Tantalocenehydridephosphorus chemistry.

Polyhedron 21 (2002) 371– 379 www.elsevier.com/locate/poly

Tantalocenehydridephosphorus chemistry. Some new complexes and crystal structures of [CpCp%TaH2(PMe2H)]PF6 (Cp=C5H5, Cp% =C5Ht2Bu(Me)2) and Cp2Ta(H)(m-PPh2,H)Fe(CO)3 Gilles Boni, Olivier Blacque, Philippe Sauvageot, Nicolas Poujaud, Claude Moı¨se, Marek M. Kubicki * Laboratoire de Synthe`se et d’Electrosynthe`se Organome´talliques – UMR 5632, Faculte´ des Sciences, Uni6ersite´ de Bourgogne, 9 A6. A. Sa6ary, BP 47870, 21078 Dijon, France Received 3 July 2001; accepted 14 November 2001

Abstract The aim of this paper is to look for a better knowledge of the behaviour of bent tantalocenes that bear hydrides, phosphorus PR2X (R=Me, Ph; X= H, lone pair) and cyclopentadienyl (Cp = C5H5, Cp%= C5Ht2Bu(Me)2, Cp*= C5Me5) ligands. An orbital control of regioselectivity of insertion of the PR2 phosphide fragment of chlorophosphines PR2Cl into the central TaH bond of trihydrides Cp2TaH3 leading to the formation of metallophosphonium cations is discussed. Neutralisation of cationic complexes with strong bases leads either to the Ta(V)–phosphide or to the Ta(III)– phosphine species depending on the nature of the cyclopentadienyl ligand; good electron donor Cp% and Cp* rings favour the formation of Ta(V)– phosphide species, while C5H5 orients neutralisation towards Ta(III)–phosphine ones. These trends are roughly confirmed by DFT calculations. The X-ray structure of the first tantalocene phosphonium ionic compound [CpCp%TaH2(PMe2H)]PF6 (Cp%=C5Ht2Bu(Me)2) as well as that of the bimetallic complex Cp2Ta(H)(m-PPh2,H)Fe(CO)3 are also reported. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tantalum; Metallocenes; Hydrides; Metallophosphines; Bimetallics; X-ray structures

1. Introduction The search for binuclear complexes containing two different transition metals still attracts great interest [1]. In particular, the chemistry of heterobimetallic compounds stabilised by bridging phosphido groups has been widely studied [2]. Our group is involved in this field and we have developed synthetic strategies starting from metalloligands. These metallophosphines are obtained by reacting d2 monohydrides Cp2M(L)H (M = Nb, Ta; L =two electron donor) [3] or d0 dihydrides Cp2MH2 (M=Mo, W) [4] with chlorophosphines PR2Cl. In keeping with our interest in phosphido derivatives, we turned our attention to the trihydrides Cp2MH3 (M= Nb, Ta; d0) which are good precursors * Corresponding author. E-mail address: [email protected] (M.M. Kubicki).

of highly reactive unsaturated intermediates [Cp2MH] and are therefore able to undergo coordination or insertion reactions with phosphines. Boni and coworkers [5] and Nikonov et al. [6] have shown that trihydrides Cp2MH3 give clean reactions with chlorophosphines PR2Cl. In this paper we report on the syntheses and properties of some new mononuclear and dinuclear complexes with TaP bonds.

2. Results and discussion

2.1. Cationic complexes It is experimentally established that chlorophosphines PR2Cl react with tantalocene (and niobocene) trihydrides (1) leading to the formal insertion of the PR2 unit into the central TaH bond with formation of

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symmetrical metallophosphonium cations [Cp2TaH2(PR2H)]+ (2) (Cp2 =(C5H5)2 (2,2), (C5H5)(C5Ht2Bu(Me)2) (2,2%), (C5H5)(C5Me5) (2,2*) and (C5Me5)2 (2*,2*); R=Me, Ph) bearing both residual hydrides in lateral positions (Scheme 1). Nikonov proposed recently a plausible general mechanism of an insertion process based on carbenoid behaviour of halophosphines [6c]. This model involves the HOMO and the LUMO of PR2Cl molecules which represent, respectively, the canonical lone pair of phosphorus (LP), and the s*(p) antibonding PCl molecular orbital. We calculated the shapes of these orbitals for PMe2Cl with a semi-empirical PM3 method [7] and depicted them on Fig. 1. Their lobes of chemical interest have a high contribution of phosphorus and form an angle close to 60°. The first step of this mechanism consists of an interaction between one of the three MH bonds of the complex with the LUMO of the PR2Cl reagent (hydride nucleophilic attack on the P atom of chlorophosphine). However, the regiochemistry of this interaction is not clear, because both the central and the lateral MH bonds of Cp2MH3 may formally operate. Nikonov assumed that it is the central MHa bond that attacks the s*(p)PCl orbital, because he knows the result of this action. The simplified energy diagrams of tantalocene trihydrides (Fig. 2), calculated at the DFT/

