Organometallics 1986, 5, 595-596 Supplementary Material Available: Tables of bond lengths, bond angles, atomic coordinates, thermal parameters, and structure factors for 5 (17 pages). Ordering information is given on any current masthead page.
595 TO UTILITY L I H I
Organometallic Chemistry of the Transition Elements. 7. A 2+ State of Bis(arene)chromium Complexes' Richard J. Markle and J. J. Lagowski" Department of Chemistry The University of Texas at Austin Austin, Texas 78712 Received October 29, 1985
Summary: A new, stable oxidation state of a bis($-arene)chromium complex is reported in THF. Two oneelectron oxidation-reduction processes are observed for bis(hexamethylbenzene)chromium(O)at - 1.630 V (reversible 1+/0 redox couple) and at -0.160 V (quasi-reversible 2+/1+ redox couple), both being measured in the presence of TBAPF, against the Ag/AgN03 couple.
ELECTROCHEMICAL CELL ASSEMBLY
Figure 1. Electrochemical cell and related assembly used for cyclic voltammetric studies.
reservoir by using standard Schlenk techniques. The THF was stored over a 1:l mole ratio of a sodium-potassium alloy and degassed by standard freeze-pump-thaw techniques. The T H F in the reservoir was kept under static The reversible or quasi-reversible oxidation-reduction vaccum at -30 "C while the cell was dried and deoxygenprocesses of bis(arene)chromium(O) complexes and their ated. corresponding bis(arene)chromium(l+) ions are wellThe supporting electrolyte, tetra-n-butylammonium Some bis(arene)chromium redox couples are hexafluorophosphate (TBAPF,), was prepared by mixing stable and sufficiently reproducible that they have been equal molar aqueous solutions of tetra-n-butylammonium recommended as references for nonaqueous solvent syshydroxide and ammonium hexafluorophosphate. The tems." Up to now attempts to oxidize bis(arene)chromium product was collected, recrystallized twice from ethanol, monocations to higher oxidation states, however, have led recrystallized once from ethyl acetate, dried under vacuum, to irreversible oxidations and/or ligand disproportionaground and dried at 140 "C,cooled, and stored in a deti~n.~,~ siccator (mp 235-237 "C, standard reference sample mp Our work in the effects of methyl substitution on mag236-238 "C). Sufficient supporting electrolyte was used netic and electrochemical properties of bis(arene)chroin all chambers of the electrochemical cell to produce a 1.5 mium(0) complexes6 has also resulted in the discovery of M solution when the appropriate amount of solvent was a new, stable, quasi-reversible oxidation state for such a introduced. complex. We report here the first example of a quasi-reThe working electrode was a platinum disk (1.950-mm versible 2+/1+ oxidation process for bis(hexamethy1diameter) mounted in Pyrex glass. The platinum surface benzene)chromium(O) which exhibits two redox potentials was polished to the point where no major imperfections in THF. could be observed at 50X magnification. Synthesis of Bis(hexamethylbenzene)chromium(O). The reference electrode was a silver wire immersed in Bis(hexamethylbenzene)chromium(O)was prepared by a solution containing TBAPF6 and saturated with AgN03, using the rotating reactor system described previ~usly.