Deborah L. Kays

Transition Metal Boryl Complexes

The article represents a review of the chemistry of transition metal complexes containing the boryl (–BX2) ligand based on the chemical literature published up to and including the calendar year 2006. A brief compendium of papers published in the year 2007 is also included. Notwithstanding the ever-growing volume of literature detailing useful organic transformations in which these complexes have been implicated (e.g. C–H activation, hydroboration, diboration), the primary focus of this article is a review of issues of structure and bonding in boryl systems. Thus, the evidence for boryl ligands acting – to a greater or lesser extent – as sigma donors and/or pi acceptors is examined at length, as are issues relating to secondary interactions (e.g with hydride co-ligands) within the metal coordination sphere, and the consequent relationship to borane sigma complexes.

Monomeric, two-coordinate Mn, Fe and Co(II) complexes featuring 2,6-(2,4,6-trimethylphenyl)phenyl ligands.

The synthesis and characterization of the monomeric, two-coordinate transition-metal complexes (2,6-Mes(2)C(6)H(3))(2)M (Mes = mesityl, 2,4,6-Me(3)C(6)H(2), M = Mn, Fe, Co) are reported; (2,6-Mes(2)C(6)H(3))(2)Co is the first structurally authenticated two-coordinate, homoleptic cobalt(II) complex featuring sigma-bonded aryl ligands.

Synthetic and reaction chemistry of heteroatom stabilized boryl and cationic borylene complexes

The synthesis, spectroscopic and structural characterization of the aryloxy and amino functionalized chloroboryl complexes (eta(5)-C(5)R(5))Fe(CO)(2)B(OMes)Cl (R = H, 2a ; R = Me, 3a) and (eta(5)-C(5)H(5))Fe(CO)(2)B(N(i)Pr(2))Cl (7a) are reported. Compound 2ais shown to be a versatile substrate for further boron-centred substitution chemistry leading to the asymmetric boryl complexes (eta(5)-C(5)H(5))Fe(CO)(2)B(OMes)ER(n) [ER(n) = OC(6)H(4)(t)Bu-4, 2c; ER(n) = SPh, 2d] with retention of the metal-boron bond. The reactivities of 2a, 3a and 7a towards the halide abstraction agent Na[BAr(f)(4)] have also been examined, in order to investigate the potential for the generation of cationic heteroatom-stabilized terminal borylene complexes. The application of this methodology to the mesityloxy derivatives and gives rise to B-F containing products, presumably via fluoride abstraction from the [BAr(f)(4)](-) counter-ion. By contrast, amino-functionalized complex 7a is more amenable to this approach, and the thermally robust terminal aminoborylene complex [(eta(5)-C(5)H(5))Fe(CO)(2)B(N(i)Pr(2))][BAr(f)(4)] (9) can be isolated in ca. 50% yield. The reactivity of 9towards a range of nucleophilic and/or unsaturated reagents has been examined, with examples of addition, protonolysis and metathesis chemistries having been established.

Chemistry of metal – boron double bonds

This article provides an overview of the recent developments in the area of transition metal complexes featuring metal-boron double bonds. In particular, emphasis has been placed on studies focused on the exploration of patterns of reactivity.

Alkaline earth complexes of silylated aminopyridinato ligands: homoleptic compounds and heterobimetallic coordination polymers.

The synthesis and characterization of magnesium and calcium complexes of sterically demanding aminopyridinato ligands is reported. The reaction of the 2-Me3SiNH-6-MeC5H3N (L(1)H), 2-MePh2SiNH-6-MeC5H3N (L(2)H), and 2-Me3SiNH-6-PhC5H3N (L(3)H) with KH in tetrahydrofuran (THF) yielded potassium salts L(1)K(thf)0.5 (1), L(2)K (2), and L(3)K(thf)0.5 (3), which, through subsequent reaction with MgI2 and CaI2, afforded the homoleptic complexes (L)2Ae(thf)n [L = L(1), Ae = Mg, n = 1 (4); L = L(2), Ae = Mg, n = 0 (5); L = L(3), Ae = Mg, n = 0 (6); L = L(2), Ae = Ca, n = 2 (7)] and heterobimetallic calciates {[(L)3Ca]K}∞ [L = L(1) (8); L = L(2) (9)]. The solid state structure of 8 reveals a polymeric arrangement in which the calciate units are interlocked by bridging potassium ions. Metalation reactions between L(1)H or L(2)H and ((n)Bu)2Mg lead to the solvent-free compounds (L)2Mg [L = L(1) (10); L = L(2) (5)]. The bridged butyl mixed-metal complex [(L(1))Li(μ2-(n)Bu)Mg(L(1))]∞ (11) was also obtained via a cocomplexation reaction with (n)BuLi and ((n)Bu)2Mg. 11, which adopts a monodimensional polymeric array in the solid state, is a rare example of an alkyl-bridged Li/Mg complex and the first complex to feature an unsupported bridging butyl interaction between two metals. Changing the cocomplexation reaction conditions, the order of reagents added to the reactions mixture, and with the use of a coordinating solvent (tetrahydrofuran) formed the magnesiate complex (L(1))3MgLi(thf) (12).

