A Reduced-Symmetry Pd2L4 Cage from a Heterotopic Dipyridyl Ligand.

Two dipyridyl ligands, L3,3 and L3,4, have been used in combination with palladium(II) in the construction of metallosupramolecular species that show anion-dependent behavior in solution. A rare example of a low-symmetry (C2h) lantern-type cage is formed in one instance, [Pd2(L3,3)4]4+, while the isomeric ligand yields a larger double-walled square complex, [Pd4(L3,4)8]8+. [Pd2(L3,3)4](NO3)4 was isolated in crystalline form revealing two anions within the interior of the C2h-symmetry cage. The cage itself is held together by hydrogen bonding between "head-to-tail" pairs of ligands that reinforces the symmetry generated by the ditopic ligands. In solution, the cage with NO3- has sharp 1H nuclear magnetic resonance (NMR) signals at room temperature, while the BF4- analogue has broad signals that sharpen at higher temperatures or upon addition of (Bu4N)(NO3), highlighting the importance of the anion in templating or otherwise influencing self-assembly in solution. Altering the substitution position of one of the pyridyl rings yields a more "open" complex, with [Pd4(L3,4)8](NO3)8 being isolated as a crystalline solid. The double-walled square complex has a greater Pd···Pd separation due to the increased angle that the pyridyl groups subtend at the core of the ligand. NMR spectroscopy and mass spectrometry studies suggest a single species in the presence of nitrate but multiple species with tetrafluoroborate.

Carbodiphosphorane-Stabilized Parent Dioxophosphorane: A Valuable Synthetic HO2P Source.

Introducing a small phosphorus-based fragment into other molecular entities via, for example, phosphorylation/phosphonylation is an important process in synthetic chemistry. One of the approaches to achieve this is by trapping and subsequently releasing extremely reactive phosphorus-based molecules such as dioxophosphoranes. In this work, electron-rich hexaphenylcarbodiphosphorane (CDP) was used to stabilize the least thermodynamically favorable isomer of HO2P to yield monomeric CDP·PHO2. The title compound was observed to be a quite versatile phosphonylating agent; that is, it showed a great ability to transfer, for the first time, the HPO2 fragment to a number of substrates such as alcohols, amines, carboxylic acids, and water. Several phosphorous-based compounds that were generated using this synthetic approach were also isolated and characterized for the first time. According to the initial computational studies, the addition-elimination pathway was significantly more favorable than the corresponding elimination-addition route for "delivering" the HO2P unit in these reactions.

Quantitative High-Field NMR- and Mass Spectrometry-Based Fatty Acid Sequencing Reveals Internal Structure in Ru-Catalyzed Deuteration of Docosahexaenoic Acid.

Ru-based catalysis results in highly unsaturated fatty acid (HUFA) ethyl esters (EE) deuterated to various extents. The products carry 2H (D) mainly at their bis-allylic positions, where they are resistant to autoxidation compared to natural HUFA and are promising as neurological and retinal drugs. We characterized the extent of deuteration at each allylic position of docosa-4,7,10,13,16,19-hexaenoic acid deuterated to completion at bis-allylic and allylic positions (D-DHA) by two-dimensional (2D) and high-field (600 and 950 MHz) NMR. In separate experiments, the kinetics of docosahexaenoic acid (DHA) EE deuteration was evaluated using Paternò-Büchi (PB) reaction tandem mass spectrometry (MS/MS) analysis, enabling deuteration to be quantitatively characterized for isotopologues (D0-D14 DHA) at each internal allylic position. NMR analysis shows that the net deuteration of the isotopologue mixture is about 94% at the bis-allylic positions, and less than 1% remained as the protiated -CH2-. MS analysis shows that deuteration kinetics follow an increasing curve at bis-allylic positions with higher rate for internal bis-allylic positions. Percent D of bis-allylic positions increases linearly from D1 to D9 in which all internal bis-allylic positions (C9, C12, C15) deuterate uniformly and more rapidly than external bis-allylic positions (C6, C18). The mono-allylic positions near the methyl end (C21) show a steep increase of D only after the D10 isotopologue has been deuterated to >90%, while the mono-allylic position near the carboxyl position, C3, deuterates last and least. These data establish detailed methods for the characterization of Ru-catalyzed deuteration of HUFA as well as the phenomenological reaction kinetics as net product is formed.

