Design of Phosphorus Centred Ligands
Different to the classical chelating diphosphane ligands, chelating the same metal centre, our Janus head ligands provide hard and soft donor sites and act as molecular staples between two different organometallic residues.[1] Nitrogen atoms in the active ligand periphery provide the coordination site to the hard co-catalytic metal while the soft central phosphorus atom donates a d-block organo- metallic moiety[2] (Fig. 1).
The bis(2-pyridyl)phosphane HP(py)2; py =2-pyridyl, undergoes tautomerism to a P(py)(pyH) species, where the hydrogen atom is pyridyl-bound. In this conformer the phosphorus atom still acts as a formal four-electron-donor with two non-bonding valence shell charge concentrations. The two lone pairs at the central phosphorus atom were localized and quantified by means of theoretical and experimental charge density investigations (Fig. 2 left). Consequently, the phosphorus atom can be addressed by two soft organometallic W(CO)5 -fragments and act as a non-A-frame μ-bridging ligand[3] (Fig. 2 right).
More recently we focus on heteroaromatic substituted phosphanes with even more donor sites. These ligands contain at least two different coordination sites different in complexation formation constants. When they are oriented to different directions they might serve as molecular staples in bi- or heterobimetallic compounds which might turn out to be useful catalysts. The (N,P,N)-ligand di(benzothiazol-2-yl)phosphane, P(bth)(Hbth) with
bth = CNS(C6H4), contains a divalent phosphorus atom because the hydrogen atom of this secondary phosphane is located at one of the ring nitrogen atoms forming an intramolecular hydrogen bridge[4]. The phosphanideP(bth)2– can be addressed by the two different coordination sites. Two different coordination motifs of the same ligand with the same metal in the same oxidation state were established. On the one hand, a dual (N,N)-coordinated iron com- plex with only 14 valence electron is accessible by the addition of [Iron(II)bis{di(trimethylsilyl)}amide] to the pure phosphane to give [Fe{(μ-bth)2P}2] (Fig. 3 left). On the other hand, an 18 valence electron complex with a strong phosphanidic character is applicable by transmetalation to give [Cp(CO)FeP(bth)2]2 (Fig. 3 right). This project is awarded a grant in the PhD program Catalysis for Sustainable Synthesis (CaSuS) funded by the Land Niedersachsen.
PhP(CH2py)2 is a neutral phosphane that may donate either via the lone pair at the P(III) atom and the N-heterocyclic nitrogen lone pairs. In addition, dual deprotonation of the phosphane can be achieved with n-butyl lithium in diethyl ether solution. This leads to a Janus head ligand in which one coordination site is provided by the two carbanionic CH bridges while the two ring nitrogen atoms and the phosphorus atom supply the second site. The ligand responds differently to the requirements of various metals[5] (Fig. 4). Those ligands are promising targets in homogeneous catalysis.
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[1] F. Baier, Z. Fei, H. Gornitzka, A. Murso, S. Neufeld, M. Pfeiffer, I. Rüdenauer, A. Steiner, T. Stey, D. Stalke J. Organomet. Chem. 2002, 661, 111.
[2] T. Stey, J. Henn, D. Stalke Chem. Commun. 2007, 413.
[3] J. Henn, K. Meindl, A. Oechsner, G. Schwab, T. Koritsanszky, D. Stalke Angew. Chem. 2010, 122, 2472; Angew.Chem. Int. Ed. 2010, 49, 2422.
[4] C. Kling, D. Leusser, T. Stey, D.Stalke Organometallics 2011, 30, 2461.
[5] I. Objartel, N. A. Pott, M. John, D.Stalke Organometallics 2010, 29, 5670.