. and tLSt.) in the ¢a~e ofl~-CF~Cbi were ( - 0.84 ±0.04, -0.46±0,14): ( - 1.04±0.1 I. - 1.16±0.37): ( - 1.67 +0.07, -3.77+0.25): ( - 1.00±0.04 kcal real ~ t _ 2, I'~ ± O.15 ¢,1 ~nol t K t ) for imidazole, pyddiue, bmmoethylamine and azide, respectively. The values of (AHt. and ~tSt.) in O~e~:a~ ~)f I~-NCCH,Cbi were found to be ( ~ 1.4 ±0.08, - 1.85 ± 0.25): ( - 1.55±0.06, - 3.25 + 0.20): ( - 1,31 ±0.10, - 3.54 + 0.30); = 0,94 ± 0,08 kcal real ~ t. = 1.98 ± 0.25 cal real * K t ) for imidazole, pyridine, bmmoethylamine and azide, respectively. These binding ~:on~t~t~t tad thermodynamic parameters allow the CF~ and NCCH= ligands to be placed in the thermodynamic trans influence series.

© 1998El~vt~ S¢iem:eS,A, All rightsre.fred. K,.v.'.rd:u Equllihflum c(m~l.nl~ Thern~yn~mi~:,4~C,~In~imld~ ¢omptc~e~;Nilf~¢m~u.~ II~a,nd. ~:omplexes

I, Intreducttm Cobinamides (vitamin B,~ derivatives in which the 5,6dtmethylbe~tmidaxole axial ligand has been chemically removed from the cobalamin molecule) are important compounds for studying the chemistry and biochemistry of B,,,. r e q u i d ~ reactions [ !-31, The comn ring does not possess a pkme of symmetry, and consequently, them is a po,~sibility of axial ligand diastereomerism when the two axial ligands am different |4--101, The 'upper', or 13-face, of cobinamide, with its upward projecting u, c and g acetamide side ~:t~ains, is less sterically hindered than the 'lower', or et-face, which is brokered by the downward projecting b, d and e pmpionamides and the ~ amidef side chain, The method

used to prepare cobinamide t'mm cobalamin depends on the selective cleavage of the pho,,~ohodiester bond of the n~leotide loop to remove the axial et-u-fibofuranosyl* C~t~.¢ e-marl: ~ 3

turbot. Tel.: + 1-614-593 173"I:ft~' + 1-614-$930148: ~tod~.~:~ts.ohiou,edu

O0.To--I@MglM$,~ ~

5,6.dimethylbenzimidazole (ot-dhazole) ' in cyanocobalmin (CNCbl) while leaving the amide functional 8mup una: ~red, Cerous(lll) hydroxide gel is one of the common methods used For the cleavage of the phosphodiester bond of dicyanocobalamin to prepare the cobinamide derivatives [ I I I. We [ 121 found an efficient and convenient method to remove the axial nucleotide from B,.. compounds by using anhydrous tdfluoromethanesulfonic acid (CF~SO~H). This method can be used for direct synthesis of I$-alkylcobinamide (RCbi) from p-alkylcobalamins (RCbl) when the organic ligand is stable to acidic conditions. Thus, I$-CF..HCbi has been pre. pared by the reaction of I$-CF:HCbl with CFsSO~H at room temperature 1 121. This method of preparation is very useful when the 13-isomer is required, which is difficult to prepare t Aldwevialion.~. cyanocobalamin, CNCbh cobinamidc, Cbi: cohalamin, C'bl: B-Y-deoxyadeo~ylcohMamin, A~k~"bl: 5,6-dimethylbenzimidazole, B~m: B-tdlt~Nmmeth)%'obinamide,I~-CF.~Cbi;13-cyanomelhylcohinamide. I~-NCCH.,Coi; methyk'obalamin, CHiChi, imidazol©, lmH; pyddine, py; hcomoethylamine, BdF.JNH,; I$-alkyk'obinamide. I$-RCbi: p-alkylcobalamin, I$-RCbl; a-dbazol¢, et-~ribofuranosyl-5,6.dimethylbenzimidazol©; Factor B is a mixture of the di¢,aeRomedc cyano(aqua)cobinamide.

