July 20, 1961
COMMUNICATIONS TO THE EDITOR
T h e reductions of Ia and Ib in the potential region of the first polarographic wave also were investigated using triangular wave cyclic voltammetry a t a hanging mercury drop electrode. Scan rates ranging from 33.3 mv./sec. to 5OO v./sec. (300 c.P.s.) were used. The reduction of Ia was reversible, giving a single cathodic and a single anodic peak under all conditions. Results obtained for the reduction of I b depended on the rate of voltage scan. At the lower rates of scan (below 1 c.P.s.) five species were detected: Ib and I b radical, I a and I a radical, and cyanide ion. The presence of I b radical was detected easily a t the lowest rates of voltage scan used, indicating that the dimerization reaction is relatively slow. At higher rates of voltage scan (>lo0 c.P.s.) only I b and I b radical were detected. Polarographically, IC axid Id showed single, PH dependent waves. Attempts to produce one electron reductions a t high pH values were not successful. At flH 12 the free base completely destroyed the material within an hour. Tetra-n-butylammonium Perchlorate Solutions in Acetonitrile.-Tile reduction of each of these salts also was investigated polarographically in acetonitrile and in each case the first wave corresponded to a reversible one electron reduction. The half wave potentials for la, Ib, IC, and Id, were -0.57, -0.79, -0.93, and -1.06 v.] respectively. These values indicate that pyridinimn ions are more easily reduced as the electron withdrawing power of the 4-substituent increases. In this aprotic solvent system further reduction was not observed for Ih, IC,or Id. In the case of l a , a second wave appeared a t - 1.06 v., since here the two electron reduction does r,ot involve hydrogen. Controlled potential electrolyses were carried out to generate macro amounts of the free radical species. The electrolysis potentials were in the diffusion current region of each salt. The coulometric current efftcieiicies indicated t h a t some minor side reactions accompanied the reduction of ICand Id. The reductim of l a proceeded very cleanly to give the expected blue free radical. As in the aqueous reductions, I b radical dimerized to give the radical derived from Ia. This material was identified by its polarographic behavior and visible spectrum. The electrode reactions in acetonitrile also were investigated using cyclic triangular wave voltammetry with a hanging mercury drop electrode. In each case the waves were reversible and only a single cathodic peak and a single anodic peak were observed over the entire range of voltage scan rates. For Ib! this indicates that the dimerization of the initially formed radical is much slower in acetoiiitrilc tliari i n water. The formation of stable free radicals on reduction of I a , IC,and Id was proved conclusively by e.p.r. spectroscopy, although quantitative comparisons of the resonance spectra were not possible.
3165
RHODIUM HEXAFLUORIDE'
Sir : Recently ruthenium hexafluoride was prepared by burning ruthenium metal in a fluorine atmosphere in a liquid nitrogen cooled quartz reactor.2 The same method now has been applied successfully to the preparation of rhodium hexafluoride] the next member of this transition series. Under similar conditions it was not possible to make the last member of the series, palladium hexafluoride. Ruff and Ascher3 were the first to report studies of the reaction between rhodium and fluorine. At 500-600' their main product was the trifluoride. They also reported the formation of a small amount of a red-brown sublimate which they presumed to be either RhF4 or RhFb. The highest rhodium fluoride t h a t Sharpe4 reported in his studies using bromine trifluoride as a fluorinating agent was the tetrafluoride. Prior to the preparation of RhFe no hexavalent rhodium compound had been prepared. Soiid rhodium hexafluoride] in bulk, is black in color; the gas is a deep red-brown, resembling ruthenium hexafluoride. Chemical analyses on different samples indicate a formula RhFs (calcd. for RhF6: F, 52.6; Rh, 47.4. Found: F, 51.8 rt 2.4; Rh, 47.S i 0.4.) Vapor density measurements a t 50 mm. pressure give a molecular 4 compared with a formula weight weight of 219 of 216.9 for RhF6. X-Ray diffraction patterns show the solid to have a pseudo-orthorhombic structure a t -23', and a cubic structure a t room t e m p e r a t ~ r e . ~Preliminary vapor pressure measurements with the solid gave the results :
*
t, "C.
p , mm.
