From a PGeP Pincer-Type Germylene to Metal Complexes Featuring Chelating (Ir) and Tripodal (Ir) PGeP Germyl and Bridging (Mn2) and Chelating (Ru) PGeP Germylene Ligands

Heavier tetrylene- and tetrylyne-transition metal chemistry: it's no carbon copy

Since the late 19th century, heavier tetrylene- and tetrylyne-transition metal chemistry has formed an important cornerstone in both main-group and organometallic chemistry alike. Driven by the success of carbene systems, significant efforts have gone towards the thorough understanding of the heavier group 14 derivatives, with examples now known from across the d-block. This now leads towards applications in cooperative bond activation, and moves ultimately towards well-defined catalytic systems. This review aims to summarise this vast field, from initial discoveries of tetrylene and tetrylyne complexes, to the most recent developments in reactivity and catalysis, as a platform to the future of this exciting, blossoming field.

Reactions of [(dmpe)2MnH(C2H4)] with hydrogermanes to form germylene, germyl, hydrogermane, and germanide complexes

Reactions of the ethylene hydride complex trans-[(dmpe)2MnH(C2H4)] (1) with secondary hydrogermanes H2GeR2 at 55-60 °C afforded the base-free terminal germylene hydride complexes trans-[(dmpe)2MnH(GeR2)] (R = Ph; 2a, R = Et; 2b). Room temperature reactions of 2a or 2b with an excess of the primary hydrogermanes H3GeR' (R' = Ph or nBu) afforded trans-[(dmpe)2MnH(GeHR')] (R' = Ph; 3a, R' = nBu; 3b) in rapid equilibrium with small amounts of 2a/b, as well as the digermyl hydride complex mer-[(dmpe)2MnH(GeH2R')2] {R' = Ph (4a) or nBu (4b)} and the trans-hydrogermane germyl complex trans-[(dmpe)2Mn(GeH2R')(HGeH2R')] {R' = Ph (5a) or nBu (5b)}. Pure 3b was isolated from the reaction of 2b with H3GenBu, whereas 3a decomposed readily in solution in the absence of free H3GePh, and a pure bulk sample was not obtained. Reactions of 1 with H3GeR' (R' = Ph or nBu) also proceeded at 55-60 °C to afford mixtures of 3a/b, 4a/b and 5a/b, accompanied by remaining 1. However, upon continued heating to consume 1, various unidentified manganese-containing intermediates were formed, ultimately affording the germanide complex [{(dmpe)2MnH}2(μ-Ge)] (6) in 17-49% spectroscopic yield. Pure trans,trans-6 was isolated in 27% yield from the reaction of 1 with H3GenBu, and it is notable that this reaction involves stripping of all four substituents from the hydrogermane. Complexes 2a, 3a, and 6 were crystallographically characterized, and the nature of the MnGe bonding in these species (as well as in 2b and 3b) was probed computationally.

Synthesis and Some Coordination Chemistry of Phosphane-Difunctionalized Bis(amidinato)-Heavier Tetrylenes: A Previously Unknown Class of PEP Tetrylenes (E = Ge and Sn)

The bis(amidinato)-heavier tetrylenes E(bzamP)2 (E = Ge (2a) and Sn (2b); bzamP = N-isopropyl-N'-(diphenylphosphanylethyl)benzamidinate), which are equipped with one heavier tetrylene (germylene or stannylene) and two phosphane fragments (one on each amidinate moiety) as coordinable groups, have been synthesized from the benzamidinum salt [H2bzamP]Cl and GeCl2(dioxane) or SnCl2 in 2:1 mol ratio. A preliminary inspection of their coordination chemistry has shown that their amidinate group can also be involved in the bonding with the metal atoms as tridentate ENP and tetradentate PENP' coordination modes have been observed for the ECl(bzamP)2 ligand of [Ir{κ3E,N,P-ECl(bzamP)2}(cod)] (E = Ge (3a) and Sn (3b); cod = η4-1,5-cyclooctadiene) and the E(bzamP)2 ligand of [Ni{κ4E,N,P,P'-E(bzamP)2}] (E = Ge (4a) and Sn (4b)), which are products of reactions of 2a and 2b with [IrCl(cod)]2 (1:0.5 mol ratio) and [Ni(cod)2] (1:1 mol ratio), respectively. These products contain a 5-membered NCNEM ring that results from the insertion of the metal M atom into an E-N bond of 2a and 2b. Additionally, while iridium(I) complexes 3a and 3b are chloridotetryl derivatives (insertion of the tetrylene E atom into the Ir-Cl bond has also occurred) that have an uncoordinated phosphane group, nickel(0) complexes 4a and 4b contain a tetrylene fragment that, maintaining the lone pair, behaves as a σ-acceptor (Z-type) ligand.

