724
Organometallics 2010, 29, 724–731 DOI: 10.1021/om900351y
Photoelectron Spectra of Phosphine Analogue Complexes of Co(CO)3NO and CpMn(CO)3 Csaba Istv an Pongor,† Zsolt Gengeliczki,†,§ Laszl o Szepes,† Frank Axe,‡ and ,‡ Balint Sztaray* †
Institute of Chemistry, E€ otv€ os Lor and University, Budapest, Hungary 1117 and ‡Department of Chemistry, University of the Pacific, Stockton, California 95211. § Current address: Department of Chemistry, Stanford University, Stanford, CA 94305. Received May 4, 2009
The electronic structure of organometallic complexes with the phosphine analogue ligands trimethylarsine and trimethylstibine was studied. Four new complexes, CpMn(CO)2AsMe3, CpMn(CO)2SbMe3, Co(CO)2NOAsMe3, and Co(CO)2NOSbMe3, were synthesized, and their He I and He II photoelectron spectra were recorded. The first ionization energies, which correspond to ionization from mainly metal d orbitals, are 6.83, 6.83, 7.58, and 7.69 eV, respectively. These numbers correspond to an approximately 1 eV uniform destabilization of the metal d orbitals, with respect to the parent carbonyls. The lone-pair orbital stabilization, with respect to the free ligands, was significant: 1.00, 0.94, 1.70, and 1.34 eV. The trends observed in these ionization energies were explained by the σ- and π-donor and π-acceptor properties of the ligands, on the basis of the changes in the hybridization of lone-pair orbitals.
Introduction Transition-metal complexes are used as catalysts in numerous reactions of biological and industrial importance. Their ligands have an important influence on the electronic structure, catalytic activity, and physical and chemical properties of the complexes, setting boundaries to their applications. Therefore, systematic studies of the electronic structure of transition-metal compounds will be indispensable in future catalyst design. Since phosphine is one of the most widely used ligands in practically important organometallic compounds, its arsenic and antimony analogues are intriguing subjects, as their effect on the electronic structure of the metal complexes may be only slightly different from that of the phosphines, allowing the fine-tuning of the charge density on the central metal. The different size and polarizability of arsenic compared to those of phosphine lead to a change in the cone angle, which can also be used to an advantage in designing new organometallic catalysts. It has been shown that diarsine or mixed phosphine-arsine ligands can exhibit higher turnover frequencies in catalytic hydroformylation reactions while retaining excellent selectivity.1 Arsine analogues of dehydrogenation catalysts have also been reported and found to be surprisingly stable.2 Even chiral arsine ligands were developed for tandem Suzuki cross-coupling-Heck reactions, and better yield and *To whom correspondence should be addressed. E-mail: bsztaray@ pacific.edu. (1) van der Veen, L. A.; Keeven, P. K.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Chem. Commun. 2000, 5, 333–334. (2) Loza, M. L.; Crabtree, R. H. Inorg. Chim. Acta 1995, 236 (1-2), 63–66. (3) Kojima, A.; Boden, C. D. J.; Shibasaki, M. Tetrahedron Lett. 1997, 38 (19), 3459–3460. pubs.acs.org/Organometallics
Published on Web 01/19/2010
enantiomeric excess were obtained in comparison to the phosphine analogue.3 Recently, triphenylarsinyl-functionalized N-heterocyclic carbene ligands have been shown to be useful ligands on a palladium center in Heck, hydro-Heck, π, σ domino-Heck, and Suzuki C-C coupling reactions.4 These examples show that there is a potential use of arsine ligands, while the lack of extensive literature can also be attributed to the bad reputation of arsenic, in general. In this work, we present the synthesis of four new complexes, CpMn(CO)2(AsMe3), CpMn(CO)2(SbMe3), Co(CO)2NO(AsMe3), and Co(CO)2NO(SbMe3), along with their UV photoelectron spectra. The previously reported photoelectron spectra of the phosphine complexes CpMn(CO)2(PMe3) and Co(CO)2NOPMe3 along with the parent carbonyls CpMn(CO)3 and Co(CO)3NO are also included for comparison. The two parent compounds are widely used as catalysts in synthetic organic chemistry;5,6 they have important use in chemical vapor deposition,7,8 and Co(CO)3NO can be used as a precursor of cobalt nanotubes.9,10 (4) Stiemke, F.; Gjikaj, M.; Kaufmann, D. E. J. Organomet. Chem. 2009, 694 (1), 5–13. (5) Roustan, J. L.; Bisnaire, M.; Park, G.; Guillaume, P. J. Organomet. Chem. 1988, 356 (2), 195–197. (6) Kubota, T.; Okamoto, H.; Okamoto, Y. Catal. Lett. 2000, 67 (2-4), 171–174. (7) Lane, P. A.; Oliver, P. E.; Wright, P. J.; Reeves, C. L.; Pitt, A. D.; Cockayne, B. Chem. Vap. Deposition 1998, 4 (5), 183. (8) Ivanova, A. R.; Nuesca, G.; Chen, X. M.; Goldberg, C.; Kaloyeros, A. E.; Arkles, B.; Sullivan, J. J. J. Electrochem. Soc. 1999, 146 (6), 2139–2145. (9) Liu, S. W.; Zhu, J. J.; Mastai, Y.; Felner, I.; Gedanken, A. Chem. Mater. 2000, 12 (8), 2205–2211. (10) Rana, R. K.; Koltypin, Y.; Gedanken, A. Chem. Phys. Lett. 2001, 344 (3-4), 256–262. (11) Lichtenberger, D. L.; Fenske, R. F. J. Am. Chem. Soc. 1976, 98 (1), 50–63. r 2010 American Chemical Society
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Both the parent compounds CpMn(CO)311,12 and Co(CO)3NO13 and many transition-metal phosphine complexes14-21 have been studied using photoelectron spectroscopy. However, their arsine and stibine derivatives were much less the focus of recent research.22,23 For the present study, the trimethyl derivatives were chosen, because the photoelectron spectra of these compounds can be wellresolved experimentally and tendencies in the spectra can be recognized.
