Synthesis of an anionic tridentate phosphinoborate and its reaction chemistry with Sn(II)

Article abstract of DOI:10.1039/a906560a

Dichlorophenylborane reacts smoothly with [Li(tmen)][CH₂PPh₂] in THF to afford, in good yields, the Li(tmen)⁺ salt of a novel monoanionic tridentate phosphinoborate, tris(diphenylphosphinomethyl)phenylborate; reaction of t

Full text of DOI:10.1039/a906560a

Synthesis of an anionic tridentate phosphinoborate and its reaction chemistry
with Sn(ii)
Alfred A. Barney, Alan F. Heyduk and Daniel G. Nocera*
Department of Chemistry, Massachusetts Institute of Technology, 6-335, 77 Massachusetts Avenue, Cambridge, MA
02139-4307, USA. E-mail: nocera@MIT.edu
Received (in Bloomington, IN, USA) 11th August 1999, Accepted 13th October 1999
Dichlorophenylborane reacts smoothly with [Li-
(tmen)][CH2PPh2] in THF to afford, in good yields, the
Li(tmen)+ salt of a novel monoanionic tridentate phosphino-
borate, tris(diphenylphosphinomethyl)phenylborate; reac-
tion of the ligand with SnCl2 produces tris(diphenylphosphi-
discerned in the 1H NMR spectrum at d 6.8–7.6 and 1.07,
respectively, as are the methyl and ethylene protons of the
Li(tmen)+ cation at d 2.18 and 2.30.
Anion
1
reacts with SnCl2 in CH2Cl2 to give
[PhB(CH2PPh2)3]SnCl 2, which was structurally character-
ized.‡ This is noteworthy in view of the small number of
structurally characterized tin(ii) phosphine complexes. The
recent classification and analysis of structural data for > 500 tin
coordination compounds6 reveals ca. 200 to contain divalent
tin. Of these, nitrogen and oxygen are the most common non-
halogen atoms found to bond with Sn(ii), with poly(pyr-
azolyl)borates figuring prominently within this classifica-
tion.7–9 However, structurally characterized monomeric
compounds containing direct ligation of phosphorus to tin are
few, and only a small number of these possess tin in its divalent
oxidation state.10–15 The solution of the single crystal X-ray
structure of 2 now adds to this abbreviated list.
The most striking feature of the ORTEP diagram shown in
Fig. 1 is that one of the phosphine arms of the ligand coordinates
the metal center only weakly. The bond distance between Sn(1)
and the two phosphorus atoms P(1) and P(3) are d[Sn(1)–P(3)]
= 2.6746(14) Å and d[Sn(1)–P(1)] = 2.690(2) Å, respectively.
These distances are slightly longer than those found in the
limited data set for Sn–P bond lengths.10,11,15–17 In contrast, the
weakly bonded phosphorus, P(2), is situated far from the metal,
d[Sn(1)–P(2)] = 3.036(2) Å, slightly beyond the standard
covalent bonding distances for tin and phosphorus.18 This
coordination of 2 is distinguished from the analogous tin(ii)
pyrazoylborate complex insofar as the long Sn–N bond is well
within the standard covalent bonding distance for a formal Sn–
N bond. Nevertheless, the distorted trigonal bipyramidal
coordination geometry about the metal, as is often observed for
tin(ii) pyrazolylborate complexes, is completed by a Sn(1)–
nomethyl) phenylboratotin(ii) chloride, which may be
3
dehalogenated to give the
h
compound, {[PhB-
(CH2PPh2)3]Sn}PF6,
tris(diphenylphosphinomethyl)-
phenylboratotin(ii) hexafluorophosphate.
By comparison to their neutral congeners,1 tridentate anionic
phosphines are rarely found within the coordination spheres of
metal ions, emerging as viable ligands only recently in the
preparation of water-soluble hydrogenation catalysts.2 In these
systems, an isolated anionic charge is introduced by sulfonyla-
tion of conventional phosphine ligands remote to the ligation
site. With the desire of adding new, anionic six-electron donor
ligands to the palette of coordination chemistry, we turned our
attention to strategies that would permit the incorporation of a
negative charge within the framework of a tridentate phosphine.
As amply demonstrated in pyrazoylborate chemistry3,4 and the
recent synthesis of [PhB(CH2SR)3]2 by Riordan and cowork-
ers,5 the borate bridgehead provides both the desired negative
charge and a scaffold to support three ligating arms with a
hybridization that sustains a fac conformation about a metal
center. We now report the synthesis and characterization of the
novel anion, tris(diphenylphosphinomethyl)phenylborate 1.
The ligand features a negative charge proximate to a highly
polarizable six-electron phosphine donor set. Within the context
of hard/soft treatments of acid/base properties, 1 should exhibit
a propensity to associate to soft metals. Along these lines, we
initially elaborate the chemistry of 1 with tin(ii), reporting the
X-ray characterization of tris(diphenylphosphinomethyl)phe-
nylboratotin(ii) chloride 2 and its structural changes upon
replacement of the coordinating chloride with an outer sphere
anion PF6.
Addition of diphenylphosphinomethide to dichlorophenyl-
borane yields a borate salt, which subsequently undergoes
substitution of its chlorides by two additional equivalents of
diphenylphosphinomethide to provide the tris(diphenylphos-
phinomethyl)phenylborate ligand in good yields (Scheme 1).†
Work up should follow immediately after the addition is
completed, as prolonged reaction times lead to lower isolated
yields. The crude compound is a pale yellow solid that can be
recovered in a purified form from CH2Cl2. The phenyl and
methylene resonances of the phosphinoborate anion are readily
Fig. 1. ORTEP representation and labeling scheme of 2 with thermal
ellipsoids drawn at the 35% probability level. Only the ipso carbons of the
phenyl rings on phosphorus are shown for clarity.
Scheme 1
Chem. Commun., 1999, 2379–2380
This journal is © The Royal Society of Chemistry 1999
2379

