Theoretical Investigations on the Agostic Interactions of the Molybdenum and Manganese Complexes

The essential participation of agostic interactions in C−H bond activation, cyclometallation and other catalytic processes has been widely observed. To quantitatively evaluate the Mo−H−C agostic interaction in the Mo β/γ- agostomers [CpMo(CO)₂(PiPr₃)]⁺ ( Mo , 1 and Mo , 2 ) and the Mn−H−C agostic interaction in the Mn α/ϵ-agostomers [(C₆H₉]Mo(CO)₃] ( Mn , 1 and Mn , 2 ), the comprehensive density functional theory (DFT) theoretical investigations were performed. Results indicated that the Mo β-agostomer 1 is only favorable by 0.5 kcal mol⁻¹ than Mo γ-agostomer 2 , and the Gibbs barrier for their interconversion was 9.1 kcal mol⁻¹. A slightly higher Gibbs barrier of 12.7 kcal mol⁻¹ for the isomerization between the Mn α/ϵ-agostomers was also obtained. The relatively strong agostic interactions in Mo β-agostomer 1 and Mn α-agostomer 1 were further verified by the AIM (Atoms-In-Molecules) analyses and the NAdOs (natural adaptive orbitals) analyses. The findings on the agostic interaction presented in this study are believed to benefit the understandings of the agostic interaction involved catalytic processes and to promote the development of new organometallic complexes.     

Studies of chelation IV. Steric influences on the chelation of ditertiary phosphine complexes of group VI metal carbonyls

The preparation of the complexes [M(CO)_n(dcpe)] [M  Cr, Mo, W; n  4, 5; dcpe is ((cyclo-C₆H₁₁)₂PCH₂)₂] is reported. Attempts to prepare [M(CO)₂(dcpe)₂] by many different methods gave only cis-[M(CO)₄(dcpe)] and [M(CO)₅(dcpe)]. Heating cis-[M(CO)₄(dcpe)] with (Me₂PCH₂)₂(dmpe) gives cis-[M(CO)₂(dmpe)₂] only. These observations are explained in terms of unfavourable intramolecular non-bonded interactions between substituents at phosphorus. The rate of chelation of [M(CO)₅(dcpe)] to give cis-[M(CO)₄(dcpe)] has been measured at various temperatures in the range 360–420 K. The activation parameters indicate the dominance of a dissociative process leading to the observed steric acceleration in the chelation step. The rate of chelation is correlated satisfactorily with the ligand cone angle; the operation of an apparent saturation effect is noted.     

Organonitrile complexes of iron(II) and Ruthenium(II): X-ray crystal structure of trans-[Fe(NCMe)2(dmpe)2](BPh4)2

The interaction of FeCl₂(dmpe)₂ [dmpe = 1,2-bis(dimethylphosphino)ethane] with RCN (R = Me or Et) gives the partially substituted complex trans-[FeCl(NCR)(dmpe)₂]Cl at room temperature, but in refluxing RCN in the presence of NaBPh₄ the product is trans-[Fe(NCR)₂(dmpe)₂](BPh₄)₂. The X-ray crystal structure of the acetonitrile complex has been determined. No reaction is observed between RuCl₂(dmpe)₂ and MeCN, although the disubstituted complex can be made in a similar way to the iron analogue. The interaction of trans-[M(NCMe)₂(dmpe)₂][BPh₄)₂ (M = Fe or Ru) with H₂ leads to the amine complexes trans-[M(H₂NEt)₂(dmpe)₂](BPh₄)₂. Although the ethylamine can be removed on refluxing in MeCN the complexes do not act as catalysts. Addition of MeCN to FeCl₂(PMe₃)₂ yields only the complex [FeCl(NCMe)(PMe₃)₂]Cl; RuCl₂(PMe₃)₄ reacts in refluxing MeCN in the presence of NaBPh₄ to give trans-[Ru(NCMe)₂(PMe₃)₄](BPh₄)₂.     

