Bond character and conformational equilibration of ethylene- and tetrafluoroethylenerhodium complexes from nuclear magnetic resonance spectra

Dinuclear iridium and rhodium complexes with bridging arylimidazolide-N(3),C(2) ligands: synthetic, structural, reactivity, electrochemical and spectroscopic studies

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.

Synthesis and dynamic behaviour of zwitterionic [M(η(6)-C6H5-BPh3)(coe)2] (M = Rh, Ir) cyclooctene complexes

The synthesis and structural characterization of zwitterionic [(η(6)-C6H5-BPh3)M(coe)2] (M = Rh, Ir) cyclooctene complexes is described. Both complexes exhibit an unusual exo-endo conformation of both cyclooctene ligands in the solid state. However, an equilibrium between the endo-endo and exo-endo rotational isomers arising from the hindered rotation about the metal-cyclooctene bond is observed in solution. Rotational barriers of around 65 kJ mol(-1) (Rh) and 84 kJ mol(-1) (Ir) have been determined by 2D EXSY NMR spectroscopy. The rotation process has also been studied by DFT calculations that showed that the dynamic behaviour is a consequence of the oscillation of the cyclooctene ligands about the metal-olefin bond instead of completing a full rotation.

Cyclopentadienylrhodium coordination to alkenylarenes

Photochemical reaction of [Rh(eta-C(5)H(5))(C(2)H(4))(2)] (5) with alkenyl benzene derivatives PhC(R(1))=CHR(2) results in the formation of four types of cyclopentadienylrhodium complexes: the mononuclear ethylene eta(2)-alkenylbenzene complexes [Rh(eta-C(5)H(5))(eta-C(2)H(4))(eta(2)-PhC(R(1))=CHR(2))] 9 a (R(1)=H, R(2)=Ph), 9 b (R(1)=Ph, R(2)=H), 9 c (R(1)=CH(3), R(2)=H), the mononuclear eta(4)-alkenylbenzene complex [Rh(eta-C(5)H(5))[beta,alpha,1,2-eta-C(6)H(5)C(Ph)=CH(2)]] (10), the dinuclear mu-eta(4):eta(4)-alkenylbenzene complex [anti-[Rh(eta-C(5)H(5))](2)[mu-beta,alpha,1,2-eta:3,4,5,6-eta-C(6)H(5)C(Ph)C=CH(2)]] (11), and the dinuclear rhodaindenyl complexes [Rh(eta-C(5)H(5))[1-3,8,9-eta-[1-(eta-C(5)H(5))]-3-R(1)-1-rhodaindenyl]] 12 a (R(1)=Ph), 12 b (R(1)=CH(3)). Reaction of 5 with triisopropenylbenzene gives the dinuclear complex [[Rh(eta-C(5)H(5))](2)(mu-beta,alpha,1,2-eta:beta',alpha',4,3-eta-C(6)H(3)[C(CH(3))=CH(2)](3))] (13). In the complexes 9, only the olefinic side chain of the alkenylbenzene binds to the metal. In the complexes 10, 11, 12, and 13, an arene nucleus coordinates to rhodium as a 1,3-diene moiety (or part thereof). The rhodaindenyl complexes 12 result from C-H activation of the alkenylbenzene at the beta and ortho positions. The crystal and molecular structures of 9 a, 9 b, 10, 11, and 12 a, b were determined. The role of 9-11 and 13 as models for intermediates during alkenylbenzene-assisted self-assembly of tricobalt clusters is discussed.

Parallel vs. perpendicular alkyne coordination in binuclear complexes. The first examples of reactivity differences in isomers differing in their alkyne coordination modes

