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

Abstract

The dimethylamine in the adducts [(HNMe(2))B(azolyl)(3)] (azolyl=methimazolyl, pyrazolyl), obtained by reaction of the azole with B(NMe(2))(3), can readily be substituted with a range of nitrogen donors to provide new charge-neutral, tripodal ligands in high yield. This observation has led to a revision of an earlier interpretation of the mechanism of the formation of these species. The donor properties of the ligands [(nmi)B(azolyl)(3)] (nmi=N-methylimidazole) have been compared with their anionic analogues [HB(azolyl)(3)](-) by synthesis of their manganese(I)-tricarbonyl complexes and comparison of their infrared nu(CO) energies. This comparison indicates that the new neutral ligands are only marginally weaker donors than the corresponding anionic hydrotris(azolyl)borate ligands. This may be explained by the ability of the attached nmi ring to stabilize a positive charge remotely from the coordinated metal, which may also account for the fact that the [(nmi)B(pyrazolyl)(3)] ligand is a substantially stronger donor than the similarly neutral tris(pyrazolyl)methane ligand.

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

“…6.65 ppm in the free ligand 3 and at 7.27/7.82 ppm in the 'mixed-sandwich' complex [{κ 3 -[S,S,S]-(N-MeImid)ZTm}Ru(II){p-cymene}] (5). 17 17 In this spectrum the NCH 3 carbon resonance appears at 86.9 ppm, which is dramatically shifted from the same resonance in [{κ 3 -[S,S,S]-(N-MeImid)ZTm}Ru(II)-{p-cymene}] (5). (30.4 ppm).…”

“…These data correspond well with previous studies of the donor properties of ZTm, Tm and Tp in manganese tricarbonyl complexes using IR-spectroscopy which found Tm and ZTm to be stronger electron donors than Tp. 17 Chen recently reported the synthesis of a homoleptic Ru(III) complex with two tris(carbene)borate ligands which was found to be more readily reduced (−630 mV) than 4 (−962 mV). This indicates that the thiolate donors in 4 are even stronger electron donors than N-heterocyclic carbene ligands.…”

“…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.…”

Synthesis and structural characterization of bis and tris(2-mercapto-1-methylbenzimidazolyl)hydroborato complexes: benzannulation promotes κ3-coordination

“…2), [7][8][9][10][11][12][13][14][15] and (iii) the synthesis of neutral L 3 counterparts. 16 Bis(mercaptoimidazolyl)hydroborato ligands, [Bm R ], 17,18 provide [S 2 ] donor arrays that have likewise found much use. 19,20 An important feature of both the [Bm R ] and [Tm R ] systems is that the properties of the ligand can be readily tailored by variation of the 1-R substituent.…”

“Twisted” scorpionates: synthesis of a tris(2-pyridonyl)borate (Thp) ligand; lessons in the requirements for successful B(L2D)3 type ligands

“…The two alternative hybridisation frameworks providing C 3 -symmetric complexes. X = sp2 hybridised atom, Y = sp 3 hybridised atom.Following the methodology we have developed for the construction of charge-neutral analogues of tripodal borate ligands bearing a neutral donor in place of the usual hydride in Tp and Tm,8,9 (HNMe 2 )tris(2pyridonyl)borate [(HNMe 2 )Thp] (1) was obtained from the reaction of 2-hydroxypyridine with B(NMe) 3 in toluene under reflux. Evolution of HNMe 2 ceased after 3 h and 1 H NMR spectroscopy showed no evidence of starting materials.…”