J . Phys. Chem. 1992, 96, 8791-8801 (4) Sundaramoorthi, R.; Kansal, K. V.; Das, B. C.; Potier, P. J. Chem. Soc., Chem. Commun. 1986, 1645, 371. (5) Schwaller, M. A.; Dodin, G.; Aubard, J. Biopolymers 1991.31, 519. (6) Aubard, J.; Lejoyeux, P.; Schwaller, M. A.; Dodin, G. J . Phys. Chem. 1990, 94, 1706. (7) Saycc, I. G.; Talenta 1972,19,831. Abello, L.; Jouini, M.; Oulaalou, M.;Poisson, R.; Lapluye, G. J. Chim. Phys. 1985, 82, 1001. ( 8 ) Adenier, A.; Aubard, J. J. Chim. Phys. 1987, 84, 921. (9) Rabinowitch, E.; Epstein, L. F. J. Am. Chem. SOC.1941, 63, 69. (10) McRae, E. G.; Kasha, M. In Physical process in radiation biology;
Augenstein, L.; Rosenberg, B.; Mason, S.F., Eds.; Academic Press: New York, 1963; pp 23-42. (1 1) Schwarz, G.; Klose, S.;Balthasar, W. Eur. J. Biochem. 1970, Z2, 454. (12) Mukeriee. P.: Ghosh. A. K. J. Am. Chem. Soc. 1970. 4. 6419. (13) Dodin,-G.'; Aubard, J.; Falque, D. J. Phys. Chem. 1987, 91, 1166. (14) Dcdin, G.; Dupont, J. J. Phys. Chem. 1987, 91, 6322. (15) Scatchard, G. Ann. N. Y.Acad. Sci. 1949, 51, 660. (16) Aubard, J.; Schwaller, M. A.; Pantigny, J.; Dodin, G.; Adenier, A. In Surfactants in solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York, in press.
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(17) Dewey, T. G.; Raymond, D.; Turner, D. H. J . Am. Chem. Soc. 1979, 12, 5822. (18) Schwarz, G.; Balthasar, W. Eur. J. Biochem. 1970, 12, 461. (19) Bernasconi, C. F. Reluxarion Kinetics; Academic Press: New York, 1976; p 15. (20) Adenier, A. Doctoral thesis, Paris, 1991. (21) Lang, J.; Tondre, C.; Zana, R.; Bauer, R.; Hoffmann, H.; Ulbricht, W. J. Phys. Chem. 1975, 79, 276. (22) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.;
Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905. (23) Kasha, M.;Rawls, H. R.; Ashraf El Bayoumi, M. Pure. Appl. Chem. 1965, 1 1 , 371. (24) Franck, H. S.;Evans, M. W. J. Chem. Phys. 1945, 13, 507. (25) Constantino, L.; Ortena, 0.;Sartorio, R.; Sylvestri, L.; Vitagliano, V. Adv. Mol. Relax. Interact. Proc. 1981, 20, 191. (26) Septinus, M.; Seiffert, W.; Zimmermann, H. W. Histochemistry 1983, 79, 443. (27) Murakami, K.; Mizuguchi, K.; Kubota, Y.; Fujisaki, Y. Bull. Chem. SOC.Jpn. 1986, 59, 3393.
Electronic Spectroscopy of Jet-Cooled Half-Sandwich Organometallic Complexes MgCSHS,MgC5H4CH3,and MgC4H4N Eric S. J. Robles,+ Andrew M. Ellis,*and Terry A. Miller* Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 4321 0 (Received: May 1 I , 1992; In Final Form: July 22, 1992)
Laser excitation and dispersed fluorescence spectra of jet-cooled half-sandwich organomagnesium radicals, MgCSH!, MgC5H4CH,,and MgC4H4N,are reported. These radicals were prepared in situ by employing a laser vaporization/photolyis technique recently developed in our laboratory. Extensive vibrational structure arising from skeletal and intraring vibrations are observed in their spectra. We have also recorded the electronic spectra of the fully-deuterated isotopomers, MgC5D5 and MgC4D4N,to assist in the vibrational assignments. Following preliminary results from ab initio calculations on MgCSH5, the electronic transition being observed here can be approximately described as being a metal ligand electron transfer. The symmetries of the ground and excited electronic states of these molecules are assigned on the basis of the theoretical data. The excitation spectrum of MgCp is found to be quite complicated, presumably because of a strong Jahn-Teller effect in the A state. From the vibrational structure of these molecules, the most likely location of the Mg atom is found to be above the ring in an d-bonding fashion. We note that this account represents the first spectroscopic observation of an organomagnesium radical.
