J . Phys. Chem. 1990, 94, 6599-6603
6599
Photophysical Behavior of Doubly Bridged d7-d7Metal-Metal Bonded Compounds: The Crystal Structure and the Excited- and Ground-State Electronic Spectra of Re,(CO),(dmpm), [dmpm = Bis(dimethylphosphino)methane] Steven J . Milder,* Department of Chemistry, University of California, Santa Cruz, California 95064
Michael P. Castellani, Timothy J. R. Weakley, David R. Tyler,* Department of Chemistry, University of Oregon, Eugene, Oregon 97403
Vincent M. Miskowski, and A. E. Stiegman* Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 I09 (Received: November 1. 1989; In Final Form: February 22, 1990)
Re2(C0)6(dmpm)2shows photophysical behavior in a rigid medium that differs dramatically from that observed in fluid solution. In a hydrocarbon glass at 77 K, metal-metal bond homolysis is suppressed and an intense phosphorescence is observed. The transient absorption spectrum, which shows only weak transitions to the red of the ground state ' ( U + U * ) transition, permits assignment of the emitting state to a '(.-u*) transition. The crystal structure of Re,(CO),(dmpm), is also reported. The ground-state electronic structure is discussed relative to the structural data.
We recently reported that the compound Re2(C0)6(dmpm)2 [dmpm = bis(dimethylphosphino)methane] exhibits phosphorescence in rigid environments (e.g., in 2-methylpentane glass = 690 nm, T = 31 p ) . ' This behavior contrasts at 77 K, A,, sharply with the dissociative photochemistry characteristic of the parent decacarbony12 and has been attributed to the steric restriction imposed by the two bridging ligands in conjunction with the rigidity imposed by the glassed ~ o l v e n t . ' , ~These conditions prevent dissociation of the metal-metal bond that results from population of the metal-metal O* level in the excited state. Presumably other distortions of the complex are also hindered by the rigid environment. In the present report, we further characterize the ground state of the title molecule with a crystal structure and a detailed analysis of the electronic absorption,spectrum. We then present 77 K transient absorption studies that conclusively establish the emissive excited state as the metal-metal 3 ( ( r - ~ * ) state.
Experimental Section The compound Re,(CO),(d~npm)~ was prepared and purified by a literature p r ~ c e d u r e . The ~ solid compound is fairly stable in air but its solutions require handling under an inert atmosphere. Crystals suitable for X-ray diffraction were obtained as clear yellow prisms, which formed from the slow evaporation of a CH2C12/heptanesolution under an indirect stream of argon in the dark. Equipment used for the measurement of emission, emission excitation, and ground-state absorption spectra has been previously described. Transient difference spectra were obtained by using a standard pump/probe technique. The samples were excited with a 7 ns, 15 mJ, 355 nm pulse from a Quanta Ray DCR-2 Nd:YAG laser. The probe beam, generated by a quartz-jacketed xenon flash lamp, was focused into a Jarrell-Ash Monospec 27 polychromater and was dispersed across a PAR 1420 optical multichannel analyzer (OMA) that was gated open for I O ns by a PAR 1302 pulser. When the excited-state transient difference spectrum was obtained, the OMA was gated open beginning 50 ns after the laser pulse. ( I ) Stiegman, A. E.; Miskowski, V. M. J . Am. Chem. Soc., 1988, 110, 405 3. (2) Meyer, T. J.; Caspar, J . V. Chem. Rev. 1985, 85, 187. (3) Stiegman, A. E.; Miskowski, V. M.; Gray, H . B. J . Am. Chem. SOC. 1986, 108, 278 1. (4) King, R. B.; Raguveer, K. S. fnorg. Chem. 1984, 23, 2482.
