Synthesis and Structural Characterization of Copper Complexes Containing "R-Substituted" Bis-7-Azaindolyl Borate Ligands.

The coordination chemistry of scorpionate ligands based on borates containing the 7-azaindole heterocycle is relatively unexplored. Thus, there is a requirement to further understand their coordination chemistry. This article outlines the synthesis and characterization of a family of complexes containing anionic flexible scorpionate ligands of the type [(R)(bis-7-azaindolyl)borohydride]- ([RBai]-), where R = Me, Ph or naphthyl. The three ligands were coordinated to a series of copper(I) complexes containing a phosphine co-ligand to form the complexes, [Cu(MeBai)(PPh3)] (1), [Cu(PhBai)(PPh3)] (2), [Cu(NaphthBai)(PPh3)] (3), [Cu(MeBai)(PCy3)] (4), [Cu(PhBai)(PCy3)] (5) and [Cu(NaphthBai)(PCy3)] (6). Additional copper(II) complexes, namely, [Cu(MeBai)2] (7) and [Cu(PhBai)2] (8), were obtained during attempts to obtain single crystals from complexes 4 and 2, respectively. Complexes 7 and 8 were also prepared independently from CuCl2 and two equivalents of the corresponding Li[RBai] salt alongside an additional complex, namely, [Cu(NaphthBai)2] (9). The copper(I) and copper(II) complexes were characterized using spectroscopic and analytical methods. Furthermore, a crystal structure was obtained for eight of the nine complexes. In all cases, the boron-based ligand was found to bind to the metal centers via a κ3-N,N,H coordination mode.

Hydroxypyridine/Pyridone Interconversions within Ruthenium Complexes and Their Application in the Catalytic Hydrogenation of CO2.

Reaction of a new ligand 6-DiPPon (6-diisopropylphosphino-2-pyridone) with 0.5 equiv of [RuCl2(p-cymene)]2 resulted in the formation of a mixture of [RuCl2(p-cymene)(κ1-P-6-DiPPon)]2 (1) and [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]Cl ([2]Cl) (where 6-DiPPin = 6-diisopropylphosphino-2-hydroxypyridine). The ratio between the two products can be controlled by the nature of the solvent. The similar reaction between 6-DiPPon and [RuCl2(p-cymene)]2 in the presence of AgOTf and Na[BArF24] (where BArF24 = [{3,5-(CF3)2C6H3}4B]-) resulted in the formation of the complexes [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]OTf, ([2]OTf) and [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]BArF24 ([2]BArF24), respectively. Reactions between complex [2]Cl, [2]OTf, or [2]BArF24 and a base (either DBU or NaOMe) resulted in the deprotonation of the hydroxyl functional group to form a novel neutral orange-colored dearomatized complex, 3. The identity of complex 3 was confirmed as [RuCl(p-cymene)(κ2-P,N-6-DiPPon*)], where 6-DiPPon* is the anionic species (6-diisopropylphosphino-2-oxo-pyridinide), which contains the deprotonated moiety. The new 6-DiPPon ligand and its corresponding air stable half-sandwich derivative ruthenium complexes 1, [2]OTf, [2]BArF24, and 3 were all isolated in good yields and fully characterized by spectroscopic and analytical methods. The interconversions between the neutral and anionic forms of the ligands 6-DiPPon, 6-DiPPin, and 6-DiPPon* offer the potential for novel secondary sphere interactions and proton shuttling reactivity. The consequences for this have been explored in the activation of H2 and the subsequent catalytic hydrogenations of CO2 into formate salts in the presence of a base.

Recent developments on the transformation of CO2 utilising ligand cooperation and related strategies.

A portfolio of value-added chemicals, fuels and building block compounds can be envisioned from CO2 on an industrial scale. The high kinetic and thermodynamic stabilities of CO2, however, present a significant barrier to its utilisation as a C1 source. In this context, metal-ligand cooperation methodologies have emerged as one of the most dominant strategies for the transformation of the CO2 molecule over the last decade or so. This review focuses on the advancements in CO2 transformation using these cooperative methodologies. Different and well-studied ligand cooperation methodologies, such as dearomatisation-aromatisation type cooperation, bimetallic cooperation (M⋯M'; M' = main group or transition metal) and other related strategies are also discussed. Furthermore, the cooperative bond activations are subdivided based on the number of atoms connecting the reactive centre in the ligand framework (spacer/linker length) and the transition metal. Several similarities across these seemingly distinct cooperative methodologies are emphasised. Finally, this review brings out the challenges ahead in developing catalytic systems from these CO2 transformations.