2
2
d 210.2 [q, J (11B–31P) 9.16 Hz], 11B{1H} NMR: d 215.45 [q, J (31P–
Cl(1) bond, which is long at 2.599(2) Å, and the stereochem-
ically active tin(ii) lone pair. The intraligand P(1)–Sn(1)–P(3)
bond angle of 81.76(4)° is markedly contracted relative to P–
Sn–Cl angles of 87.57(4) and 95.18(5)°. Finally, the local
geometry around the boron atom is approximately tetrahedral.
That all three phosphines are capable of bonding to the metal
is revealed by the solution 31P{1H} NMR spectrum of 2, which
shows only a single broad peak, centered at d 25.1. Similar
results are observed for anionic bidentate phosphine19 and
poly(pyrazolyl)borate7 complexes of tin(ii) as well. With one
coordination site of 2 occupied by chloride, the ligation of only
two phosphines is consistent with the stereochemical influence
exerted by the electron lone pair present on the tin(ii) center.
Accordingly, the removal of chloride from 2 would be expected
to open a coordination site, thus allowing the dangling
phosphine to strongly associate with the metal center. This
contention is supported by the reaction chemistry of 2 with
TlPF6. Reaction of 2 with one equivalent of thallium hexa-
fluorophosphate yields {[PhB(CH2PPh2)3]Sn}PF6 3. In contrast
to 2, the 31P{1H} NMR spectrum of 3 shows a single sharp
resonance at d +5.1 that is flanked by tin satellites. Coupling of
three equivalent phosphorus atoms to 117Sn (7.68%) and 119Sn
(8.59%) isotopes are clearly observed [1J(117/119Sn–31P) =
1330, 1270 Hz], consistent with the coordination of all three
phosphines to the metal center.
11B) 9.16 Hz)].
2: a 50-mL CH2Cl2 solution of 1 (0.4 g, 0.5 mmol) was added to a mol
equivalent of SnCl2 (0.094 g) dissolved in 10 mL of CH2Cl2. The resulting
solution was stirred overnight. A residue remained upon the vacuum
evaporation of CH2Cl2. The product was extracted away from unreacted
SnCl2 with benzene, and the resulting solution was filtered and concentrated
to a third of its original volume. Purified product was obtained by layering
the benzene filtrate with an equal volume of pentane. Compound 2 formed
over 2–3 days, after which the clear crystals were collected and dried (0.31
g, 74%). C45H41BClP3Sn; found: C, 64.01; H, 5.03; P, 10.98; requires: C,
64.37; H, 4.92; P, 11.07%. 1H NMR (C6D6): d 6.7–7.8 (m, 35H), 1.9 (br,
6H); 31P{1H} NMR (C6D6): d 25.1 (br); 11B{1H} NMR (C6D6): d 213.23
(br).
3: the chloride ligand was removed from 2 by dissolving 0.25 g (0.30
mmol) of the compound in 50 mL of acetonitrile, followed by the addition
of a mol equivalent (0.104 g) of TlPF6. The reaction mixture was stirred
overnight and then filtered to remove TlCl as a white solid. The filtrate was
removed in vacuo to afford the product, which was dried in vacuo overnight
(0.250 g, 89%). C45H41BF6P4Sn; found: C, 56.96; H, 4.55; P, 13.17;
requires: C, 56.94; H, 4.35; P, 13.05%. 1H NMR (CDCl3), d 6.7–7.8 (m,
35H), 1.9 (br, 6H); 31P{1H} NMR (CDCl3), d 5.1 [s, 1J(117/119Sn–31P)
1331.8, 1272.8 Hz]; d 2142.9 [sept,1J(31P–19F) 708.42 Hz]; 11B{1H}
NMR(CDCl3); d –11.9 (br).
‡ Crystal data for 2: C45H41BClP3Sn, M = 839.64, monoclinic, space
group P21/c, a
= 12.099(4), b = 19.699(4), c = 17.215(6), b =
103.818(10)°, U = 3984(2) Å3, Z = 4, Dc = 1.400 g cm23, T = 183(2) K,
m = 0.860 mm–1, wR2 = 0.0992 (5698 independent reflections), R1 =
0.0408 [I > 2s(I)]. CCDC 182/1450.
Considering the rich coordination chemistry of the related
tris(pyrazolyl)borate anion, a similarly diverse chemistry of 1
may be expected. The ligand is distinguished by its negative
charge and ability of 1 to adopt four- and six-electron