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I. Introduction Organomagnesiumcompounds have received considerable attention compared to other organometallic compounds mainly because of their enormous importance in organic and organometallic synthesis (for a brief summary of the synthesis, reactions, and structures of organomagnesiumcompounds, see ref 1). The most notable in this group are the so-called Grignard reagents, RMgX (where R is an alkyl group and X is a halide), which are arguably the most useful and versatile reagents known to both synthetic organic and inorganic chemists.' Among the myriad of organomagnesium compounds, bis(cyclopentadieny1)magnesium or magnesocene, Mg(C5HS),(or MgCp, for short), has been one of the most well-characterizedsince it was prepared for the first time in 1954 by Wilkinson and Cotton.2 MgCp, is established to be a monomeric, regular sandwich compound isostructural to ferrocene, FeCp,, as revealed by its Raman and infrared (IR) spectra in solution, crystalline, and molten ~ t a t e s . ~Both . ~ Cp rings are 7%0nded (the n in the 7" notation refers to the number of ~
Address correspondence to this author.
'Rohm and Haas and Phillips Petroleum Fellow. Present address:
Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, MA 02138. *NATO/SERC and Ohio State Postdoctoral Fellow. Present address: Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, England.
ligand atoms nominally attached to the metal during the bond formation) to the magnesium atom and have a staggered conformation in the solid stateSand an eclipsed conformation in the gas p h a ~ e . ~The . ~ sandwich structure of MgCp, has also been verified by ab initio c a l ~ u l a t i o n s . ~ ~ ~ MgCp, has also found application as a precursor for photolytic deposition of Mg in the electronics/semiconductor and as a catalyst for polymerization reactions.t2 It is quite likely that during such processes, organomagnesium radicals are formed as intermediatest and therefore spectroscopic identification and characterization of such transient species is potentially of great interest. Despite the seemingly important role of organomagnesium radicals, there has been no spectroscopicstudies on any organomagnesium radical in either gaseous or condensed phases prior to this work, the main reason for this being the difficulty in producing these species in a form and concentration suitable for spectroscopic characterization. In this report, we employ a pulsed laser vaporization/photolyis technique to prepare organomagnesium radicals in the gas phase and detect them using laser-induced fluorescence (LIF) spectroscopy. This is the same technique that we have used in recording the electronic spectra of several previously unobserved half-sandwich organometallic radicals, namely, the metal cyclopentadienyl (MC5H5or MCp), metal pyrrolyl (MC&N or MPy), and metal methylcyclopentadienyl (MCsH4CH3 or MMeCp)
0022-365419212096-8791$03.00/0 0 1992 American Chemical Society
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complexes of zinc, cadmium, and c a l c i ~ m . ' ~In- ~all~ these cases, TABLE I: Transition Frequencies (in cm-') for MgCp A-% the metal atom was found to be ring-bonded, Le., it is located above Excitation Swctrum the ligand ring. Furthermore, depending on the identity of the M&P MgCp-4 metal, the bonding in these molecules can vary from being mainly freuuencv" Avb freuuencv' A@ assignmentc covalent to being quite strongly ionic. The largest contributing 20216 vs 0 20216vs 0 0: factor governing the nature of the M-ring (M-R) interaction is 20340w 124 10' 20349 w 133 the energy of the valence atomic orbitals of the metal relative to 20368 vs 152 161 20388 vs 172 the highest occupied molecular orbital (HOMO) of the ligand. 20491 m 275 20478 w 262 100 For example, in the case of MCp, one has to consider the HOMO 20555 s 339 20551 s 334 4; of Cp, which is an el pair of MO's under C,, point group sym346 14' 391 20562 m 20607 s metry. If the metal valence orbitals are considerably higher in 20620 w 404 10: energy than the el orbitals of Cp, this would facilitate the transfer 20687 w 47 1 4!10(1 510 20702111 485 40160 20726 s of an electron from the metal to the Cp ring, giving rise to pre20738 m 522 20688 w dominantly ionically-bonded molecules. This is certainly the case 20759 w 543 472 3l4; for CaCp13*17 where the first ionization energy of Ca (IE = 6.1 11 20775 m 559 20707 m 491 14!16; eV)18is much lower than that of Cp (IE = 8.4 eV).19 On the other s 674 20882 m 665 4; 20890 hand, some metals have valence orbitals comparable in energy 676 4b14; 20941 m 725 20892 w to those of the Cp el HOMOS, and hence the bonding will have 21009 m 793 20968 m 752 A: a much larger covalent contribution. This was found to be the 21034 m 818 20974 m 758 B: case for ZnCp (LE. (Zn) = 9.391 eV)18and CdCp (I.E. (Cd) = 21062 m 846 4;16b 8.991 eV).'