Stopping Hydrogen Migration in Its Tracks: The First Successful Synthesis of Group Ten Scorpionate Complexes Based on Azaindole Scaffolds.

The first successful synthesis and characterization of group 10 complexes featuring flexible scorpionate ligands based on 7-azaindole heterocycles are reported herein. Addition of 2 equiv of either K[HB(azaindolyl)3] or Li[HB(Me)(azaindolyl)2] to [M(µ-Cl)(η,1η2-COEOMe)]2 leads to the formation of 2 equiv of the complexes [M{κ3- N,N,H-HB(azaindolyl)3}(η,1η2-COEOMe)] and [M{κ3- N,N,H-HB(Me)(azaindolyl)2}(η,1η2-COEOMe)] (where M = Pt, Pd; COEOMe = 8-methoxycyclooct-4-en-1-ide), respectively. In these reactions, the borohydride group is directed toward the metal center forming square based pyramidal complexes. In contrast to analogous complexes featuring other flexible scorpionate ligands, no hydrogen migration from boron is observed in the complexes studied. The fortuitous line widths observed in some of the 11B NMR spectra allow for a closer inspection of the B-H···metal unit in scorpionate complexes than has previously been possible.

Preparation and reactivity of rhodium and iridium complexes containing a methylborohydride based unit supported by two 7-azaindolyl heterocycles.

The synthesis and characterisation of a new anionic flexible scorpionate ligand, methyl(bis-7-azaindolyl)borohydride [MeBai]- is reported herein. The ligand was coordinated to a series of group nine transition metal centres forming the complexes, [Ir(MeBai)(COD)] (1), [Rh(MeBai)(COD)] (2), [Rh(MeBai)(CODMe)] (2-Me) and [Rh(MeBai)(NBD)] (3), where COD = 1,5-cyclooctadiene, CODMe = 3-methyl-1,5-cyclooctadiene and NBD = 2,5-norbornadiene. In all cases, the boron based ligand was found to bind to the metal centres via a κ3-N,N,H coordination mode. The ligand and complexes were fully characterised by spectroscopic and analytical methods. The structures of the ligand and three of the complexes were confirmed by X-ray crystallography. The potential for migration of the "hydride" or "methyl" units from boron to the metal centre was also explored. During these studies an unusual transformation, involving the oxidation of the rhodium centre, was observed in complex 2. In this case, the η4-COD unit transformed into a η1,η3-C8H12 unit where the ring was bound via one sigma bond and one allyl unit. This is the first time such a transformation has been observed at a rhodium centre.

Sequential Migrations between Boron and Rhodium Centers: A Cooperative Process between Rhodium and a Monosubstituted Borohydride Unit.

The sodium salt of a monosubstituted borohydride anion containing a 2-mercaptopyridyl unit (mp) is reported herein. This compound was coordinated to a rhodium(I) center providing the complex [Rh{κ3-H,H,S-H3B(mp)}(NBD)] (1) (where NBD = 2,5-norbornadiene) in which the boron-based ligand is coordinated to the rhodium center via the thione donor and two of the B-H bonds of the BH3 unit. Reaction of complex 1 with carbon monoxide results in the activation of the complex leading to the product of a formal intramolecular hydroboration reaction, where the NBD unit has, in effect, inserted into one of the B-H bonds. Three complexes were prepared in which the newly formed norbornenyl unit (nbe) is located at the boron center, namely, [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)2] (2), [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)(PCy3)] (3), and [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)(PPh3)] (4). The identities of the three complexes were confirmed by spectroscopic and analytical techniques. Further confirmation was obtained via structural characterization of 3. Studies confirmed that the reactivity occurs at the metal center. A metal-ligand cooperative mechanism, involving initial migration of hydride from boron to metal center, was postulated for the formation of the new complexes based on previous investigations. The newly formed norbornenyl unit then migrates from metal center to boron.

Functional group migrations between boron and metal centres within transition metal-borane and -boryl complexes and cleavage of H-H, E-H and E-E' bonds.

This feature article examines some of the recent advances in the chemistry of Z-type transition metal-borane and X-type transition metal-boryl complexes. It focuses on the employment of these boron-based functionalities acting as stores and transfer agents for functional groups such as hydrides, alkyl groups and aryl groups which can either be abstracted or delivered to the metal centre. The review also explores the rather novel reactivity involving the cleavage of H-H, E-H and E-E' bonds (where E and E' are a range of groups) across the transition metal-boron bond in such complexes. It explores the early examples of the addition of H-H across transition metal-borane bonds and describes the new transformation in the context of other known modes of hydrogen activation including classic oxidative addition and heterolytic cleavage at transition metal centres as well as Frustrated Lewis Pair chemistry. Similar reactivity involving transition metal-boryl complexes are also described particularly those which undergo both boryl-to-borane and borane-to-borohydride transformations. The delivery of hydride to the metal centre in combination with the potential to regenerate the borohydride functional group via a recharging process is explored in the context of providing a new strategy for catalysis. Finally, a light-hearted look at the analogy of the 'stinging processes' involving Trofimenko type ligands is taken one step further to determine whether it is indeed in the nature of scorpionate ligands to repeatedly 'sting' just as the real life scorpions do.

