Versatile Tautomerization of EH2-Substituted Silylenes (E = N, P, As

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Versatile Tautomerization of EH-Substituted Silylenes (E = N, P, As) in the Coordination Sphere of Nickel Terrance J. Hadlington, Tibor Szilvási, and Matthias Driess J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00159 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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Journal of the American Chemical Society

Versatile Tautomerization of EH2-Substituted Silylenes (E = N, P, As) in the Coordination Sphere of Nickel Terrance J. Hadlington,*,a Tibor Szilvási,b and Matthias Driess*,a a

Department of Chemistry, Metalorganics and Inorganic Materials, Technische Universität Berlin, Strasse des 17. Juni 135, Sekr. C2, 10623 Berlin, Germany b

Department of Chemical & Biological Engineering, University of Wisconsin—Madison, 1415 Engineering Drive, 53706, Madison, WI, United States ABSTRACT: The synthesis and tautomerization of a ‘half-parent’ aminosilylene and its heavy P- and As-analogues (TMSLSiEH2; E = N, P, As; TMSL = N(SiMe3)(2,6-iPr2C6H3)) in the coordination sphere of nickel(0) to give the corresponding side-on η2-RSi(H)=EH and RH2Si-E (‘silylpnictinidene’) nickel complexes is reported. These complexes can be accessed through salt metathesis reactions of the lithium dihydropnictides LiEH2 with the acyclic chlorosilylene nickel(0) complex 1, [TMSL(Cl)Si→Ni(NHC)2; NHC = :C[(Pri)NC(Me)]2). In addition, we report the facile E-H bond activation reactions of EH3 with 1, which furnished a silyl nickel(II) complex through NH3 activation, but phosphido and arsenido complexes in the activation of PH3 and AsH3, respectively. Notably, reaction of 1 with LiNH2 leads to the acyclic bis(amido)silylene complex [TMSL(H2N)Si→Ni(NHC)2] 5, which does not undergo N-H proton migration to silicon(II) under ambient conditions. The transformation of the P- and As-analogues of 1 furnishes directly the respective side-on Si=E Ni complexes (nickelacycles), [η2-{TMSL(H)Si=E(H)}Ni(NHC)2] (E = P, 6; E =As, 9). These nickelacycles show a vastly different stability in solutions. While 6 is stable for several days at ambient temperature, 9 undergoes further rearrangement processes within minutes of its formation. Given the high acidity of the As-H proton in 9, however, this moiety can be trapped as a highly charge separated metallated-η2-silaarsene nickel complex 12 that is best described as an [AsSiNi] nickelacycle with Si-As multiple bond character. Taken as a whole, these results give, for the first time, insights into the relative stability of the tautomeric forms of side-on silaldimine transition-metal complexes. The electronic nature and the rearrangement processes of these compounds were also investigated by quantum chemical calculations.

INTRODUCTION Since the seminal discovery of an olefin-metal π-complex in 1890, namely Zeise’s salt,1 transition-metal (TM) πcomplexes involving carbon-containing multiple bonds constitute an integral part in organometallic chemistry.2 As such, π-complexes are key in numerous indispensable chemical transformations such as hydroformylation,3 olefin metathesis,4 and Ziegler-Natta polymerization.5 Despite earlier doubts that π-bonds involving heavier maingroup elements could exist,6 this area has also flourished over the last four decades,7 since reports of isolable compounds containing π-bonding with Si or P centers in 1981.8,9 Nevertheless, respective π-complexes, or rather η2-transition-metal complexes of multiply bonded main-group analogues of olefines are comparatively rare. In fact, they can even show deviations from the classical Dewar-Chatt-Duncanson (DCD) model, as observed in the bond-strengthening of a B=B bond of diborene in the coordination sphere of Pd.10 Beyond disilene complexes, which were initially reported following the discovery of the first disilene by West and coworkers,11 η2-TM complexes involving silicon-element multiple bonds are scarce. Nevertheless, some progress has been achieved towards understanding the electronic characteristics of these complexes, with systematic

Figure 1. Literature precedents for P-hydro-, 1,2-dihydrosilapnictene (Si=E) systems and a related Si=Si transition-metal complex.

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studies leading to side-on disilene-TM π−complexes12 or metallacyclopropane complexes.13 Whilst these “heavy-alkene” transition-metal complexes have been investigated to some degree, side-on coordinated examples involving silicon-heteroatom multiple-bonds received far less attention,14 partly due to the lack of compounds containing stable multiplebonds between silicon and other main-group elements, which might allow for direct combination of silenes with electron-poor transition metal fragments. In this light, two examples of “half-parent” R2Si=EH species (i.e. E = N-Bi) have been previously reported (A and B, Figure 1).15,16 Although the Si-P bond in A was shown to hold considerable π-character,15 the substitution pattern at Si and P in B rather favored the betaine-like form, and in fact led to a highly labile Si-P bond.16 Beyond this, only two examples of structurally authenticated heavy analogues of primary aldimines with silicon (i.e. RSi(H)=EH; E = N-Bi) have been reported (Figure 1). The tungsten-coordinated phosphorus derivative C∙W(CO)5 was shown to contain little Si-P multiple bond character, whilst C, which contains a two-coordinate P center, was found to undergo head-totail dimerization in solution over time.17 The intramolecular donor-stabilized 1,2-dihydrosilaarsene D is of particular interest in that it was generated via the activation of gaseous AsH3, indicating that the 1,2-disilaarsene form of this species is the most stable tautomer.18 Computational investigations presented here, as well as experimental work previously reported by us, revealed that this is not the case for the related N and P complexes, giving some insight into the favored isomeric forms in such ‘dihydro’ species.19 To this end, matrix isolation studies involving the parent Si/P tautomeric series (i.e. :P–SiH3 ⇌ HP=SiH2 ⇌ H2P(H)Si:) also suggested that the silaphosphene form is the most favored.20 An analogous study for the reaction of Si atoms with NH3 in a cold Argon matrix indicated that the amidosilylene is formed (i.e. H2N(H)Si:), where other tautomeric forms could not be observed, Scheme 1. Tautomerization of EH2-substituted silylene-Ni complexes to corresponding η2-1,2-dihydrosilapnictene (Si=E) and silylpnictinidene complexes (E = N, P, As).