Dipyrromethane-Based PGeP Pincer Germyl Rhodium Complexes

A family of germyl rhodium complexes derived from the PGeP germylene 2,2'-bis(di-isopropylphosphanylmethyl)-5,5'-dimethyldipyrromethane-1,1'-diylgermanium(II), Ge(pyrmPi Pr2 )2 CMe2 (1), has been prepared. Germylene 1 reacted readily with [RhCl(PPh3 )3 ] and [RhCl(cod)(PPh3 )] (cod=1,5-cyclooctadiene) to give, in both cases, the PGeP-pincer chloridogermyl rhodium(I) derivative [Rh{κ3 P,Ge,P-GeCl(pyrmPi Pr2 )2 CMe2 }(PPh3 )] (2). Similarly, the reaction of 1 with [RhCl(cod)(MeCN)] afforded [Rh{κ3 P,Ge,P-GeCl(pyrmPi Pr2 )2 CMe2 }(MeCN)] (3). The methoxidogermyl and methylgermyl rhodium(I) complexes [Rh{κ3 P,Ge,P-GeR(pyrmPi Pr2 )2 CMe2 }(PPh3 )] (R=OMe, 4; Me, 5) were prepared by treating complex 2 with LiOMe and LiMe, respectively. Complex 5 readily reacted with CO to give the carbonyl rhodium(I) derivative [Rh{κ3 P,Ge,P-GeR(pyrmPi Pr2 )2 CMe2 }(CO)] (6), with HCl, HSnPh3 and Ph2 S2 rendering the pentacoordinate methylgermyl rhodium(III) complexes [RhHX{κ3 P,Ge,P-GeMe(pyrmPi Pr2 )2 CMe2 }] (X=Cl, 7; SnPh3 , 8) and [Rh(SPh)2 {κ3 P,Ge,P-GeMe(pyrmPi Pr2 )2 CMe2 }] (9), respectively, and with H2 to give the hexacoordinate derivative [RhH2 {κ3 P,Ge,P-GeMe(pyrmPi Pr2 )2 CMe2 }(PPh3 )] (10). Complexes 3 and 5 are catalyst precursors for the hydroboration of styrene, 4-vinyltoluene and 4-vinylfluorobenzene with catecholborane under mild conditions.

Dehydrogenative Double C-H Bond Activation in a Germylene-Rhodium Complex*

Transition metal tetrylene complexes offer great opportunities for molecular cooperation due to the ambiphilic character of the group 14 element. Here we focus on the coordination of germylene [(ArMes2 )2 Ge :] (ArMes =C6 H3 -2,6-(C6 H2 -2,4,6-Me3 )2 ) to [RhCl(COD)]2 (COD=1,5-cyclooctadiene), which yields a neutral germyl complex in which the rhodium center exhibits both η6 - and η2 -coordination to two mesityl rings in an unusual pincer-type structure. Chloride abstraction from this species triggers a singular dehydrogenative double C-H bond activation across the Ge/Rh motif. We have isolated and fully characterized three rhodium-germyl species associated to three C-H cleavage events along this process. The reaction mechanism has been further investigated by computational means, supporting the key cooperative action of rhodium and germanium centers.

Stannylenes based on pyrrole-phosphane and dipyrromethane-diphosphane scaffolds: syntheses and behavior as precursors to PSnP pincer palladium(II), palladium(0) and gold(I) complexes

