Inorg. Chem. 1989, 28, 4243-4250
4243
Contribution from the Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Universiti di Padova, 351 3 1 Padova, Italy, Istituto di Chimica della Universiti della Basilicata, Potenza, Italy, Istituto di Teoria e Struttura Elettronica del CNR di Roma, Rome, Italy, and Anorganisch Chemisch Laboratorium, University of Amsterdam, Amsterdam, The Netherlands
Theoretical, UV-PES, XPS, and Mossbauer Investigation of the Electronic Structure of Dinuclear Metal Carbonyl Diimine Complexes with a Metallacyclopentadienyl System R. Bertoncello,la M. Casarin,*Ja,bM. Dal Colle,Ia G. Granozzi,*?la G. Mattogno,lCF. Muller,ld U. R U S S Oand , ~ ~K. Vrieze*Jc Received M a y 22, 1989
The electronic structure of a series of metallacyclopentadienyl diiron complexes, containing a chelating 4e-donor 1,4-diaza-l,3butadiene (R-DAB) ligand in different structural arrangements, is discussed by using SCF-first-principle discrete variational (DV)-Xa calculations and gas-phase UV-photoelectron (PE), solid-state X-ray PE (XPS), and Mossbauer spectroscopies. Comparison of the obtained theoretical results with those relative to the isoelectronic unsubstituted diiron metallacyclopentadienyl complex ( Fe,(CO),(C4H4)) indicates that, even though the substitution of two terminal C O ligands with R-DAB mainly affects molecular orbitals localized on the metal atom R-DAB is bonded to, the overall bonding scheme is significantly influenced by the coordination site of R-DAB. When compared to two carbonyls, R-DAB causes a higher amount of electronic density on the metal atom to which it is directly bonded. Moreover, the involvement of n+ and n- nitrogen lone-pair combinations in the metal-nitrogen interaction is computed to be definitely stronger than in complexes where R-DAB acts as a bridging 8e donor. Remarkable differences in structural data of the investigated series are discussed and clarified on the basis of the reported theoretical results. Transition-state ionization energies reproduce very well the experimental UV-PE pattern, allowing us to be confident about the main features of the bonding scheme. Furthermore, the computed different electronic charge distributions for the two nonequivalent iron sites along the series are well in tune with both Mossbauer experimental data and binding energy values obtained by XPS. Introduction
Reactions of metal carbonyl complexes with a-diimines have in the past resulted in a very extensive chemistry.z The a-diimines show a versatile coordination behavior, and it has been found that the coordinated a-diimine ligand may easily participate not only in C-H and N-H bond formation3q4but also in C-C and N-C coupling reactions with a wide variety of unsaturated organic substrates, such as a-diimine~,~ carbodiimides (RN=C=NR),6 sulflnes (RzC=S=0),6 ketene (H2C=C=0),’ and alkynes (R’C=CR’’).* The substituted 1,4-diaza-1,3-butadiene (RN,==C,H-C,H=N,R; hereafter R-DAB) shows several coordination modes, where i t can act as a 2e ( U - N ) , ~4e (a-N, a-N’ chelating),z 4e (q2-CN, qz-CN’)? 