Synthesis and Coordination Behavior of a New Hybrid Bidentate Ligand with Phosphine and Silylene Donors

Abstract This work describes the synthesis and coordination behavior of a new mixed‐donor ligand PhC(NtBu)2SiC6H4PPh2 (1) containing both silylene and phosphine donor sites. Ligand 1 was synthesized from a reaction of ortho‐lithiated diphenylphosphinobenzene (LiC6H4PPh2) with chlorosilylene (PhC(NtBu)2SiCl). Treatment of 1 with Se and GeCl2 resulted in SiIV compounds 2 and 3 by selective oxidation of the silylene donor. This strong σ‐donor ligand induces dissociation of CuCl and PhBCl2 leading to formation of ionic complexes 4 and 5 respectively. The reaction of 1 with ZnCl2 and AlCl3 resulted in the formation of chelate complexes 5 and 7, respectively, while treatment with EtAlCl2 and GaCl3 forms monodentate complexes 8 and 9. X‐ray analysis of 4 showed that the copper is in the spiro center of the two five‐membered rings. Moreover, the copper(I)chloride has not been oxidized but dissociates to Cu+ and [CuCl2]−. All the compounds are well characterized by mass spectrometry, elemental analysis, NMR spectroscopy, and single‐crystal X‐ray diffraction studies.


Introduction
The structure, reactivity and catalytic activitieso ft he main group and transition metal complexes are strongly related to the sterica nd electronic factorso ft he coordinating ligand. Over the last few decades, highly reactive compounds with low-valents ilicon, phosphorus and otherm ain-group elements have been isolated and structurally characterized. [1][2][3][4] These compounds have attracted significant attentiond ue to their exciting electronic structures,s mall molecules activating capability at ambient conditions and catalyticp roperties in organic transformations. [1c, 5] Most of thesen ovel achievements were due to the suitable designa nd synthesis of ligands which stabilize speciesw ith low-valent elements. N-heterocyclic carbenes (NHCs), [6] cyclic alkyl(amino) carbenes (cAACs) [7] and Nheterocyclic silylenes (NHSis) [8] are the most successful and commonly used ligand systemst os tabilizec ompounds with low valent elements.
Silylenes have been effectively used as stabilizing ligands in low valent main-groupe lements, transition metals and for the metalf ree activation of robust bondsi ns mallm olecules. Few examples that illustrate the possibility to form new silylene ligandsb earing an additional coordination site were published in 2016 by S. Khan et al. They have used silylene phosphine based bidentate ligand and showed the coordination of silylene with gold. [9] Very recently,S talke et al. synthesized as idearm functionalized silylene ligand and their transition metal complexes. [10] In 2014, Driess et al. reported silylene coordinated mono-a nd dinuclear copper(I) complexes. [11] The same group has synthesized mixeds ilylene-carbene chelate ligands to stabilizet ransition metal.F urthermore they have studied the catalytic activity of A (Scheme 1). [12] Scheschkewitz et al. have shown the synthesis of ketenyl-ligated metal-silylene complexes of group 6( B). [13] Li and co-workers recently have synthesized bis(silylene)-basedS iC (sp3) Si pincerl igand and studied its coordination chemistry with Fe 0 under Ar and N 2 . [14] Tilley and others have reported chemical and catalyticp roperties of silylenec oordinated transition metalc omplexes. [15] In 2011, Kato et al. published the synthesis of as table and isolable tricoordinate silicon(II) hydride (C), stabilized by ap hosphine ligand. [16] The same research group has also delivered silylene based complexes to stabilizet ransition metal and a highly electron rich carbon(0). [17] Driess and co-workers have successfully used bis(silylene)t os tabilizez ero valent silicon and germanium,r espectively( D). [18] Very recently,t he same group reportedh omocoupling of CO and isocyanide,m ediated by abis(silylene) ligand (E). [19] Recent studies indicated that catalytic efficiencies of various catalytic reactions can be increased by silylene ligands due to their stronger s-donation ability along with cooperative effects of the divalent silicon atoms in the process of catalysis. In many cases, it wasf ound that silylenes function asp owerful sdonor ligands surpassing the activitieso fa nalogous phosphine-andN HC-based metal complexes. [20] Mixed donor ligand systemsa llow fine