Scheme 1.

Fig. 1. Shapes of the HOMO (left) and the LUMO (right) for PMe2Cl.

B3LYP level with Lanl2DZ basis[8], do not distinguish clearly which TaH bond (2a1 or b2) is preferentially responsible for the initial interaction with the s*(p)PCl orbital. The 2a1 occupied molecular orbital of trihydride is mainly built of the TaH central bond, while the filled b2 orbital contains contributions from the lateral ones. The HOMO for (1,1) is 2a1, thus indicating the reactivity of the central bond, but it lies only 0.25 eV over b2. With a good electron donor permethylated ligands (1*,1*) the b2 orbital builds the HOMO, thus suggesting a lateral interaction. However, the energy difference HOMO– HOMO-1 here is still smaller (0.15 eV) than for (1,1), and consequently, this electronic argument for lateral interaction may be contoured by steric hindrances in the transition state of insertion. If we consider the interaction of the 2a1 molecular orbital (central TaH bond of the trihydride) with the LUMO of PR2Cl, we should assume that the HOMO of PR2Cl (P lone pair) must be placed in the xz molecular plane defined by the Ta atom and the centres of the cyclopentadienyl rings (because of repulsions between the phosphorus lone pair and TaH bonds) and, consequently, should overlap with the b1 empty metallic orbital (dxz ) located in this plane. On the other hand, if the lateral TaH bond (b2) is implied in the interaction with the LUMO of PR2Cl, the lone pair of this reagent should now interact with the metallic a2 (dxy ) orbital. The dxz –LP(P) ligand interaction consists of a direct metal–ligand overlap (‘strong’ interaction), whereas that involving dxy is of lateral nature (‘weak’ interaction). Such an analysis of bonding favours a central coordination of the chlorophosphine. Moreover, the lateral interaction can lead to steric hindrances consisting of geometrical conflicts between R substituents of PR2Cl and cyclopentadienyl rings. Thus, the participation of the b1 metallic orbital (in the building of the transition state) may explain the observed regiochemistry of the insertion process. It is worth noting that the tantalocene cationic species bearing the phosphine (PMe2Ph) or the phosphite (P(OMe)3) [9] in lateral sites, obtained by protonation of Ta(III) (d2) Cp2Ta(L)H derivatives, are known and stable. However, the ligands in these last derivatives already occupy the lateral positions allowing the attack of the proton on the central lobe of the non-bonding HOMO in the d2 system [10]. In order to confirm the stereochemistry of the insertion process we have carried out the experiments with a centrally deuteriated complex (C5H5)(C5Ht2Bu(Me)2)TaH2D [11]. Reaction of this freshly prepared complex in toluene with PMe2Cl at low temperature (−78 °C) gives almost exclusively (90%) the phosphorus deuteriated salt [C5H5)(C5Ht2Bu(Me)2)TaH2(PMe2D)]Cl together with a small fraction of the metal deuteriated isotopomer [(C5H5)(C5Ht2Bu(Me)2)TaHD(PMe2H)]Cl (10%) (the ratios estimated from 1H NMR data). The

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Fig. 2. Simplified energy diagram of the frontier molecular orbitals for C5H5 (1,1) and C5Me5 (1*,1*) trihydrides.

