Isolable Diaminophosphide Boranes

Abstract Metalation of secondary diaminophosphine boranes by alkali metal amides provides a robust and selective access route to a range of metal diaminophosphide boranes M[(R2N)2P(BH3)] (M=Li, Na, K; R=alkyl, aryl) with acyclic or heterocyclic molecular backbones, whereas reduction of a chlorodiaminophosphine borane gave less satisfactory results. The metalated species were characterized in situ by NMR spectroscopy and in two cases isolated as crystalline solids. Single‐crystal XRD studies revealed the presence of salt‐like structures with strongly interacting ions. Synthetic applications of K[(R2N)2P(BH3)] were studied in reactions with a 1,2‐dichlorodisilane and CS2, which afforded either mono‐ or difunctional phosphine boranes with a rare combination of electronegative amino and electropositive functional disilanyl groups on phosphorus, or a phosphinodithioformate. Spectroscopic studies gave a first hint that removal of the borane fragment may be feasible.


Introduction
Couplingo fn ucleophilicm etal diorganophosphides I or diorganophosphideb oranes II (Scheme 1) with suitable electrophiles is one of the prime routes for the synthesiso fp hosphines. [1] The borane unit in II serves both as ap rotecting and activatinggroup that suppresses on one hand unwanted side reactions like the formation of phosphonium ions, and may on the otherh and facilitatep hosphide formation,f or example, by boosting the PH-acidity of ap hosphine precursor. [2] Typical phosphide reagents carry usuallyc hemically inert alkyls or aryls( Ia/IIa), [3] but the presence of hydrides (Ib,c/ IIb,c) [4] or substituents based on heavierg roup-14 elements (often Me 3 Si, Id [5] )i sa lso not uncommon. Primary,( Ib/IIb) parent( Ic/IIc)a nd silylated phosphides (Id)a re special because their ability to undergoe lectrophilicp ost-functionalization of reactive PÀHa nd PÀSi bondsa fter the initial metathesis step makest hem essentially polyfunctionalb uilding blocks.
In contrast to Ia-d/IIa-d,a mino-substitutedphosphide derivatives Ie/IIe have only received scarcea ttention. Like their congeners Ic,d/IIc,thesespeciescan serve as nucleophilic reagents for the synthesiso ff unctional phosphines with ap ossibility for further derivatization.H owever,w hereas the phosphorus atom in primary or silylatedp hosphinesa ccessible from Ic,d/IIc retains its nucleophilicc haracter,t he electronegative substituents in diaminophosphines generated from Ie/IIe imposee lectrophilicc haracter on the phosphorus atom transferred, which allowsf or post-functionalizationb yn ucleophilesr ather than electrophiles.I nt his respect,d iaminophosphide reagents may be considered as tools that allow coupling an electrophilic R 2 Pfragment with an electrophilics ubstrate, which makes them synthetic buildingb locks whose reactivity complements that of primary or silylatedp hosphides, respectively.
Whereas free diaminophosphides Ie remain still elusive species, [6] some progress has recently been made in the field of their complexes with borane or transition metals, respectively. Transition metal complexes [(R 2 N) 2 P(Fe(CO) 4 ]K [7] (III,R= Et, Ph) and [(Me 2 N)(Ph 3 C)P(W(CO) 5 ]K [8] (IV)c ontaining metal-stabilized mono-or diamino-phosphido ligandsw ere generated as spectroscopically detectable entities by deprotonation of PH-substituted precursor complexes and shown to react as P-centered nucleophiles. The significanceo fd iaminophosphide boranes as synthetic intermediates was first recognized by the group of Knochel, [9] who prepared diaminophosphineb oranes by elec-trophilica lkylation or arylation of an intermediary lithium diaminophosphide borane Li[1a]t hat had been generated in situ by lithium reduction of ac hlorophosphine borane precursor but was neither positivelyi dentified nor further characterized (Scheme 2a). We have recently shown that monomeric potassium (K[1a]) and lithium diethylaminophosphide borane (Li[1a]) can be alternativelya ccessed through metalation of as econdary diaminophosphine borane precursor (Scheme 2b), and reported on the first spectroscopicc haracterization of such species as well as on their transmetalation with zinc and copper halidestoafford isolable transition metal complexes. [10] Striving to further establish diaminophosphide boranes as well-defined synthetic tools, we describe here the preparation of reagents with an extended range of amino substituents, including first heterocyclic derivatives, and alkali metal ions, the isolation and crystallographic characterization of sodium and potassium dimethylaminophosphide boranes, ands elected reactions with electrophiles which extendt he application of the nucleophilic reagents to the synthesis of highly functionalized diaminosilyl phosphines and the activation of ac umulene.

Results and Discussion
Synthesis and spectroscopic characterization of diaminophosphide boranes To deepen our insighti nto the formation of diaminophosphide boranes, we testedt he routes reported for accessing Li[1a], viz. reductive metalation of the diaminochlorophosphine borane [9] and deprotonation of the secondary diaminophosphine borane, [10] with al argerr ange of structurally diverse substrates. The precursors 2b-f and 3b (Scheme 3) were prepared followingk nown procedures [9][10][11] and characterizedb ys pectroscopic and analytical data (see Experimental Section). The synthesis of 2b imposeds omep roblems, since the previously reported reactiono f( Me 2 N) 2 PCl and LiBH 4 [12] gave in our hands unsatisfactory results, and am odifieda pproachi nvolving precomplexation of (Me 2 N) 2 PCl with Me 2 S·BH 3 (see Experimental Section) furnished only inseparable mixtureso f2b with Me 2 NH·BH 3 (10-20 mol %). As the by-product did not interfere with the subsequentm etalation reactions, the mixtures were used without further purification. Reactions aiminga tt he generation of phosphides were carriedo ut in THF,d iethyle ther,o r mixtures thereof, and reactionmonitoring and product identification was generally achieved in situ by multinuclear ( 1 H, 31 P, 11 B) NMR spectroscopy.W ef ound that reliable and highly selective generation (with 90-99 %c onversion by integration of suitable NMR signals) of phosphideb oranesM [ 1b-f]w as feasible by treating the PH-substituted precursors 2b-e with a slight excess of an on-nucleophilic alkali amide such as lithium di(isopropyl)amide( LDA) or am etal hexamethyldisilazide (MHMDS = MN(SiMe 3 ) 2 ,M = Li, Na, K, Scheme 3a), respectively. The functioning of the silylamides as universally applicable bases allows selecting the alkali metal in such aw ay that subsequent metathesis of the phosphide reagent with electrophiles gives rise to an easily separable salt. Underp ractical aspects,w ec onsider KHMDS ap articularly usefulr eagent because it is commercially available in sufficientq uality,a nd potassium halides, which are formed as by-products in many follow-up reactions of the phosphides, are insoluble in common organic solvents. Metalation of 2b with NaHDMS in a THF/ether mixture and with KHMDS in the presence of a crown ether permitted also the isolation of first crystalline diaminophosphide boranes of compositionN a[1b]·THF and K[1b] (the crown ether was in this case not incorporated in the crystal), respectively.B oth products were characterized by singlecrystal X-ray diffraction studies (see further below).