tuning of the steric and electronic properties at the metal center by choosing appropriate donora toms. Such af ine tuning can have as ignificant impact on the chemo-, regio-and stereo selectivities in metal-mediated catalytic transformations of organic substrates. Our group has longstanding interest in the synthesis and reactivity of silylenes and phosphinidenes. In 2017, we reported the synthesis of cAAC anchored silylene-phosphinidenesynthesized by the reaction of silylene monochloride andc hlorophosphinidene, followed by reduction with KC 8 . [21] In the subsequent studies, we found that the reactiono fs ilylene-phosphinidene with chalcogens resulted in the selective formationo fs ilicon bonded chalcogen phosphinidenes (F). [22] Silylenes were found to be strong s-donorl igandsi nc omparison with phosphinidenes. We envisioned the synthesis of a mixed donor ligand system with phosphane and silylene donors to study its electronic properties and coordinationb ehavior.T he presentw ork results from our effort to study the reactivity of such mixed donorl igand with transition metals as well as main group elements. Herein, we presentt he synthesis of an ovel hybrid phosphane-silylene ligand (1)a nd its coordination properties with varioust ransition metalsa nd main group elements.

Results and Discussion
In 2010, our group reported ah igh yielding method for the synthesis of N,N-di(tert-butyl)amidinato chlorosilylene. [23] It is a versatile reagent, used in the synthesis of diverse silylene based ligands with different spacer lengths, specifics tructural and electronic features. We envisioned the use of chlorosilylene precursor for the synthesis of phosphane-silylene ligand system. ortho-lithiated diphenylphosphane and N,N-di(tert-butyl)amidinato chlorosilylene were treated in toluene (À78 8Ct o room temperature) and resulted in ap urple solution. The solution was filtered and concentrated to 10 mL ands tored at À30 8Ci nafreezer for crystallization. Colorless crystals of compound 1 were obtained after 24 hours (Scheme2). The 31 PNMR spectrum of compound 1 shows as ingle resonance at À11.2 ppm ( Figure S2 in the SupportingI nformation), au pfield shift of 6.2 ppm compared to the starting material( À5.0 ppm). The 29 Si NMR spectrum of compound 1 exhibits ad oublet at 18.52 ppm with a 3 J Si-P coupling of 410 Hz which is significantly downfield shifted comparedt ot he chlorosilylene (À96.8 ppm). Compound 1 crystallizes in the monoclinic space group C2/c with one single moleculeo f1 in the asymmetric unit. Analysis via single-crystal X-ray diffractiona dditionally reveals ad istorted tetrahedral environment arounds ilicon, accommodating two inequivalentn itrogen donor atoms (Si1-N1:c a. 1.86 and Si1-N2:c a. 1.87 ). Thec arbon on the phosphine moiety (C16 on Figure 1) and the lone pair of electrons on Si1 are available for further chelation.
Scheme2.Synthesis of phosphane-silylene ligand (1). An equimolar reactiono f1 with elemental seleniumi nt oluene at room temperature resulted in the selective oxidation of silylene donort of orm 2.T he 31 PNMR spectrum of 2 shows a single NMRr esonancea tÀ12.3 ppm ( Figure S6). The structure of 2 was confirmed by single-crystal X-ray diffraction. Compound 2 crystallizes in the monoclinic space group C2/c as well, with one molecule of 2 and two solventm olecules of THF in the asymmetric unit. Seleniumbinds exclusively with silicon, at 2.1330(5) bond length. The greater distance between seleniuma nd phosphorus atoms, about 5.6 ,a sw ell as the orientation of the phosphine moiety strongly invalidate the hypothesis of additional bonding ( Figure 2). Surprisingly,t he reaction of 1 with GeCl 2 did not yield ag ermaniumc omplex but resulted in the oxidation of silylene moiety to yield the dichlorinated compound 3 (Scheme 3). The 31 PNMR of 3 shows single NMR resonance at 12.2 ppm (Figure S10) whichs hows ad ownfield shift of 23.4 ppm, when compared with ligand 1.T he formation of compound 3 was furtherc onfirmed by single crystal X-ray diffraction studies, which crystallizes in the monoclinic space group P2 1 /n without any additional solventm olecules in the asymmetric unit. The penta-coordinated silicon features ad istorted environment comparable to at ransition state between trigonal bipyramidal and square pyramidal, due to the rigid structure of the amidinate ( Figure 3).
