COMMUNICATIONS TO THE EDITOR
2370
would, however, account for this discrepancy.” Furthermore, the high-field shift of r 0.2 observed for the chemical shift of proton 4 in I seems to indicate that, for the outermost protons in the molecule, solvent and concentration effects may also be very effective in altering the position of the resonance. (11) In 11, the main contribution to the low-field shift of proton 3 (and 3’) is due to the van der Waals interaction between the two protons. This effect decreases with the 6th power of the distance, and a small deviation from the coplanar arrangement of the two aromatic rings may, therefore, cause very large effects.
MELLON INSTITUTE PITTSBURGH, PENNSYLVANIA INSTITUT FC% ORGANISCHE
DER KOLN,
CHEMIE
UNIVERSIT~~T KOLN
Found: C, 63.3; H, 7.4; N, 8.3; Ni, 19.3. The crystalline complex is dark green and is very airsensitive. It decomposes at 125” in vucuo, evolving ethane and ethylene. The nmr spectra were recorded in dimethoxyethane solutions by use of a 60 Mc/sec spectrometer. As tetramethylsilane could not be used, the chemical shifts were measured by using a methyl signal of the solvent as an internal reference and then were converted into a r scale.
S. CASTELLANO
Table I: Chemical Shifts of the Ring Protons of the Ligands and Their Complexes in Dimethoxyethane Solutions. (The H. G ~ T H E R Figures in Brackets Are the Intensity Ratios.)
GERMANY
-Chemical
RECEIVED MARCH16, 1967
The Nuclear Magnetic Resonance Interpretation
Ha
H4
1.62 (1) doublet
2.29 (1) triplet
1.95 (2 ) doublet
of Diethylnickel Complexes of
shifta, H6
HE
2.80 1.46 (1) (1) quartet doublet 2.45 0.93 (1) (1) quartet doublet
Substituted Dipyridyl
Sir: In the previous paper,‘ we reported the preparation and characterization of diethyldipyridylnickel (I).
1.64 (1) singlet
, ,
.
2.80
1.45
(1)
(1)
doublet doublet
CH,
5
Ni
(1)
During the course of our study of the analogous nickel complexes of 4,4’-disubstituted 2,2’-dipyridyl, we found that our previous nmr assignments of the ring protons were incorrect. We wish to report in this communication the synthesis and the nmr interpretation of diethyl(4,4’-dimethyl 2,2’-dipyridyl)nickeI (11), and to correct the nmr assignments of the ring protons of I.
(11)
The complex (11) was prepared by the reaction of nickel acetglacetonate, 4,4’-dimethyldipyridyl, and diethylaluminum monoethoxide in ether. Anal. Calcd for C16H22N2Ni: C, 63.8; H, 7.4; N, 9.3; Nil 19.5. The Journal of Phyeical Chemistry
w7 -
N c
OCH,
, /
2.16 (1) singlet
...
2.66
1.17 (1) doublet doublet
1.97 (1) doublet
...
3.03 1.53 (1) (1) quartet doublet
2.33 (1) doublet
. .
2.84 1.30 (1) (1) quartet doublet
,
(1)
c,H/N‘c2Hs
In Figure 1, the signal a t r 2.16, which is a singlet due presumably to the small J35and J36 and/or to the poor resolution of the spectrum, is assigned to H3, because the adjacent carbon has no proton. It is most reasonable to assign the doublet a t r 1.17 t o Heand the doublet a t r 2.66 to Hg, because it is very improbable that the coordination of the ligand to nickel exerts such an effect to deshield the H5 to shift the signal to the field as low as r 1.17, and because the J M estimated G signal is ca. 6.0 cps. from the splitting of the H (1) T.Saito, Y. Uchida, A. Misono, Y. Yamamoto, K. Morifuji, and S. Ikeda, J . Am. Chem. SOC.,88, 6198 (1966).
