Synthesis and Electronic Structure of Ru2(Xap)4(Y-gem-DEE) Type Compounds: Effect of Cross-Conjugation.

Reported in this Article are the preparation and characterization of a series of new Ru2(II,III) compounds bearing one cross-conjugated σ-geminal-diethynylethene ligand (gem-DEE), namely, Ru2(Xap)4(Y-gem-DEE) (Xap = N,N'-anilinopyridinate (ap) or 2-(3,5-dimethoxy)anilinopyridinate (DiMeOap), and Y = Si(i)Pr3 (1) or H (2)) and [Ru2(ap)4]2(µ-gem-DEE) (3). Compounds 1-3 were characterized by spectroscopic and voltammetric techniques as well as the single crystal X-ray diffraction study of 2a. The X-ray structural data of 2a and the spectroscopic/voltammetric data of compounds 1 and 2 indicate that the gem-DEE ligands are similar to simple alkynyls in their effects on the molecular and electronic structures of the Ru2(Xap)4 moiety. Similar to the previously studied [Ru2(ap)4]2(µ-C2n) type compounds, dimer 3 exhibits pairwise 1e(-) oxidations and reductions, albeit the potential splits within the pair (ΔE1/2) are significantly smaller than those of [Ru2(ap)4]2(µ-C4). The electronic absorption spectra of the reduced and oxidized derivatives of 1a and 3 were determined using spectroelectrochemistry methods. No discernible intervalence charge transfer transition (IVCT) was detected in the near-IR spectrum for either 3(-) or 3(+), suggesting that the Ru2-Ru2 coupling in these mixed-valence states is weak. DFT calculations on a model compound of 3 yielded six singly occupied molecular orbitals (SOMOs), which have Ru2 contributions similar to those previously calculated for the [Ru2(ap)4]2(µ-C2n) type compounds. Among six SOMOs, SOMO-2 is the only one containing substantial dπ-π(gem-DEE) character across the entire Ru2-µ-gem-DEE-Ru2 linkage, which explains the weakened Ru2-Ru2 coupling.

Variable noninnocence of substituted azobis(phenylcyanamido)diruthenium complexes.

The synthetic chemistry of substituted 4,4'-azobis(phenylcyanamide) ligands was investigated, and the complexes [{Ru(tpy)(bpy)}2(µ-L)][PF6]2, where L = 2,2':5,5'-tetramethyl-4,4'-azobis(phenylcyanamido) (Me4adpc(2-)), 2,2'-dimethyl-4,4'-azobis(phenylcyanamido) (Me2adpc(2-)), unsubstituted (adpc(2-)), 3,3'-dichloro-4,4'-azobis(phenylcyanamido) (Cl2adpc(2-)), and 2,2':5,5'-tetrachloro-4,4'-azobis(phenylcyanamido) (Cl4adpc(2-)), were prepared and characterized by cyclic voltammetry and vis-near-IR (NIR) and IR spectroelectrochemistry. The room temperature electron paramagnetic resonance spectrum of [{Ru(tpy)(bpy)}2(µ-Me4adpc)](3+) showed an organic radical signal and is consistent with an oxidation-state description [Ru(II), Me4adpc(•-), Ru(II)](3+), while that of [{Ru(tpy)(bpy)}2(µ-Cl2adpc)](3+) at 10 K showed a low-symmetry Ru(III) signal, which is consistent with the description [Ru(III), Cl2adpc(2-), Ru(II)](3+). IR spectroelectrochemistry data suggest that [{Ru(tpy)(bpy)}2(µ-adpc)](3+) is delocalized and [{Ru(tpy)(bpy)}2(µ-Cl2adpc)](3+) and [{Ru(tpy)(bpy)}2(µ-Cl4adpc)](3+) are valence-trapped mixed-valence systems. A NIR absorption band that is unique to all [{Ru(tpy)(bpy)}2(µ-L)](3+) complexes is observed; however, its energy and intensity vary depending on the nature of the bridging ligand and, hence, the complexes' oxidation-state description.

