Synthesis,Characterization, Luminescence,and Electrochemistry of the Tetranuclear Gold(I) Amidinate Clusters,Precursors to CO Oxidation Catalysts: Au4[(ArNC(H)NAr)]4, Au4[(PhNC(Ph)NPh)]4 and Au4[PhNC(CH3)NPh)4]

A summary of the chemistry of the tetranuclear Au(I) amidinate complexes is presented. Tetranuclear Au(I) amidinate clusters are produced by the reaction of the sodium salt of a amidine ligand with the gold precursor Au(THT)Cl in a (1:1) stoichiometry. The structures of the tetranuclear Au₄[ArNC(H)NAr]₄, Ar = C₆H₄‐4‐OMe, C₆H₃‐3,5‐Cl, C₆H₄‐4‐Me, C₆H₄‐3‐CF₃, C₆F₅, C₁₀H₇ and the tetranuclear Au₄[(PhNC(Ph)NPh]₄ and Au₄[PhNC(CH₃)NPh]₄ have been characterized by X‐ray crystallography. The average Au···Au distance between adjacent Au(I) atoms is ?3.0 Å, typical of compounds having an aurophilic interaction. The four gold atoms are located at the corner of a rhomboid with the amidinate ligands bridged above and below the near plane of the four Au(I) atoms. The angles at Au···Au···Au in the cyclic units are between 70° and 116°. The tetranuclear gold(I) amidinate clusters each show different luminescence behavior. The tetranuclear clusters Au₄[(ArNC(H)NAr]₄, Ar = C₆H₄‐4‐OMe, Ar = C₆H₄‐3‐CF₃, Ar = C₆H₄‐4‐Me and Ar = C₆H₄‐3,5‐Cl are the first tetranuclear gold(I) cluster species from group 11 elements that show fluorescence at room temperature. The tetranuclear naphthyl derivative Ar = C₁₀H₇ is luminescent only at 77 K. The pentafluorophenyl derivative Ar = C₆F₅ does not show any photoluminescence in the solid state nor in the solution. The lifetimes of the naphthyl and trifluoromethylphenyl complexes are in the millisecond range indicating phosphorescent processes. Electrochemical and chemical oxidation studies of the tetranuclear Au(I) amidinate clusters are presented. The tetranuclear complexes Au₄[ArNC(H)NAr]₄, Ar = C₆H₄‐4‐OMe, Ar = C₆H₄‐4‐Me, and Ar = C₆H₃‐3,5‐Cl, show three reversible waves at 0.75, 0.95, 1.09 V vs. Ag/AgCl at a scan rate of 500 mV/s in 0.1 M Bu₄NPF₆/CH₂Cl₂ at a Pt working electrode in CH₂Cl₂. Three reversible waves at 0.87, 1.19, 1.42 V vs. Ag/AgCl at a scan rate of 100 mV/s are also observed for the tetranuclear complex Au₄[PhNC(Ph)NPh]₄ in CH₂Cl₂. The pentafluorophenyl amidinate derivative, Au₄[ArNC(H)NAr]₄, Ar = C₆F₅ shows no oxidation wave below 1.8 V. Recently it has been shown that Au₄[ArNC(H)NAr]₄ is a very effective catalyst precursor for room temperature CO oxidation.     

Structural Investigation of New Lithium Amidinates and Guanidinates

A series of potentially useful lithium amidinates and guanidinates were prepared and fully characterized. Treatment of N,N′‐diisopropylcarbodiimide with phenyllithium in diethyl ether afforded the lithium amidinate [PhC(NiPr)₂Li(OEt₂)]₂ ( 1 ). Similar treatment of N,N′‐diorganocarbodiimides R′–N=C=N–R′ [R′ = iPr, cyclohexyl (Cy)] with secondary lithium amides LiNR₂ [R₂ = Et₂, iPr₂, (CH₂)₄] followed by crystallization from THF or 1,4‐dioxane gave the lithium guanidinates [R₂NC(NR′)₂Li(S)]₂ [ 2 : R = Et, R′ = iPr, S = THF; 3 : R₂ = (CH₂)₄, R′ = iPr, S = THF; 4 : R = R′ = iPr, S = ½ 1,4‐dioxane; 5 : R₂ = (CH₂)₄, R′ = Cy, S = 1,4‐dioxane] as crystalline solids. Reaction of N‐lithioaziridine with the corresponding carbodiimides afforded solvent‐deficient [{C₂H₄NC(NiPr₂)₂}₂Li₂(THF)]₂ ( 6 ), and [C₂H₄NC(NEt)(NtBu)Li(THF)]₂ ( 7 ). Crystal structure determination revealed the presence of common ladder‐type dimeric structures for 1 – 5 . Compound 6 exists as a dimer of two ladder‐type dimers in the crystal, and 7 exhibits an unusual dimeric structure comprising an eight‐membered C₂N₄Li₂ ring.     

