The structures, bonding, and ring-opening reactions of strained cyclic carbon-based molecules form a key component of standard textbooks. In contrast, the study of strained organometallic molecules containing transition metals is a much more recent development. A wealth of recent research has revealed fascinating nuances in terms of structure, bonding, and reactivity. Building on initial work on strained ferrocenophanes, a broad range of strained organometallic rings composed of a variety of different metals, pi-hydrocarbon ligands, and bridging elements has now been developed. Such strained species can potentially undergo ring-opening reactions to functionalize surfaces and ring-opening polymerization to form easily processed metallopolymers with properties determined by the presence of the metal and spacer. This Review summarizes the current state of knowledge on the preparation, structural characterization, electronic structure, and reactivity of strained organometallic rings with pi-hydrocarbon ligands and d-block metals.
Irradiation of silicon-bridged ferrocenophane [Fe(eta-C(5)H(4))(2)SiMe(2)] (1) in the presence of substitutionally labile Lewis bases such as 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy) initiates ring-opening polymerization and oligomerization via cleavage of an iron-cyclopentadienyl bond. A distribution of cyclic polyferrocenylsilane c-PFS (PFS = [Fe(eta-C(5)H(4))(2)SiMe(2)](n)) and a series of cyclic oligomers (2(2)-2(7)) were isolated by column chromatography and fully characterized. Varying temperature and concentration were found to influence the molecular weight distribution and the ratio of polymer to oligomer products, enabling the formation of c-PFS with molecular weights >100 kDa. Cyclic polymer samples were found to possess lower hydrodynamic radii and viscosity and higher glass transition temperatures than those of their linear PFS counterparts (l-PFS) of comparable molecular weight. Compared with crystalline samples of l-PFS of similar molecular weights, c-PFS formed smaller spherulites, as observed by polarizing optical microscopy. While the wide-angle X-ray scattering (WAXS) patterns from lower molecular weight l-PFS were found to differ from those from higher molecular weight samples, those obtained for lower and higher molecular weight samples of c-PFS are identical and resemble diffraction patterns of high molecular weight l-PFS. The electrochemical behavior of each cyclic oligomer 2(2)-2(7) was studied by cyclic and differential pulse voltammetry and was found to depend on whether the oligomer contains an odd or even number of ferrocene units. In contrast to linear analogs, two reversible redox processes of varying intensities were observed for cyclic oligomers containing an even number of iron centers, while three reversible redox processes of varying intensities were observed for cyclic oligomers containing an odd number of iron centers. As the oligomer chain length increased, the electrochemical behavior of all cyclic oligomers approached that of both cyclic and linear high molecular weight polymers.
A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Fe3M(μ4-O)(μ2-OH) core that remains intact upon changing M or the oxidation state of iron. Electrochemical studies reveal that the reduction potentials (E1/2) span a window of 500 mV and depend upon the Lewis acidity of M. Using the pKa of the redox-inactive metal aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between E1/2 and acidity with a slope of ca. 70 mV per pKa unit. The current study of [Fe3MO(OH)] and previous ones of [Mn3MOn] (n = 2, 4) moieties support the generality of the above relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations, as applied to clusters of different redox-active metals.
This paper presents the synthesis and characterization of a series of pincer ligands and their Ni, Pd, Pt, and Rh complexes. The ligands under examination are based on a diarylamine which is modified either by two phosphino (-PR2) substituents in the ortho-positions (PNP ligands) or by a combination of a phosphino and an iminyl (-CH═NX) substituent (PNN ligands). The ligands can be broken down into three groups: (a) C2v-symmetric PNP ligands with identical side -PR2 donors, (b) Cs-symmetric PNP' ligands with different -PR2 side donors, and (c) PNN ligands containing a -P(i)Pr2 side donor. All of the ligands under study readily formed square-planar complexes of the types (PNZ)PdCl, (PNZ)Pd(OAc), and (PNZ)RhCO, where PNZ is the corresponding anionic tridentate pincer ligand. For select PNP ligands, (PNP)NiCl and (PNP)PtCl were also studied. The (PNZ)MCl complexes (M = Ni, Pd, Pt) underwent quasireversible oxidation in cyclic voltammetry experiments. Based on the close similarity of formal potentials for Ni, Pd, and Pt analogs, and based on the previous literature evidence, these oxidation events are ascribed primarily to the PNZ ligand, and the E1/2 values can be used to compare the ease of oxidation of different ligands. A (PNP)PdCl complex containing methoxy substituents para- to the central nitrogen underwent two quasireversible oxidations. Two mono-oxidized complexes were isolated and structurally characterized in comparison to their neutral analog, revealing minimal changes in the bond distances and angles. Several other neutral complexes were also structurally characterized. The carbonyl stretching frequency in (PNZ)RhCO complexes was used to gauge the donating ability of the various pincer ligands toward the metal. Comparison of E1/2 values for (PNZ)PdCl and νCO values for (PNZ)RhCO revealed that the two are not consistently correlated across all the studied ligands and can be tuned to different degrees through judicious ligand alteration.
A platform for investigating the impact of π-extension in benzannulated, anionic pincertype N^N-^N-coordinating amido ligands and their Pt(II) complexes is presented. Based on bis(8-quinolinyl)amine, symmetric and asymmetric proligands bearing quinoline or πextended phenanthridine (3,4-benzoquinoline) units are reported, along with their redemitting, phosphorescent Pt(II) complexes of the form (N^N-^N)PtCl. Comparing the photophysical properties of complexes of (quinolinyl)amido ligands with those of πextended (phenanthridinyl)amido analogs revealed a counter-intuitive impact of siteselective benzannulation. Contrary to conventional assumptions regarding π-extension, and in contrast to isoenergetic lowest energy absorption bands and a red shift in fluorescence from the organic proligands, a blue shift of nearly 40 nm in the emission wavelength is observed for Pt(II) complexes with more extended bis(phenanthridinyl) ligand π-systems. Comparing the ground state and triplet excited state structures optimized from DFT and TD-DFT calculations, we trace this effect to a greater rigidity of the benzannulated complexes, resulting in a higher energy emissive triplet state, rather than to a significant perturbation of orbital energies caused by π-extension.
Reactions of a series of phenyl esters with a (PNP)Rh fragment have been studied. PhO(2)CPh only underwent C-H oxidative addition (OA). PhO(2)CCF(3) chiefly underwent acyl-oxygen OA. PhO(2)CBu(t) and PhO(2)CNEt(2) initially underwent OA of an ortho-C-H bond of the phenyl group but continued thermolysis led to the phenyl-oxygen OA products.
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