~ © 1998 El,~vi~ ~k'~.'c S,A, All d~ls n:scrved. MI $0020-1593 ( ~8 ) 00120-0

M.S.A. Hmnza, K.L Bnm'n I Im)rgunk'a Chimh'a Actu 27911998) 178-185

since the normal reductive alkylation methods give a mixture of of and 13-diastereomers with varying ratios, The ratio of diastereomers varies widely with the nature of the alkylating agent during reductive alkylation of cobinamide 14], The total yield of RCbi compounds varies from 40% to 85% while the ot:13ratio of diastereomers varies from 4:96 for R = CH~ to 93:7 for R = CF3. [~-CF3Cbi has been prepared from Factor B [4,12 ! by reduction with Zn/CH3COOH followed by reaction with CF,~i, The CF,xCbi product was 93% {x-isomer and 7% 13-isomer [4] but CF2HCbi is also formed by reductive defluorination since there is an excess of reducing agent in the medium, We [7] have found that anaerobic, photoinducod isomerization can often be used for the interconvers~on of the diastereomers, so that the isomer which is difficult to prepare by tne normal reductive alkylation method can be obtained in better yield. However, ot-CF~Cbi is the only organocobinamide that does not convert to the g-isomer upon anaerobic photoinduced isomerization: instead, it undergoes decomposition only [ 8 ]. Therefore, there was a need to lind a high yield method to prepare I3-CF.~Cbi to study the equilibrium constants of this compound for ligation of different nitrogenous bases, to place the CF~ group in the known trans influence order series, and to compare it with another even weakly donating alkyl ligand, {3-NCCH:Cbi, and other alkyl groups coordinated in the 13-position of the cobinamide. Studying the equilibrium constants and the thermodynamic parameters of organoconinoids with different nitrogenous bases ( 5- and 6-membered heterocyclic compounds and primary amines) is of signiticance since the Bs,-requiring enzymes methionine synthase 113] and methylmalonyICoA inulase 114l are know, to subslitute the i.nidazole of a histidine residue for the axial 5,6-dimethylbenzimidazole upon binding m~thylcobalamin ( CH ~Cbl) and coenzyme vitamin Bla(AdoCbl), respectively. Finke el al. 1151 have delermined the equilibrium conslanls and Ihe thermodynamic par° ameters I~r the reactiotl of [3-5'-deoxyadenosylcobinanfide (D-AdoCbi) with several exogenous ~-axial bases including pyddine,4-methylpyridine.4-aminopyridine,4.dimethylam. inopyridine, and N-n|ethylimidazole. We l l6l have determined the equilibrium constants for the axial ligation reactions of methylaquacobaloxime at 25°C for a series of pyridines and aliphatic amines. The equilibrium constants and thermodynamic parameters have also been determined for the reaction of methyl, benzyl and neopentylcobinamide with series of substituted pyridines, imidazole and azide

179

OH- > CN - > vinyl > methyl > ethyl > benzyl i 18 ], where KL declines across the series. A similar trans influence order occurs for the energy of the "y-band transition (around 360 nm) in XCbl compounds [ 18 l.

~Co,--- +z ~

I

y

Co

+

v

(I)

I

Z

The equilibrium constants for the reaction of several bases with condnoid derivatives having different groups in the axial position (X) where X = H:,O, 5,6-dimethylbenzimidazole, OH-, CN, vinyl and methyl have been studied extensively [ 17-29 ] at 25°C but limited work has been carried out on the determination of thermodynamic parameters with the organocobinamides such as 13-CF3Cbiand 13-NCCH:Cbi. In this paper, a convenient and easy method for the preparation of 13-CF~Cbi from I3-CF.~Cblusing trifluoromethanesulfonic acid will be described. The equilibrium constants for the coordination of four different nitrogenous ligands with [3-CF~Cbi and I3-NCCH:Cbi have been determined spectrophotometrically as a function of temperature to evaluate the thermodynamic parameters and to compare these values with those for other organocorrinoids.

2. Experimental 2. I. Materials

Vitamin B t, (cyano¢obalmin) was purchased from Roussel. Bromoethylamine, sodium azid¢ and IrifluoromelhanesLtlt'olli¢acid were obtained from Aldrich in the highest purity available and used as received. Imidazol¢ was obtained I'rom Aldrich and recrystallized from CCI4 or benzene. Pyridine was obtained t'rom Fisher. Sodium hydrogen phosphate and sodium dihydrogen phosphate were us~ as a buffor for pit 7-8.5 and were supplied by J.T. B u r r Chemical Co. Tris(hydroxymethyl)aminomethane (Tris~, was us~:d as a buffer at pH 9 and was supplied by Sigma. ~0dium nitrate (Merck) was used to adjust the ionic strength,, Amberlite XAD-2 and SP-Sephadex were used to purity the cobinamide and were obtained from Sigma

ItTI. The trans influence/effect is the effect which a ligand ( X ), bound to a metal ion, has on the properties of the coordin,'ued trans ligand ( Y ) and on the kinetics ( trans effect) and thermodynamics (traus inlluence) {,f the replacement of Y by an incoming ligand (Z) as shown in Eq. ( I ). The trans influence/effect is a well-known phenomenon in vitamin BI,. chemistry operating at three levels; structural, thermodynamic and kinetic 118]. it has been established, forexample, that the value of KL decreases as X becomes a better G-donor and that the order of the trans influence on KL is H:O > Bzm,

2.2. Methods

UV-Vis spectra and spectrophotometric titrations were carried out on a Varian Cary 3 spectrophotometer, using I cm pathlength cells thermostated at the desired temperature + 0. I°C. The temperature was measured in the cuvettes with a thermistor device (Yellow Spring Instruments) calibrated against NBS-calibrated thermometers. The pH of solutions was measured before and alter the titration using a Radiometer PHM 64 pH meter equipped with

U,O

M.XA. Ham:a. K.L Brown I lnorganica Chimica Acta 279 (1998) 178-185

a Radiometer combined glass electrode. The pH meter was calibrated with standard buffer solutions at pH 4 and 7. Analytical HPLC was performed on a 4.6 x 75 mm Beckman C., ullrasphere column while semipreparative HPLC was performed on a I 0 x 2 5 0 mm Beckman Cs ultrasphere column, using 50 mM aqueous ammonium phosphate buffer (pH 3.0) and acetonilrile as described previously [4.30].