Copper and silver complexes bearing flexible hybrid scorpionate ligand mpBm.

The addition of flexible scorpionate ligand, [mpBm]⁻{i.e. HB(mt)2(mp), where mt = methyl-2-mercaptoimidazole and mp = 2-mercaptopyridine} to group eleven centres is reported for the first time. The coordination of this hybrid ligand to copper(I) and silver(I) centres in the presence of triphenylphosphine and trialkylphosphine co-ligands has been investigated. The trialkylphosphines coordinates to both copper and silver centres while the less basic triarylphosphine only successfully coordinates to the copper centre. Structural characterisation of [Cu{HB(mt)2(mp)}(PPh3)], [Cu{HB(mt)2(mp)}(PCy3)] and [Ag{HB(mt)2(mp)}(PCy3)] confirm κ³-SSH coordination modes for ligand where one of the mt 'arms' and the mp 'arm' of the scorpionate ligand are coordinated to the metal centre. The second mt 'arm' remains uncoordinated in all three complexes. A comparison has been made with the parent sulfur based scorpionate ligand, [Tm]⁻{HB(mt)3}.

Hydrogen atom storage upon Z-class borane ligand functions: an alternative approach to ligand cooperation.

Ligand cooperation has become an important strategy in the development of new transition metal based transformations. By using this approach some remarkable new catalytic transformations have been achieved, all in the space of only a few years. The purpose of this tutorial review is to explore the potential utilisation of ligands containing borohydride and borane functionalities as reversible hydrogen atom stores. At the heart of this review will be a discussion on hydrogen transfer reactions and the transformation between borohydride and borane moieties. An outline of the various synthetic routes to metal-borane (metallaboratrane) complexes will be provided together with a discussion of their further reactivity including key transformations such as 1,2 additions across the metal-boron bond and 'recharging' the borane functional group back to borohydride. Finally, an evaluation of the potential future applications of such reactivity will be provided.

Synthesis and characterisation of group nine transition metal complexes containing new mesityl and naphthyl based azaindole scorpionate ligands.

Two novel boron-based flexible scorpionate ligands based on 7-azaindole, Li[HB(azaindolyl)(2)(1-naphthyl)] and Li[HB(azaindolyl)(2)(mesityl)] {Li[(Naphth)Bai] and Li[(Mes)Bai] respectively}, have been prepared (mesityl = 2,4,6-trimethylphenyl). These salts have been isolated in two forms, either as dimeric structures which contain bridging hydride interactions with the lithium centres or as crystalline material containing mono nuclear bis-acetonitrile solvates. The newly formed ligands have been utilised to prepare a range of group nine transition metal complexes with the general formula [M(COD){κ(3)-NNH-HB (azaindolyl)(2)(Ar)}] (where M = rhodium, iridium; Ar = 1-naphthyl, mesityl; COD = 1,5-cyclooctadiene) and [Rh(NBD){κ(3)-NNH-HB (azaindolyl)(2)(Ar)}] (where NBD = 2,5-norbornadiene; Ar = 1-naphthyl, mesityl). These new complexes have been compared to the previously reported compounds which contain the related scorpionate ligands Li[HB(azaindolyl)(2)(phenyl)] and K[HB(azaindolyl)(3)] {Li[(Ph)Bai] and K[Tai] respectively}. Structural characterisation of the complexes [Rh(COD){κ(3)-NNH-HB (azaindolyl)(2)(mesityl)}], [Ir(COD){κ(3)-NNH-HB (azaindolyl)(2)(mesityl)}] and [Rh(NBD){κ(3)-NNH-HB (azaindolyl)(2)(naphthyl)}] confirm the expected κ(3)-NNH coordination mode for these new ligands. Spectroscopic analysis suggests strong interactions of the B-H functional group with the metal centres in all cases.

Double addition of H2 to transition metal-borane complexes: a 'hydride shuttle' process between boron and transition metal centres.

The addition of H(2) across a transition metal-borane bond is reported for the first time providing a mechanism for recharging borane functional groups to borohydride.

