Carbon-chalcogen bond formation initiated by [Al(NONDipp)(E)]- anions containing Al-E{16} (E{16} = S, Se) multiple bonds.

Multiply-bonded main group metal compounds are of interest as a new class of reactive species able to activate and functionalize a wide range of substrates. The aluminium sulfido compound K[Al(NONDipp)(S)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3), completing the series of [Al(NONDipp)(E)]- anions containing Al-E{16} multiple bonds (E{16} = O, S, Se, Te), was accessed via desulfurisation of K[Al(NONDipp)(S4)] using triphenylphosphane. The crystal structure showed a tetrameric aggregate joined by multiple K⋯S and K⋯π(arene) interactions that were disrupted by the addition of 2.2.2-cryptand to form the separated ion pair, [K(2.2.2-crypt)][Al(NONDipp)(S)]. Analysis of the anion using density functional theory (DFT) confirmed multiple-bond character in the Al-S group. The reaction of the sulfido and selenido anions K[Al(NONDipp)(E)] (E = S, Se) with CO2 afforded K[Al(NONDipp)(κ2 E,O-EC{O}O)] containing the thio- and seleno-carbonate groups respectively, consistent with a [2 + 2]-cycloaddition reaction and C-E bond formation. An analogous cycloaddition reaction took place with benzophenone affording compounds containing the diphenylsulfido- and diphenylselenido-methanolate ligands, [κ2 E,O-EC{O}Ph2]2-. In contrast, when K[Al(NONDipp)(E)] (E = S, Se) was reacted with benzaldehyde, two equivalents of substrate were incorporated into the product accompanied by formation of a second C-E bond and complete cleavage of the Al-E{16} bonds. The products contained the hitherto unknown κ2 O,O-thio- and κ2 O,O-seleno-bis(phenylmethanolate) ligands, which were exclusively isolated as the cis-stereoisomers. The mechanisms of these cycloaddition reactions were investigated using DFT methods.

Bonding and Reactivity of a Pair of Neutral and Cationic Heterobimetallic RuZn2 Complexes.

A combined experimental and computational study of the structure and reactivity of two [RuZn2Me2] complexes, neutral [Ru(PPh3)(Ph2PC6H4)2(ZnMe)2] (2) and cationic [Ru(PPh3)2(Ph2PC6H4)(ZnMe)2][BArF4] ([BArF4] = [B{3,5-(CF3)2C6H3}4]) (3), is presented. Structural and computational analyses indicate these complexes are best formulated as containing discrete ZnMe ligands in which direct Ru-Zn bonding is complemented by weaker Zn···Zn interactions. The latter are stronger in 2, and both complexes exhibit an additional Zn···Caryl interaction with a cyclometalated phosphine ligand, this being stronger in 3. Both 2 and 3 show diverse reactivity under thermolysis and with Lewis bases (PnBu3, PCy3, and IMes). With 3, all three Lewis bases result in the loss of [ZnMe]+. In contrast, 2 undergoes PPh3 substitution with PnBu3, but with IMes, loss of ZnMe2 occurs to form [Ru(PPh3)(C6H4PPh2)(C6H4PPhC6H4Zn(IMes))H] (7). The reaction of 3 with H2 affords the cationic trihydride complex [Ru(PPh3)2(ZnMe)2(H)3][BArF4] (12). Computational analyses indicate that both 12 and 7 feature bridging hydrides that are biased toward Ru over Zn.

Seven-Membered Cyclic Potassium Diamidoalumanyls.

The seven-membered cyclic potassium alumanyl species, [{SiNMes }AlK]2 [{SiNMes }={CH2 SiMe2 N(Mes)}2 ; Mes=2,4,6-Me3 C6 H2 ], which adopts a dimeric structure supported by flanking K-aryl interactions, has been isolated either by direct reduction of the iodide precursor, [{SiNMes }AlI], or in a stepwise manner via the intermediate dialumane, [{SiNMes }Al]2 . Although the intermediate dialumane has not been observed by reduction of a Dipp-substituted analogue (Dipp=2,6-i-Pr2 C6 H3 ), partial oxidation of the potassium alumanyl species, [{SiNDipp }AlK]2 , where {SiNDipp }={CH2 SiMe2 N(Dipp)}2 , provided the extremely encumbered dialumane [{SiNDipp }Al]2 . [{SiNDipp }AlK]2 reacts with toluene by reductive activation of a methyl C(sp3 )-H bond to provide the benzyl hydridoaluminate, [{SiNDipp }AlH(CH2 Ph)]K, and as a nucleophile with BPh3 and RN=C=NR (R=i-Pr, Cy) to yield the respective Al-B- and Al-C-bonded potassium aluminaborate and alumina-amidinate products. The dimeric structure of [{SiNDipp }AlK]2 can be disrupted by partial or complete sequestration of potassium. Equimolar reactions with 18-crown-6 result in the corresponding monomeric potassium alumanyl, [{SiNDipp }Al-K(18-cr-6)], which provides a rare example of a direct Al-K contact. In contrast, complete encapsulation of the potassium cation of [{SiNDipp }AlK]2 , either by an excess of 18-cr-6 or 2,2,2-cryptand, allows the respective isolation of bright orange charge-separated species comprising the 'free' [{SiNDipp }Al]- alumanyl anion. Density functional theory (DFT) calculations performed on this moiety indicate HOMO-LUMO energy gaps in the of order 200-250 kJ mol-1 .

[Ni(NHC)2 ] as a Scaffold for Structurally Characterized trans [H-Ni-PR2 ] and trans [R2 P-Ni-PR2 ] Complexes.

The addition of PPh2 H, PPhMeH, PPhH2 , P(para-Tol)H2 , PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2 ] (NHC=IiPr2 , IMe4 , IEt2 Me2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H-Ni-PR2 ] or novel trans [R2 P-Ni-PR2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4 >IEt2 Me2 >IiPr2 ) and phosphines are employed. P-P activation of the diphosphines R2 P-PR2 (R2 =Ph2 , PhMe) provides an alternative route to some of the [Ni(NHC)2 (PR2 )2 ] complexes. DFT calculations capture these trends with P-H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni-P bond. P-P bond activation from [Ni(NHC)2 (Ph2 P-PPh2 )] adducts proceeds with computed barriers below 10 kcal mol-1 . The ability of the [Ni(NHC)2 ] moiety to afford isolable terminal phosphido products reflects the stability of the Ni-NHC bond that prevents ligand dissociation and onward reaction.

Ambiphilic Al-Cu Bonding.

Copper-alumanyl complexes, [LCu-Al(SiNDipp )], where L=carbene=NHCiPr (N,N'-diisopropyl-4,5-dimethyl-2-ylidene) and Me2 CAAC (1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) and featuring unsupported Al-Cu bonds, have been prepared. Divergent reactivity observed with carbodiimides and CO2 implies an ambiphilicity in the Cu-Al interaction that is dependent on the identity of the carbene co-ligand.

Double insertion of CO2 into an Al-Te multiple bond.

We report the [Al(NONDipp)(Te)(THF)]- anion containing a terminal aluminium telluride bond. DFT calculations confirm appreciable Al-Te multiple bond character and reaction with CO2 proceeds via a double insertion to afford the previously unknown tellurodicarbonate ligand.

DFT calculations bring insight to internal alkyne-to-vinylidene transformations at rhodium PNP- and PONOP-pincer complexes.

Density Functional Theory (DFT) has been used to investigate the alkyne-to-vinylidene isomerisation reaction mediated by [Rh(PXNXP)]+ complexes (X = CH2: 2,6-bis(di-tert-butylphosphinomethyl)pyridine (PNP) and X = O: 2,6-bis(di-tert-butylphosphinito)pyridine (PONOP)) for terminal alkynes HC[triple bond, length as m-dash]CR, where R = t Bu and Ar' (3,5- t Bu2C6H3). Calculations suggest the reaction mechanism proceeds via the slippage of π-bound alkyne at the Rh centre into a Rh-alkyne σC-H complex followed by an indirect 1,2-H shift to give the Rh-vinylidene species. NBO (Natural Bond Orbital) analysis of the transition states corresponding to the latter indirect 1,2-H shift step indicates that the migrating hydrogen atom exhibits protic character and hence, the basicity of the H-accepting centre (Cß) is controlled by the substituents at that same atom and can tune the 1,2-H shift transition state. QTAIM (Quantum Theory of Atoms in Molecule) and NBO analyses of the Rh-vinylidene complexes indicate that these species exhibit a Rh ← C dative bond as well as π-back bonding from the Rh centre into the empty p z orbital of the carbene centre (Cα), showing the Rh-vinylidene complexes are Fischer type carbenes. Analysis of the alkyne and vinylidene complex HOMOs show that the equilibrium between the isomers can be tuned by the P-Rh-P bite angle of the [Rh(pincer)]+ fragment. Dictated by the nature of the pincer backbone, wider bite angles shift the equilibrium toward the formation of the Rh-vinylidene isomer (e.g., X = CH2 and R = Ar'), while tighter bite angles shift the equilibrium more to the formation of the Rh-alkyne isomer (e.g., X = O and R = Ar').