A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane.

Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H-C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H-C interactions with the geminal C-H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]- anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.

Mechanistic Studies of the Palladium-Catalyzed Desulfinative Cross-Coupling of Aryl Bromides and (Hetero)Aryl Sulfinate Salts.

Pyridine and related heterocyclic sulfinates have recently emerged as effective nucleophilic coupling partners in palladium-catalyzed cross-coupling reactions with (hetero)aryl halides. These sulfinate reagents are straightforward to prepare, stable to storage and coupling reaction conditions, and deliver efficient reactions, thus offering many advantages, compared to the corresponding boron-derived reagents. Despite the success of these reactions, there are only scant details of the reaction mechanism. In this study, we use structural and kinetic analysis to investigate the mechanism of these important coupling reactions in detail. We compare a pyridine-2-sulfinate with a carbocyclic sulfinate and establish different catalyst resting states, and turnover limiting steps, for the two classes of reagent. For the carbocyclic sulfinate, the aryl bromide oxidative addition complex is the resting state intermediate, and transmetalation is turnover-limiting. In contrast, for the pyridine sulfinate, a chelated Pd(II) sulfinate complex formed post-transmetalation is the resting-state intermediate, and loss of SO2 from this complex is turnover-limiting. We also investigated the role of the basic additive potassium carbonate, the use of which is crucial for efficient reactions, and deduced a dual function in which carbonate is responsible for the removal of free sulfur dioxide from the reaction medium, and the potassium cation plays a role in accelerating transmetalation. In addition, we show that sulfinate homocoupling is responsible for converting Pd(OAc)2 to a catalytically active Pd(0) complex. Together, these studies shed light on the challenges that must be overcome to deliver improved, lower temperature versions of these synthetically important processes.

Synthesis of Highly Fluorinated Arene Complexes of [Rh(Chelating Phosphine)]+ Cations, and their use in Synthesis and Catalysis.

The synthesis of rhodium complexes with weakly binding highly fluorinated benzene ligands is described: 1,2,3-F3 C6 H3 , 1,2,3,4-F4 C6 H2 and 1,2,3,4,5-F5 C6 H are shown to bind with cationic [Rh(Cy2 P(CH2 )x PCy2 )]+ fragments (x=1, 2). Their structures and reactivity with alkenes, and use in catalysis for promoting the Tishchenko reaction of a simple aldehyde, are demonstrated. Key to the synthesis of these complexes is the highly concentrated reaction conditions and use of the [Al{OC(CF3 )3 }4 ]- anion.

Identification of the Side Products That Diminish the Yields of the Monoamidated Product in Metal-Catalyzed C-H Amidation of 2-Phenylpyridine with Arylisocyanates.

The Ru(II)-catalyzed amidation of 2-arylpyridines with aryl isocyanates via C-H bond activation is less efficient than described previously, due to the formation of a series of side products, which were readily identified using direct infusion electrospray mass spectrometry and high-performance liquid chromatography-mass spectrometry.

Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes.

The non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C-H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh(Cy2PCH2CH2PCy2)(η:η-(H3C)CH(CH3)2][BArF4] and [Rh(Cy2PCH2CH2PCy2)(η:η-C6H12)][BArF4] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D2 occurs at all C-H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H2. These processes can be followed temporally, and modeled using classical chemical, or Johnson-Mehl-Avrami-Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [kH/kD = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C-H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C-H bond elongation in both rate-limiting transition states and suggest that the large kH/kD for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting ß-H transfer step.

Dehydropolymerization of H3B·NMeH2 To Form Polyaminoboranes Using [Rh(Xantphos-alkyl)] Catalysts.

A systematic study of the catalyst structure and overall charge for the dehydropolymerization of H3B·NMeH2 to form N-methyl polyaminoborane is reported using catalysts based upon neutral and cationic {Rh(Xantphos-R)} fragments in which PR2 groups are selected from Et, iPr, and tBu. The most efficient systems are based upon {Rh(Xantphos-iPr)}, i.e., [Rh(κ3-P,O,P-Xantphos-iPr)(H)2(η1-H3B·NMe3)][BArF4], 6, and Rh(κ3-P,O,P-Xantphos-iPr)H, 11. While H2 evolution kinetics show both are fast catalysts (ToF ≈ 1500 h-1) and polymer growth kinetics for dehydropolymerization suggest a classical chain growth process for both, neutral 11 (Mn = 28 000 g mol-1, D = 1.9) promotes significantly higher degrees of polymerization than cationic 6 (Mn = 9000 g mol-1, D = 2.9). For 6 isotopic labeling studies suggest a rate-determining NH activation, while speciation studies, coupled with DFT calculations, show the formation of a dimetalloborylene [{Rh(κ3-P,O,P-Xantphos-iPr)}2B]+ as the, likely dormant, end product of catalysis. A dual mechanism is proposed for dehydropolymerization in which neutral hydrides (formed by hydride transfer in cationic 6 to form a boronium coproduct) are the active catalysts for dehydrogenation to form aminoborane. Contemporaneous chain-growth polymer propagation is suggested to occur on a separate metal center via head-to-tail end chain B-N bond formation of the aminoborane monomer, templated by an aminoborohydride motif on the metal.

Modulation of σ-Alkane Interactions in [Rh(L2)(alkane)]+ Solid-State Molecular Organometallic (SMOM) Systems by Variation of the Chelating Phosphine and Alkane: Access to η2,η2-σ-Alkane Rh(I), η1-σ-Alkane Rh(III) Complexes, and Alkane Encapsulation.

Solid/gas single-crystal to single-crystal (SC-SC) hydrogenation of appropriate diene precursors forms the corresponding σ-alkane complexes [Rh(Cy2P(CH2) nPCy2)(L)][BArF4] ( n = 3, 4) and [ RhH(Cy2P(CH2)2( CH)(CH2)2PCy2)(L)][BArF4] ( n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as determined by single-crystal X-ray diffraction, have cations exhibiting Rh···H-C σ-interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion microenvironment. These range from chelating η2,η2 Rh···H-C (e.g., [Rh(Cy2P(CH2) nPCy2)(η2η2-NBA)][BArF4], n = 3 and 4), through to more weakly bound η1 Rh···H-C in which C-H activation of the chelate backbone has also occurred (e.g., [ RhH(Cy2P(CH2)2( CH)(CH2)2PCy2)(η1-COA)][BArF4]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment, [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF4], in which the metal center instead forms two intramolecular agostic η1 Rh···H-C interactions with the phosphine cyclohexyl groups. CH2Cl2 adducts formed by displacement of the η1-alkanes in solution ( n = 5; L = NBA, COA), [ RhH(Cy2P(CH2)2( CH)(CH2)2PCy2)(κ1-ClCH2Cl)][BArF4], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO, and NCI calculations, alongside variable temperature solid-state NMR spectroscopy, provide snapshots marking the onset of Rh···alkane interactions along a C-H activation trajectory. These are negligible in [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF4]; in [ RhH(Cy2P(CH2)2( CH)(CH2)2PCy2)(η1-COA)][BArF4], σC-H → Rh σ-donation is supported by Rh → σ*C-H "pregostic" donation, and in [Rh(Cy2P(CH2) nPCy2)(η2η2-NBA)][BArF4] ( n = 2-4), σ-donation dominates, supported by classical Rh(dπ) → σ*C-H π-back-donation. Dispersive interactions with the [BArF4]- anions and Cy substituents further stabilize the alkanes within the binding pocket.