Moreover,u sing KHMDS for deprotonation enabled both the generation of asterically congested diaminophosphide reagent like K[1c]a nd heterocyclic derivatives like K[1d], K[1e], while selectivelithiation of heterocyclic secondary diaminophosphine borane 2e was best accomplished with LDA. Metalation of the secondary diazaphospholene borane 2f turned out ab orderline case. Treatmentw ith KHMDS produced detectable amountso ft he expected, highly moisture and air sensitive phosphide K[1f], which was identified by spectroscopic data and chemical trapping with ap rotons ource.H owever,b oth deprotonation and recovery of the secondary phosphine borane were in this case unselective and produced side-products that could neither be successfully identified nor separated. We relate the difficulties encountered in the generation of K[1f]t othe unusuale lectronic structure of unprotected secondary diazaphospholenes, which displayahydride reactivity that contrasts the behavior of alkyl and arylphosphines and is associated with an "Umpolung" of the PÀHb ond. [13] Obviously, this effect is subdued by the borane coordination to make room for ap rotic reactivity that is unprecedented for diazaphospholenes,b ut the lability of the resulting anion implies that the P-H acidity remains low.
Application of other deprotonating reagents than amides gave erratic results. Metalation with n-butyl lithium,w hich had been employed in the synthesis of Li[1a], [10] enabled also preparing the sterically more congested acyclic phosphide borane Li[1c]b ut proceeded unselectively and irreproducibly with heterocyclic substrates. The use of Grignard reagents was tested with sterically uncongested diaminophosphine boranes 2a,b. While MeMgCl converted both substrates cleanly into the corresponding phosphides,EtMgBr was found to be unreactive.
Exploring the reduction of chlorophosphine boranes 3a,b [9] as an alternative to the metalation of the secondary phosphine boranes (Scheme 3b)r evealed that pure alkali metalsa re in most cases unreactive (see Ta ble 1). As ingle exception was observed for the reaction of potassium with 3b,w hicha fforded a low yield of the metalation product K[1b]a long with am ain product later identified [14] as the diphosphine bis-borane complex 4b (Scheme 4) and further unidentified by-products.F urther transformations becamef easible when soluble metal naphthalenides rather than solid metalsw ere used as reductants. However,w hiler eduction of 3a with lithium naphthalenide afforded, in accord with earlierr eports, [9] an ear quantita-tive yield of Li[1a], reactions of 3b with lithium and potassium naphthalenides gave ap roduct mixture which contained the expected metal phosphide boranes along with diphosphine bis(borane) 4b and substantial amounts of unidentified byproducts.W eh ave currently no concisee xplanation for the varying selectivity,e ven if it is tempting to relate the observed changes to different sterics of the substrates.

Crystallographic studies
Graphical representations of the results of single-crystalX -ray diffraction studies on triclinic Na(THF)[1b]( space group P1 ) and monoclinic K[1b](space group P2 1 /c)and important metrical parameters are displayed in Figures 1, 2, and S1 (Supporting Information). Full crystallographic data are included in the Supporting Information.
Crystals of Na(THF)[1b]c ontain dimeric units assembled from two anionic diaminophosphide borane and two cationic Na(THF) fragments ( Figure 1). The metal cations connect to the phosphorus atom of one and an itrogen atom of the other anion fragment in the same unit, resulting in the formation of ac entrosymmetric six-membered cyclic array with at ypical chair conformation. In addition, each metal ion features contact to two hydrides of a h 2 -bound borane unit in an eighboring dimer (NaÀH2 .38(2) to 2.39(2) )a nd aw eaker contactt o aB ÀHb ond of the adjacent borane in the same unit (Na-H Scheme4.Reactions of potassium diaminophosphide boranes with 1,2-dichloro-tetramethyldisilane 5. 