Cyclic Trinuclear Gold(I) Clusters with N,N and Unusual C,C Mixed-Ligand Bridges.

Three crystalline trinuclear gold(I) clusters, [Au3f2y] (1), [Au3fy2] (2), and [Au3y3] (3), where f = N,N'-bis(2,6-dimethylphenyl)methanimidamidate and y = dimethylendiphenylphosphinate, exhibit bridges from the N,N-formamidinate and/or from the ylide anion ligand whose P-methylene groups chelate in an unusual fashion, where the chelate CPC unit is perpendicular to the trigonal plane of the metal atoms. Assemblies 1 and 2 are the first gold(I) trinuclear clusters featuring mixed-ligand bridges from different N,N and C,C donors; 3 is a previously unknown homoleptic ylide anion cyclic trinuclear assembly. Formamidinate bridges in 1 and 2 connect gold(I) atoms at aurophilic distances of 3.084(2) and 3.0543(4) Å, whereas an out-of-plane (perpendicular) P-ylide anion bite produces AuI-AuI distances of as large as 3.900(2) Å in 3. The crystal space groups for 1 and 2 are triclinic P1̅ and that for 3 is monoclinic P21/c, with Z = 2 for 1 and 2 and Z = 4 for 3. Compounds are synthesized under Schlenk conditions at -20 °C in toluene by reacting the proper ratios of the gold(I) formamidinate [Au2f2] with the phosphorus ylide [Hy] under basic conditions (KOH), followed by extraction with ether. This synthesis also produces a dinuclear cation, [Au2f(Hy)2]+, previously reported by our group. A neutral mixed-ligand dinuclear complex, [Au2fy], was not observed. Under UV light, 1 and 2 display a bright-green luminescence at room temperature and in frozen methyltetrahydrofuran solutions under liquid nitrogen, with microsecond lifetimes. All three complexes 1-3 are characterized by their X-ray crystal structures, 1H NMR, IR, UV-visible, and luminescence spectroscopies, and elemental analysis.

Exploring the Intricacies of Weak Interactions in Metal-Metal Bonds Using an Unsymmetrical Carbonyl Precursor and a Triple-Bonded W2(6+) Paddlewheel.

Stepwise reaction of W(CO)6 with tetramethylated bicyclic guanidinate ligands, characterized by a central C(N)3 unit joining two fused six-membered rings with CH2CMe2CH2 units spanning two of the nitrogen atoms, allowed isolation of W2(µ-CO)2(µ-TMhpp)2(η(2)-TMhpp)2, 1, a precursor of W2(TMhpp)4Cl2 ( J. Am. Chem. Soc. 2013 , 135 , 17889 ; TMhpp = [(CH2CMe2CH2)2(C(N)3)]). Subsequent heating of 1 followed by reaction with TlPF6 generates [W2(TMhpp)4](PF6)2, 2. Compound 1 has an edge-sharing bioctahedral (ESBO) arrangement with a W2(µ-CO)2(4+) core having semibridging carbonyl groups, while 2 has a paddlewheel structure with a W2(6+) core spanned by four tetramethyl-substituted bicyclic guanidinate ligands. This compound also has hexafluorophosphate anions along the metal-metal bond that are nestled within methylene groups with the aid of a network of weak C-H···F interactions that prevent a close approach of the fluorine atoms to the dimetal unit. Theoretical computations were carried out on ditungsten model complexes supported by three ligand sets: bicyclic guanidinate, guanidinate, and formamidinate. The computations show that the π-accepting ability of the carbonyl groups significantly lowers the energy of the σ* orbital, and thus, the energy falls below that of the δ orbital. This information along with the diamagnetism of both 1 and 2-as shown by the sharp signals in the (1)H NMR spectra that support a lack of unpaired electrons (S = 0)-is consistent with the electronic configuration of σ(2)π(2)σ*(2)δ(2) (π(2)δ(2)) and thus a formal bond order of 2 for 1 and σ(2)π(4) for the triple-bonded W2(6+) core in 2. A comparison of the W-W bond lengths in 2, its chloro precursor W2(TMhpp)4Cl2, and the corresponding analogue W2(hpp)4Cl2 shows a substantial effect from the axially coordinated ligand, distal lone pair in determining the length of the metal-metal bond for these paddlewheel species. The importance of the ligands in tuning the energy level of the metal-metal bonds that may lead to dramatic changes in physical properties is also discussed. It is noteworthy that bicyclic guanidinates with the strongest π-donating ability push upward the energy level of the δ orbital, thus allowing the compounds to be easily oxidized.

