3258
J. Phys. Chem. 1992, 96, 3258-3265
-
transition would then unequivocally confirm that the transition we observe in this study is indeed metal ligand. As to the symmetries of the electronic states and also accurate molecular geometries, particularly the metal-ligand separation, rotationally resolved spectroscopicstudies would be highly desirable. However, given the small rotational constants expected for ZnCp and CdCp and the presence of several isotopes of Zn and Cd atoms, such experiments would be exceedingly difficult. Finally, we would like to emphasize that a number of studies of other organometallic radicals are either in progress or have recently been completed in our laboratory. As far as organozinc or organocadmium species are concerned, we have recently obtained laser-induced fluorescence spectra of the pyrrolyl (see paper 2 immediately following this one), and monomethylcyclo-
pentadieny16derivatives of these metals. We have also recorded spectra of a number of alkaline-earth-metal-containing organometallic radicals. For example, we have recorded the first gasphase electronic spectra of organomagnesium radicals. Analyses of these and other spectra are in progress and will be reported in detail elsewhere.26 Acknowledgment. We thank the National Science Foundation for support of this work via Grant No. CHE-9005963. A.M.E. gratefully acknowledges the award of NATO/SERC and Ohio State postdoctoral fellowships. E.S.J.R. greatly appreciates the award of a Rohm and Haas predoctoral fellowship. (26) Ellis, A. M.; Robles, E. S. J.; Miller, T. A., manuscript in preparation.
Electronic Spectroscopy of Jet-Cooled Half-Sandwich Organometallic Free Radicals. 2. Laser- Induced Fluorescence Study of the Pyrroiyi Complexes of Zinc and Cadmium 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 43210 (Received: November 1 , 1991; In Final Form: January 2, 1992)
Laser excitation and dispersed fluorescence spectra of the open-faced sandwich molecules zinc monopyrrolyl and cadmium monopyrrolyl, ZnC4H4Nand CdC4H4N,respectively, are reported. These organometallic free radicals were prepared in a supersonic expansion using a laser vaporization/photolysistechnique. In the excitation spectra, bands attributable to transitions to two closely spaced excited electronic states have been observed. Quite possibly, these two excited electronic states correlate with a degenerate, excited 2E,state of the corresponding metal monocyclopentadienyl radicals (see the preceding paper), but in the lower symmetry environment of the pyrrolyl complexes the electronic state degeneracy is resolved. The laser excitation and dispersed fluorescence spectra also show extensive vibrational structure, originating from both metal-ring and intra-ring vibrations.. From this structure, we have been able to determine unequivocably that both zinc and cadmium are ring-bonded rather than N-bonded in their monopyrrolyl complexes, Le., the metal atom is located above the plane of the pyrrolyl ring at equilibrium yielding C, point group symmetry. This study represents the first spectroscopic observation of these molecules.
I. Introduction In the preceding paper,l which we will refer to as paper 1, we described in detail a spectroscopic study of the half-sandwich metal cyclopentadienyl radicals ZnC5H5 and CdC5H5. Intuitively perhaps, one would expect the metal atom to be ring bonded in these complexes, i.e., to be located above the center of the cyclopentadienyl ring at equilibrium, thus yielding C,, symmetry. The spectroscopic evidence is certainly consistent with this bonding mode.’ However, if the organic cyclopentadienyl ligand were to be replaced by a less symmetric entity, the structure of the complexes would be more difficult to predict. In this work, we have extended our earlier studies to include just such a case, the heterocyclic pyrrolyl radical, C4H4N,being substituted for C5H5. Like the C5H5ligand, the pyrrolyl radical is a five-membered ring system. Furthermore, the metal pyrrolyl complexes themselves are isoelectronic with their cyclopentadienyl counterparts. However, the substitution of a N atom for a C H group in passing from the cyclopentadienyl ligand to the pyrrolyl ligand might be expected to have a very substantial effect on the way the metal atom is bound to the organic species. In a metal pyrrolyl complex having the general formula MC4H4N,where M is a metal atom, there are several binding sites at which one might reasonably expect the metal atom to be located. One possibility is that the metal sits approximately above the center of the *-electron system of the pyrrolyl ring in an analogous fashion to the cyclopentadienyl ‘Rohm and Haas Predoctoral Fellow. NATO/SERC and Ohio State Postdoctoral Fellow. Present address: Department of Chemistry, University of Leicester, University Road, Leicester LEI 7RH, U.K.
