Molecular-Scale Visualization of Steric Effects of Ligand Binding to Reconstructed Au(111) Surfaces.

Direct imaging of single molecules at nanostructured interfaces is a grand challenge with potential to enable new, precise material architectures and technologies. Of particular interest are the structural morphology and spectroscopic signatures of the adsorbed molecule, where modern probes are only now being developed with the necessary spatial and energetic resolution to provide detailed information at the molecule-surface interface. Here, we directly characterize the adsorption of individual m-terphenyl isocyanide ligands on a reconstructed Au(111) surface through scanning tunneling microscopy and inelastic electron tunneling spectroscopy. The site-dependent steric pressure of the various surface features alters the vibrational fingerprints of the m-terphenyl isocyanides, which are characterized with single-molecule precision through joint experimental and theoretical approaches. This study provides molecular-level insights into the steric-pressure-enabled surface binding selectivity as well as its effect on the chemical properties of individual surface-binding ligands.

Formation of a P162- Ink from Elemental Red Phosphorus in a Thiol-Amine Mixture.

A P162- polyphosphide dianion ink was produced by the reaction of red phosphorus with a binary thiol-amine mixture of ethanethiol (ET) and ethylenediamine (en). The polyphosphide was identified by solution 31P NMR spectroscopy and electrospray ionization mass spectrometry. This solute was compared to the reaction products of white phosphorus (P4) and other elemental pnictides in the same solvent system. The reaction of P4 with ET and en gives the same P162- polyphosphide; however, the easier handling and lower reactivity of red phosphorus highlights the novelty of that reaction. Elemental arsenic and antimony both give mononuclear pnictogen-sulfide-thiolate complexes upon reaction with ET and en under otherwise identical conditions, with this difference likely resulting from the greater covalency and tendency of phosphorus to form P-P bonds.

A Complete Triad of Zero-Valent 17-Electron Monoradicals of Group 7 Elements Stabilized by m-Terphenyl Isocyanides.

The first consistent series of mononuclear 17-electron complexes of three Group 7 elements has been isolated in crystalline form and studied by X-ray diffraction and spectroscopic methods. The paramagnetic compounds have a composition of [M0 (CO)(CNp-F-ArDArF2 )4 ] (M=Mn, Tc, Re; ArDArF2 =2,6-(3,5-(CF3 )2 C6 H3 )2 C6 H2 F) and are stabilized by four sterically encumbering isocyanides, which prevent the metalloradicals from dimerization. They have a square pyramidal structure with the carbonyl ligands as apexes. The frozen-solution EPR spectra of the rhenium and technetium compounds are clearly anisotropic with large 99 Tc and 185,187 Re hyperfine interactions for one component. High-field EPR (Q band and W band) has been applied for the elucidation of the EPR parameters of the manganese(0) complex.

Mixed-Isocyanide Complexes of Technetium under Steric and Electronic Control.

Reactions of the alkyl isocyanide fac-[Tc(CO)3(CNR)2Cl] complexes (2) (CNR = CNnBu or CNtBu) with the sterically encumbered isocyanide CNp-FArDarF2 [DArF = 3,5-(CF3)2C6H3] allow a selective exchange of the carbonyl ligands of 2 and the isolation of the mixed-isocyanide complexes mer,trans-[Tc(CNp-FArDarF2)3(CNR)2Cl] (3). Depending on the steric requirements of the residues R, the remaining chlorido ligand can be replaced by another isocyanide ligand. Cationic complexes such as mer-[Tc(CNp-FArDarF2)3(CNnBu)3]+ (4a) or mer,trans-[Tc(CNp-FArDarF2)3(CNnBu)2(CNtBu)]+ (6) have been prepared in this way and isolated as their PF6- salts. mer,trans-[Tc(CNp-FArDarF2)3(CNnBu)2(CNtBu)](PF6) represents to the best of our knowledge the first transition-metal complex with three different isocyanides in its coordination sphere. Since the degree of the ligand exchange seems to be controlled both by the electronic and steric measures of the incoming isocyanides, we undertook similar reactions with the sterically less demanding p-fluorophenyl isocyanide, CNPhpF, which indeed readily led to the hexakis(isocyanide)technetium(I) cation through an exchange of all ligands in the staring materials [Tc2(CO)6(µ-Cl)3]- or fac-[Tc(CO)3(CNR)2Cl]. The influence of the substituents at the isocyanide ligands in such reactions has been reasoned with the density functional theory-derived electrostatic potential at the accessible surface of the corresponding isocyanide carbon atoms.

