Stewart, Marzilli
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Mechanistic Investigation of Cobaloxime Complexes
(18) R. J. Doedens, J. T. Veal, and R. G. Little, lnorg. Chem., 14, 1138 (1975). (19) C. Barbeau and R. J. Dubey, Can. J. Chem., 51, 3684 (1973). (20) M. R. Churchill, B. G. DeBoer, and K. L. Kaka, h o g . Chem., 12, 1646 (1973). (21) A. Lopex-Castro and M. R. Truter, Acta Crysfallogr., 17,465 (1964). (22) R. Gerdil, Helv. Chim. Acta, 57, 489 (1974). (23) W. Branes and M. Sundaralingam. Acta Crysfallogr., Sect. E. 29, 1868
817
(1973). (24) (25) (26)
(a)J. H. Burns, W. H. Baldwin, and F. H. Fink, horg. Chem., 13, 1916(1974): (b) A. D. U. Hardy and G. A. Sim, J. Chem. Soc.,Dakon, Trans., 1900 (1972); (c)M. R. Churchill and J. Wormald, Inorg. Chem., I O , 572 (1971). W. Strohmeier and F. J. Muller, Chem. Ber., 100, 2812 (1967). At this time w e cannot rule out the possibility that the sulfonium cation is first dissociated, then dealkylated and the sulfide molecule returned to the manganese atom.
A Mechanistic Investigation of Cobaloxime Complexes Containing Good Trans Labilizing Ligands, Including Spectroscopic and Rate Comparisons Robert Charles Stewart and Luigi G. Marzilli* Contribution from the Department of Chemistry, The Johns Hopkins Unicersity, Baltimore, Maryland 21 21 8. Received March 7, 1977
Abstract: Observed and extrapolated first-order rate constants are reported for trans ligand substitution reactions in a series of six-coordinate cobalt(lI1) complexes known as cobaloximes. These complexes are of the type trans-LCo(DH)zX, where (DH) = the monoanion of bisdimethylglyoxime (HON=C(CH3)C(CH3)=NO-), and L and X are neutral and anionic ligands, respectively. The trans labilizing ability of X was sufficient to allow monitoring the substitution reactions by conventional spectrophotometric techniques at 25 OC. The nonaqueous solvent, CH2C12, was employed for these measurements. Under pseudofirst-order excess of entering ligand, L’ = 4-tert-butylpyridine, I -methylimidazole, trimethyl phosphite, trimethylphosphine, dimethylphenylphosphine, tri-n-butylphosphine, and tri-n-octylphosphine, the observed rate constants for the substitution of L = 4-cyanopyridine trans to CH3 was found to be independent of both the concentration and chemical identity of L’. The observed rate constants for the substitution of LCo(DH)2CH3 by tri-n-butylphosphine depended on L in the following order: 4trrt-butylpyridine (1.32 i 0.04 X s-l) < 4-cyanopyridine (5.7 & 1.6 X I O w 2 s-l) < triphenylphosphine (8.0 f 0.1 X IO-* s-l). The relative trans effect of a total of 1 1 different X ligands was found to increase in the following order: p S02CsH4CH3 < CHBr2 5 SC(C6Hs)j < CH2Br < p-CsHdBr < P - C ~ H ~ H ~ O
(10)
= 1 / k 3 -k (1 f K ~ [ L ] ) / ( ~ ~ K I [ L ’ ] ) ( 1 1 )
Equations 10 and I 1 have equivalent algebraic forms to eq 4 and 5 if K2[L] >> I . Ewen and Darensbourg concluded that T h e important consequence of these expressions is that for on the basis of rate retardation a distinction could not be made [L’] >> [L], the rate of the substitution reaction will be indebetween the dissociative and dissociative interchange mechapendent of both the nature and the concentration of L’, pronisms. Ewen and Darensbourg were able to suggest further, viding L’ forms a more stable complex t h a n does L. A n addion the basis of the Jensen and KiskisIo d a t a only, that an intional consequence of these expressions is that the rate of terchange mechanism was equally likely and that the poor substitution should be subject to a mass law rate retardation competitive ability of the Bu3P ligand was a consequence of effect and that from eq 5 the competition ratio ( k - l / k * ) for the low hydrogen bonding ability of Bu3P which in turn makes the reaction of the five-coordinate intermediates with L and K I relatively small for this ligand and hence leads to a larger L’ can be obtained. S u c h competition experiments, which ratio for K ~ / K Ieq, 1 I . T h e similarity in the ability of the provide information about the intermediate(s) along the resubstituted pyridine ligands and piperidine to retard the subaction coordinate, cannot be performed using N M R relaxation stitution reactions is easily explained if K l = K2. Therefore, techniques. In