complex [C5H5)(C5Ht2Bu(Me)2)TaH2(PMe2D)]Cl is stable in toluene solution under argon for several days with neither detectable H/D exchange nor the formation of isomers with the phosphine in the lateral position. The use of the new (C5Ht2Bu(Me)2) ligand [12] allowed us to grow for the first time X-ray quality crystals of a Group 5 cationic metallocene bearing a secondary phosphine ligand (2,2%,Me). The crystal structure of (2,2%,Me) is built of organometallic cations and PF6 anions. The cation has a structure similar to that reported for the neutral complex Cp2TaH2PPh2 with a terminal phosphido ligand [6b]. The geometry of the cation may be regarded as a highly distorted trigonal bipyramid or as a tetrahedron one with one edge (HH) capped with a phosphorus ligand (Fig. 3). The TaP(PMe2H) bond length of 2.533(1) A, in (2,2%,Me) is significantly shorter than that reported by Nikonov for the Ta(V) neutral complex with a terminal PPh2 ligand Cp2Ta(H)2PPh2 (2.595(3) A, ) [6b]. Although, the substituents on the P atom (Me vs. Ph) are different it seems reasonable to assume that the presence of a diffused phosphorus lone electron pair in the neutral complex, instead of the hydrogen atom in

Fig. 3. ORTEP plotting (50% probability level) of the cation in (2,2%,Me). Selected bond lengths (A, ) and angles (°): TaP1 2.533(1); TaH1 1.69(4); TaH2 1.73(6); TaCP% 2.063; TaCP 2.059; H1TaH2 135(2); H1TaP1 70(1); H2TaP1 66(2); CP%TaCP 138.8.

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(2,2%,Me), is responsible for the long TaP bond observed in the neutral complex. The pure so called ‘transition metal gauche effect’ [3d,13] cannot be applied to the observed lengthening in the neutral complex because there is no non-bonding tantalum filled orbital in a formal (Ta(V), d0) structure. However, the repulsions between the P lone pair and TaH bonds may easily operate therein.

2.2. Neutral complexes It has been observed that the cationic metallophosphonium compounds of type 2 (Group 5 and 6 metallocenes) are easily deprotonated by strong bases [3 – 6,14,15]. In the case of Group 5 metals, like Ta(V) complexes, the neutralisation may lead either to the Ta(III) –phosphine or to the Ta(V)– phosphide species (complexes 3, Scheme 2). A similar phosphine (MPPh2H) versus phosphide (MPPh2) isomerism due to the nature of the second versus third row transition metal has been observed by Baker et al. [16] for Mo/W monocyclopentadienyl Cp*Mo(PMe3)(PPh2)(PPh2H)/Cp*W(PMe3)(PPh2)2H and by Nikonov [6a,6b] for the Nb/Ta metallocene Cp2Nb(PPh2H)(H)/Cp2Ta(H)2(PPh2) series. This has been assigned to the stronger metalhydrogen bonds for third row metals as compared to those from the second row [17]. We have also observed such an isomerism after deprotonation of molybdenocene phosphonium cations [Cp2Mo(H)(PR2H)]+ (R = Me, Ph) which gives either the MoII Cp2Mo(PMe2H) or the MoIV Cp2Mo(H)(PPh2) neutral species; a process clearly depending on the nature of the coordinated secondary phosphine [15]. A similar behaviour is also observed for tantalocenes (2,2) (pathways (i) and (iii) in Scheme 2). Thus, the type of neutral complex (M, d2-phosphine) or (M, d0-phosphide) depends on the nature of M and on that of R. The isomerism observed in the case of new neutralised ‘tantalocenePMe2X (X =H or LP)’ systems depends on a third factor: the nature of the substituted

Scheme 2.

