Theoretical Study of the 15- and 17-Electron Structures of Cyclopentadienylchromium(III) and Cyclopentadienylmolybdenum(III) Complexes. Dichloride and Dimethyl Compounds

A Caged Neutral 17-Valence-Electron Iron(I) Radical [Fe(CO)2(Cl)(P((CH2)10)3P)]•: Synthetic, Structural, Spectroscopic, Redox, and Computational Studies

UV irradiation of yellow CH2Cl2 solutions of trans-Fe(CO)3(P((CH2)10)3P) (2a) and PMe3 (10 equiv) gives, in addition to the previously reported dibridgehead diphosphine P((CH2)10)3P (46%), a green paramagnetic complex that crystallography shows to be the trigonal-bipyramidal iron(I) radical trans-[Fe(CO)2(Cl)(P((CH2)10)3P)]• (1a•; 31% after workup). This is a rare example of an isolable species of the formula [Fe(CO)4-n(L)n(X)]• (n = 0-3, L = two-electron-donor ligand; X = one-electron-donor ligand). Analogous precursors with longer P(CH2)nP segments (n = 12, 14, 16, 18) give only the demetalated diphosphines, and a rationale is proposed. The magnetic susceptibility of 1a•, assayed by Evans' method and SQUID measurements, indicates a spin (S) of 1/2. Cyclic voltammetry shows that 1a• undergoes a partially reversible one-electron oxidation, but no facile reduction. The UV-visible, EPR, and 57Fe Mössbauer spectra are analyzed in detail. Complex 2a is similarly studied, and, despite the extra valence electron, exhibits a comparable oxidation potential (ΔE1/2 ≤ 0.04 V). The crystal structure shows a cage conformation, solvation level, disorder motif, and unit cell parameters essentially identical to those of 1a•. DFT calculations provide much insight regarding the structural, redox, and spectroscopic properties.

N-Heterocyclic carbene adducts to [Cp′FeI]2: synthesis and molecular and electronic structure

Adducts [Cp′FeI(NHC)] exhibit a highly anisotropic magnetic M_s = ±2 ground state resulting in unusual large spin–lattice (Orbach) relaxation barriers observed by zero-field ⁵⁷Fe Mössbauer spectroscopy.

Photoelectron spectroscopy and electronic structure calculations of d1 vanadocene compounds with chelated dithiolate ligands: implications for pyranopterin Mo/W enzymes

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the electronic structures of open-shell bent vanadocene compounds with chelating dithiolate ligands, which are minimum molecular models of the active sites of pyranopterin Mo/W enzymes. The compounds Cp2V(dithiolate) [where dithiolate is 1,2-ethenedithiolate (S2C2H2) or 1,2-benzenedithiolate (bdt), and Cp is cyclopentadienyl] provide access to a 17-electron, d1 electron configuration at the metal center. Comparison with previously studied Cp2M(dithiolate) complexes, where M is Ti and Mo (respectively d0 and d2 electron configurations), allows evaluation of d0, d1, and d2 electronic configurations of the metal center that are analogues for the metal oxidation states present throughout the catalytic cycle of these enzymes. A "dithiolate-folding effect" that involves an interaction between the vanadium d orbitals and sulfur p orbitals is shown to stabilize the d1 metal center, allowing the d1 electron configuration and geometry to act as a low-energy electron pathway intermediate between the d0 and d2 electron configurations of the enzyme.

Electronic Structure of Bent Titanocene Complexes with Chelated Dithiolate Ligands

Gas-phase photoelectron spectroscopy and density functional theory have been utilized to investigate the interactions between the p orbitals of dithiolate ligands and d orbitals of titanium in bent titanocene complexes as minimum molecular models of active site features of pyranopterin Mo/W enzymes. The compounds Cp(2)Ti(S-S) [where (S-S) is 1,2-ethenedithiolate (S(2)C(2)H(2)), 1, 1,2-benzenedithiolate (bdt), 2, or 1,3-propanedithiolate (pdt), 3, and Cp(-) is cyclopentadienyl] provide access to a formal 16-electron d(0) electronic configuration at the metal. A "dithiolate-folding-effect" involving an interaction of metal and sulfur orbitals is demonstrated in complexes with arene- and enedithiolates. This effect is not observed for the alkanedithiolate in complex 3.

Spin-state alteration from sterically enforced ligand rotation in bis(indenyl)chromium(II) complexes

