The interplay between the primary and secondary coordination spheres is crucial to determining the properties of transition metal complexes. To examine these effects, a series of trigonal bipyramidal Co-OH complexes have been prepared with tripodal ligands that control both coordination spheres. The ligands contain a combination of either urea or sulfonamide groups that control the primary coordination sphere through anionic donors in the trigonal plane and the secondary coordination sphere through intramolecular hydrogen bonds. Variations in the anion donor strengths were evaluated using electronic absorbance spectroscopy and a qualitative ligand field analysis to find that deprotonated urea donors are stronger field ligands than deprotonated sulfonamides. Structural variations were found in the Co^II-O bond lengths that range from 1.953(4) to 2.051(3) Å; this range in bond lengths were attributed to the differences in the intramolecular hydrogen bonds that surround the hydroxido ligand. A similar trend was observed between the hydrogen bonding networks and the vibrations of the O-H bonds. Attempts to isolate the corresponding Co^III-OH complexes were hampered by their instability at room temperature.

High-valent iron species are known to act as powerful oxidants in both natural and synthetic systems. While biological enzymes have evolved to prevent self-oxidation by these highly reactive species, development of organic ligand frameworks that are capable of supporting a high-valent iron center remains a challenge in synthetic chemistry. We describe here the reactivity of an Fe(II) complex that is supported by a tripodal sulfonamide ligand with both dioxygen and an oxygen-atom transfer reagent, 4-methylmorpholine-N-oxide (NMO). An Fe(III)-hydroxide complex is obtained from reaction with dioxygen, while NMO gives an Fe(III)-alkoxide product resulting from activation of a C-H bond of the ligand. Inclusion of Ca(2+) ions in the reaction with NMO prevented this ligand activation and resulted in isolation of an Fe(III)-hydroxide complex in which the Ca(2+) ion is coordinated to the tripodal sulfonamide ligand and the hydroxo ligand. Modification of the ligand allowed the Fe(III)-hydroxide complex to be isolated from NMO in the absence of Ca(2+) ions, and a C-H bond of an external substrate could be activated during the reaction. This study highlights the importance of robust ligand design in the development of synthetic catalysts that utilize a high-valent iron center.

A series of Fe^II-OH₂ complexes were synthesized with ligands based on the tetradentate sulfonamido tripod N,N',N"-[2,2',2"-nitrilotris(ethane-2,1-diyl)]-tris-({R-Ph}-sulfonamido). These complexes differ by the substituent on the aryl rings and were fully characterized, including their molecular structures via X-ray diffraction methods. All the complexes were five-coordinate with trigonal bipyramidal geometry. A linear correlation was observed between the electronic effects of each ligand, given by the Hammett constants of the para-substituents, and the potential of the Fe^II/Fe^III redox couple, which were determined using cyclic voltammetry. It was found that the range of redox potentials for the complexes spanned approximately 160 mV.

The use of 18-crown-6 (18-c-6) in place of 2.2.2-cryptand (crypt) in rare earth amide reduction reactions involving potassium has proven to be crucial in the synthesis of Ln(ii) complexes and isolation of their CO reduction products. The faster speed of crystallization with 18-c-6 appears to be important. Previous studies have shown that reduction of the trivalent amide complexes Ln(NR₂)₃ (R = SiMe₃) with potassium in the presence of 2.2.2-cryptand (crypt) forms the divalent [K(crypt)][Ln^II(NR₂)₃] complexes for Ln = Gd, Tb, Dy, and Tm. However, for Ho and Er, the [Ln(NR₂)₃]^1- anions were only isolable with [Rb(crypt)]¹⁺ counter-cations and isolation of the [Y^II(NR₂)₃]^1- anion was not possible under any of these conditions. We now report that by changing the potassium chelator from crypt to 18-crown-6 (18-c-6), the [Ln(NR₂)₃]^1- anions can be isolated not only for Ln = Gd, Tb, Dy, and Tm, but also for Ho, Er, and Y. Specifically, these anions are isolated as salts of a 1 : 2 potassium : crown sandwich cation, [K(18-c-6)₂]¹⁺, i.e. [K(18-c-6)₂][Ln(NR₂)₃]. The [K(18-c-6)₂]¹⁺ counter-cation was superior not only in the synthesis, but it also allowed the isolation of crystallographically-characterizable products from reactions of CO with the [Ln(NR₂)₃]^1- anions that were not obtainable from the [K(crypt)]¹⁺ analogs. Reaction of CO with [K(18-c-6)₂][Ln(NR₂)₃], generated in situ, yielded crystals of the ynediolate products, {[(R₂N)₃Ln]₂(μ-OC[triple bond, length as m-dash]CO)}^2-, which crystallized with counter-cations possessing 2 : 3 potassium : crown ratios, i.e.{[K₂(18-c-6)₃]}²⁺, for Gd, Dy, Ho. In contrast, reaction of CO with a solution of isolated [K(18-c-6)₂][Gd(NR₂)₃], produced crystals of an enediolate complex isolated with a counter-cation with a 2 : 2 potassium : crown ratio namely [K(18-c-6)]₂ ²⁺ in the complex [K(18-c-6)]₂{[(R₂N)₂Gd₂(μ-OCH[double bond, length as m-dash]CHO)₂]}.