*
0022-3654/92/2096-3258$03.00/0
complexes. A slight variation on this theme would be if the metal were still located above the plane of the ring but preferentially displaced toward the electron-rich nitrogen atom. Another possibility is that the metal may preferentially bond in the plane of the ring directly to the nitrogen atom, Le., it has no interaction with the pyrrolyl wring system. Such direct bonding to the N atom has been reported for alkaline earth metal amide compound~.~”Determining which of these bonding modes is actually adopted and why are interesting questions which are not just of academic interest. After all, metal pyrrole subunits are found in many chemical substances of great biological importance including heme, the chlorophylls,vitamin Biz, and the bile ~igments.~ In this paper, we present the results of a laser excitation and dispersed fluorescence spectroscopic study of ZnC4H4N and CdC4H4N,molecules, which we will refer to as ZnPy and CdPy, respectively. Neither of these molecules has been observed previously in the gas phase. In fact the only reported spectroscopic study of half-sandwich metal pyrrolyl complexes in the gas phase prior to our work was a laser-induced fluorescence study of CaPy and SrPy by Bopegedera et al.,s in which these alkaline-earth( 1 ) Robles, E. S. J.; Ellis, A. M.; Miller, T. A. J . Phys. Chem., preceding paper in this issue. (2) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. J . Phys. Chem. 1987, 91. 2779. ( 3 ) Whitham, C. J.; Jungen, Ch. J . Chem. Phys. 1990. 93, 1001. (4) Smith, K. M. Porphyrins, Corrins and Phthalocyanins. In Compre-
hensive Heterocyclic Chemistry. The Structure, Reactions, Synthesis and Uses of Heterocyclic Compounds: Bird, C . W., Cheeseman, G. W. H., Eds.; Pergamon Press: Oxford, 1984; Vol. 4, pp 377-442.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, NO.8,1992 3259
Half-Sandwich Organometallic Free Radicals. 2
CdC,H,N
12A14
0;
24;
1233;
12A24:
1$13;
12g24:
12i242
i.i
CdH mn
21700
22000
22300
22600
22900
23200
23500
23800
FREQUENCY (cm-')
F v 1. Laser excitation spectrum of ZnPy covering the A-R and 8-3 band systems. The ZnPy bands have typical full widths at half-maximum of - 4 cm-' in this spectrum. Assignments of bands arising from skeletal vibrations are given above the spectrum. More complete assignments, including those of bands due to ring vibrations, can be found in Table I. Also included in the spectrum are the 0-0 and 0-1 band systems of the ZnH A2n-X22+transition. Note that several rotational lines in the ZnH 0-0 band system have intensities which exceed the maximum shown in this spectrum.
FREQUENCY (cm.')
Figure 3. Laser excitation spectrum of CdPy. Assignments of bands arising from skeletal vibrations are given above the spectrum. More complete assignments, including those of bands due to ring vibrations, can be found in Table 111. In addition to bands due to CdPy, a number of rotational lines arising from CdH can be seen in the spectrum.
Dispersed Fluorescen_ce_Spectrum of ZnC,H,N (Pumping B - X 0; Band)
0
250
24:12:
24;12;
500
750
RELATIVE FREQUENCY (cm I )
Figure 2.. Dispersed fluorescence spectrum of ZnPy obtained by pumping the &X 0; transition at 22 253 cm-I. Contributions from scattered dye laser light have been avoided when recording this spectrum by using a detection gate opened -70 ns after the dye laser pulse. The resolution in this spectrum (fwhm), which is limited primarily by the monochromator used, is 1 1 cm-'.
-
metal pyrrolyls were prepared in a Broida oven. The Broida oven approach produces relatively hot molecules, and this seriously limits the spectral resolution attainable (typically 50 cm-' or so). In our work, we are able to avoid this problem by studying organometallic radicals in the ultracold environment of a supersonic jet expansion. This is achieved by using a laser vaporization/ photolysis technique developed in our laboratory to prepare the radicals.Iv6 With the dye laser system employed in the work described here, our resolution is in the 2-4-cm-l range for excitation spectra. With this resolution, we can readily resolve complex vibrational structure in our spectra. This is important because as will be seen later, there is extensive vibrational structure in the (5) Bopegedera, A. M.R. P.; Fernando, W. T. M.L.; Bernath, P. F. J . Phys. Chem. 1990, 94, 4476. (6) Ellis, A. M.;Robles, E. S. J.; Miller, T. A. J. Chem. Phys. 1991, 94, 1752.
