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44904-AC3
Rigid Pincer Ligands: New Prospects in Hypercoordinate Main Group Chemistry
Grant Report for the ACS PRF award #44904-AC3.
September 2006 � August 2007.
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Hypercoordinate Al complexes.1� Our investigations focused on the synthesis of five-coordinate Al complexes supported by the rigid PNP pincer ligands and the feasibility of the formation of a hypercoordinate Al alkylidenes.� Preparation of various (PNP)AlX2 complexes 2-4 was successfully accomplished via alkane elimination reactions of EtAlCl2 or trialkylaluminums with (PNP)H.� The structure of (PNP)AlCl2 (2) was determined by X-ray diffractometry and was shown to contain a five-coordinate Al center in an approximately trigonal-bipyramidal geometry.� The structures of (PNP)AlCl2 (2), (PNP)AlMe2 (3), and (PNP)AlBui2 (4) were probed by DFT computational methods (in collaboration with Prof. D. Yandulov).� DFT studies show that the five-coordinate structure is preferred for all three compounds, although the preference decreases in the substituent order of Cl > Me > Bui.� Computations were also performed to investigate whether a-abstraction from 3* to produce an aluminum alkylidene 5* is thermodynamically favorable.� The findings indicate that such a reaction is decidedly unfavorable.� It is unlikely that the high endoergonics of this transformation may be overcome by varying the Al-bound alkyl groups.
Figure 1.� Synthesis and DFT studies of PNP complexes of Al.
Dimeric PNP complex of silver(I).2� In the course of our studies, we prepared a Ag(I) derivative of the PNP ligand.� We surmised that a (PNP)Ag complex may serve as a convenient PNP transfer agent.� [(PNP)Ag]2 (6) exists as a dimer both in solution and in the solid state and can be conveniently prepared from simple Ag(I) starting materials and (PNP)H.� [(PNP)Ag]2 (6) is an air-, moisture-stable compound (unlike (PNP)Li or (PNP)Tl) and serves well to transfer the PNP ligand to divalent group 10 metals.� It is worth noting that both the Al and the Ag chemistry was performed primarily by a talented undergraduate Jessica DeMott who is presently a Ph.D. student in chemistry at Stanford.
Figure 2.� Synthesis and structure of [(PNP)Ag]2 (6).
Activation of dihydrogen, water, and ammonia by a dipalladium complex.3� We have isolated unusual PdI-PdI dimer 7 that is formed in the photochemical decomposition of (PNP)PdEt.� Curiously, this is a complex of a general formula [(PNP)M]2, but has a structure that is quite different from that of [(PNP)Ag]2 (vide supra).2� We anticipated that the reactivity of this dimer will be driven by the formation of monomeric (PNP)Pd-X compounds.� This has proven to be true in the reactions with very simple molecules.� 7 cleanly reacts with H-H, H-OH, and H-NH2 to add the H-X bonds in these molecules across the Pd-Pd bond.� This binuclear oxidative addition4 is especially remarkable for NH3.� Only recently was the first well-defined example of oxidative addition of ammonia been documented5 and ours is the only example where ammonia is split into a terminal hydride and a terminal amido moiety by two metal centers.
References.
(1) DeMott, J. C.; Guo, C.; Foxman, B. M.; Yandulov, D. V.; Ozerov, O. V. Mendeleev Commun. 2007, 17, 63 (invited contribution to the Higher Chemical College issue).
(2) DeMott, J. C.; Basuli, F.; Kilgore, U. J.; Foxman, B. M.; Huffman, J. C.; Ozerov, O. V., Mindiola, D. J. Inorg. Chem. 2007, 46, 6271.
(3) Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola, D. J.; Ozerov, O. V. J. Am. Chem. Soc. 2007, 129, 10318.
(4) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 4th ed.; Wiley-Interscience: New York, 2005, pp. 160-161.�
(5) Zhao, J.; Goldman A. S.; Hartwig J. F. Science, 2005, 307, 1080.�
