Welcoming Neighbour or Inhospitable Host? Selective Second Metal Binding in 5- and 6-Phospha-Substituted Bpy Ligands.

The controlled formation of mixed-metal bimetallics was realised through use of a fac-[Re(CO)3(N,N'-bpy-P)Cl] complex bearing an exogenous 2,4,6-trioxa-1,3,5,7-tetramethyl-8-phosphaadamantane donor at the 5-position of the bpy. The introduction of gold, silver, and rhodium with appropriate secondary ligands was readily achieved from established starting materials. Restricted rotation about the C(bpy)-P bond was observed in several of the bimetallic complexes and correlated with the relative steric bulk of the second metal moiety. Related chemistry with the 6-substituted derivative proved more limited in scope with only the bimetallic Re/Au complex being isolated.

Remote control: stereoselective coordination of electron-deficient 2,2'-bipyridine ligands to Re(I) and Ir(III) cores.

Highly diastereoselective coordination of unsymmetrical cationic 2,2'-bipyridine ligands bearing a chiral amidinium substituent to [Re(CO)3Cl] and [Ir(PhPy)2]+ cores is reported. Binding strength and stereoselectivity have been correlated with the position of the amidinium group on the bipy. The 4-, 5- and 6-substituted ligands all produce C-[Re(CO)3(LH)Cl]X selectively, while only the 4-derivative gives preferred formation of Δ-[Ir(Phpy)2(4-LH)](BF4)2.

Reactions promoted by hypervalent iodine reagents and boron Lewis acids.

Understanding the role of boranes in hypervalent iodine chemistry will open up new reactivities which can be utilised in organic synthesis. Due to similar reactivities, λ3-iodanes have presented themselves as viable alternatives for many transformations dominated by transition metals whilst mitigating some of the associated drawbacks of metal systems. As showcased by recent reports, boranes can adopt a dual role in hypervalent iodine chemistry that surpasses mere activation of the hypervalent iodine reagent. Increased efforts to harness this potential with diverse boranes will uncover exciting reactivity with high applicability across various disciplines including adoption in the pharmaceutical sciences. This review will be relevant to the wider synthetic community including organic, inorganic, materials, and medicinal chemists due to the versatility of hypervalent iodine chemistry especially in combination with borane activation or participation. We aim to highlight the development of hypervalent iodine compounds including their structure, bonding, synthesis and utility in metal-free organic synthesis in combination with Lewis acidic boranes.

A review of quantum chemical studies of Frustrated Lewis Pairs.

This review endeavours to explore the power of quantum chemistry, especially density functional theory (DFT) to unravel mechanisms underlying the fascinating field of FLP chemistry. Apart from the fundamental mechanism of hydrogen activation by FLPs, borylation, silylation, polymerization, formation of heterocycles, reaction with small gaseous molecules, alkenes, terminal alkynes have also been reviewed. The role of DFT in understanding regio-selectivity, steric effects, London dispersion and covalent Interactions in FLPs are also discussed.

A hybrid bipy-NHC ligand for the construction of group 11 mixed-metal bimetallic complexes.

An asymmetric bipy/NHC ligand L has been used to construct Au/Au, Au/Ag and Au/Cu bimetallic complexes through prior coordination of the NHC to Au(i) and subsequent introduction of the second group 11 metal ion at the bipy donor of the hybrid ligand. The complex [Au(κC-L)2]BF4,1, has been used as the precursor for the formation of [AuAg(κ-C Au,κ2-N,N'Ag-1)2](BF4)2, 2a, [AuCu(κ-C Au,κ2-N,N'Cu-1)2](BF4)2, 2b and [AuAu'(κ-CAu/Au',κ1-NAu/Au'-1)2](BF4)2, 3.

Asymmetric ketone hydroboration catalyzed by alkali metal complexes derived from BINOL ligands.

The ability of alkali metal complexes featuring functionalized BINOL-derived ligands to catalyze ketone hydroboration reactions was explored. The reduced products were formed in excellent yields and with variable enantioselectivities dependent upon the nature of the ligand and the alkali metal cation.

Twisting the arm: structural constraints in bicyclic expanded-ring N-heterocyclic carbenes.

A series of diaryl, mono-aryl/alkyl and dialkyl mono- and bicyclic expanded-ring N-heterocyclic carbenes (ER-NHCs) have been prepared and their complexation to Au(i) investigated through the structural analysis of fifteen Au(NHC)X and/or [Au(NHC)2]X complexes. The substituted diaryl 7-NHCs are the most sterically encumbered with large buried volume (%VB) values of 40-50% with the less flexible six-membered analogues having %VB values at least 5% smaller. Although the bicyclic systems containing fused 6- and 7-membered rings (6,7-NHCs) are constrained with relatively acute NCN bond angles, they have the largest %VB values of the dialkyl derivatives reported here, a feature related to the fixed conformation of the heterocyclic rings and the compressional effect of a pre-set methyl substituent.

Halide and substituent dependent structural variation in copper(i) halide complexes of 1,5,9-triphosphacyclododecanes.

The reactions of 1,5,9-triethyl-1,5,9-triphosphacyclododecane, [12]-ane-P3Et3, and 1,5,9-tri(2-propyl)-1,5,9-triphosphacyclododecane, [12]-ane-P3iPr3 with copper(i)halides produce either bimetallic species of the type [([12]-ane-P3R3)Cu(CuX2)] (X = halide) or monomeric [([12]-ane-P3R3)CuX] depending on the nature of the halide and, to a lesser extent, the macrocycle. With CuCl only bimetallic complexes are formed with one copper centre bound to the macrocycle and a second attached through a Cu-Cu bond with a mono bridging chloride. CuBr affords monomeric [([12]-ane-P3R3)CuBr] complexes when performed in a 1 : 1 M : L ratio whereas the bimetallic compound [([12]-ane-P3Et3)(CuBr)2], resulted when a 2 : 1 ratio of M : L was employed. With CuI in all ratios only monomeric complexes were obtained. The synthesised complexes have been fully characterised by spectroscopic and analytical techniques and by determination of the molecular structures by single-crystal X-ray diffractometry.

Synthesis and photophysical properties of imine borane adducts towards vapochromic materials.

A series of alkynyl aryl conjugated aldehydes and imines were prepared and their adducts with various Lewis acidic boranes have been studied via NMR, absorption, and luminescence spectroscopies in solution. The imine-B(C6F5)3 adduct showed remarkable solution stability, and was then trialled in vapochromism experiments using simple impregnated paper strips to examine the fluorescence responses.

Asymmetric Cationic Phosphines: Synthesis, Coordination Chemistry, and Reactivity.

A single enantiomer of a cationic phosphine, [α-CgPAmHMe]BF4, containing two asymmetric subunits, an amidinium group (AmH) and a phosphacycle (CgP), has been synthesized and isolated. The ligand, which is of an extremely rare class, has been coordinated to Rh(I), Au(I), Ag(I), Cu(I), and Pt(0) to enable an empirical assessment of its donor properties. Analysis of the IR stretching frequency of the carbonyl ligand in trans-[Rh(α-CgPAmHMe)2(CO)Cl](BF4)2 coupled with metric data obtained from crystal-state molecular structures of pertinent complexes confirms the strong π-accepting properties of the ligand. The integrity of the N-P bond is compromised upon addition of base to both [Cu(α-CgPAmHMe)Cl]BF4 and [Ag(α-CgPAmHMe)(OTf)]BF4 where, instead of isolating anticipated chelating and/or bridging forms of the neutral, deprotonated α-CgPAmMe derivative, decomposition products were obtained containing a phosphacycle fragment and/or amidine ligands. The fragility of the N-P bond is also evident in uncoordinated [α-CgPAmHMe]BF4 as treatment with aqueous base releases a neutral amidine fragment and generates the P-P dimer [α,α-CgPP(O)Cg]. These fortuitous observations show [α-CgPAmHMe]BF4 to be a very useful synthon for the potential production of novel asymmetric phosphines.

Synthesis and characterisation of fluorescent aminophosphines and their coordination to gold(i).

Three novel fluorescent aminophosphine ligands have been synthesised that incorporate napthyl (L1), pyrenyl (L2) and anthraquinone (L3) chromophores into their structures. The ligands react with [AuCl(tht)] (tht = tetrahydrothiophene) to give neutral complexes of the form [AuCl(L1-3)]. Solid state, X-ray crystallographic data was obtained for the anthraquinone derivative, [AuCl(L3)], and showed a distorted linear coordination geometry at Au(i). The packing structure also revealed a number of intermolecular π-π interactions that involve the anthraquinone and phenyl units of the aminophosphine ligand. 31P NMR spectroscopic data revealed δP values of +42.2 (L1), +42.1 (L2) and +26.1 (L3) ppm, which shifted downfield upon coordination to Au(i) to +64.6, +64.7, and +55.8 ppm, respectively. Supporting TD-DFT studies were able to reproduce the structure and 31P NMR chemical shifts of [AuCl(L3)] as well as rationalise the HOMO-LUMO compositions. Photophysical studies showed that the appended fluorophore dominates the absorption and emission properties for the ligands and complexes, with the anthraquinone derivatives showing visible emission at ca. 570 nm which was attributed to the intramolecular charge transfer character of the phosphinoaminoanthraquinone fragment.

Ring-Expanded N-Heterocyclic Carbenes for Copper-Mediated Azide-Alkyne Click Cycloaddition Reactions.

A series of well-defined copper(I) complexes bearing ring-expanded N-heterocyclic carbene (NHC) ligands has been applied to the azide-alkyne cycloaddition reaction. The obtained results notably showed that the six-membered NHC ligands outperform well-established five-membered ones. [CuI(Mes-6)] displayed a remarkable catalytic activity while respecting the strict criteria for click reactions.

Push and pull: the potential role of boron in N2 activation.

Recent developments in main group chemistry towards the activation and conversion of N2 have lead to the revelation that boron can greatly affect these processes. Boron is capable of acting both as a borane Lewis acid to activate metal-N2 complexes and as an ambiphilic borylene able to activate free N2. The latter example is capable of both accepting and donating electron density in a manner reminiscent of transition metal systems containing both filled and empty d-orbitals.

Amidine functionalized phosphines: tuneable ligands for transition metals.

Attachment of racemic 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphatricyclo[3.3.1.13,7]decane (α,ß-CgP) to (1R,5S)-1,8,8-trimethyl-2,4-diazabicyclo[3.2.1]oct-2-ene gave a diastereomeric mixture of a novel amidine-phosphine ligand, α,ß-CgPAm. The phosphination was completely selective for the 4-position of the bicyclic amidine and there was no subsequent 1,2-migration of the α,ß-CgP group. Methylation of the non-phosphinated nitrogen gave the amidinium salt [α,ß-CgPAmMe]BF4 as a diastereomeric mixture. The donating ability of α,ß-CgPAm and [α,ß-CgPAmMe]+ has been assessed through the synthesis and characterization of appropriate Rh(i), Au(i) and Pt(ii) complexes. As expected α,ß-CgPAm is a better net donor than the cationic derivative as shown by the magnitude of the νCO stretches in the IR spectra of the [Rh(L)(CO)(acac)]0/+ complexes and through determination of the relative energies of the HOMO and LUMO orbitals for both ligands by DFT. Attempts to resolve [Au(α,ß-CgPAmMe)Cl]BF4, [Pt(α,ß-CgPAmMe)Cl3], [Rh(α,ß-CgPAmMe)(acac)(CO)]BF4 and [Rh(α,ß-CgPAmH)(acac)(CO)]BF4 by fractional crystallization were unsuccessful as diastereomeric mixtures were obtained in every case; the structures of the last three complexes have been determined by single-crystal X-ray techniques and compared with related literature complexes.

Mono- and dimeric complexes of an asymmetric heterotopic P,CNHC,pyr ligand.