Phosphinoborane interception at magnesium by borane-assisted phosphine-borane dehydrogenation.

Reactions of B(C6F5)3 with the ß-diketiminato (BDI) alkaline-earth phosphidoborane complexes, 1a [(BDI)Ca(H3B·PPh2)] and 1b [(BDI)Mg(H3B·PPh2)]2 (BDI = [HC{C(CH3)N(2,6-iPr-C6H3)}2]-) result in the formation of phosphinodiboronate complexes 4a [(BDI)Ca(η6-toluene){H3B·PPh2·B(C6F5)3}] and 4b [(BDI)Mg{H3B·PPh2·B(C6F5)3}]. Calcium complex 4a is stable in aromatic solvents at room temperature and does not display well-defined onward reactivity at elevated temperatures. Magnesium complex 4b undergoes a room temperature transformation to provide the known hydridoborate derivative 3b [(BDI)Mg{HB(C6F5)3}] and an N,P,N'-ligated species, 5 [{HC(C(CH3)N(2,6-iPr-C6H3))2(H2BPPh2)}Mg{H3B·PPh2·B(C6F5)3}] that results from interception of the putative phosphinoborane, H2B = PPh2, by the BDI ligand backbone following B(C6F5)3-mediated hydride abstraction. NMR spectroscopic investigations were supported by DFT calculations, which suggested a mechanism involving B(C6F5)3 migration and hydride abstraction within the coordination sphere of magnesium. Interception of H2B = PPh2 by B(C6F5)3 is proposed to stabilise this species, whilst activating it towards ligand-centred nucleophilic attack. The significant stabilisation energy calculated for the Ca-π(toluene) interaction in 4a accounts for the contrasting outcomes between the two Ae-elements. The crystal structures of compounds 4a and 5 are presented and discussed.

Nucleophilic Magnesium Silanide and Silaamidinate Derivatives.

Density functional theory (DFT) calculations demonstrate that the previously reported reaction of [(BDI)Mg-n-Bu] (BDI = HC{(Me)CN-Dipp}2; Dipp = 2,6-diisopropylphenyl) with the silaborane Me2PhSi-Bpin provides the magnesium silanide derivative [(BDI)MgSiMe2Ph], through the intermediacy of a short-lived silyl-pinacolato-organoborate species. The nucleophilic character of the resultant silanide anion is assayed through a series of reactions with RN═C═NR (R = i-Pr, Cy, t-Bu) and p-tolN═C═N-p-tol. When they are performed in a strict 1:1 stoichiometry, all four reactions result in silyl addition to the carbodiimide carbon center and formation of the corresponding ß-diketiminato magnesium silaamidinate complexes. Although the performance of the reaction of [(BDI)MgSiMe2Ph] with 2 equiv of p-tolylcarbodiimide also results in the formation of a silaamidinate anion, the second equivalent is observed to engage with the nucleophilic γ-methine carbon of the BDI ligand to provide a tripodal diimino-iminoamidate ligand. This behavior is judged to be a consequence of the enhanced electrophilicity of the N-aryl-substituted carbodiimide reagent, a viewpoint supported by a further reaction with the N-isopropyl silaamidinate complex [(BDI)Mg(i-PrN)2CSiMe2Ph]. This latter reaction not only provides an identical diimino-iminoamidate ligand but also results in 2-fold insertion of p-tolN═C═N-p-tol into a Mg-N bond between the magnesium center and the silaamidinate anion.

Synthesis and reactivity of alkaline-earth stannanide complexes by hydride-mediated distannane metathesis and organostannane dehydrogenation.

