Binding N 2 , N 2 H 2 , N 2 H 4 , and NH 3 to Transition‐Metal Sulfur Sites: Modeling Potential Intermediates of Biological N 2 Fixation

In the quest for low‐molecular‐weight metal sulfur complexes that bind nitrogenase‐relevant small molecules and can serve as model complexes for nitrogenase, compounds with the [Ru(P i Pr 3 )(‘N 2 Me 2 S 2 ’)] fragment were found (‘N 2 Me 2 S 2 ’ 2− =1,2‐ethanediamine‐ N , N ′‐dimethyl‐ N , N ′‐bis(2‐benzenethiolate) 2− ). This fragment enabled the synthesis of a first series of chiral metal sulfur complexes, [Ru(L)(P i Pr 3 )(‘N 2 Me 2 S 2 ’)] with L=N 2 , N 2 H 2 , N 2 H 4 , and NH 3 , that meet the biological constraint of forming under mild conditions. The reaction of [Ru(NCCH 3 )(P i Pr 3 )(‘N 2 Me 2 S 2 ’)] ( 1 ) with NH 3 gave the ammonia complex [Ru(NH 3 )(P i Pr 3 )(‘N 2 Me 2 S 2 ’)] ( 4 ), which readily exchanged NH 3 for N 2 to yield the mononuclear dinitrogen complex [Ru(N 2 )(P i Pr 3 )(‘N 2 Me 2 S 2 ’)] ( 2 ) in almost quantitative yield. Complex 2 , obtained by this new efficient synthesis, was the starting material for the synthesis of dinuclear ( R , R )‐ and ( S , S )‐[μ‐N 2 {Ru(P i Pr 3 )(‘N 2 Me 2 S 2 ’)} 2 ] (( R , R )‐/( S , S )‐ 3 ). (Both 2 and 3 have been reported previously.) The as‐yet inexplicable behavior of complex 3 to form also the R , S isomer in solution has been revealed by DFT calculations and 2 D NMR spectroscopy studies. The reaction of 1 or 2 with anhydrous hydrazine yielded the hydrazine complex [Ru(N 2 H 4 )(P i Pr 3 )(‘N 2 Me 2 S 2 ’)] ( 6 ), which is a highly reactive intermediate. Disproportionation of 6 resulted in the formation of mononuclear diazene complexes, the ammonia complex 4 , and finally the dinuclear diazene complex [μ‐N 2 H 2 {Ru(P i Pr 3 )(‘N 2 Me 2 S 2 ’)} 2 ] ( 5 ). Dinuclear complex 5 could also be obtained directly in an independent synthesis from 1 and N 2 H 2 , which was generated in situ by acidolysis of K 2 N 2 (CO 2 ) 2 . Treatment of 6 with CH 2 Cl 2 , however, formed a chloromethylated diazene species [{Ru(P i Pr 3 )(‘N 2 Me 2 S 2 ’)}‐μ‐N 2 H 2 {Ru(Cl)(‘N 2 Me 2 S 2 CH 2 Cl’)}] ( 9 ) (‘N 2 Me 2 S 2 CH 2 Cl’ 2− =1,2‐ethanediamine‐ N , N ′‐dimethyl‐ N ‐(2‐benzenethiolate) 1− ‐ N ′‐(2‐benzenechloromethylthioether) 1− ]. The molecular structures of 4 , 5 , and 9 were determined by X‐ray crystal structure analysis, and the labile N 2 H 4 complex 6 was characterized by NMR spectroscopy.