New Routes to a Series of σ‐Borane/Borate Complexes of Molybdenum and Ruthenium

A series of agostic σ‐borane/borate complexes have been synthesized and structurally characterized from simple borane adducts. A room‐temperature reaction of [Cp*Mo(CO)₃Me], 1 with Li[BH₃(EPh)] (Cp*=pentamethylcyclopentadienyl, E=S, Se, Te) yielded hydroborate complexes [Cp*Mo(CO)₂(μ‐H)BH₂EPh] in good yields. With 2‐mercapto‐benzothiazole, an N,S‐carbene‐anchored σ‐borate complex [Cp*Mo(CO)₂BH₃(1‐benzothiazol‐2‐ylidene)] ( 5 ) was isolated. Further, a transmetalation of the B‐agostic ruthenium complex [Cp*Ru(μ‐H)BHL₂] ( 6 , L=C₇H₄NS₂) with [Mn₂(CO)₁₀] affords a new B‐agostic complex, [Mn(CO)₃(μ‐H)BHL₂] ( 7 ) with the same structural motif in which the central metal is replaced by an isolobal and isoelectronic [Mn(CO)₃] unit. Natural‐bond‐orbital analyses of 5–7 indicate significant delocalization of the electron density from the filled σ_B?H orbital to the vacant metal orbital.     

Oxo-molybdenum(IV) and tungsten(IV) complexes with phosphine and isocyanide ligands

The complexes [MOCl₂(dmpe)(PMe₃)] and [MOCl₂(dmpe)₂]Cl (M = Mo, W; dmpe = Me₂PCH₂CH₂PMe₂) have been prepared by reaction of the oxo compounds [MOCl₂(PMe₃)₃] with equivalent amounts of the dmpe ligand under appropriate conditions. The dark blue tungsten species [WOCl₂(dmpe)(PMe₃)] forms only slowly but reacts readily with more dmpe to afford [WOCl(dmpe)₂]Cl. This prevents isolation of the former in a pure form. The related isocyanide derivatives [MOCl₂(CNR)(PMe₃)₂], (M = Mo; R = CMe₃ and C₆H₁₁; M = W, R = CMe₃) have been obtained similarly by reaction of the [MOCl₂(PMe₃)₃] complexes with the stoichiometric amount of the isocyanide ligand, but attempts to prepare the carbonyl analogues, [MOCl₂(CO)(PMe₃)₂], have proved unsuccessful. The new compounds have been characterized by analytical and spectroscopic methods (IR, ¹H, ¹³C and ¹³P NMR spectroscopy).     

Synthesis and reactions of trans-dicarbonyl bis-[1,2-bis(dimethylphosphino)ethane] vanadium(O): X-ray crystal structures of trans-V(CO)2(dmpe)2 [cis-V(CO)2(dmpe)2(CH3CN)]BPh4 and cis-V(CO)2(dmpe)2(O2CEt)

Interaction of trans-VCl₂(dmpe)₂ with sodium amalgam in tetrahydrofuran under CO gives trans-V(CO)₂(dmpe)₂. The latter is oxidized by Ag⁺ in acetonitrile to give [cis-V(CO)₂(dmpe)₂(CH₃CN)]⁺, isolated as the tetraphenylborate. Interactions with acids (HX) gives neutral complexes of the type V(CO)₂(dmpe)₂X (X = Cl, MeCO₂, EtCO₂, CF₃CO₂, PhPO₂H or NH₂SO₃); the chloride can be exchanged with N₃⁻ or CN⁻ in methanol. X-ray structural studies confirm the trans stereochemistry for V(CO)₂(dmpe)₂ and the seven-coordination of V^I in both [V(CO)₂(dmpe)₂(CH₃CN)][BPh₄] and V(CO)₂(dmpe)₂(O₂CEt), which have a pseudo octahedral geometry with the two carbonyls occupying a “split” axial site. ⁵¹V NMR and other spectra are reported.     

