Oxidative Addition and ß-Hydride Elimination by a Macrocyclic Dinickel Complex: Observing Bimetallic Elementary Reactions.

The dinickel(I) complex Ni2 (tBu PONNOPONNO), featuring a planar macrocyclic diphosphoranide ligand tBu PONNOPONNO, offers a unique architectural platform for observing bimetallic elementary reactions. Oxidative addition reactions of alkyl halides produce dinickel(II) complexes of the type Ni2 (µ-R)(µ-X)(tBu PONNOPONNO). However, when R=Et ß-hydride elimination is observed to form a dinickel monohydride, with the rate dependent on the nature of X. DFT studies suggest a new mechanism for bimetallic ß-hydride elimination, where the rate dependence arises from the steric pressure imposed by the X group on the opposing trans face of the dinickel macrocycle. This work enhances understanding of bimetallic elementary reactions, particularly ß-hydride elimination, which have not been well-explored for dinuclear systems.

Bimetallic Nickel Complexes Supported by a Planar Macrocyclic Diphosphoranide Ligand.

Metal-metal cooperativity is emerging as an important strategy in catalysis. This requires appropriate ligand scaffolds that can support two metals in close proximity. Here we report nickel-promoted formation of a dinucleating planar macrocyclic ligand that can support bimetallic dinickel(II) and dinickel(I) complexes. Reaction outcomes can be tuned by variation of the substituents and reaction conditions to favour dinucleating macrocyclic, mononucleating macrocyclic or conventional pincer architectures.

Highly Adaptive Nature of Group 15 Tris(quinolyl) Ligands─Studies with Coinage Metals.

The substitution of heavier, more metallic atoms into classical organic ligand frameworks provides an important strategy for tuning ligand properties, such as ligand bite and donor character, and is the basis for the emerging area of main-group supramolecular chemistry. In this paper, we explore two new ligands [E(2-Me-8-qy)3] [E = Sb (1), Bi (2); qy = quinolyl], allowing a fundamental comparison of their coordination behavior with classical tris(2-pyridyl) ligands of the type [E'(2-py)3] (E = a range of bridgehead atoms and groups, py = pyridyl). A range of new coordination modes to Cu+, Ag+, and Au+ is seen for 1 and 2, in the absence of steric constraints at the bridgehead and with their more remote N-donor atoms. A particular feature is the adaptive nature of these new ligands, with the ability to adjust coordination mode in response to the hard-soft character of coordinated metal ions, influenced also by the character of the bridgehead atom (Sb or Bi). These features can be seen in a comparison between [Cu2{Sb(2-Me-8-qy)3}2](PF6)2 (1·CuPF6) and [Cu{Bi(2-Me-8-qy)3}](PF6) (2·CuPF6), the first containing a dimeric cation in which 1 adopts an unprecedented intramolecular N,N,Sb-coordination mode while in the second, 2 adopts an unusual N,N,(π-)C coordination mode. In contrast, the previously reported analogous ligands [E(6-Me-2-py)3] (E = Sb, Bi; 2-py = 2-pyridyl) show a tris-chelating mode in their complexes with CuPF6, which is typical for the extensive tris(2-pyridyl) family with a range of metals. The greater polarity of the Bi-C bond in 2 results in ligand transfer reactions with Au(I). Although this reactivity is not in itself unusual, the characterization of several products by single-crystal X-ray diffraction provides snapshots of the ligand transfer reaction involved, with one of the products (the bimetallic complex [(BiCl){ClAu2(2-Me-8-qy)3}] (8)) containing a Au2Bi core in which the shortest Au → Bi donor-acceptor bond to date is observed.

Cooperative approaches in catalytic hydrogenation and dehydrogenation.

Metal-ligand cooperativity (MLC) is an established strategy for developing effective hydrogenation and dehydrogenation catalysts. Metal-metal cooperativity (MMC) in bimetallic complexes is not as well understood, and to date has had limited implementation in (de)hydrogenation. Herein we use (de)hydrogenation processes as a platform to examine modes of cooperativity, with a particular focus on catalytic mechanisms. We investigate how lessons learnt from the extensive development of metal-ligand cooperative catalysts can aid the ongoing development of metal-metal cooperative catalysts.