0022-3654/90/2094-6599$02.50/0
TABLE I: Crystallographic Data for Rez(MezPCHzPMez)2(CO)6 chemical formula formula weight space group a
b C
P V Z
C16H2806P4Re2 Fm 812.7 amu T P2,ln
x
9.334 (2) 8, 15.290 (2) 8, 8.931 ( I ) A 90.079 (8)' 1274.6 (6) A' 2
d,,, fi
abs. corr. factor no. rflns (I t 3o(l)] R(F,) wR(FJ
764 23 'C 0.71069 A 2.12 g cm-3 98.9 cm-I 0.8 1-1.23 1744 0.029 0.036
The gate was moved to various times after the laser pulse to obtain the decay rate of the transient bleaching signal. The transient difference spectrum was obtained in two spectral regions, from 270 to 400 nm and from 400 to 670 nm. A Corning 0-5 1 filter (h(transmission) > 380 nm) was placed in the probe beam during determination of the spectrum from 400 to 670 nm to prevent artifacts from second-order light emerging from the spectrograph. For determination of the kinetics of the emission decay at 700 nm, the emission was focused into a Pacific Precision Instruments 0.45-111 monochrometer and wavelength selected. The resulting light was detected with a fast rise time (C2 ns) photomultiplier whose output was digitized and averaged by a Tektronix 79 12AD/404 I . The samples for the transient experiments were prepared as follows. Re2(C0)6(dmpm)2was dissolved in 3-methylpentane (Aldrich) and filtered through a coarse-fritted glass disk. The sample was placed in a brass cell with quartz windows that was placed in contact with a stainless-steel Dewar flask filled with liquid nitrogen. Data was only taken after complete glassing of the solvent. Crystallographic data are summarized in Table I and are given in more detail in the supplementary material. An air-stable yellow prism measuring 0.10 X 0.19 X 0.21 mm was attached to a glass fiber and mounted on a Rigaku AFC6R diffractometer. Approximate cell dimensions were obtained from the setting angles of 25 reflections in the 28 range of 11-16'. All cell angles were within 20 of 90', but the measured intensities of the -h,k,-1 and h,k,-l reflections corresponding to the above 25 reflections implied 2 / m rather than mmm Laue symmetry. The shell 30' I20 I 33' was then scanned in the +h,+k quadrant, and improved cell dimensions were obtained from the setting angles of 25 strong reflections. The intensities of O,k,l and O,k,-1 reflections in this shell were in good agreement, but those of general h,k,l and h,k,-1 reflections were not, consistent with monoclinic symmetry. This
0 1990 American Chemical Society
6600 The Journal of Physical Chemistry, Vol. 94, No. 1 7 , 1990 TABLE 11: Atomic Coordinates (XlO'; Re, Xlv) and Isotropic [C(41,51,61)] or Equivalent Isotropic Thermal Parameters (A2) atom X Y 2 BmC 55486 (2) 52917 (3) 4.10 (2) 36293 (3) 5.4 ( I ) 4493 (2) 7085 (3) 2886 (3) 3418 (2) 6479 (3) 5546 (3) 6 2 (1) 9.1 (5) 2792 (9) 4484 ( 5 ) 2168 (9) 9 . 2 (5) 5814 (9) 6660 (6) 985 (8) 9.4 ( 5 ) 7848 (8) 5246 (9) 6513 (6) 2740 ( 1 0) 4863 (6) 3730 ( I O ) 5.3 ( 5 ) 5577 ( I I ) 6.3 (5) 6239 (7) 1996 ( I O ) 5.9 (5) 6147 ( 6 ) 4699 ( I O ) 6885 ( I I ) 8701 (22) 4864 ( I 8 ) 1992 (29) 15 (2) 6597 (23) 3727 (18) 1480 (27) 14 (2) 7.2 (8) 7922 (14) 3830 (9) 4291 (18) 5739 (16) 2341 ( I O ) 5159 (28) 17 ( 1 ) 8020 ( 16) 3169 ( I O ) 6724 (20) 14 ( 1 ) 4312 (31) 7306 (54) 1074 ( 5 5 ) 7 (1) 6.0 (8) 8974 (41) 4746 (27) 3564 (46) 2 8 (4) 6793 (26) 3392 (17) 3554 (27) "Site occupancy factor = 0.75. bSite occupancy factor = 0.25. ' B , = ( 8 r 2 /3 ) E,x , U i , d i a *,ai-a,.
