J. Phys. Chem. 1991, 95, 7217-7220
7217
Acceptor Ability and Donor Strength of Biphenyl-like a-Mimine Ligands. A Theoretical and Gas-Phase UV Photoelectron Spectroscopic Study Vincenzo Barone,* Dipartimento di Chimica. Universith di Napoli, via Mezzocannone 4, I-80134 Napoli, Italy
Carla Cauletti, Maria Novella Piancastelli, Dipartimento di Chimica, Universith di Roma, I-001 00 Roma, Italy
Mauro Ghedini, and Marirosa Toscano Dipartimento di Chimica, Universitb della Calabria, I-87030 Arcavacata di Rende (CS), Italy (Received: June 15, 1990; In Final Form: April 26, 1991) The gas-phase photoelectron spectra of 2,2’-bipyrazine and 3,3’-bipyridazine have been recorded and interpreted by means of quantum-mechanicalcomputations. These results, together with those previously reported for 2,2’-bipyridine,2,2’-bipyrimidine, and 4,4’-bipyrimidine, and with computations of electron affinities, atomic charges, and conformational energies, allow a quantitative analysis of the trends of several physicochemical characteristicsin this class of chelating agents. Simple models have also been employed to rationalize some of these trends.
Introduction
experience on similar molecules,I2-l6the PE spectra have been interpreted by means of HAM/3 computation~l~ and the same method was used to compute the electron affinities. From another point of view, the conformation assumed upon complexation by flexible molecules often does not correspond to the equilibrium conformation of the free ligand and, in fact, is raised at the expense of strong steric repulsions. In order to better analyze this point we report also the torsional potentials obtained for all the title molecules by minimal basis set ab initio computations, which, though quite demanding in view of the molecular dimensions, avoid the well-documented shortcomings of semiempirical methcds.”J6+’gao Also the trends of charge distributions (which are the leading terms of the electrostatic interactions) were investigated at the same level.
Transition metals complexed with aromatic a-diimines are currently receiving a great deal of investigation, w h w final goals are to convert and store solar energy’V2or to provide models for intermediates in transition-metalcatalyzedreactions?*‘ In a study on new photocatalysts, Lever and Crutch1ey5Jjrecently reported the significant improvement obtained when R ~ ( b p z ) , ~(bpz + = 2,2’-bipyrazine) is used in place of Ru(bpy)32+(bpy = 2,2’-bipyridine). In a detailed study of bpz-metal complexes, the redox couples were shown, generally, to undergo a positive shift of ca. 0.5 V relative to the corresponding couples in Ru(bpy),*+. This positive shift means that the LUMO r* orbital in bpz is stabilized by about 0.5 eV with respect to bpy. Furthermore, the trends of MLCT (metal to ligand charge transfer) bands of electronic spectra are best explained assuming that bpz has a weaker u-donor strength (Le., a higher ionization potential of the highest lonepair level) with respect to bpy.SI In addition, the data recently reported about some metal complexes of 2-bpm (2-bpm = 2,2’-bipyrimidine) suggest that also the T* LUMO of this ligand is stabilized with respect to that of bpya7+ The examples reported above emphasize the role played by the dinitrogenated chelating ligand on the physical and chemical properties of such complexes and lead us to study in deeper detail the geometric and electronic structure of the above-mentioned ligands together with those of all the remaining biphenyl-like diazines possessing an inter-ring a-diimine moiety, namely bpd (bpd = 3,3’-bipyridazine) and 4-bpm (4-bpm = 4,4’-bipyrimidine). Two features are especially significant in judging the complexing ability of a ligand in terms of its electronic structure, namely the u-donor strength and the *-acceptor ability. The former characteristics is related to the ionization potentials (IPS) of lone-pair molecular orbitals, whereas the latter characteristics is related to the electron afthities (EA’S)of r* orbitals. Ionization potentials have been obtained experimentally recording the UV photoelectron (PE)spectra of bpd and bpz, which com lete the data already available for bpy,” 2-bpm, and 4 - b ~ m . lIn~ view of previous
Experimental Section 2,2’-Bipyrazine and 3,3’-bipyridazine were prepared by literature methods.6.2’ He I and He I1 photoelectron spectra were run on a Perkin Elmer PSI8 instrument equipped with a dual He I/He I1 source (Helectros Development) at temperatures between 75 and 120 OC and using Ar, CH,I, and self-ionizing He internal standards. Ionization energies were computed using Koopman’s theorem (STO-3G) or the transition-state approximation (HAM/3). In the latter case the charge -1/2 is generally removed uniformly from all the occupied valence orbitals (‘diffuse ionization”), thus giving all the ionic levels in a single SCF computation. This procedure is, however, only justified in molecules containing M O s with comparable localization properties. Since this is not the case in azabiphenyls (due to the much higher localization of lone pair (1 1) Maier, J. P.; Turner, D. W. Faraday Discuss. Chem. Soc. 1972,54, 149. (12) Barone, V.; Cauletti, C.; Lelj, F.; Piancastelli, M. N.; RUM, N. J. Am. Chem. Soc. 1902, 104,4571. (13) Barone, V.;Lelj, F.; Caulctti, C.; Piancastelli, M. N.; RUM, N. Mol. Phvs. 1983. 49. 599. 114) Barone; V.; Cauletti, C.; Commisso. L.; Lelj, F.; Piancastelli, M. N.; Russo, N . J. Chem. Res. S 1984, 338. (15) Barone. V.; Bianchi, N.; Lelj, F.; Abbate, G.; Russo, N. T H E 0 CHEM 1984,108, 35. (16) Barone, V.; Lelj, F.; Commisso, L.; Russo, N.; Cauletti, C.; Piancastclli, M. N. Chem. Phys. 1985, 96, 435. (17) Asbrink, L.; Fridh, C.; Lindholm, E. Chem. Phys. Lrrr. 1977,52,63, 69, 72. Asbrink, L.; Fridh, C.; Lindholm, E. QCPE 1980, 12, 393. (18) Barone, V.; Leu, F.; Commisso, L.; Russo, N. Terrahedron 1985,4I,
( I ) Kalyamasundaran, K. Coord. Chcm. Reu. 1982,46, 159. (2) Serpone. M.; Ponlarini, 0.;Jamicson, M. A,; Bolletta, F.; Maestri, M. Cmrd. Chem. Rev. 1983, 50,209. (3) Burton, J. T.; Puddephat, R. J.; Jones, M. L.; Ibers, J. A. Organomcrallics 1983, 2, 1487. (4) k t t , J. A.; Puddephat, R. J. fnorg. Chim. Acra 1984. 89, L27. (5) Cmtchley, R. J.; Lever, A. B. P. J . Am. Chem. Soc. 1980,102,7128. (6) Ctutchlcy, R. J.; Lever, A. B. P. Inorg. Chem. 1982, 21, 2276. (7) Ruminski, R. R.; Petenen, J. A. Inorg. Chcm. 1982, 21, 3706. (8) Overton, C.; Connor, J. A. Polyhedron 1982, I , 53. (9) Sutcliffe, V. F.; Brent Young, 0 . Polyhedron 1984, 3, 87. (IO) Dewar, M. J. S.The Molecular Orbiral Theory of Organic Chcmisfry; McGraw-Hill: New York, 1969.
0022-3654/91/2095-7217$02.50/0
.-._. 101‘4
(19) Barone, V.; Lelj, F.; Russo, N.; Toscano, M. J . Chcm. Soc., Perkin
Trans. 2 1986, 907.
(20) Barone, V.;Lelj, F.; RUM, N. Inr. J . Quantum Chcm. 1%. 29.542. (21) Lafferty, J. J.; Case, F. H. J . Org. Chcm. 1967, 32. 1591. (0
199 1 American Chemical Society
7218 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
Barone et al.
TABLE I: Coaforstrtional Clmncteristics (V, and AE in kcrl &I), ST0-X Net Atomic Clurgea (Q in millkkftrolrr) of Ortho Hydmgew (H) 8nd Nitrogem (N) for Cis (c) and Trans (t) Conformations of Biphenyl-like a-Diimine Ligandsn ligand ~N(c) qN(t) ~H(c) qH(t) A&, VI v2 v3 v4 emin, deg bPY -226 -25 I 72 95 9.5 -6.7 2.5 -2.1 -1.7 45 -121 -148 80 106 8.2 -6.3 3.2 -1.5 -1 .o 42 bpd -248 79 102 8.0 -5.9 3.1 -1.6 -1.2 42 -223 4-bpm (3.5) (-1.2) (-0.8) (44) (8.0) (-6.7) bPZ -209 -23 I 85 105 7.3 -5.1 3.1 -1.6 -1.3 41 2-bpm 0.0 0.0 1.4 0.0 -0.7 30 (47) (0.0) (-0.8) (-0.3) (0.0) (0.0) "AEn is the energy difference between cis and trans conformations. The values in brackets for 4-bpm and 2-bpm are obtained by an extended basis
set (see text).
Figure 1. Structure and labelling of aromatic a-diimines. All molecules are shown in their cis conformation (8 = OO). -40
orbitals), two separate computations were performed for each molecule removing the charge -1/2 uniformly from all the n levels in the first computation and from all the ?r levels in the second one. Analogous procedures were used in the computation of electron affinities. It has been previously shown that the computed ionization energies may strongly depend on the molecular geometry, so that the knowledge of an accurate geometry is a mandatory starting point. In view of previous experience we have used the STO-3G optimized geometries of the azabenzene rings without further modifications and an inter-ring distance of 1.50 A. The torsional potentials have been obtained spanning the angle B (that is the angle between the planes of the two rings) in steps of 30' and then fitting the results by the leading terms of the Fourier expansion n
v(e)= E(e) - ~(00) = y2cvI[i -cos ~ e ) ]
(1)
'I 1
where only even values of j are allowed for 2-bpm since E(Oo) = E( 1 goo), whereas both odd and even values are allowed for the 0 corresponds to the NN cis conother ligands (in which 0 = ' formation).
Results and Discussion The constitution and labeling of the ligands studied in this work are shown in Figure I , the conformational characteristics are summarized in Table I and Figure 2, and the ionic levels in Figures 3 and 4 and Table 11. 1. Conformations. The data reported in Table I show that all the ligands except 2-bpm have "-trans equilibrium conformations and the conformational energies at 30' and 60' suggest the existence of a secondary minimum (the only one in the case of 2-bpm) around 0 = 4 5 O . The torsional potentials of molecules with the same ortho substituents are very similar, thus confirming that modificationsat meta or para positions play only a negligible role (vide infra). Although a comprehensive analysis of the reliability of STO-3G torsional potentials for biphenyl-like molecules will be performed in a forthcoming paper, in Table I we have reported the results obtained for 2-bpm and 4-bpm using a double-f basis set augmented by polarization functions on nitrogen atoms.** The results obtained at this level are in close agreement
F i i
30"
60'
90'
120'
150'
1
2. Comparison between the torsional potentials of Cbpm (0)and
2-bpm (I).
with experimental data for 2-bpmZ3and suggest that asymmetric ligands are reasonably described by minimal basis sets, whereas some inconsistency is apparent for 2-bpm. General trends can, anyway, confidently investigated at the STO-3G level. The torsional potentials of Cbpm (also representative of bpy, bpz, and bpd) and 2-bpm obtained by an extended basis set are compared in Figure 2. The greater conformational freedom of the latter ligand is quite apparent and could have some consequence on the formation of complexes. It is remarkable that the torsional potential of the asymmetric ligands in the interval ' 0 < 8 < 90' is very similar to that of 2-bpm. On the other hand, the potential energy sensibly decreases in the interval 90' < B < 180'. This suggests that the interaction between N-N and CH-CH ortho groups are similar, whereas N-H interactions are negligible, or even attractive. 2. Ionization Potentials. The He I spectra of bpd and bpz are reported in Figure 3. We also recorded the He I1 spectra, but they do not show any appreciablevariation in relative cross sections and are, therefore, of little help in the assignment of peaks. In the lower IE (ionization energy) region (8-12 eV) eight ionic states are expected for both molecules, namely the four higher r orbitals and the for n levels essentially deriving from symmetric and antisymmetric combinations of the two higher 7 orbitals (2 and 3) and the two nitrogen lone pairs of each ring, respectively. The deeper valence MO's having IEs greater than 12 eV give rise to composite band envelopes in the PE spectra and therefore cannot be studied in detail. The assignment of the first eight states is, anyway, not trivial due to the complexity of the spectra and the overlap of several bands, so that the spectral assignment is based essentially on HAM/3 results (seeTable 11); considerationsbased on the relative band areas are used to support the analysis, but not as the main criterion, due to the different cross sections of carbon and nitrogen atomic orbitals and the complexity of band envelopes. The PE spectrum of bpd in the IE region of interest consists of two broad bands with an intensity ratio of =3:5 and maxima at 9.1 and 9.6 eV for the first band and at 11.O and 11.5 eV for ~
(22) Barone, V.; Cristineiano, P. L., submitted for publication.
I Oo
~~
~
~~~
~
~
~~~
~~
~
(23) Fernholt, L.; Romming, C.; Samdal, S.Acra Chcm. Scond. 1981, A35,
707.
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7219
Biphenyl-like a-Diimine Ligands
-E.-
cn
Y
-
4.3
V
.u
I .
b z Figure 4. Experimental first ionization potentials (lower part) and theoretical (full line HAM/3; broken line STO-3G) first electron affinities (upper part) of aromatic a-diimines. All the data are in eV. In the upper part the left scale refers to HAM/3 results and the right scale to STO-3G results. b y
8
10
12
14
16
l*€.(eVl
Figure 3. He I spectra of 3,3’-bipyridazine (bpd) and 2,2’-bipyrazine (bpz). TABLE !I: Comparison between Experimental and Theoretical IE’r of Aromatic o-Mimines bpd bvz level expt HAM/3 STO-3G level expt HAM/3 STO-3G n 8.72 8.68 n 9.08 8.58 n 9.1 8.92 9.10 T 9.3 9.31 7.52 T 9.6 9.47 8.09 n 9.9 9.44 9.43 T 10.61 9.42 T 10.6 10.34 9.32 T 11.0 10.79 9.47 T 11.08 10.35 T 11.2 10.77 9.82 n 11.47 10.68 n 11.12 10.31 n 11.5 11.73 11.52 n 11.6 11.49 10.70 T 11.9 11.55 10.43
the second one. As far as the total cross sections per electron can be considered identical in all n- and u-bands, the first three and second five ionizations should be found in different regions. This is exactly the trend found by HAM/3 computations, whereas the STO-3G results are much more erratic. A broad agreement between HAM/3 and STO-3G results is, however, obtained when considering that several studies on similar molecules have shown that u levels are destabilized at the STO-3G level by about 1 eV.I2-l6 The sequence of ionic states suggested by HAM/3 computations is the following. In the first band the first maximum accounts for the two highest n levels, and the second maximum for the highest u level. In the second band the other three nearly degenerate u levels are ionized at 11.0 eV, whereas the next maximum accounts for the ionization of the last pair of n levels. Also the PE spectrum of bpz shows two composite bands with intensity ratios 35, but with maxima at 9.3 and 9.9 eV for the
bpd
2 - b m 4-bpm
first band, and at 10.6, 11.2, 11.6, and 11.9 eV for the second band. Once again the HAM/3 results suggest that the first band accounts for two n and one u level (although the sequence is probably now n, T , n) and the second band for the other three u and two n levels. The ionization energy of the pair of degenerate n levels is essentially the same as in bpd (1 1.6 vs 1 1.5 eV), but one r level is now considerably more stable than the others. Also in this case comparable results are obtained adding a constant correction of 1 eV to STO-3G IE’s of u electrons. 3. Electron Affinities. In agreement with the greater electronegativityof nitrogen with respect to carbon, all bidiazines have electron affinities larger than bpy, albeit with considerable differences (see Figure 4). It is noteworthy that the highest EA is computed for 4-bpm, even if pyrazine is definitely a better r acceptor than ~yrimidine.2~It seems, therefore, that the presence of nitrogen centers in 4,4’-positions, the para position of the perturbed biphenyl u system, has a particularly strong effect in increasing the electron affinity. The next bidiazine in the acceptor ability scale is bpz, but different theoretical and experimental methods disagree in the relative ordering of bpd and 2-bpm. In fact, HAM/3 computations and cyclovoltammetrically determined reduction potentials suggest that bpd has a greater EA, whereas the opposite result is obtained by STO-3G (see Figure 4) and HMO25 computations. 4. Discussion. The conformational behavior of biphenyl-like molecules is usually described in terms of electrostatic, steric, and conjugative interactions. The steric constraints are essentially the same for all ligands except 2-bpm and the electrostatic interactions between the two rings are negligible or in disagreement with the subtle differences in the conformational behavior of the ligands. In fact, the torsional barriers of bpd and 4-bpm are very similar in spite of the large difference of nitrogen net charges and the torsional barrier of bpy is quite larger than that of 4-bpm in spite of similar nitrogen charges (seeTable I). The same remarks apply to conjugative interactions as measured, for instance, by r-bond orders between the inter-ring carbon atoms. As a matter of fact nearly identical bond orders are obtained for cis and trans conformations and they do not show any correlation with cis-trans energy differences and/or V, terms (u bond orders of 0.2 19.0.231, 0.222,0.239, and 0.198 are obtained at the STO-3G level for bpy, bpd, 4-bpm, bpz, and 2-bpm, respectively). In such circumstances, the only alternative model for explaining the conformational behavior of these ligands rests on the modification of bond energies originated by electron redistribution. STO-3G computations show that the electron distributions of cis and trans conformations are quite different, but the electron transfers essentially involve N (24) h i m , W.Chem. Ber. 1981,114,3789; J. Am. Chem. Soc. 1983,241, 157; 1984, 264, 317; Z. Naturforsch. 1984, 839, 801. (25) Emst, S.; Kaim, W. J . Am. Chem. Soc. 1986, 108, 3518.
7220
J. Phys. Chem. 1991, 95,1220-7228
and H ortho atoms (see Table I). In particular, there is a net N to H charge transfer in each cis conformation, while the contrary occurs in the corresponding trans form. This transfer reduces N-N and H-H repulsions in the cis conformations and increases N-H attractions in the trans forms. The involvement of only the ortho groups explains the similarity of the different ligands, and a more detailed analysis of charge transfers accounts for the fine tuning between different ligands.26 Coming to electronic characteristics, the first apparent trend is a general destabilization of the ionic levels of the azabiphenyls in respect to the parent azabenzenes. In fact each occupied M O of azabiphenyls is essentially derived from the combination of doubly occupied orbitals of the parent azabenzenes and, as is well known,I0 this kind of interaction is destabilizing. This effect is further enhanced in 2-bpm and in the cis conformations of the other ligands by electrostatic repulsions between negatively charged ortho nitrogens. The data of Figure 4 and Table I1 show that the first ionization potentials always correspond to lone-pair levels and are essentially identical in bpy, bpd, and 2-bpm, whereas bpz is a slightly worse, a 4-bpm a significantly worse, donor. This trend can be rationalized considering that the splitting of the two higher n levels is nearly constant (0.5 eV) for all ligands except bpd and 4-bpm (see also ref 12). As previously s h o ~ n , ' this ~ J ~splitting is essentially determined by through-bond interactions, which, in turn, (26) Barone, V.; Minichino, C.; Fliszar, S.;Rum, N. Can. J . Chem. 1988, 66, 1313.
are governed by the coefficient of the inter-ring carbon atoms in the n MO of the parent azabenzene. This coefficient is particularly low for 4-bpm (0.12 vs 0.24 for 2-bpm a t the STO-3G level), which, therefore, shows a pair of nearly degenerate n levels. The same situation occurs in bpd, but the higher ionization potential of the parent ring leads to a pair of degenerate n orbitals with IE's comparable to those of bpy and 2-bpm. At the same time 4-bpm is a better A acceptor than bpz, which, in turn, has a greater EA than the other ligands. This trend confirms that, as already pointed out by Crutchley and Lever: a stabilization of T* orbitals is generally compensated by a poorer u-donor strength, so that the overall complexing ability remains essentially unmodified. As previously pointed the intensities (eCT) of MLCT absorption bands show a remarkable correlation with the squared LUMO coefficient at the coordinating centers (Le., ortho nitrogen atoms). Both HAM/3 and STO-3G computations give the following order of tCT: 2-bpm < 4-bpm < bpy < bpd < bpz in agreement with H M O computations and experimental data.2s In conclusion, the combined use of HAM/3 and a b initio computations is a reliable source of quantitative physicochemical characteristics for quite large molecules. In view of the correlation of free-ligand properties with several characteristics of the complexes, this provides a reliable interpretative tool and a powerful computational screening of potentially interesting ligands. Registry No. bpy, 366-18-7; bpd, 10198-96-6; 2-bpm, 34671-83-5; 4-bpm, 2426-94-0;bpz, 10199-00-5.
Theoretlcal Study on Neutral and Anlonic Halocarbynes and Halocarbenes C.L.Cutsevt and T.Ziegler* Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N I N4 (Received: January 21, 1991)
The electronic and molecular structures of CX, CFCI, and CX2 (X = H, F, CI, Br, I), as well as the corresponding anions, have been studied by theoretical calculations. The calculations were based on density functional theory (DFT) within the local density approximation (LDA) and augmented by nonlocal exchange corrections (LDA/NL). Calculations have been carried out on the vertical electron affinity and adiabatic electron affinity of the neutrals as well as the first ionization potential of the corresponding anions. It is shown that the adiabatic corrections are more important for the halosubstituted systems than for the parent CH and CH2 molecules. The calculated properties are in good agreement with available experimental data. Many of the molecular structures and electron affinities obtained here have not yet been determined experimentally. It is concluded that all anions possess a ground state which is stable with respect to electron detachment as well as dissociation. It was further found that all the systems, with the exception of CF, CH2, and CF2, possess a second (excited) state which is stable with respect to electron detachment as well as dissociation. Calculated energies of the singlet-triplet splitting in the carbenes as well as SiF2are in good agreement with experimental data and results from high-level ab initio calculations. It is demonstrated that nonlocal corrections are crucial for accurate estimates of electron affinities.
Introduction The adiabatic electron affinity (EAd) of a molecule represents the decrease in energy of the molecular system when a single electron is added. It is in general a difficult property to obtain from experimental as well as computational techniques. The most accurate experimental estimates of electron affinities are obtained by laser photoionization of negatively charged systems. However, this technique is limited to anions that can be studied in the gas phase. It is still not routine to measure electron affinities accurately, and values obtained by alternative techniques differ often considerably, even for relatively simple molecules. Electron affinities offer also a considerable challenge from the computational point of view. In the first place, one must be able 'On leave from the Institute of Chemical Physics of the USSR Academy of Sciences, Chernogolovka, Moscow Region 142432, USSR.
0022-3654/9 1 /2095-122Q$02.50/0
to describe accurately the difference in geometry between the neutral molecule M and the corresponding anion M-.The computational method must in addition be able to account quantitatively for the difference in electron correlation energy between M and M-. This difference is in most cases substantial due to the change in the number of electron pairs. Finally, the basis set employed must be flexible enough to describe M as well as Mwith a somewhat more diffuse charge distribution. It is thus not surprising that accurate electron affinities are available only for a quite limited number of molecules.'-2 (1) Christodoulidcs, A. D.; McCorkie, D. L.; Chriatophorou, L. G. In Electron-MoleculeInreractlonc and their applicationc; Chriatophorou. L. G., Ed.;Academic Press: New York, 1984; Vol. 2. (2) (a) Gutsev, 0. L.; Boldyrev, A. I. Adu. Chem. Phys. 1985,61, 169. (b) Ziegler, T.; Gutsev, G. L. J . Compur. Chem. 1991, submitted for publication. (c) Baker, J.; Nobes, R. H.; Radom, L. J. Comput. Chcm. 1986.7,349. (d) Ziegler, T.; Gutsev, G. L. Can. J . Chcm. 1991, in press.
8 1991 American Chemical Society