The synthesis and characterization of the Schiff base complexes Fe(ii) (2M) and Fe(iii)Cl (3M), where M is a K⁺ or Ba²⁺ ion incorporated into the ligand, are reported. The Fe(iii/ii) redox potentials are positively shifted by 440 mV (2K) and 640 mV (2Ba) compared to Fe(salen) (salen = N,N'-bis(salicylidene)ethylenediamine), and by 70 mV (3K) and 230 mV (3Ba) compared to Fe(Cl)(salen), which is likely due to an electrostatic effect (electric field) from the cation. The catalytic activity of 3M towards the aerobic oxidation of allylic C-H bonds was explored. Prior studies on iron salen complexes modified through conventional electron-donating or withdrawing substituents found that only the most oxidizing derivatives were competent catalysts. In contrast, the 3M complexes, which are significantly less oxidizing, are both active. Mechanistic studies comparing 3M to Fe(salen) derivatives indicate that the proximal cation contributes to the overall reactivity in the rate determining step. The cationic charge also inhibits oxidative deactivation through formation of the corresponding Fe₂-μ-oxo complexes, which were isolated and characterized. This study demonstrates how non-redox active Lewis acidic cations in the secondary coordination sphere can be used to modify redox catalysts in order to operate at milder potentials with a minimal impact on the reactivity, an effect that was unattainable by tuning the catalyst through traditional substituent effects on the ligand.

The green compound poly[(tetra-hydro-furan)-tris-[μ-η⁵:η⁵-1-(tri-methyl-sil-yl)cyclo-penta-dien-yl]caesium(I)ytterbium(II)], [CsYb(C₈H₁₃Si)₃(C₄H₈O)] _n or [(THF)Cs(μ-η⁵:η⁵-Cp')₃Yb^II] _n was synthesized by reduction of a red THF solution of (C₅H₄SiMe₃)₃Yb^III with excess Cs metal and identified by X-ray diffraction. The compound crystallizes as a two-dimensional array of hexa-gons with alternating Cs^I and Yb^II ions at the vertices and cyclo-penta-dienyl groups bridging each edge. This, based off the six-electron cyclo-penta-dienyl rings occupying three coordination positions, gives a formally nine-coordinate tris-(cyclo-penta-dien-yl) coordination environment to Yb and the Cs is ten-coordinate due to the three cyclo-penta-dienyl rings and a coordinated mol-ecule of THF. The complex comprises layers of Cs₃Yb₃ hexa-gons with THF ligands and Me₃Si groups in between the layers. The Yb-C metrical parameters are consistent with a 4f ¹⁴ Yb^II electron configuration.

