Solid-State15N CPMAS NMR and Computational Analysis of Ligand Hapticity in Rhodium(η-diene) Poly(pyrazolyl)borate Complexes

Novel [Rh(n-diene)Tpx] complexes of sterically encumbered Tpx ligands (Tpx = Tp4Bo, diene = cod, 1; nbd, 2; Tpx = Tp4Bo,5Me, diene = cod, 3; nbd, 4; Tpx = Tpa,3Me, diene = cod, 5; nbd, 6; Tpx = Tp a*,3Me, diene = cod, 7; nbd, 8) have been prepared by treatment of [Rh(n-diene)(μ-Cl)]2 with TlTpx (Tpx in general, in detail: Tp4Bo = hydrotris(indazol-1-yl)borate, Tp 4Bo,5Me = hydrotris(5-methyl-indazol-1-yl)borate, Tpa,3Me = hydrotris(3-methyl-2H-benz[g]-4,5-dihydroindazol-2-y1)borate, Tp a*,3Me = hydrotris(3-methyl-2H-benz[g]indazol-2-yl)borate), and characterized by analytical and spectral data (IR, 1H, 11B, and 13C NMR solution). The structures adopted by [Rh(nbd)Tp4Bo] 2, [Rh(cod)Tp4Bo,5Me] 3, [Rh(nbd)Tp a,3Me] 6, [Rh(nbd)Tpa*,3Me] 8, and [Rh(nbd)Tp a*,3Me*] 8* (incorporating a borotropomeric ligand), have been investigated. Low steric hindrance between the ligands in 2 and 3 permits κ3 coordination of the pyrazolylborate while the high steric encumbrance present in 6, 8, and 8* results in κ2 ligands. The coordination modes of the ligands to the metal have also been established by 15N CPMAS studies of selected ligands and their corresponding Rh complexes. These spectroscopic data are in agreement with the 15N chemical shifts obtained by using quantum-chemical methods to assist reliable assignments of the experimental values, affording new insights into the extraction of structural information concerning the hapticity (κ2 or κ3) of the poly(pyrazolyl)borate ligands to the Rh metal.