The Chemistry of Phenylimidotechnetium(V) Complexes with Isocyanides: Steric and Electronic Factors.

Organometallic approaches are of ongoing interest for the development of novel functional 99mTc radiopharmaceuticals, while the basic organotechnetium chemistry seems frequently to be little explored. Thus, structural and reactivity studies with the long-lived isotope 99Tc are of permanent interest as the foundation for further progress in the related radiopharmaceutical research with this artificial element. Particularly the knowledge about the organometallic chemistry of high-valent technetium compounds is scarcely developed. Here, phenylimido complexes of technetium(V) with different isocyanides are introduced. They have been synthesized by ligand-exchange procedures starting from [Tc(NPh)Cl3(PPh3)2]. Different reactivity patterns and products have been obtained depending on the steric and electronic properties of the individual ligands. This involves the formation of 1:1 and 1:2 exchange products of Tc(V) with the general formulae [Tc(NPh)Cl3(PPh3)(isocyanide)], cis- or trans-[Tc(NPh)Cl3(isocyanide)2], but also the reduction in the metal and the formation of cationic technetium(I) complex of the formula [Tc(isocyanide)6]+ when p-fluorophenyl isocyanide is used. The products have been studied by single-crystal X-ray diffraction and spectroscopic methods, including IR and multinuclear NMR spectroscopy. DFT calculations on the different isocyanides allow the prediction of their reactivity towards electron-rich and electron-deficient metal centers by means of the empirical SADAP parameter, which has been derived from the potential energy surface of the electron density on their potentially coordinating carbon atoms.

Metal-Organic Frameworks with Low-Valent Metal Nodes.

Metal-organic frameworks (MOFs) are crystalline, 2- and 3-dimensional coordination polymers formed by bonding interactions between metals and multitopic organic ligands. These are typically formed using hard Lewis basic organic ligands with high oxidation state metal ions. The use of low-valent metals as structural elements in MOFs is far less common, despite the widespread use of such metals for catalysis, luminescence, and other applications. This Minireview focuses on recent advances in the field of low-valent MOFs and offers a perspective on the future development of these materials.

Crystalline Hydrogen-Bonding Networks and Mixed-Metal Framework Materials Enabled by an Electronically Differentiated Heteroditopic Isocyanide/Carboxylate Linker Group.

