Dinitrogen complexes of transition metals have attracted much attention due to their important roles in biological and abiological nitrogen fixation processes. 1 Of relevance to mechanisms of metalmediated nitrogen fixation are dinuclear complexes containing µ-η 1 :η 1 -dinitrogen ligands, which depending on the extent of metal-nitrogen π bonding may be classified into three types, namely, dinitrogen (N 2 ) 0 (A), diazenido(2-) (N 2 ) 2-(B), or hydrazido(4-) (N 2 ) 4-(C) ligand. 1,2 Cummins and co-workers demonstrated that mononuclear molybdenum tris-amide complexes are capable of splitting dinitrogen via dinitrogen-bridged dimolybdenum intermediates. 3 On the other hand, dinuclear µ-η 1 : η 1 -dinitrogen complexes are formed by coupling reactions of nitrido-osmium complexes, which may be viewed as a microscopic reverse of metal-assisted NtN cleavage. 4 Although terminal dinitrogen complexes of metalloporphyrins are well documented, there are relatively few examples of analogous complexes with bridged dinitrogen ligands. 5 Collman and coworkers reported that metal-bound ammonia can be converted to a bridged dinitrogen in a diruthenium complex of a cofacial diporphyrin via the bridged hydrazine and diazene intermediates, demonstrating the potential of metalloporphyrins as catalysts for dinitrogen reduction. 6 This prompted us to study the bonding and structure of bridged dinitrogen complexes of metalloporphyrins. Heterometallic µ-dinitrogen complexes are generally synthesized by the reactions of terminal dinitrogen complexes with another Lewis acidic metal center such as titanium(IV) and molybdenum-(IV). 7 In this connection, we set out to investigate the reactions between a rhenium(I) dinitrogen complex and metalloporphyrins. We here report on the isolation and molecular structure of the first heterometallic bridged dinitrogen complexes containing iron-(III) and chromium(III) porphyrins.Treatment of [Fe(por)(OTf)] [por ) octaethylporphyrinate OEP or tetra(p-tolyl)porphyrinate TTP] with 1 equiv of [Re(N 2 )(PMe 2 -Ph) 4 Cl] 8 led to formation of the dinitrogen-bridged Fe/Re complexes [(por)Fe(N 2 )Re(PMe 2 Ph) 4 Cl][OTf] (por ) OEP 1 or TTP 2) characterized by UV/vis and IR spectroscopies (eq 1). 9No reactions were found between [Fe(por)Cl] and [Re(N 2 )(PMe 2 -Ph) 4 Cl], suggesting that dissociation of the labile triflate ligand from the iron porphyrin is crucial for the formation of the bridged dinitrogen complexes. The KBr FT-IR spectra of 1 and 2 feature the N-N stretching modes at ca. 1803 and 1820 cm -1 , respectively, which are obviously lower than that for [Re(N 2 )(PMe 2 -Ph) 4 Cl] (1920 cm -1 ), indicating that the N-N triple bond in N 2 is weakened upon coordination to Fe(III). The ν NN for 1 and 2 are, however, lower than those for reported dinitrogen complexes of metalloporphyrins, e.g., 2112 cm -1 for [Ru 2 DPB(*Im) 2 ](µ-N 2 ) (DPB ) diporphyrinatobiphenylene tetraanion, *Im ) 1-tertbutyl-5-phenylimidazole) 6 and 2030 cm -1 for [Os(OEP)(N 2 )-(THF)]. 10 As revealed by UV/vis spectroscopy, [Fe(por)(O...

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