An asymmetric heterotopic ligand (S-N(Me)CP) containing a central bicyclic, expanded-ring NHC with one pyridyl and one phosphine exo-substituent has been synthesised and its coordination chemistry with selected late transition metals investigated. The amidinium precursor [S-N(Me)CHP]PF6 shows variable coordination modes with Ag(i), Cu(i) and Au(i) depending on the L : M ratio. The reaction of two mols of [S-N(Me)CHP]PF6 with [Cu(MeCN)4]BF4, AgBF4 or Au(THT)Cl gives the bis-ligand complexes [Cu(κ-P-N(Me)CHP)2(CH3CN)2]BF4·(PF6)2, 1, and [M(κ-P-N(Me)CHP)2]X·(PF6)2 (3: M = Ag, X = BF4; 6: M = Au, X = Cl) respectively. The 1 : 1 reaction of [S-N(Me)CHP]PF6 with AgOTf gave the head-to-tail dimer H,T-[Ag2(µ-N,P-N(Me)CHP)2(µ-OTf)2](PF6)2, 2, whereas the analogous reaction with Au(THT)Cl gave monomeric [Au(κ-P-N(Me)CHP)Cl]PF6, 5. Complex 2 was converted to H,T-[Ag2(µ-C,P-N(Me)CP)2](PF6)2, 4, upon addition of base, while 6 gave [Au(κ-C-N(Me)CP)2]Cl, 8, when treated likewise. Reaction of [S-N(Me)CHP]PF6 with Ni(1,5-COD)2 gave the oxidative addition/insertion product [Ni(κ(3)-N,C,P-N(Me)CP)(η(3)-C8H13)]PF6, 9, which converted to [Ni(κ(3)-N,C,P-N(Me)CP)Cl]PF6, 10, upon exposure of a CHCl3 solution to air. Complex 10 showed conformational isomerism that was also present in [Rh(κ(3)-N,C,P-N(Me)CP)(CO)]PF6, 14, prepared from the precursor complex [Rh(κ-P-N(Me)CHP)(acac)(CO)]PF6, 13, upon heating in C6H5Cl. [Pt(κ(3)-N,C,P-N(Me)CP)(Cl)]PF6, 12, derived from trans-[Pt(κ-P-N(Me)CHP)2(Cl)2](PF6)2, 11, was isolated as a single conformer.

Chiral carbene-borane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenations.

The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piers' borane 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-substituents (R = Me 9, Et 10, i-Pr 11) and the corresponding borenium cations 12-14 were also prepared. These cations were not active as catalysts in hydrogenation, although 9-11 were shown to undergo carbene ring expansion reactions at 50 °C to give species 15-17. The IBOX-carbene precursors 18 and 19 derived from amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts 20-23. Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr) 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-ylidene (IPhMe) and 1-tert-butyl-3-methylimidazol-2-ylidene (ItBuMe) with diisopinocampheylborane (Ipc2BH) gave chiral adducts: (IMe)(Ipc2BH) 24, (IPr)(Ipc2BH) 25, (IBnMe)(Ipc2BH) 26, (IPhMe)(Ipc2BH) 27, and (ItBuMe)(Ipc2BH) 28. Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-1H-1,2,3-triazolium, rac-29 and the 9-BBN derivative of (S)-2-amino-2'-methoxy-1,1'-binaphthalene-1,2,3-triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered.

N2S2 and N4S4 precursors to PS2 macrocycles and cyclic amidinium salts.

The cyclo-condensation of 1R,2R-diaminocyclohexane with 2,2'-(ethane-1,2-diyldisulfanediyl)dibenzaldehyde gave the 1 : 1 addition compound chxn-(im)N2S2 in high yield. When the same condensation reaction was performed with 1R,3S-diamino-1,7,7-trimethylcyclopentane as the diamine, the 2 : 2 addition compound tmcp-(im)N4S4 was obtained selectively. Reduction of the diimines gave the saturated analogues chxn-N2S2 (1) and tmcp-N4S4 (3) the former of which could be phosphorylated with PhP(NMe2)2 to give the novel 13-membered macrocycle chxn-PS2, 2. Introduction of the phenylphosphine function proved stereoselective with a preference for the N(R)/N(S) configuration at the nitrogen atoms. The coordination chemistry of the novel phosphine has been explored with Cu(i) and Mo(0) through formation of the complexes Cu(2)I, 4, and Mo(CO)3(2), 5. Extension of the phosphorylation chemistry to tmcp-N4S4 (3) proved unsuccessful but ring closure reactions of both 1 and 3 with triethylorthoformate gave cyclic amidinium salts which are potential precursors to macrocyclic N-heterocyclic carbenes.

Aspects of the coordination chemistry of rac-trans-1,2-diphosphinocyclohexane and the preparation of reinforced 9aneP3 and 9anePN2 macrocycles.