* 21 189 m 973 21 1 5 6 m 940 C!, Given that the first LE. of Mg is 7.644 eV,I8one would expect 21219 w 1003 40 21337w 1127 21306m 1090 At;+41, a larger ionic contribution to the bonding in MgR (R = Cp, Py, 21363 m 1147 21306 m 1090 C z + 14; MeCp) compared with those of ZnR and CdR. However, given 21525 w 1309 c: + 4; that magnesium is somewhat less electropositivethan calcium, the bonding in organomagnesium radicals would be expected to "To help the reader identify particular bands in the excitation specbe more covalent than those involving calcium but the degree of trum, wc use the following labels after each frequency to indicate the covalency is not obvious. Indeed, there is some controversy as intensity of a band: vs = very strong; s = strong; m = medium; w = is the a , C-C weak. bFrequency relative to the origin band. to the exact nature of bonding in organomagnesiumcompounds. stretch, v4 is the a, M g C p stretch, vl0 is the e, M g C p tilt, vI4 is the e2 For example, for the "full-sandwich" compound, MgCp,, some ring deformation, and v I 6 is the e2 ring torsion. See text for possible workers claim that the Mg-Cp bond in MgCp, is primarily of assignment for bands A: - C;. ionic nature;42s22 others regard it as covalent,3s7while a third group perhaps more realistically suggests an intermediate picture with vaporization of the metal sample and laser photolysis of the apsignificant elements of both covalent and ionic b ~ n d i n g . ~In~ . ~ ~ propriate organic precursor using an excimer laser operating on light of this controversy, it would therefore be of great interest ArF (193 nm) or KrF (248 nm) fills. The organic precursors for to determine the nature of the bonding in the half-sandwich MgCp MgCp, MgMeCp, and MgPy are, respectively, cyclopentadiene radical, and indeed of other organomagnesium radicals. (C,H,), methylcyclopentadiene (C,HsCH3), and pyrrole (C4Here, we report a detailed spectroscopic study of some halfH,NH). Fully-deuterated samples of cyclopentadiene and pyrrole sandwich complexes of magnesium, namely, MgCp, MgPy, and were also used as organic precursors to assist in the vibrational MgMeCp. These molecules are cooled in a supersonic jet exanalysis. Complete deuteration of cyclopentadiene and pyrrole pansion, which allows us to resolve extensive vibrational structure was achieved by H-D exchange reactions as described in refs 29 in their spectra. We will start our discussion with MgCp. We and 30. will show that the type of electronic transition being observed here The organic precursor vapor was seeded into a supersonic exis best described as being metal ligand and that MgCp is pansion by passing helium over the liquid sample. The precursor ring-bonded (or $-bonded, Le., the metal atom is located above was kept in a stainless steel reservoir whose temperature was the "center" of the Cp ring) and therefore has C,, symmetry. We maintained by a constant temperature bath. will then move on to the case where the Cp ring is methyl-subLaser excitation spectra were recorded by scanning a nitrostituted, Le., MgMeCp, and we will show that the Mg atom is gen-laser-pumped dye laser (Molectron DL-I1 pumped by a also ring-bonded. Finally, we will consider the case where the Molectron UV-24) and monitoring the total fluorescence with a ligand is a heterocyclic ring, namely, the'pyrrolyl radical. Py is photomultiplier tube. The signal was then transferred to a PC isoelectronic with Cp. There are at least two conceivable binding XT computer via a home-built interface, where it was averaged sites for the Mg atom in MgPy: it can be (i) ring-bonded ($), over a number of laser shots, and then stored for subsequent paralleling the case of MgCp and MgMeCp, or (ii) N-bonded analysis. For dispersed fluorescence experiments, an excimer(VI). Interestingly, there has been considerable controversy as pumped dye laser (Lumonia HD-300 pumped by a Questek Series to how the Grignard reagent PyMgX should be formulated: 2000 excimer laser) was used in pumping the desired vibronic N-MgX (91)2s-27or Py- MgX+ (vs-bonded, ionic species).25 A transition. The fluorescence was dispersed by a 0.3-m mononuclear magnetic resonance (NMR) spectroscopic study of chromator (Instruments SA HR-320) and detected by a diode PyMgXZ8showed that either configuration can be adopted by this array detector (EG & G Model 1421), which was cooled to -30 molecule, although a clear choice between these two alternatives O C . The output from the detector was then processed and stored cannot be made from the NMR data alone. However, we will by an optical multichannel analyzer (EG & G Model 1460). show that, at least in the case of MgPy, the vibrational structure Calibration of dispersed fluorescence spectra was achieved with is consistent with an $-bonded Mg atom, i.e., the metal atom is the Fe/Ne atomic lines of the Fe hollow cathode spectrum.31 located above the Py ring. We should point out that MgPy, while All excitation spectra are shown with no corrections for variinteresting purely from an academic point of view, is also extremely ations in dye laser intensity as a function of wavelength. All important biologically, serving as building blocks of the chlorodispersed fluorescence spectra were recorded without any conphylls. tributions from dye laser scattered light, which was achieved by monitoring the emission with the detector in the gated mode. 11. Experimental Section 111. Results and Discussion Details of the pulsed laser vaporization/photolysis technique The laser excitation spectra of MgCp, MgMeCp, and MgPy employed in producing MgCp, MgMeCp, and MgPy and their are shown in Figures 1, 2, and 3, while the corresponding band isotopomers have been described e 1 ~ e w h e r e . lBriefly, ~ ~ ~ ~ the ~~~ positions are given in Tables I, 11, and 111, respectively. Notice organomagnesiumradicals were prepared by simultaneous laser
