A mononuclear, terminal titanium(III) imido.

We report the first mononuclear TiIII complex possessing a terminal imido ligand. Complex [TptBu,MeTi{NSi(CH3)3}(THF)] (2) (TptBu,Me = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate) is prepared by reduction of [TptBu,MeTi{NSi(CH3)3}(Cl)] (1) with KC8 in high yield. The connectivity and metalloradical nature of 2 were confirmed by single crystal X-ray diffraction studies, Q- and X-band EPR, UV-Vis and 1H NMR spectroscopies. The d1 complex [(TptBu,Me)TiCl(OEt2)][B(C6F5)4] (3), was prepared to spectroscopically compare it to 2. Electrochemical studies of 1 and 2 reveal a reversible 1e- process, and chemical oxidants ClCPh3 or 1/2 eq. XeF2 react cleanly with 2 yielding 1 or the fluoride derivative [TptBu,MeTi{NSi(CH3)3}(F)] (4), respectively.

Tale of Three Molecular Nitrides: Mononuclear Vanadium (V) and (IV) Nitrides As Well As a Mixed-Valence Trivanadium Nitride Having a V3N4 Double-Diamond Core.

Transmetallation of [VCl3(THF)3] and [TlTptBu,Me] afforded [(TptBu,Me)VCl2] (1, TptBu,Me = hydro-tris(3-tert-butyl-5-methylpyrazol-1-yl)borate), which was reduced with KC8 to form a C3v symmetric VII complex, [(TptBu,Me)VCl] (2). Complex 1 has a high-spin (S = 1) ground state and displays rhombic high-frequency and -field electron paramagnetic resonance (HFEPR) spectra, while complex 2 has an S = 3/2 4A2 ground state observable by conventional EPR spectroscopy. Complex 1 reacts with NaN3 to form the VV nitride-azide complex [(TptBu,Me)V≡N(N3)] (3). A likely VIII azide intermediate en route to 3, [(TptBu,Me)VCl(N3)] (4), was isolated by reacting 1 with N3SiMe3. Complex 4 is thermally stable but reacts with NaN3 to form 3, implying a bis-azide intermediate, [(TptBu,Me)V(N3)2] (A), leading to 3. Reduction of 3 with KC8 furnishes a trinuclear and mixed-valent nitride, [{(TptBu,Me)V}2(µ4-VN4)] (5), conforming to a Robin-Day class I description. Complex 5 features a central vanadium ion supported only by bridging nitride ligands. Contrary to 1, complex 2 reacts with NaN3 to produce an azide-bridged dimer, [{(TptBu,Me)V}2(1,3-µ2-N3)2] (6), with two antiferromagnetically coupled high-spin VII ions. Complex 5 could be independently produced along with [(κ2-TptBu,Me)2V] upon photolysis of 6 in arene solvents. The putative {VIV≡N} intermediate, [(TptBu,Me)V≡N] (B), was intercepted by photolyzing 6 in a coordinating solvent, such as tetrahydrofuran (THF), yielding [(TptBu,Me)V≡N(THF)] (B-THF). In arene solvents, B-THF expels THF to afford 5 and [(κ2-TptBu,Me)2V]. A more stable adduct (B-OPPh3) was prepared by reacting B-THF with OPPh3. These adducts of B are the first neutral and mononuclear VIV nitride complexes to be isolated.

An Isolable Azide Adduct of Titanium(II) Follows Bifurcated Deazotation Pathways to an Imide.