Copper and Zinc Complexes of 2,7-Bis(6-methyl-2-pyridyl)-1,8-naphthyridine─A Redox-Active, Dinucleating Bis(bipyridine) Ligand.

The ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (MeL) acts as a dinucleating analogue of ubiquitous 2,2'-bipyridine ligands. Coordination of MeL to [Cu(NCMe)4]PF6 and Zn(OAc)2 led to isolation of monometallic [Zn(OAc)2(MeL)], homobimetallic [Cu2(MeL)2][PF6]2, and heterobimetallic [CuZn(µ-OAc)2(MeL)]PF6 complexes. The redox-active nature of the ligand enables access to four redox states of the complex [Cu2(MeL)2][PF6]2. DFT studies indicate that these comprise a metal-centered oxidative and ligand-centered reductive processes.

Pnictogen-Functionalised C1 Ligands: MC-ARn (n=0, 1, 2, 3).

The chemistry of transition metal carbynes, Ln M≡CR, has historically been dominated by species bearing hydrocarbyl or amino 'R' substituents, with other elements appearing only sporadically. In recent years, carbynes and related 'C1 ' species bearing other main-group substituents, particularly heavier elements of the p-block, have begun to emerge. This review details the chemistry of heavier pnictogen-functionalised C1 ligands, MCARn (A=P, As, Sb, Bi; n=0-3), including their syntheses, properties and reactivities, and how these are distinguished from more traditional carbyne complexes. Recent developments in the closely related phospha-isonitrile Ln M(CPR), cya-phosphide and cya-arside ligands, Ln M(C≡A) (A=P, As), are also discussed.

Amine-Borane Dehydropolymerization: Challenges and Opportunities.

The dehydropolymerization of amine-boranes, exemplified as H2 RB⋅NR'H2 , to produce polyaminoboranes (HRBNR'H)n that are inorganic analogues of polyolefins with alternating main-chain B-N units, is an area with significant potential, stemming from both fundamental (mechanism, catalyst development, main-group hetero-cross-coupling) and technological (new polymeric materials) opportunities. This Concept article outlines recent advances in the field, covering catalyst development and performance, current mechanistic models, and alternative non-catalytic routes for polymer production. The substrate scope, polymer properties and applications of these exciting materials are also outlined. Challenges and opportunities in the field are suggested, as a way of providing focus for future investigations.

A Tris(3-pyridyl)stannane as a Building Block for Heterobimetallic Coordination Polymers and Supramolecular Cages.

The systematic assembly of supramolecular arrangements is a persistent challenge in modern coordination chemistry, especially where further aspects of complexity are concerned, as in the case of large molecular mixed-metal arrangements. One targeted approach to such heterometallic complexes is to engineer metal-based donor ligands of the correct geometry to build 3D arrangements upon coordination to other metals. This simple idea has, however, only rarely been applied to main group metal-based ligand systems. Here, we show that the new, bench-stable tris(3-pyridyl)stannane ligand PhSn(3-Py)3 (3-Py=3-pyridyl) provides simple access to a range of heterometallic SnIV /transition metal complexes, and that the presence of weakly coordinating counter anions can be used to build discrete molecular arrangements involving anion encapsulation. This work therefore provides a building strategy in this area, which parallels that of supramolecular transition metal chemistry.

Dehydropolymerization of H3B·NMeH2 To Form Polyaminoboranes Using [Rh(Xantphos-alkyl)] Catalysts.

