Synthesis and DFT, Multinuclear Magnetic Resonance, and X-ray

Article pubs.acs.org/Organometallics

Synthesis and DFT, Multinuclear Magnetic Resonance, and X‑ray Structural Studies of Iminoacyl Imido Hydridotris(3,5dimethylpyrazolyl)borate Niobium and Tantalum(V) Complexes Miguel Galájov,† Carlos Garcıa,́ ‡ Manuel Gómez,*,‡ Pilar Gómez-Sal,‡ and Manuel Temprado§ †

Centro de Espectroscopia de RMN, ‡Departamento de Quı ́mica Orgánica y Quı ́mica Inorgánica and §Departamento de Quı ́mica Analítica, Quı ́mica Física e Ingeniería Quı ́mica, Universidad de Alcalá, Campus Universitario, E-28871 Alcalá de Henares, Spain S Supporting Information *

ABSTRACT: Reaction of alkyl imido [MTp*XR(NtBu)] (M = Nb/Ta; Tp* = HB(3,5-Me2C3HN2)3; X = Cl, R = Me (1a/ 1b), CH2CH3 (2a/2b), CH2Ph (3a/3b), CH2tBu (4a/4b), CH2SiMe3 (5a/5b), CH2CMe2Ph (6a/6b); X = R = Me (7a/ 7b)) complexes with 1 equiv of the isocyanide 2,6Me2C6H3NC takes place with migration of an alkyl group and leads to the formation of the series of chlorido or methyl imido iminoacyl derivatives [MTp*X(NtBu){C(R)NAr-κ2C,N}] (M = Nb/Ta; Ar = 2,6-Me2C6H3; X = Cl, R= Me (8a/8b), CH2CH3 (9a/9b), CH2Ph (10a/10b), CH2tBu (11a/11b), CH2SiMe3 (12a/12b), CH2CMe2Ph (13a/13b); X = R = Me (14a/ 14b)). The molecular structure of 10b was determined by X-ray diffraction methods. An irreversible endo → exo isomerization was detected by 1H NMR in compounds 10a−13a. The insertion−isomerization reaction coordinate was computed by DFT calculations.

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INTRODUCTION The development of a rich chemistry based on scorpionate- or hydridotris(pyrazolyl)borate derivatives of early transition metals in high oxidation states1 was hampered by the absence of versatile synthetic methods to yield suitable starting materials. Therefore, preliminary reports in this field pointed to facile in situ decomposition of the compounds via hydrolysis and redox reactions, B−N bond cleavage, or unexpected synthesis of ionic derivatives. However, despite these difficulties, a number of group 4 and 5 metal complexes of the [MTpX3]2−4 and [MTpX4]5,6 types, respectively, with sterically crowded and electronically flexible Tp ligands have been synthesized and their reactivity toward cationic species potentially useful in polymerization processes has been studied. In this context, we have reported the synthesis and structural studies of a series of chlorido,7 amido,7 and alkyl chlorido8 imido hydridotris(3,5-dimethylpyrazolyl)borate niobium and tantalum(V) compounds. Transfer of alkyl groups from transition metals to coordinated isocyanides is one of the more interesting organometallic reactions used in synthetic and mechanistic studies, since this is a powerful strategy for C−C bond formation under mild reaction conditions and also, at same time, the resulting metal−iminoacyl complexes are versatile reactive intermediates in many synthetic applications.9 These transformations basically depend on both the metal and the nature of its substituents: in particular, on the number of alkyl groups involved in the reaction. In this area, we have reported a systematic study of isocyanide insertion reactions into metal− carbon bonds of [MCp*Cl4−xMex] (M = Nb,10 Ta;11 Cp* = η5C5Me5; x = 1−4), which led to the isolation of iminoacyl (x = © 2014 American Chemical Society

1), azametallacyclopropane (x = 2), and alkenylamido imido (x = 3, 4) derivatives. Additionally, stable 18-electron iminoacyl compounds [MCpX(NAr){C(R)NAr}] (M = Nb, Cp = η5C5H4SiMe3; M = Ta, Cp = η5-C5Me5; X = Cl, alkyl; R = alkyl; Ar = 2,6-Me2C6H3)12 are obtained by insertion of isocyanide into M−Calkyl bonds of alkyl chlorido or dialkyl imido complexes [MCpXR(NAr)]. In contrast, the insertion in alkyl methyl imido derivatives takes place with migration of an alkyl or methyl group to give a mixture of methyl alkyliminoacyl [TaCp*Me(NAr){C(R)NAr}] and alkyl methyliminoacyl [TaCp*R(NAr{C(Me)NAr}] derivatives, respectively.12 More recently, we have demonstrated that, by treatment of toluene solutions of trialkyl imido derivatives [MR3(NtBu)] with isocyanide in a 1:2 ratio, the alkyl imido bis(iminoacyl) compounds [MR(NtBu){C(R)NAr}2] (Ar = 2,6-Me2C6H3; M = Nb,13 R = Me, CH2tBu, CH2CMe2Ph, CH2SiMe3, CH2Ph; M = Ta, 1 3 , 1 4 R = CH 2 tBu, CH 2 CMe 2 Ph, CH 2 SiMe 3 , CH2CMeCH2, CH2CHCHCH3) can be isolated in good yields. In this article, we report the synthesis of iminoacyl imido hydridotris(3,5-dimethylpyrazolyl)borate niobium and tantalum(V) complexes [MTp*X(NtBu){C(R)NAr-κ2C,N] (M = Nb, Ta; Tp* = HB(3,5-Me2C3HN2)3; X = Cl, R = Me, CH2CH3, CH2Ph, CH2tBu, CH2SiMe3, CH2CMe2Ph; X = R = Me; Ar = 2,6-Me2C6H3) obtained by insertion of 2,6dimethylphenyl isocyanide into the corresponding starting alkyl chlorido imido complex.8 An endo → exo isomerization of the iminoacyl ligand was observed by 1H NMR spectroscopy, and DFT calculations of both insertion and isomerization Received: February 27, 2014 Published: April 23, 2014 2277

dx.doi.org/10.1021/om5002028 | Organometallics 2014, 33, 2277−2286

Organometallics

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processes were carried out on model complexes. The 15N and 13 C chemical shifts of the iminoacyl species have been discussed using NBO calculated charges.

soluble in most organic solvents, including saturated hydrocarbons. IR spectra of all complexes show the characteristic absorptions for the Tp* ring (ν̅ ∼2546 (νB−H), ∼1545 (νCN), and ∼1448 (νB−N) cm−1),5k,l,15 M = Nimido (ν̅ ∼1367 cm −1),16 and iminoacyl (C(R)N at ν̅ ∼1577−1563 cm−1)12,16d,17 moieties. Methyl iminoacyl imido derivatives 14 exhibit absorptions at ν̅ ∼466 cm−1 due to the M−C stretching vibration,12,16d,17 whereas absorptions at ν̅ ∼1250 cm−1 in the trimethylsilylmethyl15d,16 complexes 12 correspond to δs(CH3). All NMR data of the imido iminoacyl complexes 8−14 confirm the existence of a metal center in a pseudooctahedral environment with a C1 symmetry in accord with the observation of three unequivalent pyrazolyl rings of the Tp* ligand. The observation of two inequivalent methyl groups for the 2,6-Me2C6H3 ring of the iminoacyl ligand is consistent with the restricted rotation of the aryl group around the N−Ci(aryl) bond, as we have observed before in alkyl bis(iminoacyl) imido derivatives.13,14 X-ray Diffraction Study of 10b. The molecular solid-state structure of complex 10b was established by an X-ray diffraction study. An ORTEP view is given in Figure 1, and a

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RESULTS AND DISCUSSION Synthesis of Chlorido and Methyl Iminoacyl Imido Complexes. Alkyl chlorido and dimethyl imido Tp* complexes [MTp*XR(NtBu)] (M = Nb/Ta; Tp* = HB(3,5Me2C3HN2)3; X = Cl, R = Me (1a/1b), CH2CH3 (2a/2b), CH2Ph (3a/3b), CH2tBu (4a/4b), CH2SiMe3 (5a/5b), CH2CMe2Ph (6a/6b); X = R = Me (7a/7b))8 react quickly with 1 equiv of 2,6-dimethyl phenylisocyanide, at room temperature in benzene or C6D6, giving the chlorido or methyl imido iminoacyl derivatives [MTp*X(NtBu){C(R)NArκ2C,N}] (Ar = 2,6-Me2C6H3; M = Nb/Ta; X = Cl, R= CH3 (8a/8b), CH2CH3 (9a/9b), CH2Ph (10a/10b), CH2tBu (11a/ 11b), CH2SiMe3 (12a/12b), CH2CMe2Ph (13a/13b), X = R = Me (14a/14b)), as a result of the insertion of the isocyanide into the M−Calkyl bond of the starting product (Scheme 1). In Scheme 1

the case of the chlorido methyl imido complexes 1a/1b, due to the impossibility of obtaining both starting complexes as pure samples, an insertion reaction in order to prepare 8a/8b was carried out by adding an equivalent amount of ArNC, for M = Nb (8a) to a mixture of [NbTp*ClMe(NtBu)] (88%) and [NbTp*Cl2(NtBu)] (10%) and for M = Ta (8b) to a mixture of [TaTp*ClMe(NtBu)] (71%), [TaTp*Cl2(NtBu)] (23%), and [TaTp*Me2(NtBu)] (6%). Furthermore, treatment of toluene solutions of 7a/7b with 2 equiv of ArNC, at 70 °C during 1 week, led to the methyl imido iminoacyl complexes 14a/14b probably because the metal center does not bind a second isocyanide molecule. This behavior is in contrast to that observed for other dimethyl or trialkyl derivatives, which in the presence of 1 equiv or an excess of ArNC gives azametallacyclopropane or bis(iminoacyl) complexes, respectively, as a result of a migration of the second methyl group to the electrophilic iminoacyl carbon atom11 or after the coordination of the second isocyanide molecule to the metal center.13,14 All iminoacyl complexes 8−14 are extremely moisturesensitive; therefore, rigorously dried solvents and handling under a dry inert atmosphere were found to be imperative for successful preparations. All of them were isolated as microcrystalline solids in good yields, are stable under argon atmosphere over a period of several weeks, and also are

