Scorpionate Ligand Research Articles

Scorpionate Complexes as Models for Molybdenum Enzymes

Scorpionate ligands have supported the development of a number of important synthetic models for pterin‐containing molybdenum enzymes. These ligands stabilize biologically relevant mononuclear oxido‐ and sulfido‐Mo(VI,V,IV) complexes capable of sustaining a wide range of biomimetic reactions, including oxygen atom, sulfur atom and coupled electron‐proton transfer reactions. Studies of scorpionate‐Mo compounds have also provided key insights into the bonding in oxido‐Mo dithiolene moieties, the orbital control of enzymatic reactions and the synthesis and behavior of biologically relevant Mo‐pterindithiolene complexes. This microreview covers advances in the synthesis and study of scorpionate complexes as models for molybdenum enzymes in the second half of the field's 30‐year history, i.e., from around 2000, the turn of the century, onwards.

Pyridylborates as a New Type of Robust Scorpionate Ligand: From Metal Complexes to Polymeric Materials

The importance of scorpionate ligands in modern coordination chemistry continues to increase, because of their outstanding versatility, tunability and user‐friendliness. Herein, we provide a short overview of recent developments in the classes of scorpionate ligands, derived from pyrazoles, triazoles, imidazoles, oxazolines, thioimidazoles and other similar systems, followed by an in‐depth discussion of a new type of robust and tunable scorpionate ligand, tris(2‐pyridyl)borates. The structure and synthesis of tris(2‐pyridyl)borate (Tpyb) ligands are discussed, and key features of coordination of Tpyb ligands with metal ions, as well as supramolecular crystal packing, transmetallation and applications in metallo‐supramolecular polymer chemistry are addressed.

Variable Borohydride Hapticity in Nickel(II) Scorpionate Complexes [(TpR,Me)Ni(ηn‐BH4)]: TpR,Me = hydrotris{3‐R‐5‐methyl‐1‐pyrazolyl}borate; R = Ph, n = 3 vs. R = Me, n = 4

A “second‐generation” scorpionate ligand was utilized to prepare the nickel(II) borohydride complex [(TpPh,Me)Ni(η3‐BH4)], wherein the borohydride was coordinated through two bridging B–H bonds, leaving two terminal B–H bonds uncoordinated as determined by X‐ray crystallography. The distorted square‐pyramidal complex is paramagnetic (S = 1), with 18 valence electrons, and contrasts with a previously reported 20‐electron pseudo‐octahedral “first‐generation” analogue, [(TpMe,Me)Ni(η4‐BH4)] (P. J. Desrochers, et al. Inorg. Chem. 2003, 42, 7945–7950). These distinct borohydride coordination modes were distinguished by FTIR spectroscopy, and rationalized by DFT calculations on simplified models, [(Tp)Ni(ηn‐BH4)] (n = 3, 4). The difference in borohydride hapticities is attributed to the steric effect of the scorpionate 3‐pyrazole phenyl substituents disposed proximally to the metal, thus demonstrating the subtle versatility of Trofimenko's scorpionate ligands in controlling ligand field geometries. The precursor complex [(TpPh,Me)Ni(κ2‐NO3)] was also prepared and characterized.

Hydroelementation of Unsaturated C–C Bonds Catalyzed by Metal Scorpionate Complexes

Tridentate scorpionate ligands have a rich history in coordination chemistry. In the past two decades, metal complexes bearing scorpionate ligands have found increasing utility as catalysts for a variety of useful transformations. This microreview focuses on the utility of scorpionate ligands in hydroelementation of carbon–carbon unsaturated bonds, focusing specifically on hydrothiolation, hydroamination, hydroalkoxylation and hydrophosphinylation of alkenes and alkynes. In each example, the solid phase and solution phase coordination and fluxionality of the scorpionate ligand will be discussed.

Mononuclear Iron‐(hydro/semi)quinonate Complexes Featuring Neutral and Charged Scorpionates: Synthetic Models of Intermediates in the Hydroquinone Dioxygenase Mechanism

