r-Metallocenylmethylium Ions and Their Isoelectronic Congeners: AComparison Based on DFT Calculations

Christian Bleiholder,†,#,‡ Frank Rominger,† and Rolf Gleiter*,†

Organisch-Chemisches Institut der UniVersitat Heidelberg, Im Neuenheimer Feld 270,D-69120 Heidelberg, Germany, and Deutsches Krebsforschungszentrum (DKFZ), Im Neuenheimer

Feld 280, D-69120 Heidelberg, Germany

ReceiVed June 22, 2008

The geometrical parameters, net natural charges, and Wiberg bond indices of a series of R-metallo-cenylmethylium ions with 18 valence electrons were calculated using density functional theory. Thetransition metals considered were Cr, Mn, Fe, Mo, Re, W, Ru, and Os. As coligands to the fulvene-metalfragment we used benzene, cyclopentadienyl, and three CO units. We also examined the fulvene-metalfragments for Co, Rh, and Ir using a cyclobutadiene coligand. Trends in bond order between the exo-carbon atom of the fulvene unit (C6) and the metal atom as well as the bending angle of C6, R, and thenet charges at C6 and the metal centers were analyzed. We found good correlation between R and themetal-C6 distance (d1). A comparison of the calculated values of R and d1 with experimental data showsgood agreement in cases where steric effects do not hamper the C6-metal interaction.

Introduction

Studies in the past 40 years on R-metallocenylmethylium ionsand their isoelectronic fulvene congeners of d6 to d9 metals1,2

have shown that the properties in solution and the solid stateare in agreement with a through-space interaction between theexomethylene carbon center of the fulvene unit (C6) and themetal. This interaction is most bonding when the C6 center isbent toward the metal, leaving the other five carbons of thefulvene ring in one plane. This metal-C6 interaction conse-quently distributes the positive charge over the ligands and themetal atom, leading to retention of configuration at the C6 atomduring nucleophilic addition at C6. The structural motif of abent R-metallocenylmethylium ion (1) is also found in systemswith two metal centers such as 2 or 3,3,4 as shown in Figure 1.

In the past 35 years several quantum chemical methodsapplying different approximations have been used to calculatethe properties of 1 and related congeners.5 The geometricalparameters of 1 that were typically reported were those thatcharacterize the distortions of the fulvene ligand with respectto a planar structure: the angles R, �, and γ and the distances∆x, d1, and d2 (Figure 2).5a It turned out, by comparing the

various structures, that the most relevant parameters are R, d1,and d2, whereas the others tend to show only rather smallchanges.2c

In this paper we present the results of quantum chemicalcalculations on a series of model systems each containing afulvene-metal segment with 18 electrons. The overall chargestate of the fulvene-metal group was changed as needed tomaintain this 18-electron count and allowed us to restrict ourcalculations to the corresponding singlets. We subdivided oursystems according to the coligands used. In the first three classes(Figure 3) we employed coligands that provide formally sixelectrons to the valence shell (benzene, 4-6; cyclopentadienyl

* To whom correspondence should be addressed. E-mail: rolf.gleiter@oci.uni-heidelberg.de.

† Universitat Heidelberg.# DKFZ.‡ Current address: Department of Chemistry and Biochemistry, University

of California, Santa Barbara, California 93106.(1) (a) Rosenblum, M.; Abbate, F. W. AdV. Chem. Ser. 1967, 62, 532–

548. (b) Cais, M. Organomet. Chem. ReV. 1966, 1, 435–454. (c) Kerber,R. C.; Ehntholt, D. J. Synthesis 1970, 449–465. (d) Watts, W. E. J.Organomet. Chem. Library 1979, 7, 399–459.

(2) (a) Kreindlin, A. Z.; Rybinskaya, M. A. Russ. Chem. ReV. 2004, 73,417–432. (b) Rybinskaya, M. I.; Kreindlin, A. Z.; Kamyshova, A. A. Russ.Chem. Bull. Int. Ed. 2002, 51, 1616–1632. (c) Gleiter, R.; Bleiholder, C.;Rominger, F. Organometallics 2007, 26, 4850–4859.

(3) (a) Sato, M.; Kudo, A.; Kawata, Y.; Saitoh, H. Chem. Commun.1996, 2, 25–26. (b) Sato, M.; Kawata, Y.; Kudo, A.; Iwai, A.; Saitoh, H.;Ochiai, S. J. Chem. Soc., Dalton Trans. 1998, 2215–2224.

(4) (a) Sato, M.; Watanabe, M. Chem. Commun. 2002, 1574–1575. (b)Sato, M.; Kubota, Y.; Kawata, Y.; Fujihara, T.; Unoura, K.; Oyama, A.Chem.-Eur. J. 2006, 12, 2282–2292.

Figure 1. Examples of bent R-metallocenylmethylium ions.

Figure 2. Definition of the structural parameters R, d1, and d2 aswell as the carbon numbering of the fulvene moiety.

Organometallics 2009, 28, 1014–10171014

10.1021/om800573u CCC: $40.75 2009 American Chemical SocietyPublication on Web 01/21/2009

anion, 7-9; and three CO groups, 10-12). In the fourth classwe used cyclobutadiene as a coligand (13-15).

Computational Details

The geometrical parameters of 4-15 were optimized using theGaussian suite of programs6 at the density functional theory (DFT)7

level of theory by applying the three-parameter hybrid functionalby Becke (B3)8 and the correlation functional suggested by Lee,Yang, and Parr (LYP).9 The 6-31G(d,p)10 basis was used for the

main atoms. For the transition metals we used the Dunning/Huzinaga valence double-� basis (D95)11 supplemented with theStuttgart/Dresden effective potentials MDF10 (Mn, Cr, Fe, Co),MWB28 (Mo, Ru, Rh), and MWB60 (Re, W, Ir, Os),12 denoted asSDD. This level of theory was chosen after considering the B3LYP/Lanl2dz, B3LYP/SDD, and B3LYP/SDD,6-31G(d,p) levels oftheory. These computations indicated an overall agreement amongthe latter two as well as to experimental structures. The B3LYP/SDD,6-31G(d,p) level of theory was then used for the computationspresented in this work. Similar benchmark calculations had alsobeen carried out for population analysis (Merz-Kollman,13

CHelpG,14 natural population analysis15 (NPA), Mulliken16), fromwhich the net charges presented in Table 1 were derived. Here,NPA was chosen for analysis of bond orders as well as atomiccharges (see Supporting Information for details). Frequency calcula-tions were performed in order to confirm the minima. The NAOdensity was used to analyze bond orders (Wiberg bond orderindex).17

Results and Discussion

In Table 1 the calculated distances between the metal centerand C6 (d1), the distances between C1 and C6 (d2), and the

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Figure 3. Definition of 4-15.

Table 1. Computed Wiberg Bond Order Indices (bond order)between the Metal Atom (M) and C6, Net Natural Charges at theMetal Centers and C6, and the Geometrical Parameters r, d1, and

d2 of 4-15

compd metal ligandformalcharge

bondorder R d1 d2

chargesq (C6) q(M)

4a Cr C6H6 0 0.57 35.85 2.27 1.41 -0.45 0.164b Mn C6H6 1 0.43 34.31 2.35 1.39 -0.33 0.294c Fe C6H6 2 0.20 16.88 2.82 1.38 -0.09 0.525a Mo C6H6 0 0.64 35.30 2.33 1.43 -0.51 0.055c Ru C6H6 2 0.36 31.89 2.48 1.39 -0.24 0.236a W C6H6 0 0.68 36.94 2.31 1.44 -0.55 0.276b Re C6H6 1 0.59 37.58 2.32 1.42 -0.42 0.246c Os C6H6 2 0.43 34.87 2.42 1.40 -0.29 0.357a Cr C5H5 -1 0.59 37.84 2.24 1.43 -0.57 0.207b Mn C5H5 0 0.53 39.50 2.21 1.41 -0.44 0.307c Fe C5H5 1 0.36 35.73 2.31 1.39 -0.31 0.468a Mo C5H5 -1 0.65 37.96 2.31 1.44 -0.57 -0.028c Ru C5H5 1 0.43 36.49 2.34 1.41 -0.35 0.219a W C5H5 -1 0.68 40.26 2.30 1.46 -0.61 0.179b Re C5H5 0 0.63 40.90 2.27 1.44 -0.50 0.219c Os C5H5 1 0.50 38.72 2.31 1.42 -0.39 0.3410a Cr (CO)3 0 0.39 32.45 2.38 1.40 -0.42 -0.6110b Mn (CO)3 1 0.28 27.96 2.49 1.38 -0.30 -0.3410c Fe (CO)3 2 0.15 10.60 2.94 1.38 -0.07 0.0811a Mo (CO)3 0 0.40 30.19 2.51 1.41 -0.44 -0.4811c Ru (CO)3 2 0.19 20.02 2.77 1.38 -0.17 -0.0712a W (CO)3 0 0.42 31.74 2.49 1.42 -0.47 -0.2612b Re (CO)3 1 0.32 29.38 2.54 1.40 -0.36 -0.1812c Os (CO)3 2 0.21 22.74 2.72 1.39 -0.20 0.0613 Co C4H4 1 0.24 30.26 2.39 1.38 -0.31 0.5414 Rh C4H4 1 0.27 27.92 2.49 1.39 -0.34 0.3915 Ir C4H4 1 0.37 33.32 2.37 1.41 -0.38 0.50

