Showing papers by "Jonas C. Peters published in 2003"

Dinitrogen chemistry from trigonally coordinated iron and cobalt platforms.

Abstract: The chemistry of low-valent iron and cobalt has been slow to develop, due largely in part to a lack of ligand scaffolds that can stabilize these species. Using the anionic tris(phosphino)borate ligand, [PhB(CH_2PiPr_2)_3]-, we have stabilized reactive complexes of the type [P_3M-L] where L can be varied from weakly-donating π-acidic ligands (e.g. N_2), to strongly π-basic ligands where multiple bonding occurs between the metal and L (e.g. NR ^2-). In addition to stabilizing a broad range of ligand types in a variety of oxidation states, the [P_3M] template has been shown to mediate the coordination and activation of dinitrogen on both iron and cobalt. The reaction chemistry of the [P_3M-L] scaffolds will be presented in addition to synthetic strategies targeting high-valent species (M^(IV), L=N ^3-).

A Low-Spin d5 Iron Imide: Nitrene Capture by Low-Coordinate Iron(I) Provides the 4-Coordinate Fe(III) Complex [PhB(CH2PPh2)3]Fe⋮N-p-tolyl

Abstract: Entry into “[PhBP3]Fe” chemistry affords a rare, pseudotetrahedral iron(I) complex, [PhBP3]Fe(PPh3), with an S = 3/2 ground state. This precursor undergoes rapid oxidation by aryl azide to produce the d5 imide [PhBP3]Fe⋮NAr (Ar = p-tolyl). The Fe(III) imide is significant in that it is low-spin and represents the first mononuclear imide of iron. Doublet [PhBP3]Fe⋮NAr reacts rapidly and quantitatively with CO at room temperature to release isocyanate and [PhBP3]Fe(CO)2. The [PhBP3]Fe(CO)2 byproduct is also a precursor to [PhBP3]Fe⋮NAr upon addition of aryl azide.

Zwitterionic and cationic bis(phosphine) platinum(II) complexes: structural, electronic, and mechanistic comparisons relevant to ligand exchange and benzene C-H activation processes.

Abstract: Structurally similar but charge-differentiated platinum complexes have been prepared using the bidentate phosphine ligands [Ph_(2)B(CH_(2)PPh_(2))_(2)], ([Ph_(2)BP_(2)], [1]), Ph_(2)Si(CH_(2)PPh_(2))_(2), (Ph_(2)SiP_(2), 2), and H_(2)C(CH_(2)PPh_(2))_(2), (dppp, 3). The relative electronic impact of each ligand with respect to a coordinated metal center's electron-richness has been examined using comparative molybdenum and platinum model carbonyl and alkyl complexes. Complexes supported by anionic [1] are shown to be more electron-rich than those supported by 2 and 3. A study of the temperature and THF dependence of the rate of THF self-exchange between neutral, formally zwitterionic [Ph_(2)BP_(2)]Pt(Me)(THF) (13) and its cationic relative [(Ph_(2)SiP_(2))Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (14) demonstrates that different exchange mechanisms are operative for the two systems. Whereas cationic 14 displays THF-dependent, associative THF exchange in benzene, the mechanism of THF exchange for neutral 13 appears to be a THF independent, ligand-assisted process involving an anchimeric, η3-binding mode of the [Ph_(2)BP_(2)] ligand. The methyl solvento species 13, 14, and [(dppp)Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (15), each undergo a C−H bond activation reaction with benzene that generates their corresponding phenyl solvento complexes [Ph_(2)BP_(2)]Pt(Ph)(THF) (16), [(Ph_(2)SiP_(2))Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (17), and [(dppp)Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (18). Examination of the kinetics of each C−H bond activation process shows that neutral 13 reacts faster than both of the cations 14 and 15. The magnitude of the primary kinetic isotope effect measured for the neutral versus the cationic systems also differs markedly (k(C6H6)/k(C6D6): 13 = 1.26; 14 = 6.52; 15 6). THF inhibits the rate of the thermolysis reaction in all three cases. Extended thermolysis of 17 and 18 results in an aryl coupling process that produces the dicationic, biphenyl-bridged platinum dimers [{(Ph_(2)SiP_(2))Pt}2(μ-η3:η3-biphenyl)][B(C6F5)4]2 (19) and [{(dppp)Pt}2(μ-η^(3):η^(3)-biphenyl)][B(C_(6)F_(5))_(4)]_(2) (20). Extended thermolysis of neutral [Ph_(2)BP_(2)]Pt(Ph)(THF) (16) results primarily in a disproportionation into the complex molecular salt {[Ph_(2)BP_(2)]PtPh_(2)}-{[Ph_(2)BP_(2)]Pt(THF)_(2)}+. The bulky phosphine adducts [Ph_(2)BP_(2)]Pt(Me){P(C_(6)F_(5))_(3)} (25) and [(Ph_(2)SiP_(2))Pt(Me){P(C_(6)F_(5))_(3)}][B(C_(6)F_(5))_(4)] (29) also undergo thermolysis in benzene to produce their respective phenyl complexes, but at a much slower rate than for 13−15. Inspection of the methane byproducts from thermolysis of 13, 14, 15, 25, and 29 in benzene-d6 shows only CH_(4) and CH3D. Whereas CH_(3)D is the dominant byproduct for 14, 15, 25, and 29, CH_(4) is the dominant byproduct for 13. Solution NMR data obtained for 13, its 13C-labeled derivative [Ph_(2)BP_(2)]Pt(^(13)CH_(3))(THF) (13-^(13)CH_(3)), and its deuterium-labeled derivative [Ph_(2)B(CH_(2)P(C_(6)D5)_(2))_(2)]Pt(Me)(THF) (13-d20), establish that reversible [Ph_(2)BP_(2)]-metalation processes are operative in benzene solution. Comparison of the rate of first-order decay of 13 versus the decay of d_(20)-labeled 13-d_(20) in benzene-d_(6) affords k_(13)/k_(13-d20) ~ 3. The NMR data obtained for 13, 13-^(13)CH_3, and 13-d_20 suggest that ligand metalation processes involve both the diphenylborate and the arylphosphine positions of the [Ph_(2)BP_(2)] auxiliary. The former type leads to a moderately stable and spectroscopically detectable platinum(IV) intermediate. All of these data provide a mechanistic outline of the benzene solution chemistries for the zwitterionic and the cationic systems that highlights their key similarities and differences.

