Tripodal borate ligands from tris(dimethylamino)borane: the first synthesis of a chiral tris(methimazolyl)borate ligand, and the crystal structure of a single diastereomer pseudo-C₃-symmetric Ru(ii) complex

Abstract

The activation of tris(dimethylamino)borane towards reaction with a chiral methimazole by N-methylimidazole has been used to prepare the first example of a chiral tris(methimazolyl)borate ligand. Coordination of this neutral ligand to Ru(II) has been achieved by reaction with [(p-cymene)RuCl(2)](2) to provide a single diastereomer complex in which the chirality of the methimazolyl substituents dictate the chirality of the bicyclo[3.3.3]cage formed by the ligand on coordination to the metal. The alternative approach to chiral tris(methimazolyl)borate ligands involving the introduction of a chiral group onto the boron atom has been explored by replacing N-methylimidazole in the above reaction by chiral oxazolines as activating bases in reaction with simple methimazole. However, although the B(NMe(2))(3) is activated to reaction with methimazole by these oxazolines, an intramolecular oxazoline ring-opening by a coordinated methimazolyl sulfur occurs and prevents the successful synthesis of these ligands.

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

“…The resulting chloride salt of the complex was then treated with (8). The positive-ion fast-atom bombardment (FAB) mass spectrum of this complex shows an ion at m/z 501.9 that corresponds to [M] + /2; the 1 H and 13 C NMR spectra are consistent with the anticipated structure and show signals for the p-cymene, dbn, and methimazolyl components.…”

“…We have previously noted the flexibility of the C-S-M angles in Tm metal complexes manifest in the variability of these angles in different complexes. 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.…”

“…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. [8] An alternative, and possibly preferable, route for the introduction of chirality into the Tm ligand is to replace the remaining B À H hydride with a chiral group. Our initial approach to this goal involved the use of [(Ipc)BCl 2 ] (Ipc = isopinocampheyl) as the boron precursor, and although its reaction with pyrazolyl sodium successfully provided the [(Ipc)B(pz) 3 ]…”

Synthesis and Structural Characterization of the Tetrakis(methimazolyl)borates Li[Bmt4] and [Ru(p‐cymene)(Bmt4)][PF6]

“…Recently Bailey and coworkers have reported the synthesis of the ligands [B(Nmethylimidazolyl)(mt) 3 ] - [11,12]. Now we were able to prepare the lithium tetrakis(methimazolyl)borate Li [3] in the melt by reaction of one equivalent of Li [BH 4 ] with four equivalents of methimazole, as shown in Scheme 1.…”

Homoleptic ‘sandwich’ complexes of substituted tris(methimazolyl)borate ligands with ruthenium, rhodium and palladium

“…1). [13][14][15][16][17][18][19][20] In all of these complexes Tm coordinates in a κ 3 -[S,S,S] mode, although there is evidence for interconversion between the κ 3 -[S,S,S] and the κ 3 -[H,S,S] modes in solution. 21 Homoleptic 'sandwich' complexes of these ligands represent another commonly observed structural motif in which the ligand coordinates as a κ 3 -[S,S,S] tripod.…”