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somewhat insinuating their instability.21 Despite the isolation of the aforementioned ‘free’ hydrosila-pnictene species, there are no reports of η2 TM complexes of such compounds. Even more challenging is the lack of any structurally characterized example of a transition-metal complex of an aldimine (RC(H)=NH), perhaps due to a propensity for dehydrogenation to give very stable cyanides. Nevertheless, one can consider the possible tautomeric forms of the related 1,2-dihydrosilapnictene species in the coordination sphere of a transition-metal (Scheme 1). Through double proton migration from E to Si (E = N, P, As) in such dihydropnictido silylene complexes, a silylpnictinidene could be formed. By the inverse process, proton migration from Si to E may regenerate the dihydropnictido silylene complex. Such tautomerization represents a currently unknown series of dihydropnictido silylene-silapnictene-silylpnictindene isomers. An example of a η2-1,2-dihydrodisilene complex has been reported by Inoue (E, Figure 1), utifragment (NHC’ = lizing the [Ni(NHC’)2] :C[(Me)NC(Me)]2).22 This remarkable complex has been described as a metallacycle, and remains the only reported example of such a 1,2-dihydrosilaalkene metal complex. This led us to speculate as to whether heavy primary aldimine fragments may be stabilized in the coordination sphere of related NHC (N-heterocyclic carbene)-coordinated nickel complex fragments. Recently, we reported the synthesis of an elusive nickel complex of a silanone (Si=O), synthesised via the Ni0 silylene complex 1, [TMSL(Cl)Si→Ni(NHC)2] (LTMS = N(SiMe3)(2,6-iPr2C6H3); NHC = :C[(iPr)NCMe]2).23 Utilizing 1, we sought to investigate synthetic methods towards the aforementioned heavy primary silaaldimine-TM complexes, in order to investigate the relative stability of possible tautomeric forms described above. Herein we report the remarkably facile addition reactions of 1 towards NH3, PH3, and AsH3, which readily proceed but do not lead to the desired metallacyclic silapnictene (Si(H)=EH) species or tautomers thereof. Therefore, the reactivity of the lithium dihydropnictides, LiEH2 (E = N-As) towards 1 was also tested, giving the desired access to nickel complexes of the first acyclic bis(amido)silylene and η2-coordinated 1,2-dihydrosilapnictenes Si(H)=EH (E = P, As). Notably, species arising from redistribution of silylpnictinidene nickel complexes have also been isolated and fully characterized. Taken as a whole, following we report the first series of dihydropnictido si[RSi(H)=EH]lylene (H2E-Si(R):)-silapnictene silylpnictinidene (RSiH2-E) tautomers with E = N, P, and As. RESULTS AND DISCUSSION Computational Assessment of Tautomerization Prior to discussing our experimental findings, and to draw a complete picture of the proton migration processes leading to the dihydropnictido silylene-silapnictene-silylpnictinidene tautomers (see Scheme 1), a Density Functional Theory (DFT) investigation into the relevant tautomerization mechanisms will be discussed (details in SI). It was found that the first H-migration from E (E = N-As) to the SiII center in the derivatives is assisted by the Ni


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Journal of the American Chemical Society ∆G kcal/mol

H

H

HE Si

NHC H E

NHC

Si

36.1

NHC

H

NHC

41.3

Ni H

Scheme 2. Synthesis of compounds 2-4, and comparison with NH3 activation by a carbene-nickel complex.

NHC Ni

Si

Ni

+40

E

E= N

26.0

NHC E

15.5 12.5

H

9.7

TMS

8.6

Si

Si NHC

H

TMS

Ni NHC

1

0.0

-10

Si

-6.5

-7.8 -10.5

NHC

NHC

HE

-20

Si

Si

E Si

Ni

Cl

NHC H 3, E = P 4, E = As

NHC Ni NHC

H SiMe 3

L H

-16.1

=P = As

N

-5.9

TMS

-20.2

NHC H

iPr

E = P or As

-17.2

Ni H

=N

E

-7.0

Ni

Tetrylene

=

0.9

-2.3

NHC

H2E

NHC

H

Pnictinidene 0

NHC 2

EH 3

L

NHC

Ni

H2N

NHC

Cl

Ni

Si

L

28.8

+20

+10

H

Cl

NHC

+30

P(Pr i) 2

Metallacycle

P(Pr i) 2

iPr

C

Figure 2. DFT-derived mechanism for the tautomerization of the dihydropnictido silylene-Ni0 complexes to the corresponding η2-silapnictene- and silylpnictinidene complexes at the B97-D/def2-SVP level of theory.

center, via a Si-(μ-H)-Ni species. This is followed by a rearrangement whereby the Si=E moiety migrates to Ni, forming an η2-complex. The second H-migration step is a concerted one-step process resulting in the corresponding pnictinidene compounds. Energetics of the DFT-derived mechanism suggest that the H-migration is unlikely for E = N due to a very high activation barrier (36.1 kcal mol-1), although the η2-complex is energetically slightly favored relative to the initial amino silylene complex by 2.2 kcal mol-1. For E = P, the two activation barriers leading to the η2-complex are thermally accessible (15.5 and 15.6 kcal mol1 ). The barrier to give the phosphinidene complex should also be thermally accessible (18.1 kcal mol-1), but this complex is found 11.3 kcal mol-1 higher in energy than the metallacyclic isomer. It is therefore unlikely that a phosphinidene complex would be directly observed. In the case of E = As, the H-migration to the η2-complex (metallacycle) is thermodynamically strongly favored, exergonic by 20.2 kcal mol-1, while the two kinetic barriers to its formation are relatively small (9.7 and 2.7 kcal mol-1), suggesting that a facile tautomerization to this species should be observed. Further isomerization to the arsinidene complex might also be possible due to the low barrier for this process (13.7 kcal mol-1). Notably, the arsinidene form is only 4.1 kcal mol-1 higher in energy than the metallacyclic η2-Si(H)=AsH complex, suggesting that such a species might be experimentally accessible. Reactions of 1 with EH3 (E = N, P, As) In our initial efforts we probed to access the desired silapnictene [Si(H)=EH] nickel complexes through E-H bond activation of EH3 molecules (E = N-As) across the coordinative Si:→Ni bond, followed by dehydrochlorination and hydride migration, or loss of H2. We found that 1 readily reacts with the EH3 species (E = N, P, As) in an exceedingly facile manner. The activation of NH3 by 1 proceeded

Ni

PPh3

P(Pr i) 2

NH 3

HC

Ni

NH 2

- PPh 3 P(Pr i) 2

Piers, 2013

Figure 3. Molecular structures of 2 (left) and 4 (right), with thermal ellipsoids at 30 % probability. Selected distances (Å) and angles (°) for 2: Ni1-Si1 2.1587(4); N1-Si1 1.761(1); N2-Si1 1.808(1); C16-Ni1 1.897(2); C27-Ni1 1.961(1); N1-Si1-Ni1 126.92(5); N2-Si1-Ni1 118.02(5); Si1-Ni1-H1a 63.9(5); Si1-Ni1-C16 96.30(5); C27-Ni1-H1a 96.1(5); C16-Ni1-C27 104.53(6). For 4: Si1-As1 2.325(1); As1-Ni1 2.3121(8); C16-Ni1 1.892(4); C27-Ni1 1.909(5); Si1-As1-Ni1 103.04(4); As1-Ni1-C16 89.9(1); As1-Ni1-C27 93.2(1); C16-Ni1-Cl1 88.2(1); C27-Ni1-Cl1 88.1(1).

