Synthesis of Cobalt-Tin and -Lead Tetrylidynes—Reactivity Study of the Triple Bond

: Tetrylidynes [TbbSn � Co(PMe 3 ) 3 ] ( 1a ) and [TbbPb � Co(PMe 3 ) 3 ] ( 2 ) (Tbb = 2,6-[CH(SiMe 3 ) 2 ] 2 -4-( t - Bu)C 6 H 2 ) are accessed for the first time via a substitu-tion reaction between [Na(OEt 2 )][Co(PMe 3 ) 4 ] and [Li-(thf) 2 ][TbbEBr 2 ] (E = Sn, Pb). Following an alternative procedure the stannylidyne [Ar*Sn � Co(PMe 3 ) 3 ] ( 1b ) was synthesized by hydrogen atom abstraction using AIBN from the paramagnetic hydride complex [Ar*SnH = Co(PMe 3 ) 3 ]

Metal-ligand multiple bonds are of interest for an investigation of the bonding parameters and for reactivity studies.In the case of the higher homologues of carbynes exhibiting a metal-carbon triple bond a variety of examples of tetrylidyne complexes with element combinations between metals M = Nb, [1] Cr, [2] Mo, [3] W, [3c,e,g,4] Mn, [5] Re, [6] Fe, [1] Os, [7] Rh, [8] Ni, [9] Pt [1] and main group elements Si, Ge, Sn and Pb have been reported. [10]Reactivity studies of tetrylidyne complexes have been reported in the case of silylidyne and germylidyne complexes.2 + 2-cycloadditon reactions, CHactivation, addition of nucleophiles at the Si-atom and addition of alcohols and α,β-unsaturated ketones have been studied. [7,11]We became interested in the chemistry of tetrylidyne complexes because we explored a dehydrogenation reaction of dihydrido bridged rhodium-tin [(Ph 3 P) 2 Rh-(μ-H) 2 SnAr*] and rhodium-lead [(Ph 3 P) 2 Rh(μ-H) 2 PbAr*] complexes to yield Rh�Sn and Rh�Pb tetrylidynes. [8]urthermore, these complexes show addition of one equivalent hydrogen in the lead case and two equivalents in the tin case.Remarkably, the hydrogen addition at the tin atom of [(Ph 3 P) 2 H 2 RhSnH 2 Ar*] is a reversible reaction at room temperature.In this publication we report on the synthesis of unprecedented cobalt tetrylidynes of tin and lead and reactivity studies of the cobalt stannylidyne with water, carbon dioxide and a Brønsted acid.
In a one-step procedure the cobalt tetrylidyne complexes (1 a, 2) were synthesized (Scheme 1) treating the anionic cobalt phosphine complex [Na(OEt ). [12]ollowing an alternative synthetic pathway, the stannylidyne 1 b was obtained by hydrogen atom abstraction from the paramagnetic hydride complex 4. Hydrides 3 and 4 were synthesized by phosphine substitution treating the cobalt complex [Co(PMe 3 ) 4 ] with low valent hydrides of germanium and tin (Scheme 1). [13]In reaction with AIBN at 40 °C the paramagnetic complex 4 was converted via hydrogen atom abstraction into the stannylidyne complex 1 b.The germanium compound 3 was also treated with AIBN, but formation of a germylidyne complex was not observed.
Crystals of 1 a, 1 b and 2 were isolated from the synthetic mixtures and the molecular structures of 1 b and 2 are shown in Figure 1 (1 a see SI), while Table 1 [14] [2.386(2) Å] and for [(Ar*CoSn) 2 ] [2.4071( 6) Å] exhibiting partial SnÀ Co double bond character. [12]In the case of the CoÀ Pb bond only a few complexes have been reported showing a range of interatomic distances of 2.761(2) Å-2.6191(4) Å. [15] The coordination sphere around the cobalt atoms in 1 and 2 can be described as a tetrahedral arrangement and is comparable with the homologous rhodium complexes. [8]The electronic structures of the complexes were evaluated using DFT calculations (wB97X-def2-SVP/TZVP(Co,Sn,Pb)).The short Co�E bond lengths and the large C1À EÀ Co angle were reproduced by these calculations (E = Sn, Pb).The HOMO, and HOMO-1 resembling the π-bonds are shown for the CoÀ Pb case in Figure 2.
In the 119 Sn NMR spectrum of 1 a, 1 b a signal at 1100, 1027 ppm and in the case of 2 a 207 Pb NMR signal at 5022 ppm were observed (Table 2).3l,8] In both cases 1 a (i-C Tbb -Sn 179.7 ppm) and 2 (i-C Tbb -Pb 248.1 ppm) the signal at highest frequency in the 13 C NMR spectrum belongs to the carbon atom attached to the Group 14 element.This high frequency shift in the 13 C NMR of tetrylidyne complexes was already documented in the literature for a variety of examples.In the case of plumbylidyne complexes signals at even larger frequencies were observed (280.6,3d,4c,8] These high frequency signals can be rationalized by the influence of the lead atom on the neighboring atoms. [17]he water addition to the Co�Sn triple bond was carried out in reaction with five equivalents of the water adduct BaCl 2 •H 2 O (Scheme 2).With smaller amounts of BaCl 2 •H 2 O we do not observe a complete product formation.The addition product of two equivalents of water [TbbSn-(OH) 2 CoH 2 (PMe 3 ) 3 ] (5) was isolated in high yield (87 %) and pale-yellow crystals suitable for X-ray diffraction were obtained from n-pentane at rt (Figure 3).Tobita and coworkers have studied the addition of alcohols to a W�Ge triple bond found in [Cp*(CO) 2 WGe{C(SiMe 3 ) 3 }] and observed formation of a GeÀ OR and WÀ H bond. [11b] The water molecules react two times with the [Co�Sn]-moiety by splitting two OÀ H bonds. Remarkably, a cobalt dihydride and a tin dihydroxide are formed.The CoÀ Sn bond is Table 1: Selected bond lengths [Å] and angles [°] of 1 a, 1 b, 2. [16] 1 a (E = Sn)    3). [18]The signal in the 119 Sn NMR spectrum at 61 ppm is indicative for an increase of the coordination number at tin in comparison to 1 a.The signal of the dihydride moiety was observed at À 15.87 ppm and exhibits tin satellites