Orthometalation of Dibenzo[1,2]quinoxaline with Ruthenium(II/III), Osmium(II/III/IV), and Rhodium(III) Ions and Orthometalated [RuNO]6/7 Derivatives
Abstract
A new family of organometallics of ruthenium(II/III), osmium(II/III/IV), and rhodium(III) ions isolated from C–H activation reactions of dibenzo[1,2]quinoxaline (DBQ) using triphenylphosphine, carbonyl, and halides as coligands is reported. The CN–chelate complexes isolated are trans-[RuIII(DBQ)(PPh3)2Cl2] (1), trans-[RuII(DBQ)(CO)(PPh3)2Cl] (2), trans-[OsIII(DBQ)(PPh3)2Br2] (3), trans-[OsII(DBQ)(PPh3)2(CO)Br] (4), and trans-[RhIII(DBQ)(PPh3)2Cl2] (5). Reaction of 1 with NO affords trans-[Ru(DBQ)(NO)(PPh3)2Cl]Cl (6+Cl–), isoelectronic to 2, with a byproduct, [Ru(NO)(PPh3)2Cl3] (7). Complexes 1–5 and 6+ were characterized by elemental analyses, mass, IR, NMR, and electron paramagnetic resonance (EPR) spectra including the single-crystal X-ray structure determinations of 1–3 and 5. The RuIII–C, RuII–C, OsIII–C, and RhIII–C lengths are 2.049(2), 2.074(3), 2.105(16), and 2.012(3) Å in 1, 2, 3, and 5. In cyclic voltammetry, 2, 3, and 4 undergo oxidation at 0.59, 0.39, and 0.46 V, versus Fc+/Fc couple, to trans-[RuIII(DBQ)(CO)(PPh3)2Cl]+ (2+), trans-[OsIV(DBQ)(PPh3)2Br2]+(3+), and trans-[OsIII(DBQ)(CO)(PPh3)2Br]+ (4+) ions. Complex 3+ incorporates an OsIV(d4 ion)–C bond. The 6+/trans-[Ru(DBQ)(NO)(PPh3)2Cl] (6) reduction couple at −0.65 V is reversible. 2+, 3+, 4+ and 6 were substantiated by spectroelectrochemical measurements, EPR spectra, and density functional theory (DFT) and time-dependent (TD) DFT calculations. The frozen-glass EPR spectrum of the electrogenerated 6 exhibits hyperfine couplings due to 99,101Ru and 14N nuclei. DFT calculations on trans-[OsIII(DBQ)(PMe3)2Br2] (3Me), St = 1/2 and trans-[OsIV(DBQ)(PMe3)2Br2]+ (3Me+), St = 0, trans-[Ru(DBQ)(NO)(PMe3)2Cl]+ (6Me+), St = 0 and trans-[Ru(DBQ)(NO)(PMe3)2Cl] (6Me), St = 1/2, authenticated a significant mixing between dOs and πaromatic* orbitals, which stabilizes MII/III/IV–C bonds and the [RuNO]6 and [RuNO]7 states, respectively, in 6+ and 6, which is defined as a hybrid state of trans-[RuII(DBQ)(NO•)(PPh3)2Cl] and trans-[RuI(DBQ)(NO+)(PPh3)2Cl] states.
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