Spectroscopic Characterization of Molybdenum Dinitrogen Complexes Containing a Combination of Di- and Triphosphine Coligands: ³¹P NMR Analysis of Five-Spin Systems

Abstract

The three molybdenum-N2 complexes [Mo(N2)(dpepp)(depe)] (1), [Mo(N2)(dpepp)(dppe)] (2), and [Mo(N2)(dpepp)(1,2-dppp)] (3), all of which contain a combination of a bi- and a tridentate phosphine ligand, were prepared and investigated by vibrational and (31)P NMR spectroscopy. As a tridentate ligand bis(2-diphenylphosphinoethyl)phenylphosphine (dpepp) has been employed. The three different bidentate ligands are 1,2-bis(diethylphosphino)ethane (depe), 1,2-bis(diphenylphosphino)ethane (dppe), and R-(+)-1,2-bis(diphenylphosphino)propane (1,2-dppp). N-N as well as metal-N vibrations of 1-3 are identified and interpreted in terms of the geometric and electronic structures of the complexes. (31)P NMR spectra are recorded and fully analyzed. Moreover, correlation spectroscopy (COSY)-45 measurements are performed to determine the relative signs of coupling constants. Special attention is directed to a detection of different isomers and their (31)P NMR, as well as vibrational spectroscopic properties. The implications of the results for the area of synthetic nitrogen fixation with phosphine complexes are discussed.

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

“…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 ]. [23] As first demonstrated by George et al and supported by our own experiments, [Mo(N 2 )(dpepp)(dppm)] can be protonated by triflic acid with retention of the pentaphosphane ligation, thus making this system a candidate for the catalytic conversion of N 2 to ammonia on the basis of Mo-phosphane systems. A corresponding mechanistic cycle has been evaluated by DFT.…”

“…Specifically, the central P donor of the dpepp ligand can coordinate in the trans or cis position to the dinitrogen ligand, which leads to the ortho-and iso forms of the complexes, respectively. [23] Both isomers can be distinguished by the degree of activation imparted to the dinitrogen ligand. In order to avoid this complication, we decided to employ tripodal ligands, which can only bind in one configuration.…”

“…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).…”

(Dinitrogen)molybdenum Complexes Supported by Asymmetric Silicon‐Centered Tripod Ligands: Steric and Electronic Influences on the Coordination of Mono‐ and Diphosphine Coligands

“…[3][4][5][6][7][8][9] In this regard, complexes that contain low-valent iron and molybdenum centers play a prominent role. [19][20][21][22][23][24] The goal of this research is to overcome drawbacks of the classic Chatt complexes, namely, (1) a breaking of metal-phosphine bonds at higher oxidation states of the central atoms, [12,25] and (2) an intrinsic loss of 50 % of catalyst per reduction cycle, which occurs when intermediate Mo I complexes undergo disproportionation. [10] Our group has been involved in spectroscopic and theoretical investigations of key steps of the Chatt cycle.…”

Molybdenum(0)‐N2 Complexes Supported by the Tripod Ligand 1,1,1‐Tris(Diphenyl‐Phosphinomethyl)Ethane: Steric Influence on the Coordination of Mono‐ and Diphosphine Coligands

“…[3][4][5][6][7][8][9] Our group has been involved in detailed experimental and theoretical investigations of Chatt-like systems for a number of years. [10][11][12] Moreover, we have explored the use of multidentate phosphine ligands to address mechanistic problems of the Chatt cycle. [10,[13][14][15] Recently we reported the synthesis of the first molybdenum dinitrogen complexes facially coordinated by the tripodal ligand 1,1,1-tris(diphenylphosphinomethyl)ethane (tdppme), [Mo(N 2 )(tdppme)(dmpm)] and [Mo(N 2 )(tdppme)(dppm)] (dppm = bis(diphenylphosphanyl)methane, dmpm = bis(dimethylphosphanyl)methane), by sodium amalgam reduction of [MoBr 3 (tdppme)] under N 2 in the presence of dmpm and dppm, respectively.…”