On the origin of apparently short carbon–carbon double bonds in transition-metal vinyl complexes

Polyhedron 18 (1999) 1717–1724

On the origin of apparently short carbon–carbon double bonds in transition-metal vinyl complexes Michael B. Hall*, Shuqiang Niu, Joseph H. Reibenspies Department of Chemistry, Texas A& M University, College Station, TX 77843, USA Received 8 September 1998; accepted 12 December 1998

Abstract A survey of carbon–carbon double bond lengths in transition metal vinyl complexes reveals a number of complexes with surprisingly short C=C bonds. Accurate calculations with restricted Hartree–Fock (RHF) and density functional theory (DFT) methods on three 1 examples with extremely short C=C double bonds, Cp 2 ZrCl(–CH=CHSiMe 3 ), Cp * Ir(PMe 3 )(C 2 H 3 )(H) and Pt(PPh 3 ) 3 (MeC=CHMe) , show that the short double bonds cannot be due to electronic or intramolecular steric effects. However, model calculations on Cp 2 ZrCl(–CH=CHSiMe 3 ) reveal that a disorder between two configurations of the C a =C b moiety could result in the apparent shortening of the C a =C b distance. The simulated structural parameters under this disorder model are completely consistent with the measured structure. A corresponding disorder model can be used to rationalize the shortening of the C=C bond in Cp * Ir(PMe 3 )(C 2 H 3 )(H) and Pt(PPh 3 ) 3 (MeC=CHMe)1 . Thus, these structures and perhaps other vinyl transition metal structures with short C=C bonds could be subject to a similar disorder problem that results in an apparent shortening of the C=C bonds.  1999 Elsevier Science Ltd All rights reserved. Keywords: Carbon-carbon double bonds; Transition-metal vinyl complexes; Density functional theory

1. Introduction Our interest in the transition metal promoted C–H activation of ethylene to form a metal hydrido vinyl complex precipitated a general interest in the structure of transition-metal vinyl complexes [1–3]. A survey of the structures of these complexes revealed a number with surprisingly short C=C bonds. The origin of this shortening could be electronic, steric, packing or statistical, and the current study was undertaken to examine this question. We will first present the results of a Cambridge Data Base (CDB) search, then a theoretical determination of the bond length expected for several free molecules that have extremely short C=C bonds and, finally, a proposal for a disorder that explains the shortening observed in the crystallography studies. Our proposed disorder model is similar to that found in dimethyl phosphino ethane complexes such as TiCl 3 (dmpe)R (R5Et, Me; dmpe5 Me 2 PCH 2 CH 2 PMe 2 ) [4]. In the X-ray refinement of TiCl 3 (dmpe)Me, no disorder could be resolved but the –(CH 2 –CH 2 )– joining the two P atoms refined to a *Corresponding author. Tel.: 11-409-845-2011; fax: 11-409-8454719. 0277-5387 / 99 / $ – see front matter PII: S0277-5387( 99 )00010-8



˚ a value considerably shorter than the distance of 1.485 A, ˚ For TiCl 3 (dmpe)Et, the disorexpected value of 1.541 A. der was resolved by a two-site model involving the twisting of the –(CH 2 –CH 2 )– unit and two ‘normal’ C–C bond distances resulted. Here, on the basis of theoretical calculations, we propose that a similar but unappreciated problem occurs in many transition-metal vinyl complexes.

2. Scientific details

2.1. Cambridge Data Base search The standard methods for searching CDB were taken from Taylor and Kennard [5]. The CDB was examined for transition metal vinyl complexes as defined in Scheme 1. The following search criteria were established before attempting the database query: (a) One and only one transition metal was allowed per hit / structure. (b) The transition metal must be singly bonded to the alpha carbon (C a) of the ethylene moiety. (c) No bond between the transition metal and the beta

1999 Elsevier Science Ltd All rights reserved.

M.B. Hall et al. / Polyhedron 18 (1999) 1717 – 1724

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Scheme 1.