R-Metallocenylmethylium Ions Organometallics, Vol. 28, No. 4, 2009 1015

bending angle R are given as defined in Figure 2. Furthermore,net charges q at the metal center and at C6 as well as the Wibergbond order index17 between the metal and C6 are compiled.

A comparison of the Wiberg bond order index (bond order)between the metal atom and C6 within the triads 4a to 4c, 6ato 6c, 7a to 7c, 9a to 9c, 10a to 10c, and 12a to 12c as well asthe dyads 5a/5c, 8a/8c, and 11a/11c shows a decrease on goingfrom left to right in the periodic table, i.e., from a to c. Thebond order increases within a period when the ligand is keptthe same, e.g., in 4a-5a-6a, 4b-5b-6b, and 4c-5c-6c, aswell as 13-14-15. These variations are independent of thecoligands (benzene, cyclopentadienyl, CO, or cyclobutadiene)attached to the metal atom. In most cases the trend found forthe bond order is also found for the bending angle R (cf. Figure2). Exceptions are 10a-11a-12a, where R is 32.45°, 30.19°,and 31.74°, respectively, and 13-14-15, where R is 30.26°,27.92°, and 33.32°. For d2, the exo-double bond of the fulvenemoiety, a slight decrease is found when going from left to rightin the periodic table and also a slight increase within one period.The distances between the metal center and carbon atoms ofthe fulvene ring (C1 to C5) vary much less than the distanceM-C6 (see Supporting Information), and no clear trend canbe established here. However, we note a relationship betweenthe distances C1-C6 and C2-C3 (for details, see SupportingInformation).

There is a good correlation (r2 ) 0.96) between the computedvalues for R and d1. We find satisfactory correlations betweenthe charge at C6 (q(C6)) and R (r2 ) 0.70), the bond order andR (r2 ) 0.76), and q (C6) and d1 (r2 ) 0.68). The predictedbending of the C6 atom of the fulvene unit toward the metaland the accompanying increase of the C1-C6 distance, togetherwith the increase of negative charge at C6 with increasing R,can be rationalized by considering the frontier orbitals of a planarmetallocenylmethylium ion or their isoelectronic neutral fulvenecomplexes. In Figure 4 we have correlated the frontier orbitalsof a planar (left) with a bent structure (right). By bending C6toward the metal (i.e., increasing value of R) the LUMO isdestabilized and the HOMO is stabilized. The resulting splittingin energy of both levels can be understood in terms of anincreased bonding interaction of the p-orbital located at the C6atom with the metal d-orbital of the HOMO and an increasedantibonding interaction between the same atomic orbitals of the

LUMO. As a result, electron density is transferred into an orbitalantibonding between C1 and C6, resulting in an increased bondlength between C1 and C6 with an increased value for thebending angle R.

Figure 5 displays R-metallocenylmethylium ions and neutralfulvene complexes whose molecular parameters have beendetermined by X-ray investigations in the solid state. Two ofthese are the unsubstituted species 8c18 and 10a,19 which havebeen examined here theoretically. The others (16-30) aresubstituted at the C6 atom of fulvene with either alkyl or arylgroups and can be correlated to one of our model compoundsshown in Figure 3 based on the metal and ligand types.