2.3. Methods of preparation Factor B. a mixture of the diasteromeric cyanoaquacobinamides was prepared from CNCbl by the triflic acid method !i21. I$-CF~Cbl was prepared as previously described [31] by bubbling CFsBr or CF31 through a solution of cob(ll)alamin obtained from CNCbi using Zn/CH3COOH as the reducing agent. The reaction was monitored by HPLC. [}-CF3Cbl was desalted on a column of Amberlite XAD-2 [ I Id] and then separated from the a-isomer by HPLC. The purified sample was dried over P2Os under vacuum for 24 h, and shown to be > 95% pure by HPLC. [$-CF~Cbi was prepared by adding 0.4 ml anhydrous CFsSOsH to 50 mg of dry [}.CF.~Cblunder a N2 atmosphere, The resulting solution was stirred for 24 h at room temperature, The reaction mixture was then poured into a solution of aqueous sodium phosphate (50 ml, 1,0 M, dibasic form, pH 9,2), The solution was loaded onto an Ambedite XAD-2 column [ lid]. and after thorough wa~ing with water and 4% ( vol,/vol,) aqueousacetoniltile to remove the: .acleotide by-products, I$-CF~Cbi was eluted with 50% (vol./vol.) aqueous acetonitrile, The solvent was removed by evaix~rao tton. Imd the concentrated ~lutlon was loaded onto a SP= ~ph~ex (Na* ;'orm? column for further purification, Aher thorough wa~ing with water, the product was eluted with 1,0 M aqueous NaCI, The total yield was about 43%. The pedty of B-CF~Cbt was > 97% by HPLC, The preparation, purification and quantitative measurements were carried out in the dark to avoid the photolysis of the light-sensit;ve organ. ocobalt cordnoids, The cobinamides were quantified spectro. photometrically by conversion to dicyanocobinamide ( e ~ - 3,04 X I(P M ~ * cm ° t) [ 32 ] by photolysis in the IX~ of KCN, I~-NCCH=C'bi was prepared as described previously [4.33 ] by adding BrCH~CN to a solution ofcob(il) inamide ob~ned from Factor B using Zn/CH~COOH as the reducing a~r:t, The reaction was monitored by HPLC. [}-NCCH:Cbi was desalted on a column of Ambedit~ XAD-2 and then ~F~cated from the a-isomer by HPLC and shown to be > 97% Pure by HPLC,

2,4, EqeillbriNm me~s~reme,¢,v A [}-RCbi solution, where R-CF~ or NCCH,, ( I 2 × I0 ~s M), dissolved in 23 ml of a suitable buffer (phosI~lte or Tris. ionic strength adjusted to 0 3 M using NaNO~), was pitted in a 1,0 cm pathlength cuvette in the thermostated

cell block of the spectrophotometer for 30 min. This solution was titrated by addition of small volumes of a concentrated stock solution of ligand, using a Hamilton syringe. The ligand solution was prepared in the same buffer used for 15-RCbi. and the ionic strength was also adjusted to 0.5 M using N a N O 3. The titrations were carried out in duplicate and were monito~d at several wavelengths where the greatest change in the absorbance took place. The values of the equilibrium constant, K, were obtained by fitting the absorbance versus concentration curve, after correction for dilution, to a binding hyperbola. Values of AH and AS were calculated from the slopes and intercepts, respectively, of plots of in K versus

liT. 3. Results and discussion

Reductive alkylation of Factor B by CF31 or CF3Br results in a mixture of a- and [3-CF3Cbi with a ratio of 93:7 and varying amounts of a- and [}-CF2HCbi due to reductive defluorination of the CF~Cbi's [4]. Unfortunately, unlike other pairs ofdiastereomcric a- and [}-RCbi's [7], anaerobic photoinduced isomerization does not convert a-CF3Cbi into IS-CF~Cbi; only decomposition results [8]. Unsuccessful attempt~ were made to im."ea~ the yield of the [}-isomer by changing the alkylating agent, the reducing agent and the reaction medium. In all ca~s an a/[~ ratio of 93:7 was obtained and both CF2HCbi isomers were also obtained whenever Zn was u~d as reducing agent. An attempt was made to eliminate the CF.,HCbi products which result from reduclive defluorination of the CF~Cbi's j 31 J by substitution at' tbrmate J 34 J I'or Zn as the reducing agent, as formate was shown not to defluorinate the CF~Cbi's. However, Factor B was inefficiently reduced by fi)nnate. In an experiment in which Factor B was reduced to cob( ll)inamide with Zn and then transl~rred away from the excess Zn by cannula to a solution of sodium formate (0.5 M), subsequent alkylation with CFJ yielded no CF, HCbi product, However, the CF.~Cbi products were still obtained in a 93:7 airs ratio, [}-CF~Cbi was obtained in sufficient amounts for experimentation by the cleavage of the phosphdiester linkag¢~of 13CFsCbl with Iriflic acid. The identity of the [}-CF~Cbi was confirmed by comparison with an authentic sample prepared by the reductive alkylation of Factor B. The difference in the UV-Vis spectra between the two isomers oftrifluoromethylcobinamide, however, is very small: the bands occur at the ~ m e wavelength but differ in molar absorptivity [41. Therefore, ratios of the absod)ance of the a-band to that of the ~/band and of that of the [}-band to that of the ",/-band were used to compare the compounds, and the HPLC retention time of [}.CF~Cbi obtained from [}-CF~Cbl was exactly the ,,rome as that of [}-CF,~Cbiobtained I'rom direct alkylation of Factor B. Four ligands (ImH, py, BrEtNH~ and N~- ) were chosen for the equilibrium studies at different temperatures and at a

