Reactions of Mesityl Azide with Ferrocene‐Based N‐Heterocyclic Germylenes, Stannylenes and Plumbylenes, Including PPh2‐Functionalised Congeners

Abstract The reactivity of ferrocene‐based N‐heterocyclic tetrylenes [{Fe(η5−C5H4−NSitBuMe2)2}E] (E=Ge, Sn, Pb) towards mesityl azide (MesN3) is compared with that of PPh2‐functionalised congeners exhibiting two possible reaction sites, namely the EII and PIII atom. For E=Ge and Sn the reaction occurs at the EII atom, leading to the formation of N2 and an EIV=NMes unit. The germanimines are sufficiently stable for isolation. The stannanimines furnish follow‐up products, either by [2+3] cycloaddition with MesN3 or, in the PPh2‐substituted case, by NMes transfer from the SnIV to the PIII atom. Whereas [{Fe(η5−C5H4−NSitBuMe2)2}Pb] and other diaminoplumbylenes studied are inert even under forcing conditions, the PPh2‐substituted congener forms an addition product with MesN3, thus showing a behaviour similar to that of frustrated Lewis pairs. The germylenes of this study afford copper(I) complexes with CuCl, including the first structurally characterised linear dicoordinate halogenido complex [CuX(L)] with a heavier tetrylene ligand L.


Introduction
Since more than a century, [1] the Staudinger reaction of organic azides RN 3 with phosphanes R' 3 P (R, R' = alkyl or aryl) has provided access to iminophosphoranes RN=PR' 3 via intermediate phosphazides RN=NÀ N=PR' 3 , which are generally unstable, but could be isolated in certain cases. [2] A significant kinetic stabilisation of phosphazides by intramolecular coordination of the P-bonded nitrogen atom to a Lewis acid was suggested by Grützmacher in 1999. [3] In view of the well-known analogy between phosphanes and N-heterocyclic carbenes (NHCs), [4] it is not surprising that organic azides can react with NHCs in a similar manner to afford cyclic guanidine derivatives NHC=NR, as was shown by Bielawski in 2005. [5] The intermediate triazenes NHC=NÀ N=NR are thermally much more robust towards N 2 extrusion than the analogous phosphazides RN=NÀ N=PR' 3 . [6] We note in this context that the synthesis of imines of the type RN=CCl 2 (i. e. isocyanide dichlorides) [7] by reaction of organic azides with the transient singlet carbene Cl 2 C [8] had been reported by Baldwin already in 1968; N 2 extrusion occurs even below room temperature in this case. [9] The heavier carbene analogues can react with organic azides in a similar fashion, giving rise to imine analogues with formal E=N (E = Si-Pb) double bonds. [10] This was first shown by Satgé in 1978, who reported the formation of the transient germanimine (Me 2 N) 2 Ge=NPh together with N 2 in the reaction of the diaminogermylene (Me 2 N) 2 Ge with PhN 3 . [11] In 1991 Meller described the first structurally characterised stable germanimines [(Me 3 Si)ArN] 2 Ge=NAr [obtained from the reaction of ArN 3 with [(Me 3 Si)ArN] 2 Ge, Ar = mesityl (Mes) or 2,6-diisopropylphenyl (Dipp)]. [12] This was followed in 1993 by the first structurally characterised stable stannanimine [(Me 3 Si) 2 N] 2 Sn=NDipp (obtained from DippN 3 and [(Me 3 Si) 2 N] 2 Sn at À 30°C). [13] The only other stable stannanimine known to date, [(Me 3 Si) 2 2 ], was reported one year later by Ando. [14] The paucity of isolable stannanimines is due to the fact that, owing to their reactive polar Sn=N bond, stannanimines readily form their head-to-tail dimers (viz. 1,3,2,4-diazadistannetidines) or give [2 + 3] cycloadducts with the organic azide to furnish stannatetrazoles, as was shown by Pinchuk and by Neumann already in the 1980s. [15] The publication of the N-heterocyclic silylene (CHNtBu) 2 Si, the first stable compound containing dicoordinate Si II , by Denk and West in 1994 [16] was followed in the same year by their report of its reaction with trityl azide, which furnished the THF complex of the silanimine (CHNtBu) 2 Si=NCPh 3 . [17] Since then, numerous reactions of organic azides with free or basestabilised silylenes furnishing free or base-stabilised silanimines have been described. [10b,c,18] Among the reactions of organic azides with heavier carbene analogues, plumbylenes have apparently been investigated least extensively. We are aware of only three studies in this context. The first study was published in 2002 by Klinkhammer, who described the reaction of [(Me 3 Si) 3 Si] 2 Pb (1) with 1-adamantyl azide in toluene at À 30°C, which furnished the triazenido Pb II complex 2 with a four-membered PbN 3 heterocycle due to migration