Mixed-metal solid-state framework materials are emerging candidates for advanced applications in catalysis and chemical separations. Traditionally, the syntheses of mixed-metal framework systems rely on postsynthetic ion exchange, metalloligands, or metal-deposition techniques for the incorporation of a second metal within a framework material. However, these methods are often incompatible with the incorporation of low-valent metal centers, which preferentially bind to electronically "soft" ligands according to the tenets of hard/soft acid/base theory. Here we present the electronically differentiated isocyanide/carboxylate heteroditopic linker ligand 1,4-CNArMes2C6H4CO2H (TIBMes2H; TIB = terphenyl isocyanide benzoate; ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H2), which is capable of selective binding of low-valent metals via the isocyano group and complexation of hard Lewis acidic metals through the carboxylate unit. This heteroditopic ligand also possesses an encumbering m-terphenyl backbone at the isocyanide function to foster coordinative unsaturation. The treatment of TIBMes2H with [Cu(NCMe)4]PF6 in a 3:1 ratio results in preferential binding of the isocyanide group to the Cu(I) center as assayed by multinuclear NMR and IR spectroscopies. IR spectroscopy also provides strong evidence for the formation of a copper(I) tris(isocyanide) complex, wherein the carboxylic acid group remains unperturbed. The addition of TIBMes2 to [Cu(NCMe)4]PF6 in a 4:1 ratio results in crystallization of the hydrogen-bonding network, [Cu(TIBMes2H)4]PF6, in which the formation of R22(8) hydrogen bonds results in a 7-fold interpenetrated diamondoid lattice structure. The preassembly of a copper(I) tris(isocyanide) complex using TIBMes2H, followed by deprotonation and the introduction of ZnCl2, generates a novel and unusual zwitterionic solid-state phase (denoted as Cu/Zn-ISOCN-5; ISOCN = isocyanide coordination network) consisting of a coordinatively unsaturated [Cu(CNR)3]+ cationic secondary building unit (SBU) and an anionic, paddlewheel-type Zn(II)-based SBU of the formulation [Cl2Zn2(O2CR)3]-. Inductively coupled plasma mass spectrometry analysis provided firm evidence for a 2:1 Zn-to-Cu ratio in the network, thereby indicating that the isocyanide and carboxylate groups selectively bind soft and hard Lewis acidic metal centers, respectively. The extended structure of Cu/Zn-ISOCN-5 is a densely packed, noninterpenetrated AB-stacked layer network with modest surface area. However, it is thermally robust, and its formation and compositional integrity validate the use of an electronically differentiated linker for the formation of mixed-metal frameworks incorporating low-valent metal centers.

Aqueous Stability and Ligand Substitution of a Layered Cu(I)/Isocyanide-Based Organometallic Network Material with a Well-Defined Channel Structure.

Isocyanide coordination networks (ISOCNs), which consist of multitopic isocyanide linker groups and transition-metal-based secondary building units (SBUs), are a promising class of organometallic framework materials for the inclusion of low-valent metal centers as primary structural components. Previously, it was demonstrated that the ditopic m-terphenyl isocyanide ligand, [CNArMes2]2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3), could provide single-metal node frameworks based on Cu(I) and Ni(0) centers. However, the relatively short linker length in [CNArMes2]2 precluded the formation of networks with significant porosity. Here, it is shown that expansion of the [CNArMes2]2 scaffold with a central phenylene spacer allows for the formation of a robust Cu(I)-based framework with a distinct and solvent accessible channel structure. This new framework, denoted Cu-ISOCN-4, is prepared as single-crystalline samples from a solvothermal reaction between [Cu(NCMe)4]PF6 and expanded linker 1,4-(CNArMes2)2C6H4. Crystallographic characterization of Cu-ISOCN-4 revealed mononuclear [Cu(THF)(CNR)3]+ structural nodes. The expanded diisocyanide linker results in fourfold interpenetrated (6,3) internal morphology. However, interpenetration in Cu-ISOCN-4 results in discrete layer domains, each of which possesses well-defined 29 × 19 Å channels along the crystallographic b axis. Thermogravimetric analysis on Cu-ISOCN-4 revealed THF solvent loss from the channels between 100-200 °C and dissociation of the Cu-coordinated THF ligand at 290 °C. The overall integrity of the network remains intact up to 400 °C, thereby signifying the robust nature of materials produced from metal-isocyanide M-C linkages. Aqueous stability studies revealed that Cu-ISOCN-4 remains chemically resistant to exposure to liquid water for several days. In addition, ligand exchange studies in both THF and aqueous solution demonstrate that the Cu-coordinated THF group in Cu-ISOCN-4 can be readily substituted with pyridine. This ligand exchange process occurs via single-crystal-to-single-crystal transformations and can also be readily monitored by infrared spectroscopy.

Side-On Coordination of Nitrous Oxide to a Mononuclear Cobalt Center.

Despite its utility as an oxygen-atom transfer reagent for transition metals, nitrous oxide (N2O) is a notoriously poor ligand, and its coordination chemistry has been limited to a few terminal, end-on κ1-N complexes. Here, the synthesis of a mononuclear cobalt complex possessing a side-on-bound N2O molecule is reported. Structural characterization, IR spectroscopy, and DFT calculations support an η2-N,N binding mode for binding of N2O to the cobalt center.

