Alkaline earth complexes of a sterically demanding guanidinate ligand

The synthesis of the guanidine MesN{C(NCy2)}N(H)Mes (LH; Mes = 2,4,6-Me3C6H2, Cy = cyclohexyl), and its use as a proligand for the synthesis of alkaline earth metal complexes are reported. Described herein are (i) an unusual Hauser base cubane, (ii) a homoleptic and a base-stabilized magnesium complex featuring the same guanidinate ligands, and (iii) the comparison of a series of alkaline earth (Mg, Ca, Sr, Ba) bis(guanidinate) complexes, which allows the opportunity to compare the changing trends in bonding as the Group is descended. The reaction between LH and MeMgI(OEt2)2 yields the Hauser base as a mixture of the tetramer [Mg4L4(μ3-I)4] (1a) and dimer [Mg2L2(μ-I)2(OEt2)2] (1b), and the reaction with two equivalents of MgnBu2 leads to the formation of four-coordinate [MgL2] (2), which features a square-planar geometry for the magnesium cation, or five-coordinate [MgL2(THF)] (3), depending on the solvent used. 1a is the first crystallographically-characterized cubane structure to consist of four LAeX (L = ligand, X = halide) units. The complexes [AeL2(THF)2] (Ae = Ca, 4; Ae = Sr, 5) and [BaL2] (6) were synthesized via redox transmetallation/ligand exchange reactions. Complex 6 is the first example of a homoleptic, monomeric barium complex of the NCN ligand family, with the structure stabilized by a number of barium-arene interactions in the solid state

Tuning coordination in s-block carbazol-9-yl complexes

1,3,6,8-Tetra-tert-butylcarbazol-9-yl and 1,8-diaryl-3,6-di(tert-butyl)carbazol-9-yl ligands have been utilized in the synthesis of potassium and magnesium complexes. The potassium complexes (1,3,6,8-tBu4carb)K(THF)4 (1; carb=C12H4N), [(1,8-Xyl2-3,6-tBu2carb)K(THF)]2 (2; Xyl=3,5-Me2C6H3) and (1,8-Mes2-3,6-tBu2carb)K(THF)2 (3; Mes=2,4,6-Me3C6H2) were reacted with MgI2 to give the Hauser bases 1,3,6,8-tBu4carbMgI(THF)2 (4) and 1,8-Ar2-3,6-tBu2carbMgI(THF) (Ar=Xyl 5, Ar=Mes 6). Structural investigations of the potassium and magnesium derivatives highlight significant differences in the coordination motifs, which depend on the nature of the 1- and 8-substituents: 1,8-di(tert-butyl)-substituted ligands gave π-type compounds (1 and 4), in which the carbazolyl ligand acts as a multi-hapto donor, with the metal cations positioned below the coordination plane in a half-sandwich conformation, whereas the use of 1,8-diaryl substituted ligands gave σ-type complexes (2 and 6). Space-filling diagrams and percent buried volume calculations indicated that aryl-substituted carbazolyl ligands offer a steric cleft better suited to stabilization of low-coordinate magnesium complexes

Iron(II) catalyzed hydrophosphination of isocyanates

The first transition metal catalyzed hydrophosphination of isocyanates is presented. The use of low-coordinate iron(II) precatalysts leads to an unprecedented catalytic double insertion of isocyanates into the P-H bond of diphenylphosphine to yield phosphinodicarboxamides [Ph2PC(=O)N(R)C(=O)N(H)R], a new family of derivatized organophosphorus compounds. This remarkable result can be attributed to the low-coordinate nature of the iron(II) centres whose inherent electron deficiency enables a Lewis-acid mechanism in which a combination of the steric pocket of the metal centre and substrate size determines the reaction products and regioselectivity

A monomeric, heterobimetallic complex with an unsupported Mg−Fe bond

The phosphinimine, trimethylsilyl-substituted BIPM ligand [BIPM = bis(iminophosphorano)methanide] has been used to stabilise CH(Ph2PNSiMe3)2MgFe(η5-C5H5)(CO)2 (1), which is a structurally authenticated complex exhibiting a direct, unsupported bond between an alkaline earth metal and a transition metal. The FTIR-measured carbonyl stretching frequencies for this complex suggest that there is a polarisation of charge from the transition metal fragment to the magnesium centre. The presence of a polar metal-metal bond in 1 is confirmed by DFT calculations, which suggest that the Mg−Fe bond is predominantly ionic in nature

Alkaline Earth Complexes of a Sterically Demanding Guanidinate Ligand: Alkaline Earth Guanidinate Complexes