Multinuclear copper(I) and silver(I) amidinate complexes: synthesis, luminescence, and CS2 insertion reactivity.

Dinuclear Cu(I) and Ag(I) complexes, Cu2[(2,6-Me2C6H3N)2C(H)]2, 1, Ag2[(2,6-Me2C6H3N)2C(H)]2, 2, Cu2[2,6-(i)Pr2C6H3N)2C(H)]2, 3, and Ag2[(2,6-(i)Pr2C6H3N)2C(H)]2, 4, were synthesized from reactions of [Cu(NCCH3)4][PF6] with Na[(2,6-R2C6H3N)2C(H)] and AgO2CCH3 with [Et3NH][(2,6-R2C6H3N2C(H)], R = Me, (i)Pr. Carbon disulfide was observed to insert into the metal-nitrogen bonds of 1 to produce Cu4[CS2(2,6-Me2C6H3NC(H)═NC6H3Me2)]4, 5, with a Cu4S8 core, which represents a rare transformation of dinuclear to tetranuclear species. Insertion is also observed with 2 and CS2, with the product likely being polymeric, 6. With the (i)Pr-derivatives, CS2 insertion was also observed, albeit at much slower rate, with 3 and 4 producing hexanuclear clusters, M6[CS2(2,6-Me2C6H3NC(H)═NC6H3Me2)]6, M = Cu, 7; Ag, 8. Complexes 1 and 5 display green luminescence, a feature not shared by their Ag(I) analogs nor with 3. Notably, oxygen acts as a collisional quencher of the luminescence from 1 and 5 at a rate faster than most metal-based quenchometric O2 sensors. For example, we find that complex 1 can be rapidly and reversibly quenched by oxygen, presenting a nearly 6-fold drop in intensity upon switching from nitrogen to an aerated atmosphere. The results here provide a platform from which further group 11 amidinate reactivity can be explored.

Manipulating magnetism: Ru25⁺ paddlewheels devoid of axial interactions.

Variable-temperature magnetic and structural data of two pairs of diruthenium isomers, one pair having an axial ligand and the formula Ru2(DArF)4Cl (where DArF is the anion of a diarylformamidine isomer and Ar = p-anisyl or m-anisyl) and the other one being essentially identical but devoid of axial ligands and having the formula [Ru2(DArF)4]BF4, show that the axial ligand has a significant effect on the electronic structure of the diruthenium unit. Variable temperature crystallographic and magnetic data as well as density functional theory calculations unequivocally demonstrate the occurrence of π interactions between the p orbitals of the chlorine ligand and the π* orbitals in the Ru2(5+) units. The magnetic and structural data are consistent with the existence of combined ligand σ/metal σ and ligand pπ/metal-dπ interactions. Electron paramagnetic resonance data show unambiguously that the unpaired electrons are in metal-based molecular orbitals.

A strong metal-to-metal interaction in an edge-sharing bioctahedral compound that leads to a very short tungsten-tungsten double bond.

An ionic edge-sharing bioctahedral (ESBO) species has been prepared having a tetramethylated bicyclic guanidinate with two fused six-membered rings characterized by a fairly flat N-C(N)-N skeleton and abbreviated as TMhpp. The anion has two W(IV) atoms bridged by two oxo groups; the metal atoms are also spanned by two bridging guanidinate ligands, and each has two monodentate chlorine atoms. The complex formulated as (H2TMhpp)2[W(µ-O)(µ-TMhpp)Cl2]2 has the shortest W-W distance (2.3318(8) Å) of any species with a σ(2)π(2) electronic configuration. The anion and cations are connected by hydrogen bonds. To unambiguously ascertain the existence of the double-bonded W2(µ-O)2 entity, density functional theory calculations and natural bond orbital analyses were done on an analogous but hypothetical species with a W2(µ-OH)2 core having trivalent tungsten atoms and a possible σ(2)π(2)δ(2) electronic configuration. The calculations decidedly support the presence of tungsten-oxo instead of tungsten-hydroxo groups and thus the existence of the double-bonded W2(µ-O)2 core. The strong bonding interaction between metal atoms is a clear indication that under certain circumstances the two octahedra in ESBO species do not behave as the sum of two mononuclear compounds.

Solubilizing the most easily ionized molecules and generating powerful reducing agents.

Two very soluble compounds having W2(bicyclic guanidinate)4 paddlewheel structures show record low ionization energies (onsets at 3.4 to 3.5 eV) and very negative oxidation potentials in THF (-1.84 to -1.90 V vs Ag/AgCl). DFT computations show the correlation from the gas-phase ionization energies to the solution redox potentials and chemical behavior. These compounds are thermally stable and easy to synthesize in high yields and good purity. They are very reactive and potentially useful stoichiometric reducing agents in nonpolar, nonprotonated solvents.

