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Journal of Organometallic Chemistry 769 (2014) 136e143

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Journal of Organometallic Chemistry

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The formation of a s-bond complex vs. an oxidation addition productin reaction of [M(CO)4(h4-nbd)] (M ¼ W, Mo) and HeEEt3(E ¼ Si, Ge, Sn): DFT optimized structures and predicted chemicalshifts of hydride ligands

Jarosław Handzlik a, **, Teresa Szyma�nska-Buzar b, *

a Faculty of Chemical Engineering and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Krak�ow, Polandb Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland

a r t i c l e i n f o

Article history:Received 21 May 2014Received in revised form23 July 2014Accepted 24 July 2014Available online 4 August 2014

Keywords:s-Bond activationMetal hydridesDFT calculationHydrosilylationHydrogermylationHydrostannation

* Corresponding author. Tel.: þ48 71 375 7221; fax** Corresponding author. Tel.: þ48 126282196; fax:

E-mail addresses: jhandz@pk.edu.pl (J. Handzlichem.uni.wroc.pl (T. Szyma�nska-Buzar).

http://dx.doi.org/10.1016/j.jorganchem.2014.07.0170022-328X/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Density functional theory calculations have been employed to understand the bonding and structuralfeatures of the key intermediate complexes formed during the catalytic cycle for the hydrogermylation,hydrosilylation and hydrostannation of norbornadiene (nbd) in the photochemical reaction of thetungsten(0) and molybdenum(0) carbonyl complex [M(CO)4(h4-nbd)] (M ¼ Mo, W) and HeEEt3 (E ¼ Si,Ge, Sn). The structure of the s-bond complexes [M(CO)3(h

2-HeEEt3)(h4-nbd)] (1) and the seven-

coordinate hydride complexes [MH(EEt3)(CO)3(h4-nbd)] (2) have been optimized. The calculatedvalues of the chemical shift for the s-bond proton in complex 1 and the terminal hydride ligand in theoxidative addition product complex 2 have been compared with recently published experimental datafor complexes of this type.

© 2014 Elsevier B.V. All rights reserved.

Introduction

The reaction of photochemically generated coordinatively un-saturated carbonyl complexes of Group 6 elements with a widerange of compounds leads to the h2-coordination of the s-bond,such as HeE (E ¼ H, C, Si, Ge, Sn), and their activation [1]. Thisprocess has been investigated since the first discovery of h2-coor-dination of dihydrogenmolecule by Kubas et al. [1s,2]. Interest in s-bond activation has been growing over the last decade owing to itssignificance for catalytic processes known as hydrogenation,hydroarylation, hydrosilylation, hydrogermylation and hydro-stannation [1a,1h,1j,1n,1p,3]. Several mechanisms have beenconsidered for these catalytic processes, but the oxidative additionof the HeE s-bond to transition metal complexes ([M]) and theformation of two s-bonds, He[M]eE, is proposed in all as the keymechanistic step (Scheme 1) [1b,1r,4].

: þ48 71 328 23 48.þ48 126282037.k), teresa.szymanska-buzar@

Although the activation of the HeSi bond by transition metalcomplexes is very well documented by 1H NMR spectroscopicstudies [1b], the activation of the analogous HeGe bond has beenobserved in only a few cases [1a,1d,1e,1r,5], and activation of theHeSn bond has been described in two papers [6].

Recent investigations carried out with molybdenum and tung-sten carbonyl complexes as catalysts suggest that the HeER3 (E¼ Si,Ge, Sn; R3 ¼ Et3, HEt2, HPh2, nBu3) bond activation could be used inthe hydrosilylation, hydrogermylation, and hydrostannation ofnorbornadiene (nbd) [1a,5,6a,7]. In the photochemical reaction ofnorbornadiene and HeER3 (E ¼ Si, Ge, Sn), in the presence of thecarbonyl complexes M(CO)6 or [M(CO)4(h4-nbd)] (M ¼ Mo, W), thehydrosilylation, hydrogermylation and hydrostannation productsof norbornadiene are formed in high yields (Scheme 2).

During these investigations, the h2-coordination or/and theoxidative addition of the HeE s-bond of silanes, germanes andstannanes to photochemically activated tungsten(0) and molyb-denum(0) carbonyl complexes, [M(CO)4(h4-nbd)], [M(CO)4(h4-cod)] (cod ¼ 1,5-cyclooctadiene) and M(CO)6, was observed by 1HNMR spectroscopy (Scheme 3) [1a,1e,1n,1o,5].

