Grandpa Bulbrook

A series of platinum­(II) complexes with the formulas Pt­(diimine)­(pip₂NCNH₂)­(L)²⁺ [pip₂NCNH₂⁺ = 2,6-bis­(piperidiniummethyl)­phenyl cation; L = Cl, Br, I, NCS, OCN, and NO₂; diimine = 1,10-phenanthroline (phen), 5-nitro-1,10-phenanthroline (NO₂phen), and 5,5′-ditrifluoromethyl-2,2′-bipyridine (dtfmbpy)] were prepared by the treatment of Pt­(pip₂NCN)­Cl with a silver­(I) salt followed by the addition of the diimine and halide/pseudohalide under acidic conditions. Crystallographic data as well as ¹H NMR spectra establish that the metal center is bonded to a bidentate phenanthroline and a monodentate halide/pseudohalide. The pip₂NCNH₂⁺ ligand with protonated piperidyl groups is monodentate and bonded to the platinum through the phenyl ring. Structural and spectroscopic data indicate that the halide/pseudohalide group (L^–) and the metal center in Pt­(phen)­(pip₂NCNH₂)­(L)²⁺ behave as Brønsted bases, forming intramolecular NH···L/NH···Pt interactions involving the piperidinium groups. A close examination of the 10 structures reported here reveals linear correlations between N–H···Pt/L angles and H···Pt/L distances. In most cases, the N–H bond is directed toward the Pt–L bond, thereby giving the appearance that the proton bridges the Pt and L groups. In contrast to observations for Pt­(tpy)­(pip₂NCN)⁺ (tpy = 2,2′;6′,2″-terpyridine), the electrochemical oxidation of deprotonated adducts, Pt­(diimine)­(L)­(pip₂NCN), is chemically and electrochemically irreversible

Cobalt POCOP Pincer Complexes via Ligand C–H Bond Activation with Co₂(CO)₈: Catalytic Activity for Hydrosilylation of Aldehydes in an Open vs a Closed System

A series of cobalt POCOP pincer complexes with the formulas {2,6-(ⁱPr₂PO)₂-4-R′-C₆H₂}­Co­(CO)₂ (R′ = H (<b>1a</b>), NMe₂ (<b>1b</b>), OMe (<b>1c</b>), CO₂Me (<b>1d</b>)), {2,6-(Ph₂PO)₂C₆H₃}­Co­(CO)₂ (<b>1e</b>), and {2,6-(^tBu₂PO)₂C₆H₃}­Co­(CO) (<b>2f</b>) have been synthesized through C–H bond activation of the corresponding pincer ligands with Co₂(CO)₈. These complexes have been demonstrated to catalyze the hydrosilylation of PhCHO with (EtO)₃SiH, which exhibits an induction period and the decreasing reactivity order <b>1b</b> > <b>1c</b> > <b>1a</b> > <b>1d</b> > <b>1e</b>. The catalytic protocol can be applied to various aldehydes with turnover numbers of up to 300. The CO ligands in the dicarbonyl complexes have been shown to exchange with ¹³CO at room temperature and partially dissociate from cobalt at high temperatures. Substitution of CO by <i>tert</i>-butyl isocyanide has been accomplished with <b>1a</b> at 50–80 °C, resulting in the formation of {2,6-(ⁱPr₂PO)₂C₆H₃}­Co­(CN^tBu)­(CO) (<b>3a</b>) and {2,6-(ⁱPr₂PO)₂C₆H₃}­Co­(CN^tBu)₂ (<b>4a</b>). The catalytic reactions are more efficient when they are carried out in an open system or if the catalysts are preactivated by the aldehydes. The structures of <b>1a</b>–<b>e</b>, <b>3a</b>, and <b>4a</b> have been studied by X-ray crystallography

Configurational Stability and Stereochemistry of P‑Stereogenic Nickel POCOP-Pincer Complexes

The P-stereogenic nickel complex {2,6-[(<i>t</i>-Bu)­(Ph)­PO]₂C₆H₃}­NiCl (<b>2</b>) has been synthesized via cyclometalation of the POCOP-pincer ligand 1,3-[(<i>t</i>-Bu)­(Ph)­PO]₂C₆H₄ (<b>1</b>) with NiCl₂. The initially isolated <b>2</b> consists of a 1:1 mixture of racemic and meso isomers that are separable through repeated crystallization and is configurationally stable even at 110 °C. Upon mixing with <i>t</i>-BuOK, the meso isomer (<b>2-</b><i><b>meso</b></i>) displays a higher ligand substitution rate than the racemic isomer (<b>2-</b><i><b>rac</b></i>), likely because its nickel center is sterically more accessible. Complex <b>2</b>, as either pure <b>2-</b><i><b>rac</b></i> or a <b>2-</b><i><b>rac</b></i>/<b>2-</b><i><b>meso</b></i> mixture, can be converted to the nickel triflate complex {2,6-[(<i>t</i>-Bu)­(Ph)­PO]₂C₆H₃}­NiOTf (<b>3</b>) or the nickel formate complex {2,6-[(<i>t</i>-Bu)­(Ph)­PO]₂C₆H₃}­NiOCHO (<b>7</b>) without epimerization at the phosphorus centers. Under a dynamic vacuum at 90 °C, decarboxylation of <b>7-</b><i><b>meso</b></i> is faster than that of <b>7-</b><i><b>rac</b></i>, suggesting that in the transition state the formato hydrogen approaches the nickel center from the axial site rather than the equatorial site. The structure of <b>2-</b><i><b>rac</b></i> has been studied by X-ray crystallography

Cobalt POCOP Pincer Complexes via Ligand C–H Bond Activation with Co₂(CO)₈: Catalytic Activity for Hydrosilylation of Aldehydes in an Open vs a Closed System

A series of cobalt POCOP pincer complexes with the formulas {2,6-(ⁱPr₂PO)₂-4-R′-C₆H₂}­Co­(CO)₂ (R′ = H (<b>1a</b>), NMe₂ (<b>1b</b>), OMe (<b>1c</b>), CO₂Me (<b>1d</b>)), {2,6-(Ph₂PO)₂C₆H₃}­Co­(CO)₂ (<b>1e</b>), and {2,6-(^tBu₂PO)₂C₆H₃}­Co­(CO) (<b>2f</b>) have been synthesized through C–H bond activation of the corresponding pincer ligands with Co₂(CO)₈. These complexes have been demonstrated to catalyze the hydrosilylation of PhCHO with (EtO)₃SiH, which exhibits an induction period and the decreasing reactivity order <b>1b</b> > <b>1c</b> > <b>1a</b> > <b>1d</b> > <b>1e</b>. The catalytic protocol can be applied to various aldehydes with turnover numbers of up to 300. The CO ligands in the dicarbonyl complexes have been shown to exchange with ¹³CO at room temperature and partially dissociate from cobalt at high temperatures. Substitution of CO by <i>tert</i>-butyl isocyanide has been accomplished with <b>1a</b> at 50–80 °C, resulting in the formation of {2,6-(ⁱPr₂PO)₂C₆H₃}­Co­(CN^tBu)­(CO) (<b>3a</b>) and {2,6-(ⁱPr₂PO)₂C₆H₃}­Co­(CN^tBu)₂ (<b>4a</b>). The catalytic reactions are more efficient when they are carried out in an open system or if the catalysts are preactivated by the aldehydes. The structures of <b>1a</b>–<b>e</b>, <b>3a</b>, and <b>4a</b> have been studied by X-ray crystallography

Mechanistic Studies of Ammonia Borane Dehydrogenation Catalyzed by Iron Pincer Complexes