cyclopentadienyl ligands. Strong electron donor Cp ligands e.g. alkyl substituted cyclopentadienyl rings, (C5Ht2Bu(Me)2) and (C5Me5) versus non-substituted C5H5 favour the formation of Ta(V)–phosphide complexes when R= Me. Both isomers are formed with mixed Cp/Cp% ligands (3,3%Me), but a sole Ta(V)–phosphide isomer is found for the mixed Cp/Cp* complex (3,3*Me). Note, however, that neutralisation of permethylated cations (2*,2*) (expected formation of Ta(V)– phosphide isomer) does not give the isolable defined product of the type (3*,3*Me). This may be due to a very electron rich nature of the expected complex. Because all neutral complexes (3), whatever their phosphine (Ta(III)) or phosphide (Ta(V)) nature, react with Group 6 and 8 metal carbonyls giving m-phosphido bimetallics [14], we focussed our attention on the relative stabilities of different Ta(V) and Ta(III) neutral species ((3,3) and (3,3*)). They have been estimated from total energies calculated for optimised geometries of extreme Ta(V) and Ta(III) cases with the DFT/ B3LYP method with Lanl2DZ basis. The results are shown on Fig. 4. The calculations indicate that for non-substituted (3,3) isomers (part a) on Fig. 4) the Ta(V)–phosphide structure should be more stable than that of Ta(III)– phosphine, but only by 0.35 eV (some 34 kJ mol − 1). Note, that the last one (Ta(III)–phosphine) is experimentally observed upon neutralisation of (2,2). In the (3,3*) series (part b) on Fig. 4), the lowest energy Ta(V) –phosphide structure is more stable than that of Ta(III)–phosphine by 0.52 eV (50 kJ mol − 1). It agrees with experimental observation. Thus, one may conclude that the calculated trends roughly correspond to the experimental ones: the presence of a good electron donor Cp ligand stabilizes the Ta(V) isomer more than the Ta(III) isomer. The Me groups of the phosphide ligand in a stable geometry of the (3,3*) system are found, with respect to the TaH2P plane, on the side of the non-substituted Cp ligand (structure x). We also looked at what happens when the Me groups are placed on the other side of this plane (rotation by 180°), in the space of the Cp* ligand (structure y). A local minimum of energy has been effectively observed for this conformation (y), lying only 0.2 eV higher with respect to the structure (x). The Ta –P distances in optimised structures (3,3) and (3,3*) remain roughly unchanged. However, one observes an increase of the Ta–P– Me angle in the (y) conformer (113.6°, Cp* –Me repulsions) with respect to the value (109.8°) calculated for (x). Thus, the P lone pair is pushed towards the unsubstituted Cp ring in (3,3*). Such a tendency may lead to an energetically (and geometrically) unfavourable structure in the presence of two permethylated Cp* ligands and, consequently, may explain the experimental difficulties encountered upon neutralisation of (2*,2*) cations attempted for the isolation of (3*,3*) metallophos-

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Fig. 4. Optimised geometries and total energies for Ta(V) – phosphide and Ta(III) – phosphine complexes. (a) (C5H5)(C5H5), (b) (C5H5)(C5Me5) series.

phides. For this last case, there are some indications on the 1H NMR spectrum of the crude material that (3*,3*) is formed, but we were not able to isolate it. It is worth noting that the attempts of optimisation of permethylated metallocene Ta(V) or Ta(III) structures (3*,3*) failed with our standard computational DFT method. Another interesting observation arising from this study concerns the TaP bond lengths. The TaP optimised distances are shorter for TaPMe2H bonds (2.58 A, ) than for TaPMe2 ones (2.68 A, ), confirming the trends observed in crystal structures of (2,2%,Me) (phosphine, 2.53 A, ) and of Cp2TaH2PPh2 (phosphide, 2.59 A, ) [6b].

2.3. Bimetallic phosphido bridged complexes The neutral complexes (3) easily react with metalcarbonyls of Group 6 M%(CO)6 (M% =Cr, Mo, W) giving rise to the monobridged Ta(m-PR2)M% (4) and the dibridged Ta (m-PR2, H) M% (5) isolated species, while with Fe(CO)5, Fe2(CO)9 or (BDA)Fe(CO)3 the dibridged structures (5) are directly formed [14]. However, no crystal structure in the series of monobridged compounds MV(H)2(m-PR2)M% has been reported to date, probably because they easily transform to the dibridged structures during crystallisation.

