guanidinato niobium complexes: synthesis and the effect of ligands on insertion reactions.

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Mixed amido-/imido-/guanidinato niobium complexes: synthesis and the effect of ligands on insertion reactions†‡ David Elorriaga, Fernando Carrillo-Hermosilla,* Antonio Antiñolo,* Isabel López-Solera, Rafael Fernández-Galán and Elena Villaseñor The new monoguanidinato complexes [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NR)(NR’)C(NMe2)}] (R = R’ = iPr, 2; R = tBu, R’ = Et, 3) were obtained by the insertion reaction of either diisopropylcarbodiimide or 1-tertbutyl-3-ethylcarbodiimide with the triamido precursor [Nb(NMe2)3(N-2,6-iPr2C6H3)] (1) bearing a bulky imido moiety. The μ-oxo derivative [{N(2,6-iPr2C6H3)}{(NiPr)2C(NMe2)}(NMe2)Nb]2(μ-O) (2a) was formed by an unexpected hydrolysis reaction of the amido niobium compound 2. Alternatively, monoguanidinato complexes [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NiPr)2C(NHR)}] (R = iPr, 4, nBu, 5) can be obtained by protonolysis of 1 with N,N’,N’’-alkylguanidines [(NHiPr)2C(NR)] (R = iPr, nBu). Compound 1 also reacts with either tert-butylisocyanide or 2,6-xylylisocyanide to give, by a migratory insertion reaction, the corresponding iminocarbamoyl compounds [Nb(NMe2)2{(NMe2)CvNR}{N(2,6-iPr2C6H3)}] (R = tBu, 6, Xy, 7). Addition of the neutral alkylguanidines to complex 6 results in a facile C–N bond cleavage at room temperature in a process directed by the formation of the stable chelate complex 4 or 5. Complex 1 reacts with heterocumulenic CS2 to produce new imido dithiocarbamato complexes [Nb(NMe2){S2C(NMe2)}2{N(2,6-iPr2C6H3)}] (8) and [Nb{S2C(NMe2)}3{N(2,6-iPr2C6H3)}] (9). These complexes do not react with alkylguanines, although new mixed guanidinato dithiocarbamato complexes [Nb(NMe2){S2C(NMe2)}{(NiPr)2C(NHiPr)}{N(2,6-iPr2C6H3)}] (10) and [Nb{(S2C(NMe2)}2{(NiPr)2C(NHiPr)}{N(2,6-iPr2C6H3)}] (11) can

Received 30th June 2014, Accepted 22nd September 2014

be obtained by reaction of complex 4 with one or two equivalents of CS2, respectively. All of the complexes were characterized spectroscopically and the dynamic behaviour of some of them was studied by

DOI: 10.1039/c4dt01975j

variable-temperature NMR. The molecular structures of 2a, 3, 6 and 10 were also established by X-ray

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diffraction studies.

Introduction Transition metal amido complexes have been employed in important catalytic reactions such as hydroamination1 or hydroaminoalkylation2 and as precursors in chemical vapour deposition (CVD) or atomic layer deposition (ALD) in order to obtain new and interesting materials.3 The chemistry of the ligands that support these reactive systems is mainly dominated by the cyclopentadienyl-based ligands but there is continued interest in obtaining suitable alternatives. The anionic N-heteroallylic ligands, such as guanidines, are good examples

Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Ciencias y Tecnologías Químicas, Universidad de Castilla-La Mancha, Campus Universitario, Ciudad Real, 13071, Spain. E-mail: [email protected], [email protected] † Dedicated to Professor A. Otero on the occasion of his 65th birthday. ‡ Electronic supplementary information (ESI) available: Table S1 with crystal data for 2a, 3, and 10. CCDC 1009639–1009641. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01975j

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of these accessible alternatives.4 These compounds have the advantage of tuning their steric demand and their electronic properties by controlled variation of the substituents of the ‘CN3’ core (Scheme 1). The electronic flexibility is based on the different contribution of the resonance forms, among which predominate the non-zwitterionic forms.5

Scheme 1

Resonance structures for monoanionic guanidinato ligands.

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One of the most direct synthetic routes to guanidinato complexes involves the insertion of carbodiimides into metal– amido bonds to give chelating ligands with small bite angles.4b Remarkably, despite the great variety of transition metal guanidinato complexes reported to date in the literature, following these procedures, examples of amido,6 imido,7 or mixed amido/imido niobium guanidinato complexes remain surprisingly rare.8 In previous years we have studied the coordination of guanidine molecules to different metal centres and the influence of these ligands on the reactivity of the resulting new organometallic complexes.9 Our work has focused primarily on the insertion of unsaturated molecules into M–C bonds. As a part of our continued efforts in this field, we recently communicated the reactivity of an imido amido niobium complex with trialkylguanidines. The unexpected facile C–N bond cleavage of an iminocarbamoyl moiety driven by the formation of a stable guanidinato complex was described.10 In this paper we report a detailed study of the synthesis of new mixed amido/imido/ guanidinato niobium complexes and their behaviour in insertion reactions with some small unsaturated molecules.

Results and discussion The small number of known amido imido niobium complexes bearing monoanionic guanidinato ligands have been synthesized by an insertion reaction of carbodiimides into a niobium–amido bond.8 Nevertheless, monoguanidinato compounds were not obtained by following this reaction scheme and, as a consequence, we decided to study the insertion reaction of a single molecule of carbodiimide with the precursor complex [Nb(NMe2)3(N-2,6-iPr2C6H3)] (1),11 in which there is significant steric hindrance around the metal. The reactions of 1 with 1 equivalent of either diisopropylcarbodiimide or 1-tertbutyl-3-ethylcarbodiimide proceeded smoothly at room temperature in toluene to provide good yields of complexes [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NR)(NR′)C(NMe2)}] (R = R′ = iPr, 2; R = tBu, R′ = Et, 3) as solids that are unstable in air (Scheme 2). The reactions with two equivalents of carbodiimide, even under reflux, led to a mixture of a monoguanidinato complex, a free ligand and a plethora of unidentified minor products, indicating the decisive influence of the bulky imido unit on the stoichiometry of the reaction. The new guanidinato complexes were characterized by spectroscopic methods and X-ray diffraction studies (details are given in the Experimental section). The 1H NMR spectra of both complexes are quite similar and indicate a pseudo-square pyramidal coordination around the metal centre, in contrast to alkylimidoniobium monoguanidinato complexes obtained by reaction of a trialkyl precursor and neutral guanidines,9a where pseudo-trigonal bipyramidal geometries were found. In addition to the signals assigned to the imido ligand, the two isopropyl groups of the guanidinato ligand appear to be equivalent and only one peak, integrating for twelve protons, was assigned to the two remaining amido groups on the metal.


