New Alkylimido Niobium Complexes Supported by Guanidinate Ligands: Synthesis, Characterization, and Migratory Insertion Reactions

A series of guanidine proligands, 2-(4-(tert-butyl)­phenyl)-1,3-diisopropylguanidine
(1), 2-(4-bromophenyl)-1,3-diisopropylguanidine (2), 2-(4-methoxyphenyl)-1,3-diisopropylguanidine (3), and 2,2′-(1,4-phenylene)­bis­(2′,3-diisopropylguanidine)
(4), has been reacted with [NbBz3(NtBu)] (5) through a protonolysis reaction to obtain new
monoguanidinate-supported dibenzyl niobium complexes, {NbBz2(NtBu)­[(4-tBuC6H4)­NC­(NiPr)­(NHiPr)]} (6), {NbBz2(NtBu)­[(4-BrC6H4)­NC­(NiPr)­(NHiPr)]} (7), {NbBz2(NtBu)­[(4-MeOC6H4)­NC­(NiPr)­(NHiPr)]} (8), and the dinuclear
complex {[NbBz2(NtBu)]2[(C6H4)­(NC­(NiPr)­(NHiPr))2]} (9). Complexes 6, 8, and 9 were structurally characterized. These neutral
complexes contain a η2-benzyl ligand coordinated
to the metal center. Insertion migratory reactions with isocyanides
resulted in the formation of bis-κ2-iminoacyl species,
{Nb­(NtBu)­(tBuNCCH2Ph)2[(4-tBuC6H4)­NC­(NiPr)­(NHiPr)]} (10), {NbBz2(NtBu) (tBuNCCH2Ph)2[(4-BrC6H4)­NC­(NiPr)­(NHiPr)]} (11), {Nb­(NtBu)­(tBuNCCH2Ph)2[(4-MeOC6H4)­NC­(NiPr)­(NHiPr)]} (12), and the dinuclear complex {[Nb­(NtBu)­(tBuNCCH2Ph)2]2[(C6H4)­(NC­(NiPr)­(NHiPr))2]} (13), when tBuNC was used. The analogous reaction using XyNC (Xy = 2,6-Me2C6H3) led to the formation of vinylamido
species, {Nb­(NtBu)­[N­(2,6-Me2C6H3)­CHCHPh]­[BzCN­(2,6-Me2C6H3)]­[(4-tBuC6H4)­NC­(NiPr)­(NHiPr)]} (14), {Nb­(NtBu)­[N­(2,6-Me2C6H3)­CHCHPh]­[BzCN­(2,6-Me2C6H3)]­[(4-BrC6H4)­NC­(NiPr)­(NHiPr)]} (15), {Nb­(NtBu)­[N­(2,6-Me2C6H3)­CHCHPh]­[BzCN­(2,6-Me2C6H3)]­[(4-MeOC6H4)­NC­(NiPr)­(NHiPr)]} (16), and {[Nb­(NtBu)­[N­(2,6-Me2C6H3)­CHCHPh]­[BzCN­(2,6-Me2C6H3)]]2[(C6H4)­[NC­(NiPr)­(NHiPr)]2]} (17), through a proposed 1,2-hydrogen shift mechanism
from an iminoacyl intermediate similar to those obtained from the
insertion of tBuNC. Complex 17 was structurally
characterized.