Homo and heteromultimetallic complexes containing a group 8 transition metal and µ-diphosphine bridging ligands involved in anticancer research: A review.

Herein, we present a comprehensive review focusing on synthetic strategies, detailed structural analysis, and anticancer activity investigations of complexes following the general formula [LnM(µ-diphosphine)M'Lm] where M = group 8 metal; M' = any transition metal; µ-diphosphine = bridging ligand; Ln and Lm = ligand spheres). Both homo- and heteromultimetallic complexes will be discussed in detail. We review in vitro, in vivo and in silico anticancer activity investigations, in an attempt to draw comparisons between the various complexes and derive structure-activity relationships (SAR). This review solely focuses on complexes falling under the general formula stated above that have been studied for their anticancer activities, other complexes falling into that scheme but which have not undergone anticancer testing are not included in this review. We compare the anticancer activities of these complexes to their mononuclear counterparts, and a positive control (cisplatin) when possible and present a summary of all existing data to date and attempt to draw some conclusions on the future development of these complexes.

Ionic mononuclear [Fe] and heterodinuclear [Fe,Ru] bis(diphenylphosphino)alkane complexes: Synthesis, spectroscopy, DFT structures, cytotoxicity, and biomolecular interactions.

Iron(II) and Ru(II) half-sandwich compounds encompass some promising pre-clinical anticancer agents whose efficacy may be tuned by structural modification of the coordinated ligands. Here, we combine two such bioactive metal centres in cationic bis(diphenylphosphino)alkane-bridged heterodinuclear [Fe2+, Ru2+] complexes to delineate how ligand structural variations modulate compound cytotoxicity. Specifically, Fe(II) complexes of the type [(η5-C5H5)Fe(CO)2(κ1-PPh2(CH2)nPPh2)]{PF6} (n = 1-5), compounds 1-5, and heterodinuclear [Fe2+, Ru2+] complexes, [(η5-C5H5)Fe(CO)2(µ-PPh2(CH2)nPPh2))(η6-p-cymene)RuCl2]{PF6} (n = 2-5) (compounds 7-10), were synthesized and characterised. The mononuclear complexes were moderately cytotoxic against two ovarian cancer cell lines (A2780 and cisplatin resistant A2780cis) with IC50 values ranging from 2.3 ± 0.5 µM to 9.0 ± 1.4 µM. For 7-10, the cytotoxicity increased with increasing Fe⋅⋅⋅Ru distance, consistent with their DNA affinity. UV-visible spectroscopy suggested the chloride ligands in heterodinuclear 8-10 undergo stepwise substitution by water on the timescale of the DNA interaction experiments, probably affording the species [RuCl(OH2)(η6-p-cymene)(PRPh2)]2+ and [Ru(OH)(OH2)(η6-p-cymene)(PRPh2)]2+ (where PRPh2 has R = [-(CH2)5PPh2-Fe(C5H5)(CO)2]+). One interpretation of the combined DNA-interaction and kinetic data is that the mono(aqua) complex may interact with dsDNA through nucleobase coordination. Heterodinuclear 10 reacts with glutathione (GSH) to form stable mono- and bis(thiolate) adducts, 10-SG and 10-SG2, with no evidence of metal ion reduction (k1 = 1.07 ± 0.17 × 10-1 min-1 and k2 = 6.04 ± 0.59 × 10-3 min-1 at 37 °C). This work highlights the synergistic effect of the Fe2+/Ru2+ centres on both the cytotoxicity and biomolecular interactions of the present heterodinuclear complexes.

Osmium Arene Germyl, Stannyl, Germanate, and Stannate Complexes as Anticancer Agents.