In reaction of M(CO)4(h4-nbd)] (M ¼ Mo, W) with the diethylsilane H2SiEt2, both the HeSi bonds interact with the metal center

Scheme 1. Activation of the HeE bond (E ¼ C, Si, Ge, Sn) by a molybdenum(0) ortungsten(0) complex [M].

J. Handzlik, T. Szyma�nska-Buzar / Journal of Organometallic Chemistry 769 (2014) 136e143 137

as was indicated by two high-field resonances (Scheme 3) [7].However, when the diphenyl silane H2SiPh2 reacted with the nor-bornadiene complex of molybdenum, coordination of only oneHeSi bond to the metal center was detected [7a], similarly as in thereaction of H2SiPh2 with M(CO)6 (M ¼ Mo, W).

The h2-coordination of the HeE s-bond of silanes, germanes andstannanes and the formation of unstable s-bond complexesobserved in the photochemical reaction of [M(CO)4(h4-nbd)](M ¼ W, Mo) (Scheme 3) was accompanied by the addition of theHeE s-bond to the double bond of norbornadiene (Scheme 2).

The photochemical reaction of M(CO)6 (M¼W,Mo)with diethylsilanes and germanes H2EEt2 (E¼ Si, Ge) leads to the formation of amixture of hydride complexes of varying stability, which wereobserved mainly by 1H NMR spectroscopy. From the mixture ofcompounds, only the binuclear complexes of the type [{M(m-h2-HeEEt2)(CO)4}2] were stable enough to isolate in crystalline form,and their molecular structure was established by single-crystal X-ray diffraction studies (Scheme 4) [1a,1e,8,9]. The chemical shift ofthe bridging hydride ligand in the latter complexes was observedat �8.33 ppm (MoeSi) [9], �7.86 ppm (MoeGe) [1e], �9.31 ppm(WeSi) [8] and �9.02 ppm (WeGe) [1a].

Only in the photochemical reaction of [W(CO)4(h4-nbd)] andHGeEt3 were two hydride signals detected at the same time by 1HNMR spectroscopy. One signal (septet, at d �7.63 ppm, 3JH-H(Et) ¼ 1.6 Hz, 1JH-W ¼ 34 Hz) was assigned to the s-bond complexon the observation of the simultaneous interaction of a protonwiththe tungsten atom and with the six methylene protons of the GeEt3moiety. The other signal (singlet at d�2.58 ppm, 1JH-W¼ 48 Hz) wasassigned to a germyl hydride complex of tungsten(II) (Scheme 3).The disappearance of these hydride signals was accompanied bythe formation of triethylgermylnorbornene with high endo-selec-tivity [1a].

This very intriguing observation needs additional explanation.The first question arises about the structure of both hydride com-plexes and the energy of their formation. Especially interesting arefactors that determine the possibility of the formation of the s-bond complex and its transformation to the oxidation additionproduct. The next question is at which point the hydride andgermyl ligands are transferred to the olefin bond coordinated to

Scheme 2. Transformation of norbornadiene into hydrosilylation, hydrogermylation orhydrostannation products in the photochemical reaction in the presence of M(CO)6 or[M(CO)4(h4-nbd)] (M ¼ Mo, W) complexes.

Scheme 3. Proposed geometry of molybdenum and tungsten carbonyl complexesformed in photochemical reactions with silanes and germanes based on NMR spec-troscopy [1a,1o,5,7].

Scheme 4. Schematic view of ligand arrangement found by X-ray diffraction studiesfor compounds of the type [{M(m-h2-HeEEt2)(CO)4}2] (M ¼ Mo, W; E ¼ Ge, Si)[1a,1e,8,9].

J. Handzlik, T. Szyma�nska-Buzar / Journal of Organometallic Chemistry 769 (2014) 136e143138

metal norbornadiene. The answers to these questions are veryimportant in investigations of the mechanism of the hydro-germylation reaction and the analogous hydrosilylation and stan-nylation reactions of norbornadiene in the presence ofphotochemically activated molybdenum and tungsten carbonylcomplexes.

