511
MAGNETIC CIRCULAR DICHROISM
Magnetic Circular Dichroism of Ferrocene and Substituted Ferrocenes: the d-d Transitions by Dennis Nielson,l Daniel Boone,2 and Henry Eyring*’ Departments of Chemistry, University of Utah, Salt Lake City, Utah 8.4119 and University of Pittsburgh, Pittsburgh, Pennsylvania (Received September 1, 1971) Publication costs assisted by the National Institutes of Health
The MCD spectra of the d-d transitions of ferrocene and some substituted ferrocenes are measured and found to fall into two distinct categories: one group of compounds giving absorption-like peaks and the other giving S shaped curves. It is found that the lowering of symmetry of the molecule is responsible for the difference.
Introduction Due to its unique structure, ferrocene has been of great interest to theoreticians, and there have been various attempts made to describe its electronic structure and explain its spectrum, but all have fallen short of being completely satisfactory. In recent years, magnetic circular dichroism has come to be recognized as a useful tool in the study of electronic structure, and it has been previously used to study the d-d transitions in f e r r ~ c e n e . ~However, the results of a preliminary investigation of the MCD spectrum of ferrocene in this laboratory were in direct conflict with this earlier work, leading to further study of the problem. An excellent discussion of MCD theory can be found in the review article by Buckingham and step hen^,^ or in more recent work.6 For the discussions to follow here, it is sufficient to know that there are three types of peaks possible in MCD spectra: an S shaped curve and two absorption-like curves, both of which may be either positive or negative in sign, and one of which is temperature dependent. The S shaped curve may occur when a given transition has a ground or excited state that is degenerate, and the point at which this type of curve crosses the base line corresponds to the absorption maximum. The first of the gaussian-shaped peaks is due to a mixing of states and is the only contribution to magnetic circular dichroism for a transition in which both the ground and excited states are nondegenerate, but which may occur for any transition. The temperature-dependent peak arises only when the ground state is degenerate, and it is due to the population difference in the states resulting from the degenerate ground state being split by the magnetic field. These three types of peaks have been called A, B, and C, respectively. The equations derived by Buckingham and Stephens4 for magnetic circular dichroism are
where @(a+) is the ellipticity through the a+j transition, and A , B , and C are the various contributions to the spectrum, Any number of the three terms may contribute for a given transition, although many times one term dominates. Ferrocene has a nondegenerate ground state, which eliminates the possibility of a contribution from C(a-+j), but its excited state for the forbidden d-d transition is thought to be degenerate. Thus, both A(a+j) and B(a-.j) may contribute to the MCD spectrum for ferrocene for this transition. One method for extracting parameters from experimental data uses the damped oscillator model.6 If [@],l and [e],, are the maximum values, including sign, of the molar ellipticity of the S shaped curve for the peaks at longer wavelength (Al) and shorter wavelength (1) University of Utah. (2) University of Pittsburgh. (3) H. Falk, Monatsh. Chem., 100, 411 (1969). (4) A. D. Buckingham and P. J. Stephens Annu. Rev. Phys. Chem., 17, 399 (1966). ( 5 ) P. N. Schatz and A. J. McCaffery, Quart. Rev., Chem. SOC.,23,4, 662 (1969); (b) D. J. Caldwell and H. Eyring, “The Theory of Optical Activity,” Wiley-interscience, New York, N. Y., 1971; (0) D. J. Caldwell, J. Thorne, and H. Eyring, Annu. Rev. Phys. Chem., 22 (1971). (6) B. Briat, et al., J. Amer. Chem. SOC.,89, 26, 7062 (1967). The Journal of Physical Chemistry, Vol. 76, No.4,1979
512
D. NIELSON, D. BOONE, AND H. EYRING ~
Table I: Data for the d-d Transitions for Those Compounds Exhibiting Spectra of the Kind Shown in Figure 1 -Absorption-
Band I --MCD--
Compound
SoIvent
Xmsx5
rmax
Amax‘
Ferrocene n-Butyl ferrocene Hydroxymethylferrocene 1 , l ’-Dihydroxymethylferrocene
Cyclohexane Cyclohexane Methanol Methanol
440 435 435 435
94 97 101 143
470 462 470 470
a
Band I1
,--
-Absorption-
B/D X
7.62 4.43 2.79 2.15
108
P--MCD--
Xmax”
emax
Xmsxa
324 324 323 323
54 53 63 68
334 334 333 333
B / D X 102
-8.7 -6.3 -11.2 -10.4
All wavelengths are in units of mp.
(A,) and S and Dif stand for their sum and difference, respectively, it is found that
1 A (Dif)/em 55.4 X
A=---
[B
1.0
(5)
0.8
+ (C/kT)J/D = - 32 !X!8
0.6
where A (band width) and Xo (wavelength of maximum absorption) are expressed in the same units (here nanometers) and r (width of half-maximum of the absorption peak) is expressed in cm-l. The dipole strength may be estimated from the formula D e 9.2 X em/Xo where em is the maximum molar extinction coefficient, and the ratios A / D and [B (C/kT)]/D can be expressed as
-
-
c
I
0.25
E
ai
e
E
0
+
AID -1.97 (Dif)/em [B ( C / k T ) ] / D’V -3.47 (S/em)
+
0.4
E
0.25
(7)
(8) However, in ferrocene itself the d-d transitions display no evidence of being S shaped, so that the equation to be used in such a cask is
t
W
Y
0.4
0.6
,
I
IO-~Q)
where [elm,,, is the maximum value of the molar ellipticity. This is the equation used to calculate the values of B/D in Table I (Figure 1).
Figure 1. Absorption (---) and MCD (-) hydroxymethyl ferrocene.