M.S.A. Hamza. K.L Brown llnorganica Chimiea A¢la 279 fl998) 178-185

pH above the pK., of the ligand, with ionic strength 0.5 M (NaNO3). The reaction between the 13-R(H20)Cbi (yellow) and the ligand (L), to girt; me red I~-R(L)Cbi can be represented by Eq. (2): &'t

{3-R(H, O)Cbi ÷ + L ~ 13-R( L)Cbi ÷

(2)

Preliminary experiments, scanning the spectrum over the range 300-600 nm, showed that at pH 7-9.5, the four bases reacted rapidly with 13-RCbi and that the equilibria were established within the mixing and measurement time. Quantitative determination of KL was carried out in duplicate experiments by spectrophotometric titration of I-2 × !O- 5 M 13-RCbi with a concentrated stock solution of the ligand to minimize the effect of dilution. There was no change in the spectra of either 13-CF.~Cbior 13-NCCH,Cbi alone in the pH range used to measure KL, suggesting that there is no indication of ionization of the axial aqua ligand in this pH range. These titrations were carried out at five different temperatures (5--45°(7) to calculate the enthalpy and entropy change associated with Eq. (2). Fig. l(a) shows the spectrophotometric titration of J3CF.~Cbi by imH at pH 8.5 and at 25°(? as a representative example. The figure shows that good isosbestic points were observed at 509, 379 and 352 nm and there is a signilicant shift in the o~, 13 and ~-bands upon formation of 13CFdhnH)Cbi. The A,,,,, wdues occurred at 540, 514 and

.-J

i).(i <

0,2

0.0 (

1 350

300

400

4SO

~00

550

~00

~,, a m

0.7 [-.--.~,--~ . . . . . • /

I

"

'

'

--'

!

t

0.5 0.4 <

i

0.3

O.I 0.0 300

350

400

450

" 500

SSO

600

~.Inm Fig. I. ( a ) UV-Vis spectra of ~-CF~Cbi in the presence of variou.~ amount.,, ( 1.5× 10 's-0.022 M) of imH at pH 8.5, 2 5 ° C . / = 0 . 5 M (NaNOa). (b) UV-Vis speclra of 13-NCCH,,Cbi in the presence of various amounts (5 × 104.-0.035 M) of hnH at pH 8.5.25°C. i= 0.5 M (N;INO~).

181

361 nm for the six-coordinate 13-CF~(ImH)Cbi. A typical spectrophotometric titration for the coordination of 13NCCH_,Cbi by ImH at 25°C and pH 8.5 is shown in Fig. 1(b). Again, good isosbestic points were observed and significant shifts in the wavelengths of the absorbance maxima were observed. Spectral changes similar to those of Fig. 1 were obtained in the case of py, BrEtNH2 and N3-, but the am~ values occurred at slightly different wavelengths from those obtained in the case of 13-CF3(ImH)Cbi or of 13NCCH2(ImH) Cbi. Significant corrections for dilution had to be made to obtain the same sort of spectra in the case of the coordination of 13-NCCH:Cbi by N3 " and BrEtNH: and the coordination of 13-CF.~Cbiby N3- where the value of KL is relatively small and :~ ,vas difficult to determine the exact wavelengths of the R(L)Cbi bands since 100% formation of I3-R(L)Cbi could not be obtained. Typical UV-Vis spectra and good isosbestic points were also obtained when the titrations of 13-RCbi with these four ligands were carried out at different temperatures (5--45°C). The ",/-band of the 13-CF3(L)Cbi products occurred at 357, 358.5, 361, and 362 nm for N~-, BrEtNH,, py and ImH, respectively. Interestingly, this is almost the same pattern as that observed when Factor B {cyano(aqua)cobinamide) was tih'ated with similar bases 1201. The ~/-bands for cyano( ligand)cobinamide were at 358,361 and 361.5 nm for the ligands BrEtNH.,, py and ImH respectively. In the case of 13-NCCH~(L)Cbi, the ~-band occurred at 372 nm for ImH and 370 nm for py and ca. 365-370 nm for BrEtNH: and N ~ . The ~-band for the coordination of vinylcobinamide by py and I m H occurs at 373 and 374 nm, respectively 120a I. Analogously, in the case of CHd L)Cbi, the ~/-band tx:curred at 375 and 378 nm for L = py and lmH respectively, but no value could be ohlained for the coordination of a primary anlin¢ with CH~Cbi o1' vinylcobinanlide since the value of K was too small to be determined 120al. These results for CH ~(L)Cbi, ~-CF~(L)Cbi and CN( L)Cbi show that the -/hand occurs at longer wavelength in the case o1' sp: hybridization (5- and 6.tnembered heterocyclic compound.~) than for sp a hybridization ( aliphauc amines), suggesting the possibility of "tr backbonding from the metal with the aromatic ligands ( vide infra). The order of increase in the wavelength agrees with the order of the trans effect ! 181. The similarity of A,,,,,~ and of the shape of the spectrum of [~-R(L)Cbi suggests a general similarity in the nature of O~-N bond, which allows direct comparison of the values of KL. The spectrophotometric titrations of 13-CF:Cbi were monitored by following the increase in absorbance at 360 nm or 545 nm or the decrease in absorbance at 348 nm. while those for ~-NCCH2Cbi were monitored by following the increase in absorbance either at 370 or 560 rim. Selected data are shown in Fig. 2. The solid lines in these curves represent the lit of Eq. (3) to the experimental data, A~=-

Ao+A.~KI.ILI

I+Kt.IL!