AdN3 (Ad = 1-adamantyl) reacts with the tetrahedral TiII complex [(TptBu,Me)TiCl] (TptBu,Me = hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate) to generate a mixture of an imide complex, [(TptBu,Me)TiCl(NAd)] (4), and an unusual and kinetically stable azide adduct of the group 4 metal, namely, [(TptBu,Me)TiCl(γ-N3Ad)] (3). In these conversions, the product distribution is determined by the relative concentration of reactants. In contrast, the azide adduct 3 forms selectively when a masked TiII complex (N2 or AdNC adduct) reacts with AdN3. Upon heating, 3 extrudes dinitrogen in a unimolecular process proceeding through a titanatriazete intermediate to form the imide complex 4, but the observed thermal stability of the azide adduct (t1/2 = 61 days at 25 °C) is at odds with the large fraction of imide complex formed directly in reactions between AdN3 and [(TptBu,Me)TiCl] at room temperature (∼50% imide with a 1:1 stoichiometry). A combination of theoretical and experimental studies identified an additional deazotation pathway, proceeding through a bimetallic complex bridged by a single azide ligand. The electronic origin of this deazotation mechanism lies in the ability of azide adduct 3 to serve as a π-backbonding metallaligand toward free [(TptBu,Me)TiCl]. These findings unveil a new class of azide-to-imide conversions for transition metals, highlighting that the mechanisms underlying this common synthetic methodology may be more complex than conventionally assumed, given the concentration dependence in the conversion of an azide into an imide complex. Lastly, we show how significantly different AdN3 reacts when treated with [(TptBu,Me)VCl].

Transition Metal Carbide Complexes.

Carbide complexes remain a rare class of molecules. Their paucity does not reflect exceptional instability but is rather due to the generally narrow scope of synthetic procedures for constructing carbide complexes. The preparation of carbide complexes typically revolves around generating LnM-CEx fragments, followed by cleavage of the C-E bonds of the coordinated carbon-based ligands (the alternative being direct C atom transfer). Prime examples involve deoxygenation of carbonyl ligands and deprotonation of methyl ligands, but several other p-block fragments can be cleaved off to afford carbide ligands. This Review outlines synthetic strategies toward terminal carbide complexes, bridging carbide complexes, as well as carbide-carbonyl cluster complexes. It then surveys the reactivity of carbide complexes, covering stoichiometric reactions where the carbide ligands act as C1 reagents, engage in cross-coupling reactions, and enact Fischer-Tropsch-like chemistry; in addition, we discuss carbide complexes in the context of catalysis. Finally, we examine spectroscopic features of carbide complexes, which helps to establish the presence of the carbide functionality and address its electronic structure.

Phosphorus and Arsenic Atom Transfer to Isocyanides to Form π-Backbonding Cyanophosphide and Cyanoarsenide Titanium Complexes.

Decarbonylation along with E atom transfer from Na(OCE) (E=P, As) to an isocyanide coordinated to the tetrahedral TiII complex [(TptBu,Me )TiCl], yielded the [(TptBu,Me )Ti(η3 -ECNAd)] species (Ad=1-adamantyl, TptBu,Me- =hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate). In the case of E=P, the cyanophosphide ligand displays nucleophilic reactivity toward Al(CH3 )3 ; moreover, its bent geometry hints to a reduced Ad-NCP3- resonance contributor. The analogous and rarer mono-substituted cyanoarsenide ligand, Ad-NCAs3- , shows the same unprecedented coordination mode but with shortening of the N=C bond. As opposed to TiII , VII fails to promote P atom transfer to AdNC, yielding instead [(TptBu,Me )V(OCP)(CNAd)]. Theoretical studies revealed the rare ECNAd moieties to be stabilized by π-backbonding interactions with the former TiII ion, and their assembly to most likely involve a concerted E atom transfer between Ti-bound OCE- to AdNC ligands when studying the reaction coordinate for E=P.

A Mononuclear and High-Spin Tetrahedral TiII Complex.

A high-spin, mononuclear TiII complex, [(TptBu,Me)TiCl] [TptBu,Me- = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(TptBu,Me)TiCl2] with KC8. Complex [(TptBu,Me)TiCl] has a 3A2 ground state (assuming C3v symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined TiII complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(TptBu,Me)TiCl(THF)], which is impervious to N2 binding. However, in the absence of THF, the TiII complex captures N2 to produce the diamagnetic complex [(TptBu,Me)TiCl]2(η1,η1;µ2-N2), with a linear Ti═N═N═Ti topology, established by single-crystal X-ray diffraction. The N2 complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two TiIII centers covalently bridged by an N22- unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(TptBu,Me)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate TiII complex, namely, [(TptBu,Me)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated TiII-containing [(ΤptBu,Me)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear TiIV complexes of the type [(TptBu,Me)Ti═E(Cl)] (with E2- = NSiMe3, N2CPh2, O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N3SiMe3, N2CPh2, N2O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.

