Coordination chemistry of poly(thioether)borate ligands

Iron(II)-alkoxide and -aryloxide complexes of a tris(thioether)borate ligand: synthesis, molecular structures, and implications on the origin of instability of their iron(II)-catecholate counterpart

The phenyltris[(tert-butylthio)methyl]borate ligand, [PhTt^tBu], has been studied extensively as a platform for coordination, organometallic, and bioinorganic chemistry, especially with 3d metals. While [PhTt^tBu]Co(3,5-DBCatH) (3,5-DBCatH is 3,5-di-tert-butylcatecholate), a Co^II-monoanionic catecholate complex, was successfully isolated to model the active site of cobalt(II)-substituted homoprotocatechuate 2,3-dioxygenase (Co-HPCD) [Wang et al. (2019). Inorg. Chim. Acta, 488, 49-55], its iron(II) counterpart, [PhTt^tBu]Fe(3,5-DBCatH), was not accessible via similar synthetic routes. Switching the nucleophile from catecholate to alkoxide or aryloxide, however, led to the successful isolation of three highly air-sensitive Fe^II-alkoxide and -aryloxide complexes, namely, (triphenylmethoxo){tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,S',S''}iron(II), [Fe(C₂₁H₃₈BS₃)(C₁₉H₁₅O)], (2), (2,6-dimethylphenolato){tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,S',S''}iron(II), [Fe(C₂₁H₃₈BS₃)(C₈H₉O)], (3), and bis{μ-tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,S':S''}bis[(phenolato-κO)iron(II)] toluene disolvate, [Fe₂(C₂₁H₃₈BS₃)₂(C₆H₅O)₂]·2C₇H₈, (4). In the solid state, compounds (2) and (3) are monomeric, with [PhTt^tBu] acting as a tridentate ligand. In contrast, compound (4) crystallizes as a dimeric complex, wherein each [PhTt^tBu] ligand binds to an iron centre with two thioethers and binds to the other iron centre with the third thioether. The molecular structures of (2)-(4) demonstrate a diversity in the binding modes of [PhTt^tBu] and highlight its potential use for assembling multinuclear complexes. In addition, the successful isolation of (2)-(4), as well as the structural information of a [PhTt^tBu] modification product, namely, bis{μ-tris[(tert-butylsulfanyl)methyl](2-oxidophenolato)borato-κO,O',S,S':O'}dicobalt(II), [Co₂(C₂₁H₃₇BO₂S₃)₂], (5), obtained from the reaction of [PhTt^tBu]CoCl with potassium monoanionic catecholate, shed light on the origin of the instability of [PhTt^tBu]Fe(3,5-DBCatH).

Abnormal N-heterocyclic Carbene Based Ni(II) π-allyl Complex towards Molecular Oxygen Activation

Abnormal N-heterocyclic carbene (aNHC) based Ni(II) π-allyl complexes (3 and 4) were synthesized starting from a Ni(0) precursor. These complexes were characterized by NMR spectroscopy, single-crystal X-ray crystallography (4) and elemental analysis data. The underlying mechanism for the formation of Ni(II) η³ -allyl complexes from a Ni(0) precursor on treatment with a free abnormal N-heterocyclic carbene in absence of any external additive or oxidant was unraveled. Later, complex 3 was exposed to O₂ gas under ambient pressure resulting in molecular oxygen activation to form a μ-hydroxo bridged Ni(II) dimer.

Ru-Catalyzed Switchable N-Hydroxyethylation and N-Acetonylation with Crude Glycerol

Highly efficient Ru-catalyzed selective C-C or C-O bond cleavage of polyols (e.g., crude glycerol) for N-hydroxyethylation or N-acetonylation of amines was achieved through the hydrogen-borrowing approach. A variety of amines were transformed to the desired amino alcohols/ketones in moderate-to-excellent yields, opening up new avenues for generation of oxygenated pharmaceuticals and fine chemicals from renewable raw materials. The use of new redox-active catalysts containing bisphosphine/thienylmethylamine ligands allows this hydrogen-borrowing system to be operated selectively under both basic and acidic conditions.

