EPR STUDIES OF 1/1 COMPLEXES OF RHODIUM(II) AND COBALT(II) PORPHYRINS WITH SIGMA-DONOR AND PI-ACCEPTOR LIGANDS - ORIGINS OF RHODIUM(II) METALLORADICAL REACTIVITY

Tetrakis(2,4,6-trimethylphenyl)porphyrinato)rhodium(II) ((TMP)Rh) and tetrakis(2,4,6-triisopropylphenyl)-porphyrinato) rhodium(II) ((TTiPP)Rh) occur as low-spin d7 complexes with EPR parameters associated with a (d(xy))2-(d(xz,yz))4(d(z)2)1 ground configuration. (por)Rh(II) species typically react as metalloradicals with a wide variety of substrates to give diamagnetic products; however, the use of increased porphyrin or substrate steric demands has permitted observation of paramagnetic 1:1 adducts. EPR spectra were used in examining features related to the electronic structure for a series of 1:1 five-coordinate complexes with nitrogen, phosphorus, arsenic, and carbon donor ligands. Each of the five-coordinate species is a low-spin d7 complex with the unpaired electron occupying a d(z)2 MO ((d(xy)d(xz)d(yz))6-(d(z)2)1 ground configuration). The ligands form 1:1 adducts that have effective axial symmetry with the exceptions of CO, which has as a bent Rh-CO unit, and ethene, which has a symmetrical pi complex structure. Donor atom spin densities are estimated from ligand hyperfine coupling. Rhodium-103 hyperfine coupling is used in evaluating the rhodium 4d(z)2 (0.67) and 5s (0.02) spin density in the ethene pi complex, which combined with the total ethene C2p (0.278) and C2s (0.008) spin densities, determined from the ligand hyperfine coupling, accounts for most of the unpaired electron. Estimates of the d(z)2 to d(xz) and d(yz) energy separations for the series of 1:1 complexes illustrate the elevation of d(z)2 by sigma donor ligands and the combined lowering of d(xz), d(yz), and d(z)2 elevation by ligands with pi acceptor ability. Adduct formation of (por)Rh. with pi acceptor ligands results in substantially larger d orbital energy separations and ligand spin densities than the strictly sigma donor ammine ligands. The d orbital energy separations and ligand spin densities of (por)Rh.L complexes are significantly greater than those observed for the corresponding (por)Co-L complexes, which is also reflected in the greatly enhanced scope of reactivity manifested by (por)Rh. species compared with the analogous Co(II) derivatives. Unusual structural and reactivity properties of the (por)Rh.CO and (por)Rh.C2H4 complexes are briefly discussed and contrasted with those of the corresponding (por)Co.L complexes.