Experimental Section
Results and Discussion
Samples of ferrocene and the various substituted ferrocenes listed in Tables I and I1 were obtained from Aldrich Chemical Co. and Research Organic/Inorganic Chemical Corp. (Sun Valley, Calif.). Solutions were made using spectral grade cycIohexane and methanol obtained from Matheson Coleman and Bell. Sample solutions ranged in concentration from approximately 8-200 mg of ferrocene or substituted ferrocene in 25 ml of solution. Measurements were made using 1.00-mm, 5.00-mm, and 1.00-cm cells. The spectra were measured on a Gary 14 spectrophotometer (uv and visible absorption) and a Cary 60 spectropolarimeter with CD attachment and super conducting solenoid capable of approximately 50,000 G, both supplied and installed by the Cary Co. All measurements were made using a field strength of 45,000 G, with one exception which is discussed below.
F ~ i l kusing , ~ a Jouan-Roussel dicrograph with a magnet attached, reported the MCD spectra of the 4400-A transition of ferrocene and some substituted ferrocenes to be dominated in all cases by A (a-j) . His work was done using a field strength of only 2830 G, which necessitated measuring the spectrum of the forbidden d-d transition of ferrocene with a highly concentrated solution. In the Cary 60, solutions whose optical density (log 10/-TtrBnsmitted) is greater than 2 give rise to artifacts which may appear to be part of the MCD spectrum. As it happens, in ferrocene one of these artifacts appears in the d-d transition, and, if the proper concentration and cell path length are used, an S shaped curve can be “created.” For example, with a 172 mg/25 ml solution of ferrocene in cyclohexane and using a 1.00-cm cell, the peak appears to be a perfect example of an A(u-tj) dominated spectrum. However, the same SO-
The J O U T of ~ Physical ~ Chemktry, Vol. 76,No. 4 , 1079
spectra of
MAGNETIC CIRCULAR DICHROISM
513
Table 11: Data for the d-d Transitions for Those Compounds Exhibiting Spectra of the Kind Shown in Figure 2
Compound
Solvent
Acetylferrocene 1,l '-Diacetylferrocene Carboxylic acid ferrocene 1,l'-Dicarboxylic acid ferrocene Benzoyl ferrocene 1,l'-Dibenzoyl ferrocene Ferrocene aldehyde Chlorocarbonyl ferrocene 1,1 '-Dichlorocarbonyl ferrocene Methyl ferrocenoate 1,1 '-Dimethyl ferrocendioate
Cyclohexane Cyclohexane MeOH MeOH Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane
All wavelengths are in units of mp.
447 463 443 447 462 471 452 455 462 438 445
-
Band I
7
--Absorption-Xmar" Emax
301 417 278 313 613 747 151 401 533 224 275
,---
MCD-7
Ala
X2(f
Xorosa"
485 505 480 485 502 520 490 495 502 475 480
415 435 410 417 433 443 418 425 433 410 415
440 457 435 440 455 467 447 455 460 434 440
",
See ref 7, p 522.
lution placed in a 5.00-mm cell now has an optical density through the transition less than 2 and yields a perfect gaussian curve. It is possible that Falk observed a similar artifact with his equipment. It might be argued that, for some reason, a t a lower field strength an S shaped curve might be observed. However, the MCD spectrum of a solution of ferrocene with an optical density less than 2 and measured a t a field strength of 3.38 X 103 G, which is close to that used by Falk, continues to yield an absorption-like curve of negative sign. It should be pointed out that the spectra obtained by Falk for compounds other than ferrocene itself are in basic agreement with our own results, but the possibility of their containing artifacts is still very real. I n the Sshaped curves obtained in this laboratory the appearance of the artifacts in the highly concentrated solutions are such that they alter position and peak height, without changing the essential form of the curves. Figures 1 and 2 are typical examples of the two types of MCD spectra observed, corresponding to the data of Tables I and 11,respectively. Of the compounds studied, those whose cyclopentadienyl rings are substituted with a carbonyl containing group give S shaped curves (see Figure 2 and Table 11). I n these same compounds, ie., those containing C=O, a peak a t -3550 8 partially overlaps the S shaped curve of the d-d transition, making the calculations of A I D and BID questionable at best. The other compounds, including ferrocene, give the absorption-like curves of Figure 1, all of which are negative in sign for the 4400 8 transition. I n Table I1 the wavelengths for emax or [e], for band I1 are listed where values can be conveniently assigned. Often the wavelength for [e], was easily discernible while that for emax was not. The data reviewed by Scott and Becker' and some later work by Armstrong, Carroll, and McGlynn* are typical of the various attempts which have been made to describe the electronic structure of ferrocene. I n most of the work discussed by Scott and Becker the bonding in ferrocene is considered to be the result of the
--Band 1 -1 --Absorptionb-Amax" emax
358
148
355
,-MCDAmax"
364 360 353 357 387 400 368 374 375 355 355
I
I
20
16
12
-
c
el, 4
8 z5
0
-8 d
E
E 4
8
12
I
I
3
4
5
10-3 (A) Figure 2 Absorption (---) and MCD (-) 1,l'-dimethyl ferrocenedicate.
spectra of
overlap of the ?r orbitals of the cyclopentadienyl rings with the orbitals of the central metal. I n this same discussion, however, the mixing of metal orbitals is taken into account in Moffitt's and Dunitz and Orgel's works. I n the paper by Armstrong, et al., the u bond structure of the cyclopentadienyl rings is taken into account in describing the bonding of ferrocene. Although all of these treatments suffer from an excess of predicted tran(7) D.R.Scott and R. 8.Becker, J . Chem. Phys., 35, 2, 516 (1961). (8) A . T. Armstrong, D. G. Carroll, and 8.P. McGlynn,
