Equilibria between α- and β-Agostic Stabilized Rotamers of Secondary Alkyl Niobium Complexes

Abstract

The isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhC&tbd1;CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Calpha-Nb-Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb-C bond allows the observation of an equilibrium between this species, 2beta, and a minor alpha-agostic rotamer 2alpha. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp'NbCl(i-Pr)(R(2)C=CMe) [Tp' = Tp(Me2), R(2) = Me (3); Tp' = Tp(Me2,4Cl), R(2) = Ph (4)]. The two diastereomers of the sec-butyl complex Tp(Me2)NbCl(sec-Bu)(MeC=CMe) (5) have been separated. In the crystal, 5CR-AS has a beta-agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single beta-agostic species is observed in solution. The other diastereomer, 5AR-CS has a beta-agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this beta-agostic methylene complex 5AR-CSbeta and a minor alpha-agostic species 5AR-CSalpha, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2beta/2alpha = 4.0 +/- 0.1 at 193 K, DeltaG(o)(193) = -2.2 +/- 0.1, DeltaH(o) = -7.4 +/- 0.1 kJ mol(-)(1), and DeltaS(o) = -27 +/- 1 J K(-)(1) mol(-)(1)), as well as kinetic parameters for the rotation about the Nb-C bond (at 193 K, DeltaG(2)= 47.5 +/- 2.5, DeltaH= 58.8 +/- 2.5 kJ mol(-)(1), and DeltaS = 59.0 +/- 10 J K(-)(1) mol(-)(1)). Upon selective deuteration of the beta-methyl protons in Tp(Me2)NbCl[CH(CD(3))(2)](PhC=CMe) (2-d(6)), an expected isotope effect that displaces the equilibrium toward the alpha-agostic rotamer is observed (K = 2-d(6)beta/2-d(6)alpha = 3.1 +/- 0.1 at 193 K, DeltaG(o)(193) = -1.8 +/- 0.1, DeltaH(o) = -8.3 +/- 0.4 kJ mol(-)(1) and DeltaS(o)= -34 +/- 2 J K(-)(1) mol(-)(1)). The anomalous values for DeltaH(o) and DeltaS(o) are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp(Me2)NbCl(i-Pr)(HC=CMe) have reproduced the energy differences between the alpha- and beta-agostic species with remarkable accuracy. Similar calculations show that Tp(Me2)NbCl(CH(2)Me)(HC=CMe) is alpha-agostic only and that Tp(5)(-)(Me)NbCl(CH(2)Me)(HC=CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is beta-agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a beta-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp(Me2). All of the se...

C–H and C–C agostic interactions in cycloalkyl tris(pyrazolyl)boratoniobium complexes

“…For the i-Pr species, one Me group occupies the wedge, and α and β agostic structures are observed when the remaing H and Me substituents, respectively, occupy the agostic site. In a later publication, 18 we extended these studies to the bulkier sec-Bu complex, and showed that similar principles could account for the more complex equilibria observed in this case. In particular, the presence of a chiral centre at the α carbon gave rise to two distinct diastereomers with very different agostic properties, the relative energies of which were again determined by the steric demands of the methyl groups.These fundamental studies have shed some light on some of the factors that control agostic bonding and, in particular, have highlighted the fact that the presence of unusual types of agostic bonding may be a result of steric factors which disfavour alternative structures rather than any intrinsically strong electronic driving force.…”

“…Nevertheless, the Nb(1)-C(1) bond of 2.159(3) Å is among the shortest we have ever seen in our series of either αor β-agostic alkyl complexes of niobium() -the corresponding value in the acyclic analogue, Tp Me2 NbCl(i-Pr)(PhC᎐ ᎐ ᎐ CMe) is 2.228(4) Å. 17,18 The Nb-Cl bond length in 2a is 2.4465(6) Å, somewhat shorter than that in Tp Me2 NbCl(i-Pr)(PhC᎐ ᎐ ᎐ CMe) (2.493(1) Å), consistent with the absence of a C-H agostic interaction. However, in previous papers we have emphasised the importance of the Cl-Nb-Cα bond angle as a structural indicator of agostic interactions, and the value of 110.85(8)Њ in 2a is still much greater than 90Њ, albeit not as large as the 122.…”

“…The 1 J CH is only slightly reduced compared to others within the cyclohexyl ring (127 Hz) giving evidence for a non-α-agostic NbCH. In the case of the isopropyl complex Tp Me2 NbCl-(i-Pr)(PhC᎐ ᎐ ᎐ CMe), 18 the α-carbon of the α-agostic rotamer resonates as a doublet centered at δ 126.4 with a 1 J CH of 100 Hz, whereas the α-carbon of the β-agostic rotamer resonates as a doublet centered at δ 72.0 with a 1 J CH of 141 Hz. Although definitive spectroscopic evidence for agostic interactions resides in a low 1 J CH , which is not observed in 4Ј, it is interesting to note a deshielded NbC Hα in the 13 C NMR spectra in our series of Closely related equilibria between αand β-agostic species have been observed in Tp Me2 NbCl(i-Pr)(PhC᎐ ᎐ ᎐ CMe) and Tp Me2 NbCl(sec-Bu)(MeC᎐ ᎐ ᎐ CMe) via coalescence techniques, and a similar barrier of 47.5 kJ mol Ϫ1 was measured for the former.…”

“…Although definitive spectroscopic evidence for agostic interactions resides in a low 1 J CH , which is not observed in 4Ј, it is interesting to note a deshielded NbC Hα in the 13 C NMR spectra in our series of Closely related equilibria between αand β-agostic species have been observed in Tp Me2 NbCl(i-Pr)(PhC᎐ ᎐ ᎐ CMe) and Tp Me2 NbCl(sec-Bu)(MeC᎐ ᎐ ᎐ CMe) via coalescence techniques, and a similar barrier of 47.5 kJ mol Ϫ1 was measured for the former. 18 Direct observation of the dynamics of agostic interactions in alkyl complexes has been largely confined to…”

Activation of propargylic alcohols by dimolybdenum tris(μ-thiolate) complexes: Influence of the substituents R in HCCCR2(OH)-vinylidene/allenylidene transformation. Reactivity of allenylidene complexes

“…The most that can be said is that the X-ray results are consistent with the chemical and spectroscopic evidence which indicates the formation of a [Mo 2 Cp 2 (lSMe) 3 (l-CHCH 2 CPh 2 H)] + cation containing an asymmetrically bridging l-C carbon atom [Mo-C 2.02(2) and 2.29(2) Å ], in agreement with an agostic interaction. It should be noted that the more common b-agostic structure [35] is not adopted by 15, perhaps for steric reasons.…”