Figure 1. ORTEP view of the molecular structure of [TaTp*Cl(NtBu){C(CH2Ph)NAr-κ2C,N}] (10b) with the atom-numbering scheme. Thermal ellipsoids are drawn at the 50% probability level.

selection of bond distances and angles is summarized in Table 1. The core structure of complex 10b exhibits a metal atom coordinated to three nitrogen atoms (with a facial coordination) of a hydridotris(3,5-dimethylpyrazolyl)borate ligand (Tp*), a chlorine atom cis to the iminoacyl ligand, and the axially located imido ligand in a distorted-octahedral geometry, obviously if we assume that the C(CH 2Ph)NAr-κ2C,N iminoacyl moiety is occupying a single coordination site. The Ta−N7 distance (1.783(4) Å) and Ta−N7−C40 angle (170.9(4)°) for the imido ligand are in the range of those reported for imido niobium and tantalum complexes.16,18 The 2278


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(5b), CH2CMe2Ph (6b); X = R = Me (7b)), the signal corresponding to the initial alkyl chlorido imido and the final chlorido imido iminoacyl complexes B were only observed. Species A and B both exhibited equal numbers of NMR signals, with the same multiplicity and similar chemical shifts that probably correspond to two isomeric structures. Several examples have been reported about the simultaneous formation of endo- and exo-iminoacyl group 4 metal complexes in high oxidation states.2 In addition, Lappert et al.20 detected an intramolecular exchange process between both isomers of [ZrCp2Cl{C(CHR2)NAr-κ2C,N}] (R = SiMe3, Ar = 4MeC6H4) by NMR spectroscopy, whereas Jordan et al.21 observed an isomeric interconversion process in the cationic complex [ZrCp2(picolyne-κ2N,C)(thf)]+ due to an exchange between free and coordinated THF. In order to resolve the spatial structures of species A and B, the isocyanide insertion in the niobium−carbon bond of complex 5a (R = CH2SiMe3) was chosen for a detailed NOE spectroscopy study, due to its very slow isomerization. After 12 h, the isocyanide had completely reacted and in the solution a mixture of A and B was present in a 3:1 ratio, whereas after 36 h, 95% of A transformed into B. All data from the 2D-NOESY spectrum and an array of 1D-NOESY spectra, mainly for the minor component, showed that the CH2SiMe3 group of A and the 2,6-dimethylphenyl substituent of B are located between the ax and eq 2 pyrazolyl rings, while the 2,6-dimethylphenyl ring of A and the CH2SiMe3 group of B are situated near MeC3 of the ax pyrazolyl ring, as shown in Figure 3.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for Complex 10b Ta−N7 1.783(4) Ta−N8 2.195(4) Ta−N1 2.407(4) Ta−N3 2.339(4) Ta−N5 2.251(4) ∠Ta−N7−C40 170.9(4) ∠Ta−N8−C50 72.2(3) θC50−N8−C60−C66 θC50−C51−C52−C57

Ta−C50 N8−C50 N7−C40 Ta−Cl B−H ∠Ta−C50−N8 ∠C50−Ta−N8

2.177(5) 1.282(6) 1.457(6) 2.468(1) 1.07(5) 73.7(3) 34.08(17) 119.7(6) 69.9(6)

Ta−N1 bond length (2.407(4) Å) for the axial pyrazolyl group trans to the imido is greater than that corresponding to the Ta−N5 bond (2.251(4) Å) for the pyrazolyl group trans to the chlorido ligand and the iminoacyl ligand (Ta−N3 2.339(4) Å), probably due to the inductive effect (I) difference5g of the imido, chlorido, and iminoacyl ligands, respectively. The average M−N distances (2.323 Å) and N−M−N angles (79.69°) formed by the tridentate Tp* ligand are on the same order as those reported for hydridotris(pyrazolyl)borate niobium and tantalum derivatives.5l,6a,d,e In the iminoacyl moiety (Figure 2), the bond distances Ta− N8 (2.195(4) Å), Ta−C50 (2.177(5) Å), and N8−C50

Figure 2. Spatial location for the iminoacyl moiety of 10b.

(1.282(6) Å) are within the range found for similar compounds for which a CN double bond and a κ2C,N coordination to the metal has been proposed.10,12,19 The θC50−N8−C60−C66 (109.6°) and θC50−C51−C52−C57 (119.7°) dihedral angles indicate that the C6H3 and C6H5 rings are fairly parallel (10°). Furthermore, the almost perpendicular orientation of the dimethylphenyl group with respect to the N8−C50 double bond indicates that π−π conjugation would not be possible. endo → exo Isomerization of the Iminoacyl Ligand. All insertion reactions were followed by 1H NMR spectroscopy, and during this process we observed that after addition of 2,6Me2C6H3NC to a C6D6 solution of [NbTp*ClR(NtBu)] (R = CH2Ph (3a), CH2tBu (4a), CH2SiMe3 (5a), CH2CMe2Ph (6a)), the first 1H NMR spectrum (1 h later) showed the resonances of the initial alkyl chlorido imido complex and the corresponding signals of the intermediate species A, which eventually evolved to the final iminoacyl species B. However, in the case of the niobium derivatives [NbTp*XR(NtBu)] (X = Cl, R = Me (1a), CH2CH3 (2a); X = R = Me (7a)) and also in all tantalum compounds [TaTp*XR(NtBu)] (X = Cl, R = Me (1b), CH2CH3 (2b), CH2Ph (3b), CH2tBu (4b), CH2SiMe3

Figure 3. Important NOE interactions in isomer A and B of complex 12a in C6D6 solution at 25 °C.

Therefore, the A and B structures correspond to the endo and exo isomers of complex 12a, respectively, and the observed transformation is really an endo → exo isomerization22 (Scheme 2). Scheme 2

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In agreement with NOE data, all chlorido imido iminoacyl complexes [MTp*Cl(NtBu){C(R)NAr-κ2C,N}] (Ar = 2,6Me2C6H3; M = Nb/Ta; X = Cl, R = Me (8a/8b), CH2CH3 (9a/9b), CH2Ph (−/10b), CH2tBu (−/11b), CH2SiMe3 (−/12b), CH2CMe2Ph (−/13b); X = R = Me (14a/14b)) and the final niobium derivatives 10a (R = CH2Ph), 11a (R = CH2tBu), 12a (R = CH2SiMe3), and 13a (R = CH2CMe2Ph) are characterized by an exo location of the κ2-iminoacyl ligand. Therefore, we propose that the formation of the endo isomer in the first step is a kinetically controlled process while the conversion into the exo isomer is the result of a thermodynamic equilibrium established between both isomers. Furthermore, the isomerization process rate in the niobium iminoacyl derivatives depends on the bulkiness of the alkyl group and decreases in the order R = CH3 ≈ CH2CH3 ≫ CH2Ph > CH2CMe2Ph ≈ CH2tBu > CH2SiMe3. DFT Theoretical Study. In order to propose the mechanism of the isocyanide insertion and the observed iminoacyl ligand isomerization, DFT calculations were performed for both processes using chlorido methyl Meimido hydridotris(3-methylpyrazolyl)borate as the truncated model structures (X = Cl, Y = Me in Figure 4).

Figure 5. Schematic mechanism of the isocyanide insertion and subsequent iminoacyl isomerization. The values in the figure correspond to the relative energies of the intermediates, transition states, and products for the niobium (blue) and tantalum (red) complexes.

2,6-dimethylphenyl moiety and one of the methyl groups of the two different pyrazolyl rings.24 NMR Spectroscopy Study. Table 2 shows 15N and 13C chemical shifts for the iminoacyl imido complexes 8−14. The δ value of 15N1 directly bonded to the boron atom has the same value (δ −156 ± 1) not only in the iminoacyl derivatives 8−14 but also in the starting alkyl chlorido 1−7,8 dichlorido,7 and amido imido7 complexes and will not be discussed. The transformation of the free isocyanide 2,6-Me2C6H3NC (13C, δ 171; 15N, δ −204) into an iminoacyl ligand produces a very important deshielding of both resonances (13C, Δδav.≈ 65 (Nb), 80 (Ta); 15N, Δδav.≈ 77 (Nb), 88 (Ta)) mainly due to the hybridization change of both atoms from sp to sp2 and the formation of new bonds with the metal atom.25 Thus, for the iminoacyl moiety of niobium (8a−14a) and tantalum (8b− 14b) complexes, the 13C signals were observed at δ 239−233 and 253−248 and the 15N resonances at δ −121 to −137 and −111 to −125, respectively. 15Nimido chemical shifts for the niobium complexes are greater Δδ = δNb − δTa= 31.5 (8−13) and 25 (14) than those of the tantalum analogues. These differences are on the same order as those observed previously by us7,8,13,14 and can be explained mainly by a variation in the diamagnetic component of the magnetic shielding constant25−27 due to the different atomic numbers of Nb (41) and Ta (73). Furthermore, 15Nimido chemical shifts depend on inductive effect ligands located in the equatorial plane of dichlorido,7 chlorido methyl (1a/1b),8 dimethyl (7a/7b),8 chlorido (8a/8b), and methyl iminoacyl (14a/14b) derivatives. The variation of the electron density in the metal atom, due to the inductive effect of the equatorial ligand, is compensated by the change in the pπ−dπ M−Nimido conjugation and, therefore, by the nitrogen atom charge. As a consequence of this, the corresponding 15Nimido chemical shift must change. Two linear correlations (see Figure 6) between the experimental δ(15Nimido) (Table 2) and NBO calculated charges (Table 3) for the imido nitrogen in the model complexes shown in Figure 4 were found for the niobium and tantalum species 8−14. In both cases, the chemical shift increases as the negative NBO charge located in this atom decreases. This behavior is in agreement with the well-known relationship Y = Aq + B, where Y is the experimental chemical shift (δ) of any magnetic active

Figure 4. Model compounds calculated by DFT and real complexes.

The mechanism predicted by DFT is similar to those proposed in earlier literature studies of CO and RNC insertion into M−alkyl bonds and also of acyl or iminoacyl isomerization processes.9a,23 The nucleophilic attack of the isocyanide at the metal atom (see Figure 5) takes place in the equatorial plane of the complex between the M−Cl and M−Me bonds and induces a lengthening in both bonds and an opening of the ∠Cl−M− Me angle (∼120°) to achieve the first transition state (TS1), in which the isocyanide carbon atom is located at a distance of 3 Å. A subsequent approach between the carbon isocyanide and metal atoms to 2.22 Å and the opening of the ∠Cl−M−Me angle to ∼131° reduces the total energy of this adduct, leading to the first intermediate species (Int1). Then, the M−C bond formation favors a concerted three-center transition state (TS2), in which a carbon and nitrogen atom hybridization change takes place with simultaneous migration of the methyl group from the metal to the carbon isocyanide atom, giving an endo κ2-iminoacyl compound. The endo → exo isomerization takes place via decoordination of the Niminoacyl to form an κ1iminoacyl complex, which undergoes subsequent rotation around the M−Ciminoacyl bond. This latter process occurs in two consecutive steps, where two local transition states (TS3′ and TS3″) have to be overcome due to the presence of the steric collisions between one of the methyl substituents of the 2280


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Table 2. Selected 15N and 13C Chemical Shifts (δ) for the Imido Iminoacyl Complexes 8−14 in C6D6 Solution at 25°C δ(15N) Tp* R CH3 CH2CH3 CH2Ph CH2tBu CH2SiMe3 CH2CMe2Ph Me Me

iminoacyl moiety

X

M

Neq 1

Neq 2

Nax

δ(15Nimido)

Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Me Me

Nb (8a) Ta (8b) Nb (9a) Ta (9b) Nb (10a) Ta (10b) Nb (11a) Ta (11b) Nb (12a) Ta (12b) Nb (13a) Ta (13b) Nb (14a) Ta (14b)

−117.4 −117.7 −117.5 −117.1 −117.7 −117.5 −116.9 −117.1 −117.8 −117.5 −116.9 −117.4 −114.8 −116.9

−128.5 −128.2 −128.8 −128.4 −128.9 −128.5 −128.7 −128.7 −128.8 −128.5 −127.7 −128.8 −123.1 −122.1

−111.3 −112.9 −111.2 −112.6 −111.3 −112.8 −111.5 −112.6 −111.8 −112.6 −111.8 −112.8 −107.9 −110.9

35.4 4.3 35 4 38 6 37 5 34.6 3.4 36.7 5.2 14.3 −10.7

13

C

234.3 250.5 236.8 253.5 233.8 249.9 234.9 251.4 233.0 248.9 234.0 249.7 238.5 251.7

15

N

−135.1 −123.3 −137.0 −125.0 −129.5 −117.0 −122.6 −112.0 −134.0 −120.0 −121.4 −111.0 −127.8 −119.8

(8−13), Δδ = −8.0, X = Me (14); 13C, Δδ (=δNb − δTa) = −16.3 ± 0.4, X = Cl (8−13); Δδ = −13.0, X = Me (14)), as we observed earlier in bis(iminoacyl) imido complexes.13,14 In spite of the limited number of iminoacyl complexes studied, this all seems to indicate that a decrease of the charge located in the carbon and nitrogen atoms of the iminoacyl moiety is accompanied by a deshielding of the corresponding resonances (see Tables 2 and 3), which would be in contradiction with the theoretical presentation of carbon25 and nitrogen26 chemical shifts. However, the increase of positive charge of the metal atom situated in an α position with respect to the iminoacyl moiety correlates with the deshielding of both resonances: δ(13C) = 43[q(M)NBO] + 200, r = 0.987; δ(15N) = 31[q(M)NBO] − 156, r = 0.952 (Figure 7). This spectral behavior of the iminoacyl moiety might be due to a bonding situation with a very small contribution of the diamagnetic component (σdia) and a very important variation of ∑BQAB and/or ΔE terms of the paramagnetic shielding tensor.27 Finally, 15N chemical shifts of the nitrogens of the Tp* ligand directly bonded to the metal did not depend on the metal center nature but on the coordination position of the pyrazolyl ring and decreased in the order Nax > Neq 2 > Neq 1, which is in accord with a larger M−Nax distance as compared to M−Neq (see Table 1).

Figure 6. Correlation between δ(15Nimido) and NBO calculated charge of imide nitrogen.

nucleus or the paramagnetic component (σpara) of the magnetic shielding constant.25−29 The different correlation parameters observed between niobium and tantalum complexes can be rationalized on the basis of the diamagnetic contribution (σdia) to the shielding constant.30 Unlike δ(15Nimido), the 15N and 13Ciminoacyl resonances in the niobium complexes are more shielded than in tantalum analogues (15N, Δδ (=δNb − δTa) = −11.7 ± 0.6, X = Cl

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CONCLUSIONS Alkyl chlorido [MTp*ClR(NtBu)] (M = Nb/Ta; R = Me (1a/ 1b), CH2CH3 (2a/2b), CH2Ph (3a/3b), CH2tBu (4a/4b),

Table 3. Selected NBO Calculated Charges for the Model Complexes total NBO charge X