with N-H bond cleavage across the Ni-Si bond, in the formation of the hydridosilylnickel(II) complex 2 (Scheme 2), akin to the previously reported activation of H2 by 1.23(a) This reaction is reminiscent of ligand assisted ammonia activation by first-row TM PCcarbeneP complexes, but is in fact the only apparent example where the H-atom is transferred to the metal center (i.e. Ni), and the NH2 fragment



to the ‘ligand’ (i.e. Si).24 The solid state structure of 2, which contains a cis-square planar geometry at Ni, would suggest little Si…H-Ni bridging interaction (Figure 3). Concordantly, the 1H NMR spectrum of 2 indicates a characteristic high-field shift for the Ni-H fragment (δ = -10.33 ppm, 2JHSi = 10.2 Hz; Figure S1) indicative of only a weak bridging hydride interaction with Si.25 No interaction of the protic NH2 fragment with the nickel hydride is observed in either the solid state or in solutions, with diagnostic signals observed in both IR and NMR spectra for the NH2 ligand. When turning our attention to PH3 and AsH3 we found that the mode of activation was quite different to that for ammonia. That is, (chloro)(hydrosilylpnictido)nickel(II) complexes 3 (E = P) and 4 (E = As) were formed (Scheme 1), through double E-H bond activation and H/Cl exchange at nickel. The generation of 3 and 4 from 1 is essentially quantitative, as ascertained through 1H NMR analysis of crude reaction mixtures, despite the seemingly complex rearrangement that occurs in their formation. The molecular structures of 3 and 4 confirmed that both exhibit trans-square planar geometry at Ni (3: Figure S43 in SI; 4: Figure 3), bearing the Cl and [{(TMSL)(H)2Si}(H)E] ligands. The 1H NMR spectrum of 3 suggests that this geometry is not fluxional in solutions, showing a sharp set of signals indicative of an H2SiPH bonding motif (PH: δ = -0.43 ppm, 1JPH = 174.0 Hz, 3JHH = 7.0 Hz; SiH2: δ = 4.86 ppm, 1JSiH = 176.8 Hz, 2JPH = 23.5 Hz, 3 JHH = 7.0 Hz; see Figures S5 and S7 in SI). The 1H NMR of 4 displays sharp signals for all H-environments, aside from the SiH2 fragment, the signals for which are considerably broadened. Its terminal AsH signal is observed as a sharp triplet at δ = -0.93 (3JHH = 6.9 Hz; Figure S8). Hydrophosphido metal species such as 3 have several literature precedents,26 but those of the arsenic complex 4 are quite rare.27 Unfortunately, compounds 2-4 could not be dehydrochlorinated or dehydrogenated. Nevertheless, the outcome of the reactions of NH3, PH3, and AsH3 with 1 reflects a different reactivity of these compounds in line with their relative E-H acidities towards a SiII→TM bond. Reaction of 1 with LiEH2 Species (E = N, P, As) Given the lack of success in the generation of metallacyclic η2-1,2-dihydrosilapnictene TM species via the activation of the parent EH3 species, we chose to investigate the use of lithium dihydropnictides taking advantage of the pendant chloride at SiII in 1. Notably, a salt metathesis/Hmigration mechanism has been utilized in the synthesis of silapnictenes previously, albeit in the absence of a TM complex fragment.17,28 The reaction of 1 with an excess of LiNH2 powder successfully led to a salt metathesis reaction at the Si-Cl moiety (Scheme 3). In line with DFT calculations (Figure 2), no tautomerization was observed for this species at ambient temperature; that is, from these reaction mixtures, we were able to isolate the acyclic bis(amido)silylene nickel complex, [TMSL(NH2)Si:→Ni(NHC)2] 5, as deep red plate-like crystals, which form deep orange solutions in benzene. The 1H NMR spectrum of 5 is very similar to that of 1, but with an additional singlet at δ = 1.89 ppm for the terminal NH2 group at silicon, which shows weak coupling to the central Si atom in the 1H,29Si HMQC NMR spectrum of 5. The29Si{1H} NMR chemical shift of 5

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Scheme 3. Synthesis of the bis(amido) silylene nickel complex 5.

Figure 4. Molecular structures of 5, with thermal ellipsoids at 30 % probability. Selected distances (Å) and angles (°) for 5: Ni1-Si1 2.1094(5); N1-Si1 1.764(2); N2-Si1 1.762(2); N1-Si1-N2 100.25(8); N1-Si1-Ni1 133.32(5); N2-Si1-Ni1 126.13(7); Si1-Ni1-C16 118.37(5); Si1-Ni1-C27 119.61; C16-Ni1-C27 119.61(7).

(δ = 120.4 ppm) is only slightly different from that for 1 (δ = 123.2 ppm), suggesting that the electronic nature at SiII shows marginal difference in these two compounds. An Xray structural analysis of crystals of 5 confirmed the Cl/NH2 metathesis in 1, leading to the first example of an “half-parent” acyclic bis(amido)silylene complex (Figure 4). The structural parameters for 5 are similar to that for 1, with a planar SiII center and a short Si-Ni interaction indicative of some multiple bond character. The two SiII-N bonds are essentially equal (d(Si1-N1) = 1.764(2) Å; d(Si1-N2) =1.762(2) Å), with the two trigonal-planar nitrogen centers co-planar with trigonal-planar Si1, suggesting some degree of N→Si π-donation from both N1 and N2 to Si1; this leads to a slight elongation of the Si-Ni bond distance in 5 relative to 1 (1: 2.075(5) Å; 5 2.1094(5) Å) due to lessened Ni→Si backbonding. The presence of the terminal NH2 group is also evident in the IR spectrum of 5, which contains characteristic N-H stretching vibrational bands at ν = 3368 and 3473 cm-1. In an attempt to overcome the high barrier to isomerization of about 36.1 kcal mol-1, samples of 5 dissolved in C6D6 were heated to 65 °C, leading to complex reaction mixtures, testament to the presumed high reactivity of a side-on silaaldimine η2-L(H)Si=N(H) nickel complex. We then postulated that borane-coordination at the NH2 ligand in 5 should lead to an increase in N-H acidity, allowing for a more facile proton migration. Addition of either BPh3 to solutions of 5, or the novel LiNH2 borane adduct, H2(Li)N→BPh3 (Figure S44), to 1 led to a complex mixture from which a few deep orange crystals of a thermally labile complex, chelating disilane NiII TMS [HN{( L)(X)Si}2]Ni(NHC)2, could be isolated (X = H or


