Molecular Alloys, Linking Organometallics with Intermetallic Hume−Rothery Phases: The Highly Coordinated Transition Metal Compounds [M(ZnR)_n] (n≥ 8) Containing Organo−Zinc Ligands

Abstract

This paper presents the preparation, characterization and bonding analyses of the closed shell 18 electron compounds [M(ZnR)(n)] (M = Mo, Ru, Rh, Ni, Pd, Pt, n = 8-12), which feature covalent bonds between n one-electron organo-zinc ligands ZnR (R = Me, Et, eta(5)-C(5)(CH(3))(5) = Cp*) and the central metal M. The compounds were obtained in high isolated yields (>80%) by treatment of appropriate GaCp* containing transition metal precursors 13-18, namely [Mo(GaCp*)(6)], [Ru(2)(Ga)(GaCp*)(7)(H)(3)] or [Ru(GaCp*)(6)(Cl)(2)], [(Cp*Ga)(4)RhGa(eta(1)-Cp*)Me] and [M(GaCp*)(4)] (M = Ni, Pd, Pt) with ZnMe(2) or ZnEt(2) in toluene solution at elevated temperatures of 80-110 degrees C within a few hours of reaction time. Analytical characterization was done by elemental analyses (C, H, Zn, Ga), (1)H and (13)C NMR spectroscopy. The molecular structures were determined by single crystal X-ray diffraction. The coordination environment of the central metal M and the M-Zn and Zn-Zn distances mimic the situation in known solid state M/Zn Hume-Rothery phases. DFT calculations at the RI-BP86/def2-TZVPP and BP86/TZ2P+ levels of theory, AIM and EDA analyses were done with [M(ZnH)(n)] (M = Mo, Ru, Rh, Pd; n = 12, 10, 9, 8) as models of the homologous series. The results reveal that the molecules can be compared to 18 electron gold clusters of the type M@Au(n), that is, W@Au(12), but are neither genuine coordination compounds nor interstitial cage clusters. The molecules are held together by strong radial M-Zn bonds. The tangential Zn-Zn interactions are generally very weak and the (ZnH)(n) cages are not stable without the central metal M.

The Reactivity of [Zn2Cp*2]: Trapping Monovalent {.ZnZnCp*} in the Metal‐Rich Compounds [(Pd,Pt)(GaCp*)a(ZnCp*)4−a(ZnZnCp*)4−a] (a=0, 2)

“…[14] Both species were not observed in our previous studies on the synthesis of [TM(ZnR) n ] compounds from [TM(GaCp*) m ] and ZnR 2 mentioned in the introduction. [11,12] The observation of free [GaCp*] and [ZnCp* 2 ] as by-products is consistent with the expectation that the formation of 1 and 2 should not involve redox reactions between TM, Ga, and Zn, but rather proceed by dissociation of {GaCp*} from [Pd-(GaCp*) 4 ] and addition of the ZnÀZn bond, that is, trapping of monovalent {CZnCp*} at electronically and coordinatively unsaturated palladium centers. This reasoning is also consis- Ellipsoids are set at 50 % probability; hydrogen atoms are omitted for clarity.…”

“…Whereas the Pd À ZnCp* distances (av. 2.448(1) ) are similar to the PdÀZn bond in [Pd(ZnCp*) 4 (ZnMe) 4 ] (2.447(1)-2.459(1) ), [11] the PdÀZnZnCp* distances are distinctly shorter (2.375(1) and 2.379(1) ). Whether this fact can be attributed only to the decreased steric demand of the {ZnZnCp*} group with respect to the {ZnCp*} ligand or also to a higher p character of the interaction between the bare Zn atom to Pd cannot be determined at this point and is part of ongoing work.…”

“…Recently, we established an access to very zinc-rich, highly coordinated [TM(ZnR) n ] compounds (TM: Group 6-10 element, n = 8-12) bridging the gap between complexes, clusters, and Hume-Rothery intermetallic phases, with the icosahedral [Mo(ZnCp*) 3 (ZnMe) 9 ] as prototype of a novel family. [11,12] The formation reaction starts from mononuclear complexes [TM(GaCp*) m ] (m = 4-6) and ZnR 2 (R = Me, Et) and involves Ga/Zn and Cp*/R exchange processes. By manual separation of the different crystals under a microscope in a glove box in the presence of solvent, samples of analytically pure 1 can be obtained, whereas it was not possible to collect substantial amounts of pure 2 suitable for C/H microanalysis or NMR spectroscopy before the crystals decompose.…”

“…Thus, the assumption of a {PdZn 12 } composition of 2 is based on the structural and computational analysis of 1 together with heuristic reasoning based on electron counting, as all homoleptic [M(ZnR) x ] and heteroleptic [M(ZnR) x (GaR) y ] compounds synthesized and characterized to date perfectly fulfill the eighteen valence-electron rule. [11,12] The coordination polyhedron of the inner {PdZn 8 } core of 2 can be best described as a slightly distorted trigonal dodecahedron (Figure 2), which is comparable to [Pd(ZnCp*) 4 (ZnMe) 4 ]. [11] The Zn À Zn bond distances of the four {ZnZnCp*} units (2.347 (3) 3+ [20,21] (dpmp = bis(diphenylphosphanylmethyl)phenylphosphane; Xyl = 2,6-dimethylphenyl) or [Ni 5 (m 5 -tpda) 4 Cl 2 ] and [Ru 5 (m 5 -tpda) 4 (NCS) 2 ]) (tpda = tripyridyldiamine).…”

Chemie von [Zn2Cp*2]: einwertiges {.ZnZnCp*}, stabilisiert in den metallreichen Verbindungen [(Pd,Pt)(GaCp*)a(ZnCp*)4−a(ZnZnCp*)4−a] (a=0, 2)

“…Da Gallium und Zink wegen ihres sehr ähnlichen Streuvermögens röntgen-strukturanalytisch nicht eindeutig unterscheidbar sind, wurde die Zuordnung von Ga/Zn in der Festkörperstruktur von 1 durch DFT-Rechnungen unterstützt. [31] Um den Metallpositionen (M) Gallium oder Zink zuordnen zu können, wurde in einem ersten Schritt die Energie für alle 28 möglichen Permutationen von zwei Gallium-und sechs Zinkatomen über die acht M-Positionen einschließlich der {MMCp*}-Einheiten auf BP86/def2-TZVPP-Niveau [32,34] (1)-2.459(1) ) sind, [11] erhält man für die Pd-ZnZnCp*-Abstände deutlich geringere Werte von 2.375(1) und 2.379(1) . Ob diese Tatsache nur am verringerten sterischen Anspruch der {ZnZnCp*}-gegenüber jenem der {ZnCp*}-Liganden oder auch an einem höheren pCharakter der Wechselwirkung zwischen dem "nackten" ZnAtom und dem Pd-Zentrum liegt, kann an dieser Stelle nicht entschieden werden und ist Gegenstand laufender Arbeiten.…”