Computational methods

Density functional theory (DFT) methods belonging to thehybrid GGA class were used in the calculations. The geometry of thecompounds was optimized with the PBE0 functional [10], whichwas successfully applied for similar investigations of molybdenumcomplexes on the basis of test calculations involving various DFTmethods [1e]. The triple-z valence def2-TZVPP basis set [11], rec-ommended for quantitative DFT and Hartree-Fock calculations [11],was applied for all the elements. The 28 and 60 innermost electronsof Mo and W, respectively, were replaced by the Stuttgart effectivecore potential [12]. Thermochemical quantities (T ¼ 298.15 K) were

Fig. 1. Optimized structures of the six-coordinate s-bond complexes [M(CO)3(h2-HeEEt3)(h4-nbd)] (1) (M ¼ W, Mo; E ¼ Si, Ge, Sn) formed in the photochemical re-action of [M(CO)4(h4-nbd)] and HeEEt3. All bond lengths are in Å.

evaluated in the rigid-rotor and harmonic-oscillatorapproximations.

In preliminary calculations, a large number of possible con-formers of the [W(CO)3(h2-HeGeEt3)(h4-nbd)] complex, whichdiffer from each other in the orientation of the ethyl groups, wereoptimized using the smaller split-valence def2-SVP basis set [11].Some most stable conformers were further recalculated with thedef2-TZVPP basis set and analogous [Mo(CO)3(h2-HeGeEt3)(h4-nbd)] structures were also analyzed. As the differences in Gibbs freeenergies and the most important geometrical parameters of theconformers are negligible, only the results for the most stableconformer of [M(CO)3(h2-HeGeEt3)(h4-nbd)] (M ¼ Mo, W) arepresented in this paper and this conformation is also taken intoaccount in the calculation of analogous [M(CO)3(h2-HeEEt3)(h4-nbd)] (1) (M ¼ W, Mo; E ¼ Si, Sn) compounds.

The natural population analysis (NPA) and the natural bondorbital (NBO) analysis [13] were applied to discuss the electronicproperties of the optimized structures of the Mo andW complexes.Atomic charges and Wiberg bond indices [14] were calculated onthe PBE0/def2-SVP level. Absolute 1H shielding values for the Moand W compounds were calculated by means of the GIAO method[15], using the LC-uPBE functional [16] combined with the def2-TZVPP basis set. This methodology was selected on the basis ofpresent 1H NMR test calculations for [{Mo(m-h2-H-GeEt2)(CO)4}2]and [{W(m-h2-H-GeEt2)(CO)4}2] compounds, also involving theM06 [17] functional (Table S1, Supplementary data), as well asbased on the previous results [1a] and tests [1e] including the PBE0method and various basis sets. The chemical shift values were ob-tained using tetramethylsilane (TMS) shielding as the reference,calculated at the same levels of theory.

All calculations were carried out with the Gaussian 09 package[18]. For the graphic presentation of the structures, the GaussView5.0 software was used [19].

Results and discussion

Optimized structures for [M(CO)3(h2-HeEEt3)(h

4-nbd)] (1) (M ¼ W,Mo; E ¼ Si, Ge, Sn)

In our continuing effort to understand the nature of s-bondactivation and its significance in catalysis, in this work we attemptto study the transformation of s-bond complexes into oxidationaddition products (Scheme 1). First, structures of the six-coordinates-bond complexes 1 were optimized by DFT calculation (Fig. 1,Table 1).

The formation of s-bond complexes, proposed as the key in-termediate species in catalytic hydrogermylation, hydrosilylationand stannylation of norbornadiene in the presence of photochem-ically activated tungsten andmolybdenum carbonyl complexes, hasbeen recently identified mainly by the NMR spectroscopic method[1a,1n,5,6a,7]. The h2-coordination of the HeE bond to the transi-tion metal can be observed in 1H NMR spectra due to the appear-ance of a high-field resonance. However, the structural propertiesof the resulting s-bond complexes in solution are sometimesdifficult to describe because of their low stability. This is the reasonwhy we decided to optimize the structure of the s-bond complex 1and calculate the chemical shifts of the proton of the h2-HeEEt3bond, and next to compare these results with the available exper-imental data.

As shown in Fig. 1, the optimized geometry for type 1 tungstenand molybdenum complexes is a distorted octahedral with theconfiguration of three carbonyl ligands close to meridional. Twocarbonyl groups are mutually trans with the OCeMeCO angle ofalmost 170�, and two carbonyl groups are mutually cis with theOCeMeCO angle of almost 90�. The coordination of the

Table 1Selected calculated bond distances R (Å), Wiberg bond indices P, angles (

�), NPA atomic charges and 1H NMR chemical shifts (d, ppm) for s-bond complexes of the type

[M(CO)3(h2-HeEEt3)(h4-nbd)] (1) (M ¼ W, Mo; E ¼ Si, Ge, Sn).

Complex MeH EeH MeE OCeMeCO EeMeH q(M) q(E) q(H) d, ppma

R P R P R P

[W(CO)3(h2-HeGeEt3)(h4-nbd)] 1.739 0.66 2.208 0.17 2.767 0.50 163.5; 88.6; 91.7 52.9 �1.22 1.51 0.06 �4.76 (�7.63)[Mo(CO)3(h2-H�GeEt3)(h4-nbd)] 1.739 0.58 1.860 0.29 2.790 0.41 167.3; 87.1; 88.5 40.8 �1.82 1.56 0.11 �7.15 (�7.25)[W(CO)3(h2-H�SiEt3)(h4-nbd)] 1.746 0.59 1.877 0.28 2.713 0.44 166.1; 86.8; 90.5 43.4 �1.20 1.59 0.05 �5.72[Mo(CO)3(h2-HeSiEt3)(h4-nbd)] 1.753 0.53 1.703 0.37 2.766 0.36 168.8; 87.3; 87.4 36.2 �1.76 1.65 0.06 �8.06[W(CO)3(h2-HeSnEt3)(h4-nbd)] 1.752 0.68 2.328 0.16 2.911 0.45 162.9; 88.7; 90.6 53.1 �1.25 1.74 0.04 �5.23[Mo(CO)3(h2-HeSnEt3)(h4-nbd)] 1.742 0.66 2.139 0.20 2.897 0.41 165.2; 87.9; 89.3 47.2 �1.90 1.78 0.11 �6.58

a In parentheses experimental values obtained from 1H NMR spectra measurement in solution [1a,5].

J. Handzlik, T. Szyma�nska-Buzar / Journal of Organometallic Chemistry 769 (2014) 136e143 139

norbornadiene ligand is arranged in two types. One of the olefinbonds is approximately trans to CO and the other is trans to the h2-HeE bond. This gives rise to a distinct (ca. 0.1Å) difference betweenthe MeColefin distances for the two olefin bonds of the nbd ligand.For the type 1 W-Ge complex, these distances are 2.405 Å, 2.412 Å

Fig. 2. Optimized structures for four isomers (2ae2d) of the hydride complex[WH(GeEt3)(CO)3(h4-nbd)] (2). All bond lengths are in Å.

and 2.310 Å, 2.313 Å, respectively, with the corresponding Wibergbond indices of 0.34, 0.33, 0.45 and 0.46. The two shorter andstronger WeC bonds are both trans to the h2-HeGe bond with theangle subtended at the tungsten atom by the midpoints of the C]Cbond and the HeGe bond of 177.5�. The two longer WeC bonds areapproximately trans to COwith the angle subtended at the tungstenatom by the midpoints of the C]C bond and the carbon atom of152.2�. This WeColefin bond difference is an effect of the better p-acidity of the CO ligand than the h2-HeE bond. The distance be-tween the H and the E atoms of the h2-HeE bond depends upon theatom E radius (Si < Ge < Sn) and for tungsten complexes increasesfrom 1.877 Å to 2.208 Å and 2.328 Å, whereas the correspondingbond indices decrease in the same order (Table 1). The same ten-dency is observed for the molybdenum complexes. In a similarorder, the WeE bond distance increases from 2.713 Å to 2.767 Åand 2.911 Å, respectively, however, the Wiberg index for the W-Gebond is highest. The MeH bond distances are almost identical inthe range of 1.739e1.753 Å, but the calculated bond indices in-crease in the order: Si < Ge < Sn. This is consistent with thementioned above decrease of the E-HWiberg indices with the atomE radius. The EeMeH angles are smaller for molybdenum com-plexes (36.2e47.2�) than for tungsten complexes (43.4e53.1�), andincrease with the increase of the atom E radius.

The calculated NPA charges on W atom are quite similar for theWeE complexes, with the most negative charge predicted forWeSn. The positive charge on Sn is clearly highest, compared to Geand Si (Table 1). According to the performed NBO analysis, 66% ofthe charge in the WeSn bond is polarized toward W. The polari-zation of the WeGe and WeSi bonds is slightly lower (61 and 63%,respectively). These results suggest a bit stronger ionic character oftheWeSn bond, compared toWeGe andWeSi. The same tendencyis observed for the Mo-E complexes, however, the calculatedcharges on Mo atom are much more negative than the charge onWatom, whereas the positive charges on E atoms are slightly higher,compared to the corresponding WeE compounds.

As can be seen in Table 1, the calculated values of chemical shiftare in acceptable agreement with those obtained experimentally.This agreement is much better for molybdenum than for tungstencomplexes. All the calculated chemical shifts of hydride for tung-sten complexes are at lower field than for the analogous molyb-denum complexes with Dd in the range 1.35e2.39 ppm.

Optimized structures for [MH(EEt3)(CO)3(h4-nbd)] (2) (M ¼ W, Mo;

E ¼ Si, Ge, Sn)

The oxidative addition of the s-bond of the h2-HeEEt3 ligand tothe metal center of complex 1 can lead to the formation of theseven-coordinate complex [MH(EEt3)(CO)3(h4-nbd)] (2) containingthe hydride ligand. The DFT calculations showed that during thetransformation of complex 1(WeGe) into 2 at least four complexes(2ae2d) can be formed (Fig. 2, Table 2).

Table 2Calculated relative Gibbs free energiesa (DG, kJ/mol), selected bond distances R (Å), Wiberg bond indices P, angles (

�), NPA atomic charges and 1H NMR chemical shifts (d, ppm)

for hydride complexes of the type [MH(EEt3)(CO)3(h4-nbd)] (2) (M ¼ W, Mo; E ¼ Si, Ge, Sn).

Complex Isomer (DG) MeH MeE OCeMeCO EeMeH q(M) q(E) q(H) d, ppm

R P R P

[WH(GeEt3)(CO)3(h4-nbd)] 2a (48) 2.049 0.12 2.818 0.48 99.2; 108.0; 113.2 120.5 �0.87 1.51 0.26 �4.562b (48) 1.792 0.58 2.805 0.51 104.8; 105.8; 116.7 90.6 �1.14 1.51 0.06 �2.362c (51) 1.792 0.56 2.791 0.52 106.2; 108.5; 113.9 92.3 �1.14 1.51 0.08 �2.642d (34) 1.731 0.63 2.739 0.65 86.4; 87.7; 168.5 130.4 �1.19 1.36 0.16 �7.95

[MoH(GeEt3)(CO)3(h4-nbd)] 2a (42) 1.964 0.17 2.775 0.51 92.8; 98.4; 130.6 142.0 �1.53 1.51 0.29 �5.152b (54) 1.767 0.62 2.777 0.49 106.3; 107.6; 113.5 94.3 �1.86 1.58 0.17 �3.20

[WH(SiEt3)(CO)3(h4-nbd)] 2a (55) 2.040 0.12 2.758 0.48 99.7; 109.7; 110.9 123.3 �0.88 1.59 0.26 �4.652b (55) 1.792 0.57 2.747 0.51 105.9; 107.0; 115.0 93.0 �1.15 1.60 0.07 �2.32

[MoH(SiEt3)(CO)3(h4-nbd)] 2a (49) 1.989 0.16 2.720 0.49 97.2; 104.3; 120.0 134.0 �1.51 1.64 0.29 �5.052b (61) 1.767 0.62 2.715 0.50 107.2; 108.4; 112.1 96.3 �1.87 1.67 0.17 �3.10

[WH(SnEt3)(CO)3(h4-nbd)] 2a (50) 2.047 0.12 2.964 0.41 98.8; 107.4; 112.2 120.7 �0.89 1.76 0.26 �4.482b (52) 1.793 0.58 2.949 0.45 104.3; 105.3; 115.8 92.0 �1.18 1.76 0.06 �2.52

[MoH(SnEt3)(CO)3(h4-nbd)] 2a (39) 1.984 0.16 2.929 0.42 95.9; 103.1; 120.9 134.4 �1.52 1.77 0.29 �5.102b (55) 1.768 0.62 2.917 0.43 105.9; 107.4; 112.7 95.8 �1.88 1.80 0.17 �3.26

a Transformation of complex 1 into the respective isomer of 2.

J. Handzlik, T. Szyma�nska-Buzar / Journal of Organometallic Chemistry 769 (2014) 136e143140

The formation of the 2ae2d complexes is moderately endoergic(DG from 34 to 51 kJ/mol). As was recently shown by the DFTcalculation (Scheme 5), the Gibbs free energy (DG ¼ 61 kJ/mol) [1e]for the oxidative addition of the s-bond HeGe of the h2-HeGeHEt2ligand of the pentacarbonyl complex [Mo(h2-HeGeHEt2)(CO)5] andthe formation of the seven-coordinate complex [MoH(Ge-HEt2)(CO)5] is distinctly higher than the value found here. Thisenergy difference is consistent with the better p-accepting abilityof carbonyl than the norbornadiene ligand and thus higher stabi-lization of s-bond complexes of the metal in a lower oxidation statethan their oxidative addition products. However, the analogoustransformation without changing the coordination number oftungsten and the formation of the six-coordinate complex oftungsten(II) is much less endoergic (DG ¼ 24 kJ/mol) [1a]. A sche-matic view of the ligand arrangement for these two complexesformed in the oxidative addition of the HeGe bond recently esti-mated by means of DFT calculations is shown in Scheme 5 [1a,1e].

The higher energy needed by the formation of the isomers 2a(DG 48 kJ/mol), 2b (DG 48 kJ/mol) and 2c (DG 51 kJ/mol) rather thanthe isomer 2d (DG 34 kJ/mol) can result from changes in theconfiguration of the three CO groups. In complexes of type 1 theconfiguration of the three carbonyl ligands is close to meridional(Table 1), while the three carbonyl groups in the isomers 2a, 2b and2c are rather cis with a facial arrangement of the three CO ligands(Table 2). Only in the most stable isomer 2d is the configuration ofthe CO groups close to meridional with the OCeMeCO angle of86.4�, 87.7� and 168.5�. Moreover, themost stable complex 2d is theone with the shortest WeH and WeGe distances, 1.731 Å and2.739 Å, respectively, the highest corresponding bond indices (0.63and 0.65) and with the larger EeWeH angle of 130.4�. The calcu-lated charge onWatom is close to the corresponding charge for W-Ge complex 1, but the positive charge on Ge is clearly lower for 2.The calculatedW-Ge NBO shows almost equal electron distributionbetweenWand Ge, indicating covalent bonding, in agreement withthe relatively highWiberg index. For themost unstable complex 2c,the WeH and W-Ge distances are longer, 1.792 Å and 2.791 Å,respectively, the corresponding bond indices are lower (0.56 and0.52) and the EeWeH angle is much smaller, 92.3�.

In complex 2a, a new CeH bond (1.158 Å, P ¼ 0.74) is predictedbetween one of the four olefin carbons of the h4-nbd ligand and thehydrogen atom derived from the h2-HeGe bond. Consequently, thepredicted positive charge on H is much higher in this case,compared to compounds 1 and 2be2d (Table 2). The charge on Watom is clearly less negative than in the case of 1 and 2be2d,

whereas the positive charge on Ge is similar, with the exception of2a. The calculated WeH distance of 2.049 Å for this bridging(agostic) hydride in 2a is significantly longer than that calculatedhere for the terminal WeH bond (1.792 Å) in complex 2b of thesame stability and the most stable complex 2d (1.731 Å) as well asthe WeH bond distance of 1.839 Å for [{W(m-h2-HeGeEt2)(CO)4}2]found recently by the DFTmethod [1a]. As expected, theWeH bondindex for 2a is very low (0.12), whereas it is in the range of0.56e0.63 for 2be2d (Table 2). The formation of a two-electronthree-center agostic bond (CeH,,,W) in 2a leads to the short-ening of one of the two WeColefin bond distances to 2.204 Å(P¼ 0.61) and the lengthening of the other WeColefin bond distanceto 2.430 Å (P ¼ 0.20), while the olefin C]C bond distance becomesdistinctly longer, at 1.524 Å (P ¼ 1.00), than the other C]C bonddistance (1.378 Å, P ¼ 1.51) of the same nbd ligand in 2a. The un-stable complex 2a can be the intermediate complex that plays acrucial role on the way of the hydride transfer from the HeGe bondto the olefin bond of the norbornadiene ligand coordinated totungsten. As was mentioned by Brookhart et al. in 1988 [20], thereare several examples of such agostic interaction, especially in hy-dride complexes containing a chelating diene in the coordinationsphere of the transition metal.

As can be expected, the calculated chemical shift of the bridginghydride ligand for complexes of type 2a are at higher field thancalculated for the terminal hydride of complexes of type 2b and 2cby ca. 2 ppm (Table 2). In 1H NMR spectra, a hydride signal ofcomplex 2a should be observed as a doublet and in 2D 1H NMRcorrelatedwith another proton at the same carbon, whose chemicalshift is predicted here at 1.87 ppm. However, in our recent exper-iments we did not detect this kind of proton signal. The protonsignal at d�2.58 ppm (1JH-W¼ 48 Hz) (Scheme 3) that was detectedin the photochemical reaction of [W(CO)4(h4-nbd)] and HGeEt3 canrather be assigned to a germyl hydride complex of tungsten(II), 2bor 2c, for which the chemical shifts of hydride ligands are predictedhere at �2.36 and �2.64 ppm, respectively (Table 2). It should benoted that the geometry of the hydride complexes 2b and 2c isdifferent only in the arrangement of ethyl groups in the trie-thylgermyl ligand (Fig. 2). It is a very probable that the hydridecomplex observed experimentally [1a] is not the seven-coordinatebut distinctly more stable six-coordinate complex easily formed inphotochemical reaction. As was shown recently, the formation ofthe six-coordinate complex of tungsten(II) is more endoergic thanthe formation of the seven-coordinate complex containing thehydride ligand (Scheme 5) [1a,1e].

Scheme 5. Schematic view of the ligand arrangement of s-bond complexes and the oxidative addition of HeGe bond products, recently estimated by means of DFT calculations[1a,1e]. In parenthesis are the chemical shift values obtained according to the methodology used in the present work.

J. Handzlik, T. Szyma�nska-Buzar / Journal of Organometallic Chemistry 769 (2014) 136e143 141

For the most stable hydride complex 2d, the chemical shift ofthe hydride ligand is predicted here at �7.95 ppm. Such high-fieldresonances were observed in the reaction of tungsten and molyb-denum norbornadiene complexes with secondary silanes: diethyland diphenyl silane (Scheme 3) [7]. For the oxidative additionproduct formed in the reaction of the norbornadiene complex oftungsten(0) and H2SiEt2, two equal-intensity doublets at �8.33(1JW-H ¼ 52 Hz) and �11.47 (1JW-H ¼ 38 Hz) ppm, on the basis of the1He183W coupling constant, were assigned to the terminal andbridging hydrides, respectively [7b]. The latter complex of tung-sten(II) was more stable than those formed in the reaction of ter-tiary silanes and germanes, and was stable enough to be fullycharacterized in solution by 1H and 13C NMR spectroscopies. Its 1HNMR data for norbornadiene ligand (d: 5.09, 3.89, 3.31, ~1.2) areclosely related to the predicted here for complex 2d (d: 5.05, 4.97,2H, CHolefin; 3.81, 3.71, 2H, CHolefin; 3.47, 3.46, 2H, CH; 1.38, 1.29, 2H,CH2) and to the corresponding 1H NMR data of the seven-coordinate norbornadiene complexes of tungsten(II) whose crys-tal structure was resolved by X-ray diffraction analyses [21]. Theanalogs to the complexes of type 2d, the seven-coordinate com-plexes [MCl(ECl3)(CO)3(h4-nbd)] (M ¼ W, Mo; E ¼ Ge, Sn), wereobtained earlier in the photochemical reaction of [M(CO)4(h4-nbd)]with tin and germanium tetrachlorides (Scheme 6).

It should be noted that in the crystal structures of all the fourseven-coordinate heterobimetallic complexes containing the MeEbond bridged by a chloride ligand, two carbonyl groups aremutually trans with the OCeMeCO angle of ca. 170� and twocarbonyl groups are mutually ciswith the OC-M-CO angle of ca. 90�.The chelating norbornadiene ligand is coordinated asymmetricallywith two longer (trans to the CO ligand) and two shorter (trans tothe Cl ligand) MeColefin distances [21].