(3)

M.S,A. Itam:a. K.L Bnm,n / Ino~anica Chimica Acto 279 (1998) 178-185

182

t

0.4

I

I

I in i

.

i

1

~1

(a)

__

0.1

0,01- . o.ooo 0.~

)

.

.

.

.

.

o.oos O.OLO ~o~s [llmH], M I

I

I

~

1

I

o.o2o

o.o~

0.20 0,15

Am

o.io 0,05

0,00 '

I

I

o,ooo o~oos o,o~o o,o~s n,ozo o,o~s o,o~ llmHI, M CFt['hl tt1 2~C, pH 8,~, I~-IL'I M, The m~lid lille i~ ~ It! .1 I~l, 1J1, (h. V ~ h t l k ~ 111tthm~'t~tt~e ttl ~ ) ~t~ o~ addilloll ,t~f ht~H Io t~;NCCI|~Chi tt~ -~'~', pH I;.~,1~0.5 M. The mdld It~ t~ a Ill of ~tl, (,A).

where the value of A, and A~ repre~nt the ab~rbance at 0~. and 100% formation of I~-R(L)Cbi, respectively, and A, is the absorbance at any 1tBandconcentration I L l. Eq. ( 31 can

be used to obtain the exact values of K and A~, where [ L I ~ - [Lho, as is the case for the modest values of KL observed, since the value of Ao is fixed and the value of Ax corresponds to the absorbance at a given concentration of the free ligand, corrected for dilution if necessary. Table 1 shows val' -~ of Ku determined for the coordination of ImH, py, BrEtNH2 and N 3 - t o I~-CF3Cbi and I~NCCH2Cbi as a function of temperature. It is clear from this table that the values of KL increase with decreasing temperature for all four ligands and q!so that the value of KL decreases in the order ImH > py > BrEtNH: > N~- in the case of I~-CF~Cbi. However, the order of KL for 13 NCCH2Cbi is lmH > py > N3- > BrEtNH:. Table 2 gives the values of KL at 25°C determined in this work. as well as literature values for KL for the replacement of H,O ( Y in Eq. ( I ) ) by various L ligands (Z in Eq. ( I ) ) tro,.v u, nine different organic and inorganic ligands (X in Eg ( I ) ). Alkyl cobinamide species with strongly donating alkyl ligands are well known to bind exogenous ligands weakly I ! 5.17.24,39.40,43.44 I. Examining the values of KL for the coordination of nitrogen containing ligands with different corrinoids having different groups (X) in the Irons position gives the following order: X = H20 > CN > CF~ > NCCH2 > vinyl > methyl. The :iftinily of imH for I~-CF~Cbi is 20 times lower than the affinity for CN(H20)Cbi (although unresolved issues of ct-I~ structural isomerism in the resulting (H:O. CN)(base)Chi remain) while it is 40 and 70 times higher than the corresponding value for vinyl and methylcobinamide and alx~ut 700 times higher than that Ibr benzylcohinamide. The same pattern has b~en uhlained when the handing of py, BrEINH, and N ~ with I~CF~Cbi is compared with lhe thai o1' uther cohinamides huvin8 ('N . M~. vinyl and I~,l~yl as u'(.,,~ liBand, but the ratios tin dilTerenl. Plots of In Ku versus I/Tare shuwn in Fig. 3. These data show thai within experimental error. 8 ~ 1 linear plots were obtained. Values of A H and AS have been calculated from the slopes and the intercepts, respectively of linear least-

Table I

Equilibriumconstantsfor the t'ormatkmof RIL)cohinam~de~" R ~ tl" NI'('It,

ImH

py

Bd~lNIt,

N,

4roll

p.~

BrEINI l,

N, I h J_: I

5

9~)0 ~: 40

1511 ± I 0

7B ± ¢~

IB ± 2

.~94 :t 1~

107 ~ '7

I11,O j. 11,7

15

t4.~O~: i0

12.4:t:1~

5~ ± 5

15± I

,~;~o~ ~

7h ~ 5

7.9 ~ 0.2

12.1~±0.9

25 ~ 4.~

700 ~ IS ,'~l) t ~1 ~11,t~- Iit

I011 ± 8 t~,~t .t 5~ ± 2

.tl~) ± 3 2fl ± I I I¢ :t" 2

12,II ~ 1t,It t) ,I ± 1t.¢~ 7 4 ± 11 7

31111± I0 232 ± 5 I ,~fl ± .~

52 ± 3 39 ~ 2 2.~ ± I

1~.2 ± 11.3 4,11 ~:11.2 .~, I :~ 0.2

II± I " 8.1~ -t 11.5 h.6 ± 11,5

I~'

?,3 'a

~,~1 "

~,,~'

4,41 ~'

"/,3 '*

I~K~ mre~,thf~5 M ¢cNaNO,), ~'atum~given ~r~ &'~ (iN, 131 J 1M *~, ~'ct" 15.~ in Rc[. ! 35 I. ,I Rel" I . ~ I . Rel'. il51. ' R~t'. i y t l . Ref. I.~1.