A series of iron bis­(phosphinite) pincer complexes with the formula of [2,6-(ⁱPr₂PO)₂C₆H₃]­Fe­(PMe₂R)₂H (R = Me, <b>1</b>; R = Ph, <b>2</b>) or [2,6-(ⁱPr₂PO)₂-4-(MeO)­C₆H₂]­Fe­(PMe₂Ph)₂H (<b>3</b>) have been tested for catalytic dehydrogenation of ammonia borane (AB). At 60 °C, complexes <b>1</b>–<b>3</b> release 2.3–2.5 equiv of H₂ per AB in 24 h. Among the three iron catalysts, <b>3</b> exhibits the highest activity in terms of both the rate and the extent of H₂ release. The initial rate for the dehydrogenation of AB catalyzed by <b>3</b> is first order in <b>3</b> and zero order in AB. The kinetic isotope effect (KIE) observed for doubly labeled AB (<i>k</i>_NH3BH3/<i>k</i>_ND3BD3 = 3.7) is the product of individual KIEs (<i>k</i>_NH3BH3/<i>k</i>_ND3BH3 = 2.0 and <i>k</i>_NH3BH3/<i>k</i>_NH3BD3 = 1.7), suggesting that B–H and N–H bonds are simultaneously broken during the rate-determining step. NMR studies support that the catalytically active species is an AB-bound iron complex formed by displacing <i>trans</i> PMe₃ or PMe₂Ph (relative to the hydride) by AB. Loss of NH₃ from the AB-bound iron species as well as catalyst degradation contributes to the decreased rate of H₂ release at the late stage of the dehydrogenation reaction

Cooperative Iron–Oxygen–Copper Catalysis in the Reduction of Benzaldehyde under Water-Gas Shift Reaction Conditions

Two Fe–Cu heterobimetallic complexes have been synthesized from the reactions of the (cyclopentadienone)iron complexes {2,5-(EMe₃)₂-3,4-(CH₂)₄(η⁴-C₄CO)}­Fe­(CO)₃ (E = Si, <b>1a</b>; E = C, <b>1b</b>) with (IPr)­CuOH (IPr = 1,3-bis­(diisopropylphenyl)­imidazol-2-ylidene). X-ray crystallographic studies show that these complexes adopt a structure featuring a bridging hydride which can be described by the formula {2,5-(EMe₃)₂-3,4-(CH₂)₄(η⁴-C₄CO)}­(CO)₂Fe­(μ-H)­Cu­(IPr) (E = Si, <b>4a</b>′; E = C, <b>4b</b>′). When they are dissolved in toluene, THF, or cyclohexane, these complexes form a rapidly equilibrated isomeric mixture of <b>4a</b>′ or <b>4b</b>′ and the terminal iron hydride {2,5-(EMe₃)₂-3,4-(CH₂)₄(η⁵-C₄COCuIPr)}­Fe­(CO)₂H (E = Si, <b>4a</b>; E = C, <b>4b</b>). The solution structure for the Me₃Si derivative is dominated by <b>4a</b>. Both <b>4a</b> and <b>4b/4b</b>′ react with HCO₂H to form a monometallic iron hydride and (IPr)­CuOCHO. They also undergo displacement of (IPr)­CuH by CO. The heterobimetallic complexes are effective catalysts for the reduction of PhCHO under water-gas shift conditions; <b>4a</b> exhibits higher activity than <b>4b/4b</b>′. Control experiments with monometallic species establish the cooperativity between a bifunctional iron fragment and a copper fragment during the catalytic reaction. A mechanistic investigation including stoichiometric reactions, various control experiments, and labeling studies has led to the proposal of two different catalytic cycles

Cooperative Iron–Oxygen–Copper Catalysis in the Reduction of Benzaldehyde under Water-Gas Shift Reaction Conditions

Two Fe–Cu heterobimetallic complexes have been synthesized from the reactions of the (cyclopentadienone)iron complexes {2,5-(EMe₃)₂-3,4-(CH₂)₄(η⁴-C₄CO)}­Fe­(CO)₃ (E = Si, <b>1a</b>; E = C, <b>1b</b>) with (IPr)­CuOH (IPr = 1,3-bis­(diisopropylphenyl)­imidazol-2-ylidene). X-ray crystallographic studies show that these complexes adopt a structure featuring a bridging hydride which can be described by the formula {2,5-(EMe₃)₂-3,4-(CH₂)₄(η⁴-C₄CO)}­(CO)₂Fe­(μ-H)­Cu­(IPr) (E = Si, <b>4a</b>′; E = C, <b>4b</b>′). When they are dissolved in toluene, THF, or cyclohexane, these complexes form a rapidly equilibrated isomeric mixture of <b>4a</b>′ or <b>4b</b>′ and the terminal iron hydride {2,5-(EMe₃)₂-3,4-(CH₂)₄(η⁵-C₄COCuIPr)}­Fe­(CO)₂H (E = Si, <b>4a</b>; E = C, <b>4b</b>). The solution structure for the Me₃Si derivative is dominated by <b>4a</b>. Both <b>4a</b> and <b>4b/4b</b>′ react with HCO₂H to form a monometallic iron hydride and (IPr)­CuOCHO. They also undergo displacement of (IPr)­CuH by CO. The heterobimetallic complexes are effective catalysts for the reduction of PhCHO under water-gas shift conditions; <b>4a</b> exhibits higher activity than <b>4b/4b</b>′. Control experiments with monometallic species establish the cooperativity between a bifunctional iron fragment and a copper fragment during the catalytic reaction. A mechanistic investigation including stoichiometric reactions, various control experiments, and labeling studies has led to the proposal of two different catalytic cycles

Roles of Hydrogen Bonding in Proton Transfer to κ^P,κ^N,κ^P‑N(CH₂CH₂P<i>ⁱ</i>Pr₂)₂‑Ligated Nickel Pincer Complexes

The nickel PNP pincer complex (^{^<i>i</i>Pr}PNP)­NiPh (^{^<i>i</i>Pr}PNP = κ^P,κ^N,κ^P-N­(CH₂CH₂P^<i>i</i>Pr₂)₂) was prepared by reacting (^{^<i>i</i>Pr}PNP)­NiBr with PhMgCl or deprotonating [(^{^<i>i</i>Pr}PN^HP)­NiPh]Y (^{^<i>i</i>Pr}PN^HP = κ^P,κ^N,κ^P-HN­(CH₂CH₂P<i>ⁱ</i>Pr₂)₂; Y = Br, PF₆) with KO<i>^t</i>Bu. The byproducts of the PhMgCl reaction were identified as [(^{^<i>i</i>Pr}PN^HP)­NiPh]Br and (^{^<i>i</i>Pr}PNP′)­NiPh (^{^<i>i</i>Pr}PNP′ = κ^P,κ^N,κ^P-N­(CHCHP^<i>i</i>Pr₂)­(CH₂CH₂P<i>ⁱ</i>Pr₂)). The methyl analog (^{^<i>i</i>Pr}PNP)­NiMe was synthesized from the reaction of (^{^<i>i</i>Pr}PNP)­NiBr with MeLi, although it was contaminated with (^{^<i>i</i>Pr}PNP′)­NiMe due to ligand oxidation. Protonation of (^{^<i>i</i>Pr}PNP)­NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C₆D₆. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [(^{^<i>i</i>Pr}PN^HP)­NiX]⁺ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl^–, HO^–, or MeO^–). Protonation of the Ni–C bond was observed at room temperature with (^{^<i>i</i>Pr}PNP)­NiMe, whereas at 70 °C with (^{^<i>i</i>Pr}PNP)­NiPh, both resulting in [(^{^<i>i</i>Pr}PN^HP)­NiCl]Cl as the final product. Protonation of (^{^<i>i</i>Pr}PNP)­NiBr was complicated by site exchange between Br^– and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate (^{^<i>i</i>Pr}PNP)­NiPh and (^{^<i>i</i>Pr}PNP)­NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including (^{^<i>i</i>Pr}PNP′)­NiBr, (^{^<i>i</i>Pr}PNP)­NiPh, (^{^<i>i</i>Pr}PNP′)­NiPh, (^{^<i>i</i>Pr}PNP)­NiMe, [(^{^<i>i</i>Pr}PN^HP)­NiPh]Y (Y = Br, PF₆, BPh₄), [(^{^<i>i</i>Pr}PN^HP)­NiPh]₂[NiCl₄], [(^{^<i>i</i>Pr}PN^HP)­NiMe]Y (Y = Cl, Br, BPh₄), [(^{^<i>i</i>Pr}PN^HP)­NiBr]­Br, and [(^{^<i>i</i>Pr}PN^HP)­NiCl]­Cl, were characterized by X-ray crystallography

Ruthenium Bis-diimine Complexes with a Chelating Thioether Ligand: Delineating 1,10-Phenanthrolinyl and 2,2′-Bipyridyl Ligand Substituent Effects

Despite the high π-acidity of thioether donors, ruthenium­(II) complexes with a bidentate 1,2-bis­(phenyl­thio)­ethane (dpte) ligand and two chelating diimine ligands (i.e., Ru­(diimine)₂­(dpte)²⁺) exhibit room-temperature fluid solution emission originating from a lowest MLCT excited state (diimine = 2,2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine, 1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 5-chloro-1,10-phenanthroline, 5-bromo-1,10-phenanthroline, 5-nitro-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline, and 3,4,7,8-tetramethyl-1,10-phenanthroline). Crystal structures show that the complexes form 2 of the 12 possible conformational/configurational isomers, as well as nonstatistical distributions of geometric isomers; there also are short intramolecular π–π interactions between the diimine ligands and dpte phenyl groups. The photoinduced solvolysis product, [Ru­(diimine)₂­(CH₃CN)₂]­(PF₆)₂, for one complex in acetonitrile also was characterized by single-crystal X-ray diffraction. Variations in the MLCT energies and Ru­(III/II) redox couple, <i>E</i>°′(Ru^3+/2+), can be understood in terms of the influence of the donor properties of the ligands on the mainly metal-based HOMO and mainly diimine ligand-based LUMO. <i>E</i>°′(Ru^3+/2+) also is quantitatively described using a summative Hammett parameter (σ_T), as well as using Lever’s electrochemical parameters (<i>E</i>_L). Recommended parametrizations for substituted 2,2′-bipyridyl and 1,10-phenanthrolinyl ligands were derived from analysis of correlations of <i>E</i>°′(Ru^3+/2+) for 99 homo- and heteroleptic ruthenium­(II) tris-diimine complexes. This analysis reveals that variations in <i>E</i>°′(Ru^3+/2+) due to substituents at the 4- and 4′-positions of bipyridyl ligands and 4- and 7-positions of phenanthrolinyl ligands are significantly more strongly correlated with σ_p⁺ than either σ_m or σ_p. Substituents at the 5- and 6-positions of phenanthrolinyl ligands are best described by σ_m and have effects comparable to those of substituents at the 3- and 8-positions. Correlations of <i>E</i>_L with σ_T for 1,10-phenanthrolinyl and 2,2′-bipyridyl ligands show similar results, except that σ_p and σ_p⁺ are almost equally effective in describing the influence of substituents at the 4- and 4′-positions of bipyridyl ligands. MLCT energies and d⁵/d⁶-electron redox couples of the complexes with 5-substituted 1,10-phenanthroline exhibit correlations with values for other d⁶-electron metal complexes that can be rationalized in terms of the relative number of diimine ligands and substituents

Neodymium(III) Complexes of Dialkylphosphoric and Dialkylphosphonic Acids Relevant to Liquid–Liquid Extraction Systems

The complexes formed during the extraction of neodymium­(III) into hydrophobic solvents containing acidic organophosphorus extractants were probed by single-crystal X-ray diffractometry, visible spectrophotometry, and Fourier-transform infrared spectroscopy. The crystal structure of the compound Nd­(DMP)₃ (<b>1</b>, DMP = dimethyl phosphate) revealed a polymeric arrangement in which each Nd­(III) center is surrounded by six DMP oxygen atoms in a pseudo-octahedral environment. Adjacent Nd­(III) ions are bridged by (MeO)₂POO^– anions, forming the polymeric network. The diffuse reflectance visible spectrum of <b>1</b> is nearly identical to that of the solid that is formed when an <i>n-</i>dodecane solution of di­(2-ethylhexyl)­phosphoric acid (HA) is saturated with Nd­(III), indicating a similar coordination environment around the Nd center in the NdA₃ solid. The visible spectrum of the HA solution fully loaded with Nd­(III) is very similar to that of the NdA₃ material, both displaying hypersensitive bands characteristic of an pseudo-octahedral coordination environment around Nd. These spectral characteristics persisted across a wide range of organic Nd concentrations, suggesting that the pseudo-octahedral coordination environment is maintained from dilute to saturated conditions