Encapsulation of Crabtree's Catalyst in Sulfonated MIL-101(Cr): Enhancement of Stability and Selectivity between Competing Reaction Pathways by the MOF Chemical Microenvironment.

Crabtree's catalyst was encapsulated inside the pores of the sulfonated MIL-101(Cr) metal-organic framework (MOF) by cation exchange. This hybrid catalyst is active for the heterogeneous hydrogenation of non-functionalized alkenes either in solution or in the gas phase. Moreover, encapsulation inside a well-defined hydrophilic microenvironment enhances catalyst stability and selectivity to hydrogenation over isomerization for substrates bearing ligating functionalities. Accordingly, the encapsulated catalyst significantly outperforms its homogeneous counterpart in the hydrogenation of olefinic alcohols in terms of overall conversion and selectivity, with the chemical microenvironment of the MOF host favouring one out of two competing reaction pathways.

Exploiting Carbonyl Groups to Control Intermolecular Rhodium-Catalyzed Alkene and Alkyne Hydroacylation.

Readily available ß-carbonyl-substituted aldehydes are shown to be exceptional substrates for Rh-catalyzed intermolecular alkene and alkyne hydroacylation reactions. By using cationic rhodium catalysts incorporating bisphosphine ligands, efficient and selective reactions are achieved for ß-amido, ß-ester, and ß-keto aldehyde substrates, providing a range of synthetically useful 1,3-dicarbonyl products in excellent yields. A correspondingly broad selection of alkenes and alkynes can be employed. For alkyne substrates, the use of a catalyst incorporating the Ampaphos ligand triggers a regioselectivity switch, allowing both linear and branched isomers to be prepared with high selectivity in an efficient manner. Structural data, confirming aldehyde chelation, and a proposed mechanism are provided.

Observation of Cationic Transition Metal-Alkane Complexes with Moderate Stability in Hydrofluorocarbon Solution.

In seeking to create more-stable transition metal-alkane complexes, we generated cationic alkane complexes of the type [(HEB)Re(CO)2(alkane)][Al(OR(f))4] (HEB = η(6)-hexaethylbenzene; alkane = cyclopentane (16) or pentane (17-19); OR(f) = perfluoro-tert-butoxy) via photolysis of the precursor complex [(HEB)Re(CO)3][Al(OR(f))4] (15) in the presence of the added alkane. The alkane complexes were generated in a hydrofluorocarbon (HFC) solvent, most often CF3CH2CF3, which is capable of simultaneously dissolving the ionic complex 15 and a small amount of alkane at low temperature (183 K). Use of the HFC solvent in tandem with the highly fluorinated, solubilizing, weakly coordinating [Al(OR(f))4](-) anion overcomes the technical difficulty of combining ionic species with alkanes in solution without the solvent molecules rapidly displacing the bound alkane ligand, as the alkanes bind in preference to the HFCs to the organometallic fragments employed in this study. The [(HEB)Re(CO)2(alkane)](+) complexes are more long-lived than the corresponding neutral alkane complexes [(HEB)W(CO)2(alkane)] and [CpRe(CO)2(alkane)] (Cp = η(5)-cyclopentadienyl), with samples of [CpRe(CO)2(cyclopentane)] decaying significantly more rapidly than [(HEB)Re(CO)2(alkane)](+) when present in the same solution. Intramolecular exchange of the methylene group bound to the metal within the cyclopentane ligand in 16 was observed at 212 K, with the 1,2 shifts appearing to be faster than 1,3 shifts.

Correction: Synthetic ramoplanin analogues are accessible by effective incorporation of arylglycines in solid-phase peptide synthesis.

[This corrects the article DOI: 10.1039/D3SC01944F.].