The complexes [Cp'₂Ln(μ-CH₃)]₂, (Cp' = C₅H₄SiMe₃; Ln = Y, Tb, Dy) were reduced to determine if these methyl-bridged complexes would form mixed valent 4f ⁿ 5d¹ Ln(II)/4f ⁿ Ln(III) compounds or bimetallic 4f ⁿ 5d¹ Ln(II) compounds containing 5d¹-5d¹ metal-metal bonds upon reduction. Reaction of the known bridged-chloride complexes, [Cp'₂Ln(μ-Cl)]₂, 1-Ln (Ln = Y, Tb, Dy), with MeLi forms the bridged-methyl complexes [Cp'₂Ln(μ-CH₃)]₂, 2-Ln, which were crystallographically characterized for Tb and Dy. KC₈ reduction of 2-Ln in the presence of 2.2.2-cryptand produced 3-Y, 3-Tb, and 3-Dy, which exhibited intense dark colors and broad absorbance peaks around 400 nm with molar extinction coefficients of 1700, 2300, and 1800 M^-1 cm^-1, respectively, which are characteristics of Ln(II) ions. The dark maroon 3-Y product had an axial electron paramagnetic resonance spectrum at 77 K (g ₁ = 1.99, A ₁ = 17.9 G; g ₂ = 2.00, A ₂ = 17.7 G) and a two-line isotropic spectrum at 273 K (g = 1.99, A = 18.4 G), which indicates that an Y(II) ion is present. Although these results are indicative of Ln(II) ions present in the solution, crystallographic evidence was not obtained to establish the structure of these complexes.

The reaction of Ni(TPAP)(COD) {where TPAP = [(NC₅H₄)CH₂]₃P(NC₂H₄)₃N} with Fe(CO)₅ resulted in the isolation of the title heterobimetallic NiFe(TPAP)(CO)₅ complex di-μ-carbonyl-tricarbon-yl[2,8,9-tris-(pyridin-2-yl-meth-yl)-2,5,8,9-tetra-aza-1-phosphabi-cyclo-[3.3.3]undeca-ne]ironnickel, [FeNi(C₂₄H₃₀N₇P)(CO)₅]. Characterization of the complex by ¹H and ³¹P NMR as well as IR spectroscopy are presented. The structure of NiFe(TPAP)(CO)₅ reveals three terminally bound CO mol-ecules on Fe⁰, two bridging CO mol-ecules between Ni⁰ and Fe⁰, and TPAP coordinated to Ni⁰. The Ni-Fe bond length is 2.4828 (4) Å, similar to that of the reduced form of the active site of NiFe hydrogenase (∼2.5 Å). Additionally, a proximal pendant base from one of the non-coordinating pyridine groups of TPAP is also present. Although involvement of a pendant base has been cited in the mechanism of NiFe hydrogenase, this moiety has yet to be incorporated in a structurally characterized synthetic mimic with key structural motifs (terminally bound CO or CN ligands on Fe). Thus, the title complex NiFe(TPAP)(CO)₅ is an unique synthetic model for NiFe hydrogenase. In the crystal, the complex mol-ecules are linked by C-H⋯O hydrogen bonds, forming undulating layers parallel to (100). Within the layers, there are offset π-π [inter-centroid distance = 3.2739 (5) Å] and C-H⋯π inter-actions present. The layers are linked by further C-H⋯π inter-actions, forming a supra-molecular framework.

The preparation and characterization of two NiII complexes are described, a terminal NiII-OH complex with the tripodal ligand tris[(N)-tertbutylureaylato)-N-ethyl)]aminato ([H3buea]3-) and a terminal Ni II-OH2 complex with the tripodal ligand N,N',N″-[2,2',2″-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3-). For both complexes, the source of the -OH and -OH2 ligand is water. The salts K2[NiIIH3buea(OH)] and NMe4[NiIIMST(OH2)] were characterized using perpendicular-mode X-band electronic paramagnetic resonance, Fourier transform infrared, UV-visible spectroscopies, and its electrochemical properties were evaluated using cyclic voltammetry. The solid state structures of these complexes determined by X-ray diffraction methods reveal that they adopt a distorted trigonal bipyramidal geometry, an unusual structure for 5-coordinate NiII complexes. Moreover, the NiII-OH and NiII-OH2 units form intramolecular hydrogen bonding networks with the [H3buea]3- and [MST]3- ligands. The oxidation chemistry of these complexes was explored by treating the high-spin NiII compounds with one-electron oxidants. Species were formed with S = 1/2 spin ground states that are consistent with formation of monomeric NiIII species. While the formation of NiIII-OH complexes cannot be ruled out, the lack of observable O-H vibrations from the putative Ni-OH units suggest the possibility that other high valent Ni species are formed.

Complexes [M(n)MST(NH3)](n-3) (M(n) = Fe(II), Fe(III), Ga(III)) were prepared and each contains an intramolecular hydrogen bonding network involving the ammonia ligand. Deprotonation of the Fe(III)-NH3 complex afforded a putative [Fe(III)MST(NH2)](-) species whose reactivity has been explored.