0 15
5 19
11 29
16 30
20 49.5
Rhodium hexafluoride] unlike the other transition metal hexafluorides, appears to decompose into fluorine and a lower fluoride a t room temperature. Preliminary investigations of the rate of this deA composition show i t to be negligible a t 0'. sample of vapor kept a t 50' was found to be 15% decomposed after three hours. The amount of fluorine produced in this experiment was equivalent to one half a mole per mole of hexafluoride decomposed. The iiiain features of the infrared spectrum are very similar to those of molybdenum hexafluoride6; this suggests that rhodium hexafluoride is similar in structure to the other hexafluorides and has the syinnietry of a regular octahedron. Two very intense bands, presumably vs and u4, have been observed, one a t 7 2 2 and the other a t 283.5 cm.-'. Evidence for a dynamic Jahn-Teller coupling such as that found in ruthenium hexafluoride2 has not been l ' o u d . These observations parallel those made for the corresponding iso-electronic compounds
(1) Based on r n r k performed under the auspices of the U.S Atomic Energy Commission. (2) H. H. Claassen, H. Selig, J. G. I l a l m , C. L. Chernick and B. Weinstock. J . A m . C h o n . SOC.. 83, 2390 ( 1 9 6 1 ) . (3) 0 . Ruff and E . Ascher. Z . anorg. Chent., 183,206 (1920). DEPARTXlENT OF CHEMISTRY TI', ? v l . SCFIWARZ A. G. Sliarpe. J . Chcm. Soc., 3444 (1950). I-VIVERSITY OF ISCON CONS IN ED\VARII M , KOSOWER (4) ( 5 ) D Northrop and S. Siegel, unpublished results. XADISON 6 , %-ISCONSIX IRVING S ~ I A I N ( G ) T . G Burke, D. F. Smith and A. H. Nielsen, J . Chcin. P h y s . , 20, 447 (1952). RECEIVED X A Y 13, 1961
3166
CObfMUNICATIONS TO THE
EDITOR
Vol. 83
osmium hexafluoride and iridium hexafluoride.’ Table I summarizes some of the properties of the 4d series hexafluorides. The volatility decreases rapidly iii going across the series, although each compound is less volatile than its counterpart in the 5d transition series. The solid-solid transition temperatures and the triple points increase regularly; the values given in the table for RhF6 are approximate extrapolations and have not been observed directly. An attempt t o measure the triple point by warming the solid in a glass capillary I ion was unsuccessful because of rapid decornpost‘ of the sample. The infrared active fundamentals v 3 and v4 observed in this series are of the same magnitude as those in the Sd series.8 Although the metal to fluorine distances are not known for the 4d series, the fact that the infrared frequencies do not vary greatly from molybdenum to rhodium indicates that the metal to fluorine bond lengths also do not vary, as has been found for the 5d series.*
(69---i5yo)of the more stable f r w s alcohol.2 These phenomena have been discussed by Dauben and his co-workers*c in terms of “steric approach control” and “product development control.” Diborane is an interesting reducing agent with reducing capabilities quite different froiii those of the basic complex hydritles.3 However, in the ])resent instance it yields predominantly t m z s - 2 niethylcyclopentariol (69%) and fmns-2-methylcpclohexanol (65 The introduction oi bulky substituents, as in disiarnylborane, niarkedly modifies tlie hydruborati~ig~ and reducing5 action of the reagent. This reagent led to the predominant formation of the cis alcohols: 787; cis-2-methylcyclopeiitanol arid 7770 cis-2-iiiethylcyclohexanol. I n seeking to enhance the steric requirements of the reagents, rve had recourse to diisopinocamphenylborane, previously utilized for the asymmetric hydroboration of olefins.6 \\-ith this reagent we realized thc formation of 94yo cis-2-methylcyclopentanol and 927; oi cis-2-rncthylcyclohexanol Thus, it has been possible to achieve a shift from TABLE I predominant !yamto nearly complete cis formation l\ZoFa TCFG 12uFs RhFa by the use of boranes with high steric requirements. \apor pressure a t 15’ 361“ 13jd 58f 38 Diisopinocamphenylborane had led previously to Solid-solid transition the asymmetric synthesis of alcohols zliu the hytemp. -9.m -5.36 f2.5’ (f7) droboration of olefins in optical purities of SO-Triple point 17.43 37“ 641 (70) It was of interest to esaniine the optical Infrared active fundapurities of the products resulting from the reducmental v Z c 741 745 735 id2 tion of ketones with this reagent. fundamental v f 264 265 275 283.5 The mixture of 2-methylcyclohexanols exhibited a 0 . Ruff and E. Ascher, Z.a m r g . Chew., 196, 419 (1931). optical activity, but because of the dXficulty in A . P. Brady, 0. E. Myers stid J. D. Claws, J . Phys. Chem., separating the two isoiiiers we were unable to 64, 588 (1960). Vnpublished results from Chis laboratory. cletermine tlie optical purities of the individual ii 11. Se!ig, C. I,. Cheriiick a n d J. G. Malm, .T. I n o i g . A’ucl. Cizenz., 111 press. e J. G. h I d m and 13. Selig, t o be published. compounds. Accordingly, we treated a series 01 Ref. 2. methyl ketones, RCOCH3, 16th this reagent with these. results ([oi]*’D, optical purity in yo, configuration) : R = ethyl, - 1.5’, 11%, K ; isopr:>pyl, (7) H. H. Claassen a n d B.Weinstock, J . Cheriz. P h y s . , 33,436 (1960). (8) H . H. Claassen, i b i d . , 30, QC8 (1959). -O.QlO, 17%, R ; f-but!;l-, +2.3’, 30(?0, S“; CEDRIC L.CHERNICK .ARGONSE SAT10Na4L LABORATORY phenyl, +6.0”, 14%, R. ARGONSE, ILLINOIS HOWARD H. CLAASSEX Ailthough the optical purities are far less than FORD MOTORCOXPANT BERXARD WEIXSTOCK those realized in the earlier hydroboration of oleSCIESTIFIC RESEARCH L.4BORATORY fins,6 they are comparable with those obtained by DEARBORS, ~IICHIGAS existing methods. detailed study of the utility RECEIVED J U N E 26, 1961 of diisopiiiocaiiipheq-lboraiiefor the prediction of absolute configuration is i n process.* In a typical reduction a sdution of 11.2 g., 100 STEREOCHEMICAL CONSEQUENCES OF KETONE REDUCTIONS BY DIBORANE AND SUBSTITUTED inmoles, of 2-methylcyclohexanone in 2 5 ml. of BORANES diglyme was added during 15 minutes to a stirred Sir: suspension of 100 mnioles of diisopinocamphenylwish to report the achievement oE steric boraneGin 200 ml. of diglyme. The reaction mixcontrol of the reduction of cyclanones by the use ture was stirred at 0’ for three hours and then left o f dialkylhoranes containing bulky substituents, oyernight a t room temperature. Fullovring oxisuch as disiamylborane and diisopinocamphenyl(2) (a) J. B. Urnland a n d hl. I . Jefraim, J . A i n Chenz. .$a