was also observed when the full data set was collected in the same quadrant of reciprocal space. The cell dimensions given in Table 1, with differing from 90' by 9a, were subsequently obtained from the refinement of 25 reflections in the range 43' I 20 I 49O. The only, but clearly defined, systematic absences were h01 for ( h I ) odd and OkO for k odd. The Patterson function readily gave the coordinates of the Re atom in PZ,/n. Other non-hydrogen atoms were located by the use of DIRDlF' and from difference synthesis. An empirical absorption correction ( D I F A B S ) ~was applied after the isotropic refinement of all non-hydrogen atoms. The large vibrational amplitudes of the carbon atoms of the dmpm ligands suggested that these atoms were disordered. Alternative positions [C(41,51,61)] for the methyl atoms C(4) and C(5) and for the methylene atom C(6) were given by a difference map. Satisfactory further refinement could be effected with site occupancy factors of 0.75 and 0.25 for the major and minor atomic positions. All non-hydrogen atoms except C(41,51,61) were allowed isotropic thermal parameters in the last cycles of full-matrix least-squares refinement converging at R = 0.029, and hydrogens were included at calculated positions on C(4,5,6,7,8). It should be noted that the structure solution and anisotropic refinement (to R = 0.037) were also carried out, without incorporation of atoms at alternative sites, using data corrected for absorption by the use of \k scans; a difference map at convergence showed more noise close to the Re atom and less well-defined alternative atomic positions. The TEXSAN' program suite, incorporating atomic scattering factors taken from ref 8, was used in all calculations.
+
Results and Discussion Crystal Structure of Re2(C0)6(dmpm)z. The crystal structure of the title compound was determined in order to facilitate the interpretation of the electronic structure of the ground and excited states. Atomic positional parameters and molecular dimensions are summarized in Tables 11 and 111 respectively, while Figure 1 shows the structure and the atom-labeling scheme (unlabeled atoms are related by an inversion center site symmetry). The molecule shows an obvious disorder as there are alternate positions for the carbon atoms in the dmpm bridge. The major and minor sites for the methylene atom, C(6) and C(61), are approximately related by reflection in the Re2P, plane; however, we have not been able to completely model the disorder in the remainder of the dmpm carbon atoms as is clear from the large thermal amplitudes. (5) Beurskens, P. TQ.Technical report 1984/ I , Crystallography Laboratory, Toernooiveld, 6525 Ed Nijmegen, Netherlands. (6) Walker, N.; Stuart, D. Acra Crystallogr. 1983, A39, 158. (7) TEXAN: Texray Program for Structural Analysis, Molecular Structures Corp.. College Station, TX, 1987. (8) Cromer, D. T.;Waber, J . T. International Tables for X-Ray Crystallography: Kynoch Press: Birmingham, England, 1974: Vol. IV, pp 71, 148.
Milder et al. TABLE 111 Re(])-Re(l*) 3.105 ( I ) Re(l)-P(I) 2.378 (2) Re(l)-P(2*) 2.365 (3) Re(l)-C(I) 1.932 (9) Re(l)-C(2) 1.87 ( I ) Re(l)-C(3) 1.96 ( I ) P(l)-C(4) 1.76 (2) Re(l*)-Re(l)-P(l) Re(l*)-Re(l)-P(2*) Re(]*)-Re(])-C(I) Re(l*)-Re(l)-C(2) Re(l*)-Re(l)-C(3) P(l)-Re(l)-P(Z*) P(I)-Re( 1)-C( I ) P(I)-Re(l)-C(2) P(I)-Re(l)-C(3) P(2*)-Re(l)-C(I) P(Z*)-Re(l)-C(2) P(2*)-Re(l)-C(3) C(I)-Re(l)-C(2) C(I)-Re(l)-C(3) C(2)-Re(l)-C(3) Re(l)-P(I)-C(4) Re(l)-P(I)-C(S) Re(I)-P(l)-C(6) Re(l)-P(l)-C(41) Re(I)-P(l)-C(51) Re(l)-P(I)-C(61)
Bond Lengths ( A ) n P(I)-C(5) 1.81 (2) P(I)-C(6) 1.82 ( I ) P(l)-C(41) 1.73 ( 5 ) P(I)-C(51) 1.84 (4) P(I)-C(61) 1.81 (3) P(2)-C(6) 1.85 ( I ) P(2)-C(7) 1.81 ( I )
P(2)-C(8) P(2)-C(61) O(I)-C(l) 0(2)-C(2) 0(3)-C(3)
Bond Angles (deg) 89.3 ( I ) C(4)-P(I)-C(5) 88.8 ( I ) C(4)-P(I)-C(6) 86.5 (2) C(S)-P(I)-C(6) 177.8 (2) C(4l)-P(l)-C(5l) 87.4 (2) C(4l)-P(l)-C(6l) 180.0 ( I ) C(5l)-P(l)-C(6l) 89.6 (3) Re(l)-P(Z*)-C(6) 93.0 (3) Re(l)-P(2*)-C(7) 88.7 (3) Re(l)-P(2*)-C(8) 91.1 (3) Re(l)-P(2*)-C(61) 88.9 (3) C(6)-P(2)-C(7) 90.4 (3) C(6)-P(2)-C(8) 93.2 (4) C(7)-P(2)-C(8) 173.7 (4) C(7)-P(2)-C(61) 93.0 (4) C(8)-P(2)-C(61) 118.2 (9) Re(1)-C(l)-O(l) 119.3 (6) Re(l)-C(2)-O(2) 116.4 (5) Re(l)-C(3)-O(3) I18 (2) P(l)-C(6)-P(2) 112 ( I ) P(l)-C(6I)-P(2) 115.6 (8)
1.80 ( I ) 1.88 (2) 1.15 ( I ) 1.16 ( I ) 1.15 ( I )
93 ( I ) 101 ( I ) IO5 (1) 105 (2)
102 (2) 102 (2) 116.4 (4) 115.4 (5) 117.0 ( 5 ) 115.9 (8) 115.9 (8) 85.8 (7) 102.0 (9) 81 (I) 118.9 (9) 177.0 (9) 177.3 (9) 175.9 (9) I 1 1 . 1 (6) i l 0 (I)
'Atoms with asterisks are at 1 - x, 1 - y, 1 - z . 2e2 (dmpm) 2 (COI 6
A
,-.
W
A
Figure 1. Molecular structure of Re2(CO),(dmpm)2.
The Re, P, and CO bond distances, with which we are primarily concerned, are well defined. The observed structure, which agrees with the one predicted on the basis of infrared and N M R spectroscopic methods: reveals a number of interesting aspects. The most striking of these is that the molecule is nearly perfectly eclipsed, which is in stark contrast to the staggered (approximately D4& structure of the parent molecule Rez(CO)lo.9 This clearly must result from the steric constraint of the two 1-dmpm ligands. (9) Churchill, M . R.; Amoh, K. N.; Wasserman, H. J. Inorg. Chem. 1981, 20, 1609.
Doubly Bridged d7-d7 Metal-Metal Bonded Compounds
The Journal of Physical Chemistry, Vol. 94, No. 17. 1990 6601
TABLE IV: Ground-State Electronic Absorption Spectral Features for Rez(CO),(dmpm)z in 2-Methylpentane Solution at Room Temperature and 77 K room temperature 77 K assignment '(da*da*) (d a+da*) '(da+da*) '(da+du*)
'
A,,
(nm) 400 342 311 289
fmax
Amax (nm)
emax
460 (sh) 12400 7650 5980
397 338 317 290
270 I6600 9800 5300
The ~ - ( d m p m ) ~unit M ~can accommodate a staggered structure if a "twist-boat" conformation of the M2P2C ring is adopted; however, a short metal-metal bond distance is required for this 250 300 350 400 450 configuration.IO For the molecule reported here, which possesses (nm) a "chair" structure in the M2P2Cbridge, the metal-metal bond Figure 2. Corrected emission excitation spectrum (monitoring emission This is, to the best of our knowledge, length is 3.105 (1) a t 700 nm) for Re2(C0)6(dmpm)z in 2-methylpentane glass a t 77 K. the longest metal-metal bond reported for a M2(dmpm)2unit. Spectral slit width is 2 nm. It is worth noting that the M-dmpm ligand, in the chair conformation, allows metal-metal distances of eclipsed compounds as short as 2.1253 (4) A, which is the bond length observed for 0'05 Mo2Cl,(dmpm),." Since the relief of ligand strain is likely to bias the M2 unit toward shorter metal-metal distances, its effect may be counterbalanced by repulsive interactions of the eclipsed equatorial CO's. However, we note that the Re-Re-C(equatorial) angles are acute, and their average angle of 87.0' is nearly identical with the 86.4' angle observed for Re2(C0)10.9This observation does not suggest much of a role for ligand-ligand repulsive interactions. The Re-P distances found here (-2.37 A) are longer than those found for the compound ~~~X-R~~(C~),(P(CH,)~(~~H~))~ (2.349 (5) A)12 but much shorter than those observed for Re2C14(dppm)2 The shorter distances (2.45 A)]' and Re2C16(dppm)2(2.44 for Re(0) compounds might reflect a n back-bonding Re-P I I I I -0.15 component in these bonds that is absent in the higher valent Re(I1) 200 300 400 500 600 700 and Re( 11) species. c ~ - R e ~ C l ~ ( d m p[dmpe e ) ~ is 1,2-bis(diWAVELENGTH (nm) methylphosphino)ethane], in which the dmpe ligands chelate individual Re(l1) atoms rather than bridge the Re2 bond, provides Figure 3. Transient difference spectrum for Re2(CO),(dmpm), in 3a counter example. In this molecule the Re-P bonds are much methylpentane solution a t 77 K. Excitation wavelength was 355 nm shorter, at 2.337 (2) 8,. Clearly, there are factors contributing (Nd:YAG 31d harmonic). The spectrum shown was taken in a 10-ns window beginning 50 ns after the peak of the laser pulse and represents to the Re-P bond length other than T back-bonding. 