A systematic study of the catalyst structure and overall charge for the dehydropolymerization of H3B·NMeH2 to form N-methyl polyaminoborane is reported using catalysts based upon neutral and cationic {Rh(Xantphos-R)} fragments in which PR2 groups are selected from Et, iPr, and tBu. The most efficient systems are based upon {Rh(Xantphos-iPr)}, i.e., [Rh(κ3-P,O,P-Xantphos-iPr)(H)2(η1-H3B·NMe3)][BArF4], 6, and Rh(κ3-P,O,P-Xantphos-iPr)H, 11. While H2 evolution kinetics show both are fast catalysts (ToF ≈ 1500 h-1) and polymer growth kinetics for dehydropolymerization suggest a classical chain growth process for both, neutral 11 (Mn = 28 000 g mol-1, D = 1.9) promotes significantly higher degrees of polymerization than cationic 6 (Mn = 9000 g mol-1, D = 2.9). For 6 isotopic labeling studies suggest a rate-determining NH activation, while speciation studies, coupled with DFT calculations, show the formation of a dimetalloborylene [{Rh(κ3-P,O,P-Xantphos-iPr)}2B]+ as the, likely dormant, end product of catalysis. A dual mechanism is proposed for dehydropolymerization in which neutral hydrides (formed by hydride transfer in cationic 6 to form a boronium coproduct) are the active catalysts for dehydrogenation to form aminoborane. Contemporaneous chain-growth polymer propagation is suggested to occur on a separate metal center via head-to-tail end chain B-N bond formation of the aminoborane monomer, templated by an aminoborohydride motif on the metal.

A General, Rhodium-Catalyzed, Synthesis of Deuterated Boranes and N-Methyl Polyaminoboranes.

The rhodium complex [Rh(Ph2 PCH2 CH2 CH2 PPh2 )(η6 -FC6 H5 )][BArF4 ], 2, catalyzes BH/BD exchange between D2 and the boranes H3 B⋅NMe3 , H3 B⋅SMe2 and HBpin, facilitating the expedient isolation of a variety of deuterated analogues in high isotopic purities, and in particular the isotopologues of N-methylamine-borane: R3 B⋅NMeR2 1-dx (R=H, D; x=0, 2, 3 or 5). It also acts to catalyze the dehydropolymerization of 1-dx to give deuterated polyaminoboranes. Mechanistic studies suggest a metal-based polymerization involving an unusual hybrid coordination insertion chain-growth/step-growth mechanism.

Iron Precatalysts with Bulky Tri(tert-butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine-Borane.

In an attempt to prepare new Fe catalysts for the dehydrocoupling of amine-boranes and to provide mechanistic insight, the paramagnetic FeII dimeric complex [Cp'FeI]2 (1) (Cp'=η5 -((1,2,4-tBu)3 C5 H2 )) was used as a precursor to a series of cyclopentadienyl FeII and FeIII mononuclear species. The complexes prepared were [Cp'Fe(η6 -Tol)][Cp'FeI2 ] (2) (Tol=C6 H5 Me), [Cp'Fe(η6 -Tol)][BArF4 ] (3) (BArF4 =[B(C6 H3 (m-CF3 )2 )4 ]- ), [N(nBu)4 ][Cp'FeI2 ] (4), Cp'FeI2 (5), and [Cp'Fe(MeCN)3 ][BArF4 ] (6). The electronic structure of the [Cp'FeI2 ]- anion in 2 and 4 was investigated by SQUID magnetometry, EPR spectroscopy and ab initio Complete Active Space Self Consistent Field-Spin Orbit (CASSCF-SO) calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 1-6 were investigated as catalysts for the dehydrocoupling of Me2 NH⋅BH3 (I) in THF at 20 °C to yield the cyclodiborazane product [Me2 N-BH2 ]2 (IV). Complexes 1-4 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra-violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 1-4 proceeded via formation of the aminoborane Me2 N=BH2 (II) as the major intermediate, whereas for 6 the linear diborazane Me2 NH-BH2 -NMe2 -BH3 (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me3 N⋅BH3 were investigated in an attempt to identify Fe-based intermediates in the catalytic reactions. The σ-complex [Cp'Fe(MeCN)(κ2 -H2 BH⋅NMe2 H][BArF4 ] was proposed to initially form in dehydrocoupling reactions involving 6 based on ESI-MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR spectroscopic evidence. The latter also suggests that these complexes function as precursors to iron hydrides which may be the true catalytic species.