Dianionic Mononuclear Cyclo-P4 Complexes of Zero-Valent Molybdenum: Coordination of the Cyclo-P4 Dianion in the Absence of Intramolecular Charge Transfer.

Relative to other cyclic poly-phosphorus species (that is, cyclo-Pn ), the planar cyclo-P4 group is unique in its requirement of two additional electrons to achieve aromaticity. These electrons are supplied from one or more metal centers. However, the degree of charge transfer is dependent on the nature of the metal fragment. Unique examples of dianionic mononuclear η4 -P4 complexes are presented that can be viewed as the simple coordination of the [cyclo-P4 ]2- dianion to a neutral metal fragment. Treatment of the neutral, molybdenum cyclo-P4 complexes Mo(η4 -P4 )I2 (CO)(CNArDipp2 )2 and Mo(η4 -P4 )(CO)2 (CNArDipp2 )2 with KC8 produces the dianionic, three-legged piano stool complexes, [Mo(η4 -P4 )(CO)(CNArDipp2 )2 ]2- and [Mo(η4 -P4 )(CO)2 (CNArDipp2 )]2- , respectively. Structural, spectroscopic, and computational studies reveal a similarity to the classic η6 -benzene complex (η6 -C6 H6 )Mo(CO)3 regarding the metal-center valence state and electronic population of the planar-cyclic ligand π system.

Photolytic Reductive Elimination of White Phosphorus from a Mononuclear cyclo-P4 Transition Metal Complex.

Elemental white phosphorus (P4 ) is well recognized as a critical precursor to organophosphorus compounds. However, regulatory constraints stemming from the toxic and pyrophoric nature of white phosphorus have significantly limited its accessibility. Herein is described a new approach to white phosphorus storage and release based on a unique example of photolytic reductive elimination of the tetrahedral P4 molecule from a mononuclear cyclo-P4 molybdenum complex. The latter functions as an air-stable, chemically-deactivated source of white phosphorus. The system features efficient photo-release of white phosphorus using inexpensive violet LED sources. Additionally, high-yield recapture of unspent white phosphorus by the molybdenum center can be achieved by post-photolysis heating at convenient temperatures.

A Highly-Reduced Cobalt Terminal Carbyne: Divergent Metal- and α-Carbon-Centered Reactivity.

Reported here is the isolation of a dianionic cobalt terminal carbyne derived from chemical reduction of an encumbering isocyanide ligand. Crystallographic, spectroscopic and computational data reveal that this carbyne possesses a low-valent cobalt center with an extensively filled d-orbital manifold. This electronic character renders the cobalt center the primary site of nucleophilicity upon reaction with protic substrates and silyl electrophiles. However, reactions with internal alkynes result in [2+2] cycloaddition with the carbyne carbon to form a new C-C bond.

Regioselective Formation of (E)-ß-Vinylstannanes with a Topologically Controlled Molybdenum-Based Alkyne Hydrostannation Catalyst.

The regioselective formation of (E)-ß-vinylstannanes has been a long-standing challenge in transition-metal-catalyzed alkyne hydrostannation. Herein, we report a well-defined molybdenum-based system featuring two encumbering m-terphenyl isocyanides that reliably and efficiently delivers (E)-ß-vinylstannanes from a range of terminal and internal alkynes with high regioselectivity. The system is particularly effective for aryl alkynes and can discriminate between alkyl chains of low steric hindrance in unsymmetrically substituted dialkyl alkynes. Catalytic hydrostannation with this system is also characterized by an electronic effect that leads to a decrease in regioselectivity when electron-withdrawing groups are present on the alkyne substrate.

A Metal-Organic Framework with Exceptional Activity for C-H Bond Amination.

The development of catalysts capable of fast, robust C-H bond amination under mild conditions is an unrealized goal despite substantial progress in the field of C-H activation in recent years. A Mn-based metal-organic framework (CPF-5) is described that promotes the direct amination of C-H bonds with exceptional activity. CPF-5 is capable of functionalizing C-H bonds in an intermolecular fashion with unrivaled catalytic stability producing >105 turnovers.