Direct evidence from electron paramagnetic resonance for additional configurations in uncommon paddlewheel Re2(7+) units surrounded by an unsymmetrical bicyclic guanidinate.

Three rare compounds have been synthesized and structurally characterized; these species have paddlewheel structures and Re(2)(7+) cores surrounded by four bicyclic guanidinates and two axial ligands along the Re-Re axis. Each possesses a formal bond order of 3.5 and a σ(2)π(4)δ(1) electronic configuration that entails the presence of one unpaired electron for each compound. The guanidinate ligands characterized by having CH(2) entities and a central C(N)(3) unit that joins two cyclic units--one having two fused 6-membered rings (hpp) and the other having a 5- and a 6-membered ring fused together (tbn)--allowed the isolation of [Re(2)(tbn)(4)Cl(2)]PF(6), 1, [Re(2)(tbn)(4)Cl(2)]Cl, 2, and [Re(2)(hpp)(4)(O(3)SCF(3))(2)](O(3)SCF(3)), 3. Because of the larger bite angle of the tbn relative to the hpp ligand, the Re-Re bond distances in 1 and 2 (2.2691(14) and 2.2589(14) Å, respectively) are much longer than that in 3 (2.1804(8) Å). Importantly, electron paramagnetic resonance (EPR) studies at both X-band (~9.4 GHz) and W-band (112 GHz) in the solid and in frozen solution show unusually low g-values (~1.75) and the absence of zero-field splitting, providing direct evidence for the presence of one metal-based unpaired electron for both 1 and 3. These spectroscopic data suggest that the unsymmetrical 5-/6-membered ligand leads to the formation of isomers, as shown by significantly broader EPR signals for 1 than for 3, even though both compounds possess what appears to be similar ideal crystallographic axial symmetry on the X-ray time scale.

An uncommon highly oxidized multiple bonded Re2(8+) species.

A long-sought metal-metal bonded species with an M(2)(8+) core has been structurally characterized. The diamagnetic compound {[Re(2)(hpp)(3)(OH)(O(3)SCF(3))](2)(µ-O)(2)}(O(3)SCF(3))(2), 2, has two σ(2)π(4) electron-poor triple-bonded Re(2)(8+) units embraced by three bicyclic guanidinate ligands (hpp) and axial OH and triflate groups; the two dimetal units are held together by bridging oxo-groups.

Synthesis and structure of a dinuclear gold(II) complex with terminal fluoride ligands.

The potential for reductive elimination of fluorine from dinuclear gold(II) for catalysis has prompted our efforts to synthesize a dinuclear gold(II) fluoride complex. This has been achieved with bis(2,6-dimethylphenyl)formamidinate bridging ligands. In order to obtain this product, it was necessary first to synthesize the corresponding dinuclear gold(II) nitrate, which reacts readily with KF in a metathesis reaction. The nitrate complex and fluoride complexes have been structurally characterized. The Au-Au distance in the dinuclear fluoride, 2.595 Å, is longer than the distance found in the analogous chloride complex, 2.567 Å. This result is consistent with the presence of a fluoride "π electron effect" on the filled Au 5d orbitals. The Raman spectrum shows an Au-Au stretch at 206 cm(-1), which agrees with Woodruff's rules and the density functional theory computational model used for modeling the complex.

Tuning the electrochemistry of Re2(6+) species with divergent bicyclic guanidinate ligands and by modification of axial π interactions.

Four Re(2)(6+) paddlewheel compounds with equatorial bicyclic guanidinate ligands and two monodentate anions in axial positions show a large change in the metal-metal distance that depends on the bite angle of the ligands and whether there are pi interactions between the dimetal unit and the axial ligands. These processes are accompanied by significant changes in the redox behavior. The two pairs of compounds that have been synthesized are Re(2)(tbn)(4)Cl(2), 1, and Re(2)(tbn)(4)(SO(3)CF(3))(2), 2, as well as Re(2)(tbo)(4)Cl(2), 3, and Re(2)(tbo)(4)(SO(3)CF(3))(2), 4, where tbn is the anion of a bicyclic guanidinate with six- and five-membered rings (1,5,7-triazabicyclo[4.3.0]non-6-ene) and tbo is an analogous species with two five-membered rings (the anion of 1,4,6-triazabicyclo[3.3.0]oct-4-ene). For both 1 and 2 as well as for 3 and 4, the metal-metal distances are shorter for the triflate species than for the chloride analogues because of the π interactions of the Cl with the π bonds of the triply bonded Re(2)(6+) cores compounded by a small but symmetry allowed interaction between the antisymmetric combination of the filled σp orbitals of the chlorine atom and the empty σ* orbital of the metal atoms. In addition there is a significant increase in the Re-Re distance from that in the six/five tbn-membered ring to the five/five-membered tbo species. Electrochemical measurements show two redox processes for each set of compounds corresponding to the uncommon Re(2)(6+) → Re(2)(7+) and Re(2)(7+) → Re(2)(8+) processes, which are strongly affected by the bite angle of the guanidinate ligand as well as the ability of the axial ligands to interact with the π orbitals of the dirhenium unit. For 1 and 3, the first redox couples are at 0.146 and 0.487 V, respectively, while for 2 and 4 these are at 0.430 and 0.698 V, respectively.