Electrophilic Activation of Osmium-Nitrido Corroles: The OsN Triple Bond as a π-Acceptor Metallaligand in a Heterobimetallic OsVIN-PtII Complex.

Presented herein is a first investigation of the chemical reactivity of osmium-nitrido corroles, which are known for their unusual thermal, chemical, and photochemical stability. Elemental chlorine perchlorinates the ß-positions of the triarylcorrole but leaves the OsN unit untouched. The OsN unit is also unaffected by a variety of other electrophilic and nucleophilic reagents. Upon photolysis, however, the anion of Zeise's salt associates with the nitrido ligand to generate an OsVI≡N-PtII complex. The very short OsN-Pt linkage [1.895(9)-1.917(8) Å] and the downfield 195Pt NMR resonance (-2702 ppm) suggest that the OsN corrole acts as a π-accepting ligand toward the Pt(II) center. This finding represents a rare example of the successful photochemical activation of a metal-ligand multiple bond that is too kinetically inert to exhibit any appreciable reactivity under thermal conditions.

Hard X-ray magnetochiral dichroism in a paramagnetic molecular 4f complex.

Magnetochiral dichroism (MΧD) originates in the coupling of local electric fields and magnetic moments in systems where a simultaneous break of space parity and time-reversal symmetries occurs. This magnetoelectric coupling, displayed by chiral magnetic materials, can be exploited to manipulate the magnetic moment of molecular materials at the single molecule level. We demonstrate herein the first experimental observation of X-ray magnetochiral dichroism in enantiopure chiral trigonal single crystals of a chiral mononuclear paramagnetic lanthanide coordination complex, namely, holmium oxydiacetate, at the Ho L3-edge. The observed magnetochiral effect is opposite for the two enantiomers and is rationalised on the basis of a multipolar expansion of the matter-radiation interaction. These results demonstrate that 4f-5d hybridization in chiral lanthanoid coordination complexes is at the origin of magnetochiral dichroism, an effect that could be exploited for addressing of their magnetic moment at the single molecule level.

Platinum(ii) as an assembly point for carbide and nitride ligands.

The sequential treatment of (Cy3P)2Cl2Ru[triple bond, length as m-dash]C with [PtCl2(C2H4)]2 and (dbm)2Cr[triple bond, length as m-dash]N affords trans-(Cy3P)2Cl2Ru[triple bond, length as m-dash]C-PtCl2-N[triple bond, length as m-dash]Cr(dbm)2, in which a platinum(ii) center is coordinated by both carbide and nitride ligands. ESR spectroscopy indicates that the carbide-nitride complex is loosely associated and that its unpaired electron remains localized mainly at the chromium center.

Molecular multifunctionality preservation upon surface deposition for a chiral single-molecule magnet.

The synthesis and characterization of a chiral, enneanuclear Mn(iii)-based, Single-Molecule Magnet, [Mn9O4(Me-sao)6(L)3(MeO)3(MeOH)3]Cl (1; Me-saoH2 = methylsalicylaldoxime, HL = lipoic acid) is reported. Compound 1 crystallizes in the orthorhombic P212121 space group and consists of a metallic skeleton describing a defect super-tetrahedron missing one vertex. The chirality of the [MnIII 9] core originates from the directional bridging of the Me-sao2- ligands via the -N-O- oximate moieties, which define a clockwise (1ΔΔ) or counter-clockwise (1ΛΛ) rotation in both the upper [MnIII 3] and lower [MnIII 6] subunits. Structural integrity and retention of chirality upon dissolution and upon deposition on (a) gold nanoparticles, 1@AuNPs, (b) transparent Au(111) surfaces, 1ΛΛ@t-Au(111); 1ΔΔ@t-Au(111), and (c) epitaxial Au(111) on mica surfaces, 1@e-Au(111), was confirmed by CD and IR spectroscopies, mass spectrometry, TEM, XPS, XAS, and AFM. Magnetic susceptibility and magnetization measurements demonstrate the simultaneous retention of SMM behaviour and optical activity, from the solid state, via dissolution, to the surface deposited species.

An Approach to Carbide-Centered Cluster Complexes.