of a (Me 3 Si) 3 Si group from Pb to N (Scheme 1, top). [19] In 2017 Song reported the reaction of the N-heterocyclic plumbylene o-C 6 H 4 (NDipp) 2 Pb (3) with mesityl azide under harsh conditions (110°C, toluene). N 2 was liberated, but no plumbanimine was observed. The reaction resulted in a donorfunctionalised plumbylene 4 with tricoordinate Pb II due to a chelating secondary amino group, which was plausibly formed by insertion of an NMes unit into a benzylic CÀ H bond, thus affording a Me 2 CÀ NHÀ Mes moiety (Scheme 1, bottom). [20] Of particular interest for the present work is Wesemann's study of the reaction of organic azides with intramolecular Lewis pairs composed of a tetrylene and a phosphane unit, which included also the plumbylene Ar*Pb[CHPh(PPh 2 )] [5, Ar* = 2,6-(2,4,6-iPr 3 C 6 H 2 ) 2 C 6 H 3 ] (Scheme 2). [21] The Pb II atom of 5 is dicoordinate. In contrast, the corresponding stannylene 6 [22] and germylene 9 [21] exhibit tricoordinate tetrel atoms due to intramolecular coordination of the P atom, thus resembling our recently reported ferrocene-based N-heterocyclic plumbylene 12, stannylene 13 and germylene 14 functionalised with a PPh 2 group (Figure 1, top). [23] Compounds 5, 6, 9 and 12-14 contain two different sites suitable for reaction with an organic azide, viz. the P atom and the tetrel atom. Wesemann observed that plumbylene 5 and stannylene 6 react with AdN 3 in the same fashion, affording addition products 7 and 8 featuring a four-membered ENPC heterocycle (E = Sn, Pb; N denotes the terminal AdN 3 nitrogen atom; Scheme 2, top). An analogous reaction had previously been reported by Ionkin for the stannylene [tBu 2 PCH 2 C-(CF 3 ) 2 O] 2 Sn, which contains a tetracoordinate Sn II atom due to intramolecular P coordination; only one equivalent of AdN 3 was consumed even under forcing conditions. [24] From a formal point of view, the products of these reactions can be described as phosphazides which are engaged in an intramolecular coordination of their P-bonded nitrogen atom to the divalent tetrel atom. A completely different behaviour was found for germylene 9, where formation of N 2 and of the corresponding germanimine 10 (kinetic product] was observed, followed by slow isomerisation of the latter to the iminophosphoranefunctionalised germylene 11 (thermodynamic product; Scheme 2, bottom). [21] The reactions of the Lewis pairs 5 and 6 with 1-adamantyl azide are reminiscent of reactions reported for several borane-phosphane (B/P) frustrated Lewis pairs (FLPs) with organic azides, where FLP addition to the terminal nitrogen atom of the azide (N) occurred, resulting in four-or five-membered heterocycles with a BNP subunit. [25] Wesemann's results inspired us to investigate the behaviour of plumbylene Scheme 1. Reactions of organic azides with plumbylenes described by Klinkhammer (top) and Song (bottom); Ad = 1-adamantyl, Mes = mesityl.

Results and Discussion
The N-heterocyclic germylene [{Fe(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }Ge] (17) was conveniently prepared analogous to the stannylene [{Fe(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }Sn] (16) [28] by reacting LiN(SiMe 3 ) 2 , [GeCl 2 (1,4-dioxane)] and [Fe(η 5 À C 5 H 4 À NHSitBuMe 2 ) 2 ] in a 2 : 1 : 1 molar ratio in THF. The product was obtained in 92 % yield and was structurally characterised by single-crystal X-ray diffraction (XRD). Pertinent metric parameters of germylene 17 and of the products of the reactions with MesN 3 obtained in this study are collected in Table 1. Data for known N-heterocyclic tetrylenes which served as starting materials for the new compounds contained in Table 1 have been included in this Table for comparison. The molecular structure of 17 is shown in Figure 2. The molecule exhibits approximate C 2 symmetry about the FeÀ Ge axis. The germanium bond lengths and angle are very similar to the values published for the N-trimethylsilyl homologue. [29] The germanium bond angle of 107.22 (7)°d etermined for 17 is essentially identical to the value reported by Lappert for the emblematic acyclic diaminogermylene [(Me 3 Si) 2 N] 2 Ge, viz. 107.1(2)°. [30] This is in line with previous observations that the 1,1'-ferrocenylene backbone of N-heterocyclic carbenes and their heavier analogues [{Fe(η 5 À C 5 H 4 À NR) 2 } [a] Dihedral angle formed by the best planes of the cyclopentadienyl rings. [b] Two independent molecules with very similar bond parameters; data arbitrarily given for molecule 1.