Bioinspired Tungsten Complexes Employing a Thioether Scorpionate Ligand

The synthesis and characterization of a series of novel tungsten complexes employing the bioinspired, sulfur-rich scorpionate ligand [PhTt] (phenyltris((methylthio)methyl)borate) are reported. Starting from the previously published tungsten precursor [WBr₂(CO)₃(NCMe)₂], a salt metathesis reaction with 1 equiv of Cs[PhTt] led to the desired complex [WBr(CO)₃(PhTt)] (1), making it the first tungsten complex employing a poly(thioether)borate ligand. Surprisingly, the reaction of [WBr₂(CO)₃(NCMe)₂] with an excess of the ligand gave complex [W(CO)₂(η²-CH₂SMe)(PhTt)] (2) with a bidentate (methylthio)methanide ligand as the major product. Thereby, phenyldi((methylthio)methyl)borane is formed, which was isolated and characterized by NMR spectroscopy. The bromido ligand in [WBr(CO)₃(PhTt)] was further substituted by the S,N-bidentate methimazole in order to make the first coordination sphere more sulfur-rich forming [W(CO)₂(mt)(PhTt)] (3). Alkyne tungsten complexes employing the sulfur-rich scorpionate ligand were accessible by reaction of [WBr₂(CO)(C₂R₂)₂(NCMe)] (R = Me, Ph) with Cs[PhTt] forming [WBr(CO)(C₂R₂)₂(PhTt- S, S')] (R = Me 4, Ph 5), with the potentially tridentate ligand coordinated only via two sulfur atoms. In the case of 4, the higher flexibility of the bidentate coordination leads to the formation of two isomers with respect to the six-membered ring formed by the tungsten center and the two coordinated sulfur atoms of the ligand. All complexes 1-5 were characterized by single-crystal X-ray diffraction analysis.

A Multifunctional Pincer Ligand for Cobalt-Promoted Oxidation by N2 O

The divalent cobalt complex of the diprotic pincer ligand bis-pyrazolylpyridine, (H₂ L)CoCl₂ , is dehydrohalogenated twice by LiN(SiMe₃ )₂ in the presence of PEt₃ to give monomeric S=1/2 LCo(PEt₃ )₂ (1), fully characterized in the solid-state and solution as a square pyramidal monomer with a long axial Co-P bond. This 17-electron species reacts in time of mixing with N₂ O to form L₂ Co₂ (μ-OPEt₃ ) (2)+3 OPEt₃ , the former the first example of phosphine oxide bridging two transition metals. The same products are formed from O₂ , and divalent cobalt persists even in the presence of excess oxidant. Species (2) catalyzes oxygen atom transfer (OAT) for generation of O=PEt₃ from PEt₃ from either N₂ O or O₂ . Bridging and terminal cobalt oxo intermediates are suggested, and the electron donor power, and potential redox activity of the dianionic pincer ligand is emphasized.

Complexes of Ni(i): a “rare” oxidation state of growing importance

The synthesis and diverse structures, reactivity (small molecule activation and catalysis) and magnetic properties of Ni(i) complexes are summarized.

A pseudotetrahedral nickel(II) complex with a tridentate oxazoline-based scorpionate ligand: chlorido[tris(4,4-dimethyloxazolin-2-yl)phenylborato]nickel(II)

Poly(pyrazol-1-yl)borates have been utilized extensively in coordination compounds due to their high affinity toward cationic metal ions on the basis of electrostatic interactions derived from the mononegatively charged boron centre. The original poly(pyrazol-1-yl)borates, christened `scorpionates', were pioneered by the late Professor Swiatoslaw Trofimenko and have expanded to include various borate ligands with N-, P-, O-, S-, Se- and C-donors. Scorpionate ligands with boron-carbon bonds, rather than the normal boron-nitrogen bonds, have been developed and in these new types of scorpionate ligands, amines and azoles, such as pyridines, imidazoles and oxazolines, have been employed as N-donors instead of pyrazoles. Furthermore, a variety of bis- and tris(oxazolinyl)borate ligands, including chiral ones, have been developed. Tris(oxazolin-2-yl)borates work as facially capping tridentate chelating ligands in the same way as tris(pyrazol-1-yl)borates. In the title compound, [Ni(C₂₁H₂₉BN₃O₃)Cl], the Ni^II ion is coordinated by three N atoms from the facially capping tridentate chelating tris(4,4-dimethyloxazolin-2-yl)phenylborate ligand and a chloride ligand in a highly distorted tetrahedral geometry. The Ni-Cl bond length [2.1851 (5) Å] is comparable to those found in a previously reported tris(3,5-dimethylpyrazol-1-yl)hydroborate derivative [2.1955 (18) and 2.150 (2) Å]. The molecular structure deviates from C_3v symmetry due to the structural flexibility of the tris(4,4-dimethyloxazolin-2-yl)phenylborate ligand.