How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands.

Although a multitude of studies have explored the coordination chemistry of classical tripodal ligands containing a range of main-group bridgehead atoms or groups, it is not clear how periodic trends affect ligand character and reactivity within a single ligand family. A case in point is the extensive family of neutral tris-2-pyridyl ligands E(2-py)3 (E=C-R, N, P), which are closely related to archetypal tris-pyrazolyl borates. With the 6-methyl substituted ligands E(6-Me-2-py)3 (E=As, Sb, Bi) in hand, the effects of bridgehead modification alone on descending a single group in the periodic table were assessed. The primary influence on coordination behaviour is the increasing Lewis acidity (electropositivity) of the bridgehead atom as Group 15 is descended, which not only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour.

Beyond Dehydrocoupling: Group†2†Mediated Boron-Nitrogen Desilacoupling.

The alkaline-earth element bis(trimethylsilyl)amides, [Ae{N(SiMe3 )2 }2 (thf)2 ] [Ae=Mg, Ca, Sr], are effective precatalysts for boron-nitrogen bond formation through the desilacoupling of amines, RR'NH (R=alkyl, aryl; R'=H, alkyl, aryl), and pinBSiMe2 Ph. This reactivity also yields a stoichiometric quantity of Me2 PhSiH and provides the first example of a catalytic main-group element-element coupling that is not dependent on the concurrent elimination of H2 .

Secondary phosphinocarbyne and phosphaisonitrile complexes.

The palladium-mediated reaction of [W(≡CBr)(CO)2(Tp*)] (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) with primary phosphines PH2R (R = Ph, Cy) affords the secondary phosphinocarbyne complexes [W(≡CPHR)(CO)2(Tp*)], deprotonation of which provides the anionic phosphaisonitrile complexes [W(CPR)(CO)2(Tp*)](-), including the structurally characterized salt [W(CPPh)(CO)2(Tp*)][K(kryptofix)].

A one-pot reduction route to bimetallic manganese 1,8-naphthyridine complexes.

Reaction of the dinucleating ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (MeL) with the MnI and MnII precursors MnBr(CO)5 and MnCl2 resulted in the formation of the monometallic complexes [MnBr(CO)3(MeL)] (1) and [MnCl2(MeL)] (3). In both cases, formation of bimetallic manganese complexes could be achieved by reduction with KC8, yielding the carbonyl-bridged complex [Mn2(CO)6(MeL)] (2) and the helicate complex [Mn2(MeL)2] (4), respectively. EPR results demonstrate that 4 represents a novel, weakly antiferromagnetically coupled homovalent dimer (J = -0.85 cm-1). The two formally Mn0 ions are both high spin (S = 3/2) and exhibit a zero-field splitting of ≈1 cm-1, suggesting reduction of the complex is substantially ligand centered, and may be better described as a MnII complex coupled to two open shell singlet ligands [MnII2(MeL2-)2]. X-ray crystallography, UV-Vis spectroscopy and DFT analysis support this finding.

Utilising a Proton-Responsive 1,8-Naphthyridine Ligand for the Synthesis of Bimetallic Palladium and Platinum Complexes.

We present four proton-responsive palladium and platinum complexes, [MCl2 (R PONNHO)] (M=Pd, Pt; R=i Pr, t Bu) synthesised by complexation of PdCl2 or PtCl2 (COD) with the 1,8-naphthyridine ligand R PONNHO. Deprotonation of [MCl2 (tBu PONNHO)] switches ligand coordination from mono- to dinucleating, offering a synthetic pathway to bimetallic PdII and PtII complexes [M2 Cl2 (tBu PONNO)2 ]. Two-electron reduction gives planar MI -MI complexes [M2 (tBu PONNO)2 ] (M=Pd, Pt) containing a metal-metal bond. In contrast to the related nickel system that forms a metallophosphorane [Ni2 (tBu PONNOPONNO)], an unusual phosphinite binding mode is observed in [M2 (tBu PONNO)2 ] containing close phosphinite-naphthyridinone P⋅⋅⋅O interactions, which is investigated spectroscopically, crystallographically and computationally. The presented proton-responsive and structurally-responsive R PONNHO and bimetallic R PONNO complexes offer a novel platform for future explorations of metal-ligand and metal-metal cooperativity with palladium and platinum.