[c] Two independent molecules, one of them showing disorder; data given for the non-disordered molecule. E] (E = C-Pb) gives rise to large bond angles at the divalent tetrel atom close to those of corresponding acyclic congeners. [27][28][29]31,32] The SnÀ N bonds of stannylene 16 are longer than the GeÀ N bonds of germylene 17 by ca. 0.2 Å, which is in accord with the difference of the covalent radii of tin (1.39 Å) and germanium (1.20 Å). [33] In turn, the Ge II bond angle of 17 is slightly wider (by 2°) than the Sn II bond angle of 16, which is in agreement with Bent's rule. [34,35] With the complete series of donor-functionalised heavier Nheterocyclic tetrylenes 12-14 and the unfunctionalised congeners 15-17 in hand, we next studied the reactivity of these six compounds towards mesityl azide (Scheme 3). Reactions were observed for all compounds except [{Fe-(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }Pb] (15). Since plumbylene 15 was inert even under rather forcing conditions (105°C, toluene) almost identical to those used by Song for o-C 6 H 4 (NDipp) 2 Pb (3; Scheme 1, bottom), we tested three additional diaminoplumbylenes in this context, viz. the acyclic congener [(Me 3 Si) 2 N] 2 Pb [36] as well as our recently reported five-and six-membered Nheterocyclic plumbylenes o-C 6 H 4 (NSiMe 3 ) 2 Pb and nap(NSiMe 3 ) 2 Pb (nap = naphthalene-1,8-diyl), [31] all containing Me 3 Si instead of SitBuMe 2 substituents to decrease steric congestion. However, these compounds proved to be equally inert. The stannylene [{Fe(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }Sn] (16) showed a smooth and swift reaction with MesN 3 (2 equiv.) at room temperature, furnishing the stannatetrazole 18 (structurally characterised by XRD, Figure 3) in 83 % yield. The use of 1 equiv. of the azide resulted in an equimolar mixture of 18 and unreacted 16. This is in line with previous reports that an initially formed stannanimine is prone to form a [2 + 3] cycloadduct with an organic azide. [13,15d,37] Stannatetrazole 18 is a spiro compound with a five-and a six-membered heterocycle connected by the Sn atom. Schulz recently confirmed computationally that closely related compounds obtained from the acyclic diaminostannylene [(Me 3 Si) 2 N] 2 Sn and aryl azides contain a tin(IV) atom with four highly polar SnÀ N single bonds instead of a tin(II) atom chelated by a tetraazabutadiene ligand. [37a] A comparison of the SnÀ N bond lengths of stannylene 16 (average value 2.06 Å, dicoordinate Sn II ) and stannatetrazole 18 (average value 2.05 Å, tetracoordinate Sn IV ) shows that obviously the increase in coordination number from two to four is compensated by the decrease in the covalent radius on going from Sn II to Sn IV . This behaviour is not unusual. For example, a comparison of the cyclic diaminostannylene Me 2 Si(NDipp) 2 Sn [38] and the corre-sponding tin(IV) spiro compound [Me 2 Si(NDipp) 2 ] 2 Sn [39] reveals essentially identical SnÀ N bond lengths of 2.06 Å for both compounds. In agreement with the few structurally characterised stannatetrazoles known to date, [37a,b] the central NÀ N bond of the N 4 unit of 18 is considerably shorter than the other two NÀ N bonds (1.27 vs. 1.39 Å), which is compatible with a double bond and adjacent single bonds, thus supporting the NÀ N=NÀ N Lewis structure advocated by Schulz. [37a] In contrast to stannylene 16, the corresponding germylene [{Fe(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }Ge] (17) afforded the germanimine [{Fe(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }Ge=NMes] (19; structurally characterised by XRD, Figure 4) in 72 % yield. [12a,40] In comparison to stannanimines, the tendency of germanimines to undergo [2 + 3] cycloadditions with organic azides is less pronounced. For example, Meller found [2 + 3] cycloadduct (i. e. stannatetrazole) formation from the stannylene [(Me 3 Si) 2 N] 2 Sn and 2,6-diethylphenyl azide at À 50°C, [13] whereas Fulton observed germanimine formation from the corresponding germylene [(Me 3 Si) 2 N] 2 Ge and the significantly less bulky phenyl azide at À 30°C and with mesityl azide even at room temperature. [40a] Germanimine 19 contains a trigonal-planar Ge IV atom (sum of angles 360°) exhibiting a short (1.71 Å) and two long GeÀ N bonds (1.83 Å), in good agreement with germanimines derived from other diaminogermylenes. [12a,40] In comparison to germylene 17, the GeÀ N single bonds of 19 are slightly shorter (by 0.04 Å). Similar to what was noted above for tin compounds 16 and 18, the increase in coordination number from two to four is obviously outbalanced by the decrease in the covalent radius on going from Ge II to Ge IV , in accord with findings of our recent systematic study addressing oxidation reactions of several homologues of 17. [41]   In contrast to [{Fe(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }Pb] (15), which was inert towards MesN 3 even under forcing conditions (see above), the donor-functionalised N-heterocyclic plumbylene 12 reacted with this azide already under fairly mild conditions (60°C, toluene), affording the addition product 20 (structurally characterised by XRD, Figure 5) with a six-membered PbNPCCN heterocycle (N denotes the terminal MesN 3 nitrogen atom). 20 may be viewed as a phosphazide stabilised by intramolecular coordination of the P-bonded nitrogen atom to the Lewis acidic Pb II atom, which is in a trigonal pyramidal bonding environment (sum of angles 274°).