Herein, we describe the synthesis, full spectroscopic characterization, DFT (density functional theory) calculations, and single-crystal X-ray diffraction analyses of a series of osmium arene σ-germyl, germanate, σ-stannyl, and stannate complexes, along with their cytotoxic (anticancer) investigations. The known dimer complexes [OsCl2(η6-C6H6)]2 (1) and [OsCl2(η6-p-cymene)]2 (2) were reacted with PPh3 to form the known mononuclear complex [OsCl2(η6-p-cymene)(PPh3)] (3) and the new complex [OsCl2(η6-C6H6)(PPh3)] (6); complex 3 was reacted with GeCl2·(dioxane) and SnCl2 to afford, by insertion into the Os-Cl bond, the neutral σ-germyl and stannyl complexes [OsCl(η6-p-cymene)(PPh3)(GeCl3)] (7) and [OsCl(η6-p-cymene)(PPh3)(SnCl3)] (11), respectively, as a mixture of enantiomers. Similarly, the reaction of complex 6 with GeCl2·(dioxane) afforded [OsCl(η6-C6H6)(PPh3)(GeCl3)] (9). Complex 2, upon reaction with 1,1-bis(diphenylphosphino)methane (dppm), formed a mixture of [OsCl2(η6-p-cymene)(κ1-dppm)] (4) and [Os(η6-p-cymene)(κ2-dppm)Cl]+Cl- (5) when prepared in acetonitrile and a mixture of 4 and the dinuclear complex [[OsCl2(η6-p-cymene)]2(µ-dppm)] (0) when prepared in dichloromethane. By utilizing either isolated 4 or a mixture of 4 and 5, the synthesis of κ2-dppm germanate and stannate salts, [OsCl(η6-p-cymene)(κ2-dppm)]+GeCl3 - (8) and [OsCl(η6-p-cymene)(κ2-dppm)]+SnCl3 - (10), were accomplished via halide-abstracting reactions with GeCl2·(dioxane) or SnCl2, respectively. All resulting complexes were characterized by means of multinuclear NMR, FT-IR, ESI-MS, and UV/Vis spectroscopy. X-ray diffraction analyses of 4, 8, 9, 10, and 11 were performed and are reported. DFT studies (B3LYP, basis set LANL2DZ for Os, and def2-TZVPP for Sn, Ge, Cl, P, C, and H) were performed on complex 9 and the benzene analogue of complex 11, 11-benzene, to evaluate the structural changes and the effects on the frontier molecular orbitals arising from the substitution of Ge for Sn. Finally, complexes 3 and 7-11 were investigated for potential anticancer activities considering cell cytotoxicity and apoptosis assays against Dalton's lymphoma (DL) and Ehrlich ascites carcinoma (EAC) malignant cancer cell lines. The complexes were also tested against healthy peripheral blood mononuclear cells (PBMCs). All cell lines were also treated with the reference drug cisplatin to draw a comparison with the results obtained from the reported complexes. The study was further corroborated with in silico molecular interaction simulations and a pharmacokinetic study.

Homo and heterobimetallic palladium and platinum complexes bearing µ-diphosphane bridges involved in biological studies.

Given the increasing reports of well-defined bimetallic molecular complexes as potential anticancer agents in the last decades, along with the prevalence of platinum in anticancer therapy, we report here a detailed survey of bimetallic platinum and palladium complexes investigated as potential anticancer agents. Specifically, we will concentrate on the synthesis, characterisation and biological (anticancer) studies of a sub-class of these agents, namely homo and heterobimetallic complexes bearing a bridging phosphane ligand of the type: [LnM1(µ-R2P(CH2)nPR2)M2Lm] (where M1 is platinum or palladium, M2 is any other transition metal, R = alkyl or aryl substituents, Ln or Lm are co-ligands, n = 1-6). We will review the in vitro and in vivo activities and any mechanistic anticancer studies of these complexes with a view of trying to delineate patterns in biological activity and structure-activity relationships (SAR). We do not include the review of bimetallic complexes in this class that have not undergone any anticancer testing, nor those that have been involved in other biological investigations unrelated to cancer studies.

25th Anniversary of Molecules-Recent Advances in Inorganic Chemistry.

Celebrating the "25th Anniversary of Molecules" with a Special Issue dedicated to "Recent Advances in Inorganic Chemistry" strengthens the renewed role that inorganic chemistry, one of the oldest chemistry divisions, has lately earned thanks to cutting-edge perspectives and interdisciplinary applications, eventually receiving the veneration and respect which its age might require [...].

A review on 1,1-bis(diphenylphosphino)methane bridged homo- and heterobimetallic complexes for anticancer applications: Synthesis, structure, and cytotoxicity.