The reaction of 1 with CuCl in toluene at room temperature leads to the dissociation of CuCl resulting in the formation of the diamagneticc omplex [L 2 Cu] + [CuCl 2 ] À (4). (Scheme 4). We use severala nalysis methods to determine the magnetic state of compound 4:adismutation of Cu I in Cu 0 andC u II would be assessed paramagnetic, whereas simple dissociation of Cu I would remaind iamagnetic. 1 Ha nd 13 CNMR measurementsa re measurable in the standard chemical shift range. Moreover,a toluene solution of 4 is found EPR silent, whichi sa lso in agreement with SQUID analysisr esults, invalidating the hypothesis of the formation of paramagnetic species.
Scheme3.Reactions of 1 with Se and GeCl 2 . À30 8Cf or 2days in af reezer.C ompound 4 crystallizes in space group P with two units of [L 2 Cu] + [CuCl 2 ] À and nine toluene molecules.T he crystal structure reveals that the copper is in the spiro center of the two five-membered rings ( Figure 4). The central coppera tom is featuring ad istorted tetrahedral geometry,p ossibly due to the steric arrangement of the phosphine phosphorusa nd the tert-butylg roups. Comparison with already reported CuÀXb ond lengths (X = P, Si, Cl;s ee the Supporting Information) also suggests that compound 4 is diamagnetic and that copperc hloride is not oxidized, but rather dissociates to Cu + and [CuCl 2 ] À .T he Si1ÀCu1 and Si2ÀCu1 distances are 2.264 and 2.273 ,w hich is comparablet oo ther SiÀ Cu distances in copper(I) complexes [24] (see the Supporting Information).T he P1ÀCu1 and P2ÀCu1 bonds are quite long, about 2.28-2.3 ,w hich is actually comparable to Cu 0 ÀPb ond lengths. However, although slightly larger than average, it still falls in the acceptable range of 2.2t o2 .4 for copper(I) complexes. [25a,b] Additionally,t he [CuCl 2 ] À unit features two short CuÀCl bonds of 2.099 and 2.104 .T his distance is closer to the one of copperm onochloride than average distances for Cu II ÀCl about 2.3 (see the Supporting Information). It is not entirely excluded that Cu I remains unoxidized but the presented analysiso verall strongly suggestst hat the obtained complex 4 is diamagnetic.
The reaction of 1 with ZnCl 2 leads to the formationo fachelate complex LZnCl 2 (L = PhC(NtBu) 2 SiC 6 H 4 PPh 2 ; 5)w here the ligand exhibits chelating mode of coordination. The 31 PNMR of 5 shows as ingler esonance at À26.7 ppm ( Figure S17) which exhibits au pfield shift of 15.5 ppm in comparison with the ligand 1.C rystals suitable for single-crystalX -ray analysisa re obtainedf rom THF solutiona fter two weeks in the freezer at À30 8C. Compound 5 crystallizes in the monoclinic space group P2 1 /c,p resenting one single molecule of 5 in the asymmetric unit. In this complex, both phosphine and silylene moieties are participating in metal coordination ( Figure 5). The Zn1ÀSi1 bond is somewhat shorter than the Zn1ÀP1 bond, indicativeo fapreference for the silylene moiety.T he slight distortion of the tetrahedral geometry around the Zn atom is due to the P1-Zn1-Si1 angle of about 858,s hortened to adapt to the backboner igid structure.
Both the complexes 4 and 5 are crystalline solids,s oluble in common organic solvents, and stable at room temperature under inert atmosphere.M elting points of complexes 4 and 5 were found to be 280 and 270 8C, respectively.
Similar to the formation of the copper complex 4,l igand 1 reactedw ith PhBCl 2 in toluene at À78 8C. This leads to aq uite unusuald issociation of PhBCl 2 into the product [LBPhCl] + [PhBCl 3 ] À (6;Scheme 5) as confirmed by single-crystal X-ray diffraction.C rystals were obtained after two weeks at room temperaturei nt oluene. Compound 6 crystallizes in the triclinic centrosymmetric space group P-1, with no residual solvent molecule in the asymmetric unit. Interestingly,t he ligand chelates the small boron atom with both coordinating moieties. The boron centerisalmost perfectly tetrahedral, and its coordination to the ligand induces ad istortion on the phosphine moiety insteado fm odifying the tetrahedral geometry on the boron atom ( Figure 6). This suggests that the small size of the boron atom enables it to fit inside of the chelating moiety, however through as hrinking of the host pocket. All bonds to boron are indeed shortert han with any other guest atom of the presents tudy (see Ta ble 1f or ad etailed comparison).
Surprised by the ability of ligand 1 in the dissociation of CuCl and PhBCl 2 ,w einvestigated the reactionw ithA ls alts for the formation of ionic aluminium compounds.
of CH 2 Cl 2 in the asymmetric unit. The aluminium atom is in a distortedt etrahedral environment:a ll Cl-Al-Cl angles are about 1098,b ut the Si-Al-Cl2 angle is remarkably smaller (98.77 (3)8), probablyt oa llow an additional weaker interaction between Al and the pending phosphine group (Figure 7). Single-crystal Xray analysis also reveals that the Al atom is boundedt ot he silylene moiety while the phosphorus donor atom is "only" oriented towards the metal. This shows as expected that silylene is as tronger s-donorl igand compared to the Pa tom of phosphane.
In the same spirit, the reaction of 1 with EtAlCl 2 and GaCl 3 in diethyle ther resulted in the formation of compounds 8 and 9. Compound 8 crystallizes in the orthorhombic space group P2 1 2 1 2 1 with one molecule of 8 in the asymmetric unit. The Al III is placed in at etrahedral environment, with ad eviation comparable to 7 (one angle is 95.53(5)8 and the others are about 1098)t ol eave some space for the phosphino group oriented towardst he aluminium atom. The distance between Al and P atoms is shorter than in 7 (3.36 and3 .59 respectively), which is probably due to the additional flexibility given by the ethyl moietyi nc omparison with the halide (Figure 8).

Conclusions
In summary,w eh ave synthesized an ew hybrid bidentate phosphane-silylene ligand and studied its coordination behavior with transition metal andm ain group precursors. Reaction of 1 with Se and GeCl 2 resulted in selectiveo xidation of silylene moiety to form complexes 2 and 3,r espectively.T he strong s-donating ligand 1 on reaction with CuCl andP hBCl 2 results in dissociation leading to the formation of compounds 4 and 6,r espectively where the ligand displays ac helating mode of coordination. In complex 5,t he phosphane moiety also participates in bonding.
The ligand 1 then offers ad ifferent coordination environment for the LAlCl 3 (7), LEtAlCl 2 (8)a nd LGaCl 3 (9)c omplexes: while the host atom is bonded to the silicon, the phosphane moiety is obviously preferably oriented towards it as well, which indicates that the two atoms are interacting together. Thus, the overall reactivity of the newly developed ligand (1) confirms that silylene is as trong s-donating ligand compared to phosphane.A ll compounds were fully characterized by single-crystal X-ray studies and various spectroscopic studies.