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NH2; Figure S45).29 This suggests that indeed the NH2 moiety in 5 can be activated in this manner, but at the present time the desired products remain elusive. Following with the related chemistry of phosphorus, 1 was reacted with Li(dme)PH2 (dme = 1,2-dimethoxyethane). Addition of a 5-fold excess of Li(dme)PH2 to a C6D6 solution of 1 at ambient temperature immediately formed a single new species. Key signals at δ = -1.64 (1JPH = 137.3 Hz; 3 JHH = 5.4 Hz) and 4.12 ppm (1JSiH = 193.0 Hz; 2JPH = 14.2 Hz) in the 1H NMR spectrum of this mixture were indicative of P-H and Si-H fragments, respectively, confirmed by 31P, 31 1 P{ H}, and 29Si,1H HMQC NMR spectroscopic experiments (Table 1; Figures S17-20). Notably, a 1JSiP coupling of 118.9 Hz could be observed. All these observations suggested that the 1,2-dihydrosilaphosphene complex 6 is formed in this reaction (Scheme 4). Repeating the reaction utilizing one equimolar amount of Li(tmeda)PH2 (tmeda = Scheme 4. Reactivity of 1 towards lithium dihydrophosphide, forming the side-on η2-1,2-dihydrosilaphosphene complex 6, and subsequent coordination of triphenylborane to form 7. NHC 5 eq. Li(dme) PH 2 or 1.1 eq. Li(tmeda) PH2

H

P

H

Si

Ni TMS

NHC

L NHC

H

P Ni

NHC

Cl Si TMS

1

NHC

TMS

Ni

L

Si

H

L

6

NHC H Ph3 B

NH2 CH3

P Ni

1. Li(tmeda) PH 2 2. Ph3 B

Si

H

NHC

TMS

L

7

tetramethylethylenediamine),30 which is considerably more soluble than Li(dme)PH2 and thus much more reactive, allowed for the isolation of 6, the first example of an η2-1,2-dihydrosilaphosphene TM complex, and indeed of any η2-TM complex of a heavy aldimineanalogue. The molecular structure of 6 confirms this (Figure 5 (a)), in which H1B on phosphorus can in fact be located in the difference Fourier map in both cis and trans geometries.31 The 1H and 31 P NMR spectra of 6 show a single environment for this PH fragment, which could indicate that cis and trans isomers are in equilibrium at ambient temperature. In order to investigate this, NMR spectra were collected for a sample of 6 dissolved in d8-THF from -60 °C to +20 °C (Figures S21-22). At -60 °C, two broad but distinct singlets can be observed in the 31P{1H} NMR spectrum (δ = -296.4 and 296.8 ppm). The 1H NMR spectrum at the same temperature contains two sets of broadened doublets relating to the P-H moiety (δ = -1.47 and -1.96 ppm; 1JPH = 143.6 and 134.7 Hz, respectively). Upon warming, these signals broaden considerably, and eventually collapse to the originally observed doublet of doublets at 20 °C. Thus, it seems as though 6 exists in a rapid cis-trans equilibrium at ambient temperature. A DFT analysis of this process suggests that this occurs through a transition state with trigonalplanar coordinate phosphorus with a barrier of 8.1 kcal mol-1. Moreover, the small difference in energy between the cis and trans forms of 6 (0.3 kcal mol-1) is in keeping with the observed equilibrium. Analysis of the features of the 1H, 29Si, and 31P NMR spectra of 6, as well as its molecular structure give valuable insights into the degree of π-character in the Si-P interaction in this complex, which would be expected to be low given the degree of charge separation in N-substituted Si=P bonded species,32 and the facile cis-trans isomerization described above. Indeed, rather than a classical DCD model, the polarization in a typical Si=P bond would lead to a better bonding description involving concomitant Ni→Si and

Figure 5. (a) Molecular structures of 6 (left) and 7 (right), with thermal ellipsoids at 30 % probability. Selected bond lengths (Å) and angles (°) for 6: P1-Si1 2.1744(8); Ni1-P1 2.3264(7); Ni1-Si1 2.2307(7); N1-Si1 1.763(2); Ni1-P1-Si1 59.31(2); P1-Si1-Ni1 63.74(2); P1Ni1-Si1 56.95(2); N1-Si1-P1 122.23(6) H1a-Si1-P1 105.75; H1b-P1-Si1 93.37. For 7: P1-Si1 2.188(1); B1-P1 2.071(4); Ni1-P1 2.2481(9); Ni1-Si1 2.2254(9); N1-Si1 1.754(3); Ni1-P1-Si1 60.20(3); P1-Si1-Ni1 61.23(3); P1-Ni1-Si1 58.57(3); N1-Si1-P1 122.59(9); B1-P1-Si1 130.6(1); H1a-Si1-P1 112.19; H1b-P1-Si1 107.15. (b) 31P (below) and 31P{1H} (above) NMR spectra for compound 6; (c) 31P (below) and 31P{1H} (above) NMR spectra for compound 7.



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Table 1. Summary of selected NMR data for previously reported compounds A-C, 3, and 6-8.a

Compound A

(trans)b

δ(31P)

δ(29Si)

1J

2J

(ppm)

(ppm)

(Hz)

PH

PH

(Hz)

1J

PSi

(Hz)

1J

HSi

(Hz)

3J

HH

(Hz)

δ1H Si-H

δ1H P-H

d(P-Si)

(ppm)

(ppm)

(Å)

123.1

249.8

123

-

157

-

-

-

5.14

A (cis)b

134.2

248.9

131

-

130

-

-

-

5.20

Bb

-293.9

101.5

143.0

-

186.4

-

-

-

-0.17

2.0712(10)

-301.4

-25.6

132.0

11.3

120.7

192.8

5.8

5.95

-1.38

-

-314.0

-22.1

194.4

-

79.0

210.1

1.4

6.03

-0.65

2.2105(4)

-174.1

-20.5

174.0

23.5

50.8

176.8

7.0

4.86

-0.43

2.2124(8)

118.9

193.0

5.4

4.12

-1.64

2.1744(8)

Cb C·W(CO)5

b

3 6

-300.2

16.5

137.3

14.2

6

(cis)c

-296.4

-

134.7

-

-

-

6

(trans)c

-296.7

-

143.6

-

-

-

-

-235.4

-13.2

268.8

21.6

-

-

5.0

-150.1

-37.8

-

-

-

-

7 8

-

a Measurements

-

collected at 298 K in C6D6 or d8-THF (see Supporting Information). Data collected at 213 K in D8-THF.