carbon (C b ) of the ethylene moiety was allowed (this was verified by monitoring the transition metal–C b distance). (d) The ethylene-containing fragment was not allowed to be involved in a cyclic structure. (e) Any structure where possible delocalization of the double bond could occur, such as dienes and enolates, was not allowed. (f) Other search criteria were: SIGFLAG 1–2 SYMCHK ON SCREEN — 35 SCREEN — 54 SCREEN — 88 SCREEN — 33

— — — —

insist insist insist insist

no-disorder no-polymers RFACTOR,510% error-free

Manual selection of hits was undertaken to ensure compliance to the pre-established criteria. X-ray structures were searched using the Cambridge Data Base package on the Vax 4000 computer in the Department of Chemistry [6].

2.2. Ab initio and density functional theory ( DFT) calculations All geometries were optimized at the restricted Hartree– Fock (RHF) [7] and DFT [8] levels; specifically, Becke’s three parameter hybrid method [9–11] and the Lee–Yang– Parr correlation functional (B3LYP) [12] were employed in all DFT calculations. The basis sets for zirconium, iridium and platinum are described by a modified version of the Hay and Wadt basis set with effective core potentials (ECPs) [13]. The modifications to the double-z basis set were made by Couty and Hall [14] and give a better representation of the (n11)p space. The result is a [3s3p2d] contracted basis set for zirconium, iridium and platinum, where the ns and np basis functions are left totally contracted but the (n11)s, (n11)p and (n)d are split (41), (41) and (21), respectively. For the CpIr(PH 3 )(C 2 H 3 )1 , Pt(PH 3 ) 3 (MeC=CHMe)1 and Cp 2 ZrCl(–CH=CHSiH 3 ) complexes, the ‘LANL2DZ’ basis sets [13,15] are used for the chlorine, carbon,

hydrogen, phosphorus and silicon atoms (BS1). The STO3G basis sets are used for the carbon and hydrogen atoms of cyclopentadienyl rings of the Cp 2 ZrCl(–CH=CHSiH 3 ) complex. For the CpIr(PH 3 )(C 2 H 3 )(H) complexes, the ECPs and basis sets of Stevens et al. [16] in double-z form are used for the carbon and phosphorus atoms, whereas the Dunning–Huzinaga (31) double-z basis set [15] is used for the hydrogen atoms (BS2). All ab initio and DFT calculations were performed using the GAUSSIAN94 program [17] on Silicon Graphics Power Challenge servers at the Supercomputer Center of Texas A and M University and Department of Chemistry, and on Silicon Graphics Indigo I and Power Indigo II workstations in our laboratory.

3. Results and discussion The CDB was searched as described above for the structural feature shown in Scheme 1. Results of the CDB search are given in Table 1 and the histogram is plotted in Fig. 1 [18–44]. The histogram does not show a normal distribution but is skewed (21.342) towards lower values. ˚ The arithmetic and geometric means are both 1.315 A, ˚ and a median of 1.325 A ˚ for with a SD equal to 0.031 A Table 1 ˚ Summary of the C a =C b bond lengths (A) Ref. code

C a =C b bond length

References

KEHLEO JANFOT JOGSAZ WIBROO SEPZAO KUMPAJ DEHGEC DAMLOS DAMLOS10 JERRUT JERRUT10 JAXREF DIDVAN JOGSED FUGWOT PIJHEV FIPHER GINGOZ JOGSIH VAXBOL CUNCOD GIRMAV MBYMOB10 PAJCEI VUNMAS BUVMEK ZIHYAQ HARPIZ CICDUN YIHYET HARPIZ

1.217 1.241 1.258 1.271 1.288 1.289 1.291 1.296 1.296 1.301 1.301 1.309 1.311 1.315 1.320 1.321 1.329 1.332 1.333 1.334 1.335 1.336 1.336 1.337 1.339 1.342 1.342 1.342 1.343 1.346 1.35

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [20] [31] [32] [33] [34] [20] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [43]

M.B. Hall et al. / Polyhedron 18 (1999) 1717 – 1724

Fig. 1. Histogram of 31 ethylene fragment double bond lengths from the Cambridge Crystallographic Data Base. Based upon the search criteria given in this work.

28 samples out of 120,000 entries. The minimum and ˚ respecmaximum values observed are 1.217 and 1.350 A, tively. The skew is surprisingly large, with one value being more than three standard deviations shorter than the mean and no values being more than one standard deviation longer. Generally, a lengthening of the C=C bond is expected because of electronic effects such as p-donation from the vinyl-ligand p bond to the transition-metal center or backdonation from the metal to the C=C p * , as illustrated in Scheme 2. However, the observed shortening of C=C bonds of several transition-metal complexes are still poorly understood. Here, we will investigate possibilities for the origin of the observed shortening of C=C bonds. We hope that this work will lead to a better appreciation of the problems encountered when studying transition-metal vinyl complexes.

3.1. The Cp2 ZrCl( –CH= CHSiMe3 ) complex The shortest observed C=C bond is found in Cp 2 ZrCl(– CH=CHSiMe 3 ) [18], where the authors examined the possibility but were unable to show that the shortening was due to three-center two-electron interaction between the

Scheme 2.

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metal center and the C a –H s-bond [18,45]. The authors of the experimental structural work realized that this bond was ‘too short’ and offer a contamination problem involving the disorder between Cp 2 ZrCl(–CH=CHSiMe 3 ) and Cp 2 ZrCl 2 as a possible explanation (see footnote 16 in ref. [18]). Although this disorder problem can lead to a shortening of the C=C distance, it will also lead to a lengthening of the Zr–C a distance because both complexes have the same Cl–Zr–L (L5Cl or vinyl) angle (96.68) [46]. In previous work, both experimental and theoretical studies showed that, in a mixture of MOCl 2 (L) 3 and MCl 3 (L) 3 (M5Mo and W), an impurity of the latter in the former results in a lengthening of the M5O distance [47]. However, the very slight difference in the Zr–C a bond distance between Cp 2 ZrCl(–CH=CHSiMe 3 ) and (t-Bu– Cp) 2 ZrCl(–CHCH 3 =CH 2 ) eliminates this possibility as the correct explanation. Here, we are going to propose a different explanation. To begin, let us examine the original refinement, which ˚ If the observed resulted in the C=C distance of 1.217 A. shortening is due to an intramolecular effect, either electronic or steric, we should be able to reproduce the shortening with the theoretical methods described above. The optimized geometries for the model compound Cp 2 ZrCl(–CH=CHSiH 3 ) in several conformations: 1a, 1b, 2 and 3 are shown in Fig. 2. For the two minimum energy rotational isomers, 1a and 1b, one might anticipate possible b- and a-CH agostic interactions, respectively. Structure 2 is the transition state for conversion of 1a to 1b, while 3 is a model with ‘forced’ angular distortion, as described below. In the RHF and DFT optimized geometries of 1a, the C b , C a , Zr, Cl and Si atoms are nearly in the same plane. The Zr–C a –C b angle is close to the normal sp 2 -hybridized angle (1208) at RHF and DFT levels. Thus, the RHF and DFT optimized geometries of 1a do not show a b -CH agostic structural feature, which is in very good agreement with the experimental X-ray result for (t-Bu–Cp) 2 ZrCl(– CHCH 3 =CH 2 ) [41]. According to previous reports, the b-CH agostic interaction in neutral alkenylzirconocene complexes only appears as a result of intramolecular steric strain [18,45]] The RHF and DFT optimized C b –C a –Zr– Cl dihedral angles of 1b are 145.7 and 140.98, respectively. In 1b, which is the conformer that most closely resembles the experimental refinement, the vinyl group twists about 408 from the Cl–Zr–C a plane. The calculated Zr–C a –H bond angle of 1b is smaller by 11 and 138 and the Zr–C a –C b angle is larger by 15 and 178 than a normal sp 2 -hybridized angle (1208) at RHF and DFT levels, respectively. The RHF and DFT optimized C a –H bond of ˚ with respect to that of 1a. Such 1b are longer by 0.004 A structural features point to the presence of a Zr–H–C a agostic interaction, as expected. However, the calculated ˚ DFT, 1.364 A) ˚ of the C a –C b bond length (RHF, 1.350 A; ˚ than that in free ethylene (RHF, of 1b is longer by 0.016 A ˚ DFT, 1.348 A). ˚ A general feature of vinyl ligands 1.334 A;

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distance for the C a –C b bond of 1b determined here is very ˚ different from the X-ray-determined distance of 1.217 A [18]. To check for a possible angular origin for the unusual distortion of the Zr–C a H=C b H–[Si] unit, the structure 1b was reoptimized with the Zr–C a –C b and C a –C b –Si angles fixed at the experimental angles of 144 and 1378, respectively (see 3 in Fig. 2). Unfortunately, this distortion only slightly changes the C a –C b bond distance, which, at ˚ (3), is still far from the experimental value of 1.346 A ˚ Therefore, this unusually short C=C bond length 1.217 A. in the X-ray structure cannot be explained by three-center two-electron interactions between the metal center and the C a –H s-bond nor by a forced, perhaps by packing, angular distortion. What then is actually responsible for this unusually short double bond? We first considered the effect of thermal motion on the experimental structure [48] These corrections are shown in Table 2; the corrected Zr–C a and ˚ respectively. It is C a –C b distances are 2.269 and 1.227 A, clear that the thermal motion corrections only slightly change the reported values [18]. We now have to suspect a possible systematic error, such as an unresolved disorder of some type. The energy difference between 1a and 1b is only about 0.6 kcal / mol at the RHF and DFT levels. The rotation barrier from 1a through 2 to 1b is only 2.2 kcal / mol at the RHF level. Thus, one could imagine the co-crystallization of 1a and 1b. This possibility was modeled by using the routine OFIT in XP (SHELXTL-PC v5.11, [48]) in a least-squares model fit. The atoms Zr, Cl and Si and the centroid of the C a =C b alkene distance of the literature structure were fitted by an orthogonal model of the calculated structure. The fit minimizes the root mean square displacement of the specified atoms. The centroid of the alkene bond distance was chosen as one of the fit parameters in order to allow for ease of trigonometric computations. A structural drawing of the best ‘fit’ is shown in Fig. 3. It is easily shown that the C a =C b bond length can be artificially shortened by the linear combination of the two calculated structures (Fig. 4). If the action is confined to the plane of the four ‘fitted’ atoms, the calculation results in a simple ‘bow tie’ trigonometric relationship between Table 2 Riding Model correction for the Zr–C a and C a –C b bond lengths of Cp 2 ZrCl(–CH=CHSiMe 3 ) Fig. 2. RHF and DFT-B3LYP optimized geometries and relative energies (kcal / mol) for the model compound Cp 2 ZrCl(–CH=CHSiH 3 ) in several conformations: 1a–b, 2 and 3.

connected to early transition metals is the lengthening of the C=C bond and the shortening of the M–C bond, effects which arise from the p-donating interaction between the vinyl and metal, as shown in Scheme 2(a). The theoretical

Zr–C a

Bond ˚ Rms i (A) ˚ b Rms ' (A) ˚ d (uncorrected) (A) ˚ d (lower limit) (A) ˚ d (upper limit) (A) ˚ d (riding model) (A) a

a

C a –C b

0.191 0.211

0.226 0.292

0.229 0.291

2.267 2.269 2.378 2.269

Rms amplitudes in Angstroms parallel to bonds. Rms amplitudes in Angstroms perpendicular to bonds.

b

0.282 0.312 1.217 1.218 1.516 1.227

M.B. Hall et al. / Polyhedron 18 (1999) 1717 – 1724

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Table 3 ˚ and angle Calculated and simulated geometrical parameters (distance in A in degrees) of Cp 2 ZrCl(–CH=CHSiMe 3 ) Disorder (1b:1a)