Phenylcyanamide ligand control of photo-induced linkage isomerism.

The photo- and electrochemically induced linkage isomerism of six new complexes [Ru(bpy)2(L)(dmso-S)](+), where dmso is dimethylsulfoxide, bpy is 2,2'-bipyridine, and L(-) is pentachloro-, 2,3,5,6-tetrachloro-, 2,4,5-trichloro-, 2,4-dichloro-, 4-chloro-, and unsubstituted phenylcyanamide anion, were investigated. The quantum yields of linkage isomerism forming the metastable [Ru(bpy)2(L)(dmso-O)](+) complexes are shown to decrease with increasing donor properties of the phenylcyanamide ligand, and it is suggested that the donor properties of the cyanamide ligand stabilize the [Ru(bpy)2(L)(dmso-S)](+) complexes in the (3)MLCT excited state. The cyclic voltammetry of these complexes showed two oxidation processes: a phenylcyanamide L(0/-) couple (an assignment supported by density functional theory (DFT) calculations) and a Ru(III/II) couple at more positive potential. Upon oxidation to Ru(III), the complexes rearranged to form Ru-O linkage isomers, and the scan rate dependent voltammograms permitted estimates of the rates of linkage isomerism.

Non-innocence of 1,4-dicyanamidobenzene bridging ligands in dinuclear ruthenium complexes.

Four dinuclear complexes, [{Ru(II)(ttpy)(bpy)}2(µ-L)][PF6]2, where bpy is 2,2'-bipyridine, ttpy is 4-(tert-butylphenyl)-2,2':6',2"-terpyridine, and L is 2,5-dimethyl-, 2,5-dichloro-, 2,3,4,5-tetrachloro- and unsubstituted 1,4-dicyanamidebenzene dianion have been synthesized and characterized. Electron paramagnetic resonance (EPR) spectroscopy of electrogenerated [{Ru(ttpy)(bpy)}2(µ-L)](3+) ions shows largely ligand centered spin and thus the complexes' oxidation states are best formulated as [Ru(II), L(•-), Ru(II)](3+). Visible-NIR and IR spectra of [{Ru(ttpy)(bpy)}2(µ-L)](3+,4+) ions were also obtained by spectroelectrochemical methods. For the [{Ru(ttpy)(bpy)}2(µ-L)](3+) ions, the significant variations in the spectra were rationalized in terms of an increased ruthenium contribution to the singly occupied molecular orbital with increasing number of chloro substituents on the bridging ligand L.

Modulation of electronic couplings within Ru2-polyyne frameworks.

Dimers of [Ru(2)(Xap)(4)] bridged by 1,3,5-hexatriyn-diyl (Xap are 2-anilinopyridinate and its aniline substituted derivatives), [Ru(2)(Xap)(4)](2)(µ-C(6)) (1), were prepared. Compounds 1 reacted with 1 equiv of tetracyanoethene (TCNE) to yield the cyclo-addition/insertion products [Ru(2)(Xap)(4)](2){µ-C≡CC(C(CN)(2))-C(C(CN)(2))C≡C} (2) and 1 equiv of Co(2)(dppm)(CO)(6) to yield the η(2)-Co(2) adducts to the middle C≡C bond, [Ru(2)(Xap)(4)](2)(µ-C(6))(Co(2)(dppm)(CO)(4)) (3). Voltammetric and spectroelectrochemical studies revealed that (i) two Ru(2) termini in 1 are sufficiently coupled with the monoanion (1(-)) as a Robin-Day class II/III mixed valence species; (ii) the coupling between two Ru(2) is still significant but somewhat weakened in 3; and (iii) the coupling between two Ru(2) is completely removed by the insertion of TCNE in 2. The attenuation of electronic couplings in 2 and 3 was further explored with both the X-ray diffraction study of representative compounds and spin-unrestricted DFT calculations.