P→Ni interactions (Scheme 4), a bonding model suggested by Apeloig et al. in a previous publication describing an η2-silaalkene platinum complex.33 The Si-P bond distance in 6 is between reported Si-P single and double bond values (d(Si1-P1) = (MBO) for the Si-P interaction (1.08), which suggests some degree of Si-P multiple bond character, whilst a much weaker interaction was found for P-Ni (0.76). The related value for the Ni-Si bond is 0.90, indicating that the Si-Ni interaction in 6 is indeed stronger than the P-Ni interaction. With regard of the frontier orbitals of 6, P-Ni bonding interactions are clear in the HOMO-3 and HOMO-5 (Figures S51 and S52), whilst the HOMO-1 (Figure S50) and HOMO-2 (S51) consist largely of non-bonding electron density at P. Charge analysis using Natural Population Analysis shows neutral Ni (-0.06), a positively charged Si center (+0.82) and a negative P center (-0.42), the latter value being in line with the considerably highfield resonance observed in the 31P NMR spectrum of 6 (calculated value: δ = -293.5 ppm). These combined experimental and computational findings suggest that there is some degree of Si-P multiple bond character in 6, although as a whole this novel complex is best described as a metallacycle. Given the facile cis-trans isomerization in 6, we sought to observe the effect of the addition of a Lewis acid to this complex. To our delight, the addition of Ph3B to a sample of 6 cleanly led to coordination at P1, forming [{(LTMS)(H)Si=P(H)(BPh3)}Ni(NHC)2], 7. This is evident in both the 1H and 31P NMR spectra of 7; disappearance of the P-H resonance and retention of the characteristic doublet of doublets relating to the presence of the Si-H moiety (2JPH = 21.6 Hz, 3JHH = 5.0 Hz) is observed in the 1H NMR spectrum, and the 31P NMR spectrum shows considerable broadening due to coordination to the quadrupolar 11B nucleus (Figure 5c). An 11B{1H} NMR analysis also showed a considerable high-field shift to δ = -8.4 ppm when compared with free Ph3B (δ = 52 ppm). Structural analysis of X-

-

-1.93

-

-1.47

2.094(5)

-

4.09

-

2.1887(9)

3.97 and 4.85

-

2.223(2)

b Values

taken from the literature, ref. 13-17. c

ray quality crystals of 7 confirmed it to be the borane adduct of a η2-silaphosphene Ni complex. Presumably due to the steric bulk of the Lewis acidic Ph3B fragment in 7, only the trans-isomer is observed in this complex (Figure 5a). The Si-P distance in 7 (d(Si1-P1) = 2.1887(9) Å) lies between those for reported Si-P single and double bonds, as in 6, again suggesting that 7 contains some degree of Si-P multiple bond character. The metallacyclic nature at the threemembered NiSiP-core of 7 is clear when observing the geometry at Si1 and P1, both of which now deviate considerably from planarity (θSi1 = 42.6°; θP1 = 30.8°), indicating considerable metallacycle character when compared with previous examples of disilene-TM complexes.12,13 Upon coordination of BPh3 to the phosphorus center in 6 a notable shift is observed in the 31P NMR spectrum (6: δ = -300.2 ppm; 7: δ = -235.4 ppm). Further, the P-H resonance in the 1H NMR spectrum of 7 is essentially unobservable due to broadening of this signal through coupling to the quadrupolar 11B nucleus. Related broadening of signals in both 31P and 31 1 P{ H} NMR spectra is also observed. Nevertheless, the 29Si satellite signals are still observable in the 1H NMR spectrum of 7, with coupling constants similar to those in 6 (Table 1). The P-H coupling constant, however, shows a remarkable increase compared to that in 6, almost doubling upon coordination (Table 1). This affect would indicate considerably more 3s-character in the P-H moiety of 7 relative to that in 6, a result of greater sp-hybridization upon addition of BPh3, which is also in line with the out-of-plane bending observed at the P1 center in 7. Indeed, a similar, albeit less pronounced effect is seen in Lewis-acid adducts of “half-parent” silaphosphene B,34 and is typically observed upon coordination of hydrophosphines to σ-acceptors.35 Although the P-H moiety in 6 might be expected to be relatively acidic, attempts to deprotonation this moiety with common bases (e.g. nBuLi, LDA, etc.; LDA = lithium diisoproylamide) were unsuccessful, and led to intractable


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Scheme 5. Rearrangement of 6 to 8, and isomerization of 8 to 8’.

sharp singlet in the 31P{1H} NMR spectrum, indicating the presence of only one phosphorus environment. The two protons on each silicon present as two multiplets in the 1H NMR spectrum of 8, through coupling to phosphorus as well as each other, due to differing substitution patterns at each nickel center in 8. Nevertheless, both couple to a single signal in the 29Si NMR spectrum (Figure S33), in keeping with the proposed formulation of 8 in solutions. The DFT-derived 29Si (-34.5 ppm) and 31P (-161.7 ppm) NMR values for 8 are also in keeping with those above.36 The molecular structure of 8’ contains a P-P single bond (d(P1-P2) = 2.170(2) Å), with Ni-P bonds (d = 2.153(2)- 2.291(1) Å) in the range of single bonds reported for related small Ni-stabilized phosphorus clusters.37 Notably, a relatively short contact between Si3 of the silyl ligand on Ni1, and P1 (d(Si3P1) = 3.028(2) Å; sum of the van der Waals radii = 4.05 Å) is perhaps an artifact of the reversible silyl-migration leading to both 8 and 8’. A DFT analysis of this isomerization process indicates that there is a negligible energy difference between 8 and 8’ of just 3.1 kcal mol-1. Such a small value is easily thermally accessible, and can also be affected by crystal packing forces, giving some explanation for the observation of these two remarkable species.38 Scheme 6. Synthesis of Li(tmeda)AsH2, and its reaction with 1 to form thermally unstable η2-1,2-dihydrosilaarsene complex 9.

Figure 6. Molecular structure of 8, with thermal ellipsoids at 30 % probability. Selected bond lengths (Å) and angles (°) for 8: P1-P2 2.170(1); Ni1-P1 2.153(1); Ni1-P2 2.291(1); Ni2-P1 2.216(1); Ni2-P2 2.283(2); P2-Si1 2.223(2); Ni1-Si3 2.214(1); P1-P2-Si1 93.24(6); Ni1-P1-Ni2 103.44(5); Ni1-P2-Ni2 97.16(5).

mixtures. However, this proton does undergo migration to Si, forming the novel bimetallic bis-phosphinidene complex 8 (Scheme 5). This process is slow, taking several days to completion at ambient temperature. We propose that 8 forms via intermittent phosphinidene nickel complex 6’ (Scheme 5), which sits slightly higher in energy than the metallacyclic isomer 6 (∆G = 11.3 kcal mol-1; Figure 2). This energetic barrier would explain the time taken for 8 to form. Following isomerization of 6 to 6’, dimerization can occur with concomitant loss of a single NHC ligand, to yield the bimetallic bis-phosphinidene complex 8 in solution (Scheme 4). Isolating deep red crystals of this complex, we found that in the solid state 8 crystallizes as an η4complex of a P-metallated diphosphene, 8’, through Ni-insertion into one P-Si bond (Figure 6). However, NMR spectroscopic analysis of re-dissolved crystals of this conformer reveal single 31P and 29Si environments, indicating that the P-Si activation process is in fact reversible, and that 8 is reformed in solution. The 31P NMR spectrum of 8 presents a broad multiplet at δ = -153 ppm, which coalesces to a