coordinating solvents, effective competition by a choice between the mechanisms is not easily made on the the solvent for intermediate(s) often leads to a n apparent basis of the Jensen and Kiskis data. In this report, we will second-order kinetic dependence for the substitution process. consider these arguments both on the basis of our earlier data’ The special ability of water to promote substitution reactions and the additional data that we report here. is no doubt associated to some extent with the hydrogen A secondary reason for carrying out this study was to exbonding capability of this solvent. Therefore, in the experiamine the relationship between the extensive information on ments reported here, we avoided any ligands which contained the spectroscopic trans influence (ground-state effect) of X N H groups. ligands” with the kinetic trans effect of X. Most spectroscopic In a preliminary report of some of the work to be described studies have involved metal centers such as Hg(I1) for which here,’ we showed that a mass-law rate retardation could be extensive kinetic information is not available or Pt(l1) which observed for cobaloximes containing such good trans labilizing undergo substitution reactions by associative and, hence, ligands as CH3- and ( C H 3 0 ) 2 P O - , and that except for trin-butylphosphine ( B u ~ P )ligands , such as (CH30)3P, C N P ~ , ~complex pathways. These metals have isotopes which allow an assessment of metal ligand bonding and metal hybridization Bupy, M e l m d , and (C6H5)3P were about equally effective in using calculations based on N M R coupling constants between retarding the rate of axial ligand substitution. In terms of a the metal nucleus and ligand nuclei. dissociative mechanism, this result suggests that the compeThere is no convenient isotope of cobalt for measuring metal tition ratio ( k - l / k z ) from eq 5 was ca. one and that the fiveto ligand spin-spin coupling. However, we have shown that the coordinate intermediate was highly reactive. W e also showed relative spectroscopic influence of X in some cobalt(II1) that, in the absence of added leaving ligand, L, the rate of compounds parallels quite closely this effect in platinum(] I ) substitution by a variety of L’ ligands was essentially indechemistry.13 Additionally, cobaloximes exhibit some chemistry pendent of both the n a t u r e and concentration of L’. which is similar to that for Pt(II).I4 Brown has pointed out that I n a later study by Jensen and Kiskis,Io rate retardation was good correlations can be found between activation parameters observed for the substitution of (piperidine)Co(DH)*CH3 by for ligand exchange and s9C0 N Q R d a t a on c o b a l ~ x i m e s . ~ substituted pyridines in HCCI3. These authors also studied the Thus, although 59C0complexes do not exhibit useful spin-spin reaction between Bu3P and (py)Co(DH)*CH3 to yield coupling, some insight into bonding can be derived from N Q R ( B u ~ P ) C O ( D H ) ~ CinH the ~ presence of added pyridine. In studies, even without recourse to the demonstrated similarity terms of eq 5 , they also found that ( k - l / k * ) was not very difbetween Co(1II) and Pt(I1) chemistry. ferent from one for most entering ligands but that Bu3P, although very nucleophilic, was a poor competitor. Results In addition to the analysis of the mechanism a s a purely dissociative process, Jensen and KiskisIo offered the following Characterization and Stoichiometry. T h e ‘H spectra of all proposal for a dissociative interchange mechanism: starting and product cobaloximes show a single resonance for the methyl groups of the dimethylglyoxime chelate system, KI indicating that the Co(DH)2 moiety is planar and that X and LCo(DH)2X L’ e LCo(DH)2X,L’ (rapid) (6) L a r e situated trans to one another. All of the axial ligand exk3 change reactions were followed by IH N M R and found to LCo(DH)zX,L’ e L ’ C O ( D H ) ~ X , L (7) proceed to completion with apparent 1 : 1 molar stoichiometry and complete retention of trans stereochemistry. More limited fast I3C N M R d a t a support the IH N M R data; in particular only L’CO(DH)~X,L L’Co(DH)*X L (8) one oxime methyl I3C N M R signal was observed for both The appropriate rate expressions for this mechanism would not starting and product complexes. N M R spectra of product permit rate retardation, and on this basis, Jensen and Kiskis complexes were identical with those where the proposed l/kobsd
+
+
Stewart, Marzilli
/ Mechanistic Investigation of Cobaloxime Complexes
Table I I 1 . O Summary of Observed and Extrapo!dted First-Order Rate Constants for the Axial Ligand Substitution of L from LCo(DH)2CH3 by L’ i n CHzCl2 at 25.0 f 0.1 OC
Leaving ligand (L) = CNpy k . s-I Obsd
ExtraDolated
6.7 f 0.9 X 5.8 f 0.3 X 6.4 f 0.4 X 5.4 x 10-2 5.5 f 0.6 X 5.7 f 1.6 X 4.9 f 0.1 x 10-2
6.3 f 1.1 X 5.8 f 0.6 X 6.0 f 1.3 X 5.7 f 0.9 X 6.0 f 0.9 X IOv2 4.5 f 1.0 x 10-2 5.0 f 1.6 X
L = (C6H5)3P BUPY MeImd (CH30)3P Bu~P
8.0 f 0.4 X IO-2 8.1 f 0.7 X 8.5 f 0.3 X 8.0 f 0.1 x
7.6 7.9 8.2 8.8
f 1.3 X f 0.6 X loT2 f 2.2 X lo-* f 2.3 x
The observed rate constants are mean values of the rate data (Table I ) taken under pseudo-first-order excess (greater than 0.2 M) of entering ligand (L’) with respect to the cobaloxime concentration (ca. 0.01 M) and without added leaving ligand (L). The extrapolated rate constants were derived from they intercept of the least-squares regression analysis of the rate data (Table I) according to eq 5. An average kohd was used for the point corresponding to no added leaving ligand ([L]/[L’] N 0). The uncertainties in the observed rate constants are the standard deviations about the mean. The uncertainties in the extrapolated rate constants are the standard deviations in the y intercept of the least-squares plot of eq 4 and are inherently large by this method. Limited data available. Table IV.“ Competition Ratios between Land L’ for the Common Intermediate, Co(DH)2CH3, i n CH2C12 at 25.0 f 0.1 OC
Entering ligand (L’)
Leaving Ligand ( L ) = CKpy k-i/kz 1.3 f 0.3 1.3 f 0.2 1.1 f 0.3 1.7 f 0.3 1.4 f 0.3 4.7 f 1.0 6.1 f 2.0 1.8 f 0.4b
L = (C6Hs)jP BuPy Melmd (CH30)3P Bu~P
0.7 f 0.2 0.6 f 0.1 0.8 f 0.3 4.0 f 1 . 1
Mass-law rate retardation data (Table I) were analyzed by eq 5 using a linear least-squares regression program. Uncertainties represent the standard propagation of errors combination of the standard deviations in the slope andy intercept of this analysis. An average kobsd was used for the point corresponding to no added leaving ligand( [L]/ [L’] N 0). From (CsH5)3P-Bupy data.
products had been prepared by alternate methods. No evidence was found for the formation of a n y disubstituted products, L’zCo(DH)*+X-, for the cobaloximes used in this work, even with high L’ ligand concentrations. Such cation species can be problematical in polar solvents such as methanol.I5 Kinetics. Treatment of the experimental absorbance vs. time rate d a t a with the standard first-order integrated expression using a linear least-squares regression analysis resulted in good (correlation coefficients > 0.99) linear fits over 3-6 half-lives (see Tables I and I1 in the microfilm edition). The kobsd in these tables were used to calculate the constants given in Tables 111-VI. Under the conditions of pseudo-first-order excess of
819
L’ and with no added leaving ligand (L), eq 5 reduces to kobsd = k1. Hence, the observed rate constants (kobsd) in Tables I11 and V represent values for the limiting rate constants (kl’s). T h e observed rate constants given in Table 111 were determined for the substitution of LCo(DH)2CH3 ( L = C N p y , ( C ~ H S ) ~ by P ) a series of different entering ligands L’. A t sufficiently high concentrations of L’ and in the absence of added L, the reaction rates were essentially independent of both the concentration and chemical identity of L’. Rates were consistently greater for the triphenylphosphine complex t h a n for the cyanopyridine compound. In addition to experiments performed in the absence of added L, a series of rate measurements were made in the presence of varying concentrations of L. T h e observed rate constants could be treated according to eq 5. Good agreement was found between observed and extrapolated rates. The rates in the forward direction for axial ligand exchange of cobaloximes containing various X groups (Table V) show a variation in rate of approximately lo4 when differences in leaving ligand are taken into account. Competition ratios calculated from these results are given in Tables IV and VI. Sample plots will be found in the microfilm edition.
Discussion Three key features emerge from the kinetic data reported in Tables 111-VI. First, the rate of the substitution reaction is independent of the concentration of the entering ligand. This finding is wholly consistent with a purely dissociative mechanism. It can also be accommodated by a dissociative interchange mechanism, if K1 [L’] is much greater than one. Second, the rate of the substitution reaction is independent of the nature of the entering ligand. Again this result is exactly in agreement with a purely dissociative mechanism. However, an interchange mechanism requires that, although the entering ligand is in the activated complex, it has no significant influence on the rate constant, k3, in eq IO. This restriction is, however, not severe since it is conceivable that the weak binding of the entering ligand may not influence the rate of the bondbreaking reaction. Third, whether the data are treated according to eq 5 or eq 1 1, a rate retardation would be expected when excess leaving ligand is present. Such a result can algebraically be used to defend either mechanistic proposal.” However, the values we have obtained for the ratios k - l / k z or K2/Kl are generally close to one. For a dissociative mechanism, this finding suggests that the five-coordinate intermediate formed is nonselective on the basis of the nucleophilicity of the incoming ligand.7 This lack of selectivity is in keeping with a highly reactive intermediate. For an interchange mechanism, this result implies that the association between the complex and the entering ligands (as well as the various and different leaving ligands employed here) is independent of the ligand. Also, since these ratios appear to be independent of X and L in the complex, the association constants ( K I and K2) would also have to be independent of the nature of the complex. This accommodation is difficult if not impossible to rationalize. T h e ligands used vary considerably in size and hydrogen bonding ability as well as in basicity. That such diverse ligands would all have nearly the same affinity for the different cobaloxime complexes used here seems highly unlikely. In particular, the hydrogen bonding ability of (CH3)3P and (Bu)3P should be quite similar but weaker than that of the N ligands. However, (CH3)3P is equally as effective as the N donor ligands in competing for the intermediate. T h e ineffectiveness of the (Bu)3P ligand to compete for the reactive five-coordinate intermediate was previously attributed by us to the steric hindrance of the dangling alkyl chain^.^ T h e greater ineffectiveness of the ( n octyl)3P ligand is consistent with this view. Also, in many of
Journal of the American Chemical Society
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1 February 1 , 1978
100.3
Table V." Summary of Observed and Extrapolated First-Order Rate Constants for the Axial Ligand Substitution of L from LCo(DH)2X by L' in CH2Cl2 at 25.0 f 0.1 OC
Leaving ligand (L) = CNpy k . s-l Extrauolated
Obsd 2.0 0.4 x 10-4 5.2 f 0.3 X 2.7 f 0.2 x 10-3 1.0 f 0.2 x 10-2
1.7 f 0.5 X 4.7 1.2 x 10-4 1.0 f 0.1 x 10-2 1.0 f 0.1 x 10-2
1.6 f 0.3 X 2.0 f 0.1 x 10-2 2.9 f 0.6 X 4.5 f 0.3 X 5.7 f 1.6 X 6.2 f 1.0 X (CH30)3P Bu~P Bu~P Bu~P
7.3 x 10-4
2.8 & 1.1
L = (C6Hd3P
7.4 f 0.4 X 8.0 f 0.1 X
7.6 f 0.7 X 2.6 f 0.3 x IO-' 7.3 f 1.7 X 8.8 f 2.3 x
L = Bupy 2.7
8.6 f 0.3 X 4.0 f 0.2 X
X lo-*
4.5 f 1.0 x 10-2 5.6 f 1.3 X IO-2
c
1.32 f 0.04 X
b
x
10-4
d
1.4 0.2 x 10-3 3.3 x 10-3 d 3.6 x 10-3 d 8.0 f 2.6 X 3.8 f 0.9 X IO-*
a The observed rate constants are mean values of the rate data (Table I1 except for X = CH3, see Table I ) taken under pseudo-first-order excess (greater than 0.2 M) of entering ligand (L') with respect to the cobaloxime concentration (ca. 0.01 M) and without added leaving ligand (L). The description of the extrapolated rate constants and the uncertainities are given in footnote a , Table I l l . Reported previously in ref 7. A 2.5 X M solution was employed. Limited data are available. d Estimated from data reported previously' where benzene was the solvent. Rates were 3.3 times slower in benzene as compared with methylene chloride, based on the data reported in this work for the methylcobaloxime complex, BupyCo(DH)zCH3.
Table VI." Competition Ratios between L and L' for the Common Intermediate, Co(DH)zX, in CH2C12 at 25.0 f 0.1 O C
X
Leaving ligand (L) = CNpy Entering ligand (L') k-i/kz
SOZC~H~CH~ CHBr2 C6HdBr C6Hs CH3 CH3(niox complex)
(CH3O)d' Bu~P Bu~P Bu~P Bu~P Bu3P
1.2 f 0.6 3.0 f 0.8 3.0 f 0.4 5.4 f 2.1 4.7 f 1.0 3.6 f 0.9
SC(C6Hs)3 C6H5 CH3
L = (C6H5)3P (CH30)3P 0.7 f 0.2 2.8 f 0.7 Bu~P 4.0 f 1.1 Bu~P
CH3 CHzC6Hs C?H
> 1 is not so likely to hold a s in the studies by Jensen and Kiskis.Io T h e key and overwhelmingly important step in the reaction is the bond-breaking step and this step is insignificantly influenced by the entering ligand. Thus, we feel that cobalt(II1) complexes a r e superior to platinum(I1) complexes for comparing reaction rates with spectroscopic parameters.
T h e relative trans labilizing effects of X ligands used in this study vary by a factor of about IO4. By utilizing information obtained in other solvents in the manner suggested by Deutsch,I5 we can make comparisons spanning a range of IO6. W e have made numerous comparisons between the rate d a t a measured here and NMR spectroscopic information collected in other studies.13-'6,17 Neither parameter which reflects metal rehybridization, the a hydrogen resonances or the 3 1 P-I H coupling (between the coordinated P and the oxime methyl groups) adequately reflects the changes in rate. T h e a hydrogen resonance is subject to a number of anomalous effectsI3 and we believe that, contrary to previous results which suggested a good ~ o r r e l a t i o n , 'this ~ parameter is not useful. These shifts for X = CH3 and C2H5 are essentially indistinguishable. However, the rates of reaction a r e appreciably different. T h e 31P-1Hcouplings, which are not subject to anomalous anisotropic effects and which more closely reflect the metal rehybridization, also suggest relatively little if any difference in the hybridization of the metal center for a number of alkyl ligands. T h e rates of substitution reactions d o vary considerably. T h e parameters which best reflect the rate d a t a a r e those which exhibit a considerable difference between the shifts or couplings in the C H 3 and C ~ H complexes. S Principal among these a r e the shift of the a C in the Bupy ligand in BupyCo(DH)2X complexes (Figure 1) and the 31P-13CN M R coupling between the coordinated phosphorus and the directly bonded C in BujPCo(DH)2X complexes. T h e r e is presently theoretical support for the contention that the chemical shifts of aromatic carbons reflect the total CT plus H electron density a t the carbon.18 Thus, it is probable that the I3C shifts a r e reflecting the ability of the Co(DH)2X moiety to withdraw electron density from the pyridine ring. In turn this shift may
Stewart, Marzilli / Mechanistic Investigation of Cobaloxirrte Complexes
821
be a measure of the ability of X to donate charge to the cobalt
as the Co(I11)-L bond is broken. Brown has been able to interpret 59CoN Q R measurements in cobaloximes in terms of the covalency of the ligands in the axial position. Such spectroscopic d a t a correlated very well with activation parameters for ligand exchange of methylcobaloximes. It would be quite interesting to extend such N Q R studies to complexes in which the labilizing ligands were varied. I n this study, those spectroscopic parameters which measured the influence of the entire X C o ( D H ) * moiety on L gave the better correlation. N Q R appears to offer a technique for examining the bonding of L to the Co center whereas most of our N M R studies primarily provide information on the effect of X on the cobalt(II1) center. DeutschI5 has studied the sulfinato ligand, SO*C6H&H3-, in an effort to determine whether this ligand or the S03*- ligand can induce trans labilization by a m x h a n i s m involving an interaction of the type 0
II --co
S
O u r spectral comparisons indicate that the ligand substitution rate for the cobaloxime containing this sulfur donor ligand is not appreciably different from that expected from extrapolation of the spectral data for such purely n donor ligands as C H 3 and CHBr2. Additionally, the competition ratios we have measured are insensitive to the following properties of the trans labilizing X group: ( a ) the nature of the donor atom (S or C); (b) the hybridization of the donor a t o m (sp3 or sp2); and (c) the effectiveness of the X group in labilizing the trans ligand. Therefore, there is no need to invoke any change in mechanism to account for the trans labilizing ability of sulfinato ligands. In conclusion, all of our data can readily be accommodated by a purely dissociative process which leads to a reactive, nonselective, five-coordinate intermediate for all complexes studied.
Experimental Section Reagents. 4-Cyanopyridine was recrystallized from a mixture of CHzCl2 and Et2O. Triphenylphosphine was recrystallized from a mixture of 95% EtOH and EtzO. All other materials were reagent grade and were used without further purification. Methylene chloride, used as the solvent in these kinetic determinations, was spectroquality grade. Prepurified nitrogen was used to deoxygenate the solutions. Caution! The alkylating agents used here may be carcinogenic. Bromoform is toxic and readily absorbed through the skin. Benzyl halides are potent lachrymators. Preparation of Complexes. The preparation of complexes used in this study and a few compounds used to obtain spectroscopic parameters are described here. The preparation and characterization of other complexes (L = Bupy: X = NO2, CH3, P(O)(OCH3)2, P(O)(OCH3)(ChH5). P(O)(C6H5)2 and L = CNpy, (C6H5)3P: X = P(0)(OCH3)2) used here may be found e l ~ e w h e r e . ’ ~ . ’ ~ , ~ ~ CNpyCo(DH)zC,jHdY(Y = H, Br, OCH3). These complexes were prepared by the procedure described by Schrauzer2’ but using = H): CIVpyCo(DH)2CI. Anal. Calcd for C ~ O H ~ ~ O(X ~N ~ CC,O51.1: H, 4.9; Co, 12.5. Found: C, 50.9; H, 5.2; Co, 12.3. Calcd for CzoH2204N6BrCo ( X = Br): C, 43.7; H, 4.0; Co, 10.7. Found: C, 44.0; H, 4.4; Co, 10.6.Calcd for C2lH2505N6Co (X = OCH3): C, 50.4; H, 5.0: Co, I I .8. Found: C, 50.1; H, 5.0; Co, I 1.8. The triphenylphosphine derivative (X = H) was prepared by dissolving the corresponding CNpy compound in CHzCl2 together with an excess of (C6H5)jP and stirring. The extent of reaction was followed by monitoring the decrease of the ‘ H NMR singlet of the CH3 groups of the (DH)2 ligand of the starting material and the corresponding increase of the doublet of the product complex. Once the reaction was complete the solvent was evaporated and the product was washed with diethyl ether to remove the excess triphenylphosphine and the displaced CNpy. The product was recrystallized from a CH2C12-EtzO mixture. Anal. Calcd for ( X = H) C32H3404N4PCo: C, 61.2; H , 5.5. Found: C, 60.1; H, 5.5.
CH2Br 0 I
1
- 4t
-6
1 L
I62
I63
8 -I3c
I64 PPM
Figure 1. Comparison of the logarithm of the observed rate constailts. Table V, for (CNpy)Co(DH)zX + (excess)L’- L’Co(DH)?X,\ + i t h the y-I3C NMR shifts of the Bupy ligand in ( B ~ ~ ~ ) C O ( D H )Points ? X . ’la~ beled ( 0 )indicate that relative rates were employed. Points labeled ( 0 and e) indicate that rates (from ref I O ) are relative to X = CH? and S02ChH4CH3 (this work). respectively. CNpyCo(DH)zR (R = CH3, CH2Br, CHBr2, p-CH2C&CN). These complexes were prepared by the procedure described by HillZ2using CNpyCo(DH)zCI and the appropriate alkylating agent (CH3I. CH2Br2, CHBr3. or p-ICH2C6HdCN). However, the NaBH4 was introduced after adding excess alkylating agent instead of adding the alkylating agent after the reduction. Also, acetone (ca. 5 mL) was added to destroy any remaining excess NaBHd. Anal. Calcd for CI~H~IO~N C,~44.0; C OH,: 5.2; Co, 14.4. Found: C, 43.9: H, 5.0; Co. 14.6. Calcd for ClsH2004N6BrCo: C, 37.0: H, 4.1: Co, 12.1. Found: C , 37.3; H, 4.2: Co, 12.4. Calcd for C ~ S H ~ ~ O ~ NC.~31.8; B ~ H, ~CO: 3.4; Co, 10.4. Found: C, 32.3; H, 3.6; Co, 10.4. Calcd for C Z ~ H ~ ~ O J N ~ C OC,’ 50.1; H ~ OH, : 5.0; Co, 11.2. Found: C, 49.7: H, ) ~ Cprepared H~ by the same 5.2; Co, 11.8. ( C ~ H ~ ) ~ P C O ( D H was procedure but using ( C ~ H ~ ) ~ P C O ( D HThe ) ~ Cmethyl I. (X = CH3) derivative may also be prepared by reaction of a 1 : I molar amount of triphenylphosphine and the “alkyl dimer”, (Co(DH)2CH3)2, in a noncoordinating solvent such as acetone or CH2C12. Anal. Calcd for C ~ T H ~ ~ O ~ NC,~57.3; PCO H,: 5.7; Co, 10.4. Found: C , 57.0; H. 5.4: Co, 10.3. BupyCo(DH)zR (R = C2H5, CHzCbHs). These complexes were also prepared by Hill‘s procedure22 but using B U ~ ~ C O ( D H and ) ~ Cthe I appropriate alkylating agent (C2H5I or BrCH2C6H5). Anal. Calcd for Cl9H3204N5Co: C, 50.3; H, 7.1; Co, 13.0. Found: C, 50.0: H. 6.8; Co, 13.0. Calcd for C24Hj404N~Co:C, 55.9: H, 6.7; Co. 1 1.4. Found: C, 56.0; H, 6.5; Co, 1 1 . I ( C ~ H ~ ) ~ P C O ( D H ) ~ S C ( C( C &~I ~H)~~ ). ~ P C O ( D H(0.01 ) ~ Cmol) I was suspended in 90% aqueous methanol (100 mL) and the pH adjusted toca. 8 with NaOH. HSC(C,jH5)3 (0.01 mol) and NaOH (0.01 mol) weredissolved with stirring in H2O (25 mL). These solutions were then mixed with stirring. The resulting solution was heated at just below the boiling point for ca. 5 min and then rotoevaporated to ca. 5O0h 01‘ the original volume. Addition of HzO (100 mL) precipitated a brown product. Anal. Calcd for C ~ ~ H ~ ~ O ~ N ~ P S CC.O62.6; . ~ HH,Z5.6. O: Found: C , 63.1; H. 5.4. Thecomplex, CNpyCo(DH)lS-r-C4Hg, uas also prepared by this procedure but using CNpyCo(DH)lCI and HS-r-CdHg. Anal. Calcd for C ~ ~ H ~ ~ O ~ N ~ S CC,O43.2: . H ZH, O 5.8: : Co, 11.8. Found: C, 43.2; H, 5.9; Co, 11.9. BupyCo(DH)I.This complex was prepared by the direct air oxida-
822
Journal of the American Chemical Society
/
100:3
/ February I , 1978
tion procedure described by Schrauzer.2' Anal. Calcd for the least-squares fit of the raw data were then plotted as 1 / k o b s d vs. (L)/(L') by a linear least-squares regression analysis. For systems C I ~ H I ~ O ~ N C, ~ I37.0; C O H, : 4.9. Found: C , 37.5; H , 4.9. BupyCo(DH)zCHzSi(CH3)3.This complex was prepared in low yield ~ mechanism and exhibiting undergoing reaction by an S N limiting ( 10%) by Hill's methodZ2but using BupyCo(DH)21. Schrauzer's mass-law rate retardation, this analysis yields 1 / k 1 as they intercept and k - l / k l k 2 as the slope. The meaning of these values will be shown method,23for preparation of the corresponding pyridine complex, was in the Results section. All uncertainties represent standard deviations unsuccessful for preparation of the Bupy complex when attempted in the y intercept of the least-squares analysis, unless otherwise by us. The yellow compound isolated in our laboratories was characnoted. terized by IH and I3C N M R showing the correct numbers of resonances in the correct ratios but no kinetic studies were done using this Supplementary Material Available: Plots of (a) the absorption material since insufficient quantities could be isolated. spectra in methylene chloride of LCo(DH)2CH3 (L = CNpy and CNpyCo(DH)zSOzC6H&H3. This complex was prepared by the Bu3P) and of the reaction mixture: (CNpy)Co(DH)zCH3 (exprocedures given by Palmer and Deutsch15 but using CNpy. Anal. ( B u ~ P ) C O ( D H ) ~ C (b) H ~ I/kobsdvs. ; [L]/[L']; and (c) cess)BulP Calcd for C~~H~@&"SCO: C, 46.0; H, 4.6; Co, 10.7. Found: C, 46.3; log kobsd vs. various N M R spectroscopic data and Tables I and 1 1 , H , 4.6; Co, 10.5. listing the experimental rate data (21 pages). Ordering information CNpyCo(niox)~CH3.(niox) represents the monoanion of 1,2-cyis given on any current masthead page. clohexanedione dioxime, -0NCC4H8CNOH. This complex and those needed for its preparation were prepared according to the procedures given for the corresponding (DH) compounds but substituting (niox) References and Notes for (DH). Anal. Calcd for C , ~ H ~ ~ O ~ C, N 49.6; ~CO H,: 5.5; Co, 12.8. Found: C, 49.3; H, 5.4; Co, 12.8. J. M. Pratt, "Inorganic Chemistry of Vitamin Bl2", Academic Press, New York, N.Y., 1972. Apparatus. The ligand exchange kinetics were monitored spectroG. N. Schrauzer, Acc. Chem. Res., 1, 97 (1968). photometrically using a Cary 14 equipped with a thermostated cell L. M. Ludwick and T. L. Brown, J. Am. Chem. Sac., 91, 5188 (1969); T. L. compartment and cell holder which maintained the reaction solution Brown, L. M. Ludwick, and R. S.Stewart, ibid., 94,384 (1972); R. J. Guschl and T. L. Brown, Inorg. Chem., 12, 2815 (1973): R. J. Guschl, R. S.Stewart, at 25.0 f 0.1 "C. ' H NMR measurements were made on a Varian and T. L. Brown, ibid., 13, 417 (1974); T. L. Brown, Acc. Chem. Res., 7, HA-IO0 spectrometer. 13CN M R spectra were recorded on a Varian 408 (1974): R. A. LaRossa and T. L. Brown, J. Am. Chem. Soc., 96,2072 CFT-20. Details and spectral assignments are reported else(1974). where,I3.16.17 K. L. Brown, D. Chernoff, D. J. Keljo, and R. G. Kallen, J. Am. Chem. Soc., 94, 6697 (1972): K. L. Brown and R. G. Kallen, ibid., 94, 1894 (1972). Kinetic Procedure. Visible spectra of methylene chloride solutions G. Tauzher, R. Dreos, G. Costa, and M. Green, J. Chem. SOC.,Chem. of each starting cobaloxime were recorded and then compared with Commun., 413 (1973); J. H. Espenson and R. Russell, Inorg. Chem., 13, the visible spectra of the solutions after adding a calculated excess (ca. 7 (1974). (This article summarizes the uncertainties associated with sub40:l) of entering ligand L' and allowing sufficient time (as estimated stitution reactions in aqueous solutions.) A. L. Crumbliss and W. K. Wilmarth, J. Am. Chem. Soc., 92,2593 (1970); by a similar N M R experiment) for the reaction to near completion. P. H. Tewari, R. H. Grover, H. K. Wilcox, and W. K. Wilmarth, Inorg. Chem., The wavelength which gave the greatest difference in absorbance was 6, 611 (1967); H. G. Tsiang and W. K. Wilmarth, ibid., 7, 2535 (1968). used to monitor the kinetics. Suitable wavelengths were in the range W. C. Trogler, R. C. Stewart, and L. G. Marzilli, J. Am. Chem. SOC.,96, 3697 (1974). The absence of significant amounts of Bu3P0 was confirmed by of 450-520 nm for the complexes studied. 13C NMR. The starting cobaloxime in each case was weighed directly into the W. D. Covey and T. L. Brown, Inorg. Chem., 12, 2820 (1973). cuvette used in the kinetic determination. I f a mass-law rate retarThe following abbreviations are used in this report: (Bu3P) = tri-n-butyldation experiment was to be done, the calculated excess of leaving phosphine, (Bupy) = 4-tert-butylpyridine, CNpy = 4-cyanopyridine. Melmd = 1-methyllmidazole, (nlox) = 1.2-cyclohexanedione dioxime. ligand was also weighed into the cuvette. In cases where the leaving F. R. Jensen and R. C. Kiskis, J. Am. Chem. Soc., 97,5820 (1975). ligand was a liquid (Melmd, (CH30)3P, Bupy) the ligand was added J. A. Ewen and D. J. Darensbourg, J. Am. Chem. Soc., 98, 4317 (1976). after addition of 2.5 mL of degassed solvent. The cuvette was placed T. G. Appleton, H. C. Clark, and L. E. Manzer, Coord. Chem. Rev., 10, 335 in the thermostated (25.0 f 0.1 "C) spectrometer and allowed to (1973). L. G. Marzilli. P. Politzer, W. C. Troaler, and R. C. Stewart, Inorq. Chem., thermally equilibrate (ca. 15 min). During this time the sample was 14, 2389 (1975). moved out of the light path to minimize a photodecomposition reaction 86, 5367 (1964); 88, 704 (1966): H. G. W, Parshall, J. Am. Chem. SOC., know to occur for certain R groups (CH3, CH2Br, CHBr2, C Z H ~ ) . ~ A. 0. Hill, K. G. Morallee, G. Pelllzer, G. Mestroni, and G. Costa, J. OrgaNext, a calculated excess of the neat entering ligand was injected using nomet. Chem., 11, 167 (1968). J. M. Palmer and E. Deutsch, Inorg. Chem., 14, 17 (1975). a microsyringe. The reaction solution was then quickly shaken and W. C. Trogler, R. C. Stewart, L. A. Epps, and L. G. Marzilli, Inorg. Chem., placed back into the spectrophotometer. Absorbance changes with 13. 1564 (1974). time were monitored. Where photodecomposition reactions were likely R I C . Stewart and L. G. Marzilli, Inorg. Chem., 16, 424 (1977). J. B. Stothers, "Carbon-13 Nuclear Magnetic Resonance Spectroscopy", absorbance readings were taken only intermittently by moving the Academic Press, New York, N.Y., 1972, Chapter 7. sample in and out of the light path. Absorbance data were collected W. C. Trogler, L, A. Epps. and L. G. Marzilii, Inorg. Chem,, 14, 2748 over at least 3 half-lives with final absorbance data taken after ca. 20 (1975). half-lives. L. G. Marzilli, J. G. Salerno, and L. A. Epps, Inorg. Chem., 11, 2050 (1972). Data Analysis. The experimental absorbance vs. time rate data were G. N. Schrauzer, Inorg. Synth., 1 I,68 (1968). treated with the standard integrated expression for a first-order proH.A. 0. Hill and K. G. Morallee, J. Chem. SOC.A, 554 (1969). cess using a linear least-squares computer program. G. N. Schrauzer and R. J. Windgassen, J. Am. Chem. Soc., 86, 3738 The pseudo-first-order rate constants obtained from the slope of (1966).
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