Protonation Of Dinitrogen Research Articles

Activation and protonation of dinitrogen at the FeMo cofactor of nitrogenase

The protonation of N2 bound to the active center of nitrogenase has been investigated using state-of-the-art density-functional theory calculations. Dinitrogen in the bridging mode is activated by forming two bonds to Fe sites, which results in a reduction of the energy for the first hydrogen transfer by 123 kJ/mol. The axial binding mode with open sulfur bridge is less reactive by 30 kJ/mol and the energetic ordering of the axial and bridged binding modes is reversed in favor of the bridging dinitrogen during the first protonation. Protonation of the central ligand is thermodynamically favorable but kinetically hindered. If the central ligand is protonated, the proton is transferred to dinitrogen following the second protonation. Protonation of dinitrogen at the Mo site does not lead to low-energy intermediates.

Bonding, activation, and protonation of dinitrogen on a molybdenum pentaphosphine complex — Comparison to trans-bis(dinitrogen) and -nitrile – dinitrogen complexes with tetraphosphine coordination

The molybdenum-N2 pentaphosphine complex [Mo(N2)(dpepp)(dppm)] (1) containing a bi- and a tridentate phosphine ligand (dpepp = PhP(CH2CH2PPh2)2, dppm = Ph2PCH2PPh2) has been prepared and characterized. By protonation of 1 with triflic acid the NNH2 complex [Mo(NNH2)(dpepp)(dppm)](CF3SO3)2 (2) is formed without loss of the pentaphosphine coordination. The electronic properties of 1 and 2 and their 15N and (or) 2H counterparts are investigated by NMR, IR, and Raman spectroscopy coupled to DFT frequency and NMR shift calculations. Force constants are evaluated from experimental frequencies and isotope shifts by the quantum chemistry based normal coordinate analysis procedure (QCB-NCA). The results for the N2 complex 1 are compared with those obtained earlier for bis(dinitrogen) and trans-nitrile dinitrogen systems. Importantly, the N2 in the pentaphosphine complex is more strongly activated compared to corresponding bis(dinitrogen) compounds and more weakly activated compared to trans-nitrile N2 systems. The activation of the NNH2 ligand in complex 2 is similar to trans-nitrile systems and weaker compared to corresponding NNH2 complexes with coordinated anions like [MoF(NNH2)(dppe)2](BF4). These results are discussed based on the relative donor–acceptor properties of the respective trans ligands, demonstrating the influence of σ donation and π acceptance on the activation of N2 in transition metal complexes.Key words: nitrogen fixation, phosphine complexes, NMR shifts, DFT calculations, normal coordinate analysis.