Inorg. Chem. 1991, 30, 4216-4218
4216
Notes Contribution from the Department of Chemistry, University of Mississippi, University, Mississippi 38677
Spectroscopic Characterization of the Octachlorodirhenate(>) Ion, [Re2C18]* Sandra K. D. Strubinger, Charles L. Hussey, and W. E. Cleland, Jr.*
Received February 21, I991
Introduction The existence of a quadruple bond between rhenium atoms in the octachlorodirhenate(II1) ion, [Re2C18j2-,was first recognized over 25 years ago. Since that time, the chemistry of dinuclear rhenium complexes possessing multiple Re-Re bonds has played a prominent role in the overall development of the field of metal-metal multiple bonds.'-" Indeed, [Re2ClE]2-is a versatile precursor for the preparation of a variety of complexes with Re2+ cores ( n = 8,6, 5, 4).lu2 While a number of dinuclear complexes possessing the Re?+ core with the $ 1 r ~ 6 ~ 6 *electronic configuration are known, the parent chloride complex, [Re2C18]*, has remained elusive. Chloroaluminate ionic liquids have been found to be excellent solvents for the study of the spectroscopic and electrochemical properties of transition-metal chloride complexes.6 Combining aluminum chloride with a quaternary ammonium chloride salt such as 1methyl-3-ethylimidazolium chloride (MeEtimCI) produces a molten salt or ionic liquid that is molten at room temperature. The Lewis acid-base properties of the melts can be altered by adjusting the ratio of aluminum chloride to quaternary ammonium chloride. Basic melts contain excess chloride ions, while acidic melts contain excess AI2CIf ions. Molten salts have several advantages for the study of transition-metal chloride complexes over conventional solvents such as water or acetonitrile, including the ability to obtain spectra of complexes free of hydrolysis or solvolysis product impurities. These advantages have been discussed in detail elsewhere.6 In a recent pub1icatio1-1,~ we reported on the electrochemistry and spectroelectrochemistry of rhenium(II1) chloride complexes utilizing a chloroaluminate molten salt as solvent, including the first successful production of stable solutions of [Re2CI8]* by bulk controlled-potential electrolytic reduction of [Re,C1812-. In this paper, we report the optical absorption and ESR spectra of [Re,C18l3-.
Experimental Section Solutions of [ Re2C1813-were prepared by bulk electrolytic reduction of [Re2CIsJz-according to the literature procedure.' Optical absorption spectra were recorded by using either a Perkin-Elmer Model 3840 Lambda Array or a Perkin-Elmer Hitachi Model 200 UV-vis spectrophotometer. Near-infrared spectra were recorded by using a Cary 17 spectrophotometer. X-Band electron spin resonance (ESR) spectra were recorded as frozen glasses at 77 K by using a Varian E4 spectrometer. ESR samples were prepared in 3 - m m quartz ESR tubes which were frozen and sealed under vacuum.
Results and Discussion During the course of our investigation of the chemistry of rhenium chloride complexes in the basic AICI,-MeEtimCI melt,5 we discovered that [Re2C1812-exhibited a reversible one-electron ( I ) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; Wiley: New York, 1982. (2) Cotton, F. A.; Walton, R. A. Strucr. Bonding 1985, 62, 1. (3) Cotton, F. A. Chem. SOC.Rev. 1983, 12, 35. (4) Trogler, W. C.; Gray, H. B. Arc. Chem. Res. 1978, 1 1 , 232. ( 5 ) Strubinger. S.K. D.; Sun, 1. W.; Cleland, W. E., Jr.; Hussey, C. L. Inorg. Chem. 1990, 29, 993. (6) Hussey, C. L. Pure Appl. Chem. 1988, 60, 1763.
0020-1669/91/ 1330-4276$02.50/0
240
610
460
900
Wavelength, nm Figure 1. Absorption spectra of dimeric rhenium chloride complexes in 49.0 mol 7% AICl,-MeEtimCI at room temperature: (-) 9.07 X 10-4M solution of ( B U , N ) ~ [ R ~ ~ C (--) I ~ ]9.53 ; X ol-' M solution of (BU,N!~[Re2Cls]after exhaustive reduction at -0.71 V; (-.-) after exhaustive M solution of [ B U , N ] ~ [ R ~under ~C~~ the] same reduction of a 5.73 X conditions. The cell path lengths were 1.00 mm. Table I. Optical Absorption Data and Possible Assignments for IM,Cln13-: M = Re, Tc
1468 (17.0)c 708 580 (2.8) 512 (2.3)
6.8 14.1 17.2 19.5
5.9 13.6 15.7 20.04
6 7r
8. 6* b* b
305 (sh, 19.0)
32.7
3 1.4d
37.3
lr
37.2
43.5
b*
6*
**
d.+i u*
u*
dd-9 LMCT
6
if
268 (75.0)
---. --- **
d9-9
LMCT LMCT
OFrom ref 7. b X in nm; Y in cm-' X 10". CPeaksin the vibrational progression at 1274, 1322, 1370, 1413, 1468 (center), 1523, 1585, 1680, 1755 nm (13 nm). dAny of the given assignments are possible for this transition. reduction and that stable solutions of [Re2CI8lf could be prepared by electrolytic reduction of [Re2C18]Z-. The [Re2CIEl3-ion is isoelectronic with its well-known technetium analogue, [Tc cl8ls-. Fortunately, the spectrum of [ T C ~ C I ~has ] ~ been reported$ along with possible assignments based on theoretical computations. The UV-vis absorption spectra of [RezC18]2- and [Re2CI8l3-are presented in Figure 1, while the near-IR spectrum of [Re2ClE13is shown in Figure 2. Data from these spectra are collected in Table I along with data and possible assignments for the isoelectronic [Tc2CI8l3-ion. Upon reduction, a subtle color change from the blue-green of [Re2C1612-to the navy blue of [Re2C18]3was observed. Figure 1 shows that the prominent band at 688 nm in the spectrum of [Re2C16]2-is completely absent from the spectrum of the reduced species and is replaced by several new features. Comparison of the spectrum of [Re2C1813-to that reported for the isoelectronic [TczCls13-ion shows that the spectra are qualitatively quite similar. Assuming that the orders of the energy levels of [Re2C1813-and [TC#&]* are approximately the (7) Cotton, F. A.; Fanwick, P. E.; Gage, L.D.; Kalbacher, B.; Martin, D. S . J. Am. Chem. SOC.1977, 99, 5642.
0 1991 American Chemical Society
Inorganic Chemistry, Vol. 30, No. 22, 1991 4211
Notes
T a b 11. Spectroscopic Properties of Rhenium Chloride Complexes with the Re?+ Core complex
626; + 66*”
-x
-
2.6’
g g
--
2.2 2.2
gi (Allb
1468 ( I 700)’ 1525 (1200j 1450 (1ooO) 1475 (1100) 1475 (1000) 1314 (2300) 1283 (2200) 146d 1375d 1350 1375 (2700) 1530 (1700) 1200 (1700) 1330 (1100) 1330 (1050) 1380 (-4000) 1410
-2.2 -2.2 -2.2 -2.2
(-350) (-350) (-350) (-350)
