Transition from a nonbonding to a bonding interaction in a tetranuclear [Mo2]2(mu-OR)4 cluster

Abnormally long-range diamagnetic anisotropy induced by cyclic d(δ)-p(π) π conjugation within a six-membered dimolybdenum/chalcogen ring

Incorporating two quadruply bonded dimolybdenum units [Mo(2)(DAniF)((3))](+) (ancillary ligand DAniF = N,N'-di-p-anisylformamidinate) with two hydroselenides (SeH(-)) gave rise to [Mo(2)(DAniF)(3)](2)(μ-SeH)(2) (1). With the molecular scaffold remaining unchanged, aerobic oxidation of 1, followed by autodeprotonation, generated [Mo(2)(DAniF)(3)](2)(μ-Se)(2) (2). The two complexes share a common cyclic six-membered Mo(2)/Se core, but compound 2 is distinct from 1 by having structural, electronic, and magnetic properties that correspond with aromaticity. Importantly, the aromatic behaviors for this non-carbon system are ascribable to the bonding analogy between the δ component in a Mo-Mo quadruple bond and the π component in a C-C double bond. Cyclic π delocalization via d(δ)-p(π) conjugation within the central unit, which involves six π electrons with one electron from each of the Mo(2) units and two electrons from each of the bridging atoms, has been confirmed in a previous work on the oxygen- and sulfur-bridged analogues (Fang, W.; et al. Chem.-Eur. J.2011, 17, 10288). Of the three members in this family, compound 2 exhibits an enhanced aromaticity because of the selenium bridges. The remote in-plane and out-of-plane methine (ArNCHNAr) protons resonate at chemical shifts (δ) 9.42 and 7.84 ppm, respectively. This NMR displacement, Δδ = 1.58 ppm, is larger than that for the oxygen-bridged (1.30 ppm) and sulfur-bridged (1.49 ppm) derivatives. The abnormally long-range shielding effects and the large diamagnetic anisotropy for this complex system can be rationalized by the induced ring currents circulating the Mo(2)/chalcogen core. By employment of the McConnell equation {Δσ = Δχ[(l - 3 cos 2θ)/3R(3)N]}, the magnetic anisotropy (Δχ = χ(⊥) - χ(||)) is estimated to be -414 ppm cgs, which is dramatically larger than -62.9 ppm cgs for benzene, the paradigm of aromaticity. In addition, it is found that the magnitude of Δχ is linearly related to the radius of the bridging atoms, with the selenium analogue having the largest value. This aromaticity sequence is in agreement with that for the chalcogen-containing aromatic family, e.g., furan < thiophene < selenophene.

Enhancement in electronic communication upon replacement of Mo-O by Mo-S bonds in tetranuclear clusters of the type [Mo2]2(mu-E-E)2 (E = O or S)

A tetranuclear cluster containing two quadruply bonded cis-Mo2(DAniF)2 units (DAniF = N,N'-di-p-anisylformamidinate) linked by four hydroxide groups (1) was obtained by hydrolysis of [Mo2(cis-DAniF)2](mu-OCH3)4. Analogous compounds linked by two bidentate bridges (o-O2C6H4 for 2, o-O2C10H6 for 3, and o-S2C6H4 for 4) were synthesized by direct assembly of the corner species precursor [Mo2(cis-DAniF)2(NCCH3)4](BF4)2 and the respective protonated ligands. All four compounds were characterized by X-ray crystallography. Cyclic voltammograms of the O-linked compound 2 and the S analogue 4 show two reversible one-electron-oxidation processes with potential separations (DeltaE(1/2)) of 474 and 776 mV, respectively. The large increase of about 300 mV in DeltaE(1/2) for the S analogue relative to that of the O compound is consistent with a large increase in electronic communication. This enhancement occurs despite the increase of ca. 0.45 A in nonbonding separation between the midpoints of the Mo2 units, which changes from 3.266 A in 2 to 3.72 A in 4, and the increase of ca. 0.4 A in M-E distances as E changes from O to S. Density functional theory calculations show that the increase in electronic communication between the metal centers in 4 is due to a superexchange pathway involving d and p orbitals in the linker E atoms that is less important in 2.

A Rare Dimer of Dimers Having Four Hydride Linkers Joining Two Quadruply Bonded Dimolybdenum Units

Hydride anions, H-, have been found to cause the assembly of dimolybdenum units [Mo2(cis-DAniF)2]2+, DAniF = N,N'-di(p-anisyl)formamidinate, forming a tetranuclear complex [Mo2(cis-DAniF)2]2(mu-H)4 (1) with an Mo4H4 core that may be described as an elongated tetrahedron in which the H atoms are along the four long edges of such tetrahedron and the Mo2 units are along the short edges. The two quadruply bonded dimolybdenum units, separated by only 2.718 A, are essentially orthogonal. This gives the shortest [Mo2]...[Mo2] distance known for complexes with multiple dimolybdenum units. DFT calculations indicate that the energy of a cuboidal isomer is only 3.8 kcal/mol above that of 1, but such an isomer has not been observed.