Scheme 2 Synthesis of niobium guanidinato complexes by carbodiimide insertion: 10 min, room temperature, toluene.

Although one peak was expected for complex 2, which has a symmetry plane, it is noteworthy that the asymmetric complex 3 would be expected to show some dynamic exchange in solution that would give rise to the equivalence of the two dimethylamido groups. However, even at very low temperatures, both groups appeared to be equivalent in the NMR spectra. In contrast, rotation of the dimethylamino group of the guanidinato ligand, generated by the insertion process, was blocked. When the temperature was decreased to −90 °C, the spectrum showed two single peaks that coalesced at −80 °C (ΔGc‡ = 9.6 kcal mol−1) and then sharpened to a single resonance at the fast exchange limit for this group. This observation suggests that the energy barrier for the proposed dynamic exchange to produce the observed equivalence of the metal– amido groups should be below this low value. These low energy barriers are justified in bidentate ligands that have the very small bite angles found for amidinate and guanidinato ligands.12 For alkyl monocyclopentadienyl amidinato complexes of group 4 metals,13 which are structurally similar to these niobium compounds, a rapid racemization mechanism in solution has been proposed and this involves ‘flipping’ of the amidinato group, a process that takes place across an intermediate with a distorted trigonal bipyramidal structure (Scheme 3). The molecular structure of 3 was determined using singlecrystals suitable for X-ray diffraction analysis. The crystals were obtained from a saturated pentane solution at −30 °C (Fig. 1). The complex is chiral, with both enantiomers present in the

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Scheme 3

Proposed mechanism for the racemization in complex 3.

Fig. 2

Thermal ellipsoid plot of 2a at 20% probability.

Table 2

Fig. 1

Thermal ellipsoid plot of 3 at 20% probability.

Table 1

Selected bond lengths (Å) and angles (°) for complex 3

Nb1–N1 Nb1–N2 Nb1–N3 Nb1–N5

1.768(3) 2.239(3) 2.167(3) 1.975(3)

Nb1–N6 N2–C13 N3–C13 N4–C13

2.002(3) 1.325(4) 1.334(4) 1.367(4)

N2–Nb1–N3 Nb1–N1–C1 Nb1–N5–C22 Nb1–N5–C23

59.2(1) 167.3(2) 134.0(3) 113.4(2)

C22–N5–C23 Nb1–N6–C24 Nb1–N6–C25 C24–N6–C25

112.4(3) 125.5(3) 124.1(2) 110.2(3)

unit cell, and crystallizes in the P21/n space group. Selected bond distances and angles are summarized in Table 1. As proposed from the NMR studies, this complex has a pseudosquare based pyramidal geometry in which the imido group occupies the axial position and the two dimethylamido and the guanidinato ligands are in the basal plane. The Nb1–N1 bond distance of 1.768(3) Å and the Nb1–N1–C1 angle of 167.3(2)° are close to those described for other aryl-imido complexes and this allows us to propose that the imido group is almost linear.14 The guanidinato ligand is coordinated to the metal centre with an N2–Nb1–N3 bite angle of 59.2(1)°. The planarity of the ‘CN3’ core is evidenced by the sum of the bond angles around C13 (360.0°). Some charge delocalization within the guanidinato ligand is proposed on the basis of the N2–C13 and N3–C13 bond lengths, with an average of 1.33(1) Å and are longer than the N4–C13 (1.367(4) Å) distance, with this unit best described through a diazaallyl resonance form. For the amido groups it was observed that the unit surrounding the nitrogen atom is flat, with bond angles close to 120°.

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Selected bond lengths (Å) and angles (°) for complex 2a

Nb1–N1 Nb1–N2 Nb1–N3 Nb1–N5 C13–N2 C13–N3 C13–N4

1.78(1) 2.25(1) 2.19(1) 1.98(1) 1.31(1) 1.35(1) 1.37(1)

Nb2–N1A Nb2–N2A Nb2–N3A Nb2–N5A C13A–N2A C13A–N3A C13A–N4A

1.78(1) 2.21(1) 2.25(1) 1.97(1) 1.34(1) 1.31(1) 1.39(1)

N2–Nb1–N3 Nb1–N1–C1 Nb1–O1–Nb2

59.5(3) 167.8(7) 153.4(3)

N2A–Nb2–N3A Nb2–N1A–C1A

59.1(3) 170.8(7)

This indicates that the lone electron pair of the nitrogen atom is involved in an additional bond with the metal atom due to its high electron deficiency. In a similar manner, recrystallization of complex 2 was attempted from pentane. After several days at −30 °C, a small number of crystals were obtained. The crystals were very weakly diffracting, we have tried data collection with several crystals but we were unable to obtain better data. However considering the importance of the structure, it was solved in spite of the aforementioned problems. The X-ray diffraction study of one of these crystals showed the presence of a new μ-oxo species (2a). The molecular structure of compound 2a was determined by X-ray diffraction and is shown in Fig. 2. Selected bond distances and angles are given in Table 2. The structure of the complex belongs to the P21/c group of the monoclinic system. In view of these results, it is possible to confirm that the insertion of the carbodiimide has taken place to form the guanidinato ligand and this allows us to confirm the geometry proposed for the parent complex 2. Compound 2 was hydrolysed by reaction with adventitious water and the subsequent rearrangement gave the observed oxygen bridge between the niobium atoms. This complex exhibits a pseudo-square pyramidal geometry around each of the metal atoms. The imido group is in the axial position and the basal plane contains the bidentate guanidinato ligand in a κ2-N,N′ coordination mode. The remaining dimethylamido ligand


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occupies the third position of the base and the last one is occupied by the µ-oxo bridge, which connects the half of the molecule that is almost identical to the one described above. The Nb1–N1 and Nb2–N1A bond distances are 1.78(1) Å, that together with the Nb1–N1–C1 and Nb2–N1A–C1A angle values of 167.8(7) and 170.8(7)°, allows us to propose that the imido ligands are almost linear, acting as four-electron donors. The bite angle for the guanidinato ligands, N2–Nb1–N3 and N2A– Nb2–N3A, has a value of 59.5(3) and 59.1(3)°, respectively. It is worth noting that the N2–Nb1 bond distance is 2.25(1) Å, whereas the Nb1–N3 bond distance is 2.19(1) Å, slightly shorter. This can be due to the presence of the µ-oxo group in the trans position to the N2 nitrogen atom. The bond distances for the ‘CN3’ system are similar (∼1.3 Å) to those of complex 3. Finally, it is necessary to emphasize the angle value for the Nb1–O1–Nb2 moiety of 153.4(3)° which implies some additional electronic donation towards the niobium atoms. As an alternative to this synthetic route, complex 1 was allowed to react with N,N′,N″-alkylguanidines, [(NHiPr)2C(NR)] (R = iPr, nBu), obtained catalytically as described previously.9c,15 Deprotonation of the guanidine by one amido ligand occurs in this reaction to give the corresponding guanidinato complexes in good yields, [Nb(NMe2)2{N(2,6-iPr2C6H3)} {(NiPr)2C(NHR)}] (R = iPr, 4, nBu, 5) (Scheme 4). The NMR spectrum of complex 4 is similar to those of complexes 2 and 3, thus showing a symmetrical pseudo-square pyramidal coordination around the metal centre. In contrast, complex 5 appears as a mixture of symmetric and asymmetric coordinated guanidinato isomers, with the latter being a mixture of enantiomers. A similar result was found previously for alkylniobium guanidinato complexes.9c Insertion reactions form a key part in the strategy for the formation of new C–C or C–X bonds. In our previous work we studied the insertion of isocyanide molecules into the M–C