Y

Cl

Cl

Cl

Me

Me

Me

Cl

ImAc

Me

ImAc

M

M

Nimido

Nb Ta Nb Ta Nb Ta Nb Ta Nb Ta

+0.77 +1.04 +0.94 +1.24 +1.14 +1.46 +0.77 +1.08 +0.84 +1.20

−0.47 −0.65 −0.52 −0.69 −0.57 −0.74 −0.51 −0.68 −0.55 −0.71

NImAc

−0.46 −0.50 −0.47 −0.51 2281

CImAc

Nax

Neq 1

Neq 2

+0.29 +0.21 +0.27 +0.19

−0.34 −0.36 −0.34 −0.35 −0.33 −0.34 −0.33 −0.34 −0.32 −0.33

−0.33 −0.36 −0.32 −0.35 −0.32 −0.34 −0.31 −0.33 −0.29 −0.32

−0.33 −0.36 −0.31 −0.34 −0.32 −0.34 −0.31 −0.33 −0.31 −0.32


Organometallics

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monitored by 1H NMR spectroscopy until total and fast formation of the chlorido imido iminoacyl complex 8a. Data for 8a are as follows. 1H NMR (C6D6; δ, ppm): 6.84 (m, 1H, Hm), 6.62 (m, 1H, Hp), 6.80 [m, 1H, Hm, C(Me)NC6H3Me2], 5.64eq 1, 5.51eq 2, 5.35ax (m, 1H, C4-H, Tp*), 3.21eq 1, 2.19ax, 2.01eq 2 (s, 3H, C3Me, Tp*), 2.78 [s, 3H, C(Me)NAr], 2.15eq 2, 2.14ax, 2.09eq 1 (s, 3H, C5Me, Tp*), 1.98 (s, 3H), 0.75 [s, 3H, C(Me)NC6H3Me2], 1.37 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 234.1 [C(Me)NAr], 152.9eq 1, 151.5eq 2, 151.2ax (C3-Me, Tp*), 144.7eq 2, 143.4eq 1, 142.6ax (C5-Me, Tp*), 142.8 (Ci), 134.4, 130.8 (Co), 129.7, 128.9 (Cm), 127.6 [Cp, C(Me)NC6H3Me2], 107.4eq 1, 107.1eq 2, 106.8ax (C4-H, Tp*), 67.4 (NCMe3), 31.8 (NCMe3), 21.6 [C(Me)NAr], 19.6, 15.9 [C(Me)NC6H3Me2], 17.9eq 1, 15.4eq 2, 14.5ax (C3-Me, Tp*), 12.9eq 1, 12.7eq 2, 12.6ax (C5-Me, Tp*). 15N NMR (C6D6; δ, ppm): 35.4 (NCMe3), −111.3ax, −117.4eq 1, −128.5eq 2 (N2-M), −135.1 [C(Me)NAr], −155.8eq 2, −157.7ax, −157.8eq 1 (N1-B). M = Ta (8b). Under rigorously anhydrous conditions, a C6D6 (0.70 mL) solution of a mixture of [TaTp*ClMe(NtBu)] (71%), [TaTp*Cl2(NtBu] (23%), and [TaTp*Me2(NtBu] (6%) was treated with a 1.43 mM solution of 2,6-Me2C6H3NC in benzene-d6 (11 μL, 0.16 mmol). The reaction was monitored by 1H NMR spectroscopy until total and fast formation of the chlorido imido iminoacyl complex 8b. Data for 8b are as follows. 1H NMR (C6D6; δ, ppm): 6.74 (m, 1H, Hm), 6.71 (m, 1H, Hm), 6.68 [m, 1H, Hp, C(Me)NC6H3Me2], 5.61eq 1, 5.47eq 2, 5.37ax (m, 1H, C4-H, Tp*), 3.21eq 1, 2.24ax, 2.09eq 2 (s, 3H, C3Me, Tp*), 2.81 [s, 3H, C(Me)NAr], 2.12ax, 2.10eq 2, 2.05eq 1 (s, 3H, C5Me, Tp*), 2.10 (s, 3H), 0.71 [s, 3H, C(Me)NC6H3Me2], 1.44 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 250.5 [C(Me)NAr], 153.4eq 1, 151.8ax, 151.3eq 2 (C3-Me, Tp*), 144.6eq 2, 143.1eq 1, 142.6ax (C5-Me, Tp*), 143.2 (Ci), 133.8, 131.6 (Co), 128.8, 128.7 (Cm), 126.4 [Cp, C(Me)NC6H3Me2], 107.6eq 1, 107.3ax, 107.1eq 2 (C4-H, Tp*), 65.7 (NCMe3), 33.4(NCMe3), 23.4 [C(Me)NAr], 19.9, 15.5 [C(Me)NC6H3Me2], 18.6eq 1, 16.2eq 2, 15.2ax(C3-Me, Tp*), 13.0eq 1, 12.8ax, 12.4eq 2 (C5-Me, Tp*). 15N (C6D6; δ, ppm): 4.7 (NCMe3), −112.6ax, −117.5eq 1, −127.5eq 2 (N2-M), −122.8 [C(Me)NAr], −156.2ax, −156.2eq 2, −157.9eq 1 (N1-B). [MTp*X(NtBu){C(R)NAr-κ2C,N}] (Ar = 2,6-Me2C6H3; M = Nb/ Ta, X = Cl, R = CH2CH3 (9a/9b), CH2Ph (10a/10b), CH2tBu (11a/ 11b), CH2SiMe3 (12a/12b), CH2CMe2Ph (13a/13b); X = R = Me (14a/14b)). In a standard experiment, under rigorously anhydrous conditions and at room temperature, ArNC (M = Nb/Ta; R = CH2CH3, 0.047/0.010 g, 0.36/0.081 mmol; R = CH2Ph, 0.022/0.038 g, 0.17/0.30 mmol; R = CH2tBu, 0.046/0.026 g, 0.35/0.20 mmol; R = CH2SiMe3, 0.024/0.039 g, 0.19/0.30 mmol; R = CH2CMe2Ph, 0.025/ 0.007 g, 0.19/0.056 mmol; X = R = Me, 0.026/0.018 g, 0.20/0.14 mmol) was added to a Schlenk tube charged with a benzene (15−20 mL) solution of [MTp*XR(NtBu)] (M = Nb/Ta; X = Cl, R = CH2CH3, 0.19/0.050 g, 0.36/0.081 mmol; R = CH2Ph, 0.10/0.20 g, 0.17/0.30 mmol; R = CH2tBu, 0.20/0.13 g, 0.35/0.20 mmol; R = CH2SiMe3, 0.11/0.20 g, 0.18/0.30 mmol; R = CH2CMe2Ph, 0.12/0.04 g, 0.19/0.057 mmol; X = R = Me, 0.10/0.08 g, 0.20/0.14 mmol) and the mixture was stirred for 12 h. The resulting solution was concentrated to ca. 5 mL and cooled to −40 °C, yielding 9−14 (M = Nb/Ta; X = Cl, R = CH2CH3 9a, 0.21 g, 87%/9b, 0.045 g, 75%; R = CH2Ph 10a, 0.10 g, 94%/10b, 0.17 g, 72%; R = CH2tBu 11a, 0.21 g, 85%/11b, 0.12 g, 77%; R = CH2SiMe3 12a, 0.09 g, 50%/12b, 0.21 g, 85%; R = CH2CMe2Ph 13a, 0.14 g, 83%/13b, 0.042 g, 88%; X = R = Me 14a, 0.10 g, 80%/14b, 0.08, 87%) as red/orange (9a/9b, 10a/10b, and 11a/11b), orange (12a/12b and 13a/13b), and white (14a/14b) microcrystalline solids. Data for 9−14 are as follows: Compound 9a. IR (KBr; ν,̅ cm−1): 3447 (m), 2969 (s), 2923 (s), 2531 (w), 1648 (m), 1547 (s), 1450 (s), 1416 (s), 1370 (m), 1232 (s), 1209 (s), 1072 (m), 1040 (m), 814 (s), 783 (m), 773 (m), 696 (w), 648 (w), 563 (w), 546 (w), 467 (w). 1H NMR (C6D6; δ, ppm): 6.80 (m, 1H), 6.62 [m, 2H, C(CH2CH3)NC6H3Me2], 5.63eq 1, 5.50eq 2, 5.36ax (m, 1H, C4-H, Tp*), 4.70 (q, 1H, JB−H = 131 Hz, BH, Tp*), 3.31, 3.03, 1.64 [ABX3, JAB = 16.2 Hz, JAX3 = JBX3 = 7.2 Hz, C(CH2CH3)NAr], 3.21eq 1, 2.28ax, 1.99eq 2 (m, 3H, C3-Me, Tp*),

Figure 7. Correlation between 13C and 15N chemical shifts with respect to NBO charges located at the metal atoms.

CH2SiMe3 (5a/5b), CH2CMe2Ph (6a/6b)) and dimethyl [MTp*Me 2 (NtBu)] (7a/7b) imido hydridotris(3,5dimethylpyrazolyl)borate niobium and tantalum(V) compounds react with 1 equiv of ArNC, yielding chlorido and methyl iminoacyl [MTp*X(NtBu){C(R)NAr-κ2C,N}] (M = Nb/Ta; Ar = 2,6-Me2C6H3; X = Cl, R= Me (8a/8b), CH2CH3 (9a/9b), CH2Ph (10a/10b), CH2tBu (11a/11b), CH2SiMe3 (12a/12b), CH2CMe2Ph (13a/13b); X = R = Me (14a/14b)) imido derivatives. An intramolecular irreversible endo → exo isomerization of the iminoacyl ligand, observed by 1H NMR spectra, was studied by DFT together with the insertion reaction. The isomerization takes place through the decoordination of the nitrogen iminoacyl atom and subsequent rotation of this ligand around the M−Ciminoacyl bond. In tantalum compounds, the 15Nimido chemical shifts are smaller than in isoelectronic niobium analogues, whereas for the iminoacyl moiety, both 13C and 15N are larger, probably due to a very small diamagnetic contribution to the magnetic shielding constant. Two linear correlations between the experimental δ(15N) and NBO calculated charges for the imido nitrogen of the niobium and tantalum species were found. Likewise, linear correlations between the 13C, 15N chemical shifts for the iminoacyl moiety and the NBO calculated charge of the metal were also obtained.

