Pas de Deux of an NO Couple: Synchronous Photoswitching from a Double-Linear to a Double-Bent Ru(NO)2 Core under Nitrosyl Charge Conservation

Abstract

The {Ru(NO)₂ }¹⁰ dinitrosylruthenium complex [Ru(NO)₂ (PPh₃ )₂ ] (1) shows photo-induced linkage isomerism (PLI) of a special kind: the two NO ligands switch, on photo-excitation, synchronously from the ground state (GS) with two almost linear RuNO functions to a metastable state (MS) which persists up to 230 K and can be populated to ≈50 %. The MS was experimentally characterised by photo-crystallography, IR spectroscopy and DS-calorimetry as a double-bent variant of the double-linear GS. The experimental results are confirmed by computation which unravels the GS/MS transition as a disrotatory synchronous 50° turn of the two nitrosyl ligands. Although 1 shows the usual redshift of the N-O stretch on bending the MNO unit, there is no increased charge transfer from Ru to NO along the GS-to-MS path. In terms of the effective-oxidation-state (EOS) method, both isomers of 1 and the transition state are Ru^-II (NO⁺ )₂ species.

Keywords: Coordination Modes; Nitrosyl Ligands; Photo-Induced Linkage Isomerism; Photophysics; Ruthenium.

Scheme 1

Linkage isomers of mononuclear mono‐ and dinitrosylmetal species.

Figure 1

Molecules of 1 in the solvent‐free crystals, 50 % probability ellipsoids at 100 K. Distances (Å) and angles (°): Ru1−N1 1.797(5), N1−O1 1.183(7), Ru1−N2 1.802(5), N2−O2 1.177(6), Ru1−P1 2.3367(15), Ru1−P2 2.3415(15); N1−Ru1−N2 141.9(2), Ru1−N1−O1 169.9(5), Ru1−N2−O2 172.8(5), P1−Ru1−P2 104.87(5).

Figure 2

Left: Photodifference map of 1 after irradiation with 590 nm light at 100 K; blue: positive values, red: negative values; contour‐lines every 0.5 e Å⁻³. Right: comparison of GS and MS structure illustrating the (l‐NO‐κN)₂ GS to the metastable (a‐NO‐κN)₂ MS configurational change.

Figure 3

Differential infrared spectra between GS and PLI state as a function of irradiation fluence Q=I t, illustrating the conversion from GS to the PLI state.

Figure 4

The transition state (TS) which connects the double‐linear GS with the double‐bent MS of 1. The TS's strongly negative vibrational mode (−835 cm⁻¹) traces the decay path if the photo‐generated MS is warmed above 230 K (see Supporting Information for an animation of the movement).

Figure 5

From bottom to top: the four orbitals that are doubly occupied in the major configuration of the GS (left) and the MS (right) in a CASSCF(8,8) approach (weight of the major 22220000 configuration in both the GS and MS: 0.76; leftmost: MO numbers, occupation and assignment with z up, x right and y into the plane for the GS; almost the same values were obtained for the MS). The topmost MOs are empty in the major configuration; three more antibonding MOs of the active space are skipped.