Herein, we review developments in synthesis, structure, and biological (anti-cancer) activities of 1,1-bis(diphenylphosphino)methane (dppm) bridged homo- and heterobimetallic systems of the type LmM(µ2-dppm)M'Ln (M and M' are transition metals which may be different or the same and Ln,m are co-ligands) since the first such reported bimetallic system in 1987 until the present time (2020). As the simplest diphosphine, dppm enables facile formation of bimetallic complexes, where, given the short spacer between the PPh2 groups, close spatial proximity of the metal centres is ensured. We concentrate on complexes bearing no M-M interaction and contrast biological activities of these complexes with mononuclear counterparts and positive control agents such as cisplatin, in an attempt to elucidate patterns in the biological activities of these complexes.

Synthesis, structure and anti-cancer activity of osmium complexes bearing π-bound arene substituents and phosphane Co-Ligands: A review.

While many examples of osmium complexes, as anti-cancer agents, have been reported and some reviews have been devoted to this topic, a particularly interesting and synthetically accessible sub-class of these compounds namely those bearing a π- bound arene and phosphane co-ligand have escaped review. These complexes have made a surprisingly late entry in the literature (2005) in terms of anti-cancer investigations. This is somewhat surprising considering the plethora of analogous complexes that have been reported for the lighter analogue, ruthenium. Herein we review all complexes, neutral and ionic, bearing the "(ƞ6-arene)Os(PR3)" moiety focusing on their synthesis, reactivity, structural features (by X-ray diffraction analysis) as well as anti-cancer biological activity. An attempt is made throughout the article to contrast these to each other and to analogous Ru systems, and a full summary of all existing in vitro biological data is presented.

From an Isolable Acyclic Phosphinosilylene Adduct to Donor-Stabilized Si=E Compounds (E=O, S, Se).

Reaction of the arylchlorosilylene-NHC adduct ArSi(NHC)Cl [Ar=2,6-Trip2C6H3; NHC=(MeC)2(NMe)2C:] 1 with one molar equiv of lithium diphenylphosphanide affords the first stable NHC-stabilized acyclic phosphinosilylene adduct 2 (ArSi(NHC)PPh2), which could be structurally characterized. Compound 2, when reacted with one molar equiv selenium and sulfur, affords the silanechalcogenones 4 a and 4 b (ArSi(NHC)(=E)PPh2, 4 a: E=Se, 4 b: E=S), respectively. Conversion of 2 with an excess of Se and S, through additional insertion of one chalcogen atom into the Si=P bond, leads to 3 a and 3 b (ArSi(NHC)(=E)-E-P(=E)Ph2, 3 a: E=Se, 3 b: E=S), respectively. Additionally, the exposure of 2 to N2O or CO2 yielded the isolable NHC-stabilized silanone 4 c, Ar(NHC)(Ph2P)Si=O.

A Persistent 1,2-Dihydrophosphasilene Adduct.

The reaction of the arylchlorosilylene-NHC adduct ArSi(NHC)Cl [Ar=2,6-Trip2 -C6 H3 ; NHC=(MeC)2 (NMe)2 C] 1 with one molar equiv of LiPH2 (.) dme (dme=1,2-dimethoxyethane) affords the first 1,2-dihydrophosphasilene adduct 2 (ArSi(NHC)(H)PH). The latter is labile in solution and can undergo head-to-tail dimerization to give [ArSi(H)PH]2 3 and "free" NHC. Further stabilization of 2 by complexation with {W(CO)5 } affords the isolable 1,2-dihydrophosphasilene-tungsten complex 4 [ArSi(NHC)(H)P(H)W(CO)5 ]. Additionally, the new 1-silyl-2-hydrophosphasilene ArSi(NHC)(H)PSiMe3 5 could be synthesized and structurally characterized. DFT studies confirmed that the SiP bond in 2 and 4 is mostly zwitterionic with drastically decreased double-bond character.

Biomimetic [2Fe-2S] clusters with extensively delocalized mixed-valence iron centers.

