Probing the factors that influence the conformation of a guanidinato ligand in [(η 5 -C 5 Me 5 )M( NN )X] ( NN = chelating N , N ′, N ′′-tri( o -substituted aryl)guanidinate(1−); X = chloro, azido and triazolato)

• Published in: New journal of chemistry 2018, Vol.42 (3), p.1853-1866
• Main Authors: Kumar, Robin, Kishan, Ram, Thomas, Jisha Mary, Chinnappan, Sivasankar, Thirupathi, Natesan
• Format: Article
• Language: English
• Subjects: Alkynes; Aromatic compounds; Chelation; Cycloaddition; Ligands; NMR spectroscopy; Single crystals; Solid state; Substitutes
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/C7NJ03766J

Three types of half sandwich complexes, namely, [(η 5 -Cp*)M( NN )X] (Cp* = C 5 Me 5 ; NN = chelating N , N ′, N ′′-tri( o -substituted aryl)guanidinate(1−) ligand; M = Rh/Ir; X = Cl ( 9–13 ), N 3 ( 14–18 ) and N 3 C 2 (C(O)OR) 2 ( 19–21 )), were prepared and isolated in moderate to good yields. The new complexes were fully characterized and further molecular structures of key complexes from each type were determined by single crystal X-ray diffraction (SCXRD). The new complexes revealed syn – syn ( 10 , 12 , 15 and 20) , anti – anti ( 13 and 16 ) and anti – syn ( 14 , 17 , 19 ·CHCl 3 and 21 ) conformations. DFT calculations were performed on four conformers of 14, which revealed the stability order as syn – syn (0.00 kcal mol −1 ) < anti – syn (0.23 kcal mol −1 ) ≪ syn – anti (3.15 kcal mol −1 ) ≪ anti – anti (5.15 kcal mol −1 ). An intra-ligand repulsive interaction between the o -substituent in two proximal aryl rings in guanidinate ligands of 14 and 17 was invoked as a driving force for the observance of the anti – syn conformer in the solid state, which is likely formed from a hypothetical syn – syn conformer via a guanidine centered rearrangement. The influence of packing forces upon the anti – syn conformation of the guanidinate ligand in 14 was also discussed. NBO analysis was carried out on the N 3 moiety of the syn – syn conformer of 14 in order to identify the likely resonance form responsible for [3+2] cycloaddition with the alkyne. Several new complexes that contain an o -substituted arylguanidinate ligand were shown to exist as a mixture of conformers in solution as revealed by NMR spectroscopy, and these conformers were shown to arise from a solid state conformer via guanidinine centered rearrangement.