The synthesis of heteroleptic complexes with calcium- and magnesium-tin bonds is described. The dimeric ß-diketiminato calcium hydride complex, [(BDI)Ca(µ-H)]2 (ICa) reacts with Ph3Sn-SnPh3 to provide the previously reported µ2-H bridged calcium stannanide dimer, [(BDI)2Ca2(SnPh3)(µ-H)] (3). Computational assessment of this reaction supports a mechanism involving a hypervalent stannate intermediate formed by nucleophilic attack of hydride on the distannane. Monomeric calcium stannanides, [(BDI)Ca(SnPh3)·OPPh3] (8·OPPh3) and [(BDI)Ca(SnPh3)·TMTHF] (8·TMTHF, TMTHF = 2,2,5,5-tetramethyltetrahydrofuran) were obtained from ICa and Ph3Sn-SnPh3, after addition OPPh3 or TMTHF. Both complexes were also synthesised by deprotonation of Ph3SnH by ICa in the presence of the Lewis base. The calcium and magnesium THF adducts, [(BDI)Ca(SnPh3)·THF2] (8·THF2) and [(BDI)Mg(SnPh3)·THF] (9·THF), were similarly prepared from [(BDI)Ca(µ-H)·(THF)]2 (ICa·THF2) or [(BDI)Mg(µ-H)]2 (IMg) and Ph3SnH. An excess of THF or TMTHF was essential in order to obtain 8·TMTHF, 8·THF2 and 9·THF in high yields whilst avoiding redistribution of the phenyl-tin ligand. The resulting Ae-Sn complexes were used as a source of [Ph3Sn]- in salt metathesis, to provide the known tristannane Ph3Sn-Sn(t-Bu)2-SnPh3 (11). Nucleophilic addition or insertion with N,N'-di-iso-propylcarbodiimide provided the stannyl-amidinate complexes, [(BDI)Mg{(iPrN)2CSnPh3}] (12) and [(BDI)Ca{(iPrN)2CSnPh3}·L] (13·TMTHF, 13·THF, L = TMTHF, THF). The reactions and products were monitored and characterised by multinuclear NMR spectroscopy, whilst for compounds 8, 9, 12, and 13·THF, the X-ray crystal structures are presented and discussed.

Carbon-Carbon Bond Forming Reactions Promoted by Aluminyl and Alumoxane Anions: Introducing the Ethenetetraolate Ligand.

[K{Al(NONDipp )}]2 (NONDipp =[O(SiMe2 NDipp)2 ]2- , Dipp=2,6-iPr2 C6 H3 ) reacts with CS2 to afford the trithiocarbonate species [K(OEt2 )][Al(NONDipp )(CS3 )] 1 or the ethenetetrathiolate complex, [K{Al(NONDipp )(S2 C)}]2 [3]2 . The dimeric alumoxane [K{Al(NONDipp )(O)}]2 reacts with carbon monoxide to afford the oxygen analogue of 3, [K{Al(NONDipp )(O2 C)}]2 [4]2 containing the hitherto unknown ethenetetraolate ligand, [C2 O4 ]4- .

A Stable Calcium Alumanyl.

A seven-membered N,N'-heterocyclic potassium alumanyl nucleophile is introduced and utilised in the metathetical synthesis of Mg-Al and Ca-Al bonded derivatives. Both species have been characterised by experimental and theoretical means, allowing a rationalisation of the greater reactivity of the heavier group 2 species implied by an initial assay of their reactivity.

Zn-Promoted C-H Reductive Elimination and H2 Activation via a Dual Unsaturated Heterobimetallic Ru-Zn Intermediate.

Reaction of [Ru(PPh3)3HCl] with LiCH2TMS, MgMe2, and ZnMe2 proceeds with chloride abstraction and alkane elimination to form the bis-cyclometalated derivatives [Ru(PPh3)(C6H4PPh2)2H][M'] where [M'] = [Li(THF)2]+ (1), [MgMe(THF)2]+ (3), and [ZnMe]+ (4), respectively. In the presence of 12-crown-4, the reaction with LiCH2TMS yields [Ru(PPh3)(C6H4PPh2)2H][Li(12-crown-4)2] (2). These four complexes demonstrate increasing interaction between M' and the hydride ligand in the [Ru(PPh3)(C6H4PPh2)2H]- anion following the trend 2 (no interaction) < 1 < 3 < 4 both in the solid-state and solution. Zn species 4 is present as three isomers in solution including square-pyramidal [Ru(PPh3)2(C6H4PPh2)(ZnMe)] (5), that is formed via C-H reductive elimination and features unsaturated Ru and Zn centers and an axial Z-type [ZnMe]+ ligand. A [ZnMe]+ adduct of 5, [Ru(PPh3)2(C6H4PPh2)(ZnMe)2][BArF4] (6) can be trapped and structurally characterized. 4 reacts with H2 at -40 °C to form [Ru(PPh3)3(H)3(ZnMe)], 8-Zn, and contrasts the analogous reactions of 1, 2, and 3 that all require heating to 60 °C. This marked difference in reactivity reflects the ability of Zn to promote a rate-limiting C-H reductive elimination step, and calculations attribute this to a significant stabilization of 5 via Ru → Zn donation. 4 therefore acts as a latent source of 5 and this operational "dual unsaturation" highlights the ability of Zn to promote reductive elimination in these heterobimetallic systems. Calculations also highlight the ability of the heterobimetallic systems to stabilize developing protic character of the transferring hydrogen in the rate-limiting C-H reductive elimination transition states.

Reductive dehydrocoupling of diphenyltin dihydride with LiAlH4: selective synthesis and structures of the first bicyclo[2.2.1]heptastannane-1,4-diide and bicyclo[2.2.2]octastannane-1,4-diide.