Persistent four-coordinate iron-centered radical stabilized by π-donation

Dinuclear iron carbonyl complex 2, which contains an elongated unsupported Fe-Fe bond, was synthesized by the reaction between Fe₂(CO)₉ and phosphinyl radical 1. Thermal Fe-Fe bond homolysis led to the generation of a four-coordinate carbonyl-based iron-centered radical, 3, which is stabilized by π-donation. Complex 3 exhibited high reactivity toward organic radicals to form diamagnetic five-coordinate Fe(ii) complexes.

Homo- and Heteropolynuclear Clusters of Phosphine Triphenolates

The synthesis and structural characterization of a series of homo- and heteropolynuclear clusters constructed with a potentially tetradentate phosphine triphenolate ligand are presented. Treatment of tris(3,5-di-tert-butyl-2-hydroxyphenyl)phosphine (H3[O3P]) with 3 equiv of nBuLi in diethyl ether at -35 °C affords hexanuclear Li6[O3P]2(OEt2)2 (1) as colorless crystals. In situ lithiation of H3[O3P] with 3 equiv of nBuLi in THF at -35 °C followed by metathetical reactions with MnCl2 or NiCl2(DME) gives crystals of forest green pentanuclear MnLi4[O3P]2(THF)3 (2) or dark brown tetranuclear Ni2Li2[O3P]2(THF)2 (3), respectively. Alkane elimination of ZnR2 (R = Me, Et) with H3[O3P] in THF at 25 °C generates high yields of colorless crystalline trinuclear Zn3[O3P]2(THF)2 (4). The cluster structures of 1-4 were all determined by single crystal X-ray diffraction studies. These molecules represent the first examples of metal complexes supported by phosphine triphenolate derivatives. The cluster 2 contains a paramagnetic core of high spin Mn(II) (S = 5/2) as indicated by solution and solid state magnetic susceptibility measurements.

Polyacryloyl hydrazide: an efficient, simple, and cost effective precursor to a range of functional materials through hydrazide based click reactions

Preparation and studies of ion exchangeable epoxy resins, stimuli responsive hydrogels, and polymer-dye conjugates have been accomplished through hydrazide based click reactions using polyacryloyl hydrazide (PAH) as the precursor. A convenient synthesis of PAH with quantitative functionality was achieved by treatment of polymethyl acrylate with hydrazine hydrate in the presence of tetra-n-butyl ammonium bromide. PAH was cured with bisphenol A diglycidyl ether (BADGE) at 60 °C to form transparent resins with superior mechanical properties (tensile strength = 2-40 MPa, Young's modulus = 3.3-1043 MPa, and ultimate elongation = 9-75%) compared to the conventional resins prepared using triethylene tetramine. The resins exhibited higher ion exchange capacities (1.2-6.3 mmol/g) compared to the commercial AHA ammonium-type (Tokuyama Co., Japan) membranes. An azo dye with aldehyde functionality was covalently attached to PAH through hydrazone linkage, and the dye labeled PAH exhibited colorimetric sensing ability for base and acids up to micromolar concentration. The swelling of the PAH based hydrogel varied in the range 4-450% depending on the pH and temperature of the medium. The hydrogels gradually released 30% of the original encapsulated dye in a period of 200 h. PAH-hydroxy naphthaldehyde conjugate released 75% of the original loading in ∼11 days at 37 °C and pH 5.0 through cleavage of the -CONHN═C- linkage. The study depicts the versatility of PAH as a precursor and inspires synthesis of a range of new materials based on PAH in the future.