(18) Barlow, S.; Cowley, A.; Green, J. C.; Brunker, T. J.; Hascall, T.Organometallics 2001, 20, 5351–5359.

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Figure 4. Correlation diagram between the frontier orbitals of aplanar (left) and a bent (right) R-metallocenylmethylium ion.

Figure 5. List of R-metallocenylmethylium ions and neutral fulvenecomplexes whose molecular structures have been determined byX-ray analysis.

1016 Organometallics, Vol. 28, No. 4, 2009 Bleiholder et al.

In Table 2 the molecular parameters R, d1, and d2 (cf. Figure2) derived experimentally for these species are compared tothose calculated for the corresponding model compound. Anexcellent correlation between the experimental and computedvalues are found for 8c and 10a and those species with smallsubstituents at C6 (17-19, 24, and 25). Large differencesbetween the experimental and computed values are particularlyobserved for 20, 29, and 30, in which cases bulky groups at theC6 atom prevent a close approach of C6 to the metal center.

Conclusions

Theoretical calculations of the structural parameters for aseries of R- metallocenylmethylium ions and related neutralfulvene complexes predict an increase of the metal-fulveneinteraction on going from the lighter to the heavier metals. Acomparison of the results obtained for these model compoundswith experimental results obtained for a group of relatedcompounds reveals good agreement when steric effects at thefulvene C6 atom are small. Besides R-metallocenylmethyliumions of CpRu and CpOs as well as the benzene(Mo) andbenzene(W) complexes with fulvene, there are no other R-met-allocenylmethylium ion species known with heavier elements.Also congeners of 2 and 3 with heavier metals and ligands otherthan Cp have not yet been investigated and should be promisingcandidates regarding their electronic properties.

Acknowledgment. Dedicated to Prof. Dr. Zvonimir B.Maksic on the occasion of his 70th birthday. We are gratefulto the Deutsche Forschungsgemeinschaft for financial sup-port. We thank Mrs. P. Kramer for typing the manuscriptand for the drawings. C.B. is grateful to the DeutschesKrebsforschungszentrum for a Ph.D. Fellowship.

Supporting Information Available: Electronic energies andzero-point vibrational energies for all calculated species, comparisonof various schemes for population analysis, additional bond lengths,and tables of Cartesian coordinates. This material is available freeof charge via the Internet at http://pubs.acs.org.

OM800573U

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Table 2. Comparison between the Experimental Parameters r, d1,and d2 (see Figure2) Derived for 8c, 10a, and 16-30 with Those ofthe Calculated Values for the Corresponding Model Systems 5-10

and 13 in Brackets

compd model R [deg] d1 [Å] d2 [Å]

8c18 (8c) 39.9 (36.5) 2.27 (2.34) 1.40 (1.41)10a19 (10a) 34.4 (32.5) 2.35 (2.38) 1.36 (1.40)165e (5a) 35.0 (35.3) 2.31 (2.33) 1.42 (1.43)175e (5a) 34.4 (35.3) 2.38 (2.33) 1.43 (1.43)185e (6a) 33.8 (36.9) 2.39 (2.31) 1.45 (1.44)195e (6a) 35.3 (36.9) 2.33 (2.31) 1.44 (1.44)2020 (7c) 6.8 (35.7) 2.96 (2.31) 1.44 (1.39)2121 (7c) 24.0 (35.7) 2.69 (2.31) 1.39 (1.39)2222 (7c) 21.1 (35.7) 2.72 (2.31) 1.42 (1.39)2323 (7c) 22.7 (35.7) 2.66 (2.31) 1.37 (1.39)2424 (8c) 38.2 (36.5) 2.27 (2.34) 1.40 (1.41)2525 (9c) 40.8 (38.7) 2.24 (2.31) 1.43 (1.42)2626 (8c) 33.9 (36.5) 2.41 (2.34) 1.40 (1.41)2727 (10a) 28.4 (32.5) 2.55 (2.38) 1.45 (1.40)2828 (10a) 23.9 (32.5) 2.63 (2.38) 1.40 (1.40)295f (10b) 7.7 (28.0) 3.05 (2.49) 1.41 (1.38)305g (13) 9.5 (30.3) 2.99 (2.39) 1.41 (1.38)

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