Communication Chemistry—A European Journal doi.org/10.1002/chem.202001168 & Platinum Complexes Boranediyl- and Diborane(4)-1,2-diyl-BridgedPlatinum A-Frame Complexes CarinaBrunecker,[a] Jonas H. Müssig,[a] Merle Arrowsmith,[a] Felipe Fantuzzi,[a, b] Andreas Stoy,[a] Julian Bçhnke,[a] Alexander Hofmann,[a] Rüdiger Bertermann,[a] Bernd Engels,[b] and Holger Braunschweig*[a] and p backbonding from the electron-rich metal centertothe Abstract: Diplatinum A-frame complexes with abridging borylene, resulting in metal–boron multiple bonding.[3] (di)boron unit in the apex position were synthesized in a Like Schrock carbenes,however,borylenesare often found single step by the doubleoxidative addition of dihalo(di)- in bridging positionsbetween two or more metal centers. 0 borane precursors at abis(diphosphine)-bridged Pt 2 com- Since the isolation of the first dinuclear bridging borylene plex. While structurally analogous to well-known m-bory- 5 complexes [m-(BY){h -(C5H4R)Mn(CO)2}2](Y=NMe2,R=H; Y= lene complexes, in which delocalized dative three-center- tBu, R= Me),[4] this class of compounds has been extensively two-electronM-B-M bonding prevails,theoretical investi- studied in terms of electronicproperties and reactivity.[3a–c] The gationsintothe natureofPt Bbonding in these A-frame À natureofM-B-M bonding in borylene-bridged dimanganese complexes show them to be rare dimetalla(di)boranes dis- complexes (I,Figure 1) was examined both experimentally and playing two electron-sharing Pt B s-bonds. This is experi- À mentally reflected in the low kinetic stability of these compounds, which are pronetoloss of the (di)boron bridgehead unit. In organometallic chemistry,transition metal carbene com- plexes are divided into two classes:a)Fischercarbenes,in Figure 1. Bonding modes in homobimetallic m-BY-bridged complexes. Cp = which R2C Mbondingisgoverned by s donation of the car- À cyclopentadienyl. bene lone pair into an empty metal orbital and p backdona- tion from afilled metal dorbitalinto the empty carbenepor- bital, and b) Schrock carbenes,inwhich bondingoccurs be- computationally.Instead of the delocalized three-center-two- [1] tween atriplet R2C: carbene and atriplet-state metal center. electron (3c2e) dative bonding expected for bridging bory- With their singletground state, which is independentofthe lenes,the topological analysis of the electrondensity distribu- nature of the substituent R,[2] and emptyporbitals, borylenes tion from alow-temperature, high-resolution X-ray diffraction (RB:) may be consideredasanalogues of Fischer carbenes. experiment suggestedtwo localized, directional two-electron Thus, the bondinginterminal borylene complexes is governed Mn Bbonds,leadingthe authors to describe these complexes À by s donation from the borylenelone pair to the metal center as dimetallaboranes (or boranediyls).[5] Calculationsbased on the quantum theory of atomsinmolecules (QTAIM) and on the electron-localization function(ELF) revealed that the calculated [a] C. Brunecker,Dr. J. H. Müssig,Dr. M. Arrowsmith, Dr.F.Fantuzzi, A. Stoy, bondingsituation—that is, boryleneversus boranediyl— Dr.J.Bçhnke, Dr.A.Hofmann, Dr.R.Bertermann, Prof. Dr.H.Braunschweig strongly depends on the choice of exchange-correlation func- Institut fürAnorganische Chemieand [5] Institute for Sustainable Chemistry &Catalysis with Boron tional. Similarcalculations on homodinuclear nickel (II)and Julius-Maximilians-UniversitätWüzburg cobalt (III) m-borylenecomplexes suggested that the dinickel Am Hubland,97074 Würzburg (Germany) complex shouldbedescribed as atrue bridging borylene E-mail:[email protected] whereasbonding in the dicobalt complex is closer to the bora- [b] Dr.F.Fantuzzi, Prof. Dr.B.Engels nediylmodel,irrespective of the choice of density functional.[7] Institut fürPhysikalische und Theoretische Chemie Julius-Maximilians-UniversitätWürzburg Additionally,complexes I–III are all stabilized by delocalization Emil-Fischer-Straße 42, 97074 Würzburg (Germany) of the metal–metal bonding molecular orbital over the empty Supporting information and the ORCID identification number(s) for the orbitals of the boron bridgeand thusfulfil the 18 valenceelec- author(s) of this articlecan be found under: tron rule. https://doi.org/10.1002/chem.202001168. Since the nature of the M-B-M bondinginbridging “bory- 2020 The Authors. Published by Wiley-VCH Verlag GmbH&Co. KGaA. lene” complexes remains complicatedtodetermine both ex- This is an open access article under the terms of the Creative Commons At- tributionLicense, whichpermits use, distributionand reproduction in any perimentally and computationally,weset out to synthesize medium, provided the original work is properly cited. true boranediylcomplexes and compare their