0:100

100:0

86:14

Measured

Zr–Cl Zr–C a C a –C b C b –Si Cl–Zr–C a Zr–C a –C b C a –C b –Si Cl–Zr–C a –C b

2.524 2.277 1.366 1.881 101.1 123.5 125.2 0.0

2.523 2.279 1.364 1.882 96.5 137.4 125.4 140.9

2.541 2.280 1.215 1.873 96.3 145.0 136.8 137.3

2.441 2.267 1.217 1.847 96.6 144.2 136.9 133.3

degree of disorder between the two configurations present in the crystal structure. Based on this model, we have simulated a structure for an 86:14 (1b:1a) disorder (see Table 3). The simulated Cl–Zr–C a , Zr–C a –C b , C a –C b – Si bond angles, and Cl–Zr–C a –C b dihedral angle are 96.3, 145.0, 136.8 and 137.38, respectively, while the C a =C b ˚ These simulated structural distance (D5) is 1.215 A. parameters are completely consistent with the measured structure.

Fig. 3. Composite stick plot of the optimized structures of 1a (dashed line) and 1b (solid line). Results of the least squares fit of the three common atoms Zr, Cl, Si, and the centroid of the C a –C b bond.

˚ and D2 (1.351 the calculated bond lengths, D1 (1.350 A) ˚ and the observed distance between similar carbon A), ˚ and D4 (0.925 atoms of the ‘fitted’ model, D3 (0.791 A) ˚ This affords the artificially shortened bond distance A). ˚ when the C a =C b bonds of 1b and 1a are (D5) of 1.051 A 50% occupied (see Fig. 4). Thus, it seems reasonable that the bond length observed in the reported structure could be a result of the disorder between two configurations of the C a =C b moiety. The disorder results in the apparent shortening of the C a =C b distance and in a slightly enlarged thermal parameter for the alkene carbon atoms as the centroids of the ellipsoidal parameters are displaced towards the disordered atom. Observed bond lengths in the ˚ are possible, depending on the range of 1.35 to 1.05 A

3.2. The Cp * Ir( PMe3 )( C2 H3 ) 1 and Cp * Ir( PMe3 )( C2 H3 )( H) complexes During our research on the ethylene C–H bond activation by the CpIr(PH 3 )(CH 3 )1 complex [2,3], we found that the Ir–vinyl complex, CpIr(PH 3 )(C 2 H 3 )1 (4a), does not show an agnostic structural feature (a close C–H???Ir interaction) but a strong p-donating interaction between the metal center and the vinyl ligand of 4a, as illustrated in Scheme 2(a). The p-donating interaction leads to a longer ˚ than that of a free C a –C b double bond (longer by 0.02 A ethylene) in the Ir–vinyl complex, as shown for 4a in Fig. 5. To elucidate the origin of this electronic effect, we optimized a corresponding Ir(III) hydrido complex, CpIr(PH 3 )(C 2 H 3 )(H) (5a) (Fig. 6). The optimized geometry of complex 5a appears to have a similarly stretched C=C bond as that of 4a. However, the X-ray crystal structure of the Ir hydrido vinyl complex, 5a, reveals an ˚ and an unexpectedly short C=C bond (about 1.29 A)

Fig. 4. Suggested mechanism for apparent shortening of the double bond distance of the vinyl ligand in the experiment.

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differences between 4a and 4b, as well as between 5a and 5b, are very small (11.24 and 10.07 kcal / mol, respectively). Thus, a co-crystallization of isomers with different rotation angles could lead to an unresolved disorder that shortens the C=C bond and enlarges the Ir–C a –C b angle, as shown in Scheme 3.

