Platinum-Molybdenum Complexes of Cyclic Tropynes, Cumulenes, and Alkynes Jerzy Klosin, Khalil A. Abboud, and W. M. Jones* Department of Chemistry, University of Florida, Gainesville, Florida 32611 Received September 14, 1995 X The bis(triphenylphosphine)platinum complexes of tropyne (1), cyclohepta-3,5-dien-1-yne (12), and cyclohepta-1,5-dien-3-yne (13) react rapidly with (η 6 -p-xylene)Mo(CO) 3 in a mixture of CD 2 Cl 2 and THF-d 8 to give the bimetallic complexes 4-6. Reduction of 4 with LiAl(O- t-Bu) 3 H or KBEt 3 H gives a mixture of 5-7 (essentially quantitative) in a ratio of 5:10:85 and 8:21:71, respectively. The major isomer contains a 1,2,3,5-cycloheptatetraene ring, a ring system that has previously been inaccessible either free or complexed to a transition metal. Hydride abstraction from 5, 6, or 7 regenerates 4. X-ray crystal structures for 5 and 7 are reported. The absolute configuration of complex 7 in the crystal was determined. Introduction In contrast to benzyne 1 and its transition metal complexes, 2 which have been studied extensively, to date published information about tropyne, benzyne’s next higher homologue, has been limited to platinum 3 (1) and zirconium 4 (2) complexes of the parent and one platinum complex of a dibenzannelated analog (3). 5 As a continuation of our work on the chemistry of 1, and its cycloheptadienyne precursors 12 and 13, we have now prepared their Mo(CO) 3 bimetallic complexes (4- 6) and have made the surprising discovery that reduc- tion of the tropyne complex gives a good yield of a new C 7 H 6 ring system. Results and Discussion Preparation of 4 by Tropyne-Arene Exchange. It has been found 6 that reaction of [Mo(η 7 -C 7 H 7 )(η 6 - C 6 H 6 )] + with nucleophilic ligands leads to displacement of benzene leaving the tropylium ligand intact. This suggests that the tropylium ion may be bonded to molybdenum more strongly than arenes. Since tropyne can be regarded as a substituted tropylium ion and since the aromatic ring can be readily displaced 7 in (η 6 -arene)- Mo(CO) 3 by more basic arenes, it occurred to us that it might be possible to use an arene exchange reaction to attach the Mo(CO) 3 fragment to the tropyne ring in 1. Indeed, reaction of the tropyne complex 1 with 1 equiv of (η 6 -p-xylene)Mo(CO) 3 at room temperature in a mixture of CD 2 Cl 2 and THF-d 8 led to an essentially instantaneous color change from red to brown-red. Comparison of the NMR spectra of this solution with the tropyne complex synthesized more conveniently by hydride abstraction from a mixture of 5 and 6 (vide infra) confirmed the essentially quantitative yield of 4 (Scheme 1). Unfortunately, we were unable to grow crystals of this red-brown solid that were suitable for X-ray diffraction. However, the bimetallic complex was completely characterized by multinuclear NMR spec- troscopy, IR, and HRMS. In the 1 H NMR, in addition to PPh 3 signals three different resonances are displayed in the range δ 5.55-6.1 ppm with the one at δ 5.55 ppm showing coupling to the 195 Pt nucleus. The multiplicity of these three signals is the same as that of 1 but each is shifted ca. 3 ppm upfield from 1. Both the 19 F{ 1 H} and the 31 P{ 1 H} NMR show singlets whereas the 195 Pt- { 1 H} NMR exhibits a triplet centered at δ -4087.6 ppm. The chemical shift of the 195 Pt nucleus is 308 ppm upfield from the corresponding resonance for complex 1. The IR spectrum displays two very strong bands (2036, 1976 cm -1 ) in the metal carbonyl region. These stretching frequencies are ca. 40 cm -1 lower than those of the Mo(CO) 3 complex of the tropylium cation which is consistent with a somewhat more electron-rich tro- pyne ring in the former [from electron donation from bis(triphenylphosphine)platinum]. The carbonyl region in the 13 C NMR shows only one peak, even at -100 °C, indicating a very low barrier for rotation of the molyb- denum tricarbonyl around the molybdenum-seven- membered-ring axis. Hydride Reduction of (PPh 3 ) 2 Pt{η 2 [(η 7 -C 7 H 5 )Mo- (CO) 3 ]} (4). Reduction of 4 with either KBEt 3 H or LiAl- (O-t-Bu) 3 H was noticeably cleaner (as shown by the 31 P NMR) than the corresponding reactions with 1 or 2, giving a mixture in which 7 is the major product (5:6:7 ) 5:10:85 for LiAl(Ot-Bu) 3 H and 8:21:71 for KBEt 3 H) (Scheme 2). Selective formation of 7 from hydride addition to the tropyne carbon indicated by arrow b was not only unexpected but was of particular interest because the resulting 1,2,3,5-cycloheptatetraene ring in X Abstract published in Advance ACS Abstracts, December 1, 1995. (1) Hoffmann, R. Q. Dehydrobenzene and Cycloalkynes; Verlag Chemie: Weinheim, Germany, 1967. (2) Bennett, M. A.; Schwemlein, H. P. Angew. Chem., Int. Ed. Engl. 1989, 28, 1320. (3) Lu, Z.; Abboud, K. A.; Jones, W. M. J. Am. Chem. Soc. 1992, 114, 10991. (4) Lu, Z.; Jones, W. M. Organometallics 1994, 13, 1539. (5) Klosin, J.; Abboud, K. A.; Jones, W. M. Organometallics 1995, 14, 2892. (6) Ashworth, E. F.; Green, J. C.; Green, M. L. H.; Knight, J.; Pardy, R. B. A. J. Chem. Soc., Dalton Trans. 1977, 1693. (7) Muetterties, E. L.; Bleeke, J. R.; Sievert, A. L. J. Organomet. Chem. 1979, 178, 197. 596 Organometallics 1996, 15, 596-603 0276-7333/96/2315-0596$12.00/0 © 1996 American Chemical Society