In the series of bimetallic dibridged complexes (mPR2,H) we have already reported two crystal structures of Group 5 (d0) metallocenes Cp2Ta(H)(m-PMe2,H)Cr(CO)4 [5b] and Cp2Nb(H)(m-PPh2,H)W(CO)4 [18], and describe here the third member in this series, the Ta–Fe complex Cp2Ta(H)(m-PPh2,H)Fe(CO)3 (5,5Fe). There is one dinuclear organometallic and one solvent (acetone) molecule in the asymmetric unit of P21/n space group. The metallic fragments have their usual pentacoordinated (Ta) and trigonal bipyramidal (Fe) geometries (Fig. 5). The Ta, H1, H2, P, Fe, C3 and O3 atoms roughly lie in the plane bisecting that of the Cp rings. The apical FeC3 (CO trans to H) distance of 1.71(2) A, in the iron polyhedron is apparently, but not significantly, shorter than the equatorial ones (1.75(2) A, ). It may suggest, however, that a slight compression of iron bipyramide is due to the donor trans effect of the hydride [19]. These distances fall in a lower part of the range of FeCO bond lengths observed in Fe(CO)n units (1.73–1.81 A, ; n=3–5) [20] indicating a rather high electron density (due to the p-back bonding) retained on the Fe(CO)3 fragment in (5,5Fe). Note, that the mean FeCO bond length reported for the structurally related electron rich discrete [cis-HFe(CO)3P(OPh)3]− anion (1.73(1) A, ) [20a] is essentially the same. The electron density on the Fe(CO)3 fragment

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neutralisation in these organometallic cations with strong bases. The proposed pathways for these reactions have been confirmed by DFT calculations. Good electron donor cyclopentadienyl ligands (Cp% and Cp*) favour the formation of Ta(V)–phosphide structures when R is also a donor (Me). A supposition that the metalphosphide (PR2) bonds are longer than the metal–secondary phosphine (PR2H) ones, because of a more diffuse nature of the phosphorus lone pair than that of the PH bond, is also confirmed by DFT calculations.

4. Experimental

Fig. 5. ORTEP drawing of (5,5Fe) (30% probability level). Selected bond lengths (A, ) and angles (°): TaFe 3.111(2); TaP 2.548(3); TaH1 1.82(13); TaH2 2.01(12); FeP 2.168(4); FeH2 1.49(12); FeC1 1.740(17); FeC2 1.757(14); FeC3 1.708(16); TaCP1 2.059; TaCP2 2.068; H1TaH2 124(5); H1TaP 58(4); H2TaP 67(3); TaPFe 82.1(1); TaH2Fe 125(3); CP1TaCP2 135.2.

is still higher in (5,5*Fe) as indicated by its w(CO) IR data (see Section 4). All these observations argue for no (or little) direct electron density exchange between the metals. Effectively, the TaFe separation in (5,5Fe) of 3.111(2) A, is longer than in Cp2Nb(m-PPh2,CO)Fe(CO)3 for which a direct NbFe bond (2.884(2) A, ) has been postulated and confirmed by EHMO calculations [3c]. Thus, the electron density exchange (bonding) between the Cp2Ta(H)(m-PPh2,H) and Fe(CO)3 fragments in (5,5Fe) operates via the bridging atoms and not through a direct TaFe bond. Such a difference may be rationalised in a frame of singular properties of ‘s’ (bridging hydride) and ‘p’ (bridging carbon (CO)) atomic orbitals which are able to orient the phases (lobes) of metal atomic orbitals in these bimetallic systems either towards the ‘in phase’ (bonding MM%) interaction (as in the case of M(m-PR2,CO)M%) or towards the opposite phases (antibonding MM%) as in the case of the hydride bridge in the M(m-PR2,H)M% system. 3. Conclusion In this contribution we attempted to clarify the crucial points of tantalocenehydridephosphorus chemical systems: the regiochemistry of insertion of PR2 (from PR2Cl) into the central TaH bond of trihydrides Cp2TaH3 (formation of symmetrical metallophosphonium cations) and the influence of substitution on the cyclopentadienyl ring on the site (TaH or PH) of

Nuclear magnetic resonance spectra were recorded on a Bruker AC 200 instrument at 200 MHz for 1H and 81 MHz for 31P. The chemical shifts are reported in ppm relative to SiMe4 (1H) and external H3PO4 (31P– {1H}). Infrared spectra were obtained with a Nicolet 250 spectrometer, with the sample in solution in thf. Elemental analyses were performed on a EA 1108 CHNS-O FISONS Instruments apparatus. All manipulations were performed under a dry nitrogen atmosphere by standard techniques. Solvents were dried and deoxygenated by using conventional procedures. Cp2TaH2(PPh2) [5a,6b], CpCp%TaH3 [12], CpCp*TaH3, Cp*2TaH3 [21] and Fe(CO)3(BDA) [22] were prepared according to literature methods.

