Towards Heteroleptic Dicoordinate CuII Complexes.

In this work we detail our efforts to systematically generate stable dicoordinate CuII complexes. Initial experiments via metathesis reactions of a bulky potassium carbazolide (RK) with copper(II) salts indeed yielded a stable product, RCuOTf (1). However, subsequent attempts to grasp systematic synthetic access to complexes of the type RCuX (X=monoanionic ligand) proved difficult as many of the complexes rapidly decomposed in solution. By using triflate-related ligands such as ethyl sulfate and bistriflimide, the additional dicoordinate copper complexes RCuOSO3 Et (2), [RCu(THF)][Cu(NTf2 )2 ] (3) and RCuNTf2 (4) could be isolated. Spectroscopic indications corroborate more CuI than CuII character in all RCuX derivatives.

Synthesis and magnetic characterization of a dinuclear complex of low-coordinate iron(II).

Low-coordinate ions possess exciting magnetic, optical, and reactive properties that may afford novel material physics. Hence, it is important to test both synthetic methods for realizing extended solids of such ions as well as the properties of smaller molecular fragments of envisioned future materials. Herein, we report the synthesis and characterization of a new dinuclear Fe species, [{(Me3Si2)2N}Fe{µ-p-{HN(SiMe3)}(C6Me4){N(SiMe3)}}2Fe{N(SiMe3)2}] (1), formed through a transamination reaction between [Fe{N(SiMe3)2}2]2 and the bulky diamine p-{HN(SiMe3)}2(C6Me4) (L). The Fe centers of this dimer assume a pseudo-trigonal-planar, three-coordinate conformation in 1, bridged by two aromatic diamines. Single-crystal X-ray diffraction, IR spectroscopy, and Mössbauer spectroscopy enable the assignment of both Fe centers as the 2+ oxidation state. Magnetic studies show that 1 displays a weak antiferromagnetic exchange interaction (J = -2.33 cm-1) and moderate zero-field splitting (D = 7.51 cm-1). Importantly, these studies demonstrate the viability of using transamination to bridge high-spin low-coordinate metal ions and hence the technique may, in the future, produce new extended structures.

Metal⋠⋠⋠F-C Bonding in Low-Coordinate Alkaline Earth Fluoroarylamides.

A set of calcium and barium complexes containing the fluoroarylamide N(C6 F5 )2 - is presented. These compounds illustrate the key role of stabilising M⋅⋅⋅F-C secondary interactions in the construction of low-coordinate alkaline earth complexes. The nature of Ca⋅⋅⋅F-C bonding in calcium complexes is examined in the light of structural data, bond valence sum (BVS) analysis and DFT computations. The molecular structures of [Ca{N(C6 F5 )2 }2 (Et2 O)2 ] (4'), [Ca{µ-N(SiMe3 )2 }{N(C6 F5 )2 }]2 (52 ), [Ba{µ-N(C6 F5 )2 }{N(C6 F5 )2 }⋅toluene]2 (62 ), [{BDIDiPP }CaN(C6 F5 )2 ]2 (72 ), [{N^NDiPP }CaN(C6 F5 )2 ]2 (82 ), and [Ca{µ-OB(CH(SiMe3 )2 )2 }{N(C6 F5 )2 }]2 (92 ), where {BDIDiPP }- and {N^NDiPP }- are the bidentate ligands CH[C(CH3 )NDipp]2 - and DippNC6 H4 CNDipp- (Dipp=2,6-iPr2 -C6 H3 ), are detailed. Complex 62 displays strong Ba⋅⋅⋅F-C contacts at around 2.85 Å. The calcium complexes feature also very short intramolecular Ca-F interatomic distances at around 2.50 Å. In addition, the three-coordinate complexes 72 and 82 form dinuclear structures due to intermolecular Ca⋅⋅⋅F-C contacts. BVS analysis shows that Ca⋅⋅⋅F-C interactions contribute to 15-20 % of the bonding pattern around calcium. Computations demonstrate that Ca⋅⋅⋅F-C bonding is mostly electrostatic, but also contains a non-negligible covalent contribution. They also suggest that Ca⋅⋅⋅F-C are the strongest amongst the range of weak Ca⋅⋅⋅X (X=F, H, Cπ ) secondary interactions, due to the high positive charge of Ca2+ which favours electrostatic interactions.

Rhodium(III) Complexes Featuring Coordinated CF3 Appendages.

The synthesis and characterisation of a homologous series of rhodium 2,2'-biphenyl complexes featuring intramolecular dative bonding of the nominally inert and weakly coordinating trifluoromethyl group are described. Presence of these interactions is evidenced in the solid state using X-ray diffraction, with Rh-F contacts of 2.36-2.45 Å, and in solution using NMR spectroscopy, through hindered C-CF3 bond rotation and the presence of time-averaged 1 JRhF and 2 JPF coupling.

Low-Coordinate Barium Boryloxides: Synthesis and Dehydrocoupling Catalysis for the Production of Borasiloxanes.

The first soluble barium boryloxides [Ba]- OB{CH(SiMe3 )2 } are presented. These mono- or dinuclear complexes feature low coordination numbers, as low as two for [Ba(OB{CH(SiMe3 )2 }2 )2 ], which is further stabilized by intra- and intermolecular Ba⋅⋅⋅H3 C agostic interactions. Barium boryloxides and the parent [Ba{N(SiMe3 )2 }2 ⋅(thf)2 ] catalyze the dehydrocoupling of borinic acids with hydrosilanes, providing borasiloxanes under mild conditions.

Selective Conversion of CO2 into Isocyanate by Low-Coordinate Iron Complexes.

Discovery of the mechanisms for selective transformations of CO2 into organic compounds is a challenge. Herein, we describe the reaction of low-coordinate Fe silylamide complexes with CO2 to give trimethylsilyl isocyanate and the corresponding Fe siloxide complex. Kinetic studies show that this is a two-stage reaction, and the presence of a single equivalent of THF influences the rates of both steps. Isolation of a thermally unstable intermediate provides mechanistic insight that explains both the effect of THF in this reaction, and the way in which the reaction achieves high selectivity for isocyanate formation.

Synthesis and Reactivity of a Neutral, Three-Coordinate Platinum(II) Complex Featuring Terminal Amido Ligation.

A crystallographically characterized three-coordinate, formally 14 electron Pt(II) complex 1 featuring terminal amido ligation is reported. Computational analysis revealed relatively weak π donation from the amide lone pair to platinum and supports a 14-electron assignment for 1. Stoichiometric reactivity studies confirmed the viability of net O-H and C-H addition across, as well as isonitrile insertion into, the terminal platinum-amido linkage of 1.

Two-coordinate Fe° and Co° complexes supported by cyclic (alkyl)(amino)carbenes.

The CAAC [CAAC=cyclic (alkyl)(amino)carbene] family of carbene ligands have shown promise in stabilizing unusually low-coordination number transition-metal complexes in low formal oxidation states. Here we extend this narrative by demonstrating their utility in affording access to the first examples of two-coordinate formal Fe(0) and Co(0) [(CAAC)2M] complexes, prepared by reduction of their corresponding two-coordinate cationic Fe(I) and Co(I) precursors. The stability of these species arises from the strong σ-donating and π-accepting properties of the supporting CAAC ligands, in addition to steric protection.

(Aminocarbene)(divinyltetramethyldisiloxane)iron(0) compounds: a class of low-coordinate iron(0) reagents.

The combined use of aminocarbene and divinyltetramethyldisiloxane (dvtms) as supporting ligands enables the access of unprecedented low-coordinate iron(0) alkene compounds [L(n)Fe(η(2):η(2)-dvtms)] (L=N-heterocyclic carbene (NHC) or cyclic (alkyl)(amino)carbene (CAAC), n=1 or 2) from the reactions of FeCl2 with alkali-metal reducing agents, free aminocarbene ligands, and dvtms. The iron(0) species deliver their {L(n)Fe(0)} fragments to perform redox reactions with Ph2SiH2, S8, Se, and DippN3, furnishing novel aminocarbene-supported iron(IV) silylene, all-ferrous iron-sulfur/selenium cubanes, and bis(imido)iron(IV) compounds. These conversions demonstrate the potential synthetic utility of the carbene-supported iron(0) complexes as a valuable class of low-coordinate iron(0) reagents.