2.69 (2) ). Interactions of this type have precedence in the structures of alkyl/arylphosphide boranes [15,17] and link in Na(THF)[1b]t he dimeric units to form one-dimensionally infinite strands aligned parallel to the crystallographic a-axis. If we count the h 2 -bound borane as one ligand,t he metal ions exhibit an irregular pseudo-[4+ +1] coordinationg eometry in which the weak "intramolecular" BÀHi nteraction is aligned roughlyo pposite to the oxygen atom of the THF ligand.T he phosphorus atom displays ad istorted tetrahedral coordination with bond angles from 103.1(1)8 (N1ÀP1ÀB1) to 118.8(1)8 (Na1ÀP1ÀB1). The PÀBa nd PÀNd istances are in the range of single bonds, with the deviation of 0.053(2) between the PÀ Nd istances reflecting the different coordination numberso f the nitrogen atoms. While the Na1ÀO1 distance of 2.311 (1) is ac lose match to the sum of covalentr adii (2.32(11) [18] )a nd may be viewed as regularc oordinative bond, the Na1ÀN1 (2.5204 (12) )a nd Na1ÀP1 distances (2.9625 (6) )p erceptibly exceed this sum (NaÀN2 .37 (10) ,N a ÀP2 .73 (12) [18] ), suggesting adescription as closeinter-ionic contacts. Crystalline K[1b]c ontains two independenta nionic units and two cations which aggregate, asi nt he sodium analogue, via K···N and K···P interactions to form strongly puckered sixmembered cyclic arrays ( Figure 2). However,w hereas the inversion symmetry in Na(THF)[1b]i mposes an head-to-taila lignment of the N,P-units that renders both sodium ions equivalent, the anion fragments in the potassium salt are arranged in ah ead-to-head fashion,w ith both phosphorus atoms binding to one and both nitrogen atoms to the otherm etal ion. Additional interaction of the P 2 -ligated potassium ion with an exocyclic dimethylaminog roup of an eighboring unit gives rise to the formation of ac oordination polymeric assembly of dimeric units with az ig-zag chain of K···N/K···P interactions and covalent PÀNb onds as backbone( Figure S1). The coordination sphere of the linking metal ion is completed by additional contacts to a h 3 -bound borane unit (H···K2 2.645 (27) (31) )f rom the second next dimericu nit in the row, which reinforce the supramolecular assembly and give rise to the formation of ad oubles tranded array of heterocyclic units extending along the crystallographic b-axis. Completion of the coordination sphere of the N 2 -coordinated potassium ioni sa ccomplished by weaker BH···K interactions (H···K1 2.69(3) to 3.04(3) )w ith borane units from adjacent, parallel strands. These secondary interactions result ultimately in the formation of two-dimensionall ayers,w hich extend in the crystallographic a,b-plane and are held together by van-der-Waals interactions. The PÀN( 1.697(2) to 1.741 (2) ) and PÀBd istances (1.935 (4) (1) )a nd N···K distances (2.849 (2) to 2.961 (2) )e xceed the reported sum of covalent radii (PÀ K3 .10(15), NÀK2 .74(13) [18] ), but the former match the distances in knownp otassium dialkyl and diarylphosphides (PÀK3 .309 AE 0.071 [19] ).

Reactionswith electrophiles
The synthetic potential of diaminophosphide boranes has so far been demonstrated in reactions under alkylation or arylation with organic electrophiles, [9] and in metatheses with transition metal salts leading to the formation of borane-supported terminal transition metal phosphides. [10] Seeking to furtherw iden the synthetic bandwidth,w ec onsidered silyl-   substituted diaminophosphines another rewarding class of target molecules.W hile the presence of two types of functional substituents with opposite polarity makest hese compounds, like the phosphides, potentially highly valuable building blocks, they are only rarely reported in the literature. [20] Known syntheses involvem ostly reactions of diaminochlorophosphines with silanides, [20a,b,e] or reductivec oupling of diaminochlorophosphines with chlorotrimethylsilane, [20c] respectively. Cross coupling of P-based nucleophiles with electrophilico rganosiliconc ompounds can thus provide an ew,c hemically complementary access route.