Neutral and anionic scorpionate ligands have been employed to generate active‐site models of hydroquinone dioxygenases (HQDOs). While the nonheme Fe center in nearly all HQDOs is coordinated to one Asp (or Glu) and two His residues, 1,2‐gentisate dioxygenase (GDO) is unique in featuring a three His triad instead. A synthetic GDO model was therefore prepared with the neutral tris(4,5‐diphenyl‐1‐methylimidazol‐2‐yl)phosphine (Ph2TIP) ligand. The gentisate substrate was mimicked with the bidentate ligand 2‐(1‐methylbenzimidazol‐2‐yl)hydroquinonate (BIHQ). X‐ray diffraction analysis of the resulting complex, [Fe(Ph2TIP)(BIHQ)]OTf (1a), revealed a distorted square‐pyramidal geometry. Structural and electrochemical data collected for 1a were compared to those previously reported for [Fe(Ph2Tp)(BIHQ)] (1b), which features an anionic hydridotris(3,5‐diphenylpyrazol‐1‐yl)borate (Ph2Tp) ligand. Oxidation of 1a and 1b provides the corresponding FeIII complexes (2a/2b) and the crystal structure of 2b is reported. Both complexes undergo reversible deprotonation to yield the brown chromophores, 3a and 3b. Detailed studies of 3a and 3b with spectroscopic (UV/Vis absorption, EPR, resonance Raman) and computational methods determined that each complex consists of a high‐spin FeII center ferromagnetically coupled to a p‐semiquinonate radical (BISQ). The (de)protonation‐induced valence tautomerization described here resembles key steps in the putative HQDO mechanism.

Extraction and coordination studies of a carbonyl-phosphine oxide scorpionate ligand with uranyl and lanthanide(III) nitrates: structural, spectroscopic and DFT characterization of the complexes.

Hybrid scorpionate ligand (OPPh2)2CHCH2C(O)Me (L) was synthesized and characterized by spectroscopic methods and X-ray diffraction. The selected coordination chemistry of L with UO2(NO3)2 and Ln(NO3)3 (Ln = La, Nd, Lu) has been evaluated. The isolated mono- and binuclear complexes, namely, [UO2(NO3)2L] (1), [{UO2(NO3)L}2(μ2-O2)]·EtOH (2), [La(NO3)3L2]·2.33MeCN (3), [Nd(NO3)3L2]·3MeCN (4), [Nd(NO3)2L2]+·(NO3)−·EtOH (5) and [Lu(NO3)3L2] (6) have been characterized by IR spectroscopy and elemental analysis. Single-crystal X-ray structures have been determined for complexes 1-5. Intramolecular intraligand π-stacking interactions between two phenyl fragments of the coordinated ligand(s) were observed in all complexes 1-5. The π-stacking interaction energy was estimated from Bader's AIM theory calculations performed at the DFT level. Solution properties have been examined using IR and multinuclear ((1)H, (13)C, and (31)P) NMR spectroscopy in CD3CN and CDCl3. Coordination modes of L vary with the coordination polyhedron of the metal and solvent nature showing many coordination modes: P(O),P(O), P(O),P(O),C(O), P(O),C(O), and P(O). Preliminary extraction studies of U(VI) and Ln(III) (Ln = La, Nd, Ho, Yb) from 3.75 M HNO3 into CHCl3 show that scorpionate L extracts f-block elements (especially uranium) better than its unmodified prototype (OPPh2)2CH2.

Stabilization of molecular lanthanide polysulfides by bulky scorpionate ligands.

Well-defined lanthanide polysulfide complexes containing S4(2-) and S5(2-) ligands, the samarium(iii) pentasulfide complex Sm(Tp(iPr2))(κ(1)-3,5-(i)Pr2Hpz)(S5) and the tetrasulfide-bridged binuclear ytterbium(iii) complex (μ-S4)[Yb(Tp(iPr2))(κ(1)-3,5-(i)Pr2Hpz)(κ(2)-3,5-(i)Pr2pz)]2 (Tp(iPr2) = hydro-tris(3,5-diisopropylpyrazolyl)borate), have been synthesized and structurally characterized by single-crystal X-ray diffraction.

Spectroscopic and Computational Investigation of Low-Spin Mn(III) Bis(scorpionate) Complexes.