Strong agostic-type interactions in ruthenium benzylidene complexes containing 7-azaindole based scorpionate ligands.

The complexes [Ru(Tai)Cl{=C(H)Ph}(PCy(3))] (4) and [Ru((Ph)Bai)Cl{=C(H)Ph}(PCy(3))] (5) [where Tai = HB(7-azaindolyl)(3) and (Ph)Bai = Ph(H)B(7-azaindolyl)(2)] have been prepared and structurally characterised. The borohydride unit is located in the coordination site trans to the chloride ligand in both complexes. The degree of interaction between the borohydride group and the metal centre was found to be significantly large in both cases. Thermolysis reactions involving complex 4 led to a dehydrogenation reaction forming [Ru(Tai)Cl{PCy(2)(η(2)-C(6)H(9))}] (6) where the benzylidene group acts as a hydrogen acceptor.

Reversible dioxygen binding in solvent-free liquid myoglobin.

The ensemble of forces that stabilize protein structure and facilitate biological function are intimately linked with the ubiquitous aqueous environment of living systems. As a consequence, biomolecular activity is highly sensitive to the interplay of solvent-protein interactions, and deviation from the native conditions, for example by exposure to increased thermal energy or severe dehydration, results in denaturation and subsequent loss of function. Although certain enzymes can be extracted into non-aqueous solvents without significant loss of activity, there are no known examples of solvent-less (molten) liquids of functional metalloproteins. Here we describe the synthesis and properties of room-temperature solvent-free myoglobin liquids with near-native structure and reversible dioxygen binding ability equivalent to the haem protein under physiological conditions. The realization of room-temperature solvent-free myoglobin liquids with retained function presents novel challenges to existing theories on the role of solvent molecules in structural biology, and should offer new opportunities in protein-based nanoscience and bionanotechnology.

Rhodium and iridium complexes containing diphenyl-2-(3-methyl)indolylphosphine: synthesis, structure and application in the catalytic transfer hydrogenation of ketones.

The synthesis and characterisation of a number of group nine complexes containing the recently reported ligand, diphenyl-2-(3-methyl)indolylphosphine, is presented herein. The complexes [RhCl(COD){PPh(2)(C(9)H(8)N)}] (1), [IrCl(COD){PPh(2)(C(9)H(8)N)}] (2), [RhCl(NBD){PPh(2)(C(9)H(8)N)}] (3) and [Rh(COD)(MeCN){PPh(2)(C(9)H(8)N)}]BF(4) (4) (where COD = 1,5-cyclooctadiene, NBD = 2,5- norbornadiene) have been structurally characterised by X-ray crystallography. The complex [Rh(2)(COD)(2){N(Me)[double bond, length as m-dash]C(H)Ph)}{PPh(2)(C(9)H(8)N)}][BF(4)](2) (8) was also isolated and structurally characterised. Complex 8 contains a '[Rh(COD)]' fragment coordinated to the aromatic ring of the indolyl group, providing the first example of a eta(6) coordination mode for this ligand. The synthesised complexes were investigated for their activity in the catalytic transfer hydrogenation of ketones and found to be moderately active catalysts.

Towards multistranded molecular wires: syntheses, structures, and reactivities of tetraplatinum bis(polyynediyl) complexes with [upper bond 1 start]Pt-C(x)-Pt-(P(CH2)3P)2-Pt-C(x)-Pt-(P(CH2)3P[upper bond 1 end])2 cores (x = 4, 6, 8).

