Synthesis, Structure, and Reactivity of Zirconium and Hafnium Imido Metalloporphyrins

The zirconium and hafnium porphyrin imido complexes (TTP)M NAr i Pr [TTP = meso -tetra- p -tolylporphyrinato dianion, M = Zr ( 1 ), Hf ( 2 ), Ar i Pr = 2,6-diisopropylphenyl] were synthesized from (TTP)MCl 2 and 2 equiv of LiNHAr i Pr . The zirconium imido complex, (TTP)Zr NAr i Pr, was also obtained from the preformed imido complex Zr(NAr i Pr )Cl 2 (THF) 2 and (TTP)Li 2 (THF) 2 . Treatment of (TTP)HfCl 2 with excess LiNH( p -MeC 6 H 4 ) resulted in the formation of a bis(amido) complex, (TTP)Hf(NH- p -MeC 6 H 4 ) 2 ( 3 ), instead of an imido complex. In the presence of excess aniline, 2 formed an equilibrium mixture of bis(amido) compounds, (TTP)Hf(NHPh)(NHAr i Pr ) and (TTP)Hf(NHPh) 2 . The nucleophilic character of the imido moiety is exhibited by its reaction with t BuNCO, producing isolable N,O-bound ureato metallacycles. The kinetic product obtained with zirconium, (TTP)Zr(η 2 -NAr i Pr C( N t Bu)O) ( 4a ), isomerized to (TTP)Zr(η 2 -N t BuC( NAr i Pr )O) ( 4b) in solution. Upon being heated to 80 °C, 4a produced the carbodiimide Ar i Pr N C N t Bu and a transient Zr(IV) oxo complex. The analogous hafnium complex (TTP)Hf(η 2 -NAr i Pr C( N t Bu)O) ( 5a ) did not eject the carbodiimide upon heating to 110 °C but isomerized to (TTP)Hf(η 2 -N t BuC( NAr i Pr )O) ( 5b ). To support the formulation of 4a and 5a as N,O bound, the complex (TTP)Hf(η 2 -NAr i Pr C( NAr i Pr )O) ( 6 ) was studied by variable-temperature NMR spectroscopy. The corresponding thio- and selenoureato metallacycles were not isolable in the reaction between 1 and 2 with t BuNCS and t BuNCSe. Concomitant formation of the metallacycle with decomposition to the carbodiimide, Ar i Pr N C N t Bu, reflects the lower C−Ch bond strength in the proposed N,Ch-bound metallacycles. Treatment of 2 with 1,3-diisopropylcarbodiimide resulted in the η 2 -guanidino complex (TTP)Hf(η 2 -NAr i Pr C( N i Pr)N i Pr) ( 7a ), which isomerized to the less sterically crowded isomer (TTP)Hf(η 2 -N i PrC( NAr i Pr )N i Pr) ( 7b ). Complexes 1, 2, 4a, 4b, and 7a were characterized by X-ray crystallography. The monomeric terminal imido compounds, 1 and 2, are isomorphous: M−N imido distances of 1.863(2) Å (Zr) and 1.859(2) Å (Hf); M−N imido −C angles of 172.5(2)° (Zr) and 173.4(2)° (Hf). The structures of the ureato complexes 4a and 4b and the guanidino complex 7a exhibit typical alkoxido and amido bond distances (Zr−N = 2.1096(13) Å ( 4a ), 2.137(3) Å ( 4b ); Zr−O = 2.0677(12) Å ( 4a ), 2.066(3) Å ( 4b ); Hf−N = 2.087(2) Å, 2.151(2) Å ( 7a )).