Spectroscopy of terminal dinitrogen complexes: nitrogen-15 and phosphorus-31 nuclear magnetic resonance

Abstract

Nitrogen-I 5 chemical shifts and coupling constants are reported for terminal dinitrogen complexes of Mo, W, Re Fe, Ru, Os, and Rh. The nitrogen shielding shows a periodic variation with the central metal, increasing with increasing atomic number of the metal across the transition-metal series, or down a particular group, the increase being steeper for the metal-bound nitrogen N, than for Ng. For complexes of metals later in the series N, is more shielded than is Ng, but there appears to be a crossover at molybdenum, and possibly at or near tungsten. The 15N shifts show evidence of interaction of adjacent dinitrogen ligands. The majority of compounds have phosphine co-ligands and their 3 ' P chemical shifts show a smaller sensitivity to change of the central metal (about half to three-quarters of that of the 15N shifts) and also show chelate effects. The preparation, n.m.r. and redox properties of the new complexes trans-[M( N,),(depe) (dppe)] (M = Mo or W; depe = Et2PCH2CH2PEt2, dppe = Ph2PCH2CH2PPh2) are reported.Dinitrogen complexes play a key role in current research into nitrogen fixation because they represent the first stage of the reduction of nitrogen gas, namely its activation towards reaction under mild conditions by co-ordination to a metal.' Such a metal-dinitrogen interaction may well occur in the nitrogenase enzyme and may be amenable to detection by spectroscopic methods.Nitrogen-I 5 n.m.r. spectroscopy, potentially a powerful technique for nitrogen fixation studies, is becoming more accessible with the advent of high-field multinuclear spectrometers, if high 15N enrichment is possible, and herein we describe a I5N n.m.r. study of a wide range of dinitrogen complexes.

Synthesis and Protonation Studies of Molybdenum(0) Bis(di­nitrogen) Complexes Supported by Diphosphine Ligands ­Containing Pendant Amines

“…Typically, the proximal or metal‐bound nitrogen atom (N p ), and the distal nitrogen atom (N d ) exhibit resonances with well‐defined chemical shifts in the 15 N NMR spectrum. However, the chemical shifts of N p and N d in some Mo‐dinitrogen complexes are nearly identical, such as those of trans ‐[Mo( 15 N 2 )(dppe) 2 ]: δ = –43.1 (N p ), δ = –42.8 (N d ) (in THF) 18. In this study, the 15 N NMR spectrum of 1‐ 15 N 2 at 25 °C exhibits coincident chemical shifts for N p and N d at δ = –42.6.…”

Rhodium(I) Pincer Complexes of Nitrous Oxide

“…Thes tructures of 2 were definitively established in DFB solution using 15 NN MR spectroscopy,a ided by samples prepared using isotopically labelled 15 N 2 O(98 % 15 N, Figure 2 and the Supporting Information). Intact coordination of N 2 O through the terminal nitrogen atom is evident by an upfield shift of Dd 43.3/37.0 for the corresponding 15 Nr esonances, comparable to that of B, [4] which exhibit 1 J RhN coupling of 28/27 Hz [14] and 2 J PN coupling of 4/5 Hz for 2a/b,respectively. Theinternal 15 Nresonances are conversely shifted downfield by Dd 15.3/14.3, but retain coupling to both 103 Rh ( 2 J RhN = 8Hz) and 31 P( 3 J PN = 1Hz), albeit with commensurate reductions in magnitude.T he 1 J NN coupling constant of free N 2 O (9 Hz) is small and appreciably reduced on complexation to rhodium;t ot he extent that it is only just resolved (1 Hz).…”

“…Only very small perturbations to the N-N and N À O bond lengths are computed on coordination (less than 0.005 ), but the associated vibrations corroborate the experimental pattern and are discernibly blue and red shifted, Figure 2. Solid-state structures (150 K) and 15 NNMR spectra ( 15 N 2 Oa tmosphere, DFB, 61 MHz, 298 K) of 2.Thermal ellipsoids drawn at 50 % (2a)and 30 %(2b)probability;minor disordered components (pincer ligand in 2b)a nd anions omitted for clarity.Selected bond lengths and angles: 2a Rh1-P2 2.2677(5) ,Rh1-P3 2.2688(5) ,Rh1-N4 1.981(2) ,N4-N5 1.108(3) ,N5-O6 1.194(3) ,Rh1-N20 2.007(2);N20-Rh1-N4 178.05(8)8 8,Rh1-N4-N5 173.4(2)8 8,N4-N5-O6 178.5(3)8 8; 2b Rh1-P2 2.282(2) ,Rh1-P3 2.288(2) ,Rh1-N4 1.962 7,N4-N5 1.111(11) ,N5-O6 1.192(11) ,Rh1-N20/N20A 2.071(7)/2.052(8) ;N20/N20A-Rh1-N41 78.1(5)/175.4(5)8 8,R h1-N4-N5 176.8(11)8 8,N 4-N5-O61 78.7 (14)8 8. [22] Angewandte Chemie Zuschriften respectively (see the Supporting Information).…”

Dynamic 15N NMR studies of iron phosphine complexes containing coordinated dinitrogen

“…Inversion-transfer-recovery experiments were carried out using library pulse sequences, incorporating a 45 ms frequency selective IBURP pulse of 1000 points designed for an inversion window of 100 Hz. Fitting of experimental data to the modified Bloch equations was performed using an iterative non-linear least-squares procedure, where Eqns (1) to (3) or (4) and (5) were integrated numerically by a successive approximation Runge-Kutta algorithm.…”

“…The spectra typically contained three resonances: two corresponding to coordinated dinitrogen (N˛for nitrogen bound to metal and Nˇfor terminal nitrogen) and one for uncoordinated molecular nitrogen in the solution. For N˛and Nˇ, the signal to higher field was assigned to N˛by comparison with previous studies 3,4 and on the basis of the expected attenuation of coupling constants j 2 J P,N j > j 3 J P,N j. …”