Dispersed Fluorescence Spectrum of CdC,H,N (Pumping B - X 0;Band)
0
200
400
600
800
RELATIVE FREQUENCY (cm I)
F i p e 4. Dispersed fluorescence spectrum of CdPy obtained by pumping the B-3 0; transition at 22065 cm-I.
spectra of both ZnPy and CdPy. As we will show, from this structure we not only are able to determine and assign the frequencies of a number of vibrational modes of these molecules but can also gain some information on the bonding site of the metal atom. 11. Experimental Section Details of the experimental apparatus used to obtain laser excitation and dispersed fluorescence spectra of the monopyrrolyl derivatives of zinc and cadmium are given in paper 1 and in ref 6. The only information necessary here concerns the preparation of the metal pyrrolyl radicals. These were prepared in a pulsed supersonic expansion by laser vaporization of the appropriate metal and simultaneous photolysis of pyrrole (C4H4NH)using an excimer laser (at 248-nm wavelength). The pyrrole vapor was seeded into a helium expansion (typical stagnation pressure of 250 psig) by flowing helium over liquid pyrrole (99%, Aldrich), the latter being maintained at a temperature of -15 OC in a stainless steel sample reservoir.
3260 The Journal of Physical Chemistry, Vol. 96, No. 8,1992
TABLE 11: Dispersed Fluorescence by Pumping ZnPy 8-3 0: Band a t 22253 cm-' freq, cm-' re1 freq, cm-' assignment
TABLE I: Transition Frequencies (em-') for ZnPy Excitation Spectrum assignment freq" 21 733 s 21 994 m 22013 s 22253 vs 22217 m 22293 m 22412 s 22507 s 22531 vs 22560 w 22 513 s 22665 m 22 690 s 22730 m 22169 m 22188 s 22 808 vs 22822 w 22845 m 22886 w 22901 m 22 921 vw 22949 m 22966 s 23007 w 23042 w 23041 w 23063 m 23072 m 23087 s 23 108 w 23 126 m 23 164 w 23 178 m 23 188 s 23 222 m 23235 m 23319 w 23 334 w 23 346 m 23 361 23378 w 23398 w 23441 w 23453 m 23467 s 23 502 m 23621 w 23659 w 23675 w 23720 w 23139 m 23714 w
00,
A-%
Avb 0
24; 12; 24: 12624; 12;
26 I 280 520 544 560
B-ic
12f24; 12;
A2
0
131, 24; 12; 827 840
159 254 278 320 412 437 477 516 535 555
12a24;
1112
- - - - -
Robles et al.
569 592 633 648 614 696 713 754 789 794 810 819 834 855 873 91 1 925 935 969 982 1066 1081 1093 1 IO8 1 I25 1145 1 I88 1200 1214 1249 1368 1406 1422 1469 1486 1521
"To help the reader to identify particular bands in the excitation spectrum, we use the following labels after each transition frequency to indicate the approximate intensity of a band: vs = very strong, = strong, m = medium, w = weak, b ~ ~ e relative q ~to the ~ origin ~ c band.
111. Results and Discussion
Laser excitation and single vibronic level dispersed fluorescence spectra of ZnPy and CdPy radicals are displayed in Figures 1-4.
22 253 22 119 22015 21 996 21 879 21 863 21 770 21 754 21 740 21 520 21 500 21 477
0 134 238 257 374 390 483 499 513 733 753 776
Pump
13: 24; 12: 13y24q 12y13: 24: 12y248 12; 12y24: 12;24q 12;
The transition frequencies of all of the bands resolved in these spectra are collected in Tables I-IV. In the excitation spectrum shown in Figure 1, in addition to vibronic bands due to ZnPy, strong ZnH A211,/2-X2Z+ and A2n3/,-X2Z+0-0 band systems at -23 260 and -23 600 cm-I, respectively, together with weaker ZnH A211,/2-X2Z+and A2113/2-X2Z+0-1 hot-band systems at -21 800 and -22 100 cm-I, respectively, can also be seen. Likewise, the strong AZIIl/2yX2Z+0-0 band system of CdH is also present in the excitation spectrum of CdPy (see Figure 3), in addition to the numerous CdPy vibronic features. The rest of the paper is organized as follows. First we discuss in detail the vibrational structure of ZnPy and CdPy and among other things will attempt to draw some conclusions from this structure about the geometries of these molecules. This will be followed