2-Ditertbutylphosphanylmethylpyrrole (H₂pyrmP^tBu₂) and 2,2'-bis(diisopropylphosphanylmethyl)-5,5'-dimethyldipyrromethane ((HpyrmPⁱPr₂)₂CMe₂) have been used to synthesize new P-donor-stabilized stannylenes in which the Sn atom is attached to one, SnCl(HpyrmP^tBu₂) (1) and Sn{N(SiMe₃)₂}(HpyrmP^tBu₂) (2), or two pyrrolyl-phosphane scaffolds, Sn(HpyrmP^tBu₂)₂ (3), or to a dipyrromethane-diphosphane scaffold, Sn(pyrmPⁱPr₂)₂CMe₂ (4). It has been found that stannylenes 3 and 4 are excellent precursors to transition metal complexes containing PSnP pincer-type ligands. Their reactions with chlorido transition metal complexes have afforded [PdCl{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (6), [PdCl{κ³P,Sn,P-SnCl(pyrmPⁱPr₂)₂CMe₂}] (7) and [Au{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (8), which contain a PSnP pincer-type chloridostannyl ligand. While complexes 6 and 7 are square-planar palladium(II) complexes, compound 8 is an uncommon gold(I) complex having a T-shaped coordination geometry with a very long Sn-Au bond (3.120 Å). The T-shaped palladium(0) complex [Pd{κ³P,Sn,P-Sn(pyrmPⁱPr₂)₂CMe₂}] (9), which contains an unprecedented PSnP pincer-type stannylene that behaves as a Z-type (σ-acceptor) ligand, has been prepared from 4 and [Pd(η³-C₃H₅)(η⁵-C₅H₅)].

Alternative Conceptual Approach to the Design of Bifunctional Catalysts: An Osmium Germylene System for the Dehydrogenation of Formic Acid

The reaction of the hexahydride OsH₆(PⁱPr₃)₂ with a P,Ge,P-germylene-diphosphine affords an osmium tetrahydride derivative bearing a Ge,P-chelate, which arises from the hydrogenolysis of a P–C(sp³) bond. This Os(IV)–Ge(II) compound is a pioneering example of a bifunctional catalyst based on the coordination of a σ-donor acid, which is active in the dehydrogenation of formic acid to H₂ and CO₂. The kinetics of the dehydrogenation, the characterization of the resting state of the catalysis, and DFT calculations point out that the hydrogen formation (the fast stage) exclusively occurs on the coordination sphere of the basic metal center, whereas both the metal center and the σ-donor Lewis acid cooperatively participate in the CO₂ release (the rate-determining step). During the process, the formate group pivots around the germanium to approach its hydrogen atom to the osmium center, which allows its transfer to the metal and the CO₂ release.

An alternative class of bifunctional catalysts can be assembled by coordination of σ-donor Lewis acids to platinum-group-metal basic fragments. In contrast to what happens with the previously reported bifunctional catalysts, this design allows enhancing the basicity of the base and the acidity of the acid. According to this, a bifunctional catalyst for the dehydrogenation of formic acid, based on an osmium(IV)-germylene cooperative system, has been prepared and the mechanism of the catalysis established.

Transition Metal Chain Complexes Supported by Soft Donor Assembling Ligands

The chemistry of discrete molecular chains constituted by metals in low oxidation states, displaying metal-metal proximity and stabilized by suitable metal-bridging, assembling ligands comprising at least one soft donor atom is comprehensively reviewed; complexes with a single (hard or soft) bridging atom (e.g., μ-halide, μ-sulfide, or μ-PR₂ etc.) as well as "closed" metal arrays (that fall in the realm of cluster chemistry) are excluded. The focus is on transition metal-based systems, with few excursions to cases combining transition and post-transition elements. Most relevant supporting ligands have neutral C, P, O, or S donor (mainly, N-heterocyclic carbene, phosphine, ether, thioether) or anionic donor (mainly phenyl, ylide, silyl, phosphide, thiolate) groups. A supporting-ligand-based classification of the metal chains is introduced, using as the classifying parameter the number of "bites" (i.e., ligand bridges) subtending each intermetallic separation. The ligands are further grouped according to the number of donor atoms interacting with the metal chain (called denticity in the following) and the column of the Periodic Table to which the set of donor atoms belongs (in ascending order). A complementary metal-based compilation of the complexes discussed is also provided in a concise tabular form.

