Photoelectron Spectroscopy and DFT Calculations of Easily Ionized Quadruply Bonded Mo 2 4+ Compounds and Their Bicyclic Guanidinate Precursors † F. Albert Cotton,* ,‡ Jason C. Durivage, § Nadine E. Gruhn, § Dennis L. Lichtenberger,* ,§ Carlos A. Murillo,* ,‡ Laura O. Van Dorn, § and Chad C. Wilkinson ‡ Department of Chemistry and Laboratory of Molecular Structure and Bonding, P.O. Box 30012, Texas A & M UniVersity, College Station, Texas 77842-3012 and Department of Chemistry, Center for Gas-Phase Electron Spectroscopy, The UniVersity of Arizona, Tucson, Arizona 85721 ReceiVed: March 23, 2006; In Final Form: April 17, 2006 A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo 2 4+ units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo 2 (TEhpp) 4 complex which has a broad first ionization band centered around 4.27 ( 0.03 eV and an ionization onset at the very low energy of 3.93 ( 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo-Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (η 5 -C 9 Me 7 ) 2 Co and (η 5 -C 5 Me 5 ) 2 Cr. Introduction It has been established that guanidinate derivatives have an extraordinary ability to lower oxidation potentials and in turn stabilize higher oxidation numbers in compounds that feature quadruply bonded units such as Cr 2 4+ , Mo 2 4+ , and W 2 4+ . 1 This capacity is also manifested in the gas phase and indeed W 2 - (hpp) 4 2 (hpp ) the anion of the bicyclic guanidine 1,3,4,6,7,8- hexahydro-2H-pyrimido[1,2a]pyrimidine) is the most easily ionized molecule known. The onset ionization energy of W 2 - (hpp) 4 (3.51 eV) 2 is even lower than that of the cesium atom (3.89 eV). 3 The dimolybdenum analogue, Mo 2 (hpp) 4 , has an onset ionization that is only slightly higher (4.01 eV) but still less than that of the francium and rubidium atoms. 2 When the noncyclic triphenylguanidinate anion is used to bridge Mo 2 4+ units, stabilization of Mo 2 5+ and even Mo 2 6+ cores is observed. 4 However, this noncyclic ligand is far less efficient at stabilizing high oxidation states in dimetal units as shown by the fact that the potential for the Mo 2 5+/4+ process is about 1.2 V more positive than for the hpp compound. 5 The reason for the ease of ionization of quadruply bonded M 2 (hpp) 4 compounds is that the π electrons of the highly basic guanidinate core strongly interact with the electrons in the M-M δ bond 2a thereby destabilizing the M 2 4+ core and favoring oxidation to the relatively uncommon M 2 5+ and even rarer M 2 6+ species. Because of such extraordinary properties of the M 2 com- pounds with these ligands and also because of the general interest in guanidinate ligands, 6 which have also been commonly used to stabilize mononuclear compounds 7 many of which are used in polymerization catalysis, 8 we decided to explore more bicyclic guanidinates to determine how changes in the geometry constraints of the fused-ring ligand system and/or addition of electron-donating substituents affect the ability of the anions to stabilize or destabilize charge in M 2 systems. It should be noted that ligand tunability, the modification of electronic structure to achieve a desired property of the molecular system, has valuable applications in many areas of science. Specifically, the ability to control the ease with which an electron can be removed from a molecule is crucial for electron-transfer reactions, catalysis, and materials applications. 9-13 We report here four bicyclic guanidinate ligands that have never been used before in coordination chemistry along with studies of some of their Mo 2 4+ species. In Figure 1, the ligand at the top is hpp, the benchmark for comparison of the modified ligands. The hpp anion has a 6,6 ring system with 12 hydrogen atoms on the 6 ancillary carbon atoms. In the middle of the figure are tbo and tbn. The first one, tbo, is the anion of 1,4,6- triazabicyclo(3.3.0)oct-4-ene. It has a 5,5 ring system containing 8 hydrogen atoms on the 4 ancillary carbon atoms. The second one, tbn, is the anion of 1,4,6-triazabicyclo(3.4.0)-non-4-ene, and it has a 5,6 ring system with 10 hydrogen atoms on the 5 ancillary carbon atoms. The bottom of the figure shows TMhpp and TEhpp. The former, TMhpp, is the anion of 3,3,9,9- tetramethyl-1,5,7-triazabicyclo[4.4.0]dec-4-ene and has a 6,6 ring system with the 4 hydrogen atoms on the carbon atom midway between the nitrogen atoms of each ring replaced with methyl groups. The latter, TEhpp, which is the anion of 3,3,9,9- tetraethyl-1,5,7-triazabicyclo[4.4.0]dec-4-ene, also has a 6,6 ring system but with the 4 hydrogen atoms on the corresponding carbon atoms replaced with ethyl groups. It should be noted that as a key feature for tbo and tbn there is a divergent angle between the bond directions from the ligating nitrogen atoms relative to hpp, while for TMhpp and TEhpp the alkyl groups have been inserted for the main purpose of enhancing solubility of the Mo 2 (bicyclic guanidinate) 4 compounds and have no first-order effect on the bite. This is † Part of the special issue “Charles B. Harris Festschrift”. * To whom correspondence should be addressed. E-mail: cotton@tamu.edu (F.A.C.); dlichten@email.arizona.edu (D.L.L.); murillo@tamu.edu (C.A.M.). ‡ Texas A&M University. § The University of Arizona. 19793 J. Phys. Chem. B 2006, 110, 19793-19798 10.1021/jp061820m CCC: $33.50 © 2006 American Chemical Society Published on Web 05/24/2006