4.1. Preparation of [CpCp*TaH2(PMe2H)]Cl (2,2*,Me) To a C6H5CH3 solution (30 ml) of Cp*CpTaH3 (0.19 g, 0.5 mmol) was added 1 equiv. of PMe2Cl. After 15 min of stirring, a white precipitate was filtered, washed with C6H5CH3 and C5H12 and dried under vacuum (yield 0.22 g, 92%). 1H NMR (200 MHz, CD3COCD3, 25 °C, TMS): l =5.69 (s, Cp), 5.29 (dh, 1J(H, P) = 348.4 Hz, 3J(H, H) = 6.5 Hz, PH), 2.19 (s, Cp*), 1.76 (dd, 2J(H, P) =10.7 Hz, 3J(H, H) =6.5 Hz, Me), − 0.58 (d, 2J(H, P) = 75.1 Hz, TaH). 31P{1H} NMR (81 MHz, CD3COCD3, 25 °C, TMS): l= −43.1 (s, PMe2H).

4.2. Preparation of CpCp*TaH2(PMe2) (3,3*,Me) A saturated aq. solution of KOH (10 ml) was added to the dried salt [Cp*CpTaH2(PMe2H)]Cl (0.24 g, 0.5 mmol). The mixture was stirred for 30 min. The product was extracted with C6H5CH3 (2×15 ml) and the organic layer was evaporated under vacuum (yield 0.2 g, 90%). 1H NMR (200 MHz, CD3COCD3, 25 °C, TMS): l= 4.99 (s, Cp), 2.02 (s, Cp*), 1.27 (d, 2J(H, P) = 2.9 Hz, Me), − 0.67 (d, 2J(H, P) = 56.1 Hz, TaH). 31 P{1H} NMR (81 MHz, CD3COCD3, 25 °C, TMS): l= − 81.3 (s, PMe2).

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4.3. Preparation of [Cp* 2TaH2(PMe2H)]PF6 (2*,2*,Me) To a C6H5CH3 solution (30 ml) of Cp*2TaH3 (0.23 g, 0.5 mmol) was added 1 equiv. of PMe2Cl. After 15 min of stirring, a white precipitate was filtered, washed with C6H5CH3 and C5H12 and dried under vacuum. The resulting white powder was dissolved in water (10 ml). One equiv. of a water solution of NH4PF6 was added. A white precipitate was instantaneously formed which was filtered washed with water a dried under vacuum (yield 0.25 g 75%). 1H NMR (200 MHz, CD3COCD3, 25 °C, TMS): l = 5.15 (dh, 1J(H, P) =344.0 Hz, 3J(H, H) = 7.1 Hz, PH), 2.06 (s, Cp*), 1.67 (dd, 2J(H, P) = 9.3 Hz, 3J(H, H) = 7.3 Hz, Me), 0.00 (d, 2J(H, P) = 77.1 Hz, TaH). 31P{1H} NMR (81MHz, CD3COCD3, 25 °C, TMS): l = − 50.2 (s, PMe2H), −146.3 (h, 1J(P, F) = 703.2 Hz, PF6).

4.4. Preparation of [Cp* 2TaH2(PPh2H)]PF6 (2*,2*,Ph)

377

ing from (2,2%,Me) (0.1 g, 0.2 mmol). The two complexes have not been separated from the mixture. The NMR data of each complex are given from the spectra of the mixture.

4.7. Cp%CpTaH(PMe2H) (60%) H NMR (200 MHz, C6D6, 25 °C, TMS): l=5.10 (dh, 1J(P, H) = 309 Hz, 3J(H, H) =6 Hz, PH), 4.30 (d, J(P, H) = 2 Hz, Cp), 3.82 (dd, J(P,H) =5Hz, J(H, H) = 3.3 Hz, CH) 3.41 (dd, J(P, H) = 3 Hz, J(H, H) =3.3 Hz, CH), 1.96 (s, Me), 1.95 (s, Me), 1.28 (s, t Bu), 1.23 (dd, 2J(P, H) =7Hz, 3J(H, H) =6 Hz, PMe2), 1.05 (dd, 2J(P, H) = 6 Hz, 3J(H, H) =6Hz, PMe2), −8.82 (d, 2J(P, H) = 22 Hz, TaH). 31P NMR (81 MHz, C6D6, 25 °C, TMS): l= − 46.8 (d, 1J(P, H) = 309 Hz, PMe2H). 1

