Dinuclear iridium and rhodium complexes with bridging arylimidazolide-N³,C²ligands: synthetic, structural, reactivity, electrochemical and spectroscopic studies

Abstract

Deprotonation of 1-arylimidazoles (aryl = mesityl (Mes), 2,6-diisopropylphenyl (Dipp)), with n-butyl lithium afforded the corresponding derivatives (1-aryl-1H-imidazol-2-yl)lithium (1a, Ar = Mes; 1b, Ar = Dipp) in good yield. Reaction of 1a with 0.5 equiv. of [Ir(cod)(μ-Cl)]2 yielded two geometrical isomers of a doubly C2,N3-bridged dinuclear complex [Ir(cod){μ-C3H2N2(Mes)-κC2,κN3}]2 (3), 3H-H, a head-to-head (H-H) isomer of CS symmetry, and 3H-T, the thermodynamically preferred head-to-tail (H-T) isomer of C2 symmetry. The metallated carbon of the 4 electron donor anionic bridging ligands has some carbene character, reminiscent of the situation in N-metallated protic NHC complexes. Displacement of cod ligands from 3H-H and 3H-T afforded the tetracarbonyl complexes [Ir(CO)2{μ-C3H2N2(Mes)-κC2,κN3}]24H-H and 4H-T, respectively. The reaction with PMe3, which gave only one complex, [Ir(CO)(PMe3){μ-C3H2N2(Mes)-κC2,κN3}]2 (5), demonstrates that the isomerization of the central core Ir[μ-C3H2N2(Mes)-κC2,κN3]2Ir from H-H to H-T on going from 4H-H to 5 is readily triggered by phosphine substitution under mild conditions. Oxidative-addition of MeI to 5 afforded the formally metal-metal bonded d(7)-d(7) complex [Ir2(CO)2(PMe3)2(Me)I{μ-C3H2N2(Mes)-κC2,κN3}2] (6). The blue [Ir(C2H4)2{μ-C3H2N2(Mes)-κC2,κN3}]2 (7) and purple [Rh(C2H4)2{μ-C3H2N2(Dipp)-κC2,κN3}]2 (9) tetraethylene complexes were also obtained with only a H-T arrangement of the bridging ligands. Although only modestly efficient in alkane dehydrogenation, complex 7 was found to be a more active pre-catalyst than 3H-T, 4H-T and 5, probably because of the favorable lability of the ethylene ligands. From cyclic voltammetry, exhaustive coulometry and spectroelectrochemistry studies, it was concluded that 3H-T undergoes a metal-based one electron oxidation to generate the mixed-valent Ir(i)/Ir(ii) system. The energy of the intervalence band for the orange dirhodium complex [Rh(cod){μ-C3H2N2(Mes)-κC2,κN3}]2 (8) is shifted toward lower energies in comparison with 3H-T, reflecting the decrease of the energy with the intermetallic distance. It was concluded from the EPR study that the Ir and Rh centres contribute substantially to the experimental magnetic anisotropy and thus to the singly occupied molecular orbital (SOMO) in the mixed-valent Ir(i)/Ir(ii) and Rh(i)/Rh(ii) systems. The molecular structures of 3H-H, 3H-T, 8 and 9 have been determined by X-ray diffraction.

Reactivity of [M2(μ‐Cl)2(cod)2] (M=Ir, Rh) and [Ru(Cl)2(cod)(CH3CN)2] with Na[H2B(bt)2]: Formation of Agostic versus Borate Complexes

“…[18] The distance between the metal centers in 1 (3.6195(9) ) and 2 (3.6749(9) )p reclude any strong bondingi nteraction between them. They are significantly longer than recently reportedd inuclear Ir I complexes,[ Ir(cod){m-C 3 H 2 N 2 (Mes)-kC 2 ,kN 3 }] 2 (HÀH3 .1844(9) and HÀT3 .1407 (2) ;H ÀHh ead to head isomer;H ÀTh ead to tail isomer) [19] and [(cod) 2 Rh 3 (m 3 -C 7 H 4 NS 2 ) 2 AgO 2 C 10 ]( ca.2.90 ). [18b] However,t he CÀNd istances in 1 (1.302 )a nd 2 (1.322 )a re slightly shortert han those of [Ir(cod){m-C 3 H 2 N 2 (Mes)-kC 2 ,kN 3 }] 2 (HÀH1 .…”

“…They are significantly longer than recently reported dinuclear Ir I complexes, [Ir(cod){μ‐C 3 H 2 N 2 (Mes)‐κC 2 ,κN 3 }] 2 (H−H 3.1844(9) Å and H−T 3.1407(2) Å; H−H head to head isomer; H−T head to tail isomer) and [(cod) 2 Rh 3 (μ 3 ‐C 7 H 4 NS 2 ) 2 AgO 2 C 10 ] (ca.2.90 Å) . However, the C−N distances in 1 (1.302 Å) and 2 (1.322 Å) are slightly shorter than those of [Ir(cod){μ‐C 3 H 2 N 2 (Mes)‐κC 2 ,κN 3 }] 2 (H−H 1.35(2) Å and H−T 1.349(5) Å) …”

“…35(2) and HÀT 1.349 (5) ). [19] To gain somei nsight into the electronic structures and the bonding, we carriedo ut the DFT calculations on 1 and 2.T he molecular orbitala nalysiso f1 and 2 show essentially identical frontier molecular orbitals with an increased HOMO-LUMO gap for 2.A dditionally,w ehave observed that the HOMO is largelyl ocalized on the metal atoms in both the cases ( Figure S11).…”

Homo‐ and Heterodinuclear Ir and Rh Imine‐functionalized Protic NHC Complexes: Synthetic, Structural Studies, and Tautomerization/Metallotropism Insights

“…In order to examinet he influence of the potentially chelating imino group on the reactivity of complex 1a,i np articular in the course of the chloride abstraction which led to products arising from tautomerization/metallotropism,w ep repared the complexes [Ir(cod)Cl{C 3 H 3 N 2 (nBu)-kN3}] (4a)a nd [Ir(cod)-Cl{C 3 H 3 N 2 (Mes)-kN3}] (4b) [20] (Mes = mesityl) (Scheme 5), with the N-butyl-and N-mesityl-substituents, respectively.…”

“…[16f] complexes 1a, [19] 2, [19] 3a + [PF 6 ] À , [19] and 4b [20] were prepared according to the literature. NMR spectra of organic compounds and complexes were recorded on aB ruker 300, 400, or 500 MHz instrument at ambient temperature and referenced by using the proton ( 1 H) or carbon ( 13 C H} NMR spectra were recorded on aB ruker Avance3 00 instrument at 121.49 MHz by using H 3 PO 4 (85 %i nD 2 O) as external standard.…”