Nature of the metal-carbon contacts in ene-diamido dº metal complexes

Letter Nature of the metal–carbon contacts in ene-diamido d0metal complexes Galindo,*aAndrea Iencoband Carlo Mealli*bAgust• n aDepartamento de Universidad de Sevilla,Aptdo 553,41071 Sevilla,Spain.Qu•mica Inorga nica, E-mail: galindo=cica.es bIstituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione,CNR, ViaJ.Nardi 39,50132 Firenze,Italy.E-mail: mealli=Ð.cnr.it Received (in Montpellier,France)29th November 1999,Accepted 23rd December 1999 Folding of ene-diamido chelates in d0metal complexes is generally attributed to a saturating pinteraction of the metal with the central C2C linkage. A MO analysis of selected models (DFT and EHMO levels) shows that the latter interaction is negligible (negative M–C overlap populations). Moreover, a detailed analysis of the orbital interactions highlights a twoelectron Ñow from Ðlled orbitals of the nitrogen atomspn toward the metal consequent to ligand folding. Conversely, it is evident that the C2Cp-electron cloud is only barely a†ected by the contacts with the metal atom. Group 4 derivatives have wide applications inCp2MX2 catalysis and they have been extensively studied.1For example, to test new reactivity and/or catalytic activity, Cp rings have been stepwise substituted for other ancillary ligands,2including amido groups.3Still, the fragments and (where M is a Group 4,4Group 55MCp(NR2) M(NR2)2 or Group 66metal) are considered metallocene-like and present interesting reactivities. Some of us have reported MO studies on the bonding capabilities of ene-diamido ligands in bridging two metals7and, previously, related 1,2-catecholates were also investigated.8Here, we focus our attention on metallocene-like d0complexes that contain a chelate enediamido ligand.¤,9 A search in the Cambridge database10 shows that the ligand is bent toward the metal.”,11 Invariably, the nitrogen atoms remain planar (the bond angles sum up to 360¡) and the actual folding occurs through the concerted torsion of the NR groupings about the CÈN bonds (ca.25È35¡). In this manner, the amount of pconjugation in the chelate is reduced with a higher localization of the intermediate C2C linkage. Importantly, the shortened MÈC distances raise the question as to whether they correspond to actual bonding. In the literature (with the exception of some discussions by Boncella,6b,12 Rheingold and Ernst,13 Petersen,14 Dieck15 and their coworkers) it is almost taken for granted that the cause of bending is pco-ordination of the C2C bond to the metal (see Scheme 1). In other terms, the classic description (r2,p) adopted for the g4bonding of butadiene to early transition Scheme 1 metals16 is extended to the ene-diamido and other comparable ligands with two additional electrons in the psystem. To understand better the bent co-ordination, MO calculations at di†erent levels of theory (DFT°,17 and EHMOÒ,18) were performed for model compounds of selected d0complexes of the Group 4È6 metals. In particular, we have considered the species 1,TiCl2[o-(iPr3SiN)2C6H4]19 (M \Ti, Zr),20M(OAr)2[(R@N)2C2R2]Cp 2 @Zr[(R@N)2C2R2],21 (M \Ta,5ahe,22 Nb23) andCp@MCl2[(R@N)2C2R2] (X \Cl12 2, and alkylW(NPh)X2[o-(Me3SiN)2C6H4] groups6ahb), all containing a bent ene-diamido ligand. Previously, MO calculations of complexes containing the planar 1,4-diazabutadiene ligand24 and the metallacyclotetraazapentadiene moiety25 have been reported, while related complexes with folded dithiolene ligands have been studied with EHMO methods.26 Full geometry optimizations were performed for the modelp compounds 1and 2, both having symmetry. For conve-Cs nience, Scheme 2 reports a selection of experimental and calculated (in parentheses) geometrical parameters. Moreover for 1, the values in italics refer to the geometry of a transition state (one negative frequency) characterized by a planar ring. Invariably, the bisector of the NÈMÈN angleMN2C2 coincides with the symmetry axis of the metal fragment [C2 for in 1and for in 2].Cl2Ti pseudo-C3W(NR)Cl2 As seen, the DFT calculations provide bent geometries for the chelate ligand. For species 2in particular, the folding (q)is on the opposite side with respect to the single NR ligand. Another conformer, with the chelate orientated toward the latter group, lies only 6.4 kcal mol~1 higher in energy. The probable transition state, with an overall planar ring,MN2C2 could not be detected, probably on account of a rather Ñat energy surface. For the same reason, the computed folding of the ligand about the NÈN vector is less pronounced than the experimental one (compare the qvalues of 157 and 131¡ in Scheme 2) while the general structural agreement is satisfactory. Conversely, the transition state between the equivalent bent conformers was fully characterized for 1and lies 12.5 kcal mol~1 above two symmetrical minima. Since our models are signiÐcantly less hindered than the experimental species, it may be safely stated that steric factors are deÐnitely of little relevance for the bending of the chelate. A qualitative MO analysis helps to elucidate the major bonding components between the diamido and metal fragments in both models 1and 2. Since the basic MO arguments are similar, we will concentrate mainly on the former case. Because of the Ðlled orbitals of the chloride ligands (6 elec-pn tron donors), a fragment can be considered asTiCl2 metallocene-like. Here, the d0metal is characterized by three low-lying frontier orbitals, one of and two of sym-b1a1 metries.27 While the former has nature (in the planedn New J.Chem., 2000, 24,73È75 73 This journal is The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000( Published on 07 February 2000. Downloaded by Gral Universidad Sevilla on 27/08/2015 09:43:52. View Article Online / Journal Homepage / Table of Contents for this issue Scheme 2 orthogonal to the ones are z2and x2[y2-likeTiCl2), a1 orbitals, respectively (zcoincides with the axis ofC2TiCl2). The basic MO structure of an isolated ene-diamido ligand has been described recently.7Its bonding capabilities arise from the two nitrogen rlone pairs (in-phase and out-of-phase combinations) and the three Ðlled pMOs. Following the observed torsions of the NR groups about CÈN linkages, the rlone pairs remain well oriented for donation into the anddn z2metal orbitals. Most critical, however, is the HOMO of the ligand (see Scheme 3), which is still distributed between the C and N atoms but which has components that are no longerpn parallel. This HOMO is very close in energy to the x2[y2 metal orbital and interact strongly with it. The resulting bonding combination, which is also the HOMO of the complex, is shown in Fig. 1 (notice how the drawings generated from DFT and EHMO calculations are consistent). Evidently, the reoriented nitrogen lobes overlap signiÐcantlypn with the lobes of x2[y2on each side of the plane**TiCl2 hence the TiÈN bonds are reinforced. It is signiÐcant that in the planar TS structure, where the latter interaction is missing, the TiÈN latter bonds are slightly longer (see Scheme 2) and the corresponding overlap populations are smaller (0.195 vs. 0.213). As another point, the carbon orbitals ofpn the ene-diamido ligand are only indirectly involved in bonding; they are still part of the ligand HOMO but do not overlap with the metal lobes. A careful analysis of the fragment orbital interactions does not provide any evidence for drifting of the C2Cp-electron cloud toward the metal. In fact, the low-lying MO of the ligand, which is mainly responsible for C2C bonding, is totally una†ected upon formation of the Scheme 3 Fig. 1 HOMO of model 1 Mbased on the structure of TiCl2[oas it results from (a) DFT and (b) EHMO calcu-(iPr3SiN)2C6H4] lations. complex. Numerically, a certain electron repulsion is conÐrmed by the values of the MÈC overlap population, which are slightly negative at any computational level. On the contrary, positive MÈC overlap population values characterize the r2,pbonding of butadiene to early transition metals.28 Lauher and Ho†mann29 pointed out long ago that the preferential upright orientation of single amido coligands in d0 complexes is due to important donation of the ÐlledCp2Mp n orbital into two adjacent lobes of the x2[y2-type orbital. Analogously, the ene-diamido chelate engages all of the x2[y2lobes in the present case. Since such a two-electron donation from nitrogen contributes most to metal saturation, the contribution of the C2Cpcloud is negligible. In conclusion, the present analysis of the bonding of the ene-diamido ligand agrees with a r2,pdescription where the pcomponent is also largely contributed by the nitrogen atoms. Thus, the valence bond picture of Scheme 1 is inadequate. A better formal description is that of Scheme 4, where the MÈN bonds are of order 1.5 and where the C2C linkage of the chelate is localized and largely preserved. Scheme 4 Acknowledgements AG appreciates the Ðnancial support from the Royal Society of Chemistry (RSC Journal Grant for International Authors) that made possible the development of this work at ISSECC (Italy). We thank Professor James M. Boncella for valuable discussions. Thanks are also due to the ““Area della Ricerca CNR di FirenzeÏÏ (in particular, Dr A. Tronconi and Mr S. Cerreti) for providing computational support. Notes and references ¤ Other nomenclature used for this ““non-innocentÏÏ type of ligand, which can be considered as the reduced form of diimine ligands, is that derived from the 1,4-diaza-1,3-butadiene moiety. See, for example, ref. 9. ” Planar diamido ligands supported by additional coordination are encountered in ref. 11(aÈc). Also, a Ti complex containing a planar 1,8-diamidonaphthalene ligand was reported in ref. 11(d). For d0complexes containing classical planar diimine ligands see ref. 11(e,f). °Ab initio calculations were performed using the Gaussian-94 program. Molecular geometries and harmonic vibrational frequencies have been determined using the density functional approach (DFT). In these calculations we have used BeckeÏs 1988 exchange functional with the correlation functional of Perdew (BP86). The standard double-zeta basis set (LANL2DZ) and e†ective core potential (ECP) was used for the metal atom. For the other atoms, DunningÏs basis set, augmented with one d polarization function for the non-hydrogen atoms, was used. ÒAll the EHMO calculations were of the extended type usingHu ckel a weighted-modiÐed WolfsbergerÈHelmholz formula. The EH parameters were taken from CACAO. The bond distances and angles used in the calculations have been taken directly from the X-ray results through the new routines of the CACAO program (ref. 18). pIn the ab initio calculations for models 1and 2, with the aim to speed the calculations, the o-phenylene group (othat links theC6H4) two N atoms of the diamido ligand was simpliÐed to a ÈCH2CHÈ carbon chain. Also, the organic groups on the nitrogen atoms (trialkylsilyl substitutents of amido and phenyl group of imido) were replaced by hydrogen atoms for simplicity. 74 New J.Chem., 2000, 24,73È75 Published on 07 February 2000. Downloaded by Gral Universidad Sevilla on 27/08/2015 09:43:52. View Article Online ** Although characterized by a di†erent ancillary fragment,L3M compound 2has a HOMO very similar to that in Fig. 1 with x2[y2 orbital involved. The latter can be described as a member of the ““t2g ÏÏ set typical of fragments (ref. 27). 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