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, 2 (7), 3406-3416
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Synthesis, Reactivity, and Catalytic Transfer Hydrogenation Activity of Ruthenium Complexes Bearing NNN Tridentate Ligands: Influence of the Secondary Coordination Sphere

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Synthesis, Reactivity, and Catalytic Transfer Hydrogenation Activity of Ruthenium Complexes Bearing NNN Tridentate Ligands: Influence of the Secondary Coordination Sphere

Jing Shi et al. ACS Omega.

Abstract

By the introduction of -OH group(s) into different position(s) of 6-(pyridin-2-ylmethyl)-2,2'-bipyridine, several NNN-type ligands were synthesized and then introduced to ruthenium (Ru) centers by reactions with RuCl2(PPh3)3. In the presence of PPh3 or CO, these ruthenium complexes reacted with NH4PF6 in CH2Cl2 or CH3OH to give a series of ionic products 5-9. The reaction of Ru(L2)(PPh3)Cl2 (2) with CO generated a neutral complex [Ru(L2)(CO)Cl2] (10). In the presence of CH3ONa, 10 was further converted into complex [Ru(L2)(HOCH3)(CO)Cl] (11), in which there was a methanol molecule coordinating with ruthenium, as suggested by density functional theory calculations. The catalytic transfer hydrogenation activity of all of these new bifunctional metal-ligand complexes was tested. Dichloride complex 2 exhibits best activity, whereas carbonyl complexes 10 and 11 are efficient for selectively reducing 5-hexen-2-one, suggesting different hydrogenation mechanisms. The results reveal the dramatic influence for the reactivity and catalytic activity of the secondary coordination sphere in transition metal complexes.

Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
(a) Active site of [Fe]-hydrogenase; (b) Szymczak’s ruthenium complex; (c) Kundu’s ruthenium complex; and (d) ruthenium complex.
Figure 2
Figure 2
Designed ligands in this paper.
Scheme 1
Scheme 1. Synthesis of Complexes 2 and 3
Figure 3
Figure 3
X-ray single-crystal structure of 4. The thermal ellipsoids are displayed at 30% probability. Hydrogen atoms have been omitted for clarity. Selected bond distances (angstrom) and angle (deg): Ru(1)–Ru(2), 2.7289(3); Ru(2)–N(1), 2.2085(17); Ru(2)–N(2), 2.2862(19); Ru(2)–N(3), 2.2150(18); Ru(1)–P(1), 2.4221(6); Ru(1)–O(1), 2.0850(16); Ru(1)–O(2), 2.0827(17); C(1)–O(1), 1.282(3); C(17)–O(2), 1.268(3); N(1)–Ru(2)–N(3), 98.77(7); N(1)–Ru(2)–N(2), 81.75(7); and N(2)–Ru(2)–N(3), 83.91(7).
Scheme 2
Scheme 2. Synthesis of Complex 4
Scheme 3
Scheme 3. Synthesis of Complexes 5 and 6
Scheme 4
Scheme 4. Synthesis of Complexes 7–9
Figure 4
Figure 4
X-ray single-crystal structure of 10. The thermal ellipsoids are displayed at 30% probability. Hydrogen atoms [except H(1)] and solvents have been omitted for clarity. Selected bond distances (angstrom) and angle (deg): Ru(1)–N(1), 2.083(4); Ru(1)–N(2), 2.021(4); Ru(1)–N(3), 2.092(4); Ru(1)–Cl(1), 2.4556(13); Ru(1)–Cl(2), 2.4416(13); Ru(1)–C(17), 1.825(6); H(1)···Cl(2), 2.115; C(1)–O(1), 1.327(7); N(2)–Ru(1)–Cl(2), 175.14(13); N(1)–Ru(1)–N(3), 168.38(17); C(17)–Ru(1)–Cl(1), 178.70(18); and O(1)–H(1)–Cl(2), 163.19.
Scheme 5
Scheme 5. Synthesis of Complexes 10 and 11
Figure 5
Figure 5
Chemoselectivity of transfer hydrogenation for complexes 7–11. Conditions: 5-hexen-2-one, 2.0 mmol (2 M in 50 mL of iPrOH); 5-hexen-2-one/iPrOK/cat. = 200/10/1; N2 (0.1 MPa); 82 °C.

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