Experimental Section Synthesis of the Samples. All manipulations were carried out under an inert nitrogen atmosphere using standard Schlenk techniques. The solvents (hexane, tetrahydrofuran, dibutyl ether, and diethyl ether) were purchased from Reanal and were freshly distilled from sodium and benzophenone before use. Co(CO)3NO and CpMn(CO)3 were obtained from Sigma-Aldrich; no further purification was needed. Trimethylarsine and trimethylstibine were synthesized according to the literature.24,25 Briefly, the appropriate trichloride was methylated using methylmagnesium iodide. The Grignard reagent was prepared in an ether type solvent, and the solution of the trihalide was added dropwise. In the case of Me3As, dibutyl ether was used, and in the case of Me3Sb, diethyl ether was used. The resulting trimethyl products were then distilled and used in the preparation of the complexes. Trimethylphosphine was purchased from Sigma-Aldrich and was used without further purification. Co(CO)2NOL (L = AsMe3, SbMe3). The following method was used for both the trimethylarsine (AsMe3) and the trimethylstibine (SbMe3) monosubstituted derivatives of Co(CO)3NO. Under an atmosphere of nitrogen, 2.5 g (14.45 mmol) of Co(CO)3NO was dissolved in 20 mL of tetrahydrofuran (THF). After the addition of a small excess of trimethylarsine or trimethylstibine (14.58 mmol), the reaction mixture was stirred for 48 h at 37 °C and for 5 days at room temperature, respectively. The solvent and the excess of the reactants were removed under reduced pressure, leaving dark red oily and dark red crystalline products, respectively. The yield was 70% or better (Table 1). Mass spectrometry has been used to prove the structure and purity of products, which was better than 95%. CpMn(CO)2L (L=AsMe3, SbMe3). The PMe3, AsMe3, and SbMe3 derivatives of CpMn(CO)3 were synthesized with the (12) Calabro, D. C.; Hubbard, J. L.; Blevins, C. H.; Campbell, A. C.; Lichtenberger, D. L. J. Am. Chem. Soc. 1981, 103 (23), 6839–6846. (13) Hillier, I. H.; Guest, M. F.; Higginso, B. R.; Lloyd, D. R. Mol. Phys. 1974, 27 (1), 215–223. (14) Gengeliczki, Z.; Sztaray, B.; Baer, T.; Iceman, C.; Armentrout, P. B. J. Am. Chem. Soc. 2005, 127 (26), 9393–9402. (15) Lichtenberger, D. L.; Jatcko, M. E. Inorg. Chem. 1992, 31, 451– 455. (16) Bursten, B. E.; Darensbourg, D. J.; Kellogg, G. E.; Lichtenberger, D. L. Inorg. Chem. 1984, 23, 4361. (17) Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt, R. J. Inorg. Chem. 1986, 25, 3675. (18) Puddephat, R. J.; Dignard-Bailey, L.; Bancroft, G. M. Inorg. Chim. Acta 1985, 96, L91. (19) Lichtenberger, D. L.; Kellogg, G. E.; Landis, G. H. J. Chem. Phys. 1985, 83, 2759. (20) Lichtenberger, D. L.; Kellogg, G. E. Acc. Chem. Res. 1987, 20, 379. (21) Beach, D. B.; Jolly, W. L. Inorg. Chem. 1986, 25, 875. (22) Flamini, A.; Semprini, E.; Stefani, F.; Cardaci, G.; Bellachioma, G.; Andreocci, M. J. Chem. Soc., Dalton Trans. 1978, 7, 695–698. (23) Schumann, H.; Frank, U.; Dumont, W. W.; Marschner, F. J. Organomet. Chem. 1981, 222 (2), 217–225. (24) Fleming, I. Science of Synthesis: Compounds of Group 15 (As, Sb, Bi) and Silicon Compounds; Georg Thieme Verlag: Stuttgart, Germany, 2003; p 15. (25) Kos, C.; Liu, J.; Irgolic, K. J. Organometallic Syntheses; Elsevier: Amsterdam, 1988; Vol. 4.
