J. A m . Chem. SOC.1983, 105, 7075-7081
7075
Investigations of the Nature of Dehydrogenation of the a-Carbon Atom in the Oxidation of Amines Coordinated to Ruthenium F. Richard Keene,*' Michael J. Ridd,' and Michael R. Snow' Contribution f r o m the Department of Chemistry and Biochemistry, James Cook University of North Queensland, Townsville, Australia 481 I , and the Department of Physical and Inorganic Chemistry, University of Adelaide, Adelaide, South Australia 5001. Received February 9, I983
Abstract: The oxidations of the amine complexes R u ( ~ ~ ~ ) ~ ( A -(where B ) ~ +A-B = ampy [2-(aminomethyl)pyridine], d2-ampy [2-(1,l -dideuterioaminomethyl)pyridine],and Meampy [2-(1-aminoethy1)pyridinel and bpy = 2,2'-bipyridine) in aqueous solution to the corresponding imine species have been studied by using flash photolysis techniques. The rate-determining step of the reaction involves removal of a hydrogen from the a-carbon atom and the concomitant intramolecular transfer of two electrons from ligand to metal. Deuterium exchange and deuterium isotope effect studies, and stereochemical arguments, are consistent with a mechanism involving either (i) a hydride transfer from C to Ru or (ii) a base (solvent water)-assisted proton abstraction. Chirality studies indicate no loss of configurational integrity about the metal center during oxidation, which favors the second proposal. The X-ray crystal structure of the more rapidly oxidized diastereoisomer of [R~(bpy)~(Meampy)]S~0~.5H~O reveals it to be the hS(AR) form, in which the methyl substituent on the a-carbon atom is equatorial and the hydrogen substituent axial.
W e recently reported3 a detailed mechanistic study of t h e two-electron oxidation of 2-(aminomethy1)pyridine (ampy) coordinated to ruthenium(I1) and we proposed a mechanistic scheme shown in eq 2-5.j T h e particular efficacy of R u in the promotion
of the dehydrogenation process appears t o be related in part to its ability t o readily attain a n oxidation state two units greater than t h e final state, allowing a low-energy pathway for the even-electron process required in t h e ligand oxidation, together with a propitious combination of redox potentials for the R u ( I I I ) / R u ( I I ) and R u ( I V ) / R u ( I I I ) couples.
I
I1
( I ) James Cook University of North Queensland. ( 2 ) University of Adelaide. (3) Ridd, M. J.; Keene, F. R. J . Am. Chem. SOC.1981, 103, 5733-5740 and references therein.
0002-7863/83/ 1505-7075$01.50/0
In this scheme, the reaction 5 is written as a single step, but it involves the transfer of two electrons from the ligand to the metal as well a s deprotonation a t t h e methylene carbon atom. A n elucidation of the nature and sequence of these events is fundamental t o t h e complete understanding of the dehydrogenation process. W e have extended the previous studies to examine the oxidation of the analogous ruthenium(I1) complexes of ligands related t o ampy where (i) t h e methylene group is deuterated ( d , - a m ~ y ) ,t~o provide mechanistic information on the deprotonation step, a n d (ii) t h e methylene group has a methyl substituent t me amp^),^ t o clarify stereochemical aspects of the reaction. We have also extended the range of p H over which the oxidation of [ R ~ ( b p y ) ~ ( a m p y )was ] ~ +studied to verify aspects of the pHrate profile predicted by the postulated mechanism.