Synthesis,Structural Characterization and Reactivity of a Bis(phosphine)(silyl) Platinum(II) Complex

Treatment of 1,2‐C₆H₄(SiH₃)(SiH₃) ( 1 ) with Pt(dmpe)(PEt₃)₂ (dmpe=Me₂PCH₂CH₂PMe₂) in the ratio of 1:1 leads to the complex {1,2‐C₆H₄(SiH₂)(SiH₂)}Pt^II (dmpe) ( 2 ), which can react with proton organic reagent bearing hydroxy group with low steric hindrance to form a tetra‐alkoxy substituted silyl platinum(II) compound ( 3 ). Compounds 2 and 3 are the very rare examples of silyl transition‐metal complexes derived from this chelating hydrosilane ligand. To the best of our knowledge, there are only 6 examples of silyl metal complexes prepared from this ligand with such structural features registered in the Cambridge Structural Database, among them, only one silyl platinum(II) compound is presented. The structures of complexes 2 and 3 were unambiguously determined by multinuclear NMR spectroscopic studies and single crystal X‐ray analysis.     

Dispersion Interaction Stabilizes Sterically Hindered Double Fullerenes

By state‐of‐the‐art quantum chemical methods, we show that for bulky functional groups like cyclohexane, [20]fullerene, dodecahedrane, and C₆₀, the attractive dispersion interaction can have a greater impact on stereochemistry than the repulsive steric effect, making the compact isomer the more stable one. In particular, for the double C₆₀ adduct of pentacene 1 , the syn isomer should be the main product instead of the anti one inferred in the original synthesis experiment (Y. Murata et al., J. Org. Chem.­ 1999 , 64, 3483). With and without dispersion interactions taken into account, the Gibbs energy difference ΔG(syn?anti) is ?6.36 and +1.15 kcal mol^?1, respectively. This study reminds us that dispersion interactions as well as electrostatic or hyperconjugation effects, etc. can lead to some unusual stereochemical phenomena.     

Molecular mechanics study of zirconocenium catalyzed isospecific polymerization of propylene

Stereocontrol energy (ΔE⁰) is investigated as a measure of enantioselectivity of ansa-zircoocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using π complex (°C) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis (1-η⁵-indenyl) - ( 1 ) and rac-ethylenebis (1-η⁵-4,5,7,8-tetrahydroindenyl) ( 2 ) zirconocenium species are isospecific in either the π-complexes or the transition states. The stereoselectivity is greater if there is α-agostic interaction; it is lowered in the cases of β and γ agostic interactions. The ¹³C-NMR steric pentad distribution indicates the poly(propylene) to be much less stereoregular than that predicted by ΔE⁰. Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss. © 1995 John Wiley & Sons, Inc.     

Synthesis,characterization and DNA binding studies of two cyclopentadienyl ruthenium(II) complexes with amino acid ligands

Two new half-sandwich cyclopentadienyl ruthenium(II) complexes containing α-amino acids, [CpRu(PPh₃)₂(Ser)] (Ser = l-serine) and [CpRu(PPh₃)(Met)] (Met = l-methionine), were synthesized and characterized by physicochemical methods. Interactions of these two complexes with calf thymus DNA were investigated by UV–Vis absorption spectroscopy, emission spectroscopy and competitive binding studies. The results indicate that both complexes can interact with DNA, leading to the damage of the double helix. [CpRu(PPh₃)₂(Ser)] binds to DNA by intercalation, while the binding mode for [CpRu(PPh₃)(Met)] is more complicated due to the formation of an EB-DNA-complex (EB = ethidium bromide). The affinity of the Met complex for DNA is stronger than that of the Ser complex, which could be due to groove–surface combination or electrostatic interaction in addition to intercalative binding.     