New Homoleptic Rare‐Earth Metal Complexes Comprising the Unsymmetrically Substituted Amidinate Ligand [MeC(NEt)(NtBu)]–

New homoleptic complexes of selected rare‐earth elements containing the unsymmetrically substituted amidinate ligand [MeC(NEt)(NtBu)]^– [= (L)^–] were synthesized and fully characterized. Treatment of in situ‐prepared Li(L) ( 1 ) with anhydrous lanthanide(III) chlorides, LnCl₃ (Ln = Sc, La, Ce, Ho), afforded three different types of amidinate complexes depending on the ionic radius of the central metal atom. The large La³⁺ formed the octa‐coordinate DME solvate La(L)₃(DME) ( 2 ). Using Ce³⁺, the octa‐coordinate “ate” complex Li(THF)[Ce(L)₄] ( 3 ) was formed. Depending on the crystallization conditions, compound 3 could be crystallized in two modifications differing in the coordination environment around Li. In the case of the smaller Sc³⁺ and Ho³⁺ ions, six‐coordinate homoleptic Sc(L)₃ ( 4 ) and Ho(L)₃ ( 5 ) were isolated. The title compounds were fully characterized by spectroscopic and analytical methods as well as single‐crystal X‐ray diffraction. With Ln = La and Ce, several by‐products incorporating lithium, chlorine and/or oxygen were also isolated and structurally characterized.     

Atom-transfer radical reactions catalyzed by a coordinatively unsaturated diruthenium amidinate, [(η-C5Me5)Ru(μ2-i-PrNC(Me)Ni-Pr)Ru(η-C5Me5)]

Atom-transfer radical cyclization (ATRC) and addition (ATRA) catalyzed by a coordinatively unsaturated diruthenium amidinate complex 4, [(η⁵-C₅Me₅)Ru(μ₂-i-PrNC(Me)Ni-Pr)Ru(η⁵-C₅Me₅)]⁺, are investigated, and their features are compared with those of atom-transfer radical polymerization (ATRP). As an example of ATRC, a cationic diruthenium amidinate 4 is found to exhibit excellent catalytic reactivity for the cyclization of N-allyl α-halogenated acetamides including an alkaloid skeleton at ambient temperature. A catalytic species generated in situ from a halide complex, (η⁵-C₅Me₅)Ru(μ₂-i-PrNC(Me)Ni-Pr)Ru(η⁵-C₅Me₅)(X) [X=Cl, Br] and sodium salts of weakly coordinating anions such as NaPF₆ and NaBPh₄ also shows high catalytic activity; this actually provides a solution for a problematic instability of 4 as the practical catalyst. The in situ-generated catalyst species 4 is also active towards the intermolecular ATRA of α,α,γ-trichlorinated γ-lactam with alkenes at rt to afford the corresponding α-alkylated γ-lactams in moderate yields. Examination of ATRP of methyl methacrylate (MMA) showed that both the isolated 4 [Y=PF₆] and in situ-generated 4 [Y=PF₆] are effective for the polymerization of MMA in the presence of 2-bromoisobutylate as the initiator. Use of the isolated catalyst results in controlled polymerization at initial stage of the reaction; in contrast, the polymerization with in situ-generated catalyst produces poly(MMA) with wide molecular weight distribution. The isolated catalyst 4 is powerful for the activation of a C-Br bond of macromolecule initiators; BrCMe₂CO₂[O(CH₂)₄]_n-n-Bu (M_n=3800; M_w/M_n=1.2) initiated ATRP of MMA even at 25 °C to afford the poly(THF)-poly(MMA) block copolymer of M_n=26,000 and M_w/M_n=1.2 with the aid of 4. The roles of the coordinatively unsaturated ruthenium species for these reactions are discussed.     