Experimental Section Measurements. Electronic spectra were recorded on a Cary 219 spectrophotometer and emission spectra on a Perkin-Elmer 3000 fluorescence spectrometer. NMR spectra were recorded on either a JEOL FX9OQ ('H) or JEOL FX6OQ ( I T ) spectrometer. Optical rotations were measured on a Perkin-Elmer 141 Polarimeter; all reported rotations were measured at h 589 nm. All electrochemicalmeasurements were made vs. the saturated sodium chloride calomel electrode (SSCE) and are uncorrected for junction potentials. The cyclic voltammetric waveform was generated by use of a Utah Electronics 0152 potentiostat coupled to a Utah Electronics 0151 sweep generator. Cyclic voltammograms were recorded on a Rikadenki RW-101 X-Y recorder. Coulometry was performed on a potentiostat built in the department. Temperature control for the cyclic voltammetry was achieved with a Lauda K4RD circulating water bath. Elemental analyses were carried out by the Australian Microanalytical Service, AMDEL, Melbourne. Materials. Ruthenium trichloride trihydrate (Matthey-Garrett), tetra-n-butylammonium bromide (Fluka, puriss), 2-(aminomethyl)pyridine (Aldrich), 2-acetylpyridine (Aldrich), ammonium hexafluorophosphate (Fluorochem), and 2,2'-bipyridine (Sigma) were used without purification. Solutions of potassium antimonyl-(+)-tartrate (BDH)were converted to the sodium form by ion exchange. Ag2(Sb2(+)tart,) was prepared by mixing aqueous solutions containing the appropriate molar ratios of K,(Sb,(+)tart,) and AgNO,, filtering the precipitate, and washing the precipitate with water, ethanol, and ether. The complexes c ~ ~ - [ R u ( b ~ ~ ) ~ C 1[Ru(b~~)~(amp~)l(PF~)~~, ~1~2H~0,~ and [Ru(bpy),(4) Abbreviations for ligands: bpy = 2,2'-bipyridine; ampy = 2-(aminomethy1)pyridine; impy = 2-(iminomethyl)pyridine; d,-ampy = 2 4 1,l-di-
deuterioaminomethy1)pyridine;d ,-impy = 2-( 1-deuterioiminomethyl)pyridine; d4-ampy = 2-(N,N,I,l-tetradeuterioaminomethyl)pyridine; Meampy = 2-( 1 aminoethy1)pyridine;hmpy = 2-(hydroxymethyl)pyridine.
0 1983 American Chemical Society
7076 J . A m . Chem. Soc.. Vol. 105, No. 24, I983
Keene. Ridd,and Snow
(impy)] (PF6)23 were prepared as described previously. description of this equipment and of the data acquisition system is given Syntheses. 2 4 1-Aminoethy1)pyridine (Meampy). The oxime of 2elsewhere.8 The experimental conditions for the kinetic study of the acetylpyridine was prepared by the standard literature procedure6 (yield oxidative dehydrogenation of [ R ~ ( b p y ) ~ ( d ~ - a m p yand ) l ~the + two dia92%; mp 118 "C). The oxime was reduced to the amine in an analogous stereoisomers of [Ru(bpy),(Meampy)12+in perchloric acid media were manner to that used for 3-(1-aminoethyl)pyridine7 (yield 81%; bp 107 as described previously for the study of [Ru(bpy),(ampy)12+,' using "C (3 mmHg)). Fe(II1) as quencher. Details of the kinetic analysis of the flash photolysis (2-( l-Aminoethyl)pyridine)bis(2,2'-bipyridine)ruthenium(II) Hexadata have been given previ~usly.~ This complex was prefluorophosphate, [R~(bpy),(Meampy)](PF~)~ For the study of the rate of oxidation of Ru(bpy)2(ampy)2+at pH 3 pared by an analogous method to that used for [R~(bpy)~(ampy)](PF~),.~ and pH 4 it was necessary to use Cu(I1) as a quencher rather than A solution of ci~-[Ru(bpy)~CI~].2H~O (5.0 g) and racemic Meampy (8.0 Fe(II1). The complex concentration was maintained at 1 X M, but g) in deaerated 1:l aqueous methanol (250 mL) was refluxed for 1 h. the quencher concentration increased to 0.1 M due to the slower The methanol was evaporated off and NH4PF6 added to the filtered quenching rate of the MLCT excited state by C U ( I I ) . ~[HC104] was solution. The resultant precipitate was washed with ice-cold water, varied from lo-, to M. No attempt at buffering was made at pH 2-propano1, and ether and dried in vacuo (yield 7.15 g, 90%). 4 to avoid possible catalytic effects on the reaction by the buffer comThe two diastereoisomeric pairs of [Ru(bpy),(Meampy)12+ were ponents. A small (