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The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8793
Electronic Spectroscopy of Organometallic Complexes
Excitation Spectrum of MaCaH4N
Excitation SDeclrum of MaCqHq
C
20150
20400
20650
20900
21150
21400
21650
Frequency (cm-1) Figure 1. Laser excitation spectrum of MgCsH5. The bandwidths of the features attributed to MgCsHs are on the order of 2.5 cm-' at FWHM. This trace also shows the A 211-X 22+1-0 band of MgH at 2080020850 cm-I. Vibrational assignments of many of the bands observed are given above the spectrum. Refer to Table I for a description of the vibrational modes of MgCsHs. Excitation Soectrum of MaCGHaCHq
20075
20300
20525
20f50
20975
Frequency
21100
21425
21650
(cm-1)
Figure 2. Laser excitation spectrum of MgCsH4CH3. In addition to MgC5H4CH3bands, the two spin-orbit components of the A 211-X 22+ 1-0 band of MgH are observed. The relatively strong bands of MgC5HS (most of which are actually off-scale) are marked with asterisks. TABLE II: Transition Frequencies (in cm-I) for MgMeCp A-k Excitation Spectrum frequency'' assignmentb AUC frequency' assignmentb 0 20503 m 17b18; 20 150 s 20275 s 125 20516 w 20 20593 s 19kO; 20319 s 169 187 20633 m 340 20337 s 20355 m 205 20648 m 18619' 20397 w 20659 m 17619: 247 20401 w 251 20676 m A + 19; 20796 m 19; 20473 m 323
AUC 353 366 443 48 3 498 509 526 646
" s = strong; m = medium; w = weak. bThe notation employed for labeling the normal modes of vibration of MgMeCp is the following: v17for the a' C-CH, deformation, v18 for the a' ring torsion, v19 for the a' metal-ligand stretch, Y~~ for the a' metal-ligand tilt, Y , ~for the a" C-CH, deformation, and v36 for the a" metal-ligand tilt. Frequency relative to the origin band.
that the band systems attributed to these molecules fall in the same spectral region but their vibrational structures are entirely different from one another. These observations clearly indicate that we are observing three different molecules. The fact that each of these spectra appear only when their respective organic precursors were introduced into the supersonic expansion provides strong evidence that the carriers of the spectra shown in Figures 1, 2, and 3 contain the Cp, MeCp, and Py moieties, respectively. As
~
zoi50
20'400
I
1
20900
21150
I
21400
I
21650
Frequency ( c m - 1 ) Figure 3. Laser excitation spectrum of MgC4H4N. Again, the A 211-X 22+1-0 band of MgH is seen here. TABLE III: Transition Frequencies (in cm-') for MgPy A-k Excitation Spectrum MgPy frequency' 20 222 vs 20358 m 20477vs 20493 m 20502 s 20565 s 20616 m 20625 vs 20637 vs 20680 s 20702 m 20823 s 20853 m 20907 s 20958 m 20967 m 20978 w 21022 m 21 186 m 21246 w 21 323 w 21440 m 21527 w
Ad 0 136 255 271 280 343 394 403 415 458 480 60 1 631 685 736 745 756 800 964 1024 1101 1218 1305
MgPY-d, frequency' Ad 20233 vs 0 134 20367m 20446m 232
20490 m 20572 s
257 339
20596 m 20628 m 20657 m
363 395 424
assignmentC
ox 131 llq 130, 24; A 12' 11813; 86 A + 13;
B 121131 110120 A + 121, 122 11812613; 8612; A + 12'13' B + 12: C 12; C + 13; D c + 12;
'vs = very strong; s = strong; m = medium; w = weak. *Frequency relative the origin band. e v8 is the a' ring deformation, vI I is the a' ring torsion, v12 is the a' Mg-Py stretch, ~ 1 is3 the a' Mg-Py tilt, and ~ 2 is 4 the a" Mg-Py tilt. See text for discussion of possible assignments for bands A-C.
will be shown shortly, the vibrational structure of these spectra is only consistent with the spectral carrier having a half-sandwich Mg-R molecular structure. In addition to the MgCp, MgMeCp, and MgPy radicals, MgH is also one of the major products formed during the laser vaporization/photolysis process, as is evident from the presence of intense MgH A zII-X bands in the excitation spectra. In Figures 1-3, the two spin-brbit components of the A 211-X 2Z+ 1-0 band of MgH are observed. The spin-orbit splitting in the A 211 state of MgH is quite small (35 ~ m - ' ) , and ) ~ therefore, on the scale shown in these figures, the two components are not easy to distinguish. In addition, much stronger MgCp bands (marked with asterisks) appear in the excitation spectrum of MgMeCp (Figure 2). Our experience has shown that the MCp features are usually quite prominent in an excitation spectrum of an MMeCp molecule, and indeed in this case, the MCp bands are dominant. Possible reasons for the formation of MCp radicals when using
8794 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
Robles et al.