The synthesis of the guanidine MesN{C(NCy2)}N(H)Mes (LH; Mes = 2,4,6-Me3C6H2, Cy = cyclohexyl), and its use as a proligand for the synthesis of alkaline earth metal complexes are reported. Described herein are (i) an unusual Hauser base cubane, (ii) a homoleptic and a base-stabilized magnesium complex featuring the same guanidinate ligands, and (iii) the comparison of a series of alkaline earth (Mg, Ca, Sr, Ba) bis(guanidinate) complexes, which allows the opportunity to compare the changing trends in bonding as the Group is descended. The reaction between LH and MeMgI(OEt2)2 yields the Hauser base as a mixture of the tetramer [Mg4L4(?3-I)4] (1a) and dimer [Mg2L2(?-I)2(OEt2)2] (1b), and the reaction with two equivalents of MgnBu2 leads to the formation of four-coordinate [MgL2] (2), which features a square-planar geometry for the magnesium cation, or five-coordinate [MgL2(THF)] (3), depending on the solvent used. 1a is the first crystallographically-characterized cubane structure to consist of four LAeX (L = ligand, X = halide) units. The complexes [AeL2(THF)2] (Ae = Ca, 4; Ae = Sr, 5) and [BaL2] (6) were synthesized via redox transmetallation/ligand exchange reactions. Complex 6 is the first example of a homoleptic, monomeric barium complex of the NCN ligand family, with the structure stabilized by a number of barium-arene interactions in the solid state

Cyclotrimerisation of isocyanates catalysed by low-coordinate Mn(II) and Fe(II) m-terphenyl complexes

Two- and three-coordinate m-terphenyl complexes of manganese and iron are efficient catalysts for the selective cyclotrimerisation of primary aliphatic isocyanates affording isocyanurates in short reaction times and under mild conditions

Catalyst-free hydrophosphinylation of isocyanates and isothiocyanates under low-added-solvent conditions

A catalyst-free, low-solvent method for the hydrophosphinylation of isocyanates and isothiocyanates is reported. A range of phosphorus nucleophiles including secondary phosphine oxides HP(O)R2 (R = Ph, iPr), phosphites HP(O)(OR)2 (R = Me, Et), and methyl phenylphosphinate were tested. The procedure tolerated isocyanates and isothiocyanates featuring a wide range of substituents and, with use of 4 equiv of 2-methyltetrahydrofuran (2-MeTHF), solid substrates can be utilized. Twenty-five compounds were prepared with improved functional group tolerance compared to previous methods allowing access to new compounds (16 are novel). Facile scale up and simple reaction conditions make this a straightforward and practical methodology for obtaining phosphorus analogues of ureas and thioureas, which are challenging to synthesize by other methods

Iron(II)-catalyzed hydroamination of isocyanates

A two-coordinate Fe(II) m-terphenyl complex acts as a precatalyst for the hydroamination of isocyanates, affording urea and biuret derivatives, with product selectivity accomplished via modification of the reaction conditions. Using a more nucleophilic amine facilitates the insertion of up to four isocyanates into the N–H bond, affording triuret and tetrauret derivatives

Catalyst-free Hydrophosphinylation of Isocyanates and Isothiocyanates under Low-Added-Solvent Conditions

A catalyst-free, low-solvent method for the hydrophosphinylation of isocyanates and isothiocyanates is reported. A range of phosphorus nucleophiles including secondary phosphine oxides HP(O)R2 (R = Ph, i Pr), phosphites HP(O)(OR)2 (R = Me, Et), and methyl phenylphosphinate are tested. The procedure tolerates isocyanates and isothiocyanates featuring a wide range of substituents and, by using four equivalents of 2-methyltetrahydrofuran (2-MeTHF), solid substrates can be utilized. Twenty-five compounds are prepared, with improved functional group tolerance compared to previous methods and allowing access to new compounds (16 are novel). Facile scale up and simple reaction conditions make this a straightforward and practical methodology for obtaining phosphorus analogues of ureas and thioureas, which are challenging to synthesize by other methods

Group 11 m-Terphenyl Complexes Featuring Metallophilic Interactions

A series of group 11 m-terphenyl complexes have been synthesized via a metathesis reaction from the iron(II) complexes (2,6-Mes2C6H3)2Fe and (2,6-Xyl2C6H3)2Fe (Mes = 2,4,6-Me3C6H2; Xyl = 2,6-Me2C6H3). [2,6-Mes2C6H3M]2 (1, M = Cu; 2, M = Ag; 6, M = Au) and [2,6-Xyl2C6H3M]2 (3, M = Cu; 4, M = Ag) are dimeric in the solid state, although different geometries are observed depending on the ligand. These complexes feature short metal–metal distances in the expected range for metallophilic interactions. While 1–4 are readily isolated using this metathetical route, the gold complex 6 is unstable in solution at ambient temperatures and has only been obtained in low yield from the decomposition of (2,6-Mes2C6H3)Au·SMe2 (5). NMR spectroscopic measurements, including diffusion-ordered spectroscopy, suggest that 1–4 remain dimeric in a benzene-d6 solution. The metal–metal interactions have been examined computationally using the Quantum Theory of Atoms in Molecules and by an energy decomposition analysis employing natural orbitals for chemical valence