Electrochemical reduction of Ru(η-arene)(κ-tris(pyrazolyl)methane) dicationic complexes

One-electron reduction of [Ru(η⁶-arene)(κ³-HCPz₃)]²⁺ complexes (arene = p-xylene, p-cymene, or hexamethylbenzene, Pz = pyrazolyl) was observed at approximately −1.4 V vs. ferrocene in nonaqueous media. Cyclic voltammetry data showed no direct evidence of a hapticity change for either the arene or the tris(pyrazolyl)methane (TPM) ligand in the electron-transfer step. The resulting 19-electron radicals underwent reactions at the arene, giving persistent cyclohexadienyl ruthenium complexes which could be oxidized back to the original complex in an overall chemically reversible process. The dominant reaction for the hexamethylbenzene-containing radical was shown to be an arene-based dimerization (k_dim = 4 (±3) × 10³ M⁻¹ s⁻¹) which occurs in competition with an H-atom capture, most likely from solvent. The behavior of the 19-electron radicals is similar to that of their all-carbocyclic mixed-sandwich analogues Ru(arene)(C₅R₅) [O.V. Gusev, M.A. Ievlev, T.A. Peganova, M.G. Peterleitner, P.V. Petrovskii, Y.F. Oprunenko, N.A. Ustynyuk, J. Organomet. Chem. 551 (1998) 93].     

The hapticity of cyclooctatetraene in its first row mononuclear transition metal carbonyl complexes: Several examples of octahapto coordination

The two cyclooctatetraene metal carbonyls that have been synthesized are the tetrahapto derivative (η⁴-C₈H₈)Fe(CO)₃ and the hexahapto derivative (η⁶-C₈H₈)Cr(CO)₃ using the reactions of cyclooctatetraene with Fe(CO)₅ and with fac-(CH₃CN)₃Cr(CO)₃, respectively. Related C₈H₈M(CO)_n (M = Ti, V, Cr, Mn, Fe, Co, Ni; n = 4, 3, 2, 1) species have now been investigated by density functional theory in order to explore the scope of cyclooctatetraene metal carbonyl chemistry. In this connection, the existence of octahapto (η⁸-C₈H₈)M(CO)_n species is predicted as long as the central metal M does not exceed the 18-electron configuration by receiving eight electrons from the η⁸-C₈H₈ ring. Thus the lowest energy structures (η⁸-C₈H₈)Ti(CO)_n (n = 3, 2, 1), (η⁸-C₈H₈)M(CO)_n (M = V, Cr; n = 2, 1), and (η⁸-C₈H₈)Mn(CO) all have octahapto η⁸-C₈H₈ rings. An exception is (η⁶-C₈H₈)Fe(CO), with a hexahapto η⁶-C₈H₈ ring and thus only a 16-electron configuration for the iron atom. Hexahapto (η⁶-C₈H₈)M(CO)_n structures are predicted for the known (η⁶-C₈H₈)Cr(CO)₃ as well as the unknown (η⁶-C₈H₈)Ti(CO)₄, (η⁶-C₈H₈)V(CO)₃, (η⁶-C₈H₈)Mn(CO)₂, and (η⁶-C₈H₈)Fe(CO)₂ with 18, 18, 17, 17, and 18 electron configurations, respectively, for the central metal atoms. There are two types of tetrahapto C₈H₈M(CO)_n complexes. In the 1,2,3,4-tetrahapto (η⁴-C₈H₈)M(CO)_n complexes two adjacent CC double bonds, forming a 1,3-diene unit similar to butadiene, are bonded to the metal atom. In the 1,2,5,6-tetrahapto (η^2,2-C₈H₈)M(CO)₃ derivatives two non-adjacent CC double bonds of the C₈H₈ ring are bonded to the metal atom. The known (η⁴-C₈H₈)Fe(CO)₃ is a 1,2,3,4-tetrahapto complex. The unknown isomeric 1,2,5,6-tetrahapto complex (η^2,2-C₈H₈)Fe(CO)₃ is predicted to lie ∼15 kcal/mol above (η⁴-C₈H₈)Fe(CO)₃. The related 1,2,5,6-tetrahapto complexes (η^2,2-C₈H₈)Cr(CO)₄, (η^2,2-C₈H₈)Mn(CO)₄, [(η^2,2-C₈H₈)Mn(CO)₃]⁻, (η^2,2-C₈H₈)Co(CO)₂, and (η^2,2-C₈H₈)Ni(CO)₂ are all predicted to be low-energy structures.     