256 averages. The Re-C(0) bonds are all much shorter than those of Re2(CO),0,9particularly the axial ones, which are 1.929 (7) 8, for a significant contribution to the metal-metal bond (which is much the decacarbonyl compared to 1.87 (1) 8,for the title compound. too long to be a simple dpz-dzz single b ~ n d ) ' ~is, made ' ~ ~ by the However, the equatorial Re-C(0) bond lengths of the present n-back-bonding interaction of the filled u symmetry metal-metal structure are quite similar to those reported for diax-Re2orbital on one center with the empty A* orbitals of the equatorial (co),(P(cH3)2(c6H5)2).12 The observed shortening of the CO ligands of the other center. Thus, the substitution of phosphine metal-C(0) bond is consistent with what has been observed for for some of the equatorial C O ligands may have weakened the a number of phosphine and arsine derivatives of Mn2(C0),014 and metal-metal bond via an electronic mechanism involving a demay be explained in terms of an increased n-back-bonding increase in the metal-CO interaction. teraction of the remaining C O ligands with the metal centers of Ground-State Electronic Spectrum. The room-temperature partially phosphine substituted compounds, as phosphines are and 77 K electronic spectra of Re2(CO),(dmpm), have been stronger u donors but much poorer n acceptors than C 0 . 1 5 presented previously.' The considerable red shift and substantial The Re2 distance in Re2(CO)6(dmpm)2is significantly longer transition (A, decrease in intensity of the metal-metal '(e-.*) than that of Re2(CO),o,though, as noted earlier, it is not obvious = 342 nm ( t = 12400) at room temperature)' from that of that this effect is steric in origin. According to theoretical studies,I6 Re2(CO)lo(A, = 310 nm ( t = 16700))18is reasonable in view of its 0.06 8, longer Re2 bond. There are three weaker absorption features at A > 270 nm (10) This is observed for the molecule Re2Cl&-dppm), [dppm = bis(diphenylphosphino)methane)], which has a relatively short metal-metal bond (Table IV) that deserve comment. We attribute the bands at distance of 2.234 (3) A. Barder, T. J.; Cotton, F. A.; Dunbar, K. R.; Powell, -315 and -290 nm to transitions of the type dn-.du*, where G. L.; Schwotzer, W.: Walton, R. A. Inorg. Chem. 1985, 24, 2550. the generic identification "daw includes all the transitions from ( I I ) Cotton, F. A.; Falvello, L. R.; H a r w d , W. S.; Powell, G. L.; Walton, the levels derived from metal dxyyr,xrsymmetry orbitals. TranR. A. Inorg. Chem. 1986, 25, 3949. (12) Harris, G. W.; Boeyens, J. C. A.; Coville, H. J. J . Chem. Soc., Dalton sitions of this type would be expected to occur at energies higher Trans. 1985. 2277. than the '(u-.u*) transition for Re2(CO)lo,'sand they have not (13) Canich, J. A . M.; Cotton, F. A.; Daniels, L. M.; Lewis, D. B. Inorg. been located experimentally. For the title molecule, transitions Chem. 1987. 26. 4046. of this type are evidently observable because (a) '(o-u*) has been (14) Hoskins; B. F.; Steen, R. J. Aust. J . Chem. 1983, 36, 683 and ref-
L
erences therein. ( I 5) Baucroft. G.M.; Dignard-Bailey, L.; Puddephat, R. J. Inorg. Chem. 1986, 25, 3675. (16) (a) Brown. D. A.; Chambers, W. J.; Fitzpatrick, N. J.; Rawlinson, R. M . J . Chem. SOC.A 1971, 72. (b) Heijser, W.; Baerends, E. J.; Ros, P. J . Chem. SOC.Symp. Faraday SOC.1980, 14, 21 I . (c) Shaik, S.; Hoffman, R.; Fisel, C. R.; Summerville, R. H. J . Am. Chem. SOC.1980, 102, 4555.
(17) (a) Miskowski, V. M.; Gray, H. B. In Understanding Molecular Properties; Avery, J., Dahl, J. P., Eds.; Reidel Publishing Co: Dordrecht, Holland, 1987. (b) Stiegman, A. E.; Tyler, D. R. Acc. Chem. Res. 1984, 17, 61. (18) Levenson, R. A.; Gray, H. B. J . Am. Chem. SOC.1975, 97, 6042.
6602
The Journal of Physical Chemistry, Vol. 94. No. 17, I990
Milder et al.
2,'
I
I
I
I
I
--
0 20
40
60
Time
80
100
(US)
Figure 4. Comparison of the rate of decay of the 700-nm emission (-) and the magnitude of the transient bleaching (V)for Re2(C0)6(dmpm)2 in 3-methylpentane at 77 K after excitation at 355 nm.
bnth red-shifted and weakened in intensity; (b) decreased equatorial ?r back-bonding due to replacement of C O by trialkylphosphine will destabilize the d r levels, lowering the energy of the d~ -da* excited states; and (c) low symmetry may intensify thc d r + d u * transitions. Detailed assignments of these two bands zro precluded by the low symmetry of the title compound. rhe very weak band at -400 nm is noteworthy in that it seems to have no analogue in the reported electronic spectrum of Re2(CO)lo. This band is not due to an impurity because it turns up prominently in the emission excitation spectrum (Figure 2). We suggest that this weak band may be the singlet-triplet absorption corresponding to the lowest energy '(d?r-.u*) excited state, perhaps that which gives the 3 15-nm absorption shoulder. Since Re,(CO),o shows no analogous feature, an assignment to 3 ( ~ u * ) can be excluded. Excited-State Electronic Spectrum. Transient difference absorption spectra, taken 50 ns after excitation at 355 nm of the title compound in 3-methylpentane glass a t 77 K, are shown in Figure 3. The decay with time of the magnitude of the strong '(u.-+cr*) ground-state bleaching signal at 338 nm, which was obtained by moving the OMA detection gate to selected times after the laser fired, is shown in Figure 4. The values are scaled to the time course of the decay of the 700-nm emission obtained after laser excitation of the title compound at 355 nm under identical conditions. Clearly, the emission and transient absorption signals decay at the same rate, and it can be concluded that excitation of Re2(C0)6(dmpm)2in glassy 3-methylpentane at 77 K gives no transients in significant yield other than the emissive stat
Excited-state absorption spectra were generated by adding the ground-state absorption spectrum, measured with the same apparatus under identical conditions, to the transient difference absorption spectrum. The best "sum", shown in Figure 5, was judged to be that in which positive or negative extrema near the ground state '(G-u*) band were minimized; this data treatment allows us to estimate excited-state extinction coefficients. It is seen that there are two very weak but reproducible absorption maxima in the visible region, at 615 nm ( t = 270) and 485 nm ( c = 250). In addition there is an intense excited-state absorption i n the U V region, with a maximum at -280 nm ( t = 14000). The visible absorptions reported here are rather similar to those reported by Brown et aLzofor the shorter lived diradicals produced LJ? laser photolysis of singly bridged compounds such as Rez(CO),idmpe) (7 = 12 ns) in fluid solution and also to the one
__ (19j Laser excitation in fluid solution at room temperature also yields ground-state bleaching, but little or no decay of this signal is observed up to 10 ms after laser excitation. It is not clear whether this signal represents permanent photochemistry or a very long-lived transient such as a n open diradical. (20) Lee, K.-W.; Hanckel, J . M.; Brown, T. L. J . Am. Chem. SOC.1986, i???. 2266
200
I
300
500
400
600
700
WAVELENGTH (nm)
Figure 5. Excited-state absorption spectrum of Re2(C0)6(dmpm)2in 3-methylpentaneat 77 K. The spectrum was obtained by using the data i n Figure 3 and correcting for ground-state bleaching; see text.
OW2)
nW2)
~
~
Figure 6. Molecular orbital diagrams showing low-energy electronic transitions for )(dp+da*) and '(da4da*) excited states of a d7-d7 M,(CO),, derivative.
recently reported for the stable, isolable, radical Re(CO),(P(C6H,1)3)2.2' The similarity of these spectra might suggest that the correct description of the transient is that of a diradical. However, the observation of emission in the visible region of the spectrum indicates that some discrete excited state must be involved. Unbridged compounds such as Re2(CO),o and diaxRe2(C0)8(P(C6H5)3)2 do not exhibit luminescence under the same conditions in which it is observed for the title compound;' therefore it seems unlikely that the luminescence emanates from an excited state generated by radical recombination. It appears that the transient spectrum may resemble that of isolated radicals simply because the excited state correlates to diradical dissociation products," and the observed excitations may be monomer-like in this excited state, even when ligand/environment constraints prevent dissociation. This situation would not be true for other types of excited states, and more detailed examination provides other reasons for an assignment of the emissive state to 3(u-+u*). As noted in our communication,' the plausible candidates for an emissive excited state were two types of triplets states. First, the excited state might , K represents any of the metal be of the type 3 ( ~ + a * )where dxlyrXyderived levels. Second, the excited state might be of the type 3 ( ~ u * )This . state is "special" in that, as explained in detail splitting is expected to be elsewhere,17the singlet-triplet (--a*) extremely large, and 3 ( u - + ~ * )is completely dissociative in the absence of strapping ligands. Figure 6 shows MO diagrams corresponding to these two situations, together with low-lying excited-state electronic transitions. Both types of excited states should show relatively low-energy, intense metal-to-ligand charge-transfer transitions of the type n*(Re,)-?r*(CO), and this is the likely assignment of the observed intense transient absorption below X = 400 nm. Both types of excited states can also show weak transitions of the type ?r(Re2)+u*(Re2),?r(Re2)-u(Re2) (21) Crocker, L. S.; Heinekey, 1989. 1 1 1 , 405.
D.M.; Schulte, G. K . J . Am. Chem. Sm.
J . Phys. Chem. 1990, 94, 6603-6607
and/or r’(Re2)+r(Re2), and such assignments are reasonable for the weak transient absorptions in the visible region. A clear distinction between the two types of excited states only arises when we consider o-+u* transitions. The PO* transition of the 3(0-u*) excited state is spin forbidden and therefore cannot give rise to an intense transient absorption band. On the other hand, the ’(n+a*)-type states have the ( O ) ~ ( U * ) ’ configuration; thus there should be a spin-allowed u+u* transition for these excited states. Note that this type of excited state retains a metal-metal bond order of I/,, which means that it will be bound regardless of ligand constraints. The energy and intensity of the excited-state PO* transition can be predicted by comparison with isoelectronic ground-state molecules that have been reduced by one electron, yielding the same ( O ) ~ ( U * ) ’configuration. While one-electron reduction of Re2(CO)loand its phosphine derivatives has been accomplished by low-temperature radiolysis and the products have been characterized by ESR methods,22no reports of the optical absorption spectra have appeared. However, the d7-d7compounds of the type Rh2(1,3-diisocyanopropane),(L):+, where L is a neutral or anionic axial ligand, have ground-state absorption spectra that are very similar to those of the d7-d7 M2(C0)’,, compounds.23 Spectral data are available for the one-electron reduced dinuclear Rh compounds.24 These ( O ) ~ ( U * ) ~ radicals show a very intense PO* transition at 450-500 nm that is red-shifted, as might be expected, from those of their present excited states to show d7-d7 dimers. We would expect 3(~--w*) intense u+u* transitions at similar wavelengths. Since the observed transient spectrum actually shows only very weak absorptions to the red of the ground state ‘ ( P O * ) transition (Figure (22) Symons, M. C. R.; Wyatt, J.; Peake, B. M.; Simpson, J.; Robinson, B. H. J . Chem. Soc., Dalton Trans. 1982, 2037. (23) Miskowski, V. M.; Smith, T. P.; Loehr, T. M.; Gray, H . B. J. Am. Chem. SOC.1985, 107, 7925. (24) (a) Miskowski, V. M.; Sigal, 1. S.; Mann, K. R.; Gray, H. B.; Milder, S. J.; Hammond, G . S.; Ryason, P. R. J . Am. Chem. SOC.1979, 101, 4383. (b) Milder, S. J.; Goldbeck, R. A.; Kliger, D. S.; Gray, H. B. J . Am. Chem. SOC.1980, 102,6761. (c) Boyd, D. C.; Matsch, P. A.; Mixa, M. M.; Mann, K. R. Inorg. Chem. 1986, / 2 5 , 3331.
6603
S), we conclude that the transient excited state must be the 3 ( ~ + u * ) state.
Energy ofthe 3 ( ~ + u * State. ) It is the 3 ( ~ u *state ) of d7-d7 metal dimers that correlates electronically to the 17-electron radical fragments that result from metal-metal bond homolysis.17 Consequently, it is the population of this triplet state that gives rise to much of the photochemistry observed for this class of metal dimers. Unfortunately, the associated singlet-triplet absorption band appears to be very weak and has never been observed in the absorption spectrum of the decacarbonyls (Mn2(CO)loor Re2(CO),,) nor is it present in the absorption or excitation spectrum of the title compound. However, the low-temperature emission (A, = 700 nm) does provide a lower limit for the energy of the relaxed 3(o+a*) state of this compound from the wavelength, -600 nm, at which the emission becomes vanishingly weak. This lower limit agrees fairly well with those derived from semiempirical calc~lations.l~~ The only other experimental bound ) from the work of Poe and coon the energy of 3 ( a + ~ *comes workers, who reported that the emissive triplet state of biacetyl (relaxed energy 19 700 cm-’ (508 nm)) was quenched by Mn2(CO),, and Re2(C0)8(P(C6H5)3)2,yielding the normal photoproducts of metal-metal dissociation. This gives an approximate ) for these comupper bound for the energy of the 3 ( ~ a *state pounds at -510 nm, which is similar to the value we estimated for Re2(CO),( dmpm),. Acknowledgment. We would like to thank Dr. G. G. Christoph for helpful discussions. D.R.T. acknowledges the National Science Foundation for support of this research. Some of the research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Supplementary Material Available: Full crystallographic details for Rez(Me2PCH2PMe2)2(CO)6 (5 pages); observed and calculated structure factors for Re2(Me2PCH2PMe2)2(CO)6 (1 2 pages). Ordering information is given on any current masthead page.
Raman Matrix Isolation Spectroscopy of Hydrogen. 4. Rotational and Vibrational Spectra of Monomeric Species and Aggregation Processes in Solid Nitrogen M. E. Alikhani and J. P. Perchard* Laboratoire de Spectrochimie MolZculaire (AssociP au CNRS, UA SOS), UniversitZ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France (Received: November 20, 1989; In Final Form: March 5, 1990)
The rotational and vibrational Raman spectra of H2, D2,and HD trapped in solid nitrogen have been recorded at 9 K. After deposition, monomeric species are identified by either one Q(0) (HD) or two Q(J), J = 0 and 1, (H2, D2)lines. The corresponding So(J) rotational transitions display a doublet pattern with a splitting ranging from 22 to 26 cm-I. After annealing at about 25 K, totally different spectra are observed. In the Q(J)region the new features behave as those obtained in the same conditions in rare gas matrices while the S o ( J ) transitions are identified as single but broad lines with frequencies close to that of the free molecules and line widths strongly decreasing upon increase in temperature. These results suggest that hydrogen, because of its mobility and its low solubility in N2, migrates in annealing conditions and forms microcrystals with spectral properties comparable to those observed for the crystalline phase.
Introduction
As a first step in matrix studies devoted to the reactivity of dihydrogen with respect to small molecules (N2,CO, C2H2)in the presence of metal atoms, we have examined in detail the Raman spectra of H2, HD, and D2 trapped in inert matrices. In three previous papers (refs I , 2, and 3, referred to as I, 11, and (1) Alikhani, M. E.; Silvi, E.; Perchard, J . P.; Chandrasekharan, V. J . Chem. Phys. 1989, 90, 5221.
0022-3654190f 2094-6603$02.50/0
111) we have examined the spectral responses of monomeric and polymeric dihydrogen species embedded in solid rare gases. The purpose of this paper is to extend the spectral analysis to the case of N z matrices, with two main goals: on the one hand to get the (2) Alikhani, M. E.; Manceron, L.; Perchard, J . P. J. Chem. Phys. 1990, 92, 22. ( 3 ) Alikhani, M. E.; Manceron, L.; Perchard, J. P. Chem. Phys. 1990,140, 51.
0 1990 American Chemical Society