The rotational orientation of cyclopentadienyl rings usually has no effect on d-orbital energy levels and splitting in transition metal complexes. With related but less symmetrical carbocyclic ligands, however, the magnetic properties of the associated complexes can be altered by the alignment of the ligands. Examples of this effect are found in substituted organochromium(II) bis(indenyl) complexes. The monosubstituted compounds (1-RC(9)H(6))(2)Cr (R = t-Bu, SiMe(3)) are prepared from the substituted lithium indenides and CrCl(2) in THF; they are high-spin species with four unpaired electrons. Their spin state likely reflects that in the unknown monomeric (C(9)H(7))(2)Cr, which is calculated to have a high-spin (S = 2) ground state in the staggered configuration (180 degrees rotation angle). However, the analogous bis(indenyl) complexes containing t-Bu or SiMe(3) groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R(2)C(9)H(5))(2)Cr) are low-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C(9)H(6)](2)Cr exists in a staggered conformation, with Cr-C (av) = 2.32(4) A. In contrast, the average Cr-C distances in [1,3-(t-Bu or SiMe(3))(2)C(9)H(5)](2)Cr are 2.22(2) and 2.20(2) A, respectively, and the rings are in a gauche configuration, with rotation angles of 87 degrees. The indenyl conformations are sterically imposed by the bulk of the t-Bu and SiMe(3) substituents. The change from a staggered to a gauche indenyl orientation lowers the symmetry of a (C(9)H(7))(2)M complex and allows greater mixing of metal and ligand orbitals. Calculations indicate that previously nonbonding pi orbitals of the indenyl anion are able to interact with the chromium d orbitals, producing bonding and antibonding combinations. The latter remain unpopulated, and the resulting increase in the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of using sterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds a potentially rich source of magnetically adjustable molecules.

Diene-Containing half-sandwich MoIII complexes as ethylene polymerization catalysts: experimental and theoretical studies

Seventeen-electron compounds of MoIII having the general formula [(eta5-C5R5)Mo(eta4-diene)X2] (R = H, Me: dieney = butadiene, isoprene, or 2,3-dimethylbutadiene: X= Cl, CH3) are a new class of ethylene polymerization catalysts. The polyethylene obtained shows a bimodal distribution, the major weight fraction being characterized by very long (M around 10(6)) and highly linear polymer chains. The newly prepared pentamethylcyclopentadienyl (Cp*) derivatives are more active than the cyclopentadienyl (Cp) derivatives, but much less active than previously investigated niobiumIII compounds having the same stoichiometry. On the other hand, the turnover frequency of the active site leading to the high molecular weight chains is at least 10 times greater than that obtained with the corresponding Nb catalyst. The reason for the low activity is explained by a difficult activation process that is attributed to the low polarity and high strength of the Mo-alkyl bond. This is confirmed by a Mulliken charge analysis of density functional theory (DFT) geometry-optimized [CpM(eta4-C4H6)(CH3)2] (M = Nb, Mo) and by the calculation of the heterolytic bond dissociation energies. DFT calculations have also been carried out on the ethylene insertion coordinate for the [CpM(eta4-C4H6)(CH3)]+ model of the presumed active site. The results indicate an equivalent activation barrier to insertion for the Nb and Mo systems. Differences in optimized geometries for the reaction intermediates are attributed to the presence of the extra electron for the Mo system. This electron opposes the formation of M-H-C agostic interactions, while it strengthens the back-bonding M-ethylene interaction, but otherwise plays no active role in the polymer chain propagation mechanism. According to the calculations, the chain propagation for the Mo system occurs entirely on the spin doublet surface, the minimum energy crossover point with the quartet surface lying at a higher energy than the transition state for insertion on the doublet surface.

Radical polymerization of styrene controlled by half-sandwich Mo(III)/Mo(IV) couples: all basic mechanisms are possible

Density functional calculations of bond dissociation energies (BDEs) have been used as a guide to the choice of metal system suitable for controlling styrene polymerization by either the stable free radical polymerization (SFRP) or the atom transfer radical polymerization (ATRP) mechanism. In accord with the theoretical prediction, CpMo(eta(4)-C(4)H(6))(CH(2)SiMe(3))(2), 2, is not capable of yielding SFRP of styrene. Still in accord with theoretical prediction, CpMo(eta(4)-C(4)H(6))Cl(2), 1, CpMo(PMe(3))(2)Cl(2), 3, and CpMo(dppe)Cl(2) (dppe = 1,2-bis(diphenylphosphino)ethane), 4, yield controlled styrene polymerization by the SFRP mechanism in the presence of 2,2'-azobisisobutyronitrile (AIBN). This arises from the generation of a putative Mo(IV) alkyl species from the AIBN-generated radical addition to the Mo(III) compound. The controlled nature of the polymerizations is indicated by linear M(n) progression with the conversion in all cases and moderate polydispersity indices (PDIs). Controlled polymerization of styrene is also given by compounds 3 and 4 in combination with alkyl bromides. These complexes then operate by the ATRP mechanism, again in accord with the theoretical predictions. Controlled character is revealed by linear increase of M(n) versus conversion, low PDIs, a stop-and-go experiment, and (1)H NMR and MALDI-TOF analyses of the polymer end groups. The same controlled polymerization is given by a "reverse" ATRP experiment, starting from AIBN and CpMo(PMe(3))(2)Cl(2)Br, 5. On the other hand, when compound 1 or 2 is used in combination with an alkyl bromide (as for an ATRP experiment), the isolated polystyrene shows by M(n), (1)H NMR, and MALDI-TOF analyses that catalytic chain transfer (CCT) radical polymerization takes place in this case. Kinetics simulations underscore the conditions regulating the radical polymerization mechanism and the living character of the polymerization. The complexes herein described are ineffective at controlling the polymerization of methyl methacrylate.