A complete series of biomimetic [2Fe-2S] clusters, [(L(Dep) Fe)2 (µ-S)2 ] (3, L(Dep) =CH[CMeN(2,6-Et2 C6 H3 )]2 ), [(L(Dep) Fe)2 (µ-S)2 K] (4), [(L(Dep) Fe)2 (µ-S)2 ][Bu4 N] (5, Bu=n-butyl), and [(L(Dep) Fe)2 (µ-S)2 K2 ] (6), could be synthesized and characterized. The all-ferric [2Fe-2S] cluster 3 is readily accessible through the reaction of [(L(Dep) Fe)2 (µ-H)2 ] (2) with elemental sulfur. The chemical reduction of 3 with one molar equivalent of elemental potassium affords the contact ion pair K(+) [2Fe-2S](-) (4) as a one-dimensional coordination polymer, which in turn reacts with [Bu4 N]Cl to afford the separate ion pair [Bu4 N](+) [2Fe-2S](-) (5). Further reduction of 4 with potassium furnishes the super-reduced all-ferrous [2Fe-2S] cluster 6. Remarkably, complexes 4 and 5 are [2Fe-2S] clusters with extensively delocalized Fe(2+) Fe(3+) pairs as evidenced by (57) Fe Mössbauer, X-ray absorption and emission spectroscopy (XAS, XES) and in accordance with DFT calculations.

Alkaline-Earth-Metal-Induced Liberation of Rare Allotropes of Elemental Silicon and Germanium from N-Heterocyclic Metallylenes.

The synthesis and striking reactivity of the unprecedented N-heterocyclic silylene and germylene ("metallylene") alkaline-earth metal (Ae) complexes of the type [(η(5)-C5Me5)2Ae←:E(N(t)BuCH)2] (3, 4, and 7-9; Ae = Ca, E = Ge 3; Ae = Sr, E = Ge 4; Ae = Sr, E = Si 7; Ae = Ba, E = Si 8; Ae = Ba, E = Ge 9) are reported. All complexes have been characterized by spectroscopic means, and their bonding situations investigated by density functional theory (DFT) methods. Single-crystal X-ray diffraction analyses of examples revealed relatively long Si-Ae and Ge-Ae distances, respectively, indicative of weak E:→Ae (E = Si, Ge) dative bonds, further supported by the calculated Wiberg bond indices , which are rather low in all cases (∼0.5). Unexpectedly, the complexes undergo facile transformation to 1,4-diazabuta-1,3-diene Ae metal complexes of the type [(η(5)-C5Me5)2Ae(κ(2)-{N(t)Bu═CHCH═N(t)Bu})] (Ae = Sr 10, Ae = Ba 11) or in the case of calcium to the dinuclear complex [(η(5)-C5Me5)2Ca←:N((t)Bu)═CHCH═((t)Bu)N:→Ca(η(5)-C5Me5)2] (12) under concomitant liberation of elemental silicon and germanium. The formation of elemental silicon and germanium is proven by inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy, selected area electron diffraction, and energy dispersive X-ray spectroscopy. Notably, the decomposition of the Si(II)→Ba complex 8 produces allo-silicon, a rare allotropic form of elemental silicon. Similarly, the analogous Ge(II)→Ba complex 9, upon decomposition, forms tetragonal germanium, a dense and rare allotrope of elemental germanium. The energetics of this unprecedented alkaline-earth-metal-induced liberation of elemental silicon and germanium was additionally studied by DFT methods, revealing that the transformations are pronouncedly exergonic and considerably larger for the N-heterocyclic germylene complexes than those of the corresponding silicon analogues.

Open-shell lanthanide(II+) or -(III+) complexes bearing σ-silyl and silylene ligands: synthesis, structure, and bonding analysis.

