399 Heteroatom Chemistry Volume 9, Number 4, 1998 Boron Heterocycles Derived from 2- Guanidinobenzimidazole Noemı ´ Andrade-Lo ´ pez, 1 Rocı ´o Cartas-Rosado, 1 Efre ´n Garcı´a-Bae ´z, 1 Rosalinda Contreras 1 and Hugo Tlahuext 2 1 Departamentode Quı´mica, Centro de Investigacio ´n y de Estudios Avanzados del IPN, Apartado Postal 14-740, Me ´xico D.F. 07000, Me ´xico 2 Facultad de Ciencias, Universidad Auto ´noma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, Cuernavaca Morelos, Me ´xico Received 6 January 1998; revised 4 February 1998 ABSTRACT: The syntheses and structure determi- nations of a series of boron heterocycles derived from 2-guanidinobenzimidazole 1 are reported. Structures of new compounds, 2-guanidino-1-methyl-benzimi- dazole 2, diphenyl-(2-guanidinobenzimidazole-N,N')- borate 3, diphenyl-(2-guanidino-1-methyl-benzimida- zole-N,N')borate 4, hydroxy-phenyl-(2-guanidino- benzimidazole-N,N')borate 5, hydroxy-phenyl-(2- guanidino-1-methyl-benzimidazole-N,N')borate 6, methoxy-phenyl-(2-guanidinobenzimidazole-N,N')bo- rate 7, isopropoxy-phenyl-(2-guanidinobenzimida- zole-N,N')borate 8, acetoxy-phenyl-(2-guanidinoben- zimidazole-N,N')borate 9, methoxy-phenyl-(2-guani- dino-1-methyl-benzimidazole-N,N')borate 10, dihy- droxy - (2 - guanidino - 1 -methyl - benzimidazole-N, N' ) borate 16, difluoro-(2-guanidinobenzimidazole- N,N')borate, 17, dihydroxy-(2-guanidino-1-benzimi- dazole-N,N')borate potassium salt 19, diphenyl-(2- guanidinium-10H-benzimidazole-N,N')borate hydro- chloride 20, methoxy-phenyl-(2-guanidinobenzi- midazole-N,N')borate hydrochloride 21, and N10-bo- rane-(diphenyl - 2 -guanidinobenzimidazole- N,N')bor- ate 22, were determined based on 1 H, 13 C, 15 N, and 11 B spectroscopy. The X-ray diffraction structures of 3–7, 19, and 20 were obtained. The formation of N3-borane Dedicated to Professor Heinrich No ¨ th on the occasion of his seventieth birthday. Correspondence to: Rosalinda Contreras.  1998 John Wiley & Sons, Inc. CCC 1042-7163/98/040399-11 adducts 11 and 12 derived from compounds 1 and 2, respectively, and the dihydride-(2-guanidinobenzimi- dazole-N,N')borate 13 and dihydride-(2-guanidino-1- methyl-benzimidazole-N,N')borate 14 were observed by 11 B NMR. The results show that 2-guanidinoben- zimidazole gives stable borate heterocycles with a de- localized p electronic system. A dynamic exchange of N–H protons was observed with preferred protonation at N-12. The new heterocycles are protonated at N-10 by acidic substances to give pyridinium-type hetero- cycles or can lose a proton to give iminium salts.  1998 John Wiley & Sons, Inc. Heteroatom Chem 9:399–409, 1998 INTRODUCTION 2-Guanidinobenzimidazole 1 is a planar delocalized ligand formed with sp 2 nitrogen and carbon atoms that gives very stable metallic complexes by coordi- nation to the lone pair of an imidazolic nitrogen atom and substitution of one acidic N–H of a guan- idinic group [1,2]. It was of interest to investigate the ability of compound 1 to form diamagnetic com- plexes by coordination with acidic boron atoms such as boron difluoride, boron diphenyl, and boron hy- droxyphenyl, which are models of metallic coordi- nation easily studied by NMR techniques. Com-