Controlled Expansion of a Strong-Field Iron Nitride Cluster: Multi-Site Ligand Substitution as a Strategy for Activating Interstitial Nitride Nucleophilicity.

Multimetallic clusters have long been investigated as molecular surrogates for reactive sites on metal surfaces. In the case of the µ4 -nitrido cluster [Fe4 (µ4 -N)(CO)12 ]- , this analogy is limited owing to the electron-withdrawing effect of carbonyl ligands on the iron nitride core. Described here is the synthesis and reactivity of [Fe4 (µ4 -N)(CO)8 (CNArMes2 )4 ]- , an electron-rich analogue of [Fe4 (µ4 -N)(CO)12 ]- , where the interstitial nitride displays significant nucleophilicity. This characteristic enables rational expansion with main-group and transition-metal centers to yield unsaturated sites. The resulting clusters display surface-like reactivity through coordination-sphere-dependent atom rearrangement and metal-metal cooperativity.

Crystalline Coordination Networks of Zero-Valent Metal Centers: Formation of a 3-Dimensional Ni(0) Framework with m-Terphenyl Diisocyanides.

A permanently porous, three-dimensional metal-organic material formed from zero-valent metal nodes is presented. Combination of ditopic m-terphenyl diisocyanide, [CNArMes2]2, and the d10 Ni(0) precursor Ni(COD)2, produces a porous metal-organic material featuring tetrahedral [Ni(CNArMes2)4]n structural sites. X-ray absorption spectroscopy provides firm evidence for the presence of Ni(0) centers, whereas gas-sorption and thermogravimetric analysis reveal the characteristics of a robust network with a microdomain N2-adsorption profile.

Terminal Iron Carbyne Complexes Derived from Arrested CO2 Reductive Disproportionation.

The encumbered tetraisocyanide dianion Na2 [Fe(CNArMes2 )4 ] reacts with two molecules of CO2 to effect reductive disproportionation to CO and carbonate ([CO3 ]2- ). When the reaction is performed in the presence of silyl triflates, reductive disproportionation is arrested by silylative esterification of a mono-CO2 adduct. This results in the formation of four-coordinate terminal iron carbynes possessing an aryl carbamate substituent owing to the direct attachment of an C(O)OSiR3 group to an isocyanide nitrogen atom. Crystallographic, spectroscopic, and computational analyses of these iron-carbon multiply bonded species reveal electronic structure properties indicative of a conformationally locked iron carbyne unit.

Oxidative-Insertion Reactivity Across a Geometrically Constrained Metal→Borane Interaction.

While interest in cooperative reactivity of transition metals and Lewis acids is receiving significant attention, the scope of known reactions that directly exploit the polarized reverse-dative σ-bond of metal-borane complexes (i.e., M→BR3 ) remains limited. Described herein is that the platinum (boryl)iminomethane (BIM) complex [Pt(κ2 -N,B-Cy2 BIM)(CNArDipp2 )] can effect the oxidative insertion of a range of unsaturated organic substrates, including azides, isocyantes, and nitriles, as well as CO2 and elemental sulfur (S8 ). In addition, alkyl migration processes available to the BIM framework allow for post-insertion reaction sequences resulting in product release from the metal center.

Robust, Transformable, and Crystalline Single-Node Organometallic Networks Constructed from Ditopic m-Terphenyl Isocyanides.

The preparation of 3D and 2D Cu(I) coordination networks using ditopic m-terphenyl isocyanides is described. The incorporation of sterically encumbering substituents enables the controlled, solid-state preparation of Cu(I) tris-isocyanide nodes with a labile solvent ligand in a manner mirroring solution-phase chemistry of monomeric complexes. The protection afforded by the m-terphenyl groups is also shown to engender significant stability towards heat as well as acidic or basic conditions, resulting in robust single-metal-node networks that can transition from 3D to 2D extended structures.