Complexes featuring lanthanide (Ln)-Si bonds represent a highly neglected research area. Herein, we report a series of open-shell Ln(II+) and Ln(III+) complexes bearing σ-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands. The reactions of the Ln(III+) complexes Cp3Ln (Ln = Tm, Ho, Tb, Gd; Cp = cyclopentadienide) with the 18-crown-6 (18-cr-6)-stabilized 1,4-oligosilanyl dianion [(18-cr-6)KSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2K(18-cr-6)] (1) selectively afford the corresponding metallacyclopentasilane salts [Cp2Ln({Si(SiMe3)2SiMe2}2)](-)[K2(18-cr-6)2Cp](+) [Ln = Tm (2a), Ho (2b), Tb (2c), Gd (2d)]. Complexes 2a-2d represent the first examples of structurally characterized Tm, Ho, Tb, and Gd complexes featuring Ln-Si bonds. Strikingly, the analogous reaction of 1 with the lighter element analogue Cp3Ce affords the acyclic product [Cp3CeSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2-Cp3Ce](2-)2[K(18-cr-6)](+) (3) as the first example of a complex featuring a Ce-Si bond. In an alternative synthetic approach, the aryloxy-functionalized benzamidinato NHSi ligand Si(OC6H4-2-tBu){(NtBu)2CPh} (4a) and the alkoxy analogue Si(OtBu){(NtBu)2CPh} (4b) were reacted with Cp*2Sm(OEt2), affording, by OEt2 elimination, the corresponding silylene complexes, both featuring Sm(II+) centers: Cp*2Sm ← :Si(O-C6H4-2-tBu){(NtBu)2CPh} (6) and Cp*2Sm ← :Si(OtBu){(NtBu)2CPh} (5). Complexes 5 and 6 are the first four-coordinate silylene complexes of any f-block element to date. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analysis. In the series 2a-2d, a linear correlation was observed between the Ln-Si bond lengths and the covalent radii of the corresponding Ln metals. Moreover, in complexes 5 and 6, notably long Sm-Si bonds are observed, in accordance with a donor-acceptor interaction between Si and Sm [5, 3.4396(15) Å; 6, 3.3142(18) Å]. Density functional theory calculations were carried out for complexes 2a-2d, 5, and 6 to elucidate the bonding situation between the Ln(II+) or Ln(III+) centers and Si. In particular, a decrease in the Mayer bond order (MBO) of the Ln-Si bond is observed in the series 2a-2d in moving from the lighter to the heavier lanthanides (Tm = 0.53, Ho = 0.62, Tb = 0.65, and Gd = 0.75), which might indicate decreasing covalency in the Ln-Si bond. In accordance with the long bond lengths observed experimentally in complexes 5 and 6, comparatively low MBOs were determined for both silylene complexes (5, 0.24; 6, 0.25) .

Synthesis of mixed silylene-carbene chelate ligands from N-heterocyclic silylcarbenes mediated by nickel.

The Ni(II) -mediated tautomerization of the N-heterocyclic hydrosilylcarbene L(2) Si(H)(CH2 )NHC 1, where L(2) =CH(CCH2 )(CMe)(NAr)2 , Ar=2,6-iPr2 C6 H3 ; NHC=3,4,5-trimethylimidazol-2-yliden-6-yl, leads to the first N-heterocyclic silylene (NHSi)-carbene (NHC) chelate ligand in the dibromo nickel(II) complex [L(1) Si:(CH2 )(NHC)NiBr2 ] 2 (L(1) =CH(MeCNAr)2 ). Reduction of 2 with KC8 in the presence of PMe3 as an auxiliary ligand afforded, depending on the reaction time, the N-heterocyclic silyl-NHC bromo Ni(II) complex [L(2) Si(CH2 )NHCNiBr(PMe3 )] 3 and the unique Ni(0) complex [η(2) (Si-H){L(2) Si(H)(CH2 )NHC}Ni(PMe3 )2 ] 4 featuring an agostic SiH→Ni bonding interaction. When 1,2-bis(dimethylphosphino)ethane (DMPE) was employed as an exogenous ligand, the first NHSi-NHC chelate-ligand-stabilized Ni(0) complex [L(1) Si:(CH2 )NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni(0) complex 6, [L(1) Si:(CH2 )NHCNi(CO)2 ], is easily accessible by the reduction of 2 with K(BHEt3 ) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada-Corriu-type cross-coupling reactions.

Unprecedented silicon(II)→calcium complexes with N-heterocyclic silylenes.