~~' the mixture being contained within a small fritted tube. Chromium metal was evaporated into neat hexamethylA second, slightly larger fritted tube surrounded the refbenzene. After workup , bis (hexamethylbenzene) chromierence electrode and contained a solution of TBAPF6 in um(0) [mp 339-340 "C dec ( l k 8 mp 400 "C dec)] was THF. The Ag/AgN03 reference electrode was thus sepcollected by sublimation [110 "C at torr]. Total yield arated from the solution containing the arene complex by was 0.294 g or 3.66%, based on metal evaporated (HRMS two concentric fritted tubes. This reference electrode was calcd 376.222 20, found 376.223 13). extremely stable with reproducible potentials. Similar Electrochemical Cell and Analyses. The electrosystems have been described by other^.^,^ chemistry of bis(hexamethy1benzene)chromium was esIn a typical experiment, the electrochemical cell and tablished by cyclic voltammetry using the apparatus ilsupporting electrolyte were assembled and then dried and lustrated in Figure l as described briefly here and in deoxygenated by heating under vacuum. After cooling, greater detail elsewherea6The solvent T H F was purified standard anaerobic and anhydrous handling techniques and dried by refluxing and distilling from a mixture of were used to incorporate the dry AgNO,, the sample inpotassium and benzophenone and transferred to a primary troduction device (and the sample), and the T H F reservoir system. The assembled apparatus was then reevacuated. (1) Paper 6 in this series: Pettijohn, T.; Shepherd, M.; Chinn, J.; The solvent (THF) was flash distilled sequentially from Lagowski, J. J., submitted for publication. the primary reservoir to the secondary reservoir, the sec(2) Valcher, S.; Casalbore, G.; Mastragostino, M. J. Electroanal. Chem. ondary reservoir was isolated, and then the T H F was Interfacial Electrochem. 1974, 51, 226. (3) Ito, N.; Saji, T.; Suga, K.; Aoyagui, S. J.Organomet. Chem. 1982, distilled from the secondary reservoir into the electro229, 43. chemical cell. Supporting electrolyte was dissolved com(4) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461. pletely and the solution allowed to equilibrate at room (5) Bailey, S.; Leung, W.; Ritchie, F. Electrochim. Acta 1985,30,861. (6) Markle, R. Master's Thesis, The University of Texas at Austin, temperature. 1985. All electrochemical measurements were made by using (7) Markle, R.; Pettijohn, T.;Lagowski, J. J. Organometallics 1985, 4, an EG&G PAR Model 175 Universal Programmer, an 1529. (8) Fischer, E.; Hafner, W. Brit. Pat. 829 574, 1960. EG&G PAR Model 173 Potentiostat/Galvanostat, and a 0276-7333/86/2305-0595$01,50/0
0 1986 American Chemical Society
596
Organometallics 1986, 5 , 596-598 Table I. Electrochemical Values for Bis(hexamethy1benzene)chromium 1 Redox Processes"
+
and 1 /2+ Room-Temperature
peak height WA
peak position V
ox. step 1+/0 2+/ 1+
+ /O
E , cathodic
E , anodic
i, cathodic
i, anodic
-1.661 -0.195
-1.600 -0.124
0.40 0.16
0.35 0.36
scan rate, m V / s 50 200
Up, mV 61 71
ipcli,,
1.03 0.44
El/%v -1.630 -0.160
"Concentration = 7.97 mM. 0.OM)v
-0.875
I -
-1.7x) 1+/0
r
-2.625
-3.m
I n
/
I I
,' 2+/1+
/'
t
I
\
I+/O
'\
,/4
+OICXJV -0038 -0175
-0312
-3450V
c--e--t---t--r -145OV-1550
i8y)
-1750 - 1 8 5 0 V
Figure 2. A typical current-voltage curve of bis(hexamethy1benzene)chromium recorded at (a) 50 mV/s over the full range exhibiting both 1+/0 and 2+/1+ redox couples, (b) 50 mV/s over the range exhibiting the 1+/0 redox couple, and (c) 200 mV/s over the range exhibiting the 2 + / 1 + redox couple.