Bonding and structural preferences of indenyl complexes: MInd2Ln (n=0–3)

A search of bis(indenyl) derivatives available in the Cambridge Crystallographic Data Centre was performed and the two main families, MInd₂ and MInd₂L_n (n=1–3), were structurally analyzed in detail. DFT calculations were performed for some relevant compounds in order to understand their electronic structure and interpret the experimental data. For MInd₂ complexes, the rotation angles between the rings show a wide range of values, depending both on the electron count and on the steric effects of the ring substituents. Hapticity change toward η³ is induced by extra electrons, but a perfect η³ coordination is never observed. For the electron deficient Cr(II) complexes, two isomers having two and four unpaired electrons are known for different substituents, and the calculated energies in models are very close, as expected. The MInd₂L₂ family is the largest one and examples of η³ coordination can be found. Both electronic and steric effects play a major role in determining the structural parameters of these species, but in the absence of bulky ring substituents, the rings are fluxional.     

Reactions of the cycloheptatrienyl complexes [MX(CO)2(η-C7H7)] (M=Mo, X=Br; M=W, X=I) with CNBu: X-ray crystal structure of [WI(CO)2(CNBu)2(η-C7H7)]

Reaction of [MX(CO)₂(η⁷-C₇H₇)] (M=Mo, X=Br; M=W, X=I) with two equivalents of CNBu^t in toluene affords the trihapto-bonded cycloheptatrienyl complexes [MX(CO)₂(CNBu^t)₂(η³-C₇H₇)] (1, M=Mo, X=Br; 2, M=W, X=I). The X-ray crystal structure of 2 reveals a pseudo-octahedral molecular geometry with an asymmetric ligand arrangement at tungsten in which one CNBu^t is located trans to the η³-C₇H₇ ring. Treatment of 2 with tetracyanoethene results in 1,4-cycloaddition at the η³-C₇H₇ ring to give [WI(CO)₂(CNBu^t)₂{η³-C₉H₇(CN)₄}], 3. The principal reaction type of the molybdenum complex 1 is loss of carbonyl and bromide ligands to afford substituted products [MoBr(CNBu^t)₂(η⁷-C₇H₇)] 4 or [Mo(CO)(CNBu^t)₂(η⁷-C₇H₇)]Br. Reaction of [MoBr(CO)₂(η⁷-C₇H₇)] with one equivalent of CNBu^t in toluene at 60°C affords [MoBr(CO)(CNBu^t)(η⁷-C₇H₇)], 5, which is a precursor to [Mo(CO)(CNBu^t)(NCMe)(η⁷-C₇H₇)][BF₄], 6, by reaction with Ag[BF₄] in acetonitrile. In contrast with the parent dicarbonyl systems [MoX(CO)₂(η⁷-C₇H₇)], complexes of the Mo(CO)(CNBu^t)(η⁷-C₇H₇) auxiliary, 5 and 6, do not afford observable η³-C₇H₇ products by ligand addition at the molybdenum centre.     

Cyclopentadienyl Complexes of the Alkaline Earths in Light of the Periodic Trends

The electron-rich cyclopentadienyl and the analogous indenyl and fluorenyl ligands (collectively denoted here as Cp’) have been impactful in stabilizing electron-deficient metal centers including the highly electropositive alkaline earths. Being in the s-block, the group 2 metals follow a major periodic variation in their atomic and ionic properties which is reflected in those Cp’ compounds. This article presents an overview of this class of compounds for all the five metals from beryllium to barium (radium is excluded for its radioactivity), highlighting their systematic variation.     

Syntheses and characterizations of methylpalladium complexes bearing a biphenyl-based bulky phosphine ligand: Weak interactions suggested by NBO and QTAIM analyses

A methylpalladium chloride complex bearing a biphenyl-based bulky phosphine ligand, ^tBu₂P(biphenyl-2-yl) (1), namely [Pd(1)(Me)Cl]₂ (=8), was synthesized by the reaction of (cod)Pd(Me)Cl with 1. A following reaction of 8 with AgOTf gave the corresponding triflate complex, Pd(1)(Me)OTf (=9). These complexes were fully characterized by NMR spectroscopy and structurally characterized by X-ray crystallographic study. In the solid state of 8, biphenyl-based phosphine ligand 1 played a role of monodentate phosphine ligand with a possible weak η¹-coordination from the phenyl ring at 2′-position of 1. On the other hand, phosphine ligand 1 in 9 showed a bidentate coordination mode consist of σ-donation of the phosphine and η²-coordination of the phenyl ring at 1′- and 2′-positions. Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) calculations also supported the existence of these weak interactions.