The first N-heterocyclic silylene (NHSi) complexes of any s-block element to date are reported for calcium: [(η(5)-C5Me5)2Ca←:Si(O-C6H4-2-(t)Bu){(N(t)Bu)2CPh}] (6) and [(η(5)-C5Me5)2Ca←:Si(N(t)BuCH)2] (7). Complexes 6 and 7 are isolable in a facile way upon reaction of the corresponding free N-heterocyclic silylenes (NHSis) with [(η(5)-C5Me5)2Ca] (2). Complexes 6 and 7 were fully characterised by spectroscopic means and the single crystal X-ray diffraction analysis of 6 is also reported. Analysis of the bonding situation by DFT methods including a Bader Atoms in molecules (AIM) analysis is also reported. The bonding interaction between the Si and Ca centres in complexes 6 and 7 can best be viewed as σ-donor-acceptor interactions, with a considerable ionic contribution in the bond. The reactivity towards the oxygen containing substrates THF and benzophenone is also discussed.

An amplified Ylidic "half-parent" iminosilane LSi═NH.

The reaction of LSiBr(NH2) (4) (L = CH[(C═CH2)CMe(NAr)2]; Ar = 2,6-iPr2C6H3) with lithium bis(trimethylsilyl)amide in the presence of pyridine or 4-dimethylaminopyridine (DMAP) resulted in the activation of the α C-H bond of pyridine or DMAP, affording the products LSi(dmap)NH2 (6) and LSi(pyridine)NH2 (7a), respectively. Remarkably, this metal-free aromatic C-H activation occurs at room temperature. The emerging aminosilanes were isolated and fully characterized. Isotope labeling experiments and detailed DFT calculations, elucidating the reaction mechanism, were performed and provide compelling evidence of the formation of the "half-parent" iminosilane 1, LSi═NH, which facilitates this transformation due to its amplified ylidic character by the chelate ligand L. Furthermore, the elusive iminosilane 1 could be trapped by benzophenone and trimethylsilylazide affording the corresponding products, 8 and 9, respectively, thereby confirming its formation as a key intermediate.

An elusive hydridoaluminum(I) complex for facile C-H and C-O bond activation of ethers and access to its isolable hydridogallium(I) analogue: syntheses, structures, and theoretical studies.

The reaction of AlBr3 with 1 molar equiv of the chelating bis(N-heterocyclic carbene) ligand bis(N-Dipp-imidazole-2-ylidene)methylene (bisNHC, 1) affords [(bisNHC)AlBr2](+)Br(-) (2) as an ion pair in high yield, representing the first example of a bisNHC-Al(III) complex. Debromination of the latter with 1 molar equiv of K2Fe(CO)4 in tetrahydrofuran (THF) furnishes smoothly, in a redox reaction, the (bisNHC)(Br)Al[Fe(CO)4] complex 3, in which the Al(I) center is stabilized by the Fe(CO)4 moiety through Al(I):→Fe(0) coordination. Strikingly, the Br/H ligand exchange reactions of 3 using potassium hydride as a hydride source in THF or tetrahydropyran (THP) do not yield the anticipated hydridoaluminum(I) complex (bisNHC)Al(H)[Fe(CO)4] (4a) but instead lead to (bisNHC)Al(2-cyclo-OC4H7)[Fe(CO)4] (4) and (bisNHC)Al(2-cyclo-OC5H9)[Fe(CO)4] (5), respectively. The latter are generated via C-H bond activation at the α-carbon positions of THF and THP, respectively, in good yields with concomitant elimination of dihydrogen. This is the first example whereby a low-valent main-group hydrido complex facilitates metalation of sp(3) C-H bonds. Interestingly, when K[BHR3] (R = Et, sBu) is employed as a hydride source to react with 3 in THF, the reaction affords (bisNHC)Al(OnBu)[Fe(CO)4] (6) as the sole product through C-O bond activation and ring opening of THF. The mechanisms for these novel C-H and C-O bond activations mediated by the elusive hydridoaluminum(I) complex 4a were elucidated by density functional theory (DFT) calculations. In contrast, the analogous hydridogallium(I) complex (bisNHC)Ga(H)[Fe(CO)4] (9) can be obtained directly in high yield by the reaction of the (bisNHC)Ga(Cl)[Fe(CO)4] precursor 8 with 1 molar equiv of K[BHR3] (R = Et, sBu) in THF at room temperature. The isolation of 9 and its inertness toward cyclic ethers might be attributed to the higher electronegativity of gallium versus aluminum. The stronger Ga(I)-H bond, in turn, hampers α-C-H metalation or C-O bond cleavage in cyclic ethers, the latter of which is supported by DFT calculations.