Scheme 4 Synthesis of niobium guanidinato complexes by protonolysis reaction: 10 min, room temperature, toluene.


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Scheme 5 Synthesis of niobium iminocarbamoyl complexes migratory insertion reaction: 16 h, room temperature, toluene.

by

bonds (M = Nb, Zr) of guanidinato-supported complexes and reported, among other things, that the properties associated with guanidinato ligands could have a marked effect on the chemistry of the resulting complexes.9a,b,d Although the migratory insertion reactions of CO or isocyanides into metal– carbon bonds have been widely reported,16 very few examples of metal-iminocarbamoyl derivatives, formed by insertion of isocyanides into metal–nitrogen bonds, have been described.17 In this context, attempts to achieve the migratory insertion products of 2–5 with tert-butylisocyanide resulted in the recovery of starting materials, even on heating at 70 °C for 16 hours. The use of 2,6-xylylisocyanide led to a complex mixture of unidentified compounds. A new strategy was adopted in which the precursor compound 1 was allowed to react with 1 equivalent of either tert-butylisocyanide or 2,6-xylylisocyanide to give, by a migratory insertion, the corresponding iminocarbamoyl compounds [Nb(NMe2)2{(NMe2)CvNR}{N(2,6-iPr2C6H3)}] (R = tBu, 6, Xy, 7) in good yield (Scheme 5). The use of an excess of isocyanide in the reaction led to the monoinsertion product and free ligand, even under reflux. The observation in the 13C[1H] NMR spectra of one peak at around δC 200 ppm and a strong signal at v ∼ 1630 cm−1 in the IR spectra are consistent with the presence of the iminocarbamoyl group.17j As described above for complex 3, the dimethylamido groups seem to be chemically equivalent in solution in the NMR spectra, even at very low temperatures. This finding can be explained by a fast formal rotation of the iminocarbamoyl ligand.12 Single-crystals of 6 suitable for X-ray analysis were obtained from a saturated solution in pentane at −30 °C and the molecular structure was determined by X-ray diffraction (Fig. 3). Selected bond lengths and angles are listed in Table 3. This complex is chiral and both enantiomers are present in the unit cell. The structure of the complex belongs to the C2/c group of the monoclinic system. The imido ligand is quasi-linear with an Nb1–N1–C1 angle of 177.3(2)° and an Nb1–N1 bond distance of 1.783(2) Å. The N2–C13 and the N3–C13 distances of 1.292(4) Å and 1.341(3) Å, respectively, reveal π delocalization in the iminocarbamoyl moiety supported by partial donation of the lone pair on NMe2 to the resonance contribution.17f Assuming that the iminocarbamoyl moiety occupies one coordination position, the niobium atom adopts a pseudotetrahedral coordination and this makes the two amido ligands

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Fig. 3

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Thermal ellipsoid plot of 6 at 20% probability. Scheme 6 Proposed mechanism for the deinsertion of isocyanide, mediated by guanidine coordination.

Table 3

Selected bond lengths (Å) and angles (°) for complex 6

Nb1–N1 Nb1–N2 Nb1–N4 Nb1–N5

1.783(2) 2.128(2) 2.006(3) 2.005(2)

Nb1–C13 N2–C13 N3–C13

2.159(2) 1.292(4) 1.341(3)

Nb1–N1–C1 N1–Nb1–N2 N1–Nb1–N5 N1–Nb1–N4 Nb1–N4–C20

177.3(2) 110.85(9) 109.2(1) 104.4(1) 127.8(2)

C20–N4–C21 Nb1–N4–C21 Nb1–N5–C22 C22–N5–C23 Nb1–N5–C23

121.4(2) 110.5(3) 130.5(2) 111.3(3) 118.2(2)

inequivalent in the solid state. To the best of our knowledge, this is the first example of an iminocarbamoyl niobium complex to be structurally characterized. The reaction of 6 with 1,2,3-trisisopropylguanidine at room temperature for 24 hours gave a mixture of the guanidinato complex 4 and free isocyanide. Deinsertion of isocyanide takes place as a result of the C–N bond cleavage at the iminocarbamoyl ligand. This process was monitored by 1H NMR spectroscopy and the presence of dimethylamine produced in a protonolysis reaction by the acidic protons of guanidine can also be observed. The peaks corresponding to the starting complex 6 disappeared and several new peaks appeared, including two peaks at δH 3.72 and 2.91 ppm, which were assigned to methyl groups of an amido ligand bonded to the metal and an amino group of an iminocarbamoyl moiety, respectively, in an intermediate complex. The intensity of these peaks increased from the beginning of the reaction and they started to disappear after approximately 5 hours with the simultaneous appearance of peaks corresponding to complex 4 and free isocyanide (Scheme 6). The long lifetime of this intermediate allowed a partial characterization by NMR spectroscopy. The 13C[1H] NMR spectrum contained the characteristic peak at δC 164.9 ppm of a chelating guanidinato ligand.6b NOESY-1D experiments revealed a pseudooctahedral disposition around the niobium atom in this intermediate. The effect of the coordination of a strong donor guanidinato ligand was proved by the reaction of complex 6 with 2-butyl-1,3-diisopropylguanidine. In this case,

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the guanidinato complex 5 and free isocyanide were obtained. This represents an example of facile C–N single bond activation. Reactions that involve the metal-mediated rupture of C–N bonds are limited to activated substrates or the use of highly reactive metal complexes.18 The deinsertion of CO from acyl ligands has been widely reported,16a,19 but the migratory insertion reaction of isocyanides is usually an irreversible process. Isocyanide deinsertion reactions of iminoacyl or iminocarbamoyl ligands in most cases involve thermal treatment to give iminoacyl-iminocarbamoyl exchanges or mixtures of equilibrium products.17g,20 This niobium iminocarbamoyl complex undergoes a soft C–N bond cleavage, presumably driven by the coordination of a very stable guanidinato chelate ligand. Similar experiments carried out with complex 7 did not lead to the corresponding guanidinato complex with the loss of isocyanide. Rather, complex mixtures were obtained that were not identified. In a similar way, reactions of complex 2 or 3 with 1,2,3-trisisopropylguanidine for 16 hours led to recovery of the starting materials. Heterocumulene molecules are building blocks for numerous substances of interest because they incorporate new functional groups in their structures with novel properties.21 As a part of our continued interest in the transformation of small unsaturated heterocumulenic molecules,22 we studied the insertion reactions of CS2 into these amido niobium complexes with a view to obtain additional information about the influence of the presence of a guanidinato ligand in the coordination sphere and to allow comparisons with the isocyanide insertion–deinsertion process discussed above. Amido complexes of metals with low oxidation states show high nucleophilicity of the nitrogen atom, due to the availability of its lone pair, towards organic electrophiles including unsaturated molecules such as CS2 to give the corresponding dithiocarbamato derivatives.23 In the case in question, the presence of strong donor ligands, such as imido or guanidinato, could reduce the donation of the lone pair of the amido ligands to the d0 metal centre. In addition, it is also of interest to study the selectivity of the reaction since the niobium–imido bond could also be reactive towards this cumulenic molecule.


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Scheme 7 Dithiocarbamato derivatives from triamido complex 1: 10 min, room temperature, toluene.

Reactions of complex 1 with two or three equivalents of CS2 were rapid and a monoinsertion product was not isolated in equimolecular reactions. Only the di- or tri-insertion products into the metal–amido ligands, namely [Nb(NMe2){S2C(NMe2)}2{N(2,6-iPr2C6H3)}] (8) and [Nb{S2C(NMe2)}3{N(2,6-iPr2C6H3)}] (9), were obtained in high yields (Scheme 7). Addition to the metal–imido moiety was not observed in any case. The IR spectra of these compounds contain a band at ca. 980 cm−1, corresponding to the v(C–S) of a dithiocarbamato unit coordinated in a bidentate fashion.24 The NMR data show a symmetric disposition of the ligands in both complexes. For complex 8, in addition to the peaks corresponding to the imido ligand, one single peak and one broad signal, at δH 2.62 and 3.96 ppm, were assigned to the methyl protons of two chemically equivalent dithiocarbamato ligands and a remaining dimethylamido ligand, respectively. At −40 °C, the rotation of this ligand was blocked and the broad signal sharpened to two single resonances (ΔGc‡ = 13.2 kcal mol−1). For complex 9, two sets of peaks, in a 1 : 2 ratio, were observed for the methyl protons of two inequivalent groups of dithiocarbamato ligands due to the addition of three CS2 molecules to the metal–amido ligands. This seven-coordinate species contains an axial imido group trans to a sulfur atom of a dithiocarbamato ligand, and an equatorial plane in which the niobium atom is surrounded by another five sulfur atoms. This bipyramidal disposition was previously found in other dialkyldithiocarbamato niobium complexes.25 Complex 8 was allowed to react with 1,2,3-trisisopropylguanidine but only starting materials were recovered, even on heating for long periods. The failure to react is probably due to the steric hindrance around the amido ligand, which hinders the required interaction with the bulky guanidine. In contrast, dithiocarbamato-guanidinato complexes were obtained by reaction of complex 4 with one or two equivalents of CS2. In this case, the presence of the bulky guanidinato ligand in the coordination sphere of the metal centre allows the formation of [Nb(NMe2){S2C(NMe2)}{(NiPr)2C(NHiPr)}{N(2,6-iPr2C6H3)}]


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Scheme 8 Dithiocarbamato guanidinato complexes from diamido complex 4: 10 min, room temperature, toluene.

(10) or the product containing two dithiocarbamato ligands, [Nb{S2C(NMe2)}2{(NiPr)2C(NHiPr)}{N(2,6-iPr2C6H3)}] (11), by carefully controlled addition of CS2 (Scheme 8). Complex 10 shows broad signals in the 1H NMR spectra, indicating that some dynamic process takes place in this case. In fact, the isopropyl moieties on the coordinated nitrogen atoms give rise to three overlapped broad peaks at room temperature. The methyl groups of the resulting dithiocarbamato and the remaining amido ligands also give rise to two broad signals, indicating that the rotation of these groups is partially blocked at this temperature. Methyl protons of the isopropyl groups on the imido ligand appear to be diastereotopic and two overlapped broad doublets were observed. The possible presence of a chiral centre at the niobium atom due to an asymmetric disposition of the four different ligands in a pseudooctahedral geometry was confirmed by means of NOESY-1D experiments and then by X-ray diffraction analysis. Single crystals of 10 suitable for X-ray analysis were obtained from a saturated solution in hexane at −30 °C and the molecular structure was determined by X-ray diffraction (Fig. 4). Selected bond lengths and angles are listed in Table 4. This complex is chiral and both enantiomers are present in the unit cell. The structure of the complex belongs to the P21/c group of the monoclinic system and it is the first example of a mixed guanidinato dithiocarbamato complex. The molecule has a pseudooctahedral geometry around the metal atom, as postulated by NMR studies. The bond distance between the nitrogen atom N1 and the Nb1 atom is 1.781(4) Å and this, together with the bond angle formed between Nb1– N1–C1 (179.5(4)°), allows us to propose that the imido ligand is almost linear and it again acts as a four-electron donor. The guanidinato ligand is located in the equatorial plane and it acts as a monoanionic chelate with a bite angle of N2– Nb1–N3 of 59.2(1)°. The bond distances in the guanidine core, N2–C13, N3–C13 and N4–C13, are different, with values of 1.337(6), 1.303(6) and 1.377(6) Å, respectively. In addition, the sum of the angles around the carbon atom C13 is 360°, which indicates the planarity of the ‘CN3’ core. The bond distance of Nb1–S2 is 2.749(1) Å, which is slightly longer than the Nb1–S1

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Fig. 4

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Thermal ellipsoid plot of 10 at 20% probability.

Scheme 9

Table 4 Selected bond lengths (Å) and angles (°) for complex 10

Nb1–S1 Nb1–S2 Nb1–N1 Nb1–N6

2.534(1) 2.749(1) 1.781(4) 2.016(5)

N2–C13 N3–C13 N4–C13 N5–C23

1.337(6) 1.303(6) 1.377(6) 1.325(7)

S1–Nb1–S2 N1–Nb1–S2 N2–Nb1–N3 Nb1–N1–C1 C23–N5–C24

66.7(1) 162.8(1) 59.2(1) 179.5(4) 123.0(5)

C24–N5–C25 C23–N5–C25 Nb1–N6–C26 Nb1–N6–C27 C26–N6–C27

115.7(5) 121.2(5) 127.6(5) 124.2(4) 108.1(6)

distance of 2.534(1) Å, probably due to the strong trans influence of the imido ligand. The dithiocarbamato ligand shows a typical small bite angle of S1–Nb1–S2 of 66.7(1)°, which is similar to those observed in other group 5 complexes of this type.25a The short N5–C23 bond distance of 1.325(7) Å, together with the planarity of the amino group, allows us to propose that there is some donation of the lone electron pair on the nitrogen atom to the delocalized system of this ligand. Similar donation was observed from the nitrogen atom of the amido ligand to the niobium centre, with a Nb1–N6 bond distance of 2.016(5) Å, which is slightly longer than those observed in the new amido complexes described above. To clarify further the fluxional behaviour present in solution, a variable-temperature 1H NMR experiment was carried out on 10 in deuterated toluene. As the temperature was lowered, the signals due to the isopropyl groups of the guanidinato ligand began to split. At −90 °C, six separate signals were observed in the region δH 1–1.4 ppm and these correspond to the diastereotopic methyl groups in a chiral complex. Rotation of the –NMe2 groups was also frozen at this temperature and two sets of two double peaks appeared for the amido and dithiocarbamato ligands. At higher temperature, the original three broad peaks of the guanidinato ligand collapsed to give two doublets. The coalescence temperature was 40 °C (ΔGc‡ = 15.4 kcal mol−1), which is very close to that found in sixcoordinate group 4 metal guanidinato or amidinato complexes.9d,13a,26 The rate constants for the interconversion of 10 at various temperatures were calculated from eqn (1) (where

17440 | Dalton Trans., 2014, 43, 17434–17444

Dynamic process proposed for complex 10.

Δν and Δνo are frequency differences (Hz) between exchangebroadened sites at temperature T and between the two sites at the slow exchange limit, respectively).27 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k ¼ π Δν0 2 Δν2 ð1Þ The activation parameters of the exchange were calculated using an Eyring plot: ΔH‡ = 0.6 kcal mol−1 and ΔS‡ = –43 eu. This fluxional process and the small ΔH‡ and negative ΔS‡ values can be explained by the Bailar torsion twist mechanism,9d whereby at high temperatures a fast racemization of 10 equilibrates the two coordinated –NiPr groups (Scheme 9). Addition of two equivalents of CS2 to a solution of complex 4 resulted in the formation of a new complex with a guanidinato and two dithiocarbamato ligands in the coordination sphere of the niobium centre, with peaks at δC 161.2 and 205.9 ppm for the central carbon atoms in the ‘CN3’ and ‘CS2N’ cores, respectively. The NMR spectrum of 11 showed sharp signals, indicating that in this case, a rigid structure is present in solution, where two dithiocarbamato ligands are equivalent and the three isopropyl moieties from a guanidinato ligand, coordinated in a chelate monoanionic fashion, are inequivalent. The presence of peaks corresponding to the imido ligand allows us to propose a heptacoordination around the metal centre in this compound (see Scheme 8).

Conclusions The use of the bulky 2,6-diisopropylphenylimido ligand allowed, for the first time, the synthesis of mixed amido/ imido/monoguanidinato complexes of niobium by an insertion reaction of carbodiimides into the niobium–amido bonds of the complex [Nb(NMe2)3(N-2,6-iPr2C6H3)] (1). Protonolysis reaction of 1 with neutral alkylguanidines opened up access to similar mixed complexes. Although these new amido guanidinato complexes failed to react with isocyanides, a migratory insertion reaction with 1 provided the corresponding iminocarbamoyl derivatives. As a result, the first iminocarbamoyl niobium complex to be structurally characterized is described.


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This compound, [Nb(NMe2)2{(NMe2)CvNtBu}{N(2,6-iPr2C6H3)}], reacted with alkylguanidines to eliminate isocyanide and form a guanidinato complex through a facile C–N bond rupture at room temperature. This elimination process, which is promoted by guanidines, did not take place in the dithiocarbamato imido complexes [Nb(NMe2){S2C(NMe2)}{(NiPr)2C(NHiPr)}{N(2,6- i Pr 2 C 6 H 3)}] and [Nb{S 2 C(NMe2 )} 2{(Ni Pr)2 C(NH iPr)}{N(2,6-iPr2C6H3)}], which were obtained by reaction of 1 with two or three equivalents of CS2. Mixed imido/amido/guanidinato/ dithiocarbamato could also be obtained by reaction of CS2 with complex 4.

Experimental section General procedures All manipulations were carried out under dry nitrogen using standard Schlenk and glovebox techniques. Solvents were purified by passage through a column of activated alumina (Innovative Technologies) and degassed under nitrogen prior to use. Microanalyses were carried out with a Perkin-Elmer 2400 CHN analyzer. FT-IR spectra were recorded on a Bruker Tensor 27 spectrophotometer. NMR spectra were recorded on a Varian FT-400 spectrometer using the standard VARIAN-FT software for NOESY-1D, COSY, g-HSQC and g-HMBC. The compounds [Nb(NMe2)3(N-2,6-iPr2C6H3)] (1),11 1,2,3-trisisopropylguanidine15,9c and 2-butyl-1,3-diisopropylguanidine15,9c were prepared according to published procedures. Synthesis of [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NiPr)2C(NMe2)}] (2). N,N′-Diisopropylcarbodiimide (77.4 μL, 0.50 mmol) was slowly added to a solution of 1 (0.20 g, 0.50 mmol) in toluene (20 mL). The reaction mixture was stirred for 10 min at room temperature. The resulting yellow solution was evaporated to dryness in vacuo. The yellow solid was redissolved in pentane and cooled to −20 °C for crystallization, to afford yellow crystals of 2. Yield: 0.23 g (86%). IR (neat, ν cm−1): 1623 (CvN), 1345 (NbvN). 1H NMR (400 MHz, C6D6): δ 1.05 (d, 12H, J = 6.4 Hz, CH(CH3)2), 1.42 (d, 12H, J = 6.9 Hz, CH(CH3)2), 2.40 (s, 6H, CN(CH3)2), 3.30 (s, 6H, N(CH3)2), 3.57 (m, 2H, CH(CH3)2), 4.38 (m, 2H, CH(CH3)2), 7.01–7.18 (m, 3H, C6H3). 13 1 C[ H] NMR: δ 24.4 (CH(CH3)2), 24.5 (CH(CH3)2), 27.7 (CH(CH3)2), 39.5 (CN(CH3)2), 47.0 (N(CH3)2), 47.2 (CH(CH3)2), 122.6, 143.5, 152.4 (C6H3), 170.4 (CN3). Anal. Calcd for C26H49N6Nb: C, 57.02; N, 15.96; H, 9.38. Found: C, 56.91; N, 16.03; H, 9.55. Synthesis of [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NEt)(NtBu)2C(NMe2)}] (3). Compound 3 was prepared by the procedure described for 2 using the following quantities: 1 (0.20 g, 0.50 mmol), 1-tertbutyl-3-ethylcarbodiimide (72.5 μL, 0.50 mmol), resulting in a yellow microcrystalline solid. Yield 0.23 g (85%). IR (neat, ν cm−1): 1626 (CvN), 1340 (NbvN). 1H NMR (400 MHz, C6D6): δ 1.03 (t, 3H, J = 7.1 Hz, CH2CH3), 1.23 (s, 9H, C(CH3)3), 1.47 (d, 12H, J = 6.8 Hz, CH(CH3)2), 1.42 (d, 12H, J = 6.9 Hz, CH(CH3)2), 2.41 (s, 6H, CN(CH3)2), 3.04 (q, 4H, J = 7.1 Hz, CH2CH3), 3.26 (s, 6H, N(CH3)2), 4.37 (m, 2H, CH(CH3)2), 7.02–7.18 (m, 3H, C6H3). 13C[1H] NMR: δ 17.8 (CH2CH3), 24.6


