Barriers to Racemization in C₃-Symmetric Complexes Containing the Hydrotris(2-mercapto-1-ethylimidazolyl)borate (Tm^Et) Ligand

Abstract

The tripodal ligands hydrotris(N-ethyl-2-mercaptoimidazol-1-yl)borate (NaTm(Et)) (1) and hydrotris(N-benzyl-2-mercaptoimidazol-1-yl)borate (NaTm(Bn)) (2), analogues of the hydrotris(N-methyl-2-mercaptoimidazol-1-yl)borate ligand (Tm) containing alternative nitrogen substituents, have been employed to examine the racemization of their C3-symmetric complexes with both four- and six-coordinate metals. The ligands react at room temperature with metal halides to provide C3-symmetric metal complexes. The syntheses of the four-coordinate complexes [Tm(Et)ZnCl] (3), [Tm(Et)CdBr] (4), [Tm(Et)HgCl] (5), [Tm(Et)CuPPh3] (6), [Tm(Et)AgPPh3] (7), and [Tm(Bn)ZnCl] (8) are reported. The six-coordinate complexes [Tm(Et)Ru(p-cymene)]Cl (9), [Tm(Et)Ru(p-cymene)]PF(6) (10), and [Tm(Et)Mn(CO)3] (11) were also synthesized. The X-ray crystal structures of 3, 4, 6, and 9 are reported. The diastereotopic nature of the ethyl and benzyl hydrogen atoms in the ligands allows the enantiomeric forms of these complexes to be distinguished by 1H NMR spectroscopy. Variable-temperature (VT) 1H NMR spectra have thus been used to investigate the energies of the racemization processes occurring in these chiral complexes. In solvents the activation energies to racemization for the four-coordinate complexes lay in the range of 53-77 kJ mol(-1). In non-donor solvents the energies are reduced and a dissociative mechanism is therefore implicated. No interconversion could be observed by VT NMR for the six-coordinate complexes in any solvent. To further explore the racemization mechanisms ab initio density functional theory calculations have been conducted on the ground- and transition-state structures of representative six-coordinate [Mn(I)] and four-coordinate [Zn(II)] complexes following a proposed nondissociative mechanism of racemization. The calculated energy barriers to racemization are 163 and 121 kJ mol(-1), respectively. It is concluded that the low-energy racemization of substitution-labile four-coordinate complexes occurs via a dissociative mechanism, while substitution-inert six-coordinate complexes experience a significantly higher barrier to racemization. Whether this is due to the operation of a dissociative mechanism with a higher activation barrier or to a nondissociative mechanism remains unknown.

A New Synthesis of Charge‐Neutral Tris‐Pyrazolyl and ‐Methimazolyl Borate Ligands

“…[6] The 148 range found for the C-SRu angles thus appears to be exceptionally large in 8 and it is a further illustration of the substantial flexibility of the M À S coordination geometry in these ligands, which we have discussed previously. [6] Tris(pyrazolyl)borate ligands: The possibility that this synthetic method may be employed for the preparation of analogous tris(pyrazolyl)borate ligands has also been explored. The substantial literature concerning the chemistry and applications of tris(pyrazolyl)borate ligands and their various derivatives attests to their significance in coordination chemistry, catalysis, and a variety of other fields.…”

“…Although this might be interpreted as weaker S À Mn interactions in 12, it has previously been noted that MÀS bond lengths in Tm complexes may vary substantially in closely related systems. [6] This is best illustrated by the fact that in the two independent molecules present in the crystal of 14, the mean MnÀS bond lengths are 2.4015 and 2.4277 (ESD as above), a difference of 0.0262 . Similarly, in the complexes [(Tm)CuA C H T U N G T R E N N U N G (PAr 3 )] (Ar = Ph, m-tolyl, p-tolyl), the mean Cu À S bond lengths differ by 0.055 , a variation that is not correlated with the steric bulk or donor properties of the phosphine ligands.…”

“…The structures of many Tm complexes are constrained by crystallographically imposed threefold symmetry, and in these all three such angles are thus equivalent; however, this is not possible when the boron-bound hydride in Tm is replaced by a donor such as (8). PF 6 À counterions not shown. Selected bond lengths and angles are provided in Table 2.…”

“…[6] Our interest in directing this chirality, with a view to exploiting Tm complexes in asymmetric catalysis, prompted our exploration of routes to tris(methimazolyl)borate ligands that will allow the introduction of chiral groups in place of the methimazolyl N-methyl groups. However, we have found that although reaction of 2-mercapto-1-benzylimidazole with tetrahydroborate salts successfully provides the corresponding Tm Bn ligand in a melt reaction, [7] the chiral 2-mercapto-1-(S)-a-methylbenzylimidazole does not undergo a similar reaction, a result that we must attribute to the increased steric bulk that results from the introduction of the a-methyl group.…”

Metallaboratranes Derived from a Triphosphanyl–Borane: Intrinsic C3 Symmetry Supported by a Z‐Type Ligand

“…Further investigations are also in progress to probe the mechanism of inversion of such helical complexes. [28] Experimental Section All reactions and manipulations were carried out in an atmosphere of dry argon by using standard Schlenk techniques. 3: TPB 1 (55 mg, 0.094 mmol) was added to a solution of [Pt(PtBu 3 ) 2 ] (51.5 mg, 0.086 mmol) in toluene (5 mL).…”