An EPR and ENDOR Investigation of a Series of Diazabutadiene–Group 13 Complexes

Paramagnetic diazabutadienegallium( II or III ) complexes, [(Ar‐DAB) 2 Ga] and [{(Ar‐DAB . )GaX} 2 ] (X=Br or I; Ar‐DAB={N(Ar)C(H)} 2 , Ar=2,6‐diisopropylphenyl), have been prepared by reactions of an anionic gallium N‐heterocyclic carbene analogue, [K(tmeda)][:Ga(Ar‐DAB)], with either “GaI” or [MoBr 2 (CO) 2 (PPh 3 ) 2 ]. A related In III complex, [(Ar‐DAB . )InCl 2 (thf)], has also been prepared. These compounds were characterised by X‐ray crystallography and EPR/ENDOR spectroscopy. The EPR spectra of all metal( III ) complexes incorporating the Ar‐DAB ligand, [(Ar‐DAB . )MX 2 (thf) n ] (M=Al, Ga or In; X=Cl or I; n =0 or 1) and [(Ar‐DAB) 2 Ga], confirmed that the unpaired spin density is primarily ligand centred, with weak hyperfine couplings to Al ( a =2.85 G), Ga ( a =17—25 G) or In ( a =26.1 G) nuclei. Changing the N substituents of the diazabutadiene ligand to tert ‐butyl groups in the gallium complex, [( t Bu‐DAB . )GaI 2 ] ( t Bu‐DAB={N( t Bu)C(H)} 2 ), changes the unpaired electron spin distribution producing 1 H and 14 N couplings of 1.4 G and 8.62 G, while the aryl‐substituted complex, [(Ar‐DAB . )GaI 2 ], produces couplings of about 5.0 G. These variations were also manifested in the gallium couplings, namely a Ga ∼1.4 G for [( t Bu‐DAB . )GaI 2 ] and a Ga ∼25 G for [(Ar‐DAB . )GaI 2 ]. The EPR spectra of the gallium( II ) and indium( II ) diradical complexes, [{(Ar‐DAB . )GaBr} 2 ], [{(Ar‐DAB . )GaI} 2 ], [{( t Bu‐DAB . )GaI} 2 ] and [{(Ar‐DAB . )InCl} 2 ], revealed doublet ground states, indicating that the GaGa and InIn bonds prevent dipole–dipole coupling of the two unpaired electrons. The EPR spectrum of the previously reported complex, [(Ar‐BIAN . )GaI 2 ] (Ar‐BIAN=bis(2,6‐diisopropylphenylimino)acenaphthene) is also described. The hyperfine tensors for the imine protons, and the aryl and tert ‐butyl protons were obtained by ENDOR spectroscopy. In [(Ar‐DAB . )GaI 2 ], gallium hyperfine and quadrupolar couplings were detected for the first time.