Orthometalation of Dibenzo[1,2]quinoxaline with Ruthenium(II/III), Osmium(II/III/IV), and Rhodium(III) Ions and Orthometalated [RuNO]^6/7 Derivatives

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 <i>trans-</i>[Ru^III(DBQ)­(PPh₃)₂Cl₂] (<b>1</b>), <i>trans-</i>[Ru^II(DBQ)­(CO)­(PPh₃)₂Cl] (<b>2</b>), <i>trans-</i>[Os^III(DBQ)­(PPh₃)₂Br₂] (<b>3</b>), <i>trans-</i>[Os^II(DBQ)­(PPh₃)₂(CO)­Br] (<b>4</b>), and <i>trans-</i>[Rh^III(DBQ)­(PPh₃)₂Cl₂] (<b>5</b>). Reaction of <b>1</b> with NO affords <i>trans-</i>[Ru­(DBQ)­(NO)­(PPh₃)₂Cl]Cl (<b>6</b>⁺Cl^–), isoelectronic to <b>2</b>, with a byproduct, [Ru­(NO)­(PPh₃)₂Cl₃] (<b>7</b>). Complexes <b>1</b>–<b>5</b> and <b>6</b>⁺ were characterized by elemental analyses, mass, IR, NMR, and electron paramagnetic resonance (EPR) spectra including the single-crystal X-ray structure determinations of <b>1</b>–<b>3</b> and <b>5</b>. The Ru^III–C, Ru^II–C, Os^III–C, and Rh^III–C lengths are 2.049(2), 2.074(3), 2.105(16), and 2.012(3) Å in <b>1</b>, <b>2</b>, <b>3</b>, and <b>5</b>. In cyclic voltammetry, <b>2</b>, <b>3</b>, and <b>4</b> undergo oxidation at 0.59, 0.39, and 0.46 V, versus Fc⁺/Fc couple, to <i>trans-</i>[Ru^III(DBQ)­(CO)­(PPh₃)₂Cl]⁺ (<b>2</b>⁺), <i>trans-</i>[Os^IV(DBQ)­(PPh₃)₂Br₂]⁺ (<b>3</b>⁺), and <i>trans-</i>[Os^III(DBQ)­(CO)­(PPh₃)₂Br]⁺ (<b>4</b>⁺) ions. Complex <b>3</b>⁺ incorporates an Os^IV(d⁴ ion)–C bond. The <b>6</b>⁺/<i>trans-</i>[Ru­(DBQ)­(NO)­(PPh₃)₂Cl] (<b>6</b>) reduction couple at −0.65 V is reversible. <b>2</b>⁺, <b>3</b>⁺, <b>4</b>⁺ and <b>6</b> 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 <b>6</b> exhibits hyperfine couplings due to ^99,101Ru and ¹⁴N nuclei. DFT calculations on <i>trans-</i>[Os^III(DBQ)­(PMe₃)₂Br₂] (<b>3</b>^Me), S_t = 1/2 and <i>trans-</i>[Os^IV(DBQ)­(PMe₃)₂Br₂]⁺ (<b>3</b>^Me+), S_t = 0, <i>trans-</i>[Ru­(DBQ)­(NO)­(PMe₃)₂Cl]⁺ (<b>6</b>^Me+), S_t = 0 and <i>trans-</i>[Ru­(DBQ)­(NO)­(PMe₃)₂Cl] (<b>6</b>^Me), S_t = 1/2, authenticated a significant mixing between d_Os and π_aromatic* orbitals, which stabilizes M^II/III/IV–C bonds and the [RuNO]⁶ and [RuNO]⁷ states, respectively, in <b>6</b>⁺ and <b>6</b>, which is defined as a hybrid state of <i>trans-</i>[Ru^II(DBQ)­(NO^•)­(PPh₃)₂Cl] and <i>trans-</i>[Ru^I(DBQ)­(NO⁺)­(PPh₃)₂Cl] states