Rhodium-phosphorus coupling

The facial isomer of the complex [RI1X3P3] has equivalent phosphorus atoms, so the spectrum is a doublet due to rhodium-phosphorus coupling, whereas the meridianal isomer has two types of phosphorus atoms, a pair mutually traws(b) and one irons to X(a) (1). 

Table 16. One-bond rhodium-phosphorus coupling constants ...

The complex ion (Figure 2.32) contains Rh2 bound cis to two phosphorus atoms (2.216 A) and more distantly to four oxygens (2.201—2.398 A), exhibiting a distortion ascribed to the Jahn-Teller effect it is paramagnetic (fi = 1.80 fiB) and exhibits an ESR spectrum (Figure 2.33) showing rhodium hyperfine coupling as the doublet for g. ... 

In these earlier studies the phosphine ligand at the rhodium(I) center has been shown to be trans to the //-h ydride with a phosphorus coupling of JHp(trans) = 30 Hz and 32 Hz, respectively. Therefore, we assign to complex 8 the structure as outlined in Scheme 12.1. 

Hz) and the phosphinite phosphorus nucleus (9.9 Hz). In the phosphorus spectra the expected couplings are with rhodium, the mutual phosphorus coupling and in the proton coupled spectra the proton coupling for the phosphorus trans to it. 

Analysis of A found C, 13.8% H, 2.1% P, 21.3% F, 39.2% and Rh, 23.6%. The mass spectrum of A showed a highest mass peak at m/z = 436. When compound A was heated to 60°C it was converted initially into an isomeric derivative B which subsequently formed a third isomer C. The H n.m.r. spectra of compounds A, B and C, measured at room temperature, are listed in the table. These spectra can be assigned by taking into account proton-proton and proton-rhodium couplings [l( " Rh) natural abundance of Rh isotope is 100%], proton-phosphorus couplings are not observed under these conditions. Interpret these data as fully as you can. Deduce the molecular structures of the complexes A, B and C and suggest a mechanism for the isomerization. 

The resulting product showed a single 31P chemical shift at 40 ppm with a XJ coupling to rhodium of 118 Hz, indicative of a rhodium (III) center. As this rhodium atom was shown to couple to two phosphorus atoms, it could be concluded that the phosphorus atoms are in a symmetrical position, as shown in Figure 11.3. The other Rh atom couples only to a phosphoms atom with a jphp of 196 Hz, indicative of a rhodium (I) center. 

When the smaller P(Me)3 ligand was used, not only the halogens but also a hydride were found in the bridging position, as was concluded from the fact that the phosphorus atom connected to the rhodium (I) center was now coupled to a hydride. 

The rhodium catalyst was recycled batch-wise four times. It was found that a short induction period occurred during the first reaction cycle. The following cycles showed a constant rate and no loss of activity was detected. A ligand-to-rhodium ratio of 5 1 led to a constant yield of 95% per cycle after 1 h. Within the four cycles a total turnover number of 1000 with a maximum turnover frequency of 234 h was achieved. The leaching of rhodium and phosphorus into the aqueous layer was determined by inductively coupled plasma atomic emission spectrometry. Rhodium leaching amounted to 14.2 ppm in the first run, then dropped to 3.6 ppm (second run) and reached values of 0.95 and 0.63 ppm in the third and fourth runs, respectively. 

The substantially larger values of M-p for phosphorus trans to chlorine compared with phosphorus trans to phosphorus correlate with shorter M-P bonds trans to chlorine for a number of metals and oxidation states tungsten(IV), rhodium(I) and (III), platinum(II) and (IV), and linear mercury(II) (15). By analogy with the discussion of the results for the platinum(II) complexes, this indicates the dominance of the (P sMSp)2 term in Equation 1 for couplings with a variety of M, but as discussed earlier it is difficult to determine the extent of variation of... 

Figure 11 shows the spectrum of the dppe system in more detail at 35° and 90°C. The chemical shift ( 54.3 ppm) and the coupling constant (P-Rh = 144 Hz) of the complexed phosphorus atoms are identical with those reported by James and Mahajan (16) for the known dppe rhodium hydride. 

The solid and solution P-31 NMR spectra of the BPPM-rhodium complex are shown in Figure 6. The solution spectrum was obtained in acetone-d6 as the solvent, and displays an unusual coupling pattern when compared with the previously published spectrum obtained in methanol-d4 (II, 12). The spectrum shown in Figure 6 displays four sets of phosphorus signals two sets of four signals are centered at 43.5... 

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