Rhodium and iridium complexes with a new scorpionate phosphane ligand

A straightforward synthesis of a new hybrid scorpionate ligand [(allyl)2B(CH2PPh2)(Pz)](-) ([A2BPN](-)) is reported. Coordination to rhodium resulted in square-planar complexes [Rh(κ(2)-A2BPN)(L)(L')] [L = L' = (1)/2cod (1,5-cyclooctadiene), CN(t)Bu, CO (6); L = CO, L' = NH3, pyridine, PPh3, PMe3] for which spectroscopic data and the molecular structure of [Rh(κ(2)-A2BPN)(CO)PPh3] (11) indicate the ligand to be κN,κP-bound to rhodium with two dangling free allyl groups. Studies in solution point out that the six-membered Rh-N-N-B-C- P metallacycle undergoes a fast inversion in all of them. The bis(carbonyl) complex 6 easily loses a CO group to give [{Rh(A2BPN)(CO)}2], a dinuclear compound in which two mononuclear subunits are brought together by two bridging allyl groups. Coordination to iridium is dominated by a tripodal κN,κP,η(2)-C═C binding mode in the TBPY-5 complexes [Ir(κ(3)-A2BPN)(L)(L')] [L = L' = (1)/2cod (3), CN(t)Bu (5), CO (7); L = CO, L' = PPh3 (13), PMe3 (14), H2C═CH2, (17), MeO2CC≡CCO2Me (dmad, 18)], as confirmed by the single-crystal structure determination of complexes 3 and 18. A fast exchange between the two allyl arms is observed for complexes having L = L' (3, 5, and 7), while those having CO and L ligands (14, 17, and 18) were found to be nonfluxional species. An exception is complex 13, which establishes an equilibrium with the SP-4 configuration. Protonation reactions on complexes 13 and 14 with HCl yielded the hydride complex [Ir(κ(2)-A2BPN)(CO)(Cl)(H)PPh3] (15) and the C-alkyl compound [ Ir{κ(3)-(allyl)B(CH2 CHCH3)(CH2PPh2)(Pz)}(Cl)(CO)PMe3] (16), respectively. The bis(isocyanide) complex 5 reacts with dmad to form [Ir(κ(2)-A2BPN)(CN(t)Bu)2(dmad)]. On the whole, the electronic density provided to the metal by the [A2BPN](-) ligand is very sensitive to the coordination mode. The basicity of the new ligand is similar to that of the Tp(Me2) ligand in the κN,κP mode but comparable to Tp if coordinated in the κN,κP,η(2)-C═C mode.

Spectroscopic and magnetic properties of an iodo Co(I) tripodal phosphine complex

Reaction of the tripodal phosphine ligand 1,1,1-tris((diphenylphosphino)phenyl)ethane (PhP3) with CoI(2) spontaneously generates a one-electron reduced complex, [(PhP3)Co(I)(I)] (1). The crystal structure of 1 reveals a distorted tetrahedral environment, with an apical Co-I bond distance of ~2.52 Å. Co(II/I) redox occurs at an unusually high potential (+0.38 V vs. SCE). The electronic absorption spectrum of 1 exhibits an MLCT peak at 320 nm (ε = 8790 M(-1) cm(-1)) and a d-d feature at 850 nm (ε = 840 M(-1) cm(-1)). Two more d-d bands are observed in the NIR region, 8650 (ε = 450) and 7950 cm(-1) (ε = 430 M(-1) cm(-1)). Temperature dependent magnetic measurements (SQUID) on 1 (solid state, 20-300 K) give μ(eff) = 2.99(6) μ(B), consistent with an S = 1 ground state. Magnetic susceptibilities below 20 K are consistent with a zero field splitting (zfs) |D| = 8 cm(-1). DFT calculations also support a spin-triplet ground state for 1, as optimized (6-31G*/PW91) geometries (S = 1) closely match the X-ray structure. EPR measurements performed in parallel mode (X-band; 0-15,000 G, 15 K) on polycrystalline 1 or frozen solutions of 1 (THF/toluene) exhibit a feature at g≈ 4 that arises from a (Δm = 2) transition within the M(S) = <+1,-1> manifold. Below 10 K, the EPR signal decreases significantly, consistent with a solution zfs parameter (|D|≈ 8 cm(-1)) similar to that obtained from SQUID measurements. Our work provides an EPR signature for high-spin Co(I) in trigonal ligation.