Finally, the reaction of mesityl azide with the donorfunctionalised germylene 14 at room temperature afforded the germanimine 22, which was found to undergo a slow isomerisation to the corresponding iminophosphorane-functionalised
The PhP 2 -functionalised germanimine 22 contains a tetracoordinate Ge IV atom due to an intramolecular coordinative GeÀ P bond (Figure 7), which is ca. 0.2 Å shorter than that of germylene 14. Analogous to germanimine 19, the Ge IV atom is involved in a short (1.73 Å) and two long GeÀ N bonds (average value 1.88 Å). These bonds are slightly elongated with respect to 19 (1.71 and 1.83 Å, see above) due to the higher coordination number of the Ge IV atom of 22, viz. four vs. three in 19. Similar to 19, Wesemann's germanimine 10, which was obtained as the kinetic product from germylene 9 and AdN 3 , also contains a tricoordinate Ge IV atom and exhibits a GeÀ N bond length of 1.71 Å; however, no coordination of the PPh 2 unit was observed in this case. [21] The iminophosphorane-functionalised germylene 23 (Figure 8), which is formed as thermodynamic product from the PPh 2 -functionalised germanimine 22 by rearrangement, contains a Ge II atom in a trigonal pyramidal bonding environment (sum of angles 300°). The molecular structure is analogous to that of the corresponding stannylene 21. The GeÀ NP bond is considerably longer (2.11 Å) than the two other GeÀ N bonds (average value 1.96 Å), in accord with the corresponding bond lengths determined for 21, when the difference of the covalent radii of Sn (1.39 Å) and Ge (1.20 Å) is taken into account. [33] The PÀ N bond lengths of both compounds are essentially identical, viz. 1.625(3) Å for 23 and 1.619(3) Å for 21; they also compare well with the value of 1.608(2) Å reported by Wesemann for the iminophosphorane-functionalised germylene 11. [21] When compared with the reactions of Wesemann's plumbylene 5 and germylene 9 with AdN 3 (Scheme 2), [21] the respective behaviour of plumbylene 12 and germylene 14 towards MesN 3 is completely analogous. However, while Wesemann's PPh 2functionalised stannylene 6 afforded an addition product (8), an iminophosphorane (21) was obtained from our PPh 2 -functionalised stannylene 13. The primary interaction of organic azides with main-group element Lewis acids has been shown to involve the C-bonded N atom. [44] In contrast, the Staudinger reaction begins with a nucleophilic attack of the phosphane PR' 3 on the terminal nitrogen atom (N) of the organic azide RN 3 . [3b,45] Slootweg recently addressed the mechanism of the reaction of RN 3 with the B/P FLP tBu 2 PCH 2 BPh 2 and found that the initial nucleophilic attack typical of a Staudinger reaction is kinetically less favourable than adduct formation of the Lewis acidic B atom with the Lewis basic C-bonded N atom. [25a] With MesN 3 and tBuN 3 , the reaction afforded the respective addition product containing a four-membered BNPC heterocycle via a six-membered ring (BNNNPC) intermediate. The inertness of the unfunctionalised plumbylenes of our study towards MesN 3 strongly indicates that the reaction of the azide occurs at the PPh 2 unit of the donor-functionalised plumbylenes 5 and 12. The fact that 5 reacts already at room temperature, while elevated temperatures are needed in the case of 12 is perfectly plausible in view of the fact that the P atom of 12 is engaged in an intramolecular coordinative bond to the Pb atom, while no such bond is present in 5, whose P atom is therefore readily available for reaction with the azide. In both cases, however, the resulting phosphazide is efficiently stabilised in this scenario by intramolecular adduct formation with the respective Lewis acidic Pb II atom. Our results obtained with the lighter congeners, viz. stannylene pair 13 and 16 and germylene pair 14 and 17, suggest that in these cases the reaction with the azide occurs at the divalent tetrel atom, leading to an E=N double bond, which is highly reactive in the case of E = Sn so that only follow-up products (18,21) were observed. Inspired in part by Breher's study of the ligand properties of the N-mesityl homologue of 17 in transition metal chemistry [46] as well as the recent report by Jambor and Herres-Pawlis on copper(I) germylene complexes in the context of lactide polymerisation, [47] we also addressed the coordination behaviour of the unfunctionalised germylene 17 and the donorfunctionalised congeners 14 and 23 towards CuCl (Scheme 4). [48]   Although an NMR spectroscopic analysis of a C 6 D 6 solution of this product revealed no significant coordination-induced signal shifts in comparison to free 17, an XRD study clearly showed that formation of a germylene-copper(I) complex had taken place and confirmed the 1 : 1 ratio already inferred from microanalytical data. While the composition of product 24 corresponds to a simple 1 : 1 complex [(17)CuCl], the solid state structure is not that simple. The Lewis structure of 24 given in Scheme 4 corresponds to the molecular structure in the crystal, which is shown in Figure 9. Pertinent metric parameters of 24 and of the other copper(I) complexes of this study are collected in Table 2.
The crystal structure of 24 exhibits four Cu I -bonded germylene moieties in the asymmetric unit. Two of them form a chlorido-bridged dimeric complex [(17)Cu(μ-Cl)] 2 containing two tricoordinate Ge II atoms (Ge1 and Ge2) as part of a diamond-shaped Cu 2 Cl 2 core. The other two moieties form a less symmetric dimer, which contains one tricoordinate (Ge4) and one tetracoordinate Ge II atom (Ge3). Only one of the two Cl atoms (Cl3) adopts a bridging position between the two Cu I atoms of this dimer. The second Cl atom (Cl4) is in a bridging position between the tetracoordinate Ge II atom Ge3 and the Cu I atom Cu4 bonded to the tricoordinate Ge II atom Ge4 of this less symmetric dimer. Instead of the diamond-shaped Cu 2 Cl 2 core of the other dimer, a five-membered heterocyclic GeCu 2 Cl 2 core is present in the less symmetric dimer. All four Cu I atoms are in a trigonal planar coordination environment with two chlorine atoms and one germanium atom as bonding partners. The Cu I atom Cu3 bonded to the tetracoordinate Ge II atom Ge3 in the less symmetric dimer is connected to one of the Cl atoms (Cl1) of the Cu 2 Cl 2 core of the symmetric dimer, thus joining the two dimeric units together and making this particular Cl atom μ 3tricoordinate. The CuÀ Cl distances of this tricoordinate Cl atom range from ca. 2.31 to 2.38 Å, while the other three Cl atoms exhibit shorter CuÀ Cl bonds (2.23-2.30 Å) due to their dicoordinate nature. The CuÀ Ge bond lengths of 24 lie in the small range from 2.25 to 2.30 Å, which compares well with other germylene complexes of tricoordinate Cu I . [47,48b,k] The fact that Cl4 is bridging a Cu I atom and a Ge II atom, instead of two Cu I atoms, suggests that the CuÀ Cl and the GeÀ Cl interactions in our system are of similar strength. Note that a chloride transfer to the germanium atom upon coordination of transition metal chlorides (MCl 2 , M = Fe, Co, Ni, Zn; CuCl), corresponding to the formation of a chlorogermyl ligand containing tetravalent germanium by Ge II insertion into the MÀ Cl bond, was recently described by Cabeza for a donor-stabilised N-heterocyclic germylene with tricoordinate Ge II due to intramolecular coordination of a PiPr 2 unit. [48d,e] The germanium atom of Cabeza's chlorogermyl copper complex is in a distorted pseudotetrahedral bonding environment, showing a distance of 0.75 Å to the plane formed by its two nitrogen atoms and the copper atom (sum of angles with respect to these three atoms: 321°).
[48e] The situation is quite different in the present case. Ge3 has a distance of only 0.23 Å from its CuN 2 plane, close to a trigonal planar arrangement (sum of angles: 356°). The bond vector formed with its additional bonding partner, Cl4, is almost perpendicular to the CuN 2 plane, in accord with a donoracceptor interaction of the chlorido ligand with the vacant ptype orbital at the Ge II atom. This notion is further supported by the Ge3À Cl4 distance of 2.60 Å. This bond is much longer than the GeÀ Cl bonds of Cu I complexes obtained from chlorogermylenes with tricoordinate Ge II due to chelating β-diketiminato or aminotropiminato units, [48h,k] which are typically 2.30 Å and thus close to the sum of the covalent radii of Ge (1.20 Å) and Cl (1.02 Å). [33] In the same vein, the ClÀ GeÀ Cu angles in these chlorogermylene complexes are ca. 120°, while the Cl4À Ge3À Cu3 angle of 24 is 94°, reflecting the approximately perpendicular orientation of the Cl4À Ge3 bond vector with respect to the CuN 2 plane as described above.  [a] Cyclopentadienyl-bonded N atoms. [b] Bond to tricoordinate Cl1. [c] Two independent molecules; data refer to the non-disordered one.

