Research Interests

It had long been assumed that exchange of coordinated and uncoordinated phosphines in dangling ligand complexes, M(CO)5(L-L) (M = Cr, Mo, W) was insignificantly slow. It was reasoned that stability with respect to chelation also implied stability with respect to phosphorus exchange. In 1994 we reported evidence that phosphorus exchange in (OC)5W[k1-PPh2CH2CH(PPh2)2] was sufficiently fast, even at room temperature, to prevent isolation of A in pure form.1

Intrigued by this highly unusual behavior, we initiated a kinetic and thermodynamic study. Our results show that isomer B is more stable than isomer A (DG298 = -3.86 kJ/mol) even though the more sterically demanding end of the phosphine ligand is coordinated in B. The entropy (DS‡ = -28.2 J K-1 mol-1) and enthalpy of activation (DH‡ = 92.6 kJ/mol) for the forward reaction strongly suggest a transition state which is associative in nature. The rate constant for the forward reaction at 55 oC is 3.7 x 10-4 s-1. For purposes of comparison we also studied phosphorus exchange in (OC)5W[k1-PPh2CH2CH2P(p-tol)2] and found that the rate constant for isomerization is 3.3 x 10-8 s-1 (four orders of magnitude slower than reaction 1), suggesting a reaction which is dissociative in nature. To account for the unusually fast rate of phosphorus exchange in reaction 1, we postulated a mechanistic model in which the transition state consists of interaction of one phosphine arm with the cis carbonyl groups of tungsten (leading to stabilization) while the second phosphine arm displaces the coordinated phosphine.2 It is significant that we have observed coupling (4JPC) in B between the phosphorus atom of the short dangling phosphine and the carbonyl groups of tungsten, suggesting a significant though-space interaction.

Presently we are continuing our thermodynamic and kinetic studies of phosphine exchange in dangling ligand complexes by (1) extending our studies to chromium and molybdenum; (2) obtaining a crystal structure of isomer B; (3) replacing the diphenylphosphino groups of the short arm of B with ditolylphosphino groups so that phosphorus exchange between the short arms may be observed; (4) comparing rates of chelation with rates of isomerization; (5) doing variable temperature C-13 NMR experiments on B and similar complexes to better understand the nature of the long range phosphorus-carbon coupling; (6) measuring DV‡ to further describe the transition state of reaction 1; (7) assessing the importance of dangling arm lengths in isomerization reactions; (8) replacing the phosphorus of the short dangling arm of B with oxygen and nitrogen for assessment of heteroatom/carbonyl interaction; (9) modeling key complexes to determine the importance of the relative stabilities of various conformations; (10) conducting a Hammett study for reaction 1 in which aryl substituents are varied to assess electronic influences; (11) examining phosphorus exchange in complexes of important chiral ditertiary phosphines such as S-PROPHOS, R,S-BPPF(R), S,S-CHIRAPHOS, and S-BINAP; and (12) by performing solvent studies to determine the extent to which the solvent influences rates of exchange and isomer stability.

These isomerization reactions are of fundamental importance because they suggest that complex reactivity may be profoundly influenced by the nature of pendant groups attached to a coordinated ligand. Although it is widely recognized that the steric repulsion between pendant groups and other ligands of a complex may significantly influence reaction rates, the importance of attractive interactions between these groups and other ligands has received little attention. It would appear that for organometallic reactions in general, it may be possible to control substitution rates by employing ligands with pendant groups which have an affinity for other ligands attached to the central metal of the complex.

Representative Recent Publications:

Keiter, R.L.; **Gamage, C. "The Combustion of White Phosphorus," J. Chem. Educ., 2001, 78, 908-910.

Keiter, R.L.; Benson, J. W.; **Jia Z., Keiter, E.A.; Brandt, D.E. "Isomerization and Chelation Studies of
(OC)5W[h1-PPh2CH2CH2 P(p-tolyl)2] and (OC)5W(h1-P(p-tolyl)2 CH2CH2 PPh2] (M = Cr, Mo, W)," Organometallics, 2000, 19, 4158.

Keiter, R.L.; Benson, J.W.; Keiter, E.A.; **Lin, W.; **Jia, Z.; *Olson, D.M.; Brandt, D.E.; *Wheeler, J.L., "Induced Acceleration of Phosphine Exchange in Metal Carbonyls by Pendant Groups of Coordinated Polyphosphines. Two Dangling Phosphine Arms Are Much Better Than One," Organometallics, 1998, 17, 4291.

Benson, J.W.; Keiter, R.L.; Keiter, E.A.; Rheingold, A.L.; Yap, G.P.A.; Mainz, V.V. "Studies of a Reluctant Ligand. An X-ray Crystallographic and NMR Spectroscopic Analysis of (OC)5W(h1-PPh2CH2PPh2) and its
(OC)5W(m1-PPh2CH2PPh2)W(CO)5 Derivative," Organometallics, 1998, 17, 4275.

Keiter, R.L.; Benson, J.W.; Keiter, E.A.; *Harris, T.A.; *Hayner, M.W.; *Mosimann, L.L.; *Karch, E.R.; *Boecker, C.A.; *Olson, D. M.; *VanderVeen, J.; Brandt, D. E.; Rheingold, A.L.; Yap, G.P.A. "Phosphorus-phosphorus Coupling Constants in Mixed Phosphine Tricarbonyl Complexes, Fe(CO)3LL'. Crystal Structure of trans-Fe(CO)3(PEt3)(PPh3)," Organometallics1997, 16, 2246.

Keiter, R.L.; Keiter, E.A.; *Boecker, C.A.; *Miller, D.R.; **Hecker, K.H. "Tricarbonylbis(phosphine)iron(0) Complexes," Inorg. Synth. 1997, 31, 210.

Benson, J.W.; Keiter, E.A.; Keiter, R.L. "On the Synthesis and 31P{1H} NMR Spectrum of (OC)5W(m-PPh2CH2PPh2)W(CO)5," J. Organomet. Chem. 1995, 495, 77.

Keiter, R.L.; Keiter, E.A.; *Olson, D.M.; *Bush, J.R.; **Lin, W.; Benson, J.W. "Isomerization of (OC)5W[PPh2CH2CH(PPh2)2]," Organometallics 1994, 13, 3752.

Anson, C.E.; Sheppard, N.; Powell, D.; Norton, J.R.; Keiter, R.L.; Fischer, W.; Johnson, B.F.G.; Lewis, J.; Bhattacharyya, K.; Knox, S.A.R.; Turner, M.L. "An Assignment of the Infrared and Raman Spectra of the Os2(m2-CHCH3) Group of [(m2-CHCH3)Os2(CO)8], and its d1 and d4 Isotopomers, as a Model for the Spectrum of an Ethylidene on a Metal Surface Pt(III)," J. Am. Chem. Soc. 1994, 116, 3058.

Huheey, J.E.; Keiter, E.A.; Keiter, R.L. Inorganic Chemistry, Principles of Structure and Reactivity, 4th ed. HarperCollins, New York, 1993.

Keiter, R.L.; Keiter, E.A.; *Rust, M.S.; *Miller, D.R.; Sherman, E.O.; **Cooper, D.W. "Oxidation of Phosphido-Bridged Complexes of Molybdenum and Tungsten. Isolation of [trans-(HPPh2)(OC)3M(m-PPh2)2M(CO)3(PPh2H)][PF6]," Organometallics 1992, 11, 487.

*Undergraduate student, **Graduate student

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