To explore the feasibility of this approach, we studied reactions of diaminophosphide boranes with commercially available 1,2-dichloro-1,1,2,2-tetramethyldisilane 5 as prototype of a potentially multifunctional silicon-based electrophile. Potassium phosphide boranes emerged as the best suited reagents under practical aspects, as the nucleophiles can easily be generated in situ by treating the secondary phosphine borane precursors with as light excess of KHMDS,a nd potassium chloride formed as by-product is easily and quantitatively removed by filtration.T he reactions of 5 with K[1c]a nd K[1e]a fforded exclusively monosubstitution products (6c,e), even when the nucleophilew as employed in excess( Scheme 4). In contrast, a clean reactionw ith K[1a]r equired employing two equivalents of the nucleophile anda fforded the expected 1,2-bisphosphino-disilane 7.T he failure to obtain analogous disubstitution products from K[1c,e]i sp resumably due to the increased steric demand of these reagents. All three products were isolated as thermally stable, moisture sensitiveo ils (6c)o rc rystalline solids (6e, 7)a nd characterized by analytical ands pectroscopic data. The 1 H, 11 B, 13 Ca nd 31 PNMR spectra are unremarkable and confirmt he proposed constitution. The 29 Si NMR spectrumo f7 is peculiar as the expected multiplet (representing the X-part of an ABXs pin system) degenerates accidentally to as imple doublet.O ne of the two expected 29 Si NMR signals of 6e stayed undetected for unknown reasons.
The molecular structures of 6e and 7 were further confirmed by single-crystal X-ray diffraction studies. Crystals of 7 ( Figure 3) contain two independent molecules which both display positional disorder in the peripheral ethyl groups (see Experimental Section). Both specimensadopt astaggered conformation of the central disilaneu nit with a transoid arrangement of the two phosphine borane units (PÀSiÀSiÀP1 76.3(1)8 and 1808, Figures 3, S2). The unit cell of 6e holds likewise two independentm olecules,b oth of which exhibit, however,anearly eclipsed conformation aroundt he SiÀSi bond. The chlorine and phosphorus atoms in one molecule adopt a gauche-orientation (Figure 4), while the peripheral atoms of the terminal SiMe 2 Cl unit in the second molecule display ad isorderi ndicating the presence of a3 5:65 mixture of two rotamers with gauche-a nd trans-aligned Cl and Pa toms ( Figure S3). The Nheterocyclic rings exhibit similart wist conformations as in borane-free diazaphospholidines [21] and diazaphospholidine oxides. [22] The PÀSi and SiÀSi distances in 7 are unremarkable and suggest the presence of normal single bonds. The PÀN distances in both complexes are shortert han in free acyclico r N-heterocyclic phosphines, [11,13,21] indicating as imilar bond strengthening upon borane coordination as had previously been observed for secondary diazaphospholidines. [11,21] The 1,2-diphosphino-disilane representing the backbone of 7 can be considered ah eavier diphosphinoethane homologue. Scattered reports on template syntheses of diphosphino-disilane transition metal complexes [23] imply that such species can in principle, like their lighter congeners, act as chelatingl igands.A iming at making an aminofunctionalized 1,2-diphosphino-disilane available for complexation studies, we set out to study de-protecting 7 by sequestering the borane units with 1,4-diazabicyclo[2.2.2]octane (DABCO). [24] Heating a hexane solution of both reactants to 60 8Cp roduced ac olorless precipitate that was identified by 11 BNMR spectroscopy as the expected bis-borane adduct of DABCO( d 11 B À10.9 ppm, 1 J BH = 99 Hz [25] ). The 31 PNMR spectrum displayed am ain signal  (d 31 P9 6.4 ppm with 56 %o ft he total integrated intensity) which lacks the characteristic broadening arising from spincoupling to aq uadrupolar 11 Bn ucleusa nd is tentatively assigned to the targeted borane-free diphosphino-disilane. In addition, signals attributable to unknown borane-free phosphorus-containing species (d 31 P9 4.2 and 94.5 ppm with integrated intensities of 9a nd 16 %, respectively)a nd bis(diethylamino)phosphine (d 31 P7 8.6 ppm, 1 J PH = 250 Hz, 19 %) wered etected, and the 11 BNMR spectrum revealed further the presence of residual 7 (the 31 PNMR signal of which was presumably buried in spectraln oise). The product distribution observed can be explained by assuming that de-protection is in principle feasible but remains incomplete and is accompanied by PÀSi bond cleavage arising from adventitioush ydrolysis. Attemptst oi solate any of the de-protected phosphorus-containing products remainedu nsuccessful.
Expecting that diaminophosphide boranes should like unprotecteds econdary diorganophosphines or -phosphides be capable of undergoing nucleophilic attack on the electrophilic carbon center of cumulenes, [26] we studied furthert he reactions of K[1a]w ith carbon dioxide and carbon disulfide. While the former gave rise to an intractable colorless solid that could as yet not be unambiguously characterized,t he reaction with CS 2 proceeded cleanly to afford as oluble speciesw hich was isolated as ar ed, air and moisture stable crystalline solid after work-up and recrystallization from ether/pentane. Analytical and MS data are in accordance with the formation of phosphinodithioformate borane 8 (Scheme5), the constitution of which was straightforwardly confirmed by as ingle-crystal X-ray diffractionstudy on ah emi-hydrate 8·0.5 H 2 O.
The crystal is composed of an array of potassium cations, complexa nionsa nd water molecules which is structured by a network of mutual non-covalent interactions with distances that are intermediate between sums of covalent and van-der-Waals radii ( Figure 5). The basic motif in this layout consists of one-dimensional stacks of alternating cations and anions stretching along the crystallographic b-axis. The connection of adjacent cations in each stackb ym 2 -bridging, 1:2k 2 S;1:2k 2 S'-coordinated anions gives rise to az ig-zag chain-like arrangement that is reinforced by weaker S···K and BH···K contactsl inking one of the sulfur atoms and the borane unit of an specific m 2bridging anion with at hird metal cation. The individual strands align parallel to each other and are mutually connected by water molecules that bind in a m 2 -bridging fashion to two metal cations in adjacent strands. The grid of K···O···K linkages thus formed connects the one-dimensional ion stackst ot wodimensional layers that extendi nt he crystallographic b,c-plane and are held together by van-der-Waals interactions between the diethylamino groups.
Beyond previous findings, sodium and potassium dimethylaminosphosphide boranes were for the first time isolated as room-temperature-stable solids. Their crystallographic characterization reveals the presence of salt-like structures that are distinguishedb yi ntimate interactions betweeni ons of opposite charge via ac ombinationo fd ative M···N and M···P and agosticM ···HÀBc ontacts. While both types of interactions are not unprecedented for aryl and alkyl phosphide boranes, the availability of additional nitrogen donor centers increases the density of the interaction network.
Last, but not least, the potential of diaminophosphide boranes to act as synthetic building blocks is illustrated by their conversion into phosphine boranes that are distinguished by a combination of electronegative amino and electropositive functional silyl groups. Initial studies indicatet hat removal of the borane units is in principle feasible, and we are currently striving to developr eliable synthetic protocols to accomplish this task.

Experimental Section
All reactions were carried out under an atmosphere of inert Argon and in flame-dried glassware if not mentioned otherwise. Solvents were dried by distillation from alkali metals (THF,E t 2 O, toluene) or by using as olvent purification system (pentane, hexane, MeCN). NMR spectra were recorded on Bruker Avance2 50 or Avance 400 instruments. Chemical shifts in 1 HNMR spectra were referenced to TMS using the residual signals of the deuterated solvent ( 1 H, d([D 8 ]THF) = 1.73 ppm; d(C 6 D 6 ) = 7.15 ppm) as secondary reference. NMR Spectra of heteronuclei were referenced using the X-scale [31] with TMS (X = 25.145020 MHz, 13 C; X = 19.867187 MHz, 29 Si), BF 3 Et 2 O( X = 32.083974 MHz, 11 B) and 85 %H 3 PO 4 (X = 40.480747 MHz, 31 P) as secondary references. Coupling constants involving boron refer to 11 Bi sotopomers, with 1:1:1:1m ultiplets being denoted as q. The product composition in metalation experiments was derived from analysis of the 1 HNMR signals of the Nsubstituents of 1b-f.E lemental analyses were carried out with an Elementar Micro Cube elemental analyzer.M ass spectra were obtained with aB ruker Daltonics Mikrotof-Q-Mass spectrometer. Phosphine boranes 2a, [10] 2e [11] and 3a [9] were prepared as reported.
X-ray diffraction studies were carried out using aB ruker Kappa Apex II diffractometer equipped with aD uo CCD-detector and a KRYO-FLEX cooling device with Mo-K a radiation (l = 0.71073 )a t T = 130(2) K. The structures were solved by direct methods (SHELXS-97 [32] )a nd refined with af ull-matrix least-squares Scheme on F 2 (SHELXL-2014 and SHELXL-97 [33] ). Semi-empirical absorption corrections were applied. Non-hydrogen atoms except disordered carbon atoms were refined anisotropically,h ydrogen atoms at boron in Na(THF)[1b], K[1b], isotropically,a nd all other hydrogen atoms with ar iding model. The peripheral atoms in the SiMe 2 Cl group of 6e and five of 16 Et-groups in 7 and two of four Etgroups in 8 are each disordered over two positions. The carbon atoms in the disordered sections were refined isotropically and with restraints. Deposition numbers 2002662, 2002663, 2002666, 2002665, and 2002664 (Na(THF)[1b], K[1b], 6e, 7, 8)c ontain the supplementary crystallographic data for this paper.T hese data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
Bis(dimethylamino)phosphine borane (2 b):M ethod A: PCl 3 (0.1 mL 1.14 mmol) was added under stirring to ac ooled (À78 8C) solution of P(NMe 2 ) 3 (0.42 mL 2.29 mmol) in Et 2 O( 50 mL). Stirring was continued while the solution was allowed to warm to ambient temperature. The solution was cooled again to À78 8Ca nd BH 3 ·SMe 2 (0.33 mL, 3.43 mmol) was added. The mixture was stirred for 1h at À78 8Ca nd allowed to warm to RT.A fter cooling once more to À78 8C, LiAlH 4 (3.43 mL 1 m solution in THF,3 .43 mmol) was added. The mixture was stirred for 1h at À78 8C, allowed to warm to RT,a nd stirred for further 24 h. Water (0.5 mL) was added and volatiles were evaporated under reduced pressure. The solid residue was extracted with hexane (20 mL). The extract was dried (Na 2 SO 4 )a nd solids were removed by filtration. Evaporation of the solvent under reduced pressure afforded 2b as colorless solid which melted around RT (yield 266 mg, 58 %). Method B: PCl 3 (0.49 mL 5.6 mmol), P(NMe 2 ) 3 (2.00 mL 11.2 mmol) and BH 3 ·SMe 2 (1.59 mL, 16.8 mmol) were reacted in Et 2 O( 30 mL) as described under A. Li[BEt 3 H] (16.8 mL 1 m solution in THF,1 6.8 mmol) was added. The mixture was allowed to warm to RT and stirred for further 18 h. Volatiles were evaporated under reduced pressure and the residue was taken up in pentane (150 mL). The resulting mixture was filtered over Celite. Evaporation of the solvent under reduced pressure gave 2b as colorless oil (yield 1.137 g, 51 %), which contained according to NMR measurements 10 to 20 %d imethylamine borane. 31  Bis(diisopropylamino)phosphine borane (2 c):L iBH 4 (0.53 mL 1 m solution in THF,0 .53 mmol) was added to as tirred solution of (iPr 2 N) 2 PCl (0.50 g, 1.8 mmol) in Et 2 O( 25 mL). The mixture was stirred for 1h.V olatiles were evaporated under reduced pressure and the residue extracted with hexane (4 20 mL). The combined extracts were filtered, the filtrate concentrated under reduced pressure, and the product isolated by crystallization at À24 8C( yield 0.37 g, 77 %). 31 P{ 1 H} NMR (C 6 D 6 ): d = 46.3 ppm (q, 1 J PB = 72 Hz); m-C 6 H 3 ), 6.91 (m, 2H, p-C 6 H 3 ), 3. 21-2.88 (m, 4H,C H 2  1,3-Bis(2,6-xylyl)-1,3,2-diazaphospholene borane (2 f):N aBH 4 (113 mg, 2.98 mmol) was added to as uspension of 2-bromo-1,3bis(2,6-xylyl)-1,3,2-diazaphospholene (746 mg, 1.98 mmol) in MeCN (50 mL). The mixture was stirred for 18 h, filtered, and evaporated to dryness. The residue was washed with pentane and dried in vacuum to afford4 77 mg (77 %) of crude 2f.F urther purification was feasible by extracting the crude product with benzene, filtration, and evaporation of the filtrate to dryness. 31   Chloro-bis(dimethylamino)phosphine borane (3 b):B H 3 ·SMe 2 (1.3 mL, 13.6 mmol) was added under stirring to ac ooled (0 8C) solution of (Et 2 N) 2 PCl (2 mL, 13.6 mmol) in Et 2 O( 10 mL). After 30 min, the solution was allowed to warm to RT and stirred for further 18 h. The solvent was then removed under reduced pressure and the residue dissolved in hexane (10 mL). After reaction control by 31 PNMR, enough BH 3 ·SMe 2 (1.3 mL, 13.6 mmol) to convert unreacted starting material and more hexane (10 mL) were added. Filtering the mixture over Celite and removal of the solvent under reduced pressure gave 3b as yellow oil (1.52 g, 66 %). The product is thermally unstable and decomposes even below room temperature;i tm ust be used immediately and cannot be stored. 31  General procedure for the metalation of secondary diaminophosphine boranes 2b,d,f with alkali metal bis(trimethylsilyl)amides:M HMDS (1.1 to 1.2 equiv., M= Li, Na, K) was added to a 0.06-0.2 m solution of the secondary diaminophosphine borane in THF or [D 8 ]THF (1 to 2mL). The solution was stirred for 10 min at RT.A na liquot of the reaction mixture was then transferred to an NMR tube and characterized by multinuclear NMR spectroscopy. Conversion to the metalation product was derived from evaluation of the integrals in 1 HNMR spectra. Deprotonation at phosphorus was in all cases confirmed by the disappearance of the characteristic signal splitting due to 1 J PH . Isolation of sodium bis(dimethylamino)phosphide borane Na [1 b]:Asolution of Na[1b]w as prepared as described above from NaHMDS (80 mg, 0.43 mmol) and 2b (49 mg, 0.36 mmol) in THF (2 mL). Storage at À24 8Cp roduced colorless, highly air and moisture sensitive crystals, which were separated by decantation (no yield determined) and characterized by NMR spectroscopy (see Ta ble 2a nd above) and as ingle-crystal X-ray diffraction study.T he high chemical sensitivity precluded obtaining as atisfactory elemental analysis.
Isolation of potassium bis(dimethylamino)phosphine borane K[1 b]:K HMDS (133 mg, 0.66 mmol) and dibenzo-18-crown-6 (216 mg, 0.60 mmol) were added to as olution of 2b (81 mg, 0.60 mmol) in am ixture of Et 2 O( 3mL) and THF (7 mL) was carried out as described above. The mixture was stirred for 5min. Storage at À24 8Cp roduced colorless, highly air and moisture sensitive crystals, which were separated by decantation (no yield determined) and characterized by NMR spectroscopy (see Ta ble 2a nd above) and as ingle-crystal X-ray diffraction study.T he high chemical sensitivity precluded obtaining as atisfactory elemental analysis.