Model RE 0074 X-4 Recorder. After the sample (0.03 g, 7.97 mM) was introduced into the electrochemical cell a cyclic voltammogram, CV, containing both redox potentials was recorded. Each redox couple was then individually investigated. Both the 1+/0 and 2+/1+ redox couples were recorded separately a t 50, 100, 200, 500, and 1,000 mV/s to test for peak position/scan rate dependency. The anodic peak position for each individual CV was essentially unaffected by and independent of the scan rate. Peak currents were dependent on the scan rate. Scan rates of each individual CV used in the determination of electrochemical parameters were chosen to maximize the peak currents and cathodic and anodic peak height parity. Figure 2 represents (a) a cyclic voltammogram (CV) recorded at a scan rate of. 50 mV/s and exhibiting both redox processes, (b) a detailed CV of the 1+/0 redox couple recorded at 50 mV/s, and (c) a detailed CV of the 2+/1+ redox couple recorded at 200 mV/s. Table I summarizes the electrochemical parameters obtained. Electron Spin Resonance. A saturated solution of bis(hexamethy1benzene)chromium in methanol was recorded on a Varian E-9 EPR spectrometer with a Varian E-101 Microwave bridge operated a t room temperature. The ESR spectrum of bis(hexamethy1benzene)chromium(1+)was consistent with that reported by B r ~ b a k e r .The ~ g value was 1.9893 (lit.9 g = 1.9867). The observation confirms that our bis(hexamethy1benzene)chromium sample forms the expected stable 1+ cation in solution. The 1+/0 and 2+/1+ redox potentials have peak separations, AE, of 61 and 71 mV, respectively. These values are well within those expected for a one-electron Nernstian process at room temperature. For example, Saji3 and Treichel" report the 1+/0 redox couple for bis(benz(9) Brubaker, C.; Li, T.; Kung, W.; Ward, D.; McCulloch, B. Organometallics 1982, 1, 1229. (10) Treichel, P.; Essenmacher, G.; Efner, H.; Klabunde, K. Inorg. Chim. Acta 1981, 48, 41.
ene)chromium in acetonitrile at 58 and 114 mV, respectively. Peak separation, the effect of the scan rate, and the ratio of cathodic peak current, ipc, to anodic peak current, ipa, form the basis of the argument for the reversibility for the 1+/0 couple of bis(hexamethy1benzene)chromium and the quasi-reversibility of the corresponding 2+/ 1+ couple. The peak current ratio for the 2+/1+ redox couple is less than unity. It is not unexpected that at moderate scan rates of 200 mV/s at room temperature that some of the 2+ cation might disproportionate before the return scan occurs because this chromium species is highly unstable under normal conditions. We can speculate that the presence of the 12 electronreleasing methyl groups on the two complexed rings provides sufficient electron density to the chromium cation to permit the formal, further oxidation of the 1+ species in a potential range that can be attained in this solvent system. Unpublished work from these laboratories on the electrochemistry of the monocations and magnetic properties of progressively methylated derivatives of bis(benzene)chromium(O)seems to support this general conclusion. Work in the extension of the range of oxidation states for bis(arene)chromium complexes continues.
Acknowledgment. We thank Mr. Ted Pettijohn for his assistance with the ESR experiments. The Robert A. Welch Foundation and the National Science Foundation provided generous support. Registry No. Bis(hexamethy1benzene)chromium (0), 1215666-0; bis(hexamethy1benzene)chromium (l+), 12243-39-9.
Influence of Dienes on the Cobalt Carbonyl Catalyzed Reactlon of Mercaptans with Carbon Monoxide Shlomo Antebi and Howard Atper"' Ottawa-Carleton Chemistry Institute Department of Chemistry, University of Ottawa Ottawa, Ontario, Canada K I N 984 Received October 29, 1985
Summary: Thio esters are obtained in good to excellent yields by the cobalt carbonyl catalyzed carbonylation of mercaptans in the presence of 2,3dimethyl- 1,&butadiene or 2,3-dimethoxy-1,&butadiene. Diene-cobalt carbonyl complexes [i.e., (diene-Co(CO),),] are probably the key catalytic species in these reactions.
Many investigations have been carried out on metalcatalyzed reactions of substrates bearing nitrogen and oxygen atoms. Examples include the cobalt carbonyl catalyzed reductive carbonylation of Schiff bases with organoboranes,l the conversion of amines to formamides John Simon Guggenheim Fellow, 1985-1986. H.; Amaratunga, S. J . Org. Chem. 1982, 47, 3593.
( 1 ) Alper,
0276-7333/86/2305-0596$01.50/0 0 1986 American Chemical Society