In order to pursue the associated chemistry of arsenic, we first developed a high-yield route to a stable LiAsH2 salt, namely Li(tmeda)AsH2, which has previously been generated from the DME derivative but not directly synthesized.39 Thus, exposure of a hexane solution of nBuLi and TMEDA to AsH3 leads to the precipitation of colorless Li(tmeda)AsH2 (Scheme 6), which can be stored in an inert atmosphere at ambient temperature for prolonged periods of time without decomposition. The X-ray crystal structure of Li(tmeda)AsH2 is very similar to that for Li(dme)PH2 (Figure S46), forming 1D chains through bridging interactions of Li with neighboring [AsH2] units.40 Li(tmeda)AsH2 is surprisingly soluble, readily dissolving in polar solvents such as THF and Et2O, as well as in unpolar solvents such as toluene. This aspect proved important for its utility in low-temperature synthetic applications, allowing for addition of THF solutions of Li(tmeda)AsH2 to solutions of 1 at -78 °C. This reaction leads to an immediate color change from dark orange-brown to yellow-orange, as with the related reaction with Li(tmeda)PH2. Quickly analyzing aliquots of this reaction mixture by 1H NMR spectroscopy indicated that a product in keeping with the η2-1,2-dihydrosilaarsene nickel complex 9 was present, with a doublet at δ = -1.37 ppm (3JHH = 5.46 Hz) attributable to the HSi-AsH fragment. However, rapid decomposition of this species was clear at ambient temperature, forming two new species. An initial recrystallization of the reaction mixture from cold THF yielded a small crop of dark red crystals,



Page 8 of 12

Figure 7. Molecular structures of 10 (left), 11 (middle), and 12 (right), with thermal ellipsoids at 30 % probability. Selected bond lengths (Å) and angles (°) for 10: As1-As1’ 2.3906(8); As1-Ni1 2.359; Si1-As1-As1’ 89.93(4); Si1As1As1’Si1’ 176.26(6). For 11: As1-Si1 2.2982(6); As1-Ni1 2.3511(5); Ni1-Si1 2.1756(6); Ni2-As1 2.3321(4); Ni1-Ni2 2.5937(4); N1-Si1 1.772(2); Ni2-As1-Si1 110.75(2); Ni1-As1-Si 67.26(1); As1-Si1-Ni1 63.34(2); As1-Ni1-Si1 60.88(2); Ni1-As1-Ni2 67.26(1); N1-Si1-As1-Ni2 173.15(6). For 12: As1-Si1 2.2404(8); As1-Ni1 2.4016(5); Si1-Si1 2.2331(9); As1-Li1 2.524(6); N1-Si1 1.790(2); N1-Si1-As1 127.29(8); Li1-As1-Si1 68.7(1); As1-Ni1-Si1 57.68(2); Ni1-As1-Si1 57.38(2); As1-Si1-Ni1 64.94(2); Li1-As1-Ni1-N1 117.1(2).

which were unexpectedly found to be the η2-diarsene nickel complex, 10 (Scheme 7, Figure 7).41 The thermal instability of compound 10 thwarted efforts to attain meaningful analytical data for this compound. Nevertheless, as with the formation of bis-phosphinidene complex 8, the initial step in the formation of 10 would involve proton migration from As to Si in 9, forming the silylarsinidene complex 9’. Subsequent formation of 10 from two equivalents of intermediary 9’ would then proceed via loss of a single [(NHC)2Ni] fragment. The facile nature of this process may be taken as circumstantial evidence that the most stable isomer for the arsenic system described is in fact the arsinidene complex, 9’, although DFT calculations suggest that this isomer is in fact 4.1 kcal mol-1 higher in energy than metallacyclic 9. Remarkably, the second crystalline product 11 (Figure 7) could be isolated from the reaction of 1 with Li(tmeda)AsH2, shedding light on the fate of the liberated [(NHC)2Ni] fragment in the formation of 10. Compound 11 represents an As-metallated η2-silaarsene nickel complex, formed through insertion of[(NHC)2Ni] into the As-H bond of intermediary 9 (Scheme 6). This intriguing compound is stable in solutions for extended periods of time, in stark contrast to closely related metallacycle 9. The Ni-H moiety in 11 can be observed in its 1H NMR spectrum at δ = -14.18 ppm, with a very weak coupling to the Si-H fragment, the signal for which can be found at δ = 5.15 ppm (1JSiH = 180.2 Hz, 3JHH =1.8 Hz). The low coupling constant for these two protons is apparently due to the hydride ligand which bridges both nickel centers, thus forming two relatively weak Ni-H bonds. Both Ni-As distances are in keeping with the mean value for single Ni-As bonds involving three-coordinate arsenic (mean: 2.342 Å; d(Ni1-As1) = 2.3514(6) Å; d(Ni2-As1) = 2.3322(4) Å), as is the Si-Ni bond. These values, alongside the bending angle at Si1 (θSi = 42.47°), would indicate that the [SiAsNi] core of 11 is best described as a metallacycle.

Scheme 7. Fragmentation of 9 leading to compounds 10 and 11, and deprotonation of 9 with LDA forming 12. TMSL

NHC As Ni TMS

L

Si

As

- Ni(NHC)2

As

NHC Ni

NHC

H

SiH2

x2

TMS

H 9'

~H

L

NHC

SiH2

10

NHC

NHC Ni

H As

Ni Ni

Si

H TMS

L

H

As

NHC

9

- NHC

Si

H

NHC

TMS

NHC

L

11

NMe 2 Me 2N

Li

NHC As Ni

LDA H

Si

TMS

L

NHC 12

Given the apparent acidity of the As-H proton in 9, which leads to its rapid isomerization in solutions at ambient temperatures, we tested whether this fragment could be directly deprotonated prior to rearrangement to 9’. Addition of a THF solution of LDA to in situ generated 9 at 78 °C indeed led to its deprotonation and thus formation of the unprecedented As-lithio η2-silaarsene nickel complex [{(TMSL)(H)Si=AsLi(tmeda)}Ni(NHC)2] 12 , with coordinated TMEDA remaining from employed Li(tmeda)AsH2 (Scheme 7, Figure 7). The solid state structure of 12 (Figure 6) contains a bridging Si-H…Li contact, leading to tetrahedral geometry at Li1 and a rather acute Si-As-Li angle of


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68.63(1)°. The high charge density at As1, due to its metallation, leads to a highly contracted Si-As bond length in 12 (δ = 2.2402(8) Å), which is essentially as short as the majority of ‘free’ Si=As bonds, including the 1,2-dihydrosilaarsene D reported by us previously (d(Si-As) = 2.214 Å).18,42 Indeed, the Si-As bond in 12 is considerably shorter than that in closely related 11, testament to the charge separation in the As-Li bond relative to the As-Ni bond. Despite this, the Ni-As bond in 12 is longer than those in 11, at large degree of multiple-bond character. The low degree of π-bonding in this moiety is evidenced by the bending angle at Si (θSi = 40.24°).

Figure 8. The Highest Occupied Molecular Orbital (HOMO) of 12.