Electronic Localization versus Delocalization Determined by the Binding of the Linker in an Isomer Pair

By using stoichiometric amounts of (C5H5)2FePF6, the isomeric neutral diamidate-bridged molecules, alpha- and beta-(DAniF)3Mo2(ArN(O)CC(O)NAr)Mo2(DAniF)3, with Ar = p-MeOC6H4, have been oxidized to give the PF6 salts of the four cations alpha1+, alpha2+, beta1+, and beta2+. All four structures have been accurately determined and, together with supporting evidence from near-IR, EPR, NMR and magnetic susceptibility measurements, it clearly establishes that in the mixed-valent alpha+ species the unpaired electron is localized over only one of the Mo2 units while the alpha2+ cation behaves as a diradical having two Mo25+ units that are essentially uncoupled. However, the beta+ species is fully delocalized, in the time scale of the experiments, with the unpaired electron being equally shared by the two Mo2 units. It displays a HOMO-1 --> SOMO transition at 4700 cm-1 (Deltanu1/2 = 2300 cm-1). Because of strong coupling, the beta2+ species is diamagnetic.

Tetraplatinum precursors for supramolecular assemblies: syntheses, crystal structures, and stereoselective self-assemblies of [Pt4(μ-OCOCH3)6(κ4-N4-DArBp)] (DArBp = 1,3-bis(arylbenzamidinate)propane)

Treatment of the cisoid-capped complex [Pt(4)(micro-OCOCH(3))(6)(kappa(4)-N(4)-DAr'Bp)] (), which was obtained from the reaction of [Pt(4)(micro-OCOCH(3))(8)] (1) with Na(2)D(t)BuPhBp (D(t)BuPhBp = 1,3-bis(para-tert-butylphenylbenzamidinate)propane), with an equimolar amount of 1,1'-ferrocenedicarboxylic acid undergoes a stereoselective self-assembly to afford an indented-quadrangle tetramer [Pt(4)(micro-OCOCH(3))(4)(kappa(4)-N(4)-D(t)BuPhBp){Fe(C(5)H(4)COO)(2)}](4) as a single diastereomer.

An isomeric pair of fluoride-bridged cyclic dimolybdenum triads

A pair of isomeric cyclic triads containing three quadruply bonded [Mo2] units, [Mo2(cis-DAniF)2]2+ (DAniF = N,N'-di-p-anisylformamidinate), bridged by six fluoride anions, has been synthesized and crystallographically characterized. For the alpha isomer, the three [Mo2] units are oriented in two orthogonal directions. Two of them are structurally equivalent and parallel to each other, but oriented perpendicular to the third one. The beta isomer is a triangle with three geometrically identical [Mo2] units, parallel to each other, as the vertices. Thus, the beta isomer possesses idealized D3h symmetry while the alpha isomer only has C2v symmetry. These two isomers do not interconvert in boiling THF or toluene or under irradiation with ultraviolet light, but oxidation of the alpha isomer first generates an alpha+ species that changes to beta+. The two isomers have very similar electrochemical behavior, both showing three reversible one-electron redox processes for the [Mo2] centers and similar potential separations (DeltaE(1/2)). The first and second redox couples are well separated (ca. 390-410 mV), while the second and third ones are separated by only about 150 mV.

Strong Electronic Communication by Direct Metal−Metal Interaction in Molecules with Halide-Bridged Dimolybdenum Pairs

Reactions of [cis-Mo2(DAniF)2(NCCH3)4](BF4)2, DAniF = N,N'-di-p-anisylformamidinate, with an excess of anhydrous Bu(n)4NX (X = Cl, Br, I), produced the halide-bridged tetranuclear clusters, [cis-Mo2(DAniF)2]2(mu-X)4, X = Cl (1), Br (2), and I (3). All three compounds show two reversible one-electron oxidation processes with potential separations (DeltaE(1/2)) between the two oxidation processes of 540, 499, and 440 mV, respectively. These DeltaE(1/2) values show that the strength of the electronic coupling between the dimetal units decreases as the Mo2...Mo2 distance increases from 1 to 2, and then to 3. The structures, EPR spectra, and near-IR (NIR) spectra of the corresponding mixed-valence species (1-PF6, 2-PF6, and 3-PF6) indicate that the clusters are electronically delocalized. Calculations at the DFT level indicate that the strong electronic communication is principally due to a direct overlap between the delta orbitals from the adjacent dimetal units.