This is the peer reviewed version of the following article: J. Abbenseth et al., Angew. Chem. Int. Ed. 2019, 58, 10971-10974, which has been published in final form at DOI: 10.1002/anie.201905130. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. Oxidative Coupling of Terminal Rhenium Pnictide Complexes Josh Abbenseth, [a] Martin Diefenbach, [b] Alexander Hinz, [c] Lukas Alig, [a] Christian Würtele, [a] Jose M. Goicoechea,* [d] Max C. Holthausen,* [b] and Sven Schneider* [a] Abstract: The isolation of rhenium(V) complexes with terminal phosphide and arsenide ligands is reported upon decarbonylation of rhenium(III) pnictaethynolates. One electron oxidation of the pnictide complexes yields Pn–Pn (Pn = P, As) coupling products that were spectroscopically and crystallographically characterized. Computational bond analysis suggests best description as {Pn2} 0 complexes that are stabilized by donor-acceptor interactions with the metal and a pyrazole ligand. Dinitrogen exhibits one of the strongest bonds known in chemistry (BDE = 225.1 kcal·mol –1 ). [1] In contrast, the heavier homologues P2 (BDE = 161.1 kcal·mol –1 ) and As2 (BDE = 91.3 kcal·mol –1 ) suffer from poor π-orbital overlap and are only stable at temperatures beyond 1500 K. [2] In the condensed phase, in situ generation and trapping of free Pn2 (Pn = P, As) is possible by extrusion from diphosphaazides (P2N–R), white phosphorous (P4), a P2-bisanthracene adduct, or dipnictide complexes (Pn2(NiLL’)2), respectively. [3] In recent years, several strategies were evaluated to stabilize Pn2 fragments, e.g. with N-heterocyclic carbene (NHC) ligands (Scheme 1, left) upon reductive coupling of PnCl3 adducts or via P2 extrusion from P4. [ 4 ] Recently, this area strongly benefitted from the introduction of heavy cyanate analogues (PnCO – ) as Pn synthons. [5] (Photo-)decarbonylative coupling of 2- phospha-/arsaethynolate precursors led to a variety of phosphanyl, [ 6 ] germanyl, [ 7 ] cyclopropenyl, [ 8 ] and stannyl [ 9 ] stabilized diphosphene/diarsene (RPn=PnR) products. Transition metal coordination provides an alternative strategy to stabilize P2 fragments. P4 activation has led to several side-on bridging complexes with {M2P2} butterfly-core (Scheme 1, center) and in rare cases end-on bridging diphosphinidene species {M=P–P=M}, [10] contrasting with the rich chemistry of end-on N2- bridged coordination compounds. [11] Decarbonylative coupling of phosphaethynolate ligands gave bridging {P2} 2– (Scheme 1, right) and {P2} 4– ligands. [3d, 12 ] Terminal phosphide species can be assumed as key intermediates for these P–P coupling reactions but direct observation of oxidative {MºP} coupling or reverse {M(P2)M} reductive cleavage remains scarce. [13] Heavy transition metal terminal pnictide complexes are currently restricted to group 5-6, [14] most of them with d 0 configuration, which prevents oxidative functionalization. [13b, 15 ] Coupling reactivity of terminal arsenides was not reported. Scheme 1. Examples for isolable diphosphene compounds that arise from PCl3 reduction (left), P4 activation (center), and PCO decarbonylation (right), respectively (Ar = 2,6-iPr2C6H3; Ar’ = 3,5-C6H3Me2; Ar’’ = 2,6-Me2C6H3). [4a,12a,16] Our group examined the mechanism of dinitrogen splitting upon reduction of rhenium(III) pincer complex ReCl2(PNP) (PNP = N{CH2CH2P t Bu2}2, 1Cl). [17] Facile nitride formation via N–N bond cleavage of [(µ-N2){ReCl(PNP)}2] emphasizes strong rhenium to pnictide multiple bonding and motivated the examination of higher homologues. 1Cl and the analogous iodide [ReI2(PNP)] (1I) were reacted with the phosphide transfer reagent [Na(diox)x]PCO. [18] Unselective conversion to several, intractable products was indicated by 31 P{ 1 H} NMR spectroscopy, probably including terminal phosphides as judged by characteristic low-field signals (dP = 1064 ppm (1Cl + PCO – ); 1092 ppm (1I + PCO – )). [18] In case of 1Cl, the carbonyl complex [ReCl2(CO)(PNP)] was found as major side product upon comparison with an original sample. [18] To prevent CO binding to the rhenium(III) halide precursor, a sixth ligand was introduced that blocks the vacant coordination site. On the other hand, a large trans-influence is expected for terminal pnictide ligands. Consequently, pnictaethynolate decarbonylation should be facilitated by coordinative unsaturation. Therefore, a hemilabile, chelating ligand was chosen as synthetic strategy, more specifically 3-(1H-pyrazol-3-yl)-pyridine ( H PyrPz) to support hemilability by pyrazolate linkage isomerism. Starting from [ReI(k 2 N 1 ,N 3 -PyrPz)(PNP)] (2), the first rhenium heavier pnictide complexes, [Re(Pn)(kN 2 -PyrPz)(PNP)] (Pn = P (3P), As (3As)), can be obtained and isolated in 60-70% yield (Scheme 2). [18] NMR spectroscopic monitoring of phosphide formation indicates an intermediate, which was assigned to [Ru(PCO)(PyrPz)(PNP)] (4P, dP(PCO) = -269 ppm) and converts to 3P above –20°C. In comparison to 3P, the sharp 1 H-NMR signals of the N-heteroaromatic ligand of 4N (r.t.) and the P P N N Ar Ar N N Me N Me Ar'' Ar'' Ir N N Me N Me Ar'' Ar'' Ir OC CO P P Nb N P N N CH 2 t Bu Ar' t BuH 2 C Ar' Ar' t BuH 2 C P Nb N N N t BuH 2 C Ar' CH 2 t Bu Ar' Ar' CH 2 t Bu N N Ar Ar 2.021(1) Å 2.2052(10) Å 2.150(2) Å [a] J. Abbenseth, L. Alig, Dr. C. Würtele, Prof. Dr. S. Schneider Georg-August-Universität, Institut für Anorganische Chemie Tammannstraße 4, 37077 Göttingen (Germany) E-mail: sven.schneider@chemie.uni-goettingen.de [b] Dr. M. Diefenbach, Prof. Dr. M. C. Holthausen Institut für Anorganische und Analytische Chemie Goethe-Universität Max-von-Laue-Strasse 7, 60438 Frankfurt am Main (Germany) E-mail: max.holthausen@chemie.uni-frankfurt.de [c] Dr. A. Hinz Karlsruhe Institute of Technology, Institute of Inorganic Chemistry Engesserstr. 15, 76131 Karlsruhe (Germany) [d] Prof. Dr. J. M. Goicoechea Department of Chemistry, University of Oxford Chemistry Research Laboratory 12 Mansfield Road, OX1 3TA, Oxford (UK) E-mail: jose.goicoechea@chem.ox.ac.uk Supporting information for this article is given via a link at the end of the document.