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202200996 The reaction of 14 with CuCl afforded a product (25) with a composition corresponding to a 2 : 1 complex [(14) 2 CuCl] according to microanalytical data, which, however, were not in accord with an analytically pure sample of such composition. The product gave rise to rather complicated NMR spectra, which were not suitable to provide conclusive evidence for the nature of the species in solution. A structural investigation by XRD revealed a trigonal-planar coordination environment of the Cu I atom, which is bonded to a P atom and two different Ge atoms, one of them carrying the Cl atom. The Lewis structure of 25 given in Scheme 4 corresponds to the molecular structure in the crystal (Figure 10). The NMR spectra obtained for 25 are compatible with such a structure also in solution. Note that the tetravalent Ge atom Ge1 is a centre of chirality. Consequently, in combination with the two different planar-chiral ferrocene moieties, four diastereomers may result. We have isolated only a single diastereomer, which was obtained as a racemic compound. Figure 10 arbitrarily shows the (R p ,R,S p ) enantiomer. The NMR spectra of the crude product do not indicate the presence of other diastereomers. In particular, the 31 P{ 1 H} NMR spectrum ( Figure S26 in the Supporting Information) exhibits only two signals, as expected for 25 with its two different phosphorus atoms. 25 was obtained in only 26 % yield. We cannot exclude, therefore, that other diastereomers were also formed in the reaction of 14 with CuCl, but remained unnoticed due to low solubility.
The quality of the crystals obtained was poor (very small crystal size and weak scattering ability), thus compromising the result of the XRD analysis performed for 25. Nevertheless, a meaningful discussion of metric parameters is possible at least for the heavy atoms. The Cu I atom is in a trigonal planar bonding environment, being coordinated by two germanium atoms and one phosphorus atom. The CuÀ P distance of 2.27 Å lies in the region typical for tricoordinate Cu I triarylphosphane complexes, comparing well with, for example, [CuX 2 (PPh 3 )][NR 4 ] (2.21 Å for X = Cl, R = Et; 2.24 Å for X = Br, R = nBu; 2.23 Å for X = I, R = nPr), [49] [CuX(PPh 3 ) 2 ] (2.27, 2.28 and 2.27 Å for X = Cl, Br, I, respectively), [50] [Cu(PPh 3 ) 3 ][BPh 4 ] (2.26-2.29 Å), [51] and [Cu{Ge-(C 6 F 5 ) 3 }(PPh 3 ) 2 ] (2.27 Å), [52] the latter germyl complex apparently being the only structurally characterised tricoordinate Cu I phosphane complex with a copper-germanium bond (CuÀ Ge 2.38 Å). Both germanium atoms of 25 are tetracoordinate and reside in a distorted pseudotetrahedral bonding environment. Their shared bonding partner is the Cu I atom, the CuÀ Ge bond length being 2.38 and 2.35 Å for Ge1 and Ge2, respectively. The intramolecular coordination of the P atom present in germylene 14 has remained intact for Ge2, but not for Ge1. Ge1 is tetravalent. The CuÀ Ge1 (2.38 Å), Ge1À Cl (2.26 Å) and Ge1À N distances (average value 1.93 Å) are similar to those reported for Cabeza's chlorogermyl copper complex (see above; CuÀ Ge 2.36 Å, GeÀ Cl 2.26 Å, GeÀ N 1.93 Å). [39] The same holds true for the distance of the Ge atom to the CuN 2 plane, which is 0.61 Å for Ge1 in 25 and 0.75 Å for Cabeza's compound (see above). However, the corresponding distance of Ge2 is only 0.29 Å. Ge2 is part of still intact germylene 14 acting as a ligand for the chlorogermyl-bonded Cu I atom. The loss of electron density at Ge2 by copper(I) complexation obviously leads to a stronger coordination of the PPh 2 moiety, as is reflected by a Ge2À P2 distance of 2.43 Å as opposed to 2.65 Å determined for the GeÀ P bond of germylene F; a much smaller, but still significant, contraction is observed for the corresponding GeÀ N bonds (average value 1.93 Å in 14 vs. 1.90 Å for Ge2 in 25). A similar effect, albeit less pronounced, has been observed by Baceiredo for a donor-stabilised germylene with tricoordinate Ge II due to intramolecular coordination of a PPh 2 unit, whose GeÀ P distance shortens from 2.43 to 2.39 Å upon complexation by a {RhCl(COD)} fragment. [53] To summarise, compound 25 contains a tetravalent germanium atom (Ge1, chlorogermyl ligand) and a divalent germanium atom (Ge2, donor-stabilised germylene ligand). To a first approximation, bonding in this complex may be rationalised, and symbolised, in the following simplified way: P:!Ge:!CuÀ Ge. [54] Finally, the reaction of the iminophosphorane-functionalised germylene 23 with CuCl cleanly afforded the germylenecopper(I) complex [(23)CuCl] (26,Scheme 4). In view of the substantial number of Cu I iminophosphorane complexes, [55] it has not been obvious a priori that this reaction leads to a Cu I germylene complex. Solution NMR spectra are in accord with the structure found in the solid state, which is shown in Figure 11. The coordination environment of the Cu I atom of 26 is linear dicoordinate (bond angle 176°). The CuÀ Cl bond length of 2.10 Å is typical for this arrangement and several structurally characterised examples of complexes [CuCl(L)] with L = carbene (CAAC, NHC) or donor-stabilised silylene, but not with L = germylene or heavier analogues, have been reported. [56] The CuÀ Ge distance of 2.25 Å corresponds to the shortest CuÀ Ge bond lengths determined for 24 (see above), where we have a combination of tricoordinate Ge II and tricoordinate Cu I . In the case of 26, we have a combination of dicoordinate Cu I and tetracoordinate Ge II , since the intramolecular coordination of the iminophosphorane N atom found for germylene 23 is also present in its copper complex 26. Cu I coordination leads to a substantial shortening of all three GeÀ N bonds. Analogous to 25 (see above), the effect is largest (0.13 Å) for the coordinative bond, which is contracted from 2.11 Å in 23 to 1.98 Å in 26, while the two other GeÀ N bonds experience a less pronounced contraction (0.08 Å on average).

Conclusion
We have compared the reactivity of the ferrocene-based Nheterocyclic tetrylenes [{Fe(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }E] [E = Pb (15), Sn (16),Ge (17)] towards mesityl azide with that of the PPh 2functionalised congeners 12-14, whose phosphorus(III) atom constitutes a second possible reaction site in addition to the respective tetrel(II) atom. Our results indicate that the reaction of this azide with the stannylenes 13 and 16 and germylenes 14 and 17 invariably occurs at the divalent tetrel atom, leading to an E=N double bond. The resulting germanimines 19 and 22 could be isolated. However, the latter isomerised readily to iminophosphorane 23 by NMes transfer from the Ge IV to the P III atom due the rather reactive Ge=N bond. In line with previous observations (see above), the reactivity of the Sn=N bond is even higher, so that only follow-up products were observed in the reactions of the stannylenes 13 and 16, namely, iminophosphorane 21 (most likely formed by NMes transfer from the Sn IV to the P III atom of the transient stannanimine) and stannatetrazol 18 (formed by [2 + 3] cycloaddition of the transient stannanimine with mesityl azide). All four unfunctionalised diaminoplumbylenes of our study, viz. the N-heterocyclic compounds o-C 6 H 4 (NSiMe 3 ) 2 Pb, nap(NSiMe 3 ) 2 Pb and [{Fe-(η 5 À C 5 H 4 À NSitBuMe 2 ) 2 }Pb] (15) as well as the acyclic congener [(Me 3 Si) 2 N] 2 Pb, proved to be inert towards MesN 3 even under forcing conditions. In contrast, the behaviour of the PPh 2 -functionalised diaminoplumbylene 12 towards mesityl azide is analogous to that of Wesemann's PPh 2 -functionalised (alkyl)(aryl)plumbylene 5, being strongly reminiscent of B/P FLPs in both cases. While 5 reacts already at room temperature, moderately higher temperatures are needed in the case of 12. This may be ascribed to the fact that the P atom of 12, in contrast to that of 5, is engaged in an intramolecular coordinative bond to the Pb atom. Consequently, plumbylene 12 exhibits a reduced frustration in comparison to 5 and thus belongs to the so-called active Lewis pairs (ALPs), which can show FLP-like behaviour due to the weakness of their coordinative bond. [57] Investigations addressing the activation of fundamentally important small molecules with 12 and closely related ALPs are underway in our laboratory. In addition, our study has demonstrated the ability of the unfunctionalised Nheterocyclic germylene 17 and its donor-functionalised relatives 14 and 23 to act as ligands for copper(I), thus underlining the potential of such ferrocene-based, and hence redox-functionalised, N-heterocyclic germylenes in coordination chemistry. [58] Notably, compound 26 is the first structurally characterised linear dicoordinate copper(I) halogenido complex [CuX(L)] with a heavier tetrylene ligand L.