by a brief discussion concerning the electronic structure of these molecules. A. Vibrational Structure and Molecular Geometries. Before we consider the vibrational structure in the spectra of ZnPy and CdPy, we first make a few comments concerning the possible geometries of these molecules since this is central to the discussion encountered throughout much of this section. If we assume for the sake of convenience that the pyrrolyl ligand is planar in these molecules, then the key question we would like to answer about their geometries is where is the metal atom located? As mentioned earlier, it is useful, though not necessarily realistic, to classify the bonding in terms of one of two limiting cases: (i) The metal bonds solely to the nitrogen atom. This bonding scheme is referred to as q i or monohapto bonding in the organometallic literature.' We can think of the molecule in this limit as being essentially a substituted metal amide radical, with the metal atom a-bonded to the pyrrolyl radical. Indeed, semiempirical INDO calculations predict a ground-state symmetry of 2Al for the pyrrolyl radical with the unpaired electron localized on the nitrogen atom (a u radical),* thereby allowing the aromatic sextet to remain unperturbed. The symmetry elements for an ql-MPy molecule are illustrated schematically in Figure 5. As can be seen in the figure, in this limit the metal atom lies in the plane of the pyrrolyl ring yielding C , point group symmetry. (ii) The metal could be bound to the pyrrolyl ligand through interactions with the Ir-orbitals on the ring. This is the same bonding mode as occurs in zinc and cadmium cyclopentadienyls. Thus, the metal would be located approximately above the center of the ring and therefore the complex would have C, symmetry. This bonding mode, which is referred to as q5 or pentahapto bonding in the organometallic literature,' is also illustrated in Figure 3. Just asin the cyclopentadienyl radical, the unpaired in the pyrrolyl in this is On the *-ring system (a A radical). TJis 7 configuration gives rise to ~the fmt excited electronic state, A2Bi, Of the p)WOlYl radical which according to the INDO calculationss is only -0.3 eV above the (7) Haiduc. 1.; Zuckerman, J. J. Basic Organometallic Chemistry; Walter de Gruyter: Berlin. 1985. ( 8 ) Koenig, T.; Wielesek, R. A,; Huntington, J. G.Tetrahedron Lett. 1974, 2283.
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3261
Half-Sandwich Organometallic Free Radicals. 2 TABLE 111: Transition Frequencies (cm-') for CdPy Excitation
Spectrum assignment freq" 21675 m 21 900 m 21 928 w 22065 vs 22 I24 m 22 146 w 22216 s 22 287 vs 22 306 s 22343 m 22365 m 22440 s 22458 w 22 509 vs 22523 s 22550 w 22564 w 22582 w 22 590 m 22608 w 22629 m 22659 s 22672 m 22 704 m 22726 m 22739 m 22781 w 22795 w 22809 m 22850 m 22867 w 22874 w 22884 w 22928 m 22948 w 22959 w 22990 s 23070 w 23 108 w 23141 w 23 I53 w 23212 m
A-2
8-2
AS
AS
0
0: 12; 24;
225 253
12; 12b24;
449 47 1
0
13; 12; 24; 12; 12324;
12: 12i24;
12; 12:24;
668 690
151 222 24 1 300 375 393 444 458 485
TABLE IV: Dispersed Fluorescence by Pumping CdPy 5% 0: Band at 22065 em-' frea. cm-l re1 frea. cm-l assianmen t 0 22 065 121 21 944 20 1 21 864 222 21 843 323 21 742 399 21 666 417 21 648 521 21 544 539 21 526 595 21 470 610 21 455 640 21 425 785 21 280 802 21 263 827 21 238 C2" -
VS
889 907
1 IO6 1120
525 543 564 594 607 639 66 1 674 716 744 785 802 809 819 863 883 894 925 1005 1043 1076 1088 1I47
"To help the reader to identify particular bands in the excitation spectrum, we use the following labels after each transition frequency to indicate the approximate intensity of a band: vs = very strong, s = strong, m = medium, w = weak. bFrequency relative to the origin band.
%*A,state (the u configuration). Clearly, both bonding modes i and ii can be adopted by the MPy molecule. The above extremes epitomize the binary choice for the metal bonding sites, either in-plane or out-of-plane. In reality of course, some form of intermediate bonding between the two extremes mentioned above could also be adopted, i.e, the metal atom has substantial interactions with both the *-ring and orbitals located mainly on nitrogen. To achieve the necessary orbital overlap, the metal atom would have to be located not above the center of the pyrrolyl ring but instead must be displaced toward the nitrogen somewhat. It is important to recognize that for any intermediate bonding scheme, the point group symmetry of the molecule will be C,. Thus,C, symmetry can only be gained if the bonding mode is of an 7' type, since only in this limit will the metal atom lie in the plane of the pyrrolyl ligand. As we will see shortly, this
o x z icayz jD/y vda')
VIJ(b2)
M-ring bend
X
,*g*
.-..;
,.+-a
vZ4(a*)
VZ4(bl)
M-ring bend
Figure 5. Schematic illustration of the symmetry elements for 7' and q5 bonding in an MPy molecule and the symmetries of the skeletal vibrations in both cases. In this figure, the pyrrolyl ring is assumed to be planar. For q' bonding, the metal atom lies in the plane of the ring and is bonded directly to the N atom. This yield C, point group symmetry. For q5 bonding, the metal atom is located above the pyrroiyl ring and consequently, the symmetry is lowered to C,.
has some important consequences for the vibrational structure expected in the spectra of ZnPy and CdPy. Since C, symmetry is encountered only in the case of bonding, we will initially classify the vibrations of ZnPy and CdPy in terms of the irreducible representations of the C, point group (a' and a"). Where necessary, correlations between the irreducible representations of the C, and C, point groups are easy to establish. For C,point group symmetry, there are 24 distinct vibrational modes for a MPy molecule with no degeneracies. These modes are illustrated schematically in Figure 6 assuming an approximately $-bonding scheme for the molecule. In describing these vibrations, it is useful to draw on experience gained in our study of zinc and cadmium cyclopentadienyls in paper 1.' There, it proved useful to categorize the vibrations of the metal cyclopentadienyls into two types: (i) skeletal modes involving motion of the metal with respect to the ligand; (ii) intra-ring modes whose
Robles et al.
3262 The Journal of Physical Chemistry, Vol. 96, No. 8, 1992
+ ? + +
+
-
Q
0
Figure 6. Schematic representation of the 24 vibrational modes of MPy. The metal M is assumed to reside above the pyrrolyl ring at equilibrium in an $-bonding mode (corresponding to C,symmetry). Arrows represent in-plane stretching and bending motions, whereas +/- illustrates out-of-plane modes.
frequencies are relatively insensitive to the identity of the metal. The same categorization turns out to be useful for the metal pyrrolyls and will be employed here. The mode numbering system employed in Figure 6 and in Table VI1 follows the usual convention9 of grouping together vibrations of a given symmetry (starting with the highest symmetry) and for a specific symmetry the subscript number increases as the fundamental frequency decreases. 1 . Skeletal Vibrations. As can be seen in Figure 6, there are three skeletal modes for a ZnPy or CdPy radical, these being one metal-ligand stretch (v12) and two bending modes (v13 and ~24). Under C, point group symmetry, v12 and 1'13 are a' modes, while ~ 2 transforms 4 as an a" irreducible representation. Consequently, while significant progressions in vI2 and 1'13 may arise in the metal pyrrolyl spectra, no such progressions would be expected for ~ 2 4 because of the non-totally-symmetric nature of this mode. However, in the case of C2, symmetry, Le., when the metal is q' bonded, the only skeletal mode that is totally symmetric is the metal-ligand stretch. Hence, if C,, symmetry occurs for both electronic states involved in an electronic transition, no significant structure arising from metal-ligand bending vibrations would be expected in the spectra. As in the case of the metal cyclopentadienyl radicals, the frequencies of the skeletal vibrations are expected to be lower than those of the ring vibrations in the metal pyrrolyl radicals. Furthermore, unlike the ring modes, the frequencies of the skeletal modes will be strongly dependent on the identity of the metal. Using this information, we can readily identify bands due to skeletal vibrations in the spectra of ZnPy and CdPy. a. ZnPy. For the excitation spectrum of ZnPy, our assignment of bands arising from skeletal vibrations is shown above the spectrum in Figure 1, as well as in Table I. The first thing to notice in Figure 1 is the existence of two electronic band systgmt. The first has an origin at 21 733 cm-' and is identified as the A-X band system since no bands to the red of this region attributable to ZnPy have been observed. ,The second electronic origin appears 520 cm-' to the _blu_eof the A-X 0; band at 22 253 cm-' and is assigned as the B-X band system. Proof that the 22253-cm-l ban! is an electronic origin rather than a vibrational band in the A-X manifold comes from the absence of any trace of second or higher members in a vibrational progression with spacings of -520
Cm.: Given the intensity of the 22 253-cm-l band relative to the A-X 0; band, at the very least the second and third members of such a progression would be expected to be very prominent in the ZnPy spectrum. Pr_ogcessions_with intervals of