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EXPERIMENTAL SECTION

All operations were carried out under a dry argon atmosphere using standard Schlenk-tube and cannula techniques or in a conventional argon-filled glovebox. Solvents were refluxed over an appropriate drying agent and distilled and degassed prior to use: benzene, C6D6, and hexane (Na/K alloy) and toluene (Na). The starting materials [MTp*XR(NtBu)]8 (M = Nb, Ta; X = Cl, R = Me, CH2CH3, CH2Ph, CH2tBu, CH2SiMe3, CH2CMe2Ph; X = R = Me) were prepared as described previously. Reagent grade 2,6-Me2C6H3NC (Aldrich) was purchased from commercial sources and was used without further purification. Samples for IR spectroscopy were prepared as KBr pellets and recorded on a Perkin-Elmer Spectrum 2000 spectrophotometer (4000−400 cm−1). NMR spectra were recorded on Mercury-300, UnityPlus-300, and VNMRS-500 spectrometers. All 2D spectra were processed by using 4K × 4K matrices. The 1H and 13C chemical shifts were referenced to the solvent signals and 15N to external CH3NO2. Microanalyses (C, H, N) were performed with a LECO CHNS 932 microanalyzer. [MTp*Cl(NtBu){C(Me)NAr-κ2C,N}] (Ar = 2,6-Me2C6H3; M = Nb (8a), Ta (8b)). M = Nb (8a). Under rigorously anhydrous conditions, a C6D6 (0.70 mL) solution of a mixture of [NbTp*ClMe(NtBu)] (88%) and [NbTp*Cl2(NtBu)] (10%) was treated with a 1.43 mM solution of 2,6-Me2C6H3NC in C6D6 (11 μL, 0.16 mmol). The reaction was 2282


Organometallics

Article

C(CH2C6H5)NAr], 107.6eq 1, 107.4ax, 107.1eq 2 (C4-H, Tp*), 65.8 (NCMe3), 42.4 [C(CH2Ph)NAr], 33.0 (NCMe3), 20.7, 15.8 [C(CH2Ph)NC6H3Me2], 18.6eq 1, 16.3eq 2, 15.9ax (C3-Me, Tp*), 13.0eq 1, 12.8eq 2, 12.5ax (C5-Me, Tp*). 15N NMR (C6D6; δ, ppm): 6.2 (TaNCMe3), −112.8ax, −117.7eq 1, −128.5eq 2 (N2-M), −117.4 [C(CH2Ph)NAr], −156.6ax, −156.8eq 1, −157.7eq 2 (N1-B). Anal. Calcd for C35H47N8BClTa (807.508): C, 52.06; H, 5.87; N, 13.94. Found: C, 52.22; H, 5.82; N, 12.87. Compound 11a. IR (CsI; ν,̅ cm−1): 3434 (m), 2952 (s), 2927 (s), 2552 (m), 1612 (m), 1548 (s), 1449 (s), 1415 (s), 1368 (m), 1227 (s), 1211 (s), 1070 (s), 1038 (s), 816 (m), 775 (m), 690 (m), 488 (m), 570 (w), 468 (w). 1H NMR (C6D6; δ, ppm): 6.85 (m, 1H), 6.82 (m, 1H), 6.61 [m, 1H, C(CH2CMe3)NC6H3Me2], 5.60eq 1, 5.47eq 2, 5.38ax (m, 1H, C4-H, Tp*), 4.40 (q, 1H, JB−H = 104 Hz, BH, Tp*), 3.22eq 1, 2.39ax, 2.08eq 2 (m, 3H, C3-Me, Tp*), 2.92, 2.56 [AB, 2H, JAB = 17.6 Hz, C(CH2CMe3)NAr], 2.14ax, 2.10eq 2, 2.09eq 1 (m, 3H, C5-Me, Tp*), 2.16 (s, 3H), 0.74 [s, 3H, C(CH2CMe3)NC6H3Me2], 1.64 [s, 9H, C(CH2CMe3)NAr], 1.42 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 235.7 [C(CH2CMe3)NAr], 152.5eq 1, 151.6eq 2, 151.1ax (C3Me, Tp*), 144.7eq 2, 142.9eq 1, 142.8ax (C5-Me, Tp*), 143.1 (Ci), 133.5, 131.4 (Co), 128.6, 128.2 (Cm), 126.2 [Cp, C(CH2CMe3)NC6H3Me2], 107.4eq 2, 106.8ax, 106.7eq 1 (C4-H, Tp*), 68.3 (NbNCMe3), 50.2 [C(CH2CMe3)NAr], 50 [C(CH2CMe3)NAr], 31.8 [C(CH2CMe3)NAr], 31.4 (NCMe3), 20.7, 15.6 [C(CH2Ph)NC6H3Me2], 18.4eq 2, 16.3ax, 16.3eq 1 (C3-Me, Tp*), 13.0eq 1, 12.8eq 2, 12.5ax (C5-Me, Tp*). 15 N NMR (C6D6; δ, ppm): 36.7 (NCMe3), −111.5ax, −116.9eq 1, −128.7eq 2 (N2-M), −122.5 [C(CH2CMe3)NAr], −156.4eq 2, −156.9ax, −157.2eq 1 (N1-B). Anal. Calcd for C35H51N8BClNb (699.477): C, 56.66; H, 7.35; N, 16.09. Found: C, 56.04; H, 7.31; N, 15.88. Compound 11b. IR (CsI; ν,̅ cm−1): 3424 (m), 2952 (s), 2927 (s), 2538 (m), 1612 (m), 1548 (s), 1449 (s), 1417 (s), 1368 (m), 1225 (s), 1211 (s), 1070 (s), 1038 (s), 816 (m), 775 (m), 690 (m), 648 (m), 570 (w), 466 (w). 1H NMR (C6D6; δ, ppm): 6.99 (m, 1H), 6.80 (m, 1H), 6.60 [m, 1H, C(CH2CMe3)NC6H3Me2], 5.58eq 1, 5.44eq 2, 5.44ax (m, 1H, C4-H, Tp*), 4.58(br, 1H, BH, Tp*), 3.22eq 1, 2.42ax, 2.17eq 2 (m, 3H, C3-Me, Tp*), 3.11, 2.38 [AB, 2H, JAB = 17.4 Hz, C(CH2CMe3)NAr], 2.22 (s, 3H), 0.74 [s, 3H, C(CH2CMe3)NC6H3Me2], 2.14ax, 2.09eq 1, 2.00eq 2 (m, 3H, C5-Me, Tp*), 1.63 [s, 9H, C(CH2CMe3)NAr], 1.48 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 251.4 [C(CH2CMe3)NAr], 153.1eq 2, 152.3eq 1, 151.6ax (C3Me, Tp*), 144.6eq 2, 142.9eq 1, 142.6ax (C5-Me, Tp*), 143.4 (Ci), 133.4, 130 (Co), 128.3, 128.1 (Cm), 126.1 [Cp, C(CH2CMe3)NC6H3Me2], 107.6eq 1, 107.2eq 2, 107.1ax (C4-H, Tp*), 66.0(NCMe3), 50.9 [C(CH2CMe3)NAr], 50.8 [C(CH2CMe3)NAr], 32.9 (NCMe3), 31.6 [C(CH2CMe3)NAr], 21.2, 15.6 [C(CH2CMe3)NC6H3Me2], 18.8eq 1, 16.7ax, 15.6eq 2 (C3-Me, Tp*), 13.1eq 1, 12.9eq 2, 12.5ax (C5-Me, Tp*). 15 N NMR (C6D6; δ, ppm): 4.63 (NCMe3), −112.2 [C(CH2CMe3) NAr], −112.6ax, −117.1eq 1, −128.7eq 2 (N2-M), −156.9eq 2, −157.2ax, −157.6eq 1 (N1-B). Anal. Calcd for C33H51N8BClTa (787.519): C, 50.33; H, 6.53; N, 14.29. Found: C, 49.93; H, 6.48; N, 14.39. Compound 12a. IR (CsI; ν,̅ cm−1): 3424 (m), 2959 (s), 2925 (s), 2546 (m), 1619 (s), 1547 (s), 1449 (s), 1416 (s), 1369 (m), 1225 (s), 1208 (s), 1070 (m), 1038 (m), 851 (m), 817 (m), 773 (m), 693 (w), 647 (w), 576 (w), 466 (w). 1H NMR (C6D6; δ, ppm): 6.89 (m, 1H), 6.80 (m, 1H), 6.60 [m, 1H, C(CH2SiMe3)NC6H3Me2], 5.63eq 1, 5.54eq 2, 5.37ax (m, 1H, C4-H, Tp*), 4.68 (q, 1H, JB−H = 107.8 Hz, BH, Tp*), 3.23eq 1, 2.40ax, 2.06eq 2 (m, 3H, C3-Me, Tp*), 2.79, 2.61 [AB, 2H, JAB = 14.2 Hz, C(CH2SiMe3)NAr], 2.16eq 2, 2.16ax, 2.08eq 1 (m, 3H, C5-Me, Tp*), 2.12 (s, 3H), 0.78 [s, 3H, C(CH2SiMe3)NC6H3Me2], 1.38 (s, 9H, NCMe3), 0.49 [s, 9H, C(CH2SiMe3)NAr]. 13 C{1H} NMR (C6D6; δ, ppm): 233.8 [C(CH2SiMe3)NAr], 152.9eq 1, 151.6eq 2, 151.2ax (C3-Me, Tp*), 144.7eq 1, 143.2eq 2, 142.8ax (C5-Me, Tp*), 143.5 (Ci), 133.9, 130.1 (Co), 128.3, 127.5 (Cm), 126.1 [Cp, C(CH2SiMe3)NC6H3Me2], 107.4eq 2, 106.9ax, 106.8eq 1 (C4-H, Tp*), 67.8(NCMe3), 31.7 (NbNCMe3), 26.8 [C(CH2SiMe3)NAr], 20.1, 15.7 [C(CH2SiMe3)NC6H3Me2], 18.5eq 1, 16.4ax, 16.1eq 2(C3-Me, Tp*), 13.2eq 2, 13.1eq 1, 12.6ax (C5-Me, Tp*), 1.69 [C(CH2CSiMe3)NAr]. 15 N NMR (C6D6; δ, ppm): 34.7 (NCMe3), −111.4ax, −117.9eq 1, −128.8eq 2 (N2-M), −133.9 [C(CH2SiMe3)NAr], −155.9ax, −155.9eq 1,

2.15eq 2, 2.15ax, 2.09eq 1 (m, 3H, C5-Me, Tp*), 2.04 (s, 3H), 0.80 [s, 3H, C(CH2CH3)NC6H3Me2], 1.38 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 236.8 [C(CH2CH3)NAr], 152.6eq 1, 151.4eq 2, 151.0ax (C3-Me, Tp*), 144.6eq 2, 143.2eq 1, 142.7ax (C5-Me, Tp*), 142.6 (Ci), 134.4, 129.5 (Co), 128.4, 128.1 (Cm), 126.1 [Cp, C(CH2CH3)NC6H3Me2], 107.3eq 1, 106.8eq 2, 106.5ax (C4-H, Tp*), 67.4 (NCMe3), 33.9 (NCMe3), 29.1 [C(CH2CH3)NAr], 18.3eq 1, 14.5ax, 14.5eq 2 (C3-Me, Tp*), 19.8, 15.6 [C(CH2CH3)NC6H3Me2], 13.1ax, 12.9eq 2, 12.6eq 1 (C5-Me, Tp*), 10.2 [C(CH2CH3)NAr]. 15N NMR (C6D6; δ, ppm): 34.7 (NCMe3), −111.3ax, −117.5eq 1, −128.8eq 2 (N2-M), −137.1 [C(CH2CH3)NAr], −155.9ax, −155.9eq 2, −157.7eq 1 (N1-B). Anal. Calcd for C30H45N8BClNb (657.396): C, 54.81; H, 6.90; N, 17.12. Found: C, 54.65; H, 6.74; N 16.89. Compound 9b. IR (KBr; ν̅, cm−1): 3421 (m), 2969 (m), 2924 (m), 2551 (m), 1619 (m), 1547 (s), 1451 (s), 1416 (m), 1370 (m), 1253 (s), 1210 (s), 1072 (m), 1041 (m), 851 (m), 783 (m), 696 (w), 648 (w), 544 (w), 469 (w). 1H NMR (C6D6; δ, ppm): 6.81 (m, 1H), 6.62 [m, 2H, C(CH2CH3)NC6H3Me2], 5.61eq 1, 5.47eq 2, 5.39ax (m, 1H, C4H, Tp*), 4.60 (q, 1H, JB−H = 126 Hz, BH, Tp*), 3.37, 2.92, 1.71[ABX3, JAB = 16.4 Hz, JAX3 = JBX3 = 7 Hz, C(CH2CH3)NAr], 3.21eq 1, 2.33ax, 2.08eq 2 (m, 3H, C3-Me, Tp*), 2.12ax, 2.10eq 2, 2.03eq 1 (m, 3H, C5-Me, Tp*), 2.11 (s, 3H), 0.70 [s, 3H, C(CH2CH3)NC6H3Me2], 1.42 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 253.8 [C(CH2CH3)NAr], 153.3eq 1, 152.2eq 2, 150.7ax (C3-Me, Tp*), 144.6eq 2, 143.1eq 1, 142.7ax (C5-Me, Tp*), 143.9 (Ci), 133.7, 129.7 (Co), 128.1, 127.8 (Cm), 125.9 [Cp, C(CH2CH3)NC6H3Me2], 107.6eq 2, 107.3ax, 107.1eq 1 (C4-H, Tp*), 65.7 (NCMe3), 33.3 (NCMe3 ), 30.1 [C(CH2CH3)NAr], 19.5, 15.2 [C(CH 2CH3)NC6H3Me2], 18.6eq 1, 16.2eq 2, 15.3ax (C3-Me, Tp*), 13.0ax, 12.8eq 1, 12.6eq 2 (C5-Me, Tp*), 10.3 [C(CH2CH3)NAr]. 15N NMR (C6D6; δ, ppm): 3.8 (NbNCMe3), −112.6ax, −117.2eq 2, −128.4eq 1 (N2-M), −125.1 [C(CH2CH3)NAr], −156.5eq 2, −156.6ax, −157.9eq 1 (N1-B). Anal. Calcd for C30H45N8BClTa (745.438): C, 48.33; H, 6.09; N, 15.10. Found: C, 48.05; H, 5.84; N, 15.20. Compound 10a. IR (CsI; ν,̅ cm−1): 3427 (s), 2966 (s), 2921 (s), 2528 (m), 1630 (m), 1544 (s), 1449 (s), 1415 (s), 1370 (m), 1229 (s), 1210 (s), 1072 (s), 1042 (s), 812 (m), 772 (m), 697 (m), 546 (w), 499 (w). 1H NMR (C6D6; δ, ppm): 8.14 (m, 2H, Ho), 7.30 (m, 2H, Hm), 7.11 [m, 1H, Hp, C(CH2C6H5)NAr], 6.80 (m, 1H, Hm), 6.60 (m, 1H, Hm), 7.10 [m, 1H, Hp, C(CH2Ph)NC6H3Me2], 5.65eq 1, 5.48eq 2, 5.40ax (m, 1H, C4-H, Tp*), 4.80, 4.10[AB, 2H, JAB = 15.3 Hz, C(CH2Ph)NAr], 4.70 (q, 1H, JB−H = 109 Hz, BH, Tp*), 3.20eq 1, 2.33ax, 1.99eq 2 (m, 3H, C3-Me, Tp*), 2.17ax, 2.15eq 2, 2.09eq 1 (m, 3H, C5-Me, Tp*), 1.85 (s, 3H), 0.80 [s, 3H, C(CH2Ph)NC6H3Me2], 1.30 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 233.8 [C(CH2Ph)NAr], 152.7eq 1, 151.6eq 2, 151.1ax (C3-Me, Tp*), 144.8eq 2, 143.3eq 1, 142.7ax (C5-Me, Tp*), 142.8 (Ci), 133.9, 131.3 (Co), 128.1, 128.2 (Cm), 126.7 [Cp, C(CH2Ph)NC6H3Me2], 130.9−126.6[C(CH2C6H5)NAr], 107.3eq 1, 106.9ax, 106.8eq 2 (C4-H, Tp*), 68.2(NCMe3), 41.6 [C(CH2Ph)NAr], 31.5(NCMe3), 20.5, 15.5 [C(CH2Ph)NC6H3Me2], 18.3eq 1, 15.8eq 2, 15.2ax(C3-Me, Tp*), 13.2eq 1, 12.9eq 2, 12.6ax (C5-Me, Tp*). 15N NMR (C6D6; δ, ppm): 37.7 (NCMe3), −111ax, −118eq 1, −128.9eq 2 (N2-M), −129.3[C(CH2Ph)NAr], −155.9eq 1, −156.5ax, −157.2eq 2 (N1-B). Anal. Calcd for C35H47N8BClNb (719.466): C, 58.43; H, 6.58; N, 15.64. Found: C, 58.50; H, 6.29; N, 15.27. Compound 10b. IR (CsI, ν̅ cm−1): 3415 (m), 2963 (s), 2919 (s), 2547 (m), 1611 (m), 1546 (s), 1448 (s), 1415(f), 1369 (m), 1252 (s), 1209 (s), 1068 (s), 1042 (s), 814 (m), 774 (m), 694 (m), 676 (m), 546 (w), 462 (w). 1H NMR (C6D6; δ, ppm): 8.22 (m, 2H, Ho), 7.36 (m, 2H, Hm), 6.83 [m, 1H, Hp, C(CH2C6H5)NAr], 7.10 (m, 1H, Hp), 6.82 (m, 1H, Hm), 6.64 [m, 1H, Hm, C(CH2Ph)NC6H3Me2], 5.62eq 2, 5.47eq 1, 5.40ax (m, 1H, C4-H, Tp*), 4.59, 4.25[AB, 2H, JAB = 15.5 Hz, C(CH2Ph)NAr], 4.40 (q, 1H, JB−H = 127 Hz, BH, Tp*), 3.24eq 1, 2.36ax, 2.08eq 2 (m, 3H, C3-Me, Tp*), 2.14ax, 2.10eq 2, 2.04eq 1 (m, 3H, C5-Me, Tp*), 1.96 (s, 3H), 0.80 [s, 3H, C(CH2Ph)NC6H3Me2], 1.36 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 249.9 [C(CH2Ph)NAr], 153.3eq 1, 152.3eq 2, 152.0ax (C3-Me, Tp*), 144.6eq 2, 142.9eq 1, 142.8ax (C5-Me, Tp*), 143.4 (Ci), 133.7, 131.2 (Co), 128.4, 128.2 (Cm), 126.6 [Cp, C(CH2Ph)NC6H3Me2], 131.3−126.6 [several phenyl, 2283