Polarographic study of Eu(III)-succinate-ethylenediamine and Eu(III)-malonate-ethylenediamine mixed systems

The mixed complexes of Eu(III) with succinate (succ^2?) and malonate (mal^2?) and ethylenediamine (en) have been studied polarographically at 25°C and at constant ionic strength, μ = 0.1 (NaNO₃) and pH 6. The reduction of the complexes in each case is quasi-reversible and diffusion-controlled. In each system three mixed complexes are formed, viz. [Eu(succ)(en)]⁺, [Eu(succ)(en)2]⁺ and [Eu(succ)₂(en)]^? with stability constants log β₁₁ = 9.2, log β₁₂ = 17.5 and log β₂₁ = 11.7; and [Eu(mal)(en)]⁺, [Eu(mal)₂(en)₂]^? and [Eu(mal)₃(en)]^3? with stability constants log β₁₁ = 11.4, log β₂₂ = 19.08 and log β₃₁ = 13.5 respectively.     

Arylation of styrene derivatives using aryliron complexes [CpFe(CO)2Ar] revealed by density functional theory calculations: Fe(II)-assisted group exchange through Fe-C bond cleavage and Fe-X bond formation

The detailed mechanism for arylation of styrene or its α-CF₃ substituted analog using aryliron complex [CpFe(CO)₂Ar] was studied using density functional theory calculations. Results of calculations show that the arylation mechanism mainly involves three steps: (1) a ligand exchange process between a CO and styrene or its derivative; (2) migration of Ar group from Fe to β-C of styrene; (3) β-H (or β-F) elimination and dissociation of the stilbene derivative from the CpFeHCO (or CpFeFCO) moiety. Both of Steps (2) and (3) experience a similar four-memberred cyclic transition state. The d_π-p_π interaction stabilizes the CC π coordinated complexes and the agostic interaction plays an important role in stabilizing intermediates and promoting elimination of the β-H (or β-F if available). For arylation of the α-CF3 substituted styrene, our calculations clarified that the dissociation of ethylene derivatives to give PF (product for β-F elimination) is kinetically and thermodynamically more favorable than to give PH (product for β-H elimination), which is the determined step for the selectivity of the final products.     

Theoretical investigation of C–HM interactions in organometallic complexes: A natural bond orbital (NBO) study

The nature of C–HM agostic interactions in model metal complexes [M²⁺(CH₂CH₃)(PH₃)_nCl] (where M = Sc, Ti, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn; n = 1, 2, 3, 4) was studied with the natural bond orbital analysis (NBO) approach using density functional theory (DFT) optimized geometries at the B3LYP/6-31G(d,p) level of theory. The effect of nature of metal, coordination number, oxidation state and ligand field effects on the agostic interaction is examined. A set of 20 crystal structures of organometallic complexes taken from the Cambridge Structural Database (CSD) was studied computationally employing AIM theory and NBO analysis, and the applicability of these methods was critically accessed in demarcating the two types of interaction.     

The mechanism of ethylene polymerization by nickel salicylaldiminato catalysts - Agostic interactions and their kinetic isotope effects

The ethylene polymerization reaction of a neutral nickel catalyst was studied by DFT calculations at the Becke3LYP/6-31G(d) level of theory. As in related cases a β-agostic bond stabilizes the nickel alkyl ground states. Transition states for the insertion of the olefin show a distinct α-agostic interaction, which has not been observed for late metal polymerization catalysts before. An ethylene-alkyl complex was identified as the resting state of the reaction. The overall barrier height of the reaction amounts to 17.54 kcal/mol, which slightly increases to 17.60 kcal/mol for the polymerization of deuterated ethylene. Therefore, a small positive kinetic isotope effect (k_H/k_D = 1.09) can be calculated, which is caused by the α-agostic interaction in the transition state. A comparison to other late metal based polymerization systems reveals that the ethylene coordination step of highly active catalysts is significantly lower in energy compared to catalysts which are only moderately active.     