The reaction of diphenyltin dihydride with LiAlH4 gives access to a set of charged tin cages as their lithium salts. Variation in the ratio of reactants provides a perstannabicyclooctane dianion and a perstannanorbornane as the di- and monoanions. These compounds can be synthesised selectively by careful stoichiometric control and have been characterised by single crystal X-ray diffractometry, NMR and UV-vis spectroscopy. Computational exploration of the electronic structures of these compounds was undertaken and, in agreement with structural and spectroscopic features, indicated significant σ-delocalisation in the tin skeletons.

A Stable Calcium Alumanyl.

A seven-membered N,N'-heterocyclic potassium alumanyl nucleophile is introduced and utilised in the metathetical synthesis of Mg-Al and Ca-Al bonded derivatives. Both species have been characterised by experimental and theoretical means, allowing a rationalisation of the greater reactivity of the heavier group 2 species implied by an initial assay of their reactivity.

Calcium stannyl formation by organostannane dehydrogenation.

Reaction of the dimeric calcium hydride, [(BDI)CaH]2 (1), with Ph3SnH ensues with elimination of H2 to provide [(BDI)Ca-µ2-H-(SnPh3)Ca(BDI)] (3) and [(BDI)Ca(SnPh3)]2 (4) alongside dismutation to Ph4Sn, H2 and Sn(0). DFT analysis indicates that stannyl anion formation occurs through deprotonation of Ph3SnH and with retention of dinuclear species throughout the reactions.

Diborane heterolysis and P(v) reduction by Ph3P[double bond, length as m-dash]O coordination to magnesium.

Reaction of a magnesium diboranate complex with triphenylphosphine oxide provides a terminal magnesium boryl, which is itself a potent reagent for the deoxygenative reduction of Ph3PO. Computational analysis with density functional theory (DFT) indicates that B-B bond activation results from initial coordination of the P[double bond, length as m-dash]O bond of the phosphine oxide to magnesium.

A computational study on the identity of the active catalyst structure for Ru(ii) carboxylate assisted C-H activation in acetonitrile.

Density Functional Theory (DFT) calculations using a consistent methodology accounting for solvation, dispersion and thermal effects have been used to study C-H activation of the simple directing group substrate 2-phenylpyridine (a-H). The computational model uses acetate (-OAc) and benzene to represent the carboxylate and arene co-ligands coordinated at a Ru organometallic complex. A variety of different mechanisms ranging from cationic to neutral, ion-paired, arene free, two substrates bound, and solvent (MeCN) coordinated have been explored. Computed results indicate that the cationic pathways from "B+", [(C6H6)Ru(OAc)(a-H)]+, and "D+ (η6)", [(η6-a-H)Ru(OAc)(a-H)]+, involve the lowest overall barriers to C-H activation. Consideration of solvent coordination leads to a complex variety of isomers and conformers. Here a neutral pathway with either one or two acetonitriles coordinated to the Ru centre give very low barriers to C-H activation.

Well-Defined Heterobimetallic Reactivity at Unsupported Ruthenium-Indium Bonds.

The hydride complex [Ru(IPr)2 (CO)H][BArF4 ], 1, reacts with InMe3 with loss of CH4 to form [Ru(IPr)2 (CO)(InMe)(Me)][BArF4 ], 4, featuring an unsupported Ru-In bond with unsaturated Ru and In centres. 4 reacts with H2 to give [Ru(IPr)2 (CO)(η2 -H2 )(InMe)(H)][BArF4 ], 5, while CO induces formation of the indyl complex [Ru(IPr)2 (CO)3 (InMe2 )][BArF4 ], 7. These observations highlight the ability of Me to shuttle between Ru and In centres and are supported by DFT calculations on the mechanism of formation of 4 and its reactions with H2 and CO. An analysis of Ru-In bonding in these species is also presented. Reaction of 1 with GaMe3 also involves CH4 loss but, in contrast to its In congener, sees IPr transfer from Ru to Ga to give a gallyl complex featuring an η6 interaction of one aryl substituent with Ru.

Experimental and Computational Studies of the Copper Borate Complexes [(NHC)Cu(HBEt3 )] and [(NHC)Cu(HB(C6 F5 )3 )].

The synthesis of the Cu-borate complexes [(6Mes)Cu(HBR3 )] featuring the unusual [HBEt3 ]- (5) and [HB(C6 F5 )3 ]- (6) ligands is described. Experimental and computational studies show both compounds feature a direct Cu-H interaction, but that while 5 is two-coordinate, 6 displays an additional, stabilizing Cu-Cipso (C6 F5 ) interaction.