Six-coordinate MnIII complexes are typically high-spin (S = 2), however, the scorpionate ligand, both in its traditional, hydridotris(pyrazolyl)borate form, Tp- and Tp*- (the latter with 3,5-dimethylpyrazole substituents) and in an aryltris(carbene)borate (i.e., N-heterocyclic carbene, NHC) form, [Ph(MeIm)3B]-, (MeIm = 3-methylimidazole) lead to formation of bis(scorpionate) complexes of MnIII with spin triplet ground states; three of which were investigated herein: [Tp2Mn]SbF6 (1SBF6), [Tp*2Mn]SbF6 (2SBF6), and [{Ph(MeIm)3B}2Mn]CF3SO3 (3CF3SO3). These trigonally symmetric complexes were studied experimentally by magnetic circular dichroism (MCD) spectroscopy (the propensity of 3 to oxidize to MnIV precluded collection of useful MCD data) including variable temperatures and fields (VTVH-MCD) and computationally by ab initio CASSCF/NEVPT2 methods. These combined experimental and theoretical techniques establish the 3A2g electronic ground state for the three complexes, and provide information on the energy of the "conventional" high-spin excited state (5Eg) and other, triplet excited states. These results show the electronic effect of pyrazole ring substituents in comparing 1 and 2. The tunability of the scorpionate ligand, even by perhaps the simplest change (from pyrazole in 1 to 3,5-dimethylpyrazole in 2) is quantitatively manifested through perturbations in ligand-field excited-state energies that impact ground-state zero-field splittings. The comparison with the NHC donor is much more dramatic. In 3, the stronger σ-donor properties of the NHC lead to a quantitatively different electronic structure, so that the lowest lying spin triplet excited state, 3Eg, is much closer in energy to the ground state than in 1 or 2. The zero-field splitting (zfs) parameters of the three complexes were calculated and in the case of 1 and 2 compare closely to experiment (lower by < 10%, < 2 cm-1 in absolute terms); for 3 the large magnitude zfs is reproduced, although there is ambiguity about its sign. The comprehensive picture obtained for these bis(scorpionate) MnIII complexes provides quantitative insight into the role played by the scorpionate ligand in stabilizing unusual electronic structures.

Theoretical investigation, biological evaluation and VEGFR2 kinase studies of metal(II) complexes derived from hydrotris(methimazolyl)borate

The reaction of soft tripodal scorpionate ligand, sodium hydrotris(methimazolyl)borate with M(ClO4)2·6H2O [MMn(II), Ni(II), Cu(II) or Zn(II)] in methanol leads to the cleavage of B–N bond followed by the formation of complexes of the type [M(MeimzH)4](ClO4)2·H2O (1–4), where MeimzH=methimazole. All the complexes were fully characterized by spectro-analytical techniques. The molecular structure of the zinc(II) complex (4) was determined by X-ray crystallography, which supports the observed deboronation reaction in the scorpionate ligand with tetrahedral geometry around zinc(II) ion. The electronic spectra of complexes suggested tetrahedral geometry for manganese(II) and nickel(II) complexes, and square-planar geometry for copper(II) complex. Frontier molecular orbital analysis (HOMO–LUMO) was carried out by B3LYP/6-31G(d) to understand the charge transfer occurring in the molecules. All the complexes exhibit significant antimicrobial activity against Gram (−ve) and Gram (+ve) bacterial as well as fungal strains, which are quite comparable to standard drugs streptomycin and clotrimazole. The copper(II) complex (3) showed excellent free radical scavenging activity against DPPH in all concentration with IC50 value of 30μg/mL, when compared to the other complexes. In the molecular docking studies, all the complexes showed hydrophobic, π-π and hydrogen bonding interactions with BSA. The cytotoxic activity of the complexes against human hepatocellular liver carcinoma (HepG2) cells was assessed by MTT assay, which showed exponential responses toward increasing concentration of complexes.

ChemInform Abstract: Chemistry of the P‐Block Elements with Anionic Scorpionate Ligands

AbstractReview: 147 refs.

Synthesis and structural characterization of two half-sandwich nickel(II) complexes with the scorpionate ligands

The synthesis and characterization of two new halfsandwich mononuclear nickel(II) complexes with the scorpionate ligands, [k3-N, N',N''-Tpt-Bu, MeNiI] (1) and [k3-N,N',N''-Tpt-Bu, MeNiNO3] (2), are reported. These complexes have been fully characterized by elemental analyses and infrared spectra. Their molecular structures were determined by single crystal X-ray diffraction. The nickel(II) ion of complex 1 is in a four-coordinate environment, in which the donor atoms are provided by three nitrogen atoms of a hydrotris(pyrazolyl) borate ligand and one iodide atom, while that of complex 2 is in a five-coordinate environment with three nitrogen atoms from a hydrotris(pyrazolyl)borate ligand and two oxygen atoms from a nitrate ion.

Observations on the Steric Impact of N‐ and S‐Donor Scorpionate Ligands

A comparison is made between the steric influences of a range of zinc hydrotris(pyrazolyl)borates and zinc hydrotris(thioimidazolyl)borates ([Zn(TpR)Cl], [Zn(TmR)Cl]; R = Me, iPr, Ph, tBu) by using inverse cone angle analysis. The study combines the crystallographic analysis of [Zn(TmiPr)Cl] and [Zn(TmPh)Cl] with the data previously deposited with the Cambridge Crystallographic Data Centre. Despite efforts to manipulate the reactive pocket around the metal centre in M(TmR) complexes, the incorporation of sterically confining substituents onto the framework seems to have a minimal effect at the metal centre unless the group attached is very large.