with a coupling constant of 39 Hz.The Sn(OH) 2 moiety, which does not tend to oligomerize under water condensation can be compared with the tin trihydroxide found by Kubiak and coworkers as a capping unit of a trinuclear nickel-cluster. [19]reating a benzene solution of the cobalt stannylidyne 1 a at rt with an excess of carbon dioxide a carbonyl coordination compound [TbbSn(CO 3 )Co(CO)(PMe 3 ) 3 ] ( 6) was isolated as the product of a redox reaction in a yield of 62 % (Scheme 3).Obviously, two CO 2 molecules react with one cobalt-tin complex under formation of one equivalent of carbon monoxide, as the product of reduction, together with a carbonate.The tin atom is oxidized and exhibits [CO 3 ] 2À coordination with SnÀ O bond lengths of 2.106(4), 2.108( 4 [19][20] The addition and reduction of CO 2 in reaction with low valent main group compounds is known from a variety of examples: amidodigermyne, [21] carbene stabilized disilicon, [22] disilyne bisphosphine adduct, [23] disilene, [24] dialumene bis(NHC) adduct [25] and a base stabilized diborene. [26]The [2+2]cycloaddition of CO 2 in reaction with a platinum germylene complex yields a GeÀ O and PtÀ C bond. [27]We therefore discuss as the first step in a possible mechanistic sequence the formation of a [2+2]-cycloaddition product, which results in the complete splitting of a CÀ O unit and formation of the carbonyl-coordination at cobalt.The intermediately formed Sn=O unit immediately reacts with a further equivalent of CO 2 under formation of the carbonate ligand (see SI).The found CoÀ Sn bond length of 2.4629(8) Å is in line with a single bond between these elements and can be compared with the distance observed in 5.The signal in the 119 Sn NMR spectrum at 178 ppm indicates an increase of the coordination number in comparison to 1 a.In the IR spectrum the CO unit exhibits a signal at 1919 cm À 1 which can be compared with carbonyl complexes [MeCo-(PMe 3 ) 3 CO] 1881 cm À 1 ; [HCo(PPh 3 ) 3 CO] 1910 cm À 1 . [28]he protonation of complex 1 a was probed in reaction with [H(OEt 2 ) 2 ][BAr F 4 ] (Scheme 4).The molecular structure of the protonated tin complex [TbbSn=CoH(PMe 3 ) 3 ][BAr F 4 ] (7 a) is shown in Figure 4 and exhibits a structure comparable with the IrÀ Sn structure [TbbSn=IrH(PMe 3 ) 3 ][BAr F 4 ] reported recently. [29]The CoÀ Sn bond [2.2907( 7) Å] is only slightly elongated in comparison to the triple bond found in 1 a. [12,14] Upon protonation, the angle C1À Sn=Co [156.8(1)°,152.1(1)°] of the cation of 7 a is reduced and comparable to the CÀ E=Ir angle found in the protonated iridium complexes [E = Ge 158.8(4)°,E = Sn 159.2(1)°]. [29]The trigonal bipyramidal coordination sphere of the cobalt atom in the cation of 7 a is also comparable with the ligand arrangement of the homologous iridium-cations [TbbE=IrH(PMe 3 ) 3 ]-[BAr F 4 ] (E = Ge, Sn). [29]Protonation of the lead complex 2 yields a protonated species exhibiting a quartet in the 1 H NMR spectrum at À 21.06 ppm showing 207 Pb satellites.We    4) is a short bond between these atoms.Literature examples exhibit short CoÀ Ge bond lengths in a range of 2.1967(6)-2.310(7)Å. [30] Cell parameters of compound 4 are almost identical to those of 3, but due to poor crystal quality of 4, structural data of 4 cannot be discussed.
In the 31 P NMR spectrum of 7 a, only one signal was observed, indicating fluxional behavior of 7 a in solution.This phenomenon is similar to the homologous iridium cations. [29]The signals in the 119 Sn NMR spectrum at 1202 ppm and 1 H NMR spectrum at À 14.76 ppm (q + satellites, CoÀ H, 2 J SnÀ H = 327 Hz, 2 J 31PÀ H = 15.7 Hz) of 7 a are in line with the molecular structure.The bonding in 7 a was by DFT calculations (wB97X-def2-SVP/TZVP-(Co,Sn)).The CoÀ Sn σ-bond is based on the Sn-lone pair σdonation into a sd-hybrid orbital at the cobalt atom and the π-backdonation stems from a cobalt d-orbital into an empty tin p-orbital.The HOMO of this cation resembles the SnÀ Co π-bond and the LUMO shows to a high extent an empty p-orbital at tin (see Figures SI56-57).The NPA charges (Sn: + 1.24, Co: À 0.46) corroborate the assignment of a cationic tin atom and are in line with the [TbbSnIrH-(PMe 3 ) 3 ]-cation (NPA = natural population analysis). [29]hus, the electronic situation of 7 a can be compared with the homologous [IrH=Sn]-cation. [29]However, the chemistry of the [CoH=Sn]-cation has not been investigated so far.
In conclusion, organodibromo stannate and plumbate [TbbEBr 2 ] À (E = Sn, Pb) react with the anionic phosphine complex [Co(PMe 3 ) 4 ] À by substitution of a phosphine ligand against a TbbE-cation thus forming the unprecedented Co�E triple bonds.Cobalt stannylidyne is also accessible via hydrogen atom abstraction from paramagnetic arylhydridostannylene cobalt complex [Ar*SnH=Co(PMe 3 ) 3 ].The cobalt stannylidyne shows a double addition of water and a redox reaction with carbon dioxide.Furthermore, the cobalt atom of the stannylidyne complex is protonated in reaction with a strong Brønsted acid and the tin-cobalt vinyl cation [TbbSn=CoH(PMe 3 ) 3 ] + was isolated.Syntheses of analogous tin and germanium cations has also been carried out by oxidizing the organohydrido tetrylene cobalt complexes [Ar*EH=Co(PMe 3 ) 3 ] (E = Ge, Sn).Thus homologous [CoH=E] vinyl cations of the highly reactive [IrH=E] cations have been made accessible and are available for reactivity studies.