structure, elec- Chem. Eur.J.2020, 26,8518 –8523 8518 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Communication Chemistry—A European Journal doi.org/10.1002/chem.202001168 tronics and reactivity to known bridging borylenes. For this purpose, we chose so-called A-framecomplexes,[8] in whichthe two metal centers are tethered by two bridging diphosphine li- gands and abridging apex ligand, and which generally show no metal-metal bonding interactions. In this communication we presentthe synthesis and characterization of aseries of boranediyl platinum A-frame complexes (IV), as well as a unique diborane(4)-1,2-diyl complex, and undertake an in- depth computational analysisofthe nature of bondinginthe Pt-B-Pt bridge. Our precursor for the desired boron-bridged platinum A- frames, complex 1,was synthesized by reacting [Pt(nbe)3] (nbe=norbornene) with 2equiv bis(dimethylphosphino)me- thane (dmpm)at08C(Scheme1). The 31PNMR spectrumof1 shows asingle resonance of higher order,similartothe litera- Scheme3.Decomposition of boranediyl complexes 2-Y to 3 and crystallo- ture-known complex [(m-dppm)3Pt2](dppm =bis(diphenylphos- graphically derived molecularstructure of 3.Thermal ellipsoidsat50% prob- phino)methane).[9] In the solid state the Pt···Pt distance of ability.Thermal ellipsoidsofligand periphery and hydrogen atomsomitted for clarity.Selectedbondlengths ()and angles (8): Pt1 Pt1’ 2.6163(4),Pt1 4.096(1) confirms the absence of Pt Pt bonding in 1.Toour À À À Br1 2.5189(5), P1-Pt1-P2’ 169.53(4),Br1-Pt1-Pt1’ 172.859(11), torsion angle knowledge, 1 is the only crystallographically characterized di- P1-Pt1-Pt1’-P2 49.41(4). phosphine-bridgeddinuclear Pt0 complex without Pt Pt bond- À ing.[10] provinghighlyunstable in both solution and the solid state (Scheme3). By employing the dimethylsulfide precursor Me2S·BBr3 instead of BBr3 the yield of 2-Br became essentially quantitative (Scheme 2b). The 11BNMR spectra of 2-Y show extremely broad resonan- ces (full width at mid-height (fwmh) 1200 to 2150 Hz) at 98 (2-Dur), 85 (2-Br)and 52 ppm (2-NMe2), in line with the in- creasingly electron-donating nature of the substituent at boron.These are significantly upfield-shifted compared to Scheme1.Synthesis of diplatinum complex 1. structurally related borylene-bridgedbimetallic complexes(m- BAr: d11B =122 to 162 ppm; m-BBr: d11B =107 to 163 ppm; m- [3b,13] Previoussyntheses of terminal platinum borylene complexes BNR2 : d11B = 70 to 119ppm). It is noteworthy,however, 11 have generally relied on the oxidative addition of RBX2 dihalo- that the BNMR resonances of platinum-containingheterobi- 0 boranes to Pt centers to generate boryl complexes of the metallicbridging borylenes, [m-BY{(MLn)(PtLm)}],are generally form [L2PtX(BXR)],which were then convertedtothe corre- 20 to 35 ppm upfield-shifted compared to the analogous ho- [11] [12] [14] sponding cationic or neutral borylenes by halide abstrac- mobimetallic complexeswithout platinum, [m-BY(MLn)2], tion or base-induced halide transfertoplatinum,respectively. owing to the strongly electron-donating nature of the Pt0 With this in mind, we carried out the room-temperature addi- center. 31 1 tion of dibromoboranes (BYBr2,Y=Dur=2,3,5,6-Me4C6H, Br, The P{ H} NMR spectra of 2-Dur, 2-Br and 2-NMe2 showed NMe )toabenzene solution of complex 1,which led to the in- singletsat 13.6, 9.3 and 5.6 ppm, respectively,with a 2 À À À stant formation of ayellow–orange precipitate (Scheme 2a). complex higher-order satellite motif. These arise from the su- Recrystallization from adichloromethane/pentane mixture at perimposed spectra of isotopomers containing no 195Pt nuclei room temperature yielded the m-borylene diplatinum A-frame (43.8%, sharpsinglet), one 195Pt nucleus (44.8 %, AA’A’’A’’’X 195 complexes 2-Dur (66%), 2-Br (34%) and 2-NMe2,the latter spin system), and two Pt nuclei (11.4%, AA’A’’A’’’XX’ spin system), based on the natural abundance of these isotopes. Based on reported analyses of higher-order spectra in doubly dppm-bridged diplatinumcomplexes,[15] the 31P{1H} NMR spec- 1 trum of 2-Br (Figure 2) yields coupling constants of JPPt = 3270 Hz, JP1Pt2 = 220 Hz, JPtPt = 510 Hz and Q=JP1P2 +JP1P3 = 42 Hz.[16] These coupling constantsare similar to those ob- servedfor cationic m-hydride[15] and m-chloride-bridged[15b] and [17] neutralCH2-bridged (m-dppm)2Pt2 A-frame complexes. The 195Pt{1H} NMR spectrum of 2-Br displays ahigher-order multip- let at 3865 ppm, additionally broadened by coupling to the À quadrupolarboron nucleus,the analysis of which confirms the 1 [18] Scheme2.Synthesis of boranediyl-bridged diplatinum A-frame complexes. JPPt coupling constant of around 3300 Hz. Chem. Eur.J.2020, 26,8518 –8523 www.chemeurj.org 8519 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA,