NMR Spectra of Cobaloximes Unexpected Nonequivalence of Methyl Groups Ben Clifford and William R. Cullen University of British Columbia, Vancouver BC V6T 1Y6, Canada
For some time now our department has been offering a course in "Inorganic Chemistry for Biological Sciences." The associated laboratory course was developed and described by Ochiai ( I ). Part of one experiment concerned with Vitamin B12 and its model compounds involves the preparation and interconversion of the 0-and a-cyanoethyl derivatives of a cohaloxime, I and 11, based on the work of Schrauzer and Windgassen ( 2 , 3 ) . CH,-CH,--CT
I
11
Although the spectroscopic properties of the two Isomers indicate that they are in fact different, there are some problems in the interpretation of the NMR spectra of the compounds, especially if the spectra are obtained on a, usually available, low field instrument. Two typical spectra are shown in Figure 1 as run on a Varian EM 360 instrument. In the p-isomer the C H 2 - C H 2 region is not well resolved and the trace is not first order. The equatorial methyl groups appear as the expected singlet and the downfield resonances associated with the axial pyridine are complex. The a-isomer is distinguished by the upfield doublet due to the CH3CHCN moiety, but the associated quartet is not obviously present. T h e equatorial methyl groups ostensibly equivalent are seen as a doublet with a chemical shift difference of 2.22-2.19 = 0.03 ppm. Students are usually puzzled by the doublet for the four ligand methyl groups and make comments like, "if more than one peak, why not four?" Gaudemer and coworkers ( 4 ) noted this phenomenon and s ~ e c u l a t e dthat it was due to the uresence of an asvmmetric ,.enter un r h r a x w I siyni~-boltdrdlidand The\ i\~undth.11 rlir I ~ ~ i r ('a-t'ttllt.K h w w ~ r Ca ~ aI mtliit~er\,I r t h t ~ dC ~ I I xvs Figure 1 . !a) The NMR spectrum of I at 60 MHz. (b) The NMR spectrum of I1at moieties show this feature which is also found in cohaloximes 60 MHz. !c)and (djare spectra of the same compounds runat 400 MHz. All solutions are made up in CDCi3 and shifts are shown relative to internal TMS. with symmetrical axial alkyl groups, e.g. CHs, associated with axial bases containing an asymmetric carbon as in CfiH&H(NH2)COOCH:, or a donor atom which, once bound, becomes asymmetric, e.g. CsH5CH2NHCH:+ It is well known that nonequivalence of, say, methyl groups is produced by a situation when a nearby carbon atom carries three different substituents as in Figure 2 (R f CH:J where the environments of the two methyl groups a and h are always different and cannot he averaged out by rapid rotation ( 5 ) .It Figure 2. An example of methyl group nonequivalence. The environments of should he noted that the carbon atom adjacent to a and b need groups a and b are always different and cannot be averaged out by rapid rotanot he an optically active asymmetric center. Thus, two restion. onances could be observed for methyl groups a and h even on a compound like R(CH~~)(CHS"C-CH(CN)-C(CH~~)(CH:rh)R. perimposable with CH:I%eing identical with CHaA but CH3A S CHsB. Similarly, comparing (b) with (c), we see that CHsB Figure 3 shows the results of applying this type of Newman projection analysis to a compound like 11. The arcs represent is identical with CH:P hut CH:IDI;CH3C. Thus, free rotation the -0-H-Obridges. Projections (a) and (d) are suof the alkyl group about the Co-C bond establishes two sets 554
Journal of Chemical Education
ca H
,
(C)
CY
H3C
CH3
H3C
CH; CN
a separation of 11.2 Hz which corresponds to a separation of -0.2 ppm. a t 60 MHz and which confirms that the doublet is due to a chemical shift difference rather than a coupling. The rest of the spectrum shows the doublet and quartet of the H-C-CH:3moiety with the latter peaks being almost buried under the resonance of the equatorial methyl groups. The peaks due to the ortho, meta, and para hydrogen atoms on the axial pyridine base are well defined, downfield. This same downfield pattern is seen in the 400 MHz spectrum of the a-isomer. The eauatorial methvl erouDs , .. . are seen as a sinelet and the resonances assuristed with the two methylene p u p s of the axial -(:H.L'H,CN are well s e ~ a r a t e dt r i ~ i e t s . In conclusion, the sikplification ofthe spectra of I a n d I1 obtained a t high field makes structure assignment a simple exercise provided students also realize the interesting consequences of the presence of asymmetric axial ligands.
Figure 3. Results of applying Newman proiection analysis to compound I1
Literature Cited
of axial methyl groups which could be expected to result in the type of doublet seen in Figure lb. The 400 MHz spectrum of the a-isomer is also shown in Figure Id. Here the doublet due to the axial methyl groups has
111 Ochiai Ei-ichiro, J. CHEW E ~ ~ ~ . , 5 0 . 6(19731. 10 191 Schrauzer,G.N.,and Windgassen, R. .J., J. Amar. Chem. Soc .89,1999 119671. 13) Schrauzer,C.N.,Accts Chem. Rescorch. 1.97 119681. 14) Nsurnkrg, M.,Duong.K. N. V..Gauderner, F.,andGsudemer.A.,CR. Arod.Sci.,Ser. C.. 270, 1801 119701. 151 Bih1a.R. H."lnterpretetionuf NMRSpectra."Plenurn Press. New Ysrk, 1965, p.74.
Volume 60 Number 7 July 1983
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