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.

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The first Chatt‐type Mo0 dinitrogen complex with a tetraphosphane ligand has been prepared and characterized by NMR as well as infrared and Raman spectroscopy. Importantly, the employed reaction route allows the stereospecific synthesis of this complex as trans‐[Mo(N2)2(meso‐prP4)] (prP4 = a tetraphos ligand with a central propylene bridge). The stereoselectivity in the reaction course is induced by the oxido‐iodido‐molybdenum(IV) precursor [Mo(O)I(prP4)]+ which directs both phenyl groups of the bridging P atoms of prP4 into a meso configuration. The paper establishes a general strategy to synthesize mononuclear Mo0 dinitrogen and related molybdenum complexes with multidentate phosphane ligands which has not been possible to date. Moreover, the obtained molybdenum tetraphos N2 complex should exhibit a higher thermodynamic stability in the reactions of the Chatt cycle of synthetic nitrogen fixation than the conventional bis(diphos) complexes, due to the linkage of the two diphosphane units by an alkyl bridge. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Reaction of the Mo(0) depe complex [Mo(N 2 H 2 )(depe) 2 (CH 3 CN)](OTf) 2 (3) (depe ) 1,2-bis-(diethylphosphino)ethane) with base gives the ethylimido complex [Mo(depe) 2 (CH 3 CH 2 N)(CH 3 -CN)](OTf) 2 (10), whereas base treatment of the corresponding dppe complex [Mo(N 2 H 2 )-(dppe) 2 (OTf)](OTf) (12; dppe ) 1,2-bis(diphenylphosphino)ethane) leads to formation of [Mo(N 2 )(dppe) 2 (CH 3 CN)] ( 14). Reaction of dinitrogen complex 14 with HBF 4 gives the ethylimido complex [Mo(dppe) 2 (CH 3 CH 2 N)(F)](BF 4 ) ( 17). The protonation of coordinated acetonitrile in complexes 3 and 14 giving the corresponding ethylimido complexes is investigated. Experimental and theoretical evidence is presented for the hypothesis that acetonitrile is only activated toward protonation at the β-carbon if it is bound along with a Lewis base like triflate or fluoride to a Mo(0) center. The activation is further influenced by the phosphine coligands (depe > dppe). DFT geometry optimizations show that the acetonitrile ligand is bent in the activated complexes, exhibiting a lone pair at C β . Protonation at this position first leads to Mo(II) methyl-azavinylidene intermediates (which cannot be isolated) and then to Mo(IV) ethylimido complexes.

The stereochemistry of prP4 (1,1,4,8,11,11-hexaphenyl-1,4,8,11-tetraphosphaundecane) and its coordination properties in mononuclear complexes are explored. The free ligand is obtained as a mixture of meso and rac diastereomers from the reaction of dilithium 1,3-bis(phenylphosphido)propane−tetrakis(tetrahydrofuran) (“Lippp”) with 2 equiv of 1-chloro-2-(diphenylphosphino)ethane. Reaction of prP4 with FeCl2 and KSCN leads to cis- and trans-[Fe(NCS)2(prP4)] complexes of the meso and the rac ligands; the composition of this mixture has been fully analyzed. The iron isothiocyanato complexes cis-α- and trans-[Fe(NCS)2(rac-prP4)] have been characterized by X-ray structure determinations. The 31P NMR spectrum of the free tetraphos ligand shows an AA‘XX‘ coupling scheme with three groups of signals centered at −20.6 and −20.7 ppm and at −12.4 ppm, which belong to the rac and meso isomers, respectively. 31P NMR spectra of the cis and trans iron isothiocyanato complexes of rac-prP4 exhibit AA‘XX‘ patterns as well, which can be reproduced by simulation. trans-[Fe(NCS)2(meso-prP4)] is identified by its 31P NMR spectrum, which is similar to that of its rac counterpart. On the basis of NMR spectroscopy it is found that the cis-α complex of the rac ligand converts into the trans-rac complex with a half-life of approximately 2 days. Infrared and Raman spectra of the cis-α and trans isomers of [Fe(NCS)2(rac-prP4)] reveal characteristic differences in the region of the C−N stretching vibrations.