4.8. Cp%CpTaH2(PMe2) (40%) H NMR (200 MHz, C6D6, 25 °C, TMS): l=4.70 (s, Cp), masked (CH), 2.08 (s, Me), 1.77 (d, 2J(P, H) = 3.7 Hz, PMe2), 1.27 (s, tBu), −0.51 (d, 2J(P, H) = 53 Hz, TaH). 31P NMR (81 MHz, C6D6, 25 °C, TMS): l = − 113.7 (d, 2J(P, H) = 53 Hz, PMe2). 1

The procedure was the same as above starting from Cp*2TaH3 (0.23 g, 0.5 mmol) and 1 equiv. of PPh2Cl (yield 0.27 g, 80%). 1H NMR (200 MHz, CDCl3, 25 °C, TMS): l =7.66 –7.30 (m, Ph), 6.86 (d, 1J(H, P) = 356.2 Hz, PH), 1.97 (s, Cp*), 0.67 (d, 2J(H, P) = 73.7 Hz, TaH). 31P{1H} NMR (200 MHz, CDCl3, 25 °C, TMS): l =2.3 (s, PPh2H), −146.3 (h, 1J(P, F) = 703.2 Hz, PF6).

4.5. Preparation of [CpCp%TaH2(PMe2H)]PF6 (2,2 %,Me) To a C6H5CH3 solution (30 ml) of Cp%CpTaH3 (0.2 g, 0.5 mmol) was added 1 equiv. of PMe2Cl. After 15 min of stirring, a white precipitate was filtered, washed with C6H5CH3 and C5H12 and dried under vacuum. To a water solution (20 ml) of this salt was added 1 equiv. of NH4PF6. A white precipitate was instantaneously formed. After 10 min of stirring, it was filtered, washed with cold water and dried under vacuum (yield 0.22 g, 72%). Recrystallization from thf gave colourless crystals. Anal. Calc. for C18H31F6P2Ta: C, 35.81; H, 5.23. Found: C, 35.74; H, 5.16%. 1H NMR (200 MHz, CD3COCD3, 25 °C, TMS): l = 5.75 (s, Cp), 5.21 (d, 3 J(H, P) = 3.5 Hz, CH), 5.54 (dh, 1J(H, P) = 360 Hz, 3 J(H, H) =6 Hz, PH), 2.33 (s, CH3), 1.79 (dd, 2J(H, P) = 9.7 Hz, 3J(H, H) =6 Hz, PMe), 1.27 (s, tBu), − 0.82 (d, 2J(H, P) = 74 Hz, TaH). 31P NMR (81 MHz, CD3COCD3, 25 °C, TMS): l = −54.0 (d, 1J(H, P) = 360 Hz, PMe2H), −141.0 (h, 1J(P, F) =708 Hz, PF6).

4.6. Preparation of CpCp%TaH(PMe2H) and CpCp%TaH2(PMe2) (3,3 %,Me) The procedure was the same as for (3,3*,Me), start-

4.9. Preparation of Cp2TaH(v-H)(v-PPh2)Fe(CO)3 (5,5,Fe) To a thf solution (30 ml) of Cp2TaH2(PPh2) (0.50 g, 1 mmol) was added 1 equiv. of Fe(CO)3(BDA). The mixture was stirred for 2 h at room temperature (the reaction was monitored by IR spectroscopy). The solvent was removed under vacuum and the crude reaction product was chromatographed on silica gel with C6H5CH3 as eluent affording a red bimetallic complex (72% yield). Anal. Calc. for C25H22FeO3PTa: C, 47.05; H, 3.47. Found: C, 47.25; H, 3.32%. IR (thf): wCO, 1989, 1940, 1916 cm − 1. 1H NMR (200 MHz, C6D6, 25 °C, TMS): l= 8.16–7.89 (m, Ph), 7.21–6.89 (m, Ph), 4.43 (s, Cp), − 1.36 (dd, 2J(H, H) = 8.9 Hz, 2J(H, P) =46.2 Hz, H), −10.11 (dd, 2J(H, H) = 8.9 Hz, 2J(H, P) = 43.3 Hz, H). 31P{1H} NMR (81MHz, C6D6, 25 °C, TMS): l=177.2 (s, PPh2). Crystals for X-ray analysis have been grown from acetone solution.

4.10. Preparation of CpCp*TaH(v-H)(v-PMe2)Fe(CO)3 (5,5*,Fe) The procedure was the same as above starting from Cp*CpTaH2(PMe2). Anal. Calc. for C20H28FeO3PTa: C, 41.12; H, 4.83. Found: C, 41.04; H, 4.81%. IR (thf): wCO, 1953, 1874, 1857 cm − 1. 1H NMR (200 MHz, C6D6, 25 °C, TMS): l=4.46 (s, Cp), 1.75 (d, 3J(H, P) = 9.8 Hz, Me), 1.70 (d, 3J(H, P) = 9.8 Hz, Me), 1.49

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Table 1 Crystallographic data for [CpCp%TaH2(PMe2H)]PF6 (2,2%Me) and Cp2Ta(H)(m-PPh2,H)Fe(CO)3,(CH3)2CO (5,5Fe) complexes Complex

(2,2%Me)

(5,5Fe)

Empirical formula Formula weight Colour, size (mm) Crystal system Space group Unit cell dimensions a (A, ) b (A, ) c (A, ) i (°) V (A, 3) Z, zcalc, Mg m−3 F(000) Radiation, l(Mo Ka), A, Linear absorption, v (mm−1) Scan q Range (°) Index ranges Reflections collected Reflections unique [I\2|(I)] Linear decay (%) Abs. corr., C scan Refinement method Data/restraints/parameters Final R indices [I\2|(I)] Goodness-of-fit F 2 zmax/zmin (e A, −3)

C18H31F6P2Ta 604.32 colourless, 0.25×0.25×0.20 monoclinic P21/c (No. 14)

C28H27FeO4PTa 695.27 red, 0.4×0.3×0.15 monoclinic P21/n (No. 14)

8.878(1) 13.303(2) 19.324(2) 97.877(8) 2260.9(5) 4, 1.775 1184 0.71073 5.052 … 2.13–26.29 05h511, −165k50, −245l5–23 4875 3697 [Rint = 0.019] −10, corrected 69.48/99.77 Full-matrix least-squares on F 2 3697/0/263 R1 = 0.026, wR2 = 0.062 1.025 0.62/−0.84

14.783(3) 11.7922(17) 17.198(4) 114.83(2) 2720.9(9) 4, 1.697 1364 0.71073 4.641 …–2q 2.30–20.59 05h514, 05k511, −165l514 2311 1931 [Rint =0.047] −28, corrected 64.5/99.78 Full-matrix least-squares on F 2 1931/0/304 R1 =0.036, wR2 =0.086 1.052 0.63/−0.57

(s, Cp*), −1.36 (dd, 2J(H, H) = 8.9 Hz, 2J(H, P) = 46.2 Hz, H), −10.11 (dd, 2J(H, H) =8.9 Hz, 2J(H, P) = 43.3 Hz, H). 31P{1H} NMR (81 MHz, C6D6, 25 °C, TMS): l = 110.0 (s, PMe2).

4.11. X-ray crystallography All measurements have been carried out at 296 K on an Enraf–Nonius CAD4 diffractometer. The pertinent crystallographic data are given in Table 1. The unit cells have been determined from 25 randomly selected reflections (CAD4-Express) [23]. Intensity data have been reduced with PROCESS contained in MolEN package [24] with neutral– atom scattering factors. Both structures were solved by direct methods (SHELXS-97) [25] and refined by using the SHELXL-97 library [26]. All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrides in both structures and the hydrogen atom bound to phosphorus in (2,2%,Me) were found from difference Fourier synthesis and refined isotropically. All other hydrogen atoms were placed in calculated positions and included in refinements in a riding model with isotropic U(H)= 1.3 Ueq(C). The acetone molecule in (5,5,Fe) was refined isotropically.

5. Supplementary material Crystallographic data for the structural analysis have been deposited with CCDC (12 Union Road, Cambridge CB2 1EZ, UK) and are available on request quoting the deposition numbers CCDC No. 147080 and 147081 for compounds 2,2%Me and 5,5Fe, respectively (fax: + 44-1223-336033; e-mail: [email protected]. ac.uk or www: http://www.ccdc.cam.ac.uk).

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