Differing coordination environments in transition metal derivatives of 1,8-bis(silylamido)naphthalene ligands.

Four low-coordinate transition metal amido complexes featuring sterically demanding 1,8-bis(silylamido)naphthalene ligands are reported. Reaction of one molar equivalent of 1,8-C(10)H(6)(NLiSiMe(3))(2) with ZnCl(2) yields the structurally authenticated dimer [1,8-C(10)H(6)(NSiMe(3))(2)Zn](2) (1), where the 1,8-bis(silylamido)naphthalene moiety is acting as both a chelating and bridging ligand. The effect on the resulting transition metal complexes of increasing the steric demands of the ligand was investigated, using the triisopropylsilyl-substituted ligand 1,8-C(10)H(6)(NSi(i)Pr(3))(2). Reaction of one molar equivalent of 1,8-C(10)H(6)(NLiSi(i)Pr(3))(2) with ZnCl(2) or FeCl(2)(THF)(1.5) yields 1,8-C(10)H(6)(NSi(i)Pr(3))(2)M(mu-Cl)Li(THF)(3) (M = Zn, 2; M = Fe, 3), respectively; the coordination of the ClLi(THF)(3) moiety to the metal center in these compounds is a rare structural motif in the coordination chemistry of the d-block elements. Analogous reaction of 1,8-C(10)H(6)(NLiSi(i)Pr(3))(2) with MnCl(2) affords the mixed-metal Li-Mn-amido complex 1,8-C(10)H(6)(NSi(i)Pr(3))(2)Li(THF)MnCl(THF) (4) which features an unusual LiMnN(2) core.

Bridging the Gap between Coordination and Cluster Compounds: Unusual Bonding Modes for Zinc

Inner virtue: The reaction of [Mo(0)(GaCp*)(6)] with ZnMe(2) yields the novel multinuclear zinc system [MoZn(12)Me(9)Cp*(3)] which geometrically resembles a classical Wade-Mingos cluster (with an interstitial molybdenum atom, see picture). In reality, its electronic structure features little direct Zn-Zn bonding, relying instead on Mo-Zn three-center, two-electron bonds spanning the body diagonals of the approximately icosahedral framework.

Structural Diversity in Alkali Metal Complexes of Sterically Demanding Carbazol-9-yl Ligands

The solid state structures of alkali metal complexes of the 1,3,6,8-tetra-tert-butylcarbazol-9-yl ((t)Bu4carb(-)) ligand are compared. Lithium complex [(t)Bu4carbLi]2 ([1]2) is a dimer in the solid state featuring a planar LiNLiN rhomboid ring, with the differing Li-N distances within the ring due to the effects of σ- and π-interactions. Recrystallization of lithium, sodium, and potassium complexes of the 1,3,6,8-tetra-tert-butylcarbazol-9-yl ligand from THF leads to the formation of (t)Bu4carbLi(THF)2 (1·2THF), (t)Bu4carbNa(THF)3 (2·3THF), and (t)Bu4carbK(THF)4 (3·4THF), respectively, in the solid state. For these THF adducts, on proceeding from lithium to sodium to potassium there is an increase in hapticity of the binding of the carbazol-9-yl ligands to the metal cations, mirroring the increasing ionic bonding character in these compounds.

Migratory insertion of [B(C6F5)2] into C–H bonds: CO promoted transfer of the boryl fragment

The reaction of (eta(5)-C5H5)Fe(CO)2B(C6F5)2 with CO has been shown to proceed via ligand substitution at the metal with accompanying transfer of the boryl fragment (via C-H insertion) to the Cp ring, thereby generating the zwitterion [eta(5)-C5H4B(C6F5)2H]Fe(CO)3 in quantitative yield.

The stabilisation of organometallic complexes using m-terphenyl ligands

The use of bulky monodentate m-terphenyl ligands in the stabilisation of d-block organometallic compounds is surveyed. Importantly, these ligands have facilitated the isolation of hitherto unknown species containing low-coordinate centres and metal-metal multiple bonds. This review reports on these advances with emphasis on the synthesis, structural characterisation and, where possible, reactivity studies of complexes featuring metal-carbon bonds between m-terphenyl ligands and the transition metals.