Note A new aqueous molybdenum cluster with a Mo 3 (l 3 -CCH 3 )(l 3 -O) trinuclear core Kris A. Pineda a , James C. Fettinger b , Jacqueline R. Houston a,⇑ a California State University, Sacramento, Department of Chemistry, 6000 J Street, Sacramento, CA 95819-6057, United States b Department of Chemistry, University of California, Davis, One Shields Ave., Davis, CA 95616, United States article info Article history: Received 11 February 2012 Received in revised form 16 March 2012 Accepted 19 March 2012 Available online 27 March 2012 Keywords: Trinuclear molybdenum cluster Alkylidyne Propionate Crystal structure Metal-oxo NMR abstract Reaction of Mo(CO) 6 with propionic acid and propionic anhydride gives the expected l 3 -propylidyne capped molybdenum cluster as well as the unanticipated l 3 -ethylidyne capped molybdenum cation with bridging propionates, [Mo 3 (l 3 -CR)(l 3 -O)(l-O 2 CCH 2 CH 3 ) 6 (H 2 O) 3 ]BrÁ4H 2 O; R = CH 3 and CH 2 CH 3 . Due to the similarity in structure, both complexes crystallize together in the triclinic crystal system upon slow evaporation of NaBr solutions (T = 277 K). Herein, we report the synthesis, characterization, and crystal- lographic study of the carbon-capped clusters. To our knowledge, this study is the first to report a trinu- clear molybdenum cation in which the l 3 -alkylidyne ligand is not the same chain length as the carboxylic acid used in the synthesis. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Led by the seminal work of Bino and Cotton, acetate-bridged tri- nuclear molybdenum cluster cations with formula [Mo 3 (l 3 -X)(l 3 - Y)(l-O 2 CCH 3 ) 6 (H 2 O) 3 ] z+ , where X = Y = O, X = Y = CCH 3 and where X = O and Y = CCH 3 , were discovered when Mo(CO) 6 was refluxed with a mixture of acetic acid and acetic anhydride [1–4]. Of these, the most intriguing molecular cation is the oxidized form of the bi- ethylidyne capped cluster (X = Y = CCH 3 ), which upon dissolution in water generates gaseous 2-butyne [5]. Mechanistically, it was proposed that the formation of 2-butyne was the result of a C–C coupling reaction in which free methyl carbyne radicals combined in solution [6]. Surprisingly, 2-butyne was not the only organic substrate found in solution, however. Other organic side-products including propyne, propene, acetaldehyde, and number of organics with short carbon chains (i.e. ethane and ethene) were detected as well. In our attempt to synthesize the bi-oxo capped propylidyne cluster, we discovered the co-crystallization of two molybdenum complexes, the mono-l 3 -CCH 2 CH 3 and the unanticipated mono- l 3 -CCH 3 capped cluster both with bridging propionate ligands ([Mo 3 (l 3 -CR)(l 3 -O)(l-O 2 CCH 2 CH 3 ) 6 (H 2 O) 3 ]BrÁ4H 2 O with R = CH 3 1 and CH 2 CH 3 2; Scheme 1). The discovery of 1 was entirely unex- pected because the propionic acid–anhydride mixture used in the synthesis should yield only the propylidyne capped cluster, as pre- viously reported by Bino and Gibson [7]. Our discovery of 1 may in fact support the carbyne radical coupling reaction in that the for- mation of the l 3 -CCH 3 capping group would require other reactive species to be present in solution besides propionic acid. 2. Results and discussion Although the original synthesis reported by Bino and Gibson [7] was only slightly modified, we isolated both the l 3 -propylidyne capped cluster and l 3 -ethylidyne capped cluster. Due to their sim- ilarity in structure, 1 and 2 crystallize in the same crystallographic site as the tetrahydrate bromide salt. Fig. 1 shows thermal ellipsoid drawings for structures 1 (a) and 2 (b), in which the C(2a) atom po- sition refers to the terminal methyl group of the l 3 -ethylidyne chain on 1 and the C(2) position refers to the methylene of the l 3 -propylidyne of 2. Both molecular cations are trinuclear with pseudo-C 3v symmetry in which each Mo(IV) ion is bound through metal–metal bonding to give a central Mo 3 unit. Above and below the Mo 3 plane are two three-coordinate capping ligands, a l 3 -CR and a l 3 -O giving a slightly distorted trigonal bipyramid. In order for the cluster cations to be monovalent, each Mo ion must be in the (IV) oxidation state, which is consistent with other trinuclear molybdenum cations [4,8,9]. Ligated to each Mo(IV) are four bridging propionate ligands and a terminal water giving rise to nine-coordinate Mo(IV) cations. The complexes crystallize together in the triclinic crystal system with space group P  1 and the following unit-cell dimen- sions; a = 11.6658(11) Å, b = 13.7602(12) Å, c = 13.7673(12) Å and 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.03.037 ⇑ Corresponding author. Address: California State University, Sacramento, Department of Chemistry, 6000 J Street, Sacramento, CA 95819-6057, United States. Tel.: +1 916 278 2583; fax: +1 916 278 4986. E-mail addresses: jcfettinger@ucdavis.edu (J.C. Fettinger), jhouston@csus.edu (J.R. Houston). Inorganica Chimica Acta 392 (2012) 485–489 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica