Application of a modified EHMO-ASED formalism to the determination of the structural parameters of organometallics

THEO CHEM Journal

of Molecular

Structure

(Theochem)

330 (1995)

191-195

Application of a modified EHMO-ASED formalism to the determination of the structural parameters of organometallics Franqois Savary, Eric Furet, Jacques Weber* Department

qf Ph.ysicul

Chemisuy,

University

Received

of Geneva,

7 January

30 quai Ernest-Ansermet,

1994; accepted

7 February

CH-1211

Genew

4, S~~itserland

1994

Abstract The parametrization of the EHMO-ASED method we have recently suggested for organometallics is shown to be also applicable, in principle without any modification, to derive the major structural parameters of second-row transition metal systems such as carbonyls or metallocenes. Furthermore, this model leads to satisfactory results when used to calculate the structure of compounds as large as (IV-methylindole)tricarbonylchromium(0) or (phenyloxdzoline)tricarbonylchromium(O) with full geometry optimization of the ligands.

1. introduction The extended-Htickel molecular orbital (EHMO) method [l] in its improved atom superposition and electron delocalization (ASED) form [2,3] has been shown to describe reasonably well the structural properties of organometallic compounds [4]. In view of the interest in having at hand simple model builders in molecular modeling applications, the ASED procedure has been recently reparametrized in such a way as to extend both its range of applicability and predictive capabilities. In particular, Calzaferri et al. [.5] and Tupper et al. [6] have shown that this approach leads to good structural predictions for small organic molecules, whereas Savary et al. [7] reported satisfactory results obtained for organometallics using a similar parametrization scheme extended to transition metal systems.

* Corresponding

author

0166-1280!95/$09.50 ~,Q 1995 Elsevier SSDI 0166-1280(94)03838-C

Science

B.V.

All rights

However, our previous study [7] was limited to structural investigations performed for first-row transition metal systems keeping frozen geometries for the ligands, i.e. metal-ligand distances and ligand-metal-ligand angles only have been optimized. In the present paper, we evaluate the performance of our EHMO-ASED formalism to second-row transition metal organometallics using the same parameters as those optimized for first-row systemsin an attempt to test their transferability. In addition, the results of full geometry optimization (ligands included) are reported for two (arene)Cr(C0)3 derivatives.

2. Formalism The model is based on the introduction of a distance-dependent Wolfsberg-Helmholz formula in off-diagonal EHMO matrix elements H,, corresponding to l-2 (metal-ligand) and reserved

192

F. Sovur~

l-3 (ligand-metal-ligand)

interactions:

H/m

= ;KAB(~$,,

et a/./J. Mol.

+fL)S,,

Smut.

(Thtwhem)

with (4

where A and B are the atoms to which orbitals p, and u belong, K and 6 are positive empirical parameters, RAB is the distance between atoms A and B, and RcovAand RcovBare their covalent radii. Our strategy in the search for optimum K and S values for first-row transition metal systems was the following [7]: in order to be efficient and of general use, the EHMO model builder is allowed to contain different sets of K, 6 parameters for the description of different metal-ligand interactions. In other words, a single set of K. 6 values should be able to lead to good predictions for a given type of metal-ligand bond distance or ligand-metalligand bond angle (i.e. carbonyl, aromatic ring. phosphine group, etc.), and it should be the same for any metal when coordinated to the same ligand. In fact, our initial parametrization has been made so as to reproduce the geometry of a single

Table 1 Structural

parameters

of organometallic

Compound’

ON’

compounds

optimized

Structural

[91

2 Nb(Cd&)(C0)4 3 4 5 6 7

MOM

[lOI [1 I]

Mo(GHs)(CO)j

1121

RU(CjHj)2 [131 Ru&H,,);[14] Ru(CO)3PCHd2

[151

Average

error

bar

I I 0 0 II II 0

2.054 (2.085) 1.960 (2.034) 1.835 (1.823) 1.700 (1.717)

0.034 A

a Distances in AngstrBms. angles in degrees. Experimental b Reference of experimental data in brackets. ‘Oxidation number of metal. d See Scheme 1. ’ CO groups located cis to Cl ligand. ‘CO group located trans to Cl l&and.

using the EHMO-ASED

method”

parameters

M-ring distance 1 TcCl(COhF’(C,H,h)2

191-195

organometallic compound of each type. n,imely WCO)~, Cr(CA)Z. Cr(G,H&, (WG)W(.:Oh, Cr(CO),P(CH,),, CrClz-, etc. [7]. This procedure allows the number of K. 6 parameters to be limited to a minimum while being flexible enough to differentiate the various types of metal-ligand interactions encountered in organometallic chemistry; it is well known, for instanct:. that carbonyl ligands are better YTacceptors than N:yclopentadienyl rings and this should somehow be reflected in the different choices of K, S paranleters. However, in view of the deliberate strateg)’ of using the same parameters for all transition metal atoms, the model is expected to be more approximate when examining the variations in metal-ligand dist Lmces within series of compounds with the same ligands such as carbonyls or metallocenes. The optimum structures are found by millimizing their total EHMO-ASED energies calculated using Eqs. (1) and (2) for the off-diagonal hamiltonian matrix elements. The algorithm useI. for minima searching is based on the use of a po\r erful sequential quadratic procedure [8]. All the tletails of the formalism and the sets of optimum K, 6 values have been reported previously [7].

(1)

KAB = 1+&e ~b(R*B-(R,,,,,-R,“,B))

330 (1995)

M-CO

M-P distance

M-Cl distance

(l.981)e (1.887)’ (2.090) (2.060) (1.905)

2.640 (2.443)

2.299 (2.506)

1.951 (1.900)

2.259 (2.339)

0.067A

0.139A

1.897 1.875 1.985 2.026 2.022

distance

values in parentheses.

Ring- Ll-(‘0 angled

II’.3

/ 120.9)

12i.O ( 126.7)

M = metal

0.207

A

5.’

F. Savary

et al./J.

Mol.

Struct.

3. Results and discussion The results of the geometry optimization of some representative second-row transition metal systems are presented in Table 1. As compared with the series of 31 first-row transition metal compounds investigated in our previous paper [7], the following trends emerge as far as the average error bars are concerned, although the number of systems calculated in this work is significantly smaller, which makes such statistics more questionable: M-ring distances are in slightly better agreement with experiment for the present systems (error bar: 0.034A vs. 0.058 A), whereas the opposite conclusion may be deduced for M-CO bond lengths (0.067 A vs. 0.039 A). These results suggest that for these structural parameters, the K, S values optimized for first-row systems seem to be also applicable to second-row transition metal compounds. The situation is different for M-P and M-Cl distances, which exhibit large discrepancies with

1.783

(Theochem)

330 (199.5)

197

191-195

experiment as reported in Table 1, and we shall come back to this point below. A closer look at Table 1 reveals that the best agreement between calculated and experimental bond distances is, in general, obtained for compounds in which the metal atom has the same oxidation number (ON) as in the system chosen for parametrization, i.e. 0 for M-(C6H6), M-CO and M-P, II for M-(CSH5) and III for M-Cl distances, respectively. This is particularly true for the M-ring distance in 5 and the M-CO bond length in 3 and 7. However, compound 6 is an exception to this statement as the calculated M-ring distance is in very good agreement with experiment in spite of the fact that the ON of Ru is II as compared with Cr(0) in Cr(C6Hg)* used for parametrization! Conversely, the predicted M-ring distance of 4 and M-CO bond lengths of 1 (cis case) and 2 exhibit a poorer agreement with experiment, probably because they have been obtained using K, 6 parameters optimized for systems in which the

x-Cr-CO(a)

: 125.9 (125.8)

x-Cr-CO(b)

: 124.5.(122.5)

x-Cr-CO(c)

: 122.7 (129.5)

(1.770)

y-x-Cr-CO(a)

Fig. 1. Structure of (N-methylindole)tricarbonylchromium(O) values in parentheses [21]).

with

the most relevant

calculated

: 60.5 (66.7)

structural

parameters

(experimental

194

F. Savar~

1.650

et al.:J.