Racemic trans-1,2-diphosphinocyclohexane (t-chxnP2) has been synthesised and its coordination chemistry to Cu(I), Ag(I), Mn(I) and Fe(II) investigated. Compounds of empirical formula [Cu(t-chxnP2)2]BF4 and [Ag(t-chxnP2)2]BF4 have been prepared as isomeric mixtures and the solid-state structure of both complexes determined by single-crystal techniques. The Cu(I) complex is a monomeric species which crystallises with one ligand bearing the R,R configuration and the other being the S,S isomer whereas the Ag(I) complex crystallises as a polymer containing both chelating and bridging t-chxnP2 ligands with 3- and 4-coordinate Ag(I) centres and argentophilic bonds. The bidentate diprimary phosphine has been coordinated to Mn(I) and Fe(II) templates and used as a P2 unit for the formation of chiral (albeit racemic) 1,4,7-triphosphamacrocycles (9aneP3). In addition 1R,2R- and 1R,2S-diaminocyclohexane (chxn) have been coordinated to Mn(I) and Fe(II) templates and similarly employed for the formation of new 1,4-diamino-7-phosphamacrocycle (9anePN2) complexes.

Manganese(I) templates for the construction of benzannulated triphosphamacrocycles.

Nine-membered 1,4,7-triphosphamacrocycles with unsaturated benzo-backbones have been prepared using the [(CO)3Mn](+) unit as a template. Two synthetic methods have been employed for the macrocyclisation both of which involve the attack of a coordinated phosphide at an activated, electrophilic ortho-fluorophenyl substituent on a neighbouring pnictide donor. Addition of base to the precursor complex fac-[(CO)3Mn(dfppb)(PhPH2)](+), 1, where dfppb = 1,2-bis[di(2-fluorophenyl)phosphino]benzene, results in the direct formation of the macrocyclic compound fac-[Mn(CO)3(tribenzo-9aneP3-Ph,Ph(F)2)](+), 3. A second precursor, namely fac-[(CO)3Mn(1,2-bpb){P(Ph(F))3}](+), 5, where 1,2-bpb = 1,2-bis(phosphino)benzene, undergoes spontaneous partial macrocyclisation when dissolved in THF to give the intermediate complex fac-[(CO)3Mn{H2PC6H4P(H)C6H4P(Ph(F))2}](+), 6, which contains a linear tridentate phosphine with the unusual combination of a primary, secondary and tertiary phosphine donor. Addition of base to 6 gives the desired macrocyclic complex fac-[Mn(CO)3(tribenzo-9aneP3-H2,Ph(F))](+), 7, which is converted in situ to the more stable dimethylated fac-[Mn(CO)3(tribenzo-9aneP3-Me2,Ph(F))](+), 8. The new complexes have been fully characterised by spectroscopic and analytical methods including single crystal X-ray structure determinations for 1, 3, 5, 6 and 8.

It's all about Me: methyl-induced control of coordination stereochemistry by a flexible tridentate N,C,N' ligand.

A chiral, tridentate, pyridyl-functionalised NHC pro-ligand, S-L(Me)-H[PF6], has been prepared diastereoselectively via a five step synthesis starting from 1R,3S-diamino-1,2,2-trimethylcyclopentane. The S prefix refers to the stereochemistry of a methyl substituted stereogenic carbon in one of the pyridyl arms which is generated by a stereoselective BH4(-) reduction of an imine precursor. The ligand has been coordinated to Rh(I) and Ir(I) to give trigonal bipyramidal complexes of the type [M(κ(3)-N,C,N'-S-L(Me))(1,5-COD)]PF6 (M = Rh, Ir) as single diastereomers. A combination of spectroscopic and X-ray techniques confirm the stereoselective formation of the thermodynamically preferred endo,endo isomer. Similar reactions with R,S-L(Me)-H[PF6] gave a mixture of endo,endo-[M(κ(3)-N,C,N'-S-L(Me))(1,5-COD)](+) and exo,exo-[M(κ(3)-N,C,N'-R-L(Me))(1,5-COD)](+). The absolute configuration at the metal is, therefore, solely dictated by the stereochemistry of the single methylpyridyl carbon. The observation of stereoselection extends to the square planar Ni(II) complex [Ni(δ-κ(3)-N,C,N'-S-L(Me))Cl](+) which is isolated as one (δ) of the two possible conformational isomers. DFT studies have been employed to explain the observed stereoselectivity with the configurations observed in the solid state being confirmed as those of lowest energy.