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Table 1. Summary of the Synthesis Results yield (%)a
E
isolated as
EMe3Co(CO)2NO As Sb
73 70
dark purple solid dark orange liquid
CpMn(CO)2EMe3 P As Sb
44 36 40 a
yellow crystals orange crystals orange crystals
Based on Co(CO)3NO or CpMn(CO)3.
following strategy. First, a carbonyl ligand was substituted for the good leaving group THF using a UV photochemical reactor equipped with a medium-pressure mercury lamp. This reactive complex (CpMn(CO)2THF) readily reacts with trimethylphosphine, trimethylarsine, or trimethylstibine to give the appropriate compound. Under an atmosphere of nitrogen in a photoreactor analogous to that described by Strongheimer,26 1 g (4.9 mmol) of CpMn(CO)3 was dissolved in 150 mL of tetrahydrofuran (THF). The mixture was cooled in an icewater bath with constant stirring. The ligand exchange reaction was initiated with UV light, and the extent of the reaction was monitored with a petroleum oil bubbler. When the bubbling stopped (after approximately 2 h), a THF solution of a small excess trimethylphosphine, trimethylarsine or trimethylstibine was added to the dark purple solution, which was then stirred for 20 h at room temperature. During this time, the color of the solution turned yellow. The solvents and excess reactants were removed under reduced pressure, resulting in a yellow crystalline material for the phosphine and a dark orange oil in the case of the arsine and stibine derivatives. These were then dissolved in 20 mL of hexane and recrystallized. CpMn(CO)3 had to be removed using sublimation at room temperature under reduced pressure (1-2 Torr). CpMn(CO)2PMe3 was isolated as yellow crystals and CpMn(CO)2AsMe3 and CpMn(CO)2SbMe3 as bright orange crystals. The yield was about 40% (Table 1). Mass spectrometry has been used to prove the composition and purity of products, which was always better than 95%. Ultraviolet Photoelectron Spectra. The He I photoelectron spectra were recorded using a custom-built ATOMKI ESA-32 instrument, which has been described in detail earlier.27 The manganese compounds were evaporated into the ionization chamber at 75 °C using the heated direct sample inlet system. The cobalt compounds, due to their higher vapor pressure, were leaked into the instrument at room temperature. The energy resolution was better than 30 meV. The spectra were calibrated against the Ar 2P3/2 peak (IE=15.759 eV) and the CO 12Σþ peak when present due to the slight thermal decomposition of the sample. The He II spectra were recorded with the same instrument, using lower helium pressure in the discharge lamp. As there was no attempt made to draw quantitative conclusions from the intensities, the spectra were not corrected for the He IIβ line. The fitting process was done using Shirley-type backgrounds and Voigt-type peaks.28,29 Quantum-Chemical Calculations. Ionization Energies. To help the interpretation of the photoelectron spectra, quantumchemical calculations have also been carried out. For the calculation of the vertical ionization energies, calculations at the DFT level of theory (B3LYP and BLYP functionals) were performed using the 6-311G(d,p) basis set for the compounds containing P (26) Strohmeier, W. Angew. Chem., Int. Ed. 1964, 3 (11), 730. (27) Csakvari, B.; Nagy, A.; Zanathy, L.; Szepes, L. Magy. Kem. Foly. 1992, 98 (10), 415–419. (28) Shirley, D. A. Phys. Rev. B 1972, 5 (12), 4709. (29) Seah, M. P.; Gilmore, I. S. Phys. Rev. B 2006, 73 (17).
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and As; the cc-pvtzp-PP30,31 and the LanL2dzdp32,33 basis sets for Sb were used as implemented in the Gaussian03 quantum-chemical package.34 Reasonable starting geometries of the complexes were optimized at smaller basis sets to serve as starting points for the DFT calculations. Relativistic effects were not considered in the calculations. According to Koopmans’ theorem,35 the negatives of the Hartree-Fock orbital energies can be taken as vertical ionization energies. For the Kohn-Sham orbital energies, however, a similar approximation is not as straightforward, because their physical meaning is widely debated. It is well-known that the negative of the energy of the highest occupied molecular orbital (HOMO) corresponds to the exact first ionization energy when an exact functional is used.36 In their recent works, Baerends and co-workers established a Koopmans-like relationship between the other ionization energies and the Kohn-Sham orbital energies.37 Politzer found that, for small organic and inorganic molecules, the approximation of ionization energies with orbital energies using nonexact functionals is more consistent than the Koopmans’ theorem.38 Similarly, reliable assignments of the photoelectron spectra of cobalt tricarbonyl nitrosyl (Co(CO)3NO) and its phosphine derivatives were given using KohnSham orbital energies, when Koopmans’ theorem failed to provide even a qualitative reproduction of the ionization energies.39 In our most recent study, we have proposed a slightly modified first-principles method that can be used for transitionmetal complexes.40 It includes a ΔDFT calculation of the first vertical ionization energy and shifting of the Kohn-Sham orbital energies to match the ΔDFT value for the first IE. Both methods (i.e. shifting to the experimentally determined first IE (A) or to the ΔDFT first IE (B)) were utilized in the present study. The ΔDFT-calculated first vertical ionization energies were consistently too low (0.15-0.20 eV for CpMn(CO)2EMe3 and 0.12-0.28 for Co(CO)2NOEMe3) in comparison to the experimental numbers; therefore, ionization energies obtained with method A are shown in the figures of the photoelectron spectra and given in the tables. In general, the BLYP calculations proved to be more in line with the experimentally determined vertical ionization energies. In the case of Co(CO)2NOSbMe3, the two different antimony basis sets performed equally well, while the LanL2DZDP basis set proved to be slightly better for the other SbMe3 complex; therefore, these values are given in Tables 3 and 5. The calculated ionization energies and Mulliken-type population analyses were used for the assignment of the photoelectron peaks. Charge Distributions. Hirschfeld and Voronoi analysis were performed with the Amsterdam Density Functional (ADF 2007.01) quantum code package41 in order to calculate the charges on the ligands. Hirshfeld charge42,43 of a certain fragment in a molecule can be obtained as the integral of the SCF charge density over space, in each point weighted by the relative (30) Metz, B.; Stoll, H.; Dolg, M. J. Chem. Phys. 2000, 113 (7), 2563– 2569. (31) Peterson, K. A. J. Chem. Phys. 2003, 119 (21), 11099–11112. (32) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82 (1), 299–310. (33) Check, C. E.; Faust, T. O.; Bailey, J. M.; Wright, B. J.; Gilbert, T. M.; Sunderlin, L. S. J. Phys. Chem. A 2001, 105 (34), 8111–8116. (34) Gaussian 03, Revision C.02; Gaussian, Inc., Pittsburgh, PA, 2004. (35) Koopmans, T. Physica 2007, 1, 104–113. (36) Perdew, J. P.; Levy, M. Phys. Rev. Lett. 1983, 51 (20), 1884–1887. (37) Chong, D. P.; Gritsenko, O. V.; Baerends, E. J. J. Chem. Phys. 2002, 116 (5), 1760–1772. (38) Politzer, P.; bu-Awwad, F. Theor. Chem. Acc. 1998, 99 (2), 83– 87. (39) Gengeliczki, Z.; Bodi, A.; Sztaray, B. J. Phys. Chem. A 2004, 108 (45), 9957–9961. (40) Gengeliczki, Z.; Pongor, C. I.; Sztaray, B. Organometallics 2006, 25 (10), 2553–2560. (41) ADF 2007.01; 2007. (42) Hirshfeld, F. L. Theor. Chim. Acta 1977, 44, 129. (43) Parr, R. G.; Ayers, P. W.; Nalewajski, R. F. J. Phys. Chem. A 2005, 109, 3957–3959.
Pongor et al. fraction of the (initial) density of that fragment in the total initial (sum-of-fragments) density. The Voronoi charge analysis44 consists of assigning the charge density in a point in space to the nearest atom. The Voronoi cell of an atom is the region in space closer to that atom than to any other, and Voronoi charge can be regarded as change in electron density upon formation of bond between two fragments of a molecule. In the cobalt compounds, the neutral cobalt atom and the neutral ligands at their positions in the equilibrium geometry of the complexes were taken as reference states. In the manganese compounds, Mnþ ion and Cp- anion were taken as reference states. In all cases, the BLYP GGA functional was utilized with the DZP basis set on hydrogen and second-row elements (C, N, O) and the TZP basis set on the metal, phosphorus, and arsenic atoms, while the ZORA TZP basis set was used on the antimony atom. The obtained charges are not accurate absolute values; the changes in the electron density, however, give a good insight into the effect of bonding formation on the electron density of the fragment. The results are summarized in Table 2.
Results and Discussion CpMn(CO)2L (L = PMe3, AsMe3, SbMe3). He I photoelectron spectra are presented in Figure 1; the experimentally determined and DFT calculated ionization energies are given in Table 3. The spectrum of the parent carbonyl complex CpMn(CO)3 is also shown for comparison. In the low-energy region of the phosphine, arsine, and stibine spectra, there are three bands, the third (from the left) of which is only present as a shoulder of the large unresolved band above 12 eV, comprised of mostly localized ligand orbitals. The first band in all three phosphine analogue complexes is shifted by approximately 1 eV with respect to the parent compound’s spectrum. The second band seems to contain three peaks, the third of which is progressively shifted to lower ionization energies from P to Sb. This peak is not only shifted to lower ionization energies but becomes substantially narrower from P to Sb. The third bands (absent in the parent spectrum) exhibit the largest shift between the three spectra, more than 1 eV from the phosphine to the stibine complex. This band comprises two overlapping peaks. In the spectrum of the PMe3 derivative, the positions of these peaks carry some uncertainty because the band partly overlaps with an experimentally unresolved broad band starting above 12 eV. The He II photoelectron spectra are also shown in Figure 1; the signal-to-noise ratio is significantly worse, due to the much lower He II intensity of the discharge lamp. Comparison of the relative band intensities in He I and He II spectra shows a large relative increase for the first two bands in the He II spectra with respect to the third band. Empirical and theoretical studies show that relative ionization crosssections from orbitals with high d character are enhanced at He II photon energy.45 This information, along with the results of the DFT calculations, allows us to assign the photoelectron spectra. Figure 2 shows representative plots of the orbitals involved; Mulliken type population analyses predict similar orbitals in all three compounds. The first band of the photoelectron spectrum can be attributed to metal d orbitals. These orbitals (44) Guerra, C. F.; Handgraaf, J. W.; Baerends, E. J.; Bickelhaupt, F. M. J. Comput. Chem. 2004, 25, 189. (45) Yeh, J. J. Atomic Calculation of Photoionization Cross-Sections and Asymmetry Parameters; Gordon and Breach: Langhorne, PA, 1993.
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Table 2. Fragment Charges in CpMn(CO)2X and Co(CO)2NOX Complexes (a) CpMn(CO)2X Complexes X = CO
X = PMe3
X = AsMe3
X = SbMe3
1.1091 -0.4433 -0.3534 0.0441
1.0999 -0.4342 -0.3515 0.0405
0.075 0.589 -0.383 0.102
0.050 0.596 -0.383 0.120
X = AsMe3
X = SbMe3
0.4640 -0.1973 -0.1978 0.1286
0.4543 -0.1966 -0.1907 0.1296
0.548 -0.229 -0.241 0.151
0.525 -0.230 -0.237 0.172
Hirschfeld Charges Mn Cp CO YMe3
1.1381 -0.3314 -0.2689
1.1367 -0.4503 -0.3542 0.0236 Voronoi Deformation Densities
Mn Cp CO YMe3
0.152 0.704 -0.285
0.112 0.580 -0.384 0.076
(b) Co(CO)2NOX Complexes X = CO
X = PMe3 Hirschfeld Charges
Co CO NO YMe3
0.4824 -0.1220 -0.1161
Co CO NO YMe3
0.588 -0.144 -0.154
0.4865 -0.1985 -0.2030 0.1147 Voronoi Deformation Densities 0.576 -0.229 -0.246 0.128
Table 3. Experimental and Calculated Ionization Energies of CpMn(CO)2EMe3 (E = P, As, Sb)a
a
Calculated at the BLYP/6-311G(d,p) (LanL2dzdp for Sb) level of theory using shifting method A (see text).
also participate in the back-donation of the d electrons of the metal center to unoccupied orbitals of ligands. With respect to the first band of the parent compound’s spectrum, these orbitals are significantly destabilized; due to the electron density being higher on the metal center than what it is in the parent carbonyl. However, there is very little change in the peak position among the PMe3, AsMe3, and SbMe3 derivatives. The second band, which is broad in the phosphine spectrum but rather narrow for the stibine complex, can be assigned to a set of three orbitals. The first two;as for the parent CpMn(CO)3;is assigned to Cp(π)-Mn(d) type orbitals (see Figure 2). The third photoelectron peak in this band can be assigned to the σ-donation of the EMe3 lone pair (also referred to as P-donor, As-donor, or Sb-donor) orbitals. This assignment is supported by the peak shapes.
The full widths at half-maximum (FWHMs) of the first two peaks in the band do not change from P to Sb; the third peak, however, becomes narrower (fwhmP = 0.70 eV, fwhmAs = 0.55 eV, fwhmSb = 0.46 eV). This is in a good agreement with the findings for the free ligands.46 This assignment is also supported by the He I/He II relative areas: the first two orbitals show significantly more d orbital participation. (Since it was impossible to fit proper peaks under the He II band envelope, in Table 3 the relative peak areas are listed together for the three orbitals. However, the shoulder corresponding to the EMe3 σ-donor orbital is (46) Szepes, L.; Nagy, A.; Zanathy, L. Photoelectron spectroscopy of organic derivatives of As, Sb and Bi. In Chemistry of Organic Arsenic, Antimony and Bismuth Compounds; Patai, S., Ed.; Wiley: Chichester, U.K., 1994; pp 265-313.
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Pongor et al.
Figure 1. He I and He II photoelectron spectra of CpMn(CO)2L (L = CO, PMe3, AsMe3, SbMe3). The symbol [ denotes DFTcalculated ionization energies (shifted BLYP).
Figure 2. GaussView plots (with 0.02 cutoff) of typical Kohn-Sham molecular orbitals assigned to the bands in the He I photoelectron spectra of CpMn(CO)2PMe3.
visibly smaller in He II than in He I.) The first ionization energies of the free EMe3 ligands remain nearly constant from P to Sb (see Table 4), because the increasing s character of the orbital counteracts the effect of the decreasing electonegativity. This is not the case in the complexes: the ionization energies of the Mn(d)-E(lp) bonds decrease from 9.79 to 9.42 eV, which obviously exceeds the change of the IE(E(lp)) in the free ligands. The stabilization (compared to that of the free ligand; see Table 4) shows a decreasing trend: in the case of PMe3, the orbital stabilization is 1.19 eV, for AsMe3, it is 1.00 eV, and for SbMe3, it is only 0.94 eV. This is somewhat surprising, as one could expect that the electron-donor ability increases with decreasing electronegativity and increasing polarizability of lone pairs. However, unlike group 16 lone pairs, the lone-pair ionization energy does not decrease significantly from P to Sb; therefore, there is no better energy match between the metal orbitals and the
Table 4. Experimental Ionization Energies and Electron Affinities of the Free Ligands EMe3 (E = P, As, Sb)a PMe3
AsMe3
SbMe3
assignment
8.48 10.3
n(X) C-X
IE 8.6 11.34
8.65 10.7 EA
-3.1 a
-2.7
Taken from ref 46.
lone pair. The explanation of this comes from the finding that the s character of the lone pair orbitals increases significantly from P to Sb,46,47 stabilizing the lone pair (47) Sztaray, B.; Szalay, P. G. J. Am. Chem. Soc. 1997, 119 (49), 11926–11932.
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Organometallics, Vol. 29, No. 4, 2010 Table 5. Stabilization of Lone Pair Orbitals
L = PMe3 L = AsMe3 L = SbMe3
PMe3b
CpMn(CO)2L
free liganda
ΔIE
9.79 9.65 9.42
8.60 8.65 8.48
1.19 1.00 0.94
CoCO2NOL
free liganda
ΔIE
8.60 8.65 8.48
2.21 1.70 1.34
L= 10.81 10.35 L = AsMe3 9.82 L = SbMe3 a From ref 46. b From ref 39.
orbitals on the ligands and leading to reduced overlap with the metal d orbitals. In a qualitative picture, one can conclude that the phosphine and phosphine analogue ligands donate their lone pair to an unoccupied d orbital of the metal center. The decreasing stabilization (compared to that of the free ligand; see Table 5) suggests that the order of σ-donor strength is PMe3 > AsMe3 > SbMe3. The third band of the spectra can be attributed to ionization from orbitals localized on the phosphine analogue ligands. These orbitals are of E-C (E=P, As, Sb) character (see Figure 2), and comparison with the He II spectra does not indicate significant d orbital contribution. Since these orbitals are very similar to those in the free ligands, the ionization energy trends follow that of uncomplexed PMe3, AsMe3, and SbMe3. The EMe3 ligands have local C3v symmetry in the complexes; therefore, the ionization energies of the E-C bonds exhibit Jahn-Teller splitting. That is why two peaks with an intensity ratio of 1:2 could be assigned to this band. CoCO2NOL (L = AsMe3, SbMe3). The other two newly synthesized compounds were also studied: their He I photoelectron spectra are presented in Figure 3, and the experimental and calculated ionization energies are given in Table 6. The He I photoelectron spectra are shown in Figure 3, along with that of the previously published phosphine analogue39 and of the parent tricarbonyl nitrosyl cobalt complex. The spectra exhibit four bands in the low-energy region, the first two of which can also be found in the spectrum of Co(CO)3NO. Unlike in the CpMn(CO)2EMe3 complexes, the second and third bands are resolved. The fwhm of the third band exhibits the same trend as in the manganese complexes: it decreases from 0.74 to 0.44 eV from P to Sb. The fourth band can be deconvoluted into two overlapping peaks with an intensity ratio of 1:2. The He II spectra of these complexes were also recorded and are presented in Figure 2. The relative intensities of the first two photoelectron bands (7.5-9.5 eV) increase significantly when compared to the parent compound, while the third and fourth bands barely rise above the background. The He II spectra along with the quantum-chemical calculations and the similarities to the previously published spectra of the phosphine analogue’s spectrum39 allows one to give detailed assignments of the spectra. The first two bands can be assigned to cobalt d orbitals participating in π bonds with the π*-acceptor type orbitals of the carbonyl and nitrosyl ligands. These photoelectron bands exhibit significant destabilization with respect to the parent carbonyl. In Co(CO)3NO, the first ionization energy
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is 8.75 eV,39 while it is 7.85, 7.58, and 7.69 eV in the case of the PMe3, AsMe3, and SbMe3 substituted complexes, respectively. This trend is analogous to that in the spectra of the CpMn(CO)2EMe3 complexes. The third band in the spectra, between 9.5 and 11.5 eV, can be attributed to the interaction of the lone-pair orbital of the EMe3 ligands with the metal center. The stabilization of these orbitals with respect to the lone pairs in the free ligands is considerable: 2.21, 1.70, and 1.34 eV for P, As, and Sb, respectively. This is in line with the trend observed for the CpMn(CO)2EMe3 complexes; however, the stabilization energies are considerably larger. This can be attributed to geometric and electronic effects: namely, the Cp ring occupies a larger space, restricting the steric arrangement of the carbonyl and EMe3 ligands, while in the case of the Co(CO)2NOEMe3 complex, the complexes assume a tetrahedral geometry with more freedom of the EMe3 ligands to coordinate to the metal. The Cp ring also donates more π electrons to the metal d orbitals, restricting their availability to the EMe3 lone pairs. The fourth band in the spectra can be attributed to the mostly localized EMe3 E-C orbitals, following the destabilization trend observed in the spectra of the free ligands.46 Again, the assumed local C3v symmetry of ligands causes the splitting of the E-C bonds with an intensity ratio of 1:2. Charge Densities. A change in ionization energies upon substitution is often interpreted in terms of electron density shifts from ligands to the metal center. In our previous paper, we claimed that, upon phosphine substitution, the electron density does not increase on the metal center but on the other carbonyl ligands.39 In order to study the effect of bond formation on the electron density of the metal center and the ligands, Hirschfeld and Voronoi density analyses were carried out on the parent compounds and their derivatives. The results are summarized in Table 2. In the CpMn(CO)2L complexes, the metal center along with the remaining carbonyl ligands and the cyclopentadienyl group always become more negative upon phosphine substitution. From P to Sb, the charge density on the carbonyl and cyclopentadienyl ligands does not change substantially, and the flow of electrons from the YMe3 ligands to the manganese center is also negligible. In terms of molecular orbitals, Mn(d) f CO(π*) and Mn(d) f Cp(π) MOs are destabilized when a carbonyl ligand is replaced with a EMe3 ligand, but further destabilization from P to Sb is not significant. This is in line with the above experimental findings. It is interesting to point out, however, that the net change of partial charge on the CpMn(CO)2 moiety is equal to the partial charge of one CO ligand in the CpMn(CO)3 complex (-0.27 according to Hirschfeld and -0.29 according to Voronoi). Therefore, this partial charge is redistributed around the metal center upon the ligand replacement. In the Co(CO)2NOL complexes, the same trends can be observed. The partial charges on the carbonyl and nitrosyl ligands become more negative upon phosphine substitution but remain nearly constant from P to Sb. In terms of molecular orbitals, it means that Co-CO and Co-NO bonds are destabilized upon replacing a carbonyl ligand with a EMe3 ligand, but only minor further destabilization can be observed. This is in line with the experimental findings: that is, the net effect of the σ-donation and the π-acidity remains largely constant from P to Sb. The main difference between the manganese and cobalt compounds is the charge density on the YMe3 ligands.
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Figure 3. He I and He II photoelectron spectra of Co(CO)2NOL (L = CO, PMe3, AsMe3, SbMe3). Spectra for L = CO, PMe3 were adapted from ref 39. The symbol [ denotes DFT-calculated ionization energies (shifted BLYP). Table 6. Experimental and Calculateda Ionization Energies of EMe3Co(CO)2NO (E = P, As, Sb) E = Pb
E = As
E = Sb
He II/He I intensity ratio
exptl
DFT
exptl
DFT
exptl
DFT
calcd orbital character
P
As
Sb
7.85 8.07 8.30
7.85 7.93 8.26
7.58 7.88 8.20
7.58 7.60 7.95
7.69 7.95 8.22
7.69 7.75 8.10
Co(d) f [CO(π*),NO(π*),X(p)]
1.00
1.00
1.00
9.07 9.40
9.04 9.08
8.74 9.02
8.74 8.79
8.75 9.03
8.85 8.91
Co(d) f [CO(π*),NO(π*),X(p)]
1.06
1.23
1.17
10.81
10.49
10.35
10.01
9.82
9.82
EMe3(lp) f Co(d)
0.45
0.60
0.66
11.99 12.48
11.44 11.45
11.32 11.78
10.70 10.71
10.65 11.15
10.29 10.31
0.36
0.40
0.27
3
E-C in EMe
a Calculated at the BLYP/6-311G(d,p) (LanL2dzdp for Sb) level of theory using shifting method A (see text). b Taken from ref 39 and included for comparison.
Again, the same trend can be observed, but the density flow from the ligands to the metal center is now comparable to the redistributed electron density arising from the removed carbonyl ligand. Approximately 50% of the partial charge increment on the Co(CO)2NO moiety is due to the electron density from the EMe3 ligands. This is in accordance with our above assumption that, due to spatial and electronic effects, the Cp ring prevents strong metal-ligand interactions in the manganese compounds. σ Donation and π Acidity of EMe3 Ligands. Phosphine ligands are known to donate their lone pair to the empty d orbitals of the transition-metal center in the complexes. They are also π acids to the extent that depends on the R group present in the PR3 ligand. The π acidity of phosphines is due to the empty antibonding orbitals between the phosphorus and carbon atoms, and it is much weaker than that of CO.40,48-50 (48) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 3rd ed.; Wiley-Interscience: New York, 2001. (49) Pacchioni, G.; Bagus, S. P. Inorg. Chem. 1992, 31 (21), 4391– 4398. (50) Lichtenberger, D. L.; Gruhn, N. E.; Renshaw, S. K. J. Mol. Struct. 1997, 405 (1), 79–86.
We have shown that stabilization of the lone pair - i.e. strength of σ donation - mildly decreases from P to Sb. Of course, the magnitude of the σ donation cannot precisely be determined only on the basis of the lone-pair stabilization, because changes in the charge distribution of the complex also have an effect on the stabilization of these orbitals. The net electron density flow from the EMe3 ligand to the metal center, based both on the calculations and the position of the first bands in the photoelectron spectra, was found to change very little from P to Sb. One can assume that this is due to the decreasing π acidity of the EMe3 ligands from P to Sb. Experimental electron affinities (Table 4) suggest that this might be the case. However, there are no such data available for SbMe3, and this assumption cannot be easily verified from the calculated charge density distributions listed in Table 2, as several other factors affect the net charge distribution. A purely experimental way to approximate the π acidity of various ligands was suggested by Lichtenberger.50 This method uses the splitting of the first three ionization energies to identify the net π-acceptor property of a ligand on an organometallic center with a high local symmetry. According to his formula, the ΔEs values for these three ligands in the CpMn(CO)2EMe3 complexes are -0.276, -0.265, and
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the order of π acidity as CO > PMe3 > AsMe3 > SbMe3. The near-quantitative agreement between the experimental ΔEs values with the Voronoi deformation densities is shown in Figure 4.
Conclusions
Figure 4. Correlation of the experimental ΔEs values in the CpMn(CO)2X complexes with the Voronoi deformation densities in NiX4 (X = CO, PMe3, AsMe3, SbMe3). Table 7. Fragment Charges in NiL4 Complexes L = CO
L = PMe3
L = AsMe3
L = SbMe3
Hirschfeld Charges Ni L
0.496 -0.124
0.482 -0.120
0.385 -0.096
0.381 -0.096
Voronoi Deformation Densities Ni L
0.579 -0.145
0.442 -0.103
0.302 -0.074
0.229 -0.057
-0.252 eV for P, As, and Sb, respectively. This trend shows that the π acidity decreases mildly from PMe3 to SbMe3. However, since there is only a local symmetry in these complexes, there can be a mixing of the orbital characters. Furthermore, this method - as it is purely experimental - does not take into account the experimentally indeterminate splitting in the tricarbonyl complex. The first three peaks in the first band of the photoelectron spectrum of CpMn(CO)3 are not resolved; therefore, the suggested value of ΔEs for the carbonyl ligand was given as -0.69 eV. However, according to Kohn-Sham orbital energies, which have been shown to be a fairly accurate approximation in a number of organometallic complexes,40 there is significant splitting even in the tricarbonyl complex. Depending on the DFT functional and the basis set, the calculated splitting is between 0.30 and 0.51 eV. With the most conservative estimate of 0.30 eV, the ΔEs value for a carbonyl is then -0.39 eV. The trend, -0.39, -0.276, -0.265, and -0.252 for CO, PMe3, AsMe3, and SbMe3, is roughly in line with the fragment charge calculations on these complexes (see Figure 4). As it is not possible to precisely divide the calculated charge densities into donor and acceptor effects, we have also investigated the π acidity of the EMe3 ligands in the NiL4 (L=CO, EMe3) model complexes. The existence of the d10 Ni(CO)4 complex is known to be due to the significant π acidity of the carbonyl ligand. Therefore, the π acidity of the EMe3 ligands can be investigated independently from the σ donation’s effect. The calculated Hirschfeld and Voronoi deformation charges are given in Table 7. In each case, there is a negative charge on the corresponding EMe3 ligand, indicating a π-acceptor interaction. According to Hirschfeld charges, PMe3 is almost as good a π acceptor as the carbonyl ligand, which is not in line with the experimental results or general consensus. The Voronoi deformation densities seem to be more reasonable, indicating
The syntheses of four new transition-metal complexes with arsine and stibine ligands were presented. The electronic structures of these complexes were studied with ultraviolet photoelectron spectroscopy and DFT calculations. The assignment of the photoelectron bands was based on the trends in the spectra, on the photoionization crosssection differences between He I and He II photon energies, and on DFT calculations of the orbital energies. Both the experiments and the calculations show a high d character in the orbitals associated with the lowest energy bands; these were assigned to metal d orbitals in back-donor type interactions with the π-acceptor ligand (CO, NO) orbitals. In the case of the CpMn(CO)2EMe3 complexes, the next band is due to interaction of π type orbitals of the cyclopentadienyl ring and d orbitals of manganese. The subsequent bands were assigned to the ionization of the E(lp)fM(d) bond and to the orbitals localized on the EMe3 ligands. From the photoelectron spectra one might conclude that the overall metal electron density hardly changes from P to Sb. The ionization energy of the E(lp)fM(d) bonds shows a decreasing stabilization of the E(lp) lone pair from P to Sb. This was explained by the increasing s character of the E(lp) lone pair in the free ligands, thus decreasing overlap with the d atomic orbitals. To further study the impact of substitution on the electronic structure of the parent compounds, Hirschfeld and Voronoi charge density analyses were carried out. It revealed that the destabilization of the M(d)fL(π) orbitals is mainly caused by the redistribution of the electron density in the CpMn(CO)2 and Co(CO)2NO moieties, but the EMe3 ligands do not donate siginificant electron density to the metal center. This is especially true for the manganese compounds. Only 10-30% of the increment of the net partial charge on the CpMn(CO)2 moiety is provided by the EMe3 ligand, while it is approximately 50% in the cobalt complexes. This is line with the higher stabilization of the E(lp) lone pair in the cobalt complexes derived from the photoelectron spectra. Due to spatial and electronic effects, the Cp ring seems to be an obstacle to the coordination of the EMe3 ligands to the metal center in the investigated manganese complexes. Although the stabilization of the lone pair - i.e. the σ donation - decreases from P to Sb, the ionization energies of the metal d orbitals along with the net charge flow from the EMe3 ligands remain nearly constant. The assumption that this is due to the π acidity of the EMe3 ligands from P to Sb was probed by determining ΔEs values, as suggested by Lichtenberger, and by charge distribution calculations on d10 NiL4 (L=CO, EMe3) model complexes. It was found that the back-donation from the filled Ni(d) orbitals to the empty P-C antibonding orbitals is not negligible and decreases in the order CO > PMe3 > AsMe3 > SbMe3. Supporting Information Available: Tables giving fitting parameters of the photoelectron spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