Synthesis and structural studies of some titanium and zirconium complexes with chiral bis(amide), amidinate or bis(amidinate) ligands

Reaction of bis(amide) sodium Na₂[(1R,2R)-(−)-1,2-(NSiMe₃)₂-C₆H₁₀] (Na₂[L¹]) with Ti(OiPr)₂Cl₂ in different conditions gave mixed-ligand complexes [Ti(OiPr)Cl][L¹] (1) or [Ti(OiPr)₂Cl]₂[L¹] (2); 2 is a dinuclear titanium example in which Ti atoms are bridged by nitrogen and oxygen atoms simultaneously forming a distorted rhombic core. Reaction of the amine-amidinate ligand (1R,2R)-(−)-1-Li[NC(Ph)N(SiMe₃)]-2-(NHSiMe₃)-C₆H₁₀(Li[L²]) or rarely linked bis(amidinate) ligand Li₂[(1R,2R)-(−)-1,2-{NC(Ph)N(SiMe₃)}₂-C₆H₁₀](Li₂[L³]) with ZrCl₄ yielded the unbridged and bridged bis(amidinate) complexes ZrCl₂[L²]₂ (3) and [ZrCl₂(THF)][L³] (4), respectively; Moreover, the reaction of (1R,2R)-(−)-1-Li[NC(Ph)N(SiMe₃)]-2-Li(NSiMe₃)C₆H₁₀(Li₂[L²]) with Ti(OiPr)₂Cl₂ gave a new type of tridentate amido-amidinate product [Ti(OiPr)₂][L²] (6), which is a distinct model compared to [Ti(OiPr)₂Cl][L²] (5) yielded from Li[L²]. All the products have been characterized by X-ray crystallography and the structural studies are presented detailedly comparing with relevant compounds.     

Synthesis and structural characterization of a homoleptic cerium(III) propiolamidinate

The first potassium salt of a propiolamidinate ligand, K[PhCC(NⁱPr)₂] (1), was prepared in 51% yield by addition of potassium phenylacetylide to N,N′-diisopropylcarbodiimide. Subsequent reaction of 1 with anhydrous cerium(III) trichloride in a molar ratio of 3:1 in THF afforded the first homoleptic lanthanide tris(propiolamidinate) derivative, [PhCC(NⁱPr)₂]₃Ce (2), in the form of bright yellow crystals in 71% yield.     

Diazaallyls of group 4 metals based on trans-1,2-diaminocyclohexane

Amination of 1-bromo-2-methylpyridine with trans-1,2-diaminocyclohexane gives the corresponding bis(aminopyridine) H₂L¹. Conversion of the same diamine to the N,N′-bis(amino-4,4-dimethylthiazoline) H₂L² is also completed in three steps. The analogous aminooxazoline is however inaccessible, although the aminocyclohexane analogue is prepared readily. The proligand H₂L¹ forms bis(aminopyridinato) alkyl complexes of the type [ZrL¹R₂] (R = CH₂Ph, CH₂Bu^t). The molecular structure of the neopentyl complex shows that the chiral backbone leads to a puckering of the N₄Zr coordination sphere, which contrasts with the related cyclohexyl-bridged Schiff-base complexes which are essentially planar. [ZrL²(CH₂Bu^t)₂] - the first aminothiazolinato complex - is formed similarly. A comparison of the structures of [ZrL¹(CH₂Bu^t)₂] and [ZrL²(CH₂Bu^t)₂] indicates that the latter has a fully delocalised N-C-N system, rather similar to a bis(amidinate). Reaction of H₂L² with [Ti(NMe₂)₄] gives [TiL²(NMe₂)₂] which appears to be C₂-symmetric like the above complexes according to NMR spectra, but has one uncoordinated thiazoline unit in the solid state. This is a result of increased ring strain at the smaller titanium metal centre.     

Carboranylamidinates

This Research Report provides an overview on synthesis, structure, and reactivity of the recently discovered carboranylamidinate ligands. Carboranylamidinate anions of the type [(o‐C₂B₁₀H₁₀C(NHR)(=NR)]^– (R = iPr, cyclohexyl) are readily accessible via addition of o‐lithiocarborane to N,N′‐carbodiimides R–N=C=N–R. They combine the highly versatile characteristics of both amidinates and carboranes in one unique ligand system. Unlike simple amidinate anions, the carboranylamidinates coordinate to metal ions not as typical N,N′‐chelating ligands but adopt an unexpected κ²C, N‐bonding mode. The free imine functionality in carboranylamidinates can be further deprotonated. The resulting dianions were demonstrated to be excellent starting materials for novel boron‐rich heterocycles incorporating e.g. Si, Sn, P, or transition metals such as Ti, Zr, Rh, and Ir. Further modification of the carboranylamidinate cage structure includes the introduction of additional functional groups like –SH or –SeH as well as the selective removal of a boron atom with formation of novel nido‐type dicarbollylamidinate ligands. An initial study already showed that transition metal carboranylamidinates are potentially useful as polymerization catalysts.