Solid state and solution structures of rhodium and iridium poly(pyrazolyl)borate diene complexes

Abstract

The structures adopted by a range of poly(pyrazolyl)borate complexes [ML2Tp(x)] [M = Rh, Ir; L2 = diene; Tp(x) = Bp' {dihydrobis(3,5-dimethylpyrazolyl)borate}, Tp' {hydrotris(3,5-dimethylpyrazolyl)borate}, Tp {hydrotris(pyrazolyl)borate}, B(pz)4 {tetrakis(pyrazolyl)borate}] have been investigated. Low steric hindrance between ligands in [Rh(eta-nbd)Tp] (nbd = norbornadiene), [Rh(eta-cod)Tp] (cod = cycloocta-1,5-diene) and [Rh(eta-nbd)Tp'] results in K3 coordination of the pyrazolylborate but [M(eta-cod)Tp'] (M = Rh, Ir) are kappa2 coordinated with the free pyrazolyl ring positioned above and approximately parallel to the square plane about the metal. All but the most sterically hindered Tp(x) complexes undergo fast exchange of the coordinated and uncoordinated pyrazolyl rings on the NMR spectroscopic timescale. For [Rh(eta-cod){B(pz)4}], [Rh(eta-dmbd)Tp'] (dmbd = 2,3-dimethylbuta-1,3-diene) and [Rh(eta-cod)Tp(Ph)] {Tp(Ph) = hydrotris(3-phenylpyrazolyl)borate} the fluxional process is slowed at low temperatures so that inequivalent pyrazolyl rings are observed. The bonding modes of the Tp' ligand (but not of other pyrazolylborate ligands) can be determined by 11B NMR and IR spectroscopy. The 11B chemical shifts (for a series of Tp' complexes) show the general pattern, kappa3 < -7.5 ppm < kappa2 and the nu(BH) stretch kappa3 > 2500 cm(-1) > kappa2. The electrochemical behaviour of the pyrazolylborate complexes is related to the degree of structural change which occurs on electron transfer. One-electron oxidation of complexes with Tp', Tp and B(pz)4 ligands is generally reversible although that of [Ir(etacod)Tp] is only reversible at higher scan rates and that of [Ir(eta-cod){B(pz)4}] is irreversible. Of the complexes with the more sterically hindered Tp(Ph) ligand, only [Rh(eta-nbd)Tp(Ph)] shows any degree of reversible oxidation. The ESR spectra of a range of Rh(II) complexes show coupling to both 14N and 103Rh nuclei in most cases but what appears to be coupling to rhodium and one hydrogen atom, possibly a hydride ligand, for the oxidation product of [Rh(eta-nbd)Tp(Ph)].

Homoleptic ‘sandwich’ complexes of substituted tris(methimazolyl)borate ligands with ruthenium, rhodium and palladium

“…30 However, in TpRh(COD) the metal adopts a distorted trigonal bipyramidal geometry with the ligand coordinated in a κ 3 -[N,N,N] mode. 31 Despite this, it was shown that a fine balance of sterics was at play with the slightly bulkier complex Tp*Rh(COD) adopting κ 2 -[N,N] coordination. The increased steric bulk of Tm ligands relative to Tp would therefore be expected to favour κ 2 -[S,S] coordination.…”

“…30 A variety of complexes of Tm with Rh(I) are known where the metal adopts either square-planar or distorted trigonal-bipyramidal geometry with the ligand coordinating as either a bidentate or a fluxional tridentate donor. 30,31 There are no published examples of heteroleptic 'half-sandwich' Rh(III) complexes with Tm coordinating in a κ 3 -[S,S,S] mode.…”

Isomerism in rhodium(i) N,S-donor heteroscorpionates: ring substituent and ancillary ligand effects

“…(The complex [Rh(cod)Tp] is trigonal bipyramidal in the solid state. 15 ) Although the nitrogen atom, N(6), of the thioxotriazolyl ring is positioned above the rhodium centre it is too far away (3.280 A ˚), and in an unfavourable orientation, to be considered bonding.…”

Predicting ESR Peaks in the 4d and 5d Transition Metal Ion Complexes by NMR, ESR and NQR Parameters: A DFT Study

“…Only a limited research work had been done in the experimental determination of number of ESR peaks in complexes of 4d and 5d metal ions [1][2][3][4][5] like Zr (III) [6][7][8] ,Hf (III) [7][8][9] ; Nb (IV) [10][11][12] , Ta (IV) 9 ; Tc (II) 9, 13, 14 , Re (II ) [15][16][17] ; Ru (III) [18][19][20][21][22][23][24] , Os (III) [18][19][20]24 ; Rh (IV) 25-27 , Ir (IV) 26,27 which formed five congeners of 2 nd and 3 rd transition series.…”