,~,51 "

14.5'

,,8.41 r

M.S.A. Hamza. K,L Bnm'n / Inorganica Chimk'a Ac'ta 279 (1998) 178-185

i 83

Table 2 Equilibrium constants for the substitution o f water ( Y in Eq, ( I ) ) by various ligands ( Z in Eq. ( I ) ) tran,~ to organic and inorganic ligands ( X in Eq. ( i ) ), 25N2 Z

X CHx

CN-

84.1 ~'

4-H.~N-py lmH Melm py N~ BrEtNH2

24.0 h 8.0 ~ 5• 2,32" 3.5 '~ 0.1 "

KcNIKin, x Kt,nnlKp~

10.5 1.09

Ado

vinyl

NCCH,~

501 s 57.5 a 18 o'h

2 . 7 9 × IOs ~

2.4 ~

301J

700 j

11.2 0 4' 0.31 a

51.5 j II.l' 6.2 ~

108 j 12 j 38.5 ~

10x ~ 3.98 X l0 '~ J 1 . 3 6 x l0 ~ 1.99 X 104 d 400" 500" 316 °

27.8 1.61

927 5.84

6.48

7250 34

I I 5b I, ~ Ref.

139h l, "ReI'. 1251, ' Ref. 140 I, ' This work, x Ref. 141

0.5 ' 1.0 ~

CF~

Ref, 135 I, h Ref. I 171; " R c f . 124 I, ,i Ref. 120aI, " Ref, I 15a I. ' Ref. " Ref. J 23 J," Ref, J 40 J , " Ref. 129 J, p Ref. [ 42 J, q Ref. [ 26 ]. I

.

I

I

i ~ L" 4

,I 2 I 0.0032

,,~

I 00034

I/T, ?

InKL

!

_

I ~ 0.0Q36

*D

K "l I

I

4

I

I

0.0032

0.00.14

•

i

0.0036

l/T, K"l Fig. 3. (a) Temperature dependence of Ki. fi)rthe coordinalionof hnH (G'). p y (Ill). BrEtNH.. ( • ) ~nd N~ ( & ) with ~-CF~Chi. (b) Tcmi~rature dependence of Ko. for the coordination of ImH ( 0 ) . py (U). BrF.INH., ( # ) and N,, ( • ) wilh ~-NCCHeCbi.

squares regressions performed on the data. These values are given in Table 3, along with the existing literature data for the formation of other R(I)Cbi's.

CN-

OH-

1.14× 10~"

1.5X 104o 2 . 8 2 × I0 ; ' '

7.6

Bzm

H:O

10': p

10 ~ r

3 . 9 × 10~ ''q 4.27 X 104 16 d 7 . 9 x IO~" 5 x 10-~'~

1 . 5 8 x IO~ "

3 . 1 6 x lO~

2.9 x I0 ~ 7.9 x 104

6.3 x 10 ~ 40

3.98X 103d

I. ' Ref.

122 I,

For the ligands and complexes studied here (Table I), log KL increases steadily with the pK,, of the axial ligand for both CF~Cbi and NCCH.,Cbi, with the exception of the primary amine ligand, BrEtNH2, for which log KL falls far below such correlations. These results are in concert with observations on the affinity of alkyl cobaloximes for such nitrogenous ligands [ 16 I. Inspection of the equilibrium constants (Table 2) for Eq. ( I ), where Y, the ligand being replaced, is water, shows that cyanide is the most tightly bound ligand among all X. This is most likely the result of significant metal-to-cyanide 'nbonding for which there is good NMR evidence both in cobalt corrinoids and in cohaloximes 148l. The equilibrium constant,sfor replacement oi' H~O by C N rise rapidly as the tran,s'ligand. X. becomes a poorer donor, much more so than any ol the nitrogenoo~ lib,ands showt't in the table.This can he seen t'ronl a ~:omparison of' the ralio of' K~.N t'or cyanide binding to gt,,tn t'or imidazole binding, which is only on the order of i0 ° for the best donors (CH~, vinyl), but rises to I()~=10Hfor the poorest donors ( Bzm. H_,O) (Table 2). This indicates that "n'-donation 1, the metal is smmgly influenced by the donor strength of the trans ligand, becoming much weaker as the trans ligand becomes a better donor. There is a similar, but very much reduced and less regular trend (albeit marred by the surprisingly low value of K,,~ for H_,OCbl) in the ratio of Kt,,,o, to K,r which increases from about I for the strongest donors, to about 50 I'or the weakest. This may suggest a small contribution of cohalt-to-ligand "rr.backbonding in the case of imidazole, a proposition recently put forward by Finke [15b], but if so. it is a very minor component compared with the case of cyanide. The thermodynamic parameters determined here (Table 3) show that ligand substitution in CF~Cbi and NCCH:Cbi is accompanied by modest negative entropy changes which vary narrowly across the series of ligands and the two complexes and show no apparent trends. The entropy changes vary much more widely, from 2cal tool t K t for CH.~(ImH)Cbi to - 14 cal tool- ' K - ' for NCCH:-

M.$.A. Ham~. K. L. Bro~s'n/ Inorxanica Chimica Acta 279 (1998) 178-185

I R4

(BrEtNH2)Cbi, but again with no evident trends. Interestingly, both the enthalpy and entropy change for py and ImH ligand substitution with NCCH2Coi ave considerably more negative than with CF3Cbi, the average differences being about 2.3+0.4 kcal real -t and 9.1± 1.5 cal real -t K-t. However, no such difference between CF3Cbi and NCCH2Cbi is observed for the ligands BrEtNH2 and N3-, for which the enthalpy and entropy changes are very similar for both cobinamides. Table 3 summarizes the existing data on the thermodynamics of ligand substitution in RCbi' s. The most extensive series of data ts available for RCbi substitution by py. To examine the effect of the donor power of R on the enthalpy and entropy changes for ligand substitution, we can use the free energy changes accompanying the intramolecular coordination of the Bzm ligand in the analogous RCbl, Eq. (4).