两种新型μ－氧桥联－双铁(III)席夫碱配合物：X-衍射分析，IR, UV-Vis, CD光谱，磁性质和电化学性质

热力学稳定的带有大环配体的μ－氧桥联－双铁配合物，由于其两个铁中心之间的有趣的电子结构和磁相互作用而受到广泛关注。μ－氧桥联－双铁席夫碱配合物，[{Fe(tbusalphn)}₂(μ-o)] (1)和[{Fe(R,R-salchxn)}₂(μ-o)] (2), 通过用咪唑或N－甲基咪唑的水溶液处理相应的单核铁氯化物，Fe(L)Cl，而获得。1和2的晶体结构通过x-射线结构分析而被确定。1属于三斜晶系，P－1空间群。2属于单斜晶系，P2₁/c空间群。由于1的配体带有庞大的叔丁基取代基，导致形成μ－氧桥联－双铁配合物时的空间拥挤，因此，其Fe-O-Fe夹角为176.5 ^o，几乎成平角。而2则由于配体上没有庞大的取代基，其Fe-O-Fe夹角为149.6^o，明显小于1的Fe-O-Fe夹角。 本文还对两种μ－氧桥联－双铁席夫碱配合物及相应的单核铁氯化物的红外光谱、紫外－可见吸收光谱及圆二色光谱性质进行了研究。与相应的单体铁配合物相比较，生成μ－氧桥联－双铁席夫碱配合物后，出现一新的红外吸收带，归属于ν_Fe-O-Fe振动。有趣的是，其数值与Fe-O-Fe夹角大小相对应。1和2除具有明显不同的Fe-O-Fe夹角外，它们的圆二色光谱却是相似的。 对1和2的磁性质研究表明，在这类化合物中两个铁(III)离子之间存在着强烈的分子内抗铁磁性偶合作用。另外，本文还采用循环伏安法对1和2的电化学性质进行了研究。     

Reactions of cis-[dicarbonylbis(1,2-bis(dimethylphosphino)ethane)metal] compounds of chromium and molybdenum with organic electron acceptors; Tetracyanoethene, 1,2,4,5-tetracyanobenzene and 1,3,5-trinitrobenzene

Reaction between cis-[Mo(CO)₂(dmpe)₂] (dmpe =Me₂PCH₂CH₂PMe₂) and organic π-acids tetracyanoethene (TCNE), 1,2,4,5-tetracyanobenzene (TCNB) and 1,3,5-trinitrobenzene (TNB) proceeds via electron transfer from the metal complex, which is oxidised to the 17-electron trans-[Mo(CO)₂(dmpe)₂]⁺ ion, to the organic acceptor which is reduced to the radical anion. The final products of the reactions are characterised ascis-[Mo{C₂(CN)₃} (CO)₂(dmpe)₂] [CN], cis-[Mo{C₆H₂(CN)₄} (CO)₂(dmpe)₂] [C₆H₂(CN)₄]₈ and [Mo(CO)₂(dmpe)₂ · 2 C₆H₃(NO₂)₃] by analysis and spectroscopic (IR, NMR, ESR) measurements which are compared with those of cis-[MoX(CO)₂(dmpe)₂]X (X = Cl, Br, I) and fac, fac-[Mo₂Cl₄(CO)₄(dmpe)₃]. The reaction of cis-[Cr(CO)₂(dmpe)₂] with TCNE gives trans-[Cr(CO)₂(dmpe)₂]⁺ [TCNE]^? only.     

Investigating pyridazine and phthalazine exchange in a series of iridium complexes in order to define their role in the catalytic transfer of magnetisation from para-hydrogen

The reaction of [Ir(IMes)(COD)Cl], [IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, COD = 1,5-cyclooctadiene] with pyridazine (pdz) and phthalazine (phth) results in the formation of [Ir(COD)(IMes)(pdz)]Cl and [Ir(COD)(IMes)(phth)]Cl. These two complexes are shown by nuclear magnetic resonance (NMR) studies to undergo a haptotropic shift which interchanges pairs of protons within the bound ligands. When these complexes are exposed to hydrogen, they react to form [Ir(H)₂(COD)(IMes)(pdz)]Cl and [Ir(H)₂(COD)(IMes)(phth)]Cl, respectively, which ultimately convert to [Ir(H)₂(IMes)(pdz)₃]Cl and [Ir(H)₂(IMes)(phth)₃]Cl, as the COD is hydrogenated to form cyclooctane. These two dihydride complexes are shown, by NMR, to undergo both full N-heterocycle dissociation and a haptotropic shift, the rates of which are affected by both steric interactions and free ligand pK_a values. The use of these complexes as catalysts in the transfer of polarisation from para-hydrogen to pyridazine and phthalazine via signal amplification by reversible exchange (SABRE) is explored. The possible future use of drugs which contain pyridazine and phthalazine motifs as in vivo or clinical magnetic resonance imaging probes is demonstrated; a range of NMR and phantom-based MRI measurements are reported.     