For comparison of the structural properties and the NMRchemical shift of the hydride ligand in tungsten and molybdenumcomplexes of type 2, isomers 2a and 2bwere chosen. The isomer 2acan be the key intermediate complex during the catalytic cycle for

Scheme 6. Oxidative addition of the s-bond CleE (E ¼ Sn, Ge) to molybdenum(0) ortungsten(0).

the hydrofunctionalization of olefin but isomer 2b may be thatobserved experimentally by 1H NMR. The chemical shift predictedhere for the hydride ligand of the complex 2b at �2.36 ppm is veryclose to the value of �2.58 ppm detected experimentally [1a]. Theoptimized structures of these isomers are shown in Figs. 2 and 3,and the selected bond distances, angles and the electronic structureparameters are given in Table 2. Themolybdenum complexes 2a areslightly more stable (DG from 39 to 49 kJ/mol) than the analogoustungsten complexes (DG from 48 to 55 kJ/mol), while for complexesof type 2b the opposite trend is observed. However, the hydridecomplexes (2a and 2b) of both metals, formed as a result of HeSnbond oxidative addition are slightly more stable than analogouscomplexes formed during HeSi bond breaking. This is agree withthe fact that in the photochemical reaction of the norbornadienecomplex of molybdenum and HSnnBu3, several signals of hydrideligands, distinctly different in terms of their 1H-117/119Sn couplingconstants, were simultaneously detected in the range from �4.6to �6.5 ppm of the 1H NMR spectrum [6a]. A comparison of thecalculated chemical shifts in Tables 1 and 2 clearly shows that thelower-field resonances are from terminal hydrides and the higher-field resonances are due to the proton of the h2-HeEEt3 ligand inthe s-bond complexes.

Conclusions

The DFT calculations presented here indeed support the viewthat the activation of the HeE (E ¼ Ge, Si, Sn) s-bonds proceeds byits h2-coordination and next by oxidative addition to tungsten(0) ormolybdenum(0). First, structures of the six-coordinate s-bondcomplexes [M(CO)3(h2-HeEEt3)(h4-nbd)] (1) (M ¼ W, Mo; E ¼ Si,Ge, Sn) were optimized by DFT calculations. The oxidative additionof the s-bonds of the h2-HeEEt3 ligand to the metal center ofcomplex 1 leads to the formation of the seven-coordinate complex[MH(EEt3)(CO)3(h4-nbd)] (2) containing a hydride ligand. The DFTcalculations showed that the transformation of complex 1(W-Ge)into 2 leads to the formation of at least four complexes (2ae2d) in amoderately endoergic (DG from 34 to 51 kJ/mol) processes. Forcomparison of the structure and the NMR chemical shifts of thehydride ligand, isomers 2a and 2b were chosen. In complexes oftype 2a the formation of a new CeH bond (1.157e1.169 Å) is pre-dicted between one of the four olefin carbons of the h4-nbd ligandand the hydrogen atom derived from the h2-HeE bond of complex1. The unstable complex 2a can be the intermediate complex thatplays the crucial role on the way of the hydride transfer from theHeE bond to the olefin bond of the norbornadiene ligand

Fig. 3. Optimized structures of the hydride complexes [MH(EEt3)(CO)3(h4-nbd)] (2) (M ¼ W, Mo; E ¼ Si, Ge, Sn) formed in the oxidative addition of the s-bond of the h2-HeEEt3ligand to a transition metal. All bond lengths are in Å.

J. Handzlik, T. Szyma�nska-Buzar / Journal of Organometallic Chemistry 769 (2014) 136e143142

coordinated to tungsten or molybdenum. The calculated value ofchemical shift for hydride in complexes of type 1 and 2 are inacceptable agreement with the available experimental data. How-ever, it should be remembered that in the experimental photo-chemical reaction not only seven but also six-coordinate complexesof tungsten(II) can be formed and detected by NMR.

Acknowledgments

Computing resources from Academic Computer Centre CYFRO-NET AGH (grants MNiSW/IBM_BC_HS21/PK/003/2013 and MNiSW/SGI3700/PK/003/2013) are gratefully acknowledged. This researchwas supported in part by PL-Grid Infrastructure.

J. Handzlik, T. Szyma�nska-Buzar / Journal of Organometallic Chemistry 769 (2014) 136e143 143

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jorganchem.2014.07.017.

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