Synthesis of 17: LiN(SiMe 3 ) 2 (395 mg, 2.36 mmol) was added to a stirred solution of [Fe(η 5 À C 5 H 4 À NHSitBuMe 2 ) 2 ] (500 mg, 1.12 mmol) in THF (8 mL). After 30 minutes [GeCl 2 (1,4-dioxane)] (260 mg, 1.12 mmol) was added and stirring was continued for 2 h. Volatile components were removed under reduced pressure. n-Hexane (6 mL) was added to the residue. Insoluble material was removed by filtration through a short pad of celite. The solvent was removed from the filtrate under reduced pressure, leaving the product as a brownish-yellow viscous oil. Yield 534 mg (92 %). Crystallisation of the product was initialised in a concentrated n-hexane solution by scratching with a glass rod. After the first crystals appeared, the flask was stored at À 40°C. The mother liquor was separated from the orange product, which was finally dried under reduced pressure. 1  Stirring was discontinued after 30 min. The solution was stored at À 40°C for crystallisation. The mother liquor was separated from the

Synthesis of 19:
A solution of mesityl azide (52 mg, 0.32 mmol) in n-hexane (1 mL) was added to a stirred solution of 17 (165 mg, 0.32 mmol) in n-hexane (5 mL). After 10 minutes the volume of the solution was reduced to ca. 3 mL. The solution was stored at À 40°C for crystallisation. The orange product was separated from the mother liquor and subsequently dried under reduced pressure. Yield 149 mg (72 %). 1

Synthesis of 20:
A solution of mesityl azide (22 mg, 0.13 mmol) in toluene (2 mL) was added to a solution of 12 (100 mg, 0.13 mmol) in toluene (3 mL). The stirred mixture was heated to 60°C for 10 h. Subsequently, volatile components were removed under reduced pressure. The crude product was dissolved in diethyl ether/nhexane (1 : 1, 2 mL) and the solution was stored at À 40°C for crystallisation. This afforded the product as red diethyl ether solvate, which was separated from the mother liquor and subsequently dried under reduced pressure. Yield 68 mg (52 %). 1

Synthesis of 24:
CuCl (30 mg, 0.30 mmol) was added to a stirred solution of 17 (120 mg, 0.23 mmol) in toluene (5 mL). After 18 h insoluble material was removed by filtration through a short pad of celite, followed by washing with toluene (1.5 mL). Volatile components were removed from the combined filtrate and washing solution. The product was extracted from the residue with n-hexane (3 × 2 mL). After filtration of the extract to remove trace amounts of insoluble material, the volume was reduced to ca. 0.5 mL. Yellow crystals were obtained after several days, which were separated from the mother liquor and subsequently dried under reduced pressure. Yield 61 mg (42 %). 1 (5 mL). After 72 h insoluble material was removed by filtration through a short pad of celite, followed by washing with toluene (1 mL). The volume of the combined filtrate and washing solution was reduced to ca. 0.5 mL. n-Hexane (0.5 mL) was added. The solvent was slowly evaporated under ambient conditions, affording the crude product as a dark brownish red microcrystalline solid, which was washed with nhexane (2 × 0.2 mL) and subsequently dried under reduced pressure. Yield 25 mg (26 %). 1  Satisfactory microanalytical data could not be obtained.

Synthesis of 26:
CuCl (10 mg, 0.10 mmol) was added to a stirred solution of 23 (67 mg, 0.09 mmol) in toluene (1 mL). After 24 h insoluble material was removed by filtration through a short pad of celite, followed by washing with toluene (1 mL). The volume of the combined filtrate and washing solution was reduced to ca. 0.5 mL. The solution was placed in a 5 mm NMR tube and was subsequently layered with n-hexane (ca. 3 mL). After two weeks yellow crystals had formed, which were separated from the mother liquor and subsequently dried under reduced pressure. Yield 51 mg (67 %). 1   X-ray crystallography: For each data collection a single crystal was mounted on a micro-mount and all geometric and intensity data were taken from this sample at 100(2) K, except for 23, which was measured at 253(2) K due to a phase transition below this temperature. Data collections were carried out either on a Stoe IPDS2 diffractometer equipped with a 2-circle goniometer and an area detector on a Stoe StadiVari diffractometer equipped with a 4circle goniometer and a DECTRIS Pilatus 200 K detector. The data sets were corrected for absorption, Lorentz and polarisation effects. The structures were solved by direct methods (SHELXT) and refined using alternating cycles of least-squares refinements against F 2 (SHELXL2014/7). [62] C-bonded H atoms were included in the models in calculated positions, heteroatom-bonded H atoms have been found in the difference Fourier lists. All H atoms were treated with the 1.2-fold or 1.5-fold isotropic displacement parameter of their bonding partner. Experimental details for each diffraction experiment are given in Table S1 in the Supporting Information.