Elucidating the effect of the nucleophilicity of the silyl group in the reduction of CO2 to CO mediated by silyl-copper(I) complexes.

The reduction of CO2 to CO with silyl-copper(I) complexes bearing various silyl groups has been investigated. The silyl-copper(I) complexes [LSi(X)Cu(IPr)] 2-5 (X = OtBu (2), OH (3), H (4), OC6F5 (5); L = CH{C=CH2}(CMe)(NAr)2, IPr = (CHNAr)2C:, Ar = 2,6-iPr2C6H3) bearing OtBu, OH, H, and OC6F5 as functional groups are readily accessible by the activation of the Cu-O and Cu-H bonds in (IPr)CuX with silylene LSi: (1). These complexes are not readily accessible by the commonly used transmetallation reaction, rendering this methodology rather unique and facile in synthesizing silicon-functionalized silyl-metal complexes. The functional groups at the silicon atoms in compounds 2-5 enable the silyl groups to feature different nucleophilicity, which affords different activities toward CO2 reduction to CO. The silyl moieties of complexes 2 and 3, containing electron-donating groups (i.e., OtBu and OH) at the silicon centers, are more nucleophilic than that of compound 4 and 5, bearing a hydride and the electron-withdrawing group OC6F5 at the silicon centers, respectively. Consistent with this observation, compounds 2 and 3 show higher activity in CO2 reduction to CO compared to compounds 4 and 5, and the latter cases are zero-order reactions with respect to 4 and 5 (4: k = 7.8×10(-6) mol L(-1) s(-1); 5: 2.7×10(-8) mol L(-1) s(-1)). This suggests that the more nucleophilic the silyl moiety in a silyl-copper(I) complex is, the higher is the efficiency in CO2 reduction to CO. In addition, the siloxyl-copper(I) complexes [LSi(X)OCu(IPr)] 6-9 [X = OtBu (6), OH (7), H (8), OC6F5 (9)] were isolated as the products from the corresponding reduction reactions. Complexes 2-4 and 6-8 were characterized by spectroscopic and structural means.

A donor-stabilized zwitterionic "half-parent" phosphasilene and its unusual reactivity towards small molecules.

The stabilization of the labile, zwitterionic "half-parent" phosphasilene 4 L'Si=PH (L'=CH[(C=CH2)CMe(NAr)2]; Ar=2,6-iPr2C6H3) could now be accomplished by coordination with two different donor ligands (4-dimethylaminopyridine (DMAP) and 1,3,4,5-tetramethylimidazol-2-ylidene), affording the adducts 8 and 9, respectively. The DMAP-stabilized zwitterionic "half-parent" phosphasilene 8 is capable of transferring the elusive parent phosphinidene moiety (:PH) to an unsaturated organic substrate, in analogy to the "free" phosphasilene 4. Furthermore, compounds 4 and 8 show an unusual reactivity of the Si=P moiety towards small molecules. They are capable of adding dimethylzinc and of activating the S-H bonds in H2S and the N-H bonds in ammonia and several organoamines. Interestingly, the DMAP donor ligand of 8 has the propensity to act as a leaving group at the phosphasilene during the reaction. Accordingly, treatment of 8 with H2S affords, under liberation of DMAP, the unprecedented thiosilanoic phosphane LSi=S(PH2) 16 (L=HC(CMe[2,6-iPr2C6H3N])2). Compounds 4 and 8 react with ammonia both affording L'Si(NH2)PH2 17, respectively. In addition, the reaction of 8 with isoproylamine, p-toluidine, and pentafluorophenylhydrazine lead to the corresponding phosphanylsilanes L'Si(PH2)NHR (R=iPr 18 a; R=C6H5-CH3 18 b, R=NH(C6F5) 18 c), respectively.

Mechanistic studies of CO2 reduction to methanol mediated by an N-heterocyclic germylene hydride.

