Metal-to-Ligand Charge Transfer (MLCT) Photochemistry of fac-Mn(Cl

Sep 19, 1996 - ... does not occur readily, does not give rise to structural rearrangement and is predicted to produce the fac complex upon back-reacti...
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15346

J. Phys. Chem. 1996, 100, 15346-15357

Metal-to-Ligand Charge Transfer (MLCT) Photochemistry of fac-Mn(Cl)(CO)3(H-DAB): A Density Functional Study Angela Rosa,† Giampaolo Ricciardi,† Evert Jan Baerends,*,‡ and Derk J. Stufkens§ Dipartimento di Chimica, UniVersita` della Basilicata, Via N. Sauro, 85, 85100 Potenza, Italy, Afdeling Theoretische Chemie, Vrije UniVersiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, and Anorganisch Chemisch Laboratorium, J. H. Van’t Hoff Research Institute, Nieuwe Achtergracht 166, UniVersiteit Van Amsterdam, 1018 WV Amsterdam, The Netherlands ReceiVed: February 12, 1996X

The title compound has low-energy Mn-3d to 1,4-diaza-1,3-butadiene (H-DAB) π* metal-to-ligand charge transfer (MLCT) excited states, which are not, by their electronic nature, Mn-CO dissociative. Their potential energy curves (PEC) exhibit Mn-COeq and Mn-COax bonding minima around Re. Loss of an equatorial CO ligand upon MLCT excitation is explained by a radiationless transition from the MLCT states to the dissociative continuum of the electronic ground state. According to the calculated PEC of the ground state, the complex will undergo a strong structural rearrangement upon equatorial CO dissociation, during which the chloride shifts to the equatorial open site. This rearrangement explains the experimentally found formation of mer-Mn(Cl)(CO)3(R-diimine) complexes upon back-reaction of their CO-loss product with CO. This mechanism of equatorial CO dissociation is very different from the usual photochemical dissociation directly from a dissociative ligand-field state or through crossing of the photoactive excited state by such a ligandfield state. In contrast, axial CO dissociation, which does not occur readily, does not give rise to structural rearrangement and is predicted to produce the fac complex upon back-reaction with CO.

1. Introduction

SCHEME 1

In recent years the photochemistry and the photophysics of complexes with low-lying metal-to-ligand charge transfer (MLCT) states have attracted considerable interest. The fact that excitation does not take place to a metal-ligand antibonding orbital, as is the case for ligand-field (LF) transitions, but to a virtual orbital that is (almost) completely located on a ligand leads to a long lifetime. Due to their long lifetimes, the MLCT states are well suited indeed for energy and electron transfer processes.1,2 However, MLCT states are not always unreactive. Recent experimental work demonstrated the photoreactivity of complexes containing an R-diimine ligand, i.e., M(CO)4(Rdiimine) (M ) Cr, Mo, W),3 M′(CO)3(R-diimine) (M′ ) Fe, Ru),4 Ni(CO)2(R-diimine),5 Mn(X)(CO)3(R-diimine) (X ) halide),6 M(R)(CO)3(R-diimine) (M ) Mn, Re; R ) methyl, ethyl, benzyl),7 and Ru(X)(R)(CO)2(R-diimine) (X ) halide, R ) isopropyl).8 In these complexes irradiation into the low-lying MLCT bands, which belong to transitions from the metal to the lowest π* orbital of the R-diimine, may give rise to homolysis of the M-X or M-R bond or photodissociation of a carbonyl ligand. An example of such remarkable photoreactivity is provided by the complexes Mn(X)(CO)3(bpy) (X ) halide, bpy ) 2,2′-bipyridine).6a Upon irradiation into the lowest energy MLCT band, the fac-Mn(X)(CO)3(bpy) complexes undergo release of a carbonyl ligand, as was established by flash photolysis and low-temperature IR spectroscopy. Irradiation of fac-Mn(X)(CO)3(bpy) did not lead to loss of the halide. According to the reaction sequence of Scheme 1, the product of this CO-loss reaction reacts back with CO, thermally and photochemically, to give exclusively the mer-Mn(X)(CO)3(bpy) isomer, which in turn transforms thermally into the fac isomer or photodecomposes into radicals X• and [Mn(CO)3(bpy)]•. The same reaction has been observed recently for the †

Universita` della Basilicata. Vrije Universiteit. § Universiteit van Amsterdam. X Abstract published in AdVance ACS Abstracts, September 1, 1996. ‡

S0022-3654(96)00425-X CCC: $12.00

related R-diimine complexes fac-Mn(X)(CO)3(iPr-PyCa) (iPrPyCa ) pyridine-2-carbaldehyde N-isopropylimine),6c while the complexes fac-Mn(X)(CO)3(iPr-DAB) (iPr-DAB ) N,N′-diisopropyl-1,4-diaza-1,3-butadiene) produced, in addition to merMn(X)(CO)3(iPr-DAB), a small amount of the fac isomer after irradiation at low temperature.6c In spite of the intense experimental work, the mechanism of the photodecomposition of the Mn(X)(CO)3(R-diimine) complexes is far from being understood. In the first place there is the fundamental question of the role of MLCT states in the observed photoreactivity. Given the usual photoactivity of LF states, it has been suggested6a that the CO-loss reaction takes place from a close-lying reactive LF state, thermally occupied after excitation into the MLCT state. The only theoretical work to date is recent CASSCF/CI calculations9 of the ground and excited states of the fac-Mn(H)(CO)3(H-DAB) complex. These © 1996 American Chemical Society

MLCT Photochemistry of fac-Mn(Cl)(CO)3(H-DAB) calculations show the MLCT states to have a clear minimum in the Mn-H coordinate, but for larger Mn-H distances they are crossed by dissociative LF states. An MLCT state that is dissociative for axial CO dissociation has also been identified9b in fac-Mn(H)(CO)3(H-DAB). Second, there are questions concerning the photochemistry of fac-Mn(X)(CO)3(R-diimine) specifically. One wonders whether axial or equatorial CO labilization occurs upon photoexcitation of fac-Mn(X)(CO)3(R-diimine) and why backreaction of the product following Mn-CO bond cleavage with CO only affords the mer isomer in the case of the bpy and iPr-PyCa complexes. It is also intriguing that no fac-X dissociation occurs, particularly since the primary photoprocess of the mer-Mn(Cl)(CO)3(bpy) isomer is characterized by X• loss (see Scheme 1) and formation of the 16-electron radical complexes [Mn(CO)3(bpy)]• which were identified at room temperature by ESR spectroscopy and which dimerize to give Mn2(CO)6(bpy)2.6 It would be difficult to answer the preceding questions and rationalize the photochemical behavior of these systems in the absence of explicit calculations of potential energy surfaces (PES) or the more readily visualized potential energy curves (PEC) along one dissociation coordinate. Such an approach has already proven its usefulness for the understanding of the photochemistry of organometallic systems.10,11b In this paper we will address some of the issues raised here. In particular we investigate how and why CO loss occurs and why this is accompanied by facfmer isomerization. To this end we have calculated, by using a density functional approach, the ground and lowest excited state potential energy curves (PEC) corresponding to the dissociation of an axial as well as an equatorial carbonyl ligand of the model complex fac-Mn(Cl)(CO)3(HDAB) (H-DAB ) 1,4-diaza-1,3-butadiene, HNdCHCHdNH). 2. Methods and Computational Details The density functional calculations reported in this paper have been carried out with the Amsterdam density functional (ADF) program package.12,13 The computational scheme is characterized by a density fitting procedure to obtain the Coulomb potential12 and by sophisticated 3D numerical integration techniques13 for the evaluation of the Hamiltonian matrix elements, including those of the exchange-correlation potential. The molecular orbitals were expanded in an uncontracted double-ζ STO basis set for the C, N, O, Cl, and H atoms with one 3d and one 2p set added, respectively, on Cl and on H. For Mn a triple-ζ 3d and double-ζ 3s, 3p, and 4s were used, augmented with one 4p STO. The cores (C, N, O: 1s; Mn, Cl: 1s2p) have been kept frozen. The Vosko-Wilk-Nusair parametrization14 of the electron gas data has been used for the local density approximation (LDA) for the exchange-correlation energy and potential. The energies of ground and excited states include Becke’s15 nonlocal corrections to the LDA exchange energy and Perdew’s16 nonlocal corrections to the LDA correlation energy. The geometry of the model complex fac-Mn(Cl)(CO)3(HDAB) (H-DAB ) 1,4-diaza-1,3-butadiene) was optimized in Cs symmetry (Figure 1), which has been established experimentally for fac-Mn(Cl)(CO)3(Ph-DAB) (Ph ) phenyl).17 The substantial agreement between theoretical and experimental geometries justifies the replacement of the bulky substituents on the R-diimine ligand by hydrogen atoms. Potential energy curves were computed for the reaction paths corresponding to Mn-COax and Mn-COeq bond breaking, with the following assumptions: (i) Cs symmetry is retained along the reaction path corresponding to Mn-COax bond elongation. Support for this assumption comes from the fact that full

J. Phys. Chem., Vol. 100, No. 38, 1996 15347

Figure 1. Optimized structure of fac-Mn(Cl)(CO)3(H-DAB) in Cs symmetry (bond distances in angstroms).

Figure 2. Contour plots of fac-Mn(Cl)(CO)3(H-DAB) orbitals in the xz plane (the orientation of the molecule is as in Figure 1): (a) 20a′ Mn-Cl σ bonding orbital; (b) 25a′ Mn-Cl σ antibonding orbital. Contour values are 0.0, ( 0.02, ( 0.1, ( 0.2, ( 0.5 [e/bohr3]1/2.

geometry optimization of the primary product Mn(Cl)(CO)2(H-DAB) gives the Cs symmetry as the most stable one (Vide infra). All geometrical parameters, except for the dissociating bond, were kept constant along the reaction pathway. This choice is justified by the fact that the frozen geometry of the five-coordinated fragment derived from fac-Mn(Cl)(CO)3(HDAB) with the axial carbonyl missing does not show significant differences with respect to the optimized one. (ii) In contrast, for the dissociation of an equatorial CO, all geometrical parameters, except for the dissociating bond, were optimized for all points on the ground state as well as those on the two lowest triplet state potential energy curves, according to the C1 symmetry assumed by the system as soon as the equatorial CO moves away. The choice to allow the system to fully relax along the reaction pathway was inspired by the fact that the optimal geometry of the five-coordinated fragment following CO loss shows remarkable rearrangements with respect to the frozen geometry. Furthermore, the dissociation process is believed to be slow in this case (Vide infra), which would allow the system to relax during CO departure. Geometry optimizations have all been performed at the LSDA level of theory by using gradient techniques.18 Excitation energies and singlet/triplet splittings have been computed according to the method of Ziegler et al.19 It is wellknown that excited states have a different status within DFT than the ground state. Gross and co-workers20 have formulated a density functional theory for excited states. However, the functionals that are defined are not known, particularly not the

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TABLE 1: One-Electron Energies and Percentage Composition (Based on Mulliken Population Analysis per MO) of the Lowest Unoccupied and Highest Occupied Orbitals of fac-Mn(Cl)(CO)3(H-DAB) in terms of Mn, COeq, COax, Cl, and H-DAB Fragmentsa MO

e(eV)

orbital character

28a′ 18a′′ 17a′′ 16a′′ 27a′ 26a′ 25a′ 15a′′ 24a′

-1.60 -1.67 -1.79 -2.09 -2.11 -2.45 -2.84 -2.97 -4.97

eg′(σ*′) eg′

eg(σ*) eg H-DAB π*

Unoccupied Orbitals 5(5σ); 36(2π*⊥) 19(5σ); 30(2π*|) 47(2π*⊥); 18(2π*|) 21(dxz) 29(2π*⊥); 20(2π*|) 17(dx2-y2) 51(2π*|); 18(2π*⊥) 21(dyz); 5(dx2-y2) 2(5σ); 22(2π*⊥); 13(2π*|) 36(dz2); 4(4pz) 2(5σ); 16(2π*⊥); 4(2π*|) 26(dxy); 5(4px); 5(dxz) 4(5s); 28(2π*|) 9(dyz) 6(2π*⊥)

14a′′ 23a′ 22a′ 13a′′ 21a′ 20a′

-5.99 6.05 -6.83 -7.11 -7.34 -7.96

Cl-pπ-dπ Cl-pπ-dπ t2g(dx2-y2) t2g (dπ + Cl-pπ) t2g (dπ + Cl-pπ) Cl-pσ

Occupied Orbitals 28(dxz) 5(2π*⊥) 16(dyz); 3(dx2-y2) 68(dx2-y2); 2(4py) 27(2p*|) 41(dxz) 13(2π*⊥) 45(dyz) 6(2π*⊥) 8(dz2); 7(4pz); 2(dyz) 7(2π*⊥)

a

Mn

COeq

16(dz2) 31(dxy)

COax 9(5σ); 34(2π*) 11(2π*) 18(2π*) 30(2π*) 10(2π*) 30(2π*) 6(5σ); 10(2π*) 16(2π*)

6(2π*) 5(2π*)

Cl

H-DAB

9(6a′′) 17(6a′′) 4(8a′) 5(8a′) 8(6a′) 15(5a′′) 78(7a′)

2(3pz) 14(3pz) 1(3px) 7(3pz) 61(3px) 70(3py) 3(3py) 34(3px) 25(3py) 69(3pz)

5(2π*) 9(2π*) 4(5σ)

6(7a′) 7(4a′′) 15(7a′) 3(7a′)

Orbital characters eg and t2g refer to the pseudooctahedral symmetry.

dependence on the ensemble weights that play a role in the theory. Application of just the existing functionals in this approach leads to rather poor agreement with experiment.21 On the other hand, the method of ref 19 has been shown to lead to quite good excitation energies and multiplet splittings for many molecules, including transition-metal complexes.19,22 It has been applied to multiplet splittings in atoms by von Barth,23 Lannoo, Baraff, and Schlu¨ter,24 and Wood.25 It should be pointed out that, for the lowest state of a given symmetry, the Kohn-Sham single-determinant treatment, which is the basis of the current method,19 has the same status as for ground states.26 This also implies that in cases where the exact wave function can only be described correctly by invoking configuration interaction, as is the case at and close to avoided crossings, the DFT energies from the single-determinant Kohn-Sham calculations are still reliable (although of course the KS determinant is not a good approximation to the wave function then). There is now considerable evidence that the present gradient-corrected functionals, like the Becke-Perdew one we are using, yield reasonably accurate transition state barriers on the ground state potential energy surface during a reaction.27 This would then be expected to hold true for an avoided crossing-type barrier on the first excited state surface of a given symmetry as well. The validity of DFT for calculating a potential energy surface for the first excited state of a given symmetry of a polyatomic molecule, where a DFT treatment is formally justified, has been recently demonstrated in the case of the photodissociation of H2O.28 In practice one has to resort to approximations, such as the generalized gradient approximation for the exchange-correlation functionals, and to the method for calculating multiplet splittings of ref 19. The use of a DFT treatment, in conjunction with these approximations, has been successfully extended to the calculation of higher excited state potential energy surfaces to model the photodissociation of Mn2(CO)10.11b We are aware, however, that the energies of the excited states that are not the lowest of a given symmetry are reliable only at a semiquantitative level. As for the present case, we will keep in mind the possibly larger errors for those excited state energies and will avoid overinterpreting them. 3. Ground State Electronic Structure As the nature of the excited states that account for the photochemical behavior of the fac-Mn(X)(CO)3(R-diimine)

TABLE 2: Percentage Contribution of Individual Atoms to the Lowest Unoccupied and Highest Occupied H-DAB Orbitals (Based on Mulliken Population Analysis per MO) MO

character

N

6a′′ 7a′

π antibonding π antibonding

Unoccupied 32(2pz) 65(2pz)

5a′′ 6a′ 4a′′ 5a′

N lone pair N lone pair π bonding π bonding

Occupied 17(2s); 49(2py); 21 (2px) 16(2s); 34(2py); 29(2px) 69(2pz) 36(2pz)

C 68(2pz) 35(2pz) 4(2px) 5(2px) 31(2pz) 64(2pz)

H

9(1s) 16(1s)

complexes depends on the character of the MOs involved in the excitations, we will give here a comprehensive description of the ground state MO composition of fac-Mn(Cl)(CO)3(HDAB). Some aspects of the electronic structure of the analogous fac-Mn(X)(CO)3(bpy) (X ) Cl, I) complexes have already been investigated6b in connection with their spectroscopic behavior in the UV/vis region. However, the π* system of the aromatic bpy ligand is not the same as that of an aliphatic R-diimine ligand such as H-DAB, and the results of this previous study cannot be applied immediately to the present fac-Mn(Cl)(CO)3(H-DAB). The composition of the highest occupied and lowest unoccupied MOs of fac-Mn(Cl)(CO)3(H-DAB) is given in Table 1, in terms of Mn, Cl, COax, COeq, and H-DAB fragment orbitals. The composition of the valence molecular orbitals of the H-DAB ligand is given separately in Table 2. In the level pattern of fac-Mn(Cl)(CO)3(H-DAB) one easily recognizes the pseudooctahedral environment of the 3d6 Mn, leading to a set of occupied “t2g” orbitals: 22a′ (dx2-y2), 13a′′ (dxz), and 21a′ (dyz), stabilized by backbonding with the CO 2π* orbitals (note the choice of axes in Figure 1). The occupied Cl pπ orbitals lie ca. 1 eV above the 3d-t2g orbitals. They are, of course, pushed up by antibonding with the t2g-dπ orbitals, forming the two nearly degenerate HOMOs (14a′′ and 23a′) both having predominantly (60-70%) halogen 3pπ lone-pair character. The t2g-dπ orbitals in turn are stabilized with respect to the unperturbed dx2-y2 22a′ by bonding interaction with the Cl pπ orbitals, forming the mostly dπ 13a′′ and 21a′. The lowest orbital in Table 1 is the Cl pz, which is, in contrast with the Cl pπ orbitals, downward shifted by a bonding σ interaction with the empty metal 3dz2 and 4pz orbitals. The virtual orbitals are particularly relevant in view of the

MLCT Photochemistry of fac-Mn(Cl)(CO)3(H-DAB)

Figure 3. Contour plots of fac-Mn(Cl)(CO)3(H-DAB) orbitals: (a) 28a′ Mn-COax σ antibonding orbital (xz plane); (b) 18a′′ Mn-COeq σ antibonding (xy plane). Contours: see caption to Figure 2.

photochemistry. We first note that the LUMO 24a′ lies only 1 eV above the HOMO and has mainly H-DAB π* character, with the H-DAB 7a′ participating for no less than 78% in this orbital. MLCT (or XLCT) transitions to this orbital will be the lowest excitations. Unlike in fac-Mn(Cl)(CO)3(bpy), where the LUMO, which also has mainly bpy π* character, is followed by two bpy π*, in fac-Mn(Cl)(CO)3(H-DAB) none of the other virtual MOs listed in Table 1 shows much H-DAB π* character. The next lowest H-DAB π* orbital, the 6a′′, lies, in fact, at quite high energy. In fac-Mn(Cl)(CO)3(H-DAB) quite a large energy gap (∼2 eV) separates the LUMO from the set of the remaining virtual orbitals of CO 2π* and Mn dxy,dz2 (“eg”) character, which extend over a range of only about 1.3 eV. We will focus our attention on the dxy, dz2-based ones, given the important role they may play in the photochemistry. It is interesting to observe that there is not a single eg set in the virtual spectrum, but the dxy,dz2 orbitals are divided over two sets, the lower 15a′′ and 25a′ and the higher 18a′′ and 28a′. We have observed this phenomenon before for the Mn(CO)5• radical and Mn2(CO)10, where it can only happen for dz2 for symmetry reasons, and we have analyzed in detail11 how this effect originates from the orbital interactions between equatorial CO 2π*, Mn 4p, Mn 3d-eg, and CO 5σ. The argument given in ref 11 for dz2 relies on the strong interaction between 4pz and 2π* of equatorial COs (perpendicular to the equatorial plane, 2π*⊥). It is also applicable to the dxy in this case, where now, due to the further lowering in symmetry, interaction exists between 4px and the in-plane 2π* orbitals of the equatorial COs (2π*|). An important result of the orbital interactions is that the lower eg set (15a′′ and 25a′) has little antibonding with the equatorial and axial CO lone pairs 5σ, respectively, whereas the higher set (18a′′ and 28a′, denoted eg′) on the contrary has strong antibonding with these CO lone pairs. The relatively high percentages of CO 5σ character in these higher orbitals (19% and 9%, respectively) already point to this strong antibonding mixing of CO 5σ, particularly when one takes into account that the percentages, being based on Mulliken gross populations per orbital, are down-scaled by the incorporation of the negative overlap populations in these numbers. The orbital expansion coefficients are even more revealing: 28a′, 0.5231dz2 - 0.5603 5σ-COax Versus 25a′, 0.6536dz2 - 0.2146 5σCOax, and 18a′′, 0.6586dxy - 0.4846(5σ-CO1eq - 5σ-CO2eq) Versus 15a′′, 0.5887dxy - 0.1078(5σ-CO1eq - 5σ-CO2eq). Also, the contour plots of the eg′ orbitals 18a′′ and 28a′ (Figure 3) demonstrate the strong antibonding in these orbitals with the equatorial and axial CO 5σ lone pairs, respectively. (For 18a′′ the picture is somewhat distorted by the strong admixture of

J. Phys. Chem., Vol. 100, No. 38, 1996 15349 the in-plane 2π*| on the equatorial COs in this orbital; see the preceding remark and the explanation for this in ref 11. For 28a′ the yz plane of drawing does not contain the equatorial COs and therefore does not make the analogous large 2π*⊥ admixture in this orbital visible.) We will denote the higher eg′ orbitals with a prime: σ*′ and dxy′ for 28a′ and 18a′′, respectively. The σ antibonding metal-CO character of these orbitals in Mn2(CO)10 proved to be very important to explain the behavior of the PECs (strongly metal-CO dissociative in excited states where the higher eg′ orbitals become occupied). The lower “dz2” of Mn(CO)5• (actually a dz2,4pz-2π*eq hybrid) is somewhat polarized toward the site opposite the axial CO, where in this case a Cl ligand is coordinated. The 25a′ does indeed have strong antibonding with the Cl pz and might be called the σ*(Mn-Cl), which is somewhat analogous to the well-known σ*(Mn-Mn) of Mn2(CO)10. One may consider this to be in line with weaker σ donor character of Cl compared to CO. The contour plot of 25a′ (Figure 2b) demonstrates the weak antibonding with axial CO and the stronger antibonding with Cl pz. Similarly, the lower dxy (15a′′) has weak antibonding with the equatorial COs, but is stronger with the H-DAB lone pairs (the 5a′′ of H-DAB). We denote the lower eg orbitals as simply σ* (25a′, antibonding with Cl pz) and dxy (15a′′, antibonding with the N lone pairs of the R-diimine). The remaining orbitals in the virtual spectrum, which are straddled by the eg and eg′ sets, consist of predominantly CO 2π* orbitals. The orbitals 26a′, 27a′, and 16a′′ have a considerable admixture of the t2g-type Mn-3d, in accordance with their interpretation as antibonding counterparts to the MnCO π bonding 21a′, 13a′′, and 22a′. These occupied t2g and unoccupied t2g*orbitals are not of special interest to us and need no further comment. 4. Excited States and Electronic Spectrum of fac-Mn(Cl)(CO)3(H-DAB) The absorption spectra of fac-M(X)(CO)3(R-diimine) (X ) Cl, Br, I; M ) Mn, Re) complexes are characterized by two rather strong bands in the UV/vis region.6a,b,29 Their position strongly depends on the R-diimine ligand and X. A shift to lower energy occurs when an aromatic R-diimine ligand such as bpy is replaced by a nonaromatic one such as R-DAB (R ) iPr, tBu, pTol, etc.) having the first available π* orbital at lower energy. On the basis of these effects, the bands have been assigned to charge transfer transitions to the R-diimine ligand. This assignment is confirmed by the rather high intensity of the bands, by their solvatochromism, and by resonance Raman spectra obtained by excitation into the lowest energy band, which show resonance enhancement of Raman intensity for the symmetrical stretching modes of the R-diimine ligand. From studies of the changes in the spectrum upon variation of the halide from Cl to I,6b,29,30 it has been concluded that the two absorption bands originate from two sets of orbitals corresponding to bonding and antibonding metal-halide (dπpπ) combinations. All of these observations can be related directly to the orbital spectrum of Table 1, where the lowest virtual level is the H-DAB π*, and the 13a′′,21a′ and 14a′′,23a′ pairs of orbitals have been identified as metal-halide π bonding and antibonding sets, respectively. The UV/vis spectrum of fac-Mn(Cl)(CO)3(iPr-DAB), taken as a prototype of fac-Mn(Cl)(CO)3(R-diimine) complexes containing an aliphatic R-diimine ligand, is displayed in Figure 4. It shows two separate absorption maxima at 526 (19011 cm-1) and 359 nm (27860 cm-1), respectively. A third maximum is present at 309 nm (32362 cm-1). The lowest energy band is relatively intense and broad. Its maximum is

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Figure 4. UV/vis spectrum of fac-Mn(Cl)(CO)3(iPr-DAB) in cyclohexane.

TABLE 3: Calculated Electronic Transition Energies (in cm-1) for fac-Mn(Cl)(CO)3DABa character

one-electron excitation

singlets state energy

triplets state energy

dxz,px(Cl)fπ*(DAB) dyz,py(Cl)fπ*(DAB) dx2-y2fπ*(DAB) py(Cl),dyzfπ*(DAB) px(Cl),dxzfπ*(DAB) dyz,py(Cl)fσ* dxz,px(Cl)fdxy dxz,px(Cl)fσ* dx2-y2,dyzfdxy σfπ*(DAB) dyz,dx2-y2f2π* dyz,py(Cl)fπ*(DAB) dx2-y2fdxy′ dyz,dx2-y2fσ*′ px(Cl),dxzfdxy py(Cl),dyzfdxy σfσ*

14a′′f24a′ 23a′f24a′ 22a′f24a′ 21a′f24a′ 13a′′f24a′ 23a′f25a′ 14a′′f15a′′ 14a′′f25a′ 23a′f15a′′ 20a′f24a′ 23a′f16a′′ 23a′f17a′′ 23a′f18a′′ 23a′f28a′ 13a′f15a′′ 21a′f15a′′ 20a′f25a′

a1A′′ b1A′ c1A′ d1A′ b1A′′ e1A′ f1A′ c1A′′ d1A′′ g1A′ e1A′′ f1A′′ g1A′′ h1A′ i1A′ l1A′ m1A′

a3A′′ a3A′ b3A′ c3A′ b3A′′ e3A′ d3A′ c3A′′ d3A′′ f3A′ e3A′′ f3A′′ g3A′′ g3A′ h3A′ i3A′ l3A′

14 786 15 883 19 718 23 920 24 640 28 417 28 576 28 892 29 089 32 618 35 365 36 765 38 400 39 341 40 424 41 184 46 675

13 998 14 782 19 360 22 724 21 502 25 128 24 223 24 125 25 988 31 000 33 706 36 673 36 846 38 328 39 920 40 757 43 923

a Orbital character is denoted with most important character in the excited state first. Often, the 23a′ and 14a′′ and the 21a′ and 13a′′ change character compared to the ground state; cf. Table 1.

shifted to the red by 100 nm with respect to the analogous facMn(Cl)(CO)3(bpy) complex,6b which is in line with the abovementioned low-energy shift of this band occurring when an aromatic R-diimine ligand is replaced by a nonaromatic one. The calculated excitation energies to the lowest 1,3A′ and 1,3A′′ excited states of fac-Mn(Cl)(CO)3(H-DAB) are reported in Table 3. We find that the lowest excited states, accessible through allowed transitions 1A′ f1A′ and 1A′f1A′′ range between 14 786 and 19 718 cm-1 and correspond to excitations from the antibonding pπ(Cl)-dπ(Mn) (14a′′,23a′) orbitals and the dx2y2 (22a′) orbital to the H-DAB π* (24a′). It is also possible that the dx2-y2 (22a′)fπ* excitation falls under the strong first band in the spectrum, but since the photochemistry is studied experimentally with irradiation into the low-energy part of this band, only the lower lying a1A′′ and b1A′ states are probably directly populated. The second absorption band, appearing in the spectrum of this complex to the blue (27 860 cm-1) of the lowest energy band, may be assigned according to the calculated excitation energies to 13a′′f24a′ and 21a′f24a′ transitions, which are from the bonding dxz(Mn)+px(Cl) and dyz(Mn)+py(Cl) orbitals to the H-DAB π*. We note that the singlet/triplet splittings corresponding to these excitations are (with one exception) relatively small, ca. 1000 cm-1, which is in line with one of the unpaired electrons being delocalized over the metal and the halide the other one being on the R-diimine ligand. Although the 13a′′ and 21a′ orbitals have, in the ground state, more Mn

dπ than Cl pπ character, these transitions cannot be classified unequivocally as MLCT, and neither can the 14a′′,23a′ to H-DAB π* transitions be considered to be XLCT. The Mn dπ/Cl pπ mixing is considerable, and moreover we have observed that in the excited states the ratio of Mn and Cl character in these orbitals reverses compared to the ground state. The lowest allowed excitations to the virtual eg set (15a′′,25a′) range between 28 417 and 29 082 cm-1, and the corresponding excited states cannot be populated upon excitation into the lowest energy band at ∼19 000 cm-1. They should contribute indeed to the band centered at ∼360 nm. The corresponding triplets range between 24 125 and 25 988 cm-1. There is a relatively large singlet/triplet splitting (3000 cm-1 on the average) due to the partial localization of the unpaired electrons on the metal center. Among these excitations, the 14a′′f25a′(σ*) and 23a′f25a′(σ*) might be photoactive with respect to the dissociation of the chlorine ligand since they populate an orbital that is MnCl σ antibonding (although they depopulate orbitals that are Mn-Cl π antibonding). Some Mn-Cl bond weakening will also occur in the 20a′f24a′(σfπ*) excited state computed at 32 618 cm-1, which depopulates a Mn-Cl σ bonding orbital. Even higher, at 46 675 cm-1, lies the allowed excited state corresponding to the σfσ* (20a′f25a′) transition, the corresponding triplet of which (at 43 923 cm-1) doubtless is photoactive with respect to Mn-Cl bond homolysis, given the the Mn-Cl bonding and antibonding nature of the σ and σ* orbitals, respectively. Due to the weakly Mn-COax σ antibonding nature of the 25a′, it is unlikely that the excited states corresponding to the 14a′′f25a′(σ*) and 23a′f25a′(σ*) excitations would lead to dissociation of an axial carbonyl ligand. Similarly, the 14a′′f15a′′ and 23a′f15a′′ excitations would hardly be expected to be photodissociative for an equatorial carbonyl ligand since, as stressed in the previous section, the 15a′′ has very little Mn-COeq σ antibonding character. Given the strongly Mn-COax and Mn-COeq σ antibonding character of 28a′ and 18a′′, respectively (see section 3), the lowest allowed LF-type excitations to states that are probably dissociative with respect to a metal-carbonyl bond are 14a′′,23a′f28a′ (dfσ*′), and 14a′′,23a′f18a′′. These excitations lie at high energy, 38 400 and 39 341 cm-1, respectively. However, we have found11b in Mn2(CO)10 that excited states of this type play a crucial role since the overlap, and therefore the destabilizing effect of the antibonding interaction, rapidly diminishes upon bond lengthening. This has the effect that these excited states precipitously lower their energy upon Mn-CO bond lengthening, so that they cross nondissociative lower excited states of the dπ,pπfσ* type rather soon. As a result, a low or no barrier for CO dissociation may result on the PES of the lower energy (dπ,pπfσ*) state. In the present case, however, we are dealing with MLCT states that are still lower than the dπ,pπfeg(σ*,dxy) states and therefore may be crossed only later (or not at all) by the descending LF states. Summarizing, a significant point to arise from our calculations is that the excited states accessible upon irradiation into the lowest energy bands would not be photoactive by themselves, neither for the dissociation of a carbonyl ligand, which is the primary reaction currently observed for this family of manganese complexes, nor for chlorine dissociation. However, release of a carbonyl ligand might still take place upon excitation in these states if crossing occurs to the potential energy surfaces of excited states that are strongly dissociative with respect to a metal-carbonyl bond. We have identified above LF-type excitations to states that are probably Mn-COax dissociative

MLCT Photochemistry of fac-Mn(Cl)(CO)3(H-DAB) CHART 1

(basically d,pfσ*′) and to states that are probably Mn-COeq dissociative (basically d,pfdxy′). Explicit calculations of the potential energy curves have been carried out to verify these possibilities and will be discussed in the next section. 5. Photodissociation of a Carbonyl Ligand Molecular Structure of the Five-Coordinated Primary Photoproducts. To facilitate the discussion of the PECs for axial and equatorial CO loss, we first consider the structures of the resulting five-coordinated intermediates. It is well established6a that, in the case of the bpy complex, the primary photoproduct following CO loss, Mn(Cl)(CO)2(bpy)(S) (S ) solvent), has a cis,cis conformation, where the two carbonyls are cis to each other and the halide and solvent ligands are cis to each other as well. This leaves us with the two possible structures depicted in Scheme 1, so the question of whether the halide occupies an axial or an equatorial position still remains. On the other hand, release of the weakly bonded solvent molecule and recombination with CO exclusively produced the mer-Mn(Cl)(CO)3(bpy) complex.6a So either the solvento species already has a mer-Cl ligand or it has a fac-Cl, but after release of the equatorial solvent and before recombinination with CO, the five-coordinated intermediate relaxes to a square pyramidal configuration with Cl, R-diimine, and CO in the basal plane. This rearrangement would occur if the square pyramidal configuration were the most stable one for this species. In the latter case, however, one would wonder why this configuration would not have been adopted by the five-coordinated intermediate immediately after the photochemical release of the carbonyl ligand. The uncertainty that surrounds the molecular structure of the primary photoproduct following CO loss has led us to conduct a comprehensive investigation of the five-coordinated intermediate, Mn(Cl)(CO)2(H-DAB). We have carried out a complete geometry optimization for a number of conformations of this species, corresponding to both axial and equatorial CO vacancies. Starting from the two basic structures 1a and 1b (see Chart 1) for Mn(Cl)(CO)2(H-DAB), corresponding to the removal of, respectively, the axial or an equatorial CO ligand, an optimization afforded the two structures given in parts A and B of Figure 5. The structure in Figure 5A has not rearranged much from the axial vacancy starting point. It has Cs symmetry and a triplet electronic state 3A′′ due to the close proximity of the two highest singly occupied orbitals. An optimization of 1a on the lowest singlet surface afforded the structure of Figure 5C with a closedshell electronic configuration, but 36 kJ/mol less stable than the triplet structure. These structures have substantially the same geometry. In contrast, optimization of the structure with an equatorial vacancy (1b) leads to strong rearrangement. The resulting

J. Phys. Chem., Vol. 100, No. 38, 1996 15351 structure of Figure 5B is characterized by pronounced bending of the halide toward the vacant equatorial site, resulting in a Cl-Mn-COax angle of 106.7°. The structure is a distorted square pyramid. It has no symmetry elements left and has a singlet ground state. We find that the Figure 5B structure is 52 kJ/mol more stable than the axial vacancy structure of Figure 5A. This difference is, however, not due to a large difference in the stabilization due to geometrical relaxation. The energy changes associated with the geometrical relaxation of the fivecoordinated fragments 1a and 1b are both large (50 and 59 kJ/ mol, respectively), but not very different. The higher stability of Figure 5B is thus primarily due to a difference in the first dissociation enthalpy between axial and equatorial COs already in the frozen structures. Taking the geometrical relaxation into account, the first dissociation enthalpy of an axial CO amounts to 174 kJ/mol, whereas the corresponding equatorial dissociation enthalpy is 122 kJ/mol. Unfortunately, direct comparison with experiment is not possible since the energetics of this complex has never been investigated. Nevertheless, it is useful to compare our data with the ∆H(Mn-CO) value of 115.1 kJ/ mol measured for the parent Mn(Cl)(CO)5 complex.32 One may wonder whether conformations other than the ones found by automatic geometry optimization starting from 1a and 1b might exist with still lower energy. We have considered two higly distorted trigonal bipyramidal (tbp) conformations of the five-coordinated intermediate, Mn(Cl)(CO)2(H-DAB), with the equatorial position occupied in turn by a carbonyl ligand or by the halide (cf. 1d and 1e). An optimization of these conformations without symmetry constraints afforded the structures displayed in Figure 5D,E, respectively. Both are much higher in energy (153 kJ/mol for Figure 5D and 89 kJ/ mol for Figure 5E) than the optimal Figure 5B structure. Thus, our DFT calculations point to the C1 structure of Figure 5B as the most stable one for the Mn(Cl)(CO)2(H-DAB) fragment. Our findings explain the exclusive occurrence of the merMn(Cl)(CO)3(H-DAB) isomer in the second reaction step of Scheme 1. Even if the solvent occupies the equatorial position, the mer-Cl structure Figure 5B will still result when it dissociates. Recombination of Figure 5B with CO thus will only afford the mer-Mn(Cl)(CO)3(H-DAB) complex, as actually experimentally found. We may also draw conclusions for the first step, the photochemical loss of CO. Since geometry optimization of the two Mn(Cl)(CO)2(H-DAB) 1a and 1b basic structures, corresponding to the removal of, respectively, the axial and an equatorial CO ligand from fac-Mn(Cl)(CO)3(HDAB), directly afforded Figure 5A, and 5B structures, it is quite reasonable to consider these structures as those adopted by the five-coordinated fragment in the primary photoproducts following the loss of CO. It should be noted, however, that the five-coordinated fragment of Figure 5A following the loss of the axial carbonyl would, after occupation of the vacant site with a solvent molecule, yield a primary photoproduct, Mn(Cl)(CO)2(R-diimine)(S), with Cl and S trans to each other, instead of the cis configuration experimentally detected. This leads to the suggestion that either axial CO dissociation is unfavorable or the Figure 5A fragment interconverts into the five-coordinated Figure 5B fragment immediately after its formation. This conversion has to be so rapid that no S is able to occupy the axial site. To check this point, we briefly investigate the Figure 5Af5B interconversion. Direct Figure 5Af5B interconversion is spin forbidden and we first would have to invoke excitation of the triplet in Figure 5A to the corresponding singlet in Figure 5C, which requires 36 kJ/mol. Another possibility would be the direct formation of the singlet in Figure 5C in the photodissociation. Next, the Figure 5C fragment will interconvert into Figure 5B along the

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Figure 5. Optimized structures for Mn(Cl)(CO)2(H-DAB): (A) Cs symmetry with an axial vacancy in its triplet ground state; (B) C1 symmetry with an equatorial vacancy in its singlet ground state; (C) Cs symmetry with an axial vacancy in the singlet state; (D) C1 symmetry in a trigonal bipyramidal-like conformation with the halide in the axial position; (E) C1 symmetry in a tbp-like conformation with the halide in the equatorial position. Energies are relative to panel B.

singlet surface, connecting the Figure 5C and 5B fivecoordinated species, provided that the energy barrier is accessible. We have computed a singlet potential energy curve for the interconversion between Figure 5C and 5B by moving one equatorial CO ligand to the vacant axial position and optimizing, at each value of the angle that the Mn-CO bond axis makes with the vertical z axis, θ, all other geometrical parameters of the system. The resulting PEC is displayed in Figure 6 together with the optimal geometries of the fragment for selected θ values. During the interconversion process, the Mn-Cl bond shortens and bends toward the newly vacant equatorial position. At θ ∼30°, the angle that the Mn-Cl bond axis makes with the vertical z axis is ∼74°, i.e., very close to that in the equatorial loss fragment, Figure 5B. The calculated barrier for conversion of the singlet axial CO loss fragment Figure 5C to the equatorial loss fragment Figure 5B is 15 kJ/mol (without zero-point correction). The barrier is early, with the maximum occurring at θ ∼70°. According to our calculations, the total energy barrier for the Figure 5Af5B interconversion amounts to 51 kJ/mol. This value does not rule out this possibility completely, but it is not likely that it would be so fast as to prevent any occupation of the axial vacancy by S. For this reason, if not for the considerably higher bond dissociation energy, one is led to consider axial CO dissociation to be less likely. This remains to be verified by explicit PEC calculations. However, before

Figure 6. Potential energy curve for the Figure 5Cf5B interconversion. θ is the angle that the Mn-CO bond makes with the vertical z axis. The optimal geometries of the fragment at selected θ angles are shown. The end points refer to the optimal trans and cis loss geometries Figure 5C and 5B, respectively.

examining the PECs for axial CO dissociation, we will first deal with the PECs for equatorial CO dissociation to investigate how equatorial CO dissociation can occur, particularly how it leads to the product Figure 5B where Cl has moved to the vacant equatorial site.

MLCT Photochemistry of fac-Mn(Cl)(CO)3(H-DAB)

A

B

Figure 7. (A) Potential energy curves for the dissociation of the MnCOeq bond in fac-Mn(Cl)(CO)3(H-DAB). The arrows show the correspondence between the Mn-COeq distances and the optimal geometries of the system shown in Figure 8. (B) Contour plot of the ground state potential energy surface as a function of the Mn-COeq elongation and the Cl bending angle τ with respect to the positive z axis. Contours (in au and relative to the ground state of fac-Mn(Cl)(CO)3(CH-DAB) at Re) are for 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.061, 0.062, 0.064, 0.068, 0.07, 0.075, 0.09, 0.1, and 0.2 au.

Potential Energy Curves for the Loss of an Equatorial Carbonyl Ligand. The calculated PECs for equatorial CO dissociation are displayed in Figure 7A. In view of the large rearrangement observed in the end product, full geometry optimization of fac-Mn(Cl)(CO)3(H-DAB) has been performed on the ground state surface, along the reaction pathway corresponding to the Mn-COeq bond elongation. The excited states have been calculated at each point by using the ground state geometries (see the following). Note that, due to the symmetry lowering to C1, all excited states have A symmetry. The curves in the figure are for the first six excited states, the lowest ones being the dxz,pxfπ* triplet and singlet (a3A and b1A) and then the dyz,pyfπ* triplet and singlet (b3A and c1A), with the triplet b3A being at Re virtually degenerate with the singlet b1A of dxz,pxfπ*, and somewhat higher we have the dx2-y2fπ* singlet and triplet (c3A and d1A). The most remarkable structural change during equatorial CO dissociation is the progressive displacement of the halide from the axial to the equatorial position. In looking at Figure 8, where the optimal C1 geometries of the molecule at certain fixed MnCOeq bond distances are shown, the bending of the Mn-Cl bond toward the equatorial position left vacant by the leaving CO is

J. Phys. Chem., Vol. 100, No. 38, 1996 15353 immediately apparent. The Cl-Mn-COax angle, which we can take as a measure of this bending, decreases from 179° initially to 138° and 106° upon elongation of the Mn-COeq bond distance from Re to 4.8 Å and ∞, respectively. The Mn-Cl bond shortens by about 0.2 Å. These structural rearrangements are responsible for the potential barrier on the ground state PEC, with a maximum at around 3.5 Å. That the bending of the MnCl bond toward the equatorial position is concomitant with the Mn-COeq bond elongation implies that, unless CO loss would occur too rapidly, the primary five-coordinated photoproduct, Mn(Cl)(CO)2(H-DAB), has already adopted a square pyramidal geometry with the halide in the equatorial plane. To get a more complete view of the rearrangement during CO departure, we have computed a 2D PES by using the Mn-COeq distance d and the Cl bending angle τ as coordinates. The PES (see Figure 7B) demonstrates that there is a simple saddle point connecting the initial and final situations, with d and τ values in agreement with Figures 7A and 8. Note that in the initial optimum geometry (d ≈ 2.0 Å, τ ≈ 8°) the Mn-Cl bending mode is much softer than the Mn-COeq stretch. It is also noteworthy that the lowest excited state PECs show a potential barrier with a maximum slightly shifted to a longer Mn-COeq distance. This feature is not simply a consequence of keeping the ground state geometry when computing the energy of the excited states along the reaction pathway. In fact, full geometry optimization of the system on the a3A (dxz,pxfπ*) and b3A (dyz,pyfπ*) surfaces afforded PECs nearly coincident with those obtained by using the ground state geometry. As inferred from Figure 7A, none of the lowest excited state PECs has a dissociative character. In fact, the asymptotic energy of the excited state PECs is sometimes only marginally higher than the energy at the equilibrium geometry, but dissociation is prevented by quite a high potential barrier (ca. 100 kJ/mol). Although COeq loss therefore cannot occur directly along the lowest excited state PECs of fac-Mn(Cl)(CO)3(H-DAB), our calculations suggest that Mn-COeq bond breaking may occur through an alternative mechanism where the topology of the ground state PEC plays a key role. According to the shape of the ground state PEC, continuum states in the Mn-COeq coordinate will exist at all energies above the asymptotic energy. According to the well-known resonance phenomenon of continuum states above a potential well,34 they will at certain energies have a resonance-enhanced amplitude in the potential well region (around Re) reminiscent of the vibrational states in the discrete part of the spectrum. Radiationless transition from the MLCT state to such a continuum state belonging to the electronic ground state PEC may lead to dissociation, either directly if the transition takes place to a continuum state above the maximum of the barrier or by thermal excitation (rather than tunneling) if the transition is to a state just below the maximum. We note that this process would, of course, occur on a much longer time scale than the rapid ejection of CO (20-200 fs)35,36 along strongly dissociative excited state PECs. This long time scale is consistent with the use of the fully relaxed ground state PEC since time is available for the presumably slow motion of the Cl to the equatorial position. The indicated slow dissociation process connected with the structural rearrangement would not occur if rapid dissociation of CO were possible along a dissociative PEC with (approximate) retention of the atomic configuration. Such dissociative curves have been calculated for axial CO dissociation in Mn2(CO)10, where low-lying excited states are not intrinsically dissociative, as neither of the present MLCT states are, but become dissociative because of crossing by strongly dissociative LF states that correspond to excitation to a strongly Mn-CO σ antibonding eg′-type orbital. We have explicitly verified whether

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Figure 8. Optimized structures of C1 symmetry for fac-Mn(Cl)(CO)3(H-DAB) at fixed Mn-COeq distances.

this type of dissociative behavior could occur in the present complexes by calculating PECs and excitation energies for the equatorial CO dissociation in the frozen geometry 1b. Figure 9 shows the ground state PEC and the excitation energies at Re and after dissociation. The σ antibonding orbital 18a′′ rapidly becomes lower in energy along the Mn-COeq coordinate, and at infinity the lowest excited state of 1b indeed has d,pfhy character, where we denote with “hy” the 18a′′-derived hybrid with large amplitude toward the vacated equatorial site. If the lowest excited states at Re were not the low-energy MLCT states, but states lying higher by say 1 eV, the asymptotically very low-lying d,pfhy excited state would offer the possibility of a dissociative lowest excited state PEC by crossing to the d,pfhy state with a low or no barrier [cf. CO dissociation in Mn2(CO)10]. However, in the present case the H-DAB π* is already a lowlying virtual orbital, so that the MLCT state is at rather low energy to begin with; the d,pfhy state does not cross with it at moderately elongated Mn-COeq distances, but only asymptotically becomes (marginally) lower in energy than the d,pfπ* state. The lowest excited state PEC therefore will not be at all dissociative in the case of (approximate) retention of the atomic configuration. The experimental observation of the isomers mer-Mn(Cl)(CO)3(bpy) and mer-Mn(Cl)(CO)3(iPr-PyCa)6a,c as the only products and mer-Mn(Cl)(CO)3(iPr-DAB) as the major product6c

after back-reaction with CO may be cited as support for the proposed mechanism of concurrent COeq departure and structural rearrangement. Occupation of the vacant axial site of the fivecoordinated product in Figure 5B by CO to form the mer-Mn(Cl)(CO)3(H-DAB) isomer should be, in principle, an easy process, since no structural rearrangements are involved. A quantitative assessment of this point would, however, require the knowledge of the energy profile of this reaction. To this purpose, the potential energy curve connecting the reactants and the product in their ground states has been computed for the back-reaction:

mer-Mn(Cl)(CO)2(H-DAB) + CO f mer-Mn(Cl)(CO)3(H-DAB) (1) During the reaction pathway, except for the Mn-CO bond that is forming, all geometrical parameters have been optimized. Reaction 1 is calculated to be exothermic by 90 kJ/mol, and there is no potential barrier between the reactants and the product. On the other hand, recombination of the fivecoordinated mer-Mn(Cl)(CO)2(H-DAB) species with CO to give back the original fac-Mn(Cl)(CO)3(H-DAB) has, according to Figure 7A, to go over a barrier of about 67 kJ/mol. The energetics of the photodissociation of an equatorial carbonyl ligand from fac-Mn(Cl)(CO)3(H-DAB) and of the

MLCT Photochemistry of fac-Mn(Cl)(CO)3(H-DAB)

Figure 9. Ground state potential energy curve for Mn-COeq dissociation in a frozen geometry, as well as a correlation diagram for the first excited states of the “reactant” [fac-Mn(Cl)(CO)3(H-DAB) at equilibrium geometry] and the CO loss “product”.

Figure 10. Potential energy curves for the dissociation of the MnCOax bond in fac-Mn(Cl)(CO)3(H-DAB).

subsequent reaction to give the mer isomer is summarized in Figure 11. Potential Energy Curves for the Loss of the Axial Carbonyl Ligand. The potential energy curves along the MnCOax coordinate for the ground state and the lowest singlet and triplet excited states that belong (at equilibrium distance) to the photochemically active band centered at 526 nm are displayed in Figure 10. Figure 10 indicates that Mn-COax bond lengthening in the 1A′ ground state of fac-Mn(Cl)(CO)3(H-DAB) leads to Mn(Cl)(CO)2(H-DAB) (a1A′) + CO (1Σ+). The a1A′ state corresponds to an electronic configuration that derives directly from the parent fac-Mn(Cl)(CO)3(H-DAB) complex by simply omitting the low-lying occupied COax orbitals from the 1-electron-level spectrum. This means that the highest occupied orbitals are again the dπ,pπ Mn-Cl antibonding orbitals, labeled 19a′ and 13a′′ (see Table 4) in the Cs symmetry of the fivecoordinated trans loss product. We note in Table 4 that the occupied orbitals can easily be recognized from our discussion

J. Phys. Chem., Vol. 100, No. 38, 1996 15355 of Table 1 for the fac-Mn(Cl)(CO)3(H-DAB) complex. However, in the virtual spectrum an interesting change has occurred: a hybrid with much dz2,4pz, 2π*eq⊥ and H-DAB π* character has come down in energy so much that it (the 20a′) is very close to the 13a′′. As a matter of fact, the triplet state a3A′′ belonging to the (13a′′)1(20a′)1 configuration drops below the singlet a1A′ belonging to (13a′′).2 The latter is the first excited state of the five-coordinated complex, lying 36 kJ/mol higher than the ground state a3A′′. The 20a′ cannot be identified with the “eg”-type 25a′ of the parent fac-Mn(Cl)(CO)3(H-DAB), which clearly survives to a considerable extent in the 21a′ of Mn(Cl)(CO)2(H-DAB), but it has much eg′-σ*′ (28a′) parentage. The precipitous lowering of this type of antibonding ligand-field orbital when the pushing-up CO ligand departs has been observed and discussed extensively for axial and equatorial CO dissociation in Mn2(CO)10.11b It is interesting to note that the difference with the case of Mn2(CO)10 is that the low-lying hybrid oriented toward the vacant site has much (41%) H-DAB π* character and is so low that a “high-spin” configuration becomes the ground state in the dissociation limit. Since the a3A′′ at Re corresponds to the MLCT excitation (dxz,px Mn-Cl antibonding orbital to H-DAB π* excitation), while asymptotically it has dxz,pxfσ*′ character, it is clear that a change in electronic character occurs during the dissociation. The shape of the a3A′′ PEC around Re shows that this MLCT state has a bonding minimum and is not dissociative. This also holds for the corresponding singlet a1A′′ and of course for the singlet-triplet pair a3A′-b1A′ corresponding to the MLCT excitation of the other π component (dyz,py Mn-Cl antibonding orbital to H-DAB π* excitation) and the MLCT excitation out of Mn dx2-y2 (b3A′-c1A′). The minimum of all of these PECs in the neighborhood of Re shows that, as expected, the electronic structure of an MLCT state does not induce dissociation. However, the curves are bent down by the “avoided crossing” with the ligand-field type strongly dissociative excited state resulting from excitation to the 28a′-σ*′ of fac-Mn(Cl)(CO)3(H-DAB) that is rapidly descending, becoming 20a′ in the trans loss fragment. This is in perfect analogy with the behavior of the σ*′ of Mn2(CO)10.11b There is strong mixing, as is evident from the fact that the “avoided crossing” does not lead to a sharp maximum in the a3A′′ PEC, but to a very broad barrier, and from the fact that in the dissociation limit there is still much H-DAB π* character in the hybrid 20a′. Of course the dissociative LF states resulting from excitation to 28a′-σ*′ of fac-Mn(Cl)(CO)3(H-DAB) have, on their way down, to cross with other A′ or A′′ states. This can be traced in detail in the electronic structure of the states as a function of R. For instance, the b3A′ and c1A′ corresponding at the equilibrium distance to 22a′f24a′ (dx2-y2fπ*) show, in the range 2.2-3.0 Å, behavior that results from crossing with the dissociative 23a′f28a′(dyzfσ*′) derived states. Their dominant character at 2.2 Å is already dyzfσ*′, becoming progressively dx2-y2fσ*′ in the range 2.5-3.0 Å due to crossing with the dissociative 22a′f28a′(dx2-y2fσ*′) derived states. The states under consideration clearly are not Mn-COax dissociative. Even the energy of the a3A′′-separated system is some 30 kJ/mol higher than the a3A′′ MLCT excited state of fac-Mn(Cl)(CO)3(H-DAB) at the equilibrium geometry. More importantly, maybe, there is also an extended barrier of ca. 50 kJ/mol on this curve. The other MLCT states are more strongly bound. Still, all of these excited states have well depths that are much smaller than that of the ground state due to the existence of the very low-lying dz2,4pz-2π*⊥,π*(H-DAB) hybrid in the trans loss five-coordinated product, making the first excitation energies in Mn(Cl)(CO)2(H-DAB), which are to this

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Figure 11. Schematic energy diagram for the photodissociation of an equatorial carbonyl ligand from fac-Mn(Cl)(CO)3(H-DAB) along the a1A ground state PEC and energy profile of the subsequent recombination reaction of the resulting fragment with CO to give the mer isomer.

TABLE 4: One-Electron Energy and Percentage Composition (Based on Mulliken Population Analysis per MO) from Spin-Restricted Calculations of the Lowest Unoccupied and Highest Occupied Orbitals of the trans Loss Mn(Cl)(CO)2(H-DAB) Fragment in Its Triplet Ground State Structure (Figure 5A) MO  (eV) 16a′′ -1.13 15a′′ -1.47 23a′ -1.79 22a′ -1.96 14a′′ -2.30 21a′ -3.76

Mn

COeq

Singly Occupied Orbitals 20a′ -5.18 20(dz2); 11(4pz); 14(2π*⊥) 8(dyz) 8(2π*⊥) 13a′′ -5.26 58(dxz) 19a′ 18a′ 12a′′ 17a′ 16a′

-5.60 -6.43 -6.94 -7.18 -8.38

Cl

Unoccupied Orbitals 17(5σ); 7(2π*⊥); 31(dxy) 36(2π*|) 71(2π*⊥); 10(2π*|) 13(dxz) 8(2π*⊥); 56(2π*|) 24(dx2-y2) 16(dyz); 6(dz2); 56(2π*⊥); 7(2π*|) 3(4pz) 24(dxy); 5(4px) 5(5σ); 50(2π*|) 15(3pz) 34(dz2); 11(dyz); 3(4pz)

Occupied Orbitals 2(2π*⊥) 35(dyz) 28(2π*|) 68(dx2-y2) 9(2π*⊥) 20(dxz) 10(2π*⊥) 29(dyz) 23(dz2); 3(4pz) 2(2π*⊥)

3(3pz)

H-DAB 7(5a′′) 8(6a′′) 11(8a′) 6(7a′) 12(5a′′) 29(7a′)

41(7a′)

30(3px) 3(4a′′) 40(3py) 21(7a′) 62(3px) 11(4a′′) 51(3py) 8(7a′); 4(5a′) 70(3pz)

hybrid, much smaller than the lowest excitations in fac-Mn(Cl)(CO)3(H-DAB) at the equilibrium geometry, which are to the H-DAB π*. We can conclude from our results that axial CO dissociation may not be efficient, and this is observed experimentally for most R-diimine complexes.6a,c The system will usually relax from an allowed excited singlet state, through nonradiative processes and intersystem crossing, to the vibrational ground state of the a3A′′ state and from there to the ground state. It is interesting to compare the present Mn-COax PECs to those found by Finger and Daniel9 for fac-Mn(H)(CO)3(HDAB). There is a clear difference, in that Finger and Daniel found a purely dissociative a3A′′ PEC and very weakly bound a3A′ and b3A′ curves. It is possible that this difference is related to the replacement of Cl by H. The highest occupied orbitals are, in our case, not pure metal d orbitals, but antibonding Clpπ Mn-dπ orbitals. These clearly lie above the dx2-y2 (see Table 1), which is nonbonding with Cl. The first MLCT excitations are indeed calculated at lower energies in our complex than in the hydrogen analogue, but the dx2-y2 to π* excitations are more

comparable. However, a more important factor that in our facMn(Cl)(CO)3(H-DAB) calculations causes the a3A′′ excited state energy at Re to lie below the asymptotic energy on the ground state curve, whereas in fac-Mn(H)(CO)3(H-DAB) it is above that limit, is the difference in asymptotic energy (i.e., bond energy of axial CO). Whereas we found 174 kJ/mol for facMn(Cl)(CO)3(H-DAB) with respect to the a3A′′ asymptotic state (203 kJ/mol with respect to the asymptotic limit of the ground state a1A′ curve), Finger and Daniel find ca. 106 kJ/mol for fac-Mn(H)(CO)3(H-DAB) (for both a3A′′ and a1A′ asymptotic states). 6. Summary From previous work on CO dissociation in Mn2(CO)10,11 we have concluded that the lowest excited states, which involved excitation into the σ*(Mn-Mn) LUMO, do not have an electronic structure that would induce Mn-CO dissociation. Mn-CO dissociation can happen because of crossing of these states with LF states that have much higher energy at equilibrium geometry but are strongly dissociative, i.e., very rapidly lower their energy upon Mn-CO bond lengthening. The introduction of a low-lying virtual ligand level, like the π* of the R-diimine ligand in the present case, introduces low-lying MLCT excitations. We have observed that these states are also not MnCO dissociative by their electronic nature. They will also be crossed by the dissociative LF states, but an important difference is that the MLCT states are at relatively low energy. They may, therefore, along the Mn-CO dissociation coordinate, curve upward before the crossing takes effect. Therefore a bonding minimum exists around Re in the PEC of the MLCT state. We have observed this type of behavior for axial CO dissociation in fac-Mn(Cl)(CO)3(H-DAB). It is interesting to note that at the molecular orbital level we do not observe a complete crossing of the LF orbital σ*′ (a dz2 hybrid with large amplitude toward the vacated axial site) with the H-DAB π*, whereas the analogous hybrid in Mn2(CO)10 did cross the σ*(Mn-Mn).9 The σ*′ does not actually lie below the H-DAB π* in the fivecoordinated product, but the LUMO of that compound is a strong mixture of the σ*′ with the H-DAB π*. Altogether axial CO dissociation may occur, but its efficiency is expected to be low. This may be a typical situation in this type of low-lying MLCT states. For equatorial CO dissociation the situation changes due to strong structural rearrangement during the CO dissociation. If that would not happen, and structure 1b of Chart 1 with an

MLCT Photochemistry of fac-Mn(Cl)(CO)3(H-DAB) equatorial vacant site were the result, a hybrid with large amplitude toward the vacated site would rapidly descend upon Mn-COeq bond lengthening, similar to what was found for axial and equatorial CO dissociation in Mn2(CO)10 and similar to what we observed here for axial CO dissociation. For equatorial CO dissociation this is an 18a′′-derived (dxy′,4px,4py, COeq-2π*|) hybrid. The LF state corresponding to occupation of such a hybrid accordingly is strongly dissociative. In the case of Mn2(CO)10, we found that for axial CO the dissociative LF state came down so rapidly that the lowest excited state PEC could not curve upward and became completely dissociative, but for equatorial CO the effect was more moderate. We therefore might expect that the low-lying MLCT states of fac-Mn(Cl)(CO)3(H-DAB) will not become dissociative for equatorial CO loss in the “frozen” geometry 1b. It has indeed been observed in calculations on 1b that the lowest excited states of 1b are not at all dissociative. So without the geometric relaxation to Figure 5B, calculated for the equatorial CO dissociation, the MLCT states probably would not have been particularly photoactive. The geometrical relaxation, however, leads to a maximum in the ground state PEC (see Figure 7A). We have noted that this opens the possibility for an alternative mechanism of Mn-COeq dissociation, where the system in the lowest MLCT excited state (which may have a long lifetime) makes a radiationless transition to a dissociative continuum state in the Mn-COeq coordinate, belonging to the electronic ground state PEC. During the dissociation of the equatorial CO, the chloride shifts to the equatorial site. This mechanism would result in the five-coordinated intermediate of Figure 5B, which leads to the exclusive formation of the mer-Cl isomer after back-reaction with CO (cf. Figure 11 for a schematic overview). This mechanism of CO dissociation is very different from the “normal” crossing of the excited state by a strongly dissociative LF state. By comparing the theoretical and experimental data, we observe that, in agreement with the preceding mechanism, the complexes fac-Mn(X)(CO)3(bpy) and fac-Mn(X)(CO)3(iPrPyCa) exclusively produce the mer isomer. The corresponding complexes fac-Mn(X)(CO)3(iPr-DAB) afford, however, a small amount of the fac complex at low temperature in addition to the mer isomer. Their relative yields are wavelength dependent and their formation is completely independent of each other. So, we are dealing here with two different primary photoprocesses, axial and equatorial CO loss. In agreement with our calculations axial CO loss is, however, rather inefficient. References and Notes (1) (a) Balzani, V.; Bolletta, F.; Scandola, F.; Ballardini, R. Pure Appl. Chem. 1979, 51, 299. (b) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159. (c) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (d) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (e) Scandola, F.; Bigozzi, C. A.; Chiorboli, C.; Indelli, M. T.; Rampi, M. A. Coord. Chem. ReV. 1990, 97, 299. (f) Balzani, V.; de Cola, L.; Prodi, L.; Scandola, F. Pure Appl. Chem. 1990, 62, 1457. (2) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (b) Ford, P. C.; Wink, D.; Di Benedetto, J. Prog. Inorg. Chem. 1983, 30, 213. (3) (a) Balk, R. W.; Snoeck, Th. L.; Stufkens, D. J.; Oskam, A. Inorg. Chem. 1980, 19, 3015. (b) van Dijk, H. K.; Servaas, P. C.; Stufkens, D. J.; Oskam, A. Inorg. Chim. Acta 1985, 104, 179. (c) Wieland, S.; Bal Reddy, K.; van Eldik, R. Organometallics 1990, 9, 1802. (d) Vı´chova´, J.; Hartl, F.; Vlcˇek, A. Jr. J. Am. Chem. Soc. 1992, 114, 10903. (e) Lindsay, E.; Vlcˇek, A., Jr.; Langford, C. H. Inorg. Chem. 1993, 32, 2269. (f) Vlcˇek, A., Jr.; Vı´chova´, J.; Hartl, F. Coord. Chem. ReV. 1994, 132, 167. (4) (a) Kokkes, M. W.; Stufkens, D. J.; Oskam, A. J. Chem. Soc., Dalton Trans. 1984, 1005. (b) van Dijk, H. K.; Stufkens, D. J.; Oskam, A. J. Am. Chem. Soc. 1989, 111, 541. (c) van Dijk, H. K.; Kok, J. J.; Stufkens, D. J.; Oskam, A. J. Organomet. Chem. 1989, 362, 163. (5) Servaas, P. C.; Stufkens, D. J.; Oskam, A. Inorg. Chem. 1989, 28, 1780. (6) (a) Stor, G. J.; Morrison, S. L.; Stufkens, D. J.; Oskam, A. Oganometallics 1994, 13, 2641. (b) Stor, G. J.; Stufkens, D. J.; Vernooijs,

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