To further shed light on this remarkable bonding situation, the electronic ground state of 12 was computationally evaluated. The MBO for the Si-As bond (1.20) suggests a considerable degree of Si-As multiple-bond character when compared to the related value for the Si-P moiety in 6 (MBO = 1.07), whilst much smaller values were found for the As-Ni (0.81) and Ni-Si (0.88) interactions. Charge analysis shows a near neutral Ni center (-0.10), a positively charged Si center (+0.64) and a negative As center (-0.62) that also indicate considerable charge separation between As and Li. The partial charges on Si and As, therefore, would lead to contraction of the bond between these two elements, as is observed in the solid state. NBO analysis of the Si-As bond is in line with the charge analysis showing only one Si-As bond and a partially filled lone pair on As (Table S4). Finally, observing DFT-derived frontier orbitals of 12 (Figures S53-55), one can observe As-Ni interactions for the HOMO-4, HOMO-5, and HOMO-6, although electron density for these orbitals largely resides on As, accounting for the negative charge density on this center. Importantly, it seems apparent that the HOMO in fact represents a weak π-interaction between As and Si (Figure 8),

(1) (a) Zeise, W. C. Von der Wirkung zwischen Platinchlorid und Alkohol, und von den dabei entstehenden neuen Substanzen. Ann. Phys., 1831, 21, 497-541; (b) Jarvis, J. A. J.; Kilbourn, B. T.; Ow-

possibly the root of multiple bond character in 12, thus suggesting that this character is not simply due to the high charge density at As and Si. It would seem that the best description for the bonding situation in unique example of an As-metallated metallacycle leans towards a metallacycle with some degree of π-bonding between As and Si. CONCLUSIONS In summary, we have taken steps towards defining the relative stability of three tautomeric forms in a TM complex of the elusive 1,2-dihydrosilapnictenes. As such, the “half-parent” bis(amido)silylene and 1,2-dihydrosilaphosphene nickel complexes have been realized, the latter being the first example of any η2-coordinated heavy aldimine analogous metal complex. We have also observed strong evidence for the formation of the phoshinidene and arsinidene isomeric forms through well-defined rearrangement processes. Finally, an As-lithio η2-silarsene nickel complex has been realized, which contains considerable SiAs double bond character despite being best describe as a metallacycle, challenging our fundamental understanding of chemical bonding in such systems. This study has therefore shed light on the first dihydropnictido silylene-silapnictene-silylpnictinidene tautomeric series with N, P and As, as well as given some insight into the electronic nature of a hitherto unknown class of main-group element TM complexes.

ASSOCIATED CONTENT Details of the synthesis and characterization data for all new compounds. NMR spectra for all new compounds. Full details for crystallographic and computational studies. Crystallographic data in CIF format (CCDC Numbers.: 18821651882177). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.

REFERENCES

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(15) Driess, M.; Block, S.; Brym, M.; Gamer, M. T. Synthesis of a “Half”-Parent Phosphasilene R2Si=PH and its Metalation to the Corresponding P-Zinciophosphasilene [R2Si=PM]. Angew. Chem. Int. Ed. 2006, 45, 2293–2296. (16) Hansen, K.; Szilvási, T.; Blom, B.; Inoue, S.; Epping, J.; Driess, M. A Fragile Zwitterionic Phosphasilene as a Transfer Agent of the Elusive Parent Phosphinidene (:PH). J. Am. Chem. Soc. 2013, 135, 11795−11798. (17) Hansen, K.; Szilvási, T.; Blom, B.; Driess, M. A Persistent 1,2-Dihydrophosphasilene Adduct. Angew. Chem. Int. Ed., 2015, 54, 15060–15063. (18) Präsang, C.; Stoelzel. M.; Inoue, S.; Meltzer, A.; Driess, M.; Metal-Free Activation of EH3 (E=P, As) by an Ylide-like Silylene and Formation of a Donor-Stabilized Arsasilene with a HSi=AsH Subunit. Angew. Chem. Int. Ed., 2010, 49, 10002 –10005. (19) Jana, A.; Schulzke, C.; Roesky, H. W. Oxidative Addition of Ammonia at a Silicon(II) Center and an Unprecedented Hydrogenation Reaction of Compounds with Low-Valent Group 14 Elements Using Ammonia Borane. J. Am. Chem. Soc. 2009, 131, 4600– 4601. (20) Glatthaar, J.; Maier, G. Reaction of Atomic Silicon with Phosphane: A Matrix-Spectroscopic Study. Angew.Chem.Int.Ed. 2004, 43, 1294–1294. (21) Chen, M.; Zheng, A.; Lu, H.; Zhou, M. Reactions of Atomic Silicon and Germanium with Ammonia: A Matrix-Isolation FTIR and Theoretical Study. J. Phys. Chem. A 2002, 106, 3077-3083. (22) Inoue, S.; Eisenhut, C.; A Dihydrodisilene Transition Metal Complex from an N-Heterocyclic Carbene-Stabilized Silylene Monohydride. J. Am. Chem. Soc. 2013, 135, 18315−18318. (23) (a) Hadlington, T. J.; Szilvási, T.; Driess, M. Silylene–Nickel Promoted Cleavage of B−O Bonds: From Catechol Borane to the Hydroborylene Ligand. Angew. Chem. Int. Ed., 2017, 56, 7470– 7474; (b) Hadlington, T. J.; Szilvási, T.; Driess, M. Synthesis of a Metallo-Iminosilane via a Silanone–Metal π-Complex. Angew. Chem. Int. Ed. 2017, 56, 14282–14286. (24) (a) Gutsulyak, D. V.; Piers, W. E.; Borau-Garcia, J.; Parvez, M. Activation of Water, Ammonia, and Other Small Molecules by PCcarbeneP Nickel Pincer Complexes. J. Am. Chem. Soc., 2013, 135, 11776–11779; (b) Brown, R. M.; Borau-Garcia, J.; Valjus, J.; Roberts, C. J.; Tuononen, H. M.; Parvez, M.; Roesler, R. Ammonia Activation by a Nickel NCN-Pincer Complex featuring a Non-Innocent N-Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium. Angew. Chem. Int. Ed., 2015, 127, 6372 –6375 (25) Schmidt, D.; Zell, T.; Schaub, T.; Radius, U. Si–H bond activation at {(NHC)2Ni0} leading to hydrido silyl and bis(silyl) complexes: a versatile tool for catalytic Si–H/D exchange, acceptorless dehydrogenative coupling of hydrosilanes, and hydrogenation of disilanes to hydrosilanes. Dalton Trans., 2014, 43, 10816-10827. (26) For some examples, see: (a) Basuli, F.; Bailey, B. C.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J. Terminal and Four-Coordinate Vanadium(IV) Phosphinidene Complexes. A Pseudo Jahn−Teller Eﬀect of Second Order Stabilizing the V−P Multiple Bond. J. Am. Chem. Soc., 2004, 126, 1924-1925; (b) Ganushevich, Y. S.; Miluykov, V. A.; Polyancev, F. M.; Latypov, S. K.; Lönnecke, P.; Hey-Hawkins, E.; Yakhvarov, D. G.; Sinyashin, O. G. Nickel Phosphanido Hydride Complex: An Intermediate in the Hydrophosphination of Unactivated Alkenes by Primary Phosphine. Organometallics, 2013, 32, 3914−3919; (c) Breit, N.C.; Eisenhut, C.; Inoue, S. Phosphinosilylenes as a novel ligand system for heterobimetallic complexes. Chem. Commun., 2016, 52, 5523—5526; (d) Hickey, A. K.; Muñoz III, S. B.; Lutz, S. A.; Pink, M.; Chen, C.-H.; Smith, J. M. Arrested α-hydride migration activates a phosphido ligand for C–H insertion. Chem. Commun., 2017, 53, 412-415.