The labile germylene hydride L(Cy)GeH is capable of activating CO2 affording the corresponding formate L(Cy)GeOCH([double bond, length as m-dash]O) (2) (L(Cy) = cyclo-C6H8-1-NAr-2-C(Ph)NAr, Ar = 2,6-iPr2C6H3). Compound and the previously reported LGeOCH([double bond, length as m-dash]O) (L = CH(MeC[double bond, length as m-dash]NAr)2, Ar = 2,6-iPr2C6H3) (2) could be further converted to methanol with the AlH3 · NMe3 alane-amine adduct as a hydrogen source upon workup with water. A plausible mechanism for the conversion of the formate complexes to methanol is proposed based on additional results from the conversion of with the milder hydride delivery agent LAlH2.

Bis-N-heterocyclic carbene (NHC) stabilized η6-arene iron(0) complexes: synthesis, structure, reactivity, and catalytic activity.

Reaction of FeCl2 with the chelating bis-N-heterocyclic carbene (NHC) bis-(N-Dipp-imidazole-2-ylidene)methylene (abbreviated {((Dipp)C:)2CH2}) (Dipp = 2,6-di-isopropylphenyl) affords the complex [FeCl2{((Dipp)C:)2CH2}] (1) in high yield. Reduction of complex 1 with excess KC8 with a 10-fold molar excess of PMe3 affords the Fe(II) complex [FeH{((Dipp)C:)2CH2}(PMe3)(η(2)-PMe2CH2)] (2) as a mixture of three stereoisomers. Complex 2, the first example of any iron(II) complex bearing mutually an NHC and PMe3 ligand, is likely obtained from the in situ, reductively generated 16 VE Fe(0) complex, [Fe{((Dipp)C:)2CH2}(PMe3)2] (2'), following intramolecular C-H activation of one of the phosphorus-bound CH3 groups. Complex 2 is unstable in aromatic solvents and forms, via a novel synthetic transformation involving intramolecular reductive elimination and concomitant PMe3 elimination, the Fe (0) arene complex [Fe{((Dipp)C:)2CH2}(η(6)-C6D6)] (4-d6) in C6D6. Complex 4-d6 represents the first example of an NHC stabilized iron (0) arene complex. The transformation from 2 to 4-d6 can be accelerated at higher temperature and at 60 °C forms immediately. Alternatively, the reduction of 1 in the presence of toluene or benzene affords the complexes [Fe{((Dipp)C:)2CH2}(η(6)-C7H8)] (3) and [Fe{((Dipp)C:)2CH2}(η(6)-C6H6)] (4), selectively and in good yields. DFT calculations characterizing the bonding situation in 3 and 4 reveal similar energies of the HOMO and LUMO orbitals, with the LUMO orbital of both complexes located on the Dipp rings of the bis-NHC. The HOMO orbital reflects a π-back-bonding interaction between the Fe(0) center and the chelating NHC ligand, while the HOMO-1 is associated with the arene interaction with the Fe(0) site. The calculations do not suggest any noninnocence of the coordinated arene in either complex. Moreover, the (57)Fe Mössbauer spectrum of 4 at 80K exhibits parameters (δ = 0.43 mm·s(-1); ΔEQ = 1.37 mm·s(-1)) which are consistent with a five-coordinate Fe(0) system, rendering 3 and 4 the first examples of well-defined authentic Fe(0)-η(6)-arene complexes of the type [Fe(η(6)-arene)L2] (L = η(1 or 2) neutral ligand, mono or bidentate). Some reactivitiy studies of 3 are also reported: The reaction of 3 with excess CO selectively yields the five-coordinate piano-stool complex [Fe{((Dipp)C:)2CH2}(CO)3] (6) in near quantitative yields, while the reaction of complex 3 with C6D6 under heating affords by toluene elimination 4-d6. The catalytic ability of 4 was also investigated with respect to amide reduction to amines, for a variety of substrates using Ph2SiH2 as a hydride source. In all cases good to excellent yields to the corresponding amines were obtained. The use of 4 as a precatalyst represents the first example of a well-defined Fe(0) complex to effect this catalytic process.