Lewis acid catalysed polymerisation of cyclopentenone.

A modest structural change of a ß-diketiminate-supported aluminium complex leads to dramatic differences in the reactivity towards cyclopentenone. While the bulkier complex efficiently executes Diels Alder transformations the smaller analogue performs unique polymerisation of this substrate. This observation appears to be unprecedented in the chemistry of Lewis acids and cyclic dienophiles as it represents a unique way to polymerise a functionalised olefin.

Synthetic ramoplanin analogues are accessible by effective incorporation of arylglycines in solid-phase peptide synthesis.

The threat of antimicrobial resistance to antibiotics requires a continual effort to develop alternative treatments. Arylglycines (or phenylglycines) are one of the signature amino acids found in many natural peptide antibiotics, but their propensity for epimerization in solid-phase peptide synthesis (SPPS) has prevented their use in long peptide sequences. We have now identified an optimized protocol that allows the synthesis of challenging non-ribosomal peptides including precursors of the glycopeptide antibiotics and an analogue of feglymycin (1 analogue, 20%). We have exploited this protocol to synthesize analogues of the peptide antibiotic ramoplanin using native chemical ligation/desulfurization (1 analogue, 6.5%) and head-to-tail macrocyclization in excellent yield (6 analogues, 3-9%), with these compounds extensively characterized by NMR (U-shaped structure) and antimicrobial activity assays (two clinical isolates). This method significantly reduces synthesis time (6-9 days) when compared with total syntheses (2-3 months) and enables drug discovery programs to include arylglycines in structure-activity relationship studies and drug development.

The preservation of sarin and O,O'-diisopropyl fluorophosphate inside coordination cage hosts.

The host-guest chemistry of O,O'-diisopropyl fluorophosphate (DFP), a phosphonofluoridate G-series chemical warfare agent simulant, was investigated in the presence of a number of octanuclear cubic coordination cage hosts. The aim was to demonstrate cage-catalysed hydrolysis of DFP at near neutral pH: however, two octanuclear coordination cages, HPEG (containing water-solubilising PEG groups) and HW (containing water-solubilising hydroxymethyl groups), were actually found to increase the lifetime of DFP in aqueous buffer solution (pH 8.7). Crystallographic analysis of DFP with a structurally related host cage revealed that DFP binds to windows in the cage surface, not in the internal cavity. The phosphorus-fluorine bond is directed into the cavity rather than towards the external environment, with the cage/DFP association protecting DFP from hydrolysis. Initial studies with the chemical warfare agent (CWA) sarin (GB) with HPEG cage in a buffered solution also showed a drastically reduced rate of hydrolysis for sarin when bound in the host cage. The ability of these cages to inhibit hydrolysis of these P-F bond containing organophosphorus guests, by encapsulation, may have applications in forensic sample preservation and analysis.

A photo-switchable molecular capsule: sequential photoinduced processes.

The metastable trilacunary heteropolyoxomolybdate [PMo9O31(py)3]3- - {PMo9}; py = pyridine) and the ditopic pyridyl bearing diarylethene (DAE) (C25H16N2F6S2) self-assemble via a facile ligand replacement methodology to yield the photo-active molecular capsule [(PMo9O31)2(DAE)3]6-. The spatial arrangement and conformation of the three DAE ligands are directed by the surface chemistry of the molecular metal oxide precursor with exclusive ligation of the photo-active antiparallel rotamer to the polyoxometalate (POM) while the integrity of the assembly in solution has been verified by a suite of spectroscopic techniques. Electrocyclisation of the three DAEs occurs sequentially and has been investigated using a combination of steady-state and time-resolved spectroscopies with the discovery of a photochemical cascade whereby rapid photoinduced ring closure is followed by electron transfer from the ring-closed DAE to the POM in the latent donor-acceptor system on subsequent excitation. This interpretation is also supported by computational and detailed spectroelectrochemical analysis. Ring-closing quantum yields were also determined using a custom quantum yield determination setup (QYDS), providing insight into the impact of POM coordination on these processes.