.H-H oot,. I I

+4+1 e~

e~

I

I

--. ~') -H

,.t

m

I.i

.4,4.1.1 ,6vi I

R

I

I

R

~o

I

+

(4)

x2o

,H,H m

(,,.

I A A w

!

m m

S

v

re%

v'~

I

0 +

m

g M

'7'7''

I

tl +¢.H

The values of - AGco decrease with increased donor power of R (Table 3). The data in Table 3 suggest that as donor power of R decreases, the enthalpy of substitution by py becomes more negative, although CF~Cbi clearly deviates from this trend (Fig. 4), for reasons which are not clear. Such a trend could simply suggest that the axial Co-N and C o O bond enthalpies vary differently with the donor strength of tile organic llgand, the former having a steeper dependen~:e, so that AH becomes more negative as the donor power of R decreases. A similar, a l t h o u g h less regular, trend ( not shown) exists for AS, with CF~Cbi again deviating badly. Such a trend is much more difficult to explain ,~ince the incoming and departing ligands remain the same t h r o u g h o u t the series

t

,q,

i

i

i

i

~ununt in I nn

!

i

!

6

"

I

|

1

t

.y. ++t

T7''

4.t I

9

I

N- N""

~"" L

i

~ i

~

,s I ]

!

;- - - - 4- +;" - ;"

•

I

•

I

,~.,..-.~,J.n~

~Gc~k ~ l real"l

~ - - ' --' ~ ,--; -~ . . . . .

..

Fig. 4. Pl,X of - ~ slope = 1,3 :t: O,l,

for nig,nd substitution I,y py in RCI+i'.,, F.,q. (2)) vs.

intercept= - 0,7 :!: 0.4, F = 0.97.

M,S,A, Hamza. K,L Brown I Inorganica Chimh'a At'ta 279 f 1998J 178-185

so that the entropic consequences of the changes in their motional freedom and solvation due to ligand substitution would be expected to be invariant 149 I.

12Ol (a) M.S,A, Hamza, Ph,D. Thesis, University of Surrey, UK, 1993:

1211

Acknowledgements This research was supported by the Donor of the Petroleum Research Fund. administrated by the American Chemical Society (Grant 28506-ACS to K.L.B.) and the Fuibright Commission ¢to M.S.A.H.).

1221 [23l 1241 1251 1261

1271

References I I I (a) A.R. Battersby, E. McDonald, in D, Dolphin led.), B,a, Vol. I, Wiley, New York, 1982, p. 107; (b) A.R. Battersby, Ace. Chem. Res. 26 { 19931 15. 121 R.H. Aheles, D. Dolphin, Ace. Chem. Res. 9 ( 19761 114. 131 {a) J. Halpern, Science 227 119851 869; lh) B,P. Hay, R.G. Finke, J. Am, Chem, Soc, 109 ( 19871 8012. 141 K.L. Brown, X, Zou, L. Sahnon, Inorg. Chem. 30 { 1991 ) 1949. 151 Y,W, Alelyunus, P,F,, Fleming, R.G, Finke, T,(;. Paganu, I..G. Marzilli, J, Am. Chem, Sac, 113 ( 1991 ) 3781. 161 K.I .. Brown, X. Zou, J. Ant, Chem. Soc, 114 ( 19921 9643. 171 X. Zou, K,I., Brou..n, C, Vaughn, Inorg. Chem. 31 ( 1992 ) 1552. 181 K,L Brown, X. Zuu, Inorg, Chem. 31 ( 1992 ) 254 I. 191 K I,, Brown, L Salmon, J.A. Kirby, Organomelallics II ( 19921 422. 1101 X. Zuu, K,L, Brown, J. Am. C'ltem. Soc. 115 ( 1993 ) 668t;. I III (a) P. Renl., Methods I,:.nl.ymul. 18 11971 ) 82; (h) W. Friedrich. K. Ilerlthau¢, Chem. lier, 89 I It1561 25117; 1¢) W. Fricdrich, K. lternhatt¢, Chettt, Bet. tS) 119571 4t~5; (d) K.I,. Brown, J.M. ltakimi. I).M. Nu,,s, Y.D. Mol|tejano. D.W. Jat'ubsel|, Inorg. ('hem. 2] { It)l'14) 14¢~.~ 1121 X, Zuu. I).R. Evans. K.I,. Browu. Illol'lLChem. 34 t 11~951 1634. 1131 la) ('. l.tt..¢llillsky-l)mvln,an. S. I1111uII~.J.T. l)r1111Ulttlnd R.(i, Malo tlle~.. M.I.. l.ndwig. S~:i¢11¢¢~(~t~ (It~94) 11~¢~9.tb) l..t', l)r¢iula11. R.(I. Ma1111¢ws.M.L.l.utlwlg. {'urr. Ol'~in. Struct. Blul..1 { 199.1) 919. i 141 F. Matu:ia, N.H. Keep. a. Nuktlgawa. P.F. I,¢adlay, S. MrSweeney. B. RanlttSS¢lt,P, Boscke, O. Dial, P.R. F,vans, Structure 4 ( 19*Jr1) 339, 1151 Ca) C.D, Gaff'. J,M. Sifovatku. R.G. Fillk¢, hlor[1. CI1¢1n. ]5 (19t)t5) 5912, (h) J.M. Simvutka, R.G, Finke. J. Ant, Chert1. So¢. 11911997 ) 3057, 1161 K.L. Bmwu, D. Chernoff, D.J. Keljo. R.G. Kalhul. J. Am. Chem. So¢. 94 ( 19721 6697. 1171 K.L. Brown, H.B. Brooks, lnorg. Chem. 3{) { 1991 ) 34211. 118] J.M. Pratt, Inorganic Chemistry of Vitamin Bt2. Academic Press. l,ondon. 1972. 1191 li..A. Betlerto11. Ph.l'}, Thesis. University of the Wqwatersaltd. Juhanesburg. South Africa. 1983.