Reactions of trans,trans-(C(6)F(5))(p-tol(3)P)(2)Pt(C[triple bond]C)(n)Pt(Pp-tol(3))(2)(C(6)F(5)) (PtC(x)Pt; x = 2n) and the 1,3-diphosphine Ph(2)P(CH(2))(3)PPh(2) (2.5 equiv) give the tetraplatinum complexes trans, trans,trans,trans-(C(6)F(5))[upper bond 1 start]Pt(C[triple bond]C)(n)Pt(C(6)F(5))(PPh(2)(CH(2))(3)Ph(2)P)(2)(C(6)F(5))Pt(C[triple bond]C)(n)Pt(C(6)F(5))(PPh(2)(CH(2))(3)Ph(2)P[upper bond 1 end])(2) ([Pt'C(x)Pt'](2); x = 4/6/8, 39%/95%/81%). Crystal structures of [Pt'C(8)Pt'](2) and two solvates of [Pt'C(6)Pt'](2) are determined. These confirm that each diphosphine spans two platinum atoms from different Pt(C[triple bond]C)(n)Pt linkages, as opposed to (1) the 1,2-diphosphine Ph(2)P(CH(2))(2)PPh(2), which under similar conditions with PtC(8)Pt affords the diplatinum bis(chelate) cis,cis-([upper bond 1 start]PPh(2)(CH(2))(2)Ph(2)P)(C(6)F(5))Pt[upper bond 1 end](C[triple bond]C)(4)[upper bond 1 start]Pt(C(6)F(5))(PPh(2)(CH(2))(2)Ph(2)P[upper bond 1 end]) (73%) or (2) alpha,omega-diphosphines with longer methylene chains, which span the platinum termini. The formulation [Pt'C(4)Pt'](2) is supported by a reaction with PEt(3) (10 equiv) to give trans,trans-(C(6)F(5))(Et(3)P)(2)Pt(C[triple bond]C)(2)Pt(PEt(3))(2)(C(6)F(5)). In [Pt'C(8)Pt'](2) and one solvate of [Pt'C(6)Pt'](2), the chains cross at 77.2 degrees-87.7 degrees angles, with the closest interchain carbon-carbon distances (3.27-3.61 A) less than the sum of the van-der-Waals radii. In the other solvate of [Pt'C(6)Pt'](2), the chains are essentially parallel, and the separation is much greater (4.96 A). UV-visible spectra show no special electronic interactions. However, cyclic voltammograms indicate irreversible oxidations, in contrast to the partially reversible oxidations of PtC(6)Pt and PtC(8)Pt. The initially formed radical cations are proposed to undergo rapid chain-chain coupling. The new complexes decompose without melting above 185 degrees C. With [Pt'C(8)Pt'](2), IR spectra indicate the formation of a new C[triple bond]C-rich substance.

Unexpected pincer-type coordination (kappa3-SBS) within a zerovalent platinum metallaboratrane complex.

The first structurally characterised zerovalent platinum complex to contain a tridentate pincer-type coordination mode (kappa(3)-SBS) is presented, raising further questions concerning the geometries and trans influence of Z-type ligands.

A new hybrid scorpionate ligand: a study of the metal-boron bond within metallaboratrane complexes.

A new boron-based hybrid scorpionate ligand based upon one 2-mercaptopyridine (mp) and two 1-methyl-imidazoyl-2-thione (mt) units, Na[HB(mt)(2)(mp)] has been prepared. This new ligand together with the recently reported ligand K[HB(mp)(3)] have been used to prepare analogues of the original metallaboratrane complex [Ru(CO)(PPh(3)){kappa(4)-SSBS-B(mt)(3)}]. The effect of tautomerisation of the ligand arms upon the electronic properties of the boron and metal centres is examined in an attempt to probe further the nature of the metal-boron bond within metallaboratrane complexes.

A new family of metallaboratrane complexes based on 7-azaindole: B-H activation mediated by carbon monoxide.

The reaction of Ir(COD)(Tai) [where Tai = {HB(7-azaindoyl)(3)}(-)] with carbon monoxide results, via a sequence of hydride migration and insertion steps, in the formation of the first complexes to contain a metal-to-boron dative interaction supported by 7-azaindole units.

A 'sting' on Grubbs' catalyst: an insight into hydride migration between boron and a transition metal.

An unusual ruthenium(ii) complex frozen at an intermediate point of hydride transfer between boron and ruthenium centres is reported.

A new family of flexible scorpionate ligands based on 2-mercaptopyridine.

A new family of flexible scorpionate ligands based on 2-mercaptopyridine is reported. The tris- and bis-substituted ligands, K[HB(mp)(3)] (1) and Na[H(2)B(mp)(2)] (2) (mp = 2-mercaptopyridine) have been prepared and fully characterised. The structural characterisation of 1 reveals an unprecedented mu(3)-kappa(3)-SS'H-eta(1)eta(1)eta(2)-kappa(2)-S''C-eta(1)eta(1)-kappa(1)-S'-eta(1) coordination mode. The coordination of both 1 and 2 to copper(I) complexes containing triphenylphosphine and tricyclohexylphosphine co-ligands is investigated suggesting kappa(3)-SSS coordination modes for [Cu{HB(mp)(3)}(PR(3))] {where R = Ph (3); R = Cy, (4)} and kappa(3)-SSH coordination modes for [Cu{H(2)B(mp)(2)}(PR(3))] {where R = Ph (5); R = Cy (6)} the latter confirmed by structural characterisation of 5. A structural comparison with the sulfur based scorpionates, HB(mt)(3) and H(2)B(mt)(2) (mt = methyl-2-mercaptoimidazole) is made in terms of the degree of tautomerisation of the heterocyclic rings.