Halide and nitrite recognizing hexanuclear metallacycle copper(II) pyrazolates.

Halide-centered hexanuclear, anionic copper(II) pyrazolate complexes [trans-Cu(6)((3,5-CF(3))(2)pz)(6)(OH)(6)X](-), X = Cl, Br, I are isolated in a good yield from the redox reaction of the trinuclear copper(I) pyrazolate complex [µ-Cu(3)((3,5-CF(3))(2)pz)(3)] with a halide source such as PPh(3)AuCl or [Bu(4)N]X, X = Cl, Br, or I, in air. X-ray structures of the anion-centered hexanuclear complexes show that the six copper atoms are bridged by bis(3,5-trifluoromethyl)pyrazolate and hydroxyl ligands above and below the six copper atom plane. The anions are located at the center of the cavity and weakly bound to the six copper atoms in a µ(6)-arrangement, Cu-X = ~3.1 Å. A nitrite-centered hexanuclear copper(II) pyrazolate complex [trans-Cu(6)((3,5-CF(3))(2)pz)(6)(OH)(6)(NO(2))](-) was obtained when a solution of [PPN]NO(2) in CH(3)CN was added dropwise to the trinuclear copper(I) pyrazolate complex [µ-Cu(3)((3,5-CF(3))(2)pz)(3)] dissolved in CH(3)CN, in air. Blue crystals are produced by slow evaporation of the acetonitrile solvent. The X-ray structure of [PPN][trans-Cu(6)((3,5-CF(3))(2)pz)(6)(OH)(6)(NO(2))] complex shows the nitrite anion sits in the hexanuclear cavity and is perpendicular to the copper plane with a O-N-O angle of 118.3(7)°. The (19)F and (1)H NMR of the pyrazolate ring atoms are sensitive to the anion present in the ring. Anion exchange of the NO(2)(-) by Cl(-) can be observed easily by (1)H NMR.

Large changes in electronic structures of Ru2(6+) species caused by the variations of the bite angle of guanidinate ligands: tuning magnetic behavior.

Syntheses and characterization of two Ru(2)(6+) paddlewheel compounds having very different magnetic behavior are reported. The compounds Ru(2)(tbn)(4)Cl(2), 1, and Ru(2)(tbo)(4)Cl(2), 2 (where tbn = the anion of 1,5,7-triazabicyclo[4.3.0]non-6-ene and tbo = the anion of 1,4,6-triazabicyclo[3.3.0]oct-4-ene), have four equatorial bicyclic guanidinate ligands and two chloride ions in axial positions. They show large disparity in Ru-Ru distances of about 0.11 A (2.389(3) and 2.499(3) A at 30 K for 1 and 2, respectively) that is attributed to the divergence in the bite angle of the ligand. Variable temperature structural data show no significant changes in the Ru-Ru distances between 30 and 213 K suggesting that the electronic structure remains unchanged in this temperature range for both compounds. Magnetic studies of 1 indicate there are two unpaired electrons at room temperature but the compound behaves as essentially diamagnetic at approximately 2 K. Compound 2 is non-magnetic across all temperatures in the range of 2 to 300 K. Density functional theory calculations suggest a pi(4)pi*(4)delta(2) electronic configuration for 2, while the magnetic behavior and structural data for 1 are consistent with a sigma(2)pi(4)delta(2)pi*(2) electronic configuration. This shows the importance of the ligand bite angle in determining the electronic configuration of the diruthenium unit and a way to tune magnetic behavior.

Proof by EPR spectroscopy that the unpaired electron in an Os(2)(7+) species is in a delta* metal-based molecular orbital.

Variable temperature structural and EPR studies are reported on the paddlewheel compound [Os(2)(hpp)(4)Cl(2)]PF(6), 1, (hpp = the anion of the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) that contains a rare M(2)(7+) species, with the goal of determining whether the unpaired electron resides in a metal- or ligand-based molecular orbital. Crystallographic studies show that the Os-Os distance in 1 remains essentially unchanged from 213 to 30 K, which is consistent with no changes in electronic structure in this range of temperature. It is noteworthy that the metal-metal distance in 1 is about 0.05 A shorter than that in the precursor Os(2)(hpp)(4)Cl(2), which is consistent with the loss of an electron in a delta* orbital. EPR spectra of 1 were measured in dilute frozen solution, powder, and single crystals. The spectra were observable only below about 50 K, with an exceptionally large line width, approximately 3,750 gauss, for a powdered sample, due to dipolar interactions and to short relaxation times. There is a very small average g value of approximately 0.750 and a cylindrical symmetry about the Os-Os bond. These data are consistent with the unpaired electron orbital having a large L value, such as that of a delta* orbital. The combination of X-ray structural data, the short relaxation time, and the magnetic data provide strong evidence that the unpaired electron in this nine-electron Os(2)(7+) species is localized in a metal-based orbital with this electron residing predominantly in a delta* orbital rather than in a pi* orbital and, thus, having an electronic configuration of sigma(2)pi(4)delta(2)delta*.

A short, unsupported CuI...CuI interaction, 2.65 A, in a dinuclear guanidine chloride complex.

The reaction of CuCl with the neutral guanidine Hhpp ligand (1,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine) yields two crystalline polymorphs of the neutral dimer, bis(1,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine) dichloro-di-copper(I) with the shortest distance known for a bridging unsupported copper(I)-copper(I) interaction.

A monomeric gold(I) carbanion complex with an uncoordinated thioether: [2-(methylsulfanyl)phenyl](triphenylphosphine)gold(I).

The title compound, [Au(C7H7S)(C18H15P)], is conformationally chiral and crystallizes from benzene-hexane as individually enantiopure crystals. This mononuclear compound has the Au(I) atom linearly bound to a triphenylphosphine P atom and to a phenyl C atom of a 2-(methylsulfanyl)phenyl group. The angle at the Au(I) atom is 175.9 (2) degrees. The linear ligand coordination about the Au(I) atom has geometric parameters inside the remarkably narrow range found for gold complexes bound by a phosphine ligand and by the ortho-C atom of a substituted phenyl group. This is the first example of gold(I) attached to a methylsulfanyl aromatic carbanion.

The neutral cluster amminehexa-mu(2)-chlorido-mu(4)-oxido-tris(1,4,6-triazabicyclo[3.3.0]oct-4-ene)tetracopper(II).

The title compound, [Cu(4)Cl(6)O(C(5)H(9)N(3))(3)(NH(3))], is a neutral conformationally chiral cluster which crystallizes under the conditions described in this paper as a racemic conglomerate. It contains four Cu(II) atoms in a tetrahedral coordination with a central O atom lying on a crystallographic threefold axis. Six chloride anions bridge the four Cu(II) atoms. Three Cu(II) atoms are bound by an N atom of a monodentate 1,4,6-triazabicyclo[3.3.0]oct-4-ene (Htbo) ligand and the remaining Cu(II) atom is bound by a terminal ammine ligand. The geometry at each copper center is trigonal bipyramidal, produced by the bound N atom of Htbo or ammonia, the O atom in the axial position, and three chloride ions in the equatorial plane. The chloride anions form an octahedron about the oxygen center. The copper-ammonia bond lies along the crystallographic threefold axis, along which the molecules are packed in a polar head-to-tail fashion.

Further study of the reaction of Fe2+ with CN-: synthesis and characterization of cis and trans [FeII,III(CN)4L2]n- complexes.

The reaction of Fe2+ with CN-, which was first performed in 1704, has been used to synthesize a new series of basic [FeII,III(CN)4L2]n- complexes, where L is a monodentate ligand. trans-Na2[FeII(CN)4(DMSO)2] and cis-[NEt4]2[FeII(CN)4(pyridine)2] are synthesized by the direct reaction of FeCl2 with 4 equiv of CN- in DMSO or pyridine. Air oxidation of the latter compound gives cis-[NEt4][FeIII(CN)4(pyridine)2]. The non-cyanide ligands in these complexes undergo facile ligand exchange reactions with solvent. Reaction of cis-[NEt4]2[FeII(CN)4(pyridine)2] with CO at room temperature gives trans-[NEt4]2[FeII(CN)4(pyridine)(CO)].