Alkyl complexes of palladium(II) with trimethylphosphine and 1,2-bis-(dimethylphosphino)ethane ligands

A number of neutral, mononuclear dialkylpalladium(II) tertiary phosphine complexes of geneal formula cis or trans-PdR₂(PMe₃)₂ and cis-PdR₂ (dmpe) [dmpe = 1,2-bis(dimethylphosphino)ethane], R = Me, CH₂Ph, CH₂CMe₂Ph, CH₂SiMe₃ have been obtained by interaction of magnesium reagents with palladium(II) acetate or trans-Pd(O₂CMe)₂(PMe₃)₂.     

Synthesis,properties, and X-ray crystal structures of two Ag(I) complexes with 2-(pyridin-4-yl)-1H-imidazole-4,5-dicarboxylic acid

Two 1-D and 3-D Ag(I) complexes involving 2-(pyridin-4-yl)-1H-imidazole-4,5-dicarboxylic acid (H₃PIDC) have been characterized by infrared spectrum, elemental analysis, and single-crystal X-ray diffraction. [Ag₂(HPIDC)]_n (1), synthesized under hydrothermal conditions, gave a 3-D framework; [Ag₂(HPIDC)(MBI)]_n (2) (MBI?=?2-methyl-1H-benzo[d]imidazole), with MBI as the second ligand, gave a 1-D zigzag chain and further formed a 3-D supramolecular structure through π···π interactions. The most interesting structural features of these complexes are the presence of C–H···Ag hydrogen bonding interactions and Ag···C weak interactions between the Ag centers and H₃PIDC. Luminescence indicates that 2 has significantly stronger fluorescent emissions than 1 in the solid state at room temperature.     

Addition of Carbon Nucleophiles to Tricarbonylchromium Complexes of 1,2-Dihydrocyclobutabenzene,Indane, 1,2,3,4-Tetrahydronaphtbalene and ortho-Xylene

3-Substituted 1,2-dihydrocyclobutabenzenes (bicyclo[4. 2. 0]octa-1,3,5-triene) are readily accessible from [Cr(CO)₃(1,2-dihydrocyclobutabenzene)] ( 1 ) via a two-step sequence which involves addition of a nucleophile and oxidation of the intermediate anionic cyclohexadienyl complex. Nucleophiles used include LiCMe₂CN (A), LiCH₂CN (B), LiC(Me)(OR)CN (C), (D), (E), LiCMe₂CO₂Me (F), and LiCH₂CO₂(t-Bu) (G). [Cr(CO)₃(Indane)] ( 2 ) also reacts highly regioselectively to give α-substitution, whereas [Cr(CO)₃(tetrahydronaphthalene)] ( 3 ) and [Cr(CO)₃(o-xylene)] ( 4 ) give mixtures of products. In several cases, the mixtures of the intermediate anionic cyclohexadienyl complexes can be equilibrated to give, after oxidation, β-substituted derivatives of 1,2,3,4-tetrahydronaphthalene and ortho-xylene selectively. EHMO calculations were carried out, and they rationalize the observed α-regioselectivity of nucleophilic addition under kinetic control, The X-ray structures of 1 and 4 are reported and in both compounds the Cr(CO)₃ group adopts in the solid state a staggered syn-conformation with respect to the substituted aromatic C-atonis.