Article pubs.acs.org/Organometallics
Natural Bond Orbital Analysis of the Electronic Structure of [LnM(CH3)] and [LnM(CF3)] Complexes Andrés G. Algarra,† Vladimir V. Grushin,*,‡ and Stuart A. Macgregor*,† †
School of Engineering and Physical Sciences, William H. Perkin Building, Heriot-Watt University, Edinburgh EH14 4AS, U.K. Institute of Chemical Research of Catalonia (ICIQ), Tarragona 43007, Spain
‡
S Supporting Information *
ABSTRACT: An analysis of the geometries and electronic structures of a series of [LnM(CX3)] species (where X = H, F) is presented, on the basis of density functional theory (DFT) and the natural bonding orbital (NBO) approach. Computed geometries show that LnM−CF3 bonds can be up to 0.1 Å shorter than the equivalent LnM−CH3 bonds, although the extent of this shortening varies considerably depending on the LnM fragment. Evidence for CF3 having a higher trans influence than CH3 is seen, but this is most apparent in systems where the LnM−CF3 bond is itself shorter. NBO calculations show that the computed charge at the metal center is usually slightly more negative (or less positive) in the [LnM(CF3)] species compared to that of its CH3 congener. Further detailed NBO analyses on the [(H3P)3Rh(CX3)] (1), trans-[(H3P)2Pt(Cl)(CX3)] (2), [(OC)5Mn(CX3)] (3), and [Pt(H)3(CX3)]2− (8) pairs indicate a significantly higher M←CX3 σ interaction when X = F. The LnM−CF3 σ bond is computed to have much higher C 2s character and is also enhanced by contributions from the C−F σ* orbitals. In contrast, any M→C−F(σ*) π back-donation is relatively weak, being at most 8% of the magnitude of M←CF3 σ interaction, while M→C−H(σ*) π back-donation is negligible in the [LnM(CH3)] congeners. The metal-based d orbitals are computed to be between 0.4 and 0.7 eV lower in energy in the [LnM(CF3)] species. Thus, CH3/CF3 replacement has two significant, apparently counterdirecting, effects, in that it both maintains and indeed can increase the electron density at the metal center, while at the same time causing a stabilization of the metal-based d orbitals. These effects account for the enhanced reactivity of [LnM(CF3)] species toward nucleophiles and form a basis for understanding the reactivity of [LnM(CF3)] species in the literature. Implications for the Pd-catalyzed trifluoromethylation of aryl halides ArX are discussed: in particular, the balance between Ar−CF3 reductive elimination from [LnPd(Ar)(CF3)] and the propensity of this species to undergo transmetalation (and hence catalyst deactivation) in the presence of [LnPd(Ar)(X)] species.
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(Hammett) and σF = +0.46 (Taft).13 This is in accord with a number of data reported for metal complexes bearing a CF3 ligand. Thus, [(salen)Co(CX 3 )], 14a [(NHC)Cu(CX 3 )], 14b and [(BOXAM)Ni(CX3)]14b are electrochemically reduced more easily and oxidized more reluctantly for X = F than for X = H. Furthermore, [(Me2PPh)2Pt(CH3)(I)] undergoes oxidative addition of RI (R = CH3, CF3), whereas [(Me2PPh)2Pt(CF3)(I)] remains unreactive under such conditions (Scheme 1).15 Some other reported observations, however, suggest strong electron donation from CF3 to the metal center. These include the long-known strong trans influence16−22 and the trans effect23−25 of the CF3 ligand, as well as the extreme fluxionality of [(Ph3P)3Rh(CF3)] in solution.22 Furthermore, computed natural bond orbital (NBO) charges for [(H3P)3Rh(CX3)] indicate that the Rh center bears a slightly larger negative charge for X = F than for X = H, despite the fact that in the CF3 derivative the carbon is strongly positively charged (+0.79)
INTRODUCTION Trifluoromethylated organic molecules, particularly those bearing the CF3 substituent on an aromatic ring, find numerous applications as active ingredients in a wide range of pharmaceuticals and agrochemicals,1−7 as well as in polymers and specialty materials.8 Metal complexes and catalysts, especially those based on Cu and Pd, are widely used for the introduction of the trifluoromethyl group into the aromatic ring. These transformations are mediated by M−CF3 species that promote or catalyze the aromatic C−CF3 bond formation. However, trifluoromethyl metal complexes exhibit reactivity and properties that in many ways differ significantly from those of their methyl analogues.9−12 A good understanding of the bonding between metal centers and the CF3 ligand is therefore critical for further developments of trifluoromethylation reactions. In most instances, M−CF3 bonds are both stronger and considerably less reactive than M−CH3 bonds. This difference between M−CF3 and M−CH3 is closely related to electronic effects of the CF3 ligand that are particularly puzzling. In organic chemistry, the CF3 group is recognized as an inductive electron acceptor, as follows from its electronic effects: e.g., σm = +0.43 © 2012 American Chemical Society
Special Issue: Fluorine in Organometallic Chemistry Received: November 9, 2011 Published: January 13, 2012 1467
dx.doi.org/10.1021/om2011003 | Organometallics 2012, 31, 1467−1476
Organometallics
Article
[(H3P)3Rh(CX3)] species (X = H, 1a; X = F, 1b), we have extended our analysis to a wider range of [LnM(CF3)]/ [LnM(CH3)] pairs, 2−7 (see Scheme 3). In several cases both
Scheme 1
Scheme 3
whereas there is a large negative charge on C of the CH3 complex (−0.96) (Scheme 2).22 A similar trend was more Scheme 2
the CH3 and CF3 species have been characterized crystallographically and so present the opportunity for a direct comparison with the computed structures. For those species bearing phosphine ligands (i.e., all but pair 3) we have computed both the full experimental system and a model system, where in the latter the substituents at phosphorus are replaced by hydrogen (while retaining the bridging ethylene units in pairs 4 and 5). The systems studied cover a range of metals and coligands, as well as a number of different coordination geometries. The inclusion of pair 3 also allows comparison with the early Fenske−Hall study on this system.33 Model pair 8 is also included, as this will form the basis of the initial NBO analysis (see below). Table 1 presents key experimental and computed structural data, along with selected computed natural charges for the [LnM(CF3)]/[LnM(CH3)] pairs 1−7, and the equivalent data for the model pair 8. For ease of analysis we initially focus on the simpler model versions of 1, 2, and 4−7, with H substituents on the phosphorus centers. In all cases the computed M−CF3 bonds are shorter than the equivalent M−CH3 bonds, although the difference does vary significantly, between 0.1 Å in pair 3 to