Synthesis of an expanded pincer ligand and its bimetallic coinage metal complexes.

An expanded pincer ligand tBu-PONNOP (2,7-bis(di-tert-butylphosphinito)-1,8-naphthyridine) has been synthesised and its coordination to coinage metals has been studied. Bimetallic complexes were produced with metal halide salts of the type [M2X2(tBu-PONNOP)] (X = Cl, M = Au, Ag, Cu; X = I, M = Cu) with a varying degree of interaction with the naphthyridyl backbone in the order Au < Ag < Cu. The salts [Ag2(tBu-PONNOP)2][BArF4]2 (ArF = 3,5-C6H3(CF3)2) and [Ag2(NCMe)2(tBu-PONNOP)]X2 (X = BArF4, PF6) were prepared, which may serve as a source of tBu-PONNOP via transmetallation.

Synthesis of tris(3-pyridyl)aluminate ligand and its unexpected stability against hydrolysis: revealing cooperativity effects in heterobimetallic pyridyl aluminates.

We report the elusive metallic anion [EtAl(3-py)3]- (3-py = 3-pyridyl) (1), the first member of the anionic tris(3-pyridyl) family. Unexpectedly, the lithium complex 1Li shows substantial protic stability against water and alcohols, unlike related tris(2-pyridyl)aluminate analogues. This stability appears to be related to the inability of the [EtAl(3-py)3]- anion to chelate Li+, which precludes a decomposition pathway involving Li/Al cooperativity.

Synthesis and coordination behaviour of aluminate-based quinolyl ligands.

The effects of moving the donor N-atom from the 2-position in lithium (2-pyridyl)- and (2-quinolyl)aluminates to the more remote position in (8-quinolyl)aluminates have been investigated by solid-state structural and DFT computational studies of the new complexes [{EtAl(2-qy)3}Li(µ-X)Li(THF)3] (X = Cl/Br 62 : 38) [(1)Li(µ-X)Li(THF)3], [{(EtAl(2-qy)3)Li}2(µ-Br)]-Li(THF)4+ [{1Li}2(µ-Br)]-Li(THF)4+, [{EtAl(2-Me-8-qy)3}Li] [(2)Li], [{Me2Al(2-Me-8-qy)2}Li(THF)] [(3a)Li(THF)], [{Me2Al(6-Me-2-py)2}Li(THF)2] [(4)Li(THF)2] and [{{EtAl(2-Me-8-qy)2}2O}(Li2THF)] (5). Increasing the remoteness of the donor N-atom from the bridgehead results in large differences in the coordination of the Li+ cations by the (8-quinolyl)aluminate anions compared to 2-quinolyl or 2-pyridyl counterparts. The results are of potential interest in understanding how the coordination sites of ligands of this type can be tuned for the coordination requirements of specific metal centres.

An experimental and theoretical study of the coordination and donor properties of tris-2-pyridyl-phosphine ligands.

The coordination characteristics and donor/acceptor properties of a series of 2-pyridyl substituted phosphine ligands have been investigated using structural, spectroscopic and DFT calculational studies. A range of different coordination modes are observed in Mo and W carbonyl complexes of tris-2-pyridyl-phosphine ligands of the type P(2-py') (2-py' = substituted or unsubstituted 2-pyridyl group), including an unprecedented example exhibiting N,N',µ2-π coordination. DFT calculations were used to assess the relative donor/acceptor properties of a range of related 2-pyridyl-phosphine ligands with respect to PPh3 and PtBu3.