3.3. The Pt( PPh3 )3 ( MeC= CHMe) 1 complex

Fig. 5. DFT-B3LYP/ BS1 optimized geometries for the model compound CpIr(PH 3 )(C 2 H 3 )1 in different conformations: 4a–b.

unusually large Ir–C a –C b angle (132.88) [25,26]. Clearly, this result cannot be explained by an electronic effect. To examine the possible packing and steric effects, we have optimized the isomers, 4b and 5b (Figs. 5 and 6). The isomers, 4b and 5b, differ from 4a and 5a by a rotation of the vinyl ligand. The calculations show that the energy

Fig. 6. DFT-B3LYP/ BS2 optimized geometries for the model compound CpIr(PH 3 )(C 2 H 3 )(H) in different conformations: 5a–b.

Since shortened C=C double bonds are also observed in several square planar complexes [19,20,24], we investigated a square planar complex, Pt(PPh 3 ) 3 (MeC=CHMe)1 , as an example. The experimental work by Stang et. al. [19] showed that two isomers (Z and E isomers) of Pt(PPh 3 ) 3 (MeC=CHMe)1 are produced by the reaction of (Me)(OTf)C=C(H)(Me) with the Pt(PPh 3 ) 4 complex. The crystal structure (refined as the Z isomer) has a shortened ˚ a normal Pt–vinyl bond (2.07 C=C bond (about 1.24 A), ˚ and an acute angle between (cis-PPh 3 )–Pt–(cis-PPh 3 ) A) (166.88). The experimental esd’s hint at a hidden problem in this structure. The optimized geometries of the model complexes, Pt(PH 3 ) 3 (trans-MeC=CHMe)1 (6a), Z isomer, and Pt(PH 3 ) 3 (cis-MeC=CHMe)1 (6b), E isomer, are shown in Fig. 7. Compared to the crystal structure of Pt(PPh 3 ) 3 (MeC=CHMe)1 , the optimized Pt–vinyl bond ˚ and (cis-PH 3 )–Pt–(cis-PH 3 ) angle (167.58) of (2.079 A) 6a are very close to the experimental values. However, the optimized C=C distances of 6a and 6b show ‘normal’ ˚ vinyl double bonds, which are longer by 0.008–0.011 A than that of free ethylene. Since the back-donating interaction between the transition-metal center and vinyl ligand can only lead to a lengthened C=C bond, as shown in ˚ in the Scheme 2(b), the shortened C=C distance (1.24 A) crystal structure is difficult to explain by any electronic mechanism. Compared to 6a, the Pt–vinyl, C a =C b and C a –C b 9 bond distances of the isomer 6b change only slightly; however, the isomerization leads to a 48 decrease in the Pt–C a =C b angle. The bulky phosphine ligand will further increase this angle in 6a because of the steric interaction between the phosphine ligands and the methyl group on the vinyl ligand. The disorder in the Pt–C a =C b angle, which results from the co-crystallization of 6a and

Scheme 3.

M.B. Hall et al. / Polyhedron 18 (1999) 1717 – 1724

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Acknowledgements We thank the Robert A. Welch Foundation (Grant A648) and the National Science Foundation (94-23271) for financial support.

References

Fig. 7. DFT-B3LYP optimized geometries for the model compound Pt(PH 3 ) 3 (MeC=CHMe)1 in different isomerizations: 6a–b. The values in parentheses are the experimental values [16].

6b, can result in a shortened C=C distance, as shown in Scheme 4.

4. Conclusion Theoretical calculations show that the extremely short C=C double bond observed in Cp * Ir(PMe 3 )(C 2 H 3 )(H), Pt(PPh 3 ) 3 (MeC=CHMe)1 and Cp 2 ZrCl(–CH=CHSiMe 3 ) cannot be due to electronic or intramolecular steric effects. In general, the calculations suggest that the metal–vinyl interactions will only lengthen the C=C bond. A disorder model that involves the co-crystallization of two (or more) nearly isoenergetic conformations, originating from rotation about the M–C a axis explains these short bonds. The skewed nature of the distribution from the Cambridge Data Base suggests that all of the vinyl transition metal complexes with short C=C bonds, a total of 16 out of 31 (see Fig. 1), may be subject to this disorder problem to some degree.

Scheme 4.

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