DOI: 10.1002/ejic.201600170 Full Paper Aluminium Hydrazides Reactivity of a Monomeric Aluminium Hydrazide towards Isocyanates and Isothiocyanates: Active Lewis Pair Behaviour versus Classical Insertion Reactions Werner Uhl,* [a] Julia Silissa Bruchhage, [a] Matthias Willeke, [a] Alexander Hepp, [a] and Jutta Kösters [a] Abstract: Hydroalumination of H 10 C 5 N–N=C(Ad) with HAliBu 2 yielded the hydrazide H 10 C 5 N–N(AliBu 2 )Ad (4; Ad = adamantyl, NC 5 H 10 = piperidinyl). Compound 4 is monomeric and contains a highly strained AlN 2 heterocycle formed by a donor–acceptor interaction between the Al atom and the -N atom of the hydrazine. Two equivalents of the hydride afforded an adduct with an HAliBu 2 molecule coordinated to 4 by an Al–N and a 3c–2e Al–H–Al bond (5). tBuN=C=O did not insert into the rela- tively weak Al←N donor–acceptor bond of 4; instead, insertion Introduction Frustrated Lewis pairs (FLPs) are a fascinating class of com- pounds and show unprecedented reactivity with the activation or coordination of various substrates. [1] Their exceptional prop- erties are based on the presence of coordinatively unsaturated Lewis acidic and basic atoms in single molecules or bimolecular systems for which the usually observed formation of Lewis acid–base adducts is prevented by steric shielding. FLPs are ac- cessible by facile routes with a wide variety of different donor and acceptor groups. [1] Recent investigations have shown that coordinative unsaturation is not a necessary prerequisite for FLP activity. [1] Monomeric or dimeric compounds having intra- or intermolecular bonding interactions between the opposite functionalities have been applied successfully in activation or complexation reactions, although cleavage of the bonds be- tween the Lewis acidic and basic centres is required as an en- ergy-consuming first step. Aluminium hydrazides are promising candidates for such studies, but they usually form stable dimers with four-, five- or six-membered Al–N heterocycles. [2] Mono- meric aluminium hydrazides are extremely rare. [3,4] One is a bis- hydrazide and was obtained by treatment of H 2 N–N(H)SiMe 3 with HAltBu 2 with dismutation, [3] and the other (1; Scheme 1) was synthesised by hydroalumination of a sterically shielded hydrazone with the same hydride. [4] Both compounds showed intramolecular Al–N interactions between the aluminium atom [a] Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, 48149 Münster, Germany E-mail: uhlw@uni-muenster.de www.uni-muenster.de/chemie.ac/uhl/index.html ORCID(s) from the author(s) for this article is/are available on the WWW under http://dx.doi.org/10.1002/ejic.201600170. Eur. J. Inorg. Chem. 2016, 2721–2730 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2721 into the shorter Al–N(amide) bond yielded five-membered AlN 3 C heterocycle 6 with an exocyclic C=O group. PhN=C=O showed a similar reaction, but the product 7 was dimeric in the solid state with two Al–O bonds. Reaction in a 1:2 ratio afforded a unique linear dimer of phenyl isocyanate, which is stabilised by the aluminium hydrazide (8). Isothiocyanates gave thiosemi- carbazides 9–11, which have Al atoms coordinated in a chelat- ing manner by formation of AlSCN heterocycles and show fasci- nating differences in their bonding situation. and the -nitrogen atom of the hydrazide to form strained three-membered AlN 2 heterocycles with relatively long Al–N donor–acceptor bonds. For compound 1 the Al–N bond lengths in the ring differed considerably [181.7(2) vs. 200.8(2) pm], and the Wiberg bond indices were calculated to 0.24 and 0.13. [4] Cleavage of the relatively weak donor–acceptor bonds resulted in a situation similar to that of FLPs, [1,5] and indeed treatment of 1 with phenyl isocyanate afforded an adduct (2; Scheme 2) in which the oxygen atom was coordinated to aluminium, while the carbonyl carbon atom was attached to the -nitrogen atom of the hydrazide. [4] The constitution of 2 resembles that of typi- cal FLP–isocyanate adducts reported in the literature. [1,6] Com- Scheme 1. Schematic drawings of monomeric di-tert-butylaluminium hydraz- ide 1 and the products obtained by insertion of phenyl isocyanate.