Zwitterionic Relatives to the Classic [(P–P)Rh(solv)2]+ Ions: Neutral Catalysts Active for H ? E Bond Additions to Olefins (E=C, Si, B)

Abstract: Formally zwitterionic bis(phosphanyl)- and bis(amino)borate rhodium(I) complexes (see picture) can catalytically mediate the hydrogenation, hydroacylation, hydroboration, and hydrosilation of double bonds. These neutral systems are shown to be highly active, even under conditions incompatible with their isostructural, but formally cationic, relatives.

The Strong-Field Tripodal Phosphine Donor, [PhB(CH2PiPr2)3]-, Provides Access to Electronically and Coordinatively Unsaturated Transition Metal Complexes

Abstract: This paper introduces a sterically encumbered, strong-field tris(diisopropylphosphino)borate ligand, [PhBPiPr3] ([PhBP^(iPr)_(3)] = [PhB(CH_2P^iPr_2)_3]^-, to probe aspects of its conformational and electronic characteristics within a host of complexes. To this end, the Tl(I) complex, [PhBP^iPr_3]Tl (1), was synthesized and characterized in the solid-state by X-ray diffraction analysis. This precursor proves to be an effective transmetallating agent, as evidenced by its reaction with the divalent halides FeCl_2 and CoX_2 (X = Cl, I) to produce the monomeric, 4-coordinate, high-spin derivatives [PhBP^iPr_3]FeCl_2 and [PhBP^iPr_3]CoX (X = Cl_3, I_4) in good yield. Complexes 2−4 were each characterized by X-ray diffraction analysis and shown to be monomeric in the solid-state. For conformational and electronic comparison within a system exhibiting higher than 4-coordination, the 16-electron ruthenium complexes {[PhBP^iPr_3]Ru(μ-Cl)}_2 (5) and {[PhBP_3]Ru(μ-Cl)}_2 (6) were prepared and characterized ([PhBP_3] = [PhB(CH_2PPh_2)_3]^-. The chloride complexes 2 and 3 reacted with excess CO to afford the divalent, monocarbonyl adducts [PhBP^iPr_3]FeCl(CO) (7) and [PhBP^iPr_3]CoCl(CO) (8), respectively. Reaction of 4 with excess CO resulted in the monovalent, dicarbonyl product [PhBP^iPr_3]CoI(CO)_2 (9). Complexes 5 and 6 also bound CO readily, providing the octahedral, 18-electron complexes [PhBP^iPr_3]RuCl(CO)_2 (10) and [PhBP_3]RuCl(CO)_2 (11), respectively. Dimers 5 and 6 were broken up by reaction with trimethylphosphine to produce the mono-PMe_3 adducts [PhBP^iPr_3]RuCl(PMe_3) (12) and [PhBP3]RuCl(PMe3) (13). Stoichiometric oxidation of 3 with dioxygen provided the 4-electron oxidation product [PhB(CH_2P(O)^iPr_2)_2(CH_2P^iPr_2)]CoCl (14), while exposure of 3 to excess oxygen results in the 6-electron oxidation product [PhB(CH_2P(O)^iPr_2)_3]CoCl (15). Complexes 2 and 4 were characterized via cyclic voltammetry to compare their redox behavior to their [PhBP_3] analogues. Complex 4 was also studied by SQUID magnetization and EPR spectroscopy to confirm its high-spin assignment, providing an interesting contrast to its previously described low-spin relative, [PhBP_3]CoI. The difference in spin states observed for these two systems reflects the conformational rigidity of the [PhBPiPr3] ligand by comparison to [PhBP_3], leaving the former less able to accommodate a JT-distorted electronic ground state.