we can employ the reported appearance potential of MgCp+ (from electron impact ionization of gaseous MgCp,) of 10.98 f 0.1 e V 8 and the measured dissociation energy of the Mg-Cp bond of -3.46 eV38to estimate the I.E. of the MgCp radical (=7.52 eV).39 Five electrons from Cp (ground valence electronic codiguration 3P3Pz .r=( of (a?)2(el”)3) and two electrons from Mg must be partitioned among the valence MO’s of MgCp. The lowest fully-filled valence MO in Figure 4 is of a, symmetry (a “a-bonding” orbital as far as the metal-ligand interaction is concerned) and is mainly a? orbital of Cp in character with some admixture of Mg 3s and 3p, AOs. The other fully-occupied MOs, le, (a ‘a-bonding” orbital) are expected to be mainly Cp e,” in character with some Mg 3p,, mixed with it. The highest occupied MO (HOMO), 2al, is a combination of the 3s and 3p, A O s of Mg and the a; orbitals of Cp, with the 3s A 0 contributing most significantly to the nature of the orbital. The valence electron configuration of MgCp is therefore_(laJ2(lel)4(2al)’,corresponding to a ground electronic state of X 2A,. Some support for this simple model comes from preliminary MCSCF ab initio calculations on MgCp.40 These calculations also predict that the ground state is ft 2Al. They also predict that the lowest excited electronic state of MgCp is of zE1symmetry, arising from an electron promotion from the fully-filled le, MO (which is mainly Cp (C 2pa) in character) to the HOMO, 2a, (which is predominantly Mg (3s) in-ch_aracter). Ab initio calOUALlTATlVE MO DIAGRAM OF M Q C ~ H S culations therefore predict that the A-X electronic transition is Figure 4. Qualitative MO diagram of MgC5H5. The relative energies approximately a metal ligand charge-transfer excitation, (in eV) of the valence orbitals of Mg, C5H5,and MgC5H5are shown here paralleling the case of ZnCp and CdCp. As we will see shortly, assuming that the first ionization energy is to a first approximation equal the spectrum of MgCp is mostly consistent with the theoretical to the negative of the orbital energy. results. In the laser excitation spectrum of the MgCp radical shown a methylcyclopentadiene precursor have been described elsein Figure 1, the-str_ong band at 20216 cm-l (see also Table I) is where. 1 6 ~ 1 7 assigned as the A-X 0; band since scans further to the red of this In the remainder of this section, we first discuss the electronic feature (as far as 15OOO cm-I) revealed no-evidence of any bands structure of MgCp, MgMeCp, and MgPy and this is followed by attributable to MgCp. To the blue of the A-X origin are several the assignment of the vibrational structure observed in their other bands of MgCp, the strongest of which is only +172 cm-l electronic spectra. from the origin. 9the basis of the MO model above, we assign A. Electronic Structure. (1) MgCp. The details of the spectra these bands to the A 2El-R 2Alelectronic transition. No spin-orbit of MgCp can be interpreted in terms of a simple, qualitative splitting is o b s e ~ e din Figure 1, which indicates that the spin-orbit molecular orbital (MO) model. An MO diagram that shows the splitting in the A 2EIstate is too small to be resolved (12 cm-I). valence M O s of MgCp along with those of the free Cp moiety Notice that the fmt strong band to the blue of the electronic origin and Mg atom is shown in Figure 4. We note that the MO cannot be attributed to the spin-orbit partner of the origin since diagram of MgCp is very similar in many respects to that of this would imply a spin-orbit splitting of 172 cm-I. This is too ZnCp.I4 Paralleling the case of ZnCp, three major assumptions large to be produced by the relatively light atoms in MgCp; for have been made. (i) We assume for the moment that the Mg atom example, the spin-orbit parameter, [3p, for atomic magnesium, is located above the center of the Cp ring ($- or ring-bonding), the heaviest of the atoms in MgCp, is only 40 cm-I,l8 which is which would render MgCp Cs, point group symmetry. As will then reduced to 35 cm-l in the corresponding A 211state of MgH.32 be shown later, the vibrational structure in the spectra of MgCp, The absence of resolvable spin-orbit splitting in the A 2EIstate MgMeCp, and MgPy provides convincing evidence to support this is easy to explain by our MO model. In this state, the unpaired assumption. (ii) We also assume that only the five C 2p7r atomic electron is in the le, MO, which we suggest is mainly C 2pr in orbitals (AO)form a basis for representing the valence MO’s of character. Consequently,the spin-orbit splitting will be largely the Cp radical (DShsymmetry) giving rise to the strongly bonding characteristic of the X 2El”state of the bare Cp radical. Since a; orbital, the weakly bonding e,” orbital, and the antibonding no spin-orbit splitting has ,,been resolved even in a rotationallye? orbital, which transform, respectively, as al, e,, and e2under resolved LIF study of the A 2El”transition of Cp, it is C,, symmetry. (iii) We also assume to a first approximation that therefore not surprising that we resolve no spin-orbit splitting in only the 3s and 3p orbitals of Mg need to be considered as valence the MgCp case. orbitals, since Mg does not have any low-lying d orbitals. Under (2) MgMeCp and MgPy. The laser excitation spectra of C,, symmetry, the Mg AO’s transform as 3s a , and 3p a , MgMeCp and MgPy are shown in Figures 2 and 3, respectively. + e,. For MgMeC we assign the band at 20 150 cm-I as the origin The relative energies of the valence orbitals of Mg, MgCp, and band of the A’LX transition, since scans to the red of this band Cp are shown in Figure 4, assuming that Koopmans approxiLeveal no other features that can be attributed to MgMeCp. The mation’, applies; Le., the ionization energy (I.E.) of an electron A-3 0; btnd (at 20 150 cm-I) of MgMeCp is shifted to the red in a given orbital (atomic or molecular) is to a first approximation of the A-X 0; band (at 20216 cm-I) of MgCp by only 66 cm-I. equal to the negative of the orbital energy. The first ionization A red-shift in the transition frequency on methyl-substitutionof energies of Mg and the Cp radical are 7.644 and 8.4 eV, rethe Cp ring relative to that of the Cp derivative is highly indicative ~pectively.’