Reversible metathesis of ammonia in an acyclic germylene-Ni0 complex

Carbenes, a class of low-valent group 14 ligand, have shifted the paradigm in our understanding of the effects of supporting ligands in transition-metal reactivity and catalysis. We now seek to move towards utilizing the heavier group 14 elements in effective ligand systems, which can potentially surpass carbon in their ability to operate via 'non-innocent' bond activation processes. Herein we describe our initial results towards the development of scalable acyclic chelating germylene ligands (viz. 1a/b), and their utilization in the stabilization of Ni0 complexes (viz. 4a/b), which can readily and reversibly undergo metathesis with ammonia with no net change of oxidation state at the GeII and Ni0 centres, through ammonia bonding at the germylene ligand as opposed to the Ni0 centre. The DFT-derived metathesis mechanism, which surprisingly demonstrates the need for three molecules of ammonia to achieve N-H bond activation, supports reversible ammonia binding at GeII, as well as the observed reversibility in the overall reaction.

Germylene-β-sulfoxide Hemilabile Ligand in Coordination Chemistry

We describe herein the synthesis of a germylene-β-sulfoxide ligand, 1, and its abilities in coordination chemistry. Treatment of 1 with metal complexes [W(cod)(CO)₄], [Mo(nbd)(CO)₄] and [Ni(cod)₂] afforded the corresponding (1)-chelated metal complexes (1)-W(CO)₄ (2a), (1)-Mo(CO)₄ (2b), and (1)-Ni(cod) (4a), clearly showing a bidentate ligation of the metal by the germanium(II) and sulfur centers. Coordination with [Ru(PPh₃)₃Cl₂] afforded an unprecedented bridged bis(ruthenium) complex 3b. In the case of 4a, the hemilability of the bidentate ligand 1 was demonstrated by sulfoxide substitution by a CO ligand.

Phosphane-functionalized heavier tetrylenes: synthesis of silylene- and germylene-decorated phosphanes and their reactions with Group 10 metal complexes

The stable phosphane-functionalized heavier tetrylenes E(tBu2bzam)pyrmPtBu2 (E = Si (1Si), Ge (1Ge); tBu2bzam = N,N'-ditertbutylbenzamidinate; HpyrmPtBu2 = ditertbutyl(2-pyrrolylmethyl)phosphane) have been prepared by reacting the amidinatotetrylenes E(tBu2bzam)Cl (E = Si, Ge) with LipyrmPtBu2. The reactions of 1Si and 1Ge with selected M0 and MII (M = Ni, Pd, Pt) metal precursors have allowed the synthesis of square-planar [MCl2{κ2E,P-E(tBu2bzam)pyrmPtBu2}] (M = Ni, Pd, Pt; E = Si, Ge), tetrahedral [Ni{κ2E,P-E(tBu2bzam)pyrmPtBu2}(cod)] (E = Si, Ge; cod = 1,5-cyclooctadiene) and triangular [M{κ2E,P-E(tBu2bzam)pyrmPtBu2}(PPh3)] (M = Pd, Pt; E = Si, Ge) complexes, showing that 1Si and 1Ge are excellent Si,P- and Ge,P-chelating ligands that, due to their large steric bulk, are able to stabilize three-coordinate Pd0 and Pt0 complexes.

A Z-type PGeP pincer germylene ligand in a T-shaped palladium(0) complex

An unusual T-shaped palladium(0) complex having a σ-acceptor (Z-type) germylene scaffold has been prepared from a dipyrromethane-based PGeP germylene.

Germylene stabilized group 12 metal complexes and their reactivity with chalcogens

This manuscript reports the first examples of germylene stabilized cadmium complexes 3, 6, and 7, and novel germylene zinc complexes 2 and 5.

A dipyrromethane-based diphosphane–germylene as precursor to tetrahedral copper(i) and T-shaped silver(i) and gold(i) PGeP pincer complexes

A dipyrromethane-based PGeP germylene has allowed the synthesis of unusual tetrahedral copper(i) and T-shaped silver(i) and gold(i) germyl complexes.

Germylone-bridged bimetallic Ir and Rh complexes

The reactions of germylone 1 with one equivalent of [M(COD)Cl]₂ (M = Ir, Rh) afforded the germylone-bridged bimetallic complexes 2 and 3, which have been spectroscopically and structurally characterized.

Germylene-sulfoxide as a potential hemilabile ligand: application in coordination chemistry

We describe here the synthesis of heteroleptic organogermylenes containing a sulfoxide donor function for their application in coordination chemistry. While complexation reaction with [W(cod)(CO)4] and [Mo(nbd)(CO)4] afforded bis(germanium)(ii) transition-metal complexes, a bidentate complex coordinated by germanium(ii) and the oxygen atom of the sulfinyl group was obtained from [Ru(PPh3)3Cl2].