Bis(phosphino)borates: A New Family of Monoanionic Chelating Phosphine Ligands

Abstract: The reaction of dimethyldiaryltin reagents Me_(2)SnR_(2) (R ) Ph (1), p-MePh (2), m,m-Me_(2)Ph (3), p-tBuPh (4), p-MeOPh (5), p-CF_(3)Ph (6)) with BCl_(3) provided a high-yielding, simple preparative route to the corresponding diarylchloroboranes R_(2)BCl (R ) Ph (10), p-MePh (11), m,m-Me2Ph (12), p-tBuPh (13), p-MeOPh (14), p-CF_(3)Ph (15)). In some cases, the desired diarylchloroborane was not formed from an appropriate tin reagent Me_(2)SnR_(2) (R ) o-MeOPh (7), o,o-(MeO)_(2)Ph (8), o-CF_(3)Ph (9)). The reaction of lithiated methyldiaryl- or methyldialkylphosphines with diarylchloroboranes or dialkylchloroboranes is discussed. Specifically, several new monoanionic bis(phosphino)borates are detailed: [Ph_(2)B(CH_(2)PPh_(2))_(2)] (25); [(p-MePh)_(2)B(CH_(2)PPh_(2))_(2)] (26); [(p-^(t)BuPh)_(2)B(CH_(2)PPh_(2))_(2)] (27); [(p-MeOPh)_(2)B- (CH_(2)PPh_(2))_(2)] (28); [(p-CF_(3)Ph)_(2)B(CH_(2)PPh_(2))_(2)] (29); [Cy_(2)B(CH_(2)PPh_(2))_(2)] (30); [Ph_(2)B(CH_(2)P{p-^(t)BuPh}_(2))_(2)] (31); [(p-MeOPh)_(2)B- (CH_(2)P{p-^(t)BuPh}_(2))_(2)] (32); [Ph_(2)B(CH_(2)P{p-CF_(3)Ph}_(2))_(2)] (33); [Ph_(2)B(CH_(2)P(BH_(3))(Me)_(2))_(2)] (34); [Ph_(2)B(CH_(2)P(S)(Me)_(2))_(2)] (35); [Ph_(2)B(CH_(2)PiPr_(2))_(2)] (36); [Ph_(2)B(CH_(2)P^(t)Bu_(2))_(2)] (37); [(m,m-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)] (38). The chelation of diarylphosphine derivatives 25-33 and 36 to platinum was examined by generation of a series of platinum dimethyl complexes. The electronic effects of substituted bis(phosphino)borates on the carbonyl stretching frequency of neutral platinum alkyl carbonyl complexes were studied by infrared spectroscopy. Substituents remote from the metal center (i.e. on boron) have minimal effect on the electronic nature of the metal center, whereas substitution close to the metal center (on phosphorus) has a greater effect on the electronic nature of the metal center.

Solution and Solid-State Spin-Crossover Behavior in a Pseudotetrahedral d7 Ion

Abstract: This Communication describes a pseudotetrahedral d7 complex, [PhBP3]Co(OSiPh3), that exhibits thermally induced spin-crossover both in solution and in the solid state. Magnetic crossover behavior is achieved by confluence of the X-type ligand and the tripodal auxiliary employed. Four-coordinate platforms of the present geometry type may offer a new approach to magnetic spin-crossover behavior distinct from their electronically related pseudooctahedral counterparts.