We report the first examples of the carbide ligand in (Cy3P)2Cl2Ru≡C (RuC) developing into a µ3 ligand toward metal centers. Conventionally, sterics exclude this coordination mode, but Fe2(CO)9 and Co2(CO)8 expel bridging CO ligands upon reaction with RuC to form trimetallic (Cy3P)2Cl2Ru═CFe2(CO)8 (RuCFe2) and (Cy3P)2Cl2Ru═CCo2(CO)7 (RuCCo2) complexes. Thus, the proximity offered by metal-metal associations in bimetallic carbonyl complexes allows the formation of trinuclear carbide complexes as verified by NMR, Mössbauer, and X-ray spectroscopic techniques.

Confluence of disparate carbido chemistries: [WRuAu2(µ-C)2Cl2(CO)2(PCy3)2(Tp*)].

The reaction of [Ru(CAuCl)Cl2(PCy3)2] with [W([triple bond, length as m-dash]CSnnBu3)(CO)2(Tp*)] (obtained from [W([triple bond, length as m-dash]CBr)(CO)2(Tp*)], nBuLi and ClSnnBu3) affords the heterotetrametallic bis(carbido) complex [WRuAu2(µ-C)2Cl3(CO)2(PCy3)2(Tp*)] in which the two distinct µ-carbido ligands adopt linear and T-shaped geometry at carbon.

Formation of the layered conductive magnet CrCl2(pyrazine)2 through redox-active coordination chemistry.

The unique properties of graphene, transition-metal dichalcogenides and other two-dimensional (2D) materials have boosted interest in layered coordination solids. In particular, 2D materials that behave as both conductors and magnets could find applications in quantum magnetoelectronics and spintronics. Here, we report the synthesis of CrCl2(pyrazine)2, an air-stable layered solid, by reaction of CrCl2 with pyrazine (pyz). This compound displays a ferrimagnetic order below ∼55 K, reflecting the presence of strong magnetic interactions. Electrical conductivity measurements demonstrate that CrCl2(pyz)2 reaches a conductivity of 32 mS cm-1 at room temperature, which operates through a 2D hopping-based transport mechanism. These properties are induced by the redox-activity of the pyrazine ligand, which leads to a smearing of the Cr 3d and pyrazine π states. We suggest that the combination of redox-active ligands and reducing paramagnetic metal ions represents a general approach towards tuneable 2D materials that consist of charge-neutral layers and exhibit both long-range magnetic order and high electronic conductivity.

Synthons for carbide complex chemistry.

The sterically accessible carbide complex, (Cy3P)Cl3Ru[triple bond, length as m-dash]C-PtCl(py)2, acts as a synthon for terminal and bridging carbide fragments that relocate to pincer and A-frame scaffolds upon ligand addition. This concept, benefitting from coordination sphere selection as the concluding step, confronts traditional synthetic strategies and broadens the scope for carbide complexes.

Weakening of Carbide-Platinum Bonds as a Probe for Ligand Donor Strengths.

We report the observation of the weakening of the RuC-Pt single bond in (Cy3P)2Cl2Ru≡C-PtCl2-L (Ru≡C-Pt-L) complexes, leading to the incipient formation of the terminal ruthenium carbide complex, (Cy3P)2Cl2Ru≡C (RuC). In the solid state, elongation of RuC-Pt bonds illustrates the degree of weakening, and in solution, decreasing platinum-carbide coupling constants and increasing carbide chemical shifts reveal weaker interaction through the carbide bridge, as the electron donating ability of L becomes progressively stronger. For the bridging carbide ligands, the chemical shifts and coupling constants to platinum are linearly dependent, and NMR data for parent RuC conform to this relationship, providing a spectroscopic means of determining the strength of the RuC-Pt linkages relative to dissociated RuC. The pliancy of the Ru≡C-Pt-L fragment with regard to the identity of L establishes the carbide-bridged complexes as remarkably wide-ranging and sensitive probes for ligand donor abilities.

Modification of σ-Donor Properties of Terminal Carbide Ligands Investigated Through Carbide-Iodine Adduct Formation.

