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

Abstract

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.

Octahedral Molybdenum(0) Monodinitrogen Complexes Facially Coordinated by the Tripodal Ligand 1,1,1‐Tris(diphenylphosphanylmethyl)ethane – Influence of Diphosphane Coligands on the Activation of N2

“…The Mo-P bond lengths of the phosphane ligands vary between 2.3986(16) and 2.4454(16) Å (Table 2) and thus are slightly shorter than those of [Mo(N 2 )(dpepp)(dppm)], which lie between 2.418(2) and 2.497(2) Å (Table 3). [22] The Mo-N bond length of 2 is 2.066(6) Å, which is slightly longer than that in [Mo(N 2 The P-Mo-P angles of 2 lie between 82.30(5) and 86.00(6)°and thus are significantly smaller than 90°, which indicates a trigonal elongation of the octahedral coordination sphere. Their values closely agree with the P-Mo-P angles in [Mo(tdppme)(CO) 3 ].…”

“…[21] The majority of data, however, have been obtained from complexes with the dpepp ligand (E = PPh). [22] These systems have been synthesized with bidentate phosphane coligands such as dppe or dppm. We have augmented this series with the synthesis and characterization of [Mo(N 2 )(dpepp)(depe)] and [Mo(N 2 )(dpepp)(1,2-dppp)] [1,2-dppp = Ph 2 PCH(CH 3 )CH 2 PPh 2 ].…”

“…[31] It is somewhat smaller as compared to that of the dppm [bis(diphenylphosphanyl)-methane] ligand in [Mo(N 2 )(dpepp)(dppm)], which exhibits a bite angle of 68.3(1)°, most likely because of steric reasons. [22] On the basis of its crystal structure, 2 was investigated by DFT. As evident from Table 3, the structural parameters derived from geometry optimization match those derived from the crystal structure fairly well.…”

“…Mo-N and Mo-P distances are about 0.1 Å too large, which has been observed before in this type of calculation. [22,23] The geometry of 2 determined by single-crystal structure analysis and DFT geometry optimization is also preserved in solution, as inferred from NMR spectroscopy. The 31 P NMR spectrum of [Mo(N 2 )(tdppme)(dmpm)] exhibits an AAЈXXЈB structure ( Figure S1).…”

Energetics and Mechanism of Ammonia Synthesis through the Chatt Cycle: Conditions for a Catalytic Mode and Comparison with the Schrock Cycle

“…Efforts to investigate existing ligand systems and develop new ones for this purpose are underway. [33,34] …”