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(CH(CH3)2), 28.0 (CH(CH3)2), 31.8 (C(CH3)3), 40.3 (CN(CH3)2), 41.7 (CH2CH3), 46.4 (N(CH3)2), 52.6 (C(CH3)3), 122.6, 143.2 (C6H3), 171.9 (CN3). Anal. Calcd for C25H49N6Nb: C, 57.02; N, 15.96; H, 9.38. Found: C, 56.86; N, 16.10; H, 9.49. Synthesis of [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NiPr)2C(NHiPr)}] (4). 1,2,3-Trisisopropylguanidine (0.09 g, 0.50 mmol) in toluene (10 mL) was added to a solution of 1 (0.20 g, 0.50 mmol) in toluene (10 mL). The reaction mixture was stirred for 10 min at room temperature. The resulting yellow solution was evaporated to dryness in vacuo. The yellow solid was redissolved in pentane and cooled to −20 °C for crystallization, to afford yellow crystals of 4. Yield: 0.22 g (83%). IR (neat, ν cm−1): 3396 (N–H), 1629 (CvN), 1336 (NbvN). 1 H NMR (400 MHz, C6D6): δ 0.87 (d, 6H, J = 6 Hz, CH(CH3)2), 1.11(d, 12H, J = 6.4 Hz, CH(CH3)2), 1.44 (d,12H, J = 5.6 Hz, CH(CH3)2), 3.31 (s, 12H, N(CH3)2), 3.49 (m, 4H, CH(CH3)2 and NH); 4.41 (m, 2H, CH(CH3)2), 7.02 (t, 1H, J = 7.7 Hz, C6H3); 7.18 (d, 2H, J = 7.7 Hz, C6H3). 13C[1H] NMR: δ 23.9 (CH(CH3)2), 24.6 (CH(CH3)2), 28.0 (CH(CH3)2), 45.4 (CH(CH3)2), 46.7 (CH(CH3)2), 46.9 (N(CH3)2), 122.7, 143.3, 152.3 (C6H3), 164.9 (CN3). Anal. Calcd for C26H51N6Nb: C, 57.76; N, 15.54; H, 9.51. Found: C, 57.68; N, 15.70; H, 9.54. Synthesis of [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NiPr)2C(NHnBu)}] (5). Compound 5 was prepared by the procedure described for 4 using the following quantities: 1 (0.20 g, 0.50 mmol), 2-butyl1,3-diisopropylguanidine (0.06 g, 0.50 mmol), resulting in a bright yellow oil. Yield 0.25 g (92%). IR (neat, ν cm−1): 3401 (N–H), 1631 (CvN), 1339 (NbvN). 1H NMR data for the mixture of isomers (symmetric and asymmetric, the latter as a racemic mixture of enantiomers) (400 MHz, C6D6, the mixture of isomers and signal overlap prevent a clear integration of the peaks): δ 0.87 (m, (CH2)3CH3), 1.09–1.14, 1.52–1.60, 3.18–3.55 (m, (CH2)3CH3), 1.11, 1.26, 1.44 (d, CH(CH3)2), 3.18–3.55 (m, CH(CH3)2), 4.38 (m, CH(CH3)2), 3.18–3.55 (m, NH), 3.31 (s, N(CH3)2), 3.33 (s, N(CH3)2), 7.00–7.20 (C6H3). 13C[1H] NMR: δ 14.4–35.2 ((CH2)3CH3), 24.3–24.8 (CH(CH3)2), 28.0, 28.2, 45.1, 45.5, 46.3 (CH(CH3)2), 46.8 (N(CH3)2), 47.0 (N(CH3)2), 122.4–152.4 (C6H3), 164.9, 165.4 (CN3). Anal. Calcd for C27H53N6Nb: C, 58.47; N, 15.15; H, 9.63. Found: C, 58.10; N, 15.32; H, 9.41. Synthesis of [Nb(NMe2)2{(NMe2)CvNtBu}{N(2,6-iPr2C6H3)}] (6). tBuNC (0.06 mL, 0.50 mmol) in toluene (10 mL) was added to a solution of 1 (0.20 g, 0.50 mmol) in toluene (10 mL). The reaction mixture was stirred for 16 h and evaporated to dryness in vacuo. The light brown oily material was redissolved in pentane and cooled to −20 °C for crystallization, to afford white crystals of 6. Yield: 0.21 g (88%). IR (neat, ν cm−1): 1632 (CvN), 1344 (NbvN). 1H NMR (400 MHz, C6D6): δ 1.28 (s, 9H, C(CH3)3), 1.36 (d, 12H, J = 6.9 Hz, CH(CH3)2), 2.78 (s, 6H, CN(CH3)2), 3.27 (s, 12H, N(CH3)2), 4.20 (m, 2H, CH(CH3)2), 6.98–7.16 (m, 3H, C6H3). 13C[1H] NMR: δ 24.3 (CN(CH3)2), 28.0 (CH(CH3)2), 30.3 (CH(CH3)2), 32.1 (C(CH3)3), 47.7 (N(CH3)2), 55.1 (C(CH3)3), 121.2, 122.4, 141.6, 152.9 (C6H3), 201.5 (CvNtBu). Anal. Calcd for C23H44N5Nb: C, 57.13; N, 14.48; H, 9.17. Found: C, 57.27; N, 14.60; H, 9.30.