The terminal carbide ligands in [(Cy3 P)2 X2 Ru≡C] complexes (X=halide or pseudohalide) coordinate molecular iodine, affording charge-transfer complexes rather than oxidation products. Crystallographic and vibrational spectroscopic data show the perturbations of iodine to vary with the auxiliary ligand sphere on ruthenium, demonstrating the σ-donor properties of carbide complexes to be tunable.

Delivering carbide ligands to sulfide-rich clusters.

The propensity of the terminal ruthenium carbide Ru(C)Cl2(PCy3)2 (RuC) to form carbide bridges to electron-rich transition metals enables synthetic routes to metal clusters with coexisting carbide and sulfide ligands. Electrochemical experiments show the Ru≡C ligand to exert a relatively large electron-withdrawing effect compared with PPh3, effectively shifting redox potentials.

Stereoretentive formylation of (S)-proline: new application of the self-regeneration of stereo-centres (SRS) principle via chelation to cobalt(III).

In a Vilsmeier-Haack-type formylation reaction the α-(dihydroxymethyl)-(S)-prolinato complex (+)578-p-[Co(tren){(RC,SN)-Pro[CH(OH)2]O}]Cl2·2H2O (22) was produced stereoselectively (85% ee) from the (S)-prolinato complex, (+)578-p-[Co(tren){(SC,SN)-ProO}]2(H3O)2(HOEt2)(O3SCF3)7 (18). Similar reaction of the (S)-alaninato complex, (-)578-p-[Co(tren)(S-AlaO)](H3O)(O3SCF3)3 (13), produced the racemate, rac-p-[Co(tren)(Ala{CH(OH)2}O)]SO4·2H2O (17). The contrasting stereochemical outcomes of the formylation reaction with 18versus13 were ascribed to the stereogenic character of the coordinated sec. amine of the (S)-prolinate chelate in 18, which serves to uphold a chiral environment during reaction, whereas reaction intermediates derived from 13 lack this stereochemical feature. The stereoselective formylation of (S)-proline, relying on coordination to an inert metal centre, as conducted here, constitutes a novel application of the concept of Self-Regeneration of Stereocentres (SRS). The α-(hydroxymethyl)-(S)-prolinato complex, (+)578-p-[Co(tren){(RC,SN)-Pro(CH2OH)O}]Cl2·2H2O (23) resulted from borohydride reduction of 22. The molecular structures of (+)578-p-[Co(tren){(RC,SN)-Pro[CH(OH)2]O}]Cl2·2H2O (22), rac-p-[Co(tren)-(Ala{CH(OH)2}O)]I2·H2O (17) and (+)578-p-[Co(tren){(RC,SN)-Pro(CH2OH)O}]Cl2·2H2O (23) were established by X-ray crystallography.

Carbide complexes as π-acceptor ligands.

The π-accepting character of a terminal carbide complex acting as a ligand is demonstrated experimentally and corroborates earlier theoretical predictions. As a result, coordination of a terminal ruthenium carbide complex to electron-rich metal centres is shown to provide a facile and versatile route to carbide-bridged heterometallic complexes. Synthesis, reactivity, spectroscopic and structural characterization are reported for heterobimetallic systems with auxiliary metals from groups 9-11: Rh(i), Ir(i), Pd(ii), Pt(ii), Ag(i), and Au(i) coordinated by [Ru(C)Cl2(PCy3)2] (RuC). This encompasses the first example of a homoleptic carbide-ligated transition metal complex: [{(Cy3P)2Cl2RuC}2Au]+. Kinetics of substitution on Pt(ii) by RuC ranks the carbide complex as having intermediate nucleophilicity. The 13C-NMR signals from the carbide ligands are significantly more shielded in the bridged heterobimetallic complexes than in the parent terminal carbide complex. Structurally, RuC forms very shorts bonds to the heterometals, which supports the notion of the multiple bonded complex acting as a π-backbonding ligand. Reactions are reported where RuC displaces CO coordinated to Rh(i) and Ir(i). A strong trans influence exerted by RuC indicates it to be a stronger σ-donor than CO. The geometries around the carbide bridges resemble those in complexes of electron-rich metals with carbonyl or bridging nitride-complex-derived ligands, which establishes a link to other strong π-acceptor ligands.