Organometallics

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−128.8eq 2 (N2-M), −156.7eq 2, −157.1ax, −157.4eq 1 (N1-B). Anal. Calcd for C38H53N8BClTa (849.59): C, 53.72; H, 6.29; N, 13.25. Found: C, 53.81; H, 6.18; N, 13.15. Compound 14a. IR (CsI; ν̅, cm−1): 3435 (s), 2962 (s), 2924 (s), 2528 (m), 1625 (m), 1545 (s), 1449 (s), 1417 (s), 1372 (m), 1235 (s), 1210 (s), 1070 (m), 1038 (m), 812 (s), 776 (m), 649 (w), 544 (w), 452(w). 1H NMR (C6D6; δ, ppm): 6.90 (m, 2H, Hm), 6.70 [m, 1H, C(Me)NC6H3Me2], 5.73eq 1, 5.55eq 2, 5.43ax (m, 1H, C4-H, Tp*), 4.70 (q, 1H, JB−H = 127 Hz, BH, Tp*), 3.02eq 1, 2.15eq 2, 1.93ax (s, 3H, C3-Me, Tp*), 2.59 [s, 3H, C(Me)NAr], 2.19eq 2, 2.18ax, 2.14eq 1 (s, 3H, C5-Me, Tp*), 2.02 (s, 3H), 0.89 [s, 3H, C(Me)NC6H3Me2], 1.37 (s, 9H, NCMe3), 1.03 (s, 3H, NbMe). 13C{1H} NMR (C6D6; δ, ppm): 238.5 [C(Me)NAr], 151.3eq 1, 151.1eq 2, 149.6ax (C3-Me, Tp*), 144.0eq 2, 143.4eq 1, 142.4ax (C5-Me, Tp*), 143.2 (Ci), 134.3, 129.5 (Co), 128.1, 128.6 (Cm), 125.8 [Cp, C(Me)NC6H3Me2], 106.9eq 1, 106.8eq 2, 106.2ax (C4-H, Tp*), 65.7 (NCMe3), 32.8 (NCMe3), 21.6 [C(Me)NAr], 19.7, 15.8 [C(Me)NC6H3Me2], 17.7eq 1, 15.4eq 2, 14.6ax (C5-Me, Tp*), 14.4 (NbMe), 12.9eq 1, 12.6ax, 12.5eq 2 (C3-Me, Tp*). 15 N NMR (C6D6; δ, ppm): 14.3 (NCMe3), −107.9ax, −114.8eq 1, −123.1eq 2 (N2-M), −127.9 [C(Me)NAr], −156.1eq 2, −157.5ax, −157.5eq 1 (N1-B). Anal. Calcd for C30H46N8BNb (622.951): C, 57.84; H, 7.44; N, 18.06. Found: C, 57.73; H, 7.25; N, 18.12. Compound 14b. IR (CsI; ν̅, cm−1): 3438 (s), 2959 (s), 2923 (s), 2532 (m), 1601 (m), 1545 (m), 1447 (s), 1417 (s), 1375 (s), 1257 (s), 1210 (s), 1067 (m), 1038 (m), 812 (s), 779 (m), 649 (w), 542 (w), 459 (w). 1H NMR (C6D6; δ, ppm): 6.90 (m, 1H, Hm), 6.85 (m, 1H, Hp), 6.70 [m, 1H, Hm, C(Me)NC6H3Me2], 5.68eq 1, 5.51eq 2, 5.45ax (m, 1H, C4-H, Tp*), 4.70 (q, 1H, JB−H = 99.7 Hz, BH, Tp*), 3.02eq 1, 2.21eq 2, 1.93ax (s, 3H, C3-Me, Tp*), 2.65 [s, 3H, C(Me)NAr], 2.16eq 1, 2.14ax, 2.08eq 2 (s, 3H, C5-Me, Tp*), 2.06 (s, 3H), 0.81 [s, 3H, C(Me)NC6H3Me2], 1.45 (s, 9H, NCMe3), 0.88 (s, 3H, TaMe). 13 C{1H} NMR (C6D6; δ, ppm): 252.1 [C(Me)NAr], 151.7eq 2, 151.1eq 1, 150.1ax (C3-Me, Tp*), 143.8eq 2, 143.4eq 1, 142.3ax (C5-Me, Tp*), 141.7 (Ci), 133.9, 129.6 (Co), 128.4, 128.1 (Cm), 125.8 [Cp, C(Me)NC6H3Me2], 106.9ax, 106.8eq 1, 106.4eq 2 (C4-H, Tp*), 64.9 (NCMe3), 33.7 (NCMe3), 23 [C(Me)NAr], 21.1(TaMe), 20.1, 15.3 [C(Me)NC6H3Me2], 18.0eq 1, 15.6eq 2, 14.9ax (C5-Me, Tp*), 12.7ax, 12.4eq 2, 12.3eq 1 (C3-Me, Tp*). 15N NMR (C6D6; δ, ppm): −10.7 (NCMe3), −110.9ax, −117.2eq 1, −122.4eq 2 (N2-M), −119.8 [C(Me) NAr], −154.1eq 2, −157.0ax, −157.1eq 1 (N1-M). Anal. Calcd for C30H46N8BTa (710.993): C, 50.68; H, 6.52; N, 15.83. Found: C, 50.59; H, 6.43; N, 15.89. Crystal Structure Determination for Compound 10b. Suitable monocrystals of compound 10b were obtained by slow crystallization from a benzene-saturated solution at −30 °C. A summary of crystal data, data collection, and refinement parameters for the structural analysis is given in Table SI-2 (Supporting Information). Crystals were glued to a glass fiber using inert polyfluorinated oil and mounted in the N2 stream in a Bruker-Nonius Kappa-CCD 4 diffractometer with area detector, and data were collected using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at low temperature (200 K). Data for compound 10b with an exposure time of 10 s per frame (three sets; 145 frames; ψ/ω scans 2.0° scan width). Raw data were corrected for Lorentz and polarization effects. The structure was solved by direct methods, completed by subsequent difference Fourier techniques, and refined by full-matrix least squares on F2 (SHELXL-97).31 Absorption correction procedures were carried out using the multiscan SORTAV program.32 Anisotropic thermal parameters were used in the last cycles of refinement for the non-hydrogen atoms. Hydrogen atoms were included from geometrical calculations and refined using a riding model in most of the cases; H1, H2a, and H2b were located in the Fourier difference maps and refined with anisotropic thermal parameters. All calculations were carried out using the WINGX system.33 Computational Details. Electronic structure calculations were carried out using the B3LYP34 density functional with the Stuttgart− Dresden MWB28 (Nb) or MWB60 (Ta) effective core potential and basis35 including a set of additional f functions for the metals and the triple-ζ quality basis set 6-311G(d,p) for other elements as implemented in the Gaussian 09 suite of programs.36 Minimum

−157.9eq 2 (N1-B). Anal. Calcd for C32H51N8BSiClNb (715.552): C, 53.71; H, 7.18; N, 15.73. Found: C, 53.51; H, 6.90; N, 15.54. Compound 12b. IR (CsI; ν,̅ cm−1): 3431 (m), 2958 (s), 2922 (s), 2551 (m), 1598 (m), 1547 (s), 1450 (s), 1416 (s), 1369 (m), 1250 (s), 1211 (s), 1070 (m), 1040 (m), 851 (s), 777 (m), 691 (w), 647(d), 562 (w), 466 (w). 1H NMR (C6D6; δ, ppm): 6.90 (m, 1H), 6.84 (m, 1H), 6.64 [m, 1H, C(CH2SiMe3)NC6H3Me2], 5.60ax, 5.51eq 2, 5.40eq 1 (m, 1H, C4-H, Tp*), 4.50 (q, 1H, JB−H = 130 Hz, BH, Tp*), 3.23eq 1, 2.46ax, 2.15eq 2 (m, 3H, C3-Me, Tp*), 2.70av [AB, 2H, JAB = 14.6 Hz, C(CH2SiMe3)NAr], 2.17 (s, 3H), 0.73 [s, 3H, C(CH2SiMe3)NC6H3Me2], 2.13ax, 2.11eq 2, 2.01eq 1 (m, 3H, C5-Me, Tp*), 1.46 (s, 9H, NCMe3), 0.53 [s, 9H, C(CH2SiMe3)NAr]. 13C{1H} NMR (C6D6; δ, ppm): 249.1 [C(CH2SiMe3)NAr], 153.4eq 1, 152.5eq 2, 152.1ax (C3Me, Tp*), 144.5eq 2, 143.1eq 1, 142.9ax (C5-Me, Tp*), 144.3 (Ci), 133.7, 131.5 (Co), 128.9, 128.4 (Cm), 126.6 [Cp, C(CH2SiMe3)NC6H3Me2], 107.6eq 1, 107.4ax, 107.2eq 2 (C4-H, Tp*), 65.9 (NCMe3), 50.9 [C(CH2CMe3)NAr], 33.2 (NCMe3), 28.7 [C(CH2SiMe3)NAr], 20.2, 15.6 [C(CH2SiMe3)NC6H3Me2], 18.9eq 1, 16.8eq 2, 16.5ax(C3-Me, Tp*), 13.0eq 1, 12.9eq 2, 12.6ax (C5-Me, Tp*), 1.49 [C(CH2SiMe3)NAr]. 15N NMR (C6D6; δ, ppm): 2.8 (NCMe3), −112.6ax, −117.5eq 1, −128.5eq 2 (N2-M), −120.3 [C(CH2SiMe3)NAr], −156.8eq 2, −156.8ax, −157.5eq 1 (N1-B). Anal. Calcd for C32H51N8BSiClTa (803.594): C, 47.83; H, 6.40; N, 14.00. Found: C, 47.58; H, 6.35; N, 13.76. Compound 13a. IR (CsI; ν,̅ cm−1): 3448 (m), 2964 (s), 2922 (s), 2550 (w), 1611 (m), 1546 (s), 1448 (s), 1415 (s), 1368 (m), 1225 (s), 1211 (s), 1069 (m), 1039 (m), 816 (s), 782 (m), 698 (m), 648 (w), 570 (w), 543 (w), 466 (w). 1H NMR (C6D6; δ, ppm): 7.41 (m, 2H, Ho), 7.00 (m, 2H, Hm), 6.7 [m, 1H, Hp, C(CH2CMe2C6H5)NAr], 7.10 (m, 1H, Hp), 6.70 (m, 1H, Hm), 6.60 [m, 1H, Hm, C(CH2CMe2Ph)NC6H3Me2], 5.62eq 1, 5.45eq 2, 5.40ax (m, 1H, C4-H, Tp*), 4.66 (q, 1H, JB−H = 119 Hz, BH, Tp*), 3.37, 2.97 [AB, 2H, JAB = 18 Hz, C(CH2CMe2Ph)NAr], 3.25eq 1, 2.42ax, 2.06eq 2 (m, 3H, C3-Me, Tp*), 2.50 (s, 3H), 2.23 [s, 3H, C(CH2CMe2Ph)NAr], 2.18ax, 2.13eq 1, 2.06eq 2 (m, 3H, C5-Me, Tp*), 2.11 (s, 3H), 0.72 [s, 3H, C(CH2CMe2Ph)NC6H3Me2], 1.45 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 234 [C(CH2CMe2Ph)NAr], 152.9−125.4 [several phenyl, C(CH2CMe2C6H5)NAr], 152.0eq 1, 150.9eq 2, 150.6ax (C3-Me, Tp*), 144.1eq 2, 142.5eq 1, 142.3ax (C5-Me, Tp*), 142.4 (Ci), 133.5, 129.2 (Co), 128.1, 127.9 (Cm), 126.4 [Cp, C(CH2CMe2Ph)NC 6 H 3 Me 2 ], 107.4 eq 1 , 106.8 eq 2 , 106.8 ax (C 4 -H, Tp*), 67.8 (NCMe3), 50.7 [C(CH2CMe2Ph)NAr], 38.3 [C(CH2CMe2Ph)NAr], 31.4 (NCMe3), 30.7, 28.3 [C(CH2CMe2Ph)NAr], 20.7, 15.2 [C(CH2CMe2Ph)NC6H3Me2], 18.4eq 1, 16.3eq 2, 16.2ax (C3-Me, Tp*), 13.2eq 1, 13.0eq 2, 12.6ax (C5-Me, Tp*). 15N NMR (C6D6; δ, ppm): 36.7 (NCMe3), −111.8ax, −116.9eq 1, −128.5eq 2 (N2-M), −120.9 [C(CH2CMe2Ph)NAr], −156.6eq 2, −157.2ax, −157.4eq 1 (N1-B). Anal. Calcd for C38H53N8BClNb (761.546): C, 59.93; H, 7.02; N, 14.78. Found: C, 59.88; H, 7.19; N, 14.55. Compound 13b. IR (CsI; ν,̅ cm−1): 3444 (m), 2962 (s), 2923 (s), 2552 (w), 1582 (m), 1548 (s), 1449 (s), 1415 (s), 1368 (m), 1252 (s), 1213 (s), 1070 (m), 1040 (m), 818 (m), 782 (s), 698 (m), 648 (w), 558 (w), 537 (w), 462 (w). 1H NMR (C6D6; δ, ppm): 7.41 (m, 2H, Ho), 7.00 (m, 2H, Hm), 6.70 [m, 1H, Hp, C(CH2CMe2C6H5)NAr], 7.10 (m, 1H, Hp ), 6.70 (m, 1H, H m), 6.50 [m, 1H, Hm, C(CH2CMe2Ph)NC6H3Me2], 5.60eq 1, 5.43eq 2, 5.42ax (m, 1H, C4-H, Tp*), 4.63 (q, 1H, JB−H = 119 Hz, BH, Tp*), 3.54, 2.90 [AB, 2H, JAB = 18 Hz, C(CH2CMe2Ph)NAr], 3.25eq 1, 2.43ax, 2.15eq 2 (m, 3H, C3-Me, Tp*), 2.50 (s, 3H), 2.21 [s, 3H, C(CH2CMe2Ph)NAr], 2.15ax, 2.09eq 2, 2.01eq 1 (m, 3H, C5-Me, Tp*), 2.11 (s, 3H), 0.72 [s, 3H, C(CH2CMe2Ph)NC6H3Me2], 1.52 (s, 9H, NCMe3). 13C{1H} NMR (C6D6; δ, ppm): 249.7 [C(CH2CMe2Ph)NAr], 152.9eq 1, 152.1eq 2, 152.1 ax (C 3 -Me, Tp*), 152.6−125.6 [several phenyl, C(CH2CMe2C6H5)NAr], 143.7 (Ci), 132.8, 129.7 (Co), 128.2, 127.9 (Cm), 126.5 [Cp, C(CH2CMe2Ph)NC6H3Me2], 143.5ax, 143.5eq 2, 142.6eq 1 (C5-Me, Tp*), 107.4eq 1, 106.9eq 2, 106.9ax (C4-H, Tp*), 66.2 (NCMe3), 51.4 [C(CH2CMe2Ph)NAr], 39.5 [C(CH2CMe2Ph)NAr], 32.6 (NCMe3), 30.9, 28.3 [C(CH2CMe2Ph)NAr], 20.7, 15.2 [C(CH2CMe2Ph)NC6H3Me2], 18.5eq 1, 16.4eq 2, 16.3ax (C3-Me, Tp*), 12.8eq 1, 12.6eq 2, 12.3ax (C5-Me, Tp*). 15N NMR (C6D6; δ, ppm): 5.2 (TaNCMe3), −110.9 [C(CH2CMe2Ph)NAr], −112.8ax, −117.4eq 1, 2284


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energy and transition state structures were optimized by computing analytical energy gradients. The obtained stationary points were characterized by performing energy second derivatives, confirming them as minima or first-order saddle points by the number of negative eigenvalues of the Hessian matrix of the energy (zero and one negative eigenvalues, respectively). The basis set superposition error (BSSE) was corrected through counterpoise calculations.37 The charge distribution in the complexes was obtained using the natural bond orbital (NBO) population partition analysis of Reed and Weinhold38 using the NBO program39 implemented in the Gaussian 09 package.36

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

1 H NMR spectrum (500 MHz) of 9a in C6D6 at 25 °C (Figure SI-1), 13C{1H} NMR spectrum (125 MHz) of 9a in C6D6 at 25 °C (Figure SI-2), 1H/13C ASAPHMQC spectrum of 9a in C6D6 at 25 °C (Figure SI-3), 1H/13C gs2hmbc spectrum of 10b in C6D6 at 25 °C (Figure SI-4), 1H/15N ghmbcAD spectrum of 13b in C6D6 at 25 °C (Figure SI-5), 2D-ROESYAD spectrum (mixN=400 ms) of 11b in C6D6 at 25 °C (Figure SI-6), 2DNOESY (mixN=600 ms) of 12a in C6D6 at 25 °C (Figure SI7), and NOESY-1d (mixN=800 ms) of 12a in C6D6 at 25 °C (Figure SI-8), selected bond lengths (Å) calculated by DFT (Table SI-1) and X-ray crystal data, data collection, and refinement parameters for the structural analysis of the complex 10b (Table SI-2, Archive SI Data), an xyz file giving XYZ coordinates for all computed species, and a CIF file giving atomic coordinates and equivalent isotropic displacement parameters, complete bond distances and angles, anisotropic displacement parameters, and hydrogen atom coordinates and isotropic parameters for 10b. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*M.G.: fax, 34-91-885.46.83; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to Prof. Obis Castaño (UAH) on the occasion of his 70th birthday. We thank the MICINN (CTQ2006-04540, CTQ2012-36966, CSD2006-00015) and the Universidad de Alcalá (GC2010-002) for their financial support of this research. C.G. is grateful to the Universidad de Alcalá for a fellowship that made this research possible. The authors are grateful to Prof. Obis Castaño (UAH) for hepful discussions and technical support.

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REFERENCES

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Organometallics

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Synthesis and DFT, Multinuclear Magnetic Resonance, and X-ray

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