2e, 2e (a-N, a-N’ bridging),2 6e (a-N, p2-N’,V ~ - C N ’ )a,n~d 8 e (a-N, a-N’, q2-CN, T ~ - C N ’ ) ~ donor. This paper is part of a systematic investigation of the electronic properties of dinuclear metallacycle complexes,1° where t h e main aim is t h a t of clarifying the bonding scheme within t h e metallacycle and the role played by the metal-metal bond in modifying metal-ligand interactions.loa Here we report a combined theoretical and experimental investigation of t h e electronic structure (a) Universita di Padova. (b) Universiti della Basilicata. (c) ITSEC, CNR. (d) University of Amsterdam. For recent review dealing with the chemistry of a-diimine ligands, see: Vrieze, K. J . Organornet. Chem. 1986, 300, 307 and references therein. Keijsper, J.; Grimberg, P.; van Koten G.; Vrieze, K.; Christophersen, M.; Stam, C. H. Inorg. Chim. Acta 1985, 102, 29. (a) Keijsper, J.; Mul, J.; van Koten, G.; Vrieze, K.; Ubbels, H. C.; Stam, C. H. Organometallics 1984,3, 1732. (b) Zcet, R.; van Wijnkoop, M.; Versloot, P.; van Koten, g.; Vrieze, K. Organometallics, in press. (a) van Koten, G.; Jastrzebski, J. T. B. H.; Vrieze, K. J . Organomet. Chem. 1983, 250, 49. (b) Staal, L. H.; Oskam, A.; Vrieze, K.; Roosendaal, E.; Schenk, H. Inorg. Chem. 1979,18, 1634. (c) Staal, L. H.; Polm, L. H.; Balk, R. W.; van Koten, G.; Vrieze, K.; Brouwers, A. M. F. W. Inorg. Chem. 1980, 19, 3343. (d) Polm, L. H.; van Koten, G.; Elsevier, C. J.; Vrieze, K.; van Santen, B. F. K.; Stam, C. H. J . Organomet. Chem. 1986, 304, 353. Keijsper, J.; Polm, L. H.; van Koten, G.; Vrieze, K.; Schagen, J. D.; Stam, C . H. Inorg. Chim. Acta 1985, 103, 137. Polm, L. H.; van Koten, G.; Vrieze, K.; Stam, C. H.; van Tunen, W. C. J. J . Chem. Soc., Chem. Commun. 1983, 1177. Staal, L. H.; van Koten, G.; Vrieze, K.; van Santen, B. F. K.; Stam, C. H. Inorg. Chem. 1981, 20, 3598. Kokkes, M. W.; Stufkens, D. J.; Oskam, A. J . Chem. SOC.,Dalton Trans. 1984, 1005. (a) Casarin, M.; Ajb, D.; Vittadini, A.; Ellis, D. E.; Granozzi, G.; Bertoncello, R.; Osella, D. Inorg. Chem. 1987, 26, 2041. (b) Casarin, M.; Ajb, D.; Granozzi, G.; Tondello, E.; Aime, S. Inorg. Chem. 1985, 24, 1241. (c) Casarin, M.; Vittadini, A.; Vrieze, K.; Muller, M.; Granozzi, G.; Bertoncello, R. J . Am. Chem. SOC.1988, 110, 1775.
of a series of ferracyclopentadienyl diiron complexes (Fez(CO),(iPr-DAB)(C4R4)) obtained from the reactions of alkynes with metal carbonyl a-diimine complexes. Along the series t h e iPr-DAB (iPr = isopropyl) ligand is present in different structural arrangements (see 1-111 in Figure l), as a result of steric properties of t h e alkynes.” The investigation has been carried out by combining SCF-first-principle discrete variational (DV)-Xa calculations with gas-phase U V photoelectron (PE), solid-state X-ray PE (XPS), and Mossbauer spectroscopies. Quite recently some of us investigated, within t h e same theoretical framework, the electronic structure of the related molecules Fe2(C0)6(C4H4) loa (IV), R u ~ ( C O )R-DAB)(p-CO) ~( *OC (V), and RU~(CO)~(R-DAB)(~-HC=CH)’~ (VI). In t h e following, an extensive use of comparisons with these theoretical results will be made in order to obtain a quantitative understanding of the different electronic behavior of t h e R-DAB ligand in different coordinative modes. Furthermore, such a comparison gives us t h e opportunity to go into the details of t h e Mossbauer spectrum of I V published more than 15 years ago.12a,b Selected contour plots ( C P s ) of some molecular orbitals (MOs) important in describing the metal-ligand interactions are reported and discussed. Moreover, in order to facilitate t h e discussion of energy level distribution, we will m a k e use of a density of states (DOS) analysis. Experimental Section Synthesis. The reported compounds were synthesized according to the published procedures.” After crystallization, their purity was checked by IR and IH N M R spectroscopies. Theroetical Method. S C F Hartree-Fock-Slater (HFS)-discrete variational (DV)-Xa c a l c ~ l a t i o n swere ~ ~ performed on a VAX-8600 computer at the computing center of the University of Padova. The approximations of the reported theoretical calculations are (i) the use of near-minimal atomic orbital (AO) basis sets, (ii) a SCC approx(1 1) (a) Muller, F.; Han, I. M.; van Koten, G.; Vrieze, K.; Heijdenrijk, D.; de Jong, R. L.; Zoutberg, M. Inorg. Chim. Acta 1989, 158, 81. (b) Muller, F. Ph.D. Thesis, Amsterdam, 1988. (12) (a) Herber, R. H.; King, R. B.; Wertheim, G. K. Inorg. Chem. 1964, 3, 101. (b) King, R. B.; Epstein, L. M.; Gowling, E. W. J . Inorg. Nucl. Chem. 1970, 32, 441. (c) Emerson, G. F.; Mahler, J. E.; Pettit, R.; Collins, R. J . Am. Chem. Soc. 1964,86,3590.(d) Collins, R.;Pettit, R. J . Am. Chem. SOC.1963, 85, 2332. (e) Greenwood, N. N.; Gibb, T. C. In Mossbauer Spectroscopy; Chapman and Hall: London, 1971; p 227. (13) (a) Averill, F. W.; Ellis, D. E. J . Chem. Phys. 1973, 59, 6412. (b) Rosen, A,; Ellis, D. E.; Adachi, H.; Averill, F. W. J. Chem. Phys. 1976, 65, 3629 and references therein. (c) Trogler, W. C.; Ellis, D. E.; Berkowitz, J. J . Am. Chem. SOC.1979, 101, 5896.
0020-1669/89/1328-4243$01.50/0 0 1989 American Chemical Society
4244 Inorganic Chemistry, Vol. 28, No. 23, 1989
Bertoncello et al.
Table I. Atomic Character from the SCC DV-Xa Calculation for Fe2(C4H4)(Me-DAB)(CO)4(I) population, % Fe Fe’ eigenvalue, MO -E,eV TSIE.eV s p d s p d (CO)., 3CO 4 8 3 0 -1 19 0 1 34a’” 3.35 1 0 2 33a’ 4.79 7.56 0 0 90 0 0 12 8 1 22a“ 5.1 1 7.88 0 0 61 0 2 6 13 6 32a‘ 5.45 8.22 1 5 3 7 0 0 1 16 26 31a’ 6.34 9.1 1 1 2 2 5 0 4 11 48 10 2 1a” 6.60 9.37 0 1 8 0 3 19 54 6 30a’ 6.70 9.47 0 1 5 0 2 19 55 6 29a’ 6.82 9.59 0 1 3 0 5 9 1 21 20a” 7.45 10.22 0 2 3 0 0 8 0 0 19a” 7.71 10.48 0 1 2 0 0 6 4 18a” 8.21 10.98 0 2 8 0 3 3 4 9 4 28a’ 8.74 11.51 0 4 5 0 2 2 21 3 27a‘ 9.36 12.13 0 0 1 0 0 0 0 17a” 9.46 12.23 0 0 25 0 0 8 22 4 1 26a’ 9.82 12.59 2 0 11 0 0 a
DAB 63 3 9 27 3 4 2 0 9 81 16 33 15 59 30
C4H4 character 3 r3*DAB 4 \
:T
11 11
55
7 58 39 58 8 30
{
r3* DAB 73* C4H4 ~2
C4H4
n-C4H4
TI
+
+ Il-DAB
C4H4
DAB - n-c.~,
Lowest unoccupied MO. C
i-Pr r
I . ,... ... ..- .. --,
IIP (RI=R4=C(CH3)2(0H), RZ=R3=H) Ilb (Rl=R3=C(CHJ)Z(OH), RtsR4.H) Ilc (Rl=R3=C(O)O(CH3), RZ=R4=H)
0’
IV 0
0 i-Yr
V
-\o
c y 0
VI
Figure 1. Schematic views of investigated molecules. The reference framework is also given. imation of the Coulomb potential, representing atoms by overlapping spherical charge distribution^,^^^ (iii) the use of the Gaspar-Kohn-Sham exchange potential,I4 (iv) neglect of relativistic effects, and (v) Slater’s transition-state (TS) formalismls to calculate the ionization energies (IE’s). Numerical A O s (through 4p on Fe, 2p on C, N, and 0, and 1s on H) obtained for the neutral atoms were used as basis functions. Due to the size of the investigated systems, orbitals Is-3p (Fe) and 1s on carbon, nitrogen, and oxygen were treated as a part of a frozen core in the molecular calculations. Gross atomic charges and bond overlap populations (OP‘s) were computed by using Mulliken’s scheme.16 The (14) (a) Gaspar, R. Acta Phys. Acad. Sci. Hung. 1954,3,263. (b) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140A, 1133. (15) Slater, J. C. Quantum Theory of Molecules and Solids. The Self Consistent Field For Molecules and Solids; McGraw-Hill: New York, 1974; Vol. 4. (16) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833.
experimental geometry” of I was idealized to C,symmetry for use in the calculations (see Figure 1). In order to save computer time, the electronic properties of the R-DAB substituents (R = isopropyl) have been simulated by replacing them with methyl groups. Moreover, despite the lack of any symmetry element in the solid-state structure of compound 11,” a SCC ground-state calculation on a prototype molecule with the R-DAB bonded to Fe’” has been run to investigate, at least semiquantitatively, differences in relative bonding schemes. The M O s have then been labeled according to the irreducible representations a’ and a”.18 Instead of the eigenvalues being displayed along an energy axis, the density of states (hereafter DOS) has been plotted as a function of energy. The component, or partial, density of states function (PDOS) for atomic basis function j is constructed according to the published procedure.19 These plots have an advantage over molecular energy level schemes because they provide insight into the disposition and composition of orbitals over a broad range of energy. UV Photoelectron Spectra. He I and He I1 excited PE spectra were measured on a Perkin-Elmer PS-18 spectrometer modified for He I1 measurements by inclusion of a hollow-cathode discharge lamp giving high output of He I1 photons (Helectros Developments). The ionization energy (IE) scale was calibrated by reference to peaks due to admitted inert gases (Xe-Ar) and to the He 1s-l self-ionization. A heated inlet probe system was adopted at 90-1 10 OC. X-ray Photoelectron Spectra. X-ray photoelectron spectra were recorded with a Vacuum Generator Ltd. ESCA 3 MK I1 spectrometer using A1 K q 2 excitation (1486.6 eV) at a residual pressure of lo-’ Pa. Samples were dusted as a thin film over a gold plate to minimize charging effects. Calibration was made by the 4f7,* gold signal at 83.7 eV. Sample charging was corrected by referencing to the CISline of carbon in the contaminating oil, taken at 285.0 eV. The accuracy of the measured binding energy was estimated at h0.2 eV. Throughout XPS measurements the samples were cooled to liquid-N2 temperature to prevent secondary decomposition effects. Peak deconvolution was performed with a Du Pont 310 curve resolver using a Gaussian shape fit. Miissbauer Spectra. Mijssbauer-effect spectra were obtained on a conventional constant-acceleration spectrometer that utilized a roomtemperature rhodium matrix cobalt-57 source. It was calibrated at room temperature with natural-abundance a-iron foil. The spectra were fitted to Lorentzian line shapes by using standard least-squares computer minimization techniques. The error analysis was carried out by using error propagation techniques. All the components of each spectrum were allowed to vary as symmetric doublets until the best fit was obtained. Results and Discussion Theoretical Results. Molecular systems I and IV are very similar indeed: they differ only in the substitution of two carbonyls (17) In such a calculation a perfectly planar Fe(CO),(C4H4) unit, with the
same Fe(C0) and Fe(C4H4)mean bond distances of I, is bonded to a Fe’(CO)(R-DAB) fragment, where no semibridging carbonyl is assumed. ( 1 8) Symmetry properties of I and I1 are herein described by the C, symmetry point group. The a’ and a” M O labeling allows us to distinguish r3*and/or rl (both a’ in symmetry) from r 2(a”) involvement in the metal-(C4H4) interaction. (19) Holland, G. F.; Ellis, D. E.: Trogler, W. C. J . Am. Chem. Soc. 1986, 108, 1884.
Inorganic Chemistry, Vol. 28, No. 23, 1989 4245
Dinuclear Metal Carbonyl Diimine Complexes Table 11. Atomic Character from the SCC DV-Xa Calculation for Fe(Me-DAB)CO population, %
MO
Fe
eigenvalue, -E, eV
lla"b
2.24 2.69 3.15 3.75 4.11 6.12 7.44
16a' 15a' 1Oa" 14a'
9a" Sa"
7.69
13a' 12a' 7a" 1 la'
8.41
8.78 8.89
" 11 refers to the xy
plane.
S
P
0 1 0 0 0 0 0 0 1 0 1
19 16
d 65 39 96 70 65 4
0 0 0 0 0
18
1 0
11 0
0 0
3 4
co 3 6 0 18 17 5 4 9 31 93 92
2c
2N 9 22 0 3 7 57 53 43 13 2
2 CH3 3 4 3 1 1 13 15 13 8 2 0
0 12 1 8 10 21 3 12 46 0 2
1
2H 1 0 0
0 7
character"
}
dxz + a 3 *
3d-like MO's a2
9
nn+
1 0 0
;5a co co
Lowest unoccupied MO.
Table 111. Atomic Character from the SCC DV-Xa Calculation for
Fe(COh
2
population, 7% eigenvalue, MO 13a'
12a'b 8a" 7a" 1 la' 1Oa'
6a" 9a' 5a" 8a' 4a" 7a'
-E,eV 3.43
4.38 4.41
6.28 6.26 6.31
9.64 9.76 9.77 10.53 10.62 10.75
Fe s
p
d
18 41 17 0 30 45 0 31 48 0 O 0 0 0 0 0 0 0
" I and 11 refer to xy
0 68 0O 64 0 0 0 6 0 4 0 5 0 11 0 8
plane.
3CO
charactef
3
24 25 21
13a'
32 3d-like M O s + a* 3352 ) 100 73% aCOll + 27% TCO,
94 49% aCOll + 45%aCO, 96 83% 5aCOll + 13% 5 a C O l 95 89 92
86% 5 a C 0 ,
23% a CO,
+ 66% a C O l l
92% 5aCOII
4
5
lla' 1 Oa'
Lowest unoccupied MO.