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Synthesis of [Nb(NMe 2 ) 2 {(NMe 2 )CvN-(2,6-Me 2 C 6 H 3 )}{N(2,6-iPr2C6H3)}] (7). Compound 7 was prepared by the procedure described for 6 using the following quantities: 1 (0.20 g, 0.50 mmol), xylylNC (0.06 g, 0.50 mmol), resulting in a yellow microcrystalline solid. Yield 0.24 g (90%). IR (neat, ν cm−1): 1638 (CvN), 1347 (NbvN). 1H NMR (400 MHz, C6D6): δ 1.38 (d, 12H, J = 7.0 Hz, CH(CH3)2), 2.07 (s, 6H, CN(CH3)2), 2.81 (s, 3H, C6H3(CH3)2), 3.14 (s, 3H, C6H3(CH3)2), 3.35 (s, 12H, N(CH3)2), 4.34 (m, 2H, CH(CH3)2), 6.90–7.22 (m, 6H, C6H3). 13 1 C[ H] NMR: δ 18.9 (CN(CH3)2), 24.4 (CH(CH3)2), 28.2 (CH(CH3)2), 31.6 (C6H3(CH3)2), 45.5 (C6H3(CH3)2), 48.0 (N(CH3)2), 121.6, 122.3, 124.7, 131.3, 142.1, 146.2, 152.7 (C6H3), 202.1 (CvN). Anal. Calcd for C27H44N5Nb: C, 61.00; N, 13.17; H, 8.34. Found: C, 60.87; N, 13.22; H, 8.26. Reaction of 6 and 1,2,3-trisisopropylguanidine The reaction was performed under an inert atmosphere in a Young-valve NMR tube. The tube was charged in a glovebox and 0.04 mmol of 6 dissolved in C6D6 and then 0.05 mmol of 1,2,3-trisisopropylguanidine dissolved in C6D6 were added. The evolution of the reaction was followed by 1H NMR spectroscopy at room temperature. A similar experiment was carried out using 2-butyl-1,3-diisopropylguanidine. The presence of free tBuNC was observed both in the 1H (δ 0.89 ppm) and 13 1 C[ H] (δ 30.2 ppm) NMR spectra. Synthesis of [Nb(NMe2){S2C(NMe2)}2{N(2,6-iPr2C6H3)}] (8). Carbon disulphide (60.1 μL, 1.00 mmol) was slowly added to a solution of 1 (0.20 g, 0.50 mmol) in toluene (10 mL). The reaction mixture was stirred for 10 min at room temperature. The resulting yellow solution was evaporated to dryness in vacuo. The yellow solid was redissolved in hexane and cooled to −20 °C for crystallization, to afford yellow crystals of 8. Yield: 0.23 g (82%). IR (neat, ν cm−1): 1331 (NbvN), 979 (CS). 1 H NMR (400 MHz, C6D6): δ 1.49 (d, 12H, J = 6.9 Hz, CH(CH3)2), 2.62 (s, 12H, CS2N(CH3)2), 3.96 (s, 6H, N(CH3)2), 4.60 (m, 2H, CH(CH3)2), 7.04–7.19 (m, 3H, C6H3). 13C[1H] NMR: δ 24.7 (CH(CH3)2), 28.7 (CH(CH3)2), 40.2 (CS2N(CH3)2), 51.4 (N(CH3)2), 58.0 (N(CH3)2), 122.6, 123.8, 141.2 (C6H3), 204.3 (CS2). Anal. Calcd for C20H35N4NbS4: C, 43.46; N, 10.14; H, 6.38. Found: C, 43.26; N, 10.25; H, 6.69. Synthesis of [Nb{S2C(NMe2)}3{N(2,6-iPr2C6H3)}] (9). Compound 9 was prepared by the procedure described for 8 using the following quantities: 1 (0.20 g, 0.50 mmol), CS2 (90.1 μL, 1.50 mmol), resulting in a yellow microcrystalline solid. Yield 0.26 g (82%). IR (neat, ν cm−1): 1339 (NbvN), 985 (CS). 1 H NMR (400 MHz, C6D6): δ 1.55 (d, 12H, J = 6.8 Hz, CH(CH3)2), 2.43 (s, 6H, CS2N(CH3)2), 2.49 (s, 6H, CS2N(CH3)2), 2.66 (s, 3H, CS2N(CH3)2), 2.70 (s, 3H, CS2N(CH3)2), 4.77 (m, 2H, CH(CH3)2), 6.94–7.11 (m, 3H, C6H3). 13C[1H] NMR: δ 25.1 (CH(CH3)2), 28.7 (CH(CH3)2), 37.5 (CS2N(CH3)2), 38.4 (CS2N(CH3)2), 40.3 (CS2N(CH3)2), 40.5 (CS2N(CH3)2), 122.7, 123.9, 147.7 (C6H3), 204.4 (CS2), 207.0 (CS2). Anal. Calcd for C21H35N4NbS6: C, 40.11; N, 8.91; H, 5.61. Found: C, 39.99; N, 8.99; H, 5.72. Synthesis of [Nb(NMe 2 ){S 2 C(NMe 2 )}{N(2,6- i Pr 2 C 6 H 3 )}{(NiPr)2C(NHiPr)}] (10). Carbon disulfide (22.2 μL, 0.37 mmol)