**~~ According to simple Hiickel MO theory, the energies of the a;, e,”, and e; a M O s of Cp are a + 28, a + (2 of a ligand-metal charge-tra_nsfer(LMCT) transition as discussed cos OM,and a (2 cos +)@, respectively, where w = 2 ~ / 5 . ~ ~in ref 16. For MgPy, the A-3 0; transition is identified to be at 20 222 cm-l, which is only 6 cm-l to the blue of that of MgCp. Since we know from the A 2A;-X 2El”electronic transition of the Cp radical that the energy separation between the a? and As will be shown in the vibrational section, the vibJation@ e,” MOs is =3.67 eV,” we can use this information to determine frequencies of the Mg-ring stretching vibration in the X and A /3 and hence deduce that the e? orbital should be -5.93 eV above electronic states of MgCp, MgMeCp, and MgPy are very similar the e,” orbital. To estimate the energy of the HOMO of MgCp, in magnitude. These observations strongly support the idea that
i.,
-
-
+
-
Electronic Spectroscopy of Organometallic Complexes the type of electronic transition we are observing in MgMeCp and MgPy is essentially the same as that in MgCp, Le., an LMCT process. However, the presence of the CH3 group in MgMeCp or the N atom in MgPy reduces the symmetry from C5, to C, in going from MgCp to MgMeCp and MgPy. The lowering of symmetry should result in the lifting of both electronic and vibrational degeneracies that are initially present in MgCp. As will be shown in the next section, the vibrational structure of MgMeCp and MgPy is consistent with this lowering of symmetry. As far as the electronic state symmetries of MgMeCp and MgPy are concerned, one would expect the following correlatio! to be fo!lowed in going from MgCp to MgMeCp and MgPy: X 2A1 X 2A' and A 2E1 2A' + 2A''. This means that two band systems are expected in the excitation spectra of MgMeCe and MgPy, resulting from the lifting of the degeneracy of the A 2El state of MgCp. We have only been able to attribute the spectra of MgMeCp and MgPy to a single excited state. There are two possibilities that could explain the apparent absence of the second excited electronic state. First, the fluorescence quantum yield from the "missing" excited state to the ground state may be too small to be observed via LIF technique. We note that in the case of pyrrolyl an! methylcyclopentadienyl derivatives of Zn and Cd,I5J6their A-X band systems are about 5 0 4 0 % weaker in intensity than their B-2 band systems. Second, notice in Figures 2 and 3 the apparent broadening and congestion of bands a450 and a600 cm-l to the blue of their respective origin bands. It is therefore conceivable that the second excited electronic state is somewhere in this spectral region, the brozdtning of bands presumably caused by overlapping of A-X and E X bands and/or predissociation of the ?J state. B. Vibrational Analysis. (1) MgCp. In ref 14, we described the 27 normal modes of vibration of a metal monocyclopentadienyl molecule with a general formula of MC5H5 and an assumed equilibrium point group symmetry of C5,. Briefly, these 27 modes can be grouped into two categories: metal-ring and intraring modes. The former group of modes are generally called skeletal modes and there are two of them, the symmetric Mg-Cp stretch, v4, and the degenerate (el) Mg-Cp bend, uI0 (the Mg-Cp bend is commonly referred to as a tilt in the organometallic literature). Being totally symmetric, there are no formal restrictions on the degree of excitation of v4 in the electronic spectrum, whereas the lower symmetry vIo mode is only allowed in even quanta of excitation in the absence of any significant vibronic coupling mechanisms. For the intraring modes, three are totally symmetric: the CH stretch (vl), the CC stretch (YJ, and the CH wag (v3), while the remaining modes are non-totally symmetric and have either a2 (4, el (v6-vg), or e2 (vll-vI6) symmetries. From symmetry arguments, only the totally symmetric (al) intraring modes are fully allowed and therefore are expected to yield the major intraring vibrational features observed in the spectra of MgCp. The rest of the intraring modes are non-totally symmetric and therefore are expected to be observed only in even quanta of excitation. However, for a doubly degenerate electronic state, which is subject to Jahn-Teller distortion, the symmetry-imposed selection rules on the potentially Jahn-Teller active modes, those of symmetry, are not very rigid. In other words, for a molecule with significant Jahn-Teller activity, observation of single quantum excitation of the e2 modes is possible. (a) Skeletal Modes. For most of the organometallic radicals we have studied previously in our laboratory, vibronic bands arising from excitation of the skeletal modes, particularly the metal-ligand stretch, tend to dominate their spectra. This observation also seems to apply to MgCp. The first three strong bands to the blue of the origin are located at 20388 (+172), 20555 (+339), and 20607 (+391) cm-' (Table I). One of these bands almost certainly corresponds to excitation of the Mg-Cp stretch, v4. Initially, we thought the most likely cagdidate was the band at 20 388 cm-l; Le., u4 is 172 cm-l in the A state, and the band at 20 555 cm-I is the overtone 4; band. However, subsequent dispersed fluorescence experiments on MgCp and laser spectroscopic studies on fuliy-deuterated MgC5D5(MgCp-d5),MgPy, and MgMeCp showed that our initial v4 assignments on MgCp were incorrect.