The reaction of [Ir2(CO)3(dppm)2] (dppm = Ph2PCH2PPh2) with dimethyl acetylenedicarboxylate (DMAD) first yields [Ir2(CO)2(μ-η1:η1-DMAD)(dppm)2] (2) in which the alkyne is bound parallel to the metal–metal axis and the diphosphines are bound in a trans arrangement at both metals. This metastable isomer slowly rearranges to the stable form, [Ir2(CO)2(μ-η2:η2-DMAD)(dppm)2] (3), in which the alkyne is now bound perpendicular to the metals and the diphosphines are bent back in a cis arrangement at both metals. The analogous species can be prepared by substituting hexafluoro-2-butyne (HFB) for DMAD; however, for the HFB adduct the isomer having the parallel geometry is seen only as a transient species; only [Ir2(CO)2(μ-η2:η2-HFB)(dppm)2] (5) was isolated. Compound 2 reacts readily with PMe3 to yield [Ir2(CO)(PMe3)(μ,-CO)(μ-η1:η1-DMAD)(dppm)2], and with CH3OSO2CF3 to yield [Ir2(CH3)(CO)2(μ-η1:η1-DMAD)(dppm)2][SO3CF3], whereas 3 reacts with neither reagent. Both 2 and 3 react with HBF4•OEt2 to yield the respective alkyne-bridged hydrides, [Ir2H(CO)2(μ-η1:η1-DMAD)(dppm)2][BF4] and [Ir2H(CO)2(μ-η2:η2-DMAD)(dppm)2][BF4], in which the gross structural features and the alkyne coordination mode of the precursor are retained in each case. The latter species rearranges readily at ambient temperature, via migratory insertion, to give the vinyl-bridged product, [Ir2(CO)2(μ-η1:η2-RC=C(H)R)(dppm)2][BF4] (R = CO2Me); however, the former is inert under these conditions, yielding the above vinyl species together with other decomposition products only upon reflux in benzene for several hours. Protonation of the perpendicular hexafluoro-2-butyne adduct also yields the corresponding vinyl product, together with decomposition products. The structure of 3, as the methylene chloride disolvate, was established by X-ray analysis. Crystal data are as follows. 3•2CH2Cl2: C60H54Cl4O6P4Ir2, monoclinic, P2/c, a = 26.088(5) Å, b = 9.896(4) Å, c = 23.954(3) Å, β = 109.27(1)°, Z = 4, R(F) = 0.038, Rw(F2) = 0.0997 (all data). Key words: diiridium alkyne complexes, parallel and perpendicular alkyne coordination.

Aryldiazenido complexes: structure, fluxionality, and properties of the iridium ethylene aryldiazenido complex [(η5-C5Me5)Ir(C2H4)(p-N2C6H4OMe)][BF4] and a comparison with the analogous nitrosyl complex [(η5-C5Me5)Ir(C2H4)(NO)][BF4]

[Cp*Ir(C2H4)(N2Ar)][BF4] (1BF4; Ar = p-N2C6H4OMe) has been synthesized by reacting [ArN2] [BF4] with Cp*Ir(C2H4)2 at low temperature. An initial electrophilic attack of the incoming diazonium ion at iridium, followed by expulsion of C2H4, is postulated to account for the mild reaction conditions that are in sharp contrast to the usual inertness of the bis(ethylene) compound toward ligand substitution. The IR and nitrogen NMR data for 1BF4 and its 15Nα derivative unambiguously establish that the ArN2 ligand has the singly bent geometry in this complex in solution. The X-ray crystal structure confirms this for the solid state, and establishes that the plane of the aryldiazenido ligand is approximately perpendicular to the plane defined by the Ir atom and the centers of mass of the Cp* and ethylene ligands. An extended Hückel molecular orbital analysis of the singly bent aryldiazenido ligand has been carried out and satisfactorily accounts for the observed orientation of the ArN2 ligand. An analysis of the variable-temperature 1H and 13C NMR of 1BF4 indicates that both restricted rotation of the C2H4 ligand and a conformational isomerization of the aryldiazenido ligand are occurring, and ΔG≠270 for the ethylene rotation barrier is estimated at ≤ 51.5 ± 0.4 kJ mol−1. This is lower than the barrier of ΔG≠353 = 68.7 ± 0.2 kJ mol−1 determined previously for the analogous nitrosyl complex [Cp*Ir(C2H4)(NO)] [BF4] (2BF4) and it is suggested that in these half-sandwich complexes both NO and ArN2 function as single-faced π-acceptors, and in these circumstances ArN2 is the better π-acceptor. The ethylene in 1BF4 is readily displaced by PPh3 to give [Cp*Ir(PPh3)(N2Ar)][BF4] (3BF4). This reacts with NaBH4 to yield Cp*IrH(PPh3)(N2Ar) (4) in which the ArN2 ligand has switched to the doubly bent geometry, on the basis of the 15Nα NMR chemical shift data. Attempts to synthesize the corresponding chloro analogue 5 resulted in only the chloride salt of the singly bent ArN2 cation 3. For example, reaction of 3BF4 with HCl yields the aryldiazene complex [Cp*IrCl(PPh3)(NHNAr)] [BF4] (6), but deprotonation of this with Et3N yields 3Cl, not 5. Compound 1BF4 crystallized in the space group P21/n with a = 8.5780(10) Å, b = 20.5310(23) Å, c = 12.0310(15) Å, β = 93.500(10)°, and Z = 4. The structure was refined to Rf = 0.0281 on the basis of 2611 observed reflections with I°> 2.5σ(Io) in the range 2θ = 0–50° (Mo-Kα). Keywords: iridium, cyclopentadienyl, aryldiazenido, nitrosyl, ethylene.