coordination modes about a highly polarizable metal center.
The combination of these properties in a singular ligand system
should find utility in the design of novel compounds and new
metal-based catalytic schemes. Along these lines, the recent
1 F. A. Cotton and B. Hong, Prog. Inorg. Chem., 1992, 40, 179.
2 C. Bianchini, P. Frediani and V. Sernau, Organometallics, 1995, 14,
5458.
3 S. Trofimenko, Acc. Chem. Res., 1971, 4, 17.
4 S. Trofimenko, Prog. Inorg. Chem., 1986, 34, 115.
5 P. J. Schebler, C. G. Riordan, I. A. Guzei and A. L. Rheingold, Inorg.
Chem., 1998, 37, 4754.
6 C. E. Holloway and M. Melnik, Main Group Met. Chem., 1998, 21,
371.
7 D. L. Reger, Synth. Lett., 1992, 469.
8 D. L. Reger, S. J. Knox, M. F. Huff, A. L. Rheingold and B. S. Haggerty,
Inorg. Chem., 1991, 30, 1754.
9 D. L. Reger, S. S. Mason, J. Takats, X W. Zhang, A. L. Rheingold and
B. S. Haggerty, Inorg. Chem., 1993, 32, 4345.
10 H. H. Karsch, A. Appelt and G. Müller, Organometallics, 1986, 5,
1664.
3
preparation of [PhB(CH2PPh2)3](H)Ir(h -C8H13) and its reac-
tion with H2SiMes2 to produce an iridium silylene20 illustrates
the unique reactivity engendered by this tridentate phosphino-
borate ligand.
The National Science Foundation Grant CHE-9817851
provides generous support for this research. We thank Jonas C.
Peters for useful discussions with regard to ligand design
strategies.
11 N. Froelich, P. B. Hitchcock, J. Hu, M. F. Lappert and J. R. Dilworth,
J. Chem. Soc., Dalton Trans., 1996, 1941.
Notes and references
12 U. Baumeister, H. Hartung, K. Jurkschat and A. Tzschach, J. Organo-
met. Chem., 1986, 304, 107.
13 A. L. Seligson and J. Arnold, J. Am. Chem. Soc., 1993, 115, 8214.
14 H. H. Karsch, A. Appelt and G. Müller, Angew. Chem., Int. Ed. Engl.,
1985, 24, 402.
15 M. Westerhausen, M. M. Enzelberger and W. Schwarz, J. Organomet.
Chem., 1995, 491, 83.
16 A. L. Balch and D. E. Oram, Organometallics, 1986, 5, 2159.
17 M. Driess, S. Martin, K. Merz, V. Pintchouk, H. Pritzkow, H.
Grützmacher and M. Kaupp, Angew. Chem., Int. Ed. Engl., 1997, 36,
1894.
18 A. H. Cowley, R. L. Geerts, C. M. Nunn and C. J. Carrano,
J. Organomet. Chem., 1988, 341, C27.
19 P. G. Harrison, Coord. Chem. Rev., 1990, 102, 234.
20 J. C. Peters, J. D. Feldman and T. D. Tilley, J. Am. Chem. Soc., in
press.
† Experimental procedures: reactions were carried out under the nitrogen
atmosphere of a dry box, which was capable of supporting a variety of
standard synthetic methodologies. Solvents were freshly prepared for
synthesis by their distillation from appropriate drying agents and by
subsequently degassing prior to use. 1H, 31P and 11B NMR spectra were
recorded on Varian Unity 300 and Mercury 300 spectrometers. Chemical
shifts for 1H, 31P, and 11B NMR are reported in ppm vs. TMS, H3PO4 (85%)
and BF3•Et2O, respectively.
1: [Li(tmen)]CH2PPh221 (3 g, 9.31 mmol), dissolved in 100 mL of THF,
was added slowly with stirring over 1.5 h to a 25 mL THF solution of
dichlorophenylborane (0.49 g, 3.1 mmol) maintained at 0 °C. The mixture
was stirred an additional 30 min whereupon the solvent was removed in
vacuo to leave an oily residue. The borate was dissolved in CH2Cl2 and the
solution was filtered to remove LiCl. Solvent removal gave 1 as a
microcrystalline solid and the product was dried in vacuo overnight (2.36 g,
82% yield). C51H57BLiN2P3; found: C, 75.5; H, 7.18; N, 3.66; P, 11.09;
requires: C, 75.75; H, 7.10; N, 3.46; P, 11.49%. 1H NMR (CD3CN): d
6.8–7.6 (m, 35H), 2.3 (s, 4H), 2.18 (s, 12H), 1.07 (br, 6H) 31P{1H} NMR:
21 N. E. Schore, L. S. Benner and B. E. Labelle, Inorg. Chem., 1981, 20,
3200.
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