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27 All examples with a 3-coordinate As center can be found in the following: (a) Ebsworth, E. A. V.; Gould, R. O.; Mayo, R. A.; Walkinshaw, M. Reactions of phosphine, arsine, and stibine with carbonylbis(triethylphosphine)iridium(I) halides. Part 1. Reactions in toluene; X-ray crystal structures of [Ir(CO)ClH(PEt3)2(AsH2)] and [Ir(CO)XH(PEt3)2(µ-ZH2)RuCl2(η6-MeC6H4CHMe2-p)](X = Br, Z = P; X = Cl, Z = As). J. Chem. Soc. Dalton Trans., 1987, 2831-2838; (b) Westerhausen, M.; Gückel, C.; Piotrowski, H.; Vogt, M. Metalation of Triisopropylsilylarsane with Bis(tetrahydrofuran)calcium-bis[tris(trimethylsilylmethyl)zincate]. Z. Anorg. Allg. Chem., 2002, 628, 735-740; (c) Roering, A. J.; Davidson, J. J.; MacMillan, S. N.; Tanskib, J. N.; Waterman, R.; Dalton Trans., 2008, 4488–4498. (28)Cui, H.; Zhang, J.; Cui, C. 2-Hydro-2-aminophosphasilene with N–Si–P π Conjugation. Organometallics, 2013, 32, 1−4. (29) For details regarding the synthesis and characterization of LiH2N→BPh3, see Supporting Information. (30) For details regarding the synthesis of TMEDA∙LiPH2, see Supporting Information. (31) H1A and H1B were located and freely refined in the structural solution for 6. (32) Nesterov, V.; Breit, N. C.; Inoue, S. Advances in Phosphasilene Chemistry. Chem. Eur. J. 2017, 23, 12014–12039. (33) Bravo-Zhivotovskii, D.; Peleg-Vasserman, H.; Kosa, M.; Molev, G.; Botoshanskii, M.; Apeloig, Y. The Direct Synthesis of a Silene–Organometallic Complex. Angew. Chem. Int. Ed. 2004, 43,745–745. (34) Hansen, K.; Szilvási, T.; Blom, B.; Driess, M. Transition Metal Complexes of a “Half-Parent” Phosphasilene Adduct Representing Silylene→ Phosphinidene→ Metal Complexes. Organometallics 2015, 34, 5703–5708. (35) Kühl, O.: Phosphorus-31 NMR Spectroscopy; Springer-Verlag: Berlin, Heidelberg, 2008. (36) Predicted NMR values for the isomer 8’ (29Si: -34.5 and -5.0 ppm; 31P: -210.3 and 160.9 ppm) deviate considerably from those observed in solution, further suggesting that 8’ does not exist in solution. (37) (a) Scherer, O. I.; Braun, J.; Wulther, P.; Heckmunn, C.; Wolmershauser, C. Phosphorus Monoxide (PO) as Complex Ligand. Angew. Chem. Int. Ed. Engl. 1991, 30, 852-854; (b) Zarzycki,

B.; Zell, T.; Schmidt, D.; Radius, U. P4 Activation at Ni0: Selective Formation of an NHC-Stabilized, Dinuclear Nickel Complex [Ni2(iPr2Im)4(μ,η2:2-P2)]. Eur. J. Inorg. Chem. 2013, 2051–2058; (c) Hierlmeier, G.; Hinz, A.; Wolf, R.; Goicoechea, J. M. Angew. Chem. Int. Ed. 2018, 57, 431–436. (38) Buntine, M. A.; Hall, V. J.; Kosovel, F. J.; Tiekink, E. R. T. Influence of Crystal Packing on Molecular Geometry: A Crystallographic and Theoretical Investigation of Selected Diorganotin Systems. J. Phys. Chem. A 1998, 102, 2472-2482. (39) (a) Becker, G.; Eschbach, B.; Mundt, O.; Reti, M.; Niecke, E.; Issberner, K.; Nieger, M.; Thelen, V.; Nöth H.; Waldhör, R.; Schmidt, M. Metal Derivatives of Molecular Compounds. IX Bis(1,2-dimethoxyethane-O,O')lithium Phosphanide, Arsanide, and Chloride - Three New Representatives of the Bis(1,2-dimethoxyethane-O,O') Lithium Bromide Type. Z. anorg. allg. Chem. 1998, 624, 469-482; (b) Herrmann, W. A.; Brauer G. Arsane, (1,2-Dimethoxyethane-O,O′ )lithium Arsanide, [1,2Bis(dimethylamino)ethane-N,N′ ]lithium Arsanide. In Phosphorus, Arsenic, Antimony and Bismuth; Karsch, H. H., Eds.; Synthetic Methods of Organometallic and Inorganic Chemistry; Thieme: Stuttgart, Germany, 1996, 3, 189-193. (40) Becker, G.; Hartmann, H.-M.; Schwarz, W. Metal Derivatives of Molecular Compounds. III. Molecular and Crystal Structure of Lithium bis(trimethylsilyl)phosphide-(dme) and of Lithium dihydrogenphosphide-(dme) Z. anorg. allg. Chem. 1989, 577, 9-22. (41) For an example of a related Pt complex, see: Jibril, I.; Frank, L. R.; Zsolnai, L.; Evertz, K.; Huttner, G. Dehydrierung von phenylarsan und seinen pentacarbonylchrom-derivativen: Abfangreaktionen für diphenyldiarsen, PHAs=AsPh. J. Organomet. Chem. 1990, 393, 213-225. (42) (a) Lee, V. Y.; Aoki, S.; Kawai, M.; Meguro, T.; Sekiguchi, A. Stibasilene Sb=Si and Its Lighter Homologues: A Comparative Study. J. Am. Chem. Soc. 2014, 136, 6243−6246; (b) Seitz, A. E.; Eckhardt, M.; Sen, S. S.; Erlebach, A.; Peresypkina, E. V.; Roesky, H. W.; Sierka, M.; Scheer, M. Different Reactivity of As4 towards Disilenes and Silylenes. Angew. Chem. Int. Ed. 2017, 56, 6655– 6659.



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Versatile Tautomerization of EH2-Substituted Silylenes (E = N, P, As

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