bonded to Fe for R-DAB. In this respect, Muller et al. have anticipated, on qualitative grounds, the different electronic behavior of R-DAB compared to that of two carbonyls in a contribution dealing with syntheses, reactivities, and solid-state structures of the title molecules.11a The importance of testing quantitatively their predictions is quite obvious. On this basis, we decided to carry out a series of ground-state DV-Xa calculations on the electronic structure of I and of the fragments Fe(CO)(R-DAB) and Fe(CO),. Charge density analyses of the relative outermost M O s are reported in Tables 1-111, respectively; in Table IV selected orbital occupation numbers and overlap populations (OP's) are collected, while in Figure 2 energy levels pertaining to the two fragments are compared. The electronic structure of I can be usefully analyzed by focusing our attention on a series of local interactions, namely (i) Fe-(R-DAB), (ii) Fe-(C4H4), (iii) Fe'-(C4H4), and (iv) Fe-Fe'. The first point deserves particular attention because it gives us the opportunity to study both the perturbations induced in IV by replacing two terminal carbonyls with R-DAB and the different electronic behavior of R-DAB in the complex I (a a-N, a-N' chelating 4e donor) and in V and VI (a a-N, a-N', q2-CN, q2-CN' bridging 8e donor) complexes.'OE The former can be easily worked out by making reference to Tables 11-IV and to Figure 2. The outstanding feature of this figure is obvious: as one passes from Fe(CO), to Fe(CO)(R-DAB) there is an overall shift of the electronic levels toward higher energy. Such an effect is the consequence of two different factors, namely (a) the better energy matching between d AO's and in-phase and out-of-phase linear combinations of nitrogen lone pairs (n+ and n-, respectively) compared to that between d AO's and carbonyl-based 5 0 levels (compare in Tables I1 and 111 the different iron participation in MO's mainly n+, n-, and 5 0 in character) and (b) the poorer a-acceptor capability of R-DAB vs that of CO's. Point b is a problem because it is counterintuitive. Actually, the energy matching between metal-based orbitals and the r3*R-DAB level is definitely better than that with CO-based a* orbitals; never-
9a"
6
7
8
9
1(
1'
-E(e
Fe(CW3
Fe(CO)( R-DAB)
Figure 2. Comparison of the energy levels for Fe(CO)3and Fe(CO)(R-
DAB) fragments.
theless, we have to remember that a,* is the only accessible R-DAB virtual level at variance with carbonyls, which, as a whole, have four empty a* orbitals, two parallel and two perpendicular to the xy plane (in our framework). In conclusion, the analysis of theoretical and data relative to Fe(CO)(R-DAB) and Fe(CO), points out that the substitution of two carbonyls with R-DAB gives rise to a higher charge density on the metal atom (see different iron charges in Table IV), in agreement with previous qualitative predictions.' la If we go on with a comparison between I and IV and keep in mind what has been just reported about the electronic structure of Fe(CO), and Fe(CO)(R-DAB), it turns out that an important difference between I and IV is the nitrogen-induced charge accumulation on the Fe atom, which will partially overcome its
4246 Inorganic Chemistry, Vol. 28, No. 23, 1989
Bertoncello et al.
Table IV
Selected Orbital Occupation Numbers 1
Fe 6.47 0.13 0.32 1.08
3d 4s 4P
Q
Fe‘ 6.51 0.05 0.37 1.07
2s 2P
c,
c,
CODA’
Na
Csb
1.42 3.26 -0.68
1.18 3.09 -0.27
1.08 3.09 -0.16
1.57 3.72 -0.29
1.36 2.76 -0.12
Fe(CO)(R-DAB) Fe 6.58 0.21 0.34 0.87
3d 4s
CBDAB 2s 2P
1.06 3.07 -0.13
N a
Cartony1
1.55 3.81 -0.36
1.36 2.78 -0.14
Fe(C0h Fe 6.56 0.03 0.47 0.94
3d 4s 4P
Q
Carbonyl 2s 2P
1.37 2.75 -0.12
Fe,(CO),(C,H,)(R-DAB)‘ Fe 6.51 0.10 0.39 1 .oo
3d 4s 4P
Q
I Fe(CO)(R-DAB) Fe(W3 Fe,(CO)4(C4H4)(R-DAB)4
11-like prototype molecule.
Fe’ 6.42 0.17 0.37 1.04
2s 2P
Fe-Fe’ 0.03
Fe-C, 0.53
0.13
0.48
1.42 3.26 -0.68
1.17 3.11 -0.28
Selected OverlaD . Populations . Fe-Nu Fe-C, Fe-C8 Fe-C,b 0.44 0.19 -0.12 0.13 0.55
Fe-Csb 0.18
C,DA’
Na
1.06 3.08 -0.14
1.57 3.69 -0.26
Nu