Using electrospray ionization-tandem mass spectrometry to explore formation and gas-phase chemistry of silver nanoclusters generated from the reaction of silver salts with NaBH4 in the presence of bis(diphenylarsino)methane.

Electrospray ionization-mass spectrometry (ESI-MS) of mixtures of AgBF4 or AgNO3 with the capping ligand bis(diphenylarsino)methane ((Ph2 As)2 CH2 = dpam) in a solution of acetonitrile revealed the formation of the following cations: [Ag(CH3 CN)(dpam)]+ , [Ag(dpam)2 ]+ , [Ag2 (Cl)(dpam)2 ]+ , and [Ag3 (Cl)2 (dpam)3 ]+ . Addition of NaBH4 to these solutions results in the formation of the cluster cations [Ag2 (BH4 )(dpam)2 ]+ , [Ag2 (BH4 )(dpam)3 ]+ , [Ag3 (H)(BH4 )(dpam)3 ]+ , [Ag3 (BH4 )2 (dpam)3 ]+ , [Ag3 (H)(Cl)(dpam)3 ]+ , and [Ag3 (I)(BH4 )(dpam)3 ]+ , as established by ESI-MS. Use of NaBD4 confirmed that borohydride is the source of the hydride in these clusters. An Orbitrap Fusion LUMOS mass spectrometer was used to explore the gas-phase unimolecular chemistry of selected clusters via multistage mass spectrometry (MSn ) experiments employing low-energy collision-induced dissociation (CID) and high-energy collision-induced dissociation (HCD) experiments. The borohydride containing clusters fragment via two competing pathways: (i) ligand loss and (ii) B-H bond activation involving BH3 loss. Density functional theory (DFT) calculations were used to calculate the energetics of the optimized structures for all precursor ions, fragment ions, and neutrals and to estimate the reaction endothermicities. Generally, there is reasonable agreement between the most abundant product ion formed and the predicted endothermicity of the associated reaction channel. The DFT calculations predicted that the novel dimer [Ag2 (BH4 )(dpam)2 ]+ has a paddlewheel structure in which the dpam and BH4 - ligands bridge both silver centers.

Bulky bis(aryl)triazenides: just aspiring amidinates? A structural and spectroscopic study.

The synthesis and molecular structures (by single crystal X-ray diffraction) of s-, p- and d-metal complexes of the sterically demanding N,N'-bis(2,6-diisopropylphenyl)triazenide are reported and the spectroscopic (NMR spectroscopy and infrared spectroscopy) and physical properties of these complexes compared to related formamidinate complexes. Through the use of infrared spectroscopy the σ-donor capacity of this ligand is demonstrated to be reduced relative to the structurally isomorphous formamidinate congener, which supports previously advanced theoretical calcluations and DFT results reported herein. These electronic differences are highlighted by the stark contrast in reaction outcomes at rhodium; where [(Dipp2N3)Rh(CO)2] (1) is an isolable, stable complex and the formamidinate complex is not. The coordination chemistry of the triazenide ligand for the s-block metal complexes (M = Li, Na, K) has been shown to give structurally isomorphous complexes to the formamidinate analogue. In contrast to the amidinate complexes, these complexes show extreme lability of coordinated, volatile Lewis-bases, which in turn-yields the highly insoluble base-free triazenide complexes. These complexes are also synthesized directly in the absence of donor solvents. This triazenide ligand has proven to be a suitable ligand for stabilising reactive main group hydrides of Group 13 (M = Ga, In) and attempts at the analogous thallium hydride complex by halide-hydride exchange are reported. Finally attempts at the synthesis of low valent main group complexes are reported ([MIL], M = In, Ga) are also reported, which yield instead disproportionation products ([MIIIXL2], M = Ga, In; [{MIIXL}2], M = Ga).