Photoinduced Reactivity of the Soft Hydrotris(6-tert-butyl-3-thiopyridazinyl)borate Scorpionate Ligand in Sodium, Potassium, and Thallium Salts.

The soft scorpionate ligand hydrotris(6-tert-butyl-3-thiopyridazinyl)borate (Tn) was found to exhibit pronounced photoreactivity. Full elucidation of this process revealed the formation of 6-tert-butylpyridazine-3-thione (PnH) and 4,5-dihydro-6-tert-butylpyridazine-3-thione (H2PnH). Under exclusion of light, no solvolytic reactions occur, allowing the development of high-yield preparation protocols for the sodium, potassium, and thallium salts and improving the yield for their derived copper boratrane complex. The photoreactivity is relevant for all future studies with electron-deficient scorpionate ligands.

Organometallic Complexes of Bulky, Optically Active, C3-Symmetric Tris(4S-isopropyl-5,5-dimethyl-2-oxazolinyl)phenylborate (ToP*)

A bulky, optically active monoanionic scorpionate ligand, tris(4S-isopropyl-5,5-dimethyl-2-oxazolinyl)phenylborate (ToP*), is synthesized from the naturally occurring amino acid l-valine as its lithium salt, Li[ToP*] (1). That compound is readily converted to the thallium complex Tl[ToP*] (2) and to the acid derivative H[ToP*] (3). Group 7 tricarbonyl complexes ToP*M(CO)3 (M = Mn (4), Re (5)) are synthesized by the reaction of MBr(CO)5 and Li[ToP*] and are crystallographically characterized. The νCO bands in their infrared spectra indicate that π back-donation in the rhenium compounds is greater with ToP* than with non-methylated tris(4S-isopropyl-2-oxazolinyl)phenylborate (ToP). The reaction of H[ToP*] and ZnEt2 gives ToP*ZnEt (6), while ToP*ZnCl (7) is synthesized from Li[ToP*] and ZnCl2. The reaction of ToP*ZnCl and KOtBu followed by addition of PhSiH3 provides the zinc hydride complex ToP*ZnH (8). Compound 8 is the first example of a crystallographically characterized optically active zinc hydride. We tested its catalytic reactivity in the cross-dehydrocoupling of silanes and alcohols, which provided Si-chiral silanes with moderate enantioselectivity.

Organonickel(IV) chemistry: a new catalyst?

With scorpionate ligands finding their way into organonickel chemistry, the state of the art of present-day nickel(IV) chemistry is highlighted. Will rapid CX coupling reactions emerge as a domain of higher-oxidation-state nickel chemistry?

Iron and Manganese Complexes of 2-Carbonyl Pyrrolyls: Scorpionate Sandwich Anions and Extended Structures

Attempts to synthesize complexes of Fe and Mn(II) with 2-amidopyrrolyl ligands (N–O) were unsuccessful, and only small amounts of the trivalent tris complexes M(N–O)3 were detected, although unusually in the case of Fe(III) a fac structure is observed. In contrast the 2-benzoylpyrrolyl systems give M(II) complexes, and in all instances thus far where Na+ is present, a scorpionate fac-[MII(N–O)3]− unit self-assembles into sandwich anions [MII(N–O)3Na(O–N)3MII]− in which the central metal is efficiently encapsulated by interdigitation of the aryl units. Extended structures are readily made through the use of a 2-(4-pyridinoyl)pyrrolylamide ligand. When Li+ is used, the scorpionate ligand is not assembled, and instead [M(N–O)2] units give rhombic 2D grids. The Fe system displays spin-crossover at 120 K.

The first scorpionate ligand based on diazaphosphole.

The reaction of PhBCl2 with 1H-1,2,4-λ(3)-diazaphosphole in the presence of NEt3 gives a new scorpionate ligand, phenyl-tris(1,2,4-diazaphospholyl)borate (PhTdap). The coordination behaviour of this ligand toward transition and non-transition metals has been comprehensively studied. In the thallium(I) complex, Tl(PhTdap), κ(2)-N,N bonding supported with intramolecular η(3)-phenyl coordination has been observed in the solid state. Tl(PhTdap) also shows unusual intermolecular π-interactions between five-membered diazaphosphole rings and the thallium atom giving infinite molecular chains in the crystal. In the square planar complex [Pd(C,N-C6H4CH2NMe2)(PhTdap)], κ(2)-bonded scorpionate has been detected in both solution and in the solid state. For other studied compounds with the central metal ion Ti(IV), Mo(II), Mn(I), Fe(II), Ru(II), Co(II), Co(III), Ni(II) and Cd(II), the κ(3)-N,N,N coordination pattern was observed. Electronic properties of PhTdap and its ligand-field strength were elucidated from UV-Vis spectra of transition-metal species. The CH/P replacement on going from tris(pyrazolyl)borate to the ligand PhTdap causes a slight increase in electronic density rendered to the central metal atom. The following order of ligand-field strength has been established: HB(3,5-Me2pz)3 < PhB(pz)3 < HB(1,2,4-triazolyl) < HB(pz)3 < PhB(1,2,4-triazolyl) < PhTdap. The crystal structures of ten metal complexes bearing the new ligand are reported. The possibility of PhTdap coordination through the phosphorus atom is also briefly discussed.

Diazoalkane complexes of ruthenium with tris(pyrazolyl)borate and bis(pyrazolyl)acetate ligands.

Diazoalkane complexes [Ru(Tp)(N2CAr1Ar2)(PPh3)L]BPh4 ( and ) [Tp = tris(pyrazolyl)borate; L = P(OMe)3, P(OEt)3; Ar1 = Ar2 = Ph; Ar1 = Ph, Ar2 = p-tolyl; Ar1Ar2 = C12H8] were prepared by allowing chloro-compounds RuCl(Tp)(PPh3)L to react with diazoalkane in the presence of NaBPh4. Acrylonitrile CH2[double bond, length as m-dash]C(H)CN reacts with diazoalkane complexes to give 3H-pyrazole derivatives [Ru(Tp){N[double bond, length as m-dash]NC(Ar1Ar2)CH(CN)CH2}(PPh3){P(OMe)3}]BPh4 and [Ru(Tp){N[double bond, length as m-dash]NC(Ar1Ar2)CH2C(H)CN}(PPh3){P(OMe)3}]BPh4 (). Diazoalkane complexes [Ru(bpza)(N2CAr1Ar2)(PPh3)2]BPh4 () [bpza = bis(pyrazolyl)acetate] were also prepared. All complexes were characterised by IR and NMR spectroscopy and X-ray crystal structure determination of [Ru(Tp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}]BPh4 (). The differences exhibited by [Ru(Tp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}](+) and [Ru(Cp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}](+), as regards coordination of the diazoalkane ligand and reactivity towards alkenes, were explained on the basis of a comparative DFT study.

Heavier group 2 metal complexes with a flexible scorpionate ligand based on 2-mercaptopyridine

Synthetic and structural details of flexible scorpionate ligand based on 2-mercaptopyridine (Bmp) supported heavier alkaline earth metal complexes with metal–sulfur bonds (metal = Sr, Ba) have been presented.

Rare‐Earth‐Metal Methyl, Amide, and Imide Complexes Supported by a Superbulky Scorpionate Ligand

The reaction of monomeric [(Tp(tBu,Me) )LuMe2 ] (Tp(tBu,Me) =tris(3-Me-5-tBu-pyrazolyl)borate) with primary aliphatic amines H2 NR (R=tBu, Ad=adamantyl) led to lutetium methyl primary amide complexes [(Tp(tBu,Me) )LuMe(NHR)], the solid-state structures of which were determined by XRD analyses. The mixed methyl/tetramethylaluminate compounds [(Tp(tBu,Me) )LnMe({μ2 -Me}AlMe3 )] (Ln=Y, Ho) reacted selectively and in high yield with H2 NR, according to methane elimination, to afford heterobimetallic complexes: [(Tp(tBu,Me) )Ln({μ2 -Me}AlMe2 )(μ2 -NR)] (Ln=Y, Ho). X-ray structure analyses revealed that the monomeric alkylaluminum-supported imide complexes were isostructural, featuring bridging methyl and imido ligands. Deeper insight into the fluxional behavior in solution was gained by (1) H and (13) C NMR spectroscopic studies at variable temperatures and (1) H-(89) Y HSQC NMR spectroscopy. Treatment of [(Tp(tBu,Me) )LnMe(AlMe4 )] with H2 NtBu gave dimethyl compounds [(Tp(tBu,Me) )LnMe2 ] as minor side products for the mid-sized metals yttrium and holmium and in high yield for the smaller lutetium. Preparative-scale amounts of complexes [(Tp(tBu,Me) )LnMe2 ] (Ln=Y, Ho, Lu) were made accessible through aluminate cleavage of [(Tp(tBu,Me) )LnMe(AlMe4 )] with N,N,N',N'-tetramethylethylenediamine (tmeda). The solid-state structures of [(Tp(tBu,Me) )HoMe(AlMe4 )] and [(Tp(tBu,Me) )HoMe2 ] were analyzed by XRD.