Comparative study of electronic and steric properties of bulky, electron-rich bisphosphinoethane, bis-NHC and phosphino-NHC chelating ligands in analogous rhodium(I) and iridium(I) COD and carbonyl complexes Marcel Brill a , Daniela Marrwitz (née Eisenhauer) a , Frank Rominger a , Peter Hofmann a, b, * a Organisch-Chemisches Institut, Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany b Catalysis Research Laboratory (CaRLa), Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 584, 69120 Heidelberg, Germany article info Article history: Received 21 January 2014 Received in revised form 27 March 2014 Accepted 2 April 2014 Keywords: Bulky ligands Bisphosphines Bis-NHC Phosphino-functionalized NHC Abnormal NHC Carbonyl complexes abstract The synthesis and characterization of rhodium(I) and iridium(I) COD (1,5-cyclooctadiene) and carbonyl complexes containing small-bite angle and bulky bisphosphine (bis(di-tert-butylphosphino)ethane ¼ dtbpe), bis-NHC (1,1 0 -di-tert-butyl-3,3 0 -methylenediimidazolin-2,2 0 -ylidene ¼ BisNHC) and phosphino- functionalized NHC (3-tert-butyl-1-(di-tert-butylphosphinomethyl)-imidazolin-2-ylidene ¼ NHCP) li- gands is described. X-ray diffraction analysis of most compounds made and spectroscopic data permitted a valuable comparison of the characteristic steric and electronic properties of these related ligand classes and also of the “abnormally” bound form of the NHCP ligand (aNHCP). While the dicarbonyl complexes indicated similar overall donating abilities for the BisNHC and NHCP ligands, dtbpe and the aNHCP ligand were shown to be significantly less or more electron donating, respectively. These complexes also allowed the evaluation of the different s-donating influences of the NHC moiety in comparison to the phosphine moiety within the hybrid NHCeP systems. Ó 2014 Elsevier B.V. All rights reserved. Introduction Since the synthesis and characterization of the first free N- heterocyclic carbene by Arduengo in 1991 [1] these compounds have quickly emerged into one of the major types of spectator li- gands used in catalysis and organometallic chemistry [2]. NHCs have become a valuable alternative or even substitute to the widely used tertiary phosphines due to excellent sdonating and poor p- acceptor abilities [3] as well as their compatibility with metals in high and low oxidation states [4]. Therefore, NHCs are often regarded as phosphine-mimics, although their very distinct to- pologies might suggest differently. In light of the increased attention gained by phosphino- functionalized NHC ligands very recently [5,6], we sought to char- acterize in detail the structural and electronic features of the bidentate hybrid ligands studied in our own group and to compare them to their parent phosphine or carbene only systems. We have been working on the development of rigid and bulky N-phosphi- nomethyl-functionalized NHCs [5f,h,i] and, among others, on N- phosphino-NHCs [5e-g,q,r,7]. Our long-standing interest in the use of similar electron-rich and bulky ligands like dtbpm (bis(di-tert- butylphosphino)methane), [8] dtpbe (bis(di-tert-butylphosphino) ethane) [9], or bis-carbene ligands [5f,10] further motivated us to gain a better understanding of the differences in steric and elec- tronic properties between these related but geometrically very different systems. The before mentioned dtbpe ligand, a frequently used bulky bisphosphine in organometallic chemistry and homo- geneous catalysis [11], represents the direct P-only analogue to the N-phosphinomethyl-NHC hybrid ligands studied in our group. Similarly, dtbpm can be viewed as the analogue to the N-phos- phino-NHC ligands which feature bite-angles as low as 65  in their metal complexes [5g]. However, in this study we will focus on the former systems (bite angles slightly larger than 80  ), since NHC- only analogues of the very small bite angle systems do not exist so far. In this paper we describe the synthesis and characterization of neutral and cationic COD and carbonyl complexes of rhodium(I) and iridium(I) containing related bulky bisphosphine, bis-NHC and * Corresponding author. Catalysis Research Laboratory (CaRLa), Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 584, 69120 Heidelberg, Germany. Tel.: þ49 6221 54 8502; fax: þ49 6221 54 4885. E-mail address: ph@uni-hd.de (P. Hofmann). Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem http://dx.doi.org/10.1016/j.jorganchem.2014.04.008 0022-328X/Ó 2014 Elsevier B.V. All rights reserved. Journal of Organometallic Chemistry xxx (2014) 1e15 Please cite this article in press as: M. Brill, et al., Journal of Organometallic Chemistry (2014), http://dx.doi.org/10.1016/ j.jorganchem.2014.04.008