-
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8795 Dispersed fluoresce ye-Spectrum of MgCsH5 (Pumping A . X 0; Band) I
4:
/I
-
ii
330
0
660
990
Relative Frequency (cm-1) Figure 5. Dispersed fluorescence spectrum of MgC HJobtained by pumping the A-k000 transition at 20216 cm-l. The 0, line is off scale by a factor of 4 to allow the weaker features to be seen more clearly. The spectral resolution of the monochromator used in these experiments in this spectral region is a1 1 cm-l. The detector was run in gated mode to bias against scattered light contributions.
a
TABLE I V MgCp ft State Vibrational Intervals (in cm-') Obtained from Dispersed Fluorescence Spectra av interval pumped transitions relative to assignB pump ment 0 : 16A 4i 14A A O
209 330 376 638
b
210 330 377 635 545
O
b
O
O
O
O
215 331b 371 633
210 210 324 330 331 375b 382 644b 642 659 704
663 708
704
657 701
996 1015
963 996 1013
959 994 1013
1012 1029
667 708 857 965
666 706 858b
1019 1037 1185
0 211 330 376 638 545 662 705 857 962 995 1014 1033 1185
00
16, 41 141
A, 4,16, 42 4,141
B, A,,t 41 4, 2, 42141 B, t 41
'The vibrational levels ppulated in the ground electronic state are shown. Refer to Table I for a description of the normal modes of vibration of MgCp observed here. bThe strongest feature in the spectrum. The dispersed fluorescence spectrum-of MgCp obtained by fixing the dye laser frequency to the A-X 0 : transition is shown in Figure 5 (refer to Table IV for line positions). The strongest feature to the red of the pump frequency, which is also the first member of the longest progression in this trace, is at 330 cm-I. We the_efore assign 330 cm-I as the Mg-Cp stretching frequency in the X state. We are quite confident about this assignment for several reasons, not least of which is _the observation that comparable vibrational intervals in the X states of MgMeCp and M g b are also attributed to their metal-ring stletching modes. If we assume that the 172-cm-I interval in the A state is due to v4, this would mean either that there is a remarkable change in Lhe-Mg-Cp distance (and therefore the strength of the bond) on A-X excitation and/or the bonding mode of the Mg atom is different between the two electronic states. However, for almost a 50% decrease in v4 in going from the X to the A state, one would not expect to see a significant intensity on the origin band. This is entirely contrary to what is seen in Figure 1. With the very regular Franck-Condon pattern in this figure, it is more likely that the Mg-Cp stretching frequency corresponts to the band at 20 555 cm-I, some 339 cm-l to the blue of the A-X origin. To obtain further support for this new assignpent, we pumped the band at 20555 (+339) cm-l and monitored the resulting emission spectrum. This spectrum is shown in Figure 6. If the 20 555-cm-l
8796 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 Dispersed Fluorescence Spectrum of MgC,H, (Pumping A - ic 4; W)
660
330
0
990
Relative Frequency (cm-l) Figure 6. Dispersed fluorescence spectrum of MgCJH5 obtained by pumping the A-f( 4; transition at 20555 cm-I.
'-'1
Excitation Spectrum of MgCsDs /6;
12
0;
20150
20375
k: ik;
4i16; 1 4 ~ ~ 1 %4;ig
20600
20825
A$4;
I$
&
21050
6;
c;!i4:
21275
21500
Frequency (cm-1) Figure 7. Laser excitation spectrum of MgC5D5.
band is the 41, band, then pumping this feature should, in general, favor the u4 progression in the ground state. As shown in Figure 6, this is indeed what is observed, with the members of the u4 progression gainin considerable intensity compared to that observed when the Oo band was pumped (Figure 5). To prove that the 339-cm-l rather than the 172-cm-' interval is due to u4, we also recorded the excitation_spectrumof MgCpd, and this is shown in Figure 7. The A-X origin band for the fully-deuterated MgCp-d5, did not show any frequency shift compared with that for nondeuterated MgCp within our spectral resolution. However, there are shifts for higher vibronic bands. The vibrational intervals for the first three bands to the blue of the origin in Figure 7 have been reduced: 172 152, 339 334, and 391 346 cm-l, in going from MgCp to MgCpd,. The 172- and 391-cm-l intervals are significantly affected by deuteration (-1 1.6 and -1 1.5% change, respectively) while the 339cm-I interval changed only by 1.5%. If we approximate the Mg-Cp stretching vibration as a pseudodiatomic oscillator, then the increase in reduced mass on deuteration for this vibration is far too small to account for the decrease in frequencies of the 172and 391-cm-' modes in MgCp. Clearly, these modes cannot be identified as uq. On the other hand, if the 339-cm-l interval in normal MgCp is due to u4, then on full deuteration of the ring, we would expect a reduction in frequency to approximately 336 cm-I, a value close to the experimental value of 334 cm-I. This therefore verifies that the 339-cm-' interval results from excitation of uq. Figure 1 also shows the presence of a band o,f medium intensity at 20491 cm-l, which is 275 cm-I from the A-X @ band. We assign this feature as being due to excitation of two quanta of the uI0mode, giving 2v10a frequency of 275 cm-I. Most likely, the
f
-
- -
Robles et al. 10; band gains intensity through Fermi rmnance interaction with the 4; band, which is only 64 cm-I to the blue of the 10; band. Furthermore, a weak band at 20 349 (+133) cm-l is also observed in Figure 1. We attribute this band to single-quantum excitation of ul0, which presumably derives its intensity from a vibronic coupling mechanism. (b) Intraring Modes. In the excitation spectrum of MgCp in Figure 1, there are several bands that cannot be assigned to the metal-ring stretching mode and yet that have intensities that are almost comparable to those of the lower frequency members of the u4 progression. It is clear that these bands arise from vibrational excitation of MgCp and are not due to other photolytic products in the supersonicjet expansion since vibronic bands of u4 built upon these *extra" bands are clearly observed. We therefore attribute these bands as being due to excitation of intraring modes. The vibrational assignments made in this section are derived mainly from a comparison of the relative frequencies of these bands (with respect to the origin band) with those of other metal cyclopentadienyl complexes as summarized in ref 14, the cyclothe full-sandwich compound MgCpz, and pentadienyl related aromatic ring species like benzene.4s We have also considered the intensity pattern in the single vibronic level fluorescence spectra obtained by pumping these intraring bands and the relative frequency shift upon full deuteration of the Cp ring. The lowest frequency feature in Figure 1 that is not a member of the main u4 progression and that is in fact the strongest feature in the spectrum is the band at +172 cm-' relative to the origin band. As discussed briefly earlier, the 172-m-l interval is reduced to 152 cm-' (-1 1.6%) on full deuteration of the Cp ring. Although this shift is quite large, it is not large enough to be attributable to a vibration consisting of "pure" hydrogenic motion. This therefore leaves a C, framework vibration and it is to this that we attribute the 172-cm-' interval. Furthermore, since the +172-cm-l band is fairly strong and this interval is already rather small, it is likely that this feature is a fundamental band. Normally, one would expect this vibration to be symmetric with respect to all symmetry elements of point group C,, for reasons outlined earlier. However, the only totally symmetric vibration involving the ring framework is the CC stretch, u2, which is expected to have a frequency in the region of 1100 cm-l.14 We therefore believe that this band is not due to 2;. However, given that the A state is attributed 2EI symmetry, then Jahn-Teller distortion of the molecule would allow singlequantum excitation of any of the six potentially Jahn-Teller active modes of e2 symmetry, of which two, the ring deformation ( ~ 1 4 ) and the ring torsion (q6) modes, are expected to have the lowest vibrational frequencies among the intraring modes. For the closed-shell species benzene, C6Ha,the frequencies of the analogous ring torsion and ring deformation modes were measured to be 243 and 365 cm-', which upon full deuteration were reduced to 208 (-14%) and 306 (-16%) cm-l, From the magnitude of the frequency of the ring torsion of C6H6 and the relative change upon de_utcration, we tentatively assign the band at 20 388 cm-l as the A-X 16; transition, which presumably deriyes its intensity from a significant Jahn-Teller distortion in the A state. Built up on this band are two members of the u4 progression, which we assign as 4;16; and 4;16; at 20726 and 21 062 cm-l, respectively. The measured spacing of -338 cm-l between the members of this progression indicates that the 20_38_8-cm-Iband is indeed of MgCp in origin and belongs to the A-X band system rather than another band system of MgCp. Because of the intensity of the 16; band, one might expect to see a strong 16; band. Examination of Figure 1 shows that at almost exactly twice the relative frequency of 16;, Le., at +339 cm-l, is an equally strong 4; band. We believe that the +339-cm-' band is due entirely to the 4; transition rather than overlapping with the 16; band because two bands are not resolved upon full deuteration. The only other alternative assignment for 16; is a band +390 cm-' from the origin, which upon full deuteration of the ring reduces to +346 cm-' (-12%), a relative change equal to that of 16;. However, if the +390-cm-I band is the 16; tran-
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8197
Electronic Spectroscopy of Organometallic Complexes
TABLE V Vibrational Frequencie (cm-I) of MgCp,MgCpda
Excitation Spectrum of MgCsHaCH3
and
MgCP,
modec v4
k
MI3CP"
A
330
339 133 391 172
VI0 v14
v16
376 211
MgCp-d,"
MgCp,b
A
z(
334 124 346 152
218 189
"This work. bReference 4.
