DOI: 10.1002/chem.200601051 Experimental and DFT Study of the Tautomeric Behavior of Cobalt- Containing Secondary Phosphine Oxides Chu-Hung Wei, Cheng-En Wu, Yi-Luen Huang, RomanG. Kultyshev, and Fung-E. Hong* [a] Introduction Trialkyl and triaryl phosphines (R 3 P, Ar 3 P) are among the most versatile and commonly employed ligands in reactions catalyzed by transition-metal complexes. [1] Recently, N-het- erocyclic carbenes (NHC) have been introduced as poten- tially effective ligands for coupling reactions. [2] Nevertheless, these types of ligands are normally either air-/moisture-sen- sitive or expensive, and this significantly limits on their syn- thetic applications. Lately, the concept of using metal-con- taining phosphines, rather than the conventional organic phosphines in various phosphine-assisted catalytic reactions, has received a great deal of attention. [3] Probably the most widely used and best known metal-containing bidentate phosphine ligands are bis(diphenylphosphino)ferrocene (dppf, 1) and its derivatives. [4] In the past few years, we have been developing another class of cobalt-containing (mono- or bidentate) phosphines, namely, 3. Each of these com- pounds was prepared by reaction of dppm-bridged dicobalt complex [(m-PPh 2 CH 2 PPh 2 )Co 2 (CO) 6 ](2) with the corre- sponding alkynyl phosphine. This reaction allows straightfor- ward access to a new class of phosphines having bulky char- acter. Interestingly, besides the commonly observed palladi- um–phosphorus bonds, unusual palladium–cobalt bonds were formed during the complexation of 3 with palladium sources. [5] Therefore, the dicobalt fragment present in metal- containing phosphines 3 does not act merely as an innocent spectator, as in the conventional organic phosphine/metal catalyst system. The unique character of 3 indeed opens a new territory for the use of metal-containing phosphines as ligands in cross-coupling reactions. The roles played by Abstract: New cobalt-containing secon- dary phosphine oxides [(m- PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {m,h-PhCC- ACHTUNGTRENNUNGP( =O)(H)(R)}] (8a :R = tBu; 8b :R = Ph) were prepared by reaction of sec- ondary phosphine oxides PhC  CP- ( =O)(H)(R) (6a :R = tBu; 6b :R = Ph) with dppm-bridged dicobalt complex [(m-PPh 2 CH 2 PPh 2 )Co 2 (CO) 6 ](2). The molecular structures of 8a and 8b were determined by single-crystal X-ray dif- fraction. Although palladium-catalyzed Heck reactions employing 8b as ligand gave satisfying results, 8a performed poorly in the same reaction. Judging from these results, a tautomeric equi- librium between 8b and its isomeric form [(m-PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {m,h- PhC  CP(OH)(Ph)}] 8b’ indeed takes place, but it is unlikely between 8a and [(m-PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {m,h-PhC CP(OH)ACHTUNGTRENNUNG(tBu)}] (8a’). The DFT studies demonstrated that reasonable activa- tion energies for the tautomeric con- versions can be achieved only via a bi- molecular pathway. Since a tBu group is much larger than a Ph group, the conversion is presumably only feasible in the case of 8bÐ8b’, but not in the case of 8aÐ8a’. Another cobalt-con- taining phosphine, namely, [(m- PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {m,h-PhC  CP- ACHTUNGTRENNUNG(NEt 2 )ACHTUNGTRENNUNG(tBu)}] (7a), and its oxidation product [(m-PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 - ACHTUNGTRENNUNG{m,h-PhCCP( =O)ACHTUNGTRENNUNG(NEt 2 )ACHTUNGTRENNUNG(tBu)}] 7a’ were prepared from the reaction of PhC  CPACHTUNGTRENNUNG(NEt 2 )ACHTUNGTRENNUNG(tBu) (5a) with 2. The molecular structures of 7a and 7a’ were determined by single-crystal X- ray diffraction. The phosphorus atom is surrounded by substituents in a tetra- hedral environment. AP N single bond (1.676(3) ) is observed in the molecular structure of 7a. Heck reac- tions employing 7a/PdACHTUNGTRENNUNG(OAc) 2 as cata- lyst system exhibited efficiency compa- rable to that of 8a/PdACHTUNGTRENNUNG(OAc) 2 . Keywords: density functional calcu- lations · Heck reaction · P ligands · palladium · tautomerism [a] C.-H. Wei, C.-E. Wu, Y.-L. Huang, Dr. R. G. Kultyshev, Prof. F.-E. Hong Department of Chemistry National Chung Hsing University Taichung 40227 (Taiwan) Fax: (+ 886) 4-2286-2547 E-mail: fehong@dragon.nchu.edu.tw Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. Chem.Eur.J. 2007, 13, 1583 – 1593 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1583 FULL PAPER