Figure 1 .
Figure 1.ORTEP of the molecular structures of 1 b and 2 (1 a see SI).Thermal ellipsoids are shown at 50 % probability level.Hydrogen atoms have been omitted.

Figure 3 .
Figure 3. ORTEP of the molecular structures of 5 and 6.Thermal ellipsoids are shown at 50 % probability level.Hydrogen atoms except CoÀ H and OÀ H have been omitted.
interpret this signal as an indicator for a hydrido-cobaltoplumbylene with a protonation at the cobalt atom [TbbPbCoH(PMe 3 ) 3 ][BAr F 4 ] (8).X-ray quality crystals of 8 have not been obtained but NMR evidence suggests formation of a similar structure to 7 and 9. Synthesis of protonated germanium 9 and tin 7 b complexes was also achieved by oxidation of 3 and 4 in reaction with [Ph 3 C]-[BAr F 4 ] (Scheme 4).The CoÀ Ge bond length of 2.1918(4) Å found in 3 (Figure 4, Table

Figure 4 .
Figure 4. ORTEP of the molecular structures of 3 and the cation of 7 a.Thermal ellipsoids are shown at 50 % probability level.Hydrogen atoms except CoÀ H have been omitted.Metal hydride positions were found in the difference-Fourier map and refined except for compound 7 a where the MÀ H distance was restrained to 1.45 Å.

1 a, 1 b and 2 are the
lists selected interatomic distances and angles.The most striking features of the molecular structures of
Scheme 3. CO 2 addition to 1 a.Scheme 4. Protonation of 1 a and oxidation of 3 and 4.