COMMUNICATIONS TO THE EDITOR
H6
I
H3
2
2371
H5
3
7
Figure 1. The nmr spectrum of the ring protons of diethyl(4,4'-dimethyl 2,2'-dipyridyl)nickel in a dimethoxyethane solution at 60 mc/sec.
abstraction from fully halogenated compounds, in particular halogenated methanes, as well as C&&Cla, CBHJ, n-C&I, and sec-C3H71. Di-t-butyl peroxide was used as thermal source of methyl radicals. Attempts by us3 to obtain similar quantitative results for ethyl chloride, neopentyl chloride, ethyl bromide, n-propyl bromide, and sec-propyl bromide were largely unsuccessful owing to complicating reactions resulting from hydrogen abstraction. The presence of hydrogen halide among the products also led us to suspect that because of facile reactions of the types CH3
+ HC1+
CHd
+ C1
CH3
+ HBr
CHI
+ Br
and In the same 'way, we synthesized diethyl(4,4'-dimethoxy 2,2'-dipyridyl)nickeI and carried out the nmr measurements of it. The data in Table I support the assignments described above of 11. From the comparison of the spectrum of I with that of 11, the assignments of the ring protons of I follows naturally. The chemical shifts of Ht, and Hg of I are probably near those of I1 because of the similarity of the structures of I and 11, and hence the doublet a t T 0.93 is assigned to Ht, and the quartet a t 7 2.45 to Hs. Consequently, the doublet a t T 1.95 is assigned to H3 and H4. As was pointed out by Castellano and Gunther12the high-field shift of He signal observed in is lacking in the specthe spectra of [Fe(di~y)3]C12~ trum of I due to the absence of the shielding effect of the adjacent ligand. We conclude, therefore, contrary to our previous presumption,' that the shielding effect of the nickel atom upon Ht, is not predominant. The high-field shift of H3of I might be due to a deviation from a complete cis coplanar structure of the two aromatic rings. This is also suggested by Castellano and Gunther.2
halogen atoms were largely replacing methyl radicals as the attacking species. We believe it important also to report that in experiments with di-t-butyl peroxide and ethyl iodide, explosions occurred when mixtures were frozen under vacuum to liquid nitrogen temperature. The reactions were carried out in a conventional static system a t 120-200", and in most cases there was an excess of alkyl halide over di-t-butyl peroxide. With alkyl chlorides, methyl chloride was only formed in trace amounts, although with the corresponding bromides methyl bromide was formed somewhat more readily. I n both instances, however, the nature of the products indicated that hydrogen abstraction was the dominant process, as noted by Tomkinson and Pritchard12and also by Alcock and Whittle4 for the reaction of trifluoromethyl radicals with methyl chloride. Typical product analyses for the reaction of ethyl chloride with di-t-butyl peroxide are shown in Table I.5 It may be observed that methane and ethylene are (2) S. M. Castellano and H. Gtlnther, J. Phys. Chem., 71, 2368 the major products. Tomkinson and Pritchard2 noted (1967). that ethylene is a by-product (ca. 0.5-la/, yield) of the (3) S. Castellano, H. Gtlnther, and S. Ebersole, ibid., 69, 4166 thermal decomposition of di-t-butyl peroxide, and (1965). DEPARTMENT OF INDUSTRIAL CHEMISTRY T. SAITO that when the peroxide is decomposed in the presence OF TOKYO M. ARAKI of large amounts of carbon tetrachloride, the amount of UNIVERSITY Y. UCHIDA ethylene formed is comparable to the ethane. ,We also HONGO, TOKYO, JAPAN A. MISONO observed a similar increase in the ethylene: ethane ratio as the initial concentration ratio of ethyl chloride to RECEIVED APRIL3, 1967
Abstraction of Halogen Atoms by Methyl Radicals
Sir: Recent reports on the abstraction of halogen atoms by methyl radicals in the gas phase's2 refer to
(1) D. M. Tomkinson, J. P.Galvin, and H. 0. Pritchard, J. Phys. Chem.,68, 541 (1964). (2) D. M. Tomkinson and H. 0. Pritchard, ibid., 70, 1579 (1966). (3) A preliminary report was given in Australian J . Chem., 18, 121 (1965). (4) W. G. Alcock and E. Whittle, Trans. Faraday SOC.,61, 244 (1965). (5) A. M. H o g g and P. Kebarle, J. Am. Chem. Soc., 86,4558 (1964)
Volume 71, Number 7 June 1067