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[311 [ 321 1331 134] 135 } 1361 1371 I.t81 13t)l

[41)l 141 I 1421 1431 1441 1451 1461 1471 1481

1491

(b) M,S,A. Hamza, J.M. Pratt, J, Chem, Soc., Dalton Trans. (1994) 1373, (c) M,S,A, Hamza, J,M. Pratt, J. Chem. See., Dalton Trans. ( 19941 1377. D.A. Baldwin, E.A. Betterton, J.M. Pratt. J, Chem. See., Dalton Trans. { 19831 2217. G.I.H. Hanania, D.H. Irvine, M.V. lrvine, J. Chem. Soc. A ( 19661 296. G.C. Hayward, H.A.O. Hill, J.M, Pratt, RJ.P. Williams, J. Chem. Soc. A ( 1971 ) 196. W.H. Pailes, H.P.C. Hogenkamp, Biochemistry 7 ( 19681 4160, D.A. Baldwin, E.A. Betterton, S.M. Chamaly, J.M, Pratt, J. Chem. Soc., Dalton Trans. ( 19851 1613. H.M. Marques, J.H. March. J.R. Mellor, O.Q. Munro, lnorg. Chim. Acla 170 (1990) 259, H.M. Marques. J.C. Bradley. L.A. Campbell. J. Chem. See.. Dalton Trans. (1992) 2019. i-t.M. Marques, J.C'. Bradley. K.L. Brown. H. Brooks, J. Chem. See.. Dalton Trans. ( 1993 ) 3475. H.M. Marques. J.C. Bradley. K.L. Brawn. H. Brooks. Inorg. Chim. Acta 209 ( 1993 ) 16 I, (a) K.L Brown, DI~. Evans, lnorg. Chem. 29 119901 2559; Ib) D.W. Jacoh,~n, R. Green, K L Brown, Melh(xls Enzymol. 123 (1986) 14. K.L, Brown. X. Zou. M. Richardson. W.P. Henry. Inorg. Chem. 3{) ( 1991 ) 4834. H.A. Barker. R.D. Smyth. H. Weisshach. J.1. Toohey. J.N. Ladd. B.E. Volcani. J. Biol. Chem. 235 (1960) 480. K.L. Brown. L. Zhou, lnorg. Chem. 35 (1994) 5032. D.E, Linn, Jr., F.,G, Gould. hlorg. Chem. 27 (1988) 1425. K,L. Brown. S. Satyanarayana. lnorg. Chim. Acta 201 (1992) 113. HM. Marqucs. T.J.F.gan. J.H. Marsh. J.R. Mellor, O.Q. Munro. inorg. Chim. Acta 166 (1989) 249. G.D. Fasman. Handh~k of Biochemistry and Molecular Biology. Vol. I, CRC Press. Ch..veland, OH, 3rd edn.. 1976. K L Brown. M, Ngamdu¢, J, Organomet, Cht;m, 243 ( 19831 339, {a) R.A. Firth, H.A,O, Hill B.IL Man, J.M, Pratt, R,G, Thorp, R.J P William,,, J, Chem, S~: A { 19bN) 2419., {b) R.A. Firlh, H,AO Ihll, JM. Pratt, R.(;. Thorp. R,,I.P, Williams, J. Chem S{~.', A {11~681 2428. R,A, Firth, It.A.~), Ihll, J,M Pratt, R,(i, Thorp, R.J,P, Wdham,~, l Chem, Sty, A 119(~9} 381, P, Gcoqle, D,H, Irvin¢. S,C, C]lauser, Ann, New York. Acad Sfi, 88 I 1960) 393. G,(', I|ayward, H,A.(~. Ilill, J,M. Pratt, N,J, Va1|,qOll.R,J,P, William,, J. Chem. Sty:, 11965 ) 6485. M.K, Cienu, J, 14alpern. J. Am, Chem. So¢, 1119{ 1987) 1238, J.D. Bregti¢.prt~:. Natl. Acad, Set. USA 62 (1969) 4¢~1, K.L. Brown, S. Peck-Slier, Inorg,. Chem. 27 11988) 3548, K.L. Brown, LM. Hakimi, D.W, Jacobsen, J. Am, Chem, So¢, 104 I 1984 ) 7894. K.L, Brown, D,R, Evans, inorg. Chem. 33 11994) 6380, I a) K,L, Brown, B.D. Gupta. Inorg. Chem, 29 { 19t~)) 3854; {b) K L Brown, S. Satyanarayana. Inorg, Chem, 31 (I~F)2) 1366. K.I., Brown. G,Z, Wu. Inorg. Chem, 33 { 1994) 4122,