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was slowly added to a solution of 4 (0.20 g, 0.37 mmol) in toluene (10 mL). The reaction mixture was stirred for 10 min at room temperature. The resulting yellow solution was evaporated to dryness in vacuo. The yellow solid was redissolved in hexane and cooled to −20 °C for crystallization, to afford yellow crystals of 10. Yield: 0.21 g (92%). IR (neat, ν cm−1): 3402 (NH), 1635 (CvN), 1341 (NbvN), 986 (CS). 1H NMR (400 MHz, C6D6): δ 0.88 (d, 3H, J = 6.3 Hz, CH(CH3)2), 1.06 (d, 3H, J = 6.3 Hz, CH(CH3)2), 1.25 (m, 6H, CH(CH3)2), 1.36 (d, 6H, J = 6.3 Hz, CH(CH3)2), 1.50 (m, 12H, CH(CH3)2), 2.79 (m, 6H, CS2N(CH3)2), 3.24 (d, 1H, J = 7.2 Hz, NH), 3.55 (m, 1H, CH(CH3)2), 3.68 (m, 1H, CH(CH3)2), 3.79 (m, 1H, CH(CH3)2), 3.93(s, 6H, N(CH3)2), 4.51 (m, 2H, CH(CH3)2), 7.02–7.18 (m, 3H, C6H3). 13C[1H] NMR: δ 23.8 (CH(CH3)2), 24.5 (CH(CH3)2), 25.0 (CH(CH3)2), 25.2 (CH(CH3)2), 27.2 (CH(CH3)2), 28.1 (CH(CH3)2), 40.7 (CS2N(CH3)2), 41.1 (CS2N(CH3)2), 46.5 (CH(CH3)2), 47.7 (CH(CH3)2), 48.0 (CH(CH3)2), 53.0 (N(CH3)2), 56.6 (N(CH3)2), 122.9, 123.2, 145.8 (C6H3), 162.9 (CN3), 205.6 (CS2). Anal. Calcd for C27H51N6NbS2: C, 52.58; N, 13.63; H, 8.33. Found: C, 52.49; N, 13.80; H, 8.55. Synthesis of [Nb{S2C(NMe2)}2{N(2,6-iPr2C6H3)}{(NiPr)2C(NHiPr)}] (11). Compound 11 was prepared by the procedure described for 10 using the following quantities: 4 (0.20 g, 0.37 mmol), CS2 (44.4 μL, 0.74 mmol), resulting in a yellow microcrystalline solid. Yield 0.20 g (80%). IR (neat, ν cm−1): 3405 (NH), 1636 (CvN), 1343 (NbvN), 984 (CS). 1H NMR (400 MHz, C6D6): δ 0.99 (d, 6H, J = 6.0 Hz, CH(CH3)2), 1.37 (d, 12H, J = 7.1 Hz, CH(CH3)2), 1.49 (m, 12H, CH(CH3)2), 2.70 (s, 3H, CS2N(CH3)2), 2.72 (s, 3H, CS2N(CH3)2), 3.11 (d, 1H, J = 10.1 Hz, NH), 3.53 (m, 1H, CH(CH3)2), 3.82 (m, 1H, CH(CH3)2), 4.01 (m, 1H, CH(CH3)2), 4.99 (m, 2H, CH(CH3)2), 7.01–7.18 (m, 3H, C6H3). 13C[1H] NMR: δ 24.2 (CH(CH3)2), 24.5 (CH(CH3)2), 25.3 (CH(CH3)2), 25.8 (CH(CH3)2), 27.6 (CH(CH3)2), 37.7 (CS2N(CH3)2), 38.8 (CS2N(CH3)2), 46.9 (CH(CH3)2), 47.1 (CH(CH3)2), 48.6 (CH(CH3)2), 122.7, 123.3, 147.4 (C6H3), 161.2 (CN3), 205.9 (CS2). Anal. Calcd for C28H51N6NbS4: C, 48.53; N, 12.13; H, 7.42. Found: C, 48.68; N, 12.25; H, 7.54. X-ray structure determination for compounds 2a, 3 and 10 Crystals of compounds 2a, 3 and 10 were mounted at low temperatures in inert oil on a glass fiber. Data were collected on a Bruker X8 APPEX II CCD-based diffractometer, equipped with a graphite monochromated MoKα (radiation source λ = 0.71073 Å). The crystal data, data collection, structural solution, and refinement parameters for the complexes are summarized in Table S1.‡ Data were integrated using SAINT28 and an absorption correction was performed with the program SADABS.29 The structures were solved by direct methods using SHELXTL,30 and refined by full-matrix least-squares methods based on F2. All non-hydrogen atoms were refined with anisotropic thermal parameters, and then were refined with an overall isotropic temperature factor using a riding model. Complex 2a is very weakly diffracting. We have tried several crystals however we were unable to obtain better data. Considering the importance of the structure, it was solved in spite of


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the aforementioned problems. C8A and C9A are in a disordered position, with 50% occupancy, for this complex. Details of the X-ray structure of 6 have been reported previously.10

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Acknowledgements We gratefully acknowledge financial support from the Ministerio de Ciencia e Innovación, Spain (grant nos. ConsoliderIngenio 2010 ORFEOCSD2007-00006 and CTQ2009-09214) and the Junta de Comunidades de Castilla-La Mancha, Spain (grant no. PCI08-0032).

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Notes and references 1 (a) K. C. Hultzsch, Adv. Synth. Catal., 2005, 347, 367; (b) T. E. Müller, K. C. Hultzsch, M. Yus, F. Foubelo and M. Tada, Chem. Rev., 2008, 108, 3795. 2 P. W. Roesky, Angew. Chem., Int. Ed., 2009, 48, 4892. 3 A. Devi, Coord. Chem. Rev., 2013, 257, 3332. 4 (a) P. J. Bailey and S. Pace, Coord. Chem. Rev., 2001, 214, 91; (b) F. T. Edelmann, Adv. Organomet. Chem., 2008, 57, 183; (c) F. T. Edelmann, Adv. Organomet. Chem., 2013, 61, 55. 5 C. Jones, Coord. Chem. Rev., 2010, 254, 1273. 6 (a) M. K. T. Tin, G. P. A. Yap and D. S. Richeson, Inorg. Chem., 1998, 37, 6728; (b) M. K. T. Tin, G. P. A. Yap and D. S. Richeson, Inorg. Chem., 1999, 38, 998; (c) M. K. T. Tin, G. P. A. Yap and D. S. Richeson, J. Chem. Soc., Dalton Trans., 1999, 2947. 7 N. Thirupathi, G. P. A. Yap and D. S. Richeson, Chem. Commun., 1999, 2483. 8 A. Baunemann, D. Bekermann, T. B. Thiede, H. Parala, M. Winter, C. Gemel and R. A. Fisher, Dalton Trans., 2008, 3715. 9 (a) D. Elorriaga, F. Carrillo-Hermosilla, A. Antiñolo, I. López-Solera, B. Menot, R. Fernández-Galán, E. Villaseñor and A. Otero, Organometallics, 2012, 31, 1840; (b) D. Elorriaga, F. Carrillo-Hermosilla, A. Antiñolo, I. López-Solera, R. Fernández-Galán, A. Serrano and E. Villaseñor, Eur. J. Inorg. Chem., 2013, 2940; (c) D. Elorriaga, F. Carrillo-Hermosilla, A. Antiñolo, F. J. Suárez, I. López-Solera, R. Fernández-Galán and E. Villaseñor, Dalton Trans., 2013, 42, 8223; (d) R. Fernández-Galán, A. Antiñolo, F. Carrillo-Hermosilla, I. López-Solera, A. Otero, A. Serrano-Laguna and E. Villaseñor, Organometallics, 2012, 31, 8360; (e) R. Fernández-Galán, A. Antiñolo, F. Carrillo-Hermosilla, I. López-Solera, A. Otero, A. Serrano-Laguna and E. Villaseñor, J. Organomet. Chem., 2012, 711, 35; (f ) R. García-Álvarez, F. J. Suárez, J. Díez, P. Crochet, V. Cadierno, A. Antiñolo, R. Fernández-Galán and F. Carrillo-Hermosilla, Organometallics, 2012, 31, 8301; (g) R. Fernández-Galán, J. A. Navarro, F. Carrillo-Hermo-


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