Mol.

Struct.

(Theochem)

330 (1995)

191-195

(1.709)

CO-Cr-CO : 89.2” (89.8”) C(a)-x-Cr-CO(a) : 29.1’ (31.7”) C(a’)-C(a)-C(b)-N : -2.3” (6.8”) C(c)-C(f)-y : 126.1’ (126.4”) C(C)-C(C)-y

Fig. 2. Structure of (phenyloxazoline)tricarbonylchromium(O) values in parentheses [22]).

metal atom has a different ON. Again, there is an exception to this statement as 4 exhibits a poor M-CO bond length, although the metal ON is 0, but this is probably due to the presence of a counterion in the crystal structure and to the anionic nature of the complex, which leads undoubtedly to a significant back-bonding donation to the CO ligand and to a concomitant decrease in MO-CO distance. However, taking into account the crudeness of the method, the present results concerning M-ring and M-CO distances are encouraging. In particular, the model seems to be able to reproduce the shortening of the trans M-Cl distance with respect to the cis one in 1, which may be attributed undoubtedly to the stronger back-bonding towards this ligand due to the trans influence of chlorine. Table 1 shows, however, that the M-P and M-Cl bond lengths seem to be much more sensitive to the ON of the metal than the other structural parameters: the calculated M-P distance in 7 is in reasonable agreement with experiment, which may be attributed to the 0 ON of Ru in this compound,

: 127.6” (121.7”)

with the most relevant

calculated

structural

parameters

(expertmental

but the same parameter is significantly overestimated in the Tc(1) complex 1. Similarly, in this compound, the calculated Tc-Cl distance is much too short due to the fact that the K, 6 parameters used have been optimized for a Cr( III ) complex. Finally, it is seen in Table 1 that ring-Iv-CO bond angles are well reproduced by the EHMOASED model, which parallels the trend pre\ lously observed [7]. As mentioned in the Introduction, a najor incentive for the development of the present model lies in the need for a simple model builder

/ d: Scheme

1.

F. Savuq~ et al./J.

Mol.

Struct.

for organometallics so as to enable us in a second step to perform a modeling of their reactivity towards electrophiles or nucleophiles. To this end. the EHMO reactivity index we have recently developed may be used as a first approach [16]. It has indeed been shown to lead to results of semiquantitative value in a large number of casesand to adequately predict the most reactive sites of these systems, even in the case of competing reaction mechanisms [17720]. As a test of the capabilities of the EHMO-ASED model builder, it was therefore interesting to fully optimize the geometry of two large organometallic systems, namely (N-methylindole)tricarbonylchromium(O) 8 and (phenyloxazoline)tricarbonylchromium(O) 9 and to compare the results with corresponding X-ray structural data [21,22]. It is seen in Figs. 1 and 2 that, asexpected on the basisof our previous calculations [7], the geometrical parameters involving the metal atom, namely Cr-ligand bond distances and ring-Cr-CO bond angles, are very well reproduced. Surprisingly, the C(a’)C(a)C(b)N torsion angle in 9 also compares well with experiment. However, as the intraligand K, S parameters have not yet been optimized for the systemsinvestigated, it is seenin Figs. 1 and 2 that the geometry of the ligands is, in general, in poorer agreement with experiment than that expected for the structural parameters involving the metal atom. However, in spite of this deficiency these results show that the EHMO-ASED method may reasonably be used as a fast model builder for organometallics. The structures so obtained can subsequently be used for a semiquantitative evaluation of their properties or as initial geometries for more elaborate conformational studies using ab initio or density functional techniques.

Acknowledgments The authors are grateful to Professors C. Daul and G. Calzaferri for helpful discussions. This work is part of projects 